Synthesis and characterisation of new polynuclear copper(I) pyrazolate complexes and their catalytic activity in the cyclopropanation of olefins

The reaction of [Cu(CH3CN)4](BF4) with racemic pyrazole-3,5-dicarboxylic acid di-sec-butyl ester (3,5-dicarbo-sec-butoxypyrazole, Hdcsbpz) or with pyrazole-3,5-di-ter-butyl (3,5-di-ter-butylpyrazole, Hdtbpz) quantitatively yields the new [Cu(dcsbpz)]4 and [Cu(dtbpz)]4 complexes, respectively. Crystals of [Cu(dcsbpz)]4 are triclinic, P1̄, a = 10.9748(7), b = 11.8399(8), c = 26.5575(17) Å, α =100.605(2), β = 90.783(2), γ = 105.362(2)°; [Cu(dtbpz)]4·CH2Cl2 is monoclinic, P21/n, a = 10.902(3), b = 19.200(3), c = 25.772(4) Å, β = 93.86(2)°. Both species contain cyclic tetrameric molecules, with the heterocyclic ligands binding in the common N,N′-exo-bidentate mode; however, the shape and geometry of the inner Cu4 moiety is remarkably different, as highlighted, for example, by the absolute values of the 1,2 and 1,3 (non-bonding) Cu⋯Cu interactions. These polynuclear copper(I) pyrazolate complexes catalyse the conversion of alkenes into the corresponding cyclopropane derivatives with interesting diastereomeric excesses. Aiming at the evaluation of their catalytic activities, a systematic study of the cyclopropanation reactions in the presence of ethyl diazoacetate has been performed.