Structural characterisation of cationic methylallyl, methylindenyl and pentamethylcyclopentadienyl nickel complexes containing the bulky phosphine 1,2-bis(diisopropylphosphino)ethane

Abstract

A novel high-yield, convenient synthetic method for the complex [Ni(COD)₂] has been developed based upon the use of dibutylmagnesium as reducing agent. The cationic complexes [Ni(η³-CH₂C(R)CH₂)(dippe)][BPh₄] (R = Me 1a or H 1b; dippe = Prⁱ₂PCH₂CH₂PPrⁱ₂) were obtained by reaction of [Ni(COD)₂] with BrCH₂C(R)CH₂ in diethyl ether, followed by treatment with dippe and NaBPh₄ in methanol. [Ni(2-MeInd)(dippe)][BPh₄] 2 (2-MeInd = 2-methylindenyl) and [Ni(C₅Me₅)(dippe)][BPh₄] 3 were respectively prepared by reaction of either [Ni(2-MeInd)Br(PPh₃)] or [Ni(C₅Me₅)Br(PPh₃)] with dippe and NaBPh₄ in methanol. The crystal structures of 1a, 2 and 3 were determined. All of the compounds show pseudo-square planar “two legged” piano stool structures. The methylallyl ligand in 1a shows η³ coordination as expected, whereas in 2 the occurrence of an allyl–ene distortion in the 2-methylindenyl ligand leads to an intermediate η³/η⁵ coordination mode. An intermediate allyl–ene/diene distortion is present in the C₅Me₅ ligand in 3, although in this case a symmetrical η⁵ coordination is observed. All compounds were also characterised in solution by NMR spectroscopy.