Generation and Properties of Co I /Ni I Species Stabilized by a Tetradentate Pyridylpyrazole Ligand: Crystal Structures of Dialkyl‐Co III Complexes

Mononuclear Co II and Ni II complexes [(L 1 )Co II (H 2 O) 2 ][ClO 4 ] 2 ( 1 ) and [(L 1 )Ni II (MeCN) 2 ][BPh 4 ] 2 ( 3 ) (L 1 = 2,2′‐[propane‐1,3‐diylbis(1 H ‐pyrazole‐1,3‐diyl)]dipyridine) display a quasireversible M II /M I redox process ( E 1/2 = –0.96 V vs. SCE, Δ E p = 110 mV for 1 ; E 1/2 = –0.71 V vs. SCE, Δ E p = 90 mV for 3 ). The dinuclear Co II complex [(L 1 ) 2 Co II 2 (μ‐O 2 CMe) 2 ][BPh 4 ] 2 ( 2 ) displays a quasireversible reductive response ( E 1/2 = –1.46 V vs. SCE, Δ E p = 120 mV), supposedly due to Co II 2 /Co I 2 redox process. For these complexes chemical (NaBH 4 ) and electrochemical reduction generate Co I and Ni I species. The existence of dark purple Ni I 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 Co I species were characterized by absorption spectroscopy. Reaction of chemically generated Co I species with CH 3 I resulted in the formation of dimethyl‐Co III complexes. Notably, Co I species generated from monomeric complex 1 led to the formation of two products: a mononuclear complex [(L 1 )Co III (CH 3 ) 2 ][ClO 4 ] ( 4 ) (minor) and a dinuclear complex [(L 1 ) 2 Co III 2 (CH 3 ) 4 ][ClO 4 ] 2 ( 5 ) (major). In contrast, when the dimeric complex 2 was reduced with NaBH 4 and reacted with CH 3 I the monomeric complex [(L 1 )Co III (CH 3 ) 2 ][BPh 4 ] ( 6 ) was isolated in reasonable yield. All three organometallic dialkyl‐Co III complexes 4 – 6 have been characterized by elemental analysis, 1 H NMR spectroscopy, and X‐ray crystallography.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)