Square Planar Cobalt(II) Hydride versus T-Shaped Cobalt(I): Structural Characterization and Dihydrogen Activation with PNP-Cobalt Pincer Complexes

The carbazole-based pincer ligand (R)((PNP)-P-Cbz)H (R = Pr-i, Bu-t) has been used for the synthesis and characterization of various low- and high-spin cobalt complexes. Upon treatment of the high-spin complexes (R)((PNP)-P-Cbz)CoCl (2(R)-(CoCl)-Cl-II) with NaHBEt3, the selective formation of cobalt(II) hydride 3(iPr)-(CoH)-H-II and T-shaped cobalt(I) complex 4(tBu)-Co-I was observed, depending on the substituents at the phosphorus atoms. For an unambiguous characterization of the reaction products, a density functional theory (DFT) supported paramagnetic NMR analysis was carried out, which established the electron configuration and the oxidation states of the metal atoms, thus demonstrating the significant impact of ligand substitution on the outcome of the reaction. A distinct one-electron reactivity was found for 4(tBu)-Co-I in the dehalogenation of (BuCl)-Bu-t and cleavage of PhSSPh. On the other hand, the Co(I )species displayed two-electron redox behavior in the oxidative addition of dihydrogen. The resulting dihydride complex 6(tBu)-Co-III(H)(2) was found to display sluggish reactivity toward alkenes, whereas the cobalt(II) hydride 3(iPr)-(CoH)-H-II was successfully employed in the catalytic hydrogenation of unhindered alkenes. The stoichiometric hydrogenolysis of 8(iPr)-(CoBn)-Bn-II at elevated pressure (10 bar) led to a rapid cleavage of the Co-C bond to yield hydride complex 3(iPr)-(CoH)-H-II. On the other hand, treatment of 2(iPr)-(CoCl)-Cl-II with phenethylmagnesium chloride directly resulted in the formation of 3(iPr)-(CoH)-H-II, indicating facile beta-H elimination of the alkene insertion product (reversibly) generated in the catalytic hydrogenation. On the basis of these observations, a mechanistic pathway involving a key sigma-bond metathesis step of the Co-II-alkyl species is proposed.