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.

本研究合成了配合物[(DAAm)Re(O)(R)]，其中DAAm为N,N-双(2-芳氨基乙基)甲胺，芳基为五氟苯基(C₆F₅)；当R为甲基时对应配合物1，当R分别为苄基(a)、4-甲基苄基(b)、4-氟苄基(c)、4-甲氧基苄基(d)时对应3a~3d，当R为苯基(Ph)时对应配合物4。将一氧化碳(CO)插入1和3a~3d的Re-R键中，分别得到乙酰基配合物2以及对应的芳基乙酰基配合物5a~5d。在相同反应条件下，5a~5d的生成速率（7小时）快于2的生成速率（72小时）。但苯基配合物4与CO未发生反应。针对不同Re-R键上的CO插入反应开展了动力学研究，确定其实验速率方程为：速率 = kobs[Re][CO]。测定了配合物1和3a的活化参数：298K下ΔG‡(298K)=24(1)。配合物1和3a的活化焓ΔH‡分别为9(1)和10(3) kcal/mol，活化熵ΔS‡分别为-49(2)和-36(4) cal/mol·K。采用M06方法进行的计算研究佐证了“CO插入反应速率取决于铼-碳键强度”的假说。综上，实验与计算数据表明，氧铼配合物中CO插入Re-R键的最可能机理为直接插入机理。