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Innovation in the development of electrochemical energy storage methods is essential if these technologies are to meet the variable needs of the electrical grid. Nonaqueous redox flow batteries represent an underdeveloped area of research in energy storage—one which has seen a recent spike in interest owing to the potential for modular, energy-dense electrochemical energy conversion. Here, we summarize our recent work, focused on the design of polyoxovanadate-alkoxide clusters [V6O7(OR)12] as a new class of charge carrier for nonaqueous energy storage. The synthetic strategies we have employed, including homoleptic ligand substitution, selective ligand functionalization, and heterometal installation, demonstrate the flexibility of this hexametalate platform, and result in significant improvement of molecular properties with relevance to flow battery energy density. The identified homoleptic surface modifications (substituting R = CH3 for R = C2H5) to the polyoxovanadate-alkoxide scaffold yield an increase in stable operating voltage window (from 0.6 V to 1.8 V), as well as an increase in the stoichiometric number of electrons that can be stored at each battery electrode (from 1 to 2). Targeted functionalization at the cluster surface with an ether-based ligand affords [V6O7(OC2H5)9(OCH2)3CCH2OC2H4OCH3], which demonstrates a 12-fold increase in solubility over its homoleptic congener, from 0.05 to 0.60 M in acetonitrile. The substitution of a titanium center into the hexametalate core to generate [V5TiO6(OCH3)13]− further increases the voltage window to 2.3 V, as new heterometal-based redox events are introduced to the profile. Through these studies, we have gained valuable insights into the molecular parameters that determine the energy storage capabilities of multimetallic charge carriers. Collectively, our results highlight new opportunities for polynuclear charge carriers, and emphasize the critical role that synthetic inorganic chemistry plays in the development of effective nonaqueous redox flow battery technologies. [ABSTRACT FROM AUTHOR]

A series of new polymer-inorganic composites has been synthesized from poly(amic acid) (PAA) and various M-MOF-74 (M = Mg, Mn, Co, Ni, Zn) frameworks using a postsynthetic ligand exchange strategy, wherein ligand moieties built into the PAA backbone are exchanged with MOF-bound ligands. The reaction is monitored by UV-vis spectroscopy in order to compare exchange rates for different metal ions, and the resulting composites are characterized by powder X-ray diffraction and scanning electron microscopy. This method of polymer-ligand exchange (PLE) hybridization has only been previously demonstrated using Zn2+-carboxylate and Zn2+-imidazolate frameworks; therefore, this work further reveals the versatility of PLE by showing that exchange is possible for several additional metal ions, as well as frameworks containing 2,5-dihydroxyterephthalate ligands. [ABSTRACT FROM AUTHOR]

A pair of organoplatinum(II) metallacycles,M1andM2, was self-assembled by combining one of two donor molecules,cis-[Pt(dhim)2(C≡CC5H4N)2] (D1; dhim = 1,3-dihexyl-2-H-imidazole-2-yelidene) and Pt(tbbpy)(C≡CC5H4N)2(D2; tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine), with an acceptor precursor, Pt(tbbpy)(OTf)2(A1; OTf = CF3SO3−), respectively. Both donor molecules exhibit an idealized 90° angle between the coordination vectors of their ethynylpyridine moieties. When mixed withA1, coordination-driven self-assembly occurs at 50 °C (4 days in acetonitrile forM1and 18 h in CH2Cl2forM2). Both metallacycles were characterized by1H NMR, FT-IR, and FT-ICR-MS techniques that support D2A2self-assembly of molecular squares. The structure of building blockD1was determined by X-ray diffraction, confirming the expected square coordination geometry and 90° orientation of the pyridyl coordination vectors. Photophysical studies ofM1andM2reveal that the metallacycles display triplet emission bands at 466 and 469 nm, respectively, that originate from transitions localized on building blocksD1andD2. This phosphorescent behavior is assigned to3ILCT and3LLCT (πC≡C* →πC≡C; ILCT = intraligand charge transfer, LLCT = ligand-to-ligand charge transfer) transitions based on previous studies of phenyl analogs toD1andD2that indicate that the ethynyl moieties dominate in their contributions to the molecular orbitals involved in absorption and emission. [ABSTRACT FROM AUTHOR]