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

Stroscio, Gautam D. and Srnec, Martin and Hadt, Ryan G. (2020) Multireference Ground and Excited State Electronic Structures of Free- versus Iron Porphyrin-Carbenes. Inorganic Chemistry, 59 (13). pp. 8707-8715. ISSN 0020-1669.

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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. In the literature, this has been framed as whether the ground state is an 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 with variable trans axial ligands. In particular, for IPCs, multireference ab initio wave function methods are more consistent with experiment and predict a mixed singlet ground state that is dominated by the CSS (Fe(II) ← {:C(X)Y}0) configuration (i.e., electrophilic carbene) but that also has a small, non-negligible contribution from an Fe(III)–{C(X)Y}−• configuration (hole in d(xz), i.e., radical carbene). In the multireference approach, the “OSS-like” excited states are metal-to-ligand charge transfer (MLCT) in nature and are energetically well above the CSS-dominated ground state. The first, lowest energy of these “OSS-like” excited states is predicted to be heavily weighted toward the Fe(III)–{C(X)Y}−• (hole in d(yz)) configuration. As expected from exchange considerations, this state falls energetically above a triplet of the same configuration. Furthermore, potential energy surfaces (PESs) along the IPC Fe–C(carbene) bond elongation exhibit increasingly strong mixings between CSS/OSS characters, with the Fe(III)–{C(X)Y}−• configuration (hole in d(xz)) growing in weight in the ground state during bond elongation. The relative degree of electrophilic/radical carbene character along this structurally relevant PES can potentially play a role in reactivity and selectivity patterns in catalysis. Future studies on IPC reaction coordinates should evaluate contributions from ground and excited state multireference character.

Item Type:Article
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URLURL TypeDescription Paper
Stroscio, Gautam D.0000-0002-0827-1062
Hadt, Ryan G.0000-0001-6026-1358
Additional Information:© 2020 American Chemical Society. Received: January 23, 2020; Published: June 8, 2020. We acknowledge Dr. Ondřej Demel for use of the DLPNO-MR-CCSD program within the ORCA package, Dr. Daniel Bı́m, Dr. Alec H. Follmer, and Mr. Ryan D. Ribson for helpful discussions, and the reviewers for critical comments during manuscript revision. G.D.S. thanks Dr. Mauricio Maldonado-Domı́nguez for guidance during early calculations. R.G.H gratefully acknowledges financial support from Caltech and the Dow Next Generation Educator Fund. The authors declare no competing financial interest.
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Dow Next Generation Educator FundUNSPECIFIED
Issue or Number:13
Record Number:CaltechAUTHORS:20200129-130658853
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Official Citation:Multireference Ground and Excited State Electronic Structures of Free- versus Iron Porphyrin-Carbenes. Gautam D. Stroscio, Martin Srnec, and Ryan G. Hadt. Inorganic Chemistry 2020 59 (13), 8707-8715; DOI: 10.1021/acs.inorgchem.0c00249
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:100989
Deposited By: Tony Diaz
Deposited On:29 Jan 2020 21:45
Last Modified:25 Jun 2021 18:01

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