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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) 8–14is 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. [ABSTRACT FROM AUTHOR]

A tridentate NCN ligand, 1, containing a bicyclic central NHC ring and two flanking pyridyl groups has been coordinated to Rh(i) and Ir(i) to give complexes of the type [M(κ3-1)(1,5-COD)]PF6(2M = Rh; 3M = Ir). In contrast to our earlier study with this ligand, the complexes have been shown to approximate to a trigonal bipyramidal geometry in the solid state and exist as an isomeric mixture in solution as determined by 1H and 13C NMR spectroscopy. Electrochemical studies revealed that both complexes undergo a 1-electron oxidation with the potential of the Rh complex 0.1V less than that of the Ir complex in CH2Cl2. Preliminary DFT studies confirm the lowest energy conformations as those seen in the solid state and show the location and energy of the HOMOs to be identical in 2and 3. Partial charge analysis shows a greater positive charge on the Ir in 3compared to the Rh in 2. Some preliminary studies of hydrogenation reactivity have shown the complexes to be efficient for both transfer and direct hydrogenation of prochiral ketones and alkenes at moderate temperatures but without any discernible enantioselectivity. [ABSTRACT FROM AUTHOR]

The preparation and characterisation of the Cr(i) complexes [Cr(CO)4L]+(L = Ph2PN(R)PPh2, Ph2P(R)PPh2), which are used as pre-catalysts for the selective oligomerization of ethylene, are reported. The electronic properties and structural features of these complexes in frozen solution have been established viacontinuous wave X-band Electron Paramagnetic Resonance (cw-EPR) and continuous wave 1H, 14N and 31P Electron Nuclear Double Resonance (cw-ENDOR) spectroscopy. The EPR spectra are dominated by the ganisotropy, with notably large PA couplings from the two equivalent 31P nuclei. The spin Hamiltonian parameters (g(gxx= gyy) > ge> g(gzz)) are consistent with a low-spin d5system possessing C2vsymmetry, with a SOMO where the metal contribution is primarily dxyfor all complexes. The isotropic Fermi contact term (Paiso, determined by EPR and ENDOR) was found to be largest for complexes containing ligands e, d, fand g, indicating that the 31P 3 s character in the SOMO is higher for the PNP type ligands than the PCP type. Subtle structural differences in the complexes were also identified through variations in the Δgshifts (identified by EPR), and through differences in the phenyl ring conformations (identified by 1H ENDOR). Attempts to correlate trends in EPR-derived parameters with data measured for catalysis using these pre-catalysts are also made, but no clear connections were found. [ABSTRACT FROM AUTHOR]

Several homogeneous palladium catalysts based on PPh3and nucleophilic carbene (NHC) ligands were screened in the telomerisation reaction of 1,3-pentadiene with methanol. A Pd(acac)2-3PPh3system showed the highest activity; initially, poor activity was observed using the Pd(0)-NHC catalysts. However, when methanol solvent/nucleophile was replaced by longer chain alcohols 1-propanol and 1-butanol, improved activity and selectivity was observed. Similarly, 1,3-hexadiene was telomerised with 1-propanol and 1-butanol with good selectivity using Pd(IMes)(dvds) as the catalyst. Pd(ii)-PPh3systems were ineffective as catalysts for the telomerisation of 1,3-hexadiene. [ABSTRACT FROM AUTHOR]

The synthesis of new functionalised 6- and 7-membered NHC (N-heterocyclic carbene) precursors bearing anisidyl or pyridine N-substituents, both symmetrically and non-symmetrically substituted is reported. Their corresponding rhodium(i) and iridium(i) complexes, M(COD)(NHC)Cl, were also prepared and characterised. The unusual Rh(iii)/Rh(i) salt, [Rh(η2-NHC-py)2Cl2][Rh(COD)Cl2], was obtained with one of the pyridyl-functionalised NHC ligands. Single-crystal X-ray analyses have been obtained for the majority of the complexes and NHC salts. The activity of these complexes was tested in the hydrogenation of a range of substrates with molecular hydrogen, including 1-cyclooctadiene and 2-methyl styrene, where they show enhanced activity and stability in comparison to non-functionalised NHC analogues, operating under exceptionally mild conditions (ambient temperature and atmospheric pressure). [ABSTRACT FROM AUTHOR]

Generation of N-heterocyclic carbene (NHC) complexes [(dmpe)M(azol-2-ylidene)R] via the oxidative addition of a series of 2-substituted azolium salts to Group-10 zerovalent metal complexes has been investigated using density functional theory (2-R = H, Me, Ph; Azole = imidazole, thiazole, oxazole; M = Ni, Pd, Pt). Overall, platinum-based pathways result in the greatest enthalpies of reaction, but due to the reactive nature of Group-10 metals bearing the 1,2-bis(dimethylphosphino)ethane (dmpe) chelate, nickel and palladium species also have little trouble proceeding to stable products in the absence of a significant barrier. Imidazolium salts were found to be the most vulnerable to oxidative addition due to their low stabilisation energies when compared to the oxazolium and thiazolium species. Activation barriers show the general trend of phenyl > methyl > hydrido with regard to the azole 2-substituent, with no observed barrier for all but one of the 2-hydrido cases. Minimal barriers were found to exist in a number of cases for activation of a C(2)–CH3 bond suggesting that synthesis of alkyl–carbene complexes may be possible via this route under certain conditions, and therefore ionic liquids based on these substituted azolium salts may be active participants in catalytic reactions. [ABSTRACT FROM AUTHOR]

Methylpalladium(ii) carbene complexes of the type [Pd(NHC)Me(P–P)]BF4 (NHC = N-heterocyclic carbene, P–P = chelating phosphine) have been synthesised, the complex [Pd(tmiy)Me(dcype)]BF4 (tmiy = 1,3,4,5-tetramethylimidazol-2-ylidene, dcype = 1,2-bis(dicyclohexylphosphino)ethane) being characterised crystallographically. Complexes bearing the tmiy ligands were shown to decompose in an analogous manner to complexes bearing monodentate phosphine ligands, with the rate of decomposition being nominally linked to the size of the chelate ring. The decomposition of these complexes in the presence of aryl halides—expected to yield Pd(Ar)X(P–P)—was studied and shown instead to yield PdX2(P–P) and [Pd(tmiy)X(P–P)]BF4. Additionally, Pd(Me)X(P–P) and Pd(Ar)X(P–P) were observed in some cases. Intermolecular cross-over reactions between the starting complex and Pd(Ar)X(P–P) were found to be the source of these unexpected products. [ABSTRACT FROM AUTHOR]

A series of diaryl, mono-aryl/alkyl and dialkyl mono- and bicyclic expanded-ring N-heterocyclic carbenes (ER-NHCs) have been prepared and their complexation to Au(i) investigated through the structural analysis of fifteen Au(NHC)X and/or [Au(NHC) 2 ]X complexes. The substituted diaryl 7-NHCs are the most sterically encumbered with large buried volume (%V B ) values of 40-50% with the less flexible six-membered analogues having %V B values at least 5% smaller. Although the bicyclic systems containing fused 6- and 7-membered rings (6,7-NHCs) are constrained with relatively acute NCN bond angles, they have the largest %V B values of the dialkyl derivatives reported here, a feature related to the fixed conformation of the heterocyclic rings and the compressional effect of a pre-set methyl substituent.

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.

A diphosphine ligand (1·HPF(6)), which is a potential precursor to a PC(NHC)P pincer, with a backbone containing two phenylene groups and a central bicyclic 4-aza-2-azoniabicyclo[3.2.1]oct-2-ene unit has been synthesised and coordinated to Pd(II) and Pt(II) to give trans-[M(κ(2)-P,P'-)Cl(2)]PF(6) where M = Pd (2) or Pt (3a). Single-crystal structure determinations of 2 and 3a show the complexes to be isostructural with the diphosphine coordinated in a trans-spanning fashion and the amidinium unit being protonated and non-coordinated. 2 and 3a react with CH(3)I to give the dimers trans-[Pd(2)(μ-1·H)(2)I(4)](PF(6))(2), 6, and trans-[Pt(2)(μ-1·H)(2)I(4)](PF(6))(2), 7, as the major products. This bridging mode of coordination of [1·H](+) is also seen in trans-[Rh(2)(μ-1·H)(1,5-COD)(2)Cl(2)]PF(6), 4, and [Pt(2)(μ-κ(2)-1·H)(dvdms)]PF(6), 5. Upon treatment with KO(t)Bu complexes and undergo deprotonation at the amidinium carbon to give trans-[M(κ(3)-P,C,P'-1)Cl]PF(6) where M = Pd (8), and Pt (9). The related trans-[Rh(κ(3)-P,C,P'-1)(CO)]PF(6) (10) is prepared directly from 1·HPF(6) and Rh(acac)(CO)(2): this and the palladium and platinum complexes 8 and 9 are isolated as isomeric mixtures as a consequence of a conformational isomerism. In situ deprotonation of 1·HPF(6) followed by addition of Ag(CF(3)SO(3)) gave S(Ag)-[Ag(κ(3)-P,C,P'-1)(CF(3)SO(3))], 11. Some preliminary studies of the reactivity of 2 and 8 in Suzuki-type reactions are reported and the Pt(0) system has been shown to be an active hydrosilylation catalyst.

An unusual example of diastereoselectivity has been observed in Cu(κ(3)-P,C,P'-1)X complexes where 1 is an asymmetric tridentate ligand containing a bicyclic NHC framework and X is a halide. When X is Cl(-), the S(Cu) isomer is formed selectively whereas when X = I(-) the R(Cu) diastereomer is preferred.

A tridentate N^C^N ligand, 1, containing a bicyclic central NHC ring and two flanking pyridyl groups has been coordinated to Rh(I) and Ir(I) to give complexes of the type [M(κ(3)-1)(1,5-COD)]PF(6) (2 M = Rh; 3 M = Ir). In contrast to our earlier study with this ligand, the complexes have been shown to approximate to a trigonal bipyramidal geometry in the solid state and exist as an isomeric mixture in solution as determined by (1)H and (13)C NMR spectroscopy. Electrochemical studies revealed that both complexes undergo a 1-electron oxidation with the potential of the Rh complex 0.1 V less than that of the Ir complex in CH(2)Cl(2). Preliminary DFT studies confirm the lowest energy conformations as those seen in the solid state and show the location and energy of the HOMOs to be identical in 2 and 3. Partial charge analysis shows a greater positive charge on the Ir in 3 compared to the Rh in 2. Some preliminary studies of hydrogenation reactivity have shown the complexes to be efficient for both transfer and direct hydrogenation of prochiral ketones and alkenes at moderate temperatures but without any discernible enantioselectivity., (This journal is © The Royal Society of Chemistry 2011)

The preparation and characterisation of the Cr(I) complexes [Cr(CO)(4)L](+) (L = Ph(2)PN(R)PPh(2), Ph(2)P(R)PPh(2)), which are used as pre-catalysts for the selective oligomerization of ethylene, are reported. The electronic properties and structural features of these complexes in frozen solution have been established via continuous wave X-band Electron Paramagnetic Resonance (cw-EPR) and continuous wave (1)H, (14)N and (31)P Electron Nuclear Double Resonance (cw-ENDOR) spectroscopy. The EPR spectra are dominated by the g anisotropy, with notably large (P)A couplings from the two equivalent (31)P nuclei. The spin Hamiltonian parameters (g(perpendicular) (g(xx) = g(yy)) > g(e) > g(parallel) (g(zz))) are consistent with a low-spin d(5) system possessing C(2v) symmetry, with a SOMO where the metal contribution is primarily d(xy) for all complexes. The isotropic Fermi contact term ((P)a(iso), determined by EPR and ENDOR) was found to be largest for complexes containing ligands e, d, f and g, indicating that the (31)P 3 s character in the SOMO is higher for the PNP type ligands than the PCP type. Subtle structural differences in the complexes were also identified through variations in the Deltag shifts (identified by EPR), and through differences in the phenyl ring conformations (identified by (1)H ENDOR). Attempts to correlate trends in EPR-derived parameters with data measured for catalysis using these pre-catalysts are also made, but no clear connections were found.

The synthesis of new functionalised 6- and 7-membered NHC (N-heterocyclic carbene) precursors bearing anisidyl or pyridine N-substituents, both symmetrically and non-symmetrically substituted is reported. Their corresponding rhodium(i) and iridium(i) complexes, M(COD)(NHC)Cl, were also prepared and characterised. The unusual Rh(iii)/Rh(i) salt, [Rh(eta(2)-NHC-py)(2)Cl(2)][Rh(COD)Cl(2)], was obtained with one of the pyridyl-functionalised NHC ligands. Single-crystal X-ray analyses have been obtained for the majority of the complexes and NHC salts. The activity of these complexes was tested in the hydrogenation of a range of substrates with molecular hydrogen, including 1-cyclooctadiene and 2-methyl styrene, where they show enhanced activity and stability in comparison to non-functionalised NHC analogues, operating under exceptionally mild conditions (ambient temperature and atmospheric pressure).

Generation of N-heterocyclic carbene (NHC) complexes [(dmpe)M(azol-2-ylidene)R] via the oxidative addition of a series of 2-substituted azolium salts to Group-10 zerovalent metal complexes has been investigated using density functional theory (2-R = H, Me, Ph; Azole = imidazole, thiazole, oxazole; M = Ni, Pd, Pt). Overall, platinum-based pathways result in the greatest enthalpies of reaction, but due to the reactive nature of Group-10 metals bearing the 1,2-bis(dimethylphosphino)ethane (dmpe) chelate, nickel and palladium species also have little trouble proceeding to stable products in the absence of a significant barrier. Imidazolium salts were found to be the most vulnerable to oxidative addition due to their low stabilisation energies when compared to the oxazolium and thiazolium species. Activation barriers show the general trend of phenyl > methyl > hydrido with regard to the azole 2-substituent, with no observed barrier for all but one of the 2-hydrido cases. Minimal barriers were found to exist in a number of cases for activation of a C(2)-CH3 bond suggesting that synthesis of alkyl-carbene complexes may be possible via this route under certain conditions, and therefore ionic liquids based on these substituted azolium salts may be active participants in catalytic reactions.

Methylpalladium(II) carbene complexes of the type [Pd(NHC)Me(P-P)]BF(4) (NHC = N-heterocyclic carbene, P-P = chelating phosphine) have been synthesised, the complex [Pd(tmiy)Me(dcype)]BF(4) (tmiy = 1,3,4,5-tetramethylimidazol-2-ylidene, dcype = 1,2-bis(dicyclohexylphosphino)ethane) being characterised crystallographically. Complexes bearing the tmiy ligands were shown to decompose in an analogous manner to complexes bearing monodentate phosphine ligands, with the rate of decomposition being nominally linked to the size of the chelate ring. The decomposition of these complexes in the presence of aryl halides-expected to yield Pd(Ar)X(P-P)-was studied and shown instead to yield PdX(2)(P-P) and [Pd(tmiy)X(P-P)]BF(4). Additionally, Pd(Me)X(P-P) and Pd(Ar)X(P-P) were observed in some cases. Intermolecular cross-over reactions between the starting complex and Pd(Ar)X(P-P) were found to be the source of these unexpected products.

New chiral imidazolinium salts with tunable steric features, based on a biisoquinoline template, have been developed and structurally characterised using single crystal X-ray crystallography. The trans PdI(2)(NHC)(2) complex was prepared by reaction of the parent H(4) imidazolinium salt with Pd(OAc)(2) in the presence of NaI, and the solid state structure determined by X-ray crystallography. The rigid, chiral, biisoquinoline geometry of the H(4) imidazolinium salt was found to be maintained upon ligand complexation. The sterically unencumbered parent biisoquinoline ligand has been found to give high conversion with modest enantioselectivity in the copper-catalysed asymmetric addition of diethylzinc to cyclohexenone.

The reductive elimination of 2-hydrocarbyl-imidazolium salts from hydrocarbyl-palladium complexes bearing N-heterocyclic carbene (NHC) ligands represents an important deactivation route for catalysts of this type. We have explored the influence that carbene N-substituents have on both the activation energy and the overall thermodynamics of the reductive elimination reaction using density functional theory (DFT). Given the proximity of the N-substituent to the three-centred transition structure, steric bulk has little influence on the activation barrier and it is electronic factors that dominate the barriers' magnitude. Increased electron donation from the departing NHC ligand acts to stabilise the associated complex against reductive elimination, with stability following the trend: Cl < H < Ph < Me < Cy < iPr < neopentyl < tBu. The intimate involvement of the carbene p pi-orbital in determining the barrier to reductive elimination means N-substituents that are capable of removing pi-density (e.g. phenyl) act to promote a more facile reductive elimination.

The influence of spectator ligand bite angle and the twist angle of the carbene on the reductive elimination of N-heterocyclic carbenes (NHCs) from palladium bis-phosphine complexes has been investigated using density functional theory. The spectator bite angle was found to have a significant influence on both the activation energy (E(act)) and the enthalpy of reaction. Widening of the bite angle was found to lower E(act) and increase the enthalpy of reaction. In contrast, rotation of the carbene with respect to the PdL(2) plane was found to have little influence on E(act). At carbene twist angles approaching 0 degrees however, relief of the increased steric strain provides a considerable driving force for the decomposition reaction.

Palladium complexes of a series of functionalised phosphines bearing the OPN donor set [2-pyCH(2)P(Ph)CH(2)(CHOCH(2)CH(2)CH(2)), 1; 2-py CH(2)P(Ph)-CH(2)CH(2)(CHOCH(2)CH(2)O), 2; 2-pyCH(2)P(Ph)CH(2)CH(2)CO(2)Me, 3; 2-pyP(Ph)CH(2)(CHOCH(2)CH(2)CH(2)), 4; 2-py = 2-pyridyl] have been prepared and characterised. Ligands 1-3 form five membered P-N chelates which is confirmed for PdCl(2) complexes of and by X-ray crystallography. O-coordination appears to be generally disfavoured although there is evidence of transient O-coordination for selected Pd complexes of 4. Palladium methyl and acetate complexes of all four ligands have been tested for catalytic activity in ethene/CO copolymerisation as well as alkoxy-carbonylation of propyne. Complexes of 1 and 4 show some activity in the copolymerisation reaction and complexes of 4 are active in the methoxy carbonylation of propyne. Unlike related pyridyl(diaryl)phosphines, 4 produces a much more stable catalyst system that does not require large excesses of ligand to maintain activity.

The oxidative addition of 1,3-dimethylimidazolium to a model Wilkinson's catalyst (RhCl(PH(3))(3)) has been studied with density functional calculations (B3LYP). According to our free energy calculations, the octahedral rhodium carbene hydride product forms from initial predissociation of a phosphine molecule to subsequently form a 5 ligand intermediate; however, results indicate that a six ligand, associative route with a concerted three-centred transition structure may also be competitive. Exchange of the phosphine molecule on the metal centre with trimethylphosphine had a significant effect in lowering the barrier to oxidative addition and decreasing the endothermicity of the reaction. Solvation was found to have a moderate effect on the overall reaction. Bulk solvent calculations reflected a relative stabilisation of reactants for both pathways, resulting in an endothermic overall reaction. A study of alternative azolium salts revealed the saturated 1,3-dimethyl-4,5-dihydroimidazolium resulted in little change to the reaction geometries or energies, while the use of 3-methylthiazolium salt significantly reduced the barrier to addition and increased the exothermicity of the reaction considerably.

A zerovalent platinum(carbene) complex with two monoalkene ligands, which is able to activate C-H bonds of imidazolium salts at room temperature to yield isolable hydrido platinum(II) bis(carbene) compounds, has been synthesised for the first time.