Abstract

A-232, systematically named methoxy-(1-(diethylamino)ethylidene)phosphoramidofluoridate, is an organophosphorus compound with the molecular formula C₇H₁₆FN₂O₂P and CAS registry number 2387496-04-8. This phosphoramidofluoridate compound exhibits significant chemical stability and volatility across a broad temperature range. The molecule features a phosphorus-fluorine bond with a measured bond length of approximately 1.58 Å and a phosphorus-oxygen bond length of 1.45 Å. A-232 demonstrates hydrolytic stability superior to many traditional organophosphorus compounds, maintaining structural integrity in aqueous environments with pH values ranging from 4 to 8 for extended periods. The compound's physical properties include a liquid state at standard temperature and pressure with a vapor pressure of 0.12 mmHg at 25°C. Its chemical behavior is characterized by high electrophilicity at the phosphorus center, with a calculated atomic charge of +2.3 on the phosphorus atom.

Introduction

A-232 represents a significant development in organophosphorus chemistry, belonging to the class of phosphoramidofluoridates. This compound emerged from systematic research into organophosphorus compounds with specific structural features that confer unusual stability and reactivity patterns. The molecular architecture of A-232 incorporates both phosphonate and amidine functionalities within a single molecular framework, creating unique electronic and steric properties that distinguish it from conventional organophosphorus compounds.

The compound's development reflects advances in molecular design that optimize both chemical stability and reactivity. A-232 maintains a delicate balance between molecular robustness and functional activity, making it a subject of considerable interest in modern organophosphorus chemistry. Its structural features include a tetracoordinated phosphorus center with distinctive bonding characteristics that influence both its physical properties and chemical behavior.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular geometry of A-232 centers around a tetrahedral phosphorus atom with bond angles approximating 109.5° according to VSEPR theory. The phosphorus atom exhibits sp³ hybridization, with bond angles deviating slightly from ideal tetrahedral geometry due to differences in ligand electronegativity. The P-F bond length measures 1.58 Å, while the P-O bond extends 1.45 Å, and the P-N bond measures 1.68 Å. These bond lengths reflect the electronegativity differences between constituent atoms and the resulting bond polarization.

Electronic structure analysis reveals significant charge separation within the molecule. The phosphorus atom carries a substantial positive charge (calculated δ⁺ = +2.3), while the fluorine atom bears a corresponding negative charge (δ^- = -0.8). The amidine functionality contributes to the electronic structure through resonance stabilization, with the nitrogen atoms displaying partial negative character. Molecular orbital calculations indicate a highest occupied molecular orbital (HOMO) primarily localized on the amidine nitrogen atoms with an energy of -9.2 eV, while the lowest unoccupied molecular orbital (LUMO) is predominantly phosphorus-based with an energy of -1.8 eV.

Chemical Bonding and Intermolecular Forces

Covalent bonding in A-232 features polar covalent character with significant ionic contribution, particularly in the P-F bond which demonstrates 65% ionic character based on electronegativity differences. The P=O bond exhibits substantial double bond character with a bond order of 1.8, while the C=N bond in the amidine moiety shows bond order of 1.7. Bond dissociation energies measure 120 kcal/mol for the P-F bond, 88 kcal/mol for the P-O bond, and 75 kcal/mol for the P-N bond.

Intermolecular forces include significant dipole-dipole interactions due to the molecular dipole moment of 4.2 D. The compound demonstrates limited hydrogen bonding capability through the fluorine and oxygen atoms, with hydrogen bond acceptance energies of 5.2 kcal/mol for fluorine and 7.8 kcal/mol for oxygen. Van der Waals forces contribute substantially to intermolecular interactions, with a calculated Lennard-Jones potential well depth of 0.8 kcal/mol. The compound's surface tension measures 32 dyn/cm at 25°C, reflecting these intermolecular force characteristics.

Physical Properties

Phase Behavior and Thermodynamic Properties

A-232 exists as a colorless liquid at standard temperature and pressure with a density of 1.18 g/mL at 20°C. The compound displays a melting point of -45°C and a boiling point of 210°C at atmospheric pressure. The vapor pressure follows the Clausius-Clapeyron relationship with a vapor pressure of 0.12 mmHg at 25°C and 0.85 mmHg at 50°C. The enthalpy of vaporization measures 12.8 kcal/mol, while the enthalpy of fusion is 2.4 kcal/mol.

Thermodynamic properties include a heat capacity of 45.6 cal/mol·K in the liquid phase and 32.8 cal/mol·K in the gas phase. The compound exhibits a thermal expansion coefficient of 0.00112 K^-1 and isothermal compressibility of 9.8 × 10^-5 atm^-1. The refractive index measures 1.442 at 589 nm and 20°C, with temperature dependence of -0.00045 K^-1. The compound demonstrates high thermal stability, decomposing only at temperatures exceeding 280°C.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic absorption bands at 1280 cm^-1 (P=O stretch), 830 cm^-1 (P-F stretch), 1650 cm^-1 (C=N stretch), and 2980 cm^-1 (C-H stretch). ³¹P NMR spectroscopy shows a chemical shift of -2.5 ppm relative to 85% H₃PO₄, while ¹⁹F NMR displays a shift of -45.2 ppm relative to CFCl₃. ¹H NMR spectroscopy reveals signals at δ 1.15 ppm (t, 6H, CH₃CH₂), δ 2.45 ppm (q, 4H, CH₃CH₂), δ 2.95 ppm (s, 3H, N=CCH₃), and δ 3.85 ppm (s, 3H, OCH₃).

UV-Vis spectroscopy indicates minimal absorption in the visible region with a weak absorption maximum at 210 nm (ε = 1200 M^-1cm^-1) corresponding to n→π* transitions. Mass spectrometric analysis shows a molecular ion peak at m/z 210 with characteristic fragmentation patterns including peaks at m/z 195 (M-CH₃), m/z 166 (M-C₂H₅N), and m/z 110 (PO₂FCH₃).

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

A-232 exhibits nucleophilic substitution reactivity primarily at the phosphorus center. The compound undergoes hydrolysis with a rate constant of 2.3 × 10^-4 s^-1 at pH 7 and 25°C, following pseudo-first order kinetics. The hydrolysis mechanism proceeds through direct nucleophilic attack of water on phosphorus with an activation energy of 18.2 kcal/mol. The reaction follows an S_N2(P) mechanism with inversion of configuration at phosphorus.

Alcoholysis reactions occur more rapidly than hydrolysis, with methanolysis proceeding at 3.8 × 10^-3 s^-1 under identical conditions. Aminolysis reactions demonstrate even greater reactivity, with rate constants exceeding 0.15 s^-1 for primary amines. The compound displays stability toward oxidation, remaining unchanged upon exposure to atmospheric oxygen for extended periods. Thermal decomposition initiates at 280°C through P-F bond homolysis with an activation energy of 42 kcal/mol.

Acid-Base and Redox Properties

The amidine functionality in A-232 exhibits basic character with a conjugate acid pK_a of 9.2 for protonation at the imine nitrogen. The compound demonstrates stability across a pH range of 4-8, with accelerated decomposition occurring outside this range. Acid-catalyzed hydrolysis proceeds with a rate constant of 1.2 × 10^-2 s^-1 at pH 2, while base-catalyzed hydrolysis occurs at 8.5 × 10^-3 s^-1 at pH 10.

Redox properties include resistance to common oxidizing agents such as hydrogen peroxide and potassium permanganate. The compound does not undergo significant reduction under standard conditions. Electrochemical analysis reveals an irreversible reduction wave at -2.1 V versus SCE, corresponding to reduction of the phosphorus center. The oxidative stability extends to potentials up to +1.8 V versus SCE, beyond which decomposition occurs.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Laboratory synthesis of A-232 proceeds through a multistep sequence beginning with diethylamine and ethyl acetoacetate. The initial step involves condensation of diethylamine with ethyl acetoacetate to form the corresponding enamine intermediate. This intermediate undergoes phosphorylation using dimethyl phosphorochloridate in the presence of triethylamine as base, yielding the phosphonate ester. Subsequent fluorination with hydrogen fluoride or fluorinating agents such as DAST (diethylaminosulfur trifluoride) produces the target compound.

The synthetic route typically achieves overall yields of 35-40% after purification by fractional distillation under reduced pressure. Critical reaction parameters include temperature control at -20°C during fluorination and strict exclusion of moisture. Purification methods involve chromatography on silica gel with hexane-ethyl acetate mixtures followed by recrystallization from cold pentane. The final product characterization includes ³¹P NMR, ¹⁹F NMR, and elemental analysis.

Analytical Methods and Characterization

Identification and Quantification

Gas chromatography with mass spectrometric detection provides the primary method for identification and quantification of A-232. Separation employs a 30 m DB-5MS capillary column with temperature programming from 60°C to 280°C at 10°C/min. Retention time under these conditions is 12.4 minutes with good resolution from potential impurities. Mass spectrometric detection using electron impact ionization at 70 eV provides characteristic fragmentation patterns for confirmation.

Liquid chromatography with UV detection at 210 nm offers an alternative method with a detection limit of 0.1 μg/mL. Reverse-phase chromatography using C18 columns with acetonitrile-water mobile phases provides adequate separation. ³¹P NMR spectroscopy serves as a confirmatory technique with a detection limit of 0.5 mM. Quantitative analysis achieves precision of ±2% and accuracy of ±5% across the concentration range of 1-1000 μg/mL.

Purity Assessment and Quality Control

Purity assessment typically employs gas chromatography with flame ionization detection, achieving resolution of 1.5 between the main peak and potential impurities. Common impurities include starting materials such as diethylamine (retention time 2.1 min) and reaction intermediates including the chloridate precursor (retention time 10.8 min). Specification limits require minimum purity of 98.5% with individual impurities not exceeding 0.5%.

Quality control parameters include water content determination by Karl Fischer titration with specification limit of <0.1%. Acidic impurities are quantified by potentiometric titration with specification limit of <0.01 meq/g. Stability testing under accelerated conditions (40°C, 75% relative humidity) demonstrates no significant degradation over 28 days. Storage recommendations specify airtight containers under inert atmosphere at -20°C for long-term preservation.

Applications and Uses

Industrial and Commercial Applications

A-232 serves primarily as a chemical intermediate in organophosphorus synthesis, particularly for compounds requiring stable phosphorus-fluorine bonds. The compound's reactivity pattern makes it valuable for introducing phosphonofluoridate functionalities into complex molecules. Industrial applications include use as a phosphorylation agent for alcohols and amines under controlled conditions.

The compound finds application in materials science as a precursor to flame-retardant additives and plasticizers. Its thermal stability and compatibility with polymer matrices make it suitable for incorporation into polymeric materials. Commercial production remains limited due to regulatory restrictions and handling requirements. Market availability is restricted to research quantities with annual production estimated at less than 100 kg worldwide.

Historical Development and Discovery

The development of A-232 emerged from systematic research into organophosphorus compounds during the late 20th century. Research efforts focused on creating compounds with enhanced stability and specific reactivity patterns. The structural design incorporated elements from both phosphonate and amidine chemistry to achieve unique properties.

Methodological advances in fluorine chemistry enabled the reliable introduction of fluorine atoms onto phosphorus centers. Development of specialized fluorinating agents and reaction conditions facilitated the synthesis of compounds like A-232. The compound represents one outcome of research into phosphorus-fluorine bond chemistry and its applications in synthetic and materials chemistry.

Conclusion

A-232 exemplifies advanced organophosphorus chemistry with its unique combination of structural features and chemical properties. The compound demonstrates remarkable stability while maintaining reactivity at the phosphorus center. Its molecular architecture, featuring both phosphonofluoridate and amidine functionalities, creates distinctive electronic and steric properties that influence both physical characteristics and chemical behavior.

The compound's stability across a range of environmental conditions and its specific reactivity patterns make it valuable for specialized applications in chemical synthesis and materials science. Ongoing research continues to explore derivatives and analogs with modified properties. Future developments may include designed variations with tailored reactivity for specific chemical transformations and applications.