The first isolable diborane(4) containing terminal BH bonds, protected by the bulky 1,1,7,7-tetramethyl-3,3,5,5-tetrapropyl-shydrindacen-4-yl (MPind) groups, was synthesized as air-and moisture-sensitive colorless crystals. The structure and bonding nature of this H-terminal diborane(4), characterized by X-ray crystallography, NMR studies, and DFT calculations, were compared to those of the previously reported isomer, a doubly H-bridged butterfly-shaped diborane(4) bearing the less bulky 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl (Eind) groups. The two distinct structures of these dihydro-diborane(4) compounds may be primarily attributed to the intramolecular steric effects of the bulky protecting groups.Over the years, many experimental and computational efforts have been directed toward studying the structures and electronic properties of diboron compounds.1 In particular, hydrogensubstituted diboron compounds have continuously attracted the attention of chemists because of their unique bonding character originating from the electron deficiency of boron.2 Elaborate calculations have indicated that the parent diborane(4), B 2 H 4 , mainly has two stable forms, i.e., an H-terminal staggered D 2d isomer and a doubly H-bridged butterfly-shaped C 2v isomer (Chart 1), with nearly the same energies.3 The D 2d isomer consists of one BB σ-bond and four terminal BH bonds in an orthogonal arrangement of the two BH 2 planes. 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).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.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 synthetic routes are shown in Scheme 1. A toluene solution of the MPind-based diborane(6), (MPind)HB(¯-H) 2 -BH(MPind) (3a), which was prepared by the reduction ...
