Alternating spin chains: Controlled assembly from bimetallic building blocks and QMC simulation of spin correlation

By using the compartmental dinucleating pyrazolate ligand HL, dinickel(ll) complexes [LNi2(mu-N-3)(acetone)(2)]X-2, (1: X=ClO4; 2: X=BPh4) and tetranickel(ll) complex [{LNi2(mu-N-3)(MeOH)}(2)](ClO4)(4) (3) have been prepared and structurally characterized. Complexes I and 2 differ in the torsion along the bridging mu-1,3-azide moiety, while the azido ligands in 3 adopt an unusual mu-1, 1,3 bridging mode to connect the two subunits. All three complexes show overall antiferromagnetic coupling and on S=0 ground state, but the torsion along the azide moiety is a determining factor for the coupling strength. Compounds I and 2 serve as preorganized building blocks for the controlled synthesis of alternating ID 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 mu-1,3-azido bridges, both within and between the bimetallic constituents. Magnetic measurements confirm on overall S=0 ground state for 4-6, and coupling parameters hove 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=J(2)J(1)(-1) places the three new systems in the HABA regime for a singlet-dimer ground state.