Structural diversities in heterometallic Mn-Ca cluster chemistry from the use of salicylhydroxamic acid: {Mn^III₄Ca₂}, {Mn^II/III₆Ca₂}, {Mn^III/IV₈Ca}, and {Mn^III₈Ca₂} Complexes with Relevance to Both High- and Low-Valent States of the Oxygen-Evolving Complex

Alaimo, Alysha A., Koumousi, Evangelia S., Cunha-Silva, Luís, McCormick, Laura J., Teat, Simon J., Psycharis, Vassilis, Raptopoulou, Catherine P., Mukherjee, Shreya, Li, Chaoran, Gupta, Sayak Das, Escuer, Albert, Christou, George and Stamatatos, Theocharis C. (2017) Structural diversities in heterometallic Mn-Ca cluster chemistry from the use of salicylhydroxamic acid: {Mn^III₄Ca₂}, {Mn^II/III₆Ca₂}, {Mn^III/IV₈Ca}, and {Mn^III₈Ca₂} Complexes with Relevance to Both High- and Low-Valent States of the Oxygen-Evolving Complex. Inorganic Chemistry, 56 (17), 10760-10774. (doi:10.1021/acs.inorgchem.7b01740).

Abstract

One-pot reactions between the [Mn₃O(O₂CPh)₆(py)_x]^+/0 triangular precursors and either CaBr₂·xH₂O or CaCl₂·6H₂O, in the presence of salicylhydroxamic acid (shaH₂), have afforded the heterometallic complexes [Mn^III₄Ca₂(O₂CPh)₄(shi)₄(H₂O)₃(Me₂CO)] (1) and (pyH)[Mn^II₂Mn^III₄Ca₂Cl₂(O₂CPh)₇(shi)₄(py)₄] (2), respectively, in good yields. Further reactions but using a more flexible synthetic scheme comprising the Mn(NO₃)₂·4H₂O/Ca(NO₃)₂·4H₂O and Mn(O₂CPh)₂·2H₂O/Ca(ClO₄)₂·4H₂O "metal blends" and shaH₂, in the presence of external base NEt₃, led to the new complexes (NHEt₃)₂[Mn^III₄Mn^IV₄Ca(OEt)₂(shi)₁₀(EtOH)₂] (3) and (NHEt₃)₄[Mn^III₈Ca₂(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 amide-iminol tautomerism of shaH₂, 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 Mn₄Ca₂ octahedron bound to an additional Mn₂ unit, 3 consists of a Mn₈ "ring" surrounding a Ca^II atom, and 4 adopts a rectangular cuboidal motif of eight Mn atoms accommodating two Ca^II 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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Contributors

Author: Laura J. McCormick

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