Ligand-centered redox in nickel(II) complexes of 2-(Arylazo)pyridine and isolation of 2-pyridyl-substituted triaryl hydrazines via catalytic n-arylation of AZO-function

Article abstract of DOI:10.1021/ic501656s

A series of nickel complexes of 2-(arylazo)pyridine have been synthesized, and the precise structure and stoichiometry of the complexes are controlled by the use of different metal precursors. Molecular and electronic structures of the isolated complexes are scrutinized thoroughly by various spectroscopic techniques, single crystal X-ray crystallography, and density functional theory (DFT). Two different classes of Ni(II) complexes are identified where the ligands bind as neutral or anion radicals in the respective metal complexes. These are shown to be chemically interconvertible, and their characterization confirmed that the redox series is entirely ligand-centered without affecting the bivalent oxidation state of the metal ion. An efficient method of Ni(II) catalyzed N-arylation of 2-(arylazo)pyridine substrates has been elaborated. The chemical reactions have led to isolation of strongly fluorescent 2-pyridyl-substituted hydrazine derivatives, which have been characterized thoroughly. Three-dimensional X-ray structure of a hydrazine molecule, 2-(2-(naphthalen-1-yl)-2-phenylhydrazinyl)pyridine, is reported. Isolated hydrazines satisfy all the prerequisites of an ideal dye with moderate absorptive property, large Stokes shift, high quantum yields, and high photostability.

Full text of DOI:10.1021/ic501656s

Article
pubs.acs.org/IC
Ligand-Centered Redox in Nickel(II) Complexes of
2‑(Arylazo)pyridine and Isolation of 2‑Pyridyl-Substituted Triaryl
Hydrazines via Catalytic N‑Arylation of Azo-Function
Debabrata Sengupta,† Pradip Ghosh,† Tanmay Chatterjee,‡ Harashit Datta,† Nanda D. Paul,†,§
and Sreebrata Goswami*,†
†Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
‡Department of Organic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
S
* Supporting Information
ABSTRACT: A series of nickel complexes of 2-(arylazo)-
pyridine have been synthesized, and the precise structure and
stoichiometry of the complexes are controlled by the use of
different metal precursors. Molecular and electronic structures
of the isolated complexes are scrutinized thoroughly by various
spectroscopic techniques, single crystal X-ray crystallography,
and density functional theory (DFT). Two different classes of
Ni(II) complexes are identified where the ligands bind as neutral
or anion radicals in the respective metal complexes. These are
shown to be chemically interconvertible, and their character-
ization confirmed that the redox series is entirely ligand-
centered without affecting the bivalent oxidation state of the
metal ion. An efficient method of Ni(II) catalyzed N-arylation of 2-(arylazo)pyridine substrates has been elaborated. The
chemical reactions have led to isolation of strongly fluorescent 2-pyridyl-substituted hydrazine derivatives, which have been
characterized thoroughly. Three-dimensional X-ray structure of a hydrazine molecule, 2-(2-(naphthalen-1-yl)-2-
phenylhydrazinyl)pyridine, is reported. Isolated hydrazines satisfy all the prerequisites of an ideal dye with moderate absorptive
property, large Stokes shift, high quantum yields, and high photostability.
industries.14,15 Moreover, 2-pyridyl-substituted hydrazines are
INTRODUCTION
■
of special importance because of their efficiency to act as
ligands that result in their potential applications in nuclear
medicines.16−20 Notably these are usually synthesized by Pd-
catalyzed N-arylation of diaryl hydrazines/hydrazones;21
however, only recently limited examples of Pd- or Au-complex
catalyzed coupling between activated azo compounds and aryl
boronic acids have been added to the literature.22−24 Herein we
have reported high yield catalytic N-arylation of 2-(arylazo)-
pyridine using a base metal nickel(II) salt as catalyst, and the
synthetic protocol leads to the isolation of a number of strongly
fluorescent triarylhydrazine dyes, which are unavailable in
literature.
In the upcoming sections we deal with synthesis and full
characterizations of the Ni-complexes along with their redox-
induced interconversions. Moreover, our journey to the
development of Ni(II)-catalyzed N-arylation of azo-function
in 2-(arylazo)pyridine substrate has also been taken up in detail.
Over the years, studies of the electronic structures of the
complexes have been the key point of exploration in the field of
redox noninnocent ligand chemistry.1−8 However, during
recent years the practiced philosophy has experienced a gradual
shift toward the application domain, primarily to nonconven-
tional ligand-based chemical reactions. For example, a number
of practical applications of transition-metal−ligand radical
complexes in catalytic and enzymatic processes have
emerged,9−12 and the synergy between metal and ligand
redox has been a subject of recent reviews.13
Out of our interest in coordination chemistry of redox
noninnocent azo-aromatics, herein we have explored redox
properties of a series of nickel complexes of 2-(arylazo)-
pyridines. Subsequent development of a potent Ni(II)-salt
catalyzed N-arylation of 2-(arylazo)pyridines, indeed, adds an
unique dimension to this domain. The key to the above
phenomenon apparently is associated with facile reduction of
coordinated azo-function of the ligand. To give a brief
introduction of catalytic potential of the reference catalytic
reaction, it must be appreciated that N-arylation reactions are
noteworthy in the context of synthesis of tri/tetra-aryl
hydrazines, which are high-valued chemical compounds
known for their extensive use in pharmaceutical and agricultural
Received: July 11, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic501656s | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Chart 1
Figure 1. Cyclic voltammograms of the complexes, [1a](ClO4)2 (green) and 2a (blue), in dichloromethane in different voltage windows: (a) +1.2 V
to −1.75 V and (b) +0.25 V to −0.50 V.
a,b
Table 1. Cyclic Voltammetric Data of the Complexes [1](ClO4)2 and 2
c
c
compd
oxidation E1/2, V (ΔEp, mV)
reduction E1/2, V (ΔEp, mV)
[1a](ClO4)2
−0.05(43), −1.03(70), −1.18(110), −1.43(90)
−0.04(40), −1.01(75), −1.15(105), −1.41(90)
−0.15(48), −1.00(80), −1.39(90)
[1b](ClO4)2
d
2a
1.01(130)
d
2b
1.05(125)
−0.12(47), −0.95(85), −1.36(95)
a
b
In a dichloromethane solution, supporting electrolyte Bu4NClO4 (0.1 M), and reference electrode Ag/AgCl. Solute concentration ca. 10−3 M.
c
E1/2 = 0.5(Epa + Epc), where Epa and Epc are anodic and cathodic peak potentials, respectively; ΔEp = Epa − Epc; scan rate = 50 mV s−1.
d
Quasireversible.
but is shifted cathodic by 0.1 V than that of the corresponding
tris-chelate. Their cyclic voltammograms are shown in Figure 1,
and the data are collected in Table 1.
RESULTS AND DISCUSSION
■
Chemical Reactions. The work began with an investigation
of redox properties of previously reported25 Ni2+ complexes of
2-(arylazo)pyridine ligands (La,b): [NiII(La,b)3](ClO4)2 and
NiII(La,b)2Cl2 (see Chart 1 for complex and ligand numbering
scheme).
The above results indicate that NiII−L complexes undergo
single step two-electron reductions as it happens26 in the
complex, [Ni(bpy)3]2+ (bpy = 2,2′-bipyridine). Exhaustive
electrolysis of [1a]2+ and 2a at −0.5 V led to a similar color
change of the test solutions. Their initial green colors became
brown on electrolysis. The brown solutions, generated as
above, are hyperactive and undergo fast chemical decom-
position to give a mixture of unidentified products.
Cyclic voltammetric experiments of the two Ni complexes,
[1a]2+ and 2a, were carried out in dichloromethane using
tetrabutylammonium perchlorate (TBAP) as the supporting
electrolyte, and the reported potentials are all referenced to the
Ag/AgCl electrode. The complexes, under study, displayed
multiple waves as shown in Figure 1a,b. The cationic complex
[1a]2+ showed four reversible/quasireversible waves in the
range 1.5 V at −0.05, −1.03, −1.18, and −1.43 V. Out of
these four reductive waves, ip for the first redox couple (at
−0.05 V) is significantly (∼2.7 times) higher than that of the
other three single-electron waves. Furthermore, peak-to-peak
separation (ΔEp) for the couple is in the range 40−43 mV (<59
mV), and the complex in principle can accept4d maximum of six
electrons. Exhaustive electrolysis at −0.5 V resulted in chemical
conversion, [1a]2+→ 3a + La (vide supra). The dichloro
complex 2a also exhibited three reversible reductive waves at
−0.15, −1.00, and −1.39 V, and another oxidative quasirever-
sible wave at +1.01 V was also observed. In this latter complex,
the reductive wave appearing at the least cathodic potential is
also characterized as a single step two-electron transfer process
The two aforesaid compounds [1a]2+ and 2a were then
subjected to chemical reductions in methanol using Zn metal
(powder) as reducing agent in inert conditions (in a glovebox).
In each case a brown solution resulted, and subsequent workup
(see Experimental Section) of the solution formed single
crystals, which were suitable for X-ray diffraction studies.
Characterization (see latter) of the products, obtained from the
above two reactions, indicated that these two are identical in all
respects with chemical composition: [NiII(La•−)2], 3a (La•−
=
one electron reduced azo anion radical). It may be worthwhile
here to compare this observation with the redox-induced
chemical transformation of [Ni(bpy)3]2+. It has been shown26
before that the latter complex undergoes single step two-
electron reduction to [Ni0(bpy)3], and is followed by a fast loss
of one bpy ligand to give an unstable tetracoordinated complex
[Ni0(bpy)2]. In the present case, however, there is an additional
B
dx.doi.org/10.1021/ic501656s | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
internal electron transfer process in [Ni0(La)2], one-electron
transfer from Ni(0) to each of the two coordinated ligands,
which produced the complex 3a. This is as expected since L
undergoes reduction more easily27 than bpy. On the basis of
the above results, a chemical reaction between a labile Ni0
precursor, [Ni0(COD)2] (COD = cyclooctadiene), and La in
1:2 molar proportion was planned, which proceeded as
anticipated with the formation of the same diradical complex,
3a in a high yield (90%). This result is parallel to our recent
knowledge4b−f on the chemical reactivities of L with electron-
rich metal precursors. The geometry of the complex 3a is
distorted tetrahedral, and it is diamagnetic (S = 0). Thorough
analyses of its electronic structure (see below) have revealed
that it is a Ni(II) complex with a high-spin d8 electron
configuration (SNi = 1) where the two spins on NiII are coupled
intramolecularly and antiferromagnetically to two coordinated
Characterization: X-ray Crystallography and Spec-
troscopy. As noted above, syntheses of the complexes
[1](ClO4)2 and 2 were known, but their X-ray structures and
redox properties were not available.25 In order to make
comparative studies, their complete characterizations are made
along with that of 3. All the complexes of La uniformly formed
excellent crystals which were used for their X-ray structure
determination. Crystallographic details of the three compounds,
[1a](ClO4)2, 2a, and 3a, are collected in Table 2, key bond
lengths and angles of these complexes are in Table 3, and
ORTEP images of all the complexes are in Figure 2.
Crystallographic details and the ORTEP of Ni(La)2I2 are
submitted as Supporting Information (Figure S2 and Table S1).
Figure 2 shows that the two complexes [1a]2+ and 2a have
approximate octahedral coordination about the nickel center,
but complex 3a adopts a distorted tetrahedral arrangement.
Complex [1a]2+ is isolated as a mer-isomer. Interestingly, the
unit cell of the latter complex, 3a, contains two geometrical
isomeric tetrahedral units (see Supporting Information, Scheme
S1). Such a possibility exists in distorted tetrahedral
complexes28 though their bond parameters remain more or
less similar. The two isomers A and B both possess C2
symmetry, but the interatomic distances between the
coordinated nitrogen are different: for example, N(py1)−
N(py2), 3.111 Å (isomer A) and 3.551 Å (isomer B). The bond
parameters of the isomers are tabulated in Supporting
Information (Table S3). Isomer A is energetically slightly
higher than isomer B by 0.69 kcal/mol. It is likely that two
isomers A and B easily interconvert in solution which resulted
1
ligand π* radicals L•− (SL = /2).
Another set of similar experiments using a p-chloro-
substituted ligand, Lb, also showed similar redox behavior for
[1b]2+ and 2b and produced [NiII(Lb•−)2], 3b, in high yields.
Interestingly, the aforementioned complexes are found to be
chemically interconvertible using appropriate reagents. For
example, the hexacoordinated complex [1]2+ undergoes facile
reduction to 3 by Zn metal in an inert atmosphere; complex 3,
on the other hand, can be reverted to [1]2+ by oxidation with 2
mol equiv of ferrocenium hexafluorophosphate in the presence
of 1 mol equiv of L. Similar oxidations of 3 with X2 (X = Cl, I),
in turn, produced the complexes NiIIL2X2, 2, in high yields.
Chemical synthesis of the complexes and their interconversions
are shown in Schemes 1 and 2, respectively.
1
in a simplified H NMR spectrum of complexes 3a,b. The N−
N bond lengths in complexes [1a](ClO4)2 and 2a lie in a
narrow window between 1.24 and 1.27 Å which is characteristic
of neutral ligands, but these two lengths are considerably
elongated in 3a: 1.337(4) and 1.325(5) Å, respectively. Bond
length elongation in 3a is an indication4 that the ligands are
reduced in this complex. Significant changes of Ni−Nazo lengths
in moving from [1a]2+/2a to 3a are also indicative of changes
in the electronic structure in [1a]2+/2a versus 3a: while the
Ni−Nazo ([1a]2+ and 2a) is longer than Ni−Npy counterparts
(av 2.17 Å vs 2.06 Å), the opposite is true in 3a (1.86 Å vs 1.92
Å). These changes of bond lengths indicate a transition on
neutral ligand in [1a]2+ and 2a, where the azo group is a weak
donor29 than Npy to an azo-anion ligand in 3a. As a whole, the
structural and physical properties are consistent with
accumulation of negative charge4d on the azo groups in 3a.
Final picture of assignment of oxidation state of the complex 3a
will be discussed in the section dealing with electronic structure
calculations.
Scheme 1. Reaction of 2-(Arylazo)pyridine Ligand with
Three Metal Precursors
Scheme 2. Redox-Driven Interconversion of Nickel Complexes
C
dx.doi.org/10.1021/ic501656s | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 2. Crystallographic Details of Complexes [1a](ClO4)2, 2a, 3a, and 4b
[1a](ClO4)2
2a
3a
4b
C21H17N3
empirical formula
C33H27N9Cl2O8Ni
807.23
C23H20N6Cl4Ni
580.94
C22H18N6Ni
425.11
molecular mass
311.38
293
T (K)
293
293
150
cryst syst
monoclinic
P21/c
monoclinic
P21/c
orthorhombic
Pna21
monoclinic
P21/c
space group
a (Å)
9.3581(7)
9.6869(7)
39.014(3)
90
9.276(5)
17.451(5)
15.923(5)
90
32.4726(16)
8.6450(4)
13.5478(7)
90
14.066(6)
7.975(4)
14.885(7)
90
b (Å)
c (Å)
α (deg)
β (deg)
93.296(2)
90
91.812(5)
90
90
99.335(14)
90
γ (deg)
V (Å3)
90
3530.9(5)
4
2577.3(18)
4
3803.2(3)
8
1647.4(13)
4
Z
Dcalcd (g/cm3)
cryst dimens (mm3)
θ range for data collection (deg)
GOF
1.519
1.497
1.485
1.255
0.12 × 0.15 × 0.19
1.0−32.1
1.04
0.14 × 0.18 × 0.20
1.7−25.7
1.01
0.09 × 0.16 × 0.18
1.3−27.5
1.06
0.08 × 0.10 × 0.12
1.47−24.89
1.025
reflns collected
unique reflns
53 081
32 145
50 725
18 737
12 353
4895
8393
2846
final R indices [I > 2σ(I)]
R = 0.0431
wR2 = 0.1074
R = 0.0374
wR2 = 0.1037
R = 0.0357
wR2 = 0.0905
R = 0.0520
wR2 = 0.1579
visible in the range δ 7.0−10.5. 13C NMR data of these are
collected in the Experimental Section, and the spectra of 3a and
3b are deposited as Supporting Information Figures S3 and
S4(b). Complexes [3a,b] are intensely brown and absorb
strongly in the 200−450 nm range which are associated with
weak and broad transition in the near IR region (λmax = 780
nm). The electronic spectra of the reported complexes, [1a,b]2+
and [2a,b], correspond25 well with the published data. Their
UV−vis spectral data are collected in the Experimental Section.
Electronic Structure Analysis. To have an insight into the
electronic structure of complex 3a we have performed series of
calculations using DFT at the B3LYP level which is
implemented at G09W program package. Two models were
primarily considered for the diamagnetism (S = 0) for the
Ni(L)2 complex, 3a: (i) high-spin NiII (SNi = 1) coupled
antiferromagnetically to two ligand radicals (SL = SL = +1/2) or
Table 3. Selected Bond Distances and Bond Angles of
Complexes [1a](ClO4)2, 2a, and 3a
experimental
3a
isomer A
calcd 3a
isomer A
param
[1a](ClO)4 2a (X = Cl)
Ni1−N1
Ni1−N3
Ni1−N4
Ni1−N6
Ni1−N7
Ni1−N9
N2−N3
N5−N6
2.0630(16)
2.1681(14)
2.0406(15)
2.0919(15)
2.0525(15)
2.1224(14)
1.259(2)
2.063(3)
2.158(3)
2.072(3)
2.143(3)
1.918(3)
1.864(3)
1.925(3)
1.870(3)
1.922
1.866
1.926
1.883
1.258 (3)
1.256(3)
1.337(4)
1.325(5)
1.334
1.331
1.255(2)
N8−N9
1.262(2)
Ni1−Cl1
Ni1−Cl2
N1−Ni1−N3
N4−Ni1−N6
N7−Ni1−N9
2.3607(15)
2.3723(16)
74.46(10)
75.02(9)
1
(ii) NiI (SNi = /2) coupled antiferromagnetically to a single
75.13(6)
76.21(6)
76.10(5)
80.81(14)
80.59(13)
80.11
80.85
unpaired electron, either localized on a single ligand or
delocalized over both. The most preferred ground-state
electronic structure of the complex 3a was found to be a
high-spin Ni(II) ion antiferromagnetically coupled to two azo-
anion radicals (La•−). The calculated bond distances derived
from the BS(2,2) calculation also matched well with the
experimental bond parameters. The selected bond distances
and bond angles are collected in Table 3. Notably, the BS(2,2)
calculated dihedral angle between the two metallacycles (plane
1 defined by Ni1−N1−C5−N2−N3 and plane 2 defined by
Ni1−N4−C16−N5−N6) in 3a is 62.61° which is in excellent
agreement with the experimental value of 59.52°. The
qualitative MO diagram (Figure 4) reveals two metal-based
singly occupied molecular orbitals (SOMOs) (dxy and dxz) in
the spin-up manifold which are coupled to two SOMOs, which
are primarily the lowest unoccupied molecular π* orbitals of
the neutral ligand, in the spin-down manifold with overlaps of
0.56 and 0.69, respectively. This indicates the existence of
strong antiferromagnetic interactions between two unpaired
electrons of metal and two ligand radicals. However, in a
tetrahedral geometry, the π* orbitals would interact with d
Cl1−Ni1−
97.48(3)
Cl2
dihedral
59.52
62.61
a
angle
a
Angle between the two planes: 1 (defined by Ni1−N1−C5−N2−
N3) and 2 (defined by Ni1−N4−C16−N5−N6).
The compound [1](ClO4)2 behaves as a 1:2 electrolyte in
acetonitrile solvent, while the solutions of 2 and 3 are
nonelectrolytic. Magnetic susceptibility measurements at 300
K for complexes [1a](ClO4)2 and 2a indicate that these are
paramagnetic with magnetic moments, 2.42 and 2.43 μB,
respectively, which are close to the spin-only value for
mononuclear NiII (S = 1) ion. The tetrahedral complexes,
[3a,b], on the other hand, are diamagnetic (S = 0) and
1
displayed highly resolved H NMR spectra. A representative
spectrum of 3a in CDCl3 is displayed in Figure 3, and that of 3b
is submitted as Supporting Information (Figure S4). In Figure
3, all seven aromatic proton resonances for one ligand are
D
dx.doi.org/10.1021/ic501656s | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 2. ORTEP representation of the complexes: (a) [1a]2+, (b) 2a, and (c) 3a (isomer A) at 50% probability ellipsoid. The counteranions, for
[1a]2+, and hydrogen atoms of all molecules are omitted for clarity.
1
Figure 3. H NMR spectrum of the complex 3a in CDCl3 solution (*1, solvent) (inset shows aromatic proton resonances).
orbitals that are orthogonal, and long-range ligand−ligand
coupling of two nearly orthogonal ligand orbitals is impossible;
hence, we have discarded28 the possibility of NiII (SNi = 0) with
two antiferromagnetically coupled ligand radicals (SL = 1/2).
Another possibility of [Ni0(La)2] was also considered for
which a restricted calculation was employed. Energy of the
optimized structure is higher than that of the broken symmetry
calculation, BS(2,2), by 4.6 kcal/mol. Moreover, metrical
parameters for the latter description are closer to the observed
structure (see Supporting Information Table S4).
Furthermore, the spin-density plot (Figure 5) derived from
broken-symmetry calculation reinforces singlet ground state
due to opposing Mulliken spin populations on Ni (1.40) and
ligands (−0.70 per L). Moreover, the presence of a net spin
density >1.0 and two approximately singly occupied d orbitals is
consistent with the high-spin NiII (SNi = 1) description. The
ligands are therefore effectively reduced by one electron (SL =
1
SL = /2) each, driving the significant elongation of two dN−N
distances relative to those in [1a]2+ and 2a (1.33 vs 1.25 Å).
Interestingly, some spin density is delocalized over the ortho
and para carbon atom of the phenyl ring, the effect of which is
manifested in the deshielding30 of aromatic proton resonances
1
in the H spectra of the complexes [3a,b].
Chemical Catalysis. From the above results it may be
concluded that NiII−L complexes are susceptible to single step
two-electron reduction processes with synergistic metal−ligand
participation and are associated with reversible chemical
transfer of ligand(s) dissociation/association in solution. In
the present context it is worth noting that it has now been
history that transition-metal complexes of noble metals like Pd,
Rh, Pt, etc. dominate the catalytic bond fusion reactions
because of their ability to participate in two-electron transfer
E
dx.doi.org/10.1021/ic501656s | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
product (Suzuki coupling). However, the reaction did not
proceed as expected, but it produced 2-pyridyl-substituted
triaryl hydrazine as the only product, and aryl halide remained
unreacted.
Consequently, an investigation of nickel-catalyzed arylation
of azo-function with aryl boronic acid was initiated. Finally, we
were successful in developing a NiII salt (Ni(ClO4)2 or NiCl2)
catalyzed N-arylation of 2-(arylazo)pyridines under mild
conditions as shown in Scheme 4. Isolated products and their
yields are collected in Table 4.
Table 4 provides information on the impact of catalyst and
temperature on the efficiency of this process. In absence of
catalyst no reaction occurs even at 450 K. The transformation
exhibits a broad scope and a high tolerance for a variety of
functional groups, as shown in Table 5. The reaction is
applicable to a wide range of substituted aryl, heteroaryl, or
polycyclic groups. Typical polycyclic groups used are naphthyl,
anthracenyl, pyrinyl, fluorinyl, etc. Furthermore, both electron-
rich and electron-poor arylboronic acids participated in the
reactions with ease. It may also be noted that the aryl−halogen
bond remained intact during the catalysis, which demonstrates
that a potential Buchwald−Hartwig or Suzuki cross-coupling
reaction cannot compete under the present conditions of
catalysis. We are particularly interested in the molecules that
contain fluorophoric groups like naphthalene, anthracene,
pyrene, fluorine, etc. to expand their application possibilities
further.14,33
Figure 4. Qualitative MO diagram of the magnetic orbitals derived
from BS(2,2) calculation of [Ni(La•−)2]. The spatial overlap (S) of the
corresponding α and β orbitals is given.
Characterization of Hydrazines: Spectroscopic and X-
ray Structure. All triaryl hydrazines, isolated as above, were
1
fully characterized by H NMR, 13C NMR, ESI-MS, and IR
spectroscopy. The N−H and N−N stretching frequencies for
these appeared34 at around 3435 and 1145 cm−1, respectively,
in the IR spectra of the compounds. These compounds
displayed multiple signals in the aromatic region. However, the
N−H proton was not detectable in CDCl3 solution. This might
be attributed35 by reversible dimerization of the 2-amino-
pyridine group in CDCl3 solution. This will lead to broadening
of the N−H resonance to that extent that it is difficult to detect
in CDCl3 solution. However, this dimerization is inhibited in
d6-DMSO solvent, because of its strong hydrogen bond
acceptor nature. Accordingly, the 1H NMR spectrum of
compound 4b (in d6-DMSO) displayed a characteristic N−H
proton signal at 9.48 ppm, which disappeared on D2O shake.
Selected spectral data of the compound are collected in the
Experimental Section, and complete details of spectral
characterizations of the compounds are submitted as
Supporting Information.
Three-dimensional X-ray structure determination of a
representative compound, 4b, is used to further authenticate
the identity of this class of triarylhydrazines. Suitable crystals
were obtained by slow evaporation of the solution of 4b in a
mixed solvent, dichloromethane−hexane (1:1) mixture. An
ORTEP of it is shown in Figure 6. Examination of N−N bond
distance, dN2−N3, 1.401(5) Å, clearly reveals that it is a single
bond, as expected for a hydrazine molecule. Moreover, the
three bond angles around N3 are N2−N3−C12 114.36(19)°,
C6−N3−C12 119.01(17)°, and C6−N3−N2 112.54(17)°,
respectively, and that around N2 also lies in the similar range
signifying that these two N atoms are sp3-hybridized. Selected
bond parameters are collected in Table 6, and crystallographic
details are presented in Table 2. As far we are aware, three-
dimensional X-ray structure of this class of compound is
unavailable in literature.
Figure 5. Spin density plot of the complex, [Ni(La•−)2], 3a, derived
from BS(2,2) calculation.
processes (oxidative addition and reductive elimination) which
in fact are the elementary steps in C−C and C−heteroatom
bond making reactions.31,32 Electron storage ability of the
reference azoaromatics, as demonstrated herein, has opened up
the scope of using these and similar 3d metal complexes in
catalytic coupling reactions.
Accordingly a stoichiometric chemical reaction, as shown in
Scheme 3, we tried to look for the corresponding C−C coupled
F
dx.doi.org/10.1021/ic501656s | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 3. Attempted C−C Coupling
Scheme 4. Ni(II)-Catalyzed Coupling of 2-(Arylazo)pyridine
with Aryl Boronic Acid
Table 5. Products of the Nickel-Catalyzed C−N Coupling
Reaction on Azo-Aromatic Compounds with Arylboronic
Acids
On the Arylation Reaction. Our present knowledge on
the above N-arylation reaction is not sufficient to assess the
mechanism as yet. However, a few observations related to the
reactions are noted below. It is envisaged from the following
observations that modified redox properties of the coordinated
2-(arylazo)pyridines play the controlling role in this trans-
formation. First, formation of a coordinated nickel complex
during the course of the reaction was established by trapping a
green product that was observed to form in the reaction
mixture by the use of higher concentration (0.5 mmol, 5 mol
%) Ni(ClO4)2 (catalyst). A green nickel complex, isolated from
the reaction mixture after 1 h, showed an identical UV−vis
spectrum as compared to that of the pure complex, [Ni-
(pap)3]2+ (see Experimental Section for details, Supporting
Information Figure S26 for spectra). This points to the notion
that the ligand L coordinates to Ni(II) salt in the catalytic cycle.
Moreover, our attempts to N-arylate two analogous azo-
aromatics, viz., azo-benzene (azbz) and azo-bispyridine (azpy),
failed under the above experimental conditions. Of the above
two azo-aromatic compounds, azbz failed to react with
NiII(ClO4)2, and no parallel chemical reduction by Zn metal
of the reported36 complex, [Ni(azpy)3]2+, was achieved by us.
We also have investigated the reaction of azpy (2 mol) with
Ni0(COD)2 in an inert atmosphere, which resulted an insoluble
material of unknown identity.
a
Table 4. Nickel-Catalyzed Coupling of Azo-Aromatics with Arylboronic Acid
d
compd
R
Ar
catalyst (mol %)
T (K)
solvent
yield (%)
bc
b
c
c
c
c
,
4a
4b
H
H
H
H
H
H
H
H
Cl
H
H
phenyl (Ph)
naphthyl
1
1
5
5
2
1
1
1
1
1
1
300
300
323
313
313
300
300
313
313
300
300
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
acetonitrile
THF
83, 81
bc
,
b
78, 75
bc
,
b
4c
9-bromoanthracyl
pyrenyl
58, 52
bc
,
b
4d
68, 66
bc
,
b
c
4e
9,9-dimethyl-fluorenyl
3-thiophenyl
p-COCH3−Ph
p-F−Ph
74, 71
b
b
4f
78
b
b
b
4g
62
b
4h
65
b
b
4i
H
68
b
b
b
4a
4a
phenyl (Ph)
phenyl (Ph)
30
b
66
a
b
c
Reaction conditions: L (1 mmol); Ar−B(OH)2 (1 mmol); catalyst, Ni(II) salt. (Ni(ClO4)2. NiCl2 (0.01 mmol), K3PO4 (1 mmol), solvent (5
d
mL), 300−323 K, 3−6 h. Isolated yield.
G
dx.doi.org/10.1021/ic501656s | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
transformations do not occur in the presence of radical
scavengers like TEMPO or BHT. With all this evidence taken
together it is concluded that the aforesaid N-arylation reaction
is highly specific for the 2-(arylazo)pyridine substrates and that
the catalytic reactions possibly occur via coordination of L to
Ni(II) and coupling of in situ generated radicals.
Properties of Triarylhydrazine. These classes of dyes, in
general, are well-known for their use.22 An ideal dye should
possess qualities38 of high absorption coefficient, high quantum
yield, large Stokes shift, and high chemical and photochemical
stability as well. For example, 4,4-difluoro-4-bora-3a,4a-diaza-s-
indacene (abbreviated as BODIPY) dye and its derivatives are
strongly UV-absorbing small molecules, which are widely
used39 to label proteins and DNA. However, most BODIPYs
have the disadvantage of small Stokes shifts. In general,
synthetic dyes with Stokes shifts >80 nm and also having high
fluorescence quantum yields are highly desirable38 to minimize
reabsorption of emitted photons.
Figure 6. ORTEP view of compound 4b at 30% probability ellipsoid.
Table 6. Bond Parameters for Compound 4b
bond param
bond distance (Å)
bond param
bond angle (deg)
The compounds, collected in Table 5, have all these desirable
properties as dyes: high quantum yields, large Stokes shift (>90
nm), and photostability as well. The photophysical properties
of these triarylhydrazines are summarized in Table 8. A
comparison of the data indicates that the presence of polycyclic
aromatic rings augments the quantum yield of the product
considerably. Thus, the quantum yield of >80% was noted in
compound 4d, which contains pyrinyl as one of the aryl groups.
The observed Stokes shift of these dyes is unusually high, lying
in the range 90−195 nm. To examine the photostability, a
solution (10−4 M solution in dichloromethane) of a
representative sample, 4b, was irradiated at two wavelengths,
255 and 364 nm, respectively, at room temperature, and its
absorption spectra were recorded periodically. The absorption
profile was unchanged after 4 h of irradiation. Thus, in addition
to thermal and air stability, this dye exhibits significant
photostability over a period of time.
N2−N3
N2−C5
N3−C6
N3−C12
1.401(2)
1.363(3)
1.436(3)
1.432(3)
N2−N3−C12
C6−N3−C12
C6−N3−N2
N3−N2−C5
114.36(19)
119.01(17)
112.54(17)
120.26(19)
We wish to add here that aryl boronic acids in the presence
of K3PO4 are known37 to bring about reduction of Ni(II)
complexes to Ni(0) even at room temperature. Moreover, the
present NiII−L complexes are susceptible to facile chemical
reduction (vide supra), and each of the above arylation
reactions invariably produced the corresponding diaryl
compound as a byproduct. Isolated yields of the biaryls were
higher in the cases of polycyclic aryl boronic acid. Considering
air-sensitivity of Ar• radical, the above reactions were also tried
in an oxygen-free atmosphere, which resulted in higher
turnover number (TON) for the reference conversion (Table
7). It may also be worthwhile to note that the above chemical
Table 7. Nickel-Catalyzed Coupling of Azo-Aromatics with
Arylboronic Acid in Inert Atmosphere
a
CONCLUSION
■
In this Article, we have introduced a complete description of
nickel complexes of azo-aromatic ligands where the ligands are
shown to be highly redox-active while the metal ion behaves as
a redox innocent center. This situation is just opposite to our
common experiences on the redox behavior of most of
transition-metal complexes of organic ligands. Our investigation
on the possible use of the Ni−L complexes as catalyst finally led
to development of an efficient catalytic cycle for N-arylation of
2-(arylazo)pyridine using Ni(II) salt as catalyst. Isolated 2-
pyridyl-substituted triarylhydrazines are all new and have
generated scopes for exploration of their properties as useful
dye. Our work on the multiple application possibilities of these
dyes is underway and will be reported in due course.
catalyst
(mol %)
b
compd
R
Ar
T (K) yield (%)
4a
4b
4c
4d
4e
H
H
H
H
H
phenyl (Ph)
naphthyl
0.5
0.5
2
300
300
323
313
313
68
56
38
42
54
9-bromoanthracyl
pyrenyl
2
9,9-dimethyl-
fluorenyl
1
a
Reaction conditions: L (1 mmol), Ar−B(OH)2 (1 mmol), Ni(ClO4)2
(0.005 mmol), K3PO4 (1 mmol), 1,4 dioxane (5 mL), 300−323 K, 3−
b
6 h. Isolated yield.
Table 8. Absorbance and Fluorescence Data for New Hydrazine Dyes
compd
absorption, nm (ϵ, M‑1 cm‑1)
emission (nm)
Stokes shift (nm) quantum yield (%) τf (ns)
4a
4b
4c
4d
4e
4f
230 (15 160), 245 (13 520), 290 (15 860)
230 (19 800), 245 (10 900), 290 (6000), 335 (2200)
255 (21 000), 380 (1470), 430 (1000)
445 (Abs = 290 nm)
485 (Abs = 290 nm)
410 (Abs = 255 nm)
440 (Abs = 350 nm)
415 (Abs = 320 nm)
420 (Abs = 285 nm)
155
195
155
90
5.15
33.8
40.8
85.6
60.0
3.38
2.87
5.17
4.47
3.89
5.02
5.21
250 (37 340), 275 (24 440), 300 (16 600), 350 (13 870), 375 (9300)
230 (29 660), 320 (29 482)
95
230 (27 900), 285 (24 500)
135
H
dx.doi.org/10.1021/ic501656s | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
225 (17 900), 315 (25 000), 410 (6500), 785 (190). Due to its high air
sensitivity, reproducible CHN data for complex 3b is not obtained.
X-ray Crystallography. Crystallographic data for complexes
[1a]2+, 2a, 3a, and 4b are collected in Table 2. Suitable X-ray quality
crystals of complexes [1a]2+ and 2a were obtained by the slow
evaporation of a dichloromethane−hexane solution of the complex,
whereas the crystals of complexes 3a and 4b were obtained by slow
diffusion of a dichloromethane solution of the complex into hexane.
All data were collected on a Bruker SMART APEX-II diffractometer,
equipped with graphite-monochromated Mo Kα radiation (λ = 0.710
73 Å), and were corrected for Lorentz polarization effects. Data for
1a(ClO4)2 follow: A total of 53 081 reflections were collected, of
which 12 353 were unique (Rint = 0.0431), satisfying the I > 2σ(I)
criterion, and were used in subsequent analysis. Data for 2a follow: A
total of 32 145 reflections were collected, of which 4895 were unique
(Rint = 0.0374). Data for 3a follow: A total of 50 725 reflections were
collected, of which 8393 were unique (Rint = 0.0357). Data for
Ni(L)2I2 follow: A total of 34 436 reflections were collected, of which
6813 were unique (Rint = 0.0299). Data for 4b follow: A total of 18 737
reflections were collected, of which 2846 were unique (Rint = 0.0520).
During refinement of 3a the observed flack parameter was 0.532 which
indicates the possible presence of racemic twin. Thus, the TWIN card
has been used before the final refinement which gives a relatively
better R factor for the structure. The structures were solved by
employing the SHELXS-97 program package41a and were refined by
full-matrix least-squares based on F2 (SHELXL-97).41b All hydrogen
atoms were added in calculated positions.
Computational Methods. All DFT calculations presented herein
were carried out using the Gaussian 09 program package.42 Geometry
optimizations were performed without imposing geometric constraints
(C1 symmetry), and stationary points were subsequently confirmed to
be minima by vibrational analysis (no imaginary frequencies). All
calculations utilized the B3LYP hybrid functional.43 The TZVP basis
set44 of triple-ζ quality with one set of polarization functions was used
on Ni. For all other atoms, C, H, N polarized 6-31G(d,p) basis sets45
were used. The broken-symmetry approach first proposed by
Ginsberg46 and Noodleman47 has been employed for all complexes.
A range of spin-polarized initial densities was tried in each case
(BS(2,2), BS(1,1)), but in all cases different guesses (with the same
total multiplicity) converged to the same minimum. Mulliken spin
densities were used for analysis of spin populations on ligand and
metal centers.48
EXPERIMENTAL SECTION
■
Materials. The salts, Ni(COD)2, Ni(ClO4)2, and all boronic acids
were purchased from Sigma-Aldrich. The salt NiCl2 was purchased
from Spectrochem, India. All other reagents and chemicals were
purchased from commercial sources and used without further
purifications. Tetrabutylammonium perchlorate was prepared and
recrystallized as reported before.27 Caution! Perchlorates have to be
handled with care and appropriate safety precautions.
Physical Measurements. PerkinElmer Lambda 950 and Perki-
nElmer LS 55 spectrophotometers were used to record UV−vis and
fluorescence spectra, respectively. Infrared spectra were obtained using
a PerkinElmer 783 spectrophotometer. 1H NMR spectra were
recorded on a Bruker Avance 300/400/500 MHz spectrometer, and
SiMe4 was used as the internal standard. A PerkinElmer 240C
elemental analyzer was used to collect microanalytical data (C, H, N).
ESI mass spectra were recorded on a micromass Q-TOF mass
spectrometer (serial no. YA 263). All electrochemical measurements
were performed using a PC-controlled PAR model 273A electro-
chemistry system. Cyclic voltammetric experiments were performed
under nitrogen atmosphere using a Ag/AgCl reference electrode, with
a Pt disk working electrode and a Pt wire auxiliary electrode, in
acetonitrile or dichloromethane containing supporting electrolyte, 0.1
M Et4NClO4 or 0.1 M Bu4NClO4, respectively. A Pt wire gauge
working electrode was used for exhaustive electrolyses. The E1/2 for
the ferrocenium−ferrocene couple under our experimental condition
was 0.39 V. The peak-to-peak separation (ΔEp) for the Fc/Fc+ (Fc+ =
ferrocenium ion) couple under this experimental conditions was 80
mV. Room temperature magnetic moment measurements for
[1a](ClO4)2 and 2a were carried out with Gouy balance (Sherwood
Scientific, Cambridge, U.K.). Emission quantum yields were obtained
by using quinine sulfate (ΦF = 0.54 in 0.1 M H2SO4) as reference.
Synthesis. The ligands La and Lb were prepared by following the
reported procedure.40 All other chemicals were of reagent grade and
used as received. Solvents were purified and dried prior to use.
Preparation and handling of air-sensitive materials were carried out
under an inert atmosphere by using standard Schlenk techniques or a
glovebox. Complexes [1](ClO4)2 and 2 were prepared and
characterized as before.25
Synthesis of Ni(La•−)2, 3a. A 133 mg portion of La, dissolved in dry
dichloromethane, was added dropwise to a freshly prepared dichloro-
methane solution containing 100 mg of Ni(COD)2 in a glovebox with
constant stirring for 4 h. During this period the color of the solution
changes from yellow to brown. The crude mass, obtained by
evaporation of solvent in vacuum, was purified by fractional
crystallization from dichloromethane and ether solvent mixture. The
precipitate was crystallized by slow diffusion of a dichloromethane
solution of the compound into hexane. Its yield and characterization
data are as follows: yield 90%, ESI-MS, m/z 425 [3a]+. 1H NMR (500
MHz, CDCl3): δ 10.16 (d, 1H, J = 6 Hz), 9.03 (d, 2H, J = 8 Hz), 7.96
(d, 1H, J = 8 Hz), 7.84 (t, 1H, J = 6 Hz), 7.72 (t, 1H, J = 7.0 Hz), 7.65
(t, 1H, J = 7.0 Hz), 7.06 (t, 2H, J = 7.5 Hz). 13C NMR (125 MHz,
CDCl3): δ 161.44, 151.37, 134.04, 131.72(2C), 128.09, 124.28,
121.67(2C), 119.77, 117.97. λmax/nm (ε/M−1 cm‑1) (dichloromethane):
230 (17 200), 325 (25 500), 415 (6200), 780 (205). Due to its high air
sensitivity, reproducible CHN data for complex 3a is not obtained.
An identical compound is also obtained by changing the solvent
from dichloromethane to hexane. In this case the compound
precipitated out during stirring. The crude solid product was filtered
and washed with diethyl ether. The residue was crystallized by slow
diffusion of a dichloromethane solution of the compound into hexane.
Yield: 85%.
Catalytic Synthesis of Trisubstituted Hydrazines. The
catalytic reactions were carried out following a general procedure. A
mixture of 2-(arylazo)pyridine (1.0 equiv), aryl boronic acid (1.0
equiv), NiIICl2 (or NiII(ClO4)2, 0.01−0.05 equiv), K3PO4 (1.0 equiv),
and 1,4 dioxane (5 mL) was stirred at 300−325 K in air for 3−5 h.
The crude product, thus obtained, was purified in yields on preparative
alumina TLC plate using ethyl acetate−hexane (1:10) mixture as
eluent. Higher concentration of catalyst and relatively higher
temperatures and reaction time were needed for polyaryl aromatic
group insertion reactions. The yields and spectral characterization of
the products are as follows.
2-(2,2-Diphenylhydrazinyl)pyridine (4a). Pale yellow solid; mp =
154−156 °C. IR (KBr): 3413, 3043, 3005, 2954, 1595, 1490,
1
1394,1142, 770 cm−1. H NMR (300 MHz, CDCl3): δ 6.71 (d, J = 5
Hz, 1H), 6.85 (d, J = 8 Hz, 1H), 7.02 (d, J = 3 Hz, 2H), 7.28 (d, J = 4
Hz, 7H), 7.47 (t, J = 7.5 Hz, 1H), 7.94 (s, 1H), 8.07 (s, 1H). 13C NMR
(75 MHz, CDCl3): δ 106.3, 115.5, 119.2 (4C), 122.8 (2C), 129.3
(4C), 138.7, 146.4 (2C), 147.8, 159.7. ESI-MS Calcd for C17H16N3 [M
+ H]+: 262.1328. Found: 262.1338. Anal. Calcd for C17H15N3: C,
78.13; H, 5.79; N 16.08. Found C, 78.09; H, 5.65; N, 15.87.
2-(2-(Naphthalen-1-yl)-2-phenylhydrazinyl)pyridine (4b). Off
white solid; mp = 185−187 °C. IR (KBr): 3400, 3049, 3009, 2953,
1593, 1491, 1392, 1146, 1280, 771 cm−1. 1H NMR (500 MHz,
CDCl3): δ 6.70 (t, J = 6 Hz, 1H), 6.85−6.91 (m, 3H), 7.08 (d, J = 8
Hz, 1H), 7.18 (t, J = 8 Hz, 2H), 7.42−7.53 (m, 3H), 7.59 (d, J = 7 Hz,
1H), 7.65 (s, 1H), 7.78 (d, J = 8.5 Hz, 1H), 7.89−7.94 (m, 2H), 8.05
(d, J = 5 Hz, 1H). 13C NMR (125 MHz, CDCl3): δ 106.8, 115.4 (2C),
115.6, 120.9, 122.9, 123.8, 126.3 (2C), 126.6, 127.3, 128.8, 129.2 (2C),
Synthesis of Ni(Lb•−)2, 3b. This was synthesized similarly as above
with an equivalent quantity of Lb in place of La. Its yield and
characterization data are as follows: yield 86%, ESI-MS, m/z 494
1
[3b]+. H NMR (500 MHz, CDCl3): δ 10.20 (d, 1H, J = 5 Hz), 8.96
(d, 2H, J = 8.5 Hz), 7.98 (d, 1H, J = 8 Hz), 7.90 (t, 1H, J = 6 Hz), 7.73
(t, 1H, J = 7.0 Hz), 7.00 (d, 1H, J = 8.5 Hz). 13C NMR (100 MHz,
CDCl3) δ 161.95, 151.68, 140.18, 134.84, 132.24(2C), 129.04,
122.92(2C), 120.00, 118.59. λmax/nm (ε/M−1 cm‑1) (dichloromethane):
I
dx.doi.org/10.1021/ic501656s | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
129.7, 135.0, 138.7, 142.8, 147.8, 149.6, 159.9. ESI-MS Calcd for
C21H18N3 [M + H]+: 312.1484. Found: 312.1596. Anal. Calcd for
C21H17N3: C, 81.00; H, 5.50; N 13.49. Found C, 80.53; H, 5.46; N,
13.46.
6.86−6.89 (m, 2H), 7.17−7.20 (m, 2H), 7.33 (d, J = 7 Hz, 1H), 7.41
(t, J = 8 Hz, 2H), 7.46−7.53 (m, 2H), 7.61 (d, J = 8 Hz, 2H), 7.88 (d,
J = 8 Hz, 1H), 7.98 (d, J = 8 Hz, 1H). 13C NMR (125 MHz, CDCl3):
δ 116.9, 118.3 (2C), 121.9 (2C), 123.8, 126.1, 126.3, 128.1, 128.7
(2C), 129.4 (2C), 134.9, 138.4, 143.8, 160.4. ESI-MS Calcd for
C17H15ClN3 [M + H]+: 296.0955. Found: 296.3767. Anal. Calcd for
C17H15ClN3: C, 69.03; H, 4.77, N, 11.99. Found C, 70.11; H, 4.90: N,
12.04.
2-(2-(10-Bromoanthracen-9-yl)-2-phenylhydrazinyl)pyridine (4c).
Pale green solid; mp = 172−174 °C. IR (KBr): 3408, 3042, 3019,
1
2943, 1591, 1497, 1382, 1270, 1143, 774 cm−1. H NMR (500 MHz,
CDCl3): δ 6.47 (t, J = 8.5 Hz, 1H), 7.46−7.53 (m, 3H), 7.60 (t, J = 9
Hz, 2H), 7.80−7.87 (m, 4H), 8.00 (d, J = 8.5 Hz, 1H), 8.25−8.27 (m,
2H), 8.31−8.33 (m, 2H), 8.45 (t, J = 8.5 Hz, 1H), 8.52 (d, J = 9 Hz,
1H). 13C NMR (75 MHz, CDCl3): δ 118.61, 122.80, 123.75 (2C),
125.43 (2C), 126.10 (2C), 127.15 (2C), 128.43 (2C), 128.92 (2C),
129.32 (2C), 130.21, 132.35, 132.58, 134.28, 137.98(2C), 138.56,
152.49, 162.95. ESI-MS Calcd for C25H19N3Br [M + H]+: 440.0684.
Found: 440.0884. Anal. Calcd for C25H18N3Br: C, 68.19; H, 4.12; N,
9.54. Found C, 68.17; H, 4.11, N, 9.52.
Isolation of [1a]2+ During the Course of the Reaction. A
mixture of 2-(arylazo)pyridine (1.0 equiv), naphthyl boronic acid (1.0
equiv), NiII(ClO4)2 (0.05 equiv), K3PO4 (1.0 equiv), and 1,4-dioxane
(5 mL) was stirred at 300 K in air for 1 h. The solvent was evaporated
in a rotary evaporator. The crude mixture thus obtained was
subsequently washed with water and ethyl acetate to remove unreacted
aryl boronic acid, 2-(arylazo)pyridine (L), base (K3PO4), and
triarylhydrazine (4b) as well. A tiny quantity of green residue was
isolated and characterized as [1a]2+ by its UV−vis spectrum.
2-(2-Phenyl-2-(pyren-1-yl)hydrazinyl)pyridine (4d). Yellowish
solid; mp = 164−166 °C. IR (KBr): 3402, 3043, 3015, 2941, 1597,
1
1491, 1385, 1278, 1148, 772 cm−1. H NMR (500 MHz, CDCl3): δ
ASSOCIATED CONTENT
* Supporting Information
■
6.63 (t, J = 7 Hz, 1H), 6.80−6.84 (m, 3H), 7.06−7.12 (m, 3H), 7.40
(d, J = 7.5 Hz, 1H), 7.92−8.08 (m, 4H), 8.09−8.16 (m, 4H), 8.31 (d, J
= 8 Hz, 1H). 13C NMR (125 MHz, CDCl3): δ 163.08, 152.60, 138.50,
137.69, 132.57, 132.32, 131.48, 131.19(2C), 129.90, 129.34, 129.04,
127.68, 127.43, 127.30, 126.71, 126.37, 126.12, 125.53, 125.37, 125.30,
125.11, 123.79, 123.56(2C), 122.82, 118.61. ESI-MS Calcd for
C27H19N3Na [M + Na]+: 408.1477. Found: 408.5035. Anal. Calcd
for C27H19N3: C, 84.13; H, 4.97, N, 10.90. Found C, 83.64; H, 4.90: N,
10.81.
S
Figures, tables, scheme, and CIF files giving crystallographic
details and selected bond parameters of the complexes
Ni(La)2I2 and two isomers of 3a, ORTEP of two isomers of
3a and Ni(La)2I2, cyclic voltammograms at different scan rates
of [1a]2+ and 2a, ORTEP representation of Ni(La)2I2, NMR
spectra of 3a,b and 4a−i, ESI-MS spectra of 3a and 4a−i, UV−
vis spectra of the complexes [1a]2+, 2a, 3a, and 4a−f,
fluorescence spectra of 4a−f, and Cartesian coordinates of
the optimized structure 3a. This material is available free of
charge via the Internet at http://pubs.acs.org.
2-(2-(9,9-Dimethyl-fluoren-2-yl)-2-phenylhydrazinyl)pyridine
(4e). Off white solid; mp = 187−189 °C. IR (KBr): 3404, 3042, 3006,
1
2952, 1583, 1498, 1396, 1287, 1151, 773 cm−1. H NMR (500 MHz,
CDCl3): δ 1.29 (s, 6H), 6.53 (t, J = 6 Hz, 1H), 6.75 (d, J = 8.5 Hz,
1H), 6.88 (t, J = 7 Hz, 1H), 7.11−7.19 (m, 7H), 7.26 (d, J = 7 Hz,
2H), 7.32 (d, J = 8.5 Hz, 1H), 7.46−7.51 (m, 2H), 7.91 (d, J = 5 Hz,
1H), 8.22 (s, 1H). 13C NMR (125 MHz, CDCl3): δ 27.2 (2C), 46.9,
106.3, 114.0, 115.4, 118.7, 118.8 (2C), 119.5, 120.6, 122.5, 122.6,
126.6, 127.0, 129.3 (2C), 134.4, 138.5, 139.0, 145.9, 146.8, 147.9,
153.5, 155.1, 159.9. ESI-MS Calcd for C26H24N3 [M + H]+: 378.1892.
Found: 378.1894. Anal. Calcd for C26H23N3: C, 82.73; H, 6.14, N,
11.13. Found C, 81.63; H, 5.82: N, 10.81.
AUTHOR INFORMATION
Corresponding Author
*E-mail: icsg@iacs.res.in.
■
Present Address
§Department of Chemistry, Indian Institutes of Engineering
Science and Technology, Shibpur, Howrah-711 103, India.
2-Phenyl-1-(pyridin-2-yl)-2-(thiophen-3-yl)hydrazine (4f). Off
white solid; mp = 143−145 °C. IR (KBr): 3414, 3048, 3016, 2957,
1582, 1497, 1393, 1285, 1147, 778 cm−1. 1H NMR (500 MHz,
CDCl3): δ 6.63−6.66 (m, 1H), 6.75−6.77 (m, 2H), 6.88−6.95 (m,
2H), 7.13−7.20 (m, 5H), 7.39−7.42 (m, 1H), 8.04 (d, J = 5 Hz, 1H).
13C NMR (125 MHz, CDCl3): δ 106.5, 109.4, 115.8, 117.2 (2C),
122.2, 122.4, 125.6, 129.3 (2C), 138.6, 145.8, 147.3, 148.0, 159.5. ESI-
MS Calcd for C15H14N3S [M + H]+: 268.0830. Found: 268.1231. Anal.
Calcd for C15H14N3S: C, 67.39; H, 4.90, N, 15.72. Found: C, 67.11; H,
4.82: N, 15.61.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The research was supported by Council for Scientific and
Industrial Research (CSIR) and Department of Science and
Technology (DST), India, funded projects 01(2714)/13/EMR-
II and SR/S1/IC/0031/2010, respectively. S.G. sincerely
thanks DST for a J. C. Bose fellowship. Crystallography was
performed at the DST funded National Single Crystal
Diffractometer Facility at the Department of Inorganic
Chemistry, IACS. D.S., P.G., T.C., and H.D. are thankful to
the Council of Scientific and Industrial Research for their
fellowship support.
1-(4-(1-Phenyl-2-(pyridin-2-yl)hydrazinyl)phenyl)ethanone (4g).
Off white solid; mp = 157−159 °C. IR (KBr): 3413, 3044, 3018,
1
2951, 1732, 1572, 1493, 1383, 1282, 1146, 771 cm−1. H NMR (500
MHz, CDCl3): δ 2.52 (s, 3H), 6.74 (s, 2H), 7.14−7.19 (m, 3H), 7.33−
7.38 (m, 4H), 7.48 (t, J = 8 Hz, 1H), 7.84 (d, J = 8 Hz, 2H), 8.10 (s,
1H). 13C NMR (125 MHz, CDCl3): δ 26.3, 106.3, 115.4 (2C), 116.1,
122.4 (2C), 125.3, 129.7 (2C), 130.3, 130.4 (2C), 138.7, 144.5, 148.0,
150.9, 159.0, 196.5. ESI-MS Calcd for C19H18N3O [M + H]+:
304.1372. Found: 304.1403. Anal. Calcd for C19H18N3O: C, 75.23; H,
5.65, N, 13.85. Found: C, 75.11; H, 4.90: N, 13.61.
REFERENCES
■
(1) Jørgensen, C. K. Coord. Chem. Rev. 1966, 1, 164−178.
(2) Chirik, P. J.; Wieghardt, K. Science 2010, 327, 794−795.
(3) (a) Shivakumar, M.; Pramanik, K.; Ghosh, P.; Chakravorty, A.
Inorg. Chem. 1998, 37, 5968−5969. (b) Pramanik, K.; Shivakumar, M.;
Ghosh, P.; Chakravorty, A. Inorg. Chem. 2000, 39, 195−199. (c) Kaim,
W. Inorg. Chem. 2011, 50, 9752−9765. (d) Sarkar, B.; Patra, S.;
Fiedler, J.; Sunoj, R. B.; Janardana, D.; Mobin, S. M.; Niemeyer, M.;
Lahiri, G. K.; Kaim, W. Angew. Chem., Int. Ed. 2005, 44, 5655−5658.
(e) Sarkar, B.; Patra, S.; Fiedler, J.; Sunoj, R. B.; Janardanan, D.; Lahiri,
G. K.; Kaim, W. J. Am. Chem. Soc. 2008, 130, 3532−3542. (f) Kaim, W.
Coord. Chem. Rev. 2001, 219−221, 463−488.
2-(4-Fluorophenyl)-2-phenyl-1-(pyridin-2-yl)hydrazine (4h). Off
white solid; mp = 151−153 °C. IR (KBr): 3423, 3054, 3028, 2957,
1579, 1492, 1388, 1286, 1143, 775 cm−1. 1H NMR (500 MHz,
CDCl3): δ 6.78−6.81 (m, 1H), 6.90−6.96 (m, 4H), 7.18−7.24 (m,
5H), 7.43−7.51 (m, 2H), 8.22 (s, 1H). ESI-MS Calcd for C17H15N3F
[M + H]+: 280.1250 Found: 280.1248. Anal. Calcd for C17H14N3F: C,
73.10; H, 5.05, N, 15.04. Found: C, 72.56; H, 4.90: N, 14.68.
2-(2-(4-Chlorophenyl)-2-phenylhydrazinyl)pyridine (4i). Off white
solid; mp = 159−161 °C. IR (KBr): 3418, 3034, 3008, 2954, 1576,
1
1498, 1393, 1292, 1145, 773 cm−1. H NMR (500 MHz, CDCl3): δ
J
dx.doi.org/10.1021/ic501656s | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(4) (a) Goswami, S.; Sengupta, D.; Paul, N. D.; Mondal, T. K.;
Goswami, S. Chem.Eur. J. 2014, 20, 6103−6111. (b) Paul, N. D.;
Rana, U.; Goswami, S.; Mondal, T. K.; Goswami, S. J. Am. Chem. Soc.
2012, 134, 6520−6523. (c) Ghosh, P.; Samanta, S.; Roy, S. K.;
Demeshko, S.; Meyer, F.; Goswami, S. Inorg. Chem. 2014, 53, 4678−
4686. (d) Samanta, S.; Ghosh, P.; Goswami, S. Dalton Trans. 2012, 41,
Hydrazone and Aryl Hydrazine. PCT Patent Application WO 03/
035604 A1, 2001.
(15) (a) Ragnarsson, U. Chem. Soc. Rev. 2001, 30, 205−213.
(b) Zhang, R.; Durkin, J. P.; Windsor, W. T. Bioorg. Med. Chem. Lett.
2002, 12, 1005−1008. (c) Lee, T.-W.; Cherney, M. M.; Huitema, C.;
Liu, J.; James, K. E.; Powers, J. C.; Eltis, L. D.; James, M. N. G. J. Mol.
Biol. 2005, 354, 1137−1151.
(16) Abrams, M. J.; Juweid, M.; TenKate, C. I.; Schwartz, D. A.;
Hauser, M. M.; Gaul, F. E.; Fuccello, A. J.; Rubin, R. H.; Strauss, H.
W.; Fischman, A. J. J. Nucl. Med. 1990, 31, 2022−2028.
(17) Rose, D. J.; Maresca, K. P.; Nicholson, T.; Davison, A.; Jones, A.
G.; Babich, J.; Fischman, A.; Graham, W.; DeBord, J. R. D.; Zubieta, J.
Inorg. Chem. 1998, 37, 2701−2716.
(18) Edwards, D. S.; Liu, S.; Harris, A. R.; Poirier, M. J.; Ewels, B. A.
Bioconjugate Chem. 1999, 10, 803−807.
(19) Harris, T. D.; Sworin, M.; Williams, N.; Rajopadhye, M.;
Damphousse, P. R.; Glowacka, D.; Poirier, M. J.; Yu, K. Bioconjugate
Chem. 1999, 10, 808−814.
2213−2226. (e) Ghosh, P.; Samanta, S.; Roy, S. K.; Joy, S.; Kramer,
̈
T.; McGrady, J. E.; Goswami, S. Inorg. Chem. 2013, 52, 14040−14049.
(f) Joy, S.; Kramer, T.; Paul, N. D.; Banerjee, P.; McGrady, J. E.;
̈
Goswami, S. Inorg. Chem. 2011, 50, 9993−10004. (g) Paul, N.;
Samanta, S.; Goswami, S. Inorg. Chem. 2010, 49, 2649−2655.
(h) Sanyal, A.; Chatterjee, S.; Castineiras, A.; Sarkar, B.; Singh, P.;
̃
́ ̌
Fiedler, J.; Zalis, S.; Kaim, W.; Goswami, S. Inorg. Chem. 2007, 46,
8584−8593. (i) Sanyal, A.; Banerjee, P.; Lee, G.-H.; Peng, S.-M.;
Hung, C.-H.; Goswami, S. Inorg. Chem. 2004, 43, 7456−7462.
(j) Samanta, S.; Singh, P.; Fiedler, J.; Zal
Inorg. Chem. 2008, 47, 1625−1633.
́ ̌
is, S.; Kaim, W.; Goswami, S.
(5) Das, A.; Scherer, T. M.; Mobin, S. M.; Kaim, W.; Lahiri, G. K.
Chem.Eur. J. 2012, 18, 11007−11018.
(20) Alberto, R.; Schibli, R.; Schubiger, A. P.; Abram, U.; Pietzsch,
H.- J.; Johannsen, B. J. Am. Chem. Soc. 1999, 121, 6076−6077.
(21) (a) Buchwald, S. L.; Wagaw, S.; Geis, O. Metal-Catalyzed
Arylations of Hydrazines, Hydrazones, and Related Substrates. U.S.
Patent US 006235936 B1, 2001. (b) Wagaw, S.; Yang, B. H.;
Buchwald, S. L. J. Am. Chem. Soc. 1998, 120, 6621−6622.
(22) (a) Muniz, K.; Iglesias, A. Angew. Chem., Int. Ed. 2007, 46,
6350−6353. (b) Lundgren, R. J.; Stradiotto, M. Angew. Chem., Int. Ed.
2010, 49, 8686−8690.
(6) (a) Lever, A. B. P. Coord. Chem. Rev. 2010, 254, 1397−1405.
(b) Lever, A. B. P.; Masui, H.; Metcalfe, R. A.; Stufkens, D. J.;
Dodsworth, E. S.; Auburn, P. R. Coord. Chem. Rev. 1993, 125, 317−
332. (c) Gorelsky, S. I.; Dodsworth, E. S.; Lever, A. B. P.; Vlcek, A. A.
Coord. Chem. Rev. 1998, 174, 469−494.
(7) (a) Pierpont, C. G. Inorg. Chem. 2011, 50, 9766−9772. (b) Gray,
H. B. Coord. Chem. Rev. 1966, 1, 156−163. (c) Dithiolene Chemistry:
Synthesis, Properties, and Applications. In Progress in Inorganic
Chemistry; Stiefel, E. I., Ed.; John Wiley and Sons: Hoboken, NJ,
2004; Vol. 52. (d) Forum Issue on Redox Noninnocent Ligands:
Inorg. Chem. 2011, 50, 9737−9740.
(23) Gu, L.; Neo, B. S.; Zhang, Y. Org. Lett. 2011, 13, 1872−1874.
(24) Arterburn, J. B.; Rao, K. V.; Ramdas, R.; Dible, B. R. Org. Lett.
2001, 3, 1351−1354.
(8) (a) Lu, C. C.; Weyhermuller, T.; Bill, E.; Wieghardt, K. Inorg.
̈
(25) Raghavendra, B. S.; Chakravorty, A. Indian J. Chem. 1976, 14A,
166−169.
Chem. 2009, 48, 6055−6064. (b) Lu, C. C.; Bill, E.; Weyhermuller, T.;
̈
Bothe, E.; Wieghardt, K. J. Am. Chem. Soc. 2008, 130, 3181−3197.
(c) Bart, S. C.; Chłopek, K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky,
E.; Neese, F.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128,
(26) Buriez, O.; Moretto, L. M.; Ugo, P. Electrochim. Acta 2006, 52,
958−964.
(27) Goswami, S.; Mukherjee, R. N.; Chakravorty, A. Inorg. Chem.
1983, 22, 2825−2832.
13901−13912. (d) Wang, M.; Weyhermuller, T.; England, J.;
̈
Wieghardt, K. Inorg. Chem. 2013, 52, 12763−12776. (e) Scarborough,
(28) Blanchard, S.; Neese, F.; Bothe, E.; Bill, E.; Weyhermu1ller, T.;
̈
C. C.; Lancaster, K. M.; DeBeer, S.; Weyhermuller, T.; Sproules, S.;
̈
Wieghardt, K. Inorg. Chem. 2005, 44, 3636−3656.
Wieghardt, K. Inorg. Chem. 2012, 51, 3718−3732. (f) Wang, M.;
(29) Dougan, S. J.; Melchart, M.; Habtemariam, A.; Parsons, S.;
Sadler, P. J. Inorg. Chem. 2006, 45, 10882−10894.
(30) (a) Lu, C. C.; Bill, E.; Weyhermuller, T.; Bothe, E.; Wieghardt,
K. Inorg. Chem. 2007, 46, 7880−7889. (b) Lu, C. C.; Bill, E.;
Weyhermuller, T.; Bothe, E.; Wieghardt, K. J. Am. Chem. Soc. 2008,
England, J.; Weyhermuller, T.; Kokatam, S.-L.; Pollock, C. J.; DeBeer,
̈
S.; Shen, J.; Yap, G. P. A.; Theopold, K. H.; Wieghardt, K. Inorg. Chem.
2013, 52, 4472−4487. (g) Wile, B. M.; Trovitch, R. J.; Bart, S. C.;
Tondreau, A. M.; Lobkovsky, E.; Milsmann, C.; Bill, E.; Wieghardt, K.;
Chirik, P. J. Inorg. Chem. 2009, 48, 4190−4200. (h) Chaudhuri, P.;
̈
̈
130, 3181−3197.
Verani, C. N.; Bill, E.; Bothe, E.; Weyhermuller, T.; Wieghardt, K. J.
̈
(31) (a) Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Angew. Chem., Int.
Ed. 2005, 44, 4442−4489. (b) Meijere, A. d.; Diederich, F. Metal-
Catalyzed Cross-Coupling Reactions; Wiley-VCH: Weinheim, 2014.
(32) (a) Hartwig, J. F. Synlett 2006, 1283−1294. (b) Buchwald, S. L.;
Mauger, C.; Mignani, G.; Scholz, U. Adv. Synth. Catal. 2006, 348, 23−
39. (c) Wolfe, J. P.; Wagaw, S.; Marcoux, J. F.; Buchwald, S. L. Acc.
Chem. Res. 1998, 31, 805−818.
(33) (a) Ma, Q. U.S. Patent US 7608568 B2, 2009. (b) Koenig, K.-H.
U.S. Patent US 3683026, 1972.
(34) Gulaczyk, I.; Krezglewski, M.; Valentin, A. J. Mol. Spectrosc.
Am. Chem. Soc. 2001, 123, 2213.
(9) (a) Chaudhuri, P.; Hess, M.; Florke, U.; Wieghardt, K. Angew.
̈
Chem., Int. Ed. 1998, 37, 2217−2220. (b) Chaudhuri, P.; Hess, M.;
Muller, J.; Hildenbrand, K.; Bill, E.; Weyhermuller, T.; Wieghardt, K. J.
̈
̈
Am. Chem. Soc. 1999, 121, 9599−9610. (c) Mukherjee, C.; Pieper, U.;
Bothe, E.; Bachler, V.; Bill, E.; Weyhermuller, T.; Chaudhuri, P. Inorg.
Chem. 2008, 47, 8943−8956.
̈
(10) (a) Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2, 270−279.
(b) Dzik, W. I.; Vlugt, J. I. V. D.; Reek, J. N. H.; Bruin, B. D. Angew.
Chem., Int. Ed. 2011, 50, 3356−3358. (c) Smith, A. L.; Hardcastle, K.
I.; Soper, J. D. J. Am. Chem. Soc. 2010, 132, 14358−14360.
(d) Praneeth, V. K. K.; Ringenberg, M. R.; Ward, T. R. Angew.
Chem., Int. Ed. 2012, 51, 10228−10234.
(11) (a) Kaim, W. Dalton Trans. 2003, 761−768. (b) Kaim, W.;
Schwederski, B. Coord. Chem. Rev. 2010, 254, 1580−1588.
(12) (a) Vlugt, J. I. V. D.; Reek, J. N. H. Angew. Chem., Int. Ed. 2009,
2003, 220, 132−136.
(35) Wu, R.; Brutschy, B. J. Phys. Chem. A 2004, 108, 9715−9720.
(36) Baldwin, D. A.; Lever, A. B. P.; Parish, R. V. Inorg. Chem. 1969,
8, 107−115.
(37) Ge, S.; Hartwig, J. F. Angew. Chem., Int. Ed. 2012, 51, 12837−
12841.
(38) Araneda, J. F.; Piers, W. E.; Heyne, B.; Parvez, M.; McDonald,
R. Angew. Chem., Int. Ed. 2011, 50, 12214−12217.
(39) Boens, N.; Leen, V.; Dehaen, W. Chem. Soc. Rev. 2012, 41,
1130−1172.
48, 8832−8846. (b) Grutzmacher, H. Angew. Chem., Int. Ed. 2008, 47,
̈
1814−1818.
(13) Luca, O. R.; Crabtree, R. H. Chem. Soc. Rev. 2013, 42, 1440−
1459.
(14) (a) Schmidt, E. W. Hydrazine and its Derivatives: Preparation,
Properties, Applications, 2nd ed.; Wiley-Interscience: Hoboken, NJ,
2001. (b) Syngenta Limited. Process for the Preparation of Aryl
(40) (a) Campbell, N.; Henderson, A. W.; Taylor, D. J. Chem. Soc.
1953, 1281−1285. (b) Priewisch, B.; Ruck-Braun, K. J. Org. Chem.
̈
2005, 70, 2350−2352.
K
dx.doi.org/10.1021/ic501656s | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(41) (a) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467−473.
(b) Sheldrick, G. M. SHELXL 97, Program for the Refinement of Crystal
Structures; University of Gottingen: Gottingen, Germany, 1997.
̈
̈
(42) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;
Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J.
B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J.
J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 09, Revision
A.02; Gaussian, Inc.: Wallingford, CT, 2010.
(43) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (b) Lee,
C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter Mater. Phys.
1988, 37, 785−789. (c) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys.
1980, 58, 1200−1211. (d) Stephens, P. J.; Devlin, F. J.; Chabalowski,
C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623−11627.
(44) Schafer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100,
̈
5829−5835.
(45) Schafer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97,
̈
2571−2577.
(46) Ginsberg, A. P. J. Am. Chem. Soc. 1980, 102, 111−117.
(47) (a) Noodleman, L.; Case, D. A.; Aizman, A. J. Am. Chem. Soc.
1988, 110, 1001−1005. (b) Noodleman, L.; Davidson, E. R. Chem.
Phys. 1986, 109, 131−143. (c) Noodleman, L.; Norman, J. G., Jr.;
Osborne, J. H.; Aizman, C.; Case, D. A. J. Am. Chem. Soc. 1985, 107,
3418−3426. (d) Noodleman, L. J. Chem. Phys. 1981, 74, 5737−5743.
(48) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1833−1840.
L
dx.doi.org/10.1021/ic501656s | Inorg. Chem. XXXX, XXX, XXX−XXX