Unusual transformation of N-arylbenzohydroxamic acids mediated by osmium. Formation of organometallic complexes of osmium(III)

Article abstract of DOI:10.1021/ic0012569

The N-arylbenzohydroxamic acids (1) undergo unusual chemical transformation upon reaction with [Os(bpy)₂Br₂] and afford a family of organometallic complexes of osmium(III) (2), where the transformed hydroxamic acids are coordinated to osmium as doubly deprotonated benzanilides.

Full text of DOI:10.1021/ic0012569

Inorg. Chem. 2001, 40, 4085-4088
4085
Chart 1
Unusual Transformation of
N-Arylbenzohydroxamic Acids Mediated by
Osmium. Formation of Organometallic
Complexes of Osmium(III)
Anindya Das,† Falguni Basuli,† Larry R. Falvello,‡ and
Samaresh Bhattacharya*,†
Department of Chemistry, Inorganic Chemistry Section,
Jadavpur University, Kolkata 700 032, India, and
Department of Inorganic Chemistry, Faculty of Science,
University of Zaragoza, E-50009 Zaragoza, Spain
ReceiVed NoVember 14, 2000
Introduction
The chemistry of transition metal hydroxamates has always
been of considerable interest largely because of their bioinor-
ganic relevance.1 In this family of hydroxamate complexes, the
iron hydroxamates have received most attention in connection
with the bacterial iron transport mechanism in the life process.2
Among the two congeners of iron, the chemistry of ruthenium
hydroxamates has received some attention,3 while that of
osmium hydroxamates appears to remain totally unexplored. In
the present work, which has originated from our interest in the
chemistry of osmium complexed by ligands of different types,4
our objective was to explore the chemistry of osmium hydrox-
amates and a group of hydroxamic acids of type 1 (see Chart
1) was chosen for this purpose. Hydroxamic acids are well-
known to bind to metal ions as bidentate O,O-donor ligands
forming stable five-membered chelate rings (2).5 Our primary
goal was restricted to the synthesis of osmium complexes
containing only one hydroxamate ligand. To satisfy the remain-
ing coordination sites of osmium, 2,2′-bipyridine (bpy) was
chosen to serve as the coligand. With this strategy in mind,
reactions of the N-arylbenzohydroxamic acids were carried out
with [Os(bpy)2Br2]. However, the product obtained from these
reactions has been found to be interestingly different from our
expectation. The hydroxamic acids undergo unusual chemical
transformation and bind to osmium as dianionic C,N-donors
(as shown in 3) affording cyclometalates of osmium(III) of type
[OsIII(bpy)2(R-R′)]+ (where R-R′ refers to the bidentate ligand
in 3). The chemistry of these complexes is described here with
special reference to their formation, structure, and properties.
† Jadavpur University.
‡ University of Zaragoza.
(1) (a) Mizukami, S.; Nagaka, K. Coord. Chem. ReV. 1968, 3, 267. (b)
Chatterjee, B. Coord. Chem. ReV. 1978, 26, 281. (c) Crumbliss, A. L.
Coord. Chem. ReV. 1990, 105, 155. (d) Kurzak, B.; Kozlowski, H.;
Farkas, E. Coord. Chem. ReV. 1992, 114, 169.
Experimental Section
Materials. Osmium tetraoxide was purchased from Arora Matthey,
Calcutta, India, and was converted to [NH4]2[OsBr6] by reduction with
hydrobromic acid.6 [Os(bpy)2Br2] was synthesized, starting from [NH4]2-
[OsBr6], by following a reported procedure.7 2,2′-Bipyridine, 4-ni-
trobenzene, 1-chloro-4-nitrobenzene, and 4-nitrotoluene were purchased
from Loba Chemie, Mumbai, India. The nitrobenzenes were converted
to the corresponding hydroxylamines by following a literature method.8
Benzoyl chloride, 4-nitrobenzoyl chloride, and 4-chlorobenzoyl chloride
were purchased from Eastgate, Whiteland, Morecambe, England. The
hydroxamic acids were prepared by acylation of the hydroxylamines
with the acid chlorides by following a published procedure.9 Purification
of acetonitrile and preparation of tetraethylammonium perchlorate
(TEAP) for electrochemical work were performed as reported in the
literature.10 All other chemicals used for the preparative works were
of reagent grade and were used without further purification.
(2) Albrecht-Gary, A.-M.; Crumbliss, A. L. In Metal Ions in Biological
Systems; Sigel, A., Sigel, H., Eds.; Marcel Dekker Inc.: New York,
1988; Vol. 35, pp 239-328.
(3) (a) Mishra, R. S. J. Indian Chem. Soc. 1967, 44, 400. (b) Mishra, R.
S. J. Indian Chem. Soc. 1969, 46, 1074. (c) Ghosh, P.; Chakravorty,
A. Inorg. Chem. 1984, 23, 2242. (d) Ghosh, P.; Chakravorty, A. J.
Chem. Soc., Dalton Trans. 1985, 361.
(4) (a) Basuli, F.; Peng, S. M.; Bhattacharya, S. Inorg. Chem. 1997, 36,
5645. (b) Basuli, F.; Peng, S. M.; Bhattacharya, S. Polyhedron 1998,
17, 2191. (c) Basuli, F.; Ruf, M.; Pierpont, C. G.; Bhattacharya, S.
Inorg. Chem. 1998, 37, 6113. (d) Basuli, F.; Peng, S. M.; Bhattacharya,
S. Polyhedron 1999, 18, 391. (e) Das, A.; Basuli, F.; Peng, S. M.;
Bhattacharya, S. Polyhedron 1999, 18, 2729. (f) Basuli, F.; Peng, S.
M.; Bhattacharya, S. Inorg. Chem. 2000, 39, 1120. (g) Das, A.; Peng,
S. M.; Bhattacharya, S. Polyhedron 2000, 19, 1227. (h) Majumder,
K.; Peng, S. M.; Bhattacharya, S. J. Chem. Soc., Dalton Trans. 2001,
284.
(5) (a) Zalkin, A.; Forrester, J. D.; Templeton, D. H. J. Am. Chem. Soc.
1966, 88, 1810. (b) Helm, D. van der; Poling, M. J. Am. Chem. Soc.
1976, 98, 82. (c) Helm, D. van der; Baker, J. R.; Eng-Nilmot, D. L.;
Hossain, M. B. Loghry, R. A. J. Am. Chem. Soc. 1980, 102, 4224. (d)
Mocherla, R. R.; Powell, D. R.; Barnes, C.; van der Helm, D. Acta
Crystallogr. 1983, C39, 868. (e) Hossain, M. B.; Jalal, M. A. F.; van
der Helm, D. Acta Crystallogr. 1986, C42, 1305. (f) van der Helm,
D.; Jalal, M. A. F.; Hossain, M. B. In Iron Transport in Microbes,
Plants and Animals; Winkelmann, G., van der Helm, D., Neilands, J.
B, Eds.; VCH Publ.: Wienheim, Germany, 1987; pp 135-165. (g)
Dietrich, A.; Powell, D. R.; Eng-Wilmot, D. L.; Hossain, M. B.; van
der Helm, D. Acta Crystallogr. 1990, C46, 816. (h) Dietrich, A.;
Fidelis, K. A.; Powell, D. R.; van der Helm, D.; Eng-Wilmot, D. L.
J. Chem. Soc., Dalton Trans. 1991, 231.
Preparation of the [Os(bpy)2(R-R′)]ClO4 Complexes. The [Os-
(bpy)2(R-R′)]ClO4 complexes were synthesized by following two
(6) Dwyer, E. P.; Hogarth, J. W. Inorg. Synth. 1957, 5, 204.
(7) Kober, E. M.; Casper, J. V.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem.
1988, 27, 4587.
(8) Vogel, A. I. A Text Book of Practical Organic Chemistry, 3rd ed.;
ELBS, Longman: London, 1968; p 197.
(9) Hanser, C. R.; Renfrow, W. B., Jr. Organic Syntheses; John Wiley:
New York, 1943; Collect. Vol. 2, p 67.
(10) (a) Sawyer, D. T.; Roberts, J. L., Jr. Experimental Electrochemistry
for Chemists; Wiley: New York, 1974; pp 167-215. (b) Walter, M.;
Ramaley, L. Anal. Chem. 1973, 45, 165.
10.1021/ic0012569 CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/30/2001

4086 Inorganic Chemistry, Vol. 40, No. 16, 2001
Table 1. Crystallographic Data for [Os(bpy)2(H-H)]ClO4
Notes
empirical formula
fw
C33H25ClN5O5Os
797.23
space group
a, Å
b, Å
orthorhombic, P212121
10.660(5)
14.990(4)
18.662(3)
2982.1(17)
4
c, Å
V, Å3
Z
λ, Å
0.710 73
cryst size, mm
T, K
0.27 × 0.12 × 0.11
297(2)
µ, mm-1
R1a
4.418
0.0558
1253
1.032
wR2b
GOFc
Figure 1. View of the [Os(bpy)2)(H-H)]+ complex.
a R1 ) ∑|Fo| - |Fc|/∑|Fo|. b wR2 ) [∑{w(Fo2 - Fc2)2}/∑{w(Fo2)}]1/2.
c GOF ) [∑(w(Fo - Fc2)2)/(M - N)]1/ 2, where M is the number of
2
Table 2. Selected Bond Distances and Bond Angles for
[Os(bpy)2(H-H)]ClO4
reflections and N is the number of parameters refined.
Bond Distances (Å)
general procedures. Yields varied in the range of 60-70%. Specific
details are given below for one complex.
Os(1)-N(5)
Os(1)-N(4)
Os(1)-C(27)
Os(1)-N(1)
Os(1)-N(3)
Os(1)-N(2)
N(5)-C(21)
C(21)-O(1)
2.041(15)
2.047(14)
2.06(2)
2.076(14)
2.091(15)
2.186(14)
1.38(2)
C(21)-C(22)
C(22)-C(27)
C(22)-C(23)
C(23)-C(24)
C(24)-C(25)
C(25)-C(26)
C(26)-C(27)
1.45(3)
1.40(3)
1.41(3)
1.36(3)
1.33(3)
1.38(3)
1.37(3)
[Os(bpy)2(H-H)]ClO4. Method A. [Os(bpy)2Br2] (100 mg, 0.15
mmol) was dissolved in 3:1 ethanol-water (40 cm3), and to it was
added N-phenylbenzohydroxamic acid (32 mg, 0.15 mmol) followed
by triethylamine (50 mg, 0.50 mmol). The solution was heated at reflux
for 8 h to produce a dark brown solution. After this solution was cooled
to room temperature, a saturated aqueous solution of NaClO4 (0.5 cm3)
was added to it followed by a large volume (∼60 cm3) of water. The
brown solution was then extracted repeatedly with dichloromethane,
and the extract was concentrated to ∼10 cm3 by evaporation under
reduced pressure. The concentrate was then triturated with 10 cm3 of
benzene causing precipitation of [Os(bpy)2(H-H)]ClO4 as a dark brown
microcrystalline solid, which was collected by filtration and dried in
air. Yield: 62%.
Method B. To a mixture of N-phenylbenzohydroxamic acid (32 mg,
0.15 mmol) and Na2CO3 (100 mg, 0.94 mmol) were added 2-meth-
oxyethanol (40 cm3) and [Os(bpy)2Br2] (100 mg, 0.15 mmol). The
resulting mixture was heated at reflux for 72 h under a N2 atmosphere
to produce a dark brown solution. After the solution was cooled to
room temperature, a saturated aqueous solution of NaClO4 (0.5 cm3)
was added to it. Upon partial evaporation [Os(bpy)2(H-H)]ClO4
precipitated as a brown solid, which was collected by filtration, washed
with ice-cold water, and dried in air. Purification of the product was
done by recrystallization from dichloromethane-benzene solution. Yield:
68%.
1.20(2)
Bond Angles (deg)
175.2(6) C(27)-Os(1)-N(2) 174.4(7)
172.0(6)
97.0(6) C(27)-Os(1)-N(3)
78.4(7) N(1)-Os(1)-N(3)
83.6(7) N(5)-Os(1)-N(2)
87.3(6) N(4)-Os(1)-N(2)
99.6(7) N(1)-Os(1)-N(2)
78.6(6) N(3)-Os(1)-N(2)
N(4)-Os(1)-N(1)
N(5)-Os(1)-N(3)
N(5)-Os(1)-N(4)
N(5)-Os(1)-C(27)
N(4)-Os(1)-C(27)
N(5)-Os(1)-N(1)
C(27)-Os(1)-N(1)
N(4)-Os(1)-N(3)
94.4(7)
97.5(6)
96.8(6)
100.1(6)
77.0(6)
90.5(6)
least-squares fit of 25 centered reflections. Data were collected on an
Enraf Nonius CAD-4 diffractometer using graphite-monochromated Mo
KR radiation (λ ) 0.710 73 Å) by ω-θ scans within the angular range
2.18 < θ < 24.98°. Three standard reflections were used to check the
crystal stability toward X-ray exposure, and they showed no significant
intensity variation over the course of data collection. X-ray data
reduction, structure solution, and refinement were done using the
SHELXS-97 and SHELXL-97 packages. The structure was solved by
the direct methods.
Physical Measurements. Microanalyses (C, H, N) were performed
using a Perkin-Elmer 240C elemental analyzer. IR spectra were obtained
on a Perkin-Elmer 783 spectrometer with samples prepared as KBr
pellets. Electronic spectra were recorded on JASCO V-570 and Hitachi
330 spectrophotometers. Solution electrical conductivities were mea-
sured using a Philips PR 9500 bridge with a solute concentration of
10-3 M. Magnetic susceptibilities were measured using a PAR 155
vibrating-sample magnetometer fitted with a Walker scientific L75FBAL
magnet. ESR spectral studies were carried out with a Varian model
109C E-line X-band spectrometer fitted with a quartz dewar for
measurements at 77 K (liquid dinitrogen). Electrochemical measure-
ments were made using a PAR model 273 potentiostat. A platinum-
disk working electrode, a platinum wire auxiliary electrode and an
aqueous saturated calomel reference electrode (SCE) were used in a
three-electrode configuration. A graphite working electrode was also
used for scanning the negative side of SCE. A platinum wire gauze
working electrode was used in the coulometric experiments. A RE 0089
X-Y recorder was used to trace the voltammograms. Electrochemical
measurements were made under a dinitrogen atmosphere. All electro-
chemical data were collected at 298 K and are uncorrected for junction
potentials.
Results and Discussion
Nine hydroxamic acids have been used in the present study,
which are abbreviated in general as H2OR-R′, where H2 stands
for the two protons that undergo dissociation upon complexation,
O stands for the hydroxy oxygen, and R and R′ stand for the
two substituents. The ligands along with their individual
abbreviations are shown in 4 (Chart 1). Reaction of these
hydroxamic acids with [Os(bpy)2Br2] proceeds smoothly in 1:3
aqueous-ethanol medium in the presence of triethylamine to
afford brown solutions of [Os(bpy)2(R-R′)]+. Reactions carried
out in refluxing 2-methoxyethanol in the presence of Na2CO3
also yield the same complexes. The complex cations have been
isolated as perchlorate salts in the solid state. Identity of these
complexes has been revealed by structure determination of a
representative member of this family, viz. [Os(bpy)2(H-H)]-
ClO4. The structure is shown in Figure 1, and selected bond
parameters are listed in Table 2.
It is very interesting to find out that the N-phenylbenzo-
hydroxamic acid has undergone oxygen loss from the NOH
function and is coordinated to osmium, via loss of two protons,
as a dianionic bidentate C,N-donor ligand forming a five-
Crystallography of [Os(bpy)2(H-H)]ClO4. Single crystals were
grown by slow diffusion of benzene into a dichloromethane solution
of the complex. Selected crystal data and data collection parameters
are given in Table 1. The unit cell dimensions were determined by a

Notes
Inorganic Chemistry, Vol. 40, No. 16, 2001 4087
membered chelate ring (as in 3) with a bite angle of 78.4(7)°.
As the coordinated ligand is equivalent to a doubly deprotonated
benzanilide, the organic transformation may be regarded as
conversion of N-phenylbenzohydroxamic acid to benzanilide.
Precedence of metal-assisted reduction of N-phenylbenzo-
hydroxamic acid to benzanilide is avilable in the literature.11
However, in all these examples the hydroxamic acid served as
a mere oxidizing agent for the metal and itself was reduced to
benzanilide, which never participated in complexation and was
aways obtained as a byproduct. In our present case, the striking
difference is that the reduced hydroxamic acid (viz. benzanilide)
remains coordinated to the oxidized metal (viz. osmium(III)),
which appears to be unprecedented in the literature. The
mechanism of the observed transformation of hydroxamic acid
is not clear to us. However, prior coordination of the hydroxamic
acid to osmium(II) followed by intramolecular redox reaction
between the coordinated hydroxamic acid and osmium(II) finally
leading to formation of the organometallic complex seems
probable. It is interesting to note that such cyclometalates of
osmium(III) are scarce in the literature.4h,12 Within the
Os(C-N) chelate, the Os-C length is comparable to the
OsIII-C distances observed by us before12b and the Os-N(5)
distance is also normal.13 The C(21)-O(1) distance (1.20(2)
Å) indicates the double bond character of the carbonyl function.
Distances within the Os(bpy)2 fragment are quite usual, as
observed in structurally characterized osmium(III) complexes
having the OsIII(bpy)2 moiety.13 However, the Os-N(2) bond,
which is trans to the Os-C bond, is noticeably longer than the
other four Os-N bonds. The OsCN5 core is significantly
distorted from ideal octahedral geometry, as reflected in the bond
parameters around osmium. As all the nine [Os(bpy)2(R-R′)]-
ClO4 complexes have been synthesized similarly and as all these
complexes exhibit similar properties (vide infra), the other eight
complexes are assumed to have similar structure as [Os(bpy)2-
(H-H)]ClO4. Elemental (C, H ,N) analytical data of all the
complexes are consistent with their compositions.
been recorded in acetonitrile solution. Each complex shows
seven absorptions, two in the near-infrared region, three in the
visible region, and two in the ultraviolet region. The two
absorptions in the near-infrared region are weak in intensity,
and these are assignable to the two possible transitions within
the three split components of the t2 orbitals, which are known
to occur at low energies.15 The absorptions in the visible region
are of high intensity, and they are probably due to allowed
ligand-to-metal charge-transfer transitions. The absorptions in
the ultraviolet region are very intense and are assignable to
transitions within the ligand orbitals.
Electrochemical properties of the [Os(bpy)2(R-R′)]ClO4
complexes have been studied in acetonitrile solution (0.1 M
TEAP) by cyclic voltammetry. Each complex shows an oxida-
tive response on the positive side of SCE and three reductive
responses on the negative side. The oxidative response, observed
within 0.76 to 0.92 V, is assigned to osmium(III)-osmium-
(IV) oxidation. This oxidation is quasi-reversible in nature,
characterized by a slightly larger anodic peak current (ipa) than
the cathodic peak current (ipc). One-electron stoichiometry of
this oxidation has been established by comparing its current
height (ipa) with that of standard ferrocene-ferrocenium couple
under identical experimental conditions. The first reductive
response, observed within -0.32 to -0.18 V, is assigned to
the osmium(III)-osmium(II) reduction. This reduction is revers-
ible, characterized by a peak-to-peak separation (∆Ep) of 60
mV, and the anodic peak current (ipa) is almost equal to the
cathodic peak current (ipc). The one-electron nature of this
reduction has been established by comparing its current heights
with the anodic peak current (ipa) of the oxidative response. In
[Os(bpy)3]2+, the osmium(II)-osmium(III) couple appears at
0.84 V.16 This large negative shift of the osmium(III)-osmium-
(II) potential upon replacing one bpy by one R-R′ ligand shows
the hard character of these R-R′ ligands and their efficiency
in stabilizing the higher oxidation states of osmium. Potentials
of both the osmium(III)-osmium(IV) and osmium(III)-os-
mium(II) couples are found to be sensitive to the nature of the
two substituents (R and R′) on the R-R′ ligands. The potentials
increase as electron-withdrawing character of the substituents
increase. Two successive one-electron reductions are displayed
by all these complexes at potentials below -1.6 V vs SCE,
which are assigned to reductions of the two bpy ligands. It is
well-known that each bpy can successively accept two electrons
in the lowest unoccupied molecular orbital.17 Hence in these
[Os(bpy)2(R-R′)]+ complexes, four successive reductions are
expected. Only two of these have been experimentally observed,
and the other two have not been observed because of solvent
cutoff.
Magnetic susceptibility measurements show that the
[Os(bpy)2(R-R′)]ClO4 complexes are one-electron paramag-
netic (µeff ) 1.87-1.93 µB), which corresponds to the +3 state
of osmium (low-spin d5, S ) 1/2) in these complexes. However,
ESR studies show that these [Os(bpy)2(R-R′)]+ complexes are
ESR-silent. ESR inactivity in low-spin d5 complexes is quite
common and is known to result from extensive mixing of the
Kramers doublets by strong spin-orbit coupling which gives
rise to short electronic relaxation time.4b,d,14 Infrared spectra of
the [Os(bpy)2(R-R′)]ClO4 complexes show many strong vibra-
tions below 1700 cm-1. The ν(CdO) stretch is uniformly
displayed as a strong band by all the complexes near 1600 cm-1
.
Two intense bands are observed near 1110 and 625 cm-1 in all
the complexes, and these are attributed to the perchlorate ion.
The [Os(bpy)2(R-R′)]ClO4 complexes are soluble in solvents
such as dichloromethane, acetonitrile, acetone, etc., producing
intense brown solutions. Conductance measurement in aceto-
nitrile solution shows that these complexes behave as 1:1
electrolytes (ΛM ) 140-150 Ω-1 cm2 M-1), as expected.
Electronic spectra of the [Os(bpy)2(R-R′)]ClO4 complexes have
The reversible character of the osmium(III)-osmium(II)
reduction, together with its relatively low potential, suggest that
the reduced complexes, viz. [OsII(bpy)2(R-R′)], should be fairly
stable. To investigate this, a representative complex, viz. [OsIII-
(bpy)2(H-H)]ClO4, has been coulometrically reduced at -0.5
V vs SCE in acetonitrile solution (0.1 M TEAP). The reduction
has been smooth and quantitative yielding a green solution
{electronic spectral data [λmax, nm (ꢀ, M-1 cm-1)]: 600 (5800),
490sh (6000), 420sh (7400), 380 (7900); sh ) shoulder}. The
voltammetric behavior of the reduced complex has been found
to be identical with that of the parent osmium(III) complex,
(11) (a) Smith, W. L.; Raymond, K. N. J. Inorg. Nucl. Chem. 1979, 41,
1431. (b) Brown, D. A.; Bogge, H.; Coogen, R.; Doocey, D.; Kemp,
T. J.; Muller, A.; Neumann, B. Inorg. Chem. 1996, 35, 1674. (c)
Farkas, E. Proc. ICCC 2000, 34, 6224.
(12) Gross, Z.; Mohammed, A. Inorg. Chem. 1996, 35, 7260.
(13) (a) Che, C.-M.; Cheng, W.-K.; Lai, T.-F.; Poon, C.-K.; Mak, T. C.
W. Inorg. Chem. 1987, 26, 1678. (b) Che, C.-M.; Lam, M. H.-W.;
Wang, R.-J.; Mak, T. C. W. J. Chem. Soc., Chem. Commun. 1990,
820.
(15) (a) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1982, 21, 3967. (b) Lahiri,
G. K.; Bhattacharya, S.; Ghosh, B. K.; Chakravorty, A. Inorg. Chem.
1987, 26, 4324.
(16) This oxidation potential has been determined by us.
(17) (a) Kahl, J. L.; Hanck, K. W.; DeArmond, K. J. Phys. Chem. 1978,
82, 540. (b) Vleck, A. A. Coord. Chem. ReV. 1982, 43, 39.
(14) Shirin, Z.; Mukherjee, R. N. Polyhedron 1992, 11, 2625.

4088 Inorganic Chemistry, Vol. 40, No. 16, 2001
Notes
except that the osmium(II)-osmium(III) couple appears as an
oxidative response. The same reduction has also been achieved
chemically by adding hydrazine to an acetonitrile solution of
the [Os(bpy)2(R-R′)]ClO4 complexes. While exposed to air,
green solutions of the [OsII(bpy)2(R-R′)] complexes undergo
oxidation to regenerate brown solutions of the [OsIII(bpy)2-
(R-R′)]+ complexes, which have been identified by their
respective electronic spectrum. This shows that the [OsII(bpy)2-
(R-R′)] complexes, though they can be generated easily by
reduction of their osmium(III) precursors, are susceptible to
aerial oxidation.
Acknowledgment. Financial assistance received from the
Department of Science and Technology, New Delhi (Grant No.
SP/S1/F33/98) and the Directorate General for Higher Education
and Scientific Research of Spain (Grant No. PB95-0792) is
gratefully acknowledged. Thanks are also due to the Third
World Academy of Sciences for financial support enabling the
purchase of an electrochemical cell system and to Dr. S. K.
Kar, Department of Chemistry, University of Calcutta, for his
help. The authors thank the reviewers for their critical comments
and suggestions which have been helpful in preparing the revised
version. F.B. thanks the University Grants Commission, New
Delhi, for her fellowship.
Conclusions
The present study shows a very unusual transformation of
N-arylbenzohydroxamic acids (1) mediated by an osmium
complex, viz. [Os(bpy)2Br2], affording a family of cyclometa-
lates of osmium(III) where the transformed hydroxamic acids
are coordinated as doubly deprotonated benzanilides (3). The
findings of the present study indicate that benzanilides should
be capable, in principle, of functioning as bidentate C,N-donor
ligands, and such a possibility is currently being explored.
Efforts are also being made to discover the right experimental
condition for binding the hydroxamic acids (1) to osmium in
the usual fashion (2).
Supporting Information Available: Figures giving the electronic
spectrum of [Os(bpy)2(Cl-Cl)]ClO4 and cyclic voltammogram of
[Os(bpy)2(H-H)]ClO4 and tables containing analytical data, electronic
spectral data, and cyclic voltammetric data of the [Os(bpy)2(R-R′)]-
ClO4 complexes and crystal data and details of structure determination,
atomic coordinates, anisotropic thermal parameters, and bond distances
and angles for [Os(bpy)2(H-H)]ClO4. This material is available free
of charge via the Internet at http://pubs.acs.org.
IC0012569