Nickel-Catalyzed NO Group Transfer Coupled with NOx Conversion.

Nitrogen oxide (NOx) conversion is an important process for balancing the global nitrogen cycle. Distinct from the biological NOx transformation, we have devised a synthetic approach to this issue by utilizing a bifunctional metal catalyst for producing value-added products from NOx. Here, we present a novel catalysis based on a Ni pincer system, effectively converting Ni-NOx to Ni-NO via deoxygenation with CO(g). This is followed by transfer of the in situ generated nitroso group to organic substrates, which favorably occurs at the flattened Ni(I)-NO site via its nucleophilic reaction. Successful catalytic production of oximes from benzyl halides using NaNO2 is presented with a turnover number of >200 under mild conditions. In a key step of the catalysis, a nickel(I)-•NO species effectively activates alkyl halides, which is carefully evaluated by both experimental and theoretical methods. Our nickel catalyst effectively fulfills a dual purpose, namely, deoxygenating NOx anions and catalyzing C-N coupling.

Alkoxide Migration at a Nickel(II) Center Induced by a π-Acidic Ligand: Migratory Insertion versus Metal-Ligand Cooperation.

Two pathways of alkoxide migration occurring at a nickel(II) center supported by a PPP ligand (PPP- = P[2-PiPr2-C6H4]2-) are presented in this Article. In the first route, the addition of a π-acidic ligand to a (PPP)Ni alkoxide species reveals the formation of a P-O bond. This reaction occurs via metal-ligand cooperation (MLC) involving a 2-electron reduction at nickel. To demonstrate a P-O bond formation, a nickel(II) isopropoxide species (PPP)Ni(OiPr) (4) was prepared. Upon addition of a π-acidic isocyanide ligand CNtBu, a nickel(0) isocyanide species (PPOiPrP)Ni(CNtBu) (6b) was generated; P-O bond formation occurred via reductive elimination (RE). When CO is present, migratory insertion (MI) occurs instead. The reaction of 4 with CO(g) results in the formation of (PPP)Ni(COOiPr) (5), representing an alternative pathway. The corresponding RE product (PPOiPrP)Ni(CO) (6a) can be independently produced from the substitution reaction of {(PPOiPrP)Ni}2(µ-N2) (3) with CO(g). While two different carbonylation pathways in 4 seem feasible, C-O bond forming migratory insertion singly occurs. Regeneration of a (PPP)Ni moiety via a P-O bond cleavage was demonstrated by treating 3 with CO2(g). The formation of (PPP)Ni(OCOOiPr) (7) clearly shows that an isopropoxide group migrates onto the bound CO2 ligand, and a P-Ni moiety is regenerated.

One metal is enough: a nickel complex reduces nitrate anions to nitrogen gas.

A stepwise reduction sequence from nitrate to dinitrogen gas at a single nickel center was discovered. A PNP nickel scaffold (PNP- = N[2-P i Pr2-4-Me-C6H3]2) emerged as a universal platform for the deoxygenation of NO x substrates. In these reactions carbon monoxide acts as the oxygen acceptor and forms CO2 to provide the necessary chemical driving force. Whereas the first two oxygens are removed from the Ni-nitrate and Ni-nitrite complexes with CO, the deoxygenation of NO requires a disproportionation reaction with another NO molecule to form NO2 and N2O. The final deoxygenation of nitrous oxide is accomplished by the Ni-NO complex and generates N2 and Ni-NO2 in a relatively slow, but clean reaction. This sequence of reactions is the first example of the complete denitrification of nitrate at a single metal-site and suggests a new paradigm of connecting CO and NO x as an effective reaction pair for NO x removal.