Oxidative addition of butanethiol and thiophenol to the *Cr(CO)3C5Me5 radical. Kinetic and thermodynamic study of a third-order reaction and its catalysis

Full text of DOI:10.1021/ja960082k

5328
J. Am. Chem. Soc. 1996, 118, 5328-5329
Kinetic data for reaction 1 were obtained in toluene solution
using a specially constructed flow cell mounted on an FTIR
microscope.8 The simple third-order equation, rate ) kobs[•Cr-
(CO)3C5Me5]2[RSH], was found true over the entire range of
available concentrations for the chromium radical and thiophenol
concentrations out to 1 M. The values of the rate contstants at
room temperature [kobs ) 23 ( 3 M-2 s-1 (R ) Ph) and 5.0 (
1 M-2 s-1 (R ) n-Bu)] showed no significant change over a
40 °C range [calculated activation parameters in the temperature
range 25-65 °C: ∆Hq ) +0.1 ( 1 kcal/mol, ∆Sq ) -52 ( 5
cal/mol deg for PhSH, and ∆Hq ) +0.2 ( 1 kcal/mol, ∆Sq )
-55 ( 6 cal/mol deg for R ) BuSH]. The near zero enthalpies
of activation and high negative entropies of activation are
consistent with other reports for third-order reactions.9 The fact
that thiophenol only reacts 5 times faster than butanethiol is
also surprising in view of a 10 kcal/mol difference in bond
strength. All of these observations point to a termolecular
transition state in which 2 mole of the chromium radical attack
the sulfur hydrogen bond.
Oxidative Addition of Butanethiol and Thiophenol
•
to the Cr(CO)3C5Me5 Radical. Kinetic and
Thermodynamic Study of a Third-Order Reaction
and Its Catalysis
Telvin D. Ju, Russell F. Lang,† Gerald C. Roper,‡ and
Carl D. Hoff*
Department of Chemistry, UniVersity of Miami
Coral Gables, Florida 33124
ReceiVed January 9, 1996
Cleavage of the sulfur-hydrogen bond at metal centers is a
key reaction in many biochemical1 as well as industrial catalytic
processes.2 In spite of the importance of thiol activation by
metals there are few thermodynamic or kinetic studies of these
reactions. Radical processes have a long history in thiol
chemistry,3 and we wish to report reaction of a stable chromium-
centered radical4 with thiols as shown in eq 1:
The proposed third-order mechanism for reaction 1 is shown
in the potential energy diagram in Figure 1. The first step is
binding of thiol to the 17-e- radical forming a proposed 19-e-
adduct.10 This pre-equilibrium is probably rapidly established
but thermodynamically disfavored. The second step is attack
of the second mole of radical on the sulfur-hydrogen bond of
the adduct leading to the termolecular transition state. The near
zero observed enthalpy of activation implies that enthalpy of
binding in the first step cancels the barrier to H atom transfer
in the second step.
Additional support for the mechanism shown in Figure 1 was
gained from study of reactions of stable 18-e- complexes of
thiols. We have recently studied reaction of W(CO)3(phen)-
(EtCN) with disulfides11 and thiophenol,12 both of which
undergo oxidative addition. In the case of butanethiol, however,
equilibrium amounts of a thiol complex12,13 are formed as shown
in eq 4:
2•Cr(CO)3C5Me5 + RSH f RS-Cr(CO)3C5Me5 +
H-Cr(CO)3C5Me5
R ) Ph, n-Bu (1)
Calorimetric measurements of reaction 1 yield absolute RS-
Cr(CO)3C5Me5 bond strengths of 35 ( 3 (R ) Ph) and 43 ( 3
(R ) n-Bu) kcal/mol.5 These values are the first direct
calorimetric measurement of a transition metal-sulfur bond
strength in solution.6 They provide a basis for determining other
metal-thiolate bond stengths by measurement of enthalpies of
•
transfer of the SR fragment.7 Two possible second-order
mechanisms for reaction 1 are shown in eqs 2 and 3:
Cr• + RSH h RS-Cr + H• + Cr• f H-Cr
Cr• + RSH h H-Cr + •SR + Cr• f RS-Cr
(2)
(3)
The Cr-SR bond strength estimates derived above can be used
to rule out mechanism 2. The RS-H bond strengths of 79 (R
) Ph) and 89 (R ) Bu) kcal/mol5 would make the first step in
eq 2 endothermic by +44 (R ) Ph) and +46 (R ) Bu) kcal/
mol. The Cr-H bond strengh of 62 kcal/mol4b can be used to
calculate that generation of free thiyl radicals as shown in eq 3
is only endothermic by +17 (R ) Ph) or +27 (R ) Bu) kcal/
mol. The low nature of the Cr-SR bond strength makes 3 the
preferred second-order path.
W(CO)3(phen)(EtCN) + BuSH h
W(CO)3(phen)(BuSH) + EtCN (4)
Addition of 2 equiv of •Cr(CO)3C5Me5 to solutions of W(CO)3-
(phen)(BuSH)/BuSH results in rapid consumption of 2 mol of
the radical on the millisecond time scale14 and produces 1 mol
of H-Cr(CO)3C5Me5 and 1 mol of W(CO)3(phen)[BuS-
† Permanent address: Coulter Electronics, Miami, Florida 33196.
‡ Permanent address: Department of Chemistry, Dickinson College,
Carlisle, Pennsylvania 17013.
(1) Molybdenum Enzymes, Cofacors, and Model Systems; Stiefel, E. I.,
Coucouvanis, D., Newton, W. E., Eds.; ACS Symposium Series 535;
American Chemical Society: Washington, D.C., 1993.
(2) Calhorda, M. J.; Hoffmann, R.; Friend, C. M. J. Am. Chem. Soc.
1991, 113, 1416.
(3) (a) Franz, J. A.; Bushaw, B. A.; Alnajjar, M. S. J. Am. Chem. Soc.
1989, 111, 268. (b) Alnajjar, M. S.; Garrossian, S. T.; Autrey, S. T.; Ferris,
K. F.; Franz, J. A. J. Phys. Chem. 1992, 96, 7037.
(4) (a) Baird, M. C. Chem. ReV. 1988, 88, 1217. (b) Kiss, G.; Zhang,
K.; Mukerjee, S. L.; Hoff, C. D. J. Am. Chem. Soc. 1990, 112, 5657. (c)
Watkins, W. C.; Jaeger, T.; Kidd, C. E.; Fortier, S.; Baird, M. C.; Kiss, G.;
Roper, G. C. J. Am. Chem. Soc. 1992, 114, 907.
(7) RS-Cr(CO)3C5Me5 and RS-Cr(CO)2(PPh3)Cp undergo rapid thiolate
exchange with other metal radicals, and some coordinatively unsaturated
metals as well: Ju, T. D.; Hoff, C. D. Unpublished results.
(8) Details of construction of the FTIR microscope reaction system will
be published later.
(9) (a) Halpern, J. Inorg. Chim. Acta. 1982, 62, 31 and references therein.
(b) The original proposal by Bodenstein of third-order reactions occurring
by an initial “sticky collison” between two of the reactants which then
collides with the third applies. See: Bodenstein, M. Z. Phys. Chem. 1922,
100, 118.
(10) (a) Two reasonable alternative mechanisms, both suggested by the
referees, would be to view the initial adduct between the chromium radical
and the thiol as a hypervalent sulfur radical rather than a 19-electron
organometallic complex and that the order of assembly of the transition
state could go through initial formation of a metal-metal dimer which then
reacts with thiol. The authors considered these alternatives also and give
preference to the 19-electron adduct on the basis of the literature data (ref
10b,c). In addition, failure to deviate from third-order kinetics at high
thiophenol concentration provides some argument that in steady state
treatment of the kinetics it is the thiol adduct that is formed first. The related
radical/dimer system [Cr(CO)3C5H5]2 reacts more slowly with thiols than
does •Cr(CO)3C5Me5. Since the former exists primarily as a dimer in solution
and the later primarily as a radical, it seems more likely that it is the initial
interaction with thiol that is important. (b) Tyler, D. R. Acc. Chem. Res.
1991, 24, 325. (c) Geiger, W. E. Acc. Chem. Res. 1995, 28, 351.
(11) Lang, R. F.; Ju, T. D.; Kiss, G.; Hoff, C. D.; Bryan, J. C.; Kubas,
G. J. Inorg. Chem. 1994, 33, 3899.
(5) (a) For reaction 1, the enthalpy of reaction in toluene solution is -18.6
( 1.8 kcal/mol for R ) Ph. Using the value of 79 kcal/mol for the PhS-H
bond strength5b and 62.34b for H-Cr(CO)3C5Me5 leads to the value of 35.3
kcal/mol. The BuS-Cr bond strength was determined from the enthalpy
of thiol/thiolate exchange: BuS-Cr + PhSH f PhS-Cr + BuSH, ∆H )
-2.5 ( 0.2 kcal/mol. The BuS-H bond is 89 kcal/mol.5c Using values for
PhSH of 795b and PhS-Cr of 35.3 (see above) leads directly to the value
of 42.8 kcal/mol. The estimates of 35 and 43 kcal/mol are considered
accurate to (3 kcal/mol. (b) Bordwell, F. G.; Zhang, X. M.; Satish, A. V.;
Cheng, J. P. J. Am. Chem. Soc. 1994, 116, 6605. (c) Brauman, J. I. In
Frontiers in Free Radical Chemistry; Pryor, W. A., Ed.; Academic Press:
New York, NY, 1980; pp 23-30.
(6) We have recently reported measurement of the W(CO)3(PCy3)2-SR
bond strength in solution; however this was relative to an agostic bond
whose exact strength is not known: Lang, R. F.; Ju, T. D.; Kiss, G.; Hoff,
C. D.; Bryan, J. C.; Kubas, G. J. J. Am. Chem. Soc. 1994, 116, 7917.
(12) Ju, T. D.; Lang, R. F.; Hoff, C. D. Unpublished results.
(13) The crystal structure of Cr(CO)5(tBuSH) has been reported recent-
ly: Darensbourg, M. Y.; Longridge, E. M.; Payne, V.; Reibenspies, J. R.;
Riordan, C. G.; Springs, J. J.; Calabrese, J. C. Inorg. Chem. 1990, 29, 2721.
S0002-7863(96)00082-0 CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 22, 1996 5329
W(CO)3(PCy3)2(H)(SR) + Cr′′ f W(CO)3(PCy3)2(SR)• +
H-Cr′′
Cr′′ ) Cr(CO)2(PPh3)C5H5 (5)
The first two steps in Scheme 1 correspond to “dissection”
of the sulfur-hydrogen bond as in eq 3 for free butanethiol,
but with the difference that the sulfur remains bound to tungsten.
The stable 18-e- complex in Scheme 1 thus serves as a model
for the proposed unstable 19-e- complex in Figure 1. Kinetic
studies show a dramatic influence of metal coordination on
the rate of H atom transfer from sulfur to metal. The first two
steps in Scheme 1 obey the second-order equation: rate )
k
obs[W(CO)3(phen)(BuSH][•Cr(CO)3C5Me5]. The value of kobs
at 25 °C , 5 × 104 M-1 s-1, is in the range of those reported for
similar H atom transfer to metal radicals.15 The 27 kcal/mol
thermodynamic barrier to cleavage of the sulfur hydrogen bond
in eq 3 has been reduced to near zero14b for the bound thiol.
This is attributed to the increase in the metal-sulfur bond
strength that occurs concomitantly with H atom abstraction as
the weak metal-thiol bond is converted to the stronger metal-
thiolate bond. Increase in the acidity of thiols upon coordination
has been demonstrated;16 however this work demonstrates that
the sulfur-hydrogen bond of thiols is effectively reduced some
25-30 kcal/mol upon coordination.
Few metals have strong enough bonds to hydrogen17 to
“single-handedly” cleave the sulfur-hydrogen bond and gener-
ate a free thiyl radical as shown in eq 3. Prior coordination to
a metal changes that picture. The large reduction in the sulfur-
hydrogen bond strength observed in the stable 18-e- complex
W(CO)3(phen)(BuSH) is proposed to be similar to that in the
postulated 19-e- complex RSH‚‚‚Cr(CO)3C5Me5. Independent
of its exact order of assembly, reaction 1 requires the presence
of all three reactants in the transition state. The weak nature
of the Cr-SR and Cr-H bonds relative to the H-SR bond is
why the termolecular mechanism is the lowest free energy
pathway in spite of a large entropic barrier: which might be
reduced in properly designed complexes. Dinuclear oxidative
addition of thiols to 2 mol of the weak chromium radical has
been shown to be catalyzed by presence of a third mol of metal
complex, (W(CO)3(phen)(EtCN)), which can be viewed as a
more efficient thiol receptor. The mechanisms proposed here
may be relevant to enzymatic and surface reactions in which
simultaneous (or near simultaneous) attack of the sulfur
hydrogen bond by two or more metals occurs.
Figure 1. Reaction profile (kcal/mol) for 2•Cr(CO)3C5Me5 + PhSH
f H-Cr(CO)3C5Me5 + PhS-Cr(CO)3C5Me5.
Scheme 1
Acknowledgment. Support of this work by the National Science
Foundation, Grant CHE-9221265, is gratefully acknowledged.
JA960082K
(14) (a) For solubility reasons, this reaction was studied in methylene
chloride solution. The rate of reaction 1 was also studied in methylene
chloride, and its rate is essentially the same as it is in toluene. Due to the
rapid nature of this reaction the high-pressure continuous flow reactor system
was used. (b) Preliminary estimates of the activation parameters for the
first two reactions in Scheme 1 are ∆Hq ) +3 ( 2 kcal/mol and ∆Sq )
-25 ( 5 cal/mol deg. The low enthalpy of activation is consistent with a
rate-determining first step which has ∆H° < 0. (c) The complex
W(CO)3(phen)[BuS-Cr(CO)3C5Me5] is assigned on the basis of its infrared
spectrum which resembles that of the seperate components: BuS-Cr-
(CO)3C5Me5 [1998, 1937, and 1916(sh) cm-1] and W(CO)3(phen)[BuS-H]
[1901 and 1785 cm-1] with the difference that the bands of the chromium
thiolate are shifted to higher wavenumber by about 4 cm-1 whereas the W
complex bands are shifted a comparable amount to lower wavenumber.
These are the shifts expected on the basis of simple complex formation
and quite different from those that would occur if oxidative addition
occurred. The same peaks are observed by mixing preformed RS-Cr-
(CO)3C5Me5 and W(CO)3(phen)(EtCN). (d) In addition to its lability, the
complex W(CO)3(phen)[BuS-Cr(CO)3C5Me5] undergoes a slow but clean
reaction with free thiol to produce H-Cr(CO)3C5Me5, CO, and W(CO)2-
(phen)(SR)2 (previously characterized11). This happens in the stoichiometric
and toward the end of the catalytic cycles.
Cr(CO)3C5Me5]. The W(O) complex of the chromium thiolate14c
can be detected in solution, but we have not been able to isolate
it.14d It is labile to ligand exchange with free butanethiol. This
provides the basis for the catalytic cycle in Scheme 1.
The first step in Scheme 1 is H atom transfer to generate 1
mol of HCr(CO)3C5Me5 and the proposed radical complex
W(CO)3(phen)(SBu•). This complex is not detected spectro-
scopically since it undergoes rapid radical combination with a
second mol of the chromium radical to form W(CO)3(phen)-
[BuS-Cr(CO)3C5Me5] in the second step. This proposal is
reasonable since we have recently prepared analogous W(I)
thiolate radical complexes6 stabilized by the presence of bulky
phosphine ligands:
(15) The second-order rate constant4b for the H atom transfer reaction,
•
H-Cr(CO)2(PPh3)Cp + Cr(CO)3C5Me5 f Cr(CO)2(PPh3)Cp + H-Cr-
(CO)3C5Me5 of 103 M-1 s-1, is actually slower than that observed for eq 6.
(16) Darensbourg, M. Y.; Liaw, W. F.; Riordan, C. G. J. Am. Chem.
Soc. 1989, 111, 8051.
(17) Simoes, J. A. M.; Beauchamp, J. L. Chem. ReV. 1990, 90, 629 and
references therein.