Copper(Ⅰ)/S_8 Reversible Reactions Leading to an End-On Bound Dicopper(Ⅱ) Disulfide Complex: Nucleophilic Reactivity and Analogies to Copper-Dioxygen Chemistry

摘要

Elemental sulfur (S_8) reacts reversibly with the copper(Ⅰ) complex [(TMPA')Cu~Ⅰ]~+ (1), where TMPA' is a TMPA (tris(2-pyridylmethyl)amine) analogue with a 6-CH_2OCH_3 substituent on one pyridyl ligand arm, affording a spectroscopically pure end-on bound disulfido-dicopper(Ⅱ) complex [{(TMPA')Cu~Ⅱ}_2(μ-1,2-S_2~(2-))]~(2+) (2) {ν_((S-S)) = 492 cm~(-1); ν_((Cu-S)sym) = 309 cm~(-1)}; by contrast, [(TMPA)Cu~Ⅰ(CH_3CN)]~+ (3)/S_8 chemistry produces an equilibrium mixture of at least three complexes. The reaction of excess PPh_3 with 2 leads to formal "release" of zerovalent sulfur and reduction of copper ion to give the corresponding complex [(TMPA')-Cu~Ⅰ(PPh_3)]~+ (11) along with S=PPh_3 as products. Dioxygen displaces the disulfur moiety from 2 to produce the end-on Cu_2O_2 complex, [{(TMPA')Cu~Ⅱ}_2(μ-1,2-O_2~(2-))]~(2+) (9). Addition of the tetradentate ligand TMPA to 2 generates the apparently more thermodynamically stable [{(TMPA)Cu~Ⅱ}_2(μ-1,2-S_2~(2-))]~(2+) (4) and expected mixture of other species. Bubbling 2 with CO leads to the formation of the carbonyl adduct [(TMPA')Cu~Ⅰ-(CO)]~+ (8). Carbonylation/sulfur-release/CO-removal cycles can be repeated several times. Sulfur atom transfer from 2 also occurs in a near quantitative manner when it is treated with 2,6-dimethylphenyl isocyanide (ArNC), leading to the corresponding isothiocyanate (ArNCS) and [(TMPA')Cu~Ⅰ(CNAr)]~+ (12). Complex 2 readily reacts with PhCH_2Br: [{(TMPA')Cu~Ⅱ}_2(μ-1,2-S_2~(2-))]~(2+) (2) + 2 PhCH_2Br → [{(TMPA')-Cu~Ⅱ(Br)}_2]~(2+) (6) + PhCH_2SSCH_2Ph. The unprecedented substrate reactivity studies reveal that end-on bound μ-1,2-disulfide-dicopper(Ⅱ) complex 2 provides a nucleophilic S_2~(2-) moiety, in striking contrast to the electrophilic behavior of a recently described side-on bound μ-η~2:η~2-disulfido-dicopper(Ⅱ) complex, [{(N_3)Cu~Ⅱ}_2(μ-η~2:η~2-S_2~(2-))]~(2+) (5) with tridentate N_3 ligand. The investigation thus reveals striking analogies of copper/sulfur and copper/dioxygen chemistries, with regard to structure type formation and specific substrate reactivity patterns.