Thermodynamic and kinetic hydricities of metal-free hydrides

Abstract

Metal-free hydrides are of increasing research interest due to their roles in recent scientific advances in catalysis, such as hydrogen activation with frustrated Lewis pairs and electrocatalytic CO₂ reduction with pyridinium and other aromatic cations. The structural design of hydrides for specific applications necessitates the correct description of their thermodynamic and kinetic prowess using reliable parameters – thermodynamic hydricity (ΔG_H⁻) and nucleophilicity (N). This review summarizes reported experimental and calculated hydricity values for more than 200 metal-free hydride donors, including carbon-, boron-, nitrogen- and silicon-based hydrides. We describe different experimental and computational methods used to obtain these thermodynamic and kinetic parameters. Furthermore, tabulated data on metal-free hydrides are discussed in terms of structure–property relationships, relevance to catalysis and contemporary limitations for replacing transition-metal hydride catalysts. Finally, several selected applications of metal-free hydrides in catalysis are described, including photosynthetic CO₂ reduction and hydrogen activation with frustrated Lewis pairs.