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)(S(Me2)N(4)(6-Me-DPEN))](BF(4)) (1), Mn(II)(S(Me2)N(4)(6-Me-DPPN))](BPh(4))·MeCN (3), Mn(II)(S(Me2)N(4)(2-QuinoPN))](PF(6))·MeCN·Et(2)O (4), and Mn(II)(S(Me2)N(4)(6-H-DPEN)(MeOH)](BPh(4)) (5)) is described, along with their magnetic, redox, and reactivity properties. These complexes are structurally related to recently reported Mn(II)(S(Me2)N(4)(2-QuinoEN))](PF(6)) (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)(S(Me2)N(4)(6-Me-DPEN)](2)-(μ-O)(BF(4))(2)·2MeOH (6), Mn(III)(S(Me2)N(4)(QuinoEN)](2)-(μ-O)(PF(6))(2)·Et(2)O (7), Mn(III)(S(Me2)N(4)(6-Me-DPPN)](2)-(μ-O)(BPh(4))(2) (8), Mn(III)(S(Me2)N(4)(QuinoPN)](2)-(μ-O)(BPh(4))(2) (9), and Mn(III)(S(Me2)N(4)(6-H-DPEN)](2)-(μ-O)(PF(6))(2)·2MeCN (10). Labeling studies show that the oxo atom is derived from (18)O(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 μ-oxo dimers 6-10 are moderately antiferromagnetically coupled, with coupling constants (2J) that fall within the expected range. Metastable intermediates, which ultimately convert to μ-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.