Hydrocarbon activation at a cationic platinum(II) diimine aqua complex under mild conditions in a hydroxylic solvent [11]

Full text of DOI:10.1021/ja984028a

1974
J. Am. Chem. Soc. 1999, 121, 1974-1975
CHCHdN(p-MeOC6H4), the oxidation resulted in intermolecular
methyl transfer reactions giving (diimine)PtIV(NCMe)(CH3)3+ and
(diimine)PtII(NCMe)(CH3)+. The latter was presumably formed
via the intermediacy of (diimine)Pt(CH3)+, an analogue of the
species believed to be responsible for the C-H activation in
Bercaw’s (tmeda)PtII system.5 In this contribution, we report that
benzene and methane C-H activation can be achieved at a related
(diimine)Pt aqua complex in a hydroxylic solvent, under mild
and neutral conditions.
Hydrocarbon Activation at a Cationic Platinum(II)
Diimine Aqua Complex under Mild Conditions in a
Hydroxylic Solvent
Lars Johansson,† Olav B. Ryan,‡ and Mats Tilset*,†
Department of Chemistry, UniVersity of Oslo
P.O.Box 1033 Blindern, N-0315 Oslo, Norway
SINTEF Applied Chemistry
The protonation of the above-mentioned (diimine)Pt(CH3)2
complexes with HOTf, HBF4, or H(Et2O)2BArf in the poorly
coordinating solvents dichloromethane and nitromethane invari-
ably led to extensive decomposition as evidenced by the plethora
of new signals that appeared in the 1H NMR spectra of the
solutions after the reactions. We have tentatively attributed this
behavior to a propensity of the in situ generated (diimine)Pt(CH3)+
species to undergo intermolecular reactions with the diimine
ligands of adjacent Pt complexes. This degradation presumably
might be avoided by appropriate protection of the N-aryl groups
of the diimine. A 3,5-bis(trifluoromethyl)phenyl substituted
diimine9 (henceforth to be denoted N-N) was selected as a new
ligand; the corresponding dimethylplatinum complex 110 was
Department of Hydrocarbon Process Chemistry
P.O.Box 124 Blindern, N-0314 Oslo, Norway
ReceiVed NoVember 23, 1998
The direct, selective conversion of alkanes into value-added
chemical products has long been a “holy grail” for chemists1 and
intensified efforts toward this goal have been made during the
past few years.2 The reaction mechanism of the classical Shilov
system, in which methane is catalytically functionalized by PtII
salts in aqueous media, has been probed in some detail.3 The
actual C-H activation step in this reaction has recently been
extensively investigated by the Bercaw group, utilizing a (tmeda)-
PtII model system.4,5 The observed exchange of labeled 13CH3
for CH3 between (tmeda)Pt(NC5F5)(CH3)+ and 13CH4 in penta-
fluoropyridine, a solvent of low basicity and coordinating ability,
demonstrated the activation of methane C-H bonds. Although
intermediates in the exchange reaction were not observed,
circumstantial evidence suggested an oxidative addition pathway
involving a PtII methane σ-complex and a PtIV methyl hydride
complex. Further demonstrating the potential for alkane activation
at Pt complexes, Goldberg and co-workers showed through the
reaction between (η2-Tp′)PtII(CH3) and various hydrocarbons
R-H that it is possible to trap the normally elusive 5-coordinated
oxidative addition product by intramolecular pyrazole coordina-
tion, ultimately producing PtIV complexes (η3-Tp′)Pt(H)(CH3)-
(R).6 Most recently, Pt-catalyzed conversion of methane to methyl
bisulfate was achieved with the complex (bpym)PtCl2 in highly
acidic media.7
straightforwardly prepared in its reaction with Pt2Me4(µ-SMe2)2.
The two CF3 groups serve to sterically as well as electronically
protect each ring against electrophilic attack by reactive cationic
Pt species. In addition, the CF3 groups also decrease the electron
density11 and thence increase the electrophilicity of the metal
center. This effect should further activate the complex with respect
to reactions with hydrocarbon C-H bonds.
For the investigation of the reactivity of derivatives of 1 toward
hydrocarbons, it was desirable to prepare 16-electron complexes
of the type [(N-N)Pt(CH3)+](L), were L is a weakly coordinated
neutral or anionic ligand. High reactivity might be achieved in a
solvent that is both inert and has a poor affinity for the
electrophilic metal center, while at the same time being sufficiently
polar to dissolve the cationic complex. For this purpose, we
selected 2,2,2-trifluoroethanol (TFE), a low-nucleophilicity but
quite strongly ionizing solvent12 that has been frequently used in
the investigations of organic nucleophilic substitution reactions.
Importantly, deuterated TFE-d3 is available, although relatively
expensive, for NMR studies.
An interesting common feature in the above examples is the
success of employing bidentate nitrogen ligands to obtain PtII
complexes that are reactive toward C-H bond activation. We
have recently8 reported a study of the one-electron oxidation
reactions of PtII complexes (diimine)Pt(CH3)2 (diimine ) ArNd
CHCHdNAr or ArNdCMeCMedNAr, with Ar ) p-MeC6H4 or
p-MeOC6H4) in acetonitrile. For the diimine (p-MeOC6H4)Nd
* Corresponding author. E-mail: mats.tilset@kjemi.uio.no.
† University of Oslo.
‡ SINTEF.
(1) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc.
Chem. Res. 1995, 28, 154.
When 1 equiv of HBF4‚Et2O is added to a dichloromethane
solution of 1 at -40 °C in the presence of ca. 3 equiv of water,
an orange suspension is formed. Removal of the solvent and
drying in vacuo gives an air-stable product that appears to be the
(2) (a) Shilov, A. E.; Shulpin, G. B. Chem. ReV. 1997, 97, 2879. (b) Stahl,
S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed. Engl. 1998, 37,
2180. (c) Bengali, A. A.; Arndtsen, B. A.; Burger, P. M.; Schultz, R. H.;
Weiller, B. H.; Kyle, K. R.; Moore, C. B.; Bergman, R. G. Pure Appl. Chem.
1995, 67, 281. (d) Crabtree, R. H. Chem. ReV. 1995, 95, 987.
(3) For leading references, see ref 2a and: (a) Luinstra, G. A.; Wang, L.;
Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Organomet. Chem. 1995, 504,
75. (b) Hutson, A. C.; Lin, M.; Basickes, N.; Sen, A. J. Organomet. Chem.
1995, 504, 69.
(9) The diimine ligand was synthesized by modification of methods for
the preparation of similar diimines: tom Dieck, H.; Svoboda, M.; Grieser, T.
Z. Naturforsch. 1981, 36B, 823. For details, see Supporting Information.
(10) (N-N)Pt(CH3)2 (1). Prepared from the diimine and Pt2(CH3)4(µ-SMe2)2
(Scott, J. D.; Puddephatt, R. J. Organometallics 1983, 2, 1643). 1H NMR
(200 MHz, dichloromethane-d2) δ 1.14 (s, 2J(195Pt-H) ) 87.2 Hz, 6 H, PtCH3),
1.35 (s, 6 H, NCMeCMeN), 7.57 (s, 4 H, ArHo), 7.89 (s, 2 H, ArHp). 19F
NMR (188 MHz, dichloromethane-d2) δ -63.14 (s, ArCF3).
(11) This is corroborated by the following reversible electrode potentials
for the first ligand-centered reductions of (ArNdCMeCMedNAr)Pt(CH3)2
(cyclic voltammetry, acetonitrile/0.1 M Bu4NPF6): Ar ) p-MeC6H4, E° )
-1.75 V;8 p-MeOC6H4, -1.76 V;8 3,5-(CF3)2C6H3, -1.42 V vs Cp2Fe/Cp2-
Fe+.
(4) Abbreviations: tmeda ) tetramethylethylenediamine; Tp′ ) hydridotris-
(3,5-dimethylpyrazolyl)borate; bpym ) 2,2′-bipyrimidine; OTf- ) triflate;
BArf- ) [3,5-(CF3)2C6H3]4B-.
(5) (a) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996,
118, 5961. (b) Holtcamp, M. W.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 1997, 119, 848. (c) Holtcamp, M. W.; Henling, L. M.; Day, M. W.;
Labinger, J. A., Bercaw, J. E. Inorg. Chim. Acta 1998, 270, 467.
(6) Wick, D. D.; Goldberg, K. I. J. Am. Chem. Soc. 1997, 119, 10235.
(7) Periana, R. A.; Taube, J. D.; Gamble, S.; Taube, H.; Satoh, T.; Fujii,
H. Science 1998, 280, 560.
(12) (a) Schadt, F. L.; Bentley, T. W.; Schleyer, P. v. R. J. Am. Chem.
Soc. 1976, 98, 7667. (b) Lowry, T. H.; Richardson, K. S. Mechanism and
Theory in Organic Chemistry, 3rd ed.; Harper and Row: New York, 1987;
pp 335-340.
(8) Johansson, L.; Ryan, O. B.; Rømming, C.; Tilset, M. Organometallics
1998, 17, 3957.
10.1021/ja984028a CCC: $18.00 © 1999 American Chemical Society
Published on Web 02/23/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 9, 1999 1975
-
-
aqua complex [(N-N)Pt(CH3)(OH2)]+(BF4 ) (2(BF4 )).13 The 1H
and 19F NMR spectra of 2 in TFE-d3 are consistent with a species
of lower symmetry than 1, and exhibit separate signals for the
equilibrium between (N-N)PtII(CH3)(π-benzene)+, (N-N)PtIV-
(H)(CH3)(C6H5)+, and (N-N)PtII(σ-CH4)(C6H5)+ intermediates.
The likely involvement of a methane σ-adduct prompted us to
investigate the possible reaction between 2(BF4-) and methane.
It was indeed found that reaction with methane occurs under mild
conditions.
two halves of the diimine ligand. The J(195Pt-CH3) coupling
2
constant of 73.3 Hz is close to the observed couplings in related
cationic species of the type (diimine)Pt(CH3)(L)+.8 However,
signals due to coordinated water could not be observed in the
NMR spectra of 2 in TFE-d3, presumably because of facile H/D
exchange with the solvent. We have considered the possibility
that BF4- or the solvent TFE-d3 might be coordinated at (N-N)-
When a solution of 2(BF4-) in TFE-d3 at 45 °C is exposed to
1
20-25 bar of 13CH4 (ca 20 equiv in solution by H NMR), the
exchange of 13CH3 for CH3 in the methyl group in 2 (eq 2) is
readily observed by 1H and 13C{1H} NMR. The Pt-CH3 singlet
undergoes a gradual replacement by a doublet, 1J(13C-H) ) 129.0
Hz, centered at the same chemical shift of δ 0.79. In the 13C NMR
spectrum, the Pt-CH3 resonance at δ -10.2, with 195Pt satellites
(1J(195Pt-C) ) 744 Hz), grows in intensity. After a reaction time
of 45 h, the extent of 13C incorporation was estimated to be ca.
43% of the total Pt-CH3 present as 2 (1H NMR integration).
When the reaction was performed in the presence of ca. 11 equiv
of added water, but under otherwise identical conditions to those
above, a substantial decrease in reaction rate was observed; after
48 h the extent of 13C incorporation in the methyl group was
merely ca. 24%. This result suggests that the methane C-H
activation at 2 requires dissociation of the aqua ligand to generate
a vacant coordination site.
-
Pt(CH3)+. Coordination of BF4 could be readily discounted
because the generation of 2 from 1 with HOTf instead of HBF4‚
Et2O gave a product 2(OTf-) with an identical 1H NMR spectrum
to that of 2(BF4-) in TFE-d3. Furthermore, the 19F NMR resonance
for 2(BF4-) appeared at the same chemical shift as the BF4- reso-
nances of coordinatively saturated BF4- salts. Further support for
coordination of water was obtained from the low-temperature
1
(-20 °C) H NMR spectrum of 2 in dichloromethane-d2 with
small amounts of added TFE.14 A signal attributed to coordinated
H2O appeared at δ 6.51 (br s, 2H). This signal was seen even
with as much as 250 equiv of TFE present, establishing that TFE
is a much poorer ligand than water. It appears likely that water
remains coordinated when TFE is the solvent as well. The ele-
mental analysis of 2(BF4-) also support that 2 is indeed (N-N)-
Pt(CH3)(OH2)+. The material 2(BF4-) as isolated from the reaction
is at least 95% pure by 1H NMR.15 When dissolved in acetonitrile,
2(BF4-) cleanly produces [(N-N)Pt(CH3)(NCMe)]+(BF4-).16
In TFE solution, 2(BF4-) readily reacts with aromatic C-H
Slow decomposition of 2, accompanied by precipitation of a
thus far unidentified red material, was observed under the
1
experimental conditions. By H NMR spectroscopy, the extent
of decomposition after 45 h was estimated at ca. 20% with or
without methane present when no water was added. Decomposi-
tion was diminished to ca. 5% with 11 equiv of water added to
the solution. There was no discernible formation of platinum black
in these reactions.
Two fundamentally different mechanisms are predominant in
the activation of alkane C-H bonds. Oxidative addition is
predominant for the late, relatively low-valent metal complexes,
whereas the σ-bond metathesis pathway is normally observed for
early, usually d0, metals. Recently, Bergman and co-workers
reported alkane activation at the cationic IrIII center Cp*Ir(PMe3)-
(CH3)+.19 Recent theoretical work has suggested that an oxidative
addition pathway via IrV intermediates is preferred in these
reactions.20 For the current (diimine)Pt system, ongoing DFT
calculations suggest that oxidative addition is preferred also in
this case.21
bonds. For example, it is quantitatively converted to the phenyl-
substituted analogue [(N-N)Pt(C6H5)(OH2)]+(BF4
)
in TFE-
- 17
d3 (2-3 h, ambient temperature) under elimination of methane
in the presence of 30 equiv of benzene (eq 1). Addition of aceto-
nitrile to the solution after the reaction cleanly gives [(N-N)Pt-
(C6H5)(NCMe)]+(BF4-).18 When C6D6 is reacted with 2(BF4-),
multiple incorporation of deuterium in the resulting methane
The C-H activation reactions that we have described at the
complex 2 appears to occur under the mildest reaction conditions
yet reported for such processes at cationic Pt complexes. The use
of a hydroxylic solvent makes these results particularly relevant
to the classical Shilov system for alkane activation and function-
alization.
1
(CH3D, CH2D2, CHD3) is observed by H NMR spectroscopy.
Similar behavior was reported in an analogous reaction between
(tmeda)Pt(CH3)(NC5F5)+ and C6D6,5b,c and by analogy, our
findings are readily explained by the occurrence of a dynamic
Acknowledgment. We gratefully acknowledge generous support from
the Norwegian Research Council, NFR (stipend to L.J.), and from Statoil
under the VISTA program, administered by the Norwegian Academy of
Science and Letters. We thank Hanne Heiberg and Ole Swang for helpful
discussions and Aud M. Bouzga for kind assistance with some NMR
experiments.
(13) [(N-N)Pt(CH3)(OH2)]+(BF4-) (2(BF4-)). 1H NMR (200 MHz, TFE-
d3) δ 0.79 (s, 2J(195Pt-H) ) 73.3 Hz, 3 H, PtMe), 1.80 (s, 3 H, NCMeC′MeN),
2.05 (s, 3 H, NCMeC′MeN), 7.58 (s, 2 H, ArHo), 7.71, (s, 2 H, Ar′Ho), 8.00
(s, 2 H, ArHp and Ar′Hp). 19F NMR (188 MHz, TFE-d3) δ -151.83 (s, 4 F,
BF4-), -63.64 (s, 6 F, ArCF3), -63.52 (s, 6 F, Ar′CF3).
(14) At ambient temperature, complex 2(BF4-) is only poorly soluble in
dichloromethane and decomposes slowly under elimination of methane to
uncharacterized products. The addition of 250 equiv of TFE improved the
solubility: 1H NMR (200 MHz, dichloromethane-d2, -20 °C) δ 0.62 (s,
2J(195Pt-H) ) 73.5 Hz, 3 H, PtMe), 1.81 (s, 3 H, NCMeC′MeN), 2.07 (s, 3
H, NCMeC′MeN), 6.51 (br s, Pt(OH2)), 7.58 (s, 2 H, ArHo), 7.70, (s, 2 H,
Ar′Ho) 7.96 (s, 2 H, ArHp and Ar′Hp).
Supporting Information Available: Synthesis, characterization, and
spectroscopic data for all new compounds (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
JA984028A
(15) Due to the high reactivity of the complex towards most solvents, we
have been unable to establish a satisfactory workup or recrystallization
procedure.
(18) [(N-N)Pt(C6H5)(NCMe)]+(BF4-). 1H NMR (200 MHz, TFE-d3) δ 2.01
(s, 3 H, PtNCMe), 2.14 (s, 3 H, NCMeC′MeN), 2.28 (s, 3 H, NCMeC′MeN),
6.65-6.75 (m, 5 H, Pt-C6H5), 7.27 (s, 2 H, ArHo), 7.65 (s, 1 H, ArHp), 7.81
(s, 2 H, Ar′Ho), 8.07 (s, 1 H, Ar′Hp). 19F NMR (188 MHz, TFE-d3) δ -152.01
(s, 4 F, BF4-), -63.63 (s, 6 F, ArCF3), -63.37 (s, 6 F, Ar′CF3). Identical to
a sample prepared from (N-N)Pt(C6H5)2 and HBF4‚Et2O in acetonitrile.
(19) (a) Arndtsen, B. A.; Bergman, R. G. Science 1996, 270, 1970. (b)
Luecke, H. F.; Bergman, R. G. J. Am. Chem. Soc. 1997, 119, 11538.
(20) (a) Strout, D. L.; Zaric, S.; Niu, S.; Hall, M. B. J. Am. Chem. Soc.
1996, 118, 6068. (b) Su, M.-D.; Chu, S.-Y. J. Am. Chem. Soc. 1997, 119,
5373. (c) Niu, S.; Hall, M. B. J. Am. Chem. Soc. 1998, 120, 6169.
(21) Heiberg, H.; Swang, O., personal communication.
(16) [(N-N)Pt(CH3)(NCMe)]+(BF4-). 1H NMR (200 MHz, TFE-d3) δ 0.71
(s, 2J(195Pt-H) ) 74.5 Hz, 3 H, PtMe), 2.02 (s, 3 H, NCMeC′MeN), 2.10 (s,
3 H, Pt(NCMe)), 2.13 (s, 3 H, NCMeC′MeN), 7.59 (s, 2 H, ArHo), 7.74, (s,
2 H, Ar′Ho), 8.01 (s, 1 H, ArHp), 8.04 (s, 1 H, Ar′Hp). 19F NMR (188 MHz,
TFE-d3) δ -152.06 (s, 4 F, BF4-), -63.64 (s, 6 F, ArCF3), -63.41 (s, 6 F,
Ar′CF3).
(17) [(N-N)Pt(C6H5)(OH2)]+(BF4-) (not isolated). 1H NMR (200 MHz,
TFE-d3) δ 1.91 (s, 3 H, NCMeC′MeN), 2.19 (s, 3 H, NCMeC′MeN), 6.65-
6.85 (m, 5 H, Pt-C6H5), 7.29 (s, 2 H, ArHo), 7.65 (s, 1 H, ArHp), 7.79 (s, 2
H, Ar′Ho), 8.05 (s, 1 H, Ar′Hp). 19F NMR (188 MHz, TFE-d3) δ -151.95 (s,
4 F, BF4-), -63.65 (s, 6 F, ArCF3), -63.48 (s, 6 F, Ar′CF3).