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)]center dot 1.5C(6)H(6), [Mn(2,2'-bipy)(mes)(2)], [Co(2,2'-bipy)(mes)(2)]center dot THF, [Ni(2,2'-bipy)(mes)(2)], [K(dibenzo-18-crown-6)center dot THF][Cr(2,2'-bipy)(mes)(2)]center dot 2THF, [K(18-crown-6)][Mn(2,2'-bipy)(mes)(2)]center dot 2THF, [K(18-crown-6)] [Mn(2,2'-bipy)(mes)(2)]center dot 0.67py center dot 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 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.