Nucleophilic Nickel and Palladium Pincer Hydroxides: A Study of Their Reactions with Dimethyl Carbonate and Other Non‐Alkylating Organic Electrophiles

Abstract

The reactions of the pincer hydroxide complexes [(iPrPCP)M(OH)] (M=Ni, Pd) with dimethyl carbonate (DMC), and a set of organic electrophiles including benzaldehyde have been investigated in the context of our ongoing investigation on the synthesis of alkyl carbonates from CO2 and alcohols. The final outcome of such reactions is diverse, but for PhCHO and DMC the first step is a mechanistically similar addition of the [M]−OH linkage across the carbonyl functionality, that leads to unstable adducts. DMC is cleaved irreversibly by both Ni and Pd hydroxides, affording the corresponding methylcarbonates [(iPrPCP)MOCO2Me] and methanol, whereas PhCHO affords benzoate complexes [[(iPrPCP)MOCO2Ph]. The main kinetic and thermodynamic features of these reactions were reproduced satisfactorily by computational DFT models. The calculations throw light on the true causes of irreversibility of DMC cleavage by nucleophilic hydroxide complexes, which is the primary cause that prevents methanol carboxylation from being catalysed by the Ni or Pd pincers.

Taming the Lewis Superacid Al(ORF)3 (RF=C(CF3)3): DFT Guided Identification of the “Stable yet Reactive” Adduct SiPr2→Al(ORF)3; Its Use as ORF‐ Abstractor from a “Ni‐ORF” complex

“…[22] Notably, a requisite for the stability of MÀ OR bonds is the lack of β-H that would favor MÀ H and carbonyl derivative formation via β-H elimination. [23] Focusing on Ni, the use of bulky alkoxide ligands, [23][24][25][26][27][28][29][30][31][32] of tridentate ligands, [25,[33][34][35][36][37][38][39] or the formation of dimer [40] have allowed isolation of stable complexes.…”

“…For example nickel-alkoxides are demonstrated to be intermediates in coupling processes (via ligand exchange or β-H abstraction, Scheme 3, top). [23,[41][42][43][44][45][46] Furthermore, insertion in NiÀ O bonds of CO 2 , CO, nitriles, ketones, isocyanate (Scheme 3, bottom); [35,36,38,39] has been established for neutral mono-alkoxide nickel complexes; as well as [OR] À ligand exchange [33] and formal reductive elimination at Ni. [37] During these studies, and as mentioned by Krossing et al, we observed that PhF!Al(OR F ) 3 adduct suffers from poor thermal stability liberating the Al(OR F ) 3 Lewis acid at room temperature, that evolves via abstraction of one of its own fluorine to form the [(R F O) 2 Al(μ-F)] 3 ring 2 (see below, Scheme 4).…”

“…Focusing on Ni, the use of bulky alkoxide ligands, [23–32] of tridentate ligands, [25,33–39] or the formation of dimer [40] have allowed isolation of stable complexes.…”

“…For example nickel‐alkoxides are demonstrated to be intermediates in coupling processes (via ligand exchange or β‐H abstraction, Scheme 3, top) [23,41–46] . Furthermore, insertion in Ni−O bonds of CO 2 , CO, nitriles, ketones, isocyanate (Scheme 3, bottom); [35,36,38,39] has been established for neutral mono‐alkoxide nickel complexes; as well as [OR] − ligand exchange [33] and formal reductive elimination at Ni. [37]…”