Structure, bonding, electronic and energy aspects of a new family of early lanthanide (La, Ce and Nd) complexes with phosphoric triamides: insights from experimental and DFT studies

A new family of isostructural early lanthanide(III) complexes (LnXPA) of the general formula Ln(XPA)(2)Cl(3)(solv)(2), where Ln = La, Ce and Nd, XPA = (4-X-C(6)H(4)NH)P(O)(NC(4)H(8)O)(2), X = H, F, Cl and Br, and solv = H(2)O and CH(3)OH, is introduced. X-ray crystallography shows that the replacement of the coordinated water by a methanol molecule may reduce the symmetry level of the unit cell from the orthorhombic crystal system and the space group Fdd2 to monoclinic and C2/c. DFT calculations, at B3LYP, PBE and B3PW91 levels, have been carried out to get a better insight into the structural, electronic and energy aspects of the compounds. The large cation attraction energy (-ΔE) values in the range 269-273 kcal mol(-1), at the B3PW91/ECP/6-311+G** level for the model complexes XPA-La(3+) with stoichiometry 1 : 1, represent new ligands XPA as efficient complexant agents for lanthanides. The electronic nature of para substituent X has no significant effect on the Ln-ligand bonding and cation affinity of the ligands XPA. The results of atoms in molecules (AIM) analysis reveal a partial covalent contribution of the Ln-ligand interaction for the models XPA-La(3+) in the absence of counterions and coordinated solvents. In the real complexes LnXPA, a closed-shell Ln-ligand interaction is established. Increasing the charge difference between nitrogen and phosphorus atoms (by ~0.06 e) associated with a weakening of the Lp(O(P))→σ*(P-N) electronic delocalization (Lp(O(P)) being the lone pair of the phosphoryl oxygen atom) may lead to an increase in partial multiple bond character of the P-N bonds in coordinated ligands, agreeing with the increase in ν(P-N) and (2)J(PH) coupling constant values. The changes in electron density (ρ) and electronic energy density (H(r)) values confirm these structural reorganizations upon complexation.