A square-planar tetracoordinate oxygen-containing Ti₄O₁₇ cluster stabilized by two 1,1'-ferrocenedicarboxylato ligands.

By introducing steric constraints into molecular compounds, it is possible to achieve atypical coordination geometries for the elements. Herein, we demonstrate that a titanium-oxo cluster [{Ti4(μ4-O)(μ2-O)2}(OPr(i))6(fdc)2], which possesses a unique edge-sharing Ti4O17 octahedron tetramer core, is stabilized by the constraints produced by two orthogonal 1,1'-ferrocenedicarboxylato (fdc) ligands. As a result, a square-planar tetracoordinate oxygen (ptO) can be generated. The bonding pattern of this unusual anti-van't Hoff/Le Bel oxygen, which has been probed by theoretical calculations, can be described by two horizontally σ-bonded 2p(x) and 2p(y) orbitals along with one perpendicular nonbonded 2p(z) orbital. While the two ferrocene units are separated spatially by the ptO with an Fe⋅⋅⋅Fe separation of 10.4 Å, electronic communication between them still takes place as revealed by the cluster's two distinct one-electron electrochemical oxidation processes.