Spectroscopic and spectromagnetic study of adenosine chloroderivatives of bivalent Co, Ni, Cu, Zn, Cd. Comparison with the corresponding adenine chloroderivatives

05&4-8539/81/090763-09$02.00/0 @ 1981Pergamon Press Ltd.

3lA,No.9.pp.763-771, 1981. Printed inGreatBritain. Spectrochimica Acto, Vol.

Spectroscopic and spectromagnetic study of adenosine chloroderivatives of bivalent Co, Ni, Cu, Zn, Cd. Comparison with the corresponding adenine chloroderivatives T. BERINGHELLI,

M. FRENLF.

MORAZZONLP.

ROMITI

and R. SERVIDA

Istituto di Chimica Generale e Inorganica, Universita di Milano, Via Venezian 21,20133 Milano, Italy (Recehed 20 February 1981) Abstract-_[M”L,CIJ (n = 1,2) (M” = Co, Ni, Cu, Zn, Cd, Hg) (L = adenine, adenosine) compounds were synthesized and the magnetic, electronic and vibrational properties of adenosine derivatives were considered in comoarison with those of adenine complexes. It was found that the ligand field symmetry is distorted tetrahedral in cobalt, zinc compounds and in CuLClr; distorted octahedral in Ni, Cd, Hg compounds and in CuL&lr. Sugar moiety of adenosine is not implied in the coordination to the metal center. Distortions from regular symmetries were recognized by ESR measurements and confirmed by vibrational analysis; it resulted an higher distorting power of adenosine than of adenine. Both monodentate and bridging N(3)-N(9) or N(l)-N(7) coordination were discussed. The M-N bond strength resulted higher for bridged coordination. ESR sensitive change of spin state were observed in CoLrClr compounds. Vibrational measurements on purine absorption modes suggest that the ligand is not strongly perturbed by the metal coordination. INTRODUCTION

A systematic analysis of the spectroscopic and spectromagnetic properties of adenosine chloro derivatives of bivalent Co, Ni, Cu, Zn, Cd has not been reported, in spite of their role as models of biochemical processes involving metal ions and nucleic acids. Recently same workers dealt with this subject, but they were only concerned with solution behaviour [I, 21 or described the title metal complexes with adenine and 9-Me adenine[3]. The results reported in[3], in our opinion cannot be extended to the adenosine complexes, because they do not account for the different distorting power in coordination geometry owing to the different electronic and structural properties of adenosine, and because they do not rule out the possible coordination of the sugar moiety. Moreover these authors discussed metal-purine complexes with anions of too variable coordinating behaviour for it could not affect all the proper-

ties of complexes. This paper reports the synthesis, the spectroscopic and spectromagnetic properties. of bis chloro adenosine derivatives of Co, Ni, Cu, Zn, Cd and it considers the contribution of different physico-chemical measurements in order to describe the electronic properties of these complexes. Comparisons are made with adenine derivatives of the same formula, when available. The aim is to clarify: (1) the coordinating behaviour of adenosine, (2) the symmetry of coordination site in purine complexes of different metals, (3) the metal-purine bond strength in purine complexes of the same metal, (4) the perturbation of the purine ligand by effect of metal coordination.

EXPERIMENTAL

Apparatus Infrared spectra were recorded in Nujol mulls on a Perkin-Elmer 621 soectroohotometer in the range 4000600 cm-‘. Far infrared spectra, in the range &O cm-‘, were recorded on Perkin-Elmer FIS-3 spectrophotometer, using polythene disks. The diffuse reflectance spectra of the powdered solids were recorded on a Beckman DK-2A spectrophotometer. Magnetic susceptibility measurements were performed by the Gouy method at room temperature; ESR spectra were recorded on polycrystalline samples by a Varian E-109 spectrometer, and the g values were standardized by DPPH. Preparation of the metal complexes The metal complexes were prepared by one of the methods listed below (see Table I). The yields varied from 75 to 95%. Preparation I The purine base, adenine or adenosine (1 mmol) in warm ethanol (30 ml) was treated with a solution of MCI2 (1 mmol) (M = Zn, Cd, Cu, Ni, Co) in ethanol (10 ml) and refluxed for 4 h. The metal complex was then filtered off, washed with ethanol, and dried in uacuo over anhydrous CaC&. These compounds are stable in air. Preparation 2 MCI2 (I mmol) (M = Zn, Cd, Hg, Cu, Ni, Co) in ethanol (10ml) was added to a warm solution containing the purine base, adenine or adenosine, (2mmol) in ethanol (40 ml). The solution was refluxed for 8 h, and cooled, whereupon the metal complex deposited. The complex was filtered off, washed with ethanol, and dried in vacua over anhydrous CaCl*. These compounds are stable in air. Preparation 3 The ourine base adenine (6 mmol), was added to a solution of CuClr . 2HsO (1 mmol) in ethanol (150 ml). This mixture was refluxed with stir&a for 6 h. The blue precipitate was filtered off, washed-with ethanol and dried in vacua over anhydrous CaClr. 763

(C5H5Ns)

2C12

-

II

II

co-

cd (Clo”, 3N504’ 2C12

Iis K5H5N5)2C’2

I

axfxmss

Cd(C,ot+3N504)C12

.

L

._,

-

--

-

>3CO

II

cmaRLEs

cd (C5H5N5) 2C’2

208

II

ccnfmus

2C12

ZnK10H13N504)

Y

.303

.I

220

210

r303

II

r,

_

-

I~

-

2.12

2aJ

II

GPELEN

co-

1.93

185

I

PALEGREEN

Zn (C5H5N5) 2C12

1.40

285

III

PNEBUE

>3cQ

1.61

275

I

PALEGRW

I

2.98

205

I

UJIaJRLESS

3.32

- .” .1 _ ._ _ -

22.14

33.47

26.66

26.51

35.70

29.50

22.16

33.53

26.66

26.86

35.69

29.60

22.70

26.70

29.87

22.14

29.14

29.78

35.40

1.07

21.70

22.39

35.90

3.28

30.69

30.23

A%

.-.:_

1.04

3.62

2.88

2.21

3.07

2.45

1.84

3.89

3.23

2.48

2.50

29.75

2.49

1.89

c.

30.03

29.90

22.81

f.

3.91

C%

Points (D.P.).

36.31

36.14

4.54

210

300

.

PALEYEUKW

“”

II

PALEYELLILM

-

11

BIm

29.98

4.31

300

300

Zn K5f~5N51 Cl2

Ni (CloHl 3N504)C12

Ni

3N504’ 2c12 >

>

II

BuJ?z

Co (C5H5N5’ 2C12

C.

Decomposition

22.6.:

(b)

4.61

,,LL(!.LR.)

CloH13N50d ~=adenosine.

D.P. C”C) th)

data (a) CSHSNS = adenine; ME!IlKxJoF PREPAPATICN

>

ELlJE

c”‘ClO”1

and analytical

I

I. Physical

Co (C5115N5K12

_(a)

Table

N%

30.92

20.87

34.42

25.83

1.91

3.82

.-A-\.“-:

25.83

19.52

II.03 15.55

2.45

4.10

2.70

1.94

20.93

17.42

3.67 4.14

34.74

26.10

17.63

35.00

21.08

34.90

26.41

c.

2.60

2.03

3.56

2.70

3.86

2.46

1.84

f.

25.22

19.80

15.58

30.39

20.56

34.17

25.78

20.41

16.42

33.42

25.50

17.68

34.10

20.91

34.80

26.35

f.

-

%

2

-__

2 F c

g B

W

9

s! P

Spectroscopic

and spectromagnetic

Preparation of the ML,,r C$ (M = Zn, Cd, Hg) doped Cu (II) complexes The complexes MLCl* (L = adenine, adenosine; M = Zn, Cd) or ML_& (L = adenine, adenosine; M = Zn, Cd, Hg) were doped by Cu(I1). Ethanol solutions containing 1 mmol of MCI,, 1 or 2 mmol of adenine or adenosine, and 10e2mmol of CuClz. 2H20 were refluxed for 4 h. The product was filtered off, washed with ethanol and dried in vacua over anhydrous CaC&. RESULTSAND DISCUSSION

Investigation of compounds of different metal ions was performed by a procedure common to all complexes reported in the present paper. Firstly the compounds of d” ions (n < 10) were investigated by electronic diffuse reflectance spectroscopy and by ESR spectroscopy in order to seek information about the symmetry of the ligand field. Then the results were compared with those deriving from the vibrational analysis, in order to confirm the previous hypothesis. For the compounds of d” ions with n = 10, only the results of vibrational spectroscopy are available. In order to decide about the coordination properties, we have compared their vibrational properties with the vibrational properties of suitable compounds of d” (n < 10) ions, which have analogous formula and the highest probability of analogous structure. Common feature to all the complexes reported in the present paper is that in solid state there is no evidence of coordinating behaviour of purine NH2 and of the sugar moiety of adenosine (as from the infrared analyses in Table 2[4]. Moreover all the reported complexes show a relevant dissociation in solution (either of DMSO, DMF and H*O). This makes meaningless all the solution measurements such as electric conductivity or NMR spectra.

765

study of adenosine chloroderivatives

Cobalt compounds Cobalt derivatives here reported have both 1: 1 and 1: 2 metal/adenine ratio. Only the 1: 2 derivative was obtained by adenosine coordination. The values of the magnetic moments show high spin state of cobalt, and a ground configuration of cubic ligand field symmetry. Electron spin resonance analysis distinguishes the bis-purine derivatives from the mono-derivative; really for bis-derivatives strong resonance lines were observed, while from the mono-derivative the resonance lines are very weak. This behaviour can be attributed to difference of distortion from the cubic ligand field. The spectra of [Co(adenine),Cl,] and [Co(adenosine)QJ are clearly visible both at room temperature and lSO”C, and show a strong resonance at low fields, characteristic[5] of the tetragonally distorted high spin cobalt compound (Fig. 1). Usually the spectra of high spin cobalt compounds are not visible, nor at liquid nitrogen temperature; provided that, by effect of distortion, an orbital singlet becomes ground state, well separated from the neighbour degenerate ones. This is the case, and the transition appearing in the ESR spectra can be attributed to transition internal to a magnetic state with fictitious S’ = l/2 spin state. The strong resonance at - 12OOG reveals axial symmetry of the 1:2 complexes and can be attributed to the perpendicular component of the magnetic tensor. The parallel one appears as much less strong resonance at -3000G. Higher distortion * for [Co(adenosine)ElJ. Magnetic tensor coiponents are given in Table 3. Ligand field spectra, whose absorption frequencies are reported in Table 4, reveal that the cubic symmetry around cobalt is a distorted

Table 2. Infrared data (cm-‘) ff

v~~-HPlU%3WIN-H)

ccmp3urd

335Gsh,33C&,

311071

335cln, 318cln ca"(adentne)Cl

339&,

2

Co"(addne)*Cl2

V(c=c) 1670s 166Ca 167Osh,

324&-+1,3180w

166%

*loiN)

6 (N-H)

16ccs

156Ca

1590s

1565m

V (M-Cl)

310s 3561315s

16608

159%

co"kd~sine)2cl2

35CCw,

341Cm,

331Crn. 3210~

1640s

159%

156%

3508314s

Ni"WenW2Cl2

335al,

32fxw,

313oll

Ni"(ademsine)C1

2

1690s

1610s

158(m

236s

324obr,33KW

1663s

15909

156511

21obr

W-N)

272m

15908

331oll ,31Cobr

v

235m 216W2oBw 225.

339cx/, 315Obr

1660s

160%

15Mm

3C4*

295s

24%

Cvradenine,*c1*

33ozkr,315Cbr

1660s

159ml

157ml

337rr 208s

23%

cu"kldemsine)c1

331Obr,31~r

1640s

159Qn

156Qn

159aI

156cnl

cu"wenine)Cl

2

2

339Obr.311Obr

Cu"wemsine)2c12 Zn"(adenirE!)Cl

2

339%

zrl"e&nine)2C12 zn"ladenosine12C12

3y1cw,34l(m,

33Wd,

324&1,

331QI

,31COsh

333Qn.

3230n,

3310s.

314all

341obr,33abr 341obr,333obr,313C4x Hg"~adePine)2C12

336(kr, 3220n,

“For adenosine compounds.

3C!8On

1660s. 317011

312Cw

1640"

338S330~

166%

1590s

295s282s

16608

1595s

356~2908

1640s

1595s

157on

16909

16038

157ml

1650s

159cm

156nn

L2uJbr

1610s

1585!n

.c2wbr

1610s

15Bh

L2cobr

168Osh, 1660s

1665s

33oe306s

2531 208s 2Omll%m

2as194s

-

766

T. BERINGHELLIet al.

000

Fig. 1. ESR spectra

of (a) [Co(adenine)#&], (b) [Co-(adenosine)zCI,] polycrystalline samples.

Table 3. ESR parameter@ CplpapD comenine)

*Cl2 2c12

2.12

5.96

2.24

5.08

2.27

2.05

D (on’,

giso

9,

A

can-'104)

2.13

Cu(adenine)C12 rfLmxericform cuwednd

at - lSO”C, on

(a) Spectra recorded at -150°C. (b) Doped with 1% Cu(I1). 911

CokIderosine)

recorded

c

116

2c12 0.1085

2.05

dimricfom

2.12

Cu(~SinelC12 Culadarpsine)2c12 Z,,tm)C12(b)

-'et

*I==~=

triplet

speCiea

nmerw2C12(b) Zn(~ine~2C12 CdLadenine)2c12

(b) b)

Cdhd.mdne)C12

(b)

Cd(~sine)2c12 Imddnd2C12

(b) Lb)

2.28

2.18

2.34

2.06

0.1153

2.06

155

2.29

2.08

2.42

2.07,

2.30

2.09

118

2.30

2.11

161

2.20

2.08

2.Y)

2.17,

192

2.02

150 116

1.95

Table 4. Electronic diffuse reflectance data (a) Symbols are in accordance to Td symmetry: v,eAze +‘Tzs); vz(‘Azr + ‘T,,(0); v,(‘Az +4T,,(P)). (b) Symbols are in accordance to Oh (c) Transitions are not symmetry: v,(~A~~+3Z’,,(3F)); u~(~A2~ --f ‘E,); v~(~Azrt+‘T,,CF)). assignable due to broad tail of the band. VI (cm-’)

CPIFCUN) Cow&l.e)C12 Coctiaenine,2c12 cobd~sine)2c12

NiMenin)2C12

N1@demslne)C12

Cu(W)C12 Cukd4m)2c12 CukdemSine)C12 cumdemsine)2c12

(a) b) I?.)

Ib)

(b)

v,br-1,

v3 cm-‘,

xm

6535,

7245,

8550

16130

4524

6211,

7560,

8470

16260

6172,

7052,

9049

15873

4444,

4629

6203,8130

12315,13605

21270,

6290, 7870

12166,13605

21186, 23041

1,900

(C)

16835

(4 (C) (=)

12048 15625

23803

Spectroscopic and spectromagnetic study of adenosine chloroderivatives

quite analogous spectra in the near infrared region, except for the energy of the metal sensitive bands. On the bases of spectroscopic and magnetic data it seems that the coordination on the cobalt is tetrahedral, whatever is the purine ligand and the metal-purine ratio. In the case of the 1:2 ratio, purine acts as monodentate ligand, while for the 1: 1 ratio it is probable that adenine acts as bridged ligand between the cobalt centers. In the later case, the bridging coordination is probably by N( 1) N(7) of the purine ring. A similar coordination was found in 9-Me adenine cobalt(U) complexes [8]. The distortion from the Td symmetry is higher for the 1:2 compounds, which clearly show G. vibrational symmetry. Temperature dependence of ESR powder spectra of the 1:2 cobaltlpurine compounds was also

tetrahedral one[6], where the degree of distortion follows the same trend proposed by ESR analysis. It must be observed (Fig. 2) that all compounds show splitting of v2 transitions, into three component transitions, as consequence of the decrease in symmetry from T.+ to Cz,. Moreover the appearing of the Y, transition confirms this distortion. Vibrational spectra of all three cobalt compounds show the M-Cl stretching absorptions in the frequency range characteristic of terminal chlorines [7]. Two well separated transitions (Table 2) are observed in the Co-bispurine adduct, while only one band is visible in the spectrum of [Co(adenine)ClJ. A single band is observed for the M-N vibration. Assignment of all these transitions was performed by considering the Zn complexes with the same formula (see later), as they show

(al

I

700

I

1000

161

I

1

1500

2000

nm

(b)

nm

Fig. 2. Electronic diffuse reflectance spectra of cobalt compounds: (-). [Co(adenine) Cl& (_._._._*_ ), [Co(adenine)zC12];(------), [Co(adenosine)&]. (a) Polycrystalline samples. (b) MgO diluted samples.

768

T. BERINGHELLI et al.

DPPH

Fig. 3. Temperature dependence of [Co(adenosine)&] ESR spectrum. The recording temperature is indicated on the left-hand side.

investigated, in the range -Ml+12O”C (Fig. 3). On increasing temperature it was observed the appearence and intensifying of a signal not assignable to an high spin compound, but to a low spin one. This type of signal is already detectable, in the adenine derivative spectrum, at - lSO”C, but in the adenosine one it appears only at +2O”C. At +12O“C the spectrum of the adenine derivative shows both signals of the high and the low spin forms, while the adenosine one does not show the signals of the high spin form. The ESR behaviour of the 1:2 cobalt purine compounds suggests an equilibrium between high and low-spin form,

caused by the mixing of electronic levels of different spin multiplicity, by spin-orbit coupling effect[9]. In this case a temperature dependent distribution of molecules among several electronic levels, even more than two, can exist. The coordination of adenine seems to favour this mixing, while the more distortive power of adenosine makes electronic levels more split. Nickel compounds

Nickel derivatives are all paramagnetic compounds, so that the presence of strong distortions from the cubic symmetry can be excluded.

Spectroscopic

and spectromagnetic

study of adenosine chloroderivatives

Copper compounds Copper compounds were obtained in both 1: 1 and 1:2 metal/purine ratios. Their are all paramagnetic complexes, with magnetic moments indicative of the d9 electronic configuration of copper. ESR spectra show in all cases strong resonance lines, as expected. However the shape of the spectrum well separates the 1: 1 from the 1:2 complexes of both adenine and adenosine ligands. The anisotropic behaviour of the g tensor is well visible in the 1:2 complexes, while it is not detectable, whatever the temperature, in the 1: 1 derivatives. The 1:2 complexes show axial symmetry and the values of their g components (table 3) are indicative of a d9, *B,((xz, YZ)~ (xy)* (z*)* (x2 - y’)) electronic ground state of copper. The 1: 1 compounds evidenced only an average value of g components, due to complete isotropic shape

No help to the symmetry determination derives from ESR spectra, because the bands are broad and very weak. Ligand field spectra (table 4) are indicative of tetragonally distorted octahedral symmetry[6], being the V, transition splitting representative of the distortion (Fig. 4). Vibrational analyses reveals bridging chlorine atoms[7] both for 1: 1 and 1:2 ratio and the assignment of M-C] and M-N vibrations was performed in this case, by comparison with the i.r. spectra of Cd and Hg derivatives (see later), which also show bridged chlorine atoms. On the basis of all spectroscopic analyses and also considering the analogy of the ligand field spectra with those of polymeric [Nipy,XJ compounds[6], we propose a polymeric structure of the derivatives through Ni-Cl chains, and one or two purines in the axial position acting as monodentate ligands.

I

700

I

loo0

769

I

1500

I

2006

"Ill

Fig. 4. Electronic diffuse reflectance spectra of nickel compounds: (-), [Ni(adenine)2C12]; (-----) [Ni(adenosine)Cl,]. (a) Polycrystalline samples. (b) MgO diluted samples.

110

T. BERINGHELLIet al.

of the spectrum; however it is very probable (see later) that a distortion from the cubic symmetry exists also in the 1: 1 compounds, and that it is averaged in the spectrum by a fast dynamic character of the magnetic distortion. The ESR spectrum of [Cu(adenine)zC1,] shows in addition resonance lines of coupled magnetic centers, interpretable in terms of Cu-Cu triplet ground state. Possible magnetic exchanges are well known for the adducts of adenine ligand [lo]. Really the value of the zero field parameter calculable from the triplet components of the spectrum of the [Cu(adenine),Cl,] (Table 3) suggests that the monomeric compound is obtained mixed to the dimeric [Cu(adenine)&12 molecular whose structure [ 1l] and magnetic behaviour [lo] were quite completely investigated. Ligand field spectra (Table 4) for 1:2 compounds are diagnostic of distorted octahedral symmetry field[6]; while for the 1: 1 compounds they suggest basically tetrahedral coordination[6]. It is really well evident the shift to lower frequency and the large decreasing of intensity, when the spectra of 1: 1 are compared with those of 1: 2 complexes. The vibrational spectra of copper-purine derivatives show absorption bands due to terminal chlorine atoms[7] (Table 2), with the exception of [Cu(adenosine)Cl,] which has a broad uninterpretable far infrared spectrum. Assignment of Y(CU-Cl) and v(Cu-N) vibrations for [Cmadenine) ClJ was performed by comparison with the spectra of cobalt and compounds. zinc [Cu(adenine)ClJ has a well detectable distortion from Td to C,, symmetry, with consequent splitting of the observed v(M-Cl) vibrations. The symmetrical shape of ESR spectrum is reasonably attributed to a dinamic character of the distortion. Far infrared spectrum of the same [Cu(adenine)Cl*] does not show any resonance at 260cm-‘, excluding that the product reported in the present paper can have the dimeric structure with bridged chlorines suggested by BROWN et al. for a compound with identical formula[l2]. A comparative survey of the far infrared spectrum of [Cu(adenine)$&] shows that the derivative contains two strongly different Cu-Cl vibrations, at 337 and 288 cm-‘, which cannot be justified by distortion symmetry alone. It seems here probable, as suggested also by ESR measurements, that the vibrations belong to the monomeric and the dimeric product with identical formula, both present in this mixture; really the comparison with the i.r. spectrum of [Cu(adenosine)&l,], which has no possibility of forming the corresponding bridged N(3)-N(9) dimeric compound, suggests that the vibration of monomeric [Cu(adenine)2C12] lie at higher frequency. On the basis of all spectroscopic results, the 1: 2

ratio copper-purine compounds appear as four coordinated compounds, where the purine acts as monodentate ligand. Distortion from the tetrahedral coordination symmetry is very high, as derives from ligand field spectra, so that a nearly planar symmetry can be proposed. Dimeric [Cu(adenine),C& has well known X-ray structure[ll]. As for the 1: 1 copper compounds the coordination symmetry is also tetrahedral but much less distortion appears. Four coordination is gained by a probable bridged coordination of the purine ligand. Zinc complexes

The electronic configuration of the zinc complexes is not suitable for magnetic or ligand field investigations; thus it seemed useful to investigate the diamagnetic compounds, doped with a paramagnetic metal ion. By considering that copper gives the highest variety of coordination symmetries in purine compounds, ZnCl, was doped with CuCl, in the preparation of complexes (see experimental). The mixtures, investigated by ESR spectroscopy, reveal for the three different complexes of zinc reported in this paper, a different symmetry of the ligand field. [Zn(adenine)&] shows the Cu(I1) ion lying in a site of axial symmetry; for [Zn(adenosine)#&] a rhombic distortion from the axial symmetry is well evident; [Zn(adenine)Cl*] shows finally very small distortion from the axial symmetry. In all three diamagnetic hosts, Cu(I1) has a ‘B, ground state configuration (Table 3); therefore tetragonal distortion is suggested. It must be observed that a triplet state of copper coexists with the doublet, in [Zmadenine) Cl,]. The coupled species reveals interaction between two Cu(I1) centers, as from the seven line splitting of the AM, = 2 transition, and it is characterized by a D value of 0.115 cm-‘. The far infrared spectra of zinc compounds are very similar to those of the corresponding cobalt complexes so that an analogous symmetry of the ligand field can be proposed. v (Zn-Cl) and v (Zn-N) absorption frequencies are in Table 2. Higher distortion from the tetrahedral symmetry is observed in [Zn(adenosine),Cl,] in agreement with the results of the ESR analyses on Zn/Cu doped compounds. Spectroscopic investigations on zinc complexes lead to assign a distorted tetrahedral coordination to all compounds, gained by halide and purine acting as monodentate ligands, in 1:2 Zn-purine compounds. As for the 1: 1 compound a bridging adenine behaviour is confirmed by the triplet state signals present in the ESR spectrum of Zn/Cu doped compound. It is unexplained why the magnetic exchange is not revealed by ESR spectrum of undiluted [Cu(adenine)ClJ.

Spectroscopic

Cadmium Either

and mercury cadmium

investigated doped

and

and spectromagnetic

complexes mercury

complexes

by ESR spectroscopy,

mixtures.

All reveal

by using

axial symmetry

were

Cu(II) of the

magnetic

tensors and copper is held in a tetragonal site, with *B, ground configuration. Magnetic tensor values are in Table 3. Vibrational spectra (Table 2) in the near infrared region, show that halide of all complexes act as a bridged ligand. Two v(M-Cl) absorptions are distinguishable, to proof that a distortion from the axial symmetry is present. The most probable structure of these compounds is a chain of MCI2 groups, axially coordinated to one or two purine bases. Interestingly no exchange coupling between the Cu(I1) centers of the diluted mixtures has been revealed by ESR analysis. CONCLUDING REMARKS

Results reported in the present paper mean that: of ligand field and the coordinating behaviour of adenine and adenosine in chloro complexes are strongly dependent on the metal ion. Similar coordination schemes have been found to relate cobalt to zinc, nickel to cadmium and mercury. Copper has really a singular behaviour, because it has been found to easy coordinate both by tetrahedral or octahedral symmetry. In general the distortions from Td or O,, symmetries are increasing with the steric hindrance of the purine ligand. @I Purine is coordinated to the metal center by one or two ring nitrogens. Really either NH2 groups and the hydroxyl groups of the sugar moiety do not interact with the metal atoms (see Table 2). Detailed proposals about the ring nitrogens implied in bonding are rarely possible, because, in the absence of X-ray analysis, they are not warranted. The compounds reported here show purine molecules acting either as monodentate or as bridged ligand; and also different bridges seem possible, through N(3)-N(9) or N(i)-N(7) of the purine ring. Where both monodentate and bridged coordinations have been proposed (cobalt and zinc complexes), it appears, from the frequency of v(M-N) vibrations, that the

(a) The symmetry

study of adenosine chloroderivatives

771

bridging bond is stronger than the monodentate one. This is a probable consequence of the higher contribution of back donation from the d metal orbitals to the 7~* system of purine, in bridged systems; as a containing proof, in some compounds bridged purine, exchange coupling processes are well visible. Coordination of adenosine lead to higher distortion of the ligand field with respect to adenine. Really the Ali values reported for Zn, Cd, Hg complexes doped Cu(I1) show that the contribution of the *A, electronic configuration to the ‘B, ground state, decreases from adenine to adenosine, as a proof of this distortion. Another proof derives from the spin equilibrium described in cobalt compounds. (c) Coordination by metal ions does not greatly affect the purine ligand. Really slight variations in the frequency of v(C=C) and v(C=N) vibrations have been found. Decreases are generally observed in the ring vibrations, as a consequence of the coordination to the metal; Ni, Cd and Hg compounds show a slight increase. These effect are probably due to different and opposing contributions of o and 7r bonding, as a consequence of the purine coordination.

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