Synthesis of some hexanuclear niobium halide cluster compounds

Abstract

A fast, efficient, and inexpensive method was developed for the preparation of Nb6Br14 •8H2O. This compound is an important starting material for the preparation of coordination compounds based on hexanuclear (Nb6Br12)n+ central cations and was available previously only in minute quantities. Potassium bromide, niobium pentabromide, and excess niobium metal were conproportionated in Vycor tubes at 720°C to give K4Nb6Br18, which upon extraction with water forms Nb6Br14•8H2o in yields up to 80%.

The new coordination compounds [(Nb6Cl12)Cl2 (CH3CN)4], [(Nb6Br12)Br2 (CH3CN)4], and [(Nb6Cl12)F2 (C5H5NO)4] have been synthesized. In attempts to prepare the adducts, (Nb6Cl12)F2L4 (L = CH3CN, DMSO, DMF), impure or nonuniform products were obtained.

The new complex, [(Nb6Cl12)Cl2 (CH3CN)4], was used as a starting material for the preparation of a product (Me4N)3 [(Nb6Cl12)Cl6]·3CH3CN with an analysed Cl:Nb ratio of 17.99:6.00. Anionic [(Nb6Cl12)Cl6]n- complexes with Cl:Nb ratios close to the ideal value of 18.00:6.00 are very difficult to obtain with Nb6Cl14•8H2O as -starting material.

Attempts to dehydrate Nb6Cl14•8H20 with thionyl chloride caused its oxidation to (Nb6Cl12)4+containing material. Impure Nb6Cl16 adducts were formed. Reaction of Nb6Cl14•8H2O with thionyl chloride and acetonitrile did not lead to the adduct Nb6Cl16 •2CH3CN, but yielded impure (Nb6Cl12 )4+-containing materials with approximate Cl:Nb ratios of 18:6 and 28:6, respectively.