Preparation, Characterization and Electronic Structure of Homoleptic diphosphacyclobutadiene complexes [M(ny4-P2C2R2)2]x- (M=Fe, Co; X=0, 1)

Wolf R, Ehlers AW, Khusniyarov MM, Hartl F, Debruin B, Long GJ, Grandjean F, Schappacher FM, Pöttgen R, Slootweg JC, Lutz M, Spek AL, Lammertsma K

Abstract

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 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.