Structural Diversities in Heterometallic Mn-Ca Cluster Chemistry from the Use of Salicylhydroxamic Acid: {MnIII4Ca2}, {MnII/III6Ca2}, {MnIII/IV8Ca} and {MnIII8Ca2} Complexes with Relevance to Both High- and Low-Valent States of the Oxygen-Evolving Complex

One-pot reactions between the [Mn3O(O2CPh)(6)(py)(x)](+/0) triangular precursors and either CaBr2 center dot xH(2)O or CaCl2 center dot 6H(2)O, in the presence of salicylhydroxamic acid (shaH(2)), have afforded the heterometallic complexes [(Mn4Ca2)-Ca-III(O2CPh)(4)(shi)(4)(H2O)(3)(Me2CO)] (1) and (pyH)[(Mn2Mn4Ca2Cl2)-Mn-II-Ca-III(O2CPh)(7)(shi)(4)(py)(4)] (2), respectively, in good yields. Further reactions but using a more flexible synthetic scheme comprising the Mn(NO3)(2)center dot 4H(2)O/Ca(NO3)(2)center dot 4H(2)O and Mn(O2CPh)(2)center dot 2H(2)O/Ca(ClO4)(2)center dot 4H(2)O metal blends and shaH(2), in the presence of external base NEt3, led to the new complexes (NHEt3)(2)[(Mn4Mn4Ca)-Mn-III-Ca-IV(OEt)(2)(shi)(10)(EtOH)(2)] (3) and (NHEt3)(4)[(Mn8Ca2)-Ca-III(CO3)(4)(shi)(8)] (4), respectively. In all reported compounds, the anion of the tetradentate (N,O,O,O)-chelating/bridging ligand salicylhydroxime (shi(3-)), resulting from the in situ metal-ion-assisted amideiminol tautomerism of shaH(2), was found to bridge both Mn and Ca atoms. Complexes 14 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 Mn4Ca2 octahedron bound to an additional Mn-2 unit, 3 consists of a Mn8 ring surrounding a CaII atom, and 4 adopts a rectangular cuboidal motif of eight Mn atoms accommodating two CaII 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.