Scope and Mechanism of Homogeneous Tantalum/Iridium Tandem Catalytic Alkane/Alkene Upgrading using Sacrificial Hydrogen Acceptors

Abstract

An in-depth investigation of a dual homogeneous catalyst system for the coupling of alkanes and alkenes based on an early-/late-transition-metal pairing is reported. The system is composed of Cp*TaCl2(alkene) for alkene dimerization and pincer-iridium hydrides for alkane/alkene transfer hydrogenation. Because there is no kinetically relevant interaction between the two catalysts, the tandem mechanism can be entirely described using the two independent catalytic cycles. The alkene dimerization mechanism is characterized by an entropically disfavored pre-equilibrium between Cp*TaCl2(1-hexene) + 1-hexene and Cp*TaCl2(metallacyclopentane) (ΔH° = −22(2) kcal/mol; ΔS° = −16(2) eu); thus, the overall rate of alkene dimerization is positive order in 1-hexene (exhibiting saturation kinetics), and increases only modestly with temperature. In contrast, the rate of 1-hexene/n-heptane transfer hydrogenation catalyzed by t-Bu[PCP]IrH4 is inverse order in 1-hexene and increases substantially with temperature. Styrene has been investigated as an alternate sacrificial hydrogen acceptor. Styrene dimerization catalyzed by Cp*TaCl2(alkene) is considerably slower than 1-hexene dimerization. The conversion of styrene/heptane mixtures by the Ta/Ir tandem system leads to three product types: styrene dimers, coupling of styrene and heptane, and heptene dimers (from heptane). Through careful control of reaction conditions, the production of heptene dimers can be favored, with up to 58% overall yield of heptane-derived products and cooperative TONs of up to 12 and 10 for Ta and Ir catalysts, respectively. There is only slight inhibition of Ir-catalyzed styrene/n-heptane transfer hydrogenation under the tandem catalysis conditions.

Dehydrogenation of n-Alkanes by Solid-Phase Molecular Pincer-Iridium Catalysts. High Yields of α-Olefin Product

“…Crystals suitable for X-ray analysis were grown by slow evaporation of a 10 mg solution of ( iPr4 PCP)Ir(C 2 H 4 ) in 1.0 ml pentane. 31 P NMR (C 6 D 6 , 161.9 MHz): 51. Synthesis of ( iPr4 PCP)IrH 4 .…”

“…Introducing H 2 to a pentane solution of ( iPr4 PCP)Ir(C 2 H 4 ) into atmosphere results in formation of ( iPr4 PCP)Ir(H 4 ) in quantitative yields as an orange solid. 31 P NMR (p-xylene-d 10 Computational. All electronic structure calculations employed the DFT method 74 and the PBE 75 exchange correlation functional.…”

“…2,4 Work with these complexes has been followed by reports of numerous catalytically active variants with the (PCP)Ir motif, [5][6][7][8][9] including other bis-phosphines, [10][11][12][13][14] bis-phosphinites (POCOP), [15][16][17][18] hybrid phosphine-phosphinites (PCOP), 19,20 arsines (AsOCOAs), 21 hybrid phosphine-thiophosphinites (PSCOP) 22 and hybrid amine-phosphinites (NCOP) 23 . In addition to simple alkane dehydrogenation, these complexes have been employed for numerous other catalytic transformations of hydrocarbons, including alkane metathesis, 6,8,9,20,[24][25][26] alkyl group metathesis, 27 dehydroaromatization, 19,28,29 alkanealkene coupling reactions, [30][31][32] borylation of alkanes 23 and the dehydrogenation of several non-alkane substrates. 22,33,34 Several pincer motifs more recently explored, such as (CCC)Ir, [35][36][37][38] (PCP)Ru 39-41 , (PCP)Os 42 , and (NCN)Ir …”

Recent Advances in Alkane Dehydrogenation Catalyzed by Pincer Complexes

“…Notably, ( tBu4 POCOP)IrH 2 (21-H 2 ) showed greater activity than ( tBu4 PCP)IrH 2 (11-H 2 ) for COA/TBE transfer dehydrogenation, although the latter was found to be significantly more active for the transfer dehydrogenation of linear alkanes [46][47][48]71]. As will be discussed later in this chapter, these phosphine, phosphinite, and mixed phosphine-phosphinite-based systems have found widespread utility as catalysts for alkane metathesis , [48,67,[71][72][73][74], alkyl group metathesis [62], dehydroaromatization reactions [61,75,76], alkane-alkene coupling reactions [77,78], and dehydrogenation of several other substrates [64,68,69,[79][80][81].…”

“…Bercaw and Labinger recently reported on alkane-alkene coupling as another approach toward upgrading light hydrocarbons [77,78]. This work is based on the fact that light hydrocarbon feeds typically contain alkenes.…”