A Synthetic Approach to Cross-Conjugated Organometallic Complexes Based on <i>geminal</i>-Diethynylethene and Co<sup>III</sup>(cyclam)

A series of Co<sup>III</sup>(cyclam) complexes ([<b>1a</b>,<b>b</b>]­Cl, [<b>2a</b>–<b>c</b>]­PF<sub>6</sub>, [<b>3</b>]­Cl<sub>2</sub>, [<b>4a</b>]­(OTf)<sub>4</sub>, [<b>4b</b>]­(PF<sub>6</sub>)<sub>2</sub>, and [<b>5</b>]­Cl<sub>2</sub>) (cyclam = 1,4,8,11-tetraazacycloctetradecane) bearing a <i>geminal</i>-diethynylethene ligand (<i>gem</i>-DEE) is reported. Syntheses of these acyclic cross-conjugated complexes were accomplished in satisfactory yields, and structural characterizations established that the geometrical feature of <i>gem</i>-DEE is largely preserved upon metalation. Combined structural and electrochemical studies suggest that the <i>gem</i>-DEE ligand, as a σ-donor, is weaker than phenylethynyl but stronger than butadiynyl in Co<sup>III</sup>(cyclam) complexes. Voltammetric analysis indicated a weak but discernible Co–Co coupling across the <i>gem</i>-DEE bridge in [<b>3</b>]­Cl<sub>2</sub> and [<b>4a</b>]­(OTf)<sub>4</sub>, while the addition of a second acetylide in the <i>trans</i> position diminished such coupling in [<b>4b</b>]­(PF<sub>6</sub>)<sub>2</sub>. 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 [<b>3</b>]­Cl<sub>2</sub> and [<b>4a</b>]­(OTf)<sub>4</sub>, the latter of which likely contributes to the weak Co–Co coupling.