Reactivity of a Dinuclear PdIComplex [Pd2(μ-PPh2)(μ2-OAc)(PPh3)2] with PPh3: Implications for Cross-Coupling Catalysis Using the Ubiquitous Pd(OAc)2/nPPh3Catalyst System

Article abstract of DOI:10.1021/acs.organomet.1c00347

[Pd^I₂(μ-PPh₂)(μ₂-OAc)(PPh₃)₂] is the reduction product of Pd^II(OAc)₂(PPh₃)₂, generated by reaction of ‘Pd(OAc)₂’ with two equivalents of PPh₃. Here, we report that the reaction of [Pd^I₂(μ-PPh₂)(μ₂-OAc)(PPh₃)₂] with PPh₃results in a nuanced disproportionation reaction, forming [Pd⁰(PPh₃)₃] and a phosphinito-bridged Pd^I-dinuclear complex, namely [Pd^I₂(μ-PPh₂){κ₂-P,O-μ-P(O)Ph₂}(κ-PPh₃)₂]. The latter complex is proposed to form by abstraction of an oxygen atom from an acetate ligand at Pd. A mechanism for the formal reduction of a putative Pd^IIdisproportionation species to the observed Pd^Icomplex is postulated. Upon reaction of the mixture of [Pd⁰(PPh)₃] and [Pd^I₂(μ-PPh₂){κ₂-P,O-μ-P(O)Ph₂}(κ-PPh₃)₂] with 2-bromopyridine, the former Pd⁰complex undergoes a fast oxidative addition reaction, while the latter dinuclear Pd^Icomplex converts slowly to a tripalladium cluster, of the type [Pd₃(μ-X)(μ-PPh₂)₂(PPh₃)₃]X, with an overall 4/3 oxidation stateperPd. Our findings reveal complexity associated with the precatalyst activation step for the ubiquitous ‘Pd(OAc)₂’/nPPh₃catalyst system, with implications for cross-coupling catalysis.

Full text of DOI:10.1021/acs.organomet.1c00347

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Article
Reactivity of a Dinuclear PdI Complex [Pd2(μ-PPh2)(μ2‑OAc)(PPh3)2]
with PPh3: Implications for Cross-Coupling Catalysis Using the
Ubiquitous Pd(OAc)2/nPPh3 Catalyst System
Neil W. J. Scott, Mark J. Ford, David R. Husbands, Adrian C. Whitwood, and Ian J. S. Fairlamb*
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ABSTRACT: [PdI2(μ-PPh2)(μ2-OAc)(PPh3)2] is the reduction product of PdII(OAc)2(PPh3)2, generated by reaction of
‘Pd(OAc)2’ with two equivalents of PPh3. Here, we report that the reaction of [PdI2(μ-PPh2)(μ2-OAc)(PPh3)2] with PPh3 results
in a nuanced disproportionation reaction, forming [Pd0(PPh3)3] and a phosphinito-bridged PdI-dinuclear complex, namely [PdI2(μ-
PPh2){κ2-P,O-μ-P(O)Ph2}(κ-PPh3)2]. The latter complex is proposed to form by abstraction of an oxygen atom from an acetate
ligand at Pd. A mechanism for the formal reduction of a putative PdII disproportionation species to the observed PdI complex is
postulated. Upon reaction of the mixture of [Pd0(PPh)3] and [PdI2(μ-PPh2){κ2-P,O-μ-P(O)Ph2}(κ-PPh3)2] with 2-bromopyridine,
the former Pd0 complex undergoes a fast oxidative addition reaction, while the latter dinuclear PdI complex converts slowly to a
tripalladium cluster, of the type [Pd3(μ-X)(μ-PPh2)2(PPh3)3]X, with an overall 4/3 oxidation state per Pd. Our findings reveal
complexity associated with the precatalyst activation step for the ubiquitous ‘Pd(OAc)2’/nPPh3 catalyst system, with implications for
cross-coupling catalysis.
dI dinuclear complexes are increasingly being adopted as
intriguing for catalytic cross-coupling, is underpinned by a
significant history of [Pd3(X)(PPh2)2(PPh3)3]X clusters dating
back to their synthesis and characterization in the late
1960s.17−20
P
effective and distinctive cross-coupling precatalysts.1−8
The Schoenebeck group has demonstrated that such
complexes, and their derivatives, display unique reactivity,
particularly with respect to controllable chemoselectivity in
cross-coupling reactions, which has very recently been
reviewed.9 Building on the important work on the activation
of ‘Pd(OAc)2’ by Amatore and Jutand,10−12 our group recently
discovered that the phosphido-bridged PdI-dinuclear complex
The reactivity of another PdI dinuclear complex, [Pd(μ-
Br)(PtBu3)]2, toward 3 equiv of PtBu3 was reported by Colacot
and Schoenebeck et al., which afforded [Pd0(PtBu3)2] as the
predominant product (Figure 1b).2
We considered that another species (e.g., PdII) might
accompany Pd0 in this type of reaction, at least transiently, as
the apparent process is of a disproportionative nature. Indeed,
controlled catalyst activation of dinuclear PdI complexes to Pd0
complexes by disproportionation has been proposed as a
rationale for their exceptional reactivity.21−23 Hazari et al.
rationalized that the in situ-formed NHC-containing dinuclear
[PdI (μ-PPh2)(μ2-OAc)(PPh3)2] 1 forms via trans-
2
PdII(OAc)2(PPh3)2 in the reaction between [PdII (OAc)6]
3
and exactly 6 equiv of PPh3 (where the Pd:PPh3 = 1:2). This is
a precatalytic Pd:PPh3 ratio often employed as an effective
catalyst system in synthetic chemistry applications. Recently,
dinuclear PdI complexes have been shown to form by reaction
between Pd(OAc)2 and dialkylbiaryl phosphines.13−15 As part
of an investigation into its role in cross-coupling catalysis, the
reactivity of 1 with electrophilic organohalides was con-
ducted.16 [PdI2(μ-PPh2)(μ2-OAc)(PPh3)2] 1 was found to
activate organohalides at room temperature to afford
tripalladium clusters, of the type [Pd3(μ-X)(μ-PPh2)2(PPh3)3]-
X, in addition to more commonly anticipated oxidative
addition PdII products (Figure 1a). The finding, while
Received: June 11, 2021
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Figure 2. 31P{1H} NMR (THF-d8, 25 °C, 203 MHz) showing
reaction of [PdI2(μ-PPh2)(μ2-OAc)(PPh3)2] 1 with nPPh3. (a)
Authentic 1; (b) [PdI2(μ-PPh2)(μ2-OAc)(PPh3)2] 1 + 1 equiv of
PPh3 after 0.5 h; (c) [PdI2(μ-PPh2)(μ2-OAc)(PPh3)2] 1 + 2 equiv of
PPh3 after 0.5 h. Note: the spectral window has been shortened, as
shown by the double diagonal lines on the chemical shift scale line.
Figure 1. (a) Formation of [PdI2(μ-PPh2)(μ2-OAc)(PPh3)2] complex
1 and reactivity with organohalides. (b, c) Examples of reactivity of
PdI−PdI dinuclear complexes.
complex(μ-allyl)(μ-Cl)Pd2(NHC)2 was activated for catalysis
via disproportion to PdIICl(η3-allyl)(NHC) and [Pd0−NHC]
(Figure 1c, NHC = N-heterocyclic carbene ligand IPr).24
Thus, we continued to pursue uncovering the distinctive
reactivity of PdI-dinuclear complexes so that the implications
for cross-coupling catalysis can be more broadly understood.
Herein, we report that addition of PPh3 to [PdI2(μ-PPh2)(μ2-
OAc)(PPh3)2] 1, induces disproportionation and subsequent
conversion to a dinuclear PdI, namely [PdI2(μ-PPh2){κ2-P,O-μ-
P(O)Ph2}(κ-PPh3)2], in addition to [Pd0(PPh3)3]. These two
different species exhibit distinct reactivity profiles toward 2-
bromopyridine in forming either mononuclear Pd species or
higher order Pd3 cluster species. The portioning between these
two types of species is important for the field of cross-coupling
catalysis to consider.
NMR resonance. Low-field resonances at δP 122.6 and 77.3
ppm, which were again present, broaden out, with both
appearing as doublets (supporting formation of a new species
2). The major, broad resonance appeared to shift marginally
upfield from δP 23.9 → 23.1 ppm, which is [Pd0(PPh3)3].
Broadened peaks at ∼ δP 25.6 and 15.6 ppm were also present,
indicating PPh3 exchange taking place in the presence of
additional phosphine. Treatment of the reaction mixture with
10 equiv of PPh3 resulted in disappearance of these broad
resonances and migration of the major singlet to δP 7.6 ppm
(externally referenced to H3PO4(aq) 85% w/w) (see
Supporting Information). The migration of this signal upfield
(toward the chemical shift of free PPh3) is consistent with the
reported behavior of [Pd0(PPh3)n] toward additional
PPh3.10,16
A bright red-orange crystal of this new species 2 was grown
by carefully layering pentane onto a cooled THF solution of
the postreaction mixture of [PdI2(μ-PPh2)(μ2-OAc)(PPh3)2] 1
and two equivalents of PPh3 (stored at −18 °C for ca. 2 days).
The crystal was subjected to single-crystal X-ray diffraction
analysis which enabled the structural elucidation of this new
species to be determined (Figure 3). It is important to note
that the asymmetric unit of crystal structural data contains two
half complexes of 2, each of which are disordered about a
center of inversion.
The single-crystal X-ray diffraction structure of [Pd2(μ-
PPh2){κ2-P,O-μ-P(O)Ph2}(κ-PPh3)] 2 shows that the P, O,
and Pd atoms all lie within approximately the same plane
(Figure 3). The Pd atoms are stabilized by bridging μ-
phosphido and μ2-phosphinito ligands. The Pd1−P4 and the
Pd2−O1 bond distances [2.328(3) and 2.150(7) Å,
respectively] are significantly less than the sum of the van
der Waals radii of the respective atoms, indicating that both O
and P atoms of the diphenylphosphinito ligand are bonded to
Pd (see DFT calculations later). The Pd1−Pd2 interatomic
distance was determined to be 2.5680(10) Å, in-keeping with
PdI−PdI bond lengths of similar PdI dinuclear com-
plexes.15,16,25−27 The P−O bond of the diphenylphosphinito
RESULTS AND DISCUSSION
■
Reactivity of [Pd2(μ-PPh2)(μ2-OAc)(PPh3)2] with PPh3.
In our first reaction, [PdI2(μ-PPh2)(μ2-OAc)(PPh3)2] 1 was
treated with one equivalent of PPh3 in THF-d8 at room
temperature. After ca. 30 min, 31P NMR spectral analysis
revealed the formation of new species (Figure 2b), which could
be compared with an authentic sample of 1 (Figure 2a). A
phosphorus triplet signal, at δP 199.6, was assigned as the
diphenylphosphido signal of unreacted 1. A loss of resolution
of the triplet was evident, as well as a slight downfield change
in chemical shift, δP 199.1 → 199.6 ppm (Figure 2b). Two new
well-resolved resonances appeared at δP 122.6 and 77.3 ppm,
with doublet (d) and doublet of doublets (dd) multiplicities,
respectively, which were found to integrate in a 1:1 ratio
(hereafter species 2; a discussion of coupling constants is
found in the text below). A major, broad resonance at δP 23.9
ppm was evident. Two broad resonances at approximately δP
26 and 15 ppm were also observed.
Upon reaction of 1 with two equivalents of PPh3, further
reaction progression was evident by 31P NMR spectroscopic
analysis after ca. 0.5 h reaction time (Figure 2c). The complete
loss of 1 was seen by the disappearance of its phosphido 31P
B
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ligand [P4−O1 = 1.550(8) Å] has some double bond-
character at 1.479(2) Å),28 which falls within the region seen
for another bridging diphenylphospinito dinuclear PdI complex
reported by Matt et al.29 The structure of 2 is desymmetrized
along the Ph3P(P3)−Pd(1)−Pd(2)−PPh3(P4) bond axis, with
the Pd1−P3 interaction being approximately 4% longer than
that of P2−P4. It is pertinent to mention that a related PCy2-
bridged-Pt complex is known.30a
Mass spectral analysis (+ve mode) of the postreaction
solution provided further evidence for the species formed
under the reaction conditions. ESI- and LIFDI-MS analysis
were complementary in confirming the presence of [Pd2(μ-
PPh2){κ2-P,O-μ-P(O)Ph2}(κ-PPh3)] 2 as the pseudomolecular
ion [M + H]+ (exact mass m/z = 1123.20) (Figure 4a, i) and
[M].+ (exact mass m/z = 1122.09), respectively, with the
correct isotopic distribution}. LIFDI-MS analysis indicated the
presence of an ion at m/z = 982.18; this is equal to the exact
mass of [Pd0(PPh3)3], detected as [Pd(PPh3)3].+ (i.e., the
radical cation), with the correct isotopic distribution (Figure
4a, i). A low-temperature 31P{1H} NMR spectrum of a sample
from the reaction of 1 with two equivalents of PPh3 (173 K,
−100 °C, THF-d8, 203 MHz) was recorded (Figure 4b, i).
Two low-field resonances at δP 123.1 and 75.6 ppm (observed
as doublets at 298 K) and two new, high-field PPh3-type
resonances at δP 27.6 and 12.0 ppm were resolved (previously
broad at 298 K), revealing fine coupling between four peaks,
each resolving as ddd peak multiplicities, giving information
important for the complete assignment of the solution
Figure 3. Structure of [Pd2(μ-PPh2){κ2-P,O-μ-P(O)Ph2}(κ-PPh3)]
(2), obtained via X-ray diffraction of a single crystal grown from THF-
d8 reaction solution of [PdI2(μ-PPh2)(μ2-OAc)(PPh3)2] 1 with two
equivalents of PPh3. Selected interatomic distances /Å: Pd1−Pd2 =
2.5680(10), Pd1−P1 = 2.360(9) Pd2−P2 = 2.275(9), Pd1−P3 =
2.316(3), Pd2−P3 = 2.210(3), P1−P4 = 2.328(3), P4−O1 =
1.550(8), P2−O1 = 2.150(7). Selected interatomic angles /°: Pd1−
P3−Pd2 = 69.093, Pd1−Pd2−O1 = 86.162, Pd2−O1−P4 = 93.688.
Note: a single molecule from the asymmetric unit cell is shown only
for clarity. H-atoms not shown.
Figure 4. Key characterization data obtained from the postreaction solution (THF-d8), containing the products of the reaction between [PdI2(μ-
PPh2)(μ2-OAc)(PPh3)2] 1 and two equivalents of PPh3. (a) Mass spectral data for ions detected in the reaction solution; [Pd2(μ-PPh2){κ2-P,O-μ-
P(O)Ph2}(PPh3)2]+., detected by ESI-MS and [Pd0(PPh3)3] detected by LIFDI-MS (right). (b) 1D 31P{1H} and 2D 31P−31P{1H} COSY NMR
spectra (202.5 MHz, CD2Cl2), recorded at 173 K of the postreaction solution.
C
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structure of 2. Each phosphorus resonance integrates in a
1:1:1:1 ratio. The P−P spin coupling constants (in Hz) for
each resonance are collated in Table 1. The four 31P
shown in Figure 5. The presence of acetic anhydride and 2, as
the other products of this type of process, gives clues as to
Table 1. JPP Coupling Constants (in Hz) for each 31P{1H}
Resonance in 2, Recorded at Low Temperature (202.5
MHz, THF-d8, 173 K)
δ/ppm
123.1
75.6
27.6
12.0
label
A
B
C
D
A
B
C
D
0
217.2
0
15.8
55.8
61.5
15.8
0
33.6
55.8
131.5
0
217.2
61.5
33.6
131.5
resonances were assigned to the four phosphorus atoms as
present in the single-crystal X-ray diffraction structure (Figure
2
3; 2,Table 1). A large JPP coupling between the two low-field
resonances A and B (217 Hz) is consistent with the trans-
arrangement of the phosphinito and phosphide ligands
3
spanning the PdI−PdI fragment. Likewise, the large JPP
Figure 5. Proposed mechanism for the disproportionation reaction
leading to formation of [PdI2(μ-PPh2){κ2-P,O-μ-P(O)Ph2}(κ-PPh3)2]
(2) and [Pd0(PPh3)3].
coupling constant observed between high-field phosphine-
type resonances (131.5 Hz, C and D) is consistent with a
trans-configuration.
3
what has happened to the resulting putative PdII fragment I
that formed during disproportionation of the starting dinuclear
PdI complex 1 (Figure 5). We propose that acetic anhydride
forms by acyl transfer to acetate at Pd, with loss of one oxygen
atom to phosphorus, in forming the phosphinito ligand. In this
system, under rigorous Schlenk conditions, acetic anhydride
can only derive from the acetate ligands of 2.
The smallest JPP coupling constant observed (15.8 Hz,
between B and C) fits with a cis-configuration, coupling
through oxygen, between the phosphinito resonance (B) and
2
one of the adjacent phosphines (C). A larger JPP coupling
constant (55.8 Hz) was measured between the phosphido
resonance (B) and the adjacent phosphine (D). The complete
connectivity of the phosphorus atoms was confirmed by
31P−31P COSY-NMR spectral analysis (Figure 4). Hence, the
31P NMR spectrum of [Pd2(μ-PPh2){κ2-P,O-μ-P(O)Ph2}(κ-
PPh3)] 2 in THF-d8 agrees with the connectivity suggested by
the single-crystal X-ray diffraction structure (Figure 3), along
with the cation detected by MS analysis, providing strong
evidence that the structure present in THF-d8 solution is 2, as
in the solid-state. In addition to 2, the presence of
[Pd0(PPh3)3] is supported by its detection (radical cation)
by LIFDI-MS analysis of the reaction solution.
It is important to acknowledge that a dinuclear PtII complex
containing a P,O-bridging phosphinito ligand was formed from
the reaction of a higher oxidation state dinuclear PtIII species
with hydroxide anion, involving nucleophilic attack of
hydroxide at the electrophilic PtIII center.30b Under our
conditions, hydroxide anion is not generated (we would
expect Ac2O to be converted to acetic acid, which was not
observed).
The transient stability of PdII fragment I could be conferred
by the acetate ligand adopting an η2-OAc binding mode;31
however, dimerization could then occur rapidly via bridging
interactions from nucleophilic κ2-diphenylphosphido ligands,32
affording dinuclear PdII complex II. The proximal relationship
of the acetate and phosphide ligands sets-up a favored P−O
bonding interaction leading to a formal reduction at Pd from
two to zero. Subsequent nucleophilic attack on the carbonyl
group linked to the phosphorus(V) center in III leads to the
generation of acetic anhydride, leaving one oxygen atom
connected to phosphorus in IV, enabling a bridging
coordination mode of O to Pd. Thus, the characterized
[PdI2(μ-PPh2){κ2-P,O-μ-P(O)Ph2}(κ-PPh3)2] complex 2 is
then formed. The redox process is driven by the formation
of a strong P−O bond, much like in the formation of Pd0 from
phosphine-ligated PdII acetate complexes (vide supra).10,12
The reaction of a PdI dimer by disproportionation, followed by
An additional piece of the jigsaw enabling full reaction
1
mapping was provided by analysis of the H and 13C NMR
spectra of the reaction mixtures (500, 125.8 MHz, respectively,
in THF-d8). For example, resonances at δH 2.15 ppm and δC
166.1 and 20.9 ppm suggested formation of acetic anhydride
(confirmed by comparison with an authentic sample).
1
Integration of the H NMR spectrum confirmed the ratio of
[Pd2(μ-PPh2){κ2-P,O-μ-P(O)Ph2}(κ-PPh3)] 2:Ac2O to be 1:1
(see Supporting Information).
Taken together, these data indicate that the room temper-
ature reaction between [PdI2(μ-PPh2)(μ2-OAc)(PPh3)2] 1 and
two equivalents of PPh3 cleanly affords [Pd2(μ-PPh2){κ2-P,O-
μ-P(O)Ph2}(κ-PPh3)] 2 and [Pd0(PPh3)3], as evidenced by
31P NMR and MS analysis, along with acetic anhydride. A
mechanistic scheme describing how the events leading to the
formation of 2, [Pd0(PPh3)3], and Ac2O have occurred is
D
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a reduction at Pd is not unprecedented. Disproportionation
followed by reduction at the PdII fragment has been observed
by Figueroa et al., but in that case, full reduction of an
isopropoxide-bridged, bulky isocyanide-stabilized PdI dinuclear
complex to Pd0 occurred, ultimately leading to the formation
of a Pd0 trimer. Interestingly, acetone and propene were
observed by Figueroa et al. as oxidized biproducts, and the Pd0
fragments combined to form a Pd3 cluster complex.33
In subsequent experiments, we found that a minor new
species crystallized from a reaction mixture where 2 was the
major species (grown by layering a THF postreaction solution
with hexane, which was stored under Ar at −18 °C). A single
crystal was subjected to X-ray diffraction analysis, showing it to
be yet another novel PdI dinuclear complex (3) (Figure 6).
Figure 6. Structure of [Pd2(μ-PPh2){κ2-O,O-μ-P(O)2Ph2}(κ-PPh3)2]
(3) obtained via a X-ray diffraction of a single crystal grown from
THF-d8 reaction solution of 1 with 2 equiv of PPh3. Selected
interatomic distances /Å Pd1−Pd2 = 2.6140(5) P1−Pd1 =
2.2901(12), Pd2−P4 = 2.3095(12), P2−Pd1 = 2.1810(12), P2−
Pd2 = 2.1888(12), Pd1−O1 = 2.162(4), Pd2−O2 = 2.186(4), O1−
P3 = 1.507(4), O2−P3 = 1.529(4). Selected interatomic angles /°
O1−P3−O2 = 120.7(2), Pd1−P2−Pd2 = 73.48(4). H-atoms not
shown.
Analogously to 1 and 2 this complex is bridged by a single μ-
diphenylphosphido ligand; however, the secondary bridging
ligand is a diphenylphosphinato ligand, which bridges through
a κ2-O2-μ2 interaction from the P(O2)PPh2 moiety. This
complex appears to be a rare example, where a phosphinato-
type ligand is coordinated to Pd via a bridging interaction
through two oxygen atoms. Moissev et al. observed the
formation of diphenylphosphinato-bridged PdII complex,
formed from the reaction of [PdII(μ2-OAc)(κ-OAc)(PPh3)]2
with molecular hydrogen in the presence of formic acid.34 We
tentatively propose the presence of 3 as a minor product to be
the result of the oxidation of 2 by trace air during the
crystallization, a process that is independent of the formation
of 2 (Figure 5).
Computational Studies for Complex 2. Computational
studies using density functional theory (DFT) calculations for
[PdI2(μ-PPh2){κ2-P,O-μ-P(O)Ph2}(κ-PPh3)2] 2 were con-
ducted using the B3LYP/DEF2SVP level of theory with an
implicit solvent model (SMD, THF implicit solvent) and
empirical dispersion corrections (GD3-BJ) (Figure 7). The
calculations reveal a short Pd−Pd bond (2.6046 Å), supporting
Figure 7. DFT calculated structure of complex 2, showing the
HOMO-1 (P1-Pd1-Pd2-O1-P2 based), HOMO (P3-Pd1-P4-O1
based) and LUMO frontier molecular orbitals.
its diamagnetic properties. The HOMO resides on the “Pd−
P−O” moiety, whereas the HOMO-1 resides primarily on the
E
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Pd−Pd centers. The LUMO can be found over the phosphide
and Pd−Pd centers. The HOMO and HOMO-1 provide clues
about the underlying reactivity of 2 toward other electrophilic
species, highlighting that oxidation of the phosphinito to
phosphinato being clearly feasible, but also direct reaction of 2
with organohalides (presumably involving HOMO-1), similar
to what we revealed for dinuclear Pd complex 1.12 Natural
bond order (NBO) analysis and calculated Wiberg Indices on
complex 2 reveal that there is a reasonable Pd1−Pd2 bonding
interaction (see Supporting Information). Wiberg indices for
Pd1 and Pd2 were determined to be 0.3167, along with a
natural atomic orbital bond order of 0.4737, indicating a partial
bond. The Pd2−O1 interaction is relatively weak in
comparison to the P4−O1 bond, with Wiberg indices of
0.2697 and 0.9477 respectively. As O1 has a calculated −1
charge, this implies the Pd2−O1 interaction could be more
electrostatic than covalent in nature.
Reactivity of the System with 2-Bromopyridine. We
next assessed the reactivity and nucleophilicity of both Pd
species − [PdI2(μ-PPh2){κ2-P,O-μ-P(O)Ph2}(κ-PPh3)2] 2 and
[Pd0(PPh3)3] (generated by reaction of two equivalents of
PPh3 with 1) toward an organohalide, namely 2-bromopyr-
idine. The reason for selecting 2-bromopyridine was that our
earlier work16 had highlighted the challenges associated with
characterizing the product(s) of the reaction mixtures of the
Pd species between more typical organohalides (e.g.,
iodobenzene). Furthermore, employing 2-bromopyridine
(having a heteroatom) is arguably like substrates that are
more typically used by the synthetic chemistry community.
A postreaction mixture of PdI2(μ-PPh2){κ2-P,O-μ-P(O)-
Ph2}(κ-PPh3)2] 2 and [Pd0(PPh3)3] was thus treated with an
excess of 2-bromopyridine in THF 25 °C. Over a period of 13
h, 31P NMR spectroscopic analysis showed the evolution of
several new phosphorus-containing species (Figure 8).
Structural assignment was possible for some of the 31P-
containing reaction products evolving under the reaction
conditions. After 20 min ( Figure 8b), the broad signal at δP
22.0 ppm, [Pd0(PPh3)3], was lost with new resonances forming
at δP 22.5 and 30.5 ppm and a broad peak at ca. δP − 4.4 ppm,
which was assigned to liberated PPh3.
The known reaction between [Pd0(PPh3)4] and 2-
bromopyridine was carried out,12 which allowed for con-
firmation of the identity of the resonances at δ 22.5 and 30.5
ppm (see Supporting Information for full details). These
resonances are associated with trans-[PdII(Br)(N,C2-pyridyl)-
(PPh3)2] and trans-[PdII(Br)(N,C2-pyridyl)(PPh3)]2, respec-
tively (4 and 5); the oxidative addition complexes of 2-
bromopyridine to [Pd0(PPh3)2], derived from [Pd0(PPh3)3],35
releasing PPh3 (as observed) in less than 10 min.36 ESI-MS
analysis confirmed the presence of complexes 4 and 5 in the
postreaction solution on the basis of observation of the
respective pseudo-molecular cations [M−Br]+.
Unexpectedly, after the addition of 2-bromopyridine,
complex 2 was seen to undergo a slower reaction, forming
[Pd3(μ-Br)(μ-PPh2)(PPh3)3]X 6, with X most likely being
diphenylphosphinite [PPh2O]− or Br−; the [Pd3(μ-Br)(μ-
PPh2)(PPh3)3]+ species was evident by 31P NMR and ESI-MS
analysis (Figure 8c). Formation of this particular trinuclear Pd
cluster relates to the reactivity of 1 towards 2-bromopyridine,
indicating [Pd3(μ-X)(μ-PPh2)(PPh3)3]X species as a thermo-
dynamic sink promoted by the bromide nucleofuge.16 We
noted that the Ac2O generated in the preceding reaction was
1
left unreacted, while 4, 5, and 6 were formed (comparing H
and 31P NMR analysis).
These findings allow us to fully delineate the reaction
pathways for 1, on reaction with two equivalents of PPh3, and
furthermore 2-bromopyridine, our chosen organohalide to
exemplify the link to cross-coupling chemistry (Figure 9).
Figure 9. Reactivity of the mixture of PdI phosphinito complex 2 and
[Pd0(PPh3)3]with 2-bromopyridine.
In conclusion, reaction of [PdI2(μ-PPh2)(μ2-OAc)(PPh3)2]
1 with two equivalents of PPh3 led to disproportionation to
[Pd0(PPh3)3] and a new complex [PdI2(μ-PPh2){κ2-P,O-μ-
P(O)Ph2}(κ-PPh3)2] 2. The presence of acetic anhydride as a
biproduct of the process indicated that the phosphinito ligand
had formed by transfer of an oxygen atom via formal acyl
transfer to acetate. The crude reaction mixture containing 2
and [Pd0(PPh3)3] reacted with 2-bromopyridine at different
rates: (1) slowly giving [Pd3(μ-Br)(μ-PPh2)(PPh3)3]X 6
(formed from 2); (2) rapidly forming typical oxidative addition
products 4 and 5 {formed from [Pd0(PPh3)3]}. These findings
reveal that the unique reactivity of dinuclear PdI complexes
Figure 8. 31P{1H} NMR data (THF-d8, 25 °C, 203 MHz), recorded
as a function of time, for the reaction between the postreaction
mixture, generated from 1 and PPh3, and subsequent reaction with 2-
bromopyridine. (a) before, (b) 20 min after, and (c) 13 h after
addition of 2-bromopyridine.
F
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Article
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enables access to different Pd speciation, with arguably
important implications for cross-coupling catalysis,37 adding
to the complexity of the ubiquitous Pd(OAc)2/nPPh3
precatalyst system.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00347.
■
sı
Detailed experimental procedures, compound character-
ization data, and additional supporting data (PDF)
DFT structures (XYZ)
Accession Codes
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CCDC 2089316 and 1894929 contains the supplementary
crystallographic 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.
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AUTHOR INFORMATION
Corresponding Author
Ian J. S. Fairlamb − Department of Chemistry, University of
York, York, North Yorkshire YO10 5DD, United Kingdom;
Email: ian.fairlamb@york.ac.uk
■
Authors
Neil W. J. Scott − Department of Chemistry, University of
York, York, North Yorkshire YO10 5DD, United Kingdom
Mark J. Ford − Bayer AG, 40789 Monheim, Germany
David R. Husbands − Department of Chemistry, University of
York, York, North Yorkshire YO10 5DD, United Kingdom
Adrian C. Whitwood − Department of Chemistry, University
of York, York, North Yorkshire YO10 5DD, United
Kingdom; orcid.org/0000-0002-5132-5468
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00347
(14) Wagschal, S.; Perego, L. A.; Simon, A.; Franco-Espejo, A.;
Tocqueville, C.; Albaneze-Walker, J.; Jutand, A.; Grimaud, L.
Formation of XPhos□Ligated Palladium(0) Complexes and Reac-
tivity in Oxidative Additions. Chem. - Eur. J. 2019, 25, 6980−6987.
(15) Montgomery, M.; O’Brien, H. M.; Méndez-Gálvez, C.;
Bromfield, C. R.; Roberts, J. P. M.; Winnicka, A. M.; Horner, A.;
Elorriaga, D.; Sparkes, H. A.; Bedford, R. B. The surprisingly facile
formation of Pd(I)−phosphido complexes from ortho-biphenylphos-
phines and palladium acetate. Dalton Trans. 2019, 48, 3539−3542.
(16) Scott, N. W.; Ford, M. J.; Schotes, C.; Parker, R. R.; Whitwood,
A. C.; Fairlamb, I. J. The ubiquitous cross-coupling catalyst system ‘Pd
(OAc)2’/2PPh3 forms a unique dinuclear Pd I complex: an important
entry point into catalytically competent cyclic Pd3 clusters. Chem. Sci.
2019, 10, 7898−7906.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to Bayer AG (Ph.D. studentship to N.W.J.S.)
for funding this study. D.R.H. is funded by an EPSRC iCASE
award (voucher number 19000077) with GlaxoSmithKline.
We thank the University of York for supporting NMR
spectrometers and X-ray equipment, and EPSRC for NMR
upgrades (EP/K039660/1). We thank Karl Heaton (MS), Dr.
Alex Heyam and Heather Fish (NMR) for their guidance and
support.
(17) Coulson, D. R. Ready cleavage of triphenylphosphine. Chem.
Commun. (London) 1968, 1530−1531.
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