Synthesis of cationic molybdenum–cobalt heterometallic clusters protected against hydrolysis by macrocyclic triazacyclononane complexes

Abstract

Four kinds of cobalt–polyoxomolybdenum clusters were synthesized by a combination of polyoxomolybdates and a Co-tacn (tacn = 1,4,7-triazacyclononane) complex. The heterometallic polynuclear clusters were obtained by the reaction of Na₂MoO₄ and [Co(tacn)(H₂O)₃](CF₃SO₃)₃·H₂O in water. The peripherals of polyoxomolybdates are capped by thermodynamically stable [Co(tacn)]³⁺ protecting groups to prevent further hydrolysis and condensation reactions. Intra-molecular hydrogen bonds between N–H groups on tacn and oxygen atoms on the molybdate core contribute to the stability of the complexes in water. Under the optimized synthetic conditions, the molar ratios of Na₂MoO₄ and [Co(tacn)(H₂O)₃](CF₃SO₃)₃·H₂O were adjusted to 1 : 0.3, 1 : 0.9, or 1 : 1.5, producing [{Co(tacn)}₂Mo₃O₁₂]·2NaCF₃SO₃·7H₂O (1), [{Co(tacn)}₄H₂Mo₇O₂₇](CF₃SO₃)₂·13H₂O (2), and [{Co(tacn)}₄H₃Mo₄O₁₇](CF₃SO₃)₅·6H₂O (3), respectively. As one of the most important factors, the adjustments of the pH values by changing the molar ratio enabled us to isolate these clusters. The presence of pyridine produces a neutral complex, [Co₂(tacn)₂(pyridine)Mo₅O₁₈]·4.5H₂O (4) containing a lacunary Lindqvist-type polyoxomolybdate. The synthesis of complexes 2–4 is also possible starting from cluster 1, implying that 1 is a precursor for the formation of 2–4 in aqueous solution. Complexes 2–4 have a common structural building block, [{Co(tacn)}₂Mo₃O₁₃], in each of the cluster units. The ¹H NMR spectra of 2 and 3 in aqueous solution show multiple splitting patterns of methylene signals on tacn ligands according to the chemical environments indicated by the crystallographic structure. The ⁵⁹Co NMR spectrum of cluster 3 in CH₃NO₂ has two signals from two different Co units with a 3 : 1 integration ratio ensuring the structural integrity in solution. The broadening of one of the signals for the ⁵⁹Co NMR spectrum of cluster 3 in water as a protic solvent reveals the presence of the deprotonation equilibrium of the acidic protons on the cluster framework in water.