Synthesis of New Azole Phosphonate Precursors for Fuel Cells Proton Exchange Membranes

Abstract

INTRODUCTIONIn an energy scenario that increasingly demands cleaner and more efficient energy sources, fuel cells are considered promising electrochemical devices since they can provide electric energy with high efficiency and low environmental impact, converting the energy stored in fuels with zero pollution levels. The proton exchange membrane fuel cells (PEMFC) are considered one of the most promising sources among the various kinds of existing fuel cells due to their high power density and high power-to-weight ratio. One of the drawbacks of current cells is related to the electrolytes currently in use, limiting their use to temperatures below 100°C when operating under water-assisted proton conduction.The temperature operation above 100°C could increase the performance of PEMFC due to a faster electrode reaction without CO poisoning of the Pt electrocatalyst, easier heating, and high energy efficiency [1][2][3][4][5][6][7][8].The proton exchange membrane is a key component for the operation of PEMFC. During recent years, the study of membrane materials have been focused in obtaining high proton conductivity, low electrical conductivity, low permeability to fuel and oxidant, good chemical and thermal stabilities, good mechanical properties, and low cost [1][2][3][4][5][6][7][8]. 236

New C‐3 Substituted 1H‐ and 2H‐Indazolephosphonic Acid Regioisomers: Synthesis, Spectroscopic Characterization and X‐Ray Diffraction Studies

“…In continuation of our long standing interest on azole heterocycles and phosphonate derivatives, [52][53][54][55] we prepared the 1Hindazolebisphosphonic acid 2 and two new isomers, 1 Nmethyl-and 2 N-methylindazolebisphosphonic acids 7 and 8, substituted at the C-3 position, from the corresponding indazolecarboxylic acid. These compounds were fully characterized, making use of their spectral properties to assign the respective regioisomer.…”

Synthetic Procedures Leading towards Aminobisphosphonates

“…A useful modification of this procedure is the application of tris(trimethylsilyl) phosphite, followed by the easy removal of ester groups by methanolysis [23,60,61]. This was used to prepare a wide variety of 1-hydroxy-1,1-bisphosphonates containing amino groups [19,23,[61][62][63]. Additionally, the use of bis((trimethylsilyl)oxy)phosphine generated from ammonium hypophosphite was applied to prepare hydroxybisphosphinic acids (also presented in Scheme 13) [64].…”

Aza-Michael Addition of Imidazole Analogues

“…As reported in some cases (e.g., tetraalkyl ethenylidenebisphosphonates), despite full conversion, the thus-obtained adduct could not be isolated (Table 10, entries 1 and 4). 92,94 Instead, the starting material 35 was recovered, sometimes accompanied by the product of consecutive Michael addition, 37. [92][93][94][95][96] This side reaction (termed 'double condensation' or 'double Michael') can be circumvented by subjecting the product to a subsequent reaction, be it removal of the α-hydrogen atom or changing the acceptor character by hydrolysis of the ester groups (Scheme 10).…”

“…92,94 Instead, the starting material 35 was recovered, sometimes accompanied by the product of consecutive Michael addition, 37. [92][93][94][95][96] This side reaction (termed 'double condensation' or 'double Michael') can be circumvented by subjecting the product to a subsequent reaction, be it removal of the α-hydrogen atom or changing the acceptor character by hydrolysis of the ester groups (Scheme 10). 93,95 Therefore, imidazole can undergo a one-pot Michael addition (in the presence of NaH or KH) combined with halogenation in the presence of previously added base and crown ether 18-C-6, leading to halo esters in 43-73% yields ( Table 10, entry 2).…”