A "hemilabile" palladium-carbon bond: Characterization and its implication in catalysis

Article abstract of DOI:10.1021/om5005435

An unusual palladium-carbon bond was identified in the crystal structure of a Pd(II) complex (VI) derived from 1-(2-diphenylphosphinophenyl)-2,5-dimethyl- 1H-pyrrole (L4). Theoretical calculations indicate that the Pd-C bond is covalent in nature. The com

Full text of DOI:10.1021/om5005435

Communication
pubs.acs.org/Organometallics
A “Hemilabile” Palladium−Carbon Bond: Characterization and Its
Implication in Catalysis
Debajyoti Saha,† Ravi Verma,‡ Devesh Kumar,‡ Shubhrodeep Pathak,§ Sourav Bhunya,§
and Amitabha Sarkar*,†
†Department of Organic Chemistry, Indian Association for the Cultivation of Science, Raja S. C. Mullik Road, Kolkata 700032, India
‡Department of Applied Physics, School for Physical Sciences, Babasaheb Bhimrao Ambedkar University, Ramabai Ambedkar Raili
Sthal Road, Lucknow 226 025, India
§Raman Centre for Atomic, Molecular and Optical Sciences, Indian Association for the Cultivation of Science, Raja S. C. Mullik Road,
Kolkata 700032, India
S
* Supporting Information
ABSTRACT: An unusual palladium−carbon bond was identified in the crystal structure of a Pd(II) complex (VI) derived from
1-(2-diphenylphosphinophenyl)-2,5-dimethyl-1H-pyrrole (L4). Theoretical calculations indicate that the Pd−C bond is covalent
in nature. The complex exhibits fluxional behavior in solution at ambient temperaturethe Pd binds alternately to C2 and C5 of
pyrrole, generating equivalent, enantiomeric structures. Theory predicts that the transition state of this interconversion probably
occurs through an unsaturated 14e complex, where the ligand binds the metal in a monodentate fashion. Comparison with
related structures reported earlier has been made, and the possible implication of such unusual bonding in the context of catalysis
of coupling reactions is discussed.
alladium is known to have rather unusual bonding
Pinteractions with an ipso carbon of an aromatic ring in
complexes with ligands featuring a β-aryl ring1 or a biaryl
motif,2,3 as revealed in their crystal structures. Such structures
are known for both Pd(0) and Pd(II) complexes, a selection of
which, viz. I−V, are presented in Figure 1. A few of these have
been scrutinized by theoretical calculations, and a covalent
bond is inferred from the existence of a typical bond critical
point between the metal and the carbon atoms.4
If we replace the ipso carbon with nitrogen as in 1-(2-
diphenylphosphino)phenyl-2,5-dimethylpyrrole (L4), the bi-
phenyl motif is technically retained and its similarity to the
reported ligands L1−L3 is evident (Figure 2).
We report herein the synthesis and structural character-
ization of a palladium(II) complex derived from ligand L4. The
unusual η1 bonding of palladium with the C2 of pyrrole in this
molecule is indeed unique. The covalent nature of the bond is
substantiated by theoretical calculations. We further established
that the structure of the complex in solution is fluxionalthe
palladium can form a bond with either of the two equivalent
carbons (C2 and C5) of pyrrole with equal facilityand
metallotropy is fast on the NMR time scale at room
Figure 1. Palladium complexes with Pd−Carene interactions.
temperature. Theory suggests that the interconversion most
likely occurs via a monoligated 14e intermediate.
Received: May 21, 2014
© XXXX American Chemical Society
A
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Organometallics
Communication
aromatic system) has been mentioned.3 Where the bonding
interaction is strong, the Pd−C bond length is short (<2.22 Å)
and deviation from planarity is significant. For structures with
Pd−C bonds larger than 2.33 Å, the deviation from planarity is
minimal. Complex VI presents another such unequivocal
example where the C2 of pyrrole connected to Pd is clearly
tetrahedral. The bond between N and C5 and the bond
between C4 and C3 of pyrrole are shorter (1.344(5) and
1.353(7) Å) with pronounced double-bond character, while the
bonds between C3 and C2 as well as between C2 and N are
longer (1.434(6) and 1.437(5) Å) with greater single-bond
character. These parameters prompted the formulation of the
zwitterionic structure VI depicted in Figure 3a. The lone pair of
nonbonded electrons of nitrogen is allowed to delocalize all the
way to C2, where increased electron density would allow σ
donation to palladium, which now attains the stable 16e
configuration.6
Figure 2. Structures of ligands.
Treatment of equimolar amounts of the ligand L4 (for details
of synthesis and characterization, see the Supporting
Information) with (CH3CN)2PdCl2 in dichloromethane
(Figure 3a) afforded the red complex VI, a suitable crystal of
which was grown from dichloromethane and light petroleum
for crystal structure determination.
The computed optimized geometry is shown in Figure 4a
(see the Supporting Information for computational details).
Figure 4. (a) Optimized geometry showing bond lengths and (b)
highest occupied molecular orbitals showing overlap between Pd and
C2.
The associated frontier molecular orbital picture clearly shows
the presence of a significant bonding overlap between
palladium and carbon of the pyrrole ring, as depicted in Figure
4b. The trend of bond length shrinkage or elongation observed
in the crystal structure is reflected in the calculated bond
lengths. Planarity around the palladium atom is also reproduced
by the calculations. In many respects, therefore, the energy-
minimized structure is similar to the crystal structure, as seen
from a comparison of structural parameters (Table 1).
Figure 3. (a) Synthesis of complex VI. (b) ORTEP diagram for the
molecular complex VI with ellipsoids at the 30% probability level.
Selected bond lengths (Å): Pd−C2, 2.169(4); C2−C3, 1.434(6); C3−
C4, 1.353(7); C4−C5, 1.386(6); C5−N, 1.344(5); N−C2, 1.437(5);
Pd−P, 2.2373(9); Pd−Cl1, 2.3626(10); Pd−Cl2, 2.3125(10).
Table 1. Bond Lengths (r), Electron Density (ρ), and the
Associated Laplacian (∇2ρ) of Representative Bonds
The ORTEP diagram (Figure 3b) reveals a near-planar
ligand arrangement at the palladium center, where the metal is
coordinated to the phosphorus and two chlorine atoms as
anticipated. In addition, an unusual bond is formed between the
C2 of pyrrole and palladium. There is no bonding interaction
between the metal center and the nitrogen atom that
substitutes for the ipso carbon in the heterocycle. Not only is
the bonding site different in this unusual structure but also
compound VI has by far one of the shortest metal−carbon
bonds (2.169(4) Å) in such a complex.5
a
b
r (Å)
ρ
∇2ρ
WBI
NAO-BO
Pd−P
2.33
2.38
2.40
2.28
0.09
0.07
0.07
0.07
−0.03
−0.04
−0.04
−0.31
0.1486
0.1603
0.1522
0.0745
0.5437
0.4791
0.4864
0.3253
Pd−Cl1
Pd−Cl2
Pd−C2
a
b
Wiberg bond index. Natural atomic orbital bond order.
Figure 5 indicates the presence of bond critical points (BCP)
In some structures of palladium η1-arene complexes, the
planarity of the arene ring with sp2-hybridized carbon atoms is
said to be conserved despite the bonding interaction.2 In the
case of other complexes, however, pyramidality of the
coordinated ipso carbon (as distinct from planarity of an
of the (3,−1) type. From Figure 5, it is clear that the BCP is
centrally situated in the Pd−C2 bond and parameters ρ and
∇2ρ (Table 1) suggest a covalent nature of the bond.7
The Mulliken population analysis places a net negative
charge on the carbon bound to palladium and a net positive
B
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Figure 7. Possible modes of interconversion between two enantio-
meric structures of VI.
Figure 5. Calculated bond critical point of (3,−1) type.
charge on the nitrogen,8 which is in agreement with our
proposed zwitterionic structure VI.
unsaturated −PPh2PdCl2 intermediate that allows for rotation
around the C−N bond is the other possibility.
The structure of complex VI implies that the symmetry of
the pyrrole ring is destroyed; thus, the two methyl groups on
pyrrole should have different chemical shift values in the proton
NMR spectrum. However, the 1H NMR spectrum of the
complex recorded at ambient temperature exhibits one six-
proton singlet at 1.46 ppm. Indeed, lowering the temperature
led to decoalescence of the six-proton singlet. At −73 °C, two
three-proton singlets are clearly seen to emerge at 1.71 and 1.04
ppm (Figure 6), which is to be expected from the nature of the
A frontier molecular orbital analysis shows that the transition
state goes through neither the η3-2-azaallyl complex nor any
Pd−N intermediate (see the Supporting Information for
computational details). In the transition state there is no
bonding overlap between the metal and the pyrrole ring
(Figure 8), which suggests that the fluxional process involves
Figure 8. (a) Transition state geometry showing bond lengths and (b)
highest occupied molecular orbitals of antibonding type.
Pd−C bond dissociation only. The free energy barrier for the
fluxional process is calculated to be 11.5 kcal mol−1. It is in
good agreement with the value of 10.42 kcal mol−1 estimated9
from DNMR data.
1
Figure 6. Low-temperature H NMR spectra of complex VI.
However curious the metal−pyrrole bond is, it appears to
have a significant effect on the catalytic efficiency of the
complex in coupling reactions of chloroarenes,10 which are
generally difficult substrates for such reactions. It is also a fact
that several such complexes cited in the references do display
significant catalytic activity.11 It can perhaps be attributed to an
unconventional, “hemilabile” participation on the part of the
presumed monodentate phosphine that stabilizes reactive
intermediates in the catalytic cycle.
Hemilabile ligands are potentially chelating ligands with one
(or more) donor that has a strong affinity for the metal center
and another which is “substitutionally labile”.12b At the
precatalyst stage or catalyst “resting” stage, the ligand acts as
a bidentate ligand and stabilizes the metal species by a
“chelating” effect. For hetero donor ligands of P, X type, where
X = O, N, etc., the metal remains bound to phosphorus
throughout the reaction; the weaker donor, O or N, dissociates
two methyl groups depicted in the crystal structure. Similarly,
two distinct and unsymmetrical pyrrole proton signals (H’s at
C3 and C4) also emerged at 8.74 and 6.13 ppm. A gradual shift
of the 31P signal of the phosphine coordinated to the metal was
also observed (see the Supporting Information). Thus, the low-
temperature NMR spectrum is consistent with the molecular
structure of complex VI.
It is worthwhile to speculate on the mechanism of the
fluxional process. The C−Pd bond is a relatively weak bond. It
can be easily disrupted, allowing for a bond rotation along the
arene−pyrrole axis leading to a similar bond formation on the
other side of the pyrrole: the two structures are enantiomeric
(Figure 7). The postulated metallotropy might involve an η3-2-
azaallyl group coordinated to palladium(II) as the transition
state between two equal-energy ground-state conformations of
η1 complexes. Formation of an open-chain, coordinatively
C
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2004, 43, 1871. (e) Chaouche, N.; Fornies, J.; Fortuno, C.; Kribii, A.;
Martín, A. J. Organomet. Chem. 2007, 692, 1168.
in the presence of the substrate molecules to trigger the
catalytic cycle. It has also been stated that “hemilability of a
chelate can be observed via fluxional processes that involve the
dissociation and recoordination of weakly bonding moieties via
intramolecular ligand exchange processes”,12a which is exactly
what we observed. It is possible that oxidative addition of ArCl
takes place when the palladium is monocoordinated, as
suggested earlier by Buchwald.4a
We cannot overlook the fact that several complexes that
catalyze coupling reaction with ArCl feature a triarylphosphine
as donor,13 not an aryldialkylphosphine. Whether and how a
weak Pd−C interaction can impart adequate electron richness
of palladium to facilitate oxidative addition across a less reactive
Ar−Cl bond remains an open question.
To sum up, we have isolated and structurally characterized an
unusual palladium(II) complex which reveals an uncommon η1
coordination of a pyrrole ring to metal that is characterized as a
covalent bond by theoretical calculations. The bond is weak,
however, as evidenced by a rapid interconversion between two
enantiomeric structures at ambient temperature. However, such
an interaction strengthens the possibility of “hemilabile”
participation of the ligand to stabilize Pd(II) intermediates
and facilitate coupling reaction of chloroarenes, notably with
triarylphosphines. This merits a closer scrutiny, a theme that is
currently being pursued in this laboratory.
̌
̌ ̌ ̌ ́
(3) (a) Kocovsky, P.; Vyskocil, S.; Cisarova, I.; Sejbal, J.; Tislerova, I.;
Smrcina, M.; Lloyd-Jones, G. C.; Stephen, S. C.; Butts, C. P.; Murray,
M.; Langer, V. J. Am. Chem. Soc. 1999, 121, 7714. (b) Dotta, P.;
Kumar, P. G. A.; Pregosin, P. S. Organometallics 2003, 22, 5345.
(4) (a) Barder, T. E.; Walker, S. D.; Martinelli, J. R.; Buchwald, S. L.
J. Am. Chem. Soc. 2005, 127, 4685. (b) Yamashita, M.; Takamiya, I.;
Jin, K.; Nozaki, K. J. Organomet. Chem. 2006, 691, 3189. (c) Barder, T.
E. J. Am. Chem. Soc. 2006, 128, 898.
(5) For comparable Cipso−Pd bond distances, see 2.336(4) Å in ref
2a, 2.676(5) Å in ref 2b, 2.338(11) Å in ref 2c, 2.374(3) Å in ref 2d,
2.391(5) Å in ref 2e, 2.187 Å in ref 3a, and 2.226(8) and 2.129(2) Å in
ref 3b.
(6) Koco
̌
vsky and Lloyd-Jones reported3a a structure that has a
tetrahedral arene carbon bound to the Pd(II) center (see structure III
in Figure 1). The lone pair of electrons on the nitrogen of the Me2N−
group is shown to delocalize (like an enamine) to increase electron
density of the ipso carbon that coordinates with the palladium ion.
The nitrogen now carries a positive charge; the palladium is shown as a
“palladate” species in conformity with its +2 oxidation state. Similar
polarizations have also been described by others.3b
(7) Zhang, L.; Ying, F.; Wu, W.; Hiberty, P. C.; Shaik, S. Chem. Eur. J.
2009, 15, 2979.
(8) Mulliken charges on different atoms of VI: Pd, 0.504499; P,
1.504727; N, 0.581628; C2, −0.346059.
(9) Eliel, E. L.; Wilen, S. H. In Stereochemistry of Organic Compounds;
Wiley: New York, 1994; p 502.
(10) Selected reviews: (a) Littke, A. F.; Fu, G. C. Angew. Chem., Int.
Ed. 2002, 41, 4176. (b) Zapf, A.; Beller, M. Chem. Commun. 2005, 431.
(c) Martin, R.; Buchwald, S. L. Acc. Chem. Res. 2008, 41, 1461.
(d) Kambe, N.; Iwasaki, T.; Terao, J. Chem. Soc. Rev. 2011, 40, 4937.
(e) Wong, S. M.; So, C. M.; Kwong, F. Y. Synlett 2012, 23, 1132.
(f) Bruno, N. C.; Tudge, M. T.; Buchwald, S. L. Chem. Sci. 2013, 4,
916.
(11) (a) See refs 3a and 4a. See also: Iwasawa, T.; Komano, T.;
Tajima, A.; Tokunaga, M.; Obora, Y.; Fujihara, T.; Tsuji, Y.
Organometallics 2006, 25, 4665. (b) Preliminary experiments show
that complex VI (2 mol %) catalyzes Suzuki−Miyaura coupling of 4-
chloroanisole and 4-chlorotoluene with phenylboronic acid with 76%
and 82% isolated yields of coupled products, respectively, in toluene at
100 °C for 6 h: Saha, D.; Sarkar, A. Unpublished results.
(12) (a) Slone, C. S.; Weinberger, D. A.; Mirkin, C. A. Prog. Inorg.
Chem. 1999, 48, 233−350. (b) Braunstein, P.; Naud, F. Angew. Chem.,
Int. Ed. 2001, 40, 680.
(13) (a) Liu, S.-Y.; Choi, M. J.; Fu, G. C. Chem. Commun. 2001,
2408. (b) Pramick, M. R.; Rosemeier, S. M.; Beranek, M. T.; Nickse, S.
B.; Stone, J. J.; Stockland, R. A., Jr.; Baldwin, S. M.; Kastner, M. E.
Organometallics 2003, 22, 523. (c) Kwong, F. Y.; Chan, K. S.; Yeung,
C. H.; Chan, A. S. C. Chem. Commun. 2004, 2336. (d) Mukherjee, A.;
Sarkar, A. Tetrahedron Lett. 2004, 45, 9525. (e) So, C. M.; Chow, W.
K.; Choy, P. Y.; Lau, C. P.; Kwong, F. Y. Chem. Eur. J. 2010, 16, 7996.
(f) Chow, W. K.; Yuen, O. Y.; So, C. M.; Wong, W. T.; Kwong, F. Y. J.
Org. Chem. 2012, 77, 3543. (g) See also ref 11a..
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, and a CIF file giving experimental details,
characterization data, including H, 13C and 31P NMR spectra
1
for the compounds prepared in this paper, and crystallographic
data for VI. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for A.S.: ocas@iacs.res.in.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank thr DST of India for financial support. D.K. is a
Ramanujan Fellow of the Department of Science and
Technology (DST), New Delhi, India. D.S. is grateful to the
CSIR of India for a Senior Research Fellowship. Crystallo-
graphic studies were performed at the facility of the
Department of Inorganic Chemistry, IACS. The authors
thank Mr. Ranjan Dutta for his valuable assistance. We thank
the reviewers for valuable suggestions and references. S.P. and
S.B. thank their mentor, Dr. Ankan Paul for his support and
encouragement.
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dx.doi.org/10.1021/om5005435 | Organometallics XXXX, XXX, XXX−XXX