Heterolytic splitting of H-X bonds at a cationic (PNP)Pd center

Article abstract of DOI:10.1039/b925265g

The (PNP)PdOTf complex is a suitable synthetic equivalent of the [(PNP)Pd]⁺ fragment in reactions with various HX substrates. The [(PNP)Pd]⁺ fragment either simply binds HX molecules as L-type ligands (X = NH₂, PCy₂, imidazolyl) or heterolytically splits the H-X bond to produce [(PN(H)P)Pd-X]⁺ (X = H, CCR, SR). DFT calculations analyze the relative energetics of the two outcomes and agree with the experimental data. Calculations also allow to assess the unobserved Pd(iv) isomer [(PNP)Pd(H)₂]⁺ and validate its unfavourability with respect to the Pd(ii) isomer [(PN(H)P)PdH]⁺. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b925265g

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PAPER
www.rsc.org/dalton | Dalton Transactions
Heterolytic splitting of H–X bonds at a cationic (PNP)Pd center†
Lauren C. Gregor,a Chung-Hsing Chen,a Claudia M. Fafard,a Lei Fan,a Chengyun Guo,a Bruce M. Foxman,a
Dmitry G. Gusevb and Oleg V. Ozerov*a,c
Received 2nd December 2009, Accepted 2nd February 2010
First published as an Advance Article on the web 3rd March 2010
DOI: 10.1039/b925265g
The (PNP)PdOTf complex is a suitable synthetic equivalent of the [(PNP)Pd]+ fragment in reactions
with various HX substrates. The [(PNP)Pd]+ fragment either simply binds HX molecules as L-type
ligands (X = NH2, PCy2, imidazolyl) or heterolytically splits the H–X bond to produce
[(PN(H)P)Pd–X]+ (X = H, CCR, SR). DFT calculations analyze the relative energetics of the two
outcomes and agree with the experimental data. Calculations also allow to assess the unobserved Pd(IV)
isomer [(PNP)Pd(H)2]+ and validate its unfavourability with respect to the Pd(II) isomer
[(PN(H)P)PdH]+.
bonds that perform addition of even unreactive C–H bonds in
Introduction
alkanes are known and include group 6 metal alkylidenes,5 group
4 metal imido complexes,6 and Ti alkylidynes.7
Addition of hydrogen-element (H–X) bonds to transition metal
complexes is of central importance in organometallic chemistry.
Addition of H–X to a metal center with formation of new
M–H and M–X bonds is a classical oxidative addition reaction
(Scheme 1).1 1,2-addition across a metal–ligand bond (M–Y) is
also possible and is typically accomplished at the expense of a
metal ligand p-bond. Classical oxidative addition requires both
a filled and an empty orbital at the same transition metal center
(or a main group element; e.g., in a carbene)2 and proceeds with
a change of oxidation state of the metal center. A 1,2-addition
utilizes a ligand-based pair of electrons and an empty orbital at the
metal and proceeds without a change in oxidation state. Addition
of H–H across a Ru–Namido bond is a key mechanistic step in the
Noyori-type hydrogenation.3,4 Examples of multiple metal ligand
In the present work, we report our investigations of addition
of various H–X substrates across a Pd–N moiety in a cationic,
three-coordinate [(PNP)Pd]+ fragment where PNP is a diary-
lamido/bis(phosphine) pincer ligand (Scheme 2).8 In contrast to
Ru amides in the Noyori cycle,3,4 Legzdins metal alkylidenes,5
Wolczanski’s metal imides,6 and Mindiola’s Ti alkylidyne,7 the
[(PNP)Pd]+ fragment should not contain a Pd–N p-bond be-
cause all orbitals of dp-symmetry at Pd are filled. Instead, it
contains a nitrogen-based lone pair (albeit delocalized over the
diarylamide p-system) and a s-symmetry empty orbital at Pd
Scheme 1 Oxidative addition to a metal center and the non-oxidative
1,2-addition.
aDepartment of Chemistry, Brandeis University, 415 South Street, Waltham,
MA 02454, USA
bDepartment of Chemistry, Wilfrid Laurier University, Waterloo, Ontario
N2L 3C5, Canada
cDepartment of Chemistry, Texas A&M University, College Station, TX
77842, USA. E-mail: ozerov@mail.chem.tamu.edu
† Electronic supplementary information (ESI) available: Coordinate tables
for the calculated structures, selected pictorial NMR spectra and represen-
tations of calculated molecular orbitals. CCDC reference numbers 756641–
756644. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/b925265g
Scheme 2 Reactions of various H–X substrates with (PNP)PdOTf (1) in
benzene or toluene.
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(corresponding to the site of the “missing” fourth ligand of a
square planar complex). The formation of a p-bond is precluded
by the orthogonality of the lone pair and the empty orbital that
is enforced by the pincer ligand, while intermolecular s-bond
formation (i.e., di- or oligomerization) is presumably unfavourable
for steric reasons. From this perspective, the [(PNP)Pd]+ fragment
is related to the so-called frustrated Lewis acid–base pairs that
has been recently reinvigorated by Stephan et al.9 Addition of
H2 to complexes of a PNP* ligand derived from deprotonated
bis(phosphinomethyl)pyridine described by Milstein et al. is
also conceptually similar.10 In the present work, we describe a
series of experiments with various H–X substrates and analyze
the heterolytic H-X splitting as well as other possible reaction
outcomes.
with PdCl2, which produced (PN(H)P)PdCl2 10 (Scheme 2).13 The
exact structure of 10 was not determined, but the presence of NH
could be inferred from the downfield resonance in the H NMR
spectrum. Thus we can tentatively view 10 as an adduct of HCl
across the N–Pd bond in [(PNP)Pd]+ (if only one chloride in 10 is
coordinated to Pd) or in (PNP)PdCl (if both chlorides in 10 are
Pd-bound).
1
Reactions with HPCy2 and imidazole led to rapid color change
from the dark blue of 1 to different shades of purple. The solubility
of the product of the reaction of imidazole with 1 (7b-OTf) was
poor and we chose to isolate the more soluble BArF salt (7b-
4
Barf) via simple salt metathesis. The products of these reactions
1
did not show any H NMR resonances in the 10-11 ppm range,
indicating the absence of H on the N of PNP. On the contrary, the
1H NMR spectrum of 6b-OTf contained a diagnostic doublet at
d 4.78 ppm with 1JHP = 352 Hz, indicating the preservation of the
P–H bond. A phosphorus-phosphorus coupling constant of 34 Hz
Results and discussion
1
was evident in the AX2 system in the 31P{ H} spectrum of 6b-OTf.
Synthesis of cationic H–X adducts
The NMR spectra of 7b-Barf and the other features of the spectra
of 6b-OTf were also consistent with a simple adduct formation; for
example, C2v symmetry was maintained. The reaction with NH3
was reported previously and gave a similar product (8b-OTf).14
We chose (PNP)PdOTf (1) as a synthon for [(PNP)Pd]+. The
synthesis of 1 was previously reported.11 Compound 1 itself is
a molecular compound in non-polar solvents, but triflate is a
weakly enough coordinated ligand that it can be displaced by
some neutral ligands. We previously showed that the 3d metal
analog, (PNP)NiOTf, reacts with acetonitrile to form a cationic
acetonitrile complex and is an active catalyst for nitrile/aldehyde
coupling.12
Deprotonation of the cationic H–X adducts
Deprotonation of complexes 2a/3a/4a/5a/6b/8b-OTf was car-
ried out in a straightforward fashion (Scheme 3). Addition of a
Brønsted base to a solution of the cationic complex resulted in
the formation of the corresponding (PNP)PdX complexes (2x-6x
and 8x). We did not attempt to optimize the strength of the base
required to deprotonate different compounds, however, it is worth
noting that the complexes containing a protonated PNP ligand
were deprotonated by rather weak bases.
Several H–X substrates (Scheme 2) were selected for the study.
Reactions were performed by adding 1.0–1.2 equiv. of a substrate
to 1 in an aromatic solvent, with the exception of H2 and NH3
which were added in excess (1 atm). In cases where displacement
of triflate did take place, the new ionic products were easily isolated
in analytical purity and good yields even on an NMR tube scale.
No change was observed by in situ NMR analysis in reactions
with pyrrole, acetylacetone, acetic acid, and ethanol, even after
the solutions were subjected to thermolysis at 100 ◦C overnight.
The reactions with H2, HCCTol, HSPh, and HSCH2Ph led to
the transformation of the dark-blue colour of 1 to colourless
(2a-OTf), yellow (3a-OTf), or red (4a-OTf, 5a-OTf). With H2
the colour change took place over 10 min; for the other three
substrates, it was complete within the time of mixing. The products
2/3/4/5a-OTf show spectroscopic characteristics consistent with
the addition of the H–X bond across the N–Pd bond. Each
displayed an N–H resonance in the d 10.5–11.5 ppm region of
1
the H NMR spectra and a signal at around -80.5 ppm in the
19F NMR spectra attributed to free triflate. The NMR spectra
also contained appropriate resonances arising from the X group
bound to Pd. Most tellingly, 2a-OTf displayed a Pd–H resonance
at d -12.05 ppm.
Scheme 3 Deprotonation of cationic HX adducts.
The (PNP)Pd fragment of 1 possesses C2 symmetry but appears
C2v symmetrical on the NMR timescale due to a combination of
torsional and rotational motions involving the ligand backbone
Complexes 2x-6x and 8x give rise to C2v-symmetric NMR
spectra that are typical for (PNP)PdX complexes. Complex 6x
1
displays an AX2 system (JPP = 21 Hz) in its 31P{ H} spectrum,
i
and the Pr groups. Addition of a proton to the N of PNP
consistent with a disposition of the dialkylphosphide ligand cis-
to the phosphines of PNP.
destroys the rotational symmetry and makes the [(PN(H)P)PdX]+
complexes appear Cs symmetrical in solution. All compounds
2/3/4/5a-OTf possess effective Cs symmetry as observed in the
1H and 13C NMR spectra at ambient temperature.
Solid state structures determined by X-ray diffraction
While we have not performed a reaction of HCl with 1 here,
several years ago we reported on the reaction of (PN(H)P) (9)
We were able to obtain quality single crystals for compounds 2a-
OTf, 3x, 6x, and 7b-Barf that were suitable for X-ray diffraction
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studies. The ORTEP representations of the molecular structures
in the solid state are given in Fig. 1–4. Compound 2a-OTf
represents the first structure of a PN(H)P ligand with any metal.
The triflate anion in 2a-OTf is not coordinated to Pd, but is
involved in a hydrogen bond with the NH proton. The other
three structures contain a PNP ligand with a central amido,
sp2-hybridized nitrogen donor. The bond distances and angles
associated with the (PNP)Pd system in 3x, 6x, and 7b-Barf are
unremarkable and are within the range of values reported for
other PNP complexes of Pd. In fact, the metrics associated with
the phosphines of PNP are similar across all four compounds:
the P–Pd–P angle varies within the ca. 160–169◦ range while the
˚
˚
average Pd–P distances range from ca. 2.27 A to ca. 2.32 A. The
differences are slight and likely caused primarily by the magnitude
of steric repulsion with the fourth ligand in the coordination plane
of Pd.
Fig. 2 ORTEP drawing (50% probability ellipsoids) of 3x showing
selected atom labelling. Hydrogen atoms are omitted for clarity. Selected
◦
˚
bond distances (A) and angles ( ) follow: Pd1–P1, 2.2574(8); Pd1–P2,
2.2746(8); Pd1–N5, 2.062(3); Pd1–C27, 1.984(3); C27–C28, 1.191(5);
P1–Pd1–P2, 168.39(3); N5–Pd1–C27, 176.79(12).
Fig. 1 ORTEP drawing (30% probability ellipsoids) of 2a-OTf showing
◦
˚
selected atom labelling. Selected interatomic distances (A) and angles ( )
follow: Pd1–P1, 2.2465(9); Pd1–P2, 2.2854(9); Pd1–N1, 2.164(3); Pd1–H2,
1.543; N1–H1, 0.86(5); O2 ◊ ◊ ◊ N1, 2.833(6); P1–Pd1–P2, 162.05(4);
N1–Pd1–H2, 176.6; N1–H1 ◊ ◊ ◊ O2, 172(4).
Fig. 3 ORTEP drawing (50% probability ellipsoids) of 6x showing
selected atom labelling. Hydrogen atom◦s are omitted for clarity. Se-
˚
lected bond distances (A) and angles ( ) follow: Pd1–P1, 2.2964(5);
Pd1–P2, 2.3410(5); Pd1–P3, 2.3494(5); Pd1–N1, 2.1186(15); P1–Pd1–P2,
160.661(17); N1–Pd1–P3, 174.34(4).
The geometry about the P3 phosphorus atom of the PCy2 unit
in (FPNP)PdMe17 is 2.0938(15) A and that in (PNP)PdCl is
˚
in 6x is distinctly pyramidal, indicative of sp3-hybridization at this
2.0258(19) A.13 The Pd–NPNP distance in 2a-OTf is notably longer
˚
˚
˚
P atom. The Pd1–P3 distance of 2.3493(5) A is comparable to the
at 2.160(4) A because the nitrogen donor in it is an amine and also
Pd–PPNP bond lengths. These facts are consistent with a purely
single bond character for the Pd–PCy2 bond. A square planar
Pd(II) center lacks empty d-orbitals of appropriate symmetry to
support a p-bond with the phosphide. Other examples of struc-
turally characterized square planar Pd(II) phosphido complexes
also display a pyramidalized phosphide phosphorus atom and
relatively long Pd–P bond distances.15,16
because it is trans to the strongly trans influencing hydride.
Computational studies and analysis
In order to assist our understanding of the reactivity preferences
of the H–X molecules under study, we carried out a computational
study of a series of relevant substrates. DFT methods were
used to optimize the geometries and obtain gas-phase energies
of either the conventional H–X adducts of [(PNP)Pd]+ or the
products of H–X addition across the N–Pd bond in [(PNP)Pd]+.
Properties computed in the gas phase for ionic species may
deviate significantly from the solution properties. However, here
we undertake a comparison between two isomeric cations where
much of the solution vs. gas phase discrepancy can be expected to
cancel out.
The trend in Pd–NPNP distances can be rationalized by invoking
trans-influence arguments. The shortest Pd–NPNP distance of
˚
˚
2.008(4) A was observed in 7b-Barf, followed by 2.062(3) A
in 3x (CCAr) and 2.1186(15) in 6x (PCy2). That the order of
-
-
apparent trans-influence is imidazole < CCR < PCy2 is quite
reasonable; this is also the order of increasing Brønsted basicity.
Interestingly, the Pd–NPNP bond length in 7b-Barf is the shortest
we have recorded so far with this type of the PNP ligand, while
that in 6x is the longest. For comparison, the Pd–NPNP distance
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Table 1 DFT calculated enthalpiesa of the isomerization reaction
[(PNP)Pd(XH)]+ → [(PN(H)P)PdX]+
#
XH
DH/kcal mol-1
1
2
3
4
5
6
7
8
9
H2
CH4
HC CH
HC CPh
MeSH
NH3
MeOH
PH3
-16.8
-7.5
≡
-5.6
≡
-2.6
-5.0
+22.5
+8.6
+0.2
+16.1
HPMe2
a Negative reaction enthalpy denotes the preference for the heterolytic
splitting isomer.
Scheme 4 Transformations among different isomers resulting from
addition of H2 to [(PNP)Pd]+ (top) or (PNP)PdOTf (1, bottom) and the
enthalpies of transformation calculated by DFT (shown over arrows).
Fig. 4 ORTEP drawing (50% probability ellipsoids) of 7b-Barf showing
selected atom labelling. Hydrogen atoms, the rotational disorder of
imidazole, the disordered toluene solvent molecule, and the BA◦rF anion
49 kcal mol-1 above 2a. If triflate is included in the calculations, a
singlet minimum for (PNP)Pd(H)2(OTf) (2c-OTf) can be located
24.5 kcal mol-1 above the ground state 2a-OTf (Scheme 4) and
15.6 kcal mol-1 above 2b-OTf. While we have only analyzed this
for the case of H2, the rather large energy differences involved
suggest that the five-coordinate Pd(IV) species, [(PNP)Pd(H)(X)]+,
are not viable products here.
4
˚
are omitted for clarity. Selected bond distances (A) and angles ( ) follow:
Pd1–P1, 2.2971(13); Pd1–P2, 2.2983(14); Pd1–N1, 2.008(4); Pd1–N2,
2.052(5); P1–Pd1–P2, 166.26(5); N1–Pd1–N2, 178.75(19).
Table 1 summarizes the enthalpic data obtained from the DFT
calculations. Because this is a comparison between two isomers of
broadly similar structure, entropic differences are likely relatively
unimportant. Our computational results are largely consistent
with the observed experimental data. Ammonia and imidazole
are computed to prefer the simple adduct while MeSH (as a
model for PhSH and PhCH2SH), H2, and the alkynes favour
heterolytic splitting. The situation is somewhat divergent with
phosphines: the two isomers arising from PH3 are calculated
to be essentially isothermic, but HPMe2 strongly favours the
simple adduct, just as observed experimentally with HPCy2.
Interestingly, the calculations also show that the hypothetical
heterolytic splitting product is preferred for CH4. Methanol is
computed to favour a simple adduct. In experimental studies,
ethanol failed to displace triflate and no adduct was observed.
We have also considered possible mechanisms for the formation
of the heterolytic splitting products. Direct transfer of proton
to N of PNP in (PNP)PdOTf from an H–X substrate appears
improbable, especially for the H–X substrates of lower Brønsted
acidity. The N in (PNP)PdCl is a weak enough base that it is
protonated by HCl only reversibly (under vacuum).13 Complex
1 is an even weaker Brønsted base and all of the substrates
involved are weak Brønsted acids. An adduct [(PNP)Pd(HX)]+
is likely initially formed via displacement of the triflate by the
H–X ligand. We investigated the intramolecular isomerization of
[(PNP)Pd(H2)]+ into [(PN(H)P)PdH]+ with the help of a potential
energy scan where the H–Pd–H angle was varied incrementally
(5◦) from 28◦ to 128◦. As it was mentioned above, no minimum
corresponding to a Pd(IV) [(PNP)Pd(H)2]+ structure was found
along this path. The highest energy point was located at 27.7 kcal
mol-1 above [(PNP)Pd(H2)]+, corresponding to an approximately
square-pyramidal transition-state dihydride structure (Scheme 4,
∠H–Pd–H = 108◦). While we cannot exclude the possibility
of Pd(IV) intermediates or of concerted H-X cleavage for all
substrates, we hypothesize that a more likely mechanism entails
On the viability of the Pd(IV) isomer and mechanism. Besides
the two isomers already discussed, another conceivable structure
for the adduct of HX to [(PNP)Pd]+ is [(PNP)Pd(H)(X)]+ with a
Pd(IV) center. We evaluated the possibility of a Pd(IV) isomer for
the case of H2 (Scheme 4). We were unable to locate a minimum
corresponding to the singlet 2c; a triplet minimum for 2c was found
to lie 32 kcal mol-1 above the dihydrogen complex 2b and thus
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Table 2 DFT calculated proton affinities at NPNP of various (PNP)PdX
compounds, as well as the Pd–N distances in (PNP)PdX and Pd–NH
distance in [(PN(H)P)PdX]+
being slightly anomalous. NH2 displays a weaker apparent trans
influence than could be expected based on its effect on the proton
affinity of NPNP
.
˚
˚
#
X
PA/kcal mol-1a
d(M–N)/A
d(M–NH)/A
N- and C-bound imidazole. Sini, Eisenstein, and Crabtree
calculated that binding of parent imidazole to [MCl3]- (M = Ni,
Pd, Pt) and to a few other Pt(II) fragments favours C-binding vs.
N-binding.18 The C-bound tautomer is the simplest N-heterocyclic
carbene. We saw no experimental evidence for the C-bound form
(7d, Scheme 6), and computed that 7d lies 15.0 kcal mol-1 higher
in enthalpy than the observed N-bound isomer 7b.
1
2
3
4
5
6
7
CH3
H
NH2
PH2
SMe
OMe
Cl
254.2
253.6
251.1
250.4
249.1
248.2
243.6
2.116
2.107
2.082
2.109
2.076
2.055
2.041
2.211
2.192
2.160
2.207
2.163
2.129
2.111
a PA = -DH of protonation.
proton transfer from a coordinated H2 (or another HX) to N of
PNP with the assistance of (catalytic) exogenous Brønsted base.
This hypothesis dovetails the computational finding (vide supra)
that triflate deprotonates 2b. We do not know the exact nature of
the exogenous base in each of the cases of heterolytic splitting,
but it is logical to suggest that triflate, free H–X, adventitious
impurities (e.g., water), or neutral (PNP)PdX compounds are
candidates of a varying degree of likelihood in each case.
Scheme 6 Isomeric N-bound (7b) and C-bound (7d) imidazole complexes
and the calculated enthalpy of isomerization (over the arrow).
Proton affinities. From another perspective, the three possible
isomers resulting from addition of H–X to [(PNP)Pd]+ are three
different outcomes of protonation of (PNP)PdX: (a) at NPNP, (b)
at X, and (c) at Pd (Scheme 5). The lack of observed Pd(IV)
products (and the DFT results concerning 2c) indicate that Pd in
(PNP)PdX is not a thermodynamically competitive protonation
site in any of the cases under consideration. Protonation at X in
(PNP)PdX appears to be preferred where X retains a lone pair
and is a relatively strong base (phosphide or amide). However, it is
necessary to point out that the variation of the basicity of X also
affects the basicity of the N site in (PNP)PdX. We have calculated
the gas phase proton affinities of the N site in several (PNP)PdX
(Table 2) and found that they span a range of over 10 kcal mol-1.
Conclusions
In conclusion, we have demonstrated that the [(PNP)Pd]+ fragment
is capable of heterolytic cleavage of certain H–X bonds, such as
in thiols, H2, or an alkyne. On the other hand, H–X cleavage was
disfavoured vs. simple coordination of H–X in cases where X- is
a relatively strong base with at least two lone pairs (RO-, R2N-,
R2P-). DFT calculations suggest that both of these outcomes are
favored over the oxidative addition of H–X to the Pd center.
Experimental
General considerations
Unless specified otherwise, all manipulations were performed
under an argon atmosphere using standard Schlenk line or
glovebox techniques. Ethyl ether, C6D6, pentane, toluene, were
dried over NaK/Ph2CO/18-crown-6, distilled or vacuum trans-
ferred and stored over molecular sieves in an Ar-filled glovebox.
Fluorobenzene was dried over and distilled from CaH2. All
other reagents and solvents were degassed and stored over 4A
molecular sieves in a glovebox prior to use. Complex 1 was
prepared according to published procedures.18 The syntheses and
characterization of complexes 2x,13 8b-OTf,14 and 8x14 have been
reported elsewhere. All other chemicals were used as received from
commercial vendors. NMR spectra were recorded on a Varian
iNova 400 (1H NMR, 399.755 MHz; 13C NMR, 100.518 MHz; 31
P
NMR, 161.822 MHz) spectrometer. Chemical shifts are reported
in d (ppm). For 1H and 13C NMR spectra, the residual solvent peak
was used as an internal reference. 31P NMR spectra were referenced
externally using 85% H3PO4 at d 0 ppm. Elemental analyses were
performed by CALI Labs, Inc. located in Parsippany, NJ, USA.
Scheme 5 Possible sites of protonation of (PNP)PdX.
Table 2 also lists the corresponding Pd–NPNP distances in
(PNP)PdX and in [(PN(H)P)PdX]+. The increase in the Pd–N
bond distance trans to X can be taken as a measure of the
trans influence of X. There is a qualitative correlation between
the increasing trans influence of X (increasing Pd–NPNP distances)
and the increasing proton affinity of N in (PNP)PdX, with NH2
Computational details
The DFT calculations were carried out using the mPW1PW91
functional implemented in Gaussian 03.19 All structures were fully
optimized twice, first using bases set BS1 and then BS2. The basis
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+
-
≡
set BS1 included: SDD (associated with the corresponding ECP)
for Pd, 6-31G(d) for all N, O, F, P, S atoms, 6-31G(p) for the
hydrogen atom of HX, 6-31G for the i-Pr groups, and 6-31G(d) for
all other atoms. The basis set BS2 included: SDD (associated with
the corresponding ECP) for Pd, 6-311+G(d) for all N, O, F, P, S
atoms and metal-bonded carbon atoms, 6-31G(p) for the hydrogen
[(PN(H)P)Pd–C CTol] OTf (3a-OTf). (PNP)PdOTf (1)
(30 mg, 0.043 mmol) was dissolved in ether and p-tolylacetylene
(5.3 mL, 0.043 mmol) was added. Immediately upon mixing the
color changed from dark blue to clear pale yellow. The reaction was
allowed to stir for 10 min at ambient temperature. The volatiles
were removed in vacuo and washed with cold pentane. (Yield:
1
=
atom of HX, and 6-31G(d) for all other atoms. The C C carbon
27 mg, 82%) H NMR (C6D6): d 11.4 (s, 1 H, N-H), 7.51 (m, 2
atoms of C2H2 and HCCPh were modelled at the 6-311+G(d)
level in BS2. The nature of all stationary points was verified by
frequency calculations at the mPW1PW91/BS1 level. All reported
enthalpies were obtained by adding thermal corrections provided
by the frequency calculations to the corresponding electronic
energies from the mPW1PW91/BS2 geometry optimizations.
H), 7.36 (d, 2 H, J = 8 Hz, Ar-H), 7.00 (m, 2 H), 6.90 (d, 2 H,
J = 8 Hz, Ar-H), 6.73 (d, 2 H, J = 8 Hz, Ar-H), 2.84 (m, 2 H,
CHMe2), 2.27 (m, 2 H, CHMe2), 2.08 (s, 3 H, p-CH3), 2.04 (s,
6 H, Ar–CH3), 1.29 (m, 12 H, CHMe2), 0.92 (m, 12 H, CHMe2).
13C{ H} (C6D6): d 148.6 (t, J = 9 Hz), 138.7, 136.3, 133.5, 132.8,
1
131.4, 129.7, 129.6, 125.8 (t, J = 3 Hz), 125.6, 120.2 (t, J = 4 Hz),
116.2 (t, J = 15 Hz), 77.6, 27.6 (t, J = 11 Hz), 24.7, 21.6, 20.9,
20.1, 18.8, 18.1. 31P{ H} NMR (C6D6): d 47.7. 19F NMR (C6D6): d
1
X-Ray crystallography
-80.6. Elem. anal. calcd for C36H48F3NO3P2PdS: C 54.04, H 6.05.
Found: C 54.12, H 5.95.
X-Ray data collection for 2a-OTf was carried out at room
temperature (low temperature apparatus was not available) on a
CAD-4 Turbo diffractometer equipped with Mo Ka radiation.20
For 3x, 6x, and 7b-Barf, All operations were performed on
a Bruker-Nonius Kappa Apex2 diffractometer, using graphite-
monochromated Mo Ka radiation. All diffractometer manipu-
lations, including data collection, integration, scaling, and ab-
sorption corrections were carried out using the Bruker Apex2
software.21 All four structures were solved by direct methods
(SIR92).22 Full-matrix least squares refinements were carried
out using the Oxford University Crystals for Windows system.23
Additional crystallographic details are available in the Electronic
Supporting Information in the form of CIF files and descriptions
of data collection, solution and refinement for each structure.
≡
(PNP)Pd–C CTol (3x). Compound 3a-OTf (60 mg,
0.075 mmol) was dissolved in toluene and K2CO3 (15 mg,
0.10 mmol) was added. The reaction was allowed to stir overnight
at ambient temperature. The solution was filtered though Celite
and the volatiles were removed under vacuum. Yellow/orange
crystals were obtained from recrystallization of the residue from
1
a 1 : 3 toluene–pentane mixture. (Yield: 32 mg, 61%) H NMR
(C6D6): d 7.78 (d, 2 H, J = 8 Hz, Ar-H), 7.59 (d, 2 H, J = 8 Hz,
Ar-H), 7.01 (d, 2 H, J = 8 Hz, Ar-H), 6.85 (m, 4 H), 2.27 (m,
4 H, CHMe2), 2.16 (s, 6 H, Ar–CH3), 2.09 (s, 3 H, p-CH3), 1.45
(dvt, 12 H, CHMe2), 1.10 (dvt, 12 H, CHMe2). 13C{ H} NMR
1
C6D6: d 162.4 (t, J = 11 Hz), 133.6, 133.2, 131.6, 129.7, 127.5,
125.4, 120.9, 116.8 (t), 116.4 (t), 25.6 (t, J = 13 Hz), 21.9, 21.0,
1
19.5, 18.8. 31P{ H} NMR C6D6: d 50.6. Elem. anal. calcd for
Preparations
C35H47NP2Pd: C 64.66, H 7.29. Found: C 64.76, H 7.44.
[(PN(H)P)PdH]+OTf- (2a-OTf). (PNP)PdOTf (1) (68 mg,
0.10 mmol) was dissolved in 3 mL C6D6 in a sealed reaction tube,
and 1 atm H2 was charged in. The reaction mixture was stirred at
ambient temperature for 10 min, and it was found that the solution
became colourless gradually. All of the volatiles were removed
under vacuum, and the pure colourless product was obtained after
recrystallization from fluorobenzene/isooctane mixture. (Yield:
[(PN(H)P)Pd-SPh]+OTf- (4a-OTf). (PNP)PdOTf (1) (50 mg,
0.073 mmol) was dissolved in toluene then benzenethiol (8.5 mL,
0.082 mmol) was added to it. The solution immediately changed
from dark blue to a dark red upon mixing. The solution was
allowed to stir for 10 min then the volatiles were removed in vacuo.
A purple/red powder was obtained by recyrstallization of the
residue from a toluene–pentane mixture. (Yield: 43 mg 78%). 1H
NMR (C6D6): d 11.3 (s, 1 H, N-H), 8.04 (d, 2 H, J = 8 Hz, Ar-H),
7.40 (d, 2 H, J = 8 Hz, Ar-H), 6.97–6.83 (m, 7 H), 2.50 (m, 2 H,
CHMe2), 2.24 (m, 2 H, CHMe2), 2.02 (s, 6 H, Ar-CH3), 1.26–1.08
1
42 mg, 60%). H NMR (C6D6): d 10.57 (s, 1 H, N-H), 7.59 (d,
2 H, Ar-H), 7.04 (s, 2 H, Ar-H), 6.94 (d, 2 H, Ar-H), 2.55 (m,
2 H, CHMe2), 2.08 (s, 6 H, Ar-Me), 1.96 (m, 2 H, CHMe2), 1.14
(app q., 12 H, J = 9 Hz, CHMe2), 0.91 (app q., 6 H, J = 9 Hz,
CHMe2), 0.64 (app q., 6 H, J = 8 Hz, CHMe2), -12.04 (s, 1H,
1
(m, 24 H, CHMe2). 13C{ H} NMR (C6D6): d 148.8 (t, J = 8 Hz,
C–N), 145.3, 138.6 (t, J = 3 Hz), 133.4 (d, J = 6 Hz), 133.0, 129.9,
129.6, 128.9, 125.9, 127.0, 27.4 (t, JC–P = 11 Hz, CHMe2), 25.4
(t, JC–P = 12 Hz, CHMe2), 20.9 (Ar-CH3), 19.0 (Ar-CH3), 18.7
1
Pd–H). 13C{ H} NMR (C6D6): d 147.8 (t, JC–P = 9 Hz), 137.3,
133.6, 133.1, 130.8 (t, JC–P = 7 Hz), 125.9, 26.5 (t, JC–P = 12 Hz),
1
23.3 (t), 20.4, 20.0, 19.3, 19.0, 18.4. 31P{ H} NMR (C6D6): d 57.4.
1
(CHMe2), 18.5 (CHMe2), 1.76. 31P{ H} NMR (C6D6): d 41.0. 19
F
19F NMR (C6D6): d -80.6.
NMR (C6D6): -80.5. Elem. anal. calcd for C33H46F3NO3P2PdS2:
C 49.90, H 5.84. Found: C 51.68, H 6.04.
(PNP)PdH (2x). (PNP)PdOTf (1) (20 mg, 29 mmol) was
dissolved in about 0.6 mL C6D6 in a J. Young tube. The solution
was frozen and the headspace evacuated and back filled with
hydrogen gas (1 atm). The reaction mixture was placed in a
50 ◦C oil bath for 3 h and the purple solution became colourless
(PNP)Pd–S-Ph (4x). Compound 4a-OTf (30 mg, 0.038 mmol)
was dissolved in ether and K2CO3 (5.7 mg, 0.040 mmol) was added.
The mixture was allowed to stir at ambient temperature overnight.
The solution was filtered through Celite to remove insolubles and
the volatiles were removed in vacuo. (Yield: 16 mg, 66%). 1H NMR
(C6D6): d 7.91 (d, 2 H, J = 8 Hz, Ar-H), 7.70 (d, 2 H, J = 2 Hz,
Ar-H), 7.05 (t, 2 H, J = 8 Hz, Ar-H), 6.91–6.80 (m, 5 H), 2.18 (m,
4 H, CHMe2), 2.13 (s, 6 H, Ar-CH3), 1.31 (dvt, 12 H, J = 8 Hz,
1
gradually. Analysis by both 31P NMR and H NMR indicated
the formation of 2a-OTf. Triethylamine (7 mL, 45 mmol) was
added and the color changed to yellow immediately. NMR analysis
indicated complete conversion to 2x.
3200 | Dalton Trans., 2010, 39, 3195–3202
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The Royal Society of Chemistry 2010
©

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was dissolved in pentane and placed in the -35 ◦C freezer. Brown
1
CHMe2), 1.07 (dvt, 12 H, J = 8 Hz, CHMe2). 13C{ H} NMR
orange crystals were collected after standing overnight (48 mg,
(C6D6): d 162.2 (t, J = 10 Hz, C–N), 149.9, 133.2, 128.0, 125.9
(t), 122.3, 120.8, 120.6, 116.6 (t, J = 5 Hz), 25.7 (t, JC–P = 12 Hz,
1
77% yield). H NMR (C6D6): d 7.64 (d, 2 H, J = 8 Hz, Ar-H),
CHMe2), 21.0 (Ar-CH3), 19.0 (CHMe2), 18.3 (CHMe2). 31P{ H}
1
6.87 (s, 2 H, Ar-H), 6.84 (d, 2 H, J = 9 Hz, Ar-H), 2.67–1.33 (m,
22 H, PCy2), 2.27 (m, 4 H, CHMe2), 2.20 (s, 6 H, Ar-Me), 1.43
(br, 12 H, CHMe2), 1.11 (br, 12 H, CHMe2). 13C NMR (C6D6): d
160.4(C–N), 132.6, 132.5, 123.9 (t), 120.4 (t, JCP = 19 Hz, C–P),
115.7, 41.4 (d, JCP = 33 Hz, C–P), 36.3 (m), 28.4 (m), 27.3, 24.6 (t,
NMR (C6D6): d 45.7. Elem. anal. calcd for C32H45NP2PdS: C
59.67, H 7.04. Found: C 59.61, H 7.12.
[(PN(H)P)Pd-SBn]+OTf- (5a-OTf). (PNP)PdOTf (1) (60 mg,
0.088 mmol) was dissolved in C6D6; then benzylthiol (12 mL,
0.10 mmol) was added and the solution changed from a bright
blue to a clear red upon mixing. The volatiles were removed under
vacuum and the solid was washed with pentane resulting in a
red/purple solid. (Yield: 59 mg, 88%) 1H NMR (C6D6): d 11.2 (s,
1 H, N-H), 7.58 (d, 2 H, J = 8 Hz), 7.41 (d, 2 H, J = 8 Hz), 7.29 (t,
2 H, J = 8 Hz), 7.03 (t, 1 H, J = 8 Hz), 6.95 (s, 2 H), 6.85 (d, 2 H,
J = 8 Hz), 3.88 (s, 2 H, S–CH2), 2.73 (m, 2 H, CHMe2), 2.19 (m,
2 H, CHMe2), 2.00 (s, 6 H, Ar-CH3), 1.31–1.14 (m, 24 H, CHMe2).
1
JCP = 11 Hz, CHMe2), 20.8, 19.7, 17.3. 31P{ H} NMR (C6D6): d
37.4 (d, 21 Hz), 17.6 (t, 21 Hz). Elem. anal. calcd for C38H62NPdP3:
C 62.33, H 8.53. Found: C 62.28, H 8.48.
[(PNP)Pd(imidazole)]+BArF - (7b-Barf). (PNP)PdOTf (1) and
4
imidazole (4.9 mg, 0.073 mmol) were dissolved in fluorobenzene
(2 mL). The solution changed in color from dark purple to a
purple pink colour. This solution was stirred for 1 h. NaBArF24
was added and solution stirred an additional hour. The solution
was filtered through a pad of Celite. Volatiles were removed under
vacuum. The residue was washed with iso-octane then dried under
vacuum resulting in a purple powder (99 mg, 92% yield). 1H NMR
(d6-acetone): d 8.62 (s, 1 H), 7.90 (br, 1 H), 7.80 (s, 8 H, BArF24),
7.68 (s, 4 H, BArF24), 7.52 (s, 1 H), 7.42 (d, 2 H, J = 8 Hz, Ar-H),
7.20 (br, 1 H), 7.14 (br, 2 H, Ar-H), 6.97 (d, 2 H, J = 8 Hz, Ar-H),
2.64 (m, 4 H, CHMe2), 2.21 (s, 6 H, Ar-Me), 1.20 (app q, 12 H,
J = 8 Hz, CHMe2), 1.08 (app q, 12 H, J = 8 Hz, CHMe2). 13C
NMR (d6-acetone): d 162.7 (q, JCB = 48 Hz, C–B), 162.5 (t, J =
11 Hz, C–N), 139.4 (imidazole), 135.6 (BArF), 133.8, 133.7, 130.8
(imidazole), 130.1 (qm, JCF = 31 Hz, C–CF3), 127.9 (t), 125.5 (q,
JCF = 272 Hz, CF3), 120.6 (imidazole), 118.5 (BArF), 117.3 (t,
JCP = 21 Hz, C–P), 117.0 (t), 24.5, (t, JCP = 12 Hz, CHMe2),
13C{ H} NMR (C6D6): d 148.8 (t, J = 9 Hz), 144.0, 138.5, 133.5,
1
129.9 (t), 129.7, 129.5, 129.3, 127.6, 127.3, 126.1, 29.5, 28.0 (t, J =
12 Hz), 25.4 (t, J = 12 Hz), 21.2, 19.7, 19.0, 18.7, 18.3. 31P{ H}
1
NMR (C6D6): d 40.6. 19F NMR (C6D6): d -80.5. Elem. anal. calcd
for C34H48F3NO3P2PdS2: C 50.53, H 5.99. Found: C 50.45, H 6.06.
(PNP)Pd-SBn (5x). Compound 5a-OTf (30 mg, 0.037 mmol)
was dissolved in a 1 : 1 toluene/ether mixture and K2CO3 (7 mg,
0.050 mmol) was added. The mixture was allowed to stir at
ambient temperature overnight and the colour changed to a lighter
orange/red. The solution was filtered through Celite to remove the
insolubles and the volatiles were removed in vacuo. (Yield: 14 mg,
57%) 1H NMR (C6D6): d 7.74 (d, 2 H, J = 8 Hz), 7.63 (d, 2 H, J =
8 Hz), 7.22 (t, 2 H, J = 8 Hz), 7.08 (t, 1 H, J = 8 Hz), 6.98 (s, 2 H),
6.80 (d, 2 H, 8 Hz), 3.88 (s, 2 H, S–CH2), 2.31 (m, 4 H, CHMe2),
2.16 (s, 6 H, Ar-CH3), 1.38 (dvt, 12 H, CHMe2), 1.11 (dvt, 12 H,
1
20.3, 18.3, 17.7. 31P{ H} NMR (d6-acetone): d 52.3. 19F NMR
(d6-acetone): d 64.9. Elem. anal. calcd for C61H56BF24N3PdP2: C
49.97, H 3.85, N 2.87. Found: C 49.78, H 3.83, N 2.91.
CHMe2). 13C{ H} NMR (C6D6): d 162.1 (t, J = 10 Hz), 146.0,
1
133.2, 132.9, 129.4, 128.8, 126.4, 125.4 (t, J = 3 Hz), 120.5 (t, J =
18 Hz), 116.4 (t, J = 6 Hz), 36.6 (t, J = 3 Hz), 25.8 (t, J = 11 Hz),
Acknowledgements
20.9, 18.9, 18.3. 31P{ H} NMR (C6D6): d 44.7. Elem. anal. calcd
1
We are grateful for the support of this work by the US National
Science Foundation (CHE-0944634), the Sloan Foundation, the
Dreyfus Foundation, and the Welch Foundation. We also thank
the National Science Foundation for the partial support of this
work through grant CHE-0521047 for the purchase of a new X-ray
diffractometer at Brandeis University. We are grateful to Yanjun
Zhu for assistance with collection of some of the NMR data.
for C33H47NP2PdS: C 60.22, H 7.20. Found: C 60.09, H 7.13.
[(PNP)Pd(HPCy2)]+OTf-
(6b-OTf). (PNP)PdOTf
(1)
(100 mg, 0.15 mmol) was dissolved in toluene (2 mL) followed by
addition of PHCy2 (30 mL, 0.15 mmol). The solution was stirred
for 20 min. Stirring was ceased and the solution was allowed
to stand overnight. Purple crystals were collected (110 mg, 86%
1
yield). H NMR (d6-acetone): d 7.05 (d, 2 H, J = 9 Hz, Ar-H),
6.96 (s, 1 H), 6.73 (d, 2 H, J = 8 Hz, Ar-H), 4.54 (d mult, 1 H,
JHP = 352, P-H), 2.63 (br, 4 H, CHMe2), 2.14–1.17 (m, 22 H,
PHCy2) 1.97 (s, 6 H, Ar-Me), 1.19 (app q, 12 H, J = 8 Hz,
CHMe2), 1.01 (app q, 12 H, J = 7 Hz, CHMe2). 13C NMR
(d6-acetone): d 160.7(C–N), 134.1, 132.3, 128.1 (t), 117.6 (t,
JCP = 20 Hz, C–P), 116.4, 37.2 (d, JCP = 25 Hz, C–P), 34.0
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3202 | Dalton Trans., 2010, 39, 3195–3202
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The Royal Society of Chemistry 2010
©