Mechanism for Site-Selective Hydroboration of C70 Fullerene with Borane by DFT-D3 Study

Abstract

We studied the hydroboration of the C₇₀ fullerene using both B3LYP-D3(BJ)/6-311G(d,p) and M06-2X-D3/6-311G(d,p) levels of theory, incorporating the empirical dispersion interaction, and Fukui index calculations. Potential energy surfaces (PESs) and Gibbs free energy surfaces (GFESs) were calculated for the pathways from four BH₃ adducts (located at the AB, CC, D, and E sites) on the C₇₀ to eight products formed by the 1,2-addition of BH₃ across the four [6,6]-ring fused bonds (AB, CC, DE, and EE) and across the two [5,6]-ring fused bonds (AA and DD). These pathways are two-step consecutive reactions. We denoted the positions on the fullerene cage as A through E, from the pole to the equator, based on the D_5h symmetry of the C₇₀ fullerene. In the first step reaction, the product ratios for the four adduct intermediates should be as the primary intermediate BH₃(D), the secondary intermediate BH₃(AB), the tertiary intermediate BH₃(CC), and the minor intermediate BH₃(E), based on the Fukui indices. In addition, in the second step reaction, transition states (TSs) from four adduct intermediates to eight product isomers, namely, BH₂(A)H (B) to BH₂(E)H (E), were obtained using the QST2 method. The calculated reaction coordinates showed exothermic reactions for all bonds except the EE bond. We also confirmed the transition states by frequency calculations and intrinsic reaction coordinate (IRC) analyses. The PESs and GFESs suggest spontaneous processes for the four isomers, of which the primary products are BH₂(A)H (B) and its isomer BH₂(B)H (A), the secondary product is BH₂(C)H (C), and the tertiary product is BH₂(D)H (D), all formed through adduct intermediates. Therefore, through the hydroboration reaction of C₇₀, we could predict and design the site selectivity of C₇₀ by controlling the energy barrier of the transition state in the second step of the reaction. This implies that we could selectively synthesize mainly BH₂(B)H (A) isomers across the AB-[6,6]-ring fused bond and also design BH₂(D)H(D) isomers across the DD-[5,6]-ring fused bond. Also, the calculations of formation rate constants can well simulate the experimental ratio of two C₇₀H₂ isomers by the hydrolysis of BH₂(A)H(B), BH₂(B)H(A), and BH₂(C)H(C) products at room temperature.