Synthesis and crystal structure of a rhodium(III) complex bearing a hypervalent phosphorus(V) ligand as a bidentate ligand and equilibrium between bidentate and monodentate structures in solution

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A new Rh(III) complex bearing a hypervalent phosphorus(V) ligand, Rf 2PRh(Cp*)Cl ( 4) (Rf=Martin ligand; o-C 6H 4C(CF 3) 2O), was obtained by the reaction of Rf 2P −Li + generated in situ with [Cp*RhCl 2] 2. X-ray structural analysis of 4 showed that the hypervalent phosphorus ligand Rf 2P acted as a bidentate ligand coordinating to the Rh center through the phosphorus and one oxygen atom of a Martin ligand. The Rh center became coordinatively saturated and was stabilized by the additional coordination of the oxygen atom. However, in solution the equilibrium between the bidentate ( 4A) and the monodentate ( 4B) species was observed ( 4A: 4B=6:1 in C 6H 6, 4:1 in CHCl 3, 2:1 in THF, 1.6:1 in CH 2Cl 2), and the monodentate species 4B became predominant in polar solvents ( 4A: 4B=1:2 in CH 3CN, 1:2.5 in CH 3NO 2, 1:3 in DMSO). Therefore, the strong Lewis acidic Rh center can be regarded as masked in the solid state but in solution it becomes unmasked to 4B, in which a solvent is coordinating to the Rh center.