Cyclopentadiene Alkylation and Nickel Complexes with Tri-, Tetra-, or Pentaisopropylcyclopentadienide or an Even Bulkier Lithium Alkylcyclopentadienide

Phase transfer catalysis has been adapted to the synthesis
of triisopropylcyclopentadiene
isomers 1 in high yield and purity. Alkylation of sodium
triisopropylcyclopentadienides with isopropyl bromide in tetrahydrofuran
has been shown to be the currently most efficient way to generate
tetraisopropylcyclopentadiene isomers 2, which give sodium
tetraisopropylcyclopentadienide (3) upon metalation.
The crucial step for the introduction of the fifth isopropyl group,
the selective attack of an iminium salt at the tetraisopropylcyclopentadienide
anion in the 5-position with generation of the corresponding 1,2,3,4-tetraisopropyl­(6-dimethylamino)­fulvene
(4), has been carried out according to a published procedure.
Addition of 1-naphthyllithium to the dimethylaminofulvene 4 yielded extremely bulky lithium dimethylamino­(1-naphthyl)­methyltetraisopropylcyclopentadienide
(7). Pure 1,2,4,1′,2′,4′-hexaisopropylnickelocene
(8) was obtained from sodium triisopropylcyclopentadienide
and nickel­(II) bromide. The tetraisopropylcyclopentadienylnickel­(II)
bromide [(C5iPr4H)­Ni­(μ-Br)]2 (9) added triphenylphosphine, trimethylphosphine,
trimethyl phosphite, and triisopropylphosphine with formation of the
diamagnetic phosphine complexes [(C5iPr4H)­NiBr­(PPh3)] (10), [(C5iPr4H)­NiBr­(PMe3)] (13), [(C5iPr4H)­NiBr­{P­(OMe)3}] (14), and [(C5iPr4H)­NiBr­(PiPr3)] (15), respectively.
The triphenylphosphine derivative 10 displays a dissociation
equilibrium with small amounts of 9 in deuteriobenzene
solution and could be converted to the methylnickel­(II) derivative
[(C5iPr4H)­NiMe­(PPh3)]
(11) or to the phenyl complex [(C5iPr4H)­NiPh­(PPh3)] (12), respectively.
Substitution reactions of the bromide 9 yielded a dinuclear
complex with two bridging 2,6-dimethylphenolate ligands, [(C5iPr4H)­Ni­(μ-2,6-OC6Me2H3)]2 (16), with potassium 2,6-dimethylphenolate
and methylene-bridged dinuclear [{(C5iPr4H)­Ni}2(μ-CH2)] (18) with methylmagnesium chloride and concomitant methane formation.
Partial hydrolysis converted the phenolate 16 to the
monohydroxy derivative [{(C5iPr4H)­Ni}2(μ-2,6-OC6Me2H3)­(μ-OH)]
(17). Crystal structure analyses have been carried out
on complexes 8, 11, 13, 14, 16–18, and 20. Lithium pentaisopropylcyclopentadienide
(6) reacted with nickel­(II) bromide dimethoxyethane adduct
in pentane to form the bromo-bridged [(C5iPr5)­Ni­(μ-Br)]2 (19), and the new
bulky dimethylamino­(naphthyl)­methyl-carrying cyclopentadienide 7 reacted with the dimethoxyethane adduct of nickel­(II) bromide
in pentane to give the diamagnetic alkylcyclopentadienylnickel bromide
monomer [{C5iPr4CH­(NMe2)­(1-naphthyl)}­NiBr] (20). The crystal structure of 20 shows no sign of electron donation from the dimethylamino
nitrogen lone pair to the nickel center. The paramagnetism of 19 is in line with similar findings for the pentamethylcyclopentadienyl,
tetraisopropylcyclopentadienyl, and 1,2,4-tri-tert-butylcyclopentadienyl analogues of 19. The diamagnetism
of 20 is most probably due to an empty dz2 orbital, which is high
in energy because of the combined electron donation from the five-membered
ring and the bromo ligand on the z axis.