The co-ordination chemistry of mixed pyridine–phenol and phenanthroline–phenol ligands; effects of π-stacking interactions and ligand rigidity on complex structures

Abstract

The crystal structures of [Cu₂L²₂(µ-MeCO₂)][PF₆], [Ni₂L²₂(µ-MeCO₂)(MeCN)₂][PF₆] and [{NiL¹(HL¹)}₂][PF₆]₂· 0.5MeCN·0.5Et₂O [HL¹ is the N,N,O-terdentate ligand 6-(2-hydroxyphenyl)-2,2′-bipyridine; HL² is the new ligand 2-(2-hydroxyphenyl)-1,10-phenanthroline, an analogue of HL¹ containing the more rigid 1,10-phenanthroline unit instead of 2,2′-bipyridyl] were determined. Both [Cu₂L²₂(µ-MeCO₂)][PF₆] and [Ni₂L²₂(µ-MeCO₂)(MeCN)₂][PF₆] contain two ML² units, in which the ligand is deprotonated and near-planar, stacked side by side such that the phenolates bridge resulting in M₂(µ-O)₂ cores in each case, with an additional bridging µ-1,3-acetate. The Ni₂(µ-O)₂ core is considerably contracted compared to the Cu₂(µ-O)₂ core, as the apical Cu–O(bridging phenolate) bonds are rather long (2.449 Å) due to Jahn–Teller distortion. The proximity of the two ML² units results in π-stacking interactions between overlapping sections of the ligands. In contrast, [{NiL¹(HL¹)}₂][PF₆]₂· 0.5MeCN·0.5Et₂O consists of two pseudo-octahedral NiL¹₂ units held together by two strong intermolecular hydrogen bonds between phenolate residues. As a result, sections of the ligands in each monomeric unit are brought into close contact resulting in additional inter-ligand stacking interactions which are optimised by considerable distortions of the ligands from planarity. There are thus two types of non-covalent intermolecular interaction in the dimer which appear to complement each other in a co-operative manner. The differences between the structures are discussed in relation to the different stereoelectronic preferences of the metal ions, and the different steric and electronic properties of the ligands.