Redox Noninnocent Azo-Aromatic Pincers and Their Iron Complexes. Isolation, Characterization, and Catalytic Alcohol Oxidation

Article

Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX

pubs.acs.org/IC

Redox Noninnocent Azo-Aromatic Pincers and Their Iron Complexes.

Isolation, Characterization, and Catalytic Alcohol Oxidation

†

‡

§

Suman Sinha, Siuli Das, Rina Sikari, Seuli Parua, Paula Brandao, Serhiy Demeshko, Franc Meyer,

̃

,

†

and Nanda D. Paul*

†

Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Botanic Garden, Howrah 711103, India

Departamento de Química/CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal

‡

§

Universitat

̈

̈ ̈ ̈

Gottingen, Institut fur Anorganische Chemie, Tammannstrasse 4, D-37077 Gottingen, Germany

*

S Supporting Information

ABSTRACT: The new redox-noninnocent azoaromatic pincers 2-

1

(

arylazo)-1,10-phenanthroline (L ) and 2,9-bis(phenyldiazo)-1,10-phe-

2

1

nanthroline (L ) are reported. The ligand L is a tridentate pincer having

NNN donor atoms, whereas L is tetradentate having two azo-N donors

2

and two N-donor atoms from the 1,10-phenanthroline moiety. Reaction of

1

2

FeCl with L or L produced the pentacoordinated mixed-ligand Fe(II)

2

1

2

complexes FeL Cl (1) and FeL Cl (2), respectively. Homoleptic

octahedral Fe(II) complexes, mer-[Fe(L ) ](ClO ) [3](ClO₄)₂and

mer-[Fe(L ) ](ClO ) [4](ClO ) , have been synthesized from the

reaction of hydrated Fe(ClO ) and L or L . The ligand L , although

2

1

2

4 2

2

4 2

1

2

4

2

having four donor sites available for coordination, binds the iron center in

a tridentate fashion with one uncoordinated pendant azo function.

Molecular and electronic structures of the isolated complexes have been

scrutinized thoroughly by various spectroscopic techniques, single-crystal

X-ray crystallography, and density functional theory. Beyond mere

characterization, complexes 1 and 2 were successfully used as catalysts

for the aerobic oxidation of primary and secondary benzylic alcohols. A

wide variety of substituted benzyl alcohols were found to be converted to the corresponding carbonyl compounds in high yields,

catalyzed by complex 1. Several control reactions were carried out to understand the mechanism of this alcohol oxidation

reactions.

INTRODUCTION

years, several new ligands were designed and synthesized with

the basic diamine/aminophenol part kept intact. The diimine

■

6

The chemistry of redox-noninnocent ligands, particularly when

they are coordinated to transition-metal ions, has been an active

area of research over the years. Redox-noninnocent ligands are

often found to play crucial roles in important natural processes

mediated by metalloenzymes. This class of ligands not only

types of ligands have also been of particular interest in recent

times due to their rich redox chemistry as well as extensive

catalytic applications. Chirik and co-workers explored several

unusual chemical transformations using base metal (particularly

Fe and Co) complexes of 2,6-bis(arylimino)pyridine where the

ligand-centered redox events play a crucial role during catalytic

1

2

oﬀers binding to the metal ions but also can participate in

electron transfer processes. Over the decades the main research

in this area was focused on understanding the ambiguous

electronic structure and bonding of such complexes. However,

recently, employing the redox-active nature of these ligands,

increasing eﬀorts are devoted to the use of these complexes to

bring about useful chemical transformations, catalysis in

7

turnover. In contrast, the chemistry of azo-aromatic ligands has

been less explored, although their redox-noninnocent character

has been known for some time. The presence of low-lying π*

orbitals makes this class of ligands susceptible toward reduction,

which indeed confers the ability to act as an electron sink

during catalytic turnover.

3

particular. Other than catalytic applications, the ligand-

Kaim and co-workers made a signiﬁcant contribution to this

centered redox events have also found several useful

applications in molecular electronics as well.

Therefore, the design and synthesis of new redox-non-

innocent ligands having multifunctional properties are of

immense interest. Among the various redox noninnocent

ligands available, the chemistry of the diamine and amino-

4

ﬁeld with various azo-aromatic ligands, particularly with the

8

2

,2′-azobipyridine ligand system. On the other hand, Goswami

and co-workers extensively studied the chemistry of 2-

Received: August 31, 2017

5

phenol ligand systems has been studied extensively. In recent

©

XXXX American Chemical Society

A

DOI: 10.1021/acs.inorgchem.7b02238

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

arylazo)pyridine and related ligand systems. A few other

Article

9

(

respectively. The experimental spectral features of the ligands

correspond well to the simulated isotopic pattern for the given

groups also studied the coordination chemistry of some further

10

1b

+

azo-aromatic ligands. However, unlike with the diamine/

aminophenol ligand systems, the chemistry with redox-active

azo-aromatics has been mainly limited to studies of structure−

formulations. The representative spectrum of [L + H ] along

with its simulated pattern are shown in the Figures S1 and S2 in

1

1a,b

the Supporting Information. The H NMR spectra of L and

8

−10

2

bonding relationships.

There are only a few reports or

L

are displayed in Figures S3 −S5 in the Supporting

attempts known where azo-aromatic ligands were exploited to

perform catalytic reactions utilizing the redox-active nature of

the azo chromophore. Recently, in a seminal work Goswami

and co-workers reported the synthesis of the tridentate azo-

aromatic pincer 2,6-bis(phenylazo)pyridine, and they have

successfully used its nickel complexes for catalytic alcohol

oxidation reactions where the azo-aromatic ligand acts as an

Information.

After successful characterization of the ligands, their

coordination behavior was explored with some chosen iron

1

a,b,2

salts. The reaction of L

(0.17 mmol) with FeCl (0.17

2

mmol) produced the new, air-stable brown neutral complexes

1

a,b

2

[Fe(L )Cl ] (1a,b) and [Fe(L )Cl ] (2) in nearly 90% yield

2

1

a,b,2

(Scheme 1). On the other hand, the reaction of L

with

11

electron reservoir during catalytic turnover.

Herein we introduce the two new redox noninnocent azo-

Scheme 1. Synthesis of the Iron Complexes

1

aromatic scaﬀolds 2-(arylazo)-1,10-phenanthroline (L ) and

2

,9-bis(phenyldiazo)-1,10-phenanthroline (L ) with the aim of

developing new azo-aromatic ligand-based catalysts for bringing

about useful chemical transformations (Figure 1). The ligand

hydrated Fe(ClO ) yielded the dicationic green air-stable

4

2

1

a,b

complexes [Fe(L ) ](ClO ) ([3a](ClO ) , [3b](ClO ) )

2

4 2

2

and [Fe(L ) ](ClO ) ([4](ClO ) ) in nearly 80% yield.

2

4 2

Elemental analysis along with positive-ion ESI mass spectra of

these complexes convincingly support their formulations (see

the Supporting Information ). For example, complex 1a showed

Figure 1. Ligands introduced in this paper.

_L1 _i_s_a_t_r_i_d_e_n_t_a_t_e_p_i_n_c_e_r_h_a_v_i_n_g_N_N_N_d_o_n_o_r_a_t_o_m_s_,_w_h_e_r_e_a_s_L²

+

an intense peak due to the molecular ion [1a] at m/z = 409

is a tetradentate ligand having two azo-N donors and two N-

donor atoms from the 1,10-phenanthroline moiety. Other than

the synthesis of the ligands, as a ﬁrst report, their coordination

chemistry with ferrous salts will be described. The ligands along

with their Fe complexes have been characterized thoroughly

using diﬀerent spectroscopic techniques in combination with

density functional theory (DFT) studies. Beyond mere

characterization, to explore some ﬁrst catalytic applications,

catalytic alcohol oxidation reactions were chosen because of

their relevance to chemical energy conversion using renewable

amu. Notably, the experimental spectral features of the

complexes correspond well to the simulated isotopic pattern

for the given formulation. A representative spectrum, that of 1a

along with the simulated spectrum, is shown in Figure S9 in the

Supporting Information. Complexes 1 and 2 have been found

to be paramagnetic in nature. The room-temperature magnetic

moment μ of complex 1a is 4.96 μ , while that of 2 is 4.97 μ .

eff

B

These experimental ﬁndings indeed indicate the presence of

high-spin Fe(II) ions. On the other hand, complexes [3a]-

(ClO ) , [3b](ClO ) , and [4](ClO ) possess an S = 0 ground

4

2

4 2

1

2

resources. Interestingly, the pentacoordinated Fe complexes 1

and 2 showed excellent catalytic activity in the oxidation of

both primary and secondary benzylic alcohols.

state, as determined by magnetic susceptibility measurements at

room temperature. Complexes [3a](ClO ) and [3b](ClO )

4

2

4 2

1

showed sharp H NMR signals in the normal range of

diamagnetic compounds; the coordinated ligands were found to

be magnetically equivalent on the NMR time scale (Figure S12

in the Supporting Information). However, the observation of

RESULTS AND DISCUSSION

Synthesis. The ligands L and L were synthesized via the

condensation of nitrosobenzene with the corresponding amino-

■

1

2

1

multiple overlapping signals in the H NMR spectrum of

1

,10-phenanthrolines under highly alkaline conditions. Reaction

[4](ClO ) points to a diﬀerent coordination mode of the two

4

2

of 2-amino-1,10-phenanthroline with nitrosobenzene in a 1:1

molar ratio produced the ligand L in nearly 70% yield, whereas

coupling of 2,9-diamino-1,10-phenanthroline with nitrosoben-

zene gave L in 60% yield. Amino-1,10-phenanthrolines were

azo chromophores in L , which was con ﬁrmed later by an X-ray

structure determination (Figure S15 in the Supporting

Information).

1

2

X-ray Crystallography. The newly synthesized ligand L¹^a

and the iron complexes have been characterized by X-ray

diﬀraction. Crystallographic parameters are collected in Table

1.

13

synthesized following an available literature procedure. After

initial workup followed by chromatographic puriﬁcation, the

identity of the newly synthesized orange ligands was conﬁrmed

by electrospray ionization (ESI) mass spectrometry. It showed

1

2

The free base ligands, L and L , are semisolid at room

temperature, but X-ray diﬀraction (XRD) quality single crystals

an intense peak originating from the molecular ion [L + H]⁺

1

b

2

+

1a

at m/z = 319 amu and [L + H] at m/z = 389 amu,

could be obtained for the perchlorate salt of L by slow

B

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1

a

1a •−

1a 2−

Table 1. Summary of Selected Experimental and Calculated Bond Lengths (Å) and Bond Angles (deg) of L , [L ] , [L ] ,

L , [L ] , [L ] , 1a, 2, [3a] , and [4] (n = 0−3)

2

2 •−

2 ••2−

n+

Fe−N1

Fe−N3

Fe−N4

N1−N2

N1−Fe−N3

N3−Fe−N4

1

a

[

HL ]ClO

X-ray

DFT

X-ray

DFT

X-ray

DFT

X-ray

DFT

X-ray

DFT

1.267(3)

1.27547

1.34088

1.37861

1.27823

1.31328

1.35962

1.290(2)

1.26492

1.276(3)

1.26408

1.28450

1.288(6)

1.28232

1.31373

1.34234

1.27797

1.300(5)

1.27783

1.32754

4

1

a

•−

2−

[

L ]

1

a

L ]

2

L

2

•−

[

L ]

2

••2−

L ]

1

a

2.149(1)

2.53472

2.225(2)

2.54879

2.12834

1.980(4)

2.11694

2.0612

2.031(1)

2.14537

2.046(2)

2.16204

1.89847

1.875(4)

1.89826

1.89448

1.89634

1.89543

1.854(3)

1.89632

1.89456

2.269(1)

2.27992

2.255(2)

2.28989

2.08110

2.054(4)

2.09471

2.09848

2.10891

2.13203

2.053(4)

2.13429

2.22246

72.35(4)

75.27154

70.75(3)

66.848

75.835

76.1(2)

75.851

76.377

76.844

76.443

77.0(2)

76.252

77.902

74.24(4)

67.26568

74.14(7)

74.642

80.692

80.4(2)

80.265

80.009

79.579

80.227

80.1(1)

79.673

78.055

2

3

2

+

[

3a]

[

3a]⁺

3a]

2.03934

2.09830

1.953(4)

2.10475

2.00960

3

+

4]

2

+

[

4]⁺

1

a

evaporation of an acetonitrile solution of L containing a drop

of perchloric acid. An ORTEP view of the molecular structure

1

a

of the cation of HL ·ClO is shown in Figure 2. The X-ray

4

1b

Figure 3. ORTEP view of FeL Cl (1b) with ellipsoids at the 50%

2

probability level. Hydrogen atoms are omitted for clarity.

1

a

Figure 2. ORTEP view of L -H with ellipsoids at the 50% probability

−

level. The ClO₄counterion and hydrogen atoms are omitted for

clarity.

(

1.267(3) Å). Notably, in similar systems, N−N distances are

considered to be an excellent indicator of the charge and the

8

,9

oxidation state of the coordinated ligand. The N−N bond

elongation sometimes reported to occur because of dπ(metal)

→ π*(L) back-bonding interactions and in some cases

complete transfer of an electron to the π* orbital of the

ligand, producing an azo-anion radical, was also found to be

responsible. Therefore, elongated N−N bonds in coordination

complexes always lead to ambiguity with respect to the

oxidation state formalism. In complex 1b, the elongated N−N

bond indeed leads to two diﬀerent limiting electronic structure

structure indeed conﬁrms the formation of the new planar azo-

1a

aromatic pincer L . It shows a trans geometry about the diaza

chromophore. The N−N distance is 1.267(3) Å, indicative of a

double bond.

Single crystals of complex 1b were obtained by slow

evaporation of its solution in a dichloromethane−hexane

solvent mixture. An ORTEP view of the molecular structure

of complex 1b with partial atom numbering scheme is displayed

in Figure 3. The geometry of the complex was found to be

intermediate between ideal trigonal-bipyramidal and square-

pyramidal geometry with a calculated τ (index of trigonality)

value of 0.50 (for an ideal trigonal-bipyramdal geometry τ = 1

and for an ideal square-pyramidal geometry τ = 0). Two N-

donor atoms N1(azo) and N4(phen) occupy the apical

positions with a N1−Fe−N4 angle of 145.15°. The N−N

bond length in the coordinated ligand was found to be slightly

elongated at 1.290(2) Å in comparison to the free ligand

II 1b

III 1b •−

possibilities, [Fe (L )Cl ] and [Fe (L ) Cl ]; as will be

2

described below, the situation was clariﬁed using diﬀerent

spectroscopic techniques combined with DFT studies.

X-ray-quality crystals for complex 2 were obtained via the

slow diﬀusion of its dichloromethane solution into hexane. An

ORTEP view of the molecular structure of complex 2 with

partial atom numbering scheme is displayed in Figure 4. The X-

ray structure conﬁrms the formation of the new tetradentate

2

ligand L and formation of its Fe complex having the molecular

C

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1

in accordance with H NMR spectroscopic solution studies.

The N−N distances were found to be 1.288(6) and 1.282(6) Å,

2

+

respectively. Given the diamagnetic nature of complex [3] as

revealed from room-temperature magnetic susceptibility

1

measurements and H NMR analysis, the N−N bond

II

1

elongation was ascribed to dπ(Fe ) → π*(L ) back-bonding

interactions. The other limiting electronic structure possibil-

IV

1 •−

2+

III 1 •−

1

2+

ities, [Fe {(L ) } ] and [Fe (L ) (L )] with S = 0

2

ground state, both having elongated N−N distances, however,

could not be excluded at this point on the basis of X-ray

5

7

crystallographic characterization. However, Fe Mo

̈

ssbauer

spectroscopic analysis along with DFT studies indeed are in

agreement with the closed-shell singlet (S = 0) [Fe (L ) ]

II 1a,b

-

2

(

ClO ) electronic structure description (cf. below).

4 2

Single crystals of complex [4](ClO ) were obtained by slow

4

2

evaporation of its solution in an acetonitrile−toluene solvent

2

mixture. The molecular structure of complex [4](ClO )

Figure 4. ORTEP view of FeL Cl (2) with ellipsoids at the 50%

4 2

2

2+

probability level. Hydrogen atoms are omitted for clarity.

consists of one [Fe(L ) ] cation and two perchlorate anions,

2

II

2

leading to the molecular formula [Fe (L ) ](ClO ) (Figure 6).

2

4 2

2

formula [FeL Cl ]. The metal ion in complex 2 is

2

pentacoordinated. The τ value was calculated to be 0.18. This

value is intermediate between trigonal-bipyramidal and square-

pyramidal geometry and is closer to the latter (ideal τ = 0). The

2

ligand L binds the central Fe center in a tridentate fashion with

one uncoordinated pendant azo chromophore. The two N-

donor atoms N1(azo) and N4(phen) occupy the apical

positions with an N1−Fe−N4 angle of 143.47°. Complex 2

shows two distinct N−N bond lengths; the azo chromophore

coordinated to the Fe center was found to be elongated

(

1.276(3) Å), whereas the uncoordinated chromophore has a

N−N distance of 1.253(2) Å.

Single crystals of complex [3b](ClO ) suitable for X-ray

4

2

diﬀraction were obtained by slow diﬀusion of its acetonitrile

solution into toluene. An ORTEP view of the molecular

structure of complex [3b](ClO ) with partial atom numbering

2

Figure 6. ORTEP view of [Fe(L ) ](ClO ) ([4](ClO ) ) with

2

4

2

4 2

−

ellipsoids at the 50% probability level. The ClO₄counterions and

hydrogens are omitted for clarity.

4

2

scheme is displayed in Figure 5. The molecular structure of

1

b

2+

complex [3b](ClO ) consists of one [Fe(L ) ] cation and

4

2

two perchlorate anions, leading to the molecular formula

Two tetradentate ligands L bind to the central Fe ion in a

II 1b

2+

[

Fe (L ) ](ClO ) . The geometry of the cation [3b] is a

tridentate meridional fashion, yielding a distorted-octahedral

geometry with one uncoordinated pendent azo chromophore

from each ligand. Complex [4]²⁺also possesses an approximate

2

4 2

distorted octahedron comprising two meridionally coordinating

ligands. The presence of an approximate (noncrystallographic)

C₂axis makes half of the molecule identical with the other half,

(noncrystallographic) C axis of symmetry. The coordinated

2

azo chromophores were found to be slightly elongated (N−N =

1

.300(5) Å), whereas the uncoordinated pendent azo

chromophores have N−N distances of 1.248(6) Å.

Magnetic Susceptibility Measurements and Mo

̈

ssba-

uer Spectroscopy. To conﬁrm the spin states and to study

the magnetic properties of the new Fe complexes, variable-

temperature magnetic susceptibility measurements were

performed on polycrystalline samples of 1b and [3b](ClO₄)₂

in the temperature range 2−295 K. The temperature

dependence of the magnetic behavior (χ T versus T plot, χ

M

being the molar magnetic susceptibility) is shown in Figure 7.

3

−1

At 295 K, the value of the product χ T (3.08 cm mol K) for

M

1

=

b is close to the spin-only value for high-spin Fe(II) with an S

2 ground state (3.00 cm mol K). As shown in Figure 7,

3

−1

χ T remains almost unaltered in the temperature range 55−

M

2

95 K; however, below 50 K χ T starts decreasing and reaches

M

3

−1

a limiting low-temperature value of 0.54 cm mol K at 2 K.

Modeling of the experimental data using an appropriate spin

Hamiltonian for zero-ﬁeld splitting and Zeeman interaction and

including a term for temperature-independent paramagnetism

1

b

Figure 5. ORTEP view of [Fe(L ) ](ClO ) ([3b](ClO ) ) with

2

4

2

4 2

−

ellipsoids at the 50% probability level. The ClO₄counterions and

−

1

hydrogens are omitted for clarity.

(TIP) provided the values g = 2.02, |D| = 15.2 cm , and TIP =

D

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Inorganic Chemistry

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at 7 K, whereas at 200 K the asymmetry almost vanishes; see

the Supporting Information) and, therefore, it can be attributed

to the spin−lattice relaxation. At 80 K complex [3b](ClO )

4

2

shows a doublet with isomer shift δ = 0.22 mm/s and

quadrupole splitting ΔE = 1.80 mm/s, typical for low-spin

Q

9

c,15

Fe(II) ions.

[

Other electronic structure possibilities such as

Fe {(L ) } ] and [Fe (L ) (L )] , having either low-

IV

1 •−

2+

III 1 •−

1

2+

2

spin Fe(IV) or low-spin Fe(III) centers coupled antiferromag-

netically with ligand radicals, would have a much lower isomer

shift or much larger quadrupole splitting, respectively.

Cyclic Voltammetry and EPR Spectroscopy. Cyclic

1

a,b

2

voltammograms of L , L , 1a, 1b, 2, [3a](ClO ) , [3b]-

4

2

(

ClO ) , and [4](ClO ) were recorded in CH CN solutions

4 2 4 2 3

containing 0.1 M [Bu N]ClO₄at 25 °C; potentials are

referenced to the saturated Ag/AgCl electrode. The results

4

1

b

2

are summarized in Table 2. The voltammograms for L , L ,

3a](ClO ) , and [4](ClO ) are displayed in Figure 9, and

[

4

2

4 2

those of 1a and 2 are shown in Figure S23 in the Supporting

Information.

Figure 7. χ T vs T plot of 1b (circles, black) and [3b](ClO )

1a

M

4

2

The ligand L shows a reversible reductive response at

(

circles, red, bottom). The solid lines represent the best ﬁts (see text

−

0.89 V along with an irreversible response at −1.34 V. The

for details).

2

ligand L , on the other hand, showed two reversible reductive

responses at −0.96 and −1.11 V. It is worth mentioning here

8

.2 × 10⁻cm mol . For [3b](ClO ) , χ T remains close to

4

3

−1

that these reduction potential values of L

1a/b

and L are

2

4

2

M

3

−1

zero over the whole temperature range (0.05 cm mol K in

the range 25−295 K) and can be simulated by assuming a small

amount of paramagnetic impurity (1.2%, S = 5/2) and TIP (3.7

10 cm mol ). These experimental data are indeed

consistent with the diamagnetic nature of complex [3b](ClO )

anodically shifted in comparison to their similar analogues 2,6-

3

4

bis(arylimino)pyridine (L ) and 2,6-bis(arylazo)pyridine) (L ),

9

c,14,15

respectively.

Complexes 1a,b showed two reductive

−4

3

−1

×

waves in the potential range −0.2 to −0.8 V. Notably the

ﬁrst reductive response is reversible, while the second response

is quasi-irreversible in nature. Complex 2 shows two irreversible

reductive waves at −0.21 and −0.81 V. The complex

4

2

1

as revealed by H NMR spectral analysis.

To get a closer look at the electronic structures and oxidation

57

state assignments of these Fe complexes, zero-ﬁeld Fe

Mossbauer spectroscopic analyses were also carried out on

the polycrystalline samples of 1b and [3b](ClO ) . At 80 K the

[3a](ClO ) shows a one-electron reversible oxidation at 0.22

4 2

̈

V, two reversible one-electron reduction processes at −0.32 and

−0.82 V, respectively, and two quasi-reversible reduction

processes at −0.93 and −1.350 V, respectively. The one-

electron nature of these processes has been conﬁrmed by

comparison of the current height with the redox response of

the ferrocene/ferrocenium couple under identical experimental

conditions and also by exhaustive electrolyses of [3a](ClO ) at

0.3 V for the reversible oxidation and at −0.4 and −0.9 V,

respectively, for the two reversible reductive responses. The

complex [4](ClO ) showed a similar one-electron oxidation at

0.15 V and multiple reversible/quasi-reversible reductive waves

4

2

spectrum of 1b shows a doublet with isomer shift δ = 0.62 mm/

s and quadrupole splitting ΔE = 1.87 mm/s (Figure 8). The

Q

isomer shift is somewhat lower than typical values for ﬁve-

coordinate high-spin Fe(II) complexes but can be explained by

the strong π-acceptor nature of the ligand system, similar to the

ﬁndings for related systems reported by Goswami and co-

4

2

9

c

workers. The even lower value of the isomer shift of 1b in

3

,4

3

comparison to the similar Fe(II) complexes FeL Cl (L =

2

4 2

4

,6-bis(arylimino)pyridine; L = 2,6-bis(arylazo)pyridine) may

be attributed to the enhanced dπ(metal) → π*(L) back-

in the potential range −0.3 to −1.5 V.

9c,14

donation in the case of 1b.

0 K is slightly asymmetric. Interestingly, the asymmetry eﬀect

is temperature dependent (a broader spectrum can be observed

The quadrupole doublet of 1b at

To gain insight into the nature of the electronic levels

associated with the reversible redox responses, electrochemi-

cally generated oxidized and reduced complexes were studied

8

Figure 8. Zero-ﬁeld ⁵⁷Fe Mo

simulations with Lorentzian doublets.

ssbauer spectra of (a) 1b (blue circles) and (b) [3b](ClO ) (purple circles) at 80 K. The solid lines represent

4 2

̈

E

DOI: 10.1021/acs.inorgchem.7b02238

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Inorganic Chemistry

Table 2. Cyclic Voltammetry Data of L , L , L , 1a, 1b, 2, [3a](ClO ) , [3b](ClO ) , and [4](ClO )

4 2

Article

a

,

b

1a

1b

2

4

2

4 2

compound

oxidation (V) (ΔE (mV))

reduction (V) (ΔE (mV))

p

1

2

a

L

−0.89 (110), −1.34

−0.87 (100), −1.31

−0.96 (75), −1.11 (85)

−0.20(71), −0.73(170)

−0.19 (75), −0.71(170)

−0.21, −0.81

b

1

2

a

b

0.80

[

3a](ClO₄)₂

3b](ClO₄)₂

4](ClO ) )

0.22 (70)

0.24 (70)

0.15 (72)

−0.32 (72), −0.82 (75), −0.93, −1.35

−0.30 (70), −0.81 (72), −0.90, −1.34

−0.33 (68), −0.81 (63), −1.00 (140), −1.17 (74), −1.48 (92)

4

a

b

−3

Conditions: solvent, CH CN; supporting electrolyte, Bu NClO (0.1 M); reference electrode, saturated Ag/AgCl. Solute concentration ca. 10

3

4

−1

M. ΔE = E − E ; scan rate 50 mV s .

p

pa

pc

similar type. However, no ¹⁴N hyperﬁne coupling is observed

9

in the EPR spectra under our experimental conditions. The

−

one-electron-reduced complex [1b] generated by exhaustive

electrolysis at −0.35 V under an argon atmosphere shows a

strong-single line EPR signal at g = 1.996 at 77 K. The one-

electron-reduced complexes [3a]⁺and [4] , generated by

exhaustive electrolysis at −0.4 V, also showed single-line EPR

spectra characteristic of organic radicals (Figure 10). The two-

electron-reduced species 3a and 4 were found to be EPR silent.

Attempts were also made to characterize the three- and four-

+

−

2−

electron-reduced complexes [4] and [4] ; however, we could

not generate them in a pure state by exhaustive electrolysis.

These experimental results are in agreement with DFT

calculations, which show that lowest unoccupied molecular

1

a

1b

2

orbitals (LUMO, LUMO+1, LUMO+2) of L , L , L , 1, 2,

3](ClO ) , and [4](ClO ) are primarily ligand centered,

[

4

2

4 2

mainly localized on the azo chromophore with partial

delocalization on the phenanthroline moiety. The reductions

in these complexes are, therefore, assigned to occur at the

ligands rather than at the metal, the oxidation state of the metal

remaining invariant. The highest occupied molecular orbitals in

[

3](ClO ) and [4](ClO ) were, however, found to be mainly

4 2 4 2

localized on the metal. The reversible oxidative waves observed

for [3](ClO₄)₂and [4](ClO₄)₂were assigned as metal-

centered oxidations.

Electronic Structures and UV−Vis Spectra. The newly

synthesized ligands and all of their Fe complexes were

thoroughly investigated using density functional theory

Figure 9. Cyclic voltammograms of L¹^a(black), L (red), [3a](ClO )

2

4

2

(

green), and [4](ClO ) (blue) in CH CN.

4 2 3

(

DFT) with the B3LYP functional. The optimized structures

by EPR spectroscopy at 77 K. The one-electron-reduced

reproduce the experimental data quite well. Selected calculated

bond lengths and angles are compared with the corresponding

experimental values in Table 1.

1a −

2 −

ligands [L ] and [L ] showed single-line EPR spectra at g =

.005 and g = 2.004, as expected for ligand-centered radicals of

2

Figure 10. EPR spectra of [L ] (black), [1b]⁻(green) and [3a] (blue) after electrochemical reduction of the precursors L , 1b, and 3a,

1

a

•−

+

1a

respectively.

F

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Inorganic Chemistry

Article

Figure 11. Spin density plots of [L ] (a), [L²]^•⁻(b), and [L²]^•^•²⁻(c).

1a •−

Figure 12. Spin density plots of 1a (a), [3a]³⁺(b), and [3a] (c).

+

The ligands L¹^aand L along with their one- and two-

2

the complexes [3a]²⁺and [4]²⁺were also well reproduced by

DFT studies. Both complexes were found to have closed-shell

singlet (S = 0) electronic conﬁgurations best described as

1

a −

2 −

1a 2−

2 2−

electron-reduced analogues [L ] , [L ] , [L ] , and [L ]

were optimized. Calculated bond lengths of the ligands in three

diﬀerent oxidation states are summarized in Table 1 , and the

molecular orbital pictures are displayed in Figures S24 and S25

in the Supporting Information. Molecular orbital analysis

clearly reveals that the lowest unoccupied MOs of L and L

are mainly centered on the azo chromophore(s). The one-

electron-reduced [L ] and [L ] have doublet ground

states. In [L ] the net spin density is primarily delocalized

over the azo chromophore(s) and phenanthroline ring, whereas

in the case of [L ] the net spin density is found to be

delocalized over the two azo chromophores. As expected, in

L ] and [L ] the N−N distances were found to be

considerably elongated in comparison to those of the neutral

L and L . The two-electron-reduced ligand [L ] has an S =

ground state with an N−N distance of 1.379 Å. On the other

hand, the two-electron-reduced analogue of L possesses an S =

ground state having one electron on each azo chromophore

with an average N−N distance of 1.356 Å. The corresponding

singlet state is 3.03 kcal mol higher in energy. Spin density

plots for [L ] , [L ] , and [L ]

II

1,2

[Fe (L ) ]; all attempts to locate open-shell singlet

2

III

1a •−

1a 2+

IV

1a •−

2+

conﬁgurations [Fe (L ) (L )] or [Fe {(L ) } ]

2

using the BS-DFT approach converged back to the closed-

shell conﬁguration. In [3a]²⁺and [4] a low-spin Fe(II) center

is coordinated by two neutral tridentate ligands in a meridional

1

a

2

2+

1

a •−

2 •−

2+

fashion with an average N−N distance of 1.282 Å (for [3a] )

1a •−

2+

and 1.278 Å (for [4] ), respectively, in good agreement with

̈

the Mossbauer spectroscopic analysis. Calculated metrical

2

•−

parameters are collected in Table 1, and MOs are shown in

the Supporting Information. Molecular orbital analysis reveals

that the high-lying occupied MOs of the complexes 1a, 2,

1

a •−

2 •−

[

2+

[

3a] , and [4] are mainly metal-centered with a small

1

a

2

1a 2−

contribution from the ligands, whereas the low-lying

unoccupied MOs are ligand-based (Figures S26 −S29 in the

Supporting Information).

0

2

1

Geometry optimizations were also carried out on the reduced

and oxidized forms of complexes [3a]²⁺and [4] . Optimized

structural parameters for the computed ground states of 3a,

2+

−1

1a •−

2 •−

2 • •2−

are shown in Figure 11.

+

3+

+

3+

[

3a] , [3a] , 4, [4] , and [4] are collected in Table 1. Both

The most stable structure of complexes 1a and 2 was found

to have an S = 2 ground state with the electronic structure

3

+

3+

the one-electron-oxidized and -reduced complexes [3a] ,[4] ,

+

II 1a,2

II

[3a] , and [3] have S = 1/2 ground states. However, spin

density analysis shows that the net spin is mainly localized on

the Fe center (Figure 12b) in the case of the one-electron-

oxidized complexes, indicating metal-centered oxidation during

description [Fe (L )Cl ]. High-spin Fe (S_F_e= 2) is

2

1

a

coordinated to two chlorido ligands and a neutral ligand L

2

1

2

or L (S

= 0) that is bound to the iron center in a tridentate

L /L

fashion. In accordance with the experimentally observed

Mossbauer parameters, the alternative electronic structure

possibility, [Fe (L ) Cl ], having a high-spin Fe (S_F_e

2

+

3+

the [3/4] → [3/4] conversion, whereas the net spin density

was found to be localized mainly on the ligands in the one-

electron-reduced analogues (Figure 12c), pointing to the

̈

III 1,2 •−

III

=

2

5

/2) center coordinated to two chlorido ligands and a one-

electron-reduced azo anion radical ligand (L

) showing antiferromagnetic coupling was found to be 3.16

kcalmol higher in energy. The spin density plot of 1a is

shown in Figure 12, and that of complex 2 is displayed in

Figure S30 in the Supporting Information. The geometries of

1

/2 •−

2+

+

1

2

ligand-centered reduction during [3,4] → [3,4] conversion.

)

(S

= 1/

L /L

2

These results are in agreement with experiment: viz., EPR data

−1

+

indicating ligand -centered radicals for complexes [3] and [4] .

The two-electron-reduced complexes 3a and 4 feature an S = 0

ground state, as observed in the EPR studies.

G

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Inorganic Chemistry

Article

1

2

The optical absorption spectra of L , L , 1, and 2 were

recorded in dichloromethane, and those of [3](ClO ) and

as the catalyst. Various reaction conditions, including bases,

solvents, and reaction temperatures, were screened. The

reaction proceeded most eﬃciently in solvents such as heptane

and toluene, whereas reactions in more polar and potentially

coordinating solvents such as MeCN, dioxane, methanol, and

DMF aﬀorded poor yields. Among the series of bases tested

4

2

[

4](ClO ) were recorded in acetonitrile. The experimental

spectral results are summarized in Table 3, and the

4 2

a

Table 3. UV−Vis−NIR Data

t

only alkali-metal tert-butoxides (MO Bu, M = Li, Na, K) were

1

compound

L¹^a

λ_m_a_x(nm) (ε (M⁻cm⁻¹))

found to be eﬀective (Table 4, entries 8−11); with other bases

249 (15100), 287 (13900, s), 354 (12700)

241 (14300), 265 (10400, s), 356 (12900)

311 (40400), 358 (30800, s)

such as K PO , K CO , Cs CO , and NaOH, only a trace

3

4

2

3

2

3

L¹^b

L²

amount of alcohol oxidation was observed. The best result was

t

obtained with 10 mol % KO Bu as base. The highest conversion

1a

240 (20300), 332 (13200), 402 (14000), 555 (1540, s), 904

of diphenylmethanol (5i) to benzophenone (5ii) was achieved

when the reaction was carried out at 75 °C in toluene for 5 h in

(1819, b) (1819)

1b

342 (11050), 403 (12500), 546 (890, s), 911 (720, b)

318 (50700), 363 (35200), 924 (2040, b)

234 (20800), 296 (16050, s), 391 (18300), 674 (3500)

304 (16000, s), 392 (16200), 681 (1970)

305 (36700), 672 (2000)

t

the presence of 10 mol % KO Bu and 3.0 mol % of the catalyst

2

(

Table 4, entry 11). Lowering the temperature or catalyst

[

3a](ClO₄)₂

3b](ClO₄)₂

loading below 3.0 mol % leads to poor conversion of the

alcohols to the corresponding aldehydes or ketones.

Among the newly synthesized iron complexes, complex 1b

having an electron-withdrawing Cl group at the para position of

the phenyl ring was found to be more eﬀective than 1a (Table

4](ClO₄)₂

a

L¹^a^,^b, L , 1a,b, and 2 in CH Cl and [3a](ClO ) , [3b](ClO ) , and

4](ClO ) in CH CN. Abbreviations: b, broad; s, shoulder.

2

4

2

4 2

[

4 2 3

4

, entry 17). Complex 2 was found to be equally eﬃcient;

however, because of its comparatively lower solubility, catalytic

reactions require longer times.

t

It is worth mentioning here that upon addition of KO Bu to a

solution of the catalyst [FeL¹Cl ] (1a,b) the initial brown

a/b

2

color of the solution instantaneously changes to reddish brown,

as observed during exhaustive electrolysis at −0.35 V.

t

Moreover, upon addition of KO Bu to a solution of the free

1

a,b

ligand L the initial orange color of the solution immediately

changes to dark orange, as observed during electrochemical

reduction of the free ligand. The alkali-metal tert-butoxides are

known to act as reducing agents; there are some reports

t

showing that MO Bu (M = Li, Na, K) reduces 1,10-

phenanthroline to generate the corresponding one-electron-

16

reduced 1,10-phenanthroline anion radical. These exper-

imental observations in our case indeed point to the possibility

t

−

of KO Bu-mediated reduction of 1b to [1b] and the

involvement of the one-electron-reduced complex [1b]⁻in

the catalytic cycle (cf. below), as recently reported by Goswami

11

and co-workers for a related system. Formation of only a trace

amount of benzophenone (5ii) in the presence of other

nonreducing bases such as K PO , K CO , Cs CO , and NaOH

Figure 13. Electronic spectra of L¹^b(black), L (blue), 1b (purple), 2

olive), [3b](ClO ) (magenta), and [4](ClO ) (orange) in CH Cl

2

(

4

2

4

2

3

4

2

3

2

3

and CH CN, respectively.

3

further indicates the possibility of the involvement of the one-

−

electron-reduced complex [1b] during catalytic turnover.

t

corresponding spectra are shown in Figure 13. The ligands

To conﬁrm the possibility of KO Bu-mediated reduction of

1

2

1

L and L show intraligand n → π and π→ π* transitions in the

1b as well as L , leading to the formation of ligand-centered

8

region 240−360 nm. Complexes 1 and 2 show strong

radicals, EPR experiments were carried out separately with

t

absorption peaks in the region 300−400 nm along with

broad bands at 911 nm (for 1b) and 924 nm (for 2). These

low-energy transitions are assigned as metal to ligand charge

transfer transitions (HOMO-1(α) → LUMO(α) (31%),

solutions containing 1:1 mixtures of KO Bu and catalyst 1b and

1

a

free ligand L , respectively (Figures S34 and S38 in the

Supporting Information). Indeed, both reaction mixtures gave

single-line EPR spectra with g = 1.997 and g = 2.004 (Figures

S34 and S38), respectively, as was observed before for the one-

2+

HOMO-1(β) → LUMO(β) (31%)). Complexes [3] and

2+

−

1a •−

[

4] show broad absorption maxima at 681 and 672 nm,

electron-reduced complexes [1b] and [L ] (Figure 10 and

17

respectively, along with intraligand charge transfer transitions in

the region 300−400 nm.

Catalytic Alcohol Oxidation. We were curious to use our

newly synthesized complexes as catalysts to bring about some

useful chemical transformations. To start with, we chose the

coordinatively unsaturated complexes 1 and 2 for performing

catalytic alcohol oxidation reactions.

Initial studies were focused on ﬁnding the optimal reaction

conditions for the catalytic alcohol oxidation reaction using

diphenylmethanol (5i) as the model substrate and complex 1a

Figures S34 and S38 ). These experimental data are in

agreement with the EPR spectra reported by Lei and co-

workers during KO Bu-mediated reduction of free 1,10-

t

16a

t

phenanthroline.

Since KO Bu is a weak reducing agent,

EPR experiments were carried out to quantify the extent of

reduction. Spin quantiﬁcation measurements using TEMPO as

the standard showed that, in dichloromethane solvent, addition

t

of 1.0 equiv of KO Bu to 1b leads to 63% conversion of 1b to

−

[1b] in 5 h and at 75 °C; using toluene as the solvent, 72% of

−

1b is converted to [1b] in 3 h. However, in case of the free

H

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Inorganic Chemistry

Article

−

a d

Table 4. Optimization of Reaction Conditions

entry

catalyst (amt (mol %))

temp (°C)

solvent

base (amt)

time (h)

yield (%)

1

2

3

4

5

6

7

8

9

1a (10)

120

75

toluene

Toluene

toluene

DMF

K PO (10 mol %)

12

5

10

3

4

1a (10)

K CO (10 mol %)

5

2

3

1a (10)

Cs CO (10 mol %)

trace

10

2

3

1a (10)

NaHCO (10 mol %)

3

1a (10)

NaOMe (10 mol %)

1a (10)

NEt (10 mol %)

3

1a (10)

NaOH (10 mol %)

trace

72

t

1a (10)

Li BuO (10 mol %)

t

1a (10)

75

Na BuO (10 mol %)

81

t

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

1a (10)

75

K BuO (10 mol %)

88

t

1a (3.0)

1a (10)

75

K BuO (10 mol %)

88

t

75

K BuO (10 mol %)

12

5

trace

5

t

1a (10)

75

EtOH

K BuO (10 mol %)

t

1a (10)

75

CH CN

K BuO (10 mol %)

<10

80

3

t

1a (10)

75

THF

K BuO (10 mol %)

t

1a (10)

75

heptane

toluene

K BuO (10 mol %)

78

t

1b (3.0)

2 (3.0)

75

K BuO (10 mol %)

92

t

60

K BuO (10 mol %)

5

60

t

75

K BuO (10 mol %)

5

68

[1b]⁻(2.0)

75

24

4

trace

95

t

75

K BuO (5.0 mol %)

75

NaOH (5.0 mol %)

NaOMe (10 mol %)

K CO (10 mol %)

4

95

75

4

61

75

4

61

2

3

75

Cs CO (10 mol %)

4

48

2

3

75

K PO (10 mol %)

4

49

3

t

4

75

K BuO (10 mol %)

12

5

<5

45

t

room temp

75

K BuO (2.0 equiv)

t

FeCl (3.0)

K BuO (10 mol %)

<5

2

t

FeCl (3.0)

75

K BuO (10 mol %)

5

3

1

t

L/L (3.0)

75

K BuO (10 mol %)

5

d

1b/[1b]⁻(3.0)

t

75

K BuO (10 mol %)

3

<5

a

b

−

c

d

Alcohol (1.0 mmol). [1b] was obtained via quantitative reduction of 1b by cobaltocene. Isolated yields after column chromatography. TEMPO

was used as radical scavenger.

1

a

1a •−

ligand, only 49% conversion of [L ] to [L ] was observed in

to benzophenone (5ii) was obtained when the reaction was

carried out at 75 °C in toluene for 4 h in the presence of 5 mol

t

5

h. To further conﬁrm the KO Bu-mediated reduction of 1b

1b

t

1b

t

−

and L , the reactions between KO Bu with 1b and L were

monitored separately using UV−vis spectroscopy(see Figures

S34, S38, and S39 in the Supporting Information). Upon

% KO Bu or NaOH, and only 2.0 mol % of [1b] is suﬃcient

(Table 4, entries 21 and 22). However, the reaction did not

occur in absence of base. These experimental results indeed

support the involvement of the one-electron-reduced complex

t

addition of 1.0 equiv of KO Bu to a solution of 1b, the

−

absorption peaks at 348 and 283 nm increase and the intensity

of the broad peak at 911 nm decreases, as was observed during

quantitative one-electron reduction of 1b by cobaltocene.

After having evidenced the involvement of the one-electron-

reduced complex [1b] as the active catalyst during catalytic

alcohol oxidation reactions, we decided to reoptimize the

[1b] as the active catalyst during catalytic alcohol oxidation

reactions. Therefore, the catalytic oxidation of benzyl alcohols

−

can be achieved either using a combination of [1b] (2.0 mol

t

%) and NaOH or KO Bu (5.0 mol %) or, alternatively, 1b (3.0

−

t

mol %) and 10 mol % of KO Bu, which other than acting as a

16a

base also acts as a reducing agent to form the active catalyst

−

catalytic reaction using the preformed [1b] as the catalyst and

[1b] in situ from 1b.

diphenylmethanol (5i) as the model substrate in toluene

Control experiments showed that no product was obtained

in the absence of catalyst 1b; only a trace amount of product

was obtained in the presence of only ligand and base (10 mol %

−

(

Table 4, entries 20−26). Pleasingly, using preformed [1b] as

the catalyst, the reaction proceeded even with bases such as

K PO , K CO , Cs CO , and NaOH, with which only a trace

t

KO Bu). Other iron sources such as FeCl and FeCl were also

3

4

2

3

2

3

2

3

amount of benzophenone was obtained as the dehydrogenated

product when parent 1b was used as the catalyst (Table 4,

entries 22−26). Reactions proceeded even with 1.0 mol % of

base loading. The highest conversion of diphenylmethanol (5i)

found to be unable to convert alcohols to the corresponding

carbonyls under the optimized reaction conditions. Only a trace

amount (<5%) of carbonyl compounds were formed in the

t

presence of 10 mol % KO Bu at 75 °C. In the presence of a

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−

a c

Table 5. Substrate Scope

a

t

b

Conditions: complex 1b (0.03 mmol), alcohol (1.0 mmol), KO Bu (10.0 mol %), reaction time 5 h. Conditions: complex 1b (0.02 mmol),

c

d

cobaltocene (0.02 mmol), alcohol (1.0 mmol), NaOH (5.0 mol %), reaction time 4 h. Isolated yields after column chromatography. The yield in

parentheses was obtained when preformed [1b] was used as the catalyst.

−

basic environment alcohols are highly susceptible to aerobic

withdrawing groups at the para position of benzyl alcohols,

albeit leading to lower yields. For example, 4-chlorobenzalde-

hyde was obtained in 85% yield from the corresponding 4-

chlorobenzyl alcohol (Table 5, entry 2). However, poor

conversion to the aldehyde was obtained in the presence of

strongly electron withdrawing groups at the para position of

benzyl alcohol. 4-Nitrobenzaldehyde was obtained in only 45%

yield from the corresponding 4-nitrobenzyl alcohol (Table 5,

entry 3).

17

oxidation even in the absence of any catalyst. Therefore, the

amount of base used during catalytic reactions should be as low

as possible. To assess the background reactivity, the catalytic

alcohol oxidation was studied with diﬀerent amounts of base

t

loading. Interestingly, use of excess KO Bu (2.0 equiv) leads to

almost 45% conversion of the alcohols to the corresponding

carbonyls at room temperature even in the absence of any Fe

catalyst (Table 4, entry 28). However, only trace amounts

(

1

<5%) of carbonyl compounds were formed in the presence of

0 mol % KO Bu even at 75 °C.

Catalytic oxidation of various substituted benzyl alcohols

Secondary alcohols were also found to be suitable substrates

for oxidation to the corresponding ketones. Diphenylmethanol

or 1-phenylethanol produced benzophenone and acetophenone

in high yields under the same optimized reaction conditions

(Table 5, entries 9 and 10). Reactions proceeded with both

electron-donating and -withdrawing groups at the phenyl ring

of 1-phenylethanol. However, lower yields of the corresponding

acetophenone were obtained in the presence of electron-

withdrawing groups (Table 5, entries 11−13). Anthracene-9-

carbaldehyde was also isolated in high yields from the

corresponding alcohol (Table 5, entry 7). Heterocyclic alcohols

also produce the corresponding aldehydes in high yield. For

example, 2-thiophenemethanol produced thiophene-2-aldehyde

in 65% yield under the same optimized conditions (Table 5,

entry 15). No reaction was observed with n-pentanol.

t

with diﬀerent electronic properties and functional groups were

studied to explore the substrate scope and versatility of the

developed catalytic methodology. Almost similar conversion of

alcohols to the corresponding aldehydes or ketones were

obtained using either the preformed one-electron-reduced

−

complex [1b] (2.0 mol %) and NaOH (5.0 mol %) as the

base or a combination of 1b (3.0 mol %) and 10 mol % KO Bu.

t

The results are summarized in Table 5. Excellent yields were

obtained with alcohols having electron-donating substituents.

The presence of electron-donating groups at the ortho, para, or

meta position of the benzyl alcohol produces the corresponding

aldehyde in high yield. Reactions also proceeded with electron-

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To explore the reaction mechanism, a few control experi-

ments were carried out. Kinetic studies of the catalytic reaction

using 1-phenylethanol as the substrate showed linear depend-

ence of the rate k _o _b _s on both substrate and catalyst

concentration (Figures S35 and S36 in the Supporting

Information). The rate of the catalytic oxidation of 1-

phenylethanol was found to rise linearly when the catalyst

loading was varied in the range 1.0−1.75 mol % with respect to

the alcohol. Similarly, a linear increase in the rate was observed

when the substrate concentration was varied over a range of 5−

2

0 equiv of alcohol with respect to the catalyst.

The coordinated azo ligand is redox active and produces an

8

−11

azo anion radical upon reduction;

therefore, to check the

involvement of ligand-centered redox as well as to conﬁrm the

formation of any organic radicals during catalytic turnover, the

catalytic reaction was performed in the presence of a radical

inhibitor. When an equivalent amount of TEMPO was added

to the reaction system, the yield of the reaction decreased

drastically (Table 4, entry 32). To gain more information about

the involvement of the coordinated azo-aromatic ligand during

catalytic turnover, stoichiometric reactions were performed

t

using 1b, 1-phenylethanol (5j), and 10 mol % KO Bu in

toluene under an argon atmosphere. Characterization of the

reaction mixture using IR spectroscopy under an argon

−1

atmosphere shows N−H stretching at 3025 and 3060 cm ,

as reported by others (Figure S40 in the Supporting

1

1,18d

Information).

To further conﬁrm the azo to hydrazo

Figure 14. Plausible mechanism for the alcohol oxidation catalyzed by

1,2

conversion during catalytic turnover, the stoichiometric

reaction was repeated with deuterated 1-phenylethanol (5j-

[

Fe(L )Cl ].

2

D ) under an argon atmosphere. Upon IR spectroscopic

2

−1

analysis of this reaction, new bands at 2310 and 2375 cm

−

characteristic of N−D stretching were observed. These

experimental ﬁndings indeed point to the involvement of the

coordinated azo-aromatic ligand during catalysis. Similar N−H

and N−D stretching frequencies were also observed when the

electron-reduced complex [1b] . The reaction proceeds via the

coordination of the deprotonated alcohol to the active catalyst

[1b] ) to form the intermediate B; the base may assist in this

step by formally trapping HCl. This is followed by hydrogen

atom abstraction from the α-carbon of the coordinated alcohol,

leading to an O-coordinated ketyl radical anion (C). In the next

step the intermediate C undergoes rapid one-electron oxidation

to aﬀord the desired carbonyl compound.

of the O-coordinated ketyl radical anion both intramolecular

metal- and ligand-centered reductions are possible. Involve-

ment of dioxygen has also been proposed in some cases. In

our case, formation of benzophenone from the stoichiometric

reaction of catalyst [1b] and diphenylmethanol (5i) in the

presence of 5.0 mol % NaOH under an argon atmosphere

−

(

stoichiometric reactions were carried out using 1-phenylethanol

−

(

5j), preformed [1b] , and 5.0 mol % NaOH.

To check the possibility of H evolution during catalytic

2

alcohol oxidation reactions intermolecular hydrogen transfer

experiments were carried out in the presence of easily reducible

substrates such as 4-methoxybenzaldehyde (5dd) and 2,4-

dimethoxybenzaldehyde (5ﬀ), respectively, in a closed

11,18

During oxidation

18

11

1

2

system. The dehydrogenation of diphenylmethanol (5i)

when it is carried out separately in the presence of 5dd and

−

5

ﬀ in a closed system under both inert and aerobic conditions

does not produce any hydrogenated product of the aldehydes

5dd or 5ﬀ). Moreover, the formation of H O was identiﬁed

−

indeed points toward the intramolecular reduction of [1b] as

(

2

11,18

11

proposed by others.

However, it is diﬃcult to comment

spectrophotometrically under aerobic conditions. Finally, to

distinguish between the one-electron (HAT) and two-electron

conclusively whether the metal- or the ligand-centered

reduction process is involved during the oxidation of the O-

coordinated ketyl radical anion. Intramolecular ligand-centered

reduction would generate a phenanthroline anion radical, which

is expected to be unstable under oxidative conditions.

Therefore, intramolecular ligand-centered reduction (gener-

ation phenanthroline radical) seems less likely and we propose

metal-centered reduction from Fe(II) to Fe(I) during the

oxidation of the O-coordinated ketyl radical anion. In the

subsequent steps, the intermediate D, thus formed, abstracts a

(

hydride transfer) processes, catalytic oxidation of cyclobutanol

12i,j

was carried out using 1b as the catalyst. Investigation of the

1

H NMR spectrum of the reaction mixture containing the

oxidation products of cyclobutanol indicates the formation of

multiple products, including 4-hydroxybutanal as the ring-

opening product (Figure S53 in the Supporting Information).

These experimental data indeed point to the one-electron

(HAT) oxidation pathway and the possibility of hydride atom

transfer seems less likely.

11

Combining all available evidence, we depict a plausible

mechanism for the present iron-catalyzed alcohol oxidation

reactions in Figure 14. On the basis of the above experimental

results along with available literature, the catalytic alcohol

oxidation reactions are believed to be catalyzed by the one-

proton form the alcohol substrate to form intermediate E,

which in the presence of dioxygen produces H with the

O

2

20

regeneration of the azo chromophore. Intramolecular electron

transfer from an electron-rich Fe(I) center to the reducible azo

9

chromophore then generates B.

K

DOI: 10.1021/acs.inorgchem.7b02238

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

1

7

609 (ν, CN), 1455 (ν, NN). Anal. Calcd for C H N : C,

CONCLUSIONS

18 12

4

■

6.04; H, 4.25; N, 19.71. Found: C, 76.09; H, 4.31; N, 19.75. ESI-MS:

In conclusion, we have reported the two new redox-

1a

+ 1

m/z 285 [L + H] . H NMR (400 MHz, CDCl ): δ 9.27 (s, 1H),

8

2H), 7.69 (s, 1H), 7.56 (s, 3H). C NMR (400 MHz, CDCl ): δ

161.61, 152.24, 150.33, 145.97, 145.26, 138.41, 136.05, 132.32, 129.36,

3

1

2

noninnocent azo-aromatic pincers L and L and synthesized

their Fe complexes with the aim of developing new azo-

aromatic ligand based catalysts to bring about useful chemical

transformations. The ligand L is tridentate, having one easily

reducible azo chromophore, whereas L is a tetradentate ligand

having two easily reducible azo functions. The coordination

behavior of the newly synthesized ligands along with the

electronic structures of all the synthesized Fe complexes were

studied thoroughly using diﬀerent spectroscopic tools coupled

with DFT computations. Interestingly, the tetradentate ligand

.44 (d, J = 8 Hz, 1H), 8.31 (d, J = 12 Hz, 1H), 8.18 (s, 3H), 7.89 (s,

13

3

1

127.61, 125.87, 123.86, 123.19, 113.51, 77.71.

1

b

1b

2

Synthesis of L . The ligand L was synthesized following the same

1a

procedure as described for L . Its yield and characterization data are

as follows. Yield: 75%. UV/vis: λ_m_a_x, nm (ε, M cm ) 241 (14285),

−1

1b

+ 1

2

65 (10370, s), 356 (12885). ESI-MS: m/z 319 [L + H] . H NMR

(400 MHz, CDCl ): δ 9.27 (s, 1H), 8.42 (d, J = 8 Hz,1H), 8.30 (d, J =

3

1

2 Hz, 1H), 8.15 (d, J = 6 Hz, 3H), 7.87 (s, 2H) 7.70 (t, J = 6 Hz,

H), 7.56(d, J = 12 Hz, 2H).

2

L shows tridentate binding with uncoordinated pendant azo

Synthesis of L . 2,9-Bis(phenyldiazo)-1,10-phenanthroline (L ) was

chromophores in complexes 2 and [4](ClO ) . The newly

synthesized via the diazotization of 2,9-diaminophenanthroline with

nitrosobenzene in highly alkaline medium for 48 h. The crude reaction

mixture was evaporated, and the solid was dissolved in dichloro-

methane and puriﬁed by column chromatography using dichloro-

4

2

synthesized Fe complexes 1 and 2 were successfully used as

catalysts for the aerobic oxidation of primary and secondary

benzylic alcohols. Mechanistic investigations show that the one-

−1

−

methane as eluent. Yield: 60%. UV/vis: λ_m_a_x, nm (ε, M cm ) 311

electron-reduced complexes [1] and [2] actually act as the

active catalysts during these catalytic alcohol oxidation

reactions. Moreover, the involvement of the coordinated

redox-noninnocent azo-aromatic ligand has also been evi-

−

1

(

40400), 358 (30760, s). IR (KBr, cm ): 1570 (ν, CN), 1450 (ν,

NN). Anal. Calcd for C H N : C 74.21; H 4.15; N, 21.64. Found:

24

16

6

2

+ 1

C, 74.25; H, 4.21; N, 21.69. ESI-MS: m/z 389 [L − H] . H NMR

(

400 MHz, CDCl ): δ 8.50 (d, J = 12, 2H), 8.23 (d, J = 12, 6H), 7.98

3

1

2

13

denced. Overall, the ligands L and L open up the possibility of

using these azo-aromatic scaﬀolds in catalysis in combination

with suitable metal ions. Our initial results in cross-coupling

reactions using 1 and 2 as catalysts are promising, and these

ﬁndings will be reported in due course.

s, 2H), 7.57 (d, J = 4, 6H). C NMR (400 MHz, CDCl ): δ 162.09,

3

152.52, 145.80, 138.86, 132.51, 129.28, 127.51, 124.07, 114.07, 77.46.

1

a

Synthesis of [FeL Cl ] (1a). A 50 mg portion (0.17 mmol) of the

2

ligand L¹was dissolved in ethanol, and 41.3 mg (0.17 mmol) of

a

FeCl ·6H O was added to it. Upon addition of the metal salt the color

2

of the solution immediately changed from orange to brown, which

gradually intensiﬁed with time. The reaction mixture was reﬂuxed for 5

h in a heating mantle. A dark precipitate was formed, which was

collected by ﬁltration of the reaction mixture. The complex

EXPERIMENTAL SECTION

■

Materials. FeCl , Fe(ClO ) , and all alcohols were purchased from

2

4 2

1

a

[

Fe(L )Cl ] was puriﬁed by fractional crystallization using a

Sigma-Aldrich. All other reagents and chemicals were purchased from

commercial sources and used without further puriﬁcations. Tetrabu-

tylammonium perchlorate was prepared and recrystallized as reported

2

dichloromethane−hexane solvent mixture. Its yield and character-

ization data are as follows: Yield: 90%. UV/vis: λmax, nm (ε, M⁻cm )

1

−1

19

before. Caution! Perchlorates have to be handled with care with

appropriate safety precautions.

240 (20260), 332 (13170), 402 (14040), 555 (1540, s), 904 (1820, b),

−1

IR (KBr, cm ): 1650 (ν CN), 1415 (ν(NN)), Anal. Calcd for

FeN : C, 52.59; H, 2.94; N, 13.63. Found: C, 52.65; H, 3.03;

N, 13.64. ESI-MS: m/z 408 [1a − H] .

Physical Measurements. UV−vis spectra were recorded with a

C

18

H12Cl

2

4

1

+

Jasco spectrometer. H NMR spectra were recorded on Bruker Avance

1

b

3

00/400/500 MHz and JEOL 400 MHz spectrometers, and SiMe was

Synthesis of [FeL Cl

] (1b). This was synthesized following the

2

4

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 AUT.MAC204 electrochemistry system. Cyclic voltam-

metry experiments were performed under a nitrogen atmosphere using

a Ag/AgCl reference electrode, with a Pt-disk working electrode and a

Pt-wire auxiliary electrode, in acetonitrile or dichloromethane

same procedure as described for 1a. Its yield and characterization data

−

1

−1

are as follows: Yield: 92%. UV/vis: λmax, nm (ε, M cm ) 342

−1

(11050), 403 (12510), 546 (885, s), 911 (720, b). IR (KBr, cm ):

1600 (ν(CN)), 1410 (ν(NN)), Anal. Calcd for C18 FeN

C, 48.53; H, 2.49; N, 12.58. Found: C, 48.47; H, 2.60; N, 12.64.

] (2). A 50 mg portion (0.12 mmol) of the

ligand L²was dissolved in ethanol, and 30.2 mg (0.12 mmol) FeCl

6H O was added to it. Upon addition of the metal salt the color of the

H

11Cl

:

4

3

2

Synthesis of [FeL Cl

2

a

·

2

containing supporting electrolyte, 0.1 M [Et N]ClO₄or 0.1 M

solution immediately changed from orange to brown, which gradually

intensiﬁed with time. The reaction mixture was reﬂuxed for 5 h in a

4

[

Bu N]ClO , respectively. A Pt-wire-gauge working electrode was used

4

for exhaustive electrolyses. Room-temperature magnetic moment

heating mantle. A dark precipitate was formed, which was collected by

2

measurements for 1 and 2 were carried out with a Gouy balance

ﬁltration of the reaction mixture. The complex [Fe(L )Cl

2

] was

(

Sherwood Scientiﬁc, Cambridge, U.K.). Temperature-dependent

puriﬁed by fractional crystallization using a dichloromethane−hexane

solvent mixture. Its yield and characterization data are as follows:

magnetic susceptibility measurements were carried out with a

Quantum-Design MPMS-XL-5 SQUID magnetometer equipped

with a 5 T magnet in the range from 295 to 2.0 K at a magnetic

ﬁeld of 0.5 T. Mo

a Rh matrix using an alternating constant acceleration Wissel

Mossbauer spectrometer operated in the transmission mode and

equipped with a Janis closed-cycle helium cryostat. Isomer shifts are

given relative to iron metal at ambient temperature. EPR spectra in the

X band were recorded with a JEOL JES-FA200 spectrometer.

−

1

−1

Yield: 90%. UV/vis: λmax, nm (ε, M cm ) 318 (50670), 363

−

1

(35165), 924 (2045, b), IR (KBr, cm ): 1600 (ν(CN)), 1435

(ν(NN)). Anal. Calcd for C24 FeN : C, 55.95; H, 3.13; N,

16.31. Found: C, 55.90; H, 3.23; N, 16.37. ESI-MS, m/z 514 [2 − H] .

ssbauer spectra were recorded with a ⁵⁷Co source in

̈

H

16Cl

2

6

+

1

a

̈

Synthesis of [Fe(L )

2

](ClO

)

4

2

([3a](ClO

mmol) of the ligand L¹was dissolved in ethanol, and 20.7 mg (0.08

mmol) Fe(ClO was added to it. Upon addition of the metal salt the

4 2

) ). A 50 mg portion (0.17

a

)

4 2

color of the solution immediately changed from orange to green. The

reaction mixture was stirred for 5 h. A green precipitate was formed,

which was collected by ﬁltration of the reaction mixture. The complex

1

a

Synthesis. Synthesis of L . 2-(Phenylazo)-1,10-phenanthroline

1a

(L ) was synthesized via diazotization of 2-amino-1,10-phenanthro-

1a

line with nitrosobenzene under highly alkaline conditions. The crude

reaction mixture was evaporated, and the product was dissolved in

dichloromethane and puriﬁed by column chromatography using

[Fe(L ) ]ClO was puriﬁed by fractional crystallization using an

2 4

acetonitrile−toluene solvent mixture. Its yield and characterization

+

data are as follows. Yield: 90%, ESI-MS: m/z 452 [2] . UV/vis: λ

,

max

−

1

−1

dichloromethane as eluent. Yield: 72%. UV/vis: λ_m_a_x, nm (ε, M

nm (ε, M cm ) 234 (20835), 296 (16050, s), 391 (18320), 674

(3510), IR (KBr, cm ): 1635 (ν(CN)), 1415 (ν(NN)), Anal.

−1

cm ) 249 (15130), 287 (13915, s), 354 (12735), IR (KBr, cm ):

L

DOI: 10.1021/acs.inorgchem.7b02238

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

M

DOI: 10.1021/acs.inorgchem.7b02238

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Calcd for C H Cl FeN O : C, 52.51; H, 2.94; N, 13.61. Found: C,

was ﬁltered; the solvent was evaporated under vacuum. The IR

36

24

2

8

+

5

2.56; H, 3.01; N, 13.63. ESI-MS: m/z 624 [3a] .

spectrum of the resultant reaction mixture showed characteristic

1

b

−1

Synthesis of [Fe(L ) ](ClO ) ([3b](ClO ) ). This was synthesized

stretching due to N−H bonds: ν(N−H) 3025 and 3060 cm .

2

4 2

following the same procedure as described for [3a](ClO ) . Its yield

A similar experiment was also performed with deuterated

4

2

and characterization data are as follows. Yield: 90%. UV/vis: λ_m_a_x, nm

diphenylmethanol (Ph CDOD) (5i-D ) following the same exper-

imental procedure as mentioned above. The IR spectrum of the

2

−

1

−1

(

ε, M cm ) 304 (16000, s), 392 (16245), 681 (1965). ESI-MS, m/z

+

1

6

1

5

2

1

93 [3b − H] . H NMR (400 MHz, CDCl ): δ 7.79 (d, J = 12 Hz,

resultant reaction mixture showed characteristic stretching due to N−

3

−1

H), 7.50 (d, J = 8 Hz, 1H), 6.73 (d, J = 12 Hz, 1H), 6.52 (m, 2H),

.58 (t, J = 8 Hz, 1H), 5.26 (s, 1H), 5.10 (m, 2H), 4.36 (d, J = 8 Hz,

D bonds: ν(N-D) 2310 and 2375 cm .

Study of Electronic Spectral Changes upon Addition of

H). ¹³C NMR (400 MHz, CDCl ): δ 195.90, 186.22, 178.48, 170.64,

t

−4

KO Bu to 1b. To a 10 molar acetonitrile solution of 1b was added a

3

t

68.84, 167.12, 162.71, 156.55, 40.13.

pinch of KO Bu. The color of the solution immediately changed from

2

Synthesis of [Fe(L ) ](ClO ) [4](ClO ) . A 50 mg portion (0.12

brown to reddish brown. The spectral changes were monitored over

2

4 2

mmol) of the ligand was dissolved in ethanol, and 16.3 mg (0.064

mmol) of the metal salt Fe(ClO ) was added to it. The color of the

time.

Kinetic Study. A 50 mL round-bottom ﬂask was charged with

appropriate amounts of alcohol, catalyst, and base. To this mixture was

added 10 mL of dry toluene, and the reaction mixture was placed in a

oil bath preheated at 75 °C. A 0.1 mL aliquot was taken from the

reaction vessel after a certain time interval and was monitored

spectrophotometrically at 290 nm with appropriate dilution.

X-ray Crystallography. Suitable X-ray-quality crystals of L ·ClO

were obtained by slow evaporation of its acetonitrile solution. X-ray-

quality crystals of 1b, 2, [3b](ClO ) , and [4](ClO ) were obtained

by either slow evaporation or slow diﬀusion of their dichloromethane−

hexane solution. The X-ray single-crystal data of L ·ClO , 1b, 2,

4

2

solution immediately changed from orange to green after addition of

the metal salt. The color of the solution gradually intensiﬁed. The

solution was reﬂuxed for 5 h. The solvent was evaporated under low

pressure. The resulting solid was dissolved in DCM and recrystallized

from a DCM−hexane mixture. Its yield and characterization data are as

−

1

−1

1a

follows. Yield: 80%. UV/vis: λ , nm (ε, M cm ) 305 (36655), 672

(

max

4

−1

2010), IR (KBr, cm ): 1595 (ν(CN)), 1435 (ν(NN)). Anal.

Calcd for C H Cl FeN O : C, 55.89; H, 3.13; N, 16.29.Found: C,

48

32

2

12

8

4

2

4 2

+

1

5

5.92; H, 3.17; N, 16.33. ESI-MS, m/z 832 [4] . H NMR (400 MHz,

1

a

CDCl ): δ 7.97 (m, 1H), 7.80 (m, 1H),7.55 (m, 5H), 7.44 (m, 2H),

3

4

6

4

.80 (t, J = 10 Hz, 1H), 6.36 (m, 14H), 5.90 (m, 2H), 5.15 (m, 4H),

.93 (m, 2H).

Catalytic Alcohol Oxidation Using 1b as the Catalyst. Under

[3b](ClO ) , and [4](ClO ) were collected with monochromated

4

2

4 2

Mo Kα radiation (λ = 0.71073 Å) on a Bruker SMART Apex II

diﬀractometer equipped with a CCD area detector at 150(2) K. The

crystals were positioned at 40 mm from the CCD, and the spots were

measured using 5 and 10 s counting times, respectively. Data reduction

aerobic conditions, 1b (3.0 mol %) and the respective alcohols (1.0

mmol) were placed in a 50 mL round-bottom ﬂask. Subsequently

t

21

KO Bu (0.1 mmol, 10 mol %) and 3 mL of dry toluene were added.

was carried out using the SAINT-NT software package. Multiscan

The reaction mixture was stirred for 5.0 h at 75 °C. After the reaction

was complete, the resulting mixture was evaporated to dryness using a

rotary vacuum evaporator and extracted with hexane. The volume of

the hexane solution was reduced and puriﬁed by column

chromatography to get the products. The pure products were

absorption correction was applied to all intensity data using the

SADABS program. The structures were solved by a combination of

2

direct methods with subsequent diﬀerence Fourier syntheses and

2

reﬁned by full-matrix least squares on F using the SHELX-2013

2

3

suite. All non-hydrogen atoms were reﬁned with anisotropic thermal

displacements. The crystal data together with reﬁnement details are

given in Table 6.

1

identiﬁed by H NMR.

Catalytic Alcohol Oxidation Using Preformed [1b]⁻as the

Catalyst. Under an argon atmosphere 8.9 mg (2.0 mol %) of 1b was

placed in a oven-dried Schlenk tube. A 5.0 mL portion of dry and

degassed toluene was added to it with stirring. To this suspension was

added 3.8 mg (1.0 equiv) of cobaltocene, and the mixture was stirred

for 10 min under an inert atmosphere. The color of the solution

changed from dark brown to reddish brown. To this solution were

added 1.0 mmol of the respective alcohol and 2.0 mg of NaOH. The

Schlenk tube was then placed in an oil bath preheated at 75 °C, and

the reaction was continued for 4.0 h in air.

Computational Details. DFT calculations presented herein were

24

carried out using the Gaussian 09 program package. Geometry

optimizations were performed without imposing geometric con-

straints. The vibrational frequency calculations were performed to

ensure that the optimized geometries represent the local minima and

that there are only positive eigenvalues. All calculations were

performed using the B3LYP functional with LANL2DZ basis set on

25

Fe and 6-31G* on C, H, N, and Cl atoms. The broken-symmetry

26,27

approach

was employed to establish the singlet state S = 0 of the

Detection of Hydrogen Peroxide during the Catalytic

1

1,20

compound(s). Mulliken spin densities were used for analysis of spin

Reactions..

The formation of H O during catalytic alcohol

28

2

populations on ligand and metal centers.

oxidation reactions was detected spectrophotometrically following the

−

gradual development of the characteristic absorption band for I₃at

3

50 nm. The catalytic oxidation of diphenylmethanol (5i) was carried

ASSOCIATED CONTENT

■

out in a round-bottom ﬂask containing 1.0 mmol of the substrate 5i,

0

t

*

S

Supporting Information

.3 mmol of catalyst 1b, and 0.1 mmol of KO Bu in 10 mL of dry

toluene. The reaction mixture was heated at 75 °C for 4 h. A 10 mL

portion of water was added to the reaction mixture, and the whole

solution was extracted three times with dichloromethane. The

This material are available free of charge via the Internet at The

Supporting Information is available free of charge on the ACS

Publications website at DOI: 10.1021/acs.inorgchem.7b02238.

separated aqueous layer was then acidiﬁed with H SO to pH 2 to

2

4

1

ESI-MS spectra of ligands and Fe complexes, H NMR

stop further oxidation. To it were added 1.0 mL of a 10% solution of

spectra, ORTEP views, EPR spectra of the electro-

generated complexes, FMOs, and Cartesian coordinates

of the optimized structures (PDF )

KI and a few drops of a 3% solution of ammonium molybdate.

−

Hydrogen peroxide oxidizes I to I , which reacts with excess I to

2

−

^f^o^r^m^I3 according to the following chemical reactions: (i) H O + 2I

2

+

−

+

2H → 2H O + I ; (ii) I (aq) + I → I . The reaction rate was slow

2 2 2 3

and increased with increasing concentrations of acid.

Accession Codes

Control Experiment for Detection of N−H/N−D Stretch-

CCDC 1550856 −1550860 contain the supplementary crystal-

lographic data for this paper. These data can be obtained free of

charge via www.ccdc.cam.ac.uk/data_request/cif , or by email-

ing data_request@ccdc.cam.ac.uk , or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

1

ing. Under an argon atmosphere, 1.0 mmol of diphenylmethanol

t

(

5i), 1.0 mmol of catalyst 1b, and 0.1 mmol of K BuO were placed in a

ﬂame-dried Schlenk tube. The tube was capped with a Teﬂon screw

cap, evacuated, and back-ﬁlled with argon. The screw cap was replaced

with a rubber septum. Then, 5 mL of dry and deoxygenated toluene

was added. The solution was stirred for 8.0 h at 75 °C. The solution

N

DOI: 10.1021/acs.inorgchem.7b02238

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

1

31, 8772−8774. (h) O’Reilly, M. E.; Veige, A. S. Trianionic pincer

AUTHOR INFORMATION

Corresponding Author

E-mail for N.D.P.: ndpaul@gmail.com.

■

and pincer-type metal complexes and catalysts. Chem. Soc. Rev. 2014,

4

K. A.; Veige, A. S. Remote Multiproton Storage within a Pyrrolide-

3, 6325−6369. (i) Nadif, S. S.; O’Reilly, M. E.; Ghiviriga, I.; Abboud,

*

ORCID

Pincer-Type Ligand. Angew. Chem., Int. Ed. 2015, 54, 15138−15142.

(4) (a) Paul, N. D.; Rana, U.; Goswami, S.; Mondal, T. K.; Goswami,

Franc Meyer: 0000-0002-8613-7862

Nanda D. Paul: 0000-0002-8872-1413

Notes

S. Azo Anion Radical Complex of Rhodium as a Molecular Memory

Switching Device: Isolation, Characterization, and Evaluation of

Current−Voltage Characteristics. J. Am. Chem. Soc. 2012, 134,

6520−6523. (b) Goswami, S.; Sengupta, D.; Paul, N. D.; Mondal,

T. K.; Goswami, S. Redox Non-Innocence of Coordinated 2-(Arylazo)

Pyridines in Iridium Complexes: Characterization of Redox Series and

an Insight into Voltage-Induced Current Characteristics. Chem. - Eur. J.

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

0

The research was supported by DST (Project: YSS/2015/

01552). S.S. thanks the IIESTS, R.S. thanks the UGC-RGNF,

2

014, 20, 6103−6111. (c) Bandyopadhyay, A.; Sahu, S.; Higuchi, M.

Tuning of Nonvolatile Bipolar Memristive Switching in Co(III)

Polymer with an Extended Azo Aromatic Ligand. J. Am. Chem. Soc.

2011, 133, 1168−1171. (d) Lee, J.; Lee, E.; Kim, S.; Bang, G. S.;

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Nitroxide Radicals as Organic Memory Elements with Both n- and p-

Type Properties. Angew. Chem., Int. Ed. 2011, 50, 4414−4418. (e) Seo,

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Am. Chem. Soc. 2008, 130, 2553−2559.

and S.P. thanks the CSIR for fellowship support. Financial

assistance from the IIESTS is acknowledged. P.B. thanks the

CICECO-Aveiro Institute of Materials, POCI-01-0145-

FEDER-007679 (FCT ref. UID/CTM/50011/2013), ﬁnanced

by national funds through the FCT/MEC and when

appropriate coﬁnanced by FEDER under the PT2020 Partner-

ship Agreement. We thank Prof. T. K. Paine (IACS, Kolkata),

Prof. K. Pramanick (JU, Kolkata), and Dr. A. Saha (JU,

Kolkata) for EPR experiments and Dr. S. Chatterjee

(5) (a) Poddel’sky, A. I.; Cherkasov, V. K.; Abakumov, G. A.

Transition metal complexes with bulky 4,6-di-tert-butyl-N-aryl(alkyl)-

o-iminobenzoquinonato ligands: Structure, EPR and magnetism.

Coord. Chem. Rev. 2009, 253, 291−324. (b) Herebian, D.;

Wieghardt, K. E.; Neese, F. Analysis and Interpretation of Metal-

Radical Coupling in a Series of Square Planar Nickel Complexes:

Correlated Ab Initio and Density Functional Investigation of

(

Srerampore College, Howrah) for helpful discussions. We

also thank all the reviewers for their constructive suggestions

during the revision period.

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Q

DOI: 10.1021/acs.inorgchem.7b02238

Inorg. Chem. XXXX, XXX, XXX−XXX

Article Doi

Redox Noninnocent Azo-Aromatic Pincers and Their Iron Complexes. Isolation, Characterization, and Catalytic Alcohol Oxidation

DOI: 10.1021/acs.inorgchem.7b02238

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Full text of DOI:10.1021/acs.inorgchem.7b02238

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