Catena-[Triaquabis(μ₂-1,4-bis(diphenylphosphoryl)butane)nitrato-κ²O-praseodymium(III)] Nitrate Monohydrate Methanol Solvate

Abstract

The bidentate ligand, 1,4-bis(diphenlyphosphoryl)butane (dppbO₂), was used to prepare a 1D polymeric Pr(III) complex which was characterised by single-crystal X-ray diffraction.

1. Introduction

The phosphine oxide moiety has had a pronounced impact on an array of chemical industries, from uses in insecticides and fungicides for agricultural means to the use in the inhibition of the corrosion of metals [1,2]. Recently, chiral phosphine oxides have also showcased their novelty in the field of organocatalysis [3,4]. The series of bis(diphenylphosphine) oxides have garnered particular interest in recent years, finding organocatalytic applications in the phosphorylation of aldehydes and the asymmetric hydrosilylation of ketimines [5,6]. The synthesis of bis(diphenylphosphine) oxides presents itself as a straightforward two-step procedure, whereby phosphorus is oxidised to bis-phosphonium salt and subsequently hydrolysed to yield bis(diphenylphosphine) oxide. The plethora of uses of bis-diphenylphosphine oxides and their relatively simple synthesis make for an appealing investigation. A further reason for the investigation of these compounds lies in their potential in lanthanoid extraction. This is attributed to the electrostatic interaction present between the lanthanoid cation and the δ- oxygen atom of the phosphine oxide [7]. The lanthanoids each possess unique chemical properties dependent on their f-orbital occupancy, giving rise to a wide range of applications, for instance, in the automotive industry within catalytic converters [8]. The isolation of lanthanoids is currently dominated by solvent extraction, a costly technique requiring multiple cycles and using copious amounts of solvent [9,10]. Hence, there is a pressing need for more efficient extractant molecules to minimise environmental impacts.

2. Results

The bidentate ligand 1,4-bis(diphenylphosphoryl)butane (dppbO₂) was prepared through a two-step synthesis following the method reported by Philp and Harris et al. [11]. Triphenylphosphine and 1,4-dibromobutane were heated under reflux in N,N-dimethylformamide (DMF). The intermediate phosphonium salt was heated under reflux in alcoholic sodium hydroxide, affording dppbO₂ as an analytically pure white powder after acidic workup (Scheme 1). The obtained melting point (259–260 °C) and ³¹P NMR data (δ_P 32.1 ppm) matched the literature. Additionally, mass spectrometry showed two major clusters in the spectrum attributed to M + Na and 2M + Na (where M = dppbO₂).

Under solvothermal conditions, dppbO₂ has been shown to form binuclear lanthanoid complexes with the general formula [(L)Ln(NO₃)₃-(μ-L)-Ln(NO₃)₃(L)] (L = dppbO₂; Ln = Pr, Nd, Sm, Dy) [12,13,14]. In these, the ligand has two chelating and bridging binding modes, for example, GEFZOH (see Figure S5) [13]. Ligands with only one or two CH₂ groups in the chain, 1,1-bis(diphenylphosphoryl)methane and 1,2-bis(diphenylphosphoryl)ethane, have shown a higher preference for the formation of chelate rings only [15]. This results from the small ligand bite angles and inner-sphere bulk as evaluated by McCann et al. through computational analysis using the de novo molecule-building software, HostDesigner, and density functional theory [16]. For example, lanthanoid complexes with 1,1-bis(diphenylphosphoryl)methane typically form six-membered chelate rings, while larger alkyl chains, e.g., butyl or hexyl, show an increase in inner-sphere bulk and larger bite angles; therefore, alternative coordination modes, such as bridging, are commonly observed [16].

By heating a 1:1 molar ratio solution of dppbO₂ and Pr(NO₃)₃·3H₂O under reflux conditions, cooling to ambient conditions, then layering with diethyl ether, crystals of 1 were formed (Scheme 2). The crystals were of suitable quality for X-ray diffraction and revealed solely non-chelating binding modes. The crystal structure showed a polymeric complex featuring eight-coordinate praseodymium centres, each coordinated by three bridging dppbO₂ ligands, three water ligands, and one bidentate nitrate ion. Alongside the additional nitrate ions necessary for charge balance, there were uncoordinated water, methanol, and dppbO₂ molecules incorporated into the structure, giving the overall formula catena-[{Pr(NO₃)(H₂O)₃-μ-dppbO₂}₂-μ-dppbO₂](NO₃)₄·(dppbO₂)·2MeOH·2H₂O.

3. Discussion

Pr(III) adopts a pseudo-hexagonal bipyramidal geometry with the ‘axial’ positions occupied by the oxygens of two phosphine oxides, giving an O1–Pr1–O2 angle of 165.5(1)°. The third coordinated phosphine oxide is positioned orthogonally to the other two, in a pseudo-equatorial position with the ‘equatorial’ to ‘axial’ phosphine oxide ligand angles being 95.7(1)° and 89.7(1)° for O2–Pr1–O3 and O1–Pr1–O3, respectively. The remaining ‘equatorial’ positions are occupied by one κ-O²-nitrate and three water molecules (Figure 1) as judged by the approximate 90° angles between the water molecules and the axial phosphine oxides (

Table 1. Selected bond lengths (Å) and angles (°) and torsions (°) for 1.

Pr1–O1	2.358(5)	P1–O1	1.504(5)
Pr1–O2	2.358(5)	P2–O2	1.503(5)
Pr1–O3	2.389(3)	P3–O3	1.496(4)
Pr1–O5	2.492(3)	P4–O4	1.497(4)
Pr1–O6	2.499(4)	N1–O11	1.265(7)
Pr1–O7	2.482(4)	N1–O12	1.269(7)
Pr1–O11	2.597(4)	O4⋯H5b	1.75(3)
Pr1–O12	2.588(4)	O4⋯H6b	1.82(4)
O4⋯O5	2.688(5)	O4⋯O6	2.733(5)
O4⋯H6b	1.82(4)	O4⋯H5b	1.75(3)
O1–Pr1–O2	165.5(1)	O5–Pr1–O6	68.6(1)
O1–Pr–O3	89.7(1)	O6–Pr1–O7	70.6(1)
O2–Pr1–O3	95.7(1)
C29–C30–C30–C29	180.0(5)	C1–C2–C3–C4	71.1(6)

).

The Pr–O bonds to the phosphine oxides are all of typical length, ranging from 2.358(5) to 2.389(3) Å, and closely resemble those of [(dppbO₂)Pr(NO₃)₃-(μ-dppbO₂)-Pr(NO₃)₃(dppbO₂)] [16]. The Pr–OH₂ bonds are also of typical length, in the range of 2.482(4)–2.499(4) Å, for example, when compared to [Pr(OH₂)₉]³⁺ which has Pr–O bonds in the range of 2.470(2)–2.583(3) Å [17]. The overall structure of 1 is a one-dimensional polymer propagating down (1 0 0). The tape- or ribbon-like polymer comprises two chains of [Pr(NO₃)(H₂O)₃-μ-dppbO₂], cross-linked by the third μ-dppbO₂ ligand. The butyl chain of the dppbO₂ ligands bridging across the width of the ribbon adopts the ‘ideal’ geometry with a C29–C30–C30–C29 torsion angle of 180.0(5)°, while the butyl chain of ligands running down the length of the ribbon are ‘kinked’ with an equivalent torsion angle of 71.1(6)°. These ribbons interact with the uncoordinated dppbO₂ via hydrogen bonds from the coordinated aqua ligands O5 and O6 (O⋯H 1.75(3) and 1.82(4) Å, O⋯O 2.688(5) and 2.733(5) Å, Figure 2), forming sheets in the (0 1 0) plane (Figure 3). The water and methanol solvates are involved in additional OH⋯O hydrogen bonds between sheets to form the overall packing.

4. Materials and Methods

All synthetic manipulations were performed in air. Glassware was dried in an oven (ca. 110 °C) prior to use. Solvents and chemicals were used as provided without further purification. IR spectra were recorded on a Perkin Elmer Spectrum Two instrument with DTGS detector and a diamond ATR attachment. The HRMS data were acquired from the University of St Andrews Mass Spectrometry Service. All NMR spectra were recorded using a Bruker Avance II 400 (MHz) spectrometer at 20 °C. The ¹³C NMR spectrum was recorded using the DEPTQ-135 pulse sequence with broadband proton decoupling. Tetramethylsilane was used as an internal standard (δ_H, δ_C 0.00 ppm) and 85% H₃PO₄ as an external standard for ³¹P NMR (δ_P 0.00 ppm). Chemical shifts (δ) are given in parts per million (ppm) relative to the TMS peak. Spectra were analysed using the MestReNova software package (version 14).

4.1. Synthesis of Bis(diphenylphosphoryl)butane (dppbO₂)

This was adapted from the literature procedure [11]. Triphenylphosphine (5.00 g, 19 mmol) and 1,4-dibromobutane (1.0 mL, 1.81 g, 8.4 mmol) were added to N,N-dimethylformamide (DMF) (10 mL) and heated under reflux for 1 h. After cooling, diethyl ether (100 mL) was added and the mixture stirred at ambient conditions for 25 min. The resultant solid was collected via vacuum filtration and washed with diethyl ether (3 × 10 mL) before being dried in vacuo. A solution of sodium hydroxide (5.00 g, 125 mmol) was added to 20 mL of 10% v/v methanol/ethanol. Distilled water (5 mL) was added to aid the dissolution of NaOH. The crude solid was added to this in one portion and the mixture heated under reflux for 1 h. The solution was poured into 2 M hydrochloric acid (100 mL) with the resultant solid collected via vacuum filtration and dried in vacuo to afford dppbO₂ (2.85 g, 75%). Mp 259–260 °C. ¹H NMR δ_H (400.3 MHz, CDCl₃, Me₄Si) 7.72–7.63 (8H, m, o-CH), 7.53–7.39 (12H, m, m-CH, p-CH), 2.28–2.18 (4H, m, α-CH), 1.77–1.65 (4H, m, β-CH). ¹³C DEPTQ NMR δ_C (100.6 MHz, CDCl₃, Me₄Si) 132.8 (d, ¹J_CP 98.5 Hz, qCipso), 131.8 (d, ⁴J_CP 2.9 Hz, p-CH), 130.7 (d, ³J_CP 9.4 Hz, m-CH), 128.7 (d, ²J_CP 11.6 Hz, o-CH), 29.4 (d, ¹J_CP 71.6 Hz, α-CH), 22.9 (dd, ³J_CP 16.0, ²J_CP 3.3 Hz, β-CH). ³¹P NMR (162.0 MHz, CDCl₃) 32.1 (s). HRMS (ES+): m/z (%) Calcd. for C₂₈H₂₉O₂P₂: 459.1643, found: 459.1630 [M + H] (25); Calcd. for C₂₈H₂₈O₂P₂Na: 481.1462, found: 481.1446 [M + Na] (95); Calcd. for C₅₆H₅₆O₄P₄Na: 939.3027, found: 939.3003 [2M + Na] (100). IR ν_max (cm⁻¹) 3056 w (ν_CH), 2924 w (ν_CH), 1437 m (ν_CP), 1179s (ν_P=O).

4.2. Synthesis of Catena-[{Pr(NO₃)(H₂O)₃-μ-dppbO₂}₂-μ-dppbO₂](NO₃)₄·(dppbO₂)·2MeOH·2H₂O

A solution of dppbO₂ (50 mg, 0.1 mmol) in methanol (10 mL) was added to a solution of praseodymium nitrate trihydrate (42 mg, 0.1 mmol) in methanol (15 mL). This was heated under reflux for 2 h. After cooling to ambient conditions, diethyl ether was layered onto the solution, and slow evaporation/diffusion overnight afforded colourless crystals of X-ray quality.

X-ray diffraction data for compound 1 were collected at 173 K using a Rigaku FR-X Ultrahigh Brilliance Microfocus RA generator/confocal optics with an XtaLAB P200 diffractometer [Mo Kα radiation (λ = 0.71073 Å)]. Data were collected (using a calculated strategy) and processed (including correction for Lorentz, polarisation, and absorption) using CrysAlisPro [18]. The structure was solved by dual-space methods (SHELXT) [19] and refined by full-matrix least-squares against F² (SHELXL-2019/3) [20]. Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were refined using a riding model except for those bound to O5-7 and O61 which were located from the difference Fourier map and refined isotropically subject to a distance restraint. Hydrogen atoms on O41 were initially placed in calculated positions before the position constraint was removed, and they were refined isotropically subject to distance restraints. For all OH hydrogen atoms, their thermal motion was allowed to ride on their parent oxygen atom. All calculations were performed using the Olex2 interface [21]. Crystal data. C₁₁₄H₁₃₆N₆O₃₆P₈Pr₂, M = 2695.86, triclinic, a = 10.0689(4), b = 13.0689(4), c = 24.8680(7) Å, α = 96.696(2), β = 92.647(2), γ = 109.058(3), Vol. = 3059.32(17) ų, space group P$\overline{1}$ (no. 2), Z = 1, 67719 reflections measured, 28754 unique (R_int = 0.0589), which were used in all calculations. The final R₁ [I > 2σ(I)] was 0.0593, and wR₂ (all data) was 0.1353. CCDC 2370838 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.

5. Conclusions

The bidentate ligand, 1,4-bis(diphenylphosphoryl)butane (dppbO₂), was shown to form an eight-coordinate complex with praseodymium(III) cations in which the dppbO₂ ligands were bridging to form one-dimensional polymers which packed together via hydrogen bonding interactions with uncoordinated dppbO₂, water, and methanol solvates.