Ruthenium complexes of 2-[(4-(arylamino)phenyl)azo]pyridine formed via regioselective phenyl ring amination of coordinated 2-(phenylazo)pyridine: Isolation of products, X-ray structure, and redox and optical properties

Article abstract of DOI:10.1021/ic0203724

Aromatic ring amination reactions in the ruthenium complex of 2-(phenylazo)pyridine is described. The substitutionally inert cationic brown complex [Ru(pap)₃](ClO₄)₂ (1) (pap = 2-(phenylazo)pyridine) reacts smoothly with aromatic amines neat and in the presence of air to produce cationic and intense blue complexes [Ru(HL²)₃](ClO₄)₂ (2) (HL² = 2-[(4-(arylamino)phenyl)azo]pyridine). These were purified on a preparative TLC plate. The X-ray structure of the new and representative complex 2c has been solved to characterize them. The results are compared with those of the starting complex, [Ru(pap)₃](ClO₄)₂ (1). The transformation 1 → 2 involves aromatic ring amination at the para carbon (with respect to the diazo function) of the pendant phenyl rings of all three coordinated pap ligands in 1. The transformation is stereoretentive, and the amination reaction is regioselective. The extended ligand HL2 coordinates as a bidentate ligand and chelates to ruthenium(II) through the pyridine and one of the azo nitrogens. The amine nitrogen of this bears a hydrogen atom and remains uncoordinated. Similarly, the amination reaction on the mixed-ligand complex [Ru(pap)(bpy)₂](ClO₄)₂ produces the blue complex [Ru(HL²)(bpy)₂](ClO₄)₂ (3) as anticipated. The reactions of [RuCl₂(dmso)₄] and [Ru(S)₂(L)₂]²⁺ (dmso = dimethyl sulfoxide, S = labile coordinated solvent, L = 2,2′-bipyridine (bpy) and pap) with the preformed HL² ligand have been explored. The structure of the representative complex [RuCl₂(HL^2a)₂] (5a) is reported. It has the chlorides in trans configuration while the pyridine as well as azo nitrogens are in cis geometry. Optical spectra and redox properties of the newly synthesized complexes are reported. All the ruthenium complexes of HL² are characterized by their intense blue solution colors. The lowest energy transitions in these complexes appear near 600 nm, which have been attributed to intraligand charge-transfer transitions. For example, the lowest energy visible range transition in [Ru(HL^2b)₃]²⁺ appears at 602 nm and its intensity is 65 510 M^-1 cm^-1. All the tris chelates show multiple-step electron-transfer processes. In [Ru(HL²)₃]²⁺, six reductions waves constitute the complete electron-transfer series. The electrons are believed to be added successively to the three azo functions. In the mixed-ligand chelates [Ru(HL²)(pap)₂]²⁺ and [Ru(HL²)(bpy)₂]²⁺ the reductions due to HL², pap, and bpy are observed.

Full text of DOI:10.1021/ic0203724

Inorg. Chem. 2003, 42, 198−204
Ruthenium Complexes of 2-[(4-(Arylamino)phenyl)azo]pyridine Formed
via Regioselective Phenyl Ring Amination of Coordinated
2-(Phenylazo)pyridine: Isolation of Products, X-ray Structure, and
Redox and Optical Properties
Chayan Das,† Amrita Saha,† Chen-Hsiung Hung,‡ Gene-Hsiang Lee,§ Shie-Ming Peng,§ and
,†
Sreebrata Goswami*
Department of Inorganic Chemistry, Indian Association for the CultiVation of Science,
Kolkata 700 032, India, Department of Chemistry, National Changhua UniVersity of Education,
Changhua, Taiwan 500, Republic of China, and Department of Chemistry, National Taiwan
UniVersity, Taipei, Taiwan, Republic of China
Received May 29, 2002
Aromatic ring amination reactions in the ruthenium complex of 2-(phenylazo)pyridine is described. The substitutionally
inert cationic brown complex [Ru(pap)3](ClO ) (1) (pap ) 2-(phenylazo)pyridine) reacts smoothly with aromatic
4 2
amines neat and in the presence of air to produce cationic and intense blue complexes [Ru(HL2)3](ClO ) (2) (HL2
4 2
) 2-[(4-(arylamino)phenyl)azo]pyridine). These were purified on a preparative TLC plate. The X-ray structure of
the new and representative complex 2c has been solved to characterize them. The results are compared with
those of the starting complex, [Ru(pap)3](ClO ) (1). The transformation 1 f 2 involves aromatic ring amination at
4 2
the para carbon (with respect to the diazo function) of the pendant phenyl rings of all three coordinated pap ligands
in 1. The transformation is stereoretentive, and the amination reaction is regioselective. The extended ligand HL2
coordinates as a bidentate ligand and chelates to ruthenium(II) through the pyridine and one of the azo nitrogens.
The amine nitrogen of this bears a hydrogen atom and remains uncoordinated. Similarly, the amination reaction on
the mixed-ligand complex [Ru(pap)(bpy)2](ClO ) produces the blue complex [Ru(HL2)(bpy)2](ClO ) (3) as anticipated.
4 2
4 2
The reactions of [RuCl2(dmso)4] and [Ru(S)2(L)2]2+ (dmso ) dimethyl sulfoxide, S ) labile coordinated solvent, L
) 2,2′-bipyridine (bpy) and pap) with the preformed HL2 ligand have been explored. The structure of the representative
complex [RuCl2(HL2a)2] (5a) is reported. It has the chlorides in trans configuration while the pyridine as well as azo
nitrogens are in cis geometry. Optical spectra and redox properties of the newly synthesized complexes are reported.
All the ruthenium complexes of HL2 are characterized by their intense blue solution colors. The lowest energy
transitions in these complexes appear near 600 nm, which have been attributed to intraligand charge-transfer
transitions. For example, the lowest energy visible range transition in [Ru(HL2b)3]2+ appears at 602 nm and its
intensity is 65 510 M-1 cm-1. All the tris chelates show multiple-step electron-transfer processes. In [Ru(HL2)3]2+,
six reductions waves constitute the complete electron-transfer series. The electrons are believed to be added
successively to the three azo functions. In the mixed-ligand chelates [Ru(HL2)(pap)2]2+ and [Ru(HL2)(bpy)2]2+ the
reductions due to HL2, pap, and bpy are observed.
Introduction
coordinated ligand 2-(phenylazo)pyridine (abbreviated as
pap). Amine fusion reactions of the above type occurring
either at ortho or at para carbon (relative to the azo fragment)
of the pendent phenyl ring (Scheme 1) were observed. For
example, when a labile [CoII(pap)3]2+ complex was reacted
with ArNH2, it produced3 a green cobalt(III) complex,
[Co(L1)2]+. This transformation involves elimination of a
This work originated from our recent interest on metal-
promoted aromatic ring amination reactions1-4 on the
* To whom correspondence should be addressed. E-mail: icsg@
mahendra.iacs.res.in. Fax: (+91) 33-473 2805.
† Indian Association for the Cultivation of Science.
‡ National Changhua University of Education.
§ National Taiwan University.
198 Inorganic Chemistry, Vol. 42, No. 1, 2003
10.1021/ic0203724 CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/10/2002

Ruthenium 2-[(4-(Arylamino)phenyl)azo]pyridines
Scheme 1
Figure 1. ORTEP representation of the cation [Ru(pap)3]2+ in 1.
bidentate pap ligand from [Co(pap)3]2+, which is followed
by ortho-amination of the rest two coordinated pap ligands.
It is to be noted here that ortho-amination of the ligand pap
leads to the formation of a bis-chelating tridentate anionic
ligand [L1]-, two of which are coordinated to the cobalt(III)
cation in its complex. It thus appeared to us that ligand
dissociation from the mediator complex might be responsible
for the above ortho-amination reaction. We thought it
worthwhile to study the above amination reaction using the
mediator metal complexes of different labilities to look for
the possible role of the metal complex in the site selectivity
of the above reaction. Although complete ortho-amination
reaction was observed in the case of [Co(pap)3]2+, we did
not have an example of complete para-amination reactions.
We, however, were successful to observe1,5 the para-amina-
tion reactions, only partially, in the cases of a rhodium(III)
and a chromium(II) complex. The results of the above reac-
tions indicated that a cationic metal complex of pap, which
is absolutely inert toward substitution, might be suitable for
a complete regioselective para-amination reaction. To achieve
this objective we chose a cationic tris-chelate [Ru(pap)3]-
(ClO4)2 as the mediator complex. The reference ruthenium
complex is known6 to be substitutionally inert. The primary
aim of this report is to present our results on the reaction of
ArNH2 with [Ru(pap)3](ClO4)2. In this case all the coordi-
nated pap ligands have undergone amination exclusively at
the para-carbon. Isolation and complete characterization of
the products follow the reactions. To have further insight
into these unusual transformations, reactions using some
chosen starting compounds were carried out, and these results
are compared and used for conclusions.
complex, [Ru(pap)3](ClO4)2 (1), was chosen as the reactant.
The synthesis of complex 1 was first reported6 in 1983, which
was subsequently modified7 by us. A high yield general and
convenient method for the synthesis of 1, based on a
transmetalation reaction, was reported. The ligand pap has
very strong affinity toward an electron-rich ruthenium
center,7,8 and the tris complexes are inert toward substitution.
The structure solution of complex 1 confirms its formulation
as well as its geometry. A view of the cationic part of 1 is
shown in Figure 1. The coordination sphere around ruthe-
nium is approximately octahedral involving the three pap
ligands in a meridional geometry. The atomic group Ru(1),
the three coordinated azo nitrogens N(3), N(6), and N(9),
and a pyridyl nitrogen, N(4), are nearly planar with maximum
deviation of 0.1793 Å. Notably, the two azo nitrogens, viz.
N(3) and N(6), lie trans to each other. The remaining two
coordinated pyridyl nitrogens, N(1) and N(7), are also in
relative trans configuration. An azo chromophore being a
strong π-acceptor, trans N(azo)-M-N(azo) grouping was
never observed8 before in the metal-pap complexes. How-
ever, the other possible facial isomer for the tris-chelate 1 is
sterically unfavorable due to crowding of three phenyl
groups. Notably, the average of Ru-N distances (Table 1)
in 1 (average 2.042(7) Å) is shorter than that observed9 in
[Ru(NH3)6]I2 (average 2.144(4) Å). The shortness of Ru-N
bond in 1 is attributed to strong dπ-pπ interactions between
ruthenium (t2) and the low-lying π* (azo) orbitals of pap.
For comparison, the reference complex, [Ru(NH3)6]2+, does
not have such π interactions. The effect of π-interactions in
1 are reflected in the long N-N length (average 1.280(9)
Å), which is appreciably longer than that observed10 in
[Hpap](ClO4) (1.258(5) Å)
Results and Discussion
B. Reactions and Products. In line with our synthetic
strategy, complex 1 was reacted with neat ArNH2 on a steam
A. Starting Complex. To study the amination reaction
on coordinated pap ligand, a substitutionally inert brown
(1) Saha, A.; Ghosh, A. K.; Majumdar, P.; Mitra, K. N.; Mondal, S.; Rajak,
K. K.; Falvello, L. R.; Goswami, S. Organometallics 1999, 18, 3772.
(2) Ghosh, A. K.; Majumdar, P.; Falvello, L. R.; Mustafa, G.; Goswami,
S. Organometallics 1999, 18, 5086.
(3) Saha, A.; Majumder, P.; Goswami, S. J. Chem. Soc., Dalton Trans.
2000, 1703.
(4) Saha, A.; Majumdar, P.; Peng, S.-M.; Goswami, S. Eur. J. Inorg.
Chem. 2000, 2631.
(5) Kamar, K. K.; Saha, A.; Castin˜eiras, A.; Hung, C.-H.; Goswami, S.
Inorg. Chem. 2002, 41, 4531.
(6) Goswami, S.; Mukherjee, R.; Chakravorty, A. Inorg. Chem. 1983, 22,
2825.
(7) Kakoti, M.; Deb, A. K.; Goswami, S. Inorg. Chem. 1992, 31, 1302.
(8) (a) Goswami, S.; Chakravarty, A. R.; Chakravorty, A. Inorg. Chem.
1983, 22, 602. (b) Goswami, S.; Chakravarty, A. R.; Chakravorty, A.
Inorg. Chem. 1982, 21, 2737. (c) Goswami, S.; Chakravarty, A. R.;
Chakrovorty, A. Inorg. Chem. 1981, 20, 2246.
(9) Stynes, H. C.; Ibers, J. A. Inorg. Chem. 1971, 10, 2304.
(10) Saha, A.; Das, C.; Goswami, S. Indian J. Chem., Sect. A 2001, 40A,
198.
Inorganic Chemistry, Vol. 42, No. 1, 2003 199

Das et al.
Table 1. Selected Bond Lengths (Å) of 1, 2c‚C7H8, and 5a
1
2c‚C7H8
5a
Ru(1)-N(1)
Ru(1)-N(3)
Ru(1)-N(4)
Ru(1)-N(7)
Ru(1)-N(6)
Ru(1)-N(9)
N(2)-N(3)
N(5)-N(6)
N(8)-N(9)
C(6)-C(7)
C(7)-C(8)
C(8)-C(9)
C(9)-C(10)
C(10)-C(11)
C(6)-C(11)
2.067(7)
2.056(7)
2.031(7)
2.027(7)
2.015(7)
2.054(7)
1.284(9)
1.276(9)
1.280(8)
1.412(11)
1.351(12)
1.398(12)
1.389(12)
1.378(11)
1.369(11)
Ru(1)-N(1)
Ru(1)-N(3)
Ru(1)-N(5)
Ru(1)-N(7)
Ru(1)-N(9)
Ru(1)-N(11)
N(2)-N(3)
2.058(4)
2.061(4)
2.046(4)
2.058(4)
2.073(4)
2.071(4)
1.296(5)
1.285(5)
1.287(6)
1.370(6)
1.427(6)
1.371(7)
1.425(7)
Ru-N(1)
Ru-N(3)
Ru-N(5)
Ru-N(7)
Ru-Cl(1)
Ru-Cl(2)
N(2)-N(3)
N(6)-N(7)
2.071(3)
1.999(3)
2.085(3)
2.013(3)
2.3755(12)
2.3937(12)
1.297(4)
1.299(4)
N(6)-N(7)
N(10)-N(11)
N(10)-C(41)
N(11)-C(42)
N(12)-C(47)
N(12)-C(48)
Scheme 2
Scheme 3
bath in air. The initial brown color of the ruthenium tris
chelate [Ru(pap)3](ClO4)2 gradually became ink-blue in 4 h
(Scheme 2). The crude product, after initial workup, was
finally purified on a preparative TLC plate. An intense blue
band of composition [Ru(HL2)3](ClO4)2 (2) was collected
in 60-70% yield. The above reaction does not proceed at
all in the presence of solvent and is sluggish in an inert
atmosphere.
may be controlled by the proper selection of the mediator
complex.
To have further insight into the ruthenium complexes of
the HL2 ligand two more synthetic reactions (Scheme 3) were
carried out using the preformed ligand HL2. The synthesis
of the mixed ligand complexes 3 and 4 could be achieved
in high yields from the reaction of [Ru(S)2(N∧N)2]2+ (S )
solvent, N∧N ) bpy, pap), generated in situ, and the
preformed HL2. The physicochemical properties of the two
samples of 3 obtained from above two different routes
(Scheme 3) are identical. The reaction of [RuCl2(dmso)4]
(dmso ) dimethyl sulfoxide) with HL2 in 1:2 molar pro-
portion produced a violet solution, which on purification
produced crystalline [RuCl2(HL2)2] (5) in 25% yield. The
filtrate contained several overlapping blue and violet bands
from which no pure product could be isolated so far. The
cationic complexes 2 and 3, isolated as above, were
formulated on the basis of their analytical as well as FAB
mass spectral measurements. The positive ion FAB mass
spectra of the two representatives 2a and 3 showed a peak
each due to the [M -2ClO4 - H]+ at m/z 924 and 686,
A similar amination reaction was also tried on the neutral
complex8c [RuCl2(pap)2], which notably was unreactive to
ArNH2. Greater electrophilicity of the cationic complexes
appears to be crucial for the amination reactions. This
proposal was further strengthened by the fact that the mixed-
ligand cationic complex8c [Ru(pap)(bpy)2](ClO4)2 (bpy )
2,2′-bipyridine) reacted smoothly with PhNH2 to produce the
blue complex [Ru(HL2a)(bpy)2](ClO4)2 (3) in a moderate
yield. It is probable that nucleophilic attack of deprotonated,
[ArNH]- at the para carbon of the pendant phenyl group of
pap is the primary and key step for this fusion reaction.
Nucleophilic substitution on an aryl ring is otherwise not
common. It is believed that the phenyl ring of pap becomes
electrophilic upon coordination. The hydride ion thus gener-
ated reacts with aerial oxygen to form hydroperoxide, which
decomposes readily under our reaction conditions. In the
aforesaid reactions, each of the coordinated pap ligands
undergoes fusion with an amine function exclusively at the
para-carbon (relative to the azo functionality) of the pendant
phenyl ring. The results have confirmed the regioselective
nature of the amination reaction. Moreover these have also
demonstrated that the site selectivity of the above reactions
6
respectively. These complexes are diamagnetic (Ru(II), t2 ).
While cationic complexes 2-4 are 1:2 electrolytes,11 complex
5 is a nonelectrolyte in CH3CN. In the resultant complexes,
the transformed ligand HL2 bears an uncoordinated donor
amine function, which is separated from a coordinated
(11) Greary, W. J. Coord. Chem. ReV. 1971, 7, 81.
200 Inorganic Chemistry, Vol. 42, No. 1, 2003

Ruthenium 2-[(4-(Arylamino)phenyl)azo]pyridines
Figure 3. ORTEP representation of the molecular complex 5a.
Figure 2. ORTEP representation of the cation [Ru(HL2c)3]2+ in 2c‚C7H8.
ligands is a further indication of the presence of strong
Ru(II)-HL2 π-bonding.
acceptor azo function by a conjugated spacer. Metal com-
plexes of such ligands having both donor and acceptor
chromophores are of current interest.12
Figure 3 shows the ORTEP and atom numbering scheme
for the compound [RuCl2(HL2a)2] (5a). The ligand HL2a is
bidentate and chelates to ruthenium through the pyridine
nitrogens N(1) and N(5) and the azo nitrogens N(3) and N(7).
The amine nitrogens N(4) and N(8) remain protonated and
uncoordinated. The binding mode of [HL2a] in 5a is similar
to that observed in 2c. The HL2 ligand lacks a 2-fold
symmetry axis, and therefore, five geometrical isomers are
possible14 for [RuCl2(HL2a)2]. As noted before, we have been
able to isolate only one of these isomers. The structural
analysis of it indicates that compound 5a has the cholrides
in trans configuration, but pyridine nitrogens as well as the
coordinated azo nitrogens are in cis geometry. Such a
configuration has been reported for two related complexes,
[RuCl2(pap)2]15 and [RuCl2(L)2]16 (L ) 1-tolyl-2-(phenylazo)-
imidazole). Strong stacking of the aromatic rings stabilizes
this configuration. Notably, the Ru-N(azo) lengths (av-
erage 2.006(3) Å) are shorter than the Ru-N(py) lengths
(average 2.078 (3) Å). This may be attributed to strong
(dπ)Ru-pπ(azo) interactions. As a consequence the N-N
length (average 1.298(4) Å) in 5a is appreciably longer than
that observed4 in the free ligand (1.264(2) Å).
The 1H NMR spectra of the tris chelates 2 and 3 are very
complex due to overlapping of unique proton resonances.
The spectrum of 5, however, is resolved. The N-H
resonance was observed4 near δ 6.0. Interestingly, each kind
of proton in HL2 gave rise to one signal in 5. Thus, the two
ligands in 5 are magnetically equivalent, for which a 2-fold
symmetry axis is required. The spectrum of a representative
5b is submitted as Supporting Information (Figure S1).
D. Optical Spectra. One of the most important observa-
tions in the IR spectra of the complexes of HL2 is the
appearance17 of ν(N-H) in the range 3420-3250 cm-1. The
dihalo complexes showed sharp bands, while those for the
tris chelates are broad. The abilities of the azo ligands to act
C. Formulation and X-ray Structure of Complexes 2c
and 5a. Compounds [Ru(HL2c)3](ClO4)2 (2c) and [RuCl2-
(HL2a)2] (5a) formed X-ray-quality crystals and were used
for structure determination. The structure analysis of 2c
indeed confirmed the regioselective fusion of ArNH2 to all
three coordinated pap ligands of [Ru(pap)3](ClO4)2 at the
para-carbon of the pendant phenyl rings. A view of the
cationic part of the molecule [Ru(HL2c)3](ClO4)2.C7H8 is
shown in Figure 2. In this complex, all three extended ligands
HL2c are identical and bind as neutral bidentate chelate. These
ligands are formed, in situ, due to fusion of (C-N bond
formation) p-anisidine function to pap at the para position
(relative to the azo fragment). The complex as a whole is
dicationic, and the crystallographic asymmetric unit also
contains two units of perchlorate and a toluene molecule as
a solvate. The geometry of the cationic complex is meridi-
onal, which is similar to that of the starting [Ru(pap)3]2+
(vide infra). Hence, the reaction is streoretentive. The chelate
bite angles of [HL2c] lie in the range 76.09(17)-76.80(17)°.
The bond distances (Table 1) indicate strong Ru(dπ)-
HL2(pπ) interactions in 2c. These are primarily evidenced
by lengthening of N-N distances in the coordinating
ligands. The N-N distance is appreciably lengthened in
[Ru(HL2c)3]2+ (average 1.289(5) Å) as compared4 to the
N-N distance (1.264(2) Å) in the uncoordinated HL2a
ligand. Coordination of HL2 to ruthenium (II) can lead to a
decrease in the N-N bond order due to both σ-donor and
π-acceptor character of the ligand, the later character having
a more pronounced10,13 effect. The relative shortness of the
Ru(II)-N(HL2) distances in 2c compared to the Ru(II)-N
distances in other ruthenium complexes9 of nitrogenous
(12) (a) Lacroix, P. G. Eur. J. Inorg. Chem. 2001, 339. (b) Bozec, H. L.;
Renouazd, T. Eur. J. Inorg. Chem. 2000, 229. (c) Dhenaut, C.; Ledoux,
I.; Samuel, I. D. W.; Zyss, J.; Bourgault, M.; Bozec, H. L. Nature
1995, 374, 339. (d) Zyss, J.; Ledoux, I. Chem. ReV. 1994, 94, 77.
(13) (a) Majumdar, P.; Kamar, K. K.; Castin˜eiras, A.; Goswami, S. Chem.
Commun. 2001, 1292. (b) Ghosh, B. K.; Mukhopadhyay, A.; Goswami,
S.; Ray, S.; Chakravorty, A. Inorg. Chem. 1984, 23, 4633. (c) Lahiri,
G. K.; Goswami, S.; Falvello, L. R.; Chakravorty, A. Inorg. Chem.
1987, 26, 3365. (d) Majumdar, P.; Peng, S.-M.; Goswami, S. J. Chem.
Soc., Dalton Trans. 1998, 1569.
(14) (a) Mitra, K. N.; Goswami, S. Inorg. Chem. 1997, 36, 1322. (b)
Choudhury, S.; Goswami, S. Polyhedron 1996, 15, 1191.
(15) Velders, A. H.; Kooijman, H.; Spek, A. L.; Haasnoot, J. G.; Vos, D.
d.; Reedijk, J. Inorg. Chem. 2000, 39, 2966.
(16) Misra, T. K.; Das, D.; Sinha, C.; Ghosh, P.; Pal, C. K. Inorg. Chem.
1998, 37, 1672.
(17) Mitra, K. N.; Choudhury, S.; Castin˜eiras, A.; Goswami, S. J. Chem.
Soc., Dalton Trans. 1998, 2901.
Inorganic Chemistry, Vol. 42, No. 1, 2003 201

Das et al.
Table 2. Optical Spectral Data
IR (KBr) (ν, cm-1
ν(CdC + CdN)
)
compd
ν(N-H)
ν(NdN)
ν(ClO4)
absa [λmax, nm (ꢀ, M-1 cm-1)]
2a
2b
2c
2d
3
4
5a
5b
3410
3390
3380
3410
3400
3420
3250
3260
1580
1590
1590
1580
1580
1580
1585
1590
1300
1295
1295
1300
1300
1290
1285
1280
1140, 640
1140, 645
1150, 630
1120, 645
1110, 630
1110, 630
593 (48 810), 496b (34 480), 282 (39 480)
602 (65 510), 494b (44 510), 284 (55 340)
608 (59 430), 498b (40 120), 284 (46 470)
595 (46 670), 500b (32 990), 284 (45 450)
569 (19 690), 500 (22 270), 283 (57 740)
603 (17 410), 509 (11 680), 370 (17 110)
606 (26 070), 482b (19 460), 290 (35 030)
620 (26 740), 484b (21 460), 290 (34 420)
a Solvent CH3CN. b Shoulder.
additional allowed transition, which is responsible for the
intense blue color of its solution. Interestingly, the mixed-
ligand compound [Ru(HL2a)(pap)2]2+ also showed a transition
at 603 nm, which is accompanied by a MLCT transition8c at
509 nm. Notably, the intensity of the 603 nm transition in
the later example is about 3-fold less intense than that
observed in [Ru(HL2a)3]2+. We thus ascribe this lowest
energy transition (near 600 nm) in the complexes of HL2 as
the intraligand charge-transfer transition.11b The ligand HL2
has the donor amine as well as the azo acceptor functions,
which are separated by a conjugated spacer. The acceptor
property of the azo-chromophore is augmented upon coor-
dination to the ruthenium center. An interesting trend with
respect to the lowest energy transitions in two pairs of the
following complexes is noted. In the order from γ-[RuCl2-
(pap)2]8c to [Ru(pap)3]2+,6 the MLCT transition shifts ap-
preciably from 635 to 492 nm. In comparison, there is only
very little shift of the lowest energy transition in moving
from 5 to 2 (Figure 4). It may be noted that the geometries
of 5 and 2 are identical with those of the two above
compounds, viz. γ-[RuCl2(pap)2] and [Ru(pap)3]2+, respec-
tively.
Figure 4. Electronic spectra of the complexes: 1 (---); 2a (s); 5a (‚‚‚).
as strong π-acceptors toward low valent metal ions are well
documented.18 The azo imine orbitals in these complexes
are strongly involved in π-interactions. Thus, the ν(NdN)
stretching frequencies in the complexes of HL2 are ap-
preciably lowered8,15 as compared to that for the free HL2.
Selected IR data are collected in Table 2.
E. Redox Properties. The redox behavior of the com-
plexes were studied by using cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) in acetonitrile (0.1 M
NEt4ClO4) in the potential range 1.8 to -2.5 V by using
platinum and glassy carbon working electrodes. The reported
potentials (Table 3) are referenced to the saturated calomel
electrode (SCE). The value for the ferrocenium-ferrocene
couple, under our experimental conditions, was 0.42 V.
All the tris chelates of HL2, reported in this work, showed
multiple reversible cathodic waves. The anodic wave how-
ever, was irreversible. The free ligand, HL2a displays two
reductive waves at -1.11 and -1.69 V, which are cathodic,
compared to pap. This may be attributed to the strong
inductive (+I) effect of the arylamino substitution in HL2.
Thus, for the tris-chelates [Ru(HL2)3]2+ six reductions, in
principle, are expected. In practice, all six reductions were
indeed observable in the tris chelates. The processes were
generally reversible except the wave for the final step, which
appears to be quasireversible or irreversible. The cyclic
voltammogram of a representative compound, 2d, is submit-
ted as a Supporting Information (Figure S2). The observation
of a complete set of six reductions in the tris chelate is rare.19
In the case of mixed pap-HL2 complex, [Ru(HL2a)(pap)2]2+
(4), all six reductions were observed, but in [Ru(HL2a)-
The solution color of all the ruthenium complexes of HL2
is blue, and these are characterized by a strong absorption
near 600 nm. The solution spectral data are collected in Table
2. The spectrum of the free ligand, HL2a, consists of two
high-energy transitions at 415 and 270 nm, respectively. The
UV-vis spectrum of the cationic tris chelate, [Ru(HL2)3]2+,
comprises a strong absorption in the range 590-610 nm,
which is associated with a shoulder near 500 nm. There is
also a strong band in the UV region. The intensity of the
lowest energy transition in these complexes is unusually high,
which vary with the change of substitution R on the ligand
HL2. For example, the intensities of the 600 nm bands of
[Ru(HL2a)3]2+ and [Ru(HL2b)3]2+ are 48 810 and 65 510 M-1
cm-1, respectively. Such highly intense transitions were noted
before12 in some related systems containing donor-acceptor
functions. Representative spectra of the parent [Ru(pap)3]2+,
[Ru(HL2a)3]2+, and [RuCl2(HL2a)2] are displayed in Fig-
ure 4 for comparison. The broad transition at 492 nm for
[Ru(pap)3]2+ was assigned to a metal-to-ligand6 charge
transfer (MLCT) transition. Comparison of the above spectra
indicates that the MLCT transition in [Ru(HL2b)3]2+ also
appeared at 494 nm. The band near 600 nm is due to an
202 Inorganic Chemistry, Vol. 42, No. 1, 2003

Ruthenium 2-[(4-(Arylamino)phenyl)azo]pyridines
Table 3. Electrochemical Dataa
compd
oxidn Ep, Vb
redn -E1/2, V (∆Ep, mV)c
∆E°ox/red, Vd
νct, Ve
2a
2b
2c
2d
3
4
5a
5b
1.35
1.25
1.15
1.41
1.28
1.33
0.17 (100), 0.45 (80), 0.89 (80), 1.53 (90), 1.84 (100), 2.04f (100)
0.25 (120), 0.52 (100), 0.96 (100), 1.59 (150), 1.99 (140), 2.12f (90)
0.19 (110), 0.48 (100), 0.91 (90), 1.53 (90), 1.93 (80), 2.09f (90)
0.11 (110), 0.40 (110), 0.84 (90), 1.48 (90), 1.86 (110), 2.10f (110)
0.51 (80), 1.20 (80), 1.57 (110), 1.77 (110)
1.52
1.50
1.34
1.52
1.79
1.39
2.09
2.06
2.04
2.08
2.18
2.06
0.06 (80), 0.40 (130), 0.85 (110), 1.48 (180), 1.79f (200), 2.14f (190)
0.72,g 0.92,g 1.49,g 1.87g
1.09
0.76h (70), 1.19
0.71,g 0.93,g 1.47,g 1.90g
a Conditions: solvent, acetonitrile; supporting electrolyte, NEt4ClO4 (0.1 M); working electrode, platinum for oxidation and glassy carbon for reduction
processes; reference electrode, SCE; solute concentration, ca. 10-3 M; scan rate, 50 mV s-1 b Anodic peak potential. The irreversible response without
.
cathodic counterpart. c Ε1/2 is calculated as the average of anodic (Epa) and cathodic (Epc) peak potentials; ∆Ep ) (Epa - Epc). An E1/2 value indicates the
redox process is reversible. d Refers to the difference in the formal potentials for the first oxidation and the first reduction processes. e Observed lowest
energy charge-transfer transitions in V (see Table 2). f Quasireversible. g Irreversible Epc. h Reversible oxidation process.
(bpy)2]2+ (3), the last two were inaccessible (see Supporting
Information Figures S4 and S3). This is not surprising since
the reductions of bpy are expected6,19 to occur at more
cathodic potentials than that of pap. It is believed that in
complex 3 the HL2a ligand is reduced completely before the
bpy reduction occurs. Each one of the tris chelates also
showed an irreversible anodic wave near 1.1-1.4 V, which
is ascribed to ligand oxidation processes. The dihalo complex
[RuCl2(HL2b)2] showed a reversible anodic response at 0.76V
followed by an irreversible wave at 1.2 V. There are multiple
irreversible cathodic responses in the range -0.70 to -1.90
V. A similar nature for the voltammogram was observed8c
for γ-[RuCl2(pap)2]. The reversible anodic wave in the
complex of the unsubstituted ligand [RuCl2(HL2a)2] (5a) was
absent; however, the pattern of the voltammogram in the
remaining potential region is similar to that observed for
[RuCl2(HL2b)2] (5b). The reversible oxidative wave might
have shifted anodic in 5a in the absence of a donor (-Me)
substituion and was thus not observed before the irreversible
wave.
the para-carbon. The generality of the fusion reaction was
established by the use of different ArNH2 species. The
products were completely characterized. The aminated ligand
HL2, thus formed, has both donor and acceptor chromophores
and shows interesting optical as well as redox properties.
Further work on similar transformations, with the use of other
aromatic amines having additional functionalities such as
4-aminopyridine, is under active search. It is anticipated that
the product of such reactions might be useful for the
construction of polymetallic systems.
Experimental Section
6
Materials. The starting complexes [Ru(pap)3](ClO4)2, [RuCl2-
(dmso)4],20 [RuCl2(bpy)2],21 and [Ru(H2O)2(pap)2](ClO4)28a and the
ligand HL2 4 were synthesized by the published procedures. Solvents
and chemicals used for synthesis were of analytical grade. The
supporting electrolyte (tetraethylammonium perchlorate) and sol-
vents for electrochemical work were obtained as before.6
Physical Measurements. A JASCO V-570 spectrometer was
used to record the electronic spectra. Infrared spectra were recorded
with a Perkin-Elemer 783 spectrophotometer. A Perkin-Elmer 240C
1
elemental analyzer was used for microanalysis. H NMR spectra
Notably, the lowest energy charge-transfer transitions in
the complexes of HL2 correlates linearly19 with ∆E°ox/red
(Table 3), where ∆E°ox/red refers to the difference in the
formal potentials for the first oxidation and the first reduction
processes.
were measured in CDCl3 with a Bruker Avance DPX 300
spectrometer, and SiMe4 was used as the internal standard.
Electrochemical measurements were done under a dry nitrogen
atmosphere on a PAR 370-4 electrochemistry system as described
before.6 All potentials in this work are referenced to the saturated
calomel electrode (SCE) and are uncorrected for junction contribu-
tion. The value for the ferrocenium-ferrocene couple under our
experimental condition was 0.42 V.
Synthesis of [Ru(HL2a)3](ClO4)2 (2a). A mixture of [Ru(pap)3]-
(ClO4)2 (0.20 g, 0.23 mmol) and PhNH2 (0.5 mL) was heated on a
steam bath for 4 h. The initial brown color of the mixture gradually
became ink-blue. The mixture was cooled and washed thoroughly
with diethyl ether for several times. Finally, it was purified on a
preparative TLC plate using a 40% chloroform-acetonitrile mixture
as the eluent. The intense blue product, thus obtained, was
recrystallized from a toluene-acetonitrile mixture. Yield: 0.19 g,
70%. Anal. Calcd for C51H42N12Cl2O8Ru: C, 54.55; H, 3.77; N,
14.97. Found: C, 54.32; H, 3.54; N, 14.76. MS: m/z 924.
Compounds 2b-2d were prepared similarly by following the
above procedure using the appropriate substituted aromatic amines
in place of PhNH2. The yields and analytical data are collected
below.
Conclusion
In this work we have achieved regioselective para-
amination at all three coordinated 2-(phenylazo)pyridine
ligands in [Ru(pap)3]2+. The role of the mediator complex
with respect to site selectivity of the amination reaction has
been established. Thus, for a labile complex, an ortho-fusion
process is favored. This seems reasonable since prior
coordination of an amine residue at the vacant site of the
metal ion would bring it in close proximity to the ortho-CH
of the pendent phenyl ring. In the absence of any such vacant
site at the metal center, as it occurs in the case of a
substitutionally inert mediator complex, ArNH2 cannot
approach the ortho-carbon and hence fusion occurs only at
(18) (a) Kharmawphlang, W.; Choudhury, S.; Deb, A. K.; Goswami, S.
Inorg. Chem. 1995, 34, 3828. (b) Krause, R. A.; Krause, K. Inorg.
Chem. 1980, 19, 2600. (c) Ackermann, M. N.; Barton, C. R.; Deodene,
C. J.; Specht, E. M.; Keill, S. C.; Schreiber, W. E.; Kim, H. Inorg.
Chem. 1989, 28, 397.
(20) Evans, I. P.; Spencer, A.; Wilkinson, G. J. Chem. Soc., Dalton Trans.
1973, 204.
(21) Giordana, P. J.; Bock, C. R.; Wrighton, M. S. J. Am. Chem. Soc. 1978,
100, 6960.
(19) Ghosh, B. K.; Chakravorty, A. Coord. Chem. ReV. 1989, 95, 239.
Inorganic Chemistry, Vol. 42, No. 1, 2003 203

Das et al.
Table 4. Crystallographic Data for 1, 2c‚C7H8, and 5a
2c‚C7H8
33H27Cl2N9O8Ru C61H56Cl2N12O11Ru C34H28Cl2N8Ru
finally purified by slow diffusion of the solution of the compound
in acetonitrile into toluene. This crystallization process was repeated
twice. Yield: 0.17 g, 63%. Anal. Calcd for C39H32N10Cl2O8Ru: C,
49.79; H, 3.43; N, 14.89. Found: C, 49.51; H, 3.61; N, 14.73.
MS: m/z 741.
1
5a
empirical formula
molecular mass
temp, K
cryst syst
space group
a, Å
C
849.61
1305.15
720.61
293(2)
293(2)
295(2)
monoclinic
P21/c
triclinic
P1h
monoclinic
P21/n
Synthesis of [RuCl2(HL2a)2] (5a). To a methanolic solution of
HL2a (0.22 g, 0.82 mmol) was added a sample of [RuCl2(dmso)4]
(0.20 g, 0.41 mmol). The mixture was refluxed on a steam bath
for 4 h. The color of the solution changed from orange yellow to
blue violet with the precipitation of a crystalline blue compound.
The reaction mixture was then allowed to cool to room temperature.
The crystalline blue product was collected by filtration, which was
washed with methanol and dichloromethane. The compound was
finally recrystallized by slow evaporation of its DMF solution.
Yield: 0.09 g, 30%. Anal. Calcd for C34H28N8Cl2Ru: C, 56.67;
H, 3.92; N, 15.55. Found: C, 56.60; H, 3.61; N, 15.41. MS: m/z
719. The filtrate from the above reaction mixture showed the
presence of several blue and violet overlapping bands on the TLC
plate, from which no pure compound could be isolated so far.
The compound [RuCl2(HL2b)2] (5b) was synthesized similarly
using HL2b in place of HL2a. Yield: 25%. Anal. Calcd for C36H32N8-
Cl2Ru: C, 57.75; H, 4.31; N, 14.97. Found: C, 57.81; H, 4.07; N,
15.27.
9.2701(13)
9.7631(14)
38.926(6)
90
93.862(3)
90
12.5390(7)
14.5309(8)
18.5892(11)
75.7620(10)
80.2070(10)
71.4670(10)
3097.1(3)
1.400
11.444(3)
21.150(4)
13.234(3)
90
106.282(18)
90
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
3515.0(9)
1.605
3074.9(11)
1.557
D
calc, Mg/m3
Z
4
2
4
cryst dimens,
0.20 × 0.08
0.61 × 0.22
0.30 × 0.16
mm3
× 0.06
× 0.06
× 0.10
θ range for data
collcn, deg
GOF
1.05-27.54
1.14-27.55
1.87-25.00
1.037
1.007
1.001
reflcns collcd
unique reflcns
final R indices
[I > 2σ(I)]
21 670
8038
R1 ) 0.0699
wR2 ) 0.1358
19 851
13 721
R1 ) 0.0689
wR2 ) 0.2097
5422
5422
R1 ) 0.0355
wR2 ) 0.0669
2b. Yield: 65%. Anal. Calcd for C54H48N12Cl2O8Ru: C, 55.67;
H, 4.15; N, 14.43. Found: C, 55.33; H, 4.46; N, 14.56.
X-ray Structure Determination. We obtained suitable X-ray-
quality crystals of compound 1 by slow diffusion of toluene into a
dichloromethane solution of the compound. The suitable X-ray-
quality crystals of 2c were grown by slow diffusion of toluene into
an acetonitrile solution of the compound. Intensity data for both
the above compounds 1 and 2c were measured on a Bruker SMART
diffractometer (Mo KR radiation, λ ) 0.710 73 Å), and data were
corrected for Lorentz-polarization effects. Both the structures were
solved by employing SHELXS-9722 program package and refined
by full-matrix least squares based on F2 (SHELXL-97).23 All
hydrogen atoms of the ligands were added in calculated positions.
X-ray-quality cryastals of 5a were obtained by slow evaporation
of a solution of 5a in dimethylformamide. The data for 5a were
collected on a Nonius CAD4 diffractometer (Mo KR radiation, λ
) 0.710 73 Å). The data were corrected for Lorentz-polarization
effects. The structure was solved by using the SHELXS-97 program
package and refined by full-matrix least-squares based on F2
(SHELXL-97). Hydrogen atoms were added in the calculated
positions. All the relevant data are given in Table 4
2c. Yield: 70%. Anal. Calcd for C54H48N12Cl2O11Ru: C, 53.47;
H, 3.99; N, 13.86. Found: C, 53.27; H, 3.88; N, 13.76.
2d. Yield: 63%. Anal. Calcd for C51H39N12Cl5O8Ru: C, 49.95;
H, 3.20; N, 13.71. Found: C, 49.73; H, 3.60; N, 13.88.
Synthesis of [Ru(HL2a)(bpy)2](ClO4)2 (3). Method A. A
mixture of [Ru(pap)(bpy)2](ClO4)2 (0.20 g, 0.25 mmol) and aniline
(0.5 mL) was heated for 5 h at 125-130 °C on an oil bath. The
resultant dull violet colored mixture was cooled and washed with
diethyl ether for several times. The crude mass was recrystallized
3 times from an acetonitrile-toluene mixture. Finally, the com-
pound was purified by preparative TLC technique using a 40%
chloroform-acetonitrile mixture as the eluent. The pure compound
was separated as a blue-violet band. Yield: 0.08 g, 36%. Anal.
Calcd for C37H30N8Cl2O8Ru: C, 50.12; H, 3.41; N, 12.64. Found:
C, 50.18; H, 3.68; N, 13.02.
Method B. A mixture of [RuCl2(bpy)2](ClO4)2 (0.20 g, 0.41
mmol) and AgNO3 (0.14 g, 0.82 mmol) was refluxed in 50 mL of
methanol for 1 h to generate [Ru(bpy)2(MeOH)2]2+. The solution
was cooled and filtered through a Whatman 42 filter paper to
remove the precipitated AgCl. Ligand HL2a (0.11 g, 0.41 mmol)
was then added to the orange filtrate, and it was further refluxed
for 1.5 h. The solution became blue-violet. It was concentrated to
15 mL, and 0.5 mL of saturated aqueous solution of NaClO4 was
added to precipitate the compound. The crude product was then
purified by slow diffusion of the solution of the compound in
acetonitrile into toluene. This process was repeated 3 times to get
pure crystalline product. Yield: 0.24 g, 65%. Anal. Calcd for
C37H30N8Cl2O8Ru: C, 50.12; H, 3.41; N, 12.64. Found: C, 49.78;
H, 3.48; N, 12.75. MS: m/z 686.
Acknowledgment. Financial support received from the
Department of Science and Technology and the Council of
Scientific and Industrial Research, New Delhi, is gratefully
acknowledged. Thanks are due to the RSIC, Lucknow, India,
for providing mass spectra. We are thankful to Dr. Samaresh
Bhattacharya for the help.
Supporting Information Available: X-ray crystallographic
details, in CIF format, of the three compounds 1, 2c, and 5a, the
1H NMR spectrum of 5b, and segmented cyclic voltammograms
of the compounds 2d, 3, and 4. This material is available free of
charge via the Internet at http://pubs.acs.org.
Synthesis of [Ru(HL2a)(pap)2](ClO4)2 (4). A mixture of
[Ru(H2O)2(pap)2](ClO4)2 (0.20 g, 0.28 mmol) and HL2a (0.08 g,
0.28 mmol), in 60 mL of dehydrated ethanol, was refluxed on a
steam bath for 4 h. The initial violet color slowly became yellowish
green with the precipitation of a gummy mass. The mixture was
cooled and filtered. The greenish filtrate was discarded. The dark
crude mass was recrystallized 4 times from a dichloromethane-
hexane mixture. The microcrystalline compound, thus obtained, was
IC0203724
(22) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.
(23) Sheldrick, G. M. SHELXL 97. Program for the refinement of crystal
structures; University of Goettingen: Goettingen, Germany, 1997.
204 Inorganic Chemistry, Vol. 42, No. 1, 2003