Molecular engineering of polyoxometalate–porphyrin ion-pair hybrids: crystallographic insight and visible-light driven photocatalytic oxidative coupling of amines

Abstract

The study demonstrates the electrostatic driven self-assembly approach to obtain molecular hybrids (POM–POR) of anionic polyoxometalate (POM) and cationic porphyrin (POR). The single crystal structure of the hybrid ion-pair POM–POR has been elucidated. The study not only provides unique opportunities to study the crystal structure of the hybrid, but also deals with the heterogenization of a homogeneous POR catalyst to study oxidative coupling of benzylamine to N-benzylidenebenzylamine under visible-light irradiation with high selectivity and stability.

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Introduction

Over the past few years, inorganic–organic hybrids have gained immense attention owing to their diverse structural and functional versatility.^1,2 Specifically, polyoxometalate (POM)–porphyrin (POR) hybrids (POM–POR) are projected as novel platforms that possess combined advantages of individual counterparts, which position them for a broad spectrum of applications, including heterogeneous photocatalysis, sensing, energy conversion and energy storage technologies, batteries and CO₂ reduction.^3–9 In such hybrid systems, for instance in photocatalysis, the POR functions as an efficient light-harvesting antenna, absorbing visible photons and generating excited states that subsequently transfer electrons to the adjacent POM cluster, which serves simultaneously as an electron reservoir and catalytic mediator.^10a–c These hybrids mitigate one of the central drawbacks, namely poor photostability of POR-based photocatalysts as POM clusters present in assemblies protect the meso positions of POR from the attack of reactive oxygen species (ROS) during photocatalysis,^11a enabling us to explore their use as visible light active photocatalysts for reactions of significant interest, for instance oxidative coupling of amines to imines that are essential intermediates for the synthesis of fine chemicals.^11b,c This photocatalytic process is of particular interest as it yields imines in a sustainable way as opposed to the conventional method of imine synthesis using a Lewis acid catalyst, dehydrating agents and harsh conditions such as high temperatures and pressures.^11b

A variety of design strategies have been developed to integrate POR with POM with the aid of covalent/non-covalent interactions.^12–15 One of the widely adopted approaches involves the tris-alkoxylation of POMs with PORs that yields discrete hybrid compounds and/or covalent organic frameworks (COFs).¹⁶ Conversely, POMs can be grafted onto metalloporphyrins or metalloporphyrins onto POMs, via pyridyl rings that establish direct covalent linkages.¹⁷ Suzuki and co-workers reported another approach to construct a polyoxotungstate (POW)–POR hybrid featuring a face-to-face stacked porphyrin dimer fastened by four lacunary POWs.¹⁸ An et al. recently reported a POM–POR hybrid via the encapsulation of POMs into POR-based porous cationic polymers.¹⁹ Ionic self-assembly manifested by electrostatic interactions between POM and POR charged moieties represents another attractive approach to obtain POM–POR hybrids.²⁰ Although electrostatic interaction-driven assembly strategies are reported in the literature, very little crystallographic evidence has been reported for ion-pair POM–POR hybrids.^15,21 It should be noted that these ion-pair hybrids are soluble in DMF and DMSO only, which poses a challenge in their crystallization process. Although a few crystallographically characterized POM–POR hybrids have been reported, the scarcity of structural information continues to impede a comprehensive understanding of their catalytic behaviour. The crystallization of these ion-pair hybrids having geometrically mismatched species, such as spherical POMs and planar POR derivatives, represents a unique supramolecular challenge to this end.

The present study discusses our approach in obtaining crystalline ion-pair hybrids via simple electrostatic interactions by rationally functionalizing POR and POM with suitable groups. The as-obtained hybrids were explored as visible light-active photocatalysts for the oxidative coupling of benzylamine (BzAM) to N-benzylidenebenzylamine (BzIM) with oxygen as the oxidant.

Results and discussion

Tetrapyridyl porphyrin (TPP) was methylated at peripheral nitrogen atoms of pyridine rings to form a cationic porphyrin. Direct combination of the tetraiodide salt of the cationic porphyrin with tetrabutylammonium hexamolybdate results in immediate precipitation of a hybrid solid as shown in Fig. 1 (section 3, SI, for more details on the synthesis of POM and POR), suggesting rapid ion exchange and formation of an insoluble and charge-balanced species. The as-obtained hybrid materials were characterized using various physicochemical characterization techniques, prior to single crystal X-ray and photocatalytic studies. The preliminary structural information of the POM–POR hybrid was obtained using ¹H NMR (Fig. S1–S3) and FTIR (Fig. S4). The FTIR spectrum of the POM–POR hybrid exhibited characteristic Mo–O stretching vibrations between 950 and 780 cm⁻¹, confirming the presence of the POM moieties, along with distinct C=C and C–N stretching bands of the porphyrin in the 1600–1400 cm⁻¹ region. The absence of peaks corresponding to the tetrabutylammonium ion in the POM–POR hybrid, typically observed in [Bu₄N]₂[Mo₆O₁₉], clearly indicates the formation of the hybrid (Fig. S4). Thermogravimetric analysis (TGA) reveals that the hybrid was thermally stable till 400 °C (Fig. S5). As evident from the FESEM images, the POM–POR hybrid possesses a one dimensional (1D) morphology (Fig. S6). The Raman spectra of POR, POM and the POM–POR hybrid are shown in Fig. S7. The spectra consist of characteristic Raman bands corresponding to POR and POM. Besides, the signatures of the POR and POM are present in the POM–POR hybrid, as expected. Notably, slight shifts in Raman bands corresponding to porphyrin are observed in the case of the POM–POR hybrid as compared to those of pristine porphyrin. This could be due to the presence of POM moieties in the proximity of porphyrin, which is in line with single crystal X-ray studies that will be discussed in the subsequent section. Further, the absorption properties of POM, POR and the POM–POR hybrid were studied using UV-vis diffuse reflectance spectroscopy (UV-vis DRS). The absorption spectrum of the POM–POR hybrid shown in Fig. S8A indicates that the hybrid exhibits enhanced visible absorption compared to pure POM. The impressive visible absorption due to the characteristic Soret (∼400 nm) and Q-bands (500–700 nm), as seen in the absorption spectra, makes them effective visible light harvesters. The optical bandgap (E_g) of the POM–POR hybrid determined from the Tauc plot is found to be 1.84 eV (Fig. S8B). All these measurements suggest the successful assembly of POR with POM via the simple electrostatic interactions.

Fig. 1 Schematic illustration of the synthesis of the POM–POR hybrid from N-methylated porphyrin and [Bu₄N]₂[Mo₆O₁₉]. The photographs indicate respective solutions. The solid hybrid obtained after electrostatic assembly is also shown.

Single crystal X-ray studies

The vapor diffusion approach was followed to obtain single crystals of ion-pair POM–POR hybrids. The solid precipitate of the hybrid was dissolved in a minimal amount of dimethylformamide (DMF) and subjected to slow vapour diffusion in a closed vial with different solvents that creates a heterogeneous solvent interface, as schematically depicted in Fig. S9. Over the course of three weeks, dark brown crystals of POM–POR suitable for home lab X-ray diffraction were isolated by diethyl ether diffusion. Single crystal X-ray analysis reveals the molecular formula of the ion-pair hybrid as [H₂TPPMe₄][Mo₆O₁₉]₂·4DMF that crystallizes in the triclinic space group P1̄ (Fig. S10A). Both the porphyrin cation and the [Mo₆O₁₉]²⁻ anion reside on inversion centres at the core of the porphyrin macrocycle and at the central oxygen of the POM, respectively. The structure consists of one tetracationic porphyrin and two hexamolybdate dianions, along with four lattice DMF molecules. The porphyrin macrocycle retains planarity, while the appended pyridinium rings exhibit significant torsion (∼134°) relative to the macrocyclic plane. Geometrical parameters of the POM, including Mo=O bond distances and O–Mo–O bond angles, are consistent with the literature values, affirming its structural integrity within the hybrid assembly.^22,23 In the extended solid-state crystal lattice, a well-ordered layered architecture is observed, wherein cationic porphyrin macrocycles and anionic [Mo₆O₁₉]²⁻ clusters alternate in a regular spatial pattern. The porphyrin units were sandwiched between [Mo₆O₁₉]²⁻ units and vice versa along the crystallographic axis as shown in Fig. 2A. Another set of [Mo₆O₁₉]²⁻ units, oriented differently, occupies the void spaces together with DMF molecules. Each porphyrin unit is surrounded by ten polyoxometalate anions, creating a tightly packed environment around the porphyrin moiety (Fig. 2B). The terminal oxygen atoms of [Mo₆O₁₉]²⁻ strongly interact with the porphyrin plane through hydrogen bonding and π interactions. The C⋯O contacts in the range of 2.951–3.173 Å and O⋯H hydrogen bonds around 2.350–2.676 Å, within van der Waals radii, stabilize the lattice packing (Fig. 2C). The dense packing of porphyrin units within the POM matrix restricts access to the C=C bonds at the meso-positions of the porphyrin core (C16 and C7 atoms) by ROS, and thus expected to effectively prevent oxidative degradation and greatly enhance the photostability of the porphyrin moieties (Fig. 2D).

Fig. 2 (A) The supramolecular arrangement of an alternating sequence of POM–POR assemblies. (B) Cationic porphyrin core surrounded by [Mo₆O₁₉]²⁻ units through O⋯H, interactions. (C) Crystal packing diagram depicting the C⋯O contacts. (D) Crystal structure highlighting dense packing at the meso positions of the porphyrin.

Hirshfeld surface analyses reveal that among all the non-covalent interactions experienced by the porphyrin macrocycle, 43.5% are dominated by C–H_(POR)⋯O_(POM) hydrogen bond contacts and 9.5% of C_(POR)⋯O_(POM) π contacts; both these interactions facilitate the tight crystal packing of the porphyrin within the POM matrix and effectively shield the meso positions of the porphyrin (Fig. S10B).

Photocatalytic studies

After the successful establishment of the crystal structure of the hybrid material, photocatalytic studies were carried out to demonstrate its ability for C–N coupling reactions under visible-light illumination using benzylamine (BzAM) as a model substrate. The products formed during the photocatalytic oxidation of BzAM were analyzed using GC-FID, GC-MS (Fig. S11 and S12) and ¹H NMR (Fig. S13). The GC-FID was utilized for the quantification studies. Prior to photocatalytic measurements using the prepared photocatalysts, control experiments were carried out. As shown in Table 1, no or negligible products were identified when the reaction was carried out without a photocatalyst, in the dark and in a N₂ environment. These measurements confirm the role of photocatalysts, light and molecular oxygen (O₂), respectively, in oxidative coupling of BzAM to BzIM (entries 1–3, Table 1). Further, the conversion of BzAM to BzIM is found to be 14.2% when POM alone was used as the photocatalyst (entry 4, Table 1). When the photocatalyst used was POR, a 91% conversion and 100% selectivity towards BzIM are noted for 30 min of illumination (entry 5, Table 1). Despite achieving good conversion and selectivity of POR towards BzIM formation, the use of POR as a photocatalyst will be challenging as the reaction occurs through homogeneous catalysis. This challenge can be circumvented using the present strategy, wherein the heterogenization of POR-based catalysts is achieved by making POM–POR hybrids via a simple approach. Additionally, synergistic interactions owing to charge transfer between POM and POR may facilitate the kinetics of the BzAM oxidation process. As expected, the POM–POR hybrid exhibits excellent photocatalytic activity with 100% conversion of BzAM and 100% selectivity towards BzIM in 30 min of light illumination (Fig. 3a) (entry 6, Table 1). Notably, the hybrid catalyst can be easily separated after the reaction unlike the pristine POR catalyst. Further, the turnover frequency (TOF) for the POM–POR hybrid is found to be 175 h⁻¹, which is comparable to the POM and POR-based photocatalysts reported in the literature (Table S2).

Table 1 Comparison of the photocatalytic activities of POM and POR-based systems for the conversion of BzAM to BzIM

Entry	Catalyst	Environment	Light	% sel.	% conv.	Yield (%)
1	No catalyst	O2	+	—	—	—
2	POM–POR	O2	—	—	—	—
3	POM–POR	N2	+	20.2	10	2.02
4	POM	O2	+	32.2	14.2	4.5
5	POR	O2	+	100	91	91
6	POM–POR	O2	+	100	100	100

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Fig. 3 (A) % Selectivity and % conversion corresponding to oxidative coupling of BzAM to BzIM obtained for POR, POM and POM–POR catalysts, (B) bar plot depicting the stability of the POM–POR hybrid for repetitive use, (C) radical scavenging experiment for oxidation of BzAM to BzIM, and (D) plausible mechanism for photocatalytic oxidation of BzAM to BzIM on the POM–POR hybrid.

The photostability of the POM–POR catalyst was examined by exposing the reaction mixture consisting of the POM–POR catalyst to light in the absence of BzAM. As shown Fig. S14, the absorption spectra before and after illumination show insignificant changes, suggesting the robustness of the POM–POR hybrid. Unlike pristine POR, the enhanced photostability of the POM–POR hybrid may be due to restricted access to meso-positions of porphyrin when it is interfaced with POM moieties as evident from the crystal structure. In other words, non-availability of meso-positions of porphyrin to ROS may be the reason for the observed photostability, in line with earlier reports.¹¹ Notably, the present study demonstrated the impressive oxidative ability of the POM–POR hybrid using O₂ as the oxidant as compared to an earlier study that reports a Ru–POM–POR hybrid catalyst for the photocatalytic oxidation of benzyl alcohols using Na₂S₂O₈ as the oxidant with poor yield (∼10%).²⁴ The robustness and repeatability of the POM–POR hybrid for the oxidative coupling of BzAM is further assessed by performing stability tests for repetitive cycles (Fig. 3B). After each run (30 min), the catalyst was recovered by centrifugation, washed with acetonitrile and dried before using it in subsequent runs. The results show that the changes in the activity of POM–POR are insignificant, suggesting the robustness of the catalyst (Fig. 3B). The catalyst recovered after 5 runs was subjected to physicochemical characterization using Raman spectroscopy, FTIR, XRD and FESEM. The data shown in Fig. S15 and S16 suggest that the POM–POR catalyst possesses excellent stability and recyclability. The robustness of the POM–POR catalyst is further validated using a hot filtration technique.^25a The data shown in Fig. S17 rule out the leaching of the catalyst during photocatalytic oxidation. Additionally, the absorption spectrum of the reaction mixture collected after catalysis does not show characteristic features corresponding to POM or POR (Fig. S18A), further proving the intactness of POM and POR moieties in the POM–POR catalyst. Unlike the POM–POR hybrid, pristine POR lacking structural protection undergoes pronounced degradation as evident from the absorbance spectra as shown in Fig. S18B.

In order to understand the photocatalytic reaction pathway and to determine the type of reactive species responsible for BzAM oxidation, various experiments were conducted in the presence of different scavengers (Fig. 3C). The yield of BzIM decreases with the addition of triethanolamine (TEOA), a hole scavenger, suggesting that holes are involved in the reaction pathway. Further, a drastic decrease in the yield of BzIM is noted when the reaction is carried out in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) and benzoquinone (BQ). These studies suggest that the reactive species such as ¹O₂ and O₂˙⁻ may be involved in driving the oxidation of BzAM as DABCO and BQ are known to act as corresponding scavengers. Notably, no change in the photocatalytic activity is observed when t-butanol was used as an ·OH radical probe, indicating that ·OH radicals are not involved in the reaction pathway (Fig. 3C). It should also be noted that the formation of hydrogen peroxide (H₂O₂) during the photocatalytic oxidation of BzAM is observed, as evident from the absorption spectra shown in Fig. S19. A clear peak around 438 nm appeared when the reaction mixture collected after 20 min of illumination is mixed with o-tolidine. Based on all these observations, the plausible mechanism for the oxidation of BzAM to BzIM is given in Fig. 3D. Upon visible-light irradiation, the POM–POR hybrid generates hole–electron pairs by photoexcitation. The photogenerated electrons reduce molecular O₂ to form species like superoxide (O₂˙⁻) and triplet O₂ (³O₂). It should be noted that the presence of heavy atoms (Mo in the present case) of POM facilitates the conversion of the triplet state (³O₂) to the singlet state (¹O₂). Simultaneously, the holes oxidize benzylamine to benzylamine cation radicals. Subsequently, O₂˙⁻ will react with the benzylamine cation radical to form phenylmethanimine and H₂O₂. In addition, the ¹O₂ species also undergoes a reaction with benzylamine forming similar products. In the final step, the benzylamine molecule undergoes nucleophilic attack on the formed phenylmethanimine intermediate yielding the final product, BzIM. Furthermore, the energetics of the POM–POR hybrid determined from the Tauc plot and Mott–Schottky measurements (Fig. S20) suggest that the catalyst possesses the lowest unoccupied molecular orbital (LUMO) at a potential more negative as compared to the redox potential of O₂/O₂˙⁻ (−0.33 V vs. NHE)^25b and the highest occupied molecular orbital (HOMO) is in a favourable position with respect to the BzAM oxidation potential (0.76 V vs. NHE).²⁶ Besides, the versatility of the POM–POR hybrid catalyst is further extended to study the oxidation of BzAM derivatives such as BzAM with electron-withdrawing and donating groups at the para position to the –CH₂NH₂ group under identical conditions (catalyst dosage, time of illumination, light source, and concentration of the substrate), as performed for the oxidation of unsubstituted BzAM. As noted from previous studies (Table S3), the oxidation of BzAM proceeds relatively faster when BzAM is connected to electron-donating groups (–CH₃, –OCH₃) as compared to that connected to electron-withdrawing groups (–Cl, –Br, –CF₃) (Table S3, entry 1–5).²⁷ Further, the oxidation of other amines such as 2-thiophenemethylamine is also carried out and the obtained results are included in Table S3 (entry 6). Thus, the study demonstrated a wide scope in exploring the POM–POR hybrid as a visible light-active photocatalyst for the oxidation of various amines to imines using O₂ as the oxidant. The ionic assembly of the POM–POR hybrid can be strategically extended to polyoxometalates with different nuclearity clusters like Keggin, Dawson, Anderson, etc. and porphyrins with various peripheral functional groups including trans A₂B₂ porphyrins along with metalloporphyrins. Such versatility enables precise tuning of absorption properties of hybrid materials to meet specific catalytic transformations.

Conclusions

In conclusion, an ion-pair POM–POR hybrid was successfully prepared through a simple electrostatic self-assembly approach. Single-crystal analysis provided the structural evidence of ion-pair POM–POR formation, revealing a layered and sandwiched arrangement stabilized by multiple non-covalent interactions. The hybrid showed strong visible-light absorption and excellent photocatalytic activity for benzylamine oxidation, achieving complete conversion and selectivity using molecular oxygen as the oxidant. The structure not only enhances light harvesting, but also protects the porphyrin from oxidative damage, thus exhibiting improved stability. This study highlights a straightforward strategy for designing robust and efficient POM–POR photocatalysts for sustainable visible-light-driven transformations. This study provides ample opportunities to extend the focus on exploring ion-pair hybrids based on metallated porphyrins with other polyoxometalates and study the oxidation of other platform molecules that has high industrial significance.

Author contributions

R. P. Bandaru: investigation, methodology, synthesis of photocatalysts, single crystal X-ray studies, data curation, formal analysis, and writing original draft. A. D. Gaonkar: investigation, methodology, photocatalytic studies, data curation, formal analysis, and writing original draft; K. Vankayala: conceptualization, investigation, writing, review & editing, funding acquisition, resources and supervision; B. K. Tripuramallu: conceptualization, investigation, writing, review & editing, funding acquisition, resources and supervision.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Data are available upon reasonable request to the corresponding authors.

Supplementary information: methods & materials; detailed experimental procedures; ¹H NMR spectra; FESEM, TGA UV-Vis DRS and Raman data of POM-POR; Hirshfeld analysis; post-catalysis characterization data; Tables S1–S3 etc. See DOI: https://doi.org/10.1039/d6cy00008h.

CCDC 2499082 contains the supplementary crystallographic data for this paper.²⁸

Acknowledgements

BKT thanks the Department of Science and Technology (SERB-TARE grant), Government of India (Project No. TAR/2023/000104) for financial support. BKT thanks Prof. Pradeepta K. Panda and his research group at the University of Hyderabad for their support to use their research facilities. KV is indebted to the ANRF (erstwhile SERB) (SRG/2020/000719), New Delhi for funding. ADG acknowledges the ANRF-DST and BITS Pilani, K K Birla Goa campus for research fellowship. We are grateful to the Central Sophisticated Instrumentation Facility (CSIF), BITS Pilani, K K Birla Goa campus for FESEM, XRD and Raman facilities.

Notes and references

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Footnote

† Both authors contributed equally.

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