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Molecular structure, characterization and reactivity of dioxo complexes formed by vanadium(V) with α-hydroxycarboxylate ligands
www.elsevier.nl/locate/ica Inorganica Chimica Acta 310 (2000) 1 – 9
Molecular structure, characterization and reactivity of dioxo complexes formed by vanadium(V) with a-hydroxycarboxylate ligands M. Biagioli a, L. Strinna-Erre a, G. Micera a,*, A. Panzanelli a, M. Zema b b
a Department of Chemistry, Uni6ersity of Sassari, I-07100 Sassari, Italy Centro Grandi Strumenti, Uni6ersita` di Pa6ia, Via Bassi 21, I-27100 Pa6ia, Italy
Received 12 April 2000; accepted 24 June 2000
Abstract The structure of the dioxovanadium(V) complexes of glycolic, lactic and malic acids Rb2[{VO2(glyc)}2] [glyc= glycolato(2 −)] (1), Cs2[{VO2(mal)}2] [mal=malato(2− )] (2), and Cs2[{VO2(lact)}2]·2H2O [lact= lactato(2−)] (3) has been studied by singlecrystal X-ray diffraction. In order to assign the features of the peculiar arrangement adopted by these kinds of complexes, the metrical details of these compounds have been examined and compared to the data available in literature for analogous complexes. The reactivity of the compounds has also been studied. In particular, the reduction of the metal ion by a biological reductant like cysteine, relevant to the role of these complexes in the constitution of the cofactor of the active site of vanadium nitrogenase, has been followed by EPR spectroscopy in an aqueous solution. In addition, the effect of peroxo complex formation on the structure of the dinuclear V2O2 cage has been analyzed by comparison with previously published results. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Vanadium(V); Oxovanadium(IV); Glycolic acid; Lactic acid; Malic acid; Vanadium nitrogenase
1. Introduction Many aspects of vanadium bio-reactivity involving the different oxidation states of the element are still unknown. The discovery of nitrogen fixing organisms which have evolved an alternative nitrogenase containing homocitrate bound to vanadium in the active site cofactor [1,2] elicited interest in vanadium(V) a-hydroxycarboxylate chemistry. The first citrate complex of vanadium(V) was reported by Djordjevic et al. in 1989 [3]. Only some years later the molecular structure of the complex was solved by single-crystal X-ray diffraction methods [4]. It was found that the citrate ligand displays a unique bidentate coordination mode to vanadium via the deprotonated hydroxyl and a unidentate carboxylate. The structure of the complex is best described as a dimer of two five-coordinate vanadium centers doubly bridged by alkoxide * Corresponding author. Tel.: + 39-079-22 9541; fax: + 39-079-21 9269.
oxygen atoms. Two oxo ligands complete the donor set at the metal centers. The same arrangement was previously described for the complex of another a-hydroxycarboxylic ligand, 2-ethyl-2-hydroxy-butanoic acid (ehba) [5]. The interaction of homocitrate (hmcitr) with vanadium(V) resulted in a compound with a similar structure [6]. The fully extended coordination mode of the ligand was considered to be of biological relevance and a role of early precursor for the nitrogenase active site was attributed to the dinuclear complex. The complex of 2-hydroxy-2-methylpropanoic or methyllactic acid (hmp), with a similar structure, has recently been published [7]. a-Hydroxycarboxylate ligands take part in the incorporation of vanadium and molybdenum in the cofactor of the active site of nitrogenases. Metal ions are adsorbed in the form of the oxoanions vanadate(V) and molybdate(VI) and are then reduced to the oxidation states III or IV. The coordination of hydroxycarboxylic ligands seems essential to the mobilization and reduction processes.
0020-1693/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 0 ) 0 0 2 6 3 - 2
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M. Biagioli et al. / Inorganica Chimica Acta 310 (2000) 1–9
Peroxo complexes are involved as intermediates of several bio-reactions, therefore the peroxo vanadium(V) complexes of a-hydroxycarboxylic ligands are currently investigated. In particular, the monoperoxo complexes of citric, malic, tartaric and glycolic acids were isolated and fully characterized [3,8 – 11]. We describe the vanadium(V) complexes of three members of this class of ligands as glycolic, lactic and malic acids. A single-crystal X-ray study has been performed on compounds isolated from aqueous solution. Rb2[{VO2(glyc)}2] [glyc =glycolato(2 − )] (1), Cs2[{VO2(mal)}2] [mal =malato(2 − )] (2), and Cs2[{VO2(lact)}2]·2H2O [lact =lactato(2 −)] (3) exhibit the same basic structure typical of all a-hydroxycarboxylate complexes. In order to assign the features of the peculiar arrangement, the structural details of the compounds have been examined and compared to each other and to those available in literature for analogous complexes. The reactivities of the complexes have also been examined. In particular, the reduction of vanadium(V) by cysteine has been followed by EPR spectroscopy in aqueous solution. In addition, the effect of peroxo coordination on the structure of the dinuclear V2O2 cage has been analyzed by comparison with previous results.
Anal. Calc. for Cs2[{VO2(lact)}2]·2H2O: C6H12Cs2O12V2: C, 11.19; H, 1.88; H2O: 5.6; Cs2O·V2O5: 69.5. Found: C, 11.58; H, 1.51; Cs2O·V2O5: 70.0; H2O: 5.0%.
2.3. Reacti6ity in water Cs2[{VO2(lact)}2]·2H2O (0.05 mmol) was dissolved in an aqueous solution (10 ml) buffered at pH 7.4 with 20 mM Hepes. L-Cysteine was added to the solution which was stirred at r.t. Different molar ratios of the vanadium compound to cysteine were investigated, namely 1:1, 1:2 and 1:10. EPR spectroscopy was used to monitor the reduction of vanadium(V) to vanadium(IV) over time. A quartz flat cell was used for the measurements at r.t. The intensity of the MI = − 1/2 transition of the isotropic spectrum was taken as an approximate measure of the VO(IV) concentration. The paramagnetic products were identified by comparison of anisotropic spectra recorded on frozen solutions with those of binary VO(IV) ligand systems. Some of them have already been examined in previous work of ours [12].
2.4. Physical measurements
Glycolic, L-lactic, D,L-malic acids, Rb2CO3, Cs2CO3, V2O5 and other reagents for both the synthetic and reactivity studies were purchased from Aldrich and used without further purification.
Elemental analyses (C and H) were obtained with a Perkin –Elmer 240 B elemental analyzer. Thermogravimetric data were obtained with a Perkin –Elmer TGS-2 apparatus in air or under a nitrogen flow. IR spectra were recorded with a FT Bruker IFS-66 interferometer using KBr (4000 –600 cm − 1) pellets. EPR spectra were recorded on a Varian E9 spectrometer at the X-band frequency (9.15 GHz) at 298 or 120 K. As usual, the samples for low-temperature measurements were added with a few drops of DMSO to ensure good glass formation in frozen solutions.
2.2. Synthesis
2.5. Crystal structure analysis
V2O5 (0.181 g, 1 mmol) and Rb2CO3 or Cs2CO3 (1.1 –1.2 mmol) were dissolved in an aqueous solution (5 ml) by heating to 40°C while stirring. The solution was cooled in an ice bath and the ligand (3 mmol) was gradually added. The reaction at r.t. yielded precipitates which were filtered off. Crystals suitable for X-ray diffractometric analysis which were formed from the mother solutions (pH 3 – 4) were kept at room temperature (r.t.) or in the refrigerator. They were filtered off, washed with EtOH, and air-dried at r.t. Rb2[{VO2(glyc)}2]: Anal. Calc. for C4H4O10Rb2V2: C, 9.91; H, 0.83; Rb2O·V2O5: 76.1. Found: C, 9.57; H, 0.89; Rb2O·V2O5: 75.5%. Cs2[{VO2(mal)}2]·2H2O: Anal. Calc. for C8H12Cs2O16V2: C, 13.13; H, 1.65, H2O, 4.9; Cs2O V2O5: 61.2. Found: C, 13.38; H, 1.64, H2O, 5.0; Cs2O·V2O5, 61.5%.
2.5.1. Crystal data Crystal data and details on the crystallographic study are reported in Table 1, whereas selected bond lengths and angles for 1, 2 and 3 are reported in Tables 2–4, respectively.
2. Experimental
2.1. Chemicals
2.5.2. Data collection and processing Unit cell parameters and intensity data were obtained on an Enraf –Nonius CAD-4 diffractometer, using graphite monochromated Mo Ka radiation. The cell dimensions were obtained by least-squares fitting of 25 centered reflections monitored in the ranges 10.24B qB18.34° for 1, 7.55B qB14.14° for 2 and 7.24BqB 14.97° for 3. Calculations were performed with the WINGX-97 software [13]. Corrections for Lp and empirical absorption were applied [14]. For crystal 3, Friedel pairs were measured to enable refinement of the Flack
M. Biagioli et al. / Inorganica Chimica Acta 310 (2000) 1–9
parameter [15] in order to confirm the absolute configuration. The three structures were solved by direct methods (SIR-92) [16], and refined by full-matrix least-squares using SHELXL-93 [17], with anisotropic displacement parameters for all non-hydrogen atoms. For crystal 1, all H atoms were seen in Fourier difference maps and refined with an isotropic displacement parameters proportional to their neighboring atoms. For crystal 2, only the H atom of the free carboxylic group was located in the DF maps and inserted in the calculations with isotropic displacement parameter proportional to that of its neighboring atom; the H atoms bound to C atoms were calculated and not refined while those of the water molecule were disregarded. The treatment of the H atoms bound to C atoms in 3 was the same as in 2. The H atoms bound
3
to O(1)W in 3 were located and inserted in the refinement with isotropic displacement parameter proportional to that of their neighboring atom while those bound to the second water molecule, which was found to be disordered, were disregarded. Atomic scattering factors and the values of Df % and Df %% were taken from International Tables for X-ray crystallography [18]. Diagrams of the molecular structures were produced by the ORTEP program [19]. It is worth noting that the E-statistics for 1 indicated its triclinic structure to be acentric. It was then solved in the atypical space group P1( . Following Marsh [20,21], an accurate analysis of the structural model revealed that the structure could be conveniently refined in the centrosymmetric space group P1( .
Table 1 Crystal data and experimental details for the compounds
Formula Molecular weight Crystal size (mm) Crystal system Space group a (A, ) b (A, ) c (A, ) h (°) i (°) k (°) V (A, 3) Z Dcalc (g cm−3) Temperature (K) Radiation, u (A, ) Monochromator v (mm−1) Scan type q Range (°) Standard reflections Maximum/minimum transmission Absorption correction method Index ranges Total Reflections measured Unique reflections Refinement type Number of parameters refined R indices a [I\2q(I)] R indices (all data) Goodness-of-fit b Extinction coefficient Flack parameter Maximum/minimum Dz (e A, −3) (Shift/e.s.d.)max
1
2
3
C4H4O10Rb2V2 484.88 0.42×0.35×0.21 triclinic P1( 6.021(1) 7.092(2) 7.982(3) 106.21(3) 107.27(2) 101.76(2) 296.83(14) 2 2.713 293 Mo Ka, 0.71073 graphite 9.761 -2q 2–35 3 every 300 reflections 0.129, 0.053 -scan −95h59, −115k511, −125l512 5216 2609 (Rint = 0.0247) F2 89 R1 = 0.0247, Rw2 = 0.0563 R1 = 0.0383, Rw2 = 0.0602 1.066 0.0294(20)
C8H12V2O16Cs2 731.86 0.21×0.21×0.18 monoclinic P21/c 9.499(3) 10.381(5) 10.000(2) 90 104.24(2) 90 955.8(6) 4 3.137 293 Mo Ka, 0.71073 graphite 4.808 -2q 2–30 3 every 300 reflections 0.421, 0.354 -scan −135h513, 05k514, 05l514 2928 2782 (Rint =0.0339) F2 131 R1 =0.0504, Rw2 =0.1340 R1 =0.0757, Rw2 =0.1484 1.049 0.0001(8)
1.004, −0.962 0.001
1.695, −2.555 0.000
C6H12Cs2O12V2 643.84 1.60×0.35×0.21 monoclinic P21 6.090(8) 7.755(4) 17.875(9) 90 99.35(9) 90 833.0(12) 2 2.423 293 Mo Ka, 0.71073 graphite 5.480 -2q 2–30 3 every 300 reflections 0.316, 0.105 -scan −85h58, −105k510, 05l525 4867 4867 (Rint =0.0000) F2 202 R1 =0.0344, Rw2 =0.0818 R1 =0.0369, Rw2 =0.0836 1.086 0.0010(3) −0.0362(261) c 1.926, −1.384 0.001
R1 =S( Fo − Fc )/S( Fo ). Rw2 = {S[w(F o2−F c2)2]/S[w(F o2)2]}1/2. Goodness-of-fit=S ={S[w(F o2−F c2)2]/(n−p)}1/2, where n is the number of reflections and p is the total number of parameters refined. c For inverted structure 0.9475(302). a
b
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Table 2 Selected interatomic distances (A, ) and angles (°) for 1 a V(1)V(2) V(1)O(1) V(1)O(4) V(1)O(5) V(1)O(1)i
3.206(2) 1.944(2) 1.614(2) 1.614(2) 1.998(2)
V(1)O(2)i O(1)V(1)i O(1)C(1) O(3)C(2)
1.979(2) 1.998(2) 1.397(2) 1.222(2)
O(5)V(1)O(4) O(5)V(1)O(1) O(4)V(1)O(1) O(5)V(1)O(2)i O(4)V(1)O(2)i O(1)V(1)O(2)i
107.87(11) 99.79(9) 100.40(10) 98.50(9) 98.90(9) 147.78(6)
O(5)V(1)O(1)i O(4)V(1)O(1)i O(1)V(1)O(1)i O(2)iV(1)O(1)i V(1)O(1)V(1)i
128.57(10) 123.53(10) 71.14(7) 76.71(7) 108.86(7)
a
Symmetry code: (i) −x, −y, −z.
Table 4 Selected interatomic distances (A, ) and angles (°) for 3
Table 3 Selected interatomic distances (A, ) and angles (°) for 2 a V(1)V(2) V(1)O(1)i V(1)O(6) V(1)O(7) V(1)O(3) V(1)O(3)i O(1)V(1)i
3.241(2) 1.977(4) 1.619(5) 1.610(4) 1.963(4) 2.016(4) 1.977(4)
O(3)V(1)i O(1)C(1) O(2)C(1) O(3)C(2) O(4)C(4) O(5)C(4)
2.016(4) 1.272(6) 1.231(7) 1.412(6) 1.213(7) 1.310(7)
O(3)V(1)O(1)i O(3)V(1)O(3)i O(6)V(1)O(3) O(7)V(1)O(3) O(7)V(1)O(6) O(7)V(1)O(1)i O(6)V(1)O(1)i
146.91(17) 70.89(16) 101.2(2) 99.2(2) 107.8(3) 95.6(2) 102.2(2)
O(6)V(1)O(3)i O(7)V(1)O(3)i O(1)iV(1)O(3)i O(2)C(1)O(1) O(4)C(4)O(5) V(1)O(3)V(1)i
121.0(2) 131.2(2) 77.07(15) 123.8(5) 123.3(6) 109.11(16)
a
group. The latter donor behaves as a monatomic bridge and binds to two metal centers. Therefore, the overall coordination at vanadium ions consists of five oxygens. One of these belongs to a carboxylate group, two are alcoholate oxygens and two terminal oxo atoms. The two oxo ligands exhibit the same bond distance from vanadium. The geometry about the metal ion is a distorted trigonal bipyramid. The equatorial plane of the bipyramid includes the two oxo oxygens and the alcoholate donor. A value of 360° is calculated for the sum of the angles (range 108 –129°) subtended at the metal ion,
Symmetry code: (i) −x+1, −y, −z+1.
V(1)V(2) V(1)O(3) V(1)O(4) V(1)O(6) V(1)O(7) V(1)O(8) V(2)O(1) V(2)O(3) V(2)O(6)
3.214(2) 1.949(3) 1.974(4) 1.998(4) 1.635(5) 1.617(4) 1.979(4) 2.004(4) 1.965(3)
V(2)O(9) V(2)O(10) C(1)O(1) C(1)O(2) C(2)O(3) C(4)O(4) C(4)O(5) C(5)O(6)
1.605(4) 1.599(5) 1.288(7) 1.205(7) 1.427(6) 1.302(8) 1.223(7) 1.406(6)
O(3)V(1)O(4) O(3)V(1)O(6) O(4)V(1)O(6) O(7)V(1)O(3) O(7)V(1)O(4) O(7)V(1)O(6) O(8)V(1)O(3) O(8)V(1)O(4) O(8)V(1)O(6) O(8)V(1)O(7) O(1)V(2)O(3) O(6)V(2)O(1)
148.2(2) 71.6(1) 77.1(2) 102.2(2) 98.8(2) 121.2(2) 99.7(2) 96.2(2) 130.9(2) 107.9(2) 76.4(2) 147.6(2)
O(6)V(2)O(3) O(9)V(2)O(1) O(9)V(2)O(3) O(9)V(2)O(6) O(10)V(2)O(1) O(10)V(2)O(3) O(10)V(2)O(6) O(10)V(2)O(9) V(1)O(3)V(2) V(2)O(6)V(1) O(2)C(1)O(1) O(5)C(4)O(4)
71.19(14) 97.8(2) 126.8(2) 101.0(2) 98.9(2) 124.5(2) 99.7(2) 108.8(3) 108.8(2) 108.4(2) 123.0(5) 123.0(5)
3. Results and discussion
3.1. Structures 3.1.1. Rb2[{VO2(glyc)}2] A perspective view of the complex is given in Fig. 1. Table 2 lists pertinent bond length and bond angle data, whereas the basic structural features of a series of a-hydroxycarboxylate compounds of V(V) are summarized in Table 5. The unit cell of Rb2[{VO2(glyc)}2] contains dinuclear [{VO2)(glyc)}2]2 − anions. The two halves of each unit are equivalent owing to a center of inversion, so that half of the formula unit comprises the asymmetric unit. There are seven shortest contacts, ranging from 2.87 to 3.08 A, , between the rubidium ions and the oxygen atoms of the anion. The glycolato(2− ) ligand exhibits a five-membered chelating mode via two oxygens, one belonging to a monodentate carboxylate and another to the adjacent deprotonated alcoholic
Fig. 1. Perspective view of the [{VO2(glyc)}2]2 − dianion. Thermal ellipsoids are drawn at 30% probability.
M. Biagioli et al. / Inorganica Chimica Acta 310 (2000) 1–9
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Table 5 Comparison of metrical details for vanadium(V) complexes of a-hydroxycarboxylate ligands a Distance (A, ) or angle (°) VO(oxo) VO(carb) VO(alc) VO%(alc) b O(oxo)VO(oxo) O(oxo)VO(alc) O(oxo)VO%(alc) O(alc)VO%(alc) O(carb)VO(alc) O(carb)VO’(alc) VOV
glyc 1.614 1.614 1.979 1.998 1.944 107.9 123.5 128.6 99.8 100.4 71.1 76.7 147.8 108.9
mal 1.610 1.619 1.977 2.016 1.963 107.8 121.0 131.2 99.2 101.2 70.9 77.1 146.9 109.1
lact c 1.617 1.635 1.974 1.998 1.949 107.9 121.2 130.9 99.7 102.2 71.6 77.1 148.2 108.8
lact c
lact+per
1.599 1.605 1.979 2.004 1.965 108.8 124.5 126.8 99.7 101.0 71.2 76.4 147.6 108.8
1.591 2.005 1.957 2.000 113.4
ehba 1.605 1.617 1.974 1.973 1.984 108.5 99.9 100.6
98.9 70.0 77.2 146.9 109.9
104.0 148.2 71.9
hmp 1.607 1.621 1.975 1.998 1.960 108.6 115.2 136.2 99.3 104.0 70.6 76.6 144.9 109.4
citr 1.611 1.623 1.980 2.013 1.957 106.4 123.0 130.5 100.1 101.5 72.0 77.3 149.2 108.0
hmcitr 1.586 1.630 1.953 1.995 1.956 108.0 123.8 128.3 99.7 100.6 71.8 77.8 149.4 108.2
a Abbreviations: glyc =Rb2[{VO2(glyc)}2] (this work); mal =Cs2[{VO2(mal)}2]·2H2O (this work); lact =Cs2[{VO2(lact)}2]·2H2O (this work); [11]; ehba = (NH4)2[{VO2(ehba)}2]·2H2O [7]; mlact = (Bu4N)2[{VO2(mlact)}2]·2H2O [7]; citr = lact+per=K2[{VO(O2)(lact)}2] K2[{VO2(citr)}2]·4H2O [4]; hmcitr =[K2(H2O)5][{VO2(hmcitr)}2]·2H2O (data refer only to the B unit owing to labeling errors in the A unit, [6]). b VO%(alc) is the bridging bond. c The columns refer to the two different coordination centers of the dinuclear unit.
which is 0.0176 A, out of this plane. The monodentate carboxylate group occupies one of the axial positions, whereas the alcoholate oxygen of the adjacent center is the other apical donor. The bonds to the terminal oxo ligands are considerably shorter than to the other oxygens, implicating a substantial double bonding. Therefore the trigonal plane of the metal ion includes two short bonds, nearly identical to each other, and one long bond. Alternatively, a distorted tetragonal –pyramidal arrangement can be assumed, involving the three glycolate oxygens and one of the oxo atoms in the basal plane. However, in this case, rather distorted basal planes are calculated with deviations from the mean plane ranging from 0.25 to 0.38 A, . The length of the VO(alcoholate) bonds (1.94 –1.98 A, ) supports that the hydroxyl groups are deprotonated. The bond of the hydroxyl oxygen to vanadium in the five-membered chelate ring is slightly longer, by approximately 0.05 A, , than the bond connecting the same atom to the adjacent metal center. This substantiates that the hydroxycarboxylate ligand displays almost the same effectiveness in bridging and chelating vanadium(V). In comparison, in the ehba complex the intrachelate VO(alcoholate) bond is slightly shorter than the intradimeric distance [5]. However, in the latter complex steric factors connected to the presence of bulky ethyl groups could prevent a closer approach of the metal ions. The V2O2 rhomboid arrangement involving bonding between the vanadium atom of a center and an oxygen from a ligand in an adjacent center is a motif often found in the chemistry of vanadium(V), particularly with ligands like 1,2-diols [22], a-hydroxycarboxylic
acids [3–11], and aminoalcohols [23]. The OVO angle in the rhomboidal cage of the glycolate complex is 71.1°. In comparison, the same angle is 104° in the ehba derivative [5]. The VOV angle is 108.9°. Noteworthy, the arrangement of the hydroxycarboxylate ligand in 1 is analogous to that established for homocitrate coordinated to the molybdenum site in the cofactor of nitrogenase [24] or to dioxovanadium(V) in a model complex [6]. The other two compounds exhibit structures similar to those of 1, as appears from the data reported below.
3.1.2. Cs2[{VO2(mal)}2] ·2H2O The molecular structure of the compound 2 is also based on a centrosymmetric dimeric arrangement (Fig. 2 and Table 3) so that the two coordination centers in the dimer are equivalent. The environment of the caesium ion is composed of nine oxygen atoms from the ligand and the water molecules with distances in the range 3.09 –3.33 A, . Only one of the carboxylic groups of malic acid is deprotonated. Therefore, the ligand is dinegatively charged. The carboxylate group adjacent to the alcoholic function is in a chelatable position and binds vanadium by closing a five-membered chelated ring. This structural feature is different from that of the monoperoxo oxovanadium(V) complex of malic acid, in which single-crystal X-ray diffraction data indicate that the second carboxylic function is deprotonated and acts as the apical donor of the metal center [7]. The lengths of the VO(alcoholate) bonds are 1.96 and 2.02 A, . Again, the shorter distance is attributable to the bridge, whereas the intrachelate VO(alcoholate) appears to be 0.05 A, longer, similar to the
M. Biagioli et al. / Inorganica Chimica Acta 310 (2000) 1–9
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VO(alcoholate) bond length in the complex of glycolic acid. The lengths of the VO(oxo) bonds are as expected, 1.61 and 1.62 A, . The coplanarity of the two oxo and alcoholic atoms is substantiated by a value of 360° for the sum of the angles subtended at the vanadium atom, which is 0.0210 A, out of the plane. The OVO angle in the rhomboidal cage is 70.9°, whereas the VOV angle is 109.1°. The free carboxylic group exhibits two different CO bonds, 1.21 and 1.31 A, , in contrast to the coordinate group that exhibits more equivalent distances of 1.23 and 1.27 A, .
Fig. 2. Perspective view of the [{VO2(mal)}2]2 − dianion. Thermal ellipsoids are drawn at 30% probability.
3.1.3. Cs2[{VO2(lact)}2] ·2H2O The molecular structure of compound 3 consists of a dimeric arrangement involving two slightly different coordination centers (Fig. 3 and Table 4). In this case in fact, no symmetry element relates the two halves of the molecule. The asymmetric unit contains one dimeric molecule, two Cs ions and two water molecules, one of which is disordered and split into two alternative positions, O(2)W and O(3)W which were both refined with occupancy factor of 0.5. The difference in the coordination bonds of vanadium in the two halves of the dimeric unit is within 0.03 A, . The caesium ions are involved in a number of contacts with the oxygen atoms of the ligand and water molecules. Eight of them are in the range 3.08 –3.38 A, for both the Cs ions. The VO(alcoholate) distances are 1.95 –1.96 A, for the bridges and 2.00 A, for the intrachelate bonds. The coplanarity of the two oxo and the alcoholic atoms yields values of approximately 360° for the sum of the angles subtended at the vanadium atoms. The metal ions are 0.0031 and 0.0089 A, out of the plane. Similarly to the complexes of glycolic and malic acids, the compound 3 exhibits the intrachelate VO(alcoholate) distance approximately 0.05 A, longer than the VO(alcoholate) bridging distance. The OVO angles are 71.2 –71.6°, whereas the VOV angles are 108 – 109°. The carboxylate group exhibits nonequivalent CO bonds 1.20 –1.22 and 1.29 –1.30 A, . 3.2. Infrared spectra
Fig. 3. Perspective view of the [{VO2(lact)}2]2 − dianion. Thermal ellipsoids are drawn at 30% probability.
Table 6 IR absorption bands of the complexes Complex
VO
COO−
COOH
OH
1
941vs 927vs 937vs 930vs 933vs
1348m,w 1664vs 1385m,w 1616vs 1349m,w 1664vs
1315m,w 1710m
2800–2500b
2 3
The assignment of the IR absorption bands of the complexes was carried out by comparison with the ligands and their sodium salts. The most significant IR features are collected in Table 6 and briefly described below. The cis couple of VO bonds exhibits distinctive symmetric and antisymmetric stretches in the range 930 –940 cm − 1. The carboxylate group taking part in metal binding is distinguished by asymmetric and symmetric stretching vibrations with splitting values D(nas − ns) in the range 231 –315 cm − 1 consistent with the monodentate coordination mode established by structural data. The position of these bands easily distinguishes the undissociated carboxylic group of malic acid in complex 2.
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Fig. 4. Reduction of [{VO2(lact)}2]2 − by cysteine: intensity of the VO(IV) EPR signal (Section 2) as a function of time and cysteine– vanadium molar ratio.
Fig. 5. High field region of the anisotropic EPR spectra recorded in the course of the reduction of [{VO2(lact)}2]2 − (cysteine–vanadium molar ratio = 2) as a function of time: (a) 40 min; (b) 110 min, (c) 210 min; (d) 1470 min. Table 7 EPR parameters of VO(IV) complexes formed upon reduction of Cs2[{VO2(lact)}2]·2H2O by cysteine Species
g
A (10−4 cm−1)
Donor atoms
I II III IV V
1.967 1.958 1.960 1.951 1.942
144 154 147 156 165
2(NH2, S−), 2(COO−, S−) (NH2, S−)+(S−, COO−) 2(COO−, O−) 2(NH2, COO−)
3.3. Comparison with the peroxo complex of lactic acid A comparison of the h2-peroxo complex of lactic acid K2[{VO(O2)(lact)}2] with its precursor 3 (Table 5) indicates that the two compounds share many structural features [11]. For instance, the VO(oxo) distance
7
in the peroxo complex is 1.59 A, , only slightly shorter than in compound 3 (160 –1.63 A, ), whereas the VO(carboxylate) bond is only slightly longer (2.00 vs. 1.974 and 1.679 A, ). Comparable VO(alcoholate) bonds are observed in the compounds, although in the peroxo derivative the intrachelate VO(alcoholate) bond is longer than the intradimeric bridge. A comparison of the geometry at the metal ions shows that four of the five oxygen atoms of the coordination polyhedron of vanadium in 3 (namely the carboxylic, two alcoholate and one of the oxo atoms) remain almost unchanged after peroxo binding. Indeed, the angle formed by the pseudo-axial donors (the carboxylate and bridging alcoholate oxygens) at the vanadium atom exhibits the same value in the two compounds. The O(alcoholate)VO(oxo) angle in the trigonal plane of the parent compound (121 –130°) becomes only slightly shorter in 3 (113.4°). Noticeably, the peroxo group replaces one of the oxo atoms in the parent compound without introducing gross changes in the overall geometry. The ligand is located almost perpendicularly to the trigonal plane, as if the axis joining the metal to the middle point of the peroxo group lies in the plane and replaces the bond of an oxo ligand. Indeed, assuming the midpoint of the peroxo group is a donor center, the sum of the angles at vanadium in the ideal trigonal plane is approximately 354°. On the other hand, the sum of the adjacent angles subtended by pseudo axial donors and peroxo oxygens at the metal ion is 203°. All these features confirm that the main effect resulting from peroxo binding is the replacement of the oxo ligand while the rest of the molecule maintains its integrity. The same features of 3 are observed in the homocitrate complex. Therefore, it is likely that the peroxidation of the hmcitr compound would give rise to similar changes.
3.4. Reduction of 6anadium(V) in aqueous solution The reduction of the V(V) complex of lactic acid by cysteine was studied in neutral aqueous solution under aerobic conditions. EPR spectra showed that, with elapsing time, vanadium(V) is reduced to EPR-active oxovanadium(IV). As shown in Fig. 4, the amount of VO(IV) increases steeply with time, reaches a maximum and then decreases slowly. The course of the reduction is very sensitive to the ratio of thiol to vanadium compound. An increasing excess of cysteine increases the amount of reduced ion and shifts the maximum extent of VO(IV) formation to longer times, consistent with a higher reductant capacity of the system. EPR spectra give evidence for monomeric oxovanadium(IV) distributed among the ligands present in the system, see Fig. 5. In particular, a number of complexes of cysteine have been identified by signals denoted as I, II and III in Fig. 5 and Table 7. These are characterized by
8
M. Biagioli et al. / Inorganica Chimica Acta 310 (2000) 1–9
coordination sets 2(NH2, S−), 2(COO−, S−) and (NH2, S−) +(S−, COO−), respectively [12]. Lactic acid forms the bis chelated species with 2x(COO−, O−) donor set (IV) [26]. With elapsing time cysteine is oxidized to cystine and the VO(IV) ions remain bound, beside to the hydroxycarboxylate ligand, to cystine. The latter ligand yields a species (V) presumably with the 2(NH2, COO−) donor set, even if the precise assignment needs further studies. In any case, the involvement of other oxidation states, e.g. EPR-inactive vanadium(III) cannot be ruled out, at least based only on our measurements. The fitting of the data in the ascending part of the kinetic curves indicate that the rate of formation of VO(IV) is of the first order with respect to vanadium(V), with kinetic constants almost independent on the cysteine to vanadium molar ratio (range: 0.010 – 0.012 min − 1). This suggests that a complex mechanism governs the reaction. Presumably, it involves the formation of an ester between vanadium(V) and cysteine, as a fast process, followed by a slow first-order decomposition of this intermediate to oxovanadium(IV) and cystine.
4. Conclusions The analysis of a series of dioxovanadium(V) compounds indicates that the V2O2 site originating from the a-hydroxycarboxylate coordination is very stable and can be considered highly conservative for this class of ligands. In this respect, the ability of the ligand to yield substantially equivalent bridging and intrachelate VO alcoholate bonds is relevant. As judged from the length of these distances, the bridging prevails over the chelating behavior. The structural integrity of the dinuclear arrangement is maintained after the interaction of the complexes with the peroxo ligand. The reaction only results in the replacement of an oxo by a peroxo group, while both the coordination features of the a-hydroxycarboxylate ligands and the dinuclear structure are retained. Ideally, the geometry around the metal ion remains the same if the axis connecting the metal ion to the center of the peroxo ligand is assumed to be a coordinate bond. On the other hand, the reduction of vanadium(V) to oxovanadium(IV) is effective in destroying the dinuclear arrangement and yielding monomeric species. The reduced metal ion remains bound to the a-hydroxycarboxylate ligand beside cysteine or cystine. These observations reinforce the proposition that the alkoxo bridged V2O2 dinuclear cage could be an effective tool adopted by organisms for the absorption and storage of vanadium(V). A rather unusual thermodynamic stability is attributed to such kind of dinuclear complexes in aqueous solution [5,25]. The integrity
maintained after the peroxidation indicates that the arrangement is robust enough to resist ligand exchange reactions. A number of biologically relevant reducing agents, including ascorbate, norepinephrine, simple thiols, cysteine and cysteine-based peptides, e.g. glutathione, which are the main components of the reductant pool of cellular systems, could be effective in the transformation of V(V) into V(IV). The reduction, beside destroying the dinuclear structure, labilizes the metal ion and increases its affinity to sulfur donors. Therefore the process is potentially very effective in promoting the inclusion of vanadium into sulfur-based clusters.
5. Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge, Crystallographic Center, CCDC No. 142410 for compound 1, No. 142411 for compound 2 and No. 142412 for compound 3. Copies of the data may be obtained free of charge form The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: + 44-1223-336033; e-mail: [email protected] or www: http://www.ccdc.cam. ac.uk).
References [1] R.D. Joeger, P.E. Bishop, CRC Crit. Rev. Microbiol. 16 (1988) 1. [2] R.N. Pau, Trends Biochem. Sci. 14 (1988) 183. [3] C. Djordjevic, M. Lee, E. Sinn, Inorg. Chem. 28 (1989) 719. [4] D.W. Wright, P.A. Humiston, W.H. Orme-Johnson, W.M. Davis, Inorg. Chem. 34 (1995) 4194. [5] T.W. Hambley, R.J. Judd, P.A. Lay, Inorg. Chem. 31 (1992) 343. [6] D.W. Wright, R.T. Chang, S.K. Mandal, W.H. Armstrong, W.H. Orme-Johnson, J. Biol. Inorg. Chem. 1 (1996) 143. [7] I. Smatanova´, J. Marek, P. Svanca´rek, P. Schwendt, Acta Crystallogr., Sect. C 54 (1998) 1249. [8] C. Djorjevic, M. Lee-Renslo, E. Sinn, Inorg. Chim. Acta 233 (1995) 97. [9] P. Schwendt, P. Svancarek, L. Kuchta, J. Marek, Polyhedron 17 (1998) 2161. [10] C. Djordjevic, M. Lee-Renslo, E. Sinn, Inorg. Chim. Acta 233 (1995) 97. [11] F. Demartin, M. Biagioli, L. Strinna-Erre, A. Panzanelli, G. Micera, Inorg. Chim. Acta 299 (2000) 123. [12] A. Dessı`, G. Micera, D. Sanna, J. Inorg. Biochem. 52 (1993) 275. [13] L.J. Farrugia, WINGX-97: A Windows Program for Crystal Structure Analysis, University of Glasgow, Glasgow, 1998. [14] A.C.T North, D.C. Philips, F.S. Mathews, Acta Crystallogr., Sect. A 24 (1968) 351. [15] H.D. Flack, Acta Crystallogr., Sect A 39 (1983) 876. [16] A. Altomare, G. Cascarano, G. Giacovazzo, A. Gualardi, J. Appl. Cryst. 26 (1993) 343. [17] G.M. Sheldrick, SHELXL-93: A Program for Crystal Structure Refinement, University of Goettingen, Germany, 1993.
M. Biagioli et al. / Inorganica Chimica Acta 310 (2000) 1–9 [18] International Tables for X-ray Crystallography, vol. 4, Kynoch, Birmingham, England, 1974, pp. 99–101 and 149– 150. [19] L.J. Farrugia, J. Appl. Cryst. (1997) 565. [20] R.E. Marsh, Acta Crystallogr., Sect. B 51 (1995) 897. [21] R.E. Marsh, Acta Crystallogr., Sect. B 55 (1999) 931. [22] S.J. Angus-Dunne, R.J. Batchelor, A.S. Tracey, F.W. Einstein, J. Am. Chem. Soc. 117 (1995) 5292. [23] W. Plass, Eur. J. Inorg. Chem. (1998) 799. [24] (a) J.T. Bolin, A.E. Ronco, T.V. Morgan, L.E. Mortenson,
.
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N.H. Xuong, Proc. Natl. Acad. Sci. USA 90 (1993) 1078. (b) J. Chen, J. Christiansen, N. Campobasso, J.T. Bolin, R.C. Tittsworth, B.J. Hales, J.J. Rehr, S.P. Cramer, Angew. Chem., Int. Ed. Engl. 32 (1993) 1592. (c) J. Kim, D.C. Rees, Science 25 (1992) 1677. (d) M.K. Chan, J. Kim, D.C. Rees, Science 260 (1993) 792. (e) J. Kim, D.C. Rees, Nature 360 (1993) 553. (f) J. Kim, D. Woo, D.C. Rees, Biochemistry 32 (1993) 7104. [25] A.S. Tracey, M.J. Gresser, K.M. Parkinson, Inorg. Chem. 26 (1987) 629. [26] G. Micera, E. Garribba, D. Sanna, unpublished results.
Molecular structure, characterization and reactivity of dioxo complexes formed by vanadium(V) with a-hydroxycarboxylate ligands M. Biagioli a, L. Strinna-Erre a, G. Micera a,*, A. Panzanelli a, M. Zema b b
a Department of Chemistry, Uni6ersity of Sassari, I-07100 Sassari, Italy Centro Grandi Strumenti, Uni6ersita` di Pa6ia, Via Bassi 21, I-27100 Pa6ia, Italy
Received 12 April 2000; accepted 24 June 2000
Abstract The structure of the dioxovanadium(V) complexes of glycolic, lactic and malic acids Rb2[{VO2(glyc)}2] [glyc= glycolato(2 −)] (1), Cs2[{VO2(mal)}2] [mal=malato(2− )] (2), and Cs2[{VO2(lact)}2]·2H2O [lact= lactato(2−)] (3) has been studied by singlecrystal X-ray diffraction. In order to assign the features of the peculiar arrangement adopted by these kinds of complexes, the metrical details of these compounds have been examined and compared to the data available in literature for analogous complexes. The reactivity of the compounds has also been studied. In particular, the reduction of the metal ion by a biological reductant like cysteine, relevant to the role of these complexes in the constitution of the cofactor of the active site of vanadium nitrogenase, has been followed by EPR spectroscopy in an aqueous solution. In addition, the effect of peroxo complex formation on the structure of the dinuclear V2O2 cage has been analyzed by comparison with previously published results. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Vanadium(V); Oxovanadium(IV); Glycolic acid; Lactic acid; Malic acid; Vanadium nitrogenase
1. Introduction Many aspects of vanadium bio-reactivity involving the different oxidation states of the element are still unknown. The discovery of nitrogen fixing organisms which have evolved an alternative nitrogenase containing homocitrate bound to vanadium in the active site cofactor [1,2] elicited interest in vanadium(V) a-hydroxycarboxylate chemistry. The first citrate complex of vanadium(V) was reported by Djordjevic et al. in 1989 [3]. Only some years later the molecular structure of the complex was solved by single-crystal X-ray diffraction methods [4]. It was found that the citrate ligand displays a unique bidentate coordination mode to vanadium via the deprotonated hydroxyl and a unidentate carboxylate. The structure of the complex is best described as a dimer of two five-coordinate vanadium centers doubly bridged by alkoxide * Corresponding author. Tel.: + 39-079-22 9541; fax: + 39-079-21 9269.
oxygen atoms. Two oxo ligands complete the donor set at the metal centers. The same arrangement was previously described for the complex of another a-hydroxycarboxylic ligand, 2-ethyl-2-hydroxy-butanoic acid (ehba) [5]. The interaction of homocitrate (hmcitr) with vanadium(V) resulted in a compound with a similar structure [6]. The fully extended coordination mode of the ligand was considered to be of biological relevance and a role of early precursor for the nitrogenase active site was attributed to the dinuclear complex. The complex of 2-hydroxy-2-methylpropanoic or methyllactic acid (hmp), with a similar structure, has recently been published [7]. a-Hydroxycarboxylate ligands take part in the incorporation of vanadium and molybdenum in the cofactor of the active site of nitrogenases. Metal ions are adsorbed in the form of the oxoanions vanadate(V) and molybdate(VI) and are then reduced to the oxidation states III or IV. The coordination of hydroxycarboxylic ligands seems essential to the mobilization and reduction processes.
0020-1693/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 0 ) 0 0 2 6 3 - 2
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Peroxo complexes are involved as intermediates of several bio-reactions, therefore the peroxo vanadium(V) complexes of a-hydroxycarboxylic ligands are currently investigated. In particular, the monoperoxo complexes of citric, malic, tartaric and glycolic acids were isolated and fully characterized [3,8 – 11]. We describe the vanadium(V) complexes of three members of this class of ligands as glycolic, lactic and malic acids. A single-crystal X-ray study has been performed on compounds isolated from aqueous solution. Rb2[{VO2(glyc)}2] [glyc =glycolato(2 − )] (1), Cs2[{VO2(mal)}2] [mal =malato(2 − )] (2), and Cs2[{VO2(lact)}2]·2H2O [lact =lactato(2 −)] (3) exhibit the same basic structure typical of all a-hydroxycarboxylate complexes. In order to assign the features of the peculiar arrangement, the structural details of the compounds have been examined and compared to each other and to those available in literature for analogous complexes. The reactivities of the complexes have also been examined. In particular, the reduction of vanadium(V) by cysteine has been followed by EPR spectroscopy in aqueous solution. In addition, the effect of peroxo coordination on the structure of the dinuclear V2O2 cage has been analyzed by comparison with previous results.
Anal. Calc. for Cs2[{VO2(lact)}2]·2H2O: C6H12Cs2O12V2: C, 11.19; H, 1.88; H2O: 5.6; Cs2O·V2O5: 69.5. Found: C, 11.58; H, 1.51; Cs2O·V2O5: 70.0; H2O: 5.0%.
2.3. Reacti6ity in water Cs2[{VO2(lact)}2]·2H2O (0.05 mmol) was dissolved in an aqueous solution (10 ml) buffered at pH 7.4 with 20 mM Hepes. L-Cysteine was added to the solution which was stirred at r.t. Different molar ratios of the vanadium compound to cysteine were investigated, namely 1:1, 1:2 and 1:10. EPR spectroscopy was used to monitor the reduction of vanadium(V) to vanadium(IV) over time. A quartz flat cell was used for the measurements at r.t. The intensity of the MI = − 1/2 transition of the isotropic spectrum was taken as an approximate measure of the VO(IV) concentration. The paramagnetic products were identified by comparison of anisotropic spectra recorded on frozen solutions with those of binary VO(IV) ligand systems. Some of them have already been examined in previous work of ours [12].
2.4. Physical measurements
Glycolic, L-lactic, D,L-malic acids, Rb2CO3, Cs2CO3, V2O5 and other reagents for both the synthetic and reactivity studies were purchased from Aldrich and used without further purification.
Elemental analyses (C and H) were obtained with a Perkin –Elmer 240 B elemental analyzer. Thermogravimetric data were obtained with a Perkin –Elmer TGS-2 apparatus in air or under a nitrogen flow. IR spectra were recorded with a FT Bruker IFS-66 interferometer using KBr (4000 –600 cm − 1) pellets. EPR spectra were recorded on a Varian E9 spectrometer at the X-band frequency (9.15 GHz) at 298 or 120 K. As usual, the samples for low-temperature measurements were added with a few drops of DMSO to ensure good glass formation in frozen solutions.
2.2. Synthesis
2.5. Crystal structure analysis
V2O5 (0.181 g, 1 mmol) and Rb2CO3 or Cs2CO3 (1.1 –1.2 mmol) were dissolved in an aqueous solution (5 ml) by heating to 40°C while stirring. The solution was cooled in an ice bath and the ligand (3 mmol) was gradually added. The reaction at r.t. yielded precipitates which were filtered off. Crystals suitable for X-ray diffractometric analysis which were formed from the mother solutions (pH 3 – 4) were kept at room temperature (r.t.) or in the refrigerator. They were filtered off, washed with EtOH, and air-dried at r.t. Rb2[{VO2(glyc)}2]: Anal. Calc. for C4H4O10Rb2V2: C, 9.91; H, 0.83; Rb2O·V2O5: 76.1. Found: C, 9.57; H, 0.89; Rb2O·V2O5: 75.5%. Cs2[{VO2(mal)}2]·2H2O: Anal. Calc. for C8H12Cs2O16V2: C, 13.13; H, 1.65, H2O, 4.9; Cs2O V2O5: 61.2. Found: C, 13.38; H, 1.64, H2O, 5.0; Cs2O·V2O5, 61.5%.
2.5.1. Crystal data Crystal data and details on the crystallographic study are reported in Table 1, whereas selected bond lengths and angles for 1, 2 and 3 are reported in Tables 2–4, respectively.
2. Experimental
2.1. Chemicals
2.5.2. Data collection and processing Unit cell parameters and intensity data were obtained on an Enraf –Nonius CAD-4 diffractometer, using graphite monochromated Mo Ka radiation. The cell dimensions were obtained by least-squares fitting of 25 centered reflections monitored in the ranges 10.24B qB18.34° for 1, 7.55B qB14.14° for 2 and 7.24BqB 14.97° for 3. Calculations were performed with the WINGX-97 software [13]. Corrections for Lp and empirical absorption were applied [14]. For crystal 3, Friedel pairs were measured to enable refinement of the Flack
M. Biagioli et al. / Inorganica Chimica Acta 310 (2000) 1–9
parameter [15] in order to confirm the absolute configuration. The three structures were solved by direct methods (SIR-92) [16], and refined by full-matrix least-squares using SHELXL-93 [17], with anisotropic displacement parameters for all non-hydrogen atoms. For crystal 1, all H atoms were seen in Fourier difference maps and refined with an isotropic displacement parameters proportional to their neighboring atoms. For crystal 2, only the H atom of the free carboxylic group was located in the DF maps and inserted in the calculations with isotropic displacement parameter proportional to that of its neighboring atom; the H atoms bound to C atoms were calculated and not refined while those of the water molecule were disregarded. The treatment of the H atoms bound to C atoms in 3 was the same as in 2. The H atoms bound
3
to O(1)W in 3 were located and inserted in the refinement with isotropic displacement parameter proportional to that of their neighboring atom while those bound to the second water molecule, which was found to be disordered, were disregarded. Atomic scattering factors and the values of Df % and Df %% were taken from International Tables for X-ray crystallography [18]. Diagrams of the molecular structures were produced by the ORTEP program [19]. It is worth noting that the E-statistics for 1 indicated its triclinic structure to be acentric. It was then solved in the atypical space group P1( . Following Marsh [20,21], an accurate analysis of the structural model revealed that the structure could be conveniently refined in the centrosymmetric space group P1( .
Table 1 Crystal data and experimental details for the compounds
Formula Molecular weight Crystal size (mm) Crystal system Space group a (A, ) b (A, ) c (A, ) h (°) i (°) k (°) V (A, 3) Z Dcalc (g cm−3) Temperature (K) Radiation, u (A, ) Monochromator v (mm−1) Scan type q Range (°) Standard reflections Maximum/minimum transmission Absorption correction method Index ranges Total Reflections measured Unique reflections Refinement type Number of parameters refined R indices a [I\2q(I)] R indices (all data) Goodness-of-fit b Extinction coefficient Flack parameter Maximum/minimum Dz (e A, −3) (Shift/e.s.d.)max
1
2
3
C4H4O10Rb2V2 484.88 0.42×0.35×0.21 triclinic P1( 6.021(1) 7.092(2) 7.982(3) 106.21(3) 107.27(2) 101.76(2) 296.83(14) 2 2.713 293 Mo Ka, 0.71073 graphite 9.761 -2q 2–35 3 every 300 reflections 0.129, 0.053 -scan −95h59, −115k511, −125l512 5216 2609 (Rint = 0.0247) F2 89 R1 = 0.0247, Rw2 = 0.0563 R1 = 0.0383, Rw2 = 0.0602 1.066 0.0294(20)
C8H12V2O16Cs2 731.86 0.21×0.21×0.18 monoclinic P21/c 9.499(3) 10.381(5) 10.000(2) 90 104.24(2) 90 955.8(6) 4 3.137 293 Mo Ka, 0.71073 graphite 4.808 -2q 2–30 3 every 300 reflections 0.421, 0.354 -scan −135h513, 05k514, 05l514 2928 2782 (Rint =0.0339) F2 131 R1 =0.0504, Rw2 =0.1340 R1 =0.0757, Rw2 =0.1484 1.049 0.0001(8)
1.004, −0.962 0.001
1.695, −2.555 0.000
C6H12Cs2O12V2 643.84 1.60×0.35×0.21 monoclinic P21 6.090(8) 7.755(4) 17.875(9) 90 99.35(9) 90 833.0(12) 2 2.423 293 Mo Ka, 0.71073 graphite 5.480 -2q 2–30 3 every 300 reflections 0.316, 0.105 -scan −85h58, −105k510, 05l525 4867 4867 (Rint =0.0000) F2 202 R1 =0.0344, Rw2 =0.0818 R1 =0.0369, Rw2 =0.0836 1.086 0.0010(3) −0.0362(261) c 1.926, −1.384 0.001
R1 =S( Fo − Fc )/S( Fo ). Rw2 = {S[w(F o2−F c2)2]/S[w(F o2)2]}1/2. Goodness-of-fit=S ={S[w(F o2−F c2)2]/(n−p)}1/2, where n is the number of reflections and p is the total number of parameters refined. c For inverted structure 0.9475(302). a
b
M. Biagioli et al. / Inorganica Chimica Acta 310 (2000) 1–9
4
Table 2 Selected interatomic distances (A, ) and angles (°) for 1 a V(1)V(2) V(1)O(1) V(1)O(4) V(1)O(5) V(1)O(1)i
3.206(2) 1.944(2) 1.614(2) 1.614(2) 1.998(2)
V(1)O(2)i O(1)V(1)i O(1)C(1) O(3)C(2)
1.979(2) 1.998(2) 1.397(2) 1.222(2)
O(5)V(1)O(4) O(5)V(1)O(1) O(4)V(1)O(1) O(5)V(1)O(2)i O(4)V(1)O(2)i O(1)V(1)O(2)i
107.87(11) 99.79(9) 100.40(10) 98.50(9) 98.90(9) 147.78(6)
O(5)V(1)O(1)i O(4)V(1)O(1)i O(1)V(1)O(1)i O(2)iV(1)O(1)i V(1)O(1)V(1)i
128.57(10) 123.53(10) 71.14(7) 76.71(7) 108.86(7)
a
Symmetry code: (i) −x, −y, −z.
Table 4 Selected interatomic distances (A, ) and angles (°) for 3
Table 3 Selected interatomic distances (A, ) and angles (°) for 2 a V(1)V(2) V(1)O(1)i V(1)O(6) V(1)O(7) V(1)O(3) V(1)O(3)i O(1)V(1)i
3.241(2) 1.977(4) 1.619(5) 1.610(4) 1.963(4) 2.016(4) 1.977(4)
O(3)V(1)i O(1)C(1) O(2)C(1) O(3)C(2) O(4)C(4) O(5)C(4)
2.016(4) 1.272(6) 1.231(7) 1.412(6) 1.213(7) 1.310(7)
O(3)V(1)O(1)i O(3)V(1)O(3)i O(6)V(1)O(3) O(7)V(1)O(3) O(7)V(1)O(6) O(7)V(1)O(1)i O(6)V(1)O(1)i
146.91(17) 70.89(16) 101.2(2) 99.2(2) 107.8(3) 95.6(2) 102.2(2)
O(6)V(1)O(3)i O(7)V(1)O(3)i O(1)iV(1)O(3)i O(2)C(1)O(1) O(4)C(4)O(5) V(1)O(3)V(1)i
121.0(2) 131.2(2) 77.07(15) 123.8(5) 123.3(6) 109.11(16)
a
group. The latter donor behaves as a monatomic bridge and binds to two metal centers. Therefore, the overall coordination at vanadium ions consists of five oxygens. One of these belongs to a carboxylate group, two are alcoholate oxygens and two terminal oxo atoms. The two oxo ligands exhibit the same bond distance from vanadium. The geometry about the metal ion is a distorted trigonal bipyramid. The equatorial plane of the bipyramid includes the two oxo oxygens and the alcoholate donor. A value of 360° is calculated for the sum of the angles (range 108 –129°) subtended at the metal ion,
Symmetry code: (i) −x+1, −y, −z+1.
V(1)V(2) V(1)O(3) V(1)O(4) V(1)O(6) V(1)O(7) V(1)O(8) V(2)O(1) V(2)O(3) V(2)O(6)
3.214(2) 1.949(3) 1.974(4) 1.998(4) 1.635(5) 1.617(4) 1.979(4) 2.004(4) 1.965(3)
V(2)O(9) V(2)O(10) C(1)O(1) C(1)O(2) C(2)O(3) C(4)O(4) C(4)O(5) C(5)O(6)
1.605(4) 1.599(5) 1.288(7) 1.205(7) 1.427(6) 1.302(8) 1.223(7) 1.406(6)
O(3)V(1)O(4) O(3)V(1)O(6) O(4)V(1)O(6) O(7)V(1)O(3) O(7)V(1)O(4) O(7)V(1)O(6) O(8)V(1)O(3) O(8)V(1)O(4) O(8)V(1)O(6) O(8)V(1)O(7) O(1)V(2)O(3) O(6)V(2)O(1)
148.2(2) 71.6(1) 77.1(2) 102.2(2) 98.8(2) 121.2(2) 99.7(2) 96.2(2) 130.9(2) 107.9(2) 76.4(2) 147.6(2)
O(6)V(2)O(3) O(9)V(2)O(1) O(9)V(2)O(3) O(9)V(2)O(6) O(10)V(2)O(1) O(10)V(2)O(3) O(10)V(2)O(6) O(10)V(2)O(9) V(1)O(3)V(2) V(2)O(6)V(1) O(2)C(1)O(1) O(5)C(4)O(4)
71.19(14) 97.8(2) 126.8(2) 101.0(2) 98.9(2) 124.5(2) 99.7(2) 108.8(3) 108.8(2) 108.4(2) 123.0(5) 123.0(5)
3. Results and discussion
3.1. Structures 3.1.1. Rb2[{VO2(glyc)}2] A perspective view of the complex is given in Fig. 1. Table 2 lists pertinent bond length and bond angle data, whereas the basic structural features of a series of a-hydroxycarboxylate compounds of V(V) are summarized in Table 5. The unit cell of Rb2[{VO2(glyc)}2] contains dinuclear [{VO2)(glyc)}2]2 − anions. The two halves of each unit are equivalent owing to a center of inversion, so that half of the formula unit comprises the asymmetric unit. There are seven shortest contacts, ranging from 2.87 to 3.08 A, , between the rubidium ions and the oxygen atoms of the anion. The glycolato(2− ) ligand exhibits a five-membered chelating mode via two oxygens, one belonging to a monodentate carboxylate and another to the adjacent deprotonated alcoholic
Fig. 1. Perspective view of the [{VO2(glyc)}2]2 − dianion. Thermal ellipsoids are drawn at 30% probability.
M. Biagioli et al. / Inorganica Chimica Acta 310 (2000) 1–9
5
Table 5 Comparison of metrical details for vanadium(V) complexes of a-hydroxycarboxylate ligands a Distance (A, ) or angle (°) VO(oxo) VO(carb) VO(alc) VO%(alc) b O(oxo)VO(oxo) O(oxo)VO(alc) O(oxo)VO%(alc) O(alc)VO%(alc) O(carb)VO(alc) O(carb)VO’(alc) VOV
glyc 1.614 1.614 1.979 1.998 1.944 107.9 123.5 128.6 99.8 100.4 71.1 76.7 147.8 108.9
mal 1.610 1.619 1.977 2.016 1.963 107.8 121.0 131.2 99.2 101.2 70.9 77.1 146.9 109.1
lact c 1.617 1.635 1.974 1.998 1.949 107.9 121.2 130.9 99.7 102.2 71.6 77.1 148.2 108.8
lact c
lact+per
1.599 1.605 1.979 2.004 1.965 108.8 124.5 126.8 99.7 101.0 71.2 76.4 147.6 108.8
1.591 2.005 1.957 2.000 113.4
ehba 1.605 1.617 1.974 1.973 1.984 108.5 99.9 100.6
98.9 70.0 77.2 146.9 109.9
104.0 148.2 71.9
hmp 1.607 1.621 1.975 1.998 1.960 108.6 115.2 136.2 99.3 104.0 70.6 76.6 144.9 109.4
citr 1.611 1.623 1.980 2.013 1.957 106.4 123.0 130.5 100.1 101.5 72.0 77.3 149.2 108.0
hmcitr 1.586 1.630 1.953 1.995 1.956 108.0 123.8 128.3 99.7 100.6 71.8 77.8 149.4 108.2
a Abbreviations: glyc =Rb2[{VO2(glyc)}2] (this work); mal =Cs2[{VO2(mal)}2]·2H2O (this work); lact =Cs2[{VO2(lact)}2]·2H2O (this work); [11]; ehba = (NH4)2[{VO2(ehba)}2]·2H2O [7]; mlact = (Bu4N)2[{VO2(mlact)}2]·2H2O [7]; citr = lact+per=K2[{VO(O2)(lact)}2] K2[{VO2(citr)}2]·4H2O [4]; hmcitr =[K2(H2O)5][{VO2(hmcitr)}2]·2H2O (data refer only to the B unit owing to labeling errors in the A unit, [6]). b VO%(alc) is the bridging bond. c The columns refer to the two different coordination centers of the dinuclear unit.
which is 0.0176 A, out of this plane. The monodentate carboxylate group occupies one of the axial positions, whereas the alcoholate oxygen of the adjacent center is the other apical donor. The bonds to the terminal oxo ligands are considerably shorter than to the other oxygens, implicating a substantial double bonding. Therefore the trigonal plane of the metal ion includes two short bonds, nearly identical to each other, and one long bond. Alternatively, a distorted tetragonal –pyramidal arrangement can be assumed, involving the three glycolate oxygens and one of the oxo atoms in the basal plane. However, in this case, rather distorted basal planes are calculated with deviations from the mean plane ranging from 0.25 to 0.38 A, . The length of the VO(alcoholate) bonds (1.94 –1.98 A, ) supports that the hydroxyl groups are deprotonated. The bond of the hydroxyl oxygen to vanadium in the five-membered chelate ring is slightly longer, by approximately 0.05 A, , than the bond connecting the same atom to the adjacent metal center. This substantiates that the hydroxycarboxylate ligand displays almost the same effectiveness in bridging and chelating vanadium(V). In comparison, in the ehba complex the intrachelate VO(alcoholate) bond is slightly shorter than the intradimeric distance [5]. However, in the latter complex steric factors connected to the presence of bulky ethyl groups could prevent a closer approach of the metal ions. The V2O2 rhomboid arrangement involving bonding between the vanadium atom of a center and an oxygen from a ligand in an adjacent center is a motif often found in the chemistry of vanadium(V), particularly with ligands like 1,2-diols [22], a-hydroxycarboxylic
acids [3–11], and aminoalcohols [23]. The OVO angle in the rhomboidal cage of the glycolate complex is 71.1°. In comparison, the same angle is 104° in the ehba derivative [5]. The VOV angle is 108.9°. Noteworthy, the arrangement of the hydroxycarboxylate ligand in 1 is analogous to that established for homocitrate coordinated to the molybdenum site in the cofactor of nitrogenase [24] or to dioxovanadium(V) in a model complex [6]. The other two compounds exhibit structures similar to those of 1, as appears from the data reported below.
3.1.2. Cs2[{VO2(mal)}2] ·2H2O The molecular structure of the compound 2 is also based on a centrosymmetric dimeric arrangement (Fig. 2 and Table 3) so that the two coordination centers in the dimer are equivalent. The environment of the caesium ion is composed of nine oxygen atoms from the ligand and the water molecules with distances in the range 3.09 –3.33 A, . Only one of the carboxylic groups of malic acid is deprotonated. Therefore, the ligand is dinegatively charged. The carboxylate group adjacent to the alcoholic function is in a chelatable position and binds vanadium by closing a five-membered chelated ring. This structural feature is different from that of the monoperoxo oxovanadium(V) complex of malic acid, in which single-crystal X-ray diffraction data indicate that the second carboxylic function is deprotonated and acts as the apical donor of the metal center [7]. The lengths of the VO(alcoholate) bonds are 1.96 and 2.02 A, . Again, the shorter distance is attributable to the bridge, whereas the intrachelate VO(alcoholate) appears to be 0.05 A, longer, similar to the
M. Biagioli et al. / Inorganica Chimica Acta 310 (2000) 1–9
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VO(alcoholate) bond length in the complex of glycolic acid. The lengths of the VO(oxo) bonds are as expected, 1.61 and 1.62 A, . The coplanarity of the two oxo and alcoholic atoms is substantiated by a value of 360° for the sum of the angles subtended at the vanadium atom, which is 0.0210 A, out of the plane. The OVO angle in the rhomboidal cage is 70.9°, whereas the VOV angle is 109.1°. The free carboxylic group exhibits two different CO bonds, 1.21 and 1.31 A, , in contrast to the coordinate group that exhibits more equivalent distances of 1.23 and 1.27 A, .
Fig. 2. Perspective view of the [{VO2(mal)}2]2 − dianion. Thermal ellipsoids are drawn at 30% probability.
3.1.3. Cs2[{VO2(lact)}2] ·2H2O The molecular structure of compound 3 consists of a dimeric arrangement involving two slightly different coordination centers (Fig. 3 and Table 4). In this case in fact, no symmetry element relates the two halves of the molecule. The asymmetric unit contains one dimeric molecule, two Cs ions and two water molecules, one of which is disordered and split into two alternative positions, O(2)W and O(3)W which were both refined with occupancy factor of 0.5. The difference in the coordination bonds of vanadium in the two halves of the dimeric unit is within 0.03 A, . The caesium ions are involved in a number of contacts with the oxygen atoms of the ligand and water molecules. Eight of them are in the range 3.08 –3.38 A, for both the Cs ions. The VO(alcoholate) distances are 1.95 –1.96 A, for the bridges and 2.00 A, for the intrachelate bonds. The coplanarity of the two oxo and the alcoholic atoms yields values of approximately 360° for the sum of the angles subtended at the vanadium atoms. The metal ions are 0.0031 and 0.0089 A, out of the plane. Similarly to the complexes of glycolic and malic acids, the compound 3 exhibits the intrachelate VO(alcoholate) distance approximately 0.05 A, longer than the VO(alcoholate) bridging distance. The OVO angles are 71.2 –71.6°, whereas the VOV angles are 108 – 109°. The carboxylate group exhibits nonequivalent CO bonds 1.20 –1.22 and 1.29 –1.30 A, . 3.2. Infrared spectra
Fig. 3. Perspective view of the [{VO2(lact)}2]2 − dianion. Thermal ellipsoids are drawn at 30% probability.
Table 6 IR absorption bands of the complexes Complex
VO
COO−
COOH
OH
1
941vs 927vs 937vs 930vs 933vs
1348m,w 1664vs 1385m,w 1616vs 1349m,w 1664vs
1315m,w 1710m
2800–2500b
2 3
The assignment of the IR absorption bands of the complexes was carried out by comparison with the ligands and their sodium salts. The most significant IR features are collected in Table 6 and briefly described below. The cis couple of VO bonds exhibits distinctive symmetric and antisymmetric stretches in the range 930 –940 cm − 1. The carboxylate group taking part in metal binding is distinguished by asymmetric and symmetric stretching vibrations with splitting values D(nas − ns) in the range 231 –315 cm − 1 consistent with the monodentate coordination mode established by structural data. The position of these bands easily distinguishes the undissociated carboxylic group of malic acid in complex 2.
M. Biagioli et al. / Inorganica Chimica Acta 310 (2000) 1–9
Fig. 4. Reduction of [{VO2(lact)}2]2 − by cysteine: intensity of the VO(IV) EPR signal (Section 2) as a function of time and cysteine– vanadium molar ratio.
Fig. 5. High field region of the anisotropic EPR spectra recorded in the course of the reduction of [{VO2(lact)}2]2 − (cysteine–vanadium molar ratio = 2) as a function of time: (a) 40 min; (b) 110 min, (c) 210 min; (d) 1470 min. Table 7 EPR parameters of VO(IV) complexes formed upon reduction of Cs2[{VO2(lact)}2]·2H2O by cysteine Species
g
A (10−4 cm−1)
Donor atoms
I II III IV V
1.967 1.958 1.960 1.951 1.942
144 154 147 156 165
2(NH2, S−), 2(COO−, S−) (NH2, S−)+(S−, COO−) 2(COO−, O−) 2(NH2, COO−)
3.3. Comparison with the peroxo complex of lactic acid A comparison of the h2-peroxo complex of lactic acid K2[{VO(O2)(lact)}2] with its precursor 3 (Table 5) indicates that the two compounds share many structural features [11]. For instance, the VO(oxo) distance
7
in the peroxo complex is 1.59 A, , only slightly shorter than in compound 3 (160 –1.63 A, ), whereas the VO(carboxylate) bond is only slightly longer (2.00 vs. 1.974 and 1.679 A, ). Comparable VO(alcoholate) bonds are observed in the compounds, although in the peroxo derivative the intrachelate VO(alcoholate) bond is longer than the intradimeric bridge. A comparison of the geometry at the metal ions shows that four of the five oxygen atoms of the coordination polyhedron of vanadium in 3 (namely the carboxylic, two alcoholate and one of the oxo atoms) remain almost unchanged after peroxo binding. Indeed, the angle formed by the pseudo-axial donors (the carboxylate and bridging alcoholate oxygens) at the vanadium atom exhibits the same value in the two compounds. The O(alcoholate)VO(oxo) angle in the trigonal plane of the parent compound (121 –130°) becomes only slightly shorter in 3 (113.4°). Noticeably, the peroxo group replaces one of the oxo atoms in the parent compound without introducing gross changes in the overall geometry. The ligand is located almost perpendicularly to the trigonal plane, as if the axis joining the metal to the middle point of the peroxo group lies in the plane and replaces the bond of an oxo ligand. Indeed, assuming the midpoint of the peroxo group is a donor center, the sum of the angles at vanadium in the ideal trigonal plane is approximately 354°. On the other hand, the sum of the adjacent angles subtended by pseudo axial donors and peroxo oxygens at the metal ion is 203°. All these features confirm that the main effect resulting from peroxo binding is the replacement of the oxo ligand while the rest of the molecule maintains its integrity. The same features of 3 are observed in the homocitrate complex. Therefore, it is likely that the peroxidation of the hmcitr compound would give rise to similar changes.
3.4. Reduction of 6anadium(V) in aqueous solution The reduction of the V(V) complex of lactic acid by cysteine was studied in neutral aqueous solution under aerobic conditions. EPR spectra showed that, with elapsing time, vanadium(V) is reduced to EPR-active oxovanadium(IV). As shown in Fig. 4, the amount of VO(IV) increases steeply with time, reaches a maximum and then decreases slowly. The course of the reduction is very sensitive to the ratio of thiol to vanadium compound. An increasing excess of cysteine increases the amount of reduced ion and shifts the maximum extent of VO(IV) formation to longer times, consistent with a higher reductant capacity of the system. EPR spectra give evidence for monomeric oxovanadium(IV) distributed among the ligands present in the system, see Fig. 5. In particular, a number of complexes of cysteine have been identified by signals denoted as I, II and III in Fig. 5 and Table 7. These are characterized by
8
M. Biagioli et al. / Inorganica Chimica Acta 310 (2000) 1–9
coordination sets 2(NH2, S−), 2(COO−, S−) and (NH2, S−) +(S−, COO−), respectively [12]. Lactic acid forms the bis chelated species with 2x(COO−, O−) donor set (IV) [26]. With elapsing time cysteine is oxidized to cystine and the VO(IV) ions remain bound, beside to the hydroxycarboxylate ligand, to cystine. The latter ligand yields a species (V) presumably with the 2(NH2, COO−) donor set, even if the precise assignment needs further studies. In any case, the involvement of other oxidation states, e.g. EPR-inactive vanadium(III) cannot be ruled out, at least based only on our measurements. The fitting of the data in the ascending part of the kinetic curves indicate that the rate of formation of VO(IV) is of the first order with respect to vanadium(V), with kinetic constants almost independent on the cysteine to vanadium molar ratio (range: 0.010 – 0.012 min − 1). This suggests that a complex mechanism governs the reaction. Presumably, it involves the formation of an ester between vanadium(V) and cysteine, as a fast process, followed by a slow first-order decomposition of this intermediate to oxovanadium(IV) and cystine.
4. Conclusions The analysis of a series of dioxovanadium(V) compounds indicates that the V2O2 site originating from the a-hydroxycarboxylate coordination is very stable and can be considered highly conservative for this class of ligands. In this respect, the ability of the ligand to yield substantially equivalent bridging and intrachelate VO alcoholate bonds is relevant. As judged from the length of these distances, the bridging prevails over the chelating behavior. The structural integrity of the dinuclear arrangement is maintained after the interaction of the complexes with the peroxo ligand. The reaction only results in the replacement of an oxo by a peroxo group, while both the coordination features of the a-hydroxycarboxylate ligands and the dinuclear structure are retained. Ideally, the geometry around the metal ion remains the same if the axis connecting the metal ion to the center of the peroxo ligand is assumed to be a coordinate bond. On the other hand, the reduction of vanadium(V) to oxovanadium(IV) is effective in destroying the dinuclear arrangement and yielding monomeric species. The reduced metal ion remains bound to the a-hydroxycarboxylate ligand beside cysteine or cystine. These observations reinforce the proposition that the alkoxo bridged V2O2 dinuclear cage could be an effective tool adopted by organisms for the absorption and storage of vanadium(V). A rather unusual thermodynamic stability is attributed to such kind of dinuclear complexes in aqueous solution [5,25]. The integrity
maintained after the peroxidation indicates that the arrangement is robust enough to resist ligand exchange reactions. A number of biologically relevant reducing agents, including ascorbate, norepinephrine, simple thiols, cysteine and cysteine-based peptides, e.g. glutathione, which are the main components of the reductant pool of cellular systems, could be effective in the transformation of V(V) into V(IV). The reduction, beside destroying the dinuclear structure, labilizes the metal ion and increases its affinity to sulfur donors. Therefore the process is potentially very effective in promoting the inclusion of vanadium into sulfur-based clusters.
5. Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge, Crystallographic Center, CCDC No. 142410 for compound 1, No. 142411 for compound 2 and No. 142412 for compound 3. Copies of the data may be obtained free of charge form The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: + 44-1223-336033; e-mail: [email protected] or www: http://www.ccdc.cam. ac.uk).
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