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PubMed1997

Mechanism of the Formation of Organic Derivatives of gamma-Zirconium Phosphate by Topotactic Reactions with Phosphonic Acids in Water and Water-Acetone Media.

The rates of the topotactic reactions between gamma-zirconium phosphate and phenylphosphonic acid in water and water-acetone mixtures at various temperatures were investigated. The slow rates of the process in aqueous medium or in water-acetone mixtures at temperatures lower than 50 degrees C were attributed to a slow interdiffusion of O(2)P(OH)(2)(-) and O(2)P(OH)(C(6)H(5))(-) groups in the interlayer region of gamma-ZrP. Similar to ion-exchange processes, the replacement begins in the external part of the interlayer region and progresses toward the central region with the formation of an advancing phase boundary. In water-acetone mixtures at temperatures higher than 60 degrees C an exfoliation of gamma-ZrP was found. Thus, the initial process is very fast since the substitution can take place directly on the surface of the exfoliated gamma-lamellae. However, after a certain degree of substitution, a flocculation of the colloidal dispersion, which slows down the rate of the further topotactic substitution, was observed. Some considerations on the topotactic substitution occurring on the surface of the exfoliated lamellae and on the mechanism of the diffusion of the exchanging species in the interlayer region are also reported.

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PubMed1998

Synthesis and Structure of a Zirconium Dinitrogen Complex with a Side-On Bridging N(2) Unit.

Reduction of Zr(O-2,6-Me(2)C(6)H(3))Cl(2)[N(SiMe(2)CH(2)PPr(i)(2))(2)] with sodium amalgam under dinitrogen yields the dinuclear zirconium dinitrogen complex {[(Pr(i)(2)PCH(2)SiMe(2))(2)N]Zr(O-2,6-Me(2)C(6)H(3))}(2)(&mgr;-eta(2):eta(2)-N(2)). Solid state structural analysis shows that the dinitrogen unit is bound in a side-on mode of coordination with the N-N bond distance at 1.528(7) Å; resonance Raman spectra show a band at 751 cm(-)(1) for nu(N-N), which is consistent with this very long bond. In addition, the N(2) ligand is hinged slightly so that the Zr(2)(&mgr;-eta(2):eta(2)-N(2)) core adopts a flattened butterfly shape rather than a completely planar core as found in other related systems. Other Zr(IV) precursors of the general formula ZrCl(2)X[N(SiMe(2)CH(2)PPr(i)(2))(2)] (X = OBu(t), OCHPh(2), NPh(2)) either decompose upon reduction under N(2) or produce mixtures of products.

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PubMed1999

Electrochemistry of Centered Hexanuclear Zirconium Halide Clusters in Ambient-Temperature Chloroaluminate Molten Salts.

Ambient temperature AlCl(3)-1-ethyl-3-methylimidazolium chloride (ImCl) molten salts, both basic (40/60 mol % AlCl(3)/ImCl) and acidic (60/40 mol % AlCl(3)/ImCl), were used in an electrochemical investigation of centered hexanuclear zirconium halide clusters. In the basic molten salt, these [(Zr(6)ZCl(12))Cl(6)](n-) (Z = Be, B, C, Mn, Fe) centered clusters exhibit the following electrochemical reactions on a glassy carbon electrode (potentials vs Al/Al(3+)): [(Zr(6)BeCl(12))Cl(6)](n)()(-) + e(-) right harpoon over left harpoon [(Zr(6)BeCl(12))Cl(6)](()(n)()(+1))(-), E(1/2) = -0.613 V (n = 4), E(1/2) = -1.085 V (n = 5); [(Zr(6)BCl(12))Cl(6)](n)()(-) + e(-) right harpoon over left harpoon [(Zr(6)BCl(12))Cl(6)](()(n)()(+1))(-), E(1/2) = -0.365 V (n = 4), E(1/2) = 0.072 V (n = 3); [(Zr(6)CCl(12))Cl(6)](n)()(-) + e(-) right harpoon over left harpoon [(Zr(6)CCl(12))Cl(6)](()(n)()(+1))(-), E(1/2) = 0.230 V (n = 3); [(Zr(6)MnCl(12))Cl(6)](4)(-) + e(-) right harpoon over left harpoon [(Zr(6)MnCl(12))Cl(6)](5)(-), E(1/2) = -0.432 V. In the acidic melt, only electrochemical reactions [(Zr(6)BeCl(12))(AlCl(4))(6)](n)()(-) + e(-) right harpoon over left harpoon [(Zr(6)BeCl(12))(AlCl(4))(6)](()(n)()(+1))(-), E(1/2) = -0.069 V (n = 5), E(1/2) = 0.504 V (n = 4), and [(Zr(6)BCl(12))(AlCl(4))(6)](4)(-) + e(-) right harpoon over left harpoon [(Zr(6)BCl(12))(AlCl(4))(6)](5)(-), E(1/2) = 0.700 V are clearly observed. These data is consistent with less systematic observations of oxidation of these clusters in solution. No unambiguous one-electron electrochemical reduction nor oxidation is observable for the Fe-centered cluster in the ionic liquids. The half-wave potentials of the above reactions of the Be-, B-, C-, and Mn-centered clusters are controlled largely by clusters' charges. This correlation of redox potentials allows a useful direct comparison with data for related hexanuclear niobium clusters in the literature.

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PubMed2000

Metal alkoxides as versatile precursors for group 4 phosphonates: synthesis and X-ray structure of a novel organosoluble zirconium phosphonate.

Reactions of Ph2P(O)(OH) and t-BuP(O)(OSiMe3)(OH) with Ti(O-i-Pr)4 in equimolar ratios gave titanium phosphonates of the type [(O-i-Pr)3Ti(mu-O)2PR1R2]2 (1, R1 = R2 = Ph; 2, R1 = t-Bu, R2 = OSiMe3) as colorless crystalline solids in moderate yields. Reactions of Ph2P(O)(OH) and the isopropoxides of zirconium and hafnium resulted in products of the composition [(O-i-Pr)3M(mu-O-i-Pr)2(mu-OPOPh2)M(O-i-Pr)2]Ph2P(O)(OH) (M = Zr (3), Hf (4)) in high yields. The compounds were characterized by 1H, 31P, and 29Si NMR, infrared (IR), and mass spectroscopic (MS) techniques. The molecular structures of 2 and 3 were confirmed by X-ray crystallography.

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PubMed2000

Addition and metathesis reactions of zirconium and hafnium imido complexes.

The zirconium and hafnium imido metalloporphyrin complexes (TTP)M = NArtPr (TTP = meso-5,10,15,20-tetra-p-tolylporphyrinato dianion; M = Zr (1), Hf; AriPr = 2,6-diisopropylphenyl) were used to mediate addition reactions of carbonyl species and metathesis of nitroso compounds. The imido complexes react in a stepwise manner in the presence of 2 equiv of pinacolone to form the enediolate products (TTP)M[OC(tBu)CHC(tBu)(Me)O] (M = Zr (2), Hf (3)), with elimination of H2NAriPr. The bis(mu-oxo) complex [(TTP)ZrO]2 (4) is formed upon reaction of (TTP)Zr = NAriPr with PhNO. Treatment of compound 4 with water or treatment of compound 2 with acetone produced the (mu-oxo)bis(mu-hydroxo)-bridged dimer [(TTP)Zr]2(mu-O)(mu-OH)2 (5). Compounds 2, 4, and 5 were structurally characterized by single-crystal X-ray diffraction.

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PubMed2000

Chloroaluminate ionic liquids as reagents for isolating soluble hexanuclear zirconium halide cluster compounds.

Ambient-temperature chloroaluminate molten salts, mixtures of aluminum trichloride (AlCl3) and 1-ethyl-3-methylimidazolium chloride (ImCl), have been used as solvents to excise and isolate centered hexanuclear zirconium halide clusters from their solid-state precursors. Cluster compounds synthesized via high-temperature reactions, KZr6CCl15 and Li2Zr6MnCl15, were dissolved into basic molten salts at 100-110 degrees C. The C-centered cluster compound, Im4Zr6CCl18, was isolated in 70% yield, and the Mn-centered cluster compound, Im5Zr6MnCl18.C7H(8).2CH3CN, was isolated in 54% yield. Im5Zr6BCl18 is efficiently oxidized by ferrocenium tetrafluoroborate, and one-electron-oxidized B-centered cluster, [(Zr6B)Cl18]4-, was isolated in 90% yield as the salt Im4Zr6BCl18.

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PubMed2001

Supported metallocene catalysts by surface organometallic chemistry. Synthesis, characterization, and reactivity in ethylene polymerization of oxide-supported mono- and biscyclopentadienyl zirconium alkyl complexes: establishment of structure/reactivity relationships.

The reactions of CpZr(CH(3))(3), 1, and Cp(2)Zr(CH(3))(2), 2, with partially dehydroxylated silica, silica-alumina, and alumina surfaces have been carried out with careful identification of the resulting surface organometallic complexes in order to probe the relationship between catalyst structure and polymerization activity. The characterization of the supported complexes has been achieved in most cases by in situ infrared spectroscopy, surface microanalysis, qualitative and quantitative analysis of evolved gases during surface reactions with labeled surface, solid state (1)H and (13)C NMR using (13)C-enriched compounds, and EXAFS. 1 and 2 react with silica(500) and silica-alumina(500) by simple protonolysis of one Zr-Me bond by surface silanols with formation of a single well-defined neutral compound. In the case of silica-alumina, a fraction of the supported complexes exhibits some interactions with electronically unsaturated surface aluminum sites. 1 and 2 also react with the hydroxyl groups of gamma-alumina(500), leading to several surface structures. Correlation between EXAFS and (13)C NMR data suggests, in short, two main surface structures having different environments for the methyl group: [Al](3)-OZrCp(CH(3))(2) and [Al](2)-OZrCp(CH(3))(mu-CH(3))-[Al] for the monoCp series and [Al](2)-OZrCp(2)(CH(3)) and [Al]-OZrCp(2)(mu-CH(3))-[Al] for the bisCp series. Ethylene polymerization has been carried out with all the supported complexes under various reaction conditions. Silica-supported catalysts in the absence of any cocatalyst exhibited no activity whatsoever for ethylene polymerization. When the oxide contained Lewis acidic sites, the resulting surface species were active. The activity, although improved by the presence of additional cocatalysts, remained very low by comparison with that of the homogeneous metallocene systems. This trend has been interpreted on the basis of various possible parameters, including the (p-pi)-(d-pi) back-donation of surface oxygen atoms to the zirconium center.

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PubMed2002

Monolayers as models for supported catalysts: zirconium phosphonate films containing manganese(III) porphyrins.

Organized monolayer films of a manganese tetraphenylporphyrin have been prepared and used as supported oxidation catalysts. Manganese 5,10,15,20-tetrakis(tetrafluorophenyl-4'-octadecyloxyphosphonic acid) porphyrin (1) has been immobilized as a monolayer film by a combination of Langmuir-Blodgett (LB) and self-assembled monolayer techniques that use zirconium phosphonate linkages to bind the molecule to the surface. Analysis by FTIR, XPS, UV-vis and polarized optical spectroscopy show that the films consist of noninteracting molecules effectively anchored and oriented nearly parallel to the surface. The monolayer films are stable to the solvent and temperature conditions needed to explore organic oxidations. The activity of films of 1 toward the epoxidation of cyclooctene using iodosylbenzene as the oxidant was compared to that of Manganese 5,10,15,20-tetrakis(pentafluorophenyl) porphyrin (2) and 1 under equivalent homogeneous conditions. The immobilized porphyrin 1 shows an enhanced activity relative to either homogeneous reaction. The main difference between 1 and 2 is the four alkyl phosphonate arms in 1 designed to incorporate the porphyrin within the films. The increased activity of immobilized 1 is a combination of the porphyrin structure, which prohibits the formation of mu-oxo dimers even in solution, and a change in conformation when anchored to the surface. The study demonstrates that careful monolayer studies can provide useful models for the design and study of supported molecular catalyst systems.

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PubMed2003

Excimer formation in the interlayer region of arene-derivatized zirconium phosphonates.

Photophysical investigations are reported for two arene-derivatized zirconium phosphonates (layered solids). The chromophore groups that are attached within the interlayer region of these materials are seen to form excimer pairs. Whereas the naphthalene-containing system exhibits both monomer and excimer fluorescence, the potentially greater overlap of chromophores for the system containing anthracene-pendant groups causes that compound to only show excimer fluorescence.

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PubMed2004

Nucleophilic character of a charge neutral, high oxidation state d0 zirconium trimethylenemethane complex.

The nucleophilic character of a charge neutral, high oxidation d0 zirconium trimethylenemethane (TMM) class of compound of general structure Cp*Zr(TMM)[N(R1)C(Me)N(R2)], 1a (R1 = R2 = i-Pr) and 1b (R1 = t-Bu, R2 = Et), is presented through documentation of its reactivity with a range of alkyl and silyl halides and triflates, including unactivated ones such as ethyl triflate. These results should contribute to efforts directed toward expanding the synthetic chemist's toolbox of synthetic methods for the construction of complex organic molecules.

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PubMed2004

Zirconium-catalyzed enantioselective [3+2] cycloaddition of hydrazones to olefins leading to optically active pyrazolidine, pyrazoline, and 1,3-diamine derivatives.

Asymmetric [3+2] cycloaddition of hydrazones to external olefins has been successfully conducted in high yields with high enantioselectivities using a chiral zirconium catalyst. These reactions open ways to synthetically and biologically important pyrazoline, pyrazolidine, and 1,3-diamine derivatives. Further, several experiments suggested that the reactions proceeded via concerted pathways.

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PubMed2004

End-group-confined chain walking within a group 4 living polyolefin and well-defined cationic zirconium alkyl complexes for modeling this behavior.

Living polymers derived from the polymerization of 1-butene using the cationic zirconium initiator, {Cp*ZrMe[N(Et)C(Me)-N(tBu)]}[B(C6F5)4] (Cp* = eta5-C5Me5) (1), have been shown to undergo end-group-confined chain walking that is competitive with direct beta-hydride elimination and chain release at -10 degrees C. The well-defined complexes, {Cp*Zr(iBu)[N(Et)C(Me)N(tBu)]}[B(C6F5)4] (2) and {Cp*Zr(2-ethylbutyl)[N(Et)C(Me)N(tBu)]}[B(C6F5)4] (3), were prepared, and each was found to possess a strong beta-hydrogen agostic interaction that is absent in the living polymer. The isotopically single- and double-labeled derivatives, {Cp*Zr(2-d-2-methylpropyl)[N(Et)C(Me)N(tBu)]}[B(C6F5)4] (2') and {Cp*Zr(1-13C-2-d-2-methylpropyl)[N(Et)C(Me)N(tBu)]}[B(C6F5)4] (2' '), were also prepared and found to undergo isotopic label scrambling at 0 degrees C. For 2' ', the observation that after scrambling each deuterium label is located on a 13C-labeled carbon atom is consistent with the Busico mechanism for chain-end epimerization rather than the Resconi mechanism. Decomposition of 3 yielded olefinic products also consistent with chain walking prior to beta-hydride elimination and chain release. Finally, the unexpected decrease in stability of the living polymer relative to that of the model complexes reveals the importance of subtle differences in steric and electronic factors in controlling beta-hydride elimination and chain release.

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PubMed2006

Living Ziegler-Natta polymerization by early transition metals: synthesis and evaluation of cationic zirconium alkyl complexes bearing beta-hydrogens as models for propagating centers.

The synthesis and characterization of a series of cationic zirconium and hafnium complexes with alkyl substituents bearing beta-hydrogens of general formula {(eta(5)-C5Me5)MR[N(Et)C(Me)N(t-Bu)]}[B(C6F5)4] [M = Zr; R = Et, n-Pr, i-Pr, n-Bu, i-Bu, and 2-ethylbutyl (5a-f) and M = Hf; R = i-Bu and t-Bu (6 and 7, respectively)] is described, including several isotopically labeled derivatives. The ability of these complexes to serve as model complexes for the living Ziegler-Natta polymerization of olefins that can be effected using the initiator 2a (R = Me in 5) has been addressed. The results obtained shed additional light on the steric and electronic factors that can contribute to the living character of a Ziegler-Natta polymerization based on an early transition metal initiator.

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PubMed2006

N-C bond formation promoted by a hafnocene dinitrogen complex: comparison of zirconium and hafnium congeners.

Nitrogen-carbon bond formation from coordinated dinitrogen has been observed upon addition of 2 equiv of PhNCO to the hafnocene dinitrogen complex, [(eta5-C5Me4H)2Hf]2(mu2,eta2,eta2-N2). The resulting product most likely arises from initial N=C cycloaddition of the first equivalent of heterocumulene, followed by carbonyl insertion of a second equivalent into the newly formed hafnium-nitrogen bond. The resulting product has considerable hafnium imido character, as evidenced by the metrical parameters determined from the solid-state structure as well as reactivity studies, whereby PhNCO, p-tolyl isocyanate, and t-BuCCH each undergo cycloaddition with the hafnium-nitrogen bond. The origin of the nitrogen-carbon-forming chemistry is likely derived from the reluctance of the hafnocene dinitrogen complex to undergo ligand-induced N2 isomerization, as was observed with the zirconium congener.

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PubMed2006

Diastereomerically-specific zirconium complexes of chiral salan ligands: isospecific polymerization of 1-hexene and 4-methyl-1-pentene and cyclopolymerization of 1,5-hexadiene.

Chiral Salan ligands were found to wrap in a highly diastereoselective manner around zirconium leading to C2-symmetric complexes of predetermined chirality at the metal. These complexes led to active polymerization of higher olefins, their activity and isospecific induction depending on the nature of the phenolate substituents.

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PubMed2000

Zirconium-mediated metathesis of imines: a study of the scope, longevity, and mechanism of a complicated catalytic system.

By kinetically stabilizing imidozirconocene complexes through the use of a sterically demanding ligand, or by generating a more thermodynamically stable resting state with addition of diphenylacetylene, we have developed transition metal-catalyzed imine metathesis reactions that are mechanistically analogous to olefin metathesis reactions catalyzed by metal carbene complexes. When 5 mol % of Cp*Cp(THF)Zr=N(t)Bu is used as the catalyst precursor in the metathesis reaction between PhCH=NPh and p-TolCH=N-p-Tol, a 1:1:1:1 equilibrium mixture with the two mixed imines p-TolCH=NPh and PhCH=N-p-Tol is generated in C(6)D(6) at 105 degrees C. The catalyst was still active after 20 days with an estimated 847 turnovers (t(1/2) 170 m; TON = 1.77 h(-1)). When the azametallacyclobutene Cp(2)Zr(N(Tol)C(Ph)=C(Ph)) is used as the catalyst precursor under similar reaction conditions, a total of 410 turnovers are obtained after 4 days (t(1/2) 170 m; TON = 4.3 h(-1)). An extensive kinetic and equilibrium analysis of the metallacyclobutene-catalyzed metathesis of PhCH=N-p-Tol and p-F-C(6)H(4)CH=N-p-F-C(6)H(4) was carried out by monitoring the concentrations of imines and observable metal-containing intermediates over time. Numerical integration methods were used to fit these data to a detailed mechanism involving coordinatively unsaturated (16-electron) imido complexes as critical intermediates. Examination of the scope of reaction between different organic imines revealed characteristic selectivity that appears to be unique to the zirconium-mediated system. Several zirconocene complexes that could generate the catalytically active "CpCp'Zr=NAr" (Cp' = Cp or Cp*) species in situ were found to be effective agents in the metathetical exchange between different N-aryl imines. N-Alkyl aldimines were found to be completely unreactive toward metathesis with N-aryl aldimines, and metathesis reactions involving the two N-alkyl imines TolCH=NPr and PhCH=NMe gave slow or erratic results, depending on the catalyst used. Metathesis was observed between N-aryl ketimines and N-aryl aldimines, but for N-aryl ketimine substrates, the catalyst resting state consists of zirconocene enamido complexes, generated by the formal C-H activation of the alpha position of the ketimine substrates.

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