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 simp le cyclohexanetriols that are models of the two coordination sites found in sugars and derivatives. Several complexes of trivalent lanthanide ions wit h cis, cis-1,3,5-trihydroxycyclohexane (L-1) and cis,cis-1,2,3-trihydroxycy clohexane (L-2) have been characterized in the solid state, and some of the m have been studied in organic solutions. With L1, Ln(L)2 complexes are obt ained when crystallization is performed from acetonitrile solutions whateve r 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)(O Tf)(2)(H2O)]-(OTf) (Ln = Nd, Eu). Lanthanum nitrate itself gives a mixed co mplex [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 sol vent and crystallization medium. With L-2, Ln(L)(2) complexes have also bee n 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 an d 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 NM R and electrospray ionization mass spectrometry experiments. Competing expe riments in complexing abilities of L-1 and L-2 with trivalent lanthanide ca tions have shown that only L-2 exhibits a small selectivity (Nd > Pr > Yb > La > Eu) in methanol.