Ruthenium-bis-terpyridine Complex with two redox-asymmetric amine substituents: Potential-controlled reversal of the direction of charge-transfer

A ruthenium-bis-terpyridine complex [Ru(NPhtpy)(Ntpy)]²⁺ (2²⁺) with two redox-asymmetric amine units has been prepared, where NPhtpy is 4-(di-p-anisylaminophen-4-yl)-2,2:6,2′-terpyridine and Ntpy is 4-(di-p-anisylamino)-2,2:6,2′-terpyridine. This complex displays two consecutive redox couples at +0.82 and +1.02 V vs Ag/AgCl, which are assigned to the N^•+/0 processes of the amine components of the NPhtpy and Ntpy ligands, respectively. The mono-oxidized complex 2³⁺ obtained by oxidative electrolysis shows the presence of the charge transfer from ruthenium(II) to the oxidized aminium radical cation of the NPhtpy ligand (MN_NPhtpyCT) around 1000 nm. In the dioxidized form (2⁴⁺), the MN_NPhtpyCT transition decreased distinctly and an opposite charge transfer from ruthenium(II) to the oxidized aminium radical cation of the Ntpy ligand (MN_NtpyCT) appeared at 1380 nm. Complexes [Ru(NPhtpy)(tpy)]²⁺ (tpy is 2,2:6,2′-terpyridine), [Ru(Ntpy)(tpy)]²⁺, and [Ru(NPhtpy)₂]²⁺ have been prepared and studied for the purpose of comparison. TDDFT calculations show that the involvement of the intraligand charge transfer from both NPhtpy and Ntpy ligands is responsible for the enhancement of the visible absorptions of these complexes with respect to [Ru(tpy)₂]²⁺. DFT and TDDFT calculations have been performed on 2³⁺ and 2⁴⁺ to provide information on the spin distributions and the nature of the near-infrared absorptions. Complex 2³⁺ shows an isotropic EPR signal at room temperature, consistent with an unpaired electron localized on the nitrogen atom.