The first silver catechol complexes: Crystal structures of triphenylphosphine stabilized silver tetra-bromo and tetra-chloro-catechols

Article abstract of DOI:10.1016/j.poly.2006.01.009

We report the solid-state structures of the first verifiable silver complexes containing bound catechol ligands, Ag(Br₄C₆(OH)O)(Ph₃P)₂ and Ag(Cl₄C₆(OH)O)(Ph₃P)₂. Only

Full text of DOI:10.1016/j.poly.2006.01.009

Polyhedron 25 (2006) 2033–2038
www.elsevier.com/locate/poly
The first silver catechol complexes: Crystal structures of
triphenylphosphine stabilized silver tetra-bromo and
tetra-chloro-catechols
a,
b,1
D.R. Whitcomb *, M. Rajeswaran
a
Eastman Kodak Company, 1 Imation Way, Oakdale, MN 55128, United States
Eastman Kodak Company, Research and Development, 1999 Lake Avenue, Rochester, NY 14650-2106, United States
b
Received 2 November 2005; accepted 3 January 2006
Available online 20 February 2006
Abstract
We report the solid-state structures of the first verifiable silver complexes containing bound catechol ligands, Ag(Br4C6(OH)O)(Ph3P)2
and Ag(Cl4C6(OH)O)(Ph3P)2. Only highly electron-withdrawing groups, such as tetra-substituted catechols coupled with triphenylphos-
phine stabilizing ligands, can be used to obtain stable silver catechol complexes. Without these features, the silver is too readily reduced
to the metal. In both cases where the halide is either chloride or bromide, only one of the catechol oxygens is coordinated to the silver
along with the ortho-halogen, which forms the chelate ring with the metal. The second phenolic group is not involved with the coordi-
nation of the metal. Additional hydrogen bonding between the catechol groups provides for the formation of dimeric species in the solid
state.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Catechol; Crystal structure; Ag-complex; Silver co-ordination; Ag-catechol
1. Introduction
with specific stabilizing groups form isolatable silver com-
plexes. Even these, to our knowledge, are limited to one
example of a silver phenol complex without additional
stabilizing ligands, the silver complex of 2,4,5-tri-chloro-
phenol [5].
Since the early days of photography, well over one hun-
dred years ago, catechol derivatives have been known to be
efficient reducing agents for silver [1], which is a critical
step in the metallic silver image process. This efficiency is
also demonstrated by the fact that catechol can be used
to quantitatively reduce Ag+ to Ag0 for analytical purposes
[2], thus, it is interesting to see that a stable silver catechol
complex has recently been claimed to have been isolated,
although not structurally characterized [3,4]. A literature
search of published data on silver catechol complexes
reveals that they are, otherwise, completely unknown. Only
the chemically simpler, less-reactive monophenol ligands
Two phenol complexes of silver have been reported that
contain stabilizing ligands. The first is the silver complex of
2,4,6-tri-chlorophenol with triphenylphosphine [6]. As in
the 2,4,5-tri-chlorophenol isomer case, the Ag–O bond is
˚
normal, 2.235(4) A; but in addition, the ortho-chlorine is
˚
found to be bonded to the silver, 3.160(2) A. While unusual
in silver–oxygen-containing complexes, such Agꢀ ꢀ ꢀhalide
interactions are not uncommon [7]. Silver complexes con-
taining Agꢀ ꢀ ꢀhalide bonds from X-organic ligands, where
X = F, Cl, Br, I, have been reported [7–12]. In these cases,
˚
the Ag–Br bonds range 2.7–3.0 A [7–9] and the Ag–Cl
*
Corresponding author.
˚
bonds range 2.5–2.9 A [7,10,11]. In the second case of the
E-mail addresses: david.whitcomb@kodak.com (D.R. Whitcomb),
phenol ligand, the 2,4,6-trinitrophenolate complex with sil-
ver utilizes ethylenediamine as a stabilizing ligand [13]. In
manju.rajeswaran@kodak.com (M. Rajeswaran).
1
Tel.: +1 585 722 5828; fax: +1 585 588 6026.
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.01.009

2034
D.R. Whitcomb, M. Rajeswaran / Polyhedron 25 (2006) 2033–2038
addition, two silver complexes containing quinone ligand,
which do not need redox stabilizing ligands, that are analo-
gous to the title complexes have also been reported. In these
Stirring at room temperature produced a nearly clear solu-
tion, which was colorless on filtration. A mixture of pink
crystals in a slight oil remained after slow evaporation.
The crystals (0.18 g) were removed after ether washing.
DSC is completely similar to the bromine complex, an exo-
therm centered at 170 ꢁC, preceded by a melting point
endotherm at 155 ꢁC. For C42H31O2AgP2Cl4 elemental C,
H (theory) C, H = 57.32 (57.34), 3.54 (3.53). The Raman
spectrum is dominated by the triphenylphosphine ligand
˚
cases, the Ag–O distances are long, 2.37–2.47 A [9,14].
In light of these reports, and as part of our continuation
of silver coordination chemistry investigation [15–19], we
have endeavored to discover what catechol structures could
be used to obtain stable, characterizable silver complexes
and to better understand silver catechol reactivity. We
now report that stable silver catechol complexes can be pre-
pared but that the ability to obtain these complexes is lim-
ited to catechols having highly electron-withdrawing
substituents (four halides, in these examples); yet they still
require triphenylphosphine ligand stabilizers. The prepara-
tion and solid-state structures of these first stable silver cat-
echol complexes, bis-triphenylphosphine-tetra-halocatechol
silver, are described below.
bands: primarily 998, 1025, and 1097 (1092 shoulder) cmꢁ1
.
Differential scanning calorimetry (DSC) was obtained
on a TAInstruments Model 910, heating at 10 ꢁC/min
under nitrogen flow (ꢂ50 mL/min).
Elemental analysis was obtained from Prevalere (8282
Halsey Road, Whitesboro, NY 13492).
Raman spectra were collected in micro mode with a JY
Horiba LabRam, equipped with a CCD dectector, at 180
backscatter with a MSPlan 100· objective. The excitation
wavelength was 785 nm with the laser power at approxi-
mately 300 mW, and the collection time was 100 s.
2. Experimental
2.1. Preparation of Ag(Br4C6(OH)O)(Ph3P)2
2.3. Single-crystal X-ray diffraction
Solution ligand exchange of the catechol with silver ace-
tate was successfully utilized to isolate the title complex
according to the following scheme:
Single-crystal structure determination by X-ray diffrac-
tion was performed on a Nonius KappaCCD [21] equipped
with a normal focus, sealed-tube X-ray source. Data reduc-
tions were performed using DENZO-SMN [22]. The struc-
tures were solved and refined by using the SHELXTL [23]
AgðO2CCH3ÞðPh3PÞ2 þ Br4C6ðOHÞ2
! AgðBr4C6ðOHÞOÞðPh3PÞ2 þ HO2CCH3
To 134 mg of Ag(O2CCH3) (Ph3P)2 [20] was added 7 mL
toluene and 88 mg tetra-bromocatechol, Br4C6(OH)2 (Al-
drich). A clear yellow solution resulted, which was layered
with an equal volume of ether. Yellow crystals (0.12 g) grew
overnight. DSC shows a single decomposition exotherm at
167 ꢁC (the starting silver acetate phosphine complex shows
this transition at 192 ꢁC). For C42H31O2AgP2Br4 elemental
C, H (theory) C, H = 47.65 (47.70), 2.81 (2.93). The Raman
spectrum is dominated by the triphenylphosphine ligand
bands: primarily 998, 1025, and 1096 cmꢁ1. The silver
tetra-bromo-catechol complex was submitted for NMR
analysis to ascertain its molecular structure in solution
(DMSO-d6). While both the proton and P-31 spectra (sup-
plementary data section) are suggestive of the proposed
hydrogen-bonded dimer structure, long term C-13 analysis,
due to inherent solution instability, is not.
Table 1
Selected bond distances and angles for Ag(Br4C6(OH)O)(Ph3P)2 and
Ag(Cl4C6(OH)O)(Ph3P)2
Ag(Br4C6(OH)O)(Ph3P)2
Ag(Cl4C6(OH)O)(Ph3P)2
Ag1–O2
Ag1–P1
Ag1–P2
C39–X1
C40–X2
C41–X3
C42–X4
Ag1–X1
2.390(3)
2.4626(12)
2.4333(11)
1.902(4)
1.896(4)
1.885(4)
1.899(4)
3.1864(6)
2.3575(19)
2.4201(7)
2.4565(8)
1.733(3)
1.736(3)
1.731(3)
1.730(3)
3.1838(9)
O1–Ag1–P2
O1–Ag1–P1
P2–Ag1–P1
123.789(127)
95.6274(127)
139.756(64)
122.66(8)
96.83(8)
139.53(5)
2.2. Preparation of Ag(Cl4C6(OH)O)(Ph3P)2
Table 2
˚
˚
Hydrogen bonds with Hꢀ ꢀ ꢀA < r(A) + 2.000 A and A–DHA > 110ꢁ,
where r is atomic radius, A is acceptor atom, D is donor atom and
\DHA is donor–hydrogen–acceptor angle
Ag2O as the deprotonating base for the catechol ligand,
in place of acetate exchange from silver acetate, was also
used to isolate the complex, according to the following
scheme:
D–H
d(D–H) d(Hꢀ ꢀ ꢀA) \DHA d(Dꢀ ꢀ ꢀA)
A
Ag(Br4C6(OH)O)(Ph3P)2
O1–H1 0.820
O1–H1 0.820
1.900
2.314
142.82 2.601
112.90 2.739
O2 [ꢁx + 2, ꢁy, ꢁz]
Ag2O þ 2Cl4C6ðOHÞ2 þ 4Ph3P
O2
! 2AgðCl4C6ðOHÞOÞðPh3PÞ2 þ H2O
To 60 mg Ag2O and 0.26 g Ph3P in 5 mL toluene was
added 128 mg tetra-chlorocatechol, Cl4C6(OH)2 (Aldrich).
Ag(Cl4C6(OH)O)(Ph3P)2
O1–H1 0.820
O1–H1 0.820
1.935
2.316
138.49 2.607
113.40 2.747
O2 [ꢁx + 2, ꢁy, ꢁz]
O2

D.R. Whitcomb, M. Rajeswaran / Polyhedron 25 (2006) 2033–2038
2035
Table 3
Using milder conditions, such as simple replacement of
the acetate from water-soluble silver acetate, still resulted
in rapid formation of metallic silver, again regardless of
the derivative. The high reactivity of these catechols, cou-
pled with the lack of silver catechol complexes reported
in the literature, indicated that a different approach was
clearly needed.
Organo-phosphines are known to act as good stabilizers
of metal ions in unusual or unstable oxidation states
[25,26]. Considering the high reactivity of Ag+ with cate-
chols, this ion might be thought of as a suitable candidate
for stabilization with phosphine-based ligands. Two
approaches were successful in the preparation of phos-
phine-stabilized silver catechol complexes, preparation of
the complex from silver acetate or its Ph3P derivative (ace-
tic acid by-product), and silver oxide (water by-product):
Details of crystal data, data collection and structure refinement for
Ag(Br4C6(OH)O)(Ph3P)2 and Ag(Cl4C6(OH)O)(Ph3P)2
Parameters
Ag(Br4C6(OH)O)- Ag(Cl4C6(OH)O)-
(Ph3P)2 (Ph3P)2
Crystal data
Empirical formula
Formula weight
Temperature (K)
C42H31AgBr4O2P2 C42H31AgCl4O2P2
1057.12
293(2)
879.28
293(2)
˚
Wavelength (A)
0.71073
0.71073
Crystal system
Space group
Unit-cell dimensions
triclinic
triclinic
ꢀ
ꢀ
P1
P1
˚
a (A)
12.3140(3)
12.8990(3)
13.5710(4)
79.4320(10)
69.3420(10)
81.6250(10)
1975.09(9)
2
12.3730(2)
12.7070(3)
13.4380(3)
79.8220(11)
68.9060(12)
81.7380(9)
1932.78(7)
2
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
Volume (A )
Ag2O þ 4Ph3P þ 2catechol ! 2AgðcatecholÞðPh3PÞ2 þ H2O
½AgðO2CCH3Þꢃ2 þ 4Ph3P þ 2catechol
Z
Dcalc (Mg/m3)
1.778
1.511
Absorption coefficient
(mmꢁ1
4.673
0.917
! 2AgðcatecholÞðPh3PÞ2 þ 2HO2CCH3
)
F(000)
1032
0.17 · 0.12 · 0.10
888
The triphenylphosphine complex of silver acetate,
Ag(O2CCH3)(Ph3P)2, is well known whose structure has
been reported [20]. Similar to the long-chain analog, C18
(stearate), AgStearate(Ph3P)2 [27], the acetate is chelated
to the silver, and the two phosphines simply fill out the
coordination sphere. For the catechol reactions, it was
found that either the Ag(O2CCH3)(Ph3P)2 can be prepared
separately, or it can be generated in situ.
Given that electron-rich catechols were found to be
exceedingly reducing for Ag+, even in the presence of the
stabilizing ligand, highly electron-withdrawing groups were
investigated as most likely to form stable silver catechol
complexes. While these halogenated catechols do reduce
silver acetate to metallic silver in the absence of the stabi-
lizing ligand, in the presence of triphenylphosphine, how-
ever, rapidly cleared solutions were formed that slowly
grew crystals suitable for X-ray structure analysis.
Crystal size (mm3)
0.32 · 0.17 · 0.07
Data collection
Diffractometer
h Range for data
collection (ꢁ)
Scan width (ꢁ)
Exposure time (s/ꢁ)
Index ranges
kappaCCD
4.19–25.99
kappaCCD
3.35–26.77
2
600
2
200
ꢁ14 6 h 6 15,
ꢁ15 6 k 6 15,
ꢁ16 6 l 6 16
21831
7694
0.0643
ꢁ15 6 h 6 15,
ꢁ16 6 k 6 16,
ꢁ16 6 l 6 17
28786
8159
0.0798
Reflections collected
Independent reflections
Rint
Completeness to h = 25.00ꢁ
Absorption correction
[24]
99.10%
multiscan
99.20%
multiscan
Refinement
Data/restraints/parameters
Goodness-of-fit on F2
7694/0/461
0.98
8159/0/461
0.965
While the silver tetra-bromo-catechol complex was suc-
cessfully obtained from silver acetate with triphenylphos-
phine as a stabilizing ligand, the tetra-chloro-catechol
complex did not form suitable crystals under these reaction
conditions. A second approach was taken to facilitate the
complex formation and crystallization. In this case, we
found that Ph3P appears to partially ‘‘dissolve’’ Ag2O,
presumably a low molecular weight oligomer of [Ph3P–
Ag–O]x, which should simply yield H2O as a byproduct.
Using this approach, well-formed crystals of the tetrachloro-
catechol complex were obtained. This complex is seen to be
completely structurally isomorphous with the bromine ana-
log, Fig. 1, including the chelate function of the catechol
via the O–Ag–Cl ring.
Final R indices [I > r2(I)], R1 0.0395
0.0387
0.0808
0.0769
0.0949
wR2
R indices (all data), R1
wR2
0.0716
0.0829
0.0848
Largest different
peak and hole (e A
0.417 and ꢁ0.618
0.323 and ꢁ0.798
ꢁ3
˚
)
suite of programs. The hydrogen atoms were incorporated
into idealized positions. Selected bond distances and angles
are shown in Table 1, and hydrogen bonding distances in
Table 2. Pertinent experimental details of crystal data, data
collection, and structure refinement are summarized in
Table 3.
3. Results and discussion
While normal Ag–O bonds between the silver and
oxygen are observed in the Ag(Br4C6(OH)O)(Ph3P)2 and
Ag(Cl4C6(OH)O)(Ph3P)2 complexes, 2.390(3) and 2.3575
(19) A, two significant observations related to the Ag–O
bonding can be made. First, unlike the normal catechol
Our attempts to prepare silver complexes of various
ortho-di-hydroxybenzenes under basic conditions rapidly
resulted in metallic silver, regardless of the substituents.
˚

2036
D.R. Whitcomb, M. Rajeswaran / Polyhedron 25 (2006) 2033–2038
a
b
Fig. 1. ORTEP diagrams of: (a) Ag(Br4C6(OH)O)(Ph3P)2, and (b) Ag(Cl4C6(OH)O)(Ph3P)2 with 50% probability of thermal ellipsoids, showing atomic
numbering schemes (hydrogen atoms omitted for clarity).
chelating functionality in which both oxygens are bound to
the same metal ion, only one of the oxygens is bound to
the silver. Second, similar to the silver–phenol complexes
noted above, there is a significant Ag–X bond involving
the halide ortho to the coordinated phenolic oxygen,
An interesting unique feature of these structures is
O–Hꢀ ꢀ ꢀO hydrogen bonding interactions that result in
crystal structure stabilization by formation of dimers as
shown in Fig. 2. The available O(1)–H is bonded, about
˚
2.60 A, to the silver bound O(2) in the adjacent molecule.
˚
˚
Ag–Br = 3.1864(6) A and Ag–Cl = 3.1838(9) A. These are
long Ag–X interactions compared to previously reported
In addition, when viewed in the solid-state crystal pack-
ing, Fig. 3, there are significant p–p interactions between
aromatic rings of a phosphine phenyl group and the cate-
˚
Ag–X–organic ligands (Ag–Br 2.7–3.0 A [7–9], Ag–Cl
˚
˚
2.5–2.9 A) [7,10,11]. The bond distances and crystallo-
chol. The stacking distance is about 3.41 A, which is similar
graphically significant orientation makes it clear that the
formation of an O–Ag–X chelated structure is more impor-
tant than that of an O–Ag–O chelated structure.
to that reported for other silver complexes [28].
Overall, the ability to prepare a stable silver catechol
complex is limited to the highly electron-withdrawing

D.R. Whitcomb, M. Rajeswaran / Polyhedron 25 (2006) 2033–2038
2037
O–Ag–X chelated structure. The second phenolic group is
not involved with the coordination of the metal. Additional
hydrogen bonding between the catechol groups provide for
the formation of dimeric species in the solid state.
Acknowledgements
Helpful discussions with our colleagues at Eastman Ko-
dak Company, P.J. Cowdery-Corvan, K. Chen-Ho, R. Ho,
L. Olson, S. Chen, B. Stwertka (who also collected the Ra-
man spectra), and W. Lenhart (also collected NMR spec-
tra) are gratefully acknowledged. We also thank the
reviewer for bringing Refs. [9] and [14] to our attention.
Appendix A. Supplementary data
Fig. 2. Packing of Ag(X4C6(OH)O)(Ph3P)2 complexes in the unit cell
showing hydrogen bonding associations between molecules forming a
dimer in the solid-state lattice.
Crystallographic data for this structure has been depos-
ited with the Cambridge Crystallographic Data Centre as
supplementary data numbers 273648 and 273649 for
Ag(Cl4C6(OH)O)(Ph3P)2 and Ag(Br4C6(OH)O)(Ph3P)2,
respectively. Copies of the data may be obtained free of
charge upon request from The Director, Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK (fax: +44 1223 336033); e-mail: deposit
@ccdc.cam.ac.uk; web: http://www.ccdc.cam.ac.uk. Sup-
plementary data associated with this article can be found,
in the online version, at doi:10.1016/j.poly.2006.01.009.
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