Sniff the different parts of the chair to determine which areas have a worse smell than others. Put some baking soda around all affected areas, putting a little more where the odor worsens. Allow the powder to rest there overnight.
Vacuum the baking soda from the chair and check if the odor is still prominent.
Spray a mixture of 1 part vinegar, 1 part water. Wipe off the vinegar as soon as you finish spraying it to prevent damage to the leather.
Spray regular water onto the leather to wash off the vinegar and wipe it off immediately.
Repeat the previous steps if the smell lingers.