Synthesis and Characterization of CoIII(TIM) Complexes Bearing Alkenyl or 1-Aza-2-cobalt-cyclopropane Moieties

Reported herein are the synthesis and characterization of mono- and bis-alkenyl Co^III(TIM) (TIM = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) complexes along with products containing a 1-aza-2-cobalt-cyclopropane unit. The trans-[Co(TIM)(C(CH₂)Ar)Cl]⁺-type (Ar = phenyl (Ph, 1a), −C₆H₄-4-^tBu (1b), and −C₆F₅ (1c)), trans-[Co(TIM)(C(CH₂)Ar)₂]⁺-type (Ar = Ph (2a) and −C₆F₅ (2c)), and trans-[Co(TIM″)(C(CH₃)Ar)Cl]⁺-type complexes (Ar = Ph (3a) and −C₆H₄-4-^tBu (3b); TIM″ is the resultant derivative of TIM) were prepared from the reaction between trans-[Co(TIM)Cl₂]PF₆ and the corresponding terminal aryl alkyne in the presence of NaBH₄. Molecular structures of 1a/c, 2a/c, and 3a were established via single-crystal X-ray diffraction studies. Characterization using ¹H NMR further confirmed the occurrence of either an alkenyl (1 and 2) or 1-aza-2-cobalt-cyclopropane (3). The absorption spectra of 1 and 3 reveal differences in the optical HOMO–LUMO gaps with well-defined d–d bands at 477 and 512 nm for 1a and 3a, respectively. Cyclic voltammograms of 1 and 2 consist of an irreversible oxidation and an irreversible reduction characteristic of the Co^III center, while those of 3 display two reduction events. Density functional theory calculations were performed to investigate the bonding and electronic structures of 1a–3a. Natural bonding orbital calculations on 3a suggest significant stabilization in donor → acceptor resonance-type interactions afforded by the 1-aza-2-cobalt-cyclopropane unit.