Forging Strong Co–C(sp²) Bonds in Co-Macrocycle: Mono- and Bis-Aryl Complexes of Co^III(TIM)

Co-alkyls/aryls supported by tetraaza macrocycles are vitamin B12 analogues and are of interest in catalytic transformations. Reported herein is the synthesis and characterization of a series of mono- and bis-aryl CoIII(TIM) (TIM = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) complexes, the latter of which are unprecedented for Co tetraaza macrocycles. The products trans-[Co(TIM)BrAr]PF6 (3) and trans-[Co(TIM)Ar2]PF6 (2) [Ar = phenyl (Ph) (a), 2-naphthyl (2-Np) (b), 4–F-C6H4 (c), 4-tBu-C6H4 (d), 4-OMe-C6H4 (e)] were generated from the reaction between trans-[Co(TIM)Br2]PF6 and arylmagnesium bromide. Upon purification in the presence of chloride, 3 readily transforms into trans-[Co(TIM)ClAr]PF6 (1), and the latter was thoroughly characterized. Molecular structures of six aryl complexes were established via single-crystal X-ray diffraction, which show elongation of the Co–C bond in the bis-aryls and bending of the TIM macrocycle due to steric effects in the mono-aryls. Cyclic voltammograms revealed one reversible reduction (Co3+/2+) for the mono-aryl complexes and two irreversible reductions for the more electron-rich bis-aryl complexes. UV–vis spectra for both the mono-aryls and bis-aryls feature multiple d–d bands with substantial charge transfer character.