A Mononuclear Fe(III) Single Molecule Magnet with a 3/2↔5/2 Spin Crossover

Abstract

The air stable complex [(PNP)FeCl2] (1) (PNP = N[2-P(CHMe2)2-4-methylphenyl]2−), prepared from one-electron oxidation of [(PNP)FeCl] with ClCPh3, displays an unusual S = 3/2 to S = 5/2 transition above 80 K as inferred by the dc SQUID magnetic susceptibility measurement. The ac SQUID magnetization data, at zero field and between frequencies 10 and 1042 Hz, clearly reveals complex 1 to undergo a frequency dependent of the out-of-phase signal and thus be a single molecular magnet with a thermally activated barrier of Ueff = 32-36 cm−1 (47 - 52 K). Variable temperature Mössbauer data also corroborate a significant temperature dependence in δ and ΔEQ values for 1, which is in agreement with the system undergoing a change in spin state. Likewise, variable temperature X-band EPR spectra of 1 reveals the S = 3/2 to be likely the ground state with the S = 5/2 being close in energy. Multi-edge XAS absorption spectra suggest the electronic structure of 1 to be highly covalent with an effective iron oxidation state that is more reduced than the typical ferric complexes due to the significant interaction of the phosphine groups in PNP and Cl ligands with iron. A variable temperature single crystal X-ray diffraction study of 1 collected between 30-300 K also reveals elongation of the Fe–P bond lengths and increment in the Cl–Fe–Cl angle as the S = 5/2 state is populated. Theoretical studies show overall similar orbital pictures except for the d(z2) orbital which is the most sensitivity to change in the geometry and bonding where the quartet (4B) and the sextet (6A) states are close in energy.

Magnetic anisotropy in two- to eight-coordinated transition–metal complexes: Recent developments in molecular magnetism

“…A c c e p t e d M a n u s c r i p t Edited June 25 experimentally difficult to restrict the coordination environments of these complexes unless conformationally rigid chelating ligands are used; thus, TBP complexes with exact D 3 symmetry are scarce [88,[92][93][94]. A small distortion from axial symmetry can significantly modify the magnetic anisotropy.…”

Diamagnetic cobalt(III)tris(o-ethylxanthate) and nickel(II)bis(o-ethylxanthate)

“…The earlier type of SMMs was based on high-spin polynuclear complexes, such as a dodecanuclear plaquette-type complex known as Mn12ac. In course of time a plethora 3d-metal complexes were identified as SMMs, including simple mononuclear systems containing Mn(III) (Grigoropoulos et al 2013), Fe(III) (Mossin et al 2012), Fe(II) , Fe(I) (Samuel et al 2014), Co(II) , Ni(II) (Lomjansky et al 2017), Ni(I) (Lin et al 2016) and Cu(II) central atoms (Frost et al 2016). Among these systems, the S-donor ligands combined with the cobalt(II) centres attract a considerable attention because of the pronounced magnetic anisotropy that supports the SMM behaviour (Zadrozny and Long 2011).…”

Experimental and Theoretical Identification of the Origin of Magnetic Anisotropy in Intermediate Spin Iron(III) Complexes

“…In the case of Fe III (d 5 ), intermediate spin ground states are observed mainly for complexes of square pyramidal coordination geometry with few exceptions found in octahedral or square planar complexes in case of suitable ligand environments 2. Although the electronic and magnetic properties of a number of mononuclear S= 3/2 Fe III complexes have been the subject of detailed experimental studies, the corresponding computational studies are limited to the determination of the spin of their ground states, and, to the best of our knowledge, the present report is the first theoretical consideration of the origin of their magnetic anisotropy 1a,1d, 3…”