Controlling Nuclearity and Stereochemistry in Vanadyl(V) and Mixed Valent V-IV/V-V Complexes of Oxido-Pincer Pyridine-2,6-dimethanol Ligands

The coordination chemistry of three oxido-pincer ligands 2,6-(HOCR2)(2)(pyridine) (H2L) based on 2,6-pyridinedimethanol [R = H (H(2)pydim), Me (H(2)pydip), Ph (H(2)pyphen)] towards vanadium(V) was explored. Reaction of NH4VO3 with the protoligands H2L gave the dinuclear complexes [(L)OV(mu-O)VO(L)]. Mononuclear anionic species [VO2(L)](-), which were isolated as alkaline metal salts were obtained from reactions of [VO(acac)(2)] (acac(-) = acetylacetonate) and H2L under basic conditions and addition of HCl to these species allowed to isolate the unprecedented oxido chlorido complexes [VOCl(L)] for pydip and pyphen. Cyclic voltammograms of the dinuclear [V2O3(L)(2)] and mononuclear [VOCl(L)] complexes show reversible V-V/V-IV reduction waves, while corresponding waves of the anionic [VO2(L)](-) are completely irreversible. The mixed-valent V-IV/V-V species [V2O3(L)(2)](center dot-) were characterized by EPR and UV/Vis spectroelectrochemistry revealing a delocalized system with a 15 line EPR spectrum and an intervalence charge transfer (IVCT) band for the bulky pyphen ligand but localized radicals in case of the pydim and pydip derivatives (8 line EPR, no IVCT). DFT calculated structures of the three derivatives show an V-O-V arrangement for [V2O3(pyphen)(2)](center dot-) of about 145 degrees ideally suited for delocalization, whereas for [V2O3(pydip)(2)](center dot-) an angle of 128 degrees was found