Synthesis, Characterization And Reactivity Studies Of Nickel Complexes Supported By A Phosphinimine-Based Pincer Ligand

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Doctor of Philosophy (PhD)
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Aminyl radical
Metal ligand multiple bonds
Nickel complexes
Nitrenium ion
Phosphinimine ligands
Pincer ligands
Inorganic Chemistry
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Xing, Xiujing

Tridentate pincer-type ligands offer both electronic and steric flexibility and have been used to stabilize highly reactive species, including late transition metal complexes with metal–ligand multiple bonds. Herein, we report a series of novel nickel complexes supported by a pincer-type bis(phosphinimine) ligand framework {HN(1,2-C6H4N=PPh2Me)2}. Metalation of the ligand onto NiCl2(dme) formed the nickel chloride species [(1,2-C6H4N=PPh2Me)2N]NiCl, which demonstrated geometric isomerism in solution. Photolysis of the azide congener [(1,2-C6H4N=PPh2Me)2N]NiN3 resulted in the C−H activation of a methyl group in the phosphinimine group, generating a nickel metallacycle complex [(1,2-C6H4N=PPh2Me)N(1,2-C6H4N=PPh2CH2)]Ni, which could also be generated upon reduction of a phenoxide complex [(1,2-C6H4N=PPh2Me)2N]NiOPh. Further deprotonation of this nickel metallacycle occurred on the second methyl group and afforded a nickel dialkyl metallacycle N(1,2-C6H4N=PPh2CH2)2Ni. Protonolysis of this complex reformed [(1,2-C6H4N=PPh2Me)N(1,2-C6H4N=PPh2CH2)]Ni, illustrating the chemical reversibility of this process. The redox non-innocence of the phosphinimine pincer ligand was then explored. Oxidation of [(1,2-C6H4N=PPh2Me)2N]NiOPh was primarily ligand-centered, as supported by EPR, UV-vis and DFT studies. Two-electron oxidation of [(1,2-C6H4N=PPh2Me)N(1,2-C6H4N=PPh2CH2)]Ni generated a diamagnetic compound, which was proposed to be a nitrenium ion bound Ni(II) complex. [(1,2-C6H4N=PPh2Me)N(1,2-C6H4N=PPh2CH2)]Ni also exhibited versatile reactivity toward H−E (E= O, S) bond activation via metal-ligand cooperation (MLC). Of particular interest is the ability of this complex to formally abstract a sulfur atom from triphenylmethyl thiol with the concomitant formation of triphenylmethane. The formation of a terminal nickel sulfide [(1,2-C6H4N=PPh2Me)2N]Ni=S as an intermediate was proposed. Experimental results suggested that the reaction mechanism involved an intermolecular S-atom transfer from the proposed nickel sulfide intermediate to [(1,2-C6H4N=PPh2Me)N(1,2-C6H4N=PPh2CH2)]Ni to form a S-atom insertion product, [(1,2-C6H4N=PPh2Me)N(1,2-C6H4N=PPh2CH2S)]Ni.

Neil Tomson
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