Carbon−Hydrogen Bond Oxidative Addition of Partially Fluorinated Aromatics to a Ni(P^<i>i</i>Pr₃)₂Synthon: The Influence of Steric Bulk on the Thermodynamics and Kinetics of C−H Bond Activation

The reaction of (P i Pr 3 ) 2 NiCl 2 with the anthracene adduct (THF) 3 Mg(η 2 -C 14 H 10 ) in THF provides the anthracene adduct (P i Pr 3 ) 2 Ni(η 2 -C 14 H 10 ). In aromatic solvents (benzene, toluene, mesitylene) a thermal equilibrium exists between the bis(phosphine)nickel(0) anthracene adduct, (P i Pr 3 ) 2 Ni(η 2 -C 14 H 10 ), and the monophosphine solvent adduct, (P i Pr 3 )Ni(η 6 -solvent). The reaction of (P i Pr 3 ) 2 Ni(η 2 -C 14 H 10 ) with 1,2,4,5-C 6 F 4 H 2 affords the C−H activation product trans -(P i Pr 3 ) 2 NiH(2,3,5,6-C 6 F 4 H). The thermodynamic C−F activation product is not obtained even after hours of heating at 100 °C. Similar reactions with 1,2,3,5-C 6 F 4 H 2 and pentafluorobenzene produce the desired C−H activation products, trans -(P i Pr 3 ) 2 NiH(2,3,4,6-C 6 F 4 H) and trans -(P i Pr 3 ) 2 NiH(C 6 F 5 ), respectively, in >95% yield. The reaction with 1,2,3,4-tetrafluorobenzene did not produce an observable C−H activation product. Unlike previously reported analogous C−H activation products with Ni(PEt 3 ) 2 synthons, the bulkier Ni(P i Pr 3 ) 2 moiety did not provide observable mononuclear or dinuclear η 2 -fluoroarene adducts. Solutions of Ni(COD) 2 with 2 equiv of triisopropylphosphine and 1,2,4,5-C 6 F 4 H 2 reacted to give the 1,5-cyclooctadiene insertion and rearrangement product, (η 3 -C 8 H 13 )Ni(P i Pr 3 )(2,3,5,6-C 6 F 4 H). The same reaction with 1,2,3,5- and 1,2,3,4-C 6 F 4 H 2 afforded analogous compounds, which demonstrates that C−H bond activation is kinetically accessible at room temperature with 1,2,3,4-tetrafluorobenzene despite the presence of a single ortho -fluorine substituent adjacent to the site of activation. The room-temperature reactions of the C−H activation products (P i Pr 3 ) 2 NiH(Ar F ) (Ar F = 2,3,5,6-C 6 F 4 H; C 6 F 5 ) with 3-hexyne provided a mixture of the alkyne adduct (P i Pr 3 ) 2 Ni(η 2 -EtC≡CEt), with the liberation of Ar F H, and the insertion product (P i Pr 3 ) 2 Ni(CEt═CHEt)(Ar F ), even in the presence of excess fluorinated aromatic Ar F H. The reaction of (P i Pr 3 ) 2 Ni(η 2 -EtC≡CEt) with 1,2,4,5-C 6 F 4 H 2 resulted in no reaction at room temperature, but heating at 50 °C provided the insertion product (P i Pr 3 ) 2 Ni(CEt═CHEt)(Ar F ) as the initial product, followed by the product of reductive elimination, 1,2,4,5-C 6 F 4 H-CEt═CHEt. In contrast, the reaction of (PEt 3 ) 2 Ni(η 2 -EtC≡CEt) with 1,2,4,5-C 6 F 4 H 2 at 80 °C slowly produced (PEt 3 ) 2 Ni(CEt═CHEt)(2,3,5,6-C 6 F 4 H), but very little 1,2,4,5-C 6 F 4 H-CEt═CHEt.

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