Insight into ferromagnetic interactions in CuII–LnIII dimers with a compartmental ligand

In the last two decades, efforts have been devoted to obtaining insight into the magnetic interactions between CuII and LnIII utilizing experimental and theoretical means. Experimentally, it has been observed that the exchange coupling (J) in CuII–LnIII systems is often found to be ferromagnetic for ≥4f7 metal ions. However, exchange interactions at sub-Kelvin temperatures between CuII and the anisotropic/isotropic LnIII ions are not often explored. In this report, we have synthesized a series of heterobimetallic [CuLn(HL)(μ-piv)(piv)2] complexes (LnIII = Gd (1), Tb (2), Dy (3) and Er (4)) from a new compartmental Schiff base ligand, N,N’-bis(3-methoxy-5-methylsalicylidene)-1,3-diamino-2-propanol (H3L). X-ray crystallographic analysis reveals that all four complexes are isostructural and isomorphous. Magnetic susceptibility measurements reveal a ferromagnetic coupling between the CuII ion and its respective LnIII ion for all the complexes, as often observed. Moreover, μ-SQUID studies, at sub-Kelvin temperatures, show S-shaped hysteresis loops indicating the presence of antiferromagnetic coupling in complexes 1–3. The antiferromagnetic interaction is explained by considering the shortest Cu⋯Cu distance in the crystal structure. The nearly closed loops for 1–3 highlight their fast relaxation characteristics, while the opened loops for 4 might arise from intermolecular ordering. CASSCF calculations allow the quantitative assessment of the interactions, which are further supported by BS-DFT calculations.

Electronic Supplementary Material (ESI) for Dalton Transactions.This journal is © The Royal Society of Chemistry 2023

Section 1: Structural information
This section consists of structural information of complex 1-4. Figure S1, S2 shows experimental IR and XRD spectra of the four complexes.Figure S3, S4 show the crystal structure of 1, 2, 4 and crystal packing diagram of 3 respectively.Table S1-S4 show crystallographic data, bond distances and shape analysis of these complexes.Section 2: DFT calculations: The BS-DFT calculated spin density distributions are shown in the Figures S5 and S6 indicating intra and inter molecular scenario respectively.

Section 3: CASSCF simulation details
Ligand field parameters obtained from CASSCF simulations are tabulated in Table S5, while computed energy levels, g values and the wavefunctions for each m j state are shown in Table S6 and S7 for CuDy and CuEr complex respectively.Section 4: Derivative-field angle map in complex 2 (CuTb) The µ-SQUID measurement setup equipped with 3D vector magnet allows angle dependent M(B) with an angular precision better than 0.1 o without manually rotating the sample.However, the external field direction is confined within the µ-SQUID plane to avoid flux due to the external field and measure magnetic signal coming only from the single crystal.

Figure S3 .
Figure S3.Crystal structures of complexes 1, 2 and 4 showing ellipsoid (30 % probability) plots together with the local coordination geometry around the Dy(III) center

Figure S5 .Figure S6 .
Figure S5.The BS-DFT calculated spin density distribution using PBE0 functional for dinuclear complexes of 1-4.The spin densities are represented by yellow/cyan surfaces calculated with a cutoff value of 0.01 e bohr -3 .Hydrogen atoms were omitted for clarity.
Figure S7 shows derivative dM/dB mapped with direction of applied field (B x , B y ) as derived from experimental M(B) curves (positive cycle) measured at different angles of external field.The observed parallel lines indicate the switching fields between antiferromagnetic (AFM) to ferromagnetic (FM) state.Only one pair of lines confirm a single easy axis for the FM/AFM ordering.

Figure S7 .
Figure S7.Derivative-field angle map (in complex 2) obtained from angle dependent M(B) loops measured by a µ-SQUID.

Table S1 .
Crystallographic data and structure refinement parameters of 1−4.

Table S3 .
SHAPE analysis of Cu II ion in complexes 1-4.

Table S4 .
SHAPE analysis of the Ln III ion in complexes 1-4.

Table S5 :
CASSCF calculated ligand field parameters for the Ln ion in CuLn systems.

Table S6 :
Computed energy levels (the ground state is set at zero) composition of the g-tensor (g x , g y , g z ) and the main components (>10%) of the wavefunction for each m j state of the groundstate multiplet 6 H 15/2 for the Dy ion in CuDy at the CASSCF level.

Table S7 :
Computed energy levels (the ground state is set at zero) composition of the g-tensor (g x , g y , g z ) and the main components (>10%) of the wavefunction for each m j state of the groundstate multiplet 4 I 15/2 for the Er ion in CuEr at the CASSCF level.