7700
J. Am. Chem. SOC.1985, 107, 7700-7705
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state 3. But some issues remain unresolved. The most refined 3 B. This calculations give 1.1 kcal/mol for the process A is too large to permit crossing by thermal activation at 4 K. Tunneling through the barrier is a distinct possibility since relatively minor hydrogen motions are required. The magnetic inequivalence of the protons in C, CH4+is about 100 G. Averaging this inequivalence by tunneling would require a rate of penetration of the barrier of a t least 3 X IO8 s-l. The calculations on CH2D2+ impose an upper limit on this frequency of ZPE(CH,H,D,D,+) - ZPE(CHIH,DID,+) 3 X 10l2s-l. These limits do not seem unreasonable in view of the barrier height and the known tunneling frequencies associated with other weakly hindered motions (e.g., 500 M H z for methyl rotors).I6 Variable-temperature studies
-
-
of the EPR spectra of CH4+and CD4+may reveal changes in the relative intensities of different hyperfine components which could be used to measure the splitting of the ground vibronic level due to tunneling. We also urge similar studies of CH2D2+and other deuterated derivatives to search for other, less stable, isomers and the onset of dynamic behavior. Acknowledgment. W e thank Dr. L. B. Knight, Jr., for stimulating our interest in this problem. We are grateful to the National Science Foundation for financial support of this work through research grants CHE-8402996 and CHE-8213329 and to David C. Spellmeyer and Frank K. Brown for assistance with graphics. Registry No. CH4+,20741-88-2;CH2D2+,61 105-67-7;CD4+,34510-
(16) Clough, S.; Poldy, F. J . Phys. 1973, C6, 1953.
Interconversion of Complexes
07-1.
p- Alkylidyne
and
p- Alkenyl
Diiron
Charles P. Casey,* Seth R. Marder, and Bruce R. Adams Contribution from the Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706. Received June 11, 1985
Abstract: The p-pentylidyne complex [ (C5H,)2(C0)2Fe2(p-CO)2(p-CCH2CH2CH2CH3)]+PF6(3) rearranged to the p-pentenyl complex [(CsHs)2(CO)2Fe2(p-CO)(p-q',~2-(E)-CH=CHCH2CH2CH3)]+PF6(4) upon heating at 88 "C in the solid state or in solution. The rate constant for the rearrangement of 3 to 4 in CD2C12solution at 88 "C is 2.9 & 0.5 X s-l (AG' = 27.1 f 0.2 kcal mol-'). Ethylidyne complex [(CsH,)2(Co)2Fe2(p-cO)(p-ccH~)]+PF6(10) gave no detectable isomerization
to [(C5H5)2(CO)2Fe2(p-CO)(~-qI,q2-CH=CH2)]+PF( (11) upon heating at 88 "C for 100 h; at this point, 50% decomposition had occurred (AG' 3 3 1 kcal mol-'). The cyclohexyl substituted carbyne complex [(C,H,),(CO),Fe,(p-CO)(p-
CCHCH2CH2CH2CH2CH2)]+PF6(12) is in rapid equilibrium with the p-alkenyl complex [(C,H,)2(CO)2Fe2(p-CO)(pq',q2-CH=CCH2CH2CH2CH2CH2)]+PF6(13) at ambient temperature (AG' = 19.9 f 0.2 kcal mol-' at -13 "C). The rate of rearrangement of alkylidyne complexes to p-alkenyl complexes is dramatically accelerated by alkyl substituents on the @ carbon of the alkylidyne group. The p-alkenyl complexes 4 and 13 exhibit a fluxional process which gives rise to a single coalesced cyclopentadienyl resonance at ambient temperature in the IH NMR. The barrier for the fluxional process for 4 is 12.9 kcal mol-' and that for 13 is 9.8 kcal mo1-l as determined from 'H NMR coalescence studies. Complex 11 exhibits two resonances for the cyclopentadienyl groups at 27 "C (AG' 2 14.7 mol-'). The rates of the fluxional process of the p-alkenyl complexes and of the rearrangement of p-alkylidyne complexes are both increased by fl-alkyl substitution and indicate increased positive charge at the @-carbonin the transition states for the respective processes.
The diiron methylidyne complex [(C5H5)2(CO)2Fe2(p-CO)(p-CH)]+PF6- (1) prepared by hydride abstraction from (C5H,)2(CO)2Fe2(p-CO)(p-CH2) (2)' with (C6H5)$+PF6- is the first compound in which a C-H unit bridges two metal centers.2 Although 1 can be stored indefinitely (>6 months) in the solid state a t -30 "C, it is extremely reactive toward nucleophiles in solution. For example, alcohols, amines, and C O add to the ~ methylidyne carbon of 1 to form isolable 1:l a d d ~ c t s .Alkenes such as 1-butene react rapidly with 1 at -50 "C in CH2C12to produce y-alkylidyne complexes such as the p-pentylidyne complex E(C5H5)2(CO)2Fe2(~-CO)(~-C(CH2),CH,)I+PF6(3); this hydrocarbation reaction proceeds by a regioselective addition of the p-C-H bond across the C=C bond.4 Alkylidyne complex 3 can ~
(1) Casey, C. P.; Fagan, P. J.; Miles, W. H. J . Am. Chem. SOC.1982,104, 1134. ( 2 ) Subsequently several other p-methylidyne complexes have been prepared, for example: (a) I[(CSHs)2Ru(p-dppm)(p-CO)(p-CH))+BF4C (Davies, D. L.; Gracey, B. P.; Guerchais, V.; Knox, S . A. R.; Orpen, A. G. J . Chem. SOC.,Chem. Commun. 1984, 841); (b) ((C,(CH,)~)(CO)FeFe(C~Hd(CO)(p-CO)(p-CH)]+PF6-(Miles, W. H. Ph.D. Dissertatlon, University of Wisconsin-Madison, 1984); and (c) ((C5HS)2(N0)2Fe2(p-CH))+PF6(Casey, C. P.; Roddick, D. M. unpublished results). (3) Casey, C. P.; Fagan, P. J.; Day, V.W. J . Am. Chem. SOC.1982,104, 7360.
also be prepared by reaction of (C5H5),(CO),Fe2 with n-BuLi followed by acidification with HPF6., We recently reported that alkylidyne complexes such as 3 rearrange to bridging alkenyl complexes upon heating.6 The nonequivalent cyclopentadienyl groups of the bridging alkenyl complexes give rise to two separate resonances at low temperature, but a fluxional process interconverts their environment and leads to a single coalesced resonance at ambient temperature. Here we report that the degree of substitution on the P-carbon of the bridging hydrocarbon group has a large effect both on the rate of rearrangement of alkylidyne complexes to bridging alkenyl complexes and on the rate of fluxionality of the bridging alkenyl complexes. Results Rearrangement of M-Akylidyne to p-Akenyl Complexes. When
a dilute CD2CI2solution of pentylidyne complex 3 was heated at 88 " C in a sealed N M R tube for 3.5 h, complete conversion to the bridging alkenyl complex, [(C,H5)2(CO)2Fe2(p-CO)(p(4) Casey, C. P.; Fagan, P. J. J . Am. Chem. SOC.1982, 104, 4950. (5) Nitay, M.; Priester, W.; Rosenblum, M. J . Am. Chem. SOC.1978, 100, 3620. (6) Casey, C. P.; Marder, S . R.; Fagan, P. J. J . Am. Chem. SOC.1983, 105, 1197.
0002-7863/85/ 1507-7700$01.50/0 0 1985 American Chemical Society
Interconversion of p -Alkylidyne and
p - Alkenyl
J . Am. Chem. SOC., Vol. 107, No. 25, 1985 7101
Diiron Complexes
rearrangement products. For example, reaction of 1 with cyq',q2-(E)-CH=CHCH2CH2cH3)]+PF6(4),was observed. The clohexene gives alkylidyne product 12 and p-alkenyl product 13. rearrangement of 3 to 4 was more conveniently carried out by Initially we considered the possibility that these products might heating solid 3 at 88 OC for 29 h under N2, which led to the have arisen from two competing pathways. The alkylidyne comisolation of 4 in 89% yield (298% conversion) after recrystalliplex 12 could have arisen from hydrocarbation. The p-alkenyl zation. complex 13 could have arisen from initial electrophilic addition The structure of complex 4 was spectroscopically deduced. In of 1 to cyclohexene to produce the intermediate carbocation I, the low-temperature 'H NMR spectrum of 4,singlets at 6 5.83 which subsequently underwent a 1,2-hydrogen migration to give and 5.62 are assigned to nonequivalent cyclopentadienyl groups, 13.' a doublet ( J = 11.8 Hz) at 6 12.06 is assigned to the proton on the a-vinyl carbon, a multiplet at 6 3.62 is assigned to the proton of the P-vinyl carbon, and multiplets at 6 2.27, 1.63, and 1.OO are assigned to the -CH2CH2CH3group. In the low-temperature 13C(IH}NMR of 4,two cyclopentadienyl resonances are seen at 6 92.6 and 89.8 and resonances due to the a- and P-vinyl carbons are observed at 6 175.4 and 96.7. In the ambient-temperature IH NMR spectrum of 4,the cyclopentadienyl resonances appear as a coalesced singlet at 6 5.67. Similar 'H and 13Cchemical shifts for the p-vinyl group of [(C,H,(CO)Fe),(p-C0)(p-~',q2-CH= CH2)]+BF4-(5) were reported by Pettit and D ~ k e . ~ . ~ The conversion of p-alkylidyne complexes with one alkyl substituent on C, to p-alkenyl complexes is rather general. Thus, [(CSH,)2(C0)2Fe,(p-CO)(p-CCH2cH~)]+PF6(6) is cleanly converted to [(C,H,)2(CO)2Fe2(p-CO)(p-q1,~2-(E)-CH= 0 CHCH3)]+PF6- (7) upon heating at 88 OC for 30 h and 13 w 20 [(C,H,)2(CO)2Fe2(p-CO)(p-C(CH2)4CH3)]+PF; (8) rearranges w [(C,H,)2(CO)2Fe2(p-CO)(p-q',q2-(E)-CH=CHto A related phenomenon was seen in the reaction of 1 with either (CH2)3CH3)]+PF6-(9) under similar conditions. cis- or trans-2-butene which led to mixtures of hydrocarbation CHz-CH,-CH, F H2- CH,-CH, product 14 and two isomeric p-alkenyl products 15 and 16 which I are the result of hydrogen migrations.' These results can also be explained in terms of two competing reactions. However, the observation that both cis- and trans-2-butene gave the same 2.3:1.0:1.5 ratio of products 14:15:16was not readily understood in terms of this mechanism. We have now discovered that alIHCOF H@ kylidyne complex 14 and p-alkenyl rearrangement products 15 and 16 rapidly equilibrate at room temperature. Consequently, the observed products could be the result of initial hydrocarbation or electrophilic addition followed by equilibration. In light of the /b\ /L , rearrangement of pentylidyne complex 3 to bridging alkenyl Fe-Fe Fe - F e 19 18 complex 4,we heated the mixture of alkylidyne 14 and p-alkenyl w w complexes 15 and 16,obtained from the reaction of 1 with cisIn dilute CD2C12solution, the rearrangement of 3 to 4 at 88.0 2-butene, at 88 "C in the hope of converting all the material to f 0.1 "C proceeds with a first-order rate constant of 2.9 0.5 p-alkenyl complexes. However, the ratio of complexes 14:15:16 X lo4 s-' which corresponds to AG*361K = 27.1 f 0.2 kcal mol-'. was not altered upon heating at 88 O C for 20 h. Similarly, the To further define the scope of this unprecedented rearrangement ratio of products from the reaction of 1 with cyclohexeneremained reaction, the ethylidyne complex [ (C,H,)2(C0)2Fe2(p-C0)(punchanged after heating at 88 OC for 27 h. These results are CCH3)]+PF6-(lo),,' was heated in CD2C12solution at 88 "C. readily explained by the rapid room-temperature equilibration of After 100 h, no detectable isomerization to [(C5H5)2(CO)2Fe2the alkylidyne and bridging alkenyl complexes which was estab(p-CO)(p-q1,v2-CH=CH2)]+PF6-was observed (
