2,4,6-Tris(2-pyridyl)-1,3,5-triazine (tptz)-Derived [RuII(tptz)(acac)(CH3CN)]+and Mixed-Valent [(acac)2RuIII{(μ-tptz-H+)-}RuII(acac)(CH3CN)]+

Abstract

Mononuclear [Ru(II)(tptz)(acac)(CH3CN)]ClO4 ([1]ClO4) and mixed-valent dinuclear [(acac)2Ru(III){(mu-tptz-Eta+)-}Ru(II)(acac)(CH3CN)]ClO4 ([5]ClO4; acac = acetylacetonate) complexes have been synthesized via the reactions of Ru(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 [1]ClO4, tptz binds with the Ru(II) ion in a tridentate N,N,N mode (motif A), whereas in [5]ClO4, tptz bridges the metal ions unsymmetrically via the tridentate neutral N,N,N mode with the Ru(II) center and cyclometalated N,C- state with the Ru(III) site (motif F). The activation of the coordinated nitrile function in [1]ClO4 and [5]ClO4 in the presence of ethanol and alkylamine leads to the formation of iminoester ([2]ClO4 and [7]ClO4) and amidine ([4]ClO4) derivatives, respectively. Crystal structure analysis of [2]ClO4 reveals the formation of a beautiful eight-membered water cluster having a chair conformation. The cluster is H-bonded to the pendant pyridyl ring N of tptz and also with the O atom of the perchlorate ion, which, in turn, makes short (C-H- - - - -O) contacts with the neighboring molecule, leading to a H-bonding network. The redox potentials corresponding to the Ru(II) state in both the mononuclear {[(acac)(tptz)Ru(II)-NC-CH3]ClO4 ([1]ClO4) >> [(acac)(tptz)Ru(II)-NH=C(CH3)-OC2H5]ClO4 ([2]ClO4) > [(acac)(tptz)Ru(II)-NH2-C6H4(CH3)]ClO4 ([3]ClO4) > [(acac)(tptz)Ru(II)-NH=C(CH3)-NHC2H5]ClO4 ([4]ClO4)} and dinuclear {[(acac)2Ru(III){(mu-tptz-H+)-}Ru(II)(acac)(NC-CH3)]ClO4 ([5]ClO4), [(acac)2Ru(III){(mu-tptz-H+(N+-O-)2)-}Ru(II)(acac)(NC-CH3)]ClO4 ([6]ClO4), [(acac)2Ru(III){(mu-tptz-H+)-}Ru(II)(acac)(NH=C(CH3)-OC2H5)]ClO4 ([7]ClO4), and [(acac)2Ru(III){(mu-tptz-Eta+)-}Ru(II)(acac)(NC4H4N)]ClO4 ([8]ClO(4))} 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 invariant. The mixed-valent Ru(II)Ru(III) species ([5]ClO4-[8]ClO4) exhibits high comproportionation constant (Kc) values of 1.1 x 10(12)-2 x 10(9), 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 the visible region and ligand-based transitions in the UV region. In spite of reasonably high K(c) values for [5]ClO4-[8]ClO4, the expected intervalence charge-transfer transitions did not resolve in the typical near-IR region up to 2000 nm. The paramagnetic Ru(II)Ru(III) species ([5]ClO4-[8]ClO4) displays rhombic electron paramagnetic resonance (EPR) spectra at 77 K (g approximately 2.15 and Deltag approximately 0.5), typical of a low-spin Ru(III) ion in a distorted octahedral environment. The one-electron-reduced tptz complexes [Ru(II)(tptz.-)(acac)(CEta3CN)] (1) and [(acac)2Ru(III){(mu-tptz-Eta+).2-}Ru(II)(acac)(CH3CN)] (5), however, show a free-radical-type EPR signal near g = 2.0 with partial metal contribution.