Synthesis, crystal structure and magnetism of new salicylamidoxime-based hexanuclear manganese(iii) single-molecule magnets

Abstract

Salicylamidoxime was used to synthesize 13 new polynuclear Mn(III) complexes. We present the crystallographic structures, the magnetic susceptibility and the magnetization measurements of eight of them (1-8) with the general formula [Mn(6)O(2)(H(2)N-sao)(6)(L)(2)(solvent)(4-6)] (L = carboxylate, chloride, 2-cyanophenolate; solvent = H(2)O, MeOH, EtOH, py). These complexes consist of two trinuclear {Mn(III)(3)(μ(3)-O)(H(2)N-sao)(3)}(+) cationic units linked together via two oximate and two phenolate oxygen atoms. All behave as single-molecule magnets, with the spin ground state varying from 4 to 12 and anisotropy energy barriers from 24 to 86 K, the latter being as high as the present record barrier in the Mn(6) complexes. DFT calculations were performed to compute the exchange magnetic coupling constants J between the metallic ions and to provide an orbital interpretation of exchange. Our results are in line with previously reported results with the parent salicylaldoxime derivatives. The Mn-N-O-Mn torsion angle appears as the main parameter controlling the J values. The critical angle where the exchange coupling between two Mn(III) switches from antiferromagnetic to ferromagnetic is 27°, less than the one found in related complexes with salicylaldoxime (30°). We propose a structural classification of the {Mn(6)} complexes in four classes depending on the coordination of the axial carboxylate. The work points out the structural flexibility of such systems, their sensitivity to solvent effects and their ability to achieve high anisotropy energy barriers by simple desolvation.

The Effect of Crystal Packing and ReIV Ions on the Magnetisation Relaxation of [Mn6]‐Based Molecular Magnets

“…The obtained g and J values are similar to those found previously by DFT calculations on [Mn 6 ] systems 23. 25, 42 Moreover, the g -factor values for Mn III and Re IV obtained from the fit fall into the typical range reported for these metal ions. For 2 , in which two cationic [Mn 6 ] 2+ complexes coexist ( 2 a and 2 b ), the fit was performed considering only one cationic entity ( 2 a ).…”

“…In that sense, theoretical simulations from the so-obtained J values were performed using fixed values of g (g Mn = 2.0 and g Re = 1.9), which are similar to those found in the literature (dotted lines in Figure 3). [23,25,42] In 1 and 2, the agreement between theoretical and experimental simulations is excellent. For this reason, even though there are a large number of independent parameters (g, J and D), tentative fittings were performed (solid lines in Figure 3), albeit with the caveat that caution should be exercised and the numbers should be taken as a guide only.…”

“…In recent years, a large body of work has involved the use of derivatised salicylaldoxime (R-saoH 2 ) [16][17][18][19][20][21][22] and salicylamidoxime (H 2 N-saoH 2 ) [23][24][25][26][27] ligands in Mn III coordination chemistry. This has led to the isolation of an extensive family of hexanuclear Mn III SMMs, many of them possessing spin ground states as large as S = 12 and energy barriers to magnetisation reversal and blocking temperatures that are the largest reported for polynuclear transition-metal complexes.…”

“…[16][17][18][19][20][21][22][23] The solution stable, magnetically tuneable [Mn 3 ] building blocks from which these hexamers are constructed are appealing candidates for building novel molecule-based magnets, because nearest neighbour magnetic exchange (J) is controlled by the twisting of the Mn-N-O-Mn torsion angle, and this in turn is controlled through simple organic chemistry through derivitisation of the oxime ligand. [16][17][18][19][20][21][22][23][24][25][26][27] Very recently, the first example of a cationic salicylamidoxime-based [Mn 6 ] 2 + SMM of formula [Mn 6 (m 3 -O) 2 (H 2 Nsao) 6 (py) 6 (EtOH) 2 ][ClO 4 ] 2 ·4 EtOH possessing a S = 12 ground state, was reported. [26,27] The preparation of this cationic [Mn 6 ] 2 + complex opens a plethora of new and appealing synthetic routes that can be easily exploited to obtain advanced, potentially multi-functional magnetic materials, since the identity and nature of the anion can be deliberately employed to control, for example, solubility, reactivity, stability and substrate specificity, but also because the cationic cluster produced can be charged balanced through the incorporation of anions that bring another physical property or functionality to the material, such as conductivity, luminescence or paramagnetism.…”

Tuning of Hula-Hoop Coordination Geometry in a Dy Dimer

“…A similar behaviour has been previously observed in a Dy dimer system [25]. Also, the axial solvent molecule contributes to this effect in terms of different energy barrier heights [29]. In order to compare the coordination geometries of the Dy ions, these were analysed using the Shape program [30][31][32].…”