Solid State and Solution Structures of Lanthanide Nitrate Complexes of Tris-(1-napthylphosphine oxide)

Coordination complexes of lanthanide metals with tris-1-naphthylphosphine oxide (Nap₃PO, L) have not been previously reported in the literature. We describe here the formation of lanthanide(III) nitrate complexes Ln(NO₃)₃L₄ (Ln = Eu to Lu) and the structures of [Ln(NO₃)₃L₂]·2L (Ln = Eu, Dy, Ho, Er) and L. The core structure of the complexes is an eight-coordinate [Ln(NO₃)₃L₂] with the third and fourth ligands H-bonded via their oxygen atoms to one of the naphthyl rings. The structures are compared with those of the analogous complexes of triphenylphosphine oxide and show that the Ln-O(P) bond in the Nap₃PO complexes is slightly longer than expected on the basis of differences in coordination numbers. The reaction solutions, investigated by ³¹P and ¹³C NMR spectroscopy in CD₃CN, show that coordination of L occurs across the lanthanide series, even though complexes can only be isolated from Eu onwards. Analysis of the ³¹P NMR paramagnetic shifts shows that there is a break in the solution structures with a difference between the lighter lanthanides (La-Eu) and heavier metals (Tb-Lu) which implies a minor difference in structures. The isolated complexes are very poorly soluble, but in CDCl₃, NMR measurements show dissociation into [Ln(NO₃)₃L₂] and 2L occurs.