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Article

Heterometallic Catecholates of Zirconium and Alkali Metals

by
Elizaveta A. Filippova
1,
Taisiya S. Sukhikh
1,
Anna A. Tychinina
2,
Ilia V. Eltsov
3,
Alexander S. Novikov
4,5,6,
Dmitriy S. Yambulatov
2,* and
Pavel A. Petrov
1,*
1
Nikolaev Institute of Inorganic Chemistry SB RAS, Lavrentieva Av. 3, 630090 Novosibirsk, Russia
2
N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninsky Prosp. 31, 119991 Moscow, Russia
3
Novosibirsk State University, 630090 Novosibirsk, Russia
4
Institute of Chemistry, Saint Petersburg State University, Universitetskaya Nab. 7/9, 199034 Saint Petersburg, Russia
5
Research Institute of Chemistry, Peoples’ Friendship University of Russia (RUDN University), Miklukho-Maklaya Str. 6, 117198 Moscow, Russia
6
Laboratory for Bio and Chemoinformatics, HSE University, Campus in Saint Petersburg, 25th Liniya, Vasilievsky Ostrov, 6, Korp. 1, 199106 Saint Petersburg, Russia
*
Authors to whom correspondence should be addressed.
Crystals 2026, 16(1), 12; https://doi.org/10.3390/cryst16010012
Submission received: 7 December 2025 / Revised: 18 December 2025 / Accepted: 22 December 2025 / Published: 24 December 2025
(This article belongs to the Special Issue Transition Metal Complexes with Heterocyclic Ligands: Structural Insights, Biological Potential, and Computational Perspectives)
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Abstract

Reaction of [Zr(η5-Cp’)2Cl2] (Cp’ = tBuC5H4) and Na2Cat36 (Cat36 = 3,6-di-tert-butylcatecholate) leads to the formation of the complexes [Na2Zr(Cat36)3(THF)2(C7H8)] (1) and [Zr(η5-Cp’)2(η1-Cp’)2] (2). Complex 1 along with its congeners [K2Zr(Cat36)3(THF)2] (3), [Li(THF)4][LiZr(Cat36)3] (4) and [Li4Zr(Cat36)4(dme)2] (5) were synthesized by the reaction of [ZrCl4(THF)2] and corresponding alkali metal catecholate M2Cat36. The complexes obtained were characterized by means of single-crystal X-ray diffraction and solution NMR spectroscopy (1H, 7Li, 13C). Relatively short Li···H contacts are present in the structures of complexes 4 and 5; nevertheless, DFT calculations have shown no covalent contribution to these interactions.

Graphical Abstract

1. Introduction

Cyclopentadienyl derivatives of early transition metals attract continuous attention for many decades [1]. They are widely used as reagents for organic synthesis (for instance, Tebbe’s [2] and Schwarz’s [3] reagents), olefin polymerization catalysts [4], and anti-cancer reagents [5,6]. More recently, luminescent properties of group 4 metallocenes were discovered [7,8,9]. The luminescence in this case is due to ligand-to-metal charge transfer (LMCT) forming stable excited triplet states [10,11]. This accounts for luminescence that is quite rare for other d0 complexes of group 4 metals. The presence of stable excited 3LMCT states for metallocenes makes them interesting candidates for use in bioimaging [12] or photoredox catalysts [13,14]. Since luminescence was found for metallocene derivatives with a wide range of π-donor ligands such as halides [15], acetylenides [7,8,9], and thiolates [16,17,18], one can expect it from catecholates of group 4 metallocenes as well. However, only scarce data on catecholate derivatives of group 4 metallocenes can be found in the literature [19,20,21]. In addition, catecholates belong to redox-active ligands capable of accepting and donating electrons reversibly. This ability brings an additional option to switch on and off the luminescence of their complexes. Beyond metallocene derivatives, catecholates and other redox-active ligands (such as amidophenolates) are widely used as precursors for complexes with interesting optical [22,23,24,25,26,27], catalytic [28,29,30,31,32] and magnetic [33,34,35,36,37] properties.
Recently, we have shown that the reaction of tert-butyl substituted dimethylzirconocene [ZrCp’2Me2] (Cp’ = η5-tBuC5H4) with 3,6-di-tert-butylcatechole H2Cat36 leads to catecholate complex [ZrCp’2(Cat36)] [20]. On the contrary, the reaction of [ZrCp’2Me2] with 3,6-di-tert-butyl-o-benzoquinone Q36 unexpectedly results in the migration of the methyl group to the cycle and the formation of the unsymmetrical catecholate [ZrCp’2(Cat36Me)] (Cat36Me = 3-tert-butyl-6-methyl-catecholate) [20]. Our initial goal was to study whether the complex [ZrCp’2(Cat36)] can be obtained via the metathesis reaction between [ZrCp’2Cl2] and alkali metal catecholates. It turned out that this reaction resulted in the formation of two novel products, namely, heterometallic catecholate [Na2Zr(Cat36)3(THF)2(C7H8)] and tetrakis(cyclopentadienyl) complex [Zr(η5-Cp’)2(η1-Cp’)2]. Consequently, we aim to synthesize other mixed-metal catecholates of Zr and alkali metals and to study their structures both in the solid state and in solution. The present work describes the reactions of [ZrCp’2Cl2] and [ZrCl4(THF)2] with alkali metal catecholates and the study of their homo- and heterometallic products.

2. Experimental Part

All reactions were carried out using standard Schlenk and vacuum line techniques. 3,6-di-tert-butyl-o-benzoquinone (Q36) was synthesized according to the literature procedure [38]. Commercially available [Cp’2ZrCl2] (Dalchem) and [ZrCl4(THF)2] (Dalchem) were used without purification. The solvents were distilled over drying agents in an argon atmosphere. NMR spectra were recorded in fire-sealed NMR tubes at ambient temperature on a Bruker Avance III 500 FT spectrometer (Bruker AXS Inc., Madison, WI, USA) with an operating frequency of 500.03 MHz for the 1H nucleus, 125.73 MHz for the 13C nucleus, and 194.29 MHz for the 7Li nucleus. Chemical shifts are given in ppm and are referenced to the solvent (C6D6, δ = 7.16 ppm for residual protons in 1H and 128.06 ppm for 13C; toluene-d8, δ = 2.08 ppm for residual protons in 1H); 7Li shifts are given relative to an external standard (1 M solution of LiCl in D2O). The IR spectra were recorded in KBr pellets (prepared in an argon-filled glove box) at room temperature by means of a Simex FT-801 Fourier spectrometer (Simex, Novosibirsk, Russia). Elemental analysis was performed with a Eurovector EuroEA3000 analyzer (EuroVector, Pavia, Italy).

2.1. Synthesis

Interaction of [ZrCp’2Cl2] with Na2Cat36. [ZrCp’2Cl2] (270 mg, 0.67 mmol) was placed into a Schlenk tube and evacuated. A solution of sodium 3,6-di-tert-butylcatecholate (prepared in situ from 147 mg (0.67 mmol) of 3,6-di-tert-butyl-o-benzoquinone and excess sodium) in ~20 mL THF was added. The resulting light-yellow suspension was heated (T = 55 °C) with stirring for two days. The solvent was removed under reduced pressure, the residue was dissolved in toluene and heated overnight at 70 °C. The bright-orange solution was separated by decanting into a two-section L-shaped tube [39], which was fire-sealed. Slow concentration led to the precipitation of colorless crystals of the [Na2Zr(Cat36)3(THF)2(C7H8)]∙C7H8 (1∙C7H8), suitable for X-ray diffraction analysis. Yield 53 mg (23%). IR (KBr, ν, cm−1): 3080 w, 2954 s, 2918 s, 2876 s, 1549 w, 1485 m, 1463 m, 1404 s, 1382 s, 1351 m, 1309 w, 1284 m, 1240 s, 1202 m, 1146 m, 1045 m, 1026 m, 976 s, 936 m, 896 w, 808 m, 790 m, 748 w, 730 w, 686 s, 652 m, 612 w. 1H NMR (C6D6): δ 1.31 (m, 8H, CH22,3 (THF)), 1.49 (s, 54H, C(CH3)3 (Cat)), 2.11 (s, 6H, CH3 (toluene)), 3.39 (m, 8H, CH21,4 (THF)), 6.91 (s, 6H, CH4,5 (Cat)), 7.01–7.15 (m, 10H, CH (toluene)). Calc. for C64H92Na2O8Zr (%): C 68.29; H 8.24. Found, %: C 68.15; H 8.55. The mother liquor after crystallization of 1 was evaporated under reduced pressure, and the residue was dissolved in pentane. The orange solution was separated from the colorless precipitate into a two-section L-shaped ampoule and fire-sealed. Slow concentration led to the formation of a red oil, which was kept at −18 °C for three weeks. Red crystals of the complex [Zr(η5-Cp’)2(η1-Cp’)2] (2), along with some amount of colorless crystals of [ZrCp’2Cl2], precipitated, suitable for X-ray diffraction analysis. The total yield of crystalline product was 143 mg (74%). IR (KBr, ν, cm−1): 2957 s, 2903 s, 2869 s, 1488 m, 1461 s, 1398 m, 1362 s, 1277 m, 1243 w, 1200 w, 1158 m, 1115 w, 1044 m, 1025 w, 916 w, 870 w, 853 w, 800 s, 723 s, 681 m, 695 w. 1H NMR (C6D6): δ 1.31 (s, 36H, CH3), 5.40 (s, 8H, CH3,4), 6.10 (s, 8H, CH2,5). 13C{1H} NMR (C6D6): δ 31.6 (CH3), 33.2 (C(CH3)), 111.0 (CH3,4), 115.4 (CH2,5), 146.4 (C1). 1H NMR (toluene-d8): δ 1.38 (s, 36H, CH3), 5.33 (m, 8H CH3,4), 6.04 (m, 8H, CH2,5).
Synthesis of [Na2Zr(Cat36)3(THF)2(C7H8)]∙C7H8 (1∙C7H8) and [Na2Zr(Cat36)3(THF)2][Na2Zr(Cat36)3(THF)]2∙THF (1′∙THF). An amount of 99 mg (0.26 mmol) of [ZrCl4(THF)2] was placed in a glove box into a Schlenk tube and evacuated. A solution of sodium 3,6-di-tert-butyl catecholate (obtained in situ from 173 mg (0.78 mmol) 3,6-di-tert-butyl-o-benzoquinone and excess sodium) in ~20 mL THF was added to it. Spontaneous heating to room temperature resulted in a pale-yellow suspension, which was heated (T = 55 °C) with stirring for two days. The solvent was removed under reduced pressure, the residue was dissolved in toluene and boiled for 24 h. The pale-blue solution was separated by decanting into an L-shaped tube and fire-sealed. Slow concentration led to the precipitation of colorless crystals of compounds 1 and 1′, suitable for X-ray diffraction analysis. Yield 166 mg (61%).
Synthesis of [K2Zr(Cat36)3(THF)4] (3). An amount of 99 mg (0.26 mmol) [ZrCl4(THF)2] was placed in a glove box into a Schlenk tube and evacuated. A solution of potassium 3,6-di-tert-butyl catecholate (obtained in situ from 174 mg (0.78 mmol) 3,6-di-tert-butyl-o-benzoquinone and excess of potassium in ~20 mL THF) was added. The resulting pale-yellow suspension was heated (T = 55 °C) with stirring for one day. The solvent was removed under reduced pressure; the residue was dissolved in toluene and boiled for 24 h. The pale-blue solution was separated by decanting into a two-section L-shaped tube and fire-sealed. Slow concentration led to the precipitation of colorless crystals suitable for X-ray diffraction analysis. Yield 196 mg (66%). IR (KBr, ν, cm−1): 3078 w, 2954 s, br, 2913 s, 2867 s, br, 1486 m, 1465 m, 1405 s, br, 1356 m, 1283 m, 1242 s, 1203 m, 1145 m, 1052 s, 1026 m, 978 s, 938 m, 922 m, 897 w, 827 w, 809 m, 788 m, 687 s, 652 m, 605 w. 1H NMR (C6D6): δ 1.38 (s, 54H, C(CH3) (Cat)), 1.41 (m, 12H, CH22,3 (THF)), 3.56 (m, 12H, CH21,4 (THF)), 6.92 (s, 6H, CH4,5 (Cat)). Calc. for C58H92K2O10Zr (%): C 62.34; H 8.30. Found, %: C 62.40; H 8.20.
Synthesis of [Li(THF)4][LiZr(Cat36)3]∙THF (4∙THF). An amount of 50 mg (0.13 mmol) [ZrCl4(THF)2] was placed in a glove box into a Schlenk tube and evacuated. A solution of lithium 3,6-di-tert-butyl catecholate (obtained in situ from 87 mg (0.40 mmol) 3,6-di-tert-butyl-o-benzoquinone and excess of lithium in ~20 mL THF) was added to it. The resulting pale-yellow suspension was heated (T = 55 °C) with stirring for three days. The light-yellow solution was separated by decanting into a two-section L-shaped tube and fire-sealed. Slow concentration led to the precipitation of colorless crystals suitable for X-ray diffraction analysis. Yield 43 mg (29%). IR (KBr, ν, cm−1): 3079 w, 2955 s, 2908 m, 2875 m, 1489 m, 1457 m, 1406 s, 1362 w, 1286 w, 1245 s, 1201 w, 1147 w, 1039 m, 978 m, 940 w, 884 w, 791 w, 809 w, 791 w, 702 m, 683 m, 653 m, 559 m, 537 m. 1H NMR (C6D6): δ 1.31 (m, 20H, CH22,3 (THF)), 1.57 (s, 54H, C(CH3)3 (Cat)), 3.45 (m, 20H, CH21,4 (THF)), 6.81 (s, 6H, CH4,5 (Cat)). 7Li NMR (C6D6): δ 0.37. Calc. for C62H100Li2O11Zr (%): C 66.15; H 8.96. Found, %: C 66.10; H 9.05.
Synthesis of [Li4Zr(Cat36)4(dme)2] (5). An amount of 49 mg (0.13 mmol) [ZrCl4(THF)2] was placed in a glove box into a Schlenk tube and evacuated. A suspension of lithium 3,6-di-tert-butyl catecholate (obtained in situ from 115 mg (0.52 mmol) 3,6-di-tert-butyl-o-benzoquinone and excess of lithium in ~20 mL 1,2-dimethoxyethane) was added to it. The resulting pale-yellow suspension was heated (T = 55 °C) with stirring for one day. The light-yellow solution was separated by decanting into a two-section L-shaped tube and fire-sealed. Slow concentration resulted in the precipitation of light-yellow crystals suitable for X-ray diffraction analysis in a yellow oil. Yield 75 mg (48%). IR (KBr, ν, cm−1): 3080 w, 2951 s, 2868 s, 1622 s, 1457 m, 1408 s, 1282 m, 1245 s, 1201 w, 1146 m, 1120 m, 1079 s, 1027 m, 977 m, 940 m, 872 w, 810 w, 785 w, 683 s, 653 s. 1H NMR (C6D6): δ 1.18 (s, 18H, C(CH3)3 (Cat)), 1.66 (s, 18H, C(CH3)3 (Cat)), 2.95 (s, 6H, CH3 (dme)), 2.98 (s, 4H, CH2 (dme)), 6.75 (s, 4H, CH4,5 (Cat)). 7Li NMR (C6D6): δ 0.86. 13C{1H} NMR (C6D6): δ 30.8 (C(CH3)3 (Cat)), 31.6 (C(CH3)3 (Cat)), 33.8 (C(CH3)3 (Cat)), 34.4 (C(CH3)3 (Cat)), 58.7 (CH3 (dme)), 71.1 (CH2 (dme)), 114.3 (CH4,5 (Cat)), 115.7 (CH4,5 (Cat)), 131.7 (C3,6 (Cat)), 132.2 (C3,6 (Cat)), 154.6 (C–O (Cat)), 156.6 (C–O (Cat)). Calc. for C64H100Li4O12Zr (%): C 65.16; H 8.55. Found, %: C 64.90; H 8.35.

2.2. Crystallography

Single-crystal XRD data were collected with a Bruker D8 Venture diffractometer equipped with a CMOS PHOTON III detector (Bruker AXS Inc., Madison, WI, USA) and IµS 3.0 source (Montel mirror optics). All experiments were carried out at 150 K using Mo Kα radiation (λ = 0.71073 Å). The φ- and ω-scan techniques were employed to measure intensities. Absorption correction was applied with the use of the SADABS program Apex3 suite (v. 2019.1-0) [40]. The crystal structures were solved using the SHELXT (v. 2014/5) [41] and were refined using the SHELXL [42] programs with the OLEX2 GUI (v. 1.5.0) [43]. Atomic thermal parameters for non-hydrogen atoms were refined anisotropically, with the exception of highly disordered THF molecules in structures 1 and 4. The positions of hydrogen atoms were calculated corresponding to their geometrical conditions and refined using the riding model, with the exception for that belonging to the methyl groups close to the Li1 atom in structure 5, which were refined freely. The crystallographic data and structure solution details are given in Table S1. CCDC 2384309–2384315 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/structures/ (accessed on 7 December 2025).

2.3. Computational Details

The DFT calculations based on the experimental X-ray geometry of 5 were carried out using the dispersion-corrected hybrid functional ωB97XD [44] with the help of the Gaussian-09 [45] program package. The second-order Douglas–Kroll–Hess scalar relativistic calculations, employing a relativistic core Hamiltonian, were carried out using the DZP-DKH basis sets [46,47,48,49] for all atoms. The topological analysis of the electron density distribution within the “atoms in molecules” (QTAIM) method, ref. [50], noncovalent interactions analysis (NCI), ref. [51], and interaction region indicator analysis (IRI) [52] were performed by using the Multiwfn program (version 3.7) [53]. The VMD program [54] was used for the visualization of NCI and IRI analysis results. The Cartesian atomic coordinates for the model structure are presented in Table S2, Supporting Information.

3. Results and Discussion

Typical methods for the functionalization of bis(cyclopentadienyl) complexes are the metathesis of ligands (most often halides) and the protonation of bis(cyclopentadienyl) complexes containing strongly basic ligands (for example, alkyls). The second route is convenient due to the easily removed by-products (often gaseous), but requires the preliminary preparation of bisalkylmetallocenes, which are usually synthesized from the corresponding dihalides [55,56]. We have previously shown that the reaction of [Cp’2ZrMe2] and H2Cat36 smoothly leads to the protonation of methyl groups and the formation of zirconocene catecholate [Cp’2Zr(Cat36)] [20]. The initial aim of subsequent work was to obtain the same complex by ligand exchange starting from Cp’2ZrCl2 and Na2Cat36.

3.1. Synthesis and Structure of the Complex [Na2Zr(Cat36)3(THF)2(C7H8)] (1)

Heating the reagents in THF resulted in the formation of a colorless precipitate (presumably NaCl) and an orange solution. Evaporation of THF, extraction of the residue with toluene, and crystallization led to the formation of colorless crystals of the complex [Na2Zr(Cat36)3(THF)2(C7H8)]∙C7H8 (1∙C7H8) in moderate yield. It was previously shown that the reaction of titanocene dichloride with potassium catecholate leads to a similar loss of cyclopentadienyl ligands [57].
Complex adopts a cage-like structure (Figure 1) resembling that of clathrochelate complexes [58]. The trigonal-prismatic environment of the Zr atom is confirmed by the SHAPE program (S = 0.483) [59]. Planes O1O3O5 and O2O4O6 are almost perfectly parallel with an angle between them of only 0.2°. The Zr–O distances are in the range 2.0362(15)–2.1256(15) Å (average 2.0917 Å). The complex 1 is a rare example of a complex bearing a six-coordinate zirconium atom in a prismatic environment, and that coordination is not determined by the rigid carbon framework of a multidentate ligand. Instead, this coordination type is stabilized by the sodium atoms that cover both triangular faces of the prism. One of them is coordinated to the three terminal oxygen atoms of the catecholate ligand and the O atom of the THF molecule. The second one is coordinated by two catecholate O atoms, as well as a THF molecule and a toluene molecule in the η6-type (Na–C 2.884(3)–3.105(3) Å, Na–Ccentr 2.663(2) Å). The Na–O bond lengths are in the range 2.2934(19)–2.4091(17) Å. For comparison, in similar heterometallic complexes of titanium and sodium with a non-rigid framework, the distances are 2.252–2.342 Å in [Na2Ti2(OH)2(Cat36)4(THF)2] [60]. Dioxolene ligands are in the catecholate state, which is confirmed by the C–O bond lengths (1.354(3)–1.369(2) Å).
Complex 1 was obtained in higher yield (61%) by the reaction of [ZrCl4(THF)2] with three equivalents of Na2Cat36. In the same synthesis, the co-crystallized complex [Na2Zr(Cat36)3(THF)2][Na2Zr(Cat36)3(THF)]2∙THF (1′∙THF) is formed as a minor impurity. Its formation indicates the relative stability of the clathrochelate-like structure. Complexes 1 and 1′ mainly differ only in the coordination environment of the sodium atoms (Figure S2). In the first molecule of the co-crystal, both sodium atoms are disordered in two positions: bound to three catecholates and to two, which apparently compensates for coordination unsaturation. The Na–O bond lengths are in the range 2.2934(19)–2.4091(17) Å. The second molecule is a dimer, which is linked through the coordination of sodium atoms to the system of the catecholate. Here, only one of the sodium atoms in each fragment is surrounded by three oxygen atoms of the catecholate and a coordinated THF molecule. The second sodium atom is coordinated to the triangular edge of the prism and the plane of the catecholate ring of the adjacent fragment. The distance between the planes of the rings in the fragment is 4.211 Å, and the distance from the sodium atom to the associated ring is 2.494 Å.
The titanium complex with unsubstituted catecholate was reported in 1920 [61]. Numerous titanium tris(catecholate) complexes were reported later, and their coordination and supramolecular chemistry were intensively studied since then [62,63,64] due to potential applications in catalysis [65,66,67] and relevance to biological systems [68]. On the contrary, only one zirconium complex bearing three catecholate ligands is known, namely, [ZrB3(cat)6(Cp*)(THF)] (cat = unsubstituted catecholate C6H4O22−) [69]. However, the coordination sphere of the Zr atom in it is extended by Cp* and THF ligands, resulting in a coordination number of 10.

3.2. Synthesis and Structure of the Complex [Zr(η5-Cp’)2(η1-Cp’)2] (2)

Evaporation of the mother liquor after crystallization of complex 1 and recrystallization of the residue from pentane led to the formation of the homoleptic complex [Zr(η5-Cp’)2(η1-Cp’)2] (2) (Figure 2).
Complex 2 crystallizes in the triclinic group P–1. The Zr–C bond lengths are 2.4107(15) and 2.4027(16) Å for ligands bound by the η1-type and lie in a wide range of 2.4960(14)–2.6226(14) Å for η5-bound Cp’. Previously, several examples of homoleptic complexes were described [MCp4] (M = Ti [70], Zr [71], Hf [72]). The only known homoleptic complex with four methylcyclopentadienyl ligands is [Zr(MeCp)4] (MeCp = MeC5H4) [73] (Table 1). It should be noted that in all cases, Zr complexes adopt structures with three η5 and one η1 cyclopentadienyl ligand, while Ti and Hf form complexes of [M(η5-Cp)2(η1-Cp)2] type. One can assume that in our case, the coordination type [Zr(η5-Cp’)3(η1-Cp’)] is not realized due to the bulkiness of the Cp’ ligand. As a consequence, the M–C bond lengths are significantly longer than those in [Hf(η5-Cp)2(η1-Cp)2] for the same reason. On the other hand, the M–C bond lengths are significantly smaller than those in [Zr(η5-Cp)3(η1-Cp)] and [Zr(η5-MeCp)3(η1-MeCp)] due to the lower coordination number.

3.3. The Structure of [Zr(η5-Cp’2)Cl2]

Along with complex 2, crystallization from pentane yielded a small number of colorless crystals of the starting compound [Zr(η5-Cp’)2Cl2], whose crystal structure deserves a comment. Its room-temperature (RT) modification, assigned to the orthorhombic space group P21212, was reported back in 1986 [75]. Our experiment at 150 K revealed the existence of a superstructure of the doubled unit cell volume (c’ = 2c) with the space group P212121. Most likely, [Zr(η5-Cp’)2Cl2] undergoes the phase transition upon cooling from RT to 150 K. The same second-order transition over a wide temperature range, 150–300 K, was observed for the isostructural titanium analog, [Ti(η5-Cp’)2Cl2] [76]. Both compounds show minor structural changes upon the phase transition. The high-temperature phase comprises the central atom residing on a crystallographic two-fold axis. On the contrary, the low-temperature phase does not possess this axis, so the central atom lies in the general position. The change in the crystal packing consists of half of the molecules shifting along the b direction by ca. 0.3 Å, while the other half does not shift noticeably (Figure S1a). The shifted molecules combine into double layers spreading along the ab plane (Figure S1b).

3.4. Synthesis and Structure of the Complex of [K2Zr(Cat36)3(THF)4] (3)

To clarify the effect of an alkali metal on the structure of heterometallic catecholates, reactions of [ZrCl4(THF)2] with M2Cat36 (M = Li, K) were carried out. It turned out that the structures of potassium- and lithium-containing complexes differ from those of their sodium counterpart. Using the potassium salt K2Cat36, a complex with the composition [K2Zr(Cat36)3(THF)4] (3) was isolated from a solution in THF in the form of large colorless or yellowish crystals (Figure 3). The environment of the Zr atom may be regarded as a highly distorted octahedron. Corresponding S values calculated with the SHAPE program are 4.383 for the octahedron and 5.964 for the trigonal prism, indicating that the experimental Zr environment fits either ideal geometry poorly. The coordination sphere of each K atom consists of two THF molecules, one catecholate oxygen atom, and a contact with the aryl ring of the adjacent catecholate. Corresponding K–C6 distances are equal to 3.003(2) and 3.194(2) Å (C6 is the centroid of the arene ring). The C–Ocat bond lengths are in the range 1.337(4)–1.362(4) Å.

3.5. Synthesis and Structure of the Complex [Li(THF)4][LiZr(Cat36)3]∙THF (4∙THF)

In the case of lithium, different structures were obtained depending on the solvent used: THF yielded [Li(THF)4][LiZr(Cat36)3]∙THF (4∙THF), whereas DME afforded [Li4Zr(Cat36)4(DME)2] (5). In both cases, the structure contains two types of lithium atoms in an oxygen environment, one of which is formally unsaturated in coordination.
Complex [Li(THF)4][LiZr(Cat36)3]∙THF (4∙THF) crystallizes in P63 space group. The Zr atom lies in a special position on the 63 screw axis. A slightly distorted trigonal-prismatic environment of the Zr atom is established by O atoms of three equivalent catecholate ligands (Figure 4). Atom Li2 caps one triangular face of the ZrO6 prism via three catecholate oxygens and is disordered by three positions. Li2–O1 bond equals 2.02(5) Å. Coordination of Li2 results in nonequivalence of C–O bonds in catecholate ligands. While C1–O1 bond lengths equal 1.321(14) Å, the C2–O2 bond (belonging to the bridging oxygen) is elongated to 1.390(13) Å. On the contrary, Zr–O bond lengths are almost equal 2.091(8) and 2.098(7) Å). Atom Li2 is formally coordinatively unsaturated with a coordination number of three. Besides three Li2–O1 bonds, Li2 forms Li∙∙∙H contacts with hydrogen atoms of the THF molecule, coordinated to Li1 with Li∙∙∙H distances of ca. 2.0–2.1 Å. Atom Li1 also lies on the 63 axis; its tetrahedral environment is built of O atoms of four THF ligands. Li1–O bond lengths equal 1.878(13) and 2.03(4) Å. Contrary to Li1, the Li2 atom does not strictly lie on the 63 screw axis and is disordered over three positions, as is the contacting THF molecule.
Reaction of [ZrCl4(THF)2] with in situ-generated Li2Cat36 in 1,2-dimethoxyethane (DME) in a ratio of 1:2 or 1:4 results in the formation of the colorless crystals of a pentanuclear complex [Li4Zr(Cat36)4(DME)2] (5). The environment around the Zr atom is a distorted square antiprism (S(D4d) = 1.0008) [77]. The dihedral angle between the planes defined by atoms O1O2O3O4 and O5O6O7O8 equals 2.6°. Zr–O bond lengths fall within the range 2.1516(11)–2.2553(11) Å. Two different types of catecholates are present in the structure coordinated to two and three Li atoms, respectively (Figure 5). Accordingly, two Li atoms (Li3 and Li4) are coordinated by two catecholate oxygens and two O atoms of the DME molecule. On the contrary, Li1 and Li2 atoms are coordinated by three catecholate oxygens only and are unsaturated in coordination. Li–O bond lengths fall within the range 1.862(3)–2.004(3) Å and 1.878(3)–2.022(3) Å for Li1/Li2 and Li3/Li4, respectively. The unsaturation of Li1 and Li2 atoms is compensated by the contacts with three H atoms of adjacent tert-butyl groups (Figure S3). The quality of the X-ray diffraction experiment allowed us to refine the positions of the hydrogen atoms closest to Li1 and Li2 (i.e., H9C, H14C, H23C, H42C, H50C, and H55C). Corresponding Li∙∙∙H distances fall within the range 2.11(2)–2.51(2) Å.
Unlike numerous examples of tris(catecholates) of group 4 metals, only a few examples are known for tetrakis(catecholates) of zirconium [78,79] and hafnium [80].

3.6. IR Spectra, 1H and 13C NMR Spectra

IR spectra of complexes 1, 3, 4, and 5 show strong bands at 1404–1408 cm−1 and 1240–1245 cm−1 along with less intense bands around 1383 cm−1 characteristic of the catecholate form of o-benzoquinone derivative [81]. 1H NMR spectra in C6D6 confirm the composition of complexes 1–5 and the catecholate state of the dioxolene ligand in them. While signals of coordinated THF in spectra of 1 and 3 are relatively narrow, that of 4 is wider, likely due to exchange between the solvent THF molecule and those coordinated to the Li atom. 7Li NMR signal for 4 (0.37 ppm) is accompanied by several minor peaks likely caused by the same reason, i.e., coexistence of several species with different numbers of coordinated THF molecules. 1H and 13C NMR spectra of complex 2 show only one set of signals, indicating that η5- and η1-Cp’ ligands rapidly exchange at room temperature. This exchange is impossible to cease even upon cooling the solution of 2 in C7D8 down to −80 °C (Figure S10). Earlier, a similar exchange in solution of [ZrCp4] was also observed upon cooling down to −60 °C [82].
1H and 13C NMR spectra of complex 5 show the signals of one asymmetrically coordinated catecholate ligand. In the aromatic part of the 1H spectrum, a wide singlet is present, referring to inequivalent protons with a small difference in chemical shifts (AB system). In the upfield area, signals of two tert-butyl groups are present at 1.66 and 1.18 ppm. Given that tert-butyl groups in the spectra of complexes 3 and 4 are present at 1.38–1.57 ppm, one can assume that the upfield signal at 1.18 ppm for complex 5 is due to agostic-like interaction of tert-butyl groups observed in the crystal structure [83]. On the other hand, 1H-13C spin–spin coupling constants for both tert-butyl groups are equal (1J = 124.7 Hz), which rules out the possibility of agostic interactions. 7Li NMR spectrum of 5 shows one signal at 0.86 ppm. Two-dimensional 1H,7Li HOESY shows no cross peaks, implying no strong covalent component in Li∙∙∙H bonding. To summarize, Li∙∙∙H contacts in complex 5 are non-covalent but rather of an electrostatic nature. Additionally, one can conclude that the asymmetry of the NMR spectra establishes the stereochemical rigidity of the D2-symmetrical eight-coordinate complex, which is preserved in solution.

3.7. Results of DFT Calculations and Topological Analysis of the Electron Density Distribution

Results of DFT calculations and topological analysis of the electron density distribution within the “atoms in molecules” (QTAIM) method [50] for a model structure based on the experimental X-ray geometry of 5 did not reveal any bond critical points (3, −1) for noncovalent interactions Li···H (Figure 6, the Poincare–Hopf relationship was satisfied, and all critical points were successfully located during the QTAIM analysis). Results of noncovalent interactions analysis (NCI) [51] and interaction region indicator analysis (IRI) [52] also revealed the absence of any noticeable interactions between the Li and H atoms in 5 (Figure 7).

4. Conclusions

To summarize, heterometallic catecholates [M2Zr(Cat36)3(THF)x] (M = Li, Na, K; Cat36 = 3,6-di-tert-butylcatecholate) and [Li4Zr(Cat36)4(dme)2] were synthesized by the reaction of [ZrCl4(THF)2] with corresponding catecholates M2Cat36. Complexes [Na2Zr(Cat36)3(THF)2(toluene)] and [Zr(η5-Cp’)2(η1-Cp’)2] (Cp’ = tBuC5H4) were synthesized by the reaction of [ZrCp’2Cl2] Na2Cat36. The complexes obtained were characterized by means of single-crystal X-ray diffraction and solution NMR spectroscopy (1H, 7Li, 13C). Complex [Li4Zr(Cat36)4(dme)2] features three-coordinated Li atoms with relative short Li···H contacts (2.11(2)–2.51(2) Å) completing the Li coordination sphere. Nevertheless, both NMR spectroscopy and QTAIM, NCI, and IRI analyses showed no covalent contribution to Li···H interactions. We plan further investigations towards the functional properties stemming from their unique architecture. The presence of redox-active catecholate centers prompts a detailed electrochemical study to map their electron transfer capabilities and reversible redox states. Furthermore, a primary focus will be the evaluation of the photophysical properties of these complexes, with particular interest in their potential emission in the near-infrared region. These planned studies aim to bridge the structural chemistry presented here with potential applications in molecular photonics with redox control.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/cryst16010012/s1: Table S1: Crystal data and structure refinement for structures 1∙C7H8, 1′∙THF, 2, 3, 4∙THF, 5, and LT-[ZrCp’2Cl2], Figure S1: (a) Overlay of a fragment of the high- (red) and low- (blue) temperature modifications of [Zr(η5-Cp’)2Cl2 (b) Overlay of crystal packings of the high- and low-temperature modifications of [Zr(η5-Cp’)2Cl2] in the setting of the superstructure. Noticeably shifted molecules are highlighted red and blue, negligibly shifted ones are highlighted black, Figure S2: Molecular structure of the complex [Na2Zr(Cat36)3(THF)2][Na2Zr(Cat36)3(THF)]2 (1′), Figure S3: Molecular structure of the complex [Li4Zr(Cat36)4(dme)2] (5), Figure S4: 1H NMR spectrum of 1 (C6D6, 298K), Figure S5: 1H NMR spectrum of 2 (C6D6, 298K), Figure S6: 13C NMR spectrum of 2 (C6D6, 298K), Figure S7: 1H NMR spectrum of 2 (THF-d8, 298K), Figure S8: 1H NMR spectrum of 2 (toluene-d8, 298K), Figure S9: 1H NMR spectrum of 2 (toluene-d8, 253K), Figure S10: 1H NMR spectrum of 2 (toluene-d8, 213K), Figure S11: 1H NMR spectrum of 3 (C6D6, 298K), Figure S12: 1H NMR spectrum of 4 (C6D6, 298K), Figure S13: 7Li NMR spectrum of 4 (C6D6, 298K), Figure S14: 1H NMR spectrum of 5 (C6D6, 298K), Figure S15: 13C{1H} NMR spectrum of 5 (C6D6, 298K), Figure S16: 1H,1H-NOESY spectrum of 5 (C6D6, 298K), Figure S17: 1H,1C-HSQC (red) and HMBC (black) spectra of 5 (C6D6, 298K), Figure S18: 7Li spectrum of 5 (C6D6, 298K), Table S2: Cartesian atomic coordinates for model structure used for DFT calculations and QTAIM, NCI and IRI analyses.

Author Contributions

Conceptualization: P.A.P.; X-ray analysis: T.S.S.; investigation and NMR spectroscopy: E.A.F. and I.V.E.; DFT calculations: A.S.N.; writing—original draft preparation: E.A.F. and P.A.P.; writing—review and editing: A.A.T., D.S.Y., and P.A.P.; project administration: P.A.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the Russian Science Foundation (project 25-73-10021), https://rscf.ru/en/project/25-73-10021/ (accessed on 7 December 2025).

Data Availability Statement

CCDC 2384309–2384315 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/structures/ (accessed on 7 December 2025).

Acknowledgments

DFT calculations and QTAIM, NCI, and IRI analyses were carried out by A.S.N. within the framework of the project “International academic cooperation” at HSE University.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Crystals 16 00012 g001
Figure 1. The molecular structure of the complex [Na2Zr(Cat36)3(THF)2(C7H8)] (1). Zirconium is depicted in blue, sodium in magenta, and oxygen in red. Thermal ellipsoids are drawn at the 30% probability. Hydrogen atoms are omitted for clarity.
Figure 1. The molecular structure of the complex [Na2Zr(Cat36)3(THF)2(C7H8)] (1). Zirconium is depicted in blue, sodium in magenta, and oxygen in red. Thermal ellipsoids are drawn at the 30% probability. Hydrogen atoms are omitted for clarity.
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Figure 2. The molecular structure of complex 2. Zirconium is depicted in blue. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.
Figure 2. The molecular structure of complex 2. Zirconium is depicted in blue. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.
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Figure 3. Molecular structure of the complex [K2Zr(Cat36)3(THF)4] (3). Zirconium is depicted in blue, potassium in magenta, and oxygen in red. Hydrogen atoms are omitted for clarity. Principal bond lengths: K–OTHF 2.604(4)–2.644(3) Å, K–Ocat 2.751(3), 2.784(3) Å.
Figure 3. Molecular structure of the complex [K2Zr(Cat36)3(THF)4] (3). Zirconium is depicted in blue, potassium in magenta, and oxygen in red. Hydrogen atoms are omitted for clarity. Principal bond lengths: K–OTHF 2.604(4)–2.644(3) Å, K–Ocat 2.751(3), 2.784(3) Å.
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Figure 4. Molecular structure of the complex [Li(THF)4][LiZr(Cat36)3] (4). Zirconium is depicted in blue, lithium in magenta, and oxygen in red. Disordered groups and hydrogen atoms (except H21a and H21b) are not shown.
Figure 4. Molecular structure of the complex [Li(THF)4][LiZr(Cat36)3] (4). Zirconium is depicted in blue, lithium in magenta, and oxygen in red. Disordered groups and hydrogen atoms (except H21a and H21b) are not shown.
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Figure 5. Molecular structure of the complex [Li4Zr(Cat36)4(dme)2] (5) Zirconium is depicted in blue, lithium in magenta, and oxygen in red. Tert-butyl substituents are replaced with methyl groups, hydrogen atoms are not shown).
Figure 5. Molecular structure of the complex [Li4Zr(Cat36)4(dme)2] (5) Zirconium is depicted in blue, lithium in magenta, and oxygen in red. Tert-butyl substituents are replaced with methyl groups, hydrogen atoms are not shown).
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Figure 6. Contour line diagram of the Laplacian of electron density distribution ∇2ρ(r), bond paths, selected zero-flux surfaces (left), and visualization of electron localization function (ELF) distribution (right) in the plane of shortest contact Li···H (2.110 Å) in the model structure based on the experimental X-ray geometry of 5. Bond critical points (3, −1) are shown in blue, nuclear critical points (3, −3)are shown in pale-brown color, ring critical points (3, +1)are shown in orange, bond paths are shown as pale-brown lines, length units re given in Å, and the color scale for the ELF map is presented in a.u.
Figure 6. Contour line diagram of the Laplacian of electron density distribution ∇2ρ(r), bond paths, selected zero-flux surfaces (left), and visualization of electron localization function (ELF) distribution (right) in the plane of shortest contact Li···H (2.110 Å) in the model structure based on the experimental X-ray geometry of 5. Bond critical points (3, −1) are shown in blue, nuclear critical points (3, −3)are shown in pale-brown color, ring critical points (3, +1)are shown in orange, bond paths are shown as pale-brown lines, length units re given in Å, and the color scale for the ELF map is presented in a.u.
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Figure 7. Visualization of the NCI (left) and IRI (right) analyses for model structure based on the experimental X-ray geometry of 5 (areas associated with weak O···H but not Li···H noncovalent interactions indicated by transparent gray rectangles with dashed black borders).
Figure 7. Visualization of the NCI (left) and IRI (right) analyses for model structure based on the experimental X-ray geometry of 5 (areas associated with weak O···H but not Li···H noncovalent interactions indicated by transparent gray rectangles with dashed black borders).
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Table 1. Principal geometrical characteristics of complex 2 and its analogs.
Table 1. Principal geometrical characteristics of complex 2 and its analogs.
FormulaSpace GroupM–CcentrM–Cσ∠(CcentrMCcentr)∠(CσMCσ)REFCODEReference
[Zr(η5-Cp’)2(η1-Cp’)2] (2)P–12.242, 2.2442.4107(15), 2.4027(16)133.587.25(6)–this work
[Zr(η5-Cp)3(η1-Cp)]P2121212.294, 2.348, 2.3492.44(2)116.4, 116.7, 119.2–CYPDZR10[74]
[Zr(η5-Cp)3(η1-Cp)]C2/c2.56(3), 2.59(2), 2.60(2)2.447(6)115, 116, 119–TCPYZR[71]
[Zr(η5-MeCp)3(η1-MeCp)]P21/n2.303, 2.353, 2.3262.513115.3, 117.7, 118.6–CIGCOM[73]
[Ti(η5-Cp)2(η1-Cp)2]P61222.0782.332129.986.3TCYPTI10[70]
[Hf(η5-Cp)2(η1-Cp)2]P–421c2.1992.38(2)130.088.2CYPDHF10[72]
[Zr(η5-Cp’)2Cl2] (RT)P212122.218–128.7–FEBNUV[75]
[Zr(η5-Cp’)2Cl2] (150K)P2121212.217, 2.222–128.9––this work
[Ti(η5-Cp’)2Cl2] (RT)P212122.090–131.1–CIZTAG04[76]
[Ti(η5-Cp’)2Cl2] (175K)P2121212.080, 2.084–131.1–CIZTAG02[76]
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Filippova, E.A.; Sukhikh, T.S.; Tychinina, A.A.; Eltsov, I.V.; Novikov, A.S.; Yambulatov, D.S.; Petrov, P.A. Heterometallic Catecholates of Zirconium and Alkali Metals. Crystals 2026, 16, 12. https://doi.org/10.3390/cryst16010012

AMA Style

Filippova EA, Sukhikh TS, Tychinina AA, Eltsov IV, Novikov AS, Yambulatov DS, Petrov PA. Heterometallic Catecholates of Zirconium and Alkali Metals. Crystals. 2026; 16(1):12. https://doi.org/10.3390/cryst16010012

Chicago/Turabian Style

Filippova, Elizaveta A., Taisiya S. Sukhikh, Anna A. Tychinina, Ilia V. Eltsov, Alexander S. Novikov, Dmitriy S. Yambulatov, and Pavel A. Petrov. 2026. "Heterometallic Catecholates of Zirconium and Alkali Metals" Crystals 16, no. 1: 12. https://doi.org/10.3390/cryst16010012

APA Style

Filippova, E. A., Sukhikh, T. S., Tychinina, A. A., Eltsov, I. V., Novikov, A. S., Yambulatov, D. S., & Petrov, P. A. (2026). Heterometallic Catecholates of Zirconium and Alkali Metals. Crystals, 16(1), 12. https://doi.org/10.3390/cryst16010012

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