Synthesis and structure of dimeric silver azooximates. Hydrogen bonding and nonbonded Ag···Ag interaction

Full text of DOI:10.1021/ic000056u

2954
Inorg. Chem. 2000, 39, 2954-2956
Synthesis and Structure of Dimeric Silver
Azooximates. Hydrogen Bonding and Nonbonded
Ag‚‚‚Ag Interaction
Sanjib Ganguly, Surajit Chattopadhyay,
Chittaranjan Sinha, and Animesh Chakravorty*
Department of Inorganic Chemistry, Indian Association for
the Cultivation of Science, Calcutta 700 032, India
Figure 1. Perspective view and atom-labeling scheme of the Ag(MeL)-
(HMeL)2 monomer in 2. All atoms are represented by their 30% thermal
probability ellipsoid.
ReceiVed January 14, 2000
Introduction
Nonbonded distances between neighboring closed-shell heavy
atoms can fall below the sum of van der Waals radii due to
relativistic effects.1 Gold(I) (5d10) compounds provide model
examples.2 The status of silver(I) (4d10) is less well defined and
is under active experimental scrutiny.3,4 The excellent affinity5,6
of azooximes for copper(I) (3d10) has prompted us to search
for the silver(I) complexes. Herein, we report the synthesis and
structures of two such compounds, both hydrogen bonded and
dimeric but with a large variation in coordination geometry. In
one of these the metal-metal contact is significantly shorter
than the van der Waals sum.
Figure 2. The [Ag(MeL)(HMeL)2]2 dimer.
Chart 1
Results and Discussion
A. Ligands and Complexes. The concerned species are listed
in Chart 1. Two azooxime ligands, HRL (R ) Me, Ph), 1, where
H refers to the dissociable oxime proton, have been used. The
reaction of ammoniacal silver hydroxide (Tollen’s reagent) with
HMeL has afforded the brown-colored dimer 2. The same
complex is deposited irrespective of the Ag+:HMeL ratio. Upon
replacement of HMeL by HPhL, the product was again a brown
dimer but of composition 3.
Even though the difference between HMeL and HPhL is only
in the substituent R, the effect of the latter on the composition
and structure (vide infra) of the complexes formed is indeed
remarkable. This highlights the flexibility that silver can exhibit
with respect to coordination number and geometry. We also
note that very few authentic silver complexes of oxime ligands
have been isolated and structural characterization is rare.7
B. Structures. The structures of both 2 and 3 have been
determined. The former is centrosymmetric, the asymmetric unit
consisting of the monomer, a view of which is shown in Figure
1. A line drawing of the dimer is depicted in Figure 2, and
selected bond parameters are listed in Table 1.
Table 1. Selected Bond Distances (Å) and Angles (deg) and Their
Estimated Standard Deviations for [Ag(MeL)(HMeL)2]2
Distances
Ag-N1
Ag-N7
Ag-N4
N2-N3
N5-N6
N8-N9
2.252(3)
2.233(3)
2.423(3)
1.257(4)
1.253(4)
1.258(4)
N1-O1
N4-O2
N7-O3
O1‚‚‚O2
O1‚‚‚O3A
Ag‚‚‚Ag
1.349(3)
1.371(4)
1.369(4)
2.566(6)
2.546(6)
3.517(1)
Angles
N1-Ag-N7
N1-Ag-N4
152.1(1)
89.6(1)
N4-Ag-N7
118.4(1)
(1) Pyykko, P. Chem. ReV. 1988, 88, 563.
(2) (a) Schmidbaur, H. Chem. Soc. ReV. 1995, 391. (b) Schmidbaur, H.
Gold Bull. 1990, 32, 11. (c) Jansen, M. Angew. Chem., Int. Ed. Engl.
1987, 26, 1098.
In 3 the whole dimer constitutes the asymmetric unit, the
two monomers (molecules 1 and 2) being metrically similar. A
view of molecule 1 and a line drawing of the dimer are shown
in Figures3 and 4, respectively. Selected bond lengths and angles
are listed in Table 2. Corresponding atoms of molecules 1 and
2 are numbered n and n + 50 such as Ag1 and Ag51.
(3) Omary, M. A.; Webb, T. R.; Assefa, Z.; Shankle, G. E.; Patterson, H.
H. Inorg. Chem. 1998, 97, 1380.
(4) Singh, K.; Long, J. R.; Stavropoulos, P. J. Am. Chem. Soc. 1997, 119,
2942.
(5) (a) Datta, D.; Chakravorty, A. Inorg. Chem. 1983, 22, 1085. (b) Pal,
S.; Bandyopadhyay, D.; Datta, D.; Chakravorty, A. J. Chem. Soc.,
Dalton Trans. 1985, 159.
(6) Dickmann, M. H.; Doedens, R. J. Inorg. Chem. 1980, 19, 3112.
(7) (a) Sommerer, S. O.; Wescott, B. L.; Jircitono, A. J.; Abboud, K. A.
Inorg. Chim. Acta 1995, 238, 149. (b) Skopenko, V.; Ponomareva,
V.; Simonov, Y.; Domasevitch, K.; Dvorkin, A. Russ. J. Inorg. Chem.
1994, 39, 1270.
a. Coordination Geometry. Coordinated oxime and azo
nitrogen atoms will be generally designated as No and Na,
respectively. In 2 each ligand is bonded to the metal solely at
the No atom, the azooxime function having a transoid disposi-
tion. The nearly perfectly planar (mean deviation 0.01 Å) AgNo
3
10.1021/ic000056u CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/06/2000

Notes
Inorganic Chemistry, Vol. 39, No. 13, 2000 2955
1.61 Å; O2-H2‚‚‚O1, 165.3°; and O3-H3, 1.03 Å;
O1A‚‚‚H3, 1.57 Å; and O3-H3‚‚‚O1A, 155.6°. The bonding
is unsymmetrical8 and is bifurcated at the deprotonated O1 site.
In 3 the intramonomer O‚‚‚O distance is ∼5.72 Å. The
intermonomer O1‚‚‚O51 and O2‚‚‚O52 lengths are ∼2.40 Å.
This is about the lowest possible limit of the hydrogen-bonded
O‚‚‚O distance below which oxygen-oxygen repulsion becomes
dominant.8 The hydrogen atom H2 was located in difference
Fourier maps at distances of 1.13 and 1.27 Å from O2 and O52,
respectively, the O2-H2‚‚‚O52 angle being 175°. Here the
hydrogen bonding is more symmetrical than that in 2, consistent
with the shorter O‚‚‚O distance.8
Figure 3. Perspective view and atom labeling scheme of the Ag(PhL)-
(HPhL) monomer (molecule 1) in 3. All atoms are represented by their
30% thermal probability ellipsoid.
c. Silver-Silver Affinity. The HMeL ligand affords 2 with
silver(I), but with copper(I) it yields the bis complex [Cu(MeL)-
(HMeL)]2, 4.5 We do not have a good explanation for this
difference, but fortunately the structures of 3 and 46 are grossly
similar, thus providing an opportunity for silver-copper com-
parison. The MN4 coordination sphere is far more squashed in
3 than in 4: the average Na-M-Na angle is 145° in 3 and
126° in 4, while the average No-M-Na angle is 139° in 3 and
123° in 4. The effect of this is that the Ag‚‚‚Ag distance in 3
(3.395(1) Å) is ∼1 Å shorter than the Cu‚‚‚Cu distance in 4
(4.377(2) Å) even though the silver(I) atom is significantly
bigger11 than the copper(I) atom. Indeed the Ag‚‚‚Ag distance
in 3 has fallen below the sum of van der Waals radii, 3.44 Å,
due to relativistic contraction.
Figure 4. The [Ag(PhL)(HPhL)]2 dimer; A, A/, B, B/ are the centroids
of the concerned phenyl rings.
The metrical difference between 3 and 4 is not due to the
difference in substituents (C-Ph in 3 and C-Me in 4). There
is no significant Ph‚‚‚Ph stacking in 3. The distances between
the centroids (A, A′, B, B′) of the C-Ph rings which appear to
lie relatively close to each other in Figure 4 are A‚‚‚B′ ) 5.92
Å, dihedral angle 25.2°, and A′‚‚‚B ) 5.50 Å, dihedral angle
25.6°.
The interdimer O‚‚‚O separation is ∼2.4 Å in both 3 and 4.6
Indeed, if not for the rigidity imposed by the very short hydrogen
bond, the Ag‚‚‚Ag distance in 3 could have been even shorter.
In 2 the Ag‚‚‚Ag separation, 3.517(1) Å, is slightly longer than
the van der Waals sum. Significantly, strong hydrogen bonds
controlling structural rigidity are more in number in 2 (four)
than in 3 (two). In these structures the strong hydrogen bonding
supports the dimeric forms, thus providing the opportunity for
pairs of silver atoms to come near each other in the first place.
The geometries assumed by the silver atoms (planar in 2 and
highly squashed tetrahedral in 3) are also conducive to close
approach. But the rigidity of the hydrogen-bonding system stiffly
restrains this closeness beyond a level. There is thus a
compromise, and in effect, relativistic contraction finds only a
mild expression in the present species.
Table 2. Selected Bond Distances (Å) and Angles (deg) and Their
Estimated Standard Deviations for [Ag(PhL)(HPhL)]2
Distances
Ag1-N6
Ag1-N4
Ag1-N1
Ag1-N3
N1-N2
O1-N3
O2-N6
N4-N5
O1‚‚‚O51
Ag‚‚‚Ag
2.319(4)
2.320(4)
2.319(4)
2.325(5)
1.259(5)
1.322(5)
1.346(5)
1.263(5)
2.414(5)
3.395(1)
Ag51-N56
Ag51-N54
Ag51-N51
Ag51-N53
N51-N52
O51-N53
O52-N56
N54-N55
O2‚‚‚O52
2.311(4)
2.338(4)
2.338(4)
2.306(4)
1.262(5)
1.342(5)
1.329(5)
1.257(5)
2.426(5)
Angles
N6-Ag1-N4
N6-Ag1-N1
N4-Ag1-N1
N6-Ag1-N3
N4-Ag1-N3
N1-Ag1-N3
68.3(2)
N56-Ag51-N54
N56-Ag51-N51
N54-Ag51-N51
N56-Ag51-N53
N54-Ag51-N53
N51-Ag51-N53
67.8(2)
127.8(2)
145.4(2)
135.9(2)
125.6(2)
67.8(2)
125.0(2)
143.7(2)
141.3(2)
126.9(2)
67.3(2)
coordination sphere has a distorted-T shape. The Ag-N4 bond
is ∼0.2 Å longer than the Ag-N7 and Ag-N1 bonds, the
N1-Ag-N7 angle being 152.1(1)°.
In 3, the azooxime function has cisoid disposition and each
ligand is chelated to the metal via No and Na atoms, affording
satisfactorily planar (mean deviation 0.02-0.05 Å) five-
membered chelate rings. The AgNo Na coordination spheres
are strongly squashed tetrahedra, vide infra. The Ag-N
distances lie within the range 2.31-2.34 Å.
b. Hydrogen Bonding. In 2 both the intramonomer
O1‚‚‚O2 and intermonomer O1‚‚‚O3A distances lie near 2.55
Å, indicating the presence of strong oxime-oximato hydrogen
bonds.8-10 The hydrogen atoms attached to O2 and O3 (Figure
1) were directly observed in the difference Fourier maps, and
the relevant bond parameters are O2-H2, 0.97 Å; O1‚‚‚H2,
Concluding Remarks
The main findings of this work will now be summarized.
The reaction of ammoniacal silver hydroxide with 1 has afforded
dimeric 2 and 3, thus providing an opportunity to scrutinize
the status of intradimer silver-silver contacts. The transoid
2
2
ligand in 2 binds via No only, the geometry of AgNo being
3
distorted-T. In 3, the cisoid ligand is chelated and the AgNo Na2
2
(10) (a) Szczepura, L. F.; Muller, J. G.; Bessel, C. A.; See, R. F.; Janik, J.
S.; Churchill, M. R.; Takeuchi, K. J. Inorg. Chem. 1992, 31, 859. (b)
Asaro, F.; Pellizer, G.; Tarangacco, L.; Costa, G. Inorg. Chem. 1994,
33, 5404. (c) Siripaisarnpipat, S.; Schlemper, E. O. Inorg. Chem. 1984,
23, 330.
(8) Chakravorty, A. Coord. Chem. ReV. 1974, 13, 1.
(11) (a) Bondi, A. J. Phys. Chem. 1964, 68, 441. (b) Bayler, A.; Schier,
A.; Bowmaker, G. A.; Schmidbaur, H. J. Am. Chem. Soc. 1996, 118,
7006. (c) Tripathi, U. M.; Bauer, A.; Schmidbaur, H. J. Chem. Soc.,
Dalton Trans. 1997, 2865.
(9) (a) Manivannan, V.; Dirghangi, B. K.; Pal, C. K.; Chakravorty, A.
Inorg. Chem. 1997, 36, 1526. (b) Ganguly, S.; Manivannan, V.;
Chakravorty, A. J. Chem. Soc., Dalton Trans. 1998, 461.

2956 Inorganic Chemistry, Vol. 39, No. 13, 2000
Notes
Table 3. Crystallographic Data for [Ag(MeL)(HMeL)2]2 and
[Ag(PhL)(HPhL)]2
tetrahedron is highly squashed. A total of four and two strong
O‚‚‚O hydrogen bonds characterize 2 and 3, respectively.
The squashing that makes the Ag‚‚‚Ag separation in 3 less
than the sum of van der Waals radii is believed to be the result
of relativistic contraction. The observed effect is relatively mild
because of the structural rigidity imposed by hydrogen bonds.
In 2, the molecular environment is more rigid and the Ag‚‚‚Ag
contact is also longer.
[Ag(MeL)(HMeL)2]2
[Ag(PhL)(HPhL)]2
empirical formula
fw
space group
a, Å
b, Å
C48H52N18O6Ag2
1192.82
P21/c
12.038(3)
10.331(3)
22.003(7)
C52H42N12O4Ag2
1114.72
P1h
8.761(4)
14.433(4)
20.570(9)
80.35(4)
77.61(4)
74.79(3)
2434(2)
2
c, Å
The complexes 2 and 3 are useful starting materials for
synthesis of azooxime chelates that are otherwise not accessible.
The chemistry based on this feature will be presented elsewhere.
R, deg
â, deg
γ, deg
V, Å3
Z
92.88(2)
2732.8(13)
2
Experimental Section
T, K
λ, Å
293(2)
0.710 73
1.450
7.80
5.00
10.28
293(2)
0.710 73
1.521
8.63
5.12
Physical Measurements. A Hitachi 330 spectrophotometer was used
to record UV-vis spectra, a Perkin-Elmer 2400 Series(II) elemental
analyzer was used to collect microanalytical data (CHN), and IR spectra
were recorded by Perkin-Elmer 783 spectrometer.
Synthesis of Ligands and Complexes. The ligands were synthesized
as previously described,12 and the complexes were prepared as described
below.
F
calcd, g cm-3
µ, cm-1
R1a [I > 2σ(I)], %
wR2b [I > 2σ(I)], %
7.84
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑w(Fo2 - Fc2)2/∑w(Fo )2]1/2
.
2
[(Phenylazo)acetaldoximato][bis(phenylazo)acetodoxime]silver-
(I), [Ag(MeL)(HMeL)2]2. A 0.05 g (0.29 mmol) portion of silver nitrate
was dissolved in 10 mL of water, and a dilute solution of sodium
hydroxide was added to it dropwise till complete precipitation of silver
hydroxide. The precipitate was dissolved by adding a minimum volume
of dilute ammonia, and the colorless solution so formed (Tollen’s
reagent) was added to a methanolic solution (20 mL) of HMeL (0.144
g, 0.88 mmol). The mass was allowed to stir for 2 h. The brown
precipitate that separated out was filtered, washed with 1:1 aqueous
methanol, and dried in vacuo over fused CaCl2. Yield: 0.158 g (90%).
Anal. Calcd for 2, C48H52N18O6Ag2: C, 48.33; H, 4.39; N, 21.14.
Found: C, 48.28; H, 4.38; N, 21.18. UV-vis (CH2Cl2, λmax, nm (ꢀ,
M-1 cm-1)): 350(12 090); 310 (14 340). IR (KBr, cm-1): νOH 2850
(br).
[(Phenylazo)benzaldoximato][(phenylazo)benzaldoxime]silver-
(I), [Ag(PhL)(HPhL)]2. This complex was prepared by the same
procedure as above starting from 0.05 g (0.29 mmol) of silver nitrate
and 0.133 g (0.59 mmol) of HPhL. Yield: 0.160 g (88%). Anal. Calcd
for 3, C52H42N12O4Ag2: C, 56.13; H, 3.62; N, 15.11. Found: C, 56.08;
H, 3.60; N, 15.09. UV-vis (CH2Cl2, λmax, nm ꢀ, M-1 cm-1)): 415
(6196); 390 (6270). IR (KBr, cm-1): νOH 2800 (br).
X-ray Structure Determination. Crystals of [Ag(MeL)(HMeL)2]2
(0.3 × 0.4 × 0.3 mm) and [Ag(PhL)(HPhL)]2 (0.3 × 0.3 × 0.4 mm)
were grown by slow diffusion of hexane into benzene solutions. For
both the complexes cell parameters were determined by the least-squares
fit of 30 machine-centered reflections (2θ ) 15-30°). Data were
collected by the ω-scan technique in the 2θ range 3-50° on a Siemens
R3m/V four-circle diffractometer with graphite-monochromated Mo
KR radiation (λ ) 0.710 73 Å). Two check reflections measured after
every 98 reflections showed no intensity reduction. Data were corrected
for Lorentz-polarization effects, and an empirical absorption correction
was performed on both sets of data on the basis of azimuthal scans13
of six reflections. For [Ag(MeL)(HMeL)2]2 6537 reflections were
collected, 6229 were unique, and 3599 satisfying I > 2σ(I) were used
for structure solution. In the case of [Ag(PhL)(HPhL)]2 the correspond-
ing numbers are 9305, 8675, and 4522, respectively.
All calculations for data reduction, structure solution and refinement
were done using programs of SHELXTL, Ver. 5.03.14 Both the
structures were solved by direct methods and were refined by full-
matrix least-squares on F2. All non-hydrogen atoms in both the
structures were refined anisotropically. A few hydrogen atoms were
located in difference Fourier maps, and the rest were included in
calculated positions. Significant crystal data are listed in Table 3.
The Cu‚‚‚Cu distance in [Cu(MeL)(HMeL)]2 was calculated using
the available atomic coordinates.6
Acknowledgment. Financial support received from the
Indian National Science Academy, the Department of Science
and Technology and Council of Scientific and Industrial
Research, New Delhi, are acknowledged. Affiliation with the
Jawaharlal Nehru Centre for Advanced Scientific Research,
Bangalore, is acknowledged.
Supporting Information Available: X-ray crystallographic files,
in CIF format, for [Ag(MeL)(HMeL)2]2 and [Ag(PhL)(HPhL)]2. This
material is available free of charge via the Internet at http://pubs.acs.org.
IC000056U
(13) North, A. C. T.; Phillips, D. C.; Mathews, F. C. Acta Crystallogr.,
Sect. A. 1968, 24, 351.
(14) Sheldrick, G. M. SHELXTL, Version 5.03, Siemens Analytical
Instruments Inc., Madison, WI, 1994.
(12) Kalia, K. C.; Chakravorty, A. J. Org. Chem. 1970, 35, 2231.