Description:
<jats:title>Abstract</jats:title><jats:p>Organometallic aryl‐cobalamins are B<jats:sub>12</jats:sub>‐derivatives featuring properties of potential ‘B<jats:sub>12</jats:sub> antivitamins’. Herein, we describe a new method for the preparation of aryl‐cobalamins using versatile diaryliodonium salts as arylation agents. Formate or sodium borohydride reduction of aquocobalamin in presence of diphenyliodonium chloride furnished Co<jats:sub>β</jats:sub>‐phenyl‐cobalamin <jats:bold>PhCbl</jats:bold> in a roughly 3:1 to 1:1 ratio with its coordination isomer <jats:bold>αPhCbl</jats:bold>, a first representative ‘base‐off’ Co<jats:sub>α</jats:sub>‐aryl‐cobalamin. The new structures were secured by detailed spectroscopic analysis, supplemented by an X‐ray crystal structure analysis of <jats:bold>PhCbl</jats:bold>. Both types of coordination isomers of the aryl‐cobalamins promise to be useful molecular tools in biomedical and biological studies.</jats:p>