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Synthesis, Structure, and Reactivity of Hydride and Phosphide Complexes of Hafnium and Zirconium

Citation

Roddick, Dean Michael (1984) Synthesis, Structure, and Reactivity of Hydride and Phosphide Complexes of Hafnium and Zirconium. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/wbgr-yd42. https://resolver.caltech.edu/CaltechTHESIS:10222018-111603289

Abstract

A series of alkyl hydride complexes Cp*2Hf(H)R (R=Et, CH2CHMe2, CH2CH2Ph; Cp* = η-C5Me5) have been prepared by reaction of Cp*2HfH2 with the appropriate olefin. The reactions of these compounds with H2 and C2H4 follow those of the previously reported zirconium isobutyl hydride, Cp*2Zr-(H)CH2CHMe2, giving Cp*2HfH2 and metallacyclopentane Cp*2Hf-CH2CH2CH2CH2, respectively, plus one equivalent alkane. All alkyl hydrides examined exhibit high thermal stability and decompose only slowly at 80°C. The thermolysis of Cp*2Hf(H)CH2CHMe2 yields a 1:1 mixture of Cp*2HfH2 and a metallacyclobutane complex Cp*2HfCH2CH(Me)CH2. A proposed mechanism for the formation of Cp*2HfCH2CH(Me)CH2 is given, involving initial γ-H abstraction from the isobutyl methyls of Cp*2Hf(H)CH2CHMe2. Cp*2HfH2 also reacts cleanly with t-butylacetylene to form a remarkably stable alkenyl hydride complex, Cp*2Hf(H)CH=CHtBu. The reactivity of Cp*2Hf(H)CH=CHtBu with H2 and C2H4 is similar to that observed for the related alkyl hydrides. The hydride complexes Cp*2Hf(H)Ph and Cp*2Hf-(H)CH2CMe3 have been conveniently prepared by the metathesis of Cp*2HfH2 with PhLi and LiCH2CMe3, respectively.

Cp*2HfH2 reacts cleanly with allene to form the π-allyl hydride Cp*2Hf(H)(η3-CH2CHCH2). 1H and 13C NMR data as well as labeling studies indicate that Cp*2Hf(H)(η3-CH2CHCH2) is highly fluxional, and isostructural to Cp*2Zr(H)(η3-CH2CHCH2). The crystal structure of Cp*2Hf(H)(η3-CH2CHCH2) is presented. The coordination of the allyl ligand is confirmed to be trihapto, with notably asymmetric Hf-C(allyl) distances (2.38, 2.48, 2.57 A). The final R index is 0.049. The hydride ligand has been reliably located, and represents the first structurally characterized example of a terminal Hf-H bond.

Cp*2Zr(H)CH2CHMe2 has been shown to react cleanly with CO to form the enolate hydride Cp*2Zr(H)OCH=CHCHMe2. A proposed mechanism for formation of Cp*2Zr(H)OCH=CHCHMe2 involves hydride migration via the initially formed acyl hydride Cp*2Zr(H)(η2-C(O)CH2CHMe2) to give a π-coordinated aldehyde complex Cp*2Zr(η2-OCHCH2CHMe2), which subsequently β-H eliminates to give the observed product. Support for the intermediacy of a π-aldehyde complex in this transformation is provided by the reaction of Cp*2M(H)CH2CHMe2 (M=Zr, Hf) with excess CO under controlled conditions to give moderately stable π-aldehyde carbonyl complexes Cp*2M(CO)(η2-OCHCH2CHMe2). Thermolysis of Cp*2Hf(CO)(η2-OCHCH2CHMe2) results in a novel coupling reaction to form the enediolate Cp*2HfOC(CH2CHMe2)CHO. The coordinatively unsaturated zirconium π-aldehyde intermediate Cp*2Zr(η2-OHCCH2-CHMe2) is also implicated in the reactions of acyl hydride Cp*2Zr(H)(η2-C(O)CH2CHMe2) with trapping substrates C2H4, MeC≡CMe, H2, and HC≡CtBu upon warming to give Cp*2Zr(OCH(-CH2CHMe2)CH2CH2), Cp*2Zr(OCH(CH2CHMe2)C(Me)=C(Me)), Cp*2Zr-(H)OCH2CH2CHMe2, and Cp*2Zr(C≡CtBu)OCH2CH2CHMe2, respectively. Carbonylation of the alkenyl hydride Cp*2Hf(H)CH=CHtBu did not yield an aldehyde complex, but rather the metallacycle Cp*2Hf(OHC=CHCH(tBu)). Mechanistic interpretations of these and related reactions are presented.

A series of mono-ring, terminal phosphide complexes of hafnium have been prepared. Reaction of Cp*HfCl3 with one equivalent or excess LiPtBu2 yields deeply-colored phosphide complexes Cp*HfCl2(PtBu2) and Cp*HfCl(PtBu2)2, respectively. Alkyl and aryl derivatives of Cp*HfCl2 (PtBu2), Cp*HfR(Y)(PtBu2) (R=Y=Me; Y=Cl, R-CH2CMe, CH2Ph, Ph), are prepared either by direct alkylation, or from the corresponding chloro-alkyls Cp*ClnR3-n. Cp*HfMe2(PtBu2) reacts slowly with H2 to form a highly insoluble methyl hydridophosphide dimer, [Cp*HfMe(µ-H)(µ-PtBu2)]2. The crystal structure of [Cp*HfMe(µ-H)(µ-PtBu2)]2 reveals a symmetric-bridged Hf2P2 core with Hf-P distances of 2.805, 2.807 A. The hydride ligands have been tentatively located and form an assymetric Hf2H2 bridge (Hf-H = 2.12, 2.33 A) orthogonal to the planar Hf2P2 moiety. The final R index is 0.066. Hydrogenolysis of alkyl derivatives Cp*HfClR(PtBu2) (R = CH2CMe3, CH2Ph) affords an analogous chlorothydridophosphide dimer, [Cp*HfCl(µ-H)(µ-Pt Bu2)]2. Treatment of Cp*HfCl(PtBu2)2 with H2 leads to rapid cleavage of Hf-P bonds and formation of [Cp*HfCl(µ-H)(µ-PtBu2]2. Cp*HfCl2(PtBu2) also reacts with H2, albeit slower, to give products derived from initially-formed Cp*HfCl2H.

The relatively low Hf-P bond energy suggested by hydrogenolysis reactions in these systems is further indicated by the reaction of Cp*HfCl2(PtBu 2) with CO to form the CO-insertion product, Cp*HfCl22-C(O)PtBu2). The crystal structure of Cp*HfCl22-C(O)PtBu2) exhibits dihapto carboxyphosphide functionality, with Hf-C = 2.203 and Hf-O = 2.117 A. The P-C(acyl) bond is 0.08 A shorter than P-C(alkyl) values, and suggests a significant P-π interaction. This has been confirmed by variable temperature studies, which reveal a substantial (10.7 kcal/mol) barrier to rotation about the P-C(acyl) bond.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Chemistry
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Collins, Terrence J.
Thesis Committee:
  • Bercaw, John E. (chair)
  • Chan, Sunney I.
  • Grubbs, Robert H.
  • Collins, Terrence J.
Defense Date:30 January 1984
Record Number:CaltechTHESIS:10222018-111603289
Persistent URL:https://resolver.caltech.edu/CaltechTHESIS:10222018-111603289
DOI:10.7907/wbgr-yd42
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:11240
Collection:CaltechTHESIS
Deposited By: Lisa Fischelis
Deposited On:22 Oct 2018 22:32
Last Modified:16 Apr 2021 23:14

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