Linkage isomerism in rhodium(I) complexes of mixed anhydrides of diphenylphosphinous and acrylic acids: crystal and molecular structure of [RhCl(PPh3)(Ph2PO2CCHCHMe)]

Abstract

Reactions of [RhCl(PPh₃)₃] with mixed anhydrides Ph₂PO₂CCRCR′R″(R = R′= H, R″= Me or CHCHMe; R = H, R′= R″= Me; R = Me, R′= H, R″= Ph) give complexes [RhCl(PPh₃)_n(Ph₂PO₂CCRCR′R″)](n= 1, R = R′= H, R″= Me or CHCHMe; n= 2, R = H, R′= R″= Me; R = Me, R′= H, R″= Ph; R = R′= H, R″= CHCHMe). For n= 1, the mixed anhydride is bound to the metal via the phosphorus atom and the double bond, whilst for n= 2 binding is through phosphorus alone. Treatment of [RhCl(PPh₃)_n(Ph₂PO₂CCRCR′R″)] with TlPF₆ or AgSbF₆ leads in all cases to complexes [Rh(PPh₃)₂(Ph₂PO₂CCRCR′R″)]⁺ in which the mixed anhydride is bound through the phosphorus atom and the carbonyl oxygen atom. Spectroscopic diagnostics for the various different bonding modes are discussed, whilst their occurrence is rationalised largely on steric grounds. The X-ray crystal structure of [RhCl(PPh₃)(Ph₂PO₂CCHCHMe)] is described along with its curious fluxional behaviour. Crystals of this complex are monoclinic (space group P2₁/n), with a= 19.954(4), b= 13.250(4), c= 11.425(1)Å, β= 91.66(2)°, and Z= 4. The structure was obtained from 3 721 observed intensities measured on an automatic diffractometer and refined to an R value of 0.0358. The complex is distorted square planar about rhodium with the co-ordination sites being occupied by the two phosphorus atoms, the chloride, and the CC of the mixed anhydride. The two phosphorus atoms are mutually cis. The CC bond length is 1.393(9)Å, and there is significant pyramidalisation at the carbon atoms of the double bond, suggesting a fairly strong interaction between the double bond and the rhodium atom.