cross -[B20H18]2– in Copper(I), Silver(I), and Cadmium(II) Complexation: Formation of [ a 2-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.