Diorganoruthenium Complexes Incorporating Noninnocent [C6H2(CH2ER2)2-3,5]22- (E=N, P) Bis-Pincer Bridging Ligands: Synthesis, Spectroelectrochemistry, and DFT Studies

The dinuclear complex [(tpy)RuII(PCP?PCP)RuII(tpy)]Cl2 (bridging PCP-PCP ) 3,3',5,5'-tetrakis(diphenylphosphinomethyl) biphenyl, [C6H2(CH2PPh2)2-3,5]2 2-) was prepared via a transcyclometalation reaction of the bis-pincer ligand [PC(H)P-PC(H)P] and the Ru(II) precursor [Ru(NCN)(tpy)]Cl (NCN ) [C6H3(CH2NMe2)2-2,6]-) followed by a reaction with 2,2':6',2''-terpyridine (tpy). Electrochemical and spectroscopic properties of [(tpy)RuII(PCP?PCP)RuII- (tpy)]Cl2 are compared with those of the closely related [(tpy)RuII(NCN-NCN)RuII(tpy)](PF6)2 (NCN?NCN ) [C6H2(CH2- NMe2)2-3,5]2 2-) obtained by two-electron reduction of [(tpy)RuIII(NCN?NCN)RuIII(tpy)](PF6)4. The molecular structure of the latter complex has been determined by single-crystal X-ray structure determination. One-electron reduction of [(tpy)RuIII(NCN?NCN)RuIII(tpy)](PF6)4 and one-electron oxidation of [(tpy)RuII(PCP?PCP)RuII(tpy)]Cl2 yielded the mixed-valence species [(tpy)RuIII(NCN?NCN)RuII(tpy)]3??and [(tpy)RuIII(PCP?PCP)RuII(tpy)]3+, respectively. The comproportionation equilibrium constants Kc (900 and 748 for [(tpy)RuIII(NCN?NCN)RuIII(tpy)]4+ and [(tpy)RuII(PCP??PCP)RuII(tpy)]2+, respectively) determined from cyclic voltammetric data reveal comparable stability of the [RuIII-RuII] state of both complexes. Spectroelectrochemical measurements and near-infrared (NIR) spectroscopy were employed to further characterize the different redox states with special focus on the mixed-valence species and their NIR bands. Analysis of these bands in the framework of Hush theory indicates that the mixed-valence complexes [(tpy)RuIII(PCP-PCP)RuII(tpy)]3+ and [(tpy)RuIII(NCN-NCN)RuII(tpy)]3+ belong to strongly coupled borderline Class II/Class III and intrinsically coupled Class III systems, respectively. Preliminary DFT calculations suggest that extensive delocalization of the spin density over the metal centers and the bridging ligand exists. TD-DFT calculations then suggested a substantial MLCT character of the NIR electronic transitions. The results obtained in this study point to a decreased metal?metal electronic interaction accommodated by the double-cyclometalated bis-pincer bridge when strong s-donor NMe2 groups are replaced by weak s-donor, p-acceptor PPh2 groups