Indian Journal of Chemi stry Vol. 45A, February 2006, pp. 370-376

Synthesis and studies of ionic chelates of hafnocene with guanine

Ekta Malhotra, N K Kaushik* & H S Malhotra t

Department of Chemistry, University of Delhi , Delhi 110007, India

Email: dr_ebhatia@yahoo.com

Received 14 JGI/uGly 2005; revised 5 November 2005

Ioni c chelates of hafnocene with guanine (I) of the type [(rr'-C5H5hHfLtX' (II) rX = CuCl" ZnCI" CdClJ , HgCl" C6H5NHNHCS2; HL = guanine] have been sy nthesized. Spectral studies (JR, UV, IH NMR and I3C NMR) indicate that the guanine moiety acts as a bidentate group, binding to the hafn ium(IV ) ion through carbonyl oxygen at C(6) and deprotonation of N(7). Conductance measurements revea l that the compou nds are I: I electrolytes. Fluorescence studies have been carried out for the complex contai ning mercury in the anionic moiety and relevant photochemical p rameters have been elucidated . The thermal behaviour of the complexes has been studi ed by thermogravimetric and differenti al thermal analytical techniques. Thennodynamic parameters of the thermal decomposition process have been computed by the Coats-Redfern method and their variat ions have been correlated with some structural parameters of the COl plexes. The ligands, as well as their hafnium(lV) complexes, ex hibi t appreciable ant ibacterial and antifungal activity against £.coli. P.aemgillosa and Z.lI1obilis bacterial strains. and versus A.awalllori and A. lliger fungal strains, respectively.

IPC Code: Int.Cl 8 C07D473/00: C07F1108; C07F3/06; C07F3/08 ; C07F3110

Transition metals are essential for the normal functioning of living organisms. Therefore, it is not surpns1l1g that transition metal coordination compounds are of great interest as potential drugs. Many complexes, including the platinum group, have been synthesized and tested in a number of biological systems. With the discovery of strongly cell-mitosis­depressing, broad spectrum, inorganic anticancer agent, cisplatin (i.e., cis-dichlorodiamineplatinum) by Rosenberg et al. I , the use of metal coordination compounds in the search for new drugs for cancer treatment started to evolve and led to the discovery of interesting results2

.5. The interaction of metal ions

with nucleic acids and nucleic acid constituents has been actively studied in recent years6

.8. Thus, interest

in the metal complexes of guanine (2-amino-l,7-diydro-6H-purin-6-one) has arisen for the following reasons. Firstly, as a result of antitumor activity of some metal complexes, there is considerable interest in the design of model complexes involving purines which could mimic the interaction of metal ions with DNA. Secondly, growing interest in studies of metal­purine complexes relates to the bonding mode of the ligand to metal ion , as they possess divergent bonding sites. In 6-oxopurines, very frequently the exocyclic oxygen 0(6) interacts with hydrogen bond donors

Tpresent Address: Department of Chemistry, S.G.T.B. Khal sa Coll ege. University of Delhi , Delhi 110007, India

within the metal complexes9.IO

. However, coordination of 6-oxopurines by an N(7), 0(6) chelation mode, as in guanine, has been frequently postul ated II and widely suggested on the basis of a number of spectroscopic and physical criteria l 2

.

Recent studies on determination of kinetic parameters from thermal data l 3

.14 prompted us to

analyse the variation in thermal stability of some metal complexes (II) in terms of their structural parameters. The present work is a sequel to our investigations on metal ion .. biomolecule interaction 15. 17.

Materials and Methods Guanine (Aldrich) and hafnocene dichloride (A lfa

Products) were used without further purification . Chlorine and sulphur were determined grav imetrica lly as silver chloride and barium sulphate, respectively. Hafnium was estimated gravi metrically as Hf02. The bactericidal activities were evaluated by cup-agar diffusion method 18 while the fungi were cultured on Czapek Dox Nutrient medium 19.

Conductance measurements were made using I .5xlO·3 M solution of the complexes in nitrobenzene using Elico conductivity bridge, type CI\Il-82. IR and UV spectra were recorded on FTIR spectrometer Spectrum 2000 and Beckman DU-64 UV-vis spectrophotometers, respectively. The IH NMR and 13C NMR spectra were recorded on Bruker AC 300

MALHOTRA el ai.:SYNTHESIS AND STUDIES OF IONIC CHELATES OF HAFNOCENE WITH GUANINE 37 1

Table I- Physical and analytical data of guanine complexes

Complex Empirical formula Decamp. /\" Found (Calcd) % Temp.

(0C) (C = 1.5

X 10-3 M) C H Hf CI N S

[(T] S-CsHshHfLt[CuCI ,r (1) Cl sHI 4NsOHfCI,Cu 2 12

[(T]s-C;HshHfLnZnCI, r (2) ClsHI4NsOHfCI}Zn 225

[(T] s-CsHshHfLnCdCI3r (3) ClsHI4N50HfCI}Cd 226

[(T]s-CsHshHfLt[HgCI3r (4) CI sHI 4NsOHfCI3Hg

+

x

(II)

spectrometer and fluorescence spectral studies were recorded on Hitachi 650-660 spectrofluorometer. Rigaku Thermoflex, model PTC-IOA, Rigaku corporation, Japan , was used for simultaneous recording of TG-DT A curves of complexes containing metal salts as anionic species, in air at a heating rate of 10° min-I.

Synthesis of hafnocene complexes

[(lls -CsHshHfLtX- (1): X = CuCh; (2): X = ZnCb; (3): X = CdCI3; (4): X .= HgCh The metal complexes (II) were prepared by adding slowly, the solution of the metal salt [CuCh.2H20 (0.043 g, 0.25 mmol) ; ZnCI 2 (0.034 g, 0.25 mmol); CdCI2 (0.046 g, 0.25 mlUol) or HgCI2 (0.068 g, 0 .25 mmol)] in acetone

175

29.9 28.54 2.32 (28.66) (2.24)

27.6 28.75 2.30 (28.57) (2.24)

29.4 26.62 2.24 (26.59) (2.08)

28.2 23.62 1.99 (23.53) (1.84 )

26.7 41.48 3.24 (41. 15) (3.30)

27.38 (27.80)

16.77 11.25 (16.92) (11.14)

16.62 11.36 (16.87) (11.11)

15.56 10.05 (15.70) (10.34)

13.55 9.36 ( 13.89) (9 .15)

15.58 9.83 (15 .27) (9 .99)

(10 mL) to a stirred solution of 0.095 g (0.25 mmol)

of bis(lls-cyclopentadienyl) hafnium(IV) dichloride and 0.038 g (0.25 mmol) of guanine (HL) (I) in acetone (20 mL). The contents were stirred for 8 h and filtered. Volume of the filtrate was reduced to about one-fourth of the original and petroleum ether was added. The precipitated complexes were filtered, dried and crystallized from acetone by the addition of a few drops of petroleum ether.

For the synthesis of [(lls -CsHshHfLt

[C6HsNHNHCS2f (5), a mixture of 0.095 g (0.25 mmol) of bis(lls-cyclopentadienyl) hafnium(lV) dichloride and 0.038 g (0.25 mmol) of guanine (HL) (I ) in water (10 mL) was stirred for 8 h. This was followed by the addition of 0.055 g (0.25 mmol) of potassium phenyldithiocarbazate lS in a minimum quantity of water. Stirring was then continued till precipitation was complete. The product was fi ltered, dried and recrystallized from acetone-petro leum ether.

Results and Discussion The isolated complexes are pure, yellow to brown

in colour and are soluble in THF, DMSO, DMF and partially soluble in water. The molar conductance measurements in nitrobenzene at 1.5x 10-3 M for these complexes fall in the range 26.5-30.2 ohm-I Cm2 mOrl , indicating that the complexes are 1: I electrolytes"O Analytical data of the complexes along with the decomposition temperature are given in Table 1.

The band at 1700 cm-I is attributed to v (C = 0)21 stretching frequency in the free li gand. The guanine li gand may chelate with the metal ion through exocyclic oxygen at C(6) and deprotonation of

372 INDIAN J CHEM, SEC A, FEBRUARY 2006

N(7)22.25, or only through N(7)26.27. The 1500-1750

cm- I portion containing the C=O, C=C and C=N stretching vibrations is where the evidence for N7/06 chelation has generally been looked for. In the prepared metal complexes, the former band appears at ca. ]630 cm·1 indicating the involvement of carbonyl oxygen at C(6) in complexation. The absorption bands at 1590 cm·1 and ]675 cm- I assigned to v (C =

N) and v (C = C) stretching frequencies, respectively, in free guanine shift to ca. 1570 cm- I and ca. 1650 cm - I, respectively , in the metal complexes, showing that N(7) is also involved in complexation. The v(N I_H) stretching frequenc/8 occurs at 3435 cm- I

in the free ligand, which shifts to 3445 cm- I in the complexes. Moreover, the two bands in the region

3350-3250 cm- I assigned to v (N - H) vibrational frequency in free guanine become weaker on complexation, suggesting a link through nitrogen. The

v (C - N) stretching vibrational frequency absorbs at 1255 cm-I in both the ligand as well as the complexes.

The v(C2-N2) stretching vibration29 which absorbs at

1637 cm- I in the free guanine appears as a shoulder at

1650 cm- I in the complexes. In potassium phenyldithiocarbazate, the following IR bands30 are

observed: the N-H stretching frequency is observed at 3150 cm - I, the asymmetric CSS stretching shows a strong split band near 1000 em - I (990, 978) indicating unidentate sulphur chelation . A strong band at 1330 cm- I is also observed which is assigned to the C-N

stretching mode of the thioureide group N-C=S. The above mentioned bands remain largely unchanged in the spectrum of complex (5) indicating that phenyldithiocarbazate behaves as an anion . The band at ca. 3100 cm- I and the absorptions at 1440, 1115, 10 17 and 800 cm - I are attributable to the TI bonded CsHs rings31. In the far-lR region, bands at ca. 370 cm-I and ca. 520 cm·1 in the metal complexes correspond to v (M - CI) and v (M - 0)32 stretching vibrations, respectively.

Thus, the guanine ligand acts as a bidentate group, being coordinated to the hafnium(IV) ion through carbonyl oxygen at C(6) and deprotonation of N(7) .

The electronic spectrum of free guanine shows an absorption band at 285 nm (log E 4.0) due to the TI-TI*

transition of chromophoric carbonyl group33, which, in case of the metal complexes, occurs at ca. 300 nm (log E ca. 5.0). This shift is attributed to the involvement of C = 0 group in complexation. The band at ca. 245 nm (log E ca. 3.9) observed in the spectra of both guanine33 and the metal complexes,

has been attributed to the TI-TI* absorption of the C = N chromophoric group. However, the intensity of this band is higher in case of the metal complexes

(logE ca. 4.5) than the free ligand (log E 3.9). The IH NMR signal of guanine appears as a singlet

at (5 8.41 [s, IH, H(8)] 34. Tn case of the metal

complexes, the signal shifts to ca. (, 8.60. This downfield shift in organohafnium(IV)-guanine complexes is attributed to the involvement of N(7) in complexation.

In the IH NMR spectra of the metal complexes, the cyclopentadienyl group appeared as a multiplet in the region (5 6.9-7.0 (m, lOH). In the complexes containing phenyldithiocarbazate group as the anionic

moiety, a multiplet in the region 8 7.7-8.1 (m , 5H) was observed due to five protons in the C6HS moiety.

In the 13C NMR spectrum of pure guanine, the signals due to C(6) carbonyl and C(8) appeared at 160.3 ppm and 148.2 ppm, respectively . However, in the complexes, the former signal shifts downfield to ca. 164.0 ppm and the latter to ca. 152.0 ppm, indicating that complexation involves deprotonation of N(7) and chelation through C(6) carbonyl. The resonance signals due to C(2), C(4) and C(5) in case of the metal complexes of guanine are observed at ca. 170.8 ppm, ca. 150.2 ppm and ca. 116.3 ppm, respectively.

In the metal complexes , a resonance signal appeared at ca. 122.2 ppm due to carbons of the cyclopentadienyl rings . In case of the complexes containing anionic phenyldithiocarbazate moiety, the resonance signals observed were: ca. 153.0 ppm C( I) ; ca. 112.1 ppm (C(2,6)); ca. 130.1 ppm (C(3 ,5)) and

ca. ] 19.2 ppm (C(4») , while the signal due to -CS2

carbon appeared at ca. 169.0 ppm.

Fluorescence studies

The compound [(l1 s-CsHshHfLlt[HgCI, r is fluorescent in nature. Hence, fluorescence studies have been carried out for it. In accordance with the Franck-Condon principle and thermal relaxation of vibrational modes, the fluorescence spectrum is observed on the red side of the absorption spectrum in approximately mirror-image relationship35. The spectrum is free from anti-Stokes effect. The pattern of the spectrum follows Levschin' s rule, indicating that the geometry of the excited state is not very different from that of the ground state'!>.

The actual radiative lifetime of the excited state. t

is smaller than the intrinsic radi ative lifetime, t o,

indicating the possibility of non-radiating energy

MALHOTRA et al.:SYNTHESIS AND STUDIES OF IONIC CHELATES OF HAFNOCENE WITH GUANINE 373

Table 2 - Fluorescence spectral parameters

Parameter

Quantum yield of fluorescence: <!>r 0.60

Quantum yield for inter-system crossing: <!> ISC 0.35

Quantum yield for internal conversion: <!> IC 0.0086

Oscillator strength : f 0.012

Actual radiative lifetime : 1: (s) 5.5xlO·8

Intrinsic radiative lifetime: 1: 0(5) 8.9xlO·8

Rate const of fluorescence emission : Kr (S· I) 0. IOxl08

Rate const for inter-system crossing, SI--7 TI : Kisc (5.1)

0.058x108

Rate const for internal conversion: Klc (S· I)

Einsteins absorption probability: Bnm

Einsteins emission probability: Amn

dissipation processes depopulating the excited state. Thus, fluorescence remains the dominant but certainly not the exclusive mode of emission. The non-radiative processes, i.e. inter-system crossing and internal conversion, compete with fluorescence. Therefore, Einstein's probability of spontaneous absorption, 8 nm,

exceeds the correspunding probability ot spontaneous emission, Am,, ' The quantum yield of fluorescence, intersystem crossing and internal conversion follow the order <1>r > <1> ISC > <1> Ie. These priorities are also established by their respecti ve rate constants37

•

The summation of the rate constants for all the photochemical and photophysical processes competing with fluorescence, "'f.Kj , equals the sum of Kisc and Klc . In the present case, there seems to be no other mode of emission, radiative or nonradiative apart from fluorescence, intersystems crossing and internal conversion36

. The oscillator strength, f, has been calculated from the relation f = 4.31 x J 0.9

f E vdv, which is valid if we assume a Lorenzian

shape for the absorption band. The factor f E vdv is

replaced by E Illax f1v, where f1v is the half band width of the absorption band36

. The relevant data are presented in Table 2.

Thermal studies

Thermal studies have been carried out for complexes containing metal salts in the anionic species. The TG curves of guanine complexes in

O.0015x108

0.39x108

O. 14x 108

100

~ '" '" <t :>:

so

%~--~I~OO~--~20~O----~3~OO~--~4~OO--~S~OO~--~600 TEMPERATURE lOCI -

Fig. I-Thermogravimetric curves of complexes: (A) [(l1 '-C,H5)2HfLt[CuCI3r , (B) [(115 -CsHshHfLnZnCl.,r. (C) [(11 5-CsHsh HfLt[CdCI3r, (D) [(115-CsHshHfLtll-IgC I3r

Fig. I indicate that the complexes finally decompose to give the oxides of hafnium and the metal in the anionic moiety. In the case of the complexes containing mercury in the anionic moiety, the observed weight loss corresponds to the formation of Hf02 only, since HgO volatilizes at such high temperatures . Figure 2 represents the DT A curves of guanine complexes, while Fig. 3 represents their linearization curves, respectively . The mass loss data of the complexes have also been recorded. Mass loss, found (calcd): (1) 52.6 (53.8); (2) SO (53.7); (3) 49.9 (SO); (4) 74 (72.5). The order of the reaction (n) ancl the activation energy (Ea) of the thermal decomposition reaction have been elucidated by the

374 INDIAN J CHEM, SEC A, FEBRUARY 2006

100,------------:--------,

-1000~-~I00~--:2~OO~--:l±OO,--....,4.1::::00,--~SOOb;--~1oOO

nMPERAlURE IOe}

Fig. 2-0T A curves of the complexes (A) [(T]s-CsHshHfLt£CuCI3r. (B) [(T]s-CsHshHfLt[ZnCI3r, (C) [(T]s-CsHshHfLt[CdCI3r, (0) [(T]5-CsHshHfLt[HgCI3r.

&-8r--------·--------...,

&·2

6_0L----~----L--.L_-....L.----L--.L---1 1-4 I'S 1-& 1·7 I·e 1·9 z·o 2' 1

Fig_ 3-Kinetic parameters for TG studies from: (A) [(T]s-CsHshHfLt[CuCI3r. (B) [(T]s-CsHshHfLt£ZnChr. (C) [(T]s-CsHshHfLt[CdCI3r. (0) [(T]s-CsHshHfLt£HgCI3r.

method of Coats and Redfern38. The entropy of

activation (S) has been calculated by the method of Zsak639

• The kinetic and thermodynamic parameters calculated from TG and DT A studies are given in Table 3.

The order of thermal decomposition reaction in all the complexes is found to be one. A comparison of the activation energy of complexes reveals that the Ea values follow the order,

[(T)s-Cs~)2HfL't[CuChr<[(T)s-CsHshHtL'nZnChr < [(T)s-CsHshHfL't [CdC13r < [(T)s-CsHshHfL't [HgChr.

This may be explained on the basis of the fact that larger cations are stabilized by larger anions. The complex cation [(TIs -CsHshHtLt is very large in size and the size of the M(II) ions in the anionic moiety varies in the order Cu(II) < Zn(II) < Cd(II) < Hg(II). In case of the complex, [(Tls-CsHshHtLt [HgChr. the large size of Hg(II) ion in the ,mionic moiety, helps in effective stabilization of the complex cation and therefore gives rise to a higher lattice energy. This makes thermal degradation of the complex relatively difficult and thus, the reaction involves a higher value of Ea. For the complex, [(.,5 -CsHshHtLt [CuChr. the small size of the Cu(Il) ion in the anionic moiety leads to a relatively poor stabilization of the complex cation and to a comparatively lower lattice energy. Thus, the thermal decomposi tion reaction involves a lower value of Ea.

It is, therefore, evident that the nature of metal ion in the anionic moiety contributes to Ithe variation in the activation energy for the thermal degradation of the complexes. The energy of activation, in tum, reflects. the kinetic lability of the complexes. The compounds with lower E{/ values are more labile as compared to those with higher Ea values.

The apparent activation entropy has a positive value for most of the complexes. Hence, thermal degradation of these complexes is a spontaneous process. Within a given series, the complex

Table 3 - Thermal data for guanine complexes

Complex Thermogravimet!I OTA Temp. Range (K) Activation energy Activation entropy T max (K) tlH (Jg .l)

(E.) (KJ mor') (S'') (JK' mor')

(1) 485-703 27.33 7.956 665 372.41

(2) 498-773 30.60 9.329 653 201.87

(3) 499-803 33.47 10.570 734 189.25

(4) 448-855 62.17 23.214 694 144.53

MALHOTRA et al.:SYNTHESIS AND STUDIES OF IONIC CHELATES OF HAFNOCENE WITH GUANINE 375

Table 4 - Anti-bacterial activity of guanine complexes

Complex

(1)

(4) HL

Cp2HfCh

Escherichia coli 25 ~g mL·1 50 ~g mL-1

+ ++

+

++

+++

++

+ Each (+) sign indicates 2 mm section of zone of inhibition.

containing Cu(II) ion in the anionic moiety has the lowest value of ~ while the one containing Hg(II) ion has the highest. Hence, the fc rmer decomposes with lowest degree of randomness, while the latter with greatest.

The TO data of the complexes is supplemented by DT A studies. The thermal effects on DT A curves are exclusively exothermic in nature. The heat of reaction (MI) for the thermal decomposition reaction has been enumerated from the DTA curves40

.4I. The temperature dependent calibration coefficient has been obtained from the Currell Equation.

Microbial assay

Table 4 gives the growth inhibition effected by complexes containing copper(II) and mercury(II) ions in the anionic moiety of the guanine complexes against pathogenic strains of Escherichia coli (E. coli), Pseudomonas aeruginosa (P.aeruginosa) and Zymomonas mobilis (Z.mobilis) using the respective ligand as the standard for comparing the activities. The samples have been screened at two concentrations (25 )..lg mL-1 and 50 )..lg mL- 1

) in DMF. The inhibitory power of the complexes was greater than the control. The complexes, in general, inhibited the growth of bacteria to a greater extent as the concentration increased. However, the complex containing Cu(II) ion in the anionic moiety was similar in its activity against (E.coli) as the free ligand.

The higher antibacterial actIvity of organohafnium(IV) ionic chelate complexes containing mercury in the anionic moiety was due to the capacity of mercurials to coordinate with sulphydryl functions of cellular proteins and enzymes,

Active enzyme

+ Hl+ - Enzyrre

S I Hg + 2H+ I S

Inacti ve enzyme

Pseudomonas aeruginosa Zymomonas mobilis

++

+++

++

+++

+++++

++

+

+++

++++ ++

+

+++

++++++

++

+

Table 5-Anti-fungal activity (percent inhibition) of guanine complexes

Complex Aspergillus niger Aspergillus awamori 25 ~g mL- 1 50 ~g mL-1 25 ~g mL- 1 50 ~g mL-1

(4) (5)

HL

55

63

45

60

60

46

45

57

30

48

55

32

The antifungal activity of complexes containing mercury and the phenyldithiocarbazate group in the anionic moiety, at concentrations 25 )..lg cm- I and 50 )..lg cm- I

, against Aspergillus niger (A.niger) and Aspergillus awamori (A.awamori) is presented in Table 5. It· has been observed that: (a) complexes containing the phenyldithiocarbazate- group in the anionic moiety were comparatively stronger antifungal agents than those containing mercury in the anionic moiety, and (b) the former complexes showed greater inhibition at lower concentrations, while the latter were most active at higher concentrations.

The mode of fungicidal action of the dithiocarbazates involves the inhibition of hexose monophosphate oxidation pathway of carbohydrate metabolism42

•

Conclusion The present work affords proof for the formation of

N7/06 chelate with a 6-oxopurine. This type of interaction has attracted considerable attention in recent years, since it has been proposed that such kind of complexes are biochemically active, as is the case with the complexes reported herein.

References I Rosenberg B, Van Camp L, Trosko J E & Mansour V H,

Nature (London), 222 (1969) 385. 2 Dass M & Livingstone S E, Br J Cancer, 37 (1978) 466. 3 Gosalvez M, Blanco M F, Vivero C & Vallee F, Eur J

Cancer, 14 (1978) 1185. 4 Kopf H & Kopf-Maier P, Angew Chern, Int Ed Engl. 18

(1979) 477. 5 Sadler P J, Chem Br, 18 (1982) 182.

376 INDIAN J CHEM,SEC A, FEBRUARY 2006

6 Kohn U, Schulz M, Goris M & Anders E, Tetrahedron: Asymmetry, 16 (2005) 2125.

7 Berlinck R G S, Kossuga M H & Nascimento A M, Sci SYllth, 18 (2005) 1077.

8 Herres S, Floerke U, Henkel G, Acta Crystal/ogr, C60 (2004) m659.

9 Sletten E & Flogstad N, Acta Crystallogr, B32 (1976) 461. \0 Marzilli L G & Kistenmacher T J, Acc Chem Res, \0 (1977)

146. II Marzilli L G, Prog Inorg Chem , 23 (1977) 255. 12 Tu A T & Heller M J, Metal Ions in Biological Systems, Vol.

I, edited by H Sigel (Marcel Dekker, New York, N.Y.), 1974.

13 Straszko J, Olszak-Hunienik M & Mozejko J, J Thenn Anal Cal, 59 (2000) 935.

14 Rosca I, Foca N, Stefancu E, Sutiman D, Cailean A, Rusu I, Sibiescu D & Vizitiu M, J Therm Anal Cal, 56 (1999) 401.

15 Malhotra E, Kaushik N K & Sodhi G S, India J Chem , 40A (200 I) 1307.

16 Malhotra E, Kaushik N K & Sodhi G S, J Therm Anal Cal, 65 (2001) 859.

17 Malhotra E, Kaushik N K & Malhotra H S, Trans Met Chem, 3 0·(2005) 69.

18 British Pharmacopaea (Pharmaceutical Press, London), 1953. p.796.

I,~;' Raper K. B & Fennel D I, The Genus Aspergillus (Wilkins ! , ; CR .. Baltimore, Maryland, U.S.A.), 1965, p.686.

20 Sodhi G S,' Sharma A K & Kaushik N K, J OrgmlOmet !,i;j;C/fell1. 238 '(1982) 177.

21 i ~}tilciu nescu D & Fruma A I, Inorg Chim Acta, 4 (1970) 287.

22 . i Szalda D J, Kistenmacher T J & Marzi lli L G, J Am Chem , So~" 98 (JIr/6) 8371. .

23 Ki~telll~acher T J, Szalda D J, Chiang C C, Rossi M & Marzilli L G, Inorg Chel11, 17 ( 1978) 2582.

24 Pnelllnatikakis G, Hadjiliadis N & Theophanides T , illOrg Chim Acta, 22 (1977) LI.

25 Crawford C A, Day E F, Saharan V P, Foiling K, Huffman J C, Dunbar K R & Christou G, J Chem Soc. Chem COl11mun (Cambridge). (1996) 1113.

26 Schimanski A, Freisinger E, Erxleben A &. Lippert B, Inor!? Chilli Acta. 283 (1998) 223.

27 Hodgson D J, Prog Inorg Chem. 23 (1977) 211. 28 Gould I R, Vincent M A & Hillier I H, Spectrochim Acta,

49A (1993) 1727. 29 Bariyanga J & Theophanides T, Inorg Chilli Acta. \08 (J 985)

133. 30 Kaul B B & Pandeya K B, J Illorg Nllc/ Chelll, 40 ( 1978)

1035. 31 Cozak D, Mardhy A, Oliver M J & Beauchamp A L, Inorg

CheJll. 25 (1986) 2600. 32 Doyle G & Tobias R S, Inorg Chem, 6 (1967) 1111. 33 Heyworth F F & Loofbourow J R, JAm Chell! Soc, 56 (1934)

1728. 34 Gosselin G, Bergogne M C, Rudder J, DeClercq E & Imbach

J L, J Med Chem, 29 (1986) 203. 35 Bowen E J & Wokes F, The Fluorescellce of Solutions

(Longmans Green & Co., London), 1953. 36 Rohatg i-Mukhmjee K K, Fundamentals of Photochemisry

(Wiley Eastern Ltd., New Delhi), 1978. 37 Calvert J G & Pitts J N, Photochemistry (John Wiley & Sons.

New York) , 1966. 38 Coats A W & Redfern J P, Nature (London), 201 (1964) 68. 39 Zsak6 J, J Phys Chem, 72 (1968) 2406. 40 Collins W E, Analytical Calorimetry edi ted by R S Porter &

J M Johnson (Plenum , New York), 1970, Vo l. 2, p. 353. 41 Currel B R, Thermal Aalysis. ed ited by R F Schwenker & P

D Garn (Academic Press, New York). 1969. Vol. 3, p. 11 85. 42 Chefurka W, £ nzymologia, 18 (1957) 209 .