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Morris RH. Reactivity umpolung (reversal) of ligands in transition metal complexes. Chem Soc Rev 2024; 53:2808-2827. [PMID: 38353155 DOI: 10.1039/d3cs00979c] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 03/19/2024]
Abstract
The success and power of homogeneous catalysis derives in large part from the wide choice of transition metal ions and their ligands. This tutorial review introduces examples where the reactivity of a ligand is completely reversed (umpolung) from Lewis basic/nucleophilic to acidic/electrophilic or vice versa on changing the metal and co-ligands. Understanding this phenomenon will assist in the rational design of catalysts and the understanding of metalloenzyme mechanisms. Labelling a metal and ligand with Seebach donor and acceptor labels helps to identify whether a reaction involving the intermolecular attack on the ligand is displaying native reactivity or reactivity umpolung. This has been done for complexes of nitriles, carbonyls, isonitriles, dinitrogen, Fischer carbenes, alkenes, alkynes, hydrides, methyls, methylidenes and alkylidenes, silylenes, oxides, imides/nitrenes, alkylidynes, methylidynes, and nitrides. The electronic influence of the metal and co-ligands is discussed in terms of the energy of (HOMO) d electrons. The energy can be related to the pKLACa (LAC is ligand acidity constant) of the theoretical hydride complexes [H-[M]-L]+ formed by the protonation of pair of valence d electrons on the metal in the [M-L] complex. Preliminary findings indicate that a negative pKLACa indicates that nucleophilic attack by a carbanion or amine on the ligand will likely occur while a positive pKLACa indicates that electrophilic attack by strong acids on the ligand will usually occur when the ligand is nitrile, carbonyl, isonitrile, alkene and η6-arene.
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Affiliation(s)
- Robert H Morris
- Department of Chemistry, University of Toronto, 80 Saint George St., Toronto, Ontario, Canada, M5S3H6.
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Rennie BE, Price JS, Emslie DJH, Morris RH. Trans Ligand Determines the Stability of Paramagnetic Manganese(II) Hydrides of the Type trans-[MnH(L)(dmpe) 2] + Where L is PMe 3, C 2H 4, or CO. Inorg Chem 2023; 62:8123-8135. [PMID: 36812512 DOI: 10.1021/acs.inorgchem.2c04432] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/24/2023]
Abstract
Paramagnetic metal hydride (PMH) complexes play important roles in catalytic applications and bioinorganic chemistry. 3d PMH chemistry has largely focused on Ti, Mn, Fe, and Co. Various MnII PMHs have been proposed as intermediates in catalysis, but isolated MnII PMHs are limited to dimeric high-spin MnII structures with bridging hydrides. In this paper, a series of the first low-spin monomeric MnII PMH complexes are generated by chemical oxidation of their MnI analogues. This series is of the type trans-[MnH(L)(dmpe)2]+/0 where the trans ligand L is PMe3, C2H4, or CO [dmpe is 1,2-bis(dimethylphosphino)ethane], and the thermal stability of the MnII hydride complexes was found to be strongly dependent on the identity of the trans ligand. When L is PMe3, the complex is the first example of an isolated monomeric MnII hydride complex. In contrast, when L is C2H4 or CO, the complexes are only stable at low temperatures; upon warming to room temperature, the former decomposed to afford [Mn(dmpe)3]+, accompanied by ethane and ethylene, whereas the latter eliminated H2, generating [Mn(MeCN)(CO)(dmpe)2]+ or a mixture of products including [Mn(κ1-PF6)(CO)(dmpe)2], depending on the reaction conditions. All PMHs were characterized by low-temperature electron paramagnetic resonance (EPR) spectroscopy, and stable [MnH(PMe3)(dmpe)2]+ was further characterized by UV-vis and IR spectroscopy, Superconducting Quantum Interference Device magnetometry, and single-crystal X-ray diffraction. Noteworthy spectral properties are the significant EPR superhyperfine coupling to the hydride (∼85 MHz) and an increase (+33 cm-1) in the Mn-H IR stretch upon oxidation. Density functional theory calculations were also employed to gain insights into the acidity and bond strengths of the complexes. MnII-H bond dissociation free energies are estimated to decrease in the series of complexes from 60 (L = PMe3) to 47 kcal/mol (L = CO).
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Affiliation(s)
- Benjamin E Rennie
- Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, Ontario M5S3H6, Canada
| | - Jeffrey S Price
- Department of Chemistry, McMaster University, Hamilton, Ontario L8S4M1, Canada
| | - David J H Emslie
- Department of Chemistry, McMaster University, Hamilton, Ontario L8S4M1, Canada
| | - Robert H Morris
- Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, Ontario M5S3H6, Canada
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Dutschke PD, Tsui BTH, von Bremen-Kühne M, Morris RH, Hahn FE. Methanol-Mediated Formation of an Iridium(III) NHC/Azolato Chelate Complex: An Experimental and Theoretical Study. Organometallics 2022. [DOI: 10.1021/acs.organomet.2c00489] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/02/2022]
Affiliation(s)
- Patrick D. Dutschke
- Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstraße 30, Münster D-48149, Germany
| | - Brian Tsz Ho Tsui
- Department of Chemistry, University of Toronto, 89 Saint George St. Toronto, Ontario M5S 3H6, Canada
| | - Maximilian von Bremen-Kühne
- Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstraße 30, Münster D-48149, Germany
| | - Robert H. Morris
- Department of Chemistry, University of Toronto, 89 Saint George St. Toronto, Ontario M5S 3H6, Canada
| | - F. Ekkehardt Hahn
- Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstraße 30, Münster D-48149, Germany
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Seo CSG, Tsui BTH, Gradiski MV, Smith SAM, Morris RH. Enantioselective direct, base-free hydrogenation of ketones by a manganese amido complex of a homochiral, unsymmetrical P–N–P′ ligand. Catal Sci Technol 2021. [DOI: 10.1039/d1cy00446h] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
Base-free direct hydrogenation of ketones using a Mn(PNP′)(CO)2 complex is more enantioselective than that of a related base-activated iron complex.
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Rennie BE, Eleftheriades RG, Morris RH. Systematic Trends in the Electrochemical Properties of Transition Metal Hydride Complexes Discovered by Using the Ligand Acidity Constant Equation. J Am Chem Soc 2020; 142:17607-17629. [PMID: 32941024 DOI: 10.1021/jacs.0c08000] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Understanding the thermodynamics of paramagnetic transition metal hydride complexes, especially of the abundant 3d metals, is important in the design of electrocatalysts and organometallic catalysts. The pKaMeCN([MHLn]+/[MLn) of paramagnetic hydrides in MeCN are estimated for the first time using the ligand acidity constant (LAC) equation where contributions to the pKaMeCN from each ligand are simply added together, with the sum corrected for effects of charge and 5d metals. The pKaLAC-MeCN([MHLn]+/MLn) of over 200 hydride complexes MHLn are used, along with their electrochemical potentials from the literature, in an uncommonly applied thermochemical cycle in order to reveal systematic trends in the redox couples MIII/II and MV/IV (M = Cr, Mo, W), MnII/I, ReVI/V and ReIV/III, MIII/II and MIV/III (M = Fe, Ru, Os), and MIII/II and MII/I (M = Co, Rh, and Ir) and allow the estimation of the bond dissociation free energies BDFE(MH) of the unoxidized hydrides MHLn and the prediction of the electrochemical potential for their oxidation. Density functional theory (DFT) calculations are used to validate the pKaLAC-MeCN values of hydrides of WIII, MnII, FeIII, RuIII, CoII, and NiIII. When a pKaLAC-MeCN is less than zero for a given complex [MHLn]+, the oxidation of MHLn is irreversible due to proton loss from the oxidized complex to the solvent. When pKaLAC-MeCN ≫ 0, the oxidation is reversible when there is no gross change in the coordination geometry upon a change in the redox state. Twenty paramagnetic hydrides prepared in bulk all have pKaLAC-MeCN > 8.
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Affiliation(s)
- Benjamin E Rennie
- Department of Chemistry, University of Toronto, 80 Saint George St., Toronto, Ontario M5S3H6, Canada
| | - Renée G Eleftheriades
- Department of Chemistry, University of Toronto, 80 Saint George St., Toronto, Ontario M5S3H6, Canada
| | - Robert H Morris
- Department of Chemistry, University of Toronto, 80 Saint George St., Toronto, Ontario M5S3H6, Canada
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Sung MMH, Prokopchuk DE, Morris RH. Phosphine-free ruthenium NCN-ligand complexes and their use in catalytic CO 2 hydrogenation. Dalton Trans 2019; 48:16569-16577. [PMID: 31560363 DOI: 10.1039/c9dt03143j] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
This work investigates the hydrogenation of carbon dioxide to formate catalysed by the phosphine-free Ru complexes Ru(OtBu)(κ3-NCN)(tBubpy) and RuH(OtBu)(κ2-NCN)(tBubpy) (OtBu = tert-butoxide, κ2-NCN = 1,3-di(2-methylpyridyl)-4,5-diphenyl-1H-imidazol-2-ylidene, where one pyridyl moiety is not coordinated to Ru, tBubpy = 4,4'-di-tert-butyl-2,2'-dipyridyl). A catalytic cycle is proposed for this reaction, supported by computational studies and the characterization of the hydride and the formate intermediates proposed to be involved. Modest catalytic turnovers are demonstrated at relatively low pressures and temperatures. The proposed rate determining step is heterolytic H2 splitting to regenerate the Ru-H complex, which has an estimated hydricity of approx. 27 kcal mol-1. The κ2-NCN ligand in the hydride complex undergoes a variety of dynamic processes as detected by EXSY spectroscopy including a pyridyl "roll-over" carbon-hydrogen - ruthenium hydride exchange, possibly occuring via a Perutz-Sabo-Etienne CAM mechanism.
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Affiliation(s)
- Molly M H Sung
- Department of Chemistry, University of Toronto, 80 Saint George St., Toronto. Ont. M5S3H6, Canada.
| | - Demyan E Prokopchuk
- Department of Chemistry, Rutgers University, 73 Warren Street, Newark, NJ 07102, USA
| | - Robert H Morris
- Department of Chemistry, University of Toronto, 80 Saint George St., Toronto. Ont. M5S3H6, Canada.
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Barlow J, Yang JY. Thermodynamic Considerations for Optimizing Selective CO 2 Reduction by Molecular Catalysts. ACS CENTRAL SCIENCE 2019; 5:580-588. [PMID: 31041377 PMCID: PMC6487447 DOI: 10.1021/acscentsci.9b00095] [Citation(s) in RCA: 66] [Impact Index Per Article: 13.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/30/2019] [Indexed: 05/17/2023]
Abstract
Energetically efficient electrocatalysts with high product selectivity are desirable targets for sustainable chemical fuel generation using renewable electricity. Recycling CO2 by reduction to more energy dense products would support a carbon-neutral cycle that mitigates the intermittency of renewable energy sources. Conversion of CO2 to more saturated products typically requires proton equivalents. Complications with product selectivity stem from competitive reactions between H+ or CO2 at shared intermediates. We describe generalized catalytic cycles for H2, CO, and HCO2 - formation that are commonly proposed in inorganic molecular catalysts. Thermodynamic considerations and trends for the reactions of H+ or CO2 at key intermediates are outlined. A quantitative understanding of intermediate catalytic steps is key to designing systems that display high selectivity while promoting energetically efficient catalysis by minimizing the overall energy landscape. For CO2 reduction to CO, we describe how an enzymatic active site motif facilitates efficient and selective catalysis and highlight relevant examples from synthetic systems.
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