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

Abstract

AbstractMononuclear CoII and NiII complexes [(L1)CoII(H2O)2][ClO4]2 (1) and [(L1)NiII(MeCN)2][BPh4]2 (3) (L1 = 2,2′‐[propane‐1,3‐diylbis(1H‐pyrazole‐1,3‐diyl)]dipyridine) display a quasireversible MII/MI redox process (E1/2 = –0.96 V vs. SCE, ΔEp = 110 mV for 1; E1/2 = –0.71 V vs. SCE, ΔEp = 90 mV for 3). The dinuclear CoII complex [(L1)2CoII2(μ‐O2CMe)2][BPh4]2 (2) displays a quasireversible reductive response (E1/2 = –1.46 V vs. SCE, ΔEp = 120 mV), supposedly due to CoII2/CoI2 redox process. For these complexes chemical (NaBH4) and electrochemical reduction generate CoI and NiI species. The existence of dark purple NiI 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∥ = 2.27 and g⟂ = 2.08). Dark greenish blue/bright green CoI species were characterized by absorption spectroscopy. Reaction of chemically generated CoI species with CH3I resulted in the formation of dimethyl‐CoIII complexes. Notably, CoI species generated from monomeric complex 1 led to the formation of two products: a mononuclear complex [(L1)CoIII(CH3)2][ClO4] (4) (minor) and a dinuclear complex [(L1)2CoIII2(CH3)4][ClO4]2 (5) (major). In contrast, when the dimeric complex 2 was reduced with NaBH4 and reacted with CH3I the monomeric complex [(L1)CoIII(CH3)2][BPh4] (6) was isolated in reasonable yield. All three organometallic dialkyl‐CoIII complexes 4–6 have been characterized by elemental analysis, 1H NMR spectroscopy, and X‐ray crystallography.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)