Stepwise Construction of Manganese–Chromium Carbonyl Chalcogenide Complexes: Synthesis, Electrochemical Properties, and Computational Studies

When trigonal-bipyramidal clusters, [PPN][E<sub>2</sub>Mn<sub>3</sub>(CO)<sub>9</sub>] (E = S, Se), were treated with Cr(CO)<sub>6</sub> and PPNCl in a molar ratio of 1:1:2 or 1:2:2 in 4 M KOH/MeCN/MeOH solutions, mono-Cr(CO)<sub>5</sub>-incorporated HE<sub>2</sub>Mn<sub>3</sub>-complexes [PPN]<sub>2</sub>[HE<sub>2</sub>Mn<sub>3</sub>Cr(CO)<sub>14</sub>] (E = S, [PPN]<sub>2</sub>[<b>1a</b>]; Se, [PPN]<sub>2</sub>[<b>1b</b>]), respectively, were formed. X-ray crystallographic analysis showed that <b>1a</b> and <b>1b</b> were isostructural and each displayed an E<sub>2</sub>Mn<sub>3</sub> square-pyramidal core with one of the two basal E atoms externally coordinated with one Cr(CO)<sub>5</sub> group and one Mn–Mn bond bridged by one hydrogen atom. However, when the TMBA<sup>+</sup> salts for [E<sub>2</sub>Mn<sub>3</sub>(CO)<sub>9</sub>]<sup>−</sup> were mixed with Cr(CO)<sub>6</sub> in a molar ratio of 1:1 in 4 M KOH/MeOH solutions and refluxed at 60 °C, mono-Cr(CO)<sub>3</sub>-incorporated E<sub>2</sub>Mn<sub>3</sub>Cr octahedral clusters [TMBA]<sub>3</sub>[E<sub>2</sub>Mn<sub>3</sub>Cr(CO)<sub>12</sub>] (E = S, [TMBA]<sub>3</sub>[<b>2a</b>]; Se, [TMBA]<sub>3</sub>[<b>2b</b>]), respectively, were obtained. Clusters <b>2a</b> and <b>2b</b> were isostructural, and each consisted of an octahedral E<sub>2</sub>Mn<sub>3</sub>Cr core, in which each Mn–Mn or Mn–Cr bond of the Mn<sub>3</sub>Cr plane was semibridged by one carbonyl ligand. Clusters <b>1a</b> and <b>1b</b> (with [TMBA] salts) underwent metal core closure to form octahedral clusters <b>2a</b> and <b>2b</b> upon treatment with KOH/MeOH at 60 °C. In addition, <b>1a</b> and <b>1b</b> were found to undergo cluster expansion to form di-Cr(CO)<sub>5</sub>-incorporated HE<sub>2</sub>Mn<sub>3</sub>-clusters [HE<sub>2</sub>Mn<sub>3</sub>Cr<sub>2</sub>(CO)<sub>19</sub>]<sup>2–</sup> (E = S, <b>3a</b>; Se, <b>3b</b>), respectively, upon the addition of 1 or 2 equiv of Cr(CO)<sub>6</sub> heated in refluxing CH<sub>2</sub>Cl<sub>2</sub>. Clusters <b>3a</b> and <b>3b</b> were structurally related to clusters <b>1a</b> and <b>1b</b>, but with the other bare E atom (E = S, <b>3a</b>; Se, <b>3b</b>) further externally coordinated with one Cr(CO)<sub>5</sub> group. The nature, cluster transformation, and electrochemical properties of the mixed manganese–chromium carbonyl sulfides and selenides were systematically discussed in terms of the chalcogen elements, the introduced chromium carbonyl group, and the metal skeleton with the aid of molecular calculations at the BP86 level of the density functional theory.