A New Family of 1D Exchange Biased Heterometal Single-Molecule Magnets: Observation of Pronounced Quantum Tunneling Steps in the Hysteresis Loops of Quasi-Linear {Mn₂Ni₃} Clusters

Abstract

First members of a new family of heterometallic Mn/Ni complexes [Mn(2)Ni(3)X(2)L(4)(LH)(2)(H(2)O)(2)] (X = Cl: 1; X = Br: 2) with the new ligand 2-{3-(2-hydroxyphenyl)-1H-pyrazol-1-yl}ethanol (H(2)L) have been synthesized, and single crystals obtained from CH(2)Cl(2) solutions have been characterized crystallographically. The molecular structures feature a quasi-linear Mn(III)-Ni(II)-Ni(II)-Ni(II)-Mn(III) core with six-coordinate metal ions, where elongated axes of all the distorted octahedral coordination polyhedra are aligned parallel and are fixed with respect to each other by intramolecular hydrogen bonds. 1 and 2 exhibit quite strong ferromagnetic exchange interactions throughout (J(Mn-Ni) ≈ 40 K (1) or 42 K (2); J(Ni-Ni) ≈ 22 K (1) or 18 K (2)) that lead to an S(tot) = 7 ground state, and a sizable uniaxial magnetoanisotropy with D(mol) values -0.55 K (1) and -0.45 K (2). These values are directly derived also from frequency- and temperature-dependent high-field EPR spectra. Slow relaxation of the magnetization at low temperatures and single-molecule magnet (SMM) behavior are evident from frequency-dependent peaks in the out-of-phase ac susceptibilities and magnetization versus dc field measurements, with significant energy barriers to spin reversal U(eff) = 27 K (1) and 22 K (2). Pronounced quantum tunnelling steps are observed in the hysteresis loops of the temperature- and scan rate-dependent magnetization data, but with the first relaxation step shifted above (1) or below (2) the zero crossing of the magnetic field, despite the very similar molecular structures. The different behavior of 1 and 2 is interpreted in terms of antiferromagnetic (1) or ferromagnetic (2) intermolecular interactions, which are discussed in view of the subtle differences of intermolecular contacts within the crystal lattice.

Crystal structure of 4-bromo-2-(1H-pyrazol-3-yl)phenol, C9H7BrN2O

“…The title pyrazole derivative contains one independent molecule in the asymmetric unit. The bond lengths and angles of the phenyl-pyrazole core are consistent with those in similar molecules [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. The dihedral angle between the pyrazole and phenyl ring is 8.6(2)°and reflects a nearly planar geometry of the molecule.…”

“…The planar conformation is stabilized by the intramolecular hydrogen bond formed between the phenol hydroxy group and the pyrazole nitrogen [O1-H1···N1: O-H 0.83(4), H···N 1.87(4), O···N 2.615(3) Å, O-H···N = 148(4)°]. This intramolecular hydrogen bond is the main structural property of different 3{5}-(2′-hydroxyaryl)pyrazoles and it ensures the co-planarity of the phenyl-pyrazole fragment in the various intramolecular environments [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. The presence of this intramolecular N-H···O hydrogen bond is related to the photostability of some hydroxyarylpyrazoles and their ability to act as UV stabilizers [9].…”

Exchange coupling and magnetic anisotropy of exchanged-biased quantum tunnelling single-molecule magnet Ni3Mn2 complexes using theoretical methods based on Density Functional Theory

“…One of these Mn 6 complexes shows the highest magnetic anisotropy barrier for a SMM. 9 Recently, Das et al 10 synthesized two Ni 3 Mn 2 complexes with Ni II and Mn III cations, [Mn 2 Ni 3 X 2 L 4 (LH) 2 (H 2 O) 2 ] (H 2 L = 2-{3-(2-hydroxyphenyl)-1H-pyrazol-1-yl} ethanol, X = Cl: 1; X = Br: 2), showing an S = 7 ground state with only ferromagnetic coupling between the paramagnetic centres (see Fig. 1).…”

“…Molecular structure of complex Mn hydroxyphenyl)-1H-pyrazol-1-yl}ethanol) 10. Violet, grey, green, light blue, red and brown spheres correspond to manganese, nickel, chlorine, nitrogen, oxygen and carbon atoms, respectively, while hydrogen atoms are omitted for clarity.2660 | Dalton Trans., 2012, 41, 2659-2666 This journal is © The Royal Society of Chemistry 2012…”

Two cyanide-bridged compounds composed of Schiff base–manganese(III) building block with dicyanamido and hexacyanocobaltate(III) as ligands: Crystal structure, thermostability and magnetic property

“…Rational design and construction of magnetic transition-metal coordination polymers, especially low-dimensional magnetic materials such as single-molecule magnets (SMMs) and singlechain magnets (SCMs), have attracted great attention because of their potential in the fields of high-density storage devices, molecular electronics and so on [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Up to now, remarkable progress has been achieved on this field, serving as the typical example of lots of relevant one-dimensional (1D) transition-metal compounds with fascinating molecular structures and magnetisms [16][17][18][19].…”