Palladation of diimidazolium salts at the C4 position: access to remarkably electron-rich palladium(II) centers

Palladation of C2-protected diimidazolium salts with Pd(OAc)2 afforded complexes comprising C4-bound N-heterocyclic dicarbene ligands. The reactivity of these complexes towards Lewis acids (AgBF4, AgOAc) and Br?nsted acids (H2SO4, H3PO4, HOAc) revealed that abnormal C4 bonding of the carbenes markedly increases the nucleophilicity of the coordinated palladium center as compared to C2 bonding. Despite its formal +2 charge, the palladium center in these complexes is best described as a Lewis base. The abnormal carbene bonding mode induces new reaction patterns such as the formation of a Pd-Ag adduct. Based on metallation studies including the palladation of a dissymmetric diimidazolium salt, a rationale for the selective activation of the C4-H bond in the diimidazolium precursor salts is proposed.     

Room‐Temperature Hydrohydrazination of Terminal Alkynes Catalyzed by Saturated Abnormal N‐Heterocyclic Carbene–Gold(I) Complexes

A number of saturated abnormal N‐heterocyclic carbene (NHC) complexes of gold, in combination with KBAr^F₄ as activator, were successfully applied in the chemoselective addition of hydrazine to alkynes. The reaction proceeds even at room temperature, which was not possible to date with gold catalysts. The reaction can be applied to a number of substituted arylalkynes. With alkylalkynes the yields are low. The saturated abnormal NHC ligands are resistant to isomerization to the saturated normal NHC coordination mode under basic reaction conditions. Under acidic conditions, a simple protonation at the nitrogen atom not neighboring the carbene center was observed and unambiguously characterized by an X‐ray crystal‐structure analysis. Computational studies confirm that such an isomerization would be highly exothermic, the observed kinetic stability probably results from the need to shift two protons in such a process.     

Comparative Analysis of Electron‐Density and Electron‐Localization Function for Dinuclear Manganese Complexes with Bridging Boron‐ and Carbon‐Centered Ligands

Bonding in borylene‐, carbene‐, and vinylidene‐bridged dinuclear manganese complexes [MnCp(CO)₂]₂X (X=B‐tBu, B=NMe₂, CH₂, C?CH₂) has been compared by analyses based on quantum theory of atoms in molecules (QTAIM), on the electron‐localization function (ELF), and by natural‐population analyses. All of the density functional theory based analyses agree on the absence of a significant direct Mn? Mn bond in these complexes and confirm a dominance of delocalized bonding via the bridging ligand. Interestingly, however, the topology of both charge density and ELF related to the Mn‐bridge‐Mn bonding depend qualitatively on the chosen density functional (except for the methylene‐bridged complex, which exhibits only one three‐center‐bonding attractor both in ??²ρ and in ELF). While gradient‐corrected functionals provide a picture with localized two‐center X? Mn bonding, increasing exact‐exchange admixture in hybrid functionals concentrates charge below the bridging atom and suggests a three‐center bonding situation. For example, the bridging boron ligands may be described either as substituted boranes (e.g., at BLYP or BP86 levels) or as true bridging borylenes (e.g., at BHLYP level). This dependence on the theoretical level appears to derive from a bifurcation between two different bonding situations and is discussed in terms of charge transfer between X and Mn, and in the context of self‐interaction errors exhibited by popular functionals.     

Zerovalent Rhodium and Iridium Silatranes Featuring Two‐Center,Three‐Electron Polar σ Bonds

Species with 2‐center, 3‐electron (2c/3e^?) σ bonds are of interest owing to their fascinating electronic structures and potential for interesting reactivity patterns. Report here is the synthesis and characterization of a pair of zerovalent (d⁹) trigonal pyramidal Rh and Ir complexes that feature 2c/3e^? σ bonds to the Si atom of a tripodal tris(phosphine)silatrane ligand. X‐ray diffraction, continuous wave and pulse electron paramagnetic resonance, density‐functional theory calculations, and reactivity studies have been used to characterize these electronically distinctive compounds. The data available highlight a 2c/3e^? bonding framework with a σ*‐SOMO of metal 4‐ or 5d_z² parentage that is partially stabilized by significant mixing with Si (3p_z) and metal (5‐ or 6p_z) orbitals. Metal‐ligand covalency thus buffers the expected destabilization of transition‐metal (TM)‐silyl σ*‐orbitals by d–p mixing, affording well‐characterized examples of TM–main group, and hence polar, 2c/3e^? σ “half‐bonds”.     

Zerovalent Rhodium and Iridium Silatranes Featuring Two‐Center,Three‐Electron Polar σ Bonds

Species with 2‐center, 3‐electron (2c/3e^?) σ bonds are of interest owing to their fascinating electronic structures and potential for interesting reactivity patterns. Report here is the synthesis and characterization of a pair of zerovalent (d⁹) trigonal pyramidal Rh and Ir complexes that feature 2c/3e^? σ bonds to the Si atom of a tripodal tris(phosphine)silatrane ligand. X‐ray diffraction, continuous wave and pulse electron paramagnetic resonance, density‐functional theory calculations, and reactivity studies have been used to characterize these electronically distinctive compounds. The data available highlight a 2c/3e^? bonding framework with a σ*‐SOMO of metal 4‐ or 5d_z² parentage that is partially stabilized by significant mixing with Si (3p_z) and metal (5‐ or 6p_z) orbitals. Metal‐ligand covalency thus buffers the expected destabilization of transition‐metal (TM)‐silyl σ*‐orbitals by d–p mixing, affording well‐characterized examples of TM–main group, and hence polar, 2c/3e^? σ “half‐bonds”.     

Synthesis,structure and catalytic polymerization activity of half‐sandwich cyclometallated iridium complexes

A series of mononuclear half‐sandwich cyclometallated iridium complexes with Schiff base ligands were synthesized in good yields. Five air‐stable C,N‐chelate mode complexes were obtained smoothly through metal‐mediated C─H bond activation. Treatments of dimeric metal complexes [Cp*IrCl₂]₂ with ligands L1–L5 afforded the corresponding C,N‐chelate mononuclear half‐sandwich iridium(III) complexes 1 – 5 . These iridium complexes exhibit high catalytic activity for norbornene polymerization. Both steric and electronic effects of the substituted groups have influences on the behaviors of the polymerization process. All complexes were characterized using infrared and NMR spectroscopies and elemental analysis. Molecular structures of complexes 1 , 2 and 5 were further confirmed using single‐crystal X‐ray analysis.     

Synthesis,Structure, and Reactivity of Ferrocenyl‐NHC ­Palladium Complexes

palladium complexes of ferrocenyl‐functionalized N‐heterocyclic carbenes with different substituents were synthesized. The molecular structures of selected complexes were determined by X‐ray diffraction and show a pseudo‐square‐planar structure with a central palladium atom surrounded by carbene, pyridine, and two chloride ligands. The influence of the different substituents on the structure and reactivity of the complexes was studied. The catalytic properties of the complexes were investigated in the Larock indolization reactions of 2‐bromoanilines with diphenylacetylene. Their performances slightly varied in this reaction, but the complex with mesityl substituent showed the best activity.     

Nitridorhenium(V) Complexes with 1,3,4‐Triphenyl‐1,2,4‐triazol‐5‐ylidene

[ReNCl₂(PPh₃)₂] and [ReNCl₂(PMe₂Ph)₃] react with the N‐heterocyclic carbene (NHC) 1,3,4‐triphenyl‐1,2,4‐triazol‐5‐ylidene (HL^Ph) under formation of the stable rhenium(V) nitrido complex [ReNCl(HL^Ph)(L^Ph)], which contains one of the two NHC ligands with an additional orthometallation. The rhenium atom in the product is five‐coordinate with a distorted square‐pyramidal coordination sphere. The position trans to the nitrido ligand is blocked by one phenyl ring of the monodentate HL^Ph ligand. The Re–C(carbene) bond lengths of 2.072(6) and 2.074(6) Å are comparably long and indicate mainly σ‐bonding between the NHC ligand and the electron deficient d² metal atom. The chloro ligand in [ReNCl(HL^Ph)(L^Ph)] is labile and can be replaced by ligands such as pseudohalides or monoanionic thiolates such as diphenyldithiophosphinate (Ph₂PS₂^?) or pyridine‐2‐thiolate (pyS^?). X‐ray structure analyses of [ReN(CN)(HL^Ph)(L^Ph)] and [ReN(pyS)(HL^Ph)(L^Ph)] show that the bonding situation of the NHC ligands (Re–C(carbene) distances between 2.086(3) and 2.130(3) Å) in the product is not significantly influenced by the ligand exchange. The potentially bidentate pyS^? ligand is solely coordinated via its thiolato functionality. Hydrogen atoms of each one of the phenyl rings come close to the unoccupied sixth coordination positions of the rhenium atoms in the solid state structures of all complexes. Re–H distances between 2.620 and 2.712Å do not allow to discuss bonding, but with respect to the strong trans labilising influence of “N^3?”, weak interactions are indicated.     

The Triple‐Bond Metathesis of Aryldiazonium Salts: A Prospect for Dinitrogen Cleavage

The {N₂} unit of aryldiazonium salts undergoes unusually facile triple‐bond metathesis on treatment with molybdenum or tungsten alkylidyne ate complexes endowed with triphenylsilanolate ligands. The reaction transforms the alkylidyne unit into a nitrile and the aryldiazonium entity into an imido ligand on the metal center, as unambiguously confirmed by X‐ray structure analysis of two representative examples. A tungsten nitride ate complex is shown to react analogously. Since the bonding situation of an aryldiazonium salt is similar to that of metal complexes with end‐on‐bound dinitrogen, in which {N₂}→M σ donation is dominant and electron back donation minimal, the metathesis described herein is thought to be a conceptually novel strategy toward dinitrogen cleavage devoid of any redox steps and, therefore, orthogonal to the established methods.     

Nickel‐Triad Complexes of a Side‐on Coordinating Distannene

NHC adducts of the stannylene Trip₂Sn (Trip=2,4,6‐triisopropylphenyl) were reacted with zero‐valent Ni, Pd, and Pt precursor complexes to cleanly yield the respective metal complexes featuring a three‐membered ring moiety Sn‐Sn‐M along with carbene transfer onto the metal and complete substitution of the starting ligands. Thus the easily accessible NHC adducts to stannylenes are shown to be valuable precursors for transition‐metal complexes with an unexpected Sn? Sn bond. The complexes have been studied by X‐ray diffraction and NMR spectroscopy as well as DFT calculations. The compounds featuring the structural motif of a distannametallacycle comprised of a [(NHC)₂M⁰] fragment and Sn₂Trip₄ represent rare higher congeners of the well‐known olefin complexes. DFT calculations indicate the presence of a π‐type Sn–Sn interaction in these first examples for acyclic distannenes symmetrically coordinating to a zero‐valent transition metal.     

Phenylimidorhenium(V) Complexes with 1,3‐Diethyl‐4,5‐dimethylimidazole‐2‐ ylidene Ligands

The phenylimidorhenium(V) complexes [Re(NPh)X₃(PPh₃)₂] (X = Cl, Br) react with the N‐heterocyclic carbene (NHC) 1,3‐diethyl‐4,5‐dimethylimidazole‐2‐ylidene (L^Et) under formation of the stable rhenium(V) complex cations [Re(NPh)X(L^Et)₄]²⁺ (X = Cl, Br), which can be isolated as their chloride or [PF₆]^? salts. The compounds are remarkably stable against air, moisture and ligand exchange. The hydroxo species [Re(NPh)(OH)(L^Et)₄]²⁺ is formed when moist solvents are used during the synthesis. The rhenium atoms in all three complexes are coordinated in a distorted octahedral fashion with the four NHC ligands in equatorial planes of the molecules. The Re–C(carbene) bond lengths between 2.171(8) and 2.221(3) Å indicate mainly σ‐bonding between the NHC ligand and the electron deficient d² metal atoms. Attempts to prepare analogous phenylimido complexes from [Re(NPh)Cl₃(PPh₃)₂] and 1,3‐diisopropyl‐4,5‐dimethylimidazole‐2‐ylidene (L^i?Pr) led to a cleavage of the rhenium‐nitrogen multiple bond and the formation of the dioxo complex [ReO₂(L^i?Pr)₄]⁺.     

Highly Robust but Surface‐Active: An N‐Heterocyclic Carbene‐Stabilized Au25 Nanocluster

Surface organic ligands play a critical role in stabilizing atomically precise metal nanoclusters in solutions. However, it is still challenging to prepare highly robust ligated metal nanoclusters that are surface‐active for liquid‐phase catalysis without any pre‐treatment. Now, an N‐heterocyclic carbene‐stabilized Au₂₅ nanocluster with high thermal and air stabilities is presented as a homogenous catalyst for cycloisomerization of alkynyl amines to indoles. The nanocluster, characterized as [Au₂₅(ⁱPr₂‐bimy)₁₀Br₇]²⁺ (ⁱPr₂‐bimy=1,3‐diisopropylbenzimidazolin‐2‐ylidene) ( 1 ), was synthesized by direct reduction of AuSMe₂Cl and ⁱPr₂‐bimyAuBr with NaBH₄ in one pot. X‐ray crystallization analysis revealed that the cluster comprises two centered Au₁₃ icosahedra sharing a vertex. Cluster 1 is highly stable and can survive in solution at 80 °C for 12 h, which is superior to Au₂₅ nanoclusters passivated with phosphines or thiols. DFT computations reveal the origins of both electronic and thermal stability of 1 and point to the probable catalytic sites. This work provides new insights into the bonding capability of N‐heterocyclic carbene to Au in a cluster, and offers an opportunity to probe the catalytic mechanism at the atomic level.     

Piperazine‐ and DABCO‐bridged dinuclear N‐heterocyclic carbene palladium complexes: synthesis,structure and application to Hiyama coupling reaction

Four dinuclear N ‐heterocyclic carbene (NHC) palladium complexes were prepared by reaction of imidazolinium salts, PdCl₂ and bridging ligands (piperazine and DABCO) in one pot or by direct cleavage of the chloro‐bridged dimeric compounds [Pd(μ ‐Cl)(Cl)(NHC)]₂ with bridging ligands. All of the complexes were fully characterized using ¹H NMR, ¹³C NMR, high‐resolution mass and infrared spectroscopies, elemental analysis and single‐crystal X‐ray diffraction. The catalytic activities of the obtained palladium catalysts towards Hiyama coupling of aryl chlorides with phenyltrimethoxysilane were investigated and the results showed that the dinuclear palladium complexes were considerably active for the coupling reaction. Copyright © 2016 John Wiley & Sons, Ltd.     

Half‐sandwich ruthenium‐based versatile catalyst for both alcohol oxidation and catalytic hydrogenation of carbonyl compounds in aqueous media

A series of half‐sandwich ruthenium‐based catalysts for both alcohol oxidation and carbonyl compounds hydrogenation have been synthesized through metal‐induced C–H bond activation based on benzothiazole ligands. The neutral ruthenium complexes 1 – 4 were fully characterized by UV–vis, NMR, IR, and elemental analysis. Molecular structures of complexes 1 and 3 were further confirmed by X‐ray diffraction analysis. All complexes exhibited high activity for the catalytic oxidation of a variety of alcohols with ^tBuOOH as oxidants to give carbonyl compounds with high yields in water. Moreover, these half‐sandwich complexes also showed high efficiency for the catalytic hydrogenation of carbonyl compounds in a methanol–water mixture. The catalyst could be reused for at least five cycles without any loss of activity. The catalytic system also worked well for various kinds of substrates with either electron‐donating or electron‐withdrawing groups.     

C4-bound imidazolylidenes: from curiosities to high-impact carbene ligands

This feature article summarizes the progress achieved thus far in using C4-bound imidazolylidenes as a new class of ligands for transition metals. Since the discovery of this unusual carbene bonding mode in 2001, various rational routes towards complexes containing C4-bound carbenes have evolved. These advances allowed for studying the impact of this new type of ligand on the transition metal center, both from a fundamental point of view as well as from a more applied perspective, in particular for catalytic applications. The promising results accomplished in this relatively short period of time demonstrate the potential of C4-bound imidazolylidenes as unique carbene ligands for inducing catalytic activity and for mediating unprecedented transformations.     

Cationic nitrile‐functionalized Ag(I)‐ and Hg(II)‐N‐heterocyclic carbene complexes of CCC,CNC, and NCN pincer‐type carbene ligands: Synthesis,crystal structures,and characterization

Three nitrile‐functionalized (benz)imid‐ zaolium derivatives with CCC, CNC, and NCN coordination pockets have been synthesized and isolated as the bromide salts and are converted into their hexafluorophosphate counterparts ( 5–7 ) by ligand metathesis. N‐heterocyclic carbene Ag(I)‐ and Hg(II)‐complexes ( 8–10 and 11–13 ) are readily formed in good to excellent yields from ligand precursors 5–7 and Ag₂O and Hg(OAc)₂, respectively. All reported compounds have been characterized by various spectral and analytical techniques, such as FTIR, ¹H, and ¹³C NMR spectroscopy and X‐ray diffraction studies. Solid‐state structures of carbene precursors 5 , 7 , and an Ag(I)–carbene complex 10 have been determined crystallographically. Single crystal X‐ray crystallography of complex 10 reveals the chelation of carbene carbons to metal center in almost a linear manner. © 2012 Wiley Periodicals, Inc. Heteroatom Chem 23:486–497, 2012; View this article online at wileyonlinelibrary.com . DOI 10.1002/hc.21041     

Organo‐ and Hydrogelators Based on Luminescent Monocationic Terpyridyl Platinum(II) Complexes with Biphenylacetylide Ligands

A series of phosphorescent terpyridyl platinum(II) complexes with ancillary biphenylacetylide ligands, namely, [(R₃tpy)PtC≡C(biphenyl)]X (R=tBu, H, or Et₂N; tpy=2,2′;6′,2′′‐terpyridyl; X is an anion) were synthesized and structurally characterized by various spectroscopic techniques and X‐ray diffraction methods. Despite a lack of long alkyl chain(s) or hydrogen‐bonding motif(s), complexes [(tpy)PtC≡C(biphenyl)]Cl and [(tBu₃tpy)PtC≡C(biphenyl)]X (X=Cl, ClO₄, PF₆, or BF₄) were found to gelate water and organic solvents, respectively. The self‐aggregation of these complexes in solutions and the resulting gels were investigated with variable‐temperature (VT) ¹H NMR spectroscopy, polarized optical microscopy, and absorption/emission spectroscopy. SEM micrographs on dry gels revealed entangled nanofibers with diameters of 20–40 nm and lengths of tens of micrometers. Powder X‐ray diffraction (PXRD) study revealed various degrees of crystallinity of these fibrillar nanostructures. The substituents on both the terpyridyl and acetylide ligands and counterion of these complexes play a profound but concerted role in tuning the intermolecular metal???metal and/or π–π interactions, and hence the gelation properties.     

Theoretical insights into the relative bonding of normal and abnormal N‐heterocyclic carbenes in [PdCl2(NHCR)2] and [PdCl2(NHCR)(aNHCR)] (R = H,Ph, Mes)

Quantum chemical insights into normal Pd‐C2(NHC^R) and abnormal Pd‐C5(aNHC^R) bonding, dominated by dispersion interactions in N‐hetereocyclic carbene complexes [PdCl₂(NHC^R)₂] ( I , R = H; II , R = Ph; III , R = Mes (2,4,6‐trimethyl)phenyl)) and [PdCl₂(NHC^R)(aNHC^R] ( IV , R = H; V , R = Ph; VI , R = Mes) have been investigated at DFT and DFT‐D3(BJ) level of theory with particular emphasis on the effects of the noncovalent interactions on the structures and the nature of Pd‐C2(NHC^R) and Pd‐C5(aNHC^R) bonds. The optimized geometries are good agreement with the experimental values. The Pd‐C bonds are essentially single bond. Hirshfeld charge distributions indicate that the abnormal aNHC^R carbene ligand is relatively better electron donor than the normal NHC^R carbene ligand. The C2 atom has larger %s contribution along Pd‐C2 bond than the C5 atom along Pd‐C5 bond. As a consequence the Pd‐C2(NHC^R) bonds are relative stronger than the Pd‐C5(aNHC^R) bonds. Thus, the results of natural hybrid orbital analysis support the key point of the present study. Calculations predict that for bulky substituent (R = Ph, Mes) at carbene, the Pd‐C2(NHC^R) bond is stronger than Pd‐C5(aNHC^R) bond due to large dispersion energy in [PdCl₂(NHC^R)₂] than in [PdCl₂(NHC^R)(aNHC^R)]. However, in case of non‐bulky substituent with small and almost equal contribution of dispersion energy, the Pd‐C2(NHC^R) bond is relative weaker than Pd‐C5(aNHC^R) bond. The bond dissociation energies are dependent on the R substituent, the DFT functional and the inclusion of dispersion interactions. Major point of this study is that the abnormal aNHCs are not always strongly bonded with metal center than the normal NHCs. Effects of dispersion interaction of substituent at nitrogen atoms of carbene ligand are found to play a crucial role on estimation of relative bonding strengths of the normal and abnormal aNHCs with metal center. © 2016 Wiley Periodicals, Inc.     

Effect of Ligand Fields on the Reactivity of O2‐Activating Iron(II)‐Benzilate Complexes of Neutral N5 Donor Ligands

Three new iron(II)‐benzilate complexes [(N4Py)Fe^II(benzilate)]ClO₄ ( 1 ), [(N4Py^Me2)Fe^II(benzilate)]ClO₄ ( 2 ) and [(N4Py^Me4)Fe^II(benzilate)]ClO₄ ( 3 ) of neutral pentadentate nitrogen donor ligands have been isolated and characterized to study their dioxygen reactivity. Single‐crystal X‐ray structures reveal a mononuclear six‐coordinate iron(II) center in each case, where benzilate binds to the iron center in monodentate mode via one carboxylate oxygen. Introduction of methyl groups in the 6‐positions of the pyridine rings makes the N4Py^Me2 and N4Py^Me4 ligand fields weaker compared to that of the parent N4Py ligand. All the complexes ( 1 – 3 ) react with dioxygen to decarboxylate the coordinated benzilate to benzophenone quantitatively. The decarboxylation is faster for the complex of the more sterically hindered ligand and follows the order 3 > 2 > 1 . The complexes display oxygen atom transfer reactivity to thioanisole and also exhibit hydrogen atom transfer reactions with substrates containing weak C?H bonds. Based on interception studies with external substrates, labelling experiments and Hammett analysis, a nucleophilic iron(II)‐hydroperoxo species is proposed to form upon two‐electron reductive activation of dioxygen by each iron(II)‐benzilate complex. The nucleophilic oxidants are converted to the corresponding electrophilic iron(IV)‐oxo oxidant upon treatment with a protic acid. The high‐spin iron(II)‐benzilate complex with the weakest ligand field results in the formation of a more reactive iron‐oxygen oxidant.     

Smooth C(alkyl)-H bond activation in rhodium complexes comprising abnormal carbene ligands

Rhodation of trimethylene-bridged diimidazolium salts induces the intramolecular activation of an alkane-type C-H bond and yields mono- and dimetallic complexes containing a formally monoanionic C,C,C-tridentate dicarbene ligand bound to each rhodium centre. Mechanistic investigation of the C(alkyl)-H bond activation revealed a significant rate enhancement when the carbene ligands are bound to the rhodium centre via C4 (instantaneous activation) as compared to C2-bound carbene homologues (activation incomplete after 2 days). The slow C-H activation in normal C2-bound carbene complexes allowed intermediates to be isolated and suggests a critical role of acetate in mediating the bond activation process. Computational modelling supported by spectroscopic analyses indicate that halide dissociation as well as formation of the agostic intermediate is substantially favoured with C4-bound carbenes. It is these processes that discriminate the C4- and C2-bound systems rather than the subsequent C-H bond activation, where the computed barriers are very similar in each case. The tridentate dicarbene ligand undergoes selective H/D exchange at the C5 position of the C4-bound carbene exclusively. A mechanism has been proposed for this process, which is based on the electronic separation of the abnormal carbene ligand into a cationic N-C-N amidinium unit and a metalla-allyl type M-C-C fragment.