Deciphering the origin of variation in the spin ground state and oxidation state of a {Mn19} cluster on a Au(111) surface: is the Au(111) surface innocent?

Surface Stabilization to Enhance Single Molecule Toroidal Behavior in {Dy3} Molecules: the Impact of Au(111), MgO, and Graphene Surfaces

Single-molecule toroids (SMTs), with vortex-like magnetic anisotropy axes, hold promise for quantum technologies, but controlling their toroidal states on the surface remains challenging. To address this, the SMT behavior of [Dy3(µ3-OH)2L3Cl(H2O)5]Cl3 (where L = ortho-vanillin) grafted onto Au(111), MgO has been studied, and graphene surfaces in pristine form (1) and with pyrene (2) and (CH2)8S (3) linkers, using periodic density functional theory and ab initio CASSCF/RASSI-SO methods. Both pristine and chemically functionalized molecules are stable on Au(111) and graphene surfaces; however, functionalization provides higher binding energies and, in some cases, enhances the SMT properties. The MgO surface, however, is found to be unsuitable as it abstracts an H atom from the molecule, leading to the loss of its SMT characteristics. The energy gap (ΔE) between the toroidal (nonmagnetic) and spin-flip (magnetic) states in complex 1 on Au(111) and graphene surfaces are 6.9 and 6.6 cm-1, respectively. Complexes 2 on Au(111) and 3 on graphene exhibit ΔE and toroidal blocking fields of 9.8 cm-1/1.2 T and 6.8 cm-1/0.83 T, respectively, representing the highest recorded values for this class of SMTs. These findings demonstrate the potential of surface stabilization to improve the functionality and applicability of SMTs in advanced quantum technologies.

Enhancing Spin-Transport Characteristics, Spin-Filtering Efficiency, and Negative Differential Resistance in Exchange-Coupled Dinuclear Co(II) Complexes for Molecular Spintronics Applications

Single-molecule spintronics, where electron transport occurs via a paramagnetic molecule, has gained wide attention due to its potential applications in the area of memory devices to switches. While numerous organic and some inorganic complexes have been employed over the years, there are only a few attempts to employ exchange coupled dinuclear complexes at the interface, and the advantage of fabricating such a molecular spintronics device in the observation of switchable Kondo resonance was demonstrated recently in the dinuclear [Co2(L)(hfac)4] (1) complex (Wagner et al., Nat. Nanotechnol. 2013, 8, 575-579). In this work, employing an array of theoretical tools such as density functional theory (DFT), the ab initio CASSCF/NEVPT2 method, and DFT combined with nonequilibrium Green Function (NEGF) formalism, we studied in detail the role of magnetic coupling, ligand field, and magnetic anisotropy in the transport characteristics of complex 1. Particularly, our calculations not only reproduce the current-voltage (I-V) characteristics observed in experiments but also unequivocally establish that these arise from an exchange-coupled singlet state that arises due to antiferromagnetic coupling between two high-spin Co(II) centers. Further, the estimated spin Hamiltonian parameters such as J, g values, and D and E/D values are only marginally altered for the molecule at the interface. Further, the exchange-coupled state was found to have very similar transport responses, despite possessing significantly different geometries. Our transport calculations unveil a new feature of the negative differential resistance (NDR) effect on 1 at the bias voltage of 0.9 V, which agrees with the experimental I-V characteristics reported. The spin-filtering efficiency (SFE) computed for the spin-coupled states was found to be only marginal (∼25%); however, if the ligand field is fine-tuned to obtain a low-spin Co(II) center, a substantial SFE of 44% was noted. This spin-coupled state also yields a very strong NDR with a peak-to-valley ratio (PVR) of ∼56 - a record number that has not been witnessed so far in this class of compounds. Additionally, we have established further magnetostructural-transport correlations, providing valuable insights into how microscopic spin Hamiltonian parameters can be associated with SFE. Several design clues to improve the spin-transport characteristics, SFE and NDR in this class of molecule, are offered.

A Theoretical Perspective to Decipher the Origin of High Hydrogen Storage Capacity in Mn(II) Metal-Organic Framework

Herein, we report a detailed periodic DFT investigation of Mn(II)-based [(Mn₄ Cl)₃ (BTT)₈ ]^3- (BTT^3- =1,3,5-benzenetristetrazolate) metal-organic framework (MOF) to explore various hydrogen binding pockets, nature of MOF…H₂ interactions, magnetic coupling and, H₂ uptake capacity. Earlier experiments found an uptake capacity of 6.9 wt % of H_2, with the heat of adsorption estimated to be ∼10 kJ/mol, which is one among the highest for any MOFs reported. Our calculations unveil different binding sites with computed binding energy varying from -6 to -15 kJ/mol. The binding of H₂ at the Mn²⁺ site is found to be the strongest (site I), with H₂ found to bind Mn²⁺ ion in a η² fashion with a distance of 2.27 Å and binding energy of -15.4 kJ/mol. The bonding analysis performed using NBO and AIM reveal a strong donation of σ (H₂ ) to the d_z ² orbital of the Mn²⁺ ion responsible for such large binding energy. The other binding pockets, such as -Cl (site II) and BTT ligands (site III and IV) were found to be weaker, with the binding energy decreasing in the order I>II>III>IV. The average binding energy computed for these four sites put together is 9.6 kJ/mol, which is in excellent agreement with the experimental value of ∼10 kJ/mol. We have expanded our calculations to compute binding energy for multiple sites simultaneously, and in this model, the binding energy per site was found to decrease as we increased the number of H₂ molecules suggesting electronic and steric factors controlling the overall uptake capacity. The calculated adsorption isotherm using the GCMC method reproduces the experimental observations. Further, the magnetic coupling computed for the unbound MOF reveals moderate ferromagnetic and strong antiferromagnetic coupling within the tetrameric {Mn₄ } unit leading to a three-up-one-down spin configuration as the ground state. These were then coupled ferromagnetically to other tetrameric units in the MOF network. The magnetic coupling was found to alter only marginally upon gas binding, suggesting that both exchange interaction and the spin-states are unlikely to play a role in the H₂ uptake. This is contrary to the O₂ uptake studied lately, where strong dependence on exchange-coupling/spin state was witnessed, suggesting exchange-coupling/magnetic field dependent binding as a viable route for gas separation.

Chiral-Induced Spin Selectivity and Non-equilibrium Spin Accumulation in Molecules and Interfaces: A First-Principles Study

Electrons moving through chiral molecules are selected according to their spin orientation and the helicity of the molecule, an effect known as chiral-induced spin selectivity (CISS). The underlying physical mechanism is not yet completely understood. To help elucidate this mechanism, a non-equilibrium Green's function method, combined with a Landauer approach and density functional theory, is applied to carbon helices contacted by gold electrodes, resulting in spin polarization of transmitted electrons. Spin polarization is also observed in the non-equilibrium electronic structure of the junctions. While this spin polarization is small, its sign changes with the direction of the current and with the handedness of the molecule. While these calculations were performed with a pure exchange-correlation functional, previous studies suggest that computationally more expensive hybrid functionals may lead to considerably larger spin polarization in the electronic structure. Thus, non-equilibrium spin polarization could be a key component in understanding the CISS mechanism.

Record High Magnetic Anisotropy in Three-Coordinate MnIII and CrII Complexes: A Theoretical Perspective

Ab initio calculations performed in two three-coordinate complexes [Mn{N(SiMe₃)₂}₃] (1) and [K(18-crown-6) (Et₂O)₂][Cr{N(SiMe₃)₂}₃] (2) reveal record-high magnetic anisotropy with the D values -64 and -15 cm^-1, respectively, enlisting d⁴ ions back in the race for single-ion magnets. A detailed spin-vibrational analysis performed of 1 and 2 suggests dominance under barrier relaxation due to the flexible coordination spheres around the metal ion. Furthermore, several in silico models were constructed by varying the nature of donor atoms based on the X-ray structure of 1 and 2, unveiling much larger anisotropy and robust single-ion magnet (SIM) characteristics for some of the models offering design clues for low-coordinate transition-metal SIMs.

Role of Ab Initio Calculations in the Design and Development of Organometallic Lanthanide-Based Single-Molecule Magnets

Single-molecule magnets based on lanthanides are very attractive due to their potential applications proposed in the area of microelectronic devices. Very recent advances in this area are due to the blend of conventional lanthanide chemistry with organometallic ligands, and several breakthrough achievements are attained with this combination. Ab initio methods based on multi-reference CASSCF calculations are playing a vital role in the design and development of such molecules. In this minireview, we aim to appraise various contributions in the area of organometallic lanthanide complexes (those containing lanthanide-carbon bonds) and describe how these robust wavefunction-based methods have played a constructive role not only in rationalizing the observed magnetic properties but also proven to be a potential predictive tool with some selected examples.