Modulating sterics in trimethylplatinum(IV) diimine complexes to achieve C-C bond-forming reductive elimination

Article abstract of DOI:10.1021/om200508k

Tuning the aryl substituents of N, N-diaryl-2,3-dimethyl-1,4-diaza-1,3- butadiene (DAB) ligands promotes the challenging C-C bond-forming reductive elimination from Pt^IV diimine complexes [(DAB)Pt(CH₃) ₃(solvent)]⁺

Full text of DOI:10.1021/om200508k

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pubs.acs.org/Organometallics
Modulating Sterics in Trimethylplatinum(IV) Diimine Complexes To
Achieve CꢀC Bond-Forming Reductive Elimination
Michael P. Lanci,†,§ Matthew S. Remy,‡ David B. Lao,† Melanie S. Sanford,*,‡ and James M. Mayer*,†
†Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States
‡Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States
S Supporting Information
b
ABSTRACT: Tuning the aryl substituents of N,N0-diaryl-2,3-dimethyl-1,4-diaza-
1,3-butadiene (DAB) ligands promotes the challenging CꢀC bond-forming reduc-
tive elimination from PtIV diimine complexes [(DAB)Pt(CH3)3(solvent)]+ (2)
under mild conditions. Experimental results and density functional calculations
indicate that 2,6-aryl substitution promotes reductive elimination by facilitating
dissociation of the coordinating solvent by close to 10 kcal molꢀ1, but too much
steric bulk inhibits the formation of 2 in the one-electron outersphere oxidation of
(DAB)Pt(CH3)2 (1).
ransition-metal-catalyzed CꢀH activation/CꢀC bond-
Scheme 1. Proposed Catalytic Cycle for Pt-Catalyzed
Methane Dimerization
T
forming reactions provide atom-economical routes for the
synthesis of commodity chemicals, pharmaceuticals, natural
products, and materials.1 Palladium-based catalysts have been
widely used for such transformations, in part because CꢀC
bond-forming reductive elimination generally proceeds effi-
ciently from Pd centers.2 The development of related Pt-cata-
lyzed reactions is attractive because PtII is well-known to effect
the CꢀH activation of substrates that are challenging using
analogous PdII species. For example, a number of PtII complexes
have been shown to promote stoichiometric and/or catalytic
activation of simple alkanes, including methane.3 However,
despite significant efforts in this area, Pt-catalyzed alkane CꢀH
functionalization reactions to generate new CꢀC bonds have not
yet been achieved.
We hypothesized that increased steric bulk in the diimine
ligand environment would facilitate reductive elimination from
[(DAB)PtIV(CH3)3(solvent)]+ (2). This hypothesis is predi-
cated on the observation that sterically encumbered diimine
ligands accelerate CH3I reductive elimination from PtIV;14
furthermore, bulky ligands have been used to promote reductive
elimination in other systems.15 In this communication we
demonstrate that modification of the DAB ligands and reaction
solvent can be used to achieve facile CꢀC reductive elimination
of ethane. In addition, we show that the diimine ligand structure
also strongly influences the oxidative disproportionation reaction
that converts 1 to 2. These studies have facilitated the identifica-
tion of a DAB ligand substitution pattern that promotes both
steps 2 and 3 in Scheme 1 efficiently at room temperature.
A series of (DAB)PtII(CH3)2 complexes (1AꢀD) have been
prepared with differently substituted aryl rings: 4-methyl (A),
One particularly exciting target for Pt-catalyzed CꢀH activa-
tion/CꢀC coupling would be the catalytic oxidative dimerization of
CH4.4 This transformation would provide a direct method for
upgrading methane to ethane, which is a key precursor to ethylene
and higher alkanes. A potential catalytic cycle for Pt-catalyzed
methane dimerization could start with methane CꢀH activation3
to form a PtIIꢀCH3 complex (Scheme 1, step 1). Oxidatively induced
disproportionation could then produce a trimethylplatinum(IV)
complex, as has been demonstrated by Bercaw5 and Tilset6 for
(DAB)PtII(CH3)2 (1; DAB = N,N0-diaryl-2,3-dimethyl-1,4-diaza-
1,3-butadiene) and by us for related Pd species7 (Scheme 1, step 2).8
Importantly, similar (DAB)PtII complexes are known to activate
methane, although typically only by methyl group exchange.9 The
final step of the catalytic cycle would then involve CꢀC bond-
forming reductive elimination from PtIV to generate ethane
(Scheme 1, step 3). Such reductive elimination is typically challenging
for PtIV methyl complexes with diimine ligands,10,11 although it is
known for related complexes with phosphine or β-diketiminate
ligands and for PdIV polymethyl adducts.10,12,13
Received: June 13, 2011
Published: June 27, 2011
r
2011 American Chemical Society
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|

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Chart 1. Yields of Reductive-Elimination Products, C2H6
and 3, from Thermolysis of 2 in Acetone-d6 for 8.5 h at 60 °C
(Scheme 2)a
Scheme 2. Mechanism for the Fluxionality of 2C,D and Their
Reductive Elimination of Ethane
aThe 1,4-diaza-1,3-butadiene ligands AꢀD are shown.
3,5-dimethyl (B), 2,6-dimethyl (C), and 2,6-diisopropyl (D)
(shown in Chart 1).16,17 The cationic PtIV solvento complexes
[(DAB)PtIV(CH3)3(solv)]+ (2AꢀD) were then generated by
treating 1AꢀD with 1 equiv of CH3I followed by AgPF6 in
1
acetone-d6 (eq 1).18,19 The H NMR spectra of 2A,B in acet-
one-d6 at ambient temperature exhibit two distinct PtꢀCH3
resonances in the expected 2:1 ratio. In contrast, 2C,D show a
single PtꢀCH3 signal, indicating that the three methyl ligands are
in fast exchange.10b This exchange is expected to proceed via the
five-coordinate cationic intermediate I-1 (Scheme 2, top).20 As
such, these data suggest that I-1 is significantly more accessible in
the more sterically crowded complexes 2C,D.
Figure 1. Reaction coordinate for C2H6 elimination (gas-phase free
energies in kcal molꢀ1) from DFT calculations.17
(with solv = acetone) were computed using the B3LYP func-
tional in Gaussian 03 and the basis sets LANL2TZ on Pt and
6-31G** on C, H, N, and O (Figure 1).23 Since it is well-known
that reductive elimination reactions from six-coordinate PtIV
trimethyl complexes proceed via initial ligand dissociation,10,24
we first calculated the free energies for loss of acetone to give I-
1B or I-1C. This step is 5.5 kcal molꢀ1 uphill for 2B but is
roughly isoergic for 2C. These results are consistent with the
NMR data discussed above, indicating much more facile loss of
solvent from 2C.
Consistent with literature precedent, compounds 2A,B are quite
thermally stable.10,11 For example, no decay was observed after
heating for 24 h in acetone-d6 at 60 °C. Even after 18 h at 100 °C,
C2H6 was not detected and only minimal (<5%) decomposition
In the transition structures (TS) for H3CꢀCH3 coupling,
the two reacting methyl groups straddle the N2Pt equatorial
plane with a C C distance of ∼1.95 Å. Distortion along the
3 3 3
was seen (Chart 1). Thus, the ΔGq
value for CꢀC bond-
imaginary mode in one direction and reoptimization of the
minimum energy geometries returned the five-coordinate
square-pyramidal complexes I-1B and I-1C. In the other direc-
60 °C
forming reductive elimination from 2A,B is >30 kcal molꢀ1
.
In contrast, the ortho-aryl-substituted diimine complexes 2C,D
quantitatively eliminate ethane over 8.5 h at 60 °C in acetone-d6,
in 96(6) and 91(6)% yields, respectively (Chart 1).21 This
reaction proceeds with concomitant formation of the correspond-
ing PtII monomethyl solvento complexes [(DAB)PtII
CH3(solv)]+ in 82(6)% (3C) and 72(5)% (3D) yield (Pt yields
are less thanquantitativedue to some competing decomposition).
tion, a slight decrease in the C C distance led to C2H6-bound
3 3 3
complexes I-2B and I-2C as local minima (Scheme 2). Displace-
ment of coordinated C2H6 with acetone then completes the
reaction sequence to form 3B or 3C.
The overall gas-phase free energy barrier for C2H6 formation
from 2C is 21.8 kcal molꢀ1 at 298 K, which is in reasonable
agreement with the experimental value (ΔGq60 °C = 25.4 kcal molꢀ1
in acetone solution). The barrier for 2B is computed to be 5.4 kcal
molꢀ1 higher than that of 2C, again consistent with experiment
1
The kinetics of CꢀC coupling were monitored by H NMR
spectroscopy. First-order rate constants were observed for the
decay of 2C ([1.6(3)] ꢁ 10ꢀ4
s
ꢀ1) and 2D ([4.2(6)] ꢁ 10ꢀ4
(ΔΔGq
> 5.6 kcal molꢀ1). The calculations show that the
s
ꢀ1) at 60 °C. These correspond to energy barriers (ΔGq60 °C) of
60 °C
25.4(1) and 24.7(1) kcal molꢀ1 for 2C,D, respectively.22
DFT calculations were conducted to gain insights into the
marked reactivity difference between the complexes with and
without 2,6-substituents on the aryl rings. Gas-phase reaction
coordinates for reductive elimination from isomeric 2B and 2C
difference in energy between the transition structures is almost
entirely due to ΔΔG° for acetone dissociation (5.5 kcal molꢀ1).
Thus, the ortho substituents accelerate reductive elimination by
facilitating access to the five-coordinate intermediate I-1, rather than
by promoting the actual CꢀC bond-forming step.
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Scheme 3. Oxidation of (DAB)PtIIMe2 (2) with AcFc+
additional steric congestion from the isopropyl groups in 1D
significantly lowers the rate of methyl transfer between platinum
centers (path a). This decreases the rate of disappearance of 1D and
allows PtꢀC bond homolysis (path b) to become competitive,
thereby providing significant quantities of CH4.
In summary, an effective ligand-based approach for promoting
CꢀC bond-forming reductive elimination from PtIV diimine
complexes under mild conditions has been described. Tuning the
steric properties of the ligand in concert with the reaction solvent
can lower the barrier for CꢀC bond formation by close to 10 kcal
molꢀ1. The lower barrier for reductive elimination is due to the
ease of accessing the five-coordinate intermediate, rather than a
greater facility for CꢀC bond formation. Furthermore, with an
appropriate choice of ligand, this reaction can be exploited in the
context of the oxidative disproportionation of PtII dimethyl
complexes. Promoting these reactions for diimine-ligated Pt
complexes is valuable, because these complexes are particularly
effective for CꢀH activation reactions. Studies are in progress to
couple these oxidative disproportionation and reductive elimina-
tion steps with methane activation. More generally, this work
provides an attractive strategy for the development of new Pt-
catalyzed CꢀC coupling reactions.
The DFT calculations indicate that CꢀC coupling from 2 should
be faster in more weakly coordinating solvents.25 Indeed, in nitro-
methane-d3, reductive elimination to C2H6 from 2C proceeds within
8 h at room temperature (k = [5.7(5)] ꢁ 10ꢀ5 sꢀ1 and ΔGq
=
25 °C
23.2(1) kcal molꢀ1). The use of CD2Cl2 resulted in quantitative
reductive elimination to C2H6 within 1 h at room temperature (k =
7.6(14) ꢁ 10ꢀ4 sꢀ1; ΔGq25 °C = 21.7(1) kcal molꢀ1).26 This order
of facility of reductive elimination, dichloromethane > nitro-
methane > acetone, parallels the lower donor ability of these
solvents.27 This is almost as fast as reductive elimination from the
1,2-bis(diphenylphosphino)ethane (dppe) complex (dppe)Pt-
(CH3)3(OTf), which occurs upon warming to room tempera-
ture in acetone (though complexes of this type have not been
shown to activate CꢀH bonds).10b Furthermore, although 2A,B
are both stable to heating in nitromethane-d3 at 60 °C (<10%
decomposition over 16 h), both undergo reductive elimination to
C2H6 when heated in CD2Cl2 at 50 °C over 48 h.
’ ASSOCIATED CONTENT
S
Supporting Information. Text, figures, and tables giving
b
experimental details, 1H NMR spectra, characterization data, and
computational details. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: mssanfor@umich.edu (M.S.S.); mayer@chem.washington.
edu (J.M.M.).
We next sought to apply these ligand effects in the two-step
oxidative disproportionation/reductive elimination sequence
proposed for methane dimerization (Scheme 1, steps 2 and 3).
The PtII dimethyl complexes 1C,D provided very different
results. Reaction of the 2,6-diisopropyl derivative 1D with 1
equiv of acylferrocenium (AcFc+) in acetone-d6 proceeded
slowly at room temperature, and the expected PtIV intermediate
2D was formed only in low yield (<20% out of a theoretical 50%
maximum yield). Furthermore, after heating for 8 h at 60 °C,
C2H6 was formed in only 17(2)% yield along with substantial
quantities of CH4. In marked contrast, the 2,6-dimethyl complex
1C reacted instantaneously and cleanly with AcFc+ at room
temperature in acetone-d6 to form 0.5 equiv of 2C and 0.5 equiv
of 3C (Scheme 3, top path). The latter then liberated C2H6 in
high yield after 8 h at 60 °C. These findings indicate that while
sterically encumbered diimine ligands are necessary for facile
solvent dissociation prior to reductive elimination, too much
steric bulk can hinder the oxidative disproportionation.
Present Addresses
§Argonne National Laboratory, 9700 South Cass Avenue, Bldg.
200, Argonne, IL 60439-4837. Tel: (630)252ꢀ2314. E-mail:
lanci@anl.gov.
’ ACKNOWLEDGMENT
We are grateful to the NSF Center for Enabling New
Technologies through Catalysis (CENTC) for financial support,
and we thank Dr. Kyle A. Grice and Prof. Karen I. Goldberg for
insightful discussions and Dr. Adam L. Tenderholt and Prof.
Thomas R. Cundari for computational support.
’ REFERENCES
The substantial differences in reactivity between 1C and 1D can
be rationalized on the basis of the mechanism in Scheme 3. Initial
1eꢀ outer-sphere oxidation by AcFc+ to give the putative PtIII
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observed upon oxidation of phosphine-ligated palladium methyl
complexes.7b,30 Path a appears to occur rapidly and cleanly for the o-
methyl-substituted aryldiimine complex 1C. In contrast, the
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