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

When trigonal-bipyramidal clusters, [PPN][E₂Mn₃(CO)₉] (E = S, Se), were treated with Cr(CO)₆ and PPNCl in a molar ratio of 1:1:2 or 1:2:2 in 4 M KOH/MeCN/MeOH solutions, mono-Cr(CO)₅-incorporated HE₂Mn₃-complexes [PPN]₂[HE₂Mn₃Cr(CO)₁₄] (E = S, [PPN]₂[1a]; Se, [PPN]₂[1b]), respectively, were formed. X-ray crystallographic analysis showed that 1a and 1b were isostructural and each displayed an E₂Mn₃ square-pyramidal core with one of the two basal E atoms externally coordinated with one Cr(CO)₅ group and one Mn–Mn bond bridged by one hydrogen atom. However, when the TMBA⁺ salts for [E₂Mn₃(CO)₉]⁻ were mixed with Cr(CO)₆ in a molar ratio of 1:1 in 4 M KOH/MeOH solutions and refluxed at 60 °C, mono-Cr(CO)₃-incorporated E₂Mn₃Cr octahedral clusters [TMBA]₃[E₂Mn₃Cr(CO)₁₂] (E = S, [TMBA]₃[2a]; Se, [TMBA]₃[2b]), respectively, were obtained. Clusters 2a and 2b were isostructural, and each consisted of an octahedral E₂Mn₃Cr core, in which each Mn–Mn or Mn–Cr bond of the Mn₃Cr plane was semibridged by one carbonyl ligand. Clusters 1a and 1b (with [TMBA] salts) underwent metal core closure to form octahedral clusters 2a and 2b upon treatment with KOH/MeOH at 60 °C. In addition, 1a and 1b were found to undergo cluster expansion to form di-Cr(CO)₅-incorporated HE₂Mn₃-clusters [HE₂Mn₃Cr₂(CO)₁₉]^2– (E = S, 3a; Se, 3b), respectively, upon the addition of 1 or 2 equiv of Cr(CO)₆ heated in refluxing CH₂Cl₂. Clusters 3a and 3b were structurally related to clusters 1a and 1b, but with the other bare E atom (E = S, 3a; Se, 3b) further externally coordinated with one Cr(CO)₅ 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.