Syntheses, crystallography and spectroelectrochemical studies of ruthenium azomethine complexes

Article abstract of DOI:10.1016/j.poly.2007.03.045

Three novel families of azomethine ligands L of the type C₆H₅N{double bond, long}NC(COCH₃){double bond, long}N(4-YC₆H₄) (6), C₆H₅N{double bond, long}NC(CO₂CH₃){double bond, long}N(4-YC₆H₄) (7) and 4-ClC₆H₄N{double bond, long}NC(CO₂CH₃){double bond, long}N(4-YC₆H₄) (8), [Y = H (a), CH₃ (b), Cl (c), Br (d), C₁₀H₇ (e), OCH₃ (f), NO₂ (g)] have been synthesized and reacted consecutively with RuCl₃, 2,2′-bipyridine (bpy) and lithium chloride in ethanol under refluxing conditions to produce three novel families of trans-[Ru^II(bpy)LCl₂] complexes. These complexes were characterized by elemental analysis, IR, ¹H NMR, UV-Vis spectroscopy and cyclic voltammetry. Crystallographic studies of two ruthenium(II) azomethine complexes (13 and 18) show that the Ru(II) ion occupies a distorted octahedral coordination sphere in which the chloride ligands are trans to each other and the azomethine and bipyridine nitrogen donor atoms are equatorially coordinated. Crystallographic, electrochemical, electronic spectral data and time-dependent DFT calculations are all consistent with azomethine ligands possessing strong π-acceptor properties. These π-acceptor properties can be tuned by a judicious choice of substituents on the azomethine ligand.

Full text of DOI:10.1016/j.poly.2007.03.045

Polyhedron 26 (2007) 3675–3685
www.elsevier.com/locate/poly
Syntheses, crystallography and spectroelectrochemical studies
of ruthenium azomethine complexes
a,
b
b
a
Mousa Z. Al-Noaimi *, Haytham Saadeh , Salim F. Haddad , Musa I. El-Barghouthi ,
c
d,
*
Mohammad El-khateeb , Robert J. Crutchley
a
Chemistry Department, Faculty of Science, The Hashemite University, Zarka, Jordan
b
Chemistry Department, Faculty of Science, University of Jordan, Amman, Jordan
Chemistry Department, Faculty of Science, Jordan University of Science and Technology, Irbid 22110, Jordan
c
d
Chemistry Department, Carleton University, Ottawa, Ontario, Canada K1S 5B6
Received 1 March 2007; accepted 30 March 2007
Available online 10 April 2007
Abstract
Three novel families of azomethine ligands L of the type C6H5N@NC(COCH3)@N(4-YC6H4) (6), C6H5N@NC(CO2CH3)@
N(4-YC6H4) (7) and 4-ClC6H4N@NC(CO2CH3)@N(4-YC6H4) (8), [Y = H (a), CH3 (b), Cl (c), Br (d), C10H7 (e), OCH3 (f), NO2
(g)] have been synthesized and reacted consecutively with RuCl3, 2,20-bipyridine (bpy) and lithium chloride in ethanol under reflux-
ing conditions to produce three novel families of trans-[RuII(bpy)LCl2] complexes. These complexes were characterized by elemental
analysis, IR, 1H NMR, UV–Vis spectroscopy and cyclic voltammetry. Crystallographic studies of two ruthenium(II) azomethine
complexes (13 and 18) show that the Ru(II) ion occupies a distorted octahedral coordination sphere in which the chloride ligands
are trans to each other and the azomethine and bipyridine nitrogen donor atoms are equatorially coordinated. Crystallographic,
electrochemical, electronic spectral data and time-dependent DFT calculations are all consistent with azomethine ligands possessing
strong p-acceptor properties. These p-acceptor properties can be ‘‘tuned’’ by a judicious choice of substituents on the azomethine
ligand.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Keywords: Ruthenium; Complexes; Azomethine; X-ray structures; Electrochemistry; Spectroelectrochemistry
1. Introduction
p-acceptor properties by changing the number of hetero-
atoms, ring size and the substituents on the heterocyclic
ring bonded to Ru(II) [3,4]. Correlations have also been
found between the charge transfer (CT) energies and
Ru(II)/Ru(III) potentials [5,6]. These correlations have
been used in the design of new ruthenium–bipyridine com-
plexes with desired spectroscopic, electrochemical, and CT
properties [7,8].
Ligands consisting of one N-heterocyclic ring with a
pendant nitrogen donor from azo function such as azo-
pyridines (I) and azo-imidazoles (II) are known as ‘arylazo’
heterocycles and are commonly named as azoimine ligands
[9]. These strong p-acceptor ligands have been used to sta-
bilize low oxidation state metals as Cu(I), Ru(II), Os(II),
Co(II), Fe(II), etc. [9].
The use of heterocyclic N-donor bidentate ligands such
as 2,20-bipyridine (bpy) and its derivatives to stabilize low
valence metal-redox states is widely studied in coordination
chemistry [1,2]. These ligands are p-acceptors and upon
coordination to Ru(II), characteristic metal-to-ligand
charge transfer bands (MLCT) arise in the visible region
of the electronic absorption spectra of the complexes. For
these bidentate ligands, it is possible to ‘‘fine-tune’’ the
*
Corresponding authors. Tel.: +962 777296245 (M.Z. Al-Noaimi);
tel.: +1 6135202600x3848; fax: +1 6135203647 (R.J. Crutchley).
E-mail addresses: manoaimi@hu.edu.jo (M.Z. Al-Noaimi), robert_
crutchley@carleton.ca (R.J. Crutchley).
0277-5387/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.03.045

3676
M.Z. Al-Noaimi et al. / Polyhedron 26 (2007) 3675–3685
solution of the appropriate hydrazonyl chloride (20 mmol).
H
N
The resulted reaction mixture was refluxed for 2 h. Con-
densing the solution followed by cooling produce a yellow
solid which was collected by filtration, washing with water
and recrystallizing from 2-propanol.
N
N
N
Ph
N
N
N
Ph
I
II
Recently, there has been an increasing interest in azome-
thine ligands, due to the photochromatic, redox active, pH
responsive and photo-redox activity of this functional
moiety [10,11]. Excellent examples of such predesigned
molecular materials are 1-arylimino-l-arylhydrazono-pro-
pan-2-one (III) [12]. The arylamidrazones (III) are new
class of bidentate ligands having a low-lying p*(CN) MOs
conjugated with p*(N@N) MOs.
2.2.1. 1-(Phenylimino)-1-(phenylhydrazono)-propan-2-one
(6a)
Yield: 2.6 g (52%). Anal. Calc. for C15H13N3O: C, 71.7;
H, 5.21; N, 16.7. Found. C, 72.3; H, 5.10; N, 16.9%. UV–
Vis in acetonitrile: kmax (emax): 352 (8.83 · 104), 294
(5.24 · 104), 214 (6.90 · 104). IR: m(N@N) 1496, m(C@N)
1
1601, m(C@O) 1668. H NMR (in CDCl3, d ppm): 6.64
(2H, d, H3), 6.92 (2H, t, H4), 7.05 (2H, d, H2), 7.31 (4H,
m, H1, HX, HY), 2.63 (3H, s, HR). M.p. is 132–134 ꢁC.
R
N
2.2.2. 1-(4-Methylphenylimino)-1-(phenylhydrazono)-
propan-2-one (6b)
N
X
N
Y
Yield: 3.0 g (57%). Anal. Calc. for C16H15N3O: C, 72.4;
H, 5.70; N, 15.8. Found: C, 72.1; H, 5.21; N, 15.5%. UV–
Vis in acetonitrile: kmax (emax): 353 (8.09 · 104), 294
(4.69 · 104), 240 (6.24 · 104). IR: m(N@N) 1478, m(C@N)
III
The conjugation shown in the structure of (III) is
expected to create an extremely strong electron acceptor
situation when they are bound to Ru(II). In this study,
we extend our synthesis of azomethine ligands to include
several new bidentate ligands and their Ru(II) complexes.
Changing the substituents (X, Y and R) is expected to vary
the p-acceptor ligands properties on these new bidentate
ligands and thus ‘‘tune’’ the electronic properties of the
ruthenium and consecutively the energy of MLCT bands.
The spectral studies, redox properties and single crystal
X-ray structures (for two representative examples) of these
new families are described in this work.
1
1599, m(C@O) 1664. H NMR (in CDCl3, d ppm): 6.58
(2H, d, H2), 6.97 (1H, t, HX), 7.07 (2H, d, H1), 7.09
(2H, d, H4), 7.27 (2H, d, H3) 2.31 (3H, s, HR), 2.63 (3H,
s, HY). M.p. is 132–134 ꢁC.
2.2.3. 1-(4-Bromophenylimino)-1-(phenylhydrazono)-
propan-2-one (6d)
Yield: 4.0 g (60%). Anal. Calc. for C15H12BrN3O: C,
54.6; H, 3.66; N, 12.7. Found: C, 54.2; H, 3.88; N,
12.5%. UV–Vis in acetonitrile: kmax (emax): 351
(8.92 · 104), 294 (5.32 · 104), 243 (6.98 · 104). IR:
1
m(N@N) 1478, m(C@N) 1596, m(C@O) 1675. H NMR (in
2. Experimental
CDCl3, d ppm): 6.51 (2H, d, H2), 6.987 (1H, t, HX), 7.1
(2H, d, H3), 7.31 (2H, t, H4), 7.35 (2H, d, H1), 2.6 (3H,
s, HR). M.p. is 144–146 ꢁC.
2.1. Materials
The reagents, aniline derivatives, a-chloroacetylacetone,
a-chloroacetoacetate, ruthenium trichloride, 2,20-bipyridine
(bpy), lithium chloride and solvents (reagent grade), were
purchased from Aldrich and were used as received. Tetrabu-
tylammonium hexafluorophosphate (TBAH), purchased
from Aldrich, was recrystallized twice from 1:1 ethanol/
water solution and then vacuum dried at 110 ꢁC. Hydrazo-
nyl chlorides were prepared according to reported proce-
dures [13]. The synthesis and the physical characterization
of the ligand, 1-(4-chlorophenylimino)-l-(phenylhydraz-
ono)-propan-2-one (6c) and the complex trans-[Ru(bpy)-
(6c)Cl2] (11) have been described previously [12]. Elemental
analysis was performed by Canadian Microanalytical Ser-
vices Ltd. (Canada).
2.2.4. 1-(Naphthylimino)-1-(phenylhydrazono)-propan-2-
one (6e)
Yield: 3.8 g (63%). Anal. Calc. for C19H15N3O: 75.7; H,
5.02; N, 13.9. Found: C, 75.6; H, 5.21; N, 13.7%. UV–Vis
in acetonitrile: kmax (emax): 358 (8.80 · 104), 294
(5.20 · 104), 210 (6.86 · 104). IR: m(N@N) 1504, m(C@N)
1
1600, m(C@O) 1673. H NMR (in CDCl3, d ppm): 6.34
(1H, d, H (naphthyl proton)), 6.91 (1H, t, HX), 7.04 (2H,
d, H (naphthyl proton)), 7.31 (3H, m, H4, H (naphthyl
proton)), 7.6 (3H, m, H3), 7.9 (1H, d, H (naphthyl pro-
ton)), 8.1 (1H, d, H (naphthyl proton)), 2.71 (3H, s, HR).
M.p. is 148–150 ꢁC.
2.2.5. 1-(4-Methyloxyphenylimino)-1-(phenylhydrazono)-
propan-2-one (6f)
2.2. General procedure for syntheses of arylamidrazones (L)
Yield: 3.1 g (55%). Anal. Calc. for C16H15N3O2: C, 68.3;
H, 5.37; N, 14.9. Found: C, 68.1; H, 5.21; N, 15.1%. UV–
Vis in acetonitrile: kmax (emax): 357 (8.75 · 104), 297
(5.0 · 104), 240 (6.78 · 104). IR: m(N@N) 1504, m(C@N)
A solution of arylamine (20 mmol) and triethylamine
(2.4 g, 24 mmol) in 5.0 mL ethanol was added to a 10 mL

M.Z. Al-Noaimi et al. / Polyhedron 26 (2007) 3675–3685
3677
1618, m(C@O) 1660. 1H NMR (in CDCl3, d ppm): 6.62 (2H,
d, H1), 6.80 (2H, d, H2), 6.92 (1H, t, HX), 7.03 (2H, t, H4),
7.25 (2H, d, H3), 3.75 (3H, s, HY), 2.75 (3H, s, HR). M.p.
is 98–100 ꢁC.
2.2.11. Methyl-1-(4-bromophenylimino)-1-
(4-chlorophenylhydrazono)-acetate (8d)
Yield: 4.7g (62%). Anal. Calc. for C15H11N3O2ClBr: C,
47.3; H, 2.91; N, 11.0. Found: C, 47.5; H, 3.20; N,
10.9%. UV–Vis in acetonitrile: kmax (emax): 340 (8.81 ·
104), 305 (5.22 · 104), 246 (6.88 · 104). IR: m(N@N) 1484,
m(C@N) 1599, m(C@O) 1712. 1H NMR (in CDCl3, d
ppm): 6.51 (2H, d, H1), 7.02 (2H, d, H4), 7.21
(2H, d, H2), 7.39 (2H, d, H3), 3.93 (3H, s, HR). M.p. is
97–99 ꢁC.
2.2.6. 1-(4-Nitrophenylimino)-1-(phenylhydrazono)-
propan-2-one (6g)
Yield: 3.1 g (51%). Anal. Calc. for C15H14N4O3: C, 60.4;
H, 4.73; N, 18.8. Found: C, 60.1; H, 4.51; N, 18.5%. UV–
Vis in acetonitrile: kmax (emax): 338 (8.81 · 104), 297
(5.20 · 104), 231 (6.85 · 104). IR: m(N@N) 1505, m(C@N)
1
1602, m(C@O) 1667. H NMR (in CDCl3, d ppm): 6.34
2.3. General procedure for the syntheses
of trans-[Ru(bpy)(L)Cl2]
(2H, d, H2), 7.02 (1H, t, HX), 7.13 (2H, d, H3), 7.31
(2H, t, H4) 8.16 (2H, d, H1), 2.60 (3H, s, HR). M.p. is
174–176 ꢁC.
Ruthenium trichloride trihydrate (261 mg, 1.0 mmol)
and (1.0 mmol) of (L) were dissolved in 100 mL of absolute
ethanol. After refluxing for 1 h, 1.0 mmol of bpy was added
to the solution. The reaction was heated for an additional
2 h then an excess amount of LiCl (500 mg, 11.8 mmol) was
added. After allowing the reaction mixture to reflux for an
additional 1 h, the solvent was then removed by rotary
evaporator. The crude product was dissolved in dichloro-
methane, filtered and washed with water to remove the
unreacted ruthenium trichloride and lithium chloride.
The filtrate was reduced to 20 mL and purified by chroma-
tography (50 · 3 cm) column containing 250 g alumina
grade (III). The first pale yellow band, thought to be the
azomethine ligand (L) was eluted with hexane. Acetone
was used to elute the second dark-red band of the product.
2.2.7. Methyl-1-(phenylimino)-1-(phenylhydrazono)-
acetate (7a)
Yield: 3.2 g (59%). Anal. Calc. for C15H13N3O2: C, 67.4;
H, 4.90; N, 15.7. Found: C, 67.1; H, 5.21; N, 15.5%. UV–
Vis in acetonitrile: kmax (emax): 335 (8.85 · 104), 291
(5.23 · 104), 234 (6.91 · 104). IR: m(N@N) 1504, m(C@N)
1
1600, m(C@O) 1696. H NMR (in CDCl3, d ppm): 6.66
(2H, d, H1), 6.9 (2H, t, H2), 7.09 (2H, t, H4), 7.25 (4H,
m, H3, HX, HY), 3.9 (3H, s, HR). M.p. is 134–136 ꢁC.
2.2.8. Methyl-1-(4-chlorophenylimino)-1-
(phenylhydrazono)-acetate (7c)
Yield: 3.1 g (51%). Anal. Calc. for C15H12N3O2Cl: C,
59.7; H, 4.01; N, 13.9. Found: C, 59.5; H, 4.21; N,
13.5%. UV–Vis in acetonitrile: kmax (emax): 345 (8.79 ·
104), 294 (5.18 · 104), 240 (6.89 · 104). IR: m(N@N), 1489
m(C@N), 1593, m(C@O) 1705. 1H NMR (in CDCl3, d
ppm): 6.54 (2H, d, H1), 6.9 (1H, t, HX), 7.09 (2H, t,
H4), 7.18 (2H, d, H2), 7.27 (2H, d, H3), 3.9 (3H, s, HR).
M.p. is 87–89 ꢁC.
2.3.1. trans-[Ru(bpy)(6a)Cl2] Æ 0.5H2O (9)
Yield: 301 mg (52%). Anal. Calc. for RuC25H21N5O-
Cl2 Æ 0.5H2O: C, 51.0; H, 3.77; N, 11.9. Found: C, 51.0;
H, 3.65; N, 11.9.%. UV–Vis in DMF: kmax (emax): 509
(13.1 · 103). 1H NMR (in CDCl3, d ppm): 6.98 (1H, t,
H80), 7.03 (1H, t, H8), 7.10 (1H, d, H9), 7.51 (6H, m,
H2, H4, H70, HX), 7.74 (1H, t, HY), 7.79 (3H, m, H3,
H7), 7.83 (1H, d, H90), 7.93 (1H, d, H6), 7.97 (1H, d,
H60), 8.06 (2H, d, H1), 2.79 (3H, s, HR).
2.2.9. Methyl-1-(phenylimino)-1-
(4-chlorophenylhydrazono)-acetate (8a)
Yield: 3.6 g (60%). Anal. Calc. for C15H12N3O2Cl: C,
59.7; H, 4.01; N, 11.8. Found: C, 59.6; H, 4.20; N,
11.6%. UV–Vis in acetonitrile kmax (emax): 340 (8.7 · 104),
275 (5.17 · 104), 211 (6.89 · 104). IR: m(N@N), 1490
m(C@N), 1598, m(C@O) 1705. 1H NMR (in CDCl3, d
ppm): 6.65 (1H, t, HX), 6.9 (2H, d, H4), 7.0 (2H, d, H2),
7.2 (2H, t, H1), 7.3 (2H, d, H3), 3.8 (3H, s, HR). m.p. is
104–106 ꢁC.
2.3.2. trans-[Ru(bpy)(6b)Cl2] (10)
Yield. 231 mg (39%). Anal. Calc. for RuC26H23N5OCl2:
C, 52.6; H, 3.91; N, 11.8. Found: C, 51.3; H, 3.19; N,
1
11.4%. UV–Vis in DMF: kmax (emax): 513 (10.5 · 103). H
NMR (in CDCl3, d ppm): 7.01 (1H, t, H8), 7.05 (1H, t,
H80), 7.23 (1H, d, H9), 7.3 (2H, d, H1), 7.4 (2H, d, H2),
7.5 (2H, t, H4), 7.72 (1H, t, HX), 7.82 (3H, m, H9, H6,
H60), 7.93 (1H, d, H70), 7.97 (1H, d, H7), 8.06 (2H, d,
H3), 2.49 (3H, s, HY), 2.78 (3H, s, HR).
2.2.10. Methyl-1-(4-chlorophenylimino)-1-
(4-chlorophenylhydrazono)-acetate (8c)
Yield: 3.7 g (55%). Anal. Calc. for C15H11N3O2Cl2: C,
53.6; H, 3.30; N, 12.5. Found: C, 53.9; H, 3.50; N, 12.2%.
UV–Vis in acetonitrile: kmax (emax): 342 (8.89 · 104), 300
(5.29 · 104), 246 (6.94 · 104). IR: m(N@N) 1488, m(C@N)
2.3.3. trans-[Ru(bpy)(6d)Cl2] Æ CH2Cl2 (12)
Yield: 311 mg (47%). Anal. Calc. for RuC25H20N5OCl2-
Br Æ CH2Cl2: C, 42.0; H, 2.98; N, 9.42. Found: C, 42.4; H,
2.69; N, 9.25%. UV–Vis in DMF: kmax (emax): 508
(11.6 · 103). 1H NMR (in CDCl3, d ppm): 6.97 (1H, t,
H80), 7.15 (1H, t, H8), 7.22 (1H, d, H9), 7.38 (2H, d,
H1), 7.5 (2H, t, H4), 7.67 (2H, d, H2), 7.71 (1H, t, HX),
1
1588, m(C@O) 1706. H NMR (in CDCl3, d ppm): 6.55
(2H, d, H2), 7.03(2H, d, H4), 7.25 (2H, d, H1), 7.38 (2H,
d, H3), 3.87(3H, s, HR). M.p. is 84–86 ꢁC.

3678
M.Z. Al-Noaimi et al. / Polyhedron 26 (2007) 3675–3685
7.76 (1H, d, H9), 7.8 (1H, t, H70), 7.87 (1H, t, H7), 7.94
(1H, d, H60), 7.99 (1H, d, H6), 8.04 (2H, d, H3), 2.07
(3H, s, HR).
48.4; H, 3.62; N, 10.3%. UV–Vis in DMF: kmax (emax):
1
499 (12.6 · 103). H NMR (in CDCl3, d ppm): 7.07 (1H, t,
H8), 7.1 (1H, t, H80), 7.21 (1H, d, H9), 7.49 (7H, m, H1,
H4), 7.58 (4H, m, H2, HY, H90), 7.87 (2H, m, H7, H70),
8.0 (2H, m, H6, H60), 8.08 (2H, d, H3), 3.88 (3H, s, HR).
2.3.4. trans-[Ru(bpy)(6e)Cl2] Æ 0.25C7H8 (13)
Yield: 294 mg (45%). Anal. Calc. for RuC29H23-
N5OCl2.0.25C7H8: C, 56.6; H, 3.86; N, 10.7. Found: C,
56.4; H, 3.52; N, 10.4%. UV–Vis in DMF: kmax (emax):
2.3.10. trans-[Ru(bpy)(8c)Cl2] (19)
Yield: 332 mg (50%). Anal. Calc. for RuC25H19N5O2Cl4:
C, 45.2; H, 2.88; N, 10.5. Found: C, 46.4; H, 3.62; N, 10.3%.
UV–Vis in DMF: kmax (emax): 501 (12.4 · 103). 1H NMR (in
CDCl3, d ppm): 7.08 (1H, t, H8), 7.17 (1H, t, H80), 7.35 (1H,
d, H9), 7.46 (4H, m, H1, H4), 7.63 (2H, d, H2), 7.79 (1H, d,
H9), 7.88 (2H, m, H7, H70), 7.94 (1H, d, H60), 8.03 (1H, d,
H6), 8.05 (2H, d, H3), 3.88 (3H, s, HR).
1
508 (10.57 · 103). H NMR (in CDCl3, d ppm): 6.74 (1H,
t, H8), 6.98 (1H, d, H80), 7.04 (1H, d, H9), 7.43 (1H, t,
H7), 7.54 (3H, m, naphtyl protons), 7.68 (1H, t, HX), 7.7
(4H, m, H7, naphtyl protons), 7.86 (3H, m, H4, H90),
7.96 (1H, d, H6), 8.03 (1H, d, H60), 8.19 (2H, d, H3),
2.72 (3H, s, HR).
2.3.5. trans-[Ru(bpy)(6f)Cl2] (14)
Yield: 257 mg (42%). Anal. Calc. for RuC26H23N5O2Cl2:
C, 51.2; H, 3.80; N, 11.5. Found: C, 51.4; H, 3.95; N,
2.3.11. trans-[Ru(bpy)(8d)Cl2] (20)
Yield: 370 mg (52%). Anal. Calc. for RuC25H19-
N5O2Cl3Br: C, 42.4; H, 2.70; N, 9.88. Found: C, 42.6; H,
3.01; N, 9.65%. UV–Vis in DMF: kmax (emax): 502
(12.0 · 103). 1H NMR (in CDCl3, d ppm): 7.09 (1H, t,
H8), 7.16 (1H, t, H80), 7.36 (1H, d, H9), 7.48 (4H, m,
H1, H4), 7.60 (1H, d, H90), 7.82 (2H, d, H2), 7.9 (2H, m,
H7, H70), 8.02 (2H, m, H6, H60), 8.05 (2H, d, H3), 3.89
(3H, s, HR).
1
11.2%. UV–Vis in DMF: kmax (emax): 514 (10.5 · 103). H
NMR (in CDCl3, d ppm):, 6.97 (1H, t, H80), 7.00 (1H, t,
H8) 7.20 (1H, d, H9), 7.26 (2H, d, H1), 7.38 (2H, d, H2),
7.48 (2H, t, H4), 7.68 (1H, t, HX)7.65 (1H, d, H90), 7.75
(2H, m, H7, H70), 7.79 (1H, d, H60), 7.94 (1H, d, H6),
8.03 (2H, d, H3), 2.74 (3H, s, HY), 2.45 (3H, s, HR).
2.3.6. trans-[Ru(bpy)(6g)Cl2] Æ CH2Cl2 (15)
2.4. Instrumentation
Yield: 309 mg (42%). Anal. Calc. for RuC25H22N6O3-
Cl2 Æ CH2Cl2: C, 43.9; H, 3.40; N, 11.8%. Found: C, 43.5;
H, 3.29; N, 11.5%. UV–Vis in DMF: kmax (emax): 504
(10.6 · 103). 1H NMR (in CDCl3, d ppm): 7.02 (1H, t, H8),
7.14 (1H, t, H80), 7.24 (1H, d, H9), 7.5 (2H, t, H4),7.65
(2H, d, H1), 7.75 (1H, d, H9), 7.8 (1H, t, HX), 7.87 (1H, t,
H70), 7.94 (1H, t, H7), 8.01 (1H, d, H60), 8.04 (1H, d, H6),
8.14 (2H, d, H2), 8.32 (2H, d, H3) 2.83 (3H, s, HR).
Cyclic voltammetry studies were performed in 99.8%
anhydrous acetonitrile (Aldrich, HPLC grade) using a
BAS CV-27W voltammetric analyzer and plotted on a
BAS XY recorder. Three electrodes were utilized in this
system, two platinum-disk working and counter (auxiliary)
electrodes (BAS, 1.6-mm diameter), and a silver wire
pseudo-reference electrode with ferrocene (0.665 V versus
NHE) as an internal reference [14]. To control the temper-
ature, a Haake D8-G refrigerated bath and circulator was
used to maintain the cell temperature at 25.0 0.1 ꢁC. Tet-
rabutylammonium hexafluorophosphate (0.1 M) was twice
recrystallized and vacuum dried at 110 ꢁC, and used as the
supporting electrolyte. Spectroscopic data were obtained
using the following instruments Cary 5 spectrophotometer;
IR spectra (KBr disk, 4000–200 cm), FT-IR JASCO model
420; H NMR spectra (Bruker (AC) 300 MHz FT NMR
spectrometer). Spectroelectrochemistry for complex 18
was performed using an optically transparent thin layer
electrochemistry (OTTLE) cell [15]. The cell had interior
dimensions of roughly 1 · 2 cm with a path length of
0.2 cm and was fitted with an Ag/AgCl reference electrode.
ITO (indium-tin oxide) coated glass served as working and
counters electrodes and was purchased from Delta-
Technologies. Spectra were obtained on Cary 5 spectro-
photometer versus appropriate background of solvent
and electrolyte at scan rate of 1800 nm/min. The potential
was controlled by the same BAS CV-27 which was used for
cyclic voltammetry. At any given potential, the system was
allowed to come to equilibrium (i ꢀ 0 lA) before the
spectrum was taken.
2.3.7. trans-[Ru(bpy)(7a)Cl2] (16)
Yield: 258 mg (43%). Anal. Calc. for RuC25H21N5O2Cl2:
C, 50.4; H, 3.55; N, 11.8. Found: C, 49.1; H, 3.49; N,
1
11.4%. UV–Vis in DMF: kmax (emax): 500 (10.6 · 103). H
NMR (in CDCl3, d ppm): 6.9 (1H, t, H8), 7.02 (1H, t,
H80), 7.2 (1H, d, H9), 7.42 (2H, t, H4), 7.54 (4H, m, HY,
H2, H90), 7.68 (1H, t, HX), 7.74 (1H, t, H70), 7.80 (1H, t,
H7), 7.88 (1H, d, H60), 7.94 (1H, d, H6), 8.00 (2H, d,
H3), 3.83 (3H, HR).
2.3.8. trans-[Ru(bpy)(7c)Cl2] (17)
Yield: 321 mg (51%). Anal. Calc. for RuC25H20N5O2Cl3:
C, 47.7; H, 3.20; N, 11.1. Found: C, 47.6; H, 3.39; N, 11.3%.
UV–Vis in DMF: kmax (emax): 500 (10.5 · 103). 1H NMR (in
CDCl3, d ppm): 6.92 (1H, t, H8), 7.12 (1H, t, H80), 7.29 (1H,
d, H9), 7.46 (7H, m, H1, H2, H4, HX), 7.56 (1H,d, H90),
7.66 (1H, t, H70), 7.73 (1H, t, H7), 7.86 (1H, d, H60), 7.89
(1H, d, H6), 7.98 (2H, d, H3), 3.85 (3H, s, HR).
2.3.9. trans-[Ru(bpy)(8a)Cl2] Æ 0.5acetone (18)
Yield: 323 mg (47%). Anal. Calc. for RuC25H20N5O2-
Cl3 Æ 0. 5C3H6O: C, 48.3; H, 3.52; N, 10.6. Found: C,

M.Z. Al-Noaimi et al. / Polyhedron 26 (2007) 3675–3685
3679
Table 1
package [16]. The Becke three parameters hybrid exchange
[17] and the Lee–Yang–Parr correlation functionals [18]
(B3LYP) were used. Time-dependent density functional
theory (TD-DFT) [19,20] was used for the calculation of
the lowest 10 singlet to singlet excitation energy.
Crystallographic data and structure refinement parameters for 13 and 18
Compound
13
18
Formula
C61.5H50Cl4N10O2Ru2 C26.5H23Cl3N5O2.5Ru
1305.06
triclinic
Molecular weight
Crystal system
Space group
658.92
monoclinic
P21/c
12.2391(11)
11.3273(10)
21.0382(19)
90
100.5510(10)
90
2867.3(4)
4
296(2)
0.71073
1.526
ꢀ
P1
2.6. Crystallography
˚
a (A)
14.6230(2)
15.6896(2)
15.6905(3)
89.3012(13)
66.9141(12)
66.5962(12)
2994.87(9)
2
298(2)
0.71073
1.447
0.734
˚
b (A)
˚
Crystallographic data of crystals of compound 13 or 18
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
were collected at 298(2) K using
a Bruker/Siemens
SMART APEX instrument (Mo Ka radiation, k =
˚
0.71073 A). Data were measured using phi and omega
3
˚
V (A )
scans of 0.5ꢁ per frame for 20 {10} s, and a full sphere of
Z
data were collected. A total of 1282 {1464} frames were
T (K)
˚
˚
k (A)
collected with a final resolution of 0.83 A. Cell parameters
qcalc (Mg/m3)
were retrieved using APEX2 [21] software and refined using
SAINTPlus on all observed reflections [22]. Data reduction
and correction for Lp and decay were performed using
the SAINTPlus software. Absorption corrections were
applied using SADABS [23]. The structure was solved by
direct methods and refined by least squares method on F2
using the SHELXTL program package [24]. The structure
l (mmꢁ1
F(000)
)
0.861
1328
1322
Crystal size (mm)
h Range (ꢁ)
Index ranges
0.36 · 0.19 · 0.12
1.43–25.25
ꢁ17 6 h 6 17,
ꢁ18 6 k 6 18,
ꢁ18 6 l 6 18
0.30 · 0.20 · 0.06
1.69–25.25
ꢁ14 6 h 6 14,
ꢁ13 6 k 6 13,
ꢁ25 6 l 6 25
45319
Number of reflections 34887
collected
ꢀ
was solved in the space group P1 for 13 and {P21/c} for
18 by analysis of systematic absences. In 13 the disordered
toluene solvent (50% occupied, modeled as a rigid group
from coordinates from the Cambridge Database) was held
isotropic. In complex 17, the disordered methyl ester group
(50%) and the acetone solvent (50%) were held isotropic.
All other non-hydrogen atoms were refined anisotropically.
No decomposition was observed during data collection for
both structures. Details of the data collection and refine-
ment of 13 and 18 are given in Table 1. Selected bond
lengths and angles for 13 and 18 are given in Table 2.
Number of
independent
10836 (0.0523)
5188 (0.0373)
5188/4/333
reflections (Rint
Data/restraints/
parameters
Goodness-of-fit
Ra1 [I > 2r(I)]
wRb2 [I > 2r(I)]
)
10836/0/694
1.038
0.0589
0.1655
1.401 and ꢁ0.586
1.073
0.0470
0.1380
1.522 and ꢁ1.312
Largest difference in
peak and hole
ꢁ3
˚
(e A
)
P
P
a
R1 = iFoj ꢁ jFci/ jFoj.
P
2.5. Computational methods
P
2
2
1=2
b
wR2 ¼ f ½wðF 2o ꢁ F c2Þ ꢂ= ½wðF 2oÞ ꢂg
.
3. Results and discussion
3.1. Synthesis
The crystallographic geometry of Ru(II) complex 18 was
used as a representative example for our calculations. All
calculations were carried out with GAUSSIAN-03 program
The new ligands (6–8) used in this study were pre-
pared in a two-step sequence starting with commercially
Table 2
˚
Selected bond lengths (A) and angles (ꢁ) for complexes 13 and 18
13
18
Conformer 1
Conformer 2
Ru1–Cl1
Ru1–Cl2
Ru1–N19
Ru1–N25
N19–N20
N20–C21
C21–N25
2.3725(16)
2.3725(14)
1.945(4)
2.154(4)
1.299(6)
1.359(7)
1.299(7)
Ru2–Cl4
Ru2–Cl3
Ru2–N54
Ru2–N60
N54–N55
N55–C56
C56–N60
2.3879(16)
2.3746(17)
1.946(4)
2.014(5)
1.323(6)
1.352(7)
1.306(7)
Ru1–Cl2
Ru1–Cl1
Ru1–N24
N23–N24
C20–N23
N19–C20
Ru1–N19
2.3539(12)
2.3904(12)
1.956(3)
1.321(5)
1.359(6)
1.438(6)
2.024(4)
N19–Ru1–N25
N19–Ru1–N1
N25–Ru1–N1
N19–Ru1–N12
N25–Ru1–N12
N1–Ru1–N12
75.55(19)
103.67(18)
175.04(18)
175.78(17)
104.84(19)
76.32(18)
N54–Ru2–N60
N54–Ru2–N47
N60–Ru2–N47
N54–Ru2–N36
N60–Ru2–N36
N47–Ru2–N36
76.23(18)
103.19(18)
169.32(19)
173.47(19)
105.38(19)
76.43(19)
N1–Ru1–N12
N24–Ru1–N19
N24–Ru1–N1
N19–Ru1–N1
N24–Ru1–N12
N19–Ru1–N12
76.64(14)
75.98(15)
103.08(14)
173.13(14)
178.46(14)
104.47(14)

3680
M.Z. Al-Noaimi et al. / Polyhedron 26 (2007) 3675–3685
Table 3
Cyclic voltammetry and electronic spectroscopy data of trans-[Ru(bpy)(L)Cl2] complexes
Complex number
Cyclic voltammetric data E1/2 (V)a
Electronic spectroscopic datab
c
f
ðEM1=2
Þ
Calculated EL(L)d
Cathodic peak maximum for Le
bpy(0/ꢁ1)
kmax (nm) (EMLCT)
L(0/ꢁ1)
L(ꢁ1/ꢁ2)
9
10
11
12
13
14
15
16
17
18
19
20
a
1.1800
1.1600
1.1700
1.2100
1.2000
1.1400
1.2300
1.2500
1.2300
1.2500
1.2600
1.2500
0.42
0.40
0.41
0.45
0.44
0.38
0.47
0.49
0.47
0.49
0.50
0.49
ꢁ0.48
ꢁ0.51
ꢁ0.49
ꢁ0.45
ꢁ0.44
ꢁ0.41
ꢁ0.44
ꢁ0.42
ꢁ0.39
ꢁ0.40
ꢁ0.41
ꢁ0.40
ꢁ0.88
ꢁ0.88
ꢁ0.85
ꢁ0.83
ꢁ0.84
ꢁ0.76
ꢁ0.80
ꢁ0.75
ꢁ0.76
ꢁ0.75
ꢁ0.77
ꢁ0.76
ꢁ1.71
ꢁ1.70
ꢁ1.60
ꢁ1.55
ꢁ1.60
ꢁ1.53
ꢁ1.60
ꢁ1.52
ꢁ1.52
ꢁ1.53
ꢁ1.58
ꢁ1.55
(510) 56.06
(513) 55.73
(511) 55.95
(508) 56.28
(506) 56.50
(514) 55.62
(504) 56.73
(500) 57.18
(500) 57.18
(499) 57.30
(499) 57.30
(500) 57.18
Solvent MeCN, supporting electrolyte Bu4NPF6 (0.1 M), scan rate 0.1 V/s, Pt-disk working electrode, Pt-wire auxiliary electrode, reference electrode
Ag at 25 ꢁC.
b
c
Solvent MeCN.
EM1=2 ¼ ðEpa þ EpcÞ=2.
h
i
X
ERuðIII=IIÞ ¼ 0:97
EL þ 0:4 in V vs: NHE
ð1Þ
d
e
f
The L(0/ꢁ1) and L(ꢁ1/ꢁ2) are irreversible. The cathodic peak maximum are reported here.
EMLCT = [28591/kmax (nm)] kcal molꢁ1
.
available aniline derivatives (1) and b-diketone or b-dike-
toester (2) as shown in Scheme 1. Reaction of the hyd-
3.2. Crystal structures
razonyl chloride
3
with the appropriate aniline
Crystallographic data for complexes 13 and 18 are
compiled in Table 1 and ORTEP figures of 13 and 18
are shown in Figs. 1 and 2, respectively. In addition,
selected inner coordination sphere bond parameters of
13 and 18 are listed in Tables 2. Complex 13 crystallizes
in two conformations with some minor differences in
bond lengths and angles (see Table 2). The presence of
two conformers is suggested to be due the crystallization
of toluene in the lattice and to crystal packing forces.
The Ru–N [N(azo): N(19) and N(54)] bond distances
derivatives (4), followed by air oxidation for the interme-
diate (5) in refluxing ethanol gave the azomethine ligands
(6–8) (Scheme 1). The structures of these ligands were
conformed by 1H NMR, mass spectra and elemental
analysis.
The synthesis of trans-[Ru(bpy)LCl2] was achieved by
the stepwise equimolar addition of ruthenium trichloride
hydrate, azoimine ligands, bpy and then an excess of lith-
ium chloride (Scheme 2).
These neutral complexes are air stable, soluble in several
organic solvents and were recrystallized easily as dark red
platelets from slowly evaporating solutions of a mixture
of dichloromethane/toluene or acetone. The structures of
˚
(average, 1.946 A) are shorter than the Ru–N
˚
[N(methine): N(25) and N(60)] (average, 2.017 A) bond
˚
distance by 0.07 A and shorter than those of Ru–N(azo)
˚
(1.971, 2.014 A) in other azo-pyridines complexes
1
the diamagnetic complexes were confirmed by H NMR
[25,26]. This shortening may be due to greater p-back
donation from Ru-d orbital to the empty p* orbital of
the azoimine ligand, and this comes as an indication that
the M–L p interaction is localized in the M–azo fragment
[9,27]. For complex (18), Ru–N(azo) bond distance is
spectra and crystallography. The chemical shifts (see Sec-
tion 2) were assigned on the basis of the literature values
and spin–spin interactions. Electron donating or withdraw-
ing substituents on L had a respective shielding or deshiel-
ding effect on chemical shifts.
The IR spectra of the ligands (see Section 2) exhibit a
sharp band at 1660–1712 cmꢁ1 corresponds to C@O
stretching frequency of the acetyl or the ester group. The
formation of the azoimine ligand was confirmed by the
intense bands at 1580–1600 and 1470–1490 cmꢁ1 which
correspond to C@N and N@N stretching bands, respec-
tively. In all complexes, m(N@N) is red-shifted by 25–
30 cmꢁ1 compared to that of the ligands, which is a good
indication of N-coordination [25].
˚
1.959 A and is shorter than Ru–N(methine) bond distance
˚
of 2.024 A with both slightly longer than the average
˚
bond distance of Ru–N(azo) (1.946 A) and Ru–
˚
N(methine) (2.017 A) of complex (13). This is probably
due to the electron withdrawing ester group which is
attached to azomethine moiety in complex (18). As shown
in Figs. 1 and 2, the Ru(II) ion of complex 13 and 18
occupies a distorted octahedral coordination sphere in
which the chloride ligands are trans to each other and
the azomethine and bipyridine nitrogen donor atoms are

M.Z. Al-Noaimi et al. / Polyhedron 26 (2007) 3675–3685
3681
O
H
X
R
N
Cl
R
X
NH2
+
N
Cl
(1)
(3)
(2)
NH2
R
N
R
N
N
Et3N
N
O2
3 +
X
N
X
Y
N
Y
Y
(5)
L (6,7,8)
(4)
L
Y
X
R
COCH3
H
a
b
c
H
6
CH3
Cl
Br
d
C10H7
OMe
NO2
e
f
g
a c
7
CO2CH3
CO2CH3
H
8
a
c d
Cl
Scheme 1.
9
9'
8'
7'
6'
Cl
8
RuCl
2-bpy
3-LiCl
1-
3
7
6
N
N
N
Y
Cl
Ru
X
N
R
N
N
Y
N
X
2
1
4
3
N
4
3
2
1
R
6-8
9-20
Scheme 2.
equatorially coordinated. In the equatorial plane, the five-
membered rings described by the coordination of an
azomethine ligand and of bpy to ruthenium have approx-
imately equivalent coordination bite-angles (Table 2) and
deviate considerably from ideal octahedral geometry.
For complexes 13 and 18, the Ru–N(azo) and Ru–
N(methine) distances are shorter than corresponding
lengths in similar reported ruthenium azo-imine complexes
(tcc-Ru(papm)2Cl2 [26] and tcc-Ru(aai-Bz)2Cl2 [25]). This
is due to the greater p-back bonding of d(Ru)–p*(azo)
and this suggests that the new azomethine bidentate
ligands are stronger p-acceptor ligands compared to
reported azo-imine ligands [25,26]. The strong p-acidity
for these new bidentate ligands are supported by the
large positive shift of the Ru(III/II) couple (Table 3) as
discussed below.
Fig. 1. Thermal ellipsoid drawing (30%) of complex 13 showing the two
independent conformations of the Ru complex and the solvent toluene
molecule (0.5 occupancy). Hydrogen atoms have been omitted for clarity.

3682
M.Z. Al-Noaimi et al. / Polyhedron 26 (2007) 3675–3685
ð1Þ
The wave at the most negative potential scanned (less than
ꢁ1.60 versus NHE) is assigned to the reduction of bpy. The
difference in the ligand reduction potentials can be ex-
plained by the greater stability of azo group orbitals com-
pared to that of bpy (see theoretical calculations below).
Both Ru(III/II) and the reduction potential for the auxil-
iary ligand were shifted to more positive potentials upon
replacing the acetyl group attached to azomethine moieties
with the more withdrawing ester group.
Lever [29] has developed an electrochemical parameter-
ization method to calculate ruthenium(III/II) couples of
complexes with octahedral geometry as shown in the fol-
lowing equation:
h
i
X
Fig. 2. Thermal ellipsoid plot (30%) of complex 18. Hydrogen atoms
omitted for clarity. The ester group is shown in only one of the disordered
conformations (acetone 0.5 occupancy).
ERuðIII=IIÞ ¼ 0:97
EL þ 0:04 in V versus NHE
ð2Þ
P
where EL is the sum of electrochemical parameters for
each ligand in the complex. The parameter EL is a measure
of the stabilizing effect a ligand has on the Ru(II) state and
so the greater the magnitude of positive EL the more posi-
tive the Ru(III/II) couple. For trans-[Ru(bpy)LCl2], the EL
of L was calculated and listed in Table 3. It is observed that
the magnitude of EL increases as the number of electron
withdrawing substituents attached to the azoimine moiety
increases. These parameters are approximately the same
as those reported for phenylazopyridine (EL = 0.40) and
tolylazopyridine (EL = 0.41) but significantly higher than
that for the weaker p-acid bpy (EL = 0.259) or for the p-do-
nor chloride (EL = ꢁ0.24) [29].
3.3. Electrochemistry
The electrochemical behavior of the complexes in aceto-
nitrile was examined by cyclic voltammetry. The results are
summarized in Table 3. As a representative example, the
cyclic voltammogram for complex 18 is shown in Fig. 3.
At positive potentials, the single wave between 1.0 and
1.30 V is assigned to a reversible Ru(III/II) couple. These
couples were determined from the average of the anodic
and cathodic peak potentials (E1/2), where E1/2
=
(Epa + Epc)/2 at scan rate of 100 mV. This couple occurs
at more positive potentials compared to many dihalide
ruthenium(II) complexes [27]. At negative potentials, three
one-electron ligand reduction waves were observed
between ꢁ0.4 and ꢁ2.0 V. The two irreversible waves
between ꢁ0.56 and ꢁ0.88 V versus NHE (cathodic wave
peak maxima) are assigned to the two electron reduction
of the azo group analogous to the azopyridine systems.
The LUMO of L can accommodate up to two electrons
and the reduction may be represented by the following
equation [26]:
3.4. Electronic structure
Theoretical calculations were performed on 18 using
GAUSSIAN-03 program package [16]. Relative percentages
of atomic contributions to the lowest unoccupied and
highest occupied molecular orbitals have been placed in
Table 4. Moreover, the isodensity plots for the HOMOs
and LUMOs orbitals of 18 are shown in Fig. 4. Results
indicate that the LUMO is constructed mainly from the
Table 4
Relative percentages of atomic contributions to the lowest unoccupied and
highest occupied molecular orbitals (LUMO and HOMO) of 18
MO
Energy (eV) Ru bpy Azo-imine ring Cl Othersa
LUMO ꢁ 4 ꢁ6.50
LUMO ꢁ 3 ꢁ6.47
LUMO ꢁ 2 ꢁ6.00
LUMO ꢁ 1 ꢁ5.43
5
3
4
5
2
1
2
20
80
94
96
3
9
6
6
17
5
41
7
0
1
24
57 26
84
4
45
31
6
2
15
3
0
12
2
73
33
61
21
10
2
HOMO
LUMO
ꢁ4.93
ꢁ2.85
LUMO + 1 ꢁ2.12
LUMO + 2 ꢁ1.39
LUMO + 3 ꢁ1.18
LUMO + 4 ꢁ0.66
a
3
0
3
2
14
0
2
5
54
Fig. 3. Cyclic voltammogram for 18 in acetonitrile 0.1 M TBAH at 25 ꢁC,
data reported in V vs. NHE with scan rate of 0.1 V/s.
Group attached to azo-imine ring (Cl-Ph, Ph, CO2CH3).

M.Z. Al-Noaimi et al. / Polyhedron 26 (2007) 3675–3685
3683
p* orbital of azoimine together with a small contribution
from p* orbital of bpy. In addition, the LUMO has 21%
metal d-orbital character which suggests significant back
donation [30]. The LUMO + 1, LUMO + 2 and
LUMO + 3 are mainly bpy in character. For the HOMOs,
the three highest energy orbitals (HOMO, HOMO ꢁ 1 and
HOMO ꢁ 2) have a significant contributions of Ru(dp)
orbitals.
The lowest 10 singlet-to-singlet spin-allowed excitation
states were taken into account for the calculation of the
electronic absorption spectrum by using TD-DFT method.
The calculated energy of excitation states and transition
oscillator strength (f) (only f > 0.005 is listed) of 18 are
shown in Table 5. The absorption spectrum of 18 was sim-
ulated using GAUSSSUM software [31] based on the obtained
TD-DFT results. Each excited state was interpolated by a
Gaussian convolution with the full-width at half-maximum
(FWHM) of 3000 cmꢁ1. In good agreement, both simu-
lated absorption spectrum of 18, Fig. 5, and experimental
spectrum, in Fig. 7, show two visible region absorptions.
The computed spectrum is for gas phase complexes and
shows a moderate red-shift to lower energy transitions
because of the absence of solvation effects. For the lower
energy band (kmax = 499 (experimental) and 563 (calcu-
lated) nm), the initial states are HOMO ꢁ 1 and
HOMO ꢁ 2 orbitals (Table 5), which have sizable contri-
butions of Ru(dp) orbitals. The LUMO possesses a signif-
icant contribution from the p* orbital of azomethine and
because of this, the lower energy band is assigned to
MLCT (Ru(dp) ! p* azomethine). The shoulder centered
at 390 (experimental) and 447 (calculated) nm is assigned
to overlapping MLCT and ligand–ligand transitions. The
MLCT transition results from Ru(dp) ! bpy(p*) (or
HOMO ! LUMO + 2), while the ligand–ligand transition
result mainly from the HOMO ꢁ 3 and HOMO ꢁ 4 which
is mainly chloride in character to the azomethine p* orbital
(LUMO).
3.5. Spectroelectrochemistry
The MLCT bands of the complexes exhibit relatively lit-
tle solvatochromism with a shift to higher values in the
hydroxylic solvents compared with nonhydroxylic solvents.
This can be attributed to significant metal–ligand orbital
mixing [30] while the small shift for MLCT to higher
energy can be attributed to more solvation of the dipolar
ground state relative to less dipolar excited state by the
polar solvent [32].
Spectroelectrochemistry of 11 and 18 in DMF solution
(Figs. 6 and 7, respectively) was performed in order to
obtain the visible electronic absorption spectra of the
Ru(III) oxidation state. Reversibility was indicated by the
maintenance of the isosbestic points in the absorption spec-
trum for both forward (oxidation) and reverse (reduction)
Fig. 4. Isodensity plots (isodensity contour = 0.04) of the HOMO and LUMO orbitals of complex 18.
Table 5
Computed excitation energies (eV), electronic transition configurations and oscillator strengths (f) for the optical transitions in the visible region of 18
(transitions with f > 0.005 are listed)
eV (nm)
f
Composition
1.911 (649)
2.203 (563)
2.577 (481)
2.776 (447)
2.797 (443)
2.804 (442)
0.0051
0.0496
0.0068
0.0083
0.0074
0.0115
91% HOMO ! LUMO + 1, 6% HOMO ! LUMO
38% HOMO ꢁ 1 ! LUMO, 33% HOMO ꢁ 2 ! LUMO, 10% HOMO ꢁ 1 ! LUMO + 1
75% HOMO ꢁ 1 ! LUMO + 1, 8% HOMO ꢁ 2 ! LUMO
65% HOMO ꢁ 3 ! LUMO, 14% HOMO ! LUMO + 2, 10% HOMO ꢁ 4 ! LUMO
77% HOMO ! LUMO + 2, 7% HOMO ꢁ 4 ! L, 6% HOMO ꢁ 3 ! LUMO
59% HOMO ꢁ 4 ! LUMO, 19% HOMO ꢁ 3 ! LUMO, 11% others

3684
M.Z. Al-Noaimi et al. / Polyhedron 26 (2007) 3675–3685
57.5
57.0
56.5
56.0
55.5
55.0
0.36 0.38 0.40 0.42 0.44 0.46 0.48 0.50 0.52
EL (V)
Fig. 5. Simulated absorption spectrum (black line) for 18 based on TD-
DFT calculations, compared to excitation energies and oscillator
strengths.
Fig. 8. The correlation between the ligand electrochemical parameter, EL,
and MLCT energy, EMLCT
.
processes and the greater than 95% recovery of the Ru(II)
complex spectrum. As shown in Figs. 6 and 7, oxidation of
these complexes results in a decrease in the intensity of the
lower energy band that we have assigned to a MLCT tran-
sition. The concomitant growth of absorbance at
k > 550 nm is assigned to chloride-to-Ru(III) (pp ! dp*)
ligand-to-metal charge transfer LMCT transitions. The
higher energy band at 368 nm (e = 9810 Mꢁ1 cmꢁ1) in the
absorption spectrum of 11+ (Fig. 6), is only a shoulder in
the spectrum of 18+ (Fig. 7), suggesting that the acetyl
group of the ligand 6c participates in the state descriptions
responsible for this absorption.
The MLCT band of a Ru(II) mononuclear complex and
its electrochemistry provide information about the relative
energies of the complex’s metal and ligand-based orbitals.
The change in dp-orbital energy can be measured experi-
mentally by measuring the potential of the Ru(III/II) cou-
ple. For the complexes of this study, acetate azomethine
ligands shift the Ru(III/II) couple approximately 100 mV
positive relative to their propanone analogues. This can
be explained by the enhanced p-acceptor properties of ace-
tate azomethine ligands which originate from the inductive
effect of the carboxy methyl ester group. The relationship
between MLCT band energy and the ligand electrochemi-
cal parameter, EL(L), for the azomethine ligands (Table
3) is revealed by Fig. 8. The linear correlation that is
observed strongly suggests that substitution of the azome-
thine ligand provides researchers with a useful tool for the
fabrication of novel ruthenium(II)–polypyridine complexes
that have specific electronic and structural properties.
20000
15000
10000
5000
0
400
500
600
700
800
wavelength (nm)
Fig. 6. Spectroelectrochemical oxidation of complex 11 in DMF, 0.1 M
TBAH, 25 ꢁC.
4. Conclusions
20000
15000
10000
5000
0
Ten ruthenium(II) complexes, trans-[Ru(bpy)LCl2],
where bpy is 2,20-bipyridine and L is a substituted azome-
thine ligand, were synthesized and characterized by crystal-
lography and spectroscopic and electrochemical methods.
Crystallographic studies of 13 and 18 show that bidentate
azomethine ligands are strong p-acceptors that coordinate
via imine and azo nitrogens. Cyclic voltammetric studies of
trans-[Ru(bpy)LCl2] complexes show reversible ruthenium-
based oxidation–reduction processes and irreversible
ligand-based redox reactions. The electronic effect of the
azomethine substituents was seen in the ruthenium and
ligand couples, which shifted to more positive values when
stronger electron-withdrawing substituents were intro-
duced. The high degree of metal-to-azomethine back dona-
400
500
600
700
800
wavelength (nm)
Fig. 7. Spectroelectrochemical oxidation of complex 18 in DMF, 0.1 M
TBAH, 25 ꢁC.

M.Z. Al-Noaimi et al. / Polyhedron 26 (2007) 3675–3685
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5. Supplementary material
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Acknowledgements
Mousa Al-Noaimi thanks the Hashemite University
(Jordan) for research support. R.J.C. is grateful to Natural
Sciences and Engineering Research Council of Canada for
financial support. The Bruker (Siemens) SMART APEX
diffraction facility was established at the University of Ida-
ho with the assistance of the NSF-EPSCoR program and
the M.J. Murdock Charitable Trust, Vancouver, WA,
USA. This work is supported in part by the Deanship of
Academic Research, The University of Jordan.
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