This journal is c The Royal Society of Chemistry 2013 Chem. Commun.

Cite this: DOI: 10.1039/c3cc40774h

C–H bond activation of ethylene by a zirconacycle†

KaKing Yan and Aaron D. Sadow*

The reaction of C2H4 and b-SiH containing azasilazirconacycle

Cp2Zr{j2-N(SiHMe2)SiHMeCH2} (3), formed via a c-abstraction reaction

of Cp2Zr{N(SiHMe2)2}H (1), follows an unusual pathway in which a rare

r-bond metathesis reaction of ethylene generates a vinyl intermediate.

That species undergoes a b-hydrogen abstraction under the reaction

conditions to form a zirconium silanimine ethylene adduct en route to

the metallacyclopentane product.

b-Hydrogen elimination and abstraction reactions are central tosynthetic organometallic chemistry and catalysis, although theseprocesses are often associated with unwanted side reactions includ-ing alkyl group decompositions, chain transfer in polymerizations,and reduction rather than bond formation in cross couplings.Inhibition of these unwanted side reactions will allow access toproductive elementary steps, such as s-bond metathesis and inser-tion, that can be used to develop new transformations. In oneapproach, ligand design strategies and catalytic reaction partnershave simply avoided b-hydrogens. However, the b-hydrogen-richtetramethyldisilazide ligand N(SiHMe2)2 has found recent applica-tion in groups 2 and 3 and f element chemistry,1,2 possibly becauseb-elimination is slow in those systems.3 Despite the rich rare earthchemistry, the reactivity of tetramethyldisilazido zirconiumcompounds is essentially unexplored.4 Early metal andrare earth b-SiH containing silazides often form agostic-typestructures,2,5,6 while Cp2Zr{N(SiHMe2)t-Bu}CH2SiMe3 reacts byH-abstraction to give a zirconium-stabilized silanimine (Scheme 1).7

N-Alkyl amido zirconocenes also react via b-hydrogen abstraction,even in the presence of g-hydrogen.8 In contrast, a key reaction ofb-hydrogen-free d0 and fnd0 [M]N(SiMe3)2 compounds is g-Habstraction to form azametallasilacyclobutanes.9

Here, we describe b- and g-hydrogen abstraction reactionsof Cp2Zr{N(SiHMe2)2}R that provide b-SiH containing zircona-silanimines and azasilazirconacyclobutanes. g-Abstraction isfavored for R = H and Me, while b-abstraction is observed whenR = Et and CHQCH2. Furthermore, the Zr–C bond in themetallocyclobutane mediates a rare and reversible C–H bondactivation of ethylene through s-bond metathesis.

Disilazido zirconium hydride and methyl compoundsCp2Zr{N(SiHMe2)2}R (R = H; Me) have dissimilar ground statestructures. Cp2Zr{N(SiHMe2)2}H (1) contains one side-on coordi-nated SiH and one terminal SiH that exchange at room temperature,while the spectroscopy of Cp2Zr{N(SiHMe2)2}Me (2) is consistentwith two classical SiHs. Still, thermolysis of 1 or 2 in a sealed tubeaffords the azasilazirconacycle Cp2Zr{k2-N(SiHMe2)SiHMeCH2}(3) with H2 or CH4 elimination through g-abstraction (eqn (1)).

ð1Þ

The 1H NMR spectrum of 3 contained resonances at4.84 ppm (1JSiH = 193.4 Hz) and 4.37 ppm (1JSiH = 204.0 Hz)assigned to inequivalent SiH moieties. Doublet of doubletresonances at 1.97 and 1.61 ppm (1 H each) were assigned tothe diastereotopic CH2. In the IR spectrum, a single nSiH bandat 2074 cm�1 was observed.

This g-H abstraction occurs in the presence of b-hydrogen, andthe observed selectivity is unusual for amides. In fact, labelingstudies of alkylamides suggest that kinetics and thermodynamicsboth favor b-abstraction over g-abstraction, based on the incorpora-tion of deuterium in only the b-position in a reversible system.10 Inan interesting contrast, H/D exchange studies indicate that earlytransition metal alkoxides react via (reversible) g-abstraction.10a

Ethylene is a small, non-polar, unsaturated substrate thatcould undergo Zr–C bond insertion. Instead, compound 3 and

Scheme 1 Hydrogen abstraction processes. Amido ligands with both b- andg-hydrogen typically react by b-hydrogen abstraction.

Department of Chemistry, Iowa State University, 1605 Gilman Hall, Ames, IA

50011, USA. E-mail: sadow@iastate.edu; Fax: +1 5152940105; Tel: +1 5152948069

† Electronic supplementary information (ESI) available: Experimental details forsynthesis of compounds. See DOI: 10.1039/c3cc40774h

Received 29th January 2013,Accepted 2nd March 2013

DOI: 10.1039/c3cc40774h

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C2H4 (1 atm) react at 150 1C over 1 week to give Cp2Zr{k2-N(SiHMe2)SiMe2CH2CH2} (4; eqn (2)).

(2)

Compound 4 is prepared by the reaction of 1 and ethylene(1 atm) at 150 1C for 1 d. In situ NMR experiments on micromolar-scale reactions of 1 and C2H4 in benzene-d6 revealed a mixture of 3,4, H2 and ethane. The ratio of 3 to 4 was 1.4 : 1 after 45 min, and0.13 : 1 after 190 min, and quantitative for 4 after 1 d. The conversionof 3 to 4 is greatly facilitated by H2, but there is no clear route togive H2 as an intermediate in the interaction of 3 and C2H4.Substoichiometric quantities of H2 do not catalyze the transfor-mation. For comparison, only starting materials are observedafter heating Cp2Zr{k2-N(SiMe3)SiMe2CH2} and ethylene at150 1C for several days. Apparently, the SiH groups are criticalto the reaction of eqn (2).

We considered two possible mechanisms for the H2-freetransformation (Scheme 2). In pathway A, Lewis acid-mediatedhydrosilylation of ethylene generates a SiEt moiety. Subsequentd-abstraction would provide the ring expansion product. In fact,olefin hydrosilylation catalyzed by B(C6F5)3 is proposed to involveSiH abstraction.11 Alternatively, pathway B is based on the knownreaction of silanimine Cp2Zr{Z2-N(t-Bu)SiMe2} and ethylene thatforms Cp2Zr{N(t-Bu)SiMe2CH2CH2}.7 C–H bond activation ofethylene by a ring-opening reaction of 3 would provide the vinylintermediate Cp2Zr{N(SiHMe2)2}CHQCH2. b-H abstraction thengenerates a silanimine intermediate. However C–H bond activa-tion of ethylene is uncommon,12 particularly for organometallicsystems known for olefin insertion. For example, the compoundCp*2Th(k2-CH2CMe2CH2) reacts with CH4 by C–H bond activationbut C2H4 inserts into a Th–C bond.12g

Additionally, vinylic organometallic compounds are often inertrelative to alkyls, and a b-abstraction reaction of Cp2Zr{N(SiHMe2)2}-CHQCH2 requires experimental justification. In support of pathwayB, b-abstraction by a zirconium acetylide was proposed in therearrangement of an alkyl acetylide to a zirconacyclopentene.13

Only starting materials and products were detected in theconversion of 3 to 4, therefore, model compounds and plausibleintermediates were studied. To investigate pathway A, we preparedCp2Zr{N(SiEtMe2)2}H (5) by reaction of Cp2ZrHCl and LiN(SiEtMe2)2

in benzene. Thermolysis of 5 for 1 h at 110 1C, or standing at rtfor 9 h,9c gives full conversion to the g-abstraction productCp2Zr{N(SiEtMe2)SiMeEtCH2} (6, eqn (3)).

(3)

Treatment of 6 at 180 1C for 5 days in a sealed tube (1 atm N2

or 1 atm C2H4) returns metallacycle 6 in quantitative NMR yield.Thus, pathway A appears unlikely. Instead, a labeling experimentsupports pathway B. Reaction of C2D4 and 1 provides 4-dn, withpartial deuteration of SiMe and the [Zr]CD2CD2Si moieties.

Reaction of 3 and the terminal alkyne HCRCSiMe3 at 150 1Cover 1 day forms Cp2Zr{N(SiHMe2)2}CRCSiMe3 (7) as a model forthe C–H bond activation of ethylene proposed in pathway B. Thistype of s-bond metathesis reaction is unusual for zirconium. Whilerare earth alkyls and terminal acetylenes readily react,14 few s-bondmetathesis reactions of 16-electron zirconium alkyl compounds andalkynes have been described. For example, CpCp*Zr{Si(SiMe3)3}Meis inert to acetylene at room temperature for 1 day, even though theZr–Si bond readily reacts with organosilanes via s-bond metathesisunder those conditions.15 Most alkynylzirconium species are formedby salt metathesis involving alkali metal acetylides16 or from reac-tions of terminal acetylenes with zirconium–heteroatom bonds.17–19

Low electron count metal alkyl compounds, however, react directlywith acetylenes.20 Zr–C bonds in strained 3-membered zirconacyclesalso form [Zr]CCR through C–H bond addition to zirconium(II)species.13,21 While Cp*2Zr(CHQCHMe)2 reacts with acetylenesto give zirconium alkynyl compounds, labelling studies impli-cate a Cp*2Zr(Z2-HCCMe) intermediate rather than a s-bondmetathesis reaction.22

Acetylide 7 is inert under the conditions that give 4 (i.e., 1 atmC2H4, 150 1C). Therefore, Cp2Zr{N(SiHMe2)2}CHQCH2 (8) wassought as an intermediate. Reaction of Cp2Zr(CHQCH2)Cl andLiN(SiHMe2)2 affords 8 as a red-brown gummy solid. The1H NMR spectrum of the solid contained the characteristicABX pattern for the zirconium vinyl group (3JHH = 19.2 Hz,3JHH = 14.4 Hz, 2JHH = 3.6 Hz). The downfield chemical shiftand large 1JSiH coupling constant (4.48 ppm, 1JSiH = 186.5 Hz)suggest classical-bonded SiH groups. We examined the thermalreactivity of 8 as the proposed intermediate in the reaction of 3and ethylene. Thermolysis of 8 in benzene-d6 at 90 1C for 4 hprovides 3 and 4 in 1 : 5 ratio (eqn (4)).

(4)

s-Bond metathesis of ethylene is the microscopic reverse ofg-abstraction by 8, and formation of 3 implies that the C–H bondactivation of C2H4 is reversible. Interestingly, only 4 is observedupon thermolysis of 8 in the presence of ethylene. However, in thepresence of excess C2D4 (ca. 3 atm), a mixture of 4 and 4-d4 areformed in a 3.4 : 1 ratio; that ratio is unchanged after 24 h at 120 1C.Additionally, the conversion of 8 to 3 and 4 is much faster than thereaction of 3 and excess ethylene, which suggests the productdistribution in eqn (4) may describe a kinetic competition betweenScheme 2 Possible H2-free pathways for conversion of 3 to 4.

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b- and g-abstraction processes. Thus, while g-abstraction isfavored with H2 and CH4 as departing groups, b-abstractionfrom [Zr]C2H3 gives Cp2Zr(Z2-C2H4){Z2-NSiMe2(SiHMe2)} as anapparent intermediate (Scheme 2).

To check if [Zr](Z2-C2H4) formation influences the relative rateof b- vs. g-abstraction, Cp2Zr{N(SiHMe2)2}Et (9) was thermalized at60 1C to afford ethane and Cp2Zr{k2-NSiMe2(SiHMe2)} (10, eqn (5)).

(5)

The 1H NMR spectrum of 10 contained a singlet (0.55 ppm) anda doublet (�0.04 ppm, 3JHH = 1.5 Hz) for the SiMe2 and SiHMe2,respectively. The unusually upfield SiH multiplet at �2.23 ppm(1JSiH = 89 Hz) is consistent with a b-agostic structure. Uponaddition of PMe3, the SiH resonance in 10 shifts dramaticallydownfield to 5.19 ppm, which suggests PMe3 disrupts the b-agosticstructure. Neither 10 nor 10�PMe3 could be isolated free ofimpurities, but both compounds react with C2H4 to afford 4,providing further support for the pathway B.

Several points are worth noting. First, the b-SiH moieties provide apathway for the chemistry described here, which is distinct from thatof N(SiMe3)2 analogues. Second, the relative reactivity towardb-abstraction decreases following the series alkyl (ethyl) > vinyl (ethenyl)c alkynyl, and we are currently testing the generality of this trend.Third, the metalation of ethylene is generally challenging becausecompounds that are capable of C–H bond activation by s-bondmetathesis are also often highly reactive toward ethylene insertionand polymerization. The chemistry of 3 with ethylene extends inter-molecular s-bond metathesis reactions involving C–H bonds for thefirst time to neutral 16-electron zirconocene compounds.

The C–H bond activation chemistry of ethylene by 3 maydepend on a combination of factors including the electronicand steric limitations of a sixteen-electron complex, the strainassociated with the four-membered metallacycle, the presenceof a b-Si–H to trap the vinyl product, and thermal stability ofcompound 3. Nevertheless, this work shows that introductionof b-SiH groups in a metallocyclobutane significantly alters theoutcome of small molecule chemistry. These results may leadto a new catalytic hydrosilylation mechanism and provide newways for C–H bond functionalization of challenging substrates.

We are grateful to the National Science Foundation (CHE-0955635, MRI-1040098 and CRIF-0946687) for financial supportof this work.

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