Martyn P. Coles

New homogeneous ethylene polymerization catalysts derived from transition metal imido precursors

SummaryNew ethylene polymerization catalysts based on half-sandwich imido complexes of the Group 5 metals and bis(imido) complexes of the Group 6 metals are described.

Transition Metal Imido Catalysts for Ethylene Polymerisation

Abstract The imido complexes CpV(N-2-MeC 6 H 4 )Cl 2 ( 1 ), Cr(N- t Bu) 2 Cl 2 ( 2 ), CpNb(N-2- t BuC 6 H 4 )Cl 2 ( 3 ) and Mo(N- t Bu) 2 Cl 2 ( 4 ) have been tested as procatalysts for the polymerisation of ethylene in combination with diethylaluminium chloride or methylaluminoxane (MAO) co-catalysts. The vanadium precursors give the highest activities but are short-lived, while the chromium system gives a long-lived catalyst of moderate activity. The niobium and molybdenum derivatives gave relatively low activities under all test conditions. The polyethylene generated by the vanadium and chromium catalysts is of high molecular weight with little branching. The dialkyl complexes Cr(N- t Bu) 2 (CH 2 Ph) 2 ( 5 ), Cr(N-2,6-Pr i 2 C 6 H 3 ) 2 (CH 2 Ph) 2 ( 6 ), Cr(N-2,6-Pr i 2 C 6 H 3 ) 2 Me 2 ( 7 ), Mo(N- t Bu) 2 (CH 2 Ph) 2 ( 8 ), Mo(N-2,6-Pr i 2 C 6 H 3 ) 2 (CH 2 Ph) 2 ( 9 ), (C 5 Me 5 )Nb(N-2,6-Pr i 2 C 6 H 3 )Me 2 ( 10 ) and (C 5 Me 5 )Ta(N- t Bu)(CH 2 Ph) 2 ( 11 ) have been prepared by treatment of the dihalide precursors with appropriate alkylating reagents and investigated as precursors to well-defined cationic alkyl catalysts. Treatment of ( 5 ) with [Ph 3 C][B(C 6 F 5 ) 4 ] affords the cationic η 2 -benzyl species [Cr(N- t Bu) 2 (η 2 -CH 2 Ph)][B(C 6 F 5 ) 4 ] ( 12 ) while its reaction with [PhNMe 2 H][B(C 6 F 5 ) 4 ] liberates toluene to give a mixture of the mono and bis(dimethylaniline) adducts [Cr(N- t Bu) 2 (η 2 -CH 2 Ph)(NMe 2 Ph)][B(C 6 F 5 ) 4 ] ( 14 ) and [Cr(N- t Bu) 2 (η 2 -CH 2 Ph)(NMe 2 Ph) 2 ][B(C 6 F 5 ) 4 ] ( 15 ). Complex 12 reacts with trimethylphosphine to give the mono-phosphine adduct [Cr(N- t Bu) 2 (η 2 -CH 2 Ph)(PMe 3 )][B(C 6 F 5 ) 4 ] ( 13 ). Solutions containing cationic species ( 12 , 14 , 15 ) are active for ethylene polymerisation in the absence of co-catalyst, affording high-molecular-weight polyethylene with relatively broad molecular-weight distributions.

Application of neutral amidines and guanidines in coordination chemistry

The aim of this perspective is to highlight the continued development and application of amidines and guanidines as neutral, N-based donor ligands in coordination chemistry. From a sporadic interest dating back to the 1980s, work on these two closely related classes of compounds has steadily grown, with examples of evermore sophisticated substitution about the common ligand framework being developed to address specific problems in a number of fields of chemistry. These compounds have shown varied coordination modes at metals from across the periodic table, and examples of their application in a number of catalytic processes are emerging as the field matures.

Facile Conversion of CO/H2 into Methoxide at a Uranium(III) Center†

The reaction of [U(-C8H6{SiiPr3-1,4}2)(-Cp*)] in toluene with CO and H2 at sub to ambient temperature and pressure affords the methoxide complex [U(-C8H6{SiiPr3-1,4}2)(-Cp*)OMe]

Living ring-opening metathesis polymerisation of amino ester functionalised norbornenes

The living ring-opening metathesis polymerisation of norbornene monomers bearing amino ester- residues, using the molybdenum initiators Mo(=CHCMe2Ph)(=N?2,6-i-Pr2C6H3)(OR)2, {R = CMe3, IA, CMe2CF3, IB, CMe(CF3)2, IC}, is reported. The monomers are derived from reactions of exo- and endo- himic anhydrides with the amino ester hydrochlorides of glycine (1), alanine (2) and isoleucine (3). The optically pure monomers derived from 2 and 3 afford optically active polymers whose optical activities are independent of cis content and molecular weight, indicating the absence of a cooperative effect between chiral centres along the chain. The polymers derived from the exo monomers show a cis-trans vinylene dependence upon the ancillary alkoxide ligands of the initiator with IA giving high trans contents and IC giving high cis. The cis/trans content for the endo polymers show relatively little or no dependence upon the initiator. Poly(exo-1) is exceptional in its ability to incorporate various hydrocarbons in the solid state, including hexane and methane.

A New Class of Linked-bis(N,N'-dialkylamidinate) Ligand: Applications in the Synthesis of Bimetallic Aluminium Complexes

The 1,4-benzenebis(N,N?-dialkylamidine) compounds, 1,4-C6H4[C{NR}{NHR}]2 (1, R=Cy; 2, R=iPr) were synthesised via addition of a stoichiometric amount of water to the 1,4-benzenebis(amidinate) species, generated from the in situ reaction between 1,4-Li2C6H4 and the appropriate carbodiimide. Alternatively, the neutral silylated derivatives, 1,4-C6H4[C{NR}{N(SiMe3)R}]2 (3, R=Cy; 4, R=iPr) are synthesised by quenching the 1,4-benzenebis(amidinate) with excess Me3SiCl. Structural characterisation of selected examples are reported, showing that the neutral NH compounds are monomeric with a E-syn:E-syn arrangement of the N-alkyl substituents, while in the silylated derivative 4, the E-anti:E-anti form is present in the solid-state. Synthesis of the bimetallic aluminium compounds 1,4-C6H4[C{NiPr}2AlMeX]2 (5, X=Cl; 6, X=Me) was achieved from AlMe2Cl using 2 by either the direct protonolysis of an Al---C bond (5), or the trans-metallation reaction of the in situ generated dilithio-salt (6). Structural characterisation of 6 revealed a monomeric complex with a perpendicular arrangement of the amidinate units with respect to the C6-group.

Reactivity of divalent germanium alkoxide complexes is in sharp contrast to the heavier tin and lead analogues.

The chemistry of β-diketiminate germanium alkoxide complexes has been examined and shown to be in sharp contrast to its heavier congeners. For instance, (BDI)GeOR (BDI = [{N(2,6-(i)Pr(2)C(6)H(3))C(Me)}(2)CH], R = (i)Pr, (s)Bu, (t)Bu) does not react with carbon dioxide to form a metal carbonate complex. Addition of aliphatic electrophiles, such as methyl iodide or methyl triflate, results in the net oxidative addition to the germanium, giving cationic tetravalent germanium complexes, [(BDI)Ge(Me)OR][X] (X = I, OTf). An examination of the contrasting reactivities of the alkoxide ligand and the germanium loan pair with Lewis acids yielded the unusual germanium(II)-copper(I) adduct, {μ(2)-Cu(2)I(2)}[(BDI)GeO(t)Bu](2). This complex not only displays a rare example of a divalent Ge-Cu bond, but is the first example in which a planar Cu(2)I(2) diamond core possesses a three-coordinate copper bound to another metal center.

UIII‐Induced Reductive Co‐Coupling of NO and CO to Form UIV Cyanate and Oxo Derivates

CO and NO give NCO and O! Sequential treatment of [U(?-C8H6{SiiPr3-1,4}2)(?-Cp*)] with CO followed by NO (or vice versa) affords a mixture of the bridging cyanate complex [{U(?-C8H6{SiiPr3-1,4}2)(?-Cp*)}2(µ-OCN)2] and the bridging oxo complex [{U(?-C8H6{SiiPr3-1,4}2)(?-Cp*)}2(µ-O)], both of which have been structurally characterised (see scheme).

The double [3+2] photocycloaddition reaction

A remarkable double [3 + 2] photocycloaddition reaction that results in the formation of fenestrane 2 from aromatic acetal 1 is reported. During the formation of 2, four carbon-carbon bonds, five new rings, and seven new stereocenters are created in a one-pot process. The reaction occurs in a sequential manner from the linear meta photocycloadduct 3, while the angular meta photocycloadduct 4 undergoes an alternative fragmentation-translocation photoreaction to afford angular tricycle 6.

An Organozinc Hydride Cluster: An Encapsulated Tetrahydrozincate?

Reaction of the alkylzinc bromide Zn(C{SiMe3}2{SiMe2hpp})Br (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) with an excess of sodium hydride gives good yields of an unusual zinc hydride cluster, in which five zinc atoms are linked by (-H) bridges