cross-[B20H18]2– in Copper(I), Silver(I), and Cadmium(II) Complexation: Formation of [a2-B20H18]4– and [B20H16]4– Tetraanions

Abstract

The cross-[B₂₀H₁₈]^2– anion, a synthetic challenge for decades, is introduced into coordination chemistry for the first time. Contrary to the well-behaved trans and iso isomers, the cross-[B₂₀H₁₈]^2– exhibits unprecedented reactivity toward d¹⁰ metal ions (Cu⁺, Ag⁺, Cd²⁺), undergoing spontaneous reduction under ambient conditions in acetonitrile affording the reduced a²-[B₂₀H₁₈]^4– anion in copper(I) and cadmium(II) complexes, and─remarkably─the previously unknown [B₂₀H₁₆]^4– species in the silver(I) system. The resulting complexes, [Ag₄(Ph₃P)₄[B₂₀H₁₆]] (1), (Et₃NH)₂[Cu₂(PPh₃)₂[a²B₂₀H₁₈]] (2), and [Ph₃PCH₂Naph][Cd₂(phen)₂(CF₃COO)[a²-B₂₀H₁₈]] (3), were characterized by single-crystal X-ray diffraction, ¹¹B NMR (only for 2), and IR spectroscopy. Density functional theory calculations at the B97MD4/def2-TZVPP level confirm a²-[B₂₀H₁₈]^4– as the thermodynamically most stable reduced form, with the strongest intercage B–B bond. This work not only establishes the cross isomer as a distinct entry point to boron cluster chemistry but also provides a remarkably simple, one-pot route to reduced boron-rich ligands.