Homoleptic Diphosphacyclobutadiene Complexes [M(η4-P₂C₂R₂)₂]x− (M=Fe, Co; x=0, 1)

摘要

The preparation and comprehensive characterization of a series of homoleptic sandwich complexes containing diphosphacyclobutadiene ligands are reported. Compounds [K([18]crown-6)(thf)₂][Fe(η4-P₂C₂tBu₂)₂] (K1), [K([18]crown-6)(thf)₂][Co(η4-P₂C₂tBu₂)₂] (K2), and [K([18]crown-6)(thf)₂][Co(η4-P₂C₂Ad₂)₂] (K3, Ad=adamantyl) were obtained from reactions of [K([18]crown-6)(thf)₂][M(η4-C₁₄H₁₀)₂] (M=Fe, Co) with tBuCP (1, 2), or with AdCP (3). Neutral sandwiches [M(η4-P₂C₂tBu₂)₂] (4: M=Fe 5: M=Co) were obtained by oxidizing 1 and 2 with [Cp₂Fe]PF₆. Cyclic voltammetry and spectro-electrochemistry indicate that the two [M(η4-P₂C₂tBu₂)₂]−/[M(η4-P₂C₂tBu₂)₂] moieties can be reversibly interconverted by one electron oxidation and reduction, respectively. Complexes 1–5 were characterized by multinuclear NMR, EPR (1 and 5), UV/Vis, and Mössbauer spectroscopies (1 and 4), mass spectrometry (4 and 5), and microanalysis (1–3). The molecular structures of 1–5 were determined by using X-ray crystallography. Essentially D_2d-symmetric structures were found for all five complexes, which show the two 1,3-diphosphacyclobutadiene rings in a staggered orientation. Density functional theory calculations revealed the importance of covalent metal–ligand π bonding in 1–5. Possible oxidation state assignments for the metal ions are discussed.