Local Effective Crystal Field Combined with Molecular Mechanics. Improved QM/MM Junction and Application to Fe(II) and Co(II) Complexes

Theoretical and computational methodologies for understanding coordination self-assembly complexes

This perspective highlights three theoretical and computational methods to capture the coordination self-assembly processes at the molecular level: quantum chemical modeling, molecular dynamics, and reaction network analysis. These methods cover the different scales from the metal-ligand bond to a more global aspect, and approaches that are best suited to understand the coordination self-assembly from different perspectives are introduced. Theoretical and numerical researches based on these methods are not merely ways of interpreting the experimental studies but complementary to them.

A quantum chemical model for a series of self-assembled nanocages: the origin of stability behind the coordination-driven formation of transition metal complexes up to [M12L24]24

Herein, we present a systematic computational model to study the electronic states and free energies of a self-assembled multi-metal complex series. By combining the previously developed model Hamiltonian approach for transition-metal complexes and the generalized Born model, the thermodynamics, optimized geometries, and electronic states of the [Pd12L24]24+ nanocage are revealed, together with [PdnLm]2n+ complex series. The effective model Hamiltonian is a theoretical method to obtain the d-electron wavefunction and potential energy including interaction energy between the transition-metal and ligands. In the present improvement, the electronic state on each transition-metal center is focused as a building unit and solved under the whole electronic field of the assembling system. We realize a reliable and systematic treatment of multi-transition-metal complexes with different sizes and charges. Consequently, our model could reproduce binding energies of the [PdnLm]2n+ complex series quantitatively as compared to density functional theory (DFT). Regarding free energy, we revealed that the assembling solute becomes unstable due to the electrostatic interaction, and effects of the solvent and counter anions mainly compensated it. Optimized geometries were also analysed. The local square-planar coordination structures around the palladium centres were characterized in the complex series. The relationships between the entire symmetrical geometries and the local coordination structures are also discussed. Finally, electronic structures of the [Pd12L24]24+ nanocage were well characterized as a single-determinant, where only dx2-y2 is unoccupied due to the ligand-field effect. We also found that the solvent polarized the electronic states of the Pd ions, whereas the counter anion suppressed the polarization. The present method realizes size-independent reliable and rapid computations, and therefore can be expected to further application studies on self-assembly dynamics.

Quantum-Admixture Model of High-Spin ↔ Low-Spin Transition for Ferrous Complex Molecules

A quantum-admixture model for the d(6) configuration ferrous complex molecules with the high-spin <--> low-spin transition has been established by using the unified crystal-field-coupling (UCFC) scheme. A general study has been made on the spin transition of octahedrally coordinated d(6) complexes, and a special application has been given to an Fe(II) compound Fe(II)(TRIM)(2)(PhCO(2))(ClO(4)). The results show the following: (i) The quantum picture of the spin transition of a d(6) system, such as Fe(II), is much more complex than a simple transition between the pure (5)T(2g) and (1)A(1g) states as usually understood. In practice, owing to spin-orbit coupling, spin is no longer a good quantum number and there is no longer a pure (5)T(2g) or (1)A(1g) state. Each of them splits into substates and each substate is a linear combination of various multiplets. The high-spin --> low-spin transition of an octahedrally coordinated d(6) ion is practically the crossover of the two lowest substates of (5)T(2g) at the critical point. (ii) At the spin-transition critical point the magnetic moment mu(eff) approximately 5.22 mu(B), which is obviously different from the simple average of the mu(eff) values of high-spin and low-spin states but near the saturation value. (iii) The calculation of the effective molecular magnetic moment mu(eff) for an octahedrally coordinated Fe(II) ion shows that the mu(eff)-T curve is in good agreement with Lemercier et al.'s experiment and both the low-spin value mu(eff) = 0.51 mu(B) and the high-spin value mu(eff) = 5.4 mu(B) are comparable with the experimental values 0.76 mu(B) and 5.4 mu(B), respectively. (iv) The T dependence of the crystal field parameter Dq in the spin-transition region is approximately linear.