Electron delocalization in mixed-valence butadienediyl-bridged diruthenium complexes

We report electrochemical and spectroelectrochemical investigations on the butadienediyl-bridged diruthenium complexes [{Ru(PPh3)2(CO)Cl}2(μ-C4H4)] (1), [{Ru(PEt3)3(CO)Cl}2(μ-C4H4)] (2), and [{Ru(PPh3)2(CO)Cl(NC5H4COOEt-4)}2(μ-C4H4)] (3). All these complexes are oxidized in two consecutive one-electron steps separated by 315 to 680 mV, depending on the co-ligands. The first oxidation is a chemically and electrochemically reversible process whereas the second varies from nearly reversible to irreversible at room temperature. We have generated and investigated the mixed-valence monocations and observed CO band shifts of ca 25 cm−1 and the appearance of new bands in the visible regime at ca 720 to 800 and 430 to 450 nm. The lower-energy band which tails into the near infrared has been assigned as a charge-resonance (or intervalence charge-transfer) absorption and used to estimate the electronic coupling parameter H AB. Our investigations point to valence delocalization for 2 + , and nearly delocalized behavior for 1 + and 3 + . Even the complex with the smallest potential splitting is, however, fully delocalized on the longer ESR timescale, as is evident from the coupling pattern of the solution spectrum. Overall IR band shifts on full oxidation and the hyperfine splittings for 1 + argue for charge and spin delocalization onto the bridging C4H4 ligand. This issue has also been addressed by quantum chemical calculations employing DFT methods. Geometry optimizations at each oxidation level reveal inversion of the C–C bond pattern from a short–long–short to a long–short–long alteration and a bis(carbenic) structure at the dication stage. All spectroscopic features such as IR band shifts, average g-values and g-tensor anisotropies are fully reproduced by the calculations

Ruthenium−Aminoallenylidene Complexes from Butatrienylidene Intermediates via an Aza-Cope Rearrangement: Synthetic, Spectroscopic, Electrochemical, Spectroelectrochemical, and Computational Studies

Ruthenium−aminoallenylidene complexes trans-[Cl(L2)2RuCCC(NR2)CH2R‘]+EF6- (4a−f; E = P, Sb, L2 = chelating diphosphine) are accessible from the respective dichloro precursors, NaEF6, butadiyne, and an allylic amine in a one-pot procedure. The reactions proceed via the primary butatrienylidene intermediate trans-[Cl(L2)2RuCCCCH2]+ and the initial addition products trans-[Cl(L2)2Ru−CCC(NR2R‘)CH2]+ 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 C3 ligand and the trigonal-planar nitrogen, indicative of its sp2 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 CN 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(L2)2RuCCC(NMe2)C4H7]+EF6- (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(L2)2RuCCCCH2]+ and trans-[Cl(L2)2RuC3{N(CH3)2}CH3]+. 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 C3. 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

Vinyl-ruthenium entities as markers for intramolecular electron transfer processes

The present account summarizes our work on mononuclear vinyl ruthenium complexes of the type RuCl(CHCHR′)(CO)(PR3)2L, divinyl-bridged diruthenium complexes {RuCl(CO)(PR3)2L}2(μ-CHCH-bridge-CHCH) and on heterobinuclear systems where only one of the two redox-active metal–organic moieties is of the vinyl ruthenium type. The favourable electrochemical properties of the {RuCl(CO)(PR3)2L(CHCH–) tag and the various spectroscopic handles offered by that unit provide detailed insights into the charge and spin delocalization over the {MCl(CO)(PR3)2L} and CHCHR′ constituents in their associated radical cations. They also offer a convenient means for measuring electronic coupling in the mixed-valent radical cations of the homo- and heterodinuclear vinyl-bridged complexes and, under favourable circumstances, on the rate of intramolecular electron transfer between the individual redox sites. Aspects of this work include examples of complexes showing time-dependent valence trapping, complexes aimed at delineating the efficiencies of through-space versus through-bond pathways for electron delocalization, complexes where electrostatic effects on the redox splitting ΔE1/2 dominate over those from the resonance contribution and systems that exhibit extensive charge and spin delocalization between two dislike endgroups despite their intrinsically different redox potentials

How to elucidate and control the redox sequence in vinylbenzoate and vinylpyridine bridged diruthenium complexes

Vinylbenzoate-bridged diruthenium complexes (RHC[double bond, length as m-dash]CH)(CO)(PiPr3)2Ru(μ-4-OOCC6H4–CH[double bond, length as m-dash]CH)RuCl(CO)(PiPr3)2 (R = Ph, 3a or CF3, 3b) and vinylpyridine-bridged (η6-p-cymene)Cl2Ru(μ-NC5H4-4-CH[double bond, length as m-dash]CH)RuCl(CO)(PiPr3)2 (3c) have been prepared from their monoruthenium precursors and investigated with respect to the sequence of the individual redox steps and electron delocalization in their partially and fully oxidized states. Identification of the primary redox sites rests on the trends in redox potentials and the EPR, IR and Vis/NIR signatures of the oxidized radical cations and is correctly reproduced by quantum chemical investigations. Our results indicate that the trifluoropropenyl complex 3b has an inverse FMO level ordering (Ru1-bridge-Ru2 > terminal vinyl-Ru1 site) when compared to its styryl substituted counterpart 3a such that the primary oxidation site in these systems can be tuned by the choice of the terminal alkenyl ligand. It is further shown that the vinylbenzoate bridge is inferior to the vinylpyridine one with regard to charge and spin delocalization at the radical cation level. According to quantum chemical calculations, the doubly oxidized forms of these complexes have triplet diradical ground states and feature two interconnected oxidized vinyl ruthenium subunits

Bridge dominated oxidation of a diruthenium 1,3-divinylphenylene complex

A divinylphenylene bridged diruthenium complex constitutes an ensemble of three coupled redox systems. Spectroelectrochemistry provides evidence that the oxidation processes are dominated by the organic bridge

Aminoallenylidene complexes of ruthenium(II) from the regioselective addition of secondary amines to butatrienylidene intermediates: a combined experimental and theoretical study of the hindered rotation around the CN-bond

Aminoallenylidene complexes trans-[Cl(dppm)2RuC3(NRR )(CH3)] are obtained from the regioselective addition of secondary amines to trans-[Cl(dppm)2Ru C C C CH2] . Unsymmetrically substituted amines give rise to Z/E isomeric mixtures. Dynamic 31P NMR spectroscopy gave an energy barrier of about 85 kJ mol 1 for rotation around the CN-bond pointing to a large contribution of the iminium alkynyl resonance form trans-[Cl(dppm)2Ru–C C– C( NRR )(CH3)] . This is also indicated by the pronounced bond length alternation within the RuC3N-chain as is revealed by X-ray structure analysis of the Z isomer of the (benzylmethyl)methylamine derivative 2d. The issue of NR2 rotation was also addressed by DFT calculations on the trans-[Cl(dhpm)2RuC3{N(CH3)2}(CH3)] model complex (dhpm = H2PCH2PH2). Upon rotation around the iminium type CN bond, the nitrogen lone pair and the π-system of the allenylidene ligand are decoupled, resulting in a significantly longer CN bond and a tetrahedrally coordinated nitrogen atom

Localised to intraligand charge-transfer states in cyclometalated platinum complexes:an experimental and theoretical study into the influence of electron-rich pendants and modulation of excited states by ion binding

The neopentyl ester of 1,3-di(2-pyridyl)benzene-5-boronic acid (dpy-B) is a useful intermediate in the divergent synthesis of N;C;N-coordinating, 1,3-di(2-pyridyl)benzene ligands, HL(n), that carry aryl substituents at the 5-position of the central ring. The platinum(ii) complexes, PtL(n)Cl, of several such ligands have been prepared, incorporating pendant anisoles, arylamines, an oxacrown, and an azacrown, all of which are strongly luminescent in solution at 298 K. The emission of the complexes is partially quenched by oxygen, and all of the compounds are very efficient sensitisers of singlet oxygen. The quantum yields of (1)O(2) formation have been measured on the basis of the intensity of the O(2)(1)Delta(g) emission at 1270 nm, and are in the range 0.25-0.65. Density functional theory (DFT) calculations have been carried out that include the effect of the solvent, on the unsubstituted complex PtL(1)Cl and on the derivatives incorporating p-dimethylaminophenyl and phenyl-15-mono-N-azacrown-5 pendants (PtL(9)Cl and PtL(12)Cl respectively). Absorption spectra have been simulated on the basis of the calculated singlet excitations: they closely resemble the experimental spectra. In particular, the DFT successfully accounts for the appearance of low-energy absorption bands that accompany the introduction of the aryl pendants, indicating the participation of the aryl group in the HOMO but not significantly in the LUMO. The calculated lowest energy triplet excitation of PtL(1)Cl is close to the observed 0-0 emission maximum of this complex in solution. Taking together data for this series of complexes and related compounds previously studied, the energies of the lowest-energy spin-allowed absorption bands are shown to correlate approximately linearly with the oxidation peak potential. The emission energies show a similar correlation in toluene, but in CH2Cl2 the value for PtL(9)Cl is anomalously low. The differing emission properties of this complex in the two solvents suggest a switch to a TICT-like state in CH2Cl2 (TICT = twisted intramolecular charge transfer), stabilised in the more polar environment. Transient DC photoconductivity measurements confirm that the dipole moment of the triplet excited state is larger in CH2Cl2 than in toluene. The azacrown PtL(12)Cl displays similar behaviour. Binding of metal ions such as Ca2+ to the azacrown unit of this complex leads to a pronounced blue shift in the emission, which can be readily understood in terms of the large increase in the TICT energy that will accompany the binding of the metal ion to the lone pair of the azacrown nitrogen atom

Redox properties of ruthenium nitrosyl porphyrin complexes with different axial ligation: Structural, spectroelectrochemical (IR, UV-Visible, and EPR), and theoretical studies

Experimental and computational results for different ruthenium nitrosyl porphyrin complexes [(Por)Ru(NO)(X)]n+ (where Por2- = tetraphenylporphyrin dianion (TPP2-) or octaethylporphyrin dianion (OEP2-) and X = H2O (n = 1,2,3) or pyridine, 4-cyanopyridine, or 4-N,N-dimethylaminopyridine (n = 1,0)) are reported with respect to their electron-transfer behavior. The structure of [(TPP)Ru(NO)(H2O)]BF4 is established as an {MNO} 6 species with an almost-linear RuNO arrangement at 178.1(3)°. The compound [(Por)Ru(NO)(H2O)]BF4 undergoes two reversible one-electron oxidation processes. Spectroelectrochemical measurements (IR, UV-vis-NIR, and EPR) indicate that the first oxidation occurs on the porphyrin ring, as evident from the appearance of diagnostic porphyrin radical-anion vibrational bands (1530 cm-1 for OEP•- and 1290 cm-1 for TPP•-), from the small shift of ∼20 cm-1 for νNO and from the EPR signal at g iso ≈ 2.00. The second oxidation, which was found to be electrochemically reversible for the OEP compound, shows a 55 cm-1 shift in νNO, suggesting a partially metal-centered process. The compounds [(Por)Ru(NO)(X)]BF4, where X = pyridines, undergo a reversible one-electron reduction. The site of the reduction was determined by spectroelectrochemical studies to be NO-centered with a ca. -300 cm-1 shift in νNO. The EPR response of the NO• complexes was essentially unaffected by the variation in the substituted pyridines X. DFT calculations support the interpretation of the experimental results because the HOMO of [(TPP)Ru(NO)(X)]+, where X = H 2O or pyridines, was calculated to be centered at the porphyrin π system, whereas the LUMO of [(TPP)Ru(NO)(X)]+ has about 50% π*(NO) character. This confirms that the (first) oxidation of [(Por)Ru(NO)(H2O)]+ occurs on the porphyrin ring wheras the reduction of [(Por)Ru(NO)(X)]+ is largely NO-centered with the metal remaining in the low-spin ruthenium(II) state throughout. The 4% pyridine contribution to the LUMO of [(TPP)Ru(NO)(py)]+ is correlated with the stability of the reduced form as opposed to that of the aqua complex.Fil: Singh, Priti. Universität Stuttgart; AlemaniaFil: Das, Atanu Kumar. Universität Stuttgart; AlemaniaFil: Sarkar, Biprajit. Universität Stuttgart; AlemaniaFil: Niemeyer, Mark. Universität Stuttgart; AlemaniaFil: Roncaroli, Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Olabe Iparraguirre, Jose Antonio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Fiedler, Jan. Czech Academy of Sciences. J. Heyrovsky Institute of Physical Chemistry; República ChecaFil: Zális, Stanislav. Czech Academy of Sciences. J. Heyrovsky Institute of Physical Chemistry; República ChecaFil: Kaim, Wolfgang. Universität Stuttgart; Alemani

Electron Transfer between Hydrogen-Bonded Pyridylphenols and a Photoexcited Rhenium(I) Complex

Two pyridylphenols with intramolecular hydrogen bonds between the phenol and pyridine units have been synthesized, characterized crystallographically, and investigated by cyclic voltammetry and UV/Vis spectroscopy. Reductive quenching of the triplet metal-to-ligand charge-transfer excited state of the [Re(CO)3(phen)(py)]+ complex (phen=1,10-phenanthroline, py=pyridine) by the two pyridylphenols and two reference phenol molecules is investigated by steady-state and time-resolved luminescence spectroscopy, as well as by transient absorption spectroscopy. Stern–Volmer analysis of the luminescence quenching data provides rate constants for the bimolecular excited-state quenching reactions. H/D kinetic isotope effects for the pyridylphenols are on the order of 2.0, and the bimolecular quenching reactions are up to 100 times faster with the pyridylphenols than with the reference phenols. This observation is attributed to the markedly less positive oxidation potentials of the pyridylphenols with respect to the reference phenols (≈0.5 V), which in turn is caused by proton coupling of the phenol oxidation process. Transient absorption spectroscopy provides unambiguous evidence for the photogeneration of phenoxyl radicals, that is, the overall photoreaction is clearly a proton-coupled electron-transfer process