Microwave-assisted synthesis, crystal structures and DFT calculations of two novel silver(I{cyrillic, ukrainian}) dimers [Ag2(μ-X)2(μ-dppm)(PPh3)2] (X = Br, I) with butterfly-shaped dinuclear cores

Article abstract of DOI:10.1016/j.molstruc.2009.04.028

Reactions of Ag₄X₄(Ph₃P)₄ (X = Br, I) with 1,3-bis(diphenylphosphine)methane (dppm) ligand under microwave heating conditions afforded two asymmetric silver(I) dimers of the type [Ag₂(μ-X)₂

Full text of DOI:10.1016/j.molstruc.2009.04.028

Journal of Molecular Structure 930 (2009) 9–14
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Microwave-assisted synthesis, crystal structures and DFT calculations of two
novel silver(I) dimers [Ag2(
dinuclear cores
l-X)2(
l-dppm)(PPh3)2] (X = Br, I) with butterfly-shaped
a
Geng-Geng Luo a, Rui-Bo Wu a,b, Di Sun a, Jian-Hua Chen a, Na Zhang a, Rong-Bin Huang a, , Li-Rong Lin ,
*
Lan-Sun Zheng a,b
a Department of Chemistry, Xiamen University, Xiamen 361005, People’s Republic of China
b State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Reactions of Ag4X4(Ph3P)4 (X = Br, I) with 1,3-bis(diphenylphosphine)methane (dppm) ligand under
microwave heating conditions afforded two asymmetric silver(I) dimers of the type [Ag2(l-X)2(l-
dppm)(PPh3)2], X = Br(1), I(2), with butterfly-shaped dinuclear cores. The molecular structures of two
compounds have been established by single-crystal X-ray diffraction and satisfactorily calculated by
using the PW91 functional considered with the DNP basis set and ECP.
Received 22 March 2009
Received in revised form 15 April 2009
Accepted 17 April 2009
Available online 3 May 2009
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Crystal structure
Silver
Butterfly-shaped
Asymmetric dimer
Microwave synthesis
DFT calculation
1. Introduction
and antibacterial properties [13,14]. In a previous work, we re-
ported an alternative route for the cubane-like compounds
Microwave-assisted synthesis [1] may dramatically reduce
reaction times and this simple and energy-efficient heating process
has recently become a rapidly developing synthetic method. This
technique is widely used in analytical chemistry [2], liquid-phase
organic syntheses [3], solid-state reactions [4], organic dry-media
reactions [5], intercalation reaction [6], and has also been found
advantageous for the preparation of inorganic hybrid materials
including many zeolites, molecular sieves and nano-sized particles
[7,8]. Despite the range of applications and advantages, the micro-
wave method which can be applied to yield crystals large enough
for single crystal X-ray analysis have not yet received great atten-
tion [9]. Acyclic diphosphines of the type Ph2P(CH2)nPPh2 known to
form a bridging bonding mode and readily lock together two or
more metal centers have been frequently used in the presence of
low oxidation state transition metal centers. Numerous silver(I)
compounds containing these diphosphines have been reported
[10,11]. These silver–phosphorus compounds have received con-
siderable attention over the past few years as a result of their
important applications in several homogeneous catalytic processes
[12], as well as their potential antitumor activity, and antifungal
Ag4X4(Ph3P)4 (X = Br, I) [15]. In the present contribution, we utilize
1,3-bis(diphenylphosphine)methane (dppm) as a bridging ligand
to react with Ag4X4(Ph3P)4 (X = Br, I) via microwave-assisted
methodology resulting in the formation of two novel dinuclear
silver(I)–phosphorus compounds with the general formula
[Ag2(l-X)2(l-dppm)(PPh3)2], where X = Br(1), I(2) (Scheme S1).
Both silver(I) compounds structurally feature novel butterfly-
shaped dinuclear cores. In addition to their structural characteriza-
tion, PW91 functional was considered using the DNP basis set and ECP
computational models for these compounds. The calculated struc-
tures provide a satisfactory description of these two title compounds.
2. Experimental
2.1. Materials and methods
All chemicals and solvents used in the syntheses were analytical
grade and used without further purification. Infrared spectra were
recorded with a Nicolet AVATAR FT-IR 360 spectrometer using the
KBr pellet technique. Elemental analyses were carried out on a CE
instruments EA 1110 elemental analyzer. The cubane-like com-
pounds Ag4Br4(Ph3P)4 and Ag4I4(Ph3P)4 were synthesized as re-
ported in the literature [15].
* Corresponding author.
E-mail address: rbhuang@xmu.edu.cn (R.-B. Huang).
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.04.028

10
G.-G. Luo et al. / Journal of Molecular Structure 930 (2009) 9–14
2.2. Synthesis of 1 and 2
generation of cubane-like structure of formula Ag4X4(Ph3P)4
(X = Cl, Br, I) as by-product which enabled an alternative route to
cubane-like compounds. A tentative route for the self-assembly
reaction was proposed therein under the so-called unit-construc-
tion method, the formation of Ag4X4(Ph3P)4 thought to undergo di-
meric assembly. Actually, in the present work, the synthesis of the
two title compounds was achieved under microwave heating con-
ditions, and in doing so, we suggest an alternative method for the
synthesis of silver(I)–phosphorus compounds. The molecular
structures of 1 and 2 are depicted in Fig. 1a and b, respectively. Se-
lected bond distances and angles with its estimated standard devi-
ations are summarized in Tables 2 and 3.
1:2 mixture of Ag4Br4(Ph3P)4 (180 mg, 0.1 mmol) and dppm
(77 mg, 0.2 mmol) was dissolved in ethanol and DMF (v/v: 4/1)
and placed in a sealed glass tube, which was then inserted into the
cavity of a microwave reactor. The reaction mixture was heated to
T = 373 K, power = 200 W, and pressure = 110 psi and maintained
at this conditions for a total of 15 min. After cooling (ca. 20 min),
the solution was filtered and allowed to stand in the darkness.
Approximately one week later, small colorless crystals of 1 were ob-
tained in 40% yield. Anal. Calcd. for C61H52Ag2Br2P4: C, 57.04, H, 4.08;
Found: C, 57.23, H, 4.41. Main IR peaks (KBr, cmꢀ1):
3044(w); (CH2) 2912(m), 2846(m); (P–Cph) 1093(m). The synthe-
t(Cph–H)
t
t
The crystal structure of 1 has shown that this compound exists as
sis of 2 was similar to that of 1, using Ag4I4(Ph3P)4 (200 mg,
0.1 mmol) in place of Ag4Br4(Ph3P)4. Colorless crystals were ob-
tained in the same way in 45% yield. Anal. Calcd. for C122H104Ag4I4P8:
C, 53.15, H, 3.80; Found: C, 52.72, H, 3.61. Main IR peaks (KBr, cmꢀ1):
an asymmetric dimer [Ag2(
l-Br)2(l-dppm)(PPh3)2], with a butter-
fly-shaped {Ag2( -Br)2} unit (Fig. 1a). Each silver atom is four-coor-
l
dinated by forming bonds with one P atom of one terminal
triphenylphosphine ligand, two doubly bridging Br atoms and one
bridging dppm ligand. The Ag–Br bond distances are in the range,
2.712(1)–2.814(1) Å with an average value of 2.758 Å. This average
value is very similar to the average ones of 2.738 and 2.780 Å,
t(Cph-H) 3049(w);
t(CH2) 2915(m), 2848(m); t(P-Cph) 1095(m).
2.3. X-ray crystallography
encounted
in
two
other
Ag(I)
dimeric
compounds
Data collections were performed on Bruker SMART Apex CCD
diffractometer with graphite monochromated Mo K radiation at
Ag2Br2(PPh3)4ꢁ2CHCl3 [17] and Ag2Br2(PPh3)2(pymtH)2 [18], for
which all the Br atoms are bridging. The four Ag–P bond distances
ranging from 2.430(1) to 2.452(1) Å give an average value of
2.444 Å, whichis very close to theestimated one of 2.44 Å for thesin-
gle Ag–P bond (1.34 Å for Ag and 1.10 Å for P) [19]. This is also similar
to the average Ag–P bond distance of 2.428 Å found in the cubane-
a
173(2) K for 1 and 2. Absorption corrections were applied by using
the multi-scan program SADABS [16a]. Structural solutions and
full-matrix least-square refinements based on F2 were performed
with the SHELXS-97 [16b] and SHELXL-97 [16c] program packages,
respectively. All the non-hydrogen atoms were refined anisotropi-
cally. The positions of all hydrogen atoms were calculated and in-
cluded in the refinement according to the riding model
approximation. Crystal data as well as details of data collection
and refinement for the compounds 1–2 are summarized in Table 1.
3. Results and discussion
3.1. Synthesis and crystal structures of compounds 1–2
In a previous paper [15], we reported on the synthesis of one
heterometal tetranuclear palladium–mercury complex with the
Table 1
Crystallographic data for compounds 1 and 2.
Compounds
1
2
Formula
Mr
Crystal system
Space group
a (Å)
Ag2C61H52Br2P4
1284.47
Monoclinic
P21/n
10.7123(2)
23.4818(5)
22.3954(4)
90
96.960(2)
90
4
5591.9(2)
1.526
2.281
Ag4C122H104I4P8
2756.89
Triclinic
P1
11.6921(3)
21.0029(5)
23.2213(6)
77.109(2)
89.157(2)
86.845(2)
2
b (Å)
c (Å)
a
(deg)
b (deg)
c
(deg)
Z
V (Å3)
5550.2(2)
1.650
1.971
Dc (g cmꢀ3
)
l
(mmꢀ1
)
F(0 0 0)
2568
2712
No. of unique reflns
No. of obsd reflns [I > 2
Parameters
10794
(I)] 5285
21633
12824
1243
r
622
GOF
0.815
0.779
Final R indices [I > 2
R indices (all data)
r
(I)]a,b
R1 = 0.0357, wR2 = 0.0670 R1 = 0.0305, wR2 = 0.0370
R1 = 0.0983, wR2 = 0.0793 R1 = 0.0743, wR2 = 0.0425
Largest difference peak and 1.384 and ꢀ0.441
0.503 and ꢀ0.503
hole (e Åꢀ3
)
Fig. 1. (a) Structure of [Ag2(
scheme. All hydrogen atoms have been omitted for clarity. Ellipsoids enclose 30% of
the electron density. (b) Structure of [Ag2( -I)2( -dppm)(PPh3)2]2 (2), with partial
numbering scheme. All hydrogen atoms have been omitted for clarity. Ellipsoids
enclose 30% of the electron density.
l-Br)2(l-dppm)(PPh3)2] (1), with partial numbering
P
P
a
R1
¼
kFoj ꢀ jFck= jFoj.
l
l
ꢂ
ꢃ
0:5
ꢀ
ꢁ
ꢀ
ꢁ
2
2
P
P
F2o ꢀ F2c
=
w
Fo2
.
b
wR2
¼
w

G.-G. Luo et al. / Journal of Molecular Structure 930 (2009) 9–14
11
Table 2
each Ag atom is in a distorted tetrahedral geometry. The Ag–P bond
distances which are in a narrow range of 2.455(1) and 2.4726(9) Å
are within the limits of the value range expected for tetrahedrally
coordinated Ag(I) [24]. The average Ag–I distance (2.899 Å) in 2 is
slightly shorter than that in [Ag3(dppm)2I2]I (2.973(9) Å) [25]. The
Selected bond distances [Å] and angles [°] for compound 1.
Ag(1)–P(1)
Ag(1)–Br(2)
Ag(2)–Br(1)
2.442(1)
2.8135(7)
2.721(1)
Ag(1)–P(3)
Ag(2)–P(2)
Ag(2)–Br(2)
2.430(1)
2.451(1)
2.784(1)
Ag(1)–Br(1)
Ag(2)–P(4)
Ag(1)–Ag(2)
2.7125(5)
2.452(1)
3.264(4)
I–Ag–I angles of central cores, Ag(l-X)Ag of 2, lie in the range of
106.82(1)–109.26(1)° while P–Ag–P angles are in a close range of
120.93(4)–121.55(4)° and are largest ones.
It should be noted that compound 2 also displays a novel but-
terfly-shaped unit adopted by iodine-bridged dimeric compound.
The I(1)–Ag(1)–Ag(2)–I(2) dihedral angle (or I(3)–Ag(3)–Ag(4)–
I(4) dihedral angle) is 34.02(3)° (or 35.61(3)°). Indeed, a search
among the Ag(I) compounds structurally characterized and con-
P(1)–Ag(1)–P(3)
126.67(4)
106.34(3)
98.97(3)
132.70(4)
109.54(3)
102.25(4)
73.84(1)
P(3)–Ag(1)–Br(1)
P(3)–Ag(1)–Br(2)
Br(1)–Ag(1)–Br(2)
P(2)–Ag(2)–Br(1)
P(2)–Ag(2)–Br(2)
Br(1)–Ag(2)–Br(2)
Ag(1)–Br(2)–Ag(2)
116.11(3)
103.50(3)
100.20(2)
109.33(3)
95.93(3)
P(1)–Ag(1)–Br(1)
P(1)–Ag(1)–Br(2)
P(2)–Ag(2)–P(4)
P(4)–Ag(2)–Br(1)
P(4)–Ag(2)–Br(2)
Ag(1)–Br(1)–Ag(2)
100.73(2)
71.34(1)
taining the P2Ag(l-X)AgP2 motif shows that in contrast to the crys-
tal structure described in the present paper, all binuclear Ag
Table 3
Selected bond distances [Å] and angles [°] for compound 2.
compounds reported so far (more than 30 examples) feature planar
Ag(1)–P(1)
Ag(1)–I(2)
Ag(2)–I(1)
Ag(3)–P(7)
Ag(4)–P(6)
Ag(4)–I(4)
2.4669(9)
2.8814(4)
2.9127(4)
2.455(1)
2.471(1)
2.9490(4)
Ag(1)–P(3)
Ag(2)–P(2)
Ag(2)–I(2)
Ag(3)–I(3)
Ag(4)–P(8)
Ag(1)–Ag(2)
2.456(1)
2.4726(9)
2.8666(4)
2.8742(4)
2.464(1)
3.0899(4)
Ag(1)–I(1)
Ag(2)–P(4)
Ag(3)–P(5)
Ag(3)–I(4)
Ag(4)–I(3)
Ag(3)–Ag(4)
2.9741(4)
2.462(1)
2.459(1)
2.9297(4)
2.8359(4)
3.1010(4)
Ag2(l-X)2 cores. Interestingly, the Agꢁ ꢁ ꢁAg distances (3.095 Å aver-
age) of 2 are slightly shorter than the above discussed Br analog of
1, showing the Agꢁ ꢁ ꢁAg distances are not only influenced by dppm
ligand but also affected by I atoms, which shows the best agree-
ment with the calculated result based on DFT.
P(7)–Ag(3)–P(5)
P(5)–Ag(3)–I(3)
P(5)–Ag(3)–I(4)
P(8)–Ag(4)–P(6)
P(6)–Ag(4)–I(3)
P(6)–Ag(4)–I(4)
P(1)–Ag(1)–P(3)
P(1)–Ag(1)–I(2)
P(1)–Ag(1)–I(1)
P(4)–Ag(2)–P(2)
P(2)–Ag(2)–I(2)
P(2)–Ag(2)–I(1)
Ag(1)–I(1)–Ag(2)
Ag(3)–I(3)–Ag(4)
121.55(4)
108.84(3)
100.55(2)
125.93(4)
107.30(2)
101.39(3)
126.47(3)
111.55(3)
99.64(2)
120.93(4)
107.19(2)
98.80(2)
63.31(1)
65.78(1)
P(7)–Ag(3)–I(3)
112.54(3)
104.80(3)
106.82(1)
110.87(3)
102.14(3)
107.32(1)
106.48(2)
103.57(3)
107.18(1)
111.54(2)
108.06(3)
109.26(1)
65.04(1)
P(7)–Ag(3)–I(4)
I(3)–Ag(3)–I(4)
P(8)–Ag(4)–I(3)
P(8)–Ag(4)–I(4)
I(3)–Ag(4)–I(4)
P(3)–Ag(1)–I(2)
P(3)–Ag(1)–I(1)
I(2)–Ag(1)–I(1)
P(4)–Ag(2)–I(2)
P(4)–Ag(2)–I(1)
I(2)–Ag(2)–I(1)
Ag(1)–I(2)–Ag(2)
Ag(3)–I(4)–Ag(4)
3.2. DFT calculations of compounds 1–2
Geometry optimizations and NBO analysis were carried out on
the model systems for compound 1, 2 and model 3 (Fig. 2). All
these calculations were performed with Gaussian 03 [26] and
DMol3 module in Material Studio programs [27] at the density
functional theory (DFT) level. In order to match the experimental
structural parameters and considering the expensive computa-
tional cost, several theoretical methods have been considered: (a)
PW91 functional considered with the DNP basis set and ECP for
Ag and Br/I atoms in the computational models. Table 4 presents
the significant structural parameters of compound 2 followed by
all these methods. (b) The generalized gradient corrected (GGA)
functional PW91[31] considered with the all-electron double
numerical basis set plus polarization functions (DNP) and density
functional semi-core pseudo-potentials (DSPP) [32] for the calcula-
tion models. (c) B3LYP functional [28] combined the 6-31G(d,p)
basis set for C, H, O and P atoms, and the LANL2DZ [29] valence ba-
sis set and effective core potentials (ECP) [30] for Ag and Br/I
atoms. It shows that the method (a) is the best description, so all
geometry optimizations for these three models were considered
with this method, and the minimal structures were also verified
by frequency analysis at the same level. All of the main structural
parameters were summarized in Table 4.
As shown in Table 4, both the Ag–Ag distances and the main
bonds of binuclear Ag compounds are very close to experiment
data with PW91 combined with DNP basis set and ECP, and the
main bond angles and dihedral angles are also considerable close.
As such, it demonstrates that for this type of substances, this meth-
od yields reasonably good agreements with the experimental
geometries. The geometric structures of compound 1 and model
3 are presented in Fig. 2. As shown in Fig. 3(a–c), the four P atoms
and two Ag atoms appear almost in the same plane, which is in
accordance with experimental structure. Interestingly, the two-
double conformation of two bromine/iodine atoms is conserved
in the calculation both for compounds 1 and 2. It should be noted
that as indicated in Fig. 3(d and e), when a phenyl group is replaced
by a methyl (model 3), the dihedral angle of both Br – the ring
plane and P3 – the ring plane apparently shows that these atoms
no longer are in a plane. So the big phenyl rings coordinate shell
of P atoms make crucial contribution to keep this plane structural
characteristic.
63.67(1)
like structure of Ag4Br4(PPh3)4 [20]. Bond angles around the two Ag
atoms are between 95.93(3) and 132.70(4)°, the largest value of
which corresponds to the P–Ag–P angle. Both of these bond angles
fit quite closely to the ranges observed in Ag4Br4(PPh3)4 [20]. The
Agꢁ ꢁ ꢁAg distance of 3.264(4) Å in 1 is considerably longer than that
in metallic silver (2.88 Å) [21], but shorter than the sum of van der
Waal’ radii of Ag (rvdw(Ag) = 1.70 Å) [22] and thus, is suggestive of
an intramolecular weak interaction between the two metal silver
centers. To our surprise, unlike the remarkable planarity with in
the Ag2Br2 moiety of the recently reported Ag2Br2(PPh3)2(pymtH)2
[18], the key structural feature of 1 is a butterfly-shaped unit
adopted by bromine-bridged. The dihedral angle between the
Br(2)–Ag(1)–Ag(2) plane and the plane of Ag(1)–Ag(2)–Br(1) is
35.00(2)°. An interesting crystal packing behavior for 1 is that the
bridging Br atoms act as hydrogen bond acceptors in weak
C(phenyl)–Hꢁ ꢁ ꢁBr hydrogen bonds involving the H atoms of phenyl
groups from dppm (Fig. S1). Their geometrical characteristic
(d(HꢁꢁꢁBr) = 2.882 Å, and \(C–HꢁꢁꢁBr) = 144.16°) are in the expected
range. These weak hydrogen-bonding interactions may be very
important to stabilize the molecules in crystalline state [23].
Unlike 1 crystallizing as monoclinic crystals in space group P21/
n, the crystals of 2 conform to the triclinic space group P1. Its struc-
ture shows the occurrence of two nearly identical dimers within
the asymmetric unit. The two independent dimers are further
linked via C–H(Ph)ꢁ ꢁ ꢁ
p
interactions (C64–Hꢁ ꢁ ꢁHC28, C51–Hꢁ ꢁ ꢁH–
C38) between phenyl rings of PPh3 and phenyl rings of dppm li-
gand leading to the formation of a tetramer C–H–Ph ring. The
molecular structure of each dimer is typical of binuclear silver(I)
system which is similar to that seen in the previous structure of
1. In each dimer, two Ag(I) ions are bridged by two Iꢀ ions and
one dppm ligand, respectively. Coordinated by PPh3 additionally,
An elaborate NBO analysis [33] of compound 1, compound 2
and model 3 was carried out. It shows that compound 2 has 263

12
G.-G. Luo et al. / Journal of Molecular Structure 930 (2009) 9–14
Fig. 2. The selected computational models.
Table 4
Structural parameters (distances [Å] and angles [°]) for compounds 1, 2, and model 3.
Structural parameters
Compound 1
Compound 2
Model 3
Exp.
Cal.a
3.336
2.482
2.506
2.698
2.626
Exp.
Cal.a
3.100
2.453
2.504
2.902
2.854
115.3
106.6
64.9
Cal.b
3.444
2.502
2.525
2.895
2.827
106.0
111.3
73.1
Cal.c
3.465
2.579
2.759
2.994
2.901
104.2
99.9
71.5
Cal.a
Ag1–Ag2
Ag1–P1
Ag1–P3
Ag1–Br1/I1
Ag1–Br2/I2
P1–Ag1–P3
P1–Ag1–Br1/I1
Ag1–Br1/I1–Ag2
Br1/I1–(P1–P2–Ag1)
Br2/I2–(P1–P2–Ag1)
P3–(P1–P2–Ag1)
3.264
2.442
2.430
2.712
2.813
126.7
106.3
73.8
3.090
2.467
2.456
2.974
2.881
126.5
99.6
3.300
2.463
2.442
2.665
2.614
106.2
137.7
76.6
114.4
104.7
77.9
ꢀ54.6
54.4
2.3
63.3
ꢀ53.7
ꢀ58.9
ꢀ55.9
ꢀ50.9
ꢀ51.9
ꢀ24.6
49.8
54.0
55.4
50.0
50.3
49.5
15.8
ꢀ6.2
ꢀ3.1
ꢀ13.6
ꢀ13.8
ꢀ33.7
a
PW91: DNP + ECP.
PW91: DNP + DSPP.
B3LYP: 6-31G(d,p) + LANL2DZ.
b
c
occupied MOs. The natural charge shown in our NBO analysis gives
the calculated net charge in compound 2 for Ag1, Ag2, I1, I2, P1 and
P3 are 0.66, 0.67, ꢀ0.52, ꢀ0.47, 1.03 and 1.27e, respectively. And a
similar charge distribution is observed in compound 1, but the
charge on Ag1 and Ag2 increases a little (0.75 and 0.73e, respec-
tively). It apparently shows that the stronger electronegativity of
Br may result in more electron biased transfer to Br atom. As a re-
sult, the Ag1–Br1–Ag2 angle in 1 would become larger than the cor-
responding Ag1–I1–Ag2 in 2. This characteristic is consistent with
the calculated result presented in Table 4. For model 3, the calcu-
lation shows that negative charge of Br1 and Br2 in model 3 is
ꢀ0.39 and ꢀ0.31, respectively, which is much lower than these
in 1.
bonds (Ag1–I1/I2, Ag1–P1/P2) which is shown in Table 4. Base on
the second order perturbation theory analysis of fock matrix in
NBO basis, we find that three P3–C bonds can stabilize the Ag1–
P3 bond about 35.3 kcal/mol totally, which comes from the charge
transfer effect. This effect is also held between Ag–P1 bond and
two P1–C bonds, but due to the accumulative result, Ag1–P3 bond
is stronger than Ag1–P1 bond. Similarly this charge shift effect
was also observed in compound 1, total bond order in Ag1 atom
of compound
1 is composed by Br1(0.1847), Br2(0.1661),
P1(0.3251), P2(0.2669), Ag2(0.0448). On the other hand, the total
bond composition of Ag1 atom in model 3 is 0.3331 with P1
and 0.3783 with P3. The three P3–C bonds in this compound sta-
bilize the Ag1–P3 bond by 15.7 kcal/mol. The HOMO and LUMO
are localized essentially over the ligand benzene ring and just a
little on the metallic center (Fig. S2). Considering the electron-
deficient environment around binuclear Ag–Br–Ag core in these
Also, NBO analysis shows the total bond order in Ag1 atom of 2
about 1.0, including respectively 0.2072 with I1, 0.2231 with I2,
0.3183 with P1, 0.2563 with P3, and only 0.0959 with Ag2. It obvi-
ously proves the bond length difference of these metal–ligand
three models, it is possible that
p electrons are advantageous to

G.-G. Luo et al. / Journal of Molecular Structure 930 (2009) 9–14
13
Fig. 3. The geometry structure of compound 1 (a–c) and model 3 (d and e). See from different view.
strengthen the Ag–P bonds in this kind of compounds. As we ob-
Acknowledgments
served from the bond order analysis, the Ag–Ag interaction is
very weak, and there is hardly overlap existence between the
two Ag atoms in all of the 263 orbitals. The further theoretical
calculations are required to understand the details about the nat-
ure of donor–acceptor interactions in this kind of unfamiliar com-
plexes, which is beyond current work.
This work was financially supported by the National Natural
Science Foundation of China (No. 20721001) and 973 Project
(Grant 2007CB815301) from MSTC. We also thank Chinese Govern-
ment Scholarship Programs.
In summary, under microwave heating conditions, we have pre-
pared and characterized two silver(I)–phosphorus compounds
which structurally feature novel butterfly-shaped dinuclear cores.
Additionally, the calculated structures based on DFT provide a sat-
isfactory description of these two compounds.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2009.04.028.
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