Rhodium, palladium and platinum complexes of tris(pyridylalkyl)amine and tris(benzimidazolylmethyl)amine N4-tripodal ligands

Abstract

To investigate the influence of a potentially N₄-tripodal amine ligand on the structure and internal exchange processes of its complexes with late transition metals, five rhodium, six palladium and two platinum complexes have been prepared from seven alkyl-bridged N-heterocyclic amine tripodal ligands: tris(2-pyridylmethyl)amine, (2-(2-pyridylethyl))bis(2-pyridylmethyl)amine, bis(2-(2-pyridylethyl))-2-pyridylmethylamine, bis(2-(2-pyridylethyl))amine, ((6-(hydroxymethyl)-2-pyridyl)methyl)bis(2-pyridylmethyl)amine, tris(2-benzimidazolylmethyl)amine (tbima) and tris(3-ethyl-2-benzimidazolylmethyl)amine. Single-crystal X-ray diffraction studies were completed for ten complexes: the d⁶-rhodium(III) complexes are octahedral with κ⁴N-bound ligands, whereas the d⁸-palladium(II) and d⁸-platinum(II) complexes are square planar, κ³N-bound by the tripodal ligand with a dangling N-donor leg, except for the unusual [Pd₂(tbima)₂Cl₂]Cl₂ dimer in which each palladium(II) ion is square planar and bound by two benzimidazole legs from one tbima ligand, one leg from the other tbima ligand and a chloride ancillary ligand. Cation bilayers are a common structural motif in the crystal structures. Variable-temperature ¹H NMR studies reveal exchange occurs between the coordinated and dangling N-donor legs in the palladium and platinum complexes. Exchange free energy (ΔG^‡_c) values have been calculated and some general rules governing the favoured complex structures and exchange pathways elucidated. The palladium(II) and platinum(II) complexes of a ligand with an pyridylethyl leg are unstable with respect to elimination of vinylpyridine.