Agostic interaction

Abstract: The impact of agostic interactions (i.e., 3-center-2-electron M-H-C bonds) on the structures and reactivity of organotransition metal compounds is reviewed.

Abstract: The selective activation and transformation of alkanes or aromatics have been a research goal for several decades.1-3 Today, research in this area is strongly motivated by the need to optimize resources and to shorten the number of steps, hence the direct and selective transformation of petrochemicals into valuable products is therefore of major interest. Any of these processes will have first to rely on the selective C-H bond activation. This has been the holy grail in molecular organometallic chemistry and homogeneous catalysis for some years;1,4,5 several success stories have appeared in the past ten years,6-9 including progress in the direct catalytic functionalization of methane into methanol.10-13 However, finding stable catalytic systems using acceptable operating conditions remains a challenge. In heterogeneous catalysis, several large scale industrial processes are based on the selective C-H bond activation of alkanes and lead to the synthesis of more valuable and functionalized products in a single step, for instance the direct oxidation of butane into maleic anhydride,14-16 the dehydrogenation of propane into propene,17,18 or the direct transformation of liquefied petroleum gas (LPG) into aromatics.19-24 C-H bond activation is in fact a ubiquitous process in heterogeneous catalysis. It involves various types of mechanisms and intermediates. C-H bond activation involving carbonium and carbenium ion intermediates3,25-27 or taking place on metal particles (supported or not) or clean metal surfaces28-35 has been extensively reviewed in the past years. Here, we will focus on processes involving C-H bond activation of alkanes and aromatics on isolated metal centers of surfaces on oxide materials leading to organometallic species and/or intermediates. The review is divided into two main sections focusing on the C-H bond activation on oxide materials and related systems (section 2) and on well-defined organometallic complexes supported on oxide materials (section 3). Section 2 is subdivided into four parts describing C-H bond activation on bulk oxides, bulk metal halides, supported transition-metal oxides, and metal-exchange zeolites.

Abstract: The phenomenon of agostic interactions is reviewed and the nature of the interaction is revisited. A historical perspective is followed by an overview of experimental techniques used to diagnose agostic behavior, and previous interpretations of agostic bonding are presented. A series of simple metal alkyl complexes is considered and a new model for the phenomenon in d(0) systems is developed which sets them apart from agostic late-transition-metal complexes. Factors such as the valence electron count and coordination number of the metal center are revealed to be unimportant in facilitating the interaction in most d(0) systems. The charge density distribution in several transition-metal alkyl complexes is explored by experimental and theoretical techniques, including the powerful "Atoms in Molecules" approach. Local charge concentrations are shown to play an important role in the agostic interaction. Finally, we demonstrate for the first time a way to manipulate and control the magnitude and disposition of such local charge concentrations, and hence the strength of agostic interactions in d(0) metal alkyl complexes.

Abstract: Palladium-catalyzed alkane arylation reactions with aryl halides are described for the preparation of 2,2-dialkyl-dihydrobenzofuran substrates. These reactions occur in excellent yield and very high selectivity for the formation of one sole product arising from a reaction at nearby methyl groups. Mechanistic and computational studies point to the involvement of a concerted, inner-sphere palladation-deprotonation pathway that is enabled by the presence of three-center agostic interactions at the transition state. This mechanism accurately predicts the experimentally observed kinetic isotope effect as well as the site selectivity and should be useful in the design of new reactions and catalysts.