) .

Indian Journal of Chemistry VoI.38A, October 1999, pp. 991-996

Template synthesis and characterisation of a copper(II) complex of a half-cyclised tetraaza ligand derived from 1,8-diaminonaphthalene,

nitroethane and formaldehyde

Nita A Lewis Department of Chemistry, University of Miami, Coral Gables, Florida 33124, USA

and

Swati Ray Energy Relearch Unit, Indian Association for the Cultivation of Science,

Calcutta 700 032, Indi a

and

, Goutam K Patra & Dipankar Datta* Department of Inorganic Chemistry, Indian Association for the Cultivation of Sc ience, Calcutta 700 032, India

Received 25 May 1999; revised 10 Allgustl999

Reaction of Cu(BF4hx1-l20 with 1,8-diaminonaphthalene, nitroethane and formaldehyde in ethanol in the presence of triethylamine yields an air sensitive copper(ll) complex of 1,3 -bis (8-aminonaphthyl-l-amino)- 2-methyl-2-nitropropane (L) as Cu(L)-(BF4h-C2I-1 S0 1-l , I. Mol ecular mechanics calculations coupled with the solution conductivity data indicate a distorted square pyramidal N4F coordination sphere for the copper atom in I with the Ouorine atom occupying the api cal position and the metal atom lifted a bit from the N4 plane. This is supported by the rhombic EPR spectra of I (g ] = 2.389. g2 = 2.069 and gl = 2.023 ; II ] = 137.9 X 10-4 cm· l , A2 = 18.1 x 10-4 cm·1 and A I = 17.7 x 10.4 cm· I ). The complex I undergoes a somewhat irreversible two-electron li gand oxidation at 0 .36 V vs SCE in dimethyl formam ide. The cyclic counterpart of I is not at all formed in the synthetic scheme adopted here.

Tetraaza macrocycles can be very conven iently synthesised from diamines or tetraamines having two

free -NH2 ends. The - NH2 ends can be joined together in the presence of a metal ion template I ike C 2+ N ' J+ b . I k ' 1-1 0 I u or 1- y various oc II1g reagents. ' ne suc 1 locking process, which can be accomplished in a single pot, is illustrated in Scheme I. For a mechanisitic understanding of Scheme I, see ref. 4. So far, Scheme I ha s been applied to aliphatic diamines only. Consequently all the tetraaza macrocycles prepared to date by following Scheme I do not have any unsaturation in the macrocyclic ring. But synthetic tetraaza macrocycles having unsaturations are of special interest in inorganic

Scheme I-M: a metal ion

chemistry because of their semblance to varIOUS naturally occurring porphyrins. In an attempt to introduce unsaturations in a tetraaza macrocyc le afforded by Scheme I , we have applied it to 1,8-diaminonaphthalene (I ,8-DAN). The results are reported here.

Materials and Methods 1,8-Diaminonaphthalene (97%) was purchased

from Lancaster (England) and was used as such. Formaldehyde solution 35% was procured from E. Merck (India) Ltd and nitroethane from Spectrochem Pvt. Ltd (India). Fresh analytical reagent grade DMF (from S. D. Fine-Chem Ltd. , India) was used directly without further purification for electrochemistry. C, H and N analyses were performed us ing a Perkin-Elmer 240011 analyse r. Copper was estimated gravimetrically as C uSCN. IR spectra (KBr di sc; 4000- 400 cm- I ) were recorded on a Perkin-Elmer 783 spectrophotometer and UV -vi s spectra on a Shimadzu

992 INDIAN JT H EM, SEC. A, OCTOBER 1999

1

UV -160A spectrophotometer. Solution conductivity was measured by a Systronics (India) direct reading conductivity meter (model 304). Thermal analys is (TG-DTA) was performed by a Sh imadzu DT-3 0 Thermal Analyser in dynamic atmosphere of dinitrogen (flow rate : 30 cm) mi n-i) wi th inert alumina as reference; the sample (partic le size within

150-200 mesh) was heated in a platinum crucible at a rate of 10DC min-i . Magnetic susceptibility was determined at room temperature by a PAR ISS vibrating sample magnetometer. The magnetometer was calibrated with Hg[Co(SCN)4] and the susceptibility data were corrected for diamagnet ism using Pascal 's constants. X-band EP R spectra were recorded . by a JEOL RE I X spectrometer. Cyclic voltammetry and coulometry were performed using EG&G PARC electrochemical analysis system (model 250/5/0) under dry nitrogen atmosphere in conventional three electrode configurations with tetraethylammonium perchlorate as the supporting electrolyte . A planar EG&G PARC G0228 platinum mill i-electrode was used as the working electrode in cyclic voltammetry. The potentials are reported here with respect to an SCE (saturated calomel electrode) and are uncorrected for liqu id j unction potentia ls . Under the experimenta l conditions emp loyed here, the ferrocene-ferrocenium coupie appears at 0.406 V vs SCE. Constant-potentia l coulometry was performed using a platinum wire gauge working e lectrode and a PAR 377 A cell system.

Synthesis and characterisation oj(l)(BF4hC2H50H Cu(BF4)2.xH20 (x = 5-6) (660 mg) and 1,8-DAN

(635 mg; 4.01 mmol) were disso lved in 50 cm3

of ethanol. To this solut ion were added nitroethane (0.29 cm3; 4.04 mmol), fo rmaldehyde ( 1.08 cm); 10.07 mmol) and five drops of triethy lamine in succession. Immediately after the additions were over, the red-brown colour of the reaction mixture became dark green, which was stirred for 3 h. The dark green

QJ

U C o

E Ul C o L I-

1800 1600 1400

:)) ( c m-1 )

\ I I I I I \ I

" \ , I \

\

I I I I

I I I I I I I ,

I 'I I ~ I I i I I ,

1200 1000

Fig. I- A compari son of the IR spectra of Cu( 1.8-DANMCI0 4)2

(broken li ne) and ( 1)(BF4hC2HsOH (full line) in the region 1800 - 1000 cn,-I . The peaks marked by asterisks are tentatively ass igned to the vibrati ons o f the ni tro group in ( 1)(B F4h.C2HsOH. See text.

compound precipitated was fi ltered, washed w ith 10 cm) of I : I ethanol-diethyl ether mixture and dried by keepi ng in air overn ight. Then it was stored under nitrogen atmosphere; yie ld, 0.965 g (74 %). This compound is analyt ica lly pure. However, it can be recrysta lli sed from dimethylfo rmamide and ethanol mixture . [Found : C. 44. 81 ; H, 4 .3 9; N, 10.09; Cu, 8.96. Calc . for C26 HJI Ns0 .1 8 2FsCu: C, 44.67; 1-1 ,4.47;

, 10.02; Cu, 9. I 0% ]. TG: ca lculated loss of ethano l, 6.59 %; found , 6.37 %. pj~lB' 1.69 (at 300 K). UV-VI S (nujol) : Alnm : 670, 570 (sh), 360. UV-V IS (DMF) : Alnm (c/dm 3 mol" cm- I) : 650sh(7,700), 552sh(9,800), 340(34,400).

Molecular Mechanics calculations These were performed with the CAChe suite of

programs available from Oxford Molecular Group

Ipc.s This program starts with the MM2 force field deve loped by Allinger6 and augments it in three ways: (I) extending the force fie ld to additional bond and atom types by includ ing weak, coordinate and ion ic bonds and atoms with hybr idisations higher than Sp3, (2) recognising conjugated and other aromatic systems, and (3) systematically applying a set of empi rical rules which esti mate mi ss ing force-field constants. The energy terms for bond stretch, bond angle, di hedra l angle, improper torsion, van der Waals, e lectrostatics and hydrogen bonding

LEW IS

994 INDIAN J CHEM, SEC. A, OCTOBER 1999

Table l--Cyclic voltammetric data for (J )(BF4h.C2HsOH in DMF at a platinum electrode'

Scan rate Epe Ep, EI/2 tJ.Ep i pe i p, ip/ ip,

5 0.294 0.409 0.351 0. 115 3.21 3.2 1 1.00 10 0.305 0.408 0.356 0. 103 4.21 4.29 0.98 50 0.290 0.399 0.344 0. 109 6.95 10.95 0.63

100 0.283 0.408 0.345 0. 125 10. 10 17.68 0. 57 200 0.292 0.427 0.359 0.135 14.74 28.42 0.52 500 0.274 0.465 0.369 0. 191 17.10 49.40 0.35

1000 0.258 0.5 11 0.384 0.253 15.70 9 1.40 0. 17

'U nits used: scan rate, mY 5- 1; various potentials, Y; i pe and ipa> 1lA. So lute concentration, 0.73 mmol dm-l Supporting e lectrolyte, 0. 1 mol dm-l in tetraethylammonium perchlorate.

-H

756 f---<

Fig. 3-X-band EPR spectra of ( 1)(BF4)2.C2HsOI-i. Upper trace,

so lid state at 77 K; lower trace, in I : I DMF-tolune g lass at 77 K. For the upper trace: g = 2.09. For the lowcr trace : g l = 2.389, g2 = 2069 and gl = 2.023; A) = 137.9 X 10-4 em-I, A2 = 18. 1 X 10-4

cm-l andA I = 17.7x 10-4 cm-l

days, it no longer di sso lves in DMF. This is not assoc iated with the loss of the ethanol molecule from (1)(BF4h.C2HsOH. It is poss ibly because of aeria l oxidation (vide infra). To maintain its solubility (stabil ity), it is stored under ni trogen. Its molar conductance in DM F (67 mho cm2 mor l) shows that it is a I: I electrolyte in so lu tion7 indicating that one of the two tetraflu oroborate anions is coordinated to the copper atom even in solution. Coordination of tetrafluoroborate to the meta l is qui te common in the chemistry of copper(" ). For examples, see refs. I I and 12 .

Si nce we have not yet been able to grow single crystals of (1)(BF4h.C2HsOH, we have tried to examine its structure by Molecular Mechanics (MM ). The mi nimum energy structure of the cation 1 is shown in Fig. 2. The cation 1 has a butterfly li ke conformation where the naphthyl fragments fo rm the two wings. The two wi ngs span on the oppos ite side of the N02 group. The meta l ion is sli ghtly above the

N4 plane moving away from the nitro group. The di spos ition of the nitro group is such that it essentially blocks orl.e side of the metal ion so that coordination of only one BF4- anion to the metal is poss ible which is consistent with the so lution conductivity of (1)(B F4)2,C2 HsOH. Thus the actua l cation in (1)(BF4)2,C2HsOH is [(I )(BF4)f where the copper(lJ ) ion has a di storted square pyramidal coordination sphere with a flu orine atom of the BF4-anion occupying the apical position and the metal ion li fted a bi t from the N4 plane. In the minimum energy MM structure of 1, the two NH hydrogens are on the same side of the N4 plane but opposite to the N02 group. Its two other possible isomers are la, where the NH hydrogens are on the same side as the N02 group, and 1 b, where the N H hydrogens are anti to each other. Our MM calculations show that l a is energetically higher than 1 by 4. 89 kca l mo r l and Ib higher than la by i3 5 kcal mOr l.

The room temperature magnetic moment of (1)(BF4h .C2HsOH corresponds to one unpaired electron. Its X-band EPR spectra in solid state at room temperature and at 77 K show a somewhat isotropic signal around g = 2.09. However, in I: I DMF-toluene glass at 77 K the signal is reso lved into a rhombic spectra (Fig. 3) with gJ = 2.3 89, g2 = 2.069 and g l = 2.023 showing clear nuclear hyperfi ne splitting fo r all the g factors. The A val ues observed (F ig. 3) are as fo llows: AJ = 137.9 X 10-4 em-I, A2 = 18.1 X 10-4 em-I and AI = 17.7 X 10-4 em-I . The ra tio (g2 - gl )/(gJ - g2), which can be regarded as a measure of rhomb icity,13 is 0. 144 suggesting a ground state intermediate between a pure d} _ / and a dz

2 ground state with more preference for the fo rmer. The order of the A values (A 3 » A2 == A I) also suggests that the ground state for the cation [(I )(BF4)t is more or less a d/ _ / one. For an approx imately dz

2 ground state,

LEWIS et al.: TEMPLATE SYNTHESIS Of A Cu{JI) COMPLEX 995

2 L.O

o 0

« =

996 INDIAN J CHEM, SEC. A, OCTOBER 1999

coordination sphere. This is fully supported by its EPR spectra. (1)(BF4)2,C2H50H undergoes a somewhat irreversible two-electron ligand oxidation at 0.36 V vs SCE in DMF.

1,8-Diaminonaphthalenes are chern ically a very important class of molecules. Their N-alkyl derivatives are known as "proton sponges" because of their unusually high pKa values. 15 For example, the pKa value of I ,8-bi s( dimethylamino )naphthalene is 12.1 while that of ammonium ion is 9.2 (incidentally, the pKa value of 1,8-DAN is 4.6 and that of aniline 5.1). This property is believed to arise from the "proximity" of the two amino groups.1 6 Moreover, poly-I,8-DAN films, generated electrochem ically, have drawn considerable attention over the years in the area of surface modified electrodes. 12 Earlier Melson and Kakazal have synthesised some bis/tri s 1,8-DAN complexes of several metal ions .17 A subsequent study worth mention ing concerns some lanthanide complexes of the I: I condensate of 1,8-DAN and 2,6-diacetylpyrid ine .18 Here for the first t ime we have tried to synthes ise a tetraaza macrocycle out of it by using Scheme I. The reason for not achieving our goal, i.e. iso lation of 2, is not understood . The geometrical di stortion present in [(1)(B F4)r (vide supra) mi ght have led to a relative orientation of the two free NH2 groups un favo urable for further cyclisatiol1 by the locking reagents used here. It is of interest to note that use of a nicke l(lI) template In our synthetic proced ure does not bring about even the half-cyc lisation.

Acknowledgement DO is grateful to the Department of Science and

Technology, Government of India, New Delhi fo r financial assistance.

References I Lindoy L F, The chemistry of macrocyclic ligand complexes

(Cambridge'Univers ity Press: Cambridge) 1989, Table 2.1. 2 Comba P, Curti s N F, Lawrcnce G A, Sargeso n A M, Skelton

B W & White A 1-1 , In org Chern, 25 (1 986) 4260 and references therein .

3 Bernh ardt P V & Hayes E J, J chern Soc, Dalton Trans. ( 1998) 3539.

4 Fabbrizzi L. Licchelli M, Lanfredi A M M, Vassal li 0 & Ugozzoli F, In org Chern, 35 (1996) 1582.

5 Oxford Molecular Group Inc., P. O. Box 4003, Beaverton, Oregon 97076, USA.

6 Allinger N L, J Am chem Soc, 99 (1977) 8127: Burkert U & Alli nger N L. Molecular mechanics (American Chemical Society, Washington. DC) 1982.

7 Bondi A. J phys Chern, 63 (1994) 441 . 8 Comba P, Hambley T W & Lawrence G A. I-Ielv chilli Acta.

68 (1985) 2332. 9 You ng C G. Broomhead J A & Boreham C J. J chern Soc.

Dalton Trans, ( 1983) 2 I 35 . 10 Geary W J, Coord chern Rev, 7 (197 1) 81. II Narayanan B & Bhadbhade M M. Acta Crysw/logr. C52

( 1996) 3049. 12 Flanagan S. Dong J, Haller K, Wang S, Scheid t W R, Scott R

1\ , Webb T R, Stanbury D M & Wilson L .I , J Am chern Soc. 119 ( 1997) 8857.

13 Ray N & Hathaway B. J chem Soc, Dal on Trans , (1980) 1105 and rcferences therein .

14 Bagheri 1\ & Natcghi M R, Indian J Chelll. 37A ( 1998) 606 and references therein .

15 Mall inson P R, Wozniak K & Smith G T, JAm chem Soc, 11 9 (1997) 11502 and references therei n.

16 Perakyl a M J, J org Chem. 6 1 (1996) 7420. 17 Kakazal A J B & Melson G A, In org clllm Acta , 4 (1970)

360: Kakazal A J B & Mdson G A. J chem Soc Pak. 3 ( 198 1) 85; Chern ribs, 95 (1981 ) 1433 26h.

18 Radecka- Paryzek W & Patroniak V, Polyhedron , 10 (199 1) 683.