Silver(I) bromide complexes of the rigid diphosphanes 1,2-bis(diphenylphosphano)benzene (dppbz) and 4,5-bis(diphenylphosphano)-9,9-dimethyl-xanthene (xantphos): Crystal structures of [Ag(μ2-Br)(dppbz)]2, [AgBr(xantphos)] and [AgBr(xa

Article abstract of DOI:10.1016/j.poly.2006.12.006

Reaction of silver(I) bromide with equimolar amounts of the rigid diphos ligands 1,2-bis(diphenylphosphano)benzene (dppbz) and 4,5-bis(diphenylphosphano)-9,9-dimethyl-xanthene (xantphos) in acetone and acetonitrile led to the corresponding chelates [Ag(μ<

Full text of DOI:10.1016/j.poly.2006.12.006

Polyhedron 26 (2007) 1634–1642
www.elsevier.com/locate/poly
Silver(I) bromide complexes of the rigid diphosphanes
1,2-bis(diphenylphosphano)benzene (dppbz)
and 4,5-bis(diphenylphosphano)-9,9-dimethyl-xanthene (xantphos):
Crystal structures of [Ag(l2-Br)(dppbz)]2, [AgBr(xantphos)]
and [AgBr(xantphos)(py2SH)]
a
b
c,
*
A. Kaltzoglou , P.J. Cox , P. Aslanidis
a
Department Chemie, Technische Universita¨t Mu¨nchen, Lichtenbergstr., 4, D-85747 Garching, Germany
School of Pharmacy, The Robert Gordon University, Schoolhill, Aberdeen AB 10 1FR, Scotland, United Kingdom
Aristotle University of Thessaloniki, Faculty of Chemistry, Inorganic Chemistry Laboratory, P.O. Box 135, GR-541 24 Thessaloniki, Greece
b
c
Received 26 September 2006; accepted 1 December 2006
Available online 14 December 2006
Abstract
Reaction of silver(I) bromide with equimolar amounts of the rigid diphos ligands 1,2-bis(diphenylphosphano)benzene (dppbz) and
4,5-bis(diphenylphosphano)-9,9-dimethyl-xanthene (xantphos) in acetone and acetonitrile led to the corresponding chelates [Ag(l2-
Br)(dppbz)]2 (1) and [AgBr(xantphos)] (2). Treatment of 1 and 2 with pyridine-2-thione (py2SH) in ethanol gave the mixed-ligand com-
plexes [AgBr(dppbz)(py2SH)] (3) and [AgBr(xantphos)(py2SH)] (4), respectively. Compounds 1, 2 and 4 have been characterized by
X-ray diffraction, establishing distorted tetrahedral or trigonal planar coordination geometries of the silver atoms.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Silver(I) complexes; Rigid diphosphanes; Pyridine-2-thione; Crystal structures
1. Introduction
metal centre. Silver(I) can exhibit coordination numbers
ranging from 2 to 4 with the preference for a given coordi-
nation number strongly depending on the nature of the
ligands. In the specific case of oligomethylene-backboned
diphosphanes of the type Ph2P(CH2)nPPh2, for instance,
the coordination behaviour (chelating or bridging mode)
primarily depends on the length of the methylene chain
between the two P donors, leading to an extensive and var-
ied family of compounds. Thus, for a given metal salt:
diphosphane stoichiometry, the adducts formed may be
monomeric chelated species, macrocyclic dimers in which
the two metal centres are linked together by both bridging
diphosphane and halide ligands, or even extended poly-
meric formations [5].
Phosphanes are particularly appropriate for stabilizing
low-valent metals ions, consequently there are numerous
diphosphane containing silver(I) complexes [1]. In recent
years, these complexes have attracted considerable atten-
tion because of their applications in several homogeneous
catalytic processes [2]. Some of these complexes have also
shown antitumour activity, as well as antifungal and anti-
bacterial properties [3,4].
It is well known that variation in coordination number
and nuclearity in complexes of closed-shell d10 metal ions
can be induced by variation of the ligands bound to the
Besides the chain length of a bridging diphosphane, its
bulkiness and flexibility have been recognized as additional
significant factors in controlling product selectivity in the
*
Corresponding author.
E-mail addresses: p.j.cox@rgu.ac.uk (P.J. Cox), aslanidi@chem.
auth.gr (P. Aslanidis).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.12.006

A. Kaltzoglou et al. / Polyhedron 26 (2007) 1634–1642
1635
formation of a multisilver complex. For instance, dinuclear
(2:2) macrocycles are the most frequently encountered
structural motifs for silver(I) salts with oxo-anions when
long-chained flexible diphosphanes are used, whereby the
rigid bis(diphenylphosphano)acetylene (dppa) selectively
forms cationic disilver complexes with triply-bridging
diphosphane units [6]. In this context, previous related
work from our laboratory has also revealed the variety of
structural motifs observed within a series of thione/phos-
phane ligated derivatives of Ag(I) halides or Ag(I) nitrate
to be dependent on the choice of the bidentate phosphane
ligand [7].
NMR spectra were recorded on a Brucker AM 300 spec-
trometer at 25 ꢁC with chemical shifts given in ppm refer-
enced to internal TMS. Melting points were measured in
open tubes with a STUART scientific instrument and are
uncorrected. Molar conductivities, magnetic susceptibility
measurements and elemental analyses for carbon, nitrogen
and hydrogen were performed as described previously [11].
Thermogravimetric analyses were performed on a SET-
SYS-1200 machine with a DTG facility in dry nitrogen
atmosphere. Sample sizes were in the range 8–10 mg and
open Pt crucibles were used. The heating rate was 10ꢁ/min.
In the quest for new materials with specialized catalytic
properties, investigations are directed at present towards
the development of new fine-tuned ligands for catalytic sys-
tems with suitable activity and selectivity. For example, the
use of sterically demanding diphosphanes as ligands in tran-
sition metal catalysed synthesis of chiral organic com-
pounds is becoming a very intense area of study [8]. The
P–M–P bite angle has been established to be an important
parameter related to the properties of the catalysts. Con-
cerning the later bite angle effect, considerable progress
has been made since the development of new diphos ligands
based on xanthene-type backbones [9]. In the course of our
systematic structural studies on mixed-ligand coordination
compounds of group 11 metals, we recently reported on
the coordination capability of two sterically demanding
and configurationally inflexible diphosphanes, namely 4,5-
bis(diphenylphosphano)-9,9-dimethylxanthene (xantphos),
the parent member of the extended xantphos ligand family,
and 1,2-bis(diphenylphosphano)benzene (dppbz) towards
copper(I) halides in the presence of heterocyclic thiones as
co-ligands [10]. As these two diphosphanes, based on rigid
aromatic backbones, produced different products to those
of their extensively studied flexible counterparts, we decided
to extend our studies to the analogous silver(I) halide com-
plexes, starting in the present paper with the structural char-
acterization of some derivatives of silver(I) bromide. In
addition, these comparative studies could provide useful
information regarding the dimerization frequently observed
within this class of complexes.
2.2. X-ray crystallographic study
Single crystals of 1 and 2 suitable for X-ray crystallo-
graphic study were grown by slow concentration over sev-
eral days at ambient temperature of their acetone and
acetonitrile mother liquid, respectively, whereas compound
4 was obtained from an ethanolic solution. X-ray diffrac-
tion data were collected on an Enraf-Nonius Kappa
CCD area-detector diffractometer.
The programs DENZO [12] and COLLECT [13] were used in
data collection and cell refinement. Details of crystal and
structure refinement are shown in Table 1. The structures
were solved using the program SIR 97 [14] and refined with
the program SHELX-97 [15]. Molecular plots were obtained
with the program ORTEP-3 [16].
2.3. Synthesis of compounds 1 and 2
The homoleptic complexes were prepared according to
the following general procedure. A suspension of silver
bromide (47 mg, 0.25 mmol) and 0.25 mmol of the appro-
priate diphosphane (111.6 mg for dppbz or 144.7 mg for
xantphos) in 60 cm3 acetone or acetonitrile was refluxed
for 2 h. The resulting mixture was filtered off and the clear
solution was kept at room temperature. Slow evaporation
of the solvent at room temperature gave the microcrystal-
line solid, which was filtered off and dried in vacuo.
2.4. [Ag(l-Br)(dppbz)]2 (1)
2. Experimental
Colourless crystals. Yield: 135 mg (85%), m.p. 298 ꢁC;
Anal. Calc. for C60H48Ag2Br2P4: C, 56.81; H, 3.81. Found:
C, 56.69; H, 3.91%. IR (cmꢀ1): 3468m, 3051w, 1714s,
2.1. Materials and instruments
Commercially available silver bromide, 1,2-bis(diphe-
nylphosphano)benzene (dppbz), 4,5-bis(diphenylphosp-
hano)-9,9-dimethyl-xanthene (xantphos) were used as
received while pyridine-2-thione (py2SH) was recrystallized
from hot ethanol prior to use (Scheme 1). All the solvents
were purified by their respective suitable methods and
allowed to stand over molecular sieves. Infra-red spectra
in the region of 4000–250 cmꢀ1 were obtained in KBr discs
with a Perkin-Elmer FT-IR Spectrum 1 spectrophotome-
ter, while a Perkin-Elmer-Hitachi 200 spectrophotometer
N
H
S
O
Ph2P
PPh2
PPh2
PPh2
xantphos
dppbz
py2SH
Scheme 1. The diphosphanes and the heterocyclic thione used as ligands,
with their abbreviations.
1
was used to obtain the electronic absorption spectra. H

1636
A. Kaltzoglou et al. / Polyhedron 26 (2007) 1634–1642
Table 1
Crystal data and structure refinement for 1, 2 and 4
1
2
4
Chemical formula
Formula weight
Temperature (K)
C60H48Ag2Br2P4 Æ C3H6O
1326.50
120(2)
0.71073
triclinic
C39H32AgBrOP2
766.37
120(2)
0.71073
monoclinic
P21/m
C44H37AgBrNOP2S Æ C2H5OH
923.59
120(2)
0.71073
monoclinic
P21/n
˚
Wavelength (A)
Crystal system
Space group
Unit cell dimensions
ꢀ
P1
˚
a (A)
11.4005(2)
11.9360(2)
21.4574(4)
105.1170(10)
102.4180(10)
94.61
2723.89(8)
2
1.617
8.7806(8)
18.1236(16)
12.2833(11)
90
107.589(4)
90
1863.3(3)
2
1.366
10.17560(10)
21.4454(3)
19.2509(2)
90
98.5760(10)
90
4153.96(8)
4
1.477
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
Volume (A )
Z
Dcalc (Mg/m3)
Absorption coefficient
2.346
1.726
1.613
(mmꢀ1
)
F(000)
1328
772
1880
Crystal size (mm3)
h Range for data
collection (ꢁ)
0.18 · 0.08 · 0.06
3.04–27.53
0.36 · 0.22 · 0.06
3.38–27.49
0.42 · 0.28 · 0.18
3.30–27.4
Index ranges
ꢀ14 6 h 6 14,
ꢀ15 6 k 6 15,
ꢀ27 6 l 6 27
51314
ꢀ11 6 h 6 11,
ꢀ23 6 k 6 22,
ꢀ15 6 l 6 15
16003
ꢀ13 6 h 6 12,
ꢀ27 6 k 6 27,
ꢀ24 6 l 6 24
57932
Reflections collected
Independent reflections 12442 [0.0435]
[Rint
4357 [0.0455]
9463 [0.0273]
]
Completeness to
99.1%
98.6%
99.6%
h = 27.47ꢁ
Maximum and minimum 0.8721 and 0.6775
0.9035 and 0.5754
0.7600 and 0.5506
transmission
Refinement method
Data /restraints/
parameters
Goodness-of-fit on F2 (S) 1.011
full-matrix least square on F2
12442/0/652
full-matrix full-matrix least square on F2 full-matrix full-matrix least square on F2
4357/21/225
9463/21/576
1.039
1.046
Final R indices
[(I > 2r(I)]
R1 = 0.0302,
R1 = 0.0652,
R1 = 0.0268,
wR2 = 0.0692
R1 = 0.0408,
wR2 = 0.0739
wR2 = 0.1860
R1 = 0.0918,
wR2 = 0.1978
wR2 = 0.0629
R1 = 0.0316,
wR2 = 0.0654
R indices (all data)
2
2
Final weighting scheme Calc w ¼ 1=½r2ðF o2Þ þ ð0:0286P2Þ
Calc w ¼ ð1=½r2ðF 2oÞ þ ð0:1153PÞ
Calc w ¼ ð1=r2ðF 2oÞ þ ð0:0263PÞ
þ2:4902Pꢁ; where P ¼ ðF 2o þ 2F c2Þ=3Þ
þ3:4918Pꢁ; where P ¼ ðF 2o þ 2F c2Þ=3Þ
2.486 and ꢀ1.136
þ3:3960Pꢁ; where P ¼ ðF 2o þ 2F c2Þ=3Þ
0.442 and ꢀ1.365
Extinction coefficient
0.00135(11)
Largest difference in peak 0.515 and ꢀ0.677
ꢀ3
˚
and hole (e A
)
1479s, 1435vs, 1216s, 1098s, 1026s, 761s, 745vs, 729s,
691vs, 506vs, 495s, 477s; UV–Vis (kmax, loge): (CHCl3);
249 (3.77), 269 (3.74), 297 (3.38). 1H NMR (CDCl3, d
ppm): 7.38–7.01 (m, 24H, 4C6H5+C6H4).
(kmax, loge): (CHCl3); 253 (3.76). 1H NMR (CDCl3, d
ppm): 7.58 (d, 1H, H3xantphos), 7.30 (m, 8H, PC6H5), 7.20
(m, 12H, PC6H5), 7.06 (dd, 1H, H3xantphos), 6.58 (d, 1H,
H1xantphos), 1.67 (s, 6H, H3Cxantphos).
2.5. [AgBr(xantphos)] Æ CH3CN (2)
2.6. Synthesis of compounds 3 and 4
Colourless crystals. Yield: 186 mg (92%), m.p. 338 ꢁC;
Anal. Calc. for C41H35AgBrNOP2: C, 60.99; H, 4.37; N,
1.73. Found: C, 60.62; H, 4.23; N, 1.57%. IR (cmꢀ1):
3437m, 3049w, 2975w, 1480m, 1435s, 1405vs, 1238s,
1097m, 1027m, 789s, 746vs, 695vs, 509vs, 457s; UV–Vis
The heteroleptic complexes were prepared according to
the following general procedure. A suspension of silver
bromide (47 mg, 0.25 mmol) and 0.25 mmol of the appro-
priate diphosphane (111.6 mg for dppbz or 144.7 mg for
xantphos) in 40 cm3 ethanol was refluxed for 2 h until a

A. Kaltzoglou et al. / Polyhedron 26 (2007) 1634–1642
1637
white precipitation formed. A solution of pyridine-2-thione
(28 mg, 0.25 mmol) in ethanol was treated with a small
quantity of pyridine and was then added dropwise to the
hot suspension and the reaction mixture was refluxed for
an additional 2 h. The resulting solution was filtered off
and the clear yellow solution was kept at room tempera-
ture. Slow evaporation of the solvent at room temperature
gave the microcrystalline solid, which was filtered off and
dried in vacuo.
the other hand, there is a well known tendency for cop-
per(I) and to a lesser extent for silver(I) to achieve the com-
mon four-coordination forming dinuclear species involving
halide ligands in a l2-X bridging mode. Thus, according to
our previous experience, particularly with dppbz, com-
pounds 1 and 2 were expected to be halide-bridged dimers
of type [Ag(l2-Br)(diphos)]2. X-ray crystal structure deter-
minations showed, however, that this is the case only for
the dppbz derivative, whereas compound 2 is a three-coor-
dinated monomer.
2.7. [AgBr(dppbz)(py2SH)] (3)
Following the one-pot two-step synthetic procedure that
has proven to produce mixed-ligand copper(I) and silver(I)
complexes virtually for any heterocyclic thione, we next
attempted the preparation of heteroleptic complexes of
the type [AgBr(diphos)(thione)]. In particular, we prepared
suspensions of compounds 1 and 2 in dry acetonitrile and
treated these with an equimolar quantity of a methanolic
solution of the appropriate thione. Heating of the mixture
at reflux for several hours did not caused, as expected, the
disappearance of the white precipitate, indicating the pres-
ence of unreacted starting material. Indeed, the white
amorphous solid isolated by filtration could be identified
as compounds 1 and 2, respectively. On the other hand,
the composition of the microcrystalline solid that was
obtained on evaporation of the yellow solution at ambient
temperature changed over time as a function of the evapo-
ration state, indicating the presence of a product mixture.
Surprisingly, only in the case of py2SH did the reaction
proceed towards the expected products, employing an eth-
anol/pyridine mixture as solvent, which may seem quite
unusual and was initially used to possibly favour deproto-
nation of the thione, thus to increase its coordination capa-
bility. It should be, however, anticipated that both
[AgBr(dppbz)(py2SH)] (3) and [AgBr(xantphos)(py2SH)]
(4) still contain the thione unit in its neutral (protonated)
form.
Pale yellow crystals. Yield: 158 mg (85%), m.p. 253 ꢁC;
Anal. Calc. for C35H29AgBrNP2S: C, 56.40; H, 3.92; N,
1.88. Found: C, 55.90; H, 4.20; N, 1.68%. IR (cmꢀ1):
3437m, 3155w, 3047w, 1614s, 1596s, 1556vs, 1479s, 1436vs,
1216m, 1135vs, 1095s, 1025m, 986m, 751vs, 730s, 694vs,
513vs, 488s, 447m; UV–Vis (kmax, loge): (CHCl3); 245
1
(4.31), 274 (4.15), 298 (4.06). H NMR (CDCl3, d ppm):
7.46 (dd, 1H, Hp6y2SH), 7.38–7.06 (m, 24H, 4C6H5+C6H4)
7.30 (overlapping t, 1H, H4py2SH), 6.68 (t, 1H, Hp5y2SH).
2.8. [AgBr(xantphos)(py2SH)] Æ C2H5OH (4)
Yellow crystals. Yield: 62 mg (27%), m.p. 279 ꢁC; Anal.
Calc. for C46H43AgBrNO2P2S: C, 59.82; H, 4.69; N, 1.51.
Found: C, 59.85; H, 4.64; N, 1.41%. IR (cmꢀ1): 3453m,
3148w, 3051w, 2963m, 1617s, 1570vs, 1480m, 1434vs,
1404vs, 1367m, 1230vs, 1134vs, 1094m, 1028m, 991m,
791m, 749vs, 729s, 694vs, 511vs, 484s, 462s; UV–Vis (kmax
,
loge): (CHCl3); 248 (4.43), 281 (4.43), 373 (3.71). 1H NMR
(CDCl3, d ppm): 7.52 (d, 1H, H3xantphos), 7.49 (dd, 1H,
H6py2SH), 7.40 (m, 8H, PC6H5), 7.32 (overlapping t, 1H,
H4py2SH), 7.24 (m, 12H, PC6H5), 7.04 (dd, 1H, H2xantphos),
6.72 (d, 1H, H1xantphos), 6.60 (t, 1H, H5py2SH), 1.63 (s, 6H,
H3Cxantphos).
All the prepared complexes are microcrystalline solids,
slightly soluble in acetonitrile and only marginally soluble
in chloroform and acetone. They are stable to air and mois-
ture and can be manipulated in air without appreciable
decomposition. Their solutions in acetonitrile exhibit no
conductivity. Room temperature magnetic measurements
confirm the expected diamagnetic nature of the compounds.
3. Results and discussion
3.1. Preparative considerations
The reaction between equimolar quantities of silver(I)
bromide and 1,2-bis(diphenylphosphano)benzene (dppbz)
or
4,5-bis(diphenyl-phosphano)-9,9-dimethyl-xanthene
3.2. Spectroscopic characterization
(xantphos) in dry acetone or acetonitrile afforded micro-
crystalline solids which, on elemental analysis, were found
to be of the composition [AgBr(diphos)] (compounds 1 and
2, respectively). It should be pointed out that both diphos-
phanes showed a strong preference for chelation at a single
metal centre in their reactions with copper(I) halides [10],
despite of the marked difference in their natural bite angles.
This coordination behaviour undoubtedly demonstrates
their strong chelating power but should be also ascribed,
to a significant degree, to the extraordinary versatility of
a d10 metal ion and therefore chelation was expected to
be preferred over bridging also in the case of silver(I). On
The solid state FT-IR spectra of the four compounds,
recorded in the range 4000–250 cmꢀ1, show all the expected
strong phosphane bands, which remain practically unshif-
ted upon coordination. Moreover the spectra of the
mixed-ligand complexes 3 and 4 contain the four character-
istic ‘‘thioamide bands’’ required by the presence of the het-
erocyclic thioamide, with shifts suggestive of its
coordination mode. ‘‘Thioamide(IV)’’, for instance, which
appears at 742 cmꢀ1 in the spectrum of the ‘‘free’’ pyri-
dine-2-thione and mainly involves contribution from the
m(C@S) vibration, undergoes a significant downward shift

1638
A. Kaltzoglou et al. / Polyhedron 26 (2007) 1634–1642
(by more than 30 cmꢀ1) upon coordination, indicating M–
S bond formation. The thione form of the coordinated het-
erocyclic thioamide is further established by the presence of
the NH stretching vibration at 3159 and 3149 cmꢀ1 for 3
and 4, respectively, in combination with the lack of an S–
H stretching band around 2560 cmꢀ1
.
1
The room temperature H NMR spectra of the com-
plexes, recorded in deuterated chloroform, are dominated
by the presence of complex multiplets in the 7.20–
7.40 ppm region due to phenyl/phenylene resonances, as
well as, in the case of compounds 3 and 4, signals attribut-
able to the aromatic protons of the pyridine-2-thione unit.
Upon coordination, the multiplets assigned to the PPh2
groups of the phosphane ligands undergo splitting due to
a small upfield shift of the ortho-positioned protons,
whereas the signals of the corresponding backbone protons
remain practically unshifted.
The electronic absorption spectra of compounds 1 and 2
in chloroform solutions appear to be mainly of intraligand
character, consisting of three or one intense sharp band
respectively (see Section 2). These bands can be attributed
to intraligand p* p transitions on the phenyl group of the
phosphane ligands, since the uncoordinated 1,2-bis(diphe-
nylphosphano)benzene and 4,5-bis(diphenylphosphano)-
9,9-dimethyl-xanthene reveal three or one strong absorptions
respectively at 245, 273 and 299 nm or 264 nm, all of them
undergoing small shifts upon coordination to Ag(I). The
spectra of complexes 3 and 4 show an additional lower
energy band that lies in the region where the free thione
absorbs, expressing a small red shift as a consequence of
the coordination to Ag(I) and should be, therefore, consid-
ered as a thione-originating intraligand transition which
possesses some MLCT character [17].
Fig. 1. TG and DTA curves for [AgBr(xantphos)] (2) and [AgBr(xant-
phos)(py2SH)] (4).
in any form. In the case of compounds 2 and 4 solvent
retention occurs at the temperature 170 ꢁC, indicated by
a weak endothermic effect. For all the compounds further
heating causes gradual sublimation of the residual AgBr
which is almost completely removed at 980 ꢁC.
3.3. Thermal decomposition
The thermal behaviour of the new compounds was
investigated using simultaneous differential thermoanalysis
(DTA) and thermogravimetry (TG) in a nitrogen atmo-
sphere. TG and DTA data curves for compounds
[AgBr(xantphos)] (2) and [AgBr(xantphos)(py2SH)] (4)
are given in Fig. 1.
3.4. Crystal structures
The X-ray crystal structures of [Ag(l2-Br)(dppbz)]2 (1),
[AgBr(xantphos)] (2) and [AgBr(xantphos)(py2SH)] (4)
(details of crystal and structure refinement are shown in
Table 1) corroborate the spectroscopic results discussed
above. Note that many heavy transition metal complexes
with xantphos or dppbz have been described in the litera-
ture, but no AgI complexes are known to be structurally
characterized by X-ray crystallography.
Complexes 1 and 2 decompose in one single stage that
starts at 330 or 390 ꢁC, respectively and extends over a tem-
perature range of 100 ꢁC. The observed mass loss notified
by the TG curve fits in each case perfectly as the loss of
the diphosphane, whereby the corresponding DTA curve
shows a strong endothermic effect. Complexes 3 and 4
show the typical decomposition pattern observed for the
analogous copper(I) halide compounds [10a,10c], generally
consisting of two main stages, the low temperature initial
stage, observed in the TG curve in the 240–280 ꢁC range,
being connected with two endothermic events in the DTA
curve and involving overall weight loss indicative of grad-
ual thermal breakdown of the pyridine-2-thione ligand. A
very strong endothermic event appears at about 380 ꢁC,
which does not affect the consistency of the compounds,
Colorless crystals of 1 suitable for X-ray crystallography
were grown from its acetone mother liquid by slow evapora-
tion at ambient temperature. The complex crystallizes in the
ꢀ
triclinic space group P1 with two formula units in the unit
cell. The asymmetric unit consists of two half-molecules as
well as one acetone molecule, which will not be further dis-
cussed as there is no evidence of any specific interaction
involving this latter solvent molecule. Respective bond
lengths and angles within the two half-molecules are slightly
different, thus two different centrosymmetric molecules (A
and B) result by suitable symmetry transformations.

A. Kaltzoglou et al. / Polyhedron 26 (2007) 1634–1642
1639
Table 2
Fig. 2 depicts a perspective of one of these molecules show-
ing the atom numbering. Bond lengths and angles around
the AgI centre are listed in Table 2.
˚
Selected bond lengths (A) and angles (ꢁ) for 1
Ag(1)–P(1)
Ag(1)–P(2)
Ag(1)–Br(1)
Ag(1)–Br(1)#
P(1)–C(1)
2.5142(6)
2.5260(6)
2.6343(3)
2.7521(3)
1.830(2)
P(1)–C(7)
P(1)–C(13)
P(2)–C(2)
P(2)–C(19)
P(2)–C(25)
1.822(2)
1.819(2)
1.839(2)
1.832(2)
1.826(2)
The crystal structure contains a center of symmetry and
features a dinuclear unit in which the two silver(I) centers
are joined by two bromine bridges to form a ‘‘diamond-
shaped’’ Ag2Br2 core with Ag–Br–Ag and Br–Ag–Br bond
angles that clearly deviate from the ideal values for sym-
metric dimers (70.5ꢁ and 109.5ꢁ) but are very close to the
ideal geometry for Y2MX2MY2 dimers according to theo-
retically proposed criteria [18]. The Ag. . .Ag separation of
P(1)–Ag(1)–P(2)
79.596(19)
121.404(16)
132.078(16)
113.198(15)
111.605(15)
115.64(8)
Ag(1)–Br(1)–Ag(1)#
Ag(1)–P(1)–C(1)
Ag(1)–P(1)–C(7)
Ag(1)–P(2)–C(2)
Ag(1)–P(2)–C(19)
P(1)–C(1)–C(2)
80.978(8)
101.27(7)
114.08(7)
100.37(7)
115.64(8)
119.61(16)
119.30(16)
P(1)–Ag(1)–Br(1)
P(2)–Ag(1)–Br(1)
P(1)–Ag(1)–Br(1)#
P(2)–Ag(1)–Br(1)#
Ag(1)–P(2)–C(19)
Br(1)–Ag(1)–Br(1)#
˚
3.4985(3) A is slightly longer than the sum of the van der
99.022(8)
P(2)–C(2)–C(1)
Waals radii [19], thus excluding any possible metal–metal
attractive interaction. It should be noted that, unlike the
remarkable non-planarity within the Cu2Br2 moiety of
the recently reported [Cu(l2-Br)(dppbz)]2 [10b], complex
1 displays a strictly planar central core commonly adopted
by halide-bridged dimeric complexes. Another noticeable
difference between the two structures arises from the P–
M–P intraligand angles, the acute P(1)–Ag(1)–P(2) bite-
angle of 79.596(19)ꢁ in the present structure being clearly
smaller than the corresponding P–Cu–P angles found in
the above mentioned copper counterpart, which certainly
can be ascribed to the different M–P bond lengths.
Symmetry transformations used to generate equivalent atoms: # ꢀx + 1,
ꢀy + 1, ꢀz + 1.
the C(7)–C(12) and C(19)–C(24) phenyl rings have their
planes in an almost parallel arrangement as a consequence
of a 3.6400(14) A p-stacking interaction between them.
Within the Ag2Br2 backbone there are two different Ag–
Br bond distances that are somewhat shorter than those
found in other tetracoordinate silver(I) phosphane com-
plexes, e.g. in [Ag(l2-Br)(pymtH)]2 [20]. Finally, the two
Ag–P distances of 2.5142(6) A and 2.5260(6) A are within
the limits of the value range expected for tetrahedrally
coordinated AgI.
Structure refinement for compound 2 was performed in
both P21 and P21/m monoclinic space groups and the
higher symmetry space group P21/m was adopted. How-
ever, disorder involving the Br atom position (split over
two sites), a poorly defined solvent molecule (occupying
˚
˚
˚
As expected, the carbon atoms of the phenylene back-
bone and the two adjacent phosphorus atoms are nearly
coplanar, but the AgPCCP five-membered ring adopts an
envelope conformation with the metal atom displaced out
of the plane containing the four diphos ligand atoms by
˚
1.2773(15) A. The two phenyl rings on each phosphorus
atom are nearly perpendicular to each other. However,
Fig. 2. Molecular structure of complex 1 with atom labels. Displacement ellipsoids are shown at the 50% probability level.

1640
A. Kaltzoglou et al. / Polyhedron 26 (2007) 1634–1642
Table 3
17% of the crystal volume, 18 electrons removed by PLATON
[21] and some residual electron density spread along the 2-
fold axis led to a structure less well defined than 1 or 4.
Fig. 3 depicts a perspective of the molecule showing the
atom labelling scheme. Selected bond lengths and angles
are listed in Table 3.
˚
Selected bond lengths (A) and angles (ꢁ) for 2
Ag(1)–P(1)
2.4478(14)
P(1)–C(6)
P(1)–C(10)
1.829(6)
1.831(6)
Ag(1)–Br(1a)
2.658(7)
P(1)–Ag(1)–P(1)#
P(1)–Ag(1)–Br(1a)
109.38(7)
125.31(4)
C(16)–P(1)–C(6)
C(6)–P(1)–C(10)
P(1)–C(6)–C(5)
P(1)–C(6)–C(7)
104.3(3)
103.1(2)
123.2(4)
118.8(4)
Contrary to the above discussed structure of the silver/
dppbz complex, the xantphos derivative is a monomer,
the inability for dimerization caused most probably by
the marked steric demands of the bulky diphos ligand,
expressed by its wide P–Ag–P bite angle versus the corre-
sponding acute bite angle in the structure of 1. Thus, the
silver(I) metal exhibits a distorted trigonal coordination
environment surrounded by two phosphorus atoms of the
chelating xantphos as well as one bromine atom, with quite
small angular deviations from the ideal trigonal value of
120ꢁ being mainly directed by the diphosphine bite angle.
The crystal structure possess a symmetry plane defined by
the C(1), C(8), C(9), O(1), Ag(1) and Br(1a) atoms, which
require the P2AgBr central core to be strictly planar. The
smallest angle within this core corresponds to the P(1)–
Ag(1)–P(1)# angle, whose value of 109.38(7)ꢁ is very close
to the ligands calculated ‘‘natural’’ bite angle of 111ꢁ [21],
indicating the extraordinary versatility of the closed shell
central metal ion. Nevertheless, some adjustment on the
part of the diphos ligand seems to be unavoidable in order
to act as a chelate, which is expressed by the folding of the
xanthene unit along its symmetry axis. In fact, the dihedral
angle between the planes of the two benzenoid rings form a
dihedral angle of 150.3(2)ꢁ whereas a value of 156.58ꢁ was
found for the ‘‘free’’ xanthene molecule [21].
Ag(1)–P(1)–C(16)
111.62(19)
Symmetry transformation used to generate equivalent atoms: x, ꢀy + 0.5,
z.
Bond lengths and angles around the AgI are listed in Table
4. Two P donors, one S donor of the heterocyclic thione
and the bromine atom form a fairly regular tetrahedron,
where the angles around the silver atom vary from
103.300(18)ꢁ for S(1)–Ag(1)–Br(1) to 117.13(3)ꢁ for P(2)–
Ag(1)–S(1). Interestingly, upon chelation the xantphos
ligand exactly retains its ‘‘natural’’ bite angle [21].
The geometrical characteristics of the structure under
investigation generally refer to those of other silver(I) com-
plexes tetrahedrally coordinated by phosphine and thione
ligands. Thus, the two Ag–P distances vary little [Ag(1)–
˚
P(1) = 2.4843(5) and Ag(1)–P(2) = 2.4994(5) A] and lie at
the upper limit of values found for a number of complexes
containing tetracoordinated silver atoms bonded to two
phosphorus atoms [7b]. The Ag–S bond distance of
˚
2.5395(5) A, coincident with the sum of the corresponding
atomic radii, is typical for this bonding arrangement.
Finally, hydrogen bonds of type N–H. . .Br and O–H. . .Br
(from the ethanol molecule) are observed, as well as a weak
intramolecular p–p stacking interaction between the C(33)
and C(38) phenyl ring of the xantphos unit and the hetero-
cyclic thione ring.
Compound 4 crystallizes in the monoclinic space group
P21/n with four formula units in the unit cell. Fig. 4 depicts
a perspective of the molecule showing the atom numbering.
Fig. 3. Molecular structure of complex 2 with disorder removed. Displacement ellipsoids are shown at the 50% probability level.

A. Kaltzoglou et al. / Polyhedron 26 (2007) 1634–1642
1641
Fig. 4. Molecular structure of complex 4 with atom labels. Displacement ellipsoids are shown at the 50% probability level.
with tetrahedrally coordinated AgI centers for dppbz and
monomeric trigonal in the case of xantphos. Both com-
plexes proved to be good precursors for the preparation
of mixed ligand monomeric complexes which contain,
besides the chelating diphos unit, a heterocyclic thioamide
bonded to the metal via the thione-S atom. According to
the overall geometries of the structures presented, the xant-
phos ligand applied can be considered as best tailored for
chelation of silver(I) centres.
Table 4
˚
Selected bond lengths (A) and angles (ꢁ) for 4
Bond length
Ag(1)–P(1)
Ag(1)–P(2)
Ag(1)–S(1)
2.4843(5)
2.4994(5)
2.5395(5)
Ag(1)–Br(1)
S(1)–C(28)
2.8587(3)
1.703(2)
Bond angle
P(1)–Ag(1)–P(2)
P(1)–Ag(1)–S(1)
P(2)–Ag(1)–S(1)
P(1)–Ag(1)–Br(1)
111.507(17)
112.602(18)
117.13(3)
P(2)–Ag(1)–Br(1)
S(1)–Ag(1)–Br(1)
C(28)–S(1)–Ag(1)
104.460(13)
103.300(18)
106.16(7)
106.564(13)
5. Supplementary material
Hydrogen bridges
N(1)–H(1)
NH(1)–Br(1)
O(2A)–H(2A)
OH(2A)–Br(1)
0.8800
2.4200
0.8400
2.7400
N(1). . .Br(1)
Br(1). . .H(1)–N(1)
O(2A). . .Br(1)
3.2945(17)
172.0
3.577(6)
173
CCDC 612754, 612755 and 612756 contain the supple-
mentary crystallographic data for 1, 2 and 4. These data
can be obtained free of charge via http://www.ccdc.cam.
ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
Br(1). . .H(2A)–O(2A)
4. Conclusion
The aim of this work was the study of the coordination
capability of the diphos ligands 1,2-bis(diphenylphosp-
hano)benzene (dppbz) and 4,5-bis(diphenylphosphano)-
9,9-dimethylxanthene (xantphos) toward silver(I) bromide.
We have found that, despite the quite different geometrical
characteristics of the two diphosphanes, xantphos provid-
ing a much larger bite angle than the acute bite angle of
dppbz, chelation was observed as the only coordination
mode for both diphos ligands. Compounds of the same
composition [AgX(diphos)], but of different geometry and
nuclearity were formed, namely a halide bridged dimer
Acknowledgements
We thank the EPSRC X-ray crystallography service at the
University of Southampton for collecting the X-ray data.
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