铑碘催化甲醇羰基化反应的理论研究

【作者】 郝茂荣；

【导师】 冯文林；

【作者基本信息】 北京化工大学 ， 应用化学， 2003， 博士

【摘要】 本论文采用有效核势能近似(ECP)从头计算方法,从理论上系统地研究了甲醇在铑碘配合物催化作用下的羰基化反应循环机理,以及它的分子反应动态学行为,以揭示甲醇羰基化反应内在的微观催化机理。获得的主要研究成果如下 :1.关于催化活性物的理论研究。按照甲醇羰基化催化循环反应的特点,催化活性物是羰化反应作用物和CO供应源,是CO转换的中间体。在HF/LANL2DZ水平上,对催化活性物可能的构型以及活性物之间的关联反应进行了理论研究,计算结果表明:(1)中心原子铑与配体一氧化碳和碘负离子间形成催化活性物的构型可能有三配位或四配位的构型,通过比较和分析可知,其它各种构型的能量都远高于四配位[Rh(CO)2I2]- 构型的能量(从100 kJ/mol到700kJ/mol不等),说明[Rh(CO)2I2]- 构型是最稳定的,而且按照配位理论,铑同其它的过渡金属又有差别,其16电子结构是比较稳定的状态,因此,可以认为[Rh(CO)2I2]- 构型是反应体系中的催化活性物。(2)催化活性物[Rh(CO)2I2]- 有顺式和反式两种构型,虽然顺式构型较反式构型的能量高出48.16KJ/mol,但是,顺式构型中CO与中心原子Rh的成键又强于反式构型,从而增强了顺式构型催化活性物的稳定些。本文的研究结果表明,顺式和反式构型的[Rh(CO)2I2]- 都能存在于反应体系中,因此,铑碘配合物[Rh(CO)2I2]-催化甲醇羰基化反应可以有顺式和反式循环两种类型。(3)研究发现,顺式和反式构型催化活性物[Rh(CO)2I2]-同时存在于反应体系中时,二者之间存在着互变关联,计算表明,这种关联是经过CO解离机理来完成的,所涉及的中间体能量最低,反应的活化位垒最小,仅为67.85kJ/mol。 2.在HF/LANL2DZ水平上,对顺式和反式铑碘配合物[Rh(CO)2I2]- 催化的甲醇羰基化反应循环中各基元反应进行了理论研究。 优化得到了反应沿基态势能面极小能量途径上的反应物、过渡态、中间体和产物的几何构型,计算了反应活化位垒,并对各基元反应进行了IRC解析。本文的主要计算结果为:(1)根据顺式和反式循环过程中各基元反应的IRC解析结果,给出了顺式和反式循环反应过程中结构变化的异同点。无论是顺式循环,还是反式循环,反应历程都经历了碘甲烷氧化加成、羰基插入、CO配位和还原消除四个基元反应步骤,仔细分析每个基元反应发现,碘甲烷氧化加成基元反应在两种催化循环中的情况基本相似,所不同的是在反应过程中<WP=5>碘甲烷向催化剂平面接近时,由于CH3I分子中的甲基和I在与中心原子Rh成键时,对原有配体的排斥作用不同,造成构型畸变的程度有所不同;对于羰基插入基元反应,无论在顺式或反式循环中,该反应的实质都是由甲基迁移来实现的;对于还原消除基元反应,产物脱离方式略有不同,在顺式循环反应中,反应开始时,首先是I原子与C原子相向摆动,互相靠近,即先逐渐形成I—C键,然后I—C键成键和C—Rh键断键同步协同进行,I原子渐离Rh原子,相反,在反式循环反应中,I—C键的形成和C—Rh键的断裂,反应一旦开始就同步地进行了,即C原子渐离Rh原子而向I原子靠拢,然后I原子渐离Rh原子;为保持顺式或反式结构特点,在羰基插入和CO配位基元反应之间还存在两个构型转变过程,然后才进行CO配位基元反应。所有基元反应的IRC解析,证明了优化所得到的中间体、过渡态都是甲醇羰基化催化循环反应的中间体、过渡态,是确实的。(2)对循环反应的各反应物、产物、中间体和过渡态位能都进行了零点能校正,从循环反应过程中能量变化的特点看出,反式和顺式循环反应中,碘甲烷氧化加成基元反应的活化位垒分别是212.88kJ/mol和216.4kJ/mol,两者比较接近;羰基插入基元反应的位垒分别是128.42 kJ/mol和114.2kJ/mol,两者的差别不大(只差14.22kJ/mol); CH3COI还原消除基元反应的位垒分别是125.32kJ/mol和162.28kJ/mol,差别最大,反式比顺式的低36.96kJ/mol;在CO配位基元反应前都发生有两个构型转变过程,它们的位垒分别是:7.64kJ/mol 、16.13kJ/mol和50.55kJ/mol、41.4kJ/mol,顺式的要大于反式的。虽然各基元反应的活化位垒有差别,但是碘甲烷氧化加成基元反应的位垒是最高的,是整个循环反应过程的速控步骤,这与实验事实相符。3.在HF/LANL2DZ水平上,对顺式和反式循环过程之间的关联性,以及其它反应过程进行了理论研究。计算结果表明,(1)顺式和反式循环过程之间存在着关联反应,在顺式和反式循环过程中的两个中间体构型(B3和ROT-INT1)之间,有一个联系二者的构型转变过渡态,所需要的活化位垒是49.82kJ/mol。显然,除催化活性物[Rh(CO)2I2]- 的顺反结构互变关联外,顺式和反式循环反应的中间体之间存在一个关联点,使顺式和反式循环关联,因此,铑碘配合物[Rh(CO)2I2]-催化甲醇羰基化反应的实现,还有一种循环过程是顺反式交替循环。(2)在不考虑循环反应过程中始终需要保持顺式或反式结构要求的情况下,顺式和反式循环过程中的两个中间体构型(B3和ROT-INT1)可和CO直接配位,然后发生CH3COI还原消除基元反应。<WP=6>本文优化得到了中间体和过渡态的构型,并给出了这两个还原消除基元反应的活化位垒分别是128.72kJ/mol,113.81kJ/mol,该值与反式循环中还原消除基元反应的活化位垒相近,但比顺式循环中的低。经IRC解析,这两个CH3COI还原消除基元反应的特点,类似于顺式循环中CH3COI还原消除?更多还原

【Abstract】 In this thesis the reaction mechanisms of methanol carbonylation catalyzed by rhodium complex have been studied by using the ab initio method with the effective core potential (ECP) approximation, and all the reaction paths of each elementary reaction in the rhodium complex catalyzed methanol carbonylation have been traced by the intrinsic reaction coordinate theory (IRC). The obtained results are summarized as follows.1. Structures and activities of catalysis reactive species The possible configurations of catalysis reactive species and the relevance between the different reaction pathways are studied at HF/LANL2DZ level. The main results are as follows: (1) The configuration of the reactive species formed between the center rhodium atom and CO and I ligands possibly has a tri-or tetra-coordination structure. The calculations indicate that the tetra-coordination configuration [Rh(CO)2I2]- has the most stable structure, and the energy of the others are higher 100 to 700 kJ/mol than that of the [Rh(CO)2I2]-. According to coordination theory, the structure of rhodium is different from other transition metal; its configuration of 16 electrons is a relative stable state. Therefore, it is thought that [Rh(CO)2I2]- configuration is a catalysis reactive species in the methanol carbonylation catalyzed by rhodium complex ; (2) The [Rh(CO)2I2]- catalysis reactive species has both trans- and cis-configuration. Although the potential of cis-configuration is higher than that of trans-one (by 48.16 kJ/mol), the coordination between CO and central Rh atom in cis-configuration is stronger than that in trans-configuration, so the stability of the reactive species with cis-configuration is enhanced. The calculated results show that the cis-and trans-configuration of [Rh(CO)2I2]- participate in the methanol carbonylation reaction simultaneously, and two reaction cycle exist in methanol carbonylation catalyzed by rhodium complex [Rh(CO)2I2]-., the <WP=8>cis-cycle and trans-cycle, respectively. (3) The further studies show that there is a linked reaction pathway, which is a switch for transition between cis- and trans-configuration [Rh(CO)2I2]- through CO decomposition mechanism. The potential barrier of the linked reaction channel is 67.85kJ/mol.2. Analysis and comparison of the reaction mechanism.Each elemental reaction has been studied at HF/LANL2DZ level for the methanol carbonylation catalyzed with cis- or trans-rhodium complex [Rh(CO)2I2]-. The geometrical structures of the reactants, intermediates, transition states and products for methanol carbonylation reaction are optimized, and the activation barriers of all the reaction are calculated, and parts of elementary reaction paths are analyzed by using the IRC theory. The main theoretical calculation results are following: (1) IRC analysis results give a comparison between the two cis- and trans-cycle processes. The two different catalytic cycles involve the four same types of reaction, the oxidative addition reaction, carbonyl insertion reaction, coordination of CO, and reductive elimination reaction. The two CH3I oxidative addition reactions in cis-catalytic cycle and trans-cycle reaction are analogous to three-center transition state. The difference of cis-reaction from trans-reaction is that, when the CH3 and I is close to the catalyst plane the different rejection to the original coordination in two cycle types will result in the different degree of structural distortion; The carbonyl insertion of elemental reaction of whether cis-cycle or trans-cycle is completed with methyl migration essentially; The product leaving mode of the reductive elimination elementary reaction in cis-cycle is different from that in trans-cycle. In the cis-reaction cycle, at beginning point, atom I and atom C are swinging and approaching each other, and then I-C bond forms and C-Rh bond breaks synchronously. On the contrary, in the trans-cycle, the atom C firstly approaches to the atom I to form I-C bond and then the atom I leaves the atom Rh to break C-Rh bond gradually; There exist two更多还原