Coplanar coordination of the smallest tetraaza macrocycle: low-spin 1,4,7,10-tetraazacyclododecane nickel(II)

The electronic spectrum of aq. blue cis-[NiL(H2O)2]2+ (L = I; m = n = p = q = 2) is typical of a high-spin octahedral complex. On adding large amts. of inert electrolyte (NaClO4) the soln. turns brown and upon heating to boiling the complex is converted to bright yellow diamagnetic low-spin square-planar [NiL]2+, but 100% conversion does not occur. The yellow species are formed much more readily with the larger rings L' (I; m = p = 2, n = q = 3) and L'' (I; m = 2, n = p = q = 3) and with the open-chain tetramine analogs of L, L' and L''. The reduced size of the low-spin Ni(II) allows it to fit in the macrocycle hole more easily. The coplanar chelation of L is thus promoted by the small size of Ni(II) and its strong preference for square coordination. The difficulty of obtaining 100% conversion to NiL2+ reflects the large steric energy spent by L on changing from a cis to a trans configuration. The energies of the d-d bands for [NiL]2+, [NiL']2+, [NiL'']2+ and their analogs with the open-chain tetramines are compared.