Deuterium labeling of the benzylic ligands shows that the decomposition of Cp*2Hf(CY2CeHs)2 (Y = H, D) proceeds primarily by a-H abstraction to give permethylhafnocene benzylidene. Trihapto coordination of the benzylic ligands Zr(OR)(CH2CsHs)3 (R = 2,6-di-tert-butylphenyl), as evidenced by the x-ray crystal structure, proves such 1r. Perdeuteration of the pentamethylcyclopentadienyl rings (2b) results in a small kinetic deuterium isotope effect (kHfkD = 1.1) and ca.
The magnitude of the deuterium isotope kinetic effects and the labeling of the toluene obtained for the thermal decomposition of compounds 2b-e indicate a major pathway involving a rate-limiting a-H abstraction process (Scheme 1), rather than direct transfer of an orlho-benzylic acid. hydrogen to the adjacent benzyl group. Cp*('75,'71-CsMe4CH2)Hfl proceeds through the rate-limiting coupling of the neopentyl ligand and a pentamethylcyclopentadienyl ring hydrogen. Steric factors should promote activation of the benzylic hydrogen, which is para rather than ortho to the substituent X is indeed para with respect to hafnium (Table 3).
Moreover, the rate of conversion of 9 (X= H) to metallacycle 5 is approximately twice that measured for the conversion of 13-15 to metallacycles 6, 7 and 16, since the concentration of reactive (sterically uncharged) C-H bonds for 9 is doublet of compounds where X ~ H. Kinetic data and activation parameters for the rearrangement of cyclopentadienyl-metalated benzyl derivs. 9 and 13-15 in resp. Similar unfavorable steric interactions were invoked to reconcile the non-involvement of the 1r system in “sigma bond metathesis”.
With one exception, all reactions were followed by monitoring the height decrease of pentamethylcyclopentadienyl.
Chapter 2
The metalated phenyl complex Cp*(?J5,?J1-CsMe3CH2)HfC5Hs has been shown to be in equilibrium with the benzene complex Cp*2Hf(?J2-C5H4). Heating benzene-d5 solutions of Cp*(?J5,'71-CsMesCH2)HfC5Hs in the presence of ethylene traps the benzene intermediate and generates the hafnacyclopentene Cp*2HfCH2CH2C5H4. The steric bulk and the absence of activated sp2 C-H bonds in this ligand have enabled the formation of stable monomeric metallocene hydride and alkyl derivatives of the early transition metals. £11 However, it has been recognized that the Cp* ligand is not limited to being a simple auxiliary ligand, but is capable of participating in reaction chemistry by metalation of its methyl-CH bonds.
Herein, some mechanistic aspects of the synthesis and reactivity of hafnium complexes containing singly or doubly metalated pentamethylcyclopentadienyl ligands are described. The metallated iodide complex Cp*('75.'71-CsMe4CH2)Hfl (1) described in Chapter 1 serves as a convenient starting material for a wide range of mono-metallated. Toluene solutions of 2 cleanly eliminate neopentane over the course of 1 day at 140 o to produce the doubly metalated permethylhafnose derivative Cp CsMe3(CH2)2)Hf (3).
To confirm the unusual structure of Cp*(f75,f71,T/1-C5Me3(CH2)2)Hf and due to the lack of structural information to metall. Metalation of the two methyl groups also results in "forward sliding" of the hafnium within the cyclopentadienyl ring system. This enhanced reactivity is apparently a consequence of the unusual bond angles and lengths of the metalated methylene groups and the resulting steric accessibility of the hafnium center.
The reaction of the metalated iodide complex 1 with acetylene proceeds in the melting of benzene to cleanly form the propylene-bridged iodide complex 5 (Eq. 5). Mechanism B involves the initial abstraction of the a-H atom of the propyl bridge with the adjacent methylene group to generate the intermediate alkylidene species B. Assuming that intermediate B can be efficiently trapped by ethylene, a rate independent of ethylene concentration will be observed.
The data are most consistent with mechanism B, that is, the rate of conversion from 6 to metallacycle 7 is controlled by the abstraction of an α-hydrogen from the propyl bridge by the adjacent methylene group. Thermolysis of the mixed alkyl complex Cp*2Hf(CH2CMe3)CH3 (8) (prepared from Cp*2Hf(CH2CMe3)l and methyl lithium), purely generates neopentane and the monometalated methyl complex. 2 e/A3 around the Hf atoms (as before, presumably residues of absorption errors), and the rest of the map was relatively flat with peaks no larger than ca.
The reactions were followed by monitoring the decrease in the intensity of the methyl resonance of 6 at 8 1.38 relative to the internal Cp2Fe. After filtration, the solution was cooled to -78 °C. The off-white precipitate was collected by filtration in ca. The resulting suspension was stirred for one hour, after which most of the tetrahydrofuran was removed under vacuum.
The reaction was followed by monitoring the decrease in intensity of the tert-butyl resonance of 4 at o 1.22 relative to internal Cp2Fe. Relative Sc-C5H5 and Sc-alkyl BDEs were estimated from the equilibration of the metalated complex. Early transition metals form very stable compounds with electronegative ligands such as halogens due to the high degree of ionic character of the M-X bond.
Furthermore, a comparison of the electronegativities of carbon, hydrogen, and various transition metals reveals that some ionic character (M 0+-R 0-; R = H, C) can be expected when M is an early transition metal. Each of the substituted benzyltetramethylcyclopentadienyl anions was prepared from the corresponding benzyl Grignard reagent and tetramethylcyclopentenone, followed by dehydration of the resulting alcohol.
Scheme I
The enthalpies of equilibrium, established by these sigma bond metathesis reactions (equation 1), allow the relative M-R and M-R' bond strengths to be determined, assuming the Benson approximation [1 0] and canceling the solvation. Interestingly, the phenyl ring is held perpendicular to the equatorial plane of the scandocendele by coupling to the cyclopentadienyl ligand, i.e. due to the high boiling points of the ligands, purification and final characterization was done by preparing the corresponding lithium salts.
Longer reaction times result in darkening of the reaction mixture and reduced lithium salt yield. Removal of the solvent in vacuo afforded 1 as an amber oil in near-quantitative yield (90-95% purity by 1H NMR). Removal of the xylene solvent (under reduced pressure) followed by water/methylene chloride workup of 24l afforded 12.6 g (95%) of 6 as off-white crystals.
Removal of petroleum ether in vacuo gave an amber oil consisting of approx. After three cycles, the contents of the bomb were transferred to the frit assembly and the benzene was replaced with petroleum ether. Removal of the xylene solvent (under reduced pressure) followed by aqueous/methylene chloride treatment gave 6.5 g (90%) of 9 as an off-white solid.
Three cycles of heating the mixture to 80°C, followed by removal of the generated dihydrogen in vacuo, yields colorless 11, which was isolated in 79% yield (0.88 g) from petroleum ether at -78°C. Filtration followed by removal of the solvent in vacuo afforded 14 as an amber oil that was essentially pure by 1 H NMR. The benzene-d5 was kept frozen at -78°C, but the rest of the tube was allowed to warm to room temperature.
The concentrations of 8 and 7 were determined from the relative heights of the resonances at o 1.92 and 2.05, respectively. In addition, the coordination of Bronsted acid with. of the metal center prior to deprotonation is likely in many cases. Due to the extreme solubility of 5, this complex could not be isolated without side products and the identity of these side products could not be firmly established.
-H) BDEs are similar, and the (it) Hf-aryl BDE does not depend on the orientation of the phenyl ring with respect to the hafnocene equatorial plane. BDE(Sc-R) (R = alkynyl, aryl, alkyl), with a slope of 0.8, not the unity value of the Bryndza/Bercaw correlation.
J complex 6-d1
Based on this inverse isotope effect, a mechanism similar to Scheme 2 is tentatively proposed, i.e., a rapid pre-equilibrium is established with hydride migrating between hafnium and the Cp* ligand, followed by rate-limiting tolyl methyl C–H addition. bond to the 14e- hafnium center. It should be emphasized that the evidence for this mechanism is limited to one inverse isotope effect; however, the proposed mechanism allows us to envision a common decomposition pathway for various Cp*2Hf(R)H derivatives. In summary, thermolysis of solutions of benzene-ds permethylhafnocene alkyl hydride complexes 1 , 5 , and 6 has been shown to result in the elimination of dihydrogen and the formation of new hafnium-containing species.
In no case does decomposition occur through the coupling of the hydride ligand with the adjacent alkyl. Kinetic isotope effects of the decomposition of Cp*2Hf(CH2C6H5)H (5), are consistent with the rate-limiting step cleavage of the ortho-benzyl C-H bond. Interestingly, an inverse isotope effect is found when the hydride ligands of 5 and 6 are deuterated, consistent with a rapid pre-equilibrium involving the transfer of the hydride ligand to the Cp* ring, followed by the rate-limiting cleavage of ' an ortho-benzyl CH bond (Scheme 2).
Unfortunately, we are unable to test for an analogous opposite isotope effect for reaction 1 because of the ease with which the hydride position of Cp*2Hf(CH2CHMe2)H (1) exchanges with Cp* hydrogens. This solution was stirred at room temperature for 2 days (Note: heating results in the incorporation of deuterium into the Cp* rings.). The contents of the bomb were transferred to a frit mount and the product (0.46 g, 47%) was isolated from -78°C petroleum ether.
The rate of decomposition of 5f was followed by monitoring the decrease in the integrated signal intensity of (Cp*-d1s)2Hf(CH2CsHs)H relative to (175-CsHs)2Fe. Reaction temperatures were maintained using constant temperature oil baths controlled by thermoregulators and observed to be constant within ±1 °C. A typical experiment involved 25 mg of hafnium complex and 6-10 mg of ferrocene dissolved in 0.4 ml of benzene-ds.
Each spectrum was recorded three times and the average peak height was used to calculate the values of k given in Tables 2-4. Errors in k represent approximately 1 standard deviation. activation parameters LI.H i and LI.S i.
![Figure 1. The molecular structure of Cp*(I'J5,1']1_CsMe4CH2)HfCH2C 6Hs (9). The themal ellipsoids are shown at the 50% probability level](https://thumb-ap.123doks.com/thumbv2/123dok/10406127.0/19.848.127.738.118.985/figure-molecular-structure-csme4ch2-hfch2c-themal-ellipsoids-probability.webp)
