A Combined Experimental and Computational Study of Unexpected C−F Bond Activation Intermediates and Selectivity in the Reaction of Pentafluorobenzene with a (PEt₃)₂Ni Synthon

The reaction of the anthracene adduct (PEt 3 ) 2 Ni(η 2 -C 14 H 10 ) with C 6 F 5 H or C 6 F 6 provides a facile route to the isolable dinuclear adducts [(PEt 3 ) 2 Ni] 2 (μ-η 2:η 2 -C 6 F 5 H) and [(PEt 3 ) 2 Ni] 2 (μ-η 2:η 2 -C 6 F 6 ). The Ni(PEt 3 ) 2 moieties reside on opposite faces of the fluoroarene ring, and bond lengths indicate considerable loss of aromaticity. Solutions of [(PEt 3 ) 2 Ni] 2 (μ-η 2:η 2 -C 6 F 5 H) with added C 6 F 5 H convert with >97% selectivity to the ortho -F activated compound tran s-(PEt 3 ) 2 NiF(2,3,4,5-C 6 F 4 H). The addition of C 6 F 5 H to solutions of [(PEt 3 ) 2 Ni] 2 (μ-η 2:η 2 -C 6 F 5 H) also provided equilibrium amounts of the mononuclear complex (PEt 3 ) 2 Ni(η 2 -C 6 F 5 H) and the C−H activation product trans- (PEt 3 ) 2 NiH(C 6 F 5 ) as intermediates. Increased C 6 F 5 H concentrations accelerate the rate of C−F bond activation. The reaction of (PEt 3 ) 4 Ni with C 6 F 5 H under similar conditions also provided spectroscopic evidence for the presence of the binuclear adduct [(PEt 3 ) 2 Ni] 2 (μ-η 2:η 2 -C 6 F 5 H), the mononuclear adduct (PEt 3 ) 2 Ni(η 2 -C 6 F 5 H), and the C−H activation product trans- (PEt 3 ) 2 NiH(C 6 F 5 ) in the reaction mixture; however, unlike the C−F bond activation observed from pure [(PEt 3 ) 2 Ni] 2 (μ-η 2:η 2 -C 6 F 5 H) and C 6 F 5 H, the reaction of Ni(PEt 3 ) 4 and C 6 F 5 H yields a mixture of C−F activation products that included the para -F and meta -F activation products as major products. This indicates that alternate mechanisms of C−F activation dominate with this source of the Ni(PEt 3 ) 2 moiety under these conditions, suggestive of a radical mechanism. Density functional theory calculations were carried out to provide insight into the 19 F NMR spectra of these complexes, as well as to gain understanding of the importance of phosphine donor choice in the thermodynamics of the interconversion of the dinuclear complexes, mononuclear complexes, and C−H activation products.

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