Synthesis, separation and structure of zinc coordination clusters: Proposal of an assembly-disassembly-reassembly process observed from ESI-MS

Abstract

In mass spectrometry, fragments with a mass higher than the original molecular ion provide valuable insights into the molecular structure and can guide the assembly and disassembly processes for chemical synthesis. Here, we report such an example and following up by modifying the solvothermal reaction conditions (temperature and time) it is possible to isolate the high mass species in crystalline form. [Zn₄L₄Cl₄] (Zn₄L₄, L = N-methylbenzimidazole-2-methanolate) has a boat-like Zn₄O₄ core but electrospray ionization mass spectrometry (ESI-MS) of the solution of its crystals shows higher mass peaks of Zn₅L₅, Zn₅L₆ and Zn₆L₆ species. Thus, both disassembly and reassembly are highly probable processes. Consequently, [Zn(HL)₂Cl₂] (Zn₁, L = N-methylbenzimidazole-2-methanolate), [Zn₄L₆Cl₂] (Zn₄L₆, L = N-methylbenzimidazole-2-methanolate) and [Zn₆L₆Cl₄(CH₃O)₂] (Zn₆L₆, L = N-methylbenzimidazole-2-methanolate) were prepared. The results of multistage ESI-MS of their dissolved crystals led to a proposed mechanism of their formation in the gas phase as follows: [Zn₃L₄] through [ZnL] → [ZnL(HL)] → [Zn(HL)₂] → [Zn₂L] → [Zn₂L₂] → [Zn₂L₃]. The mechanism was derived in conjunction with Gibbs free energies calculated using DFT of the fragments observed in the ESI-MS of Zn₄L₄, Zn₄L₆ and Zn₆L₆. This work reveals the complex of chemical reactions, involving fragmentation and unexpected combination, under mass spectrometry condition which allows one to synthesize the observed transients, leading to mechanism of formation by correlation of solid-state/solution structural information.