Electron spin resonance and electronic spectroscopy of low-spin manganese(II) complexes (C5R5)(CO)2(L)Mn with L = hydrazido(1-), arylamido, anionic nitrile and purine-type ligands

Abstract

The low-spin manganese(II) oxidation state is stabilized in complexes (C₅R₅)(CO)₂(L)Mn, R = H, CH₃, with the negatively charged and π-electron-rich nitrogen bases L = imidazolate, benzimidazolate, benzotriazolate, purinate, adeninate, 8-aza-adeninate, indolate, 4-pyridylamide, anilide, 4-tolylamide, p-phenylenediamide(1-), N,N-dimethyl-p-phenylenediamide(1-), 1,1-diphenylhydrazide(1-), dicyanamide, deprotonated malonodinitrile and t-butylmalonodinitrile. The paramagnetic compounds were obtained by synergistic oxidative deprotonation of diamagnetic manganese(I) complexes of amines. Increasing σ and π donor strength of the various anionic ligands L results in facilitated oxidative deprotonation, a considerable bathochromic shift of the ligand-to-metal charge-transfer (LMCT, dâ†�π_N) absorption band across the visible region of the spectrum, and in growing metal-to-ligand spin delocalization, as evident from diminished g factors and a(⁵⁵Mn) hyperfine coupling constants in the e.s.r. spectra. Resolved ¹⁴N and ¹H superhyperfine splitting was observed in a few instances. Electronic structures, coordination sites and molecular conformations are inferred from experimental data and from their correlation with MO calculations.