Palladium Complexes with Carbonyl, Isocyanide, and Carbene Ligands

Abstract

The first section of this chapter is focused on palladium carbonyl compounds, and encompasses theoretical and experimental studies on homoleptic carbonyls; studies related to the binding of CO to Pd surfaces; and the CO migratory insertion reaction and its role in catalysis. The latter part covers work in which Pd-carbonyl intermediates have been isolated or detected prior to insertion into a Pd-C bond. Studies investigating the energetics, kinetics and ligand effects on CO insertion are described. Most studies involve four-coordinate complexes, however, possible insertion from five-coordinate Pd-CO intermediates are also discussed. Mononuclear, bi-, tri- and higher nuclearity homometallic carbonyls are reviewed, including a section devoted to dppm complexes, in particular work on Pd3(dppm)3(CO) and derivatives. Heterometallic Pd-carbonyl dimers and clusters are also reviewed. Due to the volume of work reported in this area only general aspects covering synthesis and reactions, with reference to suitable illustrative examples are described. The second section of this chapter focuses on carbene complexes of palladium. A small subsection describes non-heteroatom carbenes and Fischer type carbenes, but by far the biggest section reviews palladium complexes of N-heterocyclic carbenes (NHC) and related ligand systems. Since the end of the 1990’s there has been an almost exponential increase in publications on Pd-NHC chemistry, largely due to the isolation of free carbenes, which allows their use as ligands in the manner of phosphines. The synthesis of palladium complexes, from free carbenes, and by other methods, is reviewed in detail. Many NHCs have been synthesized, monodentate, chelating, functionalized and chiral, and most have been complexed to palladium. NHCs and precursor imidazolium salts, have been shown to undergo several interesting reactions with palladium, including reductive elimination, migratory insertion, and oxidative addition. All aspects of this chemistry have been reviewed. A major interest in NHCs is their application in catalysis, particularly in C-C bond and C-element bond forming reactions. These reactions have been comprehensively reviewed. The final section of this chapter concentrates on palladium isocyanide complexes. Isocyanides have been compared to CO and an important part of isocyanide chemistry is their application in reactions in which they are non-innocent. Insertion of isocyanides into Pd-C bonds is analogous to insertion of CO into M-C bonds and the process has provided a wealth of interesting reactions and compounds. These will be discussed in some detail. Unlike CO, multiple isocyanide insertions are known, providing further interesting chemistry. This reactivity of isocyanides, which blurs the division between their use as ligands and their application as substrate is comprehensively reviewed. Finally insertion of isocyanides into Pd-element bonds (other than carbon) is also reviewed.