A Reduced β-Diketiminato-Ligated Ni₃H₄ Unit Catalyzing H/D Exchange

An investigation concerning the stepwise reduction of the β-diketiminato nickel(II) hydride dimer [LNi(μ-H)₂NiL], 1 (L = [HC(CMeNC₆H₃(iPr)₂)₂]⁻), has been carried out. While the reaction with one equivalent of potassium graphite, KC₈, led to the mixed valent Ni^I/Ni^II complex K[LNi(μ-H)₂NiL], 3, treatment of 1 with two equivalents of KC₈ surprisingly yielded in the trinuclear complex K₂[LNi(μ-H)₂Ni(μ-H)₂NiL], 4, in good yields. The Ni₃H₄ core contains one Ni^II and two Ni^I centers, which are antiferromagnetically coupled so that a singlet ground state results. 4 represents the first structurally characterized molecular compound with three nickel atoms bridged by hydride ligands, and it shows a very interesting chemical behavior: Single-electron oxidation yields in the Ni^II₂Ni^I compound K[LNi(μ-H)₂Ni(μ-H)₂NiL], 5, and treatment with CO leads to the elimination of H₂ with formation of the carbonyl complex K₂[LNi(CO)]₂, 6. Beyond that, it could be shown that 4 undergoes H/D exchange with deuterated solvents and the deuteride-compound 4-D₄ reacts with H₂ to give back 4. The crystal structures of the novel compounds 3−6 have been determined, and their electronic structures have been investigated by EPR and NMR spectroscopy, magnetic measurements, and DFT calculations.