An Aerobic Synthetic Approach toward Bis-Alkynyl Cobalt(III) Compounds

Reported herein is an expanded investigation into a new method for the preparation of Co(III) cyclam bis-alkynyls (cyclam = 1,4,8,11-tetraazacyclotetradecane) under aerobic, weak base conditions. Treatment of trans-[Co(cyclam)(C₂Ar)Cl]Cl-type complexes (Ar = C₆F₅ (1a), 4-C₆H₄NMe₂ (1b)) with AgOTf in MeCN resulted in the doubly charged complexes [Co(cyclam)(C₂Ar)(NCMe)](OTf)₂ (Ar = C₆F₅ (2a), 4-C₆H₄NMe₂ (2b)). These solvento complexes 2a,b undergo rapid alkynylation under aerobic conditions in the presence of an organic base and HC₂Ar' to form the symmetrical or unsymmetrical bis-alkynyl complexes trans-[Co(cyclam)(C₂Ar)(C₂Ar')](OTf) (Ar/Ar' = C₆F₅ (3a), 4-C₆H₄NMe₂ (3b); Ar = C₆F₅ and Ar' = 4-C₆H₄NMe₂ (3c), C₂Ph (3d)) in good yields. Molecular structures of the new compounds were established using single-crystal X-ray diffraction. Structural studies revealed a notable trans influence for the Co-C_α bond lengths in the unsymmetrical complex 3c with a bond length of 1.929(7) Å for the electron-withdrawing -C₂C₆F₅ ligand and 1.944(7) Å for -C₂-4-C₆H₄NMe₂. The optical HOMO-LUMO gaps for the bis-alkynyl complexes follow the trend 3a (2.83 eV) > 3d (2.77 eV) > 3c (2.70 eV) > 3b (2.64 eV). Although [Co(cyclam)(C₂R)₂]⁺ type complexes typically have irreversible electrochemical reductions, reversibility of the Co(+3/+2) couple improves in Co(III) cyclam complexes bearing more electron withdrawing substituents. Voltammetric analysis also revealed a modest NMe₂/NMe₂ coupling across the Co-alkynyl backbone in 3b, while DFT calculations identified the HOMO in 3b as the superexchange pathway for such coupling.