Probing the Effect of Axial Ligands on Easy-Plane Anisotropy of Pentagonal-Bipyramidal Cobalt(II) Single-Ion Magnets

Abstract

We herein reported the synthetic, structural, computational, and magnetic studies of four air-stable heptacoordinated mononuclear cobalt(II) complexes, namely, [Co(tdmmb)(HO)][BF] (1), [Co(tdmmb)(CN)]·2HO (2), [Co(tdmmb)(NCS)] (3), and [Co(tdmmb)(SPh)] (4) (tdmmb = 1,3,10,12-tetramethyl-1,2,11,12-tetraaza[[3](2,6)pyridino[3](2,9)-1,10-phenanthrolinophane-2,10-diene; SPh = thiophenol anion). Constrained by the rigid pentadentate macrocyclic ligand tdmmb, the Co centers in all of these complexes are in the heptacoordinated pentagonal-bipyramidal geometry. While the equatorial environments of these complexes remain very similar to each other, the axial ligands are systematically modified from C to N to O to S atoms. Analyses of the magnetic data and the ab initio calculations both reveal large easy-plane magnetic anisotropy (D > 0) for all four complexes. While the experimentally obtained D values do not show any clear tendency when the axial coordinated atoms change from C to N to O atoms (complexes 1-3), the largest value is for the heavier and softer S-atom-coordinated complex 4. Because of significant magnetic anisotropy, all four complexes are field-induced single-ion magnets. This work represents a delicate modification of the magnetic anisotropy by tuning the chemical environment of the metal centers.

A Series of Field-Induced Single-Ion Magnets Based on the Seven-Coordinate Co(II) Complexes with the Pentadentate (N3O2) H2dapsc Ligand

“…The complexes exhibit positive axial magnetic anisotropy (D) with non-zero rhombicity parameter ( Table 1). The sign and the magnitudes of D for 1-7 are in the range of previously reported those for the seven-coordinate pentagonal bipyramidal Co(II) complexes [24,25,33,34].…”

“…However, low coordinate complexes are not very stable which restricts their possible utility in different applications. At the same time, the experimental and theoretical studies of stable seven-coordinated metal centers with pentagonal bipyramidal geometry showed that such centers are highly promising as anisotropic spin carriers [16][17][18][19][20][21][22][23][24][25]. In this context, 3d-metallocomplexes with the pentadentate Schiff-base H 2 dapsc ligand (H 2 dapsc = 2,6-diacetylpyridine bis(semicarbazone), Figure 1) and its analogues are of considerable interest.…”

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

“…Furthermore, the magnetic anisotropy has been tuned by playing with donor/acceptor properties of two axial coligands, as was shown on series of Co II complexes, e.g. [Co(tdmmb)(X) 2 ] 2+/0 , where X = H 2 O, CN – , NCS – , SPh – , with the more pronounced anisotropy for the complex with SPh – ( D = 39.7 cm –1 ), or [Co(15‐pyN 3 O 2 )X 2 ], where X = Cl – , Br – and I – with the highest D = 41 cm –1 for the dibromido complex, and also on Mn II complexes [Mn(15‐pyN 3 O 2 )X], where X = Br – , I – , N 3 – , NCS – , with a small influence of axial coligands on the magnetic anisotropy (| D | < 0.7 cm –1 ).…”

“…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

“…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.…”