The synthesis, X-ray crystal structure, and solution characterization of a bis(diphosphine)-bridged dirhodium(I) complex containing a Rh2P4 six-membered ring

Abstract

The preparation of [{Rh(cod)}₂(µ-L¹)₂][BF₄]₂(1) from ([graphics omitted])₂(L¹) and [Rh(cod)₂]-BF₄(cod = cyclo-octa-1,5-diene) is described. The compound crystallizes in the monoclinic space group P2₁/c(C⁵_2h), with a= 9.650(1), b= 11.609(1), c= 21.884(3)Å, β= 117.07(1)°, and Z= 2. The structure was refined to R= 0.033 for 3 434 reflections. The solid-state structure shows two diphosphine ligands L¹ bridging two rhodium(I) atoms to form a six-membered Rh₂P₄ ring in a chair conformation. Solution studies (¹H and ¹³C-{¹H} variable-temperature n.m.r. spectra) exhibit characteristics of a ring inversion equilibrium with ΔG₂₇₁= 14.0(0.2) kcal mol^–1. Computer simulation of the ³¹P-{¹H} n.m.r. spectrum is used to analyse the internuclear couplings of the AA′A″A‴XX′ pattern (A =³¹P, X =¹⁰³Rh). The bridging diphosphine ligands exhibit a deshielding effect on the trans olefinic carbon atoms of cod in (1) which is in the range normally found for phosphine ligands.