Iron and Ruthenium σ‑Polyynyls of the General Formula [{M(dppe)Cp*}–(CC)_<i>n</i>–R]^0/+ (M = Fe, Ru): An Experimental and Theoretical Investigation

Two series of metal-polyynyl complexes of iron and ruthenium of general formula [{M­(dppe)­Cp*}–(CC)n–R]0/+ (M = Fe, Ru; R = H, Ph, SiMe3, Au­(PPh3); n = 1–3), have been synthesized, characterized, and theoretically analyzed. The results provide a comprehensive description of the effect of the length of the conjugated carbon chain and the role of the nature of the metal atom and the terminal substituent on their neutral and oxidized states. For the latter, the spin density found on the carbon chain is a source of instability; e.g., for R = Au­(PPh3), the oxidized compounds are much more accessible electrochemically than the rest of the series but are susceptible to radical attack. Of particular interest is the use of joint experimental and theoretical EPR studies, which allow elucidation of the differences of behavior within the two series. It reveals that the atomic spin density on the metal is not a sufficient criterion to evaluate EPR anisotropy but that the specific nodal properties of the frontier spin–orbitals highly influence the EPR components. The localization of the spin density on specific carbon atoms of the conjugated chain (even numbered) opens up the possibility of building extended systems by targeted radical reactions.本研究合成、表征并开展了理论分析，对象为两类铁、钌基金属-多炔基配合物，其通式为[{M(dppe)Cp*}–(C≡C)ₙ–R]^0/+（其中M=Fe、Ru；R=H、苯基、三甲基硅基（SiMe₃）、三苯基膦金（Au(PPh₃)）；n=1~3）。研究结果全面阐述了共轭碳链长度、金属原子种类与末端取代基对该类配合物中性态及氧化态的影响。针对氧化态而言，碳链上的自旋密度是其不稳定因素之一；例如当R=Au(PPh₃)时，该系列的氧化态配合物虽具备更优异的电化学可及性，但易受到自由基攻击。尤为值得关注的是，本研究结合实验与理论电子顺磁共振（EPR，Electron Paramagnetic Resonance）手段展开分析，阐明了两类配合物的行为差异。研究表明，金属原子的自旋密度并非评估EPR各向异性的充分标准，而前线自旋轨道的特定节点性质对EPR谱线组分具有显著影响。共轭碳链特定（偶数位）碳原子上的自旋密度分布，为通过靶向自由基反应构建延展型分子体系提供了可能。