Abstract

Fentin acetate, systematically named (acetoxy)(triphenyl)stannane with molecular formula C₂₀H₁₈O₂Sn and molecular mass 409.07 g·mol⁻¹, represents an organotin compound of significant historical and chemical interest. This colorless crystalline solid exhibits a melting point range of 122–124 °C and demonstrates polymeric structural characteristics in the solid state with five-coordinate tin centers. The compound previously served as an agricultural fungicide under various trade names including Brestan, though its application has diminished due to environmental and toxicological concerns. Fentin acetate displays characteristic organotin chemistry with both covalent and ionic bonding features, particularly in its acetate group coordination. Spectroscopic analysis reveals distinctive patterns in infrared, nuclear magnetic resonance, and mass spectrometry that correlate with its molecular structure. The compound's reactivity follows established patterns for triphenyltin derivatives with both nucleophilic and electrophilic character at the tin center.

Introduction

Fentin acetate, chemically designated as triphenyltin acetate, belongs to the organotin compound class characterized by direct carbon-tin bonds. This compound emerged during the mid-20th century as part of the broader development of organometallic compounds for agricultural applications. As a member of the triphenyltin family, fentin acetate exhibits the characteristic structural and reactivity patterns of organotin(IV) compounds with mixed organic and inorganic substituents. The compound's significance lies in its historical role as a fungicidal agent and its representative nature within organotin chemistry, providing insights into the coordination behavior of tin centers with oxygen-based ligands. The systematic name (acetoxy)(triphenyl)stannane follows IUPAC nomenclature conventions for organometallic compounds, prioritizing the organic substituents in alphabetical order while maintaining the stannane parent hydride nomenclature.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular structure of fentin acetate in the solid state adopts a polymeric arrangement rather than existing as discrete molecular units. Crystallographic analysis reveals five-coordinate tin centers in a distorted trigonal bipyramidal geometry, consistent with VSEPR theory predictions for tin(IV) compounds with steric crowding. The tin atom exhibits sp³d hybridization with the three phenyl groups occupying equatorial positions and the acetate oxygen atoms occupying axial positions in the coordination sphere. Bond angles at the tin center measure approximately 120° between equatorial phenyl groups and 90° between axial and equatorial positions. The electronic configuration of tin([Kr]4d¹⁰5s²5p²) permits tetravalent bonding through sp³ hybridization, though the expansion of the coordination sphere to five atoms indicates utilization of d orbitals for additional bonding.

The acetate ligand demonstrates ambidentate character, coordinating to tin through oxygen atoms with the possibility of both monodentate and bidentate coordination modes depending on crystalline environment. Tin-oxygen bond lengths typically range from 2.10–2.25 Å, consistent with covalent bonding character. The phenyl rings maintain their aromatic character with carbon-tin bond lengths of approximately 2.15 Å, slightly longer than typical carbon-carbon bonds due to the larger atomic radius of tin. Molecular orbital calculations indicate significant electron density delocalization between the tin center and the phenyl rings, with the highest occupied molecular orbitals primarily localized on the aromatic systems.

Chemical Bonding and Intermolecular Forces

Covalent bonding predominates in fentin acetate with polar character in the tin-carbon and tin-oxygen bonds. The electronegativity difference between tin (1.96) and carbon (2.55) creates bond dipoles with partial negative charge on carbon, while the tin-oxygen bond (electronegativity difference 1.24) shows even greater polarity. The molecular dipole moment measures approximately 3.5 D, primarily oriented along the tin-acetate axis. Intermolecular forces include van der Waals interactions between phenyl rings with typical π-π stacking distances of 3.5–3.8 Å. The polymeric structure in the solid state arises from bridging acetate ligands that coordinate to adjacent tin centers, creating extended networks rather than discrete hydrogen bonding or dipole-dipole interactions.

Comparative analysis with related triphenyltin compounds shows consistent bonding parameters. The tin-carbon bond energy measures approximately 310 kJ·mol⁻¹, while tin-oxygen bonds demonstrate energies near 340 kJ·mol⁻¹. These values remain consistent across the triphenyltin series regardless of the anionic ligand, indicating minimal influence of the acetate group on the fundamental tin-carbon bonding character. The acetate group itself maintains typical carbonyl (C=O) bond lengths of 1.23 Å and carbon-oxygen (C-O) bond lengths of 1.32 Å, consistent with delocalized π bonding in the carboxylate system.

Physical Properties

Phase Behavior and Thermodynamic Properties

Fentin acetate presents as a colorless crystalline solid at room temperature with orthorhombic crystal structure. The compound melts sharply at 122–124 °C with a heat of fusion measuring 28.5 kJ·mol⁻¹. No polymorphic forms have been reported under standard conditions. The density of crystalline fentin acetate measures 1.55 g·cm⁻³ at 20 °C. The compound sublimes slowly under reduced pressure (0.1 mmHg) at temperatures above 100 °C with sublimation enthalpy of 89 kJ·mol⁻¹. Thermal decomposition commences at approximately 200 °C with evolution of acetic acid and formation of hexaphenylditin as a primary decomposition product.

The specific heat capacity of solid fentin acetate measures 1.2 J·g⁻¹·K⁻¹ at 25 °C. The compound exhibits low volatility with vapor pressure less than 1×10⁻⁵ mmHg at room temperature. Solubility parameters indicate moderate solubility in organic solvents including dichloromethane (85 g·L⁻¹), acetone (72 g·L⁻¹), and benzene (64 g·L⁻¹), with minimal aqueous solubility (0.0012 g·L⁻¹ at 20 °C). The refractive index of crystalline material measures 1.62 at the sodium D line. These physical properties align with expectations for organotin compounds of similar molecular weight and structural complexity.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic vibrations diagnostic of both the triphenyltin and acetate functionalities. The carbonyl stretching frequency appears at 1655 cm⁻¹, shifted from typical acetate values due to coordination to tin. Symmetric and asymmetric COO⁻ stretches occur at 1410 cm⁻¹ and 1550 cm⁻¹ respectively, with separation (Δν) of 140 cm⁻¹ indicating predominantly monodentate coordination. Tin-carbon stretches appear in the 450–500 cm⁻¹ region, while tin-oxygen vibrations occur between 300–350 cm⁻¹. Aromatic C-H stretches appear at 3050 cm⁻¹ with out-of-plane bending at 730 cm⁻¹ and 690 cm⁻¹ characteristic of monosubstituted benzene rings.

Nuclear magnetic resonance spectroscopy provides definitive structural characterization. ¹¹⁹Sn NMR chemical shift occurs at δ -120 ppm relative to SnMe₄, consistent with five-coordinate tin environments. Proton NMR exhibits phenyl resonances as a complex multiplet centered at δ 7.5–7.7 ppm, while the acetate methyl group appears as a sharp singlet at δ 2.15 ppm. Carbon-13 NMR shows phenyl carbons between δ 128–140 ppm with ipso carbon at δ 138.5 ppm, carbonyl carbon at δ 178.5 ppm, and acetate methyl carbon at δ 22.3 ppm. Mass spectral analysis reveals molecular ion cluster centered at m/z 409 with characteristic isotope pattern for tin (Sn-120, 0.34%; Sn-118, 24.2%; Sn-119, 8.6%; Sn-120, 32.6%; Sn-122, 4.6%; Sn-124, 5.8%). Major fragmentation pathways include loss of acetate radical (m/z 351) and sequential loss of phenyl groups.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Fentin acetate demonstrates reactivity patterns characteristic of triphenyltin compounds with additional functionality from the acetate group. The compound undergoes hydrolysis in aqueous media with first-order kinetics and rate constant k = 3.2×10⁻⁴ s⁻¹ at 25 °C, producing triphenyltin hydroxide and acetic acid. This hydrolysis proceeds more rapidly in basic conditions (pH > 9) with complete conversion within minutes. The acetate group serves as a leaving group in nucleophilic substitution reactions, permitting exchange with halides, thiolates, and other anions. Second-order rate constants for halide exchange measure k₂ = 0.85 M⁻¹·s⁻¹ for chloride, 1.2 M⁻¹·s⁻¹ for bromide, and 2.8 M⁻¹·s⁻¹ for iodide in acetone solvent at 25 °C.

Thermal decomposition follows first-order kinetics with activation energy Eₐ = 105 kJ·mol⁻¹ and pre-exponential factor A = 1.5×10¹² s⁻¹. The primary decomposition pathway involves homolytic cleavage of the tin-oxygen bond with subsequent decarboxylation and recombination reactions. The compound demonstrates stability in dry air but slowly oxidizes in moist air to form tin oxides. In solution, fentin acetate exists in equilibrium between monomeric and dimeric forms depending on concentration, with association constant Kₐ = 120 M⁻¹ in benzene solution at 25 °C. The tin center acts as a Lewis acid, forming adducts with Lewis bases including pyridine (K = 350 M⁻¹) and triphenylphosphine oxide (K = 890 M⁻¹).

Acid-Base and Redox Properties

The acetate group confers mild acidity to fentin acetate with pKₐ = 4.7 for the conjugate acid in aqueous-organic mixed solvents. This value represents enhanced acidity compared to acetic acid (pKₐ = 4.76) due to the electron-withdrawing triphenyltin group. The compound functions as a weak base through the tin center, with protonation occurring on oxygen rather than tin. Redox properties involve primarily the tin(IV) center, which demonstrates resistance to reduction with reduction potential E° = -1.35 V versus SHE for the Sn(IV)/Sn(II) couple. Oxidation occurs at the phenyl rings rather than the tin center, with oxidation potential Eₚₐ = +1.25 V versus SCE in acetonitrile.

Fentin acetate maintains stability in neutral and acidic conditions but decomposes in strongly basic media through hydroxide attack on tin. The compound demonstrates limited stability in oxidizing environments, with rapid decomposition in the presence of strong oxidants including hydrogen peroxide and potassium permanganate. Electrochemical studies reveal irreversible reduction waves at -1.45 V and -1.85 V versus Ag/AgCl corresponding to sequential cleavage of phenyl groups. The compound exhibits cathodic corrosion protection properties when applied to metal surfaces, forming stable passivating layers.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The most efficient laboratory synthesis of fentin acetate proceeds through the reaction of triphenyltin hydroxide with acetic acid or acetic anhydride. Triphenyltin hydroxide (15.0 g, 0.04 mol) reacts with glacial acetic acid (2.4 g, 0.04 mol) in toluene solvent (100 mL) under reflux conditions for 2 hours. The reaction mixture undergoes azeotropic distillation to remove water, with reaction completion monitored by infrared spectroscopy disappearance of the O-H stretch at 3600 cm⁻¹. After cooling, the product crystallizes directly from the reaction mixture, yielding 15.8 g (92%) of pure fentin acetate after filtration and drying under vacuum.

An alternative route involves the reaction of triphenyltin chloride with silver acetate in acetone solvent. Triphenyltin chloride (15.7 g, 0.04 mol) and silver acetate (6.7 g, 0.04 mol) reflux in acetone (150 mL) for 4 hours with protection from light. The precipitated silver chloride removes by filtration, and the filtrate concentrates under reduced pressure. Recrystallization from hexane-dichloromethane mixture yields 14.2 g (83%) of product. This method requires careful exclusion of moisture to prevent hydrolysis side reactions. Both synthetic routes produce material of high purity (>98%) as determined by elemental analysis and spectroscopic methods.

Analytical Methods and Characterization

Identification and Quantification

Standard identification of fentin acetate employs a combination of spectroscopic techniques. Infrared spectroscopy provides definitive identification through characteristic carbonyl stretching at 1655 cm⁻¹ and the tin-carbon vibration pattern between 450–500 cm⁻¹. Nuclear magnetic resonance spectroscopy offers complementary characterization with distinctive ¹¹⁹Sn chemical shift at δ -120 ppm and proton NMR showing the acetate methyl singlet at δ 2.15 ppm. Mass spectrometry confirms molecular weight through the molecular ion cluster centered at m/z 409 with appropriate tin isotope distribution.

Quantitative analysis typically utilizes high-performance liquid chromatography with ultraviolet detection at 254 nm. Reverse-phase C18 columns with acetonitrile-water mobile phase (70:30 v/v) provide adequate separation with retention time of 6.8 minutes. The method demonstrates linear response from 0.1–100 μg·mL⁻¹ with detection limit of 0.05 μg·mL⁻¹ and quantification limit of 0.15 μg·mL⁻¹. Precision measures 2.1% relative standard deviation at 10 μg·mL⁻¹ concentration. Alternative methods include gas chromatography with mass spectrometric detection following derivatization, though this approach introduces additional complexity without significant improvement in sensitivity or selectivity.

Applications and Uses

Industrial and Commercial Applications

Fentin acetate historically served as a broad-spectrum fungicide in agricultural applications, particularly for the control of fungal diseases in potatoes, sugar beets, and rice crops. The compound demonstrated efficacy against various fungal pathogens including Phytophthora infestans, Cercospora beticola, and Pyricularia oryzae at application rates of 200–500 g·ha⁻¹. Commercial formulations typically contained 20–60% active ingredient in wettable powder or flowable concentrate forms. Market presence peaked during the 1970s and 1980s with annual production estimated at 2000–3000 metric tons worldwide before declining due to environmental concerns and regulatory restrictions.

Non-agricultural applications included use as a wood preservative against fungal decay and as an antifouling agent in marine paints. The compound demonstrated effectiveness against various wood-rotting fungi including Serpula lacrymans and Coniophora puteana at treatment levels of 0.5–1.0 kg·m⁻³. In marine applications, fentin acetate provided protection against barnacles and other fouling organisms when incorporated into paint formulations at 5–10% concentration. These applications have largely been discontinued due to the compound's persistence in the environment and toxicity to non-target organisms.

Historical Development and Discovery

The development of fentin acetate emerged from broader investigations into organotin compounds during the mid-20th century. Initial research on triphenyltin compounds began in the 1950s following the discovery of the fungicidal properties of organotin derivatives. Dutch scientists at the Nederlandse Stikstof Maatschappij (Netherlands Nitrogen Company) first reported the fungicidal activity of triphenyltin compounds in 1954, leading to the development of both the hydroxide and acetate derivatives. Fentin acetate received patent protection in 1958 and entered commercial production shortly thereafter under the trade name Brestan.

Structural characterization progressed throughout the 1960s with X-ray crystallographic studies confirming the polymeric nature of solid fentin acetate. Environmental concerns emerged during the 1970s regarding the compound's persistence and bioaccumulation potential, leading to restrictions in many countries. The 1980s brought improved understanding of organotin environmental chemistry and toxicology, resulting in further regulatory limitations. Despite its diminished commercial importance, fentin acetate remains a compound of significant interest in organometallic chemistry and environmental studies as a representative triphenyltin compound.

Conclusion

Fentin acetate represents a historically significant organotin compound with distinctive structural and chemical properties. The compound's polymeric solid-state structure with five-coordinate tin centers illustrates the complex coordination behavior possible even in seemingly simple organometallic systems. Its chemical reactivity demonstrates the interplay between organic and inorganic character, with the acetate group providing both stability and reaction pathways distinct from other triphenyltin derivatives. Although its agricultural applications have declined due to environmental concerns, fentin acetate continues to serve as a reference compound in organotin chemistry and environmental monitoring. Future research directions may include development of analytical methods for trace detection, investigation of decomposition pathways in various environments, and exploration of potential applications in materials science where its unique properties could be harnessed without environmental release.