Carbene Donor Research Articles

Modelling fundamental arene–borane contacts: spontaneous formation of a dibromoborenium cation driven by interaction between a borane Lewis acid and an arene π system

Reaction of 2,6-dimesityl pyridine (L(py)) with BBr(3) leads to the spontaneous formation of the trigonal dibromoborenium cation [L(py)·BBr(2)](+)via bromide ejection. Systematic structural and computational studies, and the reactivity displayed by a closely related N-heterocyclic carbene (NHC) donor, reveal the role played by arene-borane interactions in this chemistry. [L(py)·BBr(2)](+) features a structurally characterized (albeit weak) electrostatic interaction between the borane Lewis acid and flanking arene π systems.

Triazole‐Functionalized N‐Heterocyclic Carbene Complexes of Palladium and Platinum and Efficient Aqueous Suzuki–Miyaura Coupling Reaction

Imidazolium salts bearing triazole groups are synthesized via a copper catalyzed click reaction, and the silver, palladium, and platinum complexes of their N-heterocyclic carbenes are studied. [Ag(4)(L1)(4)](PF(6))(4), [Pd(L1)Cl](PF(6)), [Pt(L1)Cl](PF(6)) (L1=3-((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)-1-(pyrimidin-2-yl)-1H-imidazolylidene), [Pd(2)(L2)(2)Cl(2)](PF(6))(2), and [Pd(L2)(2)](PF(6))(2) (L2=1-butyl-3-((1-(pyridin-2-yl)-1H-1,2,3-triazol-4-yl)methyl)imidazolylidene) have been synthesized and fully characterized by NMR, elemental analysis, and X-ray crystallography. The silver complex [Ag(4)(L1)(4)](PF(6))(4) consists of a Ag(4) zigzag chain. The complexes [Pd(L1)Cl](PF(6)) and [Pt(L1)Cl](PF(6)), containing a nonsymmetrical NCN' pincer ligand, are square planar with a chloride trans to the carbene donor. [Pd(2)(L2)(2)Cl(2)](PF(6))(2) consists of two palladium centers with CN(2)Cl coordination mode, whereas the palladium in [Pd(L2)(2)](PF(6))(2) is surrounded by two carbene and two triazole groups with two uncoordinated pyridines. The palladium compounds are highly active for Suzuki-Miyaura cross coupling reactions of aryl bromides and 1,1-dibromo-1-alkenes in neat water under an air atmosphere.

Novel highly electron-deficient quinoxaline-annulated 1,3,2-diazagermol- and diazastannol-2-ylidenes, stabilized as LiCl adducts

Reaction of 2,3-bis(neopentylamino)quinoxaline ( 1) with nBuLi and GeCl 4 or GeCl 2·(dioxane) (molar ratio 1:2:1) in THF furnished highly moisture-sensitive Ge(IV) and Ge(II) heterocycles 2a and 3a, respectively. The quinoxaline-annulated N-heterocyclic germylene (quinNHGe) 3a is stable only in the presence of Li(THF) x and exhibits electrophilic properties associated with the strongly electron-withdrawing annulation. Coordination of chloride at Ge(II) and of Li + at nitrogen is assumed, as found in crystals of a bis(quinoxaline)-annulated eight-membered NHGe LiCl adduct. Addition of dineopentyl-benzimidazol-2-ylidene (bnNHC) provides a labile bnNHC-quinNHGe adduct 4 as indicated by strong downfield coordination shift of the NMR signal for the carbene donor atom. Attempts to grow single crystals led to decomposition and protonation of the carbene forming the bis(benzimidazolium) salt 5 with Li 2 ( THF ) 2 Cl 4 2 - anion. Introduction of 2-methoxyethyl or 2-dimethylaminoethyl side arms as chelating functional groups into the diaminoquinoxalines 6 and 7 and subsequent reaction with 2 nBuLi/GeCl 2·(dioxane) did not markedly stabilize the resulting donor-substituted quinNHGe 8 and 9. Related silicon ( 2b) and tin heterocycles ( 3c) were synthesised for comparison. The quinoxaline-annulated N-heterocyclic stannylene 3c exhibits an extreme 119Sn upfield shift compared to other N-heterocyclic stannylenes, suggesting higher coordination at tin.

Bis(imidazolin-2-ylidene-1-yl)borate Complexes of the Heavier Alkaline Earths: Synthesis and Studies of Catalytic Hydroamination

Heteroleptic complexes of the heavier alkaline earth elements, calcium and strontium, containing bis(imidazolin-2-ylidene-1-yl)borate ligands may be synthesized by deprotonation of boronium salt ligand precursors with [KN(SiMe3)2] in the presence of CaI2 or SrI2. The silylamide complexes [{H2B(ImtBu)2}M{N(SiMe3)2}(THF)n] (M = Ca, n = 1; M = Sr, n = 2), containing N-tert-butyl-substituted imidazolin-2-ylidene fragments, are stable to intermolecular solution redistribution despite some evidence of solution fluxionality in the case of M = Ca. Attempts to synthesize heteroleptic iodide-containing species led to the isolation of the homoleptic compounds [{H2B(ImtBu)2}2MTHF)n] (M = Ca, n = 1; M = Sr, n = 2) most likely because of the reduced steric demands of the halide coligand. Although extension of the “one pot” synthetic method to precursors containing N-mesityl-substituted imidazoles resulted in extensive ligand degradation, use of [Ca{N(SiMe3)2}2(THF)2] as both base and group 2 element source provided the target heteroleptic silylamide complex. X-ray diffraction analyses demonstrated that the borate ligands adopt a facial η3-binding mode in which coordination is provided through the two N-heterocyclic carbene (NHC) donors and an additional agostic-type interacton from one hydrogen of the BH2 residue. Analysis of this ligand binding by DFT methods indicates that, although the bonds from the carbon centers are similar in character to those provided by neutral NHC ligands, the bonding is supplemented by an electrostatic component which is manifested as a coordination of a hydridic BH bond. The ionic nature of the overall interaction is sufficient to maintain the metal−ligand binding in the presence of amine donors and has allowed the evaluation of the Ca and Sr complexes [{H2B(ImtBu)2}M{N(SiMe3)2}(THF)n] as precatalysts for the intramolecular hydroamination of aminoalkenes. While the calcium catalyst provides activities commensurate with our previously reported β-diketiminato complex, [CH{C(Me)N(2,6-iPr2C6H3)}Ca{N(SiMe3)2}(THF)], the strontium derivative is superior in all cases.

STRONGLY DONATING SCORPIONATE LIGANDS

The success of tris(pyrazolyl)borates has inspired the development of new scorpionate ligands that are based on other donor groups. These include borate ligands with thioimidazolyl, selenoimidazolyl, phosphine and N-heterocyclic carbene donors, ligands that provide a facial array of [S3], [Se3], [P3], and [C3] donors, respectively. These ligands are all more strongly donating than the tris(pyrazolyl)borates, with tris(carbene)borate ligands having the greatest donor strength. Despite the design similarities, strongly donating scorpionates differ in their topology and flexibility, donor properties and degradation pathways. The structural and electronic properties of these ligands are contrasted and related to their chemistry, particularly that of the transition metals. Bidentate congeners of all these ligands are also known and are briefly covered. The ability of strongly donating scorpionate ligands to stabilize metal ligand multiple bonds in late transition metal complexes is also briefly discussed.

Optically Active (Peptido‐carbene)palladium Complexes: Towards True Solid‐Phase Combinatorial Libraries of Transition Metal Catalysts

AbstractThis work explores the potential of forming transition metal catalysts by combinatorial solid‐phase peptide chemistry to produce catalysts with enzyme‐like properties of stereoselectivity, regioselectivity and even substrate selectivity. A series of new functionalised carbene precursors/donors – imidazolium salts – each containing both amino acid and carboxylic acid functionality was synthesised in solution. The readily accessible carbene precursors were incorporated within the backbones of peptides attached to PEGA resin, by standard solid‐phase peptide coupling techniques. The synthetic strategy gave easy access to both mono‐ and didentate ligand systems, providing folded structures around the central transition metal atoms, with different degrees of steric congestion and bite angles. Changing the number of incorporated amino acids between the two carbene donors facilitated variation of the properties of the complexes. Both ligand systems were complexed to palladium(II) by standard base treatment, and the Pd complexes were studied by mass spectrometry and NMR spectroscopy. The monodentate (carbene)palladium complexes were each the product of enolisation of a neighbouring carbonyl group and loss of a proton, followed by coordination of the oxy anion to the palladium atom. The established methods are suitable for the combinatorial synthesis of palladium catalysts on solid support. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

One-N, six-membered heterocyclic carbene complexes and the remote heteroatom concept

A whole library of six-membered N-heterocyclic carbene complexes of Ni(ii) and Pd(ii) were prepared by oxidative substitution. In some of these compounds the heteroatom occurs in a position distant from the carbene donor atom. Combined structural and physical data, especially (13)C NMR results, indicate carbene character in such ligands. DFT quantum mechanical calculation at the RI-BP56/SVP level allowed the extraction of valuable chemical information predicting that rNHC (r = remote) ligands would bond more strongly than their nNHC (n = normal) isomers. This result is also corroborated by calculations on the metal complexes themselves. Orbital overlap (mainly sigma) follows the order N(2)HC(5) < nN(1)HC(6) < rN(1)HC(6) when ligands derived from halo-imidazolium and halo-pyridinium salts are compared. In C-C coupling catalysis using Pd(ii) and Ni(ii) complexes, the simple one-N, six-membered carbene complexes are superior to simple two-N, five-membered examples but clear differentiation between nNHC and rNHC precatalysts in the former family, is not always possible.

Rhodium(i) and iridium(i) complexes of pyrazolyl-N-heterocyclic carbene ligands

Several Rh(I) and Ir(I) complexes containing an N-heterocyclic carbene-pyrazolyl chelate ligand have been synthesised. Determination of the single-crystal X-ray structure of the Ir(I) complex showed a novel binding mode with the iridium centre coordinated to two ligands via two carbene donors in preference to one ligand forming the entropically favoured chelate. The hydrogenation activity of several of these complexes was investigated along with that of previously synthesised Rh(I) and Ir(I) complexes containing an analogous phosphine-pyrazolyl chelate.

Modular Chelated Palladium Diaminocarbene Complexes: Synthesis, Characterization, and Optimization of Catalytic Suzuki−Miyaura Cross-Coupling Activity by Ligand Modification

A general, two-step procedure is reported for the modular synthesis of a series of palladium complexes of chelating Chugaev-type diaminocarbene ligands via metal-templated addition of hydrazines to alkylisocyanides. This method afforded high yields of (dicarbene)palladium dihalide complexes with methyl, isopropyl, cyclohexyl, and tert-butyl substituents by addition of hydrazine to the corresponding alkylisocyanide, and analogous backbone-substituted complexes were prepared by palladium-templated addition of methylhydrazine to methylisocyanide. The complexes were fully characterized by IR, 1H NMR, and 13C NMR spectroscopies. X-ray crystallographic analyses of four (dicarbene)palladium dibromide complexes revealed structural similarities with complexes of imidizole-based N-heterocyclic carbenes (NHCs), characterizing these chelating ligands as strongly donating, resonance-stabilized diaminocarbenes. To examine whether these ligands are amenable to cross-coupling catalyst optimization via systematic ligand modification, a set of 10 (dicarbene)palladium dihalide complexes was tested as precatalysts in the Suzuki−Miyaura coupling of bromobenzene with phenylboronic acid. Substantial variations in catalytic activity were observed, and a backbone-substituted palladium dicarbene complex derived from methylhydrazine was identified as the most active precatalyst. Catalyst activities did not correlate with ligand sterics, and subtle electronic perturbation of carbene donor ability by the alkyl groups is proposed to be the origin of the differences in activity. The optimized catalyst was found to give high yields in Suzuki−Miyaura cross-couplings of electron-poor aryl chlorides and a range of aryl bromides, although elevated temperatures (120 °C) were necessary. Coupling reactions conducted open to air showed little formation of homocoupling byproduct and minimal loss of yield in most cases, identifying the optimized system as a rare example of an air-tolerant Suzuki−Miyaura catalyst. This study highlights the importance of a modular ligand design in fine-tuning the activity of a homogeneous catalyst.

A Dinuclear Copper(I) Complex Containing a Bridging Chiral Tridentate Carbene Ligand

The deprotonation of the imidazolium salt (PCPH)I (1) followed by a reaction with [CuI(PPh3)] results in the formation of the dinuclear complex [Cu2I2(PCP)] (2) in 92% yield. The crystal and molecular structural analysis of 2 shows the carbene donor in an extremely rare bridging mode between the two metal centers.

Pyridinium-derived N-heterocyclic carbene ligands: syntheses, structures and reactivity of N-(2′-pyridyl)pyridin-2-ylidene complexes of nickel(II), palladium(II) and platinum(II)

Pyridinium-derived N-heterocyclic carbene complexes [( i-BIPY)M(acac)] +[OTf] − (M = Ni ( 2a), Pd ( 3a), Pt ( 4a)) ( i-BIPY = N-(2′-pyridyl)-4- tert-butylpyridin-2′-ylidene) are obtained as insoluble microcrystalline powders by cyclometallation of the ligand precursor [ i-BIPY-H] +[OTf] − (OTf = trifluormethylsulfonate) to M(acac) 2 (acac = acetylacetonate) in chlorobenzene at 160 °C. Consumption of the starting materials in the synthesis of 3a and 4a is complete after 18 h, providing ( 4a) and ( 5a) in good yields as analytically pure material after recrystallization. The reaction with Ni(acac) 2 is reversible and reaches equilibrium at approximately 75% completion. Removal by distillation of the acetylacetone produced in the cyclometallation drives the reaction to completion furnishing a good yield of pure 6a. Salt metathesis with sodium tetraphenylborate in methanol yields the tetraphenylborate salts ( 2b– 4b) that were used to grow X-ray quality crystals. Analysis of the M–O bond lengths confirms the larger trans-influence of the carbene donor compared with the pyridine donor. Ratios of the M–O(1)/M–O(2) bond lengths suggest that the carbene–metal bond strength follows the order 4b ≈ 3b > 2b. Protonation of 2a– 4a with three equivalents of pyridinium hydrochloride produces ( i-BIPY)MCl 2 in the case of palladium ( 5) and platinum ( 6), while in the case of nickel it regenerates the starting [ i-BIPY-H] + and paramagnetic nickel products.

Pyridinium-derived N-heterocyclic carbene complexes of platinum: synthesis, structure and ligand substitution kinetics.

A series of [(R-iso-BIPY)Pt(CH(3))L ](+)X(-) complexes [R-iso-BIPY = N-(2-pyridyl)-R-pyridine-2-ylidene; (R = 4-H, 1; 4-tert-butyl, 2; 4-dimethylamino, 3; 5-dimethylamino, 4); L = SMe(2), b; dimethyl sulfoxide (DMSO), c; carbon monoxide (CO), d; X = OTf(-) = trifluoromethanesulfonate and/or [BPh(4)](-)] were synthesized by cyclometalation of the [R-iso-BIPY-H](+)[OTF](-) salts 1a-4a ([R-iso-BIPY-H](+) = N-(2-pyridyl)-R-pyridinium) with dimethylplatinum-micro-dimethyl sulfide dimer. X-ray crystal structures for 1b, 2c-4c as well as complexes having bipyridyl and cyclometalated phenylpyridine ligands, [(bipy)Pt(CH(3))(DMSO)](+) (5c) and (C(11)H(8)N)Pt(CH(3))(DMSO) (6c), have been determined. The pyridinium-derived N-heterocyclic carbene complexes display localized C-C and C-N bonds within the pyridinium ligand that are indicative of carbene pi-acidity. The significantly shortened platinum-carbon distance, for "parent" complex 1b, together with NMR parameters and the nu(CO) values for carbonyl cations 1d-4d support a degree of Pt-C10 multiple bonding, increasing in the order 3 < 4 < 2 < 1. Degenerate DMSO exchange kinetics have been determined to establish the nature and magnitude of the trans-labilizing ability of these new N-heterocyclic carbene ligands. Exceptionally large second-order rate constants (k(2) = 6.5 +/- 0.4 M(-1).s(-1) (3c) to 2300 +/- 500 M(-1).s(-1) (1c)) were measured at 25 degrees C using (1)H NMR magnetization transfer kinetics and variable temperature line shape analysis. These rate constants are as much as 4 orders of magnitude greater than those of a series of structurally similar cationic bis(nitrogen)-donor complexes [(N-N)Pt(CH(3))(DMSO)](+) reported earlier, and a factor of 32 to 1800 faster than an analogous charge neutral complex derived from cyclometalated 2-phenylpyridine, (C(11)H(8)N)Pt(CH(3))(DMSO) (k(2) = 0.21 +/- 0.02 M(-1).s(-1) (6c)). The differences in rate constant are discussed in terms of ground state versus transition state energies. Comparison of the platinum-sulfur distances with second order rate constants suggests that differences in the transition-state energy are largely responsible for the range of rate constants measured. The pi-accepting ability and trans-influence of the carbene donor are proposed as the origin of the large acceleration in associative ligand substitution rate.

Tridentate Amido Carbene Ligands in Early-Transition-Metal Coordination Chemistry

Coordination of the new tridentate N-heterocyclic carbene ligand precursor (p-Me-C6H4NHCH2CH2)2N2C3H2 to Zr(IV) generates an [NCN] donor set where the N represents an amido unit and the C corresponds to the N-heterocyclic carbene. This ancillary ligand allows for the isolation and structural characterization of zirconium amido, chloro, and alkyl derivatives; the carbene donor is rendered stable to dissociation because of its central disposition between two anionic amido units.

A new entry to N-heterocyclic carbene chemistry: synthesis and characterisation of a triscarbene complex of thallium(I).

The synthesis and characterisation of a thallium(I) triscarbene complex of the chelating, tripodal carbene ligand 1,3,5-[tris(3-tert-butylimidazol-2-ylideno)methyl]-2,4,6-trimethylbenzene is reported, in which the thallium ion is coordinated by three N-heterocyclic carbene donors in a distorted trigonal planar environment.

Cyclic polycarbene ligands with crown ether structure

AbstractThe cyclization reaction of coordinated β‐functional phenyl isocyanides was used to generate coordinated anellated N‐heterocyclic carbenes. Coordinated 2‐trimethylsiloxyphenyl isocyanide reacts, after SiO bond cleavage, to yield a complex with a coordinated benzoxazol‐2‐ylidene ligand. The cyclization reaction of coordinated 2‐aminophenyl isocyanide, obtained in situ from 2‐azidophenyl isocyanide, yields the coordinated benzimidazol‐2‐ylidene. Both ylidenes can be alkylated at the nitrogen atoms. Attempts to generate four benzimidazol‐2‐ylidenes at PtII and to bridge them by N‐alkylation leading to a crown ether‐like ligand with carbene donors are presented here. Free N‐heterocyclic carbenes derived from benzimidazole with large N‐substituents exist as monomers while they dimerize to dibenzotetraazafulvalenes when substituted with sterically less‐demanding substituents at the ring nitrogen atoms. A chemical equilibrium between the monomeric carbene and its dimerized enetetraamine exists for the N,N′‐isopropyl substituted derivative. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:540–549, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10099

A pyridine bridged dicarbene ligand and its silver(I) and palladium(II) complexes: synthesis, structures, and catalytic applications

The potentially tridentate ligand 2,6-bis[(3-methylimidazolium-1-yl)methyl]pyridine dibromide reacts readily with silver(I) oxide in dichloromethane or dimethylsulfoxide to give a dinuclear silver(I)–carbene complex that was isolated as the tetrafluoroborate salt. Single crystal X-ray crystallography shows that each silver(I) ion is bridged by two ligands bonding through the carbene donors. Treatment of the silver(I) complex with suitable palladium(II) precursors gave the complexes PdCl[(CNC)]BF 4 and [PdMe(CNC)]BF 4 (CNC=2,6-bis[(3-methylimidazolin-2-yliden-1-yl)methyl]pyridine), in which the pyridyl and both carbene moieties are coordinated to a single palladium(II). The palladium(II) complexes have been fully characterised, including X-ray crystallography, and exhibit good activities in the Heck coupling reaction of 4-bromoacetophenone and n-butyl acrylate.

Reactions of (CO)5Cr:C(Me)N(CH2CH2)2 with enynes: mechanistic insight and synthetic value of changing a carbene donor group from alkoxy to dialkylamino

Reactions of the title carbene complex with 1,6- and 1,7-enynes are described and contrasted with the results of the 1-methoxyethylidene analogues