Structures and Magnetic Properties of Carbonato-Bridged Hexanuclear Ni<sup>II</sup><sub>4</sub>Ln<sup>III</sup><sub>2</sub> (Ln = Gd, Tb, Dy) Complexes Formed by Atmospheric Carbon Dioxide Fixation in the Absence of an External Base
Souvik Maity, Tanmoy Kumar Ghosh, Saki Ito, Pradip Bhunia, Takayuki Ishida, Ashutosh Ghosh
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.