Molecular and electronic structure of (η5-pentaalkyl-2,3-dihydro-1,3-diborolyl)(η5-pentamethylcyclopentadienyl)metal complexes, M = Fe, Ru

The X-ray diffraction study of the violet ruthenium sandwich derivative 2b revealed a folding along the B⋯B vector of the 1,3-diborolyl heterocycle of 40.7°, which is very similar to that in the green iron complex 1b. The molecular and electronic structure of the 1,3-diborolyl complexes of iron and ruthenium have been studied by density functional theory (DFT) with the B3LYP functional and extended triple-ζ basis sets. It is shown that the folding of the diborolyl ligand in the title complexes is due to electronic factors. The model complexes with a planar diborolyl ligand 1′ (M = Fe) and 2′ (M = Ru) are by 24.9 and 24.5 kcal/mol less stable than the equilibrium folded structures of 1 and 2, respectively. The electronic structures of 1 and 2 show similarities to those of the 18 VE species due to the participation of the σ(B–C2) bonds in the stabilizing diborolyl–metal bonding. The electronic spectra of 1 and 2 have been studied with the time-dependent DFT method. The absorptions observed in the visible range in the electronic spectrum of the title complexes are assigned as spin-allowed d–d transition with an admixture of metal to diborolyl (π∗) charge transfer (MLCT).