Synthesis, crystal structures, and deprotonation of cis- and trans-octahedral nickel(II) complexes with a 14-membered tetraaza macrocycle bearing two N-phenacyl pendant arms

The di-N-functionalized macrocycle 2,13-bis(2-phenacyl)-5,16-dimethyl-2,6,13,17-tetraazatricyclo[16.4.0.1.1807.12]docosane (H2L2) bearing two N-CH2COC6H5 groups has been prepared by the reaction of 3,14-dimethyl-2,6,13,17-tetraazatricyclo[16.4.0.07.12]docosane (L1) with phenacyl bromide. Interestingly, H2L2 reacts with Ni2+ ion to form two geometric isomers, trans-[Ni(H2L2)]2+ and cis-[Ni(H2L2)]2+. The axial Ni–O (N-CH2COC6H5 group) bond distance (2.080(2) Å) of trans-[Ni(H2L2)](ClO4)2·2DMSO is shorter than the in-plane Ni–N distances (2.096(2) and 2.100(2) Å). However, the Ni–O distances (2.105(2) and 2.124(2) Å) of cis-[Ni(H2L2)](ClO4)2·H2O are considerably longer than the Ni–N distances (2.053(2)–2.086(2) Å). Each N-CH2COC6H5 group of trans-[Ni(H2L2)]2+ and cis-[Ni(H2L2)]2+ exists as its keto form in the solid state and in various solvents. Two N-CH2COC6H5 groups of trans-[Ni(H2L2)]2+ are readily deprotonated in basic aqueous solutions, producing the enolate form trans-[Ni(L2)]. On the other hand, cis-[Ni(H2L2)]2+ undergoes deprotonation to yield cis-[Ni(HL2)]+, in which one N-CH2COC6H5 group is not deprotonated, under similar conditions.