Flexidentate Coordination Behavior and Chemical Non-Innocence of a Bis(1,3-Diphosphacyclobutadiene) Sandwich Anion.

The homoleptic 1,3-diphosphacyclobutadiene sandwich complex [Co(η -1,3-P C tBu ) ] behaved as a versatile and highly flexible metalloligand toward Ni , Ru , Rh , and Pd cations, forming a range of unusual oligonuclear compounds. The reaction of [K(thf) {Co(η -1,3-P C tBu ) }] with [Ni Cp ]BF initially afforded the σ-complex [CpNi{Co(η -1,3-P C tBu ) }(thf)] (2), which converted into [Co(η -CpNi{1,3-P C tBu -κP,κC})(η -1,3-P C tBu )] (3) below room temperature. The structure of 3 contains an unprecedented 1,4-diphospha-2-nickelacyclopentadiene moiety formed by an oxidative addition of a ligand P-C bond onto nickel. At elevated temperatures, 3 isomerized to [Co(η -CpNi{1,4-P C tBu -κ P,P})(η -1,3-P C tBu )] (4), which features a 1,3-diphospha-2-nickelacyclopentadiene unit. Transmetalation of [K(thf) {Co(η -1,3-P C tBu ) }] with [CpRuCl] (Cp=C Me ) afforded tetranuclear [(CpRu) (μ-Cl) {Co(η -1,3-P C tBu ) }] (5), in which the [Co(η -1,3-P C tBu ] anion acts as a chelate ligand toward Ru . The diphosphido complex [(CpRu) (μ,η -P )(μ,η -C tBu )] (6) was formed as a byproduct. Pure compound 6 was isolated after prolonged heating of the reaction mixture. The reaction of [K(thf) {Co(η -1,3-P C R ) }] (R=tBu; adamantyl, Ad) with [RhCl(cod)] (cod=1,5-cyclooctadiene) afforded unprecedented π-complexes [Rh(cod){Co(η -1,3-P C R ) }] (7: R=tBu; 8: R=Ad), in which one μ:η :η -P C R ligand bridges two metal atoms. The pentanuclear complex [Pd (PPh ) {Co(η -1,3-P C tBu ) } ] (10), featuring a Pd chain and a rare 1,4-diphospha-2-butene ligand, was synthesized by reacting [K(thf) {Co(η -1,3-P C tBu ) }] with cis-PdCl (PPh ) . The redox properties of selected compounds were analyzed by cyclic voltammetry, whereas DFT calculations gave additional insight into the electronic structures. The results of this study revealed several remarkable and previously unrecognized properties of the [Co(P C tBu ) ] anion.