Transmetalation of methyl groups supported by PtII-Au I bonds in the gas phase, in silico, and in solution

Article abstract of DOI:10.1021/ja110405q

We report Pt^II-to-Au^I methyl transfer reactions that occur in the gas phase and in solution. The heterobimetallic Pt ^II/Au^I complexes {[(dmpe)PtMe₂][AuPR ₃]}⁺ (R = Me (2a), Ph (2b), ^tBu (2c)), observed in the gas phase by means of electrospray ionization, were subjected to collision induced dissociation (CID) from which we could observe Pt-to-Au transmetalation along two reaction pathways involving formation of a Au-Me bond, analogous to those observed for the Pt^II/Cu^I complex recently reported. In the first pathway, neutral AuMe is generated with concomitant migration of PR₃ from Au^I to the Pt ^II center, forming cation [(dmpe)PtMe(PR₃)]⁺ (R = Me (5a) or Ph (5b)). In the second pathway, the monophosphine stays attached to the gold center, yielding cation [(dmpe)PtMe]⁺ (7) and R ₃PAuMe. Quantitative energy-resolved collision induced dissociation experiments as well as density functional theory (DFT) calculations were used to investigate the potential surface involved in the transmetalation processes. Energy barriers of 22.3 and 47.9 kcal mol^-1 for the two reaction processes of 2b and of 45.4 kcal mol^-1 for the single reaction process of 2c were obtained. Parallel reactivity is observed in THF solution, allowing for a comparison of the product distributions with those observed in the gas phase, and the postulation of simple steric control of the branching ratio between the two pathways. DFT calculations at the M06-2X//BP86/TZP level were in good agreement with the experiments.

Full text of DOI:10.1021/ja110405q

ARTICLE
pubs.acs.org/JACS
Transmetalation of Methyl Groups Supported by PtIIꢀAuI Bonds
in the Gas Phase, in Silico, and in Solution
Daniel Serra, Marc-Etienne Moret, and Peter Chen*
Laboratorium fur Organische Chemie, Eidgen€ossische Technische Hochschule ETH Zurich, Wolfgang-Pauli-Strasse 10,
Zurich CH-8093, Switzerland
S Supporting Information
b
ABSTRACT: We report PtII-to-AuI methyl transfer reactions that occur in the
gas phase and in solution. The heterobimetallic PtII/AuI complexes
{[(dmpe)PtMe2][AuPR3]}þ (R = Me (2a), Ph (2b), tBu (2c)), observed in
the gas phase by means of electrospray ionization, were subjected to collision
induced dissociation (CID) from which we could observe Pt-to-Au transme-
talation along two reaction pathways involving formation of a AuꢀMe bond,
analogous to those observed for the PtII/CuI complex recently reported. In the
first pathway, neutral AuMe is generated with concomitant migration of PR3
from AuI to the PtII center, forming cation [(dmpe)PtMe(PR3)]þ (R = Me
(5a) or Ph (5b)). In the second pathway, the monophosphine stays attached to
the gold center, yielding cation [(dmpe)PtMe]þ (7) and R3PAuMe. Quantitative energy-resolved collision induced dissociation
experiments as well as density functional theory (DFT) calculations were used to investigate the potential surface involved in the
transmetalation processes. Energy barriers of 22.3 and 47.9 kcal molꢀ1 for the two reaction processes of 2b and of 45.4 kcal molꢀ1
for the single reaction process of 2c were obtained. Parallel reactivity is observed in THF solution, allowing for a comparison of
the product distributions with those observed in the gas phase, and the postulation of simple steric control of the branching ratio
between the two pathways. DFT calculations at the M06-2X//BP86/TZP level were in good agreement with the experiments.
’ INTRODUCTION
system {[(dmpe)PtMe2]Cu(PMe3)}þ or in the case of AgBF4
which catalyzes a fast methyl scrambling in [(2,20-bpy)PtMe2].6
Gas-phase reactivity studies using electrospray ionization
tandem mass spectrometry (ESI-MS/MS) techniques have given
useful mechanistic information about fundamental catalytic
processes7,9 such as olefin metathesis,8a cyclopropanation,8b,f
reductive elimination,8c CꢀH bond activation.8d,e Having suc-
cessfully applied this method to methyl group trzansmetalation
in a PtII/CuI system5 (Figure 1), we are now motivated to obtain
more detailed insights into the factors controlling the transme-
talation reaction by varying the coinage metal and the phosphine
ligands. Herein we report experimental results in the gas phase
for related PtII/AuI systems, which clearly demonstrate that
(i) the transmetalation of a methyl group from Pt(II) to Au(I)
proceeds via a heterobimetallic cationic complex intermediate,
(ii) two competitive reaction pathways can occur in analogy to
the PtII/CuI system, and (iii) steric hindrance can be used to
control the branching ratio between competing transmetalation
pathways. We determined the energy barriers for the transme-
talation reactions by quantitative collision-induced dissociation
(CID) threshold measurements for two systems, which were
compared with activation energies obtained from DFT calcula-
tions. Remarkably experiments in solution demonstrate that the
transmetalation processes are also occurring in the condensed
The transmetalation of alkyl or aryl functionalities between
two different metal centers is a fundamental process invoked in
many cross-coupling reactions catalyzed by late transition metal1
such as, for example, the alkynyl ligand transfer from Cu(I) to
Pd(II) involved in the copper-cocatalyzed Sonogashira process,
which provides arylacetylenes or enynes.1d,2
Although the abundant occurrences of transmetalation reac-
tions are well established in coupling processes, studies on the
detailed mechanism of the step itself are uncommon. Determina-
tion of mechanistic information, such as the quantitative thermo-
chemistry, is then crucial in order to improve catalyst design,
particularly when attempting to couple catalytic cycles involving
two different metals. With the ultimate goal to functionalize
hydrocarbons catalytically, we focus our attention on the use of
heterobimetallic systems that can work cooperatively: each metal
can perform a different task for which it is optimal, leading to
novel or improved activity. Systems involving square planar d8
and electrophilic d10 metal centers are prone to form dative
metalꢀmetal bonds, which are observed in a wide range of
heterobimetallic complexes, including RhI/AgI,4a,b IrI/AgI,4a,b
IrI/AuI,4cꢀe IrI/CuI,4b RhI/CuI,4b PdII/AgI,4f,g PdII/AuI,4h PdII/
HgII,4i,k PtII/CdII,4l PtII/HgII,4i,j PtII/AgI,4h,m and PtII/CuI.4h,m
Indeed the close proximity of the two metals is a crucial condition
to observe the transmetalation process for which the organic
group is transferred as for example in the recently reported5
Received: December 1, 2010
Published: May 11, 2011
r
2011 American Chemical Society
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ARTICLE
phase, generating products similar to those observed in the gas
phase, with the branching ratio between product channels in
solution following the same trends as observed in the gas phase.
fragmentation behavior (Scheme 2). Notably, cations 2a and 2b
were found to be unstable and already undergo a fast transmetala-
tion process while preparing the solutions at low temperature
(∼ꢀ30 °C), as indicated by the high intensity of the peaks for
product ions 5a and 5b at m/z 436 and 622, respectively, in the
ESI-MS spectra (see the Supporting Information).
’ RESULTS AND DISCUSSION
Analogous to the isoelectronic PtII/CuI system,5 after mass
selection of 2a and 2b each undergo two competing reactions
upon collision-induced dissociation (CID) (Figure 2A, B),
involving the transfer of a methyl group from Pt(II) to Au(I)
(Scheme 2). In process (I), the methyl group transmetalation
occurs with simultaneous migration of the monophosphine
ligand from Au(I) to Pt(II) giving rise to the observed
[(dmpe)PtMe(PR3)]þ cation 5a/b (R = Me, Ph) along with
the neutral methylgold(I). Conversely, in process (II) migra-
tion of the monophosphine does not occur and thus the
coordinatively unsaturated complex [(dmpe)PtMe]þ (7)11 is
produced as the observable cation, along with the neutral
R3PAuMe (R = Me, Ph).
Gas-Phase Study. In this work a strong chelating ligand
bis(dimethylphosphino)ethane (dmpe) was used to stabilize
the dimethyl-platinum(II) fragment, and the stabilizing ligand
on gold was varied to probe steric and electronic effects on the
transmetalation processes. Phosphineꢀgold chloride complexes
R3PAuCl (R = Me, Ph, tBu) were reacted in acetonitrile with an
equimolar amount of [(dmpe)PtMe2] (1)10 after in situ halide
abstraction using one equivalent of NaOTf (Scheme 1). In our
ESI-MS experiment NaOTf was chosen as a weak abstracting
agent so that the transmetalation reactions do not proceed to
completion prior to the electrospray. Under these conditions 1,
R3PAuCl, and NaOTf are most likely present simultaneously in
the solution, generating a small steady-state concentration of the
bimetallic complexes during the spray. The reaction mixtures
were further diluted with acetonitrile and electrosprayed on a
Thermo Finnigan TSQ Quantum instrument, which allowed the
observation of small signals for the heterobimetallic cations 2aꢀc,
as identified by their m/z ratios (648, 834 and 774, respectively),
isotope patterns and their characteristic collision-induced
In the case of cation 2c (R = tBu), mass selection of the parent
signal followed by CID leads exclusively to reaction (II), 2c f
7 þ (tBu)3PAuMe (Figure 2C). The first pathway is presumably
hampered by the steric demands of the tris-(tert-butyl)phosphine
supporting ligand, which should disfavor the formation of cation
5c. In contrast to the PtII/CuI systems,5 the PtII/AuI hetero-
bimetallic cations react more cleanly, showing only transmetala-
tion reaction channels. They did not undergo PtꢀAu or AuꢀP
bond dissociation which would lead to additional reaction
channels (see the Supporting Information).
CID Threshold Measurements. Determination of CID
thresholds requires that the intensity of the parent ion remains
sufficient over several hours, which is an issue of sample
preparation. Therefore, to allow measurement of the more
reactive systems, freshly prepared acetonitrile solutions of
R3PAuþ (R = Me, Ph) and of complex 1 were mixed at ꢀ5 °C
in a FlowStart microreactor equipped with an ultrafast mixing
chip connected to the spray head of our spectrometer (see the
Supporting Information). Unfortunately, 2a was too reactive
to allow measurements even with this sample introduction
technique.
Figure 1. Previously reported PtIIꢀCuI systems.
Scheme 1. Generation of Ions 2aꢀc
Energy-resolved reaction cross sections were recorded on a
customized Finnigan MAT TSQ-700 spectrometer as pre-
viously described.8c The generated cation 2b was collided with
30ꢀ110 μTorr argon to monitor simultaneously the two
competing transmetalation reactions, leading to 5b and AuMe
versus 7 and Ph3PAuMe. The experimental reaction cross
section curves for both processes were extrapolated to zero
pressure to impose strict single-collision conditions, and
fitted with L-CID12 (Figure 3). As discussed previously,12 some
Scheme 2. Collision Induced Dissociation (CID) Reactions Observed for Cations 2aꢀc
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Figure 3. Zero-pressure extrapolated reactive cross sections (circles)
and L-CID fits (lines) as functions of center-of-mass collision energy for
the CID reactions 2b f 5b þ AuMe (red) and 2b f 7 þ Ph3PAuMe
(black).
In a similar fashion we performed CID threshold measure-
ments for the methyl group transmetalation in the heterobi-
metallic cation 2c, involving the more sterically hindered
tris-(tert-butyl)phosphine as supporting ligand on gold, which
afforded a single reaction channel. Figure 4 shows the zero-
pressure extrapolated cross section curve and the L-CID fit,
which provided a barrier of 45.4 ( 1.6 kcal molꢀ1, assuming
that a loose transition-state model also applies for this process
(Table 1).
DFT Calculations. A detailed investigation of the systems
2aꢀc was performed by means of density functional theory
(DFT) calculations in order to illuminate the mechanisms of the
two distinct transmetalation pathways. The involved intermedi-
ates and transition states were located at the BP86/6-31G(d,p);
Pt,Au:SDD level of theory.15,16 Subsequently, three density
functionals were considered for single-point energy evaluations:
mPW1K17 the modified PerdewꢀWang one-parameter hybrid
developed for kinetics, which was successfully employed for
several transition-metal systems;5,18ꢀ20M06-L,21ꢀ23 which was
designed to model main-group and transition-metal thermo-
chemistry, kinetics, and noncovalent interactions; and the M06-
2X23,24 functional, which was intended for main-group thermo-
chemistry, kinetics and nonbonding interactions because of its
large fraction of HartreeꢀFock exchange. Table 2 lists the
relative energies for the considered density functionals for the
intermediates and transition states (TS) involved in reaction
channels (I) and (II). Figure 5 presents the potential energy
diagram for both reaction channels, which is based on the
calculated energies obtained with the M06-2X density functional.
Included in the diagram are the optimized geometries of the
intermediates and transition states originating from the bimetal-
lic complex 2a.
Figure 2. CID spectra of A) {[(dmpe)PtMe2]Au(PMe3)}þ (2a, m/z =
648), B) {[(dmpe)PtMe2]Au(PPh3)}þ (2b, m/z = 834), and C)
{[(dmpe)PtMe2]AuP(tBu)3}þ (2c, m/z = 774) obtained at 50 V
collision offset with 0.2 mTorr argon in the collision cell. Product
peak assignment: m/z 436 [(dmpe)Pt(Me)(PMe3)]þ (5a), 622
[(dmpe)Pt(Me)(PPh3)]þ (5b), and 360 [(dmpe)Pt(Me)]þ (7).
(Insets) Experimental (black) and calculated (red) isotope patterns
of complexes 2aꢀc.
information on the nature of the transition state (TS) is necessary
for an appropriate treatment of the kinetic shift. A “tight”
transition state model should be used when the rate-limiting step
is an intramolecular rearrangement, while a “loose” TS model
applies when the rate-limiting step is a dissociation without
reverse activation barrier. The distinction arises from the fact that
the rovibrational density of states of a tight transition state closely
resembles that of the parent ion (for a large enough complex),
while a loose transition state is best viewed as two loosely bound,
orbiting fragments and therefore possesses a much larger density
of states. Chemical intuition,13 confirmed by DFT calculations,
indicated that reaction (I) is best described by a tight14 TS model
(vide infra), whereas a loose TS model should be used for reaction
(II). Therefore two-channel L-CID fits were performed accord-
ingly, affording energy barriers of 22.3 ( 0.9 kcal molꢀ1 for
reaction channel (I) (tight), and 47.9 ( 1.8 kcal molꢀ1 for
reaction channel (II) (loose) (Figure 3, Table 1).
As shown in Figure 5 (left), reaction channel (I) involves the
initial formation of a PtꢀAu methyl bridged intermediate 3,
which undergoes a migration of the CH3ꢀAuꢀP fragment from
one side of the coordination plane of Pt to the other, forming
intermediate 4 via a transition state TS3ꢀ4. In this TS a AuꢀP
bond is partially broken and a PtꢀP bond is partially formed,
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Table 1. L-CID Fitting Results (kcal molꢀ1) for the CID Reactions 2b f 5b þ AuMe, 2b f 7 þ Ph3PAuMe, and 2c f 7 þ
(tBu)3PAuMe
a Based on DFT calculation. b The L-CID fitting using a Loose TS model is described in the Supporting Information. c Two-channel fits.
Table 2. Calculated Relative Energies (kcal molꢀ1) for
Reaction Channel (I) (2aꢀc f 5aꢀc þ AuMe) (top) and
Reaction (II) (2aꢀc f 7 þ R3PAuMe) (bottom) for the
mPW1K, M06-L, and M06-2X Functionals, Evaluated with
ADF51 BP86/TZP Geometries
reaction (I)
mPW1K
system
2
3
TS3ꢀ4
4
5 þ AuMe
a
b
c
a
b
c
a
b
c
0
0
0
0
0
0
0
0
0
ꢀ1.4
ꢀ1.6
ꢀ1.0
1.2
15.9
20.2
45.9
9.4
3.4
7.3
19.3
21.1
46.9
22.5
22.7
44.8
21.1
20.9
46.9
32.7
ꢀ3.1
ꢀ3.1
18.7
3.4
M06-L
1.5
9.4
3.0
31.4
16.7
17.9
43.1
M06-2X
1.0
0.7
4.9
0.2
30.1
reaction (II)
mPW1K
system
2
3
TS3ꢀ6
6
7 þ R3PAuMe
Figure 4. Zero-pressure extrapolated reactive cross sections (circles)
and L-CID fit (line) as functions of center-of-mass collision energy for
the CID reaction 2c f 7 þ (tBu)3PAuMe.
a
b
c
a
b
c
a
b
c
0
0
0
0
0
0
0
0
0
ꢀ1.4
ꢀ1.2
ꢀ1.0
1.2
12.2
13.3
13.8
14.6
17.3
19.2
8.8
2.9
4.1
33.8
36.2
36.4
38.9
42.3
45.0
38.4
41.3
43.3
4.0
suggesting that the concerted movement of the goldꢀphosphine
moiety is the presumed reason for the tight character of the TS.
PtꢀAu bond dissociation from 4 finally leads to the observed
cationic complex 5 and neutral AuMe.
M06-L
7.8
1.5
10.3
12.1
2.8
3.0
M06-2X
1.0
At the M06-L level of theory markedly lower-energy barriers
for the transmetalation step are calculated for process (I)
(TS3ꢀ4, Table 2), suggesting that the loose product dissociation
should be rate determining throughout the scanned collision
energy range. However, fitting of both reaction cross sections of
2b with a loose TS model afforded an experimental barrier of 36.8
kcal molꢀ1 for process (I), which is 15.6 kcal molꢀ1 higher than
the calculated M06-L dissociation energy of 21.2 kcal molꢀ1 (see
the Supporting Information for detail). We therefore conclude
that the M06-L density functional underestimates the transme-
talation barrier and consequently the process 2b f 5b þ AuMe
is not appropriately represented by a loose TS model, as it was
also observed for the PtIIꢀCuI system. In contrast, for mPW1K
and M06-2X density functionals, the transition state TS3ꢀ4 and
the subsequent dissociation step to 5aꢀc þ AuMe for reaction
(I) are calculated to be similar in energy (with differences in favor
of TS3ꢀ4 of 3.4 (2a), 0.9 (2b), and 1.0 kcal molꢀ1 (2c) for
mPW1K, and of 4.4 (2a), 3.0 (2b), and 3.8 kcal molꢀ1 (2c) for
M06-2X). The rate constant of the tight transmetalation process,
via TS3ꢀ4, necessarily increases less rapidly with excess energy
than that of the subsequent loose dissociation into 5 þ AuMe
because the density-of-states rises more slowly with energy for a
tight transition state than it does for a loose one. Thus, even if the
tight transition state were to lie slightly lower on the potential
energy surface than the dissociation limit, the rate-determining
0.7
10.0
11.7
3.8
0.2
5.3
step switches from the dissociation near threshold to the
transmetalation at higher collision energies. A similar situation
was recently reported by our group for reductive elimination
from a palladium N-heterocyclic complex and it was found that a
model with a tight transition state did indeed produce the better
threshold energy.8c Indeed, the experimentally determined tight
barrier of 22.3 kcal molꢀ1 (Table 2) for the conversion of 2b into
5b þ AuMe is in good agreement with the calculated energy
obtained with mPW1K and M06-2X (21.1 and 20.9 kcal molꢀ1
respectively).
,
In the case of reaction channel (II) (Figure 5, right),
intermediate 3 undergoes dissociation of the PtꢀAu bond to
form the methyl-bridged intermediate 6 via a relatively low
energy transition state TS3ꢀ6, which involves a PtꢀCꢀAu
angle expansion around the groups to be transmetalated.
Subsequent rupture of the PtꢀC bond affords the observed
product cation 7 along with the neutral R3PAuMe complex.
The DFT calculations indicate that the latter process is rate
determining for all systems 2aꢀc and functionals considered
(Table 2). For all systems, the mPW1K density functional
provides a dissociation energy that is notably lower than with
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Figure 5. Calculated (M06-2X/TZP//BP86/6-31G(d,p);Pt,Au:SDD) potential energy surface for reaction process (I) (2aꢀc f 5aꢀc þ AuMe) and
(II) (2aꢀc f 7 þ R3PAuMe) involving system 2a, 2b, and 2c. Dashed lines indicate dissociations that proceed without a reverse activation barrier. For
clarity only the structures involving system 2a (BP86/6-31G(d,p);Pt,Au:SDD) are presented, and only the hydrogen atoms of the metal-methyl groups
are shown. Energies represented (kcal molꢀ1) are zero-point energy corrected.
either M06-L or M06-2X (by 4.6ꢀ6.9 kcal molꢀ1). The
experimentally determined barriers of 47.9 ( 1.8 kcal molꢀ1
(2b f 7 þ Ph3PAuMe) and 45.4 ( 1.6 kcal molꢀ1 (2c f 7 þ
(tBu)3PAuMe) are reasonably well reproduced by the latter two
density functionals, whereas mPW1K clearly underestimates
the energy of these dissociations. M06-2X density functional
demonstrates acceptable accuracy for both reaction channels
involved, in contrast to M06-L and mPW1K, which are showing
good accuracy either only in the case of a tight model
(mPW1K) or only in the case of a loose model (M06-L).
It may at first appear surprising that reaction (I) is not
observed for 2c even though it is predicted to have a similar
energetic barrier to reaction (II). However, one should take into
account that, for two competing reaction channels having
similar threshold energies but different transition state natures,
the rate constant for a reaction with a loose transition state will
be higher than that of a tight one because of the higher density
of states of the former. This implies for competing channels
with similar energetic barriers, the reaction over the loose
transition state will proceed faster, even to the extent that the
competing channel over a tight transition state will be un-
observable experimentally.
surfaces (PES). In order to evaluate the multireference char-
acter in our complexes, we employed the B1 parameter as proposed
by Truhlar and co-workers.25ꢀ27 To this end, BLYP and B1LYP//
BLYP dissociation energies were compared, providing B1 values
ranging from ꢀ2.7 to 11.8 kcal molꢀ1 (see the Supporting Infor-
mation), which are comparable to the ∼10 kcal molꢀ1 values
considered for single reference complexes. In our systems
M06-2X is particularly efficient in describing both observed
reaction channels presumably because the limited contribution
from the d-orbitals of gold to the chemical bonding gives rise
to substantially single reference character. Therefore the use of
M06-2X for these transition metal systems can be regarded as
legitimate.
Solution-Phase Study. To our knowledge only one PtII/AuI
complex, {[PtMe2(2,20-bpy)](AuPPh3)}þ Xꢀ (bpy = bipyri-
dine, X = NO3, PF6, BF4), related to our systems, has been
characterized by 31P NMR spectroscopy.28 Although it is a good
candidate to study the methyl transfer between Pt(II) and Au(I)
centers, this heterobimetallic complex was reported to exhibit no
transmetalation reactivity. On the other hand, the methyl-
for-chloride exchange between cis-[PtMe2(PMe2Ph)2] and
[ClAu(PMe2Ph)], leading to trans-[PtCl(Me)(PMe2Ph)2] and
[MeAu(PMe2Ph)], has been reported to occur in solution.29 It is
believed to occur via a heterobimetallic intermediate which was
neither isolated nor observed.
In this study, when complex 1 is reacted at room temperature
with a stoichiometric amount of R3PAuCl (R = Me,30 Ph,31
tBu32) in THF-d8 a clean reaction takes place leading exclusively
to the formation of methyl substituted gold complexes
R3PAuMe and neutral [(dmpe)PtMeCl] complex 9 as the sole
The good performance of M06-2X may seem surprising, as it
was originally intended for main group chemistry. Bimetallic
transition metal systems may involve significant multireference
character, to which a significant amount of static, near degeneracy, or
nondynamical correlation energy is associated.25 Since Hartreeꢀ
Fock (HF) exchange improperly treats interelectron repulsions
it is reasonable to say that hybrid functional with high HF
contribution would lead to inaccuracies in the potential energy
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Scheme 3. Reaction between 1 and R3PAuMe (R = Me, Ph, tBu)
Table 3. 31P NMR Results Involving Systems A, B, and C Depending on the Abstracting Agent
entry
1
system
abstr. agent
T (°C)
reaction time
3d
products observed
conversion (%)
n.d.a
A
ꢀ
25
9
1
Me3PAuMe
Me3PAuCl
2
3
B
ꢀ
ꢀ
25
25
3d
5d
9
n.d.a
1
Ph3PAuMe
C
9
1
64b
(tBu)3PAuMe
(tBu)3PAuCl
4
A
NaBArF
ꢀ35
10 min
5a
47c
Me3PAuMe
Me3PAuCl
9
5
6
B
NaBArF
NaBArF
ꢀ35
ꢀ35
30 min
1 h
5b
52c
68
Ph3PAuMe
9
C
9
(tBu)3PAuMe (tBu)3PAuCl
1
7
8
9
A
B
C
AgOTf
AgOTf
AgOTf
ꢀ80
ꢀ80
ꢀ80
<10 mind,e
<10 mind,e
<10 mind
5a
100
100
100
Me3PAuMe [(dmpe)PtMe(NCMe)]þ
5b
Ph3PAuMe [(dmpe)PtMe(NCMe)]þ
(tBu)3PAuMe [(dmpe)PtMe(NCMe)]þ
a The conversion could not be determined because of the poor solubility of R3PAuCl (R = Me, Ph) in THF-d8. b Based on the ratios of the 31P NMR
signals of (tBu)3PAuMe and (tBu)3PAuCl at the end of the reaction. c Yields of the isolated complex 5a and 5b: process (I) only. d The NMR tube was
prepared and brought to the NMR spectrometer at ꢀ100 °C. The sample was inserted in a spectrometer set at ꢀ95 °C and stabilized to ꢀ80 °C in about
10 min. e The reaction takes place at ꢀ100 °C upon slow addition of a THF-d8 solution of complex 1, since a fast change of color is observed.
transmetalation products (Scheme 3). Complex 9 has been
previously characterized and was synthesized for comparison
purposes.33
Ph,36 tBu37) and [(dmpe)PtMeCl] obtained during our reactions
at room temperature are analogous to the ones observed by
Puddephatt et al.,29 our reactions being different only by the fact
that slower reactions processes are observed and that no equili-
bria are taking place (see the Supporting Information).
For a better comparison with the gas-phase results (vide
supra), the reactions were performed in THF-d8 in the presence
of a chloride abstracting agent to ensure that cationic species are
involved. Two abstracting agents, NaBArF (BArF = tetrakis-
(pentafluorophenyl)borate) and AgOTf were considered38 and
the general outcome of the reactions is described in Scheme 4
and 5, while the conditions and observed products are presented
in Table 3. Systems AꢀC are defined as the reaction mixtures
involving complex 1, precursors R3PAuCl (A: R = Me, B: R = Ph,
C: R = tBu), with or without halide abstracting agent.
Monitoring of the reaction by 31P NMR (see the Supporting
Information) reveals that the reactions progresses slowly, with
both the precursors and transmetalation products being ob-
served simultaneously as the reactions proceed over days,34 this
behavior being presumably due to a slow ionization of the gold
precursors, followed by a much faster transmetalation once the
bimetallic intermediate is formed. Because of the poor solubility
of the R3PAuCl (R = Me, Ph) precursors in THF and the low
stability of the formed R3PAuMe products (R = Me, Ph),
quantitative determination of the reaction yields could be
obtained only for the system involving the more stable
(tBu)3PAuMe. Under these reaction conditions 64% of the
goldꢀmethyl complex was obtained after 5 days (Table 3,
entry 3). The transmetalation products R3PAuMe (R = Me,35
System AꢀC can engage in two competing reaction path-
ways as described in Scheme 5, the first one (I) producing the
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Scheme 4. Solution-Phase Products Observed Resulting from the Transmetalation Reactions (I) and (II)a
a The compounds with blue structures (5a, 5b, 9, and R3PAuMe (R = Me, Ph, tBu) were observed experimentally.
Scheme 5. Solution-Phase Products Observed Resulting from the Transmetalation Reactions (I) and (II)a
a The compounds with blue structures (5a, 5b, 7-NCMe, and R3PAuMe (R = Me, Ph, tBu)) were observed experimentally.
observed complexes 5a/b and the second one (II) generating
the solvent adduct 7-NCMe or complex 9 and R3PAuMe. Our
31P NMR experiments did not show any significant accumula-
tion of a bimetallic species because the transmetalation occurs
in solution faster than the formation of the bimetallic complex.
The products resulting from reactions (I) and (II) were
observed as indicated by the blue structures in Schemes 4
and 5.
Reaction with NaBArF. NaBArF was employed to abstract the
chloride from R3PAuCl (R = Me, Ph, tBu) but the reactions were
complicated by the fact that the formed NaCl could not be
removed by filtration. The reactions for all three systems (AꢀC)
were followed in solution by means of 31P NMR spectroscopy.
Low temperature 31P NMR experiments performed in THF-d8
(see the Supporting Information) showed that the transmetala-
tion process is already occurring at ꢀ80 °C, which indicates that
the reaction is dramatically accelerated relative to those per-
formed without abstracting agent (about 120 times faster) at
room temperature.
When a stoichiometric amount of “Me3PAuþBArFꢀ”39 was
mixed with 1 at ꢀ35 °C in THF-d8, the reaction occurred within
seconds with rapid formation of a black precipitate. The 31P NMR
spectrum (Figure 6, A) of the filtered reaction mixture for system
A revealed a relatively clean distribution of the products resulting
from the methyl-transfer processes involved in reaction (I)
(Scheme 4): [(dmpe)PtMe(PMe3)]þ complex 5a and the spe-
cies involved in reaction (II): Me3PAuMe, along with traces of
complex (dmpe)PtMeCl (9). Complex 9 could presumably
be generated from the solvent adduct [(dmpe)PtMe(THF)]þ
(7-THF) complex, which is formed via reaction channel (II) by
chloride trapping. However, since the THF adduct could not be
observed under our conditions, we cannot exclude a concerted
mechanism involving a nucleophilic attack of chloride on the
bimetallic species to generate 9 and Me3PAuMe. In the system
involving “Me3PAuþBArFꢀ” and 1 the reaction medium was
clean enough to integrate the methyl proton peaks of 5a,
Me3PAuMe, and 9 to provide an approximate product distribu-
tion of 0.64:0.35:0.01 (see the Supporting Information). Addi-
tionally, the air-stable [(dmpe)PtMe(PMe3)]þBArFꢀ (5a) was
isolated from the reaction mixture after purification via column
chromatography, which provided a relative yield for the process
(I) of 52%40 (Table 3, entry 7). Crystals suitable for X-ray analysis
were obtained which confirmed the structure of 5a (Figure S12,
Supporting Information)
Subsequently, the reactions were carried out in the glovebox at
a relatively low temperature (ꢀ30 °C), while the 31P NMR
spectra of the resulting solutions were acquired at room tem-
perature (Figure 6). Determination of the overall yields for the
processes turned out to be complicated by the fact that complex 9
t
plus R3PAuMe (R = Me, Ph, Bu) equilibrate with 1 and
R3PAuCl, this process being catalyzed by NaBArF. This was
confirmed by adding a stoichiomeric amount of NaBArF to a
solution of complex 9 in THF-d8 and (tBu)3PAuMe (See the
Supporting Information).
In a similar fashion, complex [(dmpe)PtMe2] (1) was reacted
with “Ph3PAuþBArFꢀ” in THF-d8 at ꢀ30 °C (system B). The
reaction appeared to be slower than for system A since the color
changed progressively from colorless to purple and finally led to
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Figure 6. 31P NMR spectra (THF-d8, 20 °C) of the reactions involving complex 1 (O in blue) and the cations Me3PAuþ (A), Ph3PAuþ (B), and
(tBu)3Auþ (C) generated in situ, using NaBArF as halide abstracting agent. The chemical shifts for the transmetalation products of process (I) are
marked with b, 1, and /, while those for reaction process (II) are labeled with and ) and 9, both in red, having the R3PAuMe complexes mentioned in
the spectra.
the formation of a black suspension in ∼10 min. After filtration,
the observation by 31P NMR (Figure 6, B) of the resulting
solution revealed the presence of [(dmpe)PtMe(PPh3)]þBArFꢀ
(5b) as the major product along with Ph3PAuMe (singlet at 47.4
ppm), complex 9 and the starting platinum complex 1. 1H NMR
did not allow the determination of a ratio of the transmetalation
products since the peaks of Ph3PAuMe overlap with the Pt-CH3
signals of 5b and 1 (see the Supporting Information). However,
5b was found to be the major reaction product and was isolated
from the reaction mixture in 47%,40 which provided a relative
yield for process (I) (Table 3, Entry 8). Crystals suitable for X-ray
analysis were also obtained for 5b, which confirmed the structure
of the complex (Figure S12, see the Supporting Information).
When 1 was reacted at room temperature with freshly
generated “(tBu)3PAuþBArFꢀ” in THF-d8 (system C), the
formation of a black precipitate and the transmetalation pro-
duct [(dmpe)PtMeP(tBu)3]þ (5c) were not observed, suggest-
ing that reaction (I) leading to AuMe and 5c is disfavored in
solution. Again the the steric bulk of the tri-(tert-butyl)pho-
sphine ligand seems to be responsible for this behavior, as it was
in the gas phase. A more detailed inspection of the 31P NMR
data (Figure 6C) reveals that only the products resulting from
reaction (II) are formed, as observed by the intense singlet peak
at 95.3 ppm for (tBu)3PAuMe and two singlets at 36.4 and 21.1
ppm for complex 9, along with unreacted complexes 1 and
(tBu)3PAuCl. Full conversion could not be reached and only
68% of the goldꢀchloride complex was converted to the
goldꢀmethyl complex based on integration of 31P NMR singlet
peaks. Attempts to observe the heterobimetallic PtII/AuI com-
plex resulting from system C failed due to its high reactivity.
Evidently the transmetalation process is already taking place
at ꢀ80 °C, as complexes (tBu)3PAuMe and 9 were observed by
low temperature 31P NMR (see the Supporting Information).
Reaction with AgOTf. The isolated gold(I) R3PAu(NC-
t
Me)þOTfꢀ (R = Me, Ph, Bu) cations were slowly added to a
precooled (ꢀ100 °C) THF-d8 solution of 1 and the reaction was
followed by 31P NMR spectroscopy at ꢀ80 °C (Scheme 5,
Figure 7).
Mixing 1 with complexes Me3PAu(NCMe)þOTfꢀ or
Ph3PAu(NCMe)þOTfꢀ, (system A and B, respectively), gener-
ated a dark precipitate even at ꢀ100 °C, while in the case of the
system C ((tBu)3PAu(NCMe)þOTfꢀ) no such decomposition
could be observed. We speculate that the decomposition
products stem from known reactions of the AuMe formed in
process (I) since in system C process (I) is shut down. Additionally
1
ethane was detected by H NMR for A and B as the sole
hydrocarbon product confirming that a reductive coupling is
taking place between gold(I) methyl species leading to ethane
and colloidal gold, as had been previously reported.41ꢀ43
System A was the most reactive and 31P NMR of the reaction
of complex 1 with Me3PAu(NCMe)þOTfꢀ, revealed complete
consumption of 1 after 10 min. The major product formed was
[(dmpe)PtMe(PMe3)]þ complex 5a (Figure 7B), which results
from process (I).44 Moreover, minor peaks stemming from
process (II) were observed, i.e. the Me3PAuMe complex exhibit-
ing a singlet at ꢀ13.8 ppm, while two small singlets at 38.1 and
21.2 ppm were observed for cation [(dmpe)PtMe(NCMe)]þ
(7-NCMe).45
The reaction of cation Ph3PAu(NCMe)þOTfꢀ and complex
1 (system B), parallels system A, forming products from both
process (I): [(dmpe)PtMe(PPh3)]þ complex 5b,44 and process
(II): Ph3PAuMe and complex 7-NCMe. A different product
distribution is observed, suggesting that reaction (II) is more
favored for system B as compared to system A; The 31P NMR
peaks for both Ph3PAuMe and 7-NCMe are more intense
(Figure 6B) than in system A.
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Figure 7. 31P NMR spectra (THF-d8, ꢀ80 °C) of the reactions involving complex 1 and the freshly prepared cations Me3PAuþ (A), Ph3PAuþ (B), and
(tBu)3Auþ (C), using AgOTf as halide abstracting agent. The chemical shifts for the transmetalation products resulting from process (I) are marked with
b, 1, and /, while the ones for reaction process (II) are labeled with ) (red) and 9 (red), having the R3PAuMe complexes mentioned in the spectra.
Unassigned signals such as at 33.0 ppm (1J(Pꢀ195Pt) = 1715 Hz) (A, B, and C) or at 14.5 ppm (1J(Pꢀ195Pt) = 1619 Hz) (B), and small singlet peaks are
marked with O (green), 0 (purple), and 3 (purple), respectively.
Finally, complex 1 was mixed with (tBu)3PAu(NCMe)þOTfꢀ
in THF-d8 at ꢀ100 °C to generate the products from system C.
In contrast to the analogous PtII/CuI bimetallic complex,5 which
could be crystallized at 4 °C, the heterobimetallic complex
resulting from system C was too reactive to be isolated; the
transmetalation products could be observed already at ꢀ80 °C
after 10 min.
The reaction of the starting compounds 1 and the cationic
gold phosphine complex was complete, leading almost exclu-
sively to the products from process (II), (tBu)3PAuMe and 7-
NCMe (Figure 6C). The 31P NMR experiments showed also a
minor signal which could not yet be assigned; The product
involved was too reactive for further characterization.46
the dissociation limits. Additionally, however, the final dissocia-
tions could become associative substitutions in solution, further
lowering the associated barriers. An attempt to include solvent
effects in our calculations (see the Supporting Information,
Figure S14) shows the expected trends, but predicts the trans-
metalation reactions to be slightly endothermic, in contradiction
with their observed irreversibility in solution, although this may
simply reflect the limitations of the continuum dielectric solvent
models used in the study, which do not include any interactions
of individual solvent molecules with the complexes. The relative
performance of various DFT methods and solvation models was
not investigated further.
The high rate of transmetalation reactions in solution may at
first appear surprising, considering the barriers measured in the
gas phase. However, one has to consider that the uncoordinated
products AuCH3 and 7 are likely solvated in solution, causing a
strong decrease in the associated barriers. Furthermore, electro-
static interactions of the ionꢀdipole or ion-induced dipole-type
typically lower the energy of gas-phase ionꢀmolecule adducts
relative to their dissociation products in solution. The screening
of the electrostatic interactions should be treated reasonably by
continuum dielectric models of solvation, which would appear in
our models as an increase in the energy of the adducts relative to
’ CONCLUSIONS
We reported the platinum-to-gold methyl transfer to occur in
both in gas phase and in solution for three systems. The
heterobimetallic complexes 2aꢀc {[(dmpe)PtMe2]AuPR3}þ
t
(R = Me (2a), Ph (2b), Bu (2c)) could be detected in the
gas phase by ESI-MS and the CID reactions of the cations
exhibited similar transmetalation reactions. Process (I) leading to
[(dmpe)PtMe(PR3)]þ (R = Me (5a), Ph (5b)) was observed,
while for process (II), complex 7, [(dmpe)PtMe]þ, was
generated.
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Gas phase reaction barriers have been measured for hetero-
bimetallic complex 2b featuring two distinct transmetalation
processes, and for 2c, for which the first pathway is not observed
due to the steric demands of the monophosphine.
Truhlar’s mPW1K functional in addition to those of the M06
family functional, M06-L and M06-2X, was used to describe the
gas-phase mechanisms involved in the transmetalation processes.
M06-2X was found to reproduce both observed reaction pro-
cesses well; its use for these heterobimetallic systems is valid due
to the low multireference character as determined by the B1
parameter.
In solution, low temperature NMR experiments showed that
transmetalation reactions are occurring already at ꢀ80 °C. Two
distinct processes can be observed depending on the monopho-
sphine gold precursor, which involves formation of the observed
products 5a/b in reaction (I), while in reaction (II) the solvent-
stabilized complex 7-NCMe or complex 9 along with R3PAuMe
is produced instead of the free coordination site complex 7.
Additionally the transmetalation products 5a and 5b from
process (I) have been characterized by X-ray diffraction analysis.
Interestingly the steric hindrance of the monophosphine sup-
porting ligand on gold can be used to control the transmetalation
reaction toward the more interesting process (II) shutting down
completely reaction (I), this effect being observed also in the gas-
phase experiments.
eluent yielded 45 mg (52% based on complex 1) of analytically pure 5a
as a pale-yellow powder. Crystals suitable for X-ray diffraction (XRD)
analysis were obtained upon slow concentration of a solution of the
complex in dichloromethane. 1H NMR (400 MHz, CD2Cl2): δ 7.75 (s,
8H, ArꢀH), 7.59 (s, 4H, ArꢀH), 1.91ꢀ1.75 (m, 4H, PCH2CH2P),
31
1.64ꢀ1.51 (m, 21H, P(CH3)2 and P(CH3)3), 0.50 (dd, 3H, 3J(Hꢀ P) =
31
14.9 Hz, 3J(Hꢀ P) = 6.7 Hz, 2J(Hꢀ195Pt) = 58.9 Hz, PtCH3). 31P{1H}
NMR (161 MHz, CD2Cl2): δ 34.2 (dd, 2J(PꢀP) = 400.7 Hz, 2J(PꢀP) =
5.1 Hz, 1J(Pꢀ195Pt) = 2448 kHz, PMe3), 26.55 (dd, 2J(PꢀP) = 18.8 Hz,
2J(PꢀP) = 4.2 Hz, 1J(Pꢀ195Pt) = 1677 Hz, PMe2 trans to PMe3), ꢀ18.8
(dd, 2J(PꢀP) = 400.7 Hz, 2J(PꢀP) = 19.1 Hz, 1J(Pꢀ195Pt) = 2531 Hz,
PMe2 cis to PMe3). 31P{1H} NMR (121 MHz, THF-d8): δ 37.9 (dd,
2J(PꢀP) = 4.8 Hz, 2J(PꢀP) = 397.8 Hz, 1J(Pꢀ195Pt) = 2418 Hz, PMe3),
28.6 (dd, 2J(PꢀP) = 4.9 Hz, 2J(PꢀP) = 18.9 Hz, 1J(Pꢀ195Pt) = 1693 Hz,
PMe2 trans PMe3), ꢀ17.7 (dd, 2J(PꢀP) = 19.8 Hz, 2J(PꢀP) = 397.5 Hz,
1J(Pꢀ195Pt) = 2482 Hz, PMe2 cis to PMe3).Anal. Calcd for
C42H40BF24P3Pt: C, 38.82; H, 3.10. Found: C, 38.91; H, 3.14.
[(dmpe)PtMe(PPh3)]þBArFꢀ (5b). To a cold (ꢀ30 °C) solution
of 1 (30 mg, 0.079 mmol) in THF-d8 was added a cold filtrated solution
of Ph3PAuCl (39 mg, 0.079 mmol) and NaBArF (70 mg, 0.079 mmol) in
THF-d8. The resulting solution was stirred at ꢀ30 °C for 15 min and
further stirred at room temperature for 1 h. The resulting black
suspension was filtered through a 0.45 μm PTFE syringe filter. 31P
NMR spectroscopy of the crude filtrate showed the presence of 5b,
Ph3PAuMe, and [(dmpe)PtMeCl] complex 9. Following the procedure
used for 5a, analytically pure 5b was obtained as a white powder (56 mg,
47% base on complex 1). Crystals suitable for XRD analysis were
obtained at 4 °C by layering a concentrated solution of the complex
in dichloromethane with hexane. 1H NMR (400 MHz, CD2Cl2): δ 7.75
(bs, 8H, ArꢀH), 7.58ꢀ7.47 (m, 19H, ArꢀH), 1.97ꢀ1.69 (m,
Our ongoing investigations will focus on the variation of the
platinum bidentate ligand and the gold stabilizing ligand, with the
aim to combine both Pt-centered CꢀH activation and transme-
talation followed by functionalization.
4
31
2
31
4H, PCH2CH2P), 1.66 (dd, 6H, J(Hꢀ P) = 2.5 Hz, J(Hꢀ P) =
10.5 Hz, J(Hꢀ195Pt) = 35.1 Hz, PCH3), 0.91 (d, 6H, J(Hꢀ P) =
3
2
31
’ EXPERIMENTAL SECTION
9.2 Hz, J(Hꢀ195Pt) = 18.0 Hz PCH3), 0.50 (dd, 3H, J(Hꢀ P) =
3
3
31
7.0 Hz, 3J(Hꢀ P) = 13.0 Hz, 2J(Hꢀ195Pt) = 57.5 Hz, PtCH3). 31P{1H}
31
General Procedures. All the reactions and manipulation were
performed under an argon atmosphere using standard Schlenk and glove-
box techniques unless stated otherwise. Solvents were distilled under
nitrogen over K/Na for hexane, ethyl ether, and tetrahydrofuran and over
CaH2 for acetonitrile and dichloromethane. All deuterated solvents for
NMR measurements were degassed via freezeꢀpumpꢀthaw cycles and
stored over activated molecular sieves (4 Å). NMR measurements were
recorded on a Varian Mercury XL 300 (1H: 300, 31P: 121 MHz) or Bruker
AV400 (1H: 400, 31P: 161 MHz) spectrometers, respectively, with
chemical shifts (δ, ppm) reported relative to tetramethylsilane, using
residual solvent peaks as internal standard for 1H NMR and 85% H3PO4 as
an external standard for 31P NMR. Multiplicities are denoted by s (singlet),
d (doublet), t (triplet), dd (double doublet), m (multiplet). Elemental
analyses were performed at the Mikrolabor of the Laboratorium f€ur
Organische Chemie of ETH Z€urich. Precursors [(dmpe)PtMe2],10
Me3PAuCl,30 Ph3PAuCl,31 (tBu)3PAuCl32 as well as compounds
Me3PAuMe,35 Ph3PAuMe,36 and (tBu)3PAuMe37 were prepared accord-
ing to literature procedures. All other starting materials were purchased in
reagent grade purity and used without further purification.
NMR (161 MHz, CD2Cl2): δ 34.4 (dd, 2J(PꢀP) = 4.3 Hz,2J(PꢀP) =
393.4 Hz, 1J(Pꢀ195Pt) = 2627 Hz, PPh3), 28.1 (dd, 2J(PꢀP) = 5.9 Hz,
2J(PꢀP) = 17.1 Hz, 1J(Pꢀ195Pt) = 1710 Hz, PMe2 trans to PPh3), 24.5
(dd, 2J(PꢀP) = 16.8 Hz, 2J(PꢀP) = 391.8 Hz, 1J(Pꢀ195Pt) = 2689 Hz,
PMe2 cis to PPh3). 31P{1H} NMR (121 MHz, THF-dd8): δ 36.6 (dd,
2J(PꢀP) = 4.4 Hz, 2J(PꢀP) = 288.5 Hz, 1J(Pꢀ195Pt) = 2609 Hz, PPh3),
29.7 (dd, 2J(PꢀP) = 4.6 Hz, 2J(PꢀP) = 16.8 Hz, 1J(Pꢀ195Pt) = 1720 Hz,
PMe2 trans PPh3), 25.7 (dd, 2J(PꢀP) = 16.9 Hz, 2J(PꢀP) = 388.7 Hz,
1J(Pꢀ195Pt)
= 2642 Hz, PMe2 cis to PPh3).Anal. Calcd for
C57H46BF24P3Pt: C, 46.08; H, 3.12. Found: C, 46.18; H, 3.39.
Mass Spectrometry. Sample Preparation. Fresh 8 mM stock
t
solutions of [(dmpe)PtMe2] and of R3PAuCl (R = Me, Ph, Bu) in
acetonitrile were prepared and stored no longer than two days
in a glovebox at ꢀ35 °C. Samples for ESI-MS were prepared by adding
10 μL of R3PAuCl stock solution of to 2 mL precooled (ꢀ35 °C)
acetonitrile containing 1 mg of NaOTf; after mixing for 5ꢀ10 min the
solution was filtered through a 0.45 μm PTFE syringe filter. To this
solution was then added 10 μL of platinum complex stock solution. The
resulting solutions were used immediately after mixing, and the samples
were stored during the experiment at ꢀ35 °C to prevent possible
decomposition.
Mass Spectrometric Measurements. Mass spectra were recorded on
a ThermoFinnigan TSQ Quantum instrument. In a drybox, the sample
was introduced into a gastight syringe which was taken out and
immediately connected to the electrospray source. The solution was
electrosprayed using a spray voltage of 5 kV and a transfer capillary
temperature of 120 °C (Capillary Offset 10 V). The tube lens voltage
was set at 100 V for all measurements. Collision-induced dissociation
(CID) spectra were measured at 50 V collision offset with argon (0.2
mTorr) as the collision gas.
[(dmpe)PtMe(PMe3)]þBArFꢀ (5a). To a cold (ꢀ30 °C) solution
of 1 (25 mg, 0.066 mmol) in THF-d8 (0.5 mL) was added a cold filtrated
solution of Me3PAuCl (20 mg, 0.066 mmol) and NaBArF (59 mg,
0.066 mmol) in THF-d8 (0.5 mL). The resulting solution was stirred
at ꢀ30 °C for 15 min and at room temperature for 1 h. The resulting
black suspension was filtered through a 0.45 μm PTFE syringe filter. 31P
NMR spectroscopy of the crude solution showed the presence of 5a,
PMe3AuMe and some traces of complex 9. Further purification was
achieved by first cooling the THF solution at ꢀ30 °C causing all Pt-
containing compounds to crystallize. Separation from the unstable
Me3PAuMe complex was realized by filtration. Since 5a is air- and
moisture stable, column chromatography over silica gel using CH2Cl2 as
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CID Threshold Measurements. Energy-resolved CID threshold mea-
surements were performed on a Finnigan MAT TSQ-700 tandem mass
spectrometer, customized as described previously.8c For the more
reactive systems 2a and 2b a FlowStart B-200 (part # FCB-200.001)
equipped with a microfluid chip reactor was connected to the spray head
of our TSQ-700 instrument. The solution of R3PAuþOTfꢀ (R =
Me, Ph) and 1 could then be mixed rapidly just before reaching the sray,
The FlowStar B-200 is also equipped with a thermocouple connected to a
temperature controller (set to ꢀ5 °C in our experiment) for mixing of the
solution at a desired temperature. The ions were electrosprayed and
transferred into a 24-pole ion guide where they were thermalized to 343 K
with argon (∼10 mTorr). The ion ofinterest was mass-selectedinthe first
quadrupole and collided with argon (30ꢀ110 μTorr) in the octopole
collision cell, and the products were mass-analyzed in the second
quadrupole. A retarding potential measurement of the kinetic energy
distribution of mass-selected ions was performed before each experiment,
yielding Gaussian distributions with fwhm between 1.3 and 1.9 eV in the
laboratory frame. The energy dependence of the reactant and product
signals were monitored simultaneously and converted to reactive cross
sections as described by Ervin et al.49 To remove multiple-collision effects
the data was recorded at several argon pressures, and the cross sections
were extrapolated to zero-collision gas pressure. To extract the activation
energies, the cross-section data were fitted using our L-CID program.12
The transition-state parameter was set to tight for the reaction 2b f 5b þ
AuMe, and to lose for 2b f 7 þ Ph3PAuMe and for 2c f 7 þ
(tBu)3PAuMe. Ancillary methyl groups, phenyls and tBu groups bounded
to phosphines, as well as PPh3 and P(tBu)3 units as a whole, were
considered free rotors (totaling 9 for 2b and 18 for 2c). For each of the
three independent data sets 15 L-CID fits were used to obtain fitted
reaction parameters and standard deviations. E0 values are given in kcal
molꢀ1 and the corresponding standard deviations include a 0.15 V
laboratory-frame uncertainty.
Computational Methods. Density functional theory (DFT)
calculations were performed with the Gaussian 0350/0947 suites of programs
as described previously,5,8a employing the BP86 exchange-correlation
functional for geometry optimization. The Stuttgart/Dresden basis set
and effective core potential were used for transition metals along with a
6-31G(d,p) basis set on all other atoms. Because of the flat character of
the potential energy surfaces encountered, the GDIIS algorithm was
used systematically and tight criteria were imposed for both geometry
and SCF convergence. The nature of each stationary point was
confirmed by a frequency analysis which also afforded the zero-point
energy (ZPE) correction; for the transition states the imaginary vibra-
tional mode corresponded to the expected reaction coordinate. This
method was found to accurately reproduce the experimental geometries
of complexes 5b and 5c obtained in this work, as well as that of the
closely related bimetallic cation {[(dmpe)PtMe2]Cu[P(tBu)3]}þ.5
Additionally, density functionals were screened with the Amsterdam
Density Functional (ADF2008)51 suite as follows: the electron density was
calculated using the BP86 functional and a TZP basis set without frozen
core, and energies were evaluated for 69 density functional using the
‘METAGGA’ and ‘HARTREEFOCK’ keywords. Relativistic effects were
treated via the scalar zeroth-order regular approximation (ZORA) form-
alism including spinꢀorbit coupling. The Gaussian BP86/6-31G(d,p);Pt,
Au:SDD zero-point energy corrections were added to the energies
calculated for mPW1K, M06-L and M06-2X and are presented in Table S5.
50 are available as SI refs 11 and 10, respectively. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
peter.chen@org.chem.ethz.ch
’ ACKNOWLEDGMENT
We thank the Swiss National Foundation for support of this
work. S.D. thanks Dr. Couzijn E. P. A. for suggestions when
writing this manuscript.
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’ ASSOCIATED CONTENT
S
Supporting Information. Experiments details, ESI-MS
b
spectra, threshold-CID measurements, X-ray structures and data
for 5a and 5b, calculated geometries and single-point energies,
evaluation of B1 parameters for 2a, DFT evaluation for the
nonobserved reactions and NMR data. Complete refs 47 and
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