Alkane coordination

Fig. 19. Molecular representation of [(( ArO)3tacn)U( cy-C6)], (4 -Bu Uc) dotted lines emphasize the ri H,C alkane coordination.

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... 

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). 

Alkane coordination and C-H bond cleavage have been extensively studied theoretically which is important because of the instability of alkane complexes— they are either are too weak to be observed or rapidly proceed to products of OA. In the latter case, computed binding energies of the transient [Pg.366]

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... 

Pyridine or bis(dimethylphosphino)alkanes coordinate via N or P to the lanthanoides as could be shown by the synthesis and structure deter-... 

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