Electronic structures of methylated azaferrocenes and their borane adducts: photoelectron spectroscopy and electronic structure calculations.

The preparation and crystal structure of a new 2,5-dimethylazaferrocene (DMAF) adduct of borane (DMAF-BH(3)) are reported. The electronic structures of 1,2,3,4,5-pentamethylazaferrocene (PMAF), its borane adduct 1,2,3,4,5-pentamethylazaferrocene-BH(3) (PMAF-BH(3)) and of DMAF-BH(3) have been studied by HeI-HeII UV photoelectron spectroscopy (UPS) and high-level DFT methods. UPS data reveal the influence of borane and methyl substitution on the electronic structures of azaferrocenes, which have been rationalized by inductive and hyperconjugative effects. UPS data also allow for the estimation of azaferrocene lone-pair stabilization upon coordination to borane, and suggest that pyridine is a slightly stronger sigma donor toward BH(3) than either azaferrocene studied. UV-Vis spectroscopic data for the radical cation of PMAF were also obtained and compared with the UPS data.