Charge-induced spin crossover in an iron(II) bis-terpyridine complex

We induced the SCO phenomenon in an iron(II) bis-terpyridyl species by introducing a positive charge through methylation on the pendant pyridyl groups of the bis-4’-(pyridin-4-yl)-2,2’:6’,2’’-terpyridine ligand. Complexes [Fe(pyterpy)2](PF6)2 (1) and [Fe(Mepyterpy)2](PF6)4 (2) were synthesized and SC-XRD analysis confirmed the proposed formulations revealing average FeN bond distances consistent with a low-spin state. Electronic and 1H NMR spectroscopies of H2O and MeCN solutions of 1 and 2 from 20°C to 80°C display the onset of a spin transition above 50°C for 2, whereas no spectral change was observed for 1. DFT calculations at the M06L/def2-SVP level in vacuum and implicit solvent (MeCN and H2O), performed for the cations of each product, yielded trends on estimated transition temperatures and energy differences that agree with the experimental results that indicate success of the proposal. Efforts are also in progress to obtain iron(III) analogs to investigate the effect of positive charge on a d⁵ system.