Nanoclusters in ionic liquids: evidence for N-heterocyclic carbene formation from imidazolium-based ionic liquids detected by (2)H NMR

The mystery of how 1,3-substituted imidazolium-based ionic liquids (ILs) can provide high stabilization for transition-metal(0) nanoclusters, that is, in the absence of the usual strongly coordinating anions, has been probed. 2H NMR product and kinetic studies of 1,3-substituted imidazolium ILs under D2 reveal that nanocluster-catalyzed H/D exchange occurs at the 2- (as well as at the 4-, 5-, and 8-) C-H positions of the imidazolium cation. The results (i) provide compelling evidence that N-heterocyclic carbene formation and ligation of nanoclusters is occurring in ILs; and (ii) argue that N-heterocyclic carbenes merit further investigation as heretofore unappreciated stabilizers of transition-metal nanoclusters.     

Tridentate, anionic tethered N-heterocyclic carbene of Pd(II) complexes

The synthesis of a series of azolium salts such as azolium iodides and chlorides having both N-anionic functional group and N-alkyl group have been developed. Reaction of azolium iodides or chlorides with Ag₂O gave the corresponding NHC-Ag complexes. It was found that the resulting NHC-Ag complexes derived from azolium iodides or chlorides differ in their physical properties. The azolium chlorides as well as azolium iodides were successfully converted into the NHC-Ag complexes, which subsequently reacted with PdCl₂(CH₃CN)₂ to give the anionic amidate/NHC-Pd complexes. Thus, a variety of the NHC-Pd complexes could be obtained from benzimidazolium and imidazolium salts.     

4-Vinylbenzyl-substituted silver(I) N-heterocyclic carbene complexes and ruthenium(II) N-heterocyclic carbene complexes: synthesis and transfer hydrogenation of ketones

4-Vinylbenzyl-substituted Ag(I) N-heterocyclic carbene (NHC) complexes and Ru(II) NHC complexes have been synthesized. The Ag(I) complexes were synthesized from the imidazolium salts and Ag₂O in dichloromethane at room temperature. The Ru(II) complexes were prepared from Ag(I) NHC complexes by transmetallation. The six 4-Vinylbenzyl-substituted Ag(I) NHC complexes and six 4-Vinylbenzyl-substituted Ru(II) NHC complexes have been characterized by spectroscopic techniques and elemental analyses. The Ru(II) NHC complexes show catalytic activity for the transfer hydrogenation of ketones.     

Searching for molecular arene hydrogenation catalysis in ionic liquids

Arene hydrogenation by homogeneous catalysts is a highly controversial area of research, with many of the mononuclear complexes shown to catalyse the reaction, being found to be pre-catalysts to nanoparticles, on closer examination. The solvent properties of ionic liquids, i.e., low nucleophilicity and high polarity, make them ideal, at least in principal, for homogeneous arene hydrogenation catalysts. In this paper, we described our attempts to prepare and study such systems, using either simple metal halides or ruthenium complexes including trinuclear ruthenium clusters as catalyst precursors.     

Silver(I)-carbene complexes/ionic liquids: novel N-heterocyclic carbene delivery agents for organocatalytic transformations

[reaction: see text] N-Heterocyclic carbene (NHC) complexes with silver were investigated as sources of unsaturated NHC carbene catalysts via thermal decomposition. The NHC complex (1-ethyl-3-methylimidazol-2-ylidene)silver(I) chloride is an ionic liquid, and was found to catalyze the ring-opening polymerization of lactide at elevated temperatures to give narrowly dispersed polylactide of predictable molecular weight. Silver-carbene complexes can also be used for the catalysis of small molecule transesterification reactions. Thermolysis of the silver complexes in the presence of CS(2) yielded the zwitterionic CS(2) adducts of the carbene, implicating the intermediacy of the free carbene in these reactions.     

Carbon dioxide hydrogenation: Efficient catalysis by an NHC-amidate Pd(II) complex

The utilization of carbon dioxide as a carbon source has long been a challenge in modern organic chemistry due to its low reactivity, yet high abundance. Herein we demonstrate the highly efficient hydrogenation of carbon dioxide into formic acid in the presence of an NHC-amidate Pd(II) complex. Excellent turnover number was observed when the catalyst was used under heterolytic conditions. This catalytic system provides a new and efficient carbon dioxide hydrogenation method.     

New N-heterocyclic carbene palladium complex/ionic liquid matrix immobilized on silica: application as recoverable catalyst for the Heck reaction

[reaction: see text] A new concept of simultaneous covalent anchoring of a N-heterocyclic carbene palladium/ionic liquid matrix on the silica surface and the application of the resulting catalyst in the Heck reaction of a variety of different haloarenes is described. The catalyst shows high thermal stability (up to 280 degrees C) and could be recovered and reused for four reaction cycles, giving a total TON congruent with 36 600. Furthermore, TEM coupled with EDX analysis indicate the formation of Pd nanoparticles within the immobilized IL layer.     

A highly efficient, recyclable catalyst for C-C coupling reactions in ionic liquids: pyrazolyl-functionalized N-heterocyclic carbene complex of palladium(II)

[reaction: see text] A hemilabile pyrazolyl-functionalized N-heterocyclic carbene complex of palladium(II) has been synthesized. It is an excellent catalyst for Heck and Suzuki cross-coupling reactions in ionic liquids.     

Cross-coupling and dehalogenation reactions catalyzed by (N-heterocyclic carbene)Pd(allyl)Cl complexes

A series of well-defined, air- and moisture-stable (NHC)Pd(allyl)Cl (NHC = N-heterocyclic carbene) complexes has been used in several catalytic reactions: Suzuki-Miyaura cross-coupling, catalytic dehalogenation of aryl halides, and aryl amination. The scope of the three processes using various substrates was examined. A general system involving the use of (IPr)Pd(allyl)Cl as catalyst and NaO(t)Bu as base has proven to be highly active for the Suzuki-Miyaura cross-coupling of activated and unactivated aryl chlorides and bromides, for the catalytic dehalogenation of aryl chlorides, and for the catalytic aryl amination of aryl triflates. All reactions proceed in short reaction times and at mild temperatures. The system has also proven to be compatible with the microwave-assisted Suzuki-Miyaura cross-coupling and catalytic dehalogenation processes, affording yields similar to those of the conventionally heated analogous reactions.     

Sulfur-functionalized N-heterocyclic carbene complexes of Pd(II): syntheses, structures and catalytic activities

N-heterocyclic carbenes (NHCs) can be easily modified by introducing functional groups at the nitrogen atoms, which leads to versatile coordination chemistry as well as diverse catalytic applications of the resulting complexes. This article summarizes our contributions to the field of NHCs bearing different types of sulfur functions, i.e., thioether, sulfoxide, thiophene, and thiolato. The experimental evidence for the truly hemilabile coordination behavior of a Pd(II) thioether-NHC complex has been reported as well. In addition, complexes bearing rigid CSC-pincer ligands have been synthesized and the reasons for pincer versus pseudo-pincer formation investigated. Incorporation of the electron-rich thiolato function resulted in the isolation of structurally diverse complexes. The catalytic activities of selected complexes have been tested in Suzuki-Miyaura, Mizoroki-Heck and hydroamination reactions.     

Synthesis of Pd particles with various shapes by ionic liquids for HFP hydrogenation catalyst

Palladium particles were simply synthesized using various ionic liquids. The morphology of the particles was significantly affected by the anion parts of the ionic liquids. Among the ionic liquids, hexafluorophosphate as an anion part was more effective in forming the palladium particles with relatively small and narrow size distribution. However, irregularly shaped palladium particles were synthesized without ionic liquid assistance. For a hexafluoropropylene hydrogenation to produce hydrofluorocarbons, palladium was impregnated on a carbon powder as a catalyst. During the preparation of the catalyst, ionic liquids were added to control the shape of the palladium on the support. After calcinations at 500 °C, all catalysts possessed the comparable crystal structure. Under identical reaction conditions, the catalyst prepared using 1-hexyl-3-methylimidazolium hexafluorophosphate was the most effective in this reaction. Hence, catalytic activity was mainly determined by the size of the palladium particles.     

Syntheses and characterizations of thiolato-functionalized N-heterocyclic carbene Pd(II) complexes with normal and mesoionic binding modes

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 studies of 1 with NaSCH(CH(3))(2) and NaBF(4) afforded the tetranuclear compound 3 with a [Pd(4)S(4)] macrocycle. All complexes have been fully characterized by multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis.     

Synthesis of ruthenium(II) N-heterocyclic carbene complexes and their catalytic activities in transfer hydrogenation of ketones

Three RuCl₂(η⁶-arene, η¹-carbene) and two RuCl₂(NHC)(arene) complexes have been prepared by the reaction of bis(1,3-dialkylperhydrobenzimidazol-2-ylidene) (1) and bis(1,3-dialkyl-4-methylzimidazolin-2-ylidene) (3) with [RuCl₂(arene)]₂ in toluene and characterized by elemental analysis, ¹H NMR, ¹³C NMR and IR spectroscopy. The catalytic activities of these complexes were examined in the transfer hydrogenation of aromatic ketones using 2-propanol as hydrogen source.     

Application of ionic liquids in palladium(II) catalyzed homogenous transfer hydrogenation

The first application of palladium(II) catalyzed homogenous transfer hydrogenations in ionic liquids is described. Cinnamic acid and its derivatives were reduced in high yields under mild conditions using ammonium formate as hydrogen donor.     

N,N'-dialkylimidazolium chloroplatinate(II), chloroplatinate(IV), and chloroiridate(IV) salts and an N-heterocyclic carbene complex of platinum(II): synthesis in ionic liquids and crystal structures

The first imidazole-type carbene complex of platinum(II), cis-(C2H4)(1-ethyl-3-methylimidazol-2-ylidene)PtCl2, has been obtained by reacting PtCl2 and PtCl4 with ethylene in the basic [EMIM]Cl/AlCl3 (1.3:1) ionic liquid (where [EMIM]+ = 1-ethyl-3-methylimidazolium) at 200 degrees C and structurally characterized (monoclinic P21/c space group, a = 10.416(2) A, b = 7.3421(9) A, c = 15.613(2) A, beta = 101.53(2) degrees, Z = 4). This complex can be regarded as a stable analogue of the pi-alkene-Pd(II)-carbene intermediate in the Heck reaction. In addition, a series of new N,N'-dialkylimidazolium salts of platinum group metals of the type [RMIM]2[MCln], where [RMIM+] = 1-alkyl-3-methylimidazolium and M = Pt(II), Pt(IV), or Ir(IV), have been prepared and characterized. The salts [EMIM]2[PtCl6] (1) and [EMIM]2[PtCl4] (2) were prepared in the ionic liquid [EMIM]Cl/AlCl3 and the salts [BMIM]2[PtCl4] (3) and [BMIM]2[PtCl6] (4) (where [BMIM]+ = 1-n-butyl-3-methylimidazolium) and [EMIM]2-[IrCl6] (5) in aqueous or acetonitrile media. From TGA measurements, salts 1-5 decompose in air in several steps eventually to form the corresponding metal, the onset of decomposition being observed at (degree C) 260 (1), 220 (2), 200 (3), 215 (4), and 210 (5). The structures of 1, 2, and 5 were determined by single-crystal X-ray analysis. The three salts crystallize in the monoclinic P21/n space group (1, a = 7.6433(9) A, b = 16.353(2) A, c = 9.213(1) A, beta = 113.56(1) degrees, Z = 2; 2, a = 8.601(1) A, b = 8.095(2) A, c = 13.977(2) A, beta = 91.75(2) degrees, Z = 2; 5, a = 10.353(2) A, b = 9.759(2) A, c = 10.371(2) A, beta = 92.98(3) degrees, Z = 2).     

Controlled synthesis of nickel(II) dihalides bearing two different or identical N-heterocyclic carbene ligands and the influence of carbene ligands on their structures and catalysis

Ni(II) dihalides bearing two different or identical NHC ligands have been prepared via a controlled indene elimination synthesis, and the former product provides a new route for the design of biscarbene Ni(II)-based catalysts. The indene elimination reaction of the indenynickel(II) complex (1-H-Ind)Ni(NHC)X (Ind = indenyl) with one equiv. of a distinct imidazolium salt at 100 °C afforded the first example of Ni(II) dihalides bearing two different NHC ligands, i.e., Ni(iPr)(IPr)X(2) [iPr = 1,3-diisopropylimidazol-2-ylidene, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), X = Cl, 1; X = Br, 2] and Ni(iPr)(IMes)Br(2) [IMes = 1,3-bis(mesityl)imidazol-2-ylidene, 3]. Alternatively, complexes 1-3 can be synthesized using a bis-indenyl Ni(II) complex (1-H-Ind)(2)Ni as starting materials via a step-by-step indene elimination at different reaction temperatures. The direct reaction of (1-R-Ind)(2)Ni (R = H or Me) with two equiv. of imidazolium salts at 100 °C afforded Ni(II) dihalides bearing two identical NHC ligands, i.e., Ni(iPr)X(2) (X = Cl, 4; X = Br, 5) and Ni(IPr)Cl(2) (6). All of these complexes were characterized by elemental analysis, NMR spectroscopy and X-ray crystallography for complexes 1-5. The two identical or different NHC ligands in complexes 1-6 changed the coordination sphere of the nickel center from a typical square-planar geometry to a slightly tetrahedral array. A preliminary catalytic study on the cross-coupling reactions of aryl Grignard reagents with aryl halides revealed that complexes 1 and 2 possess the highest activity. In comparison, complexes 3 and 6 exhibited moderate activity and the least active complexes were 4 and 5.     

Complexing of Pd(II) and Pt(II) by dithiooxamide immobilized on SiO2

It is shown that dithiooxamide immobilized on SiO₂ can bind Pd(II) and Pt(II) from aqueous chloride solutions by complexing. Values have been derived for the effective Pd(II) and Pt(II) sorption constants for dithiooxamide immobilized on SiO₂, which represent stronger binding of Pd(II) than Pt(II). L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 31 Prospekt Nauki, Kiev 252039, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 34, No. 6, pp. 366–370, November–December, 1998.