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 NiI/NiII 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 NiII and two NiI 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 NiII2NiI 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.