Sequential Hydrocarbon CâH Bond Activations by 16-Electron

Chapter 11

Sequential Hydrocarbon C-H Bond Activations by 16-Electron Organometallic Complexes of Molybdenum and Tungsten

Downloaded by IOWA STATE UNIV on September 21, 2013 | http://pubs.acs.org Publication Date: July 12, 2004 | doi: 10.1021/bk-2004-0885.ch011

Peter Legzdins and Craig B. Pamplin Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada

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Cp*M(NO)(hydrocarbyl) complexes (Cp* = η -C Me ) of molybdenum and tungsten exhibit hydrocarbyl-dependent thermal chemistry. Thus, gentle thermolysis of appropriate Cp*M(NO)(hydrocarbyl) precursors (M = Mo, W) results in the loss of hydrocarbon and the transient formation of 16electron Cp*M(NO)-containing complexes such as Cp*M(NO)(alkylidene), Cp*M(NO)(η -benzyne), Οp*Μ(ΝΟ)(η acetylene), and Cp*M(NO)(η -allene). These intermediates effect single, double, or triple activation of hydrocarbon C-H bonds intermolecularly, the first step of these activations being the reverse of the transformations by which they were generated. This Chapter summarizes the various types of C-H activations that have been effected with these organometallic nitrosyl complexes to date and presents some of the results of kinetic, mechanistic, and theoretical investigations of these processes. 2

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© 2004 American Chemical Society

In Activation and Functionalization of C—H Bonds; Goldberg, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Introduction Interest in C - H bond activations at transition-metal centers continues unabated since these processes hold the promise of permitting the more efficient utilization of the world s limited hydrocarbon resources for the production of essential chemicals (7). Our contribution to this area of chemistry began with our recent discovery that Cp*M(NO)(hydrocarbyl) complexes (Cp* = η C M e ) of molybdenum and tungsten exhibit hydrocarbyl-dependent thermal chemistry. Thus, gentle thermolysis of appropriate Cp*M(NO)(hydrocarbyl) precursors ( M = M o , W) results in loss of hydrocarbon and the transient formation of 16-electron organometallic complexes such as Cp*M(NO)(alkylidene), Cp*M(NO)^ -benzyne), Cp*M(NO)(T] -acetylene), and C p * M (NO)(n -allene). These intermediates first effect the single activation of hydro carbon C - H bonds in an intermolecular manner via the reverse of the transformations by which they were generated. Some of the new product complexes formed in this manner are stable and may be isolated. Others are thermally unstable under the experimental conditions employed and react further to effect double or triple C - H bond activations of the hydrocarbon substrates. These sequential C - H bond-activating properties of the various reactive intermediates are considered in turn in the following sections, beginning with the most straightforward Cp*M(NO)(alkylidene) systems. 5

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Cp*M(NO)(alkyIidene) Intermediates

Tungsten Complexes These intermediate complexes result from the thermal activation of bis(alkyl) precursors, the first of which to be identified was Cp*W(NO)( C H C M e ) (1). Thermal activation of 1 at 70 °C in neat hydrocarbon solutions transiently generates the highly reactive neopentylidene complex, Cp*W(NO)(=CHCMe ) (A), that can be trapped by P M e as the Cp*W(NO)(=CHCMe )(PMe ) adduct in two isomeric forms (2a-b) which differ in the orientations of the C H C M e ligand with respect to the C p * group (Scheme 1). More importantly, intermediate A can effect the single activation of solvent C - H bonds (Scheme 1) (2, 5). Thus, in neat benzene and benzene-d* it cleanly forms the corresponding phenyl complexes, Cp*W(NO)(CH CMe )(C6H ) (3) and Cp*W(NO)(CHDCMe )(C D ) (3-rf ), and in M e S i it forms the unsymmetrical bis(alkyl) complex, Cp*W(NO)(CH CMe )(CH SiMe ) (4) in high yields. Interestingly, 4 reacts further with M e S i under the thermolysis conditions, albeit 2

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186 very slowly, to form Cp*W(NO)(CH SiMe ) (5) as a second product. The relative amount of 5 gradually increases upon prolonged heating of the reaction mixture, and its formation implies the transient existence of Cp*W(NO)(=CHSiMe ) (B) as a reactive intermediate (Scheme 1). 2

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Scheme 1

Intermediate A can also effect multiple C - H bond activations even in the presence of excess P M e (Scheme 2). Thus, thermolysis of 1 in cyclohexane in the presence of P M e yields 2a-b as well as the olefin complex Cp*W(NO)(r| cyclohexene)(PMe ) (6). It is likely that the double C - H activation of cyclo hexane by A is in direct competition with trapping of A by P M e as portrayed in Scheme 3. The competitive C - H activation by an unsaturated intermediate in the presence of a strong Lewis base is remarkable, yet it has been observed before (4, 5) and is aided by low concentrations of the trapping agent relative to the cyclohexane solvent. In contrast to cyclohexane, methylcyclohexane under identical experimental conditions affords trace amounts of 2a-b and the 3

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187 Scheme 2


Q

' I^ ""j^j

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h

- Me C

j PMe

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7b (10%)

7a (90%) PMe

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^

ON....»-W^.

H

8a (91%)

8b (9%)

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exocyclic allyl hydride complex, C p * W ( N O ) ^ - C H u ) ( H ) , in two isomeric forms (7a-b) whose relative abundances are shown in Scheme 2. Ethylcyclohexane affords similar exocyclic allyl hydride products 8a-b. The mechanisms for the transformations of 1 into 7a-b or 8a-b likely mirror that of cyclohexane (Scheme 3) in that activation of a solvent C - H bond by A is followed by β-Ά activation to release the second neopentyl ligand as neopentane and form a 16e alkene complex. Unlike the cyclohexene case, however, the alkene complexes in these systems evidently do not persist long enough to be trapped by P M e . Rather, there is a third γ-Η activation by the metal center that results in the conversion of the alkene complexes into the isolable 18e allyl hydride complexes. Thus, these processes involve triple C - H bond activations of the original hydrocarbon substrates. The related benzylidene complex, Cp*W(NO)(=CHPh) (C), results from the elimination of neopentane from C p * W ( N O ) ^ - C H P h ) ( C H C M e ) at 70 °C, and it exhibits C - H activating abilities similar to those summarized above in Schemes 1 and 2 for the neopentylidene intermediate A (J). The alkylidene 7

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188 Scheme 3

PMe

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Me P 3


2a-b

complexes A , Β and C belong to a small class of compounds that intermolecularly activate C - H bonds via their addition across the M=C bond of an alkylidene intermediate (6, 7, 8). However, A , B , and C are unique in their ability to activate aliphatic C - H bonds to form isolable, well-defined products. On the basis of our studies to date it appears that a whole family of such C - H bond-activating tungsten alkylidene complexes with differing steric and electronic properties should be capable of existence.

Molybdenum Complexes In general, the thermal reactivity of Cp*Mo(NO)(CH CMe ) (9) resembles that of its isostructural tungsten analogue (vide supra), but it occurs under the unusually mild conditions of ambient temperatures and pressures (Scheme 4). For example, the reactions of 9 with tetramethylsilane or mesitylene for 30 h at 20 °C result in the clean formation of free neopentane and Cp*Mo(NO)( C H C M e ) ( C H S i M e ) (10) or C p * M o ( N O ) ( C H C M e ) ^ - C H C H - 3 , 5 - M e ) (11), respectively (9). Neither organometallic product undergoes further reactivity under these mild reaction conditions. Similarly, the reaction of 9 with C D generates predominantly Cp*Mo(NO)(CHDCMe )(C6D ) (12-4) with stereospecific deuterium incorporation at the synclinal methylene position of the neopentyl ligand (just as portrayed for 3-d in Scheme 1). 2

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Scheme 4

Mechanistic Aspects Labeling, trapping, and kinetic results indicate that the C - H activation chemistry derived from 1 proceeds through two distinct steps: (1) formation of the neopentylidene complex A via rate-determining intramolecular a - H elimination of neopentane, and (2) a 1,2-os addition of R - H across the M = C H R linkage of A . Similarly, a kinetic analysis of the reaction of Cp*Mo(NO)( C H C M e ) (9) with Q D to form Cp*Mo(NO)(CHDCMe )(C D ) (12-rf ) (Scheme 4) in the temperature range 26-40 °C reveals a first-order loss of organometallic reactant, and a linear Eyring plot affords ΔΗ* = 99(1) kJ mol" and AS* = -11(4) J mol" Κ"" (9). These parameters are very similar to those determined at higher temperatures for the tungsten analogue 1 and indicate that a similar mechanism is operative for the molybdenum system. 2

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Additional experimental and B 3 L Y P computational mechanistic studies of the intermolecular C - H activations of hydrocarbons by the tungsten alkylidene complexes have also been carried out (10). In these studies it was shown that the


190 α-deuterated derivative, Cp*W(NO)(CD CMe ) (l-d*), undergoes intra molecular H/D exchange within the neopentyl ligands, a feature consistent with the reversible formation of σ-neopentane complexes prior to neopentane elimination. Microscopic reversibility requires that such σ-complexes also be formed during the reverse process of alkane C - H activation by the alkylidene complexes. In addition, it has been found that thermolysis of Cp*W(NO)( C H C M e ) (1) at 70 °C for 40 h in a 1:1 molar mixture of tetramethylsilane and tetramethylsilane-i/i yields an intermolecular kinetic isotope effect (KIE) of 1.07(4):1. Similarly, thermolysis of 1 and C p * W ( N O ) ( C H C M e ) ^ - C H P h ) in 1:1 benzene/benzene-

Sequential Hydrocarbon CâH Bond Activations by 16-Electron

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