Raising the HOMO level of the [closo-B10H10]2− anion: apical alkyl derivatives for modern materials

Abstract

The HOMO energy of the [closo-B₁₀H₁₀]²⁻ anion is increased by placing one or two alkyl groups at the apical positions of the cluster. To accomplish this, a Pd(0) catalysed C–B cross coupling reaction of RZnCl and mono- or diiodo cluster derivatives was developed. DFT results show that these iodo derivatives are the most challenging substrates among typical closo-borane iodides characterised by the highest B–I bond strength and associated the largest σ* MO energy. The most effective catalyst for the B–I coupling reaction was BrettPhosPd-G3, which gave the alkylated products in high yields. The alkyl effect on the {B₁₀} cluster geometry was demonstrated with single crystal XRD and correlation analyses. The electronic properties of the mono- and dialkyl products were probed with electrochemistry and UV-vis spectroscopy of charge transfer ion pairs. The electrochemical results correlate with Hammett substituent parameters and DFT derived E_HOMO, while the CT band energy is proportional to the difference between the FMO energies. Material properties of such newly available alkyl derivatives of the [closo-B₁₀H₁₀]²⁻ anion were investigated with an Fe(II) complex and ionic LCs displaying CT behaviour.