Andrew J. P. White

Oligomerisation of Ethylene by Bis(imino)pyridyliron and -cobalt Complexes

A series of bis(imino)pyridyliron and -cobalt complexes [{2, 6-(CR=NAr)2C5H3N}MX2] (R=H, Me; MFe, Co; X=Cl, Br) 8-16 containing imino-aryl rings (Ar) with at least one small ortho substituent, as well as Ar=biphenyl and Ar=naphthyl, has been synthesised. Crystallographic analyses of complexes 9 (Ar=2,3-dimethylphenyl), 13 and 14 (Ar=biphenyl; X=Cl or Br, respectively) reveal a distorted trigonal-bipyramidal geometry in the solid state. These complexes, in combination with methyl aluminoxane (MAO), are active catalysts for the oligomerisation of ethylene, yielding >99 % linear α-olefin mixtures that follow a Schulz-Flory distribution. Iron ketimine (R=Me) precatalysts give the highest activities and a greater α-value than their aldimine (R=H) analogues. Cobalt precatalysts follow a similar trend, though their activities are almost two orders of magnitude lower than those of the corresponding iron catalysts. Ethylene pressure studies on cobalt precatalyst 15 reveal a first-order dependence on ethylene for both the rate of propagation and the rate of chain transfer, and a pressure independence of the α value.

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).

The Sting of the Scorpion: A Metallaboratrane

An unprecedented intramolecular B-H activation of a hydrotris(2-sulfanyl-1-methylimidazolyl)borate ligand coordinated to ruthenium has provided the first example of metallaboratranes. The remarkable ease with which the ruthenaboratrane 1 forms relative to the case with a hydrotris(pyrazolyl)borate ligand is a consequence of the lability of the chelation and the increased ring size of the chelates.

NOVEL OLEFIN POLYMERIZATION CATALYSTS BASED ON IRON AND COBALT

A new family of olefin polymerization catalysts, derived from iron and cobalt complexes bearing 2,6-bis(imino)pyridyl ligands, is described.

Study of ligand substituent effects on the rate and stereoselectivity of lactide polymerization using aluminum salen-type initiators

A series of aluminum salen-type complexes [where salen is N,N′-bis(salicylaldimine)-1,2-ethylenediamine] bearing ligands that differ in their steric and electronic properties have been synthesized and investigated for the polymerization of rac-lactide. X-ray crystal structures on key precatalysts reveal metal coordination geometries intermediate between trigonal bipyramidal and square-based pyramidal. Both the phenoxy substituents and the backbone linker have a significant influence over the polymerization. Electron-withdrawing groups attached to the phenoxy donor generally gave an increased polymerization rate, whereas large ortho substituents generally slowed down the polymerization. The vast majority of the initiators afforded polylactide with an isotactic bias; only one exhibited a bias toward heteroselectivity. Isoselectivity generally increases with increased flexibility of the backbone linker, which is presumed to be better able to accommodate any potential steric clashes between the propagating polymer chain, the inserting monomer unit, and the substituents on the phenoxy donor.

A Tricyclic Aromatic Isomer of Hexasilabenzene

Aromatic Silicon Benzene has long intrigued chemists on account of the energy stabilization, termed aromaticity, which arises from π-electron delocalization around its ring framework. A persistent question has been how such stabilization would be impacted were the carbons to be replaced by heavier atoms such as silicon. Abersfelder et al. (p. 564) have prepared a benzene analog with Si atoms in place of all six-ring carbons, but a slightly altered bonding framework in which substituents outside the ring are no longer evenly distributed. Instead, the substituents pair up at two Si sites, leaving two other ring sites with no external appendages. The resulting compound no longer has a continuous network of π-electrons, but retains a degree of aromatic stabilization involving sigma and nonbonding electrons. A structural isomer of benzene in which carbon is replaced by silicon exhibits unexpected electronic stabilization. Benzene represents the showcase of Hückel aromaticity. The silicon analog, hexasilabenzene, has consequently been targeted for decades. We now report an intensely green isomer of Si6R6 (R being 2,4,6-triisopropylphenyl) with a tricyclic structure in the solid state featuring silicon atoms with two, one, and no substituents outside the ring framework. The highly dispersed 29Si nuclear magnetic resonance shifts in solution ranging from +125 to −90 parts per million indicate an inhomogeneous electron distribution due to the dismutation of formal oxidation numbers as compared with that of benzene. Theoretical analysis reveals nonetheless the cyclic delocalization of six mobile electrons of the π-, σ- and non-bonding type across the central four-membered ring. For this alternative form of aromaticity, in principle applicable to many Hückel aromatic species, we propose the term dismutational aromaticity.

Magnesium and zinc complexes of a potentially tridentate β-diketiminate ligand

The synthesis of the unsymmetrically substituted β-diketimine, 2-(2-methoxyphenylimino)-4-(2,6-diisopropylphenylamido)pent-2-ene, (BDI-2)H, is described and its complexation chemistry with magnesium and zinc is explored. Emphasis is placed on the preparation of alkoxide and amide derivatives for the ring-opening polymerisation of lactide; their behaviour as polymerisation initiators is compared to analogous compounds supported by the N,N′-bis(2,6-diisopropylphenyl) β-diketiminate ligand, BDI-1. (BDI-2)H reacts with Me2Mg to give the bis(chelate) complex, (BDI-2)2Mg, 3. Magnesium alkyls supported by BDI-2 may be prepared by increasing the size of the alkyl group. Hence, lithiation of (BDI-2)H affords [(BDI-2)Li]2, 4; its subsequent treatment with iPrMgCl produces (BDI-2)MgiPr, 5. Aminolysis of complex 5 using iPr2NH yields the amide complex, (BDI-2)MgNiPr2, 6. Zn(NTMS2)2 and ZnEt2 react with (BDI-2)H to give (BDI-2)Zn(NTMS2), 7, and (BDI-2)ZnEt, 8, respectively. The former is converted into the siloxide complex,(BDI-2)Zn(OSiPh3), 9, upon reaction with Ph3SiOH. The chloride derivative, (BDI-2)ZnCl, 10, has also been prepared via the reaction of ZnCl2 with 4. Crystallographic analysis of compounds 3, 4, 7 and 8 reveals that the potential for (BDI-2) to bind in a tridentate manner is only realised with the more electrophilic metals Li and Mg. Compared to their (BDI-1) counterparts, complexes 6, 7 and 9 are more active, but less well-controlled, initiators for the ring-opening polymerisation of rac-lactide, a consequence of the diminished steric protection afforded by (BDI-2) relative to (BDI-1).

Supramolecular daisy chains

Our childhoods may be recalled when a self-complementary cation, endowed with both a dibenzo[24]crown-8 macroring and a secondary dialkylammonium sidearm, self-assembles to form a two-component supramolecular architecture that is reminiscent of a daisy chain (depicted schematically on the right). This daisy-chain-like superarchitecture is stabilized by a combination of [N+ -H⋅⋅⋅O] hydrogen bonds and aryl-aryl stacking interactions.

The role of bulky substituents in the polymerization of ethylene using late transition metal catalysts: a comparative study of nickel and iron catalyst systems

Abstract A series of nickel(II) and iron(II) complexes of the general formula [LMX2] containing bidentate (for M=Ni) and tridentate (for M=Fe) heterocycle-imine ligands L have been synthesized and characterized. Compared to the well-known α-diimine nickel and bis(imino)pyridine iron catalysts, these systems contain a bulky imine substituent on one side and a non-bulky N-heterocycle on the other. Depending on the ligand and the conditions used, either four- or five-coordinate complexes are obtained in the case of nickel. Iron complexes are generally five-coordinate, even with potentially tetradentate ligands. Activation of these precatalysts with MAO affords active catalyst systems for the oligomerization/polymerization of ethylene. Compared to α-diimine nickel and bis(imino)pyridine iron catalysts, both metal systems provide only half of the steric protection and consequently the catalytic activities and the degree of polymerization are significantly lower. Lower activities are attributed to a reduced stability of the active species under polymerization conditions, whereas the lower molecular weights are a result of increased β-H transfer rates. Variations within the heterocyclic component of the ligand reveal that both steric and electronic factors influence the polymerization behavior of these catalysts.

Metal‐Size Influence in Iso‐Selective Lactide Polymerization

Iso-selective initiators for the ring-opening polymerization (ROP) of rac-lactide are rare outside of Group 13. We describe the first examples of highly iso-selective lutetium initiators. The phosphasalen lutetium ethoxide complex shows excellent iso-selectivity, with a Pi value of 0.81–0.84 at 298 K, excellent rates, and high degrees of polymerization control. Conversely, the corresponding La derivative exhibits moderate heteroselectivity (Ps=0.74, 298 K). Thus, the choice of metal center is shown to be crucial in determining the level and mode of stereocontrol. The relative order of rates for the series of complexes is inversely related to metallic covalent radius: that is, La>Y>Lu.