Base‐Stabilized Diborenes: Selective Generation and η² Side‐on Coordination to Silver(I)

Abstract

(B)olefin complexes: Reductive coupling of designed monoborane precursors (see scheme; Dur=2,3,5,6-tetramethylphenyl) gives convenient access to N-heterocyclic carbene stabilized diborenes. The presence of B-B multiple bonds in the dark red diborenes is shown experimentally and theoretically. Reaction with AgCl afforded a Ag(I) species with an unprecedented, olefin-like η(2) coordination mode.

Increasing the Reactivity of Diborenes: Derivatization of NHC‐Supported Dithienyldiborenes with Electron‐Donor Groups

“…The dichlorod erivativesa ll displayed 11 BN MR shifts in the regiono f0to À1.5 ppm, similar to related (I Me )BX 2 Ar species (ca. À1.7 ppm), [8] whereas the dibromo derivatives appeared furtheru pfield, around À8ppm (Table 2), as expected from the less electron-withdrawing character of Br versus Cl.…”

“…Thus switching from PMe 3 to the better s-donor and slightly better p-acceptor I Me ,w hilst retaining mesityl or duryl anionic substituents, induces an increasei nt he energy of the HOMO concomitant with al oweringo ft he energy of the LUMO, thereby reducingt he HOMO-LUMO gap by around0 .8 eV. [11] Similarly, switching from PMe 3 to the more p-acidic chelating 1,2-bis(diphenylphosphino)methane ligand only induces al oweringo f the LUMO energy,t hereby also reducing the HOMO-LUMO gap. [12] Conversely,c hanging the anionic substituent from electron-withdrawing duryl to slightly electron-releasing isopropyl groups,a nd switching from I Me to the even better s-donor I iPr , causes a0 .9 and 1.2 eV increase in the energy of the HOMO and LUMO, respectively,t hereby making [(I iPr )B i Pr] 2 the most electron-rich diborene yet, capable even of reducing the borole MesBC 4 Ph 4 .…”

“…[3] Af ew years later our group isolated ad eep-green 1,2-dibromodiborene from the selectivet wo-electron reductiono ft he tetrabromodiborane (6) [5] The targeted synthesis of dihydrodiborenes was only recently achieved with excellents electivity and yield by the direct 1,2-hydrogenation of aN HC-stabilized diboryne, [(SI Dep Me 4 )pyrrolidin-2-ylidene). [6] Thermal decomposition of the bis-(tert-butyl)isocyanide adduct of the latter also provided the first 1,2-dicyanodiborene, [(cAAC)B(CN)] 2 ,w hichp resents two distinct 11 BN MR shifts duet oa nu nsymmetrical arrangement of the cAAC ligands, one being coplanar with, and the other orthogonal to, the dicyanodiborene core. [7] As mall range of 1,2-diaryldiborenes has been successfully synthesized using the much smaller 1,3-dimethylimidazol-2-ylidene (I Me ) ligand,b yt wo-electron reduction of the corresponding aryldichloroboranes, (I Me )BCl 2 Ar (Ar=Mes, 2,3,5,6-Me 4 C 6 H (Dur), 2-thienyl (Th), 5-trimethylsilyl-2-thienyl (Th tms ), 5-methyl-2-furyl,F igure 1B).…”

“…The two-electron reduction of 1,2-diaryl-1,2-dibromodiboranes(4), B 2 Br 2 Ar 2 (Ar=Mes, Dur,9 -anthryl) in the presence of two equivalents of PMe 3 or PEt 3 thus yieldedt he red-coloredb is(phosphino)d iborenes,[(R 3 P)BAr] 2 (R=Me, Et, Ar=Mes, Figure 1C;R =Me, Ar=9-anthryl). [11]À [13] While all hitherto mentioned diborenes present a trans-arrangement of their respective anionic and neutral ligands,t he use of chelating bis(phosphine) ligandse nabled the synthesis of the first cis-1,2-diaryldiborenes ( Figure 1C). [12] Most recently,t he group of Kinjo succeeded in isolating the first unsymmetrical diborene, supported on one side by ap articularly p-acidicc AAC ligand and on the other by I Dip (Figure 1D), [14] as well as au nique allenic diborene, stabilized by two PMe 3 ligands on the terminal boron.…”

“…[1] As ar esult, diborenes, unlike alkenes, are excellent reducinga gents, capable of acting as either i) four-electron reducinga gents towards elemental sulfur,b yf ully cleaving the B=Bd ouble bond and forming aB 2 S 3 heterocycle, [19] or ii)t woelectron reducing agentst owards the heaviere lemental chalcogens by forming three-membered B 2 Er ings (E=Se, Te ), [20] or even iii)o ne-electronr educing agents towards the tropylium cation and 1-mesityl-2,3,4,5-tetraphenylborole (MesBC 4 Ph 4 )t o form diborene radical cations. [10], [11] The substitution pattern of the diborene core plays as ignificant role in tuning this redox reactivity (Figure 2). Thus switching from PMe 3 to the better s-donor and slightly better p-acceptor I Me ,w hilst retaining mesityl or duryl anionic substituents, induces an increasei nt he energy of the HOMO concomitant with al oweringo ft he energy of the LUMO, thereby reducingt he HOMO-LUMO gap by around0 .8 eV.…”