A five-coordinate, sixteen-electron manganese(I) complex [Mn(CO)3(S,S–C6H4)]– stabilized by S,S π-donation from chelating [S,S–C6H4]2–

Abstract

The five-coordinate, sixteen-electron manganese(I) complex [N(PPh₃)₂][Mn(CO)₃(S,S–C₆H₄)] 1 was prepared from reaction of [N(PPh₃)₂][Mn(CO)₃(NH,S–C₆H₄)] and 1,2-benzenedithiol via the hexacoordinate intermediate fac-[Mn(CO)₃(S–C₆H₄SH)(NH₂,S–C₆H₄)]^–. Alternatively, oxidative addition of 1,2-benzenedithiol to [Mn(CO)₅]^–, followed by a Lewis acid–base reaction, with evolution of H₂ gas (identified by gas chromatography), led to formation of the coordinatively-unsaturated complex 1. In contrast, reaction of bis(2-pyridyl) disulfide and [N(PPh₃)₂][Mn(CO)₅] afforded hexacoordinate fac-[N(PPh₃)₂][Mn(CO)₃(S–C₅H₄–N)(S–C₅H₄N)] 2, with one anionic [S–C₅H₄N]^– ligand bound to Mn^I in a monodentate (S-bonded) manner and the other [S–C₅H₄–N]^– ligand bound in a bidentate manner (S,N-bonded). Complexes 1 and 2 have been characterized in solution by infrared spectroscopy and in the solid state by X-ray crystallography. The strong π-donating ability of the bidentate [S,S–C₆H₄]^2– ligand stabilizes the unsaturated complex 1 which has short Mn^I–S bond lengths of 2.230(1) Å (average) as a result. The existence of one π and two σ bonds between the [Mn(CO)₃]⁺ and [S,S–C₆H₄]^2– fragments, based on qualitative frontier molecular orbital analysis, also indicates that the lone-pair electrons are delocalized around the sulfur-manganese-sulfur system stabilizing the five-coordinate complex 1. The IR carbonyl stretching frequencies and the Mn^I–S bond distances of complexes 1 and [N(PPh₃)₂][Mn(CO)₃(NH,S–C₆H₄)] suggest that the relative π-donating ability of the bidentate ligands is [NH,S–C₆H₄]^2– > [S,S–C₆H₄]^2–. The Mulliken atomic charges derived from Hartree–Fock calculations roughly quantify the charge distribution in the complex [Mn(CO)₃(NH,S–C₆H₄)]^– (δ(N) = –1.14; δ(S) = –0.43; δ(Mn) = 1.14), and supports the premise that the reactions of [Mn(CO)₃(NH,S–C₆H₄)]^– with electrophiles (1,2-benzenedithiol, thiophene-2-thiol, 1,2-ethanedithiol) occur at the more electron-rich amide site, yielding charge-controlled, collision complexes.