Spectroelectrochemistry complexity

Strubinger, S.K.D., Sun, I-W, Cleland, W.E., and Hussey, C.L. (1990) Electrochemistry and spectroelectrochemistry complexes and their one-electron reduction products in a basic room-temperature chloroaluminate molten salts, Inorg. Chem. 29, 993-999. 

The first (phenoxyl)zinc(II) complexes have been prepared by using a similar strategy (143-145). Ligands containing a 1,4,7-triazacyclononane backbone and one, two, or three phenol pendent arms form very stable (phenolato)zinc(II) precursor complexes (Table II). Accordingly, their cyclic voltammograms display one, two or three reversible one-electron oxidation waves that in all cases have been shown by spectroelectrochemistry and/or EPR spectroscopy to be ligand based. 

A variety of physical methods has been used to ascertain whether or not surface ruthenation alters the structure of a protein. UV-vis, CD, EPR, and resonance Raman spectroscopies have demonstrated that myoglobin [14, 18], cytochrome c [5, 16, 19, 21], and azurin [13] are not perturbed structurally by the attachment of a ruthenium complex to a surface histidine. The reduction potential of the metal redox center of a protein and its temperature dependence are indicators of protein structure as well. Cyclic voltammetry [5, 13], differential pulse polarography [14,21], and spectroelectrochemistry [12,14,22] are commonly used for the determination of the ruthenium and protein redox center potentials in modified proteins. 

The thallium complexes show somewhat different electrochemical behavior, and reversible oxidations are observed for both n-alkyl and n-aryl thallium porphyrins, indicating that the oxidized complexes have a more stable metal—carbon bond than the gallium or indium analogs. Spectroelectrochemistry revealed that the first oxidation is porphyrin ring-centered. The first reduction is reversible and ring... 

Electrochemical data have been collected for a selection of the antimony OEP and TPP complexes including [Sb(Por)Me2] and [Sb(Por)(R)(OH)] (R = Me, Et). The complexes show one-electron oxidations and reductions at the porphyrin rings. Spectroelectrochemistry indicated that small amounts of antimony(III) products may be formed through a chemical reaction following the first reduction. " ... 

The potentially sexadentate macrocyclic ligands l,4,7-tris(3,5-dimethyl-2-hydroxybenzyl)-l,4,7-triazacyclononane (L H3), l,4,7-tris(3,5-di-t-butyl-2-hydroxybenzyl)-l,4,7-triazacyclononane (L"H3), and l,4,7-tris(3-t-butyl-5-methoxy-2-hydroxybenzyl)-l,4,7-triazacyclononane (L" H3) form the stable complexes of type [Mn L"]PF(5, [Mn L" ]PF(5 [M L "] [Mn L ]2(C104)3-(H30)(H20)3. These complexes were investigated by spectroelectrochemistry and were shown to undergo metal- and ligand-centered redox processes a phenoxyl radical Mn complex, [Mn L" ] +, was found to be accessible. 

The anions [Ru2Xg] where X = C1, Br, and =1, 2, 3, or 4, have been investigated by spectroelectrochemistry. The Ru Ru complexes exhibit delocalization of electronic charge, the Ru Ru species has a strong Ru—Ru bonding interaction, and the more oxidized systems exhibit no metal-metal bonding interaction. " ... 

A thorough electrochemical characterization of new metalloporphyrins is nowadays state of the art for the synthetic inorganic chemist. In many of the papers cited in Sects. 3 and 4, a characterization of the new complexes by cyclic voltametry and electrolysis at controlled potential has been done. Thin-layer spectroelectrochemistry is very fruitful [346]. Fortunately, apart from classical articles of Davis et al. [347], Felton et al. [292], Fuhrhop et al. [293], Buchler et al. [190], more recent reviews of Kadish et al. are available which systematically cover the field of general metalloporphyrins [294] or organometallic porphyrin complexes [306]. Therefore, a short, update of these articles will be given in the form of Table 7. For details, the reader is referred to the original literature. 

Curtis and Eisenstein355 have made a molecular orbital analysis of the regioselectivity of the addition of nucleophiles to 77-allyl complexes and on the conformation of the 773-allyl ligand in [MoX(CO)2L2(773-allyl)] type complexes. A detailed study of the chirality retention in rearrangements of complexes of the type [MX(CO)2(dppe)(rj3-C3H5)] has been made.356 Studies of the photoelectron spectra,357 electrochemical properties,358 infrared spectroelectrochemistry,359 and fast atom bombardment mass spec-... 

Spectroelectrochemistry — Many - electrode processes are complex and difficult to study quantitatively and unambiguously. The current signal from voltammetric experiments provides only very limited structural information about reaction intermediates at surfaces or in solution. In order to improve the level of quantitative and structural information available from electrochemical experiments, spectroscopic techniques are directly (or in situ ) coupled to electrochemical methods [i, ii]. 

Iron porphyrin complexes with axial (7-alkyl and (7-aryl groups have been prepared and fully characterized by several groups (17,18). Addition of a chemical oxidant to (19, 20), or electrochemical oxidation of (21), the low-spin iron(III)-alkyl (-aryl) porphyrins results in transient formation of an iron(IV) (7-alkyl (a-aryl) complex that undergoes reductive elimination to give the iron(II) N-substituted product as shown in Scheme 2. The iron(IV) intermediate has been directly observed by low temperature lH NMR spectroscopy (22) and spectroelectrochemistry (21). 

Spectroelectrochemistry [Fig. 41(a,h)] measurements showed that the reversible wave at 0.2 V involves two electrochemical processes, corresponding to the Ru (III)Ru(III)Ru(III)/Ru(III)Ru(III)Ru(II) E° = 0.21 V) and Co(III/II)P( ° = 0.07 V) redox pairs. Surprisingly, in the him, there is an inversion in the redox potentials observed in solution (Table IV), so that the peripheral clusters are reduced before the cobalt porphyrin. This fact was ascribed to axial ligand effects, in changing from the acetonitrile solution to the solid-hlm-water interface (170). This is a very important aspect since now the peripheral complexes in the reduced form can act as electron relays enhancing the catalytic activity of the cobalt porphyrin center. 

The redox noninnocence of the 2-mercapto-3,5-di-tert-butylaniline ligand has recently been investigated with nF ions. The spectroelectrochemistry of the complex displays a range of electron transfers where the monocation, the neutral species, and the mono- and dianions have been characterized. In a related manner, Wieghardt and coworkers have reported the first example of a stable N, O-coordinated o-iminobenzoquinone via air oxidation of the initial nF complex with 2-anilino-4,6-di-tert-butylphenol (94). The analogous o-iminobenzosemiquinonate 7r-radical complex was also isolated for this system and earlier for the bis-(o-immobenzosemiquinonate)nickel(II) complex. ... 

The Cp2Ti(CO)(TCNE) and Cp2Ti(CO)(TCNQ) complexes were characterized in the oxidized and reduced forms through cyclic voltammetry, EPR, IR, and UV-visible spectroelectrochemistry. While oxidation at rather low potentials yields labile carbonyltitanium(IV) species of the TCNE and TCNQ ligands, the reduction occurs stepwise at unusually negative potentials, first on the ligand (to yield coordinated TCNE and TCNQ ) and then on the metal, to form Ti(II).i ... 

From the A 1/2 = 320 mV, K om is 1.6 x 10, indicating this complex is a Robin and Day class II mixed-valence complex. Electronic absorption spectroelectrochemistry in the near infrared (NIR) shows an osminm-based intervalence charge transfer (IVCT) at 2440 cm. The extent of electronic delocalization, a, can be calcnlated nsing eqnation (16). 

Wertz provides a good inorganic example of Raman spectroelectrochemistry in which a series of ruthenium polyazine complexes were studied in varying redox states. The series of complexes and redox states studied were [Ru(bpm)3] " (n = 0 4), [Ru(bpz)(bpy)2] " [n = 0-3), [Ru(bpy)2(bpz)] " (n = 0-3), [Ru(bpz)3] " (n = 0 3), (bpm = 2,2 -bipyrimidine, bpz = 2,2 -bipyrazine, bpy = 2,2 -bipyridine) where n is the number of electrons added to the complex. Resonance Raman spectra are recorded at each redox state to investigate the identity of the redox orbital for the series of complexes. Figure 19 shows a... 

Spectroelectrochemical (EPR, UV Vis, res. Raman, and to a lesser extent IR) characterization of redox products often reveals features characteristic of reduced polypyridine ligands for ligand-localized reductions or of oxidized metal atoms for metal-centered oxidations. Stretching CO frequencies, obtained by IR spectroelectrochemistry, are an especially useful marker of a metal oxidation state in carbonyl-polypyridine complexes. 

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