Bis(diphenylphosphino)methane Dioxide Complexes of Lanthanide Trichlorides: Synthesis, Structures and Spectroscopy

Bannister, Robert D.; Levason, William; Reid, Gillian

doi:10.3390/chemistry2040060

Open AccessArticle

Bis(diphenylphosphino)methane Dioxide Complexes of Lanthanide Trichlorides: Synthesis, Structures and Spectroscopy^†

by

Robert D. Bannister

,

William Levason

and

Gillian Reid

^*

School of Chemistry, University of Southampton, Southampton SO17 1BJ, UK

^*

Author to whom correspondence should be addressed.

^†

Dedicated to Dr. Howard Flack (1943–2017).

Chemistry 2020, 2(4), 947-959; https://doi.org/10.3390/chemistry2040060

Submission received: 18 October 2020 / Revised: 5 November 2020 / Accepted: 16 November 2020 / Published: 19 November 2020

(This article belongs to the Special Issue Memorial Issue Dedicated to Dr. Howard D. Flack: The Man behind the Flack Parameter)

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Abstract

:

Bis(diphenylphosphino)methane dioxide (dppmO₂) forms eight-coordinate cations [M(dppmO₂)₄]Cl₃ (M = La, Ce, Pr, Nd, Sm, Eu, Gd) on reaction in a 4:1 molar ratio with the appropriate LnCl₃ in ethanol. Similar reaction in a 3:1 ratio produced seven-coordinate [M(dppmO₂)₃Cl]Cl₂ (M = Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb), whilst LuCl₃ alone produced six-coordinate [Lu(dppmO₂)₂Cl₂]Cl. The complexes have been characterised by IR, ¹H and ³¹P{¹H}-NMR spectroscopy. X-ray structures show that [M(dppmO₂)₄]Cl₃ (M = Ce, Sm, Gd) contain square antiprismatic cations, whilst [M(dppmO₂)₃Cl]Cl₂ (M = Yb, Dy, Lu) have distorted pentagonal bipyramidal structures with apical Cl. The [Lu(dppmO₂)₂Cl₂]Cl has a cis octahedral cation. The structure of [Yb(dppmO₂)₃(H₂O)]Cl₃·dppmO₂ is also reported. The change in coordination numbers and geometry along the series is driven by the decreasing lanthanide cation radii, but the chloride counter anions also play a role.

Keywords:

lanthanide trichloride complexes; diphosphine dioxide; coordination complexes; X-ray structures

1. Introduction

Early work viewed the chemistry of the lanthanides (Ln) (Ln = La–Lu, ≠ Pm unless otherwise indicated) in oxidation state III as very similar and often only two or three elements were examined, and the results were assumed to apply to all. More recent work has shown this to be a very unreliable approach and detailed studies of all fourteen elements (excluding only the radioactive Pm) are required to establish properties and trends [1,2]. Sometimes yttrium is also included since it is similar in size to holmium. The main changes along the series are due to the lanthanide contraction, the reduction in the radius of the M³⁺ ions between La (1.22 Å) and Lu (0.85 Å), and at some point a reduction in coordination number may be driven by steric effects, especially with bulky ligands. However, the decrease in radius also results in an increase in the charge/radius ratio along the series and this can lead to significant electronic effects on the ligand preferences. This interplay of steric and electronic effects means that changes in coordination number or ligand donor set can occur at different points along the series with different ligands. The effects are very nicely demonstrated in a recent article, which examined the changes which occurred in the series of lanthanide nitrates with complexes of 2,2′-bipyridyl, 2,4,6-tri-α-pyridyl-1,3,5-triazine and 2,2′; 6′,2”-terpyridine [2]. Tertiary phosphine oxides have proved popular ligands to explore lanthanide chemistry and the area has been the subject of a comprehensive review [3], and several detailed studies of trends along the series La-Lu have been reported [4,5,6,7]. We reported bis(diphenylphosphino)methane dioxide (dppmO₂) formed square-antiprismatic cations [La(dppmO₂)₄]³⁺ with Cl, I or [PF₆] counter ions, but lutetium gave only octahedral [Lu(dppmO₂)₂X₂]⁺ (X = Cl, I) and [Lu(dppmO₂)₂Cl(H₂O)]²⁺ [8]. Other dppmO₂ complexes reported include several types with Ln(NO₃)₃ [4], [Dy(dppmO₂)₄][CF₃SO₃]₃ [9], [Eu(dppmO₂)₄][ClO₄]₃ [10], [La(dppmO₂)₄][CF₃SO₃]₃ and [Lu(dppmO₂)₃(H₂O)][CF₃SO₃]₃ [11]. Here, we report a systematic study of the systems LnCl₃-dppmO₂ for all fourteen accessible lanthanides.

2. Materials and Methods

Infrared spectra were recorded as Nujol mulls between CsI plates using a Perkin-Elmer Spectrum 100 spectrometer over the range 4000–200 cm⁻¹. ¹H and ³¹P{¹H}-NMR spectra were recorded using a Bruker AV–II 400 spectrometer and are referenced to the protio resonance of the solvent and 85% H₃PO₄, respectively. Microanalyses were undertaken by London Metropolitan University or Medac. Hydrated lanthanide trichlorides and anhydrous LnCl₃ (Ln–Nd, Pr, Gd, Ho) were from Sigma-Aldrich and used as received. The Ph₂PCH₂PPh₂ (Sigma-Aldrich) in anhydrous CH₂Cl₂ was converted to Ph₂P(O)CH₂P(O)Ph₂ by air oxidation catalysed by SnI₄ [12].

X-Ray Experimental. Details of the crystallographic data collection and refinement parameters are given in Table 1. Many attempts were made to grow crystals for X-ray examination from a variety of solvents including EtOH and CH₂Cl₂, either by slow evaporation or layering with hexane or pentane. The crystal quality was often rather poor, and all of the structures have disordered co-solvent, either water or ethanol. No attempt was made to locate the protons on the co-solvent. Several showed disorder in one or more of the phenyl rings. Good-quality crystals used for single crystal X-ray analysis were grown from [Lu(dppmO₂)₄}Cl₂]Cl (CH₂Cl₂/hexane), [Ce(dppmO₂)₄]Cl₃, [Sm(dppmO₂)₄]Cl_3, [Gd(dppmO₂)₄]Cl₃ (EtOH), [Yb(dppmO₂)₃Cl]Cl₂, [Yb(dppmO₂)₃(H₂O)]Cl₃·dppmO₂ (EtOH), [Lu(dppmO₂)₃Cl]Cl₂ (CH₂Cl₂).

Data collections used a Rigaku AFC12 goniometer equipped with a HyPix-600HE detector mounted at the window of an FR-E+ SuperBright molybdenum (λ = 0.71073 Å) rotating anode generator with VHF Varimax optics (70 µm focus) with the crystal held at 100 K (N₂ cryostream). Structure solution and refinements were performed with either SHELX(S/L)97 or SHELX(S/L)2013 [13,14]. The crystallographic data in cif format have been deposited as CCDC 2033611-2033618.

All samples were dried in high vacuum at room temperature for several hours, but this treatment does not remove lattice water or alcohol. Heating the samples in vacuo is likely to cause some decomposition of the complexes [7] and was not applied.

[La(dppmO₂)₄]Cl₃·4H₂O and [Lu(dppmO₂)₂Cl₂]Cl·H₂O were made as described [8]. The individual new complexes were isolated as described below, with yields of 50–80%.

[Ce(dppmO₂)₄]Cl₃·6H₂O—CeCl₃·7H₂O (0.025 g, 0.067 mmol) and dppmO₂ (0.112 g, 0.268 mmol) afforded colourless crystals of [Ce(dppmO₂)₄]Cl₃·4H₂O, by concentrating the ethanolic solution and layering with n-hexane (1 mL). Required for C₁₀₀H₁₀₀CeCl₃O₁₂P₈ (2020.1): C, 59.46; H, 4.99%. Found: C, 59.50; H, 4.50%. ¹H-NMR (CD₂Cl₂): δ = 1.52 (s, H₂O) 3.60 (vbr, [8H], PCH₂P), 7.10 (s, [32H], Ph), 7.35 (m, [16H], Ph), 7.70 (m, [32H], Ph). ³¹P{¹H}-NMR (CD₂Cl₂): δ = 48.6 (s). IR (Nujol mull)/cm⁻¹: 3500 br, 1630 (H₂O), 1158, 1099s (P=O).

[Pr(dppmO₂)₄]Cl₃·6H₂O—To a solution of PrCl₃·6H₂O (0.025 g, 0.070 mmol) in ethanol (5 mL) was added a solution of dppmO₂ (0.117 g, 0.281 mmol) in ethanol (10 mL). A white powdered solid formed on slow evaporation of the ethanol. Required for C₁₀₀H₁₀₀Cl₃O₁₄P₈Pr (2020.9): C, 59.43; H, 4.99%. Found: C, 59.06; H, 4.62% ¹H-NMR (CD₂Cl₂): δ = 4.63 (m, [8H], PCH₂P), 7.19 (s, [32H], Ph), 7.44 (m, [16H], Ph), 8.19 (m, [32H], Ph). ³¹P{¹H}-NMR (CD₂Cl₂): δ = 64.0 (s). IR (Nujol mull)/cm⁻¹: 3500 br, 1630 (H₂O), 1161, 1102 (P=O).

[Nd(dppmO₂)₄]Cl₃·4H₂O—To a solution of NdCl₃·6H₂O (0.025 g, 0.070 mmol) in ethanol (5 mL) was added a solution of dppmO₂ (0.116 g, 0.279 mmol) in ethanol (10 mL). A white powdered solid formed on slow evaporation of the ethanol. Required for C₁₀₀H₉₆Cl₃NdO₁₂P₈ (1988.2): C, 60.41; H, 4.87%. Found: C, 60.41; H, 4.62%. ¹H-NMR (CD₂Cl₂): δ = 1.52 (s, H₂O) 3.66 (m, [8H], PCH₂P), 7.14 (s, [32H], Ph), 7.35 (m, [16H], Ph), 7.76 (m, [32H], Ph). ³¹P{¹H}-NMR (CD₂Cl₂): δ = 62.9 (s). IR (Nujol mull)/cm⁻¹: 3500 br, 1630 (H₂O), 1159 s, 1101 s (P=O).

[Sm(dppmO₂)₄]Cl₃·4H₂O—To a solution of SmCl₃·6H₂O (0.025 g, 0.069 mmol) in ethanol (5 mL) was added a solution of dppmO₂ (0.114 g, 0.274 mmol) in ethanol (10 mL). Colourless crystals were formed via slow evaporation of the ethanol. Required for C₁₀₀H₉₆Cl₃O₁₂P₈Sm (1994.3): C, 60.22; H, 4.85%. Found: C, 60.05; H, 4.50%. ¹H-NMR (CD₂Cl₂): δ = 2.10 (s, H₂O), 5.08 (br, [8H], PCH₂P), 7.20 (s, [32H], Ph), 7.39 (m, [16H], Ph), 7.83 (m, [32H], Ph). ³¹P{¹H}-NMR (CD₂Cl₂): δ = 35.6. IR (Nujol mull)/cm⁻¹: 3500 br, 1630 (H₂O), 1162 s, 1101 (P=O).

[Eu(dppmO₂)₄]Cl₃·4H₂O—To a solution of EuCl₃·6H₂O (0.025 g, 0.068 mmol) in ethanol (5 mL) was added a solution of dppmO₂ (0.114 g, 0.274 mmol) in ethanol (10 mL) and the solution was stirred for 20 min. The solution was then concentrated, and colourless crystals were formed through layering with n-hexane (1 mL). Required for C₁₀₀H₉₆Cl₃EuO₁₂P₈ (1995.9): C, 60.41; H, 4.87%. Found: C, 60.73; H, 4.71%. ¹H-NMR (CD₂Cl₂): δ = 2.15 (s, H₂O) 3.12 (br, [8H] PCH₂P), 7.18 (s, [32H], Ph), 7.38 (m, [16H], Ph), 7.83 (m, [32H], Ph). ³¹P{¹H}-NMR (CD₂Cl₂): δ = 25.0 (br, “free” dppmO₂), −13.4. IR (Nujol mull)/cm⁻¹: 3500 br, 1630 (H₂O), 1159, 1099 (P=O).

[Gd(dppmO₂)₄]Cl₃·4H₂O—To a solution of GdCl₃·6H₂O (0.025 g, 0.067 mmol) in ethanol (5 mL) was added a solution of dppmO₂ (0.112 g, 0.269 mmol) in ethanol (10 mL). Colourless crystals were formed through slow evaporation of the solvent. Required for C₁₀₀H₉₆Cl₃GdO₁₂P₈ (2001.2): C, 60.02; H, 4.83%. Found: C, 60.05; H, 4.86%. ¹H-NMR (CD₂Cl₂): δ = no resonance. ³¹P{¹H}-NMR (CD₂Cl₂): δ = no resonance. IR (Nujol mull)/cm⁻¹: 3500 br, 1630 (H₂O), 1160, 1099 (P=O).

[Sm(dppmO₂)₃Cl]Cl₂—To a solution of SmCl₃·6H₂O (0.025 g, 0.069 mmol) in ethanol (5 mL) was added a solution of dppmO₂ (0.086 g, 0.206 mmol) in ethanol (10 mL). The solvent was removed in vacuo and the resulting white solid was washed with cold ethanol. Colourless crystals were obtained via slow evaporation of an ethanolic solution of the product. Required for C₇₅H₆₆Cl₃O₆P₆Sm (1505.9): C, 59.80; H, 4.42%. Found: C, 59.62; H, 4.55%. ¹H-NMR (CD₂Cl₂): δ = 3.67 (br m, [6H], PCH₂P), 7.15 (br, [24H], Ph), 7.35 (m, [12H], Ph), 8.05 (m, [24H], Ph). ³¹P{¹H}-NMR (CD₂Cl₂): δ = 38.15 (s). IR (Nujol mull)/cm⁻¹: 1153 s, 1097 s (P=O).

[Eu(dppmO₂)₃Cl]Cl₂—To a solution of EuCl₃·6H₂O (0.025 g, 0.068 mmol) in ethanol (5 mL) was added a solution of dppmO₂ (0.085 g, 0.205 mmol) in ethanol (10 mL). The solvent was removed in vacuo and the resulting white solid was washed with cold ethanol. Required for C₇₅H₆₆EuCl₃O₆P₆ (1507.49): C, 59.76; H, 4.41%. Found: C, 59.71; H, 4.56%. ¹H-NMR (CDCl₃): δ = 3.66 (br, [6H], PCH₂P), 7.03 (br m, [36H], Ph), 7.87 (br, [24H], Ph). ³¹P{¹H}-NMR (CDCl₃): δ = −14.8 (s). IR (Nujol mull)/cm⁻¹: 1153 s, 1098 s (P=O).

[Gd(dppmO₂)₃Cl]Cl₂·3H₂O—To a solution of GdCl₃·6H₂O (0.025 g, 0.067 mmol) in ethanol (5 mL) was added a solution of dppmO₂ (0.084 g, 0.201 mmol) in ethanol (10 mL). The solvent was removed in vacuo and the resulting white solid was washed with cold ethanol. Required for C₇₅H₆₆Cl₃O₆P₆Gd (166.8): C, 57.49, H, 4.63%; Found: C, 57.17; H, 4.43%. ¹H-NMR (CD₂Cl₂): no resonance. ³¹P{¹H}-NMR (CD₂Cl₂): no resonance. IR (Nujol mull)/cm⁻¹: 3500 br, 1630 (H₂O), 1155 s, 1098 s (P=O).

[Tb(dppmO₂)₃Cl]Cl₂·H₂O—To a solution of TbCl₃·6H₂O (0.025 g, 0.067 mmol) in ethanol (5 mL) was added a solution of dppmO₂ (0.084 g, 0.201 mmol) in ethanol (10 cm³). The solvent was removed in vacuo and the resulting white solid was washed with cold ethanol. Required for C₇₅H₆₈Cl₃O₇P₆Tb (1532.5): C, 58.78; H, 4.47%. Found: C, 59.41; H, 4.54%. ¹H-NMR (CD₂Cl₂): δ = 1.9 (br H₂O), 3.50 (br m, [6H], PCH₂P), 5.89 (br, [36H], Ph), 7.46 (br, [24H], Ph). ³¹P{¹H}-NMR (CD₂Cl₂): δ = −29.2 (s). IR (Nujol mull)/cm⁻¹: 3500 br, 1630 (H₂O), 1153 s, 1097 s (P=O).

[Dy(dppmO₂)₃Cl]Cl₂·H₂O—To a solution of TbCl₃·6H₂O (0.025 g, 0.066 mmol) in ethanol (5 mL) was added a solution of dppmO₂ (0.083 g, 0.199 mmol) in ethanol (10 cm³). The solution was filtered then concentrated and layered with hexane (1 mL) yielding a white powdered product. Colourless crystals were formed by layering a CH₂Cl₂ solution of the product with hexane. Required for C₇₅H₆₈Cl₃DyO₇P₆ (1536.0): C, 58.64; H, 4.46%. Found: C, 58.21; H, 4.63%. ¹H-NMR (CD₂Cl₂): δ = 1.9 (vbr H₂O), 3.66 (br m, [6H], PCH₂P), 7.33 (br, [36H], Ph), 8.66 (br, [24H], Ph). ³¹P{¹H}-NMR (CD₂Cl₂): δ = 18 (vbr, s). IR (Nujol mull)/cm⁻¹: 3500 br, 1630 (H₂O), 1156 s, 1099 s (P=O).

[Ho(dppmO₂)₃Cl]Cl₂·H₂O—To a solution of HoCl₃ (0.050 g, 0.124 mmol) in ethanol (5 mL) was added a solution of dppmO₂ (0.230 g, 0.55 mmol) in ethanol (10 mL). The solvent was removed in vacuo and the resulting pale pink solid was washed with cold ethanol. Required for C₇₅H₆₈Cl₃HoO₇P₆ (1538.5): C, 58.66; H, 4.55%. Found: C, 59.41; H, 4.52%. ¹H-NMR (CD₂Cl₂): δ = 2.1 (br, H₂O), 3.72 (br s, [6H], PCH₂P), 6.78 (br, [36H], Ph), 7.68 (br, [24H], Ph)]. ³¹P{¹H}-NMR (CD₂Cl₂): δ = −13.5 (s). IR (Nujol mull)/cm⁻¹: 3500 br, 1630 (H₂O), 1154 s, 1097 s (P=O).

[Er(dppmO₂)₃Cl]Cl₂·3H₂O—To a solution of ErCl₃·6H₂O (0.025 g, 0.065 mmol) in ethanol (5 mL) was added a solution of dppmO₂ (0.082 g, 0.196 mmol) in ethanol (10 mL). The solvent was removed in vacuo and the resulting white solid was washed with cold ethanol. Required for C₇₅H₇₂Cl₃ErO₉P₆ (1576.8): C, 57.13; H, 4.60%. Found: C, 57.08; H, 4.54%. ¹H-NMR (CD₂Cl₂): δ = δ = 1.2 (br, H₂O), 3.25 (br s, [6H], PCH₂P), 5.52 (vbr, [12H], Ph), 7.15 (br s, [24H], Ph)], 7.28 (br s, [24H], Ph)]. ³¹P{¹H}-NMR (CD₂Cl₂): δ = −60.8 (s). IR (Nujol mull)/cm⁻¹: 3500 br, 1630 (H₂O), 1155 s, 1097 s (P=O).

[Tm(dppmO₂)₃Cl]Cl₂·3H₂O—To a solution of TmCl₃·6H₂O (0.025 g, 0.065 mmol) in ethanol (5 mL) was added a solution of dppmO₂ (0.081 g, 0.195 mmol) in ethanol (10 mL). The solvent was removed in vacuo and the resulting white solid was washed with cold ethanol. Required for C₇₅H₇₂Cl₃O₉P₆Tm (1578.5): C, 57.07; H, 4.60%. Found: C, 56.61; H, 4.45%. ¹H-NMR (CD₂Cl₂): δ = 3.48 (m, [6H], PCH₂P), 7.11 (br, [24H], Ph), 7.68 (br, [36H], Ph)]. ³¹P{¹H}-NMR (CD₂Cl₂): δ = −54.8 (s). IR (Nujol mull)/cm⁻¹: 3500 br, 1630 (H₂O), 1156 s, 1096 s (P=O).

[Yb(dppmO₂)₃Cl]Cl₂·H₂O—To a solution of YbCl₃·6H₂O (0.025 g, 0.065 mmol) in ethanol (5 mL) was added a solution of dppmO₂ (0.080 g, 0.194 mmol) in ethanol (10 mL). The solvent was removed in vacuo and the resulting white powder was washed with cold ethanol. Required for C₇₅H₆₈Cl₃O₇P₆Yb (1546.58): C, 58.24; H, 4.43%. Found: C, 58.73; H, 4.45%. ¹H-NMR (CD₂Cl₂): δ = 3.50 (m, [6H], PCH₂P), 6.64 (br, [24H], Ph), 7.15 (br, [36H], Ph). ³¹P{¹H}-NMR (CD₂Cl₂): δ = +9.2 (s). IR (Nujol mull)/cm⁻¹: 3500 br, 1630 (H₂O), 1154 s, 1097 s (P=O).

3. Results

The reaction of LnCl₃·nH₂O (Ln = La [8], Ce, Pr, Nd, Sm, Eu or Gd; n = 6 or 7) with four mol. equivalents of dppmO₂ in ethanol gave good yields of tetrakis-dppmO₂ complexes, [Ln(dppmO₂)₄]Cl₃. The IR and ¹H-NMR spectra show the the isolated complexes retain significant amounts of lattice water, and sometimes EtOH, which is not removed by prolonged drying of the bulk powders in vacuo. The high molecular weights make the microanalyses rather insensitive to the amount of water, but are generally consistent with a formulation [Ln(dppmO₂)₄]Cl₃·nH₂O (n = 6: Ce, Pr; n = 4: Nd, Sm, Eu, Gd), although the amount of lattice solvent probably varies with the sample and is unlikely to be stoichiometric. The presence of significant amounts of lattice solvent is common in lanthanide phosphine oxide systems [7,8,9,10], and although evident in X-ray crystal structures, it is often disordered and difficult to model. Obtaining good quality crystals of the complexes proved difficult, but crystals of the Ce, Sm and Gd salts were obtained from various organic solvents and the compositions are shown in Table 1. The crystals contain different amounts of solvent of crystallisation to the bulk samples as they were grown from different media (and crystals were not dried in vacuo). The IR spectra (Table 2) show that the υ(PO) stretch in dppmO₂ at 1187 cm⁻¹ has been lost and replaced by a new very strong and broad band ~1160 cm⁻¹ and a second band at ~ 1100 cm⁻¹, which are due to the coordinated phosphine oxide groups. The frequencies appear invariant with the lanthanide present, which may be due to small differences being obscurred by the width of the bands. In [LnCl₃(OPPh₃)₃] and [LnCl₂(OPPh₃)₄]⁺ the frequency of the υ(PO) stretch increases by ~ 10 cm⁻¹ between La and Lu [7]. The ³¹P{¹H}-NMR chemical shift of dppmO₂ at δ = +25.3 shows a high frequency shift to +33.1 in [La(dppmO₂)₄]Cl₃, whilst the corresponding spectra of the Ce, Pr, Nd and Sm complexes show larger shifts due to the presence of the paramagnetic lanthanide ion (Table 2). In contrast, although the solid [Eu(dppmO₂)₄]Cl₃ complex was isolated without difficulty, the ³¹P{¹H}-NMR spectrum shows a strong feature at δ ~ +25 (“free” dppmO₂), along with a second resonance at δ = −13.4, which may be assigned to [Eu(dppmO₂)₃Cl]Cl₂ (see below), indicating substantial dissociation of one dppmO₂ in solution; the broad resonance of the free dppmO₂ is indicative of exchange on the NMR timescale. [Gd(dppmO₂)₄]Cl₃ was isolated, and its constitution confirmed by its X-ray crystal structure, but no ¹H or ³¹P{¹H}-NMR resonances were observed, an effect seen in other gadolinium systems [6,7] and ascribed to fast relaxation by the f⁷ configuration of the metal. Attempts to isolate [Ln(dppmO₂)₄]Cl₃ complexes for Ln = Dy-Lu were unsuccessful. We note that [Dy(dppmO₂)₄][CF₃SO₃]₃ [9] was isolated with triflate counter ions, but with chloride only [Dy(dppmO₂)₃Cl]Cl₂ was produced (below). An in situ ³¹P{¹H}-NMR spectrum of CeCl₃·7H₂O + 2 dppmO₂ in CH₂Cl₂ showed a single resonance at δ = +48, which is consistent with formation of [Ce(dppmO₂)₄]³⁺, confirming the preference for formation of the tetrakis complexes early in the series, even when there is a deficit of ligand.

The X-ray structures of [Ce(dppmO₂)₄]Cl₃ (Figure 1), [Sm(dppmO₂)₄]Cl₃ (Figure 2) and [Gd(dppmO₂)₄]Cl₃ (Figure 3) show distorted square antiprismatic cations, very similar to those in [La(dppmO₂)₄][PF₆]₃ [8] and [Nd(dppmO₂)₄]Cl₃ [15]. The average Ln-O distances in this series are: La = 2.514 Å, Ce = 2.486 Å, Nd = 2.465 Å, Sm = 2.429 Å and Gd = 2.420 Å, correlating well with the decreasing Ln³⁺ radii (La = 1.216 Å, Ce =1.196 Å, Nd = 1.163 Å, Sm = 1.132 Å, Gd = 1.107 Å). The P = O bond lengths and the O-Ln-O chelate angles do not vary significantly along the series. The Ce-O(P) distances in [Ce(dppmO₂)₄]Cl₃ are markedly longer than those in [Ce(Me₃PO)₄(H₂O)₄]Cl₃ (2.372(2)-2.423(2) Å) [16], which has a distorted dodecahedral geometry with a CeO₈ donor set.

The reaction of LnCl₃·6H₂O (Ln =Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) with 3 mol. equivalents of dppmO₂ in EtOH, followed by concentration of the solution or precipitation with hexane, afforded [Ln(dppmO₂)₃Cl]Cl₂ complexes. Examination of the IR and ¹H-NMR spectra indicated these incorporated less water or ethanol lattice solvent molecules than the [Ln(dppmO₂)₄]Cl₃, and this was confirmed by the microanalyses. The Sm and Eu complexes appear largely free of solvent of crystallisation, whilst the Tb, Ho and Yb approximate to [Ln(dppmO₂)₃Cl]Cl₂·H₂O, and the Gd, Er and Tm complexes are [Ln(dppmO₂)₃Cl]Cl₂·3H₂O; again, this is likely to vary from sample to sample and with the isolation method. The IR spectra (Table 2) show the two υ(PO) bands as in the tetrakis complexes, but the higher energy bands of the tris complexes are ~ 5–10 cm⁻¹ lower in frequency than in the former. We were unable to identify υ(Ln-Cl) vibrations in the far IR spectra. The ³¹P{¹H}-NMR spectra of the [Ln(dppmO₂)₃Cl]Cl₂ show single resonances to high or low frequency of dppmO₂ depending on the fⁿ configuration of the Ln ion present (Table 2) and are generally similar to those found in other systems [5,6,7], although the magnitude of the shifts varies widely with the specific fⁿ configuration. The line broadening is also highly variable between complexes of different Ln ions. The addition of dppmO₂ to a solution of [Ln(dppmO₂)₃Cl]Cl₂ (Ln = Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) in CH₂Cl₂ showed ³¹P{¹H}-NMR resonances assignable to “free” dppmO₂ and [Ln(dppmO₂)₃Cl]Cl₂, but no new resonances that could be attributed to the formation of significant amounts of [Ln(dppmO₂)₄]³⁺. Although the resonances are broad in some cases, the observed chemical shifts are identical to those in pure [Ln(dppmO₂)₃Cl]Cl₂. For [Sm(dppmO₂)₃Cl]Cl₂ δ(³¹P{¹H}) = 38, the resonance shifts to δ = 35.6 upon addition of dppmO₂, attributable to the formation of [Sm(dppmO₂)₄]Cl₃, showing that both tris- and tetrakis-dppmO₂ complexes exist in solution for samarium in the presence of the appropriate amount of ligand.

The X-ray structures of [Er(dppmO₂)₃Cl]Cl₂ (Er-O = 2.28 Å av.) [17], [Yb(dppmO₂)₃Cl]Cl₂ (Figure 4; Yb-O = 2.28 Å av.) and [Dy(dppmO₂)₃Cl]Cl₂ (Figure S43) show pentagonal bipyramidal cations with an apical chloride. The Ln-O distances are rather variable (Er-O = 2.244(6)–2.328(6) Å; Yb-O = 2.250(2)–2.269(3) Å), but are shorter than those in the tetrakis-dppmO₂ cations, reflecting both the reduced coordination number and the smaller metal ion radii (Er = 1.062, Yb = 1.042 Å). The contraction in ionic radii is also evident in the Ln-Cl distances of 2.598(2) Å (Er) and 2.5829(9) Å (Yb). Crystals of [Dy(dppmO₂)₃Cl]Cl₂ were also obtained and show the same cation type, but during refinement, several of the phenyl rings exhibited severe disorder and the data are therefore not included here (Figure S43).

Lutetium was previously reported to form the only bis-dppmO₂ complex, [Lu(dppmO₂)₂Cl₂]Cl, in this series [8], and this has now been confirmed by the X-ray crystal structure which shows a cis-octahedral geometry (Figure 5). The Lu-O distance of 2.230 Å (av) is shorter than the Ln-O distances in the seven- or eigth-coordinate complexes, and correlates both with the reduced coordination number and the smaller radius of Lu³⁺ (1.032 Å). Treatment of a CH₂Cl₂ solution of [Lu(dppmO₂)₂Cl₂]Cl with dppmO₂ caused the ³¹P{¹H}-NMR resonance to shift from +40 to +38.3, which suggests that [Lu(dppmO₂)₃Cl]Cl₂ forms in solution. A few crystals of this product were isolated from a mixture containing excess dppmO₂. These showed a pentagonal bipyramidal dication (Figure 6). As expected, the Lu-Cl and Lu-O bond lengths are slightly longer than in the six-coordinate cation, but are shorter than the corresponding bonds in [Yb(dppmO₂)₃Cl]Cl₂, showing that the expected contraction continues along the series. The complex, [Lu(dppmO₂)₃(H₂O)][CF₃SO₃]₃, is known and its X-ray crystal structure showed seven-coordinate lutetium [11]. Although not confirmed by an X-ray structure, yttrium is reported to form a six-coordinate complex, [Y(dppmO₂)₂Cl₂]Cl [18].

A different crystal isolated from the YbCl₃-dppmO₂ reaction proved, on structure solution, to be [Yb(dppmO₂)₃(H₂O)]Cl₃·dppmO₂·12H₂O (Figure 7), which contains a seven-coordinate Yb centre coordinated to three dppmO₂ and a water molecule, with the Lu-coordinated water hydrogen-bonded to an adjacent uncoordinated dppmO₂ molecule. The geometry is best described as a very distorted pentagonal bipyramid with the water occupying an equatorial position and is similar to the geometry found in [Lu(dppmO₂)₃(H₂O)][CF₃SO₃]₃ [11]. The Yb-OH₂ distance of 2.3263(14) Å is ~ 0.05 Å longer than the Yb-O(P).

A large number of disordered solvate water molecules were also present, which proved very difficult to model, but the geometry of the ytterbium cation is clearly defined.

4. Discussion

The chemistry of dppmO₂ with lanthanides described in the previous section proves to be very systematic along the series La–Lu. For La–Gd, it was possible to isolate [Ln(dppmO₂)₄]Cl₃. Although it could be isolated in the solid state, the solution ³¹P-NMR spectroscopic data indicate that [Eu(dppmO₂)₄]Cl₃ was largely dissociated in CH₂Cl₂ solution into [Eu(dppmO₂)₃Cl]²⁺ and dppmO₂; the isolation of the tetrakis-dppmO₂ complex no doubt resulting from it being the least soluble species in an exchanging mixture in solution, although present in very minor amounts. The case of [Gd(dppmO₂)₄]Cl₃ is likely to be similar, although the fast relaxation of the f⁷ ion precluded ³¹P-NMR study. For the elements Sm-Yb, the complexes [Ln(dppmO₂)₃Cl]Cl₂ were readily isolated, but only for samarium was it possible to convert [Ln(dppmO₂)₃Cl]Cl₂ to [Ln(dppmO₂)₄]Cl₃ in CH₂Cl₂ solution by treatment with more dppmO₂. Similarly, at the end of the series, the complex isolated was [Lu(dppmO₂)₂Cl₂]Cl, for which treatment with dppmO₂ afforded a new species in solution, identified as [Lu(dppmO₂)₃Cl]Cl₂ by a structure determination from a few crystals obtained in the presence of excess dppmO₂, although a bulk sample could not be isolated [8]. The change from eight-coordination in [Ln(dppmO₂)₄]Cl₃ at the beginning of the series, to seven-coordination from Sm onwards, and finally to six-coordination at Lu, parallels the reduction in Ln³⁺ radii. Isolation of both the eight- and seven-coordinate complexes was possible only for Sm, Eu and Gd. However, one should note that the chloride counter ions also have some role, in that whilst in the LnCl₃/dppmO₂ series tetrakis-dppmO₂ species did not form beyond Gd, the complex [Dy(dppmO₂)₄][CF₃SO₃]₃ [9] has been isolated from dmf solution with triflate counter ions. The role that anions and solvents play in lanthanide chemistry is often overlooked [2], but can be critical in determining which complex is isolated from solution. For example, the reaction of LnCl₃ with Ph₃PO results in isolation of [Ln(Ph₃PO)₃Cl₃] from acetone, but [Ln(Ph₃PO)₄Cl₂]Cl from ethanol [7]. On further examination by ³¹P-NMR spectroscopy, both species were found to be present in either solvent (in varying amounts), and the form isolated reflected the least soluble complex in the particular solvent, which then precipitated from the mixture of rapidly interconverting species.

5. Conclusions

Through this synthetic, structural and spectroscopic study of the coordination of dppmO₂ to the lanthanide trichlorides, we have established where the switch from eight-, to seven-, to six-coordination at the Ln(III) centre occurs along the lanthanide series, with X-ray crystallographic authentication for representative examples. The data also reveal subtle, but systematic, variations in the spectroscopic (e.g., ν(PO)) and structural parameters across the series, reflecting the change in ionic radii, the charge:radius ratio and also the influence of the presence of the competitive chloride ions.

Supplementary Materials

The following are available online at https://www.mdpi.com/2624-8549/2/4/60/s1; Figure S1–¹H-NMR spectrum of [Ce(dppmO₂)₄]Cl₃ in CD₂Cl₂; Figure S2–³¹P{¹H} spectrum of Ce(dppmO₂)₄]Cl₃ in CD₂Cl₂; Figure S3–Infrared spectrum of [Ce(dppmO₂)₄]Cl₃ (Nujol mull); Figure S4–¹H-NMR spectrum of [Pr(dppmO₂)₄]Cl₃ in CD₂Cl₂; Figure S5–³¹P{¹H}-NMR spectrum of [Pr(dppmO₂)₄]Cl₃ in CD₂Cl₂; Figure S6–Infrared spectrum of [Pr(dppmO₂)₄]Cl₃ (Nujol mull); Figure S7—¹H-NMR spectrum of [Nd(dppmO₂)₄]Cl₃ in CD₂Cl₂; Figure S8—³¹P{¹H}-NMR spectrum of [Nd(dppmO₂)₄]Cl₃ in CD₂Cl₂; Figure S9—Infrared spectrum of [Nd(dppmO₂)₄]Cl₃ (Nujol mull); Figure S10—¹H-NMR spectrum of [Sm(dppmO₂)₄]Cl₃ in CD₂Cl₂; Figure S11—³¹P{¹H}-NMR spectrum of [Sm(dppmO₂)₄]Cl₃ in CD₂Cl₂; Figure S12—Infrared spectrum of [Sm(dppmO₂)₄]Cl₃ (Nujol mull); Figure S13—¹H-NMR spectrum of [Eu(dppmO₂)₄]Cl₃ in CD₂Cl₂ ; Figure S14—³¹P{¹H}-NMR spectrum of [Eu(dppmO₂)₄]Cl₃ in CD₂Cl₂; Figure S15—Infrared spectrum of [Eu(dppmO₂)₄]Cl₃ (Nujol mull); Figure S16—Infrared spectrum of [Gd(dppmO₂)₄]Cl₃ (Nujol mull); Figure S17—¹H-NMR spectrum of [SmCl(dppmO₂)₃]Cl₂ in CD₂Cl₂ (* = EtOH); Figure S18—³¹P{¹H}-NMR spectrum of [SmCl(dppmO₂)₃]Cl₂ in CD₂Cl₂; Figure S19—Infrared spectrum of [SmCl(dppmO₂)₃]Cl₂ (Nujol mull); Figure S20—¹H-NMR spectrum of [EuCl(dppmO₂)₃]Cl₂ in CDCl₃; Figure S21—³¹P{¹H}-NMR spectrum of [EuCl(dppmO₂)₃]Cl₂ in CDCl₃; Figure S22—³¹P{¹H}-NMR spectrum of [EuCl(dppmO₂)₃]Cl₂ + excess dppmO₂ in CDCl₃; Figure S23—Infrared spectrum of [EuCl(dppmO₂)₃]Cl₂ (Nujol mull); Figure S24—Infrared spectrum of [GdCl(dppmO₂)₃]Cl₂ (Nujol mull); Figure S25—¹H-NMR spectrum of [TbCl(dppmO₂)₃]Cl₂ in CD₂Cl₂ (* = EtOH); Figure S26—³¹P{¹H}-NMR spectrum of [TbCl(dppmO₂)₃]Cl₂ in CD₂Cl₂; Figure S27—Infrared spectrum of [TbCl(dppmO₂)₃]Cl₂ (Nujol mull); Figure S28—¹H-NMR spectrum of [DyCl(dppmO₂)₃]Cl₂ in CD₂Cl₂; Figure S29—³¹P{¹H}-NMR spectrum of [DyCl(dppmO₂)₃]Cl₂ in CD₂Cl₂; Figure S30—Infrared spectrum of [DyCl(dppmO₂)₃]Cl₂ (Nujol mull); Figure S31—¹H-NMR spectrum of [HoCl(dppmO₂)₃]Cl₂ in CD₂Cl₂ (* = EtOH); Figure S32—³¹P{¹H}-NMR spectrum of [HoCl(dppmO₂)₃]Cl₂ in CD₂Cl₂; Figure S33—Infrared spectrum of [HoCl(dppmO₂)₃]Cl₂ (Nujol mull); Figure S34—¹H-NMR spectrum of [ErCl(dppmO₂)₃]Cl₂ in CD₂Cl₂ (* = EtOH); Figure S35—³¹P{¹H}-NMR spectrum of [ErCl(dppmO₂)₃]Cl₂ in CD₂Cl₂; Figure S36—Infrared spectrum of [ErCl(dppmO₂)₃]Cl₂ (Nujol mull); Figure S37—¹H-NMR spectrum of [TmCl(dppmO₂)₃]Cl₂ in CD₂Cl₂ (* = EtOH); Figure S38—³¹P{¹H}-NMR spectrum of [TmCl(dppmO₂)₃]Cl₂ in CD₂Cl₂; Figure S39—Infrared spectrum of [TmCl(dppmO₂)₃]Cl₂ (Nujol mull); Figure S40—¹H-NMR spectrum of [YbCl(dppmO₂)₃]Cl₂ in CD₂Cl₂ (* = EtOH); Figure S41—³¹P{¹H}-NMR spectrum of [YbCl(dppmO₂)₃]Cl₂ in CD₂Cl₂; Figure S42—Infrared spectrum of [YbCl(dppmO₂)₃]Cl₂ (Nujol mull); Figure S43—The cation in [DyCl(dppmO₂)₃]Cl₂. The chloride anions and solvate molecules are omitted.

Author Contributions

Conceptualization, R.D.B., W.L. and G.R.; formal analysis, R.D.B.; investigation, R.D.B.; data curation, R.D.B.; writing—original draft preparation, R.D.B. and W.L.; writing—review and editing, R.D.B., W.L. and G.R.; supervision, W.L. and G.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The cation in [Ce(dppmO₂)₄]Cl₃. The chloride anions and solvate molecules are omitted. Selected bond lengths (Å): Ce1–O1 = 2.4874(14), Ce1–O2 = 2.4790(14), Ce1–O3 = 2.4967(13), Ce1–O4 = 2.4803(14), P1–O1 = 1.5031(14), P2–O2 = 1.5021(14), P3–O3 = 1.5018(14), P4–O4 = 1.5031(14). Chelate angle O-Ce-O = 73.1° (av).

Figure 2. The cation in [Sm(dppmO₂)₄]Cl₃. The chloride anions and solvate molecules are omitted. Selected bond lengths (Å): Sm1–O1 = 2.4160(14), Sm1–O2 = 2.4400(15), Sm1–O3 = 2.4358(14), Sm1–O4 = 2.4268(15), P1–O1 = 1.5025(15), P2–O2 = 1.5019(15), P3–O3 = 1.4961(15), P4–O4 = 1.4961(16). Chelate angle O-Sm-O = 72.9° (av).

Figure 3. The cation in [Gd(dppmO₂)₄]Cl₃. The chloride anions and solvate molecules are omitted. Selected bond lengths (Å): Gd1–O1 = 2.420(2), Gd1–O2 = 2.409(3), Gd1–O3 = 2.415(2), Gd1–O4 = 2.398(2), P1–O1 = 1.504(2), P2–O2 = 1.501(3), P3–O3 = 1.501(3), P4–O4 = 1.501(3). Chelate angle O-Sm-O = 73.1° (av).

Figure 4. The X-ray structure of [Yb(dppmO₂)₃Cl]Cl₂. The chloride anions and solvate molecules are omitted. Selected bond lengths (Å) and angles (°): Yb1–Cl1 = 2.5834(9), Yb1–O1 = 2.298(3), Yb1–O2 = 2.282(3), Yb1–O3 = 2.250(2), Yb1–O4 = 2.248(2), Yb1–O5 = 2.338(2), Yb1–O6 = 2.269(3), P–O = 1.494(3)-1.509(3), Cl1–Yb1–O4 = 173.89(9), O1–Yb1–O2 = 73.79(9), O3–Yb1–O4 = 80.64(9), O5–Yb1–O6 = 73.95(9).

Figure 5. The cation in [Lu(dppmO₂)₂Cl₂]Cl. Selected bond lengths (Å) and angles (°): Lu1–Cl1 = 2.5581(8), Lu1–Cl2 = 2.5163(7), Lu1–O1 = 2.227(2), Lu1–O2 = 2.227(2), Lu1–O3 = 2.274(2), Lu1–O4 = 2.200(2), P1–O1 = 1.510(2), P2–O2 = 1.506(2), P3–O3 = 1.513(2), P4–O4 = 1.507(2), Cl2–Lu1–Cl1 = 95.97(3), O1–Lu1–Cl1 = 97.27(6), O1–Lu1–Cl2 = 99.68(6), O1–Lu1–O2 = 81.56(8), O1–Lu1–O3 = 85.65(7), O2–Lu1–Cl2 = 91.94(5), O2–Lu1–O3 = 84.89(7), O3–Lu1–Cl1 = 87.22(6), O4–Lu1–Cl1 = 94.66(6), O4–Lu1–Cl2 = 92.55(6), O4–Lu1–O2 = 84.74(8), O4–Lu1–O3 = 81.36(7).

Figure 6. The X-ray structure of [Lu(dppmO₂)₃Cl]Cl₂. The chloride anions and solvate molecules are omitted. Selected bond lengths (Å) and angles (°): Lu1–Cl1 = 2.5604(7), Lu1–O1 = 2.341(5), Lu1–O2 = 2.268(5), Lu1–O3 = 2.268(5), Lu1–O4 = 2.297(5), Lu1–O5 = 2.354(5), Lu1–O6 = 2.227(5), P–O = 1.497(5)-1.510(5), Cl1–Lu1–O6 = 176.35(14), O1–Lu1–O2 = 73.37(17), O3–Lu1–O4 = 73.37(17), O5–Lu1–O6 = 83.45(18).

Figure 7. The cation in [Yb(dppmO₂)₃(H₂O)]Cl₃·dppmO₂·12H₂O also showing the hydrogen-bonded dppmO₂ molecule. Selected bond lengths (Å): Yb1–O3 = 2.2341(14), Yb1–O2 = 2.2899(13), Yb1–O9 = 2.3263(14), Yb1–O4 = 2.2683(13), Yb1–O6 = 2.2208(13), Yb1–O1 = 2.2328(13), Yb1–O5 = 2.2696(14), P_n–O_n (n = 1–6) = 1.5034(14)–1.5072(14), P7–O7 = 1.4924(15), P8–O8 = 1.4926(15).

Table 1. X-ray crystallographic data ^a.

Compound	[Ce(dppmO₂)₄]Cl₃·9EtOH	[Sm(dppmO₂)₄]Cl₃·9.5EtOH	[Gd(dppmO₂)₄]Cl₃·7EtOH	[Yb(dppmO₂)₃Cl]Cl₂·5EtOH	[Lu(dppmO₂)₃Cl]Cl₂·3.5CH₂Cl₂·10H₂O	[Lu(dppmO₂)₂Cl₂]Cl·CH₂Cl₂·0.5H₂O	[Yb(dppmO₂)₃(H₂O)]Cl₃·dppmO₂·12H₂O
Formula	C₁₁₈H₁₄₂ CeCl₃O₁₇P₈	C₁₁₉H₁₄₅Cl₃O_17.5P₈Sm	C₁₁₄H₁₃₀Cl₃GdO₁₅P₈	C₈₇H₁₀₂Cl₃O₁₂P₆Yb₁	C_78.5H₉₃Cl₁₀LuO₁₆P₆	C₅₁H₄₇ Cl₅LuO_4.5P₄	C₁₀₀H₁₁₄Cl₃O₂₁P₈Yb
M	2326.54	2359.80	2251.53	1805.010	2007.81	1208.49	2179.06
Crystal system	monoclinic	monoclinic	monoclinic	monoclinic	orthorhombic	orthorhombic	orthorhombic
Space group (no.)	P2/c (13)	P2/c (13)	P2/c (13)	Pc (7)	Pcca (54)	Pbca (61)	Pbca (61)
a/Å	29.5926(4	29.7348(4)	29.2352(5)	14.1964(2)	47.7209(4)	21.1303(3)	26.1035(2)
b/Å	23.2600(2)	23.1120(2)	23.1885(3)	12.9572(2)	12.7431(1)	21.7424(5)	27.6790(2)
c/Å	18.0187(2)	17.9915(3)	17.7500(3)	24.0141(3)	28.3698(2)	22.1612(3)	29.1187(2)
α/°	90	90	90	90	90	90	90
β/°	107.4810(10)	106.988(2)	107.116(2)	95.880(1)	90	90	90
γ/°	90	90	90	90	90	90	90
U/Å³	11829.9(2)	11824.8(3)	11500.1(3)	4394.05(11)	17252.0(2)	10181.4(3)	21038.8(3)
Z	4	4	4	2	8	8	8
μ(Mo-K_α)/mm^–1	0.613	0.724	0.817	1.322	1.629	2.372	1.153
F(000)	4340	4348	4676	1864	8184	4844	8984
Total number reflns	183494	181022	169524	66370	22287	75026	423866
R_int	0.0372	0.0393	0.0561	0.0354	0.0642	0.0558	0.0323
Unique reflns	30589	30564	24123	21530	22287	13145	27176
No. of params, restraints	1253, 132	1261, 35	1143, 0	849, 65	937,264	621, 5	1240, 1
R₁,wR₂ [I > 2σ(I)] ^b	0.0396, 0.0831	0.0371, 0.0788	0.0526, 0.1302	0.0346, 0.0798	0.0906, 0.1916	0.0337, 0.0708	0.0277, 0.0755
R₁, wR₂ (all data)	0.0516, 0.0870	0.0532, 0.0846	0.0650,0.1361	0.0387, 0.0814	0.0935, 0.1926	0.0552, 0.0774	0.0331, 0.0783

^a common data: T = 100 K; wavelength (Mo-K_α) = 0.71073 Å; θ(max) = 27.5°; ^b R₁ = Σ||Fo| − |Fc||/Σ|Fo|; wR₂ = [Σw(Fo² − Fc²)²/ΣwFo⁴]^1/2.

Table 2. IR and ³¹P{¹H}-NMR spectroscopic data.

Complex	δ(³¹P) ^a	υ(P=O) cm^{−1 b}
dppmO₂	+25.3	1187
[La(dppmO₂)₄]Cl₃ ^c	+33.1	1159, 1100
[Ce(dppmO₂)₄]Cl₃	+48.6	1158, 1099
[Pr(dppmO₂)₄]Cl₃	+64.0	1161, 1102
[Nd(dppmO₂)₄]Cl₃	+62.9	1159, 1101
[Sm(dppmO₂)₄]Cl₃	+35.6	1162, 1101
[Eu(dppmO₂)₄]Cl₃	−13.4 (+25 dppmO₂)	1159, 1099
[Gd(dppmO₂)₄]Cl₃	Not observed	1160, 1099
[Sm(dppmO₂)₃Cl]Cl₂	+38.0	1153, 1097
[Eu(dppmO₂)₃Cl]Cl₂	−14.8	1153, 1098
[Gd(dppmO₂)₃Cl]Cl₂	Not observed	1155, 1099
[Tb(dppmO₂)₃Cl]Cl₂	−29.2	1153, 1097
[Dy(dppmO₂)₃Cl]Cl₂	+18.0	1156, 1099
[Ho(dppmO₂)₃Cl]Cl₂	−13.5	1154, 1095
[Er(dppmO₂)₃Cl]Cl₂	−60.75	1155, 1097
[Tm(dppmO₂)₃Cl]Cl₂	−54.8	1156, 1096
[Yb(dppmO₂)₃Cl]Cl₂	+9.2	1154, 1097
[Lu(dppmO₂)₂Cl₂]Cl ^c	+40.0	1158, 1098
[Lu(dppmO₂)₃Cl]Cl₂	+38.3

^a In CD₂Cl₂ solution 298 K; ^b Nujol mull; ^c Ref. [8].

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Bannister, R.D.; Levason, W.; Reid, G. Bis(diphenylphosphino)methane Dioxide Complexes of Lanthanide Trichlorides: Synthesis, Structures and Spectroscopy. Chemistry 2020, 2, 947-959. https://doi.org/10.3390/chemistry2040060

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Bannister RD, Levason W, Reid G. Bis(diphenylphosphino)methane Dioxide Complexes of Lanthanide Trichlorides: Synthesis, Structures and Spectroscopy. Chemistry. 2020; 2(4):947-959. https://doi.org/10.3390/chemistry2040060

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Bannister, Robert D., William Levason, and Gillian Reid. 2020. "Bis(diphenylphosphino)methane Dioxide Complexes of Lanthanide Trichlorides: Synthesis, Structures and Spectroscopy" Chemistry 2, no. 4: 947-959. https://doi.org/10.3390/chemistry2040060

APA Style

Bannister, R. D., Levason, W., & Reid, G. (2020). Bis(diphenylphosphino)methane Dioxide Complexes of Lanthanide Trichlorides: Synthesis, Structures and Spectroscopy. Chemistry, 2(4), 947-959. https://doi.org/10.3390/chemistry2040060

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Bis(diphenylphosphino)methane Dioxide Complexes of Lanthanide Trichlorides: Synthesis, Structures and Spectroscopy^†

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Bis(diphenylphosphino)methane Dioxide Complexes of Lanthanide Trichlorides: Synthesis, Structures and Spectroscopy †

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Bis(diphenylphosphino)methane Dioxide Complexes of Lanthanide Trichlorides: Synthesis, Structures and Spectroscopy^†