Ring-expanded N-heterocyclic Carbene Research Articles

Ring-expanded iridium and rhodium N-heterocyclic carbene complexes: a comparative DFT study of heterocycle ring size and metal center diversity

A new series of ring-expanded six- and seven-membered N-heterocyclic carbene precursors (re-NHCs) and their transition metal complexes were synthesized. The basic properties of the synthesized materials were investigated by density functional theory (DFT). The six- and seven-membered re-NHCs were synthesized in good yield via reaction of the corresponding alkyldibromides or alkyldiiodides with N,N′-bis-(2-phenylbenzene)formamidine in the presence of K2CO3 under aerobic conditions. Complexes, represented by the formula [ML1,2(COD)Cl] (where M = Ir or Rh and L is a ring-expanded N-heterocyclic carbene ligand), were synthesized in the presence of the corresponding free carbene and iridium or rhodium metal precursors in tetrahydrofuran. All new re-NHC complexes were characterized by different analytical techniques, including NMR spectroscopy, X-ray diffraction, UV spectroscopy and elemental analysis. According to molecular electrostatic potential calculations, the electrophilic properties of the complexes were aligned, from highest to lowest, as Ir-6-DiPh, Rh-6-DiPh and Ir-7-DiPh. The HOMO, LUMO and energy gaps of the complexes were calculated by DFT. On the basis of the DFT analysis, it can be predicted that Rh-6-DiPh is the most stable complex and Ir-7-DiPh is more reactive than Ir-6-DiPh.

Three-Coordinate Iron(II) Expanded Ring N-Heterocyclic Carbene Complexes

A sterically demanding seven-membered expanded ring N-heterocyclic carbene (NHC) ligand allows access to rare examples of three-coordinate iron(II)–NHC complexes incorporating only halide coligands of the general formula [Fe(NHC)X2] (NHC = 7-DiPP; X = Br (1) Cl (2)). Reducing the steric influence of the ancillary NHC ligand through modulation of the N-aryl substituents leads to either four- or three-coordinate complexes of the general formula [Fe(NHC)Br2(THF)] (3) or [Fe(NHC)Br2] (4) (NHC = 7-Mes), dependent upon the solvent of recrystallization. The further reduction of NHC steric influence results in four-coordinate geometries at iron in the form of the dimeric species [Fe(NHC)Br(μ-Br)]2 (5) or [Fe(NHC)Br2(THF)] (6) (NHC = SDiPP), again dependent upon the solvent of recrystallization. Compounds 1–6 have been analyzed by 1H NMR spectroscopy, X-ray crystallography, elemental microanalysis, Mossbauer spectroscopy (for 1 and 3–5), and Evans method magnetic susceptibility. In addition to these measurements t...

Synthesis and Reactivity of New Copper(I) Hydride Dimers

The expanded-ring N-heterocyclic carbenes 6Dipp and 7Dipp (6Dipp = 1,3-bis(2,6-diisopropylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene and 7Dipp = 1,3-bis(2,6-diisopropylphenyl)-4,5,6,7-tetrahydro-1,3-diazepin-2-ylidene) support isolable neutral copper hydride dimers. [(6Dipp)CuH]2 reacts with 1-hexene to give (6Dipp)copper(I) hexyl by 1,2-insertion and with benzyl isonitrile to afford an η1-formimidoyl by 1,1-insertion.

Lactide polymerisation by ring-expanded NHC complexes of zinc

The reaction of the ring-expanded N-heterocyclic carbenes (RE-NHCs) with [ZnMe2] affords the structurally characterised complexes [(6-Mes)ZnMe2] (1) and [(7-Mes)ZnMe2] (2). The activity of the two complexes, along with that of the free carbenes, for the ring-opening polymerisation of rac-lactide has been probed. Even for such strongly σ-donating carbenes, dissociation from the metal centre cannot be ruled out.

Expanded-ring N-heterocyclic carbenes for the stabilization of highly electrophilic gold(I) cations.

Strategies for the synthesis of highly electrophilic Au(I) complexes from either hydride- or chloride-containing precursors have been investigated by employing sterically encumbered Dipp-substituted expanded-ring NHCs (Dipp=2,6-iPr2 C6 H3 ). Thus, complexes of the type (NHC)AuH have been synthesised (for NHC=6-Dipp or 7-Dipp) and shown to feature significantly more electron-rich hydrides than those based on ancillary imidazolylidene donors. This finding is consistent with the stronger σ-donor character of these NHCs, and allows for protonation of the hydride ligand. Such chemistry leads to the loss of dihydrogen and to the trapping of the [(NHC)Au](+) fragment within a dinuclear gold cation containing a bridging hydride. Activation of the hydride ligand in (NHC)AuH by B(C6 F5 )3 , by contrast, generates a species (at low temperatures) featuring a [HB(C6 F5 )3 ](-) fragment with spectroscopic signatures similar to the "free" borate anion. Subsequent rearrangement involves BC bond cleavage and aryl transfer to the carbophilic metal centre. Under halide abstraction conditions utilizing Na[BAr(f) 4 ] (Ar(f) =C6 H3 (CF3 )2 -3,5), systems of the type [(NHC)AuCl] (NHC=6-Dipp or 7-Dipp) generate dinuclear complexes [{(NHC)Au}2 (μ-Cl)](+) that are still electrophilic enough at gold to induce aryl abstraction from the [BAr(f) 4 ](-) counterion.

Expanded ring N-heterocyclic carbene adducts of group 15 element trichlorides: synthesis and reduction studies.

Reactions of the expanded ring N-heterocyclic carbene, 6-Dip (:C{N(Dip)CH2}2CH2, Dip = 2,6-diisopropylphenyl), with group 15 element trichlorides have yielded the monomeric complexes, [(6-Dip)ECl3] (E = P, As or Sb), two examples of which (E = P and Sb) have been crystallographically characterised. Reduction of [(6-Dip)PCl3] with KC8 yielded the unusual tetraphosphorus dicationic complex, [(6-Dip)2(μ-P4)]Cl2, the X-ray crystal structure of which shows it to be an ion-separate salt. The compound can also be prepared from the direct reaction of excess 6-Dip with PCl3. Treatment of the cyclic amidinium salt, [6-MesH]Br (6-MesH = [HC{N(Mes)CH2}2CH2](+), Mes = mesityl) with KC8, leads to reductive coupling of the heterocycle and formation of the hindered bis(hexahydropyrimidine), (6-MesH)2. An X-ray crystallographic analysis of (6-MesH)2 shows the compound to have a long central C-C bond, while an electrochemical analysis reveals it to undergo an irreversible two-electron oxidation in dichloromethane solutions.

Stereoelectronic effects in C-H bond oxidation reactions of Ni(I) N-heterocyclic carbene complexes.

Activation of O2 by the three-coordinate Ni(I) ring-expanded N-heterocyclic carbene complexes Ni(RE-NHC)(PPh3)Br (RE-NHC = 6-Mes, 1; 7-Mes, 2) produced the structurally characterized dimeric Ni(II) complexes Ni(6-Mes)(Br)(μ-OH)(μ-O-6-Mes')NiBr (3) and Ni(7-Mes)(Br)(μ-OH)(μ-O-7-Mes')NiBr (4) containing oxidized ortho-mesityl groups from one of the carbene ligands. NMR and mass spectrometry provided evidence for further oxidation in solution to afford bis-μ-aryloxy compounds; the 6-Mes derivative was isolated, and its structure was verified. Low-temperature UV-visible spectroscopy showed that the reaction between 1 and O2 was too fast even at ca. -80 °C to yield any observable intermediates and also supported the formation of more than one oxidation product. Addition of O2 to Ni(I) precursors containing a less electron-donating diamidocarbene (6-MesDAC, 7) or less bulky 6- or 7-membered ring diaminocarbene ligands (6- or 7-o-Tol; 8 and 9) proceeded quite differently, affording phosphine and carbene oxidation products (Ni(O═PPh3)2Br2 and (6-MesDAC)═O) and the mononuclear Ni(II) dibromide complexes (Ni(6-o-Tol)(PPh3)Br2 (10) and (Ni(7-o-Tol)(PPh3)Br2 (11)) respectively. Electrochemical measurements on the five Ni(I) precursors show significantly higher redox potentials for 1 and 2, the complexes that undergo oxygen atom transfer from O2.

Use of Ring-Expanded Diamino- and Diamidocarbene Ligands in Copper Catalyzed Azide–Alkyne “Click” Reactions

The two-coordinate ring-expanded N-heterocyclic carbene copper(I) complexes [Cu(RE-NHC)2]+ (RE-NHC = 6-Mes, 7-o-Tol, 7-Mes) have been prepared and shown to be effective catalysts under neat conditions for the 1,3-dipolar cycloaddition of alkynes and azides. In contrast, the cationic diamidocarbene analogue [Cu(6-MesDAC)2]+ and the neutral species [(6-MesDAC)CuCl]2 and [(6-MesDAC)2(CuCl)3] show good activity when the catalysis is performed on water.

Pd–PEPPSI-Type Expanded Ring N-Heterocyclic Carbene Complexes: Synthesis, Characterization, and Catalytic Activity in Suzuki–Miyaura Cross Coupling

The synthesis and characterization of the new six- and seven-membered Pd–PEPPSI-type N-heterocyclic carbene (NHC) complexes 3 and 4 is described. Complexes of the general formula [Pd(NHC)(3ClPy)Cl2] (NHC = 6- or 7-Mes, 3ClPy = 3-chloropyridine) are accessed via the oxidation of the well-defined parent palladium(0) complexes 1 and 2. Complexes 3 and 4 have been employed in Suzuki–Miyaura cross-coupling and catalytic dehalogenation of a range of aryl halide substrates.

Expanded‐Ring N‐Heterocyclic Carbenes Efficiently Stabilize Gold(I) Cations, Leading to High Activity in π‐Acid‐Catalyzed Cyclizations

A series of six- and seven-membered expanded-ring N-heterocyclic carbene (er-NHC) gold(I) complexes has been synthesized using different synthetic approaches. Complexes with weakly coordinating anions [(er-NHC)AuX] (X(-) = BF4(-), NTf2(-), OTf(-)) were generated in solution. According to their (13)C NMR spectra, the ionic character of the complexes increases in the order X(-) = Cl(-) < NTf2(-) < OTf(-) < BF4(-). Additional factors for stabilization of the cationic complexes are expansion of the NHC ring and the attachment of bulky substituents at the nitrogen atoms. These er-NHCs are bulkier ligands and stronger electron donors than conventional NHCs as well as phosphines and sulfides and provide more stabilization of [(L)Au(+)] cations. A comparative study has been carried out of the catalytic activities of five-, six-, and seven-membered carbene complexes [(NHC)AuX], [(Ph3 P)AuX], [(Me2S)AuX], and inorganic compounds of gold in model reactions of indole and benzofuran synthesis. It was found that increased ionic character of the complexes was correlated with increased catalytic activity in the cyclization reactions. As a result, we developed an unprecedentedly active monoligand cationic [(THD-Dipp)Au]BF4 (1,3-bis(2,6-diisopropylphenyl)-3,4,5,6-tetrahydrodiazepin-2-ylidene gold(I) tetrafluoroborate) catalyst bearing seven-membered-ring carbene and bulky Dipp substituents. Quantitative yields of cyclized products were attained in several minutes at room temperature at 1 mol % catalyst loadings. The experimental observations were rationalized and fully supported by DFT calculations.

Probing the Limits of Ligand Steric Bulk: Backbone CH Activation in a Saturated N‐Heterocyclic Carbene

The consequences of extremely high steric loading have been probed for late transition metal complexes featuring the expanded ring N-heterocyclic carbene 6-Dipp. The reluctance of this ligand to form 2:1 complexes with d-block metals (rationalised on the basis of its percentage buried volume, % Vbur , of 50.8%) leads to C-H and C-N bond activation processes driven by attack at the backbone β-CH2 unit. In the presence of Ir(I) (or indeed H(+) ) the net result is the formation of an allyl formamidine fragment, while Au(I) brings about an additional ring (re-)closure step via nucleophilic attack at the coordinated alkene. The net transformation of 6-Dipp in the presence of [(6-Dipp)Au](+) represents to our knowledge the first example of backbone C-H activation of a saturated N-heterocyclic carbene, proceeding in this case via a mechanism which involves free carbene in addition to the Au(I) centre.

Development on the Synthesis of Ring Expanded N-Heterocyclic Carbene Precursors

As a kind of novel developed ligands, N-heterocyclic carbenes (NHCs) play very important roles in the develop- ment of modern organic chemistry. Early studies on NHCs have been mainly focused on imidazole derived five membered heterocycles, due to their easy accessibility and comparably stable properties. Recently, ring expanded NHCs, such as six- or seven-membered NHCs became another research hot spot due to their steric characteristics and chemical properties. Because deprotonation of the heterocyclic onium salt precursors is by far the most commonly used method to access NHCs, the synthe- sis of the precursors remains a key factor affecting the development of ring expanded NHCs. This review will summarize the progress on the synthesis of ring expanded NHC precursors. Keywords N-heterocyclic carbene; ring expanded; precursor

Zerovalent [Pd(NHC)(Alkene)1,2] Complexes Bearing Expanded-Ring N-Heterocyclic Carbene Ligands in Transfer Hydrogenation of Alkynes

In search of more active catalysts for the transfer hydrogenation of alkynes, a series of [Pd(NHC)(MA)1,2] (8–14) and [Pd(NHC)(dvtms)] complexes (1–7), in which the NHC ancillary ligands are expanded-ring N-heterocyclic carbenes (erNHC’s), have been prepared. These very bulky, strong σ-donor ligands impart a highly constrained geometry on the complexes and in some cases enable the isolation of coordinatively and electronically unsaturated complexes (10 and 14). Their strong σ-donor character is reflected in a decrease in IR stretching frequency for the C═O bond of the maleic anhydride ligands (8–14) in comparison to their five-membered counterparts. Significantly enhanced catalytic activity in the transfer hydrogenation of 1-phenyl-1-propyne is observed using [Pd(erNHC)(dvtms)] complexes (1–7) as precatalysts. The catalysts show high initial selectivity toward (Z)-alkene. However, double-bond isomerization and over-reduction to the corresponding alkane occur when all the alkyne substrate is consumed; this feature reflects the very high efficiency of these catalysts in the transfer hydrogenation of alkynes as well as alkenes.

N-alkyl functionalised expanded ring N-heterocyclic carbene complexes of rhodium(i) and iridium(i): structural investigations and preliminary catalytic evaluation

A series of new N-alkyl functionalised 6- and 7-membered expanded ring N-heterocyclic carbene (NHC) pro-ligands 3-6 and their corresponding complexes of rhodium(I) and iridium(I), [M(NHC)(COD)Cl] 7-14 and [M(NHC)(CO)2Cl] 15-22 are described. The complexes have been characterised by (1)H and (13)C{(1)H} NMR, mass spectrometry, IR and X-ray diffraction. It is noted from X-ray diffraction studies that the N-alkyl substituents are found to orientate themselves away from the metal centre due to unfavourable steric interactions resulting in low percent buried volume (%V(bur)) values in the solid state. The heterocycle ring size is also found to dictate the spatial orientation of the N-alkyl substituents in the neopentyl functionalised derivatives 10 and 14. The 7-membered derivative 14 allows for a conformational 'twist' of the heterocycle ring with the N-alkyl substituents adopting a mutually trans configuration with respect to each other, while the more rigid 6-membered system 10 does not allow for this conformational 'twist' and consequently the N-alkyl substituents adopt a mutually cis configuration. The σ-donor function of this new class of expanded ring NHC ligand has also been probed by measured IR stretching frequencies of the [M(NHC)(CO)2Cl] complexes 15-22. A preliminary catalytic survey of the hydrogenation of functionalised alkenes with molecular hydrogen under mild conditions has also been undertaken with complex , affording an insight into the application of large ring NHC ancillary ligands bearing N-alkyl substituents in hydrogenation transformations.

Ring-Expanded N-Heterocyclic Carbene Complexes of Rhodium with Bifluoride, Fluoride, and Fluoroaryl Ligands

Thermolysis of Rh(PPh3)4H in the presence of the six-membered N-heterocyclic carbene 1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidine (6-iPr) gave the monocarbene complex Rh(6-iPr)(PPh3)2H as a 1:2 mixture of the cis- and trans-phosphine isomers 1a and 1b. This same isomeric mixture was formed as the ultimate product from treating Rh(PPh3)3(CO)H with 6-iPr at room temperature, although pathways involving both CO and PPh3 loss were observed at initial times. Treatment of 1a/1b with Et3N·3HF generated the bifluoride complex cis-Rh(6-iPr)(PPh3)2(FHF) (2a), which upon stirring with anhydrous Me4NF was converted to the rhodium fluoride complex cis-Rh(6-iPr)(PPh3)2F (3a). Thermolysis of 1a/1b with C6F6 resulted in C–F bond activation to afford a mixture of 3a and the pentafluorophenyl complex trans-Rh(6-iPr)(PPh3)2(C6F5) (5b). Complexes 1b, 2a, 3a, and 5b were structurally characterized.

Recent Developments of Expanded Ring N-Heterocyclic Carbenes

Recent Developments of Expanded Ring N-Heterocyclic Carbenes

Gold(I) Complexes Bearing Sterically Imposing, Saturated Six- and Seven-Membered Expanded Ring N-Heterocyclic Carbene Ligands

The synthesis and characterization of novel six- and seven-membered expanded ring N-heterocyclic carbene (NHC) complexes of the general formula [Au(NHC)Cl] are described. The key structural parameters of complexes 1, 2, and 4 have been evaluated by solid-state analysis and by means of the percent buried volume (%Vbur) analysis. Complex 4 is found to demonstrate the largest %Vbur value of any gold(I) NHC complex reported to date, with a value of 52.6. All complexes 1–4 have been evaluated in a preliminary catalytic survey of the hydration of internal alkynes.

Novel Intramolecular CAryl–S Bond Activation by an Electron Rich, Ring-Expanded-NHC-Rh centre: A Combined Experimental and DFT Study

The reaction of (o-MeSPh)-N-functionalized tetrahydropyrimidinium salts with KN(SiMe3)2 and [Rh(COD)Cl]2 in THF leads to the formation of a novel dimeric RhIII bis-carbene complex. The reaction involves the unexpected cleavage/oxidative addition of the aryl-sulfur bond to give dimeric metallated RhIII with bridging MeS&lt; moieties. This unusual reaction is probably a consequence of the sterically imposing structure and strong donor capacity of ring-expanded N-heterocyclic carbenes (RE-NHCs). An X-ray structure of the [(Ph,DIPP-NHC)Rh(Cl)(SMe)2] product complex has been obtained, and DFT studies were undertaken to gain an insight into the reaction pathway.

Expanded ring N-heterocyclic carbenes: A comparative study of ring size in palladium (0) catalysed Mizoroki–Heck coupling

The synthesis and characterisation of a range of new 6- and 7-membered expanded ring N-heterocyclic carbene (NHC) complexes of zero valent palladium dvtms (divinyltetramethyldisiloxane) 8-14 is reported, with the first comparative catalytic study probing the effects of large ring N-heterocyclic carbene ligands (6-Mes and 7-Mes) with their analogous 5-membered derivative (IMes) on catalytic performance. The catalytic performances of the [Pd(NHC)(dvtms)] complexes are evaluated in the Mizoroki-Heck coupling of 4-bromoacetophenone and n-butyl acrylate, providing a ready and effective comparison into the influence of ring size and steric congestion on catalytic activity in carbon-carbon cross coupling reactions.

Expanded ring N-heterocyclic carbene complexes of zero valent platinum dvtms (divinyltetramethyldisiloxane): Highly efficient hydrosilylation catalysts

The synthesis and characterisation of a series of new 6- and 7-membered NHC (N-heterocyclic carbene) complexes 7–12 of zero valent platinum dvtms (divinyltetramethyldisiloxane), [Pt(NHC)(dvtms)] are reported. A number of the complexes were investigated as catalyst precursors in the hydrosilylation of a range of unsaturated substrates, including alkynes, alkenes and a ketone with triethylsilane and bis(trimethylsiloxy)methylsilane (MD’M), at low catalyst loading (0.005 mol %); in general, the activity, and the selectivity for 1-functionalised product was found to be high.