Acta Chim. Slov. 2001, 48, 229-239.
229
NOVEL ASSOCIATIONS OF ORTHO QUINONE MONOOXIMES:
HYDROGEN BONDED DIMERS OF
4,6-DI-JJEi?r-BUTYL-l,2-BENZOQUINONE-2-MONOOXIME
Andrej Kržan*
National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia
Peter Kamaras, Michael H. Dickman, DeLanson R. Crist, Vaclav Horäk
Georgetown University, Department of Chemistry, Washington D.C. 20057, USA
Received 04-05-2000
Abstract
Unlike previously reported X-ray structures of ortho quinone monooximes, which exhibit either intramolecular hydrogen bonding or chain-like structures in the solid state, the sterically demanding 4,6-di-fert-butyl-l,2-benzoquinone-2-monooxime (DT) crystallizes as dimers. In crystals from toluene DT forms dimers held together by two hydrogen bonds in a six-membered ring commonly found for simple oximes, but not for ortho quinone monooximes. Crystals from methanol contain DT dimers held together by two bridging methanol molecules, each of which participate in three hydrogen bonds. Although phenolic and quinonoid tautomeric forms of DT coexist in solution, with nonpolar solvents favoring the aromatic phenolic tautomer, both crystals exist in the quinonoid form, independent of the solvent from which they were crystallized. These quinonoid forms are stabilized by intermolecular multi center hydrogen bonds and dipole-dipole interactions countering the effect of the loss of aromatic stabilization. With copper (II) Perchlorate, DT yielded crystals of Cu(II)(HL)(L)C104, where HL = DT. HL and L ligands are coordinated to Cu(II) in ads relationship.
Introduction
The chemistry of quinone monooximes is marked by the existence of a tautomeric equilibrium with nitrosophenols, shown below for 4,6-di-ter?-butyl-l,2-benzoquinone-2-monooxime (DT).
f-Bu-
N=0
f-Bik
f-Bu
DT (phenolic)
0	0       OH
/Jl^N-OH                 f-Bu^	IIT N
	k^
f-Bu	f-Bu
anti	syn
DT (quinonoid)	
A. Kržan P. Kamaras, M. H. Dickman, D. R. Crist, V. Horâk: Novel Associations of ortho Quinone...
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Spectroscopic methods " have revealed various structural effects on this equilibrium, and shown that, in general, quinonoid forms are favored in polar solvents, while phenolic forms are favored in nonpolar solvents.
Despite the existence of the phenolic form in solution for certain cases, X-ray crystallography has shown that all nitrosophenols reported thus far are quinonoid in the solid state " with the oximo (=N-OH) group oriented either syn or anti to the keto (-C=0) group. The syn orientation resulted in an intramolecular hydrogen bond between oximo and keto groups, while the anti form favored a chain-like arrangement of quinone monooxime molecules connected through intermolecular hydrogen bonds.
o-Quinone monooximes are known to form stable complexes with d-metal ions in which the metal ion is bound to the keto oxygen and oximo nitrogen. Numerous analytical 3 and synthetic uses were reported for these complexes. TarnopoPskii also studied the effectiveness of DT for metal ion extractions. By analysis of bond length data it was shown that a complexed metal ion causes a mesomeric shift toward the o-nitrosophenoxide form of the ligand.   '
The purpose of this work was to determine the effect of sterically demanding substituents on crystal structures obtained from polar and nonpolar solvents and on a crystalline metal complex. DT was chosen for this investigation because of strong solvent effects ' on its tautomeric equilibrium. However, DT differs from previously examined o-quinone monooximes by the presence of bulky tert-buty\ groups. The work was also an attempt to obtain an uncomplexed o-nitrosophenol crystal in the phenolic form.
Experimental
Instrumentation
IR data was collected on a Mattson GL 2020 IR spectrometer. A modified Nicolet NT 270 spectrometer operating at 270 MHz was used to collect NMR spectra, and samples were analyzed with a Fisons GC 8000 gas Chromatograph connected to a Fisons MD 800 mass spectrometer. The GC was equipped with a methyl silicone capillary column (G&W Scientific DB-5MS) and the MS with an EI source at 70 eV.
A. Kržan P. Kamaras, M. H. Dickman, D. R. Crist, V. Horâk.: Novel Associations ofortho Quinone...
Acta Chim. Slov. 2001, 48, 229-239.
231
Synthesis
4,6-di-te7t-Butyl-l,2-benzoquinone 2-monooxime was prepared from 3,5-di-tert-
90
butyl-1,2-quinone and hydroxylamine. Recrystallization from MeOH gave shiny orange crystals which upon drying became an amorphous-like orange solid; m.p. 85-87 °C; IR (KBr) Amax /cm"1 3222, 2963, 1661, 1622, 1370, 1301, 1265, 1053, 1013, 943, 889, 797; NMR ÔH (270 MHz, solvent CDC13; standard SiMe4) 1.33 (9H, s, t-Bu), 1.38 (9H, s, t-Bu), 7.5 (IH, br, ArH), 7.8 (IH, br, ArH); MS m/z 235, 220, 190, 162, 147, 134,91,57,41.
Crystals for X-ray analyses were prepared by slow solvent evaporation from a solution exposed to the atmosphere at room temperature. Orange DT crystals, obtained from methanol, changed appearance after drying and over ca. 30 days the dry solid degraded regardless of refrigeration or storage in an inert atmosphere at ambient temperature. For this reason, as soon as crystals were removed from the mother liquor, they were immediately mounted on the sample holder at liquid nitrogen temperature for X-ray analysis. Orange crystals obtained from toluene showed no such signs of change, even over a longer period at room temperature and exposure to the atmosphere.
The Cu(II) DT complex was prepared by mixing 2 equivalents of DT with 1 equivalent of Cu(C104)2 6H2O in toluene. However, crystallization from toluene did not result in useful monocrystals. Recrystalization of this product from cyclohexane gave dark violet crystals of Cu(II)(HL)(L)C104 (where HL = 4,6-di-tert-butyl-l,2-benzoquinone-2-monooxime), suitable for X-ray data collection.
X-Ray structure analysis
For DT from toluene or methanol, crystals were mounted on glass fibers and data taken at 203(2) K on a Siemens P4/R4 four-circle diffractometer equipped with a rotating Mo anode. Unit cell parameters and crystal orientation were obtained by means of least-squares refinement on at least 25 reflections. Periodic measurements of three standard reflections revealed no appreciable decay. Both structures were solved by direct
99
methods through Siemens XSCANS and SHELXTL programs.
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Acta Chim. Slov. 2001, 48, 229-239.
For the Cu complex, data were collected on a Siemens SMART CCD single-crystal diffractometer with a Mo anode and graphite monochromator. Crystals were mounted on a glass fiber and placed in a nitrogen stream at 173(2) K. A sphere of data was collected to an effective 29 value of 55 degrees, using omega scans. Routine Lorentz and polarization corrections were applied. An empirical correction was used for absorption and other systematic errors, based on measured intensities of equivalent reflections at different phi and omega values. The structure was solved by direct methods using Siemens SMART, SAINT, and SHELXTL programs.22'24 For all structures, refinements were full-matrix least-squares on F using all data, and the weighting scheme was w = 1 / [ a2 (Fo2) + ( A P)2 + ( B P) ] where P = ( Fo2 + 2 Fc2) / 3.
Results and discussion
DT crystals from toluene (DTjoi)
Crystal data for all crystals 3 as well as information about data collection and refinement are given in Table 1. Selected bond lengths of all three structures are given in Table   2.   Considerable   double   bond   localization   in benzene-ring   bond    distances    indicates    a    quinonoid structure, as confirmed by short exo C-N (1.292(2) Â) and C-0 (1.221(2) Â) bond lengths appropriate for oximo and keto groups, respectively. All atoms of the dimer in Figure 1  lie nearly in plane except for tert-buty\ groups. The oximo group is in an anti configuration with its oxygen directed away from the keto  function,  thus preventing formation   of  an   intramolecular   hydrogen   bond.   The molecules are connected through two hydrogen bonds and a weak interaction to carbonyl oxygens as indicated in Figure 1. Interatomic distances and angles of hydrogen bonds are given in Table 3.
A. Kržan P. Kamaras, M. H. Dickman, D. R. Crist, V. Horâk.: Novel Associations ofortho Quinone...
Figure 1 Hydrogen bonding between molecules of 4,6-di-tert-butyl-1,2-benzoquinone 2-monooxime derived from toluene solution. Labeled are non-hydrogen atoms of the molecule         from         the
asymmetric unit. 02A and HI atoms are from the symmetry related molecule. H atoms on the ring and t-butyl groups are not shown.
Compound / solvent
DT / toluene
DT / methanol
Cu(II)DT / toluene and cyclohexane
Empirical formula
Relative formula weight
Temperature
Wavelength
Crystal system
Space group
Unit cell parameters
Volume, Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Limiting indices
No. of reflections collected No. of independent reflections Goodness-of-fit on F" Final R indices [I>2g(I)] R indices (all data) Largest diff. peak and hole
C14H21N02
235.32
203(2) K
0.71073 A
Monoclinic
P21/n,No. 14
a =12.447(1) A a = 90°
b = 8.419(l)A p = 91.18(1)*
c= 13.431(1) A y = 90°
1407.2(2) A3, 4
1.111 Mg/m3
0.074 mm"1
512
0.5x0.3x0.3 mm
2.25 to 25.00°
-K=h<=14, -K=k<=10
-15<=1<=15
3229
2461 [R(int) = 0.0297]
1.060
Rl = 0.0397, wR2 = 0.1100
Rl = 0.0491, wR2 = 0.1134
0.169 and-0.146 eA"3
C15H25N03
267.36
203(2) K
0.71073 A
Triclinic
P-l,No. 2
a = 9.258(3) A a = 75.55(2)'
b = 9.370(3) A ß =69.33(3)°
c = 9.812(3) A y = 82.51(4)c
770.3(4) A3, 2
1.153 Mg/m3
0.079 mm"1
292
0.6 x 0.6 x 0.6 mm
2.25 to 27.51°
-l<=h<=ll,-ll<=k<=ll
-12<=1<=12
4108
3456 [R(int) = 0.0964]
1.029
Rl = 0.0530, wR2 = 0.1400
Rl =0.0710, wR2 = 0.1562
0.324 and -0.254 eA"3
C28H4,N208ClCuC35H35 678.18
173(2) K
0.71073 A
Triclinic
P-l,No. 2
a= 10.513(1) A a= 111.054(1)
b= 18.454(1) A ß = 90.302(1)°
c= 18.708(1) A y = 95.774(1)"
3366.7(3) A3, 4
1.338 Mg/m3
0.778 mm"1
1430
0.3x0.3x0.4 mm
1.19 to 28°
-14<=h<=13,-23<=k<=24
-24<=1<=24
36209
15538 [R(int) = 0.0254]
1.073
Rl =0.0559, wR2 = 0.1385
Rl =0.0705, wR2 = 0.1511
2.464 and-2.016 eA"3
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Acta Chim. Slov. 2001, 48, 229-239.
DT crystals from methanol (DTMeon)
DTMeOH also is a dimeric structure with the two DT held together by hydrogen bonding to two bridging methanol molecules as shown in Figure 2. Bond lengths in the benzene ring and exo bonds, C-N (1.297(2) Â) and C-0 (1.221(2) Â) indicate each DT to be in the quinonoid form as found for DTjoi- Both DT molecules are nearly planar and all four molecules (2 DT + 2 MeOH) are nearly coplanar. The oximo group is in an anti          configuration          preventing
intramolecular hydrogen bonding. The methanol OH group is a donor of two weak hydrogen bonds in which the keto oxygen and oximo nitrogen act as acceptors. The hydrogen bond connecting the oximo group (donor) and methanol oxygen (acceptor), on the other hand, is shorter indicating significant stabilization. Hydrogen bond parameters are given in Table 3.
In the solvents which were used the position of the tautomeric equilibrium should differ substantially, since the phenolic: quinonoid ratio is 38:62 in ethanol and 79:21 in benzene. The strong preference for the quinonoid form in the solid phase, even when recrystallized from toluene, must be due to strong intermolecular attractions not present in dilute solution.
In regard to hydrogen bonding the DT molecule has the potential alternative of forming an intramolecular hydrogen bond. ' ' This bond, although strong, would involve only two heteroatoms, leaving the third one nonbonded or perhaps weakly bonded whereas in the above discussed structures intermolecular hydrogen bonding involves all three heteroatoms in stabilizing contacts. The DTMeOH structure includes additional (stronger) intermolecular hydrogen bonds involving the methanol oxygen.
A. Kržan P. Kamaras, M. H. Dickman, D. R. Crist, V. Horâk.: Novel Associations ofortho Quinone...
Figure 2 Hydrogen bonding between molecules of        4,6-di-tert-butyl-l,2-benzoquinone        2-
monooxime and methanol molecules. Labeled are non-hydrogen atoms of the molecule from the asymmetric unit. 02M atom is from the symmetry related methanol molecule. H atoms on the ring and t-butyl groups are not shown.
Acta Chim. Slov. 2001, 48, 229-239.
235
The balance of energy contributions that govern methanol inclusion is very delicate, as shown by ease of methanol removal when crystals are exposed to ambient atmosphere at room temperature. The crystal is then in a disordered state that leads to decomposition, a phenomenon not observed in toluene-derived crystals even over longer periods. However, the fact that the interaction is sufficient to exist in the solid state indicates that intramolecular bonding is not likely in a solution with abundant solvent molecules that readily form hydrogen bonds.
Table   2.   Selected   bond   lengths   in   4,6-di-fert-butyl-l,2-benzoquinone   2-monooxime   (DT)   and Cu(II)(HL)(L)C104 (where HL=DT) crystals.
Length  Compound          DTTo|             DTMc0h                                          Cu(II)DT
Complex 1                           Complex 2
Bond			Ligand la	Ligand 2a	Ligand V	Ligand 21
0(1)-C(1)	1.221(2)	1.221(2)	1.257(3)	1.272(3)	1.250(3)	1.267(3)
0(2)-N(l)	1.3723(14)	1.367(2)	1.335(3)	1.289(3)	1.343(3)	1.288(3)
N(l)-C(6)	1.292(2)	1.297(2)	1.303(4)	1.327(3)	1.306(4)	1.327(4)
C(l)-C(2)	1.472(2)	1.472(2)	1.454(4)	1.450(4)	1.454(4)	1.454(4)
C(l)-C(6)	1.501(2)	1.499(2)	1.484(4)	1.462(4)	1.487(4)	1.454(4)
C(2)-C(3)	1.351(2)	1.341(2)	1.363(4)	1.367(4)	1.359(4)	1.366(4)
C(2)-C(0A)	1.527(2)	1.531(2)	1.534(4)	1.530(4)	1.527(4)	1.529(4)
C(3)-C(4)	1.460(2)	1.460(2)	1.469(4)	1.456(4)	1.466(4)	1.456(4)
C(4)-C(5)	1.340(2)	1.342(2)	1.357(4)	1.351(4)	1.350(4)	1.359(4)
C(4)-C(0B)	1.527(2)	1.525(2)	1.528(4)	1.534(4)	1.529(4)	1.529(4)
C(5)-C(6)	1.440(2)	1.434(2)	1.439(4)	1.426(4)	1.427(4)	1.430(4)
0(1M)-C(1M)	-	1.414(2)	-	-	-	-
Cu(l)-N(l)	-	-	1.980(2)	1.962(2)	1.973(2)	1.958(2)
Cu(l)-0(1)	-	-	2.004(2)	1.990(2)	1.998(2)	1.956(2)
Cu(l)-0(1C1)	-	-	2.149(2)	-	2.129(3)	-
Ligands 1 and 2 are the oximo (HL) and oximato (L") ligand in the copper (II) DT complexes.
DT-copper(II) Perchlorate complex
As shown in Figure 3, Cu(II) is bonded to two DT (both bidentate) with a fifth coordination site occupied by Perchlorate. One DT is in anion form giving the overall formula Cu(II)(HL)(L) CIO4 where DT is represented as a ligand HL. The geometries of the two DT ligands differ slightly as shown by values for bond lengths in Table 2. The
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Acta Chim. Slov. 2001, 48, 229-239.
asymmetric unit of this compound contains two different Cu(II)(HL)(L)C104 complex molecules and one solvent (toluene) molecule, originating from preparative work, which is disordered. Tables 2 and 3 show that bond lengths are similar in both complexes. In both cases the oximo and oximato groups of DT ligands are in a eis relationship and hydrogen bonded to each other. There is no indication of the trapped solvent affecting much the DT ligands.
Figure 3  Hydrogen bonding in Cu(II)(HL)(L)C104 complex. The ORTEP picture shows only one of the two complex molecules from the asymmetric unit.
97    98
The Cu(II)-DT complex resembles similar structures reported in the literature,   ' thus indicating little influence of bulky substituents. The 2:1  complex with square pyramidal geometry in which the metal is bound to the keto oxygen and oximo nitrogen of quinonoid ligands is the most commonly encountered arrangement among five-
90
coordinated Cu (II) complexes. However, the eis geometry of oximo groups in the present case, with a pseudo mirror plane, is unusual, since reported examples have these groups trans. The eis geometry may result from the complex having a HL and L ligands whereas reported structures have only L ligands. HL acts as a hydrogen bond donor but the hydrogen bond can form only in the eis and not in a trans geometry. It is not clear whether this cis/trans effect is in any way due to tert-butyX groups. Slight differences in geometries within ligands bound to the same metal ion were also reported in a number of similar structures.
Benzene ring bond lengths in complexed ligands show less variation than in DTtoI or DTMeOH- This shift toward the quinolic form is further confirmed by bond lengths of keto and oximo groups. These results are in agreement with the reported
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Acta Chim. Slov. 2001, 48, 229-239.
237
effect of bound metal ions causing a mesomeric shift toward the phenolic limiting structure.18
Table 3. Parameters of hydrogen bonds in crystals of 4,6-di-fer?-butyl-l,2-benzoquinone-2-monooxime.
Crystal	X-H Ya	X-H,Â	Y H, Â	X-Y, Â	2 X-H Y,0
DT / Toluene	02A-H1 Ol	0.90(2)	2.42(2)	3.245(1)	152(2)
	02A-H1 NI	0.90(2)	2.06(2)	2.792(2)	138(2)
DT / Methanol	OIM-HIM Ol	0.82(1)	2.239(8)	2.882(2)	136(1)
	OIM-HIM NI	0.82(1)	2.33(1)	3.091(2)	154.2(9)
	02-H1  Ol M	0.91(3)	1.75(3)	2.621(1)	159(3)
Cu(II)(HL)(L)C104	02-H1  02Ab	0.88(5)	1.71(5)	2.556(3)	162(4)
	02-H1  02AC	0.87(5)	1.71(6)	2.571(4)	168(5)
a Atoms marked according to notations in figures
Hydrogen bond in complex 1 c Hydrogen bond in complex 2
Conclusions
Although the quinonoid form of DT is favored in protic and polar ethanol and the phenolic in nonpolar benzene, the present results show that in the solid state DT exists in the quinonoid form whether crystals are obtained from toluene or methanol. In dilute benzene solution, stabilizing factors for the phenolic form are aromaticity and intramolecular OH—NO hydrogen bonding. Loss of aromaticity in quinonoid forms found in the solid state must be compensated for by other forces such as hydrogen bonds and increased dipole-dipole interactions in the more polar quinonoid forms.
The structure of DTmëoh is especially interesting in that it is fundamentally the same as DTtoI except that two bridging methanol molecules add two more H bonds to the dimeric structure. To our knowledge this is the first report of dimeric o-quinone monooxime structures. None of the compounds previously reported possess large bulky groups that could affect packing. Side groups in these molecules were chain-like, and several possessed strongly polar groups such as -SO3" that could affect packing through additional strong hydrogen bonds. In the structures described in this paper the tert-bu\y\
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Acta Chim. Slov. 2001, 48, 229-239.
group vicinal to the keto group hinders formation of a chain-like structure. The observed intermolecular hydrogen bonding to methanol molecules in the solid state suggests that the often postulated intramolecular hydrogen bonding of o-quinone monoximes is not likely in protic solvents due to the abundance of solvent sites for hydrogen bonding.
References and Notes
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25.   Crystallographic data (excluding structure factors) for the structures in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 150422 (Cu(II)DT), 150924 (DTtol)and 150925 (DTMeOH). Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax: +44 1223 336033 or e-mail: deposit@ccdc.cam.ac.uk).
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A. Kržan P. Kamaras, M. H. Dickman, D. R. Crist, V. Horâk: Novel Associations ofortho Quinone...
Acta Chim. Slov. 2001, 48, 229-239.                                239
28.   a) Bisi Castellani, C; Carugo, O.; Coda, h. Acta Cryst, Sect. C 1988, 44, 267; b) Bisi Castellani, C; Carugo, O.; Coda, A. Inorg. Chem. 1987, 26, 671; c) Bisi Castellani, C; Calligaris, M.; Carugo, O. Inorg. Chim. Acta 1988, 150, 203; d) Bisi Castellani, C; Gatti, G.; Mulini, R. Inorg. Chem. 1984, 23, 4004; e) McPartlin, M. Inorg. Nucl. Chem. Lett. 1973, 9, 1207; f) Carreck, P. W.; Charalambous, J.; Kensett, M. J.; McPartlin, M.; Sims, R. Inorg. Nucl. Chem. Lett. 1974,10,1A9.
29.   Hathaway, B. J. Struct. Bonding (Berlin) 1984, 57, 55-118.
Povzetek
Za razliko od objavljenih rentgenskih struktur orto kinon monooksimov, ki v trdni fazi vsebujejo intramolekulamo vodikovo vez ali imajo verigasto strukturo, sterièno ovirani 4,6-di-fcrt-butil-l,2-benzokinon-2-monooksim (DT) kristalizira kot dimer. V kristalih pridobljenih iz toluena so DT molekule povezane v dimere z dvema vodikovima vezema v šestèlenskem obroèu, ki je obièajen za enostavne oksime a ne za kinon monooksime. V kristalih pridobljenih iz metanola so DT dimeri povezani z dvema povezovalnima molekulama metanola od katerih vsaka sodeluje v treh vodikovih vezeh. Èeprav sta v raztopini prisotni fenolna in kinonska tautomerna oblika, s tem, da nepolarna topila stabilizirajo aromatièno fenolno obliko, sta obe kristalni strukturi, neodvisno od topil v katerih sta nastali, v kinonski obliki. Kinonski obliki sta stabilizirani z moènimi intermolekulamimi, veècentriènimi vodikovimi vezmi in dipol-dipol interakcijami, ki nasprotujejo vplivu izgube aromatiène stabilizacije. DT je z bakrovim (II) perkloratom tvoril kristale tipa Cu(II)(HL)(L)C104, kjer je HL = DT. HL in L sta koordinirana na Cu(II) atom v eis obliki.
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