Abstract
We report a new structure of {[Co(bpy)2(ox)][{Cu2(bpy)2(ox)}Fe(ox)3]}n·8.5nH2O NCU-1 presenting a rare ladder topology among oxalate-based coordination polymers with anionic chains composed of alternately arranged [Cu2(bpy)2(ox)]2+ and [Fe(ox)3]3− moieties. Along the a axis, they are separated by Co(III) units to give porous material with voids of 963.7 Å3 (16.9% of cell volume). The stability of this structure is assured by a network of stacking interactions and charge-assisted C-H…O hydrogen bonds formed between adjacent chains, adjacent cobalt(III) units, and alternately arranged cobalt(III) and chain motifs. The soaking experiment with acetonitrile and bromobenzene showed that water molecules (8.5 water molecules dispersed over 15 positions) are bonded tightly, despite partial occupancy. Water adsorption experiments are described by a D’arcy and Watt model being the sum of Langmuir and Dubinin–Serpinski isotherms. The amount of primary adsorption sites calculated from this model is equal 8.2 mol H2O/mol, being very close to the value obtained from the XRD experiments and indicates that water was adsorbed mainly on the primary sites. The antiferromagnetic properties could be only approximately described with the simple CuII-ox-CuII dimer using H = −J·S1·S2, thus, considering non-trivial topology of the whole Cu-Fe chain, we developed our own general approach, based on the semiclassical model (SC) and molecular field (MF) model, to describe precisely the magnetic superexchange interactions in NCU-1. We established that Cu(II)-Cu(II) coupling dominates over multiple Cu(II)-Fe(III) interactions, with JCuCu = −275(29) and JCuFe = −3.8(1.6) cm−1 and discussed the obtained values against the literature data.
Highlights
Oxalate anions act as bridges assuring the effective transfer of magnetic couplings between paramagnetic centers [1]
Tending to develop new multimetallic coordination complexes [85,86], we report the original architecture {[Co(bpy)2(ox)][{Cu2(bpy)2(ox)}Fe(ox)3]}n·8.5nH2O NCU-1, as a product of the self-assembly between the complex building block originating from the recombination of [Co(H2O)6]Cl2, [Cu(bpy)2NCS](NO3) and K3[Fe(ox)3] in aqueous solution
The molecular recognition led to the occurrence of oxalate-bridged 1D [{Cu2(bpy)2(ox)}Fe(ox)3]−∞ ladders of very rare topology, shown only very recently to crystallize with simple alkali metal cations
Summary
Oxalate anions act as bridges assuring the effective transfer of magnetic couplings between paramagnetic centers [1]. Ladder chain was detected in [Cu2(bpy)2(ox)Cr/Fe(ox)3]− 1D compounds [4,32] and the interactions were described using 0D model and a single exchange parameter because of the dominating antiferromagnetic interactions in the copper dimer over other couplings. For the description of the magnetic properties of homo- and heterometallic oxalato-based chains, the Heisenberg model was often used with a single exchange parameter [30,33]. Five exchange parameters were proposed for a trimetallic MnCuCr system showing a highly complex topology with three different ligands acting as bridges [8]. In modeling of the exchange pathways via oxalate anion connecting clusters into a chain, a classical spin approach useful for the description of systems with high spin (in this case S = 5) and relatively weak coupling between them was applied
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