Tandem assembly-disassembly-reassembly of hierarchical Co19 coordination cluster facilitated by in-situ ligand transformation: Crystallography and ESI-MS revealed mechanism

Abstract

Odd-numbered and high-nuclearity coordination clusters are extremely rare, yet they represent an intriguing subclass lacking regular repeating building blocks and high structural symmetry for understanding self-assembled multiatomic systems. Herein, the largest cobalt and polydentate ligand based cluster featuring odd-nuclearity, namely [Co₁₉(HL1)₈(L1)₁₂(L′)₂(Ac)₄]·10CH₃CH₂OH·6H₂O (1, H₂L1 = 1H-benzo[d]imidazole-2-yl)methanol, HL' = 1H-benzo[d]imidazole), was obtained with in-situ ligand transformation from H₂L1 to L′. It features a hierarchical trilayer and void-cage inside structure, consisting of central disc-shaped [Co₇L₁₀] core with two [Co₆] rings on both sides. ESI-MS of crystal 1 yields a series of more than sixteen fragments, all featuring an integrated [Co₁₉] core, suggesting stability of the polynuclear cluster in solution. During increased in-source energy from 0 to 100 eV, all MS peaks shifted to a lower m/z range, but the [Co₁₉] core remained intact, excepting for the stepwise elimination of up to three Ac⁻ anions or three L1 linkers. PXRD tracking of the reaction sediments showed the formation of a key precursor of [Co₄L₄] cubane at 3 h, and its content decreased at 6 h and vanished at 12 h, followed by the appearance of crystals 1 by the generation of a clear solution at 18 h, suggesting an initial cluster assembly-disassembly process. ESI-MS spectra analysis of both reaction sediment and solution further identify the existence of other crucial higher-nuclearity reassembled fragments of [Co₇L₁₀] disk and its expansion of [Co₁₃L₁₂(L′)₂]. A probable tandem assembly-disassembly-reassembly mechanism is put forward as [CoL₂]→[Co₄L₄]→[Co₇L₁₀]→[Co₁₃L₁₂(L′)₂]→[Co₁₉L₂₀(L′)₂]. Their evolution also indicated the ingenious synergy of coexisting organic, inorganic and in-situ generated ligands, along with diverse coordination geometries of metal ions, plays a directional role in forming odd-numbered and high-nuclearity coordination clusters. Magnetism analysis revealed antiferromagnetic coupling plays dominated role in the cluster.