The Structure and Synthesis of Plutonium(III) Chlorides from Aqueous Solution

Abstract

The preparation and structure of three trivalent plutonium chloride compounds from aqueous solution is reported. Two of the three are plutonium tetraaquatetrachloro complexes exhibiting a cis and a trans arrangement of Cl about the Pu. The identification of the coordination number of 4 with respect to Cl and the isomerism are both unprecedented in actinide solution chemistry. The third complex is a hexaaquadichloro complex of Pu(III), predicted by available thermodynamic data.

Coordination Chemistry of Homoleptic Actinide(IV)–Thiocyanate Complexes

“…When this reaction was attempted in 1 m HCl, crystalso f[ Et 4 N][Pu(H 2 O) 6 Cl 2 ]Cl 2 ·2H 2 O were isolated in lieu of the desired NCS À compound. [18] This is likely due to significant competitive ligationo fC l À as the free energy of thiocyanate complexation with actinides is very small (e.g., À0.813 kcal mol À1 for Am III ). [7e] In an effort to circumventthis, we repeated the reaction in 3.3 m trifluoromethanesulfonic acid, because the CF 3 SO 3 À anion is not expected to be ac ompetitive inner-sphere ligand in aqueous solution.…”

“…This was accomplished by reacting solutions of valence pure Pu III (prepared by addition of NH 2 OH⋅HCl to a 242 Pu IV stock solution) with [Et 4 N][SCN] (11 equiv). When this reaction was attempted in 1 M HCl, crystals of [Et 4 N][Pu(H 2 O) 6 Cl 2 ]Cl 2 ⋅ 2 H 2 O were isolated in lieu of the desired NCS − compound 18. This is likely due to significant competitive ligation of Cl − as the free energy of thiocyanate complexation with actinides is very small (e.g., −0.813 kcal mol −1 for Am III ) 7e.…”

Impact of Noncovalent Interactions on the Structural Chemistry of Thorium(IV)-Aquo-Chloro Complexes

“…The prevalence of 8–9-coordinate thorium­(IV) aquohalide complexes contrasts the coordination number of the Th IV aquo ion that crystallized in the solid state as [Th­(H 2 O) 10 ]·Br 4 . Related monomeric and dimeric actinide aquohalide compounds that only contain water or chloride in their first coordination sphere have also been reported; of particular note is the complex [Th­(H 2 O) 7 Cl 2 ]·Cl 2 ·18-crown-6·2H 2 O, which contains a structural unit related to that observed in 8 . , Other dimeric hydrolysis products have been crystallized including [Th 2 ­(μ 2 -OH) 2 ­Cl 2 ­(H 2 O) 12 ]­Cl 4 ·2H 2 O from acidic HCl solutions, (H 3 O) 4 ­[(C 2 ­H 5 ) 4 ­N] 6 ­[Th 2 ­(μ 2 -O)­Cl 4 ­(H 2 O) 12 ] 3 ­[Re 4 ­Se 4 ­(CN) 12 ] 4 from acidic HCl solutions containing a rhenium cluster salt, and [Th­(μ 2 -OH)­Cl­(H 2 O) 6 ] 2 ·Cl 4 ·18-crown-6·2H 2 O stabilized by 18-crown-6 molecules. ,, It should be noted that complete exclusion of water from the inner coordination sphere of Th is inherently difficult, consistent with the hygroscopic nature of Th salts, , yet the homoleptic ThCl 6 2– dianion has been isolated. , A similar behavior between Th IV and Pu III is noted, with monomeric plutonium­(III)-aquo-chloro complexes isolated with different counterions that yielded varying aquo/chloride ratios, including cis -Cs­[Pu­(H 2 O) 4 Cl 4 ], trans -Cs 5 [Pu­(H 2 O) 4 Cl 4 ]­Cl 4 ·2H 2 O, (Et 4 N)­[Pu­(H 2 O) 6 Cl 2 ]­Cl 2 ·2H 2 O, and (C 5 H 5 NBr) 2 [Pu­(H 2 O) 5 Cl 3 ]­Cl 2 ·2H 2 O in the solid state, whereas chloride was not observed to complex Pu III in aqueous solutions despite high chloride concentrations, as shown in X-ray absorption spectroscopic studies. − …”

Monomeric and Trimeric Thorium Chlorides Isolated from Acidic Aqueous Solution

“…Kiplinger’s Th­(H 2 O) 4 Cl 4 · (1,4-dioxane) 3 and Th­(H 2 O) 4 Cl 4 · (THF) 5 compounds exhibit a different coordination about the metal center, displaying a distorted square antiprism geometry. Related monomeric and dimeric An–aquo–halide compounds that only contain water or chloride in their first coordination sphere have also been reported. , Notable reports for monomeric Th­(IV) include the homoleptic [Th­(H 2 O) 10 ] 4+ structural unit, which was isolated from HBr solution, crystallizing with four bromide ions in the outer coordination sphere, and [Th­(H 2 O) 7 Cl 2 ] · Cl 2 · 18-crown-6 · 2H 2 O. , Monomeric Pu­(III)–aquo–chloro complexes have also been reported with varying aquo/chloride ratios and counterions including cis -Cs­[Pu­(H 2 O) 4 Cl 4 ], trans -Cs 5 [Pu­(H 2 O) 4 Cl 4 ]­Cl 4 · 2H 2 O, (Et 4 N)­[Pu­(H 2 O) 6 Cl 2 ]­Cl 2 · 2H 2 O, and (C 5 H 5 NBr) 2 [Pu­(H 2 O) 5 Cl 3 ]­Cl 2 · 2H 2 O. , Furthermore, dimeric hydrolysis products have been isolated, including [Th­(μ 2 -OH) 2 Cl 2 (H 2 O) 12 ]­Cl 4 · 2H 2 O from acidic HCl solutions and [Th­(μ 2 -OH)­Cl­(H 2 O) 6 ] 2 Cl 4 · 18-crown-6 · 2H 2 O stabilized by 18-crown-6 molecules. , …”