Field‐Supported Single‐Molecule Magnets of Type [Co(bzimpy)X₂]

Abstract

Magnetic properties of pentacoordinate complexes of the type [Co(bzimpy)X 2 ], X = Cl and Br, were investigated in DC and AC modes. The DC data shows sizeable zero-field splitting for these complexes. The AC susceptibility data confirms Cobalt(II) complexes are known as systems with a considerable magnetic anisotropy, which manifests itself in anomalies of the magnetic functions, such as vs. T, T vs. T, and M vs. B. In the case of the orbitally nondegenerate ground term, this is characterized by rather large zero-field splitting. [1] Experimental studies about the magnetic anisotropy in d-block complexes dates to the 1960s, appearing in the pioneering works of Figgis and Lewis, and later, in those of Gerloch, and Mitra, for instance. [2] The axial zero-field splitting parameter, D, for tetrahedral systems [CoL 4 ] with an 4 A 2 ground electronic term (two Kramers doublets) can adopt both positive and negative values. For hexacoordinate systems [CoL 4 X 2 ] resembling the compressed tetragonal bipyramid (D 4h symmetry, 4 A 2g ground term, two Kramers doublets), D > 0 always holds true. For the elongated tetragonal bipyramid with an 4 E g ground term (four Kramers doublets), on the contrary, the spin-Hamiltonian formalism fails, and thus, the D parameter is undefined. Systems with the geometry of a tetragonal pyramid (C 4v symmetry) can be modeled by removing one apical ligand (X) from a compressed tetragonal bipyramid to [CoL 4 X], with an increase of the bond angle X-Co-L to about 104°. In this case, the ground term is 4 E (4 Kramers doublets), and again, the spin-Hamiltonian formalism is not applicable. For the ideal trigonal bipyramid (D 3h , 4 A 1 ′ ground state, 2 Kramers doublets), the D parameter is in play. However, for large rhombicity, when |D| ≈ 3E, the sign of the D parameter stays unassigned.Determination of the D values is important from the viewpoint of single-molecule magnetism, since the D parameter en- [a]

Pentacoordinate cobalt(ii) single ion magnets with pendant alkyl chains: shall we go for chloride or bromide?

“…On the other hand, almost the same parameters of Orbach relaxation for 3 and 4 suggest that this process cannot be affected by variation of the halido ligands while the other molecular and supramolecular features of isostructural analogues remain the same. We tried to confirm this hypothesis by comparing the Co( ii ) SIMs reported herein with previously reported similar isostructural systems [Co(bbp)X 2 ]·MeOH, 21 [Co(bbp)X 2 ]·DMF, 53 and [Co(tBuBzbbp)X 2 ] 22 and cubic polymorphs of [Co(L)X]X, 54 which contain halido terminal ligands X = Cl − or Br − (where bbp = 2,6-bis(benzimidazole-1-yl)pyridine, tBuBzbbp = 2,6-bis(1-(3,5-di- tert -butylbenzyl)-1 H -benzimidazol-2-yl)pyridine and L = tris(2-pyridylmethyl)amine). Table S15† presents an overview of the ZFS and relaxation parameters obtained under the same (or similar) experimental conditions ( T , B DC ).…”

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

“…There is a clear-cut difference between these two maxima in both frequency (Figure c) and field (Figure e), indicating that they originate from two independent relaxation processes. The presence of two relaxation processes in a mononuclear cobalt­(II) complex has been reported recently. − In order to clarify the origin of these two relaxation processes, the variable-frequency (1–1500 Hz) and variable-temperature (2–6 K) ac susceptibilities at 500, 1125, 1250, and 2000 Oe were measured (Figures and S17). At 500 and 2000 Oe, a generalized Debye model for one relaxation model − was used to extract the relaxation time.…”

“…The presence of two relaxation processes in a mononuclear cobalt­(II) complex has been reported recently. − In order to clarify the origin of these two relaxation processes, the variable-frequency (1–1500 Hz) and variable-temperature (2–6 K) ac susceptibilities at 500, 1125, 1250, and 2000 Oe were measured (Figures and S17). At 500 and 2000 Oe, a generalized Debye model for one relaxation model − was used to extract the relaxation time. At 1250 Oe, where two maxima in the out-of-phase susceptibility were detected (Figure c), the experimental data were fitted using an extended Debye model − for two relaxation processes (Figure S17).…”

Water Molecule-Induced Reversible Magnetic Switching in a Bis-Terpyridine Cobalt(II) Complex Exhibiting Coexistence of Spin Crossover and Orbital Transition Behaviors

“…Under applied fields of 500 and 5000 Oe (where only one relaxation pathway is clearly dominant), the χ m ″ versus frequency plots reveal clear temperature-dependent peaks down to 1.9 K (Figures , S8, S9, and S11). The occurrence of two such relaxation pathways in other mononuclear Co­(II) SMMs has frequently been observed − and attributed to the presence of the spin–spin interactions caused by intermolecular short contacts, such as hydrogen bonds, π–π stacking, and CH−π interactions–resulting in much slower spin relaxations than occur in pure mononuclear entities. The origin of the second main group of such a spin relaxation process has been investigated by using magnetic dilution of a seven-coordinate mononuclear Co­(II) complex, [Co­(DAPBH)­(NO 3 )­(H 2 O)]­(NO 3 ) (DAPBH = 2,6-diacetylpyridinebis­(2′-pyridylhydrazone)) which shows the field-induced SMM behavior, with the diamagnetic Zn­(II) analogue by Habib and co-workers .…”