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

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, B-2(1)) and C-2v B3S2H3 (2, (1)A(1)). 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 5 pi system within a heteroatomic B3S2 ring, analogous to the pi bonding in cyclopentadiene, D-5h C5H5. The corresponding closed-shell C-2v B3S2H32- (3, (1)A(1)) 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 ((1)A) open structure. This situation is in contrast to the cyclopentadienyl anion, C5H5-, a prototypical aromatic hydrocarbon with a p sextet. The C-2v B3S2H3 (2) neutral cluster is readily obtained upon removal of one pi electron from C-2v B3S2H3- (1). The anion photoelectron spectrum of C-2v B3S2H3- (1) and the infrared absorption spectrum of C2v B3S2H3 (2) are predicted. The C-2v B3S2H3- (1) species can be stabilized in sandwich-type C-2h [(B3S2H3)(2)Fe](2-) and salt C-2h [(B3S2H3)(2)Fe]Li-2 complexes. An intriguing difference is observed between the pattern of pi 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 B3S2H30/-/2- series with charge states.