Generation and reactivity of the fragment ion [B12I8S(CN)]− in the gas phase and on surfaces

Abstract

Gaseous fragment ions generated in mass spectrometers may be employed as “building blocks” for the synthesis of novel molecules on surfaces using ion soft-landing. A fundamental understanding of the reactivity of the fragment ions is required to control bond formation of deposited fragments in surface layers. The fragment ion [B₁₂X₁₁]⁻ (X = halogen) is formed by collision-induced dissociation (CID) from the precursor [B₁₂X₁₂]²⁻ dianion. [B₁₂X₁₁]⁻ is highly reactive and ion soft-landing experiments have shown that this ion binds to the alkyl chains of organic molecules on surfaces. In this work we investigate whether specific modifications of the precursor ion affect the chemical properties of the fragment ions to such an extent that attachment to functional groups of organic molecules on surfaces occurs and binding of alkyl chains is prevented. Therefore, a halogen substituent was replaced by a thiocyanate substituent. CID of the precursor [B₁₂I₁₁(SCN)]²⁻ ion preferentially yields the fragment ion [B₁₂I₈S(CN)]⁻, which shows significantly altered reactivity compared to the fragment ions of [B₁₂I₁₂]²⁻. [B₁₂I₈S(CN)]⁻ has a previously unknown structural element, wherein a sulfur atom bridges three boron atoms. Gas-phase reactions with different neutral reactants (cyclohexane, dimethyl sulfide, and dimethyl amine) accompanied by theoretical studies indicate that [B₁₂I₈S(CN)]⁻ binds with higher selectivity to functional groups of organic molecules than fragment ions of [B₁₂I₁₂]²⁻ (e.g., [B₁₂I₁₁]⁻ and [B₁₂I₉]⁻). These findings were further confirmed by ion soft-landing experiments, which showed that [B₁₂I₈S(CN)]⁻ ions attacked ester groups of adipates and phthalates, whereas [B₁₂I₁₁]⁻ ions only bound to alkyl chains of the same reagents.