From Pyrazolate-Based Binuclear Copper(I) Complexes to Octanuclear sigma-Mesityl-Bridged mu(4)-Oxo-Cuprocuprates: Controlled Dioxygen Splitting by Organocopper Scaffolds

The synthesis of a series of new pyrazole-based binucleating compartmental ligands, 3,5-bis((RRN)-R-2-N-3)-(4-R-1)-pyrazoles (LH)-H-1-(LH)-H-6 ((LH)-H-1, R-1 = H, R-2 = Me, R-3 = 2-py(CH2); (LH)-H-2, R-1 = Ph, R-2 = Me, R-3 = 2-py(CH2); (LH)-H-3, R-1 = H, R-2 = Cy, R-3 = 2-py(CH2); (LH)-H-4, R-1 = Ph, R-2 = Cy, R-3 = 2-py(CH2); (LH)-H-5, R-1 = Ph, R-2, R-3 = 2-py(CH2), L6H, R-1 = Ph, R-2 = Me, R-3 = 8-quin), together with the X-ray crystal structure of (LH)-H-3 is reported. After deprotonation and subsequent reaction with 2 equiv of [Cu-I(CH3CN)(4)](BF4) and PMe3, (LH)-H-3 forms the stable binuclear Cu-I complex [L-3{Cu(PMe3)}(2)](BF4) (1). The analogous reaction with (LH)-H-6 and 2 equiv of tert-butyl isonitrile affords [L-6{Cu(CNtBu)}(2)](BF4) (2), 1 and 2 represent the first examples of binuclear Cu-I-pyrazolate complexes of the type [LCu21]X that have been characterized by their X-ray crystal structures. With respect to the planes spanned by the pyrazolate backbone, 1 shows a cis orientation of the PMe3 ligands, whereas 2 exhibits a trans arrangement of the tBuNC ligands. (LH)-H-1-(LH)-H-6 are shown to react with 4 equiv of mesitylcopper and stoichiometric amounts of dioxygen, leading to the formation of the unusually stable organocopper frameworks 3-8. These complexes follow a general structural principle that is best described by the heteroleptic O-centered cuprate anion [(MesCu(I))(4) (mu(4)-O)](2-) linked via four trans-oriented sigma-mesityl bridges to two flanking binuclear Cu-I-pyrazolates [(L-1-L-6)Cu-2(I)](+). Thus, 1 and 2 can also be viewed as capping binuclear Cu-I-complex units that are concealed by two ancillary PMe3 and tBuNC ligands, respectively. The exemplary reaction of 4 with an excess of dimethyl acetylenedicarboxylate (DMDAC) supports the observed cuprate features of 3-8, since after hydrolysis the corresponding (syn-)addition product MesC(CO2Me)=C(CO2Me)H (9) and the free ligand (LH)-H-2 are found as major products.