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

• Published in: Inorganic chemistry 2021-06, Vol.60 (11), p.7922-7936
• Main Authors: Gouré, Eric, Gerey, Bertrand, Astudillo, Catalina N, Pécaut, Jacques, Sirach, Selim, Molton, Florian, Fortage, Jérôme, Collomb, Marie-Noëlle
• Format: Article
• Language: English
• Subjects: Chemical Sciences
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.1c00457

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+ (Mn 3 M) (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+. Mn 3 Ca and {Mn 3 Sr} 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 (Sr 4 ), 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, Mn II 3 M, as well as in the three-electrons oxidized state, Mn III 3 M. 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.