An Unprecedented Square Planar Fe(III) Complex Exhibiting Spin Crossover Between the Spin-Admixed States.

The presence of strong spin-orbit coupling, which is comparable to the crystal field in certain ferric-porphyrin/phthalocyanine derivatives, results in a spin-admixed state between the high-spin (HS; S = 5/2) and intermediate-spin (IS; S = 3/2) states. However, there are no literature precedents for spin admixing between the IS (S = 3/2) and low-spin (LS; S = 1/2) states, which are purely governed by axial ligation. It is also true that systems showing spin-admixed behavior do not exhibit thermal equilibrium between the spin states, that is, spin crossover (SCO). In this article, we report a unique and rare square planar ferric ion complex, [Na(DME)3][Fe(L12-)2] (1), which exhibits two different physical phenomena: SCO is observed between the spin-admixed states, a phenomenon unprecedented in this context. Field- and temperature-dependent Mössbauer spectra, variable temperature X-band EPR spectra, multifrequency EPR (MF-EPR), and temperature-dependent magnetic susceptibility measurements reveal that the spin-admixed state has contributions from IS (S = 3/2) and LS (S = 1/2), unlike other reports in the literature. The detailed electronic structure of 1, along with the rationale for quantum spin admixing and SCO, is well supported by theoretical calculations.