Isolation of N-heterocyclic alkyl intermediates en route to transition metal N-heterocyclic carbene complexes: Insight into a C-H activation mechanism

Article abstract of DOI:10.1021/om301230f

An imidazolinium cation has been incorporated into an arene-linked diphosphine pincer ligand, [2]⁺, and the metalation of this ligand has been investigated via direct imidazolinium C-H activation to Pd⁰ and Pt⁰. The expected NHC-ligated metal-hydride species [5]PF ₆ (M = Pt) and 6 (M = Pd) are obtained if the halide-free imidazolinium salt [2]PF₆ is used. In contrast, treatment of the imidazolinium chloride salt [2]Cl with M(PPh₃)₄ leads to isolation of N-heterocyclic alkyl M^II species 3 (M = Pd) and 4 (M = Pt), in which the imidazolinium C-H bond remains intact. Interestingly, there are no apparent agostic interactions between the imidazolinium protons and the metal centers in 3 and 4, indicating that these species represent an unusual type of arrested C-H activation intermediate. While Pd complex 3 is thermally stable, Pt complex 4 undergoes C-H activation to afford the corresponding NHC-Pt^II-hydride species [5]Cl upon heating. Additionally, both complexes 3 and 4 undergo rapid C-H activation upon abstraction of the metal-bound halide to form 6 and [5]PF₆, respectively. The nature of the bonding in the unusual N-heterocyclic alkyl species is investigated computationally.

Full text of DOI:10.1021/om301230f

Article
pubs.acs.org/Organometallics
Isolation of N‑Heterocyclic Alkyl Intermediates en Route to
Transition Metal N‑Heterocyclic Carbene Complexes: Insight into a
C−H Activation Mechanism
Baofei Pan, Sadrach Pierre, Mark W. Bezpalko, J. Wesley Napoline, Bruce M. Foxman,
and Christine M. Thomas*
Department of Chemistry, Brandeis University, 415 South Street MS 015, Waltham, Massachusetts 02454, United States
S
* Supporting Information
ABSTRACT: An imidazolinium cation has been incorporated
into an arene-linked diphosphine pincer ligand, [2]+, and the
metalation of this ligand has been investigated via direct
imidazolinium C−H activation to Pd0 and Pt0. The expected
NHC-ligated metal-hydride species [5]PF6 (M = Pt) and 6 (M =
Pd) are obtained if the halide-free imidazolinium salt [2]PF6 is
used. In contrast, treatment of the imidazolinium chloride salt
[2]Cl with M(PPh3)4 leads to isolation of N-heterocyclic alkyl MII
species 3 (M = Pd) and 4 (M = Pt), in which the imidazolinium
C−H bond remains intact. Interestingly, there are no apparent
agostic interactions between the imidazolinium protons and the metal centers in 3 and 4, indicating that these species represent
an unusual type of arrested C−H activation intermediate. While Pd complex 3 is thermally stable, Pt complex 4 undergoes C−H
activation to afford the corresponding NHC-PtII-hydride species [5]Cl upon heating. Additionally, both complexes 3 and 4
undergo rapid C−H activation upon abstraction of the metal-bound halide to form 6 and [5]PF6, respectively. The nature of the
bonding in the unusual N-heterocyclic alkyl species is investigated computationally.
precursors or generate undesired side products.24−27 An atom-
economical pathway that has been utilized for the preparation
of NHC-metal complexes is the direct imidazolium/imidazo-
INTRODUCTION
■
Transition metal-mediated C−H bond activation is a field of
great interest for chemists in many areas including organic
synthesis and energy-related hydrocarbon functionalization.1−4
Depending on the electronic properties of the transition metal,
C−H activation reactions can be classified into five pathways,
including oxidative addition, σ-bond metathesis, metalloradical
activation, 1,2-addition, and electrophilic activation.5 Low-
valent electron-rich transition metal precursors generally favor
the oxidative addition pathway for C−H bond cleavage of RH,
resulting in the formation of M(H)(R) species and two-
electron-oxidized metal centers.6 Prior to scission of the C−H
bond, it is widely accepted that C−H bonds coordinate,
forming σ-complexes, followed by C−H bond cleavage.7,8 This
mechanism is supported by a few isolated η2 C−H agostic
metal interactions in late transition metal complexes with free
alkanes,9−14 as well as arene C−H bonds within ligand
frameworks whose proximity is enforced by additional chelating
donors.15−18 The latter examples are particularly relevant since
C−H activation to form the metal-aryl complex can often be
induced under various reaction conditions.15,18
linium C−H bond oxidative addition to low-valent late
transition metal precursors to form NHC-metal-hydride
complexes.28−32 Since no intermediates featuring monodentate
NHC precursors have been isolated, the mechanism for
imidazolium/imidazolinium C−H bond oxidative addition is
not completely clear. However, theoretical investigations by
Yates and Cavell suggest that the mechanism involves the initial
formation of an η2 C−H agostic interaction,33,34 analogous to
the canonical oxidative addition mechanism for a transition
metal-mediated C−H bond activation (Scheme 1). Moreover,
Hill and co-workers recently reported chelation-assisted double
C−H activation en route to NHC Rh and Ir complexes, and an
Scheme 1
Since the isolation of the first stable N-heterocyclic carbene
(NHC) compound by Arduengo and co-workers in 1991,19
NHCs have become a fixture as ligands in coordination
chemistry and NHC-based homogeneous catalysis.20−23 In
general, the common preparative methods for NHC-metal
complexes either require prefunctionalization of the ligand
Received: December 19, 2012
© XXXX American Chemical Society
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N-heterocyclic alkyl intermediate was isolated in the case of
Ir.35
reaction reported by Fryzuk and co-workers in which C−H
bond oxidative addition of the diisopropylphosphine-substi-
tuted analogue of [2]PF6 results in a NHC-PdII-hydride
species,40 no diagnostic hydride signal was observed in the
1H NMR spectrum of compound 3. Instead, a clear triplet
signal at δ 4.78 ppm was observed, corresponding to a proton
still bound to the central carbon, with a 3JP−H coupling constant
of 20.2 Hz. Likewise, the 13C NMR of 3 reveals a central carbon
shift at δ 85.6 ppm, which is far too upfield for a typical NHC
carbene carbon, but within the range of a regular sp3-hybridized
metal-alkyl carbon signal. The 31P NMR signal of 3 is at δ 9.54
ppm and is sufficiently shifted from that of the free ligand (δ
19.0 ppm) to suggest that both phosphine arms are bound to
Pd. These data indicate that compound 3 is an “arrested”
intermediate that has coordinated to Pd but has not undergone
C−H activation.
In an extension of our recent work featuring the coordination
chemistry of N-heterocyclic phosphenium cations incorporated
into the central position of a chelating diphosphine frame-
work,36−39 we chose to replace the central donor in our pincer
ligand with an isolobal N-heterocyclic carbene unit. A similar
ligand with diisopropylphosphine substituents was recently
reported by Fryzuk and co-workers.40 Pincer ligands have been
receiving increasing attention owing to their rigidity and ability
to stabilize reactive transition metal fragments,41−45 and pincer
ligands with NHC donors are now well-known.46,47
To extend our investigation of N-heterocyclic donor ligands
to carbenes, an imidazolinium ligand precursor with diphenyl-
phosphine-substituted o-phenylene linkers was synthesized
straightforwardly from a known diamino/diphosphine pre-
cursor.36 The coordination of this pincer-type imidazolinium
cation to low-valent Pd0 and Pt0 precursors via the
aforementioned C−H activation route leads to the isolation
of N-heterocyclic alkyl-metal species as intermediates for the
C−H bond activation process, providing valuable insight into
the mechanism of imidazolinium C−H bond oxidative addition
to transition metal centers.
As expected from the spectroscopic results, the solid-state
structure of 3 reveals that the central C−H bond of the ligand
remains intact (Figure 1). The geometry between the central
RESULTS AND DISCUSSION
Ligand Synthesis. Synthesis of the imidazolinium ligand
precursor is shown in Scheme 2. Treatment of N,N′-bis(o-
■
Scheme 2
Figure 1. Displacement ellipsoid (50%) representation of 3. For
clarity, all hydrogen atoms except the one at the central carbon are
omitted. Relevant interatomic distances (Å) and angles (deg): Pd1−
C21, 2.0747(12); Pd1−P1, 2.2950(2); Pd1−P2, 2.3150(2);
Pd1··H211, 2.57; N1−C21−Pd1, 119.17(8); N2−C21−Pd1,
103.55(8); N1−C21−N2, 101.70(10); Pd1··H211−C21, 109.8.
N−C−N plane and the C−Pd bond vector is considerably
bent, with an angle of 126°, suggesting that the nature of the
Pd-bound carbon atom is that of an X-type sp3-hybridized alkyl.
Consistent with this formulation, the C−N bonds associated
with the central carbon of 3 (1.4398(16) and 1.4439(16) Å)
are elongated relative to those in related metal-NHCs (vide
infra) or imidazolinium cations (av C−N distance 1.307 Å
obtained from Cambridge Structural Database), suggesting the
disruption of any π-donation from the nitrogen lone pairs. The
C−Pd distance (2.0747(12) Å) in 3 is slightly longer than that
in Fryzuk’s diphosphino-NHC carbene-Pd-hydride species
(2.037(8) Å), even though Fryzuk’s compound has a strongly
trans-influencing hydride ligand trans to the NHC.40 In
addition, the hydrogen atom bound to the central carbon was
located crystallographically on an electron-density difference
map. Both the long Pd···H interatomic distance (∼2.57 Å) and
large Pd−C−H angle (∼110°) suggest the absence of an
agostic C−H−M interaction in 3.48 Thus, the most accurate
classification of complex 3 is as an N-heterocyclic alkyl-PdII-Cl,
an assignment further supported by the square-planar geometry
at the Pd center.
(diphenylphosphine)phenyl)ethane-1,2-diamine36 (1) with hy-
drochloric acid followed by treatment with triethylorthoformate
affords the imidazolinium chloride salt [2]Cl as a white powder
−
after 12 h of heating at 120 °C. The corresponding PF6 salt
[2]PF6 can be obtained via anion exchange with TlPF6
(Scheme 2). The two imidazolinium ligand precursors [2]Cl
and [2]PF6 have been fully characterized by NMR spectros-
copy and elemental analysis.
Addition of [2]Cl to Pd0 and Pt0. The reaction of [2]Cl
with Pd(PPh3)4 in THF at room temperature cleanly generates
Pd complex 3, as shown in Scheme 3. In contrast to a similar
Scheme 3
Although there are no structurally characterized C−H-bound
N-heterocyclic alkyl Pd complexes in the literature for
comparison, a similar Me−C-bound N-heterocyclic alkyl Pd
compound was obtained via migratory insertion of a methyl
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group from Pd to a free NHC ligand.49 The long alkyl C−Pd
distance in the latter compound (2.085(3) Å) is comparable
with the alkyl C−Pd distance in 3 (2.0738(13) Å). As the first
fully characterized example of an imidazolinium/imidazolium-
derived N-heterocyclic alkyl metal complex with an intact C−H
bond, complex 3 represents a stable intermediate prior to the
C−H bond activation process. Several attempts to promote C−
H bond activation to generate an NHC-Pd-hydride via
thermolysis were unsuccessful: complex 3 is highly stable
upon extended refluxing in THF, CH2Cl2, and MeCN.
In contrast to Pd0, computational studies indicate that Pt0 is
favored to undergo C−H bond oxidative addition reactions
more readily due to both a lower activation barrier and greater
thermodynamic stability of the resulting PtII(H)(R) species.34
Thus, the oxidative addition of our chelating imidazolinium
cation to a Pt0 precursor was also investigated. Similar to the Pd
case, the treatment of ligand precursor [2]Cl with Pt(PPh3)4 in
THF at room temperature results in the formation of a similar
N-heterocyclic alkyl-PtII-Cl complex, 4, with the proton
remaining on the central carbon atom (Scheme 4). The 31P
Figure 2. Displacement ellipsoid (50%) representation of 4 and [5]Cl.
For clarity, all hydrogen atoms are omitted except the one at the
central NHC carbon in 4 and the Pt hydride in [5]Cl. The Cl−
counteranion in [5]Cl is also omitted for clarity. Relevant interatomic
distances (Å) and angles (deg) for 4: Pt1−C39, 2.042(3); Pt1−P1,
2.2689(8); Pt1−P2, 2.2755(8); Pt1···H391, 2.56, Pt1−C39−H391,
109.5; for [5]Cl: Pt1−C39, 2.037(2); Pt1−P1, 2.2540(6); Pt1−P2,
2.2576(6); Pt1−H1, 1.40.
Alternatively, stirring a solution of 4 for several days at room
temperature also affords [5]Cl. The presence of a Pt-bound
hydride in [5]Cl is suggested by a triplet signal at δ −3.88 ppm
with Pt satellites in the 1H NMR spectrum (1JPt−H = 851.5 Hz,
2JP−H = 24.0 Hz). In addition, a signal typical for an NHC
carbon was observed in the 13C NMR spectrum of [5]Cl at δ
Scheme 4
194.0 ppm with both Pt satellites and coupling to 31P (1JPt−C
=
2
684.6 Hz, JP−C = 11.1 Hz). Both of these diagnostic
spectroscopic features of [5]Cl are consistent with the
bis(diisopropylphosphino)-NHC-PtII-hydride species reported
by Fryzuk and co-workers (δ −4.43 ppm for the hydride in 1H
NMR; δ 196.7 ppm for the NHC carbon in 13C NMR).40
The structure of [5]Cl is further confirmed by a single crystal
X-ray diffraction study, as shown in Figure 2. The Cl−
counteranion is ∼6.8 Å away from the Pt center, confirming
its outer-sphere nature. A typical planar geometry is observed
between the NHC unit’s N−C−N plane and C−Pt bond vector
(∼178.5°). The C−Pt bond distance (2.037(3) Å) in [5]Cl is
nearly identical to the alkyl C−Pt bond distance in 4 (2.042(3)
Å), but a rigorous comparison is difficult here due to the much
stronger trans influence of H− in [5]Cl compared to that of Cl−
in 4. The most clear indication of the change in M−C bonding
between N-heterocyclic alkyl complex 4 and NHC complex
[5]Cl is the contraction of the N−C bond distances from
∼1.48 Å to ∼1.35 Å, consistent with the delocalized nitrogen π-
donation to stabilize the singlet carbene.
NMR spectrum of 4 features a singlet at δ 16.6 ppm with Pt
satellites (1JPt−P = 3120 Hz), suggesting Pt-bound phosphine
1
side arms. A 13C NMR shift at δ 66.3 ppm and a H NMR
triplet signal at δ 4.82 ppm with Pt satellites (3JP−H = 12.0 Hz,
2JPt−H = 52.0 Hz) correspond to the central carbon atom and
the proton remaining bound to the carbon atom, respectively.
These spectroscopic features are very similar to those in the Pd
compound 3, supporting the alkyl-type ligand formulation of
the central donor in compound 4.
As shown in Figure 2, the solid-state structure of 4 is quite
similar to that of 3 and features a bent geometry between the
N−C−N plane and the C−Pt bond vector (134.5°), suggesting
an sp3-hybridized central carbon atom. Again, the C−Pt bond
distance in 4 (2.0423(3) Å) is slightly longer than the C−PtII
bond distance (2.008(4) Å) reported by Fryzuk and co-workers
for the analogous NHC-Pt-hydride complex with the
diisopropyl-substituted pincer ligand.40 The hydrogen atom
on the central carbon atom was located crystallographically in
the electron-density difference map, further supporting the
metal-alkyl nature of the central N-heterocyclic carbon. Similar
to Pd derivative 3, the long Pt···H distance (∼2.56 Å) and large
Pt−C−H angle (∼110°) in 4 are outside the range for an
agostic C−H−M interaction.48
The reactions shown in Scheme 4 represent the stepwise
process by which an imidazolinium cation’s central C−H bond
is oxidatively added to a Pt0 precursor. The unusual N-
heterocyclic alkyl species 4 appears to be a direct intermediate
en route to C−H activation by Pt0 and can be isolated and fully
characterized.
Addition of [2]PF6 to Pd0 and Pt0. The C−H bond
cleavage process described for 4 could also be promoted via
halide abstraction in THF using TlPF6 to cleanly generate the
NHC-PtII-hydride complex [5]PF6 with a PF6 counteranion
−
(Scheme 5). Despite the reluctance of complex 3 to undergo
C−H activation under thermolytic conditions, treatment of 3
with TlPF6 also promoted C−H bond cleavage to generate
NHC-PdII-hydride complex 6. The presence of metal hydrides
was confirmed in the 1H NMR spectra of [5]PF6 and 6
([5]PF6: δ −3.89 ppm, 1JPt−H = 854.7 Hz, 2JP−H = 13.2 Hz; 6: δ
In contrast to Pd complex 3, which is thermally stable,
complex 4 undergoes C−H bond activation to generate the
NHC-PtII-hydride complex [5]Cl upon refluxing for 6 h in
moderately polar solvents (CH2Cl2 or MeCN, Scheme 4).
2
−5.73 ppm, JP−H = 6.0 Hz), and typical NHC carbene carbon
signals were also observed by 13C NMR spectroscopy ([5]PF6:
C
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presence of higher concentrations of PPh3 in solution, the
Scheme 5
equilibrium in Scheme 6 is shifted more toward complex [7]+,
1
resulting in a smaller JPt−H coupling constant because the
bound PPh3 donor ligand decreases the Pt-hydride σ
interaction. A very similar five-coordinate pincer-ligated
PtII(H)(PPh3) complex was recently reported using an
analogous ligand with a central N-heterocyclic phosphite
donor.39 Complex [7]+ is apparently less thermodynamically
favored than [5]PF6 and can be observed only in solution;
attempts to isolate PPh3 adduct [7]+ were unsuccessful and
always resulted in the regeneration of complex [5]PF6 in pure
crystalline form. To further probe the phosphine-binding
equilibrium, a less bulky and more electron-donating
phosphine, PMe3, was added to [5]PF6. In this case, a similar
PMe3 adduct of the NHC-PtII-hydride can be observed in
solution by NMR spectroscopy and can be isolated in the solid
state (see Experimental Section).
Computational Studies. As complexes 3 and 4 represent
isolable precursors to C−H activation of imidazolinium cations,
computational investigations were undertaken to better under-
stand the bonding in these complexes. Geometry optimizations
of complexes 4 and [5]Cl were carried out using Gaussian 09
(B3LYP/LANL2DZ),50 and the computed geometries were in
reasonable agreement with those obtained via X-ray crystallog-
raphy (see Supporting Information). As observed experimen-
tally, the Cl− counterion in the optimized structure of [5]Cl is
>3 Å from the Pt center, precluding Pt−Cl bonding
interactions. The bonding between Pt and the N-heterocycle
in complexes 4 and [5]Cl was examined using natural bond
orbital (NBO) analysis,51 and the Pt−C NBOs are shown in
Figure 4. On the basis of the contributions from each atom to
194.2 ppm; 6: 198.5 ppm). The single-crystal X-ray structure of
[5]PF6 reveals a core structure nearly identical to that of [5]Cl
(see Supporting Information). Interestingly, complex 6 does
not revert to 3 when treated with one equivalent of [Et4N]Cl
Both [5]PF6 and 6 can also be synthesized directly via
treatment of the metal precursors M(PPh3)4 (M = Pd, Pt) with
−
the pincer imidazolinium PF6 salt [2]PF6 (Scheme 5). In the
case of treatment of Pt(PPh3)4 with [2]PF6, it was observed
that free PPh3 (as a byproduct of the reaction) greatly effects
1
1
the H NMR chemical shift and JPt−H of the resulting hydride
product [5]PF6. Figure 3 illustrates these changes upon
Figure 3. Hydride region of the 1H NMR spectrum (CD2Cl2) of
[5]PF6 in the presence of varying relative concentrations of PPh3,
illustrating the effect of added PPh3 on the hydride chemical shift and
Figure 4. Pictorial representations of the Pt−C natural bond orbitals
computed for complexes 4 and [5]Cl. NBO of 4: 35.6% Pt, 64.4% C
(26.6% s/73.4% p). NBO of [5]Cl: 23.4% Pt, 76.6% C (43.7% s/
56.3% p).
1JPt−H
.
addition of varying amounts of PPh3 to isolated [5]PF6. To
explain this phenomenon, we propose the equilibrium shown in
Scheme 6, in which [5]PF6 reversibly coordinates PPh3 in
solution to form the five-coordinate phosphine adduct 7. In the
the NBO, the Pt−C NBO of 4 is more covalent (35.6% Pt/
64.4% C) than that of [5]Cl (23.4% Pt/76.6% C), as would be
expected based on the anionic alkyl nature of the carbon donor
in 4 compared to the NHC σ-donor nature of the carbon donor
in [5]Cl. Likewise, the orbital contributions in the NBO from
the carbon atom in complex 4 (26.6% s/73.4% p) are indicative
of sp3 hybridization, while there are significantly more s orbital
contributions to the Pt−C NBO (43.7% s/56.3% p) from the
carbon atom in [5]Cl.
Scheme 6
CONCLUSION
Similar to the widely accepted mechanism for C−H bond
activation by transition metal centers, C−H bond oxidative
■
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chemical shifts (in ppm) were referenced to CCl3F (−2.3 ppm).
Elemental microanalyses were performed by Complete Analysis
Laboratories, Inc., Parsippany, NJ, USA.
addition of imidazolinium cations might have also been
expected to undergo the initial formation of an agostic C−
H−M species followed by C−H bond cleavage and two-
electron oxidation of the metal center. In contrast to this
canonical mechanism, we have found that, at least in the case of
chelate-enforced imidiazolinium C−H activation, a different
mechanism is possible. Herein we have isolated intermediates
on the C−H activation pathway of a diphosphine-substituted
imidazolinium species with Pt and Pd, showing that in some
cases the initial step is not formation of a side-bound C−H
agostic interaction, but rather involves formal two-electron
oxidation of an electron-rich metal center and formation of an
N-heterocyclic alkyl intermediate species. In this particular case,
the C−H bond cleavage step occurs at a later stage.
Similar C−H activation of an imidazolinium-containing
diphosphine pincer ligand was reported by Fryzuk and co-
workers, but an N-heterocyclic alkyl species was not isolated in
their studies.40 Some key differences between the two systems
can explain the slightly different observations, namely, (1) the
isopropyl substituents on the phosphines in Fryzuk’s ligand
were electron-releasing enough to promote more rapid C−H
activation or (2) the absence of coordinating anions such as Cl−
in their studies precluded stabilization of N-heterocyclic alkyl
intermediates. It is also interesting that, although monodentate
imidazolinium and imidazolium cations have been shown to
oxidatively add to low-valent metal centers, no other reports of
N-heterocyclic alkyl intermediates on these pathways have been
reported. Here we suggest that the rigid chelating nature of our
ligand is the key to this phenomenon. Chelation of the
phosphine donor arms constrains the geometry of the N-
heterocycle such that approach to the metal center is restricted.
Another possible factor is the delocalization of the NHC
nitrogen lone pairs throughout the aromatic rings that are
forced to be in the same plane as the N-heterocycle by
chelation. We have previously suggested that such delocaliza-
tion weakens the ability of these nitrogen donors to stabilize a
singlet configuration at the central donor atom (previously an
N-heterocyclic phosphenium cation),38 and this factor could
play a role in the chemistry described herein as well.
X-ray Crystallography. All operations were performed on a
Bruker-Nonius Kappa Apex2 diffractometer, using graphite-mono-
chromated Mo Kα radiation. All diffractometer manipulations,
including data collection, integration, scaling, and absorption
corrections, were carried out using the Bruker Apex2 software.54
Preliminary cell constants were obtained from three sets of 12 frames.
Crystallographic data and refinement parameters are provided in Table
S1, and further experimental crystallographic details are described for
each compound in the Supporting Information.
Computational Details. All calculations were performed using
Gaussian0950 for the Linux operating system. Density functional
theory calculations were carried out using the B3LYP hybrid
functional, with Becke’s three-parameter exchange functional (B3)55
and the correlation functional of Lee, Yang, and Parr (LYP).56
A
mixed-basis set was employed, using the LANL2DZ(d,p) double-ζ
basis set with effective core potentials for phosphorus, chlorine, and
platinum57−59 and D95 V60 for carbon, nitrogen, and hydrogen. Using
crystallographically determined geometries as a starting point, the
geometries were optimized to a minimum, followed by analytical
frequency calculations to confirm that no imaginary frequencies were
present. NBO51 calculations were performed on the optimized
geometries of 4 and [5]Cl without including any solvation corrections.
XYZ coordinates of the optimized geometries of all computed
complexes are provided in the Supporting Information.
3-Bis(o-diphenylphosphino)phenyl-1H-imidazolinium
Chloride, [2]Cl. To a 50 mL THF solution of 1 (356 mg, 0.614
mmol) was added 2.5 mL of 1.0 M hydrochloric acid solution in
diethyl ether slowly at room temperature. The reaction mixture was
allowed to stir for 1 h, and then the volatiles were removed in vacuo.
Triethylorthoformate (15 mL) was added to the white solid residue,
and the mixture was heated to reflux under a dinitrogen atmosphere
for 12 h to ensure complete reaction. Upon completion, the reaction
mixture was cooled to room temperature. The resulting white powder
was collected via filtration, washed with diethyl ether, and further dried
1
in vacuo to yield analytically pure product. Yield: 289 mg, 75.1%. H
NMR (400 MHz, CD2Cl2): δ 8.21 (m, 2H, Ar-H), 8.14 (s, 1H, C-H),
7.52 (m, 2H, Ar-H), 7.38−7.35 (m, 14H, Ar-H), 7.32−7.28 (m, 8H,
Ar-H), 6.98 (m, 2H, Ar-H), 4.26 (s, 4H, CH2). 31P NMR (161.8 MHz,
CD2Cl2): δ 17.98 (s). 13C NMR (100.5 MHz, CD2Cl2): δ 158.7,
138.9, 134.6, 134.4, 134.3, 134.2, 131.2, 130.6, 129.9, 129.2, 128.7,
54.1. Anal. Calcd for C39H33ClN2P2: C, 74.70; H, 5.30; N, 4.47.
Found: C, 74.67; H, 5.24; N, 4.40.
Future studies will focus on establishing other unusual
reactivity patterns with these chelating ligands with the ultimate
goal of understanding the role of the chelating framework in
reaction pathways such as the C−H activation reported herein.
3-Bis(o-diphenylphosphino)phenyl-1H-imidazolinium hexa-
fluorophosphate, [2]PF6. To a suspension of TlPF6 (147 mg, 0.419
mmol) in 10 mL of dichloromethane was added white solid [2]Cl
(263 mg, 0.419 mmol). The reaction mixture was allowed to stir at
room temperature for 12 h to ensure completion, and the resulting
mixture was filtered through Celite. Removal of volatiles from the
resulting clear colorless solution in vacuo afforded a white solid as an
EXPERIMENTAL SECTION
■
General Considerations. All syntheses reported were carried out
using standard glovebox and Schlenk techniques in the absence of
water and dioxygen, unless otherwise noted. Benzene, n-pentane,
tetrahydrofuran, toluene, diethyl ether, and dichloromethane were
degassed and dried by sparging with ultra high purity argon gas
followed by passage through a series of drying columns using a Seca
Solvent System by Glass Contour. All solvents were stored over 3 Å
molecular sieves. Deuterated benzene, dichloromethane, and tetrahy-
drofuran were purchased from Cambridge Isotope Laboratories, Inc.,
degassed via repeated freeze−pump−thaw cycles, and dried over 3 Å
molecular sieves. Solvents were frequently tested using a standard
solution of sodium benzophenone ketyl in tetrahydrofuran to confirm
the absence of oxygen and moisture. Compound 1,36 Pd(PPh3)4,52 and
1
analytically pure product. Yield: 289 mg, 93.5%. H NMR (400 MHz,
CD2Cl2): δ 7.55 (m, 2H, Ar-H), 7.53 (s, 1H, C-H), 7.45−7.39 (m,
16H, Ar-H), 7.34−7.30 (m, 8H, Ar-H), 7.04 (m, 2H, Ar-H), 4.19 (s,
4H, CH2). 31P NMR (161.8 MHz, CD2Cl2): δ 16.6 (s, 2P, Ar-P),
−143.3 (septet, 1P, PF6). 13C NMR (100.5 MHz, CD2Cl2): δ 157.8,
134.9, 134.4, 134.2, 134.1, 131.3, 131.1, 130.3, 129.5, 129.4, 127.0,
53.1. Anal. Calcd for C39H33F6N2P3: C, 63.59; H, 4.52; N, 3.80.
Found: C, 63.51; H, 4.46; N, 3.73.
(N-Heterocyclic alkyl)Pd−Cl (3). Pd(PPh3)4 (207 mg, 0.179
mmol) was dissolved in 10 mL of THF, and to this stirring yellow
solution was added [2]Cl (112 mg, 0.179 mmol). The mixture became
a clear yellow solution in 5 min. The reaction was allowed to stir at
room temperature for 12 h, resulting in formation of a yellow
precipitate. The yellow solid was collected via filtration and dried in
53
Pt(PPh3)4 were synthesized using literature procedures. All other
chemicals were purchased from Aldrich, Strem, or Alfa Aesar and used
without further purification. NMR spectra were recorded at ambient
temperature unless otherwise stated on a Varian Inova 400 MHz
instrument. 1H and 13C NMR chemical shifts were referenced to
residual solvent and are reported in ppm. 31P NMR chemical shifts (in
ppm) were referenced to 85% H3PO4 (0 ppm), and 19F NMR
1
vacuo to afford analytically pure product. Yield: 95.0 mg, 72.3%. H
NMR (400 MHz, CD2Cl2): δ 7.67 (m, 4H, Ar-H), 7.50−7.32 (m,
18H, Ar-H), 6.94 (m, 2H, Ar-H), 6.88 (m, 2H, Ar-H), 6.81 (t, 2H, Ar-
H), 4.78 (t, 1H, C-H, 3JP−H = 20.2), 3.55 (m, 2H, CH2), 3.14 (m, 2H,
E
dx.doi.org/10.1021/om301230f | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
CH2). 31P NMR (161.8 MHz, CD2Cl2): δ 9.54 (s). 13C NMR (100.5
MHz, CD2Cl2): δ 150.0, 134.8, 134.3, 131.7, 130.4, 130.2, 128.5,
128.3, 120.2, 118.8, 85.6, 49.5. Anal. Calcd for C39H33ClN2P2Pd: C,
63.86; H, 4.53; N, 3.82. Found: C, 63.80; H, 4.50; N, 3.81.
Studies of PPh3 Binding to [5]PF6. Compound [5]PF6 was
dissolved in deuterated dichloromethane and transferred into a J.
Young tube. To the light yellow solution was added different amounts
1
of PPh3 (ranging from 0.5 equiv to 4 equiv), and H NMR and 31P
(N-Heterocyclic alkyl)Pt−Cl (4). Pt(PPh3)4 (72.4 mg, 0.0582
mmol) was dissolved in 10 mL of THF, and to this yellow solution was
added ligand precursor [2]Cl (36.5 mg, 0.0582 mmol). The reaction
was allowed to stir at room temperature for 1 h; then volatiles were
removed in vacuo. The residue was washed with diethyl ether to afford
a yellow solid as crude product. Single crystals suitable for X-ray
diffraction were obtained via vapor diffusion of diethyl ether into a
concentrated THF solution of 4. Yield: 35.0 mg, 73.2%. 1H NMR (400
MHz, d-THF): δ 7.63 (m, 4H, Ar-H), 7.46 (m, 4H, Ar-H), 7.36 (m,
8H, Ar-H), 7.28 (m, 6H, Ar-H), 6.84−6.71 (m, 4H, Ar-H), 6.61 (t, 2H,
Ar-H), 4.82 (dt, 1H, C-H, 3JP−H = 12.0 Hz, 2JPt−H = 52.0 Hz), 3.35 (m,
2H, CH2), 3.05 (m, 2H, CH2). 31P NMR (161.8 MHz, d8-THF): δ
16.6 (d, 1P, Ar-P, 1JPt−P = 3120 Hz). 13C NMR (100.5 MHz, CD2Cl2):
δ 151.0, 134.8, 134.3, 132.1, 130.7, 130.5, 128.4, 128.1, 118.1, 116.3,
66.3, 48.6. Anal. Calcd for C39H33ClN2P2Pt: C, 56.97; H, 4.05; N, 3.41.
Found: C, 56.91; H, 4.06; N, 3.28.
NMR spectra were recorded after addition of each amount of PPh3 to
1
the reaction. H NMR of hydride shifts (400 MHz, CD2Cl2): δ 0.0
equiv PPh3, −3.93 (1JPt−H = 851.5 Hz); 0.5 equiv PPh3, −4.39 (1JPt−H
= 835.6 Hz); 1.0 equiv PPh3, −4.80 (1JPt−H = 815.6 Hz); 1.5 equiv
PPh3, −4.97 (1JPt−H = 811.6 Hz); 2.0 equiv PPh3, −5.26 (1JPt−H
=
799.6 Hz); 4.0 equiv PPh3, −6.84 (1JPt−H = 735.6 Hz). 31P NMR shifts
(161.8 MHz, CD2Cl2): δ 0.0 equiv PPh3, 16.22 (1JPt−P = 2620 Hz); 0.5
equiv PPh3, 14.60 (2JPt−H = 2633 Hz); 1.0 equiv PPh3, 13.17 (1JPt−H
=
2646 Hz); 1.5 equiv PPh3, 12.60 (1JPt−H = 2650 Hz); 2.0 equiv PPh3,
11.71 (1JPt−H = 2699 Hz); 4.0 equiv PPh3, 6.97 (1JPt−H = 2701 Hz). All
the other resonances stay the same as those reported above for [5]PF6,
without detectable changes.
PMe3 Adduct of [5]PF6. Compound [5]PF6 (24.5 mg, 0.0298
mmol) was dissolved in 10 mL of dichloromethane, and to this light
yellow solution was added 10 equiv of PMe3. The mixture became a
yellow solution immediately. After 10 min, the volatiles were removed
in vacuo, and the residue was washed with diethyl ether (3 × 5 mL) to
afford a yellow solid. Yield: 24.7, 92.1%. 1H NMR (400 MHz,
CD2Cl2): δ 7.70 (m, 2H, Ar-H), 7.52 (t, 2H, Ar-H), 7.37 (m, 4H, Ar-
H), 7.28 (m, 16H, Ar-H), 7.11 (t, 2H, Ar-H), 6.95 (m, 2H, Ar-H), 4.36
[NHC-Pt-H][Cl], [5]Cl. Compound 4 (22.8 mg, 0.0277 mmol) was
dissolved in deuterated dichloromethane and transferred to a J. Young
tube. The yellow solution was heated at 60 °C in an oil bath. The
reaction progress was monitored by both 1H and 31P NMR
spectroscopy. After 12 h, the volatiles were removed to afford a
yellow solid as an analytically pure product. Single crystals suitable for
X-ray diffraction were obtained via vapor diffusion of diethyl ether into
a concentrated dichloromethane solution of [5]Cl. Yield: 20.8 mg,
(br, 4H, CH2), 1.10 (d, 9H, CH3), −12.71 (br, 1H, PtH, 1JPt−H = 563.7
1
Hz). 31P NMR (161.8 MHz, CD2Cl2): δ −12.1 (br, 1P, Ar-P, JPt−P
=
2754 Hz), −48.2 (br, 1P, PMe3).
1
91.2%. H NMR (400 MHz, CD2Cl2): δ 7.64−7.47 (m, 22H, Ar-H),
ASSOCIATED CONTENT
* Supporting Information
■
7.24 (t, 4H, Ar-H), 6.931 (m, 2H, Ar-H), 3.98 (s, 4H, CH2), −3.88
S
1
2
(dt, 1H, CH, JPt−H = 851.5 Hz, JP−H = 13.0 Hz),. 31P NMR (161.8
Crystallographic data and refinement parameters for 3, 4,
[5]Cl, and [5]PF6, crystallographic data in CIF format,
computational details. This material is available free of charge
via the Internet at http://pubs.acs.org.
MHz, CD2Cl2): δ 16.3 (d, 1P, Ar-P, JPt−P = 2620 Hz). 13C NMR
1
(100.5 MHz, CD2Cl2): δ 194.0, 144.5, 134.1, 134.0, 133.2, 132.1,
129.2, 128.3, 126.0, 120.3, 119.2, 50.3. Anal. Calcd for
C39H33ClN2P2Pt: C, 56.97; H, 4.05; N, 3.41. Found: C, 56.92; H,
4.19; N, 3.31.
AUTHOR INFORMATION
Corresponding Author
*E-mail: thomasc@brandeis.edu.
[NHC-Pt-H][PF6], [5]PF6. Pt(PPh3)4 (228 mg, 0.183 mmol) was
dissolved in 10 mL of THF, and to this yellow solution was added
ligand precursor [2]PF6 (135 mg, 0.183 mmol). The mixture was
allowed to stir at room temperature for 12 h to ensure reaction
completion. Volatiles were removed in vacuo, and the residue was
washed with diethyl ether to afford a yellow solid as the crude product.
Single crystals suitable for X-ray crystallography were obtained via
vapor diffusion of diethyl ether into a concentrated dichloromethane
solution of [5]PF6. Yield: 151 mg, 88.3%. 1H NMR (400 MHz,
CD2Cl2): δ 7.63−7.55 (m, 14H, Ar-H), 7.53−7.47 (m, 8H, Ar-H),
7.26 (m, 4H, Ar-H), 6.91 (m, 2H, Ar-H), 4.02 (s, 4H, CH2), −3.89
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge Brandeis University for
initially funding this project, the Brandeis MRSEC (NSF DMR-
0820492) for supporting B.P., and the National Science
Foundation under grant number CHE-1148987.
1
2
(dt, 1H, CH, JPt−H = 854.7 Hz, JP−H = 13.2 Hz). 31P NMR (161.8
MHz, CD2Cl2): δ 16.5 (d, 1P, Ar-P, 1JPt−P = 2619 Hz), −143.9 (septet,
1P, PF6, JP−F = 710 Hz). 13C NMR (100.5 MHz, CD2Cl2): δ 194.2,
1
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144.8, 134.3, 134.2, 133.4, 132.4, 129.4, 128.6, 126.3, 120.6, 119.4,
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Found: C, 50.23; H, 3.66; N, 2.97.
■
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