Palladium(II) beta-agostic alkyl cations and alkyl ethylene complexes: investigation of polymer chain isomerization mechanisms.

A series of stable dialkyl complexes of Pd, (alpha-diimine)PdR2 (alpha-diimine = aryl-substituted diimine, R = n-Pr, n-Bu, i-Bu), have been prepared via Grignard alkylation of the corresponding (alpha-diimine)PdCl2 complexes. Protonation of these dialkyl species at low temperature results in loss of alkane and formation of cationic Pd beta-agostic alkyl complexes, which have been observed as intermediates in the polymerization of ethylene and propylene by these Pd catalysts. Studies of the structure and dynamic behavior of these alkyl complexes are presented, along with the results of trapping reactions of these species with ligands such as NCMe, CO, and C2H4. Trapping with ethylene results in formation of cationic alkyl ethylene complexes which model the catalyst resting state in these systems. These complexes have been used to obtain mechanistic details and kinetic parameters of several processes, including isomerization of the alkyl ethylene complexes, associative and dissociative exchange with free ethylene, and migratory insertion rates of both primary and secondary alkyl ethylene species. These studies indicate that the overall branching observed in polyethylenes produced by these Pd catalysts is governed both by the kinetics of migratory insertion and by the equilibria involving the alkyl ethylene complexes.     

Copolymerization of ethylene with polar monomers: chain propagation and side reactions. A DFT theoretical study using zwitterionic Ni(II) and Pd(II) catalysts

Calculations utilizing anionic substituted derivates of the cationic N(wedge)N--Ni(II) and Pd(II) diimine Brookhart complex have been carried out on the barriers of ethylene and acrylonitrile insertion into a M- methyl, propyl and CH(CN)Et bond for M = Ni, Pd. The possibility of side reactions such as chelate formation with the polar functionality and oligomerization of the active species after acrylonitrile insertion are explored. The diimine ring system N--N = -NR' 'CR(1)CR(2)NR' ' with R' ' = 2,6-C(6)H(3)(i-Pr)(2) and R(1),R(2) = Me was functionalized by adding one or two anionic groups (BF(3)(-), etc.) in place of i-Pr on the aryl rings or by replacing one Me diimine backbone group (R(1)) with BH(3)(-). The objective of this investigation is computationally to design catalysts for ethylene/acrylonitrile copolymerization that have activities that are comparable to that of the cationic Ni(II) diimine or at least the Pd(II) diimine Brookhart system for ethylene homopolymerization. Complexes that might meet this objective are discussed.     

Reaction of vinyl chloride with late transition metal olefin polymerization catalysts

The reactions of vinyl chloride (VC) with representative late metal, single-site olefin dimerization and polymerization catalysts have been investigated. VC coordinates more weakly than ethylene or propylene to the simple catalyst (Me(2)bipy)PdMe(+) (Me(2)bipy = 4,4'-Me(2)-2,2'-bipyridine). Insertion rates of (Me(2)bipy)Pd(Me)(olefin)(+) species vary in the order VC > ethylene > propylene. The VC complexes (Me(2)bipy)Pd(Me)(VC)(+) and (alpha-diimine)Pd(Me)(VC)(+) (alpha-diimine = (2,6-(i)Pr(2)[bond]C(6)H(3))N[double bond]CMeCMe[double bond]N(2,6-(i)Pr(2)[bond]C(6)H(3))) undergo net 1,2 VC insertion and beta-Cl elimination to yield Pd[bond]Cl species and propylene. Analogous chemistry occurs for (pyridine-bisimine)MCl(2)/MAO catalysts (M = Fe, Co; pyridine-bisimine = 2,6-[(2,6-(i)Pr(2)[bond]C(6)H(3))N[double bond]CMe](2)-pyridine) and for neutral (sal)Ni(Ph)PPh(3) and (P[bond]O)Ni(Ph)PPh(3) catalysts (sal = 2-[C(H)[double bond]N(2,6-(i)Pr(2)-C(6)H(3))]-6-Ph-phenoxide; P[bond]O = [Ph(2)PC(SO(3)Na)[double bond]C(p-tol)O]), although the initial metal alkyl VC adducts were not detected in these cases. These results show that the L(n)MCH(2)CHClR species formed by VC insertion into the active species of late metal olefin polymerization catalysts undergo rapid beta-Cl elimination which precludes VC polymerization. Termination of chain growth by beta-Cl elimination is the most significant obstacle to metal-catalyzed insertion polymerization of VC.     

Acrylonitrile insertion reactions of cationic palladium alkyl complexes

The reactions of acrylonitrile (AN) with "L(2)PdMe+" species were investigated; (L(2) = CH(2)(N-Me-imidazol-2-yl)(2) (a, bim), (p-tolyl)(3)CCH(N-Me-imidazol-2-yl)(2) (b, Tbim), CH(2)(5-Me-2-pyridyl)(2) (c, CH(2)py'(2)), 4,4'-Me(2)-2,2'-bipyridine (d), 4,4'-(t)Bu(2)-2,2'-bipyridine (e), (2,6-(i)Pr(2)-C(6)H(3))N=CMeCMe=N(2,6-(i)Pr(2)-C(6)H(3)) (f)). [L(2)PdMe(NMe(2)Ph)][B(C(6)F(5))(4)] (2a-c) and [{L(2)PdMe}(2)(mu-Cl)][B(C(6)F(5))(4)] (2d-f) react with AN to form N-bound adducts L(2)Pd(Me)(NCCH=CH(2))(+) (3a-f). 3a-e undergo 2,1 insertion to yield L(2)Pd{CH(CN)Et}+, which form aggregates [L(2)Pd{CH(CN)Et}](n)(n)(+) (n = 1-3, 4a-e) in which the Pd units are proposed to be linked by PdCHEtCN- - -Pd bridges. 3f does not insert AN at 23 degrees C. 4a-e were characterized by NMR, ESI-MS, IR and derivatization to L(2)Pd{CH(CN)Et}(PR(3))+ (R = Ph (5a-e), Me (6a-c)). 4a,b react with CO to form L(2)Pd{CH(CN)Et}(CO)+ (7a,b). 7a reacts with CO by slow reversible insertion to yield (bim)Pd{C(=O)CH(CN)Et}(CO)+ (8a). 4a-e do not react with ethylene. (Tbim)PdMe+ coordinates AN more weakly than ethylene, and AN insertion of 3b is slower than ethylene insertion of (Tbim)Pd(Me)(CH(2)=CH(2))(+) (10b). These results show that most important obstacles to insertion polymerization or copolymerization of AN using L(2)PdR+ catalysts are the tendency of L(2)Pd{CH(CN)CH(2)R}+ species to aggregate, which competes with monomer coordination, and the low insertion reactivity of L(2)Pd{CH(CN)CH(2)R}(substrate)+ species.     

Well-defined imidotitanium alkyl cations: agostic interactions, migratory insertion vs.[2+2] cycloaddition, and the first structurally authenticated AlMe(3) adduct of any transition metal alkyl cation

The imidotitanium alkyl cations [Ti(NtBu)(Me3[9]aneN3)R]+ (R = Me (3+) or CH2SiMe3(4+)) possess either a very weak alpha-agostic or beta-Si-C agostic interactions, respectively, according to 13C and 29Si NMR and DFT studies; reaction of (4+) with iPrNCNiPr gives totally selective insertion into the Ti-alkyl bond; reaction of 3+ with AlMe3 gives the first structurally characterised AlMe3 adduct of a transition metal alkyl cation (Me3[9]aneN3 = 1,4,7-trimethyltriazacyclononane).     

Bis(imino)pyridine iron(II) alkyl cations for olefin polymerization

Treatment of the five-coordinate ferrous dialkyl complex, (iPrPDI)Fe(CH2SiMe3)2 (iPrPDI = ((2,6-CHMe2)2C6H3N=CMe)2C5H3N), with [PhMe2NH][BPh4] in the presence of diethyl ether or tetrahydrofuran furnished the corresponding alkyl cations, where the donor ligand is coordinated in the basal plane of a distorted square pyramidal iron(II) alkyl cation. Performing the same reaction with the neutral Lewis acid, B(C6F5)3, induced methide abstraction from a silicon atom followed by rearrangement to afford the base free ferrous alkyl cation, [(iPrPDI)Fe(CH2SiMe2CH2SiMe3)][MeB(C6F5)3]. This complex is active for the polymerization of ethylene and yields polymers that are of higher molecular weight and narrower polydispersity than traditional methylalumoxane-activated catalysts.     

新型Ni催化剂催化乙烯聚合的中性机理

用密度泛函理论中的B3LYP LANL2MB方法 ,研究了Ni 水杨亚胺催化剂催化乙烯聚合的中性反应机理并和阳离子活性中心的催化反应机理进行了比较 .计算结果表明 ,整个中性催化机理类似于阳离子催化机理 ,但是也有不同 .两种机理都是从带空位的活性催化剂开始 ,乙烯以垂直于催化剂平面的方式占据空位 ,为了有利于甲基的迁移 ,乙烯向甲基的方向旋转 90° ,形成四元环过渡态 .插入反应发生后 ,Ni和 β C之间形成一种氢桥键 ,协助新空位的形成 ,实现链的增长 .乙烯与中性活性中心的相互作用远远强于乙烯与阳离子活性中心的相互作用 .中性催化机理较阳离子催化机理容易引发 .阳离子催化的过渡态所需的活化能比中性催化所需的活化能低 ,表明阳离子反应机理比中性反应机理容易进行 ,甲基在中性催化过渡态中的迁移明显不同于在酸性液化过渡态中的迁移 .β agostic相互作用在中性催化反应机理中 ,在主导烷基给合物中Ni所带的电荷方面 ,起着关键性的作用 .     

Vinyl and carbene ruthenium(II) complexes from hydridoruthenium(II) precursors

The reactions of the hydrido compounds [RuHCl(CO)(L)2][L = PiPr3 (1), PCy3 (2)] with HC(triple bond)CR (R = H, Ph, tBu) afforded by insertion of the alkyne into the Ru-H bond the corresponding vinyl complexes [RuCl(CHCHR)(CO)(L)2], 3-8, which upon protonation with HBF4 gave the cationic five-coordinated ruthenium carbenes [RuCl(CHCH2R)(CO)(L)2]BF4, 9-14. Subsequent reactions of the carbene complexes with PR3(R = Me, iPr) and CH3CN led either to deprotonation and re-generation of the vinyl compounds or to cleavage of the ruthenium-carbene bond and the formation of the six-coordinated complexes [RuCl(CO)(CH3CN)2(PiPr3)2]BF4, 17, and [RuH(CO)(CH3CN)2(PiPr3)2]X, 18a,b. The acetato derivative [RuH(2-O2CCH3)(CO)(PCy3)2], 19, also reacted with acetylene and phenylacetylene by insertion to yield the related vinyl complexes [Ru(CHCHR)(kappa2-O2CCH3)(CO)(PCy3)2], 20, 21, of which that with R = H was protonated with HBF4 to yield the corresponding cationic ruthenium carbene 22. With [RuHCl(H2)(PCy3)2], 25, as the starting material, the five-coordinated chloro(hydrido)ruthenium(II) compounds [RuHCl(PCy3)(dppf)], 26(dppf = [Fe(eta5-C5H4PPh2)2]), [RuHCl[Sb(CH2Ph)3](PCy3)2], 27, and [RuHCl(CH3CN)(PCy3)2], 30, were prepared. The reactions of 27 with HCCR (R = H, Ph) gave the hydrido(vinylidene) complexes [RuHCl(CCHR)(PCy3)2], 28 and 29, whereas treatment of 30 with HC(triple bond)CPh afforded the vinyl compound [RuCl(CHCHPh)(CH3CN)(PCy3)2], 31. The molecular structures of 11(R = tBu, L = PiPr3) and 26 were determined crystallographically.     

Synthesis, electronic structure, and ethylene polymerization activity of bis(imino)pyridine cobalt alkyl cations

A new spin on polymers: the title cations comprise low-spin Co(II) centers with neutral bis(imino)pyridine chelating ligands. These complexes serve as single-component ethylene polymerization catalysts and offer insight into the mechanism of chain growth and catalyst deactivation, which occurs by forming inactive cationic bis(imino)pyridine cobalt complexes with a diethyl ether ligand.     

Preparation of stable alkyl complexes of Ni(I) and their one-electron oxidation to Ni(II) complex cations

The three-coordinate nickel(I) alkyl complexes (dtbpe)Ni(CH2CMe3) (2), (dtbpe)Ni(CH2SiMe3) (3), and (dtbpe)Ni(CH2CMe2Ph) (4) have been prepared by treatment of [(dtbpe)NiCl]2 with alkyllithium reagents. While thermally robust, they each undergo mild one-electron oxidation to give the corresponding Ni(II) complex cations [(dtbpe)Ni(CH2CMe3)+] (5), [(dtbpe)Ni(CH2SiMe3)+] (6), and [(dtbpe)Ni(CH2CMe2Ph)+] (7) as red-brown [PF6-] or [BArF4-] salts. In contrast to cationic amido and phosphido analogues that undergo alpha-deprotonation to afford imido and phosphinidene derivatives, deprotonation of 5-7 occurs at a gamma-CH3 group to give metallacyclobutane products (dtbpe)Ni(CH2CMe2CH2) (8), (dtbpe)Ni(CH2SiMe2CH2) (9), and (dtbpe)Ni(CH2CPhMeCH2) (10), not (dtbpe)Ni=CHR.     

Studies of a series of [Ni(P(R)2N(Ph)2)2(CH3CN)]2+ complexes as electrocatalysts for H2 production: substituent variation at the phosphorus atom of the P2N2 ligand

A series of [Ni(P(R)(2)N(Ph)(2))(2)(CH(3)CN)](BF(4))(2) complexes containing the cyclic diphosphine ligands [P(R)(2)N(Ph)(2) = 1,5-diaza-3,7-diphosphacyclooctane; R = benzyl (Bn), n-butyl (n-Bu), 2-phenylethyl (PE), 2,4,4-trimethylpentyl (TP), and cyclohexyl (Cy)] have been synthesized and characterized. X-ray diffraction studies reveal that the cations of [Ni(P(Bn)(2)N(Ph)(2))(2)(CH(3)CN)](BF(4))(2) and [Ni(P(n-Bu)(2)N(Ph)(2))(2)(CH(3)CN)](BF(4))(2) have distorted trigonal bipyramidal geometries. The Ni(0) complex [Ni(P(Bn)(2)N(Ph)(2))(2)] was also synthesized and characterized by X-ray diffraction studies and shown to have a distorted tetrahedral structure. These complexes, with the exception of [Ni(P(Cy)(2)N(Ph)(2))(2)(CH(3)CN)](BF(4))(2), all exhibit reversible electron transfer processes for both the Ni(II/I) and Ni(I/0) couples and are electrocatalysts for the production of H(2) in acidic acetonitrile solutions. The heterolytic cleavage of H(2) by [Ni(P(R)(2)N(Ph)(2))(2)(CH(3)CN)](BF(4))(2) complexes in the presence of p-anisidine or p-bromoaniline was used to determine the hydride donor abilities of the corresponding [HNi(P(R)(2)N(Ph)(2))(2)](BF(4)) complexes. However, for the catalysts with the most bulky R groups, the turnover frequencies do not parallel the driving force for elimination of H(2), suggesting that steric interactions between the alkyl substituents on phosphorus and the nitrogen atom of the pendant amines play an important role in determining the overall catalytic rate.     

Theoretical evaluation of ethylene insertion into chromium alkyl bonds of Cp-donor-based olefin polymerization catalysts

We propose routes for the catalytic cycle and possible termination reactions for the polymerization of ethylene with cationic chromium complexes of the type [CpCr(L)R](+) which contain donor ligands with phosphorus or nitrogen (L = PR(3) or NR(3)). We confirm the rate-determining character of the insertion of ethylene into the chromium-alkyl bond. Contrary to the situation with late transition metals, the resulting agostic isomers will readily isomerize. The termination of the polymerization reaction by β-hydrogen elimination to the chromium center and subsequent dissociation of the resulting olefin is found to require about 25 kcal/mol and to be thermodynamically much less feasible than the alternative termination process by β-hydrogen transfer to a monomer. The latter process involves spin change; two minimum-energy crossing points as well as further transition states and intermediates have been identified. Our calculations predict that adduct formation with the polymerization additive 9-BBN should be feasible both from a Cp-quinoline-based chromium catalyst and a zirconocene catalyst. However, only the latter undergoes exergonic chain transfer, which is in accordance with the experimentally observed formation of ultrahigh M(W) polyethylene when using 9-BBN as polymerization additive in combination with Cr catalysts. For the first time, quantum dynamics simulations of such open-shell systems have been performed, which give a lifetime of the Cr-alkyl complex with regard to ethylene insertion of only 500 fs. The simulations indicate that the dissociation of ethylene from the chromium center should be relatively insignificant compared to migratory insertion.     

Bulky iminophosphine-based nickel and palladium catalysts bearing 2,6-dibenzhydryl groups for ethylene oligo-/polymerization

A series of 2,6-dibenzhydryl substituted bulky Ni and Pd complexes containing P,N-chelating ligands, {[2,6-(Ph₂CH)₂-4-R-C₆H₂-N=CH-C₆H₄-2-PPh₂]MX₂; MX₂ =NiBr₂; R = Me ( Ni1 ); R = F ( Ni2 ); MX₂ =PdCl₂, R = Me ( Pd1 )}, have been prepared and used as catalyst precursors for ethylene oligo-/polymerization. Compared to the corresponding 2,6-diisopropyl Ni catalyst, these bulky Ni precatalysts activated by Et₂AlCl exhibited excellent catalytic performance toward ethylene polymerization with activity of up to 1.90 × 10⁵ g PE (mol Ni)⁻¹ h⁻¹, and result in semicrystalline PEs with high molecular weight. The catalytic performance of these bulky P,N-type complexes was significantly improved by introducing two ortho-dibenzhydryl on the N-aryl substituents. However, the formation of C₁₀–C₂₄ oligomers were generated using their palladium catalysts through ethylene oligomerization at high temperatures.     

Ethene/norbornene copolymerization with palladium(II) alpha-diimine catalysts: from ligand screening to discrete catalyst species

Sixteen palladium(II) alpha-diimine catalysts were investigated in a screening-like procedure for the copolymerization of ethene with norbornene. The resulting copolymers were characterized by (13)C NMR spectroscopy, differential scanning calorimetry, gel permeation chromatography, and viscosimetry. The degree of incorporation of norbornene in the polymer chain is very high for most of the catalysts. To validate the results achieved in the screening, two catalysts, [[ArN=CHCH=NAr]Pd(Me)(CH(3)CN)]BAr(f) (4) (1 b'; Ar=2,6-Me(2)C(6)H(3), BAr(f) (4)=B[3,5-C(6)H(3)(CF(3))(2)](4)) and [[ArN=C(CH(3))C(CH(3))=NAr]Pd(Me)(CH(3)CN)]BAr(f) (4) (2 c'; Ar=2,6-iPr(2)C(6)H(3)), were synthesized as discrete catalytically active species, and their copolymerization behavior was investigated in detail. In agreement with the screening results, 1 b' incorporates norbornene much better in the polymer chain than ethene, a property that has no analogue in metallocene catalysts.     

Synthesis and electronic structure of cationic, neutral, and anionic bis(imino)pyridine iron alkyl complexes: evaluation of redox activity in single-component ethylene polymerization catalysts

A family of cationic, neutral, and anionic bis(imino)pyridine iron alkyl complexes has been prepared, and their electronic and molecular structures have been established by a combination of X-ray diffraction, Mo?ssbauer spectroscopy, magnetochemistry, and open-shell density functional theory. For the cationic complexes, [((iPr)PDI)Fe-R][BPh(4)] ((iPr)PDI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)N═CMe)(2)C(5)H(3)N; R = CH(2)SiMe(3), CH(2)CMe(3), or CH(3)), which are known single-component ethylene polymerization catalysts, the data establish high spin ferrous compounds (S(Fe) = 2) with neutral, redox-innocent bis(imino)pyridine chelates. One-electron reduction to the corresponding neutral alkyls, ((iPr)PDI)Fe(CH(2)SiMe(3)) or ((iPr)PDI)Fe(CH(2)CMe(3)), is chelate-based, resulting in a bis(imino)pyridine radical anion (S(PDI) = 1/2) antiferromagnetically coupled to a high spin ferrous ion (S(Fe) = 2). The neutral neopentyl derivative was reduced by an additional electron and furnished the corresponding anion, [Li(Et(2)O)(3)][((iPr)PDI)Fe(CH(2)CMe(3))N(2)], with concomitant coordination of dinitrogen. The experimental and computational data establish that this S = 0 compound is best described as a low spin ferrous compound (S(Fe) = 0) with a closed-shell singlet bis(imino)pyridine dianion (S(PDI) = 0), demonstrating that the reduction is ligand-based. The change in field strength of the bis(imino)pyridine coupled with the placement of the alkyl ligand into the apical position of the molecule induced a spin state change at the iron center from high to low spin. The relevance of the compounds and their electronic structures to olefin polymerization catalysis is also presented.     

Synthesis, structures of (aminopyridine)nickel complexes and their use for catalytic ethylene polymerization

A series of α-aminopyridines in the form of (2,6-C(6)H(3)N)(R(1))(CHR(2)NR(3)R(4)) (R(1) = R(2) = H R(3) = H R(4) = (i)Pr (L1a), R(4) = (t)Bu (L1b), R(4) = Ph (L1c), R(4) = 2,6-Me(2)C(6)H(3) (L1d), R(4) = 2,6-(i)Pr(2)C(6)H(3) (L1e), R(1) = R(2) = H R(3) = R(4) = Et (L1f), R(1) = H R(2) = Me R(3) = H R(4) = (i)Pr (L2a), R(4) = Ph (L2c), R(4) = 2,6-Me(2)C(6)H(3) (L2d), R(4) = 2,6-(i)Pr(2)C(6)H(3) (L2e), R(1) = Me R(2) = H R(3) = H R(4) = 2,6-(i)Pr(2)C(6)H(3) (L3e)) and β-aminopyridines in the form of (2-C(6)H(4)N)(CH(2)CH(2)NR(1)R(2)) (R(1) = H R(2) = (i)Pr (4a), R(2) = (t)Bu (L4b), R(1) = R(2) = Et (L4f)) have been prepared. Their corresponding halonickel complexes 1a-4f are synthesized by ligand substitution from (DME)NiBr(2) and the molecular structures are characterized. Four types of coordination modes include four-coordinate mononuclear species with one ligand, five-coordinate mononuclear species with two ligands, five-coordinate dinuclear species with two ligands, and a six-coordinate polymeric framework were determined by X-ray crystallography. Using methylaluminoxanes (MAO) as the activator, the nickel complexes can catalyze ethylene polymerization under moderate pressure and ambient temperature. The activity reaches 10(5) g PE mol(-1) Ni h. The PE products with high branching and high crystallinity have M(n) ~ 10(3) with PDI < 2.     

A structure-activity study of Ni-catalyzed alkyl-alkyl Kumada coupling. Improved catalysts for coupling of secondary alkyl halides

A structure-activity study was carried out for Ni catalyzed alkyl-alkyl Kumada-type cross coupling reactions. A series of new nickel(II) complexes including those with tridentate pincer bis(amino)amide ligands ((R)N(2)N) and those with bidentate mixed amino-amide ligands ((R)NN) were synthesized and structurally characterized. The coordination geometries of these complexes range from square planar, tetrahedral, to square pyramidal. The complexes had been examined as precatalysts for cross coupling of nonactivated alkyl halides, particularly secondary alkyl iodides, with alkyl Grignard reagents. Comparison was made to the results obtained with the previously reported Ni pincer complex [((Me)N(2)N)NiCl]. A transmetalation site in the precatalysts is necessary for the catalysis. The coordination geometries and spin-states of the precatalysts have a small or no influence. The work led to the discovery of several well-defined Ni catalysts that are significantly more active and efficient than the pincer complex [((Me)N(2)N)NiCl] for the coupling of secondary alkyl halides. The best two catalysts are [((H)NN)Ni(PPh(3))Cl] and [((H)NN)Ni(2,4-lutidine)Cl]. The improved activity and efficiency was attributed to the fact that phosphine and lutidine ligands in these complexes can dissociate from the Ni center during catalysis. The activation of alkyl halides was shown to proceed via a radical mechanism.     

Neutral and cationic alkylmanganese(II) complexes containing 2,6-bisiminopyridine ligands

Manganese alkyl complexes stabilised by 2,6-bis(N,N'-2,6-diisopropyl-phenyl)acetaldiminopyridine ((iPr)BIP) have been selectively prepared by reacting suitable alkylmanganese(II) precursors, such as homoleptic dialkyls [(MnR(2))(n)] or the corresponding THF adducts [{MnR(2)(thf)}(2)] with the mentioned ligand. For R=CH(2)CMe(2)Ph or CH(2)Ph, formally Mn(I) derivatives are produced, in which one of the two R groups migrates to the 4-position of the central pyridine ring in the (iPr)BIP ligand. In contrast, a true dialkyl complex [MnR(2)((iPr)BIP)] can be isolated for R=CH(2)SiMe(3). In solution, this compound slowly evolves to the corresponding Mn(I) monoalkyl derivative. A detailed study of this reaction provides insights on its mechanism, showing that it proceeds through successive alkyl migrations, followed by spontaneous dehydrogenation. Protonation of [Mn(CH(2)SiMe(3))(2)((iPr)BIP)] with the pyridinium salt [H(Py)(2)][BAr'(4)] (Ar'=3,5-C(6)H(3)(CF(3))(2)) leads to the cationic species [Mn(CH(2)SiMe(3))(Py)((iPr)BIP)](+). Alternatively, the same complex can be produced by reaction of the pyridine complex [{Mn(CH(2)SiMe(3))(2)(Py)}(2)] with the protonated ligand salt [H(iPr)BIP](+)[BAr'(4)](-). This last reaction allows the synthesis of analogous cationic alkylmanganese(II) derivatives, when precursors of type [MnR(2)((iPr)BIP)] are not available. Treatment of these neutral and cationic (iPr)BIP alkylmanganese derivatives with a range of typical co-catalysts (modified methylaluminoxane (MMAO), B(C(6)F(5))(3), trimethyl or triisobutylaluminum) does not lead to active ethylene polymerisation catalysts.     

Ethylene polymerization by sterically and electronically modulated Ni(II) α‐diimine complexes

A series of highly active ethylene polymerization catalysts based on bidendate α‐diimine ligands coordinated to nickel are reported. The ligands are prepared via the condensation of bulky ortho‐substituted anilines bearing remote push–pull substituents with acenaphthenequinone, and the precatalysts are prepared via coordination of these ligands to (DME)NiBr₂ (DME = 1,2‐dimethoxyethane) to form complexes having general formula [ZN = C(An)‐C(An) = NZ]NiBr₂ [Z = (4‐NH₂‐3,5‐C₆H₂R₂)₂CH(4‐C₆H₄Y); An, acenaphthene quinone; R, Me, Et, iPr; Y = H, NO₂, OCH₃]. When activated with methylaluminoxane (MAO) or common alkyl aluminiums such as ethyl aluminium sesquichloride (EAS) all catalysts polymerize ethylene with activities exceeding 10⁷ g‐PE/ mol‐Ni h atm at 30 °C and atmospheric pressure. Among the cocatalysts used EAS records the best activity. Effects of remote substituents on ethylene polymerization activity are also investigated. The change in potential of metal center induced by remote substituents, as evidenced by cyclic voltammetric measurements, influences the polymerization activity. UV–visible spectroscopic data have specified the important role of cocatalyst in the stabilization of nickel‐based active species. A tentative interpretation based on the formation of active and dormant species has been discussed. The resulting polyethylene was characterized by high molecular weight and relatively broad molecular weight distribution, and their microstructure varied with the structure of catalyst and cocatalyst. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1066–1082, 2008