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4 results on '"Hannah E. Toomey"'

1. 1,2-Addition versus σ-Bond Metathesis Reactions in Transient Bis(cyclopentadienyl)zirconium Imides: Evidence for a d0 Dihydrogen Complex

Author

Doris Pun, Hannah E. Toomey, Paul J. Chirik, and Luis F. Veiros

Subjects
Inorganic Chemistry, Zirconium, Cyclopentadienyl complex, chemistry, Hydride, Organic Chemistry, Salt metathesis reaction, chemistry.chemical_element, Dihydrogen complex, Physical and Theoretical Chemistry, Photochemistry, Medicinal chemistry
Abstract

Exposure of a series of zirconocene amido and hydrazido hydride complexes, (η5-C5Me4H)2Zr(NHR)H (R = tBu, NMe2, Me, H), to 4 atm of D2 gas at 56 °C produced isotopic exchange in both the N−H and Zr...

Published
2008

2. Addition of methyl triflate to a hafnocene dinitrogen complex: stepwise n(2) methylation and conversion to a hafnocene hydrazonato compound

Author

Emil B. Lobkovsky, David Benito-Garagorri, Wesley H. Bernskoetter, Hannah E. Toomey, Ivan Keresztes, Paul J. Chirik, and Donald J. Knobloch

Subjects
Methyl triflate, Colloid and Surface Chemistry, Stereochemistry, Ligand, Chemistry, General Chemistry, Methylation, Biochemistry, Catalysis
Abstract

Treatment of the hafnocene complex bearing a strongly activated, side-on bound dinitrogen ligand, [(eta(5)-C(5)Me(4)H)(2)Hf](2)(mu(2),eta(2),eta(2)-N(2)), with two equivalents of methyl triflate yielded a mixture of products, one of which was identified as the triflato hafnocene methyl diazenide compound, (eta(5)-C(5)Me(4)H)(2)Hf(OTf)(N(2)(CH(3))), arising from methylation of one of the nitrogen atoms. This reactivity contrasts with that of the zirconocene congener, [(eta(5)-C(5)Me(4)H)(2)Zr](2)(mu(2),eta(2),eta(2)-N(2)), where methyl triflate addition yields a variety of products that lack new nitrogen-carbon bonds. The methylated hafnocene product, (eta(5)-C(5)Me(4)H)(2)Hf(OTf)(N(2)(CH(3))) provides a platform for additional transformations for the functionalized dinitrogen core. Treatment with additional methyl triflate results in a second nitrogen-carbon bond formation to yield a rare example of a triflato hafnocene hydrazonato complex. Loss of methane and formation of the hafnocene bis(triflate) accompany the transformation. Isotopic labeling studies and other experiments are consistent with a pathway involving initial methylation of the unsubstituted nitrogen in the methyl diazenido ligand followed by deprotonation by a triflate anion.

Published
2009

3. Carboxylation of an ansa-zirconocene dinitrogen complex: regiospecific hydrazine synthesis from N2 and CO2

Author

Paul J. Chirik, Hannah E. Toomey, and Donald J. Knobloch

Subjects
Steric effects, Nitrogen, Dimer, Hydrazine, Carboxylic Acids, Molecular Conformation, chemistry.chemical_element, Ligands, Biochemistry, Medicinal chemistry, Catalysis, chemistry.chemical_compound, Colloid and Surface Chemistry, Organometallic Compounds, Organic chemistry, Zirconium, Ligand, Regioselectivity, Stereoisomerism, General Chemistry, Carbon Dioxide, Hydrazines, Carboxylation, chemistry
Abstract

Addition of 2 equiv of carbon dioxide to the ansa-zirconocene dinitrogen complex resulted in selective insertion into each zirconium nitrogen bond, forming a C2 symmetric dicarboxylated diazenido compound. Treatment with excess Me3SiI furnished the ansa-zirconocene diiodide along with the N,N‘-dicarboxylated silylated hydrazine. New nitrogen−carbon bonds were also assembled by addition of methyl triflate. Tri- and tetrasubstituted hydrazines could be formed by treatment with water and Me3SiI, respectively. The regiochemistry of the N2 carboxylation is controlled by the ansa-cyclopentadienyl ligand where the sterically demanding tert-butyl substituents and the C2 symmetry of the dimer dictates the stereochemistry of CO2 insertion. These results demonstrate the ability of zirconium dinitrogen compounds to participate in heterocumulene insertion chemistry.

Published
2008

4. 1,2-Addition versus σ-Bond Metathesis Reactions in Transient Bis(cyclopentadienyl)zirconium Imides: Evidence for a d0Dihydrogen Complex.

Author

Hannah E. Toomey, Doris Pun, Paul J. Chirik, and Luis F. Veiros

Subjects
*METATHESIS reactions, *ISOMERIZATION, *ELIMINATION (Mathematics), *CHEMICAL reactions
Abstract

Exposure of a series of zirconocene amido and hydrazido hydride complexes, (η 5-C 5Me 4H) 2Zr(NHR)H (R = tBu, NMe 2, Me, H), to 4 atm of D 2gas at 56 °C produced isotopic exchange in both the N−H and Zr−H positions. In general, the relative rates of 1,2-elimination can be rationalized on the basis of ground-state effects, whereby amido compounds with the strongest N−H bonds, as judged by the corresponding free amine, undergo the slowest isotopic exchange. For the compound with the strongest N−H bond in the series, (η 5-C 5Me 4H) 2Zr(NH 2)H, the barrier for 1,2-elimination is sufficiently high such that σ-bond metathesis becomes the dominant intermolecular exchange pathway. For the other amido zirconocene hydrides, the rate constants for deuterium exchange into the N−H position are faster than for the Zr−H position. This behavior is a result of a faster intramolecular isomerization process driven by an equilibrium isotope effect favoring N−D over Zr−D bond formation. Computational studies on a related model compound, (η 5-C 5H 5) 2Zr(NH tBu)H, successfully reproduce these observations and support a pathway involving the formation of rare d 0dihydrogen complexes. [ABSTRACT FROM AUTHOR]

Published
2008
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