Synthesis,Structures, and Reactivities of Pincer‐Type Ruthenium Complexes Bearing Two Proton‐Responsive Pyrazole Arms

A new metal–ligand bifunctional, pincer‐type ruthenium complex [RuCl( L1‐H2 )(PPh₃)₂]Cl ( 1 ; L1‐H2 =2,6‐bis(5‐tert‐butyl‐1H‐pyrazol‐3‐yl)pyridine) featuring two proton‐delivering pyrazole arms has been synthesized. Complex 1 , derived from [RuCl₂(PPh₃)₃] with L1‐H2 , underwent reversible deprotonation with potassium carbonate to afford the pyrazolato–pyrazole complex [RuCl(L1‐H)(PPh₃)₂] ( 2 ). Further deprotonation of 1 and 2 with potassium hexamethyldisilazide in methanol resulted in the formation of the bis(pyrazolato) complex [Ru(L1)(MeOH)(PPh₃)₂] ( 3 ). Complex 3 smoothly reacted with dioxygen and dinitrogen to give the side‐on peroxo complex [Ru(L1)(O₂)(PPh₃)₂] ( 4 ) and end‐on dinitrogen complex [Ru(L1)(N₂)(PPh₃)₂] ( 5 ), respectively. On the other hand, the reaction of [RuCl₂(PPh₃)₃] with less hindered 2,6‐di(1H‐pyrazol‐3‐yl)pyridine ( L3‐H2 ) led to the formation of the dinuclear complex [{RuCl₂(PPh₃)₂}₂(μ₂‐ L3‐H2 )₂] ( 6 ), in which the pyrazole‐based ligand adopted a tautomeric form different from L1‐H2 in 1 and the central pyridine remained uncoordinated. The detailed structures of 1 , 2 , 3 , 3.MeOH , 4 , 5 , 6 were determined by X‐ray crystallography.     

Protic N‐Heterocyclic Carbene Versus Pyrazole: Rigorous Comparison of Proton‐ and Electron‐Donating Abilities in a Pincer‐Type Framework

Evaluation of the acidity of proton‐responsive ligands such as protic N‐heterocyclic carbenes (NHCs) bearing an NH‐wingtip provides a key to understanding the metal–ligand cooperation in enzymatic and artificial catalysis. Here, we design a CNN pincer‐type ruthenium complex 2 bearing protic NHC and isoelectronic pyrazole units in a symmetrical skeleton, to compare their acidities and electron‐donating abilities. The synthesis is achieved by direct C?H metalation of 2‐(imidazol‐1‐yl)‐6‐(pyrazol‐3‐yl)pyridine with [RuCl₂(PPh₃)₃]. ¹⁵N‐Labeling experiments confirm that deprotonation of 2 occurs first at the pyrazole side, indicating clearly that the protic pyrazole is more acidic than the NHC group. The electrochemical measurements as well as derivatization to carbonyl complexes demonstrate that the protic NHC is more electron‐donating than pyrazole in both protonated and deprotonated forms.     

Protic NNN and NCN Pincer‐Type Ruthenium Complexes Featuring (Trifluoromethyl)pyrazole Arms: Synthesis and Application to Catalytic Hydrogen Evolution from Formic Acid

NNN and NCN pincer‐type ruthenium(II) complexes featuring two protic pyrazol‐3‐yl arms with a trifluoromethyl (CF₃) group at the 5‐position were synthesized and structurally characterized to evaluate the impact of the substitution on the properties and catalysis. The increased Brønsted acidity by the highly electron‐withdrawing CF₃ pendants was demonstrated by protonation–deprotonation experiments. By contrast, the IR spectra of the carbonyl derivatives as well as the cyclic voltammogram indicated that the electron density of the ruthenium atom is negligibly influenced by the CF₃ group. Catalysis of these complexes in the decomposition of formic acid to dihydrogen and carbon dioxide was also examined. The NNN pincer‐type complex 1 a with the CF₃ group exhibited a higher catalytic activity than the tBu‐substituted analogue 1 b . In addition, the bis(CF₃‐pyrazolato) ammine derivative 4 catalyzed the reaction even in the absence of base additives.     

Coordination Chemistry of N‐Heterocyclic Nitrenium‐Based Ligands

Comprehensive studies on the coordination properties of tridentate nitrenium‐based ligands are presented. N‐heterocyclic nitrenium ions demonstrate general and versatile binding abilities to various transition metals, as exemplified by the synthesis and characterization of Rh^I, Rh^III, Mo⁰, Ru⁰, Ru^II, Pd^II, Pt^II, Pt^IV, and Ag^I complexes based on these unusual ligands. Formation of nitrenium–metal bonds is unambiguously confirmed both in solution by selective ¹⁵N‐labeling experiments and in the solid state by X‐ray crystallography. The generality of N‐heterocyclic nitrenium as a ligand is also validated by a systematic DFT study of its affinity towards all second‐row transition and post‐transition metals (Y–Cd) in terms of the corresponding bond‐dissociation energies.     

Elevated Catalytic Activity of Ruthenium(II)–Porphyrin‐Catalyzed Carbene/Nitrene Transfer and Insertion Reactions with N‐Heterocyclic Carbene Ligands

Bis(NHC)ruthenium(II)–porphyrin complexes were designed, synthesized, and characterized. Owing to the strong donor strength of axial NHC ligands in stabilizing the trans M?CRR′/M?NR moiety, these complexes showed unprecedently high catalytic activity towards alkene cyclopropanation, carbene C? H, N? H, S? H, and O? H insertion, alkene aziridination, and nitrene C? H insertion with turnover frequencies up to 1950 min^?1. The use of chiral [Ru(D₄‐Por)(BIMe)₂] ( 1 g ) as a catalyst led to highly enantioselective carbene/nitrene transfer and insertion reactions with up to 98 % ee. Carbene modification of the N terminus of peptides at 37 °C was possible. DFT calculations revealed that the trans axial NHC ligand facilitates the decomposition of diazo compounds by stabilizing the metal–carbene reaction intermediate.     

Cationic Bis‐N‐Heterocyclic Carbene (NHC) Ruthenium Complex: Structure and Application as Latent Catalyst in Olefin Metathesis

An unexpected cationic bis‐N‐heterocyclic carbene (NHC) benzylidene ether based ruthenium complex ( 2 a ) was prepared through the double incorporation of an unsymmetrical unsaturated N‐heterocyclic carbene (U₂‐NHC) ligand that bore an N‐substituted cyclododecyl side chain. The isolation and full characterization (including X‐ray diffraction studies) of key synthetic intermediates along with theoretical calculations allowed us to understand the mechanism of the overall cationization process. Finally, the newly developed complex 2 a displayed interesting latent behavior during ring‐closing metathesis, which could be “switched on” under acidic conditions.     

Ruthenium Complexes of an Abnormally Bound,Anionic N‐Heterocyclic Carbene

The abnormally bound, anionic NHC–borane complex [Ru(IDipp‐BF₃)(p‐cymene)Cl]₂ ( 4 ; IDipp‐BF₃=1,3‐(2,6‐iPr₂C₆H₃)₂‐2‐BF₃(C₃HN₂)‐4‐yl) was synthesized by transmetalation from Li[(IDipp‐BF₃)₂Ag]. Addition of donors gave species of the form [Ru(IDipp‐BF₃)(p‐cymene)(L)Cl], whereas halide abstraction with Ag(Et₂O)[B(C₆F₅)₄] gave C?H activation of the methine position of the IDipp?BF₃ ligand.     

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 [RuCl₂(cod)] (cod=1,5‐cyclooctadiene), 1,3‐diisopropylimidazolium chloride, tricyclopentylphosphonium tetrafluoroborate, and base. The second system uses the complex [RuCl₂(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.     

A Benchtop Method for Appending Protic Functional Groups to N‐Heterocyclic Carbene Protected Gold Nanoparticles

The remarkable resilience of N‐heterocyclic carbene (NHC) gold bonds has quickly made NHCs the ligand of choice when functionalizing gold surfaces. Despite rapid progress using deposition from free or CO₂‐protected NHCs, synthetic challenges hinder the functionalization of NHC surfaces with protic functional groups, such as alcohols and amines, particularly on larger nanoparticles. Here, we synthesize NHC‐functionalized gold surfaces from gold(I) NHC complexes and aqueous nanoparticles without the need for additional reagents, enabling otherwise difficult functional groups to be appended to the carbene. The resilience of the NHC?Au bond allows for multi‐step post‐synthetic modification. Beginning with the nitro‐NHC, we form an amine‐NHC terminated surface, which further undergoes amide coupling with carboxylic acids. The simplicity of this approach, its compatibility with aqueous nanoparticle solutions, and its ability to yield protic functionality, greatly expands the potential of NHC‐functionalized noble metal surfaces.     

Oxidation and β‐Alkylation of Alcohols Catalysed by Iridium(I) Complexes with Functionalised N‐Heterocyclic Carbene Ligands

The borrowing hydrogen methodology allows for the use of alcohols as alkylating agents for C?C bond forming processes offering significant environmental benefits over traditional approaches. Iridium(I)‐cyclooctadiene complexes having a NHC ligand with a O‐ or N‐functionalised wingtip efficiently catalysed the oxidation and β‐alkylation of secondary alcohols with primary alcohols in the presence of a base. The cationic complex [Ir(NCCH₃)(cod)(MeIm(2‐ methoxybenzyl))][BF₄] (cod=1,5‐cyclooctadiene, MeIm=1‐methylimidazolyl) having a rigid O‐functionalised wingtip, shows the best catalyst performance in the dehydrogenation of benzyl alcohol in acetone, with an initial turnover frequency (TOF₀) of 1283 h^?1, and also in the β‐alkylation of 2‐propanol with butan‐1‐ol, which gives a conversion of 94 % in 10 h with a selectivity of 99 % for heptan‐2‐ol. We have investigated the full reaction mechanism including the dehydrogenation, the cross‐aldol condensation and the hydrogenation step by DFT calculations. Interestingly, these studies revealed the participation of the iridium catalyst in the key step leading to the formation of the new C?C bond that involves the reaction of an O‐bound enolate generated in the basic medium with the electrophilic aldehyde.     

Color‐Tunable Electrogenerated Chemiluminescence of Ruthenium N‐Heterocyclic Carbene Complexes

Ru(II) complexes 1 – 3 bearing various N‐heterocyclic carbene (NHC) ligands were synthesized, and their photophysical, electrochemical, and electrogenerated chemiluminescence (ECL) properties were discussed to evaluate a potential of their use as multicolor ECL labels. Interestingly, they exhibited ECL emission ranging from greenish‐yellow to red both in nonaqueous and mixed aqueous solutions, which might show the potential of the Ru(II) complexes as multicolor ECL labels.     

Oxidative Enantioselective α‐Fluorination of Aliphatic Aldehydes Enabled by N‐Heterocyclic Carbene Catalysis

Described is the first study on oxidative enantioselective α‐fluorination of simple aliphatic aldehydes enabled by N‐heterocyclic carbene catalysis. N‐fluorobis(phenyl)sulfonimide serves as a an oxidant and as an “F” source. The C? F bond formation occurs directly at the α position of simple aliphatic aldehydes, thus overcoming nontrivial challenges, such as competitive difluorination and nonfluorination, and proceeds with high to excellent enantioselectivities.     

Efficient Electronic Communication of Two Ruthenium Centers through a Rigid Ditopic N‐Heterocyclic Carbene Linker

A ditopic benzobis(carbene) ligand precursor was prepared that contained a chelating pyridyl moiety to ensure co‐planarity of the carbene ligand and the coordination plane of a bound octahedral metal center. Bimetallic ruthenium complexes comprising this ditopic ligand [L₄Ru‐C,N‐bbi‐C,N‐RuL₄] were obtained by a transmetalation methodology (C,N‐bbi‐C,N=benzobis(N‐pyridyl‐N′‐methyl‐imidazolylidene). The two metal centers are electronically decoupled when the ruthenium is in a pseudotetrahedral geometry imparted by a cymene spectator ligand (L₄=[(cym)Cl]). Ligand exchange of the Cl^?/cymene ligands for two bipyridine or four MeCN ligands induced a change of the coordination geometry to octahedral. As a consequence, the ruthenium centers, separated through space by more than 10 Å, become electronically coupled, which is evidenced by two distinctly different metal‐centered oxidation processes that are separated by 134 mV (L₄=[(bpy)₂]; bpy=2,2′‐bipyridine) and 244 mV (L₄=[(MeCN)₄]), respectively. Hush analysis of the intervalence charge‐transfer bands in the mixed‐valent species indicates substantial valence delocalization in both complexes (delocalization parameter Γ=0.41 and 0.37 in the bpy and MeCN complexes, respectively). Spectroelectrochemical measurements further indicated that the mixed‐valent Ru^II/Ru^III species and the fully oxidized Ru^III/Ru^III complexes gradually decompose when bound to MeCN ligands, whereas the bpy spectators significantly enhance the stability. These results demonstrate the efficiency of carbenes and, in particular, of the bbi ligand scaffold for mediating electron transfer and for the fabrication of molecular redox switches. Moreover, the relevance of spectator ligands is emphasized for tailoring the degree of electronic communication through the benzobis(carbene) linker.     

New Route to Stabilize Ruthenium Nanoparticles with Non‐Isolable Chiral N‐Heterocyclic Carbenes

Ru nanoparticles (RuNPs) stabilized by non‐isolable chiral N‐heterocyclic carbenes (NHCs), namely SIDPhNp ((4S,5S)‐1,3‐di(naphthalen‐1‐yl)‐4,5‐diphenylimidazolidine) and SIPhOH ((S)‐3‐((1S,2R)‐2‐hydroxy‐1,2‐diphenylethyl)‐1‐((R)‐2‐hydroxy‐1,2‐diphenylethyl)‐4,5‐dihydro‐3H‐imidazoline), have been synthesized through a new procedure that does not require isolation of the free carbenes. The obtained RuNPs have been characterized by state‐of‐the‐art techniques and their surface chemistry has been investigated by FTIR and solid‐state MAS NMR upon the coordination of CO, which indicated the presence of free and reactive Ru sites. Their catalytic activity has been tested in various hydrogenation reactions involving competition between different sites, whereby interesting differences in selectivity were observed, but no enantioselectivity.     

Conjugate Umpolung of β,β‐Disubstituted Enals by Dual Catalysis with an N‐Heterocyclic Carbene and a Brønsted Acid: Facile Construction of Contiguous Quaternary Stereocenters

A sterically hindered homoenolate has been generated by the NHC‐catalyzed conjugate umpolung of β,β‐disubstituted enals and successfully employed in a facile stereoselective annulation with isatins. The strategy provides efficient access to spirocyclic oxindoles bearing two highly congested contiguous quaternary carbon centers. The use of a Brønsted acid cocatalyst was found to be crucial for guaranteeing both excellent reactivity and high stereoselectivity.     

Multicomponent Synthesis of Unsymmetrical Unsaturated N‐Heterocyclic Carbene Precursors and Their Related Transition‐Metal Complexes

A low‐cost, modular, and easily scalable multicomponent procedure affording access in good yields and excellent selectivity (up to 93 %) to a wide range of (a)chiral unsymmetrical 1‐aryl‐3‐cycloalkyl‐imidazolium salts is disclosed. Electronic and steric properties of the corresponding unsymmetrical unsaturated N‐heterocyclic carbene (U₂‐NHC) ligands were evaluated and evidenced strong electron donor ability, high steric discrimination, and modular steric demand.