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‘)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(μ-1,2-S₂^2-)]²⁺ (2) {ν_(S-S) = 492 cm^-1; ν_(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(μ-1,2-O₂^2-)]²⁺ (9). Addition of the tetradentate ligand TMPA to 2 generates the apparently more thermodynamically stable [{(TMPA)Cu^II}₂(μ-1,2-S₂^2-)]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(μ-1,2-S₂^2-)]²⁺ (2) + 2 PhCH₂Br → [{(TMPA‘)Cu^II(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 S₂^2- moiety, in striking contrast to the electrophilic behavior of a recently described side-on bound μ-η²:η²-disulfido−dicopper(II) complex, [{(N₃)Cu^II}₂(μ-η²:η²-S₂^2-)]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.