Synthesis and Reactivity of Oxorhenium(V) Methyl, Benzyl, and Phenyl Complexes with CO: Implications for a Unique Mechanism for Migratory Insertion

The complexes [(DAAm)­Re­(O)­(R)] [DAAm
= N,N-bis­(2-arylaminoethyl)­methylamine;
aryl = C6F5], 1, R = Me; 3a–d (R = benzyl, a; 4-methylbenzyl, b; 4-fluorobenzyl, c; 4-methoxybenzyl, d); and 4, R = Ph, were synthesized. CO insertion
into
the Re–R bond in 1 and 3a–d resulted in the formation of the acetyl complex, 2, and the (aryl)­acetyl complexes, 5a–d respectively. The formation of 5a–d proceeded at a faster rate (7 h) than the formation of 2 (72 h) under the same conditions. No reaction was observed however
for the phenyl complex 4 with CO. Kinetics for CO insertion
into the various Re–R bonds were examined, and the experimental
rate law was determined to be Rate = kobs[Re]­[CO]. The activation parameters for CO insertion into 1 and 3a were determined to be ΔG⧧(298 K) = 24(1). The enthalpy of activation ΔH⧧ was determined to be 9(1) and 10(3)
kcal/mol for 1 and 3a, respectively, and
the entropy of activation, ΔS⧧, was −49(2) and −36(4) cal/mol·K. Computational
studies (M06) are consistent with the hypothesis that the rate of
CO insertion is dependent on the strength of the rhenium–carbon
bond. Thus, experimental and computational data suggest that the most
likely mechanism for the insertion of CO into the Re–R bond
in oxorhenium complexes is a direct-insertion mechanism.