Anion controlled structural and magnetic diversity in unusual mixed-bridged polynuclear NiII complexes with a versatile bis(2-methoxy phenol)diamine hexadentate ligand. An experimental and theoretical magneto-structural study

Abstract

This work reports the syntheses, crystal structures, magnetic properties and DFT calculations of six novel polynuclear Ni^II compounds [Ni₂(μ-HL¹)₂(μ-N₃)]N₃·(CH₃OH)₂·2H₂O (1), [Ni₂(μ-HL¹)₂(μ-N₃)][Na₂Ni₂(μ-L¹)₂(μ-N₃)₂(CH₃OH)(N₃)]·4CH₃OH (2), [Ni₄(μ-L¹)₂(μ-N₃)₄(CH₃OH)₂]·2CH₃OH (3), [Na₂Ni₄(μ-L¹)₂(μ-OAc)₂(μ-N₃)₄(CH₃OH)₄]·2CH₃OH (4), [Ni₄(μ-L¹)₂(μ-Cl)₂(Cl)₂] (5), and [Ni₃(μ-L¹)₂(acac)₂(H₂O)₂] (6), with uncommon structures and rare mixed-bridges between Ni^II ions, which were prepared from the versatile polytopic Mannich base ligand N,N′-dimethyl-N,N′-bis(2-hydroxy-3-methoxy-5-methylbenzyl)ethylenediamine (H₂L¹). The anionic coligand (azide, chloride, acetate and acetylacetonate) and reaction conditions play crucial roles in determining the final structure of these compounds and consequently their magnetic properties. Compound 1 contains a Ni₂ cationic unit with rare di-μ-phenoxido/μ_1,1-azide triple mixed bridges whereas complex 2 is made from the same Ni₂ cationic unit as 1, cocrystallized with Na₂Ni₂ neutral units, in which double μ_1,1,1-azide bridges connect the Ni^II and Na^I anions. Complexes 3 and 4 are Ni₄ complexes with defective dicubane and linear structures, respectively. Complex 3 has two distinct types of mixed bridges, μ-phenoxido/μ_1,1,1-azido and μ_1,1-azido/μ_1,1,1-azido and a double di-μ_1,1,1-azido bridge, the latter connecting the face-sharing Ni^II ions. In 4, rare μ-phenoxido/μ_1,1-azide/syn–syn acetate triple mixed bridges connect central and terminal Ni^II atoms whereas a double μ_1,1-azide planar bridging fragment links the central Ni^II ions. Complex 5 has a defective-dicubane structure with double μ-phenoxido/μ₃-chloro mixed bridges and di-μ₃-chloro bridges, whereas complex 6 has a bent structure with very uncommon single μ-phenoxido bridges. The analysis of the magnetic properties reveals that in complexes 1–4 all magnetic pathways transmit ferromagnetic interactions leading to S = 2 ground states for 1 and 2 and S = 4 ground states for 3 and 4. In complex 5, the double μ-phenoxido/μ₃-chloro mixed bridges and di-μ₃-chloro bridges mediate antiferro- and ferromagnetic interactions, respectively, giving rise to a S = 0 ground state. Complex 6 shows antiferromagnetic interactions between the Ni^II ions through single μ-phenoxido bridging groups, leading to an S = 1 ground state. DFT calculations on the X-ray structures and model compounds were performed to support the magneto-structural data of the above compounds.