Theoretical Prediction of Spin-Crossover Temperatures in Ligand-Driven Light-Induced Spin Change Systems

High Oxidation State Iridium Mono-μ-oxo Dimers Related to Water Oxidation Catalysis

The highly active iridium "blue solution" chemical and electrochemical water oxidation catalyst obtained from Cp*IrL(OH) precursors (L = 2-pyridyl-2-propanoate) has been difficult to characterize as no crystal structure can be obtained because of the multiplicity of geometrical isomers present. Other data suggest complete loss of the Cp* ligand and the formation of a LIr-O-IrL unit. We have now developed a route to a series of well-defined Ir(IV,IV) mono-μ-oxo dimers, containing the closely related L₂Ir-O-IrL₂ unit. Unlike the catalyst, these model compounds are separable by silica gel chromatography and readily form single crystals. We report three stereoisomers with the formula ClL₂Ir-O-IrL₂Cl, which are fully characterized, including by X-ray crystallography, and are compared to the "blue solution". To the best of our knowledge, these species represent the first examples of structurally characterized dinuclear μ-oxo Ir(IV,IV) compounds without metal-carbon bonds.

Theoretical Study of Spin Crossover in 30 Iron Complexes

Iron complexes are important spin crossover (SCO) systems with vital roles in oxidative metabolism and promising technological potential. The SCO tendency depends on the free energy balance of high- and low-spin states, which again depends on physical effects such as dispersion, relativistic effects, and vibrational entropy. This work studied 30 different iron SCO systems with experimentally known thermochemical data, using 12 different density functionals. Remarkably general entropy-enthalpy compensation across SCO systems was identified (R = 0.82, p = 0.002) that should be considered in rational SCO design. Iron(II) complexes displayed higher ΔH and ΔS values than iron(III) complexes and also less steep compensation effects. First-coordination sphere ΔS values computed from numerical frequencies reproduce most of the experimental entropy and should thus be included when modeling spin-state changes in inorganic chemistry (R = 0.52, p = 3.4 × 10(-3); standard error in TΔS ≈ 4.4 kJ/mol at 298 K vs 16 kJ/mol of total TΔS on average). Zero-point energies favored high-spin states by 9 kJ/mol on average. Interestingly, dispersion effects are surprisingly large for the SCO process (average: 9 kJ/mol, but up to 33 kJ/mol) and favor the more compact low-spin state. Relativistic effects favor low-spin by ∼9 kJ/mol on average, but up to 24 kJ/mol. B3LYP*, TPSSh, B2PLYP, and PW6B95 performed best for the typical calculation scheme that includes ZPE. However, if relativistic and dispersion effects are included, only B3LYP* remained accurate. On average, high-spin was favored by LYP by 11-15 kJ/mol relative to other correlation functionals, and by 4.2 kJ/mol per 1% HF exchange in hybrids. 13% HF exchange was optimal without dispersion, and 15% was optimal with all effects included for these systems.

TD-DFT study of the light-induced spin crossover of Fe(iii) complexes

Two light-induced spin-crossover Fe(iii) compounds have been studied with time-dependent density functional theory (TD-DFT) to investigate the deactivation mechanism and the role of the ligand-field states as intermediates in this process.

Predicting the spin state of paramagnetic iron complexes by DFT calculation of proton NMR spectra

Many transition-metal complexes easily change their spin state S in response to external perturbations (spin crossover). Determining such states and their dynamics can play a central role in the understanding of useful properties such as molecular magnetism or catalytic behavior, but is often far from straightforward. In this work we demonstrate that, at a moderate computational cost, density functional calculations can predict the correct ground spin state of Fe(ii) and Fe(iii) complexes and can then be used to determine the (1)H NMR spectra of all spin states. Since the spectral features are remarkably different according to the spin state, calculated (1)H NMR resonances can be used to infer the correct spin state, along with supporting the structure elucidation of numerous paramagnetic complexes.

Charge transport properties of spin crossover systems

The study of spin crossover compounds by means of theoretical or experimental approaches has provided interesting results in recent decades. The main feature of such compounds is the change in the spin state induced by many different external stimuli, i.e. temperature, light, pressure, solvent coordination and the electric field. Spin crossover systems are potentially more useful than other magnetic molecules because their switching behaviour can occur closer to room temperature, and they are thus candidates for use in spintronic devices. Here, I review the state of the art in quantum chemical approaches to the study of such systems and discuss experiments that have focused on transport properties in single-molecule, nano-objects or thin-film spin crossover systems.