Metal complexes with alkanes

Although the direct interaction of metal complexes with alkanes under mild conditions has been demonstrated by the foregoing results, selective and catalytic functionalization of alkanes remains to be found. Success is probably more likely to be achieved by way of mild dehydrogenation (e.g., transfer of hydrogen to a suitable donor)6113 b or carbonylation rather than by oxidation. By analogy with Co(lII) oxidations of alkanes, these reactions should be facilitated by increasing the electrophilicity of the metal ion, e.g., by using the metal tri-... 

Interactions of non-reactive unsaturated metal complexes with alkanes is documented in the studies by Rayner with the fragment [W(CO)5]. TRIR was used to look at the equilibrium between the bound and ligand-free complex. These data allowed determination of the actual enthalpy for coordination of the alkane to the unsaturated metal center. As seen in Table 5, there is a substantial interaction between most alkanes and the [W(CO)5] fragment, with methane being the most weakly bound (no complexation was observed) [45]. 

Very weak adducts of metal complexes with alkanes (or alkyl groups), in which the C-H bond is directly coordinated to the metal (I-l, 1-2) or its ligands (1-3), do not necessarily lead to subsequent cleavage of the C-H bond. However,... 

In this chapter, we consider the interaction of transition metal complexes with alkanes to give alkane-derived products, where organometallic species are either the final products or are suspected intermediates. In this area only a few fundamentally different mechanistic reaction types appear to operate. 

Early reports on the reactions of transition metal complexes with alkanes were published by Shilov. Sltilov and coworkers observed H/D exchanges, oxidations, and halogenations of alkanes with platinum complexes (Equation 6.20). ° Work by Crabtree and Felkin, summarized in Equations 6.21a and 6.21b, showed that soluble complexes could be used to conduct the dehydrogenation of alkanes. " This body of results suggested that oxidative... 

These findings have stimulated enormously the search for intermolecular activation of C-H bonds, in particular those of unsubstituted arenes and alkanes. In 1982 Bergman [2] and Graham [3] reported on the reaction of well-defined complexes with alkanes and arenes in a controlled manner. It was realised that the oxidative addition of alkanes to electron-rich metal complexes could be thermodynamically forbidden as the loss of a ligand and rupture of the C-H bond might be as much as 480 kl.mol, and the gain in M-H and M-C... 

Recently the unprecedented example of stereoselective C—Si bond activation in cu-silyl-substituted alkane nitriles by bare CQ+ cations has been reported by Hornung and coworkers72b. Very little is known of the gas-phase reactions of anionic metal complexes with silanes. In fact there seems to be only one such study which has been carried out by McDonald and coworkers73. In this work the reaction of the metal-carbonyl anions Fe(CO) (n = 2, 3) and Mn(CO) (n = 3, 4) with trimethylsilane and SiH have been examined. The reactions of Fe(CO)3 and Mn(CO)4 anions exclusively formed the corresponding adduct ions via an oxidative insertion into the Si—H bonds of the silanes. The 13- and 14-electron ions Fc(CO)2 and Mn(CO)3 were observed to form dehydrogenation products (CO) M(jj2 — CH2 = SiMe2) besides simple adduct formation with trimethylsilane. The reaction of these metal carbonyl anions with SiFLj afforded the dehydrogenation products (CO)2Fe(H)(SiII) and (CO)3Mn(II)(SiII). ... 

The reactivity of a number of alkane complexes has been examined and this field has been reviewed through 1996 by Hall and Perutz. Flash photolysis of Cr(CO)6 in cyclohexane showed that solvation occurs within the first picosecond after photolysis, a fact that appears to rule out spin crossing as an important component in the dissociation of CO from Cr(CO)6. The stability of CpRe(CO)2(alkane) is particularly striking. Comparison of the rate constants for heptane solvated metal complexes with CO, Table 1, reveals that the rate constant for CpRe(CO)2(heptane) is five orders of magnitude slower than that of CpV(CO)3 (heptane). In fact, the stability of the CpRe(CO)2(alkane) complexes is so high that it has been possible to carry out low-temperature NMR on the cyclopentane complex generated by continuous photolysis of... 

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. 

Some of the first reactions of soluble metal complexes with methane occurred by a-bond metathesis. Like the first examples of oxidative addition of alkyl C-H bonds, the first examples of a-bond metathesis with alkyl C-H bonds were intramolecular. Yet, the lute-tium- and yttrium-methyl complexes, Cp MMe (M = Lu and Y) were shown by Watson to react intermolecularly with C-labeled methane to form the labeled methyl complexes and unlabeled methane at 70 °C (Equation 6.51). Related scandium compounds have now been shown to undergo similar reactions with alkanes, and a thoracyclobutane... 

More specifically, calculations have suggested that the ruthenium-alkyl complex in Equation 6.53 reacts with arene to exchange covalent ligands by a process closely related to a a-bond metathesis mechanism. Computational studies of the reactions of a simple iridium-alkyl and alkoxo complex with alkanes to generate new metal-alkyl complexes have also suggested that a mechanism is followed that involves many of the characteristics of a classic a-bond metathesis transition state. However, calculations of the mechanisms of these two processes imply that the transition state contains some degree of M-H bonding. 

The Lewis acid character of boron is responsible for the regioselective a-bond metathesis of metal-boryl complexes with alkanes. The unoccupied p orbital at boron triggers the C-H activation process, so that intramolecular arrangement places the boryl group cis to the alkyl group. 

Metal complexes prepared by reacting less than one mole of an alkan olamine with an excess of a polyhydric alcohol, such as polyethylene glycol 200—400 or glycerol, reportedly impart a greater degree of thixotropy to systems containing protective organic coUoids (501). 

Transition metal atoms react much more readily with alkenes than with alkanes because the initial interaction between the metal atom and an alkene is much less repulsive than for M+alkanes. To insert into a C-H bond of an alkane, the metal atom has to break a C-H bond and form an M-C and an M-H bond. The first step in a reaction with an alkene, however, is formation of a 7r-complex in which the C=C bond is merely weakened, not broken.119 The availability of the DCD bonding scheme (Sec. 1.1) leads... 

Note that although it may be possible for metal atoms to form a cr-complex with an alkane prior to insertion into a C-H bond, such complexes would be bound by only a few kcal/mol (comparable to dispersion forces), while metal-alkene 7r-complexes are typically bound by 20-60 kcal/mol. 

The stability of metal ion-alkane adducts such as shown in Figure 11 remains an interesting question. The bonding in such systems can be regarded as intermolecular "agostic" interactions (46). Similar adducts between metal atoms and alkanes have been identified in low-temperature matrices (47). In addition, weakly associated complexes of methane and ethane with Pd and Pt atoms are calculated to be bound by approximately 4 kcal/mol (43). The interaction of an alkane with an ionic metal center may be characterized by a deeper well than in the case of a neutral species, in part due to the ion-polarization interaction. 

The starting material is always the chalcogenol and, consequently, is more used for thiols than selenols and tellurols. There are several types of reactions depending if the starting materials are metal hydrides (hydrogen elimination), complexes with M C (alkane elimination), M-N (transamination), or M-O (hydrolysis) bonds. 

Thus, a highly reactive species is needed to make this type of bond activation reaction feasible under mild conditions. In addition, to be useful, the C-H bond activation must occur with both high chemo- and regiose-lectivity. Over the past several decades, it has been shown that transition metal complexes are able to carry out alkane activation reactions (1-5). Many of these metal-mediated reactions operate under mild to moderate conditions and exhibit the desirable chemoselectivity and regioselectiv-ity. Thus, using transition metal complexes, alkane activation can be preferred over product activation, and the terminal positions of alkanes, which actually contain the stronger C-H bonds, can be selectively activated. The fact that a hydrocarbon C-H bond has been broken in a... 

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