High-valent oxovanadium metallosupramolecular species as catalysts for the oxidation of benzyl alcohol derivatives

Abstract

Coordination-driven synthesis has been successfully utilized to prepare vanadium-based metallosupramolecular species. By systematically varying synthesis methods and reaction conditions, two series of isostructural oxovanadium(V) coordination polymers [VO(SIH)(OR)]_n (where SIH²⁻ represents salicylaldehyde isonicotinoylhydrazonate, and R corresponds to CH₃ (1β·0.25CH₃OH), C₂H₅ (2α and 2β·0.25C₂H₅OH), C₃H₇ (3α), C₄H₉ (4α and 4β), and C₅H₁₁ (5β)), were synthesized. Additionally, a phase-pure tetranuclear compound [VO(SIH)(OCH₃)]₄·4CH₃OH (1t·4CH₃OH) was also prepared. In these compounds the [VO(SIH)] units, featuring the isonicotinoylhydrazonate heterocyclic moiety, are interconnected through V–N_{isonicotinoyl} coordination bonds. This linkage enables formation of the one-dimensional (1D) zig–zag chain compounds and zero-dimensional (0D) metallocycle, as determined by single crystal X-ray crystallography. This study further explored the formation and transformation of the assemblies, thereby emphasizing the influence of the ancillary OR⁻ ligand. The compounds were also characterized by powder X-ray diffraction, chemical analysis, thermogravimetric (TGA) measurements, and spectroscopic methods (IR-ATR, UV-Vis, and NMR). The catalytic activity of vanadium coordination entities 1t and 1β was tested for the oxidation of benzyl alcohol and its derivatives, including 2-nitrobenzyl, 2-chlorobenzyl, and 2-methylbenzyl alcohol. Several oxidizing agents were used, including tert-butyl hydroperoxide (TBHP) in aqueous and decane solutions, as well as hydrogen peroxide (H₂O₂). The study also assessed the impact of different solvents, such as toluene, acetonitrile, and methanol, thereby enhancing the understanding of these systems.

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Introduction

High-valent metal–organic species are widely recognized as active oxidants in the catalytic oxidation of organic compounds, serving as a cornerstone in both academic research and industrial applications.^1–4 Their utilization requires careful consideration of several factors to optimize performance and efficiency. These factors include not only the selection of the central metal but also the choice and variation of ligands. Ligand substituents significantly influence the solubility, stability, electronic structure, and steric properties of the complex system, all of which ultimately affect catalyst performance.

Among these systems, oxovanadium complexes are particularly notable due to the advantageous characteristics of the vanadium metal center.^5–7 Vanadium exhibits a variety of oxidation states, different coordination numbers, enhanced oxophilicity, and a good Lewis acidity.^8,9 These properties make vanadium-based catalysts highly effective in promoting the oxidation of organic substrates, including saturated, aromatic, and olefinic hydrocarbons.^10–12 Despite these advantages, the development and application of high-valent oxovanadium metallosupramolecular compounds as catalysts remain relatively underexplored. High-valent vanadium complexes tend to form oxo- or alkoxo-bridged compounds, limiting their use in creating well-defined supramolecular structures.^13–15 To the best of our knowledge, there are only rare examples of VO coordination polymers or cyclic assemblies [VO(L)(L′)]_x (L, L′ = ligands) comprising nitrogen and oxygen donor ligands.^16–20

In contrast, significant progress in this area has been made for high-valent metallosupramolecular species, such as molybdenum(VI) and tungsten(VI) assemblies bearing terminal oxo co-ligands.^21–25 These species are synthesized using carefully designed organic ligands and strict control of the reaction conditions, preventing oligomerization of the highly reactive metal-oxo units through the oxygen atom.^21–24 The size, shape of these assemblies, and the functional groups of all building units are also essential and determine their properties and potential application.^25,26

From a structural perspective, one could anticipate observations of multiple crystal forms for the same metallosupramolecular assembly, however, polymorphism in metal–organic chemistry is seldom reported on; less than 0.5% of metal–organic structures in the Cambridge Structural Database have corresponding polymorphs. Moreover, the occurrence of polymorphism is unevenly distributed, particularly among first-row transition metals. Among these, iron and copper compounds exhibit the highest frequency of polymorph occurrence, aligning with their abundant entries in the database. Vanadium compounds, however, stand out as an exception compared to their periodic table neighbours, displaying a relatively high number of polymorphs compared to the total number of published crystal structures (Fig. 1).

Fig. 1 Periodic table of polymorph occurrence among crystal structures containing metal or semi-metal elements in Cambridge Structural Database. Polymorph group was defined as set of structures with the same refcode root and distances between reduced unit cell vectors of at least 0.5. Number in bold font represents number of polymorph groups (i.e. groups of crystal structures with same composition and molecular structure, but different unit cell or packing of molecules). Number in regular font represents total number of entries containing a particular element. Colour of element box (1) maps normalized polymorph occurrence or (2) is black if number of found polymorph groups is less or equal to 10.

Focusing on neutral vanadium complexes, a few studies have reported polymorphs of vanadium(III), oxovanadium(IV), (alkoxo)oxovanadium(V), and dioxovanadium(V) species. These polymorphs are associated with a variety of ligands, including pyridine derivatives,^27,28 Schiff bases,^29–31 hydrazones, and related compounds.^32–36 Additionally, polymorphism has been observed in heteropolymetallic coordination polymers containing dioxovanadium(V) ions^37,38 and vanadium complexes with porphyrin derivatives.^39–41 Interestingly, polymorphism is predominantly found in compounds lacking strong directional intermolecular interactions, such as hydrogen or halogen bonding. This suggests that vanadium compounds are prone to polymorphism in the absence of significant supramolecular synthons. However, this observation also implies that incorporating well-defined supramolecular synthons in the ligand design might hinder exploration of the polymorphic landscape.

Despite these findings, the structural chemistry of vanadium remains relatively underexplored compared to other transition metal complexes. Consequently, these insights should be regarded as preliminary rather than definitive, until more structural data is collected.

Although constructing metallosupramolecular architectures with oxovanadium(V) building blocks is more challenging, the potential of these types of compounds is highly promising and worth exploring.⁴² In this study, we therefore explored their synthesis by employing a coordination-driven self-assembly methodology as an effective tool for constructing discrete or polynuclear architectures.^43–47 The [VO(SIH)] structural unit, incorporating the salicylaldehyde isonicotinoylhydrazonate ligand (SIH²⁻) has been selected since the ligand features a heterocyclic moiety that provides a coordinating platform with a nitrogen donor atom. The choice of this ligand without additional substituents ensures that neither electron donating nor electron withdrawing groups interfere with the formation of the desired superstructures. In the presence of primary alcohols ROH (C_nH_2n+1OH, n = 1–5) building units can combine and yield metallocyclic [VO(SIH)(OR)]₄ or polymeric assemblies [VO(SIH)(OR)]_n. We also explored transformation of the assemblies by alkoxo OR⁻ ligand exchange.

By changing synthesis method and reaction conditions, two series of isostructural polymorphs were obtained (where R = CH₃ (1β·0.25CH₃OH), C₂H₅ (2α and 2β·0.25C₂H₅OH), C₃H₇ (3α), C₄H₉ (4α and 4β), and C₅H₁₁ (5β)). The structures of these oxovanadium(V) polymers and metallocyclic compound [VO(SIH)(OCH₃)]₄·4CH₃OH (1t·4CH₃OH), were elucidated using single-crystal X-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD) analysis. They were further characterized using elemental analysis, infrared (IR-ATR), UV-Vis, and NMR spectroscopic methods. The thermal stability of the assembled structures was also studied using thermogravimetric analysis (TGA).

These above mentioned findings suggest that similar strategies could be employed to advance the field of high-valent oxovanadium catalysts, opening new avenues for the development of efficient and selective oxidation processes. Due to it, we chose to investigate the oxidation of benzyl alcohol as it is extensively used as a precursor in the synthesis of various products.^48,49 Common oxidizing agents for the oxidation processes, potassium permanganate and sodium dichromate, are causing environmental issues.⁵⁰ While nitric acid is another commonly used and cost-effective oxidant its use leads to the formation of side products that are both toxic and environmentally polluting. Environmentally acceptable oxidants would be hydrogen peroxide or molecular oxygen which were tested for benzaldehyde oxidation assisted by transition metals such as Co, Fe, Sn–W, V and Au.^51–56

Our research was focused on the catalytic performance of V-coordination entities 1t and 1β whose activity was compared to the compounds [VO₂(HSIH)] (6), V₂O₅, and [MoO₂(SIH)]_n.⁵⁷ They were assessed in oxidizing benzyl alcohol and its derivatives: 2-nitrobenzyl, 2-chlorobenzyl, and 2-methylbenzyl alcohol. The oxidation of benzyl alcohol derivatives to the corresponding carbonyl compounds was explored using various oxidizing agents, including tert-butyl hydroperoxide (TBHP) in aqueous and decane solutions, as well as hydrogen peroxide (H₂O₂), while also evaluating the influence of different solvents such as toluene, acetonitrile, and methanol.

Results and discussion

Synthesis and characterization of metallocycle and two series of coordination polymers

The hydrazone derived from salicylaldehyde and isonicotinoylhydrazide was synthesized as described in the literature.⁵⁸ Supramolecular coordination complexes were afforded through the reaction of NH₄VO₃ or [VO(acac)₂], hydrazone (H₂SIH), and the corresponding primary alcohol (C_nH_2n+1OH, n = 1–5) (Table 1, Scheme 1, Procedures I and II). The solvent was evaporated leaving a dark red coloured solution from which a crystalline precipitate formed. All compounds were characterized using various analytical and spectroscopic methods and were identified as tetranuclear metallocyclic [VO(SIH)(OCH₃)]₄·4CH₃OH (1t·4CH₃OH), and infinite zig–zag chain products [VO(SIH)(OR)]_n (where R = CH₃ (1β·0.25CH₃OH), C₂H₅ (2α and 2β·0.25C₂H₅OH), C₃H₇ (3α), C₄H₉ (4α and 4β), and C₅H₁₁ (5β), Table 1). Their formation was driven by the denticity of the ligand and donor atom positions within the linking unit. The crystalline powders were analysed using PXRD and the results were then compared to the simulated powder patterns obtained from the SCXRD data (Fig. S1–S3, see ESI†). In the case of polynuclear assemblies, two series of polymorphic forms were characterized. The structures of 2α, 3α, and 4α belong to the first arrangement (Fig. S2†), while the structures of 1β·0.25CH₃OH, 2β·0.25CH₃OH, 4β, and 5β belong to the second one (Fig. S3†). The observed similarities in the powder diffractograms within each series facilitate the determination of the polymorphic type associated for each compound. Despite our attempts, we didn't obtain 1α, 3β, and 5α forms by the mentioned procedures. Following Procedure I, 4β polymorph can be isolated upon the evaporation of the reaction solution on the same day. In contrast, the 4α form is generated when the solution remains undisturbed for an extended period.

Table 1 Synthetic procedures and obtained metallosupramolecular isomer and polymorphic forms

[VO(SIH)(OR)]x	OR	x	Procedure
1t·4CH3OH	OCH3	4	I
1β·0.25CH3OH	OCH3	n	II
2α	OC2H5	n	I or III
2β·0.25C2H5OH	OC2H5	n	II
3α	OC3H7	n	I or III
4α	OC4H9	n	I or II or III
4β	OC4H9	n	I
5β	OC5H11	n	I or III

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Scheme 1 Structure of the H₂SIH ligand and synthetic pathways to oxovanadium(V) supramolecular species.

Furthermore, by immersing crystals of the tetramer [VO(SIH)(OCH₃)]₄·4CH₃OH (1t·4CH₃OH) in an appropriate coordinating solvents ROH (R = C_nH_2n+1, n = 2–5), the ancillary ligand was replaced with a different one, OR. However, in the case of ethanol and butanol, it resulted in a mixture of polymorphs 2α and 2β·0.25CH₃OH, 4α and 4β, respectively. A similar result was obtained by higher alkoxo ligand substitution when submersing crystals in methanol. Polymer and tetramer with coordinated methoxo ligand were achieved as a mixture.

We were therefore interested in studying the thermally assisted structural transformation of [VO(SIH)(OCH₃)]₄·4CH₃OH (1t·4CH₃OH) in solvent-free conditions. Consistent with findings on its thermal stability (vide infra), a crystalline sample of 1t·4CH₃OH was gradually heated from the ambient temperature up to 165 °C with a heating rate of 10 °C min⁻¹. After being held at that temperature for one hour, the amorphous residue of the thermally activated complex [VO(SIH)]_x was immersed in the corresponding ROH for one week (Procedure III). This procedure resulted in the formation of 1t, 2α, 3α, 4α, and 5β which were identified by IR-ATR and X-ray powder diffraction method (Fig. S4, see ESI†). The formation of polymers can be explained by the cleavage of the V–N_{isonicotinoyl} bonds in the metallocycle and formation of new V–N_{isonicotinoyl} bonds in solution. It is worth noting that the formation of a supramolecular architecture indirectly through the solid-state transformation of vanadium supramolecular complex has never been reported.

In the reaction between the NH₄VO₃ and hydrazone ligand in methanol, followed by the evaporation in an airstream, the [VO₂(HSIH)] (6) compound crystallized. A single-crystal X-ray study of 6 confirmed the structure to be mononuclear. However, it was previously reported (CSD refcode LAMLES).⁵⁹

Description of molecular and crystal structures

High-quality single crystals suitable for single crystal X-ray diffraction were obtained for total of 7 compounds, namely 1t·4CH₃OH, 2α, 3α, 4α, 1β·0.25CH₃OH, 2β·0.25C₂H₅OH and 5β. General and crystallographic data can be found in ESI, Tables S1–S2 and Fig. S5–S7.† Determined crystal structures represent a diverse set of metallosupramolecular assemblies based on the same building block – {VO(OR)(L)} (Fig. 2, top left). The geometry of the monomeric building unit is practically identical across the observed structures: vanadium center is situated in an expected ONO coordination environment provided by a doubly deprotonated hydrazonato ligand in its enolato-imino tautomeric form (Fig. S5†). In each compound, oligomerization or polymerization is facilitated through coordination of the nitrogen atom from an ancillary pyridyl functional group on the sixth coordination site of the oxovanadium(V) centre.

Fig. 2 [VO(OR)(L)] monomer (L = SIH) can oligomerize into porous tetramer 1t, or polymerize into polymorphs α (densely packed 1D chains) or β (porous assembly of 1D chains). Structures of α and β are shown as structural overlays of complexes with 2 and 3 different alkoxy chain lengths, respectively, emphasizing the isostructurality of the assemblies.

Tetranuclear complex 1t crystallizes in the tetragonal space group I4̄. Although the complex has a fourth order rotoinversion symmetry (Fig. 2, top right), the complex conforms to a shape of a diagonally folded square, with vanadium centers significantly shifted from the tetramer equatorial plane (Fig. S7†). Moreover, the complex molecules are stacked along the ring axis, forming large channels comprising up to 20.7% of unit cell volume. This arrangement of molecules is certainly unexpected and could represent a new moment in supramolecular metal–organic frameworks (sMOF), and the detailed investigations are underway. The contents of the void volume could not be modelled from the difference Fourier map, but according to the TGA analysis the voids are natively filled with residual solvent molecules with ratio V : CH₃OH equal to unity.

Besides this tetranuclear complex, two major groups of compounds representing (pseudo)polymorphic pairs were identified, nα and nβ, although a complete structural determination of both polymorphs was only achieved for compound 2, namely 2α and 2β·0.25C₂H₅OH.

Crystallographic analysis showed that compounds in the α form crystallize as densely packed 1D polymeric chains in space group Pbca, whereas those in the β form crystallize in space group Ibca with a different arrangement of 1D polymeric chains, but with the same 2C1 underlying topology (Fig. 2, bottom, Fig. S6†). As evidenced by the structural data, as well as PXRD data, the isolated α and β solid forms are isostructural within each group, up to a difference in the alkoxy chain. The coordination environment of vanadium ion remains consistent throughout the structures. The arrangement of chains in β polymorphs allows for significant void volume in the structures. For instance, 1β contains approximately 9.7% void volume, 2β contains about 12.7%, and 5β also has an appreciable void volume, but its determination is obstructed by disordered pentoxy chains, which partially occupy the voids. It is nevertheless inferred that β-type polymorphs generally permit the formation of channels enclosed by partially disordered alkoxy groups. As in the case of 1t, the difference Fourier map in the channels is featureless, so the final composition is inferred from the void volume, solvent molar volume and TGA analysis.

All three types of assemblies are characterized by the lack of strong or directed supramolecular interactions, as there are no suitable hydrogen bond donors. In line with the introductory findings, the lack of supramolecular interactions favours the existence of polymorphs. However, to the best of our knowledge, the α–β polymorphic system could be the first one where such a stark difference in porosity is observed. This fact, along with no significant stability differences between the polymorphs, could serve as motivation for investigation of this polymorphic system (and related systems lacking strong supramolecular interactions) as solid-state porosity-on-demand materials.

Principal component analysis

We focused on gaining a more profound comprehension of this reaction system. The reaction between NH₄VO₃ and H₂SIH (with initial reactant concentrations of 4.0 × 10⁻⁵ mol dm⁻³) was studied in methanol at ambient temperature using time-dependent UV-Vis spectroscopy (Fig. 3). The time-dependent spectra were analyzed by employing principal component analysis, a 2^nd-order tensor decomposition tool. The results indicated that two principal components could accurately represent the initial dataset (comprising 221 spectra). These components accounted for more than 98% of the total variance in the original data.

Fig. 3 Time-dependent UV-Vis spectra of a reaction mixture of NH₄VO₃ and H₂SIH in methanol at room temperature.

The first principal component accounted for 80.79% of the total variance highlighting a significant reaction involving the coordination of the hydrazonato ligand to vanadium. The second component accounted for 17.35% of the total variance, reflecting a secondary reaction occurring within the system (Fig. 4). Furthermore, the analysis of the UV spectra from the V-hydrazonato complex showed a complete conversion of the initially formed hydrazonato complex into the related dioxovanadium(V) species in very dilute methanolic solutions.

Fig. 4 Principal component scores for the reaction of NH₄VO₃ and H₂SIH in methanol at room temperature monitored in time by UV-Vis spectrophotometry.

Spectroscopic characterization

UV-Vis spectra. The UV-Vis spectra of H₂SIH and coordination complexes 1t and 1–6 were recorded in methanol and acetonitrile. The results of these spectral analyses are illustrated in Fig. S8 and S9, see ESI.† The summarized spectral data, including maximum wavelengths (λ_max) and molar absorptivity values (ε), are presented in Table S4, see ESI.†

In methanolic solution, H₂SIH exhibits distinct absorption maxima at wavelengths of 214 nm, 288 nm, and 333 nm, along with several shoulder peaks (see ESI†). The absorption band at 214 nm is associated with π → π* transitions occurring in the aromatic rings. The bands observed at 288 nm and 333 nm correspond to π → π* and n → π* transitions, respectively, which are associated with the azomethine and carbonyl functionalities.^60,61 In the spectra of methanolic solutions of complexes 1t and 1–6, these intraligand transitions are exhibited at around 230 nm and 320 nm alongside the band at about 404 nm that is assignable to charge transfer from the coordinated hydrazonato ligand to vanadium (LMCT band).⁶⁰

NMR spectra. The ¹H NMR spectra of compounds 1t and 1–5 in CD₃OD displayed a single set of well-resolved and upfield/downfield-shifted chemical resonances for the protons of coordinated SIH²⁻ unit. The azomethine H1 proton of the hydrazone-based ligand (SIH²⁻) was significantly shifted downfield by 0.28 ppm. The protons H7,9 and H6,10 of the isonicotinoyl moiety were slightly upfield and downfield shifted, respectively, in comparison to the free ligand (Scheme S1, Fig. S10, Table S4, in the ESI†). This indicates that in diluted CD₃OD solution the nitrogen atom is not coordinated to the metal ion. The phenyl protons H16 and H13 appeared as a doublet of doublets, while a triplet of doublets with a noticeable upfield shift was observed for H14 and H15.

The ¹H NMR spectra of 1–5 are similar, except for resonances attributed to free ROH. The ¹³C NMR spectra revealed deshielding of the chemical shifts for C1, C4, and C12 carbons, with maximum shifts of 3.00 ppm, 6.81 ppm, and 5.99 ppm, respectively, because of metal complexation. On the other hand, in highly diluted deuterated methanol solutions, it appears that the coordination of the pyridine ring to the coordinatively unsaturated vanadium center is not maintained. The isonicotinoyl carbons C5–C10 exhibited minimal changes in chemical shifts compared to the free ligand, with a maximum of Δδ = 0.98 ppm.

ATR-IR spectra. The O–H, =N–NH, and C=O bands (at 3200 cm⁻¹, 3180 cm⁻¹, and 1683 cm⁻¹, respectively),⁵⁸ which are characteristic of free hydrazone ligand H₂SIH, disappeared upon vanadium–ligand complexation (Fig. S11 and S12 in the ESI†). The presence of bands belonging to C=N_imine (at about 1620 cm⁻¹), C–O_hydrazonato (at about 1345 cm⁻¹), and C–O_phenolic (at about 1275 cm⁻¹) in the spectra of 1t and 1–5 indicated SIH²⁻ coordination to the metal centre via oxygen and nitrogen donor atoms.^21–23 This is further confirmed by the weaker intensity bands for V–N_imine at 750 cm⁻¹, V–O_hydrazonato at 580 cm⁻¹, and V–O_phenolato at 560 cm⁻¹. The spectra also exhibited a medium-intensity stretching band at 1600 cm⁻¹ associated with C=N of the isonicotinoyl pyridine ring moiety. This band is shifted to a lower wavenumber, when compared to free hydrazone, indicating coordination of the isonicotinic-nitrogen atom to the {VO}³⁺ core. A medium intensity band observed at 970 cm⁻¹ was assigned to N–N stretching vibrations.⁶²

Bands at about 710 cm⁻¹ characteristic for V–N bonds corroborated the metallosupramolecular complex formation. The coordination of RO⁻ in complexes 1t and 1–5 was supported by a new absorption band typical of C–O_OR at approximately 1040 cm⁻¹.⁶³ This band was missing in the spectra of the thermally activated complex [VO(SIH)]_x. It should be noted that C–O_OR stretching frequencies in metallosupramolecular species slightly differ from those in the corresponding free alcohol.

The spectra of complexes 1t and 1–5 displayed intense bands in the region of 970–956 cm⁻¹ (Fig. S11 and S12†), which were characteristic of the {VO}³⁺ core.⁶⁴ It should be noted that there is a certain similarity of spectra belonging to the same series of polymorphs.

It is worth noting that the absence of coordinated isonicotinoyl moiety and alkoxide co-ligand in the case of the mononuclear complex 6 resulted in a slightly lower intensity stretching bands at 953 cm⁻¹ and 942 cm⁻¹ belonging to the {O=V=O}⁺ core. The bands characteristic for C–O_phenolic, C=N, and C–O_hydrazone were accompanied by the broad band in the range of 2400–2600 cm⁻¹ assigned to the N–H stretching vibration of the protonated HSIH⁻ ligand.⁶⁵

Thermogravimetric analyses. Metallocycle 1t·4CH₃OH and polymers 1β·0.25CH₃OH and 2β·0.25C₂H₅OH, when left in a desiccator at room temperature for a weak, showed the gradual loss of solvent of crystallization. Whereas crystals of 1β·0.25CH₃OH and 2β·0.25C₂H₅OH were stable at −15 °C, in the case of metallocycle 1t·4CH₃OH partial desolvation of crystals occurred and yielded 1t·2CH₃OH.

The thermal stabilities of all compounds were studied using thermogravimetric analysis (Fig. S13–S17 in the ESI†) by heating the crystalline sample in the temperature range of 25–600 °C under an oxygen atmosphere. The TGA analysis of solvates showed a steady decline at the beginning of the thermogram due to the loss of the lattice solvent molecules followed by the escape of oxidized alkoxide products in the temperature ranges of 29–153 °C and 154–237 °C (for 1t·2CH₃OH), 25–77 °C and 148–256 °C (for 1β·0.25CH₃OH), and 40–144 °C and 148–259 °C (for 2β·0.25C₂H₅OH). The onset temperatures of the first step in TGA are very low, signifying the presence of weak noncovalent interactions in the solid state.

All desolvated polymeric compounds exhibited a comparable pattern of thermal decomposition steps. The mass loss process, due to the oxidation of one alkoxide for each metal–organic unit, started above 100 °C (at 148 °C (for 1β), 165 °C (for 2α), 148 °C (for 2β), 185 °C (for 3α), 135 °C (for 4α), 114 °C (for 4β), 149 °C (for 5β)) with the butoxide derivative being the most unstable and propoxide the most stable. This step was followed by the final hydrazone decomposition of the corresponding complex.

Catalytic studies. To examine the catalytic behaviour of synthesized vanadium complexes, the study utilized complexes 1t and 1β, whose activity was compared to the compounds [VO₂(HSIH)] (6), [MoO₂(SIH)]_n, [VO(SIH)]_x and V₂O₅. The objective was to evaluate the catalytic efficiency of these complexes in benzyl-, 2-nitrobenzyl-, 2-chlorobenzyl and 2-methylbenzyl alcohol oxidations. Benzyl alcohol was selected as the reference substrate due to its relatively simple structure and neutral electronic properties, providing a baseline for comparison. The inclusion of 2-nitrobenzyl and 2-chlorobenzyl alcohols was intended to explore the influence of electron-withdrawing substituents on the reactivity and behaviour of the system, with the nitro and chloro-groups serving as prototypical electron-withdrawing moieties. Conversely, 2-methylbenzyl alcohol was chosen to evaluate the effect of an electron-donating substituent, as the methyl group can impart steric and inductive effects that modulate electronic density in the aromatic ring. This systematic selection enabled a comprehensive investigation of electronic substituent effects on the substrate's chemical properties and reaction mechanisms.

Benzyl alcohol oxidation. The oxidation of benzyl alcohol was systematically studied using various oxidizing agents, including tert-butyl hydroperoxide (TBHP) in both aqueous and decane solutions, as well as hydrogen peroxide (H₂O₂). When TBHP in water was employed as the oxidant, the influence of different solvents, such as toluene, acetonitrile, and methanol, was explored to assess their impact on the reaction. For the oxidation involving hydrogen peroxide, acetonitrile was added to the reaction mixture as a co-solvent. These variations were investigated to evaluate how the choice of oxidant and solvent environment affects the efficiency and selectivity of the oxidation process. The obtained results are compiled in Fig. 5 and Table 2.

Fig. 5 Kinetic profile of benzyl alcohol conversion with catalyst 1t. Light blue presents the reaction with TBHP in decane, purple with TBHP in water, orange with TBHP in water and toluene, yellow with TBHP in water and methanol, grey with TBHP in water and acetonitrile and green with hydrogen peroxide and acetonitrile.

Table 2 Catalytic results of benzyl alcohol oxidation. Reaction conditions: time, 2 h; temperature, 80 °C, n(catalyst)/n(benzyl alcohol)/n(oxidant) = 0.1 mmol/20 mmol/40 mmol

	Catalyst	1t		1β		6		[MoO2(SIH)]n		V2O5		[VO(SIH)]x
	Time/min	120	20	120	20	120	20	120	20	120	20	120	20
a Substrate consumed at the end of the reaction.b Formed aldehyde per converted alcohol at the end of the reaction.c n(substrate) transformed/n(catalyst)/time(h) at 20 min.d n(substrate) transformed/n(catalyst) at the end of reaction.
TBHP in water
No solvent	Conversion^a	85	23	90	70	75	16	14	9	78	3	79	36
	Selectivity^b	18	74	12	33	29	77	50	31	27	2	26	60
	TOF20 min^c	140		391		92		59		42		223
	TON^d	174		168		149		30		323		163
Toluene	Conversion	68	19	63	16	50	3	5	1	30	7	—
	Selectivity	3	70	43	70	56	89	47	63	79	72
	TOF20 min	114		93		21		3		35
	TON	136		123		100		10		52
MeCN	Conversion	47	9	54	11	39	12	6	3	43	8
	Selectivity	66	90	58	85	61	35	53	39	65	63
	TOF20 min	46		61		70		17		49
	TON	83		102		75		12		82
MeOH	Conversion	53	12	52	4	47	3	10	2	45	8
	Selectivity	49	68	49	90	54	88	30	48	55	63
	TOF20 min	63		24		17		15		45
	TON	93		102		91		21		82
TBHP in decane
No solvent	Conversion	85	60	87	67	84	40	14	6	82	46	82	49
	Selectivity	16	38	13	31	16	48	68	46	20	53	19	39
	TOF20 min	346		402		226		34		282		284
	TON	163		175		161		26		169		160
H2O2
MeCN	Conversion	33	25	38	30	77	28	11	4	33	28	42	11
	Selectivity	40	42	36	38	26	49	90	99	39	48	30	76
	TOF20 min	165		197		181		23		172		67
	TON	72		81		168		22		68		85

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Catalysts 1t and 1β show consistently high conversions across various conditions, particularly with TBHP in decane (85%) and TBHP in water (>85%). These values are comparable to 6 [VO₂(HSIH)], suggesting its effectiveness. TOF_{20 min} values are among the highest (346 for 1t and 402 for 1β with TBHP in decane), indicating rapid catalytic activity. Selectivity remains moderate to low across most conditions. For instance, aldehyde selectivity for 1t is only 16% with TBHP in decane and 18% with TBHP in water. This indicates the catalyst prioritizes reactivity over specificity. Catalyst 1β performs slightly better than 1t, with TBHP in decane, it achieves a conversion of 60% and a selectivity of 38%, striking a better balance between reactivity and selectivity. TOF_{20 min} values for 1β (402 with TBHP in decane) are larger than 1t, making it a faster-acting catalyst in benzyl alcohol oxidation. The selectivity parameter is similar for 1β than 1t in aqueous TBHP-driven systems. Fig. 5 illustrates the performance comparison of the tested conditions for catalyst 1t. The data reveal that 1t exhibits the highest activity when TBHP is utilized in decane or water as the oxidant, whereas its activity is significantly reduced when H₂O₂ and MeCN are employed. Furthermore, the choice of solvent added to TBHP in water substantially affects the conversion rate, with the activity following the order: toluene > methanol > acetonitrile. However, aldehyde selectivity is the highest when TBHP in water and MeCN or MeOH were applied as oxidants. Catalyst [VO₂(HSIH)] (6) showed good conversion rates in oxidant-rich conditions, especially with TBHP in water and decane, 75 and 84%, respectively. Aldehyde selectivity in the same oxidants is moderate (13 and 29%, respectively), highlighting its preference for reactivity over product-desired aldehyde.

[MoO₂(SIH)]_n has low conversion parameters under all the tested conditions, although the aldehyde selectivity is quite high, spanning to 90% when hydrogen peroxide with the addition of acetonitrile is used as an oxidant. It can be noted that the Mo catalyst is not very active for the benzyl alcohol oxidations, no matter the oxidant chosen or solvent added to the reaction mixture.

V₂O₅ exhibits high conversion rates with TBHP in water (78%) and decane (82%). Conversion decreases in aqueous TBHP-driven systems when the solvent is added. Aldehyde selectivity with TBHP in water is 27%, while in decane is 20%. In contrast, the addition of solvent to TBHP in water increases the selectivity (79% in toluene, 65% in MeCN, 55% in MeOH). It has to be noted that the aldehyde selectivity remains the same through all the reactions and does not decrease with time, highlighting that the overoxidation of aldehyde to the corresponding carboxylic acid is not present.

[VO(SIH)]_x has been tested with TBHP in water, decane and H₂O₂. It can be observed that in TBHP benzyl alcohol oxidation is quite high, but the aldehyde selectivity remains low, while with H₂O₂ as an oxidant, the conversion of benzyl alcohol and aldehyde selectivity achieve similar values (30% of selectivity and 42% of conversion).

2-Nitrobenzyl, 2-chlorobenzyl and 2-methylbenzyl alcohol oxidations. Two different oxidants, TBHP in water and TBHP in decane were evaluated, Fig. 6 and Table 3. The reactions were monitored for 2 hours, as for some tested catalysts, no aldehyde remained in the reaction mixture beyond this time due to its oxidation to carboxylic acid, Table 3.

Fig. 6 Kinetic profile of benzyl alcohol conversion with catalyst 1t. Blue circles presents the reaction with 2-nitrobenzyl alcohol, green circles with 2-chlorobenzyl alcohol and red circles with 2-methylbenzyl alcohol. Full circles are reaction with TBHP in water and empty circles with TBHP in decane.

Table 3 Catalytic results of 2-chloro, 2-nitro and 2-methyl benzyl alcohol oxidation. Reaction conditions: time, 2 h; temperature, 80 °C, n(catalyst)/n(alcohol)/n(oxidant) = 0.0165 mmol/3.30 mmol/6.60 mmol

Catalyst	1t		1β		6		V2O5		[VO(SIH)]x
Time/min	120	20	120	20	120	20	120	20	120	20
a Substrate consumed at the end of the reaction.b Formed aldehyde per converted alcohol at the end of the reaction.c n(substrate) transformed/n(catalyst)/time(h) at 20 min.d n(substrate) transformed/n(catalyst) at the end of reaction.
2-Chloro benzyl alcohol
TBHP in water
Conversion^a	87	42	89	55	84	44	58	7	90	45
Selectivity^b	32	73	26	58	37	96	62	96	27	73
TOF20 min^c	251		395		28		44		275
TON^d	176		213		173		126		181
TBHP in decane
Conversion	88	74	87	61	88	51	86	68	89	74
Selectivity	30	42	31	40	26	58	29	39	24	43
TOF20 min	460		382		334		439		422
TON	182		180		192		184		171
2-Nitro benzyl alcohol
TBHP in water
Conversion	54	10	71	24	48	11	22	3	53	17
Selectivity	65	98	41	87	70	70	93	51	64	60
TOF20 min	55		132		66		19		93
TON	103		130		93		39		96
TBHP in decane
Conversion	76	56	78	50	78	24	71	39	79	51
Selectivity	32	48	28	42	29	57	36	69	28	49
TOF20 min	298		240		142		201		273
TON	133		125		157		122		142
2-Methyl benzyl alcohol
TBHP in water
Conversion	83	21	93	30	90	6	78	3	93	32
Selectivity	8	73	3	60	5	97	12	87	33	53
TOF20 min	117		170		27		15		185
TON	156		178		144		144		184
TBHP in decane
Conversion	93	65	95	41	86	67	93	67	93	72
Selectivity	3	7	2	22	6	11	3	15	7	8
TOF20 min	429		258		479		441		463
TON	205		200		207		205		199

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For 2-chlorobenzyl alcohol, the catalysts 1t, 1β, [VO₂(HSIH)], and [VO(SIH)]_x achieved high conversion rates (>84%) with both oxidants. Selectivity toward aldehyde at the reaction's conclusion was moderate regardless of the oxidant. However, after 20 minutes, aldehyde selectivity was notably higher with TBHP in water, peaking at 96% with [VO₂(HSIH)]. V₂O₅ exhibited a higher conversion rate with TBHP in decane compared to water (86% vs. 58%), while aldehyde selectivity was superior with TBHP in water (62%) compared to decane (29%). TON values for the catalysts 1t, 1β, [VO₂(HSIH)], and [VO(SIH)]_x are similar, no matter the oxidant used, achieving values 171-213, while V₂O₅ shows the lowest value of 126 with TBHP in water. TOF values for all the catalysts with TBHP in decane are almost two times higher than with TBHP in water.

For 2-nitrobenzyl alcohol, the catalytic activity in water is notably lower compared to decane, whereas selectivity toward the aldehyde shows the opposite trend. The oxidative conversion of aldehyde to carboxylic acid, evident from the comparison of selectivity parameters at 20 and 120 minutes, is less pronounced than in the case of 2-chlorobenzyl alcohol. This difference is likely due to the electron-withdrawing nitro group stabilizing the aldehyde, thereby reducing its susceptibility to overoxidation. Catalysts 1t, [VO₂(HSIH)], and [VO(SIH)]_x demonstrate similar performance trends. Alcohol conversions in water hover around 50%, with aldehyde selectivity at the reaction's conclusion reaching approximately 65%. In decane, alcohol conversions increase to ∼58%, while aldehyde selectivity decreases significantly to ∼30%. Catalyst 1β displays consistent activity irrespective of the oxidant, achieving alcohol conversions exceeding 70%. However, aldehyde selectivity is higher in water, reaching 42% at the reaction's end. Notably, at 20 minutes, aldehyde selectivity in water is nearly maximal, peaking at 98%. The TON values for all catalysts are lower for 2-nitrobenzyl alcohol than for 2-chlorobenzyl alcohol, with slightly higher values observed in decane. The TOF_{20 min} values, however, are significantly higher with TBHP in decane, reaching a maximum of 298 for 1t. These findings highlight the delicate interplay between catalyst, oxidant, and substrate electronic properties in determining reaction efficiency and selectivity.

A comparison of the oxidation reactions of 2-chlorobenzyl alcohol and 2-nitrobenzyl alcohol reveals that 2-nitrobenzyl alcohol is a superior substrate for selective oxidation to aldehydes. This superiority arises from the strong electron-withdrawing properties of the nitro group, which significantly increase the electrophilicity of the benzyl carbon, thereby facilitating the oxidation process under milder conditions. Furthermore, the aldehyde product derived from 2-nitrobenzyl alcohol is stabilized by the resonance and inductive effects of the nitro group, reducing its susceptibility to overoxidation into the corresponding carboxylic acid. In contrast, the weaker electron-withdrawing inductive effect of the chloro substituent in 2-chlorobenzyl alcohol renders the benzyl carbon less electrophilic, necessitating severe oxidation conditions and resulting in a less stable aldehyde product.

The conversion of 2-methylbenzyl alcohol is high, whether TBHP is used in water or decane. However, the aldehyde selectivity showed very low values. The obtained results seem to not favour 2-methylbenzyl alcohol as a substrate. The methyl group is an electron-donating group due to its inductive effect that could reduce electrophilicity at the benzyl position and destabilization of the intermediates compared to the 2-chloro- and 2-nitro-benzyl alcohol. Furthermore, the methyl group introduces significant steric hindrance near the benzyl alcohol group. As expected, the desired aldehyde is less stabilized and more prone to overoxidation to the carboxylic acid under strong oxidative conditions present in the reaction.

Experimental

General details

[VO(acac)₂], [MoO₂(SIH)]_n, and salicylaldehyde isonicotinoyl hydrazone (H₂SIH), were synthesized according to the procedures described in the literature.^57,58,66 Ammonium metavanadate, salicylaldehyde, and isonicotinoyl hydrazide were commercially available. All chemicals and solvents were used without further purification. They were purchased from Alfa Aesar or Aldrich. Details about the instruments and characterization methods (elemental and thermal analyses, ATR-IR, UV, NMR spectroscopies, PXRD, and SCXRD methods) and obtained data can be found in the ESI.† The crystals obtained from the reaction were used directly for the IR-ATR and X-ray crystallographic studies. The samples for elemental analysis were completely desolvated until constant weight and were analysed. The samples for TG measurements were transferred into a desiccator and placed in a freezer (at −15 °C).

Synthesis of the metallosupramolecular compounds

Procedure I. NH₄VO₃ (0.023 g, 0.20 mmol) was added to a solution of H₂SIH (0.051 g, 0.21 mmol) in alcohol (30 mL). The reaction mixture was refluxed for 3 h and left to stand at room temperature. The solution was concentrated under reduced pressure (see below for details). Dark red-black crystals, formed over one week, were isolated via filtration and dried to constant mass in a desiccator (at −15 °C). This procedure afforded 1t·4CH₃OH, 2α, 3α, 4α, 4β, and 5β.

Procedure II. [VO(acac)₂] (0.053 g, 0.20 mmol) and H₂SIH (0.0483 g, 0.20 mmol) were suspended in alcohol (3 mL). The reaction mixture was stirred with a glass rod in a in a nitrogen-filled glovebox for 15 minutes and the suspension was left to stand at room temperature in a closed flask. Dark red-black crystals formed over several weeks were isolated via filtration and dried to constant mass in a desiccator (at −15 °C). This procedure afforded 1β·0.25CH₃OH and 2β·0.25C₂H₅OH.

Procedure III. The crystalline sample of [VO(SIH)(OCH₃)]₄·4CH₃OH (0.02 g, 0.014 mmol) was heated for one hour at 165 °C and the obtained solid was cooled and then added to 1 mL of the corresponding primary alcohol ROH (C_nH_2n+1OH, n = 1–5). The sample of each setup was taken out after one week and immediately examined by ATR-IR spectroscopy and PXRD method. This afforded the corresponding compounds 1t, 2α, 3α, 4α, and 5β, respectively.
[VO(SIH)(OCH₃)]₄·4CH₃OH (1t·4CH₃OH). The complex was prepared by Procedure I in methanol. The solution was concentrated after two days to about 5 mL. Yield: 0.040 g; 54%.

Anal. calcd for 1t, C₅₆H₄₈N₁₂O₁₆V₄ (1348.817): C, 49.87; H, 3.59; N, 12.46. Found: C, 49.68; H, 3.27; N, 12.33%. TG for 1t·2CH₃OH: CH₃OH, 4.18 (calcd 4.54%), CH₃O, 8.53 (calcd 8.79%); V₂O₅, 25.88% (calcd 25.75%). Selected IR data (cm⁻¹): 1622, 1601 (C=N), 1349 (C–O_hydrazone), 1274 (C–O_phenolate), 1041 (C–O_alkoxo), 964 (N–N), 956 (V=O), 759 (V–N_imine), 713 (V–N_{isonicotinoyl}), 627 (V–O)_alkoxo, 582 (V–O)_hydrazonato, 563 (V–O)_phenolato.

[VO(SIH)(OCH₃)]_n·0.25CH₃OH (1β·0.25CH₃OH). The complex was prepared by Procedure II in methanol. Yield: 0.056 g; 81%. Anal. calcd for 1β, C₁₄H₁₂N₃O₄V (337.204): C, 49.87; H, 3.59; N, 12.46. Found: C, 49.68; H, 3.25; N, 12.12%. TG for 1β·0.25CH₃OH: CH₃OH, 1.97 (calcd 2.32%), CH₃O, 8.58 (calcd 8.99%); V₂O₅, 25.99% (calcd 26.34%). Selected IR data (cm⁻¹): 1616, 1600 (C=N), 1345 (C–O_hydrazone), 1279 (C–O_phenolate), 1051 (C–O_alkoxo), 973 (N–N), 959 (V=O), 750 (V–N_imine), 709 (V–N_{isonicotinoyl}), 623 (V–O)_alkoxo, 570 (V–O)_hydrazonato, 559 (V–O)_phenolato.
[VO(SIH)(OC₂H₅)]_n (2α). The complex was prepared by Procedure I in ethanol. The solution was concentrated after one day to about 20 mL and a small amount of precipitated solid filtered the next day. The product 2α was obtained from the filtrate. Yield: 0.027 g; 39%. Anal. calcd for 2α, C₁₅H₁₄N₃O₄V (351.230): C, 51.29; H, 4.02; N, 11.96. Found: C, 51.03; H, 3.86; N, 11.78%. TG for 2α: C₂H₅O, 12.56 (calcd 12.83%); V₂O₅, 25.99% (calcd 25.89%). Selected IR data (cm⁻¹): 1620, 1599 (C=N), 1349 (C–O_hydrazone), 1278 (C–O_phenolate), 1046 (C–O_alkoxo), 969 (N–N, V=O), 756 (V–N_imine), 710 (V–N_{isonicotinoyl}), 633 (V–O)_alkoxo, 587 (V–O)_hydrazonato, 562 (V–O)_phenolato.
[VO(SIH)(OC₂H₅)]_n·0.25C₂H₅OH (2β·0.25C₂H₅OH). The complex was prepared by Procedure II in ethanol. Yield: 0.063 g; 87%.

Anal. calcd for 2β, C₁₅H₁₄N₃O₄V (351.230): C, 51.29; H, 4.02; N, 11.96. Found: C, 50.98; H, 3.79; N, 11.75%. TG for 2β·0.25C₂H₅OH: C₂H₅OH, 3.12 (calcd 3.18%); C₂H₅O, 12.56 (calcd. 12.42%); V₂O₅, 24.82% (calcd 25.07%). Selected IR data (cm⁻¹): 1615, 1598 (C=N), 1344 (C–O_hydrazone), 1277 (C–O_phenolate), 1038 (C–O_alkoxo), 974 (N–N), 964 (V=O), 750 (V–N_imine), 709 (V–N_{isonicotinoyl}), 630 (V–O)_alkoxo, 574 (V–O)_hydrazonato, 560 (V–O)_phenolato.

[VO(SIH)(OC₃H₇)]_n (3α). The complex was prepared by Procedure I in 1-propanol. The solution was concentrated after one day to about 20 mL. Yield: 0.062 g; 85%. Anal. calcd for 3α, C₁₆H₁₆N₃O₄V (365.257): C, 52.61; H, 4.42; N, 11.50. Found: C, 52.45; H, 4.32; N, 11.36%. TG for 3α: C₃H₇O, 16.44 (calcd 16.18%); V₂O₅, 24.62% (calcd 24.90%). Selected IR data (cm⁻¹): 1617, 1596 (C=N), 1348 (C–O_hydrazone), 1277 (C–O_phenolate), 1049 (C–O_alkoxo), 9727 (N–N), 970 (V=O), 756 (V–N_imine), 706 (V–N_{isonicotinoyl}), 629 (V–O)_alkoxo, 590 (V–O)_hydrazonato, 562 (V–O)_phenolato.
[VO(SIH)(OC₄H₉)]_n (4α). (a) The complex was prepared by Procedure I in 1-butanol. After the reaction, the solution was left at room temperature for 5 days and evaporated to 10 mL. The resulting crystalline product was filtered the next day. Yield: 0.054 g; 71%. (b) The complex was also prepared by Procedure II in 1-butanol. Yield: 0.053 g; 70%. Anal. calcd for 4α, C₁₇H₁₈N₃O₄V (379.284): C, 53.83; H, 4.78; N, 11.08. Found: C, 52.62; H, 4.56; N, 10.84%. TG for 4α: C₄H₉O, 18.85 (Calcd. 19.28%); V₂O₅, 24.24% (Calcd. 23.98%). Selected IR data (cm⁻¹): 1614, 1598 (C=N), 1348 (C–O_hydrazone), 1277 (C–O_phenolate), 1042 (C–O_alkoxo), 972 (N–N), 969 (V=O), 754 (V–N_imine), 699 (V–N_{isonicotinoyl}), 630 (V–O)_alkoxo, 585 (V–O)_hydrazonato, 561 (V–O)_phenolato.
[VO(SIH)(OC₄H₉)]_n (4β). The complex was prepared by Procedure I in 1-butanol. After completion of the reaction, the solution was evaporated to about 10 mL the same day. The resulting crystalline product was filtered the next day. Yield: 0.023 g; 30%. Anal. calcd for 4β, C₁₇H₁₈N₃O₄V (379.284): C, 53.83; H, 4.78; N, 11.08. Found: C, 53.66; H, 4.43; N, 10.88%.

TG for 4β: C₄H₉O, 19.12 (calcd 19.28%); V₂O₅, 24.32% (calcd 23.98%). Selected IR data (cm⁻¹): 1621, 1598 (C=N), 1343 (C–O_hydrazone), 1276 (C–O_phenolate), 1060 (C–O_alkoxo), 968 (N–N), 957 (V=O), 750 (V–N_imine), 694 (V–N_{isonicotinoyl}), 629 (V–O)_alkoxo, 577 (V–O)_hydrazonato, 560 (V–O)_phenolato.

[VO(SIH)(OC₅H₁₁)]_n (5β). The complex was prepared by Procedure I in 1-pentanol. The solution was concentrated after one day to about 10 mL. Yield: 0.053 g; 67%. Anal. calcd for 5β, C₁₈H₂₀N₃O₄V (393.310): C, 54.97; H, 5.13; N, 10.68. Found: C, 54.77; H, 4.96; N, 10.48%. TG for 5β: C₅H₁₁O, 21.85 (calcd 22.16%); V₂O₅, 23.28% (calcd 23.12%). Selected IR data (cm⁻¹): 1621, 1598 (C=N), 1344 (C–O_hydrazone), 1275 (C–O_phenolate), 1040 (C–O_alkoxo), 968 (N–N), 958 (V=O), 749 (V–N_imine), 694 (V–N_{isonicotinoyl}), 629 (V–O)_alkoxo, 578 (V–O)_hydrazonato, 560 (V–O)_phenolato.

Synthesis of the mononuclear compound

[VO₂(HSIH)] (6). NH₄VO₃ (0.023 g, 0.2 mmol) was added to a solution of H₂SIH (0.057 g, 0.21 mmol) in methanol (30 mL) and the reaction mixture was left to stand at room temperature for two weeks. The obtained solution was concentrated in an airstream to a third of its volume. An orange powdered product formed at room temperature over one week. Complex 6 was isolated and dried to constant weight. Yield: 0.025 g; 38%. Anal. calcd for 6, C₁₃H₁₀N₃O₄V (323.177): C, 48.31; H, 3.12; N, 13.00. Found: C, 48.13; H, 3.02; N, 12.87%. TG for 6: V₂O₅, 27.90% (calcd 28.13%). Selected IR data (cm⁻¹): 1635, 1603 (C=N), 1351 (C–O_hydrazone), 1258 (C–O_phenolate), 969 (N–N), 953, 942 (V=O), 739 (V–N_imine), 571 (V–O_hydrazonato), 555 (V–O_phenolato).

Catalytic investigation

Reaction protocols for different alcohols.
Benzyl alcohol. A reaction mixture was prepared with benzyl alcohol (2.16 g, 20 mmol), biphenyl (0.1152 g, 0.75 mmol) as the internal standard, and catalyst (0.1 mmol). The mixture was heated to 80 °C prior to the addition of aqueous tert-butyl hydroperoxide (TBHP, 70% w/w, 5.54 mL, 40 mmol) or heated to 70 °C before the addition of hydrogen peroxide (H₂O₂) (4.12 mL, 40 mmol) in the presence of 3 mL of acetonitrile.
2-Nitrobenzyl alcohol. A reaction mixture was prepared with 2-nitrobenzyl alcohol (0.51 g, 3.3 mmol), acetophenone (0.412 g, 3.43 mmol) as the internal standard, and catalyst (0.0165 mmol). The mixture was heated to 80 °C before the addition of aqueous TBHP (70% w/w, 0.91 mL, 6.6 mmol).
2-Chlorobenzyl alcohol. A reaction mixture was prepared with 2-chlorobenzyl alcohol (0.47 g, 3.3 mmol), acetophenone (0.412 g, 3.43 mmol) as the internal standard, and catalyst (0.0165 mmol). The mixture was heated to 80 °C prior to the addition of aqueous TBHP (70% w/w, 0.91 mL, 6.6 mmol).
2-Methylbenzyl alcohol. A reaction mixture was prepared with 2-methylbenzyl alcohol (0.40 g, 3.3 mmol), dodecane (0.225 g, 1.34 mmol) as the internal standard, and catalyst (0.0165 mmol). The mixture was heated to 80 °C prior to the addition of aqueous TBHP (70% w/w, 0.91 mL, 6.6 mmol).

The reaction progress was monitored over a two-hour period by collecting 0.1 mL aliquots of the organic phase at predefined time intervals (0, 20, 40, 60, and 120 minutes). Each aliquot was diluted with 1.4 mL of diethyl ether and subjected to gas chromatography (GC) analysis to assess the conversion and formation of reaction products.

Principal component analysis

Data acquired through time-dependent UV-Vis spectrophotometry during the monitored chemical reaction were exported in ASCII format and arranged into a matrix of numerical entries. This process resulted in a 2^nd-order data tensor characterized by dimensions corresponding to the number-of-spectra × number-of-wavelengths. We then applied principal component analysis (PCA), a second-order tensor reduction technique, to decompose the dataset. PCA facilitates the identification of the best linear projections for a high-dimensional data set based on the least-squares approach. The resulting scores represent the projections of the original sample points on the principal component (PC) direction, thereby enabling a comprehensive description of the reactions in a reduced space. Consequently, each point in the score plots corresponds to one sample UV-Vis spectra. Data were mean-centered and PCA of the covariance matrix was performed utilizing the NIPALS algorithm, which was implemented within our software application, moonee.⁶⁷

Conclusions

This research contributes to a better understanding of the vanadium-based metallosupramolecular polymorphic assemblies, particularly regarding their formation and properties. Namely, two series of isostructural oxovanadium(V) coordination 1D zig–zag chain polymers were synthesized. These include [VO(SIH)(OR)]_n, with R representing different alkyl groups (R = CH₃ (1β·0.25CH₃OH), C₂H₅ (2α and 2β·0.25C₂H₅OH), C₃H₇ (3α), C₄H₉ (4α and 4β), and C₅H₁₁ (5β)). The synthesis involved combining salicylaldehyde isonicotinoylhydrazonate (SIH²⁻) and the corresponding alkoxide (OR⁻) with {VO}³⁺ units. The similarity observed in the powder diffractograms across the series allowed the identification of each compound's polymorphic type. Although detailed studies are still needed, initial results suggest that the formation of the pure phase may be controlled by the suitable reaction conditions – solution concentrations, crystallization temperatures, evaporation rates, solvent selection, and the chosen synthesis method. The occurrence of concomitant polymorphs likely arises from the capacity of the {VO}³⁺ ion and SIH²⁻ ligand to form two crystalline phases with only slight differences in formation energy. Additionally, a phase-pure metallocyclic supramolecular isomer [VO(SIH)(OCH₃)]₄·4CH₃OH (1t·4CH₃OH), was identified. This compound features the interconnection of the mononuclear units into 0D squares through V–N_{isonicotinoyl} coordination bonds, which adds to the diversity of the compounds studied. The research also emphasizes the potential of vanadium-based metallosupramolecular architectures in catalytic applications, further contributing to a better understanding of the structure–activity relationships in these complex systems.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data for 1t·4CH₃OH, 2α, 3α, 4α, 1β·0.25CH₃OH, 2β·0.25CH₃CH₂OH and 5β have been deposited at the Cambridge Structural Data base under with deposition numbers CCDC 2367532–2367538.

Author contributions

Edi Topić: investigation, writing – original draft preparation, review and editing. Josipa Sarjanović: investigation and formal analysis. Danijela Musija: investigation and formal analysis, and writing – original draft preparation. Mirna Mandarić: investigation and formal analysis. Tomica Hrenar: chemometric analysis, writing – original draft preparation, visualization. Jana Pisk: investigation, supervision, formal analysis, writing – original draft preparation. Višnja Vrdoljak: conceptualization, supervision, funding acquisition, investigation, writing – original draft preparation, visualization, review and editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Croatian Science Foundation under the project number HRZZ-IP-2022-10-7368. We acknowledge the support of project CIuK co-financed by the Croatian Government and the European Union through the European Regional Development Fund-Competitiveness and Cohesion Operational Programme (Grant KK.01.1.1.02.0016).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: (1) Powder diffraction patterns, (2) additional figures for compounds, (3) tables of selected bond distances and angles and of hydrogen bonds parameter, (4) UV-Vis spectra, (5) NMR spectra ATR-IR spectra, (6) TGA curves, (7) crystallographic data sets for the structures 1t·4CH₃OH, 2α, 3α, 4α, 1β·0.25CH₃OH, 2β·0.25C₂H₅OH and 5β. CCDC 2367532–2367538. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4ra09073j

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