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In 2007 two of us defined the σ-Complex Assisted Metathesis mechanism (Perutz and Sabo-Etienne, Angew. Chem. Int. Ed. 2007, 46, 2578-2592), that is, the σ-CAM concept. This new approach to reaction mechanisms brought together metathesis reactions involving the formation of a variety of metal-element bonds through partner-interchange of σ-bond complexes. The key concept that defines a σ-CAM process is a single transition state for metathesis that is connected by two intermediates that are σ-bond complexes while the oxidation state of the metal remains constant in precursor, intermediates and product. This mechanism is appropriate in situations where σ-bond complexes have been isolated or computed as well-defined minima. Unlike several other mechanisms, it does not define the nature of the transition state. In this review, we highlight advances in the characterization and dynamic rearrangements of σ-bond complexes, most notably alkane and zincane complexes, but also different geometries of silane and borane complexes. We set out a selection of catalytic and stoichiometric examples of the σ-CAM mechanism that are supported by strong experimental and/or computational evidence. We then draw on these examples to demonstrate that the scope of the σ-CAM mechanism has expanded to classes of reaction not envisaged in 2007 (additional σ-bond ligands, agostic complexes, sp 2 -carbon, surfaces). Finally, we provide a critical comparison to alternative mechanisms for metathesis of metal-element bonds., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Anionic two-coordinate complexes of first-row transition-metal(I) centres are rare molecules that are expected to reveal new magnetic properties and reactivity. Recently, we demonstrated that a N(SiMe3)2(-) ligand set, which is unable to prevent dimerisation or extraneous ligand coordination at the +2 oxidation state of iron, was nonetheless able to stabilise anionic two-coordinate Fe(I) complexes even in the presence of a Lewis base. We now report analogous Cr(I) and Co(I) complexes with exclusively this amido ligand and the isolation of a [Mn(I){N(SiMe3)2}2]2(2-) dimer that features a Mn-Mn bond. Additionally, by increasing the steric hindrance of the ligand set, the two-coordinate complex [Mn(I){N(Dipp)(SiMe3)}2](-) was isolated (Dipp=2,6-iPr2-C6H3). Characterisation of these compounds by using X-ray crystallography, NMR spectroscopy, and magnetic susceptibility measurements is provided along with ligand-field analysis based on CASSCF/NEVPT2 ab initio calculations., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We recently disclosed a new ruthenium-catalyzed dehydrogenative cyclization process (CDC) of diamine-monoboranes leading to cyclic diaminoboranes. In the present study, the CDC reaction has been successfully extended to a larger number of diamine-monoboranes (4-7) and to one amine-borane alcohol precursor (8). The corresponding NB(H)N- and NB(H)O-containing cyclic diaminoboranes (12-15) and oxazaborolidine (16) were obtained in good to high yields. Multiple substitution patterns on the starting amine-borane substrates were evaluated and the reaction was also performed with chiral substrates. Efforts have been spent to understand the mechanism of the ruthenium CDC process. In addition to a computational approach, a strategy enabling the kinetic discrimination on successive events of the catalytic process leading to the formation of the NB(H)N linkage was performed on the six-carbon chain diamine-monoborane 21 and completed with a (15) N NMR study. The long-life bis-σ-borane ruthenium intermediate 23 possessing a reactive NHMe ending was characterized in situ and proved to catalyze the dehydrogenative cyclization of 1, ascertaining that bis σ-borane ruthenium complexes are key intermediates in the CDC process., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

First-row two-coordinate complexes are attracting much interest. Herein, we report the high-yield isolation of the linear two-coordinate iron(I) complex salt [K(L)][Fe{N(SiMe3 )2 }2 ] (L=18-crown-6 or crypt-222) through the reduction of either [Fe{N(SiMe3 )2 }2 ] or its three-coordinate phosphine adduct [Fe{N(SiMe3 )2 }2 (PCy3 )]. Detailed characterization is gained through X-ray diffraction, variable-temperature NMR spectroscopy, and magnetic susceptibility studies. One- and two-electron oxidation through reaction with I2 is further found to afford the corresponding iodo iron(II) and diiodo iron(III) complexes., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Tuning the nature of the linker in a L~BHR phosphinoborane compound led to the isolation of a ruthenium complex stabilized by two adjacent, δ-C-H and ε-B(sp2)-H, agostic interactions. Such a unique coordination mode stabilizes a 14-electron "RuH2P2" fragment through connected σ-bonds of different polarity, and affords selective B-H, C-H, and B-C bond activation as illustrated by reactivity studies with H2 and boranes., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The Ir(III) fragment {Ir(PCy(3))(2)(H)(2)}(+) has been used to probe the role of the metal centre in the catalytic dehydrocoupling of H(3)B⋅NMe(2)H (A) to ultimately give dimeric aminoborane [H(2)BNMe(2)](2) (D). Addition of A to [Ir(PCy(3))(2)(H)(2)(H(2))(2)][BAr(F)(4)] (1; Ar(F) = (C(6)H(3)(CF(3))(2)), gives the amine-borane complex [Ir(PCy(3))(2)(H)(2)(H(3)B⋅NMe(2)H)][BAr(F)(4)] (2 a), which slowly dehydrogenates to afford the aminoborane complex [Ir(PCy(3))(2)(H)(2)(H(2)B-NMe(2))][BAr(F)(4)] (3). DFT calculations have been used to probe the mechanism of dehydrogenation and show a pathway featuring sequential BH activation/H(2) loss/NH activation. Addition of D to 1 results in retrodimerisation of D to afford 3. DFT calculations indicate that this involves metal trapping of the monomer-dimer equilibrium, 2 H(2)BNMe(2) ⇌ [H(2)BNMe(2)](2). Ruthenium and rhodium analogues also promote this reaction. Addition of MeCN to 3 affords [Ir(PCy(3))(2)(H)(2)(NCMe)(2)][BAr(F)(4)] (6) liberating H(2)B-NMe(2) (B), which then dimerises to give D. This is shown to be a second-order process. It also allows on- and off-metal coupling processes to be probed. Addition of MeCN to 3 followed by A gives D with no amine-borane intermediates observed. Addition of A to 3 results in the formation of significant amounts of oligomeric H(3)B⋅NMe(2)BH(2)⋅NMe(2)H (C), which ultimately was converted to D. These results indicate that the metal is involved in both the dehydrogenation of A, to give B, and the oligomerisation reaction to afford C. A mechanism is suggested for this latter process. The reactivity of oligomer C with the Ir complexes is also reported. Addition of excess C to 1 promotes its transformation into D, with 3 observed as the final organometallic product, suggesting a B-N bond cleavage mechanism. Complex 6 does not react with C, but in combination with B oligomer C is consumed to eventually give D, suggesting an additional role for free aminoborane in the formation of D from C., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

There have been a number of approaches developed for the catalyzed dehydrogenation of amine-boranes as potential dihydrogen sources for hydrogen storage applications in recent years. Key advances in this area have been recently made thanks to catalytic and stoichiometric studies. In this Minireview, the fate of amine-boranes upon coordination to a metal center is discussed with a particular emphasis on B-H activation pathways. We focus on the few cases in which coordination of the resulting dehydrogenated product could be achieved, which includes the coordination of aminoborane, the simplest unit resulting from dihydrogen release of ammonia-borane.

A question of coordination mode: Two new borane compounds are prepared. They act as bifunctional ligands as illustrated by their reaction with ruthenium polyhydrides which leads to the formation of two complexes (see scheme) displaying either a delta-agostic interaction of a eta(2)-B-H bond involving a trivalent boron atom or a dihydroborate ligation.

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.