Francisca N. Rein

Electronic Structure and Spectroscopy of [Ru(tpy)2]2+, [Ru(tpy)(bpy)(H2O)]2+, and [Ru(tpy)(bpy)(Cl)]+

We use a combined, theoretical and experimental, approach to investigate the spectroscopic properties and electronic structure of three ruthenium polypyridyl complexes, [Ru(tpy)(2)](2+), [Ru(tpy)(bpy)(H(2)O)](2+), and [Ru(tpy)(bpy)(Cl)](+) (tpy = 2,2:6,2-terpyridine and bpy = 2,2-bipyridine) in acetone, dichloromethane, and water. All three complexes display strong absorption bands in the visible region corresponding to a metal-to-ligand-charge-transfer (MLCT) transition, as well as the emission bands arising from the lowest lying (3)MLCT state. [Ru(tpy)(bpy)(Cl)](+) undergoes substitution of the Cl(-) ligand by H(2)O in the presence of water. Density functional theory (DFT) calculations demonstrate that the triplet potential energy surfaces of these molecules are complicated, with several metal-centered ((3)MC) and (3)MLCT states very close in energy. Solvent effects are included in the calculations via the polarizable continuum model as well as explicitly, and it is shown that they are critical for proper characterization of the triplet excited states of these complexes.

Catalytic Photooxidation of Alcohols by an Unsymmetrical Tetra(pyridyl)pyrazine-Bridged Dinuclear Ru Complex

The dinuclear complexes [(tpy)Ru(tppz)Ru(bpy)(L)](n+) (where L is Cl(-) or H(2)O, tpy and bpy are the terminal ligands 2,2:6,2-terpyridine and 2,2-bipyridine, and tppz is the bridging backbone 2,3,5,6-tetrakis(2-pyridyl)pyrazine) were prepared and structurally and electronically characterized. The mononuclear complexes [(tpy)Ru(tppz)](2+) and [(tppz)Ru(bpy)(L)](m+) were also prepared and studied for comparison. The proton-coupled, multi-electron photooxidation reactivity of the aquo dinuclear species was shown through the photocatalytic dehydrogenation of a series of primary and secondary alcohols. Under simulated solar irradiation and in the presence of a sacrificial electron acceptor, the photoactivated chromophore-catalyst complex (in aqueous solutions at room temperature and ambient pressure conditions) can perform the visible-light-driven conversion of aliphatic and benzylic alcohols into the corresponding carbonyl products (i.e., aldehydes or ketones) with 100% product selectivity and several tens of turnover cycles, as probed by NMR spectroscopy and gas chromatography. Moreover, for aliphatic substrates, the activity of the photocatalyst was found to be highly selective toward secondary alcohols, with no significant product formed from primary alcohols. Comparison of the activity of this tppz-bridged complex with that of the analogue containing a back-to-back terpyridine bridge (tpy-tpy, i.e., 6,6-bis(2-pyridyl)-2,2:4,4:2,2-quaterpyridine) demonstrated that the latter is a superior photocatalyst toward the oxidation of alcohols. The much stronger electronic coupling with significant delocalization across the strongly electron-accepting tppz bridge facilitates charge trapping between the chromophore and catalyst centers and therefore is presumably responsible for the decreased catalytic performance.

Synthesis, Structure, and Electronic Properties of a Dimer of Ru(bpy)2 Doubly Bridged by Methoxide and Pyrazolate

The heterobridged dinuclear complex cis,cis-[(bpy) 2Ru(mu-OCH 3)(mu-pyz)Ru(bpy) 2] (2+) ( 1; bpy = 2,2-bipyridine; pyz = pyrazolate) was synthesized and isolated as a hexafluorophosphate salt. Its molecular structure was fully characterized by X-ray crystallography, (1)H NMR spectroscopy, and ESI mass spectrometry. The compound 1.(PF 6) 2 (C 44H 38F 12N 10OP 2Ru 2) crystallizes in the monoclinic space group P2 1/ c with a = 13.3312(4) A, b = 22.5379(6) A, c = 17.2818(4) A, beta = 99.497(2) degrees , V = 5121.3(2) A (3), and Z = 4. The meso diastereoisomeric form was exclusively found in the crystal structure, although the NMR spectra clearly demonstrated the presence of two stereoisomers in solution (rac and meso forms at approximately 1:1 ratio). The electronic properties of the complex in acetonitrile were investigated by cyclic voltammetry and UV-vis and NIR-IR spectroelectrochemistries. The stepwise oxidation of the Ru (II)-Ru (II) complex into the mixed-valent Ru (II)-Ru (III) and fully oxidized Ru (III)-Ru (III) states is fully reversible on the time scale of the in situ (spectro)electrochemical measurements. The mixed-valent species displays strong electronic coupling, as evidenced by the large splitting between the redox potentials for the Ru(III)/Ru(II) couples (Delta E 1/2 = 0.62 V; K c = 3 x 10 (10)) and the appearance of an intervalence transfer (IT) band at 1490 nm that is intense, narrow, and independent of solvent. Whereas this salient band in the NIR region originates primarily from highest-energy of the three IT transitions predicted for Ru(II)-Ru(III) systems, a weaker absorption band corresponding to the lowest-energy IT transition was clearly evidenced in the IR region ( approximately 3200 cm (-1)). The observation of totally coalesced vibrational peaks in the 1400-1650 cm (-1) range for a set of five bpy spectator vibrations in Ru (II)-Ru (III) relative to Ru (II)-Ru (II) and Ru (III)-Ru (III) provided evidence for rapid electron transfer and valence averaging on the picosecond time scale. Other than a relatively short Ru...Ru distance (3.72 A for the crystalline Ru (II)-Ru (II) complex), the extensive communication between metal centers is attributed mostly to the pi-donor ability of the bridging ligands (pyz, OMe) combined with the pi-acceptor ability of the peripheral (bpy) ligands.

Investigation of formally zerovalent Triphos iron complexes

The reduction of Triphos [PhP(CH(2)CH(2)PPh(2))(2)] iron halide complexes has been explored, yielding formally zerovalent (κ(3)-Triphos)Fe(κ(2)-Triphos) and (κ(3)-Triphos)Fe(κ(2)-Bpy). Electrochemical analysis, coupled with the metrical parameters of (κ(3)-Triphos)Fe(κ(2)-Bpy), reveal an electronic structure consistent with a π-radical monoanion bipyridine chelate that is antiferromagnetically coupled to a low spin, Fe(I) metal center.

(4'-Ethynyl-2,2':6',2''-terpyridine)(2,2':6',2''-terpyridine)-ruthenium(II) bis-(hexa-fluoridophosphate) acetonitrile disolvate.

The title heteroleptic bis-terpyridine complex, [Ru(C15H11N3)(C17H11N3)](PF6)2·2CH3CN, crystallized from an acetonitrile solution as a salt containing two hexafluoridophosphate counter-ions and two acetonitrile solvent molecules. The RuII atom has a distorted octahedral geometry due to the restricted bite angle [157.7u2005(3)°] of the two mer-arranged N,N′,N′′-tridendate ligands, viz. 2,2′:6′,2′′-terpyridine (tpy) and 4′-ethynyl-2,2′:6′,2′′-terpyridine (tpy′), which are essentially perpendicular to each other, with a dihedral angle of 87.75u2005(12)° between their terpyridyl planes. The rod-like acetylene group lies in the same plane as its adjacent terpyridyl moiety, with a maximum deviation of only 0.071u2005(11)u2005Å from coplanarity with the pyridine rings. The mean Ru—N bond length involving the outer N atoms trans to each other is 2.069u2005(6)u2005Å at tpy and 2.070u2005(6)u2005Å at tpy′. The Ru—N bond length involving the central N atom is 1.964u2005(6)u2005Å at tpy and 1.967u2005(6)u2005Å at tpy′. Two of the three counter anions were refined as half-occupied.

(2,2′-Bi­pyridine)­chlorido­[diethyl (2,2′:6′,2′′-terpyridin-4-yl)phospho­nate]ruthenium(II) hexa­fluorido­phosphate aceto­nitrile/water solvate

The cationic complex in the title compound, [RuCl(C10H8N2)(C19H20N3O3P)]PF6·0.83CH3CN·0.17H2O, is a water-oxidation precatalyst functionalized for TiO2 attachment via terpyridine phosphonate. The The RuII atom in the complex has a distorted octahedral geometry due to the restricted bite angle [159.50u2005(18)°] of the terpyridyl ligand. The dihedral angle between the least-squares planes of the terpyridyl and bipyridyl moieties is 86.04u2005(14)°. The mean Ru—N bond length for bipyridine is 2.064u2005(5)u2005Å, with the bond opposite to Ru—Cl being 0.068u2005Å shorter. For the substituted terpyridine, the mean Ru—N distance involving the outer N atoms trans to each other is 2.057u2005(6)u2005Å, whereas the bond length involving the central N atom is 1.944u2005(5)u2005Å. The Ru—Cl distance is 2.4073u2005(15)u2005Å. The P atom of the phosphonate group lies in the same plane as its adjacent pyridyl ring, with the ordinary character of the bond between P and Ctpy [1.801u2005(6)u2005Å] allowing for free rotation of the terpyridine substituent around the P—Ctpy axis. The acetonitrile solvent molecule was refined to be disordered with two water molecules; occupancies for the acetontrile and water molecules were 0.831u2005(9) and 0.169u2005(9), respectively. Also disordered was the PF6 − counter-ion (over three positions) and one of the ethoxy substituents (with two positions). The crystal structure shows significant intra- and intermolecular H⋯X contacts, especially some involving the Cl− ligand.

Crystal structure of a mononuclear RuII complex with a back-to-back terpyridine ligand: [RuCl(bpy)(tpy–tpy)]+

In this first crystal structure of an Ru complex with 6′,6-bis(pyridin-2-yl)-2,2′:4′,4:2,2’-quaterpyridine, a ‘half’ of the ligand (one of the two terpyridyl units) is N^N^N mer-coordinated, whereas the other is free and adopts a trans,trans conformation about the interannular C—C bonds. The crystal packing features π–π stacking interactions between tpy–tpy ligands.

Structure, spectroscopy and electrochemistry of the bis(2,2′-bipyridine)(salicylato)ruthenium(II) complex

The bis(2,2′-bipyridine)(salicylato)ruthenium(II) complex has been prepared and characterized by means of single crystal X-ray diffraction, electrochemistry and resonance Raman spectroscopy. The electronic bands in the visible region have been assigned to Ru–bipy charge-transfer transitions and discussed in terms of ZINDO/S semiempirical calculations. Spectroelectrochemical measurements have been performed in order to elucidate the nature of the electrochemical waves in the cyclic voltammograms. The green complex generated by oxidation of the complex at 0.25 V has been isolated, revealing substantial ruthenium–salicylate electronic mixing, as deduced from the corresponding resonance Raman spectra. Further oxidations at 1.2 and 1.4 V have been observed and ascribed to hydroxylation of the salicylate semiquinone ligand in the complex.

Crystal structure and redox potentials of the tppz-bridged {RuCl(bpy)}+ dimer

The dinuclear complex [(bpy)(Cl)Ru(tppz)Ru(Cl)(bpy)](PF6)2 has been synthesized as a catalyst precursor and characterized by X-ray crystallography and cyclic voltammetry.