Mixed-Metal Hybrid Polyoxometalates with Amino Acid Ligands: Electronic Versatility and Solution Properties

Abstract

Eight new members of a family of mixed-metal (Mo,W) polyoxometalates (POMs) with amino acid ligands have been synthesized and investigated in the solid state and solution using multiple physical techniques. While the peripheral POM structural framework is conserved, the different analogues vary in nuclearity of the central metal-oxo core, overall redox state, metal composition, and identity of the zwitterionic α-amino acid ligands. Structural investigations reveal site-selective substitution of Mo for W, with a strong preference for Mo to occupy the central metal-oxo core. This core structural unit is a closed tetrametallic loop in the blue reduced species and an open trimetallic loop in the colorless oxidized analogues. Density functional theory calculations suggest the core as the favored site of reduction and reveal that the corresponding molecular orbital is much lower in energy for a tetra- versus trimetallic core. The reduced species are diamagnetic, each with a pair of strongly antiferromagnetically coupled Mo centers in the tetrametallic core, while in the oxidized complexes all Mo is hexavalent. Solution small-angle X-ray scattering and circular dichroism (CD) studies indicate that the hybrid POM is stable in aqueous solution on a time scale of days within defined concentration and pH ranges, with the stability enhanced by the presence of excess amino acid. The CD experiments also reveal that the amino acid ligands readily exchange with other α-amino acids, and it is possible to isolate the products of amino acid exchange, confirming retention of the POM framework. Cyclic voltammograms of the reduced species exhibit an irreversible oxidation process at relatively low potential, but an equivalent reductive process is not evident for the oxidized analogues. Despite their overall structural similarity, the oxidized and 2e-reduced hybrid POMs are not interconvertible because of the respective open- versus closed-loop arrangement in the central metal-oxo cores.

Addition of Metalated 3‐Alkyl‐Substituted Alkoxyallenes to Imines: Preparation of Tetrasubstituted 2,5‐Dihydropyrroles, Pyrrolidin‐3‐ones, and Pyrroles

“…To quantify the coloration kinetics of our system, we used the method developed by Dessapt and co-workers. The broad band in this region is comparable with those of previously reported reduced mixed metal Mo/W analogs, [23] implying that the POM has been reduced following UV irradiation, affording W 5+ addenda ions. Before light irradiation, both compounds exhibit O→W LMCT bands in the UV region (around 350 nm or 3.5 eV), consistent with the pale-pink color of the samples.…”

“…[23] Cooling the solution affords, in moderate yield, crystals that can be formulated as K 2 (GlyH) 7 14 ]·(Gly) 7 ·40H 2 O (1) upon air-drying. [23] Cooling the solution affords, in moderate yield, crystals that can be formulated as K 2 (GlyH) 7 14 ]·(Gly) 7 ·40H 2 O (1) upon air-drying.…”

“…Neither the addition of extra tungstate to the solution, nor reducing the amount of yttrium nitrate, affords compounds with a homometallic {W 4 } core. [22,23] All compounds are insoluble in both aqueous solutions and organic solvents and all subsequent characterization has been performed in the solid state. This is in direct contrast to the blue coloration due to photoreduction observed for analogous solutions that also contain molybdate.…”

“…[22,23] This electron shuttling would give rise to a larger than usual electron-hole separation in the photoinduced LMCT state and correspondingly long lifetimes of the photoreduced POM. With lifetimes of many hours to days, the photoinduced states of compounds 1 and 2 are relatively long-lived.…”

“…[20,21] In general, the photochromic response of both POM salts with organic cations and covalently functionalized hybrid organic POMs can be characterized by the kinetics of color change, the excitation energy, the stability of the photoinduced state, the fading rate, and the cyclability. [22][23][24] Herein, we present an exploration of the photocoloration properties observed for some salts of the hybrid POMs of formula [As 4 {YW 3 }W 44 Y 4 O 159 (Gly) 8 (H 2 O) 14 ] 9with different countercations (Figure 1), including primary ammonium moieties, glycinium (GlyH), benzylammonium (BAH), and 4methylbenzylammonium (MBAH), and the quaternary ammonium benzyltriethylammonium (BTEA). Neither of the two mechanisms (Scheme S1) proposed to explain photoinduced coloration have been definitively demonstrated experimentally, although DFT calculations support the Dessapt mechanism of hydrogen-atom transfer.…”