Diphenylacetylene stabilised alkali-metal nickelates: synthesis, structure and catalytic applications

Whilst low-valent nickelates have recently been proposed as intermediates in Ni-catalysed reactions involving polar organometallics, their isolation and characterisation is often challenging due to their high sensitivity and reactivity. Advancing the synthetic, spectroscopic and structural insights of these heterobimetallic systems, here we report a new family of alkyne supported alkali-metal nickelates of the formula Li(4)(solv)(n)(Ar)(4)Ni(2){μ(2):η(2),η(2)-Ph-CC-Ph} (where solv = Et(2)O, THF; Ar = Ph, o-Tol, naphthyl, 4-(t)Bu-C(6)H(4)) which can be accessed through the combination of Ni(COD)(2), Ph-CC-Ph and the relevant lithium aryl in a 2 : 1 : 4 ratio. Demonstrating the versatility of this approach, the sodium and potassium nickelates can also be accessed when using PhNa or via alkali-metal exchange with AMO(t)Bu (AM = Na, K). When employing bulky or structurally constrained aryl-lithiums, mononickel complexes of the formula Li(2)(solv)(n)(Ar)(2)Ni{η(2)-Ph-CC-Ph} are instead obtained, highlighting the structural diversity of alkali-metal nickelates bearing alkyne ligands. Expanding the catalytic potential of these systems, their ability to promote the catalytic cyclotrimerisation of diphenylacetylene to hexaphenylbenzene was explored, with mononickel compounds bearing electron rich aryl-substituents displaying the best performance.