Pentamethylcyclopentadienylrhodium complexes with tripod tetradentate ligands and bidentate ‘mixed’ ligands

Abstract

The synthesis and properties of the complexes [{Rh(C₅Me₅)}₃Cl₅L]Y [Y = Cl, BF₄, or PF₆], obtained from [Rh₂(C₅Me₅)₂Cl₄] and the appropriate tripodal tetradentate ligands, L = N(CH₂CH₂PPh₂)₃ or P(CH₂CH₂PPh₂)₃, are reported. These monoligand trimetallic compounds contain two Rh(C₅Me₅)Cl₂ units each bound to a terminal phosphorus atom of the tripod ligand and a Rh(C₅Me₅)Cl group linked to the third terminal phosphorus and to the apical donor atom, nitrogen for N(CH₂CH₂PPh₂)₃, phosphorus for P(CH₂CH₂PPh₂)₃. The Rh(C₅Me₅)Cl group in the N(CH₂H₂PPh₂)₃ derivative undergoes substitution when treated with tetrabutylammonium chloride to give [{Rh(C₅Me₅)Cl₂}₃{N(CH₂CH₂PPh₂)₃}]. The influence of steric factors in the substitution reaction has been investigated by synthesising and characterising the complexes [Rh(C₅Me₅)Cl{R′₂N(CH₂)_nPR₂}]BPh₄(n= 2, R = Ph or C₆H₁₁, R′= H, Me, or Et; n= 3, R = Ph, R′= H, Me, or Et; R = C₆H₁₁, R′= H or Me). The bidentate mixed ligands R′₂N(CH₂)_nPPh₂(n= 2, R′= Et; n= 3, R′= Me or Et) give complexes of formula [Rh(C₅Me₅)Cl{R′₂N(CH₂)_nPPh₂}₂]BPh₄, in which the ligands are monodentate through the phosphorus atom. The crystal structure of [Rh(C₅Me₅)Cl{Et₂N(CH₂)₂PPh₂}₂]BPh₄ has been determined by X-ray crystallographic analysis. The complexes [Rh(C₅Me₅)Cl{R′₂N(CH₂)_nPPh₂}₂]BPh₄(n= 2, R′= Et; n= 3, R′= Me) exist as four rotamers due to restricted rotations of the bulky phosphine substituents about the Rh–P bonds, as shown by low-temperature ³¹P n.m.r. spectroscopy.