A Family of Cross-Conjugated Polyenynes Capped by CoIII(cyclam): Syntheses, Molecular and Electronic Structures

Transition-metal complexes bridged by acyclic cross-conjugated frameworks are uncommon and can possess structurally unique nonlinear topologies and intriguing electron transport properties. Herein, we report the attachment of extended enyne scaffolds, L1 (3,6-diethynyl-2,7-dimethylocta-2,6-dien-4-yne) and L2 (5-ethynyl-6-methylhepta-5-en-1,3-diyne), to a CoIII(cyclam) unit. Mononuclear complexes Co-L1 ([Co­(cyclam)­(L1)­Cl]­PF6) and Co-L2 ([Co­(cyclam)­(L2)­Cl]­PF6) were synthesized in modest to good yields under methanolic weak-base conditions. Similarly, bimetallic compounds Co-L1-Co ([{Co­(cyclam)­Cl}2 (μ-L1)]­(PF6)2) and Co-L2-Co ([{Co­(cyclam)­Cl}2(μ-L2)]­(PF6)2) were prepared in modest yields from the reaction of [Co­(cyclam)­Cl2]Cl with Co-L1 or Co-L2, respectively. Alternatively, Co-L1-Co and Co-L2-Co were prepared directly from L1 and L2 in the presence of excess [Co­(cyclam)­Cl2]­Cl. Oxidative coupling of Co-L1 and Co-L2 yielded the respective dimeric complexes {Co-L1-} 2 ([{Co­(cyclam)­Cl}2(μ-(L1)2)]­(PF6)2) and {Co-L2-} 2 [{Co­(cyclam)­Cl}2(μ-(L2)2)]­(PF6)2) in modest yields. Both {Co-L2‑} 2 (C14) and {Co-L1-} 2 (C16) are examples of the longest bridged transition-metal enyne complexes and bridged CoIII(cyclam) complexes to date. {Co-L1-} 2 crystallized in a unique s-cis–trans–cis orientation that is stabilized by a close intermolecular π–π interaction. Cyclic voltammetry measurements of cobalt-L1 and -L2 complexes revealed isolated CoIII(cyclam) units, where redox potentials were determined by structural characteristics prior to the first intervening olefin. In contrast to the cases of shorter enyne bridged CoIII(cyclam) complexes (≤5 carbons), the LUMOs from DFT analysis do not display orbital mixing spanning the entirety of the enyne framework. This work highlights the diversity in structural and electronic properties obtained with transition-metal enyne frameworks. Such complexes are intriguing as nonlinear moieties for supramolecular assemblies and as long-range molecular insulators or switches.