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 4pz-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.
Bibliographical noteFunding Information:
The authors thank Prof. Laura Gagliardi, Prof. Chris Cramer, Dr Molly O'Hagan, Dr Adrian Houghton, and Dr Tom Autrey for helpful discussions. James Moore is acknowledged for assistance with X-ray diffraction. R. C. C. was supported by the DOE Office of Science Graduate Student Research and the UMN Doctoral Dissertation Fellowship programs. M. V. V. was supported by the NSF Graduate Research Fellowship. C. C. L. acknowledges NSF (CHE-1665010) for support of the experimental work. J. X., J. Y., and K. D. V. were supported as part of the Inorganometallic Catalyst Design Center, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences under Award DESC0012702. S. A. B., J. C. L., and A. M. A. were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Single crystal neutron diffraction experiment performed on TOPAZ used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory, under Contract No. DE-AC05-00OR22725 with UT-Battelle, LLC. X-ray diffraction experiments were performed using a crystal diffractometer acquired through an NSF-MRI award (CHE-1229400).
© 2019 The Royal Society of Chemistry.