Osmium and cobalt complexes incorporating facially coordinated N,N,O donor azo-imine ligands: Redox and catalytic properties

Article abstract of DOI:10.1007/s12039-012-0342-1

Reactions of 1-{[2-(arylazo)phenyl]iminomethyl}-2-phenol, HL [where H represents the dissociable phenolic protons of ligands and aryl groups of HL are C₆H₅ (HL¹), p-CH₃C ₆H₅ (HL²

Full text of DOI:10.1007/s12039-012-0342-1

c
J. Chem. Sci. Vol. 125, No. 1, January 2013, pp. 51–62. ꢀ Indian Academy of Sciences.
Osmium and cobalt complexes incorporating facially coordinated N,N,O
donor azo-imine ligands: Redox and catalytic properties
POULAMI PATTANAYAKa, DEBPRASAD PATRAa, JAHAR LAL PRATIHARa,
ANDREW BURROWSb, MARY F MAHONb and SURAJIT CHATTOPADHYAYa,∗
aDepartment of Chemistry, University of Kalyani, Kalyani 741235, India
bDepartment of Inorganic Chemistry, University of Bath, Claverton Down, Bath, BA27AY, UK
e-mail: scha8@rediffmail.com
MS received 25 July 2011; revised 29 August 2011; accepted 7 October 2012
Abstract. Reactions of 1-{[2-(arylazo)phenyl]iminomethyl}-2-phenol, HL [where H represents the dissoci-
able phenolic protons of ligands and aryl groups of HL are C6H5 (HL1), p-CH3C6H5 (HL2), p-ClC6H5 (HL3)]
with Os(H)(CO)(Br)(PPh3)3 in toluene and with Co(ClO4)3.6H2O in methanol afforded new facially coordi-
nated complexes of composition [(L)Os(CO)(Br)(PPh3)] and [(L)2Co]ClO4. The anionic ligand (L)−, binds the
metal in tridentate (N, N, O) manner in [(L)Os(CO)(Br)(PPh3)] and [(L)2Co]ClO4 complexes. All the Os(II)
and Co(III) complexes are diamagnetic and show characteristic 1H NMR signals and intense MLCT transitions
in the visible region. The cyclic voltammograms exhibited one quasi reversible oxidative response near 0.10 V
vs SCE for [(L)Os(CO)(Br)(PPh3)] and [(L)2Co]ClO4 complexes displayed serial reductive responses within
−0.06 to −1.61 V vs SCE. Single point DFT calculation was carried out to examine the nature of redox orbitals.
Conversion of ketones to corresponding alcohols has been studied using [(L)Os(CO)(Br)(PPh3)] as catalyst.
Keywords. 2-(Arylazo)-1-N-salicylidene phenylamine; face capped Os(II); Co(III); DFT; redox; hydrogen
transfer reaction.
1. Introduction
platinum metal complexes,22–34 but only a few of the
above mentioned ligands have been utilized for the
preparation of Os(II) complexes.35,36 As a consequence,
chemistry of Os(II) complexes with facially coordi-
nating ligands have not been explored considerably.
Coordination chemistry of osmium incorporating azo
ligands has been studied with a few bidentate (N, N
and N, O donors) and tridentate (C, N, O and N, N, N
donors) ligands.37–41 Whereas the coordination chem-
istry of osmium with tridentate N, N, O donor ligands
has not been reported so far. These background infor-
mation prompted us to study the coordination chemistry
of osmium incorporating the N, N, O donor azo-imine
ligand system, 1.
The ligands which bind the transition metal ion in a
predictable way play important role in modern coordi-
nation chemistry since they may determine the reactive
sites available at a metal centre and can modulate their
reactivity.
Fascinating chemistry of transition metal complexes
incorporating ligands that are capable of binding the
metal centre in facial manner, enhanced the interest on
the synthesis of new tridentate ligands that are suitable
for obtaining facially coordinated complexes.1–7
Facially capped piano-stool type of platinum metal
complexes have received attention due to their inter-
esting catalytic and biological activity. Conversion of
dimethyl oxalate to ethylene glycol and hydrogena-
tion of esters to yield alcohols in homogeneous media
using such metal complexes as catalyst are notable
examples.8–21
R
Although several tridentate facially coordinating li-
gands, such as 1,4,7 tri aza cyclononane, 1,4,7 trithia
cyclononane, tris pyrazolyl borate, a few scorpionate
and tripodal ligands, are known to form facially capped
HO
N
N
N
∗For correspondence
1
51

52
Poulami Pattanayak et al.
In this paper, we have described the reactions of 650 (4570), 470 (4570), 350 (19380), 255 (29850),
240 (40790). IR (KBr pellets, cm−1): νC=N1605, νN=N
1434, νCO 1920. 1H NMR (CDCl3 ppm): δ 8.27 (d, 2H),
7.94–7.92 (d, 1H), 7.62 (t, 6H), 7.53–7.42 (m, 6H),
7.42–7.39 (m, 3H), 7.32–7.25 (m, 9H), 7.17–7.15
(t,1H), 7.04 (s, 1H), 6.83 (d, 1H), 6.62 (d, 1H), 6.46 (d,
1H), 6.28 (t, 1H) ppm. E1/2 [V] (ꢀEp/mV): 1.1.
appropriate osmium and cobalt substrates with 1 to
obtain Os(II) and Co(III) complexes where the triden-
tate (N,N,O) ligand is facially coordinated. Spectro-
scopic and redox properties of new complexes have
been described. Catalytic transfer of hydrogen reactions
using newly synthesized Os(II) complexes as catalyst
have been reported. A plausible description of redox
orbitals have been ascribed on the basis of single point
DFT calculations.
2.2c and 2.2.d [(L2)Os(CO)(Br)(PPh3)] (2b) and
[(L3 )Os(CO)(Br)(PPh3 )] (2c): The complexes
[(L2 )Os(CO)(Br)(PPh3 )] was prepared fol-
lowing the same procedure as in the case of
[(L1)Os(CO)(Br)(PPh3)] using HL2 and HL3 in place of
HL1. The solvent used for thin-layer chromatographic
separation is toluene–acetonitrile (95:5 V/V) mixed
solvent.
2. Experimental
2.1 Materials
The solvents used in the reactions were of reagent
grade (obtained this from E Marck, Kolkata, India)
and were purified and dried by reported proce-
dure.42 Os(H)(CO)(Br)(PPh3)3 was prepared accord-
ing a reported procedure.43 Cobalt (III) perchlorate
was prepared by oxidation of Co (II) perchlorate
with H2O2.44 1-{[2-(Arylazo)phenyl]iminomethyl}-2-
phenol, HL [where H represents the dissociable pro-
tons upon complexation and aryl groups of HL are
phenyl for HL1; p-methyl phenyl for HL2; p-chloro
phenyl for HL3], were prepared according to a reported
procedure.45
Yield: 75% for HL2 complex [(L2)Os(CO)(Br)(PPh3)].
Anal. calc. for 2b. C 53.49, H 3.54, N 4.80. Found C
53.53, H 3.50, N 4.83%. Electronic spectrum (λmax/nm
(ε/dm2 mol−1), dichloromethane): 650 (1300), 470
(4450), 357 (18940), 260 (27370), 232 (45400). IR
(KBr pellets, cm−1): νC=N 1604, νN=N 1436, νCO 1920.
1H NMR (CDCl3 ppm): δ 8.23 (d, 2H), 7.91 (d, 1H),
7.62 (t, 6H), 7.52–7.28 (m, 8H), 7.15–7.10 (m, 2H),
7.11 (d, 1H), 6.81 (d, 1H), 6.63(d, 1H), 6.28 (t, 1H),
2.41 (s, 3H) ppm. E1/2 [V] (ꢀEp/mV): 1.12.
Yield: 70% for HL3 complex [(L3)Os(CO)(Br)(PPh3)]).
Anal. calc. for 2c. C 50.93, H 3.12, N 4.69. Found C
51.00, H 3.15, N 4.67%. Electronic spectrum (λmax/nm
(ε/dm2 mol−1), dichloromethane): 650 (1290), 468
(4440), 360 (20331), 256 (29940), 230 (49920). IR
(KBr pellets, cm−1): νC=N 1604, νN=N 1436, νCO 1927.
1H NMR (CDCl3 ppm): δ 8.30 (d, 2H), 7.92–7.91
(m, 1H), 7.78 (t, 6H), 7.61 (t, 1H ), 7.53 (d, 2H),
7.41 (t, 2H), 7.34–7.28 (m, 8H), 7.15 (t, 1H), 7.04
(s, 1H), 6.81 (d, 1H), 6.64 (d, 1H), 6.46 (d, 1H), 6.29
(t, 1H). E1/2 [V] (ꢀEp/mV): 1.19.
2.2 Syntheses of complexes
2.2a [(L)Os(CO)(Br)(PPh3)]: The [(L)Os(CO)(Br)(PPh3)]
complexes were obtained by following a general pro-
cedure. Specific details are given below for a particular
complex.
2.2b [(L1)Os(CO)(Br)(PPh3)] (2a): 2-(Phelazo)-1-N-
salicylidene phenylamine, HL1, (0.03 g, 0.102 mmol)
was dissolved in 40 ml toluene, and to it
Os(H)(CO)(Br)(PPh3)3 (0.10 g, 0.102 mmol) was 2.2e [(L)2Co]ClO4: The [(L)2Co]ClO4 complexes
added. The mixture was then heated to reflux for were obtained by following a general procedure. Spe-
8 h till the solution turned to dark brown. Evapora- cific details are given below for a particular complex.
tion of the solvent gave a dark brown residue, which
was purified by thin-layer chromatography on silica
gel. A green band separated in toluene–acetonitrile 2.2f [(L1)2Co]ClO4 (7a): To
a
solution of
(95:5,V/V) mixed solvent. From the second band Co(ClO4)3.6H2O (0.2 g, 0.429 mmol), in 40 mL
[(L1)Os(CO)(Br)(PPh3)] was isolated in pure form methanol ligand HL1 (0.5 g, 0.428 mmol) was added.
upon extracting with dichloromethane and methanol. The mixture was heated to reflux for 1 h. During
Yield: 70%. Anal. calc. for 2a. C 52.97, H 3.36, this time greenish brown solid was precipitated. The
N 4.87. Found: C 53.00, H 3.34, N 4.90%. Electronic solid product was then filtered and washed with
spectrum (λmax/nm (ε/dm2 mol−1), dichloromethane): water and a little methanol. It was recrystallized from

Facially coordinated osmium and cobalt complexes
53
dichloromethane-petroleum ether to obtain pure prod- calomel reference electrode (SCE) were used in a three-
uct. Yield: 80%. Anal. calc. for 7a. C 60.07, H 3.68, N electrode configuration. All electrochemical data were
11.06. Found C 60.00, H 3.70, N 11.10%. Electronic collected at 298 K and are uncorrected for junction
spectrum (λmax/nm (ε/dm2 mol−1), dichloromethane): potentials.
370 (19700), 285 (48200), 230 (41100). IR (KBr
pellets, cm−1): νC=N 1604, νN=N 1436, νCl−O 1083.
1H NMR (CDCl3 ppm): δ 8.02 (t, 1H), 7.90 (d, 1H),
7.82–7.79 (m, 3H), 7.49–7.44 (m, 2H), 7.37 (d, 2H),
7.24–7.18 (m, 2H), 6.71 (t, 1H), 6.09 (d, 1H) ppm. E1/2
[V] (ꢀEp/mV): −0.071 (116.4), E (ꢀEp/mV): −1.60,
−0.85.
2.4 Transformation of ketones to 20 - alcohols via
hydrogen transfer reactions
A mixture containing ketones (0.050 g, 2.8 mmol),
the osmium complex (0.0009 g, 0.0013 mmol) and
(0.0036 g, 0.0625 mmol) KOH was heated to reflux in
10 mL of i-PrOH for appropriate period of time as men-
tioned in table 3. The complex was removed as precipi-
tate from the reaction mixture by the addition of diethyl
ether followed by filtration. Subsequent neutralization
with 5 mL of 1 (M) HCl the ether layer was separated
and passed through a short path of silica gel. The prod-
uct was purified by preparative thin-layer chromatog-
raphy. The yields were calculated after isolation. The
hydrogenated products were characterized by match-
2.2g and 2.2h [(L2)2Co]ClO4 (7b) and [(L3sal)2Co]ClO4
(7c): Complexes [(L2)2Co]ClO4 and [(L3)2Co]ClO4
were prepared by a similar procedure as described
for [(L1)2Co]ClO4 using ligand HL2 and HL3 in place
of HL1.
Yield: 85%. Anal. calc. for 7b. C 60.53, H 3.88, N
10.86. Found C 60.60, H 3.90, N 10.94%. Electronic
spectrum (λmax/nm (ε/dm2 mol−1), dichloromethane):
390 (21500), 290 (45600), 230 (38600). IR (KBr pel-
1
ing the UV-Vis, IR and HNMR spectra of authentic
1
lets, cm−1): νC=N 1605, νN=N 1433, νCl−O 1089. H
samples.
NMR (CDCl3 ppm): δ 8.02 (t, 1H), 7.87 (d, 1H),
7.79 (d, 1H), 7.74 (t, 2H), 7. 34 (d, 2H), 7.29–722
(m, 1H), 7.18 (d, 2H), 6.67 (t, 1H), 6.06(d, 1H), 2.42
(s, 3H) ppm. E1/2 [V] (ꢀEp/mV): −0.081 (101.7). E
(ꢀEp/mV): −1.55, −1.05.
2.5 DFT calculations
Using the X-ray coordinates of the complex
[(L2)Os(CO)(Br)(PPh3)] and [(L1)2Co]ClO4 com-
plex ground state electronic structure calculations
have been carried out using DFT46 methods with the
Gaussian 03 program.47 Becke’s hybrid function48
with the Lee–Yang–Parr (LYP) correlation function49
was used throughout the study. We have employed a
triple-z quality all-electron basis set (TZP)50 for Os
and Co. Double-all-electron basis set51 for C, N, and
O, and a 6-31G basis set for H. LANL2DZ valence and
effective core potential functions were used. All energy
calculations were performed using the self-consistent
field ‘tight’ option of the Gaussian 03 program to
ensure sufficiently well-converged values for the state
energies.
Yield: 75%. Anal. calc. for 7c. C 57.45, H 3.40, N
10.58. Found C 7.50, H 3.35, N 10.60%. Electronic
spectrum (λmax/nm (ε/dm2 mol−1), dichloromethane):
380 (24950), 285 (52575), 230 (43400). IR (KBr pel-
1
lets, cm−1): νC=N 1603, νN=N 1432, νCl−O 1079. H
NMR (CDCl3 ppm): δ 8.34 (d, 1H), 7.78 (d, 1H), 7.34
(t, 2H), 7.27–7.23 (m, 2H), 7.16–7.11 (m, 3H), 6.85–
6.81 (m, 2H), 6.68 (d, 1H) ppm. E1/2 [V] (ꢀEp/mV):
−0.063 (122.7), E (ꢀEp/mV): −1.50, −0.75.
2.3 Physical measurements
Microanalysis (C, H, N) was performed using a
Perkin-Elmer 2400 C, H, N, S / O series II ele-
mental analyzer. Infrared spectra were recorded on a
Parkin-Elmer L120-00A FT-IR spectrometer with the
samples prepared as KBr pellets. Electronic spectra
2.6 X-ray crystallographic study
were recorded on a Shimadzu UV-1601 PC spectropho-
1
tometer. H NMR spectra were obtained on Brucker Single crystals of 2b and 7a were grown by slow evapo-
DPX 400 and Brucker 500 RPX NMR spectrometers ration of dicholoromethane-acetonitrile mixed solu-
in CDCl3 using TMS as the internal standard. Elec- tion at 298 K. Data were collected on a Brucker
trochemical measurements were made under dinitro- SMART CCD diffractometer using Mo-Kα monochro-
gen atmosphere using a PAR model VARSASTAT-II mator (λ = 0.71073). Structure solutions were per-
potentiostat. A platinum disc working electrode, a plati- formed using Shelx 97 PC version program.52 Full
num wire auxiliary electrode and an aqueous saturated matrix least square refinements on F2 were performed

54
Poulami Pattanayak et al.
using SHELXL-97 program.53 All the non-hydrogen 3. Results and discussion
atoms were refined anisotropically using full-matrix
3.1 Syntheses
least squares method. Hydrogen atoms were included
for structure factor calculations after placing them at
calculated positions. The data collection parameters and
relevant crystal data are collected in table 3. In table 3
only the cell parameters for [(L1)2Co]ClO4, 7a, are
shown since it could not be refined well due to the poor
quality of data.
Reaction of one equivalent [Os(H)(CO)(Br)(PPh3)3] with
two equivalent 1-{[2-(arylazo)phenyl]iminomethyl}-2-
phenol, 1, HL, in refluxing toluene afforded olive green
[(L)Os(CO)(Br)(PPh3)], 2, product in 70% yield after
8 h (Eq. 1).
Br
R
PPh3
CO
R
HO
Os
N
...
N
O
N N
N
Toluene
Reflux
+
Os(H)(CO)(Br)(PPh3)3
N
1
2
Compound
R
(1)
HL1
HL2
HL3
[(L)Os(CO)(Br)(PPh3)]
1a
H
CH3
Cl
1b
1c
[(L1)Os(CO)(Br)(PPh3)]
[(L2)Os(CO)(Br)(PPh3)]
H
2a
CH3
2b
Cl
[(L3)Os(CO)(Br)(PPh3)]
2c
Three complexes were prepared using HL1, HL2 and
HL3 which are nonelectrolyte, diamagnetic and soluble
in common organic solvents.
In [(L)Os(CO)(Br)(PPh3)] complexes deprotonated
tridentate (N,N,O) donor L− ligands are bound facially
and the other three coordination sites are occupied by
CO, Br and PPh3 (x-ray structure).
Earlier we have reported that the reaction of HL
with Ru(H)(CO)(PPh3)3Cl substrate afforded both the
complexes [(L)Ru(PPh3)(Cl)(CO)], 3, and [(L)2Ru], 4,
where (L)− ligands were bound to the Ru(II) centre
O
Cl
N
N
Ph3P
CO
R
N
Ru
Ru
O
N
O
N
R
N
R
N
N
N
3
4
[(L)2Ru]
[(L)Ru(CO)(Cl)(PPh3)]
in facial and meridional fashion respectively.41 Fur- [(L)2Ru], in the presence of excess free ligand, was
ther, the conversion of [(L)Ru(PPh3)(Cl)(CO)] to also reported but the mechanism of facial to meridional

Facially coordinated osmium and cobalt complexes
55
amendment of the ligand coordination could not be only the product 6 upon reaction with same ruthenium
ascertained.41 Whereas, bulkier ligands, 5, afforded substrate.41
R
Ph3P
Cl
CO
Ru
O
R
HO
N
N
N
N
N
N
6
5
Thus, the fine tuning in the ligand backbone dictated substrate [Co(H2O)6].(ClO4)3 was used for reaction
the product in the case of ruthenium substrate. In this with HL to examine the proposition. Though Os(II)
paper, we have described that subtle difference in the and Ru(II) are very close in size but Os(II) did not
characteristics of metal ion is also an important fac- afford the bis complex possibly due to less lability
tor and can be utilized to prepare the desired pro- of [(L)Os(CO)(Br)(PPh3)] than the ruthenium analogue
duct. That is, on going from ruthenium substrates to [(L)Ru(CO)(Cl)(PPh3)].
the osmium substrate Os(H)(CO)(Br)(PPh3)3 afforded
Reaction of two equivalent of HL, 1, with one
only [(L)Os(CO)(Br)(PPh3)] was isolated without any equivalent of cobalt (III) perchlorate, hexahydrate,
analogue of (L)2Ru for both the ligands, 5 and HL. Co(ClO4)3.6H2O, in refluxing methanol precipitated the
We contemplated to examine the coordination mode facially coordinated dark coloured crystalline complex
of the same ligands in the bis complexes of metal [(L)2Co]ClO4, 7, which was collected by filtration
ions that are smaller in size than Ru(II) and Os(II) (Eq. 2). A little more crop of product was isolated
with the academic interest to prepare bis complex upon concentrating the filtrate. In concurrence with our
with facially coordinated ligands. Since Co(III) is of expectation the bis complex of Co(III), [(L)2Co]ClO4,
smaller radii than Ru(II) and Os(II) in low spin octa- 7, is incorporated
hedral geometry so it was chosen. The simple metal
R
HO
N
N
O
N
N
N
N
1/2 Co (ClO4)3. 6H2O
Co
R
R
N
O
MeOH
Reflux
N
N
. ClO4
1
(2)
7
Compound
R
[(L1)2Co]ClO4
[(L2)2Co]ClO4
[(L3)2Co]ClO4
H
7a
7b
7c
CH3
Cl
with two facially coordinated tridentate (N,N,O) li- the complex cation, [(L)2Co]+, 7, was satisfied by one
gands, L−, (X-ray structure). The unipositive charge of ClO−4 ion. These complexes are soluble in common

56
Poulami Pattanayak et al.
organic solvent and the colour of the solution is green- and νN=N 1456 cm−1) signifying the coordination
ish brown. The complex is 1:1 electrolyte in acetoni- of aldimino (-CH=N-) and azo (-N=N-) nitro-
trile solution and diamagnetic signifying the low spin d6 gens.55–57 The νCO appeared near 1920 cm−1 for
configuration in octahedral ligand field. The composi- ([(L)Os(CO)(Br)(PPh3)] complexes. A broad band
tion of diamagnetic [(L)2Co]ClO4 complexes matched appeared near 1080 cm−1, which is the characteristic
well with the analytical data and total proton count in of uncoordinated ClO−4 for [(L)2Co]ClO4. The IR data
1H NMR spectrum (vide infra).
are collected in experimental section. The IR spectra of
the complexes are shown in supplementary materials
(figures S7–S12).
The 1H-NMR spectral pattern matched well
with the composition of [(L)Os(CO)(Br)(PPh3)]
and [(L)2Co]ClO4. The 1H-NMR spectra of
[(L)Os(CO)(Br)(PPh3)] complexes were recorded in
3.2 Characterization
The green solutions of [(L)Os(CO)(Br)(PPh3)] com-
plexes in dichloromethane display characteristic UV-
Vis spectra where a low energy absorption near 655 nm
is diagnostic of d–d transition. Spectral data are collated
in experimental section.
The UV–Vis spectra of the greenish brown solutions
of [(L)2Co]ClO4 in CH2Cl2 feature a relatively weak
absorption band near 670 nm for d–d transition.45,54
The [(L)2Co]ClO4 complexes show a weak absorp-
tion band near 375 nm. A representative spectrum of
[(L1)2Co]ClO4 is shown in figure 1. The UV-Vis spec-
tral data are given in the experimental section. All the
UV-Vis spectra are given in supplementary material
(figures S1–S6).
IR spectra of all the complexes were recorded
in solid KBr support. The spectra displayed νC=N
for [(L)Os(CO)(Br)(PPh3)] and [(L)2Co]ClO4 near
1604 and 1603 cm−1, respectively. The νN=N
([(L)Os(CO)(Br)(PPh3)] and [(L)2Co]ClO4, appeared
near 1434 and 1435 cm−1, respectively. These stretch-
ing frequencies are of lower energy than the corres-
ponding ligand characteristics (νC=N, ∼1615 cm−1
1
CDCl3. The H-NMR data are collected in experi-
mental section. Absence of phenolic OH signal indi-
cated the coordination of phenoxide oxygen for all
the complexes.41,56,57 The upfield shift of aldimine
protons (appeared near ∼δ 7.10 as a singlet for
[(L)Os(CO)(Br)(PPh3)]) than the ligands (∼δ 8.55)
were observed for all the complexes consistent with
the imine coordination.55–57 All other aromatic pro-
ton resonances appeared within ∼δ 8.51–δ 6.28 for
[(L)Os(CO)(Br)(PPh3)]. The fifteen protons of PPh3
appear as two multiplets within the range δ 7.53–δ 7.44
for six protons and δ 7.32–δ 7.25 for nine protons. The
aldimine and aromatic protons for [(L)2Co]ClO4 did
not resolve well due to overlapping signals. The −CH3
protons signal appears at δ 2.42 for [(L2)2Co]ClO4.
1
The H-NMR spectra of [(L)Os(CO)(Br)(PPh3)] and
[(L)2Co]ClO4 complexes are given in supplementary
materials (figures S13–S18).
3.3 X-ray structure
Single crystals of [(L2)Os(CO)(Br)(PPh3)] was grown
by slow evaporation of dichloromethane and acetoni-
trile mixed solvents. Suitable crystals were picked for
X-ray studies. A perspective view of the molecule has
been shown in figure 2 and selected bond distances and
angles are collected in table 1.
The asymmetric unit of [(L2)Os(CO)(Br)(PPh3)]
contains two molecules. The geometry and bond para-
meters are similar for both the molecules. The mole-
cular structure of each monomer molecule exhibit
octahedral coordination with facial geometry about Os,
where the monoanionic ligand, (L2)−, binds the metal
in tridentate (N,N,O) fashion. The other three ligands,
Br, CO and PPh3 complete the hexacoordination about
Os. The Os-N(azo) length [2.179(4)Å] trans to Os1-
P1 [2.33387(15)Å] is longer than the Os1-N3(aldimine)
length [2.037(16) Å] which is trans to Os1-Br1
[2.5620(6) Å]. The N(1)-N(2) length [1.273(6) Å] in
Figure 1. UV-Vis spectra of 1a (- - -), 2a(—) and
7a (· · · ·).

Facially coordinated osmium and cobalt complexes
57
Figure 3. Perspective (ball stick) view of 7a, (except
ClO4)− with atom numbering scheme. The hydrogen atoms
have been omitted for clarity.
Figure 2. Perspective view (30% probability ellipsoids) of
2b, with atom numbering scheme. The hydrogen atoms have
been omitted for clarity.
The molecular structure exhibits octahedral coordi-
nation about Co(III), where the two anionic ligand,
(L1)−, coordinated facially as tridentate N,N,O donor
through phenoxide-O, azo nitrogen and aldimino nitro-
gen. Each ligand binds the metal forming two six-
membered chelate ring. A perchlorate (ClO4)− ion sat-
isfied the unipositive charge of the complex cation,
[(L1)2Co]+. The relative orientations of donor atoms
(azo N, imine N, and phenoxide O) of two ligands are
trans, trans and trans, respectively. The X-ray structure
of [(L1)2Co]ClO4 could not be refined well due to poor
quality of data. Although the refinement was poor but
the facial coordination modes of the ligands could be
recognized.
the coordinated azo function is shorter than other Os(II)
azo complexes.37–41 The Os1 -P1 distance is longer than
other reported Os-P distances.40,41
Crystals of [(L1)2Co]ClO4, 7a, were grown by slow
diffusion of dichloromethane solution into petroleum
ether. A perspective (ball stick) view of the molecule
has been shown in figure 3. The asymmetric unit of
[(L1)2Co]ClO4 contains two molecules.
Table 1. Selected bond distances (Å) and angles (◦) for
compound 2b.
[(L2)Os(CO)(Br)(PPh3)]
Distances (Å)
Os1-Br
Os1-P1
Os1-O1
Os1-N1
Os1-N3
N3-C13
2.5620(6)
2.3387(15) N1-N2
2.091(4)
2.179(4)
2.037(5)
1.309(7)
Os1-C20
1.834(6)
1.273(6)
1.440(7)
1.423(7)
1.431(7)
1.315(7)
N1-C1
N2-C7
N3-C12
O1-C19
Angles (◦)
Br-Os1-P1
Br-Os1-O1
Br-Os1-N1
Br-Os1-N3
Br-Os1-C20
N1-Os1-N3
N1-Os1-C20
92.77(4)
88.34(10)
91.78(12)
168.79(14)
91.54(17)
78.88(17)
89.3(2)
N3-Os1-C20
Os1-O1-C19 126.4(3)
94.5(2)
N2-N1-C1
Os1-N1-N2
Os1-N1-C1
N1-N2-C7
Os1-N3-C12 114.5(3)
C12-N3-C13 116.4(5)
111.5(4)
122.5(4)
125.5(4)
121.2(5)
Figure 4. Cyclic voltammogram of the 2b complex in
dichloromethane solution (0.1 M TBAP) at scan rate of
Os1-N3-C13 128.7(4)
50 mV s−1
.

58
Poulami Pattanayak et al.
near 1.0 V in dichloromethane solution. The oxidation
3.4 Electrochemistry
has been assigned according to the couple of Eq. (3),
where [(L)Os(CO)(Br)(PPh3)]+ is the Os(III) analogue
of [(L)Os(CO)(Br)(PPh3)].
[(L)Os(CO)(Br)(PPh3)] complexes exhibit an one elec-
tron reversible oxidative cyclic voltammetric response
[(L)Os(Br)(CO)(PPh3)] +
[(L)Os(Br)(CO)(PPh3)] - e
(3)
Representative
cyclic
voltammograms
of inferred to be due to the metal centred Os(II)/Os(III)
[(L2)Os(CO)(Br)(PPh3)] has been shown in figure 4 redox couple signifying accessibility to variable redox
and all other cyclic viltammograms were given in levels of metal.
supplementary material (figures S19–S21).
The [(L)2Co]ClO4 Complexes exhibit a one elec-
To gain insight into the redox orbitals, single point tron reversible reductive cyclic voltammetric response
DFT calculations were performed using the crystallo- in the range of −0.06 to −0.12 V vs SCE and two
graphic coordinates of [(L2)Os(CO)(Br)(PPh3)]. Two irreversible reductive responses within −0.75 V to
frontier orbitals HOMO and LUMO are shown in −1.62 V vs SCE in dichloromethane solution. A rep-
figure 5a and b. From the DFT results, it appears that resentative cyclic voltammogram of [(L1)2Co]ClO4 is
the HOMO has considerable metal character, whereas shown in figure 6 and other data are given in experi-
the LUMO is essentially ligand centred. Therefore, on mental section. To gain insight into the redox orbitals
the basis of DFT result, the oxidative couple may be of the cobalt complexes, DFT restricted single point
Figure 5. (a) HOMO of 2b and (b) LUMO of 2b.
Figure 6. (a) Cyclic voltammogram of the 7a complex in dichloromethane solution (0.1 M TBAP) at scan rate of 50 mV s−1
(b) A least-squares plot of E1/2 values of 7a complexes vs. σ (Hammett substituent constant).
.

Facially coordinated osmium and cobalt complexes
59
Figure 7. (a) HOMO of 7a and (b) LUMO of 7b.
calculations were performed were carried out on process has received attention in recent years.57–59
[(L1)2Co]+ moiety using the crystallographic coordi- Several platinum metal complexes have been used
nates. Plot of HOMO and LUMO are shown in figure 7a as catalyst for the reductive conversion of ketones
and b. Although HOMO has considerable metal char- to 20 – alcohols.60–63 To the best of our knowledge,
acter but LUMO is ligand centred. As a result, reduc- use of Os(II) complexes as transfer hydrogen cata-
tive responses of [(L)2Co]ClO4 are attributed to ligand lyst have not been studied considerably.64–66 Therefore,
centred reductions. Plot of E1/2 values of [(L)2Co]ClO4, we were encouraged to examine the catalytic activity
within −0.06–0.12 V Vs SCE, against Hamett con- of [(L1)Os(CO)(Br)(PPh3)], toward transfer hydrogena-
stants of −CH3, −Cl and −H is linear indicating the tion of representative aliphatic and aromatic ketones.
ligand contribution in redox orbital.
The conversion of ketones to corresponding alcohols
were studied in 2-propanol adding catalytic amount of
[(L1)Os(CO)(Br)(PPh3)] and KOH in the reaction mix-
ture as given in Eq. (4). The results of transformations
are listed in table 2.
3.5 Transformation of ketones to 20 - alcohols
via hydrogen transfer reactions
Hydrogenation of organic molecules in homoge-
After the catalytic reactions neither the original
nous media by means of catalytic transfer hydrogen complex, [(L1)Os(CO)(Br)(PPh3)], nor the transformed
Table 2. Transformation of ketones to 20 - alcohols via hydrogen transfer reactions for 2a.
Entry
Substrate
O
Product
OH
Time (h)
Isolated yield (%)
1
2
3
1
80
75
50
O
OH
1
O
OH
1.5
O
OH
Me
Me
4
1.5
50

60
Poulami Pattanayak et al.
Table 3. Crystallographic data for 2b and 7a.
2b
7a
Chemical formula
Formula weight
Crystal system
Space group
A [Å]
C39H31BrN3O2OsP
874.77
Monoclinic
P21
9.10100(10)
18.0120(2)
20.8460(2)
90
89.8550(10)
90
C38H28CoN6O2, ClO4
759.11
Triclinic
P-1
10.6348(5)
13.7687(6)
14.5032(5)
86.013(2)
B [Å]
C [Å]
α [◦]
β [◦]
69.330(2)
78.255(2)
γ [◦ ]
λ [Å]
0.71073
3417.22(6)
1712
0.71073
1945.36(14)
864
2
V [Å3]
F(000)
Z
4
T/K
150
1.700
4.987
0.0396
0.0773
1.00
273
–
–
–
–
–
D/g/cm3
μ/mm−1
R1 (all data)
wR2[I > 2σ(I)]
GOF
complex could be isolated and identified indicating Notably, similar Ru complexes, reported earlier,41 did
the fact that the possibility of [(L1)Os(CO)(Br)(PPh3)] not exhibit the catalytic activity for such transfer hydro-
complex to be a precatalyst can not be ruled out. gen reactions.
OH
O
2a, KOH
0.0011 mmol
(4)
R
R/
R/
2-propanol
R
All the reactions were carried out in ambient con- bound. Reversible accessibility to the different redox
dition. A series of blank reactions were carried out in
the absence of complex [(L1)Os(CO)(Br)(PPh3)]. This
has ascertained the necessity of the complex during the
reaction. The yields of the alcohols obtained from all
the ketones were determined after isolation (table 3).
states for [(L)2Co]ClO4 have been attributed to lig-
and centred electron transfer processes according to the
DFT calculations.
Supplementary data
4. Concluding remarks
Figures S1–S6 shows the UV-Vis spectra, figures S7–
S12 show the IR spectra, figures S13–S18 the H
1
Reaction of tridentate N,N,O donor ligand HL
with Os(H)(CO)(Br)(PPh3)3 afforded new face
capped complexes of Os(II). The new face capped
[(L)Os(CO)(Br)(PPh3)] complexes exhibit an one elec-
tron reversible oxidative cyclic voltammetric response
near 1.0 V in dichloromethane solution. The new
Os(II) complexes are catalytically active toward
transfer hydrogen catalysis. Reaction of HL with
Co(H2O)6.(ClO4)3 afforded bis complex where both the
ligands are facially coordinated unlike the bis complex
NMR spectra and figures S19–S24 show the cyclic
voltammogram of all the complexes (these supple-
mentary figures S1–S6 can be seen in www.ias.ac.in/
chemsci.website). CCDC-764971 for 2b and CCDC-
808866 for 7a contain the supplementary crystal-
lographic data for this paper. These data can be
obtained free of charge from the Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge
CB21EZ, UK; fax: (+44)1223-336-033; or e-mail:
of Ru(II), where both the ligands are meridionally deposite@ccdc.cam.ac.uk.

Facially coordinated osmium and cobalt complexes
61
Acknowledgements
22. Chaudhuri P and Wieghardt K 1987 Prog. Inorg. Chem.
35 329
Authors thank the Department of Science and Tech-
nology (DST) (New Delhi) for funding under DST-
SERC (SR/S1/IC-0026/2007) project. The necessary
laboratory and infrastructural facility are provided by
the Department of Chemistry, University of Kalyani.
The support of DST under FIST and the University
Grants Communication (UGC) SAP program to the
Department of Chemistry, University of Kalyani is also
acknowledged.
23. Blake A J and Schröder M 1990 Adv. Inorg. Chem. 351
24. Spicer M D and Reglinski J 2009 Eur. J. Inorg. Chem.
1553
25. Iengo E, Zangrando E, Baiutti E, Munini F and Alessio
E 2005 Eur. J. Inorg. Chem. 1019
26. St.-J. Foreman M R, Hill A F, Owen G R, White A J P
and Williams D J 2003 Organometallics 22 4446
27. Garcia R, Paulo A and Santos I 2009 Inorg. Chim. Acta.
362 4315
28. Harding D J, Harding P, Kivnang J and Adams H 2010
Transition Met. Chem. 35 521
29. Trofimenko S 1993 Chem. Rev. 93 943
30. Borguet Y, Sauvage X, Zaragoza G, Demonceau A and
Delaude L 2010 Organometallics 29 6675
References
31. Wang X-Y, Shi H-T, Wu F-H and Zhang Q-F 2010
J. Mol. Struct. 982 66
32. Trofimenko S 1986 Prog. Inorg. Chem. 34 115
33. Walker J M, Cox A M, Wang R and Spivak G J 2010
Organometallics 29 6121
34. Therrien B, Vieille-Petit L and Süss-Fink G 2005 J. Mol.
Struct. 738 161
35. Puerta M C and Valerga P 1999 Coord. Chem. Rev. 193
1. Zhang J, Braunstein P and Andy Hor T S 2008
Organometallics 27 4277
2. Young C G, Malarek M S, Evans D J, Doonan C J, Wee
Lin Ng V and White J M 2009 Inorg. Chem. 48 1960
3. Poyatos M, Mata J A and Peris E 2009 Chem. Rev. 109
3677
4. Liang L-C, Cheng L-C, Tsai T-L, Hu C-H and Guo W-H
2009 Inorg. Chem. 48 5697
5. Mattsson J, Govindaswamy P, Renfrew A K, Dyson
P J, Stepnitka P, Sqss-Fink G and Therrien B 2009
Organometallics 28 4350
6. Traar P, Schröckeneder A, Judmaier M E, Belaj F,
Baumgartner J, Sachse A and Mösch-Zanetti N C 2010
Eur. J. Inorg. Chem. 5718
7. Dougan S J, Habtemariam A, McHale S E, Parsons S
and Sadler P J 2008 Proc. Natl. Acad. Sci. USA 105
11628
977
36. Das C, Peng S-M, Lee G-H and Goswami S 2002 New
J. Chem. 26 222
37. Acharyya R, Peng S-M, Lee G-H and Bhattacharya S
2003 Inorg. Chem. 42 7378
38. Gupta P, Butcher R J and Bhattacharya S 2003 Inorg.
Chem. 42 5405
39. Majumder K, Peng S M and Bhattacharya S 2001 J.
Chem. Soc. Dalton Trans. 284
40. Pratihar J L, Pattanayak P, Bhaduri S, Patra D and
Chattopadhyay S 2010 J. Chem. Res. 98
41. Pattanayak P, Pratihar J L, Patra D, Burrows A, Mohan
M and Chattopadhyay S 2010 Inorg. Chim. Acta 363
2865
42. Perrin D D and Armarego W L F 1988 Purification of
laboratory chemicals, 3rd edn.; New York: Pergamon
43. Parshall G W 1974 Inorg. Synth. XV 48 and 50
44. Trost B M and Verhoeven T R 1982 in: Comprehen-
sive organometallic chemistry, E Abel, F G A Stone, G
Wilkinson (Eds.), vol. 8; Oxford: Pergamon Press
45. Pattanayak P, Pratihar J L, Patra D, Burrows A, Mohan
M and Chattopadhyay S 2007 Eur. J. Inorg. Chem. 4263
46. Parr R G and Yang W 1989 Density functional the-
ory of atoms and molecules; Oxford: Oxford University
Press
47. Frisch M J, Trucks G W, Schlegel H B, Scuseria G E,
Robb M A, Cheeseman Jr J R, Montgomery J A, Vreven
T, Kudin K N, Burant J C, Millam J M, Iyengar S S,
Tomasi J, Barone V, Mennucci B, Cossi M, Scalmani
G, Rega N, Petersson G A, Nakatsuji H, Hada M,
Ehara M, Toyota K, Fukuda R, Hasegawa J, Ishida M,
Nakajima T, Honda Y, Kitao O, Nakai H, Klene M, Li X,
Knox J E, Hratchian H P, Cross J B, ammi R, Pomelli C,
Ochterski J W, Ayala P Y, Morokuma K, Voth G A,
Salvador P, Dannenberg J J, Zakrzewski V G, Dapprich
S, Daniels A D, Strain M C, Farkas O, Malick D K,
Rabuck A D, Raghavachari K, Foresman J B, Ortiz
J V, Cui Q, Baboul A G, Clifford S, Cioslowski J,
8. Poyatos M, Mata J A and Peris E 2009 Chem. Rev. 109
3677
9. Singh P and Singh A K 2010 Eur. J. Inorg. Chem. 4187
10. Peacock A F A, Habtemariam A, Fernandez R, Walland
V, Fabbiani F P A, Parsons S, Aird R E, Jodrell D I and
Sadler P J 2006 J. Am. Chem. Soc. 128 1739
11. Wirth S, Rohbogner C J, Cieslak M, Kazmierczak-
Baranska J, Donevski S, Nawrot B and Lorenz I P 2010
J. Biol. Inorg. Chem. 15 429
12. Boardman B, Hanton M J, Rensburg H V and Tooze R P
2006 Chem. Commun. 2289
13. van Engelen M C, Teunissen H T, De Vries J G and
Elsevier C J 2003 J. Mol. Catal. A: Chem. 206 185
14. Yin A Y, Guo X Y, Dai W-L and Fan K N 2010 Chem.
Commun. 46 4348
15. Therrien B 2009 Coord. Chem. Rev. 253 493
16. Sarfraz R A, Kazia T G, Iqbalb S, Afridia H I, Jamali
M K, Jalbani N and Arain M B 2008 Appl. Organometal.
Chem. 22 187
17. Matteoli U, Menchi G, Bianchi M and Piacenti F 1988
J. Mol. Catal. 44 347
18. Matteoli U, Menchi G, Bianchi M, Piacenti F, Ianelli S
and Nardelli M 1995 J. Organomet. Chem. 498 177
19. Matteoli U, Menchi G, Bianchi M and Piacenti F 1991
J. Mol. Catal. 64 257
20. Nomura K, Ogura H and Imanishi Y 2001 J. Mol. Catal.
A: Chem. 166 345
21. Nomura K, Ogura H and Imanishi Y 2002 J. Mol. Catal.
A: Chem. 178 105

62
Poulami Pattanayak et al.
Stefanov B B, Liu G, Liashenko A, Piskorz P, Komaromi 56. Pattanayak P, Pratihar J L, Patra D, Puranik V G and
I, Martin R L, Fox D J, Keith T, Al-Laham A, Peng C Y,
Nanayakkara A, Challacombe M, Gill P M W, Johnson 57. Saluzzo C and Lemaire M 2002 Adv. Synt. Catal. 344
B, Chen W, Wong M W, Gonzalez C, Pople J A, Gaus- 915
sian 03W, revision C.02 Gaussian, Inc., Wallingford, CT, 58. Harmon R E, Gupta S K and Brown D J 1973 Chem.
Chattopadhyay S 2008 Polyhedron 27 2209
2004.16
Rev. 73 21
48. Becke A D 1993 J. Chem. Phys. 98 5648
49. Lee C, Yang W and Parr R G 1998 Phys. Rev. B 37 785
50. Schaefer A, Hube C and Ahlrichs R 1994 J. Chem. Phys.
100 5829
59. Pintar A and Batista J 1999 Catal. Today 53 35
60. Mallat T, Bodmer M and Baiker A 1997 Catal. Lett. 44
95
61. Venkatachalam G and Ramesh R 2005 Tetr. Lett. 46
51. Schaefer A, Horn H and Ahlrichs R 1992 J. Chem. Phys.
97 2571
52. Sheldrick G M 1990 SHELXS-97, Göttingen, Germany:
University of Göttingen
5215
62. Kannan S, Ramesh R and Liu Y 2007 J. Organomet.
Chem. 692 3380
63. Pratihar J L, Bhaduri S, Pattanayak P, Patra D and
Chattopadhyay S 2009 J. Organomet Chem. 694 3401
53. Sheldrick G M 1997 SHELXL-97, Program for the
refinement of crystals structures from diffraction data, 64. Bianchini C, Peruzzini M, Farnetti E, Kagpar J and
Göttingen, Germany: University of Göttingen Graziani M 1995 J. Organomet Chem. 488 91
54. Patra D, Pattanayak P, Pratihar J L and Chattopadhyay S 65. Clapham S E and Morris R H 2005 Organometallics 24
2007 Polyhedron 26 484 479
55. Ray U, Chand B, Mostafa G, Cheng J, Lu T-H and Sinha 66. Bianchini C, Farnetti E, Graziani M, Peruzzini M and
C 2003 Polyhedron 22 2587 Polo A 1993 Organometallics 12 3753