Metal‐Only Lewis Pairs by Reversible Insertion of Ruthenium and Osmium Fragments into Metal–Boron Double Bonds

Abstract

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.

Complexation and Release of N-Heterocyclic Carbene-Aminoborylene Ligands from Group VI and VIII Metals

“…The unique robustness and purely σ-donor nature of these M–B linkages (Figure , right) is highly reminiscent of NHC-metal complexes. Indeed, reaction of 2 with the cationic coinage metal species [(IDipp)­CuOTf] results in the formation of the “metal-only Lewis pair” (MOLP) − species 6 (Scheme ), Figure ) where a Fe–Cu bond has been formed. This observation stands in contrast to known metalloborylene reactivity with coinage metals, where coordination to the borylene M–B π-bond has been described .…”

Anorganische Chemie 2014

“…Der Einsatz eines Nebengruppenmetalls eröffnete also einen Reaktionsweg, der Hauptgruppen‐FLPs verschlossen bleibt. Braunschweig et al berichteten von der Synthese rein metallischer Lewispaare aus Chrom und Osmium oder Ruthenium, die sich durch Borylentransmetallierungsreaktionen und anschließende Kristallisation isolieren lassen (Abbildung 29) 2_36. Das Borylentransferreagenz bleibt im Produkt, so dass die Bildung des Lewis‐Paars reversibel ist: Beim Lösen in Toluol bilden sich die Edukte zurück.…”

Partially and Fully Reversible Solvation-Controlled Borylene Swapping and Metal-Only Lewis Pair Formation

“…Although the mechanism of the conventional borylene transfer reaction has not been definitively determined, a number of bridging dinuclear borylene complexes have been isolated from the process . Indeed, bridging motifs appear to be highly favored with the aminoborylene ligand: BN­(SiMe 3 ) 2 ; thus, the dinuclear, nonbridging borylene MOLP complexes prepared in this earlier report were highly unusual. This MOLP formation was, therefore, presumed to be an offshoot of the conventional borylene transfer mechanism.…”

“…The MOLP-forming reactions were also found to be reversible, 5 driven in the forward direction by the crystallization of the MOLP−borylene complexes 3a and 3d from hexanes, and in the reverse by dissolution in more polar solvents. This unprecedented phenomenon suggested that the energetic profile of the reaction must be relatively flat.…”

“…4 Our recent report detailed the first osmium (3d, Figure 1), and second ruthenium (3a) terminal, base-free borylene complexes, both prepared by intermetallic borylene transfer from [Cr{BN(SiMe 3 ) 2 }(CO) 5 ] (1a). 5 However, in contrast to all previous borylene transfer processes, the group 6 metal was retained in the complex, leading to metal-only Lewis pairs (MOLPs) 6 in which a zerovalent group 8 fragment donates a pair of electrons to the group 6 pentacarbonyl fragment. Although the mechanism of the conventional borylene transfer reaction has not been definitively determined, a number of bridging dinuclear borylene complexes have been isolated from the process.…”