Magnetic Anisotropy from Easy-Plane to Easy-Axial in Square Pyramidal Cobalt(II) Single-Ion Magnets

Abstract

By elaborately employing a rigid tetrazo macrocyclic ligand, both the designed Co(II) complexes [Co(12-TMC)Py](BF 4 ) 2 (1) and [Co(12-TMC)DMF](BF 4 )Cl (2) (12-TMC = 1,4,7,10-tetramethyl-1,4,7,10tetraazacyclododecane; Py = pyridine; and DMF = N,N-dimethylformamide) adopt the distorted square pyramidal geometry with the identical equatorial site but a flexible axial ligand environment. Such a molecular design enables the realization of field-induced single-ion magnets. Importantly, the designed single-molecule magnets display axial ligand-mediated magnetic anisotropy. The axially elongated geometry in complex 1 is devoted to stabilizing a positive zero-field splitting parameter (D), while an axially compressed one in complex 2 tends to stabilize a negative D. Such an understanding was further validated by a series of complexes with the same square pyramidal geometry but different axial ligands such as Cl − and NCO − . Therefore, this study provides a new example of the delicate modification of magnetic behavior and presents fundamental insights into magneto-structural correlation.

Field induced single ion magnet behavior in CoII complexes in a distorted square pyramidal geometry

“…7). [114][115][116] Our computed D values (2 Cl > 3 Br > 1 SeCN ) well correlates with the opposite trend of the energy gap for the first excited spin-free states (2 Cl < 3 Br < 1 SeCN ) (Table 5).…”

“…Consequently, we get a positive D-value as the transition occurs between the d yz /d xz and the d xy orbitals which are of different m l levels. [114][115][116] However, local structural distortions significantly perturb the energy ordering and hence the magnitude and strength of the zero-field splitting in these complexes. Structural analysis reveals that the Co II ion is out of the basal plane in all three complexes [0.39, 0.55 and 0.54 Å for complexes 1 SeCN , 2 Cl and 3 Br respectively], as a result the d xy orbital stabilizes compared to the degenerate d yz /d xz orbitals.…”

Slow magnetic relaxation and spin crossover behavior in two mixed-valence Co(ii)/Co(iii) complexes

“…58 Such large w M T value is in the range 2.1-3.4 cm 3 mol À1 K for the highly anisotropic Co(II) ions with a considerable spin-orbit coupling contribution. 59 Upon cooling, the w M T value gradually decreases to 1.68 cm 3 mol À1 K at 2 K, indicating zerofield splitting of the ground state of the Co(II) ion. In order to gain more insight into the magnetic behavior of complex 1, the field-dependent magnetizations were measured at 2, 3 and 5 K in the range of 0-7 T (Fig.…”

Tuning of Dimensionality and Nuclearity as a Function of Ligand Field Modulation Resulting in Field-Induced Cobalt(II) Single-Ion Magnet

“…The field of SMMs started with mixed-valent Mn 12 acetate, and the first Co II -based SMM was a tetrametallic complex [Co 4 (hmp) 4 ] [hmp = hydroxymethylpyridine], while the first example of Co II SIM was a penta-coordinate complex having a square-pyramidal geometry with an acyclic N3 ligand along with two [NCS] − coligands . Though quite a few Co II -based SMMs/SIMs have been reported in numerous coordination environments over the past few years, ,− still, it is challenging to stabilize a lower coordinated Co­(II) SIM, as the high-spin Co­(II) favors the octahedral coordination environment. − …”