Pitfalls on evaluating pair exchange interactions for modelling molecule-based magnetism

Molecule-based magnetism is a solid-state property that results from the microscopic interaction between magnetic centres or radicals. The observed magnetic response is due to unpaired electrons whose coupling leads to a particular magnetic topology. Therefore, to understand the magnetic response of a given molecule-based magnet and reproduce the available experimental magnetic properties by means of statistical mechanics, one has to be able to determine the value of the JAB magnetic exchange coupling between radicals. The calculation of JAB is thus a key point for modelling molecule-based magnetism. In this Perspectives article, we will build upon our experience in modelling molecular magnetism to point out some pitfalls on evaluating JAB couplings. Special attention must be paid to the cluster models used to evaluate JAB, which should account for cooperative effects among JAB interactions and also consider the environment (counterions, hydrogen bonding) of the two radicals whose interaction has to be evaluated. It will be also necessary to assess whether a DFT-based or a wavefunction-based method is best to study a given radical. Finally, in addition to model and method, the JAB couplings have to be able to adapt to changes in the magnetic topology due to thermal fluctuations. Therefore, it is most important to appraise in which systems molecular dynamics simulations would be required. Given the large number of issues one must tackle when choosing the correct model and method to evaluate JAB interactions for modelling magnetic properties in molecule-based materials, the “human factor” is a must to cross-examine and challenge computations before trusting any result

Pitfalls on evaluating pair exchange interactions for modelling molecule-based magnetism

Molecule-based magnetism is a solid-state property that results from the microscopic interaction between magnetic centres or radicals. The observed magnetic response is due to unpaired electrons whose coupling leads to a particular magnetic topology. Therefore, to understand the magnetic response of a given molecule-based magnet and reproduce the available experimental magnetic properties by means of statistical mechanics, one has to be able to determine the value of the J(AB) magnetic exchange coupling between radicals. The calculation of J(AB) is thus a key point for modelling molecule-based magnetism. In this Perspectives article, we will build upon our experience in modelling molecular magnetism to point out some pitfalls on evaluating J(AB) couplings. Special attention must be paid to the cluster models used to evaluate J(AB), which should account for cooperative effects among J(AB) interactions and also consider the environment (counterions, hydrogen bonding) of the two radicals whose interaction has to be evaluated. It will be also necessary to assess whether a DFT-based or a wavefunction-based method is best to study a given radical. Finally, in addition to model and method, the J(AB) couplings have to be able to adapt to changes in the magnetic topology due to thermal fluctuations. Therefore, it is most important to appraise in which systems molecular dynamics simulations would be required. Given the large number of issues one must tackle when choosing the correct model and method to evaluate J(AB) interactions for modelling magnetic properties in molecule-based materials, the "human factor" is a must to cross-examine and challenge computations before trusting any result.MD, JRA, and JJN acknowledge financial support from MINECO (CTQ2017-87773-P/AEI/FEDER, UE), Spanish Structures Excellence Maria de Maeztu program (MDM-2017-0767), and Catalan DURSI (2017SGR348)

The magnetic fingerprint of dithiazolyl-based molecule magnets

Magnetic bistability in organic-radical based materials has attracted significant interest due to its potential application in electronic devices. The first-principles bottom-up study herein presented aims at elucidating the key factors behind the different magnetic response of the low and high temperature phases of four different switchable dithiazolyl (DTA)-based compounds. The drastic change in the magnetic response upon spin transition is always due to the changes in the J(AB) magnetic interactions between adjacent radicals along the -stacks of the crystal, which in turn are driven mostly by the changes in the interplanar distance and degree of lateral slippage, according to the interpretation of a series of magneto-structural correlation maps. Furthermore, specific geometrical dispositions have been recognized as a ferromagnetic fingerprint in such correlations. Our results thus show that an appropriate substitution of the chemical skeleton attached to the DTA ring could give rise to new organic materials with dominant ferromagnetic interactions

On the Overwhelming Complexity of Mechanochemical Disulphide Bond Reduction in Alkaline Solution

The coupling between mechanical stress and the reactivity of disulphide bridges has recently received a great deal of attention due to its broad relevance in biochemistry and materials science. Here, we will highlight the main findings of our computational studies on mechanochemistry of disulphide bridges, which have been carried out in the past few years in the framework of GCS Large Scale Projects. Our investigations have disclosed a very complex mechanistic scenario for the mechanochemistry of disulphides in aqueous alkaline solution. In the low-force regime, external forces play a dual role in the reduction of disulphide bridges via a bimolecular SN2 attack of a hydroxide ion at a sulphur atom. On the one hand, the external tensile force accelerates the reaction by virtue of the mechanical work performed on the system as the reaction proceeds. On the other hand, tensile forces can induce a conformational distortion of the disulphide moiety that drives the system into a spatial arrangement that is less prone to a nucleophilic attack due to steric hindrance. In the high-force regime, in turn, the tensile force gives rise to a competition between bimolecular SN2 and unimolecular C–S bond breaking mechanisms as well as to drastic changes in the free energy landscape of the system as a result of which bimolecular reaction pathways transform into pure bond-breaking processes. Our results not only provide a rationale for the enigmatic outcome of certain single-molecule force spectroscopy experiments but also suggest new experiments to continue unravelling the intricacies of the mechanochemistry of disulphide bridges

A new tetrameric Cu-II cluster with square topology exhibiting ferro- and antiferromagnetic magnetic pathways: which is which?

[Cu4L2(bpy)(4)(H2O)(3)](ClO4)(4).2.5H(2)O, 1, a new tetranuclear Cu-II cluster showing square planar geometry, formed with aspartate bridging ligand (L) has been synthesized. The global magnetic coupling is ferromagnetic but theoretical DFT/B3LYP calculations are necessary to assign which Cu-L-Cu side is ferro or antiferromagnetically coupled

Substituted m-phenylene bridges as strong ferromagnetic couplers for CuII-bridge-CuIImagnetic interactions: New perspectives

Based on a combined theoretical–experimental study, we propose that substituted m-phenylene ligands (m-N-Φ-N) can act as tuneable strong ferromagnetic couplers connecting CuII ions; a new complex presenting that bridge with J close to +15 cm−1 has been suggested and synthesized

Preparation and Structure of Three Solvatomorphs of the Polymer [Co(dbm)2(4ptz)]n: Spin Canting Depending on the Supramolecular Organization

The syntheses and X-ray structures of three isomeric 1D coordination polymers are reported. The complex [Co- (dbm)2(MeOH)2] (1) was used as a precursor in these reactions. The preparation and structure of 1 is also presented; this mononuclear complex is in the cis configuration because this allows the formation of a network of intermolecular hydrogen bonds in the solid state. Reaction of 1 with 2,4,6-tris-(4-pyridyl)-1,3,5-triazine (4ptz) yields the polymers [Co(dbm)2(4ptz)]nânTHF (2a), [Co(dbm)2(4ptz)]nâ0.75nTHFâ0.5nEt2O (2b), and [Co(dbm)2(4ptz)]nâ3nDMF (2c) in the form of zigzag chains, instead of the expected honeycomb architectures. This is because of the establishment of extended ð-ð stacking throughout these solids, which could not have occurred otherwise. Compounds 2a, 2b, and 2c are solvatomorphs, and formation of either one of them depends on the exact conditions of crystallization, which lead to significant differences in the supramolecular organization of the chains. Bulk magnetic measurements on 2a reveal weak antiferromagnetic exchange within the chains and small ordering throughout the solid that results in the manifestation of the phenomenon of spin canting, whereas for 2c the different supramolecular organization causes the antiferromagnetic exchange not to result in spin canting

Assessing the Performance of CASPT2 and DFT Methods for the Description of Long, Multicenter Bonding in Dimers between Radical Ions

The performance of a wide variety of density functionals for the description of long, multicenter bonding in dimers between radical ions has been addressed in this work. Results on interaction energies and equilibrium distances have been evaluated through pure GGA and meta-GGA, hybrid, RSH, and double hybrid functionals. Grimme’s dispersion corrections have also been assessed. All results are systematically analyzed and compared for the π-[TCNE]₂^2–, π-[TTF]₂²⁺, π-[TCNB]₂^2–, and π-[TCNP]₂^2– dimers. The DFT results are benchmarked against RASPT2 calculations based on large active spaces. It is shown that small active spaces do not quantitatively describe the interaction energy curves of these dimers. B97-D3­(BJ) turns to be the functional that best reproduces the finest RASPT2 results, while PBE-D3­(BJ), B3LYP-D3­(BJ), and M06-L also provide satisfactory results

Hydrogen bond assisted co-crystallization of a bimetallic Mn^III₂Ni^II₂cluster and a Ni^II₂cluster unit: synthesis, structure, spectroscopy and magnetism

A new bimetallic Schiff-base composite complex [Ni2(LH2)2(H2O)2Cl2][Mn2Ni2(LH)4]2Cl4(CH3OH)(1) has been synthesized by a simple one-pot reaction. The compound wasstructurally characterized by single-crystal X-ray diffraction. In thecrystal structure the dinuclear nickel units are connected to thetetranuclear Mn2Ni2 units by means ofstrong hydrogen-bonding interactions. The compound was furthercharacterized by ESI-MS, ligand-field and infrared spectroscopy. Themagnetic properties of the compound have been studied in combination withpreliminary DFT calculations, and have resulted in the successful determinationof the nature of the magnetic exchange interactions between the metal ions, andhence the coupling constants. </p

Unravelling the Key Driving Forces of the Spin Transition in π‑Dimers of Spiro-biphenalenyl-Based Radicals

Spiro-biphenalenyl (SBP) boron radicals constitute an important family of molecules for the preparation of functional organic materials. The building blocks of several SBP-based crystals are π-dimers of these radicals, in which two phenalenyl (PLY) rings face each other and the other two PLYs point away from the superimposed PLYs. The dimers of ethyl-SBP and butyl-SBP undergo a spin transition between a diamagnetic and a paramagnetic state upon heating, while other dimers exhibit paramagnetism at all temperatures. Here, we present a computational study aimed at establishing the driving forces of the spin-transition undergone by ethyl-SBP at ∼140 K. The ground state of the π-dimers below 140 K is a singlet state in which the SBP unpaired electrons are partially localized in the superimposed PLYs. Above 140 K, the unpaired electrons are localized in the nonsuperimposed PLYs. These high-temperature structures are exclusively governed by the ground triplet state because the open-shell singlet with the unpaired electrons localized in the nonsuperimposed PLYs does not feature any minimum in the potential energy surface of the system. Furthermore, we show that the electrostatic component of the interaction energy between SBP radicals in the π-dimers is more attractive in the triplet than in the singlet, thereby partially counteracting the bonding and dispersion components, which favor the singlet. This electrostatic stabilization of the triplet state is a key driving force of the spin transition of ethyl-SBP and a key factor explaining the paramagnetic response of the π-dimers of other SBP-based crystals