Moris S. Eisen

BIS(TRIMETHYLSILYL)BENZAMIDINATE ZIRCONIUM DICHLORIDES. ACTIVE CATALYSTS FOR ETHYLENE POLYMERIZATION

Abstract A study was made of the polymerization of ethylene involving the use of a series of [η4-4-RC6H4C(NSiMe3)2]2ZrCl2 as precatalysts (R = H (1); CH3 (2)). The benzamidinate zirconium dichloride complexes were prepared from ZrCl4 and the corresponding 4-RC6H4C(NSiMe3)2Li· TMEDA ligand. The structure of complex 1 was determined by a low-temperature X-ray diffraction study. “Cationic” ethylene polymerization catalysts were generated from 1 and 2 with methylalumoxane. The polymerization activity and the molecular weights of the polymers are strongly dependent on the catalyst and cocatalyst concentrations. The polymerization activity increases drastically with increase in pressure and temperature and decreases when electron-releasing groups are attached to the aromatic ring.

Formation of elastomeric polypropylene promoted by the dynamic complexes [TiCl2{N(PPh2)2}2] and [Zr(NPhPPh2)4]

Abstract The homoleptic phosphinoamide complex [Zr(NPhPPh 2 ) 4 ] ( 1 ) and the bisamido complex [TiCl 2 {N(PPh 2 ) 2 } 2 ] ( 2 ) were prepared from ZrCl 4 and four equivalents of LiNPhPPh 2 and from TiCl 4 and one equivalent of [Li(THF)N(PPh 2 ) 2 ] 2 . In the solid state, the four NPhPPh 2 ligands in 1 exhibit η 2 coordination. The ZrN 4 P 4 fragment is highly symmetrical and almost of D 2 symmetry. Hence, the complex is chiral, and the two enantiomers cocrystallize in the asymmetric unit. In solution, 1 exhibits signals for the six-coordinate complex [Zr(η 2 -NPhPPh 2 ) 2 (η 1 -NPhPPh 2 ) 2 ]. In the presence of methylalumoxane (MAO), 1 and 2 are active catalysts for the formation of high-molecular-weight elastomeric polypropylene. The formation of elastomeric polypropylene is a consequence of an epimerization mechanism of the last-inserted monomer, indicating no detachment of the growing polymer chain from the metal center during this process. Fractionation studies of all the elastomeric polymers show no atactic fractions. As corroboration for this mechanism, we have shown that these complexes are active catalysts for the isomerization and oligomerization of 1-octene, as well as for the rapid isomerization of allylbenzene to trans -methylstyrene.

Metallocene analogues containing bulky heteroallylic ligands and their use as new olefin polymerization catalysts

Abstract A series of Ti and Zr metallocene analogues containing bulky benzamidinate ligands has been prepared and fully characterized. Treatment of TiCl 4 (THF) 2 or ZrCl 4 (THF) 2 with two equivalents of the appropriate benzamidinate anions affords the bis(benzamidinato) complexes [C 6 H 5 C(NC 3 H 7 ) 2 ] 2 MCl 2 (M=Ti ( 1 ), Zr ( 2 )) and [C 6 H 5 C(NC 6 H 11 ) 2 ] 2 MCl 2 (M=Ti ( 3 ), Zr ( 4 )). The zirconium complex 2 was structurally characterized by X-ray diffraction. In a similar manner the nonafluoromesityl derivative [(CF 3 ) 3 C 6 H 2 C(NC 6 H 11 ) 2 ] 2 ZrCl 2 ( 5 ) was synthesized from ZrCl 4 (THF) 2 and Li[(CF 3 ) 3 C 6 H 2 C(NC 6 H 11 ) 2 ]. Methylation of 4 with methyllithium yields the dimethyl complex [C 6 H 5 C(NC 6 H 11 ) 2 ] 2 ZrMe 2 ( 6 ). The mixed-ligand metallocene analogues [C 6 H 5 C(NC 3 H 7 ) 2 ](C 5 Me 5 )MCl 2 (M=Ti ( 7 ), Zr ( 8 )) and [C 6 H 5 C(NC 6 H 11 ) 2 ](C 5 Me 5 )TiCl 2 ( 9 ) have been prepared by reacting (C 5 Me 5 )TiCl 3 or (C 5 Me 5 )ZrCl 3 with one equivalent of a lithium N , N ″-dialkylbenzamidinate. The polymerization of ethylene and propylene has been studied by the catalytic precatalyst complexes 1 and 2 upon reaction of an excess of methylalumoxane to obtain the active cationic complexes. The polymerization activity of the complexes is comparable to other benzamidinate ancillary containing ligands although toward shorter amounts of time due to a competitive inhibition presumably obtained by a β -hydrogen elimination from the ligand. Polymerization activity is strongly dependent on catalyst and cocatalyst concentrations and on temperature.

Organoactinides Promote the Tishchenko Reaction : The Myth of Inactive Actinide-Alkoxo Complexes

For many decades, compounds containing oxygen atoms were excluded from the actinide-catalysis field because of the high oxophilic nature of these complexes. Pursuing the conceptual question about the potential activity of actinide-oxo bonds we were surprised to find that the coupling of aromatic aldehydes catalyzed by Cp*2ThMe2 and Th(NEtMe)4 via thorium−alkoxide intermediates takes place in high yields to produce the corresponding esters. In this paper we present our breakthrough results including comprehensive mechanistic, deuterium labeling, kinetic, and thermodynamic studies. A plausible mechanism is presented taking into account as well the thermochemistry of the process.

Dehydrocoupling reactions of amines with silanes catalyzed by [(Et2N)3U][BPh4]

Abstract Dehydrocoupling reactions of primary amines RNH2 with PhSiH3 were catalyzed by [(Et2N)3U][BPh4] to give the corresponding aminosilanes PhSiH3−n(NHR)n (n=1–3), the relative yields of the products were found to be dependent on the experimental conditions and on the nature of R. For a primary silane (PhSiH3), the reactivity of RNH2 follows the order primary>secondary>tertiary. Similar dehydrocoupling reactions using secondary amines with secondary silanes were found to be less reactive. Homodehydrocoupling of the silane was found not to be a competing reaction at room temperature. The hydride [(RNH)2UH][BPh4], which is plausibly formed in the reaction of [(RNH)3U][BPh4] with PhSiH3 is a likely intermediate in the catalytic cycle.

SYNTHESIS AND OLEFIN POLYMERIZATION USING SUPPORTED AND NON-SUPPORTED GEOMETRY CONSTRAINED TITANIUM COMPLEXES

Abstract The synthesis of a constrained geometry titanium complex and its immobilization on different supports have been studied for the catalytic polymerization of ethylene, propylene and styrene. The formation of higher molecular weight polymers was found in the heterogeneous system as compared to the homogenous parent for polyethylene and polypropylene. Polymerization of propylene yielded atactic products with elastomeric properties due to long polymer chains. However, syndiotactic polymers were formed in the polymerization of styrene. Catalytic activity was found to be dependent on MAO and titanium concentration; the higher the metal concentration the lower the activity of the catalyst. The activity of the heterogeneous system was determined as a function of time, and a first-order deactivation pathway was found at room temperature. The type of solvent (toluene or CH2Cl2) also played an important role in the polymerization of ethylene and styrene, although different tendencies were found for either monomers.

Mono(imidazolin-2-iminato) Titanium Complexes for Ethylene Polymerization at Low Amounts of Methylaluminoxane

The polymerization of ethylene with titanium complexes bearing one bulky imidazolin-2-iminato ligand (L) in the presence of MAO and/or TTPB as cocatalysts have been explored. The complex LTiCl3 and its methylated forms were prepared to shed light on the nature of the active polymerization species. With some of these complexes, the best catalytic activity was obtained at an Al:Ti ratio of 8.

Synthesis, characterization and reactivity of amido titanium and zirconium complexes

Abstract The polymerization of ethylene and propylene was studied by using a series of mono- and spirocyclic coordinatively unsaturated early transition metal amides as pre-catalysts. Amino complexes were prepared either by metathesis reaction of Cp 2 ZrCl 2 and the dilithium salt of the bisamido ligand (Me 3 SiN(CH 2 ) 2 NSiMe 3 ) 2− , by transamination reaction of Zr(NMe 2 ) 4 with two equivalents of the diamine ligand (Me 3 SiNH(CH 2 ) 2 NHSiMe 3 ), and by metathesis reaction of ZrCl 4 · 2THF or TiCl 4 with equimolar amounts of the dilithium salt (Me 3 SiN(CH 2 ) 2 NSiMe 3 ) 2 . The complexes obtained were characterized by standard spectroscopic techniques. ‘Cationic’ polymerization catalysts were generated from the early transition-metal amides with methylalumoxane. Polymerization activity is dependent on catalyst and co-catalyst concentrations.

Diverse catalytic activity of the cationic actinide complex [(Et2N)3U][BPh4] in the dimerization and hydrosilylation of terminal alkynes. Characterization of the first f-element alkyne π-complex [(Et2N)2U(CCtBu)(η2-HCCtBu)][BPh4]

Abstract The cationic actinide complex [(Et 2 N) 3 U][BPh 4 ] is an active catalytic precursor for the selective dimerization of terminal alkynes. The regioselectivity is mainly towards the geminal dimer but for bulky alkyne substituents, the unexpected cis -dimer is also obtained. Mechanistic studies show that the first step in the catalytic cycle is the formation of the acetylide complex [(Et 2 N) 2 UC  CR][BPh 4 ] with the concomitant reversible elimination of Et 2 NH, followed by the formation of the alkyne π-complex [(Et 2 N) 2 UC  CR(RC  CH)][BPh 4 ]. This latter complex (R= t Bu) has been characterized spectroscopically. The kinetic rate law is first order in organoactinide and exhibits a two domain behavior as a function of alkyne concentration. At low alkyne concentrations, the reaction follows an inverse order whereas at high alkyne concentrations, a zero order is observed. The turnover-limiting step is the C  C bond insertion of the terminal alkyne into the actinideacetylide bond to give the corresponding alkenyl complex with Δ H ‡ =15.6(3) kcal mol −1 and Δ S ‡ =−11.4(6) eu. The following step, protonolysis of the uraniumcarbon bond of the alkenyl intermediate by the terminal alkyne, is much faster but can be retarded by using CH 3 C  CD, allowing the formation of trimers. The unexpected cis -isomer is presumably obtained by the isomerization of the trans -alkenyl intermediate via an envelope mechanism. A plausible mechanistic scenario is proposed for the oligomerization of terminal alkynes. The cationic complex [(Et 2 N) 3 U][BPh 4 ] has been found to be also an efficient catalyst for the hydrosilylation of terminal alkynes. The chemoselectivity and regiospecificity of the reaction depend strongly on the nature of the alkyne, the solvent and the reaction temperature. The hydrosilylation reaction of the terminal alkynes with PhSiH 3 at room temperature produced a myriad of products among which the cis - and trans -vinylsilanes, the alkene and the silylalkyne are the major components. At higher temperatures, besides the products obtained at room temperature, the double hydrosilylated alkene, in which the two silicon moieties are connected at the same carbon atom, is obtained. The catalytic hydrosilylation of (TMS)C  CH and PhSiH 3 with [(Et 2 N) 3 U][BPh 4 ] was found to proceed only at higher temperatures. Mechanistically, the key intermediate seems to be the uranium–hydride complex [(Et 2 N) 2 UH][BPh 4 ], as evidenced by the lack of the dehydrogenative coupling of silanes. A plausible mechanistic scenario is proposed for the hydrosilylation of terminal alkynes taking into account the formation of all products.

Oligomerization and hydroamination of terminal alkynes promoted by the cationic organoactinide compound [(Et2N)3U][BPh4].

The three ancillary amido moieties in the cationic complex [(Et2N)3U][BPh4] are highly reactive and are easily replaced when the complex is treated with primary amines. The reaction of [(Et2N)3U][BPh4] with excess tBuNH2 allows the formation of the cationic complex [(tBuNH2)3(tBuNH)3U][BPh4]. X-ray diffraction studies on the complex indicate that three amido and three amine ligands are arranged around the cationic metal center in a slightly distorted octahedral mer geometry. The cationic complex reacts with primary alkynes in the presence of external primary amines to primarily afford the unexpected cis dimer and, in some cases, the hydroamination products are obtained concomitantly. The formation of the cis dimer is the result of an envelope isomerization through a metal-cyclopropyl cationic complex. In the reaction of the bulkier alkyne tBuC identical to CH with the cationic uranium complex in the presence of various primary amines, the cis dimer, one trimer, and one tetramer are obtained regioselectively, as confirmed by deuterium labeling experiments. The trimer and the tetramer correspond to consecutive insertions of an alkyne molecule into the vinylic CH bond trans to the bulky tert-butyl group. The reaction of (TMS) C identical to CH with the uranium catalyst in the presence of EtNH2 followed a different course and produced the gem dimer along with the hydroamination imine as the major product. However, when other bulkier amines were used (iPrNH2 or tBuNH2) both hydroamination isomeric imines Z and E were obtained. During the catalytic reaction, the E (kinetic) isomer is transformed into the most stable Z (thermodynamic) isomer. The unique reactivity of the alkyne (TMS) C identical to CH with the secondary amine Et2NH is remarkable because it afforded the trans dimer and the corresponding hydroamination enamine. The latter probably results from the insertion of the alkyne into a secondary metal-amide bond, followed by protonolysis.