Thermodynamic and kinetic studies of H2 and N2 binding to bimetallic nickel-group 13 complexes and neutron structure of a Ni(η2-H2) adduct

Abstract

Understanding H₂ binding and activation is important in the context of designing transition metal catalysts for many processes, including hydrogenation and the interconversion of H₂ with protons and electrons. This work reports the first thermodynamic and kinetic H₂ binding studies for an isostructural series of first-row metal complexes: NiML, where M = Al (1), Ga (2), and In (3), and L = [N(o-(NCH₂PⁱPr₂)C₆H₄)₃]³⁻. Thermodynamic free energies (ΔG°) and free energies of activation (ΔG^‡) for binding equilibria were obtained via variable-temperature ³¹P NMR studies and lineshape analysis. The supporting metal exerts a large influence on the thermodynamic favorability of both H₂ and N₂ binding to Ni, with ΔG° values for H₂ binding found to span nearly the entire range of previous reports. The non-classical H₂ adduct, (η²-H₂)NiInL (3-H₂), was structurally characterized by single-crystal neutron diffraction—the first such study for a Ni(η²-H₂) complex or any d¹⁰ M(η²-H₂) complex. UV-Vis studies and TD-DFT calculations identified specific electronic structure perturbations of the supporting metal which poise NiML complexes for small-molecule binding. ETS-NOCV calculations indicate that H₂ binding primarily occurs via H–H σ-donation to the Ni 4p_z-based LUMO, which is proposed to become energetically accessible as the Ni(0)→M(III) dative interaction increases for the larger M(III) ions. Linear free-energy relationships are discussed, with the activation barrier for H₂ binding (ΔG^‡) found to decrease proportionally for more thermodynamically favorable equilibria. The ΔG° values for H₂ and N₂ binding to NiML complexes were also found to be more exergonic for the larger M(III) ions.