Hexanuclear cluster complexes of niobium and tantalum with neutral and anionic ligands

Abstract

A variety of coordination compounds based on hexa-nuclear (M6X12)n+ central cation were synthesized from hydrated niobium or tantalum halides {(M6X12)X2(H2O)4} 4H2O either by replacing water terminal ligands with other neutral ligands L or by replacing halide terminal ligands X with other anionic ligands. Water molecules appear to be difficult to replace by other neutral ligands. Through a number of neutral ligands L were tried, only a few new complexes of the general formula {(M6X12)X2L4} could b prepared.

A microscopic method was developed to check if a particular anion will replace halide terminal ligands to form crystalline hydrated (M6X12)n+ derivatives. This method showed that halide terminal ligands are easily replaced, and it proved extremely valuable in guiding subsequent full-scale preparations.

A number of well-crystallized hydrated carbonate, oxalate, toluene sulfonate, sulfate, and formate derivatives of )M6X12)n+ central cations were synthesized and characterized. Anionic ligands may be selective in their reactions. They appear to be useful for separating M6X12 cluster cations of different kinds. It was found that the oxalate ion precipitates out (M6Br12)2+ cations of niobium and tantalum as solid crystalline M6Br12C2O4 11H2O, whereas it does not react with the corresponding chloride cluster cations (M6Cl12)2+. Toluene sulfonic acid on the other hand was found to easily precipitate (Ta6Br12)3+ cations as Ta6Br12(C7H7So3) 11H2O, whereas it will react with Nb6Br12 cations only under greatly different conditions. Carboxylate anions such as formate and particularly acetate appear to be able to replace bridging halogens of M6X12 cluster cations as evidenced by products with halogen/metal ratios substantially lower than 12/6.