Activation of anisole by organoplatinum(II) complexes: Evidence for rate-determining C-H activation

Article abstract of DOI:10.1021/om200922a

A study of the basis of selectivity of C-H bond activation of anisole by electrophilic methylplatinum(II) complexes is reported. Anisole reacts with [PtXMe-(NN)] in trifluoroethanol solvent to give methane and [PtXAr(NN)], Ar = 2-, 3-, and 4-anisyl, in 90:8:2 ratio when X = HOB(C₆F ₅)₃ and NN = (2-C₅H₄N)₂CO (DPK) but not when NN = 2,2′-bipyridine. Similar results are obtained when X = triflate or when NN = (2-C₅H₄N)₂NH. Competition between reaction of anisole and anisole-d₈ with [PtXMe(NN)], X = HOB(C₆F₅)₃ and NN = DPK, in trifluoroethanol gave an isotope effect k_H/k_D = 3.6. Several 4-anisyl complexes, [PtClAr(NN)], [PtAr₂(NN)], and [PtMeAr(NN)], NN = DPK, DPA, or bipy, were prepared and reacted with HX [X = Cl, OTf, or HOB(C₆F₅)₃]. Reaction of [PtMeAr(NN)], NN = DPK or bipy, with HX gave a detectable hydride [PtXHMeAr(NN)] when X = Cl, followed by loss of methane to give [PtClAr(NN)], but only [Pt(OTf)Ar(NN)] was detected when X = OTf. Reaction with more HOTf gave anisole and [PtX ₂(NN)], X = OTf, and no isomerization of the 4-anisyl group to the more favored 2-anisyl group was observed at any stage. The similar reaction of [PtMeAr(NN)] and HOTf in CD₃OD/CD₂Cl₂ gave CH_nD_4-n (n = 0-4) and mostly 4-MeOC₆H ₄D. It is argued that the anisole C-H bond cleavage step in anisole activation, or the anisyl-H bond forming step in protonolysis, is responsible for the observed selectivity in these reactions.

Full text of DOI:10.1021/om200922a

Article
pubs.acs.org/Organometallics
Activation of Anisole by Organoplatinum(II) Complexes: Evidence for
Rate-Determining C−H Activation
Kevin J. Bonnington, Fenbao Zhang, Mahmoud M. Abd Rabo Moustafa, Benjamin F. T. Cooper,
Michael C. Jennings, and Richard J. Puddephatt*
Department of Chemistry, University of Western Ontario, London, Canada N6A 5B7
S
* Supporting Information
ABSTRACT: A study of the basis of selectivity of C−H bond activation of anisole by
electrophilic methylplatinum(II) complexes is reported. Anisole reacts with [PtXMe-
(NN)] in trifluoroethanol solvent to give methane and [PtXAr(NN)], Ar = 2-, 3-, and
4-anisyl, in 90:8:2 ratio when X = HOB(C6F5)3 and NN = (2-C5H4N)2CO (DPK) but
not when NN = 2,2′-bipyridine. Similar results are obtained when X = triflate or when
NN = (2-C5H4N)2NH. Competition between reaction of anisole and anisole-d8 with
[PtXMe(NN)], X = HOB(C6F5)3 and NN = DPK, in trifluoroethanol gave an isotope
effect kH/kD = 3.6. Several 4-anisyl complexes, [PtClAr(NN)], [PtAr2(NN)], and
[PtMeAr(NN)], NN = DPK, DPA, or bipy, were prepared and reacted with HX [X = Cl,
OTf, or HOB(C6F5)3]. Reaction of [PtMeAr(NN)], NN = DPK or bipy, with HX gave a
detectable hydride [PtXHMeAr(NN)] when X = Cl, followed by loss of methane to give
[PtClAr(NN)], but only [Pt(OTf)Ar(NN)] was detected when X = OTf. Reaction with more HOTf gave anisole and
[PtX2(NN)], X = OTf, and no isomerization of the 4-anisyl group to the more favored 2-anisyl group was observed at any stage.
The similar reaction of [PtMeAr(NN)] and HOTf in CD3OD/CD2Cl2 gave CHnD4−n (n = 0−4) and mostly 4-MeOC6H4D. It is
argued that the anisole C−H bond cleavage step in anisole activation, or the anisyl−H bond forming step in protonolysis, is
responsible for the observed selectivity in these reactions.
a
Scheme 1
INTRODUCTION
■
The selective functionalization of hydrocarbons is of great
current interest, and the activation of carbon−hydrogen bonds
by platinum(II) complexes has played an important role in this
field.1 The carbon−hydrogen bond activation can occur by
metathesis or by oxidative addition,1,2 but reactions with
reagents of the type [PtMe(S)(NN)]+, where NN is a chelating
bis(nitrogen-donor) ligand and S is a weakly coordinated
solvent molecule, with benzene generally occur by the oxidative
addition mechanism shown in Scheme 1.1−11 The key steps are
coordination of benzene to give B, C−H oxidative addition to
give C, reductive coupling to give a methane complex D, and
loss of methane to give E. Depending on the ligand, solvent,
and anion, the rate-determining step may be the coordination
of benzene, the oxidative addition of the C−H bond, or the loss
of methane.1−5 In addition, the coordination and dissociation
steps can be associative or dissociative, hydrogen exchange with
protic solvent can occur through reversible proton loss from C
to give F, and the 16-electron platinum(IV) intermediate C can
be reversibly trapped by solvent coordination to give G. The
key intermediate C can also be formed by protonation of the
methyl(phenyl)platinum(II) complex F. There is typically easy
fluxionality between coordinated CC bonds of benzene in B
or C−H bonds of methane in D so that H/D exchange between
labeled phenyl and methyl groups or between protic solvent
and methyl or phenyl groups can occur.1−4
a
NN = diimine ligand, S = solvent.
ArNCR−CRNAr, Ar = aryl, R = H or Me. For example, in
the case with Ar = 2,6-Me2C6H3, R = Me, and solvent =
CF3CH2OH, rapid exchange between intermediates B, C and D
led to almost complete H/D scrambling between labeled
methylplatinum and coordinated benzene or phenyl groups,
but with no H/D scrambling of methyl or benzene groups
with solvent, indicating that benzene coordination was the
Pioneering mechanistic work has been carried out with
Received: October 2, 2011
complexes in which the diimine ligands, NN in Scheme 1, are
Published: December 20, 2011
© 2011 American Chemical Society
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rate-determining step in conversion of A to E (Scheme 1).3a In
this system, reaction of F with H[BF4] gave A and E in 18% and
82% yields, respectively, reflecting the slightly higher barrier to
associative displacement of benzene from B compared to
methane from D (Scheme 1).3a With less sterically hindered
complexes (such as when the diimine is ArNCH−CH
NAr, with Ar = 3,5-t-Bu2C6H3), the benzene coordination is
faster and then the C−H bond cleavage step is rate deter-
mining.3d In another study, using the diimine ligand ArN
CMe−CMeNAr with Ar = 3,5(CF3)2C6H3, it was shown that
reaction of A (Scheme 1) with toluene gave tolylplatinum(II)
complexes (analogous to E) with a preference for the meta
isomer.4d In this system, reaction of any isomer (o-, m-, or p-)
of [Pt(tolyl)2(NN)] with H[BF4] gave the same mixture of
isomers of [Pt(tolyl)(S)(NN)]+, with the meta isomer pre-
dominating, indicating easy exchange between [PtH(tolyl)2-
(NN)]+ and [Pt(tolyl)(toluene)(NN)]+ isomers prior to
displacement of toluene. In the presence of acetonitrile, less
isomerization of tolyl groups was observed because the rate of
associative displacement of toluene increased.4d Several authors
have noted that activation of an aryl C−H bond of toluene
gives mostly m-tolylplatinum complexes and that parallel or
subsequent activation of a methyl C−H bond may give the
benzylplatinum isomer.1,3,4,9
Scheme 2
NN = ArNCR−CRNAr give CO stretching frequencies in
the range 2103−2116 cm−1, showing that they have a similar
range of donor abilities.3d Differences in reactivity are therefore
likely to be a result of steric effects.
The structure of complex 2a was determined and is shown in
Figure 1. The platinum center in 2a is square planar, and the
In earlier studies of reactions of Scheme 1, it was found that
with the ligand NN = 2,2′-bipyridine, bipy, or its substituted
derivatives such as 4,4′-di-tert-butyl-2,2′-bipyridine, no arene
activation was observed under mild conditions,12 whereas, with
NN = (2-C5H4)2E (E = CO, NH, or CH2), Chart 1, benzene
Chart 1. Ligand Abbreviations
Figure 1. View of the structure of complex 2a. Selected bond
parameters (distances in Å, angles in deg): Pt−N(1) 1.975(5); Pt−
N(14) 2.122(5); Pt−C(15) 2.044(7); Pt−O(21) 2.047(4); N(1)−
Pt−N(14) 88.5(2).
pyridyl groups are twisted out of the plane by 44.9° (N1 ring)
and 43.5° (N14 ring). There is a hydrogen bond between the
OH group and an ortho-fluorine atom of the HOB(C6F5)3
group with O(21)···F(42) = 2.70(1) Å. The 1H NMR spectrum
at room temperature gave a septet for the OH proton, with
J(HF) = 3 Hz, due to coupling with all six ortho-fluorine atoms,
indicating rapid rotation of the C6F5 groups and of the
B(C6F5)3 unit. However, at −80 °C, this resonance appeared as
activation was observed to give the corresponding complex E
(Scheme 1).11 In the successful reactions, toluene gave mostly
the m-tolylplatinum complex, while anisole gave mostly the
o-anisylplatinum complex.11 This article reports a detailed study
of the reactions with anisole aimed at understanding the selec-
tivity of the reaction13 and the reasons for the low reactivity
when NN = bipy.
1
a broad doublet with J(HF) = 20 Hz, as expected for the
structure of Figure 1. In addition, at room temperature the 19F
NMR spectrum contained only three resonances (o-, m-, p-F),
but each resonance split into several at low temperature as the
fluxionality was frozen out. These data show clearly that the
anion remains coordinated in solution and is not displaced by
solvent CD2Cl2.
The structure of complex 3b is shown in Figure 2. There was
disorder of the methylplatinum and platinum carbonyl groups,
and only one of the components is shown, for clarity. There are
two hydrogen bonds, with N(7)···O(20) = 2.71(1) Å and
O(20)···F(46) = 2.80(1) Å. The platinum center in 3b is
square planar, and the pyridyl groups are twisted out of the
plane by 35.5° (N1 ring) and 38.5° (N13 ring).
RESULTS AND DISCUSSION
■
Carbon−Hydrogen Bond Activation of Anisole. The
reaction of B(C6F5)3 with water impurity in organic solvents
gives H[B(OH)(C6F5)3], and this can cleave a methylplatinum
bond of the complex [PtMe2(NN)], 1,14 to give the corres-
ponding complex [PtMe{HOB(C6F5)3}(NN)], 2, according to
Scheme 2.11,12 The hydroxotris(pentafluorophenyl)borate
anion is weakly coordinating, and it was easily displaced by
carbon monoxide to give the corresponding complex [PtMe-
(CO)(NN)][ HOB(C6F5)3], 3 (Scheme 2). The carbonyl
stretching frequencies follow the sequence 3a (2119 cm−1) >
3c (2109 cm−1)12 > 3b (2105 cm−1), indicating that the
ligand donor ability follows the sequence DPA > bu2bipy >
DPK. The corresponding cations [PtMe(CO)(NN)]+ with
The carbon−hydrogen bond activation of anisole was most
conveniently carried out by reaction of complex 1a and anisole
with B(C6F5)3 in CF3CH2OH solvent. The reaction involves
formation of 2a in situ (Scheme 2) followed by reaction with
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Figure 2. View of the structure of complex 3b. Selected bond
parameters (distances in Å; angles in deg): Pt−N(1) 2.110(6); Pt−
N(13) 2.083(6); Pt−C(14A) 2.08(2); Pt−C(15A) 1.82(1); C(15A)−
O(15A) 1.14(1); N(1)PtN(13) 87.2(2).
Figure 3. View of the structure of complex o-4a. Selected bond
parameters (distances in Å; angles in deg): Pt−N(1) 1.988(4); Pt−
N(14) 2.083(4); Pt−C(21) 2.004(5); Pt−O(31) 2.056(3) Å; N(1)−
Pt−N(14) 87.6(2). H-bond distances: O(31)···O(27) 2.681(5);
O(31)···F(52) 2.786(5).
anisole accompanied by loss of methane (Scheme 3). The
product [Pt(C6H4OMe){HOB(C6F5)3}(DPK)], 4a, was
Scheme 3
act as a hydrogen bond donor to both the methoxy oxygen
atom and one of the fluorine atoms (Figure 3). It is noteworthy
that the anisyl aromatic group is almost coplanar with its trans-
pyridyl group (angle = 1°) and that the angle to the plane of
the platinum atom is 48°. The pyridyl groups are twisted out of
the platinum plane by 45o (N1 group) and 47o (N14 group).
Mechanistic Studies Using Anisole-d8. It is known that
selectivity in carbon−hydrogen bond activation can be
determined by either kinetic or thermodynamic factors, and it
is important to determine which of these factors is dominant in
the reactions of Scheme 3. The reaction of 1a with anisole-d8,
with B(C6F5)3/H2O as acid in CF3CH2OH, gave mostly 4a-d7
with only minor hydrogen content in the anisylplatinum group.
In the major ortho isomer, about 8% and 6% hydrogen
incorporation was observed at the 5- and 3-positions of the 2-
methoxyphenylplatinum complex 4a-d7, with undetectable
hydrogen content in the 4,6-positions or in the CD3O group.
This is attributed largely to H/D exchange through conven-
tional electrophilic substitution para or ortho to the activating
methoxy group, with a small preference for para, consistent
with analysis of the remaining anisole. The methane was
analyzed by GC-MS and shown to be CH4 (94%) and CH3D
(6%) with negligible CH4−nDn with n > 1. This experiment
suggests that the C−D bond cleavage is largely irreversible
under the reaction conditions and that there is no easy ex-
change between intermediates analogous to B, C, and D of
Scheme 1, which would lead to multiple H−D exchange
between the methyl and arylplatinum groups.3,4 In addition, the
observation of CH4 rather than CH3D as major product
suggests that H/D exchange between hydridoplatinum and
protic solvent is somewhat faster than methane loss (Scheme 1).
No reaction occurred between 4a and anisole-d8, or between
[PtPh{HOB(C6F5)3}(DPK)]11 and anisole, with B(C6F5)3/
H2O as acid in CF3CH2OH, showing that aryl−aryl exchange
does not occur under mild conditions. The deuterium isotope
effect on the reaction rate was determined by the reaction,
using B(C6F5)3/H2O in CF3CH2OH as acid, of complex 1a
with equimolar amounts of anisole/anisole-d8, followed by
isolated as a mixture of o-, m-, and p-isomers in a ratio of 90:8:2.
Analysis by NMR of samples taken from the reaction solution,
evaporated to dryness and redissolved in CD2Cl2, showed
similar relative amounts of the isomers. The pure ortho isomer
o-4a was obtained by crystallization. Complex 4a was also
obtained, again with similar isomer ratio, by reaction of isolated
complex 2a with anisole. In these reactions, the anion remains
coordinated in the isolated product, but otherwise the reaction
is analogous to the diimine system of Scheme 1.1,3,4 What is
important is that it must be possible to displace the solvent
(Scheme 1) or anion (Scheme 3) with the arene at inter-
mediate stages of reaction (Scheme 1).3,4 A carbon−hydrogen
bond of anisole could also be activated by reaction of 1a and
anisole with triflic acid in CF3CH2OH solvent, but it was not
possible to grow crystals of the product 4b; a similar ratio of
isomers was found (Scheme 3). Reaction of complex 1b and
anisole with B(C6F5)3 in CF3CH2OH solvent gave complex 4c,
again with the ortho isomer preferred (Scheme 3). For
comparison, activation of anisole by an organotungsten reagent
has been shown to give [WCp*(NO)(CH2-t-Bu)(C6H4OMe)]
with selectivity o:m:p = 87:7:6.13f
1
analysis of the H/D content of the isolated product by H
NMR spectroscopy. The reaction of 1a and B(C6F5)3/H2O in
CF3CH2OH with anisole gave a kinetic isotope effect kH/kD =
3.6, which suggests that the C−H cleavage step is rate
determining.2c,3,4
The structure of complex o-4a is shown in Figure 3. The
conformation adopted has the methoxy group anti to the
carbonyl group of the DPK ligand and syn to the hydroxyl
group of the B(OH)(C6F5)3 ligand. The OH group appears to
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Studies Using 4-Anisylplatinum Complexes. The
above study suggests that the C−H bond cleavage of anisole
is the rate-determining step and thus that the selectivity is
determined by kinetic factors. A further test was made on the
basis of gaining access to the reaction intermediates by
protonation of a neutral methyl(4-anisyl)platinum(II) complex
[PtMe(4-C6H4OMe)(NN)], as shown in Scheme 4.3,4 If the
groups,17 followed by reaction with DPK or bipy. Again the
reaction was not successful with DPA.
The structures of complexes 6a−6c were determined (Figure 4)
in order to compare the ligand and aryl conformations (Table 1).
Scheme 4
arene complex intermediate was formed reversibly, then the
product would be expected to contain all isomers of
[PtX(C6H4OMe)(NN)], but, if not, only the isomer [PtX(4-
C6H4OMe)(NN)] would be formed. If the selectivity was
determined by thermodynamic factors, the isomer [PtX(2-
C6H4OMe)(NN)] would be the dominant product.
The reagents needed for this study were prepared according to
Scheme 5. Reaction of [PtCl2(SMe2)2]15 with Me3Sn-4-C6H4OMe16
Figure 4. Views of the structures of the complexes (a) 6c, (b) 6a, and
(c) 6b. Selected bond parameters are in Table 1.
Scheme 5
Table 1. Selected Bond Parameters and Twist Angles for
Complexes 6a−6c
d
d
d
d
6a(1)
2.01(1)
2.286(3) 2.297(3) 2.301(1) 2.303(1) 2.285(2)
6a(2)
6b(1)
6b(2)
6c
Pt−C/Å
Pt−Cl/Å
2.00(1)
2.004(5) 2.021(5) 2.004(7)
a
Pt−N /Å
2.02(1)
2.12(1)
2.02(1)
2.11(1)
88.7(4)
57
2.017(4) 2.025(4) 1.989(6)
2.125(4) 2.125(4) 2.107(6)
b
Pt−N /Å
N−Pt−N/deg 87.6(4)
89.6(1)
89
89.0(2)
86
80.1(3)
c
Θ(aryl) /deg
49
50
47
55
6
ac
,
Θ(py) /deg
bc
42
31
35
,
Θ(py) /deg
41
28
32
3
a
b
c
trans-Cl. trans-aryl. Θ is the angle between the aromatic group and
the platinum plane. Data for two crystallographically independent
d
gave trans-[PtCl(4-C6H4OMe)(SMe2)2], 5. The trans stereo-
chemistry was proved by the presence of a single resonance
for the SMe2 protons. The aryl protons ortho to platinum
molecules.
There are many similarities in the bond parameters for the
three complexes (note that there are two independent
molecules in the structures of 6a and 6b). For example, the
Pt−N bond trans to chloride is shorter than the one trans to the
4-anisyl group as a result of different trans influences. As
expected, the N−Pt−N bite angle is smaller for the five-
membered ring of the bipy complex 6c than for the six-
membered rings of 6a and 6b. The twist of the pyridyl ligands
out of the square plane of the platinum atom followed the
3
displayed satellite peaks due to the coupling J(PtH) = 45 Hz.
Reaction of complex 5 with bidentate ligands then gave the
complexes 6a−6c in good yield. Complexes 7a and 7b were
obtained by reaction of the corresponding complex 6 with silver
triflate, but the reaction was not successful for the DPA
complex 6b. The syntheses of complexes 8a and 8b were
carried out by reaction of 5 with methyllithium in dimethyl-
sulfide solvent, to prevent scrambling of methyl and aryl
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sequence DPK (mean 45°) > DPA (mean 31°) > bipy (mean 4°)
and is much greater for the six-membered rings.11,14,18 The
twist of the anisyl group out of the square plane of the platinum
atom followed the sequence DPA (mean 87°) > bipy (55°) >
DPK (mean 53°). Only for the DPA complex 6b is the aryl
group close to orthogonal to the platinum plane (Table 1), the
conformation that is expected to give maximum aryl-platinum
π-overlap. The complexes pack in very different ways in the
lattices, and so some of the differences in conformations may
be related to packing forces. The differences in conformations
observed between nonequivalent molecules of 6a and 6b
(Table 1) support this interpretation. There is π-stacking of
bipy groups in 6c and NH···Cl intermolecular hydrogen
bonding in 6b, leading to formation of a supramolecular
polymer (Figure 5).
This was characterized by a platinum hydride resonance at δ =
1
−20.37, with J(PtH) = 1578 Hz, a methylplatinum resonance
at δ = 1.47, with 2J(PtH) = 66 Hz, and anisyl resonances at δ =
3.73 (MeO) and 6.59, 6.74 (aryl). As the solution was warmed
to 0 °C, loss of methane occurred (δ = 0.24) and complex 6c
was formed. No [PtClMe(bipy)] or free anisole were detected
1
by H NMR, showing higher selectivity than the reaction of
Scheme 1, which gave only 82% selectivity for methylplatinum
bond cleavage from F.3a If the reaction was carried out with
three equivalents of HCl, the first stage occurred in the same
way to give complex 6c, but this then reacted with more HCl to
give [PtCl2(bipy)], 10b, and anisole. The high selectivity for
methylplatinum rather than arylplatinum bond cleavage in
reactions that occur by the oxidative addition/reductive
elimination mechanism has been observed previously19 and
can be used to distinguish this mechanism from the direct SE2
cleavage mechanism.19,20 However, the case is more complex if
the C−H bond formation is not rate determining, such as when
displacement of alkane or arene from platinum is the slow
step.3a Hydridoplatinum(IV) intermediates have been observed
in several previous studies of cleavage of platinum−carbon σ-
bonds by acids,1−4,21 and five-coordinate platinum(IV)
complexes such as H (Scheme 6) are most commonly inter-
mediates in concerted reductive elimination reactions.1,3,4,21,22
The reactions of complexes 8a and 8b with triflic acid or with
B(C6F5)3/H2O were carried out under several different
conditions, including reactions in CF3CH2OD and CD3OD,
which mimicked those used in the CH activation (Scheme 3).
They gave similar results (Scheme 7), so only representative
Scheme 7
Figure 5. Supramolecular polymer formed by NH···Cl hydrogen
bonding in 6b. H-bonding distances (Å): N(7)···Cl(2) 3.384(4);
N(37)···Cl(1) 3.351(4).
The reaction of the complex [PtMe(4-C6H4OMe)(bipy)],
8b, with HCl is outlined in Scheme 6. If the reaction with one
Scheme 6
examples are described. The reaction of 8b with one equivalent
of triflic acid in CD3OD occurred rapidly to give [Pt(O3SCF3)-
(4-C6H4OMe)(bipy)], 7b, and methane as a mixture of CH4,
CH3D, CH2D2, and CHD3 (and presumably CD4) as identified
1,3,4,14,21
1
by the H NMR spectrum.
Scheme 7 shows only the
formation of CH3D, but easy equilibration between the cationic
hydridomethylplatinum(IV) and methane-platinum(II) com-
plexes can lead to extensive H/D exchange by way of 8b*
and then further reversible addition of D+.1,3,4 No significant
deuterium incorporation into the anisyl group of 7b was observed,
and no isomerization to the ortho- or meta-anisyl group occurred.
This result indicates that there is an easy equilibrium be-
tween methyl(hydrido)platinum(IV) and methane-platinum(II)
equivalent of HCl was carried out at −80 °C in CD2Cl2
1
solution and monitored by H NMR spectroscopy, the first
compound observed was the platinum(IV) hydride complex 9b.
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a
intermediates, as well as easy exchange between the hydride and
deuterium from solvent CD3OD to give multiple deuterium
incorporation into the methane, but that there is no reductive
elimination (either reversible or irreversible) involving the anisyl
group. When the reaction was carried out at −80 °C, a very
similar result was obtained and the presumed hydride
intermediate analogous to 9b was not detected. Similar reactions
of 8a gave 7a and CHnD4−n. The selectivity for methylplatinum
rather than arylplatinum bond cleavage is again very high.
Scheme 8. ( )
The reaction of 8b with excess (five equivalents) of triflic
acid in CD2Cl2/CD3OD occurred in two distinct steps. The
first step was very fast and occurred as above to give methane-
dn (n = 1−4) and 7b, but the second step occurred much more
slowly to give [Pt(O3SCF3)2(bipy)], 11b,23,24 and anisole,
which was very largely anisole-4-d1. This step was complete in
about four days at room temperature. The reaction of triflic
acid with 7b, prepared according to Scheme 5, occurred in the
same way as in the second step above. The reactions of 8a and
7a with excess triflic acid occurred similarly (Figure 6), the only
a
NN = bipy.
had limited thermal stability and decomposed to give an
unidentified black precipitate on warming the solution. Com-
plex 13 was characterized by a trimethysilylplatinum resonance
3
at δ = −0.16, with J(PtH) = 21 Hz, and by a methylplatinum
resonance at δ = 1.23, with J(PtH) = 62 Hz.26 If a solution of
2
13 at 0 °C was allowed to stand while exposed to moist air,
the anhydrous triflate complex [Pt(OTf)2(bipy)], 11b, cry-
stallized slowly and was structurally characterized (Figure 7a).
1
Figure 6. H NMR spectra from reaction of 8a with triflic acid in
CD2Cl2/CD3OD of (a) methane-dn and (b) anisole (mostly 4-
MeOC6H4D); (c) reference spectrum of anisole (MeOC6H5) in
CDCl3.
significant difference being that the reaction with 7a to give
anisole and [Pt(O3SCF3)2(DPK)], 10b,23 was complete in about
five hours at room temperature, which is significantly faster
than the reaction with 7b. Similarly, the reaction of [Pt(4-
C6H4OMe)2(bipy)]25 with triflic acid in CD2Cl2/CD3OD
occurred in two steps to give 7b and then [Pt(O3SCF3)2(bipy)],
with anisole-4-d1 as the major organic product in both steps.
The nature of the triflate complexes23 was investigated by
reaction of [PtMe2(bipy)], 12, with trimethylsilyl triflate under
varying conditions (Scheme 8). Under rigorously dry con-
ditions in CD2Cl2 solution, as monitored by 1H NMR spectro-
scopy, the reaction occurred easily at −78 °C to give a green-
black solution in which the chief product was identified as the
trimethylsilylplatinum(IV) complex [Pt(OTf)Me2(SiMe3)(bipy)],
13, by comparison of the NMR parameters with those of
similar compounds [PtXMe2(SiMe3)(bipy)].26,27 Complex 13
Figure 7. Views of the structure of (a) [Pt(O3SCF3)2(bipy)], 11b.
Selected bond parameters (distances in Å; angles in deg): Pt(1)−N(1)
1.982(5); Pt(1)−O(1) 2.054 Å; N(1)−Pt−N(1A) 81.9(3); O(1)−
Pt−O(1A) 84.1(2). (b) [Pt(OH2)2(bipy)](CF3SO3)2(H2O)2. Se-
lected bond parameters (distances in Å; angles in deg): Pt(1)−N(1)
1.976(7); Pt(1)−N(2) 1.976(7); Pt(1)−O(1) 2.042(6); Pt(1)−O(2)
2.041(6) Å; N(2)−Pt(1)−N(1) 81.9(3); O(2)−Pt(1)−O(1) 90.0(2).
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a
Scheme 9
a
NN = DPK, X = triflate, cationic complexes have a triflate anion.
When complex 12 was treated with triflic acid-d, prepared by
reaction of Me3SiOTf with CD3OD in moist solvent, a white
precipitate formed that was recrystallized to give the hydrated
complex [Pt(OH2)2(bipy)](CF3SO3)2·2H2O, 14, whose struc-
ture is shown in Figure 7b. Complex 14 forms dimer units in
the crystal through hydrogen bonding between coordinated
[O(1), O(2)] and free [O(3), O(4)] water molecules and the
1
triflate anions (Figure 7b). The H NMR spectrum of 14 in
acetone-d6 solution gave a free triflate resonance at δ(19F) =
79.0 and broad resonances for the bipy protons, which may be
indicative of dynamic exchange between coordinated water and
acetone molecules. It should be noted that the cleavage of both
methylplatinum groups from 12 (Scheme 8) occurred much
more rapidly than the cleavage of the anisyl ligand of 8b
(Scheme 7).
Theoretical Studies. Several theoretical studies have been
made on carbon−hydrogen bond activation of methane or
benzene by electrophilic platinum(II) complexes,1−4,17,21,22,28
but there has been little computational work on the selectivity
in anisole activation.13 A DFT study of the isomers of [PtMe-
(anisyl)(NN)] and of the isomers formed by protonation,
namely, [PtHMe(anisyl)(NN)]+, [PtMe(anisole)(NN)]+, and
[Pt(anisyl)(CH4)(NN)]+, with NN = bipy or DPK, has there-
fore been carried out. Results on the ground-state structures are
given in Figures 8−10 and are summarized below.
Figure 8. Calculated structures and energies (E in kJ mol−1, with
respect to J for neutral complexes or O for cationic complexes) of
complexes and cationic intermediates with NN = bipy.
kinetically. Consider the case of the bipy complex 8b. The
experimental results show that isomerization does not occur,
but, if it did, it is expected to occur by initial protonation. The
kinetic product of protonation of 8b is expected to be [PtHMe-
(4-C6H4OMe)(bipy)]+, H (Figure 8), and there is evidence for
this in the formation of 9b (Scheme 6).1−4,21 The meta and
ortho isomers of H are K and L (Figure 8) and isomerization
would be expected to occur by way of the arene isomers, which
are M and N (Figure 8), formed by reductive coupling. With
anisole, the arene can also bind through the oxygen atom to
give the ether complex O, which is calculated to be the lowest
energy isomer (Figures 8, 10). The alternative reductive
For the platinum(II) complexes [PtMe(anisyl)(NN)], the
calculated stability follows the series ortho (J, J′) > meta (I, I′) >
para (8b, 8a) for both NN = bipy and DPK, but the energy
differences are small. The equilibration of the para-anisyl
derivative 8a or 8b with its ortho and meta isomers is therefore
predicted to be thermodynamically favorable if it is allowed
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bond parameters are similar to those found experimen-
tally in a stable iridium(III) pincer ligand (PNP) complex
[IrH(2-C6H4OMe)(PNP)]+, which has Ir−C−C =
131.1° and 112.5° and Ir···O = 2.76 Å.13b
3. For the para and meta isomers, the methane complexes
[Pt(anisyl)(CH4)(bipy)]+, P and Q, lie a little lower in
energy than the corresponding complexes [PtHMe-
(anisyl)(bipy)]+, H and K, but, for the ortho isomer,
the methane complex R is higher in energy than the
corresponding hydrido(methyl)platinum(IV) complex L
(Figures 8, 10). The difference is explained by the excep-
tional stabilization of L as described above.
For the DPK complexes (Figures 9, 10) the trends are
similar, but there is one significant difference. The most stable
calculated forms of the arene π-complexes are closer to the η1
than to the η2 bonding form found with the bipy ligand. Thus,
complex U is calculated to have Pt−C2 = 2.27 Å but Pt−C1 =
3.00 Å and Pt−C3 = 2.82 Å, T has Pt−C3 = 2.30 Å but Pt−C2 =
2.76 Å and Pt−C4 = 2.97 Å, and S has Pt−C4 = 2.26 Å but Pt−
C3 = 2.80 Å and Pt−C5 = 3.05 Å. The closest Pt···H distances
of 2.51, 2.70, and 2.63 Å for U, T, and S, respectively, are too
long to indicate a bonding interaction, so these complexes have
the structure of the Wheland intermediate, which can be
described as an η1-anisole complex, rather than an η2-CH
complex. There are several possible conformers in the DPK
complexes, but, in the most stable ones (Figure 9), the arene
groups in S, T, and U are oriented toward the methylplatinum
group with the CH group roughly in the plane of the platinum
atom. This structure would not be favored for the 2,2′-
bipyridine complexes because of steric interactions.
Figure 9. Calculated structures of complexes and cationic
intermediates with NN = DPK.
The interpretation of the observed chemistry is summarized
in Scheme 8 and Figure 11, for the case with NN = DPK and
Figure 10. Relative energies (kJ mol−1 with respect to O or O′) of
cationic complexes shown in (a) Figure 8 (NN = bipy) and (b) Figure 9
(NN = DPK).
coupling is to combine the methyl and hydride groups to give
the corresponding methane complexes P, Q, and R (Figure 8).
The following features are notable in Figures 8 and 10a:
1. The arene and ether complexes [PtMe(anisole)(bipy)]+,
M, N, and O, have similar energies, they are more stable
than the corresponding complexes [PtHMe(anisyl)-
(bipy)]+ and [Pt(anisyl)(CH4)(bipy)]+, and they would
be likely to interconvert easily (the 1- or 1,2-arene
complex is calculated to have significantly higher energy).
The arene−platinum binding in M (Pt−C3 = 2.44, Pt−
C4 = 2.30 Å) and N (Pt−C2 = 2.29, Pt−C3 = 2.45 Å) is
less symmetrical than in the corresponding benzene
complexes (Pt−C = 2.30, 2.30 Å),2a but can still be
considered as η2-arene binding.3−5,22f The distortion is
rationalized in terms of the electronic effects of the
methoxy substituent, favoring the shorter Pt−C bond to
the ortho-carbon in N and to the para-carbon in M.
2. The para and meta isomers [PtHMe(anisyl)(bipy)]+, H
and K, are the least stable complexes, but the ortho
isomer L is considerably more stable (Figures 8, 10). In
L, the o-anisyl group bends to allow the oxygen atom to
be closer to platinum, as seen by different calculated
bond angles of the anisylplatinum group with Pt−C−C =
134° and 107°, compared to the respective calculated
angles of 119° and 122° in H. The distance Pt−O = 2.55 Å
in L is too long to represent a full covalent bond, but
evidently provides significant overall stabilization. These
Figure 11. Proposed energies (relative to O′, shown as inset) and
transition states of the DPK complexes shown in Scheme 8. Cationic
complexes have the triflate anion (not shown).
X− = triflate. It will be assumed that the C−H cleavage or
formation reactions occur by the oxidative addition or reductive
elimination reactions, respectively, although this is not strictly
proved.1−3 The kinetic product of protonation of complex 8a is
H′ (Scheme 8), which can undergo reductive elimination of
either methane to give the methane complex P′, via transition
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state P′†, or the anisole complex S, via transition state S†. A
third reaction is that it can undergo H/D exchange with solvent
(CD3OD in this case), which might occur by reversible
deprotonation to 8a or by an associative mechanism, such as by
a dihydrogen complex intermediate. Associative displacement
of methane from P′ or anisole from S could then give p-4b or
2d, respectively1−4 (note that the nucleophile is shown as
triflate in Scheme 8, but it could also be a solvent molecule).23
Any of these five reactions could in principle be rate deter-
mining. The observation that methane isotopomers CHnD4−n
are formed shows that the exchange between H′, H′*, and P′
must be fast relative to methane displacement from P′ to give p-
4b. The observation that anisole and complex 2d are not
formed could be explained if either the anisole displacement
from S or the reductive elimination to form S was rate deter-
mining. However, if S was formed reversibly, then isomerization
to T (Figure 9, not shown in Scheme 8) and U would also
occur and would lead eventually to formation of o-4b and to
multiple deuterium incorporation into the anisyl group, neither
of which was observed. It is therefore concluded that the
highest activation energy in the five possible reactions is the
reductive elimination to form S and that neither this step nor
the subsequent displacement of anisole occurs. The possible
relative activation energies are shown in Figure 11.29 The
selectivity for methane rather than arene formation is much
higher than when the methane or arene displacement step is
rate determining.3a
Figure 12. Calculated intermediates and transition states in the
oxidative addition of a C−H bond of anisole (left bipy, right DPK
complexes).
when NN = bipy. It seems likely, therefore, that C−H
activation by complexes “[PtMe(bipy)]+” will be possible under
optimum experimental conditions.
The mechanisms of protonolysis of 8a and of C−H
activation of anisole by complex 2d are related by microscopic
reversibility, and the C−H activation is also depicted in Scheme
8 and Figure 11. Displacement of triflate by anisole gives the
anisole complexes O′, U, T, and S, which are easily inter-
converted.30 The easiest C−H activation occurs from U to give
L′ via the transition state U†, and reductive elimination from L′
then gives methane and o-4b by way of R′† and R′ (Scheme 8).
The rate-determining step is the C−H activation, consistent
with the observed isotope effect kH/kD = 3.6.2−4,31 The
selectivity of C−H activation of arenes can be determined by
either thermodynamic or kinetic effects.1−4,13 In the present
case, kinetic control is proposed, but we note that the product
o-4b is favored by both kinetic and thermodynamic effects
(Figure 11).23
One unexplained aspect of this work is the difference
between the bipy and DPK complexes. Under mild conditions
used in this work, [PtXMe(NN)], X = OTf or HOB(C6F5)3,
reacts with arenes when NN = DPK but not when NN = bipy.
In addition the protonolysis of the anisyl−platinum bond in
[PtX(4-C6H4OMe)(NN)] occurs faster by an order of
magnitude when NN = DPK compared to NN = bipy.
However, [PtX2(NN)], X = CF3CO2, in CF3CO2D catalyzes
H/D exchange of benzene at 150 °C at similar rates when
NN = bipy or DPK, showing that C−H bond activation can
occur in both cases.1j In addition, studies using mass spectro-
metry have shown that [PtMe(C6D6)(bipy)]+ can undergo easy
H/D exchange between the methylplatinum group and the
arene, which must involve C−D bond activation.2a Although
the calculated ground-state structures of the π-arene complexes
[PtMe(anisole)(NN)]+ are different when NN = bipy (N,
Figure 12) and DPK (U, Figure 12), the calculated transition
states (N†, U†) can be considered as intermediate between the
hydrido(2-anisyl)platinum(IV) complexes (L, L′) and an η2-
CH bonded anisole complex in each case (Figure 12),2a and the
calculated activation energies are only 5−10 kJ mol−1 higher
EXPERIMENTAL SECTION
■
All reactions were carried out under nitrogen using standard Schlenk
techniques, unless otherwise specified. NMR spectra were recorded
using Varian Mercury 400 or Varian Inova 400 or 600 spectrometers.
ESI mass spectra were recorded using a Micromass LCT spectrometer,
with an injection flow rate of 20 μL/min, and were calibrated with NaI
at a concentration of 2 μg/μL in 50:50 propan-2-ol/water. The
injection flow rate of 20.0 μL/min was used throughout all
experiments. The complexes [Pt2Me4(μ-SMe2)2], [PtMe2(bipy)],
[PtMe2(DPK)], and [Pt(4-MeOC6H4)2(bipy)] were prepared by the
literature methods.16,18e,25
[PtMe{HOB(C6F5)3}(DPK)], 2a. To a solution of [PtMe2(DPK)],
1a (0.41 g, 1 mmol), in CF3CH2OH (30 mL) was added a solution
of B(C6F5)3 (0.54 g, 1 mmol) in CF3CH2OH (10 mL). The color
changed from purple to yellow-orange. The mixture was stirred for 6 h,
and the yellow precipitate of the product was separated by filtration,
washed with ether (3 × 2 mL) and pentane (3 × 10 mL), and dried
under vacuum. Yield: 86%. Anal. Calcd for C30H12BF15N2O2Pt: C,
39.03; H, 1.31; N, 3.03. Found: C, 39.54; H, 1.34; N, 3.27. NMR in
CD2Cl2: δ(1H) = 0.96 [s, 3H, 2J(PtH) = 73 Hz, Pt-Me]; 3.30 [m, 1H,
OH]; 7.50−8.80 [8H, py]. δ(19F) = −134.3 [br m, 6F, o-F]; −160.2
[t, 3F, p-F]; −165.4 [dt, 6F, m-F]. δ(13C) = −13.9 [PtMe]; 125.4,
127.4, 129.1, 129.2, 137.8, 139.1, 149.5, 149.9, 153.5, 153.9 [py];
135.6; 138.2; 146.5; 148.8 [C6F5]; 185.9 [CO].
[PtMe{HOB(C6F5)3}(DPA)], 2b. This was prepared in a similar
way from [PtMe2(DPA)] and isolated as a pale brown solid. Yield:
84%. Anal. Calcd for C29H13BF15N3OPt: C, 38.26; H, 1.44; N, 4.62.
Found: C, 38.59; H, 1.33; N, 4.47. NMR in CD2Cl2: δ(1H) = 0.86
[s, 3H, 2J(PtH) = 72 Hz, Pt-Me]; 3.70 [m, 1H, OH]; 5.70 [1H, NH];
6.38−8.51 [8H, py]. δ(19F) = −134.1 [br m, 6F, o-F]; −160.1 [t, 3F,
p-F]; −165.2 [dt, 6F, m-F].
[PtMe(DPK)(CO)]+[HOB(C6F5)3]−, 3a. Yield: 88%. NMR in
2
CD2Cl2: δ(1H) = 1.09 [s, 3H, J(PtH) = 70 Hz, PtMe]; 4.17 [m,
1H, O(H)B]; 7.42 [t, br, 2H, Py H5 and H5′]; 7.92 [d, 2H, 3J((HH) =
3
8 Hz, Py H3, H3′]; 8.03 [t, 2H, J(HH) = 7 Hz, Py H4, H4′]; 8.60
[d, br, 2H, Py H6, H6′]. δ(19F) = −136.3 [d, 6F, o-F]; −162.1 [t, br, 3F,
p-F]; −166.6 [t, 6F, m-F]. IR: 2073 cm−1 (CO). ESI-MS (m/z):
422 (M+).
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[PtMe(DPA)(CO)]+[HOB(C6F5)3]−, 3b. Yield: 91%. NMR in
3
3JPtH = 57 Hz, SMe]; 3.72 [s, 3H, OMe]; 6.64 [d, 2H, JHH = 9 Hz,
2
CD2Cl2: δ(1H) = 1.00 [s, 3H, J(PtH) = 68 Hz, PtMe]; 7.10 [t, 1H,
3
3
Hm]; 7.20 [d, 2H, JHH = 9 Hz, JPtH = 45 Hz, Ho].
Py H5]; 7.12 [t, 1H, Py H5′]; 7.46 [d, 1H, Py H4]; 7.52 [d, 1H, Py
[PtCl(4-C6H4OMe)(bipy)], 6c. A mixture of trans-[PtCl(p-anisyl)-
(SMe2)2] (100 mg, 0.216 mmol) and bipy (33.8 mg, 0.216 mmol) in
benzene (10 mL) was warmed to 50 °C for 2 days. The solvent was
evaporated, and the orange product was purified by recrystallization
from CH2Cl2/pentane. Yield: 105 mg, 98%. Anal. Calcd for
C17H15ClN2OPt: C, 41.35; H, 3.06; N, 5.67. Found: C, 40.89; H,
2.77; N, 5.27. NMR in CD2Cl2: δ(1H) = 3.80 [s, 3H, OMe]; 6.75
3
H4′]; 7.82 [t, br, 2H, Py H3, H3′]; 8.17 [d, 1H, J(PtH) = 34 Hz, Py
H6]; 8.28 [d, 1H, Py H6′]; 12.08 [s, 1H, NH]. NMR (CD2Cl2): δ(13C) =
−13.9 [s, 1J(PtC) = 520 Hz, PtMe]; 163.2 [s, 1J(PtC) = 1821 Hz, PtCO].
δ(19F) = −136.3 [d, 6F, o-F]; −161.8 [t, 3F, p-F]; −166.3 [t, 6F, m-F].
IR: 2105 cm−1 (CO). ESI-MS (m/z): 409 (M+).
[Pt(C6H4OMe){HOB(C6F5)3}(DPK)], 4a. To a solution of complex
1a (0.41 g, 1 mmol) in CF3CH2OH (30 mL) was added a solution of
B(C6F5)3 (0.54 g, 1 mmol) in CF3CH2OH (10 mL), followed by
addition of C6H5OMe (30 mmol), to give a yellow-orange solution.
After 4 days, the yellow precipitate of the product was separated by
filtration, washed with ether (3 × 2 mL) and pentane (3 × 10 mL),
and dried under vacuum. Yield: 48%. Anal. Calcd for
C36H16BF15N2O3Pt: C, 42.58; H, 1.61; N, 2.80. Found: C, 42.64; H,
1.81; N, 2.72. NMR in CD2Cl2: ortho-4a: δ(1H) = 4.01 [s, 3H, OMe];
4.81 [m, br, 1H, OH]; 6.29 [d, 1H, An H6]; 6.39 [t, 1H, An H5]; 6.68
[d, 1H, An H3]; 6.90 [dt, 1H, An H4]; 7.21−9.01 [8H, py]. δ(19F) =
−134.5 [br m, 6F, o-F]; −161.7 [t, 3F, p-F]; −166.9 [m, 6F, m-F].
meta-4a: δ(1H) = 3.75 [s, 3H, OMe]; 3.70 [m, br, 1H, OH]; 6.51 [s,
1H, An H2]; 6.56 [d, 1H, An H6]; 6.83 [t, 1H, An H5]; 6.90 [d, 1H,
An H4]; 7.25−8.97 [8H, py]. para-4a: δ(1H) = 3.72 [s, 3H, OMe];
3.69 [m, br, 1H, OH]; 6.44 [d, 2H, An H2,6]; 6.54 [d, 2H, An H3,5];
7.3−8.9 [8H, py].
[Pt(C6H4OMe)(O3SCF3)(DPK)], 4b. This was prepared similarly
but by using triflic acid. Yield: 42%. Anal. Calcd for C19H15F3N2O5PtS:
C, 35.91; H, 2.38; N, 4.41. Found: C, 35.55; H, 2.29; N, 4.06. NMR in
CD2Cl2: ortho-4b: δ(1H) = 4.04 [s, 3H, OMe]; 6.34 [d, 1H, An H6];
6.42 [t, 1H, An H5]; 6.70 [d, 1H, An H3]; 6.94 [dt, 1H, An H4]; 7.2−
9.0 [8H, py]. meta-4b: δ(1H) = 3.77 [s, 3H, OMe]; 6.51 [s, 1H, An
H2]; 6.60 [d, 1H, An H6]; 6.85 [t, 1H, An H5]; 6.93 [d, 1H, An H4];
7.2−9.0 [8H, py]. para-4b: δ(1H) = 3.73 [s, 3H, OMe]; 6.47 [d, 2H,
An H2,6]; 6.55 [d, 2H, An H3,5]; 7.2−9.0 [8H, py].
[Pt(C6H4OMe){HOB(C6F5)3}(DPA)], 4c. This was prepared as for
4a but using complex 1b. Yield: 51%. Anal. Calcd for
C35H17BF15N3O2Pt: C, 41.94; H, 1.71; N, 4.19. Found: C, 41.48; H,
1.85; N, 3.88. NMR in CD2Cl2: ortho-4c: δ(1H) = 3.95 [s, 3H, OMe];
4.73 [m, br, 1H, OH]; 6.32 [d, 1H, An H6]; 6.35 [t, 1H, An H5]; 6.60
[d, 1H, An H3]; 6.83 [dt, 1H, An H4]; 7.1−9.1 [8H, py]. δ(19F) =
−134.0 [br m, 6F, o-F]; −161.1 [t, 3F, p-F]; −167.2 [m, 6F, m-F].
meta-4c: δ(1H) = 3.77 [s, 3H, OMe]; 6.52 [s, 1H, An H2]; 6.58 [d,
1H, An H6]; 6.80 [t, 1H, An H5]; 6.90 [d, 1H, An H4]; 7.1−9.1 [8H,
py]. para-4c: δ(1H) = 3.71 [s, 3H, OMe]; 6.42 [d, 2H, An H2,6]; 6.50
[d, 2H, An H3,5]; 7.1−9.1 [8H, py].
[d, 2H, JHH = 8 Hz, Hm]; 7.27 [d, 2H, JHH = 8 Hz, JPtH = 38 Hz,
3
3
3
Ho]; 7.33 [t, 1H, H5′]; 7.73 [t, 1H, H5]; 8.05 [d, 1H, H3′]; 8.10−8.20
[m, 3H, H3,H4,H4′]; 8.69 [d, 1H, JPtH = 56 Hz, H6′]; 9.59 [d, 1H,
3
3JPtH = 15 Hz, H6]. EI-MS: m/z = 494.0 [M+].
[PtCl(4-C6H4OMe)(DPK)], 6a. This was prepared similarly but
using DPK. Yield: 74%. Anal. Calcd for C18H15ClN2O2Pt: C, 41.43; H,
2.90; N, 5.59. Found: C, 40.96; H, 2.79; N, 5.20. NMR in CD2Cl2:
δ(1H) = 3.74 [s, 3H, OMe]; 6.60 [d, 2H, JHH = 8 Hz, Hm]; 6.94
3
[d, 2H, 3JHH = 8 Hz, 3JPtH = 41 Hz, Ho]; 7.31 [t, 1H, H5′]; 7.74 [t, 1H,
H5]; 8.03 [d, 1H, H3′]; 8.10−8.17 [m, 3H, H3, H4, H4′]; 8.39 [d, 1H,
3JPtH = 60 Hz, H6′]; 9.38 [d, 1H, J
= 16 Hz, H6]. EI-MS: m/z =
3
PtH
521.0 [M+]. IR (CH2Cl2): ν(CO) 1690 cm−1.
[PtCl(4-C6H4OMe)(DPA)], 6b. This was prepared similarly but
using DPA. Yield: 65%. Anal. Calcd for C17H16ClN3OPt: C, 40.13; H,
3.17; N, 8.26. Found: C, 40.08; H, 3.48; N, 7.92. NMR in CD2Cl2:
δ(1H) = 3.72 [s, 3H, OMe]; 6.56 [d, 2H, JHH = 8 Hz, Hm]; 6.95
3
[d, 2H, JHH = 8 Hz, Ho]; 6.6 [t, 1H, H5′]; 6.95 [t, 1H, H5]; 7.17
3
[d, 1H, H3′]; 7.29 [d, 1H, H3]; 7.56 [t, 1H, H5′]; 7.62 [t, 1H, H5]; 7.94
[d, 1H, 3JPtH = 68 Hz, H6′]; 8.98 [d, 1H, 3JPtH = 15 Hz, H6]; 9.17 [s, 1H,
N-H]. ESI-MS: m/z = 507.1 [M+].
[Pt(O3SCF3)(4-C6H4OMe)(bipy)], 7b. To a solution of complex
6c (100 mg, 0.20 mmol) in thf (10 mL) was added AgO3SCF3 (52 mg,
0.20 mmol). The mixture was stirred for 30 min at 0 °C and filtered
through Celite, and the solvent evaporated. The product was extracted
with benzene, the volume was reduced, and pentane was added to
precipitate the product as a yellow powder. Yield: 38%. Anal. Calcd for
C19H15F3N2O4PtS: C, 36.84; H, 2.44; N, 4.52. Found: C, 37.17; H,
2.87; N, 4.29. NMR in CD2Cl2: δ(1H) = 3.80 [s, 3H, OMe]; 6.83
[d, 2H, 3JHH = 8 Hz, Hm]; 7.38 [d, 2H, 3JHH = 8 Hz, Ho]; 7.88 [t, 1H,
H5′]; 8.22 [t, 1H, H5]; 8.3−8.5 [m, 4H, H3, H3′,H4,H4′]; 8.48 [d, 1H,
H6′]; 8.77 [d, 1H, H6].
[Pt(O3SCF3)(4-C6H4OMe)(bipy)], 7a. This was prepared similarly
from complex 6a. Yield: 41%. Anal. Calcd for C20H15F3N2O5PtS: C,
37.10; H, 2.34; N, 4.33. Found: C, 37.21; H, 2.74; N, 4.15. NMR in
CD2Cl2: δ(1H) = 3.67 [s, 3H, OMe]; 6.53 [d, 2H, JHH = 8 Hz, Hm];
3
6.71 [d, 2H, JHH = 8 Hz, Ho]; 7.35 [t, 1H, H5′]; 7.80 [t, 1H, H5];
3
8.15−8.30 [m, 4H, H3, H3′,H4,H4′]; 8.08 [d, 1H, H6′]; 9.04 [d, 1H, H6].
[PtMe(4-C6H4OMe)(bipy)], 8b. To a solution of trans-[PtCl-
(4-C6H4OMe)(SMe2)2] (100 mg, 0.216 mmol) in Me2S (4 mL) at
−78 °C was added MeLi (0.27 mL, 1.6 M in diethyl ether, 0.433
mmol). The mixture was stirred at −78 °C for 30 min, then allowed to
warm to 0 °C. Ten drops of a cold solution of saturated, aqueous
NH4Cl was added, followed by the addition of bipy (33.8 mg,
0.216 mmol) in benzene (4 mL). The mixture was stirred at room
temperature for 8 h, the solvent was evaporated under vacuum, and
the orange product was recrystallized from CH2Cl2/pentane. Yield:
65 mg, 63%. Anal. Calcd for C18H18N2OPt: C, 45.67; H, 3.83; N, 5.92.
Found: C, 45.50; H, 3.61; N, 5.64. NMR in CD2Cl2: δ(1H) = 1.03
H/D Exchange Experiments. The reaction of 1 with anisole-d8
was carried out in a way similar to the synthesis of complex 4a, but
using the perdeuterated arene. The extent of H/D exchange at each
position of the aryl group of the product complex was determined by
integration of the corresponding resonance in the 1H NMR spectrum,
using an adjacent pyridyl resonance for calibration. In a separate
experiment, the complex 4a was dissolved in deuterated alcohol in an
NMR tube, but negligible H/D exchange was observed over several
days at room temperature under these conditions. The isotope effect
was determined by carrying out the synthesis as for 4a but using
equimolar amounts of anisole and anisole-d8. The product was isolated
followed by analysis by using 1H NMR. Aliquots were taken during the
course of the reaction, evaporated, and analyzed as a cross-check.
Me3Sn-4-C6H4OMe. This was prepared by the literature method.15
[s, 3H, 2JPtH = 86 Hz, PtMe]; 3.78 [s, 3H, OMe]; 6.73 [d, 2H, 3JH−H
=
3
3
8 Hz, Hm]; δ 7.27 [d, 2H, JHH = 8 Hz, JPtH = 65 Hz, Ho]; 7.40
2
NMR in CDCl3: δ(1H) = 0.26 [s, 9H, JSnH = 54 Hz, SnMe]; 3.80
[t, 1H]; 7.59 [t, 1H]; 8.06−8.18 [m, 4H]; 8.69 [d, 1H, 3JPH = 21 Hz];
[s, 3H, OMe]; 6.91 [d, 2H, 3JHH = 8 Hz, H3]; 7.40 [d, 2H, 3JHH = 8 Hz,
3JSnH = 43 Hz, H2].
3
9.20 [d, 1H, JPtH = 20 Hz]. EI-MS: m/z = 472.9.
[PtMe(4-C6H4OMe)(DPK)], 8a. This was prepared in a similar
way but using DPK as ligand. Yield: 45%. Anal. Calcd for
C19H18N2O2Pt: C, 45.51; H, 3.62; N, 5.59. Found: C, 45.49; H,
trans-[PtCl(4-C6H4OMe)(SMe2)2], 5. A mixture of cis/trans-
[PtCl2(SMe2)2] (1.10 g, 2.82 mmol) and Me3Sn-4-C6H4OMe (0.840 g,
3.10 mmol) in acetone (25 mL) was warmed to 50 °C with stirring
for 3 days. The resulting solution was allowed to cool to room
temperature and filtered, and the solvent was evaporated under
vacuum. The product was purified by crystallization from CH2Cl2/
pentane. Yield: 1.03 g, 79%. NMR in CD2Cl2: δ(1H) = 2.31 [s, 12H,
2
3.48; N, 5.31. NMR in CD2Cl2: δ(1H) = 0.88 [s, 3H, JPtH = 84 Hz,
3
PtMe]; 3.75 [s, 3H, OMe]; 6.60 [d, 2H, JHH = 8 Hz, Hm]; 6.96
[d, 2H, 3JHH = 8 Hz, 3JPtH = 65 Hz, Ho]; 7.32 [t, 1H, H5′]; 7.39 [t, 1H,
H5]; 8.04−8.15 [m, 4H, H3, H4, H3′, H4′]; 8.62 [d, 1H, 3JPtH = 21 Hz,
3
H6′]; 8.80 [d, 1H, JPtH = 20 Hz, H6]. EI-MS: m/z = 500.9.
315
dx.doi.org/10.1021/om200922a | Organometallics 2012, 31, 306−317

Organometallics
Article
Reactions with Acids. In a typical reaction, to a solution of
complex 8b (10 mg, 0.021 mmol) in CD2Cl2 (1 g) in an NMR tube
cooled to −78 °C was added a solution of HCl in CD2Cl2 (80 μL,
0.788 M, 0.063 mmol). An immediate color change from orange to
yellow was observed. The tube was inserted into the NMR probe
cooled to −80 °C, and NMR spectra were recorded at 20 °C intervals
as the probe was warmed slowly to room temperature. Reactions with
deuterated acids were carried out similarly but using a 1:1 CD2Cl2/
CD3OD solvent mixture.
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[Pt(OTf)Me2(SiMe3)(bipy)], 13. To a solution of Me3SiOTf
(50 μL, 0.32 mmol) in CD2Cl2 (0.5 g) in an NMR tube cooled to
−78 °C was added a solution of [PtMe2(bipy)], 12 (0.025 g, 0.064
mmol), in CD2Cl2 (1 g). An immediate color change from red to
green was observed. The tube was inserted into the NMR probe
cooled to −80 °C, and NMR spectra were recorded at 20 °C intervals
as the probe was warmed slowly to room temperature. NMR in
CD2Cl2: δ(1H) = −0.16 [s, 9H, 3J(PtH) = 21 Hz, MeSi]; 1.23 [s, 6H,
2J(PtH) = 62 Hz, MePt]; 7.74 [m, 2H, H5]; 8.20 [m, 2H, H4]; 8.23
[m, 2H, H3]; 8.84 [d, 2H, H6]. When the NMR tube was stored at
0 °C, crystals of [Pt(O3SCF3)2(bipy)], 11b, were deposited.
[Pt(bipy)(H2O)2](O3SCF3)2·(H2O)2, 14. To a stirred solution of
Me3SiOTf (250 μL, 1.6 mmol) in CH2Cl2 (3 g) at room temperature
was added CD3OD (64 μL, 1.6 mmol). The reaction mixture was then
cooled to −78 °C, and a solution of [PtMe2(bipy)], 12 (0.125 g, 0.32
mmol), in CH2Cl2 (5 g) was added dropwise. An immediate color
change from red to green was observed. The reaction mixture was
warmed slowly to room temperature. The color changed from green to
colorless with development of a white precipitate. The solvent was
decanted, and the solid was washed with CH2Cl2 twice and dried
under vacuum. Yield: 78%. NMR in (CD3)2CO: δ(1H) = 7.86−8.07
[br m, 2H, H5]; 8.47 − 8.64 [br m, 4H, H3, H4]; 8.66−8.79 [br m, 2H,
H6]; δ(19F) = −79.0 [s, 6F, CF3]. The product was crystallized from
CH2Cl2/pentane.
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X-ray Structure Determinations. Data were collected by using a
Nonius Kappa-CCD or a Bruker APEX-II diffractometer. Details of
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files. In a typical procedure, the subject crystal was mounted and
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displacement parameters for the nondisordered heavy atoms using the
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Complex 11b was disordered in two positions with the minor
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the water molecules were not located, and so both the free and
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ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic data in electronic CIF form is available free of
charge via the Internet at http://pubs.acs.org.
(14) Zhang, F.; Jennings, M. C.; Puddephatt, R. J. Organometallics
2004, 23, 1396.
AUTHOR INFORMATION
Corresponding Author
*E-mail: pudd@uwo.ca.
■
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ACKNOWLEDGMENTS
We thank the NSERC (Canada) for financial support.
■
316
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