Variable Chains Found in Mixed Transition Metal Oxyfluorides with

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Cite This: Cryst. Growth Des. 2019, 19, 3435−3444

Variable Chains Found in Mixed Transition Metal Oxyfluorides with Heterocyclic Ligands Belal Ahmed,† Hongil Jo,† and Kang Min Ok*,‡ †

Department of Chemistry, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Republic of Korea Department of Chemistry, Sogang University, 35 Baekbeom-ro, Mapo-gu, Seoul 04107, Republic of Korea

‡

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S Supporting Information *

ABSTRACT: Various chain structures of six novel metal oxyfluorides containing d0 and d10 transition metal asymmetric polyhedra, i.e., [Zn(mpz)4][TiF6] (1), [Cd(mpz)4][TiF6] (2), [Zn(mpz)3][ZrF6] (3), [Cd(pz)4]3[ZrF6][ZrF8] (4), [Zn(pz)4]2[NbOF5]2 (5), and [Cd(pz)4][NbOF5] (6; mpz = 3methylpyrazole; pz = pyrazole), were synthesized by hydrothermal methods. Compounds 1, 2, 5, and 6 consist of linear chains, whereas compounds 3 and 4 are composed of zigzag chains. Careful structural analysis suggests that the hydrogen bonding network and distinctive cis/trans-directing anionic polyhedra are responsible for the diverse chain structures. The title compounds reveal optical band gaps in the range of 5.04− 5.22 eV, which might be from the octahedral distortion of M2+ cations and the corresponding electronic transitions. Thorough spectroscopic and thermogravimetric analyses, as well as the magnitude of dipole moments and out-of-center distortions, were used to characterize the reported compounds.

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INTRODUCTION Discovering novel crystal structures containing basic building units (BBUs) of transition metal cations with individual net dipole moments has been of great interest in materials chemistry.1−9 The BBUs can be implemented in the extended frameworks of different dimensional structures with various physical and chemical properties.10−14 In particular, the combination of asymmetric BBUs of late transition-metal (LTM; Cu, Zn, Cd) cations, early transition-metal (ETM; Mo, V, Ti, etc.) cations, and organic linkers resulted in a variety of interesting oxyfluorides.15,16 In addition, depending on the employed ETM, LTM, and organic ligands, a variety of mixed metal oxyfluoride compounds revealing different morphologies such as linear, zigzag, and helical chains have been reported.17 According to the chemistry of the hard−soft acid−base (HSAB) concept, mixed metal oxyfluoride compounds are often composed of hard metal−anionic components such as [MOxF6−x]2− (M = ETM, 0 ≤ x ≤ 2) and soft metal−cationic components such as [M′L4]2+ (M′ = LTM; L = nitrogencontaining organic ligand).3,12,13 Especially, high valence metal cations (Ti4+, Zr4+, and Hf4+) tend to establish a variety of polyhedra with different coordination numbers. A few known examples revealing various geometries ranging from octahedra to dodecahedra include monomeric [ZrF6]2−, [ZrF7]3−, and [ZrF8]4−; dimeric [Zr2F124−]; oligomeric [Zr3F18]6−; and polymeric [ZrnF6n]2n−.18−20 In general, the distortion in the transition metal oxyfluorides can originate from the secondorder Jahn−Teller (SOJT) effect of polarizable d10 and d0 transition metal cations in the octahedral environments.2,21 © 2019 American Chemical Society

Crystallographically ordered transition metal oxyfluoride structures have been reported previously. An ordered structure could be obtained by providing different cationic contacts for the oxyfluoride anions such as covalent bonding to the extended framework as well as the hydrogen bonding interactions.10,13,17,22 The ETM oxyfluoride anions could show a cis/trans-directing property depending on the distribution of fluoride and oxide ligands, in which the dπ− pπ orbital interactions and the electronegativity difference of ligands are important.2,14,23 The cis/trans-directing property of ETM polyhedra could play a significant role in designing new stable crystallographic orientation of the BBUs in the oxyfluorides into either an aligned or an antialigned configuration.24−26 Considering the above-mentioned explanation, we focused our work on the mixed metal oxyfluorides especially by incorporating organic pyrazole-analogue ligands. In this study, three ordered and one disordered transition metal oxyfluoride exhibiting 1D linear chain structures as well as two 2D layered zigzag chain structures of transition metal fluorides are presented. The synthesis, crystal structure, and full characterization for the title materials are thoroughly discussed.

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EXPERIMENTAL SECTION

Caution! Hydrof luoric acid is toxic and corrosive! Received: March 9, 2019 Revised: April 30, 2019 Published: May 10, 2019 3435

DOI: 10.1021/acs.cgd.9b00307 Cryst. Growth Des. 2019, 19, 3435−3444

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Table 1. Crystal Data for Compounds 1−6 formula fw space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z T (K) λ (Å) ρcalcd (g cm−1) R(Fo)a Rw(Fo2)b

1

2

3

4

5

6

ZnTiC16H24F6N8 555.69 P42/n (no. 86) 12.3025(17) 12.3025(17) 7.9520(16) 90 90 90 1203.6(4) 2 298(2) 0.70000 1.533 0.0332 0.1043

CdTiC16H24F6N8 602.71 P21/n (no. 14) 8.1870(16) 17.228(3) 9.3110(19) 90 107.02(3) 90 1255.8(5) 2 100.35(2) 0.63000 1.594 0.0332 0.0938

ZnZrC12H18F6N6 516.91 P21/c (no. 14) 9.2130(18) 11.501(2) 17.686(4) 90 100.23(3) 90 1844.2(7) 4 297.85(2) 0.63000 1.862 0.0490 0.1130

Cd3Zr2C36H48F14N24 1602.65 P2/c (no. 13) 13.3591(2) 11.8445(2) 17.0829(3) 90 90.3540(10) 90 2703.01(8) 2 298(2) 0.71073 1.969 0.0311 0.0630

Zn2Nb2C24H32F10N16O2 1083.21 C2/c (no. 15) 20.8724(16) 24.8689(16) 15.4612(11) 90 95.707(5) 90 7985.7(10) 8 296(2) 0.71073 1.802 0.0297 0.0705

CdNbC12H16F5N8O 588.64 C2/c (no. 15) 10.4374(3) 12.9364(3) 15.4382(3) 90 93.0110(10) 90 2081.62(9) 4 296(2) 0.71073 1.878 0.0284 0.0686

R(Fo) = ∑ ||Fo| − |Fc||/∑ |Fo|. bRw(Fo2) = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2.

a

Synthesis. ZnO (99.0%, Waco), CdO (99.5%, Aldrich), TiO2 (99.8%, Aldrich), ZrO2 (99.0%, Kanto), Nb2O5 (99.5%, Alfa Aesar), C3N2H4 (pyrazole; 98%, Alfa Aesar), C4N2H6 (3-methylpyrazole) (97%, Alfa Aesar), and HF (aq. 48 wt %, J.T. Baker) were used as purchased. Hydrothermal reactions were carried out to synthesize the crystals of all the tilled compounds. ZnO (81 mg, 1 mmol), TiO2 (80 mg, 1 mmol), 3-methylpyrazole (498 mg, 6 mmol), HF (253 μL), and H2O (270 μL) were added for [Zn(mpz)4][TiF6] (1). CdO (128 mg, 1 mmol), TiO2 (80 mg, 1 mmol), 3-methylpyrazole (498 mg, 6 mmol), HF (253 μL), and H2O (540 μL) were added for [Cd(mpz)4][TiF6] (2). ZnO (81 mg, 1 mmol), ZrO2 (123 mg, 1 mmol), 3-methylpyrazole (498 mg, 6 mmol), HF (181 μL), and H2O (324 μL) were added for [Zn(mpz)3][ZrF6] (3). CdO (128 mg, 1 mmol), ZrO2 (123 mg, 1 mmol), pyrazole (408 mg, 6 mmol), HF (181 μL), and H2O (540 μL) were added for [Cd(pz)4]3[ZrF6][ZrF8] (4). ZnO (81 mg, 1 mmol), Nb2O5(266 mg, 1 mmol), pyrazole (408 mg, 6 mmol), HF (181 μL), and H2O (180 μL) were added for [Zn(pz)4]2[NbOF5]2 (5). CdO (128 mg, 1 mmol), Nb2O5 (266 mg, 1 mmol), pyrazole (408 mg, 6 mmol), HF (253 μL), and H2O (270 μL) were added for [Cd(pz)4][NbOF5] (6). Each reaction mixture of all the starting reactants was transferred into Teflon cups (23 mL) and tightly sealed in autoclaves. Then, the sealed reactors were heated to 150 °C, held for 24−72 h (180 °C for compound 2), and cooled to RT at 6 °C/h. On the basis ZnO or CdO, colorless pure phase crystals for compounds 1−6 were isolated in 60, 60, 81, 52, 57, and 78% yields, respectively. Single-Crytal X-ray Diffraction. A transparent colorless rodshaped single crystal with dimensions for compound 1 (0.116 mm × 0.141 mm × 0.351 mm) and colorless block-shaped single crystals with dimensions for compound 2 (0.033 mm × 0.064 mm × 0.384 mm), compound 3 (0.024 mm × 0.038 mm × 0.064 mm), compound 4 (0.158 mm × 0.170 mm × 0.248 mm), compound 5 (0.212 mm × 0.318 mm × 0.331 mm), and compound 6 (0.117 mm × 0.251 mm × 0.346 mm) were chosen. The data collection for the title compounds was carried out at room temperature with a Bruker SMART BREEZE diffractometer (Mo Kα radiation) with a 1K CCD area detector and at 100 and 298 K employing 0.63000 and 0.70000 Å radiation with a BL2D-SMC at the Pohang Light Source II. The crystal structures were solved and refined by SHELXS-9727 and SHELXL-97,27 respectively, in the crystallographic software package WinGX-98.28 The completeness of diffraction data collection for compounds 1−3 are 94.9, 89.9 and 96.5%, respectively, since the single axis rotation method was used to collect the diffraction data using synchrotron radiation.29−31 Crystallographic data and refinement parameters for the reported materials are summarized in Table 1. Characterization. Powder X-ray diffraction (PXRD) data were collected in the 2θ range from 5° to 70°, with a scanning step size of

0.02° and time of 0.1 s on a Bruker D8-Advance diffractometer with Cu Kα radiation of 40 kV and 40 mA. The collected PXRD patterns confirm the phase purity of the synthesized compounds (see the Supporting Information). The infrared spectra for the synthesized materials were measured in the 400−4000 cm−1 region on a Thermo Scientific Nicolet 6700 FTIR spectrometer at room temperature. UV−vis diffuse reflectance spectra were recorded at room temperature in the 200−2000 nm wavelength region by a Varian Cary 500 scan UV−vis−NIR spectrophotometer. By applying the Kubelka−Munk equation, the recorded reflectance spectra were transformed to absorbance.32,33 The thermal properties were measured up to 900 °C at a heating rate of 10 °C min−1 under an Ar gas atmosphere using a SCINCO TGA N-1000 thermal analyzer. Elemental microanalysis data collection was accomplished on a Carlo Erba EA1108 analyzer. Elemental analysis for compound 1 obsd: C, 34.58; H, 4.35; N, 20.17%; calcd: C, 34.55; H, 4.42; N, 20.11%. Compound 2 obsd: C, 31.89; H, 4.01; N, 18.59%; calcd: C, 31.82; H, 4.07; N, 18.55%. Compound 3 obsd: C, 27.88; H, 3.51; N, 16.26%; calcd: C, 27.92; H, 3.63; N, 16.15%. Compound 4 obsd: C, 26.98; H, 3.02; N, 20.98%; calcd: C, 26.93; H, 3.10; N, 20.81%. Compound 5 obsd: C, 26.61; H, 2.98; N, 20.69%; calcd: C, 23.30; H, 2.74; N, 18.03%. Compound 6 obsd: C, 24.49; H, 2.74; N, 19.04%; calcd: C, 22.88; H, 2.59; N, 17.83%.

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RESULT AND DISCUSSION Structures. [Zn(mpz)4][TiF6] (1). Compound 1 reveals a unidimensional straight chain structure consisting of TiF6 and Zn(mpz)4F2 octahedra (Figure 1a). A unique Ti4+ cation is bonded to six fluoride ligands in Ti(1)F6 octahedra with four short [1.8439(12) Å] and two long [1.8821(15) Å] Ti−F bonds along with the equatorial and axial locations, respectively. Also, the Zn2+ cation is in the octahedral coordination environment with two fluorine and four nitrogen atoms, in which the axial Zn−F and equatorial Zn−N bond lengths in the ZnN4F2 octahedra are 2.0939(15) Å and 2.1216(14) Å, respectively. The four equatorial coordination positions of the Zn2+ cation are occupied by 3-methylpyrazole ligands through nitrogen atoms, and the bond distances in the 3-methylpyrazole ligand are 1.334(16) Å [N−N], 1.328(18)− 1.347(18) Å [C−N], and 1.387(3)−1.496(2) Å [C−C]. The longer Ti−F(1) bonds in TiF6 octahedra are more nucleophilic, and the alternating [TiF6]2− anionic polyhedra 3436

DOI: 10.1021/acs.cgd.9b00307 Cryst. Growth Des. 2019, 19, 3435−3444

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between the [TiF6]2− anionic groups and 3-methylpyrazole ligands through fluorine atoms. The intrachain hydrogen bonding distances are 2.851(17)−2.944(13) Å (Figure 1c), which influence the distortions of TiF6 and ZnN4F2 octahedra. The chain structure of compound 1 may be written as neutral {[Ti(1)F4/1F2/2]− [Zn(1)(mpz)4F2/2]+}0. Calculated bond valence sums (BVSs)34−36 are 2.102, 3.934, and 0.678−0.889 for Zn2+, Ti4+, and F−, respectively. [Cd(mpz)4][TiF6] (2). Compound 2 with an infinite linear chain structure consists of TiF6 and Cd(mpz)4F2 polyhedra (Figure 2a). The Ti4+ cation in the octahedral geometry in TiF6 octahedra shows four short [1.8444(11)−1.8448(11) Å] and two long [1.8632(15) Å] Ti−F bonds along the equatorial and axial positions, respectively. Also, a unique Cd2+ cation in the Cd(1)N4F2 octahedral geometry is coordinated to two fluorine and four 3-methylpyrazole ligands through the axial Cd−F and equatorial Cd−N bonds, in which the distances are 2.2698(17) Å and 2.2799(13)−2.3156(13) Å, respectively. The 3-methylpyrazole ligands occupy the four equatorial sites of the octahedral Cd2+ center and reveal bond lengths of 1.348(2)−1.349(2) Å [N−N], 1.329(2)−1.347(2) Å [C−N], and 1.347(4)−1.506(4) Å [C−C]. The longer Ti−F(1) bonds in [TiF6]2− octahedra are associated with a more negative charge. The 1D infinite linear chains in the ab plane are built from the alternating transdirecting Cd(1)N4F2 and Ti(1)F6 octahedra, which are shared to one another through the highly nucleophilic F(1) atoms (see Figure 2b). A hydrogen bonding network is found between the [TiF6]2− anions and 3-methylpyrazole ligands through fluorine atoms with bond distances of 2.783(2)− 2.788(2) Å (Figure 2c). The formula {[Ti(1)F4/1F2/2]− [Cd(1)(mpz)4F2/2]+}0 can describe the neutral structure of compound 2. Bond valence sum (BVS) calculations34−36 for Cd2+, Ti4+, and F− reveal values of 2.186, 3.993, and 0.676− 0.933, respectively. Compounds 1 and 2 consist of crystallographically ordered TiF6 and ZnN4F2/CdN4F2 polyhedra. The Ti4+ cation is bonded to six fluoride ligands in TiF6 octahedra and located on an inversion center with almost similar Ti−F bond distances. Because of the symmetry in Ti−F bonds, no local dipole moment for TiF6 octahedra is observed (Table 2). The ordered structures for oxyfluorides 1 and 2 are obtained by providing 10 different cationic contacts for the [TiF6]2− anions, in which two interactions occur with the [Zn/ Cd(mpz)4]2+ cations and eight contacts occur with 3methylpyrazole ligands through hydrogen bonding interactions. For both compounds, the axial fluorides in the [TiF6]2− anions are coordinated to the [Zn/Cd(mpz)4]2+ cations. Each axial fluoride in the [TiF6]2− anion involves in two hydrogen bonding interactions with 3-methylpyrazole ligands [N···F, 2.944(13)−3.340(13) Å], whereas the equatorial fluoride has only one interaction [N···F, 2.784(18)−2.851(17) Å] (see Figures 1c and 2c). The difference in length of the axial and equatorial Ti−F bonds is attributed to the coordination effect. [Zn(mpz)3][ZrF6] (3). Compound 3 with a layered structure consists of ZrF7 and Zn(mpz)3F2 polyhedra (Figure 3a). A unique Zr4+ cation is covalently bonded to seven fluoride ligands in the Zr(1)F7 pentagonal bipyramids that reveal one short [1.946(3) Å], two long [2.170(2)−2.179(2) Å], and four intermediate [1.999(3)−2.094(2) Å] Zr−F bonds. The Zn(1)2+ cation in the distorted trigonal bipyramidal Zn(1)N3F2 environment is coordinated with two fluorine and three 3-methylpyrazole ligands, in which the distances are 2.069(2)− 2.163(2) Å and 1.998(4)−2.007(4) Å for the axial Zn−F and

Figure 1. (a) ORTEP drawing with 30% thermal ellipsoids for compound 1 (gray, C; purple, N; blue, Ti; cyan, Zn; green, F). Short and long bonds are denoted as s and l, respectively. (b) Ball-and-stick and polyhedral model showing a view of bonding between alternating trans-directing [TiF6]2− (blue) and ZnN4F2 (cyan) polyhedra in the bc plane. (c) Ball-and-stick model representing the hydrogen bonding network between 3-methylpyrazole ligands and anionic groups [TiF6]2− (red dotted lines). For clarity, hydrogen atoms were removed.

are bonded to Zn2+ centers through the long Ti−F bonds in a trans-coordinating fashion. The 1D linear chain structures, −[TiF6][Zn(mpz)4][TiF6][Zn(mpz)4]−, in the bc plane are constructed from the trans-coordinating Ti(1)F6 and Zn(1)N4F2 octahedra, which are linked through the fluoride anions (Figure 1b). The hydrogen bonding network is formed 3437

DOI: 10.1021/acs.cgd.9b00307 Cryst. Growth Des. 2019, 19, 3435−3444

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Table 2. Magnitude of Dipole Moments and Out-of-Center Distortion for Octahedra compound

polyhedron

dipole moment (D)a

1

Ti(1)F6 Zn(1)N4F2 Ti(1)F6 Cd(1)N4F2 Zr(1)F7 Zn(1)N3F2 Zr(1)F6 Zr(2)F8 Cd(1)N4F2 Cd(2)N4F2 Nb(1)OF5 Nb(2)OF5 Zn(1)N4OF Zn(2)N4OF Nb(1)(O/F)2F4 Cd(1)N4(O/F)2

0 0 0 0 7.14 0.87 2.46 16.94 1.25 0 1.94 1.96 0.67 0.67 0 0

2 3 4

5

6 a

Δd 0 0

0.14

0.35 0.37

0

D = debye.

The longer Zr−F(5) bonds in the [ZrF7]2− pentagonal bipyramid are highly nucleophilic. Thus, the corner-sharing of [ZrF7]2− octahedra via F(5) atoms results in the [Zr2F12]4− dimers (Figure 3b). The layered structure with zigzag chains in the ab plane are generated from the dimeric [Zr2F12]4− building units and trigonal bipyramidal ZnN3F2, which are coordinated through F(1) and F(6) ligands in a cis-directing fashion (Figure 3c). The bent F(1)−Zr(1)−F(6) bond angle of 76.71(10)° in compound 3 gives rise to a zigzag backbone structure. The hydrogen bonding network is established between the [Zr2F12]4− dimers and 3-methylpyrazole ligands through fluorine atoms. The observed intrachain hydrogen bonding distances are 2.651(4) Å−2.909(5) Å (Figure 3d). The covalent [−ZnN3F2−Zr2F12−] network exhibits 2D square nets in each layer, in which the nets stack in an offset mode to avoid steric obstruction between the 3-methylpyrazole rings. Each layer of compound 3 contains large rectangular cavities with an approximate dimension of 10.1 Å × 4.6 Å (Figure 3e). The structure of compound 3 can be considered as a neutral zigzag layer of {[Zr(1)F3/1F4/2]− [Zn(1)(mpz)3F2/2]+}0. Calculated bond valence sum (BVS)34−36 values are 2.074, 3.961, and 0.654−0.823 for Zn2+, Zr4+, and F−, respectively. [Cd(pz)4]3[ZrF6][ZrF8] (4). Compound 4 reveals another layered structure that is comprised of ZrF6, ZrF8, and Cd(pz)4F2 polyhedra (Figure 4a). Whereas the Zr(1)4+ cation in the distorted Zr(1)F6 octahedra bonded to six fluoride ligands reveals two short and four long Zr−F bonds with distances of 1.971(2)Å and 2.011(2)−2.040(2) Å, respectively, Zr(2)F8 polyhedra exhibit four long and four short Zr−F bonds with distances of 2.130(2)−2.136(2) Å and 2.104(19)− 2.112(2) Å, respectively. Two unique Cd2+ atoms are in an octahedral moiety with four pyrazole and two fluoride ligands. Four intermediate Cd−N [2.267(3)−2.325(3) Å] bonds and two long Cd−F [2.352(18)−2.397(2) Å] bonds are found in distorted CdN4F2 octahedra. The bond lengths in pyrazole ligands are 1.335(4)−1.350(5) Å [N−N], 1.3182(5)− 1.344(5) Å [C−N], and 1.329(84)−1.428(8) Å [C−C]. The longer Zr−F(1) bonds in [Zr(1)F6]2− octahedra are more nucleophilic, and the corners of the [Zr(1)F6]2− anions are coordinated to Cd2+ cations via F(1) ligands in a trans-

Figure 2. (a) ORTEP drawing of compound 2 (gray, C; purple, N; blue, Ti; cyan, Cd; green, F). (b) Ball-and-stick and polyhedral model in the ab plane showing a linear chain structure of compound 2. (c) Hydrogen bonding network is found between 3-methylpyrazole ligands and [TiF6]2− anionic groups (red dotted lines). For clarity, hydrogen atoms were removed.

equatorial Zn−N bonds, respectively. The bond lengths in 3methylpyrazole ligands occupying the equatorial sites of the distorted trigonal bipyramidal Zn2+ center are 1.347(5)− 1.356(5) Å [N−N], 1.329(6)−1.43(6) Å [C−N], and 1.367(8)−1.498(7) Å [C−C]. 3438

DOI: 10.1021/acs.cgd.9b00307 Cryst. Growth Des. 2019, 19, 3435−3444

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Figure 3. (a) ORTEP drawing of compound 3 (30% thermal ellipsoids; gray, C; purple, N; blue, Zr; cyan, Zn; green, F). Short, intermediate, and long bonds are denoted as s, i, and l, respectively. (b) Ball-and-stick model revealing the dimeric [Zr2F12]4− BBUs in compound 3. (c) Ball-and-stick and polyhedral model in the ab plane revealing zigzag layered structure of compound 3. (d) Hydrogen bonding network between 3-methylpyrazole ligands and [Zr2F12]4− dimers (red dotted lines) in compound 3. (e) 2D square nets are found in one layer of compound 3. For clarity, 3-methylpyrazole rings and hydrogen atoms were removed.

Figure 4. (a) ORTEP drawing with 30% thermal ellipsoids of compound 4 (gray, C; purple, N; blue, Zr; cyan, Cd; green, F). (b) Ball-and-stick and polyhedral model of compound 4 in the bc plane showing a corrugated layered structure with zigzag chains. (c) Hydrogen bonding network between pyrazole, [ZrF6]2−, and [ZrF8]4− polyhedra (red dotted lines). (d) 2D square nets in each layer of compound 4. For clarity, pyrazole rings and hydrogen atoms were removed. 3439

DOI: 10.1021/acs.cgd.9b00307 Cryst. Growth Des. 2019, 19, 3435−3444

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coordinating fashion. The corners of [Zr(2)F8]4− polyhedra are linked by Cd2+ cations through F(4) and F(5) atoms in a cis-directing pattern (Figure 4b). As a result, the 2D layered network containing zigzag chains in the bc plane for compound 4 is established from the trans-directing [ZrF6]2−, cis-directing [Zr(2)F8]4−, and trans-directing CdN4F2 polyhedra, which are bound to each other through the fluoride ligands (Figure 4b). The F(1)−Zr(1)−F(1) [85.27(13)°] and F(4)−Zr(2)−F(5) [85.19(9)°] bond angles are bent, which should be responsible for the zigzag chains in the structure. As seen in Figure 4c, the hydrogen bonding network is found between [ZrF6]2−, [ZrF8]4−, and pyrazole ligands through fluorine atoms. The intrachain hydrogen bonding distances range from 2.630(4) Å to 2.982(4) Å (Figure 4c). The Zr(2)F8 polyhedra contain four nonbridging fluorides, F(6) and F(7), which participate in strong hydrogen bonding interactions with pyrazole ligands. Each layer consists of distinct Zr(1)F6 and Zr(2)F8 groups that link to two and four edges of the neighboring polyhedra, respectively. The coordination of the [−CdN4F2−ZrF6− CdN4F2−ZrF8−] framework in compound 4 reveals 2D square nets with the stacked offset layered structure to avoid steric hindrance occurring between the pyrazole ligands from above and below. Rectangular cavities with approximate dimensions of 9.2 Å × 8.7 Å are observed in each layer of compound 4 (Figure 4d). Compound 4 may be expressed as a neutral layer consisting of {[Zr(1)F4/1F2/2]− [Zr(2)F4/1F4/4]2− ([Cd(1)(pz)4F2/2]+)3}0. Bond valence sum (BVS) calculations34−36 for Cd2+, Zr4+, and F− are found to be 2.029−2.081, 3.895−3.976, and 0.467−0.810, respectively. The BVS calculations for all the atoms in compound 4 give reasonable values, except for the small value for F(6) and F(7) (0.467−0.474). The lower BVS values should be attributable to the elongated Zr−F(6) [2.136(2) Å] and Zr−F(7) [2.130(2) Å] bonds in Zr(2)F8 polyhedra occurring from the hydrogen bonding interactions. [Zn(pz)4]2[NbOF5]2 (5). Compound 5 shows a linear chain structure comprised of NbOF5 and Zn(pz)4OF polyhedra (Figure 5a). The geometry of two unique Nb5+ centers is octahedral with five fluoride and one oxide ligand. One short Nb−O [1.7581(17)−1.7529(17) Å], four intermediate Nb−F [1.9233(16)−1.9442(17) Å], and one long Nb−F [2.1031(14)−2.1069(14) Å] bond result in a local C4 octahedral distortion of the NbOF5 unit. The coordination environment of Zn2+ cations is also octahedral owing to the four equatorial pyrazole ligands with Zn−N [2.117(2)− 2.162(9) Å] bonds and the two axial oxygen and fluorine atoms. The bond lengths within the pyrazole ligands are 1.334(3)−1.355(10) Å [N−N], 1.323(3)−1.381(10) Å [C− N], and 1.335(3)−1.401(5) Å [C−C]. In compound 5, the 1D infinite straight chain structures in the ab plane are constructed from the alternating transdirecting NbOF5 and ZnN4OF octahedra, which are linked via more electronegative fluoride and oxide anions as transcoordinating patterns (Figure 5b). The hydrogen bonding network is formed between the anionic polyhedra [NbOF5]− and pyrazole ligands through fluorine atoms (Figure 5c). The hydrogen bonding distances are 2.851(3)−2.997(3) Å, which influence the distortions of pyrazole rings. The linear chain of NbOF5 is completely ordered, and different cationic contacts for the [NbOF5]2− anion provide the ordered structure of compound 5.15,37 Each [NbOF5]− anion of the compound is coordinated to two [Zn(pz)4]2+ cations via oxide and fluoride ligands. In addition, the ordered structure for compound 5 should be attributed to the hydrogen bonding interactions

Figure 5. (a) ORTEP drawing with 30% thermal ellipsoids of compound 5 (gray, C; purple, N; blue, Nb; cyan, Zn; red, O; green, F). (b) Ball-and-stick and polyhedral representation of compound 5 in the ab plane viewing 1D straight chain structure. (c) The ordered structure is obtained from the coordination between trans-directing [NbOF5]2− and ZnN4FO polyhedra as well as the hydrogen bonding network between pyrazole ligands and [NbOF5]2− anions (red dotted lines). For clarity, hydrogen atoms were removed.

between fluoride and pyrazole ligands. The framework of compound 5 is delineated as a neutral straight chain of {[Nb(1)O1/2F4/1F1/2]0.5− [Zn(1)(pz)4O1/2F1/2]0.5+ [Nb(2)O1/2F4/1F1/2]0.5− [Zn(2)(pz)4O1/2F1/2]0.5+}0. BVS34−36 for Zn2+, Nb5+, O2−, and F− show values of 2.019−2.027, 4.968, 1.821−1.830, and 0.715−0.754, respectively. [Cd(pz)4][NbOF5] (6). Compound 6 exhibits another unidimensional structure consisting of NbOF5 and Cd(pz)4OF polyhedra (Figure 6a). The coordination environment of a unique Nb5+ cation shows a distorted octahedral moiety 3440

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Cd(pz)4OF, the Cd2+ center is coordinated to four pyrazole and two oxygen ligands through the two axial Cd−O [2.255(17) Å] and four equatorial Cd−N [2.310(2)− 2.327(2) Å] bonds, respectively. The bond lengths in pyrazole ligands result in values of 1.316(4)−1.336(5) Å [N−N], 1.316(4)−1.336(5) Å [C−N], and 1.323(6)−1.423(6) Å [C− C]. The straight chain structures in the ab plane are built from the alternating trans-directing [NbOF5]2− anions and CdN4OF octahedra, which are bound through more nucleophilic ligands (Figure 6b). As seen in Figure 6c, the hydrogen bonding network influencing the distortions of NbOF5 and CdN4OF octahedra is established between [NbOF5]2− anions and pyrazole ligands via fluorine atoms in which distances are 2.941(4)−3.109(4) Å (Figure 6c). The backbone of compound 6 is formulated as a neutral straight chain of {[Cd(1)(pz)4O1/2F1/2]0.5+ [Nb(1)O1/2F4/1F1/2]0.5−}0. BVS calculations34−36 show that Cd2+, Nb5+, O2−/F−, and F− have values of 2.292, 5.080, 1.400, and 0.760−0.767, respectively. Infrared Spectroscopy. Infrared spectra ensured the detailed assignment of the characteristic absorption peaks of the reported oxyfluorides.22,38−45 The bands exhibited at ca. 542−547 cm−1 might be due to the vibrations occurring from Ti−F bonds for compounds 1 and 2. The IR spectra for compounds 3 and 4 contain three strong broad overlapped peaks within the area of 432−443, 478, and 503−542 cm−1, which can be assigned to the vibrations from Zr−F bonds. The absorption peaks at ca. 923 cm−1 and 585−595 cm−1 may be due to the Nb−O and Nb−F vibrations, respectively, for compounds 5 and 6. The strong bands in the regions 761−794 and 758−795 cm−1 should be derived from the characteristic Zn−N and Cd−N vibrations, respectively. Several strong and broad IR absorption peaks in the range of 1304−1362 cm−1(N−H bending), 1537−1580 cm−1(C−H bending), 1400−1454 cm−1 (C = C stretching), and 3100−3300 cm−1 (N−H stretching) are also found, which occur from 3methylpyrazole and pyrazole ligands. Infrared spectral data for all the reported compounds are given in the Supporting Information. UV−Vis Spectroscopy. The UV spectra were measured and the optical band gaps were calculated by applying the Kubelka−Munk equation.32,33 The calculated optical band gaps are approximately 5.19, 5.18, 5.22, 5.21, 5.06, and 5.04 eV for compounds 1−6, respectively. These optical band gaps may be due to the octahedral distortion of M2+ cations for compounds. In addition, the electronic transitions between the conduction on valence band edges comprised of empty d0 orbitals on M4+/M5+ and filled d10 and on M2+ (Zn2+ and Cd2+) cations, respectively, as well as the M2+−ligand at higher energies, are responsible for the energy gaps. By the replacement of Ti4+ for Zr4+, the calculated optical absorptions for compounds 3 and 4 shift into higher energy, which matches well with the previously investigated results.46,47 All the reflectance spectra exhibiting the optical absorption edges are in the Supporting Information. Thermogravimetric Analysis. On the basis of the TGA curves in the Supporting Information, the thermal stability for all the oxyfluorides are very similar. The TGA diagrams indicate that the frameworks for the compounds are maintained to ca. 100−200 °C. Above 500 °C, the calculated (experimental) weight losses for compounds 1−6 are 72.70% (69.45%), 67.09% (64.99%), 62.35% (60.46%), 60.45% (57.20%), 51.40% (51.14%), and 48.23% (50.59%), respec-

Figure 6. (a) ORTEP drawing of compound 6 (gray, C; purple, N; blue, Nb; cyan, Cd; red, O; green, F). (b) Ball-and-stick and polyhedral models in the ab plane showing the 1D linear chain structure of compound 6. (c) Hydrogen bonding network is found in the chain. For clarity, hydrogen atoms were removed.

revealing two short Nb−O(1)/F(1) [1.9063(17) Å] and four long Nb−F [1.9180(19)−1.9216(19)Å] bonds. The oxygen and fluorine atoms in Nb(1)OF5 polyhedra are disordered with a ratio of 0.5:0.5. In the octahedral geometry of 3441

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Crystal Growth & Design tively, which correspond to the loss of fluoride, oxide, and pyrazole/3-methylpyrazole ligands. Upon further heating to 600 °C, compounds 1−4 completely decomposed to mixtures of M(II)F2 and M(IV)48 [M(II) = Zn or Cd; M(IV) = Ti or Zr], and compounds 5 and 6 decomposed to M(II)Nb2O6,49 which was confirmed by PXRD measured at variable temperatures (Supporting Information). Dipole Moments and Out-of-Center Distortions. The coordination environment of the distortive cations can be better understood by calculating the magnitude of local dipole moments and out-of-center distortion (Δd) for the octahedra.14 The calculated dipole moments for TiF6, ZrF6, ZrF7, ZrF8, NbOF5, ZnN4F2, ZnN3F2, and CdN4F2 polyhedra in compounds 1−6 are about 0.00, 2.46, 7.14, 16.94, 0.00−1.96, 0.00−0.67, 0.87, and 0.00−1.25 D, respectively. Additionally, the values of out-of-center distortion are ca. 0, 0.14, and 0− 0.37 for TiF6, ZrF6, and NbOF5 octahedra, respectively, in compounds 1, 2, 4, 5, and 6. The same metal−ligand bond distances trans to each other are responsible for the cancellation of dipole moments and out-of-center distortion for the octahedra of several oxyfluorides. The dipole moments for the ZrF6 octahedra in compound 4 are greater than that of NbOF5 octahedra in compound 5, which may be owed to the elongated Zr−F [2.040(2)−2.011(2) Å] bonds in the ZrF6 octahedra. The alignment of polar anions in the infinite chain of mixed-metal BBUs can be identified by the successive positions of the fluoride and oxide anions. The dipole moments of [NbOF5]2− anions with C4v symmetry are parallel to the F−NbO axis.17,24 The asymmetric [NbOF5]2− units in each chain of compound 5 are aligned in a polar fashion; hence, the individual local dipole moment in the linear chains can be obtained. The linear chains of the unit cell are in opposite directions from each other, which cancels their net dipole moments (Figure 7). The magnitude of dipole moments and out-of-center distortion for all the octahedra belonging to oxyfluorides are given in Table 2.

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CONCLUSIONS

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ASSOCIATED CONTENT

Article

Six new mixed metal oxyfluoride compounds, [Zn(mpz)4][TiF6] (1), [Cd(mpz)4][TiF6] (2), [Zn(mpz)3][ZrF6] (3), [Cd(pz)4]3[ZrF6][ZrF8] (4), [Zn(pz)4]2[NbOF5]2 (5), and [Cd(pz)4][NbOF5] (6), with various chain structures have been synthesized via hydrothermal reactions. Compounds 1, 2, and 5 are composed of 1D ordered linear chains, compounds 3 and 4 contain 2D layered zigzag chains, and compound 6 reveals 1D disordered linear chain structures. The compounds exhibiting linear chain structures are constructed from the alternating trans-directing octahedral moieties, [TiF6]2−/ [NbOF5]2− and [M(II)(pz/mpz)4F2]/[M(II)(pz/mpz)4OF], which are linked through more nucleophilic ligands and a hydrogen bonding network. The dimeric [Zr2F12]4− polyhedra for compound 3 and the mixed monomeric [ZrF6]2− and [ZrF8]4− polyhedra for compound 4 are coordinated to d10 cations of the extended framework in a cis-directing fashion, which leads to the formation of zigzag chain structures. Thermal properties for all the oxyfluorides are very similar, and the backbones are thermally stable up to 100−200 °C. The higher-energy absorption bands for all the reported compounds are generated from the octahedral distortion of M2+ cations and the electronic transition between the band edges consisting of d10 and d0 orbitals on M2+ and M4+/M5+ cations, respectively. The alignment of the polar anionic groups is responsible for the greater out-of-center distortions and dipole moments for [NbOF5]2− octahedra.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.9b00307. Experimental and calculated PXRD data, EDX spectra, IR spectra, UV−vis diffuse reflectance spectra, TGA diagrams, and PXRD data obtained at different temperatures (PDF) Accession Codes

CCDC 1902067−1902072 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82-2-705-7959. E-mail: [email protected]. ORCID

Hongil Jo: 0000-0002-0627-4921 Kang Min Ok: 0000-0002-7195-9089 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (grant nos. 2018R1A5A1025208 and 2019R1A2C3005530).

Figure 7. Dipole moment for each chain in a unit cell aligned in the opposite direction, which cancels the net dipole moments in compound 5. 3442

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