Steric Effects in the Self-Assembly of Palladium Complexes with Chelating Diamine Ligands

Graphical Abstract

The ratio of the trimeric and tetrameric palladium complexes has shown a strong dependence on the steric properties ofthe chelating diamines. The trimeric [Pd(en)(4,4′-bpy)]₃⁶⁺ cation has been observed for the first time.

Abstract

The influence of the steric properties of chelating diamines on the self-assembly of [Pd(N^∩N)](NO₃)₂ complexes with 4,4′-bpy as bridging ligand has been studied by ¹H NMR spectroscopy and X-ray diffraction (N^∩N = N,N,N′,N′-tetramethylethylenediamine, N,N,N′,N′-tetraethylethylenediamine, 1,3-diaminopropane, N,N′-dimethylpiperazine, homopiperazine and ethylenediamine). Based on crystallographic data, the steric bulk of the chelating amines in the [Pd(N^∩N)]²⁺ cations has been characterised in two different ways. A temperature- and concentration-sensitive trimer/tetramer equilibrium was observed by NMR spectroscopy with every diamine, in which the trimeric species dominated when the chelating amine was the bulky tetraethylethylenediamine. Although the formation of tetramers was preferred with diamines of decreasing steric demands, trimeric aggregates could be observed even with the least congested homopiperazine and ethylenediamine complexes. The distribution of the tectons between the two constituents follows the trend predicted by the steric features of the coordinated diamine ligands. Diffusion NMR spectroscopy has proved to be an efficient tool in the identification of self-assembled species. The trimer/tetramer equilibrium established with tetramethylethylenediamine as chelating ligand has been observed in solution by using wide-angle X-ray diffraction. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)