Fully Reduced and Mixed-Valent Multi-Copper Aggregates Supported by Tetradentate Diamino Bis(thiolate) Ligands

Tetradentate diamino bis( thiolate) ligands (LN2S2(2-)) with saturated linkages between heteroatoms support fully reduced [(Cu(L-N2S2))2Cu2] complexes that bear relevance as an entry point toward molecules featuring the Cu2ICu2II(mu 4-S) core composition of nitrous oxide reductase (N2OR). Tetracopper [(Cu(L-N2(SMe2)2))2Cu2] (L-N2(SMe2H) 2 = N1,N2-bis(2-methyl-2mercaptopropane)-N1,N2-dimethylethane-1,2-diamine) does not support clean S atom oxidative addition but undergoes Cl atom transfer from PhICl2 or Ph3CCl to afford [(Cu(L-N2(SMe2)2))3(CuCl)5], 14. When introduced to Cu(I) sources, the L-N2(SArH) 2 ligand (LN2(SArH) 2 = N1,N2-bis(2-mercaptophenyl)-N1,N2-dimethylethane1,2-diamine), made by a newly devised route from N1,N2-bis(2fluorophenyl)-N1,N2-dimethylethane-1,2-diamine, ultimately yields the mixed-valent pentacopper [(Cu(L-N2SAr2))3Cu2] (19), which has 3-fold rotational symmetry (D3) around a Cu2 axis. The single CuII ion of 19 is ensconced within an equatorial L-N2(SAr)2(2-) ligand, as shown by 14N coupling in its EPR spectrum. Formation of 19 proceeds from an initial, fully reduced product, [(Cu(L-N2SAr2))3Cu2(Cu(MeCN))] (17), which is C2 symmetric and exceedingly air-sensitive. While unreactive toward chalcogen donors, 19 supports reversible reduction to the all-cuprous state; generation of [19]1- and treatment with S atom donors only return 19 because structural adjustments necessary for oxidative addition are noncompetitive with outer-sphere electron transfer. Oxidation of 19 is marked by intense darkening, consistent with greater mixed valency, and by dimerization in the crystalline state to a decacopper species ([20]2+) of S4 symmetry.