Sylviane Sabo-Etienne

Coordination and dehydrogenation of amine-boranes at metal centers.

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.

Catalytic Formation of Carbon–Carbon Bonds by Activation of Carbon–Hydrogen Bonds

The direct functionalization of C–H bonds by formation of a C–C bond is an interesting and important alternative to coupling reactions involving halide derivatives. These reactions are complex but during the past few years a number of new systems have been developed which display promising scope. This microreview will briefly mention the different mechanisms leading to C–H activation and describe the new systems leading to the catalytic formation of carbon–carbon bonds.

Ruthenium-Catalyzed Hydrogenation of Nitriles: Insights into the Mechanism

Hydrogenation of benzonitrile into benzylamine is catalyzed under very mild conditions by the ruthenium bis(dihydrogen) complex RuH(2)(H(2))(2)(PCyp(3))(2), incorporating two tricyclopentylphosphines. Two key intermediates have been isolated, resulting from the activation of benzonitrile at early stages of activation, i.e., either N-coordination through the nitrile function or first hydrogenation with benzylimine formation, followed by, thanks to C-H activation, coordination at ruthenium as an orthometalated ligand.

Ruthenium-Catalyzed Reduction of Carbon Dioxide to Formaldehyde

Functionalization of CO2 is a challenging goal and precedents exist for the generation of HCOOH, CO, CH3OH, and CH4 in mild conditions. In this series, CH2O, a very reactive molecule, remains an elementary C1 building block to be observed. Herein we report the direct observation of free formaldehyde from the borane reduction of CO2 catalyzed by a polyhydride ruthenium complex. Guided by mechanistic studies, we disclose the selective trapping of formaldehyde by in situ condensation with a primary amine into the corresponding imine in very mild conditions. Subsequent hydrolysis into amine and a formalin solution demonstrates for the first time that CO2 can be used as a C1 feedstock to produce formaldehyde.

Chemistry of bis(dihydrogen) ruthenium complexes and of their derivatives

Abstract This review is devoted to the chemistry of bis(dihydrogen) complexes. Only a few bis(dihydrogen) complexes have been characterized, among which the three that have been isolated in our group, namely RuH2(H2)2(PCy3)2 1 and LRuH(H2)2 (L=hydridotris(3,5-dimethylpyrazolyl) borate, Tp*, 65; L=hydridotris(3-isopropyl-4-bromo-pyrazolyl)borate, Tp′, 66). The first part will present in detail the synthesis and characterization of RuH2(H2)2(PCy3)2. Its reactivity, as well as its use as catalyst precursor, will be extensively reviewed. The second part of this review deals with the chemistry of the hydridotris(pyrazolyl)borato ruthenium complexes. The different reactivity of these two classes of compounds is dominated by the nature of the η2-H2 coordination. The nucleophilic character of the hydride and dihydrogen ligands in 1 allows substitution and hydrogen transfer reactions, whereas the reactivity of 65 and 66 is dominated by the electrophilic character of the hydride and dihydrogen ligands.

[Ir(PCy3)2(H)2(H2BNMe2)]+ as a Latent Source of Aminoborane: Probing the Role of Metal in the Dehydrocoupling of H3B⋅NMe2H and Retrodimerisation of [H2BNMe2]2

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.

A terminal borylene ruthenium complex: from B-H activation to reversible hydrogen release.

Starting from RuHCl(H2)(PCy3)2, a terminal ruthenium mesitylborylene complex was obtained via double B-H bond activation of mesitylborane and concomitant release of dihydrogen, such a process being remarkably reversible.

Iron-Catalyzed C–H Borylation of Arenes

Well-defined iron bis(diphosphine) complexes are active catalysts for the dehydrogenative C-H borylation of aromatic and heteroaromatic derivatives with pinacolborane. The corresponding borylated compounds were isolated in moderate to good yields (25-73%) with a 5 mol% catalyst loading under UV irradiation (350 nm) at room temperature. Stoichiometric reactivity studies and isolation of an original trans-hydrido(boryl)iron complex, Fe(H)(Bpin)(dmpe)2, allowed us to propose a mechanism showing the role of some key catalytic species.

Iron-Catalyzed Reduction of CO2 into Methylene: Formation of C-N, C-O, and C-C Bonds.

We report herein the use of the (dihydrido)iron catalyst, Fe(H)2(dmpe)2, for the selective reduction of CO2 into either bis(boryl)acetal or methoxyborane depending on the hydroborane used as a reductant. In a one-pot two-step procedure, the in situ generated bis(boryl)acetal was shown to be a reactive and versatile source of methylene to create new C-N but also C-O and C-C bonds.

Two-Coordinate Iron(I) Complex [Fe{N(SiMe3)2}2]−: Synthesis, Properties, and Redox Activity†

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.