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

Abstract

Two pathways of alkoxide migration occurring at a nickel(II) center supported by a PPP ligand (PPP = P[2-PPr-CH]) 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(OPr) (4) was prepared. Upon addition of a π-acidic isocyanide ligand CNBu, a nickel(0) isocyanide species (PPP)Ni(CNBu) (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(COOPr) (5), representing an alternative pathway. The corresponding RE product (PPP)Ni(CO) (6a) can be independently produced from the substitution reaction of {(PPP)Ni}(μ-N) (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 CO(g). The formation of (PPP)Ni(OCOOPr) (7) clearly shows that an isopropoxide group migrates onto the bound CO ligand, and a P-Ni moiety is regenerated.

Addition of H2 Across a Cobalt–Phosphorus Bond

“…Metal–ligand cooperativity has been studied most extensively with metal amide/amine complexes (e.g., Noyori's catalyst), including several examples involving Co that are relevant to the work reported here . Related examples of metal–ligand cooperativity involving phosphorus are far less common . In a very recent paper by Gudat and co‐workers, a metal phosphenium complex, (uNHP Dipp )Mn(CO) 4 (where NHP R indicates a monodentate NHP + cation with N‐R substituents, “u” indicates an unsaturated backbone, and Dipp=2,6‐diisopropylphenyl), was shown to be an active catalyst for the catalytic dehydrogenation of NH 3 BH 3 .…”

Addition of H2 Across a Cobalt–Phosphorus Bond

“…Both stoichiometric and catalytic reactions involving the addition of s bonds across metal-element bonds (element = N, O, C, B, S) have become increasingly prevalent. [25][26][27][28][29][30][31][32] In av ery recent paper by Gudat and coworkers,ametal phosphenium complex, (uNHP Dipp )Mn(CO) 4 (where NHP R indicates amonodentate NHP + cation with N-Rs ubstituents," u" indicates an unsaturated backbone,a nd Dipp = 2,6-diisopropylphenyl), was shown to be an active catalyst for the catalytic dehydrogenation of NH 3 BH 3 . [21][22][23][24] Related examples of metalligand cooperativity involving phosphorus are far less common.…”

“…[21][22][23][24] Related examples of metalligand cooperativity involving phosphorus are far less common. [25][26][27][28][29][30][31][32] In av ery recent paper by Gudat and coworkers,ametal phosphenium complex, (uNHP Dipp )Mn(CO) 4 (where NHP R indicates amonodentate NHP + cation with N-Rs ubstituents," u" indicates an unsaturated backbone,a nd Dipp = 2,6-diisopropylphenyl), was shown to be an active catalyst for the catalytic dehydrogenation of NH 3 BH 3 . (uNHP Dipp -H)Mn(H)(CO) 3 could be generated through sequential addition of ah ydride and ap roton to (uNHP Dipp )Mn(CO) 4 ,and H 2 elimination could be promoted by photolysis;however, direct H 2 addition to (uNHP Dipp )Mn-(CO) 4 was not possible.…”

Nickel and Palladium Complexes with Reactive σ‐Metal‐Oxygen Covalent Bonds

“…[42] The occurrence of undesired side processes is difficult to predict, as some systems withstand more successfully than others the exchange with basic/nucleophilic reagents. [42][43][44][45][46] In our earliest attempts to prepare the Ni and Pd methoxides [( iPr PCP)M (OMe)], we tried to proceed as with hydroxides 12, but exchanging the precursor halide with sodium methoxide afforded mixtures due to incomplete conversion and competitive β-elimination, [46] the hydrides [( iPr PCP)M(H)] being identified among the products. A turnaround for this problem is to use alkoxide ligands without hydrogen atoms on the position next to oxygen, e. g. t-butoxide, [47,48] but the products can be very reactive and difficult to handle.…”

“…Phosphine ligands can be attacked by hydroxide, causing partial or complete reduction of the starting material, and strong bases can induce β‐hydrogen elimination from alkoxide through not always well understood intermolecular mechanisms, particularly in the presence of free alcohols . The occurrence of undesired side processes is difficult to predict, as some systems withstand more successfully than others the exchange with basic/nucleophilic reagents . In our earliest attempts to prepare the Ni and Pd methoxides [( iPr PCP)M(OMe)], we tried to proceed as with hydroxides 12 , but exchanging the precursor halide with sodium methoxide afforded mixtures due to incomplete conversion and competitive β‐elimination, the hydrides [( iPr PCP)M(H)] being identified among the products.…”