Oxygen atom transfer reactions from sterically encumbered brominated (oxo)manganese(V) corroles to styrene

Journal of Porphyrins and Phthalocyanines

J. Porphyrins Phthalocyanines 2015; 19: 1238–1250

DOI: 10.1142/S1088424615501059

Published at http://www.worldscinet.com/jpp/

Oxygen atom transfer reactions from sterically encumbered

brominated (oxo)manganese(V) corroles to styrene

Mian HR Mahmood^a,b, Hua-Hua Wang^a, Hai-Yang Liu*^a◊and Chi-Kwong Chang*^c

^aDepartment of Chemistry, South China University of Technology, Guangzhou 510640, P.R. China

^bDepartment of Chemistry, University of Education, Lahore 54770, Pakistan

^cDepartment of Chemistry, Michigan State University, E. Lansing, MI 48824, USA

Received 7 August 2015

Accepted 16 November 2015

ABSTRACT: Seven A₃- and trans-A₂B manganese(III) corroles (1–7-Mn) differing widely in their

electronic and steric features were synthesized and transformed to their corresponding b-brominated

manganese(III) corroles derivatives (1a–7a-Mn). Their corresponding (oxo)manganese(V) corroles

1–7-Mn(oxo) and 1a–7a-Mn(oxo) were further prepapred by treating with iodosylbenzene (PhIO). The

reactivity for the oxygen atom transfer from 1–7-Mn(oxo) to styrene followed the order of 1-Mn(oxo)

> 2-Mn(oxo) > 7-Mn(oxo) > 4-Mn(oxo) > 3-Mn(oxo) > 6-Mn(oxo) ≈ 5-Mn(oxo). The same pattern

was observed for their b-brominated analogs 1a–7a-Mn(oxo), albeit their reactivity was remarkably

higher. The steric protection of Mn^V≡O moiety by ortho–ortho′-dibromophenyl substituents was found

to enhance the stability of (oxo)manganese(V) corroles signiﬁcantly.

KEYWORDS: corrole, manganese, manganese-oxo, bromination, oxygen atom transfer reaction.

metal better than porphyrin, it is expected that some of

INTRODUCTION

its metal-oxo complexes might be stable enough to be

Biomolecules having manganese such as catalase,

isolated and be useful probe in oxygen atom transfer

superoxide dismutase (Mn-SOD), and manganese-

(OAT) reactions. Manganese corroles are proved efﬁcient

peroxidase have considerably contributed to the

understanding of manganese chemistry in many synthetic

models with reference to structural, electrochemical

in catalytic oxidation of various organic substrates [1].

b-halogenation on corrole macrocycle may also remark-

ablyincreasethecatalyticoxidationactivityofmanganese

and photophysical properties. This understanding has

corroles [12, 13]. Multiple active intermediates have

facilitated the development of manganese complexes

been suggested for the catalytic oxidation reaction of

having real applications in catalysis and biomedicines

manganese corroles, depending on various factors such

[1–4]. Manganese-oxo species have long been postulated

as electronic structure of corrole macrocycle [14–17],

as the active intermediate in manganese-mediated

steric hinderance [18], and the solvent [19, 20]. (Oxo)

oxidation of hydrocarbons [5–7]. Study on high-valent

manganese(V) corrole has been demostrated to be

manganese-oxo species still presents considerable

challenge because they are generally too reactive to

be isolated [8]. Identiﬁcation of different porphyrin

manganese-oxo species has provided great insights

active in the OAT reaction, and b-halogenated (oxo)

manganese(V) corroles exhibit higher OAT reactivity

towords alkenes [12] and sulﬁdes [21]. b-bromination

of the tetrapyrrolic macrocycle is a convenient method

into the catalytic oxidation by manganese porphyrins

to increase the catalytic oxidation activity of metallo-

[9–11]. Since corrole stabilizes high oxidation state

corroles [13] and metalloporphyrins [22]. The higher

reactivity of b-brominated (oxo)manganese(V) corrole

has been interpreted as the spin-ﬂipping effect related to

^◊ꢀSPP full member in good standing

the spin-obital interaction by DFT calculations [23, 24].

To further investigate the bromination effect on the

OAT reactivity of (oxo)manganese(V) corroles towards

*Correspondence to: Hai-Yang Liu, E-mail: chhyliu@scut.edu.

cn, tel/fax: +86 2022-236-805; Chi-Kwong Chang, E-mail:

changc@msu.edu

OXYGEN ATOM TRANSFER REACTIONS FROM STERICALLY ENCUMBERED BROMINATED (OXO)MANGANESE(V) 1239

Ar₂

Br

Ar₁

O

N

HN

N

Mn

Ar₁

N

NH HN

Br

1a-7a-Mn(oxo)

1-7

1-7-Mn(oxo)

2 Ar₁=

F

Ar₂=

Br

1 Ar₁= Ar₂=

F

CN

3 Ar₁= Ar₂=

5 Ar₁= Ar₂=

4 Ar₁= Ar₂=

Br

6 Ar₁= Ar₂=

F

Br

7 Ar₁=

Br

F

Ar₂=

F

Scheme 1. Structures of synthesized corroles and their manganese complexes

alkene, we here wish to report the synthesis of a series of

A₃- and trans-A₂B-type sterically encumbered corroles

starting from 2,6-dibromobenzaldehyde, and their

manganese complexes, as well as the OAT reaction from

corresponding (oxo)manganese(V) corroles to styrene.

RESULTS AND DISCUSSION

Synthesis

Except for hexabrominated free baseA₃-type aryl corrole

(5, 6; Scheme 1) from ortho–ortho′-dibromobenzaldehyde,

all other A₃- and trans-A₂B free base corroles could be

prepared smoothly by conventional methods [25–31]. The

optimization of synthesis ofA₃-type aryl corrole fromortho–

ortho′-dibromobenzaldehyde was ﬁnally achieved by using

BF₃∙Et₂Ocatalystinthecondensationof2,6-dibromophenyl-

substituted dipyrromethane and 2,6-dibromobenzaldehyde

and the target corroles were obtained in a satisfactory

yields of ~12% [32–35]. Manganese insertion was made

by reacting free base corroles with manganese acetate in

methanol dichloromethane mixture[12, 13, 18].

Fig. 1. EPR spectrum of 1a-Mn

state of central manganese [36]. Manganese(III)

corroles having high-spin d⁴conﬁguration (S = 2) are

EPR silent at room temperature [37, 38]. However, by

decreasing the temperature, EPR signals are observable

for manganese(III) corroles. The X-band EPR spectra of

a frozen solution of all manganese complexes 1–7-Mn

and 1a–7a-Mn (Scheme 1) in CH₂Cl₂at 88 K showed a

single line centered at g_eff= 2.0. A typical EPR spectrum

for 1a-Mn is shown in Fig. 1, and the data for all other

EPR spectroscopic characterization

of manganese(III) corroles

An EPR spectrum of manganese(III) corrole is

particularly useful to get information about the oxidation

J. Porphyrins Phthalocyanines 2015; 19: 1239–1250

1240 M. HR MAHMOOD ET AL.

manganese corroles is given in Supporting information

(Table S1, Supporting information). This may be

attributed to the presence of high-spin mononuclear

Mn(III) (S = 2) center in all these manganese(III) corroles.

The value of g = 2 for Mn(III) corrole has been assigned

as Mn(II) p-cation radical, which is EPR silent at room

temperature due to the enhanced electron spin relaxation

rate by spin-spin magnetic interactions between metal

ion and the ligand radical[38].

of these manganese(III) corrole macrocycle renders them

resistant to ring oxidations or reductions.

The cyclic voltammograms of all manganese(III)

corroles were meassured in acetonitrile solvent using

glassy carbon as working electrode, platinum wire as

counter electrode and Ag/AgCl as reference electrode.

TBAP was used as supporting electrolyte. Figure 2

shows the typical CV of Mn(III) corroles 1-Mn and

1a-Mn, while all others are shown in Figs S1 and S2.

The half wave potentials of Mn(IV)/Mn(III) couple are

summarized in Table 1. E_1/2of Mn(IV)/Mn(III) couple of

manganese(III) corroles 1–7-Mn lie in the range of 490–

850 mV depending on the nature of attached substituents

(Table 1). While the ﬁrst oxidation potential values of

1–7-Mn are signiﬁcantly higher than manganese OEC by

130–480 mV [40, 46], but still lower than manganese(III)

tetraarylporphyrins by more than 300 mV [39, 47]. The

ﬁrst oxidation potential of highly electron-rich meso-

triphenylcorrole manganese(III) 3-Mn is still higher than

Mn(III)-OEC by about 130 mV, indicating the stabilizing

effect of meso-substitution [40]. The ﬁrst oxidation

potential of ortho-sterically hindered hexabrominated

manganese(III) corroles 5-Mn and 6-Mn are remarkably

lower than electron-poor corroles 1-Mn, 2-Mn and 7-Mn

containing three, two and one electron-withdrawing

meso-pentaﬂuorophenyl groups, respectively. However,

these values are still higher than 3-Mn and 4-Mn having

meso-phenyl and meso-cyanophenyl groups, respectively.

Transformation of manganese(III) corroles to their fully

b-brominated analogs 1a–7a-Mn leads to an increase

in the ﬁrst oxidation potential by about 160–540 mV.

Variations in the ﬁrst oxidation potentials of 1–7-Mn

and 1a–7a-Mn as a function of b-pyrrole and meso-

substituents further indicate that a metal-centered redox

process is involved in all these manganese(III) corroles.

Interestingly, the increase in the ﬁrst oxidation potential

Cyclic voltammetry of manganese(III) corroles

Metallocorroles are generally harder to reduce but

easier to oxidize compared with their metalloporphyrin

analogs owing to the different formal charge on both the

macrocycles, which is -2 in porphyrins and -3 in corroles

[39]. This feature is consistent with the ability of corrole

to stabilize high-valent transition metal ions more than

the corresponding porphyrins. The electrochemistry of

manganese corroles having various b-alkyl and meso-

aryl substituents has been widely studied [37, 40, 41].

Although manganese(III) corroles are sufﬁciently

stable in air, they can be oxidized at relatively low

potential to the corresponding manganese(IV) corrole

derivatives. Various manganese(IV) corrole deriva-

tives containing OPPh₃and halide ions (Br^-, Cl^-and

I^-) as ﬁfth axial ligand have been structurally charact-

erized, indicating that corrole macrocycle can stabilize

Mn(IV) in the core [42–44]. Electronic and steric

features of corroles show remarkable inﬂuence on

the oxidation potentials of manganese(III) corroles

[17, 37, 40, 41, 45]. For example, highly electron-rich

octaethylcorrole (OEC) manganese(III), devoiding meso-

aryl substituents, is much easily oxidized as compared

to tris(pentaﬂuorophenyl) manganese(III) corrole or its

b-ﬂuoro and b-bromo analogs. The electron-poor nature

Fig. 2. CV spectra of 1-Mn and 1a-Mn in acetonitrile containing TBAP as supporting electrolyte. Scan rate 100 mV/s

J. Porphyrins Phthalocyanines 2015; 19: 1240–1250

OXYGEN ATOM TRANSFER REACTIONS FROM STERICALLY ENCUMBERED BROMINATED (OXO)MANGANESE(V) 1241

Table 1. Oxidation potentials (vs. Ag/AgCl) of manganese(III)

corroles in acetonitrile containing TBAP as supporting

electrolyte^a

however, sensitive to the electronic and steric features

of the substituents attached to the meso-phenyl groups.

The electronic spectra of octabrominated manganese(III)

corroles 1a–7a-Mn (Fig. 4) are much more similar to

their corresponding non-brominated derivatives, except

that their Soret and Q-bands are signiﬁcantly red-shifted

owing to the inductive electron-withdrawing effect of

b-bromo-substituents[21].

The electronic spectra of (oxo)manganese(V) corroles

show signiﬁcant difference from their corresponding

manganese(III) corroles owing to the change in oxidation

state of manganese accompanied with the oxygen coordi-

nation. All (oxo)manganese(V) corroles 1–7-Mn(oxo)

display characteristic split Soret band in the range of

351–413 nm and a broad Q-band around 520 nm (Fig. 5).

The corresponding b-brominated (oxo)manganese(V)

corroles 1a–7a-Mn(oxo) also exhibit the same pattern

(Fig. 6), but signiﬁcant red-shift was observed for Soret

and Q-bands[21].

Corrole E_1/2Mn^IV/Mn^IIICorrole E_1/2Mn^IV/Mn^IIIDE^b

1-Mn

2-Mn

3-Mn

4-Mn

5-Mn

6-Mn

7-Mn

0.845

0.736

0.496

0.627

0.685

0.682

0.740

1a-Mn

2a-Mn

3a-Mn

4a-Mn

5a-Mn

6a-Mn

7a-Mn

1.249

1.106

0.908

1.169

0.884

0.923

0.907

0.405

0.370

0.412

0.542

0.199

0.250

0.167

^aFc/Fc⁺couple were measured as 0.47 V under identical

conditions.^bE_1/2ofbrominatedcorroles—E_1/2ofnonbrominated

corroles.

values upon b-pyrrole bromination is less signiﬁcant

in 5a–7a-Mn than in 1a–4a-Mn, which indicates that

bromine atoms at 2,6-dibromophenyls will reduce the

effect of b-bromination on the redox potential of central

Mn atom.

Kinetic studies for oxygen atom transfer reaction

(Oxo)manganese(V) corroles are legitimate oxygen

atom transfer agents [12, 18, 21]. For studying the

kinetics of OAT reactions of various b-brominated and

-nonbrominated (oxo)manganese(V) corroles 1–7-Mn

(oxo) and 1a–7a-Mn(oxo) to organic substrates, styrene

was chosen as substrate and toluene as the solvent[19, 20].

The green colored toluene solutions of manganese(III)

corroles precursors 1–7-Mn and 1a–7a-Mn were treated

with freshly prepared iodosylbenzene (PhIO). The initial

green colored manganese(III) corroles were converted to

red pink colored (oxo)manganese(V) corroles, indicating

the transformation of Mn(III) to Mn(V) accompanied

with the oxygenation of manganese center. The excess

UV-vis spectroscopy

The electronic spectra of all manganese(III) and (oxo)

manganese(V) corroles resemble with the previously

reported manganese complexes of various peripherally

substituted corroles [12, 17, 21]. Typically, all manga-

nese(III) corroles 1–7-Mn (Fig. 3) display sharp Soret

band around 400 nm, metal-ligand charge transfer

band around 490 nm and a broad Q-band in the range

of 580–660 nm. The exact position of these bands is,

Fig. 3. UV-vis spectra of manganese(III) corroles 1–7-Mn in toluene at 20.0 0.1°C

J. Porphyrins Phthalocyanines 2015; 19: 1241–1250

1242 M. HR MAHMOOD ET AL.

PhIO was then removed by ﬂash chromotograpgy

on a short column of basic alumina to afford (oxo)-

manganese(V) corrole solution. The stability of these

(oxo)manganese(V) corroles varied signiﬁcantly

depending on the combined electronic and steric

factors. More electron-poor manganese(III) corroles

in both b-brominated and b-nonbrominated series

produced much more reactive (oxo)manganese(V)

corroles.Similarly,b-brominated(oxo)manganese(V)

corroles 1a–7a-Mn(oxo) were observed more

reactive than corresponding b-nonbrominated (oxo)-

manganese(V) corroles 1–7-Mn(oxo).

The self-decay process of (oxo)manganese(V)

corroles can easily be monitored by UV-vis

spectroscopy. From UV-vis spectral changes of

1–7-Mn^V(oxo) (Fig. 7), it is clear that all (oxo)-

manganese(V) corroles decayed with the passage

of time. This can be observed by the disappearance

of typical absorption peak corresponding to Mn^V≡O

moiety in the range between 350 to 360 nm along

with the disappearance of Q-band at about 500 nm,

as well as the appearance of absorption peaks

at about 480 nm corresponding to metal-ligand

charge transition (MLCT) band and the appearance

of broad Q-band at about 600 nm typical of

manganese(III) corroles complexes. Exceptions to

this are 3-Mn(oxo) and 4-Mn(oxo) which decayed

to their corresponding manganese(IV) corroles,

rather than manganese(III) corroles as indicated

by their UV-vis spectral changes. In contrast to

other corroles in the series, corrole macrocycle in

3-Mn(oxo) and 4-Mn(oxo) is relatively less electron-

poor. Almost similar phenomenon was observed for

1a–7a-Mn(oxo) whose UV-vis spectral changes

are shown in Fig. 8. Corresponding changes for

the transformation of (oxo)manganese(V) corroles

to manganese(III) corroles can be seen, except for

3a-Mn(oxo) and 4a-Mn(oxo). (Oxo)manganese(V)

corroles 3a-Mn(oxo) and 4a-Mn(oxo) did not display

distinct split Soret band, however, the characteristic

peak for Mn^V≡O moiety at about 385 nm can be

clearly observed whose intensity decreased with

the passage of time. A blue-shift of about 10–20 nm

during the transformation of 3a-Mn and 4a-Mn

to 3a-Mn(oxo) and 4a-Mn(oxo) also testiﬁes the

formation of their corresponding (oxo)manganese(V)

corroles. The kinetics of the self-decay of (oxo)-

manganese(V) corroles was monitored by the

absorption at left arm of the Soret band (Figs S3 and

S4, Supporting information). The self-decay rate

constants were computed from the exponential ﬁt of

the kinetic curves, and are summarized in Table 2.

After conﬁrming that all (oxo)manganese(V)

corroles 1–7-Mn(oxo) and 1a–7a-Mn(oxo) are

reasonably stable, their reactivity with reference

to oxygen atom transfer (OAT) to styrene at 20°C

in toluene was investigated. The addition of 1000

Fig. 4. UV-vis spectra of manganese(III) corroles 1a–7a-Mn in

toluene at 20.0 0.1°C

Fig. 5. UV-vis spectra of (oxo)manganese(V) corroles 1–7-Mn(oxo)

in toluene at 20.0 0.1°C

Fig. 6. UV-vis spectra of (oxo)manganese(V) corroles 1a–7a-

Mn(oxo) in toluene at 20.0 0.1°C

J. Porphyrins Phthalocyanines 2015; 19: 1242–1250

OXYGEN ATOM TRANSFER REACTIONS FROM STERICALLY ENCUMBERED BROMINATED (OXO)MANGANESE(V) 1243

Fig. 7. UV-vis spectral changes for 1–7-Mn(oxo) in toluene at 20.0 0.1°C. Inset shows the self-decay kinetic curve

J. Porphyrins Phthalocyanines 2015; 19: 1243–1250

1244 M. HR MAHMOOD ET AL.

Fig. 8. UV-vis spectral changes for 1a–7a-Mn(oxo) in toluene 20.0 0.1°C. Inset shows the self-decay kinetic curve

J. Porphyrins Phthalocyanines 2015; 19: 1244–1250

OXYGEN ATOM TRANSFER REACTIONS FROM STERICALLY ENCUMBERED BROMINATED (OXO)MANGANESE(V) 1245

Table 2. Self-decay rate constant (k) and pseudo-ﬁrst-order rate constants (k_obs) for

various (oxo)manganese(V) corroles in toluene at 20.0 0.1°C^a,b

Corrole

k/k_obs(10^-4S^-1)

Corrole

k/k_obs(10^-4S^-1)

Self-decay

Styrene

Self-decay

Styrene

1-Mn(oxo)

2-Mn(oxo)

3-Mn(oxo)

4-Mn(oxo)

5-Mn(oxo)

6-Mn(oxo)

7-Mn(oxo)

3.669

2.972

1.638

2.072

1.051

1.252

2.432

12.467

9.502

4.537

6.811

2.944

3.019

7.589

1a-Mn(oxo)

2a-Mn(oxo)

3a-Mn(oxo)

4a-Mn(oxo)

5a-Mn(oxo)

6a-Mn(oxo)

7a-Mn(oxo)

5.781

3.406

1.874

2.908

1.220

1.738

3.152

39.078

27.669

13.633

20.007

8.947

9.122

22.513

b

^aThe reaction was monitored by the absorption at left arm of the Soret band. The

decay rate constants were obtained from the exponential ﬁt of the kinetic curve.

equivalents of styrene to the toluene solutions of

electron-rich or ortho sterically hindered meso-phenyl

groups. (Oxo)manganese(V) corroles 4-Mn(oxo) and

3-Mn(oxo) represent the examples of corroles having

fairly “bare” Mn^V≡O moiety as compared with the other

corroles in the series by keeping in mind the following

order of steric hindrance Br > F > H. Both differs by the

presence of only one substituent attached at the meso-para-

phenyl, i.e. a H in 3-Mn(oxo) is replaced with a CN group

in 4-Mn(oxo). The observed difference in the reactivity

of 4-Mn(oxo) and 3-Mn(oxo) can safely be attributed

to the presence of electron-withdrawing CN group in

4-Mn(oxo), although the reactivity of both 4-Mn(oxo)

and 3-Mn(oxo) is still lower than those containing meso-

pentaﬂuorophenyl groups. (Oxo)manganese(V) corroles

6-Mn(oxo) and 5-Mn(oxo) represent the most sterically

hindered b-nonbrominated (oxo)manganese(V) corroles in

the series. Both 6-Mn(oxo) and 5-Mn(oxo) contain meso-

2,6-dibromophenyl groups, with the former containing

an additional F-substituent at the para-position. Steric

protection by 2,6-dihalophenyl groups (2,6-dichloro-

and 2,6-diﬂuorophenyls) in various (oxo)manganese(V)

corroles has been responsible for the lower reactivity

of (oxo)manganese(V) corroles towards oxygen atom

transference to thioanisoles [21]. The observed reactivity

of 6-Mn(oxo) and 5-Mn(oxo) can thus be attributed to

the steric protection by the meso-2,6-dibromophenyl

substituents. Their Mn^V≡O is the most protected and

hence the most stable amongst the investigated series of

1–7-Mn^V(oxo) corroles. From the above, it is evident that

the observed reactivity of (oxo)manganese(V) corroles

can be correlated with two aspects: Firstly, the presence

and number of meso-pentaﬂuorophenyl groups in the

(oxo)manganese(V) corroles, and Secondly, the extent of

steric protection by the bulky meso-2,6-dibromophenyl

substituents. This steric protection is present but less

efﬁcient in 7-Mn(oxo) (due to only two meso-2,6-

dibromophenyl substituents), totally lacking in case of

3-Mn(oxo) and 4-Mn(oxo), and present up to maximum in

5-Mn(oxo) and 6-Mn(oxo) corroles.

1–7-Mn(oxo) resulted in the color change of solution

from pink to green, indicating the transformation of

(oxo)manganese(V) corroles 1–7-Mn(oxo) back to

manganese(III) corroles 1–7-Mn via oxygen atom transfer

[19, 21]. The decay was remarkably accelerated in the

presence of styrene as compared with their corresponding

self-decays, consistent with the kinetics of oxygen atom

transference from (oxo)manganese(V) corroles to styrene

resulting in the formation of styrene epoxide. Further-

more, b-brominated (oxo)manganese(V) corroles 1a–7a-

Mn(oxo) exhibited higher reactivity in terms of oxygen

atom transfer to styrene than their b-nonbrominated

analogs, as might be expected on the basis of reactivity

induced by b-bromination [21, 23]. The pseudo-ﬁrst-order

rate constants were measured for all (oxo)manganese(V)

corroles 1–7-Mn(oxo) and 1a–7a-Mn(oxo) for at least

three times, and the average of these data for each (oxo)

manganese(V) corrole is depicted in Table 2. It must be

noted that the orders of self-decay and pseudo-ﬁrst-order

reaction rate constants are the same in both the series.

The rate constants for the stochiometric oxidation of

styrene to styrene epoxide by various b-nonbrominated

(oxo)manganese(V) corroles 1–7-Mn^V(oxo) revealed

the following reactivity order: 1-Mn(oxo) > 2-Mn(oxo)

> 7-Mn(oxo) > 4-Mn(oxo) > 3-Mn(oxo) > 6-Mn(oxo) ≈

5-Mn(oxo).Thispatternisconsistentwithtwomainfactors:

the electron-poor or electron-rich meso-substituents, and

the presence or absence of steric protection by these meso-

phenyl groups. The ﬁrst three members of the series differ

by the number of pentaﬂuorophenyl groups (3, 2 and 1,

respectively) in their skeleton. The pentaﬂuorophenyl

groupisthemosthighlyelectron-pooramongvariousmeso-

substituents in the above manganese corroles complexes,

and the reactivity may be related with the number of

meso-pentaﬂuorophenyl groups attached to the corrole

ring. While the remaining (oxo)manganese(V) corroles

4-Mn(oxo) to 7-Mn(oxo) lack any meso-pentaﬂuorophenyl

groups, their reactivity order can be justiﬁed by either

J. Porphyrins Phthalocyanines 2015; 19: 1245–1250

1246 M. HR MAHMOOD ET AL.

The pseudo-ﬁrst-order rate constants for the stochio-

metric oxidation of styrene by various b-brominated

(oxo)manganese(V) corroles, 1a–7a-Mn(oxo), indicated

the following order of reactivity: 1a-Mn(oxo) >

2a-Mn(oxo) > 7a-Mn(oxo) > 4a-Mn(oxo) > 3a-Mn(oxo)

> 6a-Mn(oxo) ≈ 5a-Mn(oxo).While the reactivity order of

b-brominated (oxo)manganese(V) corroles followed the

same pattern as that of corresponding b-nonbrominated

(oxo)manganese(V) corroles, a noteworthy observation

is the difference in their rate constants. The rate constant

for the oxygen atom transference from 1a–7a-Mn(oxo)

to styrene increased dramatically as compared to

1–7-Mn(oxo). The difference in the reactivity of 1a–7a-

Mn(oxo) as compared to 1–7-Mn(oxo) is correlated

with the differences in the electrochemistry of their

corresponding manganese(III) corroles 1a–7a-Mn and

1–7-Mn, respectively, and has been identiﬁed in case

of sulﬁdes [21]. b-bromination induced enhancement of

oxygen atom transfer from (oxo)manganese(V) corroles

to organic substrates is also supported from theoretical

studies [23, 24]. DFT calculations have indicated that,

while (oxo)manganese(V) corroles generally exists in

singlet states, there is only a slight difference of about

12 kcal/mol between their singlet and triplet states. As

oxygen atom transfer is more favorable in triplet state

pathway, b-bromination provides a convenient way of

spin ﬂipping (change of singlet state to triplet state) in

(oxo)manganese(V) corroles for efﬁcient OAT. DFT

calculations have also indicated that electron-withdrawing

meso-substituents can also increase the rate of OAT, but

the effect of b-bromination is more profound and can

sharply increase the rate of OAT via spin ﬂipping.

Variationsintherateconstantswithchangingelectronic

and steric effects suggest that oxygen atom transference

proceeds through direct pathway in all cases [23]. All

electron-poor (oxo)manganese(V) corroles containing

highly electron-withdrawing pentaﬂuorophenyl groups

(whether b-brominated or -nonbrominated) are more

reactive, and undergo direct oxygen atom transfer

on reaction with styrene and form styrene epoxide.

Comparatively electron-rich and sterically protected

(oxo)manganese(V) corroles are less efﬁcient for oxygen

atom transfer to styrene. While (oxo)manganese(V)

corroles 3–6-Mn(oxo) are comparatively electron-rich,

the least reactivity of 6-Mn(oxo) and 5-Mn(oxo) may be

attributed to the enhanced steric protection of Mn^V≡O by

ortho-dibromophenyl substituents.

styrene (Aladdin) was passed through short column of

basic alumina prior to use. Iodosylbenzene (PhIO) was

synthesized by a reported method [48]. The supporting

electrolyte in CV experiments, tetrabutylammonium

perchlorate, was purchased from Fluka and recrystallized

from ethanol at least three times. EPR spectra were

measured on an X-band Bruker A300 spectrometer

equipped with an Oxford Instruments liquid helium

cryostat. Mass spectra were taken on ESI/MS (Bruker

Esquire HCT plus mass spectrometer with an ESI

source) and MALDI-TOF spectra were measured on

a Bruker auto ﬂex III smart beam instrument, using

4-chloro-a-cyanocinnamic acid (CCA) as the matrix. All

experiments, unless speciﬁed otherwise, were carried out

at room temperature.

All cyclic voltammograms (CV) were performed

in acetonitrile solutions containing 0.1 M TBAP

(tetrabutylammonium perchlorate) using an Ingens

Model 1030 and manganese corroles (1 × 10^-3M) under

nitrogen atmosphere at ambient temperature. A three-

electrode system consisting of a glassy carbon working

electrode, a platinum wire counter electrode andAg/AgCl

reference electrode were employed. The scan rate was

100 mV/s. Half-wave potentials (E_1/2) for reversible or

quasi-reversible redox processes were calculated as E_1/2

=

(E_pa+ E_pc)/2, where E_paand E_pcrepresent the anodic and

cathodic peak potentials. respectively. The E_1/2value for

the ferrocenium/ferrocene couple under these conditions

was 0.47 V.

Kinetic studies were carried out using a Blue Star B

UV-vis spectrophotometer connected with a thermostat

at 20.0 0.1°C in quartz cuvette with PTFE septa. The

freshly synthesized Mn(V)-oxo corrole solution (~3 ×

10^-3M) was taken in the cuvette and placed immediately

in a thermostat cell holder in a UV-Vis spectrophotometer.

The absorbance data was collected over the range of

300–800 nm at 5-sec interval. Each measurement was

repeated three times at 20.0 0.1°C [19].

Synthesis of manganese corroles

Synthesis of b-nonbrominated manganese(III) cor-

roles. All manganese corroles were prepared by reacting

free base corroles with manganese acetate in methanol

dichloromethane mixture [12, 13, 18, 49]. The detail

procedure is given below.

Typical procedure for the synthesis of b-nonbromi-

nated manganese(III) corroles. 25 mg of corrole was

dissolved in 10 mL of dichloromethane in 100 mL round

bottom ﬂask. Subsequently, 50 mL of methanol and

15–20 folds excess of Mn(OAC)₂∙4H₂O were further

added into the ﬂask. The green colored reaction mixture

was reﬂuxed for about 2 h and the progress of the reaction

was monitored by TLC and UV-visible spectroscopy.

After the completion of reaction, the reaction mixture was

transferred to a separating funnel. Manganese corrole was

extracted in dichloromethane, washed with saturated brine

EXPERIMENTAL

General

All chemicals and solvents were purchased from

Sinopharm Chemical Reagents Co. Ltd. and used without

further puriﬁcation unless otherwise mentioned. Pyrrole

was redistilled prior to use. Commercial analytical grade

J. Porphyrins Phthalocyanines 2015; 19: 1246–1250

OXYGEN ATOM TRANSFER REACTIONS FROM STERICALLY ENCUMBERED BROMINATED (OXO)MANGANESE(V) 1247

several times and dried over anhydrous Na₂SO₄. After

evaporation of the solvent, the crude product was puriﬁed

over silica gel using dichloromethane as eluent. The yield

of manganese corroles in all cases were 80–90%.

of nitrogen gas. The green solution was evaporated

to dryness and chromatographed over silica gel using

dichloromethane as eluent. The yield of manganese

corroles in all cases were between 60–85%.

5,10,15-Tris(pentaﬂuorophenyl)corrolato manga-

nese(III), 1-Mn. Yield 90%. 22.5 mg (2.652 × 10^-5moles).

UV-vis (toluene): l_max, nm (e, × 10³M^-1.cm^-1) 411 (1.056),

478 (0.574), 599 (0.351). MS (HRMS): m/z found [M +

H]⁺848.9963; calcd. for C₃₇H₉F₁₅MnN₄⁺848.9968.

5,15-Bis(pentaﬂuorophenyl)-10-(4-bromophenyl)

corrolato manganese(III), 2-Mn. Yield 90%. 22.5 mg

(2.687 × 10^-5moles). UV-vis (toluene): l_max, nm (e, ×

10³M^-1.cm^-1) 409 (1.211), 480 (0.423), 584 (0.380), 630

(0.385). MS (MALDI-TOF): m/z found [M]⁺837.905;

calcd. for C₃₇H₁₂BrF₁₀MnN₄⁺837.945.

5,10,15-Triphenylcorrolato manganese(III), 3-Mn.

Yield 85%. 21.25 mg (3.673 × 10^-5moles). UV-vis

(toluene): l_max, nm (e, × 10³M^-1.cm^-1) 408 (1.041), 426

(0.921), 497 (0.343), 650 (0.284). MS (ESI): m/z found

[M]⁺578.9; calcd. for C₃₇H₂₃MnN₄⁺578.1.

5,10,15-Tris(4-cyanophenyl)corrolato manganese

(III), 4-Mn. Yield 88%. 22.0 mg (3.366 × 10^-5moles).

UV-vis (toluene): l_max, nm (e, × 10³M^-1.cm^-1) 413 (1.304),

443 (1.048), 496 (0.423), 648 (0.404). MS (MALDI-TOF):

m/z found [M]⁺653.062; calcd. for C₄₀H₂₀MnN₇⁺653.116.

5,10,15-Tris(2,6-dibromophenyl)corrolato

manganese(III), 5-Mn. Yield 90%. 22.5 mg (2.139 ×

10^-5moles). UV-vis (toluene): l_max, nm (e, × 10³M^-1.cm^-1)

411 (0.783), 489 (0.391), 606 (0.264). MS (ESI): m/z

found [M]⁺1051.9; calcd. for C₃₇H₁₇Br₆MnN₄⁺1051.6.

5,10,15-Tris(2,6-dibromo-4-ﬂuorophenyl)corrolato

manganese(III), 6-Mn. Yield 90%. 22.5 mg (2.034 × 10^-5

moles). UV-vis (toluene): l_max, nm (e, × 10³M^-1.cm^-1) 411

(0.911), 480 (0.501), 605 (0.245). MS (MALDI-TOF):

m/z found [M]⁺1105.447; calcd. for C₃₇H₁₄Br₆F₃MnN₄⁺

1105.559.

2,3,7,8,12,13,17,18-Octabromo-5,10,15-tris

(pentaﬂuorophenyl)corrolato manganese(III), 1a-Mn.

Yield 85%. 8.5 mg (5.744 × 10^-6moles). UV-vis (toluene):

l

_max, nm (e, × 10³M^-1.cm^-1) 402 (1.182), 423 (1.653), 493

(0.725), 614 (0.399). MS (MALDI-TOF): m/z found

[M + H]⁺1480.54; calcd. for C₃₇HBr₈F₁₅MnN₄⁺1480.273.

2,3,7,8,12,13,17,18-Octabromo-5,15-bis(penta-

ﬂuorophenyl)-10-(4-bromophenyl)corrolato manga-

nese(III), 2a-Mn. Yield 85%. 8.5 mg (5.788 × 10^-6

moles). UV-vis (toluene): l_max, nm (e, × 10³M^-1.cm^-1)

402 (0.692), 430 (0.587), 503 (0.477), 620 (0.220). MS

(MALDI-TOF): m/z found [M]⁺1467.171, [M – Br]⁺

1307.365; calcd. for C₃₇H₄Br₉F₁₀MnN₄⁺1467.223.

2,3,7,8,12,13,17,18-Octabromo-5,10,15-

triphenylcorrolato manganese(III), 3a-Mn. Yield

60%. 6.0 mg (4.959 × 10^-6moles). UV-vis (toluene):

l

_max, nm (e, × 10³M^-1.cm^-1) 400 (0.909), 499 (0.411),

586 (0.376), 656 (0.349). MS (MALDI-TOF): m/z found

[M]⁺1209.378; calcd. for C₃₇H₁₅Br₈MnN₄⁺1209.406.

2,3,7,8,12,13,17,18-Octabromo-5,10,15-tris(4-

cyanophenyl)corrolato manganese(III), 4a-Mn. Yield

65%. 6.5 mg (5.059 × 10^-6moles). UV-vis (toluene):

l

_max, nm (e, × 10³M^-1.cm^-1) 404 (1.988), 444 (1.976), 506

(0.673), 656 (0.375). MS (HRMS): m/z found [M + H]⁺

1285.4002; calcd. for C₄₀H₁₃Br₈MnN₇⁺1285.3998.

2,3,7,8,12,13,17,18-Octabromo-5,10,15-tris(2,6-

dibromophenyl)corrolato manganese(III), 5a-Mn.

Yield 60%. 6.0 mg (3.564 × 10^-6moles). UV-vis (toluene):

l

_max, nm (e, × 10³M^-1.cm^-1) 408 (0.863), 445 (0.822), 508

(0.612), 655 (0.233). MS (HRMS): m/z found [M + H]⁺

1683.8648; calcd. for C₃₇H₁₀Br₁₄MnN₄⁺1683.8710.

2,3,7,8,12,13,17,18-Octabromo-5,10,15-tris(2,6-

dibromo-4-ﬂuorophenyl)corrolato manganese(III),

6a-Mn. Yield 60%. 6.0 mg (3.454 × 10^-6moles). UV-vis

(toluene): l_max, nm (e, × 10³M^-1.cm^-1) 403 (1.193), 502

(0.624), 622 (0.378). MS (HRMS): m/z found [M]⁺

1736.8304; calcd. for C₃₇H₆Br₁₄F₃MnN₄⁺1734.8370.

2,3,7,8,12,13,17,18-Octabromo-5,15-bis(2,6-

dibromo-4-fluorophenyl)-10-(pentafluorophenyl)

corrolato manganese(III), 7a-Mn. Yield 65%. 6.5 mg

(3.936 × 10^-6moles). UV-vis (toluene): l_max, nm (e, ×

10³M^-1.cm^-1) 407 (1.255), 428 (1.044), 502 (0.731), 618

(0.344). MS (HRMS): m/z found [M]⁺1650.9791; calcd.

for C₃₇H₄Br₁₂F₇MnN₄⁺1650.9782.

5,15-Bis(2,6-dibromo-4-fluorophenyl)-10-(penta-

ﬂuorophenyl)corrolato manganese(III), 7-Mn. Yield

90%. 22.5 mg (2.206 × 10^-5moles). UV-vis (toluene):

l

_max, nm (e, × 10³M^-1.cm^-1) 411 (1.068), 480 (0.523), 607

(0.287). MS (MALDI-TOF): m/z found [M]⁺1019.661;

calcd. for C₃₇H₁₂Br₄F₇MnN₄⁺1019.702.

Synthesis of b-brominated manganese(III) corroles

All manganese corroles were converted into their

corresponding b-brominated analogs by reacting with

molecular bromine [21] according to the following

procedure.

Typicalprocedureforthesynthesisofb-brominated

manganese(III) corroles. 10 mg of manganese corrole

was dissolved in 5 mL of methanol in 50 mL round bottom

ﬂask and was stirred for about 10 min. Bromine (0.3 mL)

diluted in 10 mL of methanol was added through funnel

drop wise into the ﬂask, while the stirring was continued

overnight at room temperature. After the completion of

reaction, the excess bromine was evaporated by a stream

Synthesis of (oxo)manganese(V) corroles

All manganese(III) corroles were transformed into their

corresponding (oxo)manganese(V) corroles according

to reported procedure [12, 18]. A typical procedure is

described below:

Typical procedure for the synthesis of (oxo)

manganese(V) corroles. Stock solutions (3 × 10^-3M) of

J. Porphyrins Phthalocyanines 2015; 19: 1247–1250

1248 M. HR MAHMOOD ET AL.

manganese(III) corroles (1-Mn to 7-Mn and 1a-Mn to

7a-Mn) were prepared in volumetric ﬂask. 3 mL aliquots of

each manganese solution was treated with an appropriate

amount of PhIO in a small glass tube until the green colored

manganese(III) corrole turned to the corresponding (oxo)

manganese(V) corrole. The solution was passed through a

short column of basic alumina to ﬁlter the excess of PhIO.

The ﬁltered solution was used for characterization and

kinetic studies.

5,10,15-Tris(pentaﬂuorophenyl)corrolato manga-

nese(V)-oxo, 1-Mn(oxo). UV-vis (toluene): l_max, nm

(e, × 10²M^-1.cm^-1) 352 (4.46), 409 (4.76), 524 (1.18). MS

(ESI): m/z found [M]⁺864.03, [M – O]⁺848.84; calcd.

for C₃₇H₈F₁₅MnN₄O⁺863.98.

5,15-Bis(pentaﬂuorophenyl)-10-(4-bromophenyl)

corrolato manganese(V)-oxo, 2-Mn(oxo). UV-vis

(toluene): l_max, nm (e, × 10²M^-1.cm^-1) 353 (4.70), 408

(4.87), 528 (1.27). MS (ESI): m/z found [M + H]⁺852.93,

[M - O]⁺836.98; calcd. for C₃₇H₁₂BrF₁₀MnN₄O⁺852.95.

5,10,15-Triphenylcorrolato manganese(V)-oxo, 3-

Mn(oxo). UV-vis (toluene): l_max, nm (e, × 10²M^-1.cm^-1)

354 (4.26), 409 (4.35), 515 (1.21). MS (ESI): m/z

found [M + H]⁺595.48, [M – O]⁺580.72; calcd. for

C₃₇H₂₃MnN₄O⁺595.13.

(1.26). MS (HRMS): m/z found [M]⁺1225.4009; calcd.

for C₃₇H₁₅Br₈MnN₄O⁺1225.4012.

2,3,7,8,12,13,17,18-Octabromo-5,10,15-tris(4-

cyanophenyl)corrolato manganese(V)-oxo, 4a-Mn

(oxo). UV-vis (toluene): l_max, nm (e, × 10²M^-1.cm^-1) 384

(4.16), 545 (0.70). MS (HRMS): m/z calcd. found [M +

H]⁺1301.3964; calcd. for C₄₀H₁₃Br₈MnN₇O⁺1301.3947.

2,3,7,8,12,13,17,18-Octabromo-5,10,15-tris(2,6-

dibromophenyl)corrolato manganese(V)-oxo, 5a-Mn-

(oxo). UV-vis (toluene): l_max, nm (e, × 10²M^-1.cm^-1) 379

(4.69), 421 (5.10), 542 (1.95). MS (HRMS): m/z found

[M]⁺1698.8603; calcd. for C₃₇H₉Br₁₄MnN₄O⁺1698.8581.

2,3,7,8,12,13,17,18-Octabromo-5,10,15-tris(2,6-di-

bromo-4-ﬂuorophenyl)corrolato manganese(V)-oxo,

6a-Mn(oxo). UV-vis (toluene): l_max, nm (e, × 10²M^-1.

cm^-1) 392 (4.62), 424 (4.77), 547 (1.51).MS (HRMS): m/z

found [M]⁺1752.8332; calcd. for C₃₇H₆Br₁₄F₃MnN₄O⁺

1752.8298.

2,3,7,8,12,13,17,18-Octabromo-5,15-bis(2,6-dibromo-

4-fluorophenyl)-10(pentafluorophenyl)corrolato

manganese(V)-oxo, 7a-Mn(oxo). UV-vis (toluene): l_max

,

nm (e, × 10²M^-1.cm^-1) 392 (4.62), 424 (4.64), 540 (1.58).

MS (HRMS): m/z found [M]⁺1666.9721; calcd. for

C₃₇H₄Br₁₂F₇MnN₄O⁺1666.9732.

5,10,15-Tris(4-cyanophenyl)corrolatomanganese(V)-

oxo, 4-Mn(oxo). UV-vis (toluene): l_max, nm (e, × 10²M^-1.

cm^-1) 357 (4.26), 412 (4.63), 520 (1.36). MS (HRMS): m/z

found [M]⁺669.1100; calcd. for C₄₀H₂₀MnN₇O⁺669.1110.

5,10,15-Tris(2,6-dibromophenyl)corrolato

CONCLUSION

In conclusion, we report the synthesis of several novel

b-brominated and b-nonbrominated manganese(III)

corroles differing in their electronic and steric properties

based on meso-substitution. The transformation of

manganese(III) corroles into their corresponding (oxo)-

manganese(V) corroles on treating with PhIO, and

subsequent kinetics studies for their oxygen atom transfer

to styrene in toluene revealed signiﬁcant dependence of

pseudo-ﬁrst-order rate constants on the electronic and

steric properties of manganese corroles. The following

order of reactivity was observed for b-nonbrominated

(oxo)manganese(V) corroles: 1-Mn(oxo) > 2-Mn(oxo)

> 7-Mn(oxo) > 4-Mn(oxo) > 3-Mn(oxo) > 6-Mn(oxo) ≈

5-Mn(oxo). The rate constants for b-pyrrole-brominated

analogs followed the same pattern but signiﬁcantly larger

in all cases, indicating the effect of b-bromination. This

reactivity order is in accordance with the followings:

Firstly, electron-poor (oxo)manganese(V) corroles are

more reactive for oxygen atom transfer to styrene, and their

reactivity is directly correlated with the number of meso-

pentaﬂurophenyl groups attached to corrole. Secondly,

electron-richandstericallyunhindered(oxo)manganese(V)

corroles are less reactive t han highly electron-poor (oxo)-

manganese(V) corroles but still more reactive than

sterically hindered (oxo)manganese(V) corroles. Thirdly,

highly sterically protected (oxo)manganese(V) corroles

containing 2,6-dibromophenyl substituents are the least

reactive (oxo)manganese(V) corroles, and conversely have

the most stable Mn^V≡O moiety.

manganese(V)-oxo, 5-Mn(oxo). UV-vis (toluene): l_max

,

nm (e, × 10²M^-1.cm^-1) 355 (3.04), 412 (3.24), 527 (0.96).

MS (HRMS): m/z found [M]⁺1067.5811; calcd. for

C₃₇H₁₇Br₆MnN₄O⁺1067.5822.

5,10,15-Tris(2,6-dibromo-4-ﬂuorophenyl)corrolato

manganese(V)-oxo, 6-Mn(oxo). UV-vis (toluene): l_max

,

nm (e, × 10²M^-1.cm^-1) 354 (4.40), 413 (4.01), 525 (1.25).

MS (HRMS): m/z found [M]⁺1121.5521; calcd. for

C₃₇H₁₄Br₆F₃MnN₄O⁺1121.5539.

5,15-Bis(2,6-dibromo-4-fluorophenyl)-10-(penta-

ﬂuorophenyl)corrolato manganese(V)-oxo, 7-Mn(oxo).

UV-vis (toluene): l_max, nm (e, × 10²M^-1.cm^-1) 354 (3.80),

411 (4.26), 522 (2.34). MS (HRMS): m/z found [M]⁺

1035.6957; calcd. for C₃₇H₁₂Br₄F₇MnN₄O⁺1035.6972.

2,3,7,8,12,13,17,18-Octabromo-5,10,15-tris(penta-

ﬂuorophenyl)corrolatomanganese(V)-oxo,1a-Mn(oxo).

UV-vis (toluene): l_max, nm (e, × 10²M^-1.cm^-1) 392 (5.37),

423 (5.51), 540 (2.34). MS (HRMS): m/z found [M]⁺

1495.2589; calcd. for C₃₇Br₈F₁₅MnN₄O⁺1495.2598.

2,3,7,8,12,13,17,18-Octabromo-5,15-bis(pentaﬂuo-

rophenyl)-10-(4-bromophenyl)corrolato manganese-

(V)-oxo, 2a-Mn(oxo). UV-vis (toluene): l_max, nm (e, ×

10²M^-1.cm^-1) 388 (4.76), 423 (4.66), 544 (1.99).

MS (HRMS): m/z found [M]⁺1485.2121; calcd. for

C₃₇H₄Br₉F₁₀MnN₄O⁺1485.2154.

2,3,7,8,12,13,17,18-Octabromo-5,10,15-triphenyl-

corrolato manganese(V)-oxo, 3a-Mn(oxo). UV-vis

(toluene): l_max, nm (e, × 10²M^-1.cm^-1) 384 (3.63), 541

J. Porphyrins Phthalocyanines 2015; 19: 1248–1250

OXYGEN ATOM TRANSFER REACTIONS FROM STERICALLY ENCUMBERED BROMINATED (OXO)MANGANESE(V) 1249

Acknowledgements

19. Wang Q, Zhang Y, Yu L, Yang H, Mahmood MHR

and Liu HY. J. Porphyrins Phthalocyanines 2014;

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The ﬁnancial support from the National Natural

Science Foundation of China (21171057, 21371059) is

gratefully acknowledged.

Supporting information

Figures S1–S4 and Table S1 are given in the

supplementary material. This material is available free of

charge via the Internet at http://www.worldscinet.com/

jpp/jpp.shtml.

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Article Doi

Oxygen atom transfer reactions from sterically encumbered brominated (oxo)manganese(V) corroles to styrene

DOI: 10.1142/S1088424615501059

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Full text of DOI:10.1142/S1088424615501059

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