Planar B3S2H3− and B3S2H3 clusters with a five-membered B3S2 ring: boron–sulfur hydride analogues of cyclopentadiene

Abstract

Boron clusters can serve as inorganic analogues of hydrocarbons or polycyclic aromatic hydrocarbons (PAHs). We present herein, based upon global searches and electronic structural calculations at the B3LYP and CCSD(T) levels, the global-minimum structures of two boron–sulfur hydride clusters: C_2v B₃S₂H₃⁻ (1, ²B₁) and C_2v B₃S₂H₃ (2, ¹A₁). Both species are perfectly planar and feature a five-membered B₃S₂ ring as the structural core, with three H atoms attached terminally to the B sites. Chemical bonding analysis shows that C_2v B₃S₂H₃⁻ (1) has a delocalized 5π system within a heteroatomic B₃S₂ ring, analogous to the π bonding in cyclopentadiene, D_5h C₅H₅. The corresponding closed-shell C_2v B₃S₂H₃²⁻ (3, ¹A₁) dianion is only a local minimum. At the single-point CCSD(T) level, it is 5.7 kcal mol⁻¹ above the chain-like C₁ (¹A) open structure. This situation is in contrast to the cyclopentadienyl anion, C₅H₅⁻, a prototypical aromatic hydrocarbon with a π sextet. The C_2v B₃S₂H₃ (2) neutral cluster is readily obtained upon removal of one π electron from C_2v B₃S₂H₃⁻ (1). The anion photoelectron spectrum of C_2v B₃S₂H₃⁻ (1) and the infrared absorption spectrum of C_2v B₃S₂H₃ (2) are predicted. The C_2v B₃S₂H₃⁻ (1) species can be stabilized in sandwich-type C_2h [(B₃S₂H₃)₂Fe]²⁻ and salt C_2h [(B₃S₂H₃)₂Fe]Li₂ complexes. An intriguing difference is observed between the pattern of π sextet in C_2v B₃S₂H₃²⁻ (3) dianion and that in cyclopentadienyl anion. The present work also sheds light on the mechanism of structural evolution in the B₃S₂H₃^0/−/2− series with charge states.