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

A series of CoIII(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 CoIII(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.