SYNTHETIC, STEREOCHEMICAL AND KINETIC STUDIES ON THE COMPLEXES OF SOME NOVEL NITROGEN DONOR MACROCYCLIC LIGANDS

Abstract

A perusal oh the lite.Ka.tute. on polyaza macrocyclic com plexes shows that a lot oh work has been carried out on them during the last two decades. Mote recently, interest in these complexes has been centered on several types oh isomeric species. Although base hydrolysis studies have been carried out on diiiznent cobaltilll) macrocyclic systems, no work has been reported on the behaviour oh isomeric cobalt[lll) macrocyclic complexes . In addition very little polarographic work is reported on these complexes. Thus, the isomeric cobaltilll) complexes oh C-meso and C-rac-Me [14]ane were chosen hon- studies. A major advantage in their choice lies in the iact that the structures oh these com plexes have already been established. Alongwith base hydrolysis oh the cobaltilll) complexes oh these ligands, their chromiumilll) complexes have also been studied. Work on the polarographic behaviour oh several types oh copper ill), nickelill) and cobaltilll) macrocyclic complexes has also bee.n carried out. SYNTHESES ANV CHARACTERISATION Itomznic. Cobaltilll) Comp£exe4 Two isomeric trans-[Co iisocyclam)CI f]C104 complexes, isomeril) and isomerill), have been isolated hollowing aera tion oh a mixture oh cobalt ill) chloride and isocyclam. The isomers obtained are similar to the ones reported earlier hor C-meso and C-rac- Me [14]ane [157, 158] and have been assigned similar diastereoisomeric structures. Thiocyanato Complzxe.6 Trans[Co icyclam) iNCS)Cl]ClO has been obtained by the reaction oh the corresponding trans-dichloro complex with an equivalent amount oh KCNS [159]. This complex can be readily converted into trans- [CO. i cyclam) [WCS] (PMF] J(CIO ) and Trans-[Co iCyclam) iMCS) iOOCH)]C10 by reaction with dimethyl hormamide and hoh-m^c- ac-id respectively. In both these com plexes, VMF and h°fima^e- afie- O-bonded. These complexes show electronic spectra expected hon- a Con 0 chromophore. Chromium[111) Complexes The chromiumilll) complexes, trans- and cis- [CrlCl^]Cl have been prepared by the reaction oh [CrCl^(PMF)] with Cmeso- and C-rac-MeA 14]ane. This reaction gives a mixture oh about 90% reddish violet cis- and 10% grayish green trans complexes. The trans- isomer is soluble while the cisisomer is insoluble in methanol. The two isomers were thus separated by dissolving away the trans- isomer in methanol. These complexes show electronic spectra typical oh such chromiumilll) complexes. BASE HVVR0LVS1S Cofaa^t(III) Complexes The base hydrolysis oh isomeric cobaltilll) complexes has been studied spectrophotometrically over a pH range oh 5,0 to 1.0 in buhh^r solutions whose ionic strength was maintained constant at 0.1 M. The activation parameters &OK the base hydrolysis reactions have also been determined. The base hydrolysis reactions hollow S^l(C8) mechanism involving a hivz coordinate trigonal bipyramid intermediate. Comparision oh base hydrolysis rates show that the trans-Ill iorm (R,S,S,R) oh [CoiCyclam)Cl1 ]ClO4 and the R,S,S,R horm oh isomer a hydrolyze at similar rates while isomer* hydrolyzes at a ^aitei. The cUtftfetence* in rate could pos sibly be related to the conhormational strain energy oh the macrocycle. The folding ability oh the macrocycle, is an im portant iattOK in acid hydrolysis , does not seem to have any role in these base hydrolysis reactions. Chfiomium[lll) Comple.xz& The bast hydrolysis oh the chromiumilll) complexes was also studied spectrophotometrically. These reactions also hollow a Vcg mechanism. The cis-[Cr iLM)Cl2]Cl complex shows a remarkably high reactivity as compared to any other such complexes and the reason hor this is not very clear. The other three complexes show much lower rates, comparable to other chromiumilll) complexes. ACID HVVR0LVS1S Catbonato Complexes The carbonato complexes undergo acid catalyzed decar boxylation. The kinetics oh these reactions has been studied spectrophotometrically in acid solutions with constant ionic strength. The decarboxylation involves a two step process in which hi't-Ai the Co-0 bond breaks to open the carbonato h°i~ lowed by slow decarboxylation oh the mono dentate carbonate. A suitable mechanism hofl this has been proposed [111]. The proposed mechanism has been supported by solvent deuterium isotope ehhect studies by ho^^ou}in9 ihe reactions in VCl / V„0 solutions. Chiomium[lll) Complexes The thermal aquation iacid hydrolysis) oh chromiumilll) reactions was also i$o££owed spectrophotometrically. These reactions |fo££ow a more associative. mechanism as compared to the corresponding cobaltilll) complexes, although bond breaking is still the important ^ea^a^e oh the mechanism. The entropy oh activation shows that the hydrolysis oh these complexes proceeds with retention oh conhiguration in accord with a more associative mechanism. Comparison oh aquation rates oh various dichloro-chromiumilll) complexes show that the aquation rates exhibit dependences on the structural heatures oh the macrocycle. POLAROGRAPHIC STUV1ES The polarographic reduction oh several copperill), nickelill) and cobaltilll) complexes at a dropping mercury electrode has been studied in 0.1 M sodium perchlorate as supporting electrolyte. In the case oh copperill) complexes the values oh halh wave potentials can be correlated with ring size and the ligand hiz£d stabilization as rehlected by the position oh the d-d band appearing in their visible spectra. A similar trend is noticeable in both nickelill) and cobaltilll) complexes but it is not as perceptible as is in copperill) complexes.