Review
Nitroderivatives of Catechol: from Synthesis to Application
Kiril B. Gavazov
Department of General and Inorganic Chemistry, University of Plovdiv "Paisii Hilendatski", 24 Tsar Assen St.,
4000 Plovdiv, Bulgaria
* Corresponding author: E-mail: kgavazov@abv.bg
Received: 03-03-2011
Abstract
Nitroderivatives of catechol (NDCs) are reviewed with special emphasis on their complexes and applications. Binary, ternary and quaternary NDC complexes with more than 40 elements (aluminum, arsenic, boron, beryllium, calcium, cobalt, copper, iron, gallium, germanium, magnesium, manganese, molybdenum, niobium, rare earth elements, silicon, tin, strontium, technetium, thallium, titanium, uranium, vanadium, tungsten, zinc and zirconium) are discussed and the key characteristics of the developed analytical procedures - tabulated. The bibliography includes 206 references.
Keywords: Nitrocatechol, dinitrocatechol, complexes, spectrophotometric analysis, catabolic degradation, stability constants
1. Introduction
Nitroderivatives of catechol (NDCs) are important aromatic compounds, which contain two hydroxyl groups in ortho position and at least one nitro group directly attached to the benzene ring. NDCs belong to both the group of nitroaromatic compounds and the group of ortho diphenols. This interstitial position causes a certain inconvenience when searching for information in electronic and other databases because various names (systematic and non-systematic) have been used for these compounds
{e.g. 4-nitrocatechol could be found as 4-nitrobenzene-1,2-diol (systematic name), 4-nitropyrocatechol, 1,2-dihydroxy-4-nitrobenzene or 2-hydroxy-5-nitrophenol}. On the other hand, the data existing on NDCs are scattered in many research fields - analytical chemistry, biochemistry, coordination chemistry, environmental chemistry, enzymology, organic chemistry, pharmacology, etc. A number of review articles and books have been published concerning different aspects of nitroaromatic1-6 and phenolic compounds7-10, however, there has not been a review on NDCs. Information on NDCs (synthesis, physical and spectral characteristics, complex forming ability, partici-
Table 1. Simple NDCs in the present review
General formula
Names and abreviations
R,
R/i
R=
Ri
Mol.
mass
Melting point/ °C
3-nitrobenzene-1,2-diol	NO2 H H H 155.11	86-87 (3-nitrocatechol, 3-NC)
4-nitrobenzene-1,2-diol	H NO2 H H 155.11	174-176 (4-nitrocatechol, 4-NC)
3.4-dinitrobenzene-1,2-diol	NO2 NO2 H H 200.11	147-148 (3,4-dinitrocatechol, 3,4-DNC)
3.5-dinitrobenzene-1,2-diol	NO2 H NO2 H 200.11	166.8-167.1 (3,5-dinitrocatechol, 3,5-DNC) 2 2
4,5-dinitrobenzene-1,2-diol	H NO2 NO2 H 200.11	166.5-167.5 (4,5-dinitrocatechol, 4,5-DNC)
pation in biochemical events, applications as analytical reagents and therapeutic tools, etc.) is systematically presented here. Simple NDCs (Table 1) are considered in more details; they have been in scope of author's interest for a long time.
2. Synthesis and Production
Numerous chemical11-32 and bio-catalytic procedures33-35 for synthesis of NDCs have been described. Catechol was first nitrated by Benedikt11 using a nitration mixture of potassium nitrate and sulfuric acid. It was not recognized at first that two isomers, with different melting points, are formed: 3-NC and 4-NC. These isomers were separated by liquid-liquid extraction: 3-NC is readily soluble in petroleum ether, while 4-NC is soluble in wa-ter.12,13 A method of preparation of 3,4-DNC with 24% yield (with respect to dihydrate: 3,4-DNC.2H2O) was proposed by Rosenblatt at al.13 Several reliable methods for synthesis of the other dinitroderivative, 3,5-DNC, were reported in 1972.14-16 A higher yield (ca. 70%) was achieved by Nazarenko et al.16 Many researchers have been using this method in spite of the fact that 3,5-DNC is commercially available. 3-NC, 4,5-DNC, 4-methyl-5-NC, 4-NC and some 4-NC derivatives (4-NC sulfate, 4-NC sulfate dipotassium salt) are also commercial products. Chemical industries in several countries produce these reagents, and their price, in most cases, makes them accessible for any laboratory.
3. Participation in Catabolic Events
Simple NDCs such as 4-NC,35-49 3-NC,34 48 4-methyl-5-nitrocatechol,50-58 3-methyl-4-nitrocatechol58 and 4-nitropyrogallol59 have been detected in environmental, industrial or model systems as intermediates in biological41-59 and non-biological36-40 degradation of various nitrogen-containing compounds (nitrobenzene, 4-ni-trophenol, 2,4-dinitrotoluene, 2,6-dinitrotoluene, 2,6-dini-trophenol, azobenzene, etc.). In some cases, accumulation of NDCs (3-NC34, 4-NC35, 4-nitropyrogallol59) was observed. Some bacterial strains were isolated that could utilize 4-NC as a sole carbon, nitrogen and energy source: Rhodo-coccus wratislaviensis J360, Rhodococcus opacus J261, Pseudomonas cepacia RKJ20044 and Ochrobactrum sp. B2.62
4. Physical and Spectral Characteristics
NDCs are yellow crystalline compounds that are stable under ordinary conditions (3,4-DNC is reported somewhat hygroscopic13). They are readily soluble in various organic solvents. Some of them are soluble in water
(e.g. 4-NC, 3,4-DNC). The melting point and thermal stability of NDCs depends on the position of nitro group(s) in the catechol skeleton and the number of substituents.
3-NC,	in which OH groups and NO2 are located at neighbor carbon atoms, has the lowest melting point (Table 1). Information about the thermal behavior63 and standard molar enthalpies of combustion and sublimation64 of
4-NC	has been presented in the literature.
Acidic dissociation of aqueous NDCs has been investigated repeatedly.13,15,65-82 They are weak diprotic acids (Table 2). However, in comparison to catechol, these compounds are more acidic and less susceptible to oxidation.84 Hydrogen bonding between OH groups or between an OH group and a nitro group ortho to it has been discussed in the literature.12,75,80,84,85 Detailed information about the changes in the molecular structure of 4-NC, which take place during its deprotonation in aqueous solutions, was obtained by Cornard et al.84 The authors showed that the 4-NC ring has a pronounced aromatic character, while the ring of 4-nitrocatecholate monoanion has a quinoidal character. The C2-C3 and C5-C6 (see general formula in Table 1) bond lengths in the monoanion are significantly shorter than those of the other bonds in the benzene ring. C-N and C1-O bond lengths also become shorter. The two N-O bonds remain quasi identical but their lengths considerably increase upon first deprotonation. The second deprotonation intensifies the electronic redistribution and the benzene ring adopts a more marked quinoi-dal structure.
The spectral properties of 4-NC at different conditions correspond to the changes in the molecular structure mentioned above. Benedikt11, who first mentioned 4-NC, noted that this compound is yellow under acid conditions and red under alkaline conditions. Cooper and Tulane76 investigated in details the action of 4-NC as a titration indicator. They concluded that 4-NC could be useful for titrations of strong acids with strong bases and strong acids with weak bases, but cannot be used in titrations of weak acids or solutions containing much carbon dioxide. Absorption UV/Vis spectra of 4-NC at different conditions have been given in several papers.72,74,84-88 The spectrum in aqueous acidic solution consists of an intense broad band with two main components at 345 and 309 nm and another located at 238 nm. At higher pH a new band, which is characteristic of the mono-de-protonated form (^max = 426 nm), appears. The presence of isobestic points, at 280 and 372 nm, gives evidence of the equilibrium between the protonated and mono-de-protonated form. For pH values higher than 9, an oxidation of the catecholate function may lead to a decomposition of the molecule that prevents the observation of the doubly deprotonated form in the spectrum.84 The spectrum of this form could be recorded under inert at-mosphere.87 An absorption band with ^max at about 512 nm and isobestic point at 468 nm is characteristic for
Table 2. Literature acid dissociation constants
Compound	PKh2l	pkhl	Ionic strength/mol/l	Temp. /°C	Ref.
3-NC	6.65a, 6.59b	11.20a,11.17b	0.1 (KCl)	25	15
	6.47b	11.40b	0.1 (KCl)	25	65
	6.66b			25	13
4-NC	6.78a, 6.86b	10.64a, 10.71b	0.1 (KCl)	25	15
	6.7b	10.83b	0.1 (KCl)	25	66
	6.60a	10.74a	0.1 (KCl)	25	72
	6.70a	10.31a	1.0 (KCl)	25	72
	6.68b	10.70b	0.1 (KCl)	25	73
	6.62b	10.75b, 10.79a	0.1 (NaClO4)	25	74
	6.59b	10.75b	0.1 (KNO3)	30	75
	6.56a	11.33a	-	25	76
	6.88b, 6.74a	-	-	25	13
	6.84b	11.1b	0.1 (KNO3)	20	82
3,4-DNC	5.39b	8.27b		25	13
3,5-DNC	3.60a	9.83a	0.1 (KCl)	25	15
	3.54b	9.85b	0.1 (KCl)	25	15
	3.39b	9.69b	0.1 (KCl)	25	73
	3.37b	9.64b	0.1 (KCl)	25	81
Catechol	9.28b	13.02b	0.2 (KCl)	25	73, 83
a - obtained spectrophotometrically; b - obtained potentiometrically
Solvatochromism of 4-NC in a set of 27 common solvents, including water, was investigated by Riedel and Spange.89 The results showed that the longest-wavelenght UV/Vis absorption maximum is independent of the 4-NC concentration in the range 10-4 - 10-5 mol/L, what is an indication of lack of dye aggregation.
Raman spectra of 4-NC and its singly deprotonated form have been recorded.84,86,87 A calculation of the vibrational frequencies allowed a complete assignment of the Raman spectra of these two forms of 4-NC.84
Surprisingly, the structural and spectral information for the other NDCs found in the literature is scarce. Absorption spectra of 3,5-DNC in acidic21,90, neutral21 and alkaline21 solutions have been represented. The positions of the absorption maxima of 3-NC (300 and 400 nm), 3,4-DNC (350 nm) and 3,5-DNC (330 nm) at pH 5, 3 and 2 respectively have been reported.16
5. Complexes of NDCs and Their Analytical Application
Compounds containing a catechol moiety are well-known complex forming reagents.9,10 If attached to the catechol skeleton, a nitro group delocalizes n-electrons to satisfy its own charge deficiency. As a result, the colour, acid and complexing properties of the compound change dramatically. This may be of significance for the development of more powerful analytical procedures. In fact, two NDCs, 4-NC and 3,5-DNC, have been included in the IU-PAC list of the most important organic analytical reagents.91
It should be mentioned that NDCs chelate with many metal ions, showing relatively large stability constants for their lower basicity with respect to catechol and other catechol derivatives. This is presumably due to the resonance contribution of the nitro group to the stabilization of the chelate ring.75 However, the analytical applicability of NDCs usually do not correlate with the complex stability. For example, 3-NC forms more stable complexes than 4-NC and 3,5-DNC, but its analytical application is limited only to preparation of uranium adsorbents from seawater.92,93 Nazarenko et al.16 compared the molar ab-sorptivities (emax) of the Ge(IV) complexes with several NDCs. They found that e3_NC (3600 L mol-1 cm-1) is significantly lower than these of e3 5 DNC (11600 L mol-1 cm-1), e4_NC (19200 L mol-1 cm-1) and C34_DNC (20200 L mol-1 cm-1). This fact well explains the little application of 3-NC as reagent in spectrophotometric analysis. At the same time, the limited application of 3,4-DNC in the mentioned field is somewhat surprising. It could be attributed to difficulties which arise during its synthesis and purification and insufficient investigations on its ternary complexes.
The complexes of NDCs with more than 40 elements are described in Sections 5.1 - 5.29. The complex forming elements are listed in alphabetical order according to their chemical symbols; however, the Rare earth elements (RE) are given simultaneously (section 5.18). The application of some complexes in inorganic analysis is presented in Table 3.
When discussing the analytical application of NDCs, it should be mentioned that many enzymological studies94-101 are based on the Robinson, Smith and Wil-
liams method102, which utilizes potassium 2-hydroxy-5-nitrophenyl sulphate (nitrocatechol sulphate) as substrate, and follows the enzyme activity by the colorimetric estimation of the liberated red 4-NC anion (e510 = 1.26 x 104 L mol-1 cm-1).101,102
Another analytical application of a NDC (4,5-DNC), which is not based on complex formation, is the synthesis of the Hg(II)-selective ionophore with improved pH beha-
103
vior.
5. 1. Complexes of Al
Complex formation in aqueous solutions between Al(III) and some NDCs (H2L: 3-NC, 4-NC, 3,5-DNC) has been investigated by several authors.72,73,104-106 4-NC and 3-NC were found to form AlL+, AlL2
and AlL33- spe-
cies,104 105 while 3,5-DNC - only AlL2- and AlL33- species.73 The formation constants of the mentioned complexes (calculated at 25 °C and ionic strength 0.1 mol/L) are shown in Table 4. According to Downard et al.106 another complex is formed between Al(III) and 4-NC at pH 6-11: AlL2(OH)2-. This complex remains a minor species when a greater than 3-fold excess of 4-NC over Al(III) is maintained. In fact, the formation of AlL33- is almost quantitative at a pH close to 9 what allows Al(III) to be determined amperometrically in environmental samples106 (Table 3).
The complexation between Al(III) and 4-NC at pH = 5 was studied by Cornard et al.86 They used molecular spectroscopy combined with quantum chemical calculations. The formation of AlL(H2O)4+ and AlL2(H2O)2- was highlighted and a complete assignment of UV/Vis adsorption and Raman spectra of these complexes was proposed.
The adsorption of 4-NC onto Al(III)-bearing surfaces was investigated by Vasudevan and Stone.107 The stoichiometry of surface complex formation and the intrinsic equilibrium constants for adsorption onto Al2O3 were determined: (>SL-: log Ksintr = 14.47; >SL2-: log
Ks
: 6.58).1
Table 4. Complex formation constants for the major complex species in the aqueous Al(III) - NDC (H2L) system at 25 °C and 0.1 mol/L ionic strength obtained potentiometrically
Complexes		3-NC	4-NC	3,5-DNC
[AlL]+	logP1	= 14.7104	13.74105 13.75106 13.8973 13.372	
[AlLJ-	logP2	= 26.3104	25.39105	21.8073
			25.44106 26.3373 24.8272	
[AlL3]3-	logP3 :	= 35.81104	34.31105	31.6873
			34.38106 37.0873 33.7072	
7o.3
^ I
x §
w = -
mp
o c
C D N n
n An
abT
-
« H
3 SE
K
ft
13 =
<
hm^in co
3
Pu
-a c c M
<3 M U o
U &
m
.a &
u M C
Is
u -D
O
oj <3
o
CN
■ -a
U
13
O
> >
>
m
>
U + "
u
-5 jt (j OU ffi
o U U
a £
>
&
u + "
U
a
3 C
ra T3 -D
J? 13
S g ■
3 H
JS 'O S o
.13 <2
in
<3
S

eue c
O
oso o
ci
horn
ET
^ £
' .13 u^l U u>
""O U
C|5>
HH to ,
oo ci
* g
O &i
S3
-a
^ ,
<i «
il. .8 G %
U s
. . o
!" 8
® »S3
" ë -s
o
o
u o o
-a c
Q
W J3
o S
« M • bg ta «
< q,
uu^r
c-J csi-M
U U U
CGC
rarara >>>
zgzgzg
fH fH fH
OOO
-9
y
^ s < •
<3
^^ ^^ ^
lala'iî Jî.iî c
00Q
'fi ®
ta
s _ 's
M c
J3 ;Î3
O « 'S ^
.s
-
OOp^
IS
(U	<u
c	>
S	13
O	to
o	• J.:
.¡3	o 'a
r-
(M I
(M
O co
o
(M
.5	.0	.9	.5
8.	7.	CO	4.
ci	1	1 1 6 .5	3
8.	6.	2.	4.
<< „J
O J3
-9 "
& (U p &
%
a hJ
ITj J
O m
uu u
2r2r ^ ■4 4 4
m
pî
+
u
Z;
CO
'J3 'fi
<u <u
!3 ^
a a
P P
<u	<u
a a
«	«
pi	pi
hJ	HJ
hJ	HJ
H O b
m 2 + +
u u z;

in (M
\o
^OOOOH^
oininooin
•jg
<u
13
.a
a g
u ®
a
«
pi hJ hJ
u
H
H +
U Zi
3 8
-H U
ci ffi ^
1= ^^ B
o
B
13
.a a
o
.1	.7	.0	.0	.9
CO 1	6.	0.	7.	6. 1
	ci	.5	ci	4 .2
0.	0.	0.	0.	0.
.6	.0	.5	.4	.0
2. 1	-f .8	2. 1	CO 1	CO 1
«		.8		.5
1	1	1	2.	1
0	0	0	0	0
a.
'il 'il 'il 'il
<U (U
!3
<u <u
!3
0000
u
'J3
o .a a o .b
a a
p p
a a
p p
a
«
pi hJ hJ
O
m +
u z
Q
CO
<u	<u
a a
«	«
pi	pi
hJ	HJ
hJ	HJ
> H
h m + +
<u	<u
a a
«	«
pi	pi
hJ	HJ
hJ	HJ
H H
£
<u
13
"â	^
+3	«
&	O
«	-,¡3
pi	J3
J	J2
j	P^
h h
+ +	+
uuuu	U
zzzz	z
■'t-'t-'t-'t	4
m
pî
+
u z
CO

<u <u


I^I^I^IÎIÎIÎ
«
a
CD
^ s
(N M,
1
I
in c^
CD O
S
13
-Q
a
O
ft
«
pi J hJ
>
H +
U Z
z
o\
d
g
c n a b
b
<u
ane
*
e h lo cl
c h
lo
w
0 21
a a .a
o « .. m
ä s
m
.s »
-3
ic bi
r
o
m n
S
a £
o
c
«
-D

te
"l <3
C S
eo > -.0
iC K U
c
oe >
W £ J <3
ae
i § ft « < -a
1; o
oooo
M M M M
£ là o <
0 to ft £
.5 7.
0 3 6
.7
0.
-
2 .0 0.
O
<jy
.4 4.
-H > -H >
0 0 4
ic
tri
e
S
to ot
h ft o tr
lo
w
0 .0	.6	.2	.0
2.	2.		3.
0-2	.2-2	-2	-.5
.2 0.	0.	0.	0.
.8		.8	
1.	0	4.	5
.4-1	5.	.4-4	.7 4.
		4.	
c	c	c	c
a
to ot
h ft
o
e
ft
«
pj
J J
o
, mm
+ r/ +
Z®Z D lat D
>
b( N
n
tri et
S
to ot
h
ft g
ct
e
ft
«
pi J J
>
H +
C
-N 4-
> £
ri ri ri ri
ee
S
ee
s
oooo
ft ft
oo
ee ft ft
M «
ft ft
oo
ee
ft ft
« «
wwww wwww wwww
CCCC
ZZZZ
4- 4- 4- 4-
u ty
^ I
S * i a
ol io . at
O w
e r
» M ^ c b i
ae « O
M -o 2 1 13 C .8
r \ ^
d n & to b el Z ® , to
> Ä
h -S n, icat
, ift <u ft Ü <
« « . nn
.3 B
ot ot
fc h
ee
o c
Ji
io hi
ft ft < <
m o to ^ rt C/0
co > >
> £; voit O
05
3
ft >
A
. ; [ «
n
o
05 35 65
.3 .3 0. 0.
to to ft ft
¡3 ¡3
.3 2 1.2-2
ic
tri
e
I	§
II
O ft
^ o
8 ^
-Ö to ec n-
& o
to ',0
pi g W £ W W
O m
m Pi + +
CC NN
-D-D ,5- ,5-
3, 3,
>
^ >>
CC HH
C C nt nt
ee > >
CCCn C C C cat
nn
ee > >
£ e b,
Z
rr
o o
ooo
to to to l'y l ^^^ l-H
rrr OOO Z
.8 .5 2. 1.
0 0 4
-4
5 2 4
0 2 4
.2	6	.4		.3	.8	.1
3.	-1	7.		0. -1	8.	cK
.20.	0.	- .5 0.		.7 0.	-0.	-0.
		l				
						l
		o				C
		s			.6	H
		0	O	4	3.	"=;
		.4	Vi		-	
		0. -	25 K		.2	S
		5				.2
		.2				0.
		0.				
	c	c		c	c	c
	tric	tric		tric	tric	tric
	et	et		et	et	et
c	s	s		s	s	S
tri	to	to		to	to	to
et	ot	ot		ot	ot	ot
S	h	h		h	h	h
o	ft	ft		ft	ft	ft
ot	o	o		o	o	o
	tr	tr		tr	tr	tr
h	ct	ct		ct	ct	ct
ft	e	e		e	e	e
e	ft «	ft «		ft «	ft «	ft «
ftww
Q H + +
CCC
NNN QQQ
,5- ,5- ,5-
3, 3, 3,
www www www
H > b
H S + +
C
-N 4-
nt
+
C N
-D ,5-
3,

o	(N
3	■£
~	a
13	S
|	I
u	>n
Y	Tt
a	V
S
^	ä
<N	o

X E-h
^ IS T3 a

— ¿2 13 a
<D T Üw
> o 33
i	v
H
ffl	13 if H
o	^
^	in

•a I
Ü ffl
a
I
"S3
-c Ä &
o a
I
f-H	<N
z;	i u
s	H
13	PQ %
a	g
o
> îh
-Q
n
H3
ZS

5. 2. Complexes of As
As(V) reacts with 4-NC in the ratio of 1:3 to form a univalent complex anion, which can be extracted with a cationic dye into organic solvents.108 Nine catechol derivatives and four kinds of cationic dyes were examined. Among these reagents, 4-NC and brilliant green (BG) were found the most useful for determining trace amounts of As(V). The absorption maximum of the ternary complex in toluene is at 637 nm, and the molar absorptivity is 1.09 x 105 L mol-1 cm-1.108
The interaction between As(III) and 4-NC was investigated potentiometrically by Votava et al.109 The equilibrium constant for the equation H3AsO3 + H2L o As-L(OH)2- + H+ + H2O (where H2L = 4-NC) was determined to be pK = 9.49.
'a 5. 3. Complexes of B
Boric acid was reported to form chelates with 3-NC15, 4-NC,15'89'110-113 and 3,5-DNC.15 The equilibrium constants for the reaction B(OH)3 + H2L o BL(OH)2- + H+ + H2O (where H2L = 3-NC, 4-NC, and 3,5-DNC) were determined by spectrophotometry at an ionic strength of 0.1 mol/land 25 °C: pK = 3.46 ± 0.1, 3.76 ± 0.1 and 1.65 ± 0.12, respectively.15 Other authors reported slightly higher values for the reaction with 4-NC: 3.96 (20 °C, ionic strength 0.1 M)112 and 3.91 (25 °C, ionic strength 0.1 M).113 Pizer and Babcock110 investigated the mechanism of the complexation between boric or phenylboric acid and a series of catechol derivatives, including 4-NC. They found that the complexes of PhB(OH)2 have higher stability constants.
4-NC was used for spectrophotometric determination of boric acid.15 The sensitivity and selectivity of the proposed procedure was reported similar to that obtained with other diphenolic reagents (pyrocatechol violet, pyro-catecholocarboxilic acid, tiron), however, the color is stable for a long time (4 hours at pHopt = 8-8.5) and the optimum wavelength is in the visible range (440 nm).15
Riedel and Spange89 prepared a 1:2-complex, K[BL2], by mixing solutions of 4-NC, B(OH)3 and K2CO3. They studied the solvatochromism of the complex in 22 common solvents and noticed that hydrogen-bond donor solvents cause a hypsochromic shift of the UV/Vis bands, while increasing dipolarity/polarizability of the solvent induces a bathochromic shift.
5. 4. Complexes of Ba
Häkkinen114,115 reported the formation of two Ba2+ complexes with 4-NC (H2L), BaL and BaL22-, and one Ba2+ complex with 3-NC - BaL. The corresponding stability constants determined potentiometrically at 25 °C and an ionic strength of 0.1 mol/l (KCl) are as follows: logß1 = 2.6 and logß2 = 4.9 (for the complexes with 4-NC)114 and
logß1 = 2.71, (for the complex with 3-NC).115 The aqueous system Ba2+-3,5-DNC was investigated by the same author, however complex formation was not observed.116
5. 5. Complexes of Be
Complexation between Be(II) and NDCs (H2L:
3-NC115,	4-NC105, 3,5-DNC116) in aqueous solutions at 25 °C and an ionic strength of 0.1 mol/l (KCl) was investigated potentiometrically by Häkkinen. Formation of four complexes with 3-NC, two complexes with 4-NC and two complexes with 3,5-NC was reported. The following stability constants were calculated for these complexes: i) logß1 = 11.29, logß1h = 15.2, logß2 = 20.13 and logß2h = 25.0 (for the comp1exes of 3-NC: BeL, BeHL+, BeL22-and BeHL2-, respectively); ii) logß1 = 10.359 and logß2 = 18.273 (for the complexes of 4-NC: BeL, and BeL22-, respectively); and iii) logß1 = 8.49 and logß2 = 15.28 (for the complexes of 3,5-DNC: BeL, and BeL22-, respectively). It should be mentioned that 3,5-DNC has been regarded as a potential chelation agent for beryllium in medical and environmental systems.117 This reagent is considered capable of lowering beryllium tissue burden and environmental remediation of beryllium-contaminated environments, but may possess toxicity that would be of concern based on known data of similar nitrophenols.117
5. 6. Complexes of Ca
Ca2+ forms in aqueous medium two chelates with each of the reagents (H2L) 3-NC115, 4-NC114 and 3,5-DNC116: CaL and CaL22-. The values of their stability constants, determined at 25 °C and an ionic strength of 0.1 mol/l (KCl) are as follows: logß1 = 4.42 and logß2 = 7.5 (for the complexes of 3-NC); logß1 = 3.79 and logß2 = 7.13 (for the complexes of 4-NC); and logß1 = 3.21 and logß2 = 5.88 (for the complexes of 3,5-NC).
5. 7. Complexes of Cd
Häkkinen104,105 investigated the complex formation in the aqueous systems Cd2+ - 3-NC and Cd2+ - 4-NC. He detected CdL and CdL22- in both systems. Their stability constants were determined potentiometrically at 25 °C and an ionic strength of 0.1 mol/l (KCl): logß1 = 6.73 and logß2 = 11.87 (for the system containing 3-NC)104, and logß1 = 6.495 and logß2 = 11.279 (for the system containing 4-NC).105 Another complex, Cd(OH)L, with stability constant logß = 1.07, was detected in the system containing 3-NC.104
5. 8. Complexes of Co
Co(II) was reported to form chelates with 3-NC104,
4-NC75,118,	and 3,5-DNC81, which could be expressed with the general formulae CoL, CoL22- and CoL34- (NDC =
H2L). The values of their formation constants are shown in Table 5.
Tyson and Martell119 studied Co(II) - 4-NC catalyzed oxidation of 3,5-di-tert-butylcatechol (3,5-DTBP) to corresponding o-quinone (3,5-DTBQ): 3,5-DTBP + 0.502 ^ 3,5-DTBQ + H20. This reaction seems to be important, since it is one of the few non-enzymic reactions to give o-quinone without H202 accumulation. The absence of H202 in the above mentioned equation could be explained with the ability of Co(II) - 4-NC to catalyze its decompo-sition.119
Table 5. Complexes of Co(II) with NDC (H2L: 3-NC, 4-NC, 3,5-DNC) in aqueous solutions and their formation constants obtained potentiometrically
Complexes		3-NC	4-NC	3,5-DNC
[CoL]	logßi =	3.84a 104	7.48a 118, 7.48b 75	6.43a 81
[CoL2]2-	logß2 =	13.58a 104	12.72a 118, 12.79b 75	11.21a 81
[CoL3]4-	logß3 =	16.6a 104	15.93b 75	14.49a 81
a - at 25 °C and 0.1 mol/l ionic strength (KCl) b - at 30 °C and 0.10 mol/l ionic strength (KN03)
Table 6. Complexes of Cu(II) with NDC (H2L: 3-NC, 4-NC, 3,5-DNC) in aqueous solutions and their formation constants obtained potentiometrically
Complexes		3-NC	4-NC	3,5-DNC
[CuL] [CuL2]2-	logß1 = logß2 =	12.03a 65 22.33a 65	11.70a 66, 11.65b 75 21.10a 66, 20,93b 75	10.04a 81 17.8a 81
a - at 25 °C and 0.1 mol/l ionic strength (KCl) b - at 30 °C and 0.10 mol/l ionic strength (KN03)
5. 9. Complexes of Cu
Cu(II) forms in aqueous medium two chelates with each of the reagents 3-NC65, 4-NC6675 and 3,5-DNC81: CuL and CuL22-. The values of their formation constants are presented in Table 6. The intensive yellow coloration of the complex with 4-NC was used for determination of 4-NC in the presence of similar compounds, such as 4-ni-troanisole and 4-nitrophenol.120 Cu(II) forms also a strongly colored complex with 2-hydroxy-5-methyl-4-ni-trophenol (NMC).32 The complex formation was found independent of pH between 5.4 and 7.2. Since some enzymes (e.g. aryl-sulfate sulfohydrolase from the New Zealand mollusk) are not inhibited by low concentrations of cupric ion, the enzymatic hydrolysis of sulfate esters of NMC could be monitored continuously in the presence of cupric ions by following the formation of the yellow spe-cies.32 A direct-colouring metal precipitation method for demonstration of arylsulphatases A and B based on the reducing capacity of 4-NC was proposed by Partanen.121 In this method, 4-NC (liberated from 4-nitrocatechol sulpha-
te) reduces ferricyanide to ferrocyanide, which in turn forms a brown precipitate with copper that indicates the enzyme activity.
Several ternary complexes, which contain Cu(II) and 4-NC, were studied as well. The additional reagents used are ethylenediaminediacetic acid122, amino acids123, N,N,N',N'-tetrabenzylethylenediamine and N,N,N',N'-tetramethylethylenenediamine.124
5. 10. Complexes of Fe
3-NC115 and 4-NC66,72,125 were reported to form two and three complexes with Fe(III), respectively. Their formulae and formation constants are presented in Table 7.
Table 7. Complexes of Fe(III) with NDC (H2L: 3-NC, 4-NC) in aqueous solutions and their formation constants determined at 25 °C and 0.1 mol/l ionic strength
Complexes		3-NC	4-NC
[FeL]+	logPi =	15.71a 115	15.53a 66, 16.95b 72
[FeLJ-	logP2 =	28.92a 115	28.63a66, 29.78b 72
[FeLJ3-	logP3 =	-	38.22a 66, 39.19b 72, 40.00a 125
a - obtained by potentiometry
b - obtained by spectrophotometry and potentiometry
The interaction between Fe(III)-bearing surfaces and 4-NC were observed by Vasudevan and Stone.107 They determined the stoichiometry of surface complex formation and the intrinsic equilibrium constants for adsorption onto hematite (>SL-: log Ksintr = 13.87) and goe-tite (>SL-: log Ksmtr = 14.02; >SL2-: log Ksmtr = 7.12).
High-spin tris(nitrocatecholato)ferrate(III) complexes were synthesized and studied by Kawabata et al.126 with three 3-NC derivative antioxidants containing the following substituents in position 5: -CH = CR2, -CH2CHR2, and -CH2CR'(R)". The authors showed that nitrocatechols with a conjugation structure could sequester the chelated iron more effectively than catechol and derivatives without the conjugation.
4-NC was examined as an active site probe for non-heme iron dioxygenases.127 Several studies were concerned with complexes of Fe(III)-soybean lipoxygenases and 4-NC.128-130 Fe(III)-soybean lipoxygenase-1 yields with 4-NC a green colored 1:1 complex, which shows at pH 7.0 absorption maxima at 385 nm and 650 nm.128 The structure of soybean lipoxygenase-3 in complex with 4-NC was studied by Skrzypczak-Jankun et al.130 X-ray analysis showed 4-NC near iron with partial occupancy, blocking access to Fe but not covalently bound to it.
5. 11. Complexes of Ga
Ga(III) is known to form complexes with 4-NC131, 3,4-DNC132, 3,5-DNC132 and the nitrocatechol antioxi-
dants mentioned in the previous subsection.126 The equilibrium and kinetics for the reaction of excess Ga(III) with 4-NC (H2L) to give monochelate in aqueous solution: Ga3+ + H2L o GaL+ +2H+ was investigated over the pH range 2-3. The complex has an absorption maximum at X = 405 nm (e = 9 x 103 L mol1 cm1); however, the reaction was monitored at X = 420 nm, where the contribution of free 4-NC is less than 5%.131
Ternary ion-association complexes between Ga(III), dinitrocatechol (3,4-DNC or 3,5-DNC) and brilliant green (BG) were studied by Nazarenko et al.132 Molar absorptivity coefficients of the complexes extracted in toluene were calculated: e650 = 8 x 104 1 mol1 cm1 (for the complex with 3,4-DNC) and e650 = 7 x 104 l mol1 cm1 (for the complex with 3,5-DNC). 132 The molar ratio between the components in both complexes was found to be Ga:DNC:BG = 1:3:3.
5. 12. Complexes of Ge
Complexation between Ge(IV) and NDCs has been studied by many authors. Binary and ternary complexes with participation of 3-NC16,133, 4-NC16,67,111,133-137, 3,4-DNC16,138 and 3,5-DNC16,139 were reported and several procedures for Ge(IV) determination were propo-sed16,133,135-137,139 (Table 3). The most sensitive germanium determination was achieved with 3,5-DNC and brilliant green (e625 = 1.41 x 1051 mol-1 cm-1). However, Ge should be preliminary separated from interfering ions by LLE-extraction into CCl4 and reextraction into water.16,139 The complex has a molar ratio between the components 1:3:2 (Ge:3,5-NC:BG). All other studied ternary complexes have similar composition and could be represented with the general formula (OC+)2[Ge(NDC)3]2- (where OC+ is orga-nophilic cation, which derives from tetrazolium salt or basic dye). Some equilibrium constants (association constant - P, distribution constant - KD, extraction constant -Kex) were calculated for the LLE systems with participation of tetrazolium salts: log P = 9.6, log KD = 1.52 and log Kex = 11.12 (for the Ge(IV) - 4-NC - 2,3,5-triphenyl-tetrazolium chloride - water - chloroform system)137, and log P = 9.04, log Kd = 1.11 and log Kex = 11.15 (for the Ge(IV) - 4-NC - 3D-(4,5-dimethyl-2-tehxiazolyl)-2,5-dip-henyltetrazolium bromide - water - chloroform sys-tem).136
5. 13. Complexes of Mg
Hakkinen114-116 investigated potentiometrically the complex formation in aqueous solutions between Mg2+ and NDCs (H2L: 3-NC, 4-NC and 3,5-DNC). He detected the complexes MgL and MgL22- in all studied systems. Their stability constants were calculated to be log P1 = 5.72 and log P2 = 9.77 (for the system containing 3-NC)115, log P1 = 5.21 and log P2 = 8.85 (for the system containing 4-NC)114 and P1 = 4.53 and log P2 = 7.71 (for
the system containing 3,5-NC)116. Another complex, Mg(OH)L, with log ß = 5.8 was found in the system containing 3,5-DNC.116 All measurements were carried out at 25 °C and an ionic strength of 0.1(KCl).
5. 14. Complexes of Mn
Häkkinen65 showed that Mn(II) forms three complexes with 3-NC in aqueous solutions (MnL, MnL22- and MnHL). Their stability constants were calculated to be log ß1 = 7.22, log ß2 = 12.5 and log ß1h = 13.3 (25 °C, 0.1 mol/l KCl ionic strength).
The interaction between Mn(II) and 4-NC has been investigated by several authors.75,118,119,140 Species with 1:1 and 1:2 Mn(II) to 4-NC molar ratios were detected by Murakami et al.75 and Häkkinen.118 They reported the following values of their stability constants: log ß1 = 6.51 and log ß2 = 11.25 (30 °C, 0.10 mol/l KNO3 ionic strength)75, and log ß1 = 6.83 and log ß2 = 11.72 (25 °C, 0.1 mol/l KCl ionic strength).118 Tyson and Martell119 showed that 1:1-complex species have catalytic effect on the oxidation of catechol to o-benzoquinone and 3,5-di-tert-butylcatechol (3,5-DTBP) to corresponding o-quinones. In order to throw more light on the differences in 4-NC binding to some enzymes, such as extradiol-cleaving ca-techol dioxygenases, Reynolds et al. synthesized and compared monoanionic and dianionic 4-NC complexes of Mn(II) and Fe(II).140
soluble in water and easily extracted into organic solvents. Equilibrium constants which characterize the extraction process (extraction constant - Kex, distribution constant -Kd and association constant - P) were calculated by Dimi-trov et al.143-147 Their values are included in Table 8. Molar absorptivity coefficients of these complexes are in the range (1.94-2.41) x 104 L mol-1 cm-1 and they were successfully applied for LLE-spectrophotometric determination of Mo(VI) in ferrous metallurgy products (steels, fer-romolybdenum). A more sensitive flotation-spectrophoto-metric procedure (e = 2.1 x 105 L mol-1 cm-1) was developed for the determination of Mo(VI) in biological mate-rials.148 It is based on the formation of an ion-associate between the 3,5-DNC - Mo(VI) anionic chelate and rhodamine B: (RB+)2[MoO2(3,5-DNC)2]. The procedure was reported to be specific after a preliminary separation of molybdenum by its extraction as the a-benzoinoxime complex from 2 mol/l HCl.148149
Preparation and spectroscopic characterization were reported for monooxomolybdenum(VI) complex, MoO(4-NC)(DEASNBH), where DEASNBH2- = dianion of N-4-diethylaminosalicylidene-N'-4-nitrobenzoyl hydra-zine. The compound was prepared by replacement of an oxo group on MoO2(DEASNBH) with 4-NC. The complex exhibits a Mo = O stretching vibration (vMo = O) at 935-938 cm-1 and a series of five absorptions in the UV-Vis region including a long wave-length band at 655-658 nm which is attributed to 4-NC ^ Mo charge transfer.150
5. 15. Complexes of Mo
Mo(VI) is known to form colored 1:1 and 1:2 complex species with 4-NC (H2L), which could be represented with the following formulae141142: MoO2L, MoO(OH)L+, MoO2(OH)2L2- and MoO2L22-. The formation constant of MoO2L22-, which is the complex of analytical importance, was found to be log P2 = 22.8 ± 0.8 at 25 °C and 0.100 mol/l KCl ionic strength.142 Several ion-association complexes with participation of Mo(VI) - 4-NC anionic chelates and tetrazolium cations were studied in solutions143-147 and solid state.63 These complexes are slightly
5. 16. Complexes of Nb
Nb(V) forms colored anionic species with NDCs (4-NC and 3,5-DNC) with an Nb(V)-to-NDC ratio of 1:2 or 1:3, which can be associated with heavy organic cations such as tetrazolium cations151-157 and basic dyes.149,158 Several slightly soluble in water ternary or quaternary complexes with analytical potential were obtained. They were extracted into organic solvents151-157 or floated with cyclo-hexane.149,158 The highest sensitivity (e555 = 2.1 x 105 L mol-1 cm-1) was achieved in the last case. The following formula of the floated compound158 was suggested:
Table 8. Equilibrium constants (extraction constant - Kex, distribution constant - KD and association constant - P) for LLE-systems involving Mo(VI), 4-NC (H2L) and tetrazolium halides.
Suggested formula	Organic	Equilibrium constants			Ref.
	solvent	log Kex	log Kd	log ß	
(MTT+)2[MoO2L2]2-	CHCl3	9.6	1.12	8.5	146
(TT+)[MoO2(OH)L2]-	CHCl3	5.2	0.88	4.3	143
(INT+)2[MoO2L2]2-	C2H4Cl2	10.0	0.47	9.5	145
(TV+yMoO^J2-	C2H4Cl2	8.5	1.75	6.7	144
(BT^MoO^J2-	C2H4Cl2	5.4	-	-	147
Abbreviations: MTT+ - 3-(4,5-Dimethyl-2-thiazol)-2,5-diphenyl-2H-tetrazolium; TT - 2,3,5-Trip-henyl-2H-tetrazolium; INT - 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2H-tetrazolium; TV+ -3-(2-Naphtyl)-2,5-diphenyl-2H-tetrazolium; BT2+ - 3,3'-[3,3'-Dimetoxy(1,1'-biphenyl)-4,4'-diyl]-bis [2,5-diphenyl-2H-tetrazolium]
Table 9. Equilibrium constants (extraction constant - Kex, distribution constant - KD and association constant - P) for LLE-systems involving Nb(V), NDC (4-NC, 3,5-DNC) and tetrazolium halides
Suggested formula	Organic	Equilibrium constants			Ref.
	solvent	log Kex	log Kd	log P	
(TT+)[NbO(4-NC)2]-	CHCl3	3.25	-	-	151
(INT+)[NbO(4-NC)2]-	C2H4Cl2	3.71	0.73	3.12	152
(TV+)[NbO(4-NC)2]-	C2H4Cl2	4.53	1.61	3.01	153,154
(MTT+)3[NbO(4-NC)3]3-	CHCl3	14.0	0.82	13.2	157
(BT2+)3[NbO(4-NC)3]23-	CHCl3	8.3	0.54	7.8	156
(TT+^NbO^^P^-DNC)/-	CHCl3	9.9	1.02	8.9	155,156
Abbreviations: TT+ - 2,3,5-Triphenyl-2H-tetrazolium; INT+ - 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2H-tetrazolium; TV+ - 3-(2-Naphtyl)-2,5-diphenyl-2H-tetrazolium; MTT+ - 3-(4,5-Di-methyl-2-thiazol)-2,5-diphenyl-2H-tetrazolium; BT2+ - 3,3'-[3,3'-Dimetoxy(1,1'-biphenyl)-4,4'-diyl]-bis[2,5-diphenyl-2H-tetrazolium]
(RB+)4[(NbO)2C2O4(3,5-DNC)2] (where RB means rhodamine B). A procedure for the determination of Nb(V) in geological samples based on this complex was described.158
The molar absorptivity coefficients for the complexes with participation of tetrazolium cations154-157 are lower and may vary significantly (from e = 1.59 x 104 to e = 5.6 x 104 L mol-1 cm-1). The most sensitive procedures are based on the complexes with formulae (MTT+)3[NbO(4-NC)3] and (BT2+)3[NbO(4-NC)3]2, where MTT+ and BT2+ are the cations which derive from 3-(4,5-dimethyl-2-thia-zol)-2,5-diphenyltetrazolium bromide157 and 3,3'-[3,3'-dimetoxy(1,1'-biphenyl)-4,4'-diyl] -bis[2,5-diphenyl-2H-tetrazolium] chloride156, respectively. The 4-NC-to-Nb molar ratio in these complexes (3:1) is higher than that reported for other complexes with similar reagents (2:1) (Table 9). This is the key reason for the 2-3-fold higher molar absorptivity observed.156,157 Another reason (for the first complex only) is the ability of MTT+ to absorb light in the same spectral range where [NbO(4-NC)3]3- absorbs.157 Some equilibrium constants, which characterize the extraction of tetrazolium ion-association complexes, are shown in Table 9.
5. 17. Complexes of Ni
The complex formation between Ni2+ and the following NDCs was investigated in aqueous solutions: 3-NC104, 4-NC75118 or 3,5-DNC.81 Complexes with formulae NiL, NiL22- and NiL24- were detected potentiometrically. The values of their stability constants are shown in Table 10.
Table 10. Complexes of Ni(II) with NDCs (H2L: 3-NC, 4-NC, 3,5-DNC) in aqueous solutions and their formation constants obtained potentiometrically
Complexes		3-NC	4-NC	3,5-DNC
[NiL]	logP1 =	8.12a 104	7.2a 118, 7.82b 75	6.84a 81
[NiL/	logP2 =	13.9a 104	13.2a 118, 13.9b 75	11.94a 81
[NiLJ4	logP3 =	17.4a 104	16.9b 79	15.3a 81
a - at 25 °C and 0.1 mol/L ionic strength (KCl) b - at 30 °C and 0.10 mol/L ionic strength (KNO3)
5. 18. Complexes of Rare Earth Elements
The complex formation between rare earth ions RE(III) and NDCs (4-NC69,70,159 and 3,5-DNC160) has been studied by potentiometry69,70,159 and/or spectrophotometry.69,159,160 Bhuyan and Dubey70 determined some RE(III) - 4-NC formation constants at 25 °C and an ionic strength of 0.1, 0.15 and 0.2 mol/L (KNO3). Their ther-modynamic stability order was reported to be La(III) < Ce(III) = Pr(III) < Nd(III) < Sm(III) < Gd(III) < Y(III) < Tb(III) < Dy(III) < Ho(III). The complexes of Eu(III) and Lu(III) with the same reagent (4-NC) were investigated by Zhu et al.69 In contrast to the complexes of catechol, which have RE(III)-to-L ratios 1:1, 1:2 or 1:3 (at pH = 8.0, 10.0, and 12.0, respectively), the complexes of 4-NC contain only 1.5 catechol groups per Eu(III) and Lu(III).69 4-NC complexes of Sc(III) and Y(III) were investigated by Turkel et al.159 The following stability constants were reported at 25 °C and ionic strength 0.1 mol/L KNO3 (for the complexes of Sc) or NaClO4 (for the complexes of Y): log p1 = 13.58 (ScL+), log p2 = 27.35 (ScL2-), log p3 = 40.14 (ScL33-), log p1 = 9.83 (YL+), log p2 = 28.39 (YL2-) and log P3h = 24.39 (YHL32-).
The interaction in water-organic solvent medium between RE(III), 3,5-DNC and some basic dyes (methylene blue, Nile blue, malachite green, brilliant green, methyl
A,%
La Ce Pi Nd Pm Sm Eu Gd Tb Dy Ho Er Tu Yb Lu Y
Figure 1 Relative absorbance (ARE(III)-3_5-DNC-RB
X 100/ADy(nI)-3_5-
DNC-RB) vs. the nature of RE.160 CRE(III) = 2 X 10- mol/L, C35DNC = 1 x 10-4 mol/L, CRB = 5 X 10-5 mol/L, pH = 6.3, % = 553-5665.
violet, rhodamine B and rhodamine G) were investigated by Poluektov et al.160 The best extraction-spectrophoto-metric characteristics were obtained with rhodamine B and mixed organic solvent (benzene:izobutanol (9:1)). The molar ratio between the components was found to be RE : 3,5-DNC : RB = 1 : 4 : 1 {suggested formula (RB+) [RE(HL)4]}. The relative absorbance at X = 553-565 nm
(ARE(III)-3,5-DNC-RB X 100/ADy(III)-3,5-DNC-RB) vs. the nature of
RE is plotted in Fig. 1. It can be seen that La(III) and some other RE do not interfere at the defined optimum conditions on the determination of Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Yb(III) and Lu(III). On this basis an LLE-spectrophotometric procedure for determination of Gd(III) in the presence of La(III) was developed.160
5. 19. Complexes of Si
Quantitative data for the reaction 3H2L + Si(OH)4 o SiL32-+4H2O (where H2L is 4-NC, 3,4-DNC, catechol, or 4,5-dichlorocatechol) in aqueous solution were obtained by 1H and 29Si NMR spectroscopy.161 Mixed complexes containing catechol and 4-NC or catechol and 4,5-dichlorocatechol were obtained as well.161
Riedel and Spange89 prepared several silicates containing 4-NC ligands in order to investigate their solvatoc-hromism: sodium tris(4-nitrobenzene-1,2-diolato)silicate, tetra-n-butylammonium tris(4-nitrobenzene-1,2-diola-to)silkate, pyrrolidinium tris(4-nitrobenzene-1,2-diola-to)silicate, (3-amino-1-propyl)-bis(4-nitrobenzene-1,2-diolato)silicate and (N,N-diethyl-3-amino-1-propyl)-bis(4-nitrobenzene-1,2-diolato)silicate. The authors showed that the solvent-induced UV/Vis band shift of the negatively charged moiety of all solvatochromic dyes studied is mainly a function of the hydrogen-bond donor strength and the dipolarity/polarizability of the solvent. Hydrogen-bond donor solvents cause a hypsochromic shift of the UV/Vis band due to specific solvation of the anion site. Inversely, increasing dipolarity/polarizability of the solvent induces a bathochromic shift of the UV/Vis absorption band.89
5. 20. Complexes of Sn
The reaction between molar equivalents of dibutyl-tin(IV) oxide and some catechol derivatives (including 4-NC) were reported to give monochelated dibutyltin(IV) catecholates of the general formula Bu2SnL.162 Results of a lethality bioassay on the brine shrimp indicated that all the complexes have biological activity.162
The complex formation in aqueous medium between Sn(IV) and 3,5-DNC was investigated by Nazarenko et al.90 Only one complex, H2[SnL3], is formed in slightly acidic solution (pHopt = 2), independent from the H2L-to-Sn(IV) concentration ratio. The absorption maximum of the complex is situated at 360 nm, but a more convenient wavelength for the measurements was X = 400 nm. At this
wavelength the blank absorbed insignificantly and the molar absorptivity was satisfactory (e400 = 8.6 x 103 L mol-1 cm-1).90
A much more sensitive procedure for the determination of Sn(IV) (e630 = 1.75 x 105 L mol-1 cm-1) was proposed by the same authors.90,163 The procedure is based on the extraction of a ternary complex with 3,5-DNC (H2L) and brilliant green, (BG+)2[SnL3], into CCl4. The following other combinations of basic dye and organic solvent were tested during the optimization: methyl violet - C6H6, malachite green - CCl4, Nile blue A - C6H6, methylene blue - CHCl3, basic blue - C6H6, phenosafranine -CHCl3, rhodamine B - C6H6, and rhodamine 6G - C6H6. However, the molar absorptivity in all mentioned cases was smaller (up to 1.3 x 105 L mol-1 cm-1).90
5. 21. Complexes of Sr
The complex formation in the aqueous system Sr2+ -NDC, where NDCs (H2L) are 3-NC,115 4-NC114 and 3,5-DNC116 was investigated potentiometrically at 25 °C and 0.1 mol/L ionic strength (KCl) by Häkkinen. All mentioned reagents form complexes with a general formula SrL. Their formation constants were calculated to be log ß1 = 3.14,115 2.81114 and 2.28,116 respectively. 4-NC and 3,5-DNC form complexes with a formula SrL22- as well. The formation constants for these complexes were calculated to be log ß2 = 5.18114 and 4.54,116 respectively.
5. 22. Complexes of Tc
Tetra-n-butylammonium bis(4-nitro-1,2-benzene-diolato)oxotechnetate(V) was synthesized and its structure reported by Rochon et al.164 The geometry around the Tc atom is square pyramidal with a short Tc-O(oxo) bond in the apical position. The Tc atom lies out of the plane of the four diolato O atoms. The nitro groups are in the same planes as the catecholato ligands. The NO2 of one ligand is disordered.
5. 23. Complexes of Tl
Intensively colored ternary ion-association complexes with composition 1:2:1 are formed in the system Tl(III) - catechol derivative - basic dye.165,166 The complexes with NDCs were found less appropriate for analytical applications than the analogical complexes with tetrabro-mocatechol or catechol. The last mentioned reagent was applied for the determination of Tl(III) in alkaline salts.165
5. 24. Complexes of Ti
Complex formation in LLE systems containing Ti(IV), catechol derivative (3,5-DNC, tetrabromocatechol and tetrachlorocatechol) and basic dye (methylene blue, basic blue, gallocianine, Nile blue A, phenosafranine, bril-
liant green, Victoria blue 4R, malachite green, methyl violet) was examined by Nazarenko et al.167 The complex with 3,5-DNC and brilliant green, (BG)2[TiOL2], was reported to have the highest molar absorptivity (e640 = 2 x 105 L mol-1 cm1 in CCl4). However, tetrachlorocatechol was chosen for the development of a procedure for Ti(IV) determination in real samples.167
Two ternary ion-association complexes of Ti(IV)-4-NC anionic chelate, [TiOL2]2-, and tetrazolium cations with general formula (TS+)2[TiOL2] were studied by Ko-stova et al.168 The key equilibrium constants characterizing their extraction from water into chloroform were calculated: log Kex = 7.4, log P = 6.6 and log KD = 0.74 {for the complex with 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2H-tetrazolium}, and log Kex = 12.1, log P = 10.0 and log KD = 1.9 {for the complex with 3-(2-naphtyl)-2,5-diphenyl-2H-tetrazolium}. The associate with the second cation has better characteristics and was used for LLE determination of Ti in aluminium alloys (Table 3).168
Strong adsorption of 4-NC onto Ti(IV)-bearing surfaces was observed by Vasudevan and Stone.107 They determined the stoichiometry of surface complex formation and the intrinsic equilibrium constants for adsorption onto three Ti(IV) oxides. Araujo et al.169 compared the stability of the surface complexes of several ligands (4-NC, 2,3-dihydroxynaphtalene, catechol, gallic acid and 4-chloroca-techol) with titanium dioxide in aqueous suspensions. They found that 4-NC and 2,3-dihydroxynaphtalene form the most stable complexes, probably because of the solva-tion contribution to the overall Gibbs adsorption energy.169
5. 25. Complexes of U
The complex formation in aqueous solutions between UO22+ and 4-NC (H2L) was investigated potentio-metrically by Bartusek.82 UO2L , UO2L22- and UO2L2H-were detected in the system at different conditions. The stability constants of the first two species were calculated to be logP1 = 12.9 and logP2 = 22.7, respectively. The acidic dissociation constant of UO2L2H- was found as well (pK = 4.97). All measurements were carried out at 20 °C and an ionic strength of 0.1 mol/l (KNO3).
5. 26. Complexes of V
Yellow species are formed at mixing aqueous solutions of V(IV) or V(V) and NDCs (4-NC, 3,4-DNC or 3,5-
DNC).
The absorption maximum of the complex of
V(V) and 3,5-DNC is at 428 nm (pH = 1-3)171. The complexes of V(V) or V(IV) with 4-NC have absorption maxima at 405-410 nm in the pH region between 2 and 5.172 174 The molar V(IV)-to-4-NC ratio was determined to be 1:1.173 The complex is nonextractable in chloroform, but well extractable into n-butanol.174
Several ternary ion-associated complexes containing the mentioned NDCs and organophilic cations (deriving
from tetrazolium salts,172183 basic dyes171,184 and diphe-niylguanidinium chloride170) have been characterized. The complexes of V(V), 3,5-DNC and basic dyes were investigated by Marczenko and Lobinski.171,184 The complexes with brilliant green, (BG+)2[VO(OH)L2], and rhodamine B, (RB +)2[VO(OH)L2], have the highest analytical potential. The first one was utilized for LLE-spectrophotometric determination of traces of vanadium (about 10-5%) in alums and the second one for flotation-spectrophotometric determination of vanadium in vegetables.184
The complexes of 4-NC and tetrazolium cations are also of analytical importance. They were applied for the determination of total vanadium in steels172,179,183 and catalysts183, as well as for vanadium(IV/V) speciation in synthetic mixtures and industrial samples with low Fe and Al content.182 The speciation V(IV/V) analysis with 3-(4,5-dimethyl-2-thiazol)-2,5-diphenyl-2H-tetrazolium (MTT) is based on spectral differences at optimized reagent concentrations (Fig. 2). Some equilibrium constants, which characterize LLE of the tetrazolium ion-associated complexes, are presented in Table 11.
Figure 2. Spectra of the V(IV) - 4-NC - MTT complex, the V(V) -4-NC - MTT complex and the blank (4-NC - MTT) in chloroform182. CV(V) = CV(IV) = 2 x 10-5 mol/L, C4NC = 3.6 x 10-5 mol/L, CMTT = 1.6 x 10-4 mol/L, pH = 4.7, l = 1 cm
5. 27. Complexes of W
The complex formation in aqueous systems containing W(VI) and 3,5-DNC185 or 4-NC141 was investigated by Poluektova et al. and Natansohn et al., respectively. The complex with 3,5-DNC, H2[WO2L2], was recommended for determination of W(VI) (e400 = 1 x 104 L mol-1 cm-1). Its instability constant was calculated to be 1.8 x 10-21 (25 °C, I = 0.1 mol/L NaNO3).185
Several ternary complexes with participation of W(VI) - 4-NC or W(VI) - 3,5-DNC anionic chelates and bulky organic cations have been studied.186-188 These complexes are slightly soluble in water and easily extrac-table into organic solvents (CHCl3). Their molar absorptivity coefficients are in the range from e400 = 1.3 x 104 L mol-1 cm-1 (for the complex with antipyrine186) to 2.8 x
Table 11. Equilibrium constants (extraction constant - Kex, distribution constant - KD and association constant - P) for LLE-systems involving V(IV) or V(V), 4-NC (H2L) and tetrazo-lium salts.
Suggested formula	Organic	Equilibrium constants			Ref.
	solvent	log Kex	log Kd	log P	
(TT+)2[VOL2l2-	CHCl3	10.4	1.0	9.4	177
(TT+^VOLJ3-	CHCl3	12.1	1.10	11.0	177
(INT+)2[VOL2l2-	CHCl3	10.6	1.4	9.2	180
(INT+)3[VO2L2l3-	CHCl3	-	0.39	-	172,173
(NT2+)3[VO2L2]23-	CHCl3+n-BuOH (7:3)	-	-	8.9	178
(MTT+)2[VO2L2]2-	3 CHCl3	12.9	1.9	11.0	181
(MTT+)3[VO2L2]3-	CHCl3	19.2	1.62	17.6	183
Abbreviations: TT+ - 2,3,5-triphenyl-2H-tetrazolium; INT+ - 2-(4-iodophenyl)-3-(4-nitrop-henyl)-5-phenyl-2H-tetrazolium; MTT+ - 3-(4,5-dimethyl-2-thiazol)-2,5-diphenyl-2H-tetra-zolium
Table 12. Equilibrium constants (extraction constant - Kex, distribution constant - KD and association constant - P) for LLE-systems involving W(VI), NDC (4-NC or 3,5-DNC) and te-trazolium salts.
Suggested formula	Organic	Equilibrium constants			Ref.
	solvent	log Kex	log Kd	log P	
(TT+)2[WO2(NC)2]2-	CHCl3	10.3	1.05	9.2	187
(MTT+)2[WO2(NC)2]2-	CHCl33	10.3	1.03	9.3	188
(TV+)2[WO2(NC)2]2-	CHCl33	10.4	1.56	8.9	187
(TT+)[WO(OH)(DNC)2] -	CHCl33	10.6	1.06	9.6	187
Abbreviations: TT+ - 2,3,5-triphenyl-2H-tetrazolium; MTT+ - 3-(4,5-dimethyl-2-thiazol)-2,5-diphenyl-2H-tetrazolium; TV+ - 3-(2-naphtyl)-2,5-diphenyl-2H-tetrazolium
104 L mol-1 cm-1 (for the complex with thiazolylblue te-trazolium188). Some of them were successfully applied for the determination of W(VI) in ferrous metallurgy products.187188 Equilibrium constants (extraction constant -Kex, distribution constant - KD and association constant -P), which characterize the extraction of tetrazolium ion-association complexes were calculated by Dimitrov et
al.1
1 Their values are given in Table 12.
5. 28. Complexes of Zn
The complex formation in the aqueous systems Zn2+ - 3-NC, Zn2+ - 4-NC and Zn2+ - 3,5-DNC was investigated potentiometrically at 25 °C and an ionic strength of 0.1 mol/l (KCl) by Hakkinen.65-66-81 Complexes with general formulae ZnL and ZnL22- were detected in all systems. Their stability constants are as follows: logP1 = 8.64 and logP2 = 15.80 (for the complexes with 3-NC)65, logP1 = 8.25 and logP2 = 14.85 (for the complexes with 4-NC)66, and logP1 = 6.92 and logP2 = 12.79 (for the complexes with 3,5-DNC).81
5. 29. Complexes of Zr
Zr(IV) forms only one complex with 3,5-DNC in acidic aqueous solutions (optimum acidity 0.1-1.0 mol/L
HCl).189 This complex is electroneutral (suggested formula [Zr(OH)2L]), but not extractable into organic solvents. Its molar absorptivity was calculated to be emax = 6.1 x 103 L mol-1 cm-1 at ^max = 328.5 nm. In the presence of pyra-zolone derivatives (antipyrin or diantipyrylmethane) ternary compexes with 1:2 Zr-to-3,5-DNC ratio are formed. The molar absorptivities of these complexes are higher and bathochromic shifts are observed in their spectra: e420 = 1.5 x 104 L mol-1 cm-1 for the complex of antipyrine (Ant+)2[Zr(OH)2L2]2 and e410 = 2.06 x 104 L mol-1 cm-1 for the complex with diantipyrylmethane DAM2+ [Zr(OH)2L2]2. The complex with DAM has better characteristics, however (Ant+)2[Zr(OH)2L2]2 was used for LLE spectrophotometric analysis of real samples (Table 3). The relative standard deviation was calculated to be RSD = 1.2%.189
6. Other Applications
Except for the above-mentioned application as analytical reagents, simple NDCs are widely used as model reagents for elucidation of various environmental86142 and physiological190191 processes. Some NDCs were disclosed as important inhibitors,191-196 antioxidants,28'194'195 selectin modulating compounds197 and precursors for a
wide range of syntheses.198-202 Derivatives of 3-NC with a bulky substituent group in position 5 are well known therapeutic agents.194-196,203,204 The most important representatives of this class of compounds are nitecapone, entaca-pone and tolcapone {their systematic names are (3,4-dihy-droxy-5-nitrobenzylidene)-2,4-pentanedione; (2E)-2-cya-no-3-(3,4-dihydroxy-5-nitrophenyl)-N,N-diethylprop-2-enamide; and (3,4-dihydroxy-5-nitrophenyl)(4-methylp-henyl)methanone, respectively}. Nitecapone exerts cardioprotective205 and gastroprotective effects,195 while en-tacapone may protect from angiotensin II-induced inflammation and renal injury.206 As a highly effective inhibitor of catechol-O-methyltransferase (COMT) entacapone is used for the treatment of Parkinson's disease patients.204
7. Conclusion
NDCs are widely used in various fields including analytical and environmental chemistry, biochemistry and pharmacology. They form complexes with many metal ions. The stability order among the simple NDCs used as ligands in binary complexes follows the sequence: 3,5-DNC<4-NC<3-NC. The most important participants in this row, however, are 4-NC and 3,5-DNC. Additional investigations on complexes of these compounds, as well as on some 5-substituted derivatives of 3-NC, are worth performing. Further experiments may significantly expand the sphere of their applications.
8. Acknowledgments
The author would like to thank the Research Fund of the Plovdiv University for its long-time support (Grant Nos H13-2005, MU-11-2008, RNI09-HF-002, RS09-HF-010 and NI2011-HF-007).
9. References
1.	G. A. Olah, S. C. Narang, J. A. Olah, K. Lammertsma, Proc. Nat. Acad. Sci. USA 1982, 79 4487-4494.
2.	E. N. Izakovich, M. L. Khidekel', Russ. Chem. Rev. 1988, 57, 419-432.
3.	R. S. Padda, C. Wang, J. B. Hughes, R. Kutty, G. N. Bennett, Environ. Toxicol. Chem. 2003, 22, 2293-2297.
4.	R. Winkler, C. Hertweck, Chembiochem 2007, 8, 973-977.
5.	K.-S. Ju, R. E. Parales, Microbiol. Mol. Biol. Rev. 2010, 74, 250-272.
6.	H. Feuer (Ed.): The Chemistry of Nitro and Nitroso Group, Wiley-Interscience, New York, 1969.
7.	Z. Z. Rappoport (Ed.): The Chemistry of Phenols, Wiley-VCH, Weinheim, 2003.
8.	N. Schweigert, A. J. B. Zehnder, R. I. L., Eggen Environ. Microbiol. 2001, 3, 81-91.
9. L. Sommer, M. Bartusek, Folia Fac. Sci. Nat. UJEP Brno Chem. 1966, 7, 1-88.
10.	C. Y. Wong, J. D. Woollins, Coord. Chem. Rev. 1994, 130, 175-241.
11.	R. Benedikt, Chem. Ber. 1878, 11, 362-363.
12.	M. J. Astle, S. P. Stephenson, J Amer. Chem. Soc. 1943, 65, 2399-2402.
13.	D. H. Rosenblatt, J. Epstain, M. Levitch, J Amer. Chem. Soc., 1953, 75, 3277-3278.
14.	E. M. Kampouris, J. Chem. Soc. Perkin Trans. 11972, 10881090.
15.	E. J. Hakoila, J. J. Kankare, T. Skarp, Anal. Chem. 1972, 44, 1857-1860.
16.	V. A. Nazarenko, N. V. Lebedeva, L. I. Vinarova Zh. Anal. Khim.,1972, 27, 128-133.
17.	P. Weselsky, R. Benedikt, Monatsh. Chem. 1882, 3, 386393.
18.	R. Nietzki, F. Moll, Ber. 1893, 26, 2182-2184.
19.	E. Bamberger, M. Czerkis, J. Pract. Chem. 1903, 68, 480485.
20.	J. E. Foglesong, I. L. Newell, J Amer. Chem. Soc. 1930, 52, 834-837.
21.	V. Gold, C. H. Rochester, J. Chem. Soc. 1964, 1722-1727.
22.	D. W. Barnum, Anal. Chim. Acta 1977, 89, 157-166.
23.	E M Kampouris J. Chem. Soc. C 1971, 2577-2579.
24.	E. R. Cole, G. Cranck, H. T. H. Minh, Austr. J. Chem. 1980, 33, 675-680.
25.	G. Sloff, Rec. Trav. Chim. Pays-Bas 1935, 54, 995-1010.
26.	M. A. Iorio, Ann. Chim. 1959, 49, 370-373.
27.	J. Ehrlich, M. T. Bogert, J. Org. Chem. 1947, 12, 522-534.
28.	V. K. Daukshas, R. S. Martinkus, Z. A. Shal'tite, E. B. Udre-naite, J. Appl. Chem. USSR 1985, 58, 2520-2521.
29.	C.- J. Jiang, M.- G. Xu, W. Jiang, L.- J. Kong, Chemical Reagents 2007; DOI: CNKI:SUN:HXSJ.0.2007-12-023; http:// en.cnki.com.cn/Article_en/CJFDT0TAL-HXSJ2007120 23.htm
30.	R. B. Baudy, L. P. Greenblatt, I. L. Jirkovsky, M. Conklin, R. J. Russo, D. R. Bramlett, T. A. Emrey, J. T. Simmonds, D. M. Kowal, J. Med. Chem. 1993, 36, 331-342.
31.	J. Buckingham (Ed.): Dictionary of organic compounds, Vol. 9, Chapman & Hall, London, England, 1996, pp. 2767-2768
32 A. G. Clark, D. A. Jowett, J. N. Smith, Anal. Biochem. 1981, 118, 231-239.
33.	D. Prakash, J. Pandey, B. N. Tiwary, R. K. Jain, BMC Bio-technol. 2010, 10, 49; http://www.biomedcentral.com/conte nt/pdf/1472-6750-10-49.pdf
34.	J. Keiboom, H. Brink, J. Frankena, J. A. M. Bont, Appl. Mi-crobiol. Biotechnol. 2001, 55, 290-295.
35.	A. Fishman, Y. Tao, W. E. Bentley, T. K. Wood Biotech. Bioeng. 2004, 87, 779-790.
36.	Y. Liu, D. Wang, B. Sun, X. Zhu, J. Hazard. Mat. 2010, 181, 1010-1015.
37.	S. Chaliha, K. G. Bhattacharyya, P. Paul, J. Chem. Technol. Biotechnol. 2008, 83, 1353-1363.
38.	D. Schulte-Frohlinde, G. Reutebuch, C. Sonntag, Int. J. Rad. Phys. Chem. 1973, 5, 331-342.
39.	E. Guivarch, S. Trevin, C. Lahitte, M. A. Oturan, Environ. Chem. Lett. 2003, 1, 38-44.
40.	E. Brillas, M. A. Banos, S. Camps, C. Arias, P.- L. Cabot, J. A. Garrido, R. M. Rodrigues, New J. Chem. 2004, 28, 314-322.
41.	R. K. Jain, J. H. Dreisbach, J. C. Spain, Appl. Environ. Microbiol. 1994, 60, 3030-3032.
42.	A. Chauhan, A. K. Chakraborti, R. K. Jain, Biochem. Biophys. Res. Commun.2000, 270, 733-740.
43.	V. Kadiyala, J. C. Spain, Appl. Environ. Microbiol. 1998, 64, 2479-2484.
44.	A. Chauhan, S. K. Samanta, R. K. Jain, J. Appl. Microbiol. 2000, 88, 764-772.
45.	S. K. Samanta, B. Bhushan, A. Chauhan, R. K. Jain, Biochem. Biophys. Res. Commun. 2000, 269, 117-123.
46.	A. Schafer, H. Harms, A. J. Zehnder, Biodegradation 1996, 7, 249-255.
47.	S. B. Pakala, P. Gorla, A. B. Pinjari, R. J. Krovidi, R. Baru, M. Yanamandra, M. Merrick, D. Siddavattam, Appl. Microbiol. Biotechnol. 2007, 73, 1452-1462.
48.	B. E. Haigler, J. C. Spain, Appl. Environ. Microbiol. 1991, 57, 3156-3162.
49.	Z. Zdrahal, J. Chromatogr. A 1998, 793, 214-219.
50.	B. E. Haigler, G. R. Johnson, W.- C. Suen, J. C. Spain, J. Bacteriol. 1999, 181, 965-972.
51.	B. E. Haigler, S. F. Nishino, J. C. Spain, J. Bacteriol. 1994, 176, 3433-3437.
52.	B. E. Haigler, W.- C. Suen, J. C. Spain, J. Bacteriol. 1996, 178, 6019-6024.
53.	R. J. Spanggord, J. C. Spain, S. F. Nishino, K. E. Mortel-mans, Appl. Environ. Microbiol. 1991, 57, 3200-3205.
54.	W.- C. Suen, B. E. Haigler, J. C. Spain, J. Bacteriol. 1996, 178, 4926-4934.
55.	W.-C. Suen, J. C. Spain, J. Bacteriol. 1993, 175, 1831-1837.
56.	G. R. Johnson, R. K. Jain, J. C. Spain, J. Bacteriol. 2002, 184, 4219-4232.
57.	G. R. Johnson, R. K. Jain, J. C. Spain, Arch. Microbiol. 2000, 173, 86-90.
58.	S. F. Nishino, G. C. Paoli, J. C. Spain, Appl. Environ. Microbiol. 2000, 66, 2139-2147.
59.	S. Ecker, T. Widmann, H. Lenke, O. Dickel, P. Fischer, C. Bruhn, H. J. Knackmuss, Arch. Microbiol. 1992, 158, 149154.
60.	J. Navratilova, L. Tvrzova, E. Durnova, C. Spröer, I. Sed-lacek, J. Neca, M. Nemec, Antonie van Leeuwenhoek 2005, 87, 149-153.
61.	J. Navratilova, L. Tvrzova, J. Neca, M. Nemec, Folia Microbiol. 2004, 49, 613-615.
62.	Q. Zhong, H. Zhang, W. Bai, M. Li, B. Li, X. Qiu, J. Environ. Sci. Health, Part A 2007, 42, 2111-2116.
63.	K. Gavazov, V. Lekova, B. Boyanov, A. Dimitrov, J. Therm. Anal. Cal., 2009, 96, 249-254.
64.	M. D. R. Silva, M. A. R. Silva, G. Pilcher, J. Chem. Ther-modyn. 1986, 18, 295-300.
65.	P. Häkkinen, Finn. Chem. Lett. 1985, 12, 56-59.
66.	P. Häkkinen, Finn. Chem. Lett. 1984, 11, 9-12
67.	P. Pichet, R. L. Benoit, Inorg. Chem., 1967, 6, 1505-1509.
68.	M. A. Dhansay, P. W. Linder, J. Coord. Chem. 1993, 28, 133-145.
69.	D. H. Zhu, M. J. Kappel, K. N. Raymond, Inorg. Chim. Acta 1988, 147, 115-121.
70.	B. C. Bhuyan, S. N. Dubey, Indian J. Chem. Sec. A 1981, 20, 756-758.
71.	R. F. Jameson, M. Wilson, J. Chem. Soc. Dalton Trans. 1972, 2607-2610.
72.	V. M. Nurchi, T. Pivetta, J. I. Lachowicz, G. Crisponi, J. Inorg. Biochem. 2009, 103, 227-236.
73.	S. Giroux, S. Aury, P. Rubini, S. Parant, J.- R. Desmurs, M. Dury, Polyhedron 2004, 23 2393-2404.
74.	R. Aydin, U. Ozer, Turkish J. Chem. 1997, 21, 428-436.
75.	Y. Murakami, M. Tokunaga, Bull. Chem. Soc. Japan, 1964, 37, 1562-1563.
76.	S. R. Cooper, V. J. Tulane, Ind. Eng. Chem. Anal. Ed. 1936, 8, 210-211.
77.	F. L. Gilbert, F. C. Laxton, E. B. R. Prideaux, J. Chem. Soc. 1927, 2295-2308.
78.	F. C. Laxton, E. B. R. Prideaux, W. H. Radford, J. Chem. Soc. Trans. 1925, 127, 2499-2501.
79.	R. I. Gleb, D. A. Laufer, L. M. Schwartz, K. Wairimu, J. Chem. Eng. Data 1989, 34, 82-83.
80.	J. Sunkel, H. Staude, Ber. Bunsenges. Phys. Chem. 1968, 72, 567-573.
81.	P. Häkkinen, Finn. Chem. Lett. 1986, 13, 85-91.
82.	M. Bartusek, Collect. Czech. Chem. Commun. 1967, 32, 757-775.
83.	T. Kiss, K. Atkari, M. Jelowska-Bojczuk, P. Decock J. Coord. Chem. 1993, 29, 81-96.
84.	J.- P. Cornard, Rasmiwetti, J.- C. Merlin, Chem. Phys. 2005, 309, 239-249.
85.	J. Sunkel, H. Staude, Ber. Bunsenges. Phys. Chem. 1969, 73, 203-209.
86.	J.- P. Cornard, C. Lapouge, J.- C. Merlin, Chem. Phys. 2007, 340, 273-282.
87.	E. A. Nothnagel, R. N. Zitter, J. Phys. Chem. 1976, 80, 722-727.
88.	A. Afkhami, L. Khalafi, Anal. Chim. Acta 2006, 569, 267274.
89.	F. Riedel, S. Spange, J. Phys. Org. Chem. 2009, 22, 203211.
90.	V. A. Nazarenko, N. V. Lebedeva, L. I. Vinarova, Zh. Anal. Khim., 1973, 28, 1100-1103.
91.	L. Sommer, G. Ackermann, D. T. Burns, S. B. Savvin, Pure Appl. Chem. 1990, 62, 2147-2166.
92.	H. Nakayama, H. Taniguchi, H. Tani, Heavy metal adsorbents, Japanese Patent Number 78 23,891, (Cl. B01D15/00), Appl. 76/99,299, date of patent August 19, 1976
93.	H. Taniguchi, H. Nakayama, H. Tani, Heavy metal adsorbents, Japanese Patent Number 78 28,692, (Cl. C08G8/20), Appl. 76/102,981, date of patent August, 27, 1976
94.	D. Robinson, J. N. Smith, J. N. Spencer, R. T. Williams Bioc-hem. J. 1952, 51, 202-208.
95.	A. B. Roy, Biochem. J. 1953, 53, 12-15.
96.	A. B. Roy, Biochem. J. 1953, 55, 653-661.
97.	A. B. Roy, Biochem. J. 1956, 64, 651-657.
98.	A. A. Farooqui, B. K. Bachhawat, Biochem. J. 1972, 126, 1025-1033.
99.	H. C. Kiefer, W. I. Congdon, I. S. Scarra, I. M. Klotz, Proc. Nat. Acad. Sci. USA 1972, 69, 2155-2159.
100.	C. H. Yang, P. N. Srivastava, Biochem. J. 1976, 159, 133142.
101.	A. A. Farooqui, P. N. Srivastava, Biochem. J. 1979, 181 331-337.
102.	D. Robinson, J. N. Smith, R. T. Williams, Biochem. J. 1951, 49, lxxiv-lxxv.
103.	D.- H. Kim, M. J. Choi, S.- K. Chang Bull. Korean Chem. Soc. 2000, 21, 145-147.
104.	P. Häkkinen, Finn. Chem. Lett. 1986, 13, 15-19.
105.	P Häkkinen, Finn. Chem. Lett. 1984, 11, 151-154.
106.	A. J. Downard, R. J. Lenihan, S. L. Simpson, B. O'Sullivan, K. J. Powell, Anal. Chim. Acta 1997, 345, 5-15.
107.	D. Vasudevan, A. T. Stone, J. Coll. Int. Sci. 1998, 202, 1-19.
108.	K. Kuwada, S. Motomizu, K. Toei, Japan Analyst 1977, 26, 609-614.
109.	J. Votava, J. Havel, M. Bartusek, Scripta Fac. Sci. Nat. UJEP Brun., Chemia 1975, 5, 71-76.
110.	R. Pizer, L. Babcock, Inorg. Chem. 1977, 16, 1677-1681.
111.	R. Meilleur, R. L. Benoit, Canadian J. Chem. 1969, 47, 2569-2571.
112.	L. Havelkova, M. Bartusek, Collect. Czech. Chem. Commun. 1968, 33, 385-393.
113.	R. Wernimont, Anal. Chem. 1967, 39, 554-562.
114.	P. Häkkinen, Finn. Chem. Lett. 1985, 12, 17-20.
115.	P. Häkkinen, Finn. Chem. Lett. 1986, 13, 53-57.
116.	P. Häkkinen, Finn. Chem. Lett. 1987, 14, 15-20.
117.	M. Sutton, B. Andresen, S. R. Burastero, M. L. Chiarappa-Zucca, S. Chinn, P. Coronado, A. E. Gash, J. Perkins, A. Sawvel, S. C. Szechenyi, Modern Chemistry Techniques Applied to Metal Behavior and Chelation in Medical and Environmental Systems - final report, 2005; https://e-repor ts-ext.llnl.gov/pdf/316158.pdf (assessed: Feb 20, 2011)
118.	P. Häkkinen, Finn. Chem. Lett. 1984, 11, 59-62.
119.	C. A. Tyson, A. E. Martell, J. Am. Chem. Soc. 1972, 94, 939-945.
120.	H. Burgshat, K. J. Netter, J. Pharmaceutical Sci. 1977, 66, 60-63.
121.	S. Partanen, Histochem. J., 1984, 16, 501-506.
122.	D. S. Yogi, G. N. Kumar, M. S. Mohan, Y. L. Kumari, Proc. Indian Acad. Sci. (Chem. Sci.), 1992, 104, 443-451.
123.	P. Venkataih, M. S. Mohan, M. Rajeswari, G. Moses, Oriental J. Chem. 1996, 12, 121-125.
124.	J. Kaizer, Z. Zsigmond, I. Ganszky, G. Speier, M. Giorgi, M. Reglier, Inorg. Chem., 2007, 46, 4660-4666.
125.	G. W. Luther III, T. F. Rozan, A. Witter, B. Lewis, Geoc-hem. Trans. 2001, 2, 65; http://www.geochemicaltransacti ons.com/content/pdf/1467-4866-2-65.pdf
126.	T. Kawabata, V. Schepkin, N. Haramaki, R. S. Phadke, L. Packer, Biochem. Parmacol. 1996, 51, 1569-1577.
127.	C. A. Tyson, J. Biol. Chem, 1975, 250, 1765-1770.
128.	L. J. M. Spaaphen, J. Verhagen, G. A. Veldink, J. F. G. Vlie-genthart, Biochim. Biophys. Acta 1980, 617, 132-140.
129.	J. R. Galpin, L. G. M Tielens, G. A. Veldink, J. F. G. Vlie-genthart, J. Boldingh, FEBS Let. 1976, 69, 179-182.
130.	E. Skrzypczak-Jankun, O. Y. Borbulevych, J. Jankun, Acta Cristallogr. Sec. D 2004, 60, 613-615.
131.	C. J. Macdonald, Inorg. Chim. Acta 2000, 311, 33-39.
132.	V. A. Nazarenko, E. A. Biryuk, L. I. Vinarova, R. V. Ravit-skaya, Zh. Anal. Khim. 1981, 36 1315-1318.
133.	N. Konopic, G. Wimmer, Monatsh. Chem. 1962, 93, 14041415.
134.	M. Bartusek, Scripta Fac. Sci. Natur. UJEP Brun. Chem. 1979, 9, 23-30.
135.	V. A. Nazarenko, L. I. Vinarova, N. V. Lebedeva, R. A. Lyakh, Ukr. Khim. J. 1977, 43, 1325-1327.
136.	A. Dimitrov, S. Kostova, Bulg. Chem. Ind. 1999, 70, 88-90.
137.	A. Dimitrov, S. Kostova, A. Alexandrov, Anal. Lab. 1998, 7, 20-23.
138.	P. Bevillard, Mikrochem. Ver., Mikrochim. Acta 1952, 39, 209-215.
139.	J. Chwastowska, E. Grzegrzolka, Chem. Anal. (Warsaw) 1975, 20, 1065-1070.
140.	M. F. Reynolds, M. Costas, M. Ito, D.- H. Jo, A. A. Tipon, A. K. Whiting, L. Que, J. Biol. Inorg. Chem. 2003, 8, 263272.
141.	S. Natansohn, J. I. Krugler, J. E. Lester, M. S. Chagnon, R. S. Finocchiaro, J. Phys. Chem. 1980, 84, 2972-2980.
142.	A. L. R. Merce, C. Greboge, G. Mendes, A. S. Mangrich, J. Braz. Chem. Soc. 2005, 16, 37-45.
143.	S. Kostova, A. Dimitrov, Bulg. Chem. Ind. 2000, 71,44-46.
144.	A. Dimitrov, S. Kostova, Z. Pancheva, V. Lekova, Bulg. Chem. Ind., 2002, 73, 20-23.
145.	V. Lekova, A. Dimitrov, M. Slavova, J. Univ. Chem. Tech. Met. 2003, 38, 901-908.
146.	A. Dimitrov, V. Lekova, K. Gavazov, B. Boyanov, Cent. Eur. J. Chem. 2005, 3, 747-755.
147.	A. Dimitrov, V. Lekova, K. Gavazov, B. Boyanov, J. Anal. Chem. 2007, 62, 122-125.
148.	R. Lobinski, Z. Marczenko, Microchem. J., 1990, 42, 197-205.
149.	Z. Marczenko, M. Balcerzak, Metod'y spektrofotometrii v UF I vidimoj oblastyakh v neorganicheskom analize, BINOM. Laboratoriya znanij, Moscow, 2007.
150.	J. U. Mondal, E. Almaraz, N. G. Bhat, Inorg. Chem. Com-mum. 2004, 7, 1195-1197.
151.	A. Aleksandrov, S. Kostova, Nauchni Tr. Plovdiv Univ. Khim. 1980, 18, 11-21.
152.	A. Aleksandrov, S. Kostova, O. Navratil, Nauchni Tr. Plovdiv Univ., Khim. 1983, 21, 15-26.
153.	A. Aleksandrov, S. Kostova, Nauchni Tr. Plovdiv Univ., Khim.1986, 24, 53-66.
154.	S. Kostova, A. Aleksandrov, I. Ilieva, Nauchni Tr. Plovdiv Univ., Khim.1989, 27, 67-77.
155.	P. Racheva, V. Lekova, K. Gavazov, A. Dimitrov, Sixth International Conference of the Chemical Societies of the South-Eastern European Countries (Book of Abstracts), Sofia, Bulgaria, 2008, p. 155.
156.	V. Lekova, P. Racheva, K. Stojnova, A. Dimitrov, K. Gava-zov, Chemija 2010, 21, 106-111.
157.	P. Racheva, V. Lekova, T. Štefanova, A. Dimitrov, K. Gava-zov, Nauchni Tr. Plovdiv Univ., Khim. 2010, 37, 33-41.
158.	R. Lobinski, Z. Marczenko, Anal. Chim. Acta, 1989, 226, 281-291.
159.	N. Türkei, R. Aydin, U. Ozer, Turk. J. Chem. 1999, 23, 139152.
160.	N. S. Poluektov, R. S. Lauer, L. A. Ovchar, S. F. Potapova, Zh. Anal. Khim. 1975, 30, 1513-1517.
161.	D. F. Evans, J. Parr, E. N. Coker, Polyhedron 1990, 9, 813-823.
162.	Y. Farina, I. Baba, A. H. Othman, N. Fadzilah, R. Mohamed, Pertanika J. Sci. Technol. 1998, 6, 131-139.
163.	V. A. Nazarenko, L. I. Vinarova, N. V. Lebedeva, Zh. Anal. Khim. 1975, 30, 617-619.
164.	F. D. Rochon, R. Melanson, P. C. Kong, Acta Crys., Sec. C 1992, 48, 785-788.
165.	E. A. Biryuk, L. I. Vinarova, R. V. Ravitskaya, Fiz.-Khim. Metody' Analiza, Gorkii, USSR, 1981, pp. 31-33; Chem. Abstr. 98, 100365q
166.	L. G. Hargis, J. A. Howell, Anal. Chem. 1984, 56, 225-241R.
167.	V. A. Nazarenko, E. A. Biryuk, L. I. Vinarova, K. A. Muke-lo, Zh. Anal. Khim. 1982, 37, 252-256.
168.	S. Kostova, A. Dimitrov, A. Alexandrov, Chem. Papers 2000, 54, 66-71.
169.	P. Z. Araujo, P. J. Morando, M. A. Blesa, Langmuir 2005, 21, 3470-3474.
170.	A. I. Busev, Z. P. Karyakina, Zh. Anal Khim. 1967, 22, 1506-1509.
171.	Z. Marczenko, R. Lobinski, Talanta 1988, 35,1001-1004.
172.	Z. Simeonova, K. Gavazov, A. Alexandrov, Anal. Lab. 1998, 7, 184-188.
173.	K. Gavazov, PhD Thesis, BAS, Sofia, Bulgaria, 2000.
174.	K Gavazov, V Lekova, A Dimitrov, in S. Mladenova, Ts. Odjakova, D. Kolev, E. Nikolova (Eds.), National Scientific Conference with International Participation, Union of Scientists of Bulgaria - Branch Smolyan, (CD proceeding), Smolyan, Bulgaria, 2006, pp. 808-819.
175.	A. Dimitrov, A. Alexandrov, M. Docheva, Nauchni Tr. Plovdiv Univ., Khim. 1983, 21, 41-48.
176.	A. Dimitrov, A. Alexandrov, M. Docheva, Nauchni Tr. Plovdiv Univ., Khim. 1983, 21, 49-56.
177.	A. Dimitrov, A. Alexandrov, M. Docheva, Nauchni Tr. Plovdiv Univ., Khim. 1983, 21, 57-67.
178.	K. Gavazov, Z. Simeonova, A. Alexandrov, Nauchni Tr. Plovdiv Univ., Khim. 2002, 31, 5-11.
179.	K. Gavazov, Z. Simeonova, A. Alexandrov, Nauchni Tr. Plovdiv Univ., Khim. 2002, 31, 13-16.
180.	Z. Simeonova, K. Gavazov, A. Alexandrov, Nauchni Tr. Plovdiv Univ., Khim. 2005, 33, 5-9.
181.	P. Racheva, K. Gavazov, V. Lekova, A. Dimitrov, Nauchni Tr. Plovdiv Univ., Khim. 2008, 36, 39-44.
182.	P. Racheva, K. Gavazov, V. Lekova, A. Dimitrov, J. Iranian Chem. Res. 2008, 1, 113-121.
183.	P. Racheva, K. Gavazov, V. Lekova, A. Dimitrov, J. Anal. Chem. 2010, 65, 21-25.
184.	R. Lobinski, Z. Marczenko, Anal. Sci. 1988, 4, 629-635.
185.	E. N. Poluetkova, G. G. Shitareva, Zh. Anal. Khim. 1972, 27, 1301-1304.
186.	V. A. Nazarenko, E. N. Poluetkova, G. G. Shitareva, Zh. Anal. Khim. 1973, 28, 101-104.
187.	A. Dimitrov, A. Alexandrov, Anal. Lab. 1995, 4, 172-179.
188.	V. Lekova, K. Gavazov, A. Dimitrov, Chem. Papers 2006, 60, 283-287.
189.	E. N. Poluetkova, G. G. Shitareva, Zh. Anal. Khim. 1977, 32, 76-81.
190.	M. Ovaska, A. Yliniemela, J. Comput. Aided. Mol. Design 1998, 12, 301-307.
191.	P. Männisto, S. Kaakkola, Pharmacol. Rev. 1999, 51, 593628.
192.	P. N. Palma, M. L. Rodrigues, M. Archer, M. J. Bonifacio, A. I. Loureiro, D. A. Learmonth, M. A. Carrondo, P. Soa-res-da-Silva, Mol. Pharmacol. 2006, 70, 143-153.
193.	R. N. Patel, C. T. Hou, A. Felix, M. O. Lillard, J. Bacteriol. 1976, 127, 536-544.
194.	Y J Suzuki, M Tsuchiya, A Safadi, V E Kagan, L Packer Free Rad. Biol. Med. 1992, 13, 517-525.
195.	L. Marcocci, Y. J. Suzuki, M. Tsuchiya, L. Packer, Meth. Enzymol. 1994, 234, 526-541.
196.	P. N. Palma, M. J. Bonifacio, A. I. Loureiro, L. C. Wright, D. A. Learmonth, P. Soares-da-Silva, Drug Metabol. Disp. 2003, 31, 250-258.
197.	E. M. Aydt, R. Kranich, Aromatic nitrocatechol compounds and their use for modulating processes mediated by cell adhesion molecules, US Patent Number 7,851,501, date of patent December 14, 2010.
198.	S. W. Ng, P. Naumov, S. Chantrapromma, S. S. S. Raj, H.K. Fun, A. R. Ibrahim, G. Wojciechowski, B. Brzezinski, J. Molec. Str. 2001, 569, 139-145.
199.	S. Baniyaghoob, M. M. Najafpour, D. M. Boghaei, Spec-trochim. Acta, Part A 2010, 75, 970-977.
200.	D. Kulac, M. Bulut, A. Altindal, A. R. Ozkaya, B. Salih, O. Bekaroglu, Polyhedron 2007, 26, 5432-5440.
201.	D. P. Iga, S. Iga, A. Nicolescu, N.- A. Chira, Rev. Roum. Chim. 2010, 55, 357-363.
202.	S. Chantrapromma, A. Usman, H.- K. Fun, B.- L. Poh, C. Karalai, Acta Cryst. C 2002, 58, 675-677.
203.	L. Luukkanen, I. Kilpeläinen, H. Kangas, P. Ottoila, E. Elo-vaara, J. Taskinen, Bioconj. Chem. 1999, 10, 150-154.
204.	A. Gordin, D. J. Brooks, J. Neurol. 2007, 254, IV37-IV48
205.	N. Haramaki, D. V. Stewart, S. Aggarwal, T. Kawabata, L. Packer, Biochem. Pharmacol. 1995, 50, 839-843.
206.	T. Helkamaa, P. Finckenberg, M. Louhelainen, S. Merasto, P. Rauhala, R. Lapatto, Z. J. Cheng, I. Reenilä, P. Männistö, D. Müller, F. Luft, E. M. A. Mervaala, J. Hypertens. 2003, 21, 2353-2363.
Povzetek
Podan je pregled nitro derivatov katehola (NDCs) s posebnim poudarkom na njihovih kompleksih in aplikacijah. Predstavljeni so binarni, terciarni in kvarterni NDC kompleksi z več kot 40 elementi (aluminij, arzen, bor, berilij, kalcij, kobalt, baker, železo, galij, germanij, magnezij, mangan, molibden, niobij, redke zemeljske kovine, silicij, kositer, stroncij, tehnecij, talij, titan, uran, vanadij, volfram, cink in cirkonij), njihova uporaba v analitiki in ključne karakteristike razvitih analiznih postopkov.