1H NMR in Solution and Solid State Structural Study of Lanthanide(III) Cryptates

Abstract

We present here a detailed structural comparison, both in the solid state and in aqueous solution, of a complete series of lanthanide cryptate complexes of a Schiff base axial macrobicyclic ligand L of general formula [LnL][NO3]3·xH2O (Ln = La−Lu, Y); the macrobicyclic receptor L is an azacryptand N[(CH2)2NCH−R−CHN(CH2)2]3N (R = m-C6H2OH-2-Me-5). The crystal structures of the Ce, Nd, and Eu complexes, chemical formulae [CeL(NO3)](NO3)2·1.5H2O·0.5CH3CH2OH (3), [NdL(NO3)](NO3)2·3H2O (5), and [EuL(NO3)](NO3)2·H2O· CH3OH (7), as well as that of [YL(NO3)][Y(NO3)3(H2O)2EtOH](NO3)2.EtOH·CH3CN (16), have been determined by single-crystal X-ray crystallography. The four crystals crystallize in the triclinic space group P1̄ with Z = 2; a = 10.853(3) Å, b = 12.746(3) Å, c = 17.907(5) Å, α = 98.09(2)°, β = 89.99(2)°, γ = 96.34(2)°, for 3; a = 10.835(2) Å, b = 12.544(3) Å, c = 17.701(2) Å, α = 82.220(10)°, β = 89.240(10)°, γ = 84.45(2)° for 5; a = 10.896(2) Å, b = 12.566(4) Å, c = 17.688(3) Å, α = 81.23(2)°, β = 89.500(10)°, γ = 84.72(3)° for 7; and a = 12.723(2) Å, b = 14.047(3) Å, c = 16.943(2) Å, α = 66.07(2)°, β = 79.838(12)°, γ = 81.616(14)° for 16. In light of their crystal structures, it can be stated that all of them adopt very similar structures, with the nine-coordinated metal ion bound asymmetrically to seven donor atoms in the ligand cavity and also to two oxygen atoms of a bidentate nitrate anion. The macrobicycle cavity adapts to the lanthanide contraction, while preserving the pseudo-triple-helix conformation around the metal ion. The coordination geometry of the metal atom is best considered as a slightly distorted monocapped dodecahedron. The aqueous solution structures of the paramagnetic complexes were thoroughly characterized from the proton NMR LIS and LIR data, with particular attention to the changes induced by the lanthanide contraction, and agree quite well with the crystal structures of the Nd and Y complexes. The experimental Ln−donor distances decrease progressively along the lanthanide series both in the solid and solution structures, but no drastic structural changes occur. The gradual contraction and distortion of the coordination polyhedron along the series cause a variation of the crystal field parameter A2°<r2> and the hyperfine constants Ai of the lanthanides in the middle of the series, leading to “breaks” in the contact−pseudo-contact shift separation plots of the proton LIS values. However, this affects only slightly the geometric terms Gi of the protons and not at all their Rik ratios. The conformational rigidity of the five-membered chelate rings formed by the metal-bound ethylenediamino moieties of the bound cryptand increases upon lanthanide contraction. The ΔG⧧ value for the δ ↔ λ conformational interconversion process of those rings is 70 ± 3 kJ for the Y complex.