Bicyclic N-heterocyclic carbene (NHC) ligand precursors and their palladium complexes

Eight bicyclic amidinium precursors (3), prepared from R,S-tmcp (R,S-tmcp: (1R,3S)-diamino-1,2,2-trimethylcyclopentane) were described. Only five of the precursors (3a–e) could be converted to palladium complexes, (PdX₂(6,7-NHC)PEPPSI) (4) by treatment with PdCl₂, K₂CO₃, and pyridine (additional KBr was used for (PdBr₂(6,7-NHC)PEPPSI)). The salts and complexes were fully characterized by spectroscopic methods and X-ray crystallography.     

Oxidative addition of aryl chlorides to palladium N-heterocyclic carbene complexes and their role in catalytic arylamination

Density functional theory has been used to investigate various solvated species that may be formed from palladium bis N-heterocyclic carbene complexes, [Pd(cyclo-C{NRCH}₂)₂], (PdL₂) in benzene solution. Formation of an η²-arene complex is shown to stabilise a monocarbene species, PdL(η²-C₆H₅X), where the arene is either the solvent or a reacting aryl halide. Oxidative addition of an aryl chloride has been modelled, and the most likely transition state has been established as a PdL(arylchloride) species, with just one carbene ligand coordinated to the palladium. The catalytic cycle for aryl amination has been investigated and the oxidative addition of the aryl halide shown to be the rate determining step. Reductive elimination of the aryl amine has a lower activation energy. Oxidative addition of alkyl halides has been shown to be less favourable because of the absence of an unsaturated group, such as the aryl ring, to bond to the palladium.     

Synthesis and catalytic activity of pyrazolyl-functionalized N-heterocyclic carbene palladium complexes

Three pyrazolyl-functionalized N-heterocyclic carbene (NHC) palladium complexes based on 1-[2-(pyrazol-1-yl)phenyl]imidazole have been synthesized and characterized by physico-chemical and spectroscopic methods, and the structures of two of the complexes have been confirmed by single-crystal X-ray diffraction. The pyrazolyl-functionalized NHCs act as chelating N,C-bidentate ligands in these three complexes. Catalytic tests have proved that these complexes exhibit highly effective catalytic activity for the Suzuki–Miyaura and Mizoroki–Heck coupling reactions in water or aqueous/organic media under air. The substituents on the pyrazolyl ring exert different influences on the catalytic activity of the complexes in these coupling reactions.     

Preparation and characterization of the first pyrazole-based remote N-heterocyclic carbene complexes of palladium(II)

The first pyrazolin-4-ylidene complexes of palladium(II) have been synthesized by oxidative addition of 4-iodopyrazolium salts to Pd(2)(dba)3/PPh(3) and were fully characterized by multinuclear NMR spectroscopies, ESI mass spectrometry and X-ray diffraction studies.     

Mixed N-heterocycles/N-heterocyclic carbene palladium(II) allyl complexes as precatalysts for direct arylation of azoles with aryl bromides

A series of mixed N-heterocycles/N-heterocyclic carbene palladium(II) allyl complexes with general formula [(NHC)Pd(η³-allyl)]₂(μ₂-N-heterocycles)(BF₄)₂ were prepared in one pot based on anion metathesis of (NHC)Pd(η³-allyl)Cl complexes and then ligand replacement with N-heterocycles [N-heterocycles?=?pyrazine (pyz), 4,4′-bipyridine (bpy) and trans-4,4′-bipyridylethylene (bpe)]. The solid-state structures shown dinuclear structures with two palladium(II) centers holding together by bridged N-heterocycles. Initially investigation of the obtained complexes as precatalysts for direct CH bond arylation of azoles with aryl bromides was carried out.     

Carbamoyl-substituted N-heterocyclic carbene complexes of palladium(II): application to Sonogashira cross-coupling reactions

[reaction: see text]. The first examples of N-carbamoyl-substituted heterocyclic carbene Pd(II) complexes are described. These thermal and hydrolytically stable complexes are readily prepared from carbamoyl imidazolium salts and efficiently promote Sonogashira cross-coupling reactions under mild conditions. Cesium carbonate is also shown to be an effective base for the coupling of aryl bromides.     

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.     

Investigation of structural and biological properties of N-heterocyclic carbene silver(I) and palladium(II) complexes

abstract

Computational investigations were done on bis(1-allyl-3-benzyl-2,3-dihydro-1H-benzo[d]imidazol-2-yl)silver(I), bis(1-benzyl-3-butyl-2,3-dihydro-1H-benzo[d]imidazol-2-yl)silver(I), bis(1-allyl-3-benzyl-2,3-dihydro-1H-benzo[d]imidazol-2-yl)dibromidepalladium(II), and bis(1-benzyl-3-butyl-2,3-dihydro-1H-benzo[d]imidazol-2-yl)dibromidepalladium(II) complexes. Related complexes were optimized at different six calculation levels which are HF/6-31G(LANL2DZ), HF/6-31G(d,p)(LANL2DZ), B3LYP/6-31G(LANL2DZ), B3LYP/6-31G(d,p)(LANL2DZ), M062X/6-31G(LANL2DZ) and M062X/6-31G(d,p)(LANL2DZ) levels in vacuo. IR and NMR spectrum are calculated and examined in detail. Energy diagram of molecular orbitals, contour diagram of frontier molecular orbitals, molecular electrostatic potential maps and the harmonic surface of related molecules are examined in detail. Finally, interactions between mentioned complexes and related proteins (1BNA, 1JNX, and 2ING) are investigated in detail. As a result, it is found that biological and anti-cancer properties of silver N-heterocyclic carbene complexes are higher than those of palladium complexes.     

Synthesis of N-heterocyclic carbene rhenium(I) carbonyl complexes

Reaction of aminophosphinimine [RHN(CH(2))(2)N[double bond, length as m-dash]PPh(3)] (R = H, Et) with Re(2)(CO)(10) provided the NH-functionalized carbene rhenium complex [Re(2)(CNHCH(2)CH(2)NR)(CO)(9)] (3a, R = H, 3b, R = Et). Treatment of 3 with Br(2) provided the mono nuclear [Re(CNHCH(2)CH(2)NR)(CO)(4)Br] (1, R = H, 2, R = Et). However, NH-functionalized carbene complexes 1-3 did not undergo N-alkylation with alkyl halides to yield the N-substituted NHC complexes. The direct ligand substitution of [Re(CO)(5)Br] with a carbene donor was employed to prepare [Re(IMes(2))(CO)(4)Br] (6a, IMes(2) = 1,3-di-mesitylimidazol-2-ylidene; 6b, IMes(2) = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene). Analyses of spectroscopic and crystal data of 6a and 6b show similar corresponding data among these complexes, suggesting the saturated and unsaturated NHCs have similar bonding with Re(I) metal centers. Reduction of 6a and 6b with LiEt(3)BH yielded the corresponding hydrido complexes 7a-b [ReH(CO)(4)(IMes(2))], but not 1 and 2. Ligand substitution of 1, 6a and 6b toward 2,2'-bipyridine (bipy) was investigated. Crystal structures of 1, 3a-b, 6a-b and 7b were determined for characterization and comparison.     

N-heterocyclic carbene tethered amido complexes of palladium and platinum

With a view to applications in bifunctional catalysis, a modular cross-coupling strategy has been used to prepare amine bis(imidazolium) salts (3a and 3b) and an amine mono(imidazolium) salt (6) as precursors to chelating amido-NHC ligands. Treating the pro-ligands 3 with 3 equivalents of the bulky base KHMDS and Pd(OAc)(2) or PtCl(2)(COD) gave the four amido bis(N-heterocyclic carbene) pincer complexes [CNC-R]M-I [M = Pd (7) or Pt (8); R = i-Pr (a) or n-Bu (b)], including the first examples of platinum complexes of a CNC ligand. The reaction of 7a with AgOTf in pyridine gave the cationic complex {[CNC-i-Pr]Pd-py}OTf (9a). Heating a mixture of amine mono(imidazolium) salt 6 with PdCl(2) or K(2)PtCl(4), K(2)CO(3) and KI in pyridine at 100 °C gave the complexes [C,NH]MI(2)py [M = Pd (10) or Pt (11)], in which the amine arm of the NHC ligand is not deprotonated and does not coordinate to the metal. For a solution of 10 in 1,4-dioxane, deprotonation of the amine occurred in a biphasic reaction with aqueous KOH at 40 °C, giving the dimeric amido complex {[C,N]Pd(μ-OH)}(2) (12). The more inert Pt analogue 11 was unreactive under the same conditions. Solid-state structures of the complexes 7a, 7b, 9a, 10, 11 and 12 have been determined by single crystal X-ray diffraction.     

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.     

Anti-Markovnikov hydroaminations of styrene catalyzed by palladium(II) N-heterocyclic carbene complexes under conventional and microwave heating

Six palladium(II) complexes with benzimidazole-based N-heterocyclic carbene ligands were synthesized by transmetallation reactions between silver(I) N-heterocyclic carbene complexes and PdCl₂(PhCN)₂. The complexes were characterized by physicochemical and spectroscopic methods. The palladium complexes were tested as catalysts for intermolecular hydroamination reactions of styrene with various anilines in ionic liquids under both conventional and microwave heating. All of these complexes proved to be catalytically active in these reactions. The anti-Markovnikov addition products were selectively obtained by using 1 mol% of the palladium complex.     

Iron-catalyzed arylmagnesiation of aryl(alkyl)acetylenes in the presence of an N-heterocyclic carbene ligand

Addition of arylmagnesium bromides to aryl(alkyl)acetylenes proceeded in the presence of an iron catalyst and a N-heterocyclic carbene ligand to give high yields of the corresponding alkenylmagnesium reagents, which were transformed into tetrasubstituted alkenes by subsequent treatment with electrophiles. [reaction: see text]     

Efficient chirality switching in the asymmetric addition of indole to N-tosylarylimines in the presence of axially chiral cyclometalated bidentate N-heterocyclic carbene palladium(II) complexes

Efficient dual stereocontrol can be achieved by using axially chiral cyclometalated bidentate N-heterocyclic carbene palladium(II) complexes for the addition of indole to N-tosylarylimines simply by the adjustment of the R group on the benzene rings of the NHC–Pd(II) complexes.     

Synthesis and characterization of N-heterocyclic carbene complexes of uranium(III)

Reaction of [(((Ad)ArO)(3)tacn)U(III)] (1) or [((Me(3)Si)(2)N)(3)U(III)] (3) with tetramethylimidazol-2-ylidene (Me(4)IMC:) yields novel N-heterocyclic carbene complexes [(((Ad)ArO)(3)tacn)U(III)(Me(4)IMC:)] (2) and [((Me(3)Si)(2)N)(3)U(III)(Me(4)IMC:)] (4). Uranium complexes 2 and 4 represent the first examples of compounds with an N-heterocyclic carbene ligand coordinated to a low-valent uranium center. The paramagnetic complexes 1, 2, and 4 were characterized by (1)H NMR, UV-vis-NIR, and EPR spectroscopy as well as SQUID magnetization measurements and X-ray diffraction analyses. DFT studies indicate a significant degree of pi-bonding in the U(III)-carbene entity.     

Probing intermetallic coupling in dinuclear N-heterocyclic carbene ruthenium(II) complexes

A series of bimetallic N-heterocyclic carbene (NHC) ruthenium(II) complexes were synthesized, which comprise two [RuCl(2)(cymene)(NHC)] units that are interlinked via the NHC nitrogens by alkyl chains of different length. Electrochemical characterization revealed two mutually dependent oxidation processes for the complex with a methylene linker, indicating moderate intramolecular electronic coupling of the two metal centers (class II system). The degree of coupling decreases rapidly upon increasing the number of CH(2) units in the linker and provides essentially decoupled class I species when propylene or butylene linkers are used. Electrochemical analyses combined with structural investigations suggest a through-bond electronic coupling. Replacement of the alkyl linker with a p-phenylene group afforded cyclometalated complexes, which were considerably less stable. The electronic coupling in the methylene-linked complex and the relatively robust NHC-ruthenium bond may provide access to species that are switchable on the molecular scale.     

An air-stable palladium/N-heterocyclic carbene complex and its reactivity in aryl amination

[reaction: see text] The synthesis and characterization of [Pd(IPr)Cl(2)](2) (1), an air- and moisture-stable complex, is reported. The utilization of 1 as a catalyst for amination of aryl chlorides and bromides with a variety of amine coupling partners under mild conditions is described. The amination reactions with 1 show a remarkable insensitivity to oxygen and water, and thus the amination reactions could be performed in air on the benchtop with undried reagent grade solvents and substrates with small effects on reaction times and conversions.