The preparation and properties of mono- versus bis(carbene) Pd(II) complexes bearing unsymmetrical cyano- and ester-functionalized NHC-ligands as potential IR probes were studied in detail. Direct reaction of Pd(OAc)₂ with functionalized imidazolium salts afforded either bis(carbene) (3a, c) or mono-carbene complexes (5, 6) with a N-coordinated imidazole co-ligand. The latter were exclusively obtained with N-ethylene substituted salts, which were found to undergo N–C cleavage reaction. The milder Ag-carbene transfer reaction on the other hand was tolerable to the length of the substituents and the nature of the functional groups. All bis(carbene) complexes (3a-c, 4a-c) were obtained as a inseparable mixture of square-planar trans-anti and trans-syn rotamers. The identity, ratio and dynamic equilibrium of these rotamers have been investigated and the relatively high rotational barrier for rotamers of 3a was estimated to be about 74 kJ mol^-1 at 380 K. All eight complexes were fully characterized by NMR and IR spectroscopies, ESI mass spectrometry and X-ray single crystal and powder diffraction. A preliminary catalytic study showed that ester-functionalized complexes 4a and 4b gave rise to highly active catalyst in the double Mizoroki–Heck coupling of aryl dibromides, while the in situ ester-hydrolyzed complexes were also active in the coupling of activated aryl chlorides.

Mononuclear mixed-ligand complexes of Pd(II) containing a N,S-heterocyclic carbene (NSHC) with a secondary alkyl N-substituent and pyridyl ligand, with the general formula [PdI₂(C₁₀H₁₁NS)L] (C₁₀H₁₁NS = 3-isopropylbenzothiazolin-2-ylidene; L = pyridine, 2-aminopyridine, 3-iodopyridine and 4-tert-butyl-pyridine) have been synthesized and characterized by X-ray single-crystal crystallography. Both solution and solid-state structures, as evident from their ¹H NMR spectra and X-ray structures, show anagostic γ-hydrogen interactions of metal with methine of the substituent on the carbene or pyridyl ligand giving 5-membered-chelate-like structures.

Reaction of the bromo-bridged dimeric monocarbene complex [PdBr₂(ⁱPr₂-bimy)]₂ (1) with various isocyanides afforded a series of mixed carbene-isocyanide complexes [PdBr₂(ⁱPr₂-bimy)(CN-R)] (2a: R = Cy; 2b: R = ⁿBu; 2c: R = Xyl) as inseparable mixtures of trans- and cis-isomers. The reactivity of 2c towards nucleophiles was studied. A new mixed NHC-ADC Pd(II) complex (4) was obtained in low yield when 2c was reacted with 2,6-dimethylaniline. Reaction of 2c with hydrazine yielded a hydrazine-bridged complex (6) via ligand substitution. In addition, salt metathesis of 2c with AgO₂C₂F₃ afforded a cis arranged complex [Pd(O₂CCF₃)₂(ⁱPr₂-bimy)(CN-Xyl)] (cis-7). Most complexes were fully characterized by multinuclei NMR spectroscopies, ESI or FAB mass spectrometry and X-ray diffraction analysis.

The formation of mono- versus bis(chelate) Ni(II) complexes bearing N-heterocyclic dicarbene ligands can be controlled by the flexibility of the ligand bridge. A short methylene spacer exclusively gives rise to a dicationic bis(chelate) complex [Ni(^MeCC^meth)₂]Br₂ (1), whereas a more flexible propylene spacer affords a neutral mono-chelate complex [NiBr₂(^MeCC^prop)] (2). Complex 2 was found to autoionize very slowly to the corresponding dicationic bis(chelate) over ~45 days in d₆-DMSO. The formation of bis versus mono-chelate can be attributed to the different stabilities of the resulting metallocycles. The catalytic activity of the mono-chelate 2 was tested in the Kumada-Corriu coupling of aryl halides with aryl-magnesium reagents at ambient temperature.

Salt metathesis reaction of the dihalo-bis(carbene) complex trans-[NiBr₂(NHC)₂] (1, NHC = 1,3-diisopropylbenzimidazolin-2-ylidene) with NaN₃ in DMF at elevated temperature afforded the diazido-bis(carbene) complex trans-[Ni(N₃)₂(NHC)₂] (2) as a red crystalline solid in a yield of 78%. Complex 2 served as metal-template for the 1,3-dipolar cycloaddition of 2,6-dimethylphenylisocyanide (CN-Xyl) to the azido ligands to yield a mixed tetrazolato-carbodiimido complex trans-[Ni(CN₄-Xyl)(NCN-Xyl)(NHC)₂] (3) at ambient temperature and a dicarbodiimido complex trans-[Ni(NCN-Xyl)₂(NHC)₂] (4) at 70 °C. Reaction of alkyl isocyanides with 2 at ambient temperature gave the ditetrazolato complexes trans-[Ni(CN₄-R)₂(NHC)₂] (5, R = tert-butyl; 6, R = cyclohexyl) in good yields. A novel cationic “abnormal” tetrazolin-5-ylidene complex trans-[Ni(CN₄-^tBu,Me)₂(NHC)₂](BF₄)₂ (7, ^tBu = tert-butyl) was synthesized by direct methylation of 5 with [Me₃O]BF₄. All compounds have been fully characterized by multinuclei NMR spectroscopies and ESI mass spectrometry. The solid state molecular structures of complexes 2, 4, 5, and 7•2CH₂Cl₂ have also been confirmed by X-ray diffraction studies.

Three 4-iodopyrazolium salts with 3,5-dimethyl (3a), 3,5-diphenyl (3b) and 3,5-diisopropyl (3c) substituents, respectively, were synthesized using a modular approach. The oxidative addition of 3a-c to Pd₂(dba)₃/PPh₃ afforded products (trans-4a, cis-/trans-4b and 4c) of different geometries or connectivities indicating a dramatic substituent effect on the formation of pyrazolin-4-ylidene complexes. In addition, the reactions of 3a-c with Pd₂(dba)₃ in the presence of pyridine yielded new mixed pyrazolin-4-ylidene/pyridine complexes (5a-c). All complexes have been fully characterized by multi nuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analyses. Furthermore, an initial catalytic study in Suzuki-Miyaura and Mizoroki-Heck cross-coupling reactions also reveals a significant substituent effect on catalytic activities.

Dicarbene complexes [Pd(OAc)₂(diNHC)] (2), [Pd(O₂CCF₃)₂(diNHC)] (3) and [Pd(CNCH₃)₂(diNHC)](SO₃CF₃)₂ (4) bearing labile acetato, fluoroacetato and acetonitrile co-ligands have been synthesized via metathesis reaction of the respective precursor [PdBr₂(diNHC)] (1) with Ag-salts. All complexes are stable toward air and moisture and have been fully characterized by spectroscopic and spectrometric methods. Notably and unlike diphosphine analogues, they resist ligand disproportionation in solution. Their molecular structures have also been determined by single crystal X-ray diffraction. A preliminary catalytic study showed low activity in the hydroamination reaction, but revealed an interesting co-ligand influence.

A series of mononuclear N,N-heterocyclic carbene (NNHC) complexes of Pd^II with mixed ligands of 1,3-dibenzylbenzimidazoly-2-ylidene and solvate (dimethyl sulfoxide, CH₃CN, N,N-dimethylformamide, and pyridine) or PPh₃ were prepared and characterized by X-ray single-crystal diffraction analysis. They are more active in the Suzuki–Miyaura coupling of selected aryl bromides than their N,S-heterocyclic carbene (NSHC) analogues.

Reaction of thioether- and sulfoxide-functionalized dibenzimidazolium dibromides B⋅ 2HBr and C⋅ 2HBr with Pd(OAc)₂ afforded two new pseudo-pincer complexes cis-[PdBr₂(B-κ²C)] (1) and cis-[PdBr₂(C-κ²C)] (2), which contain pendant sulfur-functions. On the other hand, palladation of the dinitrate analogue B⋅ 2HNO₃ in the presence of 1 equiv of KBr yields the first NHC-based CSC pincer complex [PdBr(B-κ³CSC)]NO₃ (3). All three complexes have been fully characterized by multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis. Their catalytic activities in the Mizoroki-Heck reaction are reported as well.

A series of mixed-ligand, mononuclear complexes trans-[PdX₂(NSHC)(azole)] [NSHC = N,S-heterocyclic carbene; azole = imidazole, 1-(2,4,6-trimethylphenyl)-1H-imidazole, benzimidazole, benzothiazole and benzoxazole] have been prepared and characterised by X-ray single-crystal diffraction. These complexes catalyse the sp²–sp³ cross-coupling of fluoroaryl halides with arylboronic acid to give diarylmethanes with good yields and high turnovers.

The electronic parameters of 25 Werner-type and organometallic ligands have been experimentally determined and ranked on an unprecedented unified ¹³C NMR scale using safe and easily obtainable complexes of the type trans-[PdBr₂(ⁱPr₂-bimy)L]^n- (ⁱPr₂-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene; L = ligand in question) as spectroscopic probes. The methodology is based on the sensitivity of the constant ⁱPr₂-bimy carbene signal to the donor strengths of the varying co-ligands, which even allows detection of backbone and substituent effects more accurately than previous carbonyl-based systems. For the evaluation of N-heterocyclic carbenes (NHCs), a one-pot approach to novel hetero-bis(carbene) complexes bearing two different NHCs is introduced. Furthermore, the first complex of a strongly donating indazolin-3-ylidene ligand is presented. The molecular structures of ten complex-probes have been characterized by single-crystal X-ray diffraction analyses.

The reaction of [Pt^II(2-phenylpyridine)(acac)] and benzothiazolium bromide yields the N,S-heterocyclic carbene ligand trans to pyridyl while, surprisingly, a very similar N,N-heterocyclic carbene coordinates predominantly trans to the cyclometallated carbon.

Influence your neighbor: The first examples of pyrazole-based dicarbene complexes are described (for an example see figure). Remote changes in the ligand topology four bonds away from the carbon donor have substantial influences on the nuclearity of the resulting complexes.

Four imidazolium (5a/b) and benzimidazolium (6a/b) salts with hemilabile alkyl-aryl thioether functions have been prepared via a straightforward and modular pathway in order to compare their reactivities toward palladation. Reaction of 5a/b with Pd(OAc)2 gave complex product mixtures, whereas 6a/b afforded the desired bis(benzimidazolin-2-ylidene) complexes 8a/b in good yields. The difference in reactivities of benzimidazole versus imidazole derivatives was attributed to the presence of additional acidic protons at C4/5 positions of the imidazolium ring, leading to competing and unselective deprotonation reactions. The milder Ag-NHC transfer reaction, on the other hand, provided either mono- or bis(imidazolin-2-ylidene) complexes (9 or 7a/b) in good yields depending on the ligand:metal ratio. The interesting hemilability in monocarbene complex 9 was investigated by spectroscopy and thioether displacement reaction with PPh₃, yielding the mixed NHC-PPh₃ complex 10 in high yields. An initial comparative catalytic study also reveals that the mixed-donor complex 10 exhibits the highest activity among the complexes tested.

The versatile coordination chemistry of strongly donating indazolin-3-ylidene ligands as new members of the NHC family is demonstrated on 12 new PdII, AuI and RhI complexes, which were fully characterized by multinuclear NMR spectroscopies, ESI mass spectrometry and X-ray diffraction studies.

A new thiophene-functionalized benzimidazolium salt (2) has been prepared by reacting N-methylbenzimidazole with 2-bromomethylthiophene (1), which in turn was obtained by bromination of 2 thiophenemethanol with PBr₃. Subsequent reaction of salt 2 with Pd(OAc)₂ afforded the cis configured bis(carbene) Pd(II) complex (cis-3), which in solution exists as an inseparable mixture of cis-anti and cis-syn-rotamers in a 3.5:1 ratio. All new compounds have been fully characterized by spectroscopic and spectrometric methods. A preliminary catalytic study shows that cis-3 is highly active in the Suzuki-Miyaura coupling of aryl bromides with phenylboronic acid in/on water as environmentally benign reaction media.

Metathetical reaction of AgSCN with a series of trans-dihalido-bis(carbene) nickel(II) complexes in CH3CN readily afforded the novel diisothiocyanato-bis(carbene) complexes [Ni(NCS)2(NHC)2] (trans-2a, NHC = 1,3-diisopropylbenzimidazolin-2-ylidene; trans-2b, NHC = 1,3-diisobutylbenzimidazolin-2-ylidene; trans-2c, NHC = 1,3-dibenzylbenzimidazolin-2-ylidene; cis-2d, NHC = 1,3-di(2-propenyl)benzimidazolin-2-ylidene; cis-2e, NHC = 1-propyl-3-methylbenzimidazolin-2-ylidene) as greenish yellow powders in moderate to good yields. While dihalido-bis(carbene) Ni(II) complexes exclusively form trans-complexes, a trans-cis isomerization occurs upon halido-isothiocyanato exchange with complexes bearing less bulky carbene ligands, i.e. cis-2d/e. DFT calculations indicated that this isomerization can be attributed to a reduced energy difference between trans and cis isomers of diisothiocyanato complexes. All complexes have been characterized by multinuclear NMR spectroscopy, ESI mass spectrometry and X-ray diffraction analysis. A catalytic study revealed that cis-complexes generally exhibit greater activities in the Kumada-Corriu coupling reaction.

The thiolato-bridged dimeric Pd(II) benzimidazolin-2-ylidene complex 1, with a [Pd₂S₂] core, was conveniently prepared by the reaction of the thiol-functionalized benzimidazolium salt C and Pd(OAc)₂. More straightforwardly, 1 can also be synthesized by direct treatment of the thioester-functionalized benzimidazolium salt B with Pd(OAc)₂ in wet DMSO under in situ hydrolysis of the thioester function. A subsequent salt metathesis reaction of 1 with AgO₂CCF₃ afforded the mixed dicarboxylato/NHC analogue 2 in quantitative yield, leaving the sulfur bridges of the [Pd₂S₂] core unaffected despite the use of the soft Ag(I) ions. Treatment of 1 with Me₃OBF₄ resulted in an unexpected bromido abstraction of 1 leading to an unusual rearrangement/dimerization reaction to give the tetranuclear NHC complex 3, which features a [Pd₂S₂] macrocylic square with sulfur corners. These reactions demonstrate the structural diversity of the thiolato-functionalized N-heterocyclic carbene complexes and may offer access to metallo-NHC-based supramolecular architectures. A comparative catalytic study revealed the superiority of NHC/thiolato complex 2 over complexes 1 and 3 in aqueous Suzuki-Miyaura couplings at very low catalyst loading.

Three cis-chelating di-N-heterocyclic carbene palladium(II) complexes [PdX₂(diNHC)] (X = I, 1; X = SCN, 2; X = CF₃CO₂, 3) bearing different anionic co-ligands were synthesized and fully characterized. A comparison of their catalytic activities in the Mizoroki-Heck reaction and conjugate addition of arylboronic acids to cyclic enones revealed increasing efficiency in the order SCN^- < I^- < CF₃CO₂^-. The di(trifluoroacetato) complex 3 showed the best activity in both transformations highlighting the importance of co-ligands effects in catalysis. In addition, the molecular structure of an unusual poly-heteronuclear complex salt 4 is reported, which has been isolated as a byproduct in the synthesis of complex 3.

The immense success of N-heterocyclic carbenes in recent years has initiated the search for even stronger ligands leading to the discovery of abnormal and remote NHCs. This article reflects our particular interest in the coordination chemistry of pyrazolin-4-ylidenes as a contribution to this field. A modular approach to 4-iodopyrazolium salts with different substitution patterns is described, which upon oxidative addition to Pd⁰ gave rise to a library of new Pd(II) pyrazolin-4-ylidene complexes. A preliminary study showed that selected complexes are active precatalysts in Suzuki–Miyaura and Mizoroki–Heck coupling reactions. The 3,5-substituents of pyrazolin-4-ylidenes are found to have significant effects on complexation and catalytic activities. Finally, the nature of this new class of ligands is discussed, and a future direction for further explorations in this exciting field is envisioned.

Pd(II) dimers [PdI₂(rNHC)]₂ (1a/b) of pyrazole-based remote N-heterocyclic carbenes (rNHCs) have been synthesized through oxidative addition of 4-iodopyrazolium iodides (A/B) to [Pd₂(dba)₃]. Reaction of 1a with aromatic (CN-Xyl) or aliphatic (CN-Cy) isocyanides led to the template-assisted formation of novel Pd(II) dimers 8/9 bearing betainic C-imino ligands via isocyanide insertion into Pd-C_rNHC bonds and subsequent dimerization. In contrast, both isocyanides reacted with dimers [PdI₂(Me₂-indy)]₂ (2) and [PdI₂(Me₂-bimy)]₂ (3) bearing indazolin-3-ylidenes and benzimidazolin-2-ylidenes under formation of mononuclear mixed carbene/isocyanide complexes 4-7. Notably, only dimer 8 underwent further bridge-cleavage with excess isocyanide yielding the mixed C-imino/CN-Xyl complex 10, while dimer 9 remained intact. These results highlight the uniquely different reactivity of complexes with carbenes with no α-nitrogen versus those with one or two α-nitrogen atoms as a result of their decreasing donor abilities.

A series of carbene complexes [PdBr₂(ⁱPr₂-bimy)L] (C2-C13) with different types of co-ligands (L) have been tested for their catalytic activities in the carbonylative annulation of 2-iodophenol with phenylacetylene in DMF to afford the respective flavone 2a. Complex C12 with an N-phenylimidazole co-ligand showed the best activity and also afforded high yields when the substrate scope was extended to other aryl or pyridyl acetylenes. In addition, catalyst C12 was also efficient in the carbonylative annulation of 2-iodoaniline with acid chlorides giving the desirable 2-substituted 4H-3,1-benzoxazin-4-ones (4) in good yields. Additionally, this Pd-NHC complex also proved to be a very efficient catalyst for the hydroxycarbonylation of iodobenzene derivatives at low catalyst loading and under low CO pressure. These results demonstrate the versatility and efficiency of this phosphine-free Pd(II)-NHC complex in different types of carbonylations of aryl iodides under mild conditions.

Three thioether bridged diimidazolium dibromides with different steric and electronic properties have been synthesized as precursors to carbene-based CSC pincer ligands. Palladation afforded CSC Pd(II) pincer complexes for bulky and electron rich ligand systems, whereas the least donating ligand led to the formation of a pseudo-pincer complex. All complexes have been fully characterized by multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis. The catalytic activities of pincer versus pseudo-pincer complexes have been compared in the intermolecular hydroamination of alkynes with anilines as well.

Bridge cleavage reactions of the dimeric monocarbene complex [PdBr₂(ⁱPr₂-bimy)]₂ can be effectively used to end-cap pyridine containing pseudorotaxanes affording stable [2]rotaxanes.

A series of palladium(II) complexes (1-6) bearing cis-chelating homo-dicarbene ligands with varying alkyl bridges (C1-C3) and N-heterocyclic backbones (imidazole and benzimidazole) have been synthesized by reaction of Pd(OAc)₂ with the respective diazolium bromides (A•2HBr – F•2HBr) in DMSO. A comparative catalytic study employing aryl chlorides in the Mizoroki-Heck reaction revealed the superiority of methylene- and propylene-bridged dibenzimidazolin-2-ylidenes over their imidazole-derived analogues. Based on these results, two new propylene-bridged hetero-dicarbene complexes (7 and 8) were designed containing a mixed benzimidazole/imidazole derived NHC-donor set. Notably, both complexes outperformed their homo-dicarbene analogues, which may be due to the electronic asymmetry induced by hetero-dicarbene ligands. The molecular structures of complex 6 and 8 are also presented.

The thiolato-bridged dimeric Pd(II) NHC complex 1 has been synthesized from the reaction of thioester-functionalized imidazolium salt B and Pd(OAc)₂. The isolation of its interesting constitutional isomer 2 bearing both classical C(2)-bound and mesoionic C(4)-bound ligands coordinating to two different metal centers in the same complex allowed for a direct comparison of these isomeric carbenes. Reactivity study of 1 with NaSCH(CH₃)₂ and NaBF₄ afforded the tetranuclear compound 3 with a [Pd₄S₄] macrocycle. All complexes have been fully characterized by multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis.

A series of hetero-bis(carbene) complexes trans-[PdBr₂ (ⁱPr₂-bimy)(trz)] 1–4 (ⁱPr₂-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene; trz = 1,2,3-triazolin-5-ylidene) bearing the constant ⁱPr₂-bimy and varying mesoionic 1,2,3-triazolin-5-ylidenes with different N-substituents has been synthesized as complex probes. Their ¹³C NMR spectroscopic evaluation shows that mesoionic 1,2,3-triazolin-5-ylidenes are in general stronger donors than classical NHCs, while weaker than some nonclassical NHCs such as pyrazolin-3-ylidenes and mesoionic imidazolin-4-ylidenes. More important and for the first time, this methodology proves useful in establishing substituent effects in the donating abilities of 1,2,3-triazolin-5-ylidenes on a finer level. In addition, the trifluoroacetato analogues [Pd(O₂CCF₃)₂ (ⁱPr₂-bimy)(trz)] 5–7 have been synthesized through salt metathesis of 1, 2 and 4 with AgO₂CCF₃. The catalytic activities of complexes 1, 2 and 4–7 were examined in the direct arylation of pentafluorobenzene. Complexes bearing less donating trz ligands perform better in this catalysis, and trifluoroacetato complexes outperformed their bromido analogues.

A gold(I) NHC complex bearing a heteroalicyclic indazolin-3-ylidene ligand, [AuCl(Indy)] (1) (Indy = 6,7,8,9-tetrahydropyridazino[1,2-a]indazolin-3-ylidene), has been synthesized via the silver-carbene transfer method. Conversion of complex 1 to its heavier halido analogues [AuBr(Indy)] (2) and [AuI(Indy)] (3) was achieved by metathesis reactions involving LiBr and NaI in acetone, respectively. In contrast to 1 and 2, complex 3 undergoes ligand disproportionation/autoionization upon crystallization forming the solid complex salts [Au₃I₂(Indy)₄][Au₃I₄(Indy)₂] (3′) or [Au(Indy)₂][AuI₂] (3″) depending on the solvent used. This reversible process assisted by aurophilic interactions, and only occurring in the iodido complex 3, has been studied further by spectroscopic comparison with [Au(Indy)₂]BF₄ (4) and selective conversion of 3 to the gold(III) species [AuI₃(Indy)] (5). All complexes 1–5 have been fully characterized using multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis.