Department of Chemical Engineering, University of California, Berkeley, CA 94720-1462, USA.
Phys Chem Chem Phys. 2009 Nov 14;11(42):9903-11. doi: 10.1039/b906883j. Epub 2009 Aug 21.
A spectroscopic investigation of complexes used to catalyze the oxidative carbonylation of toluene to p-toluic acid was conducted. Rhodium complexes were analyzed by (103)Rh and (13)C NMR, UV-visible spectroscopy, and infrared spectroscopy. In the presence of vanadium and oxygen, the resting state of the Rh-catalyst was found to exist as a Rh(III) complex with carbonyl and trifluoroacetate ligands, consistent with the structure Rh(CO)(2)(TFA)(3). The (13)C NMR spectrum of Rh((13)CO)(2)(TFA)(3) complex exhibited a carbonyl peak with an unusual degree of shielding, which resulted in the appearance of the carbonyl peak at an unprecedented upfield position in the (13)C NMR spectrum. This shielding was caused by interaction of the carbonyl group with the trifluoroacetate ligand. In the absence of oxygen, the Rh(III) complex reduced to Rh(I), and the reduced form exhibited properties resembling the catalyst precursor. Structures and spectroscopic properties calculated using density functional theory agreed closely with the experimental results. The vanadium co-catalyst used to reoxidize Rh(I) to Rh(III) was similarly characterized by (51)V NMR and UV-visible spectroscopy. The oxidized species corresponded to (VO(2))(TFA), whereas the reduced species corresponded to (VO)(TFA)(2). The spectroscopic results obtained in this study confirm the identity of the species that have been proposed to be involved in the Rh-catalyzed oxidative carbonylation of toluene to toluic acid.
采用（103）Rh 和（13）C NMR、紫外-可见光谱和红外光谱对用于催化甲苯氧化羰基化合成对甲基苯甲酸的配合物进行了光谱研究。在钒和氧的存在下，发现 Rh-催化剂的休眠状态以具有羰基和三氟乙酸根配体的 Rh(III)配合物形式存在，与 Rh(CO)(2)(TFA)(3)的结构一致。Rh((13)CO)(2)(TFA)(3)配合物的（13）C NMR 谱显示出羰基峰具有异常程度的屏蔽，导致羰基峰在（13）C NMR 谱中出现在前所未有的场强位置。这种屏蔽是由羰基与三氟乙酸根配体的相互作用引起的。在没有氧的情况下，Rh(III)配合物还原为 Rh(I)，还原形式表现出类似于催化剂前体的性质。使用密度泛函理论计算的结构和光谱性质与实验结果非常吻合。用于将 Rh(I)氧化回 Rh(III)的钒助催化剂也通过（51）V NMR 和紫外-可见光谱进行了类似的表征。氧化态对应于(VO(2))(TFA)，而还原态对应于(VO)(TFA)(2)。本研究获得的光谱结果证实了在 Rh 催化的甲苯氧化羰基化合成对甲基苯甲酸过程中提出的参与物种的身份。
