A Synthetic Approach to Cross-Conjugated Organometallic Complexes Based on geminal-Diethynylethene and Co^III(cyclam)

A series of Co^III(cyclam) complexes ([1a,b]­Cl, [2a–c]­PF₆, [3]­Cl₂, [4a]­(OTf)₄, [4b]­(PF₆)₂, and [5]­Cl₂) (cyclam = 1,4,8,11-tetraazacycloctetradecane) bearing a geminal-diethynylethene ligand (gem-DEE) is reported. Syntheses of these acyclic cross-conjugated complexes were accomplished in satisfactory yields, and structural characterizations established that the geometrical feature of gem-DEE is largely preserved upon metalation. Combined structural and electrochemical studies suggest that the gem-DEE ligand, as a σ-donor, is weaker than phenylethynyl but stronger than butadiynyl in Co^III(cyclam) complexes. Voltammetric analysis indicated a weak but discernible Co–Co coupling across the gem-DEE bridge in [3]­Cl₂ and [4a]­(OTf)₄, while the addition of a second acetylide in the trans position diminished such coupling in [4b]­(PF₆)₂. DFT analysis revealed significant dπ–π mixing around the cobalt centers with extended π-overlap in the highest occupied orbitals and substantial σ-based mixing in the lowest unoccupied orbitals of [3]­Cl₂ and [4a]­(OTf)₄, the latter of which likely contributes to the weak Co–Co coupling.