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.