Hydrogen tunneling in protonolysis of platinum(II) and palladium(II) methyl complexes: Mechanistic implications

Full text of DOI:10.1021/ja807427d

Published on Web 12/04/2008
Hydrogen Tunneling in Protonolysis of Platinum(II) and Palladium(II) Methyl
Complexes: Mechanistic Implications
John E. Bercaw,* George S. Chen, Jay A. Labinger,* and Bo-Lin Lin
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology,
Pasadena, California 91125
Received September 25, 2008; E-mail: bercaw@caltech.edu; jal@caltech.edu
Scheme 1
The selective transformation of C-H bonds is an important area
of current research; over the past several decades, numerous examples
of C-H activation by transition metals have appeared in the literature.1
Of particular significance is the development of a number of
homogeneous catalytic systems based on late transition metals,
especially platinum2 and palladium.3 Several general mechanisms for
C-H activation by transition metal complexes have been identified,
including (1) electrophilic activation, (2) oxidative addition, (3) σ-bond
metathesis, (4) 1,2-addition to metal-ligand multiple bonds, (5) H-atom
abstraction by metal-oxo complexes, and (6) metalloradical activation.4
Kinetic hydrogen/deuterium isotope effects (KIEs) have been used
extensively by many groups to probe the nature of these reactions and
to gain key mechanistic insights.5
Table 1. KIEsa for the Protonolysis/Deuterolysis of 1d and 3 by
TFE and TFA in DCE-d4 at Various Temperatures
Abnormally large KIEs at room temperature or higher have been
observed for C-H activations involving the latter four mechanisms.5,6
To the best of our knowledge, though, reactions involving elec-
trophilic activation or oxidative addition as well as their corre-
sponding microscopic reverse processes, protonolysis of a metal
alkyl or reductive elimination, have been reported to exhibit only
normal or inverse KIEs at similar temperatures.5,7 These are the
two mechanisms generally proposed for C-H activation by PdII
and PtII.2,3,8 Herein, we report experimental observations of
abnormally large KIEs at room temperature and higher for the
protonolysis of several dimethylpalladium(II) complexes as well
as a dimethylplatinum(II) complex and offer evidence for the
occurrence of proton tunneling in these reactions.
a KIEs determined by average of 3 runs for each temperature.
at 21 °C was determined for the competitive protonolysis of 3 with
trifluoroacetic acid (TFA-d1 and TFA-d0) in dichloroethane-d4 (eq
1). No products of side reactions are detectable by NMR, nor is
deuterium incorporation into 4 observed.
Deuterolysis of dimethylpalladium(II) complexes 19 with excess
TFE-d3 (TFE-d3 ) CF3CD2OD) at room temperature (Scheme 1)
results in the liberation of methane and the corresponding (meth-
yl)(trifluoroethoxy)PdII species 2.10 A surprisingly large amount
of CH4 (∼20%, determined by integration of the two methane
isotopologue 1H NMR signals) is generated along with the expected
CH3D, considering the isotopic purity of the TFE-d3 (>99%,
KIE values greater than 10 at room temperature are generally
taken to indicate significant contribution of quantum mechanical
tunneling in proton transfer reactions.11 The presence or absence
of tunneling can also be determined from the temperature depen-
dence of the KIE.11,12 Values for the Arrhenius parameters outside
the ranges 0.5 < AH/AD < 21/2 (calculations on model systems
suggest 0.7-1.2 is a more realistic range13) and (EaD - EaH) < 1.2
kcal/mol are generally taken to demonstrate unambiguously the
involvement of tunneling. The temperature dependences of kH/kD
for the protonolysis of 1d and 3 are shown in Table 1 and Figure
1; the values indicate a tunneling pathway for the protonolysis of
the corresponding metal-carbon bonds.
Relatively few systematic studies of the temperature dependence
of kH/kD for C-H activation or the microscopic reverse have been
carried out, aside from enzymatic C-H activations.14 Temperature-
dependent KIEs typical of tunneling have been found for the
intramolecular aromatic C-H activation of Cp*2ZrPh2 via σ-bond
metathesis15 as well as in a recent study on C-H activation by an
oxoiron(IV) porphyrin radical cation.16 KIEs have been measured
for methane C-H activation by RhII porphyrin via the metalloradical
mechanism at two different temperatures,6c and parameters calcu-
1
estimated by H NMR); no significant deuterium incorporation is
observed in either the Pd-methyl or ligand (1H NMR) for 2. Thus,
the small amount of residual H+ in the solvent must be responsible
for the CH4 formed. If a 20:1 mixture of TFE-d3/TFE-d0 is used
instead, the ratio of CH4/CH3D increases to ∼1 for all cases,
indicating abnormally large kinetic isotope effects (kH/kD ∼20 at
room temperature) are operating. For 1a-c, a competing reaction
involving reductive elimination of ethane complicates determination
of precise KIE values. However, this side reaction is virtually absent
1
(<1%, estimated by H NMR) in the case of 1d, for which kH/kD
is found to be 21.4 ( 0.7 at 21 °C in dichloroethane-d4.
Based on these observations, abnormally large KIEs at room
temperature appear to be fairly general for the protonolysis of
dimethylpalladium(II) complexes. In sharp contrast, inverse or
normal KIEs have generally been observed for the corresponding
platinum(II) analogues; but (COD)PtII(CH3)2 3 (COD ) 1,5-
cyclooctadiene) is found to be an exception. A kH/kD of 17.5 ( 0.3
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10.1021/ja807427d CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
operating.18 Given the importance of C-H activation in PdII and
PtII systems, more studies (both experimental and theoretical) are
underway to investigate the correlation between proton tunneling
and the mechanisms involved.
Acknowledgment. This work was supported by BP through the
MC2 program. An NSF Graduate Research Fellowship to GSC is
gratefully acknowledged. We thank Drs. Michael W. Day and
Lawrence M. Henling for assistance with X-ray crystallography
and Dr. Jonathan S. Owen for suggestions in the preparation of
complex 1b.
Supporting Information Available: Detailed experimental data.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 1. Plot of ln(kH/kD) vs 1/T
References
Scheme 2
(1) Dyker, G. Ed. Handbook of C-H Transformations; Wiley-VCH: Weinheim,
Germany, 2005. and references therein.
(2) Lersch, M.; Tilset, M. Chem. ReV. 2005, 105, 2471–2526, and references
therein.
(3) (a) Kao, L. C.; Hutson, A. C.; Sen, A. J. Am. Chem. Soc. 1991, 113, 700–
701. (b) Lin, M. R.; Shen, C. Y.; Garcia-Zayas, E. A.; Sen, A. J. Am.
Chem. Soc. 2001, 123, 1000–1001. (c) Periana, R. A.; Mironov, O.; Taube,
D.; Bhalla, G.; Jones, C. J. Science 2003, 301, 814–818. (d) An, Z. J.;
Pan, X. L.; Liu, X. M.; Han, X. W.; Bao, X. H. J. Am. Chem. Soc. 2006,
128, 16028–16029.
(4) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507–514.
(5) (a) Ryabov, A. D. Chem. ReV. 1990, 90, 403–424. (b) Jones, W. D. Acc.
Chem. Res. 2003, 36, 140–146, and references therein.
(6) (a) Slaughter, L M.; Wolczanski, P. T.; Klinckman, T. R.; Cundari, T. R.
J. Am. Chem. Soc. 2000, 122, 7953–7975. (b) Yoshizawa, K. Coord. Chem.
ReV. 2002, 226, 251–259, and references therein. (c) Cui, W.; Wayland,
B. B. J. Am. Chem. Soc. 2004, 126, 8266–8274.
lated from those data indicate tunneling (kH/kD ) 8.2 at 296 K and
5.1 at 353 K, AH/AD ) 0.43, EaD - EaH ) 1.7 kcal mol-1). It has
been proposed, based on analysis of equilibrium isotope effects
(EIEs), that factors such as partition quotients and zero-point energy
differences can account for the large KIEs observed in the 1,2-
addition of the C-H bond across Ti-N multiple bonds,6a so that
tunneling does not necessarily need to be invoked; a similar
argument was offered for the above-cited metalloradical C-H
activation.6c In light of the temperature parameters, we would
suggest that tunneling may be a more reasonable explanation for
all of these large KIE values.
(7) (a) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996,
118, 5961–5976. (b) Romeo, R.; D’Amico, G. Organometallics 2006, 25,
3435–3446, and references therein Large KIEs have been previously
observed for other types of hydrogen transfer reactions involving inorganic
or organometallic complexes; for examples, see: (c) Whitesides, T. H.;
Neiland, J. P. J. Am. Chem. Soc. 1975, 97, 907–908. (d) Huynh, M. H. V.;
Meyer, T. J. Angew. Chem., Int. Ed. 2002, 21, 1395–1398.
(8) (a) Hill, G. S.; Rendina, L. M.; Puddephatt, R. J. Organometallics 1995,
14, 4966–4968. (b) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 1995, 117, 9371–9372. (c) Romeo, R.; Plutino, M. R.; Elding, L. I.
Inorg. Chem. 1997, 36, 5909–5916. (d) Stahl, S. S.; Labinger, J. A.; Bercaw,
J. E. Angew. Chem., Int. Ed. 1998, 37, 2180–2192. (e) Bartlett, K. L.;
Goldberg, K. I.; Borden, W. T. J. Am. Chem. Soc. 2000, 122, 1456–1465.
(f) Bartlett, K. L.; Goldberg, K. I.; Borden, W. T. Organometallics 2001,
20, 2669–2678. (g) Wik, B. J.; Lersch, M.; Tilset, M. J. Am. Chem. Soc.
2002, 124, 12116–12117.
The abnormally large KIEs and proton tunneling in the proto-
nolysis of (L-L)PdII(CH3)2 and (COD)PtII(CH3)2 suggest a unifying
mechanism. Two alternate routes have been proposed for proto-
nolysis of M-C bonds (and the corresponding microscopic reverse
reaction, C-H activation): direct protonation of the MII-C bond
or protonation at M (oxidative addition) followed by reductive
elimination (Scheme 2). Platinum(IV) hydride intermediates have
been observed at low temperatures for the protonolysis of several
diamine and diimine ligated platinum dimethyl systems, supporting
the oxidative addition mechanism.7,8 In contrast, no [PtIV-H] is
observed by 1H NMR in the protonolysis of 3 at -80 °C in CD2Cl2;
one might expect the electron-withdrawing COD ligand to disfavor
the formation of a Pt(IV) intermediate. [PdIV-H] is also relatively
unfavorable. It is also notable that no scrambling of H/D between
methyl/methane positions is observed in the systems studied (unlike
the diimine-Pt analogues17), further suggesting that [MIV-H]
intermediates are not involved.
While the possibility of the oxidative addition route cannot be
firmly excluded for the cases studied here, we suggest that they
proceed instead by direct protonation at the M-C bond and that
there is a connection between that mechanism and the observed
tunneling. We would also expect to see evidence for proton
tunneling (high KIEs at room temperature or higher) in electrophilic
C-H activation by PdII and PtII when the microscopic reverse of
this process (direct proton loss from the sigma complex) is
(9) Experimental details including syntheses and the X-ray crystal structure
of 1b (CCDC 263612) are provided in the Supporting Information.
(10) Protonolysis of dimethylpalladium(II) complexes by various alcohols has
been previously reported: (a) Kim, Y. J.; Osakada, K.; Sugita, K.;
Yamamoto, T.; Yamamoto, A. Organometallics 1988, 7, 2182–2188. (b)
Kim, Y. J.; Osakada, K.; Takenaka, A.; Yamamoto, A. J. Am. Chem. Soc.
1990, 112, 1096–1104. (c) Kapteijn, G. M.; Dervisi, A.; Grove, D. M.;
Kooijman, H.; Lakin, M. T.; Spek, A. L.; van Koten, G. J. Am. Chem.
Soc. 1995, 117, 10939–10949.
(11) (a) Bell, R. P. Chem. Soc. ReV. 1974, 4, 513–544. (b) Bell, R. P. The Proton
in Chemistry; Cornell University Press: Ithaca, NY, 1973. (c) Bell, R. P.
The Tunnel Effect in Chemistry; Chapman and Hall: London, 1980.
(12) (a) Caldin, E. F. Chem. ReV. 1969, 69, 135–156. (b) Kwart, H. Acc. Chem.
Res. 1982, 15, 401–408. (c) Melander, L.; Saunders, W. H. Reaction Rates
of Isotopic Molecules; Krieger: Malabar, FL, 1987. (d) Limbach, H.; Lopez,
J. M.; Kohen, A. Philos. Trans. R. Soc. London, Ser. B 2006, 361, 1399–
1415.
(13) Schneider, M. E.; Stern, M. J. J. Am. Chem. Soc. 1972, 95, 1517–1522.
(14) Kohen, A. In Hydrogen-Transfer Reactions; Hynes, J. T.; Klinman, J. P.;
Limbach, H. H.; Schowen, R. L. Eds.; Wiley-VCH: Weinheim, Germany,
2007; Vol. 4, pp 1311-1340.
(15) Schock, L. E.; Brock, C. P.; Marks, T. J. Organometallics 1987, 6, 232–
241.
(16) Pan, Z.; Horner, J. H.; Newcomb, M. J. Am. Chem. Soc. 2008, 130, 7776–
7777.
(17) Owen, J. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2006, 128,
2005.
(18) Computational studies on oxidative addition of methane C-H bond to Pd0
and Pt0 and their microscopic reverse have suggested a significant
contribution of proton tunneling only at low temperature: Datta, A.; Hrovat,
D. A.; Borden, W. T J. Am. Chem. Soc. 2008, 130, 2726–2727. Mamaev,
V. M.; Gloriozov, I. P.; Ishchenko, S. Y.; Simonyan, V. V.; Myshakin,
E. M.; Prisyajnyuk, A. V.; Ustynyuk, Y. A. J. Chem. Soc., Faraday Trans.
1995, 91, 3779–3782.
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