Biscarbene Complexes Research Articles

The counterion influence on the CH-activation of methane by palladium(II) biscarbene complexes – structures, reactivity and DFT calculations

Novel bridged palladium(II) biscarbene complexes with different counterions are reported: 1,1 ′-dimethyl-3,3 ′-methylene-4-diimidazolin-2,2 ′-diylidene palladium(II) bischloride ( 1) and bis(trifluoroacetate) ( 2) have been synthesized in good yields. Both complexes are active in the catalytic conversion of methane to methanol and show comparable activities to previously published NHC-catalysts. The results of the single-crystal X-ray structure determination of 1,1 ′-dimethyl-3,3 ′-methylene-4-diimidazolin-2,2 ′-diylidene palladium(II) bischloride ( 1) and 1,1 ′-dimethyl-3,3 ′-methylene-4-diimidazolin-2,2 ′-diylidene palladium (II) bis(trifluoro-acetate) 2 confirmed the structural similarity to the known corresponding palladium bromide and iodide complexes. Since free 1,1 ′-dimethyl- R-3,3 ′-methylene-4-diimidazolin-2,2 ′-diylidene are only available in low yields these compounds have been synthesized via the bromide complex, exchanging the counterion by AgCF 3COO or by exchanging the counterion of the imidazolium salt by NH 4PF 6.

Synthesis and Reactivity of New Chelate-N-Heterocyclic Biscarbene Complexes of Ruthenium

The carbene-ligand precursors methylenebis(N-alkylimidazolium) iodide (alkyl = methyl, neo-pentyl) and ethylenebis(N-methylimidazolium) chloride have been used in the preparation of several new Ru(II)-p-cymene complexes where the ligand behaves as mono- and bidentate. The molecular structures of the two biscarbene-complexes are reported. From the data reported, we can conclude that steric reasons (mainly the bisimidazolium linkers, methylene/ethylene) are the main factors determining both reactivity and synthetic difficulties of the products reported.

Biscarbene—Ruthenium Complexes in Catalysis: Novel Stereoselective Synthesis of (1E,3E)‐1,4‐Disubstituted‐1,3‐dienes via Head‐to‐Head Coupling of Terminal Alkynes and Addition of Carboxylic Acids.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Metal complexes of acyclic diaminocarbenes: links between N-heterocyclic carbene (NHC)- and Fischer-carbene complexes

Starting from free bis(diisopropylamino)carbene, only rhodium and iridium complexes could be synthesized with these carbene ligands so far. The formation of further carbene complexes of Group 18 metals failed, because of the high sensitivity of the free diaminocarbenes, as described by Alder. Here we report the synthesis of diaminocarbene complexes of Group 16 metals by reaction of several acyclic and cyclic carbenes—first synthesized by Alder—with hexacarbonyls of these metals. The reaction of bis(diisopropyl)aminocarbene with photolytically produced (ether)Cr(CO) 5 leads to a stable η 2-carbene complex, with one amino group of the carbene coordinated to the metal. The chemical and spectroscopic characteristics of this complex are described. The reaction of the same carbene with ethersubstituted Mo(CO) 6 resp. W(CO) 6 leads to unstable merely η 1-bonded complexes. In contrast to the bis(diisopropylamino)carbene, the reaction of the tetrahydropyrimidinocarbene, which was already isolated by Alder, with photolytically activated Cr(CO) 6 leads to a simply η 1-bonded complex; two of his CO ligands can be replaced with isonitrile even at ambient temperature. The analogous reaction of bis( N-piperidyl)carbene with photolytically activated Cr(CO) 6 leads to the formation of a tetracarbonylchromium complex with a completely novel chelating ligand. During all reactions of bis(diisopropylamino)carbene and tetrahydropyrimidinocarbene with the metal carbonyls their reduction to carbonyl metallates could be observed as side reaction.

Biscarbene-ruthenium complexes in catalysis: novel stereoselective synthesis of (1E,3E)-1,4-disubstituted-1,3-dienes via head-to-head coupling of terminal alkynes and addition of carboxylic acids.

The reaction of a variety of alkynes RCtbd1;CH with a variety of carboxylic acids R(1)CO(2)H, in the presence of 5% of RuCl(COD)C(5)Me(5), selectively leads to the dienylesters (1E,3E)-RCH(1)=CH(2)-CH(3)=C(R)(O(2)CR(1)). The reaction also applies to amino acid and dicarboxylic acid derivatives. It is shown that the first step of the reaction consists of the head-to-head alkyne coupling and of the formation of the metallacyclic biscarbene-ruthenium complex isolated for R = Ph and catalyzing the formation of dienylester. D-labeled reactions show that the alkyne protons remain at the alkyne terminal carbon atoms and carboxylic acid protonates the C(1) carbon atom. QM/MM (ONIOM) calculations, supporting a mixed Fischer-Schrock-type biscarbene complex, show that protonation occurs preferentially at the carbene carbon C(1) adjacent to Ru, in the relative cis position with respect to the Ru-Cl bond, to give a mixed C(1)alkyl-C(4)carbene complex in which the C(4) carbene is conjugated with the noncoordinated C(2)=C(3) double bond. This 16-electron intermediate has a weak stabilizing alpha agostic C-H bond. This most stable isomer appears to have a C(4) center more accessible to the nucleophilic addition which accounts for the experimentally observed product.

Generation of Reactive Species by One‐Electron Reduction of Fischer‐Type Carbene Complexes of Group 6 Metals and Their Use for Carbon—Carbon Bond Formation.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Generation of reactive species by one-electron reduction of Fischer-type carbene complexes of group 6 metals and their use for carbon-carbon bond formation.

Carbon-carbon bond-forming reactions mediated by one-electron reduction of Fischer-type carbene complexes of Group 6 metals were investigated. In the case of aryl- or silylcarbene complexes of tungsten, the anion radical species generated by one-electron reduction smoothly underwent addition reaction to ethyl acrylate. One-electron reduction of alpha,beta-unsaturated carbene complexes afforded biscarbene complexes by dimerization of the corresponding anion radical species at the position gamma to the metal center. In contrast, one-electron reduction of chromium phenyl- or alkylcarbene complexes gave, via carbonyl insertion, alpha-methoxyacylchromate complexes, which further underwent conjugate addition to various electron-poor olefins to give the corresponding alpha-methoxyketones.

New Pd‐Catalyzed Tandem Cyclization Processes on Group 6 Bis‐carbene Complexes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A novel synthesis of metallogermacyclopropane and molybdenum bis(iminophosphorano)carbene complexes from bisgermavinylidene

The reaction of bisgermavinylidene [(Me3SiN=PPh2)2C=Ge-->Ge=C(PPh2=NSiMe3)2] (1) with M(CO)5(THF) ( M = Cr, W, Mo) afforded the metallagermacyclopropane [(Me-3SiN=PPh2)2CGeM(CO)3[M(CO)5]] [M = W (2), Cr (3), Mo (4)]; in one of the reactions, compound 4 reacts further to give a "pincer" carbene complex [(CO)3Mo[C(Ph2P=NSi Me3)2]] (5); the X-ray structures of compounds 2 and 5 have been determined.

Synthesis and structural characterization of novel bridged platinum(II) biscarbene complexes

Novel bridged platinum(II) biscarbene complexes are reported: 1,1′-dimethyl-3,3′-methylene-4-diimidazolin-2,2′-diylidene platinum(II) ( 3) and 1,1′-dimethyl-3,3′-ethylene-4-diimidazolin-2,2′-diylidene platinum(II) complexes 4 are directly accessible in high yields starting from platinum halides. The one-pot synthesis obviates the need for multi-step reactions via metal precursors or free carbenes. An X-ray crystal structure of 1,1′-dimethyl-3,3′-methylene-4-diimidazolin-2,2′-diylidene platinum(II) dibromide ( 3b) confirmed the structural similarity to the known corresponding palladium complexes. Since free 1,1′-di- R-3,3′-methylene-4-diimidazolin-2,2′-diylidenes are only available in low yields this synthetic route provides an easy access to the corresponding carbene complexes.

Chelated Iridium(III) Bis-carbene Complexes as Air-Stable Catalysts for Transfer Hydrogenation

The new air-stable and moisture-insensitive Ir catalysts for efficient transfer hydrogenation of ketones contain a chelating bis(N-heterocyclic carbene) ligand. Most other hydrogen transfer catalysts show activity Rh > Ir, but we find Ir > Rh for these cases. Tuning of the ligand wingtip substituents, R, can greatly increase catalyst activity (R = neopentyl) or selectivity (R = isopropyl). Reactivity studies and isotopic labeling are consistent with a monohydride mechanism for the hydrogen transfer.

Bis-carbene complexes from oxidative addition of imidazolium C–H bonds to palladium(0)

Oxidative addition of the C–H bond at the 2-position of N-(2-pyridyl)imidazolium salts takes place to palladium(0) to lead to unexpected final products containing two carbenes. The formation of either cis or trans bis-carbene complexes, both identified by X-ray crystallography and spectroscopy, apparently is a consequence of the substitution pattern of the imidazolium moiety. A mechanism is suggested based on recent theoretical studies.

New Pd-catalyzed tandem cyclization processes on group 6 bis-carbene complexes.

The reaction of bis-carbene complexes 1a,b derived from catechol and Pd-catalysts gave product 3 derived from an intramolecular coupling-electrocyclization process, while nucleophilic attack-NH-carbene insertion product 6 was obtained from complexes 1c,d derived from 1,2-diaminobenzene.

The First Biscarbene Complex of a Group 4 Metal: [Zr{C(Me(2)P=NSiMe(3))(2)}(2)] We thank the Natural Sciences and Engineering Research Council of Canada, NOVA Chemicals Corp., and the University of Alberta for financial support.

Two carbene ligands bound to zirconium: Complex 1 was synthesized under mild conditions by reaction of CH2(Me2P=NSiMe3)2 with [Zr(CH2C6H5)4] in a 2:1 ratio. The resulting complex 1 is the first biscarbene complex of a Group 4 metal. Si*=SiMe3.

Pd-catalyzed inter- and intramolecular carbene transfer from group 6 metal-carbene complexes.

The use of group 6 metal-carbene complexes in inter- and intramolecular carbene transfer reactions has been studied. Thus, pentacarbonyl[(aryl)(methoxy)carbene]chromium(0) and tungsten complexes, 10, efficiently dimerize at room temperature in the presence of diverse Pd(0) and Pd(II)/Et(3)N catalysts. The effect of additives (PPh(3), AsPh(3), or SbPh(3)) on the nature and the isomeric ratio of the reaction products is negligible. The nature of the reaction products is more catalyst-dependent for metal carbenes 12 bearing alkyl groups attached to the carbene carbon. In these cases, either carbene ligand dimerization or beta-hydrogen elimination reactions are observed, depending on the catalyst. The carbene ligand dimerization reaction can be used to prepare conjugated polyenes, including those having metal moieties at both ends of the polyene system, as well as enediyne derivatives. The intramolecular carbene ligand dimerization of chromium bis-carbene complexes 28 and 30 allows the preparation of mono- and bicyclic derivatives, with ring sizes from six to nine members. For bis-carbene derivatives the beta-hydrogen elimination reaction is inhibited, provided that both metal centers are tethered by an o-xylylene group. Other alkyl complexes 32 form new mononuclear carbene complexes 37 or decompose to complex reaction mixtures. The results obtained in these reactions may be explained by transmetalation from Cr(0) to Pd(0) and the intermediacy of Pd-carbene complexes. Aminocarbene-chromium(0) complexes 15, need harsher reaction conditions to transfer the carbene ligand, and this transfer occurs only in the presence of deactivated olefins. The corresponding insertion/hydrolysis products 48 resulted in these cases. A catalytic cycle involving transmetalation from a chromacyclobutane to a palladacyclobutane is proposed to explain these results.

Synthesis and crystal structures of carbene complexes with conjugated thiophene units

Binuclear carbene complexes of chromium and tungsten containing a conjugated thiophene spacer, dithieno[3,2-b:2′,3′-d]thienylene (TTT), were synthesized according to the Fischer method for the synthesis of carbene complexes. Biscarbene complexes of the type [(CO)5M{C(OEt)TTTC(OEt)}M(CO)5] (M=Cr, 2; W, 5) were afforded together with the corresponding monocarbene complexes [M{C(OEt)TTTH}(CO)5] (M=Cr, 1; W, 4) and decomposition products [M{C(OEt)TTTC(O)OEt}(CO)5] (M=Cr, 3; W, 6), formed in the reaction of the biscarbene complexes with trace amounts of oxygen. X-ray crystallographic determinations were performed on compounds 1, 2 and 6 to assess the influence of the different substituents in the C9 position of the TTT carbene complexes on the structural features of the condensed rings. The crystallographic data of these three complexes are reported in this paper.

First chelated chiral N-heterocyclic bis-carbene complexes

[formula: see text] First chiral bidentate bis-carbene complex of Pd(II) prepared from a bis-imidazolium salt derived from (S)-2,2'-bromomethyl-1,1'-binaphthyl exists in both cis- and trans-square planar geometry. These and the corresponding trans-Ni(II) complexes are remarkably resistant to high temperature, air, water, and silica gel chromatography. The Pd complexes catalyze Heck reactions.

New Zirconium Hydrocarbyl Bis(phosphoranimino) “Pincer” Carbene Complexes

Reaction of the bis(iminodi(phenyl)phosphorano)methanes CH2(Ph2PNR)2 (R = Ad 1, SiMe3 2) with Zr(CH2Ph)4 in toluene under mild conditions gave novel electronically unsaturated bis(hydrocarbyl) carbene complexes of the formula [Zr{C(Ph2PNR)2-κ3C,N,N‘}(CH2Ph)2] (R = Ad 3, SiMe3 4) in high yields. The 13C NMR resonances for the metal-bound carbon (82.8 (3), 84.7 (4) ppm) are shifted upfield relative to the previously known group 4 carbene complexes. The crystal structure of 3 shows a short metal carbon link which is common to two fused four-membered rings which are almost coplanar. The rings are formed by chelation of the two pendant imine centers of the ligand. These complexes represent the first examples of hydrocarbylmetal carbene complexes of a group 4 metal.

Synthesis, characterization and decomposition patterns for thienylene bridged dinuclear biscarbene complexes of manganese and chromium in refluxing carbon disulphide and acetone

The dinuclear complexes [ML 3(CO) 2{C(OEt)C 4H 2SC(OEt)}M(CO) 2L 3] (M=Cr, L=CO 1; M=Mn, L 3= η 5-C 5H 4R, R=H 2 and Me 3) were synthesized from dilithiated thiophene according to the Fischer-method. The highly reactive biscarbene complexes are unstable in acetone and react to afford [ML 3(CO) 2{C(OEt)C 4H 2SC(O)OEt] (M=Cr 4, M=Mn, R=H 5 and Me 6). When refluxed in carbon disulphide, 1 affords as the only isolable product 4, whereas 2 and 3, in addition to 5 and 6, reacts with CS 2 to give the thiono ester compounds [Mn( η 5-C 5H 4R)(CO) 2{C(OEt)C 4H 2SC(S)OEt}] with R=H 7 and Me 8, respectively. Compound 1 crystallizes in the monoclinic space group C2/c with a=21.593(12), b=10.235(3), c=22.531(12) Å, β=92.06(4)° and Z=8.

Synthesis and reaction chemistry of mixed ligand methylpalladium–carbene complexes

A variety of methylpalladium carbene complexes of the type [Pd(Me)(carbene)(chelate)] [where carbene=1,3-di-methylimidazol-2-ylidene (dmiy), chelate=acetylacetonate (acac) 1, trifluoroacetylacetonate (tfac) 2, hexafluoroacetylacetonate (hfac) 3; carbene=1,3,4,5-tetramethylimidazol-2-ylidene (tmiy), chelate=acac, 6] have been synthesised and fully characterised. The bis-carbene complexes trans-[Pd(Me)(carbene) 2Cl] [carbene=dmiy, 7 and tmiy, 8] have also been isolated. The unusual cationic complex {[Pd(Me)(dmiy)(cod)] +BF 4 −} 4, in which there are three distinct Pd–carbon bonds, including cis alkyl/alkene coordination has been prepared and aspects of its chemistry investigated. The complex which slowly decomposes even at −20°C does not react via the expected migratory insertion of the methyl group with the coordinated cod but decomposes via a pathway involving the methyl group and the carbene ligand. Crystal structures of the bis-carbene complex 7 and the chloro-bridged dimer [Pd(Me)(tmiy)Cl] 2 5 reveal square planar coordination. The carbene C 3N 2 arrays are closely planar, with the ligand planes inclined at significant angles to the coordination planes of the respective complexes. For complex 7 the interplanar dihedral angle is 70.4(3)° and for 5 the angle is 65.07(8)°. Examples of the methylpalladium monocarbene and dicarbene complexes were tested in the Heck reaction and were found to be extremely active with several having turnover frequencies (TOF’s) of >20 000 and total turnover numbers (TON’s) of >100 000.