Fluxional chloro-pyrrole-Pd(ii) complex to cationic η2-pyrrole-Pd(ii) complex: subtlety in structure-directed bonding mode

Article abstract of DOI:10.1039/d1nj00866h

Fluxionality in η¹-aromatic carbocyclic-ligated Pd(ii) complexes is retained if the η¹-carbon is replaced with an N atom through “hemilabile” palladium-carbon η¹-pyrrole ligation. Herein, we report and characterize a tetra

Full text of DOI:10.1039/d1nj00866h

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Fluxional chloro-pyrrole–Pd(II) complex to
cationic g2-pyrrole–Pd(II) complex: subtlety in
structure-directed bonding mode†
Cite this: New J. Chem., 2021,
45, 10594
Received 20th February 2021,
Accepted 18th May 2021
b
Debajyoti Saha *a and Nayim Sepay
*
DOI: 10.1039/d1nj00866h
rsc.li/njc
Fluxionality in g1-aromatic carbocyclic-ligated Pd(II) complexes is type of fluxional ‘‘hemilabile’’ palladium–carbon bonding inter-
retained if the g1-carbon is replaced with an N atom through actions through Z1-pyrrole ligation in complex 1 (Fig. 1).10
‘‘hemilabile’’ palladium–carbon g1-pyrrole ligation. Herein, we Although such interactions are indeed rare, a few instances of
report and characterize a tetra-coordinated neutral chloro-pyrrole– such uncommon bonding between palladium and carbon have
Pd(II)-p-allyl complex, which shows dynamic behavior at room tem- been described by different groups.11–17 The existence of such
perature. Upon chlorine abstraction by AgSbF6, a cationic palladium ‘‘weak’’ bonds underscores the interplay of subtle orbital over-
complex is formed, where the coordination sphere of the metal is laps that are probably intimately linked with the stereochemical
Q
completed by the coordination of one C C double bond of the arrangement of atoms around the metal, i.e. dependent on the
pyrrole ring. This cationic g2-pyrrole-ligated Pd(II)-p-allyl complex specific structure of the molecule. Once such a structure is
was characterized crystallographically. The coordination mode of distorted, the bonding may weaken and/or disappear altogether.
the pyrrole in these isostructural complexes subtly changes with a In the context of our study with palladium complex 1 (Fig. 1), we
change in carbon ligand (r or p).
sought to investigate the nature of the bonding interaction
between pyrrole and palladium, where palladium is coordinated
Two or more forms of one molecule can interchange reversibly with a p-allyl ligand. In the fluxional Pd-complexes with Z1-ligation
among themselves due to molecular fluxionality, and this shown in Fig. 1, the coordination number of the metal ion is four.
process occurs through the exchange of atoms with respect to In this study, we synthesized a tetra-coordinated pyrrole–Pd(II)-allyl
symmetry-equivalent positions.1 Fluxional molecules have taken complex and explored its fluxionality. Furthermore, we extended
a special place in chemistry due to the temperature-dependent the work to the synthesis of Z2-pyrrole-ligated Pd(II) complexes
dynamic nature of their stereoelectronic properties.2 The fluxionality along with molecular orbital calculations to study the nature of
of molecules has been utilized in catalysis.3–5 Specifically, the bonding in these complexes.
fluxionality of metal-containing molecules has an influence on
catalytic activity.5,6
Palladium(II) complex 2 bearing 1-(2-diphenylphosphino
phenyl)-2,5-dimethyl-1H-pyrrole (L1) was prepared from the
In some palladium complexes, there is an uncommon dimeric complex [Pd(Z3-C3H5)Cl]2 and is shown in Scheme 1.
bonding interaction between the metal and the ipso carbon of Treatment of [Pd(Z3-C3H5)Cl]2 with the ligand L118 in dichloro-
a coordinated aromatic ring.7–9 Ligands with a b-aryl ring or a methane resulted in a yellow solution which produced a yellow
biaryl motif form such palladium complexes, which are very complex 2 (Scheme 1) in good yield after solvent evaporation.
rare in the literature. All these complexes are formed through Complex 2 was characterized in the solution phase by NMR
1
Z1-aromatic carbocyclic ligation. Some of these complexes are spectroscopy. The H NMR spectrum of complex 2 is distinctly
shown in Fig. 1 (complex I–III). Sarkar et al. modified the ligand different from the spectrum of ligand L1 and it can be recog-
by replacing the ipso carbon with nitrogen and showed another nized by the peaks of the allyl group. The single peak in the 31
P
NMR spectra (d 11.91 ppm) indicates that the reaction pre-
sented in Scheme 1 produces only complex 2.
a Department of Chemistry, Krishnagar Government College, Krishnagar,
Nadia-741 101, India. E-mail: djyoti07@gmail.com
The fluxionality of the complex can be understood from its
dynamic behavior, which was perceptible in its NMR spectra at
variable temperature (Fig. 2). The synthesized complex 2
showed a fluxional behavior in the solution phase. The two
methyl groups of the pyrrole ring of complex 2 seem to be non-
equivalent, but, at room temperature, they are in equilibrium
b Department of Chemistry, Lady Brabourne College, Kolkata-700017, India.
E-mail: nayimsepay@yahoo.com
† This paper is dedicated in the memory of Prof. Amitabha Sarkar. Electronic
supplementary information (ESI) available. CCDC 2043271 and 2043272. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d1nj00866h
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To get more clear evidence of dynamic behavior, we used
single crystal X-ray crystallography to characterize complex 2
(Fig. 3), which was crystallized from acetone-hexane. The crystal
structure of complex 2 reveals a near planar ligand arrange-
ment at the palladium center where the metal is attached to
one phosphine group, one chloride and one p-allyl group. It is
clear that the pyrrole ring is oriented orthogonal to the phenyl
ring and there is no bonding interaction with palladium
(C2–Pd: 3.49 Å and N–Pd: 3.57 Å). A hindered rotation around
the Ar–N axis at low temperature would explain the decoalescence
phenomenon involving the two methyl groups. In this crystal
structure, the pyrrole ring forms a 22.101 angle with the C–N axis,
which facilitates the aforesaid rotation of pyrrole along the C–N
axis. However, it is possible, at low temperature, that the angle
decreases and the rotation becomes restricted due to the steric
interaction between methyl groups and the allyl unit. The low
temperature separation of p-allyl protons provides evidence of
such interactions (Fig. 2).
Fig. 1 Fluxional Pd-complexes and exploration of new complexes.
Since complex 2 is already a 16e species with P, Cl and p-allyl
donors, coordination by pyrrole is no longer necessary. Had the
coordination by pyrrole been a strong one, in complex 2 the
p-allyl group would have slipped into a Z1-allyl bonding mode
to maintain the 16e configuration. We do not observe any such
p to s slippage of the allyl group.
Scheme 1 Synthesis of complex 2.
By removing the chloride ion from Pd(II) in complex 2, the
newly synthesized complex 3 would be short of two electrons,
becoming cationic. We then wished to examine whether coor-
dination by pyrrole to Pd(II) is necessary in the new complex 3,
as seen in complex 1.
Treatment of [Pd(Z3-C3H5)Cl]2 with the ligand L1 in dichloro-
methane resulted in a yellow solution that turned orange with
the addition of AgSbF6. After evaporation of the solvent under
reduced pressure, the orange solid obtained was purified by
recrystallization from acetone-hexane (Scheme 2).
The 1H NMR spectrum of the pure complex 3 reveals two
singlets at 1.89 and 1.34 ppm corresponding to methyl groups at
the 2- and 5-position of pyrrole. Pyrrole 3H and 4H also appear
as two distinct singlets at 6.16 and 6.64 ppm. The signals due to
protons on the p-allyl group appear at the expected regions as
Fig. 2 1H NMR spectra of complex 2 at different temperatures.
and appear as equivalent. The six-proton broad singlet at
1.93 ppm is assigned to the two methyl groups on pyrrole.
The protons 3H and 4H of pyrrole are indistinguishable; they
appear as a sharp two-proton singlet at 5.72 ppm. The high-
field three-proton singlet decoalesced into two signals (2.19 and
1.52 ppm) of equal intensity at À30 1C, but the singlet at
5.72 ppm revealed a small degree of line broadening (Fig. 2).
Therefore, the rotation of the C–N bond is restricted at low
temperature. At the coalescence temperature (0 1C) and below
(À20 1C), the rate of exchange was calculated and the values are
444.06 and 160 rotations, respectively (see ESI†). The relatively
broad signals of p-allyl protons at room temperature are better
resolved as the temperature is lowered.
Fig. 3 Crystal structure of complex 2 at 30% probability level (CCDC
number 2043271).†
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Scheme 2 Synthesis of complex 3.
well-separated multiplets. The signals due to other aromatic
protons are clustered between 7.07 and 7.72 ppm (for more
details, see Experimental section and ESI†). Well-separated
signals for the methyl protons as well as 3H and 4H of pyrrole
in complex 3 suggested an unsymmetrical and unwavering
disposition of the pyrrole group.
Fig. 5 (a) Optimized structures of 3; (b) highest occupied molecular
orbital of 3. Bond distances are shown in Å. Hydrogen atoms are omitted
for clarity in the molecular orbital picture.
transition metals in organometallic complexes. Harman has exten-
sively reported the chemistry of Z2-arene complexes of Os(II).24,25
Entropy controlled, intramolecular Z2-arene coordination has been
reported for palladium in mononuclear7,26–29 and dinuclear30,31
complexes or clusters.17,32,33 Organometallic complexes featuring
Z2-heteroarenes are relatively rare. Harman34 described Z2-pyrrole
complexes of Os(II) and Winter35–40 studied Z2-pyrazolate complexes
of a variety of metal ions.41 The complex 3 is the first Z2-pyrrole–
Pd(II) complex that has been isolated and structurally characterized.
Only Pregosin reported7 analogous complexes where Z2-arene
coordination is facilitated by a neighboring phosphine donor.
Molecular orbital calculations using DFT were carried out
for complex 3, which reveal Z2-pyrrole–Pd(II) bonding in the
energy-minimized structure (Fig. 5a). It is reasonable to assume
that orbital orientation for the best overlap with a s-donor will
not be the same as required by a p-donor ligand (Fig. 5b). The
bonding in complexes 1 and 3 differ in this respect. There was a
possibility of the formation of Z1-ligation of pyrrole to Pd(II) in
complex 3 just like in complex 1. To obviate this possibility, we
conceived model 4 (Fig. 6a) and optimized its structure using
DFT calculations. The optimized results showed that model 4 is
The ORTEP diagram of a crystal of complex 3 (Fig. 4)
disclosed that the bond between the pyrrole ring and palladium
is not a formal s-bond as was observed for complex 1 reported
earlier.10 The molecule has a distorted square planar arrangement
around palladium where the metal is attached to phosphine,
pyrrole and p-allyl groups. Pyrrole is bonded with palladium purely
via an Z2-type of interaction; the C2–Pd and C3–Pd bond distances
in complex 3 are nearly equal (2.386 and 2.371 Å, respectively). The
C4–Pd and C5–Pd distances are 3.099 and 3.465 Å, respectively,
which are larger than the C–Pd bond distance in Z4-coordinated
Pd complexes.19 This discarded the possibility of an Z4 coordina-
tion of pyrrole to Pd(II), leading to an 18-electron complex. Though
Z2-pyrrole coordination to other metals, e.g. Os(II), Re(I) and W(0),
has already been characterized crystallographically,20–22 this Z2-
pyrrole–Pd(II) interaction in complex 3 is reported and crystal-
lographically characterized for the first time.
It is well known that Pd(II) forms stable 16e complexes with a
large variety of ligands, where the metal ion is placed at the center
of a square-planar (or distorted square-planar) coordination
environment.23 In complex 3, in addition to 2e donation from
the phosphine and 4e donation from the p-allyl group around the
central Pd(II) ion with 8 electrons, the 16e configuration comprises
2e Z2-coordination by the pyrrole. Only a limited number of
examples exist of Z2-coordination by arenes or heteroarenes to
Fig. 6 DFT calculations of isostructural complexes showing that the
coordination mode between the pyrrole moiety and Pd(II) depends on
the s or p donor carbon ligands of the complex.
Fig. 4 The crystal structure of the molecular complex 3 at 30% probability
level. SbF6 is omitted for clarity (CCDC number 2043272).†
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30 kcal molÀ1 less stable than complex 3 and the Z1-bond in N. S. gratefully acknowledges Department of Chemistry, Lady
model 4 is larger than both Z2-bonds of complex 3 (Fig. 6a).
Brabourne College, Kolkata-700017 for research facilities. D. S.
We therefore investigated whether Z1 bonding by pyrrole gratefully acknowledges Dept. of Chemistry, Krishnagar Govt.
through C2 is allowed for isostructural complexes if one College, Krishnagar, Nadia- 741101 for research facilities. The
chlorine atom is replaced by an alkyl or an aryl group (both authors also thank Professor A. Sarkar for his encouragement
s-donors), using DFT calculations. According to theoretical and support. Crystallographic studies were performed at the
calculations, Z1 coordination of pyrrole was found to be the facility of the Department of Chemical Science, IACS, Kolkata.
preferred mode of bonding in the optimized structures of anti-
isomers (5 and 6) (Fig. 6b). Since anti-isomers were found to be
clearly more stable than syn-isomers (7 and 8) (DE8/6 = 5.3 or
Notes and references
DE7/5 = 7.3 kcal molÀ1), the syn-isomers were not observed. In
any case, theory predicts that an alkyl or aryl group at the anti-
position would favor an unusual bond-Z1-coordination of pyr-
role to the metal center. Encouraged by these results, molecular
calculations were also performed on structures 9 and 10
(Fig. 6b) to optimize their geometries (starting structural para-
meters were taken from crystallographic data of complex 1 and
Cl was replaced by a s-allyl group). Structure 9 was 4.5 kcal
molÀ1 more stable than structure 10 according to the calcula-
tion. Both structures, however, are less stable (9 by 6 kcal molÀ1
and 10 by 10.5 kcal molÀ1) than the optimized structure of
complex 3 (Fig. 5a). On one hand, this would explain why we do
not observe any trace of 9 or 10 in the 1H NMR spectrum.
On the other, theory lends support to the preference of
Z2-coordination by pyrrole in p-allyl complex 3.
1 P. R. Bunker and F. A. Matsen, Phys. Today, 1979, 32, 57–58.
2 P. R. Bunker, Fundamentals of Molecular Symmetry, CRC
Press, 2004.
3 F. A. Cotton, J. Organomet. Chem., 1975, 100, 29–41.
4 S. J. Malcolmson, S. J. Meek, E. S. Sattely, R. R. Schrock and
A. H. Hoveyda, Nature, 2008, 456, 933–937.
5 Z. Zhang, B. Zandkarimi and A. N. Alexandrova, Acc. Chem.
Res., 2020, 53, 447–458.
6 V. V. Grushin, Acc. Chem. Res., 2010, 43, 160–171.
7 P. Dotta, P. G. A. Kumar, P. S. Pregosin, A. Albinati and
S. Rizzato, Organometallics, 2004, 23, 4247–4254.
´
´
8 L. R. Falvello, J. Fornies, R. Navarro, V. Sicilia and M. Tomas,
J. Chem. Soc., Dalton Trans., 1994, 3143–3148.
´
´
9 J. Campora, J. A. Lopez, P. Palma, P. Valerga, E. Spillner and
E. Carmona, Angew. Chem., Int. Ed., 1999, 38, 147–151.
It is interesting to note that the order of the trans effect of 10 D. Saha, R. Verma, D. Kumar, S. Pathak, S. Bhunya and
the ligand is Ph 4 Me 4 allyl, but the order of energy difference in A. Sarkar, Organometallics, 2014, 33, 3243–3246.
syn/anti-complexes (Fig. 6b) is Me 4 Ph 4 allyl. This change in 11 L. R. Falvello, J. Fornies, R. Navarro, V. Sicilia and M. Tomas,
order can be explained with the help of the size of the substituents. Angew. Chem., Int. Ed. Engl., 1990, 29, 891–893.
The phenyl ring attached to Pd interacts with the Ph rings of PPh2 12 P. Kocovsk´y, S. Vyskocil, I. Cısarova, J. Sejbal, I. Tislerova,
´
´
ˇ
ˇ
ˇ
´ ˇ
´
ˇ
´
ˇ
in complex 6. This effect destabilizes the complexes somewhat,
which is responsible for the decrease of energy. Such steric
interaction is absent in the case of 5 (R = Me).
M. Smrcina, G. C. Lloyd-Jones, S. C. Stephen, C. P. Butts,
M. Murray and V. Langer, J. Am. Chem. Soc., 1999, 121,
7714–7715.
13 T. Murahashi, T. Otani, T. Okuno and H. Kurosawa, Angew.
Chem., Int. Ed., 2000, 39, 537–540.
14 S. M. Reid, R. C. Boyle, J. T. Mague and M. J. Fink, J. Am.
Chem. Soc., 2003, 125, 7816–7817.
Conclusions
To summarize, we explored bonding preferences between pyrrole
and a palladium ion in the framework of a structurally constrained,
chelating ligand environment. We have synthesized and character-
ized a tetra-coordinated pyrrole–Pd(II)-p-allyl complex which shows
dynamic behavior at room temperature. An Z2-pyrrole coordination
has been crystallographically characterized for the first time in a Pd-
p-allyl complex. Theoretical calculations reveal that the mode of
coordination of a pyrrole moiety in isostructural complexes is often
subtly dependent on the nature of carbon ligands (s or p) present in
the complex.
15 Y. Wang, X. Li, J. Sun and K. Ding, Organometallics, 2003, 22,
1856–1862.
16 P. Dotta, P. G. A. Kumar, P. S. Pregosin, A. Albinati and
S. Rizzato, Organometallics, 2003, 22, 5345–5349.
´
˜
´
17 N. Chaouche, J. Fornies, C. Fortuno, A. Kribii and A. Martın,
J. Organomet. Chem., 2007, 692, 1168–1172.
18 D. Saha, R. Ghosh, R. Dutta, A. K. Mandal and A. Sarkar,
J. Organomet. Chem., 2015, 776, 89–97.
19 S. A. Urbin, T. Pintauer, P. White and M. Brookhart, Inorg.
Chim. Acta, 2011, 369, 150–158.
20 W. H. Myers, K. D. Welch, P. M. Graham, A. Keller, M. Sabat,
C. O. Trindle and W. D. Harman, Organometallics, 2005, 24,
5267–5279.
21 K. D. Welch, P. L. Smith, A. P. Keller, W. H. Myers, M. Sabat
and W. D. Harman, Organometallics, 2006, 25, 5067–5075.
22 K. D. Welch, D. P. Harrison, M. Sabat, E. Z. Hejazi,
B. T. Parr, M. G. Fanelli, N. A. Gianfrancesco, D. S. Nagra,
W. H. Myers and W. D. Harman, Organometallics, 2009, 28,
5960–5967.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
D. S. gratefully acknowledges the generous financial support from
Science and Engineering Research Board, India (TAR/2018/000075).
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23 G. Parkin, in Comprehensive Organometallic Chemistry III, 33 S. Kannan, A. J. James and P. R. Sharp, J. Am. Chem. Soc.,
Elsevier, 2007, pp. 1–57.
1998, 120, 215–216.
24 W. D. Harman, Chem. Rev., 1997, 97, 1953–1978.
25 J. M. Keane and W. D. Harman, Organometallics, 2005, 24,
1786–1798.
26 C.-S. Li, C.-H. Cheng, F.-L. Liao and S.-L. Wang, J. Chem.
Soc., Chem. Commun., 1991, 710–712.
34 L. M. Hodges, J. Gonzalez, J. I. Koontz, W. H. Myers and
W. D. Harman, J. Org. Chem., 1993, 58, 4788–4790.
35 I. A. Guzei, A. G. Baboul, G. P. A. Yap, A. L. Rheingold, H. B.
Schlegel and C. H. Winter, J. Am. Chem. Soc., 1997, 119,
3387–3388.
´
27 J. Yin, M. P. Rainka, X.-X. Zhang and S. L. Buchwald, J. Am. 36 C. Yelamos, M. J. Heeg and C. H. Winter, Inorg. Chem., 1999,
Chem. Soc., 2002, 124, 1162–1163. 38, 1871–1878.
28 T. Iwasawa, T. Komano, A. Tajima, M. Tokunaga, Y. Obora, 37 D. Pfeiffer, B. J. Ximba, L. M. Liable-Sands, A. L. Rheingold,
T. Fujihara and Y. Tsuji, Organometallics, 2006, 25,
M. J. Heeg, D. M. Coleman, H. B. Schlegel, T. F. Kuech and
4665–4669.
C. H. Winter, Inorg. Chem., 1999, 38, 4539–4548.
´
29 W. J. Marshall and V. V. Grushin, Organometallics, 2003, 22, 38 C. Yelamos, M. J. Heeg and C. H. Winter, Organometallics,
555–562. 1999, 18, 1168–1176.
30 M. Gorlov, A. Fischer and L. Kloo, Inorg. Chim. Acta, 2003, 39 K. R. Gust, J. E. Knox, M. J. Heeg, H. B. Schlegel and
350, 449–454. C. H. Winter, Angew. Chem., Int. Ed., 2002, 41, 1591–1594.
31 U. Christmann, R. Vilar, A. J. P. White and D. J. Williams, 40 O. M. El-Kadri, M. J. Heeg and C. H. Winter, Inorg. Chem.,
Chem. Commun., 2004, 1294–1295. 2006, 45, 5278–5280.
32 L. R. Falvello, J. Fornies, C. Fortuno, A. Martın and A. P. 41 T. Arumuganathan, M. Volpe, B. Harum, D. Wurm, F. Belaj
´
˜
´
´
˜
¨
and N. C. Mosch-Zanetti, Inorg. Chem., 2012, 51, 150–156.
Martınez-Sarinena, Organometallics, 1997, 16, 5849–5856.
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