Homoleptic Diphosphacyclobutadiene Complexes [M(n⁴-P₂C₂R₂)₂]ˣ⁻ (M=Fe, Co; X=0, 1)


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-C 14H10)2] (M=Fe, Co) with tBuCï£P (1, 2), or with AdCï£P (3). Neutral sandwiches [M(η 4-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-P2C 2tBu2)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 D2d-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. Fancy a sandwich? Homoleptic sandwich anions that contain diphosphacyclobutadiene ligands result from the reaction of anthracene metalates [Fe(η4-C 14H10)2]- and [Co(η 4-C14H10)2]- with phosphaalkynes. Electrochemical and preparative investigations of [M(η4-P2C2tBu2) 2]- (M=Fe, Co) revealed that these anions are readily oxidized to neutral derivatives [M(η4-P 2C2tBu2)2]. the electronic structures of the new sandwich complexes have been elucidated by using spectroscopic techniques and density functional theory calculations. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.



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