Pairwise H₂/D₂ Exchange and H₂ Substitution at a Bimetallic Dinickel(II) Complex Featuring Two Terminal Hydrides

Area of Science:

• Organometallic Chemistry
• Bioinorganic Chemistry

Background:

• Dinuclear metal complexes offer unique reactivity for small molecule activation.
• Understanding hydride transfer mechanisms is crucial for catalysis.

Purpose of the Study:

• To synthesize and characterize novel dinuclear nickel dihydride complexes.
• To investigate the H2/D2 exchange mechanism and the reactivity of the masked dinickel(I) core.
• To explore the potential of this system for bioinspired catalysis.

Main Methods:

• Synthesis and isolation of dinuclear nickel(II) dihydride complexes.
• Crystallographic characterization.
• Nuclear Magnetic Resonance (NMR) spectroscopy (1H and 2H NMR, 2D 1H EXSY).
• Density Functional Theory (DFT) calculations.
• SQUID magnetometry.

Main Results:

• Novel dinuclear nickel(II) dihydride complexes M[LNi2(H)2] (M=Na, K) were synthesized and structurally characterized, revealing hydride ligands interacting with alkali metal cations.
• An unusual pairwise H2/D2 exchange process was observed, involving both Ni-H moieties synchronously without H/D scrambling.
• DFT calculations elucidated a facile recombination of terminal hydrides to form H2 and a dinickel(I) species, with a moderate activation barrier.
• Kinetic studies confirmed first-order dependence of H2 release on complex concentration and a significant influence of the alkali metal cation.
• The dinickel(II) dihydride complex acts as a masked dinickel(I) core, capable of releasing H2 upon substrate interaction.

Conclusions:

• Dinuclear nickel dihydride complexes serve as effective masked precursors to highly reactive dinickel(I) species.
• The observed H2/D2 exchange mechanism provides insight into hydride dynamics in bimetallic systems.
• The alkali metal cation plays a crucial role in stabilizing the dihydride core and influencing reactivity.
• This bimetallic scaffold holds promise for bioinspired catalytic applications, mimicking steps in nitrogenase function for activating small molecules.