Asymmetric Hybrid Polyoxometalates: A Platform for Multifunctional Redox‐Active Nanomaterials

Abstract Access to asymmetrically functionalized polyoxometalates is a grand challenge as it could lead to new molecular nanomaterials with multiple or modular functionality. Now, a simple one‐pot synthetic approach to the isolation of an asymmetrically functionalized organic–inorganic hybrid Wells–Dawson polyoxometalate in good yield is presented. The cluster bears two organophosphonate moieties with contrasting physical properties: a chelating metal‐binding group, and a long aliphatic chain that facilitates solvent‐dependent self‐assembly into soft nanostructures. The orthogonal properties of the modular system are effectively demonstrated by controlled assembly of POM‐based redox‐active nanoparticles. This simple, high‐yielding synthetic method is a promising new approach to the preparation of multi‐functional hybrid metal oxide clusters, supermolecular systems, and soft‐nanomaterials.

Abstract: Access to asymmetrically functionalizedp olyoxometalates is agrand challenge as it could lead to new molecular nanomaterials with multiple or modular functionality.N ow, as imple one-pot synthetic approach to the isolation of an asymmetrically functionalized organic-inorganic hybrid Wells-Dawson polyoxometalate in good yield is presented. The cluster bears two organophosphonate moieties with contrasting physical properties:achelating metal-binding group,a nd al ong aliphatic chain that facilitates solventdependent self-assembly into soft nanostructures.T he orthogonal properties of the modular system are effectively demonstrated by controlled assembly of POM-based redox-active nanoparticles.T his simple,h igh-yielding synthetic method is ap romising new approach to the preparation of multi-functional hybrid metal oxide clusters,supermolecular systems,and soft-nanomaterials.
Polyoxometalates (POMs) are anionic nanoscale metal oxide clusters comprising early transition metals in their highest oxidation states.T hey continue to attract significant interest owing to their vast structural diversity,e xcellent stability,and versatile chemical properties,making them ideal building blocks for functional materials. [1][2][3] Their capacity to undergo reversible multi-electron redox processes is of particular interest, and renders them promising photo/electrocatalysts in avariety of systems. [4,5] Thecovalent integration of organic moieties and POMs is an exciting and rapidly expanding area within the field. [6] Through the design of the individual components the intrinsic properties of the resulting hybrid systems can be fine-tuned. This allows hybrid POMs to exhibit diverse functionality [7][8][9] and facilitates their assembly into ar ange of supramolecular nanostructures for use in devices or catalytic systems. [10][11][12][13][14][15][16][17] The development of simple synthetic procedures that maximize control in the tailoring of multi-functionality in hybrid systems is an ongoing challenge.A lmost all reported bifunctionalized hybrid POMs are symmetric systems,inwhich two identical organic groups are tethered to the inorganic core.T he controlled addition of two different organic groups to create an asymmetric hybrid system would open up far greater opportunities for fine control over the function and physicochemical properties,a llowing them to be tailored towards highly specific or advanced applications.Afew key examples of asymmetrically functionalized POM hybrids have been reported but remain rare owing to the significant challenge posed by their synthesis and purification. [18] For instance,C ronin et al. recently demonstrated the use of HPLC to isolate ap recursor asymmetric Mn-Anderson hybrid in which one ligand terminates in ap rotecting group. [19] Notably,h owever, this requires demanding,e xpensive,a nd time-consuming methods (such as chromatography or slow fractional crystallization) followed by multiple synthetic steps to obtain the asymmetric product. [20][21][22] Reported examples of asymmetric hybrid POMs that exhibit complex functionalities are even fewer, [21,23] and (with the notable exception of aseries of elegant though hydrolytically unstable organotin hybrid POMs reported by Lacôte,T horimbert, and Hasenknopf) [24] are limited to the Mn-Anderson cluster type,which displays limited photo-and redox-activity compared to many POMs.C onversely,a dvanced functionalization strategies targeting clusters such as the Wells-Dawson anion, with its rich redox activity and highly tunable organic hybrid structures, [25][26][27] will provide new pathways for the isolation of advanced, multi-functional soft-nanostructures [28,29] and metal-directed extended assemblies. [30,31] Ultimately,t his could allow for the design of materials with switchable morphologies and functions in arange of different media, leading to new applications in catalysis,drug-delivery systems,orencapsulated nano-reactors. [32,33] Herein, we present as imple,i nexpensive,a nd highyielding synthetic approach to isolate the first reported example of an asymmetric bifunctional Wells-Dawson hybrid POM, obtained by exploiting the different solubilities of three hybrid POMs produced in the crude mixture.T he cluster bears ac helating metal-binding group,a nd al ong aliphatic chain unit that allows solvent-dependent selfassembly into soft nanostructures (Scheme 1). Thes imple nature of this approach (requiring no specialist lab equipment) should allow its application in the development of aw hole new class of asymmetric hybrid POMs.
Functionalization of polyoxotungstates with aryl phosphonate groups enhances their photochemical properties and can be employed to direct their self-assembly into micellar superstructures. [29,[34][35][36] Here,w ea imed to introduce two distinct, orthogonal functionalities via asymmetric functionalization and designed two suitable aryl phosphonates accordingly:( PO 3 C 21 H 16 N 3 )( TPY), at erpyridine-based ligand, and (PO 4 C 24 H 43 )( C 18 ), an aliphatic chain group (see the Supporting Information). Hybridization was then conducted based on the previously reported method. [34] In aonepot acid-catalyzed condensation reaction, one molar equivalent each of TPY and C 18 were reacted with one molar equivalent of the potassium salt of the mono-lacunary Dawson-type anion, [P 2 W 17 O 61 ] 10À (see the Supporting Information). A1 :1 solvent mixture of N,N'-dimethylformamide (DMF) and acetonitrile was used to adequately solubilize both ligands.A fter reaction completion, 31 PNMR spectroscopy indicated the presence of three species (Figure 1; Supporting Information, Figure S1), later fully characterized as ar acemic mixture of the two enantiomers of the asymmetric hybrid, K 4 (C 2 H 8 N) 2  Isolation of the desired product, 1,was achieved as follows.A large excess of ether was added to the reaction mixture upon which the solution turned cloudy.Oncentrifugation, amixture of orange-brown and green solid was collected and the yellow filtrate decanted. Removing the solvent from the filtrate Scheme 1. An asymmetric hybrid POM bearing two different organic moieties (A and B) and its tunable solvent and cation-dependent selfassembly. leaves an orange-brown crystalline solid that by 31 PNMR is identified as 3.T he remaining crude solid, amixture of 2 and 1,w as re-dissolved in acetonitrile,i nw hich 2 is sparingly soluble and can be separated by centrifugation. Finally,a n excess of ether was used to re-precipitate 1 while leaving any traces of 3 in solution. Theprocess of dissolution in minimum acetonitrile to remove any insoluble 2,f ollowed by reprecipitating 1 from the filtrate with ether, can be repeated as necessary until no signals corresponding to 2 or 3 are visible by 31 Po r 1 HNMR (Figure 1). 1 can be reliably isolated in good yield (typically 10-25 %total yield) and excellent purity.
Thec omposition and purity of 1 was confirmed by 1 HNMR, 31 PNMR, ESI-MS,e lemental (CHN) analysis, thermogravimetric (TGA) analysis,a nd FTIR (see the Supporting Information). 1 HNMR confirms the presence of both the aliphatic carbon chain and the aromatic TPY group in a1 :1 stoichiometric ratio.I nterestingly,s light shielding of the TPY phosphorus and deshielding of the C 18 phosphorus relative to their 31 Pchemical shifts in the respective symmetric hybrids suggests adegree of electronic communication via the POM core of the asymmetric structure. [37] Following successful isolation of 1,w ee xplored the unique multifunctionality of the hybrid POM system, as derived from its asymmetric structure.First, we examined the self-assembly of 1 enabled by the aliphatic chain ligand, C 18 , which imparts amphiphilic character to the hybrid POM (the POM core and TPY group acting as ap olar head-group). 1 was first dissolved in acetonitrile and then 9equivalents of water were added to facilitate spontaneous self-assembly. Dynamic light scattering (DLS) experiments on a1 .4 mm solution confirmed the formation of nanoscale assemblies of low dispersity with ahydrodynamic diameter (D h )ofapproximately 6nm(Supporting Information, Figure S18). In agreement with this,Cryo-TEM analysis of asolution of 1 in wateracetonitrile (9:1 v/v) deposited and frozen on ag rapheneoxide and holey carbon-supported Cu grid showed spherical structures with diameters of 4-7 nm (Supporting Information, Figure 2). Corresponding 1 Ha nd 31 PNMR experiments showed broadening/disappearance of peaks in aD 2 O-CD 3 CN (9:1 v/v) mixture,indicative of reduced T 2 transverse relaxation time arising from the slow tumbling of nanostructures (Supporting Information, Figure S21).
Cyclic voltammetry (CV) of 1 in a0.1m DMF solution of TBA·PF 6 revealed four distinct quasi-reversible redox processes in the potential range of À0.5 to À2.25 Vv s. Fc/Fc + (Supporting Information, Figure S10), all of which are positively shifted relative to the parent Wells-Dawson anion, [P 2 W 18 O 62 ] 6À ,a si st ypical of organophosphonate hybrid POMs. [37] Furthermore, E 1/2 potentials of 1 were found to be intermediate to those of the respective symmetric hybrids, 2 and 3,w ith redox processes in 2 more positively shifted by an average of 5mVand those of 3 more negative by an average of 20 mV,relative to the equivalent redox couples of 1 (see the Supporting Information for details).
Recently,w es howed that self-assembled micellar nanostructures formed from hybrid POM species exhibit different redox properties to their constituent molecular species. [29,35] CV experiments were conducted on 1 dissolved in a0 .1m H 2 SO 4 :CH 3 CN (9:1, v/v) mixture,a llowing for spontaneous assembly into micellar species as described above.Here,j ust two redox processes were observed between À0.5 and 0.5 V (vs.A g/AgCl) in contrast to the three seen in pure DMF (Supporting Information, Figure S15), as appears typical of micellar species of this type. [29,35] Notably,c haracteristic molecular redox behavior can be rapidly recovered by addition of DMF to the electrolyte solution, indicating the dynamic nature of this system and offering interesting potential for the development of responsive or switchable electrochemical devices.T his behavior also corresponds closely to that seen for our previously reported symmetric surfactant hybrid system, [35] and for the symmetric product 3 (Supporting Information, Figure S21).
Post-functionalization of 1 was investigated by reacting the hybrid-POM with 0.5 molar equivalents of FeCl 2 .A fter stirring the reactants at RT overnight in acetonitrile,t he product, K 7 (C 2 H 8 N) 3  Fe-1 (Figure 3a), was collected by removal of the solvent. Mass spectrometry confirmed the formation of aP OM-Fe-POM dimeric structure,and all aromatic peaks associated with the TPY groups were shifted in the 1 HNMR, indicating the presence of as ingle new species.P eak shifts of both organophosphoryl signals (C 18 downfield;T PY upfield) in the 31 PNMR spectrum of Fe-1 demonstrate adegree of electronic conjugation across the whole asymmetric system. Thed ark purple color of the product is characteristic of al ow spin d 6 Fe 2+ center, and UV/Vis absorption spectroscopy confirms the presence of ap eak at 576 nm corresponding to aF e 2+ -TPY metal-ligand charge-transfer (MLCT) band (Supporting Information, Figure S13). CV studies in DMF reveal areversible Fe 2+/3+ redox couple (E 1/2 = 0.543 Vvs. Fc/Fc + )alongside the 4q uasi-reversible POM-centered processes (Supporting Information, Figure S16). Thecombination of metal coordination and self-assembly of 1 was then investigated by dissolving Fe-1 in aw ateracetonitrile (9:1 v/v) mixture at RT.DLS studies indicated the formation of nanoscale aggregates (Supporting Information, Figure S20) with D h(avg) = 4nm, and NMR studies again showed the disappearance of the peaks in D 2 O-CD 3 CN (9:1 v/ v) solution (Supporting Information, Figure S22). CryoTEM analysis confirmed the presence of the assemblies (Figure 3b), but unlike the corresponding studies on 1,t he aggregates were of variable shape and were not hollow.This is reasonable,g iven that Fe-1 lacks the head/tail structure of 1, and is therefore unlikely to form typical micellar assemblies. Indeed, the assemblies appear to resemble those recently reported by Izzet and co-workers,w ho demonstrated the organic solvent and transition metal-dependent assembly of symmetric terpyridine-functionalized POMs into ar ange of nanoscale architectures. [30,38] It is also apparent that many of the POMs (dark areas on the micrograph) appear in pairs (Figure 3c), consistent with the expected molecular structure of Fe-1,i nw hich the POM lobes should be separated by approximately 2nm. As with 1,t he assemblies exhibited contrasting electrochemical behavior to their molecular state, which was recovered upon addition of DMF (Supporting Information, Figure S17).
In summary,w ehave isolated the first stable asymmetric Wells-Dawson hybrid POM using as imple and inexpensive procedure for the synthesis of novel multi-functional hybrid materials based on molecular metal oxides.W ehave explored the multifunctionality of these asymmetric assemblies,driven by the nature of the appended organophosphoryl groups.The orthogonal and complementary functions of the aliphatic chain (facilitating solvent-dependent self-assembly) and the chelating group (allowing transition metal binding and also influencing supramolecular interactions) allow us to tune the performance of the hybrid system through careful postfunctionalization and to obtain unique redox-active soft nanostructures.T his work provides ar obust, modular,a nd easily accessible strategy for the creative design and development of new multi-functional hybrid materials,with arange of potentially novel applications as redox-, photo-, and catalytically active soft nanomaterials.