Magnetic Exchange Interactions and Magneto-Structural Correlations in Heterobridged μ-Phenoxo-μ1,1-Azide Dinickel(II) Compounds: A Combined Experimental and Theoretical Exploration

Abstract

This investigation presents the syntheses, crystal structures, magnetic properties, and density functional theoretical modeling of magnetic behavior of two heterobridged μ-phenoxo-μ_1,1-azido dinickel(II) compounds [Ni^II₂(L¹)₂(μ_1,1-N₃)(N₃)(H₂O)]·CH₃CH₂OH (1) and [Ni^II₂(L²)₂(μ_1,1-N₃)(CH₃CN)(H₂O)](ClO₄)·H₂O·CH₃CN (2), where HL¹ and HL² are the [1 + 1] condensation products of 3-methoxysalicylaldehyde and 1-(2-aminoethyl)-piperidine (for HL¹)/4-(2-aminoethyl)-morpholine (for HL²), along with density functional theoretical magneto-structural correlations of μ-phenoxo-μ_1,1-azido dinickel(II) systems. Compounds 1 and 2 crystallize in orthorhombic (space group Pbca) and monoclinic (space group P2₁/c) systems, respectively. The coordination environments of both metal centers are distorted octahedral. The variable-temperature (2–300 K) magnetic susceptibilities at 0.7 T of both compounds have been measured. The interaction between the metal centers is moderately ferromagnetic; J = 16.6 cm^–1, g = 2.2, and D = −7.3 cm^–1 for 1 and J = 16.92 cm^–1, g = 2.2, and D(Ni1) = D(Ni2) = −6.41 cm^–1 for 2. Broken symmetry density functional calculations of exchange interaction have been performed on complexes 1 and 2 and provide a good numerical estimate of J values (15.8 cm^–1 for 1 and 15.35 cm^–1 for 2) compared to experiments. The role of Ni–N bond length asymmetry on the magnetic coupling has been noted by comparing the structures and J values of complexes 1 and 2 together with previously published dimers 3 (Eur. J. Inorg. Chem. 2009, 4982), 4 (Inorg. Chem. 2004, 43, 2427), and 5 (Dalton Trans. 2008, 6539). Our extensive DFT calculations reveal an important clue to the mechanism of coupling where the orientation of the magnetic orbitals seems to differ with asymmetry in the Ni–N bond lengths. This difference in orientation leads to a large change in the overlap integral between the magnetic orbitals and thus the magnetic coupling. DFT calculations have also been extended to develop several magneto-structural correlations in this type of complexes and the correlation aim to focus on the asymmetry of the Ni–N bond lengths reveal that the asymmetry plays a proactive role in governing the magnitude of the coupling. From a completely symmetric Ni–N bond length, two behaviors have been noted: with a decrease in bond length there is an increase in the ferromagnetic coupling, while an increase in the bond lengths leads to a decrease in ferromagnetic interaction. The later correlation is supported by experiments. The magnetic properties of 1, 2, and three previously reported related compounds have been discussed in light of the structural parameters and also in light of the theoretical correlations determined here.