Di- and tetra-nuclear mixed-valence complexes of rhodium(III,I) and rhodium(III)–iridium(I) with the 2,2′-bi-imidazolate(2–) anion (bim) as bridging ligand. Crystal structure of [{(η-C5Me5)[P(OEt)3]Rh(bim)Rh(nbd)}2][ClO4]2

Abstract

The reaction of the new mononuclear species [Rh^III(η-C₅Me₅)Cl(Hbim)](H₂bim = 2,2′-bi-imidazole) and [Rh^III(η-C₅Me₅)(Hbim)L′][ClO₄][L′= pyridine (py), NCBu^t, PPh₃, or P(OEt)₃] with the corresponding acetylacetonate (acac) complexes of formulae [M^I(acac) L₂][M = Rh; L = CO, L₂= tetrafluorobenzobarrelene (tfb)(5,6,7,8- tetrafluoro-1,4-dihydro-1,4-ethenonaphthalene) or norborna-2,5-diene (nbd). M = Ir, L₂= tfb] yields di- and tetra-nuclear mixed-valence compounds of the types [{(η-C₅Me₅)ClRh^III(bim)Rh^IL₂}₂][L = CO or L₂= tfb], [{(η-C₅Me₅) L′Rh^III(bim) Rh^I L₂}_n][ClO₄]_n[L′= py; L₂= tfb (n= 1). L′= py; L = CO or L₂= nbd (n= 2). L′= NCBu^t, PPh₃, or P(OEt)₃; L = CO, L₂= tfb or nbd (n= 2)] and [{(η-C₅Me₅)L′Rh^III(bim)Ir^I(tfb)}_n][ClO₄]_n[L′= py (n= 1), L′= PPh₃ or P(OEt)₃(n= 2)]. The structure of [{(η-C₅Me₅)[P(OEt)₃] Rh^III(bim) Rh^I(nbd)}₂][ClO₄]₂ has been determined by X- ray diffraction methods. Crystals are orthorhombic, space group F_ddd with Z= 16 in a unit cell of dimensions a= 21.337(3), b= 41.943(5), and c= 31.815(7)Å. The structure was solved by direct and Fourier methods and refined by full- matrix least squares to R= 0.075 for 2 984 observed reflections. The complex cation [{(η-C₅Me₅)[P(OEt)₃] Rh^III(bim) Rh^I(nbd)}₂]²⁺, having imposed C₂symmetry, is tetranuclear with two bim^2– ligands bridging the metal atoms. Each bim^2– anion co- ordinates to the Rh metals in an unsymmetrical tetradentate manner through its four nitrogen atoms of the two imidazolate rings, being chelated to one formally six-co-ordinated Rh atom [Rh(1)] through two N atoms and being bonded in an unidentate manner to two four-co-ordinated Rh atoms [Rh(2)], through the other two N atoms. The co-ordination around Rh(1) is completed by an η-C₅Me₅ group and by a P(OEt)₃ligand, that around Rh(2) by a nbd molecule interacting through the two double bonds. A short non-bonding Rh(2)–Rh(2′) distance [3.250(5)Å] is present in the cation.