Mechanism of Ferromagnetic Coupling in Copper(II)-Gadolinium(III) Complexes

Abstract

This paper offers the first series of state-of-the-art quantum chemical calculations (CASSCF, CASPT2, MS-CASPT2) and analytical models for the well-known problem of quasi-general ferromagnetic coupling in copper-gadolinium complexes. A system chosen from the chemical report of Costes et al. was taken as prototype. At the CASSCF level, calculated results for the experimental structure reproduced the magnetic coupling constant well (J(calcd)( )()= +7.67 cm(-)(1) vs J(exp)( )()= +7.0 cm(-)(1)). For more insight, the study molecule was further idealized by geometry optimization to C(2)(v)() symmetry. Systematic ab initio computation experiments were designed and performed. Owing to specific problems related to the non-aufbau ground configuration of the [CuL-Gd] complexes, the calculations were conducted in a nonstandard manner. We found that the qualitative mechanism of Kahn, assigned to the electron jump from 3d of Cu(II) to 5d shell of Gd(III), can be presented effectively as the cause of the phenomenon, if CASPT2 MOs are taken as magnetic orbitals. We showed that the ferromagnetic coupling is also matched and magnified by spin polarization effects over the ligand, in line with the early assumption of Gatteschi. To be distinguished from the initial hypothesis of Gatteschi, which assumed the role of 6s AO of Gd(III), we found that one 5d-type AO is actually involved in the polarization scheme. In fact, the Gatteschi and Kahn mechanisms are not mutually contradictory, but are even interconvertible with appropriate changes of the magnetic orbitals. Within C(2)(v)() symmetry of complexes, the ferromagnetic coupling can be qualitatively regarded as the preponderant influence of interaction channels exhibiting orbital orthogonality (four 3d-4f contacts) over the nonorthogonal ones (two 3d-4f contacts). The effective preponderance from ferromagnetic pathways is supported by CASPT2 results. One may explain the generality of Cu(II)-Gd(III) ferromagnetic coupling as being correlated with the large occurrence of approximate pseudo-C(2)(v)() geometry of complexes. The observed orbital regularity is lost in lower symmetries. Thus, the antiferromagnetic exceptions occur when the molecular asymmetry is advanced (e.g., owing to strong chemical nonequivalence of the donor atoms).

Ligand field density functional theory for the prediction of future domestic lighting

“…While working in the vicinity of our LFDFT developed approach, this methodological update will also help for the reliable determination of the magnetic properties of lanthanide or transition metal complexes being currently popular in theoretical research. [58][59][60][61][62][63][64][65] The formulation of the ligand field interaction, which takes into account the presence of the ligands in the coordination sphere of the lanthanide, is also the subject of intense work. 32,44 Here, the ligand field potential includes two distinct interactions the perturbation of the 4f and 5d orbitals of the lanthanide, respectively, leading to ligand field matrix of 12 by 12 elements.…”

On exchange coupling and bonding in the Gd2@C80 and Gd2@C79N endohedral dimetallo-fullerenes

“…Previously studied case of DySc 2 N@C 80 showed [4] that enhancing the basis on non-metal ions to 6-31G * does not bring significant changes to the results and also the SBKJC performs comparable to full electron basis sets in problems related to magneto-chemistry. The effective core potential was proven a good choice in previous studies of molecular magnetism, leading to a realistic reproduction of experimental data [6,10]. This is due to the reliable account of the f-type orbitals, which matters essentially in such problems.…”

“…The authors claim pioneering advances in the computational approach of lanthanide complexes, such as the control of orbital and spin population in DFT calculations [8], revealing the interaction parameters in the frame of the so-called ligand field density functional theory (LFDFT) [9]. In the multi-configuration procedures, an important methodological clue is the initialisation of selfconsistent cycles with orbitals merged from corresponding fragments, previously computed separately, namely, the free lanthanide ions and the rest of the molecule [10]. This starting procedure obeys the physical fact of a canonical space made of almost pure f orbitals, in line with their weak interaction with the environment.…”

“…In a study dedicated to the situation of quasi-generalised ferromagnetism of the Cu(II)-Gd(III) complexes, evidenced experimentally [19], we performed the first state of the art CASSCF calculations on d-f complexes at realistic scale [10], providing the underlying methodological breakthrough of the merged fragment initialisation of iterative process. The mechanisms were found in line with the qualitative guess, namely, a fraction of the spin from the d-centre arrives, by delocalisation over the ligand bridge, in the 5d orbitals of the lanthanide, conserving the spin polarisation Figure 1.…”

The angular overlap model extended for two-open-shell f and d electrons

“…[6][7][8] We mention also the situation of non-aufbau occupation [9][10][11][12] of the 4f orbitals inasmuch as convergence problems may frequently occur in the self-consistent field (SCF). The calculation of the f-d transitions in lanthanide phosphors is addressed with respect to the ligand field theory 13,14 but not restricted to its classical empirical frame.…”