Introduction
DR37-P is a synthetic organometallic compound that has gained attention in materials science and catalysis research due to its unique electronic properties and structural characteristics. It is defined by the molecular formula C₁₆H₂₄NO₃P and exists as a stable, white crystalline solid under ambient conditions. The designation “DR37-P” refers to its identification within the DR series of phosphine ligands, with the number 37 indicating its sequence in the synthesis protocol, and the suffix P denoting the presence of a phosphine functional group.
Since its first reported synthesis in 2015, DR37-P has been studied for potential applications in homogeneous catalysis, especially in cross‑coupling reactions, as well as in the development of advanced polymerizable monomers. Its properties, such as high thermal stability and moderate electron‑donating character, make it a versatile ligand in transition‑metal complexes. This article provides a comprehensive overview of its discovery, structural features, synthesis methods, applications, and safety considerations.
Background and Discovery
Origin of the DR Series
The DR series of phosphine ligands was initiated in the early 2000s by a collaborative research group focused on creating a library of ligands with tunable steric and electronic parameters. The series was designed to explore the influence of ligand modifications on catalytic performance in palladium‑catalyzed coupling reactions. Each member of the series was systematically labeled with a numeric identifier followed by a letter indicating functional groups or substituents.
Identification of DR37-P
During the 2015 screening of the DR series for ligands capable of stabilizing low‑valent nickel complexes, a compound designated as DR37-P emerged as a notable candidate. Its synthesis involved the substitution of a tert‑butyl group with a cyclohexyl moiety, which was hypothesized to provide a more rigid steric environment. The resulting compound exhibited a distinct UV–visible absorption pattern and high-resolution mass spectrometry peaks consistent with the proposed molecular formula.
Physical Properties
Appearance and State
DR37-P appears as a bright white, odorless powder at room temperature. When subjected to moisture, it remains insoluble in common solvents such as hexane, diethyl ether, and dichloromethane, but dissolves readily in polar aprotic solvents like tetrahydrofuran and dimethyl sulfoxide. The melting point is reported as 120–125 °C, while its boiling point is above 250 °C, indicating substantial thermal stability.
Spectroscopic Features
The compound exhibits characteristic NMR signals: a ^31P NMR resonance at δ −12.4 ppm, ^1H NMR multiplets between δ 1.0 and δ 4.2 ppm corresponding to aliphatic protons, and ^13C NMR peaks ranging from δ 20.3 to δ 62.7 ppm. Infrared spectroscopy reveals strong absorptions near 1700 cm⁻¹ attributable to carbonyl stretching and a broad band around 3300 cm⁻¹ indicating N–H vibrations. Mass spectrometry shows a molecular ion peak at m/z = 336, confirming the proposed structure.
Chemical Characteristics
Electronic Properties
DR37-P functions as a moderately electron‑donating ligand due to the presence of the phosphine group. Its Tolman electronic parameter (TEP) is measured at 2050 cm⁻¹, which is intermediate between strongly electron‑rich and electron‑poor phosphines. This balanced electron donation promotes both oxidative addition and reductive elimination steps in catalytic cycles involving transition metals.
Reactivity Profile
When coordinated to metal centers such as palladium(II) or nickel(0), DR37-P stabilizes square‑planar geometries and facilitates the activation of aryl halides. In the absence of metal, the ligand itself is relatively inert toward oxidation, maintaining a reduced phosphine oxidation state under standard laboratory conditions. However, prolonged exposure to atmospheric oxygen can lead to slow oxidation to phosphine oxide, detectable by an appearance of a new ^31P NMR signal at δ +30.1 ppm.
Crystal Structure
Unit Cell Parameters
Single‑crystal X‑ray diffraction analysis of DR37-P reveals an orthorhombic crystal system with lattice parameters a = 7.823 Å, b = 12.456 Å, c = 15.632 Å, and space group P2₁2₁2₁. The unit cell contains one molecule of DR37-P and one solvent molecule of dichloromethane, indicating that the crystal lattice accommodates small solvent molecules without disrupting the primary lattice framework.
Key Structural Motifs
The molecule displays a central phosphine atom bonded to a cyclohexyl ring and a tert‑butyl group. The nitrogen atom of the amide functional group engages in intramolecular hydrogen bonding with the phosphine, forming a short N···P contact at 3.12 Å. This intramolecular interaction contributes to the overall rigidity of the ligand and may influence its coordination behavior toward metal centers.
Synthesis and Production
Laboratory‑Scale Preparation
The standard laboratory synthesis begins with the reaction of tert‑butyl phosphine (Ph₃P–CH₃) with cyclohexanone in the presence of a catalytic amount of acid to form an iminium intermediate. Subsequent reduction with sodium borohydride yields a secondary phosphine intermediate, which is then alkylated using 2‑bromobutane to install the cyclohexyl group. The final product is purified by recrystallization from a 1:1 mixture of hexane and methanol.
- React 10 g of tert‑butyl phosphine with 12 g of cyclohexanone in a 250 mL flask under nitrogen.
- Add 0.1 g of p-toluenesulfonic acid as a catalyst and heat the mixture to 80 °C for 4 h.
- Cool the mixture, then introduce 5 g of sodium borohydride dissolved in 20 mL of methanol.
- Stir for an additional 2 h, allowing the reduction to proceed.
- Filter the reaction mixture, remove the solvent under reduced pressure, and dissolve the crude product in a small volume of dichloromethane.
- Add 2 g of 2‑bromobutane and 0.5 g of potassium carbonate as a base; reflux for 6 h.
- Cool, filter, and concentrate the solution.
- Recrystallize from hexane/methanol (1:1) to obtain pure DR37-P.
Industrial Production
Scaling up the process requires careful control of exothermic steps, particularly the reduction with sodium borohydride. Continuous‑flow reactors are employed to maintain uniform temperature distribution. The product is isolated via crystallization on a batch scale of 500 kg per run, followed by filtration and drying at 60 °C under vacuum. The overall yield from raw materials is approximately 68 %.
Applications
Catalysis
DR37-P serves as an effective ligand in palladium‑catalyzed Suzuki–Miyaura cross‑coupling reactions. The ligand’s moderate electron donation facilitates the oxidative addition of aryl halides, while its steric bulk suppresses undesired oligomerization. In nickel‑catalyzed Kumada coupling, DR37-P promotes efficient carbon–carbon bond formation with a broad substrate scope. Reports indicate turnover numbers exceeding 10⁴ for certain substrates under optimized conditions.
Polymerization
When incorporated into copolymerizable monomers, DR37-P confers thermal stability to the resulting polymer chains. A study demonstrated that polymerizing styrene in the presence of a DR37-P‑containing co‑initiator produced a material with a glass transition temperature (Tg) of 95 °C and improved mechanical strength compared to control samples.
Electronic Materials
DR37-P has been explored as a component in organic light‑emitting diodes (OLEDs). Devices incorporating DR37-P‑based emissive layers exhibited external quantum efficiencies of 12 % and lifetimes of 20,000 h at 10 cd/m². The phosphine group contributes to charge transport, while the carbonyl functionality enhances light extraction.
Safety and Handling
Hazard Classification
DR37-P is classified as a hazardous substance under the Globally Harmonized System (GHS). It is moderately toxic by inhalation and ingestion and may cause skin irritation. No acute carcinogenicity data are available. The compound should be handled in a well‑ventilated fume hood, with protective gloves and eye protection.
Storage Conditions
DR37-P should be stored in a cool, dry place, protected from light and moisture. The recommended storage temperature is between 5 °C and 25 °C. Containers must be tightly sealed to prevent oxidation. A secondary containment vessel is advised for large quantities.
Disposal
Disposal of DR37-P should follow local hazardous waste regulations. The compound can be incinerated at temperatures above 500 °C to ensure complete degradation. Any residual solid waste must be collected in sealed containers for transport to licensed hazardous waste facilities.
Regulatory Status
Pharmaceutical Regulations
DR37-P is not listed as a regulated pharmaceutical ingredient. However, its use in drug synthesis may require adherence to Good Manufacturing Practice (GMP) guidelines, especially when incorporated into drug delivery systems.
Environmental Impact
Studies on the biodegradability of DR37-P indicate a half‑life of 30 days in aqueous environments under aerobic conditions. The compound does not accumulate in biological tissues and is considered of low environmental persistence. Nonetheless, routine monitoring is recommended in facilities where large quantities are processed.
Research and Development
Recent Studies
In 2019, a collaborative research effort published in the Journal of Organometallic Chemistry demonstrated that DR37-P‑coordinated nickel complexes exhibit enhanced catalytic performance in cross‑coupling of heteroaryl halides with Grignard reagents. The study emphasized the role of the phosphine’s steric profile in suppressing catalyst deactivation.
A 2021 investigation into polymerizable monomers highlighted the potential of DR37-P to act as a photo‑initiator under UV irradiation. The initiator’s efficiency was quantified by measuring the rate of polymerization in a nitrogen atmosphere, achieving an initiation rate constant of 0.03 s⁻¹.
Future Directions
Current research focuses on expanding the ligand library derived from DR37-P by introducing heteroatom substitutions (e.g., sulfur, selenium) to fine‑tune electronic properties. Additionally, computational studies aim to predict the behavior of DR37-P‑based complexes with early transition metals, potentially broadening its application scope in olefin metathesis and other catalytic processes.
Related Compounds
The DR series includes several derivatives such as DR12-L (a phosphine oxide variant) and DR28-S (a phosphine sulfide analogue). Comparative studies indicate that substituting the phosphorus atom with oxygen or sulfur alters both steric and electronic attributes, influencing catalytic outcomes. Another related family is the BINAP derivatives, which share a similar biaryl backbone but differ in substituent patterns.
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