Competitive C-H bond activation and β-hydride elimination at platinum(II)

Article abstract of DOI:10.1021/ja0669629

Thermolysis of the five-coordinate Pt(IV) complex (AnIm)Pt(CH₃)₃ (1b) (AnIm = [o-C₆H₄{N(C₆H₃ⁱPr₂)}(CH=NC₆H₃ⁱPr₂)]^-) in benzene-d₆ at 60 °C yields a new Pt(II) olefin hydride complex (2b-d₂₇). For the AnIm complexes and their nacnac analogues, intermolecular arene C-H bond activation is found to be faster than intramolecular β-hydride elimination at 60 °C. In contrast, intermolecular alkane activation is slow relative to β-hydride elimination at 60 °C. Although AnIm and nacnac complexes have similar structures and spectral features, modification of the bidentate nitrogen ligand backbone from nacnac to AnIm dramatically increases the rate of ethane reductive elimination from the five-coordinate platinum(IV) complex and decreases the rate of olefin insertion at the platinum(II) olefin hydride complex. Copyright

Full text of DOI:10.1021/ja0669629

Published on Web 03/07/2007
Competitive C-H Bond Activation and â-Hydride Elimination at Platinum(II)
Susan M. Kloek and Karen I. Goldberg*
Department of Chemistry, Box 351700, UniVersity of Washington, Seattle, Washington 98195-1700
Received September 27, 2006; E-mail: goldberg@chem.washington.edu
Scheme 1
With an available open site at the metal center, â-hydride
elimination is typically a very facile reaction.1 For example, both
theoretical and experimental studies have found barriers for
â-hydride elimination/olefin insertion in diphosphinePt(II)alkyl
complexes to be less than 10 kJ/mol.2 In this contribution, we report
a system wherein intermolecular C-H bond activation of arenes
is shown to be not only competitive but actually faster than
â-hydride elimination. In contrast, intermolecular alkane C-H bond
activation was observed to be slower than intramolecular â-hydride
elimination.
Thermolysis of the five-coordinate platinum(IV) complex (nac-
nac)Pt(CH3)3 (1a) (nacnac ) [{(o-iPr2C6H3)NC(CH3)}2CH]-)3 in
benzene-d6 was reported to lead to reductive elimination of ethane,
methane, and formation of the platinum(II) olefin hydride complex
2a-d27.4 Similar chemistry has now been observed with the related
five-coordinate platinum(IV) alkyl complexes both undergo reduc-
five-coordinate complex (AnIm)Pt(CH3)3 (1b) (AnIm ) [o-C6H4-
tive elimination to form ethane, methane, and the platinum(II) olefin
i
i
{N(C6H3 Pr2)}(CHdNC6H3 Pr2)]-). It had been proposed in the
study of 1a that reversible formation of 2a was likely on the
pathway to H/D exchange and formation of 2a-d27.4 Herein, we
provide definitive evidence that H/D exchange occurs prior to
â-hydride elimination. The comparison of the reactivity of the two
ligands nacnac and AnIm in this context is also important. Although
â-diketiminate (nacnac) ligands have become popular in recent
years,5 one complication that has been observed is the sensitivity
of the central (γ) carbon to electrophilic attack.6 To protect the
ligand backbone from this type of reactivity, modified nacnac
ligands, such as the anilido-imine ligand (AnIm)7 where a phenyl
group has been incorporated into the ligand backbone, have been
developed. Despite the similar structural features and overall
reactivity shared by 1a and 1b, remarkable differences in reaction
rates were observed. Modification of the bidentate nitrogen ligand
has a significant impact on both the rate of reductive elimination
from platinum(IV) and of olefin insertion at platinum(II).
4
hydride complexes 2a-d27 and 2b-d27.
Complex 2b (prepared by thermolysis of 1b in benzene) was
characterized by 1H NMR spectroscopy and X-ray crystallography;8
the X-ray structure of 2b is shown in Figure 1. In complex 2b, the
hydride ligand is located trans to the imine nitrogen, while the
cyclometallated olefin group is located trans to the anilido nitrogen.
The solid-state structure of 2b resembles 2a4 in that both complexes
feature a cyclometallated olefin group that is tilted about 45 °C
relative to the square plane of the complex. The platinum-nitrogen
bond lengths in 2b of 2.009(4) Å (trans to olefin) and 2.081(4) Å
(trans to hydride) closely match those in 2a (2.011(4) Å (trans to
olefin), 2.089(4) Å (trans to hydride). However, the distances
between platinum and the olefinic carbons in 2b (2.126(5) and
2.116(5)Å) are slightly shorter than those of 2a (2.152(6) and 2.128-
1
(6) Å). In the H NMR spectrum of 2b, the hydride appears at
-16.93 as a doublet with platinum satellites (JPt-H ) 1254 Hz,
4JH-H ) 6 Hz), because of coupling with the imine C-H. The Pt-H
coupling value of 1254 Hz is slightly higher than in 2a, where the
hydride signal appears at -17.69 ppm with JPt-H ) 1140 Hz.
A common mechanism for the formation of 2a and 2b from 1a
and 1b, respectively, is proposed (Scheme 2). Upon heating, the
five-coordinate platinum(IV) complex undergoes reductive elimina-
tion of ethane and subsequently activates one of the ligand isopropyl
groups at either the methyl or methine position.4 Reductive
The new five-coordinate Pt(IV) complex (AnIm)Pt(CH3)3 (1b)
was prepared in 70% yield by the reaction of [Pt(CH3)3OTf]4 with
4 equiv of the potassium salt of the anilido-imine ligand in toluene.
Complex 1b was characterized by H NMR spectroscopy, X-ray
crystallography, and elemental analysis.8 The H NMR spectrum
of complex 1b (benzene-d6) displays a singlet with platinum
satellites at 1.14 ppm (JPt-H ) 73.8 Hz) integrating to 9H and cor-
responding to the three Pt-CH3 groups, indicating that the complex
is fluxional in solution.9 The analogous Pt-CH3 signal of the nacnac
derivative 1a, appears at 1.10 ppm (JPt-H ) 74.0 Hz).3
1
1
The solid-state structures of 1a3 and 1b are also similar. The
X-ray structure of 1b (Supporting Information Figure S1) reveals
that like 1a, complex 1b is square pyramidal. The platinum-methyl
distances for 1b (Pt-C1 (trans to anilido nitrogen) ) 2.056(5) Å,
Pt-C2 (trans to imine nitrogen) ) 2.047(5) Å, and Pt-C3 (trans
to open site) ) 2.016(5) Å) are comparable to those for 1a (Pt-
C1 (trans to nitrogen) ) 2.056(4) Å, and Pt-C2 (trans to open
site) ) 2.038(7) Å).
Figure 1. ORTEP drawing of 2b. (Thermal ellipsoids at 50% probability.
Non-hydridic H atoms and a molecule of cocrystallized toluene omitted
for clarity). Select bond distances (Å): Pt1-N1 ) 2.009(4), Pt1-N2 )
2.081(4), Pt1-C26 ) 2.126(5), Pt1-C27 ) 2.116(5).
Very similar chemical reactivity was observed upon thermolysis
of complexes 1a and 1b at 60 °C in benzene-d6 (Scheme 1). The
9
3460
J. AM. CHEM. SOC. 2007, 129, 3460-3461
10.1021/ja0669629 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
The reactivity of (nacnac)Pt(CH3)3 and (AnIm)Pt(CH3)3 was also
compared in alkane solvents. When the thermolysis of 1b was
conducted in cyclohexane-d12 at 60 °C for 90 h, the observed
product was the protio olefin hydride complex 2b-h27. When 1a
was heated in cyclohexane-d12 at 60 °C for 430 h, the protio olefin
hydride product 2a-h27 had incorporated less than 5% deuterium.
Since the olefin hydride products 2a-h27 and 2b-h27 exhibited little
or no deuterium incorporation from the alkane solvent, for both
nacnac and AnIm ligands, the rate of product forming â-hydride
elimination must be fast relative to alkane activation at 60 °C.
Although no isotopic exchange was observed in the 60 °C
thermolysis of 1b in cyclohexane-d12, the AnIm olefin hydride
complex 2b was capable of alkane activation at higher temperatures.
When an n-pentane solution of 2b-d27 was heated at 130 °C for 48
h, deuterium incorporated into the terminal position of n-pentane
elimination of methane (CH4 is the only isotopomer observed)
follows, producing the three-coordinate cyclometallated intermediate
A. Intermediate A can undergo a â-hydride elimination to form
the product 2a or 2b. Intermediate A is also capable of C-H bond
activation. Oxidative addition of a solvent R-D bond followed by
reductive elimination of the ligand isopropyl group leads to
deuterium exchange between the solvent and the isopropyl ligand
groups. Species A may also be generated by olefin insertion at the
platinum(II) olefin hydride product (vide infra).
2
with approximately 87% selectivity (as measured by H NMR).8
A similar selectivity was previously reported for 2a.4
In conclusion, new AnIm complexes 1b and 2b display structural
characteristics and 1H NMR features similar to their nacnac
analogues 1a and 2a. Yet, modification of the bidentate nitrogen
ligand backbone from nacnac to AnIm dramatically increases the
rate of ethane reductive elimination from the five-coordinate
platinum(IV) complex and decreases the rate of olefin insertion at
the platinum(II) olefin hydride complex. Remarkably, although
previous studies indicate â-hydride elimination and olefin insertion
reactions at Pt(II) should be facile, for these complexes at 60 °C,
intermolecular benzene C-H bond activation is clearly fast relative
to intramolecular â-hydride elimination. Intermolecular alkane C-H
bond activation, however, is slow relative to â-hydride elimination.
As unsaturated Pt(II) complexes have been shown in recent years
to be key intermediates in selective hydrocarbon oxidation reac-
tions,11 recognition of such reactivity preferences will be important
for catalyst design.
Acknowledgment. This material is based upon work supported
by the National Science Foundation under Grant No. 0137394. The
crystal structures of 1b and 2b were solved by A. G. Dipasquale
(1b) and J. B. Benedict (2b).
Supporting Information Available: Experimental procedures and
details, X-ray crystallographic data and CIF files for 1b and 2b. This
material is available free of charge via the Internet at http://pubs.acs.org.
The disappearances of both five-coordinate complexes at 60 °C
were monitored by H NMR. First-order kinetics were observed
but dramatically different rates were documented. Ethane reductive
1
elimination from nacnac five-coordinate complex 1a was found to
be nearly an order of magnitude slower (kobs ) 3.0(2) × 10-6 s-1
)
than ethane reductive elimination from AnIm five-coordinate
complex 1b (kobs ) 2.1(2) × 10-5 s-1).
As the thermolyses of 1a and 1b were monitored, it was observed
that even at early reaction times the products 2a-d27 and 2b-d27
contained deuterium in all of the isopropyl and olefinic resonances.10
To determine whether this deuterium incorporation was occurring
through fast, reversible insertion at the platinum(II) olefin hydride
complex and subsequent solvent activation (as previously pro-
posed),4 samples of olefin hydride complexes 2a-h27 and 2b-h27
were independently prepared, heated in benzene-d6 at 60 °C, and
monitored for deuterium incorporation. When heated at 60 °C in
benzene-d6 for 24 h, 2b-h27 showed no detectible deuterium
incorporation. Even after continued heating in benzene-d6 at 60 °C
for 60 h, less than 5% deuterium was incorporated. In contrast,
when 2a-h27 was heated at 60 °C in benzene-d6 for 24 h,
approximately 25% deuterium incorporation was observed. The
greater degree of deuterium incorporation in olefin hydride complex
2a (25%, as compared to less than 5% for 2b) is indicative of a
lower barrier to olefin insertion for the nacnac complex 2a than
for the AnIm complex 2b. However, for both complexes, since only
the fully deuterated olefin hydride complex is produced by the
thermolysis of the five-coordinate platinum(IV) complex under these
conditions, it is evident that at 60 °C benzene activation by
intermediate A is fast relative to â-hydride elimination to form the
products. This unusual observation likely results, at least in part,
from geometric constraints on the intramolecular â-hydride elimina-
tion reaction. Notably, as the temperature is increased, the rate of
the intramolecular â-hydride elimination would be expected to
increase relative to the intermolecular benzene activation.
References
(1) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals,
4th ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2005; 191-201.
(2) See: (a) Spencer, J. L.; Mhinzi, G. S. J. Chem. Soc., Dalton Trans. 1995,
3819. (b) Creve, S.; Oevering, H.; Coussens, B. B. Organometallics 1999,
18, 1967 and references therein.
(3) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2001, 123,
6423.
(4) Fekl, U.; Goldberg, K. I. J. Am. Chem. Soc. 2002, 124, 6804.
(5) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. ReV. 2002, 102,
3031.
(6) (a) Radzewich, C. E.; Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 1998,
120, 9384. (b) Yokota, S.; Tachi, Y.; Itoh, S. Inorg. Chem. 2002, 41,
1342. (c) Basuli, F.; Huffman, J. C.; Mindiola, D. J. Inorg. Chem. 2003,
42, 8003. (d) Brown, E. C.; Aboelella, N. W.; Reynolds, A. M.; Aullon,
G.; Alvarez, S.; Tolman, W. B. Inorg. Chem. 2004, 43, 3335. (e) Gregory,
E. A.; Lachicotte, R. J.; Holland, P. L. Organometallics 2005, 24, 1803.
(7) Hayes, P. G.; Welch, G. C.; Emslie, D. J. H.; Noack, C. L.; Piers, W. E.;
Parvez, M. Organometallics 2003, 22, 1577.
(8) See Supporting Information.
(9) Cooling a solution of 1b in CD2Cl2 to 220 K did not result in any
broadening or decoalescence of the Pt-CH3 signal.
(10) In the thermolysis of 1b, the Pt(II) olefin hydride complex having cis
imine and hydride groups (cis-2b) was observed as a kinetic product
(maximum of 40% of the observed products at 60% conversion).
Deuterium was present in all of the isopropyl and olefinic resonances,
and cis-2b reacted further to fully convert to the thermodynamic product
2b by the end of the thermolysis. See supporting information.
(11) For example, see: (a) Lersch, M.; Tilset, M. Chem. ReV. 2005, 105, 2471.
(b) Fekl, U.; Goldberg, K. I. AdV. Inorg. Chem. 2003, 54, 259.
At higher temperatures, the AnIm platinum(II) olefin hydride
complex 2b does undergo insertion and solvent activation. When
complex 2b-h27 was heated at 130 °C in benzene-d6, deuterium
incorporation was gradually observed over the course of about 8
h. Under the same conditions, deuterium incorporation into 2a-h27
was complete in 30 min, consistent with a higher barrier for olefin
insertion with the AnIm ligand than the nacnac ligand.
JA0669629
9
J. AM. CHEM. SOC. VOL. 129, NO. 12, 2007 3461