From Positive to Negative Zero-Field Splitting in a Series of Strongly Magnetically Anisotropic Mononuclear Metal Complexes

Abstract

A series of mononuclear [M(hfa)(pic)] (Hhfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; pic = 4-methylpyridine; M = Fe, Co, Ni, Zn) compounds were obtained and characterized. The structures of the complexes have been resolved by single-crystal X-ray diffraction, indicating that, apart from the zinc derivative, the complexes are in a trans configuration. Moreover, a dramatic lenghthening of the Fe-N distances was observed, whereas the nickel(II) complex is almost perfectly octahedral. The magnetic anisotropy of these complexes was thoroughly studied by direct-current (dc) magnetic measurements, high-field electron paramagnetic resonance, and infrared (IR) magnetospectroscopy: the iron(II) derivative exhibits an out-of-plane anisotropy (D = -7.28 cm) with a high rhombicity, whereas the cobalt(II) and nickel(II) complexes show in-plane anisotropy (D ∼ 92-95 cm; D = 4.920 cm). Ab initio calculations were performed to rationalize the evolution of the structure and identify the excited states governing the magnetic anisotropy along the series. For the iron(II) complex, an out-of-phase alternating-current (ac) magnetic susceptibility signal was observed using a 0.1 T dc field. For the cobalt(II) derivative, the ac magnetic susceptibility shows the presence of two field-dependent relaxation phenomena: at low field (500 Oe), the relaxation process is beyond single-ion behavior, whereas at high field (2000 Oe), the relaxation of magnetization implies several mechanisms including an Orbach process with U = 25 K and quantum tunneling of magnetization. The observation by μ-SQUID magnetization measurements of hysteresis loops of up to 1 K confirmed the single-ion-magnet behavior of the cobalt(II) derivative.

Homoleptic Mononuclear Tris‐Chelate Complexes of FeII, CoII, NiII, and ZnII Based on a Redox‐Active Imidazolyl‐2‐thione Ligand: Structural and Electrochemical Correlation

“…At room temperature the χT product is 3.71, 3.07 and 1.62 cm 3 K mol –1 , for complexes 1 to 3 , respectively. These values are higher than the expected spin only value for the χT product of 3.02, 1.89 and 1.01 cm 3 K mol –1 for mononuclear HS Fe II ( S = 2), HS Co II ( S = 3/2) and Ni II ( S = 1) complexes with g = 2.0, indicating important orbital contribution to g, as observed for other distorted mononuclear complexes of Fe, Co and Ni . Nonetheless, these values are in agreement with the HS‐state assignment performed by SC‐XRD for 1 and 2 (see above).…”

Single-ion anisotropy and exchange coupling in cobalt(ii)-radical complexes: insights from magnetic and ab initio studies

“…117 Pseudo-octahedral eld-induced SMMs have been reported with both easy-plane and easy-axis anisotropies (in the case of quenched orbital angular momentum) as well as rhombic electronic structures. 73,74,[117][118][119] Dynamic magnetic properties were measured for 1, 2(PF 6 ) and 3(PF 6 )$tol to explore the dependence of potential SMM behavior on single-ion anisotropy and exchange effects. In the absence of a direct current (DC) eld, no out-of-phase alternating current (AC) signal was observed for 1 or 2(PF 6 ), potentially due to QTM promoted by intermolecular interactions or hyperne coupling to the Co nucleus (I ¼ 7/2).…”

Multi‐Magnetic Properties of a Novel SCO [Fe(3‐OMe‐Sal2trien)][Fe(tdas)2]·CH3CN Salt

“…This facilitated identifying molecular orbitals of predominantly iron d orbital character. [44,45] The orbital analysis was straightforward for MOs containing predominant contribution of iron e g -like orbitals. Whereas the mixing of the MOs containing a predominant contribution of iron t 2g -like orbitals is much stronger in both spin states.…”