Synthesis, Structures, and Reactions of Dirhodium Complexes Bearing a 1,2-Diphenyl-3,4-diphosphinidenecyclobutene Ligand (DPCB)

A di-mu-hydroxo-dirhodium complex bearing a low-coordinate phosphorus ligand, [Rh2(mu-OH)(2)(DPCB)(2)] (2; DPCB = 1,2-diphenyl-3,4-bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobutene), is prepared from [Rh-2(mu-Cl)(2)(DPCB)(2)] (1) and KOH. Complex 2 serves as a good precursor for hydrido, complexes, affording [Rh-2(mu-H)(mu-OH)(DPCB)(2)] (3) and [Rh-2(mu-H)(2)(DPCB)(2)] (7) by the reactions with HSiMe2Ph and HCO2H, respectively. Although complex 7 is too unstable to be isolated, its formation is evidenced by a trapping experiment using 1,3-cyclohexadiene to give [Rh(eta(3)-C6H9)(DPCB)] (8). Complex 2 reacts with CO to afford [Rh-2(mu-kP,P;kP,C-DPCB)(CO)(4)] (9) and [Rh-2(mu-CO)(2)(DPCB)(2)] (10), which are interconvertible in solution. Complex 9 is the stable form under a CO atmosphere even in the presence of free DPCB, but readily converted to 10 by purging the CO gas from the solution. The X-ray structures of 2, 3, 9, 10, and related [Rh-2(mu-H)(mu-OMe)(DPCB)(2)] (4) are reported. Two Rh(DPCB) moieties in 10 are oriented orthogonal to each other in the crystal; namely, one of the Rh centers has a square-planar configuration, whereas the other is in a tetrahedral arrangement with the mu-CO and DPCB ligands, showing flexible electronic properties of DPCB.