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DOTP versus DOTA as ligands for lanthanide cations: Novel structurally characterized Ce(IV) and Ce(III) cyclen‐based complexes and clusters in aqueous solutions

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A delve into the coordination and redox chemistry of aqueous CeIV/III macrocyclic compounds was performed using the DOTA and DOTP ligands (1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid and 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetra(methylene phosphonic acid) respectively). The hydrolysis tendency of the tetravalent cation in the presence of DOTA is shown to result in the formation of highly-ordered, fluorite-like [CeIV6(O)4(OH)4(H2O)8(DOTAH)4] oxo-hydroxo structure both in solution and in the solid state. The lifetime of the analogous species formed in the presence of DOTP was found to be much shorter. Spectroscopic measurements suggest its similarity to the former. Its gradual in solution decomposition leads to the accumulation of the in cage complexes of [CeIVDOTP] and [CeIIIDOTP(H2O)], which were crystallographically characterized in the present study. The redox energetics and spectroscopic characteristics for the transition between these two in-cage complexes in aqueous solutions were studied as well. Together with the crystallographic structures of the abovementioned species, the in cage [CeIVDOTA(H2O)] complex structure is presented herein as well for the first time. An elaborative analysis of the x-ray crystallographic structural data obtained for the herein studied in cage complexes and similar structures published previously, suggests that hard bonding cyclen derived ligands are, counter-intuitively, better suited for encapsulating, and perhaps kinetically stabilize softer cations than harder ones with DOTP, marked as a possible adequate chelator for the study of the aqueous properties of LnII and AcIII cations.

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