Pyrazole-Based PCN Pincer Complexes of Palladium(II): Mono- and Dinuclear Hydroxide Complexes and Ligand Rollover C-H Activation

Article abstract of DOI:10.1021/acs.organomet.5b00355

Palladium complexes of the novel unsymmetrical phosphine pyrazole-containing pincer ligands PCN^H (PCN^H = 1-[3-[(di-tert-butylphosphino)methyl]phenyl]-1H-pyrazole) and PCN^Me (PCN^Me = 1-[3-[(di-tert-butylphosphino)methyl]phenyl]-5-methyl-1H-pyrazole) have been prepared and characterized through single-crystal X-ray diffraction and multinuclear ¹H, ¹³C{¹H}, and ³¹P{¹H} NMR spectroscopy. In preparations of the monomeric hydroxide species (PCN^H)Pd(OH), an unexpected N detachment followed by C-H activation on the heterocycle 5-position took place resulting in conversion of the monoanionic {P,C^-,N} framework into a dianionic {P,C^-,C^-} ligand set. The dinuclear hydroxide-bridged species (PCN^H)Pd(μ-OH)Pd(PCC) was the final product obtained under ambient conditions. The "rollover" activation was followed via ³¹P{¹H} NMR spectroscopy, and dinuclear cationic μ-OH and monomeric Pd^II hydroxide intermediates were identified. DFT computational analysis of the process (M06//6-31G, THF) showed that the energy barriers for the pyrazolyl rollover and for C-H activation through a σ-bond metathesis reaction are low enough to be overcome under ambient-temperature conditions, in line with the experimental findings. In contrast to the PCN^H system, no "rollover" reactivity was observed in the PCN^Me system, and the terminal hydroxide complex (PCN^Me)Pd(OH) could be readily isolated and fully characterized. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.organomet.5b00355

Article
pubs.acs.org/Organometallics
Pyrazole-Based PCN Pincer Complexes of Palladium(II): Mono- and
Dinuclear Hydroxide Complexes and Ligand Rollover C−H Activation
Wilson D. Bailey,† Lapo Luconi,‡ Andrea Rossin,‡ Dmitry Yakhvarov,§ Sarah E. Flowers,†
Werner Kaminsky,† Richard A. Kemp,*,∥,⊥ Giuliano Giambastiani,*,‡,§ and Karen I. Goldberg*,†
†Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States
‡Institute of Chemistry of Organometallic Compounds ICCOM-CNR and Consorzio INSTM, Via Madonna del Piano 10, 50019
Sesto F.no Florence, Italy
§Kazan Federal University, 420008 Kazan, Russian Federation
∥Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131, United States
⊥Advanced Materials Laboratory, Sandia National Laboratories, Albuquerque, New Mexico 87106, United States
S
* Supporting Information
ABSTRACT: Palladium complexes of the novel unsymmetrical
phosphine pyrazole-containing pincer ligands PCNH (PCNH = 1-[3-
[(di-tert-butylphosphino)methyl]phenyl]-1H-pyrazole) and PCNMe
(PCNMe = 1-[3-[(di-tert-butylphosphino)methyl]phenyl]-5-methyl-1H-
pyrazole) have been prepared and characterized through single-crystal
1
X-ray diffraction and multinuclear H, 13C{1H}, and 31P{1H} NMR
spectroscopy. In preparations of the monomeric hydroxide species
(PCNH)Pd(OH), an unexpected N detachment followed by C−H
activation on the heterocycle 5-position took place resulting in
conversion of the monoanionic {P,C−,N} framework into a dianionic
{P,C−,C−} ligand set. The dinuclear hydroxide-bridged species
(PCNH)Pd(μ-OH)Pd(PCC) was the final product obtained under
ambient conditions. The “rollover” activation was followed via 31P{1H} NMR spectroscopy, and dinuclear cationic μ-OH and
monomeric PdII hydroxide intermediates were identified. DFT computational analysis of the process (M06//6-31G*, THF)
showed that the energy barriers for the pyrazolyl rollover and for C−H activation through a σ-bond metathesis reaction are low
enough to be overcome under ambient-temperature conditions, in line with the experimental findings. In contrast to the PCNH
system, no “rollover” reactivity was observed in the PCNMe system, and the terminal hydroxide complex (PCNMe)Pd(OH) could
be readily isolated and fully characterized.
greater numbers (NCC, PNN, PCO, PCS, etc.).6 PCN-type
systems in particular are intriguing, because in (PCN)M(L)n
INTRODUCTION
■
The use of pincer-ligated transition-metal complexes as
complexes (M = transition metal; L = ancillary ligand) the
catalysts for organic transformations has grown dramatically
tridentate hybrid ligand contains both hard (N) and soft (P)
in recent years.1 The robust tridentate binding motif coupled
with the tunability of the steric and electronic parameters of
pincer ligands has proven highly effective in stabilizing and
allowing isolation of a variety of uncommon types of metal
complexes. For example, pincer ligands have been very useful in
the preparation of mononuclear late-metal complexes bearing
M−OR and M−NR2 bonds. Notably, there are significantly
fewer mononuclear late-transition-metal hydroxide, alkoxide,
donor functions, thus leading to novel and unprecedented
chemical properties.7 In such species, there is a marked
difference in the trans effect between the two different donor
arms. This difference results in the group with the weaker trans
effect (N) being more likely to dissociate from the metal center
due to its position trans to the group with a stronger trans effect
(P). The stronger M−P bond remains intact. From a
homogeneous catalysis perspective, the hemilability of the
ligand provides access to a vacant coordination site at the metal
center and so can allow for effective coordination, activation,
and amide complexes relative to their metal alkyl (M−C)
analogues.2 However, such M−OR and M−NR2 linkages are
pertinent to catalysis,3,4 and thus isolation and study of model
and transformation of substrate molecules. Such ligand
metal hydroxide, alkoxide and amide complexes are of great
hemilability has often been invoked as the critical factor
value.
when comparing the catalytic activity of (PCP)M(L)n/
The variety of available pincer ligands has increased in recent
years. While early pincer ligands were symmetric with respect
to ligand “arms” (e.g., PCP, PNP, POCOP, etc.),5 pincer-type
Received: April 30, 2015
complexes bearing unsymmetrical arms have begun to appear in
© XXXX American Chemical Society
A
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(NCN)M(L)n and (PCN)M(L)n analogues; the unsymmetrical
complex has consistently been shown to be more active than its
symmetric counterparts.6i,j,8,9
available (3-bromophenyl)methanol. The phosphine group was
introduced by transformation of the benzyl alcohol moiety into
a better leaving group (reaction ii, bromination) followed by
the phosphine nucleophilic substitution (reaction iii) as the
final synthetic step. The PCNH pincer ligand 3a was isolated as
an analytically pure and air-sensitive white solid in 48% yield
from 3-bromobenzyl alcohol (see the Experimental Section).
Synthesis of Chloro, Nitrile, and Triflate Complexes
Containing the [(PCNH)Pd]+ Fragment. The reaction of
PCNH ligand 3a with Pd(COD)Cl2 in toluene (110 °C) was
monitored through successive solution samplings and 31P{1H}
NMR spectroscopy at room temperature. Full conversion to
the PdII complex (PCNH)PdCl (4a) was observed after 4 h
with a new downfield 31P{1H} NMR signal at 95.1 ppm
(Scheme 2).
In our laboratories, we have explored a variety of both
symmetric and unsymmetrical pincer systems (PCP,10 PNP,11
PCO,6h NNC12) to stabilize and study model complexes. While
the symmetric ligand systems result in more stable complexes,
the unsymmetrical analogues have yielded higher reactivity
primarily through arm lability. In a study on late-transition-
metal PCO pincer systems, the ether O arm was found to be
labile and allowed for reductive elimination of the pincer
framework under reducing conditions.6h Ligand arm aromatic/
heteroaromatic C−H activation through σ-bond metathesis was
observed in early-transition-metal and rare-earth complexes to
give unsymmetrical NNC pincer complexes.12
The (PCNH)PdCl complex 4a was isolated as a moderately
air sensitive white microcrystalline solid and was characterized
The following is an account of a novel unsymmetrical PCN
pincer ligand system designed to relieve steric bulk around the
metal center relative to its PCP counterpart. The nitrogen
donor (pyrazolyl group) was expected to be a more strongly
binding ligand than an ether linkage (as in a related PCO
system) yet would potentially provide for hemilability at
elevated temperatures. Notably, this is also a cautionary tale
about unsymmetrical pincer ligands. Pincer ligands are often
touted for their robustness, but here we find that the
unsymmetrical members of the family can display unwanted
reactivity of their own. The metalation of two new pyrazole-
containing PCN ligand frameworks on PdII and the exploration
of the reactivity of the related chloride and hydroxide
complexes are discussed. In a hydroxide derivative, an
unexpected pyrazolyl side arm C−H bond activation was
observed. C−H addition across the Pd−OH bond on the 5-
position of the heterocycle occurred to transform the
monoanionic {P,C−,N} ligand into a dianionic {P,C−,C−}
donor. The thermodynamics are favorable, with the driving
force for the reaction to occur provided by the simultaneous
formation of a stable neutral aquo ligand that exits the metal
coordination sphere. Ligand loss generates a positive entropy
change and an overall (more) negative Gibbs energy variation
for the side arm C−H activation reaction. The process was
1
by multinuclear 31P{1H}, H, and 13C{1H} NMR spectroscopy
1
combined with single-crystal X-ray diffraction studies. The H
and 13C{1H} NMR patterns indicate that 4a possesses Cs
symmetry in solution. The most representative 1H and
13C{1H} NMR spectroscopic resonances related to the κ3-
coordinated ligand fall at lower fields in comparison with those
observed for the free ligand; the aryl carbon atom directly
bound to the PdII center shows the largest resonance shift
(from 120.1 (3a) to 150.3 (4a) ppm for PCNH). Microcrystals
suitable for X-ray analysis were grown from a concentrated
acetone solution at −30 °C. An ORTEP representation of the
crystal structure of 4a is given in Figure 1. Table S1
(Supporting Information) gives all the main crystal and
structural refinement data, and selected bond lengths and
angles are summarized in Table S2 (Supporting Information).
Complex 4a crystallizes in the orthorhombic P212121 space
group with four molecules per unit cell. The PdII center adopts
a distorted-square-planar coordination geometry (τ4 = 0.16).13
Bond lengths and angles measured within the [(PCNH)Pd]+
fragment of 4a fall in the typical range observed for related
[(PCNH)Pd]+ fragments in square-planar environments.14,9b It
is notable that the Pd−P bond in 4a (2.227(4) Å) is shorter
than those observed in symmetric (PCP)Pd(L) analogues
bearing the −PtBu2 group (mean Cambridge Structural
Database (CSD) value 2.31 Å)10,15 and that the Pd−N bond
length in 4a (2.094(10) Å) is longer than that found in
symmetric pyrazole-based (NCN)Pd(L) species (mean CSD
value 2.03 Å).16 This crystallographic evidence suggests that the
pyrazolyl group is more likely to possess hemilabile properties
in the unsymmetrical PCNH environment.
1
followed experimentally (multinuclear 31P{1H} and H NMR
spectroscopy) and then computationally modeled (DFT at the
M06//6-31G* level of theory). By protection of the ligand in
the 5-position through methylation (PCNMe), this “rollover”
reactivity was shut down and a stable terminal hydroxide
complex was isolated.
RESULTS AND DISCUSSION
■
Synthesis of the PCN-Pincer Ligand PCNH (3a). Scheme
1 summarizes the synthetic path used to obtain the PCNH
pincer ligand 3a in fairly good yields starting from commercially
It was found that the chloride ligand trans to the strongly
donating aryl backbone in 4a could be abstracted by treatment
of 4a with silver reagents (Scheme 2). The nitrile complex
[(PCNH)Pd(MeCN)](BF4) (5a) was obtained as a finely
divided white powder from the reaction of 4a with 1 equiv of
AgBF4 in CH2Cl2 in the presence of a 5-fold excess (compared
to Pd) of acetonitrile. Complex 5a was characterized by
multinuclear NMR spectroscopy and elemental analysis.
Unfortunately, attempts to obtain crystals of 5a suitable for
a
Scheme 1. Synthesis of the Pincer PCNH Ligand 3a
1
X-ray analysis were unsuccessful. The H and 13C{1H} NMR
resonances of 5a are very similar to those observed for the
parent chloride precursor 4a, except for the appearance of a
1
sharp singlet in the H NMR spectrum at 2.51 ppm (ascribed
a
to the methyl group of the coordinated CH3CN molecule).
The 31P{1H} NMR signal moves from 95.1 ppm for 4a to 98.2
ppm for 5a.
Reagents and conditions: (i) pyrazole, CuI, K2CO3, NMP, microwave
irradiation, 210 °C, 5 h, 250 W; (ii) Br3CCO2Et, PPh3, CH2Cl2, room
temperature, 0.5 h; (iii) Bu2PH, acetone, reflux, 12 h.
t
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Scheme 2. Synthesis of the Chloride (4a, 6a), Nitrile (5a), and Triflate (7a) Pincer Complexes of PCNH
Figure 2. ORTEP representation of the cationic part of the salt
{[(PCNH)Pd]2(μ-Cl)}BF4 (6a), with ellipsoids shown at 50%
probability. Hydrogen atoms, Bu groups on P, the BF4 counterion,
and crystallization solvent molecules are omitted for clarity.
Figure 1. ORTEP representation of (PCNH)PdCl (4a), with ellipsoids
shown at 50% probability. Hydrogen atoms are omitted for clarity.
t
−
Treatment of a CH2Cl2 solution of 4a with only 0.5 equiv of
AgBF4 (Scheme 2) afforded the dinuclear PdII complex
{[(PCNH)Pd]2(μ-Cl)}BF4 (6a), in the form of pale yellow
microcrystals. The NMR spectra of the chloride-bridged dimer
show the classical patterns of C2-symmetric species in solution,
whose most representative features are given by two sharp
doublets in the 13C{1H} NMR spectra: one coming from the 12
methyl groups (28.8 ppm, JPC = 3.5 Hz) of the tert-butyl
moieties and the other belonging to the ipso carbon atom σ-
bound to the palladium center (149.8 ppm, JPC = 13.3 Hz). As
expected, a downfield sharp singlet is also observed in the
31P{1H} NMR spectrum (95.1 ppm). Microcrystals suitable for
X-ray diffraction were grown by layering a concentrated
CH2Cl2 solution of the salt with cold pentane. The solid-state
structure confirmed the identity of 6a (an ORTEP
representation is given in Figure 2, and relevant bond angles
and distances are given in Table S2 (Supporting Information)).
The dimer crystallizes in the orthorhombic Pna21 space
group, with four molecules per unit cell. In the chloride-bridged
dinuclear cation, both palladium centers maintain a slightly
distorted square planar geometry. The main bond lengths and
angles in 6a are very similar to those found in the monomeric
complex 4a. Similarly to related μ-Cl monobridged cationic
dimers,17 only slightly longer Pd−Cl distances are found in 6a
(2.427(4) Å for Pd(1)−Cl(1) and 2.425(4) Å for Pd(2)−
Cl(1)) in comparison to the bond length determined in 4a
(2.388(4) Å).
The reaction of 4a with 1 equiv. of AgOTf in CH2Cl2 in the
absence of acetonitrile yields the triflate species (PCNH)Pd-
(OTf) (7a) (Scheme 2). Complex 7a was fully characterized by
multinuclear NMR spectroscopy, X-ray crystallography, and
combustion analysis. The 1H and 13C{1H} NMR spectra
display patterns very similar (albeit slightly shifted) to those
observed for the chloride analogue 4a. The 31P{1H} NMR
spectrum displayed a singlet at 94.1 ppm. Complex 7a exhibits
a typical square-planar ligand arrangement about the Pd center
with κ3-PCNH and triflate ligands coordinated. An ORTEP
representation of 7a is shown in Figure S2 (Supporting
Information). Relevant bond lengths and angles appear in
Table S2 (Supporting Information).
Formation of Hydroxide Species Containing the
[(PCNH)Pd]+ and [(PCC)Pd] Fragments: “Rollover” C−H
Bond Activation on the Pyrazolyl Side Arm. As previous
transition-metal hydroxide complexes have been synthesized
primarily through metathesis reactions with alkali-metal
C
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hydroxides,18 preparation of the desired monomeric hydroxide
complex was first attempted by addition of potassium
hydroxide (KOH) to the chloride complex 4a in tetrahydrofur-
an (THF). No reaction was observed at room temperature, and
only a slow reaction occurred to produce an array of intractable
products, including palladium black, over very long reaction
times (4 days) at elevated temperatures (60 °C). As group 10
terminal hydroxides have also been made via metathesis of
weakly bound anions rather than from the chloride precursor,19
the reactivity of the triflate complex 7a with hydroxide salts was
investigated.
temperature. In this case, reversible cleavage of the dinuclear
complex 6a may be occurring on the NMR time scale, resulting
t
in the Bu and methylene signals appearing as respectively
averaged signals. Another possibility is that free rotation about
the Pd−Cl bond of 6a occurs at room temperature, although
this option may be considered less likely given the similar steric
profiles of 6a and 8a. X-ray-quality crystals of 8a were grown
from a concentrated THF solution layered with pentane. The
solid-state structure confirmed the spectroscopic assignment of
8a as the dinuclear species {[(PCNH)Pd]2(μ-OH)}(OTf)
(Figure 4).
Addition of an excess of KOH to a THF-d8 solution of the
triflate complex 7a yielded the new product 8a, as observed by
the disappearance of the signal at 94.1 ppm and the appearance
of a new singlet at 93.3 ppm in the 31P{1H} NMR spectra
(Figure 3). However, further monitoring of this reaction
Figure 4. ORTEP representation of the cationic part of the salt
{[(PCNH)Pd]2(μ-OH)}(OTf) (8a), with ellipsoids shown at 50%
t
probability. Hydrogen atoms, Bu groups on P, and triflate counterion
are omitted for clarity.
Figure 3. 31P{1H} NMR stack of the formation of species 8a−10a
Dinuclear 8a is C2 symmetric with the two distorted-square-
planar (PCNH)Pd units bridged by a single μ-OH (τ4[Pd(1)] =
0.14; τ4[Pd(2)] = 0.16).13 Each palladium is formally PdII; one
noncoordinating triflate counterion is found in the asymmetric
unit to balance the overall positive charge of the dinuclear
complex. The structure of 8a is analogous to that of the
chloride-bridged dinuclear species 6a, with a bridging hydroxide
instead of a bridging chloride. The hydroxyl hydrogen was
refined by geometry optimization of the hydroxide ligand with
respect to the Pd(1)−O(1) bond, while leaving the Pd(2)−
over days.
showed conversion of 8a to the second species 9a with a singlet
at 91.8 ppm in the 31P{1H} NMR spectrum. A third product
10a, with two new signals at 91.1 and 71.0 ppm, then grows in
concurrently with a relative decrease of the 91.8 ppm signal due
to 9a. Some decomposition of the reaction mixture was also
observed in the reaction vessel as palladium black became
visible. When the reaction conditions were changed as
described below, characterization of the various palladium
species formed in this reaction (8a−10a) was possible.
t
O(1) bond free. The hydroxide is shielded by the bulky Bu
groups, and no hydrogen-bonding interactions were observed.
The bond lengths within the (PCNH)Pd units of 8a are similar
to those observed in the mononuclear triflate complex 7a.
While bis-μ-OH dimers are well-known for Pd, this is a rare
example of two palladium centers bridged only by a single μ-
OH ligand.20 Table S2 in the Supporting Information contains
selected bond lengths and angles for 8a.
When a THF solution of the triflate 7a was treated with a full
1 equiv of KOH, 8a was observed as a transient intermediate
and decayed into complex 9a as the major product. Complex 9a
is characterized by a singlet at 91.8 ppm in the 31P{1H} NMR
spectrum. Conversion to complex 9a was not clean, and peaks
at 91.1 and 71.0 ppm (complex 10a) in the 31P{1H} NMR
When only 0.5 equiv of KOH was added to the triflate 7a,
the new species 8a was formed as the sole product (Scheme 3).
1
The H NMR spectrum of 8a displays a peak at −1.48 ppm,
indicative of a Pd hydroxide species. The tBu protons appear as
two doublets at 1.44 and 1.51 ppm (3JPH = 14.1 and 14.3 Hz,
respectively). Integration of the doublets in comparison to the
hydroxide signal yields a 18:18:1 ratio, suggesting a dinuclear
t
species. The appearance of two signals for the Bu protons
demonstrates that these positions are diastereotopic, as would
be expected in a rigid nonplanar dinuclear complex. This
observation is in contrast with what is found for 6a, where only
t
one resonance is observed for the Bu groups at room
Scheme 3. Synthesis of the Hydroxide Pincer Complexes 8a−10a of the Ligand PCNH
D
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spectrum consistently appeared as a minor product, such that it
was not possible to isolate pure 9a. However, it was found that
the addition of water to 9a inhibited decomposition to 10a.
Thus, addition of D2O to a THF-d8 (1/4 v/v) solution of 8a
containing suspended KOH resulted in the formation of pure
9a, which could be fully characterized in solution by NMR
spectroscopy. On the basis of the intermediacy of 8a in the
formation of 9a and the 1H and 31P{1H} NMR spectral signals
assignable to 9a, complex 9a is proposed to be the
mononuclear terminal hydroxide (PCNH)Pd(OH) (Scheme
lengths within 10a are compared, a clear difference is observed.
In 10a, the hydroxide ligand is associated more with the
cationic [(PCN)Pd]+ fragment (2.0831(11) Å) than with the
neutral [(PCC)Pd] fragment (2.1077(11) Å), further support-
ing a mixed-ligand dinuclear species. It should be noted that, for
complex 8a, the hydroxide ligand is shared equally between
palladium centers within error.
The activation of a pyrazolyl C−H bond at the 5-position has
previously been observed in (poly)pyrazolyl-containing ligands
in the presence of late transition metals (e.g., Ru,21 Pd,22 Pt,23
and Ir24), and the activation of C−H bonds by palladium
fragments in several N-heterocycles has been studied computa-
tionally.25 Notably, however, the “rollover” activation to form
10a takes place under very mild conditions, in contrast to many
previously reported literature examples that require elevated
temperatures or the employment of photoirradiation. We
propose that the room-temperature “rollover” C−H activation
occurs at the monomeric hydroxide 9a, forming a neutral and
coordinatively unsaturated [(PCC)Pd] unit after water release
(i.e., loss of an aquo ligand from the palladium coordination
sphere). This is supported by the increased stability of 9a in the
presence of water (vide supra). Subsequent combination of the
[(PCC)Pd] fragment with another molecule of (yet unreacted)
9a yields the bridged species 10a (Scheme 3). When a THF
solution of a mix of 9a as the major product and 10a as a minor
impurity was left at room temperature for 6 days,
decomposition of 9a occurred, yielding 10a as the major
product, along with unidentified side products as observed by
multinuclear NMR spectroscopy. Attempts to obtain a pure
sample of 10a in solution were unsuccessful. However, by
analyzing a mixture of 9a converting to 10a before further
decomposition, signals attributed to the mixed ligand complex
10a could be, in part, made by NMR spectroscopy. Notably,
1
3). The H NMR spectrum of 9a displays a signal at −1.71
ppm (observed only in the absence of excess water),
t
attributable to the Pd−OH group, and the Bu protons appear
as a doublet at 1.43 ppm, integrating in the ratio 18H:1H in
comparison to the hydroxide signal.
Efforts to recrystallize 9a from the reaction mixture in THF
at low temperatures did yield crystals suitable for an X-ray
diffraction study, as well as amorphous material. However, as
shown in Figure 5, the crystallized product was not 9a but
Figure 5. ORTEP representation of the [(PCNH)Pd](μ-OH)[Pd-
(PCC)] dimer (10a), with ellipsoids shown at 50% probability.
1
t
the H NMR spectrum of the mixture shows four new Bu
signals (1.36, 1.39, 1.44, and 1.51 ppm), each as doublets,
indicating that the substituents are not only diastereotopic as in
complex 8a but are bound to two chemically inequivalent
phosphorus atoms. This is expected, as one phosphine arm is
trans to N and the other trans to C. This mixed-ligand
framework is also observed by four sets of doublets of doublets
for the diastereotopic methylene protons (3.08, 3.12, 3.42, and
t
Hydrogen atoms on the pincer ligands and Bu groups on P are
omitted for clarity.
rather complex 10a, the mixed-ligand hydroxide-bridged
dinuclear complex [(PCNH)Pd](μ-OH)[Pd(PCC)]. This was
confirmed, as dissolution of the isolated crystalline sample in
THF yielded spectral features that matched those displayed by
10a in the 31P{1H} NMR spectrum along with minor impurities
attributed to the amorphous solid. The structure of 10a consists
of two palladium centers bridged by a single μ-OH ligand,
similar to the case for 8a. One of the PdII centers is bound by
the {P,C−,N} ligand as observed in 8a. However, unlike 8a, the
other PdII center is bound to a dianionic tridentate {P,C−,C−}
ligand; the pyrazolyl arm has “rolled over” with C−H activation
having occurred at the 5-position. Details on how the pyrazolyl
ring and the “rollover” ring were distinguished in the solid-state
structure are presented in the Supporting Information. The
[(PCNH)Pd]+ fragment displays bond angles and distances
nearly identical with those found in the hydroxide complex 8a.
In contrast, the Pd(1)−P(1) bond in the [(PCC)Pd] fragment
is somewhat longer than the corresponding bond in the
(PCNH)Pd fragment (2.3084(4) Å vs 2.2260(4) Å). This
change is expected, as the phosphorus donor is trans to a
stronger donor when moving from neutral N to anionic C−.
Notably, no counterion was observed, which is consistent with
the dianionic PCC character of the second pincer fragment.
The Pd−O bond lengths in the cationic and neutral μ-
hydroxide dimers (8a and 10a, respectively) are very similar.
However, when the Pd(1)−O(1) and the Pd(2)−O(1) bond
t
3.49 ppm), as well as four Bu primary carbon signals in the
13C{1H} NMR spectrum (29.46, 29.51, 29.91, and 30.10 ppm;
2JPC = 3.9, 3.5, 6.0, and 6.0 Hz, respectively). Unfortunately,
due to the complexity of the mixture and the inability to isolate
10a, the other resonances in the 13C{1H} NMR spectrum could
not be assigned confidently. To explore this “rollover”
activation in more detail, DFT simulations of the reaction
were undertaken.
DFT Simulation of the “Rollover” Cyclometalation in
the Conversion of the Mononuclear Hydroxide 9a to the
Dinuclear μ-OH Complex 10a. The pyrazolyl “rollover” and
related activation of the C(5)−H pyrazolyl bond to form H2O
was examined computationally on the real system at the M06//
6-31G* level of theory. THF solvent was included in the
calculation through a continuum model (SMD), and all the
figures reported here should be considered as ΔG or ΔG⧧
values evaluated in THF (see Computational Details). A water
molecule generated by an intermolecular rearrangement of the
hydroxide complex 9a implies a (PCNH)− ↔ (PCC)2−
conversion, which is proposed on the basis of the experimental
results wherein the dinuclear complex (PCNH)Pd(μ-OH)Pd-
(PCC) (10a) was isolated. The rotation of the pyrazolyl ring
E
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around the C(2)−N(2) bond (Figure 1 for atom numbering)
within the pincer can lead to an activation of the C−H bond in
the 5-position by the palladium center, with concomitant
formation of the aquo species (PCC)Pd(H2O) (11a) as the
result of a proton transfer to the −OH group on palladium.
Beginning with the mononuclear hydroxide complex 9a,
rotation of the pyrazolyl substituent by 180° with respect to
the starting geometry yields an isomeric form of the hydroxide
(PCCH)Pd(OH) (9a′), wherein the C−H bond in the 5-
position interacts with the metal center through an agostic
interaction (optimized d[C−Pd] = 2.58 Å; d[H−Pd] = 2.29 Å;
d[C−H] = 1.09 Å). This optimized structure is drawn in Figure
6. Unsurprisingly, N-coordination is more stabilizing than a C−
H agostic interaction and the Gibbs energy of 9a′ is higher than
that of 9a by 17.0 kcal mol−1.
Figure 7. Optimized structure of TS1. Selected bond lengths and
distances (Å) are reported. H atoms on the pincer are omitted for
clarity. For the atom color code, see the caption to Figure 6.
Figure 8. Optimized structure of (PCC)Pd(H2O) (11a). Selected
bond lengths and distances (Å) are reported. H atoms on the pincer
are omitted for clarity. For the atom color code, see the caption to
Figure 6.
Figure 6. Optimized structure of (PCCH)Pd(OH) (9a′). Selected
bond lengths and distances (Å) reported. H atoms on the pincer
omitted for clarity. Atom color code: gray, C; white, H; red, O; blue,
N; purple, P; orange, Pd.
H2O] = 1.83 Å) and it is responsible for an extra stabilization of
the dinuclear product. The optimized structure of 10a···H2O is
reported in Figure 9. Evolution to the dimeric product provides
An estimation of the rotation barrier of the pyrazolyl ring was
made through a scan of the θ[C(1)−C(2)−N(2)−N(1)]
dihedral angle reaction coordinate (Scheme 4 and Figure 1 for
Scheme 4. Pincer Rearrangement around the Pd Center
through 180° Rotation of the Pyrazolyl Ring
Figure 9. Optimized structure of [(PCNH)Pd(μ-OH)Pd(PCC)]···
H2O (10a···H2O). Selected bond lengths and distances (Å) are
reported. H atoms on the pincer and tert-butyl groups on phosphorus
are omitted for clarity. For the atom color code, see the caption to
Figure 6.
atom numbering). The maximum of the energy profile along
this coordinate is located at 24.5 kcal mol−1 from the starting
geometry (at θ(C−C−N−N) ≈ 75°), and it can be reasonably
considered the “transition state” for this ligand rearrangement
(Figure S54, Supporting Information).
From 9a′, a direct proton transfer through a four-centered
transition state (TS1, Figure 7;ΔG⧧ = 20.8 kcal mol−1) leads to
the aquo complex 11a (Figure 8; ΔG(9a′/11a) = −11.4 kcal
mol−1). The dimer formation reaction 9a + 11a → 10a···H2O
is thermodynamically favored with ΔG = −12.0 kcal mol−1. In
the final structure, the water molecule engages in a hydrogen
bond with the bridging hydroxide group (optimized d[μ-HO···
the strong driving force for the reaction to take place.
Interestingly, the pyrazolyl rotation and the C−H activation
barriers are almost identical at the computational level used;
thus, the heterocycle rotation (albeit slow at ambient
temperature, owing to the relatively high ΔG⧧ found) is
accompanied by a simultaneous C−H activation on the 5-
position and dimer formation, as found experimentally.
Notably, this analysis is consistent with experimental results,
as the aquo species 11a was never detected in the course of the
F
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transformation of 9a to 10a by 31P{1H} NMR spectroscopy.
The overall Gibbs energy vs reaction coordinate profile for this
transformation is reported in Scheme 5.
Scheme 7. Synthesis of the Chloride (4b) and Triflate (7b)
Pincer Complexes of PCNMe
Scheme 5. Gibbs Energy (THF) vs Reaction Coordinate
Profile for the Formation of the Dinuclear Species 10a···H2O
Starting from the Hydroxide Complex 9a
NMR spectroscopy as well as by single-crystal X-ray diffraction.
Similar to its PCNH analogue 4a, complex 4b displays Cs
symmetry in solution. The spectroscopic signals for 4b are
nearly identical with those of 4a, with the exception of the
1
appearance of a methyl signal in the H and 13C{1H} NMR
spectra at 2.71 and 13.9 ppm, respectively, and a corresponding
loss of the pyz-H signal at the 5-position. A singlet at 93.3 ppm
is observed in the 31P{1H} NMR spectrum. An ORTEP
representation of the solid-state structure of 4b is presented in
Figure S1 in the Supporting Information. The bond distances
and angles of 4b are very similar to those of 4a.
When 4b was treated with 1 equiv of silver triflate in
methylene chloride at room temperature, AgCl precipitation
occurred over 5 h and the PdII triflate complex (PCNMe)Pd-
(OTf) (7b, Scheme 7) was formed. Complex 7b was
characterized by NMR spectroscopy, X-ray diffraction, and
elemental analysis. The 1H and 13C{1H} NMR spectra of 7b are
very similar to those of the nonmethylated analogue 7a,
differing in the appearance of a methyl resonance at 2.68 and
14.56 ppm, respectively. The 31P{1H} NMR spectrum of 7b
displays a singlet at 92.1 ppm. Similar to the case for 7a, the
solid-state structure of 7b shows a square-planar complex with
the triflate ligand directly bound to the Pd center (Figure S3 in
the Supporting Information). The physical metrics of 7b are
closely related to those of the PCNH analogue 7a, diverging
with respect to the Pd−N interaction; a shorter Pd−N distance
of 2.0779(16) Å is found in 7b in comparison to 2.112(2) Å in
7a. Table S2 in the Supporting Information gives a selection of
bond lengths and angles in the X-ray structure of 7b.
Synthesis of the Protected PCNMe Ligand and the
Corresponding Species of the [(PCNMe)Pd]+ Fragment.
Conversion of the mononuclear hydroxide complex 9a to the
rollover cyclometalated dinuclear 10a should be inhibited if the
5-position of the pyrazolyl ring is blocked. With this in mind,
the ligand PCNMe (3b), which bears a methyl group at the 5-
position of the pyrazolyl, was prepared (Scheme 6). The
a
Scheme 6. Synthesis of the Pincer PCNMe Ligand 3b
When 0.5 equiv of KOH was added to a solution of 7b in
THF (Scheme 8), formation of the new species 8b was
Scheme 8. Synthesis of the Hydroxide (8b, 9b) Pincer
a
Complexes of PCNMe
a
Reagents and conditions: (i) NaBH4, EtOH, room temperature, 1 h;
(ii) Br3CCO2Et, PPh3, CH2Cl2, room temperature, 0.5 h; (iii) tBu2PH,
acetone, reflux, 12 h.
a
PCNMe ligand 3b was synthesized following a similar
preparation from the PCNH ligand 3a, but starting from the
previously reported 3-(5-methyl-1H-pyrazol-1-yl)-
benzaldehyde.6b The phosphine arm was installed from the
benzyl bromide derivative, which after workup gave the
analytically pure off-white viscous and air-sensitive oil 3b in
52% isolated yields from the aldehyde starting complex, as
shown in Scheme 6 (see the Experimental Section).
Reagents and conditions: (i) 0.5 KOH, THF, room temperature.
observed by 31P{1H} NMR spectroscopy through the
appearance of a new singlet at 91.6 ppm. This chemical shift
variation (from the triflate species 7b at 92.1 ppm) was very
similar to the shift observed on going from 7a to 8a in the
PCNH system, suggesting that a similar μ-OH species had
formed. Indeed, 8a was identified as the symmetric dinuclear
species {[(PCNMe)Pd]2(μ-OH)}(OTf) by NMR spectroscopy
and single-crystal X-ray diffraction. The H NMR spectrum
shows a diagnostic Pd−OH signal at −1.46 ppm, and
integration of the H NMR signals was consistent with the
Metalation of 3b onto PdII to form the chloride complex
(PCNMe)PdCl (4b) was carried out in a manner analogous to
the metalation of 3a (vide supra and Scheme 7). Complex 4b
1
1
1
was characterized by multinuclear 31P{1H}, H, and 13C{1H}
G
DOI: 10.1021/acs.organomet.5b00355
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presence of 2 equiv of PCNMe ligand per hydroxide group.
Similar to 8a, the Bu protons in 8b are diastereotopic and
coordination site. Each complex is hydrogen-bound to a
symmetry-related copy of itself via two water molecules and the
hydroxyl ligand. This water-bridged mode has been observed
previously in terminal hydroxides.19,26 The [(PCNMe)Pd]+
fragment has metrics (bond angles and distances) similar to
those of the other monomeric (PCNH,Me)PdX (X = Cl, OTf)
structures described above.
The Pd−O bond length (2.078(7) and 2.080(7) Å from two
molecules in the asymmetric unit) is comparable to that found
in the handful of other monomeric PdII−OH complexes that
have been characterized in the solid state (mean CSD value
2.05 Å).27 For example, the PdII−OH bond length reported for
the pincer complex (tBuPCP)Pd(OH)···H2O was 2.095 Å.28
t
appear as two doublets at 1.43 and 1.49 ppm (3JPH = 14.0 and
14.2 Hz, respectively). The solid-state structure (Figure S4 in
the Supporting Information) displays two nearly identical
[(PCNMe)Pd]+ fragments equally sharing the bridging hydrox-
ide ligand. An outer-sphere triflate counterion is associated with
the dinuclear species. While the Pd−N distance of 8b is
comparable to that of 8a, the Pd−P bond of 8b (2.2381(11) Å)
is lengthened in comparison to 8a (2.218(2) Å). Otherwise, the
physical parameters in the solid state (bond angles and
distances) between 8b and 8a are nearly indistinguishable.
Addition of a full 1 equiv of KOH to the bridging dinuclear
hydroxide complex 8b, or addition of an excess of KOH to the
mononuclear triflate complex 7b, cleanly yielded the new
product 9b, as observed by NMR spectroscopy (Scheme 8). A
singlet at 90.2 ppm appeared with no additional signals in the
31P{1H} NMR spectrum. The similarity of the reaction and the
chemical shift change to that found for the PCNH analogue
suggests that 9b is a monomeric PdII hydroxide complex.
CONCLUSIONS
■
We have reported the synthesis and characterization of a new
unsymmetrical pincer ligand scaffold and complexation of these
novel PCN ligands to PdII. With a bulky phosphine and a less
sterically hindered pyrazolyl as the arms on the PCN pincer
ligand, both mononuclear and dinuclear PdII hydroxide
complexes were formed and characterized. Furthermore, a
pyrazolyl ring “rollover” activation at the metal center was
observed on a pyrazolyl pincer complex bearing an available C−
H bond at the 5-position of the pyrazolyl side arm. Observation
of this reactivity provides opportunities for the synthesis of new
derivatives of the dianionic tridentate (PCC)2− fragment, and it
also serves as a warning to those interested in exploiting the
“stability” of PCN-ligated structures. Notably, the use of the
methylated analogue (PCNMe) inhibits the “rollover” reactivity,
allowing for a clean isolation of the mononuclear hydroxide
species. Studies to explore the reactivity of the novel mono- and
dinuclear hydroxide complexes with small molecules of interest
in renewable energy (H2) or carbon capture and sequestration
(CO2) research are planned.
1
Indeed, a new signal was observed at −1.97 ppm in the H
NMR spectrum, corresponding to a palladium hydroxide group.
The upfield signal integrated to 1 equiv relative to the PCNMe
signals. The 13C{1H} NMR signals for this complex were also
consistent with the assignment of 9b as (PCNMe)Pd(OH).
While the reaction of the palladium triflate complex 7b or the
dinuclear bridging hydroxide complex 8b with KOH produced
9b, it was also determined that the same complex could be
prepared directly from the chloride 4b. Thus, a metathesis
reaction between 4b and an excess of KOH in THF, performed
with sonication, yielded 9b directly with no observation of a
bridging species such as 8b. As hypothesized, the mononuclear
hydroxide complex 9b was not found to undergo further
reaction (i.e. “rollover” cyclometalation) as was observed for
the PCNH analogue 9a. Thus, it was possible to purify complex
9b through crystallization, and an X-ray diffraction study was
used to confirm the solid-state structure (Figure 10).
EXPERIMENTAL SECTION
■
General Considerations and Materials Characterization. All
air- and/or moisture-sensitive reactions were performed under an inert
atmosphere in flame-dried flasks using standard Schlenk-type
techniques or in a glovebox filled with nitrogen. Tetrahydrofuran
(THF) was purified by distillation from sodium/benzophenone ketyl,
after drying over KOH or used after passage through columns of
activated alumina and molecular sieves. Benzene, n-hexane, pentane,
CH3CN, and toluene were purified by distillation from sodium/
triglyme benzophenone ketyl or were obtained by means of a MBraun
solvent purification system. Pentane and CH2Cl2 were also used after
passage through columns of activated alumina and molecular sieves.
THF-d8, benzene-d6, and toluene-d8 were dried over sodium/
benzophenone ketyl, condensed in vacuo over activated 4 Å molecular
sieves, and degassed by several freeze−pump−thaw cycles prior to use.
CD2Cl2 was dried over activated 4 Å molecular sieves or calcium
hydride. All other reagents and solvents were used as purchased from
Complex 9b crystallizes in the orthorhombic P21212 space
group, with two monomers, two water molecules, and three
disordered THF solvent molecules per asymmetric unit. The
square-planar PdII center is bound by the tridentate PCNMe
ligand with a hydroxide ligand occupying the fourth
1
commercial suppliers. H, 13C{1H}, and 31P{1H} NMR spectra were
obtained on a Bruker Avance 700, Bruker Avance 500, Bruker Avance
DRX-400, or Bruker Avance 300 MHz instrument. Chemical shifts are
reported in ppm (δ) relative to TMS, referenced to the chemical shifts
of residual solvent resonances (1H and 13C), and coupling constants
are given in Hz. 31P{1H} NMR spectra are referenced to an external
85% H3PO4 sample (0 ppm). The N, C, H elemental analyses were
carried out at ICCOM by means of a Carlo Erba Model 1106
elemental analyzer or at the CENTC Elemental Analysis Facility at the
University of Rochester. The GC/MS analyses were performed on a
Shimadzu QP2010S apparatus equipped with a column identical with
that used for GC analysis.
Figure 10. ORTEP representation of the (PCNMe)Pd(OH) complex
9b, with ellipsoids shown at 50% probability. Hydrogen atoms and tBu
groups on the pincer ligands are omitted for clarity. The Pd−HO···
H2O hydrogen bond between the hydroxyl group and the
crystallization water molecule is depicted by a yellow dotted line.
H
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X-ray Diffraction Data. X-ray diffraction intensity data were
collected on an Oxford Diffraction XcaliburPX (4a,b and 6a) equipped
with a CCD area detector or on a Bruker APEX II (7a,b, 8a,b, 9b, and
10a) diffractometer using Mo Kα radiation (λ = 0.71073 Å) at low
temperature (either T = 100 or 120 K, see Table S1 in the Supporting
Information). The data set was integrated and scaled using SAINT and
SADABS within the APEX2 software package by Bruker.29 The
program used for the data collection was CrysAlis CCD 1.171.30 Data
reduction was carried out with the program CrysAlis RED 1.171,31 and
the absorption correction was applied with the program ABSPACK
1.17. Direct methods implemented in Sir9732 were used to solve the
structures, and the refinements were performed by full-matrix least
squares against F2 implemented in SHELX97.33 All of the non-
hydrogen atoms were found from Fourier syntheses of electron density
and were refined anisotropically, while the hydrogen atoms were fixed
in calculated positions and refined isotropically with the thermal factor
depending on the atom to which they are bound (riding model), with
C···H distances in the range 0.95−1.00 Å. The geometrical calculations
were performed by PARST97.34 The details of crystallographic,
collection, and refinement data are shown in Table S1. Molecular plots
were produced by the program ORTEP3.35
C10H10N2O (174.20): C, 68.95; H, 5.79; N, 16.08. Found: C, 69.05;
H, 5.84; N, 16.17.
Synthesis of (3-(5-Methyl-1H-pyrazol-1-yl)phenyl)methanol
(1b). To a stirred solution of 3-(5-methyl-1H-pyrazol-1-yl)-
benzaldehyde (0.415 g, 2.22 mmol) in dry and degassed EtOH (10
mL) was added NaBH4 (0.126 g, 3.34 mmol) in one portion. The
reaction mixture was stirred at room temperature for 1 h before being
quenched with water (10 mL). The mixture was then extracted with
AcOEt (3 × 20 mL), and the combined organic layers were dried over
Na2SO4. Filtration and solvent removal under reduced pressure gave
the crude material, which was purified by flash chromatography (silica
gel; petroleum ether/AcOEt 60/40) to give 1b as an off-white oil
1
(0.367 g, yield 87.8%). H NMR (300 MHz, CD2Cl2, 293 K): δ 2.31
(s, 3H, CH3), 2.94 (br s, 1H, CH2OH), 4.65 (s, 2H, CH2OH), 6.19
(m, 1H, CH), 7.33−7.28 (2H, CH Ar), 7.44−7.37 (2H, CH Ar), 7.51
(m, 1H, CH Ar). 13C{1H} NMR (75 MHz, CD2Cl2, 293 K): δ 12.1
(s), 64.0 (s), 106.7 (s), 123.0 (s), 123.4 (s), 125.6 (s), 128.8 (s), 138.9
(s), 139.6 (s), 139.9 (s), 142.9 (s). Anal. Calcd for C11H12N2O
(188.23): C, 70.19; H, 6.43, N, 14.88. Found: C, 70.39; H, 6.65; N,
14.93.
General Procedure for Bromination of the Benzyl Alcohol
Intermediates 1a,b: Synthesis of 1-(3-(Bromomethyl)phenyl)-
1H-pyrazole (2a) and 1-(3-(Bromomethyl)phenyl)-5-methyl-
1H-pyrazole (2b). To a stirred solution of the selected alcohol (1a
or 1b) intermediate (5 mmol) and PPh3 (1.97 g, 7.5 mmol) in dry and
degassed CH2Cl2 (30 mL) was added Br3CCO2Et (0.93 mL, 5 mmol)
dropwise under an N2 atmosphere. After 30 min of stirring at room
temperature, the reaction mixture was quenched with water and the
formed layers were separated. The aqueous phase was washed with
CH2Cl2 (3 × 15 mL), and the combined organic extracts were dried
over Na2SO4. After solvent removal the resulting semisolid material
was washed with hot hexane (3 × 15 mL) and the organic phases were
separated from PPh3 (and OPPh3) residues upon filtration. The crude
residue was then purified by flash chromatography (silica gel;
petroleum ether/AcOEt 80/20) to give 2a,b as off-white viscous oils
in 68.5% and 67.7% isolated yields, respectively.
CCDC 1048866 (4a), 1048867 (4b), 1048868 (6a), 1061228 (7a),
1061229 (7b), 1061230 (8a), 1061231 (8b), 1061232 (9b), and
1061233 (10a) contain supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via ccdc.cam.ac.uk/community/
requestastructure.
Computational Details. Density functional theory (DFT)
calculations were performed using the Gaussian09 program (revision
C.01).36 Model structures were optimized with a M06 functional37
using the SDD/MWB10 pseudopotential and related basis set38 on the
palladium and phosphorus atoms plus a 6-31G* basis set on all the
other atoms. Introduction of diffuse functions is essential to well
reproduce conformational equilibria and experimental electron
affinities.39 An extra d-type polarization function for P and an extra
f-type function for Pd were added to the standard set.40 Gibbs energy
calculations to infer relative thermodynamic stabilities were carried out
on the real system. The initial guess geometry for the optimization was
obtained by starting from the XRD structure of the dimeric species
10a. IRC analysis41 was performed to find the two minima linked by
the related transition structure. When IRC calculations failed to reach
the minima, geometry optimizations from the initial phase of the IRC
path were performed. Frequency calculations were made on all the
optimized structures, to characterize the stationary points as minima or
TSs and calculate zero-point energies, enthalpies, entropies, and gas-
phase Gibbs energies at 298 K. Evaluation of the solvent effects was
performed through a continuum modeling of the reaction medium.
Bulk solvent effects (THF, θ = 7.42) were expressed through the SMD
Continuum Model,42 with the same basis set used for the gas phase
optimizations. The Gibbs energy in solution was calculated according
to the following simplified equation: GTHF = Ggas + (ETHF − Egas).
Synthesis of (3-(1H-Pyrazol-1-yl)phenyl)methanol (1a). To a
solution of (3-bromophenyl)methanol (1.00 g, 5.34 mmol) in N-
methylpyrrolidinone (NMP, 32 mL) were added K2CO3 (1.47 g, 10.68
mmol), CuI (0.10 g, 0.53 mmol), and pyrazole (0.38 g, 5.61 mmol) in
sequence. The reaction mixture was placed in a microwave apparatus
(CEM Discover) and heated at 210 °C for 5 h while a constant
irradiation power was maintained (250 W). The mixture was then
cooled to room temperature and filtered on a Celite pad to remove all
suspended residues, and AcOEt (50 mL) was used to wash the filter.
The collected organic phase was evaporated in vacuo to give a viscous
residue that was purified by flash chromatography (silica gel,
petroleum ether/AcOEt 60/40) to give 1a as a pale yellow oil (0.73
g, yield 78.6%). 1H NMR (300 MHz, CDCl3, 293 K): δ 3.80 (br s, 1H,
CH2OH), 4.64 (s, 2H, CH2OH), 6.42 (m, 1H, CH), 7.18 (m, 1H, CH
Ar), 7.32 (m, 1H, CH Ar), 7.50 (m, 1H, CH Ar), 7.60 (br s, 1H, CH
Ar), 7.66 (m, 1H, CH), 7.85, (m, 1H, CH). 13C{1H} NMR (75 MHz,
CDCl3, 293 K): δ 64.2 (s), 107.6 (s), 117.6 (s), 118.1 (s), 124.8 (s),
127.1 (s), 129.4 (s), 140.0 (s), 140.9 (s), 142.9 (s). Anal. Calcd for
Compound 2a. 1H NMR (300 MHz, CDCl3, 293 K): δ 4.52 (s, 2H,
CH2Br), 6.47 (m, 1H, CH), 7.30 (m, 1H, CH Ar), 7.42 (t, 1H, 3JHH
=
7.8 Hz, CH Ar), 7.60 (m, 1H, CH Ar), 7.73 (m, 1H, CH), 7.77 (br s,
1H, CH Ar), 7.93, (m, 1H, CH). 13C{1H} NMR (75 MHz, CDCl3,
293 K): δ 32.6 (s), 107.8 (s), 118.9 (s), 119.8 (s), 126.7 (s), 126.9 (s),
129.9 (s), 139.3 (s), 140.0 (s), 141.2 (s). Anal. Calcd for C10H9BrN2
(237.10): C, 50.66; H, 3.83; N, 11.82. Found: C, 50.78; H, 3.89; N,
11.90.
1
Compound 2b. H NMR (300 MHz, CD2Cl2, 293 K): δ 2.36 (s,
3H, CH3), 4.56 (s, 2H, CH2Br), 6.21 (m, 1H, CH), 7.48−7.37 (3H,
CH Ar), 7.54−7.52 (2H, CH Ar). 13C{1H} NMR (75 MHz, CD2Cl2,
293 K): δ 12.3 (s), 32.7 (s), 107.1 (s), 124.2 (s), 125.2 (s), 127.8 (s),
129.3 (s), 138.7 (s), 139.1 (s), 139.8 (s), 140.4 (s). Anal. Calcd for
C11H11BrN2 (251.12): C, 52.61; H, 4.42; N, 11.16. Found: C, 52.73;
H, 4.56; N, 11.22.
General Procedure for Phosphination of the Bromine
Intermediates 2a,b: Synthesis of 1-(3-((Di-tert-
butylphosphino)methyl)phenyl)-1H-pyrazole (3a) and 1-(3-
((Di-tert-butylphosphino)methyl)phenyl)-5-methyl-1H-pyra-
t
zole (3b). To a solution of Bu2PH (2.5 mmol) in dry and degassed
acetone (10 mL) was added an acetone solution (15 mL) of the the
selected bromine (2a or 2b) derivative (2 mmol) in one portion. The
resulting mixture was refluxed under an N2 atmosphere for 12 h.
Afterward, the cooled solution was treated with 25 mL of dry and
degassed pentane, causing the precipitation of the HBr phosphine
adduct (3a·HBr or 3b·HBr). The collected white solid was washed
with degassed pentane (3 × 15 mL) before being dissolved in 15 mL
of degassed H2O and treated with 12 mL of a degassed saturated
NaOAc solution. The aqueous phase was then extracted with dry and
degassed Et2O (3 × 20 mL), and the collected (colorless) organic
phases were dried over Na2SO4. Solvent removal gave 3a,b in the form
of an analytically pure white solid (for 3a) and off-white viscous oil
(for 3b) in 88.4% and 86.8% isolated yields, respectively.
Compound 3a. 31P{1H} NMR (121 MHz, CD2Cl2, 293 K): δ 35.2
1
3
(s). H NMR (300 MHz, CD2Cl2, 293 K): δ 1.18 (d, JPH = 10.8 Hz,
I
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2
18H, P-C(CH3)3), 2.94 (d, JPH = 2.8 Hz, 2H, Ar-−CH2-P), 6.48 (m,
1H, CH), 7.38−7.30 (2H, CH Ar), 7.48 (m, 1H, CH Ar), 7.69 (m, 1H,
CH), 7.75 (m, 1H, CH Ar), 7.98, (m, 1H, CH). 13C{1H} NMR (75
MHz, CD2Cl2, 293 K): δ 28.5 (d, JPC = 24.7 Hz, Ar-CH2-P), 29.5 (d,
P-C(CH3)3), 107.8 (s), 110.6 (s), 122.7 (d, JPC = 21.0 Hz), 126.8 (s),
126.9 (s), 140.5 (s), 143.3 (s), 143.4 (s), 150.4 (d, JPC = 13.5 Hz).
11B{1H} NMR (128 MHz, CD2Cl2, 293 K): δ −1.0. Anal. Calcd for
C20H29BF4N3PPd (535.66): C, 44.84; H, 5.46; N, 7.84. Found: C,
44.88; H, 5.50; N, 7.80.
1
JPC = 13.2 Hz, P-C(CH3)3), 31.6 (d, JPC = 22.7 Hz, P-C(CH3)3),
107.2 (s), 115.8 (s), 120.1 (d, JPC = 9.3 Hz), 126.6 (s), 127.6 (d, JPC
{[(PCNH)Pd]2(μ-Cl)}(BF4) (6a). To a stirred solution of 4a (0.100 g,
0.23 mmol) in dry and degassed CH2Cl2 (10 mL) was added 0.5 equiv
of AgBF4 (0.022 g, 0.11 mmol) under a nitrogen atmosphere. The
resulting mixture was stirred at room temperature for 5 h, and then the
solution was filtered via cannula and concentrated to approximately
one-third of its initial volume. To this concentrated solution was
added, drop by drop, dry and degassed pentane until a precipitate
formed. The final pale yellow solid was washed with several portions of
pentane and dried under vacuum for 1 h (0.062 g, 60% isolated yield).
Crystals suitable for X-ray diffraction were obtained from a
concentrated solution of the complex in a mixture of CH2Cl2/pentane
cooled at −30 °C. 31P{1H} NMR (121 MHz, CD2Cl2, 293 K): δ 95.1
=
8.5 Hz), 129.0 (s), 140.0 (s), 140.7 (s), 143.8 (d, JPC = 13.0 Hz). Anal.
Calcd for C18H27N2P (302.39): C, 71.49; H, 9.00, N, 9.26. Found: C,
71.60; H, 9.21, N, 9.30.
Compound 3b. 31P{1H} NMR (121 MHz, CD2Cl2, 293 K): δ 35.7
1
3
(s). H NMR (300 MHz, CD2Cl2, 293 K): δ 1.16 (d, JPH = 10.8 Hz,
18H, P-C(CH3)3), 2.34 (s, 3H, CH3), 2.92 (d, 2JPH = 2.8 Hz, 2H, Ar-
CH2-P), 6.20 (m, 1H, CH), 7.24−7.20 (m, 1H, CH Ar), 7.38−7.34
(2H, CH Ar), 7.45 (br s, 1H, CH Ar), 7.53 (m, 1H, CH). 13C{1H}
NMR (75 MHz, CD2Cl2, 293 K): δ 12.2 (s), 28.3 (d, JPC = 24.7 Hz,
1
Ar-CH2-P), 29.4 (d, JPC = 13.2 Hz, P-C(CH3)3), 31.7 (d, JPC = 22.7
Hz, P-C(CH3)3), 106.5 (s), 121.6 (s), 125.8 (d, JPC = 9.2 Hz), 128.6
(s), 128.7 (d, JPC = 8.4 Hz), 138.7 (s), 139.4 (s), 143.4 (d, JPC = 12.7
Hz). Anal. Calcd for C19H29N2P (316.42): C, 72.12; H, 9.24; N, 8.85.
Found: C, 72.33; H, 9.40; N, 8.92.
1
3
(s). H NMR (300 MHz, CD2Cl2, 293 K): δ 2.17 (d, JPH = 14.4 Hz,
36H, P-C(CH3)3), 3.44 (d, 2JPH = 8.7 Hz, 2H, Ar-CH2-P, H), 6.39 (m,
2H, CH), 7.11−7.16 (6H, CH Ar), 8.03−8.09 (4H, CH Ar). 13C{1H}
NMR (75 MHz, CD2Cl2, 293 K): δ 28.8 (d, JPC = 3.5 Hz, P-C(CH3)3),
34.0 (d, JPC = 30.3 Hz, Ar-CH2-P), 35.6 (d, JPC = 17.5 Hz, P-
C(CH3)3), 107.2 (s), 110.6 (s), 122.8 (d, JPC = 20.5 Hz), 126.6 (s),
139.8 (s), 142.9 (s), 144.6 (s), 149.8 (d, JPC = 13.3 Hz). 11B{1H}
NMR (96 MHz, CD2Cl2, 293 K): δ −1.1. Anal. Calcd for
C36H52BClF4N4P2Pd2 (937.87): C, 46.10; H, 5.59; N, 5.97. Found:
C, 46.20; H, 5.63; N, 5.93.
General Procedure for the Synthesis of the Chloride
Complexes 4a,b. To a stirred solution of the selected PCNR ligand
(R = H, 3a; R = Me, 3b) (0.5 mmol) in dry and degassed toluene (3
mL) was added a suspension of Pd(COD)Cl2 (0.50 mmol) in dry and
degassed toluene (4 mL) in one portion. The reaction mixture was
stirred at reflux of solvent for 4 h, and then it was cooled to room
temperature. Afterward, the solvent was removed under vacuum to
give a crude mixture as a yellow pale semisolid material. The crude
sample was washed with pentane and filtered to afford analytically pure
white crystals of 4a,b in 90% and 93% isolated yields, respectively. For
both palladium compounds, crystals suitable for X-ray diffraction
analysis were growth from concentrated acetone solutions.
(PCNH)Pd(OTf) (7a). AgOTf (33.0 mg, 0.128 mmol) was added to
a solution of 4a (51.2 mg, 0.116 mmol) in CH2Cl2 (2 mL) in the
absence of light. The suspension was stirred for 4 h and filtered
through Celite. Solvent removal yielded 7a as an off-white solid (62.7
mg, 97% yield). Recrystallization from layering pentane on a
concentrated solution of 7a in CH2Cl2 yielded clear crystals that
were suitable for X-ray crystallography. 31P{1H} NMR (202 MHz,
(PCNH)PdCl (4a). 31P{1H} NMR (161 MHz, CD2Cl2, 293 K): δ
95.0 (s). 1H NMR (400 MHz, CD2Cl2, 293 K): δ 1.48 (d, 3JPH = 14.3
2
CD2Cl2): δ 94.1 (s). 1H NMR (500 MHz, CD2Cl2): δ 1.43 (d, 3JPH
=
Hz, 18H, P-C(CH3)3), 3.41 (d, JPH = 9.3 Hz, 2H, Ar-CH2-P), 6.55
2
(m, 1H, CH), 7.06−7.15 (3H, CH Ar), 7.98−8.02 (2H, CH Ar).
14.5 Hz, 18H, P-C(CH3)3), 3.31 (d, JPH = 9.6 Hz, 2H, Ar-CH2-P),
6.55 (m, 1H, CH), 7.01 (dd, JHH = 7.6 Hz, 0.9 Hz, 1H, CH), 7.05 (d,
3JHH = 7.9 Hz, 1H, CH), 7.14 (td, JHH = 7.8, 1.3 Hz, 1H, CH), 7.98−
7.99 (m, 1H, CH), 8.32−8.33 (m, 1H, CH). 13C{1H} NMR (176
13C{1H} NMR (100 MHz, CD2Cl2, 293 K): δ 29.2 (d, JPC = 4.3 Hz,
2
1
1
P-C(CH3)3), 35.3 (d, JPC = 31.6 Hz, Ar-CH2-P), 35.6 (d, JPC = 19.7
Hz, P-C(CH3)3), 107.1 (s), 110.1 (s), 122.5 (d, 3JPC = 20.4 Hz), 125.7
3
(s), 126.0 (s), 139.3 (s), 143.7 (s), 148.6 (s), 150.3 (d, JPC = 14.0
MHz, CD2Cl2): δ 28.9 (d, JPC = 4.3 Hz, P-C(CH3)3), 33.0 (d, JPC
=
Hz). Anal. Calcd for C18H26ClN2PPd (443.26): C, 48.77; H, 5.91; N,
30.6 Hz, Ar-CH2-P), 35.7 (d, JPC = 17.6 Hz, P-C(CH3)3), 107.8 (d, JPC
= 3.7 Hz), 110.7 (s), 122.9 (d, JPC = 20.8 Hz), 126.4 (s), 126.6 (s),
139.8 (s), 142.6 (d, JPC = 2.8 Hz), 143.5 (s), 150.7 (d, JPC = 13.7 Hz).
Anal. Calcd for C19H26F3N2O3PPdS: C, 40.98; H, 4.71; N, 5.03.
Found: C, 40.97; H, 4.56; N, 4.96.
6.32. Found: C, 48.82; H, 5.97; N, 6.39.
(PCNMe)PdCl (4b). 31P{1H} NMR (161 MHz, CD2Cl2, 293 K): δ
93.3 (s). 1H NMR (400 MHz, CD2Cl2, 293 K): δ 1.48 (d, 3JPH = 14.2
Hz, 18H, P-C(CH3)3), 2.71 (s, 3H, CH3), 3.39 (d, 2JPH = 9.4 Hz, 2H,
Ar-CH2-P), 6.29 (m, 1H, CH), 7.06 (m,1H, CH), 7.13 (m,1H, CH),
7.23 (m, 1H, CH Ar), 7.91 (m, 1H, CH Ar). 13C{1H} NMR (100
[(PCNH)Pd]2(μ-OH)[OTf] (8a). Ground KOH (1.4 mg, 0.0250
mmol, 1 equiv) was added to a solution of 7a (26.7 mg, 0.0479, 2
equiv) in THF (3 mL), and the mixture was stirred for 24 h. The
golden mixture was filtered through Celite and concentrated (ca. 1
mL). The product was crystallized in a pentane-vapor diffusion
chamber at −30 °C. The isolated golden crystals of 8a were suitable
for X-ray crystallography (16.0 mg, 68% yield). 31P(1H} NMR (202
MHz, THF-d8): δ 93.3 (s). 1H NMR (500 MHz, THF-d8): δ −1.48 (s,
2
MHz, CD2Cl2, 293 K): δ 13.9 (s, CH3), 28.8 (d, JPC = 4.3 Hz, P-
C(CH3)3), 34.9 (d, 1JPC = 28.7 Hz, Ar-CH2-P), 35.0 (d, 1JPC = 17.4 Hz,
P-C(CH3)3), 108.0 (s), 111.3 (s), 121.6 (d, 3JPC = 20.4 Hz), 125.0 (s),
3
138.4 (s), 140.1 (s), 144.9 (s), 149.0 (s), 150.1 (d, JPC = 14.4 Hz).
Anal. Calcd for C19H28ClN2PPd (457.29): C, 49.90; H, 6.17; N, 6.13.
Found: C, 49.93; H, 6.21; N, 6.15.
3
[(PCNH)Pd(MeCN)](BF4) (5a). To a stirred solution of 4a (0.100 g,
0.23 mmol) in dry and degassed CH2Cl2 (5 mL) were added dry and
degassed CH3CN (23 μL, 0.44 mmol) and AgBF4 (0.065 g, 0.33
mmol) in sequence, under a nitrogen atmosphere. The resulting
mixture was stirred at room temperature for 5 h, and then the solution
was filtered via cannula and concentrated to approximately three-
fourths of its initial volume. To this concentrated solution was added,
drop by drop, dry and degassed pentane until a precipitate formed.
The final white solid was washed with several portions of pentane and
dried under vacuum for 1 h (0.090 g, 75% isolated yield). 31P{1H}
1H, Pd−OH), 1.44 (d, JPH = 14.1 Hz, 18H, P−C(CH3)3), 1.51 (d,
2
2
3JPH = 14.3 Hz, 18H, P−C(CH3)3), 3.54 (dd, JPH = 9.3 Hz, JHH
=
18.1 Hz, 2H, Ar-CH2-P), 3.58 (dd, 2JPH = 10.2 Hz, 2JHH = 18.1 Hz, 2H,
Ar-CH2-P), 6.40−6.42 (m, 2H, CH), 7.07 (dd, JHH = 7.7, 1.1 Hz, 2H,
CH), 7.11 (td, JHH = 7.7, 1.3 Hz, 2H, CH), 7.34 (dd, JHH = 7.8, 1.1 Hz,
2H, CH), 8.32−8.33 (m, 2H, CH), 8.45−8.46 (m, 2H, CH). 13C{1H}
NMR (176 MHz, THF-d8): δ 29.31 (d, JPC = 4.4 Hz, P−C(CH3)3),
29.52 (d, JPC = 4.4 Hz, P-C(CH3)3), 34.88 (d, JPC = 31.3 Hz, Ar-CH2-
P), 35.86 (d, JPC = 17.6 Hz, P-C(CH3)3), 36.13 (d, JPC = 17.6 Hz, P-
C(CH3)3), 107.56 (d, JPC = 3.3 Hz), 111.48 (s), 123.34 (d, JPC = 21.1
Hz), 126.62 (s), 128.11 (s), 140.41 (d, JPC = 1.9 Hz), 144.59 (s),
144.65 (s), 151.13 (d, JPC = 14.1 Hz). Anal. Calcd for
C37H53F3N4O4P2Pd2S: C, 45.27; H, 5.44; N, 5.71. Found: C, 46.17;
H, 5.51; N, 5.49.
1
NMR (161 MHz, CD2Cl2, 293 K): δ 98.2 (s). H NMR (400 MHz,
CD2Cl2, 293 K): δ 1.41 (d, 3JPH = 14.7 Hz, 18H, P-C(CH3)3), 2.51 (s,
2
1H, CH3CN), 3.43 (d, JPH = 9.6 Hz, 2H, Ar-CH2-P), 6.64 (m, 1H,
CH), 7.07 (m, 1H, CH Ar), 7.13 (m, 1H, CH Ar), 7.20 (m, 1H, CH
Ar), 7.90 (m, 1H, CH Ar), 8.07 (m, 1H, CH Ar). 13C{1H} NMR (100
MHz, CD2Cl2, 293 K): δ 2.9 (s, CH3CN), 28.6 (d, JPC = 4.2 Hz, P-
C(CH3)3), 33.7 (d, JPC = 31.2 Hz, Ar-CH2-P), 35.5 (d, JPC = 17.7 Hz,
(PCNH)Pd(OH) (9a) and (PCNH)Pd(μ-OH)Pd(PCC) (10a). In a
medium-walled NMR tube with a resealable Teflon pin, ground KOH
(0.8 mg, 0.0143 mmol) was added to a solution of 7a (7.0 mg, 0.0126
J
DOI: 10.1021/acs.organomet.5b00355
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
mmol) in THF-d8 (0.4 mL). The mixture was shaken intermittently
until full conversion to 8a was observed by NMR spectroscopy. An
excess of ground KOH (1.2 mg, 0.214 mmol) was then added, and the
NMR tube was rotated slowly using a NMR tube rotary device over 16
h. Full conversion of 8a to a mixture of 9a (70% yield based on
internal standard, hexamethylbenzene) and 10a was observed by NMR
spectroscopy. Compound 9a can also be prepared by the addition of
D2O (0.1 mL) to a THF-d8 (0.4 mL) solution of 8a (5.0 mg, 8.98
μmol) containing suspended KOH (0.6 mg, 10.7 μmol). Using this
procedure the resulting solution contained only 9a, as determined by
solution NMR spectroscopy.
Hz, 2H, CH), 8.34 (m, 2H, CH). 13C{1H} NMR (176 MHz, THF-d8):
δ 14.1 (s, pyz-CH3), 29.30 (d, J = 4.0 Hz, P(C(CH3)3)2), 29.58 (d, J =
4.0 Hz, P(C(CH3)3)2), 34.82 (d, J = 30.6 Hz, CH2P), 35.65 (d, J =
17.6 Hz, P(C(CH3)3)2), 36.00 (d, J = 17.7 Hz, P(C(CH3)3)2), 108.9
(d, J = 1.6 Hz, pyz-C), 112.8 (s, Ar-C), 123.14 (d, J = 20.9 Hz, Ar-C),
126.4 (s, pyz-C), 139.7 (s, pyz-C), 141.8 (s, Ar-C), 145.7 (s, Ar-C),
145.8 (s, Ar-C), 151.48 (d, J = 14.2 Hz, Ar-C). Anal. Calcd for
C39H57F3N4O4P2Pd2S: C, 46.39; H, 5.69; N, 5.55. Found: C, 47.02; H,
5.70; N, 5.26.
(PCNMe)Pd(OH) (9b). To a solution of 4a (30.0 mg, 0.0656 mmol)
in THF (5 mL) was added an excess of ground KOH (15.2 mg, 0.271
mmol). The mixture was sonicated for 2.5 h and then left for 20 h at
room temperature. The mixture was filtered through Celite and
concentrated to ca. 1 mL. The golden solution was layered with
pentane and cooled to −30 °C to yield tan crystals of 9b suitable for
X-ray diffraction (20.3 mg, 71% yield). 31P(1H} NMR (121 MHz,
THF-d8): δ 90.2 ppm (s). 1H NMR (500 MHz, THF-d8): δ −1.97 (s,
1H, Pd-OH), 1.43 (d, 3JPH = 13.7 Hz, 18H, P-C(CH3)3), 2.66 (s, 3H,
Compound 9a. 31P(1H} NMR (202 MHz, THF-d8): δ 91.9 (s). 1H
NMR (500 MHz, THF-d8): δ −1.71 (s, 1H, Pd−OH), 1.43 (d, 3JPH
=
2
13.9 Hz, 18H, P-C(CH3)3), 3.38 (d, JPH = 9.2 Hz, 2H, Ar-CH2-P),
6.49 (m, 1H, CH), 6.92−6.95 (m, 2H, CH), 7.15 (m, 1H, CH), 8.05
1
(m, 1H, CH), 8.30 (m, 1H, CH). H NMR (700 MHz, THF-d8/
D2O): δ 1.38 (d, 3JPH = 14.1 Hz, 18H, P-C(CH3)3), 3.36 (d, 2JPH = 9.3
3
Hz, 2H, Ar-CH2-P), 6.59−6.56 (m, 1H, CH), 6.93 (d, JHH = 7.5 Hz,
2
1H, CH), 6.98 (t, 3JHH = 7.6 Hz, 1H, CH), 7.18 (d, 3JHH = 7.8 Hz, 1H,
CH), 8.08 (m, 1H, CH), 8.32 (m, 1H, CH). 13C{1H} NMR (176
MHz, THF-d8/D2O): δ 29.47 (d, JPC = 4.8 Hz, P-C(CH3)3), 35.61 (d,
JPC = 31.0 Hz, Ar-CH2-P), 35.75 (d, JPC = 17.3 Hz, P-C(CH3)3),
107.98 (d, JPC = 3.5 Hz), 110.61 (s), 122.57 (d, JPC = 20.4 Hz), 125.72
(s), 127.21 (s), 140.72 (d, JPC = 2.8 Hz), 144.8 (s), 147.85 (s), 150.96
(d, JPC = 14.5 Hz). LRMS (ESI-MS) m/z: [M − OH]+ calcd for
C18H26N2PPd 407; found 407.3.
CH3), 3.36 (d, JPH = 9.2 Hz, 2H, Ar-CH2-P), 6.25 (m, 1H, CH),
3
6.91−6.93 (m, 2H, CH), 7.17 (dd, JHH = 6.1, 2.8 Hz, 1H, CH), 7.79
(m, 1H, CH). 13C{1H} NMR (126 MHz, THF-d8): δ 14.07 (s, pyz-
CH3), 29.40 (d, J = 5.0 Hz, P(C(CH3)3)2), 35.38 (d, J = 16.9 Hz),
36.16 (d, J = 28.9 Hz), 108.1 (s), 111.6 (s), 121.89 (d, J = 20.2 Hz),
124.1 (s), 138.1 (s), 140.2 (s), 146.3 (s), 150.90 (d, J = 14.8 Hz). Anal.
Calcd for C19H29N2OPPd: C, 52.00; H, 6.66; N, 6.38. Found: C,
51.89; H, 6.76; N, 6.18.
Compound 10a. 31P(1H} NMR (120 MHz, THF-d8): δ 71.0 (s, P
1
trans to C), 91.1 (s, P trans to N). H NMR (700 MHz, THF-d8): δ
ASSOCIATED CONTENT
* Supporting Information
■
−1.75 (bs, 1H, Pd-OH), 1.36 (d, J = 12.4 Hz, 9H, P−C(CH3)3), 1.39
(d, J = 12.5 Hz, 9H, P−C(CH3)3), 1.44 (d, J = 13.8 Hz, 9H, P-
C(CH3)3), 1.51 (d, J = 14.2 Hz, 9H, P-C(CH3)32), 3.08 (dd, 2JPH = 8.9
S
Tables, figures, and CIF and XYZ files giving selected
crystallographic parameters of the solved XRD structures,
NMR spectra of all the described compounds, and Cartesian
coordinates and absolute GTHF energy values of the optimized
structures of 9a, 9a′, TSrot, TS1, 10a···H2O, and 11a. The
Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.organomet.5b00355.
2
2
Hz, JHH = 17.4 Hz, 1H, Ar-CH2-P), 3.12 (dd, JPH = 8.3 Hz, JHH
=
17.4 Hz, 1H, Ar-CH2-P), 3.42 (dd, 2JPH = 9.3 Hz, 2JHH = 17.9 Hz, 1H,
2
2
Ar-CH2-P), 3.49 (dd, JPH = 9.3 Hz, JHH = 17.9 Hz, 1H, Ar-CH2-P),
6.11−6.12 (m, 1H, CH), 6.31−6.32 (m, 1H, CH), 6.60 (d, J = 7.6 Hz,
1H, CH), 6.74 (td, J = 7.6, 1.1 Hz, 1H, CH), 6.90−6.91 (m, 1H, CH),
6.91−6.93 (m, 1H, CH), 6.96−6.98 (m, 1H, CH), 7.02 (td, J = 7.7, 1.0
Hz, 1H, CH), 7.16−7.18 (m, 1H, CH), 8.17−8.19 (m, 1H, CH),
8.74−8.75 (m, 1H, CH).
AUTHOR INFORMATION
Corresponding Authors
(PCNMe)Pd(OTf) (7b). AgOTf (42.1 mg, 0.164 mmol) was added
to a solution of (PCNMe)PdCl (75.0 mg, 0.164 mmol) in CH2Cl2 (3
mL) in the absence of light. The suspension was stirred for 5 h and
filtered through Celite. Layering pentane on the CH2Cl2 solution at
room temperature gave clear light yellow crystals of 7b suitable for X-
ray crystallography (81.8 mg, 87% yield). 31P{1H} NMR (202 MHz,
■
*E-mail for R.A.K.: rakemp@unm.edu.
*E-mail for G.G.: giuliano.giambastiani@iccom.cnr.it.
*E-mail for K.I.G.: goldberg@chem.washington.edu.
1
Notes
CD2Cl2): δ 92.1 ppm (s). H NMR (500 MHz, CD2Cl2): δ 1.42 (d,
3JPH = 14.5 Hz, 18H, P-C(CH3)3), 2.68 (s, 3H, pyz-CH3), 3.29 (d, 2JPH
= 9.59 Hz, 2H, Ar-CH2-P), 6.28 (s, 1H, CH), 7.01 (d, 3JHH = 7.11 Hz,
The authors declare no competing financial interest.
3
3
1H, CH), 7.13 (t, JHH = 7.11 Hz, 1H, CH), 7.19 (d, JHH = 7.51 Hz,
ACKNOWLEDGMENTS
■
1H, CH), 8.22 (s, 1H, CH). 13C{1H} NMR (126 MHz, CD2Cl2): δ
L.L., A.R., and G.G. thank the Fondazione Cariplo (“Crystalline
Elastomers” project), the Groupe de Recherche International
(GDRI) “Homogeneous Catalysis for Sustainable Development”,
and the COST action CM1006: “EUFEN: European F-Element
Network” for supporting this work. CREA (Centro Ricerche
Energia e Ambiente) in Colle Val d’Elsa (Siena, Italy) is also
acknowledged for computational resources. The work done at
the University of Washington and the University of New
Mexico was supported by the Department of Energy (DE-
FG02-06ER15765).
2
14.56 (s, pyz-CH3), 28.96 (d, JPC = 3.9 Hz, P(C(CH3)3)2), 32.96 (d,
1JPC = 29.8 Hz, P(C(CH3)3)2), 35.49 (d, 1JPC = 17.3 Hz, CH2P), 109.3
(d, JPC = 2.75 Hz), 112.3 (s), 122.5 (d, JPC = 20.8 Hz), 126.4 (s), 140.6
(d, JPC = 2.1 Hz), 140.9 (s), 141.9 (s), 145.2 (s), 150.9 (d, JPC = 13.8
Hz). Anal. Calcd for C20H28F3N2O3PPdS: C, 42.08; H, 4.94; N, 4.91.
Found: C, 42.06; H, 4.81; N, 4.75.
[(PCNMe)Pd]2(μ-OH)[OTf] (8b). To a solution of (PCNMe)Pd-
(OTf) (14.5 mg, 0.0254 mmol) in THF (3 mL) was added ground
KOH (1.0 mg, 0.0178 mmol). The solution was stirred for 16 h, after
which it was filtered through a Teflon filter. The solution was
concentrated (ca. 1 mL), and recrystallization was accomplished by
slow diffusion of pentane into the solution at −30 °C. The dark yellow
crystals were washed with water (3 × 1 mL) and dried under vacuum,
giving 8b (11.8 mg, 92% yield). 31P{1H} NMR (202 MHz, THF-d8): δ
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91.6 ppm (s). H NMR (700 MHz, THF-d8): δ −1.46 (s, 1H, Pd-
3
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=
2
2
10.1 Hz, 2H, Ar-CH2-P), 3.58 (dd, JHH = 18.1, JPH = 10.0 Hz, 2H,
3
Ar-CH2-P), 6.20 (m, 2H, CH), 7.10 (dd, JHH = 7.7 Hz, 1.4 Hz, 2H,
3
3
CH), 7.12 (td, JHH = 7.6, 1.1 Hz, 2H, CH), 7.30 (dd, JHH = 7.7, 1.4
K
DOI: 10.1021/acs.organomet.5b00355
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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