2,4,6-Tris(2-pyridyl)-1,3,5-triazine (tptz)-Derived [Ru^II(tptz)(acac)(CH₃CN)]⁺\nand Mixed-Valent [(acac)₂Ru^III{(<i>μ</i>-tptz-H⁺)^-}Ru^II(acac)(CH₃CN)]⁺^†

Abstract

Mononuclear [Ru^II(tptz)(acac)(CH₃CN)]ClO₄ ([<b>1</b>]ClO₄) and mixed-valent dinuclear [(acac)₂Ru^III{(<i>μ</i>-tptz-<i>Η</i>⁺)^-}Ru^II(acac)(CH₃CN)]ClO₄ ([<b>5</b>]ClO₄; acac = acetylacetonate) complexes have been synthesized via the reactions of\nRu^II(acac)₂(CH₃CN)₂ and 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz), in 1:1 and 2:1 molar ratios, respectively. In\n[<b>1</b>]ClO₄, tptz binds with the Ru^II ion in a tridentate N,N,N mode (motif <b>A</b>), whereas in [<b>5</b>]ClO₄, tptz bridges the metal\nions unsymmetrically via the tridentate neutral N,N,N mode with the Ru^II center and cyclometalated N,C^- state with\nthe Ru^III site (motif <b>F</b>). The activation of the coordinated nitrile function in [<b>1</b>]ClO₄ and [<b>5</b>]ClO₄ in the presence of\nethanol and alkylamine leads to the formation of iminoester ([<b>2</b>]ClO₄ and [<b>7</b>]ClO₄) and amidine ([<b>4</b>]ClO₄) derivatives,\nrespectively. Crystal structure analysis of [<b>2</b>]ClO₄ reveals the formation of a beautiful eight-membered water cluster\nhaving a chair conformation. The cluster is H-bonded to the pendant pyridyl ring N of tptz and also with the O atom\nof the perchlorate ion, which, in turn, makes short (C−H- - - - -O) contacts with the neighboring molecule, leading\nto a H-bonding network. The redox potentials corresponding to the Ru^II state in both the mononuclear {[(acac)(tptz)Ru^II−N⋮C−CH₃]ClO₄ ([<b>1</b>]ClO₄) ≫ [(acac)(tptz)Ru^II−NHC(CH₃)−OC₂H₅]ClO₄ ([<b>2</b>]ClO₄) > [(acac)(tptz)Ru^II−NH₂−C₆H₄(CH₃)]ClO₄ ([<b>3</b>]ClO₄) > [(acac)(tptz)Ru^II−NHC(CH₃)−NHC₂H₅]ClO₄ ([<b>4</b>]ClO₄)} and dinuclear {[(acac)₂Ru^III{(<i>μ</i>-tptz-H⁺)^-}Ru^II(acac)(N⋮C−CH₃)]ClO₄ ([<b>5</b>]ClO₄), [(acac)₂Ru^III{(<i>μ</i>-tptz-H⁺(N⁺−O^-)₂)^-}Ru^II(acac)(N⋮C−CH₃)]ClO₄\n([<b>6</b>]ClO₄), [(acac)₂Ru^III{(<i>μ</i>-tptz-H⁺)^-}Ru^II(acac)(NHC(CH₃)−OC₂H₅)]ClO₄ ([<b>7</b>]ClO₄), and [(acac)₂Ru^III{(<i>μ</i>-tptz-<i>Η</i>⁺)^-}Ru^II(acac)(NC₄H₄N)]ClO₄ ([<b>8</b>]ClO₄)} complexes vary systematically depending on the electronic nature of the coordinated sixth ligands. However, potentials involving the Ru^III center in the dinuclear complexes remain more or less\ninvariant. The mixed-valent Ru^IIRu^III species ([<b>5</b>]ClO₄−[<b>8</b>]ClO₄) exhibits high comproportionation constant (<i>K</i>_c) values\nof 1.1 × 10¹²−2 × 10⁹, with substantial contribution from the donor center asymmetry at the two metal sites. Complexes display Ru^II- and Ru^III-based metal-to-ligand and ligand-to-metal charge-transfer transitions, respectively, in\nthe visible region and ligand-based transitions in the UV region. In spite of reasonably high <i>K</i>_c values for [<b>5</b>]ClO₄−[<b>8</b>]ClO₄, the expected intervalence charge-transfer transitions did not resolve in the typical near-IR region up to\n2000 nm. The paramagnetic Ru^IIRu^III species ([<b>5</b>]ClO₄−[<b>8</b>]ClO₄) displays rhombic electron paramagnetic resonance\n(EPR) spectra at 77 K (〈<i>g</i>〉 ∼ 2.15 and Δ<i>g</i> ∼ 0.5), typical of a low-spin Ru^III ion in a distorted octahedral environment.\nThe one-electron-reduced tptz complexes [Ru^II(tptz^•-)(acac)(CΗ₃CN)] (<b>1</b>) and [(acac)₂Ru^III{(<i>μ</i>-tptz-<i>Η</i>⁺)^•2-}Ru^II(acac)(CH₃CN)] (<b>5</b>), however, show a free-radical-type EPR signal near <i>g </i>= 2.0 with partial metal contribution.