Nuclear magnetic resonance, luminescence and structural studies of lanthanide complexes with octadentate macrocyclic ligands bearing benzylphosphinate groups

The solution and solid-state structures of lanthanide complexes of 1,4,7,10-tetraazacyclododecane-1,4,7,10- tetryltetramethylenetetra(benzylphosphinate), L^1a, and of its o-, m- and p-methoxybenzyl analogues (L², L³, L⁴) have been investigated by NMR, relaxometry, crystallographic and photophysical methods. It has been shown that the number of proximate water molecules is dependent on the size of the bound lanthanide ion: the complex [LaL^1a] is nine-co-ordinate and adopts a twisted square-antiprismatic structure with one bound water molecule. The analogous [YL^1a]^-, [YbL^1a]^-, [GdL^1a]^- and [EuL^1a]^- complexes also adopt a twisted square-antiprismatic geometry, but are eight-co-ordinate, with no metal-bound water. Relaxivity studies with the gadolinium complexes showed that they are purely 'outer-sphere' contrast agents, but they associate strongly with proteins leading to a pronounced relaxivity enhancement. Detailed biodistribution studies with [GdL^1a]^- revealed avid biliary uptake at low doses and a well defined tendency for the complex to be cleared more slowly from tumour tissue, allowing tissue differentiation.