Improving Redox Reversibility and Intermetallic Coupling of Co(III) Alkynyls through Tuning of Frontier Orbitals

Reported herein are the syntheses and characterizations for a series of butadiyndiyl-bridged Co^III(MPC) (MPC = 5,12-dimethyl-7,14-diphenyl-1,4,8,11-tetraazacyclotetradecane) complexes, capped with chloride ([1]²⁺), phenylacetylide ([2]²⁺), or 3,5-dichlorophenylacetylide ([3]²⁺). The MPC ligand was chosen to weaken the ligand field around the equatorial plane, allowing stronger axial coordination. Cyclic voltammetry measurement revealed the first unambiguous examples of stepwise one-electron reductions in bridged Co­(cyclam′) complexes (cyclam′ = any ligand bearing the 1,4,8,11-tetraazacyclotetradecane framework). Further voltammetric analysis using NBu₄BArF (NBu₄BArF = tetrabutylammonium tetrakis­[3,5-bis­(trifluoromethyl)­phenyl]­borate) as electrolyte resulted in well-separated reversible reduction waves with up to 160 mV separation for [2]²⁺. These features hint at the possibility of application of earth-abundant metal complexes to function as molecular wires. These complexes were also characterized with infrared and UV–vis spectroscopy, density functional theory calculations, and single-crystal X-ray diffraction ([1]­(BPh₄)₂ only).