N-Bridged Pincer Iridium Complexes for Highly Efficient Alkane Dehydrogenation and the Relevant Linker Effects

A series of iridium complexes ((POCR)-P-R'NP)-HCl (4-HCl) and ((PSCR)-P-R'NP)Ir-HCl (5-HCl) (R = substituent on P; R' = substituent on N) supported by N-linked pincer ligands, with the other linker being O- or S-atom, have been synthesized. Among them, complexes with phosphino-iPr substituents ((POCR)-P-iPr'NP)Ir (4b, 4e, and 4g) exhibit very high catalytic activity for transfer dehydrogenation (TD) of both cyclic and linear alkanes. In the prototypical TD reaction of cyclooctane (COA) with tertbutylethylene (TBE), for example, 4g affords 14 720 turnovers at 200 degrees C, which are more than twofold of that obtained by the most efficient catalyst reported so far. Moreover, these complexes are highly effective for acceptorless dehydrogenation of 1,2,3,4-tetrahydronaphthalene, giving a turnover frequency of 8.8 min(-1) for H2- production within the first 4 h. The effects of the linkers at the positions ortho to the Ir center have been elucidated by a systematic comparison of electronic and steric properties of these O/ N- and S/N-linked pincer systems to those of C/C-linked (PCP)-P-iPr, O/O-linked (POCOP)-P-iPr, and S/O-linked (PSCOP)-P-iPr systems. Examination of the structure-activity relationships reveals that alkane C-H bond addition to the 14e (pincer)Ir fragment (the ratedetermining step in the TD reaction) is more favored by (POCR)-P-iPr'NP relative to other pincers, largely due to the strong N-linker -> C(aryl) pi-donation. Compared to the parent (POCOP)-P-iPr complex with similarly pi-donating O-linkers, the energy of the alkene-bound out-of-cycle resting state is raised by (POCR)-P-iPr'NP primarily due to steric factors. Consequently, the incorporation of the N-linker exerts an overall favorable effect on the catalytic rates. In addition, we found that the thermal stability of these catalysts with different linker combinations differs significantly.