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 (L1) and cis,cis-1,2,3-trihydroxycyclohexane (L2) 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(L1)2(NO3)2](NO3) (Ln = Pr, Nd); [Ln(L1)2(NO3)H2O](NO3)2 (Ln = Eu, Ho, Yb); [Ln(L1)2(OTf)2(H2O)](OTf) (Ln = Nd, Eu). Lanthanum nitrate itself gives a mixed complex [La(L1)2(NO3)2][LaL1(NO3)4] from
acetonitrile solution while [La(L1)2(NO3)2](NO3) is obtained using dimethoxyethane as reaction solvent and
crystallization medium. With L2, Ln(L)2 complexes have also been crystallized from methanol solution [Ln(L2)2(NO3)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 L1 and L2, 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 1H NMR and electrospray ionization mass spectrometry
experiments. Competing experiments in complexing abilities of L1 and L2 with trivalent lanthanide cations have
shown that only L2 exhibits a small selectivity (Nd > Pr > Yb > La > Eu) in methanol.