A convenient general route to a series of diphosphine-bridged heterobinuclear complexes that contain rhodium and structures of the mixed-valent complexes [RhM(CO)4(Ph2PCH2PPh2)2] (M = Mn, Re)

Article abstract of DOI:10.1021/om00156a022

A convenient route to a series of dppm-bridged complexes has been developed in which the chloride anion in [RhCl(dppm)₂] is displaced by a metal carbonylate anion. This metathesis reaction is accompanied by unwinding of one or both of the chelating dppm groups to adopt a bridging arrangement and by loss of one carbonyl group. In the compounds [RhCo(CO)₃(dppm)₂], [RhMH(CO)₃(dppm)₂] (M = Fe, Ru, Os), [RhM(CO)₄(dppm)₂] (M = Mn, Re), and [RhMH(CO)₄(dppm)₂] (M = Cr, W), both diphosphines adopt a bridging arrangement, whereas in [RhFeCp(CO)(dppm)₂] and [RhMoCp(CO)₂(dppm)₂] only one dppm group bridges the metals while one remains chelating on rhodium. The structures of [RhM(CO)₄(dppm)₂] (M = Mn (5), Re (6)) have been determined by X-ray methods. The coordination geometries at Mn and Re are not octahedral, but instead (ignoring the Rh atoms), the [M(CO)₃L₂] (L = one end of dppm unit) moieties have trigonal-bipyramidal geometries, suggesting that a M(-I) formulation is appropriate. This [M(CO)₃L₂]^- fragment then functions as a pseudohalide, forming a dative M→Rh bond and giving the Rh center a square-planar arrangement consistent with a Rh(+I) formulation. The resulting M-Rh bond lengths are 2.8428 (8) ? (M = Mn) and 2.7919 (6) ? (M = Re). Compound 5 crystallizes in the triclinic space group P1 with a = 10.897 (2) ?, b = 11.046 (2) ?, c = 11.700 (2) ?, α = 68.14 (1)°, β = 67.48 (1)°, γ = 87.69 (1)°, V = 1198.7 (4) ?³, and Z = 1 and was refined to R = 0.054 and R_w = 0.082. Compound 6 crystallizes together with one THF molecule in the monoclinic space group P2₁/c with a = 12.963 (4) ?, b = 19.883 (3) ?, c = 11.277 (2) ?, β = 99.12 (2)°, V = 2870 (2) ?³, and Z = 2 and was refined to R = 0.053 and R_w = 0.088. Both compounds occupy a crystallographic inversion center so the metal carbonyl fragments are disordered, although this disorder, in reality, is readily handled crystallographically, with only two carbonyl carbons having two sets of half-occupancy positions and each metal position being composed of one half-occupancy rhodium atom and one half-occupancy manganese or rhenium atom.