Scientific paper
Synthesis and Crystal Structures of Two Novel Molybdenum(V) Complexes with Glycine
and D,L-Valine
Marina Ta{ner,a Biserka Prugove~ki,a Draginja Mrvo{-Sermek,a
Branka Korpar-Colig,a Gerald Giesterb and Dubravka Matkovic-Calogovi}
a Laboratory of General and Inorganic Chemistry, Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, 10000 Zagreb
b Institute for Mineralogy and Crystallography, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria
* Corresponding author: E-mail: dubravka@chem.pmf.hr
Received: 16-04-2008 Dedicated to the memory of Professor Ljubo Golic
a
Abstract
Two novel molybdenum(V) complexes, [Mo2O4(acac)2(gly)](EtOH)(H2O), 1, and [Mo2O4(acac)2(D,L-val)], 2, (acac = acetylacetonato, EtOH = ethanol), were synthesized by the reaction of [Mo2O3(acac)4] and amino acids glycine and d,L-valine, respectively. The complexes were characterized by elemental and IR spectral analysis. Their structures were determined by the single crystal X-ray diffraction method. Both complexes are dinuclear, with singly bonded Mo—Mo in the {Mo2O4}2+ core, a carboxylato bridging amino acid, and with each Mo atom additionally coordinated by a bidenta-te acetylacetonato ligand. Crystal data: C14H27Mo2NO12 (1) crystallizes in the monoclinic system, space group P21/n, with a = 8.439(2), b = 20.613(4), c = 12.63i9(3) À, ß= 98.410(10)°, Z = 4; C15H25Mo2NO10 (2) crystallizes in the orthor-hombic system, space group Pbca with a = 13.596(1), b = 19.346(1), c = 15.080(1) àÀ, Z^ 8.
Keywords: Molybdenum(V) complexes; crystal structure; amino acid complexes
1. Introduction
Molybdenum is an essential metal in almost all organisms and occurs in many enzymes.1 The importance of investigating molybdenum complexes with amino acids as possible molybdoenzyme model compounds was recognized as long as 40 years ago.2,3 The dinuclear fragment {Mo2O4}2+, which is also present in the two structures described here, can often be found as the building unit in the chemistry of molybdenum(V). In this fragment each molybdenum atom is bound to two bridging oxygen atoms and one terminal oxygen atom. There can also be an additional ligand such as the carboxylato group acting as a bridging n1 : n1 (O,O') ligand.45 Structures of many acetato complexes of molybdenum(V) with the {Mo2O4}2+ units as building block are known. They can be dinuclear6 or can assemble into oligonuclear species.67 Molybdenum atoms in the dinuclear complexes can also
be doubly bridged by sulphur atoms as in [Mo2O2S2(histi-dine)2]8 or [Mo2O2S2 (L-cysteine)2].9
The first crystal structures of dinuclear molybde-num(V) complexes with the {Mo2O4}2+ core and with amino acids as ligands were those of [Mo2O4 (L-his)2]-3H2O 10 and Na2[Mo2O4(L-cys)2] ■ 5H2O.11 However in these two structures the amino acid is not bridging, the L-histidinato ligand is tridentately N,N,O-bound to each Mo, while the L-cysteinato ligand is S,N,O-bound to each molybdenum atom. The structure of Na2[Mo2O4 (L-cys)2] ■ 5H2O was later redetermined by neutron dif-fraction.12 The crystal structure of another polymorph of [Mo2O4(L-his)2] ■ 3H2O was recently determined and a similar molecular structure as that of Knox and Prout was
found.13
Two structures of binuclear complexes with the {Mo2O4}2+ core and with molybdenum atoms bridged through the carboxylic atoms of the corresponding amino
acid are known. One is the structure of ^-glycine-0;0'-di-^-oxo-bis[(glycinato-^,0)oxomolybdenum(V)]14 where one glycine is p2-bridging and the other is a glycinato li-gand that is ^,0-bound to each molybdenum. The other is that of [Mo2O4(acac)2thala] ■ 3EtOH (acac = acetylaceto-nato; thala = 3-(2-thienyl)-D,L-alanine).15 Several structures that have {Mo2O4}2+ fragments linked into polynuc-lear complexes with bridging amino acids, mostly glycine or alanine, have been reported. 16-21
In this paper, we report the synthesis and crystal structures of two new dinuclear complexes of molybdenum with amino acids: di-^-oxo-^-[glycine-0;0']-bis[oxo2,4-pentanedionatomolybdate(V)] solvated with ethanol and water, [Mo2O4(acac)2(gly)] ■ (EtOH)(H2O), 1, and di-^-oxo-^-[D,L-valine-0;0']-bis[oxo(2,4-pentane-dionato)molybdate(V)], [Mo2O4(acac)2(D,L-val)], 2, (Scheme 1). Both complexes were characterized by chemical analysis and IR spectroscopy, and their crystal structures were determined by single crystal X-ray diffraction.
H3C
H3C
Mo ^Mo
CH.
CHc
-NH.
Scheme 1
R
R = H (1) R = CH(CH3)2 (2)
2. Experimental
2. 1. Materials and Methods
All solvents and chemicals were of commercial reagent grade; all reactions were carried out under an atmosphere of purified nitrogen. Complex [Mo2O3(acac)4] was prepared as described in literature.22,23 The IR spectra were recorded on a FTIR 1600 Fourier-transform spectrophotometer, using the KBr pellet technique, in the 4500-450 cm-1 region. Molybdenum was analytically determined according to the procedure given in the literatu-re.24
2.2. Synthesis and Characterization
Di-^-oxo-^ -[glycine-0;0']-bis-[oxo(2,4-pentane-dionato)molybdate(v)]-ethanol(1/1)-water(1/1), [Mo2O4 (acac)2gly](EtOH)(H2O), 1	2 4
Mo2O3(acac)4 (0.635 g, 1 mmol) was dissolved in 20 ml of boiling ethanol in a nitrogen atmosphere. The dark red mixture was hot filtered, heated again till boiling, an aqueous solution of glycine (0.075 g of glycine in 1.5 ml
of boiled water) was then added and heated in nitrogen stream for several hours. After several days at room temperature red-brown crystalline product was obtained. Anal. Calc. For Mo2O12C14H27N: C, 28.33, H, 4.55, N, 2.36%. found: C,23.2(:i, 3.31, N, 2.76%. IR(KBr), (cm-1): 1556(s); 1421(m); 964(s); 741(m).
Di-^-oxo-^- [D,L-valine-0:0'] -bis [oxo(2,4-pentane-dionato)molybdate(V)], [Mo2O4(acac)2 D,L-val], 2
0.635 g of Mo2O3(acac)4 (1 mmol) was dissolved in 35 ml ethanol, boiled under nitrogen atmosphere and than hot filtered. A solution of D,L-valine (0.117 g, 1 mmol) was added to the filtrate and heated for several hours. On the next day, light-red crystals of 2 were obtained. Anal. Calc. for Mo2O10C15H25N: C, 31.53, H, 4.37, N, 2.45%. found: C, 31.63, H, 4.28, N, 2.37%. IR(KBr), (cm-1): 1612 (s); 1431 (m); 961 (s); 743 (m).
2. 3. X-ray Structure Analysis
Single, red colored, crystals of 1 and 2 were grown by slow evaporation of the corresponding solutions obtained from the above described preparations. The X-ray diffraction intensities for compound 1 were collected at room temperature on a Philips PW1100 difractometer updated by Stoe and Cie25 using Mo-^a radiation (À = 0.7107 À) with the scan mode. Data were reduced using X-RED.26 Intensity data for compound 2 were collected at 200 K on a Nonius KappaCCD difractometer with graphite-monochromated Mo-^a radiation and the data were reduced using the DENZO27 program. The crystal structures were solved by direct methods using the SHELXS9728 program. All non-hydrogen atoms were refned anisotropically by full-matrix least-squares calculations based on F2 with the SHELXL9729 program. Hydrogen atoms in 1 could be found in the difference Fourier map, however, since the geometry was poor they were placed at ideal calculated positions (aromatic, secondary), or using the rotating group refinement (methyl, amino, hydroxyl groups) with atoms placed at density maxima. The displacement parameters were taken as 1.2 times (aromatic, secondary), and 1.5 times of their parent atoms (methyl, amino and hydroxyl groups, H2O molecule). For the H2O molecule one hydrogen atom was found in the difference Fourier map while the other was calculated using CALC-OH30 and was included in the refinement with restrained geometry. In 2 all hydrogen atoms were located in the difference Fourier maps and refined isotropically. Geometry calculations were done using PLATON31 and PARST32,33 and the structure drawings were prepared using PLATON and PLUTON programs. Crystal data and refinement details are listed in Table 1.
Compound 2 is a racemic compound. Although a mechanical mixture of two enatiomers is sometimes possible (racemic conglomerate), this was not observed in our case.
Table 1. Crystal data, data collection and structure refinement for compounds 1 and 2.
Compound	1	2
Empirical formula	C14H27Mo2NO12	C15H25Mo2NO1„
Formula weight	593.25	571.24
Crystal system	monoclinic	orthorhombic
Space group	P21/n	Pbca
Unit cell dimensions (À, °)		
a (À)	8.439(2)	13.596(1)
b (À)	20.613(4)	19.346(1)
c (À)	12.639(3)	15.080(1)
a (°)	90	90
ß (°)	98.410(10)	90
y(°)	90	90
y (À3)	2175.0(8)	3966.5(4)
Z	4	8
T (K)	293(2)	200(2)
Đcalc(g cm-3)	1.812	1.913
^ (MoKa) (mm-1)	1.212	1.318
F(000)	1192	2288
Ranges of h, k, l	-11 to 11, 0 to 29, 0 to 17	-19 to 19, -27 to 27, -21 to 21
Reflections collecteda/ unique / observed	6561/6310/2527	11490/6045/5488
Data/restrains/parameters	6310/0/271	6045/0/353
Goodness of fit on F2	0.961	1.068
R^ & wRc	0.066, 0.102	0.022, 0.054
Largest diff. peak & hole/(e À-3)	0.610, -0.747	0.442. -0.701
avalue for 2 refers to unique data but with unmerged Friedel pairs bR = S||Fo| -Fc|| / S|Fo| c wR = [Sw(Fo2 -Fc2)2/Ew(Fo2)2]1/2
The crystallographic data for compounds 1 and 2 have been deposited with the Cambridge Crystallographic Data Center as supplementary material with the deposition numbers: CCDC 682621 for 1 and 682622 for 2. Copies of the data can be obtained, free of charge via http:/www.ccdc.cam.ac.uk/const/retrieving.html.
3. Results and Discussion
By direct reaction of [Mo2O3(acac)4] with the amino acids glycine and valine, dinuclear molybdenum complexes 1 and 2 with triply bridged molybdenum atoms were prepared. The diamagnetism of both complexes together with the short Mo-Mo bond indicate presence of a metal-metal bond. IR data indicate presence of the {Mo2O4}2+ moiety. Absorption maxima that were found in the 1000-900 cm-1 and 750-700 cm-1 region are characteristic of the stretching of isolated Mo=O and the bridging Mo-O-Mo, respectively. Maxima typical for the acetylacetonate ligand and the amino acids were also found (Table 2).
3. 1. Molecular Structures of 1 and 2
Both complexes are dinuclear, with a {Mo2O4}2+ core. The Mo(p2-O)2Mo moiety is not planar. The fold angle is 165.81(16)° and 163.96(5)°, in 1 and 2, respectively. The two molybdenum atoms are bridged by the carboxy-lato group from the amino acid (glycine in 1, Fig. 1, and valine in 2, Fig. 2), and with each Mo atom additionally coordinated by a bidentate acetylacetonato ligand. Selected bond distances and angles are given in Table 3. The Cambridge Structural Database34 was searched for structures containing the {Mo2O4}2+ core, with a bridging car-boxylato group, and with two additional oxygen atoms coordinated to each molybdenum atom. The search gave 26 hits. After supressing outliers from polynuclear structures, the following ranges and mean values were found for 28 such structural fragments: Mo-Mo distances from 2.549 to 2.602 À, mean value 2.57 À; Mo=O 1.653 -1.699 À, mean 1.68 À, Mo-Ob 1.916-1.962 À, mean 1.94 À; Mo-Ocarboxyl 2.221 - 2.41b9 À, mean 2.34 À; angle Mo-Ob-Mo 81.94 - 84.77°, mean 83°. Oxygen atoms
Table 2. Characteristic IR absorption maxima (cm ') in 1 and 2
	Characteristic IR absorption maxima (cm			-1)		
	V(O-H)	V(N-H)	Vas(C=O)	Vs(C=O)	V(Mo=O)	V(Mo-O-Mo)
[Mo2O4(acac)2gly](EtOH)(H2O)	3423	3100	1556	1421	964	741
[Mo2O4(acac)2DL-val]	3448	3228	1612	1431	961	743
from the carboxylic group occupy a pair of trans sites to Mo=O, and lengthening of these Mo-O bonds is due to the known trans effect. Values in 1 and 2 correspond well to these mean values: Mo-Ob distances range from 1.9344(11)° to 1.950(4) À in 1 and 2, respectively. The Mo-Mo distances of 2.5678(10) À in 1, and 2.5687(2) À in 2 correspond to a single Mo-Mo bond.3536 The Mo-Oacac distances, ranging from 2.086(5) to 2.1038(11) À, are in good agreement with previously determined similar structures.15' 37-40 The Mo-Ocarboxyj distances fall within the above mentioned range and are from
2.2791(11) to 2.354(4) À. The overall structures are similar to that of [Mo2O4(acac)2thala]3EtOH (thala = 3-(2-thienyl)-D,L-alanine).15 Octahedra around the Mo atoms are significantly distorted with angles ranging from 74.78(4) to 106.58(6). The amino acids in both structures are in the zwitter-ionic form.
3. 2. Crystal Structure of 1
The asymmetric unit consists of the complex molecule, ethanol and H2O molecules. The protonated amino
Figure 1. A view of the complex molecule [Mo2O-(acac)2gly] with labeling of the non-hydrogen atoms and with ellipsoids at the 50% probability level.
Figure 2. A view of the complex molecule [Mo2O-(acac)2 D,L-val], 2, with labeling of the non-hydrogen atoms and with ellipsoids at the 50% probability level.
Table 3. Selected interatomic distances (À) and angles (°) in 1 and 2.
	1	2
Mo1-Mo2	2.5678(10)	2.5687(2)
Mo1-O1	1.950(5)	1.9476(11)
Mo1-O2	1.945(4)	1.9344(11)
Mo1-O3	2.086(5)	2.0906(12)
Mo1-O4	2.087(5)	2.0916(12)
Mo1-O7	2.354(4)	2.3538(12)
Mo1-O9	1.671(5)	1.6814(12)
Mo2-O1	1.935(4)	1.9405(11)
Mo2-O2	1.933(4)	1.9280(11)
Mo2-O5	2.094(5)	2.0946(11)
Mo2-O6	2.103(4)	2.1038(11)
Mo2-O8	2.287(4)	2.2791(11)
MO2-O10	1.693(5)	1.6899(12)
Mo1-O2-Mo2	82.93(16)	83.37(4)
Mo2-O1-Mo1	82.74(17)	82.70(4)
O1-Mo1-O3	85.0(2)	84.41(5)
O1-Mo1-O4	159.70(19)	156.26(5)
O1-Mo1-O7	83.04(16)	80.53(4)
O1-Mo2-O2	96.62(18)	95.93(5)
O1-Mo2-O5	83.4(2)	86.68(4)
O1-Mo2-O6	160.46(18)	160.11(4)
O1-Mo2-O8	84.48(17)	82.30(4)
O2-Mo1-O1	95.73(18)	95.48(5)
	1	2
O2-Mo1-O3	156.19(18)	157.40(5)
O2-Mo1-O4	86.37(18)	85.74(5)
O2-Mo1-O7	79.46(16)	80.45(4)
O2-Mo2-O5	156.27(18)	156.20(5)
O2-Mo2-O6	87.41(18)	84.03(5)
O2-Mo2-O8	81.07(17)	82.11(4)
O3-Mo1-O4	85.12(19)	85.55(5)
O3-Mo1-O7	77.00(17)	77.23(4)
O4-Mo1-O7	77.49(18)	76.30(4)
O5-Mo2-O6	85.32(19)	85.65(4)
O5-Mo2-O8	75.30(18)	74.78(4)
O6-Mo2-O8	77.24(17)	77.98(4)
O9-Mo1-O2	104.0(2)	105.01(5)
O9-Mo1-O1	103.8(2)	105.41(6)
O9-Mo1-O3	98.9(2)	96.74(5)
O9-Mo1-O4	95.3(3)	97.09(6)
O9-Mo1-O7	171.9(2)	171.26(5)
O10-MO2-O1	103.2(2)	105.76(5)
O10-MO2-O2	105.8(2)	106.58(6)
O10-MO2-O5	97.2(2)	95.36(5)
O10-MO2-O6	94.0(2)	93.22(5)
O10-MO2-O8	168.8(2)	167.11(5)
Figure 3. Hydrogen bonding in 1 is shown by dashed lines, b: 1-x,- y,1- z, d: 1+x, y, z.
Table 4. Hydrogen bonds in 1 and 2.
D-H-A	d(D-H) /A	d(H-A) /A	d(D-A) /A	<(DHA) /°
1 N1-H1D-O11	0.89	1.81	2.683(14)	166
N1-H1E-O2i	0.89	2.20	3.078(7)	169
N1-H1F-O2ii	0.89	2.41	2.881(7)	114
N1-H1F-O5i'i	0.89	2.32	3.110(8)	148
O11-H11A-O9i	0.87	1.92	2.648(11)	140
O11-H11B-O12	0.92	1.54	2.443(17)	165
O12-H12-O1	0.82	2.00	2.786(13)	160
2 N1-H1-O1iv	0.91(2)	1.78(3)	2.6710(19)	166(2)
N1-H2-O8	0.89(2)	2.15(2)	2.604(2)	110.7(17)
N1-H3-O6v	0.83(3)	2.25(3)	3.041(2)	161(2)
N1-H3-O10V	0.83(3)	2.63(3)	3.096(2)	117(2)
i: 1+x, y, z; ii: 1-x, - y,1- z	; iii: 1-x, - y,1-z ; iv:	-x,1-y,1-z; v: -1/2+x, y,1/2-z		
group is an excellent hydrogen bond donor and all three N1-H atoms are involved in hydrogen bonding, Table 3, Fig. 3. A molecule of [Mo2O4(acac)2gly] is linked to its centrosymmetrically related pair by the hydrogen bonds N1-H1F to O2 and O5, while the hydrogen bond N1-H1E-O2 links it to another neighbouring complex molecule. Through these hydrogen bonds the complex molecules are linked into infinite chains along the a axis. The third hydrogen links it with a H2O molecule, N1-H1D-O11. The H2O molecule is a donor toward a neighbouring complex molecule through O11-H11A ■■■O9, and to the ethanol molecule O11-H11B^O1. The ethanol molecule forms another hydrogen bond with a complex
molecule, O12-H12^O1, thus indirectly linking two complex molecules. A packing diagram is shown in Fig. 4.
3. 3. Crystal Structure of 2
There are no solvent molecules in 2, however, extensive hydrogen bonding is also found, Table 4, Fig. 5. The protonated amino group is again the hydrogen bond donor through all three hydrogen atoms, however one is intramolecular, N1-H2^O8. A hydrogen bond to the bridging atom of a molecule related by inversion center is formed through N1-H1, while N1-H3 forms two hydrogen bonds with O6 and O10 of another neighbouring molecule. In this way molecules are interlinked in planes parallel to (010), Fig. 6.
Figure 4. Packing of the molecules of [Mo2O4(acac)2gly](EtOH) (H20), 1, in the unit cell.
Figure 6. Packing of the molecules of [Mo2O4(acac)2 D,L-val], 2, in the unit cell.
Figure 5. Hydrogen bonding in 2 is shown by dashed lines, a: -x, 1-y, 1-z. The weak N1-H^^^010 hydrogen bond is not shown.
4. Acknowledgement
The research was supported by the Ministry of Science and Technology of the Republic of Croatia, Grant no. 119-1193079-1084.
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Povzetek
Sintetizirali smo dva nova molibden(V) kompleksa, [Mo2O4(acac)2(gly)](EtOH)(H2O), 1, in [Mo2O4(acac)2(D,L-val)], 2, (acac = acetilacetonato, EtOH = etanol), z reakcijo med [Mo2O3(acac)4] in amino kislinama glicin and D,L-valin. Komplekse smo okarakterizirali z elementno in IR spektralno analizo. Strukturi smo določili z rentgensko difrakcijo na mo-nokristalu. Oba kompleksa sta dinuklearna z enojno vezanim Mo-Mo v {MojO^}2^, z amino kislino vezano preko kar-boksilata ter z vsakim Mo atomom dodatno koordiniranim z bidentatnim acetilacetonato ligandom. Kristalografski podatki: C14H27Mo2NO12 (1) kristalizira v monoklinskem sistemu, prostorska skupina P21/n, a = 8.439(2), b = 20.613(4), c = 12.639(3) À, ß = 98.410(10)°, Z = 4; C15H25Mo2NO10 (2) kristalizira v ortorombičnem sistemu, prostorska skupina Pbca, a = 13.596(1), b = 19.346(1), c = 15.080(1) Àl, Z =8.