Metal-alkane coordination

Class II dependence for the activation of a chemical bond as a function of surface metal atom coordinative unsaturation is typically found for chemical bonds of a character, such as the CH or C-C bond in an alkane. Activation of such bonds usually occurs atop of a metal atom. The transition-state configuration for methane on a Ru surface illustrates this (Figure 1.13). 

Complex 26 is among the first examples of intermolecular coordination of a single B-H bond in a neutral borane to a transition metal, and these species can be regarded as models for alkane coordination. The crystal structures of borane complexes such as 26 are of interest to compare with those for other octahedral a complexes, particularly... 

In Section 1.2 we noted that the bonding in CH5+ could be described in terms of three 2c-2e C-H bonds and one 3c-2e C—H—H bond. In Section 1.3 we noted that 3c-2e C—H—M bonds could account for agostic interactions between coordinatively unsaturated metal atoms and substituent alkyl groups, and indeed for metal-alkane a complexes. Similarly, 3c-2e M—C—M bonds... 

TABLE 11.1 Representative Bond Enthalpies for Metal-Alkane Bonds for Substrates Coordinated to Metal Pentacarbonyl Fragments, M(CO)s6... 

No isolable and fully characterized examples of a transition metal complex with a coordinated alkane have been reported. With typical metal-alkane bond energies <15kcal/mol, isolation of these complexes is a substantial challenge. However, spectroscopic methods have been incorporated to directly observe alkane coordination. Initially, fast IR methods were utilized to study transient alkane coordination. More recently, novel NMR techniques have been developed to directly observe the coordination of alkanes to transition metals. Both techniques have afforded valuable insight into metal-hydrocarbon bonding. 

There are two predominant challenges to direct observation of alkanes coordinated to transition metals (1) the short-lived nature of metal/alkane complexes and (2) competition for coordination of the alkane to the metal center. Because of the weak binding energy, alkane coordination is typically short-lived. Thus, fast spectroscopy techniques are required, and these techniques are often coupled with low temperatures in order to slow processes that result in alkane dissociation. In addition to the rapid dissociation of alkanes, most organic substrates will effectively compete (kinetically and thermodynamically) with alkanes for coordination to metals. Thus, the reaction medium is an important consideration since most common solvents are better ligands than alkanes, and attempts to observe alkane coordination have been commonly performed in the gas phase, in hydrocarbon matrices, or in liquid krypton or xenon. Finally, photolysis is generally required to dissociate a ligand at low temperature to create a transient coordination site for the alkane. 

One of the potential values of the fast IR studies is to identify trends in alkane coordination. For example, through direct observation of alkane complexes and their decomposition, complexes that most strongly bind alkanes can be identified. The direct observation of a series of heptane complexes with supporting aromatic and carbonyl ligands revealed that CpRe(CO)2(alkane) complexes are relatively stable (Table 11.3).104 This was determined by monitoring the rate of disappearance of metal-alkane complexes, given as k in Table 11.3. These results provided an important lead to the first observation of an alkane complex using NMR spectroscopy (see Section 11.4.2). 

Alkanes are extremely poor acids, but on analogy with H2 binding to metal complexes, coordination to highly electrophilic M greatly enhances the acidity of the C-H bond and promotes heterolytic cleavage. Soft electrophiles such as Pt" and Hg" are ideal because they bind CH4 and other alkanes transiently even in aqueous... 

Kinetic studies of C-H reductive elimination from the alkyl-hydrido complexes bearing a d metal center have been reported [80-82], Similarly to the reactions of d metal complexes, the reductive elimination proceeds via an alkane-coordinate intermediate, as supported by the observation of an inverse kinetic isotope effect. Representative data are as follows = 0.75 for Cp2W(H)(Me) in... 

Although studies that provide information on the EIEs for the coordination, oxidative addition, and reductive elimination of C-H bonds are rare, a few examples have been reported recently in the literature. Bergman and Moore reported substantially inverse EIEs (Xh/Xd < 1) for the coordination of cyclohexane ( 0.1 at —lOO C) and neopentane ( 0.07 at —108°C) to [Cp Rh(CO)], generated from the photolysis of Cp Rh(CO)2 in Kr matrices. In contrast, Geftakis and Ball reported a normal (Ah/Aq > 1) EIE for the coordination of cyclopentane to [CpRe(CO)2] (1.33 at —93°C) (Scheme 7). In this particularly novel experiment, the alkane cr-complex was generated in situ in the NMR probe at low temperature. Resonances corresponding to bound and unbound methylenes in the H NMR spectrum were observed, making this the first transition metal alkane [Pg.549]

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