PHYSICO-CHEMICAL STUDIES ON SOME SYNTHETIC MACROCYCLIC COMPLEXES-POTENTIAL MODELS FOR NATURAL MACROCYCLIC SYSTEMS

Abstract

Macrocyclic complexes play significant chemical role in biological systems. The function of the metal ion in such systems depends on their confinement within more or less constant coordina tion environments made up of highly conjugated organic species. Metals so trapped behave differently from the corresponding more labile complexes which obviously offer a restricted role in compli cated biological,systems like haemoglobin, iron porphyrins, chlorophyll, vitamin B _ etc. The nitrogen donor synthetic macrocyclic complexes have close resemblance in their structure with their natural counter parts and can thus be considered as models for them. Curtis* pioneering work on the synthesis and characterisation of metal{I) and metal(lll) macrocyclic complexes containing the cyclic Schiff base and tetramines has orovided ample justification of this view point. The investigations incorporated in the present thesis deal with the physico-chemical properties of some new Fe(III) macrocyclic complexes. The investigations include their synthesis and spectral (including Mossbauer spectra), magnetic, polarographic and cyclic voltammetric studies. These complexes were prepared via template synthesis using o-phenylene-diamine, anhydrous ferric chloride and some diketones such as biacetyl, benzil, benzoin, acetyl acetone, l,phenyl 1,3-butanedione and 1,3 diphenyl 1,3-propanedione. The general (ii) formulae of the complexes are as follows: Pe(R4BzO [ 12] tetraene-N4)Cl2; Fe<R4Bz02[ 12] diene-N^Cl^; Fe(R4BzO [ 14] tetraene-N )C13; Fe(R,R,Bz02[ 14] tetraene~N4)Cl3 where BzO„ stands for two benzene rings, and R or R'represents methyl, phenyl or hydrogen (vide Complexes I-VII pp 22,25), The corresponding perchlorates,thiocyanates, bromides, iodides, were obtained by exchanging the chloride of the complex with the respective anions (vide Complexes VIII to XII p 26). Macrocyclic complexes, with pyridine and CH-.CN as axial ligands having the general formulae, [Fe(R4Bz02[12]tetraene-N4)(.Py) or (CH3CN)2] 01, and [Fe(R4Bz02[14] tetraene-N4)(py)2 or (CHJCKfJ CI, were similarly synthesised except that pyridine/methyl cyanide were introduced during reflux (vide Complexes XIII to XVI p 26), A typical and more general template reaction to give a macrocyclic complex may,however, be assumed to follow the follow ing type of mechanisms H-C 3 \ c=o H3C /CH3 o=c "2 H2 I-L.C - 1 VN -p n r* - c:^ - 4ft,0 1 H2C - i H2 H2 HC H3C 3=C XCH3 (iii) The above mentioned complexes were subjected to chemical analysis for c,H,N and Fe. The data are summarised in Table 2.1. The molar conductance values conform to uni-univalent formulae. Spectral and magnetic studies on these complexes provided the following information; The results of ir spectra (Table 2.2) provided evidence that the condensation of amine with tbe ketone takes place to give a cyclic structure; absorption around 3200 cm""1 due to V(nh) and the appearance of bands around 1600 cm and 1675 cm"1 coupled with the absence of absorption bands attributed to free or coordina ted NH2 or C=0"groups being taken as the proof of cyclization. The nature of the axial ligands could also be ascertained from these spectra. The far ir spectra is complex which prevents unambiguous assignment of the V (m-X) frequency. However, the multiplicity of the bands in the 300-200 cm'"" region clearly supports a cis-geometry. Magnetic data (5.7 - 6.2 B.M., Table 2.3) at room temperature has been useful in establishing the geometry of these complexes. It has been observed that the axial ligands play a very important role in deciding the spin states. Generally complexes with axial ligands like Cl", Br"", f, CNs~ are high spin while those with CHgCN and pyridine are low spin. The electronic spectra (Table 2.3) do not provide any evidence of d-d transition and, therefore, detailed investigation of ligand field strengths could not be carried out In a few cases bands have been observed in the visible region with high extinction coefficients which have been assigned to charge transfer. (iv) The Mbssbauer spectra is made up of two line spectrum with the broadening of the peak, and in some cases two distinct additional lines appear at higher velocities. The quadrupole splitting values show that the second value is almost double the first value. These observations are indicative of the existence of cis-trans isomers, the relative concentration of the trans isomer being low as comoared to the cis-isomer. The difference in the quadrupole splitting values for the cis and trans isomers arises primarily due to the electric field gradient as a result of the ligand imbalances and the consequent imbalance of the electron occupation of the d-orbitals. Mbssbauer studies provided valuable information for devising the exact procedure of preparation of the complexes. During initial preparations it was observed that all the complexes showed additional lines corresponding to iron(II) although no attempt had been made to preclude air during preparations. On standing, the intensity of the Fe(ll) lines started decreasing and finally disappeared in about 2-3 months leaving behind a spectrum due to Fe(IIl). In order to avoid the formation of Fe(ll) complexes, air was bubbled through the solutions. The separation of isomers by ion exchange chromatography gave only one band. The eluted band after evaporation was subjected to M&ssbauer spectral analysis. The spectra of the product showed it to be a major portion of the cis-isomer with some contamination by the trans-isomer. Since the Pe(lll) complex is labile and the cis-isomer is the more stable form and the Af° for the system cis ^p^ trans is very small. The inability to separate the two isomers is understandable. However, given a suitable solvent it may (v) be possible to flip over the trans form completely with the cis form. A comparison of the u.v. spectra of the complex V in CH OH and DMF shows dramatic changes (Fig.2.12). The complexity of the spectra in CH3OH indicates that the complex is a mixture of two species while the single band spectra in DMF with j\max at 284 nm could well be due to a single (cis) species. The complexes when titrated pH metrically show two inflex ion points (Fig.2.14) which correspond to the consumption of two protons. This is understandable because the high spin Fe(lll) complex being labile in character would undergo aquation, releasing proton. The reaction may be represented as, [FeL(0H2)2]3+ J^ [FeL(0H)(H93)]2+ +H+ <v [FeL(OH)(0I^)]2+ k2 „ [FeL(OH)J+ + H+ The cis-complex can undergo, slow dimerisation. This was confirmed by electronic spectra (Figs. 2.15-2.17). The absorbance shows a consistent decrease with time and the reaction is complete in about 24 h. This slow change may be associated with dimerisation and the absorbance decrease seems to be quite consistent with such a reaction. Macrocyclic complexes with attached pendant groups can be useful to prepare model compounds for complex biological systems and catalytically active macrocycles. Therefore, we planned to attach a potentially coordinating as well as other pendant grouos to the periphery of an easily preparable macrocycle. It must be noted that the dramatic increase in the reactivity of the methylene function (position 6) is unique to the cis form. In the trans form of the (vi) same macrocycle, due to a more or less planar structure, there is a high degree of delocalisation in the chelate ring and the consequent low reactivity of the methylene function. This fact has been successfully utilised to attach a dithMate and a phenylazo pendant function at the position 6 on the macrocyclic ring. Magnetic measurements (Table 3.2) show that these complexes are high spin (like other dithiolate complexes). The high spin character is supported by the Mfissbauer spectra (Table 3.3). The spectra shows a clean doublet with isomeric shift characteristic of high spin Fe(lII). A comparison of the quadru pole splitting values observed with complexes without attached pendant group shows that the values lie in the range for the cisisomers (Table 3.3). The electronic data of these complexes (Table 3.4) show that Cu(ll), Mn(ll) and Ni(ll) are in a square planar configura tion in the simple complexes, Na[Fe(Ydte Me4BzO [14]tetraene) (OH) ] v^Ecu v^ (h2o)2]+, V^ [Mn V!^ (H^^f ; V^ [Ni V^ (J^O^]"*" The complexes with 2,2* bipyridyl show spectral data which conforms to the presence of Ni(ll), Cu(ll), Mn(li) and Co(II? in octahedral environment in complexes.. (vii) V^ [Ni(bipy) (l^O^VI^ PF6; VID[Cu(bipy) (H20)2 VI4, PF6; 5 V^ [Mn(bipy) (^0)2 VID]PP6; 6. VIp [Co(bioy) (H20)2 VIQ]PF6 From the locations of the different bands (VL at 720 and 450 nm; D4 V]L 630 nm; V3_ 520 nm; 490 nm, 380 nm, 345 nm, and VL 435 nm) u5 u6 ^7 it is concluded that the Fe(lllj centre shows no d - d band due to spin forbidden-character of the d-d transition for an octahedral Fe(lll) environment. These complexes were prepared by mixing the 1:1 mol of 2+ VIp and [M(bipy) (f-^O)^ x and then precipitated by adding the NH4PF6. [Fe(phenylazo Me4BzQ2[ 14] tetraene) Cl2] PF& were prepared and the complex was obtained by maintaining the reaction conditions for diazotization of aniline and the mixing in 1:1 mol ratio of complex and diazotised aniline with constant stirring and cooling. Thus the diazotised complex VIA having the formula, [Fe (phenylazo Me4BzO [14] tetraene C17)]PF was obtained. These complexes were also subjected to chemical analysis for C,H,N,S,Fe,Ni,';o,Mn,Cu and magnetic as well as M6*ssbauer studies of these complexes were carried out for their characterisation. The electro-chemical oroperties of the complexes have been investigated by polarography and cyclic voltammetry. These were limited to simple systems, i.e., the complexes without any pendant groups attached to the 14-membered macrocycle. (viii) Almost all the complexes were insoluble or sparingly soluble in water. Amongst these only complex I was soluble in water and its reduction was studied in detail at the d.m.e. This compound gave three steps irreversible diffusion controlled waves with n=l, n*2 and n=4.. The electrode mechanism proposed for the first two steps are, 3+ — 7-t- FeLT+ + e ^ Fel/+ 2+ — o FeL +2e > Fe + L hydrolysis product The third step pertains to the reduction of the ligand for which the two mechanisms based on (H+,e,e,H+) were proposed (pp 73-75). On structural consideration (steric interaction with the macrccyclic ligand) the first mechanism was considered to be more probable. Although another reduction mechanism can also be proposed similar to the one proposed for cinnamylamine.. When we consider-C=N groups in conjugate position and hence simul taneously undergo reduction through a single four electron transfer process. (ix) Before proceeding to study the electro-reduction of these macrocyclic complexes in non-aqueous medium, a new approach, viz., reduction of the complexes in solubilized system was considered worth undertaking and the interesting results were obtained.. The waves were irreversible, diffusion controlled with large residual currents realised (Figs.4.03 - 4.04). As expected the & was shifted to more negative potential but the wave height was considerably higher in solubilized system (attributed to micellar catalysis). However, only complexes I and V gave more than one reduction step (n = 1,2,4 and n=l, n=Z respectively) while rest of the complexes underwent one reduction step FeL + e $ 2+ Fe L . The substituent effect was not marked in the case of 14-membered ring due to the increased ring/cavity size and also in terms of greater ligand field strength of the 12-membered ring cavity. Polarographic reduction in acetonitrile gave three waves for all the complexes (Table 4.o2), contrary to aqueous and solubi lized systems each wave is one electron transfer wave. Reduction of the complex in non-aqueous medium occurs as a whole and no evidence for the reduction of the free ligand is found. The electrode mechanism is as follows: FeL3+ + e" ^ Fe L2+ Fel.2+ + e™ ^ FeL+ Fe'L+ + e" t> Fe(o) + L The effect of the substituents was, however, the same as in the solubilized system. Complex I was also polarographically (x) analysed for studying the solvent composition (water, organic solvents DMSO, CH,CN and DMF). From the E, data for the third wave (Table 4.04) it was concluded that these solvents lower the rate of orotonation and hence shifts the E, to more negative 2 potential. Another factor responsible for the variation may be the decrease in the surface concentration. The cyclic voltammetric data reveal that complete voltammograms were not obtained for these complexes. No waves were observed when positive potential was applied. Two reduction peaks were obtained but the reverse peak was realised only for the first reduction step (Figs..4.07 - 4,12). The S^ was found to be independent of sweep rates, and ip/r2 was constant indicating diffusion controlled process. An examination of A Ep values (Table 4.05) for the first wave shows that the values are in the range of 60-80 mV which points out to irreversible nature of the electrode process. The first wave can be assigned to FeL + e —— FeL and the second to a two electron transfer FeL + 2e~ —jp Fe(o)+L. The results are in accordance with the d.c. polarographic data. Furthermoj_ e the fact that the Fe(lll) complexes do not show any oxidation wave lend suoport to the protonated formulation of the complexes.