Coordination, Agostic Stabilization, and C−H Bond Activation of N-Alkyl Heterocyclic Carbenes by Coordinatively Unsaturated Ruthenium Hydride Chloride Complexes

Tetracoordinated 15-Electron Ruthenium(I) in a Discrete Triruthenium Framework

This paper highlights the unique case of a tetracoordinated Ru(I) (15-electron) component in a structurally characterized discrete triruthenium setup, [(acac)2RuIIIL1(μ-RuI)L1RuII (acac)2](ClO4)2 ([3](ClO4)2, where acac = acetylacetonate; S = 1), which was formed along with the monomeric [(acac)2RuIII(L1)] ([1]ClO4; S = 1/2) and dimeric [{(acac)2RuIII}2(μ-L1)](ClO4)2 ([2](ClO4)2; S = 1) counterparts upon interaction of {Ru(acac)2} and L1 = 3,3'-dipyridin-2-yl-1,1'-bis(imidazo[1,5-a]pyridinyl).

Ruthenium Complexes of a Triphosphorus-Coordinating Pincer Ligand: Ru-P Ligand-Substituent Exchange Reactions Driven by Large Variations of Bond Energies

The reaction of [(p-cymene)RuCl₂]₂ with the triphosphine ligand bis(2-di-tert-butylphosphinophenyl)phosphine (^tBuP^HPP) results in an unusual exchange reaction in which a chloride ligand and a phosphorus-bound H atom are exchanged ("H-P/Ru-Cl exchange") to give the (chlorophosphine)ruthenium hydride complex (^tBuP^ClPP)RuHCl [1^Cl-HCl; ^tBuP^ClPP = bis(2-di-tert-butylphosphinophenyl)chlorophosphine]. Density functional theory calculations indicate that the presumed initial product of metalation, (^tBuP^HPP)RuCl₂ (1^H-Cl2), undergoes an H-P/Ru-Cl exchange via sequential P-to-Ru α-H migration to give the intermediate (^tBuPPP)RuHCl₂, followed by Ru-to-P α-Cl migration to give the observed product 1^Cl-HCl (crystallographically characterized). Dehydrochlorination of 1^Cl-HCl under a H₂ atmosphere gives (^tBuP^ClPP)RuH₄ (1^Cl-H4), which then can undergo a second dehydrochlorination and addition of H₂ to give (^tBuP^HPP)RuH₄ (1^H-H4). This reaction may proceed via the reverse of the intramolecular exchange by 1^H-Cl2, i.e., loss of H₂ from 1^Cl-H4 to give 1^Cl-H2, which could undergo Cl-P/Ru-H exchange to give (^tBuP^HPP)RuHCl (1^H-HCl). Accordingly, the thermodynamics of Cl-P/Ru-H exchange are found to be highly dependent on the nature of the ancillary anionic ligand (H or Cl), which is not directly involved in the exchange. The origin of this thermodynamic dependence can be explained in terms of the high stability of complexes (^RP^XPP)RuHCl (X = H, Cl; R = Me, ^tBu), in which the hydride is approximately trans to a vacant coordination site and the central phosphine group is approximately trans to the weak-trans-influence chloride ligand. This conclusion has general implications for five-coordinate d⁶ complexes, both pincer- and nonpincer-ligated.

Catalytic Method for the Synthesis of Deuterium-Labeled N-Heterocyclic Carbenes Enabled by a Coordinatively Unsaturated Ruthenium N-Heterocyclic Carbene Catalyst

The wide usage of N-heterocyclic carbenes (NHCs) has raised the quest for their deuterated molecules. Effective synthesis method to obtain them, however, has remained elusive. We present here a catalytic method for the preparation of deuterated NHCs, namely, the catalytic hydrogen-deuterium exchange reaction between NHCs and deuterated benzene using a coordinatively unsaturated Ru NHC catalyst. The catalytic system enables selective deuteration of the C(sp³)-H bonds of the alkyl groups on N-substituents, as well as the sterically nonhindered C(sp²)-H bonds of NHCs as demonstrated by the preparation of 16 deuterium-labeled NHCs that have a deuteration ratio on specified sites higher than 90%. The gram-scale synthesis of deuterated IMes indicated the applicability of this catalytic method. Mechanistic studies revealed that the high regio-selectivity toward those C(sp³)-H bonds on NHCs originates from the regio-selectivity of cyclometalation reactions of coordinatively unsaturated Ru NHC species.

Rate-Limiting Steps in the Intramolecular C-H Activation of Ruthenium N-Heterocyclic Carbene Complexes

Ruthenium (II) complexes with N-heterocyclic carbenes (NHC) are commonly used as efficient catalysts in hydrogenation of olefins with simultaneous intramolecular C-H activation. Using the DFT approach, we have investigated the entire hydrogenation reaction pathway for four new potential catalysts and ethylene, a model substrate. Our calculations imply that the dissociation of phosphine is the rate-limiting step of hydrogenation, contrary to recent computational results. We also found that catalysts bearing NHCs with aliphatic and aromatic side groups are energetically favorable over other aliphatic cyclohexyl-substituted NHC. To examine how electronic properties of various catalysts influence the energetic barrier in the crucial steps of the reaction, we applied the Noncovalent Interaction analysis, which allowed us to reveal crucial interactions which stabilize/destabilize important intermediates and transition states in the hydrogenation reaction.

N-Heterocyclic Carbene Complexes in C-H Activation Reactions

In this contribution, we provide a comprehensive overview of C-H activation methods promoted by NHC-transition metal complexes, covering the literature since 2002 (the year of the first report on metal-NHC-catalyzed C-H activation) through June 2019, focusing on both NHC ligands and C-H activation methods. This review covers C-H activation reactions catalyzed by group 8 to 11 NHC-metal complexes. Through discussing the role of NHC ligands in promoting challenging C-H activation methods, the reader is provided with an overview of this important area and its crucial role in forging carbon-carbon and carbon-heteroatom bonds by directly engaging ubiquitous C-H bonds.

Stoichiometric and catalytic C–F bond activation by the trans-dihydride NHC complex [Ru(IEt2Me2)2(PPh3)2H2] (IEt2Me2= 1,3-diethyl-4,5-dimethylimidazol-2-ylidene)

Multiple catalytic hydrodefluorination steps take place with thetrans-dihydride complex [Ru(IEt₂Me₂)₂(PPh₃)₂H₂] (1), taking C₆F₆to tri-, di- and mono-fluorobenzenes.

1,2,3-Triazolylidene ruthenium(ii)-cyclometalated complexes and olefin selective hydrogenation catalysis

Silver(i) 1,2,3-triazol-5-ylidenes [(RCH₂C₂N₂(NMe)Ph)₂Ag][AgCl₂] (R = Ph 3a, C₆H₂iPr₃3b, C₆H₂Me₃3c) and [(PhCH₂C₂N₂(NMe)R)₂Ag][AgCl₂] (R = C₆H₄Me 3d, C₆H₄CF₃3e) were synthesized and subsequently treated with RuHCl(PPh₃)₃ and RuHCl(H₂)(PCy₃)₂ to give cyclometallated and Ru-hydride species as the major and minor products, respectively.

Cyclometalated mono- and dinuclear Ir(III) complexes with "click"-derived triazoles and mesoionic carbenes

Orthometalation at Ir(III) centers is usually facile, and such orthometalated complexes often display intriguing electronic and catalytic properties. By using a central phenyl ring as C-H activation sites, we present here mono- and dinuclear Ir(III) complexes with "click"-derived 1,2,3-triazole and 1,2,3-triazol-5-ylidene ligands, in which the wingtip phenyl groups in the aforementioned ligands are additionally orthometalated and bind as carbanionic donors to the Ir(III) centers. Structural characterization of the complexes reveal a piano stool-type of coordination around the metal centers with the "click"-derived ligands bound either with C^N or C^C donor sets to the Ir(III) centers. Furthermore, whereas bond localization is observed within the 1,2,3-triazole ligands, a more delocalized situation is found in their 1,2,3-triazol-5-ylidene counterparts. All complexes were subjected to catalytic tests for the transfer hydrogenation of benzaldehyde and acetophenone. The dinuclear complexes turned out to be more active than their mononuclear counterparts. We present here the first examples of stable, isomer-pure, dinuclear cyclometalated Ir(III) complexes with poly-mesoionic-carbene ligands.

B-H, C-H, and B-C bond activation: the role of two adjacent agostic interactions

Tuning the nature of the linker in a L~BHR phosphinoborane compound led to the isolation of a ruthenium complex stabilized by two adjacent, δ-C-H and ε-B(sp2)-H, agostic interactions. Such a unique coordination mode stabilizes a 14-electron "RuH2P2" fragment through connected σ-bonds of different polarity, and affords selective B-H, C-H, and B-C bond activation as illustrated by reactivity studies with H2 and boranes.

Reactions of ruthenium hydrides with ethyl-vinyl sulfide

Ru-hydride precursors containing the OCO-carbene ligand react with ethyl-vinyl-sulfide to give Ru alkyl and vinyl derivatives via an initial insertion of the vinyl-fragment into the Ru–H, subsequent C–H activation and loss of diethyl sulfide.

1,2,3-Triazolylidene ruthenium(ii)(η6-arene) complexes: synthesis, metallation and reactivity

Three bis(1,2,3-triazolylidene) silver(i) complexes were reacted with [RuCl₂(p-cymene)]₂ and the ruthenium complexes [RCH₂N₂(NMe)C₂Ph]RuCl₂(p-cymene) (R = C₆H₂Me₃4a1, C₆H₂iPr₃4b1, Ph 4c1) were isolated as major products with the minor C(sp²)–H activated products [RCH₂N₂(NMe)C₂C₆H₄]RuCl(p-cymene) (R = C₆H₂Me₃4a2, C₆H₂iPr₃4b2, Ph 4c2) and [(C₆H₄)CH₂N₂(NMe)C₂Ph]RuCl(p-cymene) (4c3).

Alkali metal salts of ditopic carbanionic carbenes as reagents for the synthesis of novel complexes of group 12 and 14 metals

Reaction of the novel organo-potassium compound [:C[N(2,6-ⁱPr₂C₆H₃)]₂(CH)CK(THF)₂] (KIPr·2THF) with M[N(SiMe₃)₂]₂(M = Zn, Sn, Ge, Pb) afforded salts of the anionic three coordinate complexes [M{C(CH)[N(2,6-ⁱPr₂C₆H₃)]₂C:}{N(SiMe₃)₂}₂]⁻(M = Zn (1), Sn (2), Ge (3), Pb (4)).

Reactivity studies of iridium pyridylidenes [Tp(Me2)Ir(C(6)H(5))(2)(C(CH)(3)C(R)NH] (R=H, Me, Ph)

The reactivity of a series of iridiumpyridylidene complexes with the formula [Tp(Me2) Ir(C6 H5 )2 (C(CH)3 C(R)NH] (1 a-1 c) towards a variety of substrates, from small molecules, such as H2 , O2 , carbon oxides, and formaldehyde, to alkenes and alkynes, is described. Most of the observed reactivity is best explained by invoking 16 e(-) unsaturated [Tp(Me2) Ir(phenyl)(pyridyl)] intermediates, which behave as internal frustrated Lewis pairs (FLPs). H2 is heterolytically split to give hydridepyridylidene complexes, whilst CO, CO2 , and H2 CO provide carbonyl, carbonate, and alkoxide species, respectively. Ethylene and propene form five-membered metallacycles with an IrCH2 CH(R)N (R=H, Me) motif, whereas, in contrast, acetylene affords four-membered iridacycles with the IrC(CH2 )N moiety. C6 H5 (CO)H and C6 H5 CCH react with formation of IrC6 H5 and IrCCPh bonds and the concomitant elimination of a molecule of pyridine and benzene, respectively. Finally the reactivity of compounds 1 a-1 c against O2 is described. Density functional theory calculations that provide theoretical support for these experimental observations are also reported.

Reactivity of the latent 12-electron fragment [Rh(PiBu3)2]+ with aryl bromides: aryl-Br and phosphine ligand C-H activation

Oxidative addition of aryl bromides to 12-electron [Rh(PiBu(3))(2)][BAr(F)(4)] (Ar(F)=3,5-(CF(3))(2)C(6)H(3)) forms a variety of products. With p-tolyl bromides, Rh(III) dimeric complexes result [Rh(PiBu(3))(2)(o/p-MeC(6)H(4))(mu-Br)](2)[BAr(F)(4)](2). Similarly, reaction with p-ClC(6)H(4)Br gives [Rh(PiBu(3))(2)(p-ClC(6)H(4))(mu-Br)](2)[BAr(F)(4)](2). In contrast, the use of o-BrC(6)H(4)Me leads to a product in which toluene has been eliminated and an isobutyl phosphine has undergone C-H activation: [Rh{PiBu(2)(CH(2)CHCH(3)CH(2))}(PiBu(3))(mu-Br)](2)[BAr(F)(4)](2). Trapping experiments with ortho-bromo anisole or ortho-bromo thioanisole indicate that a possible intermediate for this process is a low-coordinate Rh(III) complex that then undergoes C-H activation. The anisole and thioanisole complexes have been isolated and their structures show OMe or SMe interactions with the metal centre alongside supporting agostic interactions, [Rh(PiBu(3))(2)(C(6)H(4)OMe)Br][BAr(F)(4)] (the solid-state structure of the 5-methyl substituted analogue is reported) and [Rh(PiBu(3))(2)(C(6)H(4)SMe)Br][BAr(F)(4)]. The anisole-derived complex proceeds to give [Rh{PiBu(2)(CH(2)CHCH(3)CH(2))}(PiBu(3))(mu-Br)](2)[BAr(F)(4)](2), whereas the thioanisole complex is unreactive. The isolation of [Rh(PiBu(3))(2)(C(6)H(4)OMe)Br][BAr(F)(4)] and its onward reactivity to give the products of C-H activation and aryl elimination suggest that it is implicated on the pathway of a sigma-bond metathesis reaction, a hypothesis strengthened by DFT calculations. Calculations also suggest that C-H bond cleavage through phosphine-assisted deprotonation of a non-agostic bond is also competitive, although the subsequent protonation of the aryl ligand is too high in energy to account for product formation. C-H activation through oxidative addition is also ruled out on the basis of these calculations. These new complexes have been characterised by solution NMR/ESIMS techniques and in the solid-state by X-ray crystallography.

Amide synthesis from alcohols and amines catalyzed by ruthenium N-heterocyclic carbene complexes

The direct synthesis of amides from alcohols and amines is described with the simultaneous liberation of dihydrogen. The reaction does not require any stoichiometric additives or hydrogen acceptors and is catalyzed by ruthenium N-heterocyclic carbene complexes. Three different catalyst systems are presented that all employ 1,3-diisopropylimidazol-2-ylidene (IiPr) as the carbene ligand. In addition, potassium tert-butoxide and a tricycloalkylphosphine are required for the amidation to proceed. In the first system, the active catalyst is generated in situ from [RuCl(2)(cod)] (cod=1,5-cyclooctadiene), 1,3-diisopropylimidazolium chloride, tricyclopentylphosphonium tetrafluoroborate, and base. The second system uses the complex [RuCl(2)(IiPr)(p-cymene)] together with tricyclohexylphosphine and base, whereas the third system employs the Hoveyda-Grubbs 1st-generation metathesis catalyst together with 1,3-diisopropylimidazolium chloride and base. A range of different primary alcohols and amines have been coupled in the presence of the three catalyst systems to afford the corresponding amides in moderate to excellent yields. The best results are obtained with sterically unhindered alcohols and amines. The three catalyst systems do not show any significant differences in reactivity, which indicates that the same catalytically active species is operating. The reaction is believed to proceed by initial dehydrogenation of the primary alcohol to the aldehyde that stays coordinated to ruthenium and is not released into the reaction mixture. Addition of the amine forms the hemiaminal that undergoes dehydrogenation to the amide. A catalytic cycle is proposed with the {(IiPr)Ru(II)} species as the catalytically active components.