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.

A photo-induced spin crossover based molecular switch and spin filter operating at room temperature

Since Venkataramani et al. (Science, 2011, 331(6016), 445-448) reported reversible, room-temperature light-induced spin crossover in Ni-porphyrin functionalized with a phenylazopyridine ligand (NiTPP-PAPy), this complex has attracted the attention of many researchers due to its potential applications in molecular-based devices. In this work, we perform a detailed study, by means of DFT and WFT methodologies, focused on the deposition of NiTP-PAPy over an Au(111) surface, followed by DFT-NEGF calculations employing a gold surface and the tip of an STM as electrodes, in order to probe the deposited complex's transport properties. Our DFT calculations show that not only the metalled porphyrin is strongly adsorbed on the surface, in both the high (HS) and low spin (LS) configurations, but also, and more importantly, photoinduced switching is preserved upon adsorption, a fact that is also confirmed through WFT and TD-DFT calculations. Moreover, our DFT-NEGF calculations indicate that the current passing through the molecular junction-like systems is much higher in the HS configuration than in the LS one, along with the fact that the current calculated in the ferromagnetic junction is highly spin-polarized. These remarkable transport properties suggest that the complex could be used as a component in molecular switches based on the total current passing through the system, modulated by light irradiation, spin filters due to the spin polarization of the carriers in the HS configuration, or even in two-step rectifiers combining the two features mentioned above, all of these operating at room temperature, giving to this complex the potential to be an active element in all kinds of future spintronic devices.

Insertion of single-ion magnets based on mononuclear Co(II) complexes into ferromagnetic oxalate-based networks

The 1 : 2 and 1 : 1 Co(ii) complexes of the L ligand (L = 6-(3,5-diamino-2,4,6-triazinyl)2,2'-bipyridine) with formulas [CoII(L)2](ClO4)2·0.5MeCN·Et2O (1) and [CoII(L)(CH3CN)2(H2O)](ClO4)2·MeCN (2) have been prepared. The structural and magnetic characterization of the two compounds shows that they contain octahedral high-spin Co(ii) and present a field-induced slow relaxation of the magnetization. 1 has been inserted into a bimetallic oxalate-based network leading to a novel achiral 3D compound of formula [CoII(L)2][MnIICrIII(ox)3]2·(solvate) (3) exhibiting ferromagnetic ordering below 4.6 K. EPR measurements suggest a weak magnetic coupling between the two sublattices.

Two-dimensional square-grid iron(ii) coordination polymers showing anion-dependent spin crossover behavior

Two Fe(ii)-based coordination polymers [Fe(tpmd)2(NCS)2]·5.5H2O (1) and [Fe(tpmd)2(NCSe)2]·7H2O (2) with the framework of square-grid type have been assembled from FeSO4·7H2O, N,N,N',N'-tetrakis(pyridin-4-yl)methanediamine (tpmd), and KNCS/KNCSe in methanol and characterized. By utilizing two pyridine groups of a tpmd ligand, 1 and 2 are formed in two-dimensional layered structures through coordination of octahedral iron(ii) ions with the tpmd to NCS-/NCSe- ligands in which they have a supramolecular isomorphous conformation. 1 shows a paramagnetic behavior between 2 and 300 K, while 2 exhibits two-step spin crossover (ca. 145 and 50 K) in the temperature range due to the coordination of NCSe- ligands. At 300 K 2 is fully high-spin state. However, at 100 K 2 becomes ca. 50% high spin and 50% low spin iron(ii) ions, which is verified by magnetic moments. In the structural analysis of 2 at 100 K, two different layers are observed with different bond distances around iron(ii) ions in which the layers are stacked alternately.

Spin-crossover compounds based on iron(ii) complexes of 2,6-bis(pyrazol-1-yl)pyridine (bpp) functionalized with carboxylic acid and ethyl carboxylic acid

New salts of the iron(ii) bppCOOH and bppCOOEt spin crossover complexes have been studied.

Iron(II) Complexes of 2,4-Dipyrazolyl-1,3,5-triazine Derivatives-The Influence of Ligand Geometry on Metal Ion Spin State

Seven [FeL₂][BF₄]₂ complex salts were prepared, where L is a 6-substituted 2,4-di(pyrazol-1-yl)-1,3,5-triazine (bpt) derivative. The complexes are all crystallographically high-spin, and exhibit significant distortions from an ideal D_2d-symmetric coordination geometry. In one case, an unusual type of metal ion disorder was observed among a cubic array of ligands in the crystal lattice. The complexes are also high-spin between 3 and 300 K in the solid state and, where measured, between 239 and 333 K in CD₃CN solution. This result is unexpected, since homoleptic iron(II) complexes of related 2,6-di(pyrazol-1-yl)pyridine, 2,6-di(pyrazol-1-yl)pyrazine, and 2,6-di(pyrazol-1-yl)pyrimidine derivatives often exhibit thermal spin-crossover behavior. Gas-phase density functional theory calculations confirm the high-spin form of [Fe(bpt)₂]²⁺ and its derivatives is stabilized relative to iron(II) complexes of the other ligand types. This reflects a weaker Fe/pyrazolyl σ-bonding interaction, which we attribute to a small narrowing of the chelate ligand bite angle associated with the geometry of the 1,3,5-triazinyl ring. Hence, the high-spin state of [Fe(bpt)₂]²⁺ centers does not reflect the electronic properties of its heterocyclic ligand donors but is imposed by the bpt ligand conformation. A high-spin homoleptic iron(III) complex of one of the bpt derivatives was also synthesized.