Synthesis, structure characterization and properties of a new oxidovanadium(IV) coordination polymer incorporating bridging (MoO₄)^2– and (Mo₈O₂₆)^4– ligands

2.1 Synthetic aspects, spectral and thermal studies

The oxidovanadium(IV) containing compound 1 was prepared by a hydrothermal reaction of molybdic acid, sodium metavanadate and 2,2′:6′,2′′-terpyridine as dark green crystalline blocks. A comparison of the experimental powder pattern of a bulk sample of 1 with that of the calculated pattern (Figure S1) reveals the formation of a phase pure product. In this reaction the N-donor serves as a structure directing ligand as well as a reducing agent. The formation of a [VO]²⁺ containing compounds starting from V⁵⁺ reactants in the presence of N-donor ligands under hydrothermal conditions is a well-known synthetic methodology in the literature [30, 32]. The presence of V⁴⁺ is confirmed by the characteristic ESR signal centred at g = 1.987 of a polycrystalline sample of 1 (Figure S2). The infrared (IR) and Raman spectra (Figures S3 and S4) of 1 exhibit several characteristic bands. The broad IR band centred at around 3410 cm⁻¹ and the signal at 1609 cm⁻¹ can be attributed to the stretching and bending vibrations respectively of the –OH moiety of lattice water. The IR spectrum also consists of peaks arising due to V–O (terminal) and various Mo–O stretching vibrations at 946, 893, 796, 725, 672, 543, 452 cm⁻¹. The intense Raman signal at 968 cm⁻¹ can be assigned for a vibration of the γ-[Mo₈O₂₆]^4– unit [33]. The TG-DTA curves of 1 exhibit an initial mass loss of 1.2% (Calcd: 1.3%) accompanied by a weak endothermic signal centered at 92 °C assignable for the loss of coordinated water (Figure S5). This is followed by an exothermic peak at 440 °C corresponding to a large mass loss of 34.5%. This value is in close agreement with the calculated value of 34.0% for loss of the organic ligand. The residual mass of 64.3% (Calcd. 65.8%) can be assigned for the formation of a phase of composition V₂Mo₅O₂₀.

2.2 Description of the crystal structure of 1

The title coordination polymer 1 [(VO(terpy))₄(MoO₄)₂(Mo₈O₂₆)]·2H₂O crystallizes in the centrosymmetric triclinic space group P 1 ‾ . Its structure consists of a crystallographically unique γ-(Mo₈O₂₆)^4– anion located on an inversion centre, an independent tetraoxidomolybdate (MoO₄)^2– dianion, two crystallographically unique (VO(terpy))²⁺ moieties and a lattice water molecule (Figure 1). Both the unique vanadium V1 and V2 species are bonded to a terminal oxido ligand O1 and O2, respectively, and are further coordinated to a terpy ligand, whose three N atoms span the meridional sites of a {VN₃O₃} octahedron. The geometric parameters of the two crystallographically unique terpyridine ligands bonded to V1 and V2 are in normal range (Table S1).

Figure 1:

The asymmetric unit of 1 showing sixfold coordination around V1 and V2. Displacement ellipsoids are drawn at 30% probability level for all the non-hydrogen atoms. Intramolecular H-bonding is shown as broken lines. For clarity, symmetry generated atoms are labelled in blue.

Symmetry code: (i) –x, –y + 1, –z + 1 (ii) –x + 1, –y + 2, –z + 2.

The first unique V⁴⁺ (V1) is further bonded to two oxygen atoms (O5 and O6) of (MoO₄)^2– while the second independent V (V2) is bonded to O4 of (MoO₄)^2– and O7 of (Mo₈O₂₆)^4– completing the {VN₃O₃} octahedron. The terminal oxido ligands O1 and O2 make the shortest V–O bonds (Table 1). Unlike the V–O distances which range from 1.5940(17) to 2.1833(16) Å for V1 (1.5998(16) to 2.1198(16) for V2), the V–N bond distances scatter in a narrow range between 2.0643(18) and 2.149(2) Å for V1 (2.0452(18) and 2.131(2) for V2). Bond valence sum of the V–N and V–O bonds for V1 and V2 were determined using the VaList program [34]. The values of 3.77 and 3.81 for V1 and V2 reveal that the unique vanadium atoms can be formulated as V(IV). The cis- as well as the trans- O–V–N or N–V–N angles deviate from ideal values and cover a wide range (76.36(8) to 101.97(7) and 152.35(8)° to 178.54(8)° for V1; 76.83(8)° to 104.11(7)° and 151.79(8)° to 177.15(9)° for V2) indicating a distortion of the {VN₃O₃} octahedron. The Mo–O bond lengths of the tetrahedral (MoO₄)^2– anion range from the shortest Mo–O3 bond of 1.7159(19) to 1.8042(16) Å for Mo–O6. The binding of O6 with V1 can explain the elongation of the Mo–O6 bond. The O–Mo–O bond angles range from 107.79(7)° to 112.53(8)° indicating a slight distortion of the {MoO₄} tetrahedron.

The tetraanionic octamolybdate (Mo₈O₂₆)^4– is well-known to exhibit polymorphism and several polymorphic modifications are well documented in the literature [35], [36], [37], [38], [39], [40], [41], [42], [43], [44]. Although the α- and β-modifications are more often encountered, other polymorphic forms like γ, δ, ε etc. are also known (Table S2). The centrosymmetric (Mo₈O₂₆)^4– unit in 1 which consists of three unique octahedrally surrounded Mo atoms (Mo2, Mo3 and Mo4) and the penta-coordinated Mo5 atom (Figure S6) is a γ-modification of (Mo₈O₂₆)^4– and the 13 unique O atoms can be classified under four types viz. i) the terminal oxido (O_t) ligands O7, O8, O12, O15, O16, O18 and O19, ii) μ₂-bridging oxido ligands O9, O10, O17 iii) μ₃-bridging O13 and O14 iv) μ₄-bridging O11. The terminal Mo–O_t bond distances range from 1.6884(16) to 1.7605(15) Å. O7 which makes the longest Mo2–O7 distance of 1.7605(15) Å has also a bond to V2, which can explain its elongation. Unlike the terminal oxygen atoms, the bridging oxido ligands exhibit longer Mo–O distances which scatter in a wider range viz. 1.7648(15)–2.2146(15) Å for μ₂-bridging ligands, and 1.8995(15)–2.3318(15) Å and 1.8973(14)–2.5049(14) Å for μ₃- and μ₄-bridging ligands respectively.

Oxidovandium(IV) compounds containing monodentate and bridging (MoO₄)^2– ligand are well documented in the literature [30, 32]. Compound 1 is unique due to the presence of two different Mo-based ligands viz. (MoO₄)^2– and (Mo₈O₂₆)^4–. To the best of our knowledge no other V⁴⁺ compound is known containing coordinated (MoO₄)^2– and (Mo₈O₂₆)^4– ligands. Zubieta and coworkers reported an example of a three-dimensional Cu(II) compound [{Cu₂(L₄)}₂{γ-Mo₈O₂₆}{MoO₄}₂]·2H₂O (entry 4 in Table S2) containing two differing molybdate ligands [37].

The μ₃-bridging (MoO₄)^2– dianion binds to two symmetry related V1 atoms via O5 and O6 and a V2 through O4 while the centrosymmetric (Mo₈O₂₆)^4– binds to V2 via O7 (Figure S7). The μ₃-bridging tridentate binding of (MoO₄)^2– results in the formation of a {V₄Mo₂O₁₂}⁴⁺ cationic unit which is composed of an eight membered heterometallic {Mo₂V₂O₄} ring containing one of the unique V atoms (V1 in the ring) with the other unique V atom (V2) protruding as handles (Figure 2).

Figure 2:

The {V₄Mo₂O₁₂}⁴⁺ cationic unit made up of an eight-membered heterometallic {Mo₂V₂O₄} ring with protruding oxidovanadium handles. The terpy ligands around V1 and V2 are not shown. Symmetry code i) –x, –y + 1, –z + 1.

A pair of {V₄Mo₂O₁₂}⁴⁺ units are bridged by a (Mo₈O₂₆)^4– ligand resulting in the formation of a polymeric chain which consists of alternating {V₄Mo₂O₁₂}⁴⁺ cations and (Mo₈O₂₆)^4– anions (Figure 3). The H atoms attached to the carbon atoms C4, C7, C9, C14, C16, C19, C24, C27, C29 and C30 of the unique terpy ligands function as hydrogen donors while the O atoms O1 and O2 attached to V1 and V2 and the O atoms of the γ-(Mo₈O₂₆)^4– unit, viz. O8, O9, O10, O14, O15, O17 and O19 function as hydrogen acceptors resulting in a total of eleven C–H⋯O bonds (Table S3) of which two are intramolecular. The intermolecular C–H⋯O bonds serve to link parallel polymeric chains. The difficulty to locate the H atoms attached to the lattice water precludes a description of the O–H⋯O interactions.

Figure 3:

A portion of an infinite chain showing alternating (Mo₈O₂₆)^4– anions and {V₄Mo₂O₁₂}⁴⁺ cations. The tridentate terpy ligands on the unique V atoms are not shown (top). Same arrangement after inclusion of the terpy ligands. The {VO₃N₃} octahedra and (MoO₄)²⁻ tetrahedra are shown in polyhedral format (bottom).

3.1 Materials and methods

All chemicals were used as received from commercial sources without any further purification. Infrared (IR) spectra of the solid samples diluted with KBr were recorded on a Shimadzu (IR Prestige-21) FT-IR spectrometer in the range 4000–400 cm⁻¹ at a resolution of 4 cm⁻¹. The Raman spectrum was recorded using a Labram HR Evolution Raman Spectrometer HORIBA Scientific at 785 nm laser radiation for excitation and laser power set to 5%. An ESR spectrum was recorded in a Jeol (JES, FA200) spectrometer using a microwave frequency of 9.5 GHz and modulation frequency of 100 kHz at Sophisticated Analytical Instruments Facility (SAIF), IIT-Madras. The elemental analysis (C, H and N) was performed on an Elementar Variomicro Cube CHNS Analyser. TG-DTA experiment was performed in air at the heating rate of 5 K/min in the temperature range of 30–700 °C on STA-409 PC simultaneous thermal analyser from Netzsch in an alumina crucible. Powder X-ray diffraction (PXRD) pattern was recorded on a Rigaku Smartlab powder diffractometer using Cu-Kα radiation. For single crystal structure determination, X-ray intensity data was collected on a Bruker D8 Quest Eco X-ray diffractometer, using graphite-monochromated Mo-Kα radiation. The structure was solved with Direct Methods using Shelxs-97 [45] and refinement was carried out against F ² using Shelxl-2016 [45]. All non-hydrogen atoms were refined anisotropically. The H atoms bonded to the aromatic carbons were located in difference Fourier maps but were positioned with idealized geometry and refined isotropically with U _iso(H) = 1.2 U _eq(C) using a riding model. The hydrogen atoms attached to the lattice water O1W could not be located. Technical details of data acquisition and selected refinement results for 1 are given in Table 2.