Chromium complexes of an isomeric N-donor ligand, 2-[(N-arylamino)phenylazo]pyridine: Amination reactions, X-ray structure, and redox properties

Article abstract of DOI:10.1021/ic0201497

The chromium chemistry of two positional isomers of the ligand 2-[(N-arylamino)phenylazo]pyridine (HL¹ and HL²) are described. While the ligand HL¹ coordinates as a bischelating tridentate N,N,N-donor, [L¹]^-, with deprotonation of the amine nitrogen, its isomer HL² coordinates as a neutral bidentate N,N-donor. The amine nitrogen in this case remains protonated. Thus the reaction of CrCl₃·nH₂O with HL¹ produced the brown cationic complex, [Cr- (L¹)₂]⁺, [1]⁺. The representative X-ray structure of [1a](ClO₄) is reported. The two azo nitrogens of the anioinc tridentate ligand approach the metal center closest with Cr(1)-N(azo) av 1.862(6) A. There is a significant degree of ligand backbone conjugation in the coordinated ligands, which resulted in shortening of the C-N distances and also in lengthening of the diazo (N=N) distances. Two synthetic approaches for the synthesis of chromium complexes of HL² are investigated. The first approach is based on the substitution reaction, wherein all the coordinated CO ligands of Cr(CO)₆ were completely substituted by the three bidentate HL² ligands to produce a violet complex [Cr(HL²)₃]. The second approach is based on para-amination reaction of coordinated 2-(phenylazo)pyridine (pap). Thus the reaction of an inert complex, [CrCl₂(pap)₂], with ArNH₂ yields a mixed ligand complex, [CrCl₂(pap)(HL²)], 3. In this reaction one of the two coordinated pap ligands in [CrCl₂(pap)₂] undergoes amination at the para carbon (with respect to the diazo function) to yield HL² in situ. This metal-promoted transformation is authenticated by the X-ray structure determination of a representative complex, [CrCl₂(pap)(HL^2a)], 3a. Notable differences in bond distances along the ligand backbones of the two coordinated ligands in 3a indicate different levels of metal-ligand overlap in this complex. All the chromium complexes of HL² are characterized by their intense blue-violet color. The frequencies of the visible range transitions in these complexes linearly correlate with the Hammett's substitution constant. Intraligand charge-transfer transitions in the visible region are believed to be responsible for the intense color. Redox properties of all these complexes are reported.

Full text of DOI:10.1021/ic0201497

Inorg. Chem. 2002, 41, 4531−4538
Chromium Complexes of an Isomeric N-Donor Ligand,
2-[(N-Arylamino)phenylazo]pyridine: Amination Reactions, X-ray
Structure, and Redox Properties
,†
Kunal K. Kamar,† Amrita Saha,† Alfonso Castin˜eiras,‡ Chen-Hsiung Hung,§ and Sreebrata Goswami*
Department of Inorganic Chemistry, Indian Association for the CultiVation of Science,
Kolkata 700 032, India, Departamento de Quimica Inorganica, UniVersidade de Santiago de
Compostela, Campus UniVersitario Sur, E-15782 Santiago de Compostela, Spain, and
Department of Chemistry, National Changhua UniVersity of Education,
Changhua, Taiwan 500, Republic of China
Received February 25, 2002
The chromium chemistry of two positional isomers of the ligand 2-[(N-arylamino)phenylazo]pyridine (HL1and HL2)
are described. While the ligand HL1 coordinates as a bischelating tridentate N,N,N-donor, [L1]-, with deprotonation
of the amine nitrogen, its isomer HL2 coordinates as a neutral bidentate N,N-donor. The amine nitrogen in this
case remains protonated. Thus the reaction of CrCl3‚nH O with HL1 produced the brown cationic complex, [Cr-
2
(L1)2]+, [1]+. The representative X-ray structure of [1a](ClO ) is reported. The two azo nitrogens of the anioinc
4
tridentate ligand approach the metal center closest with Cr(1)−N(azo) av 1.862(6) Å. There is a significant degree
of ligand backbone conjugation in the coordinated ligands, which resulted in shortening of the C−N distances and
also in lengthening of the diazo (NdN) distances. Two synthetic approaches for the synthesis of chromium complexes
of HL2 are investigated. The first approach is based on the substitution reaction, wherein all the coordinated CO
ligands of Cr(CO)6 were completely substituted by the three bidentate HL2 ligands to produce a violet complex
[Cr(HL2)3]. The second approach is based on para-amination reaction of coordinated 2-(phenylazo)pyridine (pap).
Thus the reaction of an inert complex, [CrCl2(pap)2], with ArNH yields a mixed ligand complex, [CrCl2(pap)(HL2)],
2
3. In this reaction one of the two coordinated pap ligands in [CrCl2(pap)2] undergoes amination at the para carbon
(with respect to the diazo function) to yield HL2 in situ. This metal-promoted transformation is authenticated by the
2a
X-ray structure determination of a representative complex, [CrCl2(pap)(HL )], 3a. Notable differences in bond distances
along the ligand backbones of the two coordinated ligands in 3a indicate different levels of metal−ligand overlap
in this complex. All the chromium complexes of HL2 are characterized by their intense blue-violet color. The frequencies
of the visible range transitions in these complexes linearly correlate with the Hammett’s substitution constant.
Intraligand charge-transfer transitions in the visible region are believed to be responsible for the intense color.
Redox properties of all these complexes are reported.
Introduction
ligand. Two positional isomers of this ligand, viz., HL1 and
HL2, were isolated by us by metal-promoted aromatic ring
amination reactions of the coordinated 2-(phenylazo)pyridine
ligand (pap) (Chart 1). The amination reaction is regio-
selective, occurring at ortho and para carbons in the presence
of a labile and inert metal complex mediator, respectively.
While the ortho-aminated ligand HL1 can act as a monoan-
ionic tridentate N,N,N-donor with the dissociation of the
In recent years we have been interested1-5 in the coordina-
tion chemistry of the 2-[(N-arylamino)phenylazo]pyridine
* Address correspondence to this author. E-mail: icsg@
mahendra.iacs.res.in.
† Indian Association for the Cultivation of Science.
‡ Universidade de Santiago de Compostela.
§ National Changhua University of Education.
(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.
(4) Saha, A.; Majumdar, P.; Peng, S.-M.; Goswami, S. Eur. J. Inorg.
Chem. 2000, 2631.
(3) Saha, A.; Majumdar, P.; Goswami, S. J. Chem. Soc., Dalton Trans.
2000, 1703.
(5) Das, C.; Peng. S.-M.; Lee, G.-H.; Goswami, S. New J. Chem. 2002,
26, 222.
10.1021/ic0201497 CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/23/2002
Inorganic Chemistry, Vol. 41, No. 17, 2002 4531

Kamar et al.
Chart 1
Results and Discussion
Synthesis. Two synthetic approaches were explored for
the preparation of the chromium complexes. The first one
was based on the reaction of preformed HL1 and HL2 ligands
with the suitable starting chromium salt or its complex. We
also have examined the amination reaction of coordinated
2-(phenylazo)pyridine in the two chromium complexes, viz.,
[CrCl2(pap)2] and [Cr(pap)3](ClO4), respectively. The above
compounds were reported9,11 and were shown to be substi-
tutionally inert.
The complex [Cr(L1)2](ClO4), [1](ClO4), was obtained by
heating chromium(III) chloride and the deprotonated ligand
[L1]- in a 1:2 molar proportion in absolute ethanol. The
reaction mixture became redish brown in 2 h. The crude
mass, so formed, was a mixture of a major red brown and
several minor byproducts of different colors, which were
observed on a TLC plate. The cationic complex, [Cr(L1)2]+,
[1]+, was purified by using the preparative TLC technique
and was finally isolated as its perchlorate salt in a moderate
yield. The rest of the minor bands on the TLC plate were
overlapping and we have not been successful in isolating
them in the pure state. The reaction of HL2 with Cr(CO)6
led to complete substiution10 of CO ligands from Cr(CO)6
to produce the violet compound [Cr(HL2)3], 2 in 30% yield.
Compound 2 was purified by rapid crystallization. The
solution is not very stable and dissociates HL2. The yields
of the products from the above reactions are moderate to
low. The several byproducts along with [1]+ may have
originated from the partial substitution of the inert CrCl3 or
be due to the deamination reaction of HL1. The formation
of multiple products from the reaction of [OsBr6]2- and HL1
was noted recently by us.5 However, the major product from
each of the above reactions was thoroughly purified either
on a preparative TLC plate or by repeated crystallization.
All these compounds gave consistent elemental analysis.
Notably, while the ligand HL1 coordinates as a bischelating
tridentate ligand with deprotonation of the amine nitrogen,
its isomer HL2 coordinates as a neutral bidentate N,N-donor.
The amine nitrogen in this case remains protonated.
To examine the amination reaction of coordinated 2-(phen-
ylazo)pyridine using a chromium complex mediator, we
chose the dichloro complex [CrCl2(pap)2] as the reactant.
Since the chosen complex is known to be substitutionally
inert, amination at the para carbon of the pendant phenyl
ring was anticipated.1 In line with our strategy the brown
[CrCl2(pap)2] complex was reacted with neat ArNH2 in air,
which produced an intense blue mixture in 4 h. The usual
workup followed by chromatographic purification of the
mixture yielded crystalline complex [CrCl2(pap)(HL2)] in
30-40% yield. In this reaction one of the two coordinated
pap ligands has undergone amination at the para carbon of
the pendant phenyl group of a coordinated pap ligand.
Notably, the amination reaction is regioselective and has
occurred only at the para position. This is not unexpected,
since it has already been shown that the availibility of vacant
amine proton, the para-aminated ligand, HL2, coordinates
as a neutral N,N-donor. The deprotonated anionic ligand
[L1]- has a unique combination of a strong acceptor6
azopyridine group (soft) and a strong electron donor7 amido
function (hard), which act in concert in the stabilization of
variable valence states of metal ions in the complexes. Thus
the iron complex [Fe(L1)2]n+ was isolated4 both as a cationic
[FeIII(L1)2]+ and a molecular [FeII(L1)2] complex. Moreover,
this ligand is known to stabilize uncommon low-spin states3,4
5
6
of metal ions, some examples being Mn(II) (t2 ), Fe(II) (t2 ),
5
and Fe(III) (t2 ). Extensive charge delocalization of [L1]-
along the ligand backbone in its complexes was also noted.
The neutral bidentate ligand, HL2, on the other hand, uses1
the pyridine(N) and the diazo(N) for coordination, the amine-
(N) remaining protonated and uncoordinated, which may act
as a potential donor in a charge-transfer transition. Notably,
the azopyridine function6 of HL2 is known to be an acceptor
chromophoresa property that is expected to be augmented
upon coordination to a metal ion (Lewis acid). So, intraligand
charge transfer in the metal complexes of HL2 may be
observed in the low-energy visible region. We wish to note
here that complexes of such types of ligands having both
donor and acceptor chromophores are of recent interest in
the context of synthesis of octupolar molecules8 for the study
of nonlinear optical properties.
Herein we describe the synthesis and properties of a series
of chromium(II) and chromium(III) complexes of the iso-
meric 2-[(N-arylamino)phenylazo]pyridine ligands. Optical
properties of these are markedly different than those of the
parent chromium-pap complexes,9-11 which are highlighted
in this paper.
(6) Goswami, S.; Mukherjee, R.; Chakravorty, A. Inorg. Chem. 1983, 22,
2825.
(7) (a) Bryndza, H. E.; Tam, W. Chem. ReV. 1988, 88, 1163. (b) Fryzuk,
M. D.; Montgomery, C. D. Coord. Chem. ReV. 1989, 95, 1.
(8) Bozec, H. L.; Renouard, T. Eur. J. Inorg. Chem. 2000, 229.
(9) Ferreira, V.; Krause, R. A. Inorg. Chim. Acta 1988, 145, 29.
(10) 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.
(11) Kharmawphlang, W.; Choudhury, S.; Deb, A. K.; Goswami, S. Inorg.
Chem. 1995, 34, 3828.
4532 Inorganic Chemistry, Vol. 41, No. 17, 2002

Cr Complexes of 2-[(N-Arylamino)phenylazo]pyridine
Scheme 1. Synthesis of Chromium Complexes
or labile coordination site(s) at the metal center of the
mediator complex is an essential prerequisite for the ortho-
amination reaction. In the present case there was no
dissociation of coordinated ligand(s) during the course of
the reaction and hence amination occurred only at a para
position. Moreover, we wish to note here that the amination
reaction at [CrCl2(pap)2] is sluggish and occurs only partially
(at one pap out of two). By contrast, similar type of
reactions1,4 with cationic mediator complexes such as [Co-
(pap)3]2+ and [Fe(pap)3]2+ are more facile and complete.
Cationic complexes are more electrophilic and appear to be
better mediators for the reaction. This proposal was further
strengthened12 by the fact that the molecular complex [RuCl2-
(pap)2] is totally unreactive to ArNH2, whereas the trischelate,
[Ru(pap)3](ClO4)2, is susceptible to amination. A similar
amination reaction with the trischelate [Cr(pap)3](ClO4) as
the mediator complex and aniline as the reagent also
produced a blue-violet mixture. Unfortunately, the crude
product in this case could not be purified. This decomposes
rapidly in solution and gives inconsistent analysis. The
organic ligand that was isolated from the crude mixture was
charaterized and found4 to be the para-aminated ligand HL2a.
The synthetic reactions are summarized in Scheme 1. The
above amination reactions occur only in neat aromatic amines
and in the presence of air. The reaction conditions are not
favorable for a mechanistic study. However, the site selectiv-
ity of this reaction depending on the nature of the mediator
complex has been established beyond doubt.
X-ray Structure. Figures 1 and 2 show the ORTEP and
atom numbering schemes for [Cr(L1a)2](ClO4) and [CrCl2-
(pap)(HL2a)], respectively. The structural analysis of cationic
[Cr(L1a)2]+ reveals the presence of two ligands, each of which
acts as a N,N,N-tridentate donor with deprotonation of the
amine nitrogens, viz. N(1) and N(5). The relative orientations
within the pairs of identical coordinating N-atoms of the two
ligands are cis, trans, and cis in the sequence of pyridyl-N,
azo-N, and amido-N atoms. The chelate bite angles (N)py-
Cr(1)-N(azo), N(4)-Cr(1)-N(2), and N(8)-Cr(1)-N(6)
[av 79.7(3)°] are smaller than the N(azo)-Cr(1)-N(amide)
bite angles [av 81.6(3)°]. The two azo nitrogen atoms of the
anionic tridentate ligands approach the metal center more
closely [Cr(1)-N(2)/(6), av 1.862(6) Å] than the other four
Figure 1. Molecular structure and atom numbering scheme for [Cr(L1a)2]-
(ClO4).
Figure 2. Molecular structure and atom numbering scheme for [CrCl2-
(pap)(HL2a)].
Cr(1)-N bonds. There is an indication of significant
backbone conjugation in the coordinated anionic ligand. The
ligand HL1 is a low-melting solid, and its X-ray quality
crystals could not be developed so far. However, the X-ray
structure of its isomer HL2 is reported.4 The average N-N
azo distances [1.301(7) Å] in this complex are longer than
that in HL2a [1.264(2) Å]. Moreover, the average N(azo)-
C(phenyl) distance [1.384(8) Å] is shorter than that observed
for the corresponding distance in HL.2a The amido nitrogen
atom of [L2a]- binds to the phenyl group of pap at a shorter
distance [e.g. C(7)-N(1), 1.341(9) Å] than that observed in
HL2a [1.405(2) Å]. These are all in agreement with the
proposal of strong electron delocalization along the ligand
backbone in the coordinated [L1]- anion. Similar effects were
noted1,3,4 in other complexes of [L1]- involving other 3d-
metal ions.
The structural analysis of 3a indeed authenticated the
fusion of ArNH2 to one of the coordinated pap ligands of
[CrCl2(pap)2]. In this complex, the chromium center is
surrounded by a distorted octahedral coordination environ-
(12) Das, C.; Saha, A.; Goswami, S. Unpublished results.
Inorganic Chemistry, Vol. 41, No. 17, 2002 4533

Kamar et al.
Table 1. Selected Bond Lengths and Bond Angles of [1a](ClO4) and
3a
ment of two chloride ligands, the bidentate chelating ligand,
pap, and the extended neutral ligand, HL2a, which contains
N(31), N(33), and N(34). The extended ligand has the aniline
fragment fused to the phenyl ring of coordinated pap at the
para position (relative to the azo fragment). This ligand is a
bidentate and chelates to chromium through the pyridine
nitrogen N(31) and the azo nitrogen N(33). The amine
nitrogen N(34) bears a hydrogen atom, which is in marked
opposition to the coordination mode of the corresponding
ortho isomer [L1]-. The ligand [L1]- binds as an anionic
tridentate N,N,N-donor as discussed before. The compound
has the cis moiety. Isomer description of this may be
described by considering the relative positions in the pair
N(py), N(py) and in the pair N(azo), N(azo) taken in that
order. Thus the geometry of complex 3a is trans, cis, which
is identical9 with that of the starting dihalo [CrCl2(pap)2].
Hence the amination reaction is stereoretentive.
[1a](ClO4)
3a
bond lengths (Å)
Cr(1)-N(2)
1.853(6)
Cr(1)-N(11)
Cr(1)-N(13)
Cr(1)-N(31)
Cr(1)-N(33)
Cr(1)-Cl(1)
Cr(1)-Cl(2)
N(12)-N(13)
N(32)-N(33)
C(15)-N(12)
C(35)-N(32)
C(36)-N(33)
C(16)-N(13)
2.018(4)
1.971(4)
2.065(4)
2.084(4)
2.3285(16)
2.3252(17)
1.326(5)
1.288(5)
1.372(6)
1.392(6)
1.401(6)
1.427(6)
Cr(1)-N(6)
Cr(1)-N(1)
Cr(1)-N(4)
Cr(1)-N(5)
Cr(1)-N(8)
N(8)-C(30)
N(7)-C(30)
N(6)-N(7)
N(6)-C(29)
C(29)-C(24)
N(5)-C(24)
N(5)-C(18)
1.872(6)
1.908(6)
1.928(6)
1.929(5)
1.948(6)
1.369(9)
1.355(9)
1.296(7)
1.386(8)
1.406(9)
1.371(8)
1.432(8)
bond angles (deg)
N(2)-Cr(1)-N(1)
N(6)-Cr(1)-N(5)
N(2)-Cr(1)-N(4)
N(6)-Cr(1)-N(8)
82.5(3)
80.7(3)
80.3(3)
79.1(3)
N(11)-Cr(1)-N(13)
N(31)-Cr(1)-N(33)
Cl(1)-Cr(1)-Cl(2)
C(39)-N(34)-C(42)
76.89(17)
75.74(16)
92.55(6)
129.8(5)
The chelate bite angle of pap, N(11)-Cr(1)-N(13), is
76.87(18)°, while the corresponding N(31)-Cr(1)-N(33) in
the extended ligand is 75.72(17)°. However, notable differ-
ences in bond distances along the ligand backbones of the
two coordinated ligands indicate different levels of metal-
ligand overlap in this complex. In the complexes of pap
ligands, N-N lengths has been used13,14 as a marker for the
assessment of the degree of metal(dπ)-azo(pπ) interactions.
In the structure of 3a, the N-N distance in the pap ligand,
N(12)-N(13), is 1.325(5) Å, while the corresponding
distance in the extended ligand HL2a, N(32)-N(33), is
considerably shorter, 1.287(6) Å. Moreover, the Cr(1)-N(13)
length [1.972(4) Å] is much shorter than the corresponding
Cr(1)-N(33) distance [2.084(4) Å]. In fact, the chromium-
azo bond length in the extended ligand is longer even than
the chromium-pyridyl bond lengths in the reference com-
pound. This effect may be explained by considering the
extended ligand as a p-N(H)Ph-sustituted pap ligand. The
strong electron-donating effect of the substitution is respon-
sible for the shortening of the N(34)-C(39) length
[1.371(7) Å] as compared to the corresponding N(34)-C(42)
length [1.406(7) Å]. The presence of the strongly donating
-N(H)Ph substitution at the phenyl ring makes the ligand
HL2a a weaker acceptor than the unsubstituted pap ligand.
Optical Spectra. The ability of the azo ligands, under
consideration, to act as π-acceptor toward low-valent metal
ions is documented in the literature. A diagnostic test for
the existence of the metal(dπ)-ligand(pπ) interaction is the
lengthening of N-N lengths in their metal complexes and
also the shift of νNdN to lower frequencies13,14 in moving
from the free ligands to their complexes. Notably, the νNdN
in pap, HL1, and HL2 appear at 1425, 1380, and 1390 cm-1,
respectively. These value in the complexes are appreciably
lowered (Table 2).
The solution spectral data of the complexes are collected
in Table 2. Unlike other chromium complexes11 of N-donor
ligands, the parent complexes in solutions exhibit intense
color. For example, the color of the Cr(III) complex,
[Cr(L1)2]+, is red-brown and is characterized by highly
intense visible range transitions. As noted before4 the
molecular orbitals that are involved in the low-energy
transitions of the metal complexes of [L1]- are highly
delocalized. Strong delocalization in the present chromium-
(III) complex is also obvious from its crystallographic data.
Intense low-energy transitions may thus be due to π-π*
transitions. Two more weak transitions in the near-IR region
of these chromium(III) complexes were also noted. The color
of the chromium complexes containing the HL2 ligand,
[CrCl2(pap)(HL2)], is intense blue-violet in dichloromethane.
Notably, the color of the corresponding complex, [CrCl2-
(pap)2], is brown9 in the same solvent. The spectra of the
above two dichloro complexes are displayed for comparison
(Figure 4). The additional highly intense visible range
transition near 565 nm (ꢀ, 20800 mol-1cm-1) in 3a is no
doubt responsible for the intense solution color of these
compounds. Similarly, the molecular complex [Cr(HL2a)3],
2a also showed an intense transition at 560 nm, the intensity
of which is much higher than that for the 565 nm transition
in 3. Moreover, the transition in 3 is sensitive to the nature
of substitution (R) on the aryl ring of HL2. For example, the
transition in 3 shifts red from 555 to 580 nm on changing
the para substitution from -Cl to -OCH3. The frequencies
of this transition (ν, cm-1) for 3a-d linearly correlate with
Hammett’s constant (σ).15 We wish to note here that E1/2
values for the Cr(III)/Cr(II) couple in the said complexes
are less sensitive to the nature of substitution R (vide infra).
Thus, the substitution on the aryl ring of HL2 in 3 has a
more pronounced effect on the visible range transition
energies than on the metal redox potentials. These observa-
tions, in other words, suggest that the orbitals that are
(13) (a) Ghosh, B. K.; Mukhopadhyay, A.; Goswami, S.; Ray, S.;
Chakravorty, A. Inorg. Chem. 1984, 23, 4633. (b) Seal, A.; Ray, S.
Acta Crystallogr. 1984, 40C, 929. (c) Majmudar, P.; Kamar, K. K.;
Castin˜eiras, A.; Goswami, S. Chem. Commun. 2001, 1292.
(14) (a) Krause, R. A.; Krause, K. Inorg. Chem. 1980, 19, 2600. (b) Krause,
R. A.; Krause, K. Inorg. Chem. 1982, 21, 1714. (c) Wolfgang, S.;
Strekas, T. C.; Gafney, H. D.; Krause, R. A.; Krause, K. Inorg. Chem.
1984, 23, 2650.
(15) (a) Hammett, L. P. Physical Organic Chemnistry, 2nd ed.; McGraw-
Hill: New York, 1970. (b) Mukherjee, R. N.; Rajan, O. A.;
Chakravorty, A. Inorg. Chem. 1982, 21, 785.
4534 Inorganic Chemistry, Vol. 41, No. 17, 2002

Cr Complexes of 2-[(N-Arylamino)phenylazo]pyridine
Table 2. Optical Spectral Data
IR(KBr) (υ, cm-1
υ(NdN)
)
compd
absorptiona λmax/nm (ꢀ/M-1 cm-1
)
υ(CdN)
υ(ClO4)
[Cr(L1a)2](ClO4)
1100b (980), 850b (5740), 750 (7900) 570b (9220), 510 (13640),
360b (17660)
1595
1305, 1250
1090, 620
[Cr(L1a)2]
1460b (950), 1250b (945), 860b (3000), 740b (8080), 680b (7690),
550 (12810), 480b (9370), 340b (16360)
1595
1600
1595
1310, 1265
1300, 1240
1310, 1265
[Cr(L1b)2](ClO4)
[Cr(L1b)2]
1130b (780), 880b (2810), 750 (6720), 570b (7810), 500 (11250),
360b (16250)
1090, 615
1460b (1000), 1250b (1020), 870b (3180), 740b (8550), 680b (8100),
550 (13500), 480b (10690), 340b (18500)
[Cr(HL2a)3]
850b (2630), 560 (38680), 330b (27370)
1565
1600
1585
1355, 1315
1340
1305, 1220
[Cr(pap)3]c
800b (1320), 640b (3680), 470 (13950), 330 (32110)
1415 (5315), 750b (2635), 565 (20800), 335b (17325), 275b (19930)
1010 (2470), 670b, 605 (24330), 420b, 340b
[CrCl2(pap)(HL2a)]
[CrCl2(pap)(HL2a)]+,d
[CrCl2(pap)(HL2a)]-,e
[CrCl2(pap)(HL2b)]
[CrCl2(pap)(HL2c)]
[CrCl2(pap)(HL2d)]
970 (1480), 765b, 665b, 605b, 535b, 465 (13830), 380b, 350 (20870), 305b
1415 (2485), 745b (2965), 575 (29900), 340b (26640), 265b (31970)
1400 (3105), 755b (4000), 580 (36370), 300b (37155), 255b (46480)
1425 (4370), 745b (2295), 555 (30290), 285b (39150), 245b (47535)
1590
1595
1580
1305, 1220
1295, 1220
1300, 1215
a Solvent CH3CN. b Shoulder. c Reference 10. d Generated by electrolysis at 0.50 V. e Generated by electrolysis at -0.50 V.
Figure 3. Solution electronic spectrum of [Cr(L1a)2](ClO4).
involved in the above transition in the chromium complexes
3a-d are predominantly ligand in character [N(amine) f
(azo) transition]. Furthermore, the transition, under consid-
eration, is appreciably solvatochromic. The solution colors
of 3 in hydroxilic solvents are blue while those in other
solvents are blue-violet. Complex 3 also showed a far red
band near 1415 nm, which is assigned9 to the ligand-to-metal
(LMCT) charge-transfer transition.
Redox Behavior. The electrochemical behavior of the
chromium complexes was probed in acetonitrile solution by
cyclic volammetry. Voltammetric data are collected in Table
3 and representative volammograms are displayed in Figures
5 and 6.
(a) [CrIII(L1)2](ClO4), [1](ClO4): The chromium(III)
complexes of the anionic ligand [L1]-, [Cr(L1)2](ClO4),
showed two anodic and two cathodic responses. The lowest
potential cathodic response is reversible and occurs near 0.0
V. This has been assigned to the Cr(III)/Cr(II) couple. The
anodic wave at 1.0 V is electrochemically reversible, which
follows another irreversible anodic wave at ca. 1.7 V. These
two anodic responses are attributed4,5 to ligand oxidations.
The redox process near -1.0 V showed a much larger current
height as compared to other one-electron-transfer processes.
This is attributed4 to reduction of the coordinated ligands.
A few ill-defined cathodic responses with small current
height appeared when the cathodic scan was allowed beyond
-1.2 V. These are due to electrogenerated species that are
Figure 4. Solution spectra of (a) [CrCl2(pap)2] (-), [CrCl2(pap)(HL2a)]
(‚‚‚) and (b) [Cr(pap)3] (-), [Cr(HL2a)3] (‚‚‚).
formed following the ligand reductions. Similar multiple
electron-transfer processes were also observed3,4 in the other
complexes of the anionic ligand [L1]-. Notably the observed
potentials for [1b]+ are cathodic by ca. 100 mV compared
to those of [1a]+, which is attributed to the effect of the
electron-donating methyl substituent in the ligand. Such low
potentials for the Cr(III)/Cr(II) couple in [1]+ persuaded us
to try and isolate the corresponding chromus complex, 1, in
the pure state. Fortunately, with hydrazine as the reducing
agent, we were able to isolate a representative chromium-
Inorganic Chemistry, Vol. 41, No. 17, 2002 4535

Kamar et al.
Table 3. Cyclic Voltammetric Dataa
as compared to those in [Cr(pap)3]. This may be attributed
to the strong inductive effect of p-NH(Ar) substitution in 2.
Similarly, the anodic response in 2 is more cathodic than
the corresponding two anodic responses in [Cr(pap)3]. The
E1/2 values of the [Cr(HL2)3] and [Cr(pap)3] complexes are
collected in Table 3 for comparison.
compd
oxidation E1/2(V)
reduction E1/2(V)
[Cr(L1a)2](ClO4)
[Cr(L1b)2](ClO4)
[Cr(HL2a)3]
0.96, 1.69b
0.70, 1.29b
0.31, 0.70
0.37, 1.33b
0.36, 1.25b
0.34, 1.10b
0.37, 1.40b
0.50, 1.05
0.05, -1.19
-0.19, -1.08c
-0.31, -1.11, -1.42d
-0.35, -1.12d
[CrCl2(pap)(HL2a)]
[CrCl2(pap)(HL2b)]
[CrCl2(pap)(HL2c)]
[CrCl2(pap)(HL2d)]
[Cr(pap)3]e
-0.37, -1.14d
(c) [CrCl2(pap)(HL2)], 3: Successive electron-transfer
processes are the main features for the chromium dihalo
complxes [CrCl2(pap)(HL2)], 3. Two reversible responses,
one anodic (ca. 0.35 V) and another cathodic (ca. -0.35 V),
are assigned11 due to Cr(II)/Cr(III) and Cr(II)/Cr(I) couples,
respectively. The responses for the corresponding couples9
in the parent complex, [CrCl2(pap)2], appeared at ca. 0.42
and ca. -0.30 V, respectively. Cathodic shift of these
potentials in moving from [CrCl2(pap)2] to [CrCl2(pap)(HL2)]
is primarily due to the inductive effect of the p-NH(Ar) group
in HL2. However, the effect of substitution on the ligand
HL2 has only an insignificant effect on the potentials of these
responses. In addition, an anodic (irreversible) wave at >1.30
V was also observable in 3, which, by contrast, is absent in
the [CrCl2(pap)2]. This anodic wave in 3 may thus be
assigned to ligand oxidation. An ill-defined ligand reduction
wave was observed in 3 at potentials > -1.0 V. It may be
noted here that electrochemical reductions16 of metal dihalo
complexes of 2-(arylazo)pyridine are generally ill-behaved
due to halide dissociation.
-0.38, -1.10d
-0.33, -1.15d
-0.20, -1.10, -1.48, -1.70d
a Experiments were carried out in CH3CN at 298 K with TEAP as
supporting electrolyte. The reported data correspond to a scan rate of 50
mV s-1 b Irreversible anodic response; the potential corresponds to Epa.
.
c Quasireversible cathodic response; the potential corresponds to Epc.
d Irreversible cathodic response; the potential corresponds to Epc. e Reference
10.
Figure 5. Segmented cyclic voltammogram of [Cr(L1a)2](ClO4).
To gain further insight into the nature of the strong visible
range transition in 3, we examined the complex 3a spectro-
electrochemically in acetonitrile. As the compound was
oxidized at +0.50 V, the low-energy IR band (1415 nm)
disappeared and a new broad band with much diminished
intensity appeared at 1010 nm. Interestingly the strongest
visible range transition at 565 nm in the original complex
3a is red shifted to 605 nm in [3a]+ and increased intensity
consistent with its assignment as an intraligand charge-
transfer process. The visible range transitions in the reduced
complex [3a]-, which were associated with three shoulders
at 535, 605 and 665 nm, shifted blue (465 nm) and with
much reduced intensity.
Figure 6. Segmented cyclic voltammogram of [CrCl2(pap)(HL2a)].
(II) complex, 1a (violet), in almost quantitative yield. The
compound is thermally stable but air sensitive, and is slowly
reoxidized to [1a]+ on prolong exposure to air. In solution,
complex 1a is a nonelectrolyte. It showed multiple charge
transfer in the visible region (Table 2). Under identical
conditions, the voltammogram of 1a (initial scan anodic) is
superposable on that of the corresponding [1a]+ (initial scan
cathodic) showing that the redox process, [Cr(L1)2]+
h
Conclusion
[Cr(L1)2], is chemically reversible. Upon exhaustive constant
potential oxidation of the complex, 1a at +0.25 V, the
coulomb count corresponds to one electron, the oxidized
solution is brown, and its absorption spectrum matches
quantitatively with that of [1a]+.
(b) [Cr(HL2)3], 2: The trischelate [Cr(HL2)3], 2 displayed
multiple CV responses in the potential range +1.5 to -1.5
V. Two of these are reversible anodic. There are three
cathodic responses, of which the response at the lowest
potential was irreversible. The cathodic responses are as-
signed to ligand reductions. The LUMO of the ligand, HL2,
like the parent pap ligand, can accommodate6 up to two
electrons. Therefore, successive six-electron reductions in
the tris-chelate may be expected in [Cr(HL2)3]. However,
three successive one-electron-transfer processes were ob-
served in our experimental conditions. Notably, the ligand
reductions in the tris-chelate 2 were systematically cathodic10
Two interesting classes of new chromium complexes with
two positional isomers of 2-[(N-arylamino)phenylazo]pyri-
dine have been synthesized successfully. The presence of
the combination of hard and soft binding sites in the
deprotonated [L1]- makes them excellent ligands for stabiliz-
ing the metal ion in two different oxidation states. The
coordination mode of the other isomer HL2 is totally
different. This acts as a neutral, strong π-acceptor bidentate
ligand. Interestingly, the ligand HL2 bears an uncoordinated
donor amine function, which is separated from a coordinated
acceptor azo function by a conjugated spacer. Strong
intraligand charge-transfer transitions in the complexes of
HL2 are noted. Our work on the exploration of the coordina-
tion chemistry of HL2 is in progress. Indications are strong
(16) Goswami, S.; Chakravarty, A. R.; Chakravorty, A. Inorg. Chem. 1981,
20, 2246.
4536 Inorganic Chemistry, Vol. 41, No. 17, 2002

Cr Complexes of 2-[(N-Arylamino)phenylazo]pyridine
Table 4. Crystallographic Data of the Compounds [1a](ClO4) and 3a
that some of these might turn out to be complexes with
nonlinear optical properties.
[1a](ClO4)
3a
empirical formula
molecular mass
temp [K]
cryst syst
space group
a [Å]
C34H26ClCrN8O4
698.08
293(2)
triclinic
P1h
9.1815(12)
10.3470(14)
17.220(2)
103.975(3)
91.119(3)
93.829(3)
1582.9(4)
2
C28H23Cl2CrN7
580.43
213(2)
monoclinic
C2/c
28.666(2)
11.8843(3)
17.5429(10)
90
107.729(4)
90
5692.6(5)
8
Experimental Section
The starting complexes9,11 [CrCl2(pap)2], [Cr(pap)3], and
[Cr(pap)3](ClO4) and the ligands3,4 HL1 and HL2 were prepared by
the reported method. Solvents and chemicals used for synthesis
were of analytical grade. The supporting electrolyte tetraethylam-
monium perchlorate and solvents for electrochemical work were
obtained as before.5 Perchlorate salts of metal complexes are
generally explosive. Although no detonation tendencies have been
observed, care is advised and handling of only small quantities
recommended.
b [Å]
c [Å]
R [deg]
â [deg]
γ [deg]
V [Å3]
Z
D
calc. [Mg/m3]
1.465
1.355
cryst dimen [mm3]
θ range for data
collection [deg]
GOF
0.56 × 0.13 × 0.07
0.20 × 0.15 × 0.10
Physical Measurements. A JASCO V-570 spectrometer was
used to record electronic spectra. The IR spectra were obtained
with a Perkin-Elmer 783 spectrophotometer. H NMR spectra in
1.22-27.54
5.11-64.98
1
1.022
0.957
CDCl3 were recorded on a Bruker Avance DPX 300 spctropho-
tometer with SiMe4 as internal standard. A Perkin-Elmer 240C
elemental analyzer was used to collect microanalytical data (C,H,N).
Electrochemical measurements performed under a dry nitrogen
atmosphere on a PAR model 370-4 electrochemistry system in this
work are referenced to the saturated calomel electrode (SCE) and
are uncorrected for junction contribution. The value for the
ferrocenium-ferrocene couple under our conditions is 0.40 V.
Magnetic moment measurements were carried out with a PAR 155
vibrating sample magnetometer fitted with a Walker Scientific
L75FBAL magnet. Electrical conductivity was measured by using
a Systronics Direct Reading Conductivity meter 304.
[Cr(L1a)2](ClO4), [1a](ClO4): The ligand HL1a (0.1 g, 0.36
mmol) was dissolved in 25 mL of absolute ethanol and 1-2 drops
of trimethylamine was added. To this deprotonated ligand solution
was added a methanolic solution of CrCl3‚6H2O (0.05, 0.18 mmol)
and the mixture was refluxed in a water bath for 2 h. The color of
the solution changed from orange yellow to reddish brown. The
crude product was then purified by preparative TLC. The major
reddish-brown part was separated with use of 4:1 CHCl3-CH3CN
as the eluant. The reddish-brown band was collected and 5 mL of
a dilute aqueous solution of NaClO4 was added to it. The dark
brown compound thus obtained was recrystallized from a dichlo-
romethane-hexane solvent mixture. Yield: 50%. Anal. Calcd for
C34H26N8ClO4Cr: C, 58.49; H, 3.73; N, 16.06. Found: C, 59.02;
H, 4.31; N, 16.43. Λm: 140 Ω-1 cm2 mol-1 (1 × 10-3 M in CH3-
CN). µeff(300 K): 3.80 µB. Similarly, CrCl3‚6H2O was reacted with
HL1b in a 1:2 ratio in methanolic solution in the presence of 1-2
drops of trimethylamine as a base to produce the complex [Cr-
(L1b)2](ClO4), [1b](ClO4). Yield: 45%. Anal. Calcd for C36H30N8
ClO4Cr: C, 59.55; H, 4.14; N, 15.44. Found: C, 59.87; H, 4.43;
N, 16.06. Λm: 135 Ω-1 cm2 mol-1 (1 × 10-3 M in CH3CN). µeff-
(300 K): 3.75 µB.
[Cr(HL2a)3], 2a: The ligand HL1 (0.1 g, 0.36 mmol) and
Cr(CO)6 (0.027 g, 0.12 mmol) were added to 15 mL of n-octane.
It was refluxed for 6 h. A dark compound was precipitated and the
color of the solution became yellowish-blue. The mixture was
cooled and the precipitate was collected by filtration. It was then
recrystallized from a dichloromathane-hexane solvent mixture.
Yield: 30%. Anal. Calcd for C51H42N12Cr: C, 70.02; H, 4.46; N,
19.22. Found: C, 69.76; H, 4.90; N, 19.45. µeff(300 K): diamag-
netic.
[CrCl2(pap)(HL2a)], 3a: A mixture of [CrCl2(pap)2] (0.1 g,
0.204 mmol) and aniline (0.5 mL) was heated neat on a steam bath
for 4 h. The cooled mixture was thoroughly washed with diethyl
ether. The blue-violet crude product was then purified on a
wavelengths [Å]
reflns collected
unique reflns
0.71073
10266
7069
1.54184
5716
4700
largest diff between
0.599, -0.462
0.366, -0.509
peak and hole [e Å-3
]
final R indices [I > 2σ(I)]
R1 ) 0.0569
wR2 ) 0.1002
R1 ) 0.0610
wR2 ) 0.1415
preparative silica gel TLC plate and a violet band was eluted with
chloroform as the eluant. Finally it was crystallized from a
dichloromethane-hexane solvent mixture. Yield: 30%. Anal. Calcd
for C28H23N7Cl2Cr: C, 57.93; H, 3.97; N, 16.89. Found: C, 57.10;
H, 4.15; N, 16.35. µeff(300 K): 2.83 µB. The other chromium
complexes were synthesized similarly. Their yields and analytical
data are collected below.
[CrCl2(pap)(HL2b)], 3b: Yield: 35%. Anal. Calcd for C29H25N7-
Cl2Cr: C, 58.58; H, 4.21; N, 16.50. Found: C, 58.84; H, 4.45; N,
16.77. µeff(300 K): 2.77 µB.
[CrCl2(pap)(HL2c)], 3c: Yield: 30%. Anal. Calcd for C29H25N7-
Cl2OCr: C, 57.05; H, 4.10; N, 16.07. Found: C, 58.40; H, 4.81;
N, 16.45. µeff(300 K): 2.72 µB.
[CrCl2(pap) (HL2d)], 3d: Yield: 35%. Anal. Calcd for C28H22N7-
Cl3Cr: C, 54.68; H, 3.58; N, 15.94. Found: C, 55.14; H, 3.51; N,
15.14. µeff(300 K): 2.86 µB.
Chemical reduction of [1]+ f [1]. A 25-mL dichloromethane
solution of [1a](ClO4) (0.10 g, 1.43 mmol) was stirred with 5 mL
of dilute aqueous hydrazine solution at room temperature for 30
min. The color of the solution changed from reddish brown to violet.
The above mixture was extracted with dichloromethane (2 × 10
mL), which was then dried with anhydrous sodium sulfate. The
dried solution was concentrated to half of the initial volume. Slow
addition of hexane to the above violet solution precipitated dark
microcrystals of 1a. This was further crystallized from a dichlo-
romethane-hexane solvent mixture. Yield: 95%. Anal. Calcd for
C34H26N8Cr: C, 68.23; H, 4.35; N, 18.73. Found: C, 68.44; H,
4.52; N, 18.87. Similarly, [1b](ClO4) was treated with dilute
hydrazine in dichloromethane at room temperature to produce
crystalline [Cr(L1b)2], 1b. Yield: 90%. Anal. Calcd for C36H30N8-
Cr: C, 69.00; H, 4.79; N, 17.89. Found: C, 68.87; H, 4.59; N,
18.02.
X-ray Structure Determination. The crystal data of [1a](ClO4)
and 3a are collected in Table 4.
[Cr(L1a)2](ClO4), [1a](ClO4): X-ray quality crystals (0.56 × 0.13
× 0.07 mm3) of [1a](ClO4) were obtained by slow diffusion of a
dichlromethane solution of the complex into hexane. The data were
collected on a Bruker SMART diffractometer equipped with
graphite-monochromated Mo KR radiation (λ ) 0.71073 Å). Data
Inorganic Chemistry, Vol. 41, No. 17, 2002 4537

Kamar et al.
were corrected for Lorentz-polarization effects. A total of 10266
reflections were collected, of which 7069 were unique (Rint
0.0341). The structure was solved by employing the SHELXS 97
program package17 and refined by full-matrix least squares based
on F2 (SHEXL 97).18
matrix least-squares procedure with use of anisotropic displacement
parameters18 on F2, using SHELXL 97. All the hydrogen atoms
were located in their calculated positions (CH 0.93-0.97 Å) and
were refined with use of a riding model.
)
Acknowledgment. We thank the Council of Scientific
and Industrial Research and the Department of Science and
Technology, New Delhi for financial support. The authors
are indebted to Professor M. D. Ward and Dr. R. L. Paul
for the spectroelectrochemical measurements on one of the
compounds.
[CrCl2(pap)(HL2a)], 3a: X-ray quality crystals (0.20 × 0.15 ×
0.10 mm3) of 3a were obtained by slow diffusion of a dichloro-
methane solution of the complex into hexane. Cell parameters were
obtained by least-squares fits of 25 machine-centered reflections.
Data were collected (30.537° < θ < 49.801°) on a Enraf Nonius
CAD4 automatic diffractometer19 (213 K), using Cu KR radiation
(λ ) 1.54184 Å). Data were corrected for Lorentz-polarization
effects.20 Of the 5716 reflections collected, 4700 were unique (Rint
) 0.0598) and 4700 reflections satisfying [I >2σ(I)] were used
for structure solution by direct methods17 and refined by a full-
Supporting Information Available: X-ray crystallographic
details, in CIF format, of two compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC0201497
(17) Sheldrick, G. M. Acta Crystallogr. 1990, 46A, 467.
(18) Sheldrick, G. M. SHELXL 97, Program for the refinement of crystal
structures; University of Goettingen: Goettingen, Germany, 1997.
(19) Nonius, B. V. CAD 4-Express Software, Ver. 5.1/1.2 Enraf Nonius;
Delft, The Netherlands, 1994.
(20) Kretschmar, M. GENHKL, Program for the reduction of CAD 4
Diffreactometer data; University of Tuebingen: Tuebingen, Germany,
1997.
4538 Inorganic Chemistry, Vol. 41, No. 17, 2002