H-1 NMR in solution and solid state structural study of lanthanide(III) cryptates

We present here a detailed structural comparison, both in the solid state a nd in aqueous solution, of a complete series of lanthanide cryptate complex es of a Schiff base axial macrobicyclic ligand L of general formula [LnL][N O3](3). xH(2)O (Ln = La-Lu, Y); the macrobicyclic receptor L is an azacrypt and N[(CH2)(2)N=CH-R-CH=N(CH2)(2)](3)N (R = m-C6H2OH-2-Me-5). The crystal s tructures of the Ce, Nd, and Eu complexes, chemical formulae [CeL(NO3)](NO3 )(2). 1.5H(2)O . 0.5CH(3)CH(2)OH (3), [NdL(NO3)](NO3)(2). 3H(2)O (5), and [ EuL(NO3)](NO3)(2). H2O . CH3OH (7), as well as that of [YL(NO3)][Y(NO3)(3)( H2O)(2)EtOH](NO3)(2). EtOH . CH3CN (16), have been determined by single-cry stal X-ray crystallography. The four crystals crystallize in the triclinic space group P (1) over bar with Z = 2; a = 10.853(3) Angstrom, b = 12.746(3 ) Angstrom, c = 17.907(5) Angstrom, alpha = 98.09(2)degrees, beta = 89.99(2 )degrees, gamma = 96.34(2)degrees, for 3; a = 10.835(2) Angstrom, b = 12.54 4(3) Angstrom, c = 17.701(2) Angstrom, alpha = 82.220(10)degrees, beta = 89 .240(10)degrees, gamma = 84.45(2)degrees for 5; a = 10.896(2) Angstrom, b = 12.566(4) Angstrom, 17.688(3) Angstrom, alpha = 81.23(2)degrees, beta = 89 .500(10)degrees gamma = 84.72(3)degrees for 7; and a = 12.723(2) Angstrom, b = 14.047(3) Angstrom, c = 16.943(2) Angstrom, alpha = 66.07(2)degrees, be ta = 79.838(12), gamma = 81.616(14)degrees for 16. In light of their crysta l structures, it can be stated that all of them adopt very similar structur es, 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 nit rate anion. The macrobicycle cavity adapts to the lanthanide contraction, w hile preserving the pseudo-triple-helix conformation around the metal ion. The coordination geometry of the metal atom is best considered as a slightl y distorted monocapped dodecahedron. The aqueous solution structures of the paramagnetic complexes were thoroughly characterized from the proton NMR L IS and LIR data, with, particular attention to the changes induced by the l anthanide contraction, and agree quite well with the crystal structures of the Nd and Y complexes. The experimental Ln-donor distances decrease progre ssively along the lanthanide series both in the solid and solution structur es, but no drastic structural changes occur. The gradual contraction and di stortion of the coordination polyhedron along the series cause a variation of the crystal field parameter A(2)degrees < r(2)> and the hyperfine consta nts A(i) of the lanthanides in the middle of the series, leading to "breaks " in the contact-pseudo-contact shift separation plots of the proton LIS va lues. However, this affects only slightly the geometric terms G(i) of the p rotons and not at all their R-ik ratios. The conformational rigidity of the five-membered chelate rings formed by the metal-bound ethylenediamino moie ties of the bound cryptand increases upon lanthanide contraction. The Delta G double dagger value for the delta <-> lambda conformational interconvers ion process of those rings is 70 +/- 3 kJ for the Y complex.