Quantum Chemical Modeling of Pressure‐Induced Spin Crossover in Octahedral Metal‐Ligand Complexes

Abstract

Spin state switching on external stimuli is a phenomenon with wide applicability, ranging from molecular electronics to gas activation in nanoporous frameworks. Here, we model the spin crossover as a function of the hydrostatic pressure in octahedrally coordinated transition metal centers by applying a field of effective nuclear forces that compress the molecule towards its centroid. For spin crossover in first-row transition metals coordinated by hydrogen, nitrogen, and carbon monoxide, we find the pressure required for spin transition to be a function of the ligand position in the spectrochemical sequence. While pressures on the order of 1 GPa are required to flip spins in homogeneously ligated octahedral sites, we demonstrate a fivefold decrease in spin transition pressure for the archetypal strong field ligand carbon monoxide in octahedrally coordinated Fe 2 + in [Fe(II)(NH 3 ) 5 CO] 2 + .Pressure-induced spin-flips of transition metal sites involve changes in Coulomb energy, closed shell repulsions, covalent bonding energy and crystal field energy. [1,2] Using the computational Extreme-Pressure PCM (XP-PCM) protocol, [3][4][5] it has been shown that high pressures cause drastic electronic rearrangements, causing first row transition metals with electronic configurations d n (4 � n � 8) to favor low spin configurations at high pressures. [6] In a metal-organic framework (MOF) with exposed Fe(II) sites, a distinct step as a function of atmospheric pressure has been observed in the adsorption isotherm of carbon monoxide, for which a cooperative spin-transition mechanism involving interacting iron centers in the MOF on introduction of the strong-field ligand carbon monoxide has been proposed. [7] In general, the use of MOFs in the separation of industrially relevant gases like hydrogen, nitrogen and carbon monoxide holds a lot of promise, due to the large surface areas, the thermal stability and the adjustability of various parameters of the MOFs. [8] The smallest building block of spin crossover systems is often an octahedrally coordinated transition metal center with its 3d orbitals split by the ligand field environment. Spin-flip at high pressures can be attributed to the increase in splitting of the 3d levels at the metal site such that the potential energy required to maintain a high spin configuration surpasses the spin pairing energy. [9] Using effective nuclear forces scaled by their distances from the molecular centroid we here model spin crossover in octahedral metal-ligand complexes as a function of hydrostatic pressure. We find the pressure required for spin transition to be a function of ligand position in the spectrochemical sequence [10] and demonstrate that the spin transition pressure can be tuned by an adequate choice of the ligand field. Finer quantum chemical effects at play in the process involve a change in the covalent binding energy, Pauli repulsion, and charge transfer as a function of pressure, as demonstrated with an Energy Decomposition Analysis (EDA) [11] scheme.Any external force on...

The Mechanism of Flex‐Activation in Mechanophores Revealed By Quantum Chemistry

“…In the future we plan to apply our methodology to a recently proposed mechanophore based on anthracene, [54] for which flex‐activation was tested but remained unsuccessful, with the aim of determining the optimal conditions (e. g. the pulling/deformation setup) for successful flex‐activation. Moreover, we plan to apply quantum mechanochemical models of pressure [55–58] to test the efficiency of flex‐activation in polymers, since experiments on flex‐activated mechanophores were performed under compression [24–26] . It will be insightful to juxtapose the influences of hydrostatic pressure and mechanical forces deforming the systems, which can be assumed to play a role in the process of flex‐activation.…”

Quantum chemical modeling of molecules under pressure

“…In most cases, the octahedral ligand fields described in ref. [58] were found to be unproblematic in this regard.…”

“…However, it has been found that the pressure required for spin transition is a function of the position of the ligand in the spectrochemical sequence (cf. Table 1), [58,62] with strong field ligands requiring less pressure to induce spin crossover in the central metal ion, which paves the way for a rational design and precise tailoring of transition metal complexes that show spin transition at a desired pressure. Following up on this idea, it was possible to decrease the pressure required for spin transition by an order of magnitude by tuning the ligand field.…”

“…Following up on this idea, it was possible to decrease the pressure required for spin transition by an order of magnitude by tuning the ligand field. [58] Interestingly, nitrogen ligands were found to flip from end-on coordination to side-on coordination if sufficiently high pressures are applied, which is a result of the concentric nature of the forces applied in HCFF (in analogy to G-FMPES) and the resulting effort of F I G U R E 4 A, Application of sufficiently high hydrostatic pressure in [Fe(II)(H 2 ) 6 ] 2+ with the Hydrostatic Compression Force Field (HCFF) approach leads to a switching of the spin state from high spin to low spin. The average metal-ligand distance decreases upon application of pressure (d P < d 0 ), whereas the ligand field splitting energy increases (Δ P > Δ 0 ).…”

“…Adapted with permission from ref. [58]. Copyright (2019) John Wiley and Sons the molecule to minimize its surface area.…”