Effect of linear and non-linear pseudohalides on the structural and magnetic properties of Co(ii) hexacoordinate single-molecule magnets

Abstract

A series of mononuclear hexacoordinate Co(ii) complexes with the 4-amino-3,5-bis(2-pyridyl)-1,2,4-triazole (abpt) ligand and various linear and non-linear pseudohalides, such as NCSe (selenocyanate), N{C(CN)} (1,1,3,3-tetracyano-2-azapropenide, tcap), NOC(CN) (nitrodicyanomethanide, nodcm), C(CN){C(CN)} (1,1,2,3,3-pentacyanopropenide, pcp), NONCN (nitrocyanamide, nca), and ONC(CN) (nitrosodicyanomethanide, ndcm), was prepared. X-ray analyses revealed the formation of the complexes with the general compositions [Co(abpt)(solv)]X (solv = HO and X = tcap (1), solv = HO and X = nodcm (2), solv = CHOH and X = pcp (3)) or [Co(abpt)(X)] (X = nca (4), X = NCSe (5), X = ndcm (6)). The impact of axial co-ligands (solv or X) on the magnetic properties was investigated experimentally by measuring temperature- and field-dependent static (DC) and dynamic magnetic (AC) data as well as theoretically using the CASSCF/NEVPT2, AILFT, and AOM methods. Large magnetic anisotropy was found for all complexes 1-6 and was treated either by the spin Hamiltonian or with the Hamiltonian including the orbital angular momentum. Furthermore, the AC susceptibility measurements confirmed the slow relaxation of the magnetization in a non-zero static magnetic field, thus these complexes can be classified as field-induced single-molecule magnets with an estimated energy barrier U up to 100 K.

New Cobalt(II) Field‐Induced Single‐Molecule Magnet and the First Example of a Cobalt(III) Complex with Tridentate Binding of a Deprotonated 4‐Amino‐3,5‐bis(pyridin‐2‐yl)‐1,2,4‐Triazole Ligand

“…The axial positions are occupied by two terminal trans ‐coordinated nitrogen atoms from azido ligands with the Co–N7 coordination distance of 2.121(2) Å. The Co–N bond lengths involving the pyridyl groups [Co1–N6 = 2.157(2) Å] are longer than those corresponding to the triazole rings [Co1–N2 = 2.092(2) Å], which is in good agreement with other isostructural Co II compounds containing abpt ligand , , . The angular distortions of coordination octahedron are determined by the chelate binding of abpt ligands and result in the expected substantial deviations of the chelate bite angle N2–Co1–N6 [76.75(7)°] from 90°; all cis N–Co1–N angles range from 76.75(7)° to 103.25(7)° (Table ).…”

“…The selected parameters are compared with so far published cobalt‐based SMMs comprising abpt ligand (Table ). To sum up, it is evident that various relaxation mechanisms contribute to the overall process and their identification is not an easy task, however, a better understanding of these mechanisms is crucial for the development of the single‐molecule magnets , …”

“…The Co-N bond lengths involving the pyridyl groups [Co1-N6 = 2.157(2) Å] are longer than those corresponding to the triazole rings [Co1-N2 = 2.092(2) Å], which is in good agreement with other isostructural Co II compounds containing abpt ligand. [9,10,22] The angular distortions of coordination octahedron are determined by the chelate binding of abpt ligands and result in the expected substan- (9) tial deviations of the chelate bite angle N2-Co1-N6 [76.75(7)°] from 90°; all cis N-Co1-N angles range from 76.75(7)°to 103.25(7)°( Table 1). The octahedral distortion parameter Σ = (|90i |), where i represents the twelve cis N-Co1-N angles, is equal to 58.59°.…”

“…[7] The abpt ligand was studied mainly in combination with various transition metals, due to their tendency to form coordination compounds exhibiting interesting magnetic properties, such as spin crossover, [7] as well as single-molecule magnets' behavior. [8] [9] The single-molecule magnets (SMMs) are nanomagnets showing magnetic bistability and the slow relaxation of the magnetization due to the existence of the energy barrier (U) acting against the spin momentum reversal. [10] In case of Co II complexes, the U is related to the zero-field splitting (ZFS) [11] of the ground spin state S = 3/2 as U = |D|(S 2 -1/4), where D is the axial ZFS parameter defined within the spin Hamiltonian formalism.…”

“…Recently, we have studied and described a series of mononuclear hexacoordinate Co II coordination compounds with abpt ligand and linear and non-linear pseudohalide anions, which behave as field-induced single ion magnets. [9,10] In this context, as an extension of our research, we report herein the preparation and characterization of two new complexes: bis(azido)-bis(4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) cobalt(II) complex, [Co(abpt) 2 (N 3 ) 2 ] (1) and tetrakis(azido)-bis(μ-3,5-bis(pyridin-2-yl)-4H-1,2,4-triazol-4-aminato)-(3,5-bis(pyridin-2-yl)-4H-1,2,4-triazol-4-amine)-dicobalt(III) hydrate complex, [Co 2 (abpt)(μ-abpt-H) 2 (N 3 ) 4 ]·1.12H 2 O (2). The compound 1 exhibits field-induced slow relaxation of magnetization, while the novelty of 2 consists in the first occurrence of an abpt-H bridged binuclear Co III compound as well as the first occurrence of a tridentate abpt-H ligand.…”