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 (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-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-S-2(2-))](2+) (2) {nu((S-S)) = 492 cm(-1); nu((Cu-S)sym) = 309 cm(-1)}; by contrast, [(TMPA)Cu-I(CH3CN)](+) (3)/S-8 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 SPPh3 as products. Dioxygen displaces the disulfur moiety from 2 to produce the end-on Cu2O2 complex, [{(TMPA')Cu-II}(2)(mu-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)(mu-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 PhCH2Br: [{(TMPA')Cu-II}(2)(mu-1,2-S-2(2-))](2+) (2) + 2 PhCH2Br -> [{(TMPA')Cu-II(Br)}(2)](2+) (6) + PhCH2SSCH2Ph. The unprecedented substrate reactivity studies reveal that end-on bound mu-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 mu-eta(2):eta(2)-disulfido-dicopper(II) complex, [{(N-3)Cu-II}(2)(mu-eta(2):eta(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.