Mechanistic Study of Rhodium/xantphos-Catalyzed Methanol Carbonylation

Rhodium/iodide catalysts modified with the xantphos ligand are active for the homogeneous carbonylation of methanol to acetic acid using either pure CO or CO/H2. Residues from catalytic reactions contain a Rh­(III) acetyl complex, [Rh­(xantphos)­(COMe)­I2] (1), which was isolated and crystallographically characterized. The xantphos ligand in 1 adopts a “pincer” κ3-P,O,P coordination mode with the xanthene oxygen donor trans to the acetyl ligand. The same product was also synthesized under mild conditions from [Rh­(CO)2I]2. Iodide abstraction from 1 in the presence of donor ligands (L = MeCN, CO) gives the cationic acetyl species [Rh­(xantphos)­(COMe)­I­(L)]+, whereas in CH2Cl2 migratory CO deinsertion gives [Rh­(xantphos)­(Me)­I­(CO)]+ (4), which reacts with H2 to liberate methane, as observed in catalytic reactions using syngas. A number of Rh­(I) xantphos complexes have been synthesized and characterized. Oxidative addition of methyl iodide to the cation [Rh­(xantphos)­(CO)]+ is very slow but can be catalyzed by addition of an iodide salt, via a mechanism involving neutral [Rh­(xantphos)­(CO)­I] (6). IR spectroscopic data and DFT calculations for 6 suggest the existence in solution of conformers with different Rh–O distances. Kinetic data and activation parameters are reported for the reaction of 6 with MeI, which proceeds by methylation of the Rh center and subsequent migratory insertion to give 1. The enhancement of nucleophilicity arising from a Rh- - -O interaction is supported by DFT calculations for the SN2 transition state. A mechanism for catalytic methanol carbonylation based on the observed stoichiometric reaction steps is proposed. A survey of ligand conformations in xantphos complexes reveals a correlation between P–M–P bite angle and M–O distance and division into two broad categories with bite angle 143° (trans).

采用xantphos配体（xantphos）修饰的铑/碘化铑催化剂，可在纯一氧化碳（CO）或CO/H₂混合气氛围下，有效催化甲醇的均相羰基化反应生成乙酸。催化反应后的残留体系中含有Rh(III)乙酰基配合物[Rh(xantphos)(COMe)I₂]（记为1），该配合物已被分离并通过晶体学手段完成结构表征。配合物1中的xantphos配体以“钳形”κ³-P,O,P配位模式与金属中心结合，其中氧杂蒽环的氧供体位于乙酰基配体的反位。该目标产物亦可通过温和反应条件，由前驱体配合物[Rh(CO)₂I]₂合成得到。在给体配体（L = 乙腈(MeCN)、一氧化碳(CO)）存在的条件下，从配合物1中脱去碘离子，可得到阳离子型乙酰基配合物[Rh(xantphos)(COMe)I(L)]⁺；而在二氯甲烷(CH₂Cl₂)溶剂中，配合物1会发生迁移性CO脱除反应，生成[Rh(xantphos)(Me)I(CO)]⁺（记为4），该阳离子配合物可与氢气(H₂)反应释放甲烷，这一现象与使用合成气(syngas)的催化反应中观测到的结果一致。已有多种xantphos配位的Rh(I)配合物被成功合成并完成表征。将碘甲烷(MeI)氧化加成至阳离子配合物[Rh(xantphos)(CO)]⁺的反应速率极慢，但可通过添加碘盐实现催化，其反应机理涉及中性中间体配合物[Rh(xantphos)(CO)I]（记为6）。针对配合物6的红外(IR)光谱数据与密度泛函理论(DFT, Density Functional Theory)计算结果表明，其在溶液中存在两种具有不同Rh-O键长的构象异构体。本文报道了配合物6与碘甲烷反应的动力学数据及活化参数，该反应通过铑中心的甲基化与后续迁移插入过程生成配合物1。通过针对SN2（双分子亲核取代）过渡态的DFT计算，证实了Rh---O相互作用所带来的亲核性增强效应。基于观测到的化学计量反应步骤，本文提出了一套催化甲醇羰基化的完整反应机理。对xantphos配合物的配体构象进行系统性调研后发现，P-M-P咬合角（bite angle）与M-O键长之间存在显著相关性，据此可将这类配合物分为两大类，其中一类的P-M-P咬合角为143°（反式(trans)）。