Nickel Polyene(yl) Complexes Synthesized from a Dinuclear Ni(I) Compound and (Multi)cyclic Polyenes

The low-valent Ni–Ni-bonded compound [(NiIL·–)2] (1, L = [(2,6-iPr2C6H3)NC(Me)]2) reacts with
a series of multicyclic polyenes through different redox processes,
affording seven heteroleptic complexes (2–8) containing nickel, α-diimine L, and
polyene (or polyenyl) ligands, where all three components exhibit
variable oxidation states. The reaction of 1 with 6-(dimethylamino)fulvene
or 6,6-dimethylfulvene led to the reductive dimerization of two fulvene
molecules, yielding the dinuclear complexes 2 and 3 with a bridging bis-Cp ligand. In contrast, similar reactions
with 6,6-diphenylfulvene or pentamethylcyclopentadiene (C5Me5H) only gave the adducts [LNi(η4-6,6-diphenylfulvene)] (4) or [(L·–)Ni(η4-C5Me5H)] (5) with a neutral polyene
molecule. However, the two complexes have different oxidation levels
of the nickel center and ligand L, depending on the electronic
property of the substrate. By using biphenylene, oxidative addition
of a strained C–C bond to nickel occurred to form the biphenyl
metallacycle [LNi(μ2-Ph2)]
(6). Furthermore, the reaction of 1 with
acenaphthylene or C60 afforded complexes [LNi(η2-acenaphthylene)] (7) and [LNi(η2-C60)] (8)
featuring an η2-bonded neutral polyene. The results
provide evidence of the facile electron transfer between the metal
and ligands so that the nickel center can adjust its basicity to coordinate
effectively with polyene substrates of different π-acidity.