Coligand modulated oxidative: O -demethylation of a methyl ether appended tetradentate N-ligand in Co(ii) complexes

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Varghese and N. N. Murthy, Dalton Trans., 2020, DOI: 10.1039/C9DT04609G.

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14 March 2017

Pages 3073-3412

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Coligand Modulated Oxidative O-demethylation of Methylether

Appended Tetradentate N-Ligand on Co(II) Complexes

S. S. Anjana,*^a#,B. Varghese^band N. N. Murthy*^a

Received 00th January 20xx,

Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

Two Co(II) complexes of the formula CoL^OMeX₂(X= Cl^-(1a); X= I^-(1b)), where L^OMeis 2-methoxy-N,N-bis(pyridin-2-ylmethyl)

aniline were synthesized and their structure, spectroscopy and reactivity were studied. Upon oxidation of 1a and 1b, the

ligand L^OMeundergo demethylation at the metal centre resulting in the formation of Co(III) complexes with modified

phenoxide ligand. This is the very first examples of oxidative O-demethylation reported at a Co(II) centre. The oxidative

behaviour shows striking dependance on the nature of coligands coordinated to the metal centre. The Co(II) complex 1a

with stronger chloro coligands requires a strong oxidising agent like t-BuOOH for oxidative demethylation and the

subsequent formation of a mononuclear Co(III) complex with demethylated ligand, CoL^O-Cl₂(2). On the other hand, the

complex 1b with weaker iodo coligands undergoes oxidation in presence of weak oxidant O₂to form a dihydroxo bridged

binuclear Co(III) complex [Co₂(L^O-)₂(OH)₂]²⁺(3) with modified phenoxide ligand. The oxidation of 1b to 3 is monitored and

the intermediate Co(II) iodo aqua complex, [CoL^OMeI(H₂O)]⁺and Co(II) diaqua complex, [CoL^OMe(H₂O)₂]²⁺are isolated and

characterised.

show that Cu-O₂adducts of this ligand are capable of C-H bond

oxidation of toluene and alkyl benzene.¹⁵This report indicates

that the oxidation reactions of other metal complexes with

Introduction

Modifications of coordinated ligands, such as hydroxylation of

aliphatic^1-3and aromatic C-H bonds^4,5, N-dealkylation^6-8and O-

dealkylation are often observed in the oxidation reactions of

biomimicking model complexes of non-heme enzymes, which

employ either O₂or its activated forms as oxidants. O-

dealkylation is a well studied reaction in heme chemistry; since

it is involved in the catalytic cycle of cytochrome p-450

enzymes and is responsible for the detoxification of

xenobiotics.^9-12However, reports about similar reactions in

non-heme sites are quite rare. Over the past few decades,

studies have been carried out by employing copper and iron

complexes of tetra and pentadentate N-ligands with

strategically placed alkoxo groups to accomplish metal assisted

O-dealkylation.¹³In most of these reactions, O-dealkylation

was observed as a side reaction and a clean conversion was

not achieved. Recently, Mandon et al. successfully showed the

oxidative O-demethylation at a Fe(II) complex using modified

TPA ligand; but isolation of the Fe(III) phenoxide product was

found to be difficult, owing to its sensitivity.¹⁴

similar methyl ether appended ligands may be more

interesting. Since oxidation reactions at ferrous centres are

more significant in bioinorganic point of view, we started our

studies with the ferrous complex FeL^OMeCl₂, where L^OMeis 2-

methoxy-N,N-bis(pyridin-2-ylmethyl) aniline. To our delight, it

was noted that the complex undergo oxidative O-

demethylation in presence of oxidant t-BuOOH. Demethylation

was partial and observed only for the chloro analogue.

However, the Fe(III) phenoxide product was very stable and

could be isolated and characterised both in solid and solution

state. The mechanism for O-demethylation is proposed to

proceed via a high valent Fe(IV) intermediate.¹⁶

Since methylether appended ligands show different oxidation

behavior with copper and iron, it will be of interest to see the

reaction of other metal complexes of L^OMewith oxidants. A

recent report by Kroutil et al. has showed that the cobalamin-

dependent methyl transferase enzymes are capable of

reversible C-O-ether bond formation and breakage via a Co^III

and the highly reactive Co^Icycle.¹⁷The advantage of cobalt

complexes over copper and iron analogues is their higher

stability owing to high CFSE. However, no O-demethylation

was reported at Co(II) centers of non heme complexes till date.

In this article we describe the O-demethylation of L^OMein Co(II)

Karlin et al. had studied the copper-dioxygen chemistry of a

tetradentate ligand 2-methoxy-5-tert-butyl-N,N-bis(pyridin-2-

ylmethyl) aniline (^OL) with methylether append and the results

^aDepartment of Chemistry, IIT Madras, Chennai 600 036, India, E-mail: complexes upon oxidation. Observations suggest that the

anjanacy11d044@gmail.com , nnmurthy@iitm.ac.in

coligands play an important role in reactivity, which is

^bSophisticated Analytical Instruments Facility (SAIF), IIT Madras, India

^#Present Address: Department of Chemistry, IIT Bombay, Powai 400 076

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

supplementary information available should be included here]. See

DOI: 10.1039/x0xx00000x

expected as it is known that coligands can influence

coordination number, crystal packing pattern and also

magnetic properties.^18-20The reactivity of few other redox

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inactive metal (Mn, Ni and Zn) complexes of L^OMeare also oxygen. An identical trend was observed for the Co(II)

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DOI: 10.1039/C9DT04609G

explored and the details are discussed.

complexes of TPAs with similar geometry. Though TPAs are

reported to form both five and six coordinated Co(II)

complexes with halide coligands, L^OMeexclusively formed six

coordinated complexes.^{21, 22}

Results and Discussion

Two Co(II) complexes of the formula CoL^OMeX₂(X= Cl^-, 1a; I^-,

1b) have been synthesized and characterised. On reaction with

oxidants, both these complexes underwent oxidative O-

demethylation resulting in the formation of Co(III) complexes

with phenoxide ligands (Scheme 1). The chloro complex, 1a

required a stronger oxidant like t-BuOOH for oxidation and

resulted in the formation of a mononuclear Co(III) complex,

2

whereas the iodo complex, 1b underwent demethylation in

presence of mild oxidant O₂and formed a dihydroxo bridged

binuclear Co(III) complex, 3. Cobalt was proved to be a better

choice on comparison with ferrous complexes, as the oxidation

reactions were straight forward and the Co(III)-phenoxide

complexes were stable enough to isolate and characterise.

Figure 1. Displacement ellipsoid (50% probability) plots of

Co(II) complexes CoL^OMeCl₂, 1a and CoL^OMeI₂, 1b. H atoms are

not shown for sake of clarity.

Me

UV-Vis-NIR spectra of these complexes, recorded in CH₂Cl₂,

showed well defined bands at 580 and 592 nm respectively for

1a and 1b with ε values in the range 110-160 Mol^-1cm^-1(Figure

S1). High ε values (>100 Mol^-1cm^-1) are typical of five

coordinated trigonal bipyramidal complexes. Moreover, the

splitting pattern of the peaks also matches with the spectra of

penta-coordinated complexes.²³These observations conclude

that both 1a and 1b exist as penta-coordinated trigonal

bipyramidal species in solution when a non-polar solvent like

CH₂Cl₂is used. This may be due to the dissociation of one labile

chlorine atom or due to the detachment of anisole moiety

from the coordination sphere, which subsequently lead to the

formation of a five-coordinated species.

Me

N

O

N

O

N

I

Cl

Co^II

CoCl₂

N

CoI₂

N

O

N

^OMeI

₂, 1b

CoL^OMeCl

L^OMe

CoL

₂, 1a

CH₂Cl₂

H

₂O O₂

t-BuOOH

2⁺

H

O

N

O

N

Cl

O

Co^III

^IIIN

Co

Co^III

N

O

N

O

H

CoL^O-Cl

²⁺, 3

[Co₂(L^O-)₂(OH)₂]

₂, 2

Scheme 1. Representation of oxidative demethylation of Co(II)

complexes in presence of oxidants

Spectra of the complexes were recorded in variety solvents to

understand their solution behavior and both complexes exhibit

distinct solution properties (Fig. S2). For 1a, the λ_maxwas blue

shifted to 550 nm in alcohol, on comparison with the spectrum

in CH₂Cl₂(580 nm). The ε values are in the range 50 Mol^-1cm^-1,

typical for six coordinate Co(II) complexes²⁴and it was

concluded that the solid state structure is retaining as such in

these solvents. The λ_maxwas further blue shifted to 510 nm

with much lower molar absorption coefficients (20-30 Mol^-

¹cm^-1) when H₂O was used as a solvent. This very low value and

the blue shift in absorption can be due to the formation of an

aqua complex, in which the labile halides were replaced with

water molecules. Thus it is noted that, as polarity of solvent

increases, the λ_maxgets blue shifted with lower ε values.

Similar, but more striking trend was observed for iodo complex

1b (Fig 2, S3) also. Hence it is concluded that both these

complexes exhibit solvatochromism and exist as five

coordinated species in non-polar solvents like CH₂Cl₂. Whereas

in more polar coordinating solvents like H₂O and MeOH, they

show properties of regular octahedral complexes and retain

octahedral geometry.

Sythesis and characterisation of Co(II) complexes

The precursor Co(II) complexes 1a and 1b were synthesized as

fine solids in good yield (>75%) by mixing corresponding metal

salts and ligand in 1:1 molar ratio in EtOH/MeOH under inert

atmosphere. Single crystal X-ray analysis shows that both

these complexes are mononuclear (Fig. 1, Table S1). The

molecules consist of a Co(II) centre bound by a molecule of

L^OMecoordinated in a standard tripodal tetradentate fashion.

The two other coordination sites are occupied by terminally

bound halide coligands, which are cis to each other. The metal

centre in each complex adopt an approximate octahedral

geometry with one halide trans to anisole oxygen and the

other trans to the tertiary amine nitrogen. The square plane of

octahedron was made by one of the coligands and the

nitrogen atoms from ligand. The anisole O atom from the

ligand backbone and the halide coligand trans to it occupy

axial positions. The anisole -CH₃group lies parallel to the

square plane as in the Fe(II) analogue.¹⁶The metal halide bond

lengths are non-uniform and the halide trans to the anisole

oxygen coordinate weakly due to the trans effect of ether

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

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group, which is getting detached from the metal in presence of

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DOI: 10.1039/C9DT04609G

non-polar solvents and not the coordinated halide.

This observation matches with the conclusion drawn from UV-

Vis studies, where spectra of 1a showed the characteristic

1

features of five coordinated species in CH₂Cl₂. Further, the H

NMR spectrum of 1a recorded in D₂O is identical with the

spectrum of aqua complex [CoL^OMe(OH₂)₂]⁺recorded in D₂O

(vide infra, Fig S15). Based on these observations, the solution

speciation of 1a can be depicted as in scheme 2.

2+

Me

Figure 2. UV-Vis spectra of CoL^OMeI₂, 1b in different solvents.

OH₂

N

O

N

O

Cl

Co^II

H₂O

CH₂Cl₂

N

Co^II

N

O

CH₃

In literature, UV-Vis studies of five and six coordinated Co(II)

complexes of N and O donor ligands are known to show similar

solvatochromic behaviour.²⁵Previously reported cobalt(II)

complexes of TPA also showed similar structural changes in

solvents.^{26 1}H NMR spectra of the six coordinated complex

Co(TPA)Cl₂and five coordinated [Co(TPA)Cl]⁺were identical,

which suggest that the six coordinated complex can undergo

chloride dissociation to form five coordinated species in

solution.

N

Scheme 2. Solution speciation of 1a in different solvents.

¹H NMR spectra of iodo complex 1b was recorded in CD₃OD

and was compared with that of 1a to understand the influence

of coligands on spectral features (Fig S4). Spectra were similar

with eleven sharp signals (two up-field and nine down-field) in

a wide spectral window; but some minor differences were

noticed as coligand changes. As ligand field strength of the

halide decreases, the up-field shift also decrease (-52 ppm for

1a to -37 ppm for 1b). The down-field shift and the pattern of

the signals did not show any such trend and remained more or

less same for both complexes.

Assignment of signals in the paramagnetic ¹H NMR spectrum is

challenging owing to the hyperfine shift and broadening of

signals.²⁸All signals in the ¹H NMR spectra of 1a were

1

unambiguously assigned (Fig. 4) with the help of H-¹H COSY

(Fig. S5) and T1 and T2 relaxation time measurements (Table

S3).^29-32Further, to understand the temperature dependence

of paramagnetic ¹H NMR signals, variable temperature spectra

of 1a was recorded in the temperature range -85 to +55˚C (228

- 328 K). As temperature increased, the most down-field peaks

gradually shifted up-field, whereas the up-field peaks showed

gradual down-field shift (Fig S6). Curie plots show that the

isotropically shifted resonances change linearly as a function of

T^-1(Fig S7). Magnetic susceptibility of 1a and 1b (CD₃OD, 298

K) indicate μ_effvalues of 5.2 and 5.4 μ_Brespectively, consistent

with an S = 3/2 ground state.³³

Figure 3. ¹H NMR spectra of CoL^OMeCl₂, 1a in different solvents.

More evidences for the influence of solvent on the structural

1

and spectral properties of 1a and 1b are obtained from H

NMR studies (Fig 3). The spectrum recorded in CD₃OD showed

eleven very sharp signals (two up-field and nine down-field) in

the spectral range -60 to 200 ppm. Presence of up-field shifted

signals is an indication of significant pseudo contact

contribution to isotropic shifts.²⁷Though spectral width was

much lesser in D₂O (-20 to 150 ppm), the pattern was similar

confirming that the octahedral geometry is remained more or

less same in both these solvents. However, the spectrum in

CDCl₃has only one up-field shifted signal and the other signal

was present in the diamagnetic region, indicating the

detachment of the anisole -OMe group (vide infra for signal

assignment) from the metal centre and the consequent

formation of a five coordinated Co(II) complex. Hence it is

1

Figure 4. Paramagnetic H NMR spectra of 1a in CD₃OD at RT

(500 MHz) and the assignment of signals

1

concluded from H NMR spectra that it is the methyl ether

Electrochemistry of Co(II) complexes are also investigated to

understand the feasibility of oxidation. CV of 1a, recorded in

This journal is © The Royal Society of Chemistry 20xx

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

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CH₃CN has a one-electron redox wave (E_1/2, 274 mV vs. Fc/Fc⁺) comprised of two pyridine nitrogens, phenoxide oxygen and

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corresponding to metal oxidation. CV of 1b in CH₃CN shows one terminal chloride. The other chlori^Dd^Oe^I:a¹n⁰d^.10t³e⁹r^/t^Cia^9Dry^T0a⁴m⁶⁰in^9Ge

two redox waves; one at E_1/2= 160 mV vs. Fc/Fc⁺corresponds nitrogen take the axial positions. The amine N made longer

to the redox couple I^-/I₂, and the second one at E_1/2= -209 mV bond with the metal (1.953(3) Å) on comparison with the

vs. Fc/Fc⁺indicating the one electron metal redox couple pyridines (1.9445(3) Å_av). The three trans angles around the

Co^2+/3+(Fig 5).^{34, 35}The Cu(II) complex of L^OMeexhibited similar metal center showed values of 167.03(13), 178.69(9) and

reversible redox processes with a E_1/2value of -510 mV vs. 176.68(9) indicating that the complex is less distorted on

Fc/Fc⁺. The lower E_1/2value indicates its high reactivity to O₂comparison with the Co(II) analogue, 1a and has more perfect

and the facile formation of Cu-O₂complexes. Fe(II) complex of octahedral geometry. Similar mononuclear Co(III) complexes

L^OMehas much higher E_1/2value (-55 mV vs. Fc/Fc⁺) and was with labile coligands such as Cl^-or H₂O are known as good

stable to air. Hence, the comparatively lower oxidation catalysts for phosphate ester hydrolysis.^{36, 37}

potential of Co(II) iodo complex 1b, on comparison with 1a

,

indicates that it may be more reactive to O₂.

Figure 5. CVs of 1a (left) and 1b (right) in CH₃CN with 0.1 M

nBu₄PF₆as an electrolyte. Scan rate: 0.05 V/s; glassy carbon

working electrode, Ag/AgCl/KCl(aq) reference electrode.

Figure 6. Displacement ellipsoid (50% probability) plots of

Co(III) complex [CoL^O-Cl₂, 2. H atoms, counter anions and

solvents of crystallisation are not shown for sake of clarity

As expected based on electrochemical studies, Co(II) chloro

complex 1a was stable and did not show any reactivity to air in

solid or in solution state even after exposing for longer period

of time. Reaction of 1a with stronger oxidant like H₂O₂resulted

in the decomposition of the starting material and did not bring

any oxidation. After few trials, it was found that 1a undergo

oxidation with t-BuOOH in CH₂Cl₂medium. On refluxing the

blue solution of 1a in CH₂Cl₂with two equivalents of t-BuOOH,

a parrot green solution of Co(III) complex was obtained after

Unlike choro complex 1a, 1b was not stable to air and the dull

green complex turns red on exposure to moisture, indicating

reactivity (Fig S8). Though O-demethylation was expected,

reaction of 1b with t-BuOOH in CH₂Cl₂resulted in the

decomposition of the precursor complex. Similar result was

obtained with other strong oxidants like H₂O₂also. On varying

reaction conditions, it was found that 1b undergoes oxidative

O-demethylation in presence of O₂in CH₃CN-H₂O mixture,

20 h. Based on our studies with the ferrous complexes of L^OMe

,

the possibility of oxidative O-demethylation and the formation

of a Co(III) phenoxide complex is expected, which is confirmed

by structural and spectral analysis of the product. Solvent

removal followed by washing the residue with diethyl ether,

yielding Co(III) complex

3 with phenoxide ligand. The oxidized

product was isolated as brown-red solid in 40% yield after

overnight reaction. However, no reaction was noticed in the

absence of O₂or H₂O.

the Co(III)-phenoxide complex

2 was isolated as a gray solid in

excellent yield (> 95%). The reaction was clean and took place

only under specific conditions. On using more than two

equivalents of oxidant, the complex got decomposed, whereas

less than two equivalents of oxidant leads to longer reaction

time. Solvents play an important role in the oxidative O-

demethylation and the reaction took place only in chlorinated

solvents.

Single crystal X-ray analysis shows that complex

3 has a

binuclear structure in which the Co(III) centers are held

together by two hydroxo bridges (Fig. 7, Table S4). The

complex is symmetric and has an inversion center that relates

one half to the other. The phenoxide ligand L^O-is coordinated

in a tripodal fashion around Co(III) and the octahedral

coordination is satisfied by coordination with two bridging

hydroxo groups. The pyridines make weaker bonds (1.9715(6)

Å_av) and occupy axial positions of the octahedron. The metal

atom lies on the square plane made by two hydroxo groups,

phenoxide oxygen and tertiary amine nitrogen. The amine N

forms shorter bond with the metal (1.915(5) Å) than the

pyridines. This is in stark contrast with that observed for the

Single crystal X-Ray analysis shows that

2 is mononuclear with

distorted octahedral geometry around Co(III) (Fig. 6, Table S4).

The most interesting feature is the coordination of the

phenoxide O-atom, which is formed as a result of O-

demethylation of the anisole append of L^OMeduring oxidation,

with the Co(III) centre. Except the Co-Cl bonds (2.2559(9) Å_av)

all bonds made to metal are less than 2 Å, which is typical for

Co(III) complexes. The square plane of the octahedron is

other Co(III) complex 2. The Co-Co distance is 2.929(3) Å and

O-O’ distance is 2.423(4) Å which are expected, on comparison

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

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with binuclear Co(III) complexes having similar Co^III₂(μ-OH)₂

diamond core.³⁸

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DOI: 10.1039/C9DT04609G

N

Cl

N

Cl

Co^II

Cl

N

Cl

t-BuOOH

CH₂Cl₂

Co^III

N

Co^III

O

N

O

N

O

N

O

CH₃

O

Bu^t

H

CH₃

C

1a

H₂

Cl

N

Cl

Co^III

O

N

Co^III

N

Cl

O

-Bu^tOH

-CH₂O

CH₂

N

O

2

Scheme 3. Proposed mechanism for the oxidative O-

demethylation of 1a and the formation of

2

.

Figure 7. Displacement ellipsoid (50% probability) plots of

Co(III) complex [Co₂L^O-(OH)]₂.Cl_{0.75 1.25}

.4H₂O.CH₃CN, 3. H atoms,

I

The mechanism of conversion of 1b to

3 is likely to follow a

counter anions and solvents of crystallisation are not shown

for sake of clarity

different route. It was noticed that 1b undergoes oxidation

only in presence of H₂O, suggesting the involvement of an

aqua complex as a precursor for oxidation. To understand the

mechanistic details and to identify the actual Co(II) precursor

which undergoes oxidative O-demethylation, the iodo complex

was reacted with H₂O. The red colour of complex 1b in CH₃CN

faded to orange on addition of H₂O and to very pale pink in

presence of excess H₂O. This can be an indication of the

subsequent iodide replacement by H₂O molecules and the

formation of Co(II) aqua complexes with formula

[CoL^OMe(OH₂)I](I), 1c and [CoL^OMe(OH₂)₂](I)₂, 1d. However, no

such conclusion could be obtained from UV-Vis spectra, as the

spectra remained more or less same in H₂O (a broad peak

around 500 nm, Fig 2), irrespective of the amount of solvent

used. Initial attempts to isolate and crystallize the H₂O

coordinated complexes were unsuccessful. However, by

UV-Vis spectra of Co(III) complexes

2 and 3 show well defined

d-d bands at 582 (ε, 117 Mol^-1cm^-1) and 519 nm (ε, 503 Mol^-

¹cm^-1) respectively. In addition, a high energy shoulder was

also present around 400 nm in the spectra of both complexes

(Fig. S9).^{39 1}H NMR spectra of Co(III) complexes

2 and 3 show

signals corresponding to 16 protons, whereas the ligand has 19

protons; absence of signals corresponding to three protons

provides further evidence for demethylation (Fig. S10). The IR

spectrum of dihydroxo bridged binuclear Co(III) complex

3 has

strong band at 3343 cm^-1, characteristic of the bridging

hydroxo group (Fig S11).

Proposed mechanism for O-demethylation

-

Few examples of N-dealkylation reactions were reported in

literature and these reactions are proposed to proceed via the

formation of a metal-dioxygen adduct. This metastable

intermediate attacks the N-alkyl C-H bond in a rate

determining step, resulting in the formation of dealkylated

ligand and corresponding aldehyde as byproduct.^40-43Similar

mechanism is expected to operate for O-demethylation also.

Though O-demethylation occurs for both 1a and 1b, the

difference in the nature of oxidized products confirms that the

reaction pathways are different. For 1a, demethylation occurs

only in CH₂Cl₂, in which the complex exists as a penta-

coordinated species and favors the attack of oxidant. On

reaction with t-BuOOH, the pentacoordinated 1a is postulated

to form an octahedral Co(III) tertiary butyl peroxo

intermediate. Such species are known to afford alkoxy radicals,

which can oxidise hydrocarbons.^44-47In the present case, it is

proposed that the peroxo intermediate forms a tertiary butoxy

radical, which abstract a hydrogen radical from the methoxo

group oriented in a favorable position. As a result, a cyclic

intermediate forms with the loss of a molecule of butanol. The

intermediate on further rearrangement resulted the Co(III)

exchanging the iodo counter anion with –ClO₄, both these

complexes could be crystallised as [CoL^OMe(OH₂)I](ClO₄), 1c’

and [CoL^OMe(OH₂)₂](ClO₄)₂, 1d’. The iodo-aqua complex 1c’

could be synthesised easily, whereas the formation of diaqua

complex 1d’ required high temperature and longer reaction

period. This indicates that the replacement of first iodide is

relatively facile on comparison with the removal of second

halide.⁴⁸

Crystal structures confirm that these complexes have regular

octahedral geometry around Co(II) as in 1a and 1b (Fig. 8).

However, an interesting geometric change was noticed for the

diaqua complex 1d’. In all other Co(II) complexes, L^OMewas

coordinated around the metal in a standard tripodal manner

and the N-atoms are meridionally placed. In 1d’, however, the

orientation of N- atoms of the ligand is changed and they form

the face of an octahedron maintaining the arrangement of

coligands cis to each other. Another interesting difference is

the change in bond lengths. Similar to 1a and 1b, 1c’ also have

shorter Co-N(pyridine) bonds (2.115(3) and 2.126(3) Å) and

longer Co-N(amine) bond (2.170(3) Å). But for aqua complex

1d’, the trend was reversed with longer Co-N(pyridine) bonds

(2.153(4) and 2.111(4) Å) and shorter Co-N(amine) bond

(2.097(4) Å).

complex

2 with phenoxide ligand and a molecule of

formaldehyde (Scheme 3). However, our attempts to detect

the presence of formaldehyde were hindered by longer

reaction time and comparatively higher reaction temperature.

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of H₂O confirmed that the aqua complex, 1d is the precursor

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for oxidation. When the iodo complex 1^Db^OIi^:s¹⁰t^.r¹e⁰a³⁹te^/Cd^9Dw^Ti⁰th⁴⁶H⁰⁹₂O^G

in presence of air, the diaqua complex 1d forms in situ and

reacts with O₂resulting in the Co(III) complex 3. The structural

analyses of the diiodo and aqua complexes further confirm

that the Co(II) diaqua complex is the precursor for oxidation. In

the iodo complex 1b, the –OCH₃group is positioned far away

from the coligands. However in the diaqua complex 1d’, the

anisole moiety is rearranged in such a way that it comes in

close proximity to the aqua coligands, thereby facilitating the

demethylation process.

Figure 8. Displacement ellipsoid (50% probability) plots of

Oxidation of 1b is proposed to proceed via the formation of a

metastable Co(III)-dioxygen intermediate, which abstract

hydrogen atoms from the favourably placed anisole methyl

groups.¹The mechanism then proceed via a pathway similar to

Co(II)

complexes

[CoL^OMe(OH₂)I](ClO₄),

1c’

and

[CoL^OMe(OH₂)₂](ClO₄)₂, 1d’. H atoms, counter anions and

solvents of crystallisation are not shown for sake of clarity.

that proposed for the conversion of 1a to

formation of Co(III) complex with modified phenoxide ligand

and bridging hydroxo groups (Scheme 4).

2, resulting in the

Both aqua complexes 1c’ and 1d’ are characterised in solution

by various spectral techniques. The iodo- aqua complex 1c’

showed similar solvatochromic behaviour as 1a and 1b. In

CH₂Cl₂, 1c’ exhibited parrot green color with a strong band in

the UV Vis spectrum at 614 nm (ε= 235 Mol^-1cm^-1). In addition,

two weak bands were also present in the NIR region,

confirming the formation of a five-coordinated species. On the

other hand, the complex was pale pink in EtOH and showed a

band at 540 nm with much lower molar extinction coefficient

(ε =55 Mol^-1cm^-1), which is typical for octahedral complexes

(Fig S12). Interestingly, both these solutions give similar pale

pink blocks of 1c’ on crystallization, indicating that the species

is same in solid state. No solvatochromic studies could be

carried out for the diaqua complex 1d’, as it was soluble only

in polar solvents like alcohol and H₂O, in which it show

features typical for octahedral complexes (490 nm; ε = 32 Mol^-

¹cm^-1). λ_maxfor 1c’ and 1d’, recorded in polar solvents, showed

gradual blue shift as the coligand change from weak field

iodide (540 nm) to moderate field H₂O (490 nm) (Fig S13).

3

+

2+

Me

OH₂

N

O

OH₂

I

N

I

H₂O

Co^II

OH₂

CH₃

Co^II

N

O

I

N

1d

1c

1b

4+

2+

H

N

H₃C

O₂

N

O

N

O

N

O₂

O

Co^III

N

^IIIN

Co^III

O

Co^III

Co

N

-2CH₂O

O

H

N

O

N

CH₃

3

Scheme 4. Proposed mechanism for the oxidative O-

demethylation of 1b and the formation of Co(III) complex

3

The experiments on Co(II) and Fe(II) complexes shed light on

the O-demethylation reactions of L^OMeand it is identified that

the redox properties and hard soft nature of metals play

crucial role in supporting oxidative O-demethylation. To

confirm this point, the oxidation behaviour of few other metal

complexes of L^OMeis studied. The metals (Mn, Ni and Zn) were

selected in such a way that they are less redox active and

cannot achieve higher oxidation state under normal

conditions, which indicate that they are not expected to

support O-demethylation.

1

Paramagnetic H NMR spectra of 1c‘ and 1d‘ are recorded in

CD₃OD and D₂O respectively. The spectrum of 1c’ has eleven

signals (two up-field and nine down-field) in the spectral range

-40 to 200 ppm, like the other Co(II) complexes (Fig S14).

However, the signals were broad and not well defined which

could be due to the fast electronic relaxation time. The

spectrum of diaqua complex 1d’ also showed eleven signals,

which were sharp, in a comparatively shorter spectral window

of 180 ppm (-20 to 160 ppm) (Fig S15). Magnetic susceptibility

values of 1c‘ (CD₃OD, 298 K) and 1d‘ (D₂O, 298 K) indicate μeff

values of 4.18 and 5.04 μB respectively, consistent with an S =

3/2 ground state.

Syntheses of precursor M(II) complexes were straightforward

and all of them were precipitated as stable fine powders in

excellent yield by mixing corresponding metal(II) chloride salt

and ligand in equimolar ratio in EtOH. The manganese complex

MnL^OMeCl₂, 1e was isolated as a flesh colored powder and

crystal structure shows that the complex has a geometry

identical to the Co(II) complexes 1a and 1b (Fig S16, Table S4).

To study the oxidation behaviour, the complex was refluxed in

CH₂Cl₂with two equivalent of t-BuOOH for 48 h. No visible

changes were observed and the precursor Mn(II) complex 1e

was completely crystallised out from the reaction mixture on

cooling, confirming that there is no reaction.

-

Complexes 1c’ and 1d’ with –ClO₄counter anions are stable

and could be stored for months under closed condition; but

their iodide counterparts 1c and 1d are comparatively more

reactive and subsequently get converted to the oxidised

complex, 3. However, it is noted that there is no reactivity

when the diiodo complex 1b is stored in a moisture free

environment. The inability to undergo oxidation in the absence

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

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complex, 1b with iodo coligands is not stable to air and

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NiL^OMeCl₂was isolated as sea green solid in good yield. The oxidised to Co(III) complex with demet^Dh^Oyl^Ia^{: 1}t⁰e^.d¹⁰³li⁹g^/a^Cn⁹d^DTv⁰i⁴a⁶⁰t⁹h^Ge

crystal structure of the complex is reported and it has a formation of Co(II) diaqua complexes. The incapability of

mononuclear structure with distorted octahedral geometry metals such as Cu, Mn and Ni to carry out oxidative O-

around metal centre as in 1e ⁴⁹

. The complex was characterised demethylation emphasize the importance of metal redox

by UV-Vis spectroscopy and the spectrum showed two well properties in such reactions.

defined bands at 674 (ε = 21 Mol^-1cm^-1) and 406 nm (ε = 48 Experimental Section

Mol^-1cm^-1) and a shoulder at 315 nm. Ni(II) complexes are not

expected to undergo easy oxidation because of the unstable

General Procedures

Unless otherwise stated, all solvents and chemicals used were

nature of Ni(III) oxidation products. The complex was dissolved

in CH₂Cl₂and refluxed with an equivalent of t-BuOOH for 48 h.

UV-Vis spectrum of the resultant solution showed identical

features of the initial Ni(II) complex (Fig S17). The precursor

complex 1f could be fully recovered by washing the residue

with diethyl ether.

of commercially available analytical grade, and were used as

such without further purification. Co(ClO₄)₂.6H₂O was

synthesized by reacting CoCO₃with HClO₄in aqueous medium,

followed by crystallisation from H₂O. CH₃CN was dried by

double distillation over P₂O₅and CaH₂after passing through a

long column of activated alumina. CH₂Cl₂was dried by

employing standard procedure.⁵²EtOH (99.9%) was purchased

from Hayman Ltd and used as such. The deuterated solvents

were obtained from Aldrich. Preparation and handling of air-

sensitive compounds were performed under an argon

atmosphere using standard Schlenk techniques. The UV-Vis

Reactions were repeated using upto 5 equivalents of oxidant;

but no demethylation took place in any case, indicating the

inability of Mn(II) and Ni(II) complexes to undergo oxidation.

As mentioned earlier, no demethylation is noticed for the Cu(I)

complexes of L^OMealso. For Cu²⁺, which is a borderline acid, the

coordination geometry is effectively satisfied by bonding with

soft ether O atom than hard phenoxide O-atom, which

explains the observed inertness of Cu(I) complexes to O-

demethylation whereas for Ni²⁺and Mn²⁺complexes, their

inability to achieve and stabilise higher oxidation states resist

the reaction. Co²⁺is a very suitable contestant in this aspect, as

it can easily achieve +3 oxidation state and the hard acid Co³⁺

prefers coordination with hard phenoxide oxygen atom over

ether O atom. Hence it is proven that the redox properties of

metals play significant role in the demethylation of L^OMe. The

electrochemical studies of these complexes further helps to

analyze the observed oxidation behavior. Copper complex of

L^OMehas a very low one electron redox potential (E_1/2= -510

mV Vs. Fc/Fc⁺) and undergoes metal oxidation; no O-

demethylation was noticed. On the other hand, Iron complex

has much higher one electron potential at -55 mV Vs. Fc/Fc⁺

and undergoes oxidative O-demethylation.¹⁶Cobalt complexes

with comparable CV profiles also undergo oxidative O-

demethylation as described before. The electrochemical

studies of Mn complex (1e) did not give any useful data.

However the CV of NiL^OMeCl₂, 1f shows a redox couple with a

high E_1/2value of 545 mV Vs. Fc/Fc⁺(Fig S18). From the

electrochemical analysis it is concluded that metals with low

redox potential (For Eg: Cu) support metal oxidation whereas

those with medium redox potential (Egs: Fe and Co) can assist

oxidative O-demethylation. However, metals with very high

redox potential (Eg: Ni and Mn) do not aid any type of

oxidation. It is interesting to note that the potential of the

spectra were recorded on

a

Jasco

V

650 UV-Vis

spectrophotometer or Shimadzu 3100 UV- Vis- NIR

spectrophotometer. IR spectra were recorded on a Jasco

FT/IR-4100 spectrophotometer after making pellets with KBr.

¹H and ¹³C NMR spectral data were obtained from a Bruker

400 MHz/ 500 MHz FT-NMR spectrometers at ambient

temperatures by using TMS as an external reference. All

electrochemical experiments were performed using a CH

Instrument (model CH6003D Electrochemical Analyzer). A

three electrode system was used, consisting of a platinum

(diameter of the active zone: 2.0 mm) working electrode, a

platinum wire counter electrode, and a Ag/AgCl/KCl(aq)

reference electrode. The CV measurements were performed in

dry

acetonitrile

solutions

containing

0.1

M

tetrabutylammonium hexafluorophosphate (TBAPF₆, Fluka,

recrystallized from methanol), and 2 mM substrate under N₂

atmosphere at room temperature. Scan rate of 50 mV/s were

applied for all the complexes. Ferrocene was used as an

internal standard and the E_1/2value for Fc/Fc⁺couple is found

at 450 mV vs Ag/AgCl in CH₃CN.

Preparation of ligand and complexes

L^OMe(2-methoxy-N, N-bis(pyridin-2-ylmethyl)aniline). Toluene

solution (3.0 mL) of picolinic aldehyde (4.0 g, 37.3 mmol) was

taken in an air free addition funnel and o-anisidine (4.6 g, 37.3

mmol) in toluene (3.0 mL) was added drop-wise with

continuous stirring under N₂. Formed red oil and orange

supernatant was stirred under inert atmosphere for an hour.

The supernatant was decanted and solvent removed under

reduced pressure to obtain the imine as orange oil (7.8 g,

99%). The imine was dissolved in MeOH (15 mL) and reduced

by adding NaBH₄(1.4 g, 37.0 mmol) in small portions over a

period of 24 hours under ice cold condition. After complete

addition, the solution was kept stirring overnight under N₂. The

reaction was stopped, solvent removed and excess NaBH₄was

quenched by adding 6N HCl and the reaction mixture was then

•

-

redox couple O₂/O₂in CH₃CN is -1.25 V vs. Fc/Fc⁺.^{50, 51}

In summary, we have reported the first oxidative O-

demehylation reactions at Co(II) centers by employing a

tetradentate N-ligand with anisole append (L^OMe) and halide

coligands (Cl^-or I^-). The Co(II) complex, 1a with chloro

coligands, is stable and underwent oxidative demethylation in

presence of strong oxidant like t-BuOOH. However, the Co(II)

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made basic with 4N NaOH. Extraction to CH₂Cl₂followed by solid obtained by removing solvent from the supernatant was

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DOI: 10.1039/C9DT04609G

solvent removal yielded the secondary amine as yellow orange dried well and yield noted (70 mg, 70%). Pale pink blocks were

oil (7.1 g, 90 %). The amine was dissolved in CH₃CN (40 mL) obtained after two days from CH₂Cl₂or EtOH by slow

and treated with picolyl chloride (4.3 g, 33.7 mmol) under inert evaporation. Anal.Calcd for C₁₉H₂₁ClCoIN₃O₆: C, 37.76; H, 3.17;

atmosphere. To this reaction mixture, solid Na₂CO₃(1.85 g, N, 6.95. Found: C, 37.99; H, 1.45; N, 6.21. UV-Vis (λ_max, nm; ε,

17.5 mmol) was added in small portions with continuous M^-1cm^-1): 530 (53), 1174 (7) (EtOH); 556 (146), 496 (87),

stirring. The reaction mixture was kept heating at 50^oC for 610(233) (CH₂Cl₂). FTIR (KBr, cm^-1): 3399, ν(-OH₂); 1120, 627,

-

three days. Solvent was removed and the crude product ν(ClO₄); 770, ν(pyridyl-CH). ESI-MS (m/z, H₂O): 399 (100%,

obtained was purified by column chromatography using silica [L^OMeCo(OH₂)(OH]⁺); 491 (93%, [L^OMeCo(I)]⁺); 426 (85%,

gel. Elution with petroleum ether removed the un-reacted 2° [L^OMeCo(EtOH)(OH]⁺). µ (CD₃OD, 298 K): 4.18 µB/Co(II). ¹H NMR

-

amine and picolyl chloride, whereas elution with ethyl (CD₃OD):

δ

^-31.69, 14.96, 8.79, 12.06, 17.83, 32.45, 42.91,

acetate/hexane (1:1) yielded L^OMe(1.3 g, 48%) as off-white 59.97, 73.14, 80.54, 147.39, 194.09 (CDCl₃): -27.22, -15.35, -

rectangular crystals (Rf = 0.25, silica gel, ethyl acetate/hexane 2.50, 18.02, 69.82, 80.24, 116.56, 210.31.

1:1). Yield: 48%. ¹H NMR (CDCl₃): 3.84 (s, 3H), 4.51 (s, 4H), 6.77 [Co^IIL^OMe(H₂O)₂](ClO₄)_2,1d’. The dull green precipitate of 1b

(m, 1H), 6.89 (m, 3H), 7.08 (m, 2H), 7.50 (d, 2H), 7.57 (m, 2H), (150 mg, 0.24 mmol) was suspended in EtOH and treated with

8.49 (d, 2H). ¹³C NMR (CDCl₃): 55.56 (-CH₂), 58.76 (-OCH₃), aqueous NaClO₄solution (60 mg, 0.49 mmol). The dark orange

111.72 (Ph), 120.76 (Ph), 120.80 (Py), 121.84 (Ph), 122.36 (Py), reaction mixture was heated at 60^oC for 2 hours. Cooled to

136.45 (Py), 139.30 (Ph), 149.00 (Py), 152.82 (Ph), 159.82 (Py). room temp, solvent removed and the product was extracted to

L^OMewas synthesized by Lucas et al.¹⁵, by using a slightly CH₃CN. The residue obtained from CH3CN was dissolved in

different procedure.

deionised water and heated overnight again at 60^oC. The red

Co^IIL^OMeCl₂, 1a. To a dark blue solution of CoCl₂.6H₂O (179.0 oil obtained after solvent removal on keeping closed for three

mg, 0.75 mmol) in EtOH, L^OMesolution (228.5 mg, 0.75 mmol) days yielded the diaqua complex as brown red crystalline solid

in EtOH was added drop wise with stirring at room (Yield: 87 mg, 60%). Pale pink blocks were obtained from

temperature. After stirring for 4 h at RT, the precipitated dark butanol after a week by vapour diffusion of diethyl ether.

purple powder was separated and dried (Yield: 276.0 mg, Anal.Calcd for C₁₉H₂₃Cl₂CoN₃O₁₁: C, 38.08; H, 3.87; N, 7.01.

85%). Purple needles were obtained from CH₂Cl₂overnight, by Found: C, 38.23; H, 2.28; N, 6.80. UV-Vis (λ_max, nm; ε, M^-1cm^-1;

evaporation. Anal.Calcd for C₁₉H₁₉Cl₂CoN₃O: C, 52.43; H, 4.40; H₂O/ MeOH]: 492 (32), 1074 (5). FTIR (KBr, cm^-1): 3400, ν(-

-

N, 9.66. Found: C, 52.40; H, 3.85; N, 9.58. UV-Vis (λ_max, nm; ε, OH₂); 1089, 626, ν(ClO₄); 770, ν(pyridyl-CH). ESI-MS (m/z,

M^-1cm^-1): 582 (113), 1234 (6) (CH₂Cl₂); 584 (79), 1256 (5) H₂O): 399 (100%, [L^OMeCo(OH₂)(OH]⁺); 463 (87%,

(CH₃CN); 548 (43), 1302 (6) (EtOH or MeOH); 502 (25), 1056 (6) [L^OMeCo(ClO₄)]⁺); 426 (85%, [L^OMeCo(EtOH)(OH]⁺. µ (D₂O, 298

1

(H₂O). FTIR (KBr, cm^-1): 2923, m, ν(C-C); 1605, s, ν(C=N_py); 1498, K): 5.04 µB/Co(II). H NMR (D₂O): -14.25, -7.43, 10.73, 15.51,

1444, s, ν(C=C_ar); 1249, s, ν(C–O). µ (CD₃OD, 298 K): 5.19 25.26, 26.17, 46.65, 54.72, 63.68, 146.68, 156.02.

1

µB/Co(II). H NMR (500 MHz, CD₃OD): -51.37, -25.95, 6.60, Mn^IIL^OMeCl₂, 1e. To a pale pink solution of MnCl₂.4H₂O (156.0

12.76, 28.66, 48.98, 50.40, 70.03, 76.50, 151.48, 187.89.

mg, 0.79 mmol) in EtOH, L^OMesolution (244.3 mg, 0.8 mmol) in

Co^IIL^OMeI₂, 1b. To a freshly prepared dark green solution of EtOH was added drop wise with stirring at room temperature.

CoI₂.6H₂O (80.3 mg, 0.19 mmol) in EtOH, ligand solution (59.0 After stirring for 2 h at RT, the precipitated flesh colour

mg, 0.19 mmol) in EtOH was added drop wise with stirring powder was separated and dried (Yield: 310 mg, 90%). Faint

under N₂, yielding purplish red clear solution. On stirring under pink coloured blocks were obtained from CH₂Cl₂after

N₂for overnight, orange red supernatant and grey green overnight, by evaporation. Anal. Calcd for C₁₉H₁₉Cl₂MnN₃O: C,

precipitate formed. The precipitate was separated, dried and 52.92; H, 4.44; N, 9.74. Found: C, 52.56; H, 3.95; N, 9.85.

yield of the dull green powder noted (88.5 mg, 75%). Bright NI^IIL^OMeCl₂, 1f. To a solution of NiCl₂.6H₂O (85.1 mg, 0.36

green block shaped crystals were obtained from CH₂Cl₂after a mmol) in EtOH, ligand solution (111.2 mg, 0.36 mmol) in EtOH

day, by vapour diffusing diethyl under N₂. Anal.Calcd for was added drop wise with stirring under N₂. The reaction

C₁₉H₁₉I₂CoN₃O: C, 36.92; H, 3.10; N, 6.80. Found: C, 37.08; H, mixture was stirred for two hours and the precipitated apple

2.56; N, 6.66. UV-Vis (λ_max, nm; ε, M^-1cm^-1): 360 (2495), 514 green product was separated (128.3 mg, 82%). Dark green

(96), 560 (136), 594 (143), 1446 (11) (CH₂Cl₂); 376(1289), 520 block shaped crystals were obtained from CH₃NO₂after a day,

(75), 1176 (8) (CH₃CN); 362 (673), 536 (51), 1130 (5) (MeOH); by slow evaporation. Anal.Calcd for C₁₉H₁₉Cl₂NiN₃O: C, 52.46;

370 (1583), 540 (72), 1278 (6) (EtOH). FTIR (KBr, cm^-1): 2912, H, 4.40; N, 9.66. Found: C, 52.18; H, 3.74; N, 9.39. UV-Vis

m, ν(C-C); 1604, s, ν(C=N_py); 1496, 1441, s, ν(C=C_ar); 1245, s, (λmax, nm; ε, M^-1cm^-1, CH₂Cl₂): 406 (48), 674 (21), 982 (9),

1

ν(C–O). µ (CD₃OD, 298 K): 5.40 µB/Co(II). H NMR (500 MHz, 1312 (6). FTIR (KBr, cm-1): 2930, w, ν(C-C); 1604, s, ν(C=Npy);

CD₃OD): -37.55, -20.39, 7.44, 13.78, 28.58, 40.05, 56.56, 69.99, 1446, s, ν(C=Car); 1012, s, ν(C–O). ¹H NMR (500 MHz, CD₃NO₂):

76.29, 141.40, 189.99.

2.09, 4.33, 7.15, 13.45, 22.17, 43.11, 51.25, 113.13, 138.82.

[Co^IIL^OMeI(H₂O)](ClO₄)_,1c’. The dull green precipitate of 1b Zn^IIL^OMeCl₂, 1g. To the colourless solution of ZnCl₂(101.6 mg,

(100 mg, 0.16 mmol) was suspended in EtOH and treated with 0.75 mmol) in EtOH, L^OMesolution (226.2 mg, 0.74 mmol) in

an equivalent of NaClO₄(20 mg, 0.16 mmol). The reaction same solvent was added drop wise with stirring at room

mixture was stirred overnight to obtain bright pink solution temperature. Precipitated white solid was separated and dried

and thin white precipitate which was filtered off. The pink-red (Yield: 311 mg, 95%). Anal.Calcd for C₁₉H₁₉Cl₂ZnN₃O: C, 51.67;

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

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H, 4.34; N, 9.51. Found: C, 51.88; H, 3.82; N, 9.62. ¹H NMR (400 contains a molecule of complex and half molecule of H₂O as

View Article Online

DOI: 10.1039/C9DT04609G

MHz, CDCl₃): 3.88 (3H, s), 4.93 (2H, s), 6.74 (2H, d), 7.04 (1H, solvent of crystallisation as in 1a. The Co(III) chloro complex

2

d), 7.09 (2H, t), 7.38 (2H, t), 7.44 (1H, d), 7.69 (2H, t), 9.31 (2H, was crystallised in orthorhombic system with space group

d). Pna21. The asymmetric unit consisted of a molecule of

L^O-Co^IIICl₂. H₂O, 2. To the dark blue solution of 1a (211.3 mg, complex and a H₂O molecule as solvent of crystallisation. The

0.48 mmol) in CH₂Cl₂, 6M t-BuOOH, (122 uL, 0.73 mmol) was Co(III) dihydroxo complex crystallises in orthorhombic

3

added under N₂with stirring. The reaction mixture was system with Pnma space group. The complex is highly

refluxed at 40˚C for 20 h, yielding dark green solution with symmetrical and the asymmetric unit contain just half

little grey precipitate. Solvent removed and the sticky grey molecule of complex. The charge is neutralised by 1.36 I-

residue was washed thrice with diethyl ether to remove excess atoms and 0.64 Cl- atoms. In addition, four molecules of H₂O

t-BuOOH and any butanol formed. Dried well and the yield of and a molecule of highly disordered CH₃CN also present in the

the fine grey powder was noted (195.0 mg, 95%). Parrot green lattice as solvents of crystallisation.

needles were obtained after three days from CH₃CN solution X-ray data were collected with a Bruker AXS Kappa Apex II CCD

by slow evaporation. Anal.Calcd for C₁₈H₁₆Cl₂CoN₃O: C, 51.45; diffractometer equipped with graphite monochromated Mo-

H, 3.84; N, 10.00. Found: C, 50.98; H, 3.41; N, 9.84. UV-Vis Kα (λ = 0.7107 Å) radiation. The structures were solved by

(λ_max, nm; ε, M^-1cm^-1; CH₃CN): 415 (217), 584 (117). FTIR (KBr, direct methods using SHELXT-2015, and refined by full-matrix

cm^-1): 3529, 3404, br, ν(-OH₂), 1609, m, ν(C=N_py); 1585, m, least-squares techniques using the SHELXL-2016 computer

1

1480, s, ν(C=C_ar); 1283, m, ν(C–O). H NMR (400 MHz, CD₃CN): program. All hydrogen atoms were fixed at chemically

4.63 (2H, d), 5.68 (2H, d), 6.48 (2H, t), 6.74 (1H, t), 7.35 (2H, d), meaningful positions. Molecular graphics were generated

7.52 (2H, t), 7.61 (1H, d), 7.88 (2H, t), 9.45 (2H, d). ¹³C NMR using diamond programs.

(400 MHz, CD₃CN): 58.59 (-OCH₃), 60.05 (-CH₂), 121.96 (Py),

122.77 (Py), 123.02 (ar) 127.59 (ar), 128.30 (ar), 129.22 (ar),

Conflicts of interest

There are no conflicts of interest to declare.

136.45 (Py), 139.08 (ar), 148.94 (Py), 159.82 (py).

[CoL^O-(OH)]₂.Cl_{0.75 1.25}

.4H₂O.CH₃CN, 3. The in situ prepared

I

Co(II) iodo complex 1b (166 mg, 0.27 mmol) was suspended in

CH₃CN and millipore H₂O was added till a bright red clear

solution forms. The reaction mixture was purged with O₂and

heated overnight at 60˚C. Cooled to room temperature and

Acknowledgements

S. A thanks CSIR, India for a fellowship. We gratefully

acknowledge the single-crystal X-ray diffractometer facility

funded by SAIF, IIT Madras and Dept. of Chemistry, IIT Madras.

kept open to get dark brown rectangles of

3 (125 mg, 62%)

after 3 days. Isolated product was washed well with CH₃CN

and H₂O separately, to remove impurities. Dark red needles

were obtained after overnight from CH₃CN-H₂O mixture by

evaporation. Anal.Calcd for C₃₆H₃₄Co₂I₂N₆O₄: C, 43.84; H, 3.47;

N, 8.52. Found: C, 44.28; H, 2.89; N, 8.98. UV-Vis (λ_max, nm; ε,

M^-1cm^-1; CH₃CN+H₂O): 519 (503), 375 (1313). FTIR (KBr, cm^-1):

3536, s, ν(-OH), 3343, br, ν(-OH₂), 1609, s, ν(C=N_py), 1491, s,

Notes

^‡Crystal data 1a

Orthorhombic, P2₁2₁2₁, a = 9.5758(2)

15.2197(2) , V = 2011.63(7), R_int= 0.0137, Z = 4, ρ_cald= 1.463

g cm^-3, = 1.134 mm^-1, F(000) = 908, T = 298(2) K, R(F²> 2

) =

0.0207, wR₂= 0.0514, 3421 independent reflections, 2 = 50

and 246 parameters, GOF of F²= 1.039. For 1b: C₁₉H₁₉CoI₂N₃O,

M = 618.10, monoclinic, P2₁/c, a = 12.9288(3) , b = 9.5177(2)

, c = 18.2171(5) = 108.7800(10) , V = 2122.32(9), R_int

0.0218, Z = 4,

_cald= 1.934 g cm^-3, = 3.73 mm^-1, F(000) = 1180,

T = 296(2) K, R(F²> 2

) = 0.0275, wR₂= 0.0619, 3736

independent reflections, 2 = 49.98 and 247 parameters, GOF

:

C₁₉H₁₉Cl₂CoN₃O_1.50

,

M

=

443.20,

, c =

Å

, b = 13.8028(3)

Å

1

1435 s, ν(C=C_ar); 1246, m, ν(C-O). H NMR (CD₃CN+D₂O, 400

µ

σ

MHz): 4.90 (2H, d), 5.90 (2H, d), 6.25 (1H, d), 6.53 (1H, t), 6.70

(1H, m), 6.95 (2H, t), 7.52 (2H, d), 7.65 (1H, d), 7.86 (2H, m),

7.99 (2H, d).

θ

˚

Å

,

β

˚

=

X-ray crystallography

ρ

µ

Complex 1a was crystallised in orthorhombic system with

space group P2₁2₁2₁. Asymmetric unit was formed by a

molecule of complex and half water molecule. Complex 1b was

crystallised in monoclinic system with space group P2₁/c and

the iodo aqua complex 1c was crystallised in triclinic system

with P_-1space group. The asymmetric unit of 1c consist of a

σ

θ

˚

of F²= 1.058. For 1c’: C₁₉H₂₁ClCoIN₃O₆, M = 608.67, triclinic, P-

1, a = 9.0682(3) Å, b = 11.6181(4) Å, c = 12.1412(4) Å, α =

103.0870(10)˚, β = 104.1930(10)˚, γ = 90.1940(10)˚, V =

1205.53(7), R_int= 0.0176, Z = 2, ρ_cald= 1.677 g cm^-3, μ = 2.141

mm^-1, F(000) = 602, T = 296(2) K, R(F²> 2σ) = 0.0312, wR₂=

-

molecule of complex and a –ClO₄counter anion which is

disordered. The diaqua complex 1d is crystallised in monoclinic

system with space group P2₁/c. The asymmetric unit consists

0.0774, 4420 independent reflections, 2

θ = 50˚ and 327

parameters, GOF of F²= 1.049. For 1d’: C₁₉H₂₅Cl₂CoN₃O₁₂, M =

617.25, monoclinic, P2₁/c, a = 10.8936(5) Å, b = 19.2866(10) Å,

c = 13.2468(6) Å, β = 112.981(2), V = 2562.3(2), R_int= 0.0356, Z

= 4, ρ_cald= 1.600 g cm^-3, μ = 0.944 mm^-1, F(000) = 1268, T =

293(2) K, R(F²> 2σ) = 0.0401, wR₂= 0.1019, 4555 independent

-

of a molecule of complex, two molecules of –ClO₄counter

anions which are disordered and a molecule of H₂O as solvent

of crystallisation. Just as the Co(II) chloro complex 1a, The

Mn(II) chloro complex 1e also crystallised in orthorhombic

system with space group P2₁2₁2₁. The asymmetric unit

reflections, 2

θ

= 49.2˚ and 430 parameters, GOF of F²= 1.083.

J. Name., 2013, 00, 1-3 | 9

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Journal Name

21 C. J. Davies, G. A. Solan and J. Fawcett, Polyhedron , 2004, 23

,

For 1e: C₁₉H_19.50Cl₂MnN₃O_1.25, M = 435.71, orthorhombic,

P2₁2₁21, a = 9.6360(3) Å, b = 13.9208(4) Å, c = 15.4915(5) Å, V

= 2078.04(11), Z = 4, ρ_cald= 1.393 g cm^-3, μ = 0.906 mm^-1,

F(000) = 894, T = 296(2) K, R(F²> 2σ) = 0.0237, wR₂= 0.0526,

View Article Online

3105.

DOI: 10.1039/C9DT04609G

22 S. M. Massoud, K. T. Broussard, F. A. Mautner, R. Vicente, M.

K. Saha and I. Bernal, Inorg. Chim. Acta., 2008, 361, 123

23 C. R. Easley, J. Li, A. Banerjee, M. Panda, W. W. Brennessel,

R. Loloee, E. Colacio and F. A. Chavez, Eur. J. Inorg. Chem.,

2015, 2092.

24 A. B. P. Lever, 1968, Inorganic electronic spectroscopy.

Elsevier, Amsterdam

25 P. C. Kunz, M. Börgardts and F. Mohr, Inorg. Chim. Acta.,

2012, 380, 392

26 T. M. Kooistra, K. F. W. Hekking, Q. Knijnenburg, B. Bruin, P.

H. M. Budzelaar, R. Gelder, J. M. M. Smits and A. W. Gal, Eur.

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3655 independent reflections, 2

θ = 50˚ and 251 parameters,

GOF of F²= 1.047. For

2

: C₁₈H₁₈Cl₂CoN₃O, M = 438.18,

orthorhombic, Pna2₁, a = 13.7658(5) Å, b = 15.0676(6) Å, c =

8.9198(3) Å, V = 1850.12(12), Z = 4, ρ_cald= 1.573 g cm^-3, μ =

1.234 mm^-1, F(000) = 896, T = 296(2) K, R(F²> 2σ) = 0.0245,

wR₂= 0.0599, 3098 independent reflections, 2

243 parameters, GOF of F²

1.053. For

C₃₈H₄₅Cl_0.64Co₂I_1.36N₇O₈, M = 1040.71, orthorhombic, Pnma, a =

16.0730(4) , b = 16.5200(4) , c = 16.8530(4) , V =

4474.91(19), R_int= 0.0322, Z = 4, = 1.771

_cald= 1.545 g cm^-3,

mm^-1, F(000) = 2091, T = 293(2) K, R(F²> 2

) = 0.0555, wR₂=

= 49.60 and 317

θ = 50.96˚ and

=

3:

27 J. Hyun, S. M. Park and H. So, Bull. Korean Chem. Soc., 1997,

18, 1090

Å

ρ

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28 I. Bertini, C. Luchinat and G. Parigi, 2001, Solution NMR of

Paramagnetic Molecules, Elsevier Science

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29 I. Bertini, C. Luchinat, G. Parigi and R. Pierattelli,

0.1659, 4083 independent reflections, 2

θ

˚

ChemBioChem, 2005, 6, 1536

parameters, GOF of F²= 1.071.

30 N. N. Murthy, K. D. Karlin, I. Bertini and C. Luchinat, J. Am.

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31 D. Mandon, A. Machkour, S. Goetz and R. Welter, Inorg.

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52 K. M. Kadish and J. E. Anderson, Pure & Appl. Chem., 1987,

59, 703

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DOI: 10.1039/C9DT04609G

Coligand controlled oxidative O-demethylation of a tetradentate ligand on Co(II) complexes

Article Doi

Coligand modulated oxidative: O -demethylation of a methyl ether appended tetradentate N-ligand in Co(ii) complexes

DOI: 10.1039/c9dt04609g

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