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Pincer ligands have a remarkable ability to impart control over small molecule activation chemistry and catalytic activity; therefore, the design of new pincer ligands and the exploration of their reactivity profiles continues to be a frontier in synthetic inorganic chemistry. In this work, a novel, monoanionic NNN pincer ligand containing two phosphinimine donors was used to create a series of mononuclear Ni complexes. Ligand metallation in the presence of NaOPh yielded a nickel phenoxide complex that was used to form a mononuclear hydride complex on treatment with pinacolborane. Attempts at ligand metallation with NaN(SiMe 3 ) 2 resulted in the activation of both phosphinimine methyl groups to yield an anionic, cis -dialkyl product, in which dissociation of one phosphinimine nitrogen leads to retention of a square planar coordination environment about Ni. Protonolysis of this dialkyl species generated a monoalkyl product that retained the 4-membered metallacycle. The insertion of 2,6-dimethylphenyl isocyanide (xylNC) into this nickel metallacycle, followed by proton transfer, generated a new five-membered nickel metallacycle. Kinetic studies suggested rate-limiting proton transfer (KIE ≥ 3.9 ± 0.5) from the α-methylene unit of the putative iminoacyl intermediate. 

Abstract Molecular design ultimately furnishes improvements in performance over time, and this has been the case for Rh‐ and Ir‐based molecular catalysts currently used in transfer hydrogenation (TH) reactions for fine chemical synthesis. In this report, we describe a molecular pincer ligand Al catalyst for TH, (I₂P²⁻)Al(THF)Cl (I₂P=diiminopyridine; THF=tetrahydrofuran). The mechanism for TH is initiated by two successive Al‐ligand cooperative bond activations of the O−H bonds in two molecules of isopropanol (ⁱPrOH) to afford six‐coordinate (H₂I₂P)Al(OⁱPr)₂Cl. Stoichiometric chemical reactions and kinetic experiments suggest an ordered transition state, supported by polar solvents, for concerted hydride transfer fromⁱPrO⁻to substrate. Metal‐ligand cooperative hydrogen bonding in a cyclic transition state is a likely support for the concerted hydride transfer event. The available data does not support involvement of an intermediate Al‐hydride in the TH. Proof‐of‐principle reactions including the conversion of isopropanol and benzophenone to acetone and diphenylmethanol with 90 % conversion in 1 h are described. The analogous hydride compound, (I₂P²⁻)Al(THF)H, also cleaves the O−H bond inⁱPrOH to afford (HI₂P⁻)Al(OⁱPr)H and (HI₂P⁻)Al(OⁱPr)₂, but no activity for catalytic TH was observed.