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Structural Diversities in Heterometallic Mn–Ca Cluster Chemistry from the Use of Salicylhydroxamic Acid: {Mn<sup>III</sup><sub>4</sub>Ca<sub>2</sub>}, {Mn<sup>II/III</sup><sub>6</sub>Ca<sub>2</sub>}, {Mn<sup>III/IV</sup><sub>8</sub>Ca}, and {Mn<sup>III</sup><sub>8</sub>Ca<sub>2</sub>} Complexes with Relevance to Both High- and Low-Valent States of the Oxygen-Evolving Complex
17
Citations
86
References
2017
Year
One-pot reactions between the [Mn<sub>3</sub>O(O<sub>2</sub>CPh)<sub>6</sub>(py)<sub>x</sub>]<sup>+/0</sup> triangular precursors and either CaBr<sub>2</sub>·xH<sub>2</sub>O or CaCl<sub>2</sub>·6H<sub>2</sub>O, in the presence of salicylhydroxamic acid (shaH<sub>2</sub>), have afforded the heterometallic complexes [Mn<sup>III</sup><sub>4</sub>Ca<sub>2</sub>(O<sub>2</sub>CPh)<sub>4</sub>(shi)<sub>4</sub>(H<sub>2</sub>O)<sub>3</sub>(Me<sub>2</sub>CO)] (1) and (pyH)[Mn<sup>II</sup><sub>2</sub>Mn<sup>III</sup><sub>4</sub>Ca<sub>2</sub>Cl<sub>2</sub>(O<sub>2</sub>CPh)<sub>7</sub>(shi)<sub>4</sub>(py)<sub>4</sub>] (2), respectively, in good yields. Further reactions but using a more flexible synthetic scheme comprising the Mn(NO<sub>3</sub>)<sub>2</sub>·4H<sub>2</sub>O/Ca(NO<sub>3</sub>)<sub>2</sub>·4H<sub>2</sub>O and Mn(O<sub>2</sub>CPh)<sub>2</sub>·2H<sub>2</sub>O/Ca(ClO<sub>4</sub>)<sub>2</sub>·4H<sub>2</sub>O "metal blends" and shaH<sub>2</sub>, in the presence of external base NEt<sub>3</sub>, led to the new complexes (NHEt<sub>3</sub>)<sub>2</sub>[Mn<sup>III</sup><sub>4</sub>Mn<sup>IV</sup><sub>4</sub>Ca(OEt)<sub>2</sub>(shi)<sub>10</sub>(EtOH)<sub>2</sub>] (3) and (NHEt<sub>3</sub>)<sub>4</sub>[Mn<sup>III</sup><sub>8</sub>Ca<sub>2</sub>(CO<sub>3</sub>)<sub>4</sub>(shi)<sub>8</sub>] (4), respectively. In all reported compounds, the anion of the tetradentate (N,O,O,O)-chelating/bridging ligand salicylhydroxime (shi<sup>3-</sup>), resulting from the in situ metal-ion-assisted amide-iminol tautomerism of shaH<sub>2</sub>, was found to bridge both Mn and Ca atoms. Complexes 1-4 exhibit a variety of different structures, metal stoichiometries, and Mn oxidation-state descriptions; 1 possesses an overall octahedral metal arrangement, 2 can be described as a Mn<sub>4</sub>Ca<sub>2</sub> octahedron bound to an additional Mn<sub>2</sub> unit, 3 consists of a Mn<sub>8</sub> "ring" surrounding a Ca<sup>II</sup> atom, and 4 adopts a rectangular cuboidal motif of eight Mn atoms accommodating two Ca<sup>II</sup> atoms. Solid-state direct-current magnetic susceptibility studies revealed the presence of predominant antiferromagnetic exchange interactions between the Mn centers, leading to S = 0 spin ground-state values for all complexes. From a bioinorganic chemistry perspective, the reported compounds may demonstrate some relevance to both high-valent scheme (3) and lower-oxidation-level species (1, 2, and 4) of the catalytic cycle of the oxygen-evolving complex.
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