Biicosahedral metallaboranes: aromaticity in metal derivatives of three-dimensional analogues of naphthalene

Abstract

Consideration of the well-known very stable icosahedral B₁₂H₁₂²⁻ as a three-dimensional analogue of benzene was extended by the recent synthesis of the biicosahedral B₂₁H₁₈⁻ as a three-dimensional analogue of naphthalene. The preferred structures of metallaboranes derived from B₂₁H₁₈⁻ have now been examined by density functional theory. The isoelectronic species CpNiB₂₀H₁₇ and CpCoCB₁₉H₁₇ have the 46 skeletal electrons expected by the Wade-Mingos and Jemmis rules for a structure consisting of two face-sharing fused icosahedra. The CpM units in these structures energetically prefer to be located at a meta vertex of the biicosahedron. The analogous ferraboranes CpFeB₂₀H₁₇ with only 44 skeletal electrons also have related biicosahedral structures. The presence of an agostic hydrogen atom bridging an Fe–B edge compensates for the two-electron deficiency in CpFeB₂₀H₁₇ relative to CpNiB₂₀H₁₇. The nucleus-independent chemical shift (NICS) values of these systems indicate them to be strongly aromatic.