Abstract:  Pincer ligands are a type of tridentate ligands in organometallics. In coordination, they show enhanced thermodynamic stability and tunable novel reactivity; therefore, pincer ligands are ideal in homogeneous catalysis to achieve a wide range of catalysis. In 2018, we synthesized the pseudo-aromatic PN3P*Ni-H pincer complex that shows unprecedented nucleophilicity on the ligand nitrogen arm, and reactions revealed its N-heterocycle-carbene(NHC)-like property. Based on the unique reactivity of pseudo-aromatic pincer-metal complexes. The PN3(P) pincer-metal complexes show ligand center reactivity (LCR), which compared to the other three reaction mode, is least understood and highly intriguing, and we suspect ligand center reactivity in PN3(P) pincer-metal complexes is related with their pseudo-aromaticity.

Aside from reported reactions, we are still trying to understand its unique property of H2 activation and C-H bond activation. Catalytic activation of H2 plays an important role in large-scale pharmaceutical and synthetic chemistry.  According to our current experiment the PN3P*Ni-H complex can perform H2 homolysis; however, it cannot be classified into any known type of H2 activation – the total reaction, according to our knowledge, is likely a termolecular homolysis of H2 by two PN3P*Ni-H molecules. The PN3P*Ni-H complex is also found to be capable of break C-H bond of methane, forming ethane and H2. As far as we know, there are not many examples can be classified into this type. This result further encourages us to depict the mechanism behind, since there are not many catalysts can presumably homolyze C-H bond of methane, which can expand our knowledge on hydrogen and methane utilization. The two reaction and extensive experimental entries with various (un-)saturated alkenes are unique and of great importance.

Thermodynamically, the bond dissociation Gibbs free energy of H-H bond and that of C-H bond of methane are nearly identical; therefore, any activation of the former bond is thermodynamically appropriate for the latter one. However, countless works of H2 activation are published, while the C-H activation of methane is notoriously difficult. It is easy to conclude this situation as the difference between the kinetics and bonding. The PN3P*Ni-H complex is capable of activation of these 2 bonds, regardless of their chemistry difference, exhibiting a Gibbs-free-energy-controlled result. The experimental result is profoundly astounding and evoke our curiosity of exploring the reaction mechanism behind.
Our inspection on the symmetric hydrogenated PN3P-Ni-H radical, which is the catalyst PN3P*Ni-H after addition of one hydrogen radical, exhibited an extremely short Ni-H bond in its mirror plane. Therefore, the hypothesis of pseudo-Jahn-Teller distortion is proposed to explain the observed shortening of Ni-H is a vibration averaged by time.

Fractional occupation number weighted electron density (FOD) and their corresponding orbitals are used to identify the multireference character of the radical. Followed by ab initio computational study, the most strongly-correlated orbitals are chosen, and to finally determine the bond energy, a potential energy surface scan is performed under the level of QD-SC-NEVPT2/CAS(17e,14o) for arm N-H distance, in which a manifold of conical intersections are found. The easily accessible low-lying excited states has similar bond dissociation free energy (BDFE) with 1-Hydroxy-2,2,6,6-tetramethylpiperidine (TEMPOH), which is in accordance with the experimental result of observed near equilibrium of catalyst/ H2/TEMPOH system. The resulting multi-state BDFE can potentially from energetic triangular relationships with various substrates, providing explanation towards the hydrogenation catalyzed by the PN3P*Ni-H catalyst.