Effect of solution acidity on the crystallization of polychromates in uranyl-bearing systems: synthesis and crystal structures of Rb₂[(UO₂)(Cr₂O₇)(NO₃)₂] and two new polymorphs of Rb₂Cr₃O₁₀

Abstract

Three new rubidium polychromates, Rb2[(UO2)(Cr2O7)(NO3)2] (1), γ-Rb2Cr3O10 (2) and δ-Rb2Cr3O10 (3) were prepared by combination of hydrothermal treatment at 220 °C and evaporation of aqueous solutions under ambient conditions. Compound 1 is monoclinic, P 2 1 / c $P{2}_{1}/c$ , a = 13.6542(19), b = 19.698(3), c = 11.6984(17) Å, β = 114.326(2)°, V = 2867.0(7) Å3, R 1 = 0.040; 2 is hexagonal, P 6 3 / m $P{6}_{3}/m$ , a = 11.991(2), c = 12.828(3) Å, γ = 120°, V = 1597.3(5) Å3, R 1 = 0.031; 3 is monoclinic, P 2 1 / n $P{2}_{1}/n$ , a = 7.446(3), b = 18.194(6), c = 7.848(3) Å, β = 99.953(9)°, V = 1047.3(7) Å3, R 1 = 0.037. In the crystal structure of 1, UO8 bipyramids and NO3 groups share edges to form [(UO2)(NO3)2] species which share common corners with dichromate Cr2O7 groups producing novel type of uranyl dichromate chains [(UO2)(Cr2O7)(NO3)2]2−. In the structures of new Rb2Cr3O10 polymorphs, CrO4 tetrahedra share vertices to form Cr3O10 2− species. The trichromate groups are aligned along the 63 screw axis forming channels running in the ab plane in the structure of 2. The Rb cations reside between the channels and in their centers completing the structure. The trichromate anions are linked by the Rb+ cations into a 3D framework in the structure of 3. Effect of solution acidity on the crystallization of polychromates in uranyl-bearing systems is discussed.

The Role of Acidity in the Synthesis of Novel Uranyl Selenate and Selenite Compounds and Their Structures

“…In the present study and supported by observations in recent investigations [17][18][19][20]22,23], the use of low pH conditions acts as a facile path towards novel structure generation and obtaining topology architectures that would otherwise require more intricate synthesis methods, such as the use of organic templating methods. Consequently, it is suggested then, from the observations in this investigation, that the use and presence of acid during synthesis provides a facile pathway to acquiring uranyl selenite/selenate structures with functional groups or topologies that are either novel or require more intricate methods.…”

“…The role of acidity during synthesis, as described previously, has been noted in several recent studies to act as a key determinant in controlling such chemical properties as structural motif, isomerism and also redox processes [17][18][19][20]22,23]. Nazarchuk et al [22] described that, during the evaporation of acidic solutions, the successive reduction in pH accompanied by a loss of solution resulted in distinctive organically templated uranyl sulfate structures forming which were structural dependent and proportional to the pH at which crystallisation occurs.…”

“…Examples can be seen in uranium and neptunium nitrate studies, including the identification acid controlled geometric isomerism in neptunium and uranium nitrates [17], and also uncovering the crystallization of uranium nitrates as an indicator of reaction competition [18]. The salient effect solution acidity has upon controlling structure formation in uranyl systems has also been recently observed in uranyl chromates and chromate-nitrates [19][20][21]. A further pertinent study by Nazarchuk et al [22] demonstrated that, under decreasing pH conditions (2.5 → 1.5), successive organically templated uranyl sulfates would form where their structural chemistry was dependent and proportional to the specific pH at crystallisation.…”

Secondary Role of Aliphatic and Heterocyclic Amines in the Formation of Low-Temperature Amine-Bearing U, Np, and Pu(VI) Chromates

“…The structure of pyUCr 2 O 7 represents an example of the uranyl complex containing the dichromate ion. Previously, a number of the uranyl compounds containing Cr 2 O 7 2– anions were reported. ,,− Moreover, several of these complexes with inorganic (Rb + ) and organic (guanidinium, dimethylammonium, and dimethylethylendiammonium) outer-sphere cations contain the same [(UO 2 )­(CrO 4 )­(Cr 2 O 7 )] chains as those found in the structure of pyUCr 2 O 7 . ,, Interestingly, the complex with pyridinium, (Hpy) 4 [(UO 2 ) 2 (CrO 4 ) 2 (Cr 2 O 7 )] 2 , is based on the same chains that are further linked into the 2D layers due to a μ 4 -bridging function of the dichromate ions …”