Alkanes complexes

Study on poly(pyrazol-l-yl)alkane complexes 99MI32. 

For the C-H activation sequence, the different possibilities to be considered are shown in Scheme 5 (a) direct oxidative addition to square-planar Pt(II) to form a six-coordinate Pt(IV) intermediate and (b, c) mechanisms involving a Pt(II) alkane complex intermediate. In (b) the alkane complex is deprotonated (which is referred to as the electrophilic mechanism) while in (c) oxidative addition occurs to form a five-coordinate Pt(IV) species which is subsequently deprotonated to form the Pt(II) alkyl product. 

What purpose does the alkane binding to the Pt(II) center serve For the electrophilic pathway (Scheme 5, b), this is immediately apparent, a-Alkane complexes should be considerably more acidic than free alkanes, such that deprotonation may become a viable C-H activation pathway. While the acidic character of alkane complexes has not been directly observed, it can be inferred from the measured acidity of analogous agos-tic complexes (36) and from the acidity of the a-complexes of dihydrogen (37), both of which can be regarded models for alkane complexes (see Section III.E). 

For the oxidative addition pathway, however, it is not obvious why the C-H bond cleavage reaction should be more facile if the hydrocarbon first binds in the coordination sphere of the metal (Scheme 5, c). One argument could be that the equilibrium between the Pt(II) alkane complex and the five-coordinate Pt(IV) alkyl hydride has an intrinsically low activation barrier. Insight into this question together with detailed information about the mechanisms of these Pt(II) a-complex/Pt(IV) alkyl hydride interconversions has been gained via detailed studies of reductive elimination reactions from Pt(IV), as discussed below. 

Both a-silane and a-dihydrogen complexes are useful and in some cases isolable models for the rarely observed a-alkane complexes. Unfortunately, silane complexes of Pt(II) are generally dinuclear (37,122) rendering them less desirable models for Pt(II) a-alkane complexes. In contrast, the closely related dihydrogen complexes of Pt(II) are mostly monomeric (123-126). 

There is ample evidence that the reductive elimination of alkanes (and the reverse) is a not single-step process, but involves a o-alkane complex as the intermediate. Thus, looking at the kinetics, reductive elimination and oxidative addition do not correspond to the elementary steps. These terms were introduced at a point in time when o-alkane complexes were unknown, and therefore new terms have been introduced by Jones to describe the mechanism and the kinetics of the reaction [5], The reaction of the o-alkane complex to the hydride-alkyl metal complex is called reductive cleavage and its reverse is called oxidative coupling. The second part of the scheme involves the association of alkane and metal and the dissociation of the o-alkane complex to unsaturated metal and free alkane. The intermediacy of o-alkane complexes can be seen for instance from the intramolecular exchange of isotopes in D-M-CH3 to the more stable H-M-CH2D prior to loss of CH3D. 

This mechanism clearly implicated alkane complexes as precursors to C-H activation but the IR absorptions of [Cp Rh(CO)Kr] and [Cp Rh(CO)(C6Hi2)] were not resolved and were presumed to be coincident. The temperature dependent data gave values of AH = 18 (or 22) kj mol for the unimolecular C-H (or C-D) activation step representing a normal kinetic isotope effect, kn/fco 10- However, an inverse equilibrium isotope effect (K /Kq 0.1) was found for the slightly exothermic pre-equilibrium displacement of Kr by CoHn/C Dn implying that C6Dj2 binds more strongly to the rhodium center than does C Hn-... 

Both normal and inverse KIEs have played a major role in unraveling the mechanisms of alkane activation with transition metal complexes. Alkyl hydride complexes are typical intermediates in such reactions. The loss of alkane in well-defined alkyl hydrides frequently exhibits an inverse KIE and involves a O-alkane complex.86 87 As a result, an inverse isotope effect is now taken as evidence for the intermediacy of o-alkane complexes in reductive eliminations (Scheme 8.14). 

The last step in Equation 8.131 is a reasonable approximation that associates isotope effects with the breaking/making of the carbon-hydrogen bonds and not with alkane dissociation, that is, k /k° 1. The observed isotope effect is then seen as arising from the equilibrium relating the alkyl hydride and O-alkane complexes in Scheme 8.14. 

The interrelationships between activation of H2 and other a-bonded molecules such as alkanes and silanes are highly significant because catalytic conversion of methane and other alkanes is strongly being pursued (17-19). An important question thus is whether C-H bonds in alkanes, particularly CH4, can bind to superelectrophilic metal centers to form a a alkane complex that can be split heterolytically where proton transfer to a cis ligand (or anion) takes place followed by functionalization of the resultant methyl complex (Eq. (3)). 

J. J. Schneider, Si-H and C-H activition by transition metal complexes a step towards isolable alkane complexes Angew. Chem. Int. Ed. 35,1069-76 (1996). 

Photolysis of a CO ligand from the parent compound initiates the reaction. The resultant >j3-Tp Rh(CO) complex has a coordinatively vacant site that is quickly occupied by a solvent molecule. The time scale for such a solvation process is generally on the order of 2 ps in room temperature liquids (28). The solvated 3-Tp Rh(CO)(S) (S = alkane) complex exhibits a single CO-stretching band (vCo) at 1972 cm 1 (the v = 0 1 band) as shown in Fig. 4a. For this particular system the large... 

Hall C, Perutz RN. Transition metal alkane complexes. Chem Rev 1996 96(8) 3125—3146. 

R. A. Periana, and R. G. Bergman, Isomerization of the Hydridoalkylrhodium Complexes Formed on Oxidative Addition of Rhodium to Alkane C—H Bonds. Evidence for the Intermediacy of j -Alkane Complexes, J. Am. Chem. Soc. 108, 7332-7346 (1986). 

This decomposition pathway leads to equimolar amounts of alkenes and alkanes. Complexes with /3-agostically bonded alkyl groups are likely intermediates structures of this kind may sometimes be the ground state, as in [(P—P)Pt(r72-C2H5)]+., 5... 

Coordinatively unsaturated 14- or 16-electron fragments L M, where M has a d6, ds, or dw configuration, are capable of oxidatively adding C—H bonds of arenes and alkanes and have been studied in considerable detail. Calculations suggest that the reaction proceeds via an i -alkane complex.125 More electron-rich as well as heavier transition metal centers, i.e., 3rd-row metals, facilitate C—H oxidative addition. In the case of C—H addition to Pd and Pt phosphine complexes MI, high activation barriers ( 30 kcal mol-1) have been calculated for monodentate phosphines, whereas chelating phosphines lead to values as low as 4 kcal mol-1 (M = Pt).126... 

The effect of cyclopentadienyl-ring substituents on the reactivity of the group VII half-sandwich complexes (t7 -C5R5)M(CO)2L[M = Mn and Re R = H, Me, and Et (Mn only) L = Kr and Xe] toward CO in supercritical fluid solution at room temperature has been investigated (70). The reactivity of the corresponding alkane complexes ( 7 -C5R5)Mn(CO)2(n-heptane) (R = H, Me, and Et) toward small molecules such as CO, N2, and H2 in n-heptane solution steadily increased in the order H < Me < Et (71). These results indicated that steric rather... 

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... 

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