Inductive influence of 4′-terpyridyl substituents on redox and spin state properties of iron(II) and cobalt(II) bis-terpyridyl complexes

A series of iron and cobalt bis-terpyridine (terpy) complexes were prepared with the general formula [M(R-terpy)2](PF6)2, where M represents Co(II) and Fe(II), and R is the following terpyridine substituents in order of increasing electron-withdrawing behavior [(C4H8)N, (C4H9)NH, HO, CH3O, CH3-phenyl, H, Cl, CH3SO, CH3SO2]. The complexes were prepared to investigate the extent of redox and spin state control that is attainable by simply varying the electron donating/withdrawing influence using a single substituent site on the terpyridine ligand. Cyclic voltammetry was used to assess the substituents influence on the M(III/II) redox couple. A plot of the M(III/II) redox potential (E1/2) versus the electron donating/withdrawing nature of the substituents (Hammett constants), shows a strong linear trend for both metals; however, the substituents were observed to have a stronger influence on the Fe(III/II) couple. Solution magnetic susceptibility measurements at room temperature were carried out using standard NMR methodology (modified Evans method) where all of the Fe(II) complexes exhibited a diamagnetic, low spin (S = 0) behavior. In the cobalt series where R = H for [Co(R-terpy)2]2+, the complex is known to be near the spin cross-over where the room temperature effective magnetic moment (μeff) in solution is ≈3.1 Bohr magnetons; however, in this study the μeff is observed to vary between 2.7 and 4.1 Bohr magnetons depending on the R-substituent.