Diorganoruthenium complexes incorporating noninnocent [C(6)H(2)(CH(2)ER(2))(2)-3,5](2)(2-) (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]22-) 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(CH2NMe2)2-3,5]22-) 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 sigma-donor NMe2 groups are replaced by weak sigma-donor, pi-acceptor PPh2 groups.