Si trova su / Altri legami
© We report on the isolation of a new family of μ–carboxylato–bridged metallocrown (MC) compounds by self–assembly of the recently isolated hexadentate tris(2–pyridylmethyl)amine ligand tpada2– incorporating two carboxylate units with metal cations. Twelve–membered MCs of manganese of the type 12–MC–3, namely, [{MnII(tpada)}3(M)(H2O)n]2+ (Mn3M) (M = Mn2+ (n = 0), Ca2+ (n = 1), or Sr2+ (n = 2)), were structurally characterized. The metallamacrocycles connectivity consisting in three –[Mn–O–C–O]– repeating units is provided by one carboxylate unit of the three tpada2– ligands, while the second carboxylate coordinated a fourth cation in the central cavity of the MC, Mn2+ or an alkaline earth metal, Ca2+ or Sr2+. Mn3Ca and {Mn3Sr}2 join the small family of heterometallic manganese–calcium complexes and even rarer manganese–strontium complexes as models of the OEC of photosystem II (PSII). A 8–MC–4 of strontium of the molecular wheel type with four –[Sr–O]– repeating unit was also isolated by self–assembly of the tpada2– ligand with Sr2+. This complex, namely, [Sr(tpada)(OH2)]4 (Sr4), does not incorporate any cation in the central cavity but instead four water molecules coordinated to each Sr2+. Electrochemical investigations coupled to UV–visible absorption and EPR spectroscopies as well as electrospray mass spectrometry reveal the stability of the 12–MC–3 tetranuclear structures in solution, both in the initial oxidation state, MnII3M, as well as in the three–electrons oxidized state, MnIII3M. Indeed, the cyclic voltammogram of all these complexes exhibits three–successive reversible oxidation waves between +0.5 and +0.9 V corresponding to the successive one–electron oxidation of the Mn(II) ion into Mn(III) of the three {Mn(tpada)} units constituting the ring, which are fully maintained after bulk electrolysis.
