Osmium(II) bipyridine (bpy) complexes containing O,O-donor ligands and X-ray crystal structure of the acetylacetonato(acac) complex [Os(bpy) 2(acac)](PF6)

Article abstract of DOI:10.1016/j.molstruc.2011.03.063

The new complexes [Os¹¹(bpy)₂L](PF₆) [bpy = 2,2′-bipyridine; HL = acetylacetone (Hacac), trifluoroacetylacetone (Htfacac), hexafluoroacetylacetone, dibenzoylmethane or tropolone] have been prepared and the chemical oxidation of the [Os^II(bpy) ₂L]⁺ species (L = acac or tfacac) by aqueous S ₂O82- solution in the presence of (NH₄)PF₆ gave the reddish brown complexes [Os^III(bpy)₂L](PF ₆)₂. These complexes were characterized by spectroscopic measurements and also investigated by cyclic voltammetry. The X-ray crystal structure of [Os(bpy)₂(acac)](PF₆) (1) has been determined and showed that the complex has a distorted octahedral geometry in an environment of N₄O₂ donors. The reactivity toward oxidation of benzyl alcohol by complex (1) using H₂O₂ or other co-oxidants has been studied and compared with that of the related ruthenium(II) complexes.

Full text of DOI:10.1016/j.molstruc.2011.03.063

Journal of Molecular Structure 995 (2011) 97–102
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Journal of Molecular Structure
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Osmium(II) bipyridine (bpy) complexes containing O,O-donor ligands and
X-ray crystal structure of the acetylacetonato(acac) complex [Os(bpy)2(acac)](PF6)
⇑
Ahmed M. EL-Hendawy
Chemistry Department, Faculty of Science, Damietta, Mansoura University, Egypt
a r t i c l e i n f o
a b s t r a c t
The new complexes [Os11(bpy)2L](PF6) [bpy = 2,20-bipyridine; HL = acetylacetone (Hacac), trifluoroacety-
lacetone (Htfacac), hexafluoroacetylacetone, dibenzoylmethane or tropolone] have been prepared and
Article history:
Received 22 October 2010
Received in revised form 21 March 2011
Accepted 25 March 2011
Available online 1 April 2011
the chemical oxidation of the [OsII(bpy)2L]+ species (L = acac or tfacac) by aqueous S2O2ꢀ solution in
8
the presence of (NH4)PF6 gave the reddish brown complexes [OsIII(bpy)2L](PF6)2. These complexes were
characterized by spectroscopic measurements and also investigated by cyclic voltammetry. The X-ray
crystal structure of [Os(bpy)2(acac)](PF6) (1) has been determined and showed that the complex has a
distorted octahedral geometry in an environment of N4O2 donors. The reactivity toward oxidation of
benzyl alcohol by complex (1) using H2O2 or other co-oxidants has been studied and compared with that
of the related ruthenium(II) complexes.
Keywords:
Osmium
Bipyridine complexes
O,O-donor ligands
X-ray crystal structure
Spectroscopic properties
Redox properties
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
against human ovarian cancer cell lines, some of them as potent
as cis platin and carboplatin [9].
Reaction of b-diketonates (dik) with [OsIVQ2Br2] (HQ = 8-quino-
linol) gives the osmium(III) and (IV) complexes [OsQ2(dik)]0/+ [1].
We have prepared and characterized the complexes [RuII(bpy)2
(dik)] (PF6) [2], but no work has so far been reported on similar
complexes of osmium(II), although analytical and electronic spec-
tral data have been mentioned for [OsII(bpy)2(acac)]Clꢁ3H2O [3,4].
Mixed complexes of osmium(II) with bipyridine and 2-(arylazo)
phenolate (ON) or o-keto(or aldehydo) phenolate have been
prepared from [OsII(bpy)2Br2] in the presence of NEt3; the X-ray
crystal structure has been determined for [OsII(bpy)2L]+ (L = ON
or salicyaldehyde anion) [5,6].
In the present study we report a simple route for the synthesis
of the new complexes [OsII(bpy)2L](PF6) [L = monoanions of acetyl-
acetone (acac); trifluoroacetylacetone (tfacac); hexafluoroacetylac-
etone (hfacac); dibenzoylmethane (dbm) and tropolone(trop),
2-hydoxy-2,4,6-cyclohepta-trienone] and [OsIII(bpy)2L](PF6)2 (L =
acac or tfacac). We also report their rich redox and spectroscopic
properties together with the X-ray crystal structure of [OsII(b-
py)2(acac)](PF6) (1). The reactivity of the complex (1), as a typical
example is tested towards the oxidation of alcohol, since the anal-
ogous ruthenium(II) complex [2] and [RuIII(bpy)(acac)2](PF6) [10]
have been studied in this way.
Mixed chelate copper(II) complexes patented and mark title
registered as CasiopeinasÒ are potent antineoplastic agents and
2. Experimental
have general formulas. [Cu(N–N)(O–O)]NO3 (A) and [Cu(N–N)(
a-
L
-aminoacidato)]NO3 (B), where N–N donor is an aromatic substi-
2.1. Physical measurements
tuted diimine (1,10-phenanthroline or 2,20-bipyridine) and the
O–O donor is acac or salicylaldehydate [7]. A strong relationship
between the medial inhibitory concentration (IC50) and the redox
potential (E1/2) of the complexes (A) and (B) and their derivatives
has been found; the most active complexes are the weaker oxi-
dants [8]. Recently, osmium coordination compounds and its
organometallic species have shown promising cytotoxic activity
IR spectra were measured on a Nicolet 510P FT-IR spectrometer
as paraffin mulls between CsI plates or KBr discs. 1H NMR spectra
were measured on a Bruker AT-FT 100 instrument (100 MHz). The
electronic spectra were measured on a Lambda 2S UV/Vis spectro-
photometer. EPR spectra were recorded with a Bruker ECS 106
spectrometer. Cyclic voltammetric studies were carried out on a
potentiostat/wave generator (Oxford Electrodes) using a platinum
working electrode in conjunction with a Philips PM 8043 X–Y re-
corder. The three electrode cell comprised a reference Ag, Pt
auxiliary and working electrodes, the complexes (10ꢀ3 M) in
⇑
Tel.: +20 106869447.
E-mail address: amelhendawy@yahoo.com
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.03.063

98
A.M. EL-Hendawy / Journal of Molecular Structure 995 (2011) 97–102
0.1 M (n-Bu4N)PF6 as supporting electrolyte were used. Magnetic
measurements were made on a Johnson Matthey magnetic suscep-
tibility balance. The thermal stability of the complexes was inves-
lized from MeOH to give dark purple crystals suitable for X-ray dif-
fraction. The 1H NMR data (d, ppm) in d6-DMSO:acetylacetonate
resonances, 1.76 (s, 2Me), 5.45 (s,
c-CH); bipyridine resonances,
tigated using
a
Perkin–Elmer TGA7 unit (heating rate:
6.95 (t, 2H), 7.55 (m, 6H), 7.80 (t, 2H), 8.35 (d, 2H), 8.65 (m, 4H).
A similar procedure was used to prepare the other osmium(II)
complexes [Os(bpy)2L](PF6), (2)–(5) with a 1:2 molar ratio of the
complex (C) to the O,O-donor ligand, (HL) in which the acetylace-
tone is replaced by trifluoro/hexafluoroacetylacetone, dib-
enzoylmethane or tropolone. The product yield for the complexes
were 75–80%. The 1H NMR resonance data (d, ppm) in d6-DMSO
for the complexes (2)–(5) are as follows. [Os(bpy)2(tfacac)](PF6)
10 °C minꢀ1).
2.2. X-ray crystal structure analysis
Dark purple orthorhombic crystals of [Os(bpy)2(acac)](PF6)(1)
having appropriate dimensions were measured on Bruker Kappa
CCD diffractometer equipped with graphite-monochromated
(2): trifluoroacetylacetonate, 1.78 (s, Me), 6.07 (s,
c-CH); bipyri-
Mo Ka radiation (k = 0.71073 Å), the unit cell dimensions and
dine, 7.20 (m, 2H), 7.75 (m, 6H), 8.05 (t, 2H), 8.30 (d, 1H), 8.45
(d, 1H), 8.75 (t, 2H), 8.90 (t, 2H). [Os(bpy)2(hfacac)](PF6) (3): hexa-
intensity data were measured at 298 K. The crystal data were col-
lected up to 55.34° in 2h and the structure was solved by least-
squares fit of the angular setting of strong reflections based on
F2. Program used to solve structure was SIR92 [11] while the Pro-
gram used to refine structure was maXus [12]. Integration and
scaling of the reflections were performed with the HKL Denzo-
Scalepack system of programs [13]. The non-hydrogen atoms were
refined with anisotropic thermal parameters. Crystallographic data
for (1) are summarized in Table 1.
fluoroacetylacetonate, 6.24 (s,
(m, 4H), 7.81 (m, 2H), 7.95 (m, 2H), 8.08 (m, 2H), 8.87 (m, 2H).
[Os(bpy)2(dbm)](PF6) (4): dibenzoylmethanate, 6.85 (s, -CH),
c-CH); bipyridine, 7.16 (t, 2H), 7.65
c
7.56 (m, 8H), 7.76 (d, 2H); bipyridine, 7.10 (t, 2H), 7.34 (t, 4H),
7.67 (d, 2H), 7.83 (t, 2H), 8.31 (d, 2H), 8.67 (d, 2H), 8.77 (d, 2H).
[Os(bpy)2(trop)](PF6) (5): tropolonate, 7.08 (t, 3H) 7.25 (d, 2H);
bipyridine, 7.60 (m, 6H), 7.70 (t, 2H), 7.95 (t, 2H), 8.53 (d, 2H),
8.72 (d, 2H), 8.90 (d, 2H).
2.2.1. Syntheses of complexes
2.2.1.2. [OsIII(bpy)2L](PF6)2 (L = acac, tfacac) (6), (7). Method (a): To
the evaporated filtrate (30 cm3) above, Na2S2O8 (0.48 g, 2 mmol)
in H2O (5 cm3) was added and stirred for 0.5 h, an instantaneous
brown color appears. The solution was filtered to isolate any undis-
solved material and the filtrate concentrated to 5–10 cm3 on a ro-
tary evaporator, then (NH4)PF6 (2.5 mmol) was added. The brown
precipitate was filtered off, washed carefully with EtOH and Et2O
and dried in vacuo over P2O5.
The complex (7) was similarly prepared in which trifluoroacet-
ylacetone replaces acetylacetone. The product yield for the com-
plexes (6) and (7) were 70%.
Method(b): To the complex (1) or (2) (0.25 mmol) in DMF or
Me2CO (3 cm3) was added Na2S2O8 (2.5 mmol) in H2O (5 cm3)
and NH4PF6 (1 mmol). The mixture was stirred for 15–30 min. dur-
ing which a brown precipitate formed. This was filtered off,
washed with little cold EtOH and then Et2O (yield, 75%).
2.2.1.1. [Os(bpy)2(acac)](PF6) (1). A modified method to that re-
ported for preparation of [Os(bpy)2(acac)](ClO4)ꢁ2H2O [3] was used
as follows. The complex cis-[Os(bpy)2Cl2] [14] (C) (0.2 g,
0.35 mmol) was dissolved in 50 cm3 of an H2O–EtOH mixture
(1:1) under N2, heated at 75 °C on a water bath. Acetylacetone
(0.5 cm3) and excess CaCO3 (0.2 g) were added, heating continued
for 5 h and then the solution filtered off from any unreacted cal-
cium salt. The filtrate was evaporated to 30 cm3 and the product
precipitated by addition of
(NH4)PF6 (1 mmol), filtered off and washed with H2O, Et2O and
dried in vacuo over P2O5 (yield, 75%). The product was recrystal-
a saturated aqueous solution of
Table 1
Crystal data and structure refinement parameters for [Os(bpy)2(acac)](PF6) (1).
C25H23F6N4O2OsP
746.65
298 K
0.71073Å
Orthorhombic
P 212121
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
Unit cell dimensions
a
2.2.2. Catalytic oxidation of benzyl alcohol
To 10 cm3 of dichloroethane benzyl alcohol (1 cm3, 10 mmol),
complex (1) (0.01 g, 0.013 mmol) and Aliquat 336 (0.45 g, 1 mmol)
were added. The reaction mixture was heated 70–80 °C with stir-
ring and then 30% H2O2 (7.5 cm3, 65 mmol) was added dropwise
at a constant rate over 40 min. The reddish violet color changed
into brown and the reaction continued for 80 min more, the organ-
ic layer was separated and the aqueous layer was extracted with
dichloromethane (3 ꢂ 10 cm3). The extracts were combined, dried
over anhydrous MgSO4 and evaporated to dryness. The aldehyde
content was quantified as its 2,4-dinitrophenylhydrazone deriva-
tive (equivalent to 1.43 mmol of the benzaldehyde product).
10.0508(2) Å
17.6875(3) Å
29.7659 (5) Å
90.00°
b
c
a
= b = c
5291.6 (2) Å3
8
Volume
Z
Density (calculated)
Absorption coefficient
Theta range for data collection
Index ranges
1.874 mg mꢀ3
4.95 mmꢀ1
2.910–27.485°
ꢀ12 ꢃ h ꢃ 12, ꢀ22 ꢃ k ꢃ 22,
ꢀ37 ꢃ ‘ ꢃ 37
2552
7096 [R(int) = 0.029]
None
Refinement method
full-matrix least
squares on F2
2541/0/352
Reflections collected
Independent reflections
Absorption correction
Refinement method
3. Results and discussion
3.1. Syntheses and some physico-chemical properties
Reaction of cis-[Os(bpy)2Cl2] with the O,O-donor ligands (HL) in
a 50% aqueous ethanol solution under nitrogen and reflux for 5 h
gave purple-dark microcrystals of the desired complexes isolated
as the hexafluorophosphate salts [Os(bpy)2L](PF6) ((1)–(5)) (yield,
75–80%). The presence of CaCO3 as a heterogeneous base [1,15] al-
lows completion of reaction to give pure products. Oxidation of
[Os(bpy)2L]+ species (L = acac or tfacac) by Na2S2O8 instead of FeCl3
[3] in the presence of NH4PF6 gave [Os(bpy)2L](PF6)2 (6) and (7) as
Reflections/restraints /
parameters
R indices [I > 3
r(I)]
b
0.035/0.133
R1a/wR2
1.93 and ꢀ2.58 eÅ3
Largest diff. peak and hole
P
P
a
R1
=
jjFoj ꢀ jFcjj= jFoj:
nh
i
o
P
2
b
wR2
=
wðF2o ꢀ F2c Þ
=
wF4o 1=2, where w = 1/[
r
2(F2o )].

A.M. EL-Hendawy / Journal of Molecular Structure 995 (2011) 97–102
99
Table 2
Analytical, vibrational spectroscopic and thermal gravimetric data for complexes.
a
Compound
Found (Calcd.)
KM
(X )
ꢀ1 cm2 molꢀ1
Vibrational data (cmꢀ1
)
Other bands
Thermal gravimetric
data Deco. temp.
range, °Cb,c
C
H
N
m
(C'O)
m
(C'C)
m(PF6)
[Os(bpy)2(acac)](PF6)
40.1
3.0
7.4
134
139
148
1544s,1396s
1519
s,1255s
1259m
844vs,
557s
769w
1600m, 1461s, 1163w
300–380
14.5(13.3)
260–370
18.0(19.1)
255–370
23.5(24.2)
140–365
(1)
(40.2) (3.1) (7.5)
37.4 2.4 6.8
(37.5) (2.4) (7.0)
2.1 6.5
(35.1) (2.0) (6.6)
48.1 3.0 6.3
1541s,1320m
1601s, 1478s, 1169s, 1023w,
674s
[Os(bpy)2(tfacac)](PF6)
1574s,1368m 1510m,
1257s
1539s,1319s
842vs,
556s
768w
1602m, 1453vs, 1156s
(2)
1252m
1601s, 1477s, 1167s, 1021w,
672s
[Os(bpy)2(hfacac)](PF6) 34.9
1543m,
1333s
1550s, 1318s
1520m,
1261vs
1266s
837vs,
550s
766w
1603m, 1453vs, 1156vs
(3)
1603s, 1482vs, 1170s,
1021w, 673s
[Os(bpy)2(dbm)](PF6)
145
129
220
1516vs,
1393s
1253s
842vs,
555s
1599m,1471s, 1162m
1588s, 14625s, 1168w
1608s, 1451s, 1166m
(4)
(48.3) (3.1) (6.4)
42.0 2.8 7.3
25.5(25.6)
300–375
[Os(bpy)2(trop)](PF6)
1509s, 1342s 1251s
839vs,
558s
(5)
(42.2) (2.7) (7.3)
33.6 2.5 6.2
15.5(15.4)
230–300
[Os(bpy)2(acac)](PF6)2
1531s,
1339m
1543s, 1320s
15513s,
1286m
1262m
839vs,
557s
766m
(6)
(33.7) (2.6) (6.3)
1.9 5.8
(31.7) (2.1) (5.9)
Raman bands are in italic.
1600s, 1480s, 1172s, 1043m,
672s
13.0(11.1)
150–245
[Os(bpy)2(tfacac)](PF6)2 31.5
(7)
235
1580m
1513w,
1292 s
837vs,
550s
1608s, 1448s, 1142s
16.0(16.3)
a
b
c
Refer to weight loss due to O,O-donor ligand (ꢀ1).
Underlined data refer to found weight loss (Calcd.), %.
reddish-brown complexes (product yield, ꢄ70%). Analytical data of
the complexes (Table 2) are in good agreement with their compo-
sitions. Most complexes are soluble in CH2Cl2 and CH3CN and the
molar conductivities in the latter solvent show that the complexes,
(1)–(5) behave as 1:1 electrolytes while the complexes, (6) and (7)
are 1:2 electrolytes, as expected (Table 2).
Complexes (1)–(5) were found to be diamagnetic at room tem-
perature; other complexes (6) and (7) are paramagnetic and have
magnetic moments (leff) of 1.78 and 1.92 BM, close to the spin-
only value (1.73 BM) for one unpaired electron. This suggests the
5
low-spin d5(t2g
) configuration for the osmium(III) ion in an octa-
hedral environment [6,16,17]. However, EPR studies at room tem-
perature for the solid complexes (6) and (7) show an EPR silent as
is the case for similar related osmium(III) bis-bipyridine complexes
[5,6]. This EPR inactivity in Low-spin d5 complexes is known to re-
sult from extensive mixing of the Kramers doublets by strong spin–
orbit coupling which gives rise to short electronic relaxation time
[6,18].
The thermal stabilities of the complexes have been investigated.
They are stable to 140 °C or up to 300 °C, after which temperatures
the weight loss seems to correspond to loss of the coordinated O,O-
donor lignad (ꢀ1) (Table 2). The complexes show less thermal sta-
bility than those of the analogous ruthenium(II) complexes [2]. The
thermal data (Table 2) indicate that the osmium(II) complexes, (1)
and (2) are more stable than the corresponding osmium(III) com-
plexes, (6) and (7).
Fig. 1. ORTEP diagram of [OsII(bpy)2L](PF6) (1) showing the atom-numbering
scheme, with 50% ellipsoidal propability. Hydrogen atoms are shown as smaller
spheres of arbitrary radii.
in Table 3. The OsN4O2 coordination sphere is distorted octahedral;
three near linear trans-angles (ꢄ172.9–176.4°) and twelve cis-an-
gles (ꢄ78.6–99.1°). The acetylacetonate anion (acac) is coordinated
to osmium as bidentate O,O-donor with a bite angle of 91.4°, sim-
ilar bond lengths have been observed for O3–C37 and O4–C27
[1.287(3) Å, 1.288(3) Å] and also for C32–C37 and C27–C32
3.2. Molecular and crystal structure of [Os(bpy)2(acac)](PF6)
Molecular structure of [Os(bpy)2(acac)](PF6) (1) has been
determined by X-ray crystallography. A view of the molecule is
shown in Fig 1, and selected bond distances and angles are listed

100
A.M. EL-Hendawy / Journal of Molecular Structure 995 (2011) 97–102
Table 3
charge-transfer for the complexes, (1)–(5) [5,6,23–26,27a] or li-
gand ( ) ? metal (d) charge-transfer (MLCT) for complexes (6)
Selected bond lengths and angles for [Os(bpy)2(acac)](PF6)(1).
p
and (7) [5,6,26]. It was pointed out by Lever [27b] that for a MLCT
transition involving the bpy p⁄ orbital, a molar extinction coeffi-
cient of ꢄ4000 Mꢀ1 cmꢀ1 per bpy ligand is expected. So, the bands
in the range 490–550 nm for the complexes [OsII(bpy)L](PF6) are
Bond lengths (Å)
Bond angles (°)
Os1–O3
Os1–O4
Os1–N5
Os1–N6
Os1–N7
Os1–N8
N8–C12
N8–C22
C22–C30
N5–C30
N5–C26
C12–C13
C20–C26
N6–C11
N6–C15
N7–C19
N7–C10
N7–C19
C15–C19
O3–C37
O4–C27
C32–C37
C27–C32
P2–F14
P2–F18
P2–F23
P2–F33
P2–F36
P2–F39
2.075 (2)
2.0567 (14)
2.014 (2)
2.019 (2)
2.046 (2)
2.047 (2)
1.371 (2)
1.356 (2)
1.459 (3)
1.377 (3)
1.367 (2)
1.366 (3)
1.367 (3)
1.367 (2)
1.374 (2)
1.358 (3)
1.347 (3)
1.358 (3)
1.475 (3)
1.287 (3)
1.288 (3)
1.385 (3)
1.386 (3)
1.577 (2)
1.571 (2)
1.566 (2)
1.575 (2)
1.573 (2)
1.579 (2)
O3–Os1–O4
O3–Os1–N5
O3–Os1–N6
O3–Os1–N7
O3–Os1–N8
O4–Os1–N5
O4–Os1–N6
O4–Os1–N7
O4–Os1–N8
N5–Os1–N6
N5–Os1–N7
N5–Os1–N8
N6–Os1–N7
N6–Os1–N8
N7–Os1–N8
F14–P2–F18
F14–P2–F23
F14–P2–F33
F14–P2–F36
F14–P2–F39
F18–P2–F23
F18–P2–F33
F18–P2–F36
F18–P2–F39
F23–P2–F33
F23–P2–F36
F23–P2–F39
F33–P2–F36
F33–P2–F39
F36–P2–F39
91.44 (6)
87.60 (6)
173.32 (6)
94.81 (6)
87.17 (6)
172.94 (7)
85.86 (6)
88.52 (6)
94.41 (6)
95.81 (6)
98.53 (7)
78.56 (7)
79.04 (7)
99.11 (7)
176.43 (9)
88.98 (11)
89.59 (13)
177.40 (11)
90.06 (12)
90.18 (12)
89.12 (10)
88.43 (10)
179.0 (2)
89.13 (12)
90.64 (14)
90.50 (11)
178.24 (12)
92.53 (13)
89.51 (13)
91.24 (12)
assigned to MLCT transition from the OsII(d ) to p⁄(bpy), while
p
the lower energy bands in 647–780 nm region are probably arising
from OsII(d ) to p⁄(L) transition. Each of these MLCT transitions are
p
shifted to a higher wavelength in the order of L = hfacac < tfa-
cac < acac < trop, as those found for X = CN < Br < Cl in the com-
plexes [RuIIL0X2](L0 = N4-tetradentate neutral ligand) [27c]. The
bands or shoulders below 300 nm are of the interligand
p ?
p⁄
type [25,26,28,29]. The complexes (1) and (6) showed similar spec-
tra to those previously reported for [Os(bpy)2(acac)]+ [4] and
[Os(bpy)2(acac)]2+ [30], respectively.
The 1HNMR spectra of the complexes [Os(bpy)2L](PF6) (1)-(5)
were recorded in d6-DMSO (data in Section 2.2.1) and are in accor-
dance with the structures suggested previously for analogous
ruthenium(II) complexes [2].
On the basis of different analytical and spectroscopic data above
together with the structural data of [Os(bpy)2(acac)](PF6) (1), the
structures of osmium complexes are expected to be formulated
as shown below. The non-isomeric nature of these complexes is
in contrast to those observed for [MII(PPh3)2(tfacac)2] (M = Ru or
Os) [31].
+
N
Table 4
Hydrogen bonding parameters (Å, °) for [Os(bpy)2(acac)](PF6) (1).
N
O
D–H–A
D–H (Å)
Hꢁ ꢁ ꢁA (Å)
Dꢁ ꢁ ꢁA (Å)
D–Hꢁ ꢁ ꢁA (°)
Os
N
C34–H34ꢁ ꢁ ꢁF14(i)
C29–H29ꢁ ꢁ ꢁF14(i)
C17–H17ꢁ ꢁ ꢁF18(ii)
C10–H10ꢁ ꢁ ꢁF23(ii)
C38–H38Aꢁ ꢁ ꢁF39(iii)
C32–H32ꢁ ꢁ ꢁF39(iii)
0.960(2)
0.960(2)
0.960(2)
0.960(2)
0.960(3)
0.960(2)
2.629(2)
2.702(2)
2.563(2)
2.832(2)
2.801(2)
2.817(2)
3.279(3)
3.306(3)
3.283(3)
3.583(3)
3.614(3)
3.574(3)
122.7(2)
121.4(2)
131.9(2)
135.7(2)
143.0(2)
136.39(14)
N
O
N-N = 2,2 -bipyridine
Symmetry codes: (i) 2 ꢀ x, ꢀy, ꢀz; (ii) 3/2 ꢀ x, 1/2 ꢀ y, z; (iii) 5/2 ꢀ x, 1/2 ꢀ y, z.
O-O = O,O- donor ligand (-1)
[1.385(3) Å, 1.386(3) Å] which are indicating the resonating struc-
ture of the coordinating acac ligand. Similar coordination and
structural data have been found for [OsII(bpy)2(sal)](ClO4) [sal =
O,O-donor of salicylaldhydate(ꢀ1)] [6], but a square pyramidal
structure of [Cu(bpy)(acac)(H2O)]NO3.H2O has been recently re-
ported [19]. Table 3 shows that the hexafluorophosphate anion
in the complex (1) is a slightly distorted octahedral structure;
the 12 cis-angles are near 90° (ꢄ88.4–92.5°) and the six P–F bond
lengths are similar .
3.4. Redox properties
The electron transfer properties of the complexes have been
studied by cyclic voltammetric techniques. Voltammetric data vs.
a silver electrode for 0.1 M (n-Bu4N)PF6-dichloroethane solutions
(containing 10ꢀ3 M of the complex) are presented in Table 5 and
Fig. 2 as a representative example. These are compared with
[Fe(C5H5)2]0/+, E1/2 = +0.42 V,
DE = 65 mV for the cell used under
the same conditions. The voltammogram for each of the com-
plexes, (1)–(5) shows a redox wave at positive potential (+0.07–
+0.67 V) which is assigned to the OsII/OsIII oxidation (equation be-
low), similar to that reported for [OsII(bpy)2(sal)](ClO4) [6]. The
The PFꢀ6 ions are involved in the intermolecular hydrogen bond-
ing interactions, the fluorine atoms interact with hydrogen atoms
of H–C (bpy) and H–C(acac). Medium–weak contacts of lengths
in the range 3.279(3)–3.614(3) Å have been observed (Table 4).
peak separation (DE) for each complex is close to that anticipated
for a Nernstian one electron-process (59 mV) [32], and remains
3.3. Spectroscopic properties
unaltered upon changing the scan rate (10–50 mV sꢀ1). The anodic
peak current (ipa) is almost equal to the cathodic peak current (ipc
as expected for a reversible couple.
)
The vibrational spectral data (IR and Raman, Table 2) are consis-
tent with the proposed structures for complexes and indicate that
the O,O-donor ligand (HL) is coordinated in the normal manner
through O,O-donor sites [2,10,17,20–22].
½OsIIðbpyÞ2Lꢅþ ꢀ ½OsIIIðbpyÞ2Lꢅ2þ þ e
The electronic spectra (Table 5) in (CH2Cl)2 for complexes
showed several intense absorptions for each in the visible region.
For the complexes (1) and (5) a further an irreversible peak
(Epa = +1.36 V) or quasi-reversible oxidation wave (E1/2
+1.28 V, E = 120 mV) is observed; each has the same current
a
=
These bands are probably due to either a metal (d) ? ligand (p⁄
)
D

A.M. EL-Hendawy / Journal of Molecular Structure 995 (2011) 97–102
101
Table 5
UV/Vis spectroscopic and cyclic voltammetric data for complexes.
Complex
UV/Vis data kmax
(e
, Mꢀ1 cmꢀ1
)
Voltammetric data (V)
Oxidations
Reductions
E1/2
Epa
D
E
E1/2
Epc
DE
(1)
765(4770), 525(11,180), 443(10,640)
373(11,320), 297(50,960), 247(30,600)
749(4010), 521(10,300), 438(9470)a
375(10,200), 295(48,100), 275, 244(28,800)a
+0.22
0.060
0.060
ꢀ1.28
+1.36
(2)
(3)
(4)
(5)
(6)
723(4200), 506(9500), 415(10,700)
380(8600), 296(44,050), 246(20,450)
+0.32
+0.67
+0.07
0.060
0.060
ꢀ1.73
0.060
647(5950), 490(16,800), 420(12,200)
355(8100), 293(69,000), 245(29,900)
0.060
ꢀ1.43
ꢀ1.27
ꢀ1.40
750(5600), 514(14,150), 438(17,000)
332(32,250), 297(72,850), 247(41,650)
0.060
780(3050), 706(3150), 550(9850)
438(11,850), 298(38,750), 240(32,500)
+0.08
+1.28
0.060
0.120
5105h(1500), 389(5150), 286(27,350)
252(26,000)
+0.22
+0.32
0.060
0.060
505sh, 413sh, 384(4610), 283(21,200)b
264(22,200), 250(22,500)b
(7)
700(1300), 487(4880), 402(5110)
380(5180), 291(26,380), 245(21,740)
a
In MeOH solvent Ref. [4].
In H2O solvent Ref. [30].
b
also in [Ru(bpy)n(L)3ꢀn] (L = O,N-donor ligand (ꢀ1)] [37,38]; the
OsII/OsIII redox potentials have lower values than those of the cor-
responding RuII/RuIII couples.
Comparison of the OsII/OsIII redox potentials, E1/2 (Table 5) for the
present complexes [OsII(bpy)2L](PF6), (1)–(5) with that of the
RuII/RuIII couple in the analogous ruthenium(II) complexes [2]
shows that these potentials are lower in the osmium complexes
compared to those of ruthenium. This indicates that the
osmium(II)–osmium(III) oxidation is easier than that of
ruthenium(II)–ruthenium(III) as the case for [M(bpy)2(sal)]+
(M = Ru or Os) [6,39].
The oxidation potential, E1/2 for the OsII/OsIII couple is sensitive
to the nature of the substituent in the O,O-donor ligand. It is
shifted to lower or higher values (Table 5) relative to the [OsII(b-
py)2(acac)](PF6), reflecting an increase of basicity with lower E1/2
values corresponding to O,O-donor ligand (L) in the order;
hfacac < tfacac < acac < trop ꢆ dbm
The reversibility of the OsII/OsIII couple for [OsII(bpy)2L](PF6)
(L = acac or tfacac) at relatively low positive potentials (E1/2
=
+0.22, +0.33 V) points to the possibility of the oxidized complexes
[OsIII(bpy)2L](PF6)2, (6) and (7), being stable and isolable in the so-
lid state.
At the negative potentials a reversible wave (E1/2 = ꢀ1.73 V) for
complex (2) or an irreversible cathodic peak (Epc) for the other
complexes is observed indicating a reduction of one bpy molecule
as shown in the equation below. A similar reduction wave
was found for [Os(bpy)2Cl2] (E1/2 = ꢀ1.6 V) [14,35] while in [RuII
(bpy)2L]+ two successive-one electron reductions were observed
[2,40].
Fig. 2. Cyclic voltammogram, volts versus Ag electrode, scan rate 50 mV sꢀ1 for
[OsII(bpy)2(trop)](PF6) (5). (10ꢀ3 M) in dichloroethane ꢀ0.1 M (n-Bu4N)PF6 solution.
height as that of the OsII/OsIII couple. This peak or wave is probably
arises from OsIII/OsIV oxidation as similarly found for [OsII
(bpy)2Cl2] [14] and [RuII(PPh3)2L2] [L = O,O-donor ligand(ꢀ1)]
[33,34].
½OsIIðbpyÞ2Lꢅþ þ e ꢀ ½OsIIðbpyÞðbpyꢀÞLꢅ
It is concluded that in the tris-acetylacetonato complex, the tri-
valent state of osmium is highly stabilized and has a low value of
E1/2 for the OsIII/OsII couple. This also indicates that chelation by
O,O-donors alone can not stabilize the +2 state of osmium, and in
order to have a stable complex containing O,O-donor ligand of
There is a gradual decrease in redox potentials of the OsII/OsIII
couple in the series [Os(bpy)3]2+, +0.81 V [35]; [Os(bpy)2(acac)]+,
+0.22 V; [Os(acac)3], ꢀ1.244 V [36]. This is similar to that observed
for the RuII/RuIII couple in [Ru(bpy)n(acac)3ꢀn] (n = 0,1,2,3) [10] and

102
A.M. EL-Hendawy / Journal of Molecular Structure 995 (2011) 97–102
osmium(II), strong
p
-acid ligands are needed to be in the coordina-
795957 for [OsII(bpy)2(acac)](PF6). Copy of information may be ob-
tained free of charge from The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: +44 1223 336 033; e-mail: deposit
@ccdc.com.ac.uk or http://www.ccdc.cam.ac.uk).
tion sphere, e.g. bpy ligands which are familiar stabilisers of osmiu-
m(II) and cause a positive shift to the E1/2 for OsII/OsIII couple.
A linear correlation exists on combining the OsII/OsIII oxidation
potential (E1/2) with those of the MLCT energy in both the regions
490–550 nm and 647–780 nm. For a higher potential E1/2, the
MLCT transition energy becomes higher for the complexes [OsII
(bpy)2L](PF6) in the order L = hfacac > tfacac > acac > trop. This
correlation has already been reported for many ruthenium (II)
systems [27].
Acknowledgements
I thank Prof. W.P. Griffith (Imperial College, London) for Raman
data and Qatar University for thermal analysis data.
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Crystallographic data for the structure analysis been deposited
with the Cambridge Crystallographic Data Center, CCDC No.