Stabilizing Reactive Ligands

In the various analyses given above, we considered reactive carbon species such as car-benes and allyl radicals coordinated to transition metals. This is a hallmark of organotransi-tion metal chemistry the stabilization of reactive species by coordination to a metal. Sometimes these ligands retain reactivity patterns analogous to their standard organic reactivity, but often completely new reaction pathways are induced due to the metal coordination. 

CHAPTER 12 ORGANOTRANSITION METAL REACTION MECHANISMS AND CATALYSIS 

---

Assuming that [(CH2)2(Tj-C5H4)2Ti] is indeed an inaccessible reaction intermediate as reactivity evidence has suggested, then the formation of 37 must circumvent the intermediacy of such an intermediate. Thus, Brintzin-ger and co-workers have postulated that the Ti(II) center is most likely formed while in contact with one or two stabilizing CO ligands. 

In the gas phase, ions may be isolated, and properties such as stability, metal-ligand bond energy, or reactivity determined, full structural characterization is not yet possible. There are no complications due to solvent or crystal packing forces and so the intrinsic properties of the ions may be investigated. The effects of solvation may be probed by studying ions such as [M(solvent) ]+. The spectroscopic investigation of ions has been limited to the photoelectron spectroscopy of anions but other methods such as infrared (IR) photodissociation spectroscopy are now available. 

The template-controlled generation of ylidene ligands from (3-functionalized isocyanides constitutes an alternative access to complexes with cyclic diaminocar-benes. It proceeds via the complexes with NH,NH-stabilized ylidene ligands which are easily alkylated at the heteroatoms. Such complexes are valuable and reactive intermediates for the generation of complexes with macrocyclic ligands possessing NHC donors (see Sect. 4). 

In trimethylenemethane complexes, the metal stabilizes an unusual and highly reactive ligand which cannot be obtained in free form. Trimethylenemethanetricar-bonyliron (R=H) was the first complex of this kind described in 1966 by Emerson and coworkers (Figure 1.2) [38]. It can be obtained by reaction of bromomethallyl alcohol with Fe(CO)5. Trimethylenemethaneiron complexes have been applied for [3+2]-cycloaddition reactions with alkenes [39]. 

The basis of molecular modeling is that all important molecular properties, i. e., stabilities, reactivities and electronic properties, are related to the molecular structure (Fig. 1.1). Therefore, if it is possible to develop algorithms that are able to calculate a structure with a given stoichiometry and connectivity, it must be possible to compute the molecular properties based on the calculated structure, and vice versa. There are many different approaches and related computer programs, including ab-initio calculations, various semi-empirical molecular orbital (MO) methods, ligand field calculations, molecular mechanics, purely geometrical approaches, and neural networks, that can calculate structures and one or more additional molecular properties. 

The chelate effect of tethered Cp ligands may be used to modulate the coordination sphere in order to stabilize reactive metal centres and may also serve to introduce chirality at a metal centre. A number of reviews are available.4 11... 

The development of catalytic C-H activation reactions is one of the most challenging enterprises in organic chemistry. The only complexes able to activate C-H bonds by oxidative addition are coordinatively unsaturated at the metal center. The lifetimes of these highly reactive metal species, however, are usually very short. A key issue is therefore the development of stabilizing metal ligands. In addition, it is desirable that the ligands should confer a selectivity to the functionalization reaction, such that repeated functionalization can be prevented. 

The simplest supported catalysts are mononuclear metal complexes, exemplified by industrial supported metallocene catalysts, used (with promoters) for alkene polymerization these are the so-called single-site catalysts that are finding wide industrial applications (Kristen, 1999 Kaminsky, 1999 Roscoe et al., 1998). The most common supports are metal oxides and zeolites. The metals in these complexes range from oxophilic (e.g., Zr and Ta) to noble (e.g., Rh). Supported metal complexes are stabilized by ligands—in addition to those provided by the support—such as hydride (H), hydrocarbons, and carbonyl (CO). In a typical supported metal complex, the metal is present in a positive oxidation state. Although some such complexes are relatively stable, most are, befitting their roles as catalysts, highly reactive and air- and moisture-sensitive. 

X-ray structural studies have been conducted on the zirconaaziridines 2a [20], 2b [21], 2d [44], and 2j [33] (Fig. 7). Relevant data are listed in Table 1. In each case, the stabilizing donor ligand such as THF, o-anisyl, or pyridine (referred to as L in the table) prefers to coordinate on the nitrogen side of the zirconaaziri-dine (Fig. 8), with implications for reactivity that will be discussed in Sect. 4.3.1. Evidence that such a preference is thermodynamic can be seen in the constant 2.3 1 inside outside [45] ratio for zirconaaziridine 2o regardless of the method used for its generation (Fig. 8) [32]. This inside coordination is also known to be thermodynamically favored over outside coordination for most acyl [14] and iminoacyl [32] complexes. The o-anisyl oxygen is clearly seen to coordinate on the inside in the structure of 2d (Fig. 9). 

The highest oxidation states for Ir are VI and V, stabilized by ligands such as F. IrFg (a yellow crystalline sohd, /Xeff = 2.9 /xb at 300 K) is formed by the direct fluorination of Ir metal. Molecules of IrFe are octahedral, and the stmcture has been studied in the gas phase by electron diffraction and in the sohd phase by EXAFS. IrFe is hydrolyzed by water, and reactions (1-4) illustrate its general reactivity. Reaction four represents the formation of the first binary, tripositive metal carbonyl complex. ... 

Metal compounds are often used to stabilize reactive organic fragments via complexation and intriguing alkyne ligand in this classification is benzyne. A series of iridium benzyne complexes has been made with compound (44) as an example and the reaction chemistry of coordinated benzyne has been explored. ... 

Cationic P compounds exhibit several modes of reactivity, including coordination to Lewis acids oxidation by acids, water, and alkyl chlorides and substitution of the stabilizing phosphine ligands by stronger donors (Scheme 17). Some of these P and As cations have also been shown to be useful sources of P and As ions that provide zirconium complexes (12) containing unique square-planar Pn environments (equation 22). ... 

As is evident in the above reactivity series, the cyclopentadienyl group is one of the least reactive ligands toward nucleophiles. As a result, it is widely used in organomctallic chemistry as a stabilizing ligand which will remain unreactive. In Eq. 15.124 theCp ligand remains a spectator while the alkene reacts with the Incoming nucleophile. 

---