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Self-Assembled Heterometallic Complexes by Incorporation of Calcium or Strontium Ion into a Manganese(II) 12-Metallacrown‑3 Framework Supported by a Tripodal Ligand with Pyridine-Carboxylate Motifs: Stability in Their Manganese(III) Oxidized Form

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journal contribution
posted on 20.05.2021, 20:31 by Eric Gouré, Bertrand Gerey, Catalina N. Astudillo, Jacques Pécaut, Selim Sirach, Florian Molton, Jérôme Fortage, Marie-Noëlle Collomb
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.