How does the carbon hybridization impact the rate of metal-carbon bond cleavage by an alkyne? A case study with POCOP-pincer ligated palladium hydrocarbyl complexes

The stability of group 10 metal POCOP pincer complexes: decomposition/reconstruction pathways of the pincer backbone

Organometallic chemists usually like to use the word robust to describe pincer ligand frameworks in metal pincer complexes. Although most transition metal pincer complexes are thermally stable, the pincer backbone frameworks can still decompose under certain circumstances. In order to explore the stability of the bis(phosphinite) (POCOP) pincer backbone in transition metal pincer complexes, group 10 metal POCOP pincer complexes were exposed to different nucleophilic and electrophilic conditions, respectively. It was found that the POCOP pincer backbone is stable under intermolecular nucleophilic conditions but cannot survive intramolecular nucleophilic attack; the POCOP pincer backbone is also stable under weak electrophilic conditions but the backbone can be completely destroyed by strong Lewis acids such as AlCl3. Possible decomposition/reconstruction pathways of the POCOP pincer ligand framework were proposed.

Roles of Hydrogen Bonding in Proton Transfer to κP,κN,κP-N(CH2CH2P i Pr2)2-Ligated Nickel Pincer Complexes

The nickel PNP pincer complex ( ^{i Pr}PNP)NiPh ( ^{i Pr}PNP = κ^P,κ^N,κ^P-N(CH₂CH₂P ⁱ Pr₂)₂) was prepared by reacting ( ^{i Pr}PNP)NiBr with PhMgCl or deprotonating [( ^{i Pr}PN^HP)NiPh]Y ( ^{i Pr}PN^HP = κ^P,κ^N,κ^P-HN(CH₂CH₂P ⁱ Pr₂)₂; Y = Br, PF₆) with KO ^t Bu. The byproducts of the PhMgCl reaction were identified as [( ^{i Pr}PN^HP)NiPh]Br and ( ^{i Pr}PNP')NiPh ( ^{i Pr}PNP' = κ^P,κ^N,κ^P-N(CH=CHP ⁱ Pr₂)(CH₂CH₂P ⁱ Pr₂)). The methyl analog ( ^{i Pr}PNP)NiMe was synthesized from the reaction of ( ^{i Pr}PNP)NiBr with MeLi, although it was contaminated with ( ^{i Pr}PNP')NiMe due to ligand oxidation. Protonation of ( ^{i Pr}PNP)NiX (X = Br, Ph, Me) with various acids, such as HCl, water, and MeOH, was studied in C₆D₆. Nitrogen protonation was shown to be the most favorable process, producing a cationic species [( ^{i Pr}PN^HP)NiX]⁺ with the NH moiety hydrogen-bonded to the conjugate base (i.e., Cl^-, HO^-, or MeO^-). Protonation of the Ni-C bond was observed at room temperature with ( ^{i Pr}PNP)NiMe, whereas at 70 °C with ( ^{i Pr}PNP)NiPh, both resulting in [( ^{i Pr}PN^HP)NiCl]Cl as the final product. Protonation of ( ^{i Pr}PNP)NiBr was complicated by site exchange between Br^- and the conjugate base and by the degradation of the pincer complexes. Indene, which lacks hydrogen-bonding capability, was unable to protonate ( ^{i Pr}PNP)NiPh and ( ^{i Pr}PNP)NiMe, despite being more acidic than water and MeOH. Neutral and cationic nickel pincer complexes involved in this study, including ( ^{i Pr}PNP')NiBr, ( ^{i Pr}PNP)NiPh, ( ^{i Pr}PNP')NiPh, ( ^{i Pr}PNP)NiMe, [( ^{i Pr}PN^HP)NiPh]Y (Y = Br, PF₆, BPh₄), [( ^{i Pr}PN^HP)NiPh]₂[NiCl₄], [( ^{i Pr}PN^HP)NiMe]Y (Y = Cl, Br, BPh₄), [( ^{i Pr}PN^HP)NiBr]Br, and [( ^{i Pr}PN^HP)NiCl]Cl, were characterized by X-ray crystallography.