Structural, photoluminescence, and theoretical DFT studies of?gold(I)?and silver(I) metallacycle dinuclear complexes of 1-methylbenzimidazolediphenyl phosphine (MBDP) ligand

Article abstract of DOI:10.1016/j.molstruc.2016.11.070

The synthesis and structural characterization of the bridging ligand, 1-methyl benzimidazole diphenylphosphine (MBDP) and its coordination with Au(I) and Ag(I) metal ions is reported. Two ligands coordinate to the metal ions in head to tail fashion forming a metallacycle coordination consisting of strong M-M bonding interaction with Au[sbnd]Au?=?2.808 and Ag[sbnd]Ag?=?3.017??. Linear coordination is observed for the gold, while the silver shows distorted tetrahedral arrangement. X-ray crystal data of [(MBDP)₂Au₂](CH₃CN)(BF₄)₂ (2) crystallizes in the monoclinic system with the space group of P2₁/n with cell constants of a?=?8.9993 (8) ?, b?=?19.6166 (18) ?, c?=?13.4484 (12) ?, β?=?100.966 (2)°, R?=?0.026, and R_w?=?0.064. The structure of [(MBDP)₂(CH₃CN)₄Ag₂](BF₄)₂, (3) crystallizes in the P (?)1 space group and the unit cell is a?=?10.5423 (10) ?, b?=?10.7638 (10) ?, c?=?12.3530 (12) ?; α?=?88.592 (3)°, β?=?73.097 (3)°, γ?=?84.422 (3)°, V?=?1334.8 (2) ?³ with Z?=?1, R?=?0.034 and R_w?=?0.093. The coordinating ligand as well as both dinuclear compounds reported here are luminescent in the near UV region. As supported by the DFT theoretical work, the emission is suggested to be primarily ligand based π- π* transition.

Full text of DOI:10.1016/j.molstruc.2016.11.070

Journal of Molecular Structure 1133 (2017) 374e383
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Structural, photoluminescence, and theoretical DFT studies
of gold(I) and silver(I) metallacycle dinuclear complexes of
1
-methylbenzimidazolediphenyl phosphine (MBDP) ligand
a
b, *
Darkus E. Jenkins , Zerihun Assefa
a
Chemistry Department, Savannah State University, Savannah, GA, USA
Chemistry Department, North Carolina A&T State University, Greensboro, NC 27411, USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 July 2016
Received in revised form
The synthesis and structural characterization of the bridging ligand, 1-methyl benzimidazole diphenyl-
phosphine (MBDP) and its coordination with Au(I) and Ag(I) metal ions is reported. Two ligands coor-
dinate to the metal ions in head to tail fashion forming a metallacycle coordination consisting of strong
M-M bonding interaction with AueAu ¼ 2.808 and AgeAg ¼ 3.017 Å. Linear coordination is observed for
the gold, while the silver shows distorted tetrahedral arrangement. X-ray crystal data of
21 November 2016
Accepted 22 November 2016
Available online 4 December 2016
[
(MBDP)
cell constants of a ¼ 8.9993 (8) Å, b ¼ 19.6166 (18) Å, c ¼ 13.4484 (12) Å,
¼ 0.064. The structure of [(MBDP) (CH CN) Ag ](BF
the unit cell is a ¼ 10.5423 (10) Å, b ¼ 10.7638 (10) Å, c ¼ 12.3530 (12) Å;
2
Au
2
](CH
3
CN)(BF
4
)
2
(2) crystallizes in the monoclinic system with the space group of P2
1
/n with
ꢀ
b
¼ 100.966 (2) , R ¼ 0.026, and
Keywords:
R
w
2
3
4
2
4
)
2
, (3) crystallizes in the P (ꢁ)1 space group and
Benzimidazole
Au(I) dimer
Ag(I) dimer
Phosphine
ꢀ
ꢀ
a
¼ 88.592 (3) ,
b
¼ 73.097 (3) ,
ꢀ
3
g
¼ 84.422 (3) , V ¼ 1334.8 (2) Å with Z ¼ 1, R ¼ 0.034 and R ¼ 0.093. The coordinating ligand as well
w
as both dinuclear compounds reported here are luminescent in the near UV region. As supported by the
DFT theoretical work, the emission is suggested to be primarily ligand based * transition.
2016 Elsevier B.V. All rights reserved.
p- p
©
1. Introduction
the mixed-donor species commonly used to prepare Au(I) dimers
12e14,19,20]. These ligands follow the Lewis soft acid/base
[
Phosphine ligands are well known for their ability to coordinate
to group 11 metal ions particularly to Au(I) and Ag(I) centers [1e8].
chemistry when coordinating with group 11 metals. Although N is a
lesser soft base than P, its ability to bind with Au(I) is well docu-
mented [21e25] including the work conducted by Fackler et al. on
amidinate ligands [26,27]. The ability to bring two Au(I) atoms in
close proximity is a goal for many research groups because of the
fascinating photoluminescence these types of compounds reveal.
Hence, the bridging N, P ligands not only coordinate as mono-
dentate ligand, they also have the ability to act as bidentate ligands
through the two donor atoms and bring homo- and hetero-nuclear
atoms together with short distances.
Due to their steric effects and
p-acceptor abilities, tertiary phos-
phines are one of the most studied groups of ligands for gold (I)
complexes [1e8].
Bis(diphenylphosphino)methane (dppm) and bis(dicyclohex-
ylphosphino)methane (dcpm) are representative of the diphos-
phine ligands. The dppm, dcpm and related ligands are very flexible
and capable of bringing two gold atoms as close as 2.4e3.6 Å [9e11]
with varying geometries including open and metallacycle features
with AueAu separations of 2.8e3.1 Å [12e14].
Burini and co-workers have extensively studied the N, P type
Over the past four decades attention has been given to bifunc-
tional ligands with regards to bringing two metal atoms in close
proximity [15e18]. Several types of ligands have attracted attention
including the N-, P-type ligands. Diphenylphosphinopyridine
2
ligand of (1-benzyl-2-imidazolyl) diphenyl-phosphine (BzimPh P)
to understand the behavior of coordination with various metal
systems [15e18]. They were able to utilize the monodentate and
2
bidentate feature of the BzimPh P by examining complexes with
(
dppy), dimethylphosphinopyridine (dmpp), and 2-(diphenyl-
Au(I), Ag(I), Cu(I), Rh(I), Ir(I), Hg(II), Zn(II), and Cd(II) metal ions. In
the Au (I) structure, the coordination is found to be monodentate
with a near linear geometry of the PeAueCl bond angle with the
ligand unassisted intermolecular AueAu distance of 3.306 Å.
Bachechi along with Burini and co-workers also studied the X-
phosphino)-1-benzylimidazole (BzImPh P) are representative of
2
*
Corresponding author.
E-mail address: zassefa@ncat.edu (Z. Assefa).
http://dx.doi.org/10.1016/j.molstruc.2016.11.070
022-2860/© 2016 Elsevier B.V. All rights reserved.
0

D.E. Jenkins, Z. Assefa / Journal of Molecular Structure 1133 (2017) 374e383
375
ray crystal structure of silver complexes with the same ligand [18]
and interesting geometries around the metal ions were observed.
The ligand is thought to offer stability for metal e metal interactions
including the lighter group 11 metals that possess the same abilities
to coordinate two or more metal atoms in close proximity to each
other. Rigid ligands such as dmpp are found to constrain the metal-
Schlenk flask containing 100 mL THF was added a 6.6085 g
(50 mmol) of 1-methylbenzimidazole. The solution was stirred
ꢀ
until the solid has dissolved and cooled to ꢁ78 C in acetone/dry ice
bath. To this solution 31.25 mL (50 mmol) of n-BuLi was added
slowly while stirring for 1 h. Then 9 mL (50 mmol) of chlor-
odiphenylphosphine was added drop-wise. The solution was
allowed to warm to room temperature and stirred for 3 h. The
solvent was then evacuated by vacuum suction to yield an orange
2 2 4 2
metal separation, and [dmpp Au ](BF ) was found to have an
AueAu separation of only 2.776 Å [28]. More recently, Ag(I) com-
plexes of similar ligands have been shown to exhibit argentophillic
bonding analogous to the Au(I) systems [29]. A number of other
interesting structures consisting of Cu(I) and Ag(I) metal ions with
N, P ligands have been characterized [15e18,30e33]. These com-
pounds also have interesting luminescent properties, hence further
supporting the desire for using this type of ligand.
foamy product. To this solution Et
was added and the organic layer collected. The Et
2
O and then H
2
O (50 mL each)
O was removed
2
under vacuum to produce an oily product. The product was soni-
cated for 10 min in warm hexanes. A cloudy layer was then formed,
collected, and allowed to cool. X-ray quality crystals were pro-
31
1
duced. P{ H} NMR -25.6 ppm with phosphoric acid standard.
In this manuscript we report the coordination of the methyl
benzimidazolediphenyl phosphine (MBDP) ligand to an Au(I) cen-
ter and compare the structural feature with that of Ag(I) complexes.
The structural analysis and luminescence studies are compared for
the two types of complexes as well as results from theoretical DFT
calculations reported.
2.4. Preparation of (MBDP)Au(I) chloride (1)
To a 25 mL round-bottom flask a 0.064 g (0.20 mmol) (tht)AuCl
was dissolved in 10 mL THF. To this solution a 0.064 g (0.20 mmol)
of the ligand was added and allowed to stir for 3 h. The solution was
concentrated under high pressure vacuum to approximately 3 mL
2
. Experimental
then layered with Et
night and x-ray quality crystals were formed. H NMR in CDCl
(ppm); 3.98 (s, 1H), 7.33e7.54 (m, 5H), 7.58e7.60 (m, 6H), and
2
O. The flask was placed in the freezer over-
1
3
2.1. Materials and methods
31
1
7.70e7.83 (m, 5H). The P{ H} NMR 14.59 (s).
Methylbenzimidazole, n-butyllithium (nBuLi), and chlor-
odiphenylphosphine (PPh
2
Cl) were commercially available from
2.5. Preparation of [(MBDP) Au ](CH CN)(BF ) (2)
2
2
3
4 2
the Sigma-Aldrich and used without further purification. The (tht)
Au(I) chloride (tht ¼ tetrahydrothiophene) was prepared and pu-
rified following the methods of Uson and coworkers [34]. All of the
solvents used including tetrahydrofuran (THF), acetonitrile
In a 25 mL round-bottom flask, a 0.064 g (0.20 mmol) of (tht)
AuCl was dissolved in 10 mL CH CN. A 0.040 g (0.20 mmol) of AgBF
3
4
was added to the flask and stirred for 5 min. A white precipitate
®
(
CH
3
CN), dichloromethane (DCM), diethyl ether (Et
2
O), chloroform
formed, which was filtered through a pad of Celite . To the filtrate,
(
CHCl
3
), and hexanes were degassed using a N purge.
2
the ligand MBDP (0.064 g, 0.20 mmol) was added and allowed to
stir for 3 h. The solution was concentrated under vacuum to
2.2. Physical measurements
approximately 3 mL. The solution was then layered with Et
2
O and
placed in a freezer. X-ray quality crystals were obtained after 4 days.
The infrared (IR) spectra were collected using potassium bro-
mide (KBr) pellets on a Shimadzu IRPrestige21 Fourier-Transform
2 2 3 4 4 2
2.6. Preparation of [(MBDP) Ag (CH CN) ](BF ) (3)
ꢁ1
infrared spectrophotometer, over the range 4000e400 cm
.
ꢁ1
Spectra were collected at 64 scans and a resolution of 1 cm . The
ultraviolet and visible (UVeVis) spectra were collected using a
Shimadzu UV-2401PC UVeVis spectrometer. The photo-
luminescence (PL) data were collected using a Photon Technology
International (PTI) spectrometer model QM-7/SE, equipped with a
Hamamatsu R928P photomultiplier tube with corning filters.
Excitation and emission spectra obtained at room temperature and
at 77 K with liquid nitrogen filled in the cold-finger dewar flask. The
instrument operation, data collection, and handling were all
controlled using the advanced FeliX32 software. Selection of exci-
tation and emission wavelengths are conducted by means of
computer controlled autocalibrated “QuadraScopic” mono-
chromators that are equipped with aberration corrected emission
and excitation optics.
The silver(I) tetrafluoroborate complex 3 with the MBDP ligand
(1:1 M ratio) was prepared in a 25 mL round-bottom flask covered
with aluminum foil. A 0.0397 g AgBF
4
(0.20 mmol) was dissolved in
10 mL CH CN and to this solution MBDP ligand (0.0637 g,
3
0.20 mmol) was added and allowed to stir for 3 h. The solution was
concentrated under vacuum to approximately 3 mL. The partial
removal of the solvent was followed by layering with Et O. The flask
2
was then placed in the refrigerator, where X-ray quality crystals
were formed after 4 days.
2.7. X-ray crystallography
X-ray data were collected using a Bruker AXS SMART X2S sys-
tem based diffractometer equipped with a CCD 485 Breeze detector
and a Ricor K535 power supply unit. A suitable crystal was chosen
and mounted on a pin using a micro-mount needle from Bruker
Corporation using Paratone-N oil purchased from Hampton
Research. The diffraction data for the determination of the crystal
structures were collected on CD-R using the Bruker CCD 485 Breeze
detector for cell refinement, and data reduction with Apex2 and X-
SHELL. For structural display, report and visualization the Mercury
software version 3.5.1 from CCDC 2001e2014 was used. The
structures were solved using the Apex 2 series of software program
from Bruker and by direct methods. The XSHELL program(s) was
used to refine the structures, while molecular graphics were done
using the APEX2 software.
The proton ( H) and phosphorus (31P) nuclear magnetic reso-
1
nance (NMR) spectra were recorded on a 300 MHz Varian NMR
3
00e411149 FT-NMR spectrometer. Chemical shifts (
d ppm) were
reported relative to an internal standard, tetramethylsilane (TMS)
1
for H, while 85% phosphoric acid (H
standard for the 31P NMR in CDCl
3 4
PO ) was used as an external
3
2.3. Synthesis of 1-methylbenzimidazole diphenylphosphine
(
MBDP)
The MBDP ligand was synthesized following a modified pro-
cedure of the previously reported scheme [35,37]. In a 250 mL

376
D.E. Jenkins, Z. Assefa / Journal of Molecular Structure 1133 (2017) 374e383
2.8. Computational details
4
removal of the chloride ion by the addition of the AgBF was a
necessary first step. Addition of the ligand after filtering off the AgCl
precipitate provided compound 2. Similarly, compound 3 was
The Gaussian ’09 software package [36] was used for theoretical
calculations at the Density Functional Theory (DFT) level. Geometry
optimizations and frequency calculations were computed with
Becke three-parameter exchange with the PerdewꢁWang 1991
correlation (B3PW91). The excitation energies and oscillator
strengths were performed using the optimized structures of the
ligand and the complexes without any consideration for solvent by
time-dependent (TD)-DFT with B3PW91. The predicted absorption
spectra of the optimized structures were accomplished by time-
dependent density functional theory (TD-DFT) without consider-
ation of any solvent effect. The LANL2DZ effective core potentials
and valence basis set were used to describe the valence electrons of
gold. Phosphorus, carbon, and hydrogen were described with 6-
4
prepared in 1:1 ratio using the AgBF starting material. Hence in
both metal ions, the use of a starting material with a weakly bound
and/or labile anion was a necessary condition for the metallacycle
coordination.
3.2. Solvent and temperature effect on synthetic procedure and
product stability
Synthesis of the products was attempted in several solvent
systems including THF/Et O, THF/CH CN/Et O, and CH CN. X-ray
2
3
2
3
quality crystals of (MBDP)AuCl, 1 were obtained using the THF/Et O
2
(synthesis/crystallization) solvent system. The synthetic attempt of
3
1 þ G*, 6-31G*, and 6-31G basis sets, respectively. Molecular iso-
density diagrams (Isovalues: 0.02 atomic units) of molecular orbital
estimations were created using the GaussView software
Gaussian Inc.).
2 in the THF/Et O system failed to provide the desired metallacycle
2
product and yielded only the mononuclear complex 1. The syn-
thesis of compound 2 was conducted in acetonitrile and upon
5
(
crystallization a CH CN molecule was trapped in the crystal lattice
3
as described below in the structural detail.
3
. Results and discussion
Crystals of compound 2 were not soluble in CHCl
O, but were found to be soluble in CH CN, MeOH, acetone, THF,
and CH Cl . Several attempts to remove the trapped CH CN from
3
, hexanes, and
Et
2
3
3.1. Synthesis
2
2
3
the lattice were unsuccessful including recrystallization using a
different solvent, gentle heating and lattice perturbation. These
studies were aimed at understanding the solvent effect in crystal
modulation and the nature of luminescence changes induced due
The ligand MBDP has been previously synthesized in 1982 by
the Whiteside group, and a modified version of the original syn-
thesis route [35,37] was followed, where the starting reactants
were changed to 1-methylbenzimidazole and chloro-
diphenylphosphine. Our group was able to grow X-ray quality
single crystals and reported the structure for the first time recently
to the presence of the solvent in the lattice. Gentle heating of the
ꢀ
crystals at 80 C for seven days did not remove the CH
3
CN molecule
from the lattice before noticeable changes were seen upon the
degradation of the crystals, where the pinkish color changed to
dark purple/black. The dark color is an indication of decomposition
in gold compounds. Each day for one week a single crystal was
removed from a watch glass that was placed in an oven and
analyzed using IR spectroscopic techniques for the presence of the
[
38]. However, no complexation studies or structural work has been
reported for this potentially bidentate ligand, although complexes
of the related ligand, 2-(diphenylphosphino)-1-methylimidazole
have been reported [39]. It was of much interest to understand
the effect of the benzene ring on the reactivity and coordination
ability of the phosphine-imidazole group. Particular interest for our
group has been to understand the ability of the MBDP ligand to
allow the facile formation of Au(I) and Ag(I) homo- and hetero-
bimetallic complexes that may possess interesting structural and
photophysical properties (see Scheme 1).
ꢁ
1
n
CN stretching peak at ~2250 cm . After seven days of heating the
experiment had to cease as the crystals started to turn dark indi-
cating decomposition of the material.
3.3. Structural studies
The gold complex was prepared by mixing the starting (tht)AuCl
and MDBP ligand in a 1:1 ratio. Direct addition of the ligand to the
gold starting material provided the mononuclear (MBDP)AuCl
species which shows coordination through the P atom of the ligand
with chloride atom intact in the coordination. Although the met-
allacycle coordinated compound 2 was also prepared in a 1:1 ratio,
The details of the X-ray structural feature of the MBDP ligand
were reported recently by our group [38]. The structural feature of
the mononuclear (MDBP)AuCl (1) has been conducted. Although
the details of the x-ray data of this compound has not been final-
ized to our satisfaction essential parameters of the data indicate
Scheme 1. Synthetic scheme of compound 2.

D.E. Jenkins, Z. Assefa / Journal of Molecular Structure 1133 (2017) 374e383
377
that the compound crystallizes in the space group P2
1
/c with unit
Table 1
Crystallographic Data of the MBDP based Au and Ag compounds 2, and 3.
cell parameters of a ¼ 15.650 (3) Å, b ¼ 7.4736 (12) Å, c ¼ 17.493 (3)
ꢀ
Å,
b
¼ 113.462 (5) with Z ¼ 4. The structural feature also indicates
Empirical formula
C22H22AuBF N P(2) C48H48Ag B F N P (3)
4
3
2
2
8
8
2
the lack of any significant aurophilic Au(I)eAu(I) interaction. The
AueP and AueCl distances are roughly comparable to the work of
Catalano and Horner [39] on 1-methylimidazole diphenylphos-
phine ligand.
The X-ray structural feature of complex 2 is shown in Fig. 1. The
crystallographic details are given in Table 1, while selected bond
lengths and angles are shown in Tables 2 and 3, respectively.
Compound 2 crystallizes in a monoclinic system with a space group
Formula weight
Temperature (K)
Crystal system
Space group
Unit cell dimensions
a (Å)
643.17
200 (2)
Monoclinic
1188.24
200 (2)
triclinic
P ꢁ1
P2
1
/n
8.9993 (8)
10.5423 (10)
10.7638 (10)
12.3530 (12)
88.592 (3)
73.097 (3)
84.422 (3)
1334.8 (2)
1
b (Å)
19.6166 (18)
13.4484 (12)
90
c (Å)
ꢀ
a
b
g
( )
b
90
¼ 100.966 (2)
1
of P2 /n and with four molecules present in the unit cell. The
compound features an eight-membered metallacycle ring consist-
ing of two gold, two phosphorus, two nitrogen, and two carbon
atoms. The geometry surrounding the gold is close to linear with a
3
V (Å )
1470.8 (4)
4
1.833
6.427
20345
Z
Density (calc) g/cm3
Absorption coefficient mm
Reflections collected
1.478
0.862
12591
ꢁ
1
ꢀ
PeAueN bond angle of 178.3 . The dinuclear system shows two
gold atoms at a relatively short AueAu distance of ~2.80 Å. The
bond lengths corresponding for the AueP and AueN are 2.228 and
Independent reflections R (int)
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices
4076 (0.0548)
4076/0/291
0.981
R1 ¼ 0.0264,
wR2 ¼ 0.0635
1.225 and ꢁ0.808
4657 (0.0421)
4657/0/320
1.114
0.0337
0.0929
2
.074 Å, respectively, while the bond angles for the PeAueAu, and
ꢀ
ꢀ
NeAueAu are 91.85 and 87.1 , respectively. Fig. 2 shows the
packing diagram of 2 looking along the b-axis. Although the Au
atoms appear to align along this axis in an extended fashion (with
[I > 2sigma(I)]
ꢁ
3
Largest diff. peak and hole eÅ
0.874 and ꢁ0.425
ꢀ
AueAu—Au angle of 173.3 ), the packing diagram clearly indicates
that only dimeric AueAu interactions are dominant. The AueAu
distance between the dimeric units is over 14.7 Å indicating the
absence of any significant intermolecular interaction. Hence, the
complex effectively consists of discreet dimeric units with AueAu
separation of 2.808 (4) Å. This contact is relatively short when
compared with the aurophilic interaction for the majority of the
complexes that show AueAu contacts of ~3.0 Å, albeit interactions
as small as 2.45 are also known in few instances [40]. The unco-
Table 2
Comparison of selected bond distances in compounds 2 and 3.
Compound, 2
(Ǻ)
Compound, 3
(Ǻ)
Au1 e Au2
Au1 e P1
Au1 e N2
2.808 (4)
2.228 (3)
2.074 (4)
Ag1e Ag2
Ag1e P1
Ag1 eN1
Ag1 eN3
Ag1 eN4
P1 e C1
P1 e C8
P1e C12
3.017
2.388
2.226
2.502
2.429
1.821
1.818
1.812
ordinated solvent molecules of CH
3
CN present in the structure are
P1eC1
P1eC9
P1eC15
1.805 (5)
1.814 (5)
1.836 (5)
trapped within the crystal system and occupy the spaces between
the discreet dimeric units (see Fig. 2).
The thermal ellipsoid diagram of the silver analog, 3, is shown in
Fig. 3. The packing diagram shown along the a-axis is presented in
Fig. 4. The structural details are given in Table 1 while a comparison
of the bond distances is shown in Table 2. The colorless complex, 3,
crystallizes in the triclinic space group P-1 with an inversion center
Table 3
Selected bond angles for 2 and 3.
ꢀ
ꢀ
Compound 2
Bond Angle ( )
Compound 3
Bond Angle ( )
N2eAu1eP1
N2eAu1eAu2
P1eAu1eAu2
C15eP1eAu1
C9eP1eAu1
C1eP1eAu1
C6eN2eAu1
C1eN2eAu1
178.31 (11)
87.08 (11)
91.86 (3)
114.81 (17)
112.46 (18)
108.70 (17)
122.80 (3)
130.10 (3)
N1eAg1eP1
N1eAg1eN4
P1eAg1eN4
N1eAg1eN3
P1eAg1eN3
N4eAg1eN3
N1eAg1eAg2
P1eAg1eAg2
N4eAg1eAg2
N3eAg1eAg2
147.39 (6)
99.82 (11)
109.91 (10)
98.85 (10)
96.03 (8)
86.61 (13)
85.63 (6)
80.72 (2)
91.33 (9)
175.33 (8)
midway between the silver atoms, making only half of the complex
to be crystallographiclly unique. The asymmetric unit also contains
ꢁ
one BF
4
anion, and two acetonitrile molecules bound to the silver
atom. Similar to the gold system, compound 3 features the char-
acteristic metallacycle consisting of the two Ag atoms and two
MBDP ligands that close the loop. There are two CH CN molecules
3
attached to each of the silver atoms in the dimeric unit, making a
roughly distorted tetrahedral assembly around the Ag atom. This
suggests that the Ag(I) adduct of this ligand prefers a tetrahedral
geometry as opposed to the linear geometry observed for the Au(I)
system.
The bond angle of the phosphorous, silver, and the imine ni-
trogen deviates significantly from linearity as the N (1)-Ag (1)-P (1)
ꢀ
angle is only 147.39 (6) . The N (1)-Ag (1)-N (3), N (1)-Ag (1)-N (4),
ꢀ
Fig. 1. Thermal ellipsoid diagram of compound 2 at 50% probability.
and P (1)-Ag (1)-N (3) angles are 98.85 (19), 99.82, and 96.85 (16) ,

378
D.E. Jenkins, Z. Assefa / Journal of Molecular Structure 1133 (2017) 374e383
ꢁ
Fig. 2. Packing diagram of the cation portion of compound 2 seen along the b-axis. The BF
solvent molecules of CH CN co crystallized in the lattice are shown.
4
anions that occupy the space between the dimeric units are deleted for clarity while the
3
3.4. Structural comparison
Comparison of the two complexes indicate contrasting geome-
tries in Ag(I) versus Au(I) systems. Accumulated data for silver in-
dicates that in the þ1 oxidation state, Ag(I) adopts a wide variety of
coordination geometries. For example, a 2008 review article indi-
cated that, of all the Ag(I) complexes characterized structurally and
stored in Cambridge data search, the two, three, and four coordi-
nated complexes amount to 24.2, 22.7, and 43.9%, respectively
while the remaining components consist of five, six and even seven
coordinates [45]. This variety of coordination observed for Ag(I) is
in part due to the lack of stereo chemical preference that arises from
10
a d configuration. However, distribution of coordination numbers
10
among the d ions varies with the metal indicating other factors
may also play a significant role in influencing the type of coordi-
nation. For example, the majority of the Au(I) complexes appear in a
linear two-coordinate geometry, while the large majority of Cu(I)
complexes prefer tetrahedral four coordinate geometries [46]. In
contrast, when compared to Au(I) and Cu(I) systems, the structural
geometries in Ag(I) are more evenly distributed among di, tri and
tetrakis geometries [45,46].
Fig. 3. Thermal ellipsoid diagram of the cation portion of compound
probability.
3 at 50%
Several essential attributes have been suggested as causes for
10
the large differences exhibited within the d ions of Cu(I), Ag(I),
and Au(I) systems. Previous studies conducted on homoleptic
copper(I), silver(I), and gold(I) complexes with two bisphosphine
ligands indicated that while the dppb ligand (1,2-
bis(diphenylphosphino)benzene) forms four-coordinate com-
plexes for all three cations [47], the situation with the POP (2-
respectively where N (3) and N (4) correspond to the bonding
CH CN nitrogen. As was the case in the gold system (2), complex 3
3
also forms discreet dimers with an intramolecular AgeAg separa-
tion of 3.017 Å suggesting the existence of intramolecular argen-
tophillic interactions [39e44].
(
[
diphenylphosphino) phenyl]ether) ligand is different. Both
Cu(POP) ] and [Au(POP) ] adopt a three coordinate trigonal planar
2
2
ꢀ
As it is evident from the AgeAg—Ag angle of 160.9 (Fig. 4) the
coordination with one uncoordinated phosphorus atom. Steric
hindrance caused by the large bite angle of the POP ligand was
described as the cause in limiting the coordination on the smaller
Cu(I) and Au(I) ions, while the largest among them, Ag(I), was found
to adapt a tetrahedral geometry.
Based on a theoretical work, Carvajal et al. [46] have concluded
that for the same ligand L the increase in bond distance follows the
order Cu-L < Au-L < Ag-L, which is consistent with the atomic
radius of Au to be smaller than Ag. A similar trend is apparent in the
work reported here also where the AueP bond length of 2.228 Å
discreet dimeric units are not aligned in an extended fashion and
the separation between these units are also about 7.68 Å, albeit half
the value for compound 2. The bond lengths corresponding to the
AgeP, and AgeN (1) are 2.388, 2.226 Å, respectively (N (1) corre-
sponds to the imine N). The interaction of the Ag atom with the
acetonitrile solvent is much weaker, hence the AgeN bond length is
longer by 0.203 Å when compared with that of the imine N. Hence,
the AgeN3, and AgeN4 bond lengths are 2.502, and 2.429 Å,
respectively.

D.E. Jenkins, Z. Assefa / Journal of Molecular Structure 1133 (2017) 374e383
379
ꢁ
Fig. 4. Packing diagram of compound 3, the silver MBDP dimer, where the BF
4
anions are deleted for clarity.
(Table 2) is smaller by 0.16 Å than the 2.388 Å obtained for the
3.5.2. Absorption and photoluminescence studies
AgeP bond. Similarly, the AueN bond is smaller by 0.15 Å when
compared with the AgeN bond length. The weak nature of the
silver-ligand bond [48] and the resulting weaker M-L interactions
and the crystal packing forces were considered to have a greater
influence on the geometry. These observations on Ag contrast the
linear geometry preferred for Au(I) [44,49].
In Fig. 5 is shown the absorption spectrum of the MBDP ligand
compared with that of compounds 1, 2, and 3. The compounds have
some what a similar profile with that of the ligand with only slight
difference between them. A strong band is evident ~200 nm in all
cases. In addition, the compounds have detailed features in the
250e320 nm spectral range. The ligand, compound 1, and 3 show
very similar features in this region with a broad band at ~290 nm
accompanied by a shoulder at 299e301 nm. In addition, a broader
peak ~268 nm is also evident. Compound 2 displays a slightly
different profile in this spectral region where a more defined and
sharper band is dominant at 278 nm with a broad hump around
3
3
.5. Spectroscopic studies
.5.1. NMR and vibrational studies
Several spectroscopic techniques were used to characterize the
complexes including NMR, and IR studies. For the mononuclear
1
compound 1, the H NMR spectrum collected in CDCl
3
has a similar
profile with that of the ligand with peaks at 3.98 (s, 1H), 7.33e7.54
31
1
(
m, 5H), 7.58e7.60 (m, 6H), and 7.70e7.83 (m, 5H). The P{ H}
NMR revealed a single peak at 14.59 ppm.
For compound 2, due to solubility choice the NMR data were
1
collected using the deuterated solvents CD
3
CN or DCM. The H NMR
spectrum has a similar profile with that of the ligand and com-
pound 1. The 31P NMR revealed a single peak at 27.9 ppm which is
significantly different from that of the ligand as well as the mono
31
coordinated gold (I) adduct, 1. The P NMR is ~53.5 ppm down-
field from the ligand and ~13.3 ppm down-field from the mono-
nuclear species, 1.
The Ag(I) complex, 3, was also characterized using 31P and 1
NMR in CD CN and CDCl at room temperature. The P NMR data
3 3
H
31
provided a single broad peak at ꢁ1.66 ppm while using the stan-
31
dard phosphoric acid. Previous reports [9,11,50,51] of P NMR data
for silver phosphine complexes have shown broad peaks similar to
the one found in complex 3. The broad feature is presumably due to
the dynamic behavior of the ligand scrambling reactions. A very
similar NMR behavior of a dinuclear silver complex was published
1
very recently [52]. The H NMR was in a similar range with that of
Fig. 5. Comparison of the absorbance of: a) the MBDP ligand; b) Compound 1; c)
Compound 2; and d) Compound 3. Spectral baseline are shifted for clarity. All spectra
were collected in acetonitrile solvent.
the ligand apart from small shifts. The CH
sharp peak at 2.15 ppm.
3
CN signal appears as a

380
D.E. Jenkins, Z. Assefa / Journal of Molecular Structure 1133 (2017) 374e383
2
94 nm. The molar extinction coefficients in all systems are >
The photoluminescence spectrum of the silver dimer, 3,
4
ꢁ1
ꢁ1
1
0
M
cm , hence the transitions can be assigned to the spin
p / p* within the ligand in all the systems, although the
collected at liquid nitrogen temperature is shown in Fig. 8. The
excitation spectrum was collected while monitoring the emission
at 451 nm. The excitation overall profile, although red shifted to
~320 nm (with a second, smaller broad band at 355 nm) is similar
to that observed for the free ligand (Fig. 6). The emission spectrum
which consists of vibronic components at 423, 449, 475, and 509
allowed
shift and intensity in 2 might indicate some involvement of the
metal orbitals in influencing the electronic transition.
In Fig. 6, is shown the photoluminescence spectra of the ligand.
The blue spectrum corresponds to the excitation profile where the
most intense band maximizes at 311 nm. The emission spectrum,
although highly structured, shows a broad feature centering at
ꢁ1
with an average spacing of ~1330 cm has also a similar profile to
that of the ligand.
~445 nm upon excitation at 311 nm. Several vibronic features are
evident in the 240e580 nm spectral range. The vibronic compo-
3
3
.6. Computational studies
nents are observed at 354, 372, 389, 404, 418, 427, 449, 471, and
ꢁ1
5
09 nm and an average spacing of ~1020 cm . As shown in the
.6.1. Structural comparison theoretical vs experimental
A summary of the bond lengths and bond angles of the ligand
Supplemental section (Suppl-Fig-2) compound 1 shows similar
structured spectral profile to that of the ligand.
are given in Table 4 comparing the theoretical and the single X-ray
crystal structural data of the MBDP ligand. Although the calculated
bond distances are slightly longer, the overall geometry of the
optimized structural data are in good agreement with the experi-
mental values.
Similarly, the gold dimer, 2, has a photoluminescence profile
depicted in Fig. 7. These spectra were collected at liquid nitrogen
temperature. The excitation spectrum collected while monitoring
the emission at 448 nm is shown in the figure (Fig. 7, red). The well-
defined sharper excitation band in the ligand (at 311 nm, Fig. 6)
broadens and maximizes at ~330 nm in the spectrum of 2, while the
longer wavelength region shows several structural features. The
emission spectrum of 2 is less structured compared with that of the
ligand. However, the band maximum centers around 454 nm which
is almost identical in position observed in the free ligand.
3.6.2. Electronic structures of the MBDP ligand
The frequency keyword was used to compute the vibrational
frequencies for the MBDP ligand with no imaginary frequencies
calculated. The GaussView 5 transformed the data to provide an IR
spectrum and also automation of the vibrations. Comparison of the
frequencies to the collected IR shows comparable data. The vibronic
component seen in the luminescence spectra was calculated to
ꢁ1
correspond to a frequency at ~918 cm . A calculated peak from the
theoretical work at 911 cm was animated to show that there is
ꢁ
1
Fig. 6. The PL spectra of the MBDP ligand in the solid state. a) Excitation spectrum
monitored at 445 nm, and b) Emission spectrum upon excitation at 311 nm.
Fig. 8. Excitation (red) and Emission (blue) spectra of the silver complex, 3. Spectra
were collected in the solid state at room temperature. (For interpretation of the ref-
erences to colour in this figure legend, the reader is referred to the web version of this
article.)
Table 4
Comparison of structural features from the X-ray crystal structure and theoretical
data of the MBDP ligand.
X-ray Crystal Data
Theoretical Data
Bond Angle ꢀ( )
C9eP1eC15
C15eP1eC1
C9eP1eC1
N1eC1eP1
N2eC1eP1
Bond Length (Å)
P1eC9
P1eC1
P1eC15
N1eC1
N2eC1
103.70 (8)
99.42 (8)
100.85 (8)
125.23 (15)
122.6 (3)
102.24327
101.86529
101.86318
121.13343
127.16869
1.8304 (19)
1.8321 (19)
1.823 (2)
1.394 (2)
1.312 (2)
1.89147
1.83736
1.89146
1.40198
1.33735
Fig. 7. Excitation (red) spectrum monitored at 448 nm; and emission spectrum excited
at 337 nm (right) of compound 2. Spectra were collected in the solid state at room
temperature. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)

D.E. Jenkins, Z. Assefa / Journal of Molecular Structure 1133 (2017) 374e383
381
Table 5
Partial Molecular Orbital Contributions in the Ground State of compound 1.
Contributions (%) Component (%) Contributions
Orbitals Cl
Au
Ligand
79.37
Cl
Au
Ligand
94
93
92
5
15.63
1.42s, 3.58p
x
11.19s, 3.17p, 3.21p
x y z
3.49s 21.77p 16.32p 37.79p
2.29
97.71
100
2.29s
x y z
11.41s 20.04p 49.71p 16.55p
x y z
11.52s 4.50p 11.7p 72.28p
HOMO-LUMO gap
9
1
69.29
30.7
69.29p
35.29p
z
30.70d
16.54d
90
77.17
16.54
6.28
100
x
41.88p
y
6.28s
89
100p
z
bending and stretching motion within the 1-methylbenzimidazole
ring.
The orbital occupation analysis for this ligand is provided in the
Supplemental section as Supp-Table 1. The HOMO orbital (# 78) has
the largest contribution from the phosphorus atom (P, 53%), with C
employed, it provided useful information while exploring subtle
differences in the structure and electronic contributions to the
frontier orbitals when comparing it to the ligand, MBDP. The ab-
sorption profile of 1 (Fig. 5b) shows well resolved peaks at 199, 275,
and broad shoulders at 226 and 291 nm. The PL spectrum of 1 also
shows structured emission at 414, 429, 453, 483, and 515 nm with
(
38%), and N (9%). The LUMO orbital consists of contributions from
ꢁ1
the P (8%), N (15%), and C (77%). To fully grasp the molecular orbital
contributions, visual representation for the first three LUMO's, and
the three HOMO's are shown by orbital correlation diagrams in the
Suppl-Table 1 (last column). These figures clearly indicate that the
electronic transitions proceed from the benzimidazole portion of
the ligand in the HOMOs to the phenyl rings in the LUMOs. This
an average spacing of 1212 cm . These features, while similar to
the data obtained for the ligand, show slight differences that
necessitated a closer understanding through theoretical work.
The vibronic components (spacing of 1212) in the PL spectrum
ꢁ
1
of 1 can be assigned to the calculated frequency at 1133 cm . When
this peak was animated in the GaussView software package it
displayed bending and stretching from the benzimidazole portion
of the ligand with minor H stretching on the two phenyl rings. No
motion was observed from the PeAueCl bonds. Hence, coupling of
the ligand vibration to the electronic transition can be deduced. We
viewed the excitation energies from the TD-DFT calculations to
obtain the UVeVis spectrum, shown in the Supplemental section as
(Suppl. Fig. 4).
offers support to conceptualize the transition from
p / p*, which
is believed to govern the electronic properties of this ligand.
Further analysis using the TD-DFT data of the ligand revealed
the presence of a total of three excited states contributing to the
calculated theoretical UVeVis band (Suppl-Fig. 3). The first state
corresponds to the electronic transition from orbital 78
(
HOMO) / orbital 79 (LUMO) with total contribution of 98%
calculated from the coefficient of the microstate as defined by
Gaussian [45] (Supp-Table 1). The second excited state consists of
contributions from orbitals 78 (HOMO) / 80 (2nd LUMO) and the
third excited state with contributions from the orbitals 77 (2nd
HOMO) / 79 (LUMO). These singly excited states are within the
range of comparison to the experimental spectrum seen in Fig. 5.
The theoretical calculations of the UVeVis spectrum are consistent
with the experimental spectrum, albeit with little shift. For
example the peak at 263 nm is similar to that of the experimental
It also provided information regarding the electronic transitions
that occurred from the HOMO to the LUMO. The HOMO-LUMO gap,
MO 91 and MO 92 respectively, from the theoretical calculations
ꢁ1
was approximately 40,200 cm
(249 nm). There were three
excited states that should participate in the transition. The first was
between MO 90 to MO 92 with a signal at 275 nm. The next was
from MO 91 to MO 92 with a signal at 275 nm. Finally the third
coming from MO 91 to MO 93 and MO 91 to MO 94 with a signal at
265 nm. The last contains two microstate transitions that
contribute to the 265 nm. The MO 91 to MO 93 microstate contri-
bution is 96% and the MO 91 to MO 94 contributes 3% to the
transition.
4
ꢁ1
ꢁ1
.
272 nm peak. The calculated ε for this peak is > 10 M cm
3.6.3. Electronic structures of the MBDP-gold monomer, 1
Although treatment of 1with a relative lower level of theory was
When compared to the experimental data there is a slight red

382
D.E. Jenkins, Z. Assefa / Journal of Molecular Structure 1133 (2017) 374e383
shift in the excitation energies. This is likely due to calculated gold
participation in these transitions. A better explanation comes from
the MO contribution seen in Table 5. The HOMO orbital 91 has
structure and luminescence studies of dinuclear gold(I) alkynyls of bis(di-
phenylphosphino)alkyl- and aryl-amines, Inorg. Chim. Acta 359 (2006) 3639.
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[
mainly contributions from the Au dxz (30%) and Cl p
the LUMO is totally p-orbital from C and P (p –11.5%; p
–48% remaining percentage given to s-orbital) ligand centered.
This infers a charge transfer feature to the * orbital of the ligand.
z
(60%) whereas
x
y
– 4.5%, and
[
[
p
z
p
The predicted orbital occupation for these transitions can be
seen in the orbital diagrams shown in Table 5 for LUMOþ2,
LUMOþ1, LUMO, HOMO, HOMO-1, HOMO-2 providing pictorial
representation for the electronic transition of the gold complex, 1.
Starting with LUMOþ2 shows the contributions from the Au, Cl,
and phenyl rings. The LUMOþ1 donation comes from the benz-
imidazole and phenyl ring portions of the ligand with small Au
participation. The LUMO is 100% ligand centered whereas the filling
of the HOMO is AueCl centered. The HOMO-1 is composed of
majority AueCl contributions with some contribution also. Finally,
the HOMO-2 is totally ligand centered. It should be noted that a
higher level of theory could provide more details regarding the
Au(I) behavior. However, for the purpose of this work both the
experimental and theoretical studies have provide support for the
conclusion that no metallophilic interaction between either Au(I)
or Ag(I) centers are responsible for the luminescence profiles. These
arise as a result of ligand-centered excitation and emission.
[
6] A.A. Isab, M. Fettouhi, S. Ahmad, L. Ouahab, Mixed ligand gold(I) complexes of
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[(C
a
PCH
phosphorous-sulfur
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H
5
)
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2
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2
3
[
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L
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](2þ) (M ¼ Cu-I; Ag-I) and [AuL2](þ)
[
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. Conclusion
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In this paper the synthesis and structural characterization of the
bridging ligand, 1-methyl benzimidazole diphenylphosphine
MBDP) and its coordination with Au(I) and Ag(I) ions is reported as
well as the luminescence studies and theoretical DFT calculations.
Both metals form binuclear metallacycle complexes with head-to-
tail coordination feature. Both the P and N atoms of the ligand
show a stronger interaction with Au(I) than Ag(I) as evaluated from
the bond distances. The AueP and AueN bonds are shorter by 0.16
and 0.15 Å when compared with the AgeP, and AgeN distances,
(
[
13] V.J. Catalano, A.L. Moore, Mono-, di-, and trinuclear luminescent silver(I) and
gold(I) N-heterocyclic carbene complexes derived from the picolyl-
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imidazolium tetrafluoroborate, Inorg. Chem. 44 (2005) 6558e6566.
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metal complexes with Group 13 diyl ligands ER (E ¼ B-Tl) Part X: theoretical
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3
respectively. The gold adduct 2 contains an uncoordinated CH CN
solvent in the lattice, while two solvent molecules are coordinated
to the Ag(I) in 3. Hence, a linear coordination is exhibited for the
gold in 2, whereas the coordination mode of the metal in 3 reflects a
highly distorted tetrahedral geometry. Both compounds show
structured emission in the ~450 nm region originating from the
[
[
[
[
[
15] W.A. Herrmann, V.P.W. Bohm, C.W.K. Gstottmayr, M. Grosche, C.P. Reisinger,
T. Weskamp, Synthesis, structure and catalytic application of palladium(II)
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ligand based p* LUMO orbital. This assignment is corroborated with
376e384.
the theoretical time dependent DFT calculation. Although a more
direct method such as life-time measurement could have been
useful, we have not performed these experiments due to instru-
mental limitations.
17] M.A. Rawashdeh-Omary, M.A. Omary, J.P. Fackler, R. Galassi, B.R. Pietroni,
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Acknowledgement
(2000) 392e402.
19] V. Peruzzo, C. Pretzsch, F. Tisato, M. Porchia, F. Refosco, C. Marzano, V. Gandin,
Support for this research from National Science Foundation
E. Schiller, M. Walther, H.eJ. Pietzsch, Synthesis and characterization of novel
tetrahedral
aminodiphosphines
copper(I)
and
complexes
tetradentate
comprising
tridentate
PNP-
amino-
(
CHE-0959406) and as well as support from the donors of the Pe-
troleum Research Fund of the American Chemical Society (PRF#
5441-UR3) are kindly acknowledged.
PN(X)P-substituted
diphosphines (X ¼ O, S), Inorg. Chim. Acta 387 (2012) 163e172.
5
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Appendix A. Supplementary data
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silver(I) derivatives with bridging phosphines containing an imidazole ring x-
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2016.11.
70. These data include MOL files and InChiKeys of the most
[
[
[
0
important compounds described in this article.
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