Formation and dissociation kinetics and crystal structures of nickel(II)-macrocyclic tetrathiaether complexes in acetonitrile. Comparison to nickel(II)-macrocyclic tetramines

Complex formation and dissociation rate constants have been independently d etermined for solvated nickel(II) ion reacting with eight macrocyclic tetra thiaether ligands and one acyclic analogue in acetonitrile at 25 degrees C, mu = 0.15 M. The macrocyclic ligands include 1,4,8,11-tetrathiacyclotetrad ecane ([14]aneS(4)) and seven derivatives in which one or both ethylene bri dges have been substituted by cis- or trans-1,2-cyclohexane, while the acyc lic ligand is 2,5,9,12-tetrathiatridecane (Mez-2,3,2-S-4). In contrast to s imilar complex formation kinetic studies an Ni(II) reacting with correspond ing macrocyclic tetramines in acetonitrile and N,N-dimethylformamide (DMF), the kinetics of complex formation with the macrocyclic tetrathiaethers sho w no evidence of slow conformational changes following the initial coordina tion process. The differing behavior is ascribed to the fact that such conf ormational changes require donor atom inversion, which is readily accommoda ted by thiaether sulfurs but requires abstraction of a hydrogen from a nitr ogen (to form a temporary amide), The latter process is nor facilitated in solvents of low protophilicity. The rate-determining step in the formation reactions appears to be at the point of first-bond formation for the acycli c tetrathiaether but shifts to the point of chelate ring closure (i.e., sec ond-bond formation) for the macrocyclic tetrathiaether complexes. The forma tion rate constants for Ni(II) with the macrocyclic tetrathiaethers paralle l those previously obtained for Cu(II) reacting with the same ligands in 80 %. methanol-20% water (w/w). By contrast, the Ni(II) dissociation rate cons tants show significant variations from the trends in the Cu(II) behavior. C rystal. structures are reported for the Ni(II) complexes formed with all fi ve dicyclohexanediyl-substituted macrocyclic tetrathiaethers. All but one a re low-spin species.