[Ru(bpy)3]2+ Analogues Containing an N-Heterocyclic Carbene Ligand

Diastereoselective Synthesis of Sulfoxide-Functionalized N-Heterocyclic Carbene Ruthenium Complexes: An Experimental and Computational Study

The synthesis of sulfoxide-functionalized NHC ligand precursors were carried out by direct and mild oxidation from corresponding thioether precursors with high selectivity. Using these salts, a series of cationic [Ru(II)(η6-p-cymene)(NHC-SO)Cl]+ complexes were obtained in excellent yields by the classical Ag2O transmetallation route. NMR analyses suggested a chelate structure for the metal complexes, and X-ray diffractometry studies of complexes 4 b, 4 c, 4dBArF and 4 e unambiguously confirmed the preference for the bidentate (κ2-C,S) coordination mode of the NHC-SO ligands. Interestingly, only one diastereomer, in the form of an enantiomeric pair, was observed both in 1H NMR and in the solid state for the complexes. DFT calculations showed a possible intrinsic energy difference between the two pairs of diastereomer. The calculated energy barriers suggested that inversion of the sulfoxide is only plausible from the higher energy diastereomer together with bulky substituents. Inverting the configuration at the Ru center instead shows a lower and accessible activation barrier to provide the most stable diastereomer through thermodynamic control, consistent with the observation of a single species by 1H NMR as a pair of enantiomers. All these complexes catalyse the β-alkylation of secondary alcohols. Complex 4dPF6 bearing an NHC-functionalised S-Ad group has been further studied with different primary and secondary alcohols as substrates, showing high reactivity and high to moderate β-ol-selectivities.

Chemistry of Compounds Based on 1,2,3-Triazolylidene-Type Mesoionic Carbenes

Mesoionic carbenes (MICs) of the 1,2,3-triazolylidene type have established themselves as a popular class of compounds over the past decade. Primary reasons for this popularity are their modular synthesis and their strong donor properties. While such MICs have mostly been used in combination with transition metals, the past few years have also seen their utility together with main group elements. In this paper, we present an overview of the recent developments on this class of compounds that include, among others, (i) cationic and anionic MIC ligands, (ii) the donor/acceptor properties of these ligands with a focus on the several methods that are known for estimating such donor/acceptor properties, (iii) a detailed overview of 3d metal complexes and main group compounds with these MIC ligands, (iv) results on the redox and photophysical properties of compounds based on MIC ligands, and (v) an overview on electrocatalysis, redox-switchable catalysis, and small-molecule activation to highlight the applications of compounds based on MIC ligands in contemporary chemistry. By discussing several aspects from the synthetic, spectroscopic, and application point of view of these classes of compounds, we highlight the state of the art of compounds containing MICs and present a perspective for future research in this field.

Synthesis, characterization, catalytic and biological application of half-sandwich ruthenium complexes bearing hemilabile (κ2-C,S)-thioether-functionalised NHC ligands

A series of cationic Ru(ii)(η⁶-p-cymene) complexes with thioether-functionalised N-heterocyclic carbene ligands have been prepared and fully characterized. Steric and electronic influence of the R thioether substituent on the coordination of the sulfur atom was investigated. The molecular structure of three of them has been determined by means of X-ray diffractrometry and confirmed the bidentate (κ²-C,S) coordination mode of the ligand. Interestingly, only a single diastereomer, as an enantiomeric couple, was observed in the solid state for complexes 1c, 1i and 1j. DFT calculations established a low energy inversion barrier between the two diastereomers through a sulfur pyramidal inversion pathway with R donating group while a dissociative/associative mechanism is more likely with R substituents that contain electron withdrawing group, thus suggesting that the only species observed by the ¹H-NMR correspond to an average resonance position of a fluxional mixtures of isomers. All these complexes were found to catalyse the oxydant-free double dehydrogenation of primary amine into nitrile. Ru complex bearing NHC-functionalised S-tBu group was further investigated in a wide range of amines and was found more selective for alkyl amine substrates than for benzylamine derivatives. Finally, preliminary results of the biological effects on various human cancer cells of four selected Ru complexes are reported.

Aerobic dehydrogenation of amines to nitriles catalyzed by triazolylidene ruthenium complexes with O2 as terminal oxidant

Pyridyl-substituted mesoionic triazolylidene ruthenium cymene complexes catalyze the oxidation of both aromatic and aliphatic amines to nitriles with high activity and selectivity under benign conditions using dioxygen as the terminal oxidant. Modification on the pyridyl moiety of the ligand scaffold has negligible effect on the catalytic performance, while substituents on the triazolylidene directly affect the catalytic fitness of the metal center, leading to distinct catalytic profiles. Pre-dissociation of the cymene ligand and formation of a solvento analogue further enhances the catalytic activity towards nitrile formation. Variation of reaction conditions provided valuable mechanistic insights and resulted in a highly efficient protocol for nitrile formation with maximum turnover numbers around 10 000. The turnover frequency reaches up to 400 h^-1, providing one of the fastest catalytic systems known to date for this transformation.

Synthesis, Characterization, and Properties of Tetraphenylethylene-Based Tetrakis-NHC Ligands and Their Metal Complexes

The development of highly emissive dinuclear Ag^I or Au^I complexes [M₂ L](PF₆ )₂ (L=2 a, 2 b; M=Ag, Au) derived from tetraphenylethylene (TPE)-based tetrabenzimidazolin-2-ylidene ligands is reported. The new complexes exhibit a remarkable fluorescence enhancement compared to their parent benzimidazolium salts. The quantum yield (Φ_F ) value for salt H₄ -2 a(PF₆ )₄ in dilute solution (c=10^-5  m) was found to be less than 1 %, whereas its metal complexes show Φ_F values up to 55 % at similar solution concentration. This observation can be attributed to the rigidification of the TPE skeleton upon metalation resulting in a restriction of the intramolecular rotation of the phenyl groups. Functionalization of the complexes [M₂ 2 b](PF₆ )₂ (M=Ag, Au) with terminal coumarin groups and subsequent photoirradiation yielded the complexes [M₂ 3 b](PF₆ )₂ (M=Ag, Au) bearing a new type of ligand with an unaffected TPE moiety.

Half-sandwich Iridium(III) Benzimidazole-Appended Imidazolium-Based N-heterocyclic Carbene Complexes and Antitumor Application

A series of half-sandwich iridium(III) benzimidazole-appended imidazolium-based N-heterocyclic carbene (NHC) antitumor complexes [(η⁵ -Cp^x )Ir(C^N)Cl]Cl, where Cp^x is pentamethylcyclopentadienyl (Cp*) or its biphenyl derivative (Cp^xbiph ) and C^N is a NHC chelating ligand, were successfully synthesized and characterized. The Ir^III complexes showed potential antitumor activity against A549 cells, at most three times more potent than cis-platin under the same conditions. Complexes could bind to BSA by a static quenching mode, catalyzing the change of NADH to NAD⁺ and inducing the production of reactive oxygen species (maximum turnover number, 9.8), which play an important role in regulating cell apoptosis. Confocal microscopy showed that the complexes could specifically target lysosomes in cells with a Pearson's co-localization coefficient 0.76 and 0.72 after 1 h and 6 h, respectively, followed an energy-dependent cellular uptake mechanism and damaged the integrity of lysosomes. At the same time, complexes caused a marked loss of mitochondrial membrane potential.

Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications

Mesoionic carbenes are a subclass of the family of N-heterocyclic carbenes that generally feature less heteroatom stabilization of the carbenic carbon and hence impart specific donor properties and reactivity schemes when coordinated to a transition metal. Therefore, mesoionic carbenes and their complexes have attracted considerable attention both from a fundamental point of view as well as for application in catalysis and beyond. As a follow-up of an earlier Chemical Reviews overview from 2009, the organometallic chemistry of N-heterocyclic carbenes with reduced heteroatom stabilization is compiled for the 2008-2017 period, including specifically the chemistry of complexes containing 1,2,3-triazolylidenes, 4-imidazolylidenes, and related 5-membered N-heterocyclic carbenes with reduced heteratom stabilization such as (is)oxazolylidenes, pyrrazolylidenes, and thiazolylidenes, as well as pyridylidenes as 6-membered N-heterocyclic carbenes with reduced heteroatom stabilization. For each ligand subclass, metalation strategies, electronic and steric properties, and applications, in particular, in metal-mediated catalysis, are compiled. Mesoionic carbenes demonstrate particularly high activity in (water) oxidation, hydrogen transfer reactions, and cyclization reactions. Unique features of these ligands are identified such as their dipolar structure, their specific donor properties, as well as stability aspects of the ligand and the complexes, which provides opportunities for further research.

Chemical and photochemical water oxidation catalyzed by novel ruthenium complexes comprising a negatively charged NCNHCO ligand

Pincer-type ruthenium complexes incorporating an asymmetric negatively charged imidazolylidene were prepared and capable to oxidize water chemically and photo-chemically with high stability.

Experimental and theoretical investigations on the high-electron donor character of pyrido-annelated N-heterocyclic carbenes

Rh(CO)₂Cl(NHC) complexes of dipyrido-annelated N-heterocyclic carbenes were prepared. From the C–H coupling constant of the respective imidazolium salts and the N–C–N angle of the N-heterocyclic carbene (NHC), a weaker σ-donor character than that of typical unsaturated NHCs is expected. However, the IR stretching frequencies of their Rh(CO)₂Cl complexes suggest an electron-donor character even stronger than that of saturated NHCs. We ascribe this to the extremely weak π-acceptor character of the dipyrido-annelated NHCs caused by the conjugated 14 πe⁻ system that thus allows for an enhanced Rh–CO backbonding. This extremely low π-acceptor ability is also corroborated by the ⁷⁷Se NMR chemical shift of −55.8 ppm for the respective selenourea, the lowest value ever measured for imidazole derived selenoureas. DFT-calculations of the free carbene confirm the low σ-donor character by the fact that the σ-orbital of the carbene is the HOMO−1 that lies 0.58 eV below the HOMO which is located at the π-system. Natural population analysis reveals the lowest occupation of the p_π-orbital for the saturated carbene carbon atom and the highest for the pyrido-annelated carbene. Going from the free carbene to the Rh(CO)₂Cl(NHC) complexes, the increase in occupancy of the complete π-system of the carbene ligand upon coordination is lowest for the pyrido-annelated carbene and highest for the saturated carbene.

Iridium(iii) N-heterocyclic carbene complexes: an experimental and theoretical study of structural, spectroscopic, electrochemical and electrogenerated chemiluminescence properties

Theoretical and experimental studies of a series of iridium N-heterocyclic carbene complexes.

Synthesis and catalytic alcohol oxidation and ketone transfer hydrogenation activity of donor-functionalized mesoionic triazolylidene ruthenium(II) complexes

We report on the synthesis of a variety of C,E-bidentate triazolylidene ruthenium complexes that comprise different donor substituents E (E = C: phenyl anion; E = O: carboxylate, alkoxide; E = N: pyridine at heterocyclic carbon or nitrogen). Introduction of these donor functionalities is greatly facilitated by the synthetic versatility of triazoles, and their facile preparation routes. Five different complexes featuring a C,E-coordinated ruthenium center with chloride/cymene spectator ligands and three analogous solvento complexes with MeCN spectator ligands were prepared and evaluated as catalyst precursors for direct base- and oxidant-free alcohol dehydrogenation, and for transfer hydrogenation using basic iPrOH as a source of dihydrogen. In both catalytic reactions, the neutral/mono-cationic complexes with chloride/cymene spectator ligands performed better than the solvento ruthenium complexes. The donor functionality had a further profound impact on catalytic activity. For alcohol dehydrogenation, the C,C-bidentate phenyl-triazolylidene ligand induced highest conversions, while carboxylate or pyridine donor sites gave only moderate activity or none at all. In contrast, transfer hydrogenation is most efficient when a pyridyl donor group is linked to the triazolylidene via the heterocyclic carbon atom, providing turnover frequencies as high as 1400 h(-1) for cyclohexanone transfer hydrogenation. The role of the donor group is discussed in mechanistic terms.

Towards the development of functionalized polypyridine ligands for Ru(II) complexes as photosensitizers in dye-sensitized solar cells (DSSCs)

A number of novel ruthenium(II) polypyridine complexes have been designed and synthesized for use as photosensitizers in dye-sensitized solar cells (DSSCs) due to their rich photophysical properties such as intense absorption, long-lived lifetimes, high emission quantum yields and unique redox characteristics. Many of these complexes exhibit photophysical behavior that can be readily controlled through a careful choice of ligands and/or substituents. With this perspective, we review the design and general synthetic methods of some polypyridine ligands based on bipyridine, phenanthroline, terpyridine and quaterpyridine with/without anchoring groups with a view to correlate functionality of ligand structures with the observed photophysical, electroredox and power conversion efficiency of some examples of Ru(II) polypyridyl complexes that have been reported and particularly used in the DSSCs applications. The main interest, however, is focused on showing the development of new polypyridine ligand materials containing long-range electron transfer motifs such as the alkenyl, alkynyl and polyaromatic donor functionalities.