ENHANCED NICKEL(II) CHELATION BY GEM-DIMETHYL-SUBSTITUTED MACROCYCLIC TETRATHIOETHERS

The conformational and Ni(II)-binding properties of 1,4,8,11-tetrathiocyclotetradecane (1) and derivatives bearing gem-dimethyl pairs at the 6- or at the 6- and 13-positions (2 and 3, respectively) are compared. The syntheses and crystal structures of 2, 3, and their Ni(ClO4)2 complexes are reported; analogous data for 1 and its Ni(BF4)2 complex have been in the literature for some time. All three Ni(II) complexes show very similar 14-membered ring conformations, but the metal-free macrocycles display different conformations, in the solid state. These structural data suggest that each gem-dimethyl pair progressively biases the macrocycle toward the chelating conformation. We have examined the relative Ni(II) affinities of 1-3 in CD3NO2 by means of competition experiments monitored by H-1 NMR. Tetrathioether 2 binds Ni(II) approximately 7.3 times more tightly than does 1 at room temperature, and 3 binds Ni(II) approximately 49 times more tightly than does 1. Thus, each gem-dimethyl pair leads to a 1.1 kcal/mol improvement in Ni(II) binding free energy under these conditions. We suggest that the incremental improvement in binding strength across the series 1-3 is correlated to the incremental changes in macrocycle conformation observed in the crystal structures of the metal-free thioethers.