Multireference Ground and Excited State Electronic Structures of Free- versus Iron Porphyrin-Carbenes

26 December 2019, Version 2
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Iron porphyrin carbenes (IPCs) are important reaction intermediates in engineered carbene transferase enzymes and homogeneous catalysis. However, discrepancies between theory and experiment complicate the understanding of IPC electronic structure (i.e., open- vs. closed-shell singlet (OSS vs. CSS)). Here we investigate the structurally dependent ground and excited spin state energetics of a free carbene and its IPC analogs. Only multireference ab initio wave function methods are consistent with experiment and predict a CSS ground state (Fe(II)←{:C(X)Y}0), contrary to density functional theory (DFT). The OSS is a high-lying metal-to-ligand charge transfer (MLCT) excited state that is sensitive to the nature of the axial ligand. Furthermore, potential energy surfaces (PESs) along the Fe–C bond elongation coordinate exhibit strong mixings between CSS/OSS characters, which can be an important feature for describing reaction mechanisms. Future studies on IPC reaction coordinates should evaluate contributions from ground and excited state multireference character.

Keywords

Carbenes
Catalysis
Density Functional Calculations
Iron Carbenes
Multireference ab initio calculations
Potential Energy Surfaces
Spin State Energetics

Supplementary materials

Title
Description
Actions
Title
IPC SI CR
Description
Actions
Title
IPC Structures CR
Description
Actions
Title
IPC ActiveSpacesOccupancies CR
Description
Actions

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