A theoretical investigation of the activation of propane by a rhodium catalyst

Abstract

In this work, we report a theoretical study of the activation of propane by cyclopentadienyl carbonyl rhodium, (Cp)Rh(CO), in order to investigate the selectivity of the activation process. Møller–Plesset perturbation theory up to fourth-order (MP4(SDTQ)) and PBE density functional were used along with coupled-cluster single point calculation on the MP2 optimized geometry (CCSD//MP2) for some processes. It was found that the PBE functional was found particular suitable for the calculation of structural parameters and thermal energy correction. The MP3 and PBE results were found in excellent agreement with the CCSD//MP2 results. According to our calculations, there is no kinetic or thermodynamic discrimination, which would lead to a preferable pathway for the C–H bond activation, with the Gibbs free energy differences involved in the two pathways studied, within 1–2 kcal mol⁻¹.

Introduction

The C–H bond activation and functionalization of saturated hydrocarbons is a very important process that has many important economic impacts. In particular, the intermolecular C–H bond activation of alkanes, promoted by soluble transition metal complexes, by oxidative addition mechanism under mild conditions, has been a challenge and has attracted much attention from the chemical community (for reviews on this subject, see for example: [1], [2], [3]). Since the discovery that compounds of the type (C₅Me₅)ML (M = Rh, Ir and L = CO, PMe₃) can add alkanes oxidatively [4], [5], several theoretical works have appeared in the literature, aiming to understand the energetic associated with this reaction as well as the nature of the metal–alkane interaction during the bond activation reaction [6], [7], [8], [9], [10]. These works, using methane as substrate and varied degrees of sophistication, agrees in the fact that the reaction involves the coordination of the C–H bond to the metal, leading to the formation of a σ-complex intermediate, passing through a tree-centre-like transition state. However, the energetic associated with this reaction is subject of some disagreement [10]. As an example, Ziegler et. al. [6], using DFT calculations found a barrier of 2.4 kcal mol⁻¹ for the activation of CH₄ by the compound [(C₅H₅)IrCO], with a reaction enthalpy of −33.5 to −35.9 kcal mol⁻¹. When Ir is replaced by Rh, the activation energy increases to 8.8 kcal mol⁻¹ and the reaction enthalpy less favourable at between −16.7 and −35.6 kcal mol⁻¹. Subsequently, Song and Hall have studied this same reaction at the RHF and MP2 levels of theory [7]. They found that the reaction proceeds with an energy barrier of 4.1 kcal mol⁻¹, at the MP2 level, for the [(C₅H₅)RhCO] catalytic species, with an exothermicity of 31–40 kcal mol⁻¹. For M = Ir, the reaction enthalpy obtained at the MP2 level was −69 kcal mol⁻¹. The controversial exothermicity of the oxidative addition of methane to [(C₅H₅)RhCO] was investigated by Couty et al. [10], at the MP2, MP3, MP4(SDQ) and QCISD levels of theory, using several basis set. They have found that the MPn perturbation series does not converge well and the MP2 results are only qualitative. They have also found that the MP3 energies are in good agreement with the QCISD results. A good compilation of the theoretical studies on reactions promoted by transition metal complexes, including the alkane activation, can be found in the excellent review of Niu and Hall [11]. We have reported very recently a theoretical perspective, based on quantum mechanical calculations, on the use of the [(η⁵-phospholyl)Rh(CO)₂] compound to promote C–H activation of methane [12]. The five-member ring phospholyl ligand, [C₄H₄P]⁻ is an analogue of cyclopentadienyl, in which a C–H unit is replaced by phosphorus. Our results showed that the [(η⁵-phospholyl)Rh(CO)₂] compound has a singlet ground state, with the lowest triplet state lying 55.1 kcal mol⁻¹ above it. The calculation of the electronic spectrum of this compound revealed that the MLCT charge transfer band (M → CO) which leads to CO dissociation occurs at much higher energy (5.28 and 5.54 eV) than the value for the parent Cp compound. This perspective also showed that the replacement of cyclopentadiene in the compound [(Cp)Rh(CO)₂] by the phospholyl ligand generates a compound, [(η⁵-phospholyl)Rh(CO)₂], which can be used to activate the C–H bond of alkanes with a small activation energy.

Although the theoretical calculations gave important contribution to the understanding of the energy barriers, relative stability of the species and nature of the interaction between the CH₄ and the [(Cp)M(CO)] fragment, little effort was concentrated to understand the selectivity of the alkane oxidative addition when larger alkanes are used as substrate. The selective intermolecular carbon–hydrogen activation by organometallic compounds in homogeneous solution is today a great challenge [3] and the theoretical understanding of the factors controlling such selectivity is of crucial importance for the development of new catalytic systems. In this work we address to this question studying the C–H bond activation of propane by cyclopentadienyl carbonyl rhodium complex [(Cp)Rh(CO)] by means of quantum chemical methods up to MP4(SDTQ) level of theory. When propane interacts with CpRh(CO) fragment, there are two possibilities of cleavage of the C–H bond, since in propane we have two different types of C–H bond corresponding to the internal (–CH₂–) and terminal (–CH₃) carbon atoms. Our main aim here is to assess the consistency of the theoretical methods used for the description of the propane activation reaction catalyzed and also to investigate the selectivity present on this activation process (if any), and the factors controlling such kind of preference. We report the geometrical parameters of the various species involved along the pathway for the propane C–H bond activation, that includes the reactants, intermediate species like adducts and transition state structures and the final products, along with stabilization energies and the temperature-dependent Gibbs free energy results.