Inner and Outer Sphere Coordination of Tricyclohexylphosphine oxide with Lanthanide Bromides

Complexes of lanthanide bromides with tricyclohexylphosphine oxide (Cy3PO) form three distinct structural classes. Type I complexes LnBr3(Cy3PO)3 have been structurally characterised for Ln = La, Pr, Nd, Gd and Ho and are molecular 6 coordinate with a distorted meridional octahedral arrangement. Type II complexes are based on pentagonal bipyramidal [Ln(Cy3PO)n(H2O)5]3+ structures and fall into three subsets; n = 2 and 3 have been isolated for Ln = Lu and n = 4 with Ln = La, Dy, Er and Yb. The structures of [Lu(Cy3PO)2(H2O)5]Br3•2EtOH and [Ln(Cy3PO)2(H2O)5]Br3 (Ln = Dy, Er, Yb) have been determined. When n = 2 both ligands are directly coordinated to the metal whilst complexes in which n = 4 have two ligands bonded to the metal and two hydrogen bonded to the coordinated water molecules. Analysis of the bond distances shows that the lanthanide contraction accounts for about 99% of the observed trends. The infrared spectra of the complexes give a good means of identifying the structural types produced. 31P NMR show that the Type I complexes are fluxional in solution and that the Type II complexes undergo ligand exchange between metal and H-bonded ligands. Analysis of the lanthanide induced shifts indicates that the Type II complexes are isostructural in solution.