Synthetic and kinetic studies of some transition metal complexes of macrocyclic ligands

Abstract

This thesis is divided into two sections. The first section deals with kinetic studies and the second section with synthetic and stereochemical investigations. The kinetics and mechanism of the acid-catalysed decarboxylation of a number of complexes of the general type cis-[CoN4CO3]+ (where N4 = 5,12-dimethyl-1,4,8,11-tetra-azacyclotetradeca-4,11-diene, C-meso-5,12-dimethyl-1,4,8,11-tetra-azacyclotetradecane, 1,4,7,10-tetra-azacyclododecane and 3,6-dimethyl-1,8-diamino-3,6-diazaoctane). The acid-catalysed decarboxylation of these complexes has been studied over a range of perchloric acid of nitric acid concentrations, and temperatures, 25, 35 and 45oC (except in the case of cis-carbonato (C-meso-5,12-dimethyl-1,4,8,11-tetra-azacyclotetradecane) cobalt (III) which was studied only at 35oC). The rate expression takes the form kobs = ko + k1[H]+, where the kobs is the observed first order rate constant at constant hydrogen ion concentration. The ko terms which represents the “spontaneous” or water reaction is kinetically unimportant at the acidities used in the study. The activation parameters have been determined and the mechanism of the reactions discussed. The complexes display solvent deuterium isotope effects (kD2O/kH2O) in the range of 2.1 to 2.65. The magnitude of the solvent isotope effects is consistent with a rapid pre-equilibrium protonation followed by rate-determining ring-opening, and excludes a mechanism involving concerted attack by H¬3O+. The values of ΔS+ suggest an A- type reaction to give a five-coordinate intermediate. The second part of this section deals with the kinetics of the acid-aquation of the cis-sichloro)C-meso-5,12-dimethyl-1,4,8,11-tetra-azacyclotetradecane)chromium(III) cation. The reactions were studied using 0.01 mol dm-3 nitric acid as the reaction medium. The hydrolysis occurs in two steps, cis-[CrLC1s]+ + H2O → cis-[CrLC1(H2O)]2+ + Cl- cis-[CrLC1(H2O)]2+ + H2O → cis-[CrL(H2O)2]3+ + Cl- and activation parameters for both hydrolyses obtained. Loss of the first chloride ligand is ca. 76 times faster than the second at 25oC. Possible mechanisms for these reactions are considered. The second section of the thesis deals with synthetic and stereochemical studies of macrocyclic ligands and their metal complexes. Reduction of the nickel(II) complex of C-meso-5,7,12,14-tetra-azacyclotetradeca-4,11-diene gives three diastereoisomeric tetramethylcyclams (as their nickel(II) complexes, (1), (2) and (3)). One of these complexes, believed to be (2), is formed preferentially (ca 70%) and is readily obtained isomerically pure. Molecular models indicate that the four

[diagram (1) C-meso-C-rac] [diagram (2) C-meso-C-meso] [diagram (3) C-meso-C-meso]

methyl groups in (2) are equatorial, and that the axial coordination sites are readily accessible to monodentate ligands. A number of octahedral nickel(II) complexes of (2) have been prepared and characterised. The thermodynamics of the NiL2+ + 2H2O → [NiL(H2O)22+ equilibrium has been studied in detail and values of ΔHo and ΔSo obtained. Fractional crystallisation of the nickel(II) complex of 5,6,12,13-tetramethyl-1,4,8,11-tetra-azacyclotetradeca-4,11-diene from aqueous solution led to the characterization of three isomers (arising from chiral nitrogen and carbon centres) designated a, b and c, identified by their 1H n.m.r. spectra. Equilibration of a and b in DMSO/KOH gives isomer d. N.m.r. studies confirm that the reaction proceeds via the steps, b → a → d. Cobalt(III), copper(II) and nickel(II) complexes of 1,4,7,10,13,16-hexa-azacycloctadecane have been prepared and characterised and the stereochemistry of the cobalt(III) complex discussed. The 1H n.m.r. spectrum of the cobalt(III) complex in d6-DMSO solution is consistent with the unsymm topology. This assignment is fully confirmed by 13C n.m.r. The 13C n.m.r. spectrum unequivocally establishes that the compound consists of the two NH geometrical isomers of the unsymm topology which are present in roughly equal amounts, (4) and (5).

[diagram (4)] [diagram (5)]

A number of metal(II) and cobalt(III) complexes of the macrocyclic ligand 5,12-dimethyl-7,14-di (2-thienyl)-1,4,8,11-tetra-azacyclotetradeca-4,11-diene (6) have been prepared and characterised. The spectroscopic properties of

[diagram (6)]

these complexes are consistent with a square planar geometry for the metal(II) perchlorate salts and a trans-arrangement of the unidentate ligands in the octahedral cobalt(II) complexes. The structures of all compounds were confirmed by elemental analysis, spectroscopic investigations, and in some cases by conductivity measurements.