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

Abstract

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.