Monitoring proton dissociation and solution conformation of chiral ytterbium complexes with near‐IR CD

The ytterbium complex [Yb(( S )‐THP)] 3+ (( S )‐THP = (1 S ,4 S ,7 S ,10 S ‐tetrakis(2‐hydroxypropyl)‐1,4,7,10‐tetraazacyclododecane) is investigated in solution through NMR, near‐IR absorption, and CD spectroscopy. Quantitative analysis of the paramagnetic pseudocontact NMR shift shows Λ helicity of the ligand cage around the metal. The NIR CD spectrum recorded at acidic pH is found to be very similar to that of [Yb(( R )‐DOTMA)] − (( R )‐DOTMA = (1 R ,4 R ,7 R ,10 R )‐α,α′,α′′,α′′′‐tetramethyl‐1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid), which in solution assumes a twisted square antiprism (TSA) conformation. The similarity of the NIR CD spectra is discussed, and it is the first proof of the Λ(λ,λ,λ,λ) conformation of [Yb(( S )‐THP)] 3+ . NIR CD spectra recorded in the pH range of 2–9 allow one to easily follow proton dissociation and to calculate the p K of this equilibrium in water (p K A = 6.4 ± 0.1). This value agrees well with that determined for [Lu(( S )‐THP)] 3+ using potentiometric methods. This demonstrates once again that NIR CD spectroscopy is a powerful technique for investigating the solution structure and dynamics of these complexes. Chirality 17:201–211, 2005. © 2005 Wiley‐Liss, Inc.