Abstract
Understanding H(2) binding and activation is important in the context of designing transition metal catalysts for many processes, including hydrogenation and the interconversion of H(2) with protons and electrons. This work reports the first thermodynamic and kinetic H(2) 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-(NCH(2)P(i)Pr(2))C(6)H(4))(3)](3-). Thermodynamic free energies (ΔG°) and free energies of activation (ΔG (‡)) for binding equilibria were obtained via variable-temperature (31)P NMR studies and lineshape analysis. The supporting metal exerts a large influence on the thermodynamic favorability of both H(2) and N(2) binding to Ni, with ΔG° values for H(2) binding found to span nearly the entire range of previous reports. The non-classical H(2) adduct, (η(2)-H(2))NiInL (3-H(2)), was structurally characterized by single-crystal neutron diffraction-the first such study for a Ni(η(2)-H(2)) complex or any d(10) M(η(2)-H(2)) 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 H(2) 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 H(2) binding (ΔG (‡)) found to decrease proportionally for more thermodynamically favorable equilibria. The ΔG° values for H(2) and N(2) binding to NiML complexes were also found to be more exergonic for the larger M(iii) ions.