Singlet-to-Triplet Spin Transitions Facilitate Selective 1-Butene Formation during Ethylene Dimerization in Ni(II)-MFU-4l

Abstract

Catalytic ethylene dimerization is an important chemical reaction that suffers from a lack of selectivity for the desired product, 1-butene. Metal–organic frameworks (MOFs) bearing Ni-based catalytic sites have been shown to yield record selectivity for 1-butene. Early efforts to understand this selectivity revealed that chain propagation and 2-butene formation are competitive with 1-butene, seemingly at odds with experimental evidence that these products are disfavored. Here, we present an alternative mechanism for selective 1-butene formation in the highest performing MOF, Ni­(II)-MFU-4l. Our study reveals competing electronic spin configuration pathways that intersect along the reaction coordinate. Intersystem crossing provides an explanation for the selective formation of 1-butene in the MOF. Furthermore, we explore intersystem crossing as a unique design principle for MOF catalyst design and highlight a departure from conventional molecular catalyst design paradigms.

Structure and Site Evolution of Framework Ni Species in MIL-127 MOFs for Propylene Oligomerization Catalysis

“…After the relevant Ni-propyl species is formed during the induction period on Ni-MIL-127, olefin oligomerization is proposed in the literature to follow the Cossee–Arlman mechanism on nickel catalysts as presented in Scheme . ,,,− Isomers of linear hexenes, 2-methylpentenes, 4-methylpentenes, and 2,3-dimethylbutenes are generated from the Cossee–Arlman cycle by regioselective insertion of adsorbed propylene molecules . Herein, we report the mechanism for the production of linear hexenes, while others are reported in Section S6.…”

“…22 We note that the barrier for β-hydride elimination of 130 kJ mol −1 is higher than what is reported for ethylene and butene oligomerization on Ni/UiO-66. 22,32 We attribute the larger β-hydride elimination barrier of 130 kJ mol −1 for linear hexenes due to a combination of the change in the spin state found in the nickel framework-based MOFs 51 and the specific hexene isomer that is produced. Nickelframework-based MOFs, such as Ni-MFU-4l, for olefin oligomerization have been proposed to change spin states along the reaction coordinate 51 while olefin oligomerization on nickel-supported MOFs, such as Ni/UiO-66, do not.…”

“…22,32 We attribute the larger β-hydride elimination barrier of 130 kJ mol −1 for linear hexenes due to a combination of the change in the spin state found in the nickel framework-based MOFs 51 and the specific hexene isomer that is produced. Nickelframework-based MOFs, such as Ni-MFU-4l, for olefin oligomerization have been proposed to change spin states along the reaction coordinate 51 while olefin oligomerization on nickel-supported MOFs, such as Ni/UiO-66, do not. 22,32,52 Ni-MFU-4l yields higher reaction barriers for β-hydride elimination (72 kJ mol −1 ) 51 compared to β-hydride elimination barrier on Ni/UiO-66 (48 kJ mol -1 )for ethylene oligomerization 22 where a change in the triplet to singlet spin state on Ni-MFU-4l along the reaction coordinate was attributed to contribute toward the higher observed barrier on framework-based MOFs for β-hydride elimination.…”

Computational design of metal hydrides on a defective metal–organic framework HKUST-1 for ethylene dimerization

“…In this context, it is helpful to understand the significance of ISC during ethylene adsorption and 1-butene desorption to practically tailor the catalytic performance toward ethylene dimerization. 51,59 The electronic-structural intercorrelations at different multiplicities are also observed on other H-M-DHKUST-1 except H-Cu-DHKUST-1. For instance, the agostic arrangement of b-hydride intermediate 6 on H-Co-DHKUST-1 demonstrates intercorrelation between the electronic state and structural environment.…”

Role of Molecular Simulations in the Design of Metal–Organic Frameworks for Gas-Phase Thermocatalysis: A Perspective

“…In this method, the activation barrier for an electronically diabatic reaction is estimated using two PESs and identifying the minimum energy point of intersection between them. For example, Mancuso et al 34 used MECP to compute barriers in the pathway for ethene dimerization on Ni(II)-MFU-4l catalysts, where the Ni singlet−triplet transition is a key descriptor of activity.…”