Fluoride-free Hiyama coupling by palladium abnormal N-heterocyclic carbene complexes

Article abstract of DOI:10.1039/c5dt02317c

A series of palladium complexes of the abnormal N-heterocyclic carbene ligands of the type (a-NHC)PdI₂(L) [L = NC₅H₅(1-3)b and PPh₃(1-3)c] effectively catalyzed the Hiyama coupling of aryl bromides and iodides with PhSi(OMe)₃ under the highly desired fluoride-free conditions. Interestingly enough, the pyridine based trans-(1-3)b complexes and a PPh₃ derived cis-3c complex exhibited higher yields than the related PPh₃ derived trans-(1-2)c complexes. The superior performances of the pyridine based trans-(1-3)b complexes and the PPh₃ derived cis-3c complex have been correlated to a tighter binding of the a-NHC ligand to the palladium center in these complexes, leading to a greater (a-NHC) ligand influence on the metal center partaking in the catalysis.

Full text of DOI:10.1039/c5dt02317c

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Page 1 of 46
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DOI: 10.1039/C5DT02317C
Fluoride-free Hiyama Coupling by Palladium Abnormal N-heterocyclic Carbene
Complexes
Sudipta Modak,† Manoj Kumar Gangwar,† Mitta Nageswar Rao,† Madasu Mahesh,†
Alok Ch. Kalita,† Vincent Dorcet,‡ Mayuri Arun Shejale, † Ray J. Butcher¶ and
Prasenjit Ghosh*,†
†Department of Chemistry,
Indian Institute of Technology Bombay,
Powai, Mumbai 400 076.
‡Université de Rennes 1,
Institut des Sciences chimiques de Rennes,
Campus de Beaulieu, 35042 – Rennes, France
¶ Department of Chemistry
Howard University.
Washington DC 20059, USA
Email: pghosh@chem.iitb.ac.in
Fax: +91-22-2572-3480
1

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Abstract: A series of palladium complexes of the abnormal N-heterocyclicDcOaI:r1b0e.1n03e9/C5DT02317C
ligands of the type (a-NHC)PdI2(L) [L = NC5H5 (1−3)b and PPh3 (1−3)c] effectively
catalyzed the Hiyama coupling of aryl bromides and iodides with PhSi(OMe)3 under
the much desired fluoride-free conditions. Interestingly enough, the pyridine based
trans-(1−3)b complexes and a PPh3 derived cis-3c complex exhibited higher yields
than the related PPh3 derived trans-(1−2)c complexes. The superior performances of
the pyridine based trans-(1−3)b complexes and the PPh3 derived cis-3c complex have
been correlated to a tighter binding of the a-NHC ligand to the palladium center in
these complexes leading to a greater (a-NHC) ligand influence on the metal center
partaking the catalysis.
2

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DOI: 10.1039/C5DT02317C
Introduction
Recent reports of the superior performances of some abnormal N-heterocyclic
carbene catalysts over their much-explored normal N-heterocyclic carbene
counterparts in reactions like, Suzuki coupling1, 2 and hydrogenation,3, 4 have aroused
considerable interest on these ligands.5 The abnormal N-Heterocyclic carbenes by
virtue of the carbenic center being flanked by one electronegative heteroatom are
more electron rich than the normal N-heterocyclic carbenes, in which the carbenic
center is located between two electronegative heteroatoms.6, 7 More importantly, in
many instances, much of the observed higher catalytic activities of the abnormal N-
Heterocyclic carbene complexes have been attributed to the electron richness of these
ligands.8-10
With one of our key objectives being in the development of the N-heterocyclic
carbene chemistry from the perspectives of their utility in homogeneous catalysis and
biomedical applications,11-13 we remain committed to studying the various scaffolds
of the normal and abnormal N-heterocyclic carbene ligands for the promises they
hold. In this context, we have recently reported the catalytic utility of the palladium
complexes of the abnormal N-heterocyclic carbenes derived from N-fused
heterocycles14, 15 in the much-preferred Cu-free and the amine-free Sonogashira
coupling reaction in air. Continuing further along the line, we decided to study their
utility in another potentially important but relatively unexplored C-C cross-coupling
reaction, namely the Hiyama coupling between an aryl halide and an organosilicon
nucleophile.16, 17
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The recent emphasis on the Hiyama coupling stems from the viable alternative iVtiew Article Online
DOI: 10.1039/C5DT02317C
offers to the much-popular Suzuki coupling arising from the use of non-toxic and
inexpensive organosilicon reagent as the organic nucleophile. It enjoys certain
advantages over other potential rival alternatives like the Stille coupling, which
suffers from toxicity issues associated with the use of organotin reagents.18, 19
However, despite its promise and also unlike the Suzuki coupling, which has been
extensively studied with the N-heterocyclic carbene based catalysts, the Hiyama
coupling has remained relatively unexplored. Till date, only a handful of examples of
the Hiyama coupling by the N-heterocyclic carbene based catalysts have been
reported.20-23
Towards this goal, our specific objective was in developing the PEPPSI-themed
(Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation)24-26 (a-
NHC)PdX2(NC5H5) (X = halide) type precatalysts of the abnormal N-heterocyclic
carbene ligands for the Hiyama coupling. The justification of our effort arises not
only from the absence of any such report of abnormal N-heterocyclic carbene based
catalysts for the Hiyama coupling but also for the continuation of our earlier effort on
the utility of the PEPPSI themed palladium precatalysts of the more ubiquitous
normal N-heterocyclic carbene ligands for the fluoride-free Hiyama coupling23 and
the Suzuki coupling.27 We speculated that the abnormal N-heterocyclic carbene being
more electron rich might perform better in the C-C cross-coupling reaction, and
stretching the concept further, we decided to study the even more electron rich mixed
abnormal N-heterocyclic carbene/phosphine based precatalysts of the type (a-
NHC)PdX2(PPh3) (X = halide) for the Hiyama coupling. It must be noted that
4

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some mixed phosphine/NHC complexes have been reported to exhibit superioVriew Article Online
DOI: 10.1039/C5DT02317C
performances in certain catalysis.28-32
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R2
I
R1
N
Pd PPh3
N
N
I
R2
I
Ph
R1
N
Pd N
R1 = Me, R2 = C6H10OH (1c)
R1 = Et, R2 = C6H10OH (2c)
N
N
I
Ph
R2
PPh3
R1 = Me, R2 = C6H10OH (1b)
R1 = Et, R2 = C6H10OH (2b)
R1 = Me, R2 = Ph (3b)
R1
N
Pd
I
N
N
I
Ph
R1 = Me, R2 = Ph (3c)
Figure 1.
6

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Here in the manuscript, we report a series of abnormal N-heterocyclic carbene based
palladium precatalysts of the type (a-NHC)PdI2(L) [L = NC5H5 (1−3)b and PPh3
(1−3)c] for the fluoride-free Hiyama coupling performed in the presence of NaOH as
a base under aerobic conditions.
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Results and Discussions
The three 1,2,3-triazole derived abnormal N-heterocyclic carbene ligands namely,
{(1-benzyl-3-R1-4-R2-1,2,3-triazol-5-ylidene} (R1 = Me, Et; R2 = C6H10OH, Ph) were
obtained from the corresponding 1,2,3-triazole derivatives by the direct alkylation
reactions with the methyl and ethyl iodide reagents (Scheme 1). The formation of the
1,2,3-triazolium iodide salts (1−3)a were evident from the observation of the
characteristic carbene resonances at 8.90−9.23 ppm in the 1H NMR spectrum of these
compounds. Furthermore, the reaction of (1−3)a with PdCl2 in pyridine in the
presence of an excess of KI and K2CO3 as a base yielded the pyridine bound (a-
NHC)PdI2(NC5H5) type complexes (1−3)b. Finally, with the anticipation that the
mixed phosphine/NHC complexes might exhibit superior catalytic activity,28-32 the
mixed phosphine/(a-NHC) derivatives of the type (a-NHC)PdI2(PPh3) (1−3)c were
conveniently prepared by the treatment of the pyridine derived (1−3)b complexes
with PPh3 in ca. 70−78 % yields.
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R2
R2
R1 = Me, R2 = C6H10OH (1a)
R1 = Et, R2 = C6H10OH (2a)
R1 = Me, R2 = Ph (3a)
R1
R1I
I
N
N
CH3CN
N
N
60 oC
N
N
Ph
Ph
(49-61) %
PdCl2
K2CO3
KI
N
R2
R2
R2
I
I
PPh3
R1
R1
R1
PPh3
CH2Cl2
PPh3
CH2Cl2
N
N
N
Pd
N
Pd PPh3
Pd
I
N
N
N
N
N
N
I
I
I
Ph
Ph
Ph
(70) %
(34-46) %
(77-78) %
R1 = Me, R2 = C6H10OH (1c)
R1 = Et, R2 = C6H10OH (2c)
R1 = Me, R2 = C6H10OH (1b)
R1 = Et, R2 = C6H10OH (2b)
R1 = Me, R2 = Ph (3b)
R1 = Me, R2 = Ph (3c)
Scheme 1.
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31
Quite interestingly, the P{1H} NMR spectrum of (1−3)c indicated the formation of
the trans-isomers in case of the (1−2)c complexes and a cis-isomer in case of the 3c
complex, as evidenced from the appearance of the characteristic Pd−PPh3 resonance
at 16.9−17.0 ppm for the (1−2)c complexes and at 25.2 ppm for the 3c complex, and
which are in concurrence with the related ones known in the literature (Supporting
31
Information Tables S1 and S2). The, final validation of the P{1H} NMR results
came from the X-ray diffraction studies (Figures 2−4 and the Supporting Information
Figures S50, S57 and S63 and Tables S22−S23) that confirmed the trans geometry for
the (1−2)c complexes and a cis geometry for the 3c complex. It is worth noting that
for the (NHC)PdI2(PR3) type of the mixed phosphine/NHC complexes, different
geometrical isomers have been isolated depending upon the sterics and the electronic
demands of the N-heterocyclic carbene and the phosphine ligands used for the
stabilization of these complexes (Supporting Information Tables S1 and S2). In this
context, the density functional theory study, undertaken at the B3LYP/SDD, 6-31G(d)
level of theory, revealed that the trans-isomer was more stable by 15.8−4.2 kcal/mol
than the cis-isomer for all of the pyridine based (1−3)b complexes and the phosphine
based (1−3)c complexes (Supporting Information Figure S11). Notably, higher
relative energy difference between the cis-and the trans-isomers were observed for
the pyridine based (1−3)b complexes (15.8−13.1 kcal/mol) than for the phosphine
based (1−3)c complexes (4.6−4.2 kcal/mol). Consistent with these DFT results, the
trans-isomers were isolated for all [(1−3)b and (1−2)c complexes] but one case. The
only instance, for which the cis-isomer was obtained, was for a phosphine based
complex (3c) having the lowest relative energy difference between the cis-and the
trans-forms of 4.2 kcal/mol among these pairs of the cis/trans-complexes.
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Of significant interest are the Pd−Ccarbene bond distances as they provide useful insight
on the nature of the chemical bond (Figures 2−4, Supporting Information S50, S57
and S63, Tables S22−23). In this regard, the significantly shorter Pd−Ccarbene bond
distances of [1.967(3)−1.986(5)] Å in the pyridine based (1−3)b complexes in
comparison to that of [2.006(6)−2.047(5)] Å in the phosphine based (1−3)c
complexes, is suggestive of a stronger Pd−Ccarbene bonding interaction in the former.
Furthermore, among the phosphine based (1−3)c complexes, the Pd−Ccarbene bond
distance is shorter in the cis-3c complex [2.006(6) Å] than in the trans-(1−2)c
complexes [2.047(5)−2.042(3) Å]. Consistent with the observed correlation, the
Pd−Ccarbene bond dissociation energy (De) of the pyridine based (1−3)b complexes
(80.9−81.4 kcal/mol) were indeed higher than that of the phosphine based (1−3)c
complexes (64.8−64.9 kcal/mol), as estimated at the B3LYP/SDD, 6-31G(d) level of
theory (Figure 5 and Supporting Information Figures S1). It is worth noting that the
Pd−Ccarbene bonding interaction reflects directly on the NHC influence that the ligand
exerts on the metal center of these complexes, and which in turn affects the catalysis
as would be seen later.
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Figure 2. ORTEP diagram of 1b with thermal ellipsoids shown at the 50 %
probability level. Selected bond lengths (Å) and angles (°): Pd(1)-C(6) 1.980(3),
Pd(1)-N(1) 2.091(3), Pd(1)-I(1) 2.6152(6), Pd(1)-I(2) 2.6135(6), C(6)-Pd(1)-N(1)
178.27(13), C(6)-Pd(1)-I(1) 89.93(9), N(1)-Pd(1)-I(1) 90.27(8), C(6)-Pd(1)-I(2)
88.95(9), N(1)-Pd(1)-I(2) 90.92(8), I(2)-Pd(1)-I(1) 177.550(15).
12

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Figure 3. ORTEP diagram of 1c with thermal ellipsoids shown at the 50 %
probability level. Selected bond lengths (Å) and angles (°): Pd(1)-C(1) 2.047(5),
Pd(1)-P(1) 2.3571(13), Pd(1)-I(2) 2.6145(7), Pd(1)-I(1) 2.5998(7), C(1)-Pd(1)-P(1)
176.80(14), C(1)-Pd(1)-I(2) 88.10(13), P(1)-Pd(1)-I(2) 90.42(4), C(1)-Pd(1)-I(1)
84.96(13), P(1)-Pd(1)-I(1) 96.76(4), I(1)-Pd(1)-I(2) 171.484(19).
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Figure 4. ORTEP diagram of 3c with thermal ellipsoids shown at the 50 %
probability level. Selected bond lengths (Å) and angles (°): Pd(1)-C(19) 2.006(6),
Pd(1)-P(1) 2.2804(17), Pd(1)-I(2) 2.6399(12), Pd(1)-I(1) 2.6546(9), C(19)-Pd(1)-P(1)
91.41(17), C(19)-Pd(1)-I(2) 174.76(18), P(1)-Pd(1)-I(2) 91.69(4), C(19)-Pd(1)-I(1)
86.36(16), P(1)-Pd(1)-I(1) 175.88(5), I(2)-Pd(1)-I(1) 90.79(2).
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A key design component of our trans-(a-NHC)PdI2(L) type complexes [L = NC5HV5iew Article Online
DOI: 10.1039/C5DT02317C
(1−3)b and PPh3 (1−2)c], is the “L” ligand, that is located trans to the NHC ligand.
The “L” ligand is conveniently referred to as a “throwaway” ligand owing to its
weaker binding to the metal center that arise out of a stronger -binding of the trans-
NHC ligand to the metal center. The concept was elaborately developed by Organ24,
25, 33-35
in the form of the PEPPSI themed complexes that serve as effective catalysts
for many C−C cross-coupling reactions.
From this particular perspective, the Pd−L (L = NC5H5, PPh3) bond distances in the
trans-(a-NHC)PdI2(L) type complexes (L = NC5H5, PPh3), (1−3)b and (1−2)c, are of
considerable interest. In particular, the Pd−L distances of [2.091(3)−2.104(2)] Å
observed in the pyridine based (1−3)b complexes and of [2.3515(9)−2.3571(13)] Å in
the phosphine based (1−2)c complexes are shorter than the sum of the respective
individual covalent radii [d(Pd−N) = 2.10 Å and d(Pd−P) = 2.46 Å] (Figures 2−4 and
the Supporting Information Figures S50, S57 and S63 and Tables S22−S23).36
Consistent with the “throwaway” attribute of the “L” ligand (L = NC5H5, PPh3) in
these complexes, the Pd−L (L = NC5H5, PPh3) bond dissociation energy (De) of the
pyridine based (1−3)b complexes (30.3−30.5 kcal/mol) and the phosphine based
trans-(1−2)c complexes (32.7 kcal/mol) along with its cis-3c (37.9 kcal/mol) complex
are significantly lower than the Pd−Ccarbene bond dissociation energy (De) of the
pyridine based (1−3)b complexes (80.9−81.4 kcal/mol) and the phosphine based
(1−3)c complexes (64.8−64.9 kcal/mol), as computed at the B3LYP/SDD, 6-31G(d)
level of theory (Supporting Information Tables S3−S4). Also noteworthy is the fact
that the Pd−Npyridine bond dissociation energies (De) of the pyridine based (1−3)b
complexes (30.3−30.5 kcal/mol) are slightly lower than the Pd−Pphosphine bond
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dissociation energies (De) of the phosphine based (1−3)c complexes (32.7−37.9
kcal/mol), thus suggesting that pyridine is a better throwaway ligand than PPh3 in
these complexes (Supporting Information Tables S5−S6). The success of the trans-
(NHC)PdI2(L) type complexes as catalysts is very much dependent on the ease with
which the “L” ligand makes way for the incoming substrate during catalysis, and in
line with which, we decided to carryout a comparative study of the pyridine based
(1−3)b complexes and the phosphine based (1−2)c complexes and also a cis-3c
complex for their utility in the fluoride-free Hiyama coupling.
Further understanding of the Pd−Ccarbene and the Pd−L (L = NC5H5, PPh3) interactions
in the (1−3)b and (1−3)c complexes were obtained by constructing the corresponding
molecular orbital (MO) correlation diagrams from the individual fragment molecular
orbitals (FMOs) of the constituting components (Figures 5 and 6 and Supporting
Information Figures S1−S10). A scrutiny of these molecular orbital correlation
diagram reveals that the Pd−Ccarbene molecular orbital (MO) is composed of the
individual fragment molecular orbitals (FMOs) of the (a-NHC) fragment and the
PdI2(NC5H5) fragment in case of the pyridine based (1−3)b complexes (Figure 5 and
Supporting Information Figure S1−S2) and of the (a-NHC) fragment and the
PdI2(PPh3) fragment for the phosphine based (1−3)c complexes (Supporting
Information Figure S3−S5). Along the same line, the molecular orbitals for the
Pd−Npyridine -interactions were composed of the fragment molecular orbitals (FMOs)
of the (a-NHC)PdI2 fragment and the (NC5H5) fragment for the pyridine based (1−3)b
complexes (Figure 6 and Supporting Information Figure S6−S7) while the
Pd−Pphosphine -interactions were comprised of the fragment molecular orbitals
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(FMOs) of the (a-NHC)PdI2 fragment and the (PPh3) fragment for the phosphine
based (1−3)c complexes (Supporting Information Figure S8−S10).
A closer look at these molecular orbital correlation diagrams further indicates that in
the Pd−Ccarbene -interactions in these trans-(a-NHC)PdI2(L) [L = NC5H5, PPh3] type
complexes, (1−3)b and (1−2)c, the electron donation occurs from the Ccarbene lone pair
of the a-NHC ligand on to the *-orbital of the Pd−L (L = NC5H5, PPh3) bond of the
PdI2L fragments (Figure 5 and Supporting Information Figures S1−S5). Likewise, for
the Pd−Npyridine and the Pd−Pphosphine -interactions in the trans-(a-NHC)PdI2(L) [L =
NC5H5, PPh3] type complexes, (1−3)b and (1−2)c, the electron donation takes from
the N-(pyridine) lone pair on to the *-orbital of the Pd−Ccarbene bond of the (a-
NHC)PdI2 fragment in the pyridine based (1−3)b complexes and from the P-(PPh3)
lone pair on to the *-orbital of the Pd−Ccarbene bond of the (a-NHC)PdI2 fragment in
the phosphine based (1−2)c complexes (Figure 6 and Supporting Information Figures
S6−S9).
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-4
-6
(1b)
HOMO
LUMO
11%
8%
27%
HOMO-5
-8
3%
HOMO-15
HOMO-8
34%
-10
-12
HOMO-12
Figure 5. Simplified orbital interaction diagram showing major contribution to the (a-NHC)–Pd bonding orbital HOMO-15 in 1b.
18

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(1b)
-2
-4
LUMO
5%
-6
HOMO
50%
-8
HOMO-15
9%
-10
-12
7%
HOMO-23
HOMO-26
Figure 6. Simplified orbital interaction diagram showing major contribution to the Pd–NC5H5 bonding orbitals in 1b.
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Likewise, for the structurally different cis-3c complex, the molecular orbital for the
Pd−Ccarbene -interaction represented electron donation from the Ccarbene lone pair on
to the *-orbital of the trans-Pd−I bond of the PdI2(PPh3) fragment while the
molecular orbital for the Pd−Pphosphine -interaction indicated electron donation from
the P-(PPh3) lone pair on to the *-orbital of the trans-Pd−I bond of the (a-
NHC)PdI2(PPh3) fragment (Supporting Information Figures S5 and S10).
A Natural Bond Orbital (NBO) analysis showed that the Pd−Ccarbene bond in the
pyridine based (1−3)b complexes is less polar with 68.9−69.2 % electron contribution
from the Ccarbene atom and 30.8−31.1 % electron contribution from the Pd atom than
that of the phosphine based (1−3)c complexes with 75.5−76.3 % electron contribution
from the Ccarbene atom and 23.7−24.4 % electron contribution from the Pd atom
(Supporting Information Table S20). The difference in the polarity of the Pd−Ccarbene
bond between the pyridine based (1−3)b and the phosphine based (1−3)c complexes
would reflect on the catalytic activity of these complexes as encountered later during
the catalysis study.
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(1-3)b/(1-3)c
NaOH
X
Si(OMe)3
+
R
R
R = H, Me, COCH3, NO2
X = Br, I
Equation 1.
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Quite significantly, both of the pyridine based (1−3)b complexes and the phosphine
based (1−3)c complexes successfully carried out the much-preferred fluoride-free
Hiyama coupling of the aryl bromide and the iodide substrate with an organosilicon
reagent giving the desired biaryl product (Equation 1 and Table 1). Specifically, the
Hiyama coupled products were seen for various aryl halide substrates ranging from
the activated ones namely the, 4’-bromoacetophenone, 4-bromonitrobenzene, 1-
bromo-2-nitrobenzene and 2’-bromoacetophenone to the inactivated ones like,
iodobenzene and 4-iodotoluene. The solvent optimization study yielded the
maximum conversion in a mixed medium of dioxane and water (2:1, v/v) (Supporting
Information Table S36) while the variation of base produced the best result with
NaOH (Supporting Information Table S37). After varying the catalyst stoichiometry,
the 2 mol % of the catalyst loading was chosen for the subsequent runs undertaken for
the catalysis study (Supporting Information Table S38).
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Table 1. Selected results for Hiyama cross-coupling reaction of aryl halides (ArX; X = I, Br) catalyzed by (1–3)b and (1–3)c.
yields[a]
OH
OH
OH
OH
I
I
I
I
I
PPh3
Pd
entry
reagent
Reagent
cross-coupled product
N
N
N
N
Pd
N
Pd PPh3
Pd
N
Pd PPh3
N
N
Pd
N
I
N
N
N
N
N
N
N
N
I
I
I
I
N
N
N
N
I
I
Ph
Ph
Ph
Ph
Ph
Ph
(1c)
(1b)
(2b)
(2c)
(3b)
(3c)
1
2
3
96
65
82
68
78
97
75
21
70
23
88
28
89
24
93
63
95
84
4
5
6
41
94
63
36
82
57
33
91
61
25
81
49
66
75
60
78
82
62
[a] The yields (%) were determined by GC using diethylene glycol dibutyl ether as an internal standard. Reaction conditions: 1.00 mmol of aryl halides, 1.20 mmol of PhSi(OMe)3, 1.00 mmol of diethylene glycol dibutyl
ether, 3.00 mmol of NaOH, 2 mol% of catalyst (1−3)b, (1−3)c and 6 mL of 1,4-dioxane/H2O (2:1), at 80 °C for 4 hours.
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DOI: 10.1039/C5DT02317C
Considerable ligand influence of the a-NHC ligand on the catalysis was very much
evident from the amplification of the product yields of up to ca. 37 %, observed for
the pyridine based (1−3)b and the phosphine based (1−3)c complexes relative to that
of the control runs performed with PdCl2 (Supporting Information Table S39). The
time dependence profile of the of the coupling reaction for two representative
substrates namely, 4-bromoacetophenone and PhSi(OMe)3 as catalyzed by 3b in the
presence and absence of Hg, indicated some extent of catalyst degradation occurring
in the presence of Hg resulting in comparatively lower catalysis yields ((Figures 7 and
8 and Supporting Information Table S39).37 In these time dependence Hg drop
experiments, the Hg additions were carried out at different time intervals after the
start of the reaction with the product conversions recorded before each Hg additions
and after the end of the catalysis run time of 4 hours (Figure 8). In this regard it is
worth noting that similar type of catalyst degradation has been reported for related
PEPPSI themed complexes.38-40
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Dalton Transactions
100
90
80
70
60
50
40
30
20
10
0
without Hg
with Hg
0
50
100
150
200
250
time (min)
Figure 7. A time profile overlay of the Fluoride-free Hiyama coupling of 4-bromoacetophenone and PhSi(OMe)3 as catalyzed by 3b in the
presence and absence of Hg.
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100
90
80
70
60
50
40
30
20
10
0
Hg addition after
15 min
30 min
45 min
60 min
0
50
100
time (min)
150
200
250
Figure 8. A time profile overlay of the fluoride-free Hiyama coupling of 4-bromoacetophenone and PhSi(OMe)3 as catalyszed by 3b as a
function of Hg addition at varying time intervals.
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Page 27 of 46
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT02317C
Of foremost interest is the comparison of the catalysis results of the pyridine based
(1−3)b complexes and the two types of the phosphine based trans-(1−2)c complexes
and the cis-3c complex. Quite interestingly, the pyridine based (1−3)b complexes
consistently exhibited higher yields than the phosphine based trans-(1−2)c complexes
(Entries 1−6, Table 1), and this can be ascribed to a greater (a-NHC) ligand influence
on catalysis arising from a stronger binding of the (a-NHC) ligand to the metal center
in the pyridine based (1−3)b complexes. The explanation was substantiated by the X-
ray diffraction studies that showed shorter Pd−Ccarbene bond distances in the pyridine
based (1−3)b complexes [1.967(3)−1.986(5)] Å than in the phosphine based trans-
(1−2)c complexes [2.042(3)−2.047(5)] Å (Figures 2−4, Supporting Information S50,
S57 and S63, Tables S22−S23). Additionally, the density functional theory studies
displayed higher Pd−Ccarbene bond dissociation energy (De) in the pyridine based
(1−3)b complexes (80.9−81.4 kcal/mol) than the phosphine based trans-(1−2)c
complexes (64.8−64.9 kcal/mol) (Supporting Information Tables S3−S4). In keeping
with the above observation, the density functional theory studies also showed that the
“pyridine” moiety was more labile as it exhibited a weaker Pd−Npyridine bond
dissociation energy (De) in the pyridine based (1−3)b complexes (30.3−30.5 kcal/mol)
than the corresponding Pd−Pphosphine bond dissociation energy (De) of “PPh3” moiety
in the phosphine based trans-(1−2)c complexes (32.7 kcal/mol) (Supporting
Information Tables S5−S6). All of these observations, thus, taken together, support
the notion of a greater (a-NHC) ligand influence in the pyridine based (1−3)b
complexes than that in the phosphine based trans-(1−2)c complexes, and which in the
process account for the superior activity of the pyridine based (1−3)b complexes.
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In this context it is worth mentioning that similar to our observation of superioVriew Article Online
DOI: 10.1039/C5DT02317C
performance of the NHC based percatalysts (1−3)b over the mixed phosphine/NHC
counterparts, (1−3)c, in the fluoride-free Hiyama coupling, analogous higher activities
of the hetero-bis(NHC) precatalysts over their mixed phosphine/NHC complexes have
been noted for the Cu(I)-catalyzed azide−alkyne cycloaddition (CuAAC) reaction41
and these are in contrast to the commonly observed better performances of the mixed
phosphine/NHC complexes.28-32
Additionally, of particular interest is the observation that, among the phosphine based
complexes, the cis-3c complex performed better than the trans-(1−2)c complexes,
thereby indicating that the cis-geometry of the metal complex is more favorable for
catalysis (Table 1). It is worth noting that the density functional theory studies, done
at the B3LYP/SDD, 6-31G(d) level of theory, had shown earlier that the cis-isomer
was more unstable than its trans-counterpart for all of the (1−3)b and (1−3)c
complexes (Figure S11). In this context, it can thus be said that, though the catalytic
performance of the cis-3c complex suggests that the cis-form is more promising for
catalysis, the primary challenge would however lie on the isolation of these type of
cis-complexes owing to their inherent instability relative to their trans-isomers.
Lastly, the most active of all these catalyst, i.e. the cis-3c complex, however, failed to
show much activity towards the more challenging aryl chloride substrates (Supporting
Information Table S40).
Conclusions
In summary, a series of (a-NHC)PdI2(L) [L = NC5H5, PPh3] type complexes have
been synthesized and structurally characterized. These complexes effectively
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Dalton Transactions
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DOI: 10.1039/C5DT02317C
catalyzed the much-preferred fluoride-free Hiyama coupling of the aryl bromide and
the iodide substrates with PhSi(OMe)3 in presence of NaOH as a base. The pyridine
based (1−3)b complexes exhibited superior activity than the phosphine based trans-
(1−2)c complexes and thereby giving credence to the theory that a greater (a-NHC)
ligand influence was evident in the former than in the latter. Though the catalysis
results of the phosphine based cis-3c complex looked promising, the density
functional theory study highlighted the challenges of the isolation of these types of the
cis-complexes owing to their natural instability with regard to their trans-forms.
29