Protonolysis

Protonolysis is the cleavage of a chemical bond by reaction with a proton, typically from a Brønsted acid. In inorganic and organometallic chemistry it most often refers to the reaction of a metal–carbon bond (M–R) or a metal–heteroatom bond (M–OR, M–NR2) with a proton, yielding a hydrocarbon (RH) and a regenerated metal species, such as a metal hydride (M–H) or a metal–hydroxide (M–OH).

General mechanism and scope: Protonolysis involves proton transfer to the bond, followed by heterolytic cleavage to release RH or ROH and generate a new metal fragment. The process can be concerted or stepwise and is influenced by factors such as acid strength, solvent, and the electronic and steric environment of the metal center. In organometallic catalysis, protonolysis often terminates a metal–carbon intermediate to release product and regenerate a reactive site (e.g., a metal–hydride or a different oxidation state complex).

Examples: A typical case is M–R reacting with H+, giving RH and M–H. Protonolysis of M–OR with H+ can yield ROH and M–OH. Depending on the system, protonolysis can also cleave other C–heteroatom bonds in coordinated substrates, producing the corresponding alcohols or related products and a protonated metal fragment.

Applications and context: Protonolysis is an important step in many catalytic cycles, including termination of polymerization processes and release of organic products from metal intermediates. It is distinct from hydrolysis, where water serves as the proton donor; protonolysis uses a Brønsted acid and is not inherently limited to aqueous media.