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.