Cobalt cage complexes with N₃S₃ donor sets and differing cavity sizes: A novel macrobicyclic cage with a contracted cap

Treatment of the cobalt(III) complex of the hexadentate tripodal N₃N₃ ligand ten (4,4′,4″-ethylidynetris(3-thiabutan1-amine) with propanal and paraformaldehyde under basic conditions, followed by borohydride reduction and reoxidation of the metal center, leads largely to the encapsulated (red) metal complex cation [Co(Me₂-N₃S₃sar)]₃₊ (Me₂-N₃S₃sar = 1,8-dimemyl-3,13,16-trithia-6,10,19-triazabicyclo[6.6.6]icosane). Unexpectedly, significant amounts of the homologous (yellow) complex cation [Co(Me₂-N₃S₃absar)]³⁺ (Mez-N₃S₃absar = l,8-dimethyl-3,13,16-trithia-6,10,19-triazabicyclo[6.6.5]nonadecane) were also obtained. This macrobicyclic complex has a contracted cavity resulting from a cap containing one fewer methylene units than Me₂-N₃S₃sar. The structures of both cobalt(III) complexes have been determined by X-ray crystallography. [Co(Me₂-N₃S₃sar)]Ci·ZnCl ₄·H₂O crystallizes in the cubic space group P2₁3 with Z = 4, a = 13.9683(11) Å. [Co(Me₂-N₃S₃absar)](ClO₄) ₃·0.5CH₃CN·0.5H₂O crystallizes in the triclinic space group P1̄ with Z = 4, a = 12.036(4) Å, b = 15.932(9) Å, c = 17.212(14) Å, α = 64.93(7)̊, β= 72.77(5)°, γ = 88.91(7)°. The surprising structural rearrangement is examined, along with the spectral and redox properties of both cobalt complexes. The influence of the reduced cavity size in the absar type cage is reflected in a shift of the bands in the electronic spectrum of both the cobalt(II) and cobalt(III) complexes to higher energy, and a more negative value for the Co(III/II) redox potential. The demetalation of the complexes is also described.