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In recent years 'frustrated Lewis pairs' (FLPs) have been shown to be effective metal-free catalysts for the hydrogenation of many unsaturated substrates. Even so, limited functional-group tolerance restricts the range of solvents in which FLP-mediated reactions can be performed, with all FLP-mediated hydrogenations reported to date carried out in non-donor hydrocarbon or chlorinated solvents. Herein we report that the bulky Lewis acids B(C6Cl5)x(C6F5)(3-x) (x=0-3) are capable of heterolytic H2 activation in the strong-donor solvent THF, in the absence of any additional Lewis base. This allows metal-free catalytic hydrogenations to be performed in donor solvent media under mild conditions; these systems are particularly effective for the hydrogenation of weakly basic substrates, including the first examples of metal-free catalytic hydrogenation of furan heterocycles. The air-stability of the most effective borane, B(C6Cl5)(C6F5)2, makes this a practically simple reaction method.

Fluoride coordination derivatives of niobium(V) were tested for their activation capabilities with respect to acetone and to olefins. Activation of acetone (formation of mesityloxide) was observed with NbF 4 (OMe). Several fluoride coordination derivatives of niobium(V) of different nature (neutral or ionic) and nuclearity, i.e. NbF 5 L [L = Et 2 O, 4 , thf, 5 (thf is tetrahydrofuran), MeOH, 6 , EtOH, 7 ], (NbF 4 L 2 )(NbF 6 ) [L = dmf, 8 (dmf is dimethylformamide), dme, 9 (dme is dimethoxyethane)], (NbF 4 L 4 )(NbF 6 ) [L = thf, 10 , Et 2 O, 11 , MeCN, 12 ], [S(NMe) 3 ][NbF 6 ], 13 , NbF 4 OMe, 1 , NbF 4 OPh, 3 ), NbF 3 (OPh) 2 , 14 , NbF 2 (OPh) 3 , 15 and NbF 2 (OEt) 3 , 16 , promoted the polymerization of ethylene using AlMe 3 -depleted methylaluminoxane as cocatalyst. Highly linear polyethylene was obtained. Compound 3 , upon activation with methylaluminoxane, promoted ring-opening metathesis polymerization (ROMP) of norbornene, affording polymers with a slight excess of trans content.

Sticky counterions: Low‐temperature kinetic NMR studies were performed to determine ΔH≠ and ΔS≠ values for the insertion of a single 2‐methyl‐1‐heptene molecule into a Zr-C bond of [Cp2Zr(η2‐CH2NMePh)][X] (1a: X−=MeB(C6F5)3−, 1b: B(C6F5)4−) in [D8]toluene and a 1:1 mixture of [D8]toluene and [D5]chlorobenzene. Both activation parameters critically depend on the interplay of the counterion and the solvent.

[Cp(2)ZrMe)(+) + Me(3)N + C(6)H(6)down arrow TS2Cp(2)ZrMePh + (Me(3)NH](+)down arrow TS3[Cp(2)ZrPh](+) + Me(3)N + CH(4)The C-D activation of bromobenzene-d(5), by [CP(2)*ZrMe(BrC(6)D(5)-kappa Br)][B(C(6)F(5))(4)] to form sigma-aryl complex [Cp(2)*Zr(2-BrC(6)D(4)-kappa(2)Br,C)][B(C(6)F(5))(4)] is accelerated when the decamethylzirconocene methyl cation is generated in the presence of tertiary amines. Computational studies suggest that the overall sigma-bond metathesis reaction proceeds, in the presence of tertiary amines, via initial deprotonation of bromobenzene in the coordination sphere of the metal center. In a subsequent step, the resulting neutral methyl aryl species, CP(2)*ZrMe(2-BrC(6)D(4)), reacts with the concurrently generated trialkylammonium cation to generate [Cp(2)*Zr(2-BrC(6)D(4)-kappa(2)Br,C)][B(C(6)F(5))(4)].

This paper describes a substantial enhancement of the aminopyridinato ligand stabilized early transition metal chemistry by introducing the sterically very demanding 2,6-dialkylphenyl substituted aminopyridinato ligands derived from (2,6-diisopropylphenyl)-[6-(2,6-dimethylphenyl)-pyridin-2-yl]-amine (1a-H, ApH) and (2,6-diisopropylphenyl)-[6-(2,4,6-triisopropylphenyl)-pyridin-2-yl]- amine (1b-H, Ap*H). The corresponding bis aminopyridinato zirconium dichloro complexes, [Ap(2)ZrCl(2)] (3a) and [Ap(2)*ZrCl2] (3b) and the dimethyl analogues, [Ap(2)ZrMe(2)] (4a) and [Ap(2)*ZrMe2] (4b) (Me = methyl) were synthesized, using standard salt metathesis routes. Single-crystal X-ray diffraction was carried out for the dichloro derivatives. Both zirconium metal centers have a distorted octahedral environment with a cis-orientation of the chloride ligands in 3a and a closer to trans-arrangement in 3b. The dimethyl derivatives are proven to be highly active ethylene polymerization catalysts after activation with [R2N(Me)H][B(C6F5)(4)] (R = C16H33-C18H37). During attempted co-polymerizations of alpha-olefins (propylene) and ethylene high activity and selectivity for ethylene and nearly no co-monomer incorporation was observed. Increasing the steric bulk of the ligand going from (2,6-dimethylphenyl) to (2,4,6-triisopropylphenyl) substituted pyridines, switches the catalyst system from producing long chain alpha-olefins to polymerization of ethylene in a living fashion. In contrast to the dimethyl complexes only [Ap(2)*ZrCl2] in the presence of MAO at elevated temperature gave decent polymerization activity. NMR investigations of the reaction of dichloro complexes with 25 equiv. of MAO or AlMe3 at room temperature revealed, that [Ap(2)ZrCl(2)] decomposes under ligand transfer to aluminum and formation of [ApAlMe(2)], while [Ap(2)*ZrCl2] remains almost unreacted under the same conditions. The aminopyridinato dimethyl aluminum complexes, [ApAlMe(2)] (5a) and [Ap*AlMe2] (5b) were synthesized independently and structurally characterized. The aluminum complexes 5a and b show no catalytic activity towards ethylene, when "activated" with[R2N(Me)H][B(C6F5)(4)]. (C) 2007 Elsevier B.V. All rights reserved.

Zirconocene precatalysts with sterically bulky alkyl groups were designed in order to obtain models for the propagating species in zirconocene-catalyzed alkene polymerization. Alkyllithium reagents Li(CH2CEt3) and Li(CH2CMe2CH2Ph) were prepared and utilized in methyl alkylzirconocene synthesis. Dialkyl and methyl alkylzirconocenes of the form [(eta-5-C5H5) (eta-5-C5Me5)Zr(R)2], [(eta-5-C5H5)2Zr(R)(CH3)], and [(eta-5-C5H5)(eta-5-C5Me5)Zr(R)(CH3)] (where R = CH2CMe3, CH2SiMe3, CH2CEt3,CH2CMe2CH2Ph) were synthesized and fully characterized by NMR spectroscopy and in some cases X-ray diffraction. The molecular structures determined display the bent sandwich coordination mode common for zirconocenes. The observed structural parameters are slightly perturbed by the steric influence of the bulky alkyl group. A direct examination of propagation kinetics for alkene polymerization using the zirconocenium initiator, [(eta-5-C5H5)(eta-5-C5Me5)Zr(CH2CMe3)]+[CH3B(C6F5)3]-, is reported. Propagation rate constants (kp) for the polymerization of propene and a series of 1-alkenes catalyzed by [(eta-5-C5H5)(eta-5-C5Me5)Zr(CH2CHR)nCH2CMe3)]+[CH3B(C6F5)3]- were measured by 1H NMR spectroscopy in toluene-d8 at low temperature. The kp obtained for propene and other alkenes decreases with increasing chain length and steric influence. The overall activation parameters for propagation determined from an Eyring analysis are (delta)H(activation) = 8.5(3) kcal/mol and (delta)S(activation) = -25(2) eu. The propagation rate was found to increase in the presence of [CH3B(C6F5)3]- counteranion and in a polar toluene-d8-chlorobenzene-d5 solvent system. The experimental results are most consistent with propagation mechanism that does not involve the formation of outer-sphere ions for alkene polymerization by this catalyst system. Propene initiation kinetics have been examined for a series of alkylzirconocene initiators, [(eta-5-C5H5)(eta-5-C5Me5)Zr(R)]+[CH3B(C6F5)3]- (R = CH3 (5), CH2CMe3 (1), CH2SiMe3 (4)). Measurement of ki for the neopentyl initiator reveals that the rate of initiation is on the order of propagation for this catalyst. This initiator with a polymeryl like alkyl group serves as a model of the propagating species in propene polymerization. The catalyst initiation behavior has been investigated and the observed relative rates of propene initiation are not always predicted by ground-state (zirconium-carbon bond strength or extent of ion-pairing) considerations. The catalysts 4 and 5 are both poor initiators with ki << kp and show less than 50% initiation in the presence of excess propene at -60 C in toluene-d8.

Titanium tribenzyl complexes with cyclopentadienyl-arene ligands, (η5-C5H4CMe2Ar)Ti(CH2Ph)3 (Ar = 3,5-Me2C6H3 (1), Ph (2)), were synthesized. Reaction of 1 with either B(C6F5)3 or [Ph3C][B(C6F5)4] affords the cation [(η,5η-C5H4CMe2-3,5-Me2C6H3)Ti(η-CH2Ph)2]+ (3), in which the pendant arene group and the benzyl ligands show fluxional behavior. Reaction of 2 with B(C6F5)3, leads to rapid ortho cyclometalation of the pendant arene to give the contact ion pair [(η5:η1-C5H4CMe2C6H4)Ti(η1-CH2Ph)][η6-PhCH2B(C6F5)3] (4) and toluene. Reaction of 2 with [Ph3C][B(C6F5)4] also leads to ligand ortho metalation, giving [(η5:η1-C5H4CMe2C6H4)Ti(η2-CH2Ph)][B(C6F5)4] (5) with a η2-benzyl group. Thermolysis of compounds 1 and 2 (50 °C, 50 h) results in the ortho-cyclometalated dibenzyl species (η5:η1-C5H4CMe2Ar')Ti(CH2Ph)2 (Ar' = 3,5-Me2C6H2 (6), C6H4 (7)) and toluene. The thermolysis of 1 follows first-order kinetics (k ≈ 10-5 s-1 at 333 K) with ΔH‡ = 24 ± 2 kcal mol-1 and ΔS‡ = -5 ± 5 cal mol-1 K-1. For the thermolysis of the related titanium trialkyl (η5-C5H4CMe2Ph)Ti(CH2SiMe3)3 (9), activation parameters of ΔH‡ = 21 ± 2 kcal mol-1 and ΔS‡ = -18 ± 4 cal mol-1 K-1 were found. Deuterium labeling studies with (η5-C5H4CMe2C6D5)Ti(CH2Ph)3 (2-d5) show that thermolysis of the neutral compound involves initial formation of an alkylidene intermediate, followed by o-CH addition to the Ti=C bond. In the corresponding cationic species, ortho cyclometalation proceeds via direct σ-bond metathesis.

The titanium complex Cp[1,3-(2',6' Me2C6H3) (2)(CH2N)(2)C=N] Ti(CH2Ph)(2), with a monoanionic eta(1)-iminoimidazolidide ancillary ligand, is shown to be a highly efficient catalyst for olefin polymerisation when activated with the Lewis acid B(C6F5)(3).

The titanium complex Cp[1,3-(2',6' Me2C6H3) (2)(CH2N)(2)C=N] Ti(CH2Ph)(2), with a monoanionic eta(1)-iminoimidazolidide ancillary ligand, is shown to be a highly efficient catalyst for olefin polymerisation when activated with the Lewis acid B(C6F5)(3).

Neutral and cationic titanium and zirconium diamine bis(phenolate) complexes supported by chiral ligands L-1 and L-2 are described [L-1 = (R)-6,6'-{1-(dimethylamino)propan-2-ylazanediyl}bis(methylene)-bis(2,4-di-tert-butylphenolate); L-2 = (S)-6,6'-{(1-ethylpyrrolidin-2-yl)methylazanediyl}bis-(methylene)- bis(2,4-di-tert-butylphenolate)]. Complexes [TiL(OiPr)(2)] [L = L-1 (1), L-2 (2)], [(TiLX)-X-2(OiPr)] [X = Cl (3), CH2Ph (4)], [TiL1(CH2Ph)(2)] (5), [ZrLCl2] [L = L-1 (6), L-2 (7)] and [ZrL(CH2Ph)(2)] [L = L-1 (8), L-2 (9)] are C-1-symmetric species readily prepared in moderate to high yields. Cationic compounds were obtained from complexes 5, 8 and 9 through reactions with B(C6F5)(3). The monocationic species reveal the eta(2)-binding character of the benzyl groups and noncoordinated [B(C6F5)(3)(CH2Ph)](-). Complexes 5-9 were tested in the polymerization of ethylene and propylene upon activation with modified methylaluminoxane (MMAO), B(C6F5)(3) or [NHMe2Ph](+) as cocatalysts. The [(ZrLCl2)-Cl-1]/MMAO and [ZrL1(CH2Ph)(2)]/B(C6F5)(3) systems revealed high activities in the polymerization of propylene (2.1 x 10(3) and 1.5 x 10(3) gpolmmol(cat)(-1)h(-1), respectively), leading to atatic polypropylene, whereas the titanium complexes showed no activity.