A Strain-Deformation Nexus within Pincer Ligands: Application to the Spin States of Iron(II) Complexes

The substitution of a pyrrolide ring for one (or more) pyridyl rings within the ubiquitous terpyridine (tpy, A) scaffold results in more open geometries of the pyridine-pyrrolide chelate ligands. DFT calculations (B3LYP-GD3BJ/6-31G**) demonstrate that the more open geometries of the unbound ligands are mismatched with the pinched in geometries required to chelate transition metal ions (e.g., Zn2+). The strain which builds within these ligands (Delta E-L(strain)) as they bind transition metal ions can be related to changes in a single geometric parameter: the separation between the two terminal N atoms (rho). This relationship applies more generally to other three-ringed tridentate pincer ligands, including those with different donor groups. The approach was applied to homoleptic iron(II) complexes to investigate the contribution of the steric effects operating within the ligands to the different magnetic properties, including spin crossover (SCO) activities, of these systems.