Spontaneous Isomerization [trans-B20H18]2– → [iso-B20H18]2– during Cobalt(II) Complexation with Phenanthroline

Abstract

Abstract In this work, the cobalt(II) complexation with 1,10-phenanthroline (Phen) in the presence of the [trans-B20H18]2– anion has been studied. At a Co : Phen = 1 : 2 ratio in acetonitrile, a binuclear cobalt(II) complex with bridging chlorine atoms and the boron cluster anion as a counterion [(Phen)2Co(µ-Cl)2Co(Phen)2][trans-B20H18] has been isolated. However, during slow crystallization (within a month), spontaneous isomerization of [trans-B20H18]2– into [iso-B20H18]2– is observed. It has been established by X-ray diffraction analysis that in the crystal of the final compound [(Phen)2Co(µ-Cl)2Co(Phen)2][trans-B20H18]1/3[iso-B20H18]2/3, co-crystallization of both isomeric forms of the octadecahydroeicosaborate anion is observed for the first time. The presence of the iso form of the boron cluster anion is also confirmed by IR spectroscopy data: the spectrum of the product shows a band of stretching vibrations of the BHB bridge groups at 1773 cm–1, which is absent in the trans-form of the boron cluster.