Conversion of Dinuclear, Diamagnetic [Mo^V₂O₃(NCS)₈]^4- into Mononuclear, Paramagnetic [Mo^VO(NCS)₅]^2- through Photolytic Ligand Redistribution

Dinuclear [MoV2O3(NCS)8]4- (14-) has been prepared as [NMe4]+ and [NEt4]+ salts in good yield by reduction of [MoVIO2(NCS)4]2- with 0.5 equiv of PMe2Ph in tetrahydrofuran. A Beer's law plot of the band at 513 nm (ε = 22 500 mol-1 L cm-1) from concentrations of 1 × 10-4−1 × 10-3 mol L-1 established that 14- does not participate in thermal disproportionation equilibria with mononuclear Mo(VI) and Mo(IV) species. A single crystal diffraction study has established that dark crystals containing 14- and a noncoordinating [NMe4]+ counterion crystallize from acetone as the solvate [NMe4]4[MoV2O3(NCS)8]·2Me2CO in the triclinic space group P1̄ with a = 9.455 (2) Å, b = 11.831 (2) Å, c = 12.772 (3) Å, α = 110.52 (3)°, β = 105.17 (3)°, γ = 98.63 (3)°, and Z = 1. The anion in these crystals has an anti orientation of the terminal oxo groups similar to that observed previously for a hydrate of the K+ salt. Ion pairing in the K+ salt has surprisingly modest effects on anion parameters for the anion, largely confined to increases in the bending at N of the isothiocyanate ligands. The tetraalkylammonium salts are photosensitive in CH3CN, and purple solutions become orange within hours under ambient lighting. Disappearance of the absorbance at 513 nm and appearance of a new visible absorbance at 455 nm is accompanied by well-defined isosbestic points at 483, 394, 367, and 322 nm. The photolysis product is paramagnetic (μeff = 1.86 BM in solution) and has been shown to be mononuclear [MoVO(NCS)5]2- by comparison (UV/visible, IR, combustion analysis, EPR) with a reference sample. Reproducible synthetic photolysis of [MoV2O3(NCS)8]4- requires an atmosphere of O2, consistent with an initial photochemical redistribution of the isothiocyanate ligands to form [MoVO(NCS)5]2- and a Mo(V) oxo complex which is then oxidized. The low disappearance quantum yield in the visible (Φλ>460 = 1.3 × 10-4) establishes that the reaction is not connected with the bridging oxo chromophore but probably involves initial NCS- dissociation following UV excitation (Φ310 = 1.4 × 10-3 under N2).