New Class of Scorpionate: Tris(tetrazolyl)–Iron Complex and Its Different Coordination Modes for Alkali Metal Ions

We report formation
of a new metallascorpionate ligand, [FeL3]3– (IPtz),
containing a Fe core and three 5-(2-hydroxyphenyl)-1H-tetrazole (LH2) ligands.
It features two different binding sites, oxygen and nitrogen triangles,
which consist of three oxygen or nitrogen donors from tetrazole. The
binding affinities of the complex for three alkali metal ions were
studied using UV spectrophotometry titrations. All three alkali metal
ions show high affinities and binding constants (>3 × 106 M–1), based on the 1:1 binding isotherms
to IPtz. The coordination modes of the alkali metals and IPtz in the
solid were studied using X-ray crystallography; two different electron-donor
sites show different coordination numbers for Li+, Na+, and K+ ions. The oxygen triangles have the κ2 coordination mode with Li+ and κ3 coordination mode with Na+ and K+ ions, whereas
the nitrogen triangles show κ3 coordination with
K+ only. The different binding affinities of IPtz in the
solid were manipulated using multiple metal precursors. A Fe–K–Zn
trimetallic complex was constructed by assembly of an IPtz ligand,
K, and Zn precursors and characterized using X-ray crystallography.
Oxygen donors are coordinated with the K ion via the κ3 coordination mode, and nitrogen donors are coordinated with Zn metal
by κ3 coordination. The solid-state structure was
confirmed to be a honeycomb coordination polymer with a one-dimensional
infinite metallic array, i.e., −(K–K–Fe–Zn–Fe–K)n–.