Small, beautiful and magnetically exotic: {V₄W₂}- and {V₄W₄}-type polyoxometalates

Abstract

Minimal-nuclearity vanadato-tungstate clusters in [{V^IV(dien)}₄W^VI₂O₁₄]·4H₂O (1) and [{V^IV(dien)}₄W^VI₄O₂₀]·6H₂O (2) feature cores of edge-sharing WO₆ octahedra, surrounded by a ring of four vanadyl groups. Surprisingly, the V(IV) centers in both 1 and 2 are ferromagnetically coupled, in contrast to all other known vanadato-polyoxotungstates featuring the ubiquituos V–O–W–O–V exchange pathways.

---

The chemistry of mixed-metal polyoxometalates has witnessed an impressive development during the last few decades, with synthetic and structural aspects, properties and possible applications summarized in several review articles.¹ The first mixed V–W polyanions were reported already in the 19^th century;² efficient synthesis protocols were developed for the Lindqvist-type polyanions [V_xW_6−xO₁₉]^−2−x (x = 1, 2),³ the solution stability of which is strongly pH dependent.⁴ The chemistry of mixed tungstato-vanadate compounds was further developed, resulting primarily in several compounds containing {V_xW_6−x} (x = 1–3) Lindqvist anions, where V and W atoms usually are disordered over all six metal sites.⁵ Few other small W/V complexes are known with N- and O-donor ligand environments: in [L′O(H₂O)V^IV(μ-O)W^VIO₂L]²⁺ (L = 1,4,7-triazacyclononane, L′ = 1,4,7-trimethyl-L), VN₃O₂ and WN₃O₂ moieties are μ-oxo-bridged,⁶ in [V₂O₂(μ-OMe)₂(μ-WO₄)₂(4,4′-di-tert-butyl-2,2′-bipyridine)₂], two VN₂O₃ units are bridged by two WO₄ groups.⁷

After identifying a {V₁₃W₄}-type extended Keggin structure under solvothermal conditions at high pH (ca. 12) in the presence of tris(2-aminoethyl)amine (tren),⁸ we now were able to isolate [(V(dien))₄W₂O₁₄]·4H₂O (1) and [(V(dien))₄W₄O₂₀]·6H₂O (2) (dien = diethylenetriamine, C₄H₁₃N₃) under similar conditions, where a higher reactant V : W ratio (1 : 3 vs. 1 : 4) appears to favor a smaller W nuclearity.‡ The crystal structures feature rare VN₂O₄ and VN₃O₃ moieties interconnected by edge-sharing WO₆ octahedra (Fig. 1).

Fig. 1 Combined polyhedral/ball-and-stick plots of the cluster molecules in 1 (a) and 2 (b). WO₆: grey octahedra, O: red, N: blue, C: black, V: yellow spheres. Terminal V=O vanadyl bonds are emphasized in red. H positions omitted for clarity.

Compound 1 crystallizes in the triclinic space group P1̄ (Table S1†) with all atoms located on general positions. A W₂O₁₀ core composed of two edge-sharing WO₆ octahedra connects to two VON₂ moieties (vanadyl-bidentate diene complexes) via three μ-O sites, and edge-sharing to two VON₃ units (vanadyl-tridentate fac-dien complexes). The four V sites form a planar rhomboid (V⋯V: 3.78 Å and 5.38 Å, V–V–V: 70.8°). The N⋯N distances in the VN₃O₃ octahedron are 2.732, 2.714, and 3.278 Å, and the N–N–N angle amounts to 74°. Vanadium dien complexes are rare, with only two corresponding entries, all of tridentate fac conformation, in the CSD.⁹ In 1, V–N bonds in VN₂O₄ and VN₃O₃ (2.116(4)–2.289(4) Å) exhibit a slight elongation of one V–N bond (V1–N2, Fig. S1†), caused by the trans effect. The V–O bonds (1.620(3)–2.219(3) Å) show the typical short vanadyl V=O bonds (1.633(4) and 1.620(3) Å). A database analysis (CSD) of compounds containing octahedral VN₂O₄ or VN₃O₃ units yielded a slightly smaller mean value around 1.600 Å. The W–O bonds fall into four groups: 1.752(3) Å (O_term), 1.824(3)–1.910(3) Å (μ₂-O), 2.052(3) Å (μ₃-OWV₂), and 2.356(3) Å (μ₃-OW₂V), all typical for polyoxotungstates. In 1, the [(V(dien))₄W₂O₁₄] complexes are arranged in stacks along [100] and [001], and the inter-cluster voids are occupied by crystal water molecules. Intra-cluster N–H⋯O and extensive 3D inter-cluster H bonding interactions stabilize the structure. O6_term is involved in three relatively strong H bonding contacts, which may explain the longer V=O bond, while O7_term has only one such contact (Table S2†). Bond valence sum (BVS) calculations yield values of 4.06/4.09 for V1/V2 and 5.93 for the unique W atom, in line with the formal oxidation states V⁴⁺ and W⁶⁺ in 1.

Compound 2 crystallizes in the monoclinic space group P2₁/n (Table S1†) with all unique atoms being located on general positions. Here, the cluster core consists of four edge-sharing WO₆ octahedra, forming a distorted W₄O₄ cubane. Four independent vanadyl groups each bind to a tridentate dien ligand in fac conformation and to two O atoms of neighboring WO₆ octahedra, resulting in distorted VN₃O₃ octahedral environments, with two shorter (2.127(7)–2.175(7) Å) and one longer (2.263(5)–2.318(7) Å) V–N bond, the latter trans to the terminal vanadyl O site. The resulting V₄ structure is an approximately planar square (V⋯V: 5.94–6.22 Å, root mean square deviation from ideal plane: 0.276 Å). The V–O bonds are similar to those in 1 with one short (1.610(6)–1.628(6) Å, V=O_term) and two longer bonds. The W–O bonds exhibit an identical pattern as in 1. BVS values (V: 3.97–4.17; W: 5.95–6.09) support the proposed oxidation states.

In 2, the charge-neutral clusters are arranged in the (010) plane generating channels along [010]. A similar arrangement is observed in the (100) plane, and a second channel type runs along [100]. As in 1, neighbored clusters are interlinked by N–H⋯O interactions, in addition to extensive H bonding to the crystal water molecules present in these channels.

The magnetic properties of 1 and 2 are represented in Fig. 2 as χ_mT vs. T and M_mvs. B plots. For 1, the ambient temperature (290 K) value of χ_mT is 1.50 cm³ K mol⁻¹ at 0.1 T. This value lies within the range 1.36–1.53 cm³ K mol⁻¹ expected for four non-interacting V^IV centers. Upon cooling χ_mT continuously increases up to a maximum of 1.74 cm³ K mol⁻¹ at 14 K, and subsequently drops off sharply down to 0.77 cm³ K mol⁻¹ at 2.0 K. At 2.0 K, the molar magnetization M_m as a function of the applied field B shows an inflection point at ca. 2.5 T revealing the presence of minor antiferromagnetic exchange interactions (the inflection point here indicates a change of the total spin ground state). Modeling the magnetic properties of 1 utilized the computational framework CONDON, employing a “full model” Hamiltonian,¹⁰ and assumed four identical V(IV) centers in a C_4v-symmetric ligand field, reflecting the pronounced tetragonal distortion typical for vanadyl groups. Five Heisenberg-type exchange interaction pathways between nearest-neighbor V(IV) sites (Fig. 2, inset) are characterized by three independent exchange parameters J₁ (V–O–V and V–O–W^VI–O–V), J₂ (V–O–W^VI–O–V) and J₃ (2 × V–O–W^VI–O–V). The O–W^VI–O bridges here efficiently mediate the coupling via the extended, unoccupied W 6d orbitals. For fitting purposes, the standard spin–orbit coupling constant ζ_3d = 248 cm⁻¹ is taken as a constant,¹¹ and all 10 states of a 3d¹ electron configuration are accounted for in the calculation of single ion (vanadyl) effects and Heisenberg exchange interactions (“−2J” notation), i.e. considering in total 10⁴ states. Finally, we consider the mean-field approach for potential inter-molecular interactions in the solid-state lattice. The least-squares fit (relative root mean squared error, SQ = 1.7%) yields the ligand field parameters (Wybourne notation) B²₀ = 4230 cm⁻¹, B⁴₀ = 23 250 cm⁻¹, B⁴₄ = 31 310 cm⁻¹, the exchange interaction parameters J₁ = +15.6 cm⁻¹, J₂ = –3.7 cm⁻¹, J₃ = +5.9 cm⁻¹, and the mean-field interaction parameter zJ′ = +0.1 cm⁻¹. The ligand field parameters B^k_q describe a ligand field characterized by strong tetragonal distortion generating a well-isolated Kramer's ground state doublet separated from the first excited state by more than 4000 cm⁻¹, reconfirming the almost spin-like behavior of the vanadyl groups. The exchange interaction parameters show predominant ferromagnetic exchange, and the additional antiferromagnetic exchange pathways yields a ground state characterized by S_total = 0, slightly separated (approx. 2 cm⁻¹) from the first excited S_total = 1 state, translating into M_m ≈ 2.0N_Aμ_B as reflected by the inflection point in the M_mvs. B curve. Inter-cluster interactions are almost negligible.

Fig. 2 Magnetic data of compounds 1 (top) and 2 (bottom), and coupling schemes. χ_mT vs. temperature T at 0.1 T; insets: molar magnetization M_mvs. applied field B at 2.0 K. Open circles: experimental data, red solid lines: least-squares fit.

The low-field χ_mT value of 2 at 290 K of 1.45 cm³ K mol⁻¹ falls into the expected range for four non-interacting V^IV centers. Upon cooling χ_mT increases sharply below ca. 50 K, reaching 3.57 cm³ K mol⁻¹ at 2.0 K. At 2.0 K, M_m is linear in B up to 1 Tesla, and indicates saturation for fields larger than 5 T at approximately M_m = 4N_Aμ_B, i.e. pointing to an S_total = 2 ground state, i.e. in line with dominant ferromagnetic exchange interactions in 2. In analogy to the analysis of 1 except for the coupling scheme (four V–O–W–O–V pathways characterized by a single exchange energy J), the least-squares fit (SQ = 3.2%) yields B²₀ = 120 cm⁻¹, B⁴₀ = 30 630 cm⁻¹, B⁴₄ = 29 460 cm⁻¹, J = +2.7 cm⁻¹, and the mean-field interaction parameter zJ′ = +0.1 cm⁻¹. As for 1, the ligand field parameters here correspond to a strong tetragonal distortion of the V ligand field, generating a well-isolated (ca. 6000 cm⁻¹) Kramer's ground state doublet. Note that the common V coordination geometry in 2 is significantly different from 1 (two slightly different site geometries), resulting in different ligand field parameters. The positive J reveals small ferromagnetic nearest-neighbor coupling in 2. The ground state of 2 amounts to S_total = 2, consistent with the observed saturation value of M_m ≈ 4.0N_Aμ_B. As for 1, inter-cluster coupling in 2 is almost negligible.

Compound 2 is soluble in water (0.24 mmol L⁻¹), while the solubility of 1 is extremely low. Positive-mode electrospray ionization of a 100 μM water solution of 2 results in an ESI mass spectrum exhibiting the intact cluster as the singly and doubly protonated species at m/z = 836 und 1672 (Fig. 3). The base peak of the spectrum can be assigned to [(V(dien))₄W₄O₁₉]²⁺ which is most likely formed by elimination of H₂O upon protonation of the cluster. Measurements were performed shortly after preparation of the sample solution in degassed H₂O as the cluster complex was only stable in solution over a period of 30 minutes.

Fig. 3 ESI-Q-TOF-HRMS spectrum of compound 2 (100 μM in H₂O, top); experimental isotopic pattern of dication at m/z 827 and calculated isotopic pattern of [(V(dien))₄W₄O₁₉]²⁺.

In summary, we infer from the two title compounds that the molecular growth of polyoxotungstates at pH ca. 12 appears to be impeded by coordination of VO(dien)²⁺ groups and the associated decrease in negative molecular charge, effectively stopping at {V₄W₂} and {V₄W₄} nuclearities. Comparison to species formed at similar conditions such as the {V₁₃W₄}-type polyanion emphasizes the crucial role of the employed polyamines. These clusters are among the smallest known heterometal polyoxometalates and as such demonstrate the utility of polydentate ligands such as dien in the isolation of novel polyoxometalates structures. To our great surprise, the resulting exchange pathway geometries allow for ferromagnetic coupling between neighboring vanadyl groups, in stark contrast to the usually strongly antiferromagnetic coupling present in larger vanadato-polyoxometalates featuring similar V^IV–O–M^VI–O–V^IV motifs such as the {M^VI₇₂V₃₀} Keplerate polyanions.¹²

Notes and references

1. (a) O. Oms, A. Dolbecq and P. Mialane, Chem. Soc. Rev., 2012, 41, 7497; (b) K. Y. Monakhov, W. Bensch and P. Kögerler, Chem. Soc. Rev., 2015, 44, 8443; (c) A. Proust, R. Thouvenot and P. Gouzerh, Chem. Commun., 2008, 1837; (d) A. Müller, P. Kögerler and H. Bögge, Struct. Bonding, 2000, 96, 203.
2. (a) R. Finkener, Ber. Dtsch. Chem. Ges., 1878, 11, 1638; (b) A. Rosenheim and H. Jahn, Ber. Dtsch. Chem. Ges., 1893, 26, 1191; (c) C. Friedheim, Z. Anorg. Chem., 1894, 6, 11.
3. C. M. Flynn and M. T. Pope, Inorg. Chem., 1971, 10, 2524.
4. (a) C. M. Flynn and M. T. Pope, Inorg. Chem., 1973, 12, 1626; (b) C. M. Flynn and M. T. Pope, Inorg. Chem., 1971, 10, 2745.
5. (a) L. Ouahab, S. Golhen, S. Triki, A. Łapinski, M. Golub and R. Swietlik, J. Cluster Sci., 2002, 13, 267; (b) W. Huang, L. Todaro, L. C. Francesconi and T. Polenova, J. Am. Chem. Soc., 2003, 125, 5928; (c) H. Driss, R. Thouvenot and M. Debbabi, Polyhedron, 2008, 27, 2059; (d) J.-H. Son and Y.-U. Kwon, Inorg. Chem., 2004, 43, 1929; (e) P. T. Ma, C. F. Yu, J. W. Zhao, Y. Q. Feng, J. P. Wang and J. Y. Niu, J. Coord. Chem., 2009, 62, 3117; (f) X. Wang, B. Zhou, C. Zhong and M. Ji, Cryst. Res. Technol., 2006, 41, 874; (g) C. Wang, L. Weng, Y. Ren, C. Du, B. Yue, M. Gu and H. He, Z. Anorg. Allg. Chem., 2011, 637, 472; (h) Y. Xu, J.-Q. Xu, G.-Y. Yang, T.-G. Wang, Y. Xing, Y.-H. Lin and H.-Q. Jia, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 1998, 54, 563.
6. U. Bossek, P. Knopp, C. Habenicht, K. Wieghardt, B. Nuber and J. Weiss, J. Chem. Soc., Dalton Trans., 1991, 3165.
7. S. Kodama, A. Nomoto, S. Yano, M. Ueshima and A. Ogawa, Inorg. Chem., 2011, 50, 9942.
8. M. Rasmussen, C. Näther, J. van Leusen, P. Kögerler and W. Bensch, Eur. J. Inorg. Chem., 2015, 3285.
9. (a) M.-L. Fu, G.-C. Guo, A.-Q. Wu, B. Liu, L.-Z. Cai and J.-S. Huang, Eur. J. Inorg. Chem., 2005, 3104; (b) J. Wang, C. Näther, J. Djamil and W. Bensch, Z. Anorg. Allg. Chem., 2012, 638, 1452.
10. (a) J. van Leusen, M. Speldrich, H. Schilder and P. Kögerler, Coord. Chem. Rev., 2015, 289–290, 137; (b) M. Speldrich, H. Schilder, H. Lueken and P. Kögerler, Isr. J. Chem., 2011, 51, 215.
11. J. S. Griffith, The Theory of Transition-Metal Ions, Cambridge University Press, Cambridge, 1971.
12. P. Kögerler, B. Tsukerblat and A. Müller, Dalton Trans., 2010, 39, 21.

---

Footnotes

† Electronic supplementary information (ESI) available: Experimental, crystallographical and structural details, optical properties and thermal stability data. CCDC 1475726 and 1475727. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6dt02282k

‡ Reaction of 1 mmol NH₄VO₃ and 3 mmol WO₃·H₂O in a mixture of 2 mL concentrated diethylentriamine and 2 mL water in a sealed glass tube at 130 °C afforded green rod-shaped crystals of 1 after 7 d (70% yield based on V). Orange block-shaped crystals of 2 formed under otherwise identical conditions with 1 mmol NH₄VO₃ and 4 mmol WO₃·H₂O (60% yield based on V). CCDC 1475726 (1) and 1475727 (2).

---