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

Abstract

This article reviews the chemistry of nickel and palladium complexes with terminally bound hydroxide and alkoxide ligands. The research carried out in our group is discussed in the context of the general literature. It is shown that suitable methods of synthesis, combined with the choice of adequate ligands allow the isolation of a range of stable complexes. This has enabled a detailed investigation of the chemical reactivity of the MÀ O bonds, once believed to be intrinsically weak. The elucidation of trends in thermodynam-ic stability and kinetic lability is the key for a better understanding of the reactivity of this class of compounds, that combines basic and nucleophilic properties with typical organometallic patterns, like β-hydrogen elimination. Based on reversible acid-base exchange and CO 2 insertion reactions, we discuss how the polarity of the M-OR bonds influence their relative stability, their hydrolytic sensitivity and their tendency to react with electrophiles. Scheme 3. Precedents in the chemistry of Ni(II) alkoxide and hydroxide complexes.Scheme 4. Self-aldol reaction of nickel enolates promoted by bridging alkoxide interactions. Figures in brackets stand for isolated yields of pure products. ReviewIsr. J. Chem. 2020, 60, 373 -393 Scheme 5. Dimerization-driven mechanism of the self-aldol reaction. Scheme 6. Syntheses of some of group 10 hydroxide complexes with iPr PCP pincer ligands. Figures in brackets stand for isolated yields of pure products.Review Isr. J. Chem. 2020, 60, 373 -393 Scheme 14. Proposed mechanism for the decomposition of Ni(II) alkoxides catalyzed by Ni(I) species. Scheme 15. Mechanism of decomposition and activation parameters for β-hydrogen elimination from methoxide in Ni and Pd pincer complexes. Scheme 23. Carboxylation and reversible hydrolysis of iPr PCP-based alkoxides of Ni and Pd. Scheme 24. Kinetic lability of alkylcarbonate complexes. Scheme 25. Use of reversible water-alcohol exchange to probe the thermodynamics of CO 2 into different MÀ OR bonds.

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

“…In a previous contribution, we have shown [11] that the reaction of Ni and Pd hydroxides [( iPr PCP)M(OH)] with CO leads to unstable hydroxycarbonyl complexes, [M]−COOH. These evolve in solution, undergoing partial decarbonylation to yield isolable CO 2 ‐bridged species [M]−COO−[M].The mechanism of CO insertion into Ni−OH bonds has attracted much interest because it is involved in some catalytic processes and has also relevance in biological methanogenesis [10] . Further examples of carbonylation of Ni or Pd pincer hydroxide complexes were reported later [23a,27] .…”

“…This mechanism included the formation the ion pair {( iPr PCP)M←IMe] + [O 2 CMe] − } (akin to M4 ) prior to the backside attack of the methylcarbonate anion on the activated iodomethane molecule. Unfortunately, the experimental evidence that we have collected thus far demonstrates that using methanol as solvent is still not enough to induce DMC synthesis with the pincer hydroxide complexes [10,15b] …”

“…Shortened α‐O−C bonds are frequently observed in the experimental X‐ray structures of many “normal” metal‐alkoxides including Ni and Pd pincer moieties, whose IR spectra also reveal a significant hypsochromic shift of the ν(C−O) absorption, that supports a higher bond order in comparison with the corresponding alcohols. Unfortunately, the bond shortening is underestimated by DFT calculations, [10,13] although the unsophisticated combination PBE/6‐31G* reproduces well the experimental O−C frequencies [13] . The origin of the effect did not appear clearly in the calculation outputs, but the short C−O bonds in A1 and M1 prompted us to revise the shapes of molecular orbitals (MO) in these compounds.…”

“…One of the long‐standing research topics in our group is the analogy between the chemical reactivity of metal‐carbon bonds with similar metal‐heteroatom bonds in monomeric Ni(II) and Pd(II) complexes with hydroxo, alkoxo or other similar non‐dative heteroatom ligands [10] . In our experience, the iPr PCP scaffold, which contains symmetrical CH 2 P( i Pr 2 ) arms, has an adequate balance of steric shielding to promote interesting chemical reactivity while allowing enough stability for isolation purposes.…”

Insertion of Carbon Monoxide into the Terminal Co–O Bond in a Methoxocobalt(III) Complex via a Tuneable Mechanism

“…Insertion of CO into metal–carbon bonds is a pivotal step in many of the most widely used routes for the production of carbonyl group-containing compounds in both the laboratory and the industry. − In comparison, the chemistry of metal–oxygen bonds, which in some reported cases are even more reactive toward CO than metal–carbon bonds, − has been relatively less studied and less applied in catalysis. , Two types of pathways are generally proposed for CO insertions into metal–oxygen bonds: (i) the inner-sphere migratory insertion pathway, which requires the coordination of CO at a site neighboring to the migrating group, ,− and (ii) the outer-sphere nucleophilic attack-based pathway, which is initiated by the replacement of the RO – ligand by CO, with the subsequent backbiting reaction of the freed RO – ligand with the coordinated CO accomplishing the insertion. , It is worth noting that the latter pathway is usually documented for second- and third-row transition metal–oxygen bonds (such as Pt–O and Ir–O bonds), − probably due to the relatively lower metal–oxygen bond strength, whereas it is rarely encountered with first-row transition metal complexes …”