Abstract
A family of nickel allyl complexes of the type (allyl)2Ni(L) (L = PMe3 (1), PEt3 (2), PPh3 (3), NHC (4); NHC = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydro-2H-imidazol-2-ylidene) and (2-methylallyl)2Ni(L) (L = PMe3 (5), PEt3 (6), PPh3 (7), NHC (8)) have been prepared using literature methods. Compounds 5, 7, and 8 were characterized by X-ray crystallography. Whereas compounds 5 and 7 are 18-electron species with two η3-allyl ligands, the NHC-supported complexes are 16-electron species with one η1- and one η3-allyl ligand. Using DFT we have identified the key factors in predicting whether complexes of the type (allyl)2M(L) (M = Ni, Pd) are 16- or 18-electron species. Compounds 1, 2, and 5–8 readily decompose to give a mixture of products, while compound 4 decomposes to give the unusual Ni0 species (η4-1,5-hexadiene)Ni(NHC) (9), which was characterized by X-ray crystallography. The reactions of 1–8 with CO2 were investigated. Compounds 5–8 react rapidly with CO2 at low temperature to form well-defined unidentate nickel carboxylates of the type (η3-2-methylallyl)Ni(OC(O)C4H7)(L) (L = PMe3 (10), PEt3 (11), PPh3 (12), NHC (13)). The structure of 13 was elucidated using X-ray crystallography. In contrast, compounds 1–4 do not react with CO2. We believe that the difference in reactivity between the 2-methylallyl-supported complexes and the allyl-supported complexes is related to the mechanism of CO2 insertion.
Published Version
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