Two-dimensional frameworks formed by pentagonal bipyramidal cobalt(ii) ions and hexacyanometallates: antiferromagnetic ordering, metamagnetism and slow magnetic relaxation

Abstract

We herein report the syntheses, structures, and magnetic properties of two isostructural two-dimensional (2D) coordination polymers based on a pentagonal bipyramidal Co unit [Co(TODA)] and two hexacyanometallates, namely [M(CN)][Co(TODA)]·9HO (M = Cr (1), Co (2), TODA = 1,4,10-trioxa-7,13-diazacyclopentadecane). Structure analyses show that both complexes have 2D honeycomb structures where the [Co(TODA)] units are bridged by the [M(CN)] groups through three cyano groups in the facial positions. Magnetic investigation reveals ferromagnetic coupling between the Cr and Co centres through cyanides in 1. Due to the antiferromagnetic interaction between the layers, compound 1 exhibits an antiferromagnetic ordering below 11.4 K, and shows a metamagnetic phase transition under an external dc field. Due to the disorder of the TODA ligands, compound 1 shows a spin glass behavior, which leads to slow magnetic relaxation in 1. A butterfly-shaped hysteresis loop at 1.8 K can be observed with a coercive field of 720 Oe, which is quite large for cyano-bridged Cr-Co molecular magnets. For compound 2 containing the diamagnetic [Co(CN)] unit, field-induced slow magnetic relaxation was also verified, which makes compound 2 a rare example of an SIM assembled in a 2D network. An easy-plane magnetic anisotropy with a positive D value (29.9 cm by PHI and 26.5 cm by Anisofit2.0) was deduced for hepta-coordinated Co centers. These results show the efficiency of the strategy of combining cyanometallates and pentagonal bipyramidal precursors for novel molecular magnetic materials.

Structure and Magnetism of Seven‐Coordinate FeIII, FeII, CoII and NiII Complexes Containing a Heptadentate 15‐Membered Pyridine‐Based Macrocyclic Ligand

“…geometry , . Nevertheless, several examples of linear (H 2 dabph) or macrocyclic ligands (15‐pydienN 3 O 2 , 15‐pydienN 5 , 15‐pyN 3 O 2 , tdmmb, or more recent TODA) (Figure S1) provided seven‐coordinate pentagonal bipyramidal complexes with a pentadentate ligand in the equatorial plane, which show a large easy‐axis magnetic anisotropy ( D < 0) in the case of Fe II and Ni II complexes, and on the other side, a large easy‐plane magnetic anisotropy ( D > 0) in the case of Co II complexes. Slow relaxation of magnetization was observed for [Fe(H 2 dabph)Cl 2 ] and [Fe(H 2 dabph‐NH 2 )Cl 2 ] ( τ 0 = 5 × 10 –9 s and U eff = 53 cm –1 ), for [{Ni(H 2 dabph)} 3 {W(CN) 8 } 2 ] (below the blocking temperature T N = 3.6 K, τ 0 = 1.6 × 10 –9 s, U eff = 30 K), and for many different Co II complexes , , , .…”

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

“…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.…”

“…Recently, our group has been continuously focused on the single-ion magnetism of Co II ions with pentagonal bipyramidal (PBP) geometry, and we have reported a series of mononuclear complexes [20,21], as well as 1D and 2D coordination polymers based on the PBP Co II building blocks [8][9][10]. The influence of axial ligands and magnetic exchange interactions has been carefully studied in these systems.…”

“…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.…”

Two New Three-Dimensional Pillared-Layer Co(II) and Cu(II) Frameworks Involving a [M2(EO-N3)2] Motif from a Semi-Flexible N-Donor Ligand, 5,5′-Bipyrimidin: Syntheses, Structures and Magnetic Properties

“…The M value of 0.45 Nβ at 70 kOe is far below the expected value of saturation of 2-3 Nβ for an isotropic high-spin Co(II) system, confirming the antiferromagnetic nature of Compound 1. Such field-induced magnetic transition of metamagnetism is frequently observed in layer or chain systems with strong anisotropy and competing interactions [34][35][36][37]. The hysteresis loop is obvious at 2.0 K (Figure 7, inset), which shows a remnant magnetization of 0.074 Nβ and a coercive field of 3.5 kOe, indicating that 1 has a hard magnet behavior.…”