Variable coordination of a chiral diphosphine containing an amidinium/NHC group within its backbone: μ-P,P′, κ2-P,P′ and κ3-P,C,P′ coordination modes

A diphosphine ligand (1·HPF6), which is a potential precursor to a PCNHCP 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(κ2-P,P′-1·H)Cl2]PF6 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 CH3I to give the dimers trans-[Pd2(μ-1·H)2I4](PF6)2, 6, and trans-[Pt2(μ-1·H)2I4](PF6)2, 7, as the major products. This bridging mode of coordination of [1·H]+ is also seen in trans-[Rh2(μ-1·H)(1,5-COD)2Cl2]PF6, 4, and [Pt2(μ-κ2-1·H)(dvdms)]PF6, 5. Upon treatment with KOtBu complexes 2 and 3a undergo deprotonation at the amidinium carbon to give trans-[M(κ3-P,C,P′-1)Cl]PF6 where M = Pd (8), and Pt (9). The related trans-[Rh(κ3-P,C,P′-1)(CO)]PF6 (10) is prepared directly from 1·HPF6 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·HPF6 followed by addition of Ag(CF3SO3) gave SAg-[Ag(κ3-P,C,P′-1)(CF3SO3)], 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

Monovalent chiral-at-copper complexes: halide-controlled diastereoselectivity

An unusual example of diastereoselectivity has been observed in Cu(κ3-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 SCu isomer is formed selectively whereas when X = I− the RCu diastereomer is preferred

N-Heterocyclic carbenes/imidazolium salts as substrates in catalysis: the catalytic 2-substitution and annulation of heterocyclic compounds

N-Heterocyclic carbenes (NHCs) are commonly regarded as strong donor ligands that are valuable in coordination chemistry and catalysis. Many reports describing this aspect of their chemistry have been published. The alternative view of NHCs as reaction intermediates has been little explored, and yet excellent examples exist and will be reviewed in this perspective. Group 10 hydrocarbyl complexes of NHCs [(R)(NHC)ML2 where R = H, alkyl, aryl, acyl; M = Ni, Pd, Pt] undergo a facile reductive elimination to generate M(0) and R-substituted-azolium salt as products. On the other hand, 2-Hazolium salts will oxidatively add to M(0) complexes to afford reactive NHC–M–H compounds, suitable as catalysts for selected reactions. Combining the oxidative addition and reductive elimination steps into a single cycle, in the presence of an alkene, provides of a novel and potentially exciting, atom efficient catalytic C–C bond forming process for the substitution, and annulation of heterocyclic rings

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

Transition-Metal-Catalyzed Reactions Involving Imidazolium Salt/N-Heterocyclic Carbene Couples as Substrates

Reactive Couples: Imidazolium salts react with alkenes under mild conditions in the presence of a Ni0-based catalyst to produce 2-alkyl imidazolium salts (see scheme, cod=1,5-cyclooctadiene). The mechanism involves oxidative addition of an imidazolium CH bond to M0 to give a (carbene)MH intermediate and finally bond formation between the carbene and an alkyl ligand through reductive elimination

Transition-metal-catalyzed reactions involving imidazolium salt/N-heterocyclic carbene couples as substrates

Reactive Couples: Imidazolium salts react with alkenes under mild conditions in the presence of a Ni0-based catalyst to produce 2-alkyl imidazolium salts (see scheme, cod=1,5-cyclooctadiene). The mechanism involves oxidative addition of an imidazolium CH bond to M0 to give a (carbene)MH intermediate and finally bond formation between the carbene and an alkyl ligand through reductive elimination

Metal complexes of chiral NHCs containing a fused six- and seven-membered central ring

Azolium precursors to expanded-ring NHCs incorporating a chiral bicyclic skeleton derived from (1R)-camphor with N,N′-dibenzyl or N,N′-dipyridyl substituents have been prepared and used for the synthesis of a number of metal complexes of the resultant NHCs. The bicyclic framework contains two fused rings, one of which is seven-membered while the other is six-membered. Deprotonation of (1R,5S)-2,4-dibenzyl-1,8,8-trimethyl-4-aza-2-azoniabicyclo[3.2.1]oct-2-ene hexafluorophosphate, 1a·HPF6, produces a solution of “free carbene”, 1a, that can be used for the preparation of [Ag(1a)2]OTf (2a), [Rh(1,5-COD)(1a)Cl] (3a), [Rh(CO)2(1a)Cl] (4a), and [Ir(1,5-COD)(1a)Cl] (5a) complexes. The fused expanded ring NHC ligands display properties and reactivity patterns more akin to those of five-membered NHCs rather than monocyclic six- or seven-membered NHCs. Consistent with this premise, the red Ni(II) pincer complex [Ni(η3-C8H13)(1b)]PF6, 6b, was obtained via oxidative addition of the azolium salt 1b·HPF6 to Ni(1,5-COD)2 in THF. This is the first unambiguous example of a saturated and/or expanded ring NHC oxidatively adding to a low-valent metal center. A single-crystal X-ray structure determination of 6b showed the NHC to be coordinated in a pincer fashion through the carbene and two pyridine nitrogens at a square-pyramidal nickel(II) center. One of the original 1,5-COD ligands has undergone a 1,5 to 1,3 isomerization before migratory insertion of one of the alkene groups into the Ni−H function, generated on oxidative addition of the azolium salt, to give the allylic η3-C8H13 ligand. When 6b was dissolved in CHCl3 or CH2Cl2 in air, the allyl ligand was lost and a chloride was abstracted from the solvent to give yellow square-planar [Ni(1b)Cl]PF6, 7b, the structure of which was confirmed by single-crystal X-ray techniques

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