Raising the HOMO level of the [ <i>closo</i> -B ₁₀ H ₁₀ ] ²⁻ anion: apical alkyl derivatives for modern materials

The HOMO energy of the [<i>closo</i>-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 <i>closo</i>-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 <i>E</i> _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 [<i>closo</i>-B₁₀H₁₀]^2- anion were investigated with an Fe(ii) complex and ionic LCs displaying CT behaviour.