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Recent advances in redox flow batteries have made them a viable option for grid-scale energy storage, however they exhibit low energy density. One way to boost energy density is by increasing the cell potential using a nonaqueous system. Molecular engineering has proven to be an effective strategy to develop redox-active compounds with extreme potentials but these are usually challenged by resource sustainability of the newly developed redox materials. Here, we investigate the utility of phosphine oxides as anolytes with extremely negative potentials. Specifically, we found that cyclic triphenylphosphine oxide (CPO), has a highly negative potential (-2.4 V vs Fc/Fc + ). Importantly, CPO is synthesized from triphenylphosphine oxide, a common industrial chemical waste with no commercial value. Structural and electrochemical characterization of the reduced radical anion showed that enhanced stability is due to cyclization or extended pi-conjugation. Importantly, mechanistic investigation into the decomposition of CPO under various solvents and electrochemical conditions allowed us to utilize an acetonitrile/DMF binary solvent system to enable a long-lived anolyte which exhibited no fade over 350 cycles. In summary, this work led to the development of a waste-derived cyclic phosphine oxide that exhibits excellent cycling stability making it an ideal anolyte toward the development of energy-dense RFBs.

The reaction of W(NAr)( 13 C 4 H 8 )(OSiPh 3 ) 2 ( 1 ) (NAr = 2,6-diisopropylphenylimido) with silica partially dehydroxylated at 700 °C (SiO 2-700 ) is highly dependent on the reaction conditions. The primary product of this reaction is W(NAr)( 13 C 4 H 8 )(OSiPh 3 )(OSi(O-) 3 ) ( 2 ) when the reaction is carried out in the dark. Grafting 1 onto SiO 2-700 in ambient lab light results in the formation of 2 , W(NAr)( 13 CH 2 13 CH 2 )(OSiPh 3 )(OSi(O-) 3 ) ( 4 ), and one isomer of square-pyramidal W(NAr)( 13 CH 2 13 CH( 13 Me) 13 CH 2 )(OSiPh 3 )(OSi(O-) 3 ) ( 3 ). Heating 2 to 85 °C for 6 h results in the formation of 3 , 4 , W(NAr)( 13 CH( 13 Me) 13 CH 2 13 CH 2 )(OSiPh 3 )(OSi(O-) 3 ) ( 5 ), and W(NAr)(( 13 CH 2 ) 2 13 CH( 13 Me)( 13 CH 2 ) 2 )(OSiPh 3 )(OSi(O-) 3 ) ( 6 ). Photolysis of 2 with blue LEDs (λ max = 450 nm) produces 4, both isomers of 3, 5 , and free ethylene. In the presence of excess ethylene and blue LED irradiation at 85 °C, 1 /SiO 2-700 catalyzes the direct conversion of ethylene to propylene.

Exposure of a solution of the square pyramidal tungstacyclopentane complex W(NAr)(OSiPh 3 ) 2 (C 4 H 8 ) (Ar = 2,6- i -Pr 2 C 6 H 3 ) to ethylene at 22 °C in ambient (fluorescent) light slowly leads to the formation of propylene and the square pyramidal tungstacyclobutane complex W(NAr)(OSiPh 3 ) 2 (C 3 H 6 ). No reaction takes place in the dark, but the reaction is >90% complete in ∼15 min under blue LED light (∼450 nm λ max ). The intermediates are proposed to be (first) an α methyl tungstacyclobutane complex (W(NAr)(OSiPh 3 ) 2 (αMeC 3 H 5 )), and then from it, a β methyl version. The TBP versions of each can lose propylene and form a methylene complex, and in the presence of ethylene, the unsubstituted tungstacyclobutane complex W(NAr)(OSiPh 3 ) 2 (C 3 H 6 ). The W-C α bond in an unobservable TBP W(NAr)(OSiPh 3 ) 2 (C 4 H 8 ) isomer in which the C 4 H 8 ring is equatorial is proposed to be cleaved homolytically by light. A hydrogen atom moves or is moved from C3 to the terminal C4 carbon in the butyl chain as the bond between W and C3 forms to give the TBP α methyl tungstacyclobutane complex. Essentially, the same behavior is observed for W(NCPh 3 )(OSiPh 3 ) 2 (C 4 H 8 ) as for W(NAr)(OSiPh 3 ) 2 (C 4 H 8 ), except that the rate of consumption of W(NCPh 3 )(OSiPh 3 ) 2 (C 4 H 8 ) is about half that of W(NAr)(OSiPh 3 ) 2 (C 4 H 8 ). In this case, an α methyl-substituted tungstacyclobutane intermediate is observed, and the overall rate of formation of W(NCPh 3 )(OSiPh 3 ) 2 (C 3 H 6 ) and propylene from W(NCPh 3 )(OSiPh 3 ) 2 (C 4 H 8 ) is ∼20 times slower than in the NAr system. These results constitute the first experimentally documented examples of forming a metallacyclobutane ring from a metallacyclopentane ring (ring contraction) and establish how metathesis-active methylene and metallacyclobutane complexes can be formed and reformed in the presence of ethylene. They also raise the possibility that ambient light could play a role in some metathesis reactions that involve ethylene and tungsten-based imido alkylidene olefin metathesis catalysts, if not others.

Upon addition of 5-15% PhNMe 2 H + X - (X = B(3,5-(CF 3 ) 2 C 6 H 3 ) 4 or B(C 6 F 5 ) 4 ) to Mo(NAr)(styrene)(OSiPh 3 ) 2 (Ar = N-2,6- i -Pr 2 C 6 H 3 ) in C 6 D 6 an equilibrium mixture of Mo(NAr)(styrene)(OSiPh 3 ) 2 and Mo(NAr)(CMePh)(OSiPh 3 ) 2 is formed over 36 h at 45 °C ( K eq = 0.36). A plausible intermediate in the interconversion of the styrene and 1-phenethylidene complexes is the 1-phenethyl cation, [Mo(NAr)(CHMePh)(OSiPh 3 ) 2 ] + , which can be generated using [(Et 2 O) 2 H][B(C 6 F 5 ) 4 ] as the acid. The interconversion can be modeled as two equilibria involving protonation of Mo(NAr)(styrene)(OSiPh 3 ) 2 or Mo(NAr)(CMePh)(OSiPh 3 ) 2 and deprotonation of the α or β phenethyl carbon atom in [Mo(NAr)(CHMePh)(OSiPh 3 ) 2 ] + . The ratio of the rate of deprotonation of [Mo(NAr)(CHMePh)(OSiPh 3 ) 2 ] + by PhNMe 2 in the α position versus the β position is ∼10, or ∼30 per H β . The slow step is protonation of Mo(NAr)(styrene)(OSiPh 3 ) 2 ( k 1 = 0.158(4) L/(mol·min)). Proton sources such as (CF 3 ) 3 COH or Ph 3 SiOH do not catalyze the interconversion of Mo(NAr)(styrene)(OSiPh 3 ) 2 and Mo(NAr)(CMePh)(OSiPh 3 ) 2 , while the reaction of Mo(NAr)(styrene)(OSiPh 3 ) 2 with pyridinium salts generates only a trace (∼2%) of Mo(NAr)(CMePh)(OSiPh 3 ) 2 and forms a monopyridine adduct, [Mo(NAr)(CHMePh)(OSiPh 3 ) 2 (py)] + (two diastereomers). The structure of [Mo(NAr)(CHMePh)(OSiPh 3 ) 2 ] + has been confirmed in an X-ray study; there is no structural indication that a β proton is activated through a CH β interaction with the metal. W(NAr)(CMePh)(OSiPh 3 ) 2 is also converted into a mixture of W(NAr)(CMePh)(OSiPh 3 ) 2 and W(NAr)(styrene)(OSiPh 3 ) 2 ( K eq = 0.47 at 45 °C in favor of the styrene complex) with 10% [PhNMe 2 H][B(C 6 F 5 ) 4 ] as the catalyst; the time required to reach equilibrium is approximately the same as in the Mo system.