N-heterocyclic Carbene Adducts Research Articles

How Does the Nickel Pincer Complex Catalyze the Conversion of CO2 to a Methanol Derivative? A Computational Mechanistic Study

The mechanistic details of nickel-catalyzed reduction of CO(2) with catecholborane (HBcat) have been studied by DFT calculations. The nickel pincer hydride complex ({2,6-C(6)H(3)(OP(t)Bu(2))(2)}NiH = [Ni]H) has been shown to catalyze the sequential reduction from CO(2) to HCOOBcat, then to CH(2)O, and finally to CH(3)OBcat. Each process is accomplished by a two-step sequence at the nickel center: the insertion of a C═O bond into [Ni]H, followed by the reaction of the insertion product with HBcat. Calculations have predicted the difficulties of observing the possible intermediates such as [Ni]OCH(2)OBcat, [Ni]OBcat, and [Ni]OCH(3), based on the low kinetic barriers and favorable thermodynamics for the decomposition of [Ni]OCH(2)OBcat, as well as the reactions of [Ni]OBcat and [Ni]OCH(3) with HBcat. Compared to the uncatalyzed reactions of HBcat with CO(2), HCOOBcat, and CH(2)O, the nickel hydride catalyst accelerates the H(δ-) transfer by lowering the barriers by 30.1, 12.4, and 19.6 kcal/mol, respectively. In general, the catalytic role of the nickel hydride is similar to that of N-heterocyclic carbene (NHC) catalyst in the hydrosilylation of CO(2). However, the H(δ-) transfer mechanisms used by the two catalysts are completely different. The H(δ-) transfer catalyzed by [Ni]H can be described as hydrogen being shuttled from HBcat to nickel center and then to the C═O bond, and the catalyst changes its integrity during catalysis. In contrast, the NHC catalyst simply exerts an electronic influence to activate either the silane or CO(2), and the integrity of the catalyst remains intact throughout the catalytic cycle. The comparison between [Ni]H and Cp(2)Zr(H)Cl in the stoichiometric reduction of CO(2) has suggested that ligand sterics and metal electronic properties play critical roles in controlling the outcome of the reaction. A bridging methylene diolate complex has been previously observed in the zirconium system, whereas the analogous [Ni]OCH(2)O[Ni] is not a viable intermediate, both kinetically and thermodynamically. Replacing HBcat with PhSiH(3) in the nickel-catalyzed reduction of CO(2) results in a high kinetic barrier for the reaction of [Ni]OOCH with PhSiH(3). Switching silanes to HBcat in NHC-catalyzed reduction of CO(2) generates a very stable NHC adduct of HCOOBcat, which makes the release of NHC less favorable.

Influence of multidentate N-donor ligands on highly electrophilic zinc initiator for the ring-opening polymerization of epoxides

A set of multidentate ligands have been synthesized and used to stabilize the putative highly electrophilic zinc species initiating ring-opening polymerization (ROP) of cyclohexene oxide (CHO) and propylene oxide (PO). Reaction of the bidentate C 2-chiral bis(oxazoline) ligand ( R2,R3BOX: R 2 = (4 S)- tBu, R 3 = H ( a); R 2 = (4 S)-Ph, R 3 = H ( b); R 2 = (4 R)-Ph, R 3 = (5 S)-Ph ( c)) with Zn(R 1) 2 (R 1 = Et ( 1), Me ( 2)) led to the heteroleptic three-coordinate complexes ( R2,R3BOX)ZnR 1, 1a– c and 2a, which were isolated in 92–96% yield. Next, two pyridinyl-functionalized N-heterocyclic carbene (NHC) ligands have been designed and synthesized: the 1,3-bis(2-pyridylmethyl)imidazolinium salt ( d) and the protected NHC adduct 2-(2,3,4,5,6-pentafluorophenyl)-1,3-bis(2-pyridylmethyl)imidazolidine ( e). The reaction of ligands d and e with ZnEt 2 led directly to the formation of (NHC)ZnEt(Cl) 3d complex with ethane elimination and the adduct (NHC–C 6F 5(H))ZnEt 2 4e, respectively, in high yield. In situ combinations of selected complexes 1a– c, 3d and 4e with B(C 6F 5) 3 (1 or 2 equivalents) give active systems for ROP, with high productivity (3.3–5.9 10 6 g polym. mol Zn −1 h −1) and high molecular weight ( M n up to 132 10 3 g mol −1) for CHO polymerization. Although the in situ B(C 6F 5) 3-activated zinc species were not isolated, the sterically demanding BOX ligands ( 1c > 1b > 1a) and functionalized NHC ligands seem to enhance the stability of highly electrophilic zinc complexes over ligand redistribution, allowing a better control of the cationic ROP as reflected particularly for 3d and 4e complexes by their respective efficiency (42–88%).

Synthesis and Crystal Structures of Two N-Heterocyclic Carbene Adducts of CrCl2

The reactions of CrCl2 with two N-heterocyclic carbenes (NHC) have afforded the complexes, trans-[CrCl2(NHC)2] (NHC = :C{N(C6H3Pr 2 i -2,6)C(H)}2 1 or :C{N(Pri)C(Me)}2 2). The cell parameters of the compounds are 1: P212121, a = 12.9445(4), b = 20.7364(5), c = 21.2841(6); 2: Pca21, a = 21.7583(14), b = 9.9465(8), c = 11.4414(9). The crystal structures of the complexes are the first for NHC adducts of CrCl2, and both exhibit square planar chromium centers. The synthesis and crystal structures of two N-heterocyclic carbene adducts of CrCl2, trans-[CrCl2(NHC)2] (NHC = :C{N(C6H3Pr 2 i -2,6)C(H)}2 or :C{N(Pri)C(Me)}2) are reported.

Structure and bonding in three-coordinate N-heterocyclic carbene adducts of iron(ii) bis(trimethylsilyl)amide

The molecular structures, chemical bonding and magnetochemistry of the three-coordinate iron(II) NHC complexes [(NHC)Fe{N(SiMe(3))(2)}(2)] (NHC = IPr, 2; NHC = IMes, 3) are reported.

Neutral Pentacoordinate Silicon Fluorides Derived from Amidinate, Guanidinate, and Triazapentadienate Ligands and Base-Induced Disproportionation of Si2Cl6 to Stable Silylenes

Pentacoordinate silicon fluorides L(1)SiF(3) (2a), L(2)SiF(3) (2b), and (L(3)SiF(2))(2) (2c)(2) based on amidinate (L(1) = PhC(N(t)Bu)(2)), guanidinate (L(2) = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate), and triazapentadienate (L(3) = NC(NMe(2))NC(NMe(2))NAr; Ar = 2,6-(i)Pr(2)C(6)H(3)) ligands were prepared by fluorination of the corresponding chlorosilanes L(1)SiCl(3) (1a), L(2)SiCl(3) (1b), and L(3)SiCl(2) (1c) with Me(3)SnF at ambient temperature. Compounds 1b, 1c, 2a, 2b, and (2c)(2) were characterized by (1)H, (13)C, (19)F, and (29)Si NMR spectroscopic studies. Molecular structures of 1b, 1c, 2a, and (2c)(2) were determined by single crystal X-ray structural analysis. Invariom refinement involving non-spherical scattering factors of the Hansen-Coppens multipole model was performed for 1b. Compound L(3)SiF(2) (2c) is dimeric both in the solid state and in solution, whereas its chloro-analogue 1c is monomeric. The attempted synthesis of diamidinatotetrachlorodisilane by reaction of lithium amidinate with Si(2)Cl(6) led to the formation of the silane (1a) and the silylene L(1)SiCl (3). Reaction of Si(2)Cl(6) with N-heterocyclic carbenes (NHC) afforded NHC adducts of dichlorosilylene and SiCl(4). A one pot method for the preparation of base-stabilized silylenes from Si(2)Cl(6) is discussed.

Synthesis of a stable adduct of dialane(4) (Al2H4) via hydrogenation of a magnesium(I) dimer

The desorption of dihydrogen from magnesium(II) hydride, MgH2 (containing 7.6 wt% H), is reversible. MgH2 therefore holds promise as a hydrogen storage material in devices powered by fuel cells. We believed that dimeric magnesium(I) dimers (LMgMgL, L=β-diketiminate) could find use as soluble models to aid the study of the mechanisms and/or kinetics of the hydrogenation of magnesium and its alloys. Here, we show that LMgMgL can be readily hydrogenated to yield LMg(µ-H)2MgL by treatment with aluminium(III) hydride complexes. In one case, hydrogenation was reversed by treating LMg(µ-H)2MgL with potassium metal. The hydrogenation by-products are the first thermally stable, neutral aluminium(II) hydride complexes to be produced, one of which, [{(IPr)(H)2Al}2] (IPr=:C[{(C6H3-i-Pr(2)-2,6)NCH}2]), is an N-heterocyclic carbene adduct of the elusive parent dialane4 (Al2H4). A computational analysis of this compound is presented.

Stable N‐Heterocyclic Carbene Adducts of Arylchlorosilylenes and Their Germanium Homologues

The first N-heterocyclic carbene adducts of arylchlorosilylenes are reported and compared with the homologous germanium compounds. The arylsilicon(II) chlorides SiArCl(Im-Me(4)) [Ar=C(6)H(3)-2,6-Mes(2) (Mes=C(6)H(2)-2,4,6-Me(3)), C(6)H(3)-2,6-Trip(2) (Trip=C(6)H(2)-2,4,6-iPr(3))] were obtained selectively on dehydrochlorination of the arylchlorosilanes SiArHCl(2) with 1,3,4,5-tetramethylimidazol-2-ylidene (Im-Me(4)). The analogous arylgermanium(II) chlorides GeArCl(Im-Me(4)) were prepared by metathetical exchange of GeCl(2)(Im-Me(4)) with LiC(6)H(3)-2,6-Mes(2) or addition of Im-Me(4) to GeCl(C(6)H(3)-2,6-Trip(2)). All compounds were fully characterized. Density functional calculations on ECl(C(6)H(3)-2,6-Trip(2))(Im-Me(4)), where E=Si, Ge, at different levels of theory show very good agreement between calculated and experimental bonding parameters, and NBO analyses reveal similar electronic structures of the two aryltetrel(II) chlorides. The low gas-phase Gibbs free energy of bond dissociation of SiCl(C(6)H(3)-2,6-Trip(2))(Im-Me(4)) (Delta(calcd) degrees=28.1 kJ mol(-1)) suggests that the carbene adducts SiArCl(Im-Me(4)) may be valuable transfer reagents of the arylsilicon(II) chlorides SiArCl.

N‐Heterocyclic Carbene Stabilized Digermanium(0)

Zero Ge! Reductions of an N-heterocyclic carbene (NHC) adduct of GeCl2 with magnesium(I) dimers afford a dimeric compound (see picture), which structural and theoretical studies show to contain a singlet digermanium(0) fragment :GeGe: datively coordinated by two NHC ligands. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Synthesis and Characterization of Alkynyl Complexes of Groups 1 and 2

The synthesis of N-heterocyclic carbene adducts of alkynyl lithium and magnesium is achieved, and different degrees of association are observed. Reaction of strontium amide nacnacSrN(SiMe(3))(2)(thf) (nacnac = CH(CMe2,6-iPr(2)C(6)H(3)N)(2)) with PhC[triple bond]CH in THF yields the dimeric alkynyl complex [nacnacSr(thf)(mu-C[triple bond]CPh)](2) which shows an interesting coordination geometry around the metal center. The compound retains the THF molecules, unlike its lighter congener, even in hydrocarbon solvents.

Synthesis, Characterization, and Solution Lability of N-Heterocyclic Carbene Adducts of the Heavier Group 2 Bis(trimethylsilyl)amides

A series of N-heterocyclic carbene (NHC) adducts of the heavier group 2 bis(trimethylsilyl)amides, of the general formulas [(L1)M{N(SiMe3)2}2], [(L2)Ca{N(SiMe3)2}2], and [(L1)Ca{N(SiMe3)2}Cl] (L1 = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene and L2 = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, M = Ca, Sr, and Ba), have been synthesized by either the addition of the group 2 amide to the appropriate imidazolium salt or direct addition of the carbene to the solvent-free, homoleptic, metal amide. X-ray diffraction studies of the reaction products revealed the formation of monomeric three-coordinate alkaline earth metal species in which the NHC binds via conventional σ-donation of the lone pair to the electrophilic metal center. Although 1H, 13C, and NOESY NMR experiments suggest this coordination is retained in solution, reactions with protic substrates such as 2-methoxyethylamine, diphenylamine, and di-p-tolylphosphine demonstrate the lability of the NHC under catalytically-relevant conditions. Fu...

Synthesis of Transition-metal NHC Complexes using “Protected” NHC Adduct

Since the successful isolation of N-heterocyclic carbenes (NHCs), NHCs are now fully established as an important class of ligands for the homogeneous catalysis. The most common method for the preparation of NHC complexes is direct complexation of the free NHC with the metal, in which the free NHC, either isolated or generated in situ, is produced by deprotonation of the corresponding salt as the NHC precursor. However manipulation of the free NHCs is often difficult due to their highly reactive nature toward air and moisture. The reaction of the imidazolium salt with metal precursors bearing the basic ligand such as acetate, acetylacetonate, or alkoxide is also effective for the preparation of NHC complexes, whereas this method is limited to the synthesis of the late transition metal complexes. Recently, the employment of NHC adducts as “protected” forms of the free carbene attracted much attention for new methods of the preparation of the NHC complexes. In this review, the recent developments of “protected” NHC adducts as versatile synthons for the synthesis of NHC complexes are described.

N-Heterocyclic carbene adducts of cyclopalladated ferrocenylimines: Synthesis, characterization and catalytic activity for the Suzuki–Miyaura reaction

A series of novel air-stable carbene adducts of cyclopalladated ferrocenylimines have been synthesized and characterized. The structures of these complexes were determined by single-crystal X-ray analysis. These adducts have been successfully applied to the Suzuki–Miyaura coupling reaction under aerobic conditions. Good to excellent yields were obtained in short reaction times with a lower loading of the catalyst.

F‐Block N‐Heterocyclic Carbene Complexes

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Complexes of an Anionic Gallium(I) N-Heterocyclic Carbene Analogue with Group 14 Element(II) Fragments: Synthetic, Structural and Theoretical Studies

The reactions of the anionic gallium(I) N-heterocyclic carbene (NHC) analogue, [K(tmeda)][:Ga{[N(Ar)C(H)]2}], Ar = C6H3Pri2-2,6, with the heavier group 14 alkene analogues, R2E=ER2, E = Ge or Sn, R = -CH(SiMe3)2, have been carried out. In 2:1 stoichiometries, these lead to the ionic [K(tmeda)][R2EGa{[N(Ar)C(H)]2}] complexes which exhibit long E-Ga bonds. The nature of these bonds has been probed by DFT calculations, and the complexes have been compared to neutral NHC adducts of group 14 dialkyls. The 4:1 reaction of [K(tmeda)][:Ga{[N(Ar)C(H)]2}] with R2Sn=SnR2 leads to the digallyl stannate complex, [K(tmeda)][RSn[Ga{[N(Ar)C(H)]2}]2], presumably via elimination of KR. In contrast, the reaction of the gallium heterocycle with PbR2 affords the digallane4, [Ga{[N(Ar)C(H)]2}]2, via an oxidative coupling reaction. For sake of comparison, the reactions of [K(tmeda)][:Ga{[N(Ar)C(H)]2}] with Ar'2E=EAr'2, E = Ge, Sn or Pb, Ar' = C6H2Pri3-2,4,6, were carried out and led to either no reaction (E = Ge), the formation of [K(tmeda)][Ar'2SnGa{[N(Ar)C(H)]2}] (E = Sn), or the gallium(III) heterocycle, [Ar'Ga{[N(Ar)C(H)]2}] (E = Pb). Salt elimination reactions between [K(tmeda)][:Ga{[N(Ar)C(H)]2}] and the guanidinato group 14 complexes [(Giso)ECl], E = Ge or Sn, Giso = [Pri2NC{N(Ar)}2]-, gave the neutral [(Giso)EGa{[N(Ar)C(H)]2}] complexes. All complexes have been characterized by NMR spectroscopy and X-ray crystallographic studies.

F-block N-heterocyclic carbene complexes

N-Heterocyclic carbenes (NHCs) can bind as two-electron sigma-donor ligands to lanthanide and actinide metal cations. In this review we summarise how the incorporation of an anionic group (alkoxide or amido), to form heterobidentate NHC ligands, allows the synthesis of a range of f-block NHC adducts. The tethering group also allows the lability of the NHC group, and its subsequent reactivity, to be studied. We include a brief survey of the known, structurally characterised f-element-NHC bond distances, and a range of substrates that react to displace the metal-bound NHC group.

N-Heterocyclic carbene adducts of orthopalladated triarylphosphite complexes

Carbene adducts of orthopalladated triarylphosphite complexes have been synthesised and characterised. The structures of three of these complexes were determined by single-crystal X-ray analysis. The complexes are active in the Suzuki coupling of a range of aryl bromide substrates.

The synthesis, characterisation and electronic structure of N-heterocyclic carbene adducts of PI cations

Two different syntheses of N-heterocyclic carbene complexes of P(I) cations are presented; the structural features of these phosphamethine cyanine salts, in conjunction with the results of computational investigations, provide insight into the nature of the bonding of these heavy allene analogues.

A general and versatile approach to thermally generated N-heterocyclic carbenes.

The synthesis of N-heterocyclic carbene (NHC) adducts by condensation of diamines with appropriately substituted benzaldehydes is described. This simplified approach provides the NHC adduct without first having to generate the carbene followed by its protection. These adducts undergo thermal deprotection to generate N-heterocyclic carbene in situ. Adduct decomposition temperatures were investigated as a function of catalyst structure by using thermal analysis and spectroscopic techniques. Importantly, unlike adducts derived from chloroform, the new pentafluorobenzene-based adducts are more readily prepared and are stable at room temperature. The utility of these adducts as organic catalyst precursors for living ring-opening polymerization (ROP) of lactide, transesterification reactions, and the synthesis of N-heterocyclic carbene ligated organometallic complexes is also described.

Synthesis and activity of ruthenium alkylidene complexes coordinated with phosphine and N-heterocyclic carbene ligands.

This paper reports the synthesis and characterization of a variety of ruthenium complexes coordinated with phosphine and N-heterocyclic carbene (NHC) ligands. These complexes include several alkylidene derivatives of the general formula (NHC)(PR(3))(Cl)(2)Ru=CHR', which are highly active olefin metathesis catalysts. Although these catalysts can be prepared adequately by the reaction of bis(phosphine) ruthenium alkylidene precursors with free NHCs, we have developed an alternative route that employs NHC-alcohol or -chloroform adducts as "protected" forms of the NHC ligands. This route is advantageous because NHC adducts are easier to handle than their free carbene counterparts. We also demonstrate that sterically bulky bis(NHC) complexes can be made by reaction of the pyridine-coordinated precursor (NHC)(py)(2)(Cl)(2)Ru=CHPh with free NHCs or NHC adducts. Two crystal structures are presented, one of the mixed bis(NHC) derivative (H(2)IMes)(IMes)(Cl)(2)Ru=CHPh, and the other of (PCy(3))(Cl)(CO)Ru[eta(2)-(CH(2)-C(6)H(2)Me(2))(N(2)C(3)H(4))(C(6)H(2)Me(3))], the product of ortho methyl C-H bond activation. Other side reactions encountered during the synthesis of new ruthenium alkylidene complexes include the formation of hydrido-carbonyl-chloride derivatives in the presence of primary alcohols and the deprotonation of ruthenium vinylcarbene ligands by KOBu(t). We also evaluate the olefin metathesis activity of NHC-coordinated complexes in representative RCM and ROMP reactions.