19F NMR based pH probes: lanthanide(III) complexes with pH-sensitive chemical shifts.

Experimental measurements and theoretical analysis of magnetic properties, structural dynamics and acid-base equilibria for several lanthanide(III) complexes with tetraazacyclododecane derivatives as 19F NMR chemical shift pH probes are presented; pKa values vary between 6.9 and 7.7, with 18 to 40 ppm chemical shift differences between the acidic and basic forms for Ho(III) complexes possessing T1 values of 10 to 30 ms (4.7-9.4 T, 295 K).

Enantioselective catalysis of the Henry reaction by a chiral macrocyclic ytterbium complex in aqueous media.

A chiral macrocyclic ytterbium cationic complex catalyses the nitro-aldol reaction between alpha-ketocarboxylates and nitromethane under ambient aqueous conditions, leading to the formation of for example, methyl-2-hydroxy-2-methyl-3-nitropropanoate in 96% yield and 59% enantiomeric purity. Monitoring of the paramagnetically shifted intermediate Yb species by (1)H NMR allows several different species on the catalytic cycle to be identified and is consistent with the intermediacy of stereoisomeric chelated pyruvates of differing reactivity towards the nucleophile, as well as product inhibition of turnover.

On the role of the counter-ion in defining water structure and dynamics: order, structure and dynamics in hydrophilic and hydrophobic gadolinium salt complexes.

The crystal structures of the hydrated salts of [Gd.DOTAM]3+ and its more hydrophobic derivative [Gd.]3+, bearing 4 alpha-phenylethyl groups, (both Gd and Yb salts) are reported and compared. The nature of the anion determines the degree of ordering in the lattice and the extent of hydration. These effects are correlated with the results of 17O and 1H NMR measurements of water exchange dynamics in solution. With [Gd.DOTAM]3+, structural ordering or the extent of hydration in the hydrated lattice follows the sequence Cl->Br->I- and this order also defines the water exchange rate in solution: 7.3, 19.5, 33.3x10(4) s-1 (298 K), respectively. For [Gd.]3+ salts, the measured relaxivity is determined purely by the outer sphere term and the water exchange rate at 298 K is very similar (typically 1x10(4) s-1) for chloride, bromide, iodide, acetate, triflate and nitrate salts, notwithstanding the different nature and extent of hydration found in the crystalline lattice.