Biaryl Reductive Elimination Is Dramatically Accelerated by Remote Lewis Acid Binding to a 2,2′-Bipyrimidyl-Platinum Complex: Evidence for a Bidentate Ligand Dissociation Mechanism

Article abstract of DOI:10.1021/acs.organomet.5b01003

The silicon and zinc Lewis acids Si(cat)₂ (cat = catecholato), Si(cat^F)₂ (cat^F = tetrafluorocatecholato), and Zn(C₆F₅)₂ bind to the remote ligand site of a 2,2′-bipyrimidyl-platinum diaryl complex. This platinum complex provides a platform to systematically evaluate electronic and reactivity differences triggered by Lewis acid binding. The electron density of the bipyrimidine ligand is substantially depleted upon Lewis acid binding, as evidenced by UV-vis spectroscopy and cyclic voltammetry. Biaryl reductive elimination studies allowed quantification of the effect of Lewis acid binding on reactivity, and Lewis acid binding accelerated reductive elimination rates by up to 8 orders of magnitude. Kinetics experiments in combination with DFT studies support an unusual mechanism featuring complete dissociation of the Lewis acid-coordinated bidentate bipyrimidine ligand prior to reductive elimination.

Full text of DOI:10.1021/acs.organomet.5b01003

Article
pubs.acs.org/Organometallics
Biaryl Reductive Elimination Is Dramatically Accelerated by Remote
Lewis Acid Binding to a 2,2′-Bipyrimidyl−Platinum Complex:
Evidence for a Bidentate Ligand Dissociation Mechanism
Allegra L. Liberman-Martin,† Daniel S. Levine,† Wenjun Liu,‡ Robert G. Bergman,*,† and T. Don Tilley*,†
†Department of Chemistry, University of California−Berkeley, Berkeley, California 94720, United States
‡Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
S
* Supporting Information
ABSTRACT: The silicon and zinc Lewis acids Si(cat)2 (cat =
catecholato), Si(catF)2 (catF = tetrafluorocatecholato), and
Zn(C6F5)2 bind to the remote ligand site of a 2,2′-
bipyrimidyl−platinum diaryl complex. This platinum complex
provides a platform to systematically evaluate electronic and
reactivity differences triggered by Lewis acid binding. The
electron density of the bipyrimidine ligand is substantially
depleted upon Lewis acid binding, as evidenced by UV−vis
spectroscopy and cyclic voltammetry. Biaryl reductive elimination studies allowed quantification of the effect of Lewis acid
binding on reactivity, and Lewis acid binding accelerated reductive elimination rates by up to 8 orders of magnitude. Kinetics
experiments in combination with DFT studies support an unusual mechanism featuring complete dissociation of the Lewis acid-
coordinated bidentate bipyrimidine ligand prior to reductive elimination.
NMR and UV−vis spectroscopies and cyclic voltammetry, was
used to probe electronic perturbations that occur upon Lewis
acid binding. Biaryl reductive elimination reactions provide a
benchmark for evaluation of reactivity, and tunable rate
enhancements ranging from 103 to 108 were observed in the
presence of Lewis acids. Mechanistic studies revealed an
unexpected mechanism involving complete dissociation of the
bidentate bipyrimidine ligand prior to C−C bond formation,
demonstrating the ability of remote Lewis acid binding to
profoundly alter ligand donor properties and reaction mecha-
nisms.
INTRODUCTION
■
Lewis acid activators can be used in combination with transition
metal complexes to generate highly reactive intermediates. For
example, boron and aluminum compounds are used to
irreversibly abstract hydride and alkyl ligands from a wide
range of transition metals.1 In particular, B(C6F5)3 is well known
as an activator for metallocene alkene polymerization catalysts.2
Recent research has extended the utility of Lewis acids as
activators for chemical switches that operate by reversible
abstraction of ligands. For example, alkali metal salts have been
used to alter metal−oxygen interactions in iridium and rhodium
complexes with ligands bearing aza-crown ether substituents.3 In
these cases, the Lewis acid activator operates by modifying the
coordination environment of the metal center.
In contrast to the traditional Lewis acid-promoted ligand
abstractions, binding of protons or neutral Lewis acids to the
periphery of appropriately designed ligands allows for electronic
tuning without steric modification.4,5 The ligands in these
systems feature pendant nitrogen- or oxygen-based functional
groups that are in electronic communication with the primary
metal center. These remote binding interactions offer a modular
approach to rapidly assemble a series of complexes with different
electronic properties from a common precursor, and compar-
isons of rates with and without the additive offer insight into the
electronic preferences for a reaction.
RESULTS AND DISCUSSION
■
Synthesis of bpymPtAr2 and Lewis Acid Adducts. Bis(4-
trifluoromethylphenyl)platinum-2,2′-bipyrimidine (bpymPtAr2,
1) was prepared by treatment of (4-CF3-Ph)2Pt(SEt2)2 with 2
equiv of bpym in dichloromethane. Following purification by
column chromatography on silica, yellow crystals of 1 were
obtained by layering a toluene solution with pentane at −30 °C
(for the ORTEP diagram of 1, see Supporting Information, SI).
An initial attempt to bind tris(pentafluorophenyl)borane to
the remote nitrogen site of 1 was unsuccessful, as no interaction
was observed by 1H or 19F NMR spectroscopy. This is likely due
to steric constraints enforced by the 2,2′-bipyrimidyl nitrogen
placement. Instead, silicon and zinc compounds were sought as
alternative Lewis acids that could simultaneously coordinate to
both distal nitrogen sites of 1.
Herein, we report remote Lewis acid binding to a 2,2′-
bipyrimidyl−platinum (bpym−Pt) complex. Silicon- and zinc-
based Lewis acids are demonstrated to have a strong affinity for
coordination to the bidentate binding site of the bpym−Pt
complex. A combination of methods, including 1H, 13C, and 195Pt
Received: December 11, 2015
© XXXX American Chemical Society
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Treatment of 1 with bis(catecholato)silane, Si(cat)2,6 in
toluene caused rapid formation of a red silicon−bpym−platinum
complex (2, eq 1). We have previously reported that the
bis(tetrafluorocatecholato)silane Si(catF)2 is more Lewis acidic
than the parent Si(cat)2 compound.7 For comparison with 2, the
dark brown Si(catF)2 adduct 3 was prepared by treatment of 1
with Si(catF)2(MeCN)2. Lastly, treatment of 1 with bis-
(pentafluorophenyl)zinc(η2-toluene)8 allowed isolation of a
platinum−zinc heterobimetallic complex 4 containing a bridging
bpym ligand. Red single crystals of 2 and 4 were obtained by
vapor diffusion of pentane into toluene solutions at −30 °C.
X-ray diffraction studies of 1, 2, and 4 provided solid-state
structural comparisons (Figure 1). All complexes feature the
expected square planar geometry at platinum. Compound 2
contains a hexacoordinate silicon center with approximately
octahedral geometry. The zinc center of 4 is tetrahedral, as has
been previously observed for pyridine adducts of diaryl zinc
species.9 There are negligible differences in the bond lengths of
the bpym rings, platinum−bpym bonds, and platinum−aryl
bonds of 1, 2, and 4.
NMR Spectroscopy. The bpym 1H NMR chemical shifts of
1−4 were compared to assess trends that result from Lewis acid
binding. Signals due to the bpym protons ortho to platinum for
adducts 2−4 are upfield-shifted by 0.10−0.37 ppm relative to
those for 1 (see SI). Resonances due to the protons ortho to the
remote nitrogen site of 2−4 appear downfield by 0.24−0.28 ppm
compared to the corresponding signals in the parent complex.
There is no trend in the 1H NMR chemical shift of the bpym meta
position. The 195Pt NMR signals for 1−4 occur in a small region
of the ∼6000 ppm range typically observed for organoplatinum-
Figure 1. ORTEP diagrams of complexes 2 (top) and 4 (bottom), with
all thermal ellipsoids shown at 50% probability.
(II) complexes (see SI).10 There is similarly no trend in the 13
C
NMR chemical shifts of the bpym ligand. Taken together, these
observations suggest that NMR chemical shifts are poorly
correlated with the electronic properties of the complexes in this
series. To provide additional information about electronic
changes upon Lewis acid binding, UV−vis spectroscopy and
cyclic voltammetry were employed as complementary diagnostic
techniques.
UV−Vis Spectroscopy. UV−vis spectra of 1−4 are
presented in Figure 2. An intense bipyrimidyl ligand π−π*
transition is observed in the near-UV region for all complexes
(not shown, λmax ∼300 nm). There are two diagnostic
absorptions that have been assigned as MLCT transitions for
similar platinum−bpym complexes.11 The higher of the two in
energy is not observed for the parent complex 1 (likely due to
overlap with the intense π−π* transition), but occurs between
400 and 425 nm for adducts 2−4. A lower energy absorption is
observed at 450 nm for 1 and is shifted to 525−600 nm for the
Lewis acid adducts. For both MLCT transitions, the shift in the
absorption maxima to longer wavelengths is consistent with a
significant lowering of the bpym π* orbital levels upon Lewis acid
binding.
Figure 2. Absorption spectra of compounds 1−4.
Electrochemical Experiments. Cyclic voltammetry was
performed on complexes 1−4. Silicon adducts 2 and 3 showed
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complicated, irreversible behavior upon reduction, likely due to
the redox activity of the silicon-bound catecholate ligands.12 In
at temperatures as high as 120 °C. Instead, competition
experiments were performed to determine relative binding
strengths. Mixing 3 with a 10-fold excess of Si(cat)2 yielded a 1:1
mixture of 2 and 3, allowing the free energy of Si(catF)2 binding
to 1 to be calculated as ΔG = −5.9 kcal/mol at 25 °C. Treatment
of 3 with Zn(C6F5)2 led to the quantitative formation of 4 and
free Si(catF)2, suggesting that ΔG ≤ −8.6 kcal/mol for
Zn(C5F5)2 binding to 1 at 25 °C. Therefore, the relative strength
of Lewis acid interactions with 1 increases in the order Si(cat)2 <
Si(catF)2 < Zn(C6F5)2.
contrast, for the parent complex and ZnArF adduct, 1 and 4, two
2
reversible bpym-based redox processes and one irreversible
platinum-based oxidation event were observed (Figure 3). The
Competition experiments were performed to establish the
relative strength of Lewis acid interactions with bpym versus 1
(eq 3). For all Lewis acids, there is a preference for binding to free
Figure 3. Cyclic voltammograms of 1 and 4 in o-difluorobenzene (1.0
mM with 0.1 M [nBu4N][B(C6F5)4]).
bpym rather than the platinum-bound bpym. Treatment of
Si(cat)2 with bpym and excess 1 led to a mixture of 1, 2, bpym,
and bpym−Si(cat)2 at 25 °C (ΔG = −3.4 kcal/mol; see SI for
details). In contrast, only 1 and bpym−Si(catF)2 or bpym−
Zn(C6F5)2 could be observed in the analogous experiments with
these Lewis acids. On the basis of the concentrations employed,
and assuming 99% conversion to products based on the 1H NMR
detection limits, for both Si(catF)2 and Zn(C6F5)2, ΔG ≤ −5.4
kcal/mol at 25 °C.
Biaryl Reductive Elimination. Biaryl formation from
complexes 1−4 was investigated, as this reaction represents a
crucial step in carbon−carbon coupling reactions that often
utilize group 10 catalysts.13 We and others have shown that
electron-deficient platinum complexes undergo facile reductive
elimination compared to more electron-rich analogues.5e,14
Therefore, this elementary step is a benchmark for the impact
of remote Lewis acid binding on platinum-based reactivity.
Biaryl reductive elimination was exceptionally slow from 1,
requiring heating to 225 °C in nitrobenzene-d5 to produce 4,4′-
bis(trifluoromethyl)-1,1′-biphenyl, free bpym, and insoluble
platinum(0) material. Coordination of Zn(C6F5)2 promoted
reductive elimination, with biaryl formation occurring at 140 °C
with first-order kinetics and kobs = (1.5 0.1) × 10−5 s−1.15
The addition of bis(trimethylsilyl)acetylene (BTMSA)
provided a lower energy pathway to reductive elimination (eq
4). Kinetics experiments in the initial rate regime indicated that
first bpym reduction event (bpym0/bpym1−) for 4 is 600 mV less
negative than that observed for complex 1. Similarly, a shift of
400 mV was observed for the second bpym reduction event
(bpym1−/bpym2−). In addition, the platinum-based oxidation
requires a ∼+100 mV greater potential upon remote zinc
binding. These electrochemical data indicate that the electron
density of the bpym ligand is substantially depleted upon zinc
coordination, but the electronic perturbation at the platinum
center is modest.
1
Lewis Acid Binding Constants. Variable-temperature H
NMR experiments were performed to determine the relative
strength of Lewis acid interactions in complexes 2−4 (eq 2). For
complex 2, partial, reversible dissociation of Si(cat)2 occurs upon
mild heating (40 °C), leading to a shift in the observed
bipyrimidyl resonances due to averaging on the NMR time scale.
Equilibrium constants were calculated based on the observed 1H
NMR chemical shift of the bpym resonances as a function of
temperature. From van’t Hoff analysis of the equilibrium
constants between 30 and 80 °C, the enthalpy and entropy for
binding of Si(cat)2 to 1 were measured as ΔH = −18.2 1.5
kcal/mol and ΔS = −47 4.7 eu (ΔG = −4.2 kcal/mol, Keq
=
∼1200 M−1 at 25 °C).
In contrast to the behavior of Si(cat)2, dissociation of Si(catF)2
and Zn(C6F5)2 from 3 and 4, respectively, could not be observed
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biaryl formation from 1 proceeded slowly in the presence of
excess BTMSA at 150 °C (kobs = (1.2
exhibiting first-order decay of 1 and concomitant appearance of
the biaryl product. The reaction was determined to exhibit a first-
order dependence on the concentration of BTMSA (Figure 4),
consistent with an associative mechanism.5e
trapped as the soluble alkyne adduct [Pt2(μ-Me3SiCCSiMe3)-
(Me3SiCCSiMe3)2] (5).16 At higher temperatures, this alkyne
complex was unstable, and the platinum slowly precipitated as a
black solid.
0.1) × 10−6 s−1),
In all reactions, disappearance of 2−4 was first order in the
initial rate regime, and no intermediates were observed. The
reductive elimination exhibited a first-order dependence on the
concentration of BTMSA and was inverse first order in bpym-
Si(cat)2 and bpym-Si(catF)2 for complexes 2 and 3, respectively.
It is fortuitous that under the reaction conditions complexes 2
and 3 remain intact in the presence of added bpym-Si(cat)2 and
bpym-Si(catF)2 (i.e., binding of a second equivalent of Lewis acid
to bpym does not compete with binding to the bpym−platinum
complex). In contrast, experiments with 4 in the presence of
added bpym-Zn(C6F5)2 were complicated by preferential
dissociation of Zn(C6F5)2 from 4 to generate 1 and the bis(zinc)
adduct (C6F5)2Zn-bpym-Zn(C6F5)2. For 2−4, addition of excess
Lewis acid had no effect on reaction rate, suggesting that the
Lewis acid does not dissociate from bpym during the reaction.
A proposed mechanism is shown in Scheme 1. First, there is
associative ligand exchange of bpym for BTMSA to generate a
Scheme 1. Reductive Elimination Mechanism
Figure 4. Effect of bis(trimethylsilyl)acetylene (top) and bpym
(bottom) on the biaryl reductive elimination from 1 at 150 °C. A
standard error of <5% is estimated from experiments performed in
triplicate.
Next, experiments were performed to probe whether bpym
dissociation occurs prior to reductive elimination. We have
previously found that reductive elimination from a borane-
activated bipyrazine (bpyz) platinum complex was not inhibited
by addition of excess bpyz ligand.5e Surprisingly, the rate of
reductive elimination from 1 slowed dramatically upon the
addition of bpym. Quantitative kinetics experiments were
performed, and a linear dependence was observed in a plot of
kobs versus 1/[bpym] (Figure 4), indicating that reductive
elimination is inverse first-order in bpym. Therefore, different
reaction profiles are observed in the alkyne-promoted biaryl
reductive elimination reactions from the similar bipyrazine− and
2,2′-bipyrimidine−platinum complexes.
three-coordinate platinum−alkyne complex (A). This must be
reversible to account for the inhibition observed in experiments
with added bpym. Biaryl reductive elimination occurs from the
(BTMSA)PtAr2 complex (A) in the rate-determining step to
generate 4,4′-bis(trifluoromethyl)-1,1′-biphenyl, which is fol-
lowed by trapping of platinum with additional alkyne to form 5.
Applying the steady-state approximation to the concentration of
intermediate (BTMSA)PtAr2, the rate expression for the
mechanism shown in Scheme 1 is given in eq 5. If k−1[bpym]
≫ k2, this equation can be simplified to the form shown in eq 6.
These expressions account for the observed first-order depend-
ences for platinum and BTMSA as well as the inverse first-order
behavior for bpym.
Complexes 2−4 also underwent reductive elimination in the
presence of BTMSA, with widely varying temperatures required
to induce C−C bond formation at rates convenient for
monitoring by NMR spectroscopy. Biaryl elimination occurred
at 120 °C for the Si(cat)2 adduct 2, 70 °C for the Si(catF)2 adduct
3, and 45 °C for the Zn(C6F5)2 adduct 4 (see below for
quantitative rate constant comparisons). For reactions at
temperatures below 100 °C, the platinum(0) product was
k1k2[Pt][BTMSA]
rate =
k−1[bpym] + k2
(5)
Keqk2[Pt][BTMSA]
k1k2[Pt][BTMSA]
k−1[bpym]
rate =
=
[bpym]
(6)
D
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The complete dissociation of the bidentate bpym ligand in this
reductive elimination reaction is unusual, although exchange of
monodentate ligands is a well-precedented mechanism. For
example, carbon−silicon reductive elimination from cis-PtMe-
(SiPh3)(PMePh2)2 in the presence of diphenylacetylene
proceeds by displacement of one phosphine ligand by alkyne.17
Additionally, aryl−allyl reductive elimination from palladium(II)
complexes was found to involve exchange of a triphenylarsine
ligand for an alkene.18 In contrast, with bidentate ancillary
ligands, dissociation of one donor group is a more common
pathway, as has been observed in studies of reductive elimination
from octahedral platinum(IV) complexes.19 Dissociation of
bidentate ligands is often overlooked as a potential mechanism.
Biaryl formation from 1 was investigated by DFT calculations
to determine the mechanism of ligand substitution (associative vs
dissociative) and to calculate the expected barrier for biaryl
reductive elimination (Figure 5). The barrier for direct bpym
Figure 6. Eyring analysis of reductive elimination from 1 in the presence
of BTMSA from 140 to 180 °C. A standard error of <5% is estimated
based on triplicate experiments at one temperature.
Table 1. Comparison of Reductive Elimination Rates from 1−
4
complex
Lewis acid
kobs (s−1
)
temp (°C) relative rate
1
2
3
4
none
(1.2 0.1) × 10−6
(8.7 0.1) × 10−5
(1.9 0.1) × 10−4
(4.8 0.1) × 10−4
150
120
70
1
Si(cat)2
Si(catF)2
Zn(C6F5)2
1.5 × 103
1.2 × 106
1.2 × 108
45
established, or both. The value of Keq can be calculated based on a
Boltzmann distribution with the energy difference between 1 and
A from the DFT calculations and taking into account the effects
of Lewis acid coordination from the binding constant measure-
ments. For Si(cat)2, the change in equilibrium constant accounts
for an 80-fold rate acceleration for reductive elimination, while
the remaining 20-fold rate acceleration is due to kinetic effects
(see SI for details). Therefore, changes to both the location of the
equilibrium position and the kinetics of the ligand exchange step
due to Lewis acid binding give rise to the observed rate
acceleration.
Figure 5. Reaction energy profile diagram (298 K, 1 atm).
dissociation from (bpym)PtAr2 (1) was determined to be 52.0
kcal/mol, while associative interchange, involving the concerted
exchange of bpym for an incoming alkyne, was found to have a
much lower barrier of 33.6 kcal/mol (favored mechanism shown
in Scheme 1). This supports an associative ligand exchange
process, as would be expected for a square planar d8 platinum
complex.20
It is fortuitous that complexes 1−4 display the same kinetics
profiles, as the conserved mechanism allows for quantification of
relative rates. Eyring analysis was performed for the reaction of 1
and excess BTMSA from 140 to 180 °C (Figure 6). The
temperature dependence of the observed rate constant afforded
activation parameters of ΔH⧧ = +31.6 0.96 kcal/mol and ΔS⧧
= −11.4 2.2 eu. The experimentally determined free energy of
CONCLUSIONS
■
Coordination of silicon and zinc Lewis acids to the remote
binding site of a platinum−bipyrimidine complex was inves-
tigated. Spectroscopic and electrochemical studies evidence
substantial depletion of electron density from the bipyrimidine
ligand upon Lewis acid binding. These electronic effects are also
manifested in the rates of biaryl reductive elimination from these
complexes, with tunable rates based on Lewis acid selection.
Mechanistic experiments and DFT calculations unexpectedly
revealed complete dissociation of the Lewis acid-coordinated
bpym ligand prior to reductive elimination. The combined
experimental and theoretical results indicate that bidentate
ligand dissociation is kinetically favored by Lewis acid
coordination (i.e., bpym is more labile with Lewis acids
bound). However, a thermodynamic driving force also favors
ligand dissociation, due to stronger binding of the Lewis acids to
free bpym than to the platinum-bound bpym ligand. Combined,
these effects drastically modify the rate of biaryl elimination by
factors of 103 to 108.
activation (ΔG⧧
= +35.0
1.6 kcal/mol) matches the
298 K
calculated value of 33.6 kcal/mol within error.
Initial rates of reductive elimination for complexes 2−4 were
measured in triplicate in the presence of 0.35 M BTMSA (Table
1). To determine relative rates versus 1, estimated rate constants
for reductive elimination from 1 at 120 °C (k = 5.8 × 10−8 s−1),
70 °C (k = 1.6 × 10−10 s−1), and 45 °C (k = 4.2 × 10−12 s−1) were
calculated by extrapolation using the measured Eyring
parameters. By comparing measured rate constants for 2−4
with extrapolated values for 1, relative rates due to Lewis acid
binding were calculated for 2 (1.5 × 103 at 120 °C), 3 (1.2 × 106
at 70 °C), and 4 (1.2 × 108 at 45 °C).
We sought to determine whether the Lewis acid binding leads
to rate acceleration by shifting the bpym and alkyne exchange
equilibrium, changing the rate at which the equilibrium is
E
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b01003.
■
S
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DFT results and additional experimental details (PDF)
X-ray data (CIF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: rbergman@berkeley.edu.
*E-mail: tdtilley@berkeley.edu.
■
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Director, Office of Science,
Office of Basic Energy Sciences of the U.S. Department of
Energy, under contract no. DE-AC02-05CH11231 (T.D.T.) and
the National Science Foundation under award no. CHE-
0841786 (R.G.B.). We also acknowledge the National Institutes
of Health for funding of the ChexRay X-ray crystallographic
facility (College of Chemistry, University of California, Berkeley)
under grant number S10-RR027172, the Berkeley College of
Chemstry NMR facility under grant SRR023679A, and the
National Science Foundation for funding the Molecular Graphics
and Computation Facility under grant number CHE-0840505.
D.S.L. would like to acknowledge support of an NSF Graduate
Research Fellowship. Electrochemical measurements were
performed at the Joint Center for Artificial Photosynthesis, a
DOE Energy Innovation Hub, supported through the Office of
Science of the U.S. Department of Energy under award number
DE-SC0004993 (W.L.). The authors thank Mark C. Lipke for
useful discussions on silicon Lewis acids, Michael I. Lipschutz
and Micah S. Ziegler for assistance with X-ray diffraction, and
Teera Chantarojsiri for assistance with UV−vis spectroscopy.
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DOI: 10.1021/acs.organomet.5b01003
Organometallics XXXX, XXX, XXX−XXX