Synthesis and Characterization of New N-Alkylamino-3,5-diphenylpyrazole Ligands and Reactivity Toward PdII and PtII. Study of the cis-trans Isomerization

In this paper, the synthesis and characterization of two new N-alkylaminopyrazole ligands, 1-[2-(ethylamino)ethyl]-3, 5-diphenylpyrazole (dpea) and 1-[2-(octylamino)ethyl]-3,5-diphenylpyrazole (dpoa) are reported. The reaction of these ligands with [MCl2(CH3CN)(2)] (M=Pd-II, Pt-II) affords the following square planar complexes: cis-[MCl2(NN')](M=Pd-II: NN' = dpea, 1; dpoa, 2; M = Pt-II: NN' = dpea, 3; dpoa, 4). Reaction of [PdCl2(CH3CN)(2)] and dpea or dpoa in 1:2M:NN' molar ratio, in the presence of NaBF4, yields complexes [Pd(NN')(2)](BF4)(2) (NN' = dpea, [5](BF4)(2)); dpoa, [6](BF4)(2)). The solid-state structures of complexes 1, 3, and [5](BF4)(2) have been determined by single-crystal X-ray diffraction methods. In complexes 1 and 3, the dpea ligand is coordinated through the Npz and N-amino atoms to the metallic centre, which completes its coordination with two chlorine atoms in a cis disposition. For complex [5](BF4)(2), the crystal structure consists of cations involving a [Pd(N-pz)2(N-amino)(2)](2+) core with a cis disposition of the two dpea ligands in a square-planar geometry and BF-4 anions. Theoretical calculations were carried out to optimize the geometries of the cis and trans isomers of the [Pd(dpea)(2)](2+) cation and of the [Pd(dpea)(2)](BF4)(2) complex. The results show that the trans isomer is the most stable for [Pd(dpea)(2)](2+), in contrast with the cis stereochemistry observed in the crystal structure of [Pd(dpea)(2)](BF4)(2). The calculations also predict that in acetonitrile solution, the dissociation of this complex into the corresponding ions is thermodynamically favourable. The cis- trans isomerization process of [Pd(dpea)(2)](2+) in acetonitrile solution has been studied by NMR spectroscopy at different temperatures. These experimental results confirm that the trans isomer is the thermodynamically most stable form of the complexes [5](BF4)(2) and [6](BF4)(2).