Theoretical Study of Ring Exchange in the Borocenium Cation, [B(C 5 R 5 ) 2 ] + (R = H, Me)

Geometries and energetics of the borocenium cation, [BCp 2 ] +, and the decamethylborocenium cation, [BCp* 2 ] +, have been studied by using density functional theory (DFT) and ONIOM methods. Possible structures with different hapticities were considered for [BCp 2 ] + at the B3LYP/6-31+G(d) level, and for [BCp* 2 ] + at the ONIOM(B3LYP/6-31+G(d):HF/STO-3G) level. In the ground-state structure of [BCp* 2 ] + one ring is monohapto and one ring is pentahapto (η 1 /η 5 ), in good agreement with experiment. The energy difference between the η 1 /η 5 ( A ) and η 5 /η 5 ( I ) structure of [BCp 2 ] + is 54.5 kcal/mol at the B3LYP/6-311+G(2d,p)//B3LYP/6-31+G(d)+ZPC level, which is in contrast to the small energy difference (∼2 kcal/mol) found between the η 1 /η 5 and η 5 /η 5 structures in the isoelectronic Cp 2 Be system. The ring exchange process for the [BCp 2 ] + cation has an activation energy barrier of 14.7 kcal/mol where the hapticity of one ring gradually decreases (η 5 → η 2 (TS) → η 2 → η 1 (TS) → η 1 ) and the hapticity for the other ring gradually increases (η 1 → η 1 (TS)→ η 2 → η 2 (TS) → η 5 ). GIAO/B3LYP/6-31+G(d,p) calculations for the [BCp* 2 ] + 11 B and 13 C chemical shifts are in excellent agreement with experiment. The decamethyl-substituted Cp donates more electron density to boron compared to Cp resulting in a 5.4 ppm downfield 11 B chemical shift for [BCp* 2 ] + relative to [BCp 2 ] + .