Enhancing the Stability of Aromatic PCN Pincer Nickel Complexes by Incorporation of Pyridine as the Nitrogen Side Arm

Abstract

New PCNPy pincer nickel complexes have been synthesized through a short synthetic route. Incorporating pyridine as the nitrogen side arm facilitated the C–H activation in the PCN ligand and allowed the cyclometallation with nickel to take place at room temperature. Pyridine also enhanced the stability of β‐hydrogen‐containing alkyl complexes. Also, the symmetric NCN nickel complex with pyridine side arms was successfully obtained giving a rare example of such type of complexes to be prepared through direct C–H activation. Furthermore, preliminary results showed that the (PCNPy)Ni–Br is active in Kumada coupling reactions particularly the coupling of aryl halides with aryl Grignard reagents.

Oxidative Addition and β‐Hydride Elimination by a Macrocyclic Dinickel Complex: Observing Bimetallic Elementary Reactions

“…While this differs from previously calculated bimetallic β‐hydride elimination mechanisms, [25,26] the involvement of both nickel sites is clearly important due to the generation of a free coordination site on Ni1 by migration of the bridging co‐ligand to Ni2 (reminiscent of the bimetallic mechanisms of oxidative addition discussed in the Introduction, Figure 1a), [19,21,22] as well as the short Ni1−H and Ni2−H distances in TS2 for each pathway. The importance of a free coordination site for facilitating β‐hydride elimination is well‐known in mononuclear complexes, and free coordination sites are typically formed through a hemilabile ligand or through ligand abstraction [11,46,47,62] . In this example, this is mediated by the dinuclear nature of the system.…”

Nonsymmetrical Benzene–Pyridine-Based Nickel Pincer Complexes Featuring Borohydride, Formate, Ethyl, and Nitrosyl Ligands

“…A similar reaction was reported by Wendt with the related complex [Ni( tBu PCN CH2 )-(η 1 -Et)]. 17 In the solid state, under the exclusion of moisture and air, both complexes are stable at room temperature for several days.…”

“…On consideration of nonprecious first-row transition metals, the scientific landscape of nickel pincer complexes is quite comprehensive and is generally dominated by symmetrical monoanionic ECE as well as ENE pincer, ligands with E being two identical donor groups that feature an aromatic anionic benzene or neutral pyridine backbone, repsectively. , In the past decade, nonsymmetrical PCN pincer ligands − became available, with nickel analogues appearing in the literature (Chart ). − These kinds of pincer ligands offer new possibilities due to the incorporation of a soft phosphorus and a hard nitrogen donor. A higher probability for the nitrogen donor to dissociate from the metal center has been reported, resulting in a vacant coordination site, making this ligand an interesting counterpart to its symmetrical relatives for homogeneous catalysis. , …”

Hard to Handle: Extremely Labile Hydrido Nickel(II) Complexes of Cyclometalated C∧N∧N and N∧C∧N Ligands

“…47 In the recently studied trans-C-X configured [Ni(PyPhPy)(X)] (X = Cl, Br, I) complexes containing a N ∧ C ∧ N cyclometalated ligand, the trans-influence of the strongly σ-donating carbanionic phen-2-ide group (Ph − ) labilizes markedly the Ni−X bond. 53,54 This bonding situation might suppress intramolecular concerted RE reactivity for carbanionic coligands as R = alkyl, aryl, or H but rather allows for intermolecular R or H transfer or radical eliminations.…”

“… , The former provides a C ∧ N ∧ N coordination to a square planar Ni­(II) center in complexes of the type [Ni­(Phbpy)­(X)] and some derivatives (X = Br, CF 3 , CH 3 ) and R­(Ph)­(R′bpy) have been explored recently for their optical and electrochemical properties. − The X = Br complexes are rather stable in contrast to the corresponding CF 3 and CH 3 derivatives exhibiting facile reductive elimination (RE) that yields F 3 C–Phbpy and H 3 C–Phbpy due the cis -orientation of the two carbanionic groups . In the recently studied trans -C-X configured [Ni­(PyPhPy)­(X)] (X = Cl, Br, I) complexes containing a N ∧ C ∧ N cyclometalated ligand, the trans -influence of the strongly σ-donating carbanionic phen-2-ide group (Ph – ) labilizes markedly the Ni–X bond. , This bonding situation might suppress intramolecular concerted RE reactivity for carbanionic coligands as R = alkyl, aryl, or H but rather allows for intermolecular R or H transfer or radical eliminations.…”