Effects of the halogenido ligands on the Kumada-coupling catalytic activity of [Ni{tBuN(PPh2)2-κ2P}X2], X = Cl,Br, I,complexes

Novel nickel(ii) complexes bearing (^tbutyl)bis(diphenylphosphanyl)amine and different halogenido ligands, [Ni(P,P)X₂] = [Ni{^tBuN(PPh₂)₂-κ²P}X₂], (X = Cl, Br, I) are prepared, characterized by IR and NMR spectroscopy, mass spectrometry and X-ray crystallography, and tested as catalysts in the Kumada cross-coupling reaction of model substituted iodobenzenes and p-tolylmagnesium bromide. The data obtained together with DFT calculations indicate that these new catalysts operate in the Ni(i)–Ni(iii) mode. The highest catalytic activity and selectivity are exhibited by [Ni(P,P)Cl₂], which is most easily reduced by the used Grignard reagent to the Ni(i) state. This process is much more energy demanding in the case of the bromido and iodido complexes, causing the appearance of the induction period. [Ni(P,P)Cl₂] is also very active in the cross-couplings of substrates with iodine atoms sterically shielded by ortho substituents. The data obtained are in good accordance with the described positive effect of the increased electron-releasing power of N-substituents R′ on the overall catalytic performance of [Ni{R′N(PPh₂)₂-κ²P}X₂] complexes.

Novel nickel(ii) complexes [Ni(P,P)X₂] = [Ni{^tBuN(PPh₂)₂-κ²P}X₂], X = Cl, Br, I, are prepared, characterized by IR and NMR spectroscopy, mass spectrometry and X-ray crystallography, and tested as catalysts in the Kumada cross-coupling reaction.

In recent years, the chemical and catalytic properties of transition metal complexes bearing N-functionalized bis(diphenylphosphanyl)amine ligands, R′N(PPh₂)₂, have been under consideration.^1,2 For instance, chromium complexes with this type of ligand are known to oligomerize various olefins.^3–8 In addition, a large number of [M{R′N(PPh₂)₂-κ²P}X₂] complexes, M = Ni, Pd, Pt; X = Cl, Br, I (see Scheme 1), exhibiting small P–M–P bite angles, were recently reviewed.² Selected palladium(ii) complexes bearing X = Cl,^9–14 Br,^14,15 I,^14,16 catalyze the Suzuki–Miyaura and Heck coupling reactions. Some structurally characterized Ni(ii) analogous complexes bearing X = Cl,^17–28 Br,^18,29–37 I,^18,36,38 catalyze polymerization of norbornene^20,21 or oligomerization (X = Br,^32,34 I,³⁸) and polymerization (X = Br²⁹) of ethene. It should be stressed that nickel(ii) complexes of this family are only moderately active catalysts in the Suzuki–Miyaura reaction.³⁵ On the other hand, they exhibit a considerable catalytic activity and acceptable selectivity in the Kumada coupling reaction.^23,35Open in a separate windowScheme 1General structure of the studied complexes [M(P,P)X₂], M = Ni, Pd, Pt; X = Cl, Br, I; R'' = ((S)-CHMePh), (CH₂)₃Si(OCH₃)₃, ^tBu.Kumada coupling is one of the most important C–C coupling reactions³⁹ for a wide range of purposes, including pharmaceutical applications.⁴⁰ Although palladium-based complexes are mostly the first choice catalysts for this coupling,^41–43 complexes of other transition metals such as iron,^44,45 and nickel^46,47 are also used. We have already investigated the catalytic activity of [Ni{R′N(PPh₂)₂-κ²P}X₂], R′ = (S)-CHMePh; X = Cl, Br,³⁵ and R′ = (CH₂)₃Si(OMe)₃; X = Cl,²³ in homogeneous systems to extend the scope of nickel(ii) catalysts in this reaction. The latter catalyst has also been anchored onto mesoporous molecular sieves, thus providing an active heterogenized catalyst.²³ In homogeneous catalytic reactions, both catalysts bearing R′ = (S)-CHMePh) showed a substrate conversion (68% for X = Cl and 63% for X = Br),³⁵ significantly lower compared to that of the catalyst with R′ = (CH₂)₃Si(OMe)₃ and X = Cl (79%).²³ These results suggested that the increased electronegativity of coordinated halogenido ligands and the increased electron-donating power of the R′ moiety have positive effects on the catalytic efficiency of this type of nickel(ii) complexes. In the work presented herein, the effects exerted by the identity of halogenido ligands X⁻ and the R′ moiety on catalytic activity and selectivity were further assessed by exploring three novel complexes, [Ni{^tBuN(PPh₂)₂-κ²P}X₂], X = Cl, Br, I, henceforth referred to as [Ni(P,P)X₂], bearing the strongly electron-releasing ^tbutyl (^tBu) group as R′.