Thiocyanate Linkage Isomerism in a Ruthenium Polypyridyl Complex

Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520-8107, USA.
Inorganic Chemistry (Impact Factor: 4.76). 11/2011; 50(23):11938-46. DOI: 10.1021/ic200950e
Source: PubMed


Ruthenium polypyridyl complexes have seen extensive use in solar energy applications. One of the most efficient dye-sensitized solar cells produced to date employs the dye-sensitizer N719, a ruthenium polypyridyl thiocyanate complex. Thiocyanate complexes are typically present as an inseparable mixture of N-bound and S-bound linkage isomers. Here we report the synthesis of a new complex, [Ru(terpy)(tbbpy)SCN][SbF(6)] (terpy = 2,2';6',2''-terpyridine, tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine), as a mixture of N-bound and S-bound thiocyanate linkage isomers that can be separated based on their relative solubility in ethanol. Both isomers have been characterized spectroscopically and by X-ray crystallography. At elevated temperatures the isomers equilibrate, the product being significantly enriched in the more thermodynamically stable N-bound form. Density functional theory analysis supports our experimental observation that the N-bound isomer is thermodynamically preferred, and provides insight into the isomerization mechanism.

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Available from: Wendu Ding, Apr 07, 2014
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    • "The study of the kinetic and thermodynamic aspects of electron transfer to generate phenoxyl radicals bearing bulky groups in the ortho-and para-positions may help to understand the different biological roles of phenols. The photochemistry and photophysics of transition metal complexes containing d 6 electronic configuration, particularly Ruthenium polypyridyl complexes ([Ru(NN) 3 ] 2+ ), have attracted the chemists in the design of light-driven water splitting photoanodes (Brimblecombe et al., 2010; Li et al., 2010; White et al., 2012; Young et al., 2012), molecular probes (Gill and Thomas, 2012; Liao et al., 2012; Tan et al., 2013; Tan et al., 2012), construction of solar cells (Adewale et al., 2012; Chitra et al., 2013; Sannino et al., 2013; Brewster et al., 2011; Tuikka et al., 2011), artificial photosynthesis (Kalyanasundaram and Graetzel, 2010) sensors (Cui et al., 2008; Schmittel and Lin, 2008), molecular machine devices (Balzani et al., 2009; Li et al., 2008) and organic light emitting diodes (Chi and Chou, 2010). This is due to the combination of excellent photophysical and electrochemical properties such as luminescence in solution at room temperature, moderate quantum yield and excited state lifetime, spectroscopically distinguishable metal redox states, tunable electronic properties, ability to undergo energy and electron transfer processes and chemical stability (Campagna et al., 2007; Kavan et al., 2008; Lee et al., 2003; Lo et al., 2008; Siebert et al., 2011; Sun et al., 2010). "
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    ABSTRACT: The photoredox reactions of biologically important phenols (p-coumaric acid, ferulic acid, thymol, quercetin and gallic acid) with the excited state [Ru(nbpy)3]2+ (nbpy = 4,4′-dinonyl-2,2′-bipyridine) complex proceed through photoinduced electron transfer reaction in DMSO and have been studied by luminescence quenching technique. The complex shows absorption and emission maximum at 457 and 628 nm and it shows a lifetime of 804 ns in DMSO. The excited state reduction potential of the complex in DMSO is 0.72 V vs Ag/Ag+. The dynamic nature of quenching is confirmed from the ground state absorption studies. The reductive quenching of [Ru(nbpy)3]2+ by phenolate ions has been confirmed from transient absorption spectra and from the linear variation of log kq vs oxidation potential of the phenols. The quenching rate constant, (kq) is highly sensitive to the availability of active phenolate ions, oxidation potential of the polyphenols, the free energy change (ΔG0) of the reaction and to the electron transfer distance between the complex and the quencher. Structural effects seem to play an important role in the photoinduced electron transfer reactions in DMSO.
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    ABSTRACT: Freeze-frame: Octahedral and square-planar structural isomers, representing the two "end states" in a hemilabile ligand bond-breaking isomerization reaction, have been characterized in solution by spectroscopic methods and in the solid state by X-ray crystallography (see picture: Ni green, C gray, P orange, N blue, S yellow).
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    ABSTRACT: Complexes [Ru(bpy-R)(2)(NCS)(2)], where R = H (1), 4,4'-(CO(2)Et)(2) (2), 4,4'-(OMe)(2) (3), and 4,4'-Me(2) (4), were studied by spectroelectrochemistry in the UV-vis and IR regions and by in situ electron paramagnetic resonance (EPR). The experimental information obtained for the frontier orbitals as supported and ascertained by density functional theory (DFT) calculations for 1 is relevant for the productive excited state. In addition to the parent 1, the ester complex 2 was chosen for its relationship to the carboxylate species involved for binding to TiO(2) in solar cells; the donor-substituted 3 and 4 allowed for better access to oxidized forms. Reflecting the metal-to-ligand (Ru → bpy) charge-transfer characteristics of the compounds, the electrochemical and EPR results for compounds 1-4 agree with previous notions of one metal-centered oxidation and several (bpy-R) ligand-centered reductions. The first one-electron reduction produces extensive IR absorption, including intraligand transitions and broad ligand-to-ligand intervalence charge-transfer transitions between the one-electron-reduced and unreduced bpy-R ligands. The electron addition to one remote bpy-R ligand does not significantly affect the N-C stretching frequency of the Ru(II)NCS unit. Upon oxidation of Ru(II) to Ru(III), however, the single N-C stretching band exhibits a splitting and a shift to lower energies. The DFT calculations serve to reproduce and understand these effects; they also suggest significant spin density on S for the oxidized form.
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