Download Chapter 19 C-H Bond Activation with Neutral Platinum Methyl

ACS Symposium Series 885, Activation and Functionalization of C-H Bonds,
Karen I. Goldberg and Alan S. Goldman, eds. 2004.
Chapter 19
C-H Bond Activation with Neutral Platinum Methyl
Complexes
Carl N. Iverson,1 Charles A. G. Carter1, John D. Scollard,2 Melanie
A. Pribisko,1 Kevin D. John1, Brian L. Scott1, R. Tom Baker,1,* John
E. Bercaw,2,* and Jay A. Labinger2,*
1
Los Alamos Catalysis Initiative, Chemistry Division, MS J514,
Los Alamos National Laboratory, Los Alamos, NM 87545
2
Department of Chemistry, California Institute of Technology, Pasadena
CA 91125
The selective metal-catalyzed oxidation of alkanes to alcohols
offers immense opportunities for saving energy and reducing
waste in the petroleum and chemical manufacturing industries.
Robust transition metal complexes with chelating nitrogen
ligands show great promise as homogeneous catalysts for this
reaction, but a practical system has yet to be identified. In this
work a variety of neutral platinum methyl complexes with
bidentate anionic N,N- and N,C-donor ligands were prepared.
Ligand backbones included one, two, and three-atom bridges
between the donor atoms. Redox properties of the new
complexes were investigated using competition studies with I2
and evaluating equilibrium constants between divalent methyl
complexes and their tetravalent diiodides. The ease of
oxidation was amidinate > β-diketiminate > iminopyrrolide.
Studies of benzene C-H bond activation using iminopyrrolide
platinum complexes were consistent with rate determining
benzene association and revealed a novel geometric effect on
the rate for iminopyrrolide ligands.
© 2004 American Chemical Society
319
320
Introduction
Shilov et al. first demonstrated Pt(II)-catalyzed oxidation of alkanes to
alcohols and alkyl chlorides some 30 years ago with simple platinum salts(1). In
spite of the limitations imposed by the Pt(IV) stoichiometric oxidant and poor
catalyst lifetime due to Pt metal precipitation, this mild (120°C) alkane
functionalization is notable for its selectivity (1º > 2º > 3º) that is opposite that
of radical processes. Much effort has been expended to elucidate the mechanism
of this transformation(2-3). The first step is proposed to involve electrophilic
activation of an alkane C–H bond by a Pt(II) salt to form a metal alkyl complex.
Subsequent oxidation of the Pt(II) alkyl to Pt(IV) is followed by nucleophilic
attack at the Pt(IV) alkyl by water or chloride to form the alcohol or alkyl halide
and regenerate the starting Pt(II) salt.
More recently, Periana et al. reported that solutions of bipyrimidine
platinum dichloride in boiling sulfuric acid selectively oxidize methane to
methylbisulfate in high one-pass yields(4-5). Subsequent studies showed that
nitrogen-ligated, cationic platinum methyl complexes with diamine or diimine
ligands undergo facile C-H bond activation reactions in non-coordinating
solvents(6,7). While neutral platinum dimethyl complexes with these ligands
could also be oxidized with dioxygen in methanol to give Pt(IV) alkyl
products(8), cationic Pt methyl complexes derived from methane C-H bond
activation proved more difficult to oxidize(9). As a result, platinum methyl
complexes with monoanionic nitrogen-based chelates were investigated for their
ability to activate C-H bonds and undergo facile oxidation reactions(10). The
first example, reported by Goldberg et al., involved a tris(3,5dimethylpyrazolyl)borate (Tp’) platinum complex that oxidatively adds alkane
C-H bonds to afford a Pt(IV) methyl alkyl hydride complex stabilized in a sixcoordinate geometry by the tridentate Tp’ ligand(11). More recently, the same
group used a bulky β-diketiminate ligand to support the first five-coordinate
Pt(IV) methyl compound(12), and further work with Tp ligands by the groups of
Templeton and Keinan afforded a rare η2-benzene complex and showed
reversible H/D exchange of methyl and hydride positions with alcohol solvent,
respectively(13,14).
Finally,
Peters
et
al.
have
developed
a
bis(phosphinoalkyl)borate ligand that has also shown C–H bond activation
capabilities when ligated to platinum(15).
In this work a variety of neutral platinum methyl complexes ligated
with bidentate anionic N,N- and N,C-donor ligands were prepared. Ligand
backbones included one,- two,- and three-atom bridges between the donor
atoms. Several of the resulting complexes were then compared for their ability
to activate C-H bonds and their ease of oxidation to Pt(IV) methyl products.
© 2004 American Chemical Society
321
Preparation and Characterization of Platinum Methyl
Complexes with Anionic Bidentate Ligands
The ligands 1-5 used in this study were prepared by standard literature
procedures(16-20). Reactions of PtMeCl(cod) (6, cod = 1,5-cyclooctadiene)
Cy
N
Ar
Ar
N
N
Ar
N
Li
N N
Li
N
Cy
N H
1
N
S
K
N N
Ar
i
2a: R = 4-MeO-Ph
Ph2B
3: R = 2,6- Pr2-Ph
2b: R = 4-CF3-Ph
4a: R = 4-MeO-Ph
4b: R = 4-Cl-Ph
5
2c: R = 3,5-(CF3-)2-Ph
i
2d: R = 2,6- Pr2-Ph
2e: R = 4-Me-Ph
with ligands 1, 5 or deprotonated 2 afforded five-coordinate platinum methyl
complexes 7-9 (Eqs. 1-3).
Cy
N
(cod)Pt(Me)Cl
+
Me
Li
Pt N
N
THF
N
Cy
1
6
Cy
Cy
7
Ar
Me
N
(cod)Pt(Me)Cl
+
N H
(2)
Pt N
N
Ar
KOtBu
6
(1)
THF
2a
8a: Ar = 4-MeO-Ph
Me
N N
(cod)Pt(Me)Cl
+
Ph2B
K
THF
Pt
N N
6
5
© 2004 American Chemical Society
9
N N BPh
2
N N
(3)
322
These complexes are all proposed to have a pseudo trigonal bipyramidal
geometry with an apical methyl group, based on the similar values of JHPt (73,
78 Hz; Table I) and the molecular structure of 7 as determined by X-ray
diffraction (Figure 1).
Figure 1. Molecular Structure of PtMe(cod)(CyNCMeNCy), 7
Table I. 1H NMR chemical shifts and Pt-H Coupling Constants (Hz)
of Divalent Platinum Methyl Complexes
Complex
Solvent
δ Pt-Me, 2JHPt
δ Pt-SMe2, 3JHPt
7
M
0.70, 73
8a
C
0.63, 78
9
A
0.76, 73
13
B
1.13, 83
1.73, 52
14a
B
1.31, 79
1.52, 51
14b
B
1.26, 80
1.43, 51
14c
B
1.22, 81
1.45, 52
15a
B
0.74, 73
1.73, 57
15b
B
0.58, 73
1.68, 58
15d
B
0.42, 73
1.71, 58
16
B
1.72, 84
1.58, 29
17
B
1.30, 79
18a
B
0.33, 72
1.48, 50
18b
M
-0.20, 78
2.01, 46
19
B
0.38, 75
2.05, 54
A = acetone-d6, B = C6D6, C = CDCl3, M = CD2Cl2
© 2004 American Chemical Society
323
Four coordinate platinum methyl complexes 13-19 were prepared by a
similar salt metathesis route from PtMeCl(SMe2)2 10 or via protonolysis of a
methyl group in [PtMe2(µ-SR2)]2 (11, R = Me; 12, R= Et) by the N-H bond of
the neutral ligand (Eqs. 4-9). With the exception of the bulky 2,6diisopropylphenyl-substituted example, the unsymmetrical iminopyrrolide
ligands 2 gave both stereoisomers from the metathesis route, but a single isomer
from the protonation route. Complexes 14 and 15 are labeled cis and trans,
respectively, according to the relationship of the anionic methyl and pyrrolide
ligands. Reaction of 11 with ligand 3 yielded a methyl complex with an N,Cchelate arising from orthometallation of the β−C-H bond of the thiophene
ring(21). The analogous diethylsulfide dimer, 12, also reacts with the neutral
iminopyrollides via deprotonation (eq. 8), albeit at a much slower rate, to
produce the cis product 17.
Characterization of complexes 14 and 15 was facilitated by the
magnitude of JHPt for both Pt-Me and SMe2 groups (Table I) that reflected the
donor nature of the trans ligand. The two-bond H-Pt coupling constant for the
Pt-Me group in 14 (trans to the weaker σ-donor imine) averaged 80 Hz vs. 73
Hz for 15 with Pt-Me trans to pyrrolide. Likewise, the coupling constants for the
SMe2 resonances were 58 Hz in 14 vs. 51 Hz for 15. Although comparisons with
other complexes were complicated by the different backbone lengths, the
coupling constant differences between the symmetrical, three atom bridge βdiketiminate and bis(pyrazolyl)borate ligands were consistent with the higher
donor power of the former.
Pt(Me)Cl(SMe2)2
Cy
Cy
N
+
Li
N
THF
N
Cy
10
Pt(Me)Cl(SMe2)2
+
THF
2-Li
Me
13
Ar
N
SMe2
Pt
N
Me
14a: Ar = 4-MeO-Ph
14c: Ar = 4-CF3-Ph
Ar
N
+
Me
Pt
N
SMe2
15a: Ar = 4-MeO-Ph
15c: Ar = 4-CF3-Ph
i
15e: Ar = 2,6- Pr2-Ph
© 2004 American Chemical Society
(4)
Cy
Li
N
10
N
1
Ar
N
SMe2
Pt
(5)
324
Ar
N
Ar
N
THF
[PtMe2(µ-SMe2)]2 +
N H
11
14a: Ar = 4-MeO-Ph
14c: Ar = 4-CF3-Ph
Ar
N
+
S
11
Ar
- CH4
Me
S
Ar
N
C6H6
+
Li
Ar
N
THF
N
Ar
K
5
© 2004 American Chemical Society
Me
18a: Ar = 4-MeO-Ph
18b: Ar = 4-Cl-Ph
N N
10
(9)
Ar
4
N N
SMe2
Pt
N
+ Ph2B
Me
N
Ar
Pt(Me)Cl(SMe2)2
(8)
17: Ar = 4-MeO-Ph
N
10
SEt2
Pt
- CH4
2a
+
(7)
i
N H
Pt(Me)Cl(SMe2)2
SMe2
Pt
16: Ar = 2,6- Pr2-Ph
Ar
N
12
N
CH2Cl2
3
[PtMe2(µ-SEt2)]2
(6)
Me
N
2a,c
[PtMe2(µ-SMe2)]2
SMe2
Pt
THF
N N
Ph2B
SMe2
Pt
N N
19
Me
(10)
325
Complexes 13, 16 and 17 were also characterized by X-ray diffraction
(Figure 2) and the effect of different chelate ligands on geometric parameters of
the Pt-Me and Pt-SMe2 moieties was examined. The Pt-C bond distances (Table
II) are nearly invariable among the complexes; however, the Pt-S bond in 16 is
~0.1 Å longer than in either 13 or 17. A related structure reported by Templeton
has bond lengths similar to those observed in 13 and 17(22). The Pt-C and Pt-S
bonds in Tp'PtMe(SMe2) are 2.049 and 2.249Å, respectively. Steric interactions
between the 2,6-diisopropylphenyl substituent of the imine and the dimethylsulfide ligand, due to their mutual cis arrangement, is a likely reason for this
lengthening. The longer bond is also reflected in the 3JHPt value of 29 Hz for 16
in comparison to ~51 Hz for complexes 14.
a)
b)
c)
Figure 2. Molecular Structures of a) PtMe(SMe2)(CyNCMeNCy), 13,
b) PtMe(SMe2)[2-(N-[2,6-(i-Pr)2-Ph]imino)thiophenide], 16, and
c) PtMe(SEt2)[2-(N-[4-MeO-Ph]imino)pyrollide], 17.
© 2004 American Chemical Society
326
Table II. Selected bond lengths (Å) for complexes 13,16 and 17
Complex
Pt-Me
Pt-SMe2
13
2.048(6)
2.238(2)
16
2.041(6)
2.352(2)
17
2.077(9)
2.249(2)
Redox Properties of Neutral Platinum Methyl Complexes
In previous work we determined the ease of oxidation of nitrogenligated platinum complexes from competition reactions with I2 and also
evaluated equilibrium constants between divalent methyl complexes and their
tetravalent diiodides(9). Complexes 13-15 and 18 all reacted cleanly with iodine
to afford the trans diiodides 20-23 (Eq. 11, complexes 21 and 22 are produced
from 14 and 15, respectively). Complex 19 did not react with iodine at room
temperature over several days. Presumably, the steric bulk from the two phenyl
rings attached to boron in addition to dimethyl substitution of the pyrazole rings
prevents I2 from reacting with the platinum metal center(23). The reduction of
electron density in the tetravalent diiodides (vs. the Pt(II) starting compounds) is
reflected in the greatly reduced values of JHPt for the SMe2 resonances (Table
III)(24). Competition reactions with pairs of complexes indicated that the ease of
oxidation of the divalent platinum methyl complexes is 13 > 18 > 15 > 14.
N
SMe2
Pt
N
+ I2
N I SMe2
Pt
N I Me
20-23
C 6D 6
Me
13-15, 18
20: N-N =
Cy
N
N
Cy
(11)
Ar
Ar
N
21, 22: N-N =
(cis, trans)
23: N-N =
N
N
N
Ar
This ordering was also reflected in estimates of the equilibrium constants
between pairs of divalent methyl complexes and the corresponding tetravalent
© 2004 American Chemical Society
327
diiodides (Eq. 12). Representative equilibrium constants are given in Table IV.
The reaction of 13 with 23, for example, shows the equilibrium greatly favors
Table III. 1H NMR chemical shifts and Pt-H Coupling Constants (Hz)
of Tetravalent Methylplatinum Diiodides in C6D6
Complex
20
21a
21b
22a
22b
23a
δ Pt-Me, 2JHPt
2.65, 70
2.79 ,70
2.74 ,71
2.30, 67
2.14, 67
1.87, 67
δ Pt-SMe2, 3JHPt
2.05, 31
1.72, 32
1.68, 33
2.05, 36
1.99, 38
1.66, 32
the amidinate ligated Pt(IV) complex as 23 converts completely to complex 18b
as judged by 1H NMR spectroscopy. In accordance with electronic substitution
effects of the aryl group, the more nucleophilic complex 14a (p-OMe
substituted) favors oxidation over 14b (p-CF3 substituted). Also, from these
equilibrium oxidation studies it would appear that the trans products (15) of the
iminopyrrolide ligated complexes are more electron rich than the corresponding
cis products (14).
N'
SMe2
+
Pt
N'
Me
N I SMe2
Pt
N I Me
C 6D 6
I SMe2
Pt
+
N' I Me
N'
N
SMe2
Pt
N
Table IV. Equilibrium Constants for Reactions of Pt(II)
with Pt(IV) Complexes in C6D6
Equilibrium system
[20][18a]/[13][23a]
[20][14a]/[13][21a]
[23a][14a]/[18a][21a]
[21a][14b]/[14a][21b]
[21b][15b]/[14b][22b]
© 2004 American Chemical Society
Q
≥ 13
≥ 400
3.0
2.8
2.9
Me
(12)
328
Electrophilic Activation of Benzene C-H Bonds with Neutral
Platinum Methyl Complexes
The reactivity of platinum methyl complexes 13-15, 18 and 19 with
benzene at 85°C was monitored by 1H NMR spectroscopy (eq. 13). While most
of the complexes reacted sluggishly (13, 14a,b, 18b) or not at all (19),
iminopyrrolide complexes 15a,b,d activated solvent C-D bonds to form a
platinum phenyl product with concomitant formation of CH3D within hours.
N'
SMe2
Pt
N
C6D6, 85 oC
- CH3D
Me
N'
SMe2
(13)
Pt
N
Ph-d5
13-15, 18, 19
Electronic effects on the rate of benzene C-H bond activation (cf. 15b
is ca. four times faster than 15a) were consistent with an electrophilic reaction.
More surprising was the ca. 80-fold faster reactions observed for 15 in which the
Pt-Me is trans to the pyrrolide nitrogen, vs. isomeric 14 with Pt-Me trans to the
imine nitrogen (Scheme 1). We suggest that rate-determining associative
displacement of SMe2 by benzene accounts for these differences(25) and the
greater electronegativity of the ligand in 14b and 15b would tend to favor
associative displacement.
Scheme 1
4-CF3-Ph
4-CF3-Ph
N
N
Me
Pt
SMe2
N
t1/2
o
C6H6, 85 C
- CH3D
N
Ph
Pt
SMe2
N
14b:14 days
4-OMe-Ph
N
N
Me
SMe2
SMe2
Me
N
4-OMe-Ph
N
t1/2
Pt
+
15b: 1 hr
Pt
4-CF3-Ph
+
15a:4 hr
© 2004 American Chemical Society
4-OMe-Ph
Pt
N
SMe2
Me
14a: 3 days
o
N
C6H6, 85 C
- CH3D
Ph
Pt
N
SMe2
329
An associative mechanism is favored in systems with little steric
hindrance and also where the leaving group is trans to a poor donor(26).
However, the trigonal bipyramidal intermediate for such displacement in trans
complexes 15 would have the better π-acceptor imine ligand equatorial and the
better σ-donor pyrrolide axial, the strongly preferred conformation (Scheme 2).
Slower displacement in the cis isomer must pass through a higher-energy
intermediate and is accompanied by isomerization to the thermodynamically
preferred cis isomer of the platinum phenyl product.
Scheme 2
Ar
Ar
N
Me
Pt
L
SMe2
N
N
Me
Pt
N
L
trans, 15
favored
Ar
Ar
N
Pt
N
SMe2
Me
cis, 14
L
N
Pt
N
SMe2
SMe2
L
Me
disfavored
A series of SMe2/SMe2-d6 ligand exchange reactions were performed
with complexes 13-15, 18, and 19. The rate of sulfide exchange correlates well
with the observed trend in solvent C-H bond activations. For instance, when a
1:1 mixture of 14b and 15b was treated with SMe2-d6 the resonance for
coordinated SMe2 in 15b disappeared within one hour whereas the
corresponding resonance in 14b required 5 days for exchange to occur. Slow
exchange is also observed for complexes 13 and 18a. The borate complex 19 is
again the slowest in terms of reactivity, not showing any appreciable exchange
at room temperature after several days. In addition, the presence of excess
sulfide isomerizes 15b to the more stable thermodynamic complex 14b.
Sulfide exchange is also observed between complexes in a crossover
experiment involving co-thermolysis of 14e and 17 in C6D6 (Eq. 14); however,
the process is very sluggish with respect to the complex/ligand exchange
reactions. The mixture requires heating at 85 °C for 48 hours before significant
© 2004 American Chemical Society
330
(~ 35%) sulfide exchange is observed. C–D bond activation of the solvent was
also observed but it occurs at an even slower rate than the ligand crossover.
Me
N
Pt
OMe
SMe2 +
Me
N
N
Pt
N
Me
SEt2
85 oC
N
Pt
Me
OMe
N
SEt2 +
Pt
Me
N
N
SMe2
(14)
Me
In contrast to these sluggish reactions, reaction of 12 with bulky
iminopyrrole 2d in perdeuterobenzene at 25°C over several hours afforded two
equivalents of methane isotopomers and trans-phenyl complex 24 (Eq. 15). Ca.
10% of trans methyl complex 15e was also formed and methyl groups of the
isopropyl substituents were partially deuterated(27). In this reaction we propose
Ar
Ar
N
N
C 6D 6
1/2 [PtMe2(µ-SMe2)]2 +
Pt
NH
12
N
2d
C 6D 5
SMe2
+ 2 CH4-xDx (15)
24d: Ar = 2,6-iPr2Ph
+ 10% 15d
that benzene C-H bond activation must precede chelation since preformed
complexes 15 only react with benzene at elevated temperature. The fact that
only trans products are observed in Eq. 15 is presumably a consequence of steric
interactions between the SMe2 ligand and the ortho-isopropyl substituents.
Conclusions
Neutral platinum methyl complexes have been shown to be easily
oxidized to tetravalent complexes and to activate benzene C-H bonds under mild
conditions. In this study, the inherent reactivity of N,N-chelated platinum methyl
complexes toward hydrocarbon C-H bonds was masked by the need to substitute
strongly coordinating dialkylsulfide ligands. Use of unsymmetrical
iminopyrroles and iminopyrrolide ligands shed some light on this process and
further work is in progress(28) to identify nitrogen-ligated systems for alkane
functionalization outside of strong acid media.
© 2004 American Chemical Society
331
Acknowledgements
Akzo Nobel, Inc., the US Department of Energy’s Office of Industrial
Technologies, the Laboratory-Directed Research and Development program
at Los Alamos, and BP are gratefully acknowledged for support and CNI
thanks Los Alamos for a Director’s Funded Postdoctoral Fellowship.
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Oxidative addition of methyl iodide to a similar square planar Pt(II)
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Skelton, B.W.; White, A.H. J. Organomet. Chem. 1992, 433, 223-229.
Only a small reduction in 2JHPt values is seen for the Pt-Me group protons of
complexes 20-23. These values, however, are typical for methyl groups
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© 2004 American Chemical Society
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A recent paper detailed volume of activation studies with respect to the
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Similar exchange has been observed in a related system: Fekl, U.;
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Iverson, C. N.; Carter, C. A. G.; Scott, B. L.; Baker, R. T.; Scollard, J. D.;
Day, M. W.; Labinger, J. A.; Bercaw, J. E., submitted for publication.
© 2004 American Chemical Society