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 (MNNPhtpyCT) around 1000 nm. In the dioxidized form (24+), the MNNPhtpyCT transition decreased distinctly and an opposite charge transfer from ruthenium­(II) to the oxidized aminium radical cation of the Ntpy ligand (MNNtpyCT) 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.

本研究制备了一种含有两个氧化还原不对称胺单元的双三联吡啶钌配合物[Ru(NPhtpy)(Ntpy)]²⁺(22+)，其中NPhtpy为4'-(4-二对茴香氨基苯基)-2,2':6',2''-三联吡啶，Ntpy为4'-(二对茴香氨基)-2,2':6',2''-三联吡啶。该配合物在相对于银/氯化银（Ag/AgCl）参比电极的+0.82 V和+1.02 V电位处呈现两个连续的氧化还原峰，分别归属于NPhtpy与Ntpy配体中胺组分的N•+/0氧化还原过程。通过氧化电解制备得到的单氧化态配合物23+在约1000 nm波长处，出现了从钌(II)中心向NPhtpy配体氧化胺鎓自由基阳离子的电荷转移吸收（MNNPhtpyCT）。在双氧化态形式(24+)中，MNNPhtpyCT跃迁显著减弱，同时在1380 nm波长处出现了从钌(II)中心向Ntpy配体氧化胺鎓自由基阳离子的反向电荷转移吸收（MNNtpyCT）。为进行对比研究，本研究还制备并表征了[Ru(NPhtpy)(tpy)]²⁺（tpy为2,2':6',2''-三联吡啶）、[Ru(Ntpy)(tpy)]²⁺以及[Ru(NPhtpy)₂]²⁺三种配合物。含时密度泛函理论（Time-Dependent Density Functional Theory, TDDFT）计算结果表明，相较于[Ru(tpy)₂]²⁺，NPhtpy与Ntpy配体参与的配体内电荷转移过程，是该类配合物可见光区吸收强度增强的关键原因。针对23+和24+开展了密度泛函理论（Density Functional Theory, DFT）与含时密度泛函理论计算，以获取其自旋分布及近红外吸收性质的相关信息。配合物23+在室温下呈现各向同性电子顺磁共振（Electron Paramagnetic Resonance, EPR）信号，与未成对电子定域于氮原子的理论结论一致。