Multi-phosphine-chelated iron-carbide clusters via redox-promoted ligand exchange on an inert hexa-iron-carbide carbonyl cluster, [Fe6(μ6-C)(μ2-CO)4(CO)12]2−

Abstract

We report the reactivity, structures and spectroscopic characterization of reactions of phosphine-based ligands (mono-, di- and tri-dentate) with iron-carbide carbonyl clusters. Historically, the archetype of this cluster class, namely [Fe₆(μ₆-C)(μ₂-CO)₄(CO)₁₂]²⁻, can be prepared on a gram-scale but is resistant to simple ligand substitution reactions. This limitation has precluded the relevance of iron-carbide clusters relating to organometallics, catalysis and the nitrogenase active site cluster. Herein, we aimed to derive a simple and reliable method to accomplish CO → L (where L = phosphine or other general ligands) substitution reactions without harsh reagents or multi-step synthetic strategies. Ultimately, our goal was ligand-based chelation of an Fe_n(μ_n-C) core to achieve more synthetic control over multi-iron-carbide motifs relevant to the nitrogenase active site. We report that the key intermediate is the PSEPT-non-conforming cluster [Fe₆(μ₆-C)(CO)₁₆] (2: 84 electrons), which can be generated in situ by the outer-sphere oxidation of [Fe₆(μ₆-C)(CO)₁₆]²⁻ (1: closo, 86 electrons) with 2 equiv. of [Fc]PF₆. The reaction of 2 with excess PPh₃ generates a singly substituted neutral cluster [Fe₅(μ₅-C)(CO)₁₄PPh₃] (4), similar to the reported reactivity of the substitutionally active cluster [Fe₅(μ₅-C)(CO)₁₅] with monodentate phosphines (Cooke & Mays, 1990). In contrast, the reaction of 2 with flexible, bidentate phosphines (DPPE and DPPP) generates a wide range of unisolable products. However, the rigid bidentate phosphine bis(diphenylphosphino)benzene (bdpb) disproportionates the cluster into non-ligated Fe₃-carbide anions paired with a bdpb-supported Fe(II) cation, which co-crystallize in [Fe₃(μ₃-CH)(μ₃-CO)(CO)₉]₂[Fe(MeCN)₂(bdpb)₂] (6). A successful reaction of 2 with the tripodal ligand Triphos generates the first multi-iron-chelated, authentic carbide cluster of the formula [Fe₄(μ₄-C)(κ3-Triphos)(CO)₁₀] (9). DFT analysis of the key (oxidized) intermediate 2 suggests that its (μ₆-C)Fe₆ framework remains fully intact but is distorted into an axially compressed, ‘ruffled’ octahedron distinct from the parent closo cluster 1. Oxidation of the cluster in non-coordinating solvent allows for the isolation and crystallization of the CO-saturated, intact closo-analogue [Fe₆(μ₆-C)(CO)₁₇] (3), indicating that the intact (μ₆-C)Fe₆ motif is retained during initial oxidation with [Fc]PF₆. Overall, we demonstrate that redox modulation beneficially ‘bends’ Wade-Mingo's rules via the generation of electron-starved (non-PSEPT) intermediates, which are the key intermediates in promoting facile CO → L substitution reactions in iron-carbide-carbonyl clusters.