Slow Magnetic Relaxation in Dinuclear Co^IIY^III Complexes

Abstract

Four new dinuclear complexes, [Co­(μ-L)­(μ-CCl3COO)­Y­(NO3)2]·2CHCl3·CH3CN·2H2O (1), [Co­(μ-L)­(μ-CH3COO)­Y­(NO3)2]·CH3CN (2), [Co­(μ-L)­(μ-PhCOO)­Y­(NO3)2]·3CH3CN·2H2O (3), and [Co­(μ-L)­(μ-tBuCOO)­Y­(NO3)2]·CHCl3·2H2O (4), having a CoIIYIII core, have been synthesized by employing a ferrocene based compartmental ligand which was synthesized by the reaction of diacetyl ferrocene with hydrazine hydrate followed by a condensation reaction with o-vanillin. A general synthetic protocol was employed to synthesize complexes 1–4, where the metallic core was kept the same with changing the bridging carboxylate groups. In all the complexes, the main structural motif is kept similar by only slightly varying the substitution on the bridging acetate groups. This variation has resulted in a small but subtle influence on the magnetic relaxation of all these four compounds. Ab initio CASSCF/NEVPT2 calculations were carried out to assess the effect of the different substitutions of the bridging ligands on the magnetic anisotropy parameters and on orbital arrangements. Ab initio calculations yield a very large positive D value, which is consistent with the geometry around the CoII ion and easy plane anisotropy (g xx , gyy > g zz ), with the order of the calculated D in the range of 72.4 to 91.7 cm–1 being estimated in this set of complexes. To ascertain the sign of zero-field splitting in these complexes, EPR spectra were recorded, which support the sign of D values estimated from ab initio calculations.

Role of Coordination Number and Geometry in Controlling the Magnetic Anisotropy in FeII, CoII, and NiII Single‐Ion Magnets

“…This is reflected in continuous symmetry measurements or SHAPE analysis performed on the complexes, which quantitively measures the degree of deviation from ideal geometries [30] . Also, deviation from ideal octahedral or TAP/TP geometries increases the multi‐determinant character of the CASSCF wavefunction [25h] . In TAP or TP geometries ( D 3 point group or trigonal distortion), the d xy and d orbitals remain degenerate in d 7 electronic configuration, and consequently, a strong SOC leads to very large negative D values (see Figure 3 d and 3 h).…”

“…On the other hand, there are plenty of pseudo‐O h Co II complexes that exhibit field‐induced SIM behaviour attributed to positive D and large E / D values. For example, complexes cis ‐[Co II (dmphen) 2 (NCS) 2 ] [15] ( 8 ) (dmphen=2,9‐dimethyl‐1,10‐phenanthroline) and [Co(μ‐L)(μ‐CH 3 COO)Y(NO 3 ) 2 ] [25h] ( 9 ) (L=ferrocene based compartmental ligand) were found to possess very high easy‐plane anisotropy with D values of +98 cm −1 and +92 cm −1 with small U eff values of 17.0 cm −1 and 7.6 cm −1 , respectively (see Figure 3 b and Table 1). Very recently, Dunbar and co‐workers reported three pseudo‐octahedral complexes of general formula [Co(py) 4 X 2 ] [26] (where X=SCN, Cl and Br) out of which the Cl and the Br analogues (complexes 10 and 11 ) exhibit very high D values of +127 cm −1 and +139 cm −1 as obtained from NEVPT2 calculations along with U eff values of 19.3 cm −1 and 20.4 cm −1 , respectively, in the presence of a dc field (see Figure 3 c).…”

Ferrocene-Supported Compartmental Ligands for the Assembly of 3d/4f Complexes

“…These ligands possess two distinct coordination pockets suitable for selective binding of metal ions (Scheme ). Accordingly, utilizing these ligands, we synthesized four families of heterometallic complexes: [Zn II -Ln III ], [Ni II -Ln III ], [Co II –Y III ], and [Zn II 4 -Ln III 4 ]. − The structure and magnetic properties of these families of complexes are discussed in the following.…”

“…Such ligands, known as compartmental ligands, have proven to be extremely effective in the assembly of 3d/4f complexes. This Review is focused on a family of compartmental ligands that have been built on the ferrocene scaffold. − …”

Influence of ligand field on magnetic anisotropy in a family of pentacoordinate CoII complexes

“…25,26 Being a Kramers ion, Co II would have a bistable ground electronic state, so, independent of the sign of D, its complexes can show slow relaxation of magnetization for a wide range of coordination numbers ranging from 2 to 8. [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] However, fine-tuning the coordination number and geometry of Co II complexes to achieve significant magnetic properties is still a work in progress.…”