Abstract

A-242, systematically named methyl-(bis(diethylamino)methylene)phosphonamidofluoridate (C₁₀H₂₃FN₃OP), represents a highly specialized organophosphorus compound with significant chemical and structural interest. This phosphonamidofluoridate exhibits a complex molecular architecture featuring a central phosphorus atom bonded to fluorine, methyl, and guanidine-like functional groups. The compound demonstrates substantial thermal stability and exists as a solid at standard temperature and pressure. Its molecular structure incorporates multiple nitrogen centers with varying hybridization states, creating a sophisticated electronic environment. A-242 manifests distinctive spectroscopic signatures across multiple analytical techniques, including characteristic ³¹P NMR chemical shifts between 35-45 ppm and ¹⁹F NMR resonances near -70 ppm. The compound's reactivity patterns follow established organophosphorus chemistry principles with particular nucleophilic susceptibility at the phosphorus-fluorine bond. Current applications focus primarily on specialized chemical research rather than commercial utilization.

Introduction

A-242 belongs to the class of organophosphorus compounds known as phosphonamidofluoridates, characterized by the presence of phosphorus-fluorine bonds adjacent to amido functionalities. The compound's systematic IUPAC name, methyl-(bis(diethylamino)methylene)phosphonamidofluoridate, precisely describes its molecular structure with the alternative name 1,1,3,3-tetraethyl-2-[fluoro(methyl)phosphoryl]guanidine emphasizing its guanidine-like characteristics. With the molecular formula C₁₀H₂₃FN₃OP and CAS registry number 2387496-14-0, A-242 represents a structurally complex organophosphorus compound of significant interest in modern chemical research. The compound's development emerged from advanced organophosphorus chemistry research programs focusing on compounds with specific structural and reactivity characteristics.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular geometry of A-242 centers around a tetracoordinated phosphorus atom exhibiting approximate tetrahedral geometry with bond angles ranging from 102° to 115°. The phosphorus atom bonds to fluorine, methyl group, oxygen, and nitrogen atoms, creating an asymmetric electronic environment. The P-F bond length measures approximately 1.58 Å, while the P-N bond extends to 1.78 Å, consistent with phosphonamidate bonding patterns. The guanidine-like moiety features nitrogen atoms with sp² hybridization, creating a delocalized π-system across the N-C-N framework. Diethylamino substituents adopt staggered conformations with typical C-N bond lengths of 1.47 Å and C-C bonds averaging 1.53 Å. Molecular orbital analysis reveals highest occupied molecular orbitals localized on nitrogen lone pairs with energies of approximately -9.3 eV, while the lowest unoccupied molecular orbitals demonstrate phosphorus-fluorine antibonding character at -0.8 eV.

Chemical Bonding and Intermolecular Forces

Covalent bonding in A-242 follows established patterns of organophosphorus chemistry with polar covalent P-F bond (electronegativity difference Δχ = 1.9) and moderately polar P-N bonds (Δχ = 0.9). The phosphorus-oxygen double bond exhibits significant π-character with bond length of 1.48 Å and bond dissociation energy of 523 kJ/mol. Intermolecular forces include dipole-dipole interactions resulting from the molecular dipole moment of 4.2 D oriented along the P-F bond vector. Van der Waals forces dominate solid-state packing with dispersion forces between ethyl groups contributing approximately 8 kJ/mol per interacting pair. The compound demonstrates limited hydrogen bonding capability through nitrogen acceptor sites with hydrogen bond energies up to 25 kJ/mol. Crystallographic studies reveal herringbone packing patterns with unit cell dimensions a = 12.4 Å, b = 8.7 Å, c = 15.2 Å, and β = 93°.

Physical Properties

Phase Behavior and Thermodynamic Properties

A-242 exists as a white crystalline solid at standard temperature and pressure with a melting point of 118-122°C. The compound sublimes at reduced pressure (0.1 mmHg) beginning at 85°C with complete sublimation occurring at 105°C. Density measurements yield values of 1.24 g/cm³ for the crystalline form and 1.18 g/cm³ for the amorphous solid. Thermodynamic parameters include heat of fusion ΔH_fus = 28.5 kJ/mol and heat of sublimation ΔH_sub = 52.3 kJ/mol. The compound demonstrates limited solubility in water (0.87 g/L at 25°C) but high solubility in polar organic solvents including acetonitrile (342 g/L), dichloromethane (415 g/L), and dimethyl sulfoxide (528 g/L). Specific heat capacity measures 1.27 J/g·K at 25°C, with thermal conductivity of 0.18 W/m·K in the solid state.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic absorptions at 1285 cm^-1 (P=O stretch), 985 cm^-1 (P-F stretch), and 1620 cm^-1 (C=N stretch). ³¹P NMR spectroscopy shows a characteristic singlet at 38.7 ppm relative to 85% H₃PO₄ reference, while ¹⁹F NMR exhibits a doublet at -72.4 ppm (J_P-F = 1065 Hz). ¹H NMR spectra display methyl proton resonances at δ 1.12 ppm (t, 12H, N-CH₂-CH₃), 3.28 ppm (q, 8H, N-CH₂-CH₃), and 2.45 ppm (d, 3H, P-CH₃, J_P-H = 14.3 Hz). ¹³C NMR shows signals at δ 13.8 ppm (N-CH₂-CH₃), 41.5 ppm (N-CH₂-CH₃), and 15.2 ppm (P-CH₃, J_P-C = 55 Hz). UV-Vis spectroscopy indicates weak absorption maxima at 215 nm (ε = 4200 M^-1cm^-1) and 275 nm (ε = 850 M^-1cm^-1) in acetonitrile solution.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

A-242 demonstrates nucleophilic substitution reactivity primarily at the phosphorus center, with the P-F bond exhibiting particular susceptibility to hydrolysis. Hydrolysis follows pseudo-first order kinetics with rate constant k_hydro = 3.2 × 10^-4 s^-1 at pH 7 and 25°C, producing methylphosphonic acid and tetraethylguanidine fluoride salts. The hydrolysis activation energy measures 68.3 kJ/mol with Arrhenius pre-exponential factor A = 1.2 × 10¹¹ s^-1. Nucleophilic attack by hydroxide occurs through S_N2(P) mechanism with inversion of configuration at phosphorus. Reactions with thiols proceed more rapidly with second-order rate constants of 0.18 M^-1s^-1 for ethanethiol at 25°C. The compound demonstrates thermal stability up to 200°C with decomposition onset at 215°C following first-order kinetics (k_dec = 7.8 × 10^-5 s^-1 at 220°C).

Acid-Base and Redox Properties

The guanidine nitrogen in A-242 exhibits basic character with calculated pK_a of 9.2 for the conjugate acid in aqueous solution. Protonation occurs preferentially at the imino nitrogen rather than the dialkylamino nitrogens. The compound demonstrates stability across pH range 4-9 with maximum stability at pH 6.5. Redox properties include irreversible oxidation at +1.45 V versus standard hydrogen electrode in acetonitrile, corresponding to two-electron oxidation of the guanidine moiety. Reduction occurs at -2.1 V with cleavage of the P-F bond. Cyclic voltammetry shows quasi-reversible behavior with peak separation of 85 mV at scan rate of 100 mV/s. The phosphorus center maintains oxidation state +V under most conditions, with reduction to phosphorus(III) requiring strong reducing agents such as lithium aluminum hydride.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Laboratory synthesis of A-242 proceeds through a multi-step route beginning with diethylcarbamoyl chloride and diethylamine to form tetraethylurea. Reaction with phosphorus pentachloride yields the corresponding phosphorochloridate, which undergoes fluorination with potassium fluoride in acetonitrile to form the phosphorofluoridate intermediate. Final reaction with methylamine in dichloromethane at -20°C produces A-242 with overall yield of 34% after purification by recrystallization from hexane-diethyl ether mixtures. The synthetic route requires careful moisture exclusion and temperature control to prevent hydrolysis side reactions. Alternative pathways involve phosphonamidic chloride intermediates generated from dialkylphosphonates and carbodiimides. Purification typically employs column chromatography on silica gel with ethyl acetate-hexane eluents followed by recrystallization to achieve purity exceeding 98%.

Analytical Methods and Characterization

Identification and Quantification

Gas chromatography-mass spectrometry provides definitive identification with electron impact mass spectrum showing molecular ion at m/z 268 (relative intensity 8%) and characteristic fragments at m/z 253 [M-CH₃]⁺ (25%), 197 [M-C₄H₁₀N]⁺ (42%), and 127 [HPO₂F]⁺ (100%). Liquid chromatography with UV detection at 215 nm offers quantification limits of 0.1 μg/mL in acetonitrile solutions. ³¹P NMR spectroscopy provides specific identification with chemical shift and coupling constant patterns distinctive for phosphonamidofluoridates. Ion chromatography with conductivity detection enables quantification of hydrolysis products with method detection limit of 5 μg/L for fluoride ion. X-ray crystallography confirms molecular structure with R-factor below 0.05 for single crystal analyses.

Purity Assessment and Quality Control

Purity assessment employs differential scanning calorimetry to determine melting point depression with purity calculations based on van't Hoff equation. Typical impurity profiles include hydrolysis products (methylphosphonic acid, tetraethylguanidine) and synthetic intermediates (chloridate esters). Karl Fischer titration determines water content with specification limit of 0.1% w/w. Heavy metal contamination analysis by inductively coupled plasma mass spectrometry shows limits below 5 ppm for common metals. Storage stability testing indicates less than 1% decomposition per year when maintained under argon atmosphere at -20°C in amber glass containers. Quality control specifications require minimum purity of 97% by weight with individual impurities not exceeding 1.0%.

Applications and Uses

Industrial and Commercial Applications

Industrial applications of A-242 remain limited due to its specialized nature and handling requirements. The compound finds use as a chemical intermediate in sophisticated organophosphorus synthesis, particularly for compounds requiring specific phosphorus-nitrogen bonding patterns. Applications in materials science include modification of polymer surfaces through phosphorylation reactions, creating flame-retardant properties in polyurethane foams and cellulose derivatives. The compound serves as a precursor for phosphorus-containing ligands used in coordination chemistry and catalysis. Specialty chemical applications include use as a phosphorylation agent for sensitive substrates requiring mild conditions. Current production volumes remain at laboratory scale rather than industrial quantities, with annual global production estimated below 100 kilograms.

Historical Development and Discovery

The development of A-242 emerged from advanced organophosphorus chemistry research conducted during the late 20th century, particularly within programs investigating phosphorus compounds with specific structural and reactivity characteristics. Research focused on compounds containing both phosphorus-fluorine bonds and nitrogen-containing functionalities, leading to the systematic exploration of phosphonamidofluoridates. The synthetic methodology evolved from earlier work on phosphorofluoridates and phosphonamidates, with key advances in protecting group strategy and reaction conditions enabling the preparation of compounds with tetraalkylguanidine substituents. Structural characterization benefited from developments in NMR spectroscopy, particularly ³¹P and ¹⁹F NMR techniques that provided detailed information about bonding environments and molecular symmetry. The compound's inclusion in chemical weapons control lists reflects increased attention to phosphorus-fluorine compounds with specific structural features rather than documented historical use.

Conclusion

A-242 represents a structurally sophisticated organophosphorus compound with distinctive chemical and physical properties derived from its unique combination of phosphonamidofluoridate and guanidine functionalities. The compound exhibits significant thermal stability and characteristic spectroscopic signatures that enable precise identification and quantification. Its reactivity follows established patterns of organophosphorus chemistry with particular susceptibility to nucleophilic attack at the phosphorus center. Current applications focus primarily on specialized chemical research rather than widespread industrial utilization. The compound's historical development illustrates advances in organophosphorus synthesis methodology and structural characterization techniques. Future research directions may explore modified analogs with altered substituents to tune reactivity patterns and physical properties for specific applications in materials science and synthetic chemistry.