Metal-centered Oxidation Research Articles

Nitro–ruthenium(ii)–arylazoimidazoles: synthesis, spectra, crystal structure and electrochemistry of dinitro-bis{1-alkyl-2-(arylazo)imidazole}ruthenium(ii). Nitro–nitroso derivatives and reactivity of the electrophilic {Ru-NO}6system

Silver ion assisted aquation of blue cis,trans,cis-RuCl2(RaaiR′)2 (4–6) leads to solvento species, blue–violet cis,trans,cis-[Ru(OH2)2(RaaiR′)2](ClO4)2 [RaaiR′ = p-R–C6H4–NN–C3H2–NN-1-R′ (1–3), abbreviated as N,N′-chelator where N(imidazole) and N(azo) represent N and N′, respectively; R = H (a), Me (b), Cl (c) and R′ = Me (1/4/7/10), CH2CH3(2/5/8/11), CH2Ph (3/6/9/12)] that have been reacted with NO2− in warm ethanol resulting violet dinitro complexes of the type, Ru(NO2)2(RaaiR′)2 (7–9). The structure in one case, [Ru(NO2)2(HaaiCH2Ph)2] (9a), has been established by X-ray diffraction as the cis-Ru(NO2)2 motif along with trans-N,N and cis-N′,N′ dispositions of the chelator N atoms around the coordination sphere. The nitrite complexes are useful synthons of electrophilic nitrosyls, and on triturating the compounds 7b–9b with conc. HClO4 nitro–nitrosyl derivatives, [Ru(NO2)(NO)(MeaaiR′)2](ClO4)2 (10b–12b), are isolated. The solution structure and stereoretentive transformation in each reaction step have been established by 1H NMR results. All the complexes exhibit strong MLCT transitions in the visible region. They are redox active and display one metal-centred oxidation and successive ligand-based reductions. The redox potentials of Ru(III)/Ru(II) (E1/2M) of 10b–12b are anodically shifted by ∼0.2 V as compared to those of dinitro precursors 7b–9b. The ν(NO) > 1900 cm−1 strongly suggests the presence of linear Ru–N–O bonding. The electrophilic behaviour of metal bound nitrosyl has been proved in one case (12b) by reacting with a bicyclic ketone, camphor, containing an active methylene group and an arylhydrazone with an active methine group, and the heteroleptic tris-chelates thus formed are characterised.

Syntheses, redox and UV–Vis spectroelectrochemical properties of mono- and dinuclear tris(pyrazolyl)borato-oxomolybdenum(IV) complexes with pyridine ligands

Reaction of [Mo VI(Tp Me,Me)(O) 2Cl] with a variety of pyridine-based ligands [pyridine (py), 4,4′-bipyridine (bpy), 4-phenylpyridine (phpy) and 1,2′-bis(4-pyridyl)ethene (bpe)] in toluene in the presence of Ph 3P affords the mononuclear oxo-Mo(IV) complexes [Mo(Tp Me,Me)(O)Cl(L)] (L=py, phpy or monodentate bpy; abbreviated as Mo(py), Mo(phpy) and Mo(bpy), respectively) and the dinuclear complexes [{Mo(Tp Me,Me)(O)Cl} 2(μ-L)] (L=bpy, bpe; abbreviated as Mo 2 (bpy), Mo 2 (bpe), respectively). The complex Mo 2 (bpy), together with the by-product [{Mo(Tp Me,Me)(O)Cl} 2(μ-O)], have been crystallographically characterised. Electrochemical studies on the oxo-Mo(IV) complexes reveal the presence of reversible Mo(IV)/Mo(V) couples at around −0.3 V versus ferrocene/ferrocenium in every case. For the dinuclear complexes Mo 2 (bpy) and Mo 2 (bpe) these redox processes are coincident, indicating that they are largely metal-centred and not significantly delocalised across the bridging ligand. In contrast, Mo 2 (bpe) alone shows two reversible reductions, separated by 320 mV; these could be described as ligand-centred reductions of the bpe bridge, or as Mo(IV)/Mo(III) couples which—because of their separation—are substantially delocalised onto the bridging ligand. UV–Vis spectroelectrochemical studies using an OTTLE cell at 243 K revealed that oxidation of the complexes results in spectral changes (collapse of the Mo(IV) d–d transitions, loss in intensity of the Mo→pyridine MLCT transition) consistent with the formation of a Mo(V) state following metal-centred oxidation, but that one-electron reduction of Mo 2 (bpe) results in appearance of numerous intense transitions more characteristic of a ligand radical following ligand-centred reduction.

A new polytopic bis-diazacrown-ether-polypyridine ligand and its complexes with Zn(II) salts and mononuclear and dendritic Ru(II) precursors. Synthesis, absorption spectra, redox behavior, and luminescence properties.

The new polytopic receptor 1 containing two terpyridine, one phenanthroline, and two diazacrown-ether sites has been prepared using a modular approach. Such a new species contains several pieces of information in its structure which can be processed by different metal ions to give different supramolecular inorganic architectures. Actually, reaction of 1 with Zn(CH(3)COO)(2) in methanol, and subsequent anion exchange, afforded the intramolecular ring-type [Zn(1)](2+) complex, which appears to be formed by a self-assembling reaction. A different synthetic approach, stepwise synthesis, allowed us to synthesize the two multicomponent compounds [(bpy)(2)Ru(mu-1)Ru(bpy)(2)](4+) (Ru2; bpy = 2,2'-bipyridine) and [[(bpy)(2)Ru(mu-2,3-dpp)](2)Ru(mu-1)Ru[(mu-2,3-dpp)Ru(bpy)(2)](2)](12+) (Ru6; 2,3-dpp = 2,3-bis(2-pyridyl)pyrazine). The absorption spectra and luminescence properties of 1 and [Zn(1)](2+) are dominated by pi --> pi transitions and excited states. The absorption spectra of the ruthenium compounds are dominated by ligand-centered (LC) bands in the UV region and metal-to-ligand charge-transfer (MLCT) bands in the visible. The latter compounds undergo several reversible metal-centered oxidations and ligand-centered reductions in the potential window investigated (-2.0/+2.0 V versus SCE) and exhibit MLCT luminescence in both acetonitrile fluid solution at room temperature and in butyronitrile rigid matrix at 77 K. Both the redox and photophysical properties of Ru2 and Ru6 can be assigned to specific subunits of the multicomponent structures. The data indicate that the [Ru(bpy)(2)](2+) and the dendritic [Ru[(mu-2,3-dpp)Ru(bpy)(2)](2)](6+) fragments appended to the polytopic 1ligands behave as independent components of the multicomponent arrays.

Tris(pyridinealdoximato)metal complexes as ligands for the synthesis of asymmetric heterodinuclear Cr(III)M species [M = Zn(II), Cu(II), Ni(II), Fe(II), Mn(II), Cr(II), Co(III)]: a magneto-structural study.

Reactions of the LCr(III) unit with an in situ prepared M(PyA)(3)(n-) ion, where L represents 1,4,7-trimethyl-1,4,7-triazacyclononane and PyA(-) is the monoanion of pyridine-2-aldoxime, yield heterodinuclear complexes of general formula [LCr(III)(PyA)(3)M](2+/3+) as perchlorate salts, where M = Cr(II) (1), Mn(II) (2), low-spin Fe(II) (3), Ni(II) (4), Cu(II) (5), Zn(II) (6), and low-spin Co(III) (7). These compounds contain three oximato anions as bridging ligands. The hexadentate ligand with the identical donor atoms, tris(2-aldoximato-6-pyridyl)phosphine, P(PyA)(3), has been employed to prepare a second Cr(III)Ni(II) species 8, whose magnetic properties differ significantly from those of 4. Complexes 1-8 have been characterized on the basis of elemental analysis, mass spectrometry, IR, UV-vis, Mössbauer, and EPR spectroscopies, and variable-temperature (2-295 K) magnetic susceptibility measurements. They are isostructural in the sense that they all contain a terminal Cr(III) ion in a distorted octahedral environment, CrN(3)O(3), and a second six-coordinated metal ion M in a mostly trigonal prismatic MN(6) geometry. The crystal structures of the perchlorate salts of 2-5, 7, and 8 have been determined by X-ray crystallography at 100 K. The structures consist of mixed-metal Cr(III)M(II) and Cr(III)Co(III) complexes with a geometry in which two pseudooctahedral polyhedra are joined by three oximato (=N-O(-)) groups, with an intramolecular Cr.M(Co) distance in the range of 3.4-3.7 A. The cyclic voltammograms of the complexes reveal ligand oxidation and reduction processes, and in addition, metal-centered oxidation processes have been observed. X-band EPR spectroscopy has been used to establish the electronic ground state of the heterodinuclear complexes. Analysis of the susceptibility data indicates the presence of weak exchange interactions, both ferro- and antiferromagnetic, between the paramagnetic centers. A qualitative rationale on the basis of the Goodenough-Kanamori rules is provided for the difference in magnetic behaviors.

SYNTHESIS AND ELECTROCHEMICAL PROPERTIES OF CYCLOPENTADIENYLIRON(II) COMPLEXES WITH BIS(DIPHENYLPHOSPHINO)AMINE AS LIGAND

The synthesis and properties of new cationic iron(II) complexes of general formula [(η5-C5H5)FeL(η2-dppa)]A [A=I−, L = CO(1); A = BF4, L = CO(2) CH3CN(4), η1-dppa(5); dppa = NH(PPh2)2] are described. The carbonyl complex [(η5-C5H5)Fe(CO)(η2-dppa)]BF4 is deprotonated to give the neutral complex [(η5-C5H5)Fe(CO){η2-(PPh2)2N}](3). All complexes have been characterized by elemental analysis and IR and NMR spectroscopies. Cyclic voltammetry of complexes 1–5 shows a diverse redox chemistry in acetonitrile solution. While the reduction of 1 and 2 leads to the formation of a dinuclear Fe(I) complex, 4 and 5 form mononuclear species of Fe(I); oxidation of metal centers of 1 and 2 is not observed and in complexes 3 and 4 the metal centers are oxidized at potentials < 1. Complex 5 in acetonitrile solution is transformed into complex 4.

Metal binding studies with macrocyclic hexathioethers: Transition metal complexes of 22S6 and 20S6

AbstractThe two macrocyclic hexathioethers, 1,4,7,12,15,18‐hexathiacyclodocosane (22S6) and 1,4,7,11,14,17‐hexathiacycloeicosane (20S6) have been examined with respect to their complexation behavior towards a variety of transition metals ions. In all of these complexes, the two ligands generate relatively strong ligand field, but their field strengths fall between the smaller ring and stronger field hexathioether ligands (such as 18S6) and larger ring and weaker field ligands, such as 24S6. The stability of the complexes towards hydrolysis parallels their reduced field strengths with first row transition metal complexes of both ligands showing enhanced sensitivity to solvolysis reactions compared to the analogous 18S6 complexes. In general, the electrochemical behavior of the hexathioether complexes shows metal centered oxidations which occur at higher potentials than those complexes involving trithioethers. We suggest that the hexathioether ligand is less able to expand and contract during the reduction and oxidation of the metal center. The complex [Pd(22S6)]2+ does not display the unusual spectroscopic and electrochemical properties observed in the analogous 18S6 complex.

Valence-tautomerism in high-valent iron and manganese porphyrins.

Iron and manganese hemes are "high-valent" when the valence state of the metal exceeds III. Redox chemistry of the high valent metal complexes involves redistribution of holes and electrons over the metal ion and the porphyrin and axial ligands, defined as valence tautomerism. Thus, catalytic pathways of heme-containing biomolecules such as peroxidases, catalases and cytochromes P450 involve valence tautomerism, as do pathways of biomimetic oxygen transfer catalysis by manganese porphyrins, robust catalysts with potential commercial value. Determinants of the site of electron abstraction are key to understanding valence tautomerism. In model systems, metal-centered oxidation is supported by hard anionic axial ligands that are also strongly pi-donating, such as oxo, aryl, bix-methoxy and bis-fluoro groups. Manganese(IV) is more stable than iron(IV) and metal-centered one-electron oxidations occur with weaker pi-donating axial ligands such as bisazido, -isocyanato, -hypochlorito and bis chloro groups. Virtually all known high-valent iron porphyrin complexes oxidized by two-electrons above the ferric state are coordinated by the strongly pi-donating oxo or nitrido ligands. In all well-characterized oxo complexes, iron is in the ferryl state and the second oxidizing equivalent resides on the porphyrin. Complexes with iron(V) have not been definitively characterized. One-electron oxidation of oxomanganese(IV) porphyrin complexes gives the oxomanganese(IV) porphyrin pi-cation redicals. In aqueous solution, oxidation of Mn(III) complexes of tetra cationic N-methylpyridiniumylporphyrin isomers by monooxygen donors yields a transient oxomanganese(V) species.

Extensive redox series in dinuclear and dendritic Ru(II) complexes

The electrochemical behavior of a dinuclear and a hexanuclear dendritic ruthenium(II) bipyridine complexes with 2,3-bis(2-pyridyl)pyrazine (2,3-dpp) as bridging ligands has been investigated in highly purified liquid SO 2 or N, N-dimethylformamide (DMF) solutions. The compounds have general formula [Ru n (bpy) n+2 (2,3-dpp) n−1 ] 2 n+ , where n=2 or 6; bpy is 2,2′-bipyridine. The wide anodic and cathodic potential windows explored in liquid SO 2 and DMF, at low temperature, (up to ca. 4.3 and −3.1 V vs. SCE, respectively) have allowed the observation of several metal-centered oxidations and ligand-centered oxidations and reductions, leading to the most extensive redox series so far reported, comprising up to 26 reversible ligand-centered reductions and 14 reversible metal- or ligand-centered oxidations, for a total of 40 redox processes in the hexanuclear complex. The redox standard potentials for overlapping processes in multielectron waves have been obtained from the analysis of the voltammetric curves and their digital simulation, and the localization of all the redox processes and an evaluation of the mutual interactions between the redox centers have been obtained. These results allowed to clarify the important role played by bridging ligands in mediating the interactions between the equivalent redox sites. Furthermore, the localization of the redox sites and the determination of relative standard potentials allowed the mapping of MLCT excited states within the dendrimeric structure, through the correlation between the absorption spectroscopic data and the relevant voltammetric data.

Metal- and ligand-centered monoelectronic oxidation of mu-nitrido[((tetraphenylporphyrinato)manganese)phthalocyaninatoiron)], [(TPP)Mn-N-FePc]. X-ray crystal structure of the Fe(IV)-containing species [(THF)(TPP)Mn-N-FePc(H(2)O)](I(5))02THF.

The reaction of mu-nitrido[((tetraphenylporphyrinato)manganese)(phthalocyaninatoiron)], [(TPP)Mn-N-FePc], with I(2) in THF develops with the formation of two different species, i.e., [(THF)(TPP)Mn-N-FePc(H(2)O)](I(5)).2THF (I) and [(TPP)Mn(IV)-N-Fe(III)Pc](I(3)) (II). On the basis of single-crystal X-ray work and Mössbauer, EPR, Raman, and magnetic susceptibility data, I, found to be isostructural with the corresponding Fe-Fe complex, is shown to contain a low-spin triatomic Mn(IV)=N=Fe(IV) system (metal-centered oxidation). Data at hand for II (Mössbauer, EPR, Raman) show, instead, that oxidation takes place at one of the two macrocycles, very likely TPP (ligand-centered oxidation). The same cationic fragment present in I, and containing the Mn(IV)=N=Fe(IV) bond system, is also obtained when (TPP)Mn-N-FePc is allowed to react in THF with (phen)SbCl(6) (molar ratio 1:1). There are indications that the use of (phen)SbCl(6) in excess (2:1 molar ratio), in benzene, probably determines further oxidation with the formation of a species showing the combined presence of the Mn(IV)-Fe(IV) couple and of a pi-cation radical.

New ruthenium(II) and osmium(II) trinuclear dendrons. Synthesis, redox behavior, absorption spectra, and luminescence properties

The new trinuclear dendrons [Cl2Os{(μ-2,3-dpp)Ru(bpy)2}2]4+3, Cl2OsRu2 (bpy = 2,2′-bipyridine; 2,3-dpp = 2,3-bis(2-pyridyl)pyrazine), [Cl2Os{(μ-2,3-dpp)Os(bpy)2}2]4+4, Cl2OsOs2, and [(bpy)Ru{(μ-2,3-dpp)Os(bpy)2}2}6+6, bpyRuOs2 have been synthesized, and their redox properties, absorption spectra, and luminescence properties studied, together with the same properties of the species [Cl2Ru{(μ-2,3-dpp)Ru(bpy)2}2]4+1, Cl2RuRu2, [Cl2Ru{(μ-2,3-dpp)Os(bpy)2}2]4+2, Cl2RuOs2, and [(bpy)Ru{(μ-2,3-dpp)Ru(bpy)2}2}6+5, bpyRuRu2. All the compounds undergo several reversible metal-centered oxidation and ligand-centered reduction processes which are assigned to specific subunits of the trinuclear structure. Analysis of the reduction patterns of the various compounds allows one to obtain information on the extent of ligand–ligand interactions mediated by the central metals. The compounds show very intense ligand-centered absorption bands in the UV region and intense metal-to-ligand charge-transfer (MLCT) bands in the visible region, which can be assigned to transitions involving specific subunits of the dendron structure. All except 3 and 4 exhibit MLCT luminescence both at room temperature in acetonitrile fluid solution and in a MeOH–EtOH 4 ∶ 1 (v/v) rigid matrix at 77 K. The luminescence properties are dominated by the photophysical properties of the subunit(s) in which the lowest-lying excited state of any structure is localized, suggesting that fast intercomponent energy transfer takes place in all the compounds.

Dendritic Supramolecular Assembly with Multiple Ru(II) Tris(bipyridine) Units at the Periphery: Synthesis, Spectroscopic, and Electrochemical Study

A supramolecular assembly with eight peripheral ruthenium(II) tris(bipyridine), [Ru(bpy)3]2+, units covalently linked to a carbosilane dendrimer platform has been synthesized. 1H NMR and MALDI-TOF mass spectrometry confirm the target structure. Spectroscopic and electrochemical studies disclose that the identical [Ru(bpy)3]2+ units in this system interact neither in the ground state nor in the excited state. In acetonitrile solutions of identical molar concentration of [Ru(bpy)3]2+ units, both the dendrimer and reference monomeric [Ru(bpy)2(4-octoxy-2,2‘-bipyridine)]2+ exhibit identical absorption and emission spectra. Cyclic voltammetry reveals that the dendrimer and monomer possess the same redox potentials of the metal-centered oxidation process [Ru(bpy)3]2+/3+ and the first ligand-centered reduction process [Ru(bpy)3]1+/2+. The abnormal redox peaks of the [Ru(bpy)3]0/1+ and [Ru(bpy)3]1-/0 transitions of the dendrimer are attributed to the accumulation of neutral dendrimer on the electrode. A prelimina...

Oxygen ligand exchange at metal sites – implications for the O 2 evolving mechanism of photosystem II

The mechanism for photosynthetic O 2 evolution by photosystem II is currently a topic of intense debate. Important questions remain as to what is the nature of the binding sites for the substrate water and how does the O–O bond form. Recent measurements of the 18O exchange between the solvent water and the photogenerated O 2 as a function of the S-state cycle have provided some surprising insights to these questions (W. Hillier, T. Wydrzynski, Biochemistry 39 (2000) 4399–4405). The results show that one substrate water molecule is bound at the beginning of the catalytic sequence, in the S 0 state, while the second substrate water molecule binds in the S 3 state or possibly earlier. It may be that the second substrate water molecule only enters the catalytic sequence following the formation of the S 3 state. Most importantly, comparison of the observed exchange rates with oxygen ligand exchange in various metal complexes reveal that the two substrate water molecules are most likely bound to separate Mn III ions, which do not undergo metal-centered oxidations through to the S 3 state. The implication of this analysis is that in the S 1 state, all four Mn ions are in the +3 oxidation state. This minireview summarizes the arguments for this proposal.

The Proton/Electron Coupling Ratio at Heme a and CuA in Bovine Heart Cytochrome c Oxidase

Measurements of the H(+)/heme a, Cu(A) ratios for proton-electron coupling at these centers (redox Bohr effect) in CO-inhibited cytochrome c oxidase purified from bovine heart mitochondria, both in the soluble state and reconstituted in liposomes, are presented. In the soluble oxidase, the H(+)/heme a, Cu(A) ratios were experimentally determined upon oxidation by ferricyanide of these centers as well as upon their reduction by hexammineruthenium(II). These measurements showed that in order to obtain H(+)/heme a, Cu(A) ratios approaching 1, one-step full oxidation of both metal centers by ferricyanide had to be induced by a stoicheiometric amount of the oxidant. Partial stepwise oxidation or reduction of heme a and Cu(A) did produce H(+)/heme a, Cu(A) ratios significantly lower or higher than 1, respectively. The experimental H(+)/heme a, Cu(A) ratios measured upon stepwise reduction/oxidation of the metals were reproduced by mathematical simulation based on the coupling of oxido-reduction of both heme a and Cu(A) to pK shifts of common acid-base groups. The vectorial nature of the proton-electron coupling at heme a/Cu(A) was analyzed by measuring pH changes in the external bulk phase associated with oxido-reduction of these redox centers in the CO-inhibited oxidase reconstituted in liposomes. The results show that the proton release associated with the oxidation of heme a and Cu(A) takes place in the external aqueous phase. Protons taken up by the oxidase upon rereduction of the centers derive, on the other hand, from the inner space. These results provide evidence supporting the view that cooperative proton-electron coupling at heme a/Cu(A) is involved in the proton pump of the oxidase.

Copper(II) complexes of inverted porphyrin and its methylated derivatives.

The inverted porphyrins 2-aza-5,10,15,20-tetraphenyl-21-carbaporphyrin (CTPPH2) and its methylated derivatives 2-aza-2-methyl-5,10,15,20-tetraphenyl-21-carbaporphyrin (2-NCH3CTPPH) and 2-aza-2-methyl-5,10,15,20-tetraphenyl-21-methyl-21-carbaporphyrin (2-NCH3-21-CH3CTPPH) stabilize the rare organocopper(II) complexes (CTPP)CuII (1), (2-NCH3CTPP)CuII (2), (CTPPH)CuIIX (3-X), (2-NCH3CTPPH)CuIIX (4-X) (X = Cl-, TFA), and (2-NCH3-21-CH3CTPP)CuIICl (5). The EPR spectra recorded for 1, 2, 4, and 5 revealed typical features diagnostic of the copper(II) electronic structure. The superhyperfine coupling pattern indicates a presence of three nitrogen donors in the first coordination sphere. An addition of HX acid to 1 and 2 to yield the species 3-X and 4-X. The reaction mechanism includes protonation of the inner C(21) carbon accompanied by an axial coordination of anion. Methylation of (2-NCH3CTPP)CuII (2) with methyl iodide resulted in formation of (2-NCH3-21-CH3CTPP)CuIICl (5) which implies an existence of a sigma-carbanion-copper(II) bond in 2. The 2H NMR investigations carried out for the pyrrole deuterated derivatives (CTPP-d7)CuII, (2-NCH3-21-CH3CTTP-d7)CuIICl, and the methyl deuterated (2-NCH3-21-CD3CTPP)CuIICl one confirmed independently the copper(II) electronic structure with the considerable dx2-y2 metal orbital contribution to the SOMO. The redox properties of copper(II) inverted porphyrins were studied by the cyclic and differential pulse voltammetry. The halfwave potentials indicate a metal-centered oxidation of 1 (390 mV) and 2 (343 mV). The dimethylated homologue 5 reveals the reduction process at -224 mV attributed to the CuII/CuI transformation.

Organometallic complexes for nonlinear optics: Part 19. Syntheses and molecular quadratic hyperpolarizabilities of indoanilino–alkynyl–ruthenium complexes

The terminal alkyne 4-HCCC 6H 4N CCHC tBuC(O)C tBuC H ( 1) and ruthenium complex derivatives trans-[Ru(CC-4-C 6H 4N CCH=C tBuC(O)C tBuC H}Cl(dppm) 2] ( 2) and [Ru{CC-4-C 6H 4N CCHC tBuC(O)C tBuC H}(PPh 3) 2(η-C 5H 5)] ( 3) have been synthesized. An X-ray structural study of 3 reveals the expected equivalent CC bond lengths of the phenyl and alternating CC and CC bond lengths of the quinonal ring in the indoanilino–alkynyl ligand; there is a dihedral angle of 47.59° between the phenyl and quinonal rings, probably a result of ortho-hydrogen repulsion. Metal-centred oxidation potentials of 2 and 3 are similar to those of ‘extended chain’ 4-nitroaryl–alkynyl complex analogues. Irreversible quinonal ring-centred reductions occur at significantly more negative potentials than the quasi-reversible reductions in their nitro-containing analogues. Quadratic optical nonlinearities by hyper-Rayleigh scattering at 1064 nm for 2 (417×10 −30 esu) and 3 (658×10 −30 esu) are both large, but resonance enhanced. Two-level-corrected nonlinearities for these complexes (124×10 −30, 159×10 −30 esu, respectively) are also large, despite the presence of electron-donating tert-butyl groups reducing the efficiency of the (formally) electron-accepting quinonal ring in these donor-bridge-acceptor complexes.

Organometallic complexes for nonlinear optics: Part 20. Syntheses and molecular quadratic hyperpolarizabilities of alkynyl complexes derived from ( E)-4,4′-HCCC 6H 4NNC 6H 4NO 2

The syntheses of the alkyne ( E)-4,4′-HCCC 6H 4NNC 6H 4NO 2 ( 1) and alkynyl complexes L n M{( E)-4,4′-CCC 6H 4NNC 6H 4NO 2} [L n M= trans-[RuCl(dppm) 2] ( 2), Ru(PPh 3) 2(η-C 5H 5) ( 3), Au(PPh 3) ( 4)] are reported. A structural study of 2 reveals E stereochemistry about the azo-linkage. Electrochemical data for the ruthenium complexes reveal that the azo-linkage in complexes 2 and 3 perturbs the metal-centred oxidation potential compared to all other alkynyl complexes of similar composition. Quadratic optical nonlinearities by hyper-Rayleigh scattering (HRS) at 1064 nm are very large for 2 and 3, but resonance-enhanced. Comparison of HRS data for 4 with those of Au{( E)-4,4′-CCC 6H 4XCHC 6H 4NO 2}( PPh 3 ) (X=CH, N) reveals that complex 4 has a significantly larger quadratic nonlinearity than its ene- or imino-linked analogues.

Metal-assisted unusual hydroxylation at the carbon atom of the triazine ring in dinuclear ruthenium(II) and osmium(II) complexes bridged by 2,4,6-tris(2-pyridyl)-1,3,5-triazine: synthesis, structural characterization, stereochemistry, and electrochemical studies.

The reaction of cis-[M(bpy)2Cl2] (M = Ru(II), and Os(II) with 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) in refluxing ethanol-water resulted in the formation of dinuclear complexes of the composition [(M(bpy)2)2(tptz-OH)](PF6)3.nH2O (n = 1 for Ru and n = 0 for Os). In this reaction an unusual metal-induced hydroxylation at the carbon atom of the triazine ring of bridged tptz occurred. However, hydroxylation did not occur in the corresponding mononuclear complexes under similar reaction condition. A comparative study revealed that sufficient electrophilicity on the carbon atom and free movement of the attached pyridyl ring promoted the hydroxylation reaction. The hydroxylated dinuclear complexes exist in two stereoisomeric forms, a rac form (delta delta/lambda lambda) and a meso form (delta lambda/lambda delta). Both diastereoisomers have been isolated in pure form and characterized. The molecular structures of the rac form of Ru(II) complex (3-II) and meso form of the Os(II) complex (4-I) have been established by single-crystal X-ray studies. Crystal data: complex 3-II, monoclinic, C2/c, a = 24.584(7) A, b = 14.309(4) A, c = 41.044(13) A, beta = 92.84(2) degrees, V = 14420.0(7) A3, Z = 8, R = 0.179, wR2 = 0.479; complex 4-I, triclinic, P1, a = 13.444(7) A, b = 14.576(5) A, c = 19.641(7) A, alpha = 98.21(3) degrees, beta = 101.67(4) degrees, gamma = 105.80(4) degrees, V = 3546.0(3) A3, Z = 2, R = 0.093, wR2 = 0.279. The poor data quality of 3-II did not allow anisotropic refinement of non-hydrogen atoms except Ru and P. A PLUTO drawing of this compound is given only to support the molecular structure. 1H NMR data have been used to characterize the diastereoisomers. The dinuclear complexes exhibit unusual electrochemical behavior; cathodic shifts of the metal-centered oxidations and ligand-based first reduction compared to mononuclear complexes have been observed. There is a splitting in the metal-centered oxidation potentials, indicating strong electronic communication between the metal centers. Comproportionation constants (Kcom) of the mixed-valence species have been calculated; the values are in the range 6.03 x 10(4)-4.7 x 10(6). It appears that a metal-metal interaction occurred by an electron-transfer mode across the low-lying pi* orbital of the bridged tptz.

Absorption Spectra, Photophysical Properties, and Redox Behavior of Stereochemically Pure Dendritic Ruthenium(II) Tetramers and Related Dinuclear and Mononuclear Complexes.

The absorption spectra, luminescence properties, and redox behavior of stereochemically pure, dendritic Ru(II) tetramers have been studied. Furthermore, the investigation has also been performed on stereochemically resolved dinuclear complexes of the same family and on racemic forms of their mononuclear precursors and models. The complexes studied are the racemic species [(phen)(2)Ru(1,10-phenanthroline-5,6-dione)](PF(6))(2) (A, phen = 1,10-phenanthroline), [(phen)(2)Ru(1,10-phenanthroline-5,6-diamine)](PF(6))(2) (B), [Ru(1,10-phenanthroline-5,6-dione)(3)](PF(6))(2) (C), [(phen)(2)Ru(tpphz)](PF(6))(2) (1, tpphz = tetrapyrido[3,2-a:2',3'-c:3' ',2' '-h:2' ',3' '-j]phenazine), [(phen)(2)Ru(&mgr;-tpphz)Ru(phen)(2)](PF(6))(4) (2), and [{(phen)(2)Ru(&mgr;-tpphz)}(3)Ru](PF(6))(8) (4), the stereochemically pure dinuclear species DeltaDelta-[(phen)(2)Ru(&mgr;-tpphz)Ru(phen)(2)](PF(6))(4) (DeltaDelta-2), LambdaLambda-[(phen)(2)Ru(&mgr;-tpphz)Ru(phen)(2)](PF(6))(4) (LambdaLambda-2), and DeltaLambda-[(phen)(2)Ru(&mgr;-tpphz)Ru(phen)(2)](PF(6))(4) (DeltaLambda-2), and the stereochemically pure dendritic tetranuclear complexes [(Delta-(phen)(2)Ru(&mgr;-tpphz))(3)-Delta-Ru](PF(6))(8) (Delta(3)Delta-4), [(Delta-(phen)(2)Ru(&mgr;-tpphz))(3)-Lambda-Ru](PF(6))(8) (Delta(3)Lambda-4), and [(Lambda-(phen)(2)Ru(&mgr;-tpphz))(3)-Lambda-Ru](PF(6))(8) (Lambda(3)Lambda-4). All the complexes exhibit reversible metal-centered oxidation processes: the mononuclear complexes undergo a one-electron oxidation within the potential range +1.30 to +1.70 V vs SCE, whereas the dinuclear complexes undergo a two-electron oxidation at about +1.35 V and the tetranuclear compounds undergo a three-electron process at about +1.35 V followed by a one-electron process at +1.46 V. On reduction, each compound undergoes several reversible or quasireversible ligand-centered reductions within the potential window investigated (+2.00/-1.80 V vs SCE). The absorption spectra of the complexes exhibit intense ligand-centered (LC) bands in the UV region (epsilon up to 10(6) M(-)(1) cm(-)(1)) and moderately intense metal-to-ligand charge-transfer (MLCT) bands in the visible region (epsilon in the range 10(4)-10(5) M(-)(1) cm(-)(1)). All the complexes are luminescent both in fluid acetonitrile solution at room temperature (lambda(max) in the range 600-720 nm) and in MeOH/EtOH 4:1 (v/v) rigid matrix at 77 K (lambda(max) in the range 560-620 nm), except C which is luminescent only at 77 K. In all the cases, luminescence decays are monoexponential with lifetimes in the range 10(-)(5)-10(-)(8) s. Energy transfer occurs in the dendritic tetranuclear complexes from the central chromophore to the peripheral ones. For the oligonuclear tpphz-containing complexes, luminescence at room temperature and at 77 K originates from different MLCT states. When the experimental uncertainties are taken into account, the absorption spectra, luminescence properties, and redox behavior of the various stereoisomers studied here are practically undistinguishable one another. Comparison of our results with the photophysical results reported for other stereochemically pure luminescent multimetallic arrays is attempted.

Manganese-quinizarin-pyrazinecarboxylic acid mixed-ligand complex in aprotic media. Formation of a manganese(III)- semiquinone binuclear species

The monoanion of pyrazincarboxylic acid (PcA−), the dianion of quinizarine (Qz=) and manganese(II) yield a soluble deep-blue complex in dimethylsulphoxide whose MnII:Qz=:PcA− stoichiometry has been established as 1:1:1. This mixed-ligand complex is oxidized in two steps, each involving one equivalent of charge per complex present, as indicated by controlled-potential electrolyses done at +0.30V versus s.c.e. and +0.50 V versus s.c.e.. The association between the metal ion and the ligands prevail and the oxidations finally produce a dark-red complex which possesses the same stoichiometry as the original deep-blue species. In the latter the metal ion is present in oxidation state +3 with the quinizarine dianion as the corresponding semiquinone of the oxidized quinizarine. The monoanion of pyrazincarboxylic acid remains unchanged. Controlled-potential electrolysis at −0.20 V versus s.c.e. of a solution of the mixed-ligand complex indicates that it is binuclear, generating a MnII-MnIIImixed-valence species. The latter is, in turn, reduced at −1.60 V versus s.c.e. producing probably a mononuclear of manganese(II) species. If manganese(II) is combined with the semiquinone of quinizarine the metal ion exhibits the␣magnetic characteristic of manganese(III) and the␣semiquinone is reduced to the quinizarine dianion, indicating that the mixed-ligand formed exhibits intramolecular charge-transfer. This is a good example of a binuclear species accumulating four oxidation equivalents after oxidation of both metal centers and the quinizarine ligands.

Redox Chemistry of Dimethylplatinum(II) Diimine Complexes. Oxidatively Induced Pt−Me Transfer between Transient Platinum(III) Cation Radicals

The redox chemistry of the series of Pt(II) diimine complexes L2PtMe2 (1; L2 = Ar−NCRCRN−Ar, where Ar/R = 4-MeC6H4/H (a), 4-MeOC6H4/H (b), 4-MeC6H4/Me (c), 4-MeOC6H4/Me (d)), with particular emphasis on the oxidation processes, has been studied in detail. As seen by cyclic voltammetry, 1a−d undergo two successive, reversible one-electron reductions at the diimine ligands and an irreversible, metal-centered one-electron oxidation. The oxidation of 1b has been investigated in some detail. Chemical oxidation of 1b with Cp2Fe+PF6- in acetonitrile yields a near 1:1 ratio of the corresponding Pt(II) and Pt(IV) cations L2Pt(NCMe)Me+ (2b) and fac-L2Pt(NCMe)Me3+ (3b). Controlled-potential electrolysis of 1b yields mixtures of 2b and 3b in a 1:1 ratio, as well as the cis,cis (4b) and one cis,trans (5b) isomer of the dicationic Pt(IV) complexes L2Pt(NCMe)2Me22+. The percentage of the dications 4b and 5b depended on the electrode potential. A mechanism involving methyl group transfer between two transient Pt(III) int...