Copper(I)/S(8) 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')Cu(II)}2(mu-1,2-S2(2-)) (2) {nu(S-S) = 492 cm(-1); nu(Cu-S)sym = 309 cm(-1)}; by contrast, (TMPA)Cu(I)(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')Cu(I)(PPh3) (11) along with S=PPh3 as products. Dioxygen displaces the disulfur moiety from 2 to produce the end-on Cu2O2 complex, {(TMPA')Cu(II)}2(mu-1,2-O2(2-) (9). Addition of the tetradentate ligand TMPA to 2 generates the apparently more thermodynamically stable {(TMPA)Cu(II)}2(mu-1,2-S2(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')Cu(I)(CNAr) (12). Complex 2 readily reacts with PhCH2Br: {(TMPA')Cu(II)}2(mu-1,2-S(2)(2-) (2) + 2 PhCH2Br --> {(TMPA')Cu(II)(Br)}2 (6) + PhCH2SSCH2Ph. The unprecedented substrate reactivity studies reveal that end-on bound mu-1,2-disulfide-dicopper(II) complex 2 provides a nucleophilic S2(2-) moiety, in striking contrast to the electrophilic behavior of a recently described side-on bound mu-eta(2):eta(2)-disulfido-dicopper(II) complex, {(N3)Cu(II)}(2)(mu-eta(2):eta(2)-S2(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.