MLCT1 SPECTRA AND CARBON-MONOXIDE BINDING TO IRON(II) DIFLUORO(DIOXIMATO)BORATE COMPLEXES IN WEAK DONOR SOLVENTS

A variety of Fe(dioxBF2)2W2 complexes, where diox equals dimethylglyoximate, naphthoquinone dioximate, and benzoquinone dioximate and W is a weak donor ligand, including tetrahydrofuran (thf), N,N-dimethylformamide (dmf), dimethyl sulfoxide (dmso), tetrahydrothiophene (tht), and a variety of nitriles (benzonitrile, acrylonitrile, phthalonitriles, cyanopyridines, tetracyanoethylene (tcne)), are described. Low-energy MLCT bands are observed in the visible spectrum for axial ligands with low reduction potentials (tcne, 922 nm; nitrophthalonitrile, 549 nm; methylpyrazinium, 687 nm; tetrachlorophthalonitrile, 530 nm; 4-cyanopyridine, 470 nm), and these are compared with pentacyanoferrate analogues. The kinetics of the reaction of these systems with CO were determined by flash photolysis in neat solvents or in toluene solution in the presence of excess W. The order of lability for FeN4W2 in either toluene or neat W is thf > dmf > dmso > nitriles greater-than-or-equal-to tht >> py. Carbon monoxide binding systems with rate and equilibrium constants spanning over 6 orders of magnitude are described.