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.     

Preparation of a series of Ru(II) complexes with N-heterocyclic carbene ligands for the catalytic transfer hydrogenation of aromatic ketones

The reaction of [RuCl(2)(p-cymene](2) with Ag-N-heterocyclic carbene (NHC) complexes yields a series of [(p-cymene)Ru(NHC)] complexes (2a-f). All synthesised compounds were characterized by elemental analysis, NMR spectroscopy and the molecular structure of 2a was determined by X-ray crystallography. All complexes have been tested as catalysts for the transfer hydrogenation of aromatic ketones, showing excellent activity in this reaction.     

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.     

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 and anticancer properties of gold(I) and silver(I) N-heterocyclic carbene complexes

Bis(1,3-dimethylimidazol-2-ylidene)silver(I) nitrate and bis(4,5-dichloro-1,3-dimethylimidazol-2-ylidene)silver(I) nitrate were prepared by reacting the corresponding imidazolium nitrate salts with silver oxide. Bis(1,3-dimethylimidazol-2-ylidene)gold(I) nitrate and bis(4,5-dichloro-1,3-dimethylimidazol-2-ylidene)gold(I) nitrate salts were prepared via transmetallation of their silver precursors with chloro dimethylsulfide gold. The anticancer properties were determined using NCI-H460 lung cancer cells. Efficacy was established by comparison of the gold and silver compounds with cisplatin.     

Binuclear ruthenium(II) pyridazine complex catalyzed transfer hydrogenation of ketones

A convenient and general method of synthesis of binuclear ruthenium(II) pyridazine complex was reported. The synthesized complex was characterized by analytical and spectral methods. The structure of the complex was confirmed by X-ray diffraction technique and was found to be an efficient catalyst for the transfer hydrogenation of ketones with excellent conversions in the presence of isopropanol/KOH at 82 °C. The effect of solvents, bases, and different catalyst/substrate ratio for the reaction was also investigated.     

Application of Ni(II) complexes of air stable Schiff base functionalized N-heterocyclic carbene ligands as catalysts for the transfer hydrogenation of aliphatic ketones

Abstract

New air stable N-heterocyclic carbene functionalized Schiff base ligands (L) of the type 2-[-2-[3-(R)imidazol-1-yl]ethyliminomethyl]phenol [R?=?methyl (2), 2-pyridylmethyl (3)] were synthesized and characterized by NMR, IR, MS, and CHN analysis. Single crystal X-ray structural analysis of their Ni(II) complexes revealed square planar arrangement of the chelating ligands coordinated in tridentate (2, C^N^O) and tetradentate (3, N^C^N^O) modes around the metal. The three new isolated and fully characterized complexes were utilized as catalysts for the catalytic transfer hydrogenation of aliphatic ketones in 2-propanol as solvent and source of hydrogen. Based on 0.2?mol% catalyst concentration, the complexes showed activity for aliphatic ketones and 100% conversion (turnover number of 500) for cyclohexanone and all the aromatic ketones tested.     

Half-sandwich ruthenium(II) complexes containing 4-substituted aniline derivatives: structural characterizations and catalytic properties in transfer hydrogenation of ketones

Transition Metal Chemistry - Four half-sandwich Ru(II) complexes (1)–(4) with the general formulae [Ru(η6-p-cymene)(L)Cl2] were synthesized by the reaction of one equivalent of the...     

Asymmetric transfer hydrogenation of ketones catalyzed by ruthenium(II) complexes bearing a chiral phosphinoferrocenyloxazoline ligand

The catalytic activity in asymmetric transfer hydrogenation of ketones using octahedral and half-sandwich (η⁵-indenyl and η⁶-arene) ruthenium(II) complexes containing the chiral ligand (4S)-2-[(S_p)-2-(diphenylphosphino)ferrocenyl]-4-(isopropyl)oxazoline (FcPN) has been explored. Catalytic studies with complex fac-[RuCl₂{η²(P,N)-FcPN}(PMe₃)₂] (1) show excellent TOF values (9600 h⁻¹). Experiments in the presence of free FcPN, which lead to an increase in conversion rates and ee values when the catalyst is complex [Ru(η⁵-C₉H₇){κ²(P,N)-FcPN}(PPh₃)][PF₆] (4) have been carried out. The characterization of the new complexes mer-trans-[RuCl₂{P(OMe)₃}₂{κ²(P,N)-FcPN}] and of the water-soluble complexes fac- and mer-trans-[RuCl₂(PTA)₂{κ²(P,N)-FcPN}] is also reported.     

N-heterocyclic carbene complexes of Zn(II): synthesis, X-ray structures and reactivity

The synthesis of six novel zinc (II) mono(N-heterocyclic carbene) complexes is described. 1,3-Bis(mesityl)-imidazol-2-ylidene was reacted with the zinc salts ZnX₂ (X=Cl, CH₃COO, PhCOO, and PhCH₂COO) to yield the corresponding monomeric Zn-NHC complex ZnCl₂(NHC)(THF) (1) and dimeric [Zn(OOCCH₃)₂(NHC)]₂ (2), [Zn(OOCPh)₂(NHC)]₂ (3), [Zn(OOCCH₂Ph)₂(NHC)]₂ (4) (NHC=1,3-bis(mesityl)-imidazol-2-ylidene). Reaction of 1 with 2 equivalents of silver trifluoromethanesulfonate yielded monomeric Zn(O₃SCF₃)₂(NHC)(THF) (5), reaction of 1 with sodium {[R(+)-α-2-(1-phenyl-ethylimino)-methyl]-phenolate} yielded monomeric ZnCl(OC₆H₄-2-CHN(CHPhCH₃)(NHC) (6). Compounds 1, 4-6 were structurally characterized by X-ray analysis. Selected compounds were investigated for their activity in the copolymerization of carbon dioxide with cyclohexene oxide as well as in the ring-opening polymerization of cyclohexene oxide and ε-caprolactone.     

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.     

Dinuclear N-heterocyclic carbene complexes of silver(I), derived from imidazolium-linked cyclophanes

The synthesis of four new silver complexes of bidentate N-heterocyclic carbenes, derived from imidazolium-linked cyclophanes, has been achieved via a simple complexation reaction of the imidazolium-linked cyclophanes with the basic metal source Ag(2)O. The cyclophane structures contain two imidazolyl links between ortho-, meta- and mixed ortho/meta-substituted aromatic rings. The new silver carbene systems are thermally stable and have been characterised by NMR spectroscopy and X-ray crystallography. Three of the complexes have a dimeric structure of the form [L(2)Ag(2)](2+) in the solid state that is rigid on the NMR timescale in solution. The fourth complex has a neutral structure of the form [L(AgBr)(2)], the NMR studies suggesting some lability of the L-Ag bonding in solution.     

N-heterocyclic carbene Pt(II) complexes from caffeine: synthesis, structures and photoluminescent properties

The caffeine-derived N-heterocyclic carbene (NHC) complex [Pt(II)(C^N)(NHC)Cl] (C^N = 2-phenylpyridine), 4 has the opposite stereochemistry and a shorter Pt-C(carbene) bond compared to that of an analogous benzimidazole-derived N,N-heterocyclic carbene (NNHC) Pt complex 2. These suggest a lower trans influence of pyridyl N compared to cyclometallated carbon and an increased Pt-NHC π-backbonding because of decreased π-donation resulting from conjugation to the electron deficient pyrimidine of caffeine. Complex 4 has a lower emission quantum yield (Φ) and is blue-shifted into the green region of the visible spectrum relative to non-carbene Pt(II) cyclometalated complex 5.     

Asymmetric N-heterocyclic carbene benzimidazolium salts and their silver(I) complexes: potential as ionic liquid crystals

The synthesis and characterisation of a homologous series of monodentate benzimidazolium salts, 1–4 and their mononuclear silver(I)–NHC (where NHC = N-heterocyclic carbene) complexes, 5–8, are reported. The benzimidazolium salts were prepared from the N-alkylation of 1-methyl-benzimidazole with alkyl halides of varying carbon chain lengths. The mono silver(I)-NHC complexes, 5–8, were prepared by the reaction of the benzimidazolium salts with Ag₂O. All the synthesised compounds were fully characterised by ¹H-nuclear magnetic resonance (¹H-NMR), ¹³C-NMR and fourier-transform infrared (FTIR) spectroscopy. The molecular structures of compounds 3·PF₆, 4·PF₆, 7 and 8 were elucidated through single-crystal X-ray diffraction analyses. We postulate that the attachment of long alkyl chains to the heterocyclic core of 1-methyl benzimidazole could induce mesophase formation. The liquid crystalline behaviour of the benzimidazolium salts was investigated by polarised optical microscope and differential scanning calorimetry. Salts 3 and 4 were found to be thermotropic liquid crystals which exhibited a smectic A phase. However, upon complexation with silver(I) ions, all the Ag(I)–NHC complexes are found to be non-mesogenic.     

Asymmetric transfer hydrogenation of ketones catalyzed by phosphine-rhodium(I) and -iridium(I) complexes

The asymmetric hydrogen transfer from propan-2-ol to prochiral ketones is effectively catalyzed by diphosphine complexes of iridium and rhodium. The influence of the reaction conditions on the activity and selectivity of the catalysts has been investigated.     

Mechanism of the hydrogenation of ketones catalyzed by trans-dihydrido(diamine)ruthenium II complexes

The complexes trans-RuH(Cl)(tmen)(R-binap) (1) and (OC-6-43)-RuH(Cl)(tmen)(PPh(3))(2) (2) are prepared by the reaction of the diamine NH(2)CMe(2)CMe(2)NH(2) (tmen) with RuH(Cl)(PPh(3))(R-binap) and RuH(Cl)(PPh(3))(3), respectively. Reaction of KHB(sec)Bu(3) with 1 yields trans-Ru(H)(2)(R-binap)(tmen) (5) while reaction of KHB(sec)Bu(3) or KO(t)Bu with 2 under Ar yields the new hydridoamido complex RuH(PPh(3))(2)(NH(2)CMe(2)CMe(2)NH) (4). Complex 4 has a distorted trigonal bipyramidal geometry with the amido nitrogen in the equatorial plane. Loss of H(2) from 5 results in the related complex RuH(R-binap)(NH(2)CMe(2)CMe(2)NH) (3). Reaction of H(2) with 4 yields the trans-dihydride (OC-6-22)-Ru(H)(2)(PPh(3))(2)(tmen)(6). Calculations support the assignment of the structures. The hydrogenation of acetophenone is catalyzed by 5 or 4 in benzene or 2-propanol without the need for added base. For 5 in benzene at 293 K over the ranges of concentrations [5] = 10(-)(4) to 10(-)(3) M, [ketone] = 0.1 to 0.5 M, and of pressures of H(2) = 8 to 23 atm, the rate law is rate = k[5][H(2)] with k = 3.3 M(-1) s(1), DeltaH++ = 8.5 +/- 0.5 kcal mol(-1), DeltaS++ = -28 +/- 2 cal mol(-1) K(-1). For 4 in benzene at 293 K over the ranges of concentrations [4] = 10(-4) to 10(-3) M, [ketone] 0.1 to 0.7 M, and of pressures of H(2) = 1 to 6 atm, the preliminary rate law is rate = k[4][H(2)] with k = 1.1 x 10(2) M(-1) s(-1), DeltaH++ = 7.6 +/- 0.3 kcal mol(-1), DeltaS++ = -23 +/- 1 cal mol(-1) K(-1). Both theory and experiment suggest that the intramolecular heterolytic splitting of dihydrogen across the polar Ru=N bond of the amido complexes 3 and 4 is the turn-over limiting step. A transition state structure and reaction energy profile is calculated. The transfer of H(delta+)/H(delta-) to the ketone from the RuH and NH groups of 5 in a Noyori metal-ligand bifunctional mechanism is a fast process and it sets the chirality as (R)-1-phenylethanol (62-68% ee) in the hydrogenation of acetophenone. The rate of hydrogenation of acetophenone catalyzed by 5 is slower and the ee of the product is low (14% S) when 2-propanol is used as the solvent, but both the rate and ee (up to 55% R) increase when excess KO(t)Bu is added. The formation of ruthenium alkoxide complexes in 2-propanol might explain these observations. Alkoxide complexes [RuP(2)]H(OR)(tmen), [RuP(2)] = Ru(R-binap) or Ru(PPh(3))(2), R= (i) Pr, CHPhMe, (t)Bu, are observed by reacting the alcohols (i)PrOH, phenylethanol, and (t)BuOH with the dihydrides 5 and 6, respectively, under Ar. In the absence of H(2), the amido complexes 3 and 4 react with acetophenone to give the ketone adducts [RuP(2)]H(O=CPhMe)(NH(2)CMe(2)CMe(2)NH) in equilibrium with the enolate complexes trans- [RuP(2)](H)(OCPh=CH(2))(tmen) and eventually the decomposition products [RuP(2)]H(eta(5)-CH(2)CPhCHCPhO), with the binap complex characterized crystallographically. In general, proton transfer from the weakly acidic molecules dihydrogen, alcohol, or acetophenone to the amido nitrogen of complexes 3 and 4 is favored in two ways when the molecule coordinates to ruthenium: (1) an increase in acidity of the molecule by the Lewis acidic metal and (2) an increase in the basicity of the amido nitrogen caused by its pyramidalization. The formato complexes trans-[RuP(2)]H(OCHO)(tmen) were prepared by reacting the respective complex 4 or 5 with formic acid. The crystal structure of RuH(OCHO)(PPh(3))(2)(tmen) displays similar features to the calculated transition state for H(delta+)/H(delta-) transfer to the ketone in the catalytic cycle.     

Transfer hydrogenation of unfunctionalised alkenes using N-heterocyclic carbene ruthenium catalyst precursors

Transfer hydrogenation of unfunctionalised and aliphatic alkenes in iPrOH/KOH is efficiently catalysed by an olefin-tethered N-heterocyclic carbene ruthenium complex, which also catalyses double bond migration as a competitive and considerably faster process.