Impact of Halogenido Coligands on Magnetic Anisotropy in Seven-Coordinate Co(II) Complexes

Abstract

The structural and magnetic features of a series of mononuclear seven-coordinate Co complexes with the general formula [Co(L)X], where L is a 15-membered pyridine-based macrocyclic ligand (3,12,18-triaza-6,9-dioxabicyclo[12.3.1]octadeca-1(18),14,16-triene) and X = Cl (1), Br (2), or I (3), were investigated experimentally and theoretically in order to reveal how the corresponding halogenido coligands in the apical positions of a distorted pentagonal-bipyramidal coordination polyhedron may affect the magnetic properties of the prepared compounds. The thorough analyses of the magnetic data revealed a large easy-plane type of the magnetic anisotropy (D > 0) for all three compounds, with the D-values increasing in the order 35 cm for 3 (I), 38 cm for 1 (Cl), and 41 cm for 2 (Br). Various theoretical methods like the Angular Overlap Model, density functional theory, CASSCF/CASPT2, CASSCF/NEVPT2 were utilized in order to understand the observed trend in magnetic anisotropy. The D-values correlated well with the Mayer bond order (decreasing in order Co-I > Co-Cl > Co-Br), which could be a consequence of two competing factors: (a) the ligand field splitting and (b) the covalence of the Co-X bond. All the complexes also behave as field-induced single-molecule magnets with the spin reversal barrier U increasing in order 1 (Cl) < 2 (Br) < 3 (I); however, taking into account the easy-plane type of the magnetic anisotropy, the Raman relaxation process is most likely responsible for slow relaxation of the magnetization. The results of the work revealed that the previously suggested and fully accepted strategy employing heavier halogenido ligands in order to increase the magnetic anisotropy has some limitations in the case of pentagonal-bipyramidal Co complexes.

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

“…The dependence of D and E/D on d (M–OH) is depicted in Figure . The d ‐parameters are large and positive for a Co II model complex within whole range of d (Co–OH) values, the D is increasing with decreasing ligand field in the axial positions, which agrees with our previous AOM prediction . The rhombicity is very small down to d (Co–OH) = 2.1 Å, and then starts to increase up to almost E/D = 1/3, which suggest that for very strong axial ligand field, the D could be negative.…”

“…In general, there are several processes in solid state responsible for the observation of the slow relaxation of the magnetization: one‐phonon direct process, two‐phonon Raman and Orbach processes and quantum tunnelling. Recently, we have shown that temperature‐dependence of the relaxation times for seven‐coordinate Co II complexes can be fitted either with the Orbach or Raman relaxation process , . In case of 3 , large and positive D together with negligible E/D ratio means that the easy‐plane type of the magnetic anisotropy is present, and therefore the existence of the spin reversal barrier U eff = | D |×( S 2– 1/4) is hard to imagine.…”

“…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 ).…”

“…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 , , , . 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.…”

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

“…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.…”