As a part of a broader study directed towards helical coordination compounds with benzenedithiolate donors, we have synthesized the bis(benzenedithiol) ligands 1,2-bis(2,3-dimercaptobenzamido)ethane (H₄-1) and 1,2-bis(2,3-dimercaptophenyl)ethane (H₄-2). Both ligands form dinuclear complexes with Ni^II, Ni^III and, after air-oxidation, Co^III ions under equilibrium conditions. Complexes (NEt₄)₄[Ni^II₂(1)₂] (11b), (NEt₄)₂[Ni^III₂(1)₂] (13), and Na₄-[Ni^II₂(2)₂] (14) were characterized by X-ray diffraction. In all complexes, two square-planar [Ni(S₂C₆H₃R)₂] units are linked in a double-stranded fashion by the carbon backbone and they assume a coplanar arrangement in a stairlike manner. Cyclic voltammetric investigations show a strong dependence of the redox potential on the type of the ligand. The substitution of 1^4- for 2^4- on nickel (-785 mV for 11b versus -1130 mV for 14, relative to ferrocene) affects the redox potential to a similar degree as the substitution of nickel for cobalt (-1160 mV for [Co₂(1)₂]^2-/[Co₂(1)₂]^4-, relative to ferrocene). The redox waves display a markedly less reversible behavior for complexes with the shorter bridged ligand 2^4- compared to those of 1^4-.

The tris(benzene-1,2-dithiolato) complex [W^VI(C₆H₄S₂-1,2)₃] 1 was synthesized from [W(CH₃)₆] and C₆H₄(SH)₂-1,2 in diethyl ether. Crystals of [W^VI(C₆H₄S2-1,2)₃] were obtained from a saturated dichloromethane solution at room temperature. The X-ray crystal structure analysis revealed that the tungsten atom in 1 is coordinated in an almost perfect trigonal-prismatic fashion with W-S distances between 2.3724(14) Å and 2.3840(14) Å.

The ortho-lithiation of benzene-1,2-dithiol with three equivalents of nBuLi afforded mono- as well as di-C-lithiated intermediates. The surprising kinetically controlled formation of di-C-lithiated species offers the opportunity to obtain dimercaptobenzoic and -terephthalic acid derivatives in a one pot synthesis in reasonable yields. Both compounds are versatile building blocks for the synthesis of macrocycles containing arylenedithiol units. Focusing on novel terephthalic acid derivatives, two preparation routes, via amide and via alkyl linkage, respectively, have been elaborated. The reaction sequence (i) sulfur protection using either isopropyl or benzyl groups, (ii) transformation of the carboxylic function into an amide or an alkyl group and (iii) subsequent removal of the protection groups afforded the terephthaldiamidedithiol 6 and the 1,2-bis(2,3-dimercaptophenyl)ethane 11.

The red title compound was synthesized by the metathesis reaction of [FeCl₂(dppe)] (dppe = Ph₂P(CH₂)₂PPh₂) and NaS-tBu in THF. The X-ray structure analysis revealed a dinuclear complex with two iron(II) centers coordinated in a distorted tetrahedral fashion by two bridging thiolates, one bridging dppe molecule and one terminal chloro ligand.

Mixed carbene-carboxylate complexes of Palladium(II) have been prepared by reacting {1,1’-dimethyl-3,3’methylene-diimidazoline-2,2’-diylidene}palladium(II) diiodide 1 with AgO₂CR, where R = CF₃, CF₂CF₃ and CF₂CF₂CF₃. In this manner, {1,1’-dimethyl-3,3’methylenediimidazoline-2,2’-diylidene}palladium(II) bis(trifluoroacetate) (2), {1,1’-dimethyl-3,3’methylenediimidazoline-2,2’-diylidene}palladium(II) bis(pentafluoropropionate) (3) and {1,1’-dimethyl-3,3’methylenedi-imidazoline-2,2’-diylidene}palladium(II) bis(heptafluorobutyrate) (4) were obtained. All three complexes were fully characterized by ¹H-, ¹³C- and ¹⁹F-NMR spectroscopy as well as ESI mass spectrometry. X-ray crystal structure analyses of complexes 3 and 4 reveal mononuclear species with a square planar metal center coordinated by a cis-chelating dicarbene and two monodentate carboxylate ligands. The results show that the introduction of a cis-chelating N,N-heterocyclic carbene ligand stabilizes the palladium-carboxylate moiety effectively.

Reaction of N,N’-dimethylbenzimidazolyl iodide A with Pd(OAc)₂ in DMSO gives selectively trans-bis(N,N’-dimethylbenzimidazoline-2-ylidene) palladium(II) diiodide (trans-2) in 77% yield. The selective formation of the trans-coordination isomer and thus the cis-trans rearrangement is driven by the insolubility of trans-2 in DMSO. X-ray single-crystal diffraction analysis and ¹³C NMR spectroscopy confirm the trans-geometry of the square planar Pd(II) complex. Catalytic studies show that cis-1 and trans-2 are highly efficient in the Mizoroki-Heck coupling reaction of aryl bromides and activated aryl chlorides both in DMF and [N(n-C₄H₉)₄]Br as ionic liquid. The catalytic activities of Pd(II) complexes with N-heterocyclic carbene ligands derived from benzimidazole are comparable to their imidazole-derived analogues.

A series of N,N’-dialkylimidazolium bis(nonafluorobutane-1-sulfonyl)imides was synthesized in high yields by quaternization of imidazole derivatives with various readily available alkylating reagents, followed by anion exchange with highly stable and non-hygroscopic potassium bis(nonafluorobutane-1-sulfonyl)imide. The latter was obtained by an improved method starting from ammonium chloride and nonafluorobutane-1-sulfonyl fluoride. The quaternary imidazolium salts thus obtained constitute a new subfamily of thermally stable and remarkably hydrophobic ionic liquids with melting points in the range 0–40 °C and solubilities in water and organic solvents (aromatic hydrocarbons, dialkyl ethers) in the range of 0.5–1.5 wt%. The ionic liquids can be easily purified from ionic byproducts (e.g., halogenide salts) by aqueous extraction followed by thorough drying in a high vacuum without loss of yield. Due to the above features, these new ionic fluids may be considered as promising recyclable media in repeated catalytic processes.

The syntheses of tert-alkyl nitroso compounds RCH₂CMe₂N=O from commercially available tert-alkyl amines RCH₂CMe₂NH₂ proceed cleanly via the intermediacy of the benzoyl derivatives RCH₂CMe₂NHOC(O)Ph and the corresponding hydroxylamines RCH₂CMe₂NHOH. Since the intermediates require no purification in the course of the transformations, the overall yields of the isolated crystalline nitroso dimers (75–80% for R = H, 75% for R = Me and 66% for R = Me₃C) are based on the corresponding amine precursors. In the latter case (R = Me₃C), significant steric demands and hydrophobicity of Me₃CCH₂CMe₂ group necessitate the application of more efficient reagents and conditions on the debenzoylation and oxidation steps. The syntheses are perfectly suitable for scale-up and were successfully performed on up to 500-mmol scale.

The reaction of Ni(OAc)₂ with the benzimidazolium salts 1,3-dimethylbenzimidazolium iodide (1), 1,3-bis(2-propenyl)benzimidazolium bromide (2), 1,3-dipropylbenzimidazolium bromide (3), 1-(2-propenyl)-3-methylbenzimidazolium bromide (6), and 1-propyl-3-methylbenzimidazolium iodide (7) in molten [Bu₄N]X (X = Br, I, BF₄) as ionic liquid afforded novel square-planar nickel(II) bis(benzimidazolin-2-ylidene) complexes of the general formula trans-[NiX₂(NHC)₂] (X = I, NHC = 1,3-dimethylbenzimidazolin-2-ylidene, 8; X = Br, NHC = 1,3-bis(2-propenyl)benzimidazolin-2-ylidene, 9; X = Br, NHC = 1,3-dipropylbenzimidazolin-2-ylidene, 10; X = Br, NHC = 1-(2-propenyl)-3-methylbenzimidazolin-2-ylidene, 11; X = I, NHC = 1-propyl-3-methylbenzimidazolin-2-ylidene, 12). X-ray diffraction studies of 8-12 reveal a square-planar coordination geometry for all complexes with the carbene ligands arranged in a trans fashion.

Mixed dicarboxylato-bis(carbene) complexes of palladium(II) have been prepared by reacting cis-diiodo-bis(N,N’-dimethylbenzimidazolin-2-ylidene)palladium(II) (cis-A) with AgO₂CR, where R ) CH₃, CF₃, and CF₂CF₃. In this manner, cis-diacetato-bis(N,N’-dimethylbenzimidazolin-2-ylidene)palladium(II) (1), cis-di(trifluoroacetato)-bis(N,N’-dimethylbenzimidazolin-2-ylidene)palladium(II) (2), and cis-di(pentafluoro-propionato)-bis(N,N’-dimethylbenzimidazolin-2-ylidene)palladium(II) (3) were obtained. Complexes 1-3 were fully characterized by multinuclear NMR spectroscopies as well as ESI mass spectrometry. X-ray crystal structure analyses of the first mixed carboxylato/benzimidazolin-2-ylidene complexes reveal mononuclear species with a square-planar palladium(II) center coordinated by two monodentate carbene and two monodentate carboxylato ligands in a cis arrangement. The cis configuration was found to be thermodynamically favored in this type of complexes. The results also show that the introduction of N-heterocyclic carbene ligands stabilizes the palladium-carboxylate moiety effectively, thus preventing both reductive decomposition and autoionization processes. A preliminary catalytic study revealed that complexes 1-3 are highly active in the Mizoroki-Heck coupling of aryl bromides and activated aryl chlorides.

The reaction of allyl bromide with benzothiazole under neat conditions furnished 3-(2-propenyl)-benzothiazolium bromide, (H-1)Br, in high yield. Attempts to synthesize the corresponding carbene dimer by deprotonation of (H-1)⁺ led to the isolation of the rearrangement product 2,3-di(2-propenyl)-2’,3’-dihydro-2,2’-bisbenzothiazole (2). The reaction of [Ir(μ-OMe)(cod)]₂ with the salt (H-1)Br unexpectedly afforded [IrBr(cod)(benzothiazole)] (3) (cod = 1,5-cyclooctadiene), which contained an N-coordinated unsubstituted benzothiazole ligand. The formation of carbene complexes of Ir^I with the benzothiazolin-2-ylidene ligand could be achieved via precoordination of the allyl substituent of (H-1)⁺ to [Ir(cod)(MeCN)₂]BF₄ and subsequent deprotonation at the C2 position of (H-1)⁺ by addition of base. The use of NaH as external base yielded the square planar Ir^I complex 4 with an N-propyl-substituted carbene ligand, while deprotonation with KOtBu gave the five-coordinated Ir^I complex [IrBr(cod)(η²-1)], 5. Displacement of the cod ligand in complex 4 by two CO ligands afforded the complex [IrBr(CO)₂(NHC)] (NHC = 3-propylbenzothiazolin-2-ylidene), 6, which allowed an estimation of the σ-donor capabilities of benzothiazolin-2-ylidene ligands. Compounds 1-6 have been characterized spectroscopically, and the molecular structures of (H-1)Br and 2-5 were determined by X-ray diffraction.

The sterically bulky carbene precursor 1,3-diisopropylbenzimidazolium bromide (ⁱPr₂-bimyH+Br-) (A) has been prepared by an improved method in 84% yield. Reaction of A with Pd(OAc)₂ and NaBr gave the dimeric Pd(II) benzimidazolin-2-ylidene complex [PdBr₂(ⁱPr₂-bimy)]₂ (1), which can be easily cleaved by CH₃CN, another equivalent of salt A, and triphenylphosphine to afford the novel benzannulated monocarbene complexes trans-[PdBr₂(CH₃CN)( ⁱPr₂-bimy)] (2), (ⁱPr₂-bimyH)[PdBr₃(ⁱPr₂-bimy)] (3), trans-[PdBr₂(ⁱPr₂-bimy)(Ph₃P)] (trans-4), and cis-[PdBr₂(ⁱPr₂-bimy)(Ph₃P)] (cis-4), respectively. All compounds have been fully characterized by multinuclei NMR spectroscopies and mass spectrometries (FAB, ESI). X-ray diffraction studies on single crystals of 1-3 and cis-4 revealed a square planar geometry and a fixed orientation of the N-isopropyl substituents with the C-H group pointing to the metal center to maximize C-H⋅⋅⋅Pd interactions. The large downfield shift of the C-H protons in the ¹H NMR spectrum compared to the precursor A indicates that these C-H⋅⋅⋅Pd interactions are retained in solution and better described as weak hydrogen bonds, rather than as agostic interactions. Furthermore, the molecular structures of especially complexes 2 and 3 clearly show a bending of the bromo ligands toward the carbene carbon atom in order to maximize intramolecular C_carbene⋅⋅⋅Br interactions. The nature of these interactions can be attributed to a form of back-bonding to the formally vacant p-orbital of the C_carbene atom with the electron density originating from the bromo ligands’ lone pairs. A detailed study on the trans-cis isomerization of the mixed NHC-phosphine complexes 4 revealed that a cis arrangement in such complexes is thermodynamically favored. Furthermore, a preliminary catalytic study shows that complex 1 is highly active in the Suzuki-Miyaura coupling of aryl bromides and chlorides in pure water as environmentally benign solvent.

The truely hemilabile nature of a novel thioether-functionalized N-heterocyclic carbene ligand is demonstrated in a range of Pd(II) complexes.

Under solventless conditions, benzothiazole reacts with benzyl bromide to give near-quantitative yield of the salt N-benzylbenzothiazolium bromide (A), which is a convenient air-stable heterocyclic carbene precursor. Treatment of A with Pd(OAc)₂ in CH₃CN affords the bis(carbene) complex cis-[PdBr₂(NHC)₂] (1) (NHC = N-benzylbenzothiazolin-2-ylidene). In DMSO, this reaction yields an unprecedented dinuclear N,S-heterocyclic carbene complex, [PdBr₂(NHC)]₂ (2). Complex 2 undergoes bridge cleavage reactions with CH₃CN and DMF to give the mononuclear and solvated monocarbene complexes trans-[PdBr₂(NHC)(Solv)] [Solv = CH₃CN (3) and DMF (4)]. All compounds have been fully characterized by ¹H and ¹³C NMR spectroscopy, ESI or FAB mass spectrometry, and elemental analysis. The molecular structures of A and 1-4 have been determined by X-ray single-crystal diffraction. The catalytic activities of 1-4 toward Mizoroki-Heck coupling reactions of aryl bromides with tert-butyl acrylate are described and compared.

The bis(benzene-o-dithiol) ligands H₄-1, H₄-2 and H₄-3 react with [Ti(OC₂H₅)₄] to give dinuclear triple-stranded helicates [Ti₂L₃]^4- (L = 1^4-, 2^4-, 3^4-). NMR spectroscopic investigations revealed that the complex anions possess C₃ symmetry in solution. A crystal structure analysis for (PNP)₄[Ti₂(2)₃] ((PNP)₄[14]) confirmed the C₃ symmetry for the complex anion in the solid state. The complex anion in Li(PNP)₃[Ti₂(1)₃] (Li(PNP) ₃[13]) does not exhibit C₃ symmetry in the solid state due to the formation of polymeric chains of lithium bridged complex anions. Complexes [13]^4- and [14]^4- were obtained as racemic mixtures of the Δ,Δ and Λ,Λ isomers. In contrast to that, complex (PNP)₄[Ti₂(3)₃] ((PNP)₄[15]) with the enantiomerically pure chiral ligand 3^4- shows a strong Cotton effect in the CD spectrum, indicating that the chirality of the ligands leads to the formation of chiral metal centers. The o-phenylene diamine bridged bis(benzene-o-dithiol) ligand H₄-4 reacts with Ti⁴⁺ to give the dinuclear double-stranded complex Li₂[Ti₂(4)₂(μ-OCH₃)₂] containing two bridging methoxy ligands between the metal centers. The crystal structure analysis and the ¹H NMR spectrum of (Ph₄As)₂[Ti₂(4)₂(μ-OCH₃)₂] ((Ph₄As)₂[(16]) reveal C₂ symmetry for the anion [Ti₂(4) ₂(μ-OCH₃)₂]^2-. For a comparative study the dicatechol ligand H₄-5, containing the same o-phenylene diamine bridging group as the bis(benzene-o-dithiol) ligands H₄-4 was prepared and reacted with [TiO(acac)₂] to give the dinuclear complex anion [Ti₂(5) ₂(μ-OCH₃)₂]^2-. The molecular structure of (PNP)₂[Ti₂(5)₂(μ-OCH₃)₂] ((PNP)₂[17]) contains a complex anion which is similar to [16]^2-, with the exception that strong N-H⋅⋅⋅O hydrogen bonds are formed in complex anion [17]^2-, while N-H⋅⋅⋅S hydrogen bonds are absent in complex anion [16]^2-.

The first pyrazolin-4-ylidene complexes of palladium(II) have been synthesized by oxidative addition of 4-iodopyrazolium salts to Pd₂(dba)₃/PPh₃ and were fully characterized by multinuclear NMR spectroscopies, ESI mass spectrometry and X-ray diffraction studies.

Thermolysis of {(n-C₄H₉)₄N[Mn^IICr^III(C₂O₄)₃]}_n, 1 at 500 °C for 10 h gives spinel Mn_1.5Cr_1.5O₄ which was characterized by powder XRD, SEM, FTIR and elemental analysis. The thermal conversion occurs by an internal redox process at ca. 400 °C in one step (TGA). It offers an alternative molecular source for an intermetallic oxide. Thermolysis of {[Ba^II₆ (H₂O)₁₇][Cr^III(C₂O₄)₃]₄}⋅ 7H₂O, 2 under similar conditions gives a mixture of Ba^IICr^VIO₄ and Ba^IICO₃. These results suggested that the oxalate ligand in a heterometallic complex may be a convenient source for oxides in intermetallic composites and that, under suitable conditions, both metals in the heterometallic complexes can be transferred to the intermetallic oxides. It suggested that composite metal oxides can be generated from hetero- and inter-metallic oxalato complexes at relatively low temperatures, which could serve as a convenient route for the preparation of technologically important composites.

The bis(N,N’-diisopropylbenzimidazolin-2-ylidene)Pd(II) complexes trans-[PdBr₂(ⁱPr₂-bimy)₂] (trans-1) and trans-[PdI₂(ⁱPr₂-bimy)₂] (trans-2) have been prepared in good yields by in situ deprotonation of the corresponding N,N’-diisopropylbenzimidazolium salt (ⁱPr₂-bimyH+X-) (A: X = Br, B: X = I) with Pd(OAc)₂ in DMSO at elevated temperature. Salt metathesis of trans-1 or trans-2 with AgO₂CCF₃ in refluxing CH₃CN afforded the novel mixed carbene-carboxylato complex cis-[Pd(O₂CCF₃)₂(ⁱPr₂-bimy)₂] (cis-3). This halo/trifluorocarboxylato ligand substitution can be regarded as a selective method for the synthesis of cis-configured bis(carbene) complexes. All compounds have been fully characterized by multinuclei NMR spectroscopies and ESI mass spectrometry. X-ray diffraction studies on single crystals of trans-1, trans-2 and cis-3 revealed a square planar geometry and a fixed orientation of the N-isopropyl substituents with the C-H protons pointing to the metal center to maximize rare C-H⋅⋅⋅Pd preagostic interactions. These interactions are also retained in solution as indicated by the large downfield shift of the isopropyl C-H protons in the ¹H NMR spectrum compared to those in precursor salts A or B. A preliminary catalytic study revealed that all complexes are highly active in the Mizoroki-Heck coupling of aryl bromides and chlorides. However, these complexes gave slower conversions as compared to catalysts with less bulky benzimidazolin-2-ylidenes. This is most likely due to the steric bulk of the ligands, which hamper a fast reductive formation of catalytically active Pd(0) species.

3-(2-propenyl)benzothiazolium bromide (A) provided a direct and simple entry to Pd(II) complexes with N,S-heterocyclic carbene (NSHC) ligands functionalized with an allyl pendant with hemilabile potential. Addition of salt A to Pd(OAc)₂ eliminated HOAc and afforded the bis(carbene) complexes cis [PdBr₂(NHSC)₂] (cis-1) (NSHC = 3-(2-propenyl)benzothiazolin-2-ylidene) and trans [PdBr₂(NHSC) ₂] (trans-1) along with the monocarbene complexes [PdBr₂(NSHC)] (2) and trans-[PdBr₂(NSHC)(benzothiazole-κN)] (3) as minor side products. Salt-metathesis of cis-1 with AgO₂CCF₃ yielded the mixed dicarboxylato-bis(carbene) complex cis-[Pd(O₂CCF₃)₂(NSHC)₂](4). Complexes cis-1, trans-1 and 4 were characterized by multinuclear NMR spectroscopies, ESI mass spectrometry and elemental analysis. The molecular structures of complexes cis-1, 2 and 3 have been determined by X-ray single crystal diffraction. Complexes cis-1 and 4 as well as a in situ mixture of Pd(OAc)₂ and salt A were found to be active towards Suzuki-Miyaura coupling of aryl bromides and activated aryl chlorides giving good conversions.

The reaction of PtBr₂ with NaOAc and 1,3-diisopropylbenzimidazolium bromide (A) in DMSO afforded the mixed monocarbene-DMSO complex cis-[PtBr₂(DMSO)(ⁱPr₂-bimy)] (cis-1) and the bis(carbene) complex trans-[PtBr₂(ⁱPr₂-bimy)₂] (trans-2). The DMSO ligand in cis-1 can be easily replaced by stronger donors such as triphenylphosphine and pyridine to give novel benzannulated monocarbene complexes trans-[PtBr₂(ⁱPr₂-bimy)(PPh₃)] (trans-3), cis-[PtBr₂(ⁱPr₂-bimy)(PPh₃)] (cis-3) and trans-[PtBr₂(ⁱPr₂-bimy)(Pyridine)] (trans-4), respectively. All compounds have been fully characterized by multinuclei NMR spectroscopies and mass spectrometry (ESI, FAB). X-ray diffraction studies on single crystals of cis-1, trans-2, cis-3 and trans-4 revealed a square planar geometry and a fixed orientation of the N-isopropyl substituents with the C-H protons pointing to the metal center to maximize interesting and rare C-H⋅⋅⋅Pt preagostic interactions. These interactions are also retained in solution as indicated by the large downfield shift of the isopropyl C-H protons in the ¹H NMR spectrum compared to that in the precursor salt A.

The Lewis acidic pincer with a labile triflate ligand, viz. [Pd(OTf)(PCP)] (PCP = ^-CH(CH₂CH₂PPh₂)₂) 1 was prepared from [PdCl(PCP)] with AgOTf. It reacts readily with neutral bidentate ligands [L = 4,4'-bipyridine (4,4'-bpy) and 1,1-bis(diphenylphosphino)ferrocene (dppf)] to give dinuclear PCP pincers [{Pd(PCP)}₂(μ-L)][OTf]₂ (L = 4,4′-bpy, 2; dppf, 3). [PdCl(PCP)] also reacts with 4-mercaptopyridine in the presence of KOH to give a Lewis basic pincer with a free pyridine functional group [Pd(4-Spy)(PCP)] 4. Its metalloligand character is exemplified by the isolation of an asymmetric dinuclear double-pincer complex [{Pd(PCP)}₂(μ-4-Spy)][PF₆] 6 bridged by an ambidentate pyridinethiolato ligand. Complexes 1, 2, 3, 4 and 6 have been characterized by single-crystal X-ray diffraction analyses.

Reaction of the bromo-bridged dimeric monocarbene complex [PdBr₂(ⁱPr₂-bimy)]₂ (1) with various bidentate N-heterocycles afforded novel linear dinuclear Pd(II) complexes {[PdBr₂(ⁱPr₂-bimy)]₂(μ-L)} {μ-L = 4,4’-bipyridine (2); μ-L = 4,4’-bipyridylethane (3); μ-L = 4,4’-bipyridylethylene (4)}. The mononuclear counterpart [PdBr₂(ⁱPr₂-bimy)(Pyridine)] (5) has also been synthesized by reaction of 1 with pyridine. All compounds have been fully characterized by multi nuclei NMR spectroscopies, FAB mass spectrometry and X-ray diffraction analyses. Molecular structures of 2-5 show a fixed orientation of the C-H protons in the N-isopropyl substituents towards the metal center suggesting interesting C-H⋅⋅⋅Pd preagostic interactions. These interactions are retained in solution as indicated by the large downfield shift of these C-H protons in the ¹H NMR spectrum. UV-Vis and CV studies revealed that the dinuclear complexes 2-4 have similar electrochemical behavior to the mononuclear species 5.

Two mono-cationic complexes of pyrazole-derived remote carbene ligands of the type trans [PdI(rNHC)(PPh₃)₂]⁺OTf^- {rNHC = 2-ethyl-3,5-dimethyl-1-phenylpyrazolin-4-ylidene (6a); rNHC = 1-ethyl-2,3,5-trimethylpyrazolin-4-ylidene (6b)}, were prepared by oxidative addition of 4 iodo-1,2,3,5-tetrasubstituted pyrazolium triflate salts to [Pd₂(dba)₃]/PPh₃ in good yields. Both compounds were fully characterized by multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis. A comparative study on the aqueous Suzuki-Miyaura catalytic activities of these cationic complexes with their previously reported neutral counterparts reveals the superiority of the former.

The reaction of Pd(OAc)₂ with 1,3-dibenzylbenzimidazolium bromide (A) and 1-propyl-3-methylbenzimidazolium iodide (B) afforded the dihalo-bis(carbene) complexes cis-[PdBr₂(Bz₂-bimy)₂] (1) and cis-[PdI₂(Pr,Me-bimy)₂] (2), respectively. Halide substitution of 1 and 2 with AgO₂CCH₃ gave the mixed diacetato-bis(carbene) complexes cis-[Pd(O₂CCH₃)₂(Bz₂-bimy)₂] (3) and cis-[Pd(O₂CCH₃)₂(Pr,Me-bimy)₂] (4). In situ deprotonation of isopropylthiol with the mixed carbene-carboxylato complexes 3 and 4 yielded the novel dipalladium complexes [Pd₂(μ-ⁱPr-S)₂(Bz₂-bimy)₄](BF₄)₂ (5) and [Pd₂(μ-ⁱPr-S)₂(Pr,Me-bimy)₄](BF₄)₂ (6) with a [Pd₂S₂] core solely supported by N-heterocyclic carbenes. All compounds have been fully characterized by multinuclei NMR spectroscopies and ESI mass spectrometry. The solid state molecular structures of complexes 2, 3, 5 and 6 have also been confirmed by X-ray diffraction studies.

The reaction of [AuCl(SMe₂)] with in situ generated [AgCl(ⁱPr₂-bimy)] (ⁱPr₂-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene), which in turn was obtained by the reaction of Ag₂O with 1,3-diisopropylbenzimidazolium bromide (ⁱPr₂-bimyH⁺Br^-, A), afforded the monocarbene Au(I) complex [AuCl(ⁱPr₂-bimy)] (1). Subsequent reaction of 1 and the ligand precursor ⁱPr₂-bimyH⁺BF₄^-, (B) in acetone in the presence of K₂CO₃ yielded the bis(carbene) complex [Au(ⁱPr₂-bimy)₂]BF₄ (2) as a white powder in 80% yield. The oxidative addition of elemental iodine to complex 2 gave the bis(carbene) Au(III) complex trans-[AuI₂(ⁱPr₂-bimy)₂]BF₄ (3) as an orange-red powder in 92% yield. All complexes 1-3 have been fully characterized by multinuclear NMR spectroscopies, ESI mass spectrometry, elemental analysis, and X-ray single crystal diffraction. Complexes 1 and 2 adopt a linear geometry around metal centers as expected for d¹⁰ metals. The geometry around the Au(III) metal center in 3 is essentially square-planar with two carbene ligands in trans-position to each other. Complex 3 shows absorption and photoluminescence properties owing to a ligand to metal charge transfer.

Novel Pd(II) mixed N,S-heterocyclic carbene (NSHC)-phosphine complexes of the general formula [PdBr₂(NSHC)(PR₃)] were obtained from bridge cleavage of dinuclear NSHC complexes of type [PdBr₂(NSHC)]₂ [NSHC = 3-benzylbenzothiazolin-2-ylidene and 3-propylbenzothiazolin-2-ylidene] with triphenylphosphine, tricyclohexylphosphine and 2-diphenylphosphanyl-pyridine. All complexes have been fully characterized by ¹H and ¹³C NMR spectroscopy, ESI mass spectrometry and elemental analysis. The X-ray crystal structures of complexes 3–8 are reported. The complexes exhibit moderate to good catalytic activity in the Suzuki–Miyaura coupling reaction of aryl bromides and chlorides.

Pincer PCP-Pd(II) complex [PdCl(PCP)] (1) (PCP = ^-CH(CH₂CH₂PPh₂)₂) reacts with AgNO₃ to give [Pd(NO₃)(PCP)] (2). Similar reaction with AgBF₄ gives the aqua complex [Pd(OH₂)(PCP)][BF₄] (3) and the dinuclear complex [{Pd(PCP)}₂(μ-Cl)][BF₄] (4) with singly bridging chloro ligand. All new complexes were characterized by NMR spectroscopy, ESI-MS and single-crystal X-ray diffraction. Complex 1 and the triflate complex [Pd(OTf)(PCP)] (5) are active towards Suzuki–Miyaura coupling between aryl bromides and phenyl boronic acid.

Nickel(II) bis(benzimidazolin-2-ylidene) complexes of the general formula [NiBr₂(NHC)₂] (NHC = 1,3-dibenzylbenzimidazolin-2-ylidene, 7; NHC = 1,3-diisopropylbenzimidazolin-2-ylidene, 8; NHC = 1,3-dibenzhydrylbenzimidazolin-2-ylidene, 9; NHC = 1,3-diisobutylbenzimidazolin-2-ylidene, 10; NHC = 1-isopropyl-3-benzylbenzimidazolin-2-ylidene, 11; NHC = 1-benzhydryl-3-benzylbenzimidazolin-2-ylidene, 12) have been prepared and fully characterized by spectroscopic methods and single-crystal X ray structure analyses. All complexes adopt a square-planar geometry with nickel as the crystallographic inversion center and a trans arrangement of the carbene ligands. For complexes 11 and 12 bearing unsymmetrically substituted ligands, only the trans-anti configuration was found in the solid state. In addition, the structures of 8, 9, 11 and 12 reveal a fixed orientation of the N-isopropyl and N benzhydryl substituents with the C-H groups pointing to the nickel(II) center to maximize rare intramolecular C-H⋅⋅⋅Ni anagostic or preagostic interactions. The large downfield shift of these C-H protons in the ¹H NMR spectrum compared to their precursor salts indicates that these interactions are retained in solution. Preliminary catalytic studies show that complexes 7-12 are active in the Ullmann-coupling of bromobenzene and 4-bromoanisole. In particular, complexes 8, 9 and 12 with sterically more demanding ligands exhibit the best catalytic activities. The coupling reaction was found to be successful when carried out in neat [Bu₄N]Br as ionic liquid, but not in dry DMF nor in DMF with [Bu₄N]Br as an additive.

A novel atom-efficient catalytic reaction, which converts imidazolium salts, with N-butenyl, N-substituted butenyl, and N-pentenyl substituents, into five- and six-membered fused-ring imidazolium and thiazolium salts has been developed. The reaction proceeds through azolium, C2-H, oxidative addition to Ni(0) followed by intramolecular insertion of the N-alkenyl double bond into the Ni hydride to give an intramolecularly bound carbene-Ni-alkyl intermediate. Reductive elimination of the linked carbene and alkyl groups gives the fused-ring azolium products and regenerates the Ni(0) catalyst. Products are potential building blocks for the synthesis of pharmaceuticals and novel ionic liquids. For example, 1,7-dimethyl-6,7-dihydro-5H-pyrrole[1,2-R]imidazolium bromide (2f), a five-membered fused-ring imidazolium salt, is formed from the catalytic ring closing of 1-butenyl-3-methylimidazolium bromide (1f). The reaction proceeds at moderate temperatures (50 °C) to give the products in high yield and selectivity. The catalyst was formed in situ from Ni(COD)₂ plus added ligand L (where L = IMes, SMes, IPr, SPr, 4,5-Me₂IPr, PPh₃, PCy₃, PCy₂(Biphenyl), P^tBu₃) in DMF.

Mixed-ligand N,S-heterocyclic carbene (NSHC) complexes, trans-[PdBr₂(NSHC)(Py)] (NSHC = 3-benzyl- or 3-propyl-benzothiazolin-2-ylidene), have been obtained from bridge-cleavage reactions of the dinuclear complex, [Pd(μ-Br)Br(NSHC)]₂, in pyridine at room temperature. Use of neutral N-bidentate donors (L = pyrazine, 1,2-bis(4-pyridyl)ethane, 4,4’-bipyridine and trans-1,2-bis(4-pyridyl)-ethylene) yields the dinuclear spacer-bridged [Pd₂Br₄(NSHC)₂(μ-L)] complexes. The X-ray single-crystal structures of the pyridyl, bridging pyrazine and 1,2-bis(4-pyridyl)ethane complexes are reported. These air-stable complexes are active in the Suzuki–Miyaura coupling reactions of selected aryl bromides. The dinuclear complexes are generally more active than their mononuclear pyridyl analogues. The benzyl derivatives consistently outperform the n-propyl counterparts.

Reaction of the sterically bulky 1,3-dibenzhydrylbenzimidazolium bromide (Bh₂-bimyH⁺Br) (A) with Pd(OAc)₂ in DMSO yielded a mono(carbene) Pd(II) complex (1) with a N-bound benzimidazole derivative, which resulted from an unusual NHC rearrangement reaction. Reaction of A with Ag₂O, on the other hand, cleanly gave the Ag(I) carbene complex [AgBr(Bh₂-bimy)] (2), which has been used as a carbene-transfer agent to prepare the acetonitrile complex trans-[PdBr₂(CH₃CN)(Bh₂-bimy)] (3). Dissociation of acetonitrile from complex 3 and subsequent dimerization afforded the dinuclear Pd(II) complex [PdBr₂(Bh₂-bimy)]₂ (4) in quantitative yield. All complexes were fully characterized by multinuclear NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis. Furthermore, the catalytic activity of complex 4 in aqueous Suzuki-Miyaura cross-coupling reactions was studied and compared with that of its previously reported less bulky analogue [PdBr₂(ⁱPr₂-bimy)]₂.