Synthesis and Characterization of 6-(1,3-dimethylureido) dibenzo[c,e][1,2]oxaphosphinine 6-oxide ^†

Abstract

The compound 6-(1,3-dimethylureido)dibenzo[c,e][1,2]oxaphosphinine 6-oxide was synthesized in a single step, under mild reaction conditions, from the commercially available H-phosphinate 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and 1,3-dimethylurea in the presence of a mild oxidant and triethylamine. The reaction led to the formation of a P–N bond between the phosphoryl group and one of the urea nitrogen atoms. The compound was spectroscopically characterized, and thermal properties were investigated.

1. Introduction

H-Phosphinate 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) is an organophosphorus derivative widely investigated in the field of non-halogenated flame retardants for plastics [1,2]. Among the possible derivatizations of DOPO, the formal replacement of the P-bonded hydrogen atom with nitrogen substituents leads to the formation of phosphonamidates that exhibit peculiar flame-retardant properties thanks to the P-N synergism [3]. For instance, in the condensed phase, phosphorus derivatives promote the formation of a carbonaceous char layer, and nitrogen-containing compounds may promote stability and intumescence in the layer. A common approach to obtaining DOPO-based phosphonamidates is based on the synthesis of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-chloride (DOPO-Cl), followed by the displacement of the chlorine atom with a suitable amine. DOPO-Cl can be formed through the Atherton–Todd reaction between DOPO and carbon tetrachloride, but alternative chlorinating agents such as sulfuryl dichloride, trichlorocyanuric acid, chlorine gas, and N-chlorosuccinimide can be employed [4,5,6,7,8,9,10,11,12].

Based on the knowledge developed by our group on the chemistry of DOPO and related organophosphorus species [13,14,15,16] and patented alternative approach for the preparation of DOPO derivatives with P–N bonds, prepared under mild conditions without chlorinated reactants [17,18], in this paper, we report the synthesis and characterization of 6-(1,3-dimethylureido)dibenzo[c,e][1,2]oxaphosphinine 6-oxide (DOPO-N^urea), obtained from DOPO and 1,3-dimethylurea. The thermal behaviour of the new compound was also investigated.

2. Experimental Section

2.1. Materials

9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) was purchased from Fluorochem (Glossop, UK) and used without further purification. 1,3-Dimethylurea (DMU), other organic reactants, and iodine were Merck (Darmstadt, Germany) products, used as received.

2.2. Characterizations

Carbon, hydrogen, and nitrogen elemental analyses were performed using an Elementar (Langenselbold, Germany) Unicube microanalyzer. ATR-IR spectra were recorded with a Perkin-Elmer (Shelton, CT, USA) Spectrum Two spectrophotometer equipped with diamond ATR. Mono- and bidimensional NMR spectra were collected with a Bruker Avance 400 instrument (Billerica, MA, USA) operating at 400.13 MHz of ¹H resonance. ¹H and ¹³C NMR spectra were referenced to tetramethylsilane, while ³¹P NMR chemical shifts were referred to external 85% H₃PO₄ in water. Absorption spectra were recorded with a Yoke (Fengxian, China) 6000Plus double-beam spectrophotometer. Steady-state and time-resolved luminescence measurements were carried out with an Edinburgh Instruments (Livingston, UK) FS5 spectrofluorometer. Melting points were registered using a FALC (Treviglio, Italy) 360 D instrument equipped with a camera. Thermogravimetric analyses were performed under N₂ flow with a Perkin-Elmer TGA4000 instrument. Differential scanning calorimetry measurements were carried out under N₂ with a Mettler Toledo DSC 3.

2.3. Synthesis of DOPO-N^urea

The reaction was carried out in a MBraun (Garching bei München, Germany) MB10 glovebox filled with N₂. In a typical preparation, DOPO (0.500 g, 2.3 mmol) was dispersed in 25 mL of anhydrous dichloromethane. Triethylamine (650 μL, 4.7 mmol) was added to the reaction mixture at room temperature. After stirring at room temperature until the complete dissolution of DOPO (about 10 min), DMU (0.203 g, 2.3 mmol) was added. Solid iodine (0.584 g, 2.3 mmol) was introduced into small aliquots, and the solution was left under stirring overnight at room temperature. The solvent was evaporated at reduced pressure, and the product was dissolved with two aliquots of toluene (15 mL each). The by-product triethylammonium iodide was separated by centrifugation, and toluene was removed by evaporation at reduced pressure. The addition of diethyl ether (10 mL) caused the formation of a white powder, which was collected by filtration, washed two times with 5 mL of diethyl ether, and dried under vacuum. Yield: 83% (0.577 g).

Characterization of DOPO-N^urea: Anal. calcd for C₁₅H₁₅N₂O₃P (302.26 g mol⁻¹,%): C, 59.60; H, 5.00; N, 9.27. Found (%): C, 59.36; H, 5.05; N, 9.23. ATR-IR (cm⁻¹): 3315 ν_NH, 1687 ν_CO, 1288 ν_P = _O + δ_NH. ¹H NMR (CDCl₃, 300 K) δ 8.07–7.96 (m, 2H, arom-CH+NH), 7.93 (d, 1H, J_HH = 7.9 Hz, arom-CH), 7.74 (dd, 1H, J_PH = 15.9 Hz, J_HH = 7.6 Hz, arom-CH), 7.67 (d, 1H, J_HH = 7.6 Hz, arom-CH), 7.47 (td, 1H, J_HH = 7.6 Hz, J_PH = 3.2 Hz, arom-CH), 7.34 (t, 1H, J_HH = 7.7 Hz, arom-CH), 7.22 (t, 1H, J_HH = 7.7 Hz, arom-CH), 7.18 (d, 1H, J_HH = 8.3 Hz, arom-CH), 2.85 (d, 3H, J_HH = 4.5 Hz, N(H)-CH₃), 2.62 (d, 3H, J_PH = 9.2 Hz, N(P)-CH₃). ³¹P{¹H} NMR (CDCl₃, 300 K) δ 14.5 (s). ¹³C{¹H} NMR (CDCl₃, 300 K) δ 156.2 (d, J_PC = 7.4 Hz, CO), 149.7 (d, J_PC = 7.5 Hz, arom-C_ipso), 136.9 (d, J_PC = 7.3 Hz, arom-C_ipso), 133.8 (d, J_PC = 2.2 Hz, arom-CH), 130.8 (s, arom-CH), 129.8 (d, J_PC = 9.6 Hz, arom-CH), 128.8 (d, J_PC = 15.4 Hz, arom-CH), 124.9 (s, arom-CH), 124.9 (s, arom-CH), 123.7 (d, J_PC = 12.0 Hz, arom-CH), 121.6 (d, J_PC = 174.5 Hz, arom-C_ipso), 120.8 (d, J_PC = 3.1 Hz, arom-C_ipso), 120.3 (d, J_PC = 7.2 Hz, arom-CH), 31.0 (d, J_PC = 4.6 Hz, N(P)-CH₃), 27.5 (s, N(H)-CH₃). UV-VIS (CH₂Cl₂, 298 K, nm): <320, 302 sh, 291, 269, 260. PL (solid, λ_excitation = 255 nm, nm): 383 (FWHM = 5800 cm⁻¹). τ (λ_excitation = 280 nm, λ_emission = 385 nm, ns): 3.7.

2.4. Computational Simulations

Computational simulations were carried out with the r²SCAN-3c method [19], based on the meta-GGA r²SCAN functional combined with a tailor-made triple-ζ Gaussian atomic orbital basis set and D4 and geometrical counter-poise corrections for London dispersion and basis set superposition error. The C-PCM implicit solvation model was added, considering dichloromethane as a continuous medium [20]. IR simulations were carried out using the harmonic approximation. The software used was ORCA version 5.0.3 [21].

3. Results and Discussion

DOPO-N^urea was synthesized with high yield and purity by reacting under mild conditions a solution containing DOPO, DMU, and two equivalents of triethylamine with stoichiometric iodine, according to the reaction in Scheme 1. The salt by-product of the reaction was easily separated from DOPO-N^urea thanks to the different solubility in toluene.

Elemental analysis data agree with the proposed formulation. The ATR-IR spectrum shows a band at 3315 cm⁻¹ attributed to the N-H stretching, while the ν_CO vibration falls at 1687 cm⁻¹. The P=O stretching was assigned to a band at 1288 cm⁻¹ thanks to the simulation of the IR spectrum carried out on the DFT-optimized geometry of the compound (Figure 1). The computed data revealed that the ν_P=O vibration is combined with the bending of the N–H bond, thanks to the presence of an intramolecular hydrogen bond involving the two fragments (with computed H–O and N–H distances equal to 1.914 and 1.014 Å, respectively). The computed geometry revealed that the two N–C bonds are markedly different, since the distance involving the P-substituted nitrogen atom is 1.424 Å, 0.078 Å longer than the C(O)-NHMe one (1.346 Å). Considering other geometrical parameters, the two phosphorus–oxygen bond distances are 1.491 and 1.627 Å, with the shortest one corresponding the phosphoryl fragment. The phosphorus centre is roughly tetrahedral, with the τ₄ parameter [22] being equal to 0.93.

The formation of DOPO-N^urea was confirmed by NMR analysis. In particular, the high-frequency region of the ¹H NMR spectrum showed eight resonances attributable to the biphenyl fragment between 8.1 and 7.1 ppm. The NH resonance overlaps around 8.0 ppm. The two non-equivalent methyl groups fall at 2.85 and 2.62 ppm. The first one is a doublet because of the coupling with the N-bonded hydrogen atom (J_HH = 4.5 Hz), while the second couples with the ³¹P isotope (J_PH = 9.2 Hz), as shown by the ¹H{³¹P} NMR spectrum. No resonance due to hydrogen atoms directly bonded to the phosphorus atom was observed. Only one sharp signal at 14.5 ppm is present in the ³¹P{¹H} NMR spectrum (Figure 2). The ¹³C{¹H} NMR spectrum is composed of twelve resonances between 180.0 and 120.0 ppm, with four corresponding to C_ipso carbon atoms. A doublet at 156.2 ppm with J_PC coupling constant equal to 7.4 Hz was assigned to the carbonyl carbon atom. The two methyl signals fall at 31.0 and 27.5 ppm, and only the first one shows the coupling with the phosphorus centre, with J_PC equal to 4.6 Hz (Figure 3). The NMR data unequivocally indicate the formation of one P–N bond between DOPO and DMU, and only one isomer of the final product is present in CDCl₃ solution.

For what concerns other spectroscopic characterizations, DOPO-N^urea absorbs radiation for wavelengths shorter than 320 nm. Excitation with UV light causes a wide emission centred in the near-ultraviolet range (λ_max = 383 nm), attributed to fluorescent decay on the basis of the excited-state lifetime, in the nanosecond range.

DOPO-N^urea melts without decomposition slightly above 140 °C, a result in line with a sharp endothermic DSC peak centred at 147 °C. The compound starts losing volatile compounds at temperatures above 150 °C. The first decomposition process ends around 240 °C with a residual mass of about 81% (Figure 4). According to recent studies on the thermal decomposition of organic ureas [23], the mass loss is coherent with a proton transfer and elimination of a methyl isocyanate molecule: (C₁₂H₈O₂P)-NMeC(O)NHMe → (C₁₂H₈O₂P)-NHMe + O=C=NMe. The TGA curve indicates that the intermediate compound is unstable and further decompositions with mass loss occur at higher temperatures (Figure 4). The flame-retardant properties of DOPO-N^urea in bio-based polymers are currently under investigation.