Mechanism of Catalytic Hydration of Nitriles with Hydrotris(pyrazolyl)borato (Tp) Ruthenium Complexes

The aquo-amido complexes TpRu(PPh3)(H2O)(NHC(O)R) (R = Me, Ph), which can be prepared by refluxing a THF solution of TpRu(PPh3)(RCN)H containing excess water or more conveniently by reacting TpRu(PPh3)(RCN)Cl with NaOH in THF in the presence of water, are found to be active for catalytic hydration of nitriles to amides. The catalysis proceeds via a mechanism that is distinctly different from the common ones involving intramolecular nucleophilic attack of a hydroxo (or aquo) ligand or external attack of a hydroxide ion (or water) at the carbon atom of the η1-coordinated nitrile to form the metal amide intermediate and subsequent protonation of amido ligand by an adjacent aquo ligand or solvent water. The new mechanism involves the intermediacy of a relatively stable complex containing a chelating N-imidoylimidato ligand; ring-opening nucleophilic attack of this ligand by water is the product-generating step. Formation of the N-imidoylimidato complex from TpRu(PPh3)(H2O)(NHC(O)R) involves several steps. The initial one is displacement of the H2O ligand by a nitrile molecule to yield the nitrile-amido species TpRu(PPh3)(RCN)(NHC(O)R). This is followed by an unusual linkage isomerization of the N-bonded amido ligand to an O-bonded imido, which then undergoes nucleophilic attack at the carbon atom of the nitrile ligand in the complex; facile 1,3-proton shift between the nitrogen atoms on the resulting ring completes the reaction. The N-imidoylimidato complexes TpRu(PPh3)(κ2-N,O-NHCMeNCMeO), TpRu(PPh3)(κ2-N,O-NHCPhNCPhO), and TpRu(PPh3)(κ2-N,O-NHCMeNCPhO) were independently prepared, and the molecular structure of TpRu(PPh3)(κ2-N,O-NHCPhNCPhO) was determined by X-ray crystallography. To study the feasibility of the proposed mechanism for nitrile hydration with the aquo-amido complexes, theoretical calculations were performed at the Becke3LYP level of DFT theory to examine the whole catalytic cycle. It is learned that there is a substantially high barrier for the hydrolysis of the highly stable N-imidoylimidato complex, a step involving the ring-opening nucleophilic attack of this ligand by water, and this is probably the reason for the requirement of a relatively high reaction temperature.

水合酰胺基配合物TpRu(PPh3)(H2O)(NHC(O)R)（R = 甲基(Me)、苯基(Ph)）可通过两种方法制备：其一为将含过量水的TpRu(PPh3)(RCN)H的四氢呋喃(THF)溶液回流；更简便的方法则是在水存在下，于THF中使TpRu(PPh3)(RCN)Cl与氢氧化钠(NaOH)反应。该配合物可高效催化腈类水合生成酰胺。

该催化反应的机理与经典机理截然不同：经典机理通常是氢氧根配体（或水合配体）发生分子内亲核进攻，或是氢氧根离子（或水分子）对η¹配位腈的碳原子进行外部亲核进攻，进而生成金属酰胺中间体，随后由相邻的水合配体或溶剂水分子对酰胺配体质子化。

而本研究提出的新机理则涉及一种相对稳定的螯合N-酰亚胺酰亚胺酸酯配体配合物作为中间体；水分子对该配体进行开环亲核进攻即为产物生成的关键步骤。

由TpRu(PPh3)(H2O)(NHC(O)R)生成N-酰亚胺酰亚胺酸酯配合物的过程包含多步反应：第一步为腈分子取代H2O配体，得到腈-酰胺物种TpRu(PPh3)(RCN)(NHC(O)R)；随后发生罕见的键联异构化：以N配位的酰胺配体异构为以O配位的亚胺配体，该配体随即对配合物内腈配体的碳原子进行亲核进攻；最终，生成的环上氮原子间发生快速的1,3-质子转移，完成整个反应循环。

研究人员独立合成了三种N-酰亚胺酰亚胺酸酯配合物：TpRu(PPh3)(κ2-N,O-NH=CMeN=CMeO)、TpRu(PPh3)(κ2-N,O-NH=CPhN=CPhO)以及TpRu(PPh3)(κ2-N,O-NH=CMeN=CPhO)，并通过X射线晶体衍射测定了TpRu(PPh3)(κ2-N,O-NH=CPhN=CPhO)的分子结构。

为验证该水合酰胺基配合物催化腈类水合反应机理的可行性，研究人员采用密度泛函理论(DFT)的Becke3LYP方法对整个催化循环进行了理论计算。结果表明，高度稳定的N-酰亚胺酰亚胺酸酯配合物的水解步骤（即水分子对该配体进行开环亲核进攻的步骤）存在显著较高的能垒，这或许正是该反应需要相对较高温度的原因。