Organometallics 1995, 14, 4366-4373
4366
Group and Site Selective a-Bond Metathesis Reactions of CH~SCCH~CH~~ with [2,2-D2]Propane, [ 1,I, 1,4,4,4=Ds]-n-Butane, [2-D]Isobutane, and n-Pentane? Kevin C. Crellin, Serge Geribaldi,* M. Widmer,§ and J. L. Beauchamp" Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125 Received February 15, 1995@ Fourier transform ion cyclotron resonance mass spectrometry has been used to examine the reactions of CH3ScCH2CH3+ with [Dllmethane, ethane, [2,2-D2lpropane, [1,1,1,4,4,4Dsl-n-butane, [2-Dlisobutane, n-pentane, and [Dlzl-n-pentane. C H ~ S C C H ~ C His~ +not observed to react with methane or ethane, but for propane, n-butane, isobutane, and n-pentane o-bond metathesis with ethane elimination is the initial and dominant reaction observed, with further dehydrogenation of the resulting products occurring a s additional reaction channels. For propane, n-butane, and isobutane, no methane elimination is observed. For n-pentane, methane elimination is a minor reaction channel. For propane, n-butane, and isobutane, the initial o-bond metathesis involves predominantly the primary C-H bonds of the hydrocarbon. These processes are facile a t room temperature and occur with little or no activation energy. Measured total bimolecular rate constants with [2,2Dzlpropane, [1,1,1,4,4,4-Ds]-n-butane, [2-Dlisobutane, and n-pentane are 0.87 x 0.98 x 10-lo, 1.7 x and 6.4 x 10-lo cm3 s-l molecule-', respectively. With the butanes and pentane a second intramolecular metathesis reaction follows the initial addition t o yield a metallacycle product. In accordance with earlier theoretical predictions these metathesis reactions appear to proceed via a n allowed four-center mechanism similar to that of a 2, 2, cycloaddition. The higher reactivity of the metal-ethyl bond compared to the metalmethyl bond and the observed C-H bond specificity are also in agreement with theoretical predictions.
+
Scheme 1
Introduction Previous studies in our laboratory have revealed the exceptional reactivity of Sc(CD&+ with small a1kanes.l A four-center cycloaddition mechanism was invoked for a-bond metathesis occurring at the metal center (Scheme l), in accord with the prediction of Steigenvald and Goddard that certain organometallic systems containing an extremely acidic (in the Lewis sense) metal center should exhibit this kind of reactivitye2 These reactions of Sc(CD&+ with small alkanes do not involve oxidative addition mechanisms since Sc+ has only two valence electrons, which precludes the formation of more than two strong a bonds.3 The reactions also exhibit a high degree of site specificity, with attack at the primary position of a small alkane favored over attack at secondary or tertiary positions.l This result agrees well with theoretical predictions of the enthalpies of these reactions as calculated by Perry and G0ddard.l Similar site specificity has also been observed for reactions of ' Contribution No. 9052. Present Address: Universite de Nice-Sophia Antipolis, Laboratoire de Chemie Physique Organique, 06108 Nice Cedex 2,France. 4 Present Address: Zyma SA, DT-QC, CH-1260 Nyon, Switzerland. Abstract published in Advance ACS Abstracts, July 15, 1995. (1)Crellin, K.C.; Geribaldi, S.; Beauchamp, J. L. Organometallics @
1994,13, 3733. (2)Steigerwald, M.L.; Goddard, W. A., 111. J . Am. Chem. SOC. 1984,
MX+
MX+
+
+
yz
Oxidative Addition
YZ
Four-Center Sigma Bond Metathesis
M+-X
pM+-
]
+
-MY+
']-
My+
+
XZ
xz
j(
Co+ with propane in the gas phase5 and for reactions of iridium complexes in solution.6 Though the reactivity of Sc(CD3)2+has been investigated, no studies have yet been done on the reactivity of mixed ligand systems such as CH3ScCH2CH3+. By examining the reactions of CH3ScCH2CH3+, we can investigate how the reactivity changes as larger ligands are introduced to the metal center. We can also attempt to elucidate the factors that favor the reactivity of one ligand over that of another ligand and observe the effect that a second and larger ligand might have on the selectivity previously seen in smaller systems.l The focus of the present work is to examine the relative reactivities of the metal-methyl and metal-ethyl bonds. Theoretical results of Perry and Goddard indicate that the Sc+-C bond in Sc+-CH3 is approximately 5 kcal
106, 308.
(3)(a) Tolbert, M.A.; Beauchamp, J. L. J . Am. Chem. SOC. 1984, 106, 8117.(b) Beauchamp, J. L.; van Koppen, P. A. M. Energetics of Organometallic Species; Simoes, J. A. M., Ed.; Kluwer Academic
Publishers: Dordrecht, The Netherlands, 1992. (4)Perry, J. K.; Goddard, W. A., 111. J . Am. Chem. SOC. 1994,116,
5013.
(5)van Koppen, P. A. M.; Brodbelt-Lustig, J . ; Bowers, M. T.; Dearden, D. V.; Beauchamp, J. L.; Fisher, E. R.; Armentrout, P. B. J . Am. Chem. SOP. 1991,113, 2359. (6) (a) Wax, M. J.; Stryker, J . M.; Buchanan, J. M.; Kovac, C. A.; Bergman, R. G. J . Am. Chem. SOC. 1984,106,1121.(b) Buchanan, J. M.; Stryker, J. M.; Bergman, R. G. J . Am. Chem. SOC.1986,108,1537.
0276-733319512314-4366$09.00/0 0 1995 American Chemical Society
Selective a-Bond Metathesis Reactions
Organometallics, Vol. 14, No. 9, 1995 4367
mol-l stronger than the Sc+-C bond in SC+-CHZCH~.~ From this we can estimate that the process shown in reaction 1should be about 5 kcal mo1-l more endothermic than the process shown in reaction 2. This suggests
c
CH3 + S C + - C H ~ C H ~
(1)
CH3CH2 + Sc+-CH3
(2)
CH~-SC+-CH~CH~
that the methyl and ethyl groups should behave differwith ~ + small alently for reactions of C H ~ S C C H ~ C H kanes. It might also be suspected that replacement of one of the methyl groups by an ethyl group could affect the reactivity of the remaining methyl group compared t o the reactivity observed for methyl groups in Sc(CD3)2+. Synergistic effects of one ligand upon another metal-ligand bond strength within the same complex have been observed in the gas phase even for linear, two-ligand metal complexes.' Thus it is possible that CH3ScCHzCH3+ may exhibit a trans influence,8with the ethyl group affecting the bond energy or other properties of the remaining Sc+-CH3 bond. Since Sc(CD&+ shows a preference for attack at the primary positions of small alkanes,' we have used labeled compounds to see if any site selectivity is observed in the reactions of CH3ScCH2CH3+ with hydrocarbons. In this study we report the results of the reaction of C H ~ S C C H ~ C H with ~ + [D41methane, ethane, [2,2-d2lpropane, [1,1,1,4,4,4-D&butane, [2-Dlisobutane, and n-pentane.
Experimental Section Reactions were investigated with Fourier transform ion cyclotron resonance spectrometry (FT-ICR),of which a number of reviews are a ~ a i l a b l e . Only ~ details relevant to these experiments are outlined here. A l - i r ~trapping .~ cell is located between the poles of a Varian 15-in. electromagnet maintained at 1.0 T. Data collection is accomplished with an IonSpec Omega 386 FT-ICR data system and associated electronics. Neutral gases are introduced into the cell by separate leak valves, and their pressures are measured with a SchultzPhelps ion gauge calibrated against a n MKS 390 HA00001SP05 capacitance manometer. Uncertainties in absolute pressures are estimated to be &20%. Labeled [2,2-Dz]propane (98%D), [1,1,1,4,4,4-D&n-butane(98% D), and [2-Dlisobutane (98% D) were obtained commercially from Merck Sharp and Dohme. Labeled [D&methane (99% D) and labeled [Dlzl-npentane (98%D) were obtained commercially from Cambridge Isotope Laboratories. All reactant gases utilized were purified by freeze-pump-thaw cycling. Sc+ ions were produced by laser ablation of a scandium metal target with an Nz laser at 337.1 nm.'Jo The reactant ion was generated by reaction of Sc+ with n-pentane, and unwanted ions were ejected from the cell using double resonance techniques1' andor frequency sweep excitation.lz Re(7) Kappes, M. M.; Jones, R. W.; Staley, R. H. J. Am. Chem. SOC. 1982,104,888. ( 8 ) (a) Huheey, J. E. Inorganic Chemistry, 3rd ed.; Harper and Row: New York, 1983;Chapter 11. (b) Hartley, F. R. Chem. SOC.Rev. 1973,2, 163.(c) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Rev. 1973,IO, 335. (9)(a) Marshall, A. G. Acc. Chem. Res. 1986,18,316. (b) Comisarow, M. B. Anal. Chim. Acta. 1985,178,1. (10)(a) Azzaro, M.; Breton, S.; Decouzon, M.; Geribaldi, S . Int. J . Mass Spectrom. Ion Proc. 1993,128,1. (b) Cody, R.B.; Burnier, R. C.; Reents, W. D., Jr.; Carlin, T.J.; McCrery, D. A.; Lengel, R. K.; Freiser, B. S. Int. J. Mass Spectrom. Ion Phys. 1980,33, 37. (11)Anders, L.R;Beauchamp, J. L.;Dunbar, R. C.; Baldeschwieler, J. D. J . Chem. Phys. 1966,45,1062. (12)Comisarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974,26, 489.
actionsof isolated CH&cCHzCHs+ with n-pentane were examined first. Some experiments were performed with mixed samples of n-pentane and [Dlzl-n-pentane in order to confirm the presence of certain structures. These mixed reagent experiments will be discussed in more detail in the Results and Discussion sections. The pentane pressures used in these experiments were in the range (2-3) x lo-' Torr. Labeled methane, ethane, labeled propane, labeled n-butane, or labeled isobutane were then added along with the n-pentane, and reactions due t o the additional alkane were observed. The pressure of the additional methane or ethane used was about 1 x 10+ Torr, while the pressure of the additional alkanes larger than ethane were typically in the range (3-7) x Torr. Rate constants were determined in a straightforward manner, from slopes of semilog plots of the decay of reactant ion abundance versus time and the pressure of the neutral reactant. The reported rate constants are averages of several different sets of experimental data taken at different pressures of the neutral gases. Errors are estimated to be f 2 0 % due to uncertainties in absolute pressure determination. All experiments were performed at ambient temperature. To insure accurate product distributions, all distributions given in the next section were determined at short reaction times, before any subsequent secondary reactivity could affect the observed product distribution.
Results Reaction of Sc+with n-Pentane: Generation of C&ScCH&&+. A typical mass spectrum showing the products of the reaction of Sc+ with n-pentane is presented in Figure l a . Figure l b shows a typical semilogarithmic plot of the decay of the Sc+ abundance with time and the temporal variation of product ion abundances following the isolation of Sc+. The initial processes observed are reactions 3-7. The metallacycle f% CHySc+-CH2CH3 + CHzCHz
&
tH2
(3)
(4)
sc+
P
+CH4
product of reaction 6, ion 11, has been previously of the reaction of Sc+ with labeled 0 b s e r v e d . l ~Studies ~~ n-butane show that 1,3 dehydrogenation of the n-butane t o form ScC4&+ is observed, which is consistent with the formation of II.3a In addition, the previously observed reaction of labeled isobutane with Sc(CD&+ leads t o the formation of ion 111, which is the only structure consistent with the observed mass peak.l The postulated metallacycle product of reaction 4, ion I, is
I
analogous to the metallacycle formed in reaction 6 and to ion 111. Reactions 5 and 7 are observed when the nascent products of reactions 4 and 6 dehydrogenate. For the reaction of Sc+ with n-pentane, the total
Crellin et al.
4368 Organometallics, Vol. 14, No. 9, 1995 bimolecular rate constant k is 1.2 x cm3 s-l molecule-l and the reaction efficiency k/kmo is 0.85.13 CH~SCCHZCH~+ can react further with n-pentane as shown in reactions 8-18. A typical semilogarithmic plot
Id2
b)
0.5 ++CH3-SC%SH11
+ DHP
-tCH3CHZ-Sc+-C~H11 + DHP
kp
k)
+ (2C2H6 or C3Hg + CH4)
sc+
+V-+DHP
(14)
+ CH3CH3 + CH4 sc+ 0
0.2
0.4
0.6
0.8
1
Reaction Time (sec)
of the decay of the CH~SCCHZCH~+ abundance with time and the temporal variation of product ion abundances following the isolation of CH3ScCHzCH3+ is shown in Figure 2. It is postulated that reaction 16 occurs when the vibrationally excited products of reactions 8 or 11 have enough energy for a second intramolecular metathesis reaction, resulting in the observed metallacycle product. The neutral products of reactions 14 and 15 are uncertain. Reactions 9, 10, 12, 13, 15, 17, and 18 are observed when the nascent products of reactions 8, 11, 14, and 16 dehydrogenate. The reaction of CH3ScCHzCH3+ with n-pentane will be discussed in more detail in the next section. Since C H ~ S C C ~is Hthe ~ ~predominant + product of the reaction of CH3ScCHzCH3+ with n-pentane, it was also isolated so that its reactivity with n-pentane could be examined. A semilogarithmic plot of the temporal variation of ion abundances following the isolation of C H ~ S C C ~ is H shown ~ ~ + in Figure 3. Reactions 19-23 CH~-SC+-C~HII +
,-%
+CH4+CHs(CH2)3CH3 (19) sc+
are the initial processes observed, and it appears that adduct formation with the neutral n-pentane generates enough energy to either collisionally or chemically (13)Collision rates are calculated using ADO theory: Su,T.; Bowers, M. T.Int. J. Mass Spec. Ion Phys. 1973,12, 347.
Figure 1. (a) Products of the reaction of Sc- with npentane. Spectrum taken 450 ms after isolation of Sc-. Peaks at mlz 141 and 143 correspond to dehydrogenated CH3CHZScC5Hllf. The main peaks above mlz 145 correspond to ScRz', R = alkyl. The peaks at m l z 61 and 103 correspond to ScO+ and ScOC3He+,respectively. The peaks at mlz 71, 75, and 85 correspond to ScCzH2+,Sc(CH3)2+, and CH3ScCCH+,respectively. (b) Semilogarithmic plot of the decay of the Sc+ abundance with time for a single experimental run. The line is a fit to the data. The constant Sc+ abundance observed in the first 100-ms "induction period" is most likely due to translationally excited Sc+ions that must cool down before they can react. The temporal variation of the product ion distribution for the reaction of Sc+ with n-pentane is also shown. The relative intensities of the Sc+-alkyl products involving n-pentane include the contribution from secondary dehydrogenated products which are associated with the main Sc+-alkyl peaks and formed by subsequent unimolecular reactions of the primary Sc+-alkyl products. Note the lack of deviation from linearity of the data, which suggests that little or no excited Sc+ is present. activate the CH&C5H11+ ion. This strongly suggests that C H ~ S C C ~ is, H ~in~fact, + a precursor to the metallacycle species and supports our postulate that the metallacycle of reaction 16 results from vibrationally excited C H ~ S C C ~ions. H ~ ~At + longer reaction times, the abundance of both metallacycle products decrease and the formation of ScC,H,+ products with n I7 and m I 12 is observed. Reactions 20, 21, and 23 are observed when the nascent products of reactions 19 and 22 dehydrogenate. For the reaction of CH3ScC5H1lf with n-pentane, the total bimolecular rate constant k is 1.6 x cm3 s-l molecule-l and the reaction efficiency k l k m o is 0.15.13 Reaction 3 was utilized to generate the CH3ScCHzCH3+ ion, which was then isolated as shown in Figure 4a for further study of its reactivity. Reaction of C&ScCH2C&+ with n-Pentane. Elucidation of Structures. Reactions 8-18 are observed with n-pentane (see Figures 2 and 4b). Both single loss
Selective a-Bond Metathesis Reactions
Organometallics, Vol. 14, No. 9,1995 4369 a)
_"-I
/.
0
I
100,
-R- -0.5 1-
Y
-g
-1 0
: 160 200
40
80
mh
-1.5
0
0.6
0.4
0.2
0.8
1
Reaction Time (sec)
Figure 2. Semilogarithmic plot of the decay of the CH3ScCHzCH3+abundance with time for a single experimental run. The line is a fit to the data. The temporal variation of the product ion distribution for the reaction of CH3ScCHzCH3+with n-pentane is also shown. The relative intensities of the Sc'-alkyl products involving n-pentane include the contribution from secondary dehydrogenated products which are associated with the main Sc+-alkyl peaks and formed by subsequent unimolecular reactions of the primary Sc+-alkyl products. Note the lack of deviation from linearity of the data, which suggests that little or no excited CH3ScCHzCH3+is present. DHP I Dehydrogenated PrOducl
0.5
+
+CH,CH2-Sc*-C,Hil
DH
1
0
and the total bimolecular rate constant k is 6.5 x cm3 s-l molecule-l and the reaction efficiency kIkmo is 0.56.13 Reactions of C H ~ S C C H ~ C Hand ~ + CD3ScCD2CD3+ with n-pentanel[Dl2l-n-pentanemixtures were also observed to provide corroborating evidence that the major product is in fact C H ~ S C C ~ and H ~ not ~ + CH3CH2SCC~HS+, both of which have the same mass. When CH3ScCH&H3+ was isolated and reacted with labeled n-pentane, only reaction 24 was observed. Reaction 25
-0.5
2 W
z-0"
-1
-1.5
-2
1 0
mlz
Figure 4. (a) Demonstration of our ability to isolate the CH3ScCHZCH3+ ion from unwanted products of the reaction of Sc+with n-pentane. Spectra taken 400 ms affer generation of Sc+. (b) Products of the reaction of CH3ScCHzCH3+ with n-pentane. Spectrum taken 400 ms after the isolation of CH3ScCHzCH3+.Peaks at m I z 141 and 143 correspond to dehydrogenated CH~CHZSCC~HII~. The main peaks above m I z 145 correspond to ScRz+,R = alkyl. The peaks at m/z 61 and 103 correspond to ScO+ and ScOCsHs+, respectively. The peaks at m l z 71,75,and 85 correspond to ScCzHz+, SC(CH~)Z+, and CH3ScCCH+,respectively.
U
0.5
1
Reaction Time (sec)
1.5
2
C H ~ - S C + - C H ~ G+H ~
-> C H ~ - S C + - C ~1 D+ICH3CDH2 (24) Figure 3. Semilogarithmic plot of the decay of the CH3CDs(CD2)3CD3 ScC5H11+ abundance with time for a single experimental run. The line is fit to the data. The temporal variation of X-> CH~CH~-SC+-C + ~C2D3H3 D~ (25) product ion distribution for the reaction of C H ~ S C C ~ H ~ ~ + with n-pentane is also shown. The relative intensities of was not seen (Figure 5a). Analogously, when CD3the Sc+-alkyl products involving n-pentane include the ScCD2CD3+ was isolated and reacted with unlabeled contribution from secondary dehydrogenated products n-pentane, only reaction 26 was observed. Reaction 27 which are associated with the main Sc+-alkyl peaks and formed by subsequent unimolecular reactions of the priC D ~ - S C + - C D ~ C+D ~ mary Sc'-alkyl products. -> CD~-SC+-CJHI I + CD3CD2H (26) and multiple loss of hydrogen are observed from all the CWCHzWH3 nascent products of reactions 8,11,14,and 16. For the X--, C D ~ C D Z - S C + - C+~CH ~ D ~ H J(27) reaction of C H ~ S C C H ~ C H with ~ + n-pentane the total cm3 s-l bimolecular rate constant k is 6.4 x did not occur (Figure 5b). These observations support molecule-l and the reaction efficiency k l k m o is 0.55.13 our assertion that the major product of the reaction In order to determine if a n isotope effect is present we between C H ~ S C C H ~ Cand H ~ +n-pentane is C H ~ S C C ~ H ~ ~ + . Reaction of C&ScCH&&+ with Methane and also observed the reaction of CD3ScCD2CD3+ with [DE& Ethane. The possible exchange reactions of CH3ScCH2n-pentane. The reaction with labeled n-pentane shows CH3+ with CD4 were not observed. We estimate that the same product distribution as in the unlabeled case,
c
Crellin et al.
4370 Organometallics, Vol. 14, No. 9, 1995
0
m/z
b,
'O01
CH3-Sc:*H2CH3
1
CH3Sc*-CD2CH,CH2CD3
>.
120
140
120
160
'
,
rio
'
180
mh
for the reaction of CH3ScCH2CH3+ with methane the total bimolecular rate constant k is 11 x cm3 s-l molecule-l. The possible exchange reactions of CD3ScCD2CD3+ with CH3CH3 were also not observed. Labeled n-pentane was used since we did not possess labeled ethane. We estimate that for the reaction of C H ~ S C C H ~ C with H ~ +ethane the total bimolecular rate cm3 s-l molecule-'. constant is less than 1 x Reaction of C&ScCH&Hs+ with Propane. Reactions 28 and 29 were observed with labeled propane CH~-SC+-CH~C +H ~ 90% >
€ >'oI
C H ~ - S C + - C H ~ C D ~+CCH3CH3 H~
(28)
CH~-SC+-CD(CHJ)~ + CH3CDH2
(29)
(see Figure 6a), but reactions in which the methyl group instead of the ethyl group was exchanged were not observed. For the reaction of C H ~ S C C H ~ C H with ~+ labeled propane, the total bimolecular rate constant k is 0.87 x cm3 s-l molecule-l and the reaction efficiency k l k m o is 0.08.13 Reaction of CH&cCH&Hs+ with n-Butane. Reactions 30-32 were observed with labeled n-butane (see Figure 6b). Methyl exchange reactions were not seen. It is postulated that reaction 32 occurs when the CH~-SC+-CH~C +H~
m , C H ~ - S C + - C D ~ ( C H ~+) ~CH~CDHZ CD~ (30)
f
CD3(CH2)2CD3
'5
CH~-SC+-CH(CD~)CH~CD~ + CHjCHj
l-
3 a " -
Figure 5. (a) Products of the reaction of CH3ScCHzCH3+ with a mixture of n-pentane and [Dlzl-n-pentane between mlz 120 and 160. Spectrum taken 600 ms after the isolation of CH3ScCHzCH3+.Note the absence of any CH3CH2ScC4Dg+product. (b) Products of the reaction of CD3ScCDzCD3+ with a mixture of n-pentane and [Dlzl-npentane between mlz 120 and 160. Spectrum taken 700 ms after the isolation of CD3ScCDzCD3+.Note the absence of any CD3CDzScC4Hgf product.
CH3CD2CH3
.-
(3 I )
vibrationally excited product of reaction 30 or 31 has enough energy for a second intramolecular metathesis
:L 3
0 prrrrm 040
80
40
BO
160
200 200
0
m/z
Figure 6. (a) Products of the reaction of CH3ScCHzCH3+ with [2,2-Dz]propane. Spectrum taken 250 ms after the isolation of CH3ScCHzCH3+. The main peaks above mlz 125 correspond to ScRz+,R = alkyl. The peak at mlz 61 corresponds to ScO+. The peaks at mlz 71, 75, and 85 correspond to ScCzHz', Sc(CH3)2+, and CH~SCCCH',respectively. (b) Products of the reaction of CH~SCCH~CH~+ Spectrum taken 250 ms with [1,1,1,4,4,4-D,jl-n-butane. after the isolation of CH3ScCHzCH3-. The main peaks above m / z 125 correspond to ScRz+, R = alkyl. The peaks at mlz 61 and 73 correspond to ScO+ and ScCzH4+, respectively. (c) Products of the reaction of CH3ScCHzCH3with [2-Dlisobutane. Spectrum taken 250 ms after the isolation of CH3ScCHzCH3+. The main peaks above mlz 125 correspond to ScRz', R = alkyl. The peak at mlz 61 corresponds to ScO+. The peaks at mlz 71 and 75 correspond to ScCzHz+ and Sc(CH&-, respectively. reaction, resulting in the metallacycle product. It should be noted that the labeling of the hydrocarbon products shown in reaction 32 assumes that the initial metathesis occurs as shown in reaction 30. However, since reaction 31 is also observed as a minor reaction pathway, hydrocarbon products with the opposite labeling are also present but in significantly smaller quantities. For the reaction of C H ~ S C C H ~ C with H ~ + labeled n-butane, the total bimolecular rate constant k is 0.98 x cm3 s-l molecule-l and the reaction efficiency k l k m o is 0.09.13 Reaction of C&ScCH&Hs+ with Isobutane. Reactions 33 and 34 were observed with labeled isobutane (see Figure 612). No reactivity is observed at the tertiary site of isobutane. Reaction 34 occurs when the vibra-
Selective a-Bond Metathesis Reactions
Organometallics, Vol. 14, No. 9, 1995 4371
Table 1. Summary of the Observed Reactivity of CH3ScCH2CHs+with the Small Alkanes Examined in This Study"
methane ethane propane n-butane isobutane
-
a
s-1)
W~OII)
no reaction
E, (reaction occurs) energy of the reactants (upper curve of Figure 8). However, both CH3ScCH2CH3+ and Sc(CD3)2+ react Figure 8. Schematic of a general potential energy surface with alkanes larger than ethane. In these cases the showing how the initial electrostatic potential well can energy of the transition state is below the energy of the affect the height of a barrier relative to the energy of the reactants so that no net barrier to reaction exists, as reactants. Deepening the initial electrostaticwell can lower the barrier to reaction below the energy of the reactants. shown by the lower curve in Figure 8. Our observation The upper curve would be appropriate for the reaction of that both CH3ScCHzCH3+ and Sc(CD&+ react with CH3ScCHaCH3' with methane or ethane. The lower curve alkanes larger than ethane is in accordance with the would be appropriate for the reaction of CH3ScCH&H3+ prediction of Steigenvald and Goddard.2 with propane, the isomeric butanes or n-pentane. Reaction of Sc+and n-Pentaneand Reaction of Subsequent Product Ions with n-Pentane. The Overall Reaction Efficiencies. The largest rate reaction of Sc+ with n-pentane produced a complex constant we observed was for the reaction of CH3ScCH2mixture of products. At long times many high-mass CH3+ with n-pentane. The rate constant for reaction RScR+ products, where R and R are alkyl groups, were with pentane was about 3.8 times larger than that for observed. In order to determine which ions were the reaction with isobutane, while the rate constant for precursors of these species, various product ions of the reaction with isobutane was about 1.7 times larger than reaction of Sc+ with n-pentane were isolated and althat for reaction with n-butane and about 2.0 times lowed to react in turn with n-pentane. CH3ScCH2CH3+ larger than that for reaction with propane. Ethane and reacted with n-pentane to produce mostly CH3ScC5H1lf, methane are not observed to react at all with CH3ScCHzCH~CHZSCC~H I, ~11, ~ +and , small amounts of RScR+ CH3+. This trend is analogous to the trend seen with products of higher mass which result from subsequent Sc(CD3)2+.' The increase in reaction efficiency seen as reactions of the main product ions. C H ~ S C C ~ also H~~+ the alkane reacting with C H ~ S C C H ~ C Hincreases ~' in reacted with n-pentane to produce mostly RScR+ where size can be accounted for by the deeper potential wells R and R are larger than ethyl. What is of interest here which result from the increased polarizabilities of the is that C H ~ S C C H ~ C H reacted ~ + with n-pentane via a larger alkanes. This makes it easier t o overcome o-bond metathesis involving only C-H bond activation, intrinsic barriers to a-bond metathesis (Figure 8). as was seen previously for the reaction of Sc(CD&+ with These results are reminiscent of the reactions of Co+ small a1kanes.l The reaction of CH3ScCH2CH3+ with with hydrocarbons, where larger hydrocarbons deepen alkanes smaller than n-pentane, as observed in this the electrostatic potential well for the Co+-alkane study, also involved only C-H bond activation. But the adduct, more strongly binding the alkane to Co+ and facts that C H ~ S C C ~ H can ~ ~produce + RScR+ where providing more energy for chemical a c t i v a t i ~ n . ~ J ~ either R or R is not a pentyl group and that I1 can be Furthermore, the kinetic efficiencies observed for CH3formed from C H ~ S C C ~ seem H ~ ~ t+o suggest that some ScCH2CH3+reacting with propane, n-butane, and isobintermolecular and intramolecular C-C bond activation utane are lower than for Sc(CD&+ reacting with the is occurring. same a1kanes.l This observation suggests that, compared to the energy of the reactants, the energy of the Observation of the reaction of C H ~ S C C ~ H with ~~+ transition state for the reaction of CH3ScCHzCH3+with n-pentane also provided us with direct evidence that propane, n-butane or isobutane is higher than the intramolecular o-bond metathesis does occur. Metalenergy of the transition state for the reaction of Sclacycles I and I1 were both observed t o form from (CD3)2+with the same alkane. This effect could be due isolated C H ~ S C C ~ H These ~ ~ + . products could only be to extra steric interactions introduced by the larger ethyl observed if intramolecular o-bond metathesis were group or a possible trans effecta of the methyl and ethyl occurring. This supports our postulate that excited CH3groups upon each other's bond strength with Sc+. ScR+ ions, where R = n-butyl, isobutyl, or pentyl, can Similar synergistic effects in two-ligand gas phase perform a second intramolecular a-bond metathesis metal-ion complexes have been previously ~ b s e r v e d . ~ reaction to form metallacyclic products. The group specificity observed in the reactions of CH3ScCH2CH3+ with propane, n-butane, isobutane, and Acknowledgment. We wish t o thank J. K. Perry n-pentane is another example of how the replacement and Professor W. A. Goddard I11 for their theoretical of one methyl group with an ethyl group synergistically work on this ~ y s t e m .This ~ work was supported by the affects the reactivity of the remaining methyl group National Science Foundation under Grant CHE-9108318, compared t o the reactivity observed for methyl groups by a grant from AMOCO, and by the Office of Naval in Sc(CD&+. Research. The observation that ethane and methane do not react OM9501261 with CH3ScCH2CH3+ appears to be in accordance with
E,> E' (no reaction)
