Detection and characterization of mononuclear Pd(I) complexes supported by N2S2 and N4 tetradentate ligands

Article abstract of DOI:10.1021/acs.inorgchem.0c01938

Palladium is a versatile transition metal used to catalyze a large number of chemical transformations, largely due to its ability to access various oxidation states (0, I, II, III, and IV). Among these oxidation states, Pd(I) is arguably the least studied, and while dinuclear Pd(I) complexes are more common, mononuclear Pd(I) species are very rare. Reported herein are spectroscopic studies of a series of Pd(I) intermediates generated by the chemical reduction at low temperatures of Pd(II) precursors supported by the tetradentate ligands 2,11-dithia[3.3](2,6)pyridinophane (N2S2) and N,N′-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane (^tBuN4): [(N2S2)Pd^II(MeCN)]₂(OTf)₄ (1), [(N2S2)-Pd^IIMe]₂(OTf)₂ (2), [(N2S2)Pd^IICl](OTf) (3), [(N2S2)Pd^IIX](OTf)₂ (X = tBuNC 4, PPh₃ 5), [(N2S2)Pd^IIMe(PPh₃)](OTf) (6), and [(^tBuN4)Pd^IIX₂](OTf)₂ (X = MeCN 8, tBuNC 9). In addition, a stable Pd(I) dinuclear species, [(N2S2)Pd^I(μtBuNC)]₂(ClO₄)₂ (7), was isolated upon the electrochemical reduction of 4 and structurally characterized. Moreover, the (^tBuN4)Pd^I intermediates, formed from the chemical reduction of [(^tBuN4)Pd^IIX₂](OTf)₂ (X = MeCN 8, tBuNC 9) complexes, were investigated by EPR spectroscopy, X-ray absorption spectroscopy (XAS), and DFT calculations and compared with the analogous (N2S2)Pd^I systems. Upon probing the stability of Pd(I) species under different ligand environments, it is apparent that the presence of soft ligands such as tBuNC and PPh₃ significantly improves the stability of Pd(I) species, which should make the isolation of mononuclear Pd(I) species possible.

Full text of DOI:10.1021/acs.inorgchem.0c01938

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Detection and Characterization of Mononuclear Pd(I) Complexes
Supported by N2S2 and N4 Tetradentate Ligands
Jia Luo, Giang N. Tran, Nigam P. Rath, and Liviu M. Mirica*
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ABSTRACT: Palladium is a versatile transition metal used to
catalyze a large number of chemical transformations, largely due to
its ability to access various oxidation states (0, I, II, III, and IV).
Among these oxidation states, Pd(I) is arguably the least studied, and
while dinuclear Pd(I) complexes are more common, mononuclear
Pd(I) species are very rare. Reported herein are spectroscopic studies
of a series of Pd(I) intermediates generated by the chemical
reduction at low temperatures of Pd(II) precursors supported by the tetradentate ligands 2,11-dithia[3.3](2,6)pyridinophane
(N2S2) and N,N′-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane (tBuN4): [(N2S2)PdII(MeCN)]2(OTf)4 (1), [(N2S2)-
PdIIMe]2(OTf)2 (2), [(N2S2)PdIICl](OTf) (3), [(N2S2)PdIIX](OTf)2 (X = tBuNC 4, PPh3 5), [(N2S2)PdIIMe(PPh3)](OTf)
(6), and [(tBuN4)PdIIX2](OTf)2 (X = MeCN 8, tBuNC 9). In addition, a stable Pd(I) dinuclear species, [(N2S2)PdI(μ-
tBuNC)]2(ClO4)2 (7), was isolated upon the electrochemical reduction of 4 and structurally characterized. Moreover, the
(
tBuN4)PdI intermediates, formed from the chemical reduction of [(tBuN4)PdIIX2](OTf)2 (X = MeCN 8, tBuNC 9) complexes, were
investigated by EPR spectroscopy, X-ray absorption spectroscopy (XAS), and DFT calculations and compared with the analogous
(N2S2)PdI systems. Upon probing the stability of Pd(I) species under different ligand environments, it is apparent that the presence
of soft ligands such as tBuNC and PPh3 significantly improves the stability of Pd(I) species, which should make the isolation of
mononuclear Pd(I) species possible.
investigation of mononuclear Pd(I) complexes. In this regard,
reported herein are spectroscopic studies of a series of Pd(I)
intermediates generated by the chemical reduction of Pd(II)
precursors supported by the tetradentate ligands 2,11-
dithia[3.3](2,6)pyridinophane (N2S2) and N,N′-di-tert-butyl-
2,11-diaza[3.3](2,6)pyridinophane (tBuN4): [(N2S2)-
PdII(MeCN)]2(OTf)4 (1), [(N2S2)PdIIMe]2(OTf)2 (2),
[(N2S2)PdIICl](OTf) (3), [(N2S2)PdIIX](OTf)2 (X =
tBuNC 4, PPh3 5), [(N2S2)PdIIMe(PPh3)](OTf) (6), and
[(tBuN4)PdIIX2](OTf)2 (X = MeCN 8, tBuNC 9, Scheme 1).
In addition, a stable Pd(I) dinuclear species, [(N2S2)PdI(μ-
tBuNC)]2(ClO4)2 (7), was isolated upon the electrochemical
reduction of 4. Moreover, the (tBuN4)PdI intermediates,
formed from the chemical reduction of [(tBuN4)PdIIX2](OTf)2
(X = MeCN 8, tBuNC 9, Scheme 2) complexes, were
investigated by EPR spectroscopy, X-ray absorption spectros-
copy (XAS), and DFT calculations and compared with the
analogous (N2S2)PdI systems. Upon probing the stability of
Pd(I) species under various ligand environments (N2S2 and
INTRODUCTION
■
Palladium is a versatile transition metal used to catalyze a large
number of chemical transformations, largely due to its ability
to access various oxidation states (0, I, II, III, and IV). The
Pd(0) and Pd(II) systems are by far the most extensively
studied for catalytic applications,1,2 and the chemistry of high-
valent Pd(III) and Pd(IV) complexes has also received
significant attention in the past few years.3−11 By comparison,
the reports describing Pd(I) complexes are far less, although
Pd(I) species have been proposed to play important roles in
radical atom transfer and cross-coupling reactions.12−22 While
several reports describe the detection of transient mononuclear
Pd(I) species,23−28 only one structurally characterized
mononuclear Pd(I) cationic species [PdI(tBu3P)2]+ has been
reported independently by two research groups;29,30 however,
no further reactivity studies were reported for this complex,
even though it decomposes in solution at room temperature.
An earlier report also described an isolated mononuclear Pd(I)
complex, although the presence of a redox noninnocent ligand
complicates the unambiguous Pd center oxidation state
assignment.31 In addition, most of the isolated Pd(I)
complexes reported to date are dinuclear Pd(I) species
containing a Pd(I)−Pd(I) bond and thus are closed-shell
systems.
Received: June 30, 2020
Given our long-term interest in developing catalytic
transformations employing odd-electron Pd(I) and Pd(III)
species,32−38 we are actively targeting the synthesis and
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Scheme 1. Synthesis of (N2S2)PdII and (N2S2)PdI Complexes and Proposed Decomposition
Scheme 2. Synthesis of (tBuN4)PdII and (tBuN4)PdI Complexes and Proposed Dimerization of 8 in Solution
Figure 1. ORTEP representations of the dications of 4, 5, and 6, shown with 50% thermal ellipsoids and with the triflate counteranions and H
atoms omitted for clarity. Selected bond distances (Å) for 4: Pd1−C15 1.935(9), Pd1−S1 2.349(3), Pd1−S2 2.336(2), Pd1−N1 2.041(7), Pd1···
N2 2.573(7), N3−C15 1.157(10); for 5: Pd1−P1 2.2670(9), Pd1−S1 2.3256(9), Pd1−S2 2.3344(9), Pd1−N2 2.082(3), Pd1−N1 2.459(3); and
for 6: Pd1−N1 2.1662(14), Pd1−N2 2.1492(13), Pd1−C1 2.0466(15), Pd1−P1 2.2495(4).
B
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tBuN4), it is apparent that the presence of soft ligands such as
tBuNC and PPh3 significantly improves the stability of Pd(I)
species, which should make the isolation of mononuclear Pd(I)
species possible.
interacting at all) with the Pd center in the axial position.
Such a conformation change process could be facilitated by the
presence of a coordinating solvent, as seen previously.39 The
addition of PPh3 to a MeCN solution of 2 gives [(N2S2)-
PdIIMe(PPh3)](OTf) (6), isolated as almost-colorless crystals.
The X-ray crystal structure of 6 reveals a square-planar
coordination at the Pd(II) center, with cis-positioned methyl
and PPh3 groups, as well as a κ2 conformation for N2S2
(Figure 1).35,41,42 The bond lengths of Pd−C (2.047 Å) and
Pd−Nave (2.158 Å) are similar to (N2S2)PdIIMe2, and the Pd−
P bond distance (2.250 Å) is also comparable to other
reported values.43,44 This conformation change of N2S2 from
κ3 to κ2 in the presence of PPh3 in 6 further confirms the
possibility of a conformational change observed in the UV−vis
spectra of 5.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Pd(II) Precursors.
The series of complexes [(N2S2)PdII(MeCN)]2(OTf)4 (1),
[(N2S2)PdIIMe]2(OTf)2 (2), and [(N2S2)PdIICl](OTf) (3)
were synthesized by following the previously reported
procedures.37,39 The addition of 2 equiv tBuNC or PPh3 to
a MeCN solution of 1 generated complexes [(N2S2)-
PdII(tBuNC)](OTf)2 (4) and [(N2S2)PdII(PPh3)](OTf)2
(5) complexes, respectively, which were isolated as red crystals
(Scheme 1). The X-ray crystal structures of 4 and 5 reveal that
Pd(II) centers are in a distorted square pyramidal geometry
with both tBuNC and PPh3 ligands replacing the MeCN
molecule in the equatorial position, while the N2S2 ligand
Generation and Characterization of (N2S2)PdI Inter-
mediates. Cyclic voltammetry (CV) studies were performed
at room temperature for the (N2S2)PdII complexes to show
that complexes 1−6 exhibit reduction potentials of −0.40 to
−1.35 V vs Fc/Fc+ in MeCN (Figures S1−S6), suggesting that
the corresponding Pd(I) species should be accessible upon
reduction with a chemical reductant. Therefore, these Pd(II)
complexes were reacted with 1 equiv of the appropriate
reducing agents in MeCN at −35 °C to generate the
corresponding Pd(I) intermediates: CoCp2 was used for
complexes 1−5, and CoCp*2 was used for 6. Excitingly, all
reaction mixtures of the reduction of complexes 1−6 exhibit
axial or pseudoaxial signals in their EPR spectra, suggesting the
presence of odd-electron species (Figure 3). Based on the
simulation of the EPR data, similar superhyperfine coupling
constants to the axial pyridine N atom in the gz region were
detected for the [(N2S2)PdIX]n+ (X = Me 2-PdI, Cl 3-PdI, n =
remains in the unusual κ3 conformation (Figure 1).39
A
significant intramolecular π−π interaction between two
pyridine rings was observed, with the pyridine ring centroid-
centroid interplanar distance of 3.44 and 3.65 Å for 4 and 5,
respectively.40 However, in contrast to the dinuclear complexes
1 and 2, the Pd−Pd d8−d8 interactions in 4 and 5 were no
longer observed in the solid state, likely due to the steric effect
of the tBuNC and PPh3 groups. The Pd−Save bond lengths of
2.343 Å for 4 and 2.330 Å for 5 are comparable to those of 1
and 2, while the tilt angle of the pyridine ring vs the Pd−Nax
bond in 5 of 52.8° is smaller than that in 4 of 54.9°, consistent
with the shorter bond length of Pd−Nax (2.459 Å) in 5 than
that of 4 (2.573 Å). Interestingly, the UV−vis spectrum of a
freshly prepared MeCN solution of 5 (0.9 mM) exhibits an
absorption band at 503 nm, which slowly decays over the
course of several hours (Figure 2), and a diluted solution of 5
0; X = MeCN 1-PdI, tBuNC 4-PdI, n = 1) complexes,
1
2
2
suggesting a (dx −y
)
ground state for the Pd(I) centers
(Figure 3 and Table 1), which should still adopt a square-
pyramidal geometry with the N2S2 ligand in the κ3
conformation. The slightly rhombic EPR spectrum of 2-PdI
could indicate that there is a slight distortion in the equatorial
plane. By comparison, the EPR spectrum of the [(N2S2)-
PdI(PPh3)]+ species 5-PdI shows no discernible superhyperfine
coupling constants to the N atom(s), implying that upon
reduction to Pd(I) the coordination environment has changed
from square-pyramidal to a trigonal geometry, with N2S2
ligand adopting a κ2 conformation and the two N atom donors
and the P atom binding in the equatorial plane, while the two S
atoms are likely no longer coordinated to the Pd center.
Similarly, no superhyperfine coupling to the N or P atoms was
observed for the (N2S2)PdIMe(PPh3) species 6-PdI, in
agreement with the κ2 conformation of N2S2 in 6. Overall,
all observed g values are consistent with previously reported
Pd(I) d9 species with the gave > 2.0023,23−28 confirming that
the unpaired electron resides on the Pd center rather than on
the ligand(s).31
These Pd(I) complexes exhibit limited thermal stability and
decompose with different rates even at low temperatures (for
example, see Figure S9).45 Spin integration of the EPR spectra
of these Pd(I) species against a standard reveals that they form
in yields from 5% to 98%, following the general trend that the
presence of soft ligands such as tBuNC and Ph3P further
stabilizes the Pd(I) species, while formation of dinuclear,
diamagnetic Pd(I) complexes can further diminish the
observed EPR signal (vide infra). Moreover, some of these
transient Pd(I) species 1-PdI to 9-PdI seem to persist for
Figure 2. UV−vis spectra of 5 in MeCN. Inset: zoomed-in UV−vis
spectrum in the 400−800 nm range for a freshly prepared 0.09 mM
MeCN solution of 5.
(0.09 mM) does not exhibit the 503 nm absorption band. The
change of the UV−vis spectrum of 5 in solution suggests that
the binding mode of the N2S2 ligand could change from a κ3
conformation to a κ2 conformation, with the PPh3 ligand and
the two N donor atoms positioned in the equatorial plane,
while the two S atoms are weakly interacting (or not
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Figure 3. Experimental and simulated EPR spectra of reaction mixtures of (N2S2)PdII complexes and reducing agents at 77 K in 1:3 MeCN:PrCN
frozen glass: (a−e) 1−5 + CoCp2; (f) 6 + CoCp*2.
Table 1. Parameters Used for the Simulation of EPR Spectra of Pd(I) Species and Their Yields
g values
superhyperfine coupling (G)
b
species
gx
gy
gz
A(x,x)
A(y,y)
A(z,z)
yield (%)
% EPR signal left at RT after given time
a
1-PdI
2-PdI
3-PdI
4-PdI
5-PdI
6-PdI
8-PdI
9-PdI
2.0023
1.9983
2.0023
2.0023
1.9986
2.0019
1.9965
1.9982
2.0023
2.0085
2.0023
2.0023
2.0023
2.0032
2.0063
2.0080
2.0870
2.0770
2.0857
2.0820
2.0993
2.0713
2.0853
2.0810
ND
ND
ND
ND
ND
ND
ND
ND
ND
A1N = 19.0
A1N = 14.5
A1N = 16.0
A1N = 19.0
ND
ND
ND
ND
26
80
23
12
90
77
5
25% after 30 min
40% after 30 min
23% after 30 min
11% after 1 min
60% after 30 min
20% after 5 min
10% after 1 min
25% after 5 min
ND
ND
ND
ND
ND
ND
ND
98
a
b
ND = not detected. Determined by spin integration of the EPR signal against a standard.
Figure 4. (a) UV−vis spectra of the reduction of 4 (0.12 mM, black dotted line) with 1 equiv CoCp2 in MeCN at −35 °C: (thick blue solid line)
spectrum at 1 min after adding CoCp2; (thick green dashed line) spectrum after 5 min; (thin solid lines) time interval = 0.4 min. (b) Kinetic fitting
of the absorbance decay at 452 nm. Squares: 0.8−1.4 min; circles: 1.4−6 min; solid line: first-order decay fitting with the rate constant k = 0.656
min−1 (R2 = 0.998).
D
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different lengths of time at room temperature (Table 1),
lending support to our continuing efforts to isolate these Pd(I)
complexes, although so far we could not crystallize these
species.
To further investigate the reactivity of these Pd(I)
complexes, 1H NMR was employed to monitor at room
temperature the reduction of 2 and 6 by CoCp2 and CoCp*2,
respectively (Figures S10 and S11), and to determine the
nature of the final products. These results reveal that except for
the formation of a small amount of (N2S2)PdIIMe2 (<10%),
likely formed upon ligand exchange,37,39 the majority of the
products are Pd(0) black, free N2S2 ligand, and small amounts
of unidentified decomposition products,45 suggesting that
these Pd(I) intermediates decompose by disproportionation to
(N2S2)Pd(II) species and Pd(0) and free N2S2 ligand
(Scheme 1) along with other ligand exchange reactions.
Similar decomposition pathways have been observed pre-
viously by us for both (RN4)Pd and (N2S2)Pd systems.32−39
Moreover, the only reported isolated mononuclear Pd(I)
complex [PdI(tBu3P)2]+ was also shown to undergo unin-
dentified decomposition pathways.30
We have also monitored the chemical reduction of the
(N2S2)PdII complexes by UV−vis spectroscopy. The UV−vis
spectra of the reaction mixture of 4 and CoCp2 in MeCN at
−35 °C exhibit an absorption band at 452 nm (Figure 4),
suggesting the formation of a Pd(I) intermediate 4-PdI, which
was further confirmed by the corresponding EPR spectrum
(Figure 3d). Both UV−vis and EPR spectra show that the
corresponding signals of the 4-PdI species decrease within
several minutes.45 Monitoring of the decay of the absorbance
band at 452 nm implies reveals a a first-order decay process
with a rate constant of 0.656 min−1, and this instability of the
4-PdI complex at −35 °C prevented any isolation attempts.
Interestingly, the bulk electrolysis reduction of 4 under an
inert atmosphere at room temperature yields a robust
diamagnetic Pd(I) dinuclear complex [(N2S2)PdI(μ-
tBuNC)]2(ClO4)2 (7), which was isolated as orange crystals.
The X-ray crystal structure of 7 shows that each Pd(I) center
adopts a pseudo-square-pyramidal geometry, with one N2S2
ligand binding in a κ3 comformation, two bridging tBuNC
ligands, and one Pd(I)−Pd(I) bond: the two N donor atoms
and the two tBuNC ligands define the equatorial plane, while
one S donor atom weakly interacts in the axial position (Figure
5). To the best of our knowledge, there is only one other
tBuNC bridged complex that has been reported to date:
[(tBuNC)2CuI2(dppm)2(μ-CNtBu)](BF4)2.46 The bond
length of CN of the bridged tBuNC group in 7 is 1.162 Å,
slightly longer than the reported bridged C−N bond length
(1.141 Å) in the Cu(I) complex but significantly shorter than
the average bond distance of a CN double bond (1.279
Å).47 Given that this C−N bond length it is also comparable to
that of the terminal tBuNC ligand in 4 (1.157 Å) and the other
reported complexes with terminal tBuNC groups,48,49 it
suggests that the bonding interaction between the tBuNC
ligand and the Pd center should not be defined as a “PdII−
(tBuNC−)” system containing a redox noninnocent tBuNC
ligand. Therefore, the observed diamagnetism of 7 is likely due
to the antiferromagnetic coupling between the two Pd(I)
centers, with the bond distance of Pd(I)−Pd(I) (2.7416 Å)
comparable to but slightly longer (0.04−0.14 Å) than other
reported Pd(I)−Pd(I) bonding interactions.16,50,51 While the
Pd−Nave bond length is comparable to other (κ2-N2S2)PdII
complexes, the Pd−S bond (2.6588 Å) is significantly longer
Figure 5. ORTEP representation of the dication in 7, shown with
50% thermal ellipsoids and the perchlorate counteranions and H
atoms omitted for clarity. Selected bond distances (Å): Pd1−N1
2.220(2), Pd1−N2 2.217(2), Pd1−C15 1.994(3), Pd1−C15_i
2.076(3), Pd1−S1 2.6588(8), Pd1−Pd1_i 2.7416(5), N3−C15
1.162(4).
than that in 4, likely due to the conformation change of N2S2
between 4 in which the two S atoms interact in the equatorial
plane, while in 7 one S atom occupies an axial position with an
elongated bonding interaction. In addition, the Pd−Cave bond
distance (2.035 Å) is slightly longer than that in 4 as a result of
the bridging ligation of tBuNC. Because the dinuclear Pd(I)
complex 7 was formed upon the reduction of the mononuclear
Pd(II) complex 4, we wanted to probe whether the formation
of 7 involves the transient formation of the 4-PdI species at
room temperature; however the analysis of the electrolysis
solution by EPR at different time points did not reveal the
presence of paramagnetic species, suggesting that on the time
scale of the electroreduction process at room temperatute any
4-PdI species formed will dimerize rapidly to form 7. This can
also explain the low 12% yield obtained by spin integration of
the EPR signal for the in situ formed 4-PdI species at −78 °C
(Table 1).
Characterization of (tBuN4)PdI Species. Because the
tBuN4 ligand was shown to stabilize paramagnetic Pd(III)
complexes,32 we also wanted to probe the possibility of
employing such a ligand system for stabilizing Pd(I) species as
a means to support a potential Pd(I)/Pd(III) catalytic cycle for
chemical transformations. The [(tBuN4)PdII(tBuNC)2](OTf)2
complex (9) was obtained as pink crystals by reacting
[(tBuN4)PdII(MeCN)2](OTf)2 (8)52 with 2 equiv of tBuNC
in MeCN (Scheme 2). The X-ray crystal structure reveals that
the Pd(II) center has a distorted octahedral coordination
geometry, similar to the structure of 8:52 the tBuN4 ligand
adopts a κ4 conformation with an axial elongation for the two
amine N donor atoms, while the two tBuNC ligands are cis-
positioned in the equatorial place along with the two pyridine
N donor atoms (Figure 6). The average Pd−Npy bond length
(2.061 Å) and the average Pd···NtBu distance (2.612 Å) are
consistent with the previously reported values for 8,52 while the
average Pd−CNtBu distance (1.950 Å) is slightly shorter that
the average Pd−NCMe distance in 8, likely due to the stronger
donating ability of tBuNC vs MeCN.
Upon the chemical reduction of 8 and 9 by 1 equiv of
CoCp2 at −35 °C in MeCN:PrCN (1:3, v:v), the [(tBuN4)-
PdIX2]+ species (X = MeCN, 8-PdI; X = tBuNC, 9-PdI) were
observed by EPR, although the EPR signal for 8-PdI was quite
faint (Figure 7 and Table 1). The axial pattern of the EPR
1
2
2
spectra suggests that both Pd(I) centers exhibit a (dx −y
)
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oxidation state for a systematic series of Pd complexes.53,54
Because the EPR spectrum of 8-PdI reveals a weak signal
(corresponding to a 5% yield based on spin integration, Table
1), to further probe the presence of a Pd(I) center, we have
obtained XAS spectra for MeCN solutions of 8-PdI generated
upon one-electron reduction at low temperature, the Pd(I)
reference compound [PdI2(MeCN)6]2+,55 and the Pd(II)
precursor [(tBuN4)PdII(MeCN)2]2+ (8) and the Pd(III)
species [(tBuN4)PdIII(MeCN)2]3+ formed upon electrochem-
ical oxidation of 8 (Figure 8).52 Interestingly, an increase in the
edge energy is observed with increasing oxidation state of the
Pd center (24352, 24354, and 24355.5 eV for Pd(I), Pd(II),
and Pd(III), respectively), confirming that in this case the XAS
edge energy correlates with the formal oxidation state
assignment. Moreover, the edge energy of 8-PdI matches
very closely the edge energy observed for [PdI2(MeCN)6]2+,
supporting the formation of either mononuclear or dinuclear
Pd(I) species upon the reduction of 8. The presence of a weak
EPR signal for 8-PdI could be also due to the formation of a
dinuclear Pd(I) species either containing a Pd−Pd bond
commonly observed for Pd(I) complexesor in which the two
Pd(I) centers are antiferromagnetically coupled through
bridging ligand(s), similar to 7 (vide supra). It is important
to note that these XAS studies were employed mainly to probe
the overall formal oxidation state of the resulting Pd species
upon the one-electron reduction of 8, and the results strongly
suggest the formation of Pd(I) species, without subsequent
disproportionation or oxidation of these Pd(I) species. While
these studies do not provide structural information for the
various Pd(I) species formed in situ, they lead us to suggest
that one approach to further stabilize mononuclear Pd(I)
species and to disfavor dimerization could be the use of
bulkier, sterically hindered ancillary or exogenous ligands.
Figure 6. ORTEP representation of the cation of 9, shown with 50%
thermal ellipsoids and the triflate counteranions and H atoms omitted
for clarity. Selected bond distances (Å) for 9: Pd1−N1 2.063(2),
Pd1−N2 2.059(2), Pd1−C23 1.946(3), Pd1−C28 1.954(3), Pd1···
N3 2.610, Pd1···N4 2.614.
ground state, and the absence of N superhyperfine coupling
from the tBuN groups to the Pd(I) center implies that upon the
one-electron reduction the interaction with the axial N atoms
is no longer present; that is, the conformation of the tBuN4
ligand might change from κ4 to κ2. To further probe this
conformational change, we have performed DFT calculations
for the 4-PdI, κ4-9-PdI, and κ2-9-PdI species and analyzed the
interactions between the axial N atoms and the Pd(I) centers
(Table 2).45 When comparing the atomic orbital contributions
to the frontier molecular orbitals of these complexes, we
obtained for 4-PdI a 5% contribution from the axial pyridine N
atom to the β lowest unoccupied molecular orbital (LUMO),
supporting the observed superhyperfine coupling to the axial N
in the EPR spectrum of 4-PdI. By comparison, for both κ4-9-
PdI, and κ2-9-PdI species in which the tBuN4 ligand adopts κ4
and κ2 conformations, respectively, no contribution to the β
LUMO from the axial N atomic orbitals was observed (Table
2), supporting the absence of any appreciable superhyperfine
coupling to the axial N atoms. In addition, the EPR signal
intensity for 9-PdI (98% yield by spin integration) is
significantly stronger than 8-PdI (5% yield, Table 1 and Figure
7), suggesting that the presence of tBuNC stabilizes to a
greater extent the Pd(I) center.
CONCLUSION
■
In summary, reported herein is a thorough spectroscopic
characterization of a series of (N2S2)PdI and (tBuN4)PdI
complexes. We have shown that the Pd(I) oxidation state is
accessible for all the studied Pd(II) complexes via either
chemical or electrochemical reduction. EPR, UV−vis, and X-
ray absorption spectroscopy provide direct evidence for the
presence of Pd(I) intermediates, for which the stability was
further improved with the assistance of a series of soft isonitrile
and phosphine ligands. The proposed structures based on the
simulations of the EPR data were further corroborated by DFT
calculations. Moreover, an uncommon diamagnetic Pd(I)−
Palladium K-edge X-ray absorption spectroscopy (XAS),
which involves excitation of Pd 1s electrons, has been shown to
be an effective qualitative means of probing the formal
Figure 7. Experimental and simulated EPR spectra of reaction mixtures of (tBuN4)PdII complexes 8 and 9 with 1 equiv of CoCp2 at 77 K in 1:3
MeCN:PrCN frozen glass: (a) 8 + CoCp2; (b) 9 + CoCp2. The parameters used for the simulations can be found in Table 1.
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Table 2. Comparison of the β Lowest Unoccupied Molecular Orbitals (LUMOs) of 4-PdI, κ4-9-PdI, and κ2-9-PdI Complexes
(ppm): 1.66 (s, 9H, tBu), 4.62 (d, J = 16.5 Hz, 4H, CH2), 5.08 (d, J =
16.5 Hz, 4H, CH2), 7.24 (d, J = 7.8 Hz, 4H, Py Hmeta), 7.62 (t, J = 8.1
Hz, 2H, Py Hpara). UV−vis (MeCN; λ, nm (ε, M−1 cm−1)): 327
(3200), 497 (200). Elemental analysis: found, C 34.06, H 2.98, N
6.40%; calculated C21H23F6N3O6PdS4·0.5CH3CN, C 33.76, H 3.16, N
6.26%. ESI-MS (m/z): 462.0314, calculated for [(N2S2)-
PdII(tBuNC)-H+]+: 462.0290.
[(N2S2)PdII(PPh3)](OTf)2, 5. 2 equiv PPh3 (10.2 mg, 39.0 μmol)
was dissolved in 2 mL of MeCN and added to an MeCN solution (2
mL) of [(N2S2)PdII(MeCN)]2(OTf)4 (28.1 mg, 19.5 μmol) while
stirring. The solution changed color from brown to red immediately.
After 1 h, the mixture was set up for crystallization with anhydrous
diethyl ether vapor diffusion at room temperature. A needle-shaped
red crystalline product formed after 1−2 days. Yield: 29.9 mg, 82%.
1H NMR (CD3CN, 300 MHz), δ (ppm): 4.42 (d, J = 16.5 Hz, 4H,
CH2), 4.58 (d, J = 16.2 Hz, 4H, CH2), 7.18 (d, J = 7.8 Hz, 4H, Py
Hmeta), 7.59 (t, J = 7.8 Hz, 2H, Py Hpara), 7.6−7.9 (m, 15H, Ph). UV−
vis (MeCN; λ, nm (ε, M−1 cm−1)): 317 (sh, 6600), 392 (sh, 1360),
503 (630). Elemental analysis: found, C 43.66, H 2.96, N 4.00%;
Figure 8. Edge region of the X-ray absorption spectra for 8-PdI (or a
calculated C34H29F6N2O6PPdS4·0.5CH3CN, C 43.71, H 3.20, N
dinuclear PdI species generically labeled “[(tBuN4)PdI(MeCN)]+”),
3.64%. ESI-MS (m/z): 321.0275, calculated for [(N2S2)-
the Pd(I) reference compound [PdI2(MeCN)6]2+,55 [(tBuN4)-
PdII(MeCN)2]2+ (8), and the Pd(III) species [(tBuN4)-
PdIII(MeCN)2]3+.52
PdII(PPh3)]2+: 321.0273.
[(N2S2)PdIIMe(PPh3)](OTf), 6. A 0.5 mL MeCN solution of 2 equiv
of PPh3 (8.2 mg, 31.2 μmol) was added dropwise to an MeCN
solution (2 mL) of [(N2S2)PdIIMe]2(OTf)2 (17.0 mg, 15.6 μmol)
while stirring. The solution changed color from orange to light yellow
immediately. After 0.5 h, the mixture was set up for crystallization
with diethyl ether vapor diffusion at room temperature (RT). Square-
Pd(I) dinuclear species with two bridging tBuNC ligands was
isolated via electrochemical reduction of 4 and structurally
characterized. Taking advantage of these results, we are
currently further optimizing the reactions conditions and the
ligands employed for isolating a range of mononuclear Pd(I)
complexes, with the goal of investigating in detail their
stoichiometric reactivity and catalytic applications.
shaped colorless crystalline formed after 1−2 days. Yield: 15.0 mg,
1
31
1
60%. H NMR (CD3CN, 300 MHz), δ (ppm): 0.75 (d, J P− H = 3 Hz,
3H, Me), 3.94 (br, 2H, CH2), 4.48 (br, 2H, CH2), 4.92 (br, 2H,
CH2), 5.30 (br, 2H, CH2), 7.00 (br, 2H, Py Hmeta), 7.4−7.6 (m, 15H,
Ph), 7.77 (br, 1H, Py Hpara). Another set of Py Hmeta and Hpara peaks
are buried under the peak corresponding to the Ph groups. UV−vis
(MeCN; λ, nm (ε, M−1 cm−1)): 292 (1300), ∼320 (sh, 430), ∼406
(sh, 20). Elemental analysis: found, C 50.67, H 3.61, N 3.42%;
calculated C34H32F3N2O3PPdS3, C 50.59, H 4.00, N 3.47%. ESI-MS
(m/z): 657.0784, calculated for [(N2S2)PdIIMe(PPh3)]+: 657.0779.
[(N2S2)PdI(μ-TBuNC)]2(ClO4)2, 7. Electrochemical reduction of
[(N2S2)PdII(tBuNC)](OTf)2 (27.7 mg, 36.3 μmol) was performed
in 10 mL of 0.1 M (Bu4N)ClO4 in MeCN at RT under N2. The color
changed from red to yellowish orange during the electrolysis. After the
charge corresponding to 1e− reduction has passed, the electrolysis was
stopped and the solution was concentrated to ∼1 mL under vacuum.
The resulting orange solution was stored at −35 °C for several days.
Orange needle-shaped crystals were collected by filtration and washed
EXPERIMENTAL DETAILS
■
Ligand and Metal Complex Syntheses. Reagents and
Materials. All chemicals were commercially available from Aldrich,
Fisher, or Strem Chemicals and were used as received without further
purification. 2,11-Dithia[3.3](2,6)pyridinophane (N2S2),37,41,56
N,N′-ditertbutyl-2,11-diaza[3.3](2,6)pyridinophane (tBuN4),57
[(N2S2)PdII(MeCN)]2(OTf)2 (1),39 [(N2S2)PdIIMe]2(OTf)2
(2),39 [(N2S2)PdIICl](OTf) (3),37 and [(tBuN4)PdII(MeCN)2]-
(OTf)2 (8)52 were prepared according to the literature procedures.
Solvents were purified prior to use by passing through a column of
activated alumina using an MBraun solvent purification system. On
the basis of their reduction potential,58 either cobaltocene (CoCp2) or
bis(pentamethyl)cyclopentadienyl cobalt(II) (CoCp*2) was used as a
reducing agent.
1
with pentane. Yield: 6.3 mg, 30%. H NMR (CD3CN, 300 MHz), δ
[(N2S2)PdII(tBuNC)](OTf)2, 4. 2 equiv of tBuNC (8.1 μL, 70.2
μmol) was added via a microsyringe to an MeCN solution (2 mL) of
[(N2S2)PdII(MeCN)]2(OTf)4 (50.4 mg, 35.0 μmol) while stirring.
The solution changed color from brown to red immediately. After 1 h,
the solvent was removed under vacuum, and the resulting red oil was
stored at −20 °C for 1 h to give a needle-shaped red crystalline
product. Yield: 52.8 mg, 89%. 1H NMR (CD3CN, 300 MHz), δ
(ppm): 0.75 (s, 18H, tBu), 4.55 (d, J = 15.9 Hz, 8H, CH2), 4.95 (br, J
= 15.9 Hz, 8H, CH2), 7.30 (d, J = 6.9 Hz, 8H, Py Hmeta), 7.70 (t, br, J
= 7.2 Hz, 4H, Py Hpara). UV−vis (MeCN; λ, nm (ε, M−1 cm−1)): 365
(3400). Elemental analysis: found, C 42.12, H 4.13, N 8.90%;
calculated C40H48Cl2N6O8Pd2S4·1.5CH3CN, C 42.53, H 4.36, N
8.65%. ESI-MS (m/z): 463.0359, calculated for [(N2S2)-
PdI(tBuNC)]+: 463.0368.
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[(tBuN4)PdII(tBuNC)2](OTf)2, 9. 2 equiv of tBuNC (5.4 μL, 47.7
μmol) was added through a microsyringe to an MeCN solution (1
mL) of [(tBuN4)PdII(MeCN)2](OTf)2 (20.1 mg, 24.0 μmol) while
stirring. The solution changed color from blue to pink immediately.
After 30 min, the solvent was removed, and the resulting pink
on the crystallographic coordinates of the cations of 4, 9, and a
proposed conformation of κ2-9, respectively. The ground state wave
functions were investigated by analysis of the frontier MOs, and the
atomic contributions to MOs were calculated by using the program
Chemissian.68
1
X-ray Absorption Spectroscopy (XAS) Studies. The XAS measure-
ments were conducted at Argonne National Laboratory (Argonne,
IL), at the Advanced Photon Source (ring energy = 7.0 GeV) on the
10-ID beamline of the Materials Research Collaborative Access Team
(MRCAT). The samples were loaded into a custom-designed,
chemically resistant PEEK cell fitted with a cap with a Swagelok
VCR fitting and a hand-tightened O-ring seal.53 The temperature- and
air-sensitive samples were loaded under a nitrogen blanket in a dry ice
bath. The solution measurements were made in emission mode. The
experiments utilized a cryogenically cooled double-crystal Si(111)
monochromator in conjunction with a glass-coated mirror to
minimize the presence of harmonics The monochromator was
scanned continuously from −200 to +800 eV relative to the Pd k-
edge energy. Scans were taken with a Pd0 foil reference (24352.6 eV)
for energy correction during data analysis (see below).
XAS Spectral Processing and Analysis. The program Athena
(Athena Ravel REF) was used to process the XANES spectra. The
sample and Pd foil reference spectra were imported, and background
removal and normalization was performed. Then, the Pd foil reference
for every sample spectrum was energy corrected by selecting the
maximum in the first derivative, finding the zero crossing in the
second derivative, and setting the energy of the zero crossing to
24352.6 eV. The energy-corrected Pd foil reference spectra were then
aligned to have all sample data on the same absolute energy grid for
analysis purposes.
X-ray Diffraction Studies. Crystals of X-ray diffraction quality were
obtained by slow anhydrous diethyl ether vapor diffusion into
acetonitrile solutions. Suitable crystals of appropriate dimensions were
mounted on Mitgen loops in random orientations. Preliminary
examination and data collection were performed by using a Bruker
Kappa Apex-II charge coupled device (CCD) detector system single
crystal X-ray diffractometer equipped with an Oxford Cryostream LT
device. Data were collected by using graphite monochromated Mo K
radiation (= 0.71073 Å) from a fine focus sealed tube X-ray source.
Preliminary unit cell constants were determined with a set of 36
narrow frame scans. Typical data sets consist of a combination of and
φ scan frames with typical scan width of 0.5 and counting time of 15−
30 s/frame at a crystal-to-detector distance of ∼4.0 cm. The collected
frames were integrated by using an orientation matrix determined
from the narrow frame scans. Apex II and SAINT software packages
were used for data collection and data integration.70 Analysis of the
integrated data did not show any decay. Final cell constants were
determined by global refinement of reflections from the complete data
set. Data were corrected for systematic errors by using SADABS based
on the Laue symmetry using equivalent reflections.70
crystalline solid was dried under vacuum. Yield: 22.0 mg, 100%. H
NMR (CD3CN, 300 MHz), δ (ppm): 1.46 (s, 18H, tBu-NC), 1.47 (s,
18H, tBu-N4), 3.57 (d, J = 17.1 Hz, 4H, CH2), 4.64 (d, J = 17.4 Hz,
4H, CH2), 7.31 (d, J = 8.1 Hz, 4H, Py Hmeta), 7.85 (t, J = 8.1 Hz, 2H,
Py Hpara). UV−vis (MeCN; λ, nm (ε, M−1 cm−1)): 489 (70), 343 (sh,
250). Elemental analysis: found, C 44.08, H 4.93, N 8.98%; calculated
C34H50F6N6O6PdS2, C 44.23, H 5.46, N 9.10%. ESI-MS (m/z):
229.0833, calculated for [(tBuN4)PdII]2+: 229.0831; 270.6199,
calculated for [(tBuN4)PdII(tBuNC)]2+: 270.6199; 312.1567, calcu-
lated for [(tBuN4)PdII(tBuNC)2]2+: 312.1566; 773.2652, calculated
for [(tBuN4)PdII(tBuNC)2](OTf)+: 773.2652.
Physical Measurements. General Methods. 1H (300.121 MHz)
NMR spectra were recorded on a Varian Mercury-300 spectrometer.
Low-temperature (−20 °C) 1H (600 MHz) NMR spectra were
recorded on a Varian Unity Inova-600 spectrometer. Chemical shifts
are reported in ppm and referenced to residual solvent resonance
peaks. Abbreviations for the multiplicity of NMR signals are s
(singlet), d (doublet), t (triplet), q (quartet), sep (septet), m
(multiplet), and br (broad). UV−vis spectra were recorded on a
Varian Cary 50 Bio spectrophotometer. EPR spectra were recorded
on a JEOL JES-FA X-band (9.2 GHz) EPR spectrometer in
MeCN:PrCN (v:v = 1: 3) frozen glass at 77 K. Simulation of EPR
spectra was performed by using WinEPR SimFonia v.1.25. Elemental
analyses were performed by the Columbia Analytical Services Tucson
Laboratory. ESI-MS experiments were performed on a Bruker Maxis
QTOF mass spectrometer with an electron spray ionization source.
ESI massspectrometry was provided by Washington University Mass
Spectrometry Resource, a NIH Research Resource (Grant
P41RR0954).
Electrochemical Measurements. Electrochemical grade (Bu4N)-
ClO4 purchased from Aldrich was used as the supporting electrolyte.
Cyclic voltammetry was performed with a BASi EC Epsilon
electrochemical workstation or a CHI 660D electrochemical analyzer.
Electrochemical measurements were performed under a flow of
nitrogen, and the analyzed solutions were deaerated by purging with
nitrogen. A glassy carbon electrode (GCE, d = 1.6 mm) was used as
the working electrode, while a Pt wire was used as the auxiliary
electrode. The nonaqueous reference electrode containing Ag/0.01 M
AgNO3 in 0.1 M Bu4NClO4/MeCN was calibrated against Fc; the
potential of the Fc+/Fc couple vs Ag/0.01 M AgNO3/0.1 M
Bu4NClO4/MeCN reference electrode is +0.105 V.
EPR Studies of the In Situ Formation of the Pd(I) Species. An
EPR tube was charged with a solution of Pd complex in MeCN:PrCN
(v:v = 1: 3) and cooled to −78 °C in a dry ice/acetone bath. Another
solution containing 1 equiv of chemical reducing agent (CoCp2 or
CoCp*2) in the same solvent mixture was quickly added via a
microsyringe, followed by a quick shake of the tube, and then the
sample was frozen in the liquid nitrogen. An initial EPR spectrum was
taken at 77 K. The sample was then carefully warmed for 10−30 s to
allow for the complete reaction to occur and refrozen; the EPR
spectrum was recorded again, or the sample was warmed for longer
times to probe the thermal stability of the generated Pd(I) species
(Figure S9).
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01938.
1
Reactivity studies of reduction of 2 and 6 by H NMR,
time-dependent EPR spectra of the mixture of 4 and
CoCp2, computational details, and X-ray crystal
structure data (PDF)
Computational Studies. The density functional theory (DFT)
calculations were performed with the program package Gaussian 09.59
The B3LYP functional was employed,60,61 and the Stevens (CEP-
31G)62,63 valence basis set and effective core potential were used for
Pd,64,65 which have been shown previously to reproduce well
experimental parameters of Pd complexes,66,67 and have been used
by us previously and shown to predict well the electronic properties of
paramagnetic Pd complexes.32−38 We have also employed the B3LYP-
D3 and M06 functionals and other electron core potential basis sets;
however, the calculated atomic contribution to the frontier MOs
varied only slightly. The geometry optimization calculations used
were performed for the cations of 4-PdI, κ4-9-PdI, and κ2-9-PdI, based
Accession Codes
CCDC 1569571−1569575 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
H
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Article
(14) Chen, X. Y.; Pu, M. P.; Cheng, H. G.; Sperger, T.;
Schoenebeck, F. Arylation of Axially Chiral Phosphorothioate Salts
by Dinuclear Pd-I Catalysis. Angew. Chem., Int. Ed. 2019, 58, 11395−
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AUTHOR INFORMATION
■
Corresponding Author
Liviu M. Mirica − Department of Chemistry, University of
Illinois at Urbana−Champaign, Urbana, Illinois 61801, United
States; orcid.org/0000-0003-0584-9508; Email: mirica@
illinois.edu
(16) Vilar, R.; Mingos, D. M. P.; Cardin, C. J. Synthesis and
structural characterization of [Pd2(P-Br)2(PBut3)2], an example of a
palladium(I)-palladium(I) dimer. J. Chem. Soc., Dalton Trans. 1996,
4313−4314.
Authors
Jia Luo − Department of Chemistry, Washington University, St.
Louis, Missouri 63130-4899, United States
Giang N. Tran − Department of Chemistry, University of Illinois
at Urbana−Champaign, Urbana, Illinois 61801, United States
Nigam P. Rath − Department of Chemistry and Biochemistry,
University of Missouri, St. Louis, Missouri 63121-4400, United
States
(17) Hruszkewycz, D. P.; Wu, J.; Hazari, N.; Incarvito, C. D.
Palladium(I)-bridging allyl dimers for the catalytic functionalization of
CO2. J. Am. Chem. Soc. 2011, 133, 3280−3283.
(18) Aufiero, M.; Proutiere, F.; Schoenebeck, F. Redox Reactions in
Palladium Catalysis: On the Accelerating and/or Inhibiting Effects of
Copper and Silver Salt Additives in Cross-Coupling Chemistry
Involving Electron-rich Phosphine Ligands. Angew. Chem., Int. Ed.
2012, 51, 7226−7230.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01938
(19) Proutiere, F.; Aufiero, M.; Schoenebeck, F. Reactivity and
Stability of Dinuclear Pd(I) Complexes: Studies on the Active
Catalytic Species, Insights into Precatalyst Activation and Deactiva-
tion, and Application in Highly Selective Cross-Coupling Reactions. J.
Am. Chem. Soc. 2012, 134, 606−612.
(20) Kalvet, I.; Bonney, K. J.; Schoenebeck, F. Kinetic and
Computational Studies on Pd(I) Dimer-Mediated Halogen Exchange
of Aryl Iodides. J. Org. Chem. 2014, 79, 12041−12046.
(21) Yin, G. Y.; Kalvet, I.; Schoenebeck, F. Trifluoromethylthiolation
of Aryl Iodides and Bromides Enabled by a Bench-Stable and Easy-
To-Recover Dinuclear Palladium(I) Catalyst. Angew. Chem., Int. Ed.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Department of Energy’s BES Catalysis Science
Program (DE-SC0006862) for financial support. We also
thank Drs. Jeffrey T. Miller and Ryan C. Nelson (Argonne
National Lab) for assistance with obtaining the XAS data and
Dr. Jason Schultz for assistance with the EPR measurements.
(22) Simpson, Q.; Sinclair, M. J. G.; Lupton, D. W.; Chaplin, A. B.;
Hooper, J. F. Oxidative Cross-Coupling of Boron and Antimony
Nucleophiles via Palladium(I). Org. Lett. 2018, 20, 5537−5540.
(23) Fujiwara, S.; Nakamura, M. Electron Spin Resonance of Pd(I).
2. Gamma-Irradiated Single Crystals of K2pdcl4 and (Nh4)2pdcl4. J.
Chem. Phys. 1971, 54, 3378−3380.
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