Synthesis, Electronic Structure, and Reactivity of Strained Nickel-, Palladium-, and Platinum-Bridged [1]Ferrocenophanes

The group 10 bis(phosphine)metalla[1]ferrocenophanes, [{Fe(η5-C5H4)2}M(Pn-Bu3)2] [M = Ni (4a), Pd (4b), and Pt (4c)], have been prepared by the reaction of Li2[Fe(η5-C5H4)2]·tmeda (5, tmeda = N,N,N′,N′-tetramethylethylenediamine) with trans-[MCl2(Pn-Bu3)2] [M = Ni (trans-6a) and Pd (trans-6b)] and cis-[PtCl2(Pn-Bu3)2] (cis-6c), respectively. Single crystal X-ray diffraction revealed highly tilted, strained structures as characterized by α angles of 28.4° (4a), 24.5° (4b), and 25.2° (4c) and a distorted square planar environment for the group 10 metal center. UV/visible spectroscopy and cyclic voltammetry indicated that all three compounds had smaller HOMO−LUMO gaps and were more electron-rich in nature than ferrocene and other comparable [1]ferrocenophanes. DFT calculations suggested that these differences were principally due to the electron-releasing nature of the M(Pn-Bu3)2 metal−ligand fragments. Attempts to induce thermal or anionic ring-opening polymerization of 4a−c were unsuccessful and were complicated by, for example, competing ligand dissociation processes or unfavorable chain propagation. In contrast, these species all reacted rapidly with acids effecting clean extrusion of the bis(phosphine)metal fragment. Carbon monoxide inserted cleanly into one of the palladium−carbon bonds of 4b to afford the ring-expanded, acylated product [{Fe(η5-C5H4)(η5-C5H4)(CO)}Pd(Pn-Bu3)2] (10). The nickel analogue 4a, however, afforded [Ni(CO)2(Pn-Bu3)2] whereas the platinum-bridged complex 4c was inert. Remarkably, all compounds 4a−c were readily oxidized by elemental sulfur to afford the [5,5′]bicyclopentadienylidene (pentafulvalene) complexes [{η4:η0-C5H4(C5H4)}M(Pn-Bu3)2] [M = Ni (11a)] and [(η2-C10H8)M(Pn-Bu3)2] [M = Pd (11b) and Pt (11c)] by a formal 4-electron oxidation of the carbocyclic ligands. Compounds 11b and 11c represent the first examples of [5,5′]bicyclopentadienylidene as a neutral η2-ligand. The relative energies of η2-coordination with respect to that of η4:η0 bonding were investigated for 11a−c by DFT calculations.

第10族双(膦)金属[1]铁卟啉芳烃，如[Fe(η5-C5H4)2]M(Pn-Bu3)2 [M = Ni (4a), Pd (4b), 和 Pt (4c)]，是通过Li2[Fe(η5-C5H4)2]·tmeda (5，tmeda = N,N,N′,N′-四甲基乙二胺)与trans-[MCl2(Pn-Bu3)2] [M = Ni (trans-6a) 和 Pd (trans-6b)]以及cis-[PtCl2(Pn-Bu3)2] (cis-6c)分别反应制备的。单晶X射线衍射揭示了高度倾斜、受压的结构，特征为α角分别为28.4° (4a)、24.5° (4b)和25.2° (4c)，以及第10族金属中心的扭曲正方形平面环境。紫外/可见光谱和循环伏安法表明，这三种化合物均具有较小的HOMO−LUMO能隙，其电子丰富程度高于铁卟啉和其他可比的[1]铁卟啉芳烃。DFT计算表明，这些差异主要归因于M(Pn-Bu3)2金属−配体片段的电子释放性质。尝试诱导4a−c的热或阴离子开环聚合均未成功，且受到例如竞争性配体解离过程或不利链增长等复杂因素的影响。相反，这些物种均能与酸迅速反应，实现双(膦)金属片段的干净挤出。一氧化碳干净地插入4b中的钯−碳键之一，得到环扩大的酰化产物[{Fe(η5-C5H4)(η5-C5H4)(CO)}Pd(Pn-Bu3)2] (10)。然而，镍类似物4a得到[Ni(CO)2(Pn-Bu3)2]，而铂桥联复合物4c则表现出惰性。值得注意的是，所有化合物4a−c均易于被元素硫氧化，通过羰基环状配体的形式4电子氧化，得到[5,5′]双环戊二烯基iden (五富瓦烯)复合物[{η4:η0-C5H4(C5H4)}M(Pn-Bu3)2] [M = Ni (11a)]和[(η2-C10H8)M(Pn-Bu3)2] [M = Pd (11b) 和 Pt (11c)]。化合物11b和11c代表了[5,5′]双环戊二烯基iden作为中性η2配体的第一个例子。通过DFT计算研究了11a−c中η2配位相对于η4:η0键合的相对能量。