A DFT Study on the Stabilization of the B≡B Triple Bond in a Metallaborocycle: Contrasting Electronic Structures of Boron and Carbon Analogues

The electronic structure of (η5‐Cp)2Zr(NH2‐BB‐NH2) (3 b) suggests that it could be a candidate for having a boron–boron triple bond in the cyclic system; however, computational studies shows that 3 b is a very high energy isomer on its potential energy surface. Replacement of amines with tricoordinate nucleophilic boron groups (η5‐Cp)2Zr[B(PH3)2‐BB‐B(PH3)2] (3 c) reduces the relative energy dramatically. The B≡B triple bond arises through the donation of two electrons from the metal fragment, ZrCp2, to the in‐plane π‐bonding orbital of the central B–B unit. Strong σ‐donating and chelating bis‐phosphine ligands (Me2P(CH2)nPMe2), which stabilize donor–acceptor bonding interaction in gem‐diborene L2B‐BBr2 (10), would be a good choice along the synthetic path towards 3 d, (η5‐Cp)2Zr[B4(Me2P(CH2)3PMe2)2]. A comparison of the energetics of the reaction leading to a cyclic boryne system (3 d), with the linear boryne isomer [(B2NHCPh)2] shows that the angle strain from cyclization is compensated by stabilization from the metal.