Palladium and Platinum Acyl Complexes and Their Lewis Acid Adducts. Experimental and Computational Study of Thermodynamics and Bonding

Article abstract of DOI:10.1021/acs.organomet.5b00507

The interactions of Pt and Pd acyl complexes with Lewis acids are investigated through experiment and computation. Variable-temperature NMR studies indicate BF₃ association with trans-[(PPh₃)₂(CO)Pt(COCH₂Ph)][BAr^F₄] (3; BAr^F₄ = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) at 298 K is endergonic (ΔG° = 2.0 kcal mol^-1) yet exothermic (ΔH° = -3.4 kcal mol^-1), suggesting a Lewis basicity comparable to or greater than aldehydes and ketones. Despite the accelerating effect of Lewis acids in the formation of the Pd congener trans-[(PPh₃)₂(CO)Pd(COCH₂Ph)][BAr^F₄] (4), no evidence for adduct formation was obtained. DFT (M06-L/def2-TZVP/QZVP) suggests that BF₃/trans-[(PPh₃)₂(CO)M(COCH₂Ph)]⁺ adduct formation is more favored for M = Pt than M = Pd. Natural bond orbital analysis shows that upon Lewis acid coordination, the acyl C-O bond is weakened, the natural charge of the acyl C is more positive, and the π_{acyl C-O} orbital is lowered in energy relative to other unoccupied orbitals. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.organomet.5b00507

Article
pubs.acs.org/Organometallics
Palladium and Platinum Acyl Complexes and Their Lewis Acid
Adducts. Experimental and Computational Study of
Thermodynamics and Bonding
Danmin Chen, Michael R. Gau, and Graham E. Dobereiner*
Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States
S
* Supporting Information
ABSTRACT: The interactions of Pt and Pd acyl complexes with Lewis
acids are investigated through experiment and computation. Variable-
temperature NMR studies indicate BF3 association with trans-
[(PPh3)2(CO)Pt(COCH2Ph)][BArF ] (3; BArF = tetrakis(3,5-bis-
4
4
(trifluoromethyl)phenyl)borate) at 298 K is endergonic (ΔG° = 2.0
kcal mol−1) yet exothermic (ΔH° = −3.4 kcal mol−1), suggesting a Lewis
basicity comparable to or greater than aldehydes and ketones. Despite the
accelerating effect of Lewis acids in the formation of the Pd congener
trans-[(PPh3)2(CO)Pd(COCH2Ph)][BArF ] (4), no evidence for adduct
4
formation was obtained. DFT (M06-L/def2-TZVP/QZVP) suggests that
BF3/trans-[(PPh3)2(CO)M(COCH2Ph)]+ adduct formation is more
favored for M = Pt than M = Pd. Natural bond orbital analysis shows that upon Lewis acid coordination, the acyl C−O
bond is weakened, the natural charge of the acyl C is more positive, and the π*acyl C−O orbital is lowered in energy relative to
other unoccupied orbitals.
organic carbonyls and acyls.6 Early metal η2-acyls are highly
INTRODUCTION
■
electrophilic at the acyl carbon,7 and Lewis acid O-coordination
to η1-acyl complexes should similarly polarize the acyl carbon
and decrease the LUMO (π*CO) energy. Rare, late transition
metal η2-acyl complexes have reduced electrophilicity due to
dπ/π*CO interactions.7 However, such overlap is minimized in
η1 coordination shown in Chart 1, bottom.
Lewis acids promote metal-centered reactivity through ligand-
based interactions with transition metal complexes,1 such as
facilitating migratory insertion of CO.2 Shriver and co-workers
established that Lewis acids can accelerate CO insertion into
metal−carbon bonds and stabilize the resulting acyl complexes
through association to the acyl oxygen.3 Since Shriver’s seminal
work, only a handful of reports4 address the thermodynamic
stability of Lewis acid−acyl adducts. Moreover, Lewis acids are
seldom added to transition-metal-catalyzed carbonylations,5
despite the likelihood of acyl intermediates in these reactions.
A Lewis acid should increase acyl electrophilicity for the
same reasons that Lewis acids influence organic carbonyl
reactivity. Chart 1 shows illustrations comparing the LUMO of
A lower LUMO energy could (a) make π*CO more available
for reactions with exogenous nucleophiles and (b) promote
internal attack by nucleophilic coligands, such as amines or
alkyl ligands. The latter phenomenon resembles the accelerat-
ing effect of ligand−Lewis acid interactions on reductive
elimination, for which there are both stoichiometric8 and
catalytic examples, such as hydrocyanation.9
We stress that this qualitative picture is based on an
understanding of Lewis acid interactions with aldehydes and
ketones. Shriver and co-workers3 rationalized the C−O acyl
bond lengthening of CpFeL2(COR) upon BF3 binding as a
resonance effect, shortening the M−C bond and increasing
Fischer carbene character. Later theoretical work by Green and
co-workers10 provided a more nuanced picture: the HOMO
and LUMO of iron acyls are both influenced by Lewis acid
binding, with sigma and pi effects each contributing to
lengthening of the C−O bond. Clearly the viewpoint in
Chart 1 is insufficient to fully describe the variety of orbital
interactions in organometallic settings.
Chart 1. LUMO Illustrations for Lewis Acid-Coordinated
Ketones and Organotransition Metal Acyl Complexes
Received: June 16, 2015
© XXXX American Chemical Society
A
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The present study aims to understand the bonding of Lewis
acid−acyl adducts as a foundation for investigations of catalytic
processes, much as previous studies11 of Lewis acid−organic
carbonyl adducts have led to improved catalysts. Our focus in
this work is the chemistry of Pd and Pt acyl complexes, thought
to be participants in various catalytic cycles, including
hydroformylation12 and carbonylation.13 Evidence from experi-
ment suggests a Pt acyl complex binds more strongly to Lewis
acids than its Pd congener. This is reinforced by computational
thermochemistry data and natural bond orbital (NBO) analysis.
On the basis of the relative integrations of 2 and 3 (1.4:1
after several days at room temperature) and assuming [CO] =
0.024 M, estimated from the known solubility of CO in DCM16
(mole fraction, 298 K, 1 atm = 1.54 × 10−3; [DCM] = 15.62
M) the equilibrium constant for formation of 3 from 2 is K =
3.0 × 10 at 1 atm CO and 298 K. Therefore, ΔG° = −0.3 kcal
mol−1.
A solution phase infrared spectrum of the above reaction
mixture (1 atm CO, DCM) features a band at 2097 cm−1, the
only absorption in the range characteristic of cationic trans-
platinum carbonyl complexes (2050−2150 cm−1).17 The νCO
absorptions of either 2 or 3 may absorb at this frequency and
potentially overlap. A broad band at 1609 cm−1 is presumably
the carbonyl group of the acyl ligand of 3. For comparison, Sen
and co-workers18 reported absorptions for trans-[Pt-
(PPh3)2(CO)(COPh)]BF4 at 2070 cm−1 (terminal CO) and
1625 cm−1 (acyl CO), while Stang and co-workers15 indicated
absorptions at 2108 and 1658 cm−1 for trans-[Pt(PPh3)2(CO)-
(η1-(CO)(methyl propanoate))]OTf (a compound similarly
unstable toward loss of CO).
RESULTS AND DISCUSSION
■
In Situ Preparation of Acyl Complexes. Yamamoto13
and co-workers studied the mechanism of Pd-catalyzed
carbonylation of benzyl chlorides by reacting cationic benzyl
Pd and Pt complexes with CO to form acyl complexes in situ.
1a ([(PPh3)2Pt(CH2Ph)][BArF ]; BArF = tetrakis(3,5-bis-
4
4
(trifluoromethyl)phenyl)borate) and 1b ([(PPh3)2Pd-
(CH2Ph)][BArF ]) (Chart 2) were prepared via abstraction
4
Lewis Acids in Pt Carbonylation. When 1a is reacted
with 1 atm of CO in the presence of BF3Et2O (Scheme 2), the
Chart 2. Cationic Benzyl Complexes
[(PPh3)2Pt(CH2Ph)][BArF ] (1a) and
4
[(PPh3)2Pd(CH2Ph)][BArF ] (1b)
4
Scheme 2. Proposed Coordination of BF3 to trans-
[(PPh3)2(CO)Pt(COCH2Ph)][BArF ] (3)
4
of Cl− from (PPh3)2M(CH2Ph)Cl using NaBArF (see
4
Experimental Section). The crystal structure obtained of 1b is
similar to η3-benzyl complexes featuring bisphosphines
(Xantphos, DPEphos, and DPPF) (see the Supporting
Information).14
resonance for acyl complex 3 is shifted downfield, suggesting an
interaction with BF3; the chemical shifts of 2 are unchanged.
Conversion of 2 to 3 is incomplete over several days. A
downfield shift of acyl resonances upon Lewis acid complex-
ation has been seen previously.4d,19 At high concentrations of
BF3Et2O relative to Pt, the peak is shifted as far as 3.05 ppm
(Figure S7), while for [Pt] ≫ [BF3Et2O] the peak does not
shift appreciably (Figure S8).
Treatment of 1a with CO (1 atm) in CD2Cl2 (Scheme 1)
forms a mixture of trans-[(PPh3)2(CO)Pt(CH2Ph)][BArF ] (2)
4
Scheme 1. Reaction of [(PPh3)2Pt(CH2Ph)][BArF ] (1a)
4
with CO (1 atm)
To assess the strength of Lewis acid−acyl binding, Stimson
and Shriver previously determined Lewis acid−acyl equilibrium
binding constants exclusively from quantitative IR studies,20
while dynamic NMR studies underpin prior explorations of
Lewis acid−carbonyl binding.21 Two NMR-based approaches
were used here to calculate thermodynamic parameters for BF3
coordination to 3. First, for a given concentration of BF3Et2O
and 3, the chemical shift of the diethyl ether methylene peak at
room temperature (Figure S9) was established relative to free
diethyl ether (3.43 ppm in DCM) and BF3Et2O (4.20 ppm).
This approach, which assumes rapid exchange, was validated by
Gajewski and Ngernmeesri for BF3 complexation to carbonyl
compounds,22 and provides ΔG = 1.8 0.1 kcal mol−1 at 293
K for the coordination of BF3 to the acyl oxygen of 3.
The BF3 acyl complex is under slow exchange conditions at
253 K (Figure S9). Since both free Et2O and complexed
BF3Et2O resonances are observed, the relative integrations of
the two peaks provide an equilibrium constant K = 0.1 at 253
K. A van’t Hoff analysis (253−273 K; Table S1) provided
thermodynamic parameters: ΔH° = −3.4 1.0 kcal mol−1, ΔS°
= −0.018 0.100 kcal mol−1 K−1, ΔG° = 2.0 1.0 kcal mol−1.
The acyl oxygen’s Lewis basicity can be estimated from the
enthalpy of Lewis base−BF3 binding in dichloromethane.23 On
the basis of this scale, the Pt acyl complex 3 has greater Lewis
basicity (−ΔH° = 22.4 kcal mol−1; see the Supporting
and acyl complex trans-[(PPh3)2(CO)Pt(COCH2Ph)][BArF ]
4
(3). Conversion to 3 is incomplete over several days at 1 atm
CO. Since both are unstable outside of a CO atmosphere, the
crude mixture of 2 and 3 was characterized via NMR
spectroscopy under a CO atmosphere. A resonance at 2.95
1
ppm in the H NMR spectrum (CD2Cl2, 298 K; Figure S5) is
assigned to the CH2 methylene protons of the (Pt-COCH2Ph)
moiety of 3; the equivalent protons of 2 (Pt-CH2Ph) are at 2.74
ppm. This downfield shift upon carbonylation is consistent with
the behavior of analogous Pd complexes.13 An HMBC
experiment provides evidence for the proposed assignment
(Figure S6). The 1H NMR 2.95 ppm resonance exhibits
coupling to a 13C NMR signature at 224.62 ppm, consistent
with the acyl CO of a cationic Pt(II) complex.15 The trans
configuration of 2 and 3 is confirmed by the presence of
singlets, each with 195Pt satellites, visible in the 31P NMR
spectrum (CD2Cl2, 298 K; 18.92 ppm, 2; 13.12 ppm, 3), as well
1
as the triplet H NMR coupling pattern of the benzyl protons
of 2 (3JP,H = 10.0 Hz).
B
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Table 1. Structural Parameters for 3, 3-bf3, 4, and 4-bf3 and Relevant X-ray Precedent
compound
M−Cacyl
M−P (av) M−Ccarbonyl CO (acyl) CO (carbonyl) Cacyl−M−Ccarbonyl Pt−CO
trans-[Pt(COCO2Me)(CO)(PPh3)2][BF4]24
trans-[PtCl(COC6H13)(PPh3)2]25
2.04(1)
2.02(1)
2.002(10)
2.082
2.34
1.96(1)
1.18(2)
1.22(1)
1.200(15)
1.205
1.11(2)
176.6(6)
174.8(1)
2.30
26
[Pt(PNP)(COCH2CH3)]BF4
2.28
3 (computed)
2.383
2.396
2.403
2.405
1.978
1.953
2.031
1.996
1.137
1.135
1.134
1.132
175.55
175.15
176.06
173.29
172.49
177.18
172.64
175.47
3-bf3 (computed)
4 (computed)
2.049
1.250
2.060
1.196
4-bf3 (computed)
2.026
1.244
Information) than many functional groups, including nitriles
(13−14 kcal mol−1), aldehydes and ketones (16−19 kcal
mol−1), esters (14−18 kcal mol−1), and ethers (18−22 kcal
mol−1), while most amines and amides are more Lewis basic
than 3.
Despite the exothermicity of Lewis acid binding, the reaction
is endergonic at ambient temperature. Data obtained by
Gajewski and Ngernmeesri22 suggest that the entropy differ-
ence for the reaction of BF3Et2O with an aldehyde is negligible.
Metal complex−BF3 association might inhibit rotation of
ligands on the Pt center or otherwise limit the dynamic
behavior of 3.
A side reaction forms toluene in varying amounts when 1a is
in the presence of Lewis acid, regardless of CO pressure. In the
presence of protic solvent, Lewis acids generate Bronsted acids
capable of Pt−C protonolysis,27 the presumed mechanism here.
Other mechanisms are conceivable,28 but the present reaction
requires Lewis acid. Attempts to slow or stop toluene formation
by rigorous exclusion of water (silylation of glassware with
bis(trimethylsilyl)acetamide,29 drying solvents with various
dessicants) failed. The rate and extent of this side reaction
does not appear to influence the above binding data.
not attempted due to the very low concentration of acyl
complex. With the addition of Lewis acids (BF3Et2O, B(C6F5)3,
AlCl3) the conversion is more rapid, as expected.3 Unlike the Pt
complex 3, the acyl chemical shift does not move appreciably
upon addition of Lewis acid, precluding an estimation of
1
binding strength by H NMR. Toluene is again a side product
when Lewis acids are used.
Computational Study. The coordination of Lewis acids to
acyl complexes was investigated further using DFT. Calcu-
lations were performed using the M06-L functional34 and
Ahlrichs/Weigend basis functions (def2-TZVP/QZVP),35
known to be an accurate, efficient method for evaluating
organometallic thermochemistry.36 The geometry-optimized
structure of 3 (in dichloromethane via implicit SMD37
solvation) demonstrates bond lengths and angles analogous
to crystallographic data for similar structures (Table 1, Figure
1A).
Various structures of 3-BF3 are possible depending on the
site of BF3 binding. Coordination to the acyl oxygen with B cis
to Pt (Figure 1B) provided a stable minimum without
Other Lewis acids (AlCl3, B(C6F5)3) have similar effects on
the reaction of 1a with CO, but side reactions complicate
analysis. With 20 equiv of B(C6F5)3 added to 1a (under 1 atm
of CO) the acyl resonance shifts downfield (1H NMR, CD2Cl2,
298 K, 3.05 ppm, Δδ 0.10), and the benzyl aromatic resonances
shift upfield. Cooling the solution amplifies the effect (e.g., 253
K acyl resonance @ 3.15 ppm, Δδ 0.20). The resonance
corresponding to 2 does not show temperature dependence. An
enthalpically favored but entropically disfavored interaction
between 3 and B(C6F5)3 is consistent with the above. There is
no spectroscopic evidence for Ph3P:B(C6F5)3,30 but unassigned
resonances (1H NMR, 2.92 ppm; 31P NMR, 8.27 ppm) are
observed. Excess AlCl3 causes an immediate color change from
yellow to dark purple and an increase in the formation of
toluene, as well as various other side products.
Lewis acid coordination to other sites, such as the O of the
CO ligand or directly to the Pt(II)31 center, may occur, but
they have not been observed here. All NMR signals of Pt CO
complex 2 are unaffected by BF3, suggesting a lack of
substantive interactions with carbonyl or Pt. Known reactions
32
27
of BF3 and B(C6F5)3 with Pt(II) complexes occur without
apparent Pt−B coordination. Another potential competing
process, abstraction of PPh3 by BF3, should be apparent via 31
P
NMR (CD2Cl2, 298 K: 11.8 ppm33), but no signal
corresponding to Ph3P:BF3 was observed.
Pd Reaction. In contrast to the Pt congener, reaction of 1b
with 1 atm of CO does not result in immediate formation of an
1
acyl complex. A single new species grows in by H NMR (δ
3.32) over several days, presumably trans-[(PPh3)2(CO)Pd-
Figure 1. (A) Optimized DFT structure of 3. (B) Optimized DFT
structure of 3-bf3.
(COCH2Ph)][BArF ] (4). IR spectroscopy on the sample was
4
C
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constraints. Attempts to find optimized geometries for trans
coordination to the acyl oxygen, Pt−B coordination (at an axial
position), or BF3 coordination to the oxygen atom of the
carbon monoxide (carbonyl) ligand were unsuccessful, with
Lewis acid coordination apparently disfavored relative to
complete dissociation. Therefore, the following analysis was
performed on the structure 3-bf3 shown in Figure 1B.
the charge on the acyl and carbonyl carbons increases upon
Lewis acid coordination, while the charge on the oxygen of the
acyl becomes more negative. For both 3 and 3-bf3, the
carbonyl carbon has a greater natural charge than the acyl
carbon.
The occupancy and energy for selected NBOs are shown in
Table 3. The antibonding σ*acyl C−O and π*acyl C−O orbitals are
stabilized by coordination of BF3, their occupancy is increased,
and the acyl antibonding orbitals become more polarized.
Other orbitals, including those of the carbon monoxide ligand,
are little changed by comparison. An energy level diagram
(Figure 2) shows the influence of BF3 on the acyl orbitals
According to the structures of 3 and 3-bf3, the largest
changes in bond length upon BF3 coordination are a
lengthening of the acyl C−O bond (1.205 Å to 1.250 Å;
Table 1) and a shortening of the Pt−Cacyl bond (2.082 Å to
2.049 Å). These changes mirror findings by Green and co-
workers for the BF3−CpFeL2(COR) system.10 A decrease in
the Pt−Ccarbonyl bond upon BF3 binding suggests a weakening
trans influence on coordination. The Pt−CO angle
approaches linearity upon BF3 binding. Optimized structures
of the Pd congeners 4 and 4-bf3 show the same general trends.
To study the thermodynamics of the reaction of 3 with
BF3Et2O, frequency calculations were performed on optimized
structures of 3, 3-bf3, BF3Et2O, and BF3. The difference in total
electronic energy, ΔE, is −2.5 kcal mol−1. The thermochemistry
obtained via frequency calculations (ΔH° = −2.7 kcal mol−1,
ΔS° = −0.005 kcal mol−1 K−1, ΔG° = −1.2 kcal mol−1) is
reasonably consistent with experiment (ΔH° = −3.4 kcal
mol−1, ΔS° = −0.018 kcal mol−1 K−1, ΔG° = 2.0 kcal mol−1;
vide supra). Thermochemistry for the analogous reaction of 4
(ΔE = 0.6 kcal/mol; ΔH° = 0.3 kcal mol−1, ΔS° = −0.005 kcal
mol−1 K−1, ΔG° = 1.0 kcal mol−1) indicates that BF3
coordination to the Pd congener 4 is less favorable.
To assist in understanding the influence of BF3 on the
orbitals of 3, a natural bond orbital analysis (NBO 6.0) was
performed on the computed structures of both 3 and 3-bf3. In
the following we discuss changes to the acyl ligand upon BF3
binding using the carbon monoxide (carbonyl) ligand as
reference. A natural population analysis (Table 2) shows that
Figure 2. Energy level diagram (NBO) for 3 and 3-bf3. Blue labels are
used for orbitals on the acyl ligand. The n label refers to lone pairs; for
example, nPt‑1 refers to the platinum lone-pair NBO with the highest
energy.
Table 2. Natural Population Analysis of 3 and 3-bf3
relative to the remainder of the metal complex. Effects on
orbital energies are mostly localized to the acyl ligand, e.g., a
lowering of π*acyl C−O relative to π*carbonyl C−O. Figure 3 shows
representative illustrations of preorthogonalized NBOs of 3-
bf3, which have the general shapes of organic carbonyl orbitals.
As expected for a square planar complex,38 two hyperbonds
(three-center, four-electron bonds) are present in both 3 and 3-
bf3, each corresponding to mutually trans ligand pairs (Table
natural charge
atom
3
3-bf3
Δ
Pt
0.248
0.397
0.294
0.431
0.046
0.034
C (acyl)
C (carbonyl)
O (acyl)
0.484
0.524
0.040
−0.575
−0.420
−0.598
−0.402
−0.023
0.018
O (carbonyl)
Table 3. Selected Natural Bond Orbitals (NBOs), Occupancies, and Energies
3
3-bf3
NBO
occupancy
energy (au)
occupancy
energy (au)
σacyl C−O
πacyl C−O
nacyl O
1.99
1.98
1.81
−1.075
−0.363
−0.250
−0.014
0.543
1.99
1.98
1.67
−1.071
−0.408
−0.503
−0.064
0.430
π*acyl C−O
σ*acyl C−O
ncarbonyl O
πcarbonyl C−O
π*carbonyl C−O
nPt
0.12 (69.9% C, 30.1% O)
0.19 (77.5% C, 22.5% O)
0.022 (67.8% C, 32.2% O)
0.03 (70.3% C, 29.7% O)
1.98
2.00
0.13
1.98
1.95
1.86
1.83
−0.752
−0.476
−0.011
−0.172
−0.275
−0.271
−0.296
1.98
2.00
0.14
1.98
1.96
1.86
1.83
−0.760
−0.479
−0.028
−0.174
−0.290
−0.288
−0.314
D
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Pt acyl carbonyl compounds are known to undergo
nucleophilic attack at the carbonyl rather than the acyl,18,24
while selective Pd acyl attack is expected in carbonylation
reactions.13a Comparing the natural charges of Ccarbonyl vs Cacyl
in 3 and 4 yields a straightforward explanation. The Pt acyl
carbon is much less positively charged than the Pt carbonyl and
thus less prone to attack. Contrast this with the Pd acyl and
carbonyl carbons, which have similar natural charges.
CONCLUSIONS
■
The present work shows the influence of Lewis acids on Pd and
Pt organotransition metal acyl complexes. Changes in NBO
energy and occupancy upon Lewis acid coordination support
an increase in electrophilic character of the acyl ligand. Future
studies will explore the influence of Lewis acids on acyl
reactivity.
EXPERIMENTAL SECTION
■
General Methods. Unless otherwise specified, all manipulations
were performed under a dry N2 atmosphere using Schlenk techniques
or a Vacuum Atmospheres inert atmosphere glovebox. Infrared spectra
were collected using a Thermo Fisher Nicolet iS5 spectrometer with a
sealed liquid cell (International Crystal Laboratories SL-3, 0.1 mm,
CH2Cl2, 298 K). Analytical data were obtained from the CENTC
Elemental Analysis Facility at the University of Rochester, funded by
NSF CHE-0650456. NMR spectra were collected on Bruker Avance
1
III 500 MHz and DRX 500 MHz instruments. H NMR chemical
shifts (δ, ppm) are referenced to residual protiosolvent resonances,
and 13C NMR chemical shifts are referenced to the deuterated solvent
peak.39 31P NMR chemical shifts are referenced using an 85% H3PO4
external standard (δ = 0). Dichloromethane, pentane, and toluene
were purified using a commercial solvent purification system. Benzene,
tetrahydrofuran, diethyl ether, and benzyl chloride were dried over a
column of activated alumina and vacuum distilled. All deuterated
NMR solvents (Cambridge Isotope Laboratories) were dried over
activated 4 Å molecular sieves for 48 h before use. Sodium tetrakis(3,5-
bistrifluoromethyl)phenylborate (NaBArF ) was prepared using the
4
procedure of Yakelis and Bergman.40 Tris(pentafluorophenyl)borane
(B(C6F5)3) was purified via sublimation (100 mTorr, 90 °C) prior to
use. Other chemicals were used as received from commercial suppliers.
Carbon monoxide (Airgas, > 99.3% purity) was used directly as
supplied.
Figure 3. PNBOs of (A) π*acyl C−O; (B) σ*acyl C−O; and (C)
π*carbonyl C−O of 3-bf3.
4). For the ωC−Pt−C bond, the contribution from the Pt−Cacyl
relative to a Pt−Ccarbonyl decreases from 58.3% in 3 to 55.3% in
3-bf3. The contributions from each Pt−P (2c/2e) bond to the
ωP−Pt−P hyperbond remain equivalent on BF3 coordination.
The acyl NBOs of 4 and 4-bf3 are qualitatively similar in
energy and occupancy (Tables S2 and S3). The largest
differences between the Pd and Pt systems are the reduced
energy of the highest occupied nM NBO (4, −0.225 au; 4-bf3,
−0.239 au) and the natural charges of Cacyl (4, 0.47; 4-bf3,
0.48), Oacyl (4, −0.55; 4-bf3, −0.60), and M (4, 0.27; 4-bf3,
0.32).
Both experiment and computation support a metal depend-
ence on Lewis acid−acyl binding affinity, but no simple
explanation presents itself. Pt complex 3 and Pd complex 4
undergo similar bond length changes on BF3 binding, and the
NBO analysis of each is similar. The acyl O of 3 does have a
more negative charge than that of 4, suggesting a greater affinity
for Lewis acids.
trans-(PPh3)2Pt(CH2Ph)Cl. This known compound41 was prepared
in a one-pot, two-step sequence. Pt(PPh3)4 was synthesized from
K2[PtCl4] as previously reported42 and, without purification,
immediately treated with excess degassed benzyl chloride. After
several hours of heating at 75 °C, the reaction flask was cooled to
room temperature and the volatiles were removed in vacuo. The crude
residue was extracted with toluene and filtered through Celite. The
resulting solution was recrystallized from toluene/n-pentane. 1H NMR
(500 MHz, CD2Cl2): δ 7.62 (m, 12H, ArH), 7.42 (m, 6H, ArH), 7.36
(m, 12H, ArH), 6.80 (m, 1H, ArH), 6.70 (t, 3JH,H = 7.6 Hz, 2H, ArH),
6.40 (d, 3JH,H = 7.0 Hz, 2H, ArH), 2.27 (t, 3JP,H = 7.9 Hz, 2H, CH2Ph).
1
31P{1H} NMR (202 MHz, CD2Cl2): δ 27.47 (s, JPt,P = 1625 Hz).
[(PPh3)2Pt(CH2Ph)][BArF ] (1a). To a 20 mL scintillation vial in
4
the glovebox were added (PPh3)2Pt(CH2Ph)Cl (465 mg, 0.549
mmol), NaBArF (495 mg, 0.559 mmol, 1.02 equiv), and DCM (10
4
mL). The mixture was allowed to stir for 48 h. After filtration through
Celite, the solution was concentrated and transferred to a 4 mL vial.
Layering the solution with n-pentane resulted in bright yellow crystals
Table 4. Hyperbonds in 3 and 3-bf3
hyperbond
3
3-bf3
ωC−Pt−C
ωP−Pt−P
58.3% Pt−Cacyl/41.7% Pt−Ccarbonyl
49.7% Pt−P/50.3% Pt−P
55.3% Pt−Cacyl/44.7% Pt−Ccarbonyl
49.8% Pt−P/50.2% Pt−P
E
DOI: 10.1021/acs.organomet.5b00507
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
over several days, which were dried in vacuo. Yield: 689 mg, 0.412
mmol, 75%; mp 179 °C. 1H NMR (CD2Cl2, 500 MHz): δ 7.71 (s, 8H,
BArF ), 7.55 (s, 4H, BArF ), 7.46−7.16 (m, 24H, ArH), 6.95−6.81 (m,
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00507.
■
S
4
4
3
3
8H, ArH), 6.68 (t, JH,H = 7.6 Hz, 1H, p-CH2Ph), 6.54 (d, JH,H = 6.8
Hz, 2H, o-CH2Ph), 2.53 (m, 1H, CH2Ph). 31P{1H} NMR (202 MHz,
1
1
CD2Cl2): δ 24.01 (s, JPt,P = 1660 Hz), 21.68 (s, JPt,P = 2700 Hz).
Anal. Calcd for C75H49BF24P2Pt: C, 53.81; H, 2.95. Found: C, 53.66;
H, 2.89.
NMR spectra, computational thermochemistry, NBO
analysis of 4 and 4-bf3, and crystal structure report for
trans-(PPh3)2Pd(CH2Ph)Cl. This known compound43 was pre-
pared through a known procedure. To a yellow suspension of
Pd(PPh3)4 (2.77 g, 2.40 mmol) in degassed benzene was added an
excess of degassed benzyl chloride (10 mL) via cannula. After 30 min,
the resulting dark yellow solution was reduced in vacuo. Diethyl ether
(40 mL) was added to precipitate a yellow solid. Crude yield: 1.33 g,
1.76 mmol, 73%. Recrystallization from DCM/pentane provided
yellow material with spectra consistent with literature values.43
[(PPh3)2Pd(CH2Ph)][BArF ] (1b) (PDF)
4
Crystallographic Information File for [(PPh3)2Pd-
(CH2Ph)][BArF ] (1b) (CIF)
4
Cartesian coordinates for all computed structures (XYZ)
AUTHOR INFORMATION
Corresponding Author
*E-mail: dob@temple.edu.
Notes
■
[(PPh3)2Pd(CH2Ph)][BArF ] (1b). To a 20 mL scintillation vial in
4
the glovebox were added (PPh3)2Pd(CH2Ph)Cl (85 mg, 0.11 mmol),
NaBArF (99 mg, 0.11 mmol, 1.0 equiv), and DCM (3 mL). The
4
mixture was allowed to stir for 2 h. After filtration through Celite, the
solution was concentrated and transferred to a 4 mL vial. Layering the
solution with toluene and cooling resulted in yellow-orange crystals,
which were dried in vacuo and washed with toluene and pentane.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
1
This research was supported in part by the National Science
Foundation through major research instrumentation grant
number CNS-09-58854. We thank Boulder Scientific for a gift
of B(C6F5)3 and Dr. Charlie DeBrosse for performing and
analyzing the HMBC NMR experiment. G.E.D. thanks Dr.
Christine Hahn (Texas A&M−Kingsville) for helpful dis-
cussions.
Yield: 138 mg, 0.087 mmol, 79%; mp 136 °C (dec). H NMR (500
MHz, CD2Cl2): δ 7.72 (s, 8H, BArF ), 7.55 (s, 4H, BArF ), 7.49−6.69
4
4
3
(m, 35H, ArH), 2.99 (t, JP,H = 4.4 Hz, 2H, CH2Ph). 31P NMR (202
MHz, CD2Cl2): δ 33.75 (s), 23.32 (s). Anal. Calcd for
C75H49BF24P2Pd: C, 56.82; H, 3.12. Found: C, 56.56; H, 3.11.
Reaction of [(PPh3)2Pt(CH2Ph)][BArF ] (1a) with 1 atm of CO.
4
To a J. Young-style NMR tube were added 20 mg [(PPh3)2Pt-
(CH2Ph)][BArF ] and 0.5 mL of DCM-d2. The tube was attached to a
4
Schlenk manifold, subjected to three freeze−pump−thaw cycles (in a
liquid N2 bath), backfilled with CO (1 atm), and sealed. The starting
compound is immediately converted to trans-[(PPh3)2(CO)Pt-
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DOI: 10.1021/acs.organomet.5b00507
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