Electron donating substituent effects on redox and spin state properties of iron(II) bis-terpyridyl complexes

The synthesis of iron(II) terpyridyl (trpy) complexes was achieved with the general formula [Fe(trpy)(2-x)((R)(3)trpy)(x)] (PF6)(2), where R represents CH3O and (Et)(2)N. Both the methoxy and diethylamino substituents are electron donating in nature, where the latter is a new derivative of terpyridine. Cyclic voltammetry was used to assess the substituents' influence on the Fe(III/II) couple in which an average shift in potential (Delta E-1/2) of 0.06 volts per methoxy group, and 0.18 volts for each diethylamino group, was exhibited. Solution magnetic susceptibility measurements were carried out with standard NMR methodology. All of the Fe(II) complexes, except [Fe((Et2N)(3)trpy)(2)](2+) exhibited an expected low spin (S = 0) behavior. The [Fe((Et2N)(3)trpy)(2)](2+) complex was observed to have a room temperature paramagnetic moment (mu(eff)) of 1.6 BM indicating thermal population of the high spin (S=2) state. Since [Co(trpy)(2)](2+) is known to be near the spin crossover (mu(eff) = 3.2 BM), the [Co((CH3O)(3)trpy)(2)](2+) was prepared in order to measure the influence methoxy substituents would have on mu(eff); a significant shift was observed (mu(eff) = 4.2 BM). The results of this study indicate that electron donating para-substituents on terpyridine affect the metal-ligand pi-bonding interactions (by increasing the ligand's pi-donor properties at the expense of weakening its pi-acceptor capabilities) to a greater extent than the sigma-interactions. This results in a decrease of the ligand held splitting. (C) 1998 Elsevier Science S.A. All rights reserved.