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

Abstract

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-Cα 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.