Borylene: eine aufstrebende Verbindungsklasse

Abstract

Freie Borylene (R‐B:) konnten bislang lediglich spektroskopisch in der Gasphase oder in Tieftemperatur‐Matrices charakterisiert werden. In der letzten Zeit gelang es jedoch, einige Mono‐ und Di(Lewis‐Base)‐stabilisierte Borylene zu isolieren. In beiden Verbindungen ist das Boratom durch die formale Oxidationsstufe +I gekennzeichnet, während es sich in klassischen Organoborverbindungen in der Oxidationsstufe +III befindet. Mono(Lewis‐Base)‐stabilisierte Borylene sind isoelektronisch zu Singulett‐Carbenen, und ihre Reaktivität ähnelt teilweise der von Übergangsmetallen, etwa bei der Aktivierung kleiner Moleküle (wie H2) und der Koordination weiterer Liganden. Di(Lewis‐Base)‐Borylen‐Addukte sind isoelektronisch zu Aminen und Phosphanen. Im Unterschied zu Boranen, die als Elektronenakzeptoren fungieren, sind diese Spezies elektronenreich und eignen sich als Liganden für Übergangsmetalle.

Metal‐Free Nitrogen Fixation at Boron

“…Chemie and B(C 6 F 5 ) 3 , [13] which is surprisingly sensitive towards N 2 loss even at room temperature,c ontrasting the behavior of (Ph 3 P) 2 N 2 .T his finding clearly shows that judicious choice of Lewis acids and bases offers are ap otential avenue to metal-free capture of N 2 . Against conventional wisdom, hypovalent boron species, namely borylenes R-B with am onovalent boron center, [14] possess alone pair (LP) of electrons in combination with two unoccupied orbitals at boron, asituation mimicking TMs with filled and empty metal-centered dorbitals.Borylenes must be stabilized by one or two Lewis donors (L), such as Nheterocyclic carbenes (NHCs), cyclic alkyl amino carbenes (CAACs), [8b] and CO,orinthe coordination sphere of aTM. This allows judicious adjustment of the energy of the orbitals on boron and therefore opens the door for the challenging task of nitrogen fixation (Scheme 2, right).…”

C−C and C−N Bond Activation, Lewis‐Base Coordination and One‐ and Two‐Electron Oxidation at a Linear Aminoborylene

“…The first stable metal‐free borylene, synthesized in 2010 by Bertrand, [11] relied on the electronic and steric stabilization of two cyclic alkyl(amino)carbene (CAAC) ligands, acting as strong σ donors and π acceptors, [12,13] and thus enabling the delocalization of the excess electron density over the entire CAAC−B−CAAC π framework. Since then, numerous symmetrical and unsymmetrical tricoordinate borylene derivatives (LL′BR, L,L′=neutral donor ligands) with various R substituents (e. g., H, F, Cl, Br, CN, NCS, aryl, boryl) have been isolated [14,15] . Their reactivity, however, remains often hampered by excessive steric hindrance around the tricoordinate boron center.…”

Electron‐Precise Dicationic Tetraboranes: Syntheses, Structures and Rearrangement to an Alkylidene Borate‐Borenium Zwitterion and a 1,3‐Azaborinine

“…The last two decades have witnessed a surge in the use of carbenes, notably N‐heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (CAACs), as stabilizing ligands in the synthesis of low‐valent boron species [33–34] . While monoboranes and diboranes(4) have proven to be versatile building blocks for novel chemical compounds, [35–36] very little is known about electron‐precise oligoboranes (B 3 −B 6 ) due to the lack of reliable supporting ligands during the course of functionalization and the synthetic difficulties associated with their precursors [25,37–38] . Early work by Nöth and coworkers demonstrated that halogenation of tetraboranes could be achieved to give a variety of dihalotetraboranes (B 4 X 2 R 4 ), which showed promise as candidates for more complex structures [25] .…”