Synthesis, structure and computational studies of a cationic T-shaped Pd-complex

Article abstract of DOI:10.1039/c3nj41145a

Reaction of [(cod)Pd(Me)(thf)][SbF₆] or [(cod)Pd(Me)(MeCN)] [B(Ar_F)₄] (cod = 1,5-cyclooctadiene, B(Ar _F)₄ = [3,5-(F₃C)₂C₆H ₃]₄B) with one or two equivalents of tBu₃P gives [(tBu₃P)₂Pd(Me)]⁺ (3) exclusively. The two sterically encumbered tBu-groups prevent solvent coordination. In addition, this formally three coordinate complex is stabilized by a γ-agostic interaction in solid state, whereas solution NMR studies confirm that this interaction is rather weak. The strength of the γ-agostic interaction was evaluated using density functional theory (DFT) calculations. Surprisingly, [(tBu₃P)₂Pd(Me)]⁺ is unreactive toward CO, H₂, C₂H₄ and norbornene.

Full text of DOI:10.1039/c3nj41145a

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Synthesis, structure and computational studies of a
cationic T-shaped Pd-complex†
Marc D. Walter,*a Peter S. Whiteb and Maurice Brookhart*b
Cite this: NewJ.Chem., 2013,
37, 1128
Reaction of [(cod)Pd(Me)(thf)][SbF6] or [(cod)Pd(Me)(MeCN)][B(ArF)4] (cod
= 1,5-cyclooctadiene,
B(ArF)4
=
[3,5-(F3C)2C6H3]4B) with one or two equivalents of tBu3P gives [(tBu3P)2Pd(Me)]+ (3)
Received (in Victoria, Australia)
17th December 2012,
exclusively. The two sterically encumbered tBu-groups prevent solvent coordination. In addition, this
formally three coordinate complex is stabilized by a g-agostic interaction in solid state, whereas solution
NMR studies confirm that this interaction is rather weak. The strength of the g-agostic interaction was
evaluated using density functional theory (DFT) calculations. Surprisingly, [(tBu3P)2Pd(Me)]+ is unreactive
toward CO, H2, C2H4 and norbornene.
Accepted 22nd January 2013
DOI: 10.1039/c3nj41145a
www.rsc.org/njc
[(tBu3P)PdCl(Me)] (1)/Ag[B(ArF)4] system which forms the dimer
[(tBu3P)2Pd2(Me)2Cl][B(ArF)4] (2) upon chloride abstraction
Introduction
Vacant coordination sites on transition metal complexes are
a prerequisite for many classic organometallic reactions.1
Consequently, unsaturated metal complexes are proposed
intermediates in catalytic cycles such as hydrogenation, poly-
merization, cross-coupling reactions and C–H bond activation
and functionalization.1,2 An interesting sub-class in this context
are 3-coordinate, T-shaped d8-transition metal complexes
which are relevant in cross-coupling reactions or in the stabili-
zation of unusual molecules.3–12 Consequently such species
have been probed by many computational investigations.13–16
However, truly T-shaped, 3-coordinate complexes have remained
elusive with the exception of Pt–boryl complexes such as trans-
[(Cy3P)2Pt{B(Fc)Br}][B(ArF)4] (B(ArF)4 = [3,5-(F3C)2C6H3]4B);17,18 in
fact most of the formally 3-coordinate complexes are stabilized
by (weak) agostic interactions.3–6,9–12,16,19–22
(Scheme 1). The dimer acts as a precatalyst but requires
the dissociation into [(tBu3P)PdCl(Me)] and [(tBu3P)Pd(Me)]+
(the active species) prior to polymerization.20 While this cata-
lyst system can polymerize certain functionalized norbornenes
in a living manner with moderate activity, the polymeriza-
tion of the more reactive unfunctionalized NB is non-living
as judged from the polymerization data.20 Catalyst archi-
tecture A would overcome this dissociation problem. The
cationic nature of A would likely lead to a stabilizing intra-
molecular g-agostic interaction between the tBu groups and the
cationic Pd center. In this contribution we report on our
attempts to synthesize A, the synthesis of [(tBu3P)2Pd(Me)]+
(3), which is a rare example of a formally 3-coordinate cationic
Pd complex, and some reactivity and computational studies
concerning 3.
In the course of our investigations on the vinyl addition
polymerization of norbornene (NB)23–25 we began to investigate
whether one could synthesize a cationic species of the
type [(tBu3P)Pd(Me)(solvent)]+ (A) which would provide a
cationic Pd complex with a very open coordination sphere
while overcoming the limitations of the originally introduced
a
¨
Institut fur Anorganische und Analytische Chemie,
¨
Technische Universitat Braunschweig, Hagenring 30, 38106 Braunschweig,
Germany. E-mail: mwalter@tu-bs.de; Fax: +49 531-391-5309
b Department of Chemistry, University of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-3290, USA. E-mail: brookhart@email.unc.edu
† Electronic supplementary information (ESI) available: Computational energies
and coordinates, crystallographic data, calculated structures (mol-files). CCDC
915278. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3nj41145a
Scheme 1
c
1128 New J. Chem., 2013, 37, 1128--1133
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Results and discussion
Synthesis
Complex 1 was synthesized, and NMR experiments in CD2Cl2
showed that on addition of AgSbF6 or NaB(ArF)4 only the dimer
2 was formed. On addition of 20 equivalents of NB polymeriza-
tion occurs, while most of 2 remains unreacted. This suggests
that the dissociation of 2 is responsible for the slow initiation
(relative to propagation) of this catalyst system and only for the
less reactive functionalized NB derivatives does the rate of
polymerization become competitive to the rate of initiation.
From this study it was concluded that [(tBu3P)Pd(Me)(solvent)]+
(A) would overcome this limitation. We therefore attempted to
perform the chloride abstraction in coordinating solvents such
as diethyl ether to stabilize the cationic intermediate as a mono
solvent adduct, [(tBu3P)Pd(Me)(OEt2)]+. However, these attempts
were unsuccessful.
Fig. 1 ORTEP diagram for 3-SbF6 (ellipsoids drawn at the 50% probability level).
The [SbF6]À counter anion and most H-atoms were omitted for clarity. Selected
bond distances (Å) and angles (1): Pd–P1 2.3483(15), Pd–P2 2.4353(15),
Pd–C25 2.029(6), Pd–C11 2.900(2), P1–Pd–P2 173.40(5), P1–Pd–C25 91.4(2),
P2–Pd–C25 95.1(2).
Consequently, an alternative starting material was investi-
gated in which the chloride group has already been removed
and therefore should overcome the formation of the chloride
dimer, 2. In this context, [(cod)Pd(Me)(MeCN)][B(ArF)4] (cod =
1,5-cyclooctadiene), which is readily prepared in situ from
[(cod)Pd(Me)(Cl)] on addition of Na[B(ArF)4] in a mixture of
CH2Cl2 and MeCN was chosen.26 However, on addition of tBu3P
only the bis(phosphine) adduct 3-B(ArF)4 was isolated regardless
of the reaction conditions and the stoichiometry (Scheme 2).
Complex 3-B(ArF)4 was obtained as green crystals that are air
stable. The 1H NMR spectrum of 3-B(ArF)4 shows, in addition to
the B(ArF)4 counteranion signals, only resonances due to tBu3P
and Me in the correct ratio of 36 : 3. No additional solvent
coordination to the cationic Pd center is observed. In addition,
3-B(ArF)4 exhibits a single 31P{1H} NMR resonance (d 64.4) and
two 1H NMR resonances for the Me and CMe3 groups at
anion [B(ArF)4]À for [SbF6]À. The starting material of choice was
[(cod)Pd(Me)(thf)][SbF6], which is readily prepared from
[(cod)Pd(Me)(Cl)] on addition of AgSbF6 in tetrahydrofuran.27
Addition of tBu3P in tetrahydrofuran yielded 3-SbF6 and single
crystals were grown from a saturated CH2Cl2 solution at
À25 1C.‡ Fig. 1 shows an ORTEP representation of the mole-
cular structure of 3-SbF6, and selected bond distances and
angles are given in the caption. At first glance this molecule may
be regarded as a 3-coordinate, T-shaped cationic [(tBu3P)2Pd(Me)]+
complex. However, a closer inspection of the structure reveals
an interaction (2.900(7) Å) between the cationic Pd atom and
C11 on P1. This value is within the usual range for agostic
interactions observed for Pd.5,20
Therefore, the cationic complex 3 exhibits a similar stabi-
lizing g-agostic interaction to that of neutral precursor 1 to give
a 4-coordinate square-planar Pd center. The solution behavior
indicated that this g-agostic interaction is rather weak and thus
the reaction chemistry of 3 might provide additional insight
into the strength of this interaction. Surprisingly, 3 is unreac-
tive toward H2, C2H4, and norbornene, and no adduct for-
mation was observed with the sterically undemanding ligand,
CO (even at low temperature). In addition, no significant
change in the chemical shifts of 3 is observed on addition of
a good s-donor such as MeCN thus ruling out any coordina-
tion. We also attempted to protonate 3 with [H(OEt2)2][B(ArF)4],
but no reaction occurs similar to the previous results on
3
d 2.12 (t, JPH = 4.4 Hz) and d 1.54 (JPH = 6.4 Hz), respectively.
This suggests equivalent P and tBu groups on the NMR time
scale at ambient temperature; no decoalescence was observed
on cooling of the sample to À80 1C. The low solubility of this
molecule prevented variable temperature (VT) NMR studies at
even lower temperatures. The solution behavior is consistent
with a 3-coordinate Pd(II) complex which might be stabilized by
an additional weak agostic interaction. The related cationic
[Pt(Me)(iPr3P)2]+ complex behaves similarly, but undergoes
acid-catalyzed intramolecular CH-activation.12
Solid state structure
[(PONOP)PdMe][B(ArF)4] (PONOP
=
2,6-(tBu2PO)2C5H3N).28
To evaluate the presence of agostic interactions we attempted
to obtain a molecular structure using single crystal X-ray
diffraction data. Unfortunately, crystals of 3-B(ArF)4 were not
suitable for X-ray diffraction, so we decided to exchange the
Furthermore, no intramolecular CH-bond activation was
observed as was seen in the case of [Pt(Me)(iPr3P)2]+.12
‡ Crystal data for 3-SbF6: C25H57F6P2PdSb, Mw
= 761.80, monoclinic, a =
14.5334(6) Å, b = 14.2796(5) Å, c = 15.2101(5) Å, a = 90.001, b = 92.956(3)1, g =
90.001, V = 3152.4(2) Å3, T = 100(2) K, space group P2/a, Z = 4, MoKa, 9649
reflections measured, 6987 independent reflections. The final R1 value was 0.0564
(I 4 2s(I)). The final wR(F2) value was 0.1785 (I 4 2s(I)). The final R1 value was
0.0807 (all data). The final wR(F2) value was 0.1953 (all data). The goodness of fit
on F2 was 1.079.
Scheme 2
c
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Table 1 Comparison between the calculated and experimental structures of
complexes 1 and 3
Computational studies
In view of this unexpected (and disappointing) reaction chemistry
of 3, we decided to quantify the strength of the g-agostic inter-
actions in complexes 1, 3, 4 and 5 (Fig. 2). These Pd-complexes
can be grouped into neutral (1), mono-cationic (3) and di-cationic
(4 and 5) species. So the strength of the g-agostic interaction can
be evaluated as a function of complex charge. In addition, the
calculations on 5 provide structural insights into a hypothetical
s-methane complex and these results were compared to
[(PONOP)Pd(CH4)]2+ (6).
1
2
Exp.
Calc.
Exp.
Calc.
Complex
Pd–P
2.245(3)a
2.271a
2.348(2)a
2.435(2)b
2.029(6)
2.387a
2.398b
2.063
Pd–CH3
Pd–Cl
1.985(5)
2.336(3)
2.84
2.059
2.332
3.212
2.637
PdÁ Á Ág-C
2.90
3.320
2.748
PdÁ Á Ág-H
a
b
P-atoms attached to the g-agostic Me group. P-atoms without
g-agostic Me interaction.
Previous DFT studies have shown that for alkane com-
plexes weak non-covalent (dispersion) interactions need to be
considered.29 Hence, we decided to use the dispersion-
corrected B97 functional, B97D (see Experimental section for determined structures. Therefore, we will focus our discussions
details).30 For comparison, we have also performed these only on the B97D results, but the PBE0 data are given in the
calculations with the PBE0 functional which we have used ESI.† Although the computed and experimental structures
previously for Rh and Ir pincer complexes.28 The computed agree reasonably well, the agostic interaction observed in solid
molecular geometries at the B97D level of theory are shown in state appears to be stronger (as judged by a shorter PdÁ Á ÁC
Fig. 2 and a comparison between calculated and experimental distance) than in the calculated gas-phase structures. In addi-
structures for 1 and 3 is given in Table 1. Overall, the B97D tion, a small rotation barrier (DE‡ = 9.5 kcal molÀ1) along the
results are in much better agreement with the experimentally dihedral angle Pd–P1–C9–C11 (see ESI† for details) is computed
Fig. 2 Molecular geometries of 1, 3, 4, 5 and 6 optimized at the B97D/6-311G(d,p) (C, H, N, O, P) and SDD (Pd) level.
c
1130 New J. Chem., 2013, 37, 1128--1133
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which suggests only a weak g-agostic interaction which is Conclusions
consistent with the solution VT NMR studies. Hence we attri-
bute the differences between the computed and crystal struc-
tures to crystal packing effects.
Reaction of [(cod)Pd(Me)(thf)][SbF6] (cod = 1,5-cyclooctadiene)
with one or two equivalents of tBu3P gives [(tBu3P)2Pd(Me)]-
[SbF6] exclusively. The sterically demanding tBu-groups prevent
additional solvent coordination and stabilize this formally
three coordinate complex by a g-agostic interaction in solid
state, whereas solution studies confirm that this interaction is
rather weak. Unfortunately, the sterically demanding tBu-groups
in combination with the g-agostic interaction render [(tBu3P)2-
Pd(Me)]+ rather unreactive and consequently no reactivity with
CO, H2, C2H4 and norbornene was observed. The strength of
this g-agostic interaction was evaluated using relaxed force-
constants and compared to a series of neutral and cationic
Pd-complexes.
For strong covalent bonds the bond dissociation enthalpy
(BDE) and other indirect methods are frequently used to get an
estimate for the intrinsic strengths of an individual bond,31
but for the weaker agostic interactions this approach fails
and only approximate values are proposed.32 Hence, for
agostic interactions the estimated values may range from 1 to
10 kcal molÀ1 33–36
In addition, the separation between real
.
values and artifacts can be exceedingly difficult. To over-
come these limitations the concept of compliance constants
is a useful tool to describe the mechanical strength of an
individual bond or a non-covalent interaction,37,38 which is,
in principle, measurable by atomic force microscopy (AFM)
of single molecules.39 Furthermore, the generalized com-
pliance matrix allows for a straightforward comparison of
different molecular surroundings and has already been suc-
cessfully applied to a-agostic interactions.40 In an attempt to
rationalize our experimental observations we extended this
concept to g-agostic and s-methane interactions for which
rather weak bonding interactions are expected. Unperturbed
C–H bonds such as in CH4 or PtBu3 have relaxed force
constants in the range of 4.8–5.2 N cmÀ1, and any strong
agostic interaction should have a significant impact on this
value. Table 2 lists the relaxed force constants for several Pd–X
bonds or interactions. A closer inspection of the relaxed force
constants for 1 and 3 also confirms that the interaction
between the neutral or cationic Pd-atom and the g-C or g-CH
bonds are weak. Hence, no significant perturbation of the
relaxed force constants is observed. The situation gets more
interesting for the hypothetical dicationic Pd-complexes 4
and 5. In these molecules much stronger g-agostic interactions
are expected, and this is confirmed by computed compliance
constants. The two dicationic s-CH4 complexes 5 and 6 show
only weak methane binding and the calculated compliance
constants for 6 predict a slightly stronger bond for this s-CH4
complex. Computations also show that CH4 loss from the
hypothetical dicationic Pd complex 4 proceeds essentially
barrierless.
Experimental details
General considerations
All reactions, unless otherwise stated, were conducted under an
atmosphere of dry, oxygen free argon using standard high-
vacuum, Schlenk, or drybox techniques. Argon was purified by
passing through a BASF R3-11 catalyst (Chemalog) and 4 Å
1
molecular sieves. H, 13C{1H}, 31P{1H} and 13C DEPT135 NMR
spectra were recorded on a Bruker DRX 500 MHz, a Bruker DRX
1
400 MHz, or a Bruker 400 MHz AVANCE spectrometer. H and
13C chemical shifts are referenced relative to residual CHCl3
(d 7.24 for 1H), CH(D)Cl2 (d 5.32 for 1H), CHCl2F (d 7.47 for 1H),
C6HD5 (d 7.15 for 1H), 13CD2Cl2 (d 53.8 for 13C), 13CDCl3(d 77.0
for 13C), 13CDCl2F (d 104.2 for 13C) and 13C6D6 (d 128.0 for 13C);
31P chemical shifts are referenced to an external standard of
H3PO4. Probe temperatures were calibrated using methanol as
previously described.41 Elemental analyses were carried out by
Robertson Microlit Laboratories of Madison, NJ.
Materials
All solvents were deoxygenated and dried by passage over
columns of activated alumina.42,43 CD2Cl2, purchased from
Cambridge Laboratories, Inc., was dried over CaH2, vacuum
transferred to a Teflon sealable Schlenk flask containing 4 Å
molecular sieves, and degassed via three freeze–pump–thaw
cycles. [H(OEt2)2][B(ArF)4],44 Na[B(ArF)4],45 [(cod)Pd(Me)(Cl)],46
and [(cod)Pd(Me)(thf)][SbF6]27 were synthesized according to
literature methods. All other reagents were purchased from
Aldrich, Acros, Alpha Aesar or Strem Chemicals and used as
received.
Table 2 Computed relaxed force constants (in N cmÀ1) for complexes 1, 3–6
Complex
1
3
4
5
6
Pd–P
1.63a
1.03a
1.10b
1.72
1.12a
0.98b
1.33/1.33a
1.47b
3-B(ArF)4
Pd–CH3
1.76
PdÁ Á ÁCH4
PdÁ Á ÁCHH3
PdÁ Á Ág-C
PdÁ Á Ág-H
0.19
0.53
0.31
0.48
0.26
0.35
Under an argon atmosphere a Schlenk tube was charged with
[(cod)Pd(Me)(Cl)] (0.132 g, 0.5 mmol) and Na[B(ArF)4] (0.44 g,
0.5 mmol). After cooling the flask to À40 1C CH2Cl2 (10 mL) and
MeCN (10 mL) were added using a syringe. The reaction
mixture was stirred while warming to À20 1C, resulting in the
formation of [(cod)Pd(Me)(NCMe)][B(ArF)4] accompanied by
precipitation of NaCl. After allowing the precipitate to settle,
the solution of the NCMe adduct was transferred via a cannula
0.27
0.15
0.23
0.12
0.34/0.34
0.46/0.47
C(CH3)
g-CHÁ Á ÁPd
4.78
5.00
4.90
5.03
2.83
5.10
2.70/4.81
4.83/5.26
CH2 (ave.)
a
b
P-atoms attached to the g-agostic Me group. P-atoms without
g-agostic Me interaction.
c
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(0.202 g, 1 mmol) dissolved in a mixture of MeCN (4 mL) and
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3
(s, 8H, o-Ar, B(ArF)4), 7.16 (s, 4H, p-Ar, B(ArF)4), 2.12 (t, JPH
=
4.4 Hz, 3H, Pd–CH3), 1.54 (vt, 36H, JP–H = 6.4 Hz, C(CH3)3).
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d 162.1 (q, 38 Hz, ipso-Ar, B(ArF)4), 135.2 (o-Ar, B(ArF)4), 128.8
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Found: C, 49.37; H, 4.93.
´
´
´
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was stirred for 5 min at 0 1C and pentane (20 mL) was added.
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of solution. The mother liquor was decanted and the product was
dried under dynamic vacuum. Yield: 0.25 g (0.33 mmol, 66%).
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slow degradation was observed accompanied by the formation of
metallic Pd. Single crystals were grown by slow pentane diffusion
into a concentrated CH2Cl2 solution at À30 1C. Anal. calcd for
C25H47F6P2PdSb: C, 39.41; H, 7.54. Found: C, 39.22; H, 7.32.
˜
´
´
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¨
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Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (DFG) 25 M. D. Walter, P. S. White and M. Brookhart, Chem. Commun.,
for financial support through the Emmy-Noether program 2009, 6361.
(WA 2513/2-1) and the National Science Foundation (NSF) 26 D. J. Tempel, L. K. Johnson, R. L. Huff, P. S. White and
(CHE-1010170).
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