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

Abstract

By using the compartmental dinucleating pyrazolate ligand HL, dinickel(II) complexes [LNi2(micro-N3)(acetone)2]X2 (1: X = CIO4; 2: X = BPh4) and tetranickel(II) complex [{LNi2(micro-N3)(MeOH)2](CI04)4 (3) have been prepared and structurally characterized. Complexes 1 and 2 differ in the torsion along the bridging micro-1,3-azide moiety, while the azido ligands in 3 adopt an unusual micro-1,1,3 bridging mode to connect the two subunits. All three complexes show overall antiferromagnetic coupling and an S = 0 ground state, but the torsion along the azide moiety is a determining factor for the coupling strength. Compounds 1 and 2 serve as preorganized building blocks for the controlled synthesis of alternating 1D polymeric structures 4-6 by replacement of their labile acetone ligands by additional azido ligands. Due to the modular synthetic approach, 4-6 can be described as Heisenberg antiferromagnetic systems with inherent bond alternation (HABA), whereby the organic ligand framework ensures that the individual nickel/azido chains are well isolated in the crystal lattice. Like their precursors, 4-6 are mainly distinguished by torsion along the micro-1,3-azido bridges, both within and between the bimetallic constituents. Magnetic measurements confirm an overall 5 = 0 ground state for 4-6, and coupling parameters have been deduced from quantum Monte Carlo simulations. The two J values for the alternating 1D chains can be clearly assigned on the basis of the magnetostructural correlations established for the bimetallic building blocks. The alternation ratio gamma = J2J1(-1) places the three new systems in the HABA regime for a singlet-dimer ground state.

Tetranuclear complexes in molecular magnetism: Targeted synthesis, high-field EPR and pulsed-field magnetization

“…Spin correlation in the various 1D nickel-azido systems has been analyzed by Quantum Monte Carlo simulations, and assignment of the two different J 1 and J 2 values was based on comparison with the magnetic characteristics found for the bimetallic constituents. This again emphasizes the advantages of the modular building block approach [72,73].…”

Coordination Versatility of Pyrazole‐Based Ligands towards High‐Nuclearity Transition‐Metal and Rare‐Earth Clusters

“…Variation of the side arm chain lengths gives some control over metal-metal separation, while the number and type of side arm donor sites allow the determination of electronic and coordinative properties. An important part of this research has been developed by Meyer and co-workers, [111][112][113][114][115][116][117][118][119][120][121][122][123][124][125][126] who have studied the coordination ability of the 3,5-disubstituted pyrazole ligands with chelating side arms, mainly with N-donor atoms. [11] The main part of the review was dedicated to dinuclear compounds.…”

“…[11] The main part of the review was dedicated to dinuclear compounds. [115][116][117][118][119][120] Another synthetic approach employed the reaction of dinuclear complexes with labile co-ligands and with azides. An important part of this research has been developed by Meyer and co-workers, [111][112][113][114][115][116][117][118][119][120][121][122][123][124][125][126] who have studied the coordination ability of the 3,5-disubstituted pyrazole ligands with chelating side arms, mainly with N-donor atoms.…”

“…An important part of this research has been developed by Meyer and co-workers, [111][112][113][114][115][116][117][118][119][120][121][122][123][124][125][126] who have studied the coordination ability of the 3,5-disubstituted pyrazole ligands with chelating side arms, mainly with N-donor atoms. [115,123] The similar central core of the tetranuclear nickel(II) structures is formed by dinuclear nickel(II) units, [Ni 2 L] 3+ , which are connected by azide and carboxylate bridges ( Figure 10). Tetranuclear nickel(II) compounds with the general formula [Ni 4 L 2 ] 6+ were synthesized with stoichiometric amounts of the metal salt, the pyrazole-based ligand (HL = HL2c-HL2h, see Figure 9), the base and an appropriate amount of the bridging ligand, such as azide and/or carboxylate.…”

“…These ligands differ in the chain lengths of the chelating side arms and the number of donor sites. [115][116][117][118][119][120] Table 2 shows the values of the magnetic exchange interactions for the tetranuclear nickel(II) compounds discussed above. [115][116][117][118][119][120] Another synthetic approach employed the reaction of dinuclear complexes with labile co-ligands and with azides.…”