Cp Zr Research Articles

Ionization of Cp2 ZrMe2 and Lewis Bases by Methylaluminoxane: Computational Insights.

The interactions of the Lewis bases CO, octamethyltrisiloxane (OMTS) and 2,2'-bipyridine (bipy) with a sheet model for the principal activator (MeAlO)16 (Me3 Al)6 (16,6) in hydrolytic methylaluminoxane (MAO) were investigated by DFT. These studies reveal that OMTS and bipy form adducts with Me3 Al prior to methide abstraction by 16,6 to form the ion-pairs [Me2 Al(κ2 -L)][16,6] (5: L=OMTS, 6: L=bipy, [16,6]- =[(MeAlO)16 (Me3 Al)6 Me]- ) while CO simply binds to a reactive edge site without ionization. The binding and activation of Cp2 ZrMe2 with 16,6 to form both neutral adducts 1 Cp2 ZrMe2 ⋅16,6 and contact ion-pairs 4 and 7, both with formula [Cp2 ZrMe][μ-Me(MeAlO)16 (Me3 Al)6 ], featuring terminal and chelated MAO-anions, respectively was studied by DFT. The displacement of the anion with either excess Cp2 ZrMe2 or Me3 Al was also studied, forming outer-sphere ion-pairs [(Cp2 ZrMe)2 μ-Me][16,6] (2) and [Cp2 Zr(μ-Me)2 AlMe2 ][16,6] (3). The theoretical NMR spectra of these species were compared to experimental spectra of MAO and Cp2 ZrMe2 and found to be in good agreement with the reported data and assignments. These studies confirm that 16,6 is a very suitable model for the activators present in MAO but highlight the difficulty in accurately calculating thermodynamic quantities for molecules in this size regime.

Formation of a Strong Heterogeneous Aluminum Lewis Acid on Silica.

Al(OC(CF3 )3 )(PhF) reacts with silanols present on partially dehydroxylated silica to form well-defined ≡SiOAl(OC(CF3 )3 )2 (O(Si≡)2 ) (1). 27 Al NMR and DFT calculations with a small cluster model to approximate the silica surface show that the aluminum in 1 adopts a distorted trigonal bipyramidal coordination geometry by coordinating to a nearby siloxane bridge and a fluorine from the alkoxide. Fluoride ion affinity (FIA) calculations follow experimental trends and show that 1 is a stronger Lewis acid than B(C6 F5 )3 and Al(OC(CF3 )3 )(PhF) but is weaker than Al(OC(CF3 )3 ) and i Pr3 Si+ . Cp2 Zr(CH3 )2 reacts with 1 to form [Cp2 ZrCH3 ][≡SiOAl(OC(CF3 )3 )2 (CH3 )] (3) by methide abstraction. This reactivity pattern is similar to reactions of organometallics with the proposed strong Lewis acid sites present on Al2 O3 .

Selective Reduction of Secondary Amides to Imines Catalysed by Schwartz's Reagent.

The partial reduction of amides is a challenging transformation that must overcome the intrinsic stability of the amide bond and exhibit high chemoselective control to avoid overreduction to amine products. To address this challenge, we describe a zirconium-catalysed synthesis of imines by the reductive deoxygenation of secondary amides. This reaction exploits the excellent chemoselectivity of Schwartz's reagent (Cp2 Zr(H)Cl) and utilises (EtO)3 SiH as a mild stoichiometric reductant to enable catalyst turnover. The reaction generally proceeds with high yields (19 examples, 51 to 95 % yield) and tolerates a variety of functional groups (alkene, ester, nitro, etc.). Stoichiometric mechanistic investigations suggest the regeneration of the active [Zr]-H catalyst is achieved through the metathesis of Si-H and Zr-OR σ-bonds.

Microstructure and mechanical performance of zirconium, manufactured by selective laser melting

Additive manufacturing as an innovative material fabrication technique has been attracting increasing attention from academia and industry in recent years. However, intensive studies on additive manufacturing of zirconium and its alloy are very scarce due to the challenge of fabrication technology. Herein, commercially pure zirconium (CP–Zr) components with high mechanical properties in terms of strength and plasticity were successfully manufactured by selective laser melting (SLM) under an argon atmosphere. The microscopic characteristics and mechanical properties of as-processed CP-Zr were comprehensively investigated by the means of optical microscope (OM), transmission electron microscope (TEM) and universal tensile testing machine. The characterization experiments indicated that the as-built microstructure of CP-Zr was composed of a large amount of ultra-fine α' martensite locating inside the columnar prior β grains, and massive dislocations and twins distributing in martensite or matrix. Based on the tensile testing, the as-built samples achieved a great ultimate tensile strength of 746.3 MPa and a high elongation of 27.1%. Due to the formation of ultra-fine microstructure and the synergistic effect of dislocations and twins, the strength and plasticity of as-built objects are 96% and 69% higher than the wrought CP-Zr cubes required in ASTM-523, respectively. In this work, a near-net shape method was innovatively proposed for fabricating zirconium and its alloy components. As well, a perspective insight was provided for investigating the strengthening and deformation mechanism of SLM-manufactured CP-Zr.

Effects of laser surface alloying with Cr on microstructure and hardness of commercial purity Zr

Pulsed laser was utilized to treat a commercial purity Zr (CP Zr) sheet coated with Cr by brush plating to achieve Cr alloying on its surface. Microstructural characteristics and hardness of the laser-treated surface were investigated by use of electron channeling contrast imaging, electron backscatter diffraction, energy dispersive spectrometry techniques and microhardness measurement. Results show that after the laser surface alloying (LSA) with Cr, five zones with distinctly different microstructural characteristics are presented from the surface to the matrix: two melted zones (MZs), two solid-state phase transformation zones (SSPTZs) and the unchanged matrix. The MZ-1 and the MZ-2 are featured by equiaxed and columnar structures with internal substructures inside them, respectively. The SSPTZ-1 is fully comprised of martensitic plates transformed from complete β domains while the SSPTZ-2 contains both martensitic plates and bulk α grains due to cooling from α+β domains. The Cr alloying mainly occurs in the two melted zones (MZ-1 and MZ-2) while essentially not in the SSPTZs. Compared with the initial microstructure, many low angle boundaries appear in untransformed bulk α grains in the SSPTZ-2, which is related to thermal stresses induced by the laser surface treatment. Hardness measurements show that the LSA with Cr can effectively increase the surface hardness of the CP Zr, with the maximum value (468 HV) to be ∼2.4 times that of the matrix. Such hardening could be mainly attributed to solid-solution strengthening of Cr in α-Zr and significant grain refinement.

Visiting the Limits between a Highly Strained 1‐Zirconacyclobuta‐2,3‐diene and Chemically Robust Dizirconacyclooctatetraene

The reaction of the allene precursor Li2 (Me3 SiC3 SiMe3 ) with [Cp2 ZrCl2 ] (Cp=cyclopentadienyl) was examined. The selective formation of hitherto unknown linear, allene-bridged dizirconocene complexes [(Cp2 ZrCl)2 {-μ-(Me3 Si)C3 (SiMe3 )-}] and [(Cp2 Zr)2 {-μ-(Me3 Si)C3 (SiMe3 )-}2 ] was observed. Upon σ coordination of the allenediyl unit to {Cp2 Zr}, pyrophoric Li2 (Me3 SiC3 SiMe3 ) is tamed stepwise to yield a surprisingly robust 1,5-dizirconacyclooctatetra-2,3,6,7-ene with cumulated double bonds. This complex is unexpectedly inert against moisture, air, water and acetone. Surprisingly, it degrades under MS conditions to give the highly strained 1-zirconacyclobuta-2,3-diene. All compounds isolated have been fully characterised and the molecular structures are discussed. The stability and reactivity of these complexes are rationalised by DFT computations.

Coupling and Dearomatization of Pyridines at a Transient η2 -Cyclopropene/Bicyclobutane Zirconocene Complex.

This paper reports on stereospecific coupling reactions between an η2 -cyclopropene ligand and pyridine derivatives, which are preferred to alternative C-H bond activation reactions. The dicyclopropylzirconocene complex [Cp2 Zr(c-C3 H5 )2 ] (1) eliminates cyclopropane to generate the η2 -cyclopropene/bicyclobutane intermediate [Cp2 Zr(η2 -c-C3 H4 )] (A). A does not activate any pyridine C-H bonds, but rather pyridine inserts into a Zr-C bond of A, yielding an azazirconacycle with a dearomatized pyridyl group [Cp2 Zr{κ2 -N,C8 -(2-c-C3 H4 )-C5 H5 N}] (2). Kinetic data, isotopelabelling experiments, and DFT calculations indicate that the rate-determining step of this stereospecific reaction is cyclopropane elimination, and that the stability of the intermediate [Cp2 Zr(η2 -c-C3 H4 )(NC5 H5 )] (A-py) governs the selectivity of the reaction. Complex 2 tautomerizes to [Cp2 Zr{κ2 -N,C8 -(2-C3 H5 )-C5 H4 N}] (6) through a base-catalyzed proton migration accompanied by cyclopropyl opening and restoration of conjugation within the zirconacycle.

Reactions of 2-Substituted Pyridines with Titanocenes and Zirconocenes: Coupling versus Dearomatisation.

Reactions of group 4 metallocene sources with 2-substituted pyridines were investigated to evaluate their coordination type between innocent and reductive dearomatisation as well as to probe the possibility for couplings. A dependence on the cyclopentadienyl ligands (Cp, Cp*), the metals (Ti, Zr), and the substrates (2-phenyl-, 2-acetyl-, and 2-iminopyridine) was observed. While 2-phenylpyridine is barely reactive, 2-acetylpyridine reacts vigorously with the Cp-substituted complexes and selectively with their Cp* analogues. With 2-iminopyridine, in all cases selective reactions were observed. In the isolated [Cp2 Ti], [Cp2 Zr], and [Cp*2 Zr] compounds the substrate coordinates by its pyridyl ring and the unsaturated side-chain. Subsequently, the pyridine was dearomatised, which is most pronounced in the [Cp*2 Zr] compounds. Using [Cp*2 Ti] leads to the unexpected paramagnetic complexes [Cp*2 TiIII (N,O-acpy)] and [Cp*2 TiIII (N,N'-impy)]. This highlights the non-innocent character of the pyridyl substrates.

Interaction of the Negishi reagent Cp2ZrBun 2 with 1,4-bis(tert-butyl)butadiyne

The interaction of the Negishi reagent Cp2ZrBun2 with 1,4-bis(tert-butyl)butadiyne ButC≡C-C≡CBut leads to four products: a five-membered zirconacyclocumulene complex Cp2Zr(η4-ButC4But) (2) synthesized earlier by another method, the previously unknown seven-membered zirconacyclocumulene Cp2Zr[η4-ButC4(But)-C(C2But)=CBut] (3) as well as small amounts of the zirconocene binuclear butatrienyl complex Cp2(Bun)Zr(ButC4But)Zr(Bun)Cp2 (4), and the dimeric acetylide [Cp2ZrC≡CBut]2 (5). The structure of complexes 2–5 was established by X-ray diffraction studies.

Zirconium-Catalyzed Carboalumination of α-Olefins and Chain Growth of Aluminum Alkyls: Kinetics and Mechanism

A mechanism based on Michaelis-Menten kinetics with competitive inhibition is proposed for both the Zr-catalyzed carboalumination of α-olefins and the Zr-catalyzed chain growth of aluminum alkyls from ethylene. AlMe(3) binds to the active catalyst in a rapidly maintained equilibrium to form a Zr/Al heterobimetallic, which inhibits polymerization and transfers chains from Zr to Al. The kinetics of both carboalumination and chain growth have been studied when catalyzed by [(EBI)Zr(μ-Me)(2)AlMe(2)][B(C(6)F(5))(4)]. In accord with the proposed mechanism, both reactions are first-order in [olefin] and [catalyst] and inverse first-order in [AlR(3)]. The position of the equilibria between various Zr/Al heterobimetallics and the corresponding zirconium methyl cations has been quantified by use of a Dixon plot, yielding K = 1.1(3) × 10(-4) M, 4.7(5) × 10(-4) M, and 7.6(7) × 10(-4) M at 40 °C in benzene for the catalyst species [rac-(EBI)Zr(μ-Me)(2)AlMe(2)][B(C(6)F(5))(4)], [Cp(2)Zr(μ-Me)(2)AlMe(2)][B(C(6)F(5))(4)], and [Me(2)C(Cp)(2)Zr(μ-Me)(2)AlMe(2)][B(C(6)F(5))(4)] respectively. These equilibrium constants are consistent with the solution behavior observed for the [Cp(2)Zr(μ-Me)(2)AlMe(2)][B(C(6)F(5))(4)] system, where all relevant species are observable by (1)H NMR. Alternative mechanisms for the Zr-catalyzed carboalumination of olefins involving singly bridged Zr/Al adducts have been discounted on the basis of kinetics and/or (1)H NMR EXSY experiments.

General route for the synthesis of terminal phosphanylphosphido complexes of Zr(IV) and Hf(IV): Structural investigations of the first zirconium complex with a phosphanylphosphido ligand

Reactions of R 2P–P(SiMe 3)Li with [Cp 2MCl 2] (M = Zr, Hf) in hydrocarbons yield the related terminal phosphanylphosphido complexes [Cp 2M(Cl){(Me 3Si)P–PR 2-κ P 1}] (R = i Pr and t Bu). The solid state structures of [Cp 2M(Cl){P(SiMe 3)–P iPr 2-κ P 1}] (M = Zr, Hf) were established by single crystal X-ray diffraction studies. The phosphido-P atoms adopt almost planar geometries and the phosphanyl P atoms adopt pyramidal geometries. The reaction of a mixture of (Me 3Si) 2PLi and Ph 2P–P(SiMe 3)Li with [CpZrCl 3] in toluene yields the dinuclear complex [Cp 2Zr 2Cl 5(Ph 2PPPSiMe 3)(Li THF DME)].

Unusual bond activation processes in the reaction of group 4 cyclopentadienyl alkyne complexes with azobenzene

The alkyne complexes [Cp′ 2M(L)( η 2-Me 3SiC 2SiMe 3)] (Cp′ = substituted or unsubstituted cyclopentadienyl; M = Ti, Zr, Hf; L = Py, THF) can serve as metallocene precursors by substitution of the alkyne molecule with other ligands. The reactions of the unsubstituted cyclopentadienyl complexes [Cp 2Zr(THF)( η 2-Me 3SiC 2SiMe 3)] ( 1) and [Cp 2Ti( η 2-Me 3SiC 2SiMe 3)] ( 2) with azobenzene were investigated. In the first case the diazene complex [Cp 2Zr(THF)(N 2Ph 2)] ( 3) was obtained by alkyne exchange. In the reaction of the titanium complex 2 a N N bond cleavage of azobenzene and a C–H activation of the cyclopentadienyl ligand were observed and the dinuclear imido bridged compound 4 was formed. This mixed valence complex is bridged additionally by a cyclopentadienyl ligand in a η 1: η 5-coordination mode which is very unusual for titanium complexes. The molecular structures of both compounds were confirmed by X-ray crystallography and compared to former structural data shown in literature.

Selective synthesis of α-methylenyl zirconacyclopentene via cross-coupling of alkyne and allene

α-Methylenyl zirconacyclopentenes are synthesized regio- and stereoselectively via reductive intermolecular cross-coupling of alkynes and allenes promoted by zirconocene species ‘Cp 2Zr’. An interesting reductive intramolecular coupling of the α-methylenyl zirconacyclopentene has been observed in the presence of DMAD/CuCl, resulting in the generation of cyclobutene with an exocyclic double bond. Polysubstituted 1,4-dienes can be given with high selectivity and good yields.

Preparation of organozirconium aromatic compounds in mixed aqueous-organic solvents: X-ray structures of [Cp 2ZrCl(μ 2-O′,O′′C-C 6H 5)], [(CpZr) 3(μ 2-O′,O′′C-C 6H 3Cl 2) 3(μ 3-OH)(μ 2-OH) 3](Cl 2C 6H 3COO) 2, and [(CpZr) 3(μ 2-O′,O′′C-C 6H 4Cl) 3(μ 3-OH)(μ 2-OH) 3]Cl 2·CH 2Cl 2

A new and facile method is presented for the synthesis of zirconocene carboxylate compounds, in which zirconocene dichloride (Cp 2ZrCl 2) is dissolved in 1 M aqueous HCl solution and the requisite ligand is dissolved in an organic solvent. Five such compounds [Cp 2ZrCl(μ 2-O′,O′′C-C 6H 5)] ( 1), [Cp 2ZrCl(μ 2-O′,O′′C-C 6H 3Cl 2)] ( 2), [Cp 2Zr(μ 2-O′,O′′C-C 6H 3(OH)Cl) 2] ( 3), [Cp 2Zr(μ 2-O′,O′′C-C 6H 3(OH)(NO 2)) 2] ( 4), and [Cp 2Zr(μ 2-O′,O′′C-C 6H(OH)Cl 3) 2] ( 5) have been obtained by this method. The effect of pH on the stability of Cp 2ZrCl 2 in 1 M HCl solution has been investigated by UV/vis spectrophotometry and 1H NMR spectrometry. The results showed that the aqueous Cp 2ZrCl 2 solutions became less stable with increasing pH, liberating cyclopentadiene. Accordingly, at higher pH (∼7), two trinuclear zirconium monocyclopentadienyl compounds, [(CpZr) 3(μ 2-O′,O′′C-C 6H 3Cl 2) 3(μ 3-OH)(μ 2-OH) 3](Cl 2C 6H 3COO) 2 ( 6) and [(CpZr) 3(μ 2-O′,O′′C-C 6H 4Cl) 3(μ 3-OH)(μ 2-OH) 3]Cl 2·CH 2Cl 2 ( 7), were obtained. All compounds 1– 7 have been characterized by FT-IR, 1H NMR spectra and elemental analysis. In all of the compounds, the aromatic acid acts as a bidentate ligand in coordinating to the zirconium; both chelating and bridging modes are observed. X-ray crystallographic studies on 1, 6, and 7 have revealed that the geometries at zirconium are distorted octahedral in 6 and 7, and distorted trigonal-bipyramidal in 1.

Synthetic studies towards a new scaffold, spirobicycloimidazoline

A new scaffold consisting of a carbocycle and a substituted imidazoline in an orthogonal arrangement was synthesized as a potential specific inhibitor of glycosidases. The spirobicycloimidazoline, (5 R,6 R,7 R,8 R)-8-(hydroxymethyl)-2-phenyl-1,3-diazaspiro[4.4]non-1-ene-6,7-diol, was synthesized from methyl 2- O- p-methoxybenzyl-3,4-di- O-benzyl-α/β- d- gluco-6-enopyranoside via (1 R,2 S,3 S,4 R,5 S)-3,4-bis(benzyloxy)-2-(4-methoxybenzyloxy)-5-vinyl-cyclopentanol. The ring contraction of the 6-enopyranoside in the presence of zirconocene equivalent (‘Cp 2Zr’) reagent gave exclusively the corresponding cyclopentanol without cleavage of the PMB protecting group. In the course of the study, a new α-mannosidase inhibitor, (1 R,2 R,3 R,5 R)-5-amino-3-hydroxymethyl-cyclopentane-1,2-diol, was also discovered.

Remarkable Lewis acid effects on polymerization of functionalized alkenes by metallocene and lithium ester enolates

Drastic effects of Lewis acids E(C 6F 5) 3 (E = Al, B) on polymerization of functionalized alkenes such as methyl methacrylate (MMA) and N, N-dimethyl acrylamide (DMAA) mediated by metallocene and lithium ester enolates, Cp 2Zr[OC(O i Pr) CMe 2] 2 ( 1) and Me 2C C(O i Pr)OLi, are documented as well as elucidated. In the case of metallocene bis(ester enolate) 1, when combined with 2 equiv. of Al(C 6F 5) 3, it effects highly active ion-pairing polymerization of MMA and DMAA; the living nature of this polymerization system allows for the synthesis of well-defined diblock and triblock copolymers of MMA with longer-chain alkyl methacrylates. In sharp contrast, the 1/2B(C 6F 5) 3 combination exhibits low to negligible polymerization activity due to the formation of ineffective adduct Cp 2Zr[OC(O i Pr) CMe 2] +[O C(O i Pr)CMe 2B(C 6F 5) 3] − ( 2). Such a profound Al vs. B Lewis acid effect has also been observed for the lithium ester enolate; while the Me 2C C(O i Pr)OLi/2Al(C 6F 5) 3 system is highly active for MMA polymerization, the seemingly analogous Me 2C C(O i Pr)OLi/2B(C 6F 5) 3 system is inactive. Structure analyses of the resulting lithium enolaluminate and enolborate adducts, Li +[Me 2C C(O i Pr)OAl(C 6F 5) 3] − ( 3) and Li +[Me 2C C(O i Pr)OB(C 6F 5) 3] − ( 4), coupled with polymerization studies, show that the remarkable differences observed for Al vs. B are due to the inability of the lithium enolborate/borane pair to effect the bimolecular, activated-monomer anionic polymerization as does the lithium enolaluminate/alane pair.

Stereoselective synthesis of unsymmetrical conjugated dienes and trienes utilizing silacyclobutenes

Treatment of the different kinds of alkynyl-substituted dialkynyldiarylsilanes with zirconocene–ethylene complex Cp 2Zr(CH 2 CH 2) followed by acidification with 3 N HCl gave regio- and stereoselectively the corresponding silacyclobutenes in good yields. Desilylation of the silacyclobutenes with tetrabutylammonium fluoride afforded stereoselectively unsymmetrical conjugated (1 E,3 E)-dienes and -trienes (R 1 or R 2 = 1-cyclohexenyl) in excellent yields.

Effect of solvent on reactions of Cp 2Zr{(μ-H) 2BHR} 2 and Cp 2ZrH{(μ-H) 2BHR} (R = CH 3, Ph) with B(C 6F 5) 3

The effect that a solvent has on reactions of Cp 2Zr{(μ-H) 2BHR} 2 and Cp 2ZrH{(μ-H) 2BHR} (R = CH 3, Ph) with B(C 6F 5) 3 has been studied. From the reaction in benzene the metathesis product Cp 2Zr{(μ-H) 2B(C 6F 5) 2} 2, 2, was isolated. In the case of diethyl ether, different hydride abstraction products, including [Cp 2Zr(OEt 2){(μ-H) 2BHPh}][HB(C 6F 5) 3], 3, [Cp 2Zr(OEt 2){(μ-H) 2BHCH 3}][HB(C 6F 5) 3], 4, [Cp 2Zr(OEt 2){(μ-H) 2BH 2}][HB(C 6F 5) 3], 5, and [Cp 2Zr(OEt)(OEt 2)][HB(C 6F 5) 3], 6, were isolated depending on the starting zirconocene complex. The diethyl ether molecules of 3– 6 are weakly coordinated to Zr and displaced in THF solution. Isolation of 3 and 4 is attributed to their fast precipitation from the reaction mixture, which prevented further reactions from occurring. In addition to the hydride abstraction, a hydride metathesis was also involved in the formation of 5. Time-elapsed 11B NMR studies indicate that 3 and 4 are the intermediates on the pathway to 5 and 6. The molecular structures of 2– 6 were determined by single-crystal X-ray diffraction.

Generation and reaction of heteroaromatic zirconocene: synthetic application to polycyclic heterocycles

Heteroaromatic zirconocene intermediates were generated by the reaction of ‘Cp 2Zr’ with alkoxymethyl-(TMS-ethynyl)-indole, -benzofuran or -benzothiophene derivatives under mild conditions in moderate to good yields. Copper-catalyzed C–C bond formation of the zirconocene intermediate with allylic halides gave allylation products, which were transformed into heterocyclic dienes through enyne metathesis. Preliminary Diels–Alder reaction of the dienes with DMAD showed notable site selectivity.

Synthesis, structure and properties of the Cp 2Zr{CH(SiMe 3) 2} + cation

The generation and properties of the Cp 2Zr{CH(SiMe 3) 2} + cation are described. An X-ray crystallographic analysis shows that the carborane salt [Cp 2Zr{CH(SiMe 3) 2}][HCB 11Me 5Br 6] contains an agostic Zr–μ-Me–Si interaction in the solid state. Low temperature NMR spectra of the borate salt [Cp 2Zr{CH(SiMe 3) 2}][B(C 6F 5) 4] show that this interaction is retained in solution. Variable temperature NMR spectra establish that the SiMe 2(μ-Me) and unbound SiMe 3 units of Cp 2Zr{CH(SiMe 3) 2} + exchange by a “pivot” process involving partial rotation around the Zr–CH(SiMe 3) 2 bond, with a barrier of Δ G ‡ = 9.2(1) kcal/mol at −89 °C. Cp 2Zr{CH(SiMe 3) 2} + does not coordinate alkenes or alkynes.