Article pubs.acs.org/IC
On the Origin of the Differences in Structure Directing Properties of Polar Metal Oxyfluoride [MOxF6−x]2− (x = 1, 2) Building Units Romain Gautier,† Régis Gautier,‡ Kelvin B. Chang,† and Kenneth R. Poeppelmeier*,† †
Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, United States Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS − Ecole Nationale Supérieure de Chimie de Rennes, 11 allée de Beaulieu, CS 50837, 35708 Rennes cedex 7, France
‡
S Supporting Information *
ABSTRACT: In oxyfluoride chemistry, the [MOxF6−x]2− anions (M = transition metal) are interesting polar building units that may be used to design polar materials, but their polar vs antipolar orientations in the solid state, which directly depend on the interactions between O2−/F− ligands and the extended structure, remain difficult to control. To improve this control, these interactions were assessed through crystallization of five related [MOxF6−x]2− (M = Ti4+, V5+, Mo6+, W6+) anions with organic molecules. The hybrid organic−inorganic compounds, (4,4′-bpyH2)TiF6 (1), (enH2)MoO2F4 (2), (4-hpyH)2MoO2F4·H2O (3), (4,4′-bpyH2)WO2F4 (4), and (4,4′-bpyH2)VOF5 (5), exhibit isolated [MOxF6−x]2− anions in a hydrogen bond network. The analysis of these crystal structures in combination with DFT calculations elucidate how differences in structure directing properties of these anions arise when π-overlap between O 2p orbitals and M d orbitals is weak and significantly affected by an increase of the energy of the d orbitals from 3d to 5d.
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[TaOF5]2− anions always direct the coordination through trans ligands but [MoO2F4]2− and [VOF5]2− anions always direct the coordination through cis ligands.11−13 On the other hand, [WO2F4]2− anions can direct the coordination through either cis or trans ligands.7,11,14 Hybrid crystal structures with polar [MoO2F4]2−, [WO2F4]2−, and [VOF5]2− anions isolated in hydrogen bond networks were targeted to investigate their directing properties. Incorporation of organic molecules to isolate the BBUs had two goals: To order the anion into a hydrogen bond network as previously reported,7 and to quantify the nucleophilicity of the O2−/F− ligands with only hydrogen bonds. Thus, the isolation of the units allowed for a reliable comparison between anions.15 New insights about the long-range ordering, the covalent and ionic contributions of metal−ligand bonds, and the structure directing properties of the anions were also identified and discussed on the basis of the crystal structures determined by single-crystal X-ray diffraction, and DFT calculations on the isolated anions.
INTRODUCTION Structural building units that exhibit a net dipole moment have an important role in solid-state chemistry owing to their potential to exhibit interesting properties. The individual net dipole moments can be induced by the off-centering of transition metals in octahedral [MOxF6−x]2− anionic basic building units (BBUs) (with x = 0 for M = Ti, x = 1 for M = V, Nb, Ta, and x = 2 for M = Mo, W).1−4 Long-range orientational ordering and a maximized alignment of the BBUs must be achieved to optimize physical properties, including piezoelectricity, ferroelectricity, nonlinear optical activity, and/or pyroelectricity. In previous works, we reported strategies to order the anionic units using a mixture of transition metals, matching hard/soft cations and anions or hydrogen bonding, but the control of the alignment of polar BBUs remains a challenge.5−7 Better control of the alignment of BBUs can be gained if the interactions between the polar BBUs and their environments are understood. Anions have a specific structure directing property: Anions are always coordinated to their environment through two of the O2−/F− ligands in either cis or trans positions. We previously showed that the structure directing properties of the anion is important to order the BBUs into an aligned or antialigned configuration.8,9 These structure directing properties were also recently shown to be important in the design of frustrated lattices.10 The geometry of the anion and the electronic effects play important roles on the nucleophilicities of the ligands and lead to different modes of bonding with the extended structures. For these reasons, [NbOF5]2− and © XXXX American Chemical Society
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EXPERIMENTAL SECTION
Caution! Hydrof luoric acid is toxic and corrosive! It must be handled with extreme caution and the appropriate protective gear.16−18 This paper reports the addition of hydrofluoric acid into a solution containing ethylenediamine, which is highly exothermic! Received: November 6, 2014
A
DOI: 10.1021/ic5026735 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Table 1. Crystal Data, Structure Solutions, and Refinements for (4,4′-bpyH2)TiF6 (1), (enH2)MoO2F4 (2), (4-hpyH)2MoO2F4· H2O (3), (4,4′-bpyH2)WO2F4 (4), and (4,4′-bpyH2)VOF5 (5) with Hydrogen Atoms Localized Using the Riding Model space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) T (°K) Z maximum θ (deg) λ(Mo Kα) (Å) ρcalc. (g·cm−3) R1 wR2 goodness-of-fit
1
2
3
4
5
Ibam 7.0960(5) 11.8410(6) 13.1480(9)
P1̅ 5.8949(2) 7.7512(3) 8.5602(3) 94.8475(15) 91.5028(15) 99.4202(15) 384.16(2) 100(2) 2 27.5 0.71073 2.3 0.018 0.070 1.39
C2/c 14.0496(5) 9.5290(3) 13.2486(5)
P21/c 7.1281(2) 23.6372(6) 6.8472(2)
P21/c 6.9059(5) 23.5460(18) 6.9082(5)
121.024(2)
93.013(1)
93.806(5)
1519.98(9) 100(2) 4 30.1 0.71073 1.889 0.020 0.055 1.09
1152.08(6) 100(2) 4 30.5 0.71073 2.595 0.023 0.059 1.17
1120.84(14) 100(2) 4 30.8 0.71073 1.897 0.065 0.164 1.10
1104.79(12) 100(2) 4 36.3 0.71073 1.924 0.054 0.140 1.12
Synthesis. TiO2 (99.9%, Sigma-Aldrich), MoO3 (99.5%, SigmaAldrich), WO3 (99.8%, Alfa-Aesar), V2O5 (99.6%, Alfa-Aesar), 4,4′bipyridine (bpy, 98%, Sigma-Aldrich), ethylenediamine (en, 99%, Sigma-Aldrich), 4-hydroxypyridine (4-hpy, 95%, Sigma-Aldrich), and hydrofluoric acid (49% HFaq by weight, Sigma-Aldrich) were used as received. Reagent amounts of deionized water were also used in the synthesis. All reagents were sealed in Teflon pouches and placed into a 125 mL Parr autoclave with a backfill of 40 mL of distilled water.19 The autoclave was heated to 150 °C for 24 h and cooled to room temperature at 6 °C/h. The single crystals were recovered by vacuum filtration in air. Compound 1. (4,4′-bpyH2)TiF6 single crystals (1) were synthesized by adding 1.5 × 10−3 mol (0.120 g) of TiO2, 2.56 × 10−3 mol (0.400 g) of 4,4′-bipyridine, 5.5 × 10−3 mol (0.10 mL) of deionized water, and 2.780 × 10−2 mol (1.00 mL) of 48% aqueous HF to a Teflon pouch (phase pure/41.6% yield based on TiO2). Compound 2. (enH2)MoO2F4 single crystals (2) were synthesized by adding 1.69 × 10−3 mol (0.243 g) of MoO3, 3 × 10−3 mol (0.200 mL) of ethylenediamine, 5.5 × 10−3 mol (0.10 mL) of deionized water, and 2.780 × 10−2 mol (1.00 mL) of 48% aqueous HF to a Teflon pouch (phase pure/53.2% yield based on MoO3). Compound 3. (4-hpyH)2MoO2F4·H2O single crystals (3) were synthesized by adding 4.05 × 10−3 mol (0.583 g) of MoO3, 5.15 × 10−3 mol (0.490 g) of 4-hydroxypyridine, and 1.668 × 10−2 mol (0.60 mL) of 48% aqueous HF to a Teflon pouch (mix of crystals and MoO3 powder/ 36.3% yield based on MoO3). Compound 4. (4,4′-bpyH2)WO2F4 single crystals (4) were synthesized by adding 1.69 × 10−3 mol (0.391 g) of WO3, 2.56 × 10−3 mol (0.400 g) of 4,4′-bipyridine, 5.5 × 10−3 mol (0.10 mL) of deionized water, and 2.780 × 10−2 mol (1.00 mL) of 48% aqueous HF to a Teflon pouch (mix of crystals with WO3 powder/48.5% yield based on WO3). Compound 5. (4,4′-bpyH2)VOF5 single crystals (5) were synthesized by adding 0.75 × 10−3 mol (0.136 g) of V2O5, 2.56 × 10−3 mol (0.400 g) of 4,4′-bipyridine, 5.5 × 10−3 mol (0.10 mL) of deionized water, and 2.780 × 10−2 mol (1.00 mL) of 48% aqueous HF to a Teflon pouch (phase pure/37.3% yield based on V2O5). Crystallographic Determination. Single-crystal diffraction experiments were conducted at 100 K using a Bruker-APEX II CCD diffractometer with monochromated Mo Kα radiation (λ = 0.71073 Å and crystal-to-detector distance: 60 mm). Data integration was performed using the SAINT-V7.23A program.20 Absorption corrections were made using SADABS (for compounds (1), (2), (3), and (4)) or TWINABS (for compound (5)).21,22 The structure was determined by direct methods and completed by Fourier difference syntheses using SIR97 and then refined using SHELXL-97 within the WinGX suite.23−25 Additional symmetry elements were checked using the program PLATON.26 Anisotropic displacement parameters and hydrogen
atoms were refined. Hydrogen atoms of organic and water molecules were included in the refinement model as riding atoms in idealized positions (C−H = 0.93 Å and N−H = 0.86 Å in aromatic compounds; C−H = 0.97 Å, N−H = 0.89 Å, and O−H = 0.82 Å in nonaromatic compounds; O−H = 0.80 Å in water molecules; Uiso(H) = 1.5Ueq(C or N)). Crystallographic data are reported in Table 1. Computational Method. DFT calculations were carried out using the ADF software (version 2010.02)27 with the PBE functional.28 Scalar relativistic corrections and spin−orbit coupling were considered with a ZORA approach.29 An all-electron quadruple-ζ basis set was used. Geometry optimizations and natural bond order (NBO) charge calculations were performed on the uncoordinated anions [VOF5]2−, [NbOF5]2−, [TaOF5]2−, [MoO2F4]2−, and [WO2F4]2−. Geometry optimizations were carried out with symmetry constraints (C4v for [VOF5]2−, [NbOF5]2−, and [TaOF5]2− and C2v for [MoO2F4]2− and [WO2F4]2−). The binding between MF5 and O2− fragments has been analyzed using the energy decomposition scheme of the ADF package, which is based on the Morokuma30,31 and Ziegler and Rauk32−34 methods. On the basis of these studies, the bond energy ΔE between the fragments can be decomposed as
ΔE = ΔEprep + ΔE int where ΔEprep is the energy required to promote the free fragments from their equilibrium structure in the electronic ground state into that of the molecule. The interaction energy, ΔEint, is the difference between the energy of the molecule and energies of the two fragments of the molecule and can be decomposed into three main components
ΔE int = ΔEelstat + ΔE Pauli + ΔEorb where ΔEelstat gives the electrostatic interaction energy between the fragments, which is calculated with a frozen electron density distribution. The size of electrostatic attraction depends on the topology of the charge distribution. The ΔEPauli, which represents the exchange repulsion or Pauli repulsion, takes into account the destabilizing twoorbital three- or four-electron interactions between the occupied orbitals of both fragments. The orbital interaction, ΔEorb, is the interaction between the occupied and virtual orbitals of the two fragments. The orbital interaction, which depends on the energy values and the special distribution of the interacting orbitals, is always attractive.
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RESULTS Structural Description. Compound 1. (4,4′-bpyH2)TiF6 compound (1) crystallizes in the centrosymmetric space group Ibam. The bipyridinium cations create a hydrogen bond network in which the isolated [TiF6]2− anions are localized (Figure 1a). B
DOI: 10.1021/ic5026735 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 1. View of the hydrogen bonding in the structures of (a) (4,4′-bpyH2)TiF6 (1), (b) (enH2)MoO2F4 (2), (c) (4-hpyH)2MoO2F4·H2O (3), (d) (4,4′-bpyH2)WO2F4 (4), and (e) (4,4′-bpyH2)VOF5 (5).
Compound 2. The ethylenediammonium molybdenum oxyfluoride (enH2)MoO2F4 (2) crystallizes in the centrosymmetric space-group P1̅. The polar [MoO2F4]2− anions are
The bond lengths between Ti1 and F atoms vary from 1.8488(16) Å to 1.8878(19) Å. C
DOI: 10.1021/ic5026735 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Table 2. Summary of DFT Calculations for Group V Elements charges of ligands
distances (Å)
angle (deg)
M
M
O
Ftrans
Fequ.
MO
M−Ftrans
M−Fequ.
OM−F
V Nb Ta
+1.42 +2.12 +2.29
−0.51 −0.85 −0.96
−0.67 −0.72 −0.74
−0.51 −0.64 −0.65
1.636 1.771 1.793
1.996 2.113 2.113
1.885 1.998 1.992
96.4 95.1 95.6
Table 3. Summary of DFT Calculations for Group VI Elements charges of ligands
distances (Å)
angles (deg)
M
M
O
Ftrans
Fequ.
MO
M−Ftrans
M−Fequ.
OMO
OM−Ftrans
Mo W
+1.82 +2.11
−0.66 −0.78
−0.66 −0.68
−0.59 −0.60
1.744 1.765
2.064 2.066
1.981 1.971
101.2 100.9
89.8 90.2
distortion of the transition metal “M” in their octahedral environment. Two categories of distortions are usually distinguished: the primary distortion that arises from dπ−pπ metal−oxide orbital interactions and the weaker secondary distortions that arises from interactions between the [MOxF6−x]2− anion and its environment.35 The identity of the ligand (either oxide or fluoride) cannot be directly distinguished by X-ray crystallography because of the similar scattering factors. A comparison of metal−ligand bond lengths, however, makes this identification possible (Figure 2). For the [VOF5]2− anion, the VO bond length is typically ∼1.60 Å, while V−Ftrans and V−Feq lengths are typically about 2.10 and 1.80 Å, respectively. For the [MO2F4]2− anion, the M O bond lengths are typically ∼1.70 Å for M = Mo6+ and ∼1.75 Å for M = W6+, while the M−Ftrans length is ∼2.05 Å. The M−Feq bond lengths are typically ∼1.93 Å for M = Mo6+ and ∼1.91 Å for M = W6+.7,13,14 The long-range disorder in the orientation of [MOxF6−x]2− anions can also be described as the long-range disorder of O2−/ F− ligands. From a crystallographic viewpoint, disorder results in equivalent metal−ligand bond lengths and makes the distinction between oxide and fluoride impossible. Orientational Ordering of the Polar Anions in Hydrogen Bond Network. Different strategies such as matching hard/soft cations and anions, hydrogen bonding, using mixed transition metals, or using two anions have been reported in the literature to order the O2−/F− anions.5−7 Each of these methods increases the anisotropy of the anion’s environment. Moreover, the crystallographic ordering of the ligands occurs if the valences of anionic ligands and cations match.9 Heier et al. suggested that hydrogen bonding could restrict the orientation of the [WO2F4]2− anions into a Cu(py)2(H2O)2WO2F4 structure.7,14 However, no information is available to explain how the hydrogen bonding allows for the matching of ligands nucleophilicity with the environment. To our knowledge, very few crystal structures were determined with orientational disorder when the anions are isolated into hydrogen bond networks. The analysis of structures 1−5 reveals two features responsible for the O2−/F− ordering: (a) the high coordination number of each O2−/F− ligand with hydrogen atoms and (b) the flexibility of the hydrogen bonding. The large ratio cation (proton)/anion allows one to better match the negative charge of the O2−/F− anions to an equivalent cationic environment. Thus, the number of interactions between each ligand of the anions and hydrogen atoms (
On the Origin of the Differences in Structure Directing Properties of Polar Metal Oxyfluoride [MOxF6−x]2− (x = 1, 2) Building Units Romain Gautier,† Régis Gautier,‡ Kelvin B. Chang,† and Kenneth R. Poeppelmeier*,† †
Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, United States Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS − Ecole Nationale Supérieure de Chimie de Rennes, 11 allée de Beaulieu, CS 50837, 35708 Rennes cedex 7, France
‡
S Supporting Information *
ABSTRACT: In oxyfluoride chemistry, the [MOxF6−x]2− anions (M = transition metal) are interesting polar building units that may be used to design polar materials, but their polar vs antipolar orientations in the solid state, which directly depend on the interactions between O2−/F− ligands and the extended structure, remain difficult to control. To improve this control, these interactions were assessed through crystallization of five related [MOxF6−x]2− (M = Ti4+, V5+, Mo6+, W6+) anions with organic molecules. The hybrid organic−inorganic compounds, (4,4′-bpyH2)TiF6 (1), (enH2)MoO2F4 (2), (4-hpyH)2MoO2F4·H2O (3), (4,4′-bpyH2)WO2F4 (4), and (4,4′-bpyH2)VOF5 (5), exhibit isolated [MOxF6−x]2− anions in a hydrogen bond network. The analysis of these crystal structures in combination with DFT calculations elucidate how differences in structure directing properties of these anions arise when π-overlap between O 2p orbitals and M d orbitals is weak and significantly affected by an increase of the energy of the d orbitals from 3d to 5d.
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[TaOF5]2− anions always direct the coordination through trans ligands but [MoO2F4]2− and [VOF5]2− anions always direct the coordination through cis ligands.11−13 On the other hand, [WO2F4]2− anions can direct the coordination through either cis or trans ligands.7,11,14 Hybrid crystal structures with polar [MoO2F4]2−, [WO2F4]2−, and [VOF5]2− anions isolated in hydrogen bond networks were targeted to investigate their directing properties. Incorporation of organic molecules to isolate the BBUs had two goals: To order the anion into a hydrogen bond network as previously reported,7 and to quantify the nucleophilicity of the O2−/F− ligands with only hydrogen bonds. Thus, the isolation of the units allowed for a reliable comparison between anions.15 New insights about the long-range ordering, the covalent and ionic contributions of metal−ligand bonds, and the structure directing properties of the anions were also identified and discussed on the basis of the crystal structures determined by single-crystal X-ray diffraction, and DFT calculations on the isolated anions.
INTRODUCTION Structural building units that exhibit a net dipole moment have an important role in solid-state chemistry owing to their potential to exhibit interesting properties. The individual net dipole moments can be induced by the off-centering of transition metals in octahedral [MOxF6−x]2− anionic basic building units (BBUs) (with x = 0 for M = Ti, x = 1 for M = V, Nb, Ta, and x = 2 for M = Mo, W).1−4 Long-range orientational ordering and a maximized alignment of the BBUs must be achieved to optimize physical properties, including piezoelectricity, ferroelectricity, nonlinear optical activity, and/or pyroelectricity. In previous works, we reported strategies to order the anionic units using a mixture of transition metals, matching hard/soft cations and anions or hydrogen bonding, but the control of the alignment of polar BBUs remains a challenge.5−7 Better control of the alignment of BBUs can be gained if the interactions between the polar BBUs and their environments are understood. Anions have a specific structure directing property: Anions are always coordinated to their environment through two of the O2−/F− ligands in either cis or trans positions. We previously showed that the structure directing properties of the anion is important to order the BBUs into an aligned or antialigned configuration.8,9 These structure directing properties were also recently shown to be important in the design of frustrated lattices.10 The geometry of the anion and the electronic effects play important roles on the nucleophilicities of the ligands and lead to different modes of bonding with the extended structures. For these reasons, [NbOF5]2− and © XXXX American Chemical Society
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EXPERIMENTAL SECTION
Caution! Hydrof luoric acid is toxic and corrosive! It must be handled with extreme caution and the appropriate protective gear.16−18 This paper reports the addition of hydrofluoric acid into a solution containing ethylenediamine, which is highly exothermic! Received: November 6, 2014
A
DOI: 10.1021/ic5026735 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Table 1. Crystal Data, Structure Solutions, and Refinements for (4,4′-bpyH2)TiF6 (1), (enH2)MoO2F4 (2), (4-hpyH)2MoO2F4· H2O (3), (4,4′-bpyH2)WO2F4 (4), and (4,4′-bpyH2)VOF5 (5) with Hydrogen Atoms Localized Using the Riding Model space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) T (°K) Z maximum θ (deg) λ(Mo Kα) (Å) ρcalc. (g·cm−3) R1 wR2 goodness-of-fit
1
2
3
4
5
Ibam 7.0960(5) 11.8410(6) 13.1480(9)
P1̅ 5.8949(2) 7.7512(3) 8.5602(3) 94.8475(15) 91.5028(15) 99.4202(15) 384.16(2) 100(2) 2 27.5 0.71073 2.3 0.018 0.070 1.39
C2/c 14.0496(5) 9.5290(3) 13.2486(5)
P21/c 7.1281(2) 23.6372(6) 6.8472(2)
P21/c 6.9059(5) 23.5460(18) 6.9082(5)
121.024(2)
93.013(1)
93.806(5)
1519.98(9) 100(2) 4 30.1 0.71073 1.889 0.020 0.055 1.09
1152.08(6) 100(2) 4 30.5 0.71073 2.595 0.023 0.059 1.17
1120.84(14) 100(2) 4 30.8 0.71073 1.897 0.065 0.164 1.10
1104.79(12) 100(2) 4 36.3 0.71073 1.924 0.054 0.140 1.12
Synthesis. TiO2 (99.9%, Sigma-Aldrich), MoO3 (99.5%, SigmaAldrich), WO3 (99.8%, Alfa-Aesar), V2O5 (99.6%, Alfa-Aesar), 4,4′bipyridine (bpy, 98%, Sigma-Aldrich), ethylenediamine (en, 99%, Sigma-Aldrich), 4-hydroxypyridine (4-hpy, 95%, Sigma-Aldrich), and hydrofluoric acid (49% HFaq by weight, Sigma-Aldrich) were used as received. Reagent amounts of deionized water were also used in the synthesis. All reagents were sealed in Teflon pouches and placed into a 125 mL Parr autoclave with a backfill of 40 mL of distilled water.19 The autoclave was heated to 150 °C for 24 h and cooled to room temperature at 6 °C/h. The single crystals were recovered by vacuum filtration in air. Compound 1. (4,4′-bpyH2)TiF6 single crystals (1) were synthesized by adding 1.5 × 10−3 mol (0.120 g) of TiO2, 2.56 × 10−3 mol (0.400 g) of 4,4′-bipyridine, 5.5 × 10−3 mol (0.10 mL) of deionized water, and 2.780 × 10−2 mol (1.00 mL) of 48% aqueous HF to a Teflon pouch (phase pure/41.6% yield based on TiO2). Compound 2. (enH2)MoO2F4 single crystals (2) were synthesized by adding 1.69 × 10−3 mol (0.243 g) of MoO3, 3 × 10−3 mol (0.200 mL) of ethylenediamine, 5.5 × 10−3 mol (0.10 mL) of deionized water, and 2.780 × 10−2 mol (1.00 mL) of 48% aqueous HF to a Teflon pouch (phase pure/53.2% yield based on MoO3). Compound 3. (4-hpyH)2MoO2F4·H2O single crystals (3) were synthesized by adding 4.05 × 10−3 mol (0.583 g) of MoO3, 5.15 × 10−3 mol (0.490 g) of 4-hydroxypyridine, and 1.668 × 10−2 mol (0.60 mL) of 48% aqueous HF to a Teflon pouch (mix of crystals and MoO3 powder/ 36.3% yield based on MoO3). Compound 4. (4,4′-bpyH2)WO2F4 single crystals (4) were synthesized by adding 1.69 × 10−3 mol (0.391 g) of WO3, 2.56 × 10−3 mol (0.400 g) of 4,4′-bipyridine, 5.5 × 10−3 mol (0.10 mL) of deionized water, and 2.780 × 10−2 mol (1.00 mL) of 48% aqueous HF to a Teflon pouch (mix of crystals with WO3 powder/48.5% yield based on WO3). Compound 5. (4,4′-bpyH2)VOF5 single crystals (5) were synthesized by adding 0.75 × 10−3 mol (0.136 g) of V2O5, 2.56 × 10−3 mol (0.400 g) of 4,4′-bipyridine, 5.5 × 10−3 mol (0.10 mL) of deionized water, and 2.780 × 10−2 mol (1.00 mL) of 48% aqueous HF to a Teflon pouch (phase pure/37.3% yield based on V2O5). Crystallographic Determination. Single-crystal diffraction experiments were conducted at 100 K using a Bruker-APEX II CCD diffractometer with monochromated Mo Kα radiation (λ = 0.71073 Å and crystal-to-detector distance: 60 mm). Data integration was performed using the SAINT-V7.23A program.20 Absorption corrections were made using SADABS (for compounds (1), (2), (3), and (4)) or TWINABS (for compound (5)).21,22 The structure was determined by direct methods and completed by Fourier difference syntheses using SIR97 and then refined using SHELXL-97 within the WinGX suite.23−25 Additional symmetry elements were checked using the program PLATON.26 Anisotropic displacement parameters and hydrogen
atoms were refined. Hydrogen atoms of organic and water molecules were included in the refinement model as riding atoms in idealized positions (C−H = 0.93 Å and N−H = 0.86 Å in aromatic compounds; C−H = 0.97 Å, N−H = 0.89 Å, and O−H = 0.82 Å in nonaromatic compounds; O−H = 0.80 Å in water molecules; Uiso(H) = 1.5Ueq(C or N)). Crystallographic data are reported in Table 1. Computational Method. DFT calculations were carried out using the ADF software (version 2010.02)27 with the PBE functional.28 Scalar relativistic corrections and spin−orbit coupling were considered with a ZORA approach.29 An all-electron quadruple-ζ basis set was used. Geometry optimizations and natural bond order (NBO) charge calculations were performed on the uncoordinated anions [VOF5]2−, [NbOF5]2−, [TaOF5]2−, [MoO2F4]2−, and [WO2F4]2−. Geometry optimizations were carried out with symmetry constraints (C4v for [VOF5]2−, [NbOF5]2−, and [TaOF5]2− and C2v for [MoO2F4]2− and [WO2F4]2−). The binding between MF5 and O2− fragments has been analyzed using the energy decomposition scheme of the ADF package, which is based on the Morokuma30,31 and Ziegler and Rauk32−34 methods. On the basis of these studies, the bond energy ΔE between the fragments can be decomposed as
ΔE = ΔEprep + ΔE int where ΔEprep is the energy required to promote the free fragments from their equilibrium structure in the electronic ground state into that of the molecule. The interaction energy, ΔEint, is the difference between the energy of the molecule and energies of the two fragments of the molecule and can be decomposed into three main components
ΔE int = ΔEelstat + ΔE Pauli + ΔEorb where ΔEelstat gives the electrostatic interaction energy between the fragments, which is calculated with a frozen electron density distribution. The size of electrostatic attraction depends on the topology of the charge distribution. The ΔEPauli, which represents the exchange repulsion or Pauli repulsion, takes into account the destabilizing twoorbital three- or four-electron interactions between the occupied orbitals of both fragments. The orbital interaction, ΔEorb, is the interaction between the occupied and virtual orbitals of the two fragments. The orbital interaction, which depends on the energy values and the special distribution of the interacting orbitals, is always attractive.
■
RESULTS Structural Description. Compound 1. (4,4′-bpyH2)TiF6 compound (1) crystallizes in the centrosymmetric space group Ibam. The bipyridinium cations create a hydrogen bond network in which the isolated [TiF6]2− anions are localized (Figure 1a). B
DOI: 10.1021/ic5026735 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 1. View of the hydrogen bonding in the structures of (a) (4,4′-bpyH2)TiF6 (1), (b) (enH2)MoO2F4 (2), (c) (4-hpyH)2MoO2F4·H2O (3), (d) (4,4′-bpyH2)WO2F4 (4), and (e) (4,4′-bpyH2)VOF5 (5).
Compound 2. The ethylenediammonium molybdenum oxyfluoride (enH2)MoO2F4 (2) crystallizes in the centrosymmetric space-group P1̅. The polar [MoO2F4]2− anions are
The bond lengths between Ti1 and F atoms vary from 1.8488(16) Å to 1.8878(19) Å. C
DOI: 10.1021/ic5026735 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Table 2. Summary of DFT Calculations for Group V Elements charges of ligands
distances (Å)
angle (deg)
M
M
O
Ftrans
Fequ.
MO
M−Ftrans
M−Fequ.
OM−F
V Nb Ta
+1.42 +2.12 +2.29
−0.51 −0.85 −0.96
−0.67 −0.72 −0.74
−0.51 −0.64 −0.65
1.636 1.771 1.793
1.996 2.113 2.113
1.885 1.998 1.992
96.4 95.1 95.6
Table 3. Summary of DFT Calculations for Group VI Elements charges of ligands
distances (Å)
angles (deg)
M
M
O
Ftrans
Fequ.
MO
M−Ftrans
M−Fequ.
OMO
OM−Ftrans
Mo W
+1.82 +2.11
−0.66 −0.78
−0.66 −0.68
−0.59 −0.60
1.744 1.765
2.064 2.066
1.981 1.971
101.2 100.9
89.8 90.2
distortion of the transition metal “M” in their octahedral environment. Two categories of distortions are usually distinguished: the primary distortion that arises from dπ−pπ metal−oxide orbital interactions and the weaker secondary distortions that arises from interactions between the [MOxF6−x]2− anion and its environment.35 The identity of the ligand (either oxide or fluoride) cannot be directly distinguished by X-ray crystallography because of the similar scattering factors. A comparison of metal−ligand bond lengths, however, makes this identification possible (Figure 2). For the [VOF5]2− anion, the VO bond length is typically ∼1.60 Å, while V−Ftrans and V−Feq lengths are typically about 2.10 and 1.80 Å, respectively. For the [MO2F4]2− anion, the M O bond lengths are typically ∼1.70 Å for M = Mo6+ and ∼1.75 Å for M = W6+, while the M−Ftrans length is ∼2.05 Å. The M−Feq bond lengths are typically ∼1.93 Å for M = Mo6+ and ∼1.91 Å for M = W6+.7,13,14 The long-range disorder in the orientation of [MOxF6−x]2− anions can also be described as the long-range disorder of O2−/ F− ligands. From a crystallographic viewpoint, disorder results in equivalent metal−ligand bond lengths and makes the distinction between oxide and fluoride impossible. Orientational Ordering of the Polar Anions in Hydrogen Bond Network. Different strategies such as matching hard/soft cations and anions, hydrogen bonding, using mixed transition metals, or using two anions have been reported in the literature to order the O2−/F− anions.5−7 Each of these methods increases the anisotropy of the anion’s environment. Moreover, the crystallographic ordering of the ligands occurs if the valences of anionic ligands and cations match.9 Heier et al. suggested that hydrogen bonding could restrict the orientation of the [WO2F4]2− anions into a Cu(py)2(H2O)2WO2F4 structure.7,14 However, no information is available to explain how the hydrogen bonding allows for the matching of ligands nucleophilicity with the environment. To our knowledge, very few crystal structures were determined with orientational disorder when the anions are isolated into hydrogen bond networks. The analysis of structures 1−5 reveals two features responsible for the O2−/F− ordering: (a) the high coordination number of each O2−/F− ligand with hydrogen atoms and (b) the flexibility of the hydrogen bonding. The large ratio cation (proton)/anion allows one to better match the negative charge of the O2−/F− anions to an equivalent cationic environment. Thus, the number of interactions between each ligand of the anions and hydrogen atoms (
