Generation and Properties of CoI/NiI Species Stabilized by a Tetradentate Pyridylpyrazole Ligand: Crystal Structures of Dialkyl-CoIII Complexes

Mononuclear Co-II and Ni-II complexes [(L-1)Co-II(H2O)(2)][ClO4](2) (1) and [(L-1)Ni-II(MeCN)(2)][BPh4](2) (3) (L-1 = 2,2'-[propane-1,3-diylbis(1H-pyrazole-1,3-diyl)]dipyridine) display a quasireversible M-II/M-I redox process (E-1/2 = -0.96 V vs. SCE, Delta E-p = 110 mV for 1; E-1/2 = -0-71 V vs. SCE, Delta E-p = 90 mV for 3). The dinuclear Co-II complex [(L-1)(2)Co-2(II)(mu-O2CMe)(2)][BPh4](2) (2) displays a quasireversible reductive response (E-1/2 = -1.46 V vs. SCE, Delta E-p = 120 mV), supposedly due to Co-2(II)/Co-I(2) redox process. For these complexes chemical (NaBH4) and electrochemical reduction generate COI and Ni-I species. The existence of dark purple Nil species has been authenticated by its UV/Vis spectral feature (crystal-field transition at 901 nm and metal-to-ligand charge-transfer transition at 537 nm) and axial EPR spectrum (g(parallel to) = 2.27 and g(perpendicular to) = 2.08). Dark greenish blue/bright green Co-I species were characterized by absorption spectroscopy. Reaction of chemically generated Cot species with CH3I resulted in the formation of dimethyl-Co-III complexes. Notably, Col species generated from monomeric complex 1 led to the formation of two products: a mononuclear complex [(L-1)Co-III(CH3)(2)][ClO4] (4) (minor) and a dinuclear complex [(L-1)(2)CO2III(CH3)(4)][ClO4](2) (5) (major). In contrast, when the dimeric complex 2 was reduced with NaBH4 and reacted with CH3I the monomeric complex [(L-1)Co-III-(CH3)(2)][BPh4] (6) was isolated in reasonable yield. All three organometallic dialkyl-Co-III complexes 4-6 have been characterized by elemental analysis, H-1 NMR spectroscopy, and X-ray crystallography. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)