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.

双(膦)基金属桥联[1]二茂铁藩（bis(phosphine)metalla[1]ferrocenophanes）类化合物[{Fe(η⁵-C₅H₄)₂}M(Pⁿ-Bu₃)₂] [M = Ni (4a)、Pd (4b) 及 Pt (4c)]，可分别通过Li₂[Fe(η⁵-C₅H₄)₂]·tmeda（5，tmeda = N,N,N′,N′-四甲基乙二胺）与反式-[MCl₂(Pⁿ-Bu₃)₂] [M = Ni (trans-6a) 与 Pd (trans-6b)]以及顺式-[PtCl₂(Pⁿ-Bu₃)₂] (cis-6c) 反应制备得到。单晶X射线衍射（Single crystal X-ray diffraction）表征显示，这类化合物具有高度倾斜的张力结构，其α夹角分别为28.4°（4a）、24.5°（4b）与25.2°（4c），且第10族金属中心处于畸变的平面四配位环境中。紫外-可见光谱（UV/visible spectroscopy）与循环伏安法（cyclic voltammetry）分析表明，相较于二茂铁及其他同类[1]二茂铁藩类化合物，这三种化合物的HOMO-LUMO能隙更小，且本质上更富电子。密度泛函理论计算（DFT, Density Functional Theory calculations）结果显示，上述差异主要源于M(Pⁿ-Bu₃)₂金属-配体片段的给电子特性。尝试通过热引发或阴离子引发方式实现4a~4c的开环聚合（ring-opening polymerization）均未成功，且伴随配体解离（ligand dissociation）竞争反应、链增长（chain propagation）不利等问题。与之相反，这类化合物均可与酸快速反应，实现双(膦)基金属片段的洁净脱除。一氧化碳（carbon monoxide）可对4b的一条钯-碳键进行精准插入，得到环扩张的酰基化产物[{Fe(η⁵-C₅H₄)(η⁵-C₅H₄)(CO)}Pd(Pⁿ-Bu₃)₂]（10）。而镍类似物4a则生成[Ni(CO)₂(Pⁿ-Bu₃)₂]，铂桥联配合物4c则无反应活性。值得注意的是，所有4a~4c均可被单质硫快速氧化，通过碳环配体形式上的四电子氧化过程，得到[5,5′]二环戊二烯亚基（富瓦烯，pentafulvalene）配合物[{η⁴:η⁰-C₅H₄(C₅H₄)}M(Pⁿ-Bu₃)₂] [M = Ni (11a)] 以及[(η²-C₁₀H₈)M(Pⁿ-Bu₃)₂] [M = Pd (11b) 与 Pt (11c)]。其中配合物11b和11c是首例以中性η²配体形式存在的[5,5′]二环戊二烯亚基化合物。通过密度泛函理论计算，我们探究了11a~c中η²配位与η⁴:η⁰成键模式的相对能量差异。