Reactivity, Formation, and Solubility of Polyoxometalates Probed by Calorimetry

Abstract

Room temperature calorimetry methods were developed to describe the energy landscapes of six polyoxometalates (POMs), Li-U24, Li-U28, K-U28, Li/K-U60, Mo132, and Mo154, in terms of three components: enthalpy of dissolution (ΔHdiss), enthalpy of formation of aqueous POMs (ΔHf,(aq)), and enthalpy of formation of POM crystals (ΔHf,(c)). ΔHdiss is controlled by a combination of cation solvation enthalpy and the favorability of cation interactions with binding sites on the POM. In the case of the four uranyl peroxide POMs studied, clusters with hydroxide bridges have lower ΔHf,(aq) and are more stable than those containing only peroxide bridges. In general for POMs, the combination of calorimetric results and synthetic observations suggest that spherical topologies may be more stable than wheel-like clusters, and ΔHf,(aq) can be accurately estimated using only ΔHf,(c) values owing to the dominance of the clusters in determining the energetics of POM crystals.

Calorimetric Study of Functionalized Uranyl Peroxide Nanoclusters and Their Monomeric Building Block

“…In the current study we determined Δ H diss , Δ H f ° (aq) , and Δ H f ° (c) values for the uranyl lithium triperoxide monomer (Li−UT) and four UPCs (LiRb−U@U 24 , LiNa−U 24 Pp 12 , NaK−U 24 Pp 12 , and U 60 Ox 30 ; Pp=pyrophosphate, Ox=oxalate; see Figure 1 and Table 1). This study reveals how the energy landscape of previously measured topologies (U 24 and U 60 ) [13a] is impacted by counter cations and the substitution of pyrophosphate and oxalate for hydroxo bridges. Additionally, the enthalpy results for Li−UT allow us to calculate the enthalpy of reaction for the formation of Li−U 24 and Li−U 28 from their monomeric building blocks.…”

“…The derived positive Δ H diss values measured for UPCs revealed that their dissolution process is entropically driven and determined Δ H f ° (aq) values indicate that hydroxo bridges stabilize cluster topologies in comparison to those containing only peroxo bridges. The enthalpy of reaction (Δ H rxn ) for the conversion of Li−U 28(aq) to Li−U 24(aq) , previously described in the literature, [12] was found to be favorable and thus Li−U 28 is kinetically favored whilst Li−U 24 is thermodynamically favorable [13a] …”

“…The affinity of counter cations to the topological windows (squares, pentagons, and hexagons) of UPCs impacts their aqueous solubility and crystallization [13–14] . Recent studies have provided insights into formation mechanisms with the isolation of an intermediate building block, the uranyl pentamer (U 5 ), while synthesizing K−U 28 from potassium uranyl triperoxo monomer (K−UT); [15] using precursor clusters to form clusters never synthesized before; [9c] and converting clusters from one to another [13a,16] …”

“…Revealing the thermodynamic energy landscape of POMs will provide a more comprehensive understanding of the formation and fate of the clusters and has three components: (1) measurement of the enthalpy of dissolution of POMs (Δ H diss ) (equal but opposite to enthalpy of crystallization), (2) measurement of the enthalpy of formation of aqueous POMs (Δ H f ° (aq) ), and (3) determination of the enthalpy of formation of POM crystals (Δ H f ° (c) ). Δ H f ° (c) has been reported for several UPCs derived from high‐temperature drop solution calorimetry, but recently developed calorimetric methods yielded the first measurements of Δ H diss and Δ H f ° (aq) of UPCs [13a,17] . The derived positive Δ H diss values measured for UPCs revealed that their dissolution process is entropically driven and determined Δ H f ° (aq) values indicate that hydroxo bridges stabilize cluster topologies in comparison to those containing only peroxo bridges.…”

The Renascence of One Ancient Recipe for Synthesizing Luminescent Cs4PbBr6 Microcrystals

“…Inspired by the observation and a thorough search of previous literatures, [11] we tested different threads, including cotton, plastic, and copper, and discovered a general method to produce Cs 4 PbBr 6 crystals using threads as seeds. The crystal growth process was investigated by calorimetry and real-time scanning electron microscopy (SEM), [12] which showed that the growth of Cs 4 PbBr 6 in solution is an exothermic enthalpy driving process, and the surface energy calculations further supported the formation of rhombohedra shape. [7a,13] In this work, the obtained microcrystals show strong green luminescence with an absolute PL quantum yield (PLQY) of 31.5%, which was used to fabricate high-performance white and green LEDs.…”