Dehydrogenation of amines in aryl-amine functionalized pincer-like nitrogen-donor redox non-innocent ligands via ligand reduction on a Ni(ii) template

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DOI: 10.1039/D0DT00466A

Dehydrogenation of amines in aryl-amine functionalized pincer-

like nitrogen-donor redox non-innocent ligands via ligand

reduction on Ni(II) template

Received 00th January 20xx

Accepted 00th January 20xx

Manas Khatua,^bBappaditya Goswami^band Subhas Samanta*^a

DOI: 10.1039/x0xx00000x

We have synthesized

a series of new redox non-innocent azo aromatic pincer-like ligands: 2-(phenylazo)-6-

(arylaminomethyl)pyridine (HL^a–c: HL^a= 2-(phenylazo)-6-(2,6-diisopropylphenylaminomethyl)pyridine, HL^b= 2-(phenylazo)-

6-(2,6-dimethylphenylaminomethyl)pyridine, HL^c= 2-(phenylazo)-6-(phenylaminomethyl)pyridine), in which one side arm

is an arylaminomethyl moiety and the other arm is a 2-phenylazo moiety. Nickel(II) complexes, 1–3, of these ligands HL^a–c

were synthesized in good yield (approximately 70%) by the reaction of ligands : (NiCl₂.6H₂O) in 1:1 molar ratio in methanol.

The amine donor in each of the ligands HL^a–cbinds to the Ni(II) centre without deprotonation. In the solid state, complex 3

is a dimer; in solution it exists as monomer 3a. Reduction of acetonitrile solutions of each of the complexes 1, 2 and 3a,

separately, with cobaltocene (1 equivalent), followed by exposure of the solution to air, resulted in the formation of new

complexes 7, 8 and 9, respectively. Novel free ligands L^xand L^yhave also been isolated, in addition to complexes 7 and 8,

from the reaction of complexes 1 and 2, respectively. Complexes 7–9 and free ligands L^xand L^yhave been formed via a

dehydrogenation reaction of the arylaminomethyl side arm. The mechanism of the reaction was thoroughly investigated

using a series of studies, including cyclic voltammetry, EPR, UV-Vis spectral studies and density functional theory (DFT)

calculations. The results of these studies suggest a mechanism initiated by ligand reduction followed by dioxygen

activation. A Cl⁻/I⁻scrambling experiment revealed that dissociation of the chloride ligand(s) was associated with one-

electron reduction of the ligand (azo moiety) in each of the complexes 1, 2 and 3a. The dissociated chloride ligand(s) were

reassociated with the metal following the dehydrogenation reaction to yield the final products. All of the newly

synthesized compounds were fully characterized using a variety of physicochemical techniques. Single-crystal X-ray

structures of the representative compounds were determined to confirm the identities of the synthesized molecules.

arylaminomethyl group-functionalized pincer-like redox non-

innocent azo aromatic ligands HL^a–cand their dehydrogenative

imine formation reaction (Scheme 1a) on a nickel(II) template via

Introduction

Chemical transformations by transition metal complexes containing

redox non-innocent ligand(s) have attracted considerable research

interest in recent times.^1,2Research activity in this area is to be

welcomed, as this type of ligand is expected to expand the scope of

utilizing redox-inert metals in catalysis and because cheaper first-

row transition metal complexes can be utilized in various chemical

transformations, etc.^2c,3For example, Chirik et al., as well as others,

have shown that Fe and Co complexes containing bis(imino)

pyridine based ligands are very efficient at catalysing many novel

chemical transformations involving solely the imine-based redox

process.^2a,3Contemporarily, worldwide, many research groups have

also been involved in exploring other transition metal complexes

containing such redox non-innocent ligand(s) in various kind of

chemical transformations.^2c,4–8Numerous potentially redox non-

innocent azo aromatic nitrogen-donor ligands and their transition

metal complexes are also known.^9–12Some of these complexes are

reported to be active in chemical transformations involving azo-

ligand reduction (azo reduction).

(a)

(b)

R

N

M

X

based redox events.^9b–dHerein, we report

a

new group of

R

R = H, Me, etc

A

B

Reported (Ref. 2a, 3, 8)

Reported (Ref. 9b-d)

^a.Department of Chemistry,

Indian Institute of Technology Jammu,

Jammu 181221 India.

Scheme 1. (a) Ni complexes of the ligands (HL^a–c) and their chemical transformation; (b)

similar reported ligands and their complexes.

E-mail: subhas.samanta@iitjammu.ac.in

^b.Department of Chemical Sciences,

Indian Institute of Science Education and Research Kolkata,

Mohanpur-741246, India

† Footnotes relating to the title and/or authors should appear here.

Electronic Supplementary Information (ESI) available: [details of any

supplementary information available should be included here]. See

DOI: 10.1039/x0xx00000x

The synthesized azo aromatic pincer-like ligands HL^a–cin their

respective nickel complexes, 1–3 (Scheme 1a), possess structures in

which the 6-arylaminomethyl side arm acts as a σ-donor moiety to

the metal (reduced by two protons and two electrons compared to

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the phenylazo moiety), whereas the 2-phenylazopyridine part acts (i) Synthesis

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as both a σ-donor and a π-acceptor moiety.^10a,13,14Compared to the Our work began with the synthesis of the li^Dga^On^I:d¹s⁰H^.10L^a³^–⁹^c^/,^Da⁰s^Ds^Th⁰o⁰w⁴n⁶⁶i^An

ligands HL^a–cin their corresponding complexes, 1, 2 and 3, the Scheme 3. This involved preparation of compound A by the reaction

related reported pincer nitrogen donors in their complexes A and B of nitrosobenzene and 6-methyl-2-aminopyridine in alkaline

(Scheme 1b) have extended π-conjugation in the coordinating conditions following the literature procedure,²⁹followed by

backbone of the ligands, allowing these to act only as σ-donor- and bromination of the methyl group in A using N-bromosuccinimide

π-acceptor-type ligands.^{2a,3,8,9b–d}Complexes containing such and azobisisobutyronitrile (AIBN) in CCl₄solvent to isolate

pincer/pincer-like non-innocent ligands have been found to show compound B and, finally, substitution of the bromide of the

reactivity in various kinds of chemical transformations.^{2a,3,8,9b–d,15–17}bromomethyl group of B with the corresponding aromatic amine.

Recently, Rohde and co-workers have reported interesting ligand- The ligands HL^a–cwere isolated in approximately 70% yield.

centred reactivity with dioxygen in the pincer Ni(II) complex

containing

a

one-electron reduced redox non-innocent

bis(imino)pyridine radical ligand.¹⁸As azo aromatic ligands are

known for their redox non-innocence,^9–12we were interested to

explore the reactivity of complexes 1, 2 and 3 upon reduction in air.

The dimeric complex 3 in the solid state was converted to its

monomer 3a in acetonitrile solution. It was found that reduction of

the acetonitrile solutions of each of the complexes 1, 2 and 3a by

cobaltocene (1 equivalent) led to ligand reduction (N=N bond),

which, subsequently, in the presence of air promoted

dehydrogenation of the arylaminomethyl side arm to yield

complexes 7, 8 and 9, respectively. The novel free ligands L^xand L^y

were also isolated, in addition to complexes 7 and 8, from the

reaction of complexes 1 and 2, respectively (Scheme 1a).

Scheme 3. Synthesis of the ligands HL^a–c

Compounds A and B and ligands HL^a–cwere characterized

satisfactorily by positive ion ESI-MS, IR and NMR spectral analysis

(Figures S1–S15). The characterization data are available in the

experimental section. The infra-red (IR) spectra of the ligands HL^a–c

showed strong absorption bands in the range 1430–1507ꢀcm^–1,

which were assigned to the N=N bond stretching frequency.¹⁰In the

positive ion ESI-MS spectra, the ligands HL^a, HL^band HL^cshowed

The dehydrogenation reaction of coordinated amines^19–25in the

presence of air has been reported by Curtis et al.²⁶and is still under

active investigation.^25,27,28Notably, in all of these cases,^19–25first,

metal oxidation leads to the formation of the complex in a higher

intense peaks at m/z

= 373.24, 317.17 and 288.14ꢀamu,

oxidation state, accompanied by

(Scheme 2), which then undergoes

a

superoxide radical anion

base-promoted

respectively, corresponding to the [MH]⁺ion, where M represents

the ligand. Their simulated spectra matched precisely with their

corresponding experimental spectra (Figures S13–S15). The NMR

spectra also fully corroborated the identity of the ligands HL^a–c. For

example, in the ¹H NMR, the ligand HL^a(Figure S5) showed a

doublet signal for 12H at 1.28ꢀppm, which was assigned to the

methyl groups of the isopropyl moieties. A multiplet signal for 2H at

3.45ꢀppm was assigned to the tertiary C–H protons of the isopropyl

groups, and a singlet signal at 4.42ꢀppm was assigned to the protons

of the CH₂group attached to the pyridine ring. Thus, it is evident

that the 2,6-diisopropylaniline moiety is indeed present in the

ligand HL^a. In the aromatic region of the spectrum, we found signals

for 11 protons that matched exactly with the number of aromatic

protons expected in the ligand HL^a. The signal for the N–H proton

was observed at 4.01ꢀppm.

a

dehydrogenation reaction to yield the final imine complex.^19–28In

contrast, our experiment has shown that the ligand can also initiate

such an oxidation reaction without changing the oxidation state of

the metal. As far as we are aware, the involvement of a ligand-

based redox process in the dehydrogenation of amines has not

been reported in the literature (Scheme 2).

The synthesized ligands HL^a–cwere reacted individually with

hydrated Ni(II)Cl₂.6H₂O at a ratio of 1:1 metal:ligand in methanol to

yield the corresponding nickel complexes, 1–3 (Scheme 4). In a

series of experiments, one equivalent of Ni(II)Cl₂.6H₂O was mixed

with each of the ligands HL^a–cin methanol, upon which the reaction

mixture immediately turned reddish-brown. Each mixture was

refluxed for 1.5ꢀh, and then the solvent was removed under

reduced pressure. Extraction of the crude product using

dichloromethane and its slow diffusion into hexane solvent resulted

in the formation of reddish-brown crystals of complexes 1–3,

respectively, in approximately 70% yield. Further treatment of each

of these synthesized complexes, 1, 2 and 3, with one equivalent of

Scheme 2. Reported reaction vs. our experiments

Results and discussion

(A) The ligands (HL^a–c) and their nickel complexes

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pyridine resulted in complexes 4,

approximately 85% yield (Scheme 4).

5 and 6, respectively, in

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DOI: 10.1039/D0DT00466A

Figure 1. ORTEP representations and atom numbering schemes for ligand HL^c(50%)

and complexes 1, 3 and 6 at 50%, 30% and 50% probability levels, respectively.

Scheme 4. Synthesis of complexes 1–6

The Ni–N bond lengths in complex 1 are in the range 1.950–

2.200ꢀÅ, and are typical of metal–nitrogen bond lengths in

complexes containing neutral nitrogen-donor ligands.^30,31A d_N1–N2

value of 1.267(3) Å in complex 1 is indicative of an unreduced azo

(N=N) bond.^{11,12,31–33}A d_C12–N4value of 1.490(3)ꢀÅ clearly indicates

the single-bond character of the CH₂–NH bond.^{15–17,34–37}

Each of the complexes 1–6 was characterized convincingly (see

below, Experimental section). Microanalytical CHN data matched

exactly with the formulae of complexes 1–6. In their IR spectra,

these complexes showed the N=N stretching frequency¹⁰in the

range 1430–1490ꢀcm^–1and the N–H stretching frequency¹⁵in the

range 3200–3300ꢀcm^–1. All of the complexes 1, 2 and 4–6 are

paramagnetic (two unpaired electrons), as evidenced by their room

temperature magnetic susceptibility in the range μ_eff= 2.76–

2.81ꢀB.M. at 298ꢀK. Compound 3 is dinuclear in the solid state; its

room temperature magnetic susceptibility was found to be

4.33ꢀB.M.

Whereas X-ray structure analysis of complex

2 showed

qualitatively similar geometry to that of complex 1 (Figure S16),

complex 3 crystallized as a dimer bridged by two chloride ligands,

with each of the metal ions being hexacoordinated. Overall,

complex 3 has C₂crystallographic symmetry that makes one half of

the molecule identical to the other half. The d_N1–N2and d_C12–N4

values were almost equal to those of complex 1, indicating

unreduced azo and reduced C–N bonds in complex 3 as well.

X-ray structure determination of complex 6 showed that it

crystallized as a mononuclear complex with a hexacoordinated

geometry. A pyridine ligand coordinated trans to the N3 donor

atom of the pyridine moiety of ligand HL^c.

The structural parameters calculated from density functional

theory (DFT) also closely matched with the crystallographically

determined parameters. Selected experimental and DFT-calculated

bond lengths and bond angles are given in Table 1. The optimized

structure of ligands HL^a, HL^cand complexes 1, 3 and 6 are shown in

Figures S17–21.

(ii) Crystal structure

Ligand HL^cand complexes 1–3 and 6 were characterized by single-

crystal X-ray structure determination, for which suitable crystals of

the compounds were grown by slow diffusion of dichloromethane

solutions of the compounds into hexane. ORTEP representations of

the structures of ligand HL^cand complexes 1, 3 and 6 are shown in

Figure 1, and selected bond parameters are presented in Table 1.

X-ray crystallographic analysis of ligand HL^crevealed that the

phenylazo moiety combined with the spacer pyridine ring has a

more planar structure than the arylaminomethyl moiety. A d_N1–N2

value of 1.254(3)ꢀÅ and a d_C12–N4value of 1.429(3)ꢀÅ (Table 1) are

indicative of the double- and single-bond characters, respectively,

of these two bonds.

The coordination geometry of complex 1 is best described as a

distorted trigonal bipyramid in which the ligand HL^acoordinates as

a tridentate ligand, with the N_azoatom (N1) and N_amineatom (N4)

occupying the apical positions and an N1–Ni–N4 angle of 154.82°.

The other two coordination sites are occupied by the two terminal

chloride ligands (Figure 1).

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Table 1. Experimental and DFT-calculated bond lengths and angles of complexes 1, 3, 6 and ligand HL^c.

Complex 1

Complex 3

Complex 6

Ligand HL^c

Bonds and

angles

Bond lengths (Å) and bond

angles (°)

Experimental

1.267(3)

2.151(2)

2.193(2)

1.960(2)

---

DFT

1.266

2.253

2.255

2.018

---

Experimental

1.259(2)

2.232(2)

2.213(2)

1.979(1)

---

DFT

1.262

Experimental

1.265(2)

2.134(2)

2.218(2)

1.977(1)

2.050(1)

1.488(2)

2.396(6)

2.431(6)

75.33(6)

155.04(6)

79.74(6)

109.0(1)

113.4(2)

116.5(2)

DFT

1.261

2.248

2.255

2.021

2.138

1.481

2.427

2.459

73.756

149.9

77.482

107.19

111.24

115.56

Experimental

DFT

1.255

---

N1–N2

1.254(3)

Ni1–N1

2.302

2.240

2.033

---

Ni1–N4

---

Ni1–N3

---

Ni1–N5

---

C12–N4

1.491(4)

2.238(7)

2.277(8)

75.48(8)

154.82(8)

79.44(8)

110.1(2)

111.9(2)

116.0(2)

1.485

2.347

2.270

74.28

152.9

78.62

109.32

113.3

116.80

1.471(3)

2.334(6)

2.347(5)

74.31(6)

152.91(6)

78.82(6)

108.9(1)

112.4(2)

115.7(2)

1.487

2.387

2.422

72.52

151.08

78.60

106.18

111.68

118.83

1.429(3)

---

1.453

---

Ni1–Cl1

Ni1–Cl2

---

N1–Ni1–N3

N1–Ni1–N4

N3–Ni1–N4

C12–N4–Ni1

N4–C12–C11

N3–C11–C12

---

The corresponding reduction potential of the free ligand HL^a

appeared at −1.149(65)ꢀV as a reversible cathodic wave (Figure

2).

(B) Dehydrogenation reactions

(i) Chemical reactions

Recently, transition metal complexes containing pincer or

pincer-like redox non-innocent ligands have been exploited for

various kinds of reactions involving ligand-based redox

processes.^1–12For example, Rohde and co-workers have

reported the interesting reactivity of dioxygen with the one-

electron reduced bis(imino)pyridine pincer ligand in its Ni(II)

complex.¹⁸As azo aromatic ligands have been reported to be

potentially redox non-innocent,^9–12we were interested to

explore the reactivity of complexes 1–3 upon reduction in air.

Though complex 3 is dinuclear in the solid state, it exists as

monomer 3a in solution [see below, Scheme 5b and Section

B(iii)]. As all of the chemical reactions were carried out in

solution, complex 3a instead of 3 was considered in this

reaction.

Examination of the redox properties of complexes 1 and 3a

using cyclic voltammetry showed that there are indeed facile

irreversible reductive waves for both complexes at

approximately −0.3ꢀV (Figure 2). The DFT-calculated MOs of the

representative ligand HL^aand its complex 1 (Figure 2) showed

that the LUMOs are localized on the azo pyridine moiety,

indicating that the irreversible reductions observed at

approximately −0.3ꢀV in complexes 1 and 3a might be due to azo

pyridine reduction.

Figure 2. Cyclic voltammogram of ligand HL^aand complexes 1 and 3 in acetonitrile

(left) and molecular orbitals of HL^aand complex 1 (right).

Interestingly, in the EPR spectrum, the electrogenerated

complex [1]^.−showed a sharp single line spectrum (Figure 3) at g

= 1.998 in an argon atmosphere at 90ꢀK, confirming that ligand-

centred reduction is associated with the reduction of complex 1.

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Upon one-electron reduction of complex 1, a square planar Ni(II)

complex with an Sꢀ=ꢀ½ ground state was formed via dissociation

of one of the chloride ligands. Formation of a similar square

planar Ni(II) complex with an Sꢀ=ꢀ½ ground state via dissociation

of one of the chloride ligands has been reported by Rohde et al.

quantitative conversion to complex 9 _{View Article O}w_nlia_nes

containing L^z

observed, and no free ligand L^zwas foun^Dd^O(S^I:c¹h⁰e^.1m⁰³e⁹5^/Db⁰).^DT00466A

It is to be noted here that, in a separate chemical reaction,

when the acetonitrile solutions of complexes 1 and 2 were

treated with excess metallic Zn powder (3 equivalents) in air,

complexes 7 and 8, respectively, were isolated in almost

quantitative yield. No free imine ligands L^xand L^ywere observed

(Scheme 6). Zn powder is a mild reducing agent.^9c,dThus, it

reduced complexes 1 and 2 to [1]^.−and [2]^.−, respectively.

Subsequently, these reduced complexes [1]^.−and [2]^.−

underwent aerial dehydrogenation to yield complexes 7 and 8,

respectively. We also found that the yield of complex 7

increased to approximately 75% when excess anhydrous ZnCl₂(5

equivalents) was added to a solution of complex 1 reacting with

cobaltocene (1 equivalent) in air (Scheme 6). The yield of

complex 7 was found to be comparatively low, approximately

55%, when KCl (5 equivalents) was added instead of anhydrous

for one-electron reduction of

bis(imino)pyridine Ni(II) complex.¹⁸

a

trigonal bipyramidal

ZnCl₂in another separate reaction of complex

1 with

cobaltocene (1 equivalent). We added anhydrous ZnCl₂and KCl

in these two separate chemical reactions to increase the

concentration of chloride ligand in the reaction medium. An

increase in the yield of complex 7 with increasing concentration

of chloride ligand indicates that the dissociated chloride

ligand(s) again reassociated with the metal following the

dehydrogenation reaction to yield the final products (see below,

Mechanistic investigations).

Figure 3. Transient EPR spectrum (X-band) upon one-electron reduction of complex

1 at 90ꢀK in tetrahydrofuran.

As ligand reduction was observed in the EPR spectrum, we

therefore planned for the reaction of complexes 1, 2 and 3a

using a reductant. Reduction of acetonitrile solutions of each of

complexes 1,

2 and 3a, separately, with cobaltocene (1

equivalent) and their subsequent exposure to air showed a clean

dehydrogenation reaction of the arylaminomethyl side arm in all

of these complexes (Scheme 5).

Scheme 6. Reaction with Zn powder.

Our efforts to isolate the deprotonated complexes of 1 and

3a by the reaction of 1 and 3a with NaOtBu in acetonitrile were

unsuccessful. The reaction of 1 with NaOtBu (1 equivalent)

resulted in the complete decomposition of 1 (Scheme S22). In

comparison, the identical reaction with 3a yielded complex 9

(Scheme S22), along with the free ligand, HL^c. It is known that

NaOtBu can act as a reductant,^38,39in addition to its strong basic

nature. Thus, it may participate partly in reducing the ligand in

3a to facilitate the dehydrogenation reaction.

The microanalytical CHN data matched exactly with the

formulations of the nickel complexes, 7–9, (see below,

Experimental section). In the IR spectra, complexes 7–9 showed

the N=N stretching frequency at around 1450ꢀcm⁻¹and the C=N

stretching frequency at around 1575ꢀcm⁻¹. Compounds 7 and 8

are paramagnetic (two unpaired electrons), as evidenced by

Scheme 5. Reaction with Cp₂Co^IIand dioxygen

In the case of the reaction of complex 1, the free ligand L^x

was isolated as the major product (60% yield), and complex 7

was isolated as the minor product (35% yield, Scheme 5a). In the

reaction of complex 2, the isolated free ligand L^ywas the minor

product (25% yield), and complex 8 was the major product (65%

yield, Scheme 5a). For the reaction of complex 3a, almost

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their room temperature magnetic susceptibility, μ_eff= 2.78ꢀ and

2.80ꢀB.M at 298ꢀK, respectively. Compound 9 is dinuclear, and its

room temperature magnetic susceptibility was found to be

4.54ꢀB.M.

4, and ORTEP plot of complex 8 can be found in t_Vh_iee_ws_Au_rtp_icp_leo_Ort_ni_ln_ing_e

information file (Figure S29). Suitable crystals for X-ray structure

determination of complexes, 7–9 and ligand L^xwere grown by

slow diffusion of a dichloromethane solution of the compounds

into hexane.

X-ray structural analysis of compounds 7 and 8 showed a

trigonal bipyramidal geometry, with N_azo(N1) and N_imine(N4)

occupying the apical positions; complex 9 is a dimer bridged by

two chloride ligands, with each metal centre having distorted

DOI: 10.1039/D0DT00466A

The isolated new ligands L^xand L^ywere also characterized

satisfactorily by positive ion ESI-MS, NMR and IR spectral studies

(Figures S23–S28). In the IR spectra, the ligands L^xand L^yshowed

strong absorption bands at 1456 and 1472ꢀcm⁻¹, respectively,

assigned to the N=N bond stretching frequency.¹⁰Strong

absorption bands at 1589 and 1597ꢀcm⁻¹for ligands L^xand L^y,

respectively, were assigned to the HC=N stretching frequency.³¹

In the positive ion ESI-MS, ligands L^xand L^yshowed molecular

ion peaks at m/z = 371.22 and 315.16ꢀamu, respectively, due to

the [MH]⁺ion, where M represents the ligand (Figures S27–S28).

Their simulated spectra matched exactly with their

corresponding experimental spectra. The identities of the

ligands were further clearly established by their NMR spectra

(Figures S23–S26). For example, in the ¹H NMR spectra, ligand L^x

showed a doublet signal for 12H at 1.19ꢀppm, which was

assigned to the methyl group of the isopropyl moieties. A

multiplet signal for 2H at 3.01ꢀppm was assigned to the tertiary

C–H proton of the isopropyl groups, and the singlet signal at

8.51ꢀppm was assigned to the imine C–H proton. In the aromatic

region of the spectrum, we found signals for 11 protons that

matched exactly with the number of aromatic protons expected

for ligand L^x.

octahedral geometry. Complex

9 has C₂crystallographic

symmetry; thus, one half of the molecule is identical to the

other half. As complex 8 is structurally qualitatively similar to

complex 7, only complex 7 is discussed below. The d_C12–N4values

for complexes 7 and 9 were 1.281(4) and 1.280(4)ꢀÅ, respectively

(Table 2), which were assigned to the CH=N bond.²⁶Similarly,

the d_N1–N2values for complexes 7 and 9 were 1.266(4) and

1.264(4)ꢀÅ, respectively (Table 2), clearly indicating the

unreduced state of the N=N bond. Thus, the coordinated ligands

L^xand L^zin complexes 7 and 9, respectively, have one side arm

containing an azo chromophore and the other side arm

containing an imine chromophore. The corresponding d_C12–N4

and d_N1–N2values for the free ligand L^xwere 1.266(2) and

1.241(2)ꢀÅ, respectively. Slight elongation of these two bonds in

complexes 7 and 9 compared to the free ligand may be due to

dπ(Ni)–π*(ligand) back-bonding. A comparison of the overlaid

structures of complexes 3 and 9 indeed showed a significant

difference in geometry. The aminomethyl group (C12–N4) in

complex 3 is significantly out of the plane, whereas it is in the

plane of the molecule in complex 9 (Figure 5a).

(ii) Crystal structure

Final authentication of the dehydrogenation products came

from the X-ray crystallographic structure determination of

complexes 7–9 and ligand L^x. These molecular structures have

confirmed the chemical reactions depicted in Schemes 5 and 7.

ORTEP plots of complexes 7, 9 and ligand L^xare shown in Figure

7

9

L^x

Figure 4. ORTEP representations and atom numbering schemes for complexes 7, 9 and ligand L^xat the 50% probability level

(a)

(b)

(c)

Figure 5. (a): Overlay structures of complexes 3 (red) and 9 (blue); (b, c): core structures of complexes 3 and 9.

6 | J. Name., 2012, 00, 1-3

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Table 2. Experimental and DFT-calculated bond lengths and angles of complexes 7, 9 and ligand L^x.

View Article Online

DOI: 10.1039/D0DT00466A

Complex 7

Complex 9

Ligand L^x

Bonds and angles

Bond lengths (Å) and bond angles (°)

Experimental

1.268 (3)

2.214 (2)

2.166(2)

1.958 (2)

1.280(3)

2.274(6)

2.248(6)

----

DFT

1.266

Experimental

1.264(2)

DFT

1.264

Experimental

DFT

1.257

N1–N2

Ni1–N1

1.241(2)

----

2.292

2.276

2.310(17)

2.215(16)

1.985(16)

1.280(3)

2.330

2.243

2.032

1.285

2.376

2.416

2.542

71.90

147.70

76.50

110.97

119.15

114.50

----

Ni1–N4

----

Ni1–N3

2.006

----

C12–N4

1.282

1.266(2)

----

1.275

----

Ni1–Cl1

2.856

2.325(6)

Ni1–Cl2

2.310

2.372(6)

----

Ni1–Cl2'

----

2.440(6)

----

N1–Ni1–N3

N1–Ni1–N4

N3–Ni1–N4

C12–N4–Ni1

N4–C12–C11

N3–C11–C12

73.98(7)

151.46(7)

77.71(7)

111.9(1)

118.0(2)

112.8(2)

72.956

149.63

76.80

72.68(6)

----

149.48(6)

77.06(6)

----

109.517

119.63

114.529

110.66(14)

118.80(19)

113.85(18)

----

The UV-Vis spectrum of the free ligand HL^ashowed a weak

absorption at 452ꢀnm (εꢀ=ꢀ614ꢀM⁻¹ꢀcm⁻¹) and a high intensity

absorption at 317ꢀnm (εꢀ=ꢀ18739ꢀM⁻¹ꢀcm⁻¹). In complex 1, the

low-energy absorption at 452ꢀnm of ligand HL^adisappeared, and

the high-energy transition at 317ꢀnm red-shifted to 353ꢀnm

(εꢀ=ꢀ15141ꢀM⁻¹ꢀcm⁻¹). Even though complex 3 is dimeric in the

solid state, it showed a spectrum similar to that of the

DFT calculations were performed on the representative

imine complexes 7 and 9 as well as ligand L^x. The DFT-calculated

structural parameters closely matched the crystallographically

determined parameters. Selected experimental and DFT-

calculated bond lengths and bond angles are given in Table 2.

The optimized structures of ligand L^xand complexes 7 and 9 are

shown in Figures S30–S32 and the MOs of complexes 7–9 are

shown in Figures S33–S38.

mononuclear complex

1

in solution. Spectra of HL^aand

complexes 1 and 3 are shown in Figure S39. Even in the variable

temperature UV-Vis spectral studies in acetonitrile solution,

none of its spectral features changed significantly. In cyclic

voltammetry, both the amine complexes 1 and 3 showed similar

voltammograms (Figure 2, see above). Thus, it is concluded that

the solid-state form of the binuclear complex 3 is converted to

its mononuclear analogue 3a in solution.

(iii) Comparison of structures 3 and 9 as solids and in solution

Even though both complexes

3 and 9 crystallized as

chlorobridged dimers in the solid state, there are noticeable

differences in the Ni(II)-bridging chloride bond lengths. The

symmetry-generated pair of bonds, Ni1–Cl2', Ni1'–Cl2 are

2.614(5)ꢀÅ and the Ni1–Cl2, Ni1'–Cl2' are 2.234(6)ꢀÅ in complex 3

(Figure 5b). The corresponding symmetry-generated pair of

In comparison, the dinuclear azo imine complex 9 has a

completely different spectrum in acetonitrile solution compared

to the monomeric complex 7. While the mononuclear complex 7

showed only one intense absorption at 358ꢀnm, dimer 9 showed

two prominent absorption peaks at 365 and 300ꢀnm (Figure

S39). In the variable temperature UV-Vis spectral study,

interestingly, it showed a slight shift of the equilibrium towards

the mononuclear form, as identified by the appearance of a new

peak at 320ꢀnm, accompanied by the disappearance of the peaks

at 365 and 300ꢀnm (Figure 6). Notably, on cooling the solution,

the original spectrum of complex 9 was regenerated very slowly.

bond lengths in complex

9 are 2.440(6) and 2.372(6)ꢀÅ,

respectively (Figure 5c). The Ni1–Cl2', Ni1'–Cl2 pair of bonds in

complex 9 are significantly shorter than those in complex 3,

indicating stronger bridging of the Cl⁻ligands in complex 9

compared to complex 3. Thus, complex 3 exists as monomer 3a

(Scheme 5b), whereas complex 9 retains its identity as a dimer

in acetonitrile solution. This is also reflected in the spectral

properties of the complexes in solution.

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shown in Figure 7. Identification of the iodide ligands in

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complex 10 as well as both iodide a^Dn^Od^I:c¹h^0.l¹o⁰r³id^9/e^D0li^Dg^Ta⁰n⁰d⁴s⁶⁶in^A

complex 11 indeed confirms the Cl⁻/I⁻exchange reaction, as

shown in Scheme 7. When excess KI (5 equivalents) was

added, complex 10 was isolated as the sole product

(Scheme 7). Notably, complex 3a/9 without reduction by

cobaltocene (1 equivalent) did not result in any Cl⁻/I⁻

exchange product over the same timescale (Scheme 7).

Figure 6. Variable temperature UV-Vis spectra of complex 9 (enlarged part is

shown in inset).

The dimeric structure of complex 9 in acetonitrile solution

was further probed by means of cyclic voltammetry. The

mononuclear complex 7 showed one irreversible reduction at

−0.25ꢀV, whereas complex 9 showed two reversible reductions

at −0.28(70) and −0.53(75)ꢀV. These two successive reductions

are associated with the reduction of the two azo pyridine

moieties in complex 9. Representative voltammograms of

complexes 7 and 9 are shown in the supporting information file

(Figure S40).

(iv) Mechanistic investigations

We carried out further experimental investigations in

order to gain insight into the mechanisms of the chemical

transformations that these ligands and complexes undergo.

To confirm the fact that the chloride ligand(s) indeed

dissociated from complexes 1, 2 and 3a upon one-electron

reduction, and that the dissociated chloride ligand(s) again

reassociated with the metal following the dehydrogenation

reaction to yield the respective final products 7, 8 and 9, we

studied the halide scrambling reaction by adding potassium

iodide (KI) to the reaction. The representative complex 3a

was used in this study (Scheme 7). Complex 3a was reduced

by cobaltocene (1 equivalent) to generate complex [3a]^.−.

Subsequently, 1 equivalent of solid KI was added, and the

solution was stirred for 1 h. The mixture was then exposed

to air for the dehydrogenation reaction to occur. Following

crystallization of the crude products, complex 11 was

isolated as the major product (45% yield), and complex 10

was isolated as the minor product (20% yield, Scheme 7).

Analysis of the X-ray structures of complexes 10 and 11

showed that complex 10 is mononuclear (Figures S41),

containing two iodide ligands with trigonal bipyramidal

geometry, whereas complex 11 is a dimer bridged by two

chloride ligands, and each of the metal atoms has distorted

octahedral geometry. In addition to ligand L^zand the

bridging chlorides, sixth coordination site in each of the

metal atoms has occupied by a mixture of terminal iodide

and chloride ligands with occupancy of approximately 0.69

iodide and 0.31 chloride. An ORTEP of the compound 11 is

Scheme 7. Chloride/iodide exchange reaction

Figure 7. An ORTEP of the complex 11 at 50% probability level

The reduced complex [3a]^.−was unreactive in an inert

atmosphere, even on heating the solution. Complexes 1, 2

and 3a were inert towards (2,2,6,6-tetramethylpiperidin-1-

yl)oxyl (TEMPO^.), O2^.−and Ph₃CCl (Scheme 8). It was also

observed that reaction of TEMPO^./O₂^.−with the one-

electron reduced partners [1]^.−, [2]^.−and [3a]^.−of complexes

1, 2 and 3a, respectively, in an inert atmosphere did not

result in any dehydrogenation products; instead, the

respective unreduced complexes 1, 2 and 3a were isolated

as the major products, along with some minor unidentified

compounds. However, exposure of each of the reduced

complexes [1]^.−, [2]^.−and [3a]^.−in acetonitrile solution to air

8 | J. Name., 2012, 00, 1-3

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led to dehydrogenation of the arylaminomethyl side arm,

indicating involvement of the putative reactive

intermediate ‘A’ (Scheme 8) in this reaction.^22c

Generally, during the oxidation of the aminomethyl moiety

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to the imine moiety in the metal com^Dpl^Oe^Ix^:e¹s⁰,^.1t⁰h³e^9/Dco⁰o^Dr^Td⁰i⁰n⁴a⁶te^6Ad

superoxide radical anion first abstracts a hydrogen atom to form

a complex containing the methylamino radical ligand and a

peroxide ligand.^19–28Then, the peroxide ligand is either released

as H₂O₂from this intermediate complex following the

abstraction of a proton^{19–21,23–28}or it participates further in the

oxidation of the methylamino radical moiety, resulting in a

hydroxo complex containing an imine ligand.^22cFollowing

significant efforts on our part, we were unable to identify any

trace of H₂O₂. It was found that Zn(OH)₂was generated from the

reaction of complex 1 with metallic Zn powder (Scheme 6), even

when dry air was used. Formation of Zn(OH)₂was confirmed by

the IR spectrum of the crude product following extraction of

.

.−

Scheme 8. Comparison of the reactions of PPh₃CCl/O₂/TEMPO and the

complex 7.

A

broad stretching frequency at 3465ꢀcm⁻¹

Cp₂Co^II/O₂-mediated reaction

characteristic of the Zn–OH moiety was observed (Figure S44).

Similarly, in the reaction of complex 3a with 1 equivalent of

cobaltocene, cobaltocenium hydroxide, [Cp₂Co]OH, was isolated

as the by-product (Scheme 5b). A broad stretching frequency at

3287ꢀcm⁻¹in the IR spectrum characteristic of [Cp₂Co]OH was

observed⁴⁰(Figure S45). The electron count showed that, in

general, this reaction is a three-electron oxidation starting from

the one-electron reduced complexes [1]^.−, [2]^.−and [3a]^.−.

Formation of a hydroxo complex by three-electron transfer in

To confirm the reaction of aerial oxygen with the one-

electron reduced complexes [1]^.−, [2]^.−and [3a]^.−, the

chemical transformation of one of them, [3a]^.−, in air was

monitored by time-dependent UV-Vis spectral studies, and

the spectral changes are shown in Figure 8. Complex [3a]^.−

showed three prominent transitions at 331, 430 and

595ꢀnm in the absence of air. Upon exposure of a solution

of [3a]^.−to air, the peaks at 430 and 595ꢀnm disappeared

and the peak at 331ꢀnm increased in intensity. Notably, the

spectrum of the solution showed only one transition at

331ꢀnm that was very similar to the spectrum of the

mononuclear dehydrogenated complex 7. Significantly, the

isolated complex 9, the end product of the reaction of [3a]^.−

in air, is a binuclear complex (see above, Crystal structure).

Thus, it is proposed that, initially, the mononuclear complex

12 was formed in the dehydrogenation reaction (Scheme

5b), which then dimerized slowly to form the comparatively

more stable binuclear complex 9. Notably, the reflux of an

acetonitrile solution of complex 9 with Zn powder resulted

in a significant quantity of mononuclear complex 12, as

evidenced by the increase in intensity of the band at

322ꢀnm in the UV-Vis spectrum (Figure S42). Following 6ꢀh

of reflux, crystallization of the crude mixture resulted in

two different types of crystals. Their characterization by X-

ray structure determination confirmed the formation of the

mononuclear complex 12 (Figure S43).

oxygen in

a similar dehydrogenation reaction has been

reported.^22cMoreover, multiple-electron transfer to dioxygen

resulting in O–O bond cleavage and formation of a hydroxo

complex in other chemical reactions have also been reported.⁴¹

Based on these observations, it is proposed that dissociation

of the chloride ligand(s) was indeed associated with one-

electron reduction of the complexes. Following the

dehydrogenation reaction, the dissociated chloride ligand(s)

were again reassociated with the metal to yield the final

dehydrogenated complexes (Schemes 5 and 7). The one-

electron reduced intermediate complexes [1]^.−, [2]^.−and [3a]^.−

activated dioxygen to facilitate the dehydrogenation reaction,

and the superoxide radical anion generated from complexes

[1]^.−, [2]^.−and [3a]^.−participated in the reaction while remaining

coordinated to the metal (Scheme 8).^22cThis active putative

coordinated superoxide radical anion picked up a hydrogen

atom from the NH position of the ligands,^22cforming a Ni(II)

peroxo complex (Scheme 9). The metal-coordinated peroxide

participated further in the oxidation of the amino radical ligand,

leading to the formation of a Ni(II) hydroxo complex and a free

hydroxyl radical^22c(Scheme 9). This Ni(II) hydroxo intermediate,

upon exchange of its hydroxide ligand with the dissociated

chloride ligand, generated the final dehydrogenated complexes

(Scheme 9). Furthermore, as the hydroxyl radical is a highly

reactive species, it might have decomposed the intermediate

hydroxo complex to liberate free ligands L^xand L^yin the reaction

of complexes 1 and 2, respectively, with cobaltocene (Scheme

9). Oxidation of the metal complexes by the hydroxyl radical has

been reported in the literature.⁴²Notably, metal-promoted

amine oxidation in transition metal complexes has been studied

for quite some time,^19–28and is still under active

investigation.^25,27,28In all of these examples, metal oxidation

initiates the reaction. However, such reactions, where ligand

Figure 8. Time-dependent UV-Vis spectrum of complex [3a]^.−in acetonitrile upon

exposure to air

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reduction promotes dehydrogenation of the amine to an imine

has not been reported in the literature.

their coordination chemistry with transition metal ions, as well

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as their reactivity, is ongoing and will be ^Dre^Op^I:o¹r⁰te^.1d⁰³in^9/d^Du⁰e^DTc⁰o⁰u⁴rs⁶e⁶.^A

Ar

N

Experimental

N

n

o

i

t

i

s

o

N

Materials and reagents

p

m

H

o

Ph

c

O

e

[Cp₂Co]Cl

+

y

D

b

Ar

Ni

Ar

Ni

Ar

NH

Ar

NH

Other products

N

OH

Cl

O₂

H

O₂

Cl

All reagents were of analytical grade purity, commercially available,

and used without further purification. The solvents used were purified

and dried using standard procedures. Tetrabutylammonium perchlorate

was prepared and recrystallised as reported earlier^12b. Perchlorate salts

are potential explosives and should be handled with care.

O₂

N

Ni

N

Ni Cl

ition

mpos

Deco

Cl

N

b

y

OH

Ph

[Cp₂Co]Cl

+

[Cp₂Co]Cl

C

c

[Cp₂Co]Cl

l ^-

OH

-

e

O

x

H

h

r

e

a

n

g

e

c

t

i

o

n

N

Ar

Ph

Ni

Instrumentation and methods

Cl

+

[Cp₂Co]OH

Other products

Cyclic voltammetry was conducted in 0.1 M Bu₄NClO₄solutions

by using a three-electrode configuration (Pt working electrode,

Pt counter electrode, and Ag/AgCl reference electrode) and PC-

controlled PAR model 273A electrochemistry system.

A

PerkinElmer model 2400 elemental analyser was used to obtain

microanalytical data (C, H, and N). The ESI mass spectra of the

samples were recorded on a maxis impact UHR spectrometer

(Bruker Daltonics) and Q-Tof micro (WATERS) mass

1

spectrometer. H and ¹³C NMR spectra were recorded using 400

MHz JEOL and Bruker 500 MHz spectrometers, respectively.

Chemical shifts were reported as δ-values relative to the internal

reference of tetramethylsilane (TMS) and solvent peak for ¹H

NMR and ¹³C NMR, respectively. IR spectroscopic data were

recorded using the PerkinElmer Spectrum RX I spectrometer

with KBr pellets. UV-vis spectra were obtained using a Cary 8454

UV-vis spectrophotometer. Spectroscopic grade solvents were

used for spectroscopic and electrochemical studies. X-band EPR

Scheme 9. Plausible reaction pathways

Conclusions

In conclusion, a series of redox non-innocent azo aromatic

pincer-like ligands: 2-(phenylazo)-6-

spectra were recorded using

a

Bruker Biospin EMX^micro

(arylphenylaminomethyl)pyridine (HL^a–c) have been synthesized

in which the aminomethyl group is reduced by two electrons

and two protons from its iminopyridine congener, whereas the

2-phenylazo fragment is oxidized by two electrons and two

protons from its hydrazo congener. In their nickel(II) complexes,

1–3, the aryl aminomethyl moiety binds the Ni(II) centre without

deprotonation. In the solid state, complex 3 is a dimer, while it

exists as monomer 3a in solution. In all of these three

complexes, 1, 2 and 3a, the aryl aminomethyl moiety undergoes

a dehydrogenative imine formation reaction in air via the

reduction of the azo pyridine moiety in the same ligand to yield

the new complexes 7, 8 and 9, respectively. In addition, free

ligands L^xand L^yhave also been isolated from the reaction of

complexes 1 and 2, respectively. The reaction mechanism was

investigated thoroughly by cyclic voltammetry, EPR, UV-Vis

spectroscopic studies, Cl⁻/I⁻exchange reaction, as well as DFT

calculations. Though transition metal mediated oxidative

dehydrogenative imine formation reactions have been the

subject of continuous study for quite some time, to date, only

metal oxidation has been found to initiate the dehydrogenation

reaction. As far as we are aware, such a dehydrogenation

reaction via ligand reduction has not been reported in the

literature. Moreover, this reaction has resulted in a new type of

pincer-like ligand, L^xand L^y, in which one side arm has a redox

non-innocent azo chromophore, while another side arm has a

redox non-innocent imine chromophore.³Our efforts to explore

spectrometer. Room-temperature magnetic moment was

measured using the Gouy balance (Sherwood Scientific,

Cambridge, UK).

Synthesis of the Compound A

Compound A was synthesised using the procedure reported in

the literature.²⁹To the warm solution of 50% sodium hydroxide

(50 mL), 6-methyl-2-aminopyridine (2 g) was added. The

resulting solution was then gently heated, and benzene (5 mL)

was added to it. Nitrosobenzene (2.5 g) was added for 10 min

with stirring. The mixture was stirred at 70°C for 8 h. The

reaction mixture was extracted using benzene (2 × 100 mL) to

obtain a solution that was heated with charcoal, filtered, and

concentrated to 100 mL under

a reduced pressure. The

concentrate was purified using column chromatography with

silica gel (100–200 mesh size) as a stationary phase and toluene

as an eluent. The yield and characterisation data of the resultant

compound A are as follows: Yield: 70%, ESI-MS: m/z 198 amu

[MH]⁺,¹H NMR (400 MHz, CDCl₃): 8.02-8.00 (m, 2H ); 7.74 ( t, 1H,

J= 7.64 Hz); 7.57 ( d, 1H, J = 7.9 Hz ); 7.51-7.49 ( m, 3H); 7.25 (d,

1H, J= 7.1 Hz); 2.68 ( s, 3H ); ¹³C NMR (400MHz,CDCl₃):

162.96(C); 158.80(C-H); 152.51(C-H); 138.62(C-H); 132.22(C-H);

129.26(C); 125.18(C-H); 123.78(C-H); 110.95 (C-H); 24.60 (CH₃);

IR: 1421 cm^-1(N=N); UV-vis: λ, 321 nm; (ε= 19649 M^-1cm^-1).

Synthesis of the Compound B

A total of 700 mg (3.55 mmol) of compound A was dissolved in

15 mL of carbon tetrachloride in a 100 mL RB flux. A catalytic

amount of AIBN and 0.812

g

(4.61 mmol) of N-

10 | J. Name., 2012, 00, 1-3

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bromosuccinamide were added to it, and the mixture was

refluxed for 12 h. The crude mass obtained from the

evaporation of carbon tetrachloride was dissolved in a minimum

volume of dichloromethane, and the product was isolated using

column chromatography with stationary-phase silica gel (100–

200 mesh size) and toluene as the eluent. The yield and

characterisation data are as follows: Yield: 60%, ESI-MS: m/z 276

amu [MBr⁷⁹],278 amu [MBr⁸¹], ¹H NMR(400MHz,d₆-DMSO): 8.08

(t, 1H, J=7.8Hz ); 7.99-7.96 (m, 2H); 7.75 (d, 1H, J=7.64Hz); 7.67-

7.64 (m, 4H); 4.83 (s, 2H); ¹³C NMR (400MHz,CDCl₃): 162.95 (C);

157.09 (C); 152.42 (C-H); 139.64 (C-H); 132.66 (C-H); 129.39 (C);

125.34 (C-H); 123.93 (C-H); 112.61 (C-H); 33.41 (CH₂); IR: 1417

cm^-1(N=N); UV-vis: λ, 319 nm; (ε= 18595 M^-1cm^-1).

Synthesis of Complex 1

View Article Online

DOI: 10.1039/D0DT00466A

In 10 mL of methanol, 200 mg (0.53 mmol) of ligand HL^awas

dissolved in a round bottom flask. In 5 mLof methanol, 191.12

mg (0.80 mmol) of NiCl₂.6H₂O was dissolved, and the resulting

solution was then added dropwise to the ligand solution present

in the round bottom flask. During addition, the colour of the

reaction mixture immediately changed from red to reddish-

brown. After 1.5 h of reflux, the solvent evaporated, and the

crude mass thus obtained was dried under high vacuum. The

product was then extracted with a dichloromethane solvent and

layered with hexane for slow diffusion, and crystalline

compound 1 was isolated. Its yield and characterisation data are

as follows: Yield: 70%, Anal. Calcd. for C₂₄H₂₈Cl₂N₄Ni: C, 57.53; H,

5.43; N, 11.18; Found C, 57.51; H, 5.46; N, 11.11; IR: 1438 cm^-

¹(N=N); 3268 cm^-1(2° NH); UV-vis: ε_354nm, 16209 M^-1cm^-1.

Synthesis of Ligand, HL^a

In 10 mL of THF, 300 mg (1.09 mmol) of compound B was

dissolved in a 100 mL round bottom flask. A total of 1.35 g (7.63

mmol) of 2, 6-diisopropyl aniline and 1 equivalent of dry sodium

carbonate with respect to 2, 6-diisopropyl aniline was added and

stirred at reflux temperature for 14 h. The crude mass, obtained

after THF evaporation, was dissolved in a minimum volume of

dichloromethane, and excess 2, 6-diisopropyl aniline was

evaporated by heating overnight at 60°C–70°C. Subsequently,

the crude mass was extracted using dichloromethane and

loaded on a preparative silica gel TLC plate for purification. A

solvent mixture of hexane: ethyl acetate (9:1) was used as an

eluent to isolate ligand HL^ain its pure state. Its yield and

characterization data are as follows: Yield: 70%, ESI-MS: m/z

373.24 amu [MH]⁺; ¹H NMR(400MHz,CDCl₃): 8.10 (d, 2H, J=7.6

Hz); 7.83 (t, 1H, J=8.4 Hz); 7.71 (d, 1H, J=7.6 Hz); 7.54 (m, 4H);

7.14 (m, 3H); 4.42 (s, 2H); 4.01 (br, s, 1H); 3.45 (m, 2H); 1.28 (d,

12H, J=6.8Hz);¹³C NMR (400MHz, d₆-DMSO): 162.12 (C); 159.68

(C); 151.88 (C); 143.19 (C-H); 142.44 (C); 139.27 (C-H); 132.49

(C); 129.65 (C-H); 123.84 (C-H); 123.53 (C-H); 123.31 (C-H);

122.97 (C-H); 112.00 (C-H); 56.05 (CH₂); 27.02 (C-H); 24.25 (CH₃);

IR: 1430 cm^-1(N=N); 3316 cm^-1(2° NH); UV-vis: ε_{319 nm}, 18843 M^-1

cm^-1; ε_{444 nm}, 734 M^-1cm^-1.

Synthesis of Complex 2

Complex 2 was synthesised using the procedure similar to

complex 1 synthesis. Its yield and characterisation data are as

follows: Yield: 68.1%, Anal. Calcd. for C₂₀H₂₀Cl₂N₄Ni₁: C, 53.98; H,

4.30; N, 12.59; Found C, 53.92; H, 4.31; N, 12.56; IR: 1450 cm^-

¹(N=N); 3268 cm^-1(2° NH); UV-vis: ε_356nm, 20035 M^-1cm^-1.

Synthesis of Complex 3

Complex 3 was synthesised by following the procedure similar to

that of complex 1 synthesis. Its yield and characterisation data

are as follows: Yield: 72.1%, Anal. Calcd. for C₃₆H₃₂Cl₄N₈Ni₂: C,

51.73; H, 3.86; N, 13.41; Found C, 51.77; H, 3.81; N, 13.46; IR:

1492 cm^-1(N=N); 3205 cm^-1(2° NH); UV-vis: ε_350nm, 41693 M^-1cm^-

1

.

Synthesis of Complex 4

In 5 mL of dry acetonitrile, 30.4 mg (0.06 mmol) of complex 1

was dissolved. Subsequently, 1 equivalent (0.06 mmol) of

pyridine was added to the complex solution. During addition,

the colour of the reaction mixture immediately changed from

reddish-brown to greenish-brown. After 3-h stirring at room

temperature, the solvent was evaporated under the reduced

pressure. Thus, the crude mass was obtained and dissolved in

acetonitrile. The pure compound was isolated through ether

diffusion into the acetonitrile solution of the crude product. Its

yield and characterisation data are as follows: Yield: 83%, Anal.

Calcd. for C₂₉H₃₃Cl₂N₅Ni: C, 59.93; H, 5.72; N, 12.05; Found C,

59.95; H, 5.70; N, 12.09; IR: 1432 cm^-1(N=N); 3285 cm^-1(2° NH);

UV-vis: ε_{352 nm}, 36856 M^-1cm^-1.

Synthesis of Ligand, HL^b

Ligand HL^bwas synthesised following the procedure similar to

that followed for HL^asynthesis. The yield and characterisation

data of ligand HL^bare as follows: Yield: 71.4%, ESI-MS: m/z

1

317.17 amu [MH]⁺; H NMR(400MHz,CDCl₃): 8.08-8.06 (m, 2H);

7.80 (t, 1H, J = 7.7 Hz); 7.67 (d, 1H, J = 7.9 Hz); 7.58-7.54 (m, 3H);

7.39 (d, 1H, J = 7.5 Hz); 7.01 (d, 2H, J = 7.5 Hz); 6.84 (t, 1H, J = 7.4

Hz); 4.46 (s, 2H); 2.34 (s, 6H);¹³C NMR (400MHz, d₆-DMSO):

162.4 (C); 160.4 (C); 151.8 (C); 148.2 (C-H); 139.3 (C-H); 132.4

(C); 129.6 (C-H); 128.9 (C-H); 122.9 (C-H); 119.9 (C-H); 116.0 (C-

H); 112.2 (C-H); 111.3 (C-H); 48.6 (CH₂); IR: 1441 cm^-1(N=N); 3310

cm^-1(2° NH); UV-vis: ε_315nm, 18716 M^-1cm^-1; ε_454nm, 811 M^-1cm^-1

Synthesis of Complex 5

Synthesis of Ligand, HL^c

Complex 5 was synthesised by following the procedure similar to

that of complex 4 synthesis. Yield: 85%, Anal. Calcd. for

C₂₅H₂₅Cl₂N₅Ni: C, 57.18; H, 4.80; N, 13.34; Found C, 57.13; H,

4.89; N, 13.33; IR: 1443 cm^-1(N=N); 3290 cm^-1(2° NH); UV-vis: ε₃₅₈

_nm, 22856 M^-1cm^-1.

Ligand HL^cwas synthesised following the procedure similar to

that followed for HL^asynthesis. Its yield and characterisation

data are as follows: Yield: 68.93%, ESI-MS: m/z 288.14 amu

[MH]⁺; ¹H NMR(400MHz,CDCl₃):8.07-8.05 (m, 2H);7.84 (t, 1H, 7.9

Hz); 7.67 (d, 1H, 7.9 Hz); 7.57-7.53 (m, 3H); 7.50 (d, 1H, 7.5 Hz);

7.19 (t, 2H, 7.6 Hz); 6.75-6.69 (m, 3H); 4.64 (s, 2H); ¹³C NMR

(400MHz, d₆-DMSO): 162.42 (C); 160.48 (C); 151.86 (C); 148.25

(C-H); 139.38 (C-H); 132.51 (C); 129.66 (C-H); 128.98 (C-H);

122.92 (C-H); 119.99 (C-H); 116.09 (C-H); 112.28 (C-H); 111.39

(C-H); 48.35 (CH₂); IR: 1507 cm^-1(N=N); 3385 cm^-1(2° NH); UV-vis:

ε_312nm, 37247 M^-1cm^-1; ε_445nm,1922 M^-1cm^-1

Synthesis of Complex 6

Complex 6 was synthesised following the procedure similar to

that of complex 4 synthesis. For the reaction, two equivalents

(0.18 mmol) of pyridine with respect to complex 3 were used.

The yield and characterisation data of complex 6 are as follows:

Yield: 87%, Anal. Calcd. for C₂₃H₂₁Cl₂N₅Ni: C, 55.69; H, 4.06; N,

This journal is © The Royal Society of Chemistry 20xx

J. Name., 2013, 00, 1-3 | 11

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ARTICLE

Journal Name

14.12; Found C, 55.62; H, 4.08; N, 14.10; IR: 1446 cm^-1(N=N);

utilised. No free ligand L^zemerged from the reaction mixture.

View Article Online

3224 cm^-1(2° NH); UV-vis: ε_{351 nm}, 36856 M^-1cm^-1.

The yield and characterisation data of complex 9 are as follows:

DOI: 10.1039/D0DT00466A

Yield: 90%, Anal. Calcd. for C₃₆H₂₈Cl₄N₈Ni₂: C, 51.98; H, 3.39; N,

13.47; Found C, 51.91; H, 3.39; N, 13.46; IR: 1488 cm^-1(N=N);

1589 cm^-1(CH=N); UV-vis: ε_{315 nm}, 30548 M^-1cm^-1; ε_{371 nm}, 27214

M^-1cm^-1.

Synthesis of the Complex 7 and Ligand L^x

In 10 mL of dry degassed acetonitrile, 100 mg (0.19 mmol) of

compound

1 was dissolved in a 25-mL Schlenk tube.

After the crystallisation of complex 9, the precipitate was

Subsequently, one equivalent of cobaltocene (37.6 mg, 0.19

mmol) was added under the inert atmosphere maintained with

argon. The colour of the reaction mixture immediately changed

from reddish-brown to deep brown. After 2.5 h of stirring at

room temperature, the reaction mixture was exposed to air. The

colour of the reaction mixture changed to light brown. The

solution was then evaporated under the reduced pressure. The

crude mass thus obtained was dissolved in dichloromethane and

crystallised through a slow diffusion of the dichloromethane

solution into hexane. After one week, a crop of block-shaped

crystals was separated from the mixture. Moreover, a light

green precipitate was deposited in the solution along with the

crystals. The collected crystals were of complex 7. The solution

phase was decanted in a 100-mL round bottom flask, and the

precipitate was discarded. The solvent was evaporated under

reduced pressure. The crude product obtained after solvent

evaporation was extracted thrice using 5 mL of diethyl ether.

The combined ether solution was filtered through a cellite pad,

and the filtrate was collected. The solvent was evaporated under

reduced pressure to obtain pure ligand L^x. Their yield and

characterisation data are as follows:

characterised using IR spectroscopy.

A broad stretching

frequency obtained at 3287 cm⁻¹was the characteristic of

[Cp₂Co]OH (Figure S30).

Chloride/Iodide exchange reaction: Synthesis of Complex 10

and 11

In 10 mL of dry degassed acetonitrile, 42.3 mg (0.05 mmol) of

complex 3 was dissolved in a 25-mL Schlenk tube under argon

atmosphere in a Schlenk line. 19 mg (2 equivalent, 0.1 mmol) of

solid cobaltocene was added to it maintaining the inert

atmosphere. During addition, the colour of the solution

immediately changed from brown to deep reddish-brown. The

reaction mixture was stirred for 15 min. Afterwards, 8.4 mg of KI

(1 equivalent) was added in acetonitrile. The reaction mixture

was stirred for 1 h at room temperature. The reaction mixture

was then exposed to air. The colour of the solution changed

from deep brown to considerably light reddish-brown. The

solvent was then evaporated, and the crude mass was dried

under high vacuum. The product obtained was dissolved in

dichloromethane and stored for the slow diffusion of hexane.

After three days, a crop of block-shaped crystals was separated

from the mixture as a minor product (complex 10). The solution

was decanted and evaporated under reduced pressure followed

by crystallisation through the slow diffusion of hexane into the

dichloromethane solution, which resulted in complex 11

formation in 5 days. Their isolated yield and characterisation

data are as follows:

Complex 7: Yield: 35%. Anal. Calcd. for C₂₄H₂₆Cl₂N₄Ni: C, 57.64;

H, 5.24; N, 11.20; Found C, 57.62; H, 5.25; N, 11.17; IR: 1449 cm^-

¹(N=N); 1573 cm^-1(CH=N); UV-vis: ε_{359 nm}= 16209 M^-1cm^-1.

Ligand L^x: Yield: 60%. ESI-MS: m/z 371.22 amu [MH]⁺; ¹H

NMR(400MHz,CDCl₃): 8.51(s,1H); 8.46(d,1H); 8.09-8.08(m,3H);

7.93(d,1H);

7.55(d,3H);

7.21-7.13(m,3H);

3.01(m,2H);

1.19(d,12H), ¹³C NMR (400MHz, CDCl₃): 163.30(C); 162.55(C);

154.36(C-H); 152.30(C); 148.20(C); 139.14(C-H); 137.21(C);

132.62(C-H), 129.30(C-H); 124.68(C-H); 123.83(C-H); 123.15(C-

H); 122.66(C-H); 114.68(C-H); 28.06(C-H); 23.52(CH₃), IR: 1456

cm^-1(-N=N-); 1589 cm^-1(-CH=N-).

Complex 10: Yield: 20%, Anal. Calcd. for C₁₈H₁₄I₂N₄Ni: C, 36.10;

H, 2.36; N, 9.36; Found C, 36.08; H, 2.38; N, 9.31; IR: 1400 cm^-

¹(N=N); 1586 cm^-1(CH=N); UV-vis: ε_{313 nm}, 21497 M^-1cm^-1; ε_{368 nm}

,

20725 M^-1cm^-1.

Synthesis of Complex 8 and Ligand L^y

Complex 11: Yield: 45%, Anal. Calcd. for C₃₈H₃₂Cl_6.64I_1.36N₈Ni₂:

C,40.54; H, 2.86; N, 9.95; Found C, 40.68; H, 2.75; N, 10.01; IR:

1414 cm^-1(N=N); 1595 cm^-1(CH=N); UV-vis: ε_{313 nm}, 21497 M^-1

cm^-1; ε_{368 nm}, 20725 M^-1cm^-1.

Complex 8 and Ligand L^ywere synthesised by following the

procedure similar to that followed for complex 7 and ligand L^x

syntheses. Their yield and characterisation data are as follows:

When the identical reaction was conducted with excess KI (5

equivalents), only complex 10 was isolated.

Complex 8: Yield: 65%, Anal. Calcd. for C₂₀H₁₈Cl₂N₄Ni: C,54.10;

H,4.09; N, 12.62; Found C, 54.06; H, 4.01; N, 12.58; IR: 1465 cm^-

¹(N=N); 1589 cm^-1(CH=N); UV-vis: ε_{359 nm}, 19027 M^-1cm^-1.

Dehydrogenation reaction with metallic Zn-powder

Ligand L^y: Yield: 25%. ESI-MS: m/z 315.16 amu [MH]⁺, 337.14

amu [MNa]⁺; ¹H NMR(400MHz,CDCl₃): 8.54 (s,1H); 8.46(d,1H);

8.09-8.08(m.2H); 8.03(t,1H); 7.93(d,1H); 7.56-7.54(t,3H);

7.10(d,2H); 7.02-6.98(t,1H); 2.20(s,6H); ¹³C NMR (400MHz,

CDCl₃): 162.85 (C); 154.21 (C); 152.11 (C-H); 149.89 (C); 138.90

(C-H); 132.41 (C-H); 129.10 (C-H); 128.07 (C-H); 126.69 (C);

124.12 (C-H); 123.62 (C-H); 132.33 (C-H); 114.82 (C-H); 18.22

(CH₃); IR: 1472 cm^-1(N=N); 1597 cm^-1(-CH=N-).

In 10 mL of acetonitrile solution, 100 mg (0.199 mmol) of

compound 1 was dissolved in a 25-mL round bottom flask. To

the solution, 38.8 mg (3 equivalents) of Zn powder was added.

The mixture was stirred for 2 h at room temperature. The

solvent was then evaporated under reduced pressure and the

crude product was extracted using dichloromethane. The

dichloromethane extract was concentrated and layered with

hexane in a 10-mL beaker for slow diffusion. After 7 days,

compound 7 was isolated as crystals with a 95% yield.

Synthesis of Complex 9

The characterisation results of compound 7 obtained from UV-

vis spectrum and CHN analysis demonstrated that it was the

same product that was obtained from the reaction of compound

1 with cobaltocene in air.

Complex 9 was synthesised by following the procedure similar to

that of complex 7 synthesis. In this reaction, one equivalent of

cobaltocene with respect to the complex 3a (Scheme 5b) was

12 | J. Name., 2012, 00, 1-3

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Dalton Transactions

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Journal Name

ARTICLE

After the extraction of compound 7 from the crude reaction

mixture by using dichloromethane, the residue present in the

25-mL round bottom flask was collected, and its IR spectrum

was obtained, which showed a strong absorption at 3465 cm⁻¹

refined using the ShelXL⁴⁶refinement package with least-

squares minimisation, in Olex2 refine⁴³. The suitab^Vle^iewhi^Ag^rh^tic-^lq^eu^Oaⁿl^liⁱⁿty^e

DOI: 10.1039/D0DT00466A

single crystals of complexes 3, 9, and 10 were mounted on the

goniometer head by using loops and collected on a Bruker

SMART APEX-II diffractometer equipped with graphite-

monochromated Mo Kα radiation (λ = 0.71073 Å) and were

corrected for Lorentz polarisation effects. Data reduction was

performed using Bruker SAINT software.⁴⁷Structures were

investigated by employing the ShelXT programme package⁴⁵and

were refined using either ShelXL⁴⁶with full-matrix least-squares

method or Olex2⁴⁸. All hydrogen atoms were calculated and

fixed using ShelXL after the hybridisation of all non-hydrogen

atoms.⁴⁹Refinement details and relevant explanations

(wherever applicable) are included in the individual CIFs. Table 3

presents all crystallographic parameters for compounds HL^c, L^x,

1, 3, 6, 7, 9, and 11, and Table S1 presents those for compounds

2, 8, 10, and 12. Table 1 and 2 present the selected bond lengths

and angles, respectively. All the data were deposited in

www.ccdc.cam.ac.uk. The CCDC no. of the crystals reported here

are 1887721(1), 1887736(2), 1887737(3), 1887738(6),

Furthermore, the same reaction was observed for compound 2.

Dehydrogenation reaction of the complex 1 with cobaltocene

in the presence of excess anhydrous ZnCl₂

In 10 mL of dry degassed acetonitrile, 100 mg (0.19 mmol) of

compound

1 was dissolved in the 25-mL Schlenk tube.

Subsequently, one equivalent of cobaltocene (37.6 mg, 0.19

mmol) was added by maintaining the inert atmosphere with

argon. After 2.5 h of stirring at room temperature, anhydrous

ZnCl₂(5 equivalents) was added and stirred for another 10 min.

Afterwards, the reaction mixture was exposed to air and the

solution was evaporated under the reduced pressure. The crude

mass obtained was dissolved in dichloromethane and

crystallised through the slow diffusion of the dichloromethane

solution into hexane. After one week, a crop of block-shaped

crystals was separated from the mixture. Moreover, light green

precipitate deposited in the solution along with crystals. Crystals

were collected as complex 7 with approximately 75% yield. The

solution phase was decanted in a 100-mL round bottom flask,

and the precipitate was discarded. The solvent was evaporated

under reduced pressure. Thus, the crude product obtained after

solvent evaporation was extracted thrice using 5 mL of diethyl

ether. The combined ether solution was filtered through the

cellite pad, and the filtrate was collected. The solvent was

evaporated under the reduced pressure to obtain pure ligand L^x.

Ligand L^xwas isolated as a minor yield (10%).

1887739(7),

1964891(8),

1887740(9),

1887741(10),

1887742(11), 1887743(12), 1887744(HL^c), and 1887745(L^x).

Computational Details

All computational calculations were performed in Gaussian 09

program package (revision A.02).⁵⁰The complete geometry

optimisation of ligands (HL^a, HL^c, and L^x) and nickel complexes

(1, 3, 6, 7–9) was conducted using the density functional theory

method at the RB3LYP and UB3LYP level of theory.⁵¹The 6-31+G

basis set was used for C, H, N, and Cl atoms, and the LANL2DZ

basic set with effective core potential was used for Ni atoms.^52-54

Vibrational frequency calculations were performed to ensure

that the optimised geometries represented the local minima,

and only positive Eigenvalues were obtained. An isosurface

value of 0.04 was used for the visualisation of Kohn-Sham

molecular orbitals.

Synthesis of Complex 12

In 10 mL of dry acetonitrile, 40.2 mg (0.048 mmol) of complex 9

was dissolved, and 15.7 mg (5 equivalents, 0.24 mmol) of zinc

powder was added. During addition, the colour of the reaction

mixture immediately changed from reddish-brownish to light

green. The reaction mixture was then refluxed for 2.5 h, and the

colour changed to deep brown. After solvent evaporation, the

crude mass was dried under high vacuum. The product was then

dissolved in dichloromethane and layered with hexane for slow

diffusion. After one week, the crop of block-shaped and rod-

shaped crystals was separated. The structure of block-shaped

crystals was determined using X-ray diffraction, and the crystals

exhibited the mononuclear form. The characterisation data of

complex 12 are as follows: Anal. Calcd. for C₁₈H₁₄Cl₂N₄Ni:

C,51.98; H, 3.39; N, 13.47; Found C, 51.91; H, 3.31; N, 13.41;IR:

1442 cm^-1(N=N); 1586 cm^-1(CH=N); UV-vis: ε_{357 nm}, 18985 M^-1

cm^-1.

Acknowledgements

We are thankful to DST (Department of Science and Technology)

for DST-Inspire research grant: DST/INSPIRE Faculty

Award/2014/CH-163. SS is thankful to DST for fellowship

support. MK is thankful to UGC for fellowship support. BG is

thankful to DST and IISERK for fellowship support. Authors are

thankful to Dr. Somnath Dey and Dr. Biswajit Bhattacharya for

their help to improve the X-ray crystal structure model. The

authors are also grateful to the reviewers for their critical

comments and suggestions to improve the quality of the

manuscript. We are also thankful to the anonymous

crystallographer for helping us to finalise some of the crystal

structure.

X-ray Crystallography

For diffraction, the high-quality suitable single crystals of

compounds HL^c, L^x, 1, 2, 6, 7, 8, 11, and 12 were coated with

fomblin oil, fixed on a loop, and mounted on a goniometer

under N₂flow at a temperature of 100 K. The data were

collected in a Agilent SuperNova, (Dual, Cu at zero, Eos)

diffractometer by using either a monochromatic Mo Kα source

(λ = 0.71073 Å) or a monochromatic Cu Kα source (λ = 1.5406 Å).

Data reduction was performed with CrysAlisPRO, Agilent

Technologies, Version 1.171.37.34. Single-crystal structures

were refined using Olex2⁴³software package, with the

Superflip⁴⁴or ShelXT⁴⁵structure solution programme by using

Charge flipping or intrinsic phasing method. All structures were

Conflicts of interest

There are no conflicts to declare.

J. Name., 2013, 00, 1-3 | 13

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ARTICLE

Journal NameRTICLE

View Article Online

DOI: 10.1039/D0DT00466A

Table 3. Crystallographic Data of the ligand HL^c, L^xand complexes 1, 3, 6, 7, 9 and 11.

Parameters

HL^c

L^x

1

3

6

7

9

11

CCDC no.

empirical formula

1887744

C₁₈H₁₆N₄

1887745

C₂₄H₂₆N₄

1887721

C₂₄H₂₈Cl₂N₄Ni

1887737

C₃₆H₃₂Cl₄N₈

Ni₂

1887738

C₂₃H₂₁Cl₂N₅

Ni

1887739

C₂₄H₂₆Cl₂N₄Ni C₃₆H₂₈Cl₄N₈Ni₂

1887740

1887742

C₃₈H₃₂Cl_6.64I_1.36

N₈Ni₂

formula wt.

crystal system

space group

a (Å)

b (Å)

c (Å)

α (deg)

β (deg)

γ (deg)

Cell Volume

288.35

orthorhombic

Pna2₁

29.279(4)

10.4375(13)

4.6429(6)

90.00

1418.9(3)

370.49

Triclinic

P -1

8.6058(4)

10.9184(4)

11.7427(4)

68.670(3)

83.291(4)

89.451(3)

1020.09(7)

502.09

Triclinic

P-1

9.4290(5)

10.4037(8)

12.8727(8)

107.115(7)

90.214(5)

100.707(6)

1183.54(13)

835.88

Monoclinic

P 2₁/n

9.6504(4)

14.5816(5)

12.5608(4)

90.00

98.294(2)

90

497.06

Triclinic

P -1

9.1097(5)

10.1284(5)

12.2167(7)

90.864(4)

100.473(4)

101.390(4)

1085.07(10)

500.10

Monoclinic

P 2₁/n

19.0222(4)

12.3486(2)

20.4706(4)

90.00

90.436(2)

90.00

4808.35(16)

831.88

Monoclinic

P 2₁/n

12.3109(15)

9.8915(13)

15.806(2)

90.00

108.517(7)

90.00

1825.1(4)

1126.11

Monoclinic

P 2₁/c

10.4316(3)

15.7887(4)

13.0106(3)

90.00

100.576(2)

90.00

1749.05(11)

2106.46(9)

R-factor

Z

4.32

4

3.61

2

3.75

2

2.81

2

2.78

2

4.68

8

2.94

2

2.71

2

T (K)

µ (mm^─1)

100.00(10)

0.654

100.00(10)

0.562

100.00(10)

1.064

296.15(10)

1.423

100.00(10)

1.162

100.00 (10)

3.345

296.15(10)

1.363

100.03(11)

13.134

ρ_calcd(g cm^-3)

F (000)

1.350

608.0

1.206

396.0

1.409

524.0

1.587

856.0

1.521

512.0

1.382

2080.0

1.514

848.0

1.775

1114.0

2θ range (deg)

6.04 to 132.6

8.14 to

3.32 to 54.24

4.3 to 55.26

4.1 to 52.74

6.32 to

3.68 to 55.76

8.62 to 132.42

132.22

132.72

Data/restraints/

parameters

R₁, wR₂[I >2σ(I)]

2470/1/199

3526/0/257

4489/0/287

4056/0/290

4441/0/284

8387/0/567

4307/0/282

0.0294,0.0599

0.0488,0.0672

3692/0/257

0.0271,0.0697

0.0289,0.0707

0.0432,

0.1050

0.0475,

0.1092

1.072

0.24/-0.23

0.0361,

0.0972

0.0383,

0.0991

1.055

0.0375,

0.0811

0.0444,

0.0862

1.041

0.49/ -0.37

0.0281,0.0609 0.0278,0.0620

0.0423,0.0663 0.0319,0.0648

0.0468,

0.1267

0.0492,

0.1297

1.039

0.66/ -0.64

R₁, wR₂(all data)

GOF on F²

largest difference

in peak and hole

(e Å^-3)

1.047

0.35/-0.41

1.057

0.32/-0.33

1.030

0.31/-0.33

1.066

0.82,-0.64

0.43/

-0.23

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Article Doi

Dehydrogenation of amines in aryl-amine functionalized pincer-like nitrogen-donor redox non-innocent ligands via ligand reduction on a Ni(ii) template

DOI: 10.1039/d0dt00466a

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