Homoleptic diphosphacyclobutadiene complexes [M(η(4)-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(η(4)-P(2)C(2)tBu(2))(2)] (K1), [K([18]crown-6)(thf)(2)][Co(η(4)-P(2)C(2)tBu(2))(2)] (K2), and [K([18]crown-6)(thf)(2)][Co(η(4)-P(2)C(2)Ad(2))(2)] (K3, Ad = adamantyl) were obtained from reactions of [K([18]crown-6)(thf)(2)][M(η(4)-C(14)H(10))(2)] (M = Fe, Co) with tBuC[triple bond]P (1, 2), or with AdC[triple bond]P (3). Neutral sandwiches [M(η(4)-P(2)C(2)tBu(2))(2)] (4: M = Fe 5: M = Co) were obtained by oxidizing 1 and 2 with [Cp(2)Fe]PF(6). Cyclic voltammetry and spectro-electrochemistry indicate that the two [M(η(4)-P(2)C(2)tBu(2))(2)](-)/[M(η(4)-P(2)C(2)tBu(2))(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.