Synthesis and 1H NMR studies of paramagnetic nickel(ii) complexes containing bis(pyrazolyl)methaneligands with dendritic substituents

Abstract

The substituted bis(pyrazolyl)methane ligands RCH(3,5-Me₂pz)₂ (R = SiMe₃ (1), CH₂Ph (2), G1 (3), G2 (4), and G3 (5); Gn = Fréchet-type dendritic wedges of generation n) have been prepared starting from H₂C(3,5-Me₂pz)₂. Reaction of these didentate ligands with [NiBr₂(DME)] is a straightforward procedure that allows the synthesis of the nickel(II) complexes [NiBr₂{RCH(3,5-Me₂pz)₂}] (6–10). The molecular structure of compound 7 (R = CH₂Ph) has been determined by X-ray diffraction studies. The nickel centre coordinates two bromine and two nitrogen atoms in a tetrahedral environment, and the metallacycle Ni(NN)₂C adopts a boat conformation with the benzyl group in an axial position. ¹H NMR studies have been carried out to characterize these paramagnetic nickel compounds in solution. Valuable information about the disposition of the ligands and dendritic wedges in solution has been obtained thanks to the influence of the paramagnetic centre on the proton resonances.