4998
Journal of the American Chemical Society
bromide, [ a ] 2 5 0.00. ~ When (R)-(-)-a-deuteriobenzyl chloride was used under the same conditions, the recovered chloride was 90.2% racemic. Benzylchloro-1,2-bis(diphenylphos~hinoethane)palladium(I~). To a stirred suspension of 3.5 g (5.0 mmol) of dichlorobis(tripheny1phosphine)palladium(I I) and 4.0 g ( I O mmol) of I ,2-bis(diphenylphosphinoethane) in 100 ml. of degassed ethanol was added 1.00 g (37.8mmol) of sodium borohydride, and the suspension was stirred a t 25 "C under argon until the gas evolution stopped (4 h). The yellow I ,2-bis(diphenylphosphinoethane)bis(tripheny1phosphine)paila-
d i ~ m ( O )isolated , ~ ~ by filtration under argon, was washed with ethanol and dried in vacuo. To a solution of this complex in 80 m L of degassed benzene under argon was added 3.04g (24.0mmol) of benzyl chloride. After being stirred at 25 "C for 36 h, the mixture was filtered and the solvent was removed by evaporation. Pentane-ether (2:l) was added, and the yellow benzylchloro-1,2-bis(diphenylphosphinoethane)palladium(ll), isolated by filtration, was washed with pentaneand dried in vacuo to give 2.00 g (63.5% based on dichlorobis(triphenyIphosphine)palladium(l1)) of product: mp 184-186 "C dec; ' H NMR (CDCl3) 6 8.05-6.70(m. 25 H,phenyls), 3.20 (d of d , J = 4 Hz,2 H,PhCH2), 2.50(distorted d, J = 3 Hz, 4 H, P - CH2); 13C NMR (CDCI3) 6 134.434-127.385(m), 23.163 (s), 21.669 (s), 20.207 (s). Anal. (C33H31ClP2Pd)C, H,CI: calcd, 5.62;found, 6.17;P: calcd, 9.81; found, 10.94.
Acknowledgment. This research was supported by the Department of Energy, Office of Basic Sciences, Contract No. ER-78-S-02-4903, A000. References and Notes (1)(@E.Negishi, A. 0. King, and N. Okukado, J. Org. Chem., 42, 1821 (1977).
/
I01:17
/ August
15, 1979
and references therein. (b) A. Sekiya and N. Ishikawa, J. Organornet. Chem., 118,349 (1976),and references therein. (2)E. Negishi, N. Okukado, A. 0. King, D. E. VanHorn, and B. I. Spiegel, J. Am. Chem. Soc., 100,2254 (1978),and references therein. (3)D. Milstein and J. K. Stille, J. Am. Chem. SOC., preceding paper in this issue.
(4)D. Milstein and J. K. Stille, J. Am. Chem. Soc., 100,3636 (1978). (5)(a) D. F. Hoeg and D. I. Lusk, J. Am. Chem. Soc., 86,928(1964);(b) D.F. Hoeg and D. I. Lusk, J. Organomet. Chem., 5 , 1 (1966). (6)C. J. Cardin, D. J. Cardin. and M. F. Lappert, J. Chem. Soc., Dalton Trans.. 767 (1977). (7)C. Eaborn, K. J. Odell, and A. Pidcock, J. Chem. SOC.,Dalton Trans.,357 (1978). (8)P. FittonandE. A. Rick, d Organomet. Chem., 28, 287 (1971). The u values are based on D. H. McDaniel and H. C. Brown, J. Org. Chem., (9) .. 23,420 (1958). (10)M. Foa and L. Cassar, J. Chem. Soc., Dalton Trans., 2572 (1975). (11) J. K. Stille and K. S. Y. Lau. Ace. Chem. Res., 10,434(1977). (12)A. V. Kramer and J. A. Osborn, J. Am. Chem. Soc., 96, 7832 11974). (13)(a) K. S. Y. Lau, P. K. Wong, and J. K. Stille, J. Am. Chem. Soc., 98,5832 (1976);(b)J. K. Stilleand K. S. Y. Lau, ibid., 98,5841(1976);(c) Y. Becker and J. K. Stille, ibid., 100,838 (1978). (14)(a) J. A. Osborn, "Organob'ansition Metal Chemistry", Y. lshii and M. Tsutsui, Ed., Plenum Press, New York, 1975,p 65.(b) A. V. Kramer, J. A. Labinger. J. S. Bradley, and J. A. Osborn, J. Am. Chem. Soc., 96, 7145 (1974). (15) D. G. Morrell and J. K. Kochi, J. Am. Chem. Soc., 97, 7262 (1975). (16)Theee is basedupontheabsoluter0tation.[cu]~~~f1.20f 0.1', calculated from observations made by A. Streitweiser, Jr.. and J. K. Wolfe, Jr., J. Am. Chem. Soc.. 79, 903 (1957)(also see ref 3). (17)(RH-)-a-Deuterioethylbenzene has an absolute rotation of [(YIz5D-0.73 f 0.06': A. Streitweiser, Jr., J. R. Wolfe, Jr., and W. D. Schaeffer, Tetrahedron, 6, 338 (1959). (18)D. Seyferth and F. G. A. Stone, J. Am. Chem. Soc., 79, 515 (1957). (19)D. R. Coulson, lnorg. Syn., 13, 121 (1972). (20)P. Fitton, J. E. McKeon, and B. C. Ream, Chem. Commun., 370 (1969). (21)R. C. Weast, "Handbook of Chemistry and Physics", 56th ed.,CRC Press, Cleveland, 1975,p C368. (22) M. M. Midland, A. Tramomtano, and S. A. Zderic. J. Am. Chem. Soc., 99,
5211 11977). (23)J.-F. Fauvaique and A. Jutand, Bull. Soc. Chim. Fr., 765 (1976).
Reactions of Atomic Metal Ions with Alkyl Halides and Alcohols in the Gas Phase J. Allison and D. P. Ridge* Contribution f r o m the Department of Chemistry, Unicersity of Delaware, Newark, Delaware 19711. Received January 7 , 1979
Abstract: The chemistry of L.i+, Na+, Fe+. Co+, and Ni+ with alkyl halides and alcohols in the gas phase as elucidated by ion cyclotron resonance spcctroscapy is described. The alkali metal ions induce dehydrohalogenation and dehydration of alkyl h a lides and alcohols. rcspectively. Reactivity of R-X decreases w i t h increasing D(R+-X-) indicating a mechanism involving charge generation a t the carbon. A potential energy surface with a barrier associated w i t h such a charge generation is proposed and discussed quantitatively. From halide transfer reactions of R X to alkali metal ions it is deduced that AHr0?98( I-adamantyl cation) = 160.7 f 3.0kcal/mol and AHr029~(C7H7+)5 2 15.1 f 3.0kcal/mol. Fe+, Co+, and Ni+, formed by electron impact on Fe(CO)s, Co(NO)(C0)3,and Ni(C0)4, respectively, dehydrate alcohols and dehydrohalogenate alkyl halides by a mcchanism completely different f r o m the alkali metal ion reactions. Oxidative addition of M+ to R X giving R M X + is postulated. In some cases M R + and M X + products are observed. Limits on several (M+-X) bond strengths are deduced. It is deduced from observed reactions that the proton affinities of Fc and FeO are 5203 and 2227 kcal/mol, rcspectively. Reactions with the alcohols and alkyl halides of ions other t h a n M + formed by electron impact on the carbonyls are also described. Reactions with the neutral metal carbonyls of a number of organic ions formed from the alkyl halides and alcohols are described.
The gas-phase chemistry of simple organic molecules with ions formed by electron impact on metal carbonyls has been the subject of several recent communications from our laboratory.' We have reported limits on metal to carbon bond strengths in simple transition-metal alkyls.' W e have also reported direct evidence for metal insertion and p hydrogen shift mechanisms in the reactions of the atomic metal ions with alkyl halides and alcohols.2 We provide here a more nearly complete description of the ion chemistry observed in gaseous mixtures of Fe(C0)5, Co(NO)(CO)3, and N i ( C 0 ) J with simple alcohols and alkyl halides. 0002-7863/79/1501-4998$01 .OO/O
There have also been several reports on the gas-phase chemistry of alkyl halides with alkali metal We report here a mechanistic study of the alkali metal ion induced dehydrohalogenation of alkyl halides and the alkali metal ion induced dehydration of alcohols. An unusual generalization emerges from this study relating reaction rates to thermodynamic properties of the reactants. The reactions of the transition-metal ions and the alkalimetal ions show superficial similarities. A closer examination reveals marked differences in the mechanism and energetics of the reactions of the two classes of metal ions. We attempt 0 1979 American Chemical Society
4999
Allison, Ridge / Reactions of Atomic Metal Ions with Alkyl Halides and Alcohols Table I. Reactions of Alkali Metal Ions reaction
obsd a
AH,
AHexch?'
kcal/mol
kcal/mol 71.5
no no no no no no no
71.3 61.9 58.1
no 50.4
no no no no no no no no no no no no no no no no no no yes no yes ( 13%) yes (87%) Yes no no no Yes no yes (85%) yes ( 1 5%) Yes no yes no Yes no Yes
50. I 48.4 47.5 44.4 44.2 38.3 36.7 34.8 29.0 24.5 23.1 21.5 20.1 16.9
I .7 -1.3 -1.9 (D(RX-M+)
+ AHexch)
(24)
This assumption is essentially the same as that commonly used to rationalize the linear free energy relationships observed i n the kinetics of many common r e a ~ t i 0 n s . The l ~ condition for reaction to occur then becomes E* > -(D(u-M+)
+ AH^^^^)
I n the case of a bimolecular reaction between M+ and RX where E* = D(RX-M+) the condition for reaction to occur becomes ( 1 - cu)D(RX-M+) > aAHeXch.Our observations suggest that for reactions of Li+ and N a + with alkyl chlorides to occur AHcxchmust be less than -30 kcal/mol. The available data indicates that D ( R X - M I ) and hence E* in these cases is 20-40 kcal/mol. The data are thus consistent with eq 24 if CY 0.4-0.5 This same value of a seems to apply both to alcohols and alkyl halides. A testable prediction of this relation is that Li+ should dehydrate i - C J H 7 0 H with an activation energy of less than 5 kcal/mol. Attempts to produce product from the Li+ i-C3H70H reaction by using the double-resonance oscillator to increase the translational energy of Li+ were unsuccessful, however. A further consequence of the proposed model has to do with the structure of species of the type RX-M+. If such species are formed with relatively little internal energy (e.g., by a displacement reaction between C,,Hrn-M+ and RX), then the barrier will prevent rearrangement of the complex into HX-M+-C,,Hz,. That is, the RX-M+ configuration of the complex is at least a local minimum in the potential surface and stable. An experimentally observed consequence of this fact is the isotopic composition of the products of the reaction (eq 25). The manner in which the isotopic distribution of
+
X?+ (C,D,35C1)Li(C,Hs)+ (t-c40135ci),i+
+
t-c,~~37Ci
+ H37C1
4
h
(25)
(C4Hg3'C1)Li(C,D8)+
+ D
~
~
products was determined is described in the Results section (see account following reactions I6 and 17). The transition state in the reaction must be symmetric with respect to the CI atoms and the butyl groups. This suggests that the reactant complex has the indicated structure, (t-C4Dy35CI)Li+. Another observation consistent with the proposed model is that dehydrohalogenation is the predominant reaction mode even at relatively high collision energies. This is indicated by the data in Table X.Is The reaction pathway for dehydrohalogenation and that for halogen exchange are the same until MX and R+ begin to separate. It might be expected that from this point on even energetic collisions would follow the lowenergy dehydrohalogenation exit channel if the complex lives long enough for proton transfer from R+ to MX to occur. The
c
5005
Mf R X
Figure 3. Schematic representation of the potential surface for the reaction of a transition-metal ion with a n alkyl halide.
Table X. Variation of Product Distribution for z - C ~ H ~Reaction CI with Li+ Enerev emitter bias," V
iGHxLi+ll[C4Hs+l
12 20
8.3 5.2
40
I .I
" The positive bias (relative to source potential) on a filament bearing a P-eucryptite (Li+ emitter) bead inside the source of a Du Pont I I O mass spectrometer. The bead was 1 cm from the exit slit on the axis of the ion optics. The repeller was at source potential. The ratio of the m/z 63 peak height to the m/z 57 peak height with a Torr of I - C ~ H ~inC the I source. pressure of -2 X lifetime of the complex is probably much enhanced by the electrostatic attraction between R + and MCI (p(LiC1) = 7.21 D) even at high collision energies. While not all exothermic reactions are observed, it may be assumed that all observed reactions are exothermic or thermoneutral. This'is because i n our system reactions will be observed only if their rate constants are within one or two orders of magnitude of the reactant collision frequencies. Limits on several ionic heats of formation may be deduced from reactions noted in Table I . Of particular interest are the limits on the heat of formation of the I-adamantyl cation deduced from reactions of Li+ with adamantyl halides. If it is assumed that the observed chloride transfer from 1 -adamantyl chloride is exothermic and the unobserved bromide transfer from 1 adamantyl bromide is endothermic, we arrive a t A H I . ~ I-adamantyl ?~~( cation) = 160.7 f 3.0 k ~ a l / m o l , ~ ~ . ~ ~ cation) which compares favorably with A H I . ~ ~1 -adamantyl ~x( = 160 k c a l / r n ~ l " . ~ deduced ~ from the appearance potential of CloH15+from 1-adamantyl bromide (9.79 eV).IXA limit ~ AHvo29~(C7H-ii) 5 21 5. I f 3.0 kcal/mol may be deduced from the observation of chloride transfer froin benzyl chloride to Li+ and is consistent with a critically evaluated value AHf02y8(C7H7+) = 214 kcal/mol in the literature.I2 Transition Metal Atomic Ions. The reactions of the transition metal atomic ions M+ with alkyl halides and alcohols can be best accounted for by a metal insertion mechanism. The potential siirface for this mechanism is schematically represented in Figure 3. The essential feature of this diagram is the absence of a barrier similar to that in Figure 2 . The evidence indicates that i n most instances species with the R M X + stoichiometry also have that structure. This, in turn, suggests that in most instances the RX-M+ configuration is not a local minimum and that there is little, if any, energy barrier to the metal insertion. The formation of both MCH3+ and M I + products in reactions of CH31 supports a CH3MI+ intermediate. The best
5006
Journal of the American Chemical Society
evidence that R M X + is the lowest energy structure for ions of that stoichiometry, however, comes from the reactions of low-energy R M X + species such as (see Table VI): Feco+
+
C H ~ O H -+ F ~ C H ~ O H++ co
-+
F e C H 3 0 ~ ++ C D ~ H
Only CH3 is lost in the second step, no CD3, suggesting an activated complex asymmetric with respect to the methyl groups. Methanol attack on a C H 3 0 H - F e + species would surely lead to a complex symmetric with respect to the methyl groups. Hence we propose that F e C H 3 0 H + is actually CH3FeOH+. Similar conclusions can be derived from the reactions of FeCH3I+ with CD3I (see Table 111). Note also that CzHs is displaced from FeC2H50H+ by C l H s O H and O H is displaced from the resulting Fe(OH)(CZHjOH)+ ion by C 2 H j O H (Table VI). W e can set a limit on the depth of the well, E*, in Figure 2 i n some cases. As noted above, if the reaction YCO+
+
FCY
-3
RMx+
+ co
is observed it must be exothermic or thermoneutral since reactions must go a t nearly the collision rate to be observed. I n order for this reaction to be exothermic the following must be true: E*
I
AH )'M(
+
3Hf(RX)
-
AHf(RMX)+ > D(M+-CO).
D( Fe+-CO) and D(Ni+-CO) have been reliably measured to be 6 2 and 48 kcal/mol, r e s p e ~ t i v e l yHence . ~ ~ E* is greater than 58 kcal/mol for Fe+ reacting with CH31, CH3Br, CZHSI, C H 3 0 H , C2HjOH, and i - C j H 7 0 H . E* is similarly greater than 48 kcal/mol for Ni+ reacting with the same compounds. In the presence of a P hydrogen atom, metal insertion can lead to dehydrohalogenation in the way shown in Scheme 111. We suggest in Figure 2 that there is no energy barrier between 1 and 2. The data supporting this are the product distributions observed for reactions between the metal ions and CD3CHlI. HI and DI are eliminated i n approximately the statistical 2:3 ratio (see Table IV). This suggests that interconversion between l and 2 is very rapid in the excited complex. Such rapid interconversion scrambles the H atoms and the D atoms so that loss of HI or DI occurs statistically. The success with which the hydrogen atom shift competes with hydrogen iodide loss in the excited complex eliminates the possibility of any significant energy barrier between 1 and 2. Further evidence that the metal insertion and P-H atom shift occur without intermediate energy barriers comes from reactions of M R X + species formed by low-energy processes. Consider, for example, the following series of reactions (see Table V): ~ o ( N o ) ( C o ) ++ i-C,H7C1
~ o ( ~ o ] c 3 ~ 7 3+5 c31~,37c1 c~
+
Co(NOlC3H7C1+ + CO
C3H737C1C~(NO)(C,H,)
+ H3'Cl
Only the 37CI-containing ion is formed from the indicated isotopic variants of the reactants of the second reaction. This is in marked contrast to the stoichiometrically similar reaction of t-C4HgCILi+ with t-C4H9CI. This suggests that the Co(NO)C3H7CI+ complex formed in the first step exists as (CI)Co(NO)(C,H7)+ or (CI)(H)Co(NO)(C3H6)+ rather than C O ( N O ) ( C ~ H ~ C I )This + . complex has less energy, of course, than the same species formed by direct combination of C o ( N O ) + and i-C3H7CI; nevertheless, the metal insertion Scheme 111
/
101:17
August 15, 1979
and H atom shift appears to occur without hindrance. We note also that Ni(CO)+ and Co(CO)+ react according to (see Table IV) M(CO)+
F ~ O H C D ~ O H+ + C H ~
/
+CZD~I
+
M(CzD4)'
+ DI + CO
which again suggests that even relatively low energy complexes find the metal insertion hydrogen shift pathway quite accessible. The isopropyl chloride (Table V) and isopropyl alcohol (Table VII) both react with the transition metal ions according to ti+
+ i-c 3H 7x
4
C3H,+ + MX.
This suggests the possibility of a mechanism similar to that of the alkali metal ion reactions: attack a t X and heterolytic cleavage of the C-X bond. The products might just as readily result from a metal insertion process, however. The formation of C3H7+ and M X rather than M X + and C3H7 may be dictated by the low ionization potential of i-C3H7 (7.36 eV).'O If the i-C3H7 ionization potential is lower than that of MX, then C3H7+ and M X are the lowest energy products of the decomposition of C3H7MXt. The ionization potentials of the M X species are not known but the ionization potentials of the metal atoms Fe, Co, and Ni (7.87, 7.86, and 7.64 eV, respectively") are slightly higher than that of i-C3H7. This is in contrast to the ionization potentials of methyl and ethyl radicals (9.84 and 8.38 eV, respectivelyI2), which are considerably in excess of the metal ionization potentials. Methyl and ethyl cations, of course, are not observed to be products of any of the metal ion reactions. In the reactions discussed so far, Ni+ shows a notably different behavior than that of Fe+ and Co+. The metal tends to retain the halogen more often in Ni+ reactions. In the reactions of M + with CH31, MI+ is 50% of the product in the case of Fe+, 73% in the case of Co+, and 100% in the case of Ni+ (Table 111). I n the reactions of M+ with C ~ D S IM , I + and MDI+ are 14% of product in the case of Fe+, 22% in the case of Co+, and 68% in the Case of Ni+ (Table IV). In the reactions of M+ with i-C3H7CI, MCI is 37% of the product in the case of Fe+, 35% of the product in the case of Co+, and 91% of the product in the case of Nit (Table V). These data suggest that Ni+ has a higher affinity for halogens and perhaps a lower affinity for alkyl groups than do Fe+ and Co+. This is perhaps related to the fact that the ionization potential of Ni (7.64 eVI2) is slightly less than those of Co (7.86 eV") and Fe (7.87 eV"). The difference is small, only 5 kcal/mol, but may be related to the difference in the bonding capabilities of the ions. Limits on heats of formation deduced from the observed reactions of transition-metal ions are noted in Table XI. Very little thermodynamic information is available on species of the type MX and MX+ and none on species of the type M R + where M is a transition metal. As we discuss elsewhere' the Co+-CH3 and Fe+-CH3 bonds are evidently somewhat stronger than the few transition metal carbon bondsjor which bond strengths are in the literature. A value of D(Ni-CI) = 83 f 10 kcal/mol has been reportedz1in good agreement with the limit we obtain. Values of D(Ni-I) = 70 kcal/mol" and D(Mn-I) = 68 kcaI/mol2I have also been reported and compare reasonably well for the limits we deduce for D(M+-l). Note that we deduce a lower limit on the proton affinity of FeO ( P A ( F e 0 ) > 227 kcal/mol). Very little information is available on the basicity of metal oxides. The upper limit obtained on the proton affinity of atomic Fe (203 kcal/mol) is the proton affinity reported for Fe(CO)5 (203 kcal/mo18b) and considerably less than our lower limit on P A ( F e 0 ) . Transition Metal Carbonyl Ions. As noted above and in Table VIII, metal-containing ions with two or more carbonyls generally undergo displacement reactions. In a few instances
Allison, Ridge
1 Reactions
5007
of Atomic Metal ions with Alkyl Halides and Alcohols
Table XI. Thermochemical Inferences from Metal Ion Reactions
M+
+ I-C~HTCI
M+
+ i-C3H70H
+ i-C3H70H M+ + CH31 Ni+ + CH31 M+
M+ iCHlBr
---jtc
C3H7+ + MCI
M = Fe, Co, Ni
C3H7+ -I MOH
M = Fe,Co
-c
MCH3+ t I MI+ C H j NiCH3+ I MCH3+ Br MBr+ CH3 FeOH+ CH3
+
+
+
--c ++ -
M = Fe, Co
M = Fe, Co, Ni
M
= Fe, Co, Xi M = Fe, Co, Ni
MH++I
M = Co, Ni M = Fe, Co, Ni
MI++ H
M = Fe, Co, Ni
7T
M + + HI
D(M+-CI) > 168,h D(Fe-CI) > 7 I ," D(Co-CI) > 7 1 , h D(Ni-CI) > 76" D(M+-OH-) > 220,h D(Fe-OH) > 123,h D(Co-OH) > 1 23:h D(Ni+-OH-) < 220," D(Ni-OH) < 128* D(M+-CH3) > 56 D(M+-I) > 56 D(Ni+-CH3) < 56 D(M+-CH3) < 69 D(M+-Br) > 69, D(Fe+-OH) > 91, P A ( F e 0 ) 2227'' D ( M + - O H ) < 91 D(b.+-H) < 71, PA(Fe) < 203, PA(Co) < 203, PA(Ni) < 208 D(M+-l) > 71
C3H7+ + NiOH
+ CH30H M+ + CH3OH e MOH+ + CH3 Fc+
inferences,[' kcal/mol
react ion
' I Inferred by assuming reactions observed to be exothermic and reactions not observcd to be endothermic. Except as noted additional heats of formation needed to calculate bond strengths obtained from ref 12. A H I . ~ ~ ~ x ( ~ -which C ~ Hwas ~ + determined ) to be I87 kcal/mol from IP(i-C3H7) = 7.36 eV (ref 20) and AHl(i-C3H7) = 17.6 kcal/mol (ref 12). ' Calculated using AHf029s(FeO) = 60 i 5 kcal/mol from citation in ref I I .
M(C0)2+ ions enter into dehydration or dehydrohalogenation reactions. Two trends emerge in the displacement reactions. First, alcohols tend to displace two carbonyls more than the halides do. This suggests that the alcohols bind the metals more strongly than the halides do. Second, the possibility that an alkyl halide will displace two carbonyls tends to increase with the size of the alkyl group. Such a trend is not clearly evident for the alcohols. This suggests that the binding energy between the metals and alkyl halides tends to increase with the size of the alkyl group. The reactions of Fe(CO)3+ and Ni(C0)3+ (Table V l I l ) deviate from the general pattern. In addition to displacing a carbonyl, CH31 reacts with these ions to form Fe(CO)3I+ and Ni(C0)31+, respectively. This suggests that the metal carbonyl bonds in the M(CO)31+ species are relatively stable or perhaps that the CH3-M(C0)31+ bonds a r e relatively unstable. Alkyl Cations and Proton Transfer. The neutral metal carbonyls react with ions formed by electron impact on the alkyl halides and alcohols to give two sets of products (Table 1X): -8
AH'
+
!
