A Stable Doubly Hydrogen-Bridged Butterfly-Shaped Diborane(4) Compound

Abstract

In contrast to the common multiple bonding between carbon atoms, multiply bonded boron compounds have still been a synthetic challenge due to the electron deficiency of boron. We now report that a stable doubly hydrogen-bridged diborane(4), EindB(μ-H)2BEind, is produced by the two-electron oxidation of a hydrogen-substituted diborane(4) dianion [Li+(thf)]2[Eind(H)BB(H)Eind]2−, where Eind denotes the 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl. The X-ray crystallography reveals a short B−B distance of 1.4879(7) Å in comparison with the normal B−B single bond length (1.72 Å), the presence of two hydrogen atoms bridged perpendicular to the B−B bond with a butterfly shape having a dihedral angle of the two BHB triangles of 113(1)°, and a linear geometry around the B−B bond with a C−B−B bond angle of 178.92(4)°. These structural data, experimental electron density analysis, and computational studies confirm the 3-fold bonding (a σ and two π-like bonds) between the two boron atoms incorporating the two bridging hydrogen atoms.

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

“…8 This is in sharp contrast to the fact that the crystals of the Eind-based 1b(br) remain stable at room temperature for more than one year in the absence of air. 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.…”

“…The major structural parameters for 1a(tm) are summarized in Table 1, together with those of 1b(br) for comparison. 5 The BB bond distance of 1.673(2) ¡ in 1a(tm) is distinctly longer than 1.4879(7) ¡ observed in 1b(br) (Entry 1). 5 An essentially The structure of 1a(tm) in solution (benzene-d 6 ) was characterized by 1 H, 11 B, and 13 C NMR spectroscopies.…”

“…5 The BB bond distance of 1.673(2) ¡ in 1a(tm) is distinctly longer than 1.4879(7) ¡ observed in 1b(br) (Entry 1). 5 An essentially The structure of 1a(tm) in solution (benzene-d 6 ) was characterized by 1 H, 11 B, and 13 C NMR spectroscopies. The spectral profiles of 1a(tm) are in sharp contrast to those of 1b(br) (see Supporting Information (SI)).…”

“…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).…”

“…The C 2v isomer is considered to have one BB σ-bond and two BHB 3c2e bonds, which are closely related to a canonical form as a doubly proton-bridged dianionic BB triple bond. 4 We recently obtained rather serendipitously, for the first time, a stable doubly H-bridged diborane(4), (¯-H) 2 (Eind)BB(Eind) (1b(br)), 5 containing the fused-ring bulky Eind groups (Eind: 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl) 6 as a protecting group (Scheme 1). 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).…”