Niobium oxyiodide clusters from heterogeneous solid-state reactions: the example of Li₃Nb₇O₅I₁₄(NbI₆)

Abstract

No fewer than nine niobium oxyiodide phases have been synthesised and characterised from the reaction between NbI₄ and Li₂O in the presence of Li₂(CN₂) under closed-system conditions at temperatures between 400 °C and 500 °C. Reactions in this system proceed through diffusion-controlled solid–solid pathways as well as gas-phase reactions that extend beyond equilibrium, which enables the concurrent formation of multiple niobium oxyiodide compounds. The crystal structures of these products display a range of architectures based on different niobium oxide clusters, including the [Nb₇O₅] core, which is a characteristic structural motif in the structure of Li₃Nb₇O₅I₁₄(NbI₆), the compound examined in this report.

---

Introduction

Metal-rich halide cluster compounds of group 5 and 6 transition metals are well known. Among binary niobium iodides, well known representatives are Nb₆I₁₁, Nb₃I₈, NbI₃, and NbI₄.^1–3 These compounds are typically synthesised through metallothermic reactions involving binary metal halides. For example, Nb₆I₁₁ is commonly prepared from Nb₃I₈ and metallic niobium at elevated temperatures,¹ whereas Nb₃I₈ itself is obtained from reactions between niobium powder and niobium pentaiodide.²

The extension of the metal-rich systems with chalcogenide or pnictide anions has greatly broadened this chemistry, leading to the formation of numerous heteroanionic cluster compounds. These compounds can be regarded as architectures in which anions are either substituted or structurally integrated, often necessitating distinct coordination environments. Variations in connectivity and structural motifs are evident, for example, in the structures of one-dimensional compounds such as Nb₆I₉S and ANb₃Br₇S (A = Rb and Cs).⁴ Further structural diversity is observed in chalcogenide-containing compounds such as Nb₃TeI₇,⁵ Nb₄Se₄I₄,⁶ and Nb₇S₂I₁₉,⁷ as well as in layered pnictide analogues like Nb₄PnX₁₁ (Pn = N and P; X = Cl, Br, and I).⁸ Beyond differences in the number of Nb atoms constituting the cluster cores, these compounds display pronounced variations in their electronic structures and properties.

Niobium oxyiodides such as NbOI₃, NbO₂I, and the metal-rich NbOI₂ have been known for a long time and studied intensively.⁹ Condensed niobium oxide iodides are highly volatile and can be crystallised only within narrow equilibrium conditions. This highlights the sensitivity of these systems to parameters such as temperature, pressure, and composition, all of which profoundly influence the formation of distinct niobium oxyiodide phases.

The semiconducting compounds NbOX₂ (X = Cl, Br, and I) can be synthesised from Nb powder, Nb₂O₅, and NbX₅ and have been studied for applications such as photocatalytic water splitting,¹⁰ 2D dielectric capacitors,¹¹ and ferroelectric materials.^11,12 Metallothermic reduction remains a conventional and efficient route for preparing NbOX₂ and related metal-rich metal halides. An alternative approach, introduced by L. Messerle et al., uses bismuth as a “non-conventional” reducing agent.¹³ Additional soft reduction pathways have been reported using transition metals M = Cr, Mn, Fe, Co, or Ni.^14,15 Owing to their lower electropositivity, these reduction agents facilitate the formation of multiple intermediate compounds with progressively decreasing oxidation states, as has been exemplified by the reduction cascade from WCl₆ to WCl₂, which illustrates a distinctive reduction mechanism.¹⁵ Following this mechanism, the reductant metal (M) is first incorporated and then released as MCl_x.

Lithium carbodiimide is known to serve as a reactant delivering (NCN)²⁻ ions in solid-state metathesis (SSM) reactions with metal halides, yielding metal carbodiimides.¹⁶ However, when it is reacted with more electropositive metal halides, Li₂(CN₂) instead acts as a reducing agent. This has been demonstrated by its reaction with NiCl₂, which produces metallic nickel along with C₃N₄.¹⁷ In other combinations, Li₂(CN₂) exhibits only a weaker reducing effect.

Transferring these findings, we explored the Nb–O–I system by reacting NbI₄ with Li₂O in the presence of Li₂(CN₂). Our previous results demonstrated that interactions among NbI₄, Li₂O, and Li₂(CN₂) generate a heterogeneous system in which local temperature and pressure variations strongly affect the phase formation.¹⁸ As a result of these investigations, we have already identified a family of Nb₄O_12−x compounds, namely Nb₄OI₁₂, two modifications of Nb₄OI₁₁, and Nb₄OI₁₀, all featuring rectangular [Nb₄O] cluster cores.¹⁹ From the same reaction system, we also obtained Nb₅O₄I₁₁ featuring a [Nb₅O₄] cluster core.²⁰ Subsequent reactions produced Li₃Nb₇O₅I₁₅ and Nb₈O₅I₁₇(NbI₅), which contain [Nb₇O₅] and [Nb₈O₅] cluster cores, respectively; the latter is extended by an additional niobium atom.¹⁸ More recently, we reported two new compounds Nb₆O₃I₁₅ and Nb₁₁O₆I₂₄, forming three-dimensional and one-dimensional assemblies of their crystal structures.²¹ The compound presented in this work, Li₃Nb₇O₅I₁₄(NbI₆), is proposed as the competing phase to the formation of Li₃Nb₇O₅I₁₅.

Results and discussion

The Nb–O–I system was investigated by reacting NbI₄ with Li₂O in the presence of Li₂(CN₂), loaded into fused silica ampoules. Both the previously reported reactions and the one described here were conducted with minor adjustments to the relative amounts of starting materials. The temperature programs applied to the silica ampoules were varied within narrow limits, while the overall reaction volume was kept constant. A natural temperature gradient along the ampoule was consistently evidenced by the occurrence of small amounts of volatile NbOI₂ as a side-phase, which was typically found at the cooler end of the ampoule.

Essentially all niobium oxyiodide compounds discussed in the Introduction section are formed close to 500 °C, appearing in the reaction vessel in different combinations with each other. These compounds occur as crystalline materials, including well-formed single-crystals, distributed across several regions in the silica ampoule. Their spatial distribution highlights the crucial role of gas-phase species in transporting both, reactants and products throughout the system.

Only two compounds, the previously reported Li₃Nb₇O₅I₁₅ ¹⁸ and the new Li₃Nb₇O₅I₁₄(NbI₆), were obtained at the bottom of the silica ampoule. These compounds are likely formed without the involvement of a gas-phase transport process. In addition, Li₃Nb₇O₅I₁₄(NbI₆) is produced at significantly lower temperatures and appears consistently in all reactions, although in varying quantities. At 450 °C, it is observed as a black, poorly crystalline powder. Unfortunately, the crystallinity and the crystal size of this compound were insufficient to fully characterise the compound by single-crystal X-ray analysis with acceptable refinement values. Therefore, the crystalline powder of Li₃Nb₇O₅I₁₄(NbI₆) was analysed by powder X-ray diffraction, and the crystal structure was refined by Rietveld analysis. The corresponding diffraction pattern showing the final structure refinement is presented in Fig. 1, and relevant refinement parameters are summarised in Table 1.

Fig. 1 Powder X-ray diffraction pattern subjected to Rietveld refinement for the crystal structure of Li₃Nb₇O₅I₁₄(NbI₆) at 298 K, with the experimental (red) and calculated (black) intensities, Bragg positions (green), and the difference curve (blue) shown.

Table 1 Selected crystallographic data and Rietveld refinement values of Li₃Nb₇O₅I₁₄(NbI₆)

	Li3Nb7O5I14(NbI6)
CCDC	2493985
Crystal system	Orthorhombic
Space group	Imma
Temperature (K)	298
Unit cell dimensions (Å)	a = 13.5176(1)
	b = 13.4466(2)
	c = 23.7811(3)
Volume (Å^3)	4322.56(9)
Z	4
Wavelength, Cu-Kα, (Å)	1.54060
μ (mm^−1)	128.992
Reflections collected	1856
Parameters	261
RBragg	2.734
Rp/Rwp	3.143/4.2119
X^2	1.3461

---

Overall, this system generates a range of niobium oxyiodides, but these phases are thermally metastable, as evidenced by their transformation into NbOI₂ at temperatures exceeding 500 °C.

The structural refinement of the crystalline powder of Li₃Nb₇O₅I₁₄(NbI₆) by means of Rietveld refinement reveals a close relationship with the structure of Li₃Nb₇O₅I₁₅. Li₃Nb₇O₅I₁₄(NbI₆) was found to crystallise orthorhombically in the space group Imma, featuring four crystallographically distinct niobium sites and three distinct oxygen sites in the structure. The localisation and refinement of lithium ions in the structure was fostered by their presence in special Wyckoff positions 4e of Li1 and Li2, and 4f of Li3.

The crystal structure of Li₃Nb₇O₅I₁₄(NbI₆) is based on a [Nb₇O₅] cluster core (Fig. 2) that has been previously reported as a basic building block in other crystal structures of niobium oxyiodides, obtained in the same reaction system.¹⁸ The niobium-to-niobium distances within the [Nb₇O₅] cluster core of Li₃Nb₇O₅I₁₄(NbI₆) range between 2.71(1) Å and 3.097(9) Å, which are in good accordance with the corresponding distances in Li₃Nb₇O₅I₁₅ (Table 2). This type of cluster core is surrounded by iodine atoms, which can be assigned as inner (i), outer (a), and outer–outer bridging (a–a) ligands when following the nomenclature that has been originally introduced for octahedral cluster compounds.²²

Fig. 2 Structure of the [Nb₇O₅] cluster core inside Li₃Nb₇O₅I₁₄(NbI₆), with labelled niobium atoms.

Table 2 Selected interatomic distances in Li₃Nb₇O₅I₁₄(NbI₆) and Li₃Nb₇O₅I₁₅

Distances/Å	Li3Nb7O5I14(NbI6)	Li3Nb7O5I15 ^18
Nb1–Nb2	2.821(7)	2.8050(4)
Nb1–Nb3	3.097(9)	3.0667(6)
Nb2–Nb3	2.71(1)	2.7883(7)
Nb–O	1.902(6)–2.254(9)	1.9464(6)–2.247(6)
Nb–I^i	2.825(9)–2.842(5)	2.7528(5)–2.7908(3)
Nb–I^a	2.934(9)	2.8761(5)
Nb–I^a–a	2.92(1)–2.994(8)	2.9105(7)–3.0224(5)
Nb–I of [NbI6]^−	Nb4–I4: 2.57(1); Nb4–I6: 2.598(7); Nb4–I7: 2.756(6); Nb4–I8: 3.09(2)	—
Li–I^a	2.722(5)–2.95(2)	2.904(6)–2.97(1)
Li–I^a–a	2.750(6)	3.17(1)
Li–I to other iodides	Li1–I4: 2.83(1); Li2–I8: 2.9(3); Li3–I8: 3.218(5)	Li–I5: 2.64(2)–2.76(2)

---

The surrounding of the cluster core with iodine atoms (Fig. 3) coincides with the pattern obtained in Li₃Nb₇O₅I₁₅,¹⁸ with relevant distances summarised in Table 2. Besides four inner Iⁱ and outer I^a, there are twelve surrounding I^a–a iodine atoms making up a [Nb₇O₅I₄ⁱI₄^aI_12/2^a–a] cluster (Fig. 3) in the structure of Li₃Nb₇O₅I₁₄(NbI₆). The connectivity between adjacent clusters is established by six pairs of I^a–a bridges departing from each of the six corners of niobium atoms of the [Nb₇O₅] core, to build up a three-dimensional network, indicated by broken-off bonds in Fig. 3. The connectivity pattern in this network creates open channels to host molecular (NbI₆)⁻ units with average Nb–I distances of ∼2.73 Å, displayed as a blue polyhedron in Fig. 4. The incorporation of (NbI₅) molecules has been previously reported in Nb₈O₅I₁₇(NbI₅) with an average Nb–I distance of ∼2.66 Å.¹⁸ An example of a (TaBr₆)⁻ ion is represented in the structure of the octahedral cluster compound (Ta₆Br₁₅)(TaBr₆)_0.86 with an average Ta–Br distance of ∼2.53 Å.²³

Fig. 3 Iodide coordination of the [Nb₇O₅] cluster core with the connectivity implied by the broken bonds. Niobium atoms are coloured in blue, oxygen in red and iodide in pink.

Fig. 4 Section of the crystal structure viewed along the a-direction, with (NbI₆)⁻ units shown as blue octahedra.

Lithium ions in the crystal structure of Li₃Nb₇O₅I₁₄(NbI₆) are refined in nearly square-pyramidal (Li1 and Li2) and distorted octahedral (Li3) iodide environments (Fig. 5). A similar square-pyramidal environment of lithium ions was also observed in the structure of Li₃Nb₇O₅I₁₅, here with an average Li–I distance of 2.85 Å. The octahedrally surrounded Li3 atom shows an average Li–I distance of 3.05 Å.

Fig. 5 Iodide coordination of the lithium atoms inside Li₃Nb₇O₅I₁₄(NbI₆).

The crystal structure of Li₃Nb₇O₅I₁₄(NbI₆) displays a close resemblance to that of Li₃Nb₇O₅I₁₅ when substituting the (NbI₆)⁻ with one I⁻ ion. This incorporation of (NbI₆)⁻ as an anion clarifies that Li₃Nb₇O₅I₁₄(NbI₆) can also be described as a double salt, which leads to the formula Li₂[Nb₇O₅I₁₄]·Li[NbI₆]. In this respect, Li₃Nb₇O₅I₁₅ could be as well depicted as Li₂Nb₇O₅I₁₄·LiI.

In spite of the similarity of local structures and bridging iodine atoms in Li₃Nb₇O₅I₁₅ and Li₃Nb₇O₅I₁₄(NbI₆) there are clear differences in the connectivity patterns between clusters in both structures (Fig. 6).

Fig. 6 Comparison of the cluster arrangement in Li₃Nb₇O₅I₁₅ (top) and Li₃Nb₇O₅I₁₄(NbI₆) (bottom) with the created cavities for I⁻ (violet balls) and (NbI₆)⁻ ions (violet polyhedra). Two iodide bridges, which connect the central cluster with six and four adjacent clusters, respectively, are illustrated with pink dashed lines.

For Li₃Nb₇O₅I₁₅, this results in a structural arrangement where each [Nb₇O₅] cluster is surrounded by six adjacent clusters via iodine bridges. In contrast, in Li₃Nb₇O₅I₁₄(NbI₆), the [Nb₇O₅] cluster core connects only to four adjacent clusters via iodine bridges.

Conclusion

The series of previously known metal-rich niobium oxyiodides is complemented by the basic compound Li₃Nb₇O₅I₁₄(NbI₆), which appears as a major phase at 450 °C, but is transformed at slightly higher temperatures into other newly discovered niobium oxyiodide compounds. The crystal structures of Li₃Nb₇O₅I₁₄(NbI₆) and Li₃Nb₇O₅I₁₅ show a close structural relationship to each other, and both phases appear as bottom phases in the reaction vessel, compared to other compounds in this system.

The discovery of the large series of compounds and crystal structures arises from the heterogeneous reaction between NbI₄ and Li₂O, with Li₂(CN₂) serving as a mild reduction agent. These reaction conditions permit niobium, oxygen, and iodine to coexist as gas phase species (e.g. as NbI_x, NbOI₂, I₂, etc.). The composition of the gas phase is thought to be decisive for the formation of multiple compounds in one and the same reaction, allowing formation conditions for several phases, possibly under locally marginal temperature and pressure gradients, or cooling rates, as would be likely expected for non-equilibrium conditions. This finding and recent developments in the field of niobium oxyiodides expand the knowledge beyond the previously reported compounds NbOI₃, NbO₂I and NbOI₂ by the newly reported examples Nb₄OI₁₀, Nb₄OI₁₁ (two modifications), Nb₄OI₁₂, Nb₅O₄I₁₁, Li₃Nb₇O₅I₁₅, Nb₆O₃I₁₅, Nb₈O₅I₁₇(NbI₅), Nb₁₁O₆I₂₄, and Li₃Nb₇O₅I₁₄(NbI₆). Some of these metal-rich compounds have been reported to exhibit semiconducting behaviour, although in practice their applications are limited by their moisture sensitivity, which is characteristic for all reported niobium oxyiodides.

Experimental section

All manipulations of starting materials and products were performed in a glovebox under dry argon with moisture and oxygen levels below 1 ppm. Li₂O (ABCR, 95%) was used as purchased. NbI₄ was synthesised as described in the literature.²⁴ Li₂(CN₂) was synthesised as described before.¹⁶

Synthesis of Li₃Nb₇O₅I₁₄(NbI₆)

Li₃Nb₇O₅I₁₄(NbI₆) was synthesised from NbI₄, Li₂O and Li₂(CN₂). For this purpose, NbI₄ (160.8 mg, 0.268 mmol), Li₂O (2 mg, 0.067 mmol) and Li₂(CN₂) (7.2 mg, 0.135 mmol) were encapsulated into a fused and evacuated silica ampoule with a length of 3 cm and a volume of about 1.5 cm³. The ampoule was heated in a box furnace from room temperature to 450 °C at a rate of 0.1 °C min⁻¹. The holding time was 24 h before the reaction mixture was cooled to room temperature at a rate of 10 °C min⁻¹. A block like, polycrystalline material of Li₃Nb₇O₅I₁₄(NbI₆) was obtained at the bottom of the ampoule in the powder form, besides LiI and an amorphous phase, produced by the decomposition of Li₂(CN₂). The product was washed with dry acetonitrile to remove coproduced LiI. The estimated yield of Li₃Nb₇O₅I₁₄(NbI₆) was about 70%, intermingled with finely divided niobium powder, which is inappropriate for compositional chemical analysis. The crystalline powder appears black and decomposes in moist air.

Powder X-ray diffraction

PXRD patterns of products were obtained using a Stadi-P (STOE, Darmstadt) powder diffractometer with germanium monochromated Cu-K_α1 radiation (λ = 1.5406 Å) and a Mythen 1K detector.

Conflicts of interest

The authors declare no conflicts of interest.

Data availability

Crystallographic data have been deposited at the CCDC under 2493985.²⁵ The data that support the findings of this study are available on request from the corresponding author, H.-J. Meyer.

Acknowledgements

Funding by the Deutsche Forschungsgemeinschaft through grant ME 914/32-1 is gratefully acknowledged.

References

1. A. Simon, H.-G. von Schnering and H. Schäfer, Z. Anorg. Allg. Chem., 1967, 355, 295–310.
2. A. Simon and H.-G. von Schnering, J. Less-Common Met., 1966, 11, 31–46.
3. P. W. Seabaugh and J. D. Corbett, Inorg. Chem., 1965, 4, 176–181; L. F. Dahl and D. L. Wampler, Acta Crystallogr., 1962, 15, 903–911.
4. H.-J. Meyer and J. D. Corbett, Inorg. Chem., 1991, 30, 963–967; H.-J. Meyer, Z. Anorg. Allg. Chem., 1994, 620, 863–866; F. Grahlow, F. Strauß, M. Scheele, M. Ströbele, A. Carta, S. F. Weber, S. Kroeker, C. P. Romao and H.-J. Meyer, Phys. Chem. Chem. Phys., 2024, 26, 11789–11797.
5. M. D. Smith and G. J. Miller, J. Alloys Compd., 1998, 281, 202–205.
6. H. B. Yaich, J. C. Jegaden, M. Potel, M. Sergent, A. K. Rastogi and R. Tournier, J. Less-Common Met., 1984, 102, 9–22.
7. G. J. Miller and J. Lin, Angew. Chem., Int. Ed. Engl., 1994, 33, 334–336.
8. M. Ströbele, O. Oeckler, M. Thelen, R. F. Fink, A. Krishnamurthy, S. Kroeker and H.-J. Meyer, Inorg. Chem., 2022, 61, 17599–17608.
9. J. Rijnsdorp and F. Jellinek, J. Less-Common Met., 1978, 61, 79–82; S. Hartwig and H. Hillebrecht, Z. Naturforsch., 2007, 62, 1543–1548; S. Hartwig and H. Hillebrecht, Z. Anorg. Allg. Chem., 2007, 634, 115–120.
10. L. Pan, Y.-L. Wan, Z.-Q. Wang, H.-Y. Geng and X.-R. Chen, J. Appl. Phys., 2023, 134, 085105.
11. Y. Jia, M. Zhao, G. Gou, X. C. Zeng and J. Li, Nanoscale Horiz., 2019, 4, 1113–1123.
12. C. Liu, X. Zhang, X. Wang, Z. Wang, I. Abdelwahab, I. Verzhbitskiy, Y. Shao, G. Eda, W. Sun, L. Shen and K. P. Loh, ACS Nano, 2023, 17, 7170–7179.
13. V. Kolesnichenko, D. C. Swenson and L. Messerle, Inorg. Chem., 1998, 37, 3257–3262.
14. M. Ströbele, A. Mos and H.-J. Meyer, Inorg. Chem., 2013, 52, 6951–6956; A. Mos, M. Ströbele and H.-J. Meyer, Z. Anorg. Allg. Chem., 2015, 641, 1722–1727; A. Mos, M. Ströbele and H.-J. Meyer, J. Cluster Sci., 2015, 26, 187–198.
15. A. Mos, C. Castro, S. Indris, M. Ströbele, R. F. Fink and H.-J. Meyer, Inorg. Chem., 2015, 54, 9826–9832.
16. R. Srinivasan, M. Ströbele and H.-J. Meyer, Inorg. Chem., 2003, 42, 3406–3411.
17. K. Gibson, M. Ströbele, B. Blaschkowski, J. Glaser, M. Weisser, R. Srinivasan, H. J. Kolb and H.-J. Meyer, Z. Anorg. Allg. Chem., 2003, 629, 1863–1870.
18. J. Beitlberger, M. Ströbele, P. Schmidt, C. P. Romao and H.-J. Meyer, Dalton Trans., 2025, 54, 14376–14383.
19. J. Beitlberger, M. Ströbele, F. Strauß, M. Scheele, C. P. Romao and H.-J. Meyer, Eur. J. Inorg. Chem., 2024, 27, e202400329; J. Beitlberger, M. Martin, M. Scheele, P. Schmidt, M. Ströbele and H.-J. Meyer, Dalton Trans., 2025, 54, 5486–5493.
20. F. Grahlow, J. Beitlberger, M. Martin, E. Juriatti, H. Peisert, M. Scheele, M. Ströbele, C. P. Romao and H.-J. Meyer, Dalton Trans., 2025, 54, 16593–16604.
21. J. Beitlberger, M. Martin, M. Scheele, M. Matas, C. P. Romao, M. Ströbele and H.-J. Meyer, arXiv, 2026, preprint, arXiv:0262.16011,  DOI:10.48550/arXiv:2602.16011.
22. H. Schäfer and H.-G. von Schnering, Angew. Chem., 1964, 76, 833–849; A. Simon, H.-G. von Schnering, H. Wöhrle and H. Schäfer, Z. Anorg. Allg. Chem., 1965, 339, 155–170; B. Baján and H.-J. Meyer, Z. Naturforsch., 1995, 50, 1373–1376.
23. K. Habermehl, A.-V. Mudring and G. Meyer, Eur. J. Inorg. Chem., 2010, 2010, 4075–4078.
24. G. Brauer, Handbuch der präparativen anorganischen Chemie, Enke, 1975.
25. CCDC 2493985: Experimental Crystal Structure Determination, 2026,  DOI:10.25505/fiz.icsd.cc2pq64s.

---