KINETIC AND SYNTHETIC STUDIES ON COBALT (III), NICKEL (II) AND COPPER (II) MACROCYCLIC COMPLEXES

Abstract

It is evident from the literature that very little work has been done on tetraaza 14-membered macrocycles carrying ethyl group as substituents on the ring or macrocycles which have a cis configuration. Some work reported earlier on these macrocycles deals with the synthesis of these macrocycles in the diene form and the characterization of their complexes. It was thus thought worthwhile to prepare Et4Me2 [14]diene and cis-Me6[ 14]diene by reported methods, subject them to reduction and carry out the following further work on these macrocycles : SYNTHESIS OF LIGANDS AND THEIR COMPLEXES Et4Me2[14]anes and their Complexes Et Me [14]diene was prepared by the non template condensation of 1,2-diaminoethane and ethylmethyIketone in which the C=N groups occupy the trans position. The diene was reduced with sodium borohydride to the corresponding saturated tetraaza macrocycle. The product has two carbon chiral centers at position 5 and 12 which can give two isomers labeled as LI and L2 which were separated by fractionation from xylene. The infrared spectra of LI and L2 show a sharp uNH band at about 3250 cm"1. These spectra also show subtle differences in the 1350-700 cm"1 region. Both isomers readily form complexes with nickel (II) copper(II) and cobalt( 111) . On coordination, four additional N-chiral centers are created. As a result of N-chirality, several isomers of the complexes are possible but only a few are expected to be stable enough to be isolated in the solid state. I have isolated two isomers of each metal with both LI and L2 and established their stereochemistry. Nickel(II) Complexes LI and L2 with nickel(II) give yellow or orange-yellow square planar, [NiL]2+ complexes. The infrared spectra of these complexes show a u NH band around 3200 cm and the perchlorate band around 1100 and 610 cm" . The spectra further show marked differences in the finger print region. The electronic spectra of these complexes show a d-d band around 21400 cm"1, which can be assigned to A > ^B transition. The high value of the extinction coefficient (40-95 dm3 mo 1"\m~1 )for this band shows that these comp lexes exist in almost 100% square planar configuration. Steric hindrance of the four bulky ethyl groups present on the macrocyclic skeleton prevents formation of octahedral species. Strain energy calculations with tetramethyl cyclam by Hambley show that the isomer with all NH groups lying on the same side of N1N. plane has the lowest strain energy and sh ould be the most stable. Thus, the first isomer isolated in these preparations should have this (R,R,R,R) configur ation. On the same basis the second isomer should have an R,S,S,R configuration. The p.m.r. spectra of LlA complex shows two methyl singlets at 1.65 and 1.12 ppm (3 protons each) due to the methyl component of the chiral ethyl groups. This shows that the chiral groups should have an equatoria1-axial arrangement which is present in diastereoisomers (i), (iii), (v) and (x) (Fig. 4 )• Structure (i) has an R,R,R,R configuration which by analogy should have least strain energy and hence is assigned to LlA complex. Structure (vi) has a trans-III configuration. The next most stable form LIB is assigned this configuration. The p.m.r. spectra of L2A complex shows one methyl singlet at 1.53 ppm which suggests a diequatorial arrange ment is present in diastereoisomers (v) (viii) and (x) (Fig. 5). L2A is assigned structure (x) as it has R,R,R,R arrangement. The p.m.r. spectra of L2B show a methyl singlet at 1.21 and 1.59 ppm which suggests an equatoria1-axial arrangement. This arrangement is present in diastereoisomers (ii), (iv) and (vi). Since trans-III is the next stable form, present in (iv), L2B is assigned this configuration. Cobalt(III) Complexes The trans- dichloro cobalt( III) complexes have been prepared by aeration of a mixture of cobalt(II) and the ligand solutions. Both LI and L2 give two N-chiral diastereoisomers. The electronic spectra show a broad d-d band around 1500 cm"1, which is typical of atrans CoN4 Cl„ chromophore. The infrared soectra of these shows the u NH band around 3200 cm an; the perchlorate band around 1100 and 610 cm These also show subtle differences in the 1500-700 cm 1 1 reg ion. The N-chirality of these diastereoisomers can also be assigned on the basis of their p.m.r. spectra as in the case of nickel(II) complexes. Copper(II) Complexes Ll and L2 give the square planar copper(II) complexes which analyse to the general formula [CuL](C10 ) . With both Ll and L2 two diastereoisomers have been obtained by using fractional crystallization during the preparation of the complexes. The infrared spectra of these complexes show uNH bands around 3200 cm"1 and UCIO4 bands around 1100 and 610 cm" These spectra also show subtle differences from each other in the 1500-800 cm"1 region as has already been noted for the corresponding nicke 1(11) complexes. Since the copper(II) complexes are also square planar like the corresponding nieke 1(11) complexes, these complexes by analogy should have the same structures as assigned to the nickel(II) diastereoisomers of Ll and L2. The electronic spectra of the complexes show a broad d-d band in the visible region which can be assigned to the overlap of a, -> b, , b0 lg lg 2g -> b, and e ig g > b ig transitions of square planar copper(II). Cis-Me.-[14]anes and their Complexes o The template condensation of 1,2-diamino ethane with acetone in presence of nickel(II) yields both the cis- and trans- forms. The cis- form has a much lower solubility than the trans- form (isomer A) and hence the two can be fractionally crystallized from water to a very high degree of isomeric purity. The macrocyclic complex [Ni(Cis-Me^[ 14] diene)](C10 ) has been labeled as isomer B while the trans form as isomer A. The cis- complex was reduced with sodium borohydride and nickel(II) from the reduced complex was knocked off by treating with excess sodium cyanide. The free macrocycle was recovered by extraction with benzene and the product is a mixture of C-meso and C-rac forms. The two isomers were separated from the product by fractional crystallization from xylene. The infrared spectra of the parent complex shows the (C=N) band at 1560 cm"1. This band is not present in the spectra of L3 and L4. L3 shows the uNH band at 3260 cm"1 and L4 at 3300 cm .The infrared spectra show marked differ ences in the 1400-700 cm"1 region indicating the two fractions to be distinct. L3 and L4 readily form complexes with nickel(II), cobalt(III) and copper(II). In each case again two dia stereoi somers have been isolated. Nickel(II) Complexes Both L3 and L4 give yellow or orange colored, square planar nickel(II) complexes, which analyse to the general formula ,[N iL ](C10J „. Their infrared spectra show the oNH band at 3200 cm"1. The spectra of the diastereoisomers show subtle differences in the finger print region. The elect ronic spectra of these complexes show a single d-d band in the region of 21600 cm"1 which compares favorably with the absorption bands reported for other polyaza macrocyclic complexes. Like Ll and L2, both L3 and L4 on complexation are capable of giving 10 diastereoisomers for each of the isomers. However, only a few of them are stable in solid state. The p.m.r. spectra of nickel (II) complexes of cis-Me [14]anes have been identified easily by multiplets in the range of 2.0 to 3.0 ppm due to the methylene protons of the carbon chain and singlet and doublets in the range of 0.90 to 1.8 ppm due to the methyl groups. The most prominent feature of these p.m.r. spectra are the two sharp singlets due to the gem-dimethyl groups appearing around 1.00 and 1.7 ppm. In addition two doublets due to the chiral methyl groups also appears in this region. The gem-dimethyl group singlets can be assigned to their axial and equatorial components. The doublet arises due to the coupling of the .methyl resonance with the single proton on the adjacent carbon atom. The p.m.r. spectrum of [Ni(L4A)](C104 )g shows two singlets at 1.20 and 1.85 ppm due to the equatorial and axial components of the gem-dimethyl groups. The spectrum further shows one doublet at 1.6 ppm which is due to a diaxial arrangement of the chiral methyl groups. This shows that the complex should have a trans-III structure which places both the chiral methyl in adiaxial geometry. The second isomer, [Ni(L4B)](C104)2 should then have a N-rac configuration. The same criteria would hold good for [Ni(L3A)](C104)2 and [Ni(L3B)](ClO^g. Cobalt(III) Complexes Both L3 and L4 give the green colored trans-dich loro complexes, [CoLCl^, and with both L3 and L4 two diastereoisomers have been obtained. The electronic spectra of these complexes show a broad d-d band around 15000 cm" as expected. The infrared spectra of these complexes show the appearance of the uNH band around 3200 cm" and perchlorate band around 1100 and 610 cm" . These spectra also show marked differences in the 1400 -800 cm" region. The p.m.r. spectra of these complexes show similar trends to what have been discussed in the case of nickel(II) complexes. In cobalt(III) complexes of similar ligands, the axial methyl appears upfield as compared to the equatorial methyl group. The p.m.r. spectra of [Co(L3A)C1 2 ]C104 show two singlets at 1.35 and 1.7 ppm due to the axial and equatorial components of gem-dimethyl groups. Each signal integrates to 6H. In addition the spectra shows a doublet at 1.55 ppm corresponding to 6H and is assigned to a diequatorial chiral methyl group arrangement. This pattern assigns the structure to the trans-III form. The p.m.r. spectra of [Co(L3B) C1 ]C 10. show two singlets at 1.35 and 1.65 ppm (6H each) assigned to gemdimethyl groups. It also shows a doublet at 1.55 ppm (6H) which is assigned to diequatorial methyls. The same criteria would hold good for [Co(L4A)C 1 ]C 10 and [Co(L4B )C1 ]C10 . Copper(II) Complexes L3 and L4 form square planar copper(II) complexes which analyse to the general formulae, [CuL](C10^)2. Here also two diastereoisomers have been isolated in each case. The electronic spectra of each complex shows a broad d-d band as expected for square planar copper(II) complexes. These complexes also give distinct infrared spectra as expected and show the uNH band around 3200 cm"].Copper(11) complexes by analogy should have the same structures as assigned to the nieke 1(11) diastereoisomers of L3 and L4. ACID HYDROLYSIS In the acid hydrolysis (also called aquation) of the cobalt(III) complexes, it has been shown that the reaction follows a dissociative mechanism. Here the bond breaking step is usually rate determining step. It has been shown that acid hydrolysis of cobaIt(111) with polyaza macrocycles also follows a similar mechanism. In addition large variations in rate are observed with a change in ring size or even when the nitrogen chirality is changed. These variations in the acid hydrolysis rates have been inter preted in terms of strain energy of the macrocycle and also on the ability of the macrocycle to fold. In this work the kinetic studies on the acid hydrolysis of the diastereo isomers of cobalt(III) complexes with Ll, L2, L3 and L4 have been carried out. 10 A macrocycle can fold along the diagonal of the N» plane if the two NH groups placed on this diagonal lie on the same side of the N» plane. In the case of R,R,R,R isomer, there are two fold axis available so that the macrocycle can fold along each fold axis in either direction. Either way, the folding is going to produce a steric compression on one of the trans- chlorides. This compression accelerates its rate of aquation as we already know that in aquation bond breaking is the predominant rate determin ing step. The rate of aquation of the present coba11(111) macrocyclic complexes show that isomer A's aquate at a lower rate as compared to the isomer B's. Isomer A's have been assigned the trans-III form and hence have no fold axis. Isomer B's have a fold axis and the rate enhancement could be accounted for by the folding of the macrocycle towards the leaving group. Complexes which show a positive value of entropy of activation aquate with inversion of configuration. The present complexes show a negative entropy of activation. If the macrocycle can fold, the intermediate would resemble a trigonal bipyramid and would result in inversion, while distortion towards a trigonal bipyramid is possible so that in such cases even if the macrocycle can fold, retention of configuration would be observed. The rate of aquation would be decided by the ease with which the distorted trigonal 11 bipyramid intermediate is achieved and this is the main driving force behind the reaction. POLAROGRAPHY The polarographic technique provides a yery convenient method for studying redox reactions. The early work on the redox behaviour of macrocyclic complexes had mainly dealt with 14-membered unsaturated systems while later studies have dealt with several types of saturated systems also. All the complexes studied give well defined, diffusion controlled one electron transfer reduction waves. The waves are irreversible in nature. The low values of k0^ u, the formal rate of reduction, also support this conclusion. In the case of nickel(II) complexes, a regular cathodic shift in half wave potential in changing over from the transstructured macrocycles to the cis- ones is observed. This type of behaviour has been correlated with the basicity of the donor nitrogens. General Introduction 12 The coordination chemistry of metalloporphyrins is of continuing interest due to their role in energy transport and respiration system and also due to the wide variety of metal ions that can be bound (or chelated) by the porphyrins. Synthetic macrocycles are similar to the porphyrins in the sense that they have similar coordination sites to porphyrins available for coordination. Synthetic polyaza macrocycles have same unique characteristics which makes this worthy of exploring their new synthetic techniques and properties. Some of the features which contribute to interest in these systems are: 1. A kinetic inertness, both to the formation of the complexes from the ligand and metal ion and the reverse, i.e. their dissociation involving extraction of the metal ion from the ligand. 2. They can stabilize unusual oxidation states such as Cu(I), Cu(III), Ag(II), Ag(III), Ni(III) etc. which are otherwise of transient nature. 3. These complexes have high thermodynamic stability, e.g. the tetraaza macrocycles may have orders of magnitude higher formation constant values than for similar non macrocyclie 1igands . As a result, a large amount of work have been done on the coordination chemistry of polyaza macrocylic complexes during the last two decades [1-10]. 13 The synthetic methods for the synthesis of the polyaza macrocycles can be divided into two categories : (a) Template methods (b) Non template methods These methods involve direct synthesis of macrocylic complexes from the linear precursors in the presence of metal ions [11-22]. The utilization of metal ions to control the steric course of reactions is designed to produce macrocyclic ligands and it has been realized that the metal ion may serve as a template to organize the course of complex multi step reactions. Such processes are known as template reactions [23]. :n 14 The best example of such a template reaction directly related to biological materials, has been reported by Johnson, Kay and Rodrigo [24] for syntheses of corrin like rings [I]. It involves the reaction of 5,5'-bi-[5 '-bromo - 3,3•-diethyl-4.4'-dimethyl dipyromethane)-pa 1ladium(11) with formaldehyde (II). The characteristic feature of the reaction is the linking of coordinated amine groups by a three carbon bridge with acetone as the carbonyl moiety, the bridging group is diacetone amine, imine formed from the acetone molecules.....................