The Electronic Properties of Ni(PNN) Pincer Complexes Modulate Activity in Catalytic Hydrodehalogenation Reactions

Abstract

Three chloronickel(II) complexes of PNN‐ pincer ligands with pyrazolyl and diphenylphosphino donors appended to different arms of diarylamido anchors were prepared and fully characterized. The three derivatives (1‐OMe, 1‐Me, 1‐CF3) differ only by the identity of the para‐aryl substituent on the pyrazolyl arm with 1‐OMe being 310 mV easier to oxidize than 1‐CF3. All three complexes are competent catalysts for hydrodehalogenation reactions of 1‐bromooctane and a variety of aryl halides in dimethylacetamide using NaBH4 as both base and hydride source. Comparative studies using diverse substrates showed that catalytic activity correlates with electron donor properties; 1‐OMe was superior to the other two. Deuterium labeling studies verified NaBD4 as the deuteride source and excluded solvent‐assisted radical pathways.

Recent Advances in Chemistry of Unsymmetrical Phosphorus-Based Pincer Nickel Complexes: From Design to Catalytic Applications

“…Research gains new unprecedented complexes with unexpected properties in the past few years. The development in this field seems to be very promising [ 109 , 124 , 125 , 153 , 154 ].…”

“…Recently, Gardinier and Wang demonstrated synthesis of unsymmetrical PNN pincer nickel complexes 34a − 34c with pyrazolyl and diphenylphosphino donor groups ( Scheme 16 ) [ 124 ]. These complexes were tested in hydrodehalogenation reactions of 1-bromooctane and different aryl halides in using sodium borohydride as a hydride source and as a base.…”

Applicability of Nickel-Based Catalytic Systems for Hydrodehalogenation of Recalcitrant Halogenated Aromatic Compounds

“…Solvents of a greater coordinating power, such as N,N-dimethylformamide, Ni(II) chloride and NaBH4 reportedly constitute a soluble hydrogenation catalyst [51] (Figure 1 and Scheme 7). Recently, a series of Ni(NNP) pincer complexes with HDH activity in co-action of excess of NaBH4 as a reductant was described by Wang and Gardinier [61]. This system (especially in case if R = methoxy or methyl) was recognized as effective mainly for hydrodebromination of Ar-Brs and hydrodeiodination of Ar-Is but is very slow in case of HDC of Ar-Cls in N,N-dimethylacetamide (DMA) even at 80 °C [61] (Scheme 10).…”

“…Shteingarts and Adonin described effective co-action of Zn powder as the reductant and NiX 2 together with appropriate ligand (2,2 -bipyridyl (bipy) or phenanthroline (phen)) for partial aromatic hydrodefluorination in case of polyfluorinated aromates [60,61]. Using this Ni-based HDH system, (the hydride complex HNiXL n probably operates as the reductant [65,66]) (Scheme 13).…”

“…Scheme 10. HDH of Ar-Br or Ar-I using Ni(NNP) complex and NaBH4 and the proposed reaction pathway[61].…”