Mechanism of Heterolysis of H₂ by an Unsaturated d⁸ Nickel Center: via Tetravalent Nickel?

Abstract

Collision of H(2) with the unusual nickel complex, (PNP)Ni(+), where PNP = ((t)Bu(2)PCH(2)SiMe(2))(2)N, forms a rare dihydrogen complex of the d(8) configuration which then rearranges to heterolytically cleave the H-H bond. Experimental studies support a short H/H distance in the coordinated diatomic, and DFT calculations show that the transition state for heterolysis, in spite of the fact that this involves an amide nitrogen located trans to the H(2), has the H/H bond fully split, and has all the geometric features of Ni(IV), but this is a local maximum, not a minimum.

Molecular Electrocatalysts for the Oxidation of Hydrogen and the Production of Hydrogen – The Role of Pendant Amines as Proton Relays

“…In contrast to most other metals, dihydrogen complexes of Ni are very rare; Caulton has recently discovered a Ni(H 2 ) complex, characterized by NMR spectroscopic measurements and DFT calculations. [36] Cleavage of the H-H bond leads to 9, a Ni 0 complex with two protonated amine ligands that can be observed spectroscopically. The two electrons of H 2 have reduced the Ni II to Ni 0 , and the two protons have been transferred to the basic sites on the pendant amines.…”

Thermally stable N2 and H2 adducts of cationic nickel(ii)

“…The only reported molecular example is a [PNP]Ni (H 2 ) + intermediate detected at low temperatures by Caulton and co-workers; the H 2 subsequently undergoes intramolecular heterolytic cleavage. 4 Due to the lack of stable Ni(H 2 ) complexes and the relative scarcity of nickel hydrides, 1 iron is often proposed to be the site of dihydrogen binding and heterolytic activation in [NiFe] hydrogenases. 5,6 However, certain EPR, X-ray diffraction and theoretical studies have been interpreted to suggest that H 2 is instead activated at a five-coordinate, EPR-silent Ni center in the enzyme active site.…”

Coordination of a Hemilabile Pincer Ligand with an Olefinic Backbone to Mid-to-Late Transition Metals

“…The geometry of the metal center is pseudo-T-shaped, as deduced from the PCuP angle of 162.34 (4) o . This angle is smaller than the corresponding angle in [(PNP tBu )-Cu][SbF 6 ], which has a value of 172.44(3)°. 26 The olefin appears to be bound to the metal center; however the CuC distances of 2.340(3) and 2.337(3) Å are longer than those in previously observed copper olefin complexes, which show values around 2.0 Å.…”

“…Analytically pure 5 was obtained from a concentrated toluene solution layered with n-pentane at −35°C (23. 6 a NMR spectra were recorded in CDCl 3 . b C 6 D 6 was used as the NMR solvent.…”

“…Most hydrogen atoms were omitted for clarity. Selected distances (Å) and angles (deg): Thermal-ellipsoid (50% probability level) representation of (tPCHCHP)CoCl(6). Most hydrogen atoms were omitted for clarity.…”