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=CH^tBu. The reactivity of Cp^*₂Hf(H)CH=CH^tBu 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≡C^tBu 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≡C^tBu)OCH₂CH₂CHMe₂, respectively. Carbonylation of the alkenyl hydride Cp^*₂Hf(H)CH=CH^tBu 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 LiP^tBu₂ yields deeply-colored phosphide complexes Cp*HfCl₂(P^tBu₂) and Cp*HfCl(P^tBu₂)₂, respectively. Alkyl and aryl derivatives of Cp*HfCl₂ (P^tBu₂), Cp*HfR(Y)(P^tBu₂) (R=Y=Me; Y=Cl, R-CH₂CMe, CH₂Ph, Ph), are prepared either by direct alkylation, or from the corresponding chloro-alkyls Cp*Cl_nR_3-n. Cp*HfMe₂(P^tBu₂) reacts slowly with H₂ to form a highly insoluble methyl hydridophosphide dimer, [Cp*HfMe(µ-H)(µ-P^tBu₂)]₂. The crystal structure of [Cp*HfMe(µ-H)(µ-P^tBu₂)]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(P^tBu₂) (R = CH₂CMe₃, CH₂Ph) affords an analogous chlorothydridophosphide dimer, [Cp*HfCl(µ-H)(µ-P^t Bu₂)]₂. Treatment of Cp*HfCl(P^tBu₂)₂ with H₂ leads to rapid cleavage of Hf-P bonds and formation of [Cp*HfCl(µ-H)(µ-P^tBu₂]₂. Cp*HfCl₂(P^tBu₂) 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(P^tBu ₂) with CO to form the CO-insertion product, Cp*HfCl₂ (η²-C(O)P^tBu₂). The crystal structure of Cp*HfCl₂(η²-C(O)P^tBu₂) 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.

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