Indian Journal of Chemistry Vol. 44A, January 2005, pp. 7~-!9

Oxygenation reactions of cobalt(II) tetraen chelates generated on fl ash photolysis and

continuous photolysis of cobalt(III) complexes in different solvents

Chelli Janardhana*, S Jagadeeswara Rao, V Venkatesh, J Sai Kiran & R Sai Sathish

Department of Chemistry, Sri Sathya Sai Institute of Hi gher Learn ing, (Deemed University), Prashanthi Nilayam 515 134,

. Andhra Pradesh, India Emai l: chellij anardhana @7-ediffmail.com

Received 23 March 2004; revised 15 October 2004

Photo lysis of ([Co( tetraen)C1 j(CI0 4) 2 and [Co(tetraen)N02]­

(C I0 4h chelates in 0.1 M aq HC I04 , methanol and ace tonitrile media yie lds coba lt(lI ) chelates. Oxygenatio n reac tions o f cobalt(l l) bound to tetraen, {N-(2-aminoethy l)-N'- [2-[(2-aminoethyl)aminojethyll-I ,2-ethanedi amine) . have been studied by continuous and flash photolysis techniques. Quantum yie lds and excited state deac tivation pathways for cobalt(lll ) - amine complexes have been investi gated and a sche me for the exci ted state processes suggested. A co mpari son of the quantum y ie lds in different solvent media ind icates the ro le o f solvent on the quantum yie ld for the photoredox process.

IPC Code: Int. CI. 7 C07 F 15/06

Activation of molecular oxygen by amine complexes of cobalt(II ) has earlier been studied by fl ow techniques and mechanistic det.ai Is have been sugges ted '. Flash photolysis2

.3 studies have suggested

a two-step mechani sm for the formation of the final product, J.l-peroxo complex and rate constants for the individual processes have been determined . We have found that oxygen activation by cobalt(I1) amine species, photo produced from the cobalt(l II ) complexes, opens up feasibilities to find new species during the course of the reac tion and also more defined mechani stic information on the oxygenation reaction. Photochemical reactions of cobalt(IIf) complexes reveal that charge transfer processes occur from the charge-transfer-to-metal excited states leading to the reduction of the metal centre and the oxidation of the ligand4

• Due to the labile nature of the cobalt(1l) complexes, substitution of the solvent in the coordination sphere occurs rapidly in less than millisecond time scale. It is , therefore, possible to investiga te the oxygenation reactions by fl ash photolysis of the oxygenated solutions of cobalt(III) complexes and observing the intermediates as

transients and investigating the s pec~ ra l detail s and decay kinetics5.

Experimental The solvents, methanol and acetonitrile, used in this

study were of HPLC grade. Water was doubly distilled over potass ium permanganate before use. All the chemicals used in the inves tigation were of analar grade. ([Co(tetraen)CI](CI04h. 1, and [Co(tetraen)­N02](CI04h, 2, were prepared and charac terized by known methods6

.

[Co (tetraen) CI](CI04h A mixture of cobaltous chloride (8 . 1. g), tetraen (5.4

ml), sodium chloride (4 g), HCI ( I ml) and di sti lled water (75 ml) was taken in a two-necked round bottom flask and air oxidized for 8 hrs. After air ox idation, the solution was filtered and the filtrate was evaporated to half of its volume. The mixture was cooled to 5°C and perchloric ac id was added dropwise untill the appearance of crystals . The solution was kept in an ice bath for about 4 hrs and filtered. The precipitated complex was washed with methanol and ether. The crude product was recrystalli sed from hot water and air-dried.

[Co(tetraen)NOz](CI04h The complex was prepared by the above method

using a mixture of cobaltous nitrate exahydrate (IO g), tetraen (6.5 g), nitric ac id ( 1.08 g) and sodi um nitrite (4.72 g) in di still ed water (75 ml). To ensure complete precipitation of the complex, the solution was cooled to below 5°C and acetone was added. Sometimes some amount of cobalt(II) remained unox idised in the precipitate. Cobalt(III) complex was obtained in pure form by precIpI tation using methanol. The crys tals thus obtained were filtered, washed with methanol and ether, and air-dried.

The complexes were characterized by UV-vis and IR spectra. The complexes were ana lyzed for the elements, carbon, hydrogen and ni trogen. CHN analys is: For CoCgH2J N60 IOCI2: Found (%): C, 19.29; H, 4.64; N, 16.92. Calc. (%) : C, 19.48 ; H, 4.69 ; N, 17.04. For CoCsH23N5CbOs: Found (%): C, 19.76; H, 4.76; N, 14.35. Calc. (%): C, 19.9 1; H, 4.80 ; N, 14 .5\. The values obtained are within 2% error of the

NOTES 77

calculated values. Electronic absorption spectral measurements were made on a Hitachi-320 double beam spectrophotometer (U-2000) in the range 200 to 600 nm.

The light intensity m the continuous photolysis experiments were determined at the particular wavelength using ferrioxalate actinometr/. Continuous photolysis experiments were carried out using Pen-ray lamps (Ultraviolet Products) of 254 nm and 365 nm wavelength.

The quantum yields for cobalt(II) ion complexes were determined by the Kitson's method6

. The flash photolysis experiments were carried out using conventional flash photolysis equipment (KN020 -Applied Photo physics, U.K) .

Results and discussion The absorption spectra of the complexes are shown

in Fig. I, which indicate low energy ligand field bands with low intensity. In the case of complex 1, the ligand field bands are well separated from the charge transfer bands since the cobal t(III)-chloride charge transfer to metal (CTTM) transition occurs at high energy. However, in the case of complex 2, the CTTM transition occurs at low energy overlapping with the ligand field transition as the nitro li gand is more easi ly oxidisable as compared to the chloride ligand.

Quantum yield measurements and excited state deactivation pathways for cobalt(III) - amine complexes

In the present investigation, photolysis of the [Co(tetraen)N02]2+ complex was carried out by irradiating in the charge transfer band region. Cobalt(II) ion was found to be a product on irradiation with 254 nm and 365 nm radiation . Photolysis of the complex leads to the reduction of the metal centre and the corresponding oxidation of the easily oxidisable nitro ligand.

[Co (tetraen) N02] 2+ -----I~~[Co(tetraen)]2+ + N02

The high oxidation number of the central metal ion and the reducing nature of co-ordinated acido ligands in the cobalt(III)-amine complexes cause the complexes to undergo photo redox reactions from the CTTM excited state. The redox reactions usually involve electron transfer from the ligand to the central metal followed by the decomposition of the unstable cobalt(II)-amine complex to the aquo complex.

4.5

4.0

"'"' 0 3.5 .s: ·c ~ 3.0

a

'" .0

'" a 2.5 (5 g

2.0 b/)

.!2

1.5

<\ ..... .. ~ b 'V . \

150 200 250 300 350 400 450 500 550 600 650

Wavelength (om)

Fig. I-Electronic absorption spectrum of the cobalt (lll) all1ines in 0.1 M HCI04 • [(a, ....... [Co (tetraen) N02](CI04h; b-- [Co (tetraen) CI](CI04h] .

Table I--Quantum yield for the formation of cobalt (11 ) ion on continuolls photolysis"

Complex So lvent

[Co(tetraen)N02] 2+ O.IM HCI04

Methanol Acetonitrile

[Co(tetraen)CI] 2+ 0.IMHCI04

Methanol Acetonitrile

Wavelength of irradi ation. nm

254 365

0.09 0.04 0.02 0.02 0.04 0.02

0.01 0.04 0.03

"The values are average of four irradiations

The quantum yields for the formation of cobalt(II) ion were measured by irradiating the complex in 0.1 M HC104, acetonitrile or methanol as solvent and the results are shown in Table 1. It was found that the quantum yield of cobalt(I1) was high in 0.1 M HCI04

as compared to the other solvents. Increase in the quantum yield is due to the fact that at low pH the intermediate cobalt(II)-amine complex, which is labile, undergoes rapid aquation to produce cobaltous ion. On the other hand in non-aqueous solvents the cobalt(II)-amine produced in photolysis reacts with the dissolved oxygen to revert back to the higher oxidation state before aquation of the reduced metal ion takes place.

The quantum yield for the formation of cobalt(II) from [Co(tetraen)N02 f+ ion was found to be less

78 INDIAN J CHEM, SEC A, JANUARY 2005

than that for [Co(NH3)sN02f+ ion and [Co(trien)(N02ht ion at 254 nm; the values are 0.10, 0.51 and 0.11 respectively for these complexes. This decrease in quantum yield is presumably due to increase in the size of the amine chelate, which stabilizes the cobalt(II) complex. The decrease in quantum yield with increase in the wavelength of the radiation has been observed for other similar complexes as well; for [Co(NH3)sN02f+, <p Co(ll) was found to be 0.51, 0.41, 0.31 and 0.12 for irradiation at 254,313,365 and 442 nm respectively 7 .

Adamson4.7 proposed the radical pair model for the

photoreaction of the cobalt complexes. According to the model, {COli L,X O} and {COli LS,X O} are the geminate and the solvent separated radical species and /). is the energy difference between the energy of the absorbed photons and the minimum energy required to cleave the COII1_X- bond. The extra energy is suggested to appear as the kinetic energy to form the secondary radical species.

Quantum yields for the redox reaction yielding cobalt(II) for the chelates investigated are an order of magnitude lower as compared to the ammine complex. Chelation of the cobalt(III) complex renders the cobaJt(II) formed in photolysis less labile which facilitates the recombination of the geminate radicals to give back the parent complex or the linkage isomer additionally in the case of the complex 2. Indeed earlier investigation also indicates lowering of photoredox quantum yield for chelates of cobalt(III) ion8

.

The nitro ligand is unique as it has two binding modes- via nitrogen and via oxygen. On photolysis,

CT·-~~---~,--~ , , , 1 ,~",---;-__ 3CT"

: : ~ ........ I Solvent~ ..... 1. I • .. .. , 2+

~:,~'_.'-'/_...J..~_\_ ,7'~""'11 Co.q

~·-g-L-[L..I-CO-!lU~'--O-2'- \ ~ '\ [LCo", + N02

--[L~OlaONO) 1 [Lcol~O- )

1 2

----...... Photo-excitation Products

- - - - - - - - - - - - - - - .. Relaxation(Photophysical)

Scheme I

the isomerisation of the nitro isomer to the nitrite also occurs to some extent7

• The photoredox and the photoisomerisation reactions have been found to be competitively formed from a common precursor. This precursor has been postulated to be the radical species, which dissociates to form redox products or combines to give the linkage isomer.

The quantum yield for linkage isomerisation was very low (<p < 1 0-3) as compared to that for the cobalt(II) as the nitrito isomer is kinetically labile and thermodynamically also not very stable reverting back to the stable nitro complex in solution.

A generalized Scheme 1 is proposed for the excited state processes of these complexes.

Oxygenation studies on flash photolysis Cobalt(II)-amine chelates are comparatively long­

lived species8 in the non-aqueous solvents and interact on flash photolysis with the dissolved molecul ar oxygen present in the solution to give the Il-superoxo complexes.

Oxygenation reactions of 1 and 2 were investigated particularly to understand the role of photo-oxidized ligand in solution. Photo-redox reactions of the complex produce, the same cobalt(H) amine in both cases and the kinetics of the oxygenation reaction would indicate the role of the photo-oxidized ligand if any on the oxygenation reaction .

Conventional flash photolysis of the complex in acetonitrile and methanol showed a growth in absorbance in the region 400-420 mn. In deaerated solution and in acidic aqueous solution no such transient is seen indicating that the dissolved oxygen has not reacted with the cobalt(II)-amine complex formed in the photolysis reaction. Cobalt(lI) bound to ammonia ligand is kinetically unstable and in the acidic aqueous medium gets converted rapidly to cobalt(lI) aquo ion in microsecond timescale before any substitution or oxygenation reaction can occur. In the case of amine chelates however, in non-aqueous medium the cobalt(I1)- amine complex is able to react with molecular oxygen to give the oxygenated cobalt(III) species as the chelate stabilizes the complex in the time scale investigated. It is known from the literatures that the oxygenated cobalt(lIl) species exists as the mononuclear superoxo, binuclear superoxo or the binuclear peroxo complexes5

. The transient formed is attributed to the reaction between the photochemically produced cobalt:(Il) complex and the dissolved oxygen to give mononuclear superoxo

NOTES 79

[LCoD] + (h ---. [LCo W_ (h 1

Solvent [LCo w-02- -Cow L] -. [LCo W _(h-2-Co W L]

Scheme 2

0.20

0.15

~~1 1'~ S' ,2 .& • - -. -3 , .... L....,._-__ -_--1

!!l ° '0

-0 >

time (sec)

0.10

0.05

!O 15 20 25 30 35

Time (sec)

Fig. 2--Conventional flash photolysis of [Co(tetraen)N02t 2 ion in air-equilibrated acetonitrile solution at 20°e. [Decay of the transient (400 nm)) . Inset: kinetic analysis of the decay)

cobalt(III) species9, which has absorption maxima at

400 and 520 nm. The absorption spectra of the transient recorded

two secs after the flash showed an absorption maxima at around 700 nm. This was attributed to the formation of a binuclear Il-superoxo complex by the reaction of the mononuclear superoxo complex with the starting cobalt(IIJ)-complex2

.,o.

The rate of dimerisation was found to be low as compared to the other simpler amines because of the chelate formation by the bulky tetraen, which leads to the stabi lity of the mononuclear species. The binuclear Il-superoxo complex also decays to form the Il-peroxo complex (see Scheme 2). This decay was studied at 400 nm. The plot of log of the difference in change in absorbance versus time was linear, showing that the reaction is first order (Fig. 2 Inset). Thus, it is concluded that the solvent itself at room temperature

Table 2-Flash photolysis of the cobalt (III) amine chelates. Rate constants (k, S-I) for the decay of transients at a monitoring wavelength of 420 nm and temp 20°e.

Complex k, S- I in

Methanol Acetonitrile

[Co(tetraen)NOz)z+ 0.44 0.22

[Co(tetraen)CI)z+ 0.07 0.06 0.22 0 .25

brought about this redox reaction, with very low redox potential.

In the case of the complex 2, the rate constant for the oxygenation reaction does not seem to be affected by the solvent while in the case of the complex 1, the rate constants are presumably affected for the oxygenation reaction in different solvents as seen in Table 2.

Acknowledgement We are grateful to Prof P Natarajan, Director,

National Centre for Ultra Fast Processes, Taramani Campus, Madras University , Chennai, for providing the necessary fac ilities to carry out the flash photolysis studies in the centre_

References I Wilkins R G, Kinetics and Mechanism of Reactions of

Transition Metal Complexes, (VCH Publishers, New York), 2nd Edn, 1993.

2 Dhanasekaran T & Natarajan P, JAm Chem Soc, 114 ( 1992) 4621 .

3 Prakash H & Natarajan P, Res Chem Int, 29(4) (2003) 349. 4 Horvath 0 & Stevenson K L, Charge Transfer

Photochemistry of Coordination Compound.\·, (VCH Publishers, New York), 1993.

5 Ramamurthy P, Radhakrishnan A, Vijayraghavan R & Natarajan P, Indian J Chem, 29A (1990) I.

6 Dhanasekaran T, Photochemistry of Some Coba/t(lIl) and Nickel( 1/) Coordination Compounds and the Reactions of the Photoprodllcts with MoleCll/ar O~\ygen, Ph.D Thesis, University of Madras, (1995).

7 Balzani V, Sabbatini N & Scandola F, Chem Rev. 86 (1986). 3 19.

8 Pina F, Maestri M, Ballardini R, Mullazani Q G, Angelantonio M D & Balzani V. Inorg Chelll, 25 (1986). 4249 .

9 Natarajan P & Raghavan NY, J Am Chelll Soc. 102 ( 1980) 4518.

10 Parkanyi C, Yeh huang L, Chu S & Jeffries A T. Czech Chem Comlll. 6 1 (3) (1996).342.