Solid state and solution studies of lanthanide(III) complexes of cyclohexanetriols, models of the coordination sites found in sugars

This report covers studies in trivalent lanthanide complexation by two simple cyclohexanetriols that are models of the two coordination sites found in sugars and derivatives. Several complexes of trivalent lanthanide ions with cis, cis-1,3,5-trihydroxycyclohexane (L-1) and cis,cis-1,2,3-trihydroxycyclohexane (L-2) have been characterized in the solid state, and some of them have been studied in organic solutions. With L1, Ln(L)2 complexes are obtained when crystallization is performed from acetonitrile solutions whatever the nature of the salt (nitrate or triflate) [Ln(L-1)(2)(NO3)(2)] (NO3) (Ln = Pr, Nd); [Ln(L-1)(2)(NO3)H2O](NO3)(2) (Ln = Eu, Ho, Yb); [Ln(L-1)(2)(OTf)(2)(H2O)]-(OTf) (Ln = Nd, Eu). Lanthanum nitrate itself gives a mixed complex [La(L-1)(2)(NO3)(2)] [LaL1(NO3)(4)] from acetonitrile solution while [La(L-1)(2)(NO3)(2)](NO3) is obtained using dimethoxyethane as reaction solvent and crystallization medium. With L-2, Ln(L)(2) complexes have also been crystallized from methanol solution [Ln(L-2)(2)-(NO3), (Ln = Pr, Nd, Eu). Single-crystal X-ray diffraction analyses are reported for these complexes. Complex formation in solution has been studied for several triflate salts (La, Pr, Nd, Eu, and Yb) with L-1 and L-2, respectively in acetonitrile and in methanol. In contrast to the solid state, both structures Ln(L) and Ln(L)2 equilibrate in solution, as was demonstrated by low-temperature H-1 NMR and electrospray ionization mass spectrometry experiments. Competing experiments in complexing abilities of L-1 and L-2 with trivalent lanthanide cations have shown that only L-2 exhibits a small selectivity (Nd > Pr > Yb > La > Eu) in methanol.