Elucidating the Reactivity of Vicinal Dicarbenoids: From Lewis Adduct Formation to B-C Bond Activation.

The reactivity of the diaminoacetylene Pip-C≡C-Pip (Pip=piperidyl=NC5 H10 ) towards phenyldichloro- and triphenylborane is presented. In the case of the less Lewis acidic PhBCl2 , the first example of a double Lewis adduct of a vicinal dicarbenoid is reported. For the more Lewis acidic triphenylborane, coordination to the bifunctional carbene leads to a mild B-C bond activation, resulting in a syn-1,2-carboboration. Ensuing cis/trans isomerization yields a novel ethylene-bridged frustrated Lewis pair (FLP). The compounds were characterized using multinuclear NMR spectroscopy, structural analysis, and mass spectrometry. Reactivity studies of both isomers with the N-heterocyclic carbene 1,3-dimethylimidazol-2-ylidene (IMe) aided in elucidating the proposed isomerization pathway. DFT calculations were carried out to elucidate the reaction mechanism. The rather low free energy of activation is consistent with the observation that the reaction proceeds smoothly at room temperature.

Fundamental Differences between Group 8 Metals: Unexpected Oxidation State Preferences and Mechanisms in Ruthenium Borylene Complex Formation.

The reaction of the salts K[Ru(CO)3 (PMe3 )(SiR3 )] (R=Me, Et) with Br2 BDur or Cl2 BDur (Dur=2,3,5,6-Me4 C6 H) leads to both boryl and borylene complexes of divalent ruthenium, the former through simple salt elimination and the latter through subsequent CO loss and 1,2-halide shift. The balance of products can be altered by varying the reaction conditions; boryl complexes can be favored by the addition of CO, and borylene complexes by removal of CO under vacuum. All of these products are in competition with the corresponding (aryl)(halo)(trialkylsilyl)borane, a reductive elimination product. The Ru(II) borylene products and the mechanisms that form them are distinctly different from the analogous reactions with iron, which lead to low-valent borylene complexes, highlighting fundamental differences in oxidation state preferences between iron and ruthenium.

Mild and Complete Carbonyl Ligand Scission on a Mononuclear Transition Metal Complex.

The complete scission of the carbon-oxygen bond of carbon monoxide, while frequently observed on bulk metals and with bimetallic and cluster transition metal complexes, is unknown with monometallic systems. Reaction of a zerovalent iron bis(borylene) complex with a cyclic (alkyl)(amino)carbene revealed a highly selective intramolecular cleavage of the C-O bond of a carbonyl ligand at room temperature, leading to the formation of a highly unusual iron complex containing a base-stabilized (bora)alkylideneborane ligand. DFT investigation of the reaction mechanism suggested that the two Lewis acidic borylene boron atoms cooperate to cleave the C-O multiple bond.

Lewis Acid Binding and Transfer as a Versatile Experimental Gauge of the Lewis Basicity of Fe(0) , Ru(0) , and Pt(0) Complexes.

A number of zerovalent ruthenium tri- and tetracarbonyl complexes of the form [Ru(CO)5-n Ln] (n=1, 2) with neutral phosphine or N-heterocyclic carbene donor ligands have been treated with the Lewis acids GaCl3 and Ag(+) to form a range of metal-only Lewis pairs (MOLPs). The spectroscopic and structural parameters of the adducts are compared to each other and to related iron carbonyl based MOLPs. The Lewis basicity of the original Ru(0) complexes is gauged by transfer experiments, as well as through the degree of pyramidization of the bound GaCl3 units and the Ru-M bond lengths. The work shows the benefits of the MOLP concept as one of the few direct experimental gauges of metal basicity, and one that can allow comparisons between metal complexes with different metal centers and ligand sets.

1,2-Halosilane vs. 1,2-alkylborane elimination from (boryl)(silyl) complexes of iron: switching between borylenes and silylenes just by changing the alkyl group.

Reaction of different combinations of aryl(dihalo)boranes and trialkylsilyl iron metallates, a route previously used to prepare a terminal iron arylborylene complex, is found to lead to three distinct new reaction outcomes, including unselective decomposition, an inert iron(II) (boryl)(silyl) complex, and a dinuclear bis(µ-silylene) complex. The latter result is to our knowledge the first example of a 1,2-alkylborane elimination, in contrast to the facile and ubiquitous 1,1-alkylborane elimination observed from (alkyl)(boryl) transition metal complexes, and is also a novel route to bridging silylene complexes.

Metal-only Lewis pairs by reversible insertion of ruthenium and osmium fragments into metal-boron double bonds.

New metal-only Lewis pairs (MOLPs: Ru→Cr and Os→Cr) are prepared by the insertion of a zerovalent ruthenium or osmium complex into chromium-boron double bonds of borylene complexes. The reaction creates new borylene complexes (the first ever for osmium), and is crystallization-controlled; re-dissolving the complexes results in regeneration of the starting materials. A mechanism is proposed based on DFT calculations, along with a computational study of the unusual MOLPs.