Characterization and Dioxygen Reactivity of a New Series of Coordinatively Unsaturated Thiolate-Ligated Manganese(II) Complexes

The synthesis, structural, and spectroscopic characterization of four new coordinatively unsaturated mononuclear thiolate-ligated manganese (II) complexes ([Mn-II ((SN4)-N-Me2 (6-Me-DPEN))] (BF4) (1), [Mn-II((SN4)-N-Me2(6-Me-DPPN))](BPh4)center dot MeCN (3), [Mn-II((SN4)-N-Me2(2-QuinoPN))] (PF6)center dot MeCN center dot Et2O (4), and [Mn-II((SN4)-N-Me2(6-H-DPEN)(MeOH)](BPh4) (5)) is described, along with their magnetic, redox, and reactivity properties. These complexes are structurally related to recently reported [Mn-II(((SN4)-N-Me2(2-QuinoEN))](PF6) (2) (Coggins, M. K.; Kovacs, J. A. J. Am. Chem. Soc. 2011, 133, 12470). Dioxygen addition to complexes 1-5 is shown to result in the formation of five new rare examples of Mn(III) dimers containing a single, unsupported oxo bridge: Mn-III((SN4)-N-Me2(6-Me-DPEN)](2)-(mu-O) (BF4)(2)center dot 2MeOH (6), [Mn-III((SN4)-N-Me2(QuinoEN)](2)-(mu-O)(PF6)(2)center dot Et2O (7), [Mn-III((SN4)-N-Me2(6-Me-DPPN)](2)-(mu-O)(BPh4)(2) (8), [Mn-III((SN4)-N-Me2(QuinoPN)](2)-(mu-O)(BPh4)(2) (9), and [Mn-III((SN4)-N-Me2(6-H-DPEN)](2)-(mu-O)(PF6)(2)center dot 2MeCN (10). Labeling studies show that the oxo atom is derived from O-18(2). Ligand modifications, involving either the insertion of a methylene into the backbone or the placement of an ortho substituent on the N-heterocyclic amine, are shown to noticeably modulate the magnetic and reactivity properties. Fits to solid-state magnetic susceptibility data show that the Mn(III) ions of mu-oxo dimers 6-10 are moderately antiferromagnetically coupled, with coupling constants (2J) that fall within the expected range. Metastable intermediates, which ultimately convert to mu-oxo bridged 6 and 7, are observed in low-temperature reactions between 1 and 2 and dioxygen. Complexes 3-5, on the other hand, do not form observable intermediates, thus illustrating the effect that relatively minor ligand modifications have upon the stability of metastable dioxygen-derived species.