Impact of Electronic Modification of the Chelating Benzylidene Ligand in cis-Dichloro-Configured Second-Generation Olefin Metathesis Catalysts on Their Activity

Grudzień K., Żukowska K., Malińska M., Woźniak K., Barbasiewicz M.

Derivatives of the Hoveyda-Grubbs complex bearing S-, Br-, I-, and N-coordinating naphthalene ligands were synthesized and characterized with NMR and X-ray studies. Depending on the arrangement of the coordinating sites on the naphthalene core, the isomeric catalysts differ in activity in model metathesis reactions. In particular, complexes with the RuCH bond adjacent to the second aromatic ring of the ligand suffer from difficulties experienced on their preparation and initiation. The behavior most probably derives from steric hindrance around the double bond and repulsive intraligand interactions, which result in abnormal chemical shifts of benzylidene protons observed with (1) H NMR. Furthermore EXSY studies revealed that the halogen-chelated ruthenium complexes display an equilibrium, in which major cis-Cl2 structures are accompanied with small amounts of isomeric forms. In general, contents of the minor forms, measured at 80 °C, correlate with the observed activity trends of the catalysts, although some exceptions complicate the mechanistic picture. We assume that for the family of halogen-chelated metathesis catalysts the initiation mechanism starts with the cis-Cl2 ⇌trans-Cl2 isomerization, although further steps may become rate-limiting for selected systems.

van Lierop B.J., Fogg D.E.

While attempts to incorporate secondary phosphine ligands into the first-generation Grubbs catalyst RuCl2(PR3)2(═CHPh) (GI, R = Cy) were frustrated by decomposition (for HPCy2) or bulk (for HPtBu2)...

Barbasiewicz M., Malińska M., Błocki K.

A modified Hoveyda–Grubbs metathesis catalyst incorporating a chelating 2-iodo-3-nitrobenzylidene ligand is described. The complex displays very little activity in model metathesis reactions at rt, while easily initiates at slightly elevated temperatures. Latent behavior of the catalyst is attributed to a through-space electron-donating effect of the nitro group, which stabilizes the chelate by the formation of NO2 → I → Ru triad.

Varnado C.D., Jr., Rosen E.L., Collins M.S., Lynch V.M., Bielawski C.W.

A redox-switchable ligand, N,N'-dimethyldiaminocarbene[3]ferrocenophane (5), was synthesized and incorporated into a series of Ir- and Ru-based complexes. Electrochemical and spectroscopic analyses of (5)Ir(CO)2Cl (15) revealed that 5 displayed a Tolman electronic parameter value of 2050 cm(-1) in the neutral state and 2061 cm(-1) upon oxidation. Moreover, inspection of X-ray crystallography data recorded for (5)Ir(cis,cis-1,5-cyclooctadiene)Cl (13) revealed that 5 was sterically less bulky (%V(Bur) = 28.4) than other known diaminocarbene[3]ferrocenophanes, which facilitated the synthesis of (5)(PPh3)Cl2Ru(3-phenylindenylid-1-ene) (18). Complex 18 exhibited quasi-reversible electrochemical processes at 0.79 and 0.98 V relative to SCE, which were assigned to the Fe and Ru centers in the complex, respectively, based on UV-vis and electron pair resonance spectroscopic measurements. Adding 2,3-dichloro-5,6-dicyanoquinone over the course of a ring-opening metathesis polymerization of cis,cis-1,5-cyclooctadiene catalyzed by 18 ([monomer]0/[18]0 = 2500) reduced the corresponding rate constant of the reaction by over an order of magnitude (pre-oxidation: k(obs) = 0.045 s(-1); post-oxidation: k(obs) = 0.0012 s(-1)). Subsequent reduction of the oxidized species using decamethylferrocene restored catalytic activity (post-reduction: k(obs) = up to 0.016 s(-1), depending on when the reductant was added). The difference in the polymerization rates was attributed to the relative donating ability of the redox-active ligand (i.e., strongly donating 5 versus weakly donating 5(+)) which ultimately governed the activity displayed by the corresponding catalyst.

Khan R.K., Torker S., Hoveyda A.H.

Rationally designed Ru-based catalysts for efficient Z-selective olefin metathesis are featured. The new complexes contain a dithiolate ligand and can be accessed in a single step from commercially available precursors in 68–82% yield. High efficiency and exceptional Z selectivity (93:7 to >98:2 Z:E) were achieved in ring-opening metathesis polymerization (ROMP) and ring-opening/cross-metathesis (ROCM) processes; the transformations typically proceed at 22 °C and are operationally simple to perform. Complete conversion was observed with catalyst loadings as low as 0.002 mol %, and turnover numbers of up to 43 000 were achieved without rigorous substrate purification or deoxygenation protocols. X-ray data and density functional theory computations provide support for key design features and shed light on mechanistic attributes.

Barbasiewicz M., Błocki K., Malińska M., Pawłowski R.

A series of modified Hoveyda-Grubbs catalysts incorporating a chelating iodo-benzylidene ligand were prepared and characterized. The presence of electron-withdrawing ring substituents in the para position to the iodide was found to decrease the catalytic activity, revealing that dissociation of the Ru···I-Ar bond is not the rate-determining step.

Falivene L., Poater A., Cazin C.S., Slugovc C., Cavallo L.

A DFT analysis of the strength of the Ru-halide bond in a series of typical olefin metathesis (pre)catalysts is presented. The calculated Ru-halide bond energies span the rather broad window of 25-43 kcal mol(-1). This indicates that in many systems dissociation of the Ru-halide bond is possible and is actually competitive with dissociation of the labile ligand generating the 14e active species. Consequently, formation of cationic Ru species in solution should be considered as a possible event.

Bantreil X., Poater A., Urbina-Blanco C.A., Bidal Y.D., Falivene L., Randall R.A., Cavallo L., Slawin A.M., Cazin C.S.

The synthesis of the four olefin metathesis precatalysts Caz-1a–d, featuring the NHC ligand N,N′-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (SIMes) and four different phosphites (P(OiPr)3, P(OPh)3, P(OEt)3, and P(OMe)3), is reported. The complexes are readily synthesized from commercially available [RuCl2(3-phenylinden-1-ylidene)(pyridine)(SIMes)] (Ind-III) in yields of up to 88%. These complexes adopt an unusual cis configuration between the phosphite and the NHC ligands. NMR experiments and computational studies confirm that the cis complexes are thermodynamically favored in comparison to their trans counterparts. In addition, the isomerization from trans to cis occurs via a mononuclear and non-dissociative mechanism. Among the four precatalysts, cis-Caz-1a, featuring a P(OiPr)3 ligand, displays the highest activity in ring-closing metathesis and cross-metathesis transformations. Experiments at low catalyst loadings demonstrated the potential of this catalyst, allowing better conversions than with c...

Barbasiewicz M., Michalak M., Grela K.

Coordination, not insertion: New ruthenium benzylidenes with a chelating halogen atom were easily prepared and showed excellent stability and activity as metathesis catalysts (see figure). Structure–activity studies reveal that strength of the ruthenium–halogen interaction can be tuned across a wide range to set up a family of latent to active catalysts.

Pump E., Fischer R.C., Slugovc C.

The current article discusses the lability of the chloride ligands in cis-dichloro ruthenium benzylidene complexes featuring the NHC ligand N,N′-bis[2,4,6-(trimethyl)phenyl]imidazolin-2-ylidene] (SIMes) and an ester-chelating benzylidene ligand. A simple halide exchange reaction with iodide salts was used to assess the halide's lability in this type of complex. The starting compound bearing the cis-dichloro stereochemistry (cis-1) underwent rapid and selective exchange of the chloride trans to the SIMes ligand (cis-2). The substitution of the chloride cis to the NHC ligand for an iodide was found to be much slower, and the corresponding cis-diiodo compound could be isolated and characterized (cis-3). The latter compound undergoes fast and selective exchange of the iodide trans to the NHC ligand when an excess of a chloride source is added, and the fourth member of the family, bearing a chloride trans to the NHC ligand and a iodide cis to the NHC ligand (cis-4), was obtained. The four compounds have been f...

Nuñez-Zarur F., Poater J., Rodríguez-Santiago L., Solans-Monfort X., Solà M., Sodupe M.

Hoveyda–Grubbs complexes are commonly used catalysts in alkene metathesis. They are precursors of the catalytic species and need to be activated before entering into the real catalytic cycle. This activation can determine in a large extent the performance of the catalyst and thus, it is important to understand the factors that influence their activation and stability. In the present work, the electronic structure of 15 different Hoveyda–Grubbs precatalysts is analyzed by means of DFT quantum chemical calculations. Electronic delocalization measures and aromaticity indices have been used to study the influence of the Hoveyda ligand substituents on the Ru⋯O interaction and the metal carbene bond, and to analyze whether or not the 5-membered metal containing ring exhibits metalloaromaticity. Results show that these complexes do not exhibit any metalloaromaticity, although delocalization measures indicate that there is a certain π electron delocalization on the Hoveyda ligand. On the other hand, present study shows that the Ru⋯O interaction is mainly electrostatic and that the influence of the substituents not only depends on the changes occurred at the O atom but also on the metal site. Substituents para to the alkoxy group (meta to the carbene) mainly tune the electronic structure of the chelating ligand in such a way that electron donor substituents strengthen the Ru⋯O interaction whereas electron withdrawing ones induce the reverse effect. However, substituents in para to the carbene (meta to the alkoxy) mainly tune the electronic structure of the ruthenium carbene bond modifying the Ru δ - C ene δ + bond polarization and producing the opposite effect.

Nuñez-Zarur F., Solans-Monfort X., Rodrı́guez-Santiago L., Sodupe M.

A detailed exploration of the three proposed mechanisms (associative, dissociative, and interchange) for the activation of Grubbs–Hoveyda-type precatalysts is performed, using DFT (B3LYP) calculations. The effects induced by the nature of the reacting alkene, the bulk of the chelating alkoxy group, and the presence of substituents in the Hoveyda ligand are taken into account. Results show that, while the associative mechanism has always high energy barriers, neither the dissociative nor the interchange mechanism can be ruled out for the first step of the activation process. In fact, the preference for one or the other mechanism seems to be influenced, to a large extent, by the nature of the chelating alkoxy group in such a way that small OR groups tend to favor the interchange pathway. Moreover, for all considered Grubbs–Hoveyda-type precatalysts, the highest transition structure corresponds to the Hoveyda ligand decoordination at the end of the cross-metathesis process. It is worth noting that this is observed regardless of the initial alkene coordination pathway (dissociative or interchange), precursor nature, and substrate and, thus, the rate-determining transition structure in all considered cases is the final alkene decoordination process. In contrast, the highest transition structure for the activation process of the phosphine-containing complexes is the initial phosphine dissociation, for which the reacting alkene is not yet involved. Overall, although the interchange mechanism may also have a role, the present calculations show that the different sign of the experimentally measured activation entropies is more likely associated with a change in the nature of the rate-determining transition structure rather than in a change of the nature of the elementary steps.

Leitgeb A., Mereiter K., Slugovc C.

Upon comparative NMR spectroscopy studies, the isomerization of newly disclosed dichloro [κ2(C,O)-2-(N-propylaminocarbonyl)benzylidene][1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene]ruthenium from trans-dichloro configuration (SPY-5-31) to its thermodynamically more favored cis-dichloro derivative (with SPY-5-34 stereochemistry) was found to proceed via dissociation of one chloride ligand to form an intermediate cationic species. Furthermore, the performance of the trans- and the cis-dichloro derivatives as catalysts in ring-closing metathesis and as initiators in ring-opening metathesis polymerization was studied.

Abbas M., Slugovc C.

The cross-metathesis of methyl oleate and oleylamine with ethyl acrylate has been studied using three different ruthenium-based second-generation catalysts. Emphasis was on determining the smallest amount of catalyst necessary for full conversion of technical grade educts. Conversion of methyl oleate is best performed in the presence of 10 equiv. ethyl acrylate without use of additional solvent whereas oleylamine cannot be used directly and protection of the amino group is necessary. An easy protection procedure for oleylamine with acetic anhydride was developed and the best results for cross-metathesis of N-acetyloleylamine were obtained in toluene solution. .

Songis O., Slawin A.M., Cazin C.S.

The use of a mixed phosphite/N-heterocyclic carbene bearing ruthenium precursor permits the synthesis and characterisation of unprecedented four-coordinate Ru(II) and Ru(III) cationic complexes adopting an unusual sawhorse structure. The cationic Ru(II) complex performs very effectively on challenging substrates at high temperature in very short reaction times and low catalyst loadings.

Antonova Alexandra S., Zubkov Fedor I.

Catalytic olefin metathesis using Hoveyda-Grubbs type ruthenium complexes is a powerful tool for creating complex molecules possessing a variety of practically useful properties. This method is also applied for obtaining modern polymer materials from low-demand petroleum products. Among all ruthenium complexes containing five- or six-membered chelate rings, the commercially available HG-II catalyst is the most common. In addition, other Hoveyda-Grubbs type complexes, which include a Het→Ru donor–acceptor bond in the chelate ring, often exhibit metathesis activity equal to or superior to that of HG-II. This review considers second-generation N-heterocyclic ruthenium carbene Hoveyda-Grubbs type complexes with donor–acceptor bonds such as O→Ru, S→Ru, Se→Ru, N→Ru, P→Ru and Hal→Ru in the chelate ring. Methods of preparation, analysis of stability and catalytic activity of such complexes are compared, and examples of the application of these organometallic ruthenium derivatives in the synthesis of practically relevant products are provided. The literature from 2010 to 2023 is summarized, making this review useful for a broad audience of chemists working in heterocyclic and organometallic chemistry, as well as practitioners involved in the production of catalysts and polymers.The bibliography includes 174 references.

Tarannam N., Alassad N., Lemcoff N.G., Kozuch S.

Yu W., Zhou Y., Zhao Y., Bai W.

Reactions of ReCl3(PMePh2)3 with N-(2-ethynylphenyl)formamides gave the first rhenaindole complexes 1–3, which were characterized by elemental analysis and HRMS. While the reaction of ReCl3(PMePh2)3 with N-(2-ethynylphenyl)acetamide produced bi(metallacyclopropene) complex 4. The delocalized structure of azarhenacycle 1 was confirmed by X-ray diffraction analysis. The UV–Vis absorption spectra of 1–3 were examined to study the substituent effect on the benzo unit.

Alassad N., Nechmad N.B., Phatake R.S., Reany O., Lemcoff N.G.

In this work, the structure, latency, and activity of twelve sulfur-chelated ruthenium precatalysts were studied by systematically varying their ligand shell.

Pump E., Poater A., Bahri-Laleh N., Credendino R., Serra L., Scarano V., Cavallo L.

The examination of cross metathesis reactions leading to the desired product has been conducted to uncover computationally the origin of the chemo-, regio- and stereoselectivity. The comparison between the relative stabilities of all involved intermediates and products, together with the transition states, links to the probability for the respective pathway. Particularly, the respective transition states for each reaction tune the regio- and stereoselectivity because they define the energy barriers needed to be overcome to form the new olefin as final product. The broad range of studied reactions with the 2nd generation Grubbs catalysts allows concluding in detail the points to pay attention and thus helps to understand the chemo-, regio- and stereoselectivity in new olefin metathesis reactions. Here, a web-server joins all these mechanistic insights which is intended to support future predictive olefin metathesis catalysis.

Wei W., Jia G.

The coordination chemistry of ruthenium, osmium, technetium and rhenium with carbon-based ligands covering the literature from 2003 to 2018 is discussed. These metals form a vast number of coordination compounds with carbon-based ligands ranging from a single carbon atom (a carbido ligand) to polydentate hydrocarbon chains, and from monodentate organometallic ligands such as carbenes to multidentate ligands such as pincers and macrocycles. Particular focus of this review is given to syntheses, properties and applications of complexes with popular carbon-based ligands such as monodentate N-heterocyclic carbenes (NHCs), cyclometallated bidentate ligands, tridentate pincers and porphyrin-like macrocycles.

Gupta S., Su S., Zhang Y., Liu P., Wink D.J., Lee D.

Metallaaromatics constitute a unique class of aromatic compounds where one or more transition metal elements are incorporated into the aromatic system, the parent of which is metallabenzene. One of the main concerns about metallabenzenes generally deals with the structural characterization related to their relative aromaticity compared to the carbon archetype. Transition metal-containing metallabenzenes are also implicated in certain catalytic processes such as alkyne metathesis polymerization; however, these transition metal-based metallaaromatic compounds have not been developed as a catalyst. Herein, we describe an effective strategy to generate diverse arrays of ruthenabenzenes and demonstrated them as an aromatic equivalent of the Grubbs-type ruthenium alkylidene catalysts. These ruthenabenzenes can be prepared via an enyne metathesis and metallotropic [1,3]-shift cascade process to form alkyne-chelated ruthenium alkylidene intermediates followed by spontaneous cycloaromatization. The aromatic nature of these complexes was confirmed by spectroscopic and X-ray crystallographic data, and the mechanistic pathways for the cycloaromatization process were studied by DFT calculations. These ruthenabenzenes display robust catalytic activity for metathesis and other transformations, which illustrates that metallabenzenes are not only compounds of structural and theoretical interests but also are a novel platform for new catalyst development.

Zachmann R.J., Fürstner A.

Gupta S., Sabbasani V.R., Su S., Wink D.J., Lee D.

A variety of heteroatom-chelated ruthenium alkylidenes have been developed as metathesis-active catalysts. Alkene-chelated ruthenium alkylidenes, however, have not been considered as a viable alter...

Bao X., Li Y., Bai W., Zhou Y., Wang Y., Sun Y., Jiang J.

Facile syntheses of the first cyclopropametalla-2-benzopyrylium complexes containing fused metallapyrylium and metallacyclopropene units.

Segalovich-Gerendash G., Rozenberg I., Alassad N., Nechmad N.B., Goldberg I., Kozuch S., Lemcoff N.G.

Herein we show the design and synthesis of an electron rich, sulfoxide-chelated, ruthenium benzylidene. In contrast to previously reported sulfoxide-chelated ruthenium benzylidenes, this complex is...

Pirovano V., Marchetti M., Carbonaro J., Brambilla E., Rossi E., Ronda L., Abbiati G.

We prepared a small library of polarity-sensitive fluorescent dyes characterized by an isocoumarin core properly functionalized with a conjugated push-pull system. The key step of the synthesis is based on a regio-selective silver(I)/p-TSA co-catalyzed cyclization of 2-alkynylbenzoates recently optimized in our laboratory. The photophysical properties of isocoumarin-based D-π-A systems have been investigated and a rationale was proposed based on their dipole moments and Hammett constants of the ED and EW groups involved.

Johns A.M., Fiamengo B.A., Herron J.R., Bourg J., Doppiu A., Karch R., Pederson R.L.

Novel sulfoxide-ligated ruthenium complexes were prepared by reacting second-generation metathesis precatalysts with p-toluenesulfonyl chloride in the presence of a small excess of sulfoxide. (SIMes)Ru(S-DMSO)(Ind)Cl2 (M54) and (SIMes)Ru(S-DMSO)(CHPh)Cl2 (M54a) were characterized crystallographically and, in agreement with NMR spectroscopy, were found to adopt an unusual cis-dichloro configuration. Despite having traditionally latent geometry, the new complexes were found to be highly reactive precatalysts for routine metathesis transformations. Additionally, the robustness, scalability, and industrial utility of M54 as a ruthenium synthon are demonstrated.

Luque-Urrutia J., Gimferrer M., Casals-Cruañas È., Poater A.

Grudzień K., Trzaskowski B., Smoleń M., Gajda R., Woźniak K., Grela K.

N-Phenylpyrrole-2,6-diisopropylphenyl ruthenium complex and its perbrominated derivative are active in ring-closing metathesis at 80 °C, but inactive at room temperature.