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 determined for solvated nickel(II)
ion reacting with eight macrocyclic tetrathiaether ligands and one acyclic analogue in acetonitrile at 25 °C, μ =
0.15 M. The macrocyclic ligands include 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4) and seven derivatives in
which one or both ethylene bridges have been substituted by cis- or trans-1,2-cyclohexane, while the acyclic
ligand is 2,5,9,12-tetrathiatridecane (Me2-2,3,2-S4). In contrast to similar complex formation kinetic studies on
Ni(II) reacting with corresponding macrocyclic tetramines in acetonitrile and N,N-dimethylformamide (DMF),
the kinetics of complex formation with the macrocyclic tetrathiaethers show no evidence of slow conformational
changes following the initial coordination process. The differing behavior is ascribed to the fact that such
conformational changes require donor atom inversion, which is readily accommodated by thiaether sulfurs but
requires abstraction of a hydrogen from a nitrogen (to form a temporary amide). The latter process is not 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 acyclic tetrathiaether but shifts to the point of chelate ring closure (i.e., second-bond formation) for the macrocyclic tetrathiaether complexes. The formation rate constants for Ni(II) with the
macrocyclic tetrathiaethers parallel 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 constants show significant variations from
the trends in the Cu(II) behavior. Crystal structures are reported for the Ni(II) complexes formed with all five
dicyclohexanediyl-substituted macrocyclic tetrathiaethers. All but one are low-spin species.