Chemists triple down on metal helicates

Chemists have come far in using metal-assisted self-assembly of organic building blocks to construct molecules with three-dimensional architectures. For example, a team led by supramolecular chemistry specialists Pradyut Ghosh of the Indian Association for the Cultivation of Science and Christoph A. Schalley of Free University of Berlin has been matching various metal salts with a multidentate ligand it developed that is made up of a 2,2'-bipyridine core flanked by triazolyl-pyridine groups. This combination leads to formation of unusual trimetallic triple-stranded helicate complexes, (L₃M₃)⁶⁺, where L is the ligand and M is Fe, Co, Ni, Cu, or Zn or a combination of two of the metals. In continuing the work, the team has now systematically used electrospray ionization mass spectrometry to study the formation mechanism and thermodynamic stabilities of the helicates when mixtures of metals are used. The researchers have found that some metals outcompete others for binding sites, leading some of the helicates to be interconverted into others. In one case, they observed formation of (L₃FeZnCu)⁶⁺ (shown), the first example of such a complex containing three different metals. However, this trinuclear species only appears as an intermediate on the way to (L₃Cu₃)⁶⁺, which is the most stable isolable helicate (Inorg. Chem. 2017, DOI: 10.1021/acs.inorgchem.7b01980). Ghosh and Schalley say the magnetic properties of the trimetallic helicates will be of interest for developing smart materials, in particular for the versions having a combination of diamagnetic and paramagnetic metals.