Carboxylation of the Ni–Me Bond in an Electron-Rich Unsymmetrical PCN Pincer Nickel Complex

Abstract

The synthesis of a new unsymmetrical PCN ligand bearing tert-butyl groups on the phosphorus atom and isopropyl groups on the nitrogen donor atom is presented. It reacts with the commercially available Ni­(DME)­Br2 precursor to offer the corresponding t‑BuPCNi‑Pr pincer nickel bromide complex 1 together with a paramagnetic species, which was characterized as a tetrahedral nickel complex. Complex 1 reacts with MeMgCl to give the corresponding methyl complex 3. Carboxylation of complex 3 using 4 atm of CO2 gave the PCN nickel acetate complex 4 under mild reaction conditions comparable to those for the corresponding palladium complexes with PCP ligands.

Ligand and solvent effects on CO2 insertion into group 10 metal alkyl bonds

“…It has traditionally been challenging to perform kinetics experiments on CO 2 insertion into group 10 alkyl complexes either because of the instability of the complexes 14 or the harsh conditions required for insertion. 16,17 In contrast, the insertion of CO 2 into ( t Bu PBP)Pd(CH 3 ) under mild conditions enables kinetics experiments to be performed. Initially, 1 H or 31 P NMR spectroscopy was used to monitor the reaction of ( t Bu PBP)Pd(CH 3 ) with CO 2 as a function of time (Fig.…”

“…16 Changing the pincer ligand from t Bu PCP to a system containing an sp 3 -hybridized carbon donor or replacing one of the phosphine arms of the pincer ligand with a hemi-labile nitrogen donor only leads to small changes in the rate of reaction. 17 A notable example of a group 10 methyl complex that does not contain a pincer ligand and inserts CO 2 at room temperature is an unusual anionic palladium dimethyl complex, with a pendant Lewis acid, prepared by Jordan and co-workers. 7 Although this system inserts CO 2 rapidly it requires careful handling as it is unstable at room temperature.…”

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

“…[ 10–12,18 ] Also increased reactivity in CO 2 insertion has been reported. [ 23 ] The hemilability of unsymmetric pincer complexes is sometimes important but it also counteracts the stability of these complexes and can facilitate their decomposition, as reported by us and others for PCN pincer complexes. [ 12,18,22 ] Introducing a relatively strongly binding nitrogen side arm could enhance the stability of the PCN complexes.…”

“…Despite the fast expansion of the field of pincer complexes, [1][2][3][4][5][6][7][8][9] those with unsymmetric pincer ligands have received limited attention compared to the symmetric ones in part as a result of the long synthetic routes used to prepare the unsymmetric ligand scaffolds. [10][11][12][13][14][15][16][17][18][19][20][21][22][23] However, unique reactivity has sometimes been documented for these ligands, and for PCN ligands such reactivity includes selective C-C vs. C-H bond activation and hemilability through de-coordination of the weak side arm of the unsymmetric pincer ligand. [10][11][12]18] Also increased reactivity in CO 2 insertion has been reported.…”

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

“…Later, Moussa and co-authors presented the synthesis of an unsymmetrical PCN pincer complex of nickel bearing tertbutyl groups on the phosphorus atom and isopropyl groups on the nitrogen donor atom (complex 18 in Scheme 7 ) [ 98 ]. The corresponding methyl complex 19 was also obtained from 18 through a transmetalation reaction with MeMgCl.…”