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The divinylphenylene-bridged diruthenium complexes (E,E)-[{(P i Pr 3) 2 (CO)ClRu} 2 (µ-HCdCHC 6 H 4 CHd CH-1,3)] (m-2) and (E,E)-[{(P i Pr 3) 2 (CO)ClRu} 2 (µ-HCdCHC 6 H 4 CHdCH-1,4)] (p-2) have been prepared and compared to their PPh 3-containing analogues m-1 and p-1. The higher electron density at the metal atoms increases the contribution of the metal end groups to the bridge-dominated occupied frontier orbitals and stabilizes the various oxidized forms with respect to those of m-1 and p-1. This has been confirmed and quantified electrochemically, because the two reversible oxidation waves were observed at considerably lower potentials than for the PPh 3 complexes. Owing to their greater stability, the one-and two-electronoxidized forms m-2 n+ and p-2 n+ of both complexes could be generated and spectroscopically characterized inside an optically transparent thin layer electrolysis cell. UV/vis/near-IR and ESR spectroelectrochemistry indicates that the oxidation processes are centered at the organic bridging ligand. σ-Bonded divinylphenylenes thus constitute an unusual class of "noninnocent" ligands for organometallic compounds. Electronic transitions observed for the mono-and dioxidized forms closely resemble those of donorsubstituted phenylenevinylene compounds, including oligo(phenylenevinylenes) (OPVs) and poly-(phenylenevinylene) (PPV) in the respective oxidation states. Strong ESR signals and nearly isotropic g tensors are observed for the monocations in fluid and frozen solutions. The metal contribution to the redox orbitals is illustrated by a shift of the CO stretching bands to notably higher energies upon stepwise oxidation. The shifts strongly exceed those observed for the PPh 3 containing, six-coordinated species (E,E)-[{(PPh 3) 2 (CO)Cl(L)Ru} 2 (µ-HCdCHC 6 H 4 CHdCH)] n+ (L) substituted pyridine). IR spectroelectrochemistry reveals the presence of two electronically different transition-metal moieties in m-2 + , while they resemble each other more closely in p-2 +. Differences in electronic coupling are illustrated by the charge distribution parameters calculated from the spectra. Bulk electrolysis experiments confirm the results from the in situ spectroelectrochemistry and the overall stoichiometry of the redox processes. Quantum-chemical calculations were performed in order to provide insight into the nature and composition of the frontier orbitals. The electronic transitions observed for the neutral forms were assigned by TD DFT. IR frequencies calculated for m-2 and p-2 in their various oxidation states retrace the experimental observations. They fail, however, in the case of m-2 + , where a symmetrical structure is calculated, as opposed to the distinctly asymmetric electron distribution observed by IR spectroscopy. Geometryoptimized structures were calculated for all accessible oxidation states. The structural changes following stepwise oxidation agree well with the experimental findings: e.g., a successive low-energy shift of the CdC stretching vibration of the bridge. The radical cation m-...

This review is a cautionary note against the often purported direct relation between the half-wave potential splitting ΔE 1/2 (or ΔE°) for stepwise, consecutive electron transfer from systems featuring two or more identical redox sites and the true electronic coupling H AB and charge or spin distribution in the ground state of intermittently formed mixed-valent (MV) systems. Several examples where these different quantities go in parallel are contrasted with other examples where this is not the case. Different kinds of such “non-conformist” behavior are outlined with the aid of representative examples. These include cases of fairly strong electronic couplings and large degrees of ground-state delocalization despite small values of ΔE 1/2sometimes just above the statistical limit or even below thatas well as examples for just the opposite behavior of no detectable electronic coupling despite appreciable electrochemical half-wave potential splitting. The crucial roles of the nominal bridges that interconnect the individual redox sites and of the environment (solvent, supporting electrolyte) in determining ΔE 1/2 and H AB are emphasized. We also seek to provide some guidelines for the practitioner as to how to discriminate between these various types of behaviors and how to determine the strength of the electronic coupling between the redox sites.

We herein describe a systematic account of mononuclear ruthenium vinyl complexes L-{Ru}-CH=CH-R where the phosphine ligands at the (PR'3)2Ru(CO)Cl={Ru} moiety, the coordination number at the metal (L = 4-ethylisonicotinate or a vacant coordination site) and the substituent R (R = nbutyl, phenyl, 1-pyrenyl) have been varied. Structures of the enynyl complex Ru(CO)Cl(PPh3)2(eta1:eta2-nBuHC=CHCCnBu), which results from the coupling of the hexenyl ligand of complex 1a with another molecule of 1-hexyne, of the hexenyl complexes (nBuCH=CH)Ru(CO)Cl(PiPr3)2 (1c) and (nBuCH=CH)Ru(CO)Cl(PPh3)2(NC5H4COOEt-4) (1b), and of the pyrenyl complexes (1-Pyr-CH=CH)Ru(CO)Cl(PiPr3)2 (3c) and (1-Pyr-CH=CH)Ru(CO)Cl(PPh3)3 (3a-P) have been established by X-ray crystallography. All vinyl complexes undergo a one-electron oxidation at fairly low potentials and a second oxidation at more positive potentials. Anodic half-wave or peak potentials show a progressive shift to lower values as pi-conjugation within the vinyl ligand increases. Carbonyl band shifts of the metal-bonded CO ligand upon monooxidation are significantly smaller than is expected of a metal-centered oxidation process and are further diminished as the vinyl CH=CH entity is incorporated into a more extended pi-system. ESR spectra of the electrogenerated radical cations display negligible g-value anisotropies and small deviations of the average g-value from that of the free electron. The vinyl ligands thus strongly contribute to or even dominate the anodic oxidation processes. This renders them a class of truly "non-innocent" ligands in organometallic ruthenium chemistry. Experimental findings are fully supported by quantum chemical calculations: The contribution of the vinyl ligand to the HOMO increases from 46% (Ru-vinyl delocalized) to 84% (vinyl dominated) as R changes from nbutyl to 1-pyrenyl.

Regio- and stereoselective insertion of the terminal ethynyl functions of 4-ethynylstilbene, the E and Z isomers of 4,4'-bis(ethynylphenyl)ethene and a backbone-rigidified cyclohexenyl derivative of the Z isomer into the Ru-H bond of the complex RuClH(CO)(P(i)Pr(3))(2) provides the corresponding vinyl ruthenium complexes, which have been characterized spectroscopically and by X-ray crystallography. Large red shifts of the UV/vis absorption bands evidence efficient incorporation of the vinyl metal subunit(s) into the conjugated π-system. All complexes oxidize at low potentials. The various oxidized forms of all complexes were generated and characterized by UV/vis/NIR, IR and EPR spectroscopies. These studies indicated electrocatalytic Z→E isomerization of the oxidized Z-distyrylethene complex Ru-Z2, which is prevented in its backbone-rigidified derivative Ru-Z2fix. The radical cations of the E and the configurationally stable cyclohexene-bridged Z-derivatives are spin-delocalized on the EPR time scale but charge-localized on the faster IR time scale. The degree of ground-state charge delocalization in the mixed-valent state has been quantified by the incremental shifts of the Ru-CO bands upon stepwise oxidation to the radical cations and the dications and was found to be remarkably large (19% and 9%) considering redox splittings ΔE(1/2) of just 49 or 74 mV. Quantum chemical studies with various levels of sophistication reproduce our experimental results including the electronic spectra of the neutral complexes and the intrinsically localized nature of the radical cations of the dinuclear complexes.

The structure of five-coordinate Ru(II) complexes RuHCl(CO)(P i Pr 3) 2 , 1, RuCl 2 (CO)(P i Pr 3) 2 , 2, and Ru(Ph)Cl(CO)(P t Bu 2 Me) 2 , 12, are reported. All three of these complexes have square-based pyramid geometry with the strongest σ-donor ligand trans to the vacant site. These 16-electron complexes do not show bona fide agostic interactions. This is attributed to the strong trans influence ligand (H, CO, and Ph) and π-donation of the Cl, which is further supported by the fact that two agostic interactions are present in the Clremoval product of 12, i.e., the four-coordinate [RuPh(CO)L 2 ]BAr′ 4 (L) P t Bu 2 Me, Ar′) 3,5-C 6 H 3 (CF 3) 2), 16. Structural comparison of 16 and 12 reveals that removal of Cldoes not change the remaining ligand arrangements but creates two low-lying LUMOs for agostic interactions, which persist in solution as evidenced by IR spectroscopy. Reactions of 16 with E-H (E) B, C(sp)) bonds cleave the Ru-Ph bond and form Ru-E/H bonds by different mechanisms. The reaction with catecholborane gives [RuH(CO)L 2 ]BAr′ 4 , which further reacts with catecholborane to give [Ru(BR 2)(CO)L 2 ]BAr′ 4. However, the reaction with Me 3 SiCCH undergoes a multistep transformation to give a PhCCSiMe 3-and Me 3 SiCCH-coupled product, the mechanism of which is discussed. Reaction of RuCl 2 (CO)L 2 with 1 equiv MeLi affords RuMeCl(CO)L 2 , 5, which further reacts with MeLi forming RuMe 2 (CO)L 2 , 7. Variable-temperature 13 C{ 1 H} NMR spectra reveal the two methyls in 7 are inequivalent and exchange by overcoming an energy barrier of 6.8 kcal/mol at-30°C. The chloride of 5 can be removed to give [RuMe(CO)L 2 ]BAr′ 4 .

Ruthenium-aminoallenylidene complexes trans-[Cl(L 2) 2 RuCCC(NR 2)CH 2 R′] + EF 6-(4af; E) P, Sb, L 2) chelating diphosphine) are accessible from the respective dichloro precursors, NaEF 6 , butadiyne, and an allylic amine in a one-pot procedure. The reactions proceed via the primary butatrienylidene intermediate trans-[Cl(L 2) 2 RudCdCdCdCH 2 ] + and the initial addition products trans-[Cl(L 2) 2 Ru-CtCC(NR 2 R′)dCH 2 ] + via an Aza-Cope type rearrangement. Amine adducts have been isolated for (dimethylamino)-2-pentyne (3f) and 1-methyl-1,2,5,6-tetrahydropyridine (3g). The former cleanly converts to its aminoallenylidene isomer upon warming. All products have been characterized by various spectroscopic techniques, including NMR, IR, and UV/vis spectroscopy and cyclic voltammetry; complex 4b was also characterized by X-ray crystallography. Most notable are the considerable bond length alternations along the unsaturated C 3 ligand and the trigonalplanar nitrogen, indicative of its sp 2 character. Aminoallenylidene complexes of this type are best described as a hybrid between true cumulenic and iminium alkynyl resonance forms, with major contributions of the latter, as is also evident from the high energy barriers for rotation around the iminium type CdN bond. The effect of the electron density on the metal on the spectroscopic and electrochemical properties of the cations in 4 has been probed for the dimethylallylamine-derived complexes trans-[Cl(L 2) 2 RuCCC(NMe 2)C 4 H 7 ] + EF 6-(4a-c), which only differ in the nature of the chelating diphosphine ligand. Aminoallenylidene complexes 4 undergo reversible one-electron oxidation. In contrast, their reduction is irreversible at room temperature but partially reversible at temperatures between 233 and 195 K. The spectroscopic changes accompanying oxidation were monitored by in situ UV/ vis, IR, and EPR techniques. DFT calculations have been performed on the model complexes trans-[Cl(L 2) 2 RudCdCdCdCH 2 ] + and trans-[Cl(L 2) 2 RuC 3 {N(CH 3) 2 }CH 3 ] +. Our results explain the regioselectivity of nucleophilic addition to the proposed butatrienylidene intermediate and the spectroscopic and electrochemical properties of aminoallenylidene complexes 4. Both orbital and steric effects are equally important in the regioselective addition to C 3. The calculations further indicate primarily metal-based oxidation and ligand-based reduction of complexes 4, in accordance with experimental observations. They also let us assign the experimental UV/vis bands and the two main IR absorptions in the 2000-1500 cm-1 region.

International audienceIn this work, we describe the preparation and the properties of the novel bis(vinylphenylene)-bridged diruthenium complexes {Ru(CO)(η2-O2C-p-C6H4SAc)(PiPr3)2}2(μ-CH═CH-C6H4-CH═CH-1,3 and -1,4) (6 and 7), the bis(ethynylphenylene)-bridged complex trans-[AcS-p-C6H4-C≡C-Ru(dppe)2-C≡C-p-C6H4-C≡C-Ru(dppe)2-C≡C-p-C6H4-SAc] (11), the bis(1-ethynyl-4-vinylphenylene)-bridged triruthenium complex trans-[{Ru(dppe)2}{−C≡C-p-C6H4-CH═CH-Ru(CO)(η2-O2C-p-C6H4SAc)(PiPr3)2}2] (8), and the monometallic congeners Ru(CH═CH-p-C6H4SAc)(CO)(η2-O2C-p-C6H4SAc)(PiPr3)2 (4) and trans-[Ru(dppe)2(−C≡C-p-C6H4-SAc)2] (10). These mono-, bi-, and trimetallic complexes feature terminal acetyl-protected thiol functions for covalent binding to gold surfaces or for bridging the gaps of gold nanoelectrodes. All complexes display low oxidation potentials, and IR studies of the neutral complex 8 and of its various oxidized forms 8n+ indicate the high vinyl/ethynyl bridging ligand contribution to the oxidation processes and complete charge delocalization in all available oxidation states (n = 1–3). Strong delocalization of the relevant occupied frontier MOs over the entire π-conjugated {Ru}–bridge–{Ru′}–bridge–{Ru} backbone is also supported by DFT calculations on the parent complexes V8 and V8OMe. The benzoate ligand bearing the functional group for gold binding is outside the conjugation path and insulates the wirelike central portion of these molecules from their periphery. Upon insertion into molecular junctions, these molecules are expected to enhance sequential tunneling and to facilitate Coulomb blockade behavior. They will thus contribute to our understanding of structure–property relationships for metal-containing molecular wires