Transition-metal catalysis coordinative unsaturation

The ability of the nitrosyl ligand to behave as an electron pair reservoir has also been considered to play an important part in certain catalytically active systems. The vacant site provided by isomerization of the ligand could enable an unsaturated organic molecule to enter the transition metal s coordination sphere, thus forming an active intermediate. Examples of catalysis by nitrosyl complexes include the hydrogenation of alkenes by Rh(NO)L3 species and the dimerization of dienes in the presence of Fe(CO)2(NO)2 or Fe(n-C3Hs)(CO)2NO. Certain molybdenum dinitrosyl complexes, such as MoCljfNOljfPPhjlj, have also been found to provide very efficient alkene dismutation catalysts. ... 

Indeed, the application of transition-metal catalysis in organic synthesis is built around many such formalisms. In addition to the formal oxidation state, these include coordinative unsaturation, coordination number, and coordination geometry, hydride formalism and the 18-electron and 16-18-electron rules, also referred to as electron bookkeeping. 

Eormalisms in transition metal catalysis Uniqueness of transition metals Oxidation state of a metal Coordinative unsaturation, coordination number, and coordination geometry Ligands and their roie in transition metal catalysis... 

In spite of some declining industrial interest, the last 5 years have seen an unusual academic interest in the catalytic properties of the metal carbonyls. This has been part of a wider surge of interest in the organometallic chemistry of the transition metals and its application to homogeneous catalysis. Reactions such as Ziegler polymerization, the Oxo reaction, and the Wacker process are but a few of the many reactions of unsaturated molecules catalyzed in the coordination sphere of transition metal complexes (20). These coordination catalyses have much in common, and the study of one is often pertinent to the study of the others. 

A convenient tool for understanding organometallic catalysis mechanisms is the 16 and 18 electron rule, whereby valence electrons are counted in order to ascertain whether or not complexes are coordinatively unsaturated. An 18 electron complex possesses an inert gas configuration and must first undergo dissociation to achieve the coordinative unsaturation necessary for reactivity. The number of valence electrons for various transition metals is readily seen from their position in the periodic table (e.g., Mn has 7, Fe has 8). The number counted for a particular metal is independent of its oxidation state. 

Asymmetric catalysis undertook a quantum leap with the discovery of ruthenium and rhodium catalysts based on the atropisomeric bisphosphine, BINAP (3a). These catalysts have displayed remarkable versatility and enantioselectivity in the asymmetric reduction and isomerization of a,P- and y-keto esters functionalized ketones allylic alcohols and amines oc,P-unsaturated carboxylic acids and enamides. Asymmetric transformation with these catalysts has been extensively studied and reviewed.81315 3536 The key feature of BINAP is the rigidity of the ligand during coordination on a transition metal center, which is critical during enantiofacial selection of the substrate by the catalyst. Several industrial processes currently use these technologies, whereas a number of other opportunities show potential for scale up. 

Oligomerization and polymerization catalysis by metal complexes comprises three steps initiation, propagation, and termination. Chain growth proceeds at a coordinatively unsaturated see Coordinative Saturation Unsaturation) center having a metal carbon or metal hydride see Hydride Complexes of the Transition Metals) bond, nsuaUy generated by the interaction of a metal complex with an activating species such as an alkylaluntinum cocatalyst. The first insertion of an alkene monomer into the metal carbon or metal hydride bond (chain initiation) is followed by repeated insertions... 

When transition-metal cations are resident within an empty zeolite, they may interact further by a covalent mechanism with a suitable guest. For example, an electron pair from a guest may delocalize into an empty d orbital on the cation. Such interactions are not only stronger, but may be more disruptive to the electronic structure of the guest. They are more demanding in their geometrical and stereochemical requirements, and may play a specific role in further reaction and catalysis. For example, unsaturated homonuclear organic bonds have been found to coordinate sideways to ions such as Ag" ", Co2+, or Mn2+ within a zeolite. 

The discovery that the nitrosyl ligand is capable of binding to transition metals in two isomeric valence forms1 is one of the most dramatic recent developments in organometallic chemistry. Since bent NO donates 2 fewer electrons to the metal than the linear isomer does, linear-bent tautomerism raises the possibility of coordinative unsaturation and catalysis.2 In fact, complexes of nitric oxide are receiving increasing attention as catalysts,3 since they are more reactive than the corresponding carbonyls.4... 

Catalysis of the [2+2+2] cycloaddition of alkynes by transition metal complexes has been extensively exploited for the synthesis of complex organic molecules [30-34]. The accepted mechanism for this transformation, shown in Scheme 10, involves coordination of two alkyne molecules to the metal centre followed by oxidative coupling to form the coordinatively unsaturated metallocyclo-pentadiene 49, which can coordinate a third molecule of alkyne to afford 50. Insertion of the alkyne in a metal-carbon bond of this complex leads to met-allocycloheptadiene 51, and reductive elimination then affords cyclotrimer 52 and regenerates the catalytic species. Alternatively, the transformation of 49 into 52 might involve a Diels-Alder reaction giving intermediate 53, followed by reductive elimination [35]. 

Pd catalysts have two important dimensions (1) as a late transition metal catalyst and (2) as a Lewis acid catalyst. There is no need to describe the former catalysis, which is usually based on the Pd(0)-Pd(II) redox catalytic system. The latter Lewis acid catalysis, despite the soft metal, is based on the empty orbital of the coordinatively unsaturated 16-electron Pd(II) species. 

Most of the above uses rely on physisorption rather than strong chemical bond formation. Chemisorption, however, is of particular importance in heterogeneous catalysis, where it is a necessary precursor to reaction. Chemisorption sites include Bronsted and Lewis acid sites as well as coordinatively unsaturated transition metal cations, either within the framework or as charge-balancing cations outside the frameworks. Their catalytic activity is discussed further in the following Chapters 8 and 9. 

As illustrated in Figure 10.28 in heterogeneous catalysis for nanosized metal particles, one distinguishes three types of transition-metal particle-size-dependent behavior. When activation of a-type CH bonds is rate controlhng, the rate of reaction normalized per surface atom, TOF (turn over frequency) tends to increase with a decrease in particle size. This is the behavior according to curve II. It is due to the relative increase in the fraction of more reactive coordinatively unsaturated surface atoms. 

Pincer ligands, that is, tridentate Hgands that enforce meridional geometry upon complexation to transition metals, result in pincer complexes which possess a unique balance of stability versus reactivity [3]. Transition-metal complexes of bulky, electron-rich pincer ligands have found important appHcations in synthesis, bond activation, and catalysis [4, 5]. Among these, pincer complexes of Pr-PNP (2,6-bis-(di-iso-propylphosphinomethyl)pyridine), Bu-PNP (2,6-bis-(di-terPbutyl-phosphinomethyl)pyridine), and PNN ((2-(di-tert-butylphosphinomethyl)-6-diethyl-aminomethyl)pyridine), PNN-BPy (6-di-tert-butylphosphinomethyl-2,2 -bipyridine) ligands exhibit diverse reactivity [6-8]. These bulky, electron-rich pincer ligands can stabilize coordinatively unsaturated complexes and participate in unusual bond activation and catalytic processes. 

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