Transition metal complexes of benzil-α-monoxime (BMO); X-ray structure determination of Co(BMO)3

Article abstract of DOI:10.1016/S0277-5387(00)00429-0

Cobalt(III), nickel(II) and copper(II) complexes of benzil-α-monoxime (BMO) have been synthesized and characterized by elemental analyses, Conductivity, magnetic susceptibility measurements, IR and electronic spectral data. The molar conductivity data show them to be non-electrolytes. The monoanionic bidentate behavior of BMO is inferred from IR spectral studies. The electronic spectral data suggest planar geometry for nickel(II) and copper(II) complexes. The nickel complex assumes an octahedral structure in a coordinating solvent (N,N-dimethylformamide (DMF)). The [Co(BMO)₃] complex has been isolated as a single crystal and its structure was determined by X-ray diffraction. X-ray data confirms the presence of a fac-octahedral configuration for this complex. Electrochemical studies suggest that the cobalt complex undergoes two single one-electron reduction steps corresponding to Co(III) → Co(II) and Co(II) → Co(I). (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(00)00429-0

www.elsevier.nl/locate/poly
Polyhedron 19 (2000) 1687–1692
Transition metal complexes of benzil-a-monoxime (BMO); X-ray
structure determination of Co(BMO)3
P. Sambasiva Reddy, K. Hussain Reddy *
Department of Chemistry, Sri Krishnade6araya Uni6ersity, Anantapur 515 003, India
Received 28 January 2000; accepted 8 May 2000
Abstract
Cobalt(III), nickel(II) and copper(II) complexes of benzil-a-monoxime (BMO) have been synthesized and characterized by
elemental analyses, conductivity, magnetic susceptibility measurements, IR and electronic spectral data. The molar conductivity
data show them to be non-electrolytes. The monoanionic bidentate behavior of BMO is inferred from IR spectral studies. The
electronic spectral data suggest planar geometry for nickel(II) and copper(II) complexes. The nickel complex assumes an
octahedral structure in a coordinating solvent (N,N-dimethylformamide (DMF)). The [Co(BMO)3] complex has been isolated as
a single crystal and its structure was determined by X-ray diffraction. X-ray data confirms the presence of a fac-octahedral
configuration for this complex. Electrochemical studies suggest that the cobalt complex undergoes two single one-electron
reduction steps corresponding to Co(III)“Co(II) and Co(II)“Co(I). © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Transition metal complexes; Electronic spectral data; Oximes; Benzil-a-monoxime
1. Introduction
2. Experimental
Oximes are widely recognized ligands [1,2]. In gen-
eral, monoxime is known to coordinate in three differ-
ent ways (structures I, II and III). In structure III
monoxime acts as a bridging ligand. Among carbonyl
oximes diacetylmonoxime is the most studied ligand.
Ni(II), Cu(II) and Co(III) complexes of diacetyl
monoxime have been reported [3–5]. There is consider-
able confusion in the literature [6] about the nickel
complexes of benzil-a-monoxime (BMO). Because of
this, no report is available on Co(III), Ni(II) and Cu(II)
complexes of BMO. In continuation of our ongoing
research [7–11] on the metal complexes of benzil-based
derivatives, we report herein the synthesis and charac-
terization of metal complexes with BMO. The structure
of Co(BMO)3 is determined by X-ray single-crystal
diffraction. Variety in structural features of BMO metal
complexes is presented in this paper.
2.1. Materials and methods
Benzil and hydroxylamine hydrochloride were of
reagent grade. Solvents used for the synthesis of ligands
and their metal complexes were redistilled. All other
chemicals were of AR grade and used as supplied.
2.2. Preparation of ligand
BMO was prepared according to the procedure de-
scribed previously [12].
2.3. Preparation of complexes
Attempts to prepare a cobalt(II) complex of BMO
were not successful. Cobalt(III), nickel(II) and cop-
per(II) complexes were prepared by mixing a hot
methanolic solution of metal chlorides (1 mol) and
ligand (2 mol). To the boiling solution of the ligand
(0.01776 mol) in methanol (100 cm3) was added metal
chloride dissolved in a minimum quantity of methanol
and refluxed for 30 min. NaOAc (2 g) was added to the
* Corresponding author. Tel.: +91-8554-55367-73; fax: +91-
8554-29043.
0277-5387/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0277-5387(00)00429-0

1688
P. Sambasi6a Reddy, K. Hussain Reddy / Polyhedron 19 (2000) 1687–1692
Table 1
tion of the complex in dimethylformamide solvent.
Intensity data for a dark orange crystal with approxi-
mate dimensions 0.3×0.2×0.2 mm were measured at
r.t. 273(2) K on an Enraf–Nonius CAD4 diffractometer
using monochromated Mo Ka radiation (u=0.71073
Analytical data of ligand and their metal complexes
Ligand/complex Color
(m.p. ° C)
Percentage a of
Carbon Hydrogen
Nitrogen
,
A) with a scan mode of ꢀ–2q. No decomposition of the
BMO
light pink 74.00
(140–141) (74.66)
orange red 68.20
(195–197) (68.94)
light green 65.95
(215–218) (66.40)
dark green 65.67
(175–177) (65.75)
4.57
(4.80)
4.15
(4.10)
3.72
(3.75)
3.85
6.50
(6.20)
5.70
(5.74)
5.48
(5.53)
5.49
crystal occurred during the data collections. The Co
atom was located from an E-map . The other non-hy-
drogen atoms were determined with successive differ-
ence Fourier syntheses. Final refinement by full-matrix
least-squares methods was carried out with anisotropic
thermal parameters for non-hydrogen atoms. The hy-
drogen atoms, included in the refinement were located
in succeeding difference Fourier synthesis after the non-
hydrogen atoms were refined anisotropically.
[Co(BMO)3]
[Ni(BMO)2]
[Cu(BMO)]n
(3.91)
(5.47)
a Calculated values are in parentheses.
3. Results and discussion
reaction mixture (in the case of the copper complex
CH3ONa is added) and refluxed for another 30 min.
Crystalline complexes which separated from the
solution were collected by filtration, washed with hot
water and a small quantity of methanol and dried
under vacuum over CaCl2. The analytical data of the
ligand and their complexes are presented in Table 1.
The analytical data of the ligand (Table 1) is consis-
tent with its proposed molecular formula. All the com-
plexes are insoluble in water and methanol, but readily
soluble (except copper complex) in CHCl3, DMF and
DMSO. The copper complex is sparingly soluble in
DMF and DMSO. The physico-chemical and analytical
data for all the complexes are given in Table 1. The
analytical data (Table 1) are consistent with the pro-
posed molecular formula. The molar conductivity data
reveal the nonelectrolytic nature of all the complexes.
Cobalt(III) and nickel(II) complexes are diamagnetic
and the magnetic moment of the copper(II) complex is
found to be 1.24 BM indicating the presence of one
unpaired electron. The value is subnormal because it is
quite less than the experimental magnetic moment (1.9–
2.1 BM) values. It may suggest the existence of poly-
meric species.
2.4. Physical measurements
The elemental microanalyses were carried out at the
RSIC, CDRI, Lucknow, India. The molar conductance
of the complexes in N,N-dimethylformamide (DMF)
(1×10−3 M) solution were measured at 28°C with a
Systronic model 303 direct reading conductivity bridge.
Magnetic measurements were carried out in the poly-
crystalline state on a PAR model 155 vibrating sample
magnetometer operating at a field strength between 2
and 8 KG. High purity Ni metal (saturation moment 55
emu g−1) was used as a standard. The electronic spectra
were recorded in DMF and CHCl3 with a Shimadzu
UV-160 A spectrophotometer. The FT-IR spectra were
recorded in the range 4600–50 cm−1 with a JASCO
FT/IR-5300 in KBr and polyethylene medium. 1H
NMR spectra were obtained using a JEOL-GSX-400
NMR instrument at r.t. The voltammetric measure-
ments were performed on Bio-analytical Systems (BAS)
CV-27 assembly in conjunction with an X-Y recorder.
These measurements were made on the degassed (N2
bubbling for 5 min) solution in dimethylformamide
(1×10−3 M) containing 0.1 M tetraethylammonium
perchlorate as the supporting electrolyte. The system
consisted, working (glass carbon), an auxiliary (Pt wire)
and reference (Ag/AgCl) electrodes.
3.1. Electronic spectra
The electronic spectra (Table 2) of the cobalt complex
are recorded in CHCl3 and DMF. The most intense
band in the highest energy region has been assigned to
p–p* transition of ligand. The next lowest energy band
found at 27 397 cm−1 (in CHCl3), 27 777 cm−1 (in
DMF) has been assigned to charge transfer transitions
[13].
In the electronic spectrum of the nickel complex in
DMF, three bands are observed at 10 152 cm−1 (w1);
17 094 (w2); 22 772 cm−1 (w3) assigned to A2g(F)“
3
3
3
3T2g(F); A2g(F)“3T1g(F) and A2g(F)“3T1g(P) transi-
tions [14], respectively. The ratio of w1:w2 lies at 1.68 as
required for an octahedral Ni(II) complex. The ligand
field parameters, field splitting energy (10 Dq), Racah
interelectronic repulsion parameter i, covalent factor
and ligand field stabilization energy (LFSE) have been
calculated and presented in Table 3. The electronic data
(in DMF) suggest an octahedral geometry for the nickel
2.5. X-ray crystallography and structure solution
A diffraction quality crystal of cobalt(III) complex
was grown at room temperature (r.t.) by slow evapora-

P. Sambasi6a Reddy, K. Hussain Reddy / Polyhedron 19 (2000) 1687–1692
1689
Table 2
Electronic spectral band (cm−1) for the metal complexes of BMO
Complex
CHCl3
DMF
Intraligand and charge transfer
d–d
Intraligand and charge transfer
d–d
[Co(BMO)3]
[Ni(BMO)2]
33 222
27 397
33 222
27 777
33 898
28 901
27 777
16 806
16 949
34 482
28 901
27 777
17 094
10 152
[Cu(BMO)]n
33 557
23 333
24 390
16 863
Table 3
Electronic spectral data and ligand field parameters for the nickel complex
b
a
Method of evaluation
w1 (cm−1
)
w2 (cm−1
)
w3 (cm−1
)
B
B35
lV
10 Dq w2−w1 w2/w1 LFSE (K cal−1 mol−1
)
Observed
Calculated
10 152
17 094
19 702
27 777
25 168
10 152 6942 1.68 29.00
10 152 9550 1.94
10 Dq
961 0.923 2607
a Difference in the observed and calculated value of frequencies.
b Ratio of the free ion and the complex.
Table 4
Important IR spectral bands and their assignments
Ligand/complex
w(OH)
w(CꢀO)
w(CꢀN)
w(NꢁO)
wasy(CꢁC6H5)
wsym(CꢁC6H5)
w(MꢁO)
w(MꢁN)
BMO
3389
1647
1597
1589
1583
1597
1576
1562
1556
1211
1277
1269
1271
1493
1493
1446
1439
1373
1377
1346
1336
–
–
[Co(BMO)3]
[Ni(BMO)2]
[Cu(BMO)]n
–
–
–
490
459
456
445
424
422
complex. DMF is a known coordinating solvent and
presumably coordinates nickel in both axial positions to
complete the octahedral structure.
the free ligand. This band is absent in the spectra of all
complexes suggesting the deprotonation of oxime pro-
ton in complex formation. The downward shift of
w(CꢀN) and w(CꢀO) bands in the spectra of all com-
plexes suggests the coordination of the metal ion
through the azomethine nitrogen and the oxygen atom
of the CꢀO group. The non ligand bands in the 456–
490 and 422–445 cm−1 regions are tentatively assigned
to w(MꢁO) and w(MꢁN) stretching vibrations [16],
respectively.
The electronic spectrum of the nickel complex is also
recorded in chloroform. The spectral behavior in chlo-
roform is in agreement with the square-planar coordi-
nation of nickel. The electronic spectrum of the copper
complex is recorded in DMF covering a 50 000–
125 000 cm−1 range. The absorption band is observed
at 33 330 cm−1 is due to p–p*/charge transfer transi-
tion. However, the low intensity band is observed at
2
1
16 863 cm−1. This band is assigned to the B1g“2A1g
3.3. H NMR spectra
transition [14,15] suggesting the presence of square-pla-
nar geometry.
Cobalt(III) and nickel(II) complexes are character-
ized by high resolution (400 MHz) H NMR spectra in
1
3.2. IR spectra
CDCl3 solvent. In the case of the ligand a singlet is
observed in the low field region of the spectra at l 8.227
ppm, but it is absent in the cobalt and nickel com-
plexes, which indicates the absence of an oxime proton
in these complexes.
Important IR spectral bands of BMO and its com-
plexes are given in Table 4. A broad band of medium
intensity is observed at 3389 cm−1 in the spectrum of

1690
P. Sambasi6a Reddy, K. Hussain Reddy / Polyhedron 19 (2000) 1687–1692
Table 5
Cyclic voltammetric data of cobalt, nickel and copper complexes (10−3 M) in DMF containing 0.1 M TEAClO4 at 1.0 V/S−1 at glassy carbon
electrode (Temp. 26°C)
Complex
Scan rate
100
Redox couple
Epc (V)
Epa (V)
−0.18
DE (mV)
340
E1/2 (V)
0.35
−ic/ia
[Co(BMO)3]
III/II
II/I
II/I
−0.52
−0.128
−1.20
+0.28
[Ni(BMO)2]
[Cu(BMO)]n
100
100
III/II
+0.55
270
0.415
0.651
3.4. Cyclic 6oltammetric studies
3.5. X-ray crystal and molecular structure of the
cobalt(III) complex of BMO
The cyclic voltammograms for cobalt, nickel and
copper complexes are recorded in dimethylformamide.
The cyclic voltammetric data for all the complexes are
given in Table 5.
The cyclic voltammogram of the present cobalt com-
plex has a single reversible electron [Co(III)“Co(II)]
couple with E1/2= −0.35 V. This complex also has an
irreversible cathodic peak at Epc= −1.28 V, corre-
sponding to Co(II)“Co(I).
The cyclic voltammogram data for the copper com-
plex show a single reduction wave at E1/2= +0.415 V
and have a quasi reversible behavior [17] as indicated
by the non-equivalent current intensity of cathodic and
anodic peaks (−ic/ia=0.615 at 100 mV s−1). The
reversible couple is assigned to Cu(III)“Cu(II), which
is similar to values reported earlier [18–21].
Electronic spectral data together with magnetic mo-
ment data suggest an octahedral structure of the
[Co(BMO)3] complex. Thus, it is a tris(benzil-a-monox-
imato)cobalt(III) complex.
The crystallographic data and data collection details
for[Co(BMO)3] are summarized in Table 6. The molec-
ular structure of [Co(BMO)3] is shown in Fig. 1 and the
important bond lengths and bond angles are listed in
Table 7.
From Fig. 1, it can be seen that cobalt is six-coordi-
nate with three nitrogen atoms [N(1), N(2), N(3)] and
three oxygen atoms [O(2), O(4), O(6)], thus giving an
octahedral configuration from three unsymmetrically
bonding bidentate ligands.
As expected, the three imino nitrogen atoms give
stronger coordination bonds. This is evident by shorter
,
CoꢁN bond lengths (1.868–1.901 A). The CoꢁO bond
Table 6
Crystal data collection parameter for [Co(BMO)3]
Empirical formula
Crystal color
C42H30CoN3O6
orange
Formula weight
Temperature (K)
731.62
293(2)
No isomers are possible for tris-chelated complexes
with symmetrical ligands. However, cobalt complexes
with unsymmetrical ligands give facial and meridonial
isomers.
,
u (A)
0.71073
monoclinic
P21/c
Crystal system
Space group
,
a (A)
18.165(2)
10.0260(12)
19.962(4)
90
98.638(13)
90
3594.3(9)
4
1.352
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
3
,
V (A )
Z
Dcalc (mg cm−3
)
v(Mo Ka) (cm−1
F(000)
)
0.530
1512
Crystal (dimension mm)
Index ranges: h, k, l
Number of reflections collected
Unique reflections
0.3×0.2×0.2
−21–21, 0–11, 0–23
6288
The present ligand, BMO is unsymmetrical ligand.
Now the question is whether the Co(BMO)3 has facial
configuration or meridonial configuration. The 1H
NMR spectrum of Co(BMO)3 gave no information
about the configuration of the complex. Hence it is
considered worthwhile to study the complex using the
X-ray diffraction technique. The results obtained in the
X-ray structure determination of the complex are pre-
sented herein.
6288
Absorption correction
Max. and min. transmission
Final R, Rw
Psi scan
0.99 and 0.9
R1=0.0547,
wR2=0.1395
R1=0.0949,
wR2=0.1534
0.0000(4)
0.523 and −0.555
R indices (all data)
Extinction coefficient
Largest difference peak and hole (e A
−3
,
)

P. Sambasi6a Reddy, K. Hussain Reddy / Polyhedron 19 (2000) 1687–1692
1691
Table 7
,
Selected bond lengths (A) and angles (°) for [Co(BMO)3] with e.s.d
values in parentheses a
CoꢁN(1)
CoꢁN(2)
CoꢁN(3)
CoꢁO(4)
CoꢁO(2)
CoꢁO(6)
O(1)ꢁN(1)
O(2)ꢁC(8)
O(3)ꢁN(2)
O(4)ꢁC(22)
O(5)ꢁN(3)
O(6)ꢁC(36)
N(1)ꢁC(1)
N(2)ꢁC(15)
N(3)ꢁC(29)
C(15)ꢁC(22)
C(29)ꢁC(36)
1.868(3)
1.881(3)
1.901(3)
1.906(2)
1.918(2)
1.933(3)
1.266(4)
1.279(4)
1.236(4)
1.300(4)
1.275(4)
1.301(4)
1.328(4)
1.364(4)
1.295(4)
1.443(5)
1.434(5)
Fig. 1. Molecular structure of [Co(BMO)3] with numbering scheme.
cobalt(III) complexes containing nitrogen and oxygen
donor atoms.
,
N(1)ꢁCoꢁN(2)
N(1)ꢁCoꢁN(3)
N(2)ꢁCoꢁN(3)
N(1)ꢁCoꢁO(4)
N(2)ꢁCoꢁO(4)
N(3)ꢁCoꢁO(4)
N(1)ꢁCoꢁO(2)
N(2)ꢁCoꢁO(2)
N(3)ꢁCoꢁO(2)
O(4)ꢁCoꢁO(2)
N(1)ꢁCoꢁO(6)
N(2)ꢁCoꢁO(6)
N(3)ꢁCoꢁO(6)
O(4)ꢁCoꢁO(6)
O(2)ꢁCoꢁO(6)
C(8)ꢁO(2)ꢁCo
C(22)ꢁO(4)ꢁCo
C(36)ꢁO(6)ꢁCo
O(1)ꢁN(1)ꢁC(1)
O(1)ꢁN(1)ꢁCo
C(1)ꢁN(1)ꢁCo
O(3)ꢁN(2)ꢁC(15)
O(3)ꢁN(2)ꢁCo
C(15)ꢁN(2)ꢁCo
O(5)ꢁN(3)ꢁC(29)
O(5)ꢁN(3)ꢁCo
C(29)ꢁN(3)ꢁCo
N(1)ꢁC(1)ꢁC(8)
N(1)ꢁC(1)ꢁC(2)
O(2)ꢁC(8)ꢁC(1)
O(2)ꢁC(8)ꢁC(9)
N(2)ꢁC(15)ꢁC(22)
N(2)ꢁC(15)ꢁC(16)
O(4)ꢁC(22)ꢁC(15)
O(4)ꢁC(22)ꢁC(23)
N(3)ꢁC(29)ꢁC(36)
N(3)ꢁC(29)ꢁC(30)
O(6)ꢁC(36)ꢁC(29)
O(6)ꢁC(36)ꢁC(37)
94.79(14)
96.91(12)
94.48(12)
177.52(12)
83.71(11)
85.19(11)
82.56(11)
88.75(12)
176.76(13)
95.43(9)
90.38(13)
174.27(11)
82.43(11)
91.21(10)
94.38(11)
112.4(2)
111.2(2)
110.9(2)
121.1(3)
122.8(2)
116.1(2)
120.5(3)
123.9(2)
115.5(3)
123.3(3)
119.8(2)
116.5(2)
110.9(3)
119.3(3)
117.7(3)
114.8(3)
109.0(3)
122.1(3)
119.0(3)
115.1(3)
111.0(3)
120.5(3)
118.8(3)
116.4(3)
The distances of C(1)ꢁC(8) (1.424 A), C(15)ꢁC(22)
,
,
(1.443 A) and C(29)ꢁC(36) (1.434 A) are shorter than
the normal CꢁC single bond and larger than the CꢀC,
suggesting the delocalization of electrons in the com-
plexed ligand systems.
The ligand acts as an uninegative unsymmetrical
bidentate ligand. The oxime hydrogen is absent in the
structure. The ligands are arranged in facial configura-
tion at the metal center.
4. Supplementary material
Supplementary data are available on request from
The Director, CCDC, 12 Union Road, Cambridge,
CB2 1EZ, UK (fax: +44-1223-336033; e-mail: de-
posit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.
ac.uk) by quoting the deposition number CCDC
138847.
Acknowledgements
P.S.R. acknowledges support from the CSIR (New
Delhi) who provided the financial support in the form
of a senior research fellowship.
References
[1] M. Akbar Ali, S.F. Livingstone, Coord. Chem. Rev. 13 (1974)
101.
[2] A. Chakravorty, Coord. Chem. Rev. 13 (1974) 1.
[3] T.W.J. Taylor, E.K. Ewbank, J. Chem. Soc., Lond. (1926) 2818.
[4] A. Okac, M. Simek, Collect. Czech. Chem. Commun. 15 (1950)
445.
a Symmetry transformations used to generate equivalent atoms.
[5] D.A. White, J. Chem. Soc. A. (1971) 233.
[6] J.V. Quagliano, D.H. Wilkins, in: J.C. Bailor Jr. (Ed.), Chem-
istry of the Coordination Compounds, Reinhold, New York,
1956, p. 672.
,
lengths fall in the 1.906–1.933 A range. The CoꢁN and
CoꢁO bond lengths are comparable to those of the

1692
P. Sambasi6a Reddy, K. Hussain Reddy / Polyhedron 19 (2000) 1687–1692
[7] A. Krishna, K. Hussain Reddy, D. Venkata Reddy, Inorg.
Chim. Acta 177 (1990) 13.
[14] J. Bjerrum, C.J. Balahauisen, C.K. Jorgenson, Acta Chem.
Scand. (1984) 1273.
[8] K. Hussain Reddy, Y. Lingappa, Transition Met. Chem. 19
(1994) 487.
[15] W. Byers, A.B.P. Lever, R.D. Parish, Inorg. Chem. 7 (1968)
1835.
[9] M. Radhakrishna Reddy, K. Mohana Raju, K. Hussain Reddy,
Indian J. Chem. 35A (1996) 677.
[10] K. Hussain Reddy, M. Radhakrishna Reddy, Y. Lingappa,
Indian J. Chem. 35A (1996) 775.
[11] K. Hussain Reddy, M. Radhakrishna Reddy, K. Mohana Raju,
Indian J. Chem. 38A (1999) 299.
[12] A.I. Vogel, Text Book of Practical Organic Chemistry, 4th ed.,
ELBS, UK, 1984, p. 811.
[16] K. Nakamoto, Infrared and Raman Spectra of Inorganic and
Coordination Compounds, Wiley-Interscience, New York, 1978.
[17] R.S. Nicholson, I. Shain, Anal. Chem. 36 (1964) 706.
[18] R. Shukla, S. Mandal, P.K. Bharadwaj, Polyhedron 12 (1993)
83.
[19] X.H. Bu, Z.H. Zhang, X. Cao, S. Ma, Y. Tichen, Polyhedron 16
(1997) 3525.
[20] A. Pasini, E. Bermini, M. Scaglia, Polyhedron 15 (1996) 4461.
[21] S. Djebbar-Sid, Benali-Baitich, J.P. Deloume, Polyhedron 16
(1997) 2175.
[13] A.B.P. Lever, Inorganic Electronic Spectroscopy, 2nd ed., El-
sevier, Amsterdam, 1984.
.