An experimental and theoretical magneto-structural study of polynuclear NiII complexes assembled from a versatile bis(salicylaldehyde)diamine polytopic ligand

Abstract

Six novel Ni^II complexes, ranging from mononuclear to tetranuclear, have been prepared from the polytopic symmetrical Mannich base ligand N,N′-dimethyl-N,N′-bis(2-hidroxy-3-formyl-5-bromo-benzyl)ethylenediamine (H₂L) and different anionic coligands: [Ni(H₂L)(NO₃)(H₂O)]NO₃·H₂O (1), [Ni₂(μ-L)(acac)₂(H₂O)]·CH₃CN (2), [Ni₂(μ-L)(μ-OAc)(NCS)] (3), [Ni₃(μ-L)₂(μ-OH₂)₂(H₂O)(CH₃CN)](NO₃)₂·4CH₃CN (4), [Ni₄(μ-L)₂(μ-OAc)₂(μ-OCH₃)₂]·6H₂O·2CH₃OH (5) and [Ni₄(μ-L)₂(μ-OAc)₂(μ-N₃)₂]·2H₂O·CH₃OH (6). These complexes have been characterized by single crystal X-ray diffraction, magnetic measurements and DFT theoretical calculations. The structural analysis of these complexes reveals that the anionic coligand and reaction conditions play a fundamental role in determining their final structures and magnetic properties. Compound 1 contains a monomeric cationic unit with the nickel ion coordinated in the external O₄ site of the compartmental ligand H₂L, which acts in a neutral zwitterionic form. Complexes 2 and 3 are dinuclear Ni₂ neutral entities, in which the Ni^II ions are connected through two μ-phenoxido bridging groups. The Ni(O)₂Ni bridging fragment in 2 is almost planar, whereas in 3 is bent due to the additional presence of a syn–syn acetate bridge connecting the Ni^II atoms. Complex 4 has a bent trinuclear structure with double μ-phenoxido/μ-water bridges between the central and terminal nickel atoms. Complexes 5 and 6 are Ni₄ complexes with defective dicubane structures, in which triple μ-phenoxido/μ_1,1,1-X/syn–syn acetate and double μ-phenoxido/μ_1,1,1-X mixed bridges connect central and terminal Ni^II atoms, whereas double μ_1,1,1-X bridging ligands link the central Ni^II ions (X = methoxido and azido groups for 5 and 6, respectively). Magnetic susceptibility measurements reveal that complex 2 shows a moderate antiferromagnetic interaction between the Ni^II ions through the double di-μ-phenoxido bridge, leading to a S = 0 ground state. Compared to 2, complex 3 shows a much weaker magnetic exchange interaction due the counter-complementarity effect provoked by the additional presence of the syn–syn acetate bridging group, as well as the non-planarity of the bridging fragment. In complex 4, the double μ-phenoxido/μ-water mixed bridges lead to very weak antiferromagnetic interactions between the central and external Ni^II ions. Overall ferromagnetic interactions are found for 3, 5 and 6, although in 5 not all the magnetic pathways transmit ferromagnetic interactions. A detailed analysis of the magnetic exchange interactions transmitted through the different pathways as well as DFT calculations on the X-ray structures of compounds 2–6 were performed to support the magneto-structural data of these compounds.