Thermodynamic Modulation of Dihydrogen Activation Through Rational Ligand Design in Ge^II-Ni⁰ Complexes

A family of chelating aryl-functionalized germylene ligands has been developed and employed in the synthesis of their corresponding 16-electron Ni⁰ complexes (^PhiPDippGeAr·Ni·IPr; ^PhiPDipp = {[Ph₂PCH₂Si(ⁱPr)₂](Dipp)N}^-; IPr = [{(H)CN(Dipp)}₂C:]; Dipp = 2,6-ⁱPr₂C₆H₃). These complexes demonstrate the ability to cooperatively and reversibly activate dihydrogen at the germylene-nickel interface under mild conditions (1.5 atm H₂, 298 K). We show that the thermodynamics of the dihydrogen activation process can be modulated by tuning the electronic nature of the germylene ligands, with an increase in the electron-withdrawing character displaying more exergonic ΔG₂₉₈ values, as ascertained through NMR spectroscopic Van't Hoff analyses for all systems. This is also shown to correlate with experimental ³¹P NMR and UV/vis absorption data as well as with computationally derived parameters such as Ge-Ni bond order and Ni/Ge NPA charge, giving a thorough understanding of the modulating effect of ligand design on this reversible, cooperative bond activation reaction. Finally, the utility of this modulation was demonstrated in the catalytic dehydrocoupling of phenylsilane, whereby systems that disfavor dihydrogen activation are more efficient catalysts, aligning with H₂-elimination being the rate-limiting step. A density functional theory analysis supports cooperative activation of the Si-H moiety in PhSiH₃.