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

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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 = Pri 2PCH2CH2PPri 2) 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.

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