NMR study of solution dynamics of LnIII complexes of monophosphorus H4dota analogues

Lanthanides have several specific properties which cannot be found for other elements in the periodic table. Among various applications of lanthanides, complexes of LnIII ions are used in medicine, e.g., as contrast agents in MRI, as luminescent probes or as radiopharmaceuticals, where their specific properties are important. These complexes must be kinetically inert to prevent release of highly toxic "free" LnIII ions. This requirement is fulfilled with pre-organized ligands such as analogues of H4dota (1,4,7,10- tetraazacyclododecane-1,4,7,10-tetraacetic acid). Many of important properties of LnIII complexes of H4dota, such as relaxivity, isomerism and fluxionality, depend on the solution dynamics of the complexes. However, the knowledge of this solution dynamics is limited for LnIII complexes of H4dota derivatives with phosphonate or phosphinate pendant arms. Recently, a new dynamical process where phosphonate oxygen atoms interchange through a bidentate phosphonate intermediate ("a phosphonate rotation") has been proposed by DFT calculations but unconfirmed experimentally. To prove the process experimentally, solution dynamics of LnIII complexes of monophosphonate and monophosphinate derivatives of H4dota was investigated. Especially, to examine the "P-rotation", 17 O NMR spectroscopy was used..