Bipyridyl/carbazolate silver(I) and gold(I) N-heterocyclic carbene complexes: A systematic study of geometric constraints and electronic properties

Article abstract of DOI:10.1002/aoc.5335

A series of silver(I) and gold(I) carbene complexes of the type [M(L)(2,2′-bipyridine)][PF₆] (L = 1-benzyl-3-(2-pyridylmethyl)benzimidazolylidene; M = Ag (1); M = Au (3)) and [M(L)(carbazole)] (M = Ag (2); M = Au (4)) were synthesized and analy

Full text of DOI:10.1002/aoc.5335

Received: 16 February 2019
DOI: 10.1002/aoc.5335
Revised: 21 June 2019
Accepted: 13 October 2019
F U L L P A P E R
Bipyridyl/carbazolate silver(I) and gold(I) N-heterocyclic
carbene complexes: A systematic study of geometric
constraints and electronic properties
Sirsendu Das Adhikary1 | Ambarish Mondal2 | Hemanta K. Kisan2
|
Christopher W. Bielawski3,4,5
| Joydev Dinda2
1School of Applied Science, Haldia
Institute of Technology, Haldia, 721657,
West Bengal, India
2Department of Chemistry, Utkal
University, Vani Bihar, Bhubaneswar,
751004, Odisha, India
3Center for Multidimensional Carbon
Materials (CMCM), Institute for Basic
Science (IBS), Ulsan, 44919, Republic of
Korea
4Department of Chemistry, Ulsan
National Institute of Science and
Technology (UNIST), Ulsan, 44919,
Republic of Korea
A series of silver(I) and gold(I) carbene complexes of the type [M(L)(2,20-
bipyridine)][PF6] (L 1-benzyl-3-(2-pyridylmethyl)benzimidazolylidene;
=
M = Ag (1); M = Au (3)) and [M(L)(carbazole)] (M = Ag (2); M = Au (4)) were
synthesized and analyzed using a range of spectroscopic and crystallographic
techniques. Inspection of the solid-state structures of 1, 2 and 4 revealed a
number of intermolecular noncovalent interactions. In the solid-state structure
adopted by 1, π–π and Ag–Ag interactions directed the complexes to orient in a
head-to-tail fashion. The photophysical properties were found to be influenced
by the ancillary ligands in solution as well as in the solid-state. Calculations
were performed to support the aforementioned structural and optoelectronic
assignments.
5Department of Energy Engineering,
Ulsan National Institute of Science and
technology (UNIST), Ulsan, 44919,
Republic of Korea
K E Y W O R D S
N-heterocyclic carbene, gold(I)–NHC, silver(I)–NHC, photoluminescent, carbazole
Correspondence
Joydev Dinda, Department of Chemistry,
Utkal University, Vani Bihar,
Bhubaneswar 751004, Odisha, India.
Email: dindajoy@yahoo.com
Funding information
Ministry of Education and the National
Research Foundation, Grant/Award
Numbers: IBS-R019-D1, IBS-R019; Science
and Engineering Research Board, Grant/
Award Number: EMR/2016/007022
1 | INTRODUCTION
their electrophilic carbene analogues.[2] NHCs are also
strong σ-electron donors, similar to phosphines, and offer
In 1991, Arduengo and co-workers successfully isolated
1,3-di(adamantyl)imidazol-2-ylidene as the first stable N-
heterocyclic carbene (NHC).[1] Since this hallmark event,
it has been determined that significant electron density
resides on the carbene nuclei of NHCs, which are typi-
cally nucleophilic and thus can be distinguished from
a number of advantages when compared to other ligands
due, in part, to a high thermal stability and unique struc-
tural parameters.[3] Moreover, NHCs have been shown to
bind to a broad range of transition metals in high as well
as low oxidation states.[4] Particular interest in Ag(I) and
Au(I) supported by NHCs has grown over the past several
Appl Organometal Chem. 2019;e5335.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5335

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DAS ADHIKARY ET AL.
decades because such complexes hold potential for use in
a range of medicinal,[5] electronic[6] and materials
applications.[7–9] Due to such versatility and accessibility,
NHC ligands represent an attractive scaffold for con-
structing new complexes.[10]
Ag2O in dichloromethane at room temperature afforded
[Ag(1-benzyl-3-(2-pyridylmethyl)benzimidazolylidene)
Cl] ([Ag(L)Cl]).[17] The [Ag(L)(bpy)][PF6] complex 1 was
synthesized by treating [Ag(L)Cl] with an equimolar
quantity of [Ag(bpy)2][PF6] in acetonitrile at room tem-
perature. The [Au(L)(bpy)][PF6] complex 3 was obtained
by transmetallating 1 with Au(SMe2)Cl.[18] Alternatively,
the same complex was obtained upon introducing [Au(L)
Cl] to [Ag (bpy)2][PF6].
The formation of 1 and 3 were independently con-
firmed in part using multi-nuclear NMR spectroscopy
and other analytical techniques. The presence of the
carbene nuclei was supported by the appearances of 13C
NMR signals at 163.9 and 166.7 ppm for 1 and 3, respec-
tively. Further support for the structural assignment of
1 was obtained from an X-ray diffraction analysis and
will be described in more detail below. As X-ray-quality
single crystals of 3 remained elusive, an optimized molec-
ular structure was calculated and will also be discussed
below.
Complex 2 was synthesized by treating [Ag(L)Cl] with
cbz in a basic mixture of dichloromethane and water.
After isolation and evaporation of the organic solvent,
the residual solid was recrystallized from dic-
hloromethane and diethyl ether to afford 2. Trans-
metallation of 2 with Au(SMe2)Cl afforded 4. Likewise,
treatment of [Au(L)Cl] under basic conditions afforded
the same complex. Multi-nuclear NMR spectroscopy,
X-ray crystallography and other analytical techniques
were used to elucidate the structures and to assess the
purities of 2 and 4. Collectively, complexes 1–4 were pre-
pared in good yields, isolated as colorless to light yellow
solids and were found to be stable under ambient
conditions.
2,20-Bipyridine (bpy) and its derivatives have also
been the subject of numerous studies in coordination
chemistry due to their abilities to form chelates as well as
their useful photophysical properties.[11–13] Likewise,
carbazole (cbz), a strong σ-donor commonly used in
hole-transporting applications, has also been extensively
studied as a ligand.[14] The integration of bpy or cbz was
hypothesized to influence the fundamental electronic
and photophysical properties displayed by metal com-
plexes supported by NHCs. For example, bpy was
expected to enhance π-backbonding interactions and thus
electronic delocalization, whereas cbz was expected to
place more electron density on the coordinated metal.[14]
Finally, since there are relatively few known metals that
are supported by NHCs and bpy[12,13] or cbz[14] ligands,
we envisioned enriching our understanding of such types
of complexes through the synthesis of new derivatives.
2 | RESULTS AND DISCUSSION
The synthetic procedures used to prepare the target com-
plexes are summarized in Scheme 1.[15] The requisite
NHC precursor, 1-benzyl-3-(2-pyridylmethyl)benzimi-
dazolium chloride, was prepared from 1-benzylben-
zimidazole and picolylchloride hydrochloride in ethanol
under basic conditions.[16] Treatment of the salt with
In order to unambiguously confirm the structural
assignment of 1, single crystals were obtained by slowly
diffusing diethyl ether into a saturated acetonitrile solu-
tion of 1 at room temperature. The solid-state structure
adopted by 1 is depicted in Figure 1; selected crystallo-
graphic parameters and bond parameters are listed in
Tables 1 and 2, respectively. Complex 1 crystallized in the
triclinic space group p-1. The Ag–carbene bond distance,
Ag(1)–C(1), was measured to be 2.089(2) Å, consistent
with that reported for
a bis(benzimidazolylidene)
silver(I) hexafluorophosphate (2.086(3) Å)[17] and less
than the sum of the covalent radii of the constituent car-
bon and silver atoms (2.111 Å). The Ag(1)–N(4) and
Ag(1)–N(5) bond distances were measured to be
2.337(2) and 2.272(2) Å, respectively, and the N(1)–C(1)–
N (2) bond angle was measured to be 105.9(2)ꢀ; these
values were comparable to analogues reported in the lit-
erature.[19] The N(4)–Ag(1)–N(5), N(4)–Ag(1)–C(1) and
N(5)–Ag(1)–C(1) angles were measured at 71.68(8)ꢀ,
SCHEME 1 Syntheses of complexes 1–4 (R = benzyl;
R0 = 2-picolyl)

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FIGURE 1 Single-crystal X-ray
structure of 1 (right) and optimized
geometry of 3 (left). Hydrogen atoms
and counter anions have been removed
for clarity
TABLE 1 Summary of crystal X-ray structure parameters
1
2
4
Empirical formula
Formula weight
Temperature
Crystal system
Unit cell dimensions
a (Å)
C30H25AgN5,F6P
C32H25AgN4
573.43
C32H25AuN4
662.53
708.39
295
293(2)
293(2)
Triclinic
Monoclinic
Monoclinic
9.2368(2)
13.1310(3)
13.3482(4)
115.9631(9)
94.5730(8)
96.8450(13)
1429.35(6)
2
24.744(14)
10.057(5)
21.621(10)
90.00
26.942(7)
9.772(2)
21.448(5)
90.00
b (Å)
c (Å)
α (ꢀ)
β (ꢀ)
97.404(18)
90.00
109.788(9)
90.00
γ (ꢀ)
Volume
5336(5)
8
5313(2)
8
Z
Calcd density (Mg m−3
Absorption coefficient
θ range (ꢀ)
)
1.646
1.428
1.656
0.831
0.783
5.565
2.85–30.00
8215
1.66–23.51
3912
1.61–25.00
3413
No. of reflns collected
No. of ind. reflns
Goodness-of-fit on F2
6209
19669
4679
1.051
1.122
1.013
Final R indices [I > 2σ(I)]
R1 = 0.0459
R1 = 0.0678
R1 = 0.0833
wR2 = 0.1228
wR2 = 0.1704
wR2 = 0.2126
R indices (all data)
R1 = 0.0673
R1 = 0.0865
R1 = 0.1043
wR2 = 0.1325
wR2 = 0.1868
wR2 = 0.2413
TABLE 2 Selected bond lengths and bond angles measured in the solid-state structures of 1 and 3 along with calculated values
Bond lengths (Å)
1
Bond angles (ꢀ)
1
3
3
Expt
Calcd
Calcd
2.0101
2.1185
—
Expt
105.9(2)
Calcd
105.81
133.06
156.33
70.21
Calcd
106.64
174.34.
—
Ag(1)–C(1)
Ag(1)–N(4)
Ag(1)–N(5)
C(1)–N(1)
C(1)–N(2)
2.089(2)
2.337(2)
2.272(2)
1.351(3)
1.347(3)
2.1074
2.3223
2.4132
1.3614
1.3582
N(1)–C(1)–N(2)
C(1)–Ag(1)–N(4)
C(1)–Ag(1)–N(5)
N(4)–Ag(1)–N(5)
139.14(9)
148.47(9)
71.68(8)
1.3601
1.3575
—

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DAS ADHIKARY ET AL.
139.14(9)ꢀ and 148.47(9)ꢀ, respectively. The smallest
angle reflects chelation of the bpy ligand with the Ag cen-
ter. The imidazole ring and bpy were observed to be
nearly coplanar with a dihedral angle of 6.22ꢀ. As shown
in Figure 2, the crystal packing diagram revealed that the
complexes arranged in a head-to-tail fashion due to the
formation of π–π and Ag–Ag interactions.[20] The Ag–Ag
distance (3.130 Å) was measured to be shorter than the
sum of the constituent van der Waals radii (4.26 Å)[21a]
yet longer than a typical Ag–Ag covalent bond (2.611 Å).
[21b]
TABLE 3 Selected bond lengths (Å) and bond angles (ꢀ)
measured in the solid-state structures of 2 and 4 along with
calculated values
Ag–cbz (2)
Au–cbz (4)
Expt
Calcd
2.084
Expt
Calcd
2.011
M(1)–C(1)
M(1)–N(4)
C(1)–N(1)
C(1)–N(2)
2.073(6)
2.072(5)
1.370(8)
1.341(9)
107.0(5)
1.979(14)
1.997(11)
1.347(18)
1.419(19)
106.8(12)
2.063
1.364
2.026
1.362
1.358
1.366
N(1)–C(1)–
105.83
106.07
The calculated structure of 3 revealed that the geome-
try about the Au center was nearly linear (C1–Au1–
N43 = 174.34ꢀ) where one nitrogen atom of the bpy
ligand was coordinated to Au and the other remained as
a free base. The Au–Ccarbene distance was measured
(2.0101 Å) to be similar to analogous distances reported
for other Au–NHC complexes.[15,19] All other bond
parameters calculated for 3 were similar to the experi-
mental values determined for 1. For example, the
N3–C1–N4 angle (106.64ꢀ) is only slightly larger than the
corresponding experimentally determined value for
1 (105.9(2)ꢀ). Collectively, these data support the veracity
of the computational method employed.
N(2)
C(1)–M(1)–
N(4)
174.79(19) 172.23
105.9(5) 106.38
177.8(5)
174.25
106.78
C(21)–N(4)–
C(32)
105.3(11)
nearly coplanar with a dihedral angle of 9.0ꢀ. The Ag–
Ccarbene distance (2.073(6) Å) is similar to that of other
reported Ag(I)–NHC structures[19] and less than the sum
of the covalent radii of the constituent carbon and silver
atoms (2.111 Å). The C(1)–N(1) and C(1)–N(2) bond
lengths were measured to be 1.370(8) and 1.341(9) Å,
respectively. The Ag–Ncbz bond distance (Ag(1)–
N(4) = 2.072(5) Å) was measured to be shorter than the
Ag(1)–Nbpy distance found in complex 1, which may
reflect a relatively strong bonding interaction between
the Ag(I) center and the cbz ligand.
Single crystals suitable for X-ray analysis were grown
by slowly diffusing diethyl ether into a saturated acetoni-
trile solution of 4 at room temperature. The structure of
4 is depicted in Figure 3. Selected crystallographic param-
eters and bond parameters are listed in Tables 1 and 3,
Single crystals suitable for X-ray analysis were grown
by slowly diffusing diethyl ether into a saturated acetoni-
trile solution of
2
at room temperature. The
corresponding structure of the complex is depicted in
Figure 3. Selected crystallographic parameters and bond
parameters are listed in Tables 1 and 3, respectively.
Complex 2 crystallized in the monoclinic space group
C2/c. The geometry of the Ag(I) center was found to be
linear as indicated by the C(1)–Ag(1)–N(4) angle of
174.79(19)ꢀ. The imidazole and carbazolate rings were
FIGURE 2 Packing view of
1 showing C–H–π, π–π and Ag–Ag
interactions
FIGURE 3 ORTEP view (50%
probability) of 2 (left) and single-crystal
X-ray structure of 4 (right). Hydrogen
atoms have been removed for clarity

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respectively. The Au–Ccarbene (1.979(14) Å) and Au–Ncbz
(1.997(11) Å) distances were measured to be shorter than
the analogous values measured for 2 and comparable to
values reported in the literature.[14] The Au–Ccarbene dis-
tance is less than the sum of the covalent radii of the con-
stituent Au (1.66 Å) and C (1.70 Å) atoms.[19] The C(1)–
Au(1)–N(4) angle was measured to be 177.8(5)ꢀ.
Comparing the Ag–Ccarbene or Au–Ccarbene bond dis-
tances measured in 1 versus 3 (2.086(3) versus 2.073(6) Å,
respectively) or 2 versus 4 (1.992(14) versus 1.979(14) Å,
respectively) revealed that the carbazole unit had a lim-
ited effect on the M–Ccarbene bond length. For example,
the Ag–Ccarbene bond distance (2.073(6) Å) measured in
the solid-state structure of 2 was only slightly shorter
than the analogous distance measured in [Ag(L)2][PF6]
(2.086(3) Å).[17] Similarly, the Au–Ccarbene bond distance
(1.979(14) Å) measured in the solid-state structure of
4 was only slightly shorter than the analogous distance
measured in [Au(L)2][PF6] (1.992(14) Å).[15] Likewise,
the C–N bond distances found in the solid-state
structures of 2 and 4 were nearly identical to those mea-
sured in [M(L)2][PF6] complexes (M = Ag or Au).
Absorption and luminescence data were recorded for
complexes 1–4 in CH3CN or in the solid-state. As shown
in Figures 4–6 and summarized in Table 4, all of the com-
plexes exhibited weak absorption bands in the range
355–370 nm, which were assigned to metal-to-ligand
charge transfer (MLCT) processes.[12] The complexes
exhibited absorption maxima between 240 and 280 nm,
which were assigned to intra-ligand (IL) ð–ð* transitions.
The highest energy transitions displayed by 2 and 4 were
blue-shifted and centered at ca 230 nm. Complex 4 also
displayed two intense red-shifted bands at 306 and
317 nm, which may be due to extended conjugation. All
of the complexes displayed high-energy emissions
between 400 and 440 nm upon excitation at 280–295 nm.
In the solid-state, the emission bands displayed by
complexes 1 and 2 appeared at 525–545 nm whereas
those exhibited by 3 and 4 were measured at 525–570 nm
(as presented in Table 4). While the NHC ligand (L) and
FIGURE 4 Emission (right) and excitation (left) spectra recorded for various complexes (indicated) in acetonitrile at room temperature
FIGURE 5 Emission (right) and
excitation (left) spectra recorded for
2 and 4 in acetonitrile at room
temperature

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DAS ADHIKARY ET AL.
FIGURE 6 Emission spectra of
various complexes and free carbazole
(indicated) as measured in the solid-state at
room temperature
TABLE 4 Photoluminescence data recorded for 1–4
centers and the NHCs. For comparison, the LUMOs of
the same complexes were calculated to localize on the
bpy units. As such, MLCT from the metal centers to the
bpy ligands or ILCT from the NHC to the bpy ligands can
be expected.[12]
For comparison, the HOMOs of 2 and 4 were cal-
culated to localize primarily on cbz, whereas the NHC
dominated the LUMOs. As such, ILCT should proceed
from the cbz unit to the NHC ligand. The presence of
a ligand-to-ligand charge transfer band in the visible
spectrum may be expected on the basis of the calcu-
lated electronic structure. Indeed, the color of the com-
plex was found to be off-white to light yellow and the
only absorption found in the UV–visible spectra stem-
med primarily from transitions that were observed in
the UV region (Figures 4 and 5).
The chemical reactivities of the complexes were
compared on the basis of chemical hardness (η) values,
as postulated by Parr and Pearson, who described a
relationship between η and the energy gap (ΔE) of the
frontier molecular orbitals (η = (ELUMO − EHOMO)/2).[23]
From the data in Table 5, one may conclude that the
Au complex 3 is more reactive than the Ag complex 1,
presumably due to an open coordinate site in the for-
mer. NBO analyses confirmed that the natural charges
on the Ag atoms in 1 and 2 ranged between +0.511
and +0.456, respectively (see also the Wiberg bond
indices (WBIs) listed in Table 6). However, the charges
on the Au atoms in 3 and 4 were calculated to range
between +0.334 and +0.322, respectively. The relative
contributions may stem from the NHC and the ancil-
lary ligands. Indeed, the former was calculated to sig-
nificantly contribute (99%) to the frontier molecular
orbitals of the requisite complexes. As expected, the
Au–Ccarbene bonds were found to be shorter than
Ag–Ccarbene bonds and were consistent with values
reported in the literature.[24]
a,b
a,b
a,c
Complex
λex
λem
430
440
415
422
λem
540
567
515
523
[NHC–Ag–bpy]+, 1
[NHC–Au–bpy]+, 3
[NHC–Ag–cbz], 2
[NHC–Au–cbz], 4
280
280
285
285
aData were recorded at room temperature and the values reported are in
nanometers (nm).
bData were recorded in solution.
cData were recorded in the solid-state.
NHC–Ag(I)–Cl are non-luminescent, d10 configurations
of silver or gold ions can be expected to combine with the
π* orbitals of the ligands to produce MLCT and/or IL
transitions, which result in luminescence.[12] In the solid-
state, complex 1 exhibits luminescence at 545 nm and
2 at 570 nm; similarly, complex 3 exhibits luminescence
at 525 nm and 4 at 527 nm. In other words, solid-state
emission is red-shifted when compared to that measured
in solution. The origin of the emissions observed at
532 and 575 nm for 1 and 2 may be due to metal-
perturbed IL transitions, whereas the high-energy emis-
sion at 325–430 nm may stem from IL π–π* transitions.
Regardless, the ancillary ligands (bpy versus cbz)
appeared to play a prominent role in determining the
intrinsic photophysical properties displayed by the
complexes.[14]
In order to further support the structural assignments
described above, complexes 1–4 were examined using
density functional theory (DFT) at the B3LYP level (see
Figures 7–9 and 4–6). A natural bond orbital (NBO) anal-
ysis was also conducted to elucidate the electronic struc-
tures of the complexes.[22] As summarized in Tables 2
and 3, agreement between the calculated structures and
the solid-state structures was excellent. The HOMOs cal-
culated for 1 and 3 are localized primarily on the metal

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FIGURE 7 Frontier molecular
orbitals calculated for 1 (left) and
3 (right)
FIGURE 8 Frontier molecular
orbitals calculated for 2 (left) and
4 (right)
The analysis of the molecular orbitals of complex 1 in
the dimeric form provided support for the formation of
intermolecular AgꢁꢁꢁAg interactions. The overlapping of
dz2 orbitals on the silver centers revealed a strong
bonding interaction, although an antibonding interaction
was also observed (Figure 9). Plots of the molecular
orbitals for the dimeric form of 1 revealed that the Ag
atoms constitute a major contribution to a HOMO with a

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DAS ADHIKARY ET AL.
FIGURE 9 Depiction of the
calculated Ag–Ag bonding (HOMO-26)
and antibonding (HOMO-21) orbitals in
1
TABLE 5 Chemical hardness (η) values calculated for 1–4
TABLE 7 Calculated bond order for dinuclear complex
Complex
Chemical hardness (η)
2.08169
Dinuclear complex
Bond order
WBI
1
2
3
4
0.120
0.311
0.266
2.16537
NAO
1.31432
MO
1.35282
TABLE 6 Calculated WBI values
Compound
1
WBI (bond order)
[Ag(1)–C(1)]: 0.5189
[Ag(1)–N(4)]: 0.1433
[Ag(1)–N(5)]: 0.1837
[Ag(1)–C(1)]: 0.4937
[Ag(1)–N(4)]: 0.3932
[Au(1)–C(1)]: 0.7042
[Au(1)–N(4)]: 0.3297
[Au(1)–N(5)]: 0.0577
[Au(1)–C(1)]: 0.6693
[Au(1)–N(4)]: 0.4750
AgꢁꢁꢁAg couple. In the case of the NAO method, the
AgꢁꢁꢁAg bond strength was found to be approximately
one-fourth that of a typical covalent bond. For compari-
son, the bond strength was found to be one-third that of
a typical covalent bond using the MO bond order
method.
2
3
3 | CONCLUSIONS
4
The synthesis, structures and photophysical properties
of a series of Ag(I) and Au(I) complexes supported by
1-benzyl-3-(2-pyridylmethyl)benzimidazolylidene
and
either bpy- or cbz-based ligands were described. X-ray
crystallography revealed that π–π and Ag–Ag interac-
tions governed the structures of the aforemen-
tioned complexes in the solid-state. Such observations
are expected to enrich to crystal engineering design
parameters used to create higher order structures. The
electronic and optical properties of the complexes were
also examined and found to depend on the ancillary
ligands as well as on the constituent metals. Many of
the aforementioned assignments were corroborated by
a series of DFT calculations which were in good
agreement with the experimental data. Collectively,
integrating bpy or cbz ligands into NHC-supported
complexes is expected to facilitate refinement of
moderate involvement of the NHC and bpy fragments.
The contributions of the Ag atoms and the NHC compo-
nents in the other calculated molecular orbitals were
found to be similar, even though a contribution that orig-
inated from the bpy ligand was identified.
The strength of the AgꢁꢁꢁAg interactions was assessed
using DFT and the bond order was calculated using three
methods: WBIs, natural atomic orbital (NAO) bond order
and molecular orbital (MO) bond order. The
corresponding bond orders were calculated as follows:
WBI, 0.1201; NAO, 0.3112; MO, 0.2660 (as presented in
Table 7). The positive value of the bond order is consis-
tent with the existence of a net bonding interaction in the

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metal complexes used in contemporary electronics
applications.
the precipitate was removed via filtration, and then the
solvent was evaporated. The residual solid was rec-
rystallized from CH3CN–Et2O to afford the desired com-
plex as a colorless solid (69 mg, 0.098 mmol, 70% yield).
1H NMR (DMSO-d6, 300 MHz, 25ꢀC, δ, ppm): 8.62 (d,
J = 7.6 Hz, bpy, 4H), 8.58 (m, bpy, 4H), 8.57 (d,
J = 7.6 Hz, py, 2H), 7.68 (m, py, 2H), 7.45 (d, J = 7.6 Hz,
bzimi, 2H), 7.43–7.41 (m, bzimi, 2H), 7.37 (d, J = 6.8 Hz,
bzyl, 2H), 7.32–7.26 (m, bzyl, 3H), 5.87 (s, CH2, 2H), 5.83
4 | EXPERIMENTAL
4.1 | General procedures
Reagents were purchased from Sigma-Aldrich and used
without further purification. All manipulations were car-
ried out under ambient atmosphere unless otherwise
stated. All solvents were distilled over appropriate drying
agents. NMR spectra were recorded using Bruker
400 spectrometers at 25ꢀC with tetramethylsilane as an
internal standard. Absorption and emission spectra were
recorded using Shimadzu UV-1601 and PerkinElmer LS
55 spectrometers, respectively. The complexes [Ag(L)Cl]
and [Au(L)Cl] reported earlier were prepared as previ-
ously described.[17]
13
(s, CH2, 2H). C NMR (DMSO-d6, 100.5 MHz, 25ꢀC, δ,
ppm): 163.9, 154.3, 151.5, 149.3, 149.1, 148.7, 148.5,148.3,
137.5, 137.4, 127.3, 126.2, 123.3, 122.1, 120.78, 117.8,
117.5, 116.1, 115.7, 114.3, 113.1, 112.5, 46.4, 45.6. Anal.
Calcd for C30H25N5AgPF6 (%): C, 50.85; H, 3.53; N, 9.89;
found (%): C, 50.68; H, 3.49; N, 9.80.
4.4 | Synthesis of complex 2
A dichloromethane (20 ml) solution of complex [Ag(L)
Cl] (141 mg, 0.33 mmol) was treated with carbazole
(55 mg, 0.33 mmol) followed by NaOH (0.5 N, 2 ml) and
Bu4NCl (0.07 mmol). The resulting mixture was stirred
for 2 days. Afterward, 50 ml of deionized water was
added, and the resulting mixture was stirred for an addi-
tional hour. The organic component was extracted into
dichloromethane and then concentrated to give a yellow
residue.[14] Recrystallization from CH3CN–Et2O afforded
the desired complex as a white solid (160.5 mg,
4.2 | Synthesis of 1-benzyl-3-(2-
pyridylmethyl)-1H-benzimidazolium
chloride
The synthesis of this salt of has been previously
reported.[15] An alternative procedure is as follows.
1-Benzylbenzimidazole (500 mg, 2.4 mmol), 2-picol-
ylchloride hydrochloride (316 mg, 1.93 mmol) and
NaHCO3 (211 mg, 2.51 mmol) were refluxed in EtOH for
28 h. The residual solvent was then removed, and the
residual gummy mass was triturated with dic-
hloromethane. Recrystallization from dichloromethane
and Et2O afforded the desired compound as a colorless
solid (564 mg, 1.68 mmol, 70% yield). 1H NMR (400 MHz,
CD3CN, δ, ppm): 9.38 (s, NCHN, 1H), 8.48 (d, 1H,
J = 4.8 Hz, pyridine-H), 7.82 (d, 2H, J = 8.0 Hz), 7.73 (m,
1H), 7.53–7.44 (m, 5H), 7.38–7.32 (m, 3H), 7.23–7.22 (m,
1H), 5.79 (s, 2H, CH2), 5.63 (s, 2H, CH2). 13C NMR
(100 MHz, DMSO-d6, δ, ppm): 153.28, 149.2, 143.6, 137.3,
132.1, 131.5, 130.7, 129.0, 128.8, 127.9, 127.8, 126.6, 123.5,
123.4, 114.2, 112.9, 52.1, 51.2. Anal. Calcd for C20H18N3Cl
(%): C, 71.54; H, 5.37; N, 14.31; found (%): C, 71.38; H,
5.33; N, 14.20.
1
0.28 mmol, 85% yield). H NMR (DMSO-d6, 300 MHz,
25ꢀC, δ, ppm): 8.54 (d, J = 7.6 Hz, py, 2H), 8.01 (d,
J = 4.8 Hz, cbz, 2H), 7.78 (m, cbz, 4H), 7.68 (m, py, 2H),
7.48 (d, J = 5.8 Hz, cbz, 2H), 7.45 (d, J = 7.6 Hz, bzimi,
2H), 7.42–7.38 (m, bzimi, 2H), 7.36 (d, J = 6.8 Hz, bzyl,
2H), 7.28–7.26 (m, bzyl, 3H), 5.87 (s, CH2, 2H), 5.80 (s,
13
CH2, 2H). C NMR (DMSO-d6, 100.5 MHz, 25ꢀC, δ,
ppm): 162.8, 152.2, 144.6, 140.0, 139.5, 132.7, 131.3, 127.5,
126.6, 125.4, 124.4, 123.9, 121.6, 119.5, 114.7, 113.3, 112.5,
110.6, 109.7, 109.9, 108.6, 46.4, 45.6. Anal. Calcd for
C32H25AgN4 (%): C, 66.97; H, 4.36; N, 9.77; found (%): C,
66.69; H, 4.31; N, 9.62.
4.5 | Synthesis of complex 3
After adding the complex [Au(L)(Cl)] (90 mg, 0.17 mmol)
to 10 ml of dichloromethane, an acetonitrile solution of
[Ag(bpy)2]PF6 (96 mg, 0.17 mmol) was introduced
dropwise at room temperature; the formation of a white
precipitate (AgCl) was immediately observed. After 2 h,
the precipitate was removed through filtration, and then
the solvent was evaporated. The residual solid was rec-
rystallized from CH3CN–Et2O to afford the desired
4.3 | Synthesis of complex 1
After adding the complex [Ag(L)Cl] (60 mg, 0.14 mmol)
to 10 ml of dichloromethane, an acetonitrile solution of
[Ag(bpy)2][PF6] (77 mg, 0.14 mmol) was introduced
dropwise at room temperature; the formation of a white
precipitate (AgCl) was immediately observed. After 2 h,

10 of 11
DAS ADHIKARY ET AL.
complex as a colorless solid (96 mg, 0.12 mmol, 70%
yield). Alternatively, [Ag(L)(bpy)][PF6] (90 mg,
0.17 mmol) was mixed with 15 ml of acetonitrile. Adding
an acetonitrile solution of Au(SMe2)Cl (90 mg,
0.17 mmol) dropwise resulted in the immediate precipita-
tion of a white solid (AgCl). Following a similar proce-
dure to that described above afforded the desired
complex in a comparable yield. 1H NMR (DMSO-d6,
300 MHz, 25ꢀC, δ, ppm): 8.64 (d, J = 7.6 Hz, bpy, 4H),
8.61 (m, bpy, 4H), 8.59 (d, J = 7.6 Hz, py, 2H), 7.67
(m, py, 2H), 7.46 (d, J = 7.6 Hz, bzimi, 2H), 7.44–7.41
(m, bzimi, 2H), 7.38 (d, J = 6.8 Hz, bzyl, 2H), 7.31–7.28
(m, bzyl, 3H), 5.89 (s, CH2, 2H), 5.83 (s, CH2, 2H). 13C
NMR (DMSO-d6, 100.5 MHz, 25ꢀC, δ, ppm): 166.7, 154.5,
151.8, 149.5, 149.3, 148.9, 148.7, 148.4, 137.6, 137.5, 127.4,
126.3, 123.4, 122.3, 120.8, 118.0, 117.6, 116.5, 115.8, 114.6,
113.5, 112.6, 46.5, 45.8. Anal. Calcd for C30H25N5AuPF6
(%): C, 45.17; H, 3.14; N, 8.78; found (%): C, 44.85; H,
3.10; N, 8.70.
given in Table 1. X-ray data were collected with a CCD
diffractometer using graphite-monochromated Mo Ka
radiation (0.71073 Å). The structures were solved by
direct methods and refined on F2 using all corrections
using the SHELX-97 program.[25] The non-hydrogen
atoms were anisotropically located. Hydrogen atoms that
were not bound to imidazolium-C2 atoms were placed in
calculated positions and assigned with an isotropic dis-
placement parameter of 0.08.
4.8 | Computational methods
The complexes were fully optimized in the gas phase at
the B3LYP-D3 level of theory[26] using the Gaussian
09 Revision D.01 quantum chemical program.[27] The sil-
ver and gold atoms were treated with the def2-TZVP
basis set, which replaced the core 28 and 60 electrons,
respectively, with an effective core potential and a triple-
zeta valence basis set.[28] All other elements such as H, C
and N were treated with the 6-31G** basis set.[29] Hes-
sian indices were used to elucidate the nature of the sta-
tionary states. NBO analyses were carried out on the
optimized complexes using the B3LYP-D3/def2-TZVP
(Ag, Au), 6-31G** (C, H, N) level of theory.[30]
4.6 | Synthesis of complex 4
A dichloromethane (50 ml) solution of complex [Au(L)
(Cl)] (191 mg, 0.38 mmol) was treated with carbazole
(60 mg, 0.36 mmol) followed by NaOH (0.5 N, 2 ml) and
Bu4NCl (0.07 mmol). The resulting mixture was stirred
for 2 days. Afterward, 50 ml of deionized water was
added, and the resulting mixture was stirred for an addi-
tional hour. The organic component was extracted into
dichloromethane and then concentrated to give a yellow
residue. Recrystallization from CH3CN–Et2O afforded the
desired complex as a white solid (211 mg, 0.32 mmol,
85% yield). 1H NMR (DMSO-d6, 300 MHz, 25ꢀC, δ, ppm):
8.58 (d, J = 7.6 Hz, py, 2H), 8.03 (d, J = 4.8 Hz, cbz, 2H),
7.79 (m, cbz, 4H), 7.67 (m, py, 2H), 7.50 (d, J = 5.8 Hz,
cbz, 2H), 7.46 (d, J = 7.6 Hz, bzimi, 2H), 7.43–7.40 (m,
bzimi, 2H), 7.38 (d, J = 6.8 Hz, bzyl, 2H), 7.30–7.28 (m,
bzyl, 3H), 5.89 (s, CH2, 2H), 5.82 (s, CH2, 2H). 13C NMR
(DMSO-d6, 100.5 MHz, 25ꢀC, δ, ppm): 164.4, 152.6, 144.8,
140.1, 139.7, 132.8, 131.4, 127.6, 126.8, 125.5, 124.6, 124.0,
121.7, 119.7, 114.8, 113.4, 112.7, 110.8, 109.8, 110.0, 108.7,
46.5, 45.8. Anal. Calcd for C32H25AuN4 (%): C, 58.01; H,
3.78; N, 8.46; found (%): C, 57.70; H, 3.74; N, 8.38.
SUPPORTING INFORMATION
AVAILABLE
Crystallographic data for complexes 1, 2 and 4 are avail-
able free of charge at http://www.ccdc.cam.ac.uk.
NOTES
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
J.D. is grateful to the DST, Government of India for
financial support through project EMR/2016/007022.
C.W.B. acknowledges the Institute for Basic Science
(grant IBS-R019) as well as the BK21 Plus Program as
funded by the Ministry of Education and the National
Research Foundation of Korea for support. J.D. is grateful
to Mr. P. Mitra for solid-state structural studies.
ORCID
4.7 | X-ray crystal structure
determination
Christopher W. Bielawski https://orcid.org/0000-0002-
0520-1982
Joydev Dinda https://orcid.org/0000-0002-8780-6300
Single crystals suitable for X-ray data collection were
grown by the slow diffusion of diethyl ether into a satu-
rated acetonitrile solution of 1, 2 or 4. The crystal data
and details of the data collections for the complexes are
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How to cite this article: Das Adhikary S,
Mondal A, Kisan HK, Bielawski CW, Dinda J.
Bipyridyl/carbazolate silver(I) and gold(I)
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