Copper(I)/S₈ Reversible Reactions Leading to an End-On Bound Dicopper(II) Disulfide Complex: Nucleophilic Reactivity and Analogies to Copper−Dioxygen Chemistry

Elemental sulfur (S8) reacts reversibly with the copper(I) complex [(TMPA‘)CuI]+ (1), where TMPA‘ is a TMPA (tris(2-pyridylmethyl)amine) analogue with a 6-CH2OCH3 substituent on one pyridyl ligand arm, affording a spectroscopically pure end-on bound disulfido−dicopper(II) complex [{(TMPA‘)CuII}2(μ-1,2-S22-)]2+ (2) {ν(S-S) = 492 cm-1; ν(Cu-S)sym = 309 cm-1}; by contrast, [(TMPA)CuI(CH3CN)]+ (3)/S8 chemistry produces an equilibrium mixture of at least three complexes. The reaction of excess PPh3 with 2 leads to formal “release” of zerovalent sulfur and reduction of copper ion to give the corresponding complex [(TMPA‘)CuI(PPh3)]+ (11) along with SPPh3 as products. Dioxygen displaces the disulfur moiety from 2 to produce the end-on Cu2O2 complex, [{(TMPA‘)CuII}2(μ-1,2-O22-)]2+ (9). Addition of the tetradentate ligand TMPA to 2 generates the apparently more thermodynamically stable [{(TMPA)CuII}2(μ-1,2-S22-)]2+ (4) and expected mixture of other species. Bubbling 2 with CO leads to the formation of the carbonyl adduct [(TMPA‘)CuI(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‘)CuI(CNAr)]+ (12). Complex 2 readily reacts with PhCH2Br: [{(TMPA‘)CuII}2(μ-1,2-S22-)]2+ (2) + 2 PhCH2Br → [{(TMPA‘)CuII(Br)}2]2+ (6) + PhCH2SSCH2Ph. The unprecedented substrate reactivity studies reveal that end-on bound μ-1,2-disulfide−dicopper(II) complex 2 provides a nucleophilic S22- moiety, in striking contrast to the electrophilic behavior of a recently described side-on bound μ-η2:η2-disulfido−dicopper(II) complex, [{(N3)CuII}2(μ-η2:η2-S22-)]2+ (5) with tridentate N3 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.