Aqueous redox flow batteries with organic active materials offer an environmentally benign, tunable, and safe route to large-scale energy storage. Development has been limited to a small palette of organics that are aqueous soluble and tend to display the necessary redox reversibility within the water stability window. We show how molecular engineering of fluorenone enables the alcohol electro-oxidation needed for reversible ketone hydrogenation and dehydrogenation at room temperature without the use of a catalyst. Flow batteries based on these fluorenone derivative anolytes operate efficiently and exhibit stable long-term cycling at ambient and mildly increased temperatures in a nondemanding environment. These results expand the palette to include reversible ketone to alcohol conversion but also suggest the potential for identifying other atypical organic redox couple candidates.

Aqueous organic redox flow batteries are a promising technology for large-scale energy storage. The stability of the redox active organic molecules is increasingly being recognized as one of the major...

A phosphinite-based dinickel pincer complex, {2,3,5,6-( i Pr2PO)4C6}­Ni2Cl2, is synthesized by heating a mixture of 1,2,4,5-tetrahydroxybenzene, i Pr2PCl, NiCl2, and Et3N at 150 °C in a microwave reactor or refluxing 1,2,4,5-( i Pr2PO)4C6H2 with NiCl2 and Et3N at 175–250 °C in a flask. Substitution of chlorides by hydrides (from LiAlH4 or LiEt3BH) and methyl groups (from MeLi) generates {2,3,5,6-( i Pr2PO)4C6}­Ni2H2 and {2,3,5,6-( i Pr2PO)4C6}­Ni2Me2, respectively. Protonation of the methyl complex with HCO2H gives {2,3,5,6-( i Pr2PO)4C6}­Ni2(OCHO)2. Spectroscopic and crystallographic data of these Janus POCOP pincer complexes are compared to those of the analogous mononuclear complexes {2,6-( i Pr2PO)2C6H3}­NiX (X = Cl, H, Me, and OCHO). The hydride complex {2,3,5,6-( i Pr2PO)4C6}­Ni2H2 reacts with phenylacetylene and CO2 to give insertion products. It also catalyzes the reduction of CO2 with catecholborane (HBcat) to yield CH3OBcat and catBOBcat with a turnover frequency of 7.5 min–1. The stoichiometric and catalytic activities of {2,3,5,6-( i Pr2PO)4C6}­Ni2H2 are compared to those of {2,6-( i Pr2PO)2C6H3}­NiH.

A nickel hydride complex, {2,6-( i Pr2PO)2C6H3}­NiH, has been shown to catalyze the coupling of RCHO and R′OH to yield RCO2R′ and RCH2OH, where the aldehyde also acts as a hydrogen acceptor and the alcohol also serves as the solvent. Functional groups tolerated by this catalytic system include CF3, NO2, Cl, Br, NHCOMe, and NMe2, whereas phenol-containing compounds are not viable substrates or solvents. The dehydrogenative coupling reaction can alternatively be catalyzed by an air-stable nickel chloride complex, {2,6-( i Pr2PO)2C6H3}­NiCl, in conjunction with NaOMe. Acids in unpurified aldehydes react with the hydride to form nickel carboxylate complexes, which are catalytically inactive. Water, if present in a significant quantity, decreases the catalytic efficiency by forming {2,6-( i Pr2PO)2C6H3}­NiOH, which causes catalyst degradation. On the other hand, in the presence of a drying agent, {2,6-( i Pr2PO)2C6H3}­NiOH generated in situ from {2,6-( i Pr2PO)2C6H3}­NiCl and NaOH can be converted to an alkoxide species, becoming catalytically competent. The proposed catalytic mechanism features aldehyde insertion into the nickel hydride as well as into a nickel alkoxide intermediate, both of which have been experimentally observed. Several mechanistically relevant nickel species including {2,6-( i Pr2PO)2C6H3}­NiOC­(O)­Ph, {2,6-( i Pr2PO)2C6H3}­NiOPh, and {2,6-( i Pr2PO)2C6H3}­NiOPh·HOPh have been independently synthesized, crystallographically characterized, and tested for the catalytic reaction. While phenol-containing molecules cannot be used as substrates or solvents, both {2,6-( i Pr2PO)2C6H3}­NiOPh and {2,6-( i Pr2PO)2C6H3}­NiOPh·HOPh are efficient in catalyzing the dehydrogenative coupling of PhCHO with EtOH.

A new bidentate secondary phosphine oxide (SPO) was synthesized from diphenyl ether via ortho-lithiation, phosphorylation with PhP(Cl)NEt2, and hydrolysis in an acidic medium. Nickel(0) species ligated with this new SPO was established as a more effective catalyst than Ni(0)-Ph2P(O)H for the cross-coupling of aryl iodides with aryl thiols.

The nickel PNP pincer complex (iPrPNP)NiPh (iPrPNP = κP,κN,κP-N(CH2CH2PiPr2)2) was prepared by reacting (iPrPNP)NiBr with PhMgCl or deprotonating [(iPrPNHP)NiPh]Y (iPrPNHP = κP,κN,κP-HN(CH2CH2PiPr2)2; Y = Br, PF6) with KOtBu. The byproducts of the PhMgCl reaction were identified as [(iPrPNHP)NiPh]Br and (iPrPNP′)NiPh (iPrPNP′ = κP,κN,κP-N(CH=CHPiPr2)(CH2CH2PiPr2)). The methyl analog (iPrPNP)NiMe was synthesized from the reaction of (iPrPNP)NiBr with MeLi, although it was contaminated with (iPrPNP′)NiMe due to ligand oxidation. Protonation of (iPrPNP)NiX (X = Br, Ph, Me) with various acids, such as HCl, water, and MeOH, was studied in C6D6. Nitrogen protonation was shown to be the most favorable process, producing a cationic species [(iPrPNHP)NiX]+ with the NH moiety hydrogen-bonded to the conjugate base (i.e., Cl–, HO–, or MeO–). Protonation of the Ni–C bond was observed at room temperature with (iPrPNP)NiMe, whereas at 70 °C with (iPrPNP)NiPh, both resulting in [(iPrPNHP)NiCl]Cl as the final product. Protonation of (iPrPNP)NiBr was complicated by site exchange between Br– and the conjugate base and by the degradation of the pincer complexes. Indene, which lacks hydrogen-bonding capability, was unable to protonate (iPrPNP)NiPh and (iPrPNP)NiMe, despite being more acidic than water and MeOH. Neutral and cationic nickel pincer complexes involved in this study, including (iPrPNP′)NiBr, (iPrPNP)NiPh, (iPrPNP′)NiPh, (iPrPNP)NiMe, [(iPrPNHP)NiPh]Y (Y = Br, PF6, BPh4), [(iPrPNHP)NiPh]2[NiCl4], [(iPrPNHP)NiMe]Y (Y = Cl, Br, BPh4), [(iPrPNHP)NiBr]Br, and [(iPrPNHP)NiCl]Cl, were characterized by X-ray crystallography.

A Diphenyl Ether Derived Bidentate Secondary Phosphine Oxide as a Preligand for Nickel-Catalyzed C-S Cross-Coupling Reactions. -A new bidentate secondary phosphine oxide, only the second of the kind bearing a covalent backbone, is synthesized from diphenyl ether via ortho-lithiation, phosphorylation with PhP(Cl)NEt 2, and hydrolysis in an acidic medium. Nickel(0) species ligated with this new ligand is established as an efficient catalyst for cross-coupling reactions of aryl iodides with aryl thiols. -(WELLALA, N. P. N.; GUAN*, H.; Org. Biomol. Chem. 13 (2015) 44, 10802-10807, http://dx.