On the solvatochromic properties of the oxalate-bridged complexes [(tBuCO2)3M2]2(μ-O2C2O2) where M=Mo or W

The room temperature electronic absorption spectra of the oxalate bridged MM quadruply bonded complexes [(tBuCO2)3M2]2(μ-O2C2O2), where M = Mo or W have been recorded in H2O, THF:H2O mixtures, THF, CH2Cl2, toluene, DMSO, aniline, toluene saturated with N,N-dimethylaniline and ethanol. The strong absorptions in the visible region of the electronic absorption spectra assignable to the metal-to-ligand (bridge) charge transfer are shown to be highly solvent dependent. Those samples prepared in H2O, CH2Cl2 and toluene are shown to comprise of a suspension of microcrystalline particles ranging in size from 100 nm to 5 μm. Individual particles were found by scanning electron microscopy to have an aspect ratio of ∼10:1, all being needle shaped. The spectra in THF, EtOH, aniline, DMSO and toluene–N,N-dimethylaniline all show similar vibronic progressions and are attributed to discrete solvated molecular species. The spectra recorded in aniline are notably red-shifted which is proposed to arise from a combination of hydrogen bonding and Lewis base stabilization of the photoexcited state.