Structures and Magnetic Properties of Carbonato-Bridged Hexanuclear NiII4LnIII2 (Ln = Gd, Tb, Dy) Complexes Formed by Atmospheric Carbon Dioxide Fixation in the Absence of an External Base

Abstract

The reaction of the mononuclear Ni(II) building block [NiL] of an asymmetrically dicondensed N2O3 donor Schiff base ligand, N-salicylidene-N′-3-methoxysalicylidene-1,3-propanediamine (H2L), with LnCl3·6H2O (Ln = Gd, Tb, Dy) in a 2:1 molar ratio leads to the formation of three hexanuclear carbonato-bridged Ni4Ln2 complexes formulated as [Ni4Ln2(CO3)2Cl2(L)2(L′)2(CH3CN)2]·4CH3CN·2H2O (Ln = Tb (2), Dy (3)), where H2L′ = N,N′-bis(salicylidene)-1,3-propanediamine. In complex 1, the unsymmetrical nature of the Schiff base remains intact; however, in 2 and 3, between the two metalloligands coordinated to each Ln, one is the precursor [NiL] of the unsymmetrical ligand whereas the other [NiL′] of the symmetrical ligand, formed by an in situ aldehyde-exchange reaction during complex formation with Ln salts. The Ln(III) centers are octacoordinated with a biaugmented-trigonal-prismatic geometry in all three complexes. In complex 1, half of the Ni(II) centers possess a distorted octahedral geometry while the other half has a square-pyramidal geometry; all of the Ni(II) centers in complexes 2 and 3 have a distorted-octahedral geometry. A magnetic study clarifies that the Ni–Ln magnetic exchange couplings are ferromagnetic. Though the complexes consist of a hexanuclear [Ln2Ni4] system, the magnetic properties have been analyzed on a trinuclear Ni–Ln–Ni basis. The exchange parameter 2JGd–Ni/kB in 1 was estimated to be +0.61 to +0.84 K across the double-oxide bridges. Complexes 2 and 3 exhibited an indication of single-molecule magnets in an ac susceptibility study.