Agostic C-H complexes

Pd-catalysed chelate-directed acetoxylation of meta -substituted arenes has been studied.61 Many substituted groups are tolerated by this process and the reaction shows a high degree of regioselectivity for the less sterically hindered ortfto-position. For example, 2-(3-nitrophenyl)pyridine forms 2-(2-acetoxy-3-nitrophenyl)pyridine. Finally, density functional calculations62 on the palladium acetate-promoted cyclomet-allation of dimethylbenzylamine suggest that reaction occurs via an agostic C-H complex rather than a Wheland intermediate. An intramolecular H-transfer to a coordinated acetate via a six-membered transition state follows. 

The first step is coordination of the ethylene through its n orbital. The ethylene is trans to Cl with the C=C bond in the Cl-Ru-H plane. Facile migratory insertion (AE = 7.6 kcal.mol 1) of the coordinated ethylene in the Ru-H bond leads to an alkyl intermediate 6.2 kcal.mol 1 less stable than the n ethylene complex. The alkyl intermediate has a strong P C-H agostic interaction as illustrated by the unusually long agostic C-H bond (1.221 A) which helps to stabilize the unsaturation in the formally 14-electron alkyl intermediate. 

Agostic interactions, i. e., the three-center bonds related to structure 51 [26, 44-49], were noted earlier by Green and Brookhart and have been cited above in the methoxycarbonylation chemistry (Figure 1.9). These bonds are often characterized by low frequency (hydride-like) proton chemical shifts, and/or substantially reduced /( C, H) values. Often, it is necessary to cool the NMR sample in order to freeze the equilibrium. Complex 52 represents a nice example of an agostic C-H bond, with relevance to polymerization chemistry [47]. 

The recent discovery of many metal complexes having t/2-H2 ligands or agostic C—H bonds has focused attention on a type of bonding that had hitherto seemed to be a domain of early main group elements and metal clusters three-center, two-electron bonds. As more and more examples are found, it has become obvious that such bonds are probably more widespread among transition metal compounds than had been commonly assumed and that it is about time to recognize the features the various species have in common. 

In all complexes of the CpL2Mn(H)SiR3 family, except 4, the silane is bonded to the metal only by the three-center bond (type K). Contrary to this, the agostic C—H bond in all known complexes with M—H—C three-center bonds is part of a ligand, which is also classically bonded to the same metal at another coordination site (type L ), either by a covalent bond or a 7r-bond (as in 21) or by a metal-metal bond (as in 22). Although this difference probably has no influence on the principal characteristics of the three-center bond, it may be very important with respect to the stability... 

Arene)(e/ido-dicyclopentadiene)M(0) complexes [M = Ru (211), Os (212)] are prepared by reduction of lc or Id with 2-propanol and Na2C03 in the presence of dicyclopentadiene (Scheme 15). Protonation of derivatives 211 and 212 with HPF6 leads to cations 213 and 214 which are stabilized by an agostic C—H—M bond, as shown by the X-ray structure of the osmium species (214). The cations 213 and 214 react with CN(t-Bu) to give complexes of type 215 in which the agostic C—H—M is no longer observed (102). 

The cationic hexamethylbenzene ruthenium complex 254 can be depro-tonated by treatment with t-BuOK to give the ruthenium complexes 255 and 256 in which the r 4-o-xylylene ligand is coordinated via its exocyclic diene (Scheme 23). Protonation of 255 gives the dication 15c, whereas protonation of 256 produces the fluxional tj3-benzyl derivative 257 which is stabilized by an agostic C—H bond (156,157). 

Early metal complexes often avoid the 18e configuration - Me3TaCl2 is ostensibly lOe, for example. This can provide coordinative unsaturation at the metal Schrock carbene complexes with <18e commonly have agostic C H bonds see Alkane Carbon-Hydrogen Bond Activation, Dihydrogen Complexes Related Sigma Complexes). When... 

Wadepohl etal.103 also investigated the hydroboration reactions of Fischer carbyne metal complexes [W(=CR)(Tp )(CO)2]. The boryl-metal complexes [W(Tp )(CO)2 ri2-B(R/)CH2R ] (R = Et or Ph, R = CgH4Me-4 or Me) has been prepared. A 3-agostic C-H- -M interaction is present between the metal and the boryl ligand.103... 

In nearly all cases, these complexes decompose rapidly and irreversibly at or near room temperature because of the weak H2 binding on such CO-rich metals, where less backbonding is present. Their instability is exacerbated because the 16e product of H2 dissociation is extremely reactive since it is not stabilized by internal agostic C-H interactions or solvent binding (hydrocarbon solvents are even more weakly bound than H2). The rate of room-temperature dissociation of H2 from Cr(CO)5(H2) in hexane is actually slower than that for many stable species. Thus, this complex and others like it might otherwise be stable under H2. One such complex initially presumed to be unstable, CpMn(H2)(CO)2, was in fact isolated as a solid from supercritical C02 (scC02) at room temperature in a flow reactor by rapid expansion of the scC02 (Scheme 5.21).163 164... 

The precursor complex can be a formally 16e species possessing an agostic C-H interaction158 that is in effect displaced by the incoming H2 ligand [Eq. (3.4)]. The... 

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