Single-ion magnet behaviour in mononuclear and two-dimensional dicyanamide-containing cobalt(ii) complexes

Abstract

Three cobalt(ii) complexes of formulae [Co(dca)2(bim)4] (), [Co(dca)2(bim)2]n () and [Co(dca)2(bmim)2]n () [dca = dicyanamide, bim = 1-benzylimidazole and bmim = 1-benzyl-2-methylimidazole] were prepared and structurally analyzed by single-crystal X-ray crystallography. Compound is a mononuclear species where the cobalt(ii) ion is six-coordinate with four bim molecules in the equatorial positions [Co-Nbim = 2.1546(15) and 2.1489(15) Å] and two trans-positioned dca ligands [Co-Ndca = 2.1575(18) Å] in the axial sites of a somewhat distorted octahedral surrounding. The structures of and consist of two-dimensional grids of cobalt(ii) ions where each metal atom is linked to the other four metal centres by single dca bridges exhibiting the μ1,5-dca coordination mode [Co-Ndca = 2.190(3)-2.220(3) () and 2.127(3)-2.153(3) Å ()]. Two trans-coordinated bim ()/bmim () molecules achieve the six-coordination around each cobalt(ii) ion [Co-Nbim = 2.128(3)-2.134(4) Å () and Co-Nbmim = 2.156(3)-2.163(39) Å ()]. The values of the cobalt-cobalt separation through the single dca bridges are 8.927(2) and 8.968(2) Å in and 8.7110(5) and 8.7158(5) Å in . Magnetic susceptibility measurements for in the temperature range of 2.0-300 K reveal that these compounds behave as magnetically isolated high-spin cobalt(ii) ions with a significant orbital contribution to the magnetic moment. Alternating current (ac) magnetic susceptibility measurements for show a frequency dependence of out-of-phase susceptibility under static applied fields in the range of 500-2500 G, a feature which is characteristic of the single-ion magnet behaviour (SIM) of the Co(ii) ion in them. The values of the energy barrier for the magnetic relaxation (Ea) are 5.45-7.74 (), 4.53-9.24 () and 11.48-15.44 cm(-1) (). They compare well with those previously reported for the analogous dca-bridged 2D compound [Co(dca)2(atz)2]n () (Ea = 5.1 cm(-1) under an applied static field of 1000 G), which was the subject of a previous report.

A Cobalt Supramolecular Triple-Stranded Helicate-based Discrete Molecular Cage

“…S6C). This value, while higher than the estimated spin-only value of 1.88 (emu K mol −1 ) for S  = 3/2, still falls within an acceptable range when compared to other experimentally observed high-spin octahedral Co (II) ions with an orbital angular momentum contribution4243444546. Upon cooling, χ M T continuously decreases to a value of 1.73 (emu K mol −1 ) at 6.4 K. The chemical states of the Co species in 1 are also investigated by X-ray photoelectron spectroscopy (XPS) (Supplementary Fig.…”

Field-Induced Single-Ion Magnet Behaviour in Two New Cobalt(II) Coordination Polymers with 2,4,6-Tris(4-pyridyl)-1,3,5-triazine

“…In addition, to gain a clear scope on reported non-isolated Co II SIMs, the magnetic properties of these systems, including magnetic anisotropy parameters and energy barriers, are compiled in Table S3. From this table, we noticed that the single-ion magnetism in 1D, 2D, and 3D Co II coordination polymers with local Oh symmetry only exhibit an easy-plane type of magnetic anisotropy [11][12][13][14][15][16][17][18]. The situation in the present compounds further support this phenomenon.…”

“…At 300 K, the χ M T values for 1 and 2 are 2.826 and 3.103 cm 3 ·mol −1 ·K, respectively, which are significantly higher than the spin-only value for a high-spin Co II ion with g = 2 (1.875 cm 3 ·mol −1 ·K). These high values can be attributed to the orbit contribution and significant spin-orbit coupling (SOC) and are comparable to the reported values (2.1-3.8 cm 3 ·mol −1 ·K) for high-spin Co II centres in the distorted octahedral geometry [11][12][13][14][15][16][17][18]. Upon cooling, the χ M T curves decrease very slowly down to the vicinity of 50 K and then abruptly decrease to 2 K, which is typical of anisotropic mononuclear Co II complexes [4].…”

“…For 1, the linear variation of the logarithm of relaxation times at the whole temperature region indicates a dominating Orbach relaxation. It is worth noting that such phenomena have also been observed in some other 2D Co II based SIMs with octahedral local geometry [9][10][11][12][13][14][15][16]. However, multiple relaxation pathways are strongly suggested by the curvature of the Arrhenius plot of 2.…”

“…Intense efforts have been devoted to the design and synthesis of isolated 3d SIMs with coordination numbers ranging from two to eight in various geometries [6]. Furthermore, the field-induced SIM behaviour has also been explored in some one-dimensional (1D) [7,8], two-dimensional (2D) [9][10][11][12][13][14][15][16], and even three-dimensional (3D) Co II coordination frameworks [17,18], as long as the magnetic coupling between the spin centres can be ignored. The magnetic anisotropic spin centres in the frameworks can be well isolated by organic linkers or diamagnetic cyanometallates, and thus can be regarded as isolated single-ion magnetic centres.…”