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

摘要

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 center dot H2O (1), [Ni-2(mu-L)(acac)(2)(H2O)]center dot CH3CN (2), [Ni-2(mu-L)(mu-OAc)(NCS)] (3), [Ni-3(mu-L)(2)(mu-OH2)(2)(H2O)(CH3CN)](NO3)(2)center dot 4CH(3)CN (4), [Ni4(mu-L)(2)-(mu-OAc) 2(mu-OCH3)(2)]center dot 6H(2)O center dot 2CH(3)OH (5) and [Ni-4(mu-L)(2)(mu-OAc)(2)(mu-N-3)(2)]center dot 2H(2)O center dot 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 O-4 site of the compartmental ligand H2L, which acts in a neutral zwitterionic form. Complexes 2 and 3 are dinuclear Ni-2 neutral entities, in which the Ni-II ions are connected through two mu-phenoxido bridging groups. The Ni(O)(2)Ni 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 NiII atoms. Complex 4 has a bent trinuclear structure with double mu-phenoxido/mu-water bridges between the central and terminal nickel atoms. Complexes 5 and 6 are Ni-4 complexes with defective dicubane structures, in which triple mu-phenoxido/mu(1,1,1)-X/syn-syn acetate and double mu-phenoxido/mu(1,1,1)-X mixed bridges connect central and terminal Ni-II atoms, whereas double mu(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-mu-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 mu-phenoxido/mu-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.