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Abstract
The controlled catalytic functionalization of alkanes via the activation of C-H bonds is a significant challenge. Although C-H activation by transition metal catalysts is often suggested to operate via intermediate σ-alkane complexes, such transient species are difficult to observe due to their instability in solution. This instability may be controlled by use of solid/gas synthetic techniques that enable the isolation of single-crystals of well-defined σ-alkane complexes. Here we show that, using this unique platform, selective alkane C-H activation occurs, as probed by H/D exchange using D2, and that five different isotopomers/isotopologues of the σ-alkane complex result, as characterized by single-crystal neutron diffraction studies for three examples. Low-energy fluxional processes associated with the σ-alkane ligand are identified using variable-temperature X-ray diffraction, solid-state NMR spectroscopy, and periodic DFT calculations. These observations connect σ-alkane complexes with their C-H activated products, and demonstrate that alkane-ligand mobility, and selective C-H activation, are possible when these processes occur in the constrained environment of the solid-state.
Highlights
Functionalizations that operate via outer sphere or radical pathways, such as carbene[17] or oxo transfer reactions,[18] these processes are proposed to proceed via direct coordination of the alkane C−H bond with the metal center, engaging in a 3center 2-electron interaction, i.e. a σ-alkane complex.[4,19−21] From such σ-complexes flow mechanistically distinct C−H activation pathways: σ-bond metathesis, C−H oxidative cleavage, and electrophilic activation (Scheme 1)
As, due to a combination of strong nonpolar C−H bonds and steric interactions from alkyl groups, alkanes are poor ligands, coordinating only weakly to metal centers.[1,25]. Their direct observation generally relies upon generation in solution and detection in situ using low-temperature NMR spectroscopy and time-resolved infrared spectroscopy (TR-IR); either by photogeneration of the alkane complex by loss of, for example, a CO ligand or direct protonation of an Rh−alkyl bond.[26−30] Such observations build upon earlier matrix isolation experiments.[19]
These selectivity patterns indicate a significant and unexpected mobility associated with the NBA−alkane ligand in the solid-state crystalline environment. This has been probed by variable-temperature singlecrystal X-ray diffraction and solid-state NMR spectroscopy experiments, in concert with periodic DFT calculations that map out pathways for fluxional processes in the solid-state
Summary
The controlled functionalization of alkanes via the activation of C−H bonds is of significant importance to the development of new methodologies that enable complexity to be introduced into simple fossil or bioderived natural resources or alreadysophisticated molecules.[1−7] Catalytic methodologies using transition metal fragments offer the potential to dictate selectivity and reduce energetic barriers to such processes, for example, the selective dehydrogenation of alkanes to give olefins,[8−10] the upgrading of low-value light alkanes to highermolecular weights for use as transportation fuels,[11−14] or the functionalization of alkanes to give valuable synthetic equivalents for further derivatization.[15,16] Aside from C−H functionalizations that operate via outer sphere or radical pathways, such as carbene[17] or oxo transfer reactions,[18] these processes are proposed to proceed via direct coordination of the alkane C−H bond with the metal center, engaging in a 3center 2-electron interaction, i.e. a σ-alkane complex.[4,19−21] From such σ-complexes flow mechanistically distinct C−H activation pathways: σ-bond metathesis, C−H oxidative cleavage, and electrophilic activation (Scheme 1). Examples where a σ-alkane complex is observed which undergoes C−H activation are even less common Such species have been implicated from isotope labeling experiments,[22] dynamic processes in which an alkyl hydride is in rapid equilibrium with a σ-alkane complex,[34−36] or by direct observation using TR-IR that shows they are formed and consumed by C−H oxidative cleavage over very short, e.g. nanosecond, time scales[24,37−39] (Scheme 2). These selectivity patterns indicate a significant and unexpected mobility associated with the NBA−alkane ligand in the solid-state crystalline environment. This has been probed by variable-temperature singlecrystal X-ray diffraction and solid-state NMR spectroscopy experiments, in concert with periodic DFT calculations that map out pathways for fluxional processes in the solid-state
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