TSpace Research Repository tspace.library.utoronto.ca Brønsted-Lowry Acid Strength of Metal Hydride and Dihydrogen Complexes Robert H. Morris Version Post-print/accepted manuscript Citation Morris, R. H. Chemical Reviews 2016, 116, 8588–8654. (published version) http:/doi.org//10.1021/acs.chemrev.5b00695 Copyright / License Publisher’s Statement This document is the Accepted Manuscript version of a Published Work that appeared in final form in Chemical Reviews, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://pubs.acs.org/doi/abs/10.1021/acs.chemrev.5b00695. How to cite TSpace items Always cite the published version, so the author(s) will receive recognition through services that track citation counts, e.g. Scopus. If you need to cite the page number of the author manuscript from TSpace because you cannot access the published version, then cite the TSpace version in addition to the published version using the permanent URI (handle) found on the record page. This article was made openly accessible by U of T Faculty. Please tell us how this access benefits you. Your story matters. Brønsted‐Lowry Acid Strength of Metal Hydride and Dihydrogen Complexes Robert H. Morris* Department of Chemistry, University of Toronto, 80 Saint George St. Toronto, Ontario, M5S3H6, Canada Transition metal hydride complexes are usually amphoteric, not only acting as hydride donors, but also as Brønsted‐Lowry acids. A simple additive ligand acidity constant equation (LAC for short) allows the LAC estimation of the acid dissociation constant Ka of diamagnetic transition metal hydride and dihydrogen complexes. It is remarkably successful in systematizing diverse reports of over 450 reactions of acids with metal complexes and bases with metal hydrides and dihydrogen complexes, including LAC catalytic cycles where these reactions are proposed or observed. There are links between pKa and THF DCM MeCN pKa , pKa , pKa for neutral and cationic acids. For the groups from chromium to nickel, tables are LAC provided that order the acidity of metal hydride and dihydrogen complexes from most acidic (pKa ‐18) LAC to least acidic (pKa 50). Figures are constructed showing metal acids above the solvent pKa scales and organic acids below in order to summarize a large amount of information. Acid‐base features are analyzed for catalysts from chromium to gold for ionic hydrogenations, bifunctional catalysts for hydrogen oxidation and evolution electrocatalysis, H/D exchange, olefin hydrogenation and isomerization, hydrogenation of ketones, aldehydes, imines, and carbon dioxide, hydrogenases and their model complexes, and palladium catalysts with hydride intermediates. CONTENTS 1 Introduction .......................................................................................................................................... 3 2 Experimental and theoretical methods for determining acid strengths of transition metal hydrides 4 2.1 Measurement of pKa for acids in solvents with medium to high dielectric constants ................. 5 2.1.1 Acetonitrile ............................................................................................................................ 5 2.1.2 Water and methanol ............................................................................................................. 8 2.1.3 Dimethylsulfoxide ................................................................................................................. 9 2.2 Low dielectric constant solvents ................................................................................................... 9 2.2.1 Tetrahydrofuran .................................................................................................................. 10 2.2.2 Dichloromethane ................................................................................................................ 11 2.2.3 pKa of dihydrogen. ............................................................................................................... 11 2.3 Comparing pKa values of acids in different solvents ................................................................... 11 3 Additive ligand acidity constant (LAC) method used for ordering acid strengths .............................. 12 4 Specific observations according to the group. .................................................................................... 14 4.1 Titanium group ............................................................................................................................ 14 4.2 Vanadium group .......................................................................................................................... 14 4.3 Chromium group ......................................................................................................................... 14 4.4 Manganese group ....................................................................................................................... 21 1 4.5 Iron group ................................................................................................................................... 26 4.5.1 Very acidic dihydrogen complexes ..................................................................................... 26 4.5.2 Hydrogenase and model complexes ................................................................................... 26 4.5.3 Phosphine carbonyl hydride complexes ............................................................................. 26 4.5.4 Cyclopentadienyl complexes ............................................................................................... 27 4.5.5 Complexes with nitrogen donors ........................................................................................ 30 4.5.6 Complexes with several phosphorus donors ...................................................................... 31 4.5.7 Neutral, weak hydride acids ................................................................................................ 32 4.5.8 Other group 8 hydride complexes. ..................................................................................... 33 4.5.9 Effect of the metal on acidity .............................................................................................. 33 4.6 Cobalt group ................................................................................................................................ 40 4.6.1 Cyclopentadienyl complexes ............................................................................................... 40 4.6.2 Dicationic hydride acids ...................................................................................................... 41 4.6.3 Monocationic hydride acids ................................................................................................ 42 4.6.4 Neutral and anionic hydride acids ...................................................................................... 45 4.7 Nickel group ................................................................................................................................ 51 4.7.1 Dicatonic hydride acids ....................................................................................................... 51 II + 4.7.2 Monocationic hydride acids [M HL4] ................................................................................. 53 4.7.3 Other complexes ................................................................................................................. 54 5 General observations of the Data in Section 4 ................................................................................... 59 6 Implications for catalytic processes .................................................................................................... 64 6.1 Ionic hydrogenation using dihydrogen gas ................................................................................. 64 6.1.1 Ketone and aldehyde hydrogenation ................................................................................. 65 6.1.2 Hydrogenolysis of alcohols ................................................................................................. 68 6.1.3 Water and aldehyde shift to hydrogen and carboxylic acid ............................................... 68 6.1.4 Carboxylic acid and ester hydrogenation............................................................................ 68 6.1.5 Imine and heterocycle hydrogenation ................................................................................ 69 6.1.6 Iminium and enamine hydrogenation ................................................................................ 73 6.1.7 Reductive amination ........................................................................................................... 75 6.1.8 Alkene and alkyne hydrogenation ...................................................................................... 76 6.2 Ionic transfer hydrogenation ...................................................................................................... 77 6.3 Hydrogen/deuterium exchange .................................................................................................. 78 6.4 Olefin hydrogenation vs isomerization ....................................................................................... 79 2 6.5 Carbon‐hydrogen bond functionalization ................................................................................... 79 6.6 Metal‐ligand