An Electron‐Precise, Tetrahedral μ₃ Boride Complex

Abstract

Recent research into the transition-metal chemistry of boron has unveiled a remarkable ability of boron-based ligands to adopt a wide diversity of different coordination modes ( Figure 1). Classifications have been made by the number of substituents at boron and by the number of transition metals attached to the boron center. [1] Experimental and theoretical studies clearly identified a classical bonding situation with two-center-two-electron (2c2e) bonds for most borane (I) and boryl (II) complexes. By contrast, borders between electron-precise and non-classical structures tend to become blurred moving to borylene (III-V) or boride (VI) ligands.Various synthetic strategies have enabled the isolation of terminal (mono-) and m 2 /m 3 bridging (di-and trinuclear) borylene complexes, which usually require kinetic stabilization by sterically demanding ligands at the transition-metal center, or by bulky and/or electron-donating substituents at boron. [2] Trinuclear m 3 bridging borylene complexes still remain rather exotic species, and only few examples have been structurally authenticated so far. Thus, Fehlner and Rheingold reported on the synthesis of a phosphacobaltaborane (1; Scheme 1) featuring a m 3 -BPh ligand. [3] Later on, Suzuki et al. succeeded in the generation of the trinuclear ruthenium species 2 (R = H, CN), albeit without structural authentication by X-ray diffraction. [4] Reaction of the B-H derivative with protic reagents (MeOH, EtOH) resulted in the loss of H 2 and formation of the corresponding alkoxyborylenes 2 (R = OMe, OEt), which implies a hydridic nature of the BÀH moiety. Consequently, homotrimetallic borylenes 2 might also be regarded as trimetalloborate complexes, thus emphasizing the difficulties associated with the exact description of such uncommon species. However, the borylene ligands in 1 and 2 always act as two-electron donors within a non-classical cluster structure. Therefore, an electronprecise bonding situation is only amenable in the absence of any covalently bound exohedral ligand at the boron center of the M 3 B core.At best, this is achieved by formal removal of the B-bound substituent R resulting in trinuclear m 3 boride complexes in which boron is exclusively coordinated to transition-metal fragments. Several examples for this family of compounds have been realized to date, featuring varying structures (Tshaped, Y-shaped, trigonal) with variable degrees of metalboron bonding. [1, 5] However, the transition from m 3 borylene to m 3 boride complexes involves rehybridization of the boron center from sp 3 to either sp (T-shaped, Y-shaped) or sp 2 (trigonal) and loss of the tetrahedral M 3 B structure. For instance, trinuclear 3, which closely approaches an ideal trimetalloboride structure with an electron-precise bonding situation, is essentially trigonal-planar (sum of angles: 359.38). [5e] These results clearly imply that saturation of the fourth valence at boron by a non-covalent interaction is required to make a tetrahedral, electron-precise m 3 boride complex feasible...

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

“…Previous TD-DFT calculations by our group on I Mesupported diaryldiborenes showed that, independento ft he natureo ft he aryl group, the highest wavelength absorption band alwaysc orresponds to the HOMO-LUMO transition, p(BÀB)!p(NHC), the 483-502nma bsorption band to the p(BÀB)!p(Aryl/NHC)t ransition and the lowest wavelength absorptionb and to the s(BÀB)!p(NHC) transition. [8,9] Based on these calculations,w epropose as imilara ssignment to the three absorption bands of each of the UV/Vis spectra presented in Figure 5, as outlined in Ta ble 4.…”

“…Unlike their solutions, which were deep purple (or pink in the case of [(I iPr )BTh Me2 ] 2 ), single crystalso f (13) 129.0(3) (C1,B1,B1',C10') [a] 0.0(3) 2.7(4) 0.7(5) 0.9 (7) 4.2(3) 3.7(2), 5.0(2) 0.1(5), 0.6(5) (B1',B1,C10,S1) [b] 0.9(2) 0.3(5) 20.6(3)6.4(4)51.2(3) 5.1(2), 22.7(2) 0.6(5), 1.8(5) (B1',B1,C1,N1) [c] 89.3(3)77.9(3) 76.5(3)83.0(5) 51.7(3) 72.9(2),6 5.1(2) 82.4(4), 79.3(4) (C14,N3,C13,S1) [d] -----73. 585(4)-1.593(5) ), [8] [9] indicating significant B=Bd ouble bond character.T he BÀC NHC bonds (1.582(2)-1.603(3) )a re considerably shortened compared to the borane precursors (1.613(3)-1.632(3 ), as expected when movingf rom sp 3 -t os p 2 -hybridized boron.E xcept in [(I iPr )BTh tms ] 2 ,t he NHC ligandsa re rotatedn ear orthogonally with respect to the central diborene plane (torsion angles: 70.6(2)-89.3(3)8)i no rder to minimize steric interactions with the thiophene substituents. The BÀC Th bonds, all around 1.56 in length, are 0.02-0.03 shorter than in the borane precursors, whichp oints to some degree of p-electron delocalization over the 1,2-dithienyldiborene framework.…”

“…[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). [8,9] Diboreneso ft he form [(I Me )BAr] 2 are typically red (Ar=Mes, Dur,5 -methyl-2-furyl) or deep purple (Ar= Th, Th tms )i ns olution. Interestingly,t he thienyl-and furyl-substituted diborenes show ac oplanar arrangemento ft he heteroaromatic rings with the diborenec ore, thus enabling p-electron delocalization over the entire 1,2-bis(heteroaryl)diborene framework, [8] whereas the mesityl and duryl groups rotate into orthogonal positions, presumably to relieve the steric strain caused by their ortho-methyl substituents.…”

“…Interestingly,t he thienyl-and furyl-substituted diborenes show ac oplanar arrangemento ft he heteroaromatic rings with the diborenec ore, thus enabling p-electron delocalization over the entire 1,2-bis(heteroaryl)diborene framework, [8] whereas the mesityl and duryl groups rotate into orthogonal positions, presumably to relieve the steric strain caused by their ortho-methyl substituents. [9] The only 1,2-dialkyldiborene isolated to date remains the highly electron-rich [(I iPr )BiPr] 2 (I iPr =1,3-diisopropylimidazol-2-ylidene). [10] In contrast, phosphine-stabilized diborenes are currently only accessible startingf rom ap re-formed boron-boron bond.…”

“…[15] With such al imited number of combinations of anionic substituents and neutral donor ligandsc apable of successfully stabilizing diborenes, there is still little known about their reactivity.L ike their alkener elatives, [LBR] 2 diborenes form p-adducts with late transition metals of groups 11 and 12. [9,16] However, while alkene hydroboration or cycloaddition reactions require ac atalyst, these reactions may proceed uncatalyzed with diborenes.B is(heteroaryl)diborenes of the form [(I Me )BAr] 2 ,f or example,w ere found to undergo spontaneoush ydroboration with catecholborane (Ar=Th tms ,5 -methyl-2-furyl) [8] and 9-BBN (Ar=Th, Th tms ), [17] while the phosphine-stabilized [(Me 3 P)BMes] 2 underwent photo-induced [2+ +2] cycloaddition with acetylene. [18] Overall,h owever,t he range of existing diborenes has provens urprisingly ineffective in the activation of small molecules.…”