A Synthetic Approach to Cross-Conjugated Organometallic Complexes Based on geminal-Diethynylethene and CoIII(cyclam)
journal contributionposted on 12.10.2016, 11:50 by Sean N. Natoli, Tyler J. Azbell, Phillip E. Fanwick, Matthias Zeller, Tong Ren
A series of CoIII(cyclam) complexes ([1a,b]Cl, [2a–c]PF6, Cl2, [4a](OTf)4, [4b](PF6)2, and 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 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 Cl2 and [4a](OTf)4, the latter of which likely contributes to the weak Co–Co coupling.