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–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.