Towards Covalent Photosensitizer-Polyoxometalate Dyads-Bipyridyl-Functionalized Polyoxometalates and Their Transition Metal Complexes

Abstract

A triol-functionalized 2,2′-bipyridine (bpy) derivative has been synthesized and used for the tris-alkoxylation of polyoxometalate (POM) precursors. The resultant POM-bpy conjugates of the Wells–Dawson- and Anderson-type feature a C–C bond as a linkage between the POM and bpy fragments. This structural motif is expected to increase the hydrolytic stability of the compounds. This is of particular relevance with respect to the application of POM-bpy metal complexes, as photocatalysts, in the hydrogen-evolution reaction (HER) in an aqueous environment. Accordingly, Rh(III) and Ir(III) complexes of the POM-bpy ligands have been prepared and characterized. These catalyst-photosensitizer dyads have been analyzed with respect to their electrochemical and photophysical properties. Cyclic and square-wave voltammetry, as well as UV/vis absorption and emission spectroscopy, indicated a negligible electronic interaction of the POM and metal-complex subunits in the ground state. However, emission–quenching experiments suggested an efficient intramolecular electron-transfer process from the photo-excited metal centers to the POM units to account for the non-emissive nature of the dyads (thus, suggesting a strong interaction of the subunits in the excited state). In-depth photophysical investigations, as well as a functional characterization, i.e., the applicability in the HER reaction, are currently ongoing.

{P2V3W15}-Polyoxometalates Functionalized with Phthalocyaninato Y and Yb Moieties

“…Polyoxometalate (POM)-based organic–inorganic hybrid compounds constitute a large part of stimuli-responsive materials with promising applications in photo/electrocatalysis, molecular magnetism, and energy storage. − In particular, their development is pursued due to high interest for photoinduced charge transfer between transition-metal/lanthanide complex units and POM. The effective attachment of the former to the polyoxoanion unit can be effectively realized via organic functional groups and linkages, which are structurally exposed at the molecular periphery of POM through well-established functionalization and postfunctionalization mechanisms. ,,− For instance, the formation of the molecular heterometal metal-to-POM charge transfer (MPCT) chromophore with a short-lived charge-separated state has been observed in the compound K 15 NaP 4 W 35 O 124 {Re­(CO) 3 } 2 ·37H 2 O and its organic-solvent-soluble analog ( n Bu 4 N) 15 [HP 4 W 35 O 124 ]­{Re­(CO) 3 } 2 ·0.3CH 2 Cl 2 , which consist of a [Re­(CO) 3 ] + unit bound directly to the lacunary Wells–Dawson-type POM [α 2 -P 2 W 17 O 61 ] 10– .…”

“…Photoinduced long-lived charge-separated states were reported for the photosensitizer-POM hybrids separated by a π-conjugated spacer; examples include Ru­(II)-bis­(terpyridine)- or Ru­(II)­(tpy) 2 [PW 11 O 39 Ge] 4– (Keggin-type), , related cyclometalated Ir­(III) complexes-POMs (Keggin- or Wells–Dawson-type), and noble metal-free bodipy fluorophores linked to Si- or Sn-functionalized Keggin-type POMs . A through-space mechanism of electron charge transfer was described for tris­(alkoxo)-Csp 3 -derivatized POMs (a Wells–Dawson-type [P 2 V 3 W 15 O 62 ] 9– or an Anderson-type POM) linked by a bipyridine group to Re­(I)-, , Rh­(III)-, and Ir­(III)-complexes , or by an amido functionality to the Zn­(II) tetraphenylporphyrin complex . Advances in the area of charge-transfer materials based on photosensitizer-POMs hybrid assemblies are comprehensively discusssed in the reviews by Santoni and Keyes …”

Cations Modulated Assembly of Triol-Ligand Modified Cu-Centered Anderson-Evans Polyanions

“…Compared with the terminal O atoms in Anderson-Evans polyanion, the bi-or tri-bridging O atoms (µ 2 -or µ 3 -O) have a higher reactive activity and can be replaced by some organic species with hydroxyl groups under specific circumstances to form various decoration types (Figure S1, Supplementary Materials) [4][5][6]. Through this method, different organic functional groups can be introduced into the inorganic skeleton, which not only enriches the structural figures of the final adducts but also integrates the properties of both parts, resulting in novel functionalities [7][8][9][10][11]. Because the organic functional groups have different hydrophilic and hydrophobic properties, they need to be introduced into polyanions under different solvent environments, such as water or organic solvents [12,13], while due to the use of TBA 4 Mo 8 O 26 (TBA = tetrabutylammonium cation) as the Mo source in most cases, triol-ligand modified Anderson-Evans polyanions are mostly prepared in organic solvents such as CH 3 CN, CH 3 OH and N,Ndimethylformamide, which limits their applications [14,15].…”