Synthesis and properties of rhodium(I) chloranilate and 2,5-dihydroxy-1,4-benzoquinonate complexes. Crystal structures of the binuclear [Rh2(u-CA)(cod)2] and tetranuclear [Rh4(u-CA)2(cod)4] complexes (CA =Chloranilate Anion)

Resumen

If one treats [Rh2(μ-OMe)2(diolefin)2] {diolefin = cycloocta-1,5-diene (cod), norborna-1,5-diene (nbd) or tetrafluoro-[5,6]bicyclo[2.2.2]octa-2,5,7-triene (tfb)} with chloranilic acid (H2CA) or 2,5-dihydroxy-1,4-benzoquinone (H2-DHBQ) in a 1:1 ratio, the binuclear complexes [Rh2(μ-L)(diolefin)2] (L = CA2-, DHBQ2-) can be obtained. The complex [Rh(acac)(cod)J (acac = acetylacetonate) reacts with H2CA to give the tetranuclear complex [Rh4(μ-CA)2(cod)4]. The complex [Rh2(μ-DHBQ)(cod)2] undergoes two sequential reversible one-electron reductions at a platinum electrode in dichloromethane. The crystal structures of the complexes Rh4(μ-CA)2(cod)4] (1) and [Rh2(μ-CA)(cod)2] (2) have been determined by X-ray diffraction methods. Crystal data for 1: space group P21/n, a = 15.284 (6) Å, b = 8.976 (4) Å, c = 16.553 (7) Å, β = 112.40 (2)°, Z = 2, 2086 reflections, R = 0.0253. Crystal data for 2: space group C2/c, a = 19.310 (7) Å, b = 6.608 (3) Å, c = 19.283 (8) Å, β = 117.06 (2)°, Z = 4, 1526 reflections, R = 0.0624. In the centrosymmetric tetranuclear complex 1 each CA2- dianion, in the o-quinone form, displays a novel coordination mode interacting with three rhodium atoms via three σ-bonds through oxygens and two π-bonds through two adjacent double bonds. The coordination around each Rh atom is completed by a cod molecule interacting in the usual manner through the two double bonds. If the midpoints of the double bonds interacting with the metals of the CA2- and cod ligands are taken into account, two Rh atoms are in a square planar and two are in a square pyramidal arrangement. The CA2- ligand, involved in a very complicate bonding with three metals, has lost the planarity, the six-membered ring showing a "boat" conformation with the two planar moieties tilted by 31.6 (3)°. In the centrosymmetric binuclear complex 2, the strictly planar CA2- ligand, in the more common p-quinone form, chelates to two Rh centers through the oxygen atoms as a tetradentate ligand. The coordination around each Rh atom is completed by a cod ligand. © 1993 American Chemical Society.