Trapping of a Pseudotetrahedral Co^IIO₄ Core in Mixed-Valence Mixed-Geometry [Co₅] Coordination Aggregates: Synthetic Marvel, Structures, and Magnetism

Abstract

A systematic one-step one-pot multicomponent reaction of Co­(ClO4)2·6H2O, H3L (2,6-bis­((2-(2-hydroxyethylamino)­ethylimino)­methyl)-4-methylphenol), and readily available carboxylate salts (RCO2Na; R = CH3, C2H5) resulted in the two structurally novel coordination aggregates [CoIICoIII 4L2(μ1,3-O2CCH3)2(μ-OH)2]­(ClO4)4·4H2O (1) and [CoIICoIII 4L2(μ1,3-O2CC2H5)2(μ-OH)­(μ-OMe)]­(ClO4)4·5H2O (2). At room temperature, reactions of H3L in MeOH with cobalt­(II) perchlorate salts led to coassembly of initially formed ligand-bound {CoII 2} fragments following aerial oxidation of metal centers and bridging by in situ generated hydroxido/alkoxido groups and added carboxylate anions. Available alkoxido arms of the initially formed {L­(μ1,3-O2CCH3)­(μ-OH/OMe)­Co2}+ fragments were utilized to trap a central CoII ion during the formation of [Co5] aggregates. In the solid state, both complexes have been characterized by X-ray crystallography, variable-temperature magnetic measurements, and theoretical studies. Both 1 and 2 show field-induced slow magnetic relaxation that arises from the single pseudo-T d CoII ion present. The structural distortion leads to an easy-axis magnetic anisotropy (D = −31.31 cm–1 for 1 and −21.88 cm–1 for 2) and a small but non-negligible transverse component (E/D = 0.11 for 1 and 0.08 for 2). The theoretical studies also reveal how the O–Co–O bond angles and the interplanar angles control D and E values in 1 and 2. The presence of two diamagnetic {Co2(μ-L)} hosts controls the distortion of the central {CoO4} unit, highlighting a strategy to control single-ion magnetic anisotropy by trapping single ions within a diamagnetic coordination environment.

Magnetic Anisotropy in CoIIX4 (X=O, S, Se) Single‐Ion Magnets: Role of Structural Distortions versus Heavy Atom Effect

“…It is to be noted here that we have not considered another important torsional angle parameter of X‐Co‐X‐Y ( ω ) as the ligands here are bidentate in nature and therefore ω does not vary much across the structures. It is observed that the axial and rhombic anisotropies ( D and E / D ) strongly depend on angle 2 θ and dihedral angle θ d …”

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 facial folding of the imine-thioether-alcohol (NSO) arms of L 3À wasv ital for the tetrahedral dispositiono ff our oxygen atoms around the central Co II center.T he four Co III centers( Co1, Co2, Co3 and Co4) stay in distorted octahedral NSO 4 coordination geometry ( Figure 3). (2)8 range reported for analogousc omplex in our previousw ork [10] signifying greater amount of folding in presenceo fS .T he O-Co-O angles aroundt he central tetrahedral Co II ion span from 100.9(3)-133.4(3)8.The Houser's geometry index t 4 (t 4 = [3608À(a + b)]/1418; a and b being the two largesta ngles) for the central Co II ion is 0.83 indicating at slightly distortion of the T d geometry. [23] For ap erfect tetrahedral environment t 4 value is 1.00 while for an ideal square planar geometry the value is 0.00.…”

“…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. [10] Thisc an be substantiated from the variation of C-X-C (X = N, S) bond angles with the larger size of Sc ompensating for any structural strains introduced by the change in bridging carboxylate.…”

“…[19] The ether Oa longw ith the alcoholic OH remained uncoordinated and dangling. In our previous attempt to synthesize polynuclear aggregates based on cobalt ions we had utilized the ligand 2,6-bis((2-(2-hydroxyethylamino)ethylimino)methyl)-4-methyl-phenol with X = N. [10] Reactions of the ligand with Co(ClO 4 ) 2 ·6 H 2 Oi nt he presence of externally added NaCOOR (R = Me, CH 2 Me) resultedi nm ixed valence pentanuclear aggregates arising from the entrapment of ac entral Co II ion by the initially formed {Co III 2 (m-L)(m-OH/OMe)} fragments. ( 2)h aving similar molecular structures with X = N. It is most likely that Sb eing larger in size as compared to Nw ill introduce more flexibility in the alcohol arm in turn affectingt he amounto fd istortion in the tetrahedral geometry aroundt he trapped Co II ion and hence the magnitude of D. The synthesis, growth of crystals, X-ray structured eterminations,m agnetic properties and catechol oxidation activity of these complexes are described and discoursed.…”

Phosphine Oxide Ligand Based Tetrahedral CoII Complexes with Field‐induced Slow Magnetic Relaxation Behavior Modified by Terminal Ligands

“…This is responsible for the result that 1 possesses a higher | E / D | value. As has been shown in other pseudotetrahedral Co II complexes, deviation from regular tetragonal would result in more mixing at the ground‐state wave function, and therefore a higher | E / D | value …”