Photochemical reactions of dioxygen containing metalloporphyrins

Coo~di~~on

Chemistry Reviews, I32

(I 994)235-242

235

Photochernical Reactions of Dioxygen containing Metalloporphyrins M. Hoshino, K Yamamoto, J. P. L.iHis,T. Chijimatsu, and 5. Wzawa The Institute of Physical and Chemical Research (RIKEN), Wako, Mama, 351-01, Japan

ABSTRACT

Optical absorption and Jaser pboto~ysis studies reveal a similarity in the nature and reactivity in the peroxo dioxygen metalloporphyrin complexes of Ti, and MO. This reactivity

is compared with that observed

for the superoxo dioxygen

Co(D)

tetraphenylporphyrin complex.

A. INTRODUCT’iON

The reversible fixation and the activation of dioxygen by metal complexes have attracted considerable attention as systems to model biological oxygen transport and oxidase

function

85 well as to develop homogene~s

oxidation catalysts.

analogs of the heterogeneous

J.n a number of systems dioxygen metal complexes have been

implicated as intermediates in these processes (~2).

The structures of dioxygen

complexes with one metal center are roughly divided into two types; the peroxo and superoxo type complexes (3,4). Titanium(IV) end molybdenum(Vl) porphyrins form the pcroxo complexes in which the central metal binds to both of the two oxygen atoms (5). Cobalt@) porphyrins are known to form&e superoxo form iu which the metal binds to only one of the oxygen atoms (6). In particular, tbe photochemistry of metahopqhyrindioxygen complexes has been used extensively to aid in the understanding of the mechanisms of storage and aanslxn?ation of oxygen by hemoproteins (7). 0010~8545@4/$26.00 SSDI 00 I O-8545 (94) 02029-I

0 1994 - Elsevier Sequoia. All rights resewed

236

Previous preferentially

studies

that

undergo the photodissociation

photocleavage 02TiIv(TPP), photoproducts

of the oxygen-oxygen

metalloporphyrin of dioxygen.

were identified

similar

dioxygen

derivative

tetraphenylporphyrin,

02TiN(TTP)

as oxotitanium(IV)porphyrin

in which

the

N-oxide, O=Ti(TPP)O

(8).

of bisperoxomolybdenum(VI)tetraphenylporphyrin, reactivity.

Co(II)tetrapheny@rphyrin

The

photochemistry

was investigated

complexes

The present paper presents the

bond in peroxotitianium(IV)

and the paratolyiporphyrin

The photochemistry exhibits

indicated

of the

04MoV1TPP

dioxygen

adduct

of

for comparison.

B.ExPERIMENTAL

All reagents

were purchased

with the highest

purity possible

from Wako

Chemicals and used as received. Oxygen gas was pumhased from Niion Sanso and used as received.

Steady state photolyses were conducted using a high pssure

W USH250D) and a cutoff filter (h ~32Onm).

N-Methyldiphenyiamine

mercury lamp (250 was used as the

actinometer. The

peroxotitianium(IV)tettaphenylporphyrin,

phenylporphyrin,

02Ti”(TPP),

and bisperoxo-molybdenum(VI)tetraphenyiporphyrin,

cobalt(II)tetraOqMoVITPP

were synthesized and purified according to literature methods (45~). Lases photolysis was conducted using the second and third harmonics of a NdYAG laser with a detection

system

previously

described

(9).

Low temperature

measurements were done using a cryostat (Oxford Instruments Model DN 1020).

C. RESULTS AND DISCUSSION

The absor@ion spectrum of 02TiIv(TPP)

in benzene exhibits absorption peaks

at 423 (E= 3.3 x 105 Mm1 cm-l) and 547 run (E= 1.8 x lo4 Me1 cm-l). Upon photolysis, the spectrum gradually changes with new peaks at 438,566, and 606 nm and an isosbestic point at 557 run. The photolysis of 02TiIVCrpP)

in both aerated and deaerated benxene

237

yields the photoproduct. photoreaction. photoproduct.

This indicates that atmospheric

Irradiation

of 02TiN(TPP)

The photoreaction

oxygen is not involved in the

in toluene

at 77 K gave the same

seems to be unimolecular

since diffusion is suppressed

at 77 K. The preparative photolysis of 02’I’iW(TPP) in benzene or in chloroform a&ded O+TiW(TPp)O

with ca. 80% yield.

slowly photodecomposed The 02TiN(IPP)

quantum

It was found that solutions

of the photoproduct

to yield O=TiN(TPP)

and an unidentified brown polymer.

yields,

formation

a’, for the

of the photo~oduct

from

in benzene were found to be cb< 1O’3 at 545 nm and @=0.095 f 0.005 at

425, 320 and 310 ML This result indicates that excitation

of the Soret and other short

wavelength bands gives rise to the formation of the photoProduct. 02Ti~~~

A benzene solution of

was subjected to laser pulses (2Ons in duration) at 355nm.

The transient

spectrum observed at 50 ns after a pulse did not decay over a few ms. Since the transient spectrum

and the difference

spectrum,

obtained

by subtracting

the spectrum

of

are in good agreement, the photoreaction

of

02Tirv(TPP)

from that of the photoproduct,

02TiW(TPP)

is concluded to occur within 50 ns after the pulse.

A solution triphenylphosphinc. phosphine

of photoproduct,

O=TiW(TPP)O,

The photoproduct

to quantitatively

and ~phenylph~hine

in benzene

in benzene

was reacted with

was found to react with triphenyl-

form oxoti~~ium~te~aphenyi~hy~n,

oxide.

O=l?(TPp),

When 6.6 x 10e2M ~~enylphos~ne

is added into the

solution of the photoproduct the spectrum gradually changed to that of o-“riTv(TPP) isosbestic obtained photo~ct

with

points (Figure 1). The pseudo first order rate constant for the reaction was to bc 9.2 x 104 s-1 at room temperature.

This finding

suggests that the

has an oxygen atom which is abstracted by ~phenyl~~ne. The IR spectrum of 02TiN(I’PP)

the photoproduct

shows a vo_o at 900 cm-1 in KBr. However

displays no vo_o in the region of 900 cm-l.

that the oxygen-oxygen

bond of 02TirvcrpP,

The lH Nh4R spectrum of the ~oto~~ct

This observation indicates

is photodissociated

in the photoproduct.

showed a stnmg temperature dependence.

238

1.2

1

0

440

480

520

Wavelength Figure 1. U.V.1 Visible spectra of O=l;TV(TPP)O phosphine (6.6 ~10~ M) in benzene.

560

600

640

(nm) (4.0x lOA M) with added triphenyl

Spectra taken at t=o (1) 1 (2), 2.5 (3) and 12 h (4)

This behavior can be rationalized by a structure in which the porphyrin N-oxide is formed. The formation of porphyrin K-oxide would cause one of the pyrrole rings to bend out of plane and allow rotation of the phenyl rings. The photoproduct is therefore identified to be oxotitaniumt&raphenylporPhyrin

N-oxide (Figure 2(10)). The initial photochemical

event

of OzTiwvCrpP) is supposed to be the fission of either the Ti-0 or the O-O bond. The cleavage of a TM bond leads to the formation of the superoxo complex

According to an

earlier study of the superoxo complex of tita.nium(lU)tetraphenylporphyrin,

the superoxo

complex readily changes to 02TiTV(TPP) photochemical

(1 I). Tt is, therefore, suggested that the

fission of the Xi-0 bond does not give O&V(TPP)O.

239

Figure 2 Schematic representation

The photochemistry

of the 02Ti(N)porphyrin

for the formation of the photoproduct,

photoproduct.

O=T?(TPP)O,

is described

below (Scheme 1).

Scheme 1

The oxygen-oxygen intramolecular

bond in 02TiN(TPP)

is initially photodissociated,

oxygen atom transfer to yield O-riW(TPP)O.

followed by

Since, O=TiWCrpP)O

has

an oxygen bound to the Pyrrole nitrogen; an oxygen atom can then be readily abstracted by triphenyphosphine

leading to the formation of OCTv(TPP)

and triphenylphosphine

oxide. It has been reported that 04MowTPP undergoes photoreaction to give 02Mo =TPP the Photodissociation wavelengths excitation

of dioxygen

in both aerated and degassed benzene and dioxygen (11). The quantum yields for

were determined

to be 0.054 f 0.005 at excitation

shorter than 450 mu (Figure 3). of the Q-band,

No photoreaction was observed as was the case of 02T1 *IV(TPP). No photoproducts

observed that could be assigned to an intramolecular case of 02TiN(TPP),

the initial photochemical

oxygen transfer product.

upon were

As in the

event is presumed to be the fission of one

240

of the oxygen-oxygen

bonds in 04MokPP

lea&g

to the facile dissociation of dioxygen

(Scheme 2).

d-o Scheme 2

0.1

1

0.78

0.07 a E a E 0.05 3 4 C.

0.12 0.0 -0.1

I 101

it 0.02 $ 0.0 I

I

I

15

20

25

Wavenumber

Figure 3.

Absorption

spectrum

formation of the photoproduct, O~MO~TPP,

02CoucrpP),

in 2-methyltetrahydrof

30

( 10 3 c me’)

of 04Mo vITPP

Cobalt(E)tetraphenylporphyrin,

-0.01

I

and the quantum

yield (0) for the

in benzene solution.

CoU(TPP), forms a superoxo dioxygen adduct, at 77 K (7).

Low temperature

laser flash

photolysis studies were carried out iu oxygen saturated solutions. The transient spectrum, obtained with excitation at both 355 and 532 um, is in good agreement with the difference spectrum, obtained by subtmcting

the spectrum of 02-Co~P)

result implies that the 02-CoT4TpP)

photodissociates

from C&(TPP).

02 upon laser irradiation

This with

241

either the second, 532 nm or the thiid, 355 nm, harmonic of a Nd-YAG laser. This observation indicates that dioxygen

dissociation occurs as a wavelength

independent

D. CONCLUSION Peroxotitanium(IV)tetraphenylporphyrin

has been shown to preferentially cleave

the oxygen-oxygen upon irradiation to yield a porphyrin N-oxide product. Both Ti(lV) and Mo(IV)tetraphenylporphyrin photochemistry.

dioxygen

complexes

exhibited

wavelength

dependent

Though the MO complex does not form the N-oxoporphyrin product,

the photoproduct can be rationalized by a reaction scheme in which the oxygenoxygen bond is cleaved.

In comparison, the cobalt(II)tetraphenylporphyrin

releases dioxygen upon irradiation.

dioxygen complex

From these results, we believe that the primary

photochemical process in these metalloperoxoporphyrins

is to activate the coordinated

dioxygen.

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Acknowledgment

The Dr. J. P. Lillis Agency

of

Japan

through

a postdoctoral

would

and

the

like

to thank

National

fellowship

the Science

Science at RIKEN.

and Technology

Foundation

for

support