Phenol was discovered in 1834 by Friedlieb Ferdinand Runge.... Injections of phenol were used as a means of individual execution by the Nazis during the Second World War. It was originally used by the Nazis in 1939 as part of Action T4.
Thus saith Wikipedia.
Was this the first known use of phenol to purposefully execute individuals? I am not only asking about its use as an official means of execution, but about anybody who purposefully used phenol to execute an individual, whether sanctioned by a government or not.