Boron–Boron σ-Bond Formation by Two-Electron Reduction of a H-Bridged Dimer of Monoborane

Abstract

Diborane(6) as a H-bridged dimer of monoborane can be converted cleanly by two-electron reduction into diborane(6) dianion, which is isoelectronic with ethane, through BÀB σ-bond formation when each boron atom has a bulky ligand on it. The existence of the BÀB σ bond is supported by the X-ray molecular structure [BÀB bond length of 1.924(3) Å], NMR studies, magnetic susceptibility measurements, and DFT calculations. Stepwise hydride abstraction reactions of the diborane(6) dianion produce the corresponding H-bridged diborane(5) anion and doubly H-bridged diborane(4) without BÀB bond scission.

An Isolable Diborane(4) Compound with Terminal B–H Bonds: Structural Characteristics and Electronic Properties

“…5 The molecular structures of 2a and 1a(tm) were confirmed by X-ray crystallography, as shown in Figures 1 and 2, respectively. The dianion 2a has essentially the same structure as previously reported 2b, 7 providing the second example of a thermally stable well-defined diborane(6) dianion derivative. One different point to be mentioned is that the BB bond length (1.905(3) ¡) for 2a is somewhat shorter than that (1.924(3) ¡) for 2b, 7 which is considerably longer than the typical B(sp 3 )B(sp 3 ) single bond length (1.76 ¡), 9,10 because of the electrostatic Coulombic repulsion between the two anionic boron moieties.…”

“…We now report that the introduction of somewhat bulkier MPind groups (MPind: 1,1,7,3,5, 6 leads to the selective formation of (MPind)HBBH(MPind) (1a(tm)) (Scheme 1), as the first example of an isolable H-terminal staggered diborane(4) compound. In this paper, we discuss the structural features and electronic properties of 1a(tm) in comparison to those of the H-bridged isomer 1b(br).…”

“…Soon after, we developed a new logical method for the selective formation of 1b(br), which involves BB σ-bond formation via the two-electron reduction of the readily available H-bridged dimer of monoborane 3b (Scheme 1). 7 The X-ray crystallographic analysis of 1b(br) revealed a butterflyshaped B 2 (¯-H) 2 structure with an almost completely linear CB BC core and a very short BB bond of 1.4879(7) ¡. These structural data, experimental electron density analysis, and DFT computations suggest a threefold bonding character (one σ and two π-like bonds) between the two boron atoms incorporating the two bridging hydrogen atoms.…”

Weakly Stabilized Primary Borenium Cations and Their Dicationic Dimers

“…17 A substantially shorter B···B distance has been established for a structure containing a B–B bond (X-ray,1.49Å; neutron diffraction, 1.48Å). 18 Hydrogens near the central boron atoms were located on difference Fourier maps and were allowed to refine isotropically as independent atoms. The exact location of the B–H hydrogens is uncertain due to their inherently low contribution to the electron density map.…”

Deprotonation of a Seemingly Hydridic Diborane(6) To Build a B−B Bond

“…[8][9][10][11] Specifically,o ur group generated the compound K[1H] from diborane 1H 2 [12,13] through ar eduction/hydride elimination sequence (Scheme 2). [8][9][10][11] Specifically,o ur group generated the compound K[1H] from diborane 1H 2 [12,13] through ar eduction/hydride elimination sequence (Scheme 2).…”