The Ligand Field of the Azido Ligand: Insights into Bonding Parameters and Magnetic Anisotropy in a Co(II)–Azido Complex

Abstract

The azido ligand is one of the most investigated ligands in magnetochemistry. Despite its importance, not much is known about the ligand field of the azido ligand and its influence on magnetic anisotropy. Here we present the electronic structure of a novel five-coordinate Co(II)-azido complex (1), which has been characterized experimentally (magnetically and by electronic d-d absorption spectroscopy) and theoretically (by means of multireference electronic structure methods). Static and dynamic magnetic data on 1 have been collected, and the latter demonstrate slow relaxation of the magnetization in an applied external magnetic field of H = 3000 Oe. The zero-field splitting parameters deduced from static susceptibility and magnetizations (D = -10.7 cm(-1), E/D = 0.22) are in excellent agreement with the value of D inferred from an Arrhenius plot of the magnetic relaxation time versus the temperature. Application of the so-called N-electron valence second-order perturbation theory (NEVPT2) resulted in excellent agreement between experimental and computed energies of low-lying d-d transitions. Calculations were performed on 1 and a related four-coordinate Co(II)-azido complex lacking a fifth axial ligand (2). On the basis of these results and contrary to previous suggestions, the N3(-) ligand is shown to behave as a strong σ and π donor. Magnetostructural correlations show a strong increase in the negative D with increasing Lewis basicity (shortening of the Co-N bond distances) of the axial ligand on the N3(-) site. The effect on the change in sign of D in going from four-coordinate Co(II) (positive D) to five-coordinate Co(II) (negative D) is discussed in the light of the bonding scheme derived from ligand field analysis of the ab initio results.

Magnetic anisotropy in two- to eight-coordinated transition–metal complexes: Recent developments in molecular magnetism

“…The presence of magnetic anisotropy in these complexes was further demonstrated by the non-saturation of the field-dependent magnetization at low temperature and in a high field [93,98]. The magnetization data were reproduced well, with D = -6.2 cm -1 and -2.5 cm -1 when assuming E = 0 and g iso = 2.24 and 2.22 for 62 and 63, respectively.…”

“…The diluted complex 62 exhibited magnetization blocking below 1 K with butterflyshaped magnetic hystereses [93]. Interestingly, although the complex 64 had similar magnetic anisotropy (D = -10.7 cm -1 and E = -2.3 cm -1 ), it exhibited slow relaxation of its magnetization with an effective energy barrier for magnetization reversal U eff = 19.7 cm -1 [98].…”

“…However, by also considering the symmetry, detailed theoretical investigations were performed of high-spin Co II complexes with TBP geometry and C 3v symmetry, which demonstrated the following: (i) the axial symmetry imposed by coordinating ligands leads to the stabilization of easy-axis anisotropy, (ii) the absolute value of the D parameter is regulated by the energy difference between the (d xz , d yz ) and (d xy , d x2-y2 ) orbital sets (Scheme 5), and (iii) the presence of strong  donating ligands at axial positions and weak  donating ligands at equatorial positions yields an overall magnetic anisotropy that is high in magnitude and negative in sign [93]. Hence, penta-coordinated A c c e p t e d M a n u s c r i p t [98], which incorporate high-spin Co II with TBP geometry and pseudo-C 3v symmetry (Fig. 21).…”

Synthesis and Characterization of 4‐, 5‐, and 6‐Coordinate Tris(1‐ethyl‐4‐isopropylimidazolyl‐κN)phosphine Cobalt(II) Complexes

“…The D values for 2 and 3 agree well with those previously observed for tetrahedral complexes, [43,44] whereas the value for 1, even though somewhat high, is similar to that observed for pentacoordinate high-spin square-pyramid Co II complexes. [45,46] The change from Cl to Br on passing from 2 to 3 affects the ligand field around the Co II ion and, as a result, the values of D and g, [47] and this change can be, at least in part, responsible of the small differences between these parameters observed for 2 and 3.…”

Effect of Low Spin Excited States for Magnetic Anisotropy of Transition Metal Mononuclear Single Molecule Magnets

“…In this way, experimentally derived parameters as zero-field splitting D and E, effective g-factors, and Racah and ligand field parameters can be directly compared with their calculated counterparts. In many cases, data from magnetic measurements is not enough to fit a consistent parameter set in a univocal way, which can be estimated by electronic structure methods [22][23][24].…”