Coordination behaviour of new dipyridylpyrazole ligands towards ZnCl2 and PdCl2 fragments. Crystalline structural characterization and multinuclear NMR studies as evidence of linkage and conformational isomers

Abstract

A series of novel N,N,N-donor pyrazole based ligands of the type pypz^R(n)py (R(n) = C₆H₄OC_nH_2n+1, n = 1, 12–18) (1–5) bearing two pyridine substituents at the 1- and 3-positions and an alkyloxyphenyl group at the 5-position have been prepared and characterized. The X-ray structure of pypz^R(1)py is presented. Coordination of those ligands to MCl₂ fragments (M = Zn, Pd) gives rise to complexes of the type [MCl₂(pypz^R(n)py)] (M = Zn, Pd; R(n) = C₆H₄OC_nH_2n+1; n = 1, 12–18) (6–15) which were characterized and studied in the solid state and in solution. In these compounds the ligands were highly versatile as they were metal coordinated in a N,N-bidentate, N,N,N-tridentate fashion or N,N-bidentate fashion with an additional interaction via the third active nitrogen, this different behaviour depending on the metal fragment as well as on the state (solid or in solution) in which the complexes are. The X-ray structures of [ZnCl₂(pypz^R(1)py)] (6) and [PdCl₂(pypz^R(1)py)] (11) have also been determined showing distorted trigonal bipyramid and square-planar geometries at the zinc and palladium centres, respectively. In both cases the coordination environment has been confirmed by ¹³C-and ¹⁵N-NMR studies in solid state. In particular, two binuclear M–M stereoisomers “Z”- and “U”-shaped may be established for the Pd derivative. The ¹H and ¹³C-NMR studies in CDCl₃ solution of (6) and (11) indicate the presence of monomeric species which exhibit the same or different bidentate coordination of the ligand, respectively, than that observed in solid state. However, DMSO-d⁶ causes a total dissociation of the ligand in Zn derivatives, while for the related Pd complexes two monomeric linkage isomers (N2,N1′PdCl2 and N2,N1′′PdCl2) were detected.