Spontaneous resolution of heterometallic complex: cis-[Co(NH3)4(H2O)2][Fe(ox)3]·2H2O – A rare example for labile iron(III) complex

Sorption and Magnetic Properties of Oxalato-Based Trimetallic Open Framework Stabilized by Charge-Assisted Hydrogen Bonds

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 ų (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 Cu^II-ox-Cu^II 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 J_CuCu = −275(29) and J_CuFe = −3.8(1.6) cm⁻¹ and discussed the obtained values against the literature data.

Structural, Electrical, and Magnetic Versatility of the Oxalate-Based [CuFe] Compounds Containing 2,2':6',2″-Terpyridine: Anion-Directed Synthesis

The heterodimetallic [CuFe] compounds [Cu^II₄(terpy)₄Cl₅][Fe^III(C₂O₄)₃]·10H₂O (1;terpy = 2,2':6',2''-terpyridine), [Cu^II₂(H₂O)₂(terpy)₂(C₂O₄)][Cu^IIFe^III(CH₃OH)(terpy)(C₂O₄)₃]₂ (2), and {[Cu₂^IIFe^III(H₂O)(terpy)₂(C₂O₄)_7/2]·6H₂O}_n (3) were obtained using building block approach, from reaction of aqueous solution of [Fe(C₂O₄)₃]^3- and a methanol solution containing Cu²⁺ ions and terpy by the layering technique. Interestingly, by changing only the anion of the starting salt of copper(II), Cu(NO₃)₂·3H₂O instead of CuCl₂·2H₂O, an unexpected change in the type of bridge, oxalate (2 and 3) versus chloride (1), was achieved, thus affecting the overall structural architecture. Two polymorphs of 3D coordination polymer [Cu^IIFe^II₂(H₂O)(terpy)(C₂O₄)₃]_n (4), crystallizing in the triclinic (a) and monoclinic (b) space groups, were formed hydrothermally, depending on whether CuCl₂·2H₂O or Cu(NO₃)₂·3H₂O was added to the water, besides K₃[Fe(C₂O₄)₃]·3H₂O and terpy, respectively. Under hydrothermal conditions iron(III) from initial building block is reduced to the divalent state, creating 2D honeycomb [Fe^II₂(C₂O₄)₃]_n^2n- layers, which are bridged by [Cu(H₂O)(terpy)]²⁺ cations. Compounds were investigated by single-crystal X-ray diffraction, IR, and impedance spectroscopies, magnetization measurements, and density functional theory (DFT) calculations. In compounds 1 and 2, 0D magnetism is observed, with 1 having a ground-state spin of 1 due to different interactions through chloride bridges of Cu²⁺ ions in tetramer [Cu^II₄(terpy)₄Cl₅]³⁺ and 2 showing strong antiferromagnetic coupling of Cu²⁺ ions mediated by oxalate ligand in [Cu^II₂(H₂O)₂(terpy)₂(C₂O₄)]²⁺ and weak ones between Cu²⁺ and Fe³⁺ ions through oxalate bridge in [Cu^IIFe^III(CH₃OH)(terpy)(C₂O₄)₃]^-. Polymer 4 exhibits antiferromagnetic phase transition at 25 K: The [Fe^II₂(C₂O₄)₃]_n^2n- layers are antiferromagnetically ordered, and a small amount of interlayer interaction is transferred through [Cu(H₂O)(terpy)]²⁺ cations via O_ox-Cu-O_ox bridges. Additionally, compounds 1 and 2 are electrical insulators, while 4a and 4b show proton conductivity.

Solvent-assisted structural conversion involving bimetallic complexes based on the tris(oxalato)ferrate(iii) unit with the green → blue → red crystal color sequence

Three new complexes forming a dynamic system and given by the following formulae: [Cu₂(bpy)₄Fe(ox)₃]NO₃·H₂O 1, [Fe(bpy)₃]₂[Fe(ox)₃]NO₃·10H₂O 2 and [Cu(bpy)₃]₂[Fe(ox)₃]NO₃·10H₂O 3 (bpy - 2,2'-bipyridine, ox - oxalate), were synthesized from a methanol-water mixture or water, and characterized structurally, spectroscopically and magnetically. Compound 1 contains trinuclear [(bpy)₂Cu(μ-ox)Fe(ox)(μ-ox)Cu(bpy)₂]⁺ cations, while 2 and 3 can be classified as isomorphous ionic compounds, with alternately arranged hydrophobic and hydrophilic layers of mononuclear complex ions. The green crystals of 1 are perfectly stable in air, whereas in selected solvents they undergo irreversible solvent-assisted recrystallization towards red crystals of 2, which is also accompanied by the appearance of mononuclear blue copper complexes with oxalate, 2,2'-bipyridine and aqua ligands, already described in the literature. The above crystallization/recrystallization processes indicate variable solution contents. The whole effect is accelerated by both the increased temperature and day light irradiation, however, different products from the pool prevail under various conditions. The observed transformation can be understood in terms of thermodynamic and kinetic control, involving the known photo-activity of [Fe(ox)₃]^3- moieties and the effect of quadruple aryl embrace (QAE) on the stability of the crystal network. Considering the presence of Fe^III-ox-Cu^II connectivity in 1 we performed detailed magnetic studies and theoretical calculations for this compound. Due to the strong asymmetry of Cu-O bonds the antiferromagnetic coupling is rather weak, with J_Cu-Fe being ca. -3.4 cm^-1 (using Hamiltonian of the type H = -J_Cu-Fe(S_Cu1S_Fe + S_FeS_Cu2) -J_Cu-Cu(S_Cu1S_Cu2)). We found that these values are very close to those predicted by B3LYP/6-311G* calculations.