mer and fac isomerism in tris chelate diimine metal complexes

Abstract

In this perspective, we highlight the issue of meridional (mer) and facial (fac) orientation of asymmetrical diimines in tris-chelate transition metal complexes. Diimine ligands have long been the workhorse of coordination chemistry, and whilst there are now good strategies to isolate materials where the inherent metal centered chirality is under almost complete control, and systematic methodologies to isolate heteroleptic complexes, the conceptually simple geometrical isomerism has not been widely investigated. In systems where the two donor atoms are significantly different in terms of the σ-donor and π-accepting ability, the fac isomer is likely to be the thermodynamic product. For the diimine complexes with two trigonal planar nitrogen atoms there is much more subtlety to the system, and external factors such as the solvent, lattice packing and the various steric considerations play a delicate role in determining the observed and isolable product. In this article we discuss the possibilities to control the isomeric ratio in labile systems, consider the opportunities to separate inert complexes and discuss the observed differences in their spectroscopic properties. Finally we report on the ligand orientation in supramolecular systems where facial coordination leads to simple regular structures such as helicates and tetrahedra, but the ability of the ligand system to adopt a mer orientation enables self-assembled structures of considerable beauty and complexity.

Imposing control on self-assembly: rational design and synthesis of a mixed-metal, mixed-ligand coordination cage containing four types of component

“…Our initial choice of metal ions is Ru( ii ) for the kinetically inert vertices, given the straightforward and well-established synthesis and purification of stable tris-chelate complexes as their pure fac and mer isomers; 10,11 and Cd( ii ) for the kinetically labile vertices to facilitate 1 H NMR analysis. If we prepare mer -[RuL 3 ] 2+ units (blue in Fig.…”

“…In conclusion, the stepwise synthetic methodology for preparation of heterometallic cages based on initial preparation of kinetically inert fragments, for which we reported the first examples recently, 11 has been substantially extended. We have performed a rational two-step synthesis of the hexadecanuclear mixed-metal, mixed-ligand cage [Ru 4 Cd 12 (L ph ) 12 (L naph ) 12 ](PF 6 ) 7 (BF 4 ) 25 , which contains two types of ligand and two types of metal ion, all at pre-determined positions within the superstructure that are dictated by the synthesis.…”

Geometric isomerism in coordination cages based on tris-chelate vertices: a tool to control both assembly and host/guest chemistry

“…It is well documented that post-synthetic separation of mer and fac isomers is generally not a trivial process; 8 following preparation of [Ru(L Bn ) 3 ](PF 6 ) 2 as the expected 3 : 1 mixture of mer and fac isomers, this became increasingly obvious. Reported methods of such isomeric separation by Piguet 21 and Fletcher 22 by chromatography or fractional crystallisation were unsuccessful.…”

A Tripodal Ruthenium(II) Polypyridyl Complex with pH Controlled Emissive Quenching

“…The formation of tris‐diimine podand type structures with Ru II is however more challenging than with first row transition metal complexes. Due to the stability of the 4d 6 low‐spin configuration, the kinetically, rather than the thermodynamically, favored product is typically isolated, and there can also be complications of mer verses fac coordination geometries . This often results in a mixture of species, with high order nuclearity, difficult purification procedures and low yields.…”

“…Due to the stability of the 4d 6 low-spin configuration, the kinetically, rather than the thermodynamically, favored product is typically isolated, and there can also be complications of mer verses fac coordination geometries. [18] This often results in a mixture of species, with high order nuclearity, difficult purification procedures and low yields. For example, a tris(bipyridine-imidazolium) ligand reported by Sato, gave a 91 % yield with Fe 2+ , but only 46 % with Ru 2+ , whilst a more recent example Nabeshima and co-workers show similar differences, [1l] and the systems reported by Oyler et al, [19] and Weizman et al [20] only achieved yields in the region of 25 % despite using high reaction temperatures.…”