Mixed-Oxidation Divanadium(IV,V) Compound with Ligand Asymmetry: Electronic and Molecular Structure in Solution and in the Solid State

Abstract

Reaction of [V(IV)OL(1)(Im)] (H(2)L(1) = S-methyl-3-((2-hydroxyphenyl)methyl)dithiocarbazate) with [V(V)OL(OCH(3))] allows isolation of (ImH)[L(1)OV-(&mgr;-O)-VOL] complexes 2 (H(2)L = H(2)L(2) = S-methyl-3-((5-bromo-2-hydroxyphenyl)methyl)dithiocarbazate) and 3 (H(2)L = H(2)L(1)), one of which (2) has ligand asymmetry not previously known in this type of complex. In the solid state, (ImH)[L(1)OV-(&mgr;-O)-VOL(2)] (2) provides an example of a divanadium(IV,V) compound with a syn angular [V(2)O(3)](3+) core structure that exhibits crystallographically imposed mirror symmetry due to static disorder. Crystals of 2 are orthorhombic, space group Pnma, with a = 10.740(2) Å, b = 18.912(4) Å, c = 17.163(4) Å, and Z = 4. In toluene at room temperature, both 2 and 3 have 8-line EPR spectra, characteristic of trapped-valence structure. When acetonitrile is added to these solutions, the spectra reveal 15-line features with asymmetric distortions that smooth out with the lowering of temperature. This probably has its origin in a solvent-dependent equilibrium involving two magnetically inequivalent structural forms of the divanadium(IV,V) compound, with syn angular and anti linear structures of the [V(2)O(3)](3+) core. Variable temperatures (298-220 K) (51)V NMR spectroscopic studies in solution also support this view. In acetonitrile, both 2 and 3 exhibit an intervalence transfer band in the near-IR region at ca. 970 nm (epsilon, 1600 and 1480 M(-)(1) cm(-)(1) for 2 and 3, respectively) and they undergo one-electron reversible oxidation at ca. 0.40 V (vs SCE) due to the V(IV)V(V)/V(V)V(V) couple.

An Unsymmetrical Mixed‐Valent Divanadium(IV/V) Complex

“…The origin of this broad shoulder is presumably a combination of more than one ligand-field transitions. [19][20][21][22][23][24] Following this weak absorption, a shoulder at 330 nm and a strong absorption at 275 nm are observed. These are assigned to the ligand-to-metal charge transfer and ligandcentered transitions.…”

“…Observation of such two V=O stretching modes for trapped-valent complexes of {V 2 O 3 } 3+ core was previously reported. [19,24] The electronic spectrum of the complex in methanol solution displays a low intensity broad shoulder centered at 535 nm. The origin of this broad shoulder is presumably a combination of more than one ligand-field transitions.…”

“…Structurally characterized complexes containing the {OV(µ-O)VO} 3+ core are very few. [13][14][15][16][17][18][19][20][21][22][23][24] In all these complexes, the metal ions are bridged by only the oxo group and they are symmetric with respect to the ligands bonded to them. Only in one case do the two ligands attached to the two metal centers differ in the substituents attached to them.…”

“…[13][14][15][16][17][18][19][20][21][22][23][24] In all these complexes, the metal ions are bridged by only the oxo group and they are symmetric with respect to the ligands bonded to them. Only in one case do the two ligands attached to the two metal centers differ in the substituents attached to them.[24] The reported structures have been scrutinized [19,24] for a correlation between the geometry of the {OV(µ-O)VO} 3+ core and its electronic structure. It has been found that the anti linear configuration of the core facilitates a very effective overlap of the d xy metal orbitals through the oxo bridge p x orbital and, hence, a complete delocalization of the unpaired 3d electron over the two vanadium centers.…”

“…[19] Complexes with the syn bent {OV(µ-O)-VO} 3+ core are rare and they have localized electronic structures. [18,22,24] The lone example for a complex of {OV(µ-O)VO} 3+ with a configuration in between syn and anti also has a localized electronic structure. [20] It is interesting to note that on the EPR timescale, except for very few cases, [19,21,24] valence-trapped species display delocalized .…”