Chelate Complexes of 3d Transition Metal Ions – A Challenge for Electronic-Structure Methods?

20 May 2024, Version 2
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Different electronic-structure methods were assessed for their ability to predict two important properties of the industrially relevant chelating agent nitrilotriacetic acid (NTA): its selectivity with respect to six different first-row transition metal ions and the spin-state energetics of its complex with Fe(III). The investigated methods encompassed density functional theory (DFT), the random phase approximation (RPA), coupled cluster (CC) theory, the auxiliary-field quantum Monte-Carlo (AFQMC) method, as well as the complete active space self-consistent field (CASSCF) method and the respective on-top methods second-order N-electron valence state perturbation theory (NEVPT2) and multiconfiguration pair-density functional theory (MC-PDFT). Different strategies for selecting active spaces were explored and the density matrix renormalization group (DMRG) approach was used to solve the largest active spaces. Despite somewhat ambiguous multi-reference diagnostics, most methods gave relatively good agreement with experimental data for the chemical reactions connected to the selectivity, which only involved transition-metal complexes in their high-spin state. CC methods yielded the highest accuracy followed by range-separated DFT and AFQMC. We discussed in detail that even higher accuracies can be obtained with NEVPT2, under the prerequisite that consistent active spaces along the entire chemical reaction can be selected, which was not the case for reactions involving Fe(III). A bigger challenge for electronic-structure methods was the prediction of the spin-state energetics, which additionally involved lower spin states that exhibited larger multi-reference diagnostics. Conceptually different, typically accurate methods ranging from CC theory via DMRG-NEVPT2 in combination with large active spaces to AFQMC agreed well that the high-spin state is energetically significantly favored over the other spin states. This was in contrast to most DFT functionals and RPA which yielded a smaller stabilization and some common DFT functionals and MC-PDFT even predicting the low-spin state to be energetically most favorable.

Keywords

Transition Metals
Chelating Agents
Multi-Reference Active Space Methods
Quantum Monte Carlo
Density Matrix Renormalization Group Theory
Coupled Cluster
Density Functional Theory
Random Phase Approximation

Supplementary materials

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Supporting Information
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CBS extrapolation; frozen core approximation; influence of HF exchange in DFT on spin-state energetics; energies of reactions with varying amounts of water ligands; geometrical structure of Fe(III)-hexaaqua; adding TM-ligand covalency to active space for the selectivity; angles in Fe(III)-NTA; orientation of Fe(III)-NTA; spin-state energetics of Fe(III)-NTA for all methods; experimental Gibbs free energies for the selectivity; absolute energies of optimized structures for each spin state and all TM-NTA complexes; Gibbs free energies for addition of second water ligand to TM-NTA complexes; Gibbs free energy corrections from thermodynamical analysis and solvation corrections for selectivity; Gibbs free energies for selectivity using CCSD(T) in combination with ROHF and UHF orbitals; DLPNO-CCSD(T1) results for the selectivity; DLPNO-CCSD(T), DLPNO-CCSD(T)TPSS, and AFQMC results for the selectivity using different basis sets; MC-PDFT results for selectivity using fully-translated functionals; isosurface plots of orbitals of Mn(II)-NTA selected by SOE; RMSEs and MaxAEs for selectivity for all methods; coordinates
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Supporting Information 2
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Total energies in Hartree for all methods and systems, Gibbs free energies for the selectivity in kJ/mol
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