Stable Lewis Base Adducts of Tetrahalodiboranes: Synthetic Methods and Structural Diversity

Abstract

A series of 22 new bis(phosphine), bis(carbene), and bis(isonitrile) tetrahalodiborane adducts has been synthesized, either by direct adduct formation with highly sensitive B2X4 precursors (X=Cl, Br, I) or by ligand exchange at stable B2X4(SMe2)2 precursors (X=Cl, Br) with labile dimethylsulfide ligands. The isolated compounds have been fully characterized using NMR spectroscopy, elemental analysis, and, for 20 of these compounds, single‐crystal X‐ray diffraction, revealing an unexpected variation in the bonding motifs. In addition to the classical B2X4L2 diborane(4) bis‐adducts, certain more sterically demanding carbene ligands induce a halide displacement which led to the first halide‐bridged monocationic diboron species, [B2X3L2]A (A=BCl4, Br, I). Furthermore, low‐temperature 1:1 reactions of B2Cl4 with sterically demanding N‐heterocyclic carbenes led to the formation of kinetically unstable mono‐adducts, one of which was structurally characterized. A comparison of the NMR spectra and structural data of new and literature‐known bis‐adducts shows several trends pertaining to the nature of the halides and the stereoelectronic properties of the Lewis bases employed.

Spontaneous trans‐Selective Transfer Hydrogenation of Apolar Boron–Boron Double Bonds

“…X-ray crystallographic analysis of single crystals of compounds 1, 2,and 4 revealed in all three cases the doubly basestabilized 1,2-dihydrodiboranes resulting from aformal transaddition of H 2 to the B=Bdouble bond, that is,the meso forms for 1 and 2 and the (R,R)/(S,S)f orm for 4 ( Figure 2). The boron-bound hydrogen atoms,w hich were located in the Fourier difference map and freely refined, are positioned trans to each other in all three cases (dihedral angles (H1,B1,B2,H2): 1a 180; 2a 163(2); 4 177(1) 8 8), resulting in an eclipsed conformation of the boron substituents.S ince repeated crystallizations of 1 and 2 only afforded single crystals of the meso forms,t hese were assigned to the major diastereomers formed, 1aand 2a,respectively.The BÀBbond length in 1a (1.793 (6) )i sw ithin the range expected for bis(base)-stabilized diboranes [23] and similar to that reported by Robinson for the related (IiPr Me ) 2 B 2 H 2 Ph 2 (1.796 (3) , IiPr Me = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene). [24] Those in 2a (1.817(4) )a nd 4 (1.8171 (19) )a re slightly longer, presumably due to the increased steric bulk provided by the 5-trimethylsilyl-2-thienyl and IiPr ligands (IiPr = 1,3diisopropylimidazol-2-ylidene),a sw ell as the strain of the ferrocenediyl chelate,respectively.…”

A Facile Synthesis of Robinson's NHC‐Stabilised Diborane(4)

“…N‐heterocyclic carbenes (NHCs), to sp 2 ‐sp 3 diboranes. [ 1,4,5 ] The application of NHCs in boron chemistry [ 6 ] has furthermore led to the synthesis and isolation of a range of unusual low oxidation state molecules, [ 7 ] including diadducts of diboron species such as B 2 H 4 , [ 8 ] B 2 H 2 , [ 8,9 ] and B 2 , [ 10 ] see Figure 1 for selected examples involving the sterically demanding NHC IDip [= (HCNDip) 2 C:, Dip = 2,6‐ i Pr 2 C 6 H 3 ]. The products 1 – 3 were obtained from reduction reactions involving very strong alkali metal‐based reducing agents and the BH moieties in 1 and 2 resulted from uncontrolled hydrogen abstraction reactions likely of solvent molecules.…”

Dibortetraiodid (B2I4) ist im Festkörper ein Polymer aus sp3‐hybridisiertem Bor

“…Unsere anschließenden Untersuchungen bezüglich der Reaktivität von B 2 I 4 offenbarten deutliche Unterschiede zwischen diesem hochreaktiven Molekül und den leichteren Dibortetrahalogeniden, was einerseits durch die Reaktivität gegenüber niedervalenten Metallkomplexen ersichtlich ist, andererseits kann B 2 I 4 (wie auch B 2 Br 4 ) zwar als Vorstufe für die Synthese von Dihalogendiboranen eingesetzt werden, die Produkte unterscheiden sich allerdings drastisch von den entsprechenden Bromderivaten . Weiterhin wurde seither die Quaternisierung der Boratome in B 2 I 4 mittels zweifacher Addition von Lewis‐Basen (unter Bildung von Addukten der Form B 2 I 4 L 2 ) beziehungsweise von Halogenidanionen (unter Bildung des Dianions [B 2 I 6 ] 2− ) erreicht …”