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

摘要

Four kinds of cobalt-polyoxomolybdenum clusters were synthesized by a combination of polyoxomolyb-dates 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)center dot 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)center dot H2O were adjusted to 1 : 0.3, 1 : 0.9, or 1 : 1.5, producing [{Co(tacn)}(2)Mo3O12]center dot 2NaCF3SO3 center dot 7H2O (1), [{Co(tacn)}(4)H2Mo7O27](CF3SO3)(2)center dot 13H(2)O (2), and [{Co(tacn)}(4)H3Mo4O17](CF3SO3)(5)center dot 6H(2)O (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, [Co-2(tacn)(2)(pyridine) Mo5O18]center dot 4.5H(2)O (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)}(2)Mo3O13], in each of the cluster units. The H-1 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 Co-59 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 Co-59 NMR spectrum of cluster 3 in