Complexes in which a sigma-H--E bond (E=H, B, Si, C) acts as a two-electron donor to the metal center are called sigma complexes. Clues that it is possible to interconvert sigma ligands without a change in oxidation state derive from C--H activation reactions effecting isotope exchange and from dynamic rearrangements of sigma complexes (see Frontispiece). Through these pathways, metathesis of M--E bonds can occur at late transition metals. We call this process sigma-complex-assisted metathesis, or sigma-CAM, which is distinct from the familiar sigma-bond metathesis (typical for d(0) metals and requiring no intermediate) and from oxidative-reductive elimination mechanisms (inherently requiring intermediates with changed oxidation states and sometimes involving sigma complexes). There are examples of sigma-CAM mechanisms in catalysis, especially for alkane borylation and isotope exchange of alkanes. It may also occur in silylation and alkene hydrogenation.

The σ‐CAM Mechanism: σ Complexes as the Basis of σ‐Bond Metathesis at Late‐Transition‐Metal Centers

Photoredox catalysis is a tool enabling a wide variety of chemical reactions with high selectivity under mild conditions of visible light. In this Review, we summarize recent experiments which use heterogeneous, covalent, metal-free semiconductors with adjustable reactivity to drive such reactions. This class started with mesoporous graphitic carbon nitride, then continued with poly(heptazine imides), but is meanwhile extended to other polymers and solid-state organics with conjugated electron system. Because of the high thermal and chemical stability, as well as adjustable conduction and valence band positions, the reaction space could be expanded to many organic reactions, such as photocatalytic synthesis of organosulfur compounds, or unconventional halogenation and cyanation reactions. Performance of carbon nitrides and homogeneous systems in certain reactions was compared in the present Review.

Heterogeneous Organocatalysis for Photoredox Chemistry

The formation and stability of (arene)Cr(CO)3 species inside two highly porous materials—UiO-66, which is a recently synthesized metal−organic framework, and a cross-linked poly(styrene-co-divinylbenzene) resin—are investigated in detail by means of complementary spectroscopic techniques and theoretical calculations. In particular, Fourier transform infrared (FT-IR), ultraviolet-visible (UV−vis), and X-ray absorption near-edge structure (XANES) spectroscopies, coupled with theoretical calculations, allow the formation of the (arene)Cr(CO)3 species to be followed in situ, starting from the Cr(CO)6 precursor, monitoring the changes in the vibrational and electronic properties of the materials. EXAFS spectroscopy gives the structural evidence of the functionalized unities. Finally, the photoinduced reactivity of Cr(CO)3 in UiO-66 is also explored, by following the substitution of one CO ligand with a N2 molecule. The overall presented data would become the starting point for the development of a systematic p...

Functionalization of UiO-66 Metal−Organic Framework and Highly Cross-Linked Polystyrene with Cr(CO)3: In Situ Formation, Stability, and Photoreactivity

The discovery of dihydrogen complexes, L
 n
 M(H2), pointed to direct transfer of hydrogen from coordinated H2 ligands to substrates as an operable pathway in catalysis both in homogeneous and heterogeneous systems. Sigma complexes, L
 n
 M(η2-H–X) (X=H, Si, C, etc), are indeed relevant in hydrogenation as well as silane alcoholysis and methane conversion.

Catalytic Processes Involving Dihydrogen Complexes and Other Sigma-bond Complexes

Der σ‐CAM‐Mechanismus: σ‐Komplexe als Schlüssel der σ‐Bindungsmetathese bei späten Übergangsmetallen

This paper presents a new method for investigating the mechanisms of homogeneously catalyzed reactions involving gases, particularly H2. We show how the combination of polyethylene (PE) matrices and high pressure−low temperature (HPLT) experiments can be used to provide new mechanistic information on hydrogenation processes. In particular, we show how we are able to generate reaction intermediates at low temperature, and then to extract the contents of the PE film at room temperature to characterize the organic products using GC-MS. We have used our new technique to probe both the hydrogenation of dimethyl fumarate (DF), using Fe(CO)4(η2-DF) as the catalytic species, and the hydrogenation of norbornadiene (NBD), using (NBD)M(CO)4 (M = Cr or Mo) as the catalytic species. Irradiation of Fe(CO)4(η2-DF) in a PE matrix at 150 K resulted in the formation of an intermediate complex tentatively assigned Fe(CO)3(η4-DF). Warming this complex to 260 K under H2 leads to the formation of Fe(CO)3(η2-DF)(η2-H2). Further...

A New Approach To Studying the Mechanism of Catalytic Reactions: An Investigation into the Photocatalytic Hydrogenation of Norbornadiene and Dimethylfumarate Using Polyethylene Matrices at Low Temperature and High Pressure

Michael J. Carrott

Papers

A New Approach To Studying the Mechanism of Catalytic Reactions: An Investigation into the Photocatalytic Hydrogenation of Norbornadiene and Dimethylfumarate Using Polyethylene Matrices at Low Temperature and High Pressure