Synthetic strategy of diflurophosphate-bridged bimetallic N-heterocyclic carbene complexes: Synthesis, structures and photoluminescence of picolyl-substituted alkylbenzimidazolylidene ligands

Article abstract of DOI:10.1016/j.ica.2011.12.007

Starting from the proligands 1-methyl-3- picolylbenzimidazoliumhexaflurophosphate (1) and 1-benzyl-3- picolylbenzimidazoliumhexaflurophosphate (2) Ag(I)-NHC complexes, 1-methyl-3-picolylbenz-imidazolylidenesilver(I)hexaflurophosphate (3) and 1-benzyl-3-picolylbenzimidazolylidenesilver(I)hexaflurophosphate (4) have been synthesized. PO₂F₂ bridged Ag(I)-bimetallic system (5 and 6) developed from 3 and 4 after treatment of AgPF₆ and water. 1-Methyl-3-picolyl-benzimidazolylidene-gold(I)hexaflurophosphate(7) and 1-benzyl-3-picolylbenzimidazolylidenegold(I)hexaflurophosphate (8) have been synthesized by Ag-carbene transmetallation method. Further reaction of AgPF ₆ with 7 and 8 build up the heterobimetallic Ag(I)-Au(I) systems (9 and 10); significantly, PO₂F₂ unit holds two heterobimetallic Ag(I)-Au(I) units together. Solid state structures of 3, 5, 8 and 9 have been determined by X-ray diffraction studies. Closed shell d ¹⁰-d¹⁰ interactions are ascertained by the luminescence of bimetallic systems 7-10 upon irradiation of UV light.

Full text of DOI:10.1016/j.ica.2011.12.007

Inorganica Chimica Acta 384 (2012) 239–246
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Inorganica Chimica Acta
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Synthetic strategy of diflurophosphate-bridged bimetallic N-heterocyclic
carbene complexes: Synthesis, structures and photoluminescence
of picolyl-substituted alkylbenzimidazolylidene ligands
Sirsendu Das Adhikary a, Lalmohan Jhulki a, Saikat Seth b, Arjama Kundu c, Valerio Bertolasi d,
Partha Mitra e, Ambikesh Mahapatra c, Joydev Dinda a,
⇑
a School of Applied Science, ‘‘Applied Synthetic Chemical Research Lab.’’, Haldia Institute of Technology, Haldia, Purba Medinipur, West Bengal 721657, India
b Department of Physics, M. G. Mahavidyalaya, Bhupatinagar, Purba Medinipur, West-Bengal 721425, India
c Department of Chemistry, Jadavpur University, Jadavpur, Kolkata 700032, West Bengal, India
d Dipartimento di Chimica and Centro di Strutturistica Diffrattometrica, Universita’ di Ferrara, Via L. Borsari, 46, Italy
e Department of Inorganic Chemistry, Indian Association for the Cultivation of Sciences, Jadavpur, Kolkata 700032, West Bengal, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 July 2011
Received in revised form 16 November 2011
Accepted 6 December 2011
Available online 14 December 2011
Starting from the proligands 1-methyl-3-picolylbenzimidazoliumhexaflurophosphate (1) and 1-benzyl-3-
picolylbenzimidazoliumhexaflurophosphate (2) Ag(I)–NHC complexes, 1-methyl-3-picolylbenz-imidazol-
ylidenesilver(I)hexaflurophosphate (3) and 1-benzyl-3-picolylbenzimidazolylidenesilver(I)hexafluro
phosphate (4) have been synthesized. PO2F2 bridged Ag(I)-bimetallic system (5 and 6) developed from 3
and 4 after treatment of AgPF6 and water. 1-Methyl-3-picolyl-benzimidazolylidene-gold(I)hexaflurophos-
phate(7) and 1-benzyl-3-picolylbenzimidazolylidenegold(I)hexaflurophosphate (8) have been synthesized
by Ag–carbene transmetallation method. Further reaction of AgPF6 with 7 and 8 build up the heterobime-
tallic Ag(I)–Au(I) systems (9 and 10); significantly, PO2F2 unit holds two heterobimetallic Ag(I)–Au(I) units
together. Solid state structures of 3, 5, 8 and 9 have been determined by X-ray diffraction studies. Closed
shell d10–d10 interactions are ascertained by the luminescence of bimetallic systems 7–10 upon irradiation
of UV light.
Keywords:
Imidazolium salts
N-Heterocyclic carbenes (NHCs)
Ag(I)–NHCs
Au(I)–NHCs
Ag(I)–Au(I)-bimetal
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
transition metals where most of metal-N-heterocyclic carbenes
participate in catalysis [11–24]. A large number of N-heterocyclic
N-Heterocyclic carbenes (NHCs) [1–5] are defined as singlet
carbenes, in which the divalent carbenic carbons are directly asso-
ciated at least one nitrogen atom within the heterocycles. Wanz-
lick originally suggested their existence in the early 1960s [6]
and in 1991 Arduengo and co-workers successfully first isolate
the free NHC 1,3-di(adamantyl)imidazol-2-ylidene by deprotona-
tion of its imidazolium precursor [7]. Contrary to other carbenes,
which are generally found to be electrophilic, N-heterocyclic carb-
enes are electron-rich nucleophilic scaffold in which the carbene
center enjoy the benefits from the stabilization associated with
carbene complexes have also been synthesized and structurally
characterized and their potential uses as medicine, and lumines-
cent materials have been studied [25–34].
Nonbonding interactions between closed-shell metals particu-
larly d10–d10 interaction (e.g. Ag(I)–Ag(I), Ag(I)–Au(I), Au(I)–Au(I),
Cu(I)–Cu(I)–NHC, etc.) often produce materials with interesting
luminescent properties [28,35–39]. The incorporation of a dissim-
ilar d10 metal, like silver(I) or gold(I), produces a dipole moment
that can strengthen the d10–d10 metal–metal interactions, and of-
ten, shorter intermetallic separations are observed within these
heterometallic systems [37–39]. Investigations into mixed-metal
interactions are also becoming more popular, and many examples
of complexes containing short argento-aurophilic contacts of imi-
dazolylidenes have been reported [37–39]; strong argentophilic
interactions leading to polymerizations of bis-carbene benzimidaz-
olylidene systems have also been achieved [40–42] recently. In
many of these systems mixed-metal separations are shorter than
the corresponding distance in the homometallic analogs [37–39].
The shortened separation and increased stability is attributed to
both the
ter of the nitrogen centers; in one word NHCs are stabilized by
‘push–pull’ mechanism [8–10]. Due to their strong -electron-
r-electron-withdrawing and p-electron-donating charac-
r
donating properties they are treated as alternative of phosphine
even though more credible than phosphine. Because of their un-
ique electronic commitment they can bind with a broad range of
⇑
Corresponding author. Tel.: +91 3224 252900/252850; fax: +91 3224 252800.
E-mail address: dindajoy@yahoo.com (J. Dinda).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.12.007

240
S.D. Adhikary et al. / Inorganica Chimica Acta 384 (2012) 239–246
the opening of a dipolar interactions between the dissimilar metals
augmenting the dispersion forces [43,44] believed to be the prime
attractive force maintaining these assemblies.
as colorless solids. As shown in Scheme 1, deprotonation of the imi-
dazolium salts with Ag2O in acetonitrile at room temperature
yielded [Ag(1-H)2]PF6, (3) and [Ag(2-H)2]PF6 (4); treatment of AgPF6
with [Ag(1-H)2]PF6, (3) and [Ag(2-H)2]PF6 (4) in acetonitrile and
water mixture yielded [Ag2(1-H)2 (PO2F2)](PF6) (5) and [Ag2(2-H)2
(PO2F2)](PF6) (6), respectively. Gold complexes, [Au(1-H)2]PF6, (7)
and [Au(2-H)2]PF6, (8) were obtained from the clean carbene trans-
fer reactions of the silver–NHC complexes (3 and 4) by Au(SMe2)Cl.
Treatment of AgPF6 with [Au(1-H)2]PF6, (7) and [Au(2-H)2]PF6, (8) in
acetonitrile and water mixture yielded [AuAg(1-H)2 (PO2F2)](PF6)
(9) and [AuAg(2-H)2 (PO2F2)](PF6) (10). Synthesis and characteriza-
tion of the complexes have been done according to known procedure
[28,35–39]. These precursors and carbene complexes have been
characterized by NMR spectroscopy and elemental analyses; their
spectra were consistent with their proposed formulation. Though
the bromide and chloride salts of proligand 1 and 2 already reported
[45,46], but PF6 salts are presently reported by us, where carbenic
protons (9.10 and 9.38 ppm for 1 and 2, respectively) less downfield
shifted than halide precursors (9.90 and 12.04 ppm for 1 and 2,
respectively). The methylene and methyl resonances appear at sin-
glet at 5.77 and 4.10 ppm, respectively for proligand 1 where as
the boyt the methylene resonances appear at 5.79 and 6.89 ppm in
case of 2.
Being inspired by these recent findings, we selected 1-methyl-
3-picolylbenzimidazoliumh-exaflurophosphate and 1-benzyl-3-
picolylbenzimidazoliumhexaflurophosphate [48,49] as proligands
to synthesize the mononuclear of Ag(I) and Au(I) complexes. Reac-
tion of azolium salts with silver bases like Ag2O, Ag2CO3, and
AgOAc [35,36,46–53] leads to Ag–NHC; Ag-transmetallation by
Au(SMe2)Cl produce Au(I)–NHC [35–39]. Use of excess Ag(I) source
like AgPF6 or simple Ag2O towards the same complex produce the
bimetallic system through the use of non-coordinated pyridine-N
[37,39]. The achievements of these synthetic routes, structural fea-
tures, and applications of the coinage metal–NHC complexes has
been summarized by Youngs and coworkers [35] and Lin [36],
respectively and the bimetallic systems highlighted by Catalano
[28,37–39]. Benzimidazolin-2-ylidenes are novel class of NHC li-
gands that are easily accessible from 1-alkylbenzimidazole with
2-picolylchloride hydrochloride; [45,46], a series of preparation,
structures, and reactions of benzannulated NHCs have been well
studied by Hahn’s group and other [51–63]; however, compared
to their imidazolylidene analogs, benzimidazolin-2-ylidenes com-
plexes are less studied [63–71]. Our current research interest fo-
cused in designing and preparation of multinuclear metal
complexes containing direct metal–metal interactions, using
1-alkylbenzimidazolin-2-ylidenes which are proven to be good
electron donating ligands. Strategically we have used PO2F2 as a
bridging ligand to bring structural diversity in the polymer.
The formation of Ag- and Au–NHC complexes were manifested
by the absence of the 1HNMR resonances for the acidic NCHN pro-
tons at 10.21–10.86 ppm, where the resonance signals of 1 and 2
proligands were found. The Ag(I) bis-carbene complex (3), causes
the most downfield resonance to disappear and minimally shifts
the methylene resonances downfield to 5.85 ppm. In case of Ag–
NHC complexes 3 and 4, the
a-pyridyl proton signal appears at
8.25–8.24 ppm and Ccarbene signal appers at 187.38–188.21 ppm,
this values are very consistence with the reported benzimidazoly-
lidene systems [45,46,51–63] where as, in case of Au(I)–NHC com-
plexes (7 and 8) 0.11–0.13 ppm down field shift is observed than
the sister Ag complexes. The 1HNMR and 13CNMR chemical shifts
of known Ag/Au–NHC complexes depends upon the ancillary li-
gands, in present cases spectra show only one set of resonance sig-
nals assigned to the ligand; this result indicates that the complexes
in solution may lose one molecule of ligand by solvent acetonitrile
or maintain C2 symmetry (supported by solid state structures of 3
and 8). In case of all the bimetallic complexes down field shift in
2. Result and discussion
2.1. Spectroscopic discussion
Two imidazolium salts, 1-methly-3-(2-picilyl)benzimidazolium
hehaflur-ophosphate, 1 and 1-benzyl-3-picilylbenzimidazoliumhehafl
urophosphate, 2 have been prepared straightforwardly from of 2-pic-
olylchloridehydrochloride and 1-methylbenzlimidazole or 1-ben-
zylbenzlimidazole in refluxing dioxane using Na2CO3 as external
base and subsequent treatment of the resultant imidazolium chlo-
rides salt with NH4PF6 in aqueous media [45,46]. They were isolated
PF6
+
NN
R
R= -CH3; 1
R = -CH2Ph; 2
N
Ag2O
PF6
PF6
PF
6
PF
6
N
N
NN
N
N
AgPF6
H2O
N
N
R
F
F
P
F
R
N
R
N
Ag
N
N
Au(SMe2)Cl
N
R
R
AgPF
6
Ag
N
Ag
N
Au
N
O P
F
O
Au
O
Ag O
N R
H2O
N
R N
N
NN
N
R N
R= -CH3; 3
R = -CH2Ph; 4
R= -CH3; 9
R = -CH2Ph; 10
R= -CH3; 5
R = -CH2Ph; 6
R= -CH3; 7
R = -CH2Ph; 8
Scheme 1. Synthesis of Ag(I) and Au(I) monometallic and bimetallic complexes.

S.D. Adhikary et al. / Inorganica Chimica Acta 384 (2012) 239–246
241
1HNMR (0.02–0.06 ppm) and 13CNMR (0.91–0.25 ppm) spectra is
observed than the mononuclear complexes. For the two Ag(I)–
Ag(I) bimetallic complexes, the resonance signals assignable to
the carbenic carbons were observed at 188.36- and 181.29 ppm
for 5 and 6, respectively which is slightly down shifted (ca
1 ppm) in comparison to mononuclear complex 3 and 4; this
may be due to Ag–Ag bonding and additionally O- coordination
of PO2F2. Similar facts were observed in cases of Ag(I)–Au(I) bime-
tallic system 9 and 10 where the signal of carbenic carbons ca
0.95 ppm down shifted in comparison to mononuclear complexes
7 and 8. The maximum wave length of absorption (kmax) of the
Ag(I)–Ag(I) bimetallic complexes (7 and 8) appears in the range
of 275–277 nm with a broad band in the range of 256–262 nm
where as in case of Ag(I)–Au(I) heterobimetallic complexes (9
and 10) kmax appears at 290–295 nm with a broad band at 255–
260 nm.
Fig. 1. ORTEP view of single crystal X-ray structure of 3 (50% probability) (hydrogen
atoms, anion PF6 and solvent molecules have been removed for clarity).
directed towards the same side of the coordination plane i.e. adopt
the syn configuration.
The molecular structure of 5 is shown in Fig. 2 and crystallo-
graphic data listed in Table 1. The molecule crystallizes as triclinic
P1 space group. Two silver atoms are bound together by two 1-
2.2. Single crystal X-ray structural description
ꢀ
methyl-3-(2-picolyl)benzimidazolylidene ligands forming a metal-
locycle. Unlike 3, where the pyridine groups are remain non-coor-
dinated but in 5, 1-methyl-3-(2-picolyl)benzimidazolylidene acts
as both Ccarb and Npy donor bidentate ligand. Each silver ion is bico-
ordinated through a pyridine-N of one ligand and carbene carbon
of benzimidazolylidene group of another ligand. We have strategi-
cally added AgPF6 with one drop of water in acetonitrile media to
generate PO2F2 insitu; hydrolysis of PF6ꢀ by water produce PO2F2 is
a reported example [74]. Each Ag(I) is coordinated by one O of
PO2F2 unit, and there is strong Ag–Ag bond leading to tetra coordi-
nated geometry around Ag. There is strong AgꢁꢁꢁAg interaction evi-
dent by 2.867 Å AgꢁꢁꢁAg separation; shorter than the similar
pyrimidine substituted imidazolylidene system 3.0318(5) Å [75].
The Ag–Ccarb and Ag–Npy bond distances are 2.113(4) and
2.236(4) Å, respectively (listed in Table 3) and consistent with
Complexes 3, 5, 8 and 9 have been further characterized by
X-ray diffraction analysis. As shown in Fig. 1, complex 3 is
mononuclear and crystallized as monoclinic C2 space group. The
crystallographic parameters are listed in Table 1. The central silver
is bicoordinated by two benzimidazolylidenes in a linear fashion.
The pyridine groups do not interact with the central Ag(I) atom.
The Ag(1)–C(1) bond distance 2.070(2) Å is consistence with the
other similar Ag-benzimidazolylidenes e.g. N,N0-dibutylbenzimi-
dazolylidene silver chloride (Ag–Ccarb = 2.096(4)–2.091(4)) [62],
less than the sum of the covalent radii of carbon and silver (sum
of covalent radii of Ag(I) and carbon is 2.111 Å) [72,73]. The
selected bond parameters of 3 are listed in Table 2. The two benz-
imidazolylidene rings coordinated to the silver atom are nearly
coplanar as evidenced by the small dihedral angle of 1.44°. Another
structural feature of the compound is that the two methyl groups
Table 1
X-ray crystallographic data of 3, 5, 8 and 9.
3
5
8
9
Empirical formula
Formula weight
Crystal system
Space group
T (K)
C
28H26AgF6N6P
C28H26Ag2N6PF6PF2O2C2H3N
949.28
C40H34AuF6N6P
940.67
C56H52Ag2Au2F12N12O6P4
699.39
monoclinic
C2/c
1950.65
monoclinic
C2/c
triclinic
triclinic0
ꢀ
ꢀ
P1
P1
200.0
295
150(2)
293(2)
Cell dimensions
a (Å)
b (Å)
9.907(1)
23.430(3)
12.112(2)
90
100.23(1)
90
2766.9(6)
4
1.679
10.6096(5)
10.7752(5)
16.4584(8)
71.4554(18)
82.638(2)
81.271(2)
1756.76(14)
2
8.398(4)
10.715(5)
10.988(5)
70.076(12)
85.924(12)
68.550(10)
863.6(7)
1
13.1078(18)
21.795(3)
22.354(3)
90.00
97.667(2)
90.00
6329.0(15)
4
2.047
c (Å)
a
(°)
b (°)
c
(°)
Volume(Å3)
Z
Dcalc (Mgmꢀ3
)
1.795
1.809
Absorption coefficient
0.856
1.288
4.379
5.428
(l)
F(000)
1408
940
464
3760
Crystal size (mm)
Theta range for data
collection
0.13 ꢂ 0.39 ꢂ 0.40
0.22 ꢂ 0.24 ꢂ 0.50
0.09 ꢂ 0.13 ꢂ 0.19
0.06 ꢂ 0.18 ꢂ 0.29
1.71–30.00
3.40–26.50
1.5–25.0
1.5–25.0
Index ranges
ꢀ13 6 h 6 13; ꢀ32 6 k 6 32;
ꢀ17 6 l 6 15
ꢀ13 6 h 6 13; ꢀ12 6 k 6 13;
ꢀ19 6 l 6 20
ꢀ9 6 h 6 9; ꢀ12 6 k 6 12;
ꢀ13 6 l 6 12
ꢀ13 6 h 6 14;ꢀ23 6 k 6 23;ꢀ24 6 l 6 23
Total/unique/observed 21417/4129/3197
data
15542/7195/5606
7621/3019/2312
23910/4107/3069
Goodness of fit (GOF)
1.684
1.054
1.026
1.046
Final R indices
R1 = 0.0352, wR2 = 0.0373
R1 = 0.0531, wR2 = 0.0531
R1 = 0.0790, wR2 = 0.1894
R1 = 0.0797, wR2 = 0.1968
[I > 2r(I)]
R indices (all data)
R1 = 0.0352, wR2 = 0.0373
R1 = 0.0646, wR2 = 0.1506
R1 = 0.1018, wR2 = 0.2074
R1 = 0.1048, wR2 = 0.2146

242
S.D. Adhikary et al. / Inorganica Chimica Acta 384 (2012) 239–246
Table 2
Selected bond lengths (Å) and angles (°) of 3.
Table 3
Selected bond lengths (Å) and angles (°) of 5.
Bond lengths
Bond angles
Bond lengths
Bond angles
Ag(1)–C(1) = 2.070(2)
N(1)–C(1) = 1.347(2)
N(2)–C(1) = 1.346(2)
N(1)–C(2) = 1.379(2)
Ag(1)–C(1) = 2.070(2)
N(1)–C(1)–N(2) = 105.9(1)
N(1)–C(1)–Ag(1) = 128.1(1)
N(2)–C(1)–Ag(1) = 125.3(1)
C(1)–Ag(1)–C(1) = 175.1(1)
C(1)–N(1)–C(2) = 111.0(1)
Ag(1A)–Ag(1ai) = 2.8674(8)
C(1A)–Ag(1a) = 2.113(4)
N(3ai)–Ag(1a) = 2.236(4)
N(1a)–C(1a) = 1.370(6)
N(2a)–C(1a) = 1.354(6)
C(1a)–Ag(1a)–N(3ai) = 143.65(17)
N(1a)–C(1a)–N(2a) = 105.6(4)
N(1a)–C(1a)–Ag(1a) = 122.5(3)
N(2a)–C(1a)–Ag(1a) = 131.3(3)
72.13(12),116.76(10), 116.69(17), respectively. In molecule 5,
PO2F2 moiety acts as bridging ligand, out of two oxygen of PO2F2
one oxygen bind with one Ag-atom, another single charge of Ag–
Ag bimetallic unit balanced by PF6. The bridging PO2F2 form an infi-
nite 1D polymer chain (shown in Fig. 3). Best of our knowledge this
is the first example of diflurophosphate bridged [76] N-heterocy-
clic carbene complex.
Slow diffusion of diethyl ether into acetonitrile solution of com-
plex 8 in dark leads to colorless crystals after 3–4 days. The molec-
ular structure of 8 is established by single crystal X-ray diffraction
studies as depicted in Fig. 4. Complex 8 crystallizes in the triclinic
space group C2/c with one cationic complex part and PF6 as coun-
ter anion. On contrary to Ag(I) complex 3, in the Au(I) complex 8,
where two 1-benzyl-3-(2-picolyl)benzimidazolylidene ligands
bonded with Au(I) in nearly linear through the Ccarb–Au(I)–Ccarb
bonding and two picolyl groups remain as anti configuration. This
syn and anti configuration depends upon energy, the molecule
adopt the particular configuration where the energy is minimum
this is proved by theoretical energy calculation [77]. The Au(1)–
C(1) bond distance is 1.992(14) Å; shorter than the less of summa-
tion of vander Wall radii of Au(I) and carbon (summation of vander
Wall radii of Au(I) and carbon is 2.108 Å) [78]. The linear geometry
around Au in complex 8 with Ccarb–Au–Ccarb angles of 180.0(6)°
more perfect than similar Ccarb–Ag–Ccarb system (175.1(1)°) of
complex 3. The selected bond parameters are listed in Table 4. In
the molecule 8, the acute angle N(1)–C(1)–N(2) = 104.6(12)° con-
sistence with the benzimidazolylidene system [46].
Fig. 2. ORTEP view of single crystal X-ray structure of 5 (50% probability) (hydrogen
and anion PF6 has been removed for clarity).
those of silver–NHC complexes [28,37–39] of the similar pyridine
functionalized imidazolylidene systems having Ag–Ccarb and Ag–
Npy distance is 2.032(13)–2.088(10) and 2.232(9) Å, respectively.
[37–39] The Ag(1)–O1 bond distance 2.402(5) within covalent radii
of Ag(I) and Oxygen. There is strain in the molecule supported by
C(1A)–Ag(1)A–N(3A) bond angle [143.65(17)°] heavily deviated
from the linearity. After rearranging from linear geometry in com-
plex 3 the Ag(I) adopts the tetrahedral geometry in 5 supported by
the C(1A)–Ag(1A)–O(1), N(3A)–Ag1A–O1, C1A–Ag1A–Ag1A, N3A–
Ag1A–Ag1A, O1–Ag1A–Ag1A bond angles 113.5(2), 94.4(2),
Fig. 3. One dimensional polymer of 5 through PO2F2 bridge (PF6 has been removed for clarity).

S.D. Adhikary et al. / Inorganica Chimica Acta 384 (2012) 239–246
243
Table 5
Selected bond lengths (Å) and angles (°) of 9.
Bond lengths
Bond angles
Au(1)–Ag(1) = 2.7910(17)
Au(1)–C(1) = 2.043(19)
Au(1)–C(15) = 2.043(19)
Ag(1)–N(6) = 2.237(16)
Ag(1)–N(3) = 2.275(16)
C(1)–N(2) = 1.32(2)
C(15)–Au(1)–Ag(1) = 79.8(5)
C(1)–Au(1)–Ag(1) = 96.7(5)
N(6)–Ag(1)–N(3) = 121.4(5)
N(6)–Ag(1)–Au(1) = 109.6(4)
N(3)–Ag(1)–Au(1) = 102.5(4)
N(2)–C(1)–N(1) = 105.5(16)
N(2)–C(1)–Au(1) = 129.2(13)
N(1)–C(1)–Au(1) = 125.3(14)
C(1)–N(1) = 1.39(2)
N(1)–C(2) = 1.39(2)
Fig. 4. ORTEP view of single crystal X-ray structure of 7 (50% probability) (hydrogen
atoms and anion PF6 have been removed for clarity).
Table 4
Selected bond lengths (Å) and angles (°) of 8.
Bond lengths
Bond angles
Au(1)–C(1) = 1.992(14)
N(1)–C(1) = 1.358(18)
N(2) –C(1) = 1.363(18)
N(1)–C(2) = 1.386(18)
Au(1)–C(1) = 1.992(14)
N(1)–C(1)–N(2) = 104.6(12)
N(1)–C(1)–Au(1) = 126.8(10)
N(2)–C(1)–Au(1) = 128.1(10)
C(1)–Au(1)–C(1) = 180.0(6)
C(1)–N(1)–C(2) = 111.8(12)
The molecular structure of 9 is established by the X-ray diffrac-
tion studies that depicted in Fig. 5. The Au(I)–Ag(I) bimetallic com-
plex crystallizes in monoclinic space group C2/c. The asymmetric
unit of 9 contains a two fold axis of symmetry passing through
the center of the. In the asymmetric unit of 9 two Au(I)–Ag(I) bime-
tallic unit bridged by PO2F2 bridging ligand which is produced in-
stitu from AgPF6 and water. In it’s asymmetric unit two
independent silver and gold atoms which are bonded together by
two 1-methyl 3-(2-picolyl)benzimidazolylidene ligands. The linear
coordination geometry of Au(I) is described as Ccarb–Au(I)–Ccarb
boding through two 1-methyl-3-(2-picolyl)benzimidazolylidene
ligands; where Ag is seized by two pyridine-N of two different li-
gands which are remain non-coordinated in 7 and 8. There is con-
siderable structural differences is observed in Ag–Ag bimetal (5)
with Ag–Au (9) bimetal; in Ag–Ag bimetal each Ag bears the
Fig. 6. Emission spectra of the bimetallic complexes 5 and 9 in acetonitrile at room
temperature after excitation at 260 and 290 nm, respectively.
Npy–Ag–Ccarb bonding connectivity where as in Ag–Au bimetal
the Ag bears the Npy–Ag–Npy bonding connectivity and Au carries
the Ccarb–Au–Ccarb bonding connectivity. There is one similarity in
structure between 5 and 9; both cases PO2F2 unit bridge through
Ag(I). Though thorough studies needed to make a clear conclusion;
but it is assumed in solution when AgPF6 is added, the bonding
connectivity of Ag(I)–carbene is reorganized from structure 3 to
structure 5; but in Au(I)–Ag(I) bimetallic system bonding connec-
tivity of Au(I) remain unaltered as 8, only Ag(I) is sized by two pyr-
idine-N. In the molecule 9 Ccarb–Au–Ccarb bond angle is 175.5(6)°
and Npy–Ag–Npy bond angle is and 121.4(5)°, heavily deviated from
linearity. The Au–C(1), Au–C(15), Ag(1)–N(3) and Ag(1)–N(6) bond
distances are 2.043(19),1.98(2), 2.237(16), 2.275(16) Å, respec-
tively (listed in Table 5) and consistent with those of the known
Au(I)–Ag(I) NHC bimetallic complexes [37–39]. The bridging
PO2F2 possess the Ag(1)–O(1) bond distance 2.256(17) Å. The
Au(I)–Ag(I) distance [(2.7910(17) Å] in 9 is shorter than similar
Ag(1)–Au(1) separation (3.0318(5) Å) in 1-methyl-3-picolylimi-
dazoline system [37–39], this may be due to less
ure of benzimidazolylidene systems.
r-donating nat-
2.3. Luminescence studies
The photoluminescent properties of complexes 7–10 have been
studied in acetonitrile at room temperature. The emission of 5 and
9 is shown in Fig. 6. All the bimetallic complexes display strong
luminescence; the Ag(I)–Ag(I) bimetallic systems 5 and 6 show
the high energy emission at 325–330 nm upon excitation at
260–262 nm; where as the heterobimetallic complexes 9 and 10
show emission at 365–370 nm with a less intense hump at
Fig. 5. ORTEP view of single crystal X-ray structure of 9 (50% probability) (hydrogen
atoms and anion PF6 have been removed for clarity).

244
S.D. Adhikary et al. / Inorganica Chimica Acta 384 (2012) 239–246
550–552 nm upon excitation at 290–295 nm. The red shifts of
emission band in case of heterobimetallic systems (9 and 10)
incomparision to homobimetallic systems (5 and 6) could be as-
cribed to the existence of much stronger intermolecular
served, filtered, washed with water and dried over vacuo. Yield
was 0.49 g, 1.12 mmol, 58%.
1H NMR (400 MHz, CD3CN): d = 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):
d = 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. ppm.
Anal. Calc. for C20H18N3PF6: C, 53.93; H, 4.05; N, 9.44. Found: C,
53.71; H, 4.07; N, 9.32%.
Ag(I)–Au(I) and ring
p–p interactions [28,35–40]. The range of
luminescence observed is consistence with the reported similar
bis-carbene benzimidazolylidene systems [79–83]. The lumines-
cence of these bimetallic complexes may be originated from the
mixed excitation states [46]; either intraligand or metal-centered
electronic transfer processes can be possible. Further work and
more studies required to clarify the nature of the emission
properties.
3.2.3. Synthesis of silver complex 3
Silver oxide (0.02 g, 0.08 mmol) and proligand, 1 (0.06 gm,
0.16 mmol) were taken in dry acetonitrile and the colorless solu-
tion was stirred for 3 h. in dark, then the solution was filtered
through celite and solvent was removed to get 3 as solid mass,
yield was 0.08 g, 0.12 mmol, 74%.
3. Experimental
3.1. General procedures
1H NMR (400 MHz, CD3CN): d = 8.51 (d, J = 5.4 Hz, H, pyridine-
H), 7.99 (t, H, Ar-H), 7.81 (d, J = 8.0 Hz, H, Ar-H), 7.73 (d, J =
8.0 Hz, H, Ar-H), 7.65 (d, J = 8.8 Hz, H, pyridine-H), 7.53 (t,
J = 7.8 Hz, H, pyridine-H), 7.50 (t, J = 7.4 Hz, H, pyridine-H), 5.85
(s, 4H, NCH2-pyridine), 4.07 (s, 3H, NCH3) ppm. 13C NMR
(400 MHz, CD3CN): d = 187.36, 153.94, 150.89, 139.08, 134.87,
133.23, 126.83, 124.60, 111.82, 54.41, 35.25 ppm. Anal. Calc. for
AgC28H26N6PF6: C, 48.07; H, 3.72; N, 12.02. Found: C, 47.24; H,
3.33; N, 11.59%.
All the reagents 1-methylbenzimidazole, Ag2O, 2-poicolylchlo-
ride hydrochloride, were purchased from Sigma–Aldrich, and used
without further purification. Au(SMe2)Cl was prepared according
to literature [84]. All manipulations were carried out under open
atmosphere otherwise stated. All solvents were distilled over
appropriate drying agents and N2-saturated prior to use. NMR
spectra were measured on a Bruker 400 and 300 spectrometers
at 25 °C with tetramethylsilane as an internal standard. Electronic
and emission spectra of the complexes were obtained on a Shima-
dzu UV-1601 and Perkin–Elmer-LS 55 luminescence spectrometer,
respectively.
3.2.4. Synthesis of silver complex 4
The synthesis and characterization of the complex reported ear-
lier. [45].
3.2. Synthesis of imidazolium salts and metal complexes
3.2.5. Synthesis of silver–silver bimetallic complex 5
The colorless complex 3 (0.07 g, 0.10 mmol) was taken in 10 ml
acetonitrile; AgPF6 (0.03 g, 0.103 mmol) and one drop of water
were added with stirring in the solution of 3 and stirring is contin-
ued for 2 h in dark at room temperature. Then the solution was fil-
tered through celite and solvent was removed to get 5 as solid
mass, yield was 0.07 g, 0.07 mmol, 72%.
3.2.1. Synthesis of proligand 1
Synthesis of the bromide salt of the proligand 1 is reported ear-
lier [45], we have modified the procedure to get the PF6 salt of 1. 1-
methylbenzimidazole (0.4 g, 3.03 mmol) and 2-picolylchloride
hydrochloride (0.5 g, 3.05 mmol) and NaHCO3 (0.27 g, 3.21 mmol)
were taken in EtOH and reflux for 24 h. Then the solvent was re-
moved and the gummy mass was triturated with dichloromethane,
the colorless solid was extracted in dichloromethane and recrystal-
lised from dichloromethane and Et2O to get the solid mass. The so-
lid mass was redisolved in minimum water and aqueous saturated
KPF6 was added, immediate precipitation was observed, filtered,
washed with water and dried over vacuo. Yield was 0.67 gm,
1.82 mmol, 60%.
1H NMR (400 MHz, CD3CN): d = 8.53 (d, J = 5.6 Hz, H, pyridine-H),
8.05 (t, H, Ar-H), 7.84 (d, J = 8.1 Hz, H, Ar-H), 7.75 (d, J = 8.1 Hz, H, Ar-
H), 7.66 (d, J = 8.7 Hz, H, pyridine-H), 7.55 (t, J = 7.6 Hz, H, pyridine-
H), 7.52 (t, J = 7.5 Hz, H, pyridine-H), 5.89 (s, 4H, NCH2-pyridine),
4.13 (s, 3H, NCH3) ppm. 13C NMR (400 MHz, CD3CN): d = 188.27,
154.2, 151.12, 139.41, 134.95, 133.32, 127.02, 124.2, 112.2, 54.47,
35.5 ppm. Anal. Calc. for C28H26Ag2N6P2F8O2: C, 37.00; H, 2.86; N,
9.25. Found: C, 36.35; H, 2.69; N, 9.13%.
1H NMR (400 MHz, CD3CN): d = 9.10 (s, 1 H, NCHN), 8.53 (d,
J = 5.2 Hz, 1H, pyridine-H), 7.90–7.82 (m, 3H, Ar-H), 7.75–7.66
(m, 2 H, pyridine-H), 7.56 (d, J = 8.4 Hz, 1 H, pyridine-H), 7.39–
7.36 (m, H, Ar-H), 5.77 (s, 2 H, NCH2-pyridine), 4.10 (s, 3H, NCH3)
ppm. 13C NMR (400 MHz, CD3CN): d = 153.11, 149.57, 141.82,
137.28, 131.89, 131.22, 126.83, 126.75, 123.61, 122.53, 113.32,
113.08, 51.24, 33.13 ppm. Anal. Calc. for C14H14N3PF6: C, 45.53;
H, 3.79; N, 11.38. Found: C, 45.23; H, 3.53; N, 11.05%.
3.2.6. Synthesis of silver–silver bimetal complex 6
Complex 6 was prepared similar to 5; complex 4 (0.07 g,
0.08 mmol) was taken in 10 ml acetonitrile and AgPF6 (0.02 g,
0.08 mmol) was added to same and the solution was stirred for
3 h, rest of the procedure as like as 5. Yield was 0.06 g, 0.06 mmol,
70%.
1H NMR (400 MHz, DMSO-d6): d = 8.66 (d, J = 4.2 Hz, 1H, pyri-
dine), 7.95 (m, 1H), 7.78 (m, 1H), 7.73 (m, 2H), 7.47–7.38 (m, 5H),
7.33–7.28 (m, 3H), 5.98 (s, 2H,CH2,), 5.79 (s, 2H,CH2,). 13C NMR
(100 MHz, DMSO-d6): d = 188.29, 156.31, 150.22, 137.92, 137.07,
134.43, 133.81, 129.31, 129.32, 128.51, 127.72, 124.81, 124.71,
123.82, 122.72, 112.91, 53.90, 52.5. Anal. Calc. for C28H26Ag2N6P2-
F8O2: C, 45.28; H, 3.21; N, 7.92. Found: C, 44.45; H, 2.89; N, 7.56%.
3.2.2. Synthesis of proligand 2
Synthesis of the chloride salt of the proligand 2 is reported ear-
lier [45] but, we have synthesized the same in different way. 1-
benzylbenzimidazole (0.4 g, 1.92 mmol) and 2-picolylchloride
hydrochloride (0.32 g, 1.92 mmol) and NaHCO3 (0.16 g, 1.93 mmol)
were taken in EtOH and reflux for 28 h. Then the solvent was re-
moved and the gummy mass was triturated with dichloromethane,
the colorless solid was extracted in dichloromethane and recrystal-
lized from dichloromethane and Et2O to get the solid mass. The so-
lid mass was redisolved in minimum water and aqueous saturated
KPF6 solution was added, immediate white precipitation was ob-
3.2.7. Synthesis of gold complex 7
Proligand, 1 (0.07 g, 0.19 mol) and silver oxide (0.02 g, 0.09 mol)
were added to acetonitrile. The mixture was protected from light
and stirred for 4 h. at room temperature. Then the solution was fil-
tered through celite, and Au(SMe2)Cl (0.028 g, 0.09 mol) was added

S.D. Adhikary et al. / Inorganica Chimica Acta 384 (2012) 239–246
245
to the filtrate and stirred for 3 h. in dark at room temperature. The
resulting mixture was filtered through celite. The solvent was re-
moved to get (7) as solid mass, again recrystalized from acetoni-
trile/diethyl ether, yield was 0.11 g, 0.14 mmol, 73%. 1H NMR
(400 MHz, CD3CN): d = 8.55 (d, J = 5.5 Hz, H, pyridine-H), 7.95 (t,
H, Ar-H), 7.83 (d, J = 8.0 Hz, H, Ar-H), 7.75 (d, J = 8.1 Hz, H, Ar-H),
7.66 (d, J = 8.7 Hz, H, pyridine-H), 7.52 (t, J = 7.7 Hz, H, pyridine-
H), 7.51 (t, J = 7.5 Hz, H, pyridine-H), 5.86 (s, 4H, NCH2-pyridine),
4.08 (s, 3H, NCH3) ppm. 13C NMR (400 MHz, CD3CN): d = 188.17,
153.87, 150.91, 139.11, 134.87, 133.26, 126.85, 124.63, 111.81,
54.44, 35.29 ppm. Anal. Calc. for AgC28H26N6PF6: C, 42.63; H,
3.29; N, 10.65. Found: C, 42.01; H, 3.01; N, 10.29%.
collected at room temperature using a Bruker APEX-II CCD diffrac-
tometer with graphite monochromated Mo K radiation (graphite
a
monochromated Mo Ka radiation, k = 0.71073 Å). The crystallo-
graphic data and details of the data collections for 3, 5, 8 and 9
are given in Table 1. The structures were solved by direct methods
and refined on F2 using all reflections with SHELX-97 [85]. Data
reduction was carried out using the program Bruker SAINT [86] for
5. The structures of 2 and 3 were solved by direct methods and re-
fined by the full-matrix least-square technique on F2 with aniso-
tropic thermal parameters to describe the thermal motions of all
non hydrogen atoms using the programs SHELXS97 and SHELXL97
[85] and refined using full-matrix least-squares with all non-
hydrogen atoms anisotropically and hydrogens included on calcu-
lated positions, riding on their carrier atoms. All calculations were
performed using SHELXL-97 [85], PARST [87] and WINGX [88] programs.
3.2.8. Synthesis of gold complex 8
Preparation of complex is similar to 7, Ligand, 2 (0.07 g,
0.16 mol) and Silver oxide (0.02 g, 0.08 mol) were added to aceto-
nitrile and stirred for 4 h. at room temperature. Then the solution
was filtered through celite and Au(SMe2)Cl (0.023 g, 0.08 mol)
was added to the filtrate and stirred for 3 h, rest of the procedure
as like as 7; yield was 0.114 g, 0.12 mmol, 76%. 1H NMR
(400 MHz, DMSO-d6): 1H NMR (400 MHz, DMSO-d6): d = 8.57(d,
J = 4.4 Hz, pyridine, 1H),7.95 (m, 1H), 7.78 (m, 1H), 7.75 (m, 2H),
7.45–7.39 (m, 5H),7.36–7.28 (m, 3H), 5.98 (s, CH2, 2H), 5.79 (s,
CH2, 2H). 13C NMR (400 MHz, DMSO-d6): d = 188.20, 155.5,
149.9, 137.7, 136.7, 134.3, 133.7129.2, 129.1, 128.4, 127.7124.7,
124.6, 123.7, 122.7, 112.8, 53.7, 52.3. Anal. Calc. for C40H34AuF6N6P:
C, 51.06; H, 3.61; N, 8.94. Found: C, 50.72; H, 3.67; N, 8.77%.
5. Conclusion
This article presents a structural survey of the metal-N-hetero-
cyclic carbenes of Ag(I) and Au(I) mononuclear and homo and het-
ero binuclear complexes of 1-alkyl-3-picolylbenzimidazolylidenes
ligands. The Ag(I) and Au(I) complexes 3 and 8 adopt the Ccarb
–
M–Ccarb linear geometry keeping the pyridyl-N uncoordinated.
The homo bimetallic Ag(I)–Ag(I) complex 5 bears the Ccarb–Ag–
Npy bonding connectivities where as Ag(I)–Au(I) hetero-bimetallic
system 9 bears Ccarb–Au–Ccarb and Npy–Ag–Npy bonding connectiv-
ities. We have strategically added AgPF6 and water to produce
PO2F2 insitu, which acts as bridging ligand. In case of Ag(I)–Ag(I)
bimetallic system, PO2F2 helps to form a infinite ID polymer where
as in case of Ag(I)–Au(I) hetero-bimetallic system, PO2F2 bridge be-
tween to bimetallic units. We carefully studies the electronic and
luminescent properties of both the bimetals at room temperature,
the spectral data supports the fact of dipolar difference between
M–M bonds.
3.2.9. Synthesis of silver–gold bimetal complex 9
Complex 7 (0.07 g, 0.09 mmol) is taken in 6–7 ml acetonitrile,
an acetonitrile solution of AgPF6 (0.02 g, 0.09 mmol) and water
(one drop) were added dropwise under stirring condition to the
earlier solution in dark at room temperature. After 2 h the solution
was filtered through celite and solvent was removed to get 9 as so-
lid mass, yield was 0.06 g, 0.06 mmol, 72%. 1H NMR (300 MHz,
CD3CN, 25 °C): d = 8.61(d, 3J = 5.4 Hz, 2 H, pyridine-H), 8.09 (t, 2
H, Ar-H and pyridine-H,), 7.91 (d, 2 H, Ar-H and pyridine-H), 7.83
(d, 2H, Ar-H and pyridine-H), 7.74 (d, 2J = 14.0 Hz, 2 H, NCH2-pyr-
idine), 7.64 (t, J = 7.9 Hz, H, pyridine-H), 7.59 (t, J = 7.6 Hz, H, pyri-
dine-H), 5.91 (d, 2J = 14.0 Hz, 4 H, NCH2-pyridine), 4.15 (s, 6 H,
NCH3) ppm. 13C NMR (400 MHz, CD3CN): d = 189.12, 154.04,
150.91, 139.3, 134.98, 133.3, 126.9, 124.3, 111.9, 54.56,
35.31 ppm. Anal. Calc. for C28H26AgAuN6P2O2F8: C, 33.70; H, 2.61;
N, 8.43. Found: C, 33.21; H, 2.45; N, 8.33%.
Acknowledgements
J.D. thanks to Department of Science and Technology, DST, India
for financial support under SERC Fast Track Young Scientist
Scheme SR/FT/CS-046/2009 and Prof. Surajit Chattopadhyay, Kaly-
ani University for constant encouragement.
Appendix A. Supplementary material
CCDC 771630, 827581, 827583 and 827582 contain the supple-
mentary crystallographic data for complexes 3, 5, 8 and 9, respec-
tively. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.ica.2011.12.007.
3.2.10. Synthesis of silver–gold bimetal complex 10
Preparation of complex 10 is similar to 9, complex 8 (0.07 g,
0.075 mmol), AgPF6 (0.02 g, 0.075 mol) and water (one drop) were
mix together, rest of the procedure as similar as 9. Yield was 0.06 g,
0.05 mmol, 70%. 1H NMR (400 MHz, DMSO-d6): 1HNMR (400 MHz,
DMSO-d6): d = 8.61(d, J = 4.6 Hz, pyridine, 1H),7.98 (m, 1H), 7.77
(m, 1H), 7.77 (m, 2H), 7.47–7.42 (m, 5H),7.39–7.31 (m, 3H), 6.02
(s, CH2, 2H), 5.81 (s, CH2, 2H). 13C NMR(400 MHz, DMSO-d6):
d = 189.16, 154.91, 150.12, 137.81, 136.82, 134.43, 134.07,129.23,
129.11, 128.41, 127.74, 124.73, 124.64, 123.81, 122.72, 112.82,
53.78, 52.43. Anal. Calc. for C28H26AgAuN6P3O2F8: C; 41.78; H,
2.96; N, 7.31. Found: C, 41.23; H, 2.85; N, 7.16%.
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