Crystal Structure of Bis(2,4,6-trimethylphenyl)-phosphine Oxide

Abstract

The single crystal structure of bis(2,4,6-trimethylphenyl)phosphine oxide has been determined. All interatomic distances and angles can be considered normal. The aryl substituents adopt an intermediate configuration when compared to both sterically unhindered (e.g., diphenylphosphine oxide) and congested (e.g., bis(2,4,6-tri-tert-butylphenyl)phosphine oxide) secondary phosphine oxides, illustrating the influence of steric congestion on the molecular structure.

1. Introduction

The importance of organophosphorus compounds is illustrated by their widespread applications in areas of organic [1,2], inorganic [3,4], medicinal [5,6,7,8], and material [9] chemistries. Over the past decade, the synthetic potential of secondary phosphine oxides has become increasingly well-established. These compounds are precursors to phosphinoyl radicals; they are effective reagents in phosphorylation reactions [10,11,12] due to the accessible P–H bond dissociation energies (ca. 80 kcal mol⁻¹) [13], and also participate in olefin addition reactions [14,15]. Given the utility of these compounds, it seemed pertinent to report the single crystal structure of the title compound bis(2,4,6-trimethylphenyl)phosphine oxide, 1.

2. Results and Discussion

Crystals of the title compound were grown by pentane evaporation, and a crystal structure was obtained (Figure 1). As expected based on spectroscopic data [16] and related secondary phosphine oxides [16,17,18,19], the organophosphorus compound 1 exhibits a tetracoordinate geometry. The hydrogen atom H1 was located in the difference map and allowed to freely refine giving a P1–H1 distance of 1.319(17) Å. The P1–O1 distance measured 1.4854(13) Å and the P1–C1 and P1–C10 distances were 1.8151(18) Å and 1.8162(18) Å, respectively. The mesityl rings exhibit a twist of 31.93(16)° (measured as the dihedral angle between C1–C6–C10–C15) that is intermediate in magnitude when compared to diphenylphosphine oxide [17] (1.2°, relatively unhindered) and bis(2,4,6-tri-tert-butylphenyl)phosphine oxide [19] (52.4°, extremely congested), illustrating the influence of steric congestion on the molecular structure of the compound. In the crystal, intermolecular O∙∙∙H–C contacts were identified between O1, H16C, and H17B with interatomic distances of 2.561(2) Å and 2.653(2) Å, respectively (Figure 2). In the crystal, C–H∙∙∙π interactions were also observed between C6–H8B (2.879(3) Å) and C12–H18C (2.842(3) Å), which resulted in a stacking of the mesityl rings (Figure 2).

The NMR spectroscopic data in CDCl₃ for the title compound were consistent with previous reports [16]. The ¹H NMR spectrum contained resonances at 6.86 (d, ⁴J_HP = 3.7 Hz), 2.39, and 2.28 ppm for the meta-aromatic, ortho-methyl, and para-methyl positions of the mesityl groups, respectively. The P-H resonance was identified at 8.55 ppm as a doublet with a coupling constant of 476 Hz, which is typical for P(V) ¹J_HP constants [18]. Additional evidence was given by the coalescence of this doublet into a singlet at 8.55 ppm in the ¹H{³¹P} NMR spectrum. The ³¹P{¹H} NMR spectrum contained one resonance at 10.1 ppm. All resonances in the ¹³C{¹H} NMR spectrum were observed as doublets. The aromatic region contained resonances at 141.9 (⁴J_CP = 2.7 Hz), 141.7 (²J_CP = 10.4 Hz), 130.5 (³J_CP = 11.0 Hz), and 126.4 ppm (¹J_CP = 100.1 Hz) for the para, ortho, meta, and ipso carbon environments, respectively. Methyl environments were identified at 21.1 (⁵J_CP = 1.0 Hz) and 20.8 ppm (³J_CP = 7.8 Hz) for the ortho and para positions, respectively.

3. Materials and Methods

3.1. Materials

All reagents and non-deuterated solvents were purchased from Sigma-Aldrich (Oakville, ON, Canada) and used without further purification. NMR solvents were purchased from Cambridge Isotope Laboratories, Inc. (Tewksbury, MA, USA) and stored over 4 Å molecular sieves for a minimum of one day prior to use.

3.2. Instrumentation

3.2.1. General Remarks

NMR spectra were recorded at 298 K on a 300 MHz spectrometer (Bruker, Milton, ON, Canada) and are reported in ppm. ¹H-NMR spectra were collected in deuterated solvents and referenced internally to tetramethylsilane (TMS, δ = 0 ppm). The ¹³C{¹H} NMR spectrum was referenced internally to CDCl₃ relative to TMS (δ = 0 ppm). ³¹P NMR chemical shifts were referenced to an external standard of 85% phosphoric acid (δ = 0 ppm). Coupling constants are reported in Hz and given as absolute values. A sample of 1 was prepared as a KBr pellet for IR spectroscopy, and measured using a Bruker ALPHA FT-IR spectrometer. High-resolution mass spectrometry was performed on a micrOTOF from Bruker Daltonics with sample preparation as follows: A sample of 1 was dissolved in dichloromethane (HPLC grade) and diluted (>10,000 times) with methanol (HPLC grade). The sample was introduced to the mass spectrometer via syringe pump at a flow rate of 2 μL min⁻¹ and ionized (electrospray ionization, ESI, positive mode). ESI conditions: spray voltage applied to the ESI needle = 4.0 kV; dry gas flow rate = 4 L min⁻¹; nebulizer gas pressure = 1 Bar; source temperature = 180 °C. The melting point of 1 was recorded on an Electrothermal MEL-Temp 3.0 using a glass capillary sealed under inert conditions and is uncorrected.

3.2.2. X-ray Crystallography

Under inert conditions, crystals were prepared for mounting by suspending them in paratone-N oil on a microscope slide. A single crystal was attached to the tip of an appropriately sized MiTeGen loop with paratone-N oil and cooled to 125 K. Measurements were made on a Bruker APEX2 CCD-equipped diffractometer (30 mA, 50 mV) using monochromated Mo Kα radiation (λ = 0.71073 Å) at 125 K. The initial orientation and unit cell were indexed [20] using a least-squares analysis of a random set of reflections collected from three series of 0.5° wide scans, 10 seconds per frame, and 12 frames per series that were well distributed in reciprocal space. For data collection, the frame length was adjusted to 5 seconds per frame to give a predicted resolution of 0.65 Å, and five ω-scan frame series were collected with 0.5° wide scans and 366 frames per series at varying 2θ, ω, and φ angles (2θ = −28°, ω = −28°, φ = 0°, 90°, 180°, 270°; 2θ = 28° ω = 28°, φ = 0°) and a φ-scan with 0.5° wide scans and 720 frames (2θ, ω, and φ = 0°). Cell refinement and data reduction were performed with the Bruker SAINT software [21] which corrects for beam inhomogeneity, possible crystal decay, and Lorentz and polarization effects. Data processing and a multi-scan absorption correction was applied using APEX3 software package [21]. The structure was solved using direct methods [22] and all non-hydrogen atoms were refined anisotropically using the ShelXLE [23] graphical user interface and SHELXL [24]. Hydrogen atoms (excluding H1) were included at geometrically idealized positions and were fixed (Ar-H) or in the case of methyl groups, the dihedral angle of the idealized tetrahedral CH₃ fragment was allowed to refine. Hydrogen atoms were placed in calculated positions using an appropriate riding model and coupled isotropic temperature factors. H1 was located in the difference map and allowed to freely refine its position, with a coupled isotropic temperature factor of −1.2 to P1. Figures were made using Ortep-3 for Windows [25] and Mercury [26].

3.3. Synthesis

The title compound was prepared from the hydrolysis of bis(2,4,6-trimethylphenyl)phosphorus chloride according to a previously published literature procedure [16]. ³¹P-NMR (CDCl₃): δ 10.1 ppm (d, ¹J_PH = 476 Hz). ¹H-NMR (CDCl₃): δ 8.55 (d, ¹J_HP = 476 Hz, 1H, P-H), 6.86 (d, ⁴J_HP = 3.7 Hz, 4H, m-Ar), 2.39 (s, 12H, o-CH₃), 2.28 ppm (s, 6H, p-CH₃). ¹³C{¹H} NMR (CDCl₃): δ 141.9 (d, ⁴J_CP = 2.7 Hz, para-Ar), 141.7 (d, ²J_CP = 10.4 Hz, ortho-Ar), 130.5 (d, ³J_CP = 11.0 Hz, meta-Ar), 126.4 (d, ¹J_CP = 100.1 Hz, ipso-Ar), 21.1 (d, ⁵J_CP = 1.0 Hz, para-CH₃), 20.8 ppm (d, ³J_CP = 7.8 Hz, ortho-CH₃). IR (KBr pellet): ν 3024 (m), 2957 (vs), 2917 (vs), 2853 (vs), 2737 (m), 2361 (vs, P-H), 2175 (w), 2149 (w), 1950 (w), 1920 (w), 1890 (w), 1764 (w), 1730 (w), 1605 (vs), 1558 (s) 1454 (vs) 1411 (vs), 1378 (vs), 1290 (m), 1274 (w), 1246 (s), 1190 (vs), 1170 (vs), 1082 (vs), 1034 (vs), 1003 (vs), 980 (vs), 959 (m), 914 (s), 850 (vs), 804 (w), 716 (m), 639 (vs), 612 (s), 572 (s), 562 (s), 545 (m), 517 (m), 505 (m), 440 (vs), 426 cm⁻¹ (vs). m.p.: 135.5–141.3 °C. HRMS (ESI TOF): [M + Na⁺]⁺ found 309.1391. [C₁₈H₂₃PO + Na⁺]⁺ requires 309.1384. Crystal data for C₁₈H₂₃OP (M = 286.33 g mol⁻¹): triclinic, space group P-1 (no. 2), a = 7.9123(6) Å, b = 8.2998(6) Å, c = 12.4165(9) Å, α = 99.613(4)°, β = 108.264(4)°, γ = 91.343(4)°, V = 760.94(10) ų, Z = 2, T = 125(2) K, μ(MoKα) = 0.174 mm⁻¹, Dcalc = 1.250 g cm⁻³, 9674 reflections measured (1.757° ≤ 2Θ ≤ 29.210°), 3786 unique (Rint = 0.0373, Rsigma = 0.0553) which were used in all calculations. The final R1 was 0.0467 (I > 2σ(I)) and wR2 was 0.1176 (all data). CCDC 1571849 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html.