Electronic Structure and Magneto-Structural Correlations Study of Cu₂UL Trinuclear Schiff Base Complexes: A 3d–5f–3d Case

Abstract

The magnetic properties of trinuclear Schiff base complexes M 2 AnL i (M II = Zn, Cu; An IV = Th, U; L i = Schiff base; i = 1−4, 6, 7, 9), exhibiting the [M(μ-O) 2 ] 2 U core structure with adjacent M1•••U and M2•••U and nextadjacent M1•••M2 interactions, featuring 3d-5f-3d subsystems, have been investigated theoretically using relativistic ZORA/B3LYP computations combined with the broken symmetry (BS) approach. Bond order and natural population analyses reveal that the covalent contribution to the bonding within the Cu−O−U coordination is important thus favoring superexchange coupling between the transition metal and the uranium magnetic centers. The calculated coupling constants J CuU between the Cu and U atoms, agree with the observed shift from the antiferromagnetic (AF) character of the L 1,2,3,4 complexes to the ferromagnetic (ferro) of the L 6,7,9 ones. The structural parameters, i.e., the Cu•••U distances and the Cu−O−U angles, as well as the electronic factors driving the magnetic couplings are discussed. The analyses are supported by the study of the mixed ZnCuUL i and Cu 2 ThL i systems, where in the first complex the Cu II (3d 9 ) ion is replaced by the diamagnetic Zn II (3d 10 ) one, whereas in the second complex the U IV (5f 2 ) paramagnetic center is replaced by the diamagnetic Th IV (5f 0 ) one.

Exchange Couplings and Magneto‐Structural Correlations of Trinuclear MII‐UIV‐MII (MII=Co, Ni, Cu) Complexes

“…[41,42] In all cases, the starting molecular structures for the geometry optimizations are derived from the structures of the M 2 UL (M II = Co II , Ni II ) complexes in the reported Xray data. [32][33][34] As successfully used previously, [35] the geometry optimization of the M 2 UL complexes was carried out by fixing the [M(μ-O b ) 2 ] 2 U core as it is in the X-ray structure. However, we noted that the full geometry optimization of the whole complex including the M 2 O 4 U core, leads to an overestimation of the exchange coupling J constant, and even a slight distortion of the core M 2 O 4 U geometry from the X-ray structure affects drastically the magnetic properties.…”

“…For the computation of the BS, BS1, and BS2 energies, a singlepoint calculation using the molecular orbitals (MOs) of the HS structure has been used as a starting guess, changing the spin on the corresponding metal center in each case. [35] It must be kept in mind that the energy differences between the different multiplets HS, BS, BS1, and BS2 states, is often smaller than ~0.5 kcal/mol (i. e. ~170 cm À 1 ) [58] so that a very good accuracy of the evaluation of the coupling constant J, must be insured to avoid numerical round-up errors.…”

“…[24] Moreover, this AF to Ferro shift, when passing from the L 1 to L 9 diimino-backbone, correlates with the decrease of the uranium orbitals weight (covalency) in the magnetic [Cu(μ-O)] 2 U core, limiting the superexchange responsible for the AF Cu II À U IV character. [35] A few years afterward, S.A. Kozimor et al [36,37] reported the magnetic properties of the trinuclear pyrazolate-chloro-bridged M II (μ-Cl)U IV (M II = Co, Ni, Cu, and Zn) complexes of the formula (cyclam)M[(μ-Cl)U IV (Me 2 Pz) 4 ] 2 (cyclam = 1,4,8,11-tetraazacyclotetradecane; Me 2 Pz À = 3,5-dimethylpyrazolate). Strong Ferro coupling (15 cm À 1 < J UÀ M < 48 cm À 1 ) is observed for the Co IIcontaining species (À 2J convention).…”

Magneto‐structural correlations of oximato‐bridged dinuclear copper(II) complex: A theoretical perspective

“…The density functional theory combined with the broken symmetry approach (DFT‐BS) by Noodleman et al [40, 41] was used to evaluate J values. This method is well‐established for estimating the magnetic coupling constants for various dinuclear and multicore complexes [42–64]. In this study, all calculations were performed in ORCA 5.0.3 software [65, 66], using a tight SCF convergence (1 × 10 −8 Eh in energy).…”

The resolution of the weak-exchange limit made rigorous, simple and general in binuclear complexes

“…39 Many seminal studies show the use of configuration interaction schemes to calculate magnetic properties. [40][41][42][43][44][45][46][47] It is worth noting that density functional theory (DFT) is equally involved in rationalizing magnetic data, [48][49][50][51][52][53][54] as well as single-reference spin-flip approaches, [55][56][57] and more recently coupled-cluster methods. [58][59][60] In principle, the low-energy ab initio spectrum offers sufficient detail for calculating anisotropy parameters and spin-orbit correction to the exchange interaction if a SOCI is performed, 22,33 as well as EPR parameters and wT profiles provided that the Zeeman interaction is treated.…”