Synthesis and characterization of a family of M super(2+) complexes supported by a trianionic ONO super(3-) pincer-type ligand: towards the stabilization of high-spin square-planar complexes

High-spin square-planar molecular compounds are rare. In an effort to access this unique combination of geometry and spin state, we report the synthesis of a series of M(ii) compounds stabilized by a trianionic pincer-type ligand, highlighting the formation of a high-spin square-planar Co(ii) complex. Low-temperature, variable-frequency EPR measurements reveal that the ground electronic state of the Co(ii) analogue is a highly anisotropic Kramers doublet (effective gvalues 7.35, 2.51, 1.48). This doublet can be identified with the lowest doublet of a quartet, S= 3/2 spin state that exhibits a very large ZFS, D greater than or equal to 50 cm super(-1). The observation of an effective gvalue considerably greater than the largest spin-only value 6, demonstrates that the orbital angular moment is essentially unquenched along one spatial direction. Density Functional Theory (DFT) and time-dependent DFT calculations reveal the electronic configurations of the ground and excited orbital states. A qualitative crystal field description of the g sub(eff) tensor shows that it originates from the spin-orbit coupling acting on states obtained through the transfer of a beta electron from the doubly occupied xyto the singly-occupied {xz/yz} orbitals.