Abstract

2-Diphenylphosphinobenzoic acid (CAS: 17261-28-8) is an organophosphorus compound with the molecular formula C₁₉H₁₅O₂P and a molar mass of 306.30 g·mol⁻¹. This white crystalline solid exhibits a melting point range of 174-181 °C and a density of 1.278 g·cm⁻³. The compound combines both phosphine and carboxylic acid functional groups, making it a versatile bidentate ligand in coordination chemistry. Its primary significance lies in catalytic applications, particularly in industrial processes such as the Shell higher olefin process. The molecule demonstrates solubility in polar organic solvents including alcohols, acetone, and dichloromethane, while being insoluble in water. Structural analysis reveals a twisted molecular geometry with the phosphorus atom adopting a pyramidal configuration.

Introduction

2-Diphenylphosphinobenzoic acid represents a class of hybrid organophosphorus compounds that bridge organic and coordination chemistry. This compound belongs to the family of functionalized phosphines, specifically those containing both phosphine and carboxylic acid moieties on an aromatic framework. The simultaneous presence of a soft phosphine donor and a hard carboxylic acid group creates unique coordination properties that have been exploited in various catalytic systems. The compound was first synthesized in the mid-20th century as researchers explored the coordination chemistry of phosphine-containing carboxylic acids. Its development coincided with the growing interest in chelating ligands for transition metal catalysis. The structural motif of combining electron-donating phosphorus atoms with potentially coordinating carboxyl groups has proven particularly valuable in homogeneous catalysis, where ligand design critically influences catalytic activity and selectivity.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular structure of 2-diphenylphosphinobenzoic acid features a benzoic acid scaffold with a diphenylphosphino group substituted at the ortho position. The phosphorus center exhibits a pyramidal geometry consistent with sp³ hybridization, with bond angles approximately 109.5° around the phosphorus atom. The P-C bond lengths measure typically 1.85-1.87 Å, while the C-C bonds in the aromatic systems range from 1.39-1.41 Å. The carboxylic acid group displays characteristic bond lengths with C=O at 1.21 Å and C-O at 1.32 Å. The torsion angle between the benzoic acid plane and the phenyl rings attached to phosphorus ranges from 45-65°, creating a twisted molecular conformation that influences its coordination behavior. The phosphorus atom possesses a formal oxidation state of +3 and carries a lone pair of electrons in an sp³ hybrid orbital, providing both σ-donor and π-acceptor capabilities in metal coordination.

Chemical Bonding and Intermolecular Forces

Covalent bonding in 2-diphenylphosphinobenzoic acid follows typical patterns for aromatic phosphines and carboxylic acids. The phosphorus-carbon bonds exhibit bond dissociation energies of approximately 305 kJ·mol⁻¹, while the carbon-oxygen bonds in the carboxyl group demonstrate energies around 360 kJ·mol⁻¹ for C=O and 385 kJ·mol⁻¹ for C-O. Intermolecular forces include strong hydrogen bonding between carboxylic acid groups with association energies of 25-30 kJ·mol⁻¹, creating dimeric structures in the solid state. Van der Waals interactions between phenyl rings contribute additional stabilization with energies of 5-10 kJ·mol⁻¹. The molecular dipole moment measures approximately 3.2 D, primarily oriented from the electron-deficient carboxyl carbon toward the electron-rich phosphorus center. The compound demonstrates moderate polarity with a calculated octanol-water partition coefficient (log P) of 3.8, indicating greater affinity for organic solvents than aqueous environments.

Physical Properties

Phase Behavior and Thermodynamic Properties

2-Diphenylphosphinobenzoic acid presents as a white crystalline solid at room temperature. The compound melts sharply between 174-181 °C with a heat of fusion of 28.5 kJ·mol⁻¹. No boiling point is typically reported due to decomposition above 250 °C. The density measures 1.278 g·cm⁻³ at 20 °C. The crystal structure belongs to the monoclinic system with space group P2₁/c and unit cell parameters a = 12.45 Å, b = 7.89 Å, c = 15.32 Å, and β = 112.5°. The specific heat capacity at 25 °C is 1.2 J·g⁻¹·K⁻¹. The refractive index of crystalline material is 1.623 at 589 nm. The compound sublimes slowly under reduced pressure (0.1 mmHg) at temperatures above 150 °C. Solubility characteristics include high solubility in dichloromethane (125 g·L⁻¹), acetone (98 g·L⁻¹), and methanol (45 g·L⁻¹), but negligible solubility in water (<0.1 g·L⁻¹) and hexane (<0.5 g·L⁻¹).

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic vibrations including O-H stretching at 3050-2500 cm⁻¹ (broad), C=O stretching at 1685 cm⁻¹, aromatic C=C stretching at 1580 and 1480 cm⁻¹, and P-C stretching at 1435 cm⁻¹. ¹H NMR spectroscopy (CDCl₃) shows aromatic protons between δ 7.2-8.2 ppm and the carboxylic acid proton at δ 12.1 ppm. ¹³C NMR displays signals for the carbonyl carbon at δ 172.5 ppm, aromatic carbons between δ 128-142 ppm, and ipso carbons attached to phosphorus at δ 135.2 ppm (d, J_CP = 12 Hz). ³¹P NMR exhibits a characteristic signal at δ -12.5 ppm relative to phosphoric acid standard. UV-Vis spectroscopy shows absorption maxima at 265 nm (ε = 12,500 M⁻¹·cm⁻¹) and 295 nm (ε = 8,200 M⁻¹·cm⁻¹) corresponding to π→π* transitions in the aromatic systems. Mass spectrometry demonstrates a molecular ion peak at m/z 306 with major fragments at m/z 227 [M-COOH]⁺, m/z 199 [M-C₆H₅O₂]⁺, and m/z 77 [C₆H₅]⁺.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

2-Diphenylphosphinobenzoic acid exhibits dual reactivity patterns characteristic of both tertiary phosphines and carboxylic acids. The phosphine moiety undergoes oxidation to phosphine oxide with oxidizing agents such as hydrogen peroxide (k = 0.15 M⁻¹·s⁻¹ at 25 °C) or air (half-life of 48 hours in solution). The carboxylic acid group participates in typical acid-base reactions with pK_a = 3.8 in water, forming carboxylate salts with bases. Esterification reactions proceed with rate constants comparable to benzoic acid derivatives (k_ester = 2.3 × 10⁻⁴ M⁻¹·s⁻¹ with methanol). The compound acts as a bidentate ligand toward transition metals, forming chelate complexes with stability constants log β₂ = 8.2 for Pd(II) and log β₂ = 7.8 for Pt(II). Decomposition occurs above 250 °C through decarboxylation followed by oxidation of the phosphine group. The compound demonstrates stability in neutral and acidic conditions but undergoes slow hydrolysis of the P-C bond under strongly basic conditions at elevated temperatures.

Acid-Base and Redox Properties

The carboxylic acid group exhibits a pK_a of 3.8 in aqueous solution, making it a moderately strong organic acid. The phosphine moiety demonstrates weak basicity with protonation occurring at phosphorus (pK_a of conjugate acid = -2.3). Redox properties include oxidation of phosphine to phosphine oxide with E_1/2 = +0.85 V versus SCE for the P(III)/P(V) couple. The compound shows no significant reduction waves within the accessible potential window of common solvents. Buffer capacity is maintained in the pH range 3-5 due to the carboxylic acid group. The phosphine group reduces certain metal ions including Au(III) to Au(0) and Ag(I) to Ag(0) with second-order rate constants of 0.08 M⁻¹·s⁻¹ and 0.03 M⁻¹·s⁻¹ respectively. Stability in oxidizing environments is limited with half-life of 2 hours in 3% H₂O₂ solution, while reducing conditions do not affect the carboxylic acid group.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The primary laboratory synthesis involves nucleophilic aromatic substitution between sodium diphenylphosphide and the sodium salt of 2-chlorobenzoic acid. The reaction proceeds in anhydrous tetrahydrofuran or dimethoxyethane under nitrogen atmosphere at 65-70 °C for 12-16 hours. The sodium diphenylphosphide reagent is prepared in situ from diphenylphosphine and sodium amide in liquid ammonia or from chlorodiphenylphosphine and sodium metal. Typical yields range from 65-75% after purification by recrystallization from ethanol-water mixtures. Alternative routes include palladium-catalyzed cross-coupling between diphenylphosphine and 2-iodobenzoic acid using Pd(PPh₃)₄ catalyst with yields up to 82%. The product is purified by column chromatography on silica gel using ethyl acetate/hexane mixtures or by recrystallization from toluene. The compound is typically characterized by melting point, ³¹P NMR spectroscopy, and elemental analysis.

Analytical Methods and Characterization

Identification and Quantification

Identification of 2-diphenylphosphinobenzoic acid is reliably achieved through ³¹P NMR spectroscopy, which shows a characteristic singlet between δ -10 to -15 ppm. Complementary techniques include infrared spectroscopy with diagnostic bands at 1685 cm⁻¹ (C=O stretch) and 1435 cm⁻¹ (P-C stretch). High-performance liquid chromatography with UV detection at 265 nm provides quantitative analysis with a detection limit of 0.1 μg·mL⁻¹ using C18 reverse-phase columns and acetonitrile/water mobile phases. Titration with standardized sodium hydroxide solution determines acid content with precision of ±0.5%. Mass spectrometric analysis confirms molecular weight through the molecular ion at m/z 306. X-ray crystallography provides definitive structural characterization, particularly for metal complexes. Elemental analysis typically yields carbon 74.50%, hydrogen 4.94%, phosphorus 10.12% with acceptable error margins of ±0.3%.

Purity Assessment and Quality Control

Purity assessment typically involves determination of acid content by potentiometric titration, phosphine content by ³¹P NMR spectroscopy against an internal standard, and chromatographic analysis for organic impurities. Common impurities include triphenylphosphine oxide (detectable by ³¹P NMR at δ +25 ppm), benzoic acid, and diphenylphosphine. Acceptable purity for catalytic applications exceeds 97% with metal content below 10 ppm. Quality control specifications include melting point range of 174-181 °C, specific rotation [α]_D²⁰ = 0° (achiral), and absorbance ratio A₂₆₅/A₂₉₅ = 1.52±0.05. The compound is stable for at least two years when stored under argon at -20 °C protected from light. Solutions in organic solvents remain stable for several weeks when kept under inert atmosphere but gradually oxidize upon exposure to air.

Applications and Uses

Industrial and Commercial Applications

2-Diphenylphosphinobenzoic acid serves primarily as a ligand precursor in catalytic systems for industrial processes. Its most significant application is in the Shell higher olefin process (SHOP) where it forms palladium complexes that catalyze the oligomerization of ethylene to higher α-olefins. The chelating P,O coordination mode modifies the electronic properties of the metal center, influencing both activity and selectivity toward linear olefins. Additional industrial applications include use in hydroformylation catalysts where the combination of phosphine and carboxylate coordination tunes the catalyst's electronic properties and solubility characteristics. The compound finds use in specialty chemical synthesis as a building block for more complex ligands through functionalization of the carboxylic acid group. Market demand remains steady at approximately 5-10 metric tons annually worldwide, with primary manufacturers located in Europe and the United States. Production costs range from $200-500 per kilogram depending on purity and quantity.

Research Applications and Emerging Uses

In research settings, 2-diphenylphosphinobenzoic acid functions as a versatile ligand for constructing heterobimetallic complexes where the carboxylate group can bridge metal centers while the phosphine coordinates to another metal. Recent applications include development of metal-organic frameworks (MOFs) with phosphine functionality for gas storage and separation. The compound serves as a precursor for photoactive coordination compounds with potential applications in solar energy conversion. Emerging research explores its use in asymmetric catalysis when converted to chiral derivatives through esterification with chiral alcohols or amidation with chiral amines. The ligand demonstrates activity in carbon-carbon bond forming reactions including Suzuki-Miyaura and Heck couplings when complexed with palladium. Patent literature discloses applications in electrochemical sensors where the phosphine group facilitates electron transfer while the carboxylate provides anchoring to electrode surfaces. Ongoing investigations explore its potential in nanomaterials synthesis and as a component of functionalized surfaces for catalysis.

Historical Development and Discovery

The development of 2-diphenylphosphinobenzoic acid emerged from broader investigations into functionalized phosphines during the 1950s and 1960s. Early work focused on understanding how substituents on phosphorus influenced the electronic and steric properties of phosphine ligands. The specific combination of phosphine and carboxylic acid functionalities gained attention when researchers recognized the potential for creating chelating ligands with mixed donor atoms. The compound's synthesis was first reported in the chemical literature around 1965, coinciding with growing interest in tailor-made ligands for homogeneous catalysis. The discovery of its utility in the Shell higher olefin process during the 1970s marked a significant advancement in industrial catalysis. Methodological improvements in synthesis and purification developed throughout the 1980s enabled higher purity material for research applications. The late 1990s saw expanded interest in its use for constructing molecular materials and coordination polymers. Recent developments focus on derivatization strategies to create libraries of related ligands for high-throughput catalyst screening.

Conclusion

2-Diphenylphosphinobenzoic acid represents a structurally interesting and functionally versatile organophosphorus compound that bridges traditional domains of organic and coordination chemistry. Its unique combination of soft phosphine and hard carboxylic acid donors creates distinctive coordination behavior that has been exploited in numerous catalytic applications, particularly in industrial olefin oligomerization processes. The compound's well-characterized physical and chemical properties, straightforward synthesis, and stability under standard handling conditions contribute to its continued utility in both industrial and research settings. Future research directions likely include further exploration of its derivatives for asymmetric catalysis, development of functionalized materials incorporating its molecular framework, and investigation of its behavior in non-traditional solvent systems. The compound continues to serve as a valuable building block in ligand design and catalyst development, demonstrating the enduring importance of fundamental organophosphorus chemistry in advanced chemical applications.