A Baker−Figgis Isomer of Conventional Sandwich Polyoxometalates. H₂Na₁₄[Fe^III₂(NaOH₂)₂(P₂W₁₅O₅₆)₂], a Diiron Catalyst for Catalytic H₂O₂-Based Epoxidation

Abstract

The Wells-Dawson-derived sandwich-type polyoxometalates (POMs) are a versatile and robust group of compounds with applications in catalysis, 1-5 molecular magnetism, 6,7 and other areas. These compounds (formula [M 4 (H 2 O) 2 (P 2 W 15 O 56 )] n-where M ) Co II , Mn II , Cu II , Zn II , and Fe III ) are isostructural with a central planar tetrameric M 4 unit bound to two trivacant R-[P 2 W 15 O 56 ] 12-units. 8-13 We report here a new type of synthesis which results in a new type of sandwich POM. This complex, H 2 Na 14 [Fe III 2 -(NaOH 2 ) 2 (P 2 W 15 O 56 ) 2 ] (Na1), is a structural isomer of the conventional Wells-Dawson and B-Keggin-derived sandwich POMs. It differs in the way the two R-[P 2 W 15 O 56 ] 12-units are bound to the central unit in a manner analogous to Baker-Figgis (cap rotation) isomers (e.g., R-versus -[XM 12 O 40 ] n-and Rversus -[P 2 W 18 O 62 ] 6-), and it contains only two central d-electron metals. This structurally novel diiron POM, unlike previously reported Fe III -containing sandwiches 1,8,14 and [FePW 11 O 39 ], 15 is an effective catalyst for H 2 O 2 -based epoxidation, exhibiting selectivity and rates that approach the Neumann/Khenkin systems. Reaction of R-[P 2 W 15 O 56 ] 12-with 2.0 equiv of Fe(II) in aqueous NaCl followed by air oxidation produces Na1. 16 The synthesis of 1 requires Fe(II), H 2 O, and Na + . Use of Fe(III) in place of Fe(II) produces only the conventional Wells-Dawson sandwich POM, [(Fe III OH 2 ) 2 (Fe III ) 2 (P 2 W 15 O 56 ) 2 ] 12-(2). If the Fe(II)/R-[P 2 W 15 O 56 ) 2 ] 12-mixture is phase transferred from H 2 O to CH 2 Cl 2 using tetra-n-butylammonium chloride (TBACl) prior to air oxidation, only TBA2 is formed. In the preparation of 1, the new dark green air-sensitive complex, [Fe II 4 (P 2 W 15 O 56 ) 2 ] 16-, forms initially (the Fe/P/W ratios are 2/2/15, and the IR spectrum is virtually identical to that of [Zn II 4 (P 2 W 15 O 56 ) 2 ] 16-). 13 During the O 2 oxidation process, the iron atoms with terminal aqua ligands exchange with the sodium atoms in solution. While 2 is stable over the pH range in which Na1 is prepared (pH 4.5-5), 2 is not observed as an intermediate in the synthesis of 1.The X-ray structure 17 reveals that only two d-electron metals reside in the central unit. The other positions in this central unit are occupied by two weakly bound seven-coordinate Na + ions. 18 Each sodium ion is ligated by three of the oxygens from each of the trivacant R-[P 2 W 15 O 56 ] 12-units and by a weakly bound terminal water molecule. In contrast, four d-block metals (and/or Zn) reside in the central tetrameric unit in all published WellsDawson and B-Keggin-derived sandwich POMs.

Supramolecular Structures of Titanium(IV)‐Substituted Wells–Dawson Polyoxotungstates

“…16À , and in this compound the two equivalent exterior positions are occupied by sodium ions in the solid state. [17] Hill et al also showed that this polyanion reacts with Cu 2 or Co 2 ions in aqueous solution leading to a trisubstituted, mixed-metal sandwich-type polyanion. [18] Very recently the same authors described the tri-iron(iii)-substituted polyanion, [Fe 2 (FeOH 2 )(NaOH 2 )(P 2 W 15 O 56 ) 2 ] 14À .…”

“…This is further supported by the observation that the di-and trisubstituted species (with two and one sodium centers in the vacancies, respectively) are more stable than their corresponding tetrasubstituted analogues. [17,19,21] Polyanion 1 represents the first lacunary sandwich-type polyanion that does not contain a sodium ion substituting for the first-row transition metals. Here the lacunary sites are surrounded and stabilized by ammonium ions and/or water molecules in the solid state (and most likely also in solution).…”

Rational Synthesis, Structure, Magnetism and Electrochemistry of Mixed Iron–Nickel‐Containing Wells–Dawson‐Fragment‐Based Sandwich‐Type Polyoxometalates

“…The first example of a sandwich-type species with less than four transition metals was described by Hill et al [30] The diiron(III)-substituted polyanion [(NaOH 2 ) 2 2 ], in which the two equivalent interior positions are occupied by Fe III centres, and the two equivalent exterior positions are occupied by sodium ions in the solid state. It was also shown that this polyanion reacts with Cu 2+ or Co 2+ ions in aqueous solution, leading to a trisubstituted mixed-metal sandwich-type polyanion [31] with partial decomposition of one of the [X 2 W 15 O 56 ] 12-fragments (two tungsten atoms nearest the vacant site in the central unit are now replaced by Cu or Co atoms).…”

Selective Oxidation of Sulfides to Sulfones by H2O2 Catalyzed by Fe-Substituted Sandwich Type Polyoxometalate

“…Sandwich-type compounds (one or more transition metals bonded between two POM lacunary fragments) represent the largest subclass [32]. Hill reported that Sandwich-type polyoxometalates have been referred to as oxidatively stable inorganic analogues of metalloporphyrins and used as catalysts for a range of organic oxidations [33,34] [36][37][38][39][40][41][42]. Meanwhile, the role of the ''belt'' and ''cap'' (Keggin with different heteroatoms) tungsten atoms in their catalytic activity is not obvious enough and such reactions are still rare and lack generality.…”