Electronic effect of substituent groups on the oxidation and reduction of bis(2,2′:6′,2″-terpyridine)ruthenium

Abstract

A series of bis(tpy)ruthenium(II) (tpy = differently substituted 2,2′:6′,2″-terpyridine) were prepared. The substituents were chosen to range from electron donating (pyrrolidinyl, hydroxy, tert-butyl, methoxy, phenyl) to electron withdrawing (Cl, Br). Electrochemistry results show that the complex with the most electron donating group in the series, [Ru(4′-(N-pyrrolidinyl)-2,2′:6′,2″-terpyridine)2]2+, oxidizes at 0.363 V vs the redox couple of ferrocene (Fc/Fc+). The oxidation of [Ru(4′-chloro-2,2′:6′,2″-terpyridine)2]2+, containing the electron withdrawing chlorine group, is much higher, 0.992 V vs Fc/Fc+. Theoretical density functional theory calculations confirmed that the oxidation and reduction of bis(tpy)ruthenium(II) are metal and ligand based, respectively. Graphs comparing the experimentally measured redox potentials and theoretically calculated energies, charges, potentials and various global and local reactivity parameters, all show linear relationships associated with the donating/withdrawing nature of the substituents as quantified by Hammett constants. All these relationships may be utilized to quantify the electronic influence of substituent groups on the redox chemistry of bis(tpy)ruthenium(II) complexes. Linear relationships obtained may also be utilized in the design of bis(tpy)ruthenium(II) type of molecules with a required oxidation or reduction potential. DFT calculations also indicate the potential utilization of these complexes as dye sensitizers in dye-sensitized solar cells.