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Mixed-ligand nickel(II) complexes of 2,2′-bipyridine and 1,10-phenanthroline with 3,4-dimercaptotoluene as quenchers of 1O2
Journal
of Photochemistry
and Photobiology,
A:
Chemistry,
48 (1989)
249
- 257
249
MIXED-LIGAND NICKEL( II) COMPLEXES OF 2,2’-BIPYRIDINE AND l,lO-PHENANTHROLINE WITH 3,4_DIMERCAPTOTOLUENE AS QUENCHERS OF ‘02 S. SHUKLA
and T. S. SRIVASTAVAT
Department (India)
of Chemistry,
Received
November
8,1988;
Indian
Institute
in revised
of Technology,
form
February
Powai, Bombay
400 076
15,1989)
summary Mixed-ligand complexes of nickel(I1) with 2,2’-bipyridine (bipy) and 3,4dimercaptotoluene (DMT) and with l,lO-phenanthroline (phen) and 3,4dimercaptotoluene were prepared and characterized. These complexes give sharp ‘H nuclear magnetic resonance (NMR) spectra in dimethyl-d, sulphoxide which suggests that they are diamagnetic and square planar. They show five absorption bands in the region 250 - 600 nm. One of these bands shows a bathochromic shift of about 50 nm when the solvent is changed from CH,OH to CHC13. They also act as physical quenchers of the photo-oxidation of 2,2,6,6-tetramethyl-4-piperidinol sensitized by haematoporphyrin IX with quenching rate constants of 6.8 X lo9 and 1.1 X 10” M-’ s-l for [Ni(bipy)(DMT)] and [Ni(phen)(DMT)] respectively. These values approach the quenching rate constant (1.1 X 10” M-’ s-l) of a diamagnetic square planar complex of bis( di-n-butyldithiocarbamato)nickel(II) which is a singlet molecular oxygen quencher.
1. Introduction Nickel(I1) chelates act as quenchers of singlet molecular oxygen ( ‘02) [l - 63. Diamagnetic square planar nickel(I1) complexes are very efficient quenchers of ‘02 with efficiencies approaching that of a diffusion-controlled process. In these nickel(I1) complexes the best quenchers have the nickel atom coordinated to four sulphur atoms and throughout the diamagnetic group the order of efficiency is S4 > N4 > N202 [5]. These diamagnetic square planar nickel(I1) complexes show much higher efficiency than the paramagnetic nickel(I1) complexes of tetrahedral geometry [ 3 - 51. This work describes the preparation and characterization of diamagnetic square planar nickel(I1) complexes of the type [Ni(NN)(DMT)] (where TAuthor
to whom
lOlO-6030/89/$3.50
correspondence
should
be addressed.
@ Elsevier
Sequoia/Printed
in The Netherlands
250
NN is 2,2’-bipyridine or l,lO-phenanthroline and DMT is the dianion of 3,4dimercaptotoluene). The quenching efficiencies of these complexes in dimethylformamide (DMF) were obtained using haematoporphyrin IX as photosensitizer and 2,2,6,6-tetramethyl-4-piperidinol as a chemical trap of lo*.
2. Experimental details
2.1. Materials [Ni(bipy)(DMT)] (where bipy is 2,2’-bipyridine) was prepared as follows [7]. Nickel chloride hexahydrate (1 mmol) and 3,4dimercaptotoluene (2 mmol) were refluxed in water for 90 min, when black crystals of Ni(DMT)2 were obtained. These crystals were filtered, recrystallized from chloroform and dried in a vacuum desiccator over anhydrous calcium chloride. Ni(DMT)* (1 mmol) and 2,2’-bipyridine (1 mmol) were then refluxed in 50 ml of toluene for 90 min, when black-purple crystals of [WbiwNDM’Ql were obtained. They were filtered, recrystallized from chloroform, and dried in a vacuum desiccator over anhydrous calcium chloride. Analysis: calculated for C1,Hi4N2NiSZ: C, 55.33%; H, 3.80%; N, 7.59%; found: C, 54.97%; H, 4.28%; N, 7.20%. [ Ni(phen)( DMT)] (where phen is 1 ,lO-phenanthroline) was prepared following the procedure given for [Ni(bipy)(DMT)] using l,lO-phenanthroline instead of 2,2’-bipyridine. Analysis: calculated for C i9Hi4N2NiS2: C, 58.06%; H, 3.56%; N, 7.13%; found: C, 58.55%; H, 3.61%; N, 6.79%. Bis(di-n-butyldithiocarbamato)nickel(II) was prepared by the method given in ref. 8. Nickel chloride hexahydrate, 2,2’-bipyridine and l,lO-phenanthroline monohydrate were obtained from Glaxo (India). 2,2,6,6-Tetramethyl-4piperidinol and 3,4dimercaptotoluene were obtained from Fluka (Switzerland). Haematoporphyrin IX was obtained from Sigma (U.S.A.). All compounds were used as received. Other chemical used were of analytical grade. The reagent grade solvents were purified before use by the standard procedures [ 91.
2.2. Measurements
The physical methods used have been described elsewhere [ 10 - 121.
2.3. Irradiation
procedures
The irradiation of solutions was carried out on a merry-go-round apparatus as described previously [ 10,111.
251
3. Results 3.1. Characterization of [Ni(bipy)(DMT) J and [Ni(phen)(DMT)] The chemical analyses of the nickel(U) complexes suggest that they are mixed-&and complexes containing diimine as one ligand and 3,4dimercaptotoluene as the other ligand. The low molar conductance values of 8.5 and 8.1 cm2 52-l for [Ni(bipy)(DMT)] and [Ni(phen)(DMT)] respectively in DMF (10e3 M) suggest that they are non-electrolytes [13]. The IR spectra of these nickel(H) complexes show that the characteristic bands of 2,2’-bipyridine and l,lO-phenanthroline are shifted to higher frequencies on coordination. The v(S-H) vibration observed in free 3,4-dimercaptotoluene at 2540 cm-’ is not present in the nickel(I1) complexes, suggesting the binding of both sulphur atoms of the ligand to nickel(I1) [14]. The ‘H nuclear magnetic resonance (NMR) signals of both ligands in [Ni(NN)(DMT)] in (CD3)#0 are observed and they can be interpreted by following the peaks assigned for [Pt(bipy)(DMT)] [ 151. The integrated areas obtained from the protons of the diimine ligand and from the protons of the 3,4-dimercaptotoluene are in the desired ratio for the complex. The sharp ‘H NMR signals suggest that the complexes are diamagnetic with a square planar geometry. The electronic absorption spectra of the mixed-ligand nickel(I1) complexes in DMF show five bands as shown in Table 1. The four bands in the region 260 - 364 nm have been assigned on the basis of the assignments for [M(NN)ClJ (where M2+ = PdZf and Pt2+) [16,17]. Thus bands 2 and 4 given in Table 1 are tentatively assigned to charge-transfer transitions from TABLE
1
Electronic absorption 1, lo-phenanthroline)
Complex
spectral data of in various solvents
Band maxima
[Ni(NN)(DMT)]
( w h ere NN is 2,2’-bipyridine
(nm)
Band 1
Band 2
Band 3
Band 4
Band 5
542 (0.46)a 532 520 570
360 sh
308 (2.13)
270 (2.96)
260 sh
550 (0.49) 531 524 577
364 (0.78)
315 sh
294 sh
268 (3.19)
[NWpy)WWI In DMF In DMSO In CH30H In CHCIJ [ Ni( phen)( DMT)] In DMF In DMSO In CH30H In CHC13
or
DMF, dimethylformamide; DMSO, dimethyl sulphoxide; sh, shoulder. aExtinction coefficients (1 mol-’ cm-’ x 1 OT4) are given in parentheses.
252
the metal d orbital to the K antibonding orbitals of the diimine ligands. Bands 3 and 5 are tentatively assigned to intraligand n-r* transitions of the diimine ligands. In these complexes, an additional strong band (band 1) is observed in the visible region. This band is highly solvent dependent and shows a bathochromic shift with a decrease in polarity of the solvent as shown in Table 1. This band is attributed to a charge-transfer transition from the highest occupied molecular orbital of the 3,4dimercaptotoluene dianion to the lowest unoccupied molecular orbital of the diimine ligand via the metal [7,18]. 3.2. Photo-oxidation of 2,2,6,6-tetramethyl-4-piperidinol sensitized by haematoporphyrin IX: detemtination of the rate constant of chemical quenching of ‘02 Molecular-oxygen-saturated DMF solutions containing a constant concentration of haematoporphyrin IX (5 X 10d5 M) and various concentrations of 2,2,6,6-tetramethyl-4-piperidinol (NH) in the range 10-l - 2 X lop3 M were irradiated with light of wavelength 300 - 800 nm (see Section 2.3) for 1 h. The amount of nitroxide radical (NO) produced in each case was measured by the electron paramagnetic resonance (EPR) method [lo, 121. Values of 1/ [NO] were then plotted against 1/ [NH] as shown in Fig. 1. The slope and intercept were obtained from this plot. The rate constant k, of chemical quenching of ‘02 was calculated using eqn. (2) (see Section 4) [ 19,201, taking the rate constant lzd of quenching of ‘02 by DMF molecules as 1.4 X lo5 s-l [3]. The 12,value obtained is 5.0 X lo7 M-l s-l.
01 0 100
200 [NH]-'
300
400
500
600
(M-')-
Fig. 1. Plot of the reciprocal concentration of nitroxide radical produced us. the reciprocal concentration of 2,2,6,6-tetramethyl-4-piperidinol (NH) using haematoporphyrin IX (5 x 1 O-’ M) as photosensitizer.
253
3.3. Determination of the quenching rate constant for [Ni(bipy)(DMT)], [Ni(phen)(DMT)] and bis(di-n-butyldithiocarbamato)nickel(II) Molecular-oxygen-saturated DMF solutions containing constant concentrations of haematoporphyrin IX (2 X 10ms M) and two concentrations of [Ni(bipy)(DMT)] (10e4 M or lo-’ M) with varying concentrations of 2,2,6,6tetramethyl-4-piperidinol (10-l - 2 X 1O-3 M) were irradiated with light of wavelength 300 - 800 nm for 1 h. The amount of nitroxide radical produced in each case was measured by the EPR method [ 10,121. Values of l/[ NO] were then plotted against l/[NH] for the two different concentrations of [Ni(bipy)(DMT)] (10e4 M and lo-’ M). Two linear plots were obtained, which meet at one point on the y axis as shown in Fig. 2. The values of the of [Niquenching rate constant k, for 10d4 M and lo-’ M concentrations (bipy)(DMT)] were calculated using eqn. (4) (see Section 4) [20]. The values are 8.5 X lo9 and 5.1 X lo9 M-’ s-l respectively. The average k, value is 6.8 X lo9 M-’ s-l. The experiments described above were repeated using [Ni(phen)(DMT)] instead of [Ni(bipy)(DMT)]. The linear plots of l/[NO] us. l/[NH] for 10e4 M and lops M concentrations of [Ni(phen)(DMT)] are given in Fig. 3. The values of k, for 10m4 M and lops M concentrations of [Ni(phen)-
160
/ (b)
t
140
120
t
t “:
I 100
0
x
00
‘I ‘;
I-61
60
L5 40
20
.b
200
400 [NH]-'
600 CM-')--
800
1000
‘(0)
/
X
L
0
/
I
t
0r 0
I 200
1 400
I
600
I 800
I 1ooc
[NH]-'CM-')--
Fig. 2. Plots of the reciprocal concentration of nitroxide radical produced us. the reciprocal concentration of NH in the presence of lo-’ M (a) and 10m4 M (b) concentrations of [ Ni( bipy)( DMT)]. Fig. 3. Plots of the reciprocal concentration of nitroxide radical produced us. the reciprocal concentration of NH in the presence of lob5 M (a) and 10v4 M (b) concentrations of [Ni(phen)(DMT)].
254
0, 0
200
LOO
600
800
(M-l) [NH]-’ Fig. 4. Plot of the reciprocal concentration of nitroxide radical produced us. the reciprocal concentration of NH in the presence of 10v6 M concentration of bis(di-n-butyldithiocarbamato)nickel(II).
(DMT)] are 1.2 X 10” and 1.1 X 10” MB1 s-i respectively. The average value of lz, is 1.1 X 1Oro M-r s-l. The experiments were also repeated using bis(di-n-butyldithiocarbamato)nickel(II). A linear plot of l/[NO] us. l/[NH] was obtained for the nickel complex (10e6 M) (see Fig. 4). The value of Iz, obtained from this plot is 1.1 X 10” M-’ s-‘. This value is comparable with the reported value of 1.7 X ~O’“M-‘S-~ in isooctane [3].
4. Discussion The rate constant 12, of production of nitroxide radicals in the reaction of 2,2,6,6-tetramethyl-4-piperidinol with ‘02 in ethanol is known [19]. However, the rate constant of production of nitroxide radicals in this reaction in DMF is not known and thus has been determined for the first time. The mechanism and the kinetic equation derived on the basis of this mechanism [ 191 for the photo-oxidation of 2,2,6;6-tetramethyl-4-piperidinol using haematoporphyrin IX as photosensitizer are given below. hv
s-
‘S
255
kr
‘02 + NH -
‘O,+Q
HNO:, -
NO
kcl
-302+Q
S is haematoporphyrin IX as photosensitizer (its first excited state is ‘S and its first excited triplet state is 3S), hv is the energy of a photon, ISC is the intersystem crossing, ET is the energy transfer, kd is the rate constant of quenching of ‘02 by DMF solvent molecules, k, is the rate constant of chemical quenching of IO2 by reduced spin label 2,2,6,6-tetramethyl-4piperidinol (NH) to produce nitroxide radical (NO) and k, is the rate constant of physical quenching of ‘02 by the nickel(I1) complex (Q). The amount of nitroxide radical produced is given [ 19,201 as
(1)
&j=&jjl+k&)
In eqn. (1) the amount of nitroxide radical produced will be proportional to [NH] at a limiting value of k, [NH]. In our experiments the intensity of light and the quantum yield of triplet formation of haematoporphyrin IX are constant, and therefore a plot of l/[NO] (or l/(d[NO]dt)) us. l/[NH] is linear as shown in Fig. 1. The ratio between the slope and the intercept of this linear plot is given [19,20] as 12, =
kd
(2)
Taking the rate constant kd of quenching of ‘02 in DMF solvent as 1.4 X lo5 s-l [3], the value of k, in DMF is calculated as 5.0 X 10’ M-l s-’ (see Section 3.2). If the last step in parentheses is also included in the mechanism, the amount of nitroxide radical produced in the presence of nickel(I1) complexes is given [20] as
-=_1 [NOI
1
[‘O,l
(3)
When l/[NO] (or l/(d[NO]/dt)) is plotted us. l/[NH], a linear p.lot is obtained as shown in Figs. 2 and 3. The slope obtained from this linear plot is given as
256 k, =
slope intercept
k, [&3 -
k, c&l
(4)
Taking k, = 1.4 X 10s se1 and k, = 5.0 X 10’ M-’ s-l, k, can be calculated from eqn. (4). The k, values thus calculated for [Ni(phen)(DMT)], [Ni(bipy)(DMT)] and bis(di-n-butyldithiocarbamato)nickel(II) are 1.1 X lOlo, 6.8 X lo9 and 1.1 X 10” M-’ s-’ respectively (see Section 3.3). These k, values suggest that the nickel(D) complexes quench ‘02 with an efficiency approaching that of a diffusion-controlled process. The physical quenching of ‘02 by the nickel(I1) complexes is supported by the unchanged intensity of the absorption spectra of the nickel(I1) complexes in the UV and visible regions after irradiation of their DMF solutions for 1 h with light of wavelength 300 - 800 nm in the presence of haematoporphyrin IX as monitored by difference spectroscopy. The physical quenching of IO2 in the photooxidation reaction is further supported by the data in Figs. 2 and 3. In these figures two linear plots with different slopes meeting at one point on the y axis are obtained for two different concentrations of the nickel complex and this rules out the radical mechanism [ 31. The physical quenching of ‘0, by the nickel(I1) complexes seems to involve a spin-allowed energy transfer to produce a low-lying triplet ligand field state of the complexes [21,22]. 2,2,6,6-Tetramethyl-4-piperidinol acts as a specific chemical quencher of IO2 and does not react with the superoxide ion, hydrogen peroxide or hydroxyl radicals [12,23]. The variation in the k, values for [Ni(bipy)(DMT)] and [Ni(phen)(DMT)] suggests that the size of the diimine ligand does not have much effect. The square planar geometry of the nickel(I1) complexes seems to have a major controlling effect, In this geometry ‘02 has an unhindered approach to the nickel atom from above or below the plane formed by the coordinating atoms and thus can easily interact with an orbital perpendicular to this plane [5]. The factors which determine the relative efficiency of a complex in quenching ‘02 are less obvious [ 53. The best quenchers have a nickel atom coordinated to four sulphur atoms in a diamagnetic square planar complex. The present work on square planar nickel complexes containing a nickel atom coordinated to two nitrogen and two sulphur atoms suggests that these complexes approach the efficiency of nickel complexes containing a nickel atom coordinated to four sulphur atoms in the quenching of ‘Oz. Thus the order of efficiency throughout the diamagnetic group is S4 > N,S2 > N4 > N202.
5. Conclusions This study suggests that [Ni(bipy)(DMT)] and [Ni(phen)(DMT)] can act as physical quenchers of ‘Oz. These diamagnetic square planar complexes which have a nickel atom coordinated to two nitrogen and two sulphur
257
atoms can approach the ‘02 quenching ability of the best quenchers have a nickel atom coordinated to four sulphur atoms.
which
Acknowledgment We thank the DST research.
(Government
of India) for financial
support of this
References 1 D. J. Carlsson,
Sot.,
94 (1972)
G.
D. Mendenhall,
T. Suprunchuk
and
D. M. Wiles,
J. Am.
Chem.
8960.
2 D. J. Car&on, 3 4
5 6 7 8 9 10
T. Suprunchuk and D. M. Wiles, Can. J. Chem., 52 (1974) 3728. D. Bellus, in B. Ranby and J. F. Rabek (eds.), Singlet Oxygen: Reaction with Organic Wiley-Interscience, New York, 1978, pp. 68 - 110. Compounds and Polymers, H. Furue and K. E. Russell, Can. J. Chem., 56 (1978) 1595. B. M. Monroe and J. J. Mrowca, J. Phys. Chem., 83 (1979) 591. M. Botsivali, D. F. Evans, P. H. Missen and M. W. Upton, J. Chem. Sot., Dalton Trans., (1985) 1147. T. R. Miller and I. G. Dance, J. Am. Chem. Sot., 95 (1973) 6970. R. Kellner and G. S. Nikolov, J. Inorg. Nucl. Chem., 43 (1981) 1183. A. I. Vogel, A Textbook of Practical Organic Chemistry, Longman, London, 1956, 3rd edn., pp, 163 - 179. S. Shukla, S. S. Kamath and T. S. Srivastava, J. Photochem. Photobiol., A: Chem., 44
(1988) 11 12 13 14 15 16
143.
S. Shukla, S. S. Kamath and T. S. Srivastava, J. Photochem. PhotobioL, A: Chem., 47 (1989) 287. Y. Lion, E. Gardin and A. Van de T;orst, Photochem. Photobiol., 31 (1980) 305. W. J. Geary, Coord. Chem. Rev., 7 (1971) 81. K. Nakamoto, Infrared Specfra and Inorganic and Coordination Compounds, WileyInterscience, New York, 1963, pp. 189 - 220. L. Kumar, K. H. Puthraya and T. S. Srivastava, Inorg. Chim. Acta, 86 (1984) 173. P. M. Gidney, R. D. Gillard and B. T. Heaton, J. Chem. SOL, Dalton Trans., (1973)
132. 17 18 19
20 21 22 23
L. N. Minochkina, V. I. Fadeeva and N. B. Zorob, Anal. Lett., 12 (1979) 451. 23 (1984) 506. A. Vogler, H. Kunkely, J. Hlavatsh and A. Merz, fnorg. Chem.. S. Cannistraro, G. Jori and A. Van de Vorst, Photochem. Photobiol., 27 (1978) 517. A. Zweig and W. A. Henderson, Jr., J. PoIym. Sci., Polym. Chem. Ed., I1 (1975) 717. C. S. Foote, in H. H. Wasserman and R. W. Jurray (eds.), Singlet Oxygen, Academic Press, New York, 1979, pp. 160 - 161. D. F. Evan, J. Chem. Sot., Chem. Commun., (1980) 1134. A. Rigo, E. Argese, R. Stevanto, E. F. Orsega and P. Viglino, Znorg. Chem. Acta, 24 (1977) 171.
of Photochemistry
and Photobiology,
A:
Chemistry,
48 (1989)
249
- 257
249
MIXED-LIGAND NICKEL( II) COMPLEXES OF 2,2’-BIPYRIDINE AND l,lO-PHENANTHROLINE WITH 3,4_DIMERCAPTOTOLUENE AS QUENCHERS OF ‘02 S. SHUKLA
and T. S. SRIVASTAVAT
Department (India)
of Chemistry,
Received
November
8,1988;
Indian
Institute
in revised
of Technology,
form
February
Powai, Bombay
400 076
15,1989)
summary Mixed-ligand complexes of nickel(I1) with 2,2’-bipyridine (bipy) and 3,4dimercaptotoluene (DMT) and with l,lO-phenanthroline (phen) and 3,4dimercaptotoluene were prepared and characterized. These complexes give sharp ‘H nuclear magnetic resonance (NMR) spectra in dimethyl-d, sulphoxide which suggests that they are diamagnetic and square planar. They show five absorption bands in the region 250 - 600 nm. One of these bands shows a bathochromic shift of about 50 nm when the solvent is changed from CH,OH to CHC13. They also act as physical quenchers of the photo-oxidation of 2,2,6,6-tetramethyl-4-piperidinol sensitized by haematoporphyrin IX with quenching rate constants of 6.8 X lo9 and 1.1 X 10” M-’ s-l for [Ni(bipy)(DMT)] and [Ni(phen)(DMT)] respectively. These values approach the quenching rate constant (1.1 X 10” M-’ s-l) of a diamagnetic square planar complex of bis( di-n-butyldithiocarbamato)nickel(II) which is a singlet molecular oxygen quencher.
1. Introduction Nickel(I1) chelates act as quenchers of singlet molecular oxygen ( ‘02) [l - 63. Diamagnetic square planar nickel(I1) complexes are very efficient quenchers of ‘02 with efficiencies approaching that of a diffusion-controlled process. In these nickel(I1) complexes the best quenchers have the nickel atom coordinated to four sulphur atoms and throughout the diamagnetic group the order of efficiency is S4 > N4 > N202 [5]. These diamagnetic square planar nickel(I1) complexes show much higher efficiency than the paramagnetic nickel(I1) complexes of tetrahedral geometry [ 3 - 51. This work describes the preparation and characterization of diamagnetic square planar nickel(I1) complexes of the type [Ni(NN)(DMT)] (where TAuthor
to whom
lOlO-6030/89/$3.50
correspondence
should
be addressed.
@ Elsevier
Sequoia/Printed
in The Netherlands
250
NN is 2,2’-bipyridine or l,lO-phenanthroline and DMT is the dianion of 3,4dimercaptotoluene). The quenching efficiencies of these complexes in dimethylformamide (DMF) were obtained using haematoporphyrin IX as photosensitizer and 2,2,6,6-tetramethyl-4-piperidinol as a chemical trap of lo*.
2. Experimental details
2.1. Materials [Ni(bipy)(DMT)] (where bipy is 2,2’-bipyridine) was prepared as follows [7]. Nickel chloride hexahydrate (1 mmol) and 3,4dimercaptotoluene (2 mmol) were refluxed in water for 90 min, when black crystals of Ni(DMT)2 were obtained. These crystals were filtered, recrystallized from chloroform and dried in a vacuum desiccator over anhydrous calcium chloride. Ni(DMT)* (1 mmol) and 2,2’-bipyridine (1 mmol) were then refluxed in 50 ml of toluene for 90 min, when black-purple crystals of [WbiwNDM’Ql were obtained. They were filtered, recrystallized from chloroform, and dried in a vacuum desiccator over anhydrous calcium chloride. Analysis: calculated for C1,Hi4N2NiSZ: C, 55.33%; H, 3.80%; N, 7.59%; found: C, 54.97%; H, 4.28%; N, 7.20%. [ Ni(phen)( DMT)] (where phen is 1 ,lO-phenanthroline) was prepared following the procedure given for [Ni(bipy)(DMT)] using l,lO-phenanthroline instead of 2,2’-bipyridine. Analysis: calculated for C i9Hi4N2NiS2: C, 58.06%; H, 3.56%; N, 7.13%; found: C, 58.55%; H, 3.61%; N, 6.79%. Bis(di-n-butyldithiocarbamato)nickel(II) was prepared by the method given in ref. 8. Nickel chloride hexahydrate, 2,2’-bipyridine and l,lO-phenanthroline monohydrate were obtained from Glaxo (India). 2,2,6,6-Tetramethyl-4piperidinol and 3,4dimercaptotoluene were obtained from Fluka (Switzerland). Haematoporphyrin IX was obtained from Sigma (U.S.A.). All compounds were used as received. Other chemical used were of analytical grade. The reagent grade solvents were purified before use by the standard procedures [ 91.
2.2. Measurements
The physical methods used have been described elsewhere [ 10 - 121.
2.3. Irradiation
procedures
The irradiation of solutions was carried out on a merry-go-round apparatus as described previously [ 10,111.
251
3. Results 3.1. Characterization of [Ni(bipy)(DMT) J and [Ni(phen)(DMT)] The chemical analyses of the nickel(U) complexes suggest that they are mixed-&and complexes containing diimine as one ligand and 3,4dimercaptotoluene as the other ligand. The low molar conductance values of 8.5 and 8.1 cm2 52-l for [Ni(bipy)(DMT)] and [Ni(phen)(DMT)] respectively in DMF (10e3 M) suggest that they are non-electrolytes [13]. The IR spectra of these nickel(H) complexes show that the characteristic bands of 2,2’-bipyridine and l,lO-phenanthroline are shifted to higher frequencies on coordination. The v(S-H) vibration observed in free 3,4-dimercaptotoluene at 2540 cm-’ is not present in the nickel(I1) complexes, suggesting the binding of both sulphur atoms of the ligand to nickel(I1) [14]. The ‘H nuclear magnetic resonance (NMR) signals of both ligands in [Ni(NN)(DMT)] in (CD3)#0 are observed and they can be interpreted by following the peaks assigned for [Pt(bipy)(DMT)] [ 151. The integrated areas obtained from the protons of the diimine ligand and from the protons of the 3,4-dimercaptotoluene are in the desired ratio for the complex. The sharp ‘H NMR signals suggest that the complexes are diamagnetic with a square planar geometry. The electronic absorption spectra of the mixed-ligand nickel(I1) complexes in DMF show five bands as shown in Table 1. The four bands in the region 260 - 364 nm have been assigned on the basis of the assignments for [M(NN)ClJ (where M2+ = PdZf and Pt2+) [16,17]. Thus bands 2 and 4 given in Table 1 are tentatively assigned to charge-transfer transitions from TABLE
1
Electronic absorption 1, lo-phenanthroline)
Complex
spectral data of in various solvents
Band maxima
[Ni(NN)(DMT)]
( w h ere NN is 2,2’-bipyridine
(nm)
Band 1
Band 2
Band 3
Band 4
Band 5
542 (0.46)a 532 520 570
360 sh
308 (2.13)
270 (2.96)
260 sh
550 (0.49) 531 524 577
364 (0.78)
315 sh
294 sh
268 (3.19)
[NWpy)WWI In DMF In DMSO In CH30H In CHCIJ [ Ni( phen)( DMT)] In DMF In DMSO In CH30H In CHC13
or
DMF, dimethylformamide; DMSO, dimethyl sulphoxide; sh, shoulder. aExtinction coefficients (1 mol-’ cm-’ x 1 OT4) are given in parentheses.
252
the metal d orbital to the K antibonding orbitals of the diimine ligands. Bands 3 and 5 are tentatively assigned to intraligand n-r* transitions of the diimine ligands. In these complexes, an additional strong band (band 1) is observed in the visible region. This band is highly solvent dependent and shows a bathochromic shift with a decrease in polarity of the solvent as shown in Table 1. This band is attributed to a charge-transfer transition from the highest occupied molecular orbital of the 3,4dimercaptotoluene dianion to the lowest unoccupied molecular orbital of the diimine ligand via the metal [7,18]. 3.2. Photo-oxidation of 2,2,6,6-tetramethyl-4-piperidinol sensitized by haematoporphyrin IX: detemtination of the rate constant of chemical quenching of ‘02 Molecular-oxygen-saturated DMF solutions containing a constant concentration of haematoporphyrin IX (5 X 10d5 M) and various concentrations of 2,2,6,6-tetramethyl-4-piperidinol (NH) in the range 10-l - 2 X lop3 M were irradiated with light of wavelength 300 - 800 nm (see Section 2.3) for 1 h. The amount of nitroxide radical (NO) produced in each case was measured by the electron paramagnetic resonance (EPR) method [lo, 121. Values of 1/ [NO] were then plotted against 1/ [NH] as shown in Fig. 1. The slope and intercept were obtained from this plot. The rate constant k, of chemical quenching of ‘02 was calculated using eqn. (2) (see Section 4) [ 19,201, taking the rate constant lzd of quenching of ‘02 by DMF molecules as 1.4 X lo5 s-l [3]. The 12,value obtained is 5.0 X lo7 M-l s-l.
01 0 100
200 [NH]-'
300
400
500
600
(M-')-
Fig. 1. Plot of the reciprocal concentration of nitroxide radical produced us. the reciprocal concentration of 2,2,6,6-tetramethyl-4-piperidinol (NH) using haematoporphyrin IX (5 x 1 O-’ M) as photosensitizer.
253
3.3. Determination of the quenching rate constant for [Ni(bipy)(DMT)], [Ni(phen)(DMT)] and bis(di-n-butyldithiocarbamato)nickel(II) Molecular-oxygen-saturated DMF solutions containing constant concentrations of haematoporphyrin IX (2 X 10ms M) and two concentrations of [Ni(bipy)(DMT)] (10e4 M or lo-’ M) with varying concentrations of 2,2,6,6tetramethyl-4-piperidinol (10-l - 2 X 1O-3 M) were irradiated with light of wavelength 300 - 800 nm for 1 h. The amount of nitroxide radical produced in each case was measured by the EPR method [ 10,121. Values of l/[ NO] were then plotted against l/[NH] for the two different concentrations of [Ni(bipy)(DMT)] (10e4 M and lo-’ M). Two linear plots were obtained, which meet at one point on the y axis as shown in Fig. 2. The values of the of [Niquenching rate constant k, for 10d4 M and lo-’ M concentrations (bipy)(DMT)] were calculated using eqn. (4) (see Section 4) [20]. The values are 8.5 X lo9 and 5.1 X lo9 M-’ s-l respectively. The average k, value is 6.8 X lo9 M-’ s-l. The experiments described above were repeated using [Ni(phen)(DMT)] instead of [Ni(bipy)(DMT)]. The linear plots of l/[NO] us. l/[NH] for 10e4 M and lops M concentrations of [Ni(phen)(DMT)] are given in Fig. 3. The values of k, for 10m4 M and lops M concentrations of [Ni(phen)-
160
/ (b)
t
140
120
t
t “:
I 100
0
x
00
‘I ‘;
I-61
60
L5 40
20
.b
200
400 [NH]-'
600 CM-')--
800
1000
‘(0)
/
X
L
0
/
I
t
0r 0
I 200
1 400
I
600
I 800
I 1ooc
[NH]-'CM-')--
Fig. 2. Plots of the reciprocal concentration of nitroxide radical produced us. the reciprocal concentration of NH in the presence of lo-’ M (a) and 10m4 M (b) concentrations of [ Ni( bipy)( DMT)]. Fig. 3. Plots of the reciprocal concentration of nitroxide radical produced us. the reciprocal concentration of NH in the presence of lob5 M (a) and 10v4 M (b) concentrations of [Ni(phen)(DMT)].
254
0, 0
200
LOO
600
800
(M-l) [NH]-’ Fig. 4. Plot of the reciprocal concentration of nitroxide radical produced us. the reciprocal concentration of NH in the presence of 10v6 M concentration of bis(di-n-butyldithiocarbamato)nickel(II).
(DMT)] are 1.2 X 10” and 1.1 X 10” MB1 s-i respectively. The average value of lz, is 1.1 X 1Oro M-r s-l. The experiments were also repeated using bis(di-n-butyldithiocarbamato)nickel(II). A linear plot of l/[NO] us. l/[NH] was obtained for the nickel complex (10e6 M) (see Fig. 4). The value of Iz, obtained from this plot is 1.1 X 10” M-’ s-‘. This value is comparable with the reported value of 1.7 X ~O’“M-‘S-~ in isooctane [3].
4. Discussion The rate constant 12, of production of nitroxide radicals in the reaction of 2,2,6,6-tetramethyl-4-piperidinol with ‘02 in ethanol is known [19]. However, the rate constant of production of nitroxide radicals in this reaction in DMF is not known and thus has been determined for the first time. The mechanism and the kinetic equation derived on the basis of this mechanism [ 191 for the photo-oxidation of 2,2,6;6-tetramethyl-4-piperidinol using haematoporphyrin IX as photosensitizer are given below. hv
s-
‘S
255
kr
‘02 + NH -
‘O,+Q
HNO:, -
NO
kcl
-302+Q
S is haematoporphyrin IX as photosensitizer (its first excited state is ‘S and its first excited triplet state is 3S), hv is the energy of a photon, ISC is the intersystem crossing, ET is the energy transfer, kd is the rate constant of quenching of ‘02 by DMF solvent molecules, k, is the rate constant of chemical quenching of IO2 by reduced spin label 2,2,6,6-tetramethyl-4piperidinol (NH) to produce nitroxide radical (NO) and k, is the rate constant of physical quenching of ‘02 by the nickel(I1) complex (Q). The amount of nitroxide radical produced is given [ 19,201 as
(1)
&j=&jjl+k&)
In eqn. (1) the amount of nitroxide radical produced will be proportional to [NH] at a limiting value of k, [NH]. In our experiments the intensity of light and the quantum yield of triplet formation of haematoporphyrin IX are constant, and therefore a plot of l/[NO] (or l/(d[NO]dt)) us. l/[NH] is linear as shown in Fig. 1. The ratio between the slope and the intercept of this linear plot is given [19,20] as 12, =
kd
(2)
Taking the rate constant kd of quenching of ‘02 in DMF solvent as 1.4 X lo5 s-l [3], the value of k, in DMF is calculated as 5.0 X 10’ M-l s-’ (see Section 3.2). If the last step in parentheses is also included in the mechanism, the amount of nitroxide radical produced in the presence of nickel(I1) complexes is given [20] as
-=_1 [NOI
1
[‘O,l
(3)
When l/[NO] (or l/(d[NO]/dt)) is plotted us. l/[NH], a linear p.lot is obtained as shown in Figs. 2 and 3. The slope obtained from this linear plot is given as
256 k, =
slope intercept
k, [&3 -
k, c&l
(4)
Taking k, = 1.4 X 10s se1 and k, = 5.0 X 10’ M-’ s-l, k, can be calculated from eqn. (4). The k, values thus calculated for [Ni(phen)(DMT)], [Ni(bipy)(DMT)] and bis(di-n-butyldithiocarbamato)nickel(II) are 1.1 X lOlo, 6.8 X lo9 and 1.1 X 10” M-’ s-’ respectively (see Section 3.3). These k, values suggest that the nickel(D) complexes quench ‘02 with an efficiency approaching that of a diffusion-controlled process. The physical quenching of ‘02 by the nickel(I1) complexes is supported by the unchanged intensity of the absorption spectra of the nickel(I1) complexes in the UV and visible regions after irradiation of their DMF solutions for 1 h with light of wavelength 300 - 800 nm in the presence of haematoporphyrin IX as monitored by difference spectroscopy. The physical quenching of IO2 in the photooxidation reaction is further supported by the data in Figs. 2 and 3. In these figures two linear plots with different slopes meeting at one point on the y axis are obtained for two different concentrations of the nickel complex and this rules out the radical mechanism [ 31. The physical quenching of ‘0, by the nickel(I1) complexes seems to involve a spin-allowed energy transfer to produce a low-lying triplet ligand field state of the complexes [21,22]. 2,2,6,6-Tetramethyl-4-piperidinol acts as a specific chemical quencher of IO2 and does not react with the superoxide ion, hydrogen peroxide or hydroxyl radicals [12,23]. The variation in the k, values for [Ni(bipy)(DMT)] and [Ni(phen)(DMT)] suggests that the size of the diimine ligand does not have much effect. The square planar geometry of the nickel(I1) complexes seems to have a major controlling effect, In this geometry ‘02 has an unhindered approach to the nickel atom from above or below the plane formed by the coordinating atoms and thus can easily interact with an orbital perpendicular to this plane [5]. The factors which determine the relative efficiency of a complex in quenching ‘02 are less obvious [ 53. The best quenchers have a nickel atom coordinated to four sulphur atoms in a diamagnetic square planar complex. The present work on square planar nickel complexes containing a nickel atom coordinated to two nitrogen and two sulphur atoms suggests that these complexes approach the efficiency of nickel complexes containing a nickel atom coordinated to four sulphur atoms in the quenching of ‘Oz. Thus the order of efficiency throughout the diamagnetic group is S4 > N,S2 > N4 > N202.
5. Conclusions This study suggests that [Ni(bipy)(DMT)] and [Ni(phen)(DMT)] can act as physical quenchers of ‘Oz. These diamagnetic square planar complexes which have a nickel atom coordinated to two nitrogen and two sulphur
257
atoms can approach the ‘02 quenching ability of the best quenchers have a nickel atom coordinated to four sulphur atoms.
which
Acknowledgment We thank the DST research.
(Government
of India) for financial
support of this
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