Degree Type

Dissertation

Date of Award

2013

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

Aaron D. Sadow

Abstract

A series of organometallic compounds containing the tris(dimethylsilyl)methyl ligand are described. The potassium carbanions KC(SiHMe2)3 and KC(SiHMe2)3TMEDA are synthesized by deprotonation of the hydrocarbon HC(SiHMe2)3 with potassium benzyl. KC(SiHMe2)3TMEDA crystallizes as a dimer with two types of three-center-two-electron K-H-Si interactions. Homoleptic Ln(III) tris(silylalkyl) complexes containing β-SiH groups M{C(SiHMe2)3}3 (Ln = Y, Lu, La) are synthesized from salt elimination of the corresponding lanthanide halide and 3 equiv. of KC(SiHMe2)3. The related reactions with Sc yield bis(silylalkyl) ate-complexes containing either LiCl or KCl. The divalent calcium and ytterbium compounds M{C(SiHMe2)3}2L (M = Ca, Yb; L = THF2 or TMEDA) are prepared from MI2 and 2 equiv of KC(SiHMe2)3. The compounds M{C(SiHMe2)3}2L (M = Ca, Yb; L = THF2 or TMEDA) and La{C(SiHMe2)3}3 react with 1 equiv of B(C6F5)3 to give 1,3-disilacyclobutane {Me2Si-C(SiHMe2)2}2 and MC(SiHMe2)3HB(C6F5)3L, and La{C(SiHMe2)3}2HB(C6F5)3, respectively. The corresponding reactions of Ln{C(SiHMe2)3}3 (Ln = Y, Lu) give the β-SiH abstraction product [{(Me2HSi)3C}2LnC(SiHMe2)2SiMe2][HB(C6F5)3] (Ln = Y, Lu), but the silene remains associated with the Y or Lu center. The abstraction reactions of M{C(SiHMe2)3}2L (M = Ca, Yb; L = THF2 or TMEDA) and Ln{C(SiHMe2)3}3 (Ln = Y, Lu, La) and 2 equiv of B(C6F5)3 give the expected dicationic M{HB(C6F5)3}2L (M = Ca, Yb; L = THF2 or TMEDA) and dicationic mono(silylalkyl) LnC(SiHMe2)3{HB(C6F5)3}2 (Ln = Y, Lu, La), respectively.

Salt metathesis reactions of Cp2(NR2)ZrX (X = Cl, I, OTf; R = t-Bu, SiHMe2) and lithium hydrosilazide ultimately afford hydride products Cp2(NR2)ZrH that suggest unusual β-hydrogen elimination processes. A likely intermediate in one of these reactions, Cp2Zr[N(SiHMe2)t-Bu][N(SiHMe2)2], is isolated under controlled synthetic conditions. Addition of alkali metal salts to this zirconium hydrosilazide compound produces the corresponding zirconium hydride. However as conditions are varied, a number of other pathways are also accessible, including C-H/Si-H dehydrocoupling, γ-abstraction of a CH, and β-abstraction of a SiH. Our observations suggest that the conversion of (hydrosilazido)zirconocene to zirconium hydride does not follow the classical four-center β-elimination mechanism.

Elimination and abstraction reactions dominate the chemistry of ligands containing β-hydrogen. In contrast, Cp2Zr{N(SiHMe2)2}H and Cp2Zr{N(SiHMe2)2}Me undergo selective γ-CH bond activation to yield the azasilazirconacycle Cp2Zr{κ2-N(SiHMe2)SiHMeCH2}, even though there are reactive β-hydrogen available for abstraction. The β-SiH groups in metallacycle provide access to new pathways for sixteen-electron zirconium alkyl compounds, in which Cp2Zr{κ2-N(SiHMe2)SiHMeCH2} undergoes a rare σ-bond metathesis reaction with ethylene. The resulting vinyl intermediate undergoes β-hydrogen abstraction to reform ethylene and a silanimine zirconium species that reacts with ethylene to give a metallacyclopentane as the isolated product. The pendent β-SiH in metallocycle also reacts with paraformaldehyde through an uncatalyzed hydrosilylation to form an exocyclic methoxysilyl moiety, while the zirconium-carbon bond in metallocycle is surprisingly inert toward formaldehyde. Still, the Zr-C moiety in metallocycle is available for chemistry, and it interacts with the carbon monoxide and strong electrophile B(C6F5)3 to provide Cp2Zr[κ2-OC(=CH2)SiMeHN(SiHMe2)] and Cp2Zr[N(SiHMe2)SiHMeCH2B(C6F5)3]. Finally, the frustrated Lewis-pair 2,6-lutidine-B(C6F5)3 adduct reacts with the intra-cyclic SiH to give a transient 2,6-lutidine-stabilized silicon cation [Cp2ZrCH2SiMe(2,6-Me2 -NC6H3)N(SiMe2H)][HB(C6F5)3] that slowly rearranges to give Cp2Zr[N(SiHMe2)SiHMeCH2B(C6F5)3] and free 2,6-lutidine.

Finally, we also demonstrated a β-elimination of a cationic zirconocene disilazide compound [Cp2ZrN(SiHMe2)2]+ that is facilitated by DMAP (4-N,N-dimethylaminopyridine) to give [Cp2ZrH{N(SiHMe2)(SiMe2DMAP)}]+. A formal insertion reaction of a Zr-R group of Cp2ZrN(SiHMe2)2R (R = H, alkyl, halide, alkoxide) into a silaimine, formed by reaction of the zirconocene silazide and B(C6F5)3, to give [Cp2Zr{N(SiHMe2)(SiRMe2)]+. Thus, we also show the application of the β-elimination reaction in hydrosilylation of ketones and aldehydes.

Copyright Owner

KaKing Yan

Language

en

File Format

application/pdf

File Size

359 pages

Included in

Chemistry Commons

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