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|Title:||PHYSICO-CHEMICAL STUDIES OF SOME NEW MACROCYCLIC COMPLEXES|
|Abstract:||The naturally occurring macrocyclic complexes like chlorophyll, vitamin B, , haemoglobin, porphyrins etc. have been known for quite some time- These macrocyclic complexes perform vital functions in the various biological processes. It is important to note that the significant role played by metal ions in living systems depends on their confinement in an approximately planar, tetradentate, totally enclosed frame work. The chemistry of the synthetic macrocyclic complexes, stems from the condensation reaction between ethylene diamine with acetone to give 5,7,7,12,14,14 hexamethyl 1,4,8 »H tetraasacyclotetradeca 4,11 diene generally abbreviated as Me^p.4] diene- A large number of such compounds are now known and they form an important class of compounds in themselves* These nitrogen donor synthetic macrocyclic complexes have close resemblance in their structure with their natural counterparts and can thus be considered as models for them. Although a large amount of work has been done on the macrocyclic complexes during the last decade, evidently a large number of areas can be identified in this field which have received scant attention. Most of the work has been centered around the copper(ll), nickel(ll), cobalt(lll) and iron(ll) complexes of 12- to 14-membered macrocycles. Very little has been done either to prepare macrocycles with larger rings or to attach pendant groups to the already 11 known macrocyclic structures. Preparation and characterisa tion of macrocycles with large rings could be useful to generalise the characteristics which determine the stability of these complexes. Attachment of pendant groups (carrying different functional moities) to the macrocyclic structure would be useful in investigating the effect of such structures on the dynamic properties of the metal ion. In the present studies an attempt has been made to investigate both the above mentioned aspects to add to our existing knowledge on macrocyclic complexes. With this aim in view, the following studies on the chemistry of macrocyclic complexes were carried out. a) Preparation and characterisation of two new 16- and 17-membered macrocycles (capable of giving 8- and 9-rnembered chelate rings) and their complexes with copper(ll), nickel(ll), cobalt(lll) and chromiumdll). Some of their properties have also been investigated. b) A few macrocycles with different attached pendant functional groups were synthesised and their metal complexes characterised. These studies have been extended to include a few bimetallic centered complexes* iii a ) Copperdl), nickeldl), C3>balt(lll) and chromium(lll) complexes of 1.4.ft .11 cvclohexadecane and 1.4.8,11 cvcloheptadecane A large number of tetraaza macrocycles and their metal complexes have been prepared and studied, during the last fifteen years. Most of the work has been done on 12- to 14-membered macrocycles which form the most stable complexes and it has been observed that the 14-membered cavity is a best fit for cobalt(lll) and to a very large extent for nickel(H) as well. It has been observed that these complexes persist in acid solution for months, while in case of macrocycles carrying bigger chelate rings, the stability is markedly decreased. In order to investigate the effect of such larger chelate rings on the stability of the macrocyclic complexes, two new 16- and 17-membered macrocycles have been prepared. The tetratosylated linear tetramine was condensed with bistosylated diol (in case of 16- membered macrocycles, 1.5 pentane diol and in the case of 17-membered macrocycles 1.6 hexane diol was used) in DMF to give the cyclised product. The condensation product was then hydrolysed with conc.H?SO, to give the free macrocycle- Both the ligands undergo complexation quite easily with copper(ll), nickel(ll), cobalt(lll) and chromium(lll). The cobalt(lll) complexes are quite stable and undergo axial ligand substitution reactions by the usual metathesis procedures* IV The infrared spectra of copper(ll) and nickel(ll) complexes shows a sharp NH band around 3200 cm and perchlorate band at 1100 and 620 cm . Since there is no splitting in the vasym (Cl-0) band at 1100 cm , it is concluded that perchlorate is present in the ionic state in the solid complex. The infrared spectra of cobalt(lll) complexes show bands corresponding to the macrocycle and the appropriate axial ligands. The skeletal vibrations of the macrocycle in 750-1500 cm region, reflect changes in the geometry n-f of the macrocycle. A comparison of the spectra of [CoLYJ (Y=glycine or acac) with that of L£OL Cl^J reflects the difference between the folded geometry in the former cases and the planar geometry in the latter. Bands observed in the trans-chloro complexes are either shifted or not observed at all in the cis-complexes. The characteristic bands due to chelated acetyl acetone ligand appear around 1570 cm " and 1520 cm . The glycine complex shows a broad band at 1340 cm assigned to the asymmetric stretching vibration of the coordinated COC and another band at 1600 cm due to symmetric stretching of COO group. The unidentate ligand N0„ , which occurs in two complexes shows the characteristic bands due to Vasym(NOp)j vsyrn(NO^) and 6(N02). In the case of chromium(lII) complexes, the cisand trans- isomers can be easily distinguished on the basis of their infrared spectra. The pink coloured cis-isomer, cis-[CrLCl2~| CI and cis- [crLClg*] CIO. , show two v(NH) -1 -1 bands appearing around 3150 cm and 3200 cm while the light green coloured trans-isomer, trans-J_Cr( J_ 16 J ane N, )C12J CI and trans-Ccr(Q.63 ane N, JClg^ClO^, show a single v(NH) band around 3200 cm" . Conductometric measurement on copper(ll) and nickel(ll) complexes in aqueous solutions show that the complexes are 2:1 electrolytes. The ultraviolet spectra of copper(ll) complexes show a single band corresponding to N-Cu charge transfer. This shows that all the four nitrogens are equivalent in these complexes. The spectra of nickel(ll) complexes show a broad band at 22,222 cm"1 for £Ni(|16J ane \ *1(cl\ >2and 21,733 cm"1 for CNi(p.7] ane N, )J(C10^)2 due to combination of ^a —» 3i< (P) transition of octahedral nickel(ll)? *g Ig and A-, —r? B, transition of square planar nickel(ll). The extinction coefficient of these two complexes are low and show temperature dependance, which suggests an octahedral planar equilibrium. The equilibrium is endothermic and shifts towards the planar form with increase in temperature or addition of-inorganic salts like NaCIO, which reduces the free water concentration 2+ 2+ £>iL(H20)2 2 .jpfe CNiLJ * 2H2° vi In the case of an analogous Complex, c-meso-5,12 dimethyl 1,4,8,11 tetraazacyclotetradecane, only a single d-d band is observed at 21410 cm (e-64 dnr mol cm ). For this complex, it has been evaluated that at 25 C, there is ca 65*/. of the octahedral species and 35*/> of the planar species. Such equilibria exhibit values of AH in the range of 7»1 to 6. 2KJ mol"1 and AS°Qft of the order of 20 to 140 J mol" over a temperature range of 20-50°C. The electronic spectra of cobalt(lll) complexes, trans- [CoLCl2 J CIO, , show a typical d-d band corresponding to the CoN,Cl2 chromophore. Anumber of other trans- jjboLX2 J CIO, and cis-|1boLYj (CIO, )2 complexes, prepared by ligand metathesis reactions show typical bands corresponding to CoN,, CoN^Cl, CoNj-0 or CoN.Op chromophores. The position of these bands supports the assignment of trans- and cis-configuration to these complexes. These values are comparable with other literature values for similar complexes. The spectra of chromium(ill) complexes can be used to distinguish the cis- and trans-geometry of these complexes. The more symmetrical trans- isomers have low extinction coefficients of <^30 and the lowest d-d band appears in the 17,500-16,900 cm range, while the less symmetrical cis-isomers have much higher extinction coefficients (of the order of 80-120) and the lowest d-d band occurs at 18,900-17,500 cm" . The present complexes also show absorption bands in these regions in accord with their vii assigned geometries. The macrocycles are capable of forming a novel sequence of chelate rings 5,6,5,8 and 5,6,5,9 on undergoing complexation.The complexes of these ligands carry four chiral nitrogen centres,which can yield as many as ten Ndiastereoisomers- Molecular models show that out of the possible ten diastereoisomers, only four are virtually free from torsional strain as well as have minimum hydrogenhydrogen interaction in the chelate rings. The ligands are expected to exist in their most stable trans- form (cyclam form) with an R,S,S,R configuration. Kinetics of acid-catalysed dissociation of copper(ll) complexes have also been studied. The present copper(ll) complexes undergo very rapid dissociation in acidic solutions. A plot of k - vs hydrogen ion concentration shows that initially there is a linear dependence of ^obs on CH 3 but at higher hydrogen ions concentrations the reaction becomes independent of £h j concentration. The plots show an intercept corresponding to a solvolytic path associated with th-ese dissociation reactions. The acid-catalysed path leading to dissociation is consistent with the following kinetic scheme. CCuL-^,2+ »H++ ^-fKr-* Or- uG-i3+ 2+ Cculh]5-1' —*-» cu2+ ♦ Clh 1 The overall rate expression for this mechanism can viii be represented as kobs ~ kc wcS'J i+kCh+: where kQ is the solvolytic route. The above equation can be written as 1 I 1 1 «WV " kK C^l K which means that a double reciprocal plot of l/(k , -k ) vs ,„_ .„... should be linear. The present results show that on this plot is linear for both the complexes. A general mechanism proposed for the acid-catalysed dissociation of linear polyamine complexes may be adopted for these complexes. It is observed that the macrocyclic complexes dissociate at a lower rate as compared to the linear tetraamine complexes. The dissociation mechanism involves some angular expansions of the bond angles in the chelate rings. This is followed by a slow cleavage of the second copper nitrogen bond, which is assumed to be rate determining. Breaking of subsequent copper nitrogen bonds resulting in complete dissociation is a very rapid process. Copper(ll) complexes usually show square planar or tetragonally distorted octahedral structure, for which broad bands are observed due to overlap of a, -^b„ , lg lg b2->b and Dpcf* bla trans;i-'tions ln the visible region. The \__v for these broad bands gives an approximate value of IX lODq.^ and the value of A. r corresponds to the field created by the macrocycles in the square plane. The 'position of this absorption band can therefore be used as a measure of the ligand field. It has been observed by Bhattacharya that the ligand experiences a strain in arranging itself at the corners of a square plane around the copper(ll) ion when the 15- and 16-membered rings are considered. Spectral data studies on a variety of copper(ll) complexes have shown that cyclam, with the 5,6,5,6 chelate ring sequence forms the most stable complex* A comparison of the \ values * * max for a large number of copper(ll) complexes shows that the 5,6,5 chelate ring sequence is the most favoured geometry for complexation as it places the four nitrogens in a preoriented geometry favourable for coordination. Similar conclusions can be drawn for the nickel(ll) complexes. Polarographic studies on the copper(ll) complexes in aqueous media shows that the complexes undergo a one electron, reversible reduction at the d.m-e. The reversible reduction of these complexes can be assigned to Cu^ —> Cu couple* A comparison of E, ,„ values of the copper(ll) complexes shows a systematic variation in their values with ring size. The values clearly show that the reduction potential decreases with increase in ring size. o) Copper(ll). nickel(ll), cobalt(ll). cobalt(lll) and palladiumdl) complexes of macrocycles carrying pendant functional groups- The naturally occurring macrocyclic complexes carry pendant functional groups which act as structural components to the coordination sites. In order to synthesise the macrocyclic complexes with pendant groups, the simplest route is to first synthesise the simple macrocyclic ligand and then attach suitable functional groups. The pendant functional groups can be substituted either at carbon or nitrogen atoms. Both types of reaction have been observed and they give distinctly different types of complexes. In the present studies an attempt has been made to attach the CS„ group to the appropriate carbon on the macrocyclic frame work to give 1,1 dithiolate derivative of macrocycle. The 1,1 dithio late is capable of reacting with metals like copper(ll), nickel(ll), palladiumdl) and metal bipyridyl complexes and diazotised aniline and benzidine. The infrared spectra of these complexes shows the presence of C=C, C-S and M-S bands corresponding to 1600 cm"1, 1100, 840 and 620 cm" and 390 cm respectively. It is evident that the macrocycle is bound to the added metal ion through the two 1,1 sulphur atoms* In case of bipyridyl complexes the infrared spectra shows absorption in the 1600-1400 cm-1 region corresponding to the ring stretching vibration. The infrared spectra of aniline and benzidine xi complexes shows bands for N=N and the aromatic groups in addition to the C-S, ON and NH groups of the macrocycles. The results of magnetic measurements show that nickel(II) and palladiumdl) complexes are diamagnetic while the copper(H) complex is paramagnetic corresponding to one unpaired spin. The magnetic data of bipyridyl complexes is in accord with the octahedral geometry of the bipyridyl part of the complex- The spectral data is an accord with these geometries. The complexes with aniline and benzidine are also diamagnetic The electronic spectra shows bands in the visible region due to the diazotised moities. The results of these studies show that diazonium ions can be conveniently attached to the methylene sites of macrocyclic complexes as reported by Goddard and Joslin. The reaction is very fast even at the low temperatures used for diazotisation reaction. It is observed that the macrocyclic complex [>i Me2Q.3ldienell reacts with diazotised aniline and diazotised p-aminobenzoic acid in the ratio of ltl« The infrared spectra of these complexes shows the v(n=N) band in 1600-1580 cm"1 region. In addition, both complexes show bands assignable to v(C-H), v(N=N) and ring vibrations due to C-C and C=C stretchings of the phenyl groups. The H.n.m.r. spectra shows typical bands corresponding to -CH^, -OMJ*,-NH-and the aromatic moities of the complex. xii Further investigations under this section deal with complexes derived from cyclam and C-rac and C-meso Me2cyclam. The synthesis of tetra cyanoethylcyclam was first referred to briefly by Barefield but detailed investigations on the nickel(ll) complex of tetra cyanoethylcyclam was reported by Wainwright who observed the weak coordinative interaction of the four cyano groups with the metal. Thus in effect a high spin five coordinated nickel(ll) complex is formed. The cyanoethylation of both C-rac and C-meso Me2cyclam proceeds quantitatively on refluxing with acrylonitrile as reported for cyclam. Infrared spectra of these ligands show no band for v(NH) and show a sharp single band around 2240 cm due to C=3N. Both these ligands readily yield complexes with copper(ll), nickel(ll)s cobalt(ll) in methanolic/ethanolic solutions. A cobalt(lll) complex has also been successfully isolated with tetra cyanoethyl cyclam- The infrared spectra of nickel(ll) complex of cyanoethyl cyclam shows two bands corresponding to C^N at 2250 cm (same as in the free ligand) and another weak band at 2282 cm" which shows the coordination of four nitrile groups with the nickel(ll) ion and this leads to the formation of high spin five coordinate complex, while in the case of C-meso and C-rac, the infrared spectra shows a sharp single C=N band at 2240 cm" . This shows that there is no ligation of the nitrile groups with the central xiii metal ion. The infrared spectra of C-rac complex shows the splitting of perchlorate band at 1080, 1110 and -l , -1 1120 cm in addition to the sharp CIO, band at 620 cm which corresponds to presence of coordinated CIO, in the solid state. In the case of C-meso complex, the infrared spectra does not provide any evidence for coordinated perchlorate due to steric effects of the two C-meso methyl groups* Both the complexes are 211 electrolyte in CH,CN. 3 Both copper(ll) complexes of C-rac and C-meso show a single infrared band for C=N at around 2240 cm . There is no shifting of this band in comparison to the free ligand, indicating that there is no ligation of the nitrile groups with copper(ll)- These complexes also show the splitting of perchlorate band at 1100 cm which indicates the presence of coordinated perchlorate in the solid state* Both the copperdi) complexes are 2.1 electro lyte in CH^CN. The polarographic waves of the copperdi) complexes show reversible reduction through a one electron transfer process in which copper(ll) is reduced to copper(l)< The infrared spectra of cobalt(ll) complexes of C-rac shows a single band for C=N which clearly shows the non-coordination of the nitrile groups. The complexes are non-electrolytes in CH,CN which indicate that both the bromides occupy the two axial sites. The magnetic data xiv shows that the complexes are high spin with three unpaired spins. A green coloured cobalt(lll) complex of tetra cyanoethyl cyclam has been isolated which shows a single absorption band for C=N at 2250 cm"1 and is diamagnetic. The visible spectra of the complex shows a broad band at 15,384 cm"1 which is consistent with, a cobalt(III) species a N, Br, chromophore. 4 ^|
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