Mechanistic Investigations on Hydrogenation, Isomerization and Hydrosilylation Reactions Mediated by a Germyl-Rhodium System

We recently disclosed a dehydrogenative double C-H bond activation reaction in the unusual pincer-type rhodium-germyl complex [(ArMes)2ClGeRh] (ArMes=C6H3-2,6-(C6H2-2,4,6-Me3)2). Herein we investigate the catalytic applications of this Rh/Ge system in several transformations, namely trans-semihydrogenation of internal alkynes, trans-isomerization of olefins and hydrosilylation of alkynes. We have compared the activity and selectivity of this catalyst against other common rhodium precursors, as well as related sterically hindered rhodium complexes, being the one with the germyl fragment superior in terms of selectivity towards E-isomers. To increase this selectivity, a tandem catalytic protocol that incorporates the use of a heterogeneous catalyst for the trans-semihydrogenation of internal alkynes has been devised. Kinetic mechanistic investigations provide important information regarding the individual catalytic cycles that comprise the overall trans-semihydrogenation of internal alkynes.

Dehydrogenative Double C-H Bond Activation in a Germylene-Rhodium Complex*

Transition metal tetrylene complexes offer great opportunities for molecular cooperation due to the ambiphilic character of the group 14 element. Here we focus on the coordination of germylene [(ArMes2 )2 Ge :] (ArMes =C6 H3 -2,6-(C6 H2 -2,4,6-Me3 )2 ) to [RhCl(COD)]2 (COD=1,5-cyclooctadiene), which yields a neutral germyl complex in which the rhodium center exhibits both η6 - and η2 -coordination to two mesityl rings in an unusual pincer-type structure. Chloride abstraction from this species triggers a singular dehydrogenative double C-H bond activation across the Ge/Rh motif. We have isolated and fully characterized three rhodium-germyl species associated to three C-H cleavage events along this process. The reaction mechanism has been further investigated by computational means, supporting the key cooperative action of rhodium and germanium centers.

Cooperativity in Transition Metal Tetrylene Complexes

Cooperative reactivity between transition metals and ligands, or between two metals, has created significant opportunities for the development of new transformations that would be difficult to carry out with a single metal. Here we explore cooperativity between transition metals and divalent heavier group 14 elements (tetrylenes), a less-explored facet of the field of cooperativity. Tetrylenes combine their strong σ-donor properties with an empty p-orbital that can accept electron density. This ambiphilicity has allowed them to form metal tetrylene and metallotetrylene complexes that place a reactive site adjacent to the metal. We have selected examples to demonstrate what has been achieved so far regarding cooperative reactivity, as this already spans metal-, tetrylene- or multi-site-centred bond cleavage, cycloaddition, migration, metathesis, and insertion. We also highlight some challenges that need to be overcome for this cooperativity to make it to catalysis.

Strained and Reactive Donor/Acceptor-Supported Metallasilanone

A novel stable donor/acceptor-supported Mn^I -metallasilanone 3 was synthesized. The intramolecular silanone-Mn^I interaction induces a highly strained three-membered cyclic structure, leading to an exceptionally high reactivity of 3 as a donor/acceptor complex of silanone. Indeed, metallasilanone 3 readily reacts with various small molecules such as H₂ or ethylene gas in mild conditions.

An H-Substituted Rhodium Silylene

Divergent reactivity of organometallic rhodium(I) complexes, which led to the isolation of neutral rhodium silylenes, is described. Addition of PhRSiH₂ (R=H, Ph) to the rhodium cyclooctene complex (^iPr NNN)Rh(COE) (1-COE; ^iPr NNN=2,5-[iPr₂ P=N(4-iPrC₆ H₄ )]₂ N(C₆ H₂ )^- , COE=cyclooctene) resulted in the oxidative addition of an Si-H bond, providing rhodium(III) silyl hydride complexes (^iPr NNN)Rh(H)SiHRPh (R=H, 2-SiH₂ Ph; Ph, 2-SiHPh₂ ). When the carbonyl complex (^iPr NNN)Rh(CO) (1-CO) was treated with hydrosilanes, base-stabilized rhodium(I) silylenes κ² -N,N-(^iPr NNN)(CO)Rh=SiRPh (R=H, 3-SiHPh; Ph, 3-SiPh₂ ) were isolated and characterized using multinuclear NMR spectroscopy and X-ray crystallography. Both silylene species feature short Rh-Si bonds [2.262(1) Å, 3-SiHPh; 2.2702(7) Å, 3-SiPh₂ ] that agree well with the DFT-computed structures. The overall reaction led to a change in the ^iPr NNN ligand bonding mode (κ³ →κ² ) and loss of H₂ from PhSiRH₂ , as corroborated by deuterium labelling experiments.

Entrapment of THF-Stabilized Iridacyclic IrIII Silylenes from Double H-Si Bond Activation and H2 Elimination

The reaction of H₃ SiR (R=Ph, nBu) with cationic η₅ -C₅ Me₅ - (Cp*) and benzo[h]quinolinyl-based iridacycle [1 b]⁺ gives rise to new [(IrH)→SiRH₂ ]⁺ adducts. In the presence of THF these adducts readily undergo elimination of H₂ gas at subambient temperature to form THF-stabilized metallacyclic Ir^III silylene complexes, which were characterized in situ by NMR spectroscopy, trapped in minute amounts by reactive crystallization, and structurally characterized by XRD. Theoretical investigations (static DFT-D reaction-energy profiling, ETS-NOCV) support the promoting role of THF in the H₂ elimination step and the consolidation of the Ir-to-Si interaction in the spontaneous (ΔG<0) formation of Ir silylenes in the presence of THF. Mechanistic insights indicate that the Ir silylene species arising from the [1 b]⁺ /phenylsilane system are relevant catalytic species in the hydrodefluorination of fluoroalkanes.