Stereochemistry and near-infrared circular dichroism of a chiral Yb complex

The near-infrared CD spectrum of ytterbium complexed by DOTMA, [DOTMA = (1R,4R,7R, 10R)-alpha, alpha', alpha ", alpha'''-tetramethyl-1,4,7, 10-tetraazacyclododecane-1,4,7,10-tetraacetic acid)] is presented. This is the first report of a near-infrared CD spectrum in solution of the f-f transitions of ytterbium in a complex whose solution structure and dynamics are completely defined. The spectrum shows several well-resolved transitions around 980 nm with very high dissymmetry factors. In water and in methanol, there are two diastereomeric conformations of YbDOTMA in equilibrium, Lambda(delta delta delta delta) or n and Lambda(lambda lambda lambda lambda) or p, being the latter always in a large excess. The relative proportion depends on temperature and solvent composition, but the CD spectrum is rather insensitive to these parameters, which demonstrates that neglecting the minor form contribution is an acceptable approximation. On lowering the temperature, the presence of hot bands is highlighted, which leads to a tentative assignment of the transitions. The absorption counterpart compares very well with that of YbDOTA, an achiral analogue, largely studied and well understood.