Room Temperature Acceptorless Alkane Dehydrogenation from Molecular σ-Alkane Complexes

Area of Science:

• Organometallic Chemistry
• Catalysis
• Solid-State Chemistry

Background:

• Non-oxidative catalytic dehydrogenation of light alkanes via C-H activation is challenging due to high endothermicity and unfavorable thermodynamics.
• Alkanes are poor ligands, making their binding to metal centers prior to C-H activation difficult.
• High temperatures or sacrificial hydrogen acceptors are typically required to overcome thermodynamic barriers.

Purpose of the Study:

• To demonstrate a novel method for activating alkane C-H bonds under mild conditions using solid-state molecular organometallic chemistry (SMOM-chem).
• To prepare and characterize well-defined alkane sigma-complexes.
• To investigate the dehydrogenation mechanism and kinetics of these complexes.

Main Methods:

• Synthesis of rhodium-alkane sigma-complexes ([Rh(Cy2PCH2CH2PCy2)(alkane)][BArF4]) via solid/gas single-crystal to single-crystal transformation of precursor alkene complexes.
• Solid-gas H/D exchange experiments with D2 to probe C-H bond reactivity.
• Variable temperature solid-state nuclear magnetic resonance (NMR) spectroscopy and periodic density functional theory (DFT) calculations to study fluxional processes and reaction mechanisms.
• Kinetic studies, including kinetic isotope effects (KIEs), and modeling using classical chemical kinetics or Johnson-Mehl-Avrami-Kolmogorov (JMAK) models.

Main Results:

• Well-defined isobutane and cyclohexane sigma-complexes were successfully prepared and characterized.
• Spontaneous, acceptorless dehydrogenation of alkane sigma-complexes to alkene complexes occurred at 298 K under vacuum or Ar-flow.
• Kinetic isotope effects (kH/kD = 3.6(5) and 10.8(6)) for cyclohexane dehydrogenation indicated C-H activation as the rate-determining step.
• Periodic DFT calculations supported experimental findings, predicting activation barriers and elucidating the mechanism involving C-H bond elongation and beta-H transfer.

Conclusions:

• Solid-state molecular organometallic chemistry (SMOM-chem) enables the preparation of alkane sigma-complexes and their subsequent dehydrogenation under mild conditions.
• The study provides direct experimental evidence for C-H activation in alkane dehydrogenation, supported by kinetic and computational data.
• This approach offers a promising new avenue for catalytic functionalization of alkanes, overcoming thermodynamic limitations.