Stabilizing a NiII-aqua complex via intramolecular hydrogen bonds: Synthesis, structure, and redox properties

Hydrogen bonds within the secondary coordination sphere are effective in controlling the chemistry of synthetic metal complexes. Coupling the capacity of hydrogen bonds with those of redox-active ligands offers a promising approach to enhance the functional properties of transition metal complexes. These qualities were successfully illustrated with the [NNN](3-) pincer ligand N,N'-(azanediylbis(2,1-phenylene))bis(2,4,6-triisopropyl-benzene-sulfonamido ([ibaps](3-)) through the preparation of the Ni-II-OH2 complex, [Ni-II(ibaps)(OH2)](-). The [ibaps](3-) ligand contains two appended sulfonamido groups that support the formation of intramolecular hydrogen bonds. The bulky 2,4,6-triisopropylphenyl rings are necessary to ensure that only one ligand binds to a single metal ion. The molecular structure of the complex shows a square planar N3O primary coordination sphere and two intramolecular hydrogen bonds involving the aqua ligand. Electrochemical measurements in acetonitrile revealed two oxidation events at potentials below that of the ferrocenium/ferrocene couple. Oxidation with 1 equiv of ferrocenium produced the one-electron oxidized species, [Ni(ibaps)(OH2)]. Experimental and computational studies support this assignment.