A Homologous Series of First-Row Transition-Metal Complexes of 2,2′-Bipyridine and their Ligand Radical Derivatives: Trends in Structure, Magnetism, and Bonding

The organometallic first-row transition-metal complexes [M­(2,2′-bipy)­(mes)2] (M = Cr (1), Mn (2), Co (4), Ni (5); 2,2′-bipy = 2,2′-bipyridine; mes = 2,4,6-Me3C6H2) were reacted with potassium and a suitable alkali-metal sequestering agent to yield salts of the anionic species [M­(2,2′-bipy)­(mes)2]−. The neutral parent compounds and their corresponding anionic congeners were characterized by single-crystal X-ray diffraction in [Cr­(2,2′-bipy)­(mes)2]·1.5C6H6, [Mn­(2,2′-bipy)­(mes)2], [Co­(2,2′-bipy)­(mes)2]·THF, [Ni­(2,2′-bipy)­(mes)2], [K­(dibenzo-18-crown-6)·THF]­[Cr­(2,2′-bipy)­(mes)2]·2THF, [K­(18-crown-6)]­[Mn­(2,2′-bipy)­(mes)2]·2THF, [K­(18-crown-6)]­[Mn­(2,2′-bipy)­(mes)2]·0.67py·0.67tol, [K­(2,2,2-crypt)]­[Co­(2,2′-bipy)­(mes)2], and [K­(2,2,2-crypt)]­[Ni­(2,2′-bipy)­(mes)2]. These species, along with the previously reported neutral and anionic iron complexes [Fe­(2,2′-bipy)­(mes)2]0/– (3/3–), form a homologous series of compounds which allow for an in-depth study of the interactions between metals and ligands. Single-crystal X-ray diffraction data, DFT calculations, and various spectroscopic and magnetic measurements indicate that the anionic complexes (1––5–) can be best formulated as M­(II) complexes of the 2,2′-bipyridyl radical anion. These findings complement recent studies which indicate that bond metric data from single-crystal X-ray diffraction may be employed as an important diagnostic tool in determining the oxidation states of bipyridyl ligands in transition-metal complexes.