Hysteretic Magnetic Bistability Based on a Molecular Azide Switch

Abstract

Dedicated to Professor Gottfried Huttner on the occasion of his 68th birthdayMaterials that exhibit hysteretic bistability at temperatures not too far from ambient conditions are expected to have great potential for future memory or sensing applications. [1] The two relatively stable states should be clearly distinguished by their physical properties, such as their magnetic or optical characteristics. Thermal magnetic hysteresis with an abrupt and rapid property change around room temperature is particularly rare for molecule-based materials, [2] with spincrossover compounds providing the most prominent examples.[3] More recently, magnetic hysteresis has also been observed for some molecular organic radicals [4] or transition metal coordination compounds, [5,6] where switching between two states occurs through a thermal phase transition with orientational changes in the 3D crystal lattice. Herein, we report thermal hysteresis of the magnetic susceptibility for single crystals of the molecular dinickel(ii) complex [LNi 2 (N 3 ) 3 ] (1), in which a m 1,3 -azido ligand functions as an on/off switch for the intramolecular antiferromagnetic coupling between the two metal ions.Pyrazolate-based compartmental ligands such as L À , which bears thioether side-arms, have previously proven suitable as dinucleating scaffolds for the synthesis of preorganized azidonickel(ii) complexes that can serve as modules for the assembly of oligonuclear species or 1D extended chain compounds; [7] the bimetallic complex 1 was prepared as a potential building block in this context. Single crystals of 1 were grown from acetone/hexane, and an initial X-ray crystallographic analysis was carried out at 133 K (Figure 1).[8] As anticipated, both nickel ions in the C 2 -symmetric array are nested within their respective ligand compartment (with two sulfur and one nitrogen donors from the ligand side-arm) and are spanned by the bridging pyrazolate. A m 1,3 -azido bridge is found within the bimetallic pocket, and an additional terminal azide fills the remaining site at each of the six-coordinate metal ions.Since 1 did not exhibit any unusual structural features at this stage, the temperature dependence of its magnetic susceptibility was highly unexpected. The product c m T for a polycrystalline sample of 1 is depicted in Figure 2 for both decreasing and increasing temperature. The value of 1.82 cm 3 K mol À1 at 300 K is somewhat lower then the spinonly value expected for two uncoupled high-spin nickel(ii) ions (2.14 cm 3 K mol À1 for g = 2.07), and decreases only slightly upon lowering the temperature to 220 K. At 215 K, however, an abrupt drop of c m T occurs (centered at T fl = 212 K), followed by a much more rapid decline at lower temperatures; below 50 K c m T tends towards zero. The lowtemperature data are indicative of strong antiferromagnetic coupling and an S = 0 ground state, while coupling is apparently only weak above 220 K. c m T shows an analogous behavior when measured as a function of increasing temperature, but exhibits therma...

Alternating Spin Chains: Controlled Assembly from Bimetallic Building Blocks and QMC Simulation of Spin Correlation

“…[36] Magnetostructural correlations for m-1,3-azido-bridged dinickel(II) complexes have revealed that two geometric parameters, the Ni-N-N angles and the dihedral angle f along the azide ligand, are of major importance. [17,19,32] For an Ni-NNN-Ni torsion angle of f= 1808, antiferromagnetic coupling is predicted to have a maximum at Ni-N-N angles around 1088 and to decrease at larger angles. On the other hand, for all Ni-N-N angles maximum coupling is expected for a torsion of 1808 (or 08).…”

“…In its deprotonated form L À this ligand was previously shown to nest two high-spin (S = 1) nickel(II) ions in its binding sites and to afford a variety of nickel(II) azido complexes. [21,23,31,32] These may feature unusual azide binding modes in oligonuclear aggregates [21,23] as well as a certain geometrical flexibility of the central azide ion that is hosted between the two metal ions of the bimetallic array, [32] which led us to expect a series of related yet distinct 1D systems from a single dinickel(II) precursor.…”

“…Such geometrical flexibility is in line with the most recent observation of an azide moiety behaving as a temperature-dependent toggle switch in a related pyrazolate-based dinickel(II) complex. [32] In both 1 and 2 the sixth coordination site of each nickel centre is occupied by an acetone solvent molecule. Complex 3, which was crystallized from methanol, has only one solvent molecule per bimetallic entity, while sixfold coordination at the second metal ion is achieved by dimerization through two azide N atoms to give a tetranuclear species.…”

Siamese‐Twin Porphyrin: A Pyrazole‐Based Expanded Porphyrin Providing a Bimetallic Cavity

“…Hence, a vanishing antiferromagnetic exchange contribution (J antiferro ) to the overall coupling term (J = J ferro + J antiferro ) is observed, rendering the system ferromagnetically coupled. Although the torsion-angle-dependent change of the sign of the metal-metal interaction from antiferro-to ferromagnetic was observed for several bridging ligands, [23][24][25][26] the realization of ferromagnetic coupling is unprecedented for bridging pyrazolate units.…”

Siamesisches Zwillingsporphyrin: ein Pyrazol‐expandiertes Porphyrin mit Bindungstaschen für Dimetallkomplexe

“…Ein Torsionswinkel-abhängiger Vorzeichenwechsel der Metall-Metall-Wechselwirkung von antiferromagnetisch zu ferromagnetisch wurde schon bei verschiedenen Brückenliganden beobachtet, [23][24][25][26] ist für Pyrazolatbrücken aber beispiellos. Zusammenfassend haben wir ein neuartiges Pyrazol-expandiertes Porphyrin 9 H 4 und dessen Kupfer(II)-Homodimetallkomplex 9·Cu 2 hergestellt.…”