An Investigation of Some Aspects of the Coordination Chemistry of Synthetic Macroheterocyclic Ligands
The preparation of a range of fully saturated, unsubstituted pentaazamacrocycles is described. The macrocycles vary in ring size from fifteen to twenty members, and comprise every possible arrangement of dimethylene and trimethylene linkages between five nitroqens in a monocyclic arrangement. A new linear homologue of tetraethylene pentamine with trimethylene linkages between nitrogens is also reported. The copper(II) and nickel(II) complexes of these amines have been prepared; the conductivity and spectral properties have been determined in order to investigate their stereochemistry. The nickel(II) complexes of the two largest macrocycles appear to be five-coordinate both in the solid state and in solution. The remainder of the complexes are either five-coordinate (as the perchlorate salts in the solid state or in non-coordinating solvents) or six-coordinate (with a coordinated nitrate). Cobalt(III) complexes of the fifteen to eighteen membered macrocycles have been prepared with a variety of ligands occupying the sixth coordination site. Ligand field parameters have been derived from the electronic spectra of the complexes. The stereochemistry of the complexes and their behaviour on ligand substitution have been investigated principally by 13C n.m.r. Only a few of the numerous possible isomers of each species were formed. The structures of [Co(1, 4, 7, 10, 14-pentaazacycloheptadecane) Cl]Br0.33 Cl1.67. H2O and [Co(1, 4, 7, 11, 15-pentaazacyclooctadecane)Br]Br2, which were determined by single-crystal x-ray diffraction studies, are described. The spontaneous aquation rates of the bromo complexes have been investigated semi-quantitatively, and found to span many orders of magnitude. The most labile bromo complex [Co(1, 4, 8, 11, 15-pentaazacyclooctadecane)Br]Br2 spontaneously aquates in a matter of seconds at room temperature. The increasing strain and steric crowding caused by successive replacement of five-membered chelate rings by six-membered chelate rings, or by simply altering the sequence of five- and six-membered chelate rings is manifested in a progressive increase in the instability of the complexes. In the case of the nineteen- and twenty-membered macrocycles, this crowding and strain results in the formation of stable five-coordinate cobalt(II) complexes; for these ligands, no stable complexes were formed with the smaller cobalt(III) cation. The acid-dissociation kinetics of the copper(II) complexes have been examined in nitric acid at 298 K. A variable temperature study has also been performed on the complex of l, 4, 7, 10, 14-pentaazacycloheptadecane in order to determine the activation parameters. The complexes are labile by comparison with most tetraazamacrocyclic complexes. The dissociation reactions are first-order in complex concentration, but the acid-dependence varies. The observed rate constant is second-order in hydrogen ion concentration for the complex of 1, 4, 7, 10, 13-pentaazacyclopentadecane, first-order in hydrogen ion concentration for 1, 4, 7, 10, 14-pentaazacycloheptadecane and takes the form kobs = a[H+]2/(l+b[H+]2) for the complex of 1, 4, 7, 10, 13-pentaazacyclohexadecane. For the remainder of the complexes, the observed rate constant takes the form kobs = (c[H+] + d[H+]2)/(e + [H+]). Possible mechanisms that are consistent with the above behaviour are presented.