TEMPLATE SYNTHESIS OF 2,11-DIMETHYL-3,12-DIETHYL / PROPYL /
PHENYL-1,4,10,13-TETRAAZACYCLOOCTADECA-1,3,10,12-TETRAENE
COMPLEXES OF Mg(II), Ca(II), Sr(II) AND Ba(II)

R N. PRASAD*, SEEMA GUPTA

Department of Chemistry, University of Rajasthan, Jaipur - 302004, India
e - mail : prasadraghunandan @ yahoo.com
(Received: December 12, 2001 - Accepted: September 10, 2002)

SUMMARY

2+2 Cyclocondensation of 1,5-diaminopentane with b-diketones viz. 2,3-pentanedione, 2,3- hexanedione or 1-phenyl -1,2- propanedione in the presence of alkaline earth metal ions as templates yield a series of hexacoordinated complexes of the type [ML(X₂)] ( where L=N₄ macrocyle having 18-membered ring and X = Cl or NCS) The resulting complexes have been characterized by elemental analyses, conductance measurements, IR and ¹H NMR spectra.

INTRODUCTION

In the recent years there has been a lot of interest in the field of macrocyclic complexes because of their close resemblance with naturally occurring biological systems. A variety of macrocycles viz. crown ethers, cryptands and azamacrocycles have been synthesized. Azamacrocycles have strong tendency to form stable complexes with transition metals^1-3) . Bhoon and Singh⁴⁾ have synthesized Ni(II) complexes of tetraazamacrocyles by the condensation of _-diketones with diamines. Cr(III), Fe(III), Co(II), Zn(II) and Cd(II) complexes of azamacrocycles derived from _-diketones and diaminoalkanes have been reported from our laboratories^5-13). Drew et al.¹⁴⁾ have synthesized pentagonal bipyramidal Mg(II) complexes by the template condensation of 2,6-diacetylpyridine with triethylenetetraamine. Fenton and coworkers¹⁵⁾ have reported Ba(II) complex of a 24-membered macrocycle derived from 2,6-diacetylpyridine and N,N-bis(2-aminoethyl)-2-methoxyethylamine. Mg(II) complexes of tetraazamacrocyles derived from 2+2 cyclocondensation reactions of _-diketones with aliphatic diamines having even number of carbon atoms have been reported from our laboratories ^16,17) In the present paper synthesis of Mg(II), Ca(II), Sr(II) and Ba(II) complexes of 18- membered macrocycles 2,11-dimethyl-3,12-diethyl / propyl / phenyl - 1,4,10,13-tetraazacyclooctadeca-1,3,10,12-tetraene derived from _- diketones and an aliphatic diamine containing odd number of carbon atoms i.e. 1,5-diaminopentane is described.

EXPERIMENTAL

Materials

2,3-Pentanedione (Fluka), 2,3-hexanedione (Aldrich), 1-phenyl-1,2-propanedione (Aldrich) and 1,5-diaminopentane (Aldrich) were used as such. MgCl₂.6H₂O (BDH), Ca(NO₃)₂.4H₂O (BDH), SrCl₂.6H₂O (E. Merck) and BaCl₂.2H₂O (Glaxo) were of AR grade. Methanol, ethanol and butanol were distilled before use.

Synthesis of Mg(II) complexes of tetraazamacrocycles

MgCl₂.6H₂0 ( 0.98 mmol) was dissolved in ~ 20 ml of n-butanol. A butanolic solution of 2,3-pentanedione (1.96 mmol) was added. To this, 1,5-diaminopentane (1.96 mmol in 20 ml n- butanol) was added drop by drop with constant stirring. Stirring was continued for ~ 6 hrs. The solid separated was filtered, washed with butanol and dried in vacuo. Similarly, the reactions of MgCl₂.6H₂O were carried out with 1,5- diaminopentane and 2,3- hexanedione or 1-phenyl-1,2-propanedione and Mg(II) complexes of 18- membered tetraazamacrocycles were isolated.

Synthesis of Ca(II) complexes of tetraazamacrocycles

To the butanolic solution of Ca(NO₃)₂.4H₂O ( 0.87 mmol in ~ 20 ml), a butanolic solution of 2,3-pentanedione (1.74 mmol) was added. To this, 1,5-diaminopentane (1.74 mmol) was added. As no solid appeared a butanolic solution of potassium thiocyanate (1.74 mmol) was added. The contents were stirred for ~ 6 hrs and the temperature was maintained at 40^oC throughout the course of the reaction. Solid obtained was filtered, washed with butanol and dried under vacuo. Similar reactions of Ca(NO₃)₂.4H₂O were carried out with 1,5- diaminopentane and 2,3- hexanedione or 1-phenyl-1,2-propanedione.

Synthesis of Sr(II) complexes of tetraazamacrocycles

To the methanolic solution of SrCl₂.6H₂O (1.74 mmol in ~ 20 ml) a solution of 2,3-pentanedione (3.48 mmol in methanol) was added. To this reaction mixture, methanolic solution of 1,5-diaminopentane (3.48 mmol) was added with continuous stirring. As no solid appeared a methanolic solution of potassium thiocyanate (3.48 mmol) was added. Reaction contents were stirred for ~ 10 hrs. Solid obtained was filtered, washed with methanol and dried under vacuo. Similarly, the reaction of SrCl₂.6H₂O with 1,5- diaminopentane and 2,3- hexanedione was carried out.

Synthesis of Ba(II) complexes of tetraazamacrocycles

BaCl₂.2H₂O (2.02 mmol) was dissolved in ethanol-water (1:1) and ethanolic solution of 2,3-pentanedione (4.04 mmol) was added. To this, 1,5-diaminopentane (4.04 mmol in ethanol) was added drop by drop with continuous stirring. As no solid appeared ethanolic solution of potassium thiocyanate (4.04 mmol) was added with continuous stirring. After half an hour solid appeared. Stirring was continued for ~10 hrs. The solid obtained was filtered, washed with ethanol and dried in vacuo. Similarly, the reaction of BaCl₂.2H₂O was carried out with 1,5-diaminopentane and 2,3-hexanedione and Ba(II) complex of 18- membered tetraazamacrocycle was isolated.

Analytical methods and physical measurements

Magnesium, calcium and strontium were determined volumetrically using EDTA, barium gravimetrically as barium sulphate, nitrogen by kjeldahl's method and chlorine gravimetrically as AgCl. Infrared spectra were recorded as KBr pallets in the region 4000-400 cm^-1 on a Nicolet Magna 550 FT IR Spectrophotometer and ¹H NMR spectra were recorded in DMSO-d₆ on Zeol FX 90 Q FT NMR Spectrometer using TMS as a reference. Conductances were measured using a Systronics Direct Reading Conductivity Meter-304.

RESULTS AND DISCUSSION

The reactions of metal chlorides or nitrates with 1,5-diaminopentane and different _ -diketones such as 2,3-pentanedione, 2,3-hexanedione or 1-phenyl-1,2-propanedione in 1:2:2 molar ratios result in the formation of M(II) tetraazamacrocyclic complexes (I) according to the following general scheme:

M = Mg; X= Cl	R2	R1
M= Ca, Sr, Ba; X=NCS	CH3	C2H5
	CH3	C3H7
	CH3	C6H4

The resulting macrocyclic complexes are yellow to brown solids.and are insoluble in chloroform and nitromethane but soluble in DMSO. The characteristics and analyses of the complexes are given in Table I.

TABLE I. Analyses and characteristics of M(II) complexes of tetraazamacrocycles

Complex	Colour and	% Analysis
	Temp. of decomposition	M found (Calc.)	N found (Calc.)	Cl found (Calc.)
[ Mg(Me2Et2[18] tetraene N4 ) Cl2 ]	Yellow 235	5.58 (5.68)	12.88 (13.09)	16.70 (16.57)
[ Mg(Me2Pr2[18] tetraene N4 ) Cl2 ]	Yellow 220	5.11 (5.33)	12.38 (12.29)	15.29 (15.55)
[ Mg(Me2Ph2[18] tetraene N4 ) Cl2 ]	Yellow 140	4.64 (4.63)	10.63 (10.69)	15.44 (15.53)
[ Ca (Me2Et2 [18] tetraene N4 ) (NCS)2 ]	Yellow 205	7.94 (8.20)	11.45 (11.46)
[ Ca (Me2Pr2[18] tetraene N4 ) (NCS)2 ]	Yellow 240	7.71 (7.75)	10.79 (10.84)
[ Ca (Me2Ph2 [18] tetraene N4 ) (NCS)2 ]	Olive green 170	6.77 (6.85)	9.55 (9.57)
[ Sr (Me2Et2[18] tetraene N4 ) (NCS)2 ]	Brownish yellow 180	16.26 (16.33)	10.41 (10.44)
[Sr (Me2Pr2[18] tetraene N4 ) (NCS)2 ]	Brown 195	15.51 (15.52)	9.92 (9.92)
[Ba (Me2Et2[18] tetraene N4 ) (NCS)2 ]	Brown 195	23.41 (23.43)	9.55 (9.55)
[Ba (Me2Pr2[18] tetraene N4 ) (NCS)2 ]	Brown 220	22.34 (22.36)	9.09 (9.12)

Infrared spectra

Important infrared absorption bands of the complexes are given in Table II.

TABLE II. Molar conductances and important IR bands (cm^-1) of M(II) complexes of tetraazamacrocycles

Complex	n C=N	n NCS	n C=C phenyl	C-H phenyl bending	Molar cond. (ohm-1cm2mol-1)
[Mg(Me2Et2[18]tetraeneN4)Cl2]	1600				62
[Mg(Me2Pr2[18]tetraeneN4)Cl2]	1600				67
[Mg(Me2Ph2[18]tetraeneN4)Cl2]	1600		1400	720	96
[Ca(Me2Et2[18]tetraeneN4)(NCS)2]	1560	2070			0171
[Ca(Me2Pr2[18]tetraeneN4)(NCS)2]	1600	2070			0175
[Ca(Me2Ph2[18]tetraeneN4)(NCS)2]	1600	2070	1380	720	0126
[Sr(Me2Et2[18]tetraeneN4)(NCS)2]	1600	2070			27
[Sr(Me2Pr2[18]tetraeneN4)(NCS)2]	1600	2070			22
[Ba(Me2Et2[18]tetraeneN4)(NCS)2]	1580	2070			14
[Ba(Me2Pr2[18]tetraeneN4(NCS)2]	1600	2050			15

No absorption band was observed at 1700 cm^-1 or 3200-3400 cm^-1 indicating the absence of residual >C=O or NH₂ group^18,19). All the complexes show a strong absorption band in the region 1560-1600 cm^-1 assigned to the coordinated >C=N group. Drew and coworkers²⁰⁾ reported a strong band at 1600-1610 cm^-1 characteristic of the coordinated C=N group in Ca(II), Sr(II) and Ba(II) complexes of macrocycles derived from 2+2 cyclocondensation of 2,5-diformylfuran with o-phenylenediamine. For TIM complex of Co(II),n C=N bands have been reported at 1550-1600 cm^{-1 21)}. Complexes of macrocycles containing phenyl ring show absorption bands at 1380-1400 cm^-1 due to the n C=C of phenyl group. These complexes exhibit bands at 720 cm^-1 assignable to C-H out of plane bending of phenyl groups. In Ni(II), Co(II) and Fe(II) complexes of MePhTIM, Eggleston and Jackels ²²⁾ have reported intense bands at 700 cm^-1 and 750 cm^-1 due to aromatic C-H out of plane modes. Bands at 2050-2070 cm^-1 are assigned to N-bonded thiocyanate stretching vibrations in thiocyanate complexes of Ca(II), Sr(II) and Ba(II). Appearance of these bands at lower wave number in the complexes as compared to free thiocyanate (2100 cm^-1) supports the coordination of thiocyanate group to the metal atom through nitrogen. Fenton and Cook²³⁾ have reported N-bonded thiocyanate stretching frequencies at 2063, 2073 and 2081 cm^-1 in Ca(II), Sr(II) and Ba(II) complexes respectively of the macrocyle 3,15,21-triaza-6,9,12-trioxa-bicyclo[15.3.1] heneicosa-1(21),2,5,17,19-pentaene. The chloro complexes [MgLCl₂] exhibit a band at 400 cm^-1 which may be assigned to coordinated chloro group²⁴⁾. Absorption bands in the region 2900-3000 cm^-1 are assigned to aliphatic C-H stretching absorptions. Jackels et al. ²²⁾ have reported similar bands in the region 2900-3000 cm^-1 in Co(MePhTIM).

Conductance measurements

Molar conductances of the complexes in DMSO (10^-3 M solutions) are recorded in Table II. All the complexes are non-electrolytic in nature which indicates that both chloro or thiocyanato groups are coordinated to the metal atom. Thus, the metal atoms are hexacoordinated in these complexes. The crystal structure of [Mg(urea)₄)H₂O)₂]Br₂, in which Mg²⁺ cation is octahedrally surrounded by four urea molecules and two water molecules,has been reported ²⁵⁾. However, in some complexes the higher values of conductances may be due to the replacement of weakly coordinated chloro or thiocyanato groups by the solvent molecules as such type of complexes are generally hexacoordinated as reported earlier ¹⁷⁾.

Nuclear magnetic resonance spectra

The structures of complexes of tetraazamacrocycles were confirmed by ¹H NMR spectra. d ppm values of the protons in the ¹H NMR spectra of the macrocyclic complexes are given in Table III. In KIM (II) the b-CH₂ protons have been reported to appear as a triplet at d 3.64 ppm and a-CH₂ protons as a quintet at d 2.11ppm ²⁶⁾. In free diamines, the a -CH₂ protons appear as a triplet at d 2.69-2.79 ppm while b and other CH₂ protons appear as a quintet at d 1.33-1.60 ppm ²⁷⁾. The downfield shift of KIM protons as compared to the free diamines is due to deshielding by by p electrons of C=N

 TABLE III. ¹H NMR Chemical shifts (d , ppm) of tetraazamacrocyclic complexes

Complex	Amine residue		Ketone residue			Aromatic Protons
	a - CH2	b - CH2	CH3a	CH3C	CH2b
[Mg(Me2Et2[18]tetraeneN4)Cl2]	2.71t	1.44b	0.0.87	t 2.09s	2.37m
[Mg(Me2Pr2[18]tetraeneN4)Cl2]	2.71t	1.42b	0.87	t 2.00s	2.37m
[Mg(Me2Ph2[18]tetraeneN4)Cl2]	2.74t	1.42m	2.05s			7.51b,7.97b
s = singlet, t = triplet, m = multiplet, b = broad

KIM

bond. The free macrocycle, though not isolated, would have exhibited these resonances at almost similar positions as in KIM. In the macrocyclic complexes _ -CH₂ protons exhibit a triplet at _ 2.71-2.74 ppm (J = 6-7 Hz) and other methylene protons of amine residue are observed as a broad peak at _ 1.42 ppm. Upfield shifting of _ - and _ -CH₂ protons in macrocyclic complexes as compared to KIM supports the coordination of nitrogen to the metal atom.

In the complex (III) of the macrocycle derived from 2,3-pentanedione, the CH₃^a protons of the ketone moiety give rise to a triplet at _ 0.87 ppm while the CH₂^b protons of ethyl group appear as a multiplet at _ 2.37 ppm, CH₃^c protons exhibit a singlet at _ 2.09 ppm. In free 2,3-pentanedione (CH₃^cCOCOCH₂^bCH₃^a) the CH₃^c protons give a singlet at _ 2.35 ppm, CH₃^a protons exhibit a triplet at _ 1.06 ppm while CH₂^b protons give a quartet at _ 2.74 ppm. The high field shifting of CH₃^c protons and CH₂^b protons in the complexes confirms the coordination of the nitrogen of macrocyle to the metal atom. However, the CH₃^a protons of the ethyl group are very slightly shifted as these are away from the C=N group and are not affected much by coordination. In the complexes of the

macrocycles derived from 2,3-hexanedione, the peaks of the protons of ketone residue are almost at the same positions as in the complexes of macrocycles derived from 2,3-pentanedione.

In the NMR spectrum of Mg(II) complex (IV) of the macrocycle derived from 1-phenyl-1,2- propanedione and 1,5- diaminopentane the CH₃^c protons give a singlet at _ 2.05 ppm. Jackels et al. ²⁸⁾ have reported the methyl signal at _ 2.15-2.26 ppm in ¹H NMR spectrum of Zn(II) complexes of MePhTIM in CD₃NO₂. The aromatic protons give very weak broad signals at _ 7.51 and 7.97 ppm.

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