The elusive structure of Pd2(dba)3. Examination by isotopic labeling, NMR spectroscopy, and X-ray diffraction analysis: Synthesis and characterization of Pd2(dba-Z)3 complexes

Article abstract of DOI:10.1021/ja403259c

Pd⁰₂(dba)₃ (dba = E,E-dibenzylidene acetone) is the most widely used Pd⁰ source in Pd-mediated transformations. Pd⁰₂(dba-Z)₃ (Z = dba aryl substituents) complexes exhibit remarkable and differential catalytic performance in an eclectic array of cross-coupling reactions. The precise structure of these types of complexes has been confounding, since early studies in 1970s to the present day. In this study the solution and solid-state structures of Pd⁰₂(dba)₃ and Pd ⁰₂(dba-Z)₃ have been determined. Isotopic labeling (²H and ¹³C) has allowed the solution structures of the freely exchanging major and minor isomers of Pd⁰ ₂(dba)₃ to be determined at high field (700 MHz). DFT calculations support the experimentally determined major and minor isomeric structures, which show that the major isomer of Pd⁰ ₂(dba)₃ possesses bridging dba ligands found exclusively in a s-cis,s-trans conformation. For the minor isomer one of the dba ligands is found exclusively in a s-trans,s-trans conformation. Single crystal X-ray diffraction analysis of Pd⁰₂(dba)₃· CHCl₃ (high-quality data) shows that all three dba ligands are found over two positions. NMR spectroscopic analysis of Pd⁰ ₂(dba-Z)₃ reveals that the aryl substituent has a profound effect on the rate of Pd-olefin exchange and the global stability of the complexes in solution. Complexes containing the aryl substituents, 4-CF ₃, 4-F, 4-t-Bu, 4-hexoxy, 4-OMe, exhibit well-resolved ¹H NMR spectra at 298 K, whereas those containing 3,5-OMe and 3,4,5-OMe exhibit broad spectra. The solid-state structures of three Pd⁰ ₂(dba-Z)₃ complexes (4-F, 4-OMe, 3,5-OMe) have been determined by single crystal X-ray diffraction methods, which have been compared with Goodson's X-ray structure of Pd⁰₂(dba-4-OH) ₃.

Full text of DOI:10.1021/ja403259c

Article
pubs.acs.org/JACS
The Elusive Structure of Pd2(dba)3. Examination by Isotopic Labeling,
NMR Spectroscopy, and X‑ray Diffraction Analysis: Synthesis and
Characterization of Pd2(dba-Z)3 Complexes
Anant R. Kapdi,† Adrian C. Whitwood, David C. Williamson, Jason M. Lynam, Michael J. Burns,
Thomas J. Williams, Alan J. Reay, Jordan Holmes, and Ian J. S. Fairlamb*
Department of Chemistry, University of York, Heslington, York, North Yorkshire, YO10 5DD, United Kingdom
S
* Supporting Information
ABSTRACT: Pd0 (dba)3 (dba = E,E-dibenzylidene acetone)
2
is the most widely used Pd0 source in Pd-mediated
transformations. Pd0 (dba-Z)3 (Z = dba aryl substituents)
2
complexes exhibit remarkable and differential catalytic
performance in an eclectic array of cross-coupling reactions.
The precise structure of these types of complexes has been
confounding, since early studies in 1970s to the present day. In
this study the solution and solid-state structures of Pd0 (dba)3
2
and Pd0 (dba-Z)3 have been determined. Isotopic labeling (2H
2
and 13C) has allowed the solution structures of the freely exchanging major and minor isomers of Pd0 (dba)3 to be determined at
2
high field (700 MHz). DFT calculations support the experimentally determined major and minor isomeric structures, which
show that the major isomer of Pd0 (dba)3 possesses bridging dba ligands found exclusively in a s-cis,s-trans conformation. For the
2
minor isomer one of the dba ligands is found exclusively in a s-trans,s-trans conformation. Single crystal X-ray diffraction analysis
of Pd0 (dba)3·CHCl3 (high-quality data) shows that all three dba ligands are found over two positions. NMR spectroscopic
2
analysis of Pd0 (dba-Z)3 reveals that the aryl substituent has a profound effect on the rate of Pd-olefin exchange and the global
2
stability of the complexes in solution. Complexes containing the aryl substituents, 4-CF3, 4-F, 4-t-Bu, 4-hexoxy, 4-OMe, exhibit
well-resolved 1H NMR spectra at 298 K, whereas those containing 3,5-OMe and 3,4,5-OMe exhibit broad spectra. The solid-state
structures of three Pd0 (dba-Z)3 complexes (4-F, 4-OMe, 3,5-OMe) have been determined by single crystal X-ray diffraction
2
methods, which have been compared with Goodson’s X-ray structure of Pd0 (dba-4-OH)3.
2
processes,11 and their effects supported by theoretical studies.12
INTRODUCTION
■
The dba-Z ligands also influence transmetalation and reductive
Pd0 (dba)3 is the most widely used Pd0 precursor complex in
2
elimination steps13 and reduce the rate of β-hydrogen
synthesis and catalysis.1 Since early studies in the 1970s,
elimination in “PdII-σ-alkyl” complexes.14 This latter finding
determination of the precise structure of Pd0 (dba)3 has proven
2
confounding vide infra. Dba acts as an olefin, enone,2 or 1,4-
facilitates intramolecular Heck reactions of alkenyl tethered
alkyl bromides to give exo-cyclopentene products (providing
dba-4-OMe is used). Furthermore, dba-4-Z (Z = CF3 or OMe)
dien-3-one ligand and is often noninnocent3 (i.e., involved in
key steps) in catalytic processes.4 [Pd0(η2-dba)Ln] (L =
ligands influence regiochemical effects in the cross-coupling of
phosphine or N-heterocyclic carbene) complexes are formed
aryl halides with allylic sililoate salts.15 The design of dba-
in catalytic processes, generated by addition of ligand to
structures incorporating thienyl (th) rings affords Pd0 (th-dba)3
Pd0 (dba)3 in situ;5 dba-ligation affects the concentration of
2
Pd02Ln species available for oxidative addition with organo-
halides, the first committed step in cross-coupling processes,
and can act positively in regulating the concentration of Pd0Ln
when reactive organohalides are used (e.g., ArI) and reduce Pd
agglomeration which can form large inactive Pd0 clusters.6
complexes, allowing oxidative addition rates to be controlled.16
Pd2(dba-4-OH)3 and Pd2(dba-4-OAc)3 complexes have been
adapted for use in aqueous Suzuki polycondensation
reactions.17
In this paper we detail the synthesis and characterization of
several Pd0 complexes containing symmetrical aryl-substituted
‘dba-Z’ ligands. In order to achieve this task it has been
necessary to study the solution and solid-state structures of the
Recent selected examples where Pd0 (dba)3 has been employed
2
as a powerful catalyst system are given in Scheme 1, which
range from a remarkable macrocyclization,7 N1-selective
arylation of unsymmetrical imidazoles,8 and a highly selective
polycondensation process.9
parent complex, Pd0 (dba)3, and unambiguously determine its
2
structure by use of isotopic labeling (2H and 13C). The solid-
By electronically tuning the aryl groups of dba (e.g., dba-Z,10
where Z = OMe, t-Bu, H, F, CF3), the noninnocent behavior
has been exploited experimentally in several cross-coupling
Received: April 5, 2013
© XXXX American Chemical Society
A
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Scheme 1. Powerful Pd2(dba)3-Mediated Coupling Processes
Figure 1. Three conformations for dba (I−III) and the solution
structure(s) of Pd0 (dba)3. Bold lines denote strong olefin bonding
2
interactions (f > e > d), and dashed lines denote weak olefin bonding
interactions (c > b > a). (A) and (B) are the isomers proposed as most
likely in the Kawazura study.
value of 0.5 ppm and in the s-trans form a maximum value of
1.5 ppm.
state characterization of a series of Pd2(dba-Z)3·CH2Cl2
complexes (Z = 3,5-OMe, 4-OMe, and 4-F) has been
conducted by X-ray diffraction. The study is placed into
context with that reported for Pd2(dba-4-OH)3,17 specifically
the importance associated with intermolecular H-bonding
stabilizing one isomeric form (with no apparent conformational
disorder) of the dba-backbone in the dinuclear Pd0 complexes.
For several Pdn(dba-Z)n+1 complexes it has been possible to
probe their solution behavior and stability, by 1H NMR
spectroscopy, which is comparable to the spectroscopic data for
Pd2(dba)3.
Deformation toward the s-skew form causes Δαβ to increase
for the s-cis conformer, whereas it decreases for the s-trans
conformer. By extrapolation coordinated olefins with a small
Δαβ value (δ 0.069−0.186 ppm) were proposed to be in the s-
cis form, whereas those with a large value (δ 0.631−0.965 ppm)
were in a s-trans form. However, our recent work20 on ‘dba-
coordinated’ metal complexes (Rh, Pd, Pt, and Cu)21 show that
these ranges are underestimated, leaving the structural
connectivity reliant on NOE contacts and a low-resolution
1H NMR spectrum. It is clear that there are few structures
containing palladium that are more spectroscopically con-
founding than ‘Pd0 (dba)3’.
RESULTS AND DISCUSSION
■
2
Structure of Pd0 (dba)3 in CDCl3 and Additional
In a study reported in 2012, Zalesskiy and Ananikov
2
Context. In 1978 the solution behavior of Pd0 (dba)3·CHCl3
reported22 high-field (600 MHz) NMR spectroscopic data for
2
[hereafter ‘Pd0 (dba)3’ unless reference is made to an
‘Pd0 (dba)3’ as a mixture of two isomersthe first time a
2
2
alternative solvate] was valiantly investigated by Kawazura
second ‘minor’ isomer was reported in solution.23 Through a
combination of COSY, LR-COSY, HSQC, HMBC, and
NOESY measurements, the authors proposed that the dba
ligands in Pd2(dba)3 were in fact found in a s-cis,s-cis
conformation, standing in stark contrast with the Kawazura
and co-workers18 by a combination of H NMR spectroscopy
1
(at 100 MHz), 2H labeling, and computational simulation. This
work showed that each olefin in Pd0 (dba)3 can be found in a
2
distinct chemical environment. That is six olefins coordinate to
two chemically unique Pd0 atoms, with each dba acting as a “η2-
olefin,η2-olefin” bridging ligand. To explain why six olefins are
observed spectroscopically one needs to consider that: (i) dba
can exist in one of three 1,4-dien-3-one conformations (I−III,
Figure 1); (ii) the preferred conformation for dba in binding
Pd0 in the dinuclear complex, Pd2(dba)3, being s-cis,s-trans (II);
and (iii) the strength of the η2-olefin-metal bonding
interactions, via coordination through s-cis and s-trans olefins,
is different; the proposed solution-state structure of ‘Pd2(dba)3’
(one of two possible isomers) is shown in Figure 1.
1
study. H DOSY measurements confirmed that the major and
minor isomeric species had identical diffusion coefficients, i.e.,
two forms of Pd0 (dba)3. Following on from work on Pd0 (dba-
2
2
Z)3 complexes, we were aware of their intricate solution NMR
spectra (see later), due to their inherent low symmetry, dba-Z
lability, and inter- and intramolecular exchange. On symmetry
reasons alone it seemed unlikely that a s-cis,s-cis conformation
would be preferred, i.e., on the NMR time scale all three ligands
and associated olefins (six) would be expected to bind Pd0 to
the same extent, e.g., as averaged signals with only two α- and
β-proton environments, which is not the case (free exchange
occurs at ca. 298 K). The s-cis,s-cis conformations were
discounted for these reasons in the Kawazura study.18 Given
these details, an independent high-field NMR spectroscopic
study on the structure of Pd0 (dba)3 in solution (CDCl3) was
The Kawazura study18 relied on two pieces of information:
(i) NOE proton contacts between neighboring olefins within
two of the three coordinating dba ligands; and (ii) the chemical
shift difference between α- and β-protons within individual
olefins (Δαβ = δβ − α). This latter information can provide an
indication of the enone conformation in dba.19 For example in
the noncoordinated dba the s-cis form has a minimum Δαβ
conducted. We selected Pd02(dba)3·dba as from experience
2
some free dba ligand is observed spectroscopically in solutions
B
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of Pd0 (dba)3·solvent (solvent = CHCl3, CH2Cl2, or benzene).
this solution to 238 K, where negligible inter- or intramolecular
exchange occurs (established by EXSY experiments), resulted
in the broad aromatic protons shifting to higher chemical shift
(three broad signals at ca. δ 7.0, 7.55 and 7.85), leaving 12
resolved Pd0-olefin protons signals.
2
At this point it is worth noting that Pd0 (dba)3 is potentially
2
sensitive in solution to photooxidation [λmax = 520 nm
(MLCT) in CHCl3],24 in addition to thermal degradation. A
1H NMR spectrum (700 MHz) of Pd0 (dba)3·dba [hereafter
2
The three dba ligands thus bridge two Pd0 atoms in an
unsymmetrical manner, e.g., “η2-olefin,η2-olefin”, involving the
coordination of six independent alkenes, i.e., three distinct dba
ligands. Shown by open circles are the olefin protons attributed
Pd0 (dba)3] in alumina-filtered CDCl3 reveals the Pd0-olefin
2
bonding interactions (Figure 2).
to six unique olefins in Pd0 (dba)3. In both spectra run at 298
2
and 238 K, a minor species was also observed, akin to that
reported by Zalesskiy and Ananikov.22 Highlighted by black
circles are eight olefin protons (Figure 2), which belong to the
minor species (ratio of major/minor isomeric species 1:0.23 at
298 K).
The connectivity of each alkene in both major and minor
isomers needed to be confirmed by a combination of NMR
experiments and isotopic labeling (deuterium and 13C; Figures
3 and 4). The latter was necessary to ascertain the position of
1
Figure 2. 700 MHz H NMR spectra of Pd0 (dba)3·dba at 298 K
2
(bottom) and 238 K (top) (in CDCl3, referenced to residual CHCl3 at
δ 7.26). Key: closed black square, free dba; closed gray circle, ortho-aryl
protons; open circles, olefin protons in major species; closed black
circles, olefin protons in minor species.
Free dba ligand is observed in the 1H NMR spectrum
(highlighted by black squares). Proton integration reveals the
stoichiometry as Pd0 (dba)3·dba (in agreement with the C,H
2
composition indicated by elemental analysis). Two broad
signals, indicative of an exchange process, are observed at ca. δ
6.35−6.65 ppm which are the shielded ortho-protons of the
aromatic groups (ca. 12 H) connecting the 1,4-dienone
Figure 4. 13C Isotopically labeled Pd0 (dba)3 complexes synthesized
2
for this study (black circle indicates the position of 13C).
moieties of three dba ligands in Pd0 (dba)3 (indicated by
the α- and β-protons, which could allow each olefin to be
2
gray circles). These broad signals overlap with resolved olefin
protons in the same chemical shift region (at 298 K). Cooling
located accurately. The isotopically labeled dba ligands and Pd0
complexes were prepared in an identical manner to Pd0 (dba)3
2
Figure 3. 700 MHz 1H NMR spectra (at 298 K) of Pd0 (D(2,4)-dba-PhD5)3, Pd0 (D(1,5)-dba-PhD5)3, and Pd0 (dba-PhD5)3 (red and green colors
2
2
2
1
used to depict the α- and β-protons in the major isomer). Note that H−2H spin−spin couplings are not observed in these spectra.
C
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1
on small scale (typically 100 mg) (Figure 4). The H NMR
spectra of the deuterium-labeled Pd0 complexes are shown in
Figure 3.
isomers, and also reveals valuable 13C−13C spin−spin coupling
information, e.g., in Pd0 (13C(2,4)-dba)3.
2
For the major and minor species of Pd0 (dba)3 there are six
2
1
independent olefins, confirming their isomeric relationship. In
Considering the major isomer then, the H NMR spectrum
for Pd0 (dba-PhD5)3 shows the α-protons (red) and β-protons
the major isomer the carbonyl moieties [located from
2
Pd0 (13C(3)-dba)3] are all found in different chemical environ-
(green), which were located by comparison with the spectra of
2
Pd0 (D(1,5)-dba-PhD5)3 and Pd0 (D(2,4)-dba-PhD5)3. The
ments and shifted between δ 4.5−8.4 from the free dba ligand
(δ 189.2). The minor isomer exhibits a similar coordination
shift for two of the three dba ligands (δ 5.3 and 6.4). However,
the remaining CO signal exhibits a shift of only δ 0.2, which
is very close to the free ligand. This latter signal could belong to
a dba contaminant, rather than the minor isomer, leaving the
remaining dba CO overlapping with the major isomer of the
2
2
protons do not exhibit spin−spin coupling to deuterium due
to exchange at 298 K (in combination with small quadrupolar
effects). The spectra for Pd0 (D(2,4)-dba-PhD5)3 and Pd0 (D-
2
2
(1,5)-dba-PhD5)3 also reveal additional minor isomeric proton
signals, shown in light green for the former complex and orange
for the latter complex. These isomers were overlapping with the
Pd0 (13C(3)-dba)3.
major species in the spectrum of Pd0 (dba-PhD5)3 [and
2
2
unlabeled Pd0 (dba)3].
The α-carbons (six) in the major isomer are in different
2
2
As the 2H labeling had been successful in locating the α- and
chemical environments shifted to δ 84.4−94.5, and JCC
β-protons in the major isomer of Pd0 (dba)3 (and most of them
coupling is observed between olefins within the same dba
ligand. Three α-carbons can be grouped δ 84.4−85.5 and the
remaining α-carbons δ 89.3−94.5. The magnitude of the
coupling across the six α-carbons is 15.8−16.2 Hz ( 0.1 Hz).
Similar carbon signals are seen for the minor isomer; indeed the
average coordination shifts from the free ligand are similar in
both major and minor isomers (37.6 and 36.4 ppm,
respectively). The coordination shift difference between the
lowest and highest chemical shift signals was 10.1 and 8.0, for
the major and minor isomers, respectively.
2
in the minor isomer), we anticipated that 13C labeling would
provide valuable 13C NMR spectroscopic data; due to the
limited solubility of Pd0 (dba)3 in CDCl3 (ca. 5 mg/mL) it was
2
deemed necessary to synthesize 13C labeled analogues. The
13C{1H} NMR spectra of Pd0 (13C(3)-dba)3, Pd0 (13C(2,4)-
2
2
dba)3, and Pd0 (13C(1,5)-dba)3 are shown in Figure 5, and the
2
spectroscopic data is collated in Table 1. 13C labeling shows the
positions of all the 13C nuclei within the 1,4-dien-3-one
backbone of the three dba ligands, in both major and minor
The β-carbons (six) in the major isomer are also in different
chemical environments shifted to δ 85.5−111.2. Three β-
carbons can be grouped δ 85.5−97.5 and the remaining β-
carbons δ 107.7−111.2. Four β-carbon signals for the minor
isomer can be grouped δ 88.0−98.1, with the remaining β-
carbons δ 111.4−113.0. The average coordination shifts from
the free ligand are similar in both major and minor isomers
(43.3 and 44.3 ppm, respectively). The coordination shift
difference between the lowest and highest chemical shift signals
was 25.7 and 25.0 ppm for the major and minor isomers,
respectively. Interestingly, four of the β-carbon signals in the
minor isomer have a coordination shift >45 ppm, whereas only
three of the β-carbon signals are >45 ppm in the major isomer.
It is clear from these 13C NMR spectra that the β-carbons are
more responsive to coordination to Pd0 than the α-carbons.
EXSY experiments on this isotopically labeled derivative show
that the exchange mechanism is complex (see SI for spectra).
Spectrum D (Figure 5) shows the rapid exchange of added
13C(1,5)-dba to Pd0 (13C(3)-dba)3 (in CDCl3).
2
In keeping with the Kawazura study three of the olefins (H1
through to H6) are involved in strong binding to Pd0 (denoted
by ‘S’ in Table 1), and three olefins (H7−H12) are involved in
weak binding (denoted by ‘W’ in Table 1). This was originally
proposed by considering the average olefin chemical shifts.18 It
is noticeable that all the S olefins are found in a s-trans
conformation with respect to the carbonyl group, whereas W
olefins are found in a s-cis conformation. Thus, the ‘S’ olefins
have average chemical shifts of δ 5.18 (olefin f), 5.45 (olefin e),
and δ 5.56 (olefin d) and ‘W’ olefins δ 6.43 (olefin c), δ 6.51
(olefin b), and δ 6.72 (olefin a). A 13C−COSY experiment
confirmed the connection of α-carbons (2JCC coupled), which
along with 1H−13C HSQC experiments (see SI) allowed all the
α- and β-carbons to be connected to their respective protons.
1
1JCH couplings (ca. 157−160 Hz) are observed in H NMR
Figure 5. 13C{1H} NMR spectra (at 300 K, 125 MHz) of: (A)
spectra of both Pd0 (13C(2,4)-dba)3 and Pd0 (13C(1,5)-dba)3,
Pd0 (13C(3)-dba)3; (B) Pd0 (13C(2,4)-dba)3; (C) Pd0 (13C(1,5)-
2
2
2
2
2
dba)3; (D) 13C{1H} NMR spectrum following addition of 1 equiv of
although no change in the magnitude of coupling as a function
of chemical environment was recorded.
13C(1,5)-dba to a CDCl3 solution of Pd0 (13C(3)-dba)3.
2
D
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1
Table 1. Combined H and 13C NMR Spectroscopic Data for the Major Isomer of Pd0 (dba)3 in CDCl3
2
δ
/
olefin binding
conformer/olefin (a−f)/dba
a
b
b
β−α c
d
e
f
g
h
i
olefin
δα/ppm
4.9 (H1)
δβ/ppm
ppm
δave αβ
J/Hz
3
NOE
strength
HMQC
(I−III)
H1−H5
C1−C5
H2−H4
C2−C4
H3−H6
C3−C6
5.9 (H5)
1.0
12.6
0.36
1.2
5.45
91.0
5.18
84.7
5.56
92.9
6.51
98.7
12.7 JH1−H5
H5β-H8α
S
S
S
C5−H5···H1
C4−H4···H2
C6−H6···H3
s-trans, e, I
s-trans, f, II
s-trans, d, III
2
84.9 (C1) 97.5 (C5)
5.0 (H2) 5.36 (H4)
15.8 JC1−C8
3
12.4 JH2−H4
H4β-
H10α
2
84.4 (C2) 85.5 (C4)
5.16 (H3) 5.96 (H6)
94.5 (C3) 91.8 (C6)
16.2 JC2−C10
3
0.8
13.0 JH3−H6
H6β-H7α
H7α-H6β
H8α-H5β
2
−2.7
0.65
18.5
15.8 JC3−C7
3
H7−H12 6.18 (H7) 6.83 (H12)
C7−C12
13.2 JH7−H12
2
W
W
C12−H12···H7
C9−H9···H8
s-cis, b, III
s-cis, c, I
89.6 (C7) 108.1
(C12)
15.8 JC7−C3
3
H8−H9
C8−C9
6.38 (H8) 6.48 (H9)
85.5 (C8) 107.7 (C9)
0.1
6.43
96.4
6.72
13.2 JH8−H9
2
22.2
0.09
15.8 JC8−C1
H10−
6.67
(H10)
6.76 (H11)
13.4
3JH10−H11
H11
H10α-
H4β
C11−
W
s-cis, a, II
2
H11···H10
C10−C11 89.3 (C10) 111.2
(C11)
21.8
100
16.2 JC10−C2
a
b
Protons numbered from the lowest chemical shift (H1) to highest chemical shift (H12). Position of α- and β-protons was located from the 1H and
c
d
13C NMR spectra of the H and 13C labeled Pd0 (dba)3 complexes. Chemical shift difference between α- and β-protons. Average chemical shift
2
2
e
between α- and β-protons. Spin−spin coupling constants determined from 1D 1H and 13C NMR spectra [with assistance from 1H−COSY and 13C-
f
g
HMQC spectra (at 298 K), see SI]. Determined by NOE measurements (quantitative; magnitude of NOE, ca. 10%). Strength of olefin binding: for
the strongest binding average 1H and 13C chemical shifts of α- and β-protons δH 5.18−5.56 (δC 84.7−92.9) and weakest binding δH 6.43−6.72 (δC
h
96.4−100). The 1H−13C correlations were determined by an HMQC experiment with Pd0 (13C(1,5)-dba)3 described as 1JCH and 2JCH, e.g., 1JC5H5
2
i
1
1
and JC5H1. Conformation of alkene connected to the CO group (determined by NOE), specific olefin (from H−COSY) lettered according to
average 1H and 13C chemical shifts of α- and β-protons (the olefin with the weakest binding to Pd0 is denoted ‘a’ and that with the strongest binding
‘f’. 13C−13C COSY (125 MHz) correlated signals for Pd0 (13C(2,4)-dba)3 are 94.5 (C3) and 89.6 (C7) (conformer III); 89.3 (C10) and 84.4 (C2)
2
(conformer II); 85.5 (C8) and 84.9 (C1) (conformer I).
With the data above in hand one can employ HSQC/
HMQC and NOESY experiments to fully connect the protons
and carbon environments in the individual dba ligands. The full
solid state (denoted isomer M in Table 2). Isomer M has the
lowest relative energy (set to 0) of the four isomeric structures.
Altering the conformation of one dba ligand in isomer M, i.e.,
the conformation of one enone (s-cis ↔ s-trans) gives isomers
N and O. The isomer containing a s-trans,s-trans ligand is lower
in relative energy than the isomer containing a s-cis,s-cis ligand.
The structure containing all s-trans enones at Pd1 and all s-cis
enones at Pd2 has the highest relative energy of the series. The
order of relative energy (stability) is M < N < O ≪ P.
Therefore, the most likely isomers, in comparison with the
NMR spectroscopic data presented above, are M and N, i.e.,
structural connectivity of the major isomer of Pd0 (dba)3 could
2
be essentially deduced from the 1H−COSY and NOESY data of
the unlabeled Pd0 (dba)3 (at 238 K, where there is negligible
2
inter- or intramolecular exchange) (Figure 6 and Table 1). The
NOE interactions (cross-peaks) provide unequivocal proof that
each ligand in the major isomer is s-cis,s-trans. Specifically, H5β-
H8α, H4β-H10α, and H6β-H7α (summarized in Figure 7).
The spectroscopic data for the minor isomer of Pd0 (dba)3
2
are collated in the SI (summarized in Figure 8). Four of the
olefins (H1′ through to H8′) are involved in strong binding to
Pd0 (δave αβ < 5.5 ppm), and two olefins (H9′-H12′) are
involved in weak binding (detected by 13C−1H correlations,
HMQC, having very small αβ chemical shift separations).
The NOE data for the unlabeled complex allowed two of the
olefins in one dba ligand to be paired, that is the β-protons
H5′β and H7′β, which is therefore s-trans,s-trans. 2D NOESY
major and minor isomers of Pd0 (dba)3, respectively (Figure 9).
2
It is interesting to note that the olefins occupying a s-trans
conformation in isomers M and N are more strongly bound to
Pd0 in both isomeric structures, which is in keeping with the
synergic (back-bonding) arguments that the s-trans olefin
protons are more shielded than the s-cis olefins.
Single Crystal X-ray Structure of Pd0 (dba)3·CHCl3.
2
There have been several X-ray crystal structure determinations
25
on the Pd0 (D(2,4)-dba-PhD5)3 compound showed the same
of Pd0 (dba)3·solvent (solvent = CH2Cl2 and CHCl3;34 for
2
2
NOE cross-peak between the β-protons H5′ and H7′. A weak
correlation is seen for H8′β with a proton at d 6.76. Due to
resolution issues we could not fully deduce the proton−carbon
connectivity for the minor isomer from HMQC or HSQC
experiments (see SI). The 13C−13C COSY (125 MHz)
the former two ligands were found to be s-cis,s-trans and one
ligand s-cis,s-cis, whereas the latter showed all ligands to be s-
cis,s-trans).26 The ‘best structure’ was reported by Pregosin and
co-workers27 in the late 1990s, which indicated that two of the
dba ligands were disordered over two positions (around the
1,4-dien-3-one moieties),28 that is complicated by conforma-
spectrum of Pd0 (13C(2,4)-dba)3 in CDCl3 showed well-
2
resolved correlated signals: 93.6 and 89.1; 92.3 and 85.6; 88.9
and 86.2.
̀
tional changes in the dba ligand (vis-a-vis s-cis and s-trans
geometrical issues). Following many attempts over the last 10
years we are pleased to have accomplished a high-quality X-ray
DFT Calculations. We estimate that there are 64 potential
isomers of Pd0 (dba)3. Considering the substantial computa-
structure determination of Pd0 (dba)3·CHCl3 (the crystallized
2
2
tional effort needed to determine accurate structures for all of
Pd0 (dba)3·CHCl3 was dissolved in fresh CHCl3 and layered
2
these isomers, DFT calculations were conducted on the four
with hexane affording small purple crystals), giving final R
indexes [I ≥ 2σ (I)] R1 = 0.0333, wR2 = 0.0720 (monoclinic,
P21/n space group, Z = 4). The data show that all three ligands
are disordered over two positions each, with a ratio of 79:21
most likely isomers of Pd0 (dba)3 (Table 2). The starting point
2
used for the calculations was Goodson’s X-ray structure of
Pd0 (dba-4,4′-OH)3,17 which shows only one isomer in the
2
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Figure 7. Proton connectivity in the major isomer of Pd0 (dba)3,
2
established by COSY and NOESY experiments.
Figure 8. Proton connectivity in the minor isomer of Pd0 (dba)3,
2
established by COSY and NOESY experiments. Note that it was not
possible to accurately assign the positions of 4 protons, which have not
been numbered, but they are weakly coordinated.
Table 2. Relative Energies for Pd0 (dba)3 Isomers by DFT
2
isomer
ΔE/kJmol−1
ΔG298.15/kJmol−1
Pd2(s-Z,s-Z,s-E), Pd1(s-E,s-E,s-Z), M
Pd2(s-E,s-E,s-Z), Pd1(s-E,s-E,s-Z), N
Pd2(s-Z,s-E,s-Z), Pd1(s-Z,s-Z,s-E), O
Pd2(s-Z,s-Z,s-Z), Pd1(s-E,s-E,s-E), P
0
3
0
5
16
26
16
23
dba-Z ligands with varying aryl substituents were prepared by
the method described by Ishii in 1970 (procedure A, Figure
11).30 A large stock solution of Na2PdCl4 in MeOH was used
for the synthesis of several Pdx(dba-Z)x+1 complexes. The
required dba-Z ligand (3 equiv) was added to a MeOH solution
of Na2PdCl4 and heated to 60 °C until complete dissolution.
The exact composition of this mixture remains unknown. A PdII
complex, e.g., NanPdCln+2(dba-Z)2, is a likely structure for the
intermediate species, which is subsequently reduced. Six equiv
of NaOAc were added to this solution, assisting the reduction
process in alcoholic solvent (PdII → Pd0). On cooling solutions
to ambient temperature the complexes precipitate as brown-
maroon or purple solids (note: Pd black is formed in all cases
usually deposited on the glassware surface). Following
precipitation from solution, which usually occurs at 60 °C,
the complexes are filtered (at ambient temperature), washed
with MeOH and water, and then dried in vacuo (on a glass frit);
small quantities of acetone or acetone/water (1:2, v/v) can be
used to improve the purity of the complexes (note: for the
majority of the complexes some product dissolution was
observed using acetone alone, which lowers the overall yields).
A series of Pd2(dba-Z)3·L complexes were prepared in this
1
Figure 6. 700 MHz spectra of Pd0 (dba)3 (CDCl3, 238 K). (A) H−
2
1
COSY; (B) H NOESY spectrum (correlations for major isomer).
(Figure 10; note the s-cis olefins are shown bonded in green
and s-trans olefins shown bonded in orange). Both isomeric
forms are shown for clarity (A and B). The Pd(1)−Pd(2) bond
distance is 3.244 Å. The average s-cis CC bond distances in
isomer 1 is 1.3603 Å. For the s-trans CC bonds the average
distance is 1.393 Å. It is noticeable from the C−Pd bond
distances that the s-cis CC bonds are coordinating in an
asymmetric fashion (average Cα−Pd = 2.247 Å, Cβ−Pd =
2.287 Å), whereas the s-trans CC bonds are more symmetric
(average Cα−Pd = 2.228 Å, Cβ−Pd = 2.229 Å), indicating that
there is more back-bonding in the s-trans CC bonds.29
Synthesis and Characterization of Pd0 (dba-Z)3 Com-
2
plexes. The Pd2(dba-Z)3·L complexes containing different
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Figure 10. X-ray structure of Pd0 (dba)3·CHCl3: (A) isomer 1; (B)
Figure 9. DFT calculated structures of (A) the major isomer of
Pd0 (dba)3 (M) and (B) the minor isomer of Pd0 (dba)3 (N) (most
likely isomers in solution). The bonds shown in green highlight the s-
cis olefins and those shown in orange the s-trans olefins.
2
isomer 2. Thermal ellipsoids set to 50% (not shown for the phenyl
groups for reasons of clarity; H-atoms not shown). The fragmented
bond denotes the Pd(1)···Pd(2) bond. The bonds shown in green
highlight the s-cis olefins and those shown in orange the s-trans olefins
(arbitrary atom numbering used).
2
2
manner in acceptable yields (Table 3). The purity of the
complexes was ascertained principally by element analysis
(based on carbon and hydrogen percentage compositions).
While one can ascertain the Pd:dba ratio for Pd2dba3·dba in
1
CDCl3 by H NMR spectroscopy, we believe it is not suitable
for assaying the purity of Pd2(dba-Z)3·L complexes, as they
degrade to give Pd nanoparticles/particles in CDCl3 (i.e., the
outcome is dependent on the time between dissolution and
spectroscopic analysis). This degradation process is well-known
Figure 11. Synthesis of symmetrical Pd2(dba-Z)3·L complexes.
Table 3. Yields of Pd2(dba-Z)3·L Complexes
for Pd0 (dba)3 in certain solvents31 or reducing conditions,
Pd2(dba-Z)3·L (L = dba-Z or H2O)
yield/%
2
being a valuable precursor to Pd nanoparticles.32
Pd2(dba-3,4,5-OMe)3
59
53
72
70
75
80
69
Pd2(dba-3,5-OMe)3·dba-3,5-OMe
Pd2(dba-4-OMe)3
For reactions where the dba-Z ligand was not fully dissolved,
Pd0 particle formation occurred quite readily (inferred by
elemental analysis and low carbon and hydrogen composition
and the appearance of fine particulate Pd black).33
It is pertinent to mention that several of these Pd2(dba-Z)3·L
complexes react cleanly with PPh3 (in DMF) to afford Pd0(η2-
dba-Z)(PPh3)2 complexes, which have been characterized by
NMR spectroscopic analysis and cyclic voltammetric meth-
ods.11b
We have also tested the synthesis of the Pd2(dba-Z)3·L
complexes by the method described by Ishii in 1974 (procedure
B),34,35 from PdCl2 [dba-Z ligand (3.3 equiv) and NaOAc (ca.
8 equiv) in MeOH, heated for 5 min at 50 °C, then PdCl2 (1
equiv) added with heating for 4 h]. The Pd2(dba-4,4′−
OHexyl)3·dba-4,4′−OHexyl complex was prepared by this
procedure. Dissolution in CHCl3 showed that ‘Pd2(dba-4,4′−
OHexyl)3’ was as stable as the parent complex (TEM analysis
showed negligible Pd nanoparticle formation). Pd2(dba-4-t-
Pd2(dba-4-OHexyl)3·dba-4-OHexyl
Pd2(dba-4-t-Bu)3
Pd2(dba-4-F)3·H2O
Pd2(dba-4-CF3)3·dba-4-CF3
Bu)3·dba-4-t-Bu was also synthesized via procedure B in a
similar yield (78%).
1H NMR Spectroscopic Analysis of Pd2(dba-Z)3
1
Complexes. H NMR spectra of the Pd2(dba-Z)3 complexes
in CDCl3 (ca. 5 mg in 0.7 mL) were obtained (Figure 12).
Complexes where Z = OMe possess limited solubility in CDCl3
(especially 3,4,5-OMe and 3,5-OMe derivatives, spectra A and
B, Figure 12). Moreover, within hours these complexes degrade
1
in CDCl3 (liberating free dba-Z ligand). Nevertheless a H
NMR spectral overlay shows that all the complexes exhibit
diagnostic Pd0-olefin interactions (ca. 5−7 ppm), akin to
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Figure 12. 1H NMR spectra of Pd0 (dba-Z)3 in CDCl3 at 300 K (500
2
MHz, unless otherwise specified): (A) Pd0 (dba-3,4,5-OMe)3; (B)
2
Figure 13. Selected 13C-MAS spectra [100.56 MHz (recycle delays ca.
2−5 s, spin rate ca. 10 kHz)] of the Pd2(dba-Z)3 complexes: (A)
Pd0 (dba-3,4,5-OMe)3; (B) Pd0 (dba-4-OMe)3; (C) Pd0 (dba-4-t-
Pd0 (dba-3,5-OMe)3; (C) Pd0 (dba-4-OMe)3; (D) Pd0 (dba-4-OHex-
2
2
2
yl)3 (note 400 MHz spectrum); (E) Pd0 (dba-4-t-Bu)3; (F)
2
Pd0 (dba)3; (G) Pd0 (dba-4-F)3; (H) Pd0 (dba-4-CF3)3.
2
2
Bu)3; (D) Pd0 (dba)3; (E) Pd02(dba-4-F)3; (F) Pd0 (dba-4-CF3)3.
2
2
2
2
2
2
Peaks colored orange are attributed as Pd-olefin carbon signals and red
are attributed carbonyl signals.
Pd0 (dba)3. That is in most cases there are 12 olefinic protons
2
as expected in these dinuclear Pd complexes. 1H COSY spectra
of several complexes exhibit proton−proton correlations that
X-ray Structures of Selected Pd2(dba-Z)3·L Com-
plexes. Three Pd2(dba-Z)3 complexes (Z = 3,5-OMe, 4-
OMe, and 4-F) could be crystallized from a CH2Cl2 solution
layered with diethyl ether at room temperature. As with
Pd2(dba)3·CHCl3 we were able to obtain X-ray diffraction data
for which structural refinement was possible. For example
Pd2(dba-3,5-OMe)3·CH2Cl2 crystallized in the P21/c space
group (monoclinic, Z = 4) (Figure 14). The two alkenes in
each ligand coordinating both Pd0 centers are essentially
eclipsed. Interestingly, the complex contains one dba-3,5-OMe
ligand that can be found over two positions (occupancy =
0.682(3):0.318(3) or ca. 2.1:1). Similar disorder was noted for
Pd2(dba-H)3·CH2Cl2 vide supra. The phenyl moieties and
positions of the carbonyl groups in both conformers were
resolved in the case of Pd2(dba-3,5-OMe)3·CH2Cl2. In essence,
for each conformer there is a strict requirement for these
moieties to exhibit different orientations. The Pd···Pd distance
is 3.2314(5) Å, which compares with 3.244 Å in our X-ray
structure of Pd2(dba-H)3·CHCl3 and is indicative of a
nonbonding interaction; the electron count on each Pd0 center
is 16. The tightly packed structure of Pd2(dba-3,5-
OMe)3·CH2Cl2 is also illustrated in Figure 14, which shows
that each Pd0 atom is effectively shielded from solvent (or other
ligands) in the solid state. There is also significant steric
congestion between the 3,5-methoxy substituents in neighbor-
ing dba-3,5-OMe ligands.
are similar to Pd0 (dba)3. We did not observe spectral
2
broadening for Pd2(dba-4-OMe)3 and Pd2(dba-4-OHexyl)3.
Some broadening is observed for Pd2(dba-4-CF3)3, although
the signals for the major isomer are relatively sharp. Minor
isomers for several of these complexes (uncharacterized) are
1
evidenced by the small peaks in the H NMR spectra, akin to
Pd2(dba)3.
It is interesting to note that the position (chemical shift) of
the olefin protons is affected to a certain degree by the remote
aryl substituents, indicative of electronic communication in the
complexes and in keeping with the electronic differences
observed at the olefins in dba-Z derivatives.12,36 Particularly
noticeable is the shift of the most shielded olefin protons, H1
and H2, in spectra C and D in Figure 12.
The limited solubility of the Pd0 (dba-Z)3 complexes
2
prevented their solution 13C NMR spectra from being obtained.
However, the solid-state 13C MAS spectra of a number of
complexes gave some useful information (Figure 13). The
parent complex, Pd0 (dba)3, exhibits 13C signals for Pd-olefins
2
(ca. δ 80−105 ppm) and carbonyl moieties (ca. δ 180 ppm).
Spinning side-bands are observed on the outer edges of the
spectrum. For Pd0 (dba-4-F)3 the intrinsic line width is quite
2
high (spectral resolution is relatively limited), which could be a
result of fluorine being in the system. For all other complexes
there is a hint that the intrinsic line widths are relatively low,
i.e., there are a high number of overlapping lines with similar
shifts that limits the resolution, evidence for ligand
inequivalence in each complex and multiple structures (i.e.,
conformers) in the asymmetric unit. It is important to
Considering the major isomer of [Pd2(dba-3,5-
OMe)3·CH2Cl2], it is instructive to compare the CC bond
lengths of the coordinated ligand to each Pd center (Figure 15).
For Pd1 the CC bond, C(52A)−C(53A), orientated s-trans
to the CO group is significantly longer than the other two
CC bonds, one orientated s-cis, C(10)−C(11), and the other
emphasize that the solid-state structures of the Pd0 (dba-Z)3
2
complexes mirror that of Pd0 (dba)3 by 13C MAS NMR.
2
H
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each palladium center there is a s-trans CC bond that is
substantially longer than the other CC bonds, indicative of
back-bonding. The free ligand dba-3,5-OMe (see SI for X-ray
structure) was found to (fortuitously) adopt a s-cis,s-trans
conformation in a single crystal, with both CC bonds
showing the same distance (within error). The longest CC
bond, C(52A)−C(53A) in the complex is ca. 0.09 Å longer
than the same bond in the free ligand.
The X-ray diffraction data for Pd2(dba-4-OMe)3·CH2Cl2
(monoclinic, P21/n space group, Z = 4) and Pd2(dba-4-
F) ·CH Cl (triclinic, P1, Z = 2) show that dba-Z ligand
̅
3
2
2
disorder is seen about the 1,4-dien-3-one moieties. In the case
of Pd2(dba-4-F)3·CH2Cl2 each dba-4-F ligand is disordered
over two positions; all could be satisfactorily modeled in a
similar manner to Pd0 (dba)3·CHCl3 (vide supra). The solvate
2
molecules of crystallization appear to be crucial. In this context
the X-ray structure37 of Pd0 (dba-4,4′−OH)3 requires further
2
examination. In this structure extensive H-bonding interactions
between neighboring hydroxyls and carbonyl groups lock and
stabilize the structure (Figure 16), which leads to only one
isomeric form being observed in the solid-state. This is a unique
case and stands in contrast to the other related structures
detailed in this paper.
Figure 14. Top: X-ray structure of Pd2(dba-3,5-OMe)3·CH2Cl2.
Hydrogens and 1 CH2Cl2 molecule omitted for clarity; one dba-3,5-
OMe ligand is found over 2 positions [i.e., C(49A)-C(50A) and
C(52A)-C(53A)] (only one ligand position shown). The bonds
shown in green highlight the s-cis olefins and those shown in orange
the s-trans olefins (arbitrary atom numbering used). Bottom: Space-
filling models (each ligand has been colored red, blue and green to
allow differentiation), left - side-on perspective and right - end-on
perspective.
Figure 16. H-bonding network seen for Pd0 (dba-4,4′-OH)3. Key H-
2
bonding interactions (shown as blue fragmented bonds): O(2)···H-
(7A)−O(7) = 1.689 Å, O(5)···H(4A)−O(4) = 1.819 Å, O(7)···H-
(9A)−O(9) = 1.910 Å, O(8)···H(6A)−O(6) = 1.806 Å. The bonds
shown in green highlight the s-cis olefins and those shown in orange
the s-trans olefins.
Figure 15. Selected bond lengths (X-ray diffraction data) of the olefins
in the free ligand, dba-3,5-OMe, and Pd2(dba-3,5-OMe)3·CH2Cl2. We
note that C49a−C50a is a particularly short CC bond (larger
estimated standard deviations are observed within the disordered
ligand). The bonds shown in green highlight the s-cis olefins and those
shown in orange the s-trans olefins.
CONCLUSION
■
The solution and solid-state structures of Pd0 (dba)3 and
2
Pd0 (dba-Z)3-type complexes have been comprehensively
2
determined. In the case of the former parent complex isotopic
labeling (2H and 13C) was necessary to determine the solution
structures of the freely exchanging major and minor isomers of
Pd0 (dba)3; in the case of the major isomer of Pd0 (dba)3 this is
s-trans, C(31)−C(32). The latter s-cis and s-trans CC bonds
are identical (within the error limits).
2
2
For Pd2 it is apparent that the CC bond orientated s-trans
to the CO group, C(7)−C(8), is significantly longer than the
two CC bonds orientated s-cis, C(28)−C(29) and C(49A)-
C(50A) (for the latter, foreshortening of this CC bond can
be explained in part by excessive thermal motion). Thus, for
unambiguous. It consists of three s-cis,s-trans ligands, where the
s-cis olefins are weakly bound to Pd0 and the s-trans olefins
strongly bound. Asymmetric dba coordination results, with one
Pd0 atom coordinated to two s-trans olefins and one s-cis olefin,
and the second Pd0 atom coordinated to two s-cis olefins and
I
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one s-trans olefin. The minor isomer of Pd0 (dba)3 possesses a
however the X-ray diffraction data quality was poor, and a reasonable
structure could not be determined.
2
s-trans,s-trans ligand and the chemical shifts of the β-protons
indicate that four of the six olefins are s-trans and strongly
bound to both Pd0 centers, with other two olefins being s-cis.
This study brings about a resolution to the confounding
molecular structure18,22 of this ubiquitous complex in solution.
Theoretical calculations (DFT) support the feasibility of the
experimentally determined major and minor isomeric structures
Characterization Data for Pd2(dba-Z)3·dba-Z. In all cases the
minor isomer could not be fully assigned, therefore the proton signals
given are for the major isomer. The aromatic regions are broad in all
cases; overlapping signals are observed for both major and minor
isomeric forms for each complex. In all cases where resolved olefin
proton signals are observed, we propose that the ligands adopt a s-cis,s-
trans conformation, i.e., having a similar structure to the major isomer
in solution. The major isomer of Pd0 (dba)3 contained three
of Pd0 (dba)3. The reactions were typically run using 0.1−0.25 g of
2
2
PdCl2.
bridging dba ligands found exclusively in a s-cis,s-trans
[Pd0 (dba-4-F)3·H2O]. Yield = 80%; Mp 134−136 °C; UV (THF,
conformation. The minor isomer of Pd0 (dba)3 reveals that
2
2
1
3
nm) 518 (d−d); H NMR 4.82 (1H, d, JHH = 12.5 Hz, H1), 4.87
(1H, d, JHH = 12.5 Hz, H2), 5.03 (1H, d, JHH = 13 Hz, H3), 5.30
(1H, d, JHH = 12.5 Hz, H4), 5.85 (1H, d, JHH = 13.5 Hz, H5), 5.89
(1H, d, JHH = 13 Hz, H6), 6.03 (1H, d, JHH = 13.5 Hz, H7), 6.22
one dba ligand is found exclusively in a s-trans,s-trans
3
3
conformation. A high-resolution X-ray structure determination
3
3
of Pd0 (dba)3·CHCl3 shows that all three dba ligands are
2
3
3
disordered over two positions, which is an issue associated with
the conformational flexibility around the 1,4-dien-3-one moiety
in the solid-state.
3
3
(1H, d, JHH = 13 Hz, H8), 6.41 (1H, d, JHH = 13.5 Hz, H9), 6.44−
7.03 (ca. 27 H, ArH, 3 olefin protons H10−H12); Anal. calcd for
C51H38F6O4Pd2 (1041.68), C 58.80, H 3.68; Found, C 58.95, H 4.10.
Crystals suitable for X-ray analysis were grown from a CH2Cl2 solution
of the complex (ca. 20 mg) layered with diethyl ether.
NMR spectroscopic analysis of Pd0 (dba-Z)3 reveals that the
2
aryl substituent has a profound effect on the rate of Pd-olefin
exchange (evidenced by the relative sharpness of their 1H NMR
spectra), in addition to their stability in solution (e.g., in
[Pd0 (dba-4-t-Bu)3]. Yield = 75%; Mp 139−140 °C; UV (THF,
2
nm) 533 (d-d); 1H NMR 4.91 (1H, d, 3JHH = 13 Hz, H1), 4.95 (1H, d,
3JHH = 12.5 Hz, H2), 5.09 (1H, d, JHH = 13 Hz, H3), 5.30 (1H, d,
3
CDCl3). The solid-state structures of three Pd0 (dba-
2
3JHH = 12 Hz, H4), 5.87 (1H, d, JHH = 13.5 Hz, H5), 5.91 (1H, d,
3
Z)3·solvent complexes (4-F, 4-OMe, 3,5-OMe) have been
3JHH = 13 Hz, H6), 6.12 (1H, d, 3JHH = 13 Hz, H7), 6.31 (1H, d, 3JHH
determined by single crystal X-ray diffraction methods. As with
= 13.5 Hz, H8), 6.37 (1H, d, 3JHH = 13.5 Hz, H9), 6.60 (1H, d, 3JHH
=
=
Pd0 (dba)3·CHCl3, varying degrees of dba-Z disorder were
2
13.5 Hz, H10), 6.65 (1H, d, 3JHH = 13.5 Hz, H11), 6.74 (1H, d, 3JHH
observed, where the Z-substituent affects the distribution of
isomers produced, an observation especially noticeable in the
solid-state.
13.5 Hz, H12), 6.39−6.54 (ca. 6H, br m, overlapping with olefin
protons H9−H10), 6.93−7.21 (ca. 12H, br m). Note: t-Bu protons
overlapping with free ligand at δ 1.34; ESI-MS: m/z 821.1 [Pd(dba-4-
t-Bu)2 + Na]+ (6), 929.1 [Pd2(dba-4-t-Bu)2Na]+ (58); Anal. calcd for
C75H90O3Pd2 (1252.36), C 71.93, H 7.24; Found, C 72.31, H 6.77.
Crystals were grown from a CH2Cl2 solution of the complex (ca. 20
mg) layered with diethyl ether, however these gave poor quality X-ray
diffraction data (a cif file is available from the authors).
EXPERIMENTAL SECTION
■
General details for this section (NMR, MS, IR, UV−vis, X-ray,
elemental analysis, TEM) are detailed in the SI. Important synthetic
procedures and characterization data are presented below. Deuterium-
labeled Pd0 (D(2,4)-dba-PhD5)3·D(2,4)-dba-PhD5, Pd0 (D(1,5)-dba-
[Pd0 (dba-4-OMe)3]. Yield = 72%; Mp 141−143 °C; UV (THF,
2
1
3
2
2
nm) 533 (d−d); H NMR 4.78 (1H, d, JHH = 12.5 Hz, H1), 4.79
(1H, d, 3JHH = 12 Hz, H2), 4.91 (1H, d, 3JHH = 13 Hz, H3), 5.24 (1H,
d, 3JHH = 12 Hz, H4), 5.84 (1H, d, 3JHH = 12.5 Hz, H5), 5.85 (1H, d,
PhD5)3·D(1,5)-dba-PhD5, and Pd0 (dba-PhD5)3·dba-PhD5 complexes
2
have been previously reported.18 Details of the 13C-labeled dba
compounds, and all other dba-Z ligands, are given in the SI (including
representative NMR spectra).
3JHH = 12.5 Hz, H6), 5.99 (1H, d, JHH = 13 Hz, H7), 6.16 (1H, d,
3
3JHH = 13 Hz, H8), 6.33 (1H, d, JHH = 13 Hz, H9), ca. 6.1−7.5 (ca.
3
General Synthesis of Pd2(dba)3·L and Pd2(dba-Z)3·L (L = dba/
dba-Z or solvate). Procedure A: NaCl (3.28 g, 56 mmol) was added
to a solution of PdCl2 (4.97 g, 28 mmol) in methanol (140 mL) and
stirred at ambient temperature under an inert atmosphere for 24 h.
The dark-brown solution was then filtered through a plug of cotton
wool and concentrated in vacuo to approximately one-half of its
original volume. The solution was warmed to 60 °C, and then dba (88
mmol, 3.1 equiv) was added (or dba-Z). The resulting mixture was
stirred at 60 °C for 15 min, and then sodium acetate (13.78 g, 0.168
mol, 6 equiv) was added. The mixture was allowed to cool to ambient
temperature (with no external cooling) and stirred for 2 h until a dark-
red/purple precipitate was observed, which was filtered and washed
with methanol (2 × 100 mL), water (2 × 50 mL), and finally acetone
(2 × 5 mL). The product was partially dried under suction. The solid
was transferred to a Schlenk flask and slowly dried by passage of
nitrogen gas (with stirring) overnight. This gave the desired complexes
as maroon/purple microcrystalline solids. These reactions work on a
scale as low as ca. 0.1 g PdCl2, in addition to using aliquots from stock
solutions of Na2PdCl4 in methanol.
27H, br m, overlapping with olefin protons H10−H12). Note:
Overlapping methoxy signals at δ 3.79, 3.80, and 3.82 (broad signals);
ESI-MS: m/z (%) = 716.9 [Pd(dba-4-OMe)2 + Na]+ (6), 824.9
[Pd2(dba-4-OMe)2Na]+ (66), 929.9 [Pd3(dba-4-OMe)2Na]+ (61),
1035.8 [Pd4(dba-4-OMe)2Na]+ (84); Anal. calcd for C57H54O9Pd2
(1095.87), C 62.47, H 4.97; Found, C 62.03, H 4.95. Crystals suitable
for X-ray analysis were grown from a CH2Cl2 solution of the complex
(ca. 20 mg) layered with diethyl ether.
[Pd0 (dba-4-OHexyl)3·dba-4-OHexyl]. Yield = 70%; Mp 104−107
2
1
3
°C; UV (CDCl3, nm) 540 (d−d); H NMR 4.79 (1H, d, JHH = 12.5
Hz, H1), 4.80 (1H, d, 3JHH = 12 Hz, H2), 4.93 (1H, d, JHH = 13 Hz,
3
3
3
H3), 5.25 (1H, d, JHH = 12 Hz, H4), 5.85 (1H, d, JHH = 12.5 Hz,
H5), 5.87 (1H, d, 3JHH = 13 Hz, H6), 6.01 (1H, d, 3JHH = 13 Hz, H7),
3
3
6.18 (1H, d, JHH = 13.5 Hz, H8), 6.35 (1H, d, JHH = 13.5 Hz, H9),
6.41−6.80 (ca. 27H, br m, overlapping with olefin protons H10−
H12). Note: Overlapping alkyl protons with free ligand at 0.92, 1.35,
1.48, 1.80, and 3.98 (broad signals). LIFDI-MS: m/z (%) = 1516.9
[Pd2(dba-4-OHexyl)3]•+ (100); 1516.66 (calcd); Anal. calcd for
C116H152O12Pd2 (1515.66), C 71.4, H 7.85; Found, C 71.61, H 7.76.
Procedure B: This procedure is similar to that reported by Ishii and
[Pd0 (dba-4-CF3)3·dba-4-CF3·H2O]. Yield = 69%; Mp 151−152 °C;
co-workers34 (PdCl2, dba or dba-Z, NaOAc in MeOH at 40−50 °C)
2
and by our group16 for Pd0 (thn-dba)3·thn-dba derivatives.
UV (THF, nm) 534 (d−d); 1H NMR 4.95 (1H, d, 3JHH = 13 Hz, H1),
2
3
3
5.02 (1H, d, JHH = 12.5 Hz, H2), 5.19 (1H, d, JHH = 13 Hz, H3),
Characterization data for Pd2(dba)3·CHCl3: Anal. calcd for
C52H43O3Pd2Cl3 (1135.09), C 60.34, H 4.19; Found, C 60.25, H
4.3. Single crystals of this pure material were grown from a saturated
CHCl3 solution layered with hexane, which gave purple rod-like
crystals, from which one single crystal was selected for X-ray
diffraction. Also noted were the formation of hexagon crystals,
3
3
5.39 (1H, d, JHH = 12.5 Hz, H4), 5.88 (1H, d, JHH = 13 Hz, H5),
5.95 (1H, d, 3JHH = 13 Hz, H6), 6.12 (1H, d, 3JHH = 13 Hz, H7), 6.35
(1H, d, 3JHH = 13.5 Hz, H8), 6.45 (1H, d, 3JHH = 13.5 Hz, H9), 6.41−
6.80 (ca. 15H, br m, overlapping with olefin protons H10−H12),
7.27−7.58 (ca. 12H, br m); ESI-MS: m/z (%) = 846 [Pd1(dba-4-
J
dx.doi.org/10.1021/ja403259c | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
CF3)2]+ (10), 975 [Pd2(dba-4-CF3)2Na]+ (15); Anal. calcd for
C36H26F12O3Pd (865.01), C 52.76, H 3.03; Found, C 52.72, H 2.98.
for providing the X-ray cif file for Pd0 (dba-4,4′-OH)3. Prof.
2
Cortlant Pierpont is thanked for sharing his X-ray diffraction
[Pd0 (dba-3,5-OMe)3·dba-3,5-OMe]. Due to the broadness of the
data analysis on Pd0 (dba)3. The Royal Society is thanked for
2
2
proton signals the proton integrals are not reliable; selected data is
therefore quoted (i.e., the peaks observed). The observed spin−spin
coupling constants are averaged because of the exchange on the NMR
time scale (due to intra- and intermolecular exchange). Yield = 53%;
Mp 134−135 °C; UV (THF, nm) 530 (d−d); 1H NMR 4.94 (1H, d,
12 Hz, H1), 5.04 (2H, d, 12 Hz, H2/H3), 5.29 (2H, d, 12 Hz, H4/
H5), 5.78 (1H, d, 12 Hz, H6), ca. 6.2−7.5 (ca. 24 H, overlapping with
olefin protons H7−H12). Note: Overlapping methoxy signals at δ
3.57−3.77 (broad signals). The free ligand exhibits a methoxy signal at
3.84; ESI-MS: m/z (%) = 838.9 [Pd1(dba-3,5-OMe)2 + Na]+ (14),
944.9 [Pd2(dba-3,5-OMe)2Na]+ (100); Anal. calcd for C42H44O10Pd
(815.21), C 61.88, H 5.44; Found, C 62.55, H 5.19. Crystals suitable
for X-ray analysis were grown from a CH2Cl2 solution of the complex
(ca. 20 mg) layered with diethyl ether.
funding a University Research Fellowship (I.J.S.F.). The
research leading to these results has received funding from
the Innovative Medicines Initiative Joint Undertaking under
grant agreement no. 115360, resources of which are composed
of financial contribution from the European Union’s Seventh
Framework Programme (FP7/2007-2013) and EFPIA compa-
nies’ in kind contribution (PhD studentship for A.J.R.). We
gratefully acknowledge use of the Esquire ESI-MS instrument
in the York Centre of Excellence in Mass Spectrometry, which
was created thanks to a major capital investment through
Science City York, supported by Yorkshire Forward with funds
from the Northern Way Initiative. We thank EPSRC for
funding (PhD funding for T.J.W., grant code EP/P505178/1;
Computational Cluster, grant code EP/H011455/1).
[Pd0 (dba-3,4,5-OMe)3]. Due to the broadness of the proton
2
signals, the proton integrals are not reliable; selected data is therefore
quoted (i.e., the peaks observed). Yield = 59%; Mp 194−197 °C; UV
1
(THF, nm) 529 (d−d); H NMR 4.92 (2H, d, 11 Hz, H1/H2), 5.05
REFERENCES
■
(2H, d, 11 Hz, H3/H4), 5.21−5.31 (broad signal), 5.30−5.42 (broad
signal), 5.82−5.93 (broad signal), 6.02−6.10 (broad signal), 6.17−6.55
(broad signal), 6.65−6.76 (broad signal). Note: Overlapping methoxy
signals at 3.62, 3.68, 3.80, and 3.83 (broad signals). The liberated
ligand exhibits methoxy signals at 3.90 and 3.92; Anal. calcd for
C69H78O21Pd2 (1456.19), C 56.91, H 5.40; Found, C 57.31, H 5.46.
Details of DFT Calculations. All calculations were performed
using the TURBOMOLE V5.10 package using the resolution of
identity (RI) approximation.38 Initial optimizations were performed at
the (RI-)BP86/SV(P) level, followed by frequency calculations at the
same level. All minima were confirmed as such by the absence of
imaginary frequencies. Energies, geometries, and vibrational frequen-
cies are presented in the SI. Single-point calculations on the
(RI‑)BP86/SV(P) optimized geometries were performed using the
hybrid PBE0 functional and the flexible def2-TZVPP basis set. The
(RI‑)PBE0/def2-TZVPP SCF energies were corrected for their zero
point energies (ΔE), thermal energies, and entropies (obtained from
the (RI-)BP86/SV(P)-level frequency calculations at 298.15 K,
ΔG298.15). In all calculations, a 28 electron quasi-relativistic ECP
replaced the core electrons of Pd. No symmetry constraints were
applied during optimizations.
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ASSOCIATED CONTENT
* Supporting Information
■
S
All other synthetic procedures, characterization data (repre-
sentative spectra), details, and X-ray diffraction details. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
ian.fairlamb@york.ac.uk
■
(8) Ueda, S.; Su, M.; Buchwald, S. L. J. Am. Chem. Soc. 2012, 134,
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Present Address
†Department of Chemistry, Institute of Chemical Technology,
Matunga, Mumbai-400019, India
Notes
The authors declare no competing financial interest.
́
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ACKNOWLEDGMENTS
■
Organometallics 2006, 25, 1795−1800. (c) Fairlamb, I. J. S.; Kapdi, A.
R.; Lee, A. F.; McGlacken, G. P.; Weissburger, F.; de Vries, A. H. M.;
Schmieder-van de Vondervoort, L. Chem.Eur. J. 2006, 12, 8750−
We are grateful to Heather Fish, Prof. Simon Duckett, and Dr.
Ryan Mewis for assistance with several NMR experiments. Ms.
Qiong Xu and Mr. Josh Bray are thanked for their contributions
8761. For related work with ionic chalcones, see: (d) Bauerlein, P. S.;
̈
to this project (particularly on the stability of Pd0 (dba)3 and
2
Fairlamb, I. J. S.; Jarvis, A. G.; Lee, A. F.; Muller, C.; Slattery, J. M.;
̈
Pd0 (dba-4-Ohexyl)3 complexes). Dr. Petr Sehnal is thanked
Thatcher, R. J.; Vogt, D.; Whitwood, A. C. Chem. Commun. 2009,
2
for crystallizing dba-3,5-OMe. We thank Prof. Felix Goodson
5734−5736.
K
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2
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L
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