Tracking the Formation of a Polynuclear Co16 Complex and Its Elimination and Substitution Reactions by Mass Spectroscopy and Crystallography

We present the syntheses and structures of the biggest chiral cobalt coordination cluster, [Co-16(L)(4)(H3L)(8)-(N-3)(6)](NO3)(2)center dot 16H(2)O center dot 2CH(3)OH (1, where H4L = S,S-1,2-bis-(1H-benzimidazol-2-yl)-1,2-ethanediol). 1 consists of two Co4O4 cubes (Co-4(L)(2)(H3L)(2)) alternating with Co-2(EO-N-3)(2)Co-2 (Co-4(L)(2)(H3L)(2)(N-3)(2)), bridged by the benzimidazole and azide nitrogen atoms to form a twisted ring. The ligand adopts both cis and trans forms, and all the rings have the same chiralilty. ESI-MS of 1 from a methanol solution of crystals reveals the fragment [Co-16(L)(4)(H3L)(8)(N-3)(6)+2H](4+), suggesting the polynuclear core is stable in solution. ESI-MS measurements from the reaction solution found smaller fragments, [Co-4(H3L)(4)-H](3+), [Co-4(H3L)(4)-2H](2+), [Co-4(H3L)(4)(N-3)(2)](2+), and [Co-2(H3L)(2)](2+), and ESI-MS from a methanol solution of the solid deposit found in addition the Co-16 core. These results and the dependence on the synthesis time allow us to propose the process for the formation of 1, which opens up a new way for the direct observation of the ligand-controlled assembly of clusters In addition, the isolation of [Co-4(H3L)(4)](NO3)(4)center dot 4H(2)O (2) consisting of separate Co4O4 cubes with the ligands being only cis in crystalline form supports the proposal. Interestingly, N-3(-) is replaced by either CH3O- or OH-, and this is the first time that high resolution ESI-MS is successfully utilized to examine both the step-by-step elimination and substitution of inner bridging ligands in such a high nuclear complex. Increasing the voltage results in stepwise elimination of azide from the parent cluster. The preliminary magnetic susceptibility of 1 indicates ferromagnetic cubes antiferromagnetically coupled to the squares within the duster, though in a field of 2.5 kOe, weak and slow relaxation is observed below 4 K.