Lanthanide nitrate complexes with triethylphosphine oxide. Solid state and solution properties

Complexes between lanthanide nitrates and triethylphosphine oxide have been prepared and fall into two distinct categories. With the lighter lanthanides Ln(NO3)3(Et3PO)3 (Ln = La, Ce, Pr, Nd, Eu) are formed whilst for the heavier metals mixtures of Ln(NO3)3(Et3PO)3 and Ln(NO3)3(Et3PO)2 are isolated. The structures of Ln(NO3)3(Et3PO)3 (Ln = La, Ce, Eu) have been determined by single crystal X-ray crystallography and show that all the complexes are 9-coordinate and have a pseudo mer-octahedral arrangement if the bidentate nitrates are envisaged as monodentate ligands. The conductivity in dichloromethane and acetonitrile indicates that the complexes are non-conducting. NMR spectra in CD2Cl2 solution are consistent with a rapid interconversions of the inequivalent phosphorus environments. For the Yb and Lu complexes static structures can be assigned to peaks in the low temperature 31P NMR spectra which also show evidence of a pseudo fac-isomer and a 1:2 complex. The formation of mixtures of 1:2 and 1:3 complexes for the heavier lanthanides is discussed in terms of the balance between steric effects and the basicity of the phosphine oxide ligands.

Graphical abstractThe complexes Ln(NO3)3(Et3PO)3 crystallise from reaction mixtures with lighter lanthanides. Mixtures of Ln(NO3)3(Et3PO)3 and Ln(NO3)3(Et3PO)2 form with the heavier lanthanides.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► In this paper we report the coordination chemistry of Et3PO with lanthanide nitrates. ► Complexes with composition Ln(NO3)3(Et3PO)3 form for Ln = La–Eu. ► Crystal structures show 9-coordinate metals with 3 bidentate nitrates and phosphine oxides. ► Mixtures of Ln(NO3)3(Et3PO)3 and Ln(NO3)3(Et3PO)2 are isolated for Ln = Tb–Lu. ► VT NMR shows the presence of pseudo mer- and fac-isomers and Ln(NO3)3(Et3PO)2.