Cis-trans Isomerism in mixed-ligand ruthenium(II) complexes containing bis(phosphine) and azoimine ligands

Article abstract of DOI:10.1016/j.poly.2012.04.044

This study presents the syntheses and characterization of mixed-ligand azoimine-diphosphine ruthenium complexes of the general formula [RuCl ₂(Azo)(P-P)] (C1-C5) {Azo = C₆H₅NNC(COCH ₃)NC₆H₅; P-P = 1,2-bis(dimethylphosphino)ethane (dmpe) (C1), cis-1,2-bis(diphenylphosphino)ethylene (depe) (C2), 1.1′-bis(diphenylphosphino)methane (dppm) (C3), 1,2-bis(diphenylphosphino) benzene (dbpe) (C4), 1,2-bis(di(pentaflurophenyl)phosphino)ethane (F-dppe) (C5)}. These complexes were synthesized via the reaction of RuCl₃, azoimine and the diphosphine ligands in ethanol solutions. The X-ray structure of C2 reveals a cis-dichloro geometry, in spite of the bulky P-P ligand, and this is attributed to the presence of an intramolecular π-π interaction between benzene rings (on the Azo and P-P ligands) [centroid-centroid distances = 3.596 and 3.654 A?; dihedral angle between ring planes = 13.2(3) and 15.4(4)° for the two independent molecules]. In contrast, the X-ray structure of C1, with a less bulky dmpe ligand in which no Ph rings are available for π-π bonding, revealed a trans-dichloro geometry. In addition to the X-ray structures of C1 and C2, this work presents and discusses the spectroscopic (IR, UV-Vis, ¹H NMR and ³¹P NMR) and electrochemical (cyclic voltammetry) behavior of C1-C5.

Full text of DOI:10.1016/j.poly.2012.04.044

Polyhedron 42 (2012) 66–73
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
cis–trans Isomerism in mixed-ligand ruthenium(II) complexes containing
bis(phosphine) and azoimine ligands
a,
Mousa Al-Noaimi a, , Mahmoud Sunjuk , Mohammad El-khateeb b,1, Salim F. Haddad c, Alaa Haniyeh a,
⇑
⇑
Murad AlDamen c
a Department of Chemistry, The Hashemite University, Zarqa, Jordan
b Faculty of Science and Arts at Alkamil, King Abdulaziz University, Alkamil 21931, Saudi Arabia
c Chemistry Department, Faculty of Science, University of Jordan, Amman, Jordan
a r t i c l e i n f o
a b s t r a c t
Article history:
This study presents the syntheses and characterization of mixed-ligand azoimine-diphosphine ruthe-
nium complexes of the general formula [RuCl2(Azo)(P–P)] (C1–C5) {Azo = C6H5N@NC(COCH3)@NC6H5;
P–P = 1,2-bis(dimethylphosphino)ethane (dmpe) (C1), cis-1,2-bis(diphenylphosphino)ethylene (depe) (C2),
1.10-bis(diphenylphosphino)methane (dppm) (C3), 1,2-bis(diphenylphosphino)benzene (dbpe) (C4),1,
2-bis(di(pentaflurophenyl)phosphino)ethane (F-dppe) (C5)}. These complexes were synthesized via the
reaction of RuCl3, azoimine and the diphosphine ligands in ethanol solutions. The X-ray structure of C2
reveals a cis-dichloro geometry, in spite of the bulky P–P ligand, and this is attributed to the presence
of an intramolecular p–p interaction between benzene rings (on the Azo and P–P ligands) [centroid–cen-
troid distances = 3.596 and 3.654 Å; dihedral angle between ring planes = 13.2(3) and 15.4(4)° for the two
independent molecules]. In contrast, the X-ray structure of C1, with a less bulky dmpe ligand in which no
Received 10 March 2012
Accepted 30 April 2012
Available online 17 May 2012
Keywords:
Ruthenium
Complexes
Azomethine
Diphosphine ligands
X-ray structures
Electrochemistry
Ph rings are available for p–p bonding, revealed a trans-dichloro geometry. In addition to the X-ray struc-
tures of C1 and C2, this work presents and discusses the spectroscopic (IR, UV–Vis, 1H NMR and 31P NMR)
and electrochemical (cyclic voltammetry) behavior of C1–C5.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
aromatic ring (Ar) on the electronic properties of the ruthenium
center. The tuning of the electronic properties of the ruthenium
Metal complexes containing phosphine ligands have important
industrial applications [1]. The ability of bis(dialkylphosphi-
no)alkane ligands to donate or accept electrons from a metal center
can be easily tuned by changing the alkyl groups bonded to the
phosphorus atoms [1–6]. This change affects the activity, basicity,
selectivity and stability of catalytic processes via ligand electronic
and/or steric properties [1,5–8]. Tolman made a practical and use-
ful separation between the electronic and the steric effects of these
ligands and he was able to measure these effects [2].
Mixed-ligand diamine-bis(diphosphine) ruthenium(II) com-
plexes have received much attention in recent years due to their
remarkable performance in selective [9–11] and asymmetric [12–
14] hydrogenation of unsaturated carbonyl compounds. Previ-
ously, we synthesized a family of complexes of the general type
cis-[RuII(dppe)LCl2] {L = C6H5N@NC(COCH3)@NAr, dppe = Ph2P
(CH2)2PPh2} [15,16] to study the effect of the substituents of the
center was monitored by the change in the energy of the MLCT
bands and the redox properties. Herein, we describe the synthesis
of mixed-ligand azoimine-bis(dialkylphosphine) ruthenium(II)
complexes of the general formula [RuCl2(Azo)(P–P)] (C1–C5)
{Azo = C6H5N@NC(COCH3)@NC6H5, P–P = 1,2-bis(dimethylphosph-
ino)ethane (dmpe) (C1), cis-1,2-bis(diphenylphosphino)ethylene
(depe) (C2), 1.10-bis(diphenylphosphino)methane (dppm) (C3),
1,2-bis(diphenylphosphino)benzene (dbpe) (C4), 1,2-bis(di(penta-
flurophenyl)phosphino)ethane (F-dppe) (C5)}. In addition, this
work presents and discusses the spectroscopic (IR, UV–Vis, 1H
NMR and 31P NMR) and electrochemical behavior of these com-
plexes. The X-ray structures of complexes C1 and C2 are presented.
2. Experimental
2.1. Materials
The reagents, ruthenium trichloride hydrate, 1,2-bis(dim-
ethylphosphino)ethane (dmpe), cis-1,2-bis(diphenylphosphino)
ethylene (depe), 1.10-bis(diphenylphosphino)methane (dppm),
1,2-bis(diphenylphosphino)benzene (dbpe), 1,2-bis(di(pentaflur-
ophenyl)phosphino)ethane (F-dppe) and tetrabutylammonium
⇑
Corresponding authors. Tel.: +962 5 390 3333x4315; fax: +962 5 390 3349.
E-mail addresses: manoaimi@hu.edu.jo (M. Al-Noaimi), mahmoud.sunjuk@hu.
edu.jo (M. Sunjuk).
1
Permanent address: Chemistry Department, Faculty of Science and Arts, Jordan
University of Science and Technology, Irbid 22110, Jordan.
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.04.044

M. Al-Noaimi et al. / Polyhedron 42 (2012) 66–73
67
hexafluorophosphate (TBAH), and the solvents (reagent grade)
2.7. cis-[RuCl2(Azo)(F-dppe)] (C5)
were purchased from Aldrich and were used as received. The syn-
thesis and the physical characterization of the azoimine ligand
(Azo) have been described previously [17].
Yield 235 mg, 20%. Anal. Calc. for RuCl2C41H13N3P2F20O: C,
41.82; H, 1.11; N, 3.57. Found: C, 41.64; H, 1.07; N, 3.83%. UV–
Vis in acetonitrile: kmax/nm (
e
max/Mꢁ1 cmꢁ1): 340 (8.78 ꢀ 103),
2.2. Preparation of [RuCl2(Azo)(P–P)] (C1–C5); general procedure
396 (8.87 ꢀ 103), 513 (6.20 ꢀ 103). IR/cmꢁ1
: m(N@N) 1478,
m
(C@N) 1612, m
(C@O) 1709. 31P NMR (CDCl3, d ppm): 95.41 (d),
Ruthenium trichloride trihydrate (0.26 g, 1.0 mmol) and azoim-
ine (Azo) (0.25 g, 1.0 mmol) were dissolved in 100 mL of absolute
ethanol under argon. After refluxing for 1 h, 1.0 mmol of the
bis(diphosphino) ligand (P–P) was added to the solution. The reac-
tion was heated for an additional 3 h, then the solvent was re-
moved by a rotary evaporator. The crude product was dissolved
in dichloromethane and purified by chromatography (50 ꢀ 3 cm)
on silica gel. The first pale yellow band of the azoimine ligand
(Azo) was eluted with dichloromethane. A mixture of acetone/
dichloromethane (1:1) was used to elute the second dark-red band
of the product. Complexes C1 and C2 were recrystallized easily as
dark red platelets from slowly evaporating solutions of
dichloromethane.
106.81 (d). 1H NMR (DMSO, d ppm): 2.26 (s, COCH3), 1.75 (m,
4H, F-dppe), 7.11 (d, 2H, H4), 7.21 (t, 2H, H5), 7.28 (t, 1H, H6),
7.35 (t, 2H, H2), 7.45 (t, 1H, H1), 7.54 (d, 2H, H3).
2.8. Instrumentation
Electrochemical measurements were performed in acetonitrile
(Aldrich, HPLC grade) using a Volta Lab model PGP201 with a plat-
inum disk working electrode (1.6-mm diameter), a platinum wire
counter electrode and a silver wire pseudo-reference electrode.
Ferrocene (0.665 V versus NHE) was used as an internal reference
[18]. The temperature was controlled (at 25.0 0.1 °C) by a Haake
D8-G refrigerator. Tetrabutylammonium hexafluorophosphate
(0.10 M) was twice recrystallized and vacuum dried at 120 °C,
and used as the supporting electrolyte. IR spectra were measured
on a FT-IR JASCO model 420 spectrophotometer. Nuclear magnetic
resonances (1H and 31P NMR) spectra were measured on a Bruker-
Avance 400 MHz spectrometer at 400 and 161.3 MHz, respectively.
All chemical shifts are reported in ppm downfield of TMS (1H) or
85% phosphoric acid (31P) and referenced using the chemical shifts
of residual solvent resonances. Elemental analyses were carried
out on a Euro vector E.A.3000 instrument using copper sample-
tubes.
2.3. Trans-[RuCl2(Azo)(dmpe)] (C1)
Yield 257 mg, 45%. Anal. Calc. for RuCl2C21H29N3P2O: C, 43.99;
H, 5.10; N, 7.33. Found: C, 44.12; H, 5.30; N, 7.36%. UV–Vis in ace-
tonitrile: kmax/nm
(
e
max/Mꢁ1 cmꢁ1): 326 (8.05 ꢀ 103), 375
(5.14 ꢀ 103) 547 (5.68 ꢀ 103). IR/cmꢁ1
: m(N@N) 1488, m(C@N)
1618, m
(C@O) 1702. 31P NMR (CDCl3, d ppm): 43.04 (d), 47.15 (d).
1H NMR (DMSO, d ppm): 7.14 (d, 2H, H3), 7.38 (m, 3H, H2, H1),
7.58 (m, 5H, H4, H5, H6), 2.56 (s, COCH3), 1.52 (m, 4H, dmpe),
0.90 (d, 6H, dmpe), 0.82 (d, 6H, dmpe).
2.9. X-ray crystallography
2.4. cis-[RuCl2(Azo)(depe)] (C2)
Suitable crystals of complexes C1 and C2 were selected and at-
tached to oil dipped fibers. The data were collected at 100(1) K for
C1 using a Gemini E Ultra/Oxford Diffractometer equipped with an
E O S CCD detector and a Cryojet XL low temperature device, and at
291(2) K for C2 using an Oxford Diffraction Xcaliburs. For C1 and
C2, CrysAlisPro software [19] was used for data collection and
reduction. Both structures were solved by direct methods, using
the program OLEX2 [20], followed by Fourier synthesis, and refined
on F2 with SHELXL-97 [21]. The hydrogen atoms were positioned
constrained and assigned isotropic thermal parameters of 1.2 times
the riding atoms. A summary of the crystallographic data and
structure refinement parameters for C1 and C2 is given in Table 1.
Yield 286 mg, 35%. Anal. Calc. for RuCl2C41H35N3P2O: C, 57.22;
H, 4.27; N, 4.94. Found: C, 57.30; H, 4.34; N, 5.14%. UV–Vis in ace-
tonitrile: kmax/nm
(
e
max/Mꢁ1 cmꢁ1): 315 (14.69 ꢀ 103), 385
(6.92 ꢀ 103), 506 (6.50 ꢀ 103). IR/cmꢁ1
: m(N@N) 1475, m(C@N)
1623, m
(C@O) 1701. 31P NMR (CDCl3, d ppm): 74.70 (d), 76.45 (d).
1H NMR (DMSO, d ppm): 2.64 (s, COCH3), 6.49 (t, 2H, H5), 6.69
(d, 2H, H4), 6.83 (t, 1H, H1), 6.93 (m, 5H, depe), 7.15 (m, 15 H,
depe), 7.47 (m, 3H, H3, H6), 7.56 (t, 2H, depe), 7.71 (d, 2H, H2).
2.5. cis-[RuCl2(Azo)(dppm)] (C3)
ꢀ
Yield 340 mg, 40%. Anal. Calc. for RuCl2C40H35N3P2Oꢂ0.5 CH2Cl2:
The C2 structure was solved in the P1 space group. There are
C, 59.49; H, 4.37; N, 5.20. Found: C, 59.30; H, 4.77; N, 5.23%. UV–
four molecular units, C41H35Cl2N3OP2Ru, in the cell, with two mol-
ecules per asymmetric unit. There was also a region around the
center of inversion with residual peaks that seemed to be due to
disordered water molecules. Molecule analysis by the program
PLATON [22] using the squeeze routine indicated a void space of
210 Å3. On squeezing out this disordered region, the refinement
converged at R1 = 0.0631, S = 1.080 and the largest electron density
peak of 0.401 Å3 was located 0.627 Å3 off Ru1. No decomposition
was observed during data collection. Anisotropic least-squares
refinement of non-H atoms was applied. The crystal structure of
the complex C2 is depicted in Fig. 2. Crystal data after void squeeze
is given in Table 1 and selected bond distances and angles are given
in Table 3.
The C1 structure was solved in the P1211 space group. There
are two molecular units, C21H29Cl2N2OP2Ru, in the asymmetric
unit. The crystal was found to be a pseudo-merohedral twin. Twin
law 2-fold rotation about the a-axis (or equivalently about the c-
axis) was applied. The volume fraction of the twin domains is
approximately in the ratio 72:28. The diffraction images show
some spots to be badly split. The mosaic spot spread was very
Vis in acetoinitrile: kmax/nm (
e
max/Mꢁ1 cmꢁ1): 307 (8.15 ꢀ 103),
385 (7.16 ꢀ 103), 508 (6.99 ꢀ 103). IR/cmꢁ1
: m(N@N) 1486,
m
(C@N) 1624, m
(C@O) 1712. 31P NMR (CDCl3, d ppm): 4.75 (d),
8.43 (d). 1H NMR (DMSO, d ppm): 2.58 (s, COCH3), 4.45 (d, 2H,
dppm), 6.48 (t, 2H, H5), 6.69 (t, 2H, H2), 6.85 (m, 5H, dppm),
7.05 (m, 5H, dppm), 7.12 (m, 4H, H3, H4), 7.29 (t, 1H, H1), (7.33
(m,5H, dppm), 7.58 (m, 5H, dppm), 7.96 (t, 1H, H1).
2.6. cis-[RuCl2(Azo)(dbpe)] (C4)
Yield 304 mg, 35%. Anal. Calc. for RuCl2C45H37N3P2O: C, 62.15;
H, 4.29; N, 4.83. Found: C, 62.23; H, 4.07; N, 4.53%. UV–Vis in ace-
tonitrile: kmax/nm
(
e
max/Mꢁ1 cmꢁ1): 306 (6.42 ꢀ 103), 368
(6.64 ꢀ 103), 509 (6.71 ꢀ 103). IR/cmꢁ1
: m(N@N) 1486, m(C@N)
1619, m
(C@O) 1711. 31P NMR (CDCl3, d ppm): 69.70 (d), 75.69 (d).
1H NMR (DMSO, d ppm): 2.26 (s, COCH3), 6.49 (m, 4H, dbpe),
6.72 (d, 2H, H4), 4.86 (m, 5H, dbpe), 6.95 (m, 3H, H5, H6), 7.01 (t,
2H, H2), 7.15 (d, 2H, H4), 7.26 (m, 3H, dbpe), 7.02(t, 1H, H1),
7.35 (m, 6H, dbpe), 7.45 (m, 3H, dbpe), 7.62 (m, 3H, dbpe).

68
M. Al-Noaimi et al. / Polyhedron 42 (2012) 66–73
Table 1
Crystal data and structure refinement for complexes C1 and C2.
Complex
C1
C2
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C
21H29Cl2N3OP2Ru
C41H35Cl2N3OP2Ru
819.63
291
573.38
100.0
0.7107
monoclinic
P1211
0.7107
triclinic
ꢀ
P1
Unit cell dimensions
a (Å)
b (Å)
9.9695(6)
22.2516(13)
10.8470(6)
2406.2(2)
11.8545(5)
14.7083(5)
23.9846(8)
3993.7(3)
c (Å)
Volume (AÅ3)
0
Z
4
4
Density (calculated) (Mg/m3)
Absorption coefficient (mmꢁ1
F(000)
1.583
1.025
1168
1.363
0.641
1672
)
Crystal size (mm3)
Theta range for data collection (°)
0.30 ꢀ 0.27 ꢀ 0.19
0.35 ꢀ 0.23 ꢀ 0.15
3.76–25.03
2.88–25.03
Index ranges
ꢁ11 6 h 6 11
ꢁ26 6 k 6 26
ꢁ12 6 l 6 12
24225
ꢁ14 6 h 6 13
ꢁ17 6 k 6 17
ꢁ28 6 l 6 25
35447
Reflections collected
Independent reflections
Completeness to theta = 25.03°
Maximum and minimum transmission
Refinement method
8448 [Rint = 0.0587]
99.6%
0.8259 and 0.7498
full-matrix least-squares on F2
8448/1/302
14089 [Rint = 0.0435]
99.8%
0.401 and 0.627
full-matrix least-squares on F2
14089/0/902
1.080
Data/restraints/parameters
Goodness-of-fit on F2
1.116
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0965, wR2 = 0.2350
R1 = 0.1014, wR2 = 0.2380
2.318 and ꢁ1.942
R1 = 0.0631, wR2 = 0.1228
R1 = 0.0909, wR2 = 0.1332
0.78 and ꢁ0.84
Largest difference in peak and hole (e Åꢁ3
)
R1
=
R
||Fo| ꢁ |Fc||/
R
|Fo|; wR2 = {
R
[w(F2o ꢁ F2c )2]/
R .
[w(F2o)2]}1/2
Fig. 1. 31P{1H} NMR (300 MHz, CDCl3, 25 °C) spectra for complex C1.
wide, so the scan width was dropped to 0.5 degrees; even so, there
were about 18% of overlapped pixels. The R1 value dropped from
about 0.18 to about 0.10, S = 1.116 when the second (merohedral)
twin component was introduced into the refinement. The data
quality did not allow anisotropic refinement for all non-hydrogen
atoms. The crystal structure of the complex C1 is depicted in
Fig. 1 and selected bond distances and angles are given in Table 2.
3. Results and discussion
3.1. Synthesis
A previous study showed that the products of the reaction of the
bulky diphosphine ligand dppe with substituted azoimine ligands
(Azo) and RuCl3 are cis-[RuCl2(Azo)(dppe)] complexes [15]. The

M. Al-Noaimi et al. / Polyhedron 42 (2012) 66–73
69
Table 2
Selected bond lengths (Å) and angles (°) for complex C1.
Complex C1
Molecule (1)
Molecule (2)
Bond lengths (Å)
Ru(1)–N(1)
Ru(1)–N(3)
Ru(1)–P(1)
Ru(1)–Cl(1)
Ru(1)–P(2)
Ru(1)–Cl(2)
N(1)–N(2)
2.08(2)
Ru(31)–N(31)
Ru(31)–N(33)
Ru(31)–P(31)
Ru(31)–Cl(31)
Ru(31)–Cl(32)
Ru(31)–P(32)
N(31)–N(32)
N(31)–C(31)
N(32)–C(37)
1.990(14)
2.051(11)
2.363(5)
2.364(6)
2.364(5)
2.392(5)
1.30(2)
2.101(14)
2.348(6)
2.364(6)
2.378(5)
2.378(6)
1.22(2)
N(2)–C(7)
N(3)–C(7)
1.40(2)
1.18(2)
1.45(2)
1.44(2)
Bond angles (°)
N(1)–Ru(1)–N(3)
N(1)–Ru(1)–P(1)
N(3)–Ru(1)–P(1)
N(1)–Ru(1)–Cl(1)
N(3)–Ru(1)–Cl(1)
P(1)–Ru(1)–Cl(1)
N(1)–Ru(1)–P(2)
N(3)–Ru(1)–P(2)
P(1)–Ru(1)–P(2)
Cl(1)–Ru(1)–P(2)
N(1)–Ru(1)–Cl(2)
N(3)–Ru(1)–Cl(2)
P(1)–Ru(1)–Cl(2)
Cl(1)–Ru(1)–Cl(2)
P(2)–Ru(1)–Cl(2)
71.1(7)
101.7(6)
172.8(4)
97.5(6)
93.2(4)
88.6(2)
175.2(6)
104.4(4)
82.75(17)
84.3(2)
91.1(6)
93.2(4)
85.8(2)
170.6(2)
87.5(2)
N(31)–Ru(31)–N(33)
N(31)–Ru(31)–P(31)
N(33)–Ru(31)–P(31)
N(31)–Ru(31)–Cl(31)
N(33)–Ru(31)–Cl(31)
P(31)–Ru(31)–Cl(31)
N(31)–Ru(31)–Cl(32)
N(33)–Ru(31)–Cl(32)
P(31)–Ru(31)–Cl(32)
Cl(31)–Ru(31)–Cl(32)
N(31)–Ru(31)–P(32)
N(33)–Ru(31)–P(32)
P(31)–Ru(31)–P(32)
Cl(31)–Ru(31)–P(32)
Cl(32)–Ru(31)–P(32)
74.5(5)
100.5(4)
174.8(4)
93.0(4)
93.3(4)
85.45(19)
93.9(4)
Fig. 2. Thermal ellipsoid drawing (30%) of complex C1 showing the two indepen-
dent molecules of the Ru complex.
The 1H NMR spectrum of C1 shows a multiplet at 1.52 ppm
which is assigned to the four protons of the two CH2 groups of
the dmpe ligand. The two doublets at 0.82 and 0.90 ppm are as-
signed to the 12 protons of the 4-methyl groups of the dmpe li-
gand, while the protons of the acetyl group of the Azo ligand
appear as a singlet at 2.56 ppm. The aromatic region in the 1H
NMR spectra of complexes C2–C5 consists of several coupled mul-
tiplets due to the aromatic protons of the phenyl rings of the
diphosphine and the azoimine ligands.
92.8(4)
89.0(2)
171.8(2)
175.9(4)
101.5(3)
83.51(18)
87.9(2)
85.6(2)
The 31P{1H} NMR spectrum of trans-[RuCl2(dmpe)(Azo)] (C1)
(Fig. 1) shows two doublets which are assigned to the two types
of phosphorus atoms, indicating an AMX spin system. These dou-
blets appeared as broad bands at 43.04 and 47.15 ppm due to the
coupling of the two phosphorus atoms of the P–P ligand. It is sug-
gested that the shielded signal refers to the phosphorus atom trans
to the imine nitrogen of the azoimine ligand and the deshielded
signal refers to the phosphorus atom trans to the azo nitrogen atom
[23]. Moreover, complexes C2–C5 have magnetic non-equivalent
phosphorus atoms (P trans to Cl and P trans to the nitrogen of
the Azo ligand), which appear as two doublets. It is also suggested
that the shielded signal refers to the phosphorous trans to the
nitrogen of the azoimine and the deshielded signal refers to the
phosphorous atom trans to the Cl atom [23], which is consistent
with a structure in which the chlorides are mutually in cis-posi-
tions. It is well known that the P–P chelates possess three major
characteristics: electron-donating ability, steric properties and a ri-
gid P–M–P angle, often called the ‘‘bite angle’’ [8,24]. For the four-
membered chelate complex C3, the phosphorus resonates at the
formation of the thermodynamically stable products (cis-isomer)
was supported by an X-ray structure determination [15,16]. Here-
in, the sequential addition of equimolar amounts of azoimine
(Azo), different bis(diphosphine) ligands (P–P) and RuCl3 in ethanol
as the solvent gives the air-stable trans-[RuCl2(dmpe)(Azo)] (C1)
and cis-[RuCl2(P-P)(Azo)] (C2–C5) (Scheme 1). Although both cis
and trans isomers of [RuCl2(Azo)(P–P)] (C1–C5) could be excepted
as products of these syntheses, purification by silica gel showed
only one product. The isolated products are expected to be of a
trans-geometry for C1 and cis-geometry for C2–C5, as proved by
X-ray crystal structure determination. Complexes C1–C5 have been
characterized by elemental analysis, infrared spectroscopy, 1H and
31P{1H} NMR spectroscopy, as well as X-ray structure determina-
tion for complexes C1 and C2.
The IR spectra of the complexes C1–C5 show three sets of char-
acteristic absorptions in the ranges 1701–1712, 1618–1624 and
1475–1488 cmꢁ1, which can be assigned to C@O of the acetyl
group, C@N and N@N double bonds, respectively.
1
2
Cl
Cl
3
Cl
P
P
P
N
N
N
1- RuCl3
2- P-P
N
N
N
N
Ru
P
or
Ru
Cl
N
H3COC
H3COC
N
H3COC
4
5
6
C1
C2-C5
P-P ligand
Complex
dmpe
depe
dppm
dbpe
F-dppe
C1
C2
C3
C4
C5
Scheme 1. The chemical structures of the ligand (Azo) and the complexes C1–C5.

70
M. Al-Noaimi et al. / Polyhedron 42 (2012) 66–73
3.2. Crystal structures
X-ray crystal structures of complexes C1 and C2 were obtained
and the molecular structures are shown in Figs. 2 and 3, respec-
tively. Selected bond lengths and angles are given in Tables 2
and 3. There are two independent but structurally similar mole-
cules in the asymmetric unit. For complex C1 (Fig. 2), the donor
atoms around the Ru(II) center occupy cis:cis:trans N,N:P,P:Cl,Cl
positions with average bit angles for N–Ru–N and P–Ru–P of
72.8° and 83.13°, respectively. On the other hand, for complex C2
(Fig. 3), the donor atoms occupy cis:cis:cis N,N:P,P:Cl,Cl positions
with average bite angles for N–Ru–N, P–Ru–P and Cl–Ru–Cl of
76.50°, 83.59° and 91.41°, respectively. These angles indicate a dis-
torted octahedral coordination geometry in both complexes.
The azoimine (Azo) ligand in C1 is chelating via its N1 (azo) and
N3 (imine) in molecule 1 and N31 (azo) and N33 (imine) in mole-
Fig. 3. Thermal ellipsoid drawing (30%) of complex C2 showing the two indepen-
dent molecules of the Ru complex.
cule 2, thus forming five-membered chelate rings with average
0
Ru(II)–N(azo) and Ru(II)–N(imine) distances of 2.035 and 2.076 ÅA,
respectively. The dmpe ligand is also chel0ating via its two P-atoms,
with an average Ru–P distance of 2.356 ÅA. The average Ru–P (Ru–
highest field, whereas the resonances of the five-membered ring
containing complexes C1, C2, C4 and C5 are observed at lower
fields [25]. For the five-membered chelates C1, C2, C4 and C5, the
C1 resonance is found at the lowest field. The difference in the
chemical shift can be explained by the C–P–C bond angle [24]; as
the cone angle increases, the phosphorus chemical shift moves up-
field. The effect of the double bond between the two P–P atoms on
the electronic delocalization makes the phosphorus atoms more
deshielded and the peaks appear at 74.70 and 76.45 ppm for C2
and at 69.70 and 75.69 ppm for C4. A similar effect has been ob-
served in the series [Pd(S2CNEt2)(P–P)]+ [26], trans-[RuCl2(P–P)2]
[27] and fac-[RuCl3(NO)(P–P)] [28]. Increasing the electron with-
drawing ability of the substituent on the phenyl ring (as in C5) de-
creases the electron density on the phosphorus atom and thus
shifts the 31P{1H} NMR peaks to higher values, 95.41 and
106.81 ppm compared to cis-[RuCl2(Azo)(dppe)] [15].
P2 and Ru-P32) distance trans to Ru–N(azo) (Ru-N1 and Ru-N31)
0
of 2.385 ÅA is longer than the Ru–P distance trans to Ru–N(imine)
0
of 2.356 ÅA. The two chloride ligands 0are trans to each other with
an average Ru–Cl distance of 2.368 ÅA. The Ru-N, Ru-P and Ru-Cl
bond lengths are within the well established range of Ru(II) com-
plexes containing these atoms [29–31]. For complex C2 (Fig. 3),
the azoimine (Azo) is chelating via its N1A (azo) and N3A (imine)
in molecule A and N1B (azo) and N3B (imine) in molecule B, with
average Ru(II)–N(azo) and Ru(II)–N(imine) distances of 1.969 and
0
2.111 ÅA, respectively. This observation suggests that there is a
strong back donation to the azoimine ligand and this back donation
is localized in the azo group [32]. However, it is interesting to note
that the extent of the
etry of the [RuCl2(Azo)(P–P)] complexes. For cis-C2 the azo group,
which is trans to the -donor chloride ligand, has an average Ru–
p-back donation is also affected by the geom-
p
0
N(azo) bond length of 1.969 ÅA, and this is slightly shorter than
0
the corresponding bond of trans-C1 (2.035 ÅA). In contrast, the aver-
0
age N@N(azo) bond length of C2 (1.303 ÅA) is similar to that in cis-
[RuCl2(dppe)(Azo)] (C6) [15], but slightly longer than that in C1
0
0
Table 3
(1.260 ÅA). The average Ru(II)–Cl2 distance (2.392 ÅA) trans to the
0
Selected bond lengths (Å) and angles (°) for complex C2.
strong
to better a
Recently,
p
-acceptor N(azo) is longer than Ru(II)–Cl1 (2.441 ÅA), trans
-donor and weak -acceptor phosphorus atom.
interactions have been revealed as important fac-
r
p–p
p
Complex C2
Molecule (A)
Molecule (B)
tors in deciding ligand arrangements and molecular structures
Bond lengths (Å)
Ru(1A)–N(1A)
Ru(1A)–N(3A)
Ru(1A)–P(1A)
Ru(1A)–P(2A)
Ru(1A)–Cl(2A)
Ru(1A)–Cl(1A)
N(1A)–N(2A)
N(3A)–C(1A)
N(2A)–C(1A)
[33]. It is important to note that a cis or trans geometry of the com-
plexes C1–C5 depends upon the presence of a p–p interaction be-
1.967(4)
2.117(4)
Ru(1B)–N(1B)
Ru(1B)–N(3B)
Ru(1B)–P(1B)
Ru(1B)–P(2B)
Ru(1B)–Cl(2B)
Ru(1B)–Cl(1B)
N(2B)–N(1B)
N(3B)–C(1B)
N(2B)–N(1B)
1.971(4).
2.105(4)
2.3273(14)
2.3465(14)
2.4018(14)
2.4414(14)
1.314(6)
2.3208(15)
2.3454(14)
2.3809(15)
2.4393(14)
1.321(6)
1.312(7)
1.357(7)
1.293(7)
1.359(7)
Bond angles (°)
N(1A)–Ru(1A)–N(3A)
N(1A)–Ru(1A)–P(1A)
N(3A)–Ru(1A)–P(1A)
N(1A)–Ru(1A)–P(2A)
N(3A)–Ru(1A)–P(2A)
P(1A)–Ru(1A)–P(2A)
N(1A)–Ru(1A)–Cl(2A)
N(3A)–Ru(1A)–Cl(2A)
P(1A)–Ru(1A)–Cl(2A)
P(2A)–Ru(1A)–Cl(2A)
N(1A)–Ru(1A)–Cl(1A)
N(3A)–Ru(1A)–Cl(1A)
P(1A)–Ru(1A)–Cl(1A)
P(2A)–Ru(1A)–Cl(1A)
Cl(2A)–Ru(1A)–Cl(1A)
76.62(18)
99.20(13)
100.15(12)
102.99(14)
176.33(12)
83.53(5)
171.27(14)
94.88(13)
84.14(5)
N(1B)–Ru(1B)–N(3B)
N(1B)–Ru(1B)–P(1B)
N(3B)–Ru(1B)–P(1B)
N(1B)–Ru(1B)–P(2B)
N(3B)–Ru(1B)–P(2B)
P(1B)–Ru(1B)–P(2B)
N(1B)–Ru(1B)–Cl(2B)
N(3B)–Ru(1B)–Cl(2B)
P(1B)–Ru(1B)–Cl(2B)
P(2B)–Ru(1B)–Cl(2B)
N(1B)–Ru(1B)–Cl(1B)
N(3B)–Ru(1B)–Cl(1B)
P(1B)–Ru(1B)–Cl(1B)
P(2B)–Ru(1B)–Cl(1B)
Cl(2B)–Ru(1B)–Cl(1B)
76.39(18)
99.39(12)
98.34(12)
103.33(13)
178.02(13)
83.64(5)
169.55(13)
93.88(13)
85.68(5)
85.35(5)
86.25(5)
84.98(13)
83.40(12)
175.02(5)
92.92(5)
84.70(12)
85.05(12)
175.19(5)
92.97(5)
Fig. 4. A view of the crystal structure of C2, showing the intramolecular Cl---H–C
intramolecular interactions.
92.11(5)
90.71(5)

M. Al-Noaimi et al. / Polyhedron 42 (2012) 66–73
71
Fig. 5. A view of the crystal structure of C2, showing the intramolecular
p–p
intramolecular interaction between one phenyl ring of the bis-diphenylphosphine
ligand and one phenyl ring of the azoimine ligand.
tween one of the phenyl rings of the bis-diphenylphosphine ligand
and one of the phenyl rings of the azoimine ligand, and not on the
steric hindrance (size) of the diphosphine ligands. If ligand volume
was the only factor which determines the complex stereochemis-
try, then large diphosphine ligands would render trans isomers be-
cause the ligand steric interactions are minimized in this
arrangement. For C2, There are Cl---H–C intermolecular (Fig. 4)
and p–p intramolecular interactions (Fig. 5). The p–p intramolec-
ular interaction between one phenyl ring of the bis-diphenylphos-
phine ligand and one phenyl ring of the azoimine ligand is possible
due to the disposition and proximity of the rings. This p–p interac-
Fig. 6. Cyclic voltammogram for complex C1 in acetonitrile 0.1 M TBAH at 25 °C,
data reported in V vs. NHE with scan rate of 0.1 V/s
ing results are summarized in Table 4. As a representative example,
the cyclic voltammogram for complex C1 is shown in Fig. 6. Two
one-electron reduction steps are observed in the range ꢁ0.12 to
ꢁ0.71 V versus NHE, and these are localized at the azo group, anal-
ogous to the azopyridine systems [31]. Our concern here is the
quasi-reversible single oxidation wave which lies between 1.15
and 1.52 V. The multi scan CV of the complexes in this region indi-
cated that these complexes are electrochemically stable. This wave
is assigned to Ru(III/II) and occurs at more positive potentials com-
pared to the analogous dihalo ruthenium(II) complexes [34]. The
large anodic shift for this family (compared to the [RuCl2(Azo)(bpy)]
complex [35]) results from the decrease of electron density trans-
ferred to the metal by the diphosphine (P–P) ligand [36].
tion is maximized in the cis chlorine configuration (C2–C5) (Fig. 4).
The normal to the plane of the ring including C1A1 to C6A1 (cen-
troid X1A) makes an angle of 13.2(3)° with the normal to the plane
of the ring including C1A6 to C6A6 (centroid X1B), and the
0
X1A...X1B separation is 3.596 ÅA. The normal to the plane of the ring
including C1B2 to C6B2 (centroid X1C) makes an angle of 15.5(4)°
with the normal to the plane of the ring including C1B3 to C6B3
(centroid X1D), and the X1C...X1D separation is 3.654 ÅA. Complex
C1 has a dmpe ligand (no Ph rings) and, therefore, has a trans struc-
ture since a p–p interaction is not possible. The crystal structure of
On comparing the Ru(III/II) redox couple for the trans-complex
C1 with the cis-complexes C2-C5, there is a decrease in the oxida-
tion potential by 300 mV on going from the trans-complex C1 to
the cis-complexes C2–C4 and 100 mV on going to the cis complex
C5. The decrease in Ru(III/II) redox couple for the trans-complex
C1 relative to cis-complexes (C2-C5) is well established in Ru(II)
chemistry [37]. For the trans-complex C1, the competition between
0
C2 reveals two Cl---H-C intramolecular interactions with Cl---C
0
distances of 3.415 and 3.603 ÅA.
dmpe and the azoimine ligands for the
p-electrons of the ruthe-
3.3. Electrochemistry
nium atom makes the metal electron-rich and decreases the oxida-
tion potential compared to the cis-complexes C2–C5 [38].
For the series cis-[RuCl2(Azo)(P-P)] (C2–C5), the differences in
the Ru(III/II) values are related to the stability of the diphosphine
complexes. Consequently, the less stable complex is easier to oxi-
dize. A similar behavior was observed for the [Ru(Cp)(P–P)(NO2)]
series (P–P = dppe, dppm or dmpe) [36] in which the ruthe-
nium(III/II) redox potential shifted to more anodic potentials when
the more stable complex (P–P = dppe, dppm) is replaced by the less
stable [Ru(Cp)(dmpe)(NO2)] complex [38]. The large decrease in
the oxidation potential for the less stable complex C5 compared
to the more stable complexes C2–C4 and cis-[RuCl2(Azo)(dppe)]
[15] is due to the strong electron withdrawing fluorine atoms in
the phenyl rings which reduces the basicity of the P–P ligands,
and consequently the stability of C5.
The electron-transfer behavior of the complexes in acetonitrile
solution was examined by cyclic voltammetry and the correspond-
Table 4
Cyclic voltammetry and electronic spectroscopic data of cis-[RuCl2(P-P)(Azo)] (C2–C5)
and cis-[RuCl2(dppe)(Azo)] (C6) [15].
b
d
Complexesa
Ru(III/II)
Azo(0/ꢁ)c
(
D
E)
kmax (nm) e,energy (eV)e
C1
C2
C3
C4
C5
C6
1.15
1.45
1.52
1.49
1.25
1.50
ꢁ0.45
ꢁ0.28
ꢁ0.12
ꢁ0.22
ꢁ0.17
ꢁ0.28
1.60
1.73
1.64
1.71
1.42
1.78
547,2.267
506, 2.450
508, 2.440
506, 2.450
513, 2.417
504, 2.460
a
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.
3.4. Absorption spectra
b
Ru(III/II) = (E°pa + E°pc)/2.
The cathodic peak maximum.
c
d
D
E° = Ru(III/II) ꢁ Azo(0/ꢁ).
UV–Vis electronic absorption spectra of all the complexes were
measured in acetonitrile. Fig. 7 shows the electronic spectra of
e
MLCT = [1239.8/kmax(nm)] eV.

72
M. Al-Noaimi et al. / Polyhedron 42 (2012) 66–73
Fig. 7. UV–Vis spectrum for complex C1 in acetonitrile.
4. Conclusion
2.47
2.46
2.45
2.44
2.43
2.42
2.41
This work presents a study on the dependence of the product
geometry (cis versus trans) on the P–P ligand of the reaction of
RuCl3 and azoimine with different alkyl and aryl diphosphine (P–
P) ligands. Based on the X-ray structures, the
p–p intramolecular
stabilizing interaction is an important factor in deciding ligand
arrangements and molecular structures. This stabilizing interac-
tion is maximized in the aryl diphosphine ligands (P–P = depe,
dppm, dbpe, F-dppe) which results in cis-[RuCl2(P-P)(Azo)] as the
main product. For the alkyl bis(phosphine) ligand (P–P = dmpe),
the main product is the trans one. There is a decrease in oxidation
potential going from cis-[RuCl2(P–P)(Azo)] (C2, C3 and C4) to the
trans-complex. The differences in the E1/2 values for the cis-family
C2–C5 are related to the stability of the bis(phosphine) complexes.
Consequently, the less stable complex C5 is easier to oxidize. The
chemical shifts of the 31P{1H} NMR spectra are also related to the
C–P–C bond angle, electron-donating ability of the P–P chelates,
the steric properties and the rigid P–M–P bite angle.
1.40
1.45
1.50
1.55
1.60
1.65
1.70
1.75
1.80
ΔE (V)
Fig. 8. Linear correlation between MLCT band energies and
P)(Azo)] (C2–C5) and cis-[RuCl2(dppe)(Azo)] (C6) [15].
E° = Ru(III/II)-Azo(0/ꢁ) in
volts. The equation of the line is (EMLCT = 0.11 E° + 2.25).
DE° for cis-[RuCl2(P-
D
Acknowledgments
D
The authors would like to thank the Hashemite University (Jor-
dan). Thanks are also extended to Dr. Christopher Ceccarelli, Appli-
cations Scientist, Agilent Technologies Inc., Blacksburg, VA 24060.
complex C1, and the changes in the kmax values are tabulated in Ta-
ble 1, based on the change in the bis(diphosphine) ligand. Complex
C1, as representative example, has three bands: two transitions at
k = 326 and 375 nm, which are assigned as ligand–ligand charge
transfer (LLCT) type, and a transition in the visible region at
547 nm which is assigned to a MLCT transition [15,16].
Appendix A. Supplementary material
CCDC 847801 and 859317 contain the supplementary crystallo-
graphic data for C1 and C2. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
The difference in the two successive redox responses at poten-
tials positive and negative to NHE [
D
E° = Ru(III/II) ꢁ Azo(0/ꢁ)] may
be correlated with the low energy MLCT [t(Ru) ? p⁄(azomethine)]
transition [39]. Since the electronic excitation may be considered
as an intramolecular redox process [40], the spin-allowed MLCT
transition is expected to be linearly related to
DE°. The least
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