Synthesis and structure of tridentate bis(phosphinic amide)-phosphine oxide complexes of yttrium nitrate. Applications of 31P,89Y NMR methods in structural elucidation in solution

Abstract

The synthesis and characterisation of a tridentate ligand containing two diphenylphosphinic amide side-arms connected through the ortho position to a phenylphosphine oxide moiety and the 1 : 1 and 2 : 1 complexes formed with yttrium nitrate are reported for the first time. The free ligand (R_P1*,S_P3*)-11 is obtained diastereoselectively by reaction of ortho-lithiated N,N-diisopropyl-P,P-diphenylphosphinic amide with phenylphosphonic dichloride. Complexes [Y((R_P1*,S_P3*)-11)(NO₃)₃] and [Y((R_P1*,S_P3*)-11)₂(NO₃)](NO₃)₂ were isolated by mixing ligand 11 with Y(NO₃)₃·6H₂O in acetonitrile at room temperature in a ligand to metal molar ratio of 1 : 1 and 2 : 1, respectively. The 1 : 1 derivative is the product of thermodynamic control when a molar ratio of ligand to yttrium salt of 1 : 1 is used. The new compounds have been characterised both as the solid (X-ray diffraction) and in solution (multinuclear magnetic resonance). In both yttrium complexes the ligand acts as a tridentate chelate. The arrangement of the two ligands in the 2 : 1 complex affords a pseudo-meso structure. Tridentate chelation of yttrium(III) in both complexes is retained in solution as evidenced by ⁸⁹Y NMR data obtained via³¹P,⁸⁹Y-HMQC, and ⁸⁹Y,³¹P-DEPT experiments. The investigation of the solution behaviour of the Y(III) complexes through PGSE NMR diffusion measurements showed that average structures in agreement with the 1 : 1 and 1 : 2 stoichiometries are retained in acetonitrile.