Activation of H–E (E = H, Si, B, C) σ-Bond by Iridium and Rhodium Pincer Complexes: Implications in Catalytic (De)hydrogenation Reactions

Abstract

Methane is a major constituent of natural gas and crude oil. Due to its abundance, methane is attractive from the standpoint of its utilization and conversion into value-added products.1 This process involves activation and the subsequent cleavage of the very strong C–H bond (dissociation energy, 105 kcal/mol) followed by its functionalization.2 The activation and the cleavage of the C–H bond could be achieved via its binding to a transition metal fragment, in the so-called -methane complex. 2b,3 Formation and study of -methane complex is fraught with difficulty due to the weak binding energy (8-14 kcal/mol) of the C–H bond to the metal center.3 Therefore, to study and understand the binding and activation of the C–H bond of methane, often the interaction of other σ-bonds like H–H in H2, Si–H in silanes, and B–H in boranes are employed as models. Thus, the study of the binding and activation of these σ-bonds act as valuable model systems for binding and activation of the C–H bond of methane and other alkanes.3 In this work, H–E (E = H, Si, B, C) σ-bond activation by iridium and rhodium pincer complexes have been studied. In the study with Ir complexes,4 Abstraction of the chloride from an Ir(III) hydrido chloride complex, [Ir(H)(Cl)tBu4(PNCNP)] {tBu4(PNCNP) = 2,6-C6H3(NHPtBu2)2} under H2 afforded a cationic bis-σ-H2 complex, [Ir(H)(ɳ2-H2)2tBu4(PNCNP)]+. On the other hand, introduction of H2 to an Ir(I) 14-electron fragment, [IrtBu4(PNCNP)], generated in-situ by a reaction of [Ir(H)(Cl)tBu4(PNCNP)] with KOtBu resulted in the oxidative addition of two moles of H2 to form an iridium tetrahydride complex, [Ir(H)4tBu4(PNCNP)]. This complex was also formed in a reaction of the Ir(I) complex, [IrtBu4(PNCNP)], with H3B·NMe3 via B–H bond activation. Reaction of the Ir(I) complex, [IrtBu4(PNCNP)], with SiHEt3 resulted in the oxidative addition of the Si–H bond to afford an Ir(III) silyl hydride complex, [Ir(H)(SiEt3)tBu4(PNCNP)]. Reaction of a Rh(III) hydrido chloride complex, [Rh(H)(Cl)tBu4(PNCNP)] with NaBH4 gave a Rh(III) borohydride complex, [Rh(H)(К2-BH¬4)tBu4(PNCNP)].4 This species undergoes B–H bond activation to afford a neutral Rh(I) σ-H2 complex, [Rh(ɳ2-H2)tBu4(PNCNP)] which could be isolated in the solid state. Its HD isotopomer [Rh(ɳ2-HD)tBu4(PNCNP)] could be prepared from the corresponding borodueteride complex, [Rh(H)(К2-BD-4)tBu4(PNCNP)]. The σ-H2 complex was also obtained by two alternative routes. The binding of other small molecules such as N2, CH3CN, CO, and O2 to a 14-electron Rh(I) fragment, [RhtBu4(PNCNP)], has also been studied. Reactions of this Rh fragment with silane and borane did not afford the expected -complexes. This was traced to the high nucleophilicity of the rhodium center. Therefore, to reduce the nucleophilicity of the rhodium center, a different pincer ligand was introduced, iPr4(PN3P) {iPr4(PN3P) = 2,5-NC5H3(NHPiPr2)2}, in place of tBu4(PNCNP). Therefore, a Rh(I) monochloride complex, [Rh(Cl)iPr4(PN3P)], and a Rh(III) trichloride complex, [Rh(Cl)3iPr4(PN3P)], were synthesized. From the studies on the activation of the σ-bonds, the work was integrated with catalysis. Homogeneous dehydrogenation and hydrogenation of 1,2,3,4-tetrahydroisoquinoline and isoquinoline (N-heterocyclic compounds), respectively, are important processes towards the design and development of catalysts for reversible hydrogen storage in liquid organic hydrogen carriers (LOHCs). In many reported processes, iridium pincer complexes have been employed as catalysts for these reactions in the presence of a strong base and a sacrificial acceptor. In the present study, base-free, acceptorless, and reversible dehydrogenation-hydrogenation of N-heterocycles in a single reaction medium catalyzed by the Ir(III) hydrido chloride pincer complex, [Ir(H)(Cl)tBu4(PNCNP)], has been carried out.5 This is particularly significant as it simplifies the catalytic system by eliminating the need for additional additives. The results of these studies will be presented in detail.