Trans influence of cyanide on the structural and electronic properties of a series of organocobaltIII(TIM) complexes

Abstract

The trans influence of anionic ligands on the nature of the cobalt–carbon bond in trans-[Co(TIM)RX]⁺-type complexes (R is a C-bound ligand and TIM is 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) was probed through the incorporation of cyanide as the second monoanionic axial ligand (X). Salt metathesis reactions between KCN and the corresponding halide precursors readily generate trans-[Co(TIM)(C(CH₂)Ph)(CN)]⁺ (1), trans-[Co(TIM′′)(C(CH₃)C₆H₄-4-^tBu)(CN)]⁺ (2, TIM′′ is the reduced TIM derivative following the formation of an aza-cobalt-cyclopropane moiety), and trans-[Co(TIM)(CH₃)(CN)]⁺ (3). The molecular structures of 1 and 2 were established via single-crystal X-ray diffraction studies. FT-IR spectroscopy revealed a red-shift in the ν(CN) stretching frequency from 1 to 3 to 2. The absorption spectra of 1–3 exhibit d–d bands which are substantially blue-shifted from their halide precursors as a result of the enhanced ligand field imbued by the cyanide. Cyclic voltammograms of 1 and 3 consist of an irreversible Co^4+/3+ oxidation and an irreversible Co^3+/2+ reduction. In 2, a reversible Co-centered reduction is observed at −1.75 V vs. Fc^1+/0. Also reported are the crystal structures of trans-[Co(TIM)(CN)₂]PF₆ (4), trans-[Co(TIM)(CH₃)I]PF₆ (5) and trans-[Co(TIM)(CH₃)₂]PF₆ (6). The π-accepting capabilities of the cyanide reduce the dπ(Co)-π*(imine) backdonation in 4, while the solely strong σ-donating nature of the –CH₃ ligands in 6 exerts a large trans influence and significantly increases the Co-TIM interactions.