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 [{Ti-4(mu(4)-O)(mu(2)-O)(2)}(OPri)(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 sigma-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 center dot center dot center dot Fe separation of 10.4 angstrom, electronic communication between them still takes place as revealed by the cluster's two distinct one-electron electrochemical oxidation processes.