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)

Controlling pairing of π-conjugated electrons in 2D covalent organic radical frameworks via in-plane strain

Controlling the electronic states of molecules is a fundamental challenge for future sub-nanoscale device technologies. -conjugated bi-radicals are very attractive systems in this respect as they possess two energetically close, but optically and magnetically distinct, electronic states: the open-shell antiferromagnetic/paramagnetic and the closed-shell quinoidal diamagnetic states. While it has been shown that it is possible to statically induce one electronic ground state or the other by chemical design, the external dynamical control of these states in a rapid and reproducible manner still awaits experimental realization. Here, via quantum chemical calculations, we demonstrate that in-plane uniaxial strain of 2D covalently linked arrays of radical units leads to smooth and reversible conformational changes at the molecular scale that, in turn, induce robust transitions between the two kinds of electronic distributions. Our results pave a general route towards the external control, and thus technological exploitation, of molecular-scale electronic states in organic 2D materials. Controlling the electronic states of molecules is a fundamental challenge for future sub-nanoscale device technologies but the external dynamical control of these states still awaits experimental realization. Here, via quantum chemical calculations, the authors demonstrate that in-plane uniaxial strain of 2D covalently linked arrays of radical units induces controlled pairing of pi -conjugated electrons in a reversible way

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

Thermal spin crossover in Fe(ii) and Fe(iii). Accurate spin state energetics at the solid state

The thermal spin crossover (SCO) phenomenon refers to an entropy-driven spin transition in some materials based on d(6)-d(9) transition metal complexes. While its molecular origin is well known, intricate SCO behaviours are increasingly common, in which the spin transition occurs concomitantly to e.g. phase transformations, solvent absorption/desorption, or order-disorder processes. The computational modelling of such cases is challenging, as it requires accurate spin state energies in the solid state. Density Functional Theory (DFT) is the best framework, but most DFT functionals are unable to balance the spin state energies. While a few hybrid functionals perform better, they are still too expensive for solid-state minima searches in moderate-size systems. The best alternative is to dress cheap local (LDA) or semi-local (GGA) DFT functionals with a Hubbard-type correction (DFT+U). However, the parametrization of U is not straightforward due to the lack of reference values, and because ab initio parametrization methods perform poorly. Moreover, SCO complexes undergo notable structural changes upon transition, so intra- and inter-molecular interactions might play an important role in stabilizing either spin state. As a consequence, the U parameter depends strongly on the dispersion correction scheme that is used. In this paper, we parametrize U for nine reported SCO compounds (five based on Fe-II, 1-5 and four based on Fe-III, 6-9) when using the D3 and D3-BJ dispersion corrections. We analyze the impact of the dispersion correction treatments on the SCO energetics, structure, and the unit cell dimensions. The average U values are different for each type of metal ion (Fe(II)vs. Fe-III), and dispersion correction scheme (D3 vs. D3-BJ) but they all show excellent transferability, with mean absolute errors (MAE) below chemical accuracy (i.e. MAE <4 kJ mol(-1)). This enables a better description of SCO processes and, more generally, of spin state energetics, in materials containing Fe-II and Fe-III ions

Nanomechanics of bidentate thiolate ligands on gold surfaces

The effect of the chain length separating sulfur atoms in bidentate thiols attached to defective gold surfaces on the rupture of the respective molecule-gold junctions has been studied computationally. Thermal desorption always yields cyclic disulfides. In contrast, mechanochemical desorption leads to cyclic gold complexes, where metal atoms are extracted from the surface and kept in tweezer-like arrangements by the sulfur atoms. This phenomenon is rationalized in terms of directional mechanical manipulation of Au-Au bonds and Au-S coordination numbers. Moreover, the flexibility of the chain is shown to crucially impact on the mechanical strength of the junction.Fil: Zoloff Michoff, Martin Eduardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; ArgentinaFil: Ribas Arino, Jordi. Ruhr Universität Bochum; Alemania. Universidad de Barcelona; EspañaFil: Marx, Dominik. Ruhr Universität Bochum; Alemani

Unraveling the role of water in the stereoselective step of aqueous proline-catalyzed aldol reactions

A multiscale computational study was performed with the aim of tracing the source of stereoselectivity and disclosing the role of water in the stereoselective step of propionaldehyde aldol self-condensation catalyzed by proline amide in water, a reaction that serves as a model for aqueous organocatalytic aldol condensations. Solvent mixing and hydration behavior were assessed by classical molecular dynamics simulations, which show that the reaction between propanal and the corresponding enamine takes place in a fully hydrated environment. First-principles molecular dynamics simulations were used to study the free-energy profile of four possible reaction paths, each of which yields a different stereoisomer, and high-level static first-principles calculations were employed to characterize the transition states for microsolvated species. The first solvation shell of the oxygen atom of the electrophilic aldehyde at the transition states contains two water molecules, each of which donates one hydrogen bond to the nascent alkoxide and thereby largely stabilizes its excess electron density. The stereoselectivity originates in an extra hydrogen bond donated by the amido group of proline amide in two reaction paths

Structural and magnetic diversity based on different imidazolate linkers in Cu(II)-azido coordination compounds

This Article reports the syntheses, structural characterization, and magnetic studies of four different Cu(II)-azido compounds based on imidazole or substituted imidazole ligand. The compounds, [Cu<SUB>2</SUB>(μ<SUB>1,1</SUB>-N<SUB>3</SUB>)<SUB>2</SUB>(EtimiH)<SUB>4</SUB>(ClO<SUB>4</SUB>)<SUB>2</SUB>] (1) (EtimiH = 2-ethylimidazole), [Cu<SUB>2</SUB>(&#x00B5;-Meimi–)(MeimiH)<SUB>2</SUB>(&#x00B5;<SUB>1,1</SUB>-N<SUB>3</SUB>)<SUB>2</SUB>(&#x00B5;<SUB>1,3</SUB>-N<SUB>3</SUB>)]<SUB>n</SUB> (2) (MeimiH = 2-methylimidazole; &#x00B5;-Meimi– is the bridging mononegative anion of 2-methylimidazole), [Cu<SUB>2</SUB>(&#x00B5;-imi–)(imiH)<SUB>2</SUB>(&#x00B5;<SUB>1,1</SUB>-N<SUB>3</SUB>)<SUB>2</SUB>(&#x00B5;<SUB>1,3</SUB>-N<SUB>3</SUB>)]<SUB>n</SUB> (3), and [{Cu<SUB>2</SUB>(&#x00B5;<SUB>1,1</SUB>-N<SUB>3</SUB>)<SUB>2</SUB>(&#x00B5;<SUB>1,3</SUB>-N<SUB>3</SUB>)(&#x00B5;-imi–)(imiH)<SUB>3</SUB>}·H<SUB>2</SUB>O]<SUB>n</SUB> (4) (imiH = imidazole; &#x00B5;-imi– = bridging mononegative anion of imidazole), have been synthesized by the self-assembly of Cu(II) salts, azide ion, and the corresponding imidazole bridging ligands. By changing the substitution on the second linker (imidazole or substituted imidazole) and varying synthetic conditions, diverse structural and magnetic features have been achieved in compounds 1–4. Compound 1 has a double end-on azido bridged dinuclear core, while the other compounds (2–4) have 2D networks. Compound 2 and 3 contain 1D chains with alternate &#x00B5;<SUB>1,1</SUB>-N<SUB>3</SUB> and &#x00B5;-Meimi– bridging, and such chains are further connected through a &#x00B5;<SUB>1,3</SUB>-N<SUB>3</SUB> bridge to result in the formation of the 2D network. Compound 4 is a novel 2D coordination polymer consisting of a zigzag 1D coordination chain having (&#x00B5;<SUB>1,3</SUB>-N<SUB>3</SUB>)<SUB>2</SUB>, &#x00B5;-imi–, and (&#x00B5;<SUB>1,3</SUB>-N<SUB>3</SUB>)<SUB>2</SUB> bridging groups and the chains undergo bridging through a &#x00B5;<SUB>1,3</SUB>-N<SUB>3</SUB> group resulting in the 2D network. Temperature dependent magnetic measurements show diverse magnetic properties of 1–4. Such versatile magnetic behaviors have been correlated to the respective bridging mode of azide and the corresponding imidazole bridging ligands

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