Transition Metal Chromophores Research Articles

Synthesis of redox derivatives of lysine and their use in solid-phase synthesis of a light-harvesting peptide

Redox-active amino acids were synthesized for incorporation into peptide assemblies to study photoinitiated electron or energy transfer. 4′-Methyl-2,2′-bipyridine-4-carboxylic acid was obtained in 72% yield by consecutive SeO 2 and Ag 2O oxidation without isolation of intermediates. The side chain ε-amino group of Boc- l-lysine methyl ester or γ-carboxyl group of Boc- l-glutamic acid α-methyl ester was coupled to a redox moiety (transition-metal chromophore, electron donor, electron acceptor, metal ligand, or triplet-energy transmitter) using 4-(dimethylamino)pyridine, (1-benzotriazoleoxy)tris(dimethylamino)phosphonium hexafluorophosphate, N-methylmorpholine, and 1-hydroxybenzotriazole. Use of one equivalent of 4-(dimethylamino)pyridine provided the amide coupling product in 80–97% isolated yield. Selective hydrolysis of the methyl esters with lithium hydroxide provided the redox Boc-amino acids in 70–98% yield. These redox modules are suitable for solid-phase assembly of light-harvesting peptides, as illustrated by the synthesis of the partially α-helical 11-residue redox triad that contains a phenothiazine electron donor, a ruthenium(II)tris(bipyridine) chromophore, and an anthraquinone electron acceptor. Upon laser excitation at 420 nm, the peptide triad underwent photoinduced electron transfer to create a charge-separated state with a lifetime of 53 ns and decayed with a first-order rate constant of 1.9 × 10 8 s −1.

Spectroelectrochemistry of nickel(II) complexes of N,N′-bis(salicylaldehyde)–o-phenylenediamine and N,N-bis(2-hydroxy- 1-naphthaldehyde)–o-phenylenediamine using an optically transparent thin-layer electrode

The Schiff base nickel(II) complexes of N,N′-bis(salicylaldehyde)–o-phenylenediamine and N,N-bis(2-hydroxy-1-naphthaldehyde)–o-phenylenediamine provide an opportunity for the spectroelectrochemical investigation of the azomethazine and transition-metal chromophores using a gold-mesh optically-transparent thinlayer electrode (OTTLE) in protic solvents. Cyclic voltammetry of these compounds showed a well ordered electrochemical behaviour with reductions in both the metal and ligand. All complexes showed two well defined electrode couples, NiI/NiII and NiII/NiIII. The dependence of the electrochemical parameters on the solvent, supporting electrolyte, scan rate and the nature of the ligands is discussed. Application of the OTTLE cell technique to the study of the redox chemistry of the nickel(II) pigments tested proved efficient for the spectral and electrochemical characterization of the various oxidation states. The spectropotenotiostatic experiments provided redox potentials, n values, close to those obtained by controlled potential coulometry and in agreement with those values predicted from the bulk cyclic voltammetry. The estimated standard electrode potential (E0) values agreed well with those obtained by Nernstian analysis, and facilitated the acquisition of E and n values, which are not readily obtained by conventional techniques. This was possible even for quasireversible reactions where the peak separation does not yield accurate n values. Exhaustive electrolysis of the small volume of solution in the OTTLE ensured the quantitative generation of a particular redox state without interference from extraneous redox reactions. The effect of an additional aromatic ring is to provide an additional redox couple. The electrochemical and spectral behaviours were in good agreement, which suggests that the phenomenon of colour fading in the complex and electrochemical changes produce similar trends.

A cellular ligand-field model for ‘l-l’ spectral intensities

The transition energies and intensity distributions in the ligand-field spectra of K2PtCl4 and K2PtBr4 are quantitatively reproduced within the cellular ligand-field approach. The energy and intensity parameters provide a realistic and consistent account of the electron distribution in these third row transition-metal chromophores. The intensity arises dynamically through the molecular vibration modes. The parameters for the three ungerade bending modes in the molecular D 4h symmetry are of similar magnitude, a result that is consistent with the nearly equal frequencies of these vibrations. Analyses of the transition energies of the analogous palladium(II) complexes are consistent with the studies of the platinum(II) species.

Calculation and electronic description of quadratic hyperpolarizabilities. Toward a molecular understanding of NLO responses in organotransition metal chromophores

This contribution explores the use of the computationally efficient, chemically-oriented INDO electronic structure model (ZINDO) in concert with perturbation theory to relate molecular quadratic hyperpolarizabilities to molecular architecture and electronic structure in transition metal chromophores. The ZINDO-derived second-order nonlinear optical responses are found to be in excellent agreement with the experiment for a variety of ferrocenyl and (arene)chromium tricarbonyl derivatives

Photoinduced Intramolecular Electron Transfer in RE(I) Chromophore-Quencher Complexes: Rate Dependence in the Inverted Region and the Use of a Rigid Organic Spacer

Abstract Rates for intramolecular electron transfer (kET) were determined in two molecular systems in which an electron donor is covalently bonded to a transition metal chromophore of the type [(diimine)ReI(CO)3L]+. In one system the dependence of kET on ΔGET was examined and in another the rates for charge separation and recombination across a rigid organic spacer were determined.

A transition-metal chromophore as a new, sensitive spectroscopic tag for proteins. Selective covalent labeling of histidine residues in cytochromes c with chloro(2,2':6'2"-terpyridine)platinum(II) chloride

Reactivity and selectivity of Pt(trpy)Cl/sup +/ toward proteins are studied with cytochromes c from horse and tuna as examples. The new transition-metal reagent is specific for histidine residues at pH 5. The reaction, facile one-step displacement of the Cl/sup -/ ligand by imidazole, produces good yield. The binding sites, His 26 and His 33 in the horse protein and His 26 in the tuna protein, are identified by UV-vis spectrophotometry and by peptide-mapping experiments. Model complexes with imidazole, histidine, histidine derivatives, and histidine-containing peptides are prepared and characterized. The covalently attached Pt(trpy)/sup 2 +/ labels allow easy separation of the protein derivatives by cation-exchange chromatography. The labels do not perturb the conformation and reduction potential of cytochrome c, as shown by UV-vis spectrophotometry, cyclic voltammetry, differential-pulse voltammetry, EPR spectroscopy, and /sup 1/H NMR spectroscopy. The selectivity of Pt(trpy)Cl/sup +/ is entirely opposite from that of PtCl/sub 4//sup 2 -/ although both of them are platinum(II)-chloro complexes. Owing to an interplay between the steric and electronic effects of the terpyridyl ligand, the new reagent is unreactive toward methionine (a thio ether) and cystine (a disulfide), which are otherwise highly nucleophilic ligands, but very reactive toward imidazole, which is otherwise a relativelymore » weak ligand. Unusual and useful selectivity of preformed transition-metal complexes toward proteins evidently can be achieved by a judicious choice of ancillary ligands.« less

Stereospecific induction

The first resolution of an inorganic transition metal complex by Werner (1911), over fifty years ago, made possible the study of the optical activity of the metal chromophore. In his classic studies, Werner (1912 a ) resolved the cis -bis-ethylene-diamine and tris-ethylenediamine systems of tervalent cobalt. Since the absolute configuration of no dissymmetric complex was known, early studies were devoted mainly to correlating the configurations of structurally related complexes with one another. A number of rules were proposed for this purpose, the most notable of which (Werner 1912 b ) asserted that when a common resolving reagent was employed, similar complexes forming the less soluble diasterioisomers had the same absolute configuration. In addition, it was tacitly assumed that structurally similar complexes of the same central metal would give similar rotatory dispersion and circular dichroism spectra. Thus, the bis -ethylenediamine (en) complexes of cobalt (III) of the type cis -[Co en 2 X 2 ] n + were shown (Mathieu 1936) to have rotatory dispersion and circular dichroism spectra, which in most cases were consistent with the diasteriomeric solubility criterion. It was only with the re-emergence of the crystal field theory (Bethe 1929), the determination of the absolute configuration of the tris-ethylenediamine cobalt (III) ion [Co en 3 ] 3+ by X-ray diffraction (Saito, Nakatsu, Shiro & Kuroya 1957), and the use (Corey & Bailar 1959) of optically active bidentate ligands of known absolute configuration that the transition metal chromophore had a reasonable spectroscopic base and in the case of tris-bidentate systems an absolute stereo­-chemical reference point. As yet, no absolute configuration of a bis -bidentate system has been determined by the X-ray method, and there seems no simple chemical method by which the tris- and bis-bidentate complexes can be related; although recently (McCaffery, Mason & Norman 1965) a spectroscopically based relation has been put forward.