David S. McGuinness

Olefin Oligomerization via Metallacycles: Dimerization, Trimerization, Tetramerization, and Beyond

Recent advances in the area of Olefin Oligomerization via metallacycles that include dimerization, trimerization, tetramerization, and beyond, are reviewed. Studies have found that metallacyclopentane decomposition to 1-butene many not be particularly facile due to the absence of metallacycle expansion. Follow-up studies concentrated on the N-H functionality and the Cr oxidation state and role of MAO show that activities and selectivities to 1-hexene are similar to the original Cr(III) complexes. Nenu and Weckhuysen prepared silica-supported triazacyclohexane complexes, by treating the reduced Phillips polymerization catalyst with triazacyclohexane ligands in dichloromethane. The influence of N-aryl functionality investigated by Killian et al. shows that the selectivity was mainly dependent upon the steric bulk attached to nitrogen, and less so on the groups basicity.

Palladium(II) complexes containing mono-, bi- and tridentate carbene ligands. Synthesis, characterisation and application as catalysts in CC coupling reactions

Abstract Palladium complexes of functionalised heterocyclic carbene complexes have been synthesised. Treatment of imidazolium salts with Ag2O yields AgI(carbene)2 complexes, which act as carbene transfer agents when reacted with palladium salts. In this manner, [Pd(Me)(1-(2-ethylpyridyl)-3-methylimidazolin-2-ylidene)Cl]2 (4a) and Pd(Me)(1-benzyl-3-methylimidazolin-2-ylidene)2Cl (4c) have been prepared from PdMeCl(cod) (cod=1,5-cyclooctadiene) and the appropriate silver complex. Similarly, the reaction of a Ag(carbene)2 complex with PdCl2(MeCN)2 gives Pd(1-benzyl-3-methylimidazolin-2-ylidene)2Cl2 (4b). The tridentate carbene complex [Pd(Me)(1,3-di(2-picolyl)imidazolin-2-ylidene)]BF4 (6a) is synthesised via the in situ reaction of the imidazolium salt with Ag2O, followed by PdMeCl(cod) and AgBF4, whilst [PdCl{1,3-bis(diisopropyl-2-ethylamino)imidazolin-2-ylidene}]BF4 (6c) is synthesised in an identical manner from PdCl2(MeCN)2. The chelated complexes [1,1′-dimethyl-3,3′-(1,2-xylylene)diimidazolin-2,2′-diylidene]Pd(II) dibromide (5a), [1,1′-dimethyl-3,3′-(1,3-xylylene)diimidazolin-2,2′-diylidene]Pd(II) dibromide (5b) and Pd(imidazoliophane)Br2 (5c) have been synthesised via the reaction of the appropriate imidazolium salt with Pd(OAc)2. X-ray crystal structures of the imidazolium salt, 1,3-di(2-picolyl)imidazolium chloride (1f) and the complex [Pd(Me)(tetramethylimidazolin-2-ylidene)2Cl] (2) are reported. Complex 2 shows square planar coordination with the two carbene ligands trans to each other. The carbene ligands are inclined at 65.3° to the coordination plane. Several complexes proved to be highly stable and efficient catalysts for intermolecular Heck and Suzuki coupling reactions, giving turnover numbers of up to 980 000 (Heck) and 177 500 (Suzuki).

Synthesis and reaction chemistry of mixed ligand methylpalladium–carbene complexes

Abstract A variety of methylpalladium carbene complexes of the type [Pd(Me)(carbene)(chelate)] [where carbene=1,3-di-methylimidazol-2-ylidene (dmiy), chelate=acetylacetonate (acac) 1 , trifluoroacetylacetonate (tfac) 2 , hexafluoroacetylacetonate (hfac) 3 ; carbene=1,3,4,5-tetramethylimidazol-2-ylidene (tmiy), chelate=acac, 6 ] have been synthesised and fully characterised. The bis-carbene complexes trans -[Pd(Me)(carbene) 2 Cl] [carbene=dmiy, 7 and tmiy, 8 ] have also been isolated. The unusual cationic complex {[Pd(Me)(dmiy)(cod)] + BF 4 − } 4 , in which there are three distinct Pd–carbon bonds, including cis alkyl/alkene coordination has been prepared and aspects of its chemistry investigated. The complex which slowly decomposes even at −20°C does not react via the expected migratory insertion of the methyl group with the coordinated cod but decomposes via a pathway involving the methyl group and the carbene ligand. Crystal structures of the bis-carbene complex 7 and the chloro-bridged dimer [Pd(Me)(tmiy)Cl] 2 5 reveal square planar coordination. The carbene C 3 N 2 arrays are closely planar, with the ligand planes inclined at significant angles to the coordination planes of the respective complexes. For complex 7 the interplanar dihedral angle is 70.4(3)° and for 5 the angle is 65.07(8)°. Examples of the methylpalladium monocarbene and dicarbene complexes were tested in the Heck reaction and were found to be extremely active with several having turnover frequencies (TOF’s) of >20 000 and total turnover numbers (TON’s) of >100 000.

Novel Cr-PNP complexes as catalysts for the trimerisation of ethylene

Cr(III) complexes of tridentate PNP ligands have been prepared and evaluated as catalysts for ethylene trimerisation, with several giving high activity and excellent selectivity towards 1-hexene when activated with methylaluminoxane.

Reduction of a Chelating Bis(NHC) Palladium(II) Complex to [{μ‐bis(NHC)}2Pd2H]+: A Terminal Hydride in a Binuclear Palladium(I) Species Formed under Catalytically Relevant Conditions

Catalytic processes featuring N-heterocyclic carbene (NHC) ligands have been extensively studied following the isolation of free imidazol-2-ylidenes An important reaction class is the base-assisted palladium-mediated C–C/N coupling, such as the Sonogashira, Buchwald–Hartwig, and Mizoroki–Heck reactions

Probing the effects of ligand structure on activity and selectivity of Cr(III) complexes for ethylene oligomerisation and polymerisation.

A series of distorted octahedral Cr(III) complexes containing tridentate S-, S/O- or N-donor ligands comprised of three distinct architectures: facultative {S(CH(2)CH(2)SC(10)H(21))(2) (L(1)) and O(CH(2)CH(2)SC(10)H(21))(2) (L(2))}, tripodal {MeC(CH(2)S(n)C(4)H(9))(3) (L(3)), MeC(CH(2)SC(10)H(21))(3) (L(4))} and macrocyclic {(C(10)H(21))[9]aneN(3) (L(5)), (C(10)H(21))(3)[9]aneN(3) (L(6)), with [9]aneN(3)=1,4,7-triazacyclononane} are reported and characterised spectroscopically. Activation of [CrCl(3)(L)] with MMAO produces very active ethylene trimerisation, oligomerisation and polymerisation catalysts, with significant dependence of the product distribution upon the ligand type present. The properties of the parent [CrCl(3)(L)] complexes are probed by cyclic voltammetry, UV-visible, EPR, EXAFS and XANES measurements, and the effects upon activation with Me(3)Al investigated similarly. Treatment with excess Me(3)Al leads to substitution of Cl ligands by Me groups, generation of an EPR silent Cr species (consistent with a change in the oxidation state of the Cr to either Cr(II) or Cr(IV)) and substantial dissociation of the neutral S and S/O-donor ligands.

Unprecedented C–H bond oxidative addition of the imidazolium cation to Pt0: a combined density functional analysis and experimental study

Density functional studies predict that oxidative addition of an imidazolium salt to a Pt0 complex is an exothermic process, and consistent with this proposal, the reaction of the 1,3-dimethylimidazolium cation with Pt(PPh3)4 yields a hydrido–PtII carbene complex.

A survey of pendant donor-functionalised (N,O) phosphine ligands for Cr-catalysed ethylene tri- and tetramerisation

In this study three classes of ligands are explored for ethylene tri-/tetramerisation in conjunction with chromium and both triethylaluminium (AlEt3) and methylaluminoxane (MAO) co-catalysts. Hydrazine based ligands containing an N–H functionality [PN(NH)P], analogous to Rosenthals previously reported PNPNH system, show selectivity towards 1-hexene and 1-octene formation in conjunction with AlEt3, and act as PNP tetramerisation analogues when MAO is employed. PNP ligands containing non-protic pendant donor moieties generally show poor activity and selectivity when AlEt3 is employed as an activator, however when MAO is used good activities and selectivities are achieved. n-Propylcyclopentane and 2-propenylcyclopentane, and higher homologues, are produced during catalysis when oxygen is the donor atom. The formation of such products is discussed with respect to the generation of methylenecyclopentane and methylcyclopentane by PNP based tetramerisation catalysts. Simple phosphine ligands containing O–H functionalisation are also explored and it was shown that the catalyst selectivity is highly dependent on both the activator and structural features of the ligand employed.

Ethylene polymerisation and oligomerisation with arene-substituted phenoxy-imine complexes of titanium: investigation of multi-mechanism catalytic behaviour

A range of unsubstituted (1,2) and 6-substituted (3-5) ortho-phenoxy-imine ligands have been prepared and converted to their silyl ether derivatives (6-10). Reaction of silyl ethers with TiCl(4)(thf)(2) in the case of the unsubstituted species yields bis-ligated complexes while the substituted species react cleanly to yield complexes of the form [Ti(O^NR)Cl(3)(thf)]. In most cases the complexes have been characterised by X-ray crystallography. Testing of the complexes for ethylene oligomerisation and polymerisation has been undertaken employing alkylaluminium co-catalysts (AlEt(3), MAO). In all cases the predominant product formed is polyethylene however careful analysis of the liquid phase reveals a complex process by which 1-butene is most likely formed via Cossee mechanism while 1-hexene results from a metallacyclic process.

A DFT mechanistic study on ethylene tri- and tetramerization with Cr/PNP catalysts: single versus double insertion pathways

The mechanism of ethylene trimerization and tetramerization with a chromium-diphosphinoamine (Cr-PNP) catalyst system has been studied by theoretical (DFT) methods. Two representative ligands have been explored, namely Ph2 PN(Me)PPh2 and (o-MeC6 H4 )2 PN(Me)P(o-MeC6 H4 )2 . Calculations on the former ligand reveal how a combination of single and double ethylene insertion mechanisms may lead to 1-hexene, 1-octene and the major side products (cyclopentanes and n-alkanes). For the latter ligand, introduction of o-alkyl substitution leads to a more sterically congested active species, which suppresses the available pathways for tetramerization and side product formation. Hence, the high selectivity of o-aryl substituted PNP ligands for trimerization can be rationalized.