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.

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 tpada ^2- incorporating two carboxylate units with metal cations. Twelve-membered MCs of manganese of the type 12-MC-3, namely, [{Mn ^II (tpada)} ₃ (M)(H ₂ O) _n ] ²⁺ ( Mn ₃ M ) (M = Mn ²⁺ ( n = 0), Ca ²⁺ ( n = 1), or Sr ²⁺ ( 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 tpada ^2- ligands, while the second carboxylate coordinated a fourth cation in the central cavity of the MC, Mn ²⁺ or an alkaline earth metal, Ca ²⁺ or Sr ²⁺ . 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 tpada ^2- ligand with Sr ²⁺ . This complex, namely, [Sr(tpada)(OH ₂ )] ₄ ( Sr ₄ ), does not incorporate any cation in the central cavity but instead four water molecules coordinated to each Sr ²⁺ . 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.