Low‐Symmetry Uranyl Pyrophosphate Cage Clusters

Abstract

Self-assembly of complex molecules is a key attribute of nature. The symmetry and beauty of such self-assembled molecules is striking, as they sometimes adopt the shape of high-symmetry Platonic or Archimedean solids.[1] Self-assembly of inorganic molecules can result in large high-symmetry molecules or clusters as well, such as C 60 buckminsterfullerene with icosahedral symmetry, [2] the "giant wheel" molybdenum oxide containing 154 Mo cations with approximate symmetry D 7d , [3,4] and the Keplerate containing 72 Mo and 30 Fe cations with approximate icosahedral symmetry. [5] Overall, self-assembly of clusters based upon simpler building blocks usually results in higher-symmetry clusters, and we are unaware of any such cluster that contains no symmetry.Polyoxometalates, anionic metal oxide clusters with potential applications in catalysis, medicine, and materials science [6][7][8] exhibit a broad range of structures and compositions. [9][10][11] Most are transition-metal based, although this family of materials is now being extended into the actinide series. Several years ago a cluster containing a polyoxometalate core with six uranium atoms was reported.[12] More recently, squares formed from four uranyl peroxide polyhedra were incorporated into a P 6 W 36 system.[13] Our group has created 16 clusters containing from 16 to 60 uranyl peroxide polyhedra, [14][15][16][17][18][19] and 10 uranyl-pyrophosphate and uranyl-oxalate hybrid clusters. [20][21] Most are cages, although bowland crown-shaped clusters built from uranyl polyhedra have also been created.[18] Nanoscale clusters of uranyl polyhedra self-assemble through shared peroxide edges because the uranyl-peroxide-uranyl interaction is covalent and inherently bent.[22] These cage clusters self-assemble in aqueous solutions under ambient conditions over a wide range of pH, and are topologically varied with combinations of squares, pentagons, and hexagons. Most adopt high-symmetry isomers, and we have argued that symmetry is a major consideration in determining the topology of the cluster. [17,19] We are interested in the factors that dictate the size, shape, topology, and symmetry of cage clusters built using uranyl polyhedra. In the course of our extensive combinatorial synthesis effort in search of uranyl peroxide cage clusters, we isolated a highly unusual cluster with an odd number of uranyl polyhedra arranged in a low-symmetry topology. The cluster, designated U 45 Py 23 , consists of 45 uranyl polyhedra and 23 pyrophosphate groups. It was crystallized to facilitate single-crystal X-ray diffraction analysis of its structure.Cluster U 45 Py 23 is a highly complex distorted dumb-bell shape and is 31.4 by 21.5 in length and width, measured from the edges of bounding O atoms (Figure 1). Each U 6 + cation is present as a uranyl ion, and these are further coordinated by peroxide groups and O atoms of pyrophosphate groups. This results in both hexagonal and pentagonal bipyramids that are capped by the O atoms of the uranyl ions. The U À O Ur and U À O eq bond le...

Topologically and geometrically flexible structural units in seven new organically templated uranyl selenates and selenite–selenates

“…The diversity of polyhedral units found in uranyl compounds is unique, starting from isolated complexes [1][2][3] and clusters [4][5][6][7] to cage compounds [8][9][10][11] and nanotubules [12][13][14]. Selenium-containing uranyl oxysalts are known for both monoand divalent inorganic cations [15][16][17][18][19], as well as for organic template molecules of various charge and structure [20][21][22][23][24].…”

Water-soluble multi-cage super tetrahedral uranyl peroxide phosphate clusters

“…11,12 The bonding between the peroxide and uranyl units includes covalent interactions that favor a bent conguration consistent with curvature of the cage, [13][14][15][16] and "yl" oxygen atoms truncate the cage on both the inside and outside. Although peroxide is important for formation of uranyl cage clusters, other uranyl bridges in addition to peroxide include hydroxyl, 11,[17][18][19][20][21][22][23][24][25] pyrophosphate, 18,20,25,26 phosphite, 27 nitrate, 18,28 methylenediphosphonate, 20 and oxalate groups. 19,29 More than 40 uranyl peroxide clusters have been reported that consist of single cages, and core-shell cages in two cases.…”

Cage clusters built from uranyl ions bridged through peroxo and 1-hydroxyethane-1,1-diphosphonic acid ligands

“…We are interested in incorporating a variety of organic and inorganic species in uranyl peroxide cage clusters because this may expand the properties and functions of these clusters. Previous efforts included incorporating pyrophosphate, [21][22][23] oxalate acid, 24 and diphosphonic acid. 22 Diphosphonic acid differs from pyrophosphate in that the two P cations are bridged through a methane group, rather than an O atom.…”