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

The homoleptic 1,3-diphosphacyclobutadiene sandwich complex [Co(η4 -1,3-P2 C2 tBu2 )2 ]- behaved as a versatile and highly flexible metalloligand toward Ni2+ , Ru2+ , Rh+ , and Pd2+ cations, forming a range of unusual oligonuclear compounds. The reaction of [K(thf)2 {Co(η4 -1,3-P2 C2 tBu2 )2 }] with [Ni2 Cp3 ]BF4 initially afforded the σ-complex [CpNi{Co(η4 -1,3-P2 C2 tBu2 )2 }(thf)] (2), which converted into [Co(η4 -CpNi{1,3-P2 C2 tBu2 -κP,κC})(η4 -1,3-P2 C2 tBu2 )] (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(η4 -CpNi{1,4-P2 C2 tBu2 -κ2 P,P})(η4 -1,3-P2 C2 tBu2 )] (4), which features a 1,3-diphospha-2-nickelacyclopentadiene unit. Transmetalation of [K(thf)2 {Co(η4 -1,3-P2 C2 tBu2 )2 }] with [Cp*RuCl]4 (Cp*=C5 Me5 ) afforded tetranuclear [(Cp*Ru)3 (μ-Cl)2 {Co(η4 -1,3-P2 C2 tBu2 )2 }] (5), in which the [Co(η4 -1,3-P2 C2 tBu2 ]- anion acts as a chelate ligand toward Ru2+ . The diphosphido complex [(Cp*Ru)2 (μ,η2 -P2 )(μ,η2 -C2 tBu2 )] (6) was formed as a byproduct. Pure compound 6 was isolated after prolonged heating of the reaction mixture. The reaction of [K(thf)2 {Co(η4 -1,3-P2 C2 R2 )2 }] (R=tBu; adamantyl, Ad) with [RhCl(cod)]2 (cod=1,5-cyclooctadiene) afforded unprecedented π-complexes [Rh(cod){Co(η4 -1,3-P2 C2 R2 )2 }] (7: R=tBu; 8: R=Ad), in which one μ:η4 :η4 -P2 C2 R2 ligand bridges two metal atoms. The pentanuclear complex [Pd3 (PPh3 )2 {Co(η4 -1,3-P2 C2 tBu2 )2 }2 ] (10), featuring a Pd3 chain and a rare 1,4-diphospha-2-butene ligand, was synthesized by reacting [K(thf)2 {Co(η4 -1,3-P2 C2 tBu2 )2 }] with cis-PdCl2 (PPh3 )2 . 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(P2 C2 tBu2 )2 ]- anion.