STUDIES ON THE TRANSITION METAL COMPLEXES OF TETRAAZAMACROCYCLIC LIGANDS

Abstract

The importance of synthetic macrocyclic complexes as models for their natural counterparts is now well recognised. However, a clear understanding of the basic properties of the metal ions in these systems is still ' lacking due to the structural complexities of these ligands. It is, therefore, quite evident that the design and syn thesis of small molecules and ions that are capable of per forming the functions identified with metal containing natural products is a multifaceted problem. It can be expected that research efforts in this direction will develop the chemistry implied by the functions of metal ions in natural systems and will contribute to the understanding of biochemistry as well as provide informations in the basic area of metal ion chemistry. The importance of cyclam'and its derivatives lies in the fact that they have a basic framework similar to natural macrocyclic complexes. The alkyl substituted macrocycles and their complexes are interesting from a stereo chemical viewpoint also. A comprehensive survey of literature' on the complexes of methyl substituted fourteen membered macrocycles with a tetraaza donor system shows that a considerable amount of work has been carried out on their synthesis and reactions. Some stereochemical studies have been reported on di-, tetra-, penta- and hexa-methyl substituted fourteen membered rings. In the case of Me2[l4]anes and Me6Ll4janes, several isomeric species have been isolated X and characterised. In contrast, relatively negligible work has been reoorted on the octamethyl substituted tetraaza rings. An attempt has, therefore, been made to carry out systematic studies on the isolation and characterisation o f isomeric MeJl4]anes and their complexes. The work '8 carried out during the course of these studies can be broadly divided into three portions. (a) Preparation of 3,10-C-meso-Me8Ll4]diene by a nontemplate method, reduction of the diene to MeQ[l4]ane, isolation of C-chiral isomers of this ligand and their characterisation. (b) Synthesis of copper(ll), nickel(Il), cobalt(lll) and rhodium(IIl) complexes of isomeric Me8[l4]anes, separation of their N-chiral isomers, their characterisation and stereochemistry. (c) Aquation kinetic studies on the cobalt(lll) complexes. Synthesis The ligand, 3,10-C-meso-Me8[l4]diene has been prepared by the condensation of 1,2-propanediamine with acetone. Reduction of this diene with NaBH4 gives three C-chiral isomers. These isomers have been separated by fractionation from xylene. These isomeric ligands easily undergo complexation with copper(ll), nickel(ll), cobalt(lll) and rhodium(lll).. On complexation, each isomer further yields N-chiral isomers, which have been separated by sequential precipitation during synthesis. The isomeric ligands and their complexes have been characterised on the basis of 13 their infrared, visible, ultraviolet, mass, C nmr and 1„ 4. H nmr soectra. Mes[l4]anes The non-template method adopted for the synthesis of the ligand MeR[l4]diene gives a completely stereospecific ligand 3,10-C-meso-Meg[l4]diene. Attempts to isolate the 3,10-C-rac-isomer were unsuccessful. Reduction of 3,10-Cmeso- Mep[l4]diene with NaBH. generates two carbon chiral centres at position 5 and 12 and as a consequence three possible isomers labelled as LA, Lg and Lq could be isolated by fractionation from xylene. All the isomers show distinct m.pts. and differences in solubilities in some common solvents. Isomeric purity of each is-omer has been checked by TLC. Their infrared spectra show a sharp fNH band at about 3,200 cm" . Comparison of these spectra (Fig.1,p.58) shows subtle differences in the 1350-700 crn~~ region. This fact has been used to check the reproducibility of the synthetic procedures developed. Mass spectra A detailed analysis of the mass spectra of all the isomers has been carried out. These isomers show the same fragmentation pattern but intensities of ion peaks are different. All the isomers show a peak at m/e 312 (Fig. 2-4, p. 62-64) corresponding to molecular ion. An addit ional peak at m/e 313 corresponding to M+l peak is observed in case of Ln. The coincidence of observed intensity of this peak with the calculated intensity gives an additional evidence for correct molecular weight. The base peak at m/e 157 occurs by the double a-cleavage. The detailed mass spectral study shows that the prominent mode of fragment ation is a-cleavage, removal of an alkene by C-N bond rupture followed by rearrangement of hydrogen. Removal of methyl and hydrogen radicals are as commonly observed for other amines. The detailed fragmentation pattern is pres ented in scheme 4 and 5 (p.68,69,71,72,74). N.m.r. spectra of Meg[l4]anes H nmr spectra The parent macrocycle, 3,10-C-rneso-Me8[l4]diene undergoes reduction with the retention of 3R, 10S configur ation, while two C-chiral centres are created at position 5 and 12 which leads to the possibility of formation of three C-chiral isomers (Scheme 2, p.56 ). The configuration of the possible isomers have been assigned on the basis of 1H nmr spectra. The axial or equatorial arrangement of the various methyl substituents has been made by examining molecular models of each isomer. In each model structure, the ring is placed in a'chair form with minimum hydrogenhydrogen interactions and two adjacent carbons in a gauche conformation. The 400 mHz H nmr spectrum of LA (Fig.5a,p.77 ) shows a sharp singlet at 1.6 ppm corresponding to the equatorial components of the gem-dimethyl group. An unresolved multiplet at 1.7 ppm has been assigned to the overlap of the singlet of the axial components of the gem-dimethyl group with the doublet arising out of the chiral methyls at C3, C5, C1Q and C±2 positions. This type of pattern for L. is suggestive of a completely C-meso structure in which C3> C&> C1Q and C12 methyls are all equatorial with C3R* C&,S; C1Q,s; C12,R configuration (Structure(i), Scheme 2,p.56 ). A similar assignment has been reported for [Ni(teta)j(C104)2 where teta has a C-meso configuration. Isomer Lg shows (Fig.5b,p.77 ) two singlets (each for 6H) at 1.08 and 1.16 ppm corresponding to axial and equatorial gem-dimethyls and the two doublets (each for 6H) at 0.99 and 1.03 ppm can be assigned to a diaxial-diaquatorial arrangement where C3, C,Q and C^, C,2 should be pairwise equivalent. The C,-, C,2 methyls occupy a rac-configuration with C3,RJ C5,R; C1q,S; C12»R chiralities (Structure(ii), Scheme 2,p.56). Isomer Lp shows (Fig.5c,p.77 ) two singlets at 1.07 and 1.11 ppm for gem-dimethyl groups and three doublets at 0.99, 1.08 and 1.14 ppm corresponding to 6, 3 and 3 protons for the four chiral methyls. The doublet at 0.99 ppm can be assigned to a pair of eqivalent equatorial methyls and at lo08 ppm for an equatorial methyl. The doublet at 1.14 ppm is assigned to an axial methyl. This is also suggestive of a C-meso configuration for C,-, C,2 methyls in an arrangement opposite to the one already assigned to L. and the chiral centres have C3,RJ C^,R; C1Q,S; C,2S configuration (Structure(iii), Scheme 2,p. 56').