Synthesis of Oxorhenium Acetyl and Benzoyl Complexes Incorporating Diamidopyridine Ligands: Implications for the Mechanism of CO Insertion

A series of oxorhenium alkyl, phenyl, and vinyl complexes
of the
form [(DAP)­Re­(O)­(R)] (R = aryl, vinyl, alkyl) was reported, and their
reactivity with CO was examined. The methyl complex 5a reacts with CO at a significantly faster rate (2.5 h) than the phenyl
complex 7a (24 h). Computational (B3PW91) studies reveal
that although the acyl complex is the least stable (ΔG353 = −11.2 kcal/mol) with respect to
CO insertion compared to the benzoyl complex (ΔG353 = −14.5 kcal/mol), the activation energy for
CO insertion is lower for the methyl complex (ΔG⧧353 = 14.6 kcal/mol) than for the phenyl
complex (ΔG⧧353 = 17.4 kcal/mol). This is consistent with the previously proposed
mechanism, where CO inserts directly into the Re–R bond without
prior formation of a CO adduct. The X-ray crystal structures of complexes 6, 7a, 8a, and 9a are
reported.