Magnetic Anisotropy of Tetrahedral Co^II Single-Ion Magnets: Solid-State Effects

Abstract

This study reports the static and dynamic magnetic characterization of two mononuclear tetrahedral Co complexes, [Co{PrP(E)NP(E)Pr}], where E = S (CoS) and Se (CoSe), which behave as single-ion magnets (SIMs). Low-temperature (15 K) single-crystal X-ray diffraction studies point out that the two complexes exhibit similar structural features in their first coordination sphere, but a disordered peripheral Pr group is observed only in CoS. Although the latter complex crystallizes in an axial space group, the observed structural disorder leads to larger transverse magnetic anisotropy for the majority of the molecules compared to CoSe, as confirmed by electron paramagnetic resonance spectroscopy. Static magnetic characterization indicates that both CoS and CoSe show easy-axis anisotropy, with comparable D values (∼-30 cm). Moreover, alternating-current susceptibility measurements on these Co complexes, magnetically diluted in their isostructural Zn analogues, highlight the role of dipolar magnetic coupling in the mechanism of magnetization reversal. In addition, our findings suggest that, despite their similar anisotropic features, CoS and CoSe relax magnetically via different processes. This work provides experimental evidence that solid-state effects may affect the magnetic behavior of SIMs.

Magnetic Anisotropy and Field‐induced Slow Relaxation of Magnetization in Tetracoordinate CoII Compound [Co(CH3‐im)2Cl2]

“…Usually, the tetracoordinate Co II compounds of the composition [Co(L N/P ) 2 (L1) 2 ], (where L N/P = a monodentate N/P-donor ligand, L1 = halogenido or pseudohalogenido ligands), adopt values of D parameters ranging from −14 to +6 cm −1 [17]. It must be noted that much larger values of D were observed for compounds involving monodentate ligands with soft donor atoms such as S or Se [18], bidentate N-donor ligands [19], and bidentate S/Se-donor ligands [20,21]. The aforementioned correlation [9] on the anisotropy of the [Co(L N/P ) 2 (L1) 2 ] compounds involves N/P-Co-N/P ( α ) and N/X-Co-N/X angles ( β , N or X are the donor atoms from the L1 ligands) and it reads: δ = 2 α Td – ( α + β ), where α Td is the angle of the ideal tetrahedron (109.5°).…”

Entrapment of a Pseudo‐Tetrahedral CoII Center by Thioether Sulfur Bound {Co2(μ‐L)} Fragments: Synthesis, Field‐Induced Single‐Ion Magnetism and Catechol Oxidase Mimicking Activity

“…The CoÀNa nd CoÀSs eparations were in the ranges of 1.889(7)-1.908(7) and 2.194(2)-2.203(3) ,r espectively.T he Co III ÀOb ond lengths from the bridging phenoxido, hydroxido, propanoato groups within the {Co 2 (m-L)(m-OH)(m-O 2 CC 2 H 5 )} fragments were in 1.879(5)-1.937(5) range. Withint he flattened tetrahedral cavity (Houser's t 4 = 0.82) the Co II ÀOb onds were longera t1 .989(6)-2.043 (5) .T he O-Co-O angles around this ion span from 97.2 (19) to 134.3(2)8 The C-S-C anglesw ere found within 102.0(5)-104.4(5)8 range which is similart ot hat in 1.I nc omparison, the C-N-C angles for the analogous compound previously reported by us fall in 109.3(2)-114.5(3)8 range which deviates considerably compared to that recorded for CH 3 CO 2 À bridges (113.0(6)-117.3(2)8). [10] The very similar t 4 value compared to 1 confirmed almost negligible effect on the compression of the tetrahedral geometry aroundc entral Co II due to bridging of the ligand bound Co III centers by the C 2 H 5 CO 2 À groups in place of CH 3 CO 2 À .I no ur previous investigation the variation in t 4 values were greater for analogous compounds.…”

“…All calculations were performed using the ORCA 4.0.0 program package on the X-ray crystal structures. [50] def2-TZVP basis set was employed on Co, N, Oa nd Sw hile def2-SVP basis set was employed on Ca nd H. [51][52][53] State averaged complete active space self-consistent field (SA-CASSCF) calculations were performed on 1 and 2 using these basis sets with the active space comprising of seven de lectrons of Co in five do rbitals i.e., CAS (7,5). 10 quartet and 40 doublet roots were computed using this active space in the CI (configuration interaction) step.…”

“…Self-assembling of ligand bound fragments can result in visual-ly pleasing shapes and structures of the compounds which can be interesting in the areaso fm olecular magnetism and magneto-caloric materials. [3][4][5][6] Room-temperature synthesis, characterization and crystallization of 3d metal ion aggregates have received considerable attention in recent years. Many of these aggregates often possess al arge ground-state spin (S)a nd sizeable magnetic anisotropy (D)g iving rise to sizeable barrier for reversal of magnetization, U eff .W ithin these multimetallic aggregates either all the centers are paramagnetic or in the middle of several diamagnetic centers one or more of them could be paramagnetic.…”

From Paramagnetic to Single‐Molecule Magnet Behaviour in Heterobimetallic Compounds Containing the Tetrakis(thiocyanato‐κN)cobaltate(II) Anion

“…From a magnetic point of view, the tetrahedral geometry is an interesting environment for the cobalt(II) ion, as demonstrated by the [Co(SPh) 4 ] 2– complex (HSPh = benzenethiol), which was the first example reported of a mononuclear 3d transition‐metal complex that exhibits single‐ion magnet (SIM) behaviour in the absence of an applied magnetic field . Since then, other examples of four‐coordinate cobalt(II) compounds exhibiting slow magnetic relaxation either in the absence or under an applied dc field have been investigated, the relaxation processes involved being more complicated than expected, and a deep understanding of the fine structure is required. Very recently, the relaxation dynamics of the magnetisation in two double salts of formula [K(18‐crown‐6)][Co(NCS) 4 ] and [Ba(18‐crown‐6)(H 2 O) 3 ][Co(NCS) 4 ] containing the mononuclear tetrakis(thiocyanato‐ κN )cobaltate(II) unit have been investigated in detail .…”