Synthesis and Reactivity of N‑Heterocyclic PSiP Pincer Iron and Cobalt Complexes and Catalytic Application of Cobalt Hydride in Kumada Coupling Reactions

The new N-heterocyclic σ-silyl
pincer ligand HSiMe­(NCH2PPh2)2C6H4 (1) was designed. A series of tridentate
silyl pincer Fe and Co complexes were prepared. Most of them were
formed by chelate-assisted Si–H activation. The typical iron
hydrido complex FeH­(PMe3)2(SiMe­(NCH2PPh2)2C6H4) (2) was obtained by Si–H activation of compound 1 with Fe­(PMe3)4. The combination of compound 1 with CoMe­(PMe3)4 afforded the Co­(I)
complex Co­(PMe3)2(SiMe­(NCH2PPh2)2C6H4) (3).
The Co­(III) complex CoHCl­(PMe3)­(SiMe­(NCH2PPh2)2C6H4) (5)
was generated by the reaction of complex 1 with CoCl­(PMe3)3 or the combination of complex 3 with HCl. However, when complex 3 was treated with
MeI, the Co­(II) complex CoI­(PMe3)­(SiMe­(NCH2PPh2)2C6H4) (4),
rather than the Co­(III) complex, was isolated. The catalytic performance
of complex 5 for Kumada coupling reactions was explored.
With a catalyst loading of 5 mol %, complex 5 displayed
efficient catalytic activity for Kumada cross-coupling reactions of
aryl chlorides and aryl bromides with Grignard reagents. This catalytic
reaction mechanism is proposed and partially experimentally verified.