Increasing Structural Dimensionality of Alkali Metal Fluoridotitanates(IV).

Abstract

Reactions between AF (A = Li, Na, K, Rb, Cs) and TiF4 (with starting n(AF):n(TiF4) molar ratios in the range from 3:1 to 1:3) in anhydrous hydrogen fluoride yield [TiF6]2-, [TiF5]-, [Ti4F19]3-, [Ti2F9]-, and [Ti6F27]3- salts. With the exception of the A2TiF6 compounds, which consist of A+ cations and octahedral [TiF6]2- anions, all of these materials arise from the condensation of TiF6 units. The anionic part in the crystal structures of A[TiF5] (A = K, Cs) and A[TiF5]·HF (A = Na, K, Rb) is composed of infinite ([TiF5]-)∞ chains built of TiF6 octahedra sharing joint vertices. Each structure shows a slightly different geometry of the ([TiF5]-)∞ chains. The crystal structure of Na[Ti2F9]·HF is constructed from polymeric ([Ti2F9]-)∞ anions that appear as two parallel infinite zigzag chains comprising TiF6 units, where each TiF6 unit of one chain is connected to a TiF6 unit of the other chain through a shared fluorine vertex. Slow decomposition of single crystals of K4[Ti8F36]·8HF and Rb4[Ti8F36]·6HF ( Shlyapnikov , I. M. ; et al. Chem. Commun. 2013 , 49 , 2703 ) leads to the formation of [Ti2F9]- (Rb) and [Ti6F27]3- (K, Rb) salts. The former displays the same ([Ti2F9]-)∞ double chain as in Na[Ti2F9]·HF, while the anionic part in the latter, ([Ti6F27]3-)∞, represents the first example of a three-dimensional network built of TiF6 octahedra. The ([Ti6F27]3-)∞ anion was also found in [H3O]3[Ti6F27]. The crystal structure determination of Cs3[Ti4F19] revealed a new type of polymeric fluoridotitanate(IV) anion, ([Ti4F19]3-)∞. Similar to the ([Ti2F9]-)∞ anion, it is also built of zigzag double chains comprising TiF6 units. However, in the former there are fewer connections between TiF6 units of two neighboring chains than in the latter.