Crystalline Metaphosphate Acid Salts: Synthesis in Organic Media, Structures, Hydrogen-Bonding Capability, and Implication of Superacidity

Abstract

Metaphosphate acids cannot be thoroughly studied in aqueous media because their acidity is leveled by the solvent, and the resulting metaphosphates are susceptible to acid-catalyzed hydrolysis. Exploration of metaphosphate acid chemistry has now been made possible with the development of a general synthetic method for organic media soluble metaphosphate acids. Protonation of the [PPN](+) salts ([PPN](+) = [N(PPh3)2](+)) of tri-, tetra-, and hexametaphosphates results in five new metaphosphate acids, [PPN]2[P3O9H] (2), [PPN]4[(P4O12)3H8] (3), [PPN]4[P6O18H2]·2H2O (4), [PPN]3[P6O18H3] (5), and [PPN]2[P6O18H2(H3O)2] (6), obtained in yields of 80, 71, 66, 88, and 76%, respectively. Additionally, our synthetic method can be extended to pyrophosphate to produce [PPN][P2O7H3] (7) in 77% yield. The structural configurations of these oxoacids are dictated by strong hydrogen bonds and the anticooperative effect. Intramolecular hydrogen bonds are observed in 2, 4, and 5 and the previously reported [PPN]2[P4O12H2] (1), while intermolecular hydrogen bonds are observed in 3, 6, and 7. The hydrogen bonds in 3-7 possess short distances and are classified as low-barrier hydrogen bonds. Gas-phase acidity computations reveal that the parent tri- and tetrametaphosphoric acids are superacids. Their remarkable acidity is attributable to the stabilization of their corresponding conjugate bases via intramolecular hydrogen bonding.

Lighting up solid states using a rubber

“…The length of each of the two hydrogen bonds is 1.842 Å (Fig. 3b), which is much shorter than typical hydrogen bonds [50][51][52][53] . Such a short distance indicates a unique topochemical probability in which a proton transfer can be more easily facilitated within the dimer.…”

N-donor Base Adducts of P2O5 as Diphosphorylation Reagents

“…2 Synthesis and Characterization of Compounds In a glovebox, [PPN] 4 (1.36 g, 0.72 mmol) was suspended in 10 mL of anhydrous acetone. (Note: At a concentration less than 1 g/mL, precipitation does not occur in the following step.)…”

N-donor Base Adducts of P2O5 as Diphosphorylation Reagents

“…The last several years have seen an explosion of new reagents and methodologies for the synthesis of oligophosphorylated molecules, due to their prevalence in biological systems. [1][2][3][4][5][6] This field has slowly developed since Khorana's pioneering first synthesis of adenosine triphosphate (ATP) in 1954, 7 and an improved synthesis of ATP and of other nucleoside triphosphate (NTP) derivatives has been a major area of focus in synthetic oligophosphorylation. 8 Many of the recent developments have focused on reagents for the simultaneous addition of three phosphate units, via trimetaphosphate ((P 3 O 9 ) 3-, TriMP) in order to minimize the number of synthetic steps to convert a nucleoside into an NTP.…”

“…9 The first effective TriMP based methodology was that of Taylor, utilizing the activation of TriMP with sulfonyl chloride reagents. 4,[10][11][12][13] These seminal papers have been followed up more recently with our own group's 1 as well as Jessen's 3,14 isolable triphosphorylation reagents. By utilizing substrates B. X-ray crystal structure of 1 with thermal ellipsoids set at 50% probability and cations omitted (red for oxygen, orange for phosphorus, yellow for sulfur, grey for carbon).…”

“…The current investigation began by repeating the earlier triphosphorylation work of Taylor where the TBA (tetrabutylammonium) salt of TriMP is activated with mesitylenesulfonyl chloride and nucleophilic N-donor bases, such as DABCO (1,4-diazabicyclo[2.2.2]octane), pyridine, and N -methylimidazole. 4,[10][11][12][13] The putative mesitylenesulfonyl TriMP species was unisolated in Taylor's reports. However, in an analogous reaction of the PPN (bis(triphenylphosphine)iminium) salt of TriMP and tosyl chloride we were able to isolate and crystallographically characterize tosyl TriMP (1, Figure 1).…”