Coupled reaction equilibria enable the light-driven formation of metal-functionalized molecular vanadium oxides

The introduction of metal sites into molecular metal oxides, so-called polyoxometalates, is key for tuning their structure and reactivity. The complex mechanisms which govern metal-functionalization of polyoxometalates are still poorly understood. Here, we report a coupled set of light-dependent and light-independent reaction equilibria controlling the mono- and di-metal-functionalization of a prototype molecular vanadium oxide cluster. Comprehensive mechanistic analyses show that coordination of a Mg2+ ion to the species {(NMe2H2)2[VV12O32Cl]}3- results in formation of the mono-functionalized {(NMe2H2)[(MgCl)VV12O32Cl]}3- with simultaneous release of a NMe2H2+ placeholder cation. Irradiation of this species with visible light results in one-electron reduction of the vanadate, exchange of the second NMe2H2+ with Mg2+, and formation/crystallization of the di-metal-functionalized [(MgCl)2VIVVV11O32Cl]4-. Mechanistic studies show how stimuli such as light or competing cations affect the coupled equilibria. Transfer of this synthetic concept to other metal cations is also demonstrated, highlighting the versatility of the approach.

Early studies by Streb and co-workers have used a placeholder strategy for the predictable metal-functionalization of POVs.This approach uses the dodecanuclear species {(NMe 2 H 2 ) 2 [V 12 O 32 Cl]} 3-, (= (DMA) 2 {V 12 }, DMA = dimethyl ammonium), where two vacant metal coordination sites are blocked by hydrogen-bonded DMA cations.Insitu exchange of these cations with a variety of metals is possible, leading to the mono-or (more rarely) di-metal-functionalized species ({MV 12 } and {M 2 V 12 }, respectively), and applications ranging from (light-driven) catalysis 30,31 to energy storage 32,33 .Note that for most of the di-metal-functionalized {V 12 } species were only accessible as mixed V IV/V oxidation state clusters [33][34][35][36] .Recently, a similar synthetic approach to metal-functionalized molybdates has been pioneered by Yamaguchi, Suzuki and co-workers.They used pyridine moieties to coordinatively stabilize and functionalize lacunary polyoxomolybdates which are otherwise difficult to access 18,37 .
Most often, temperature, solvent, solution acidity and type of metal salt are the key synthetic parameters varied to facilitate metalfunctionalization of POVs 24 .In contrast, the use of light, i.e. photons with energies in the visible spectral range has rarely been discussed as a systematic control parameter to trigger POM and POV functionalization.This is surprising, as the (visible) light photoactivity of POMs is well-documented, and the light-induced excitation of O→M ligand-tometal charge-transfer transitions is an easy tool to selectively access reduced POM species 8,[38][39][40][41] .The concept has been pioneered by Yamase and co-workers who demonstrated that previously unknown mixed-valent POMs and POVs can be accessed photochemically in the presence of suitable electron donors, e.g., organic amines [42][43][44] .POV chemistry is particularly sensitive to photoinduced reactions triggered by visible light, as POVs typically show higher visible light absorption compared with tungstates and molybdates 38 .This approach has recently been explored by Liu and co-workers who demonstrated the visible-light-assisted synthesis of mixed-valent POVs 45 .Here, we demonstrate how light-dependent, coupled reaction equilibria can be used to selectively target the partial reduction and metalfunctionalization of {V 12 }, leading to a di-magnesium functionalized species, {Mg 2 V 12 }, as an intriguing model compound for future studies, e.g., in electrochemical energy storage 8,33 .

Synthesis and characterization
The starting point was our study into the design of magnesium(II)functionalized {V 12 } as molecular models for Mg ion batteries, where solid-state magnesium vanadates are under investigation as active electrode materials 46 .Initial experiments to functionalize {V 12 } with Mg 2+ were performed by reacting Mg 2+ with (DMA) 2 {V 12 } in acetonitrile under the standard placeholder-functionalization conditions described above 13,34 .Despite the extensive variation of the reaction and isolation conditions, reproducible formation of Mg-functionalized {V 12 } was not possible.Systematic study of the key reaction parameters showed that visible light irradiation and oxygen-free conditions were required to access the target compound, (nBu 4 N) 4 [(MgCl) 2 V 12 O 32 Cl] (={Mg 2 V 12 }, 1).Diffusion crystallization using diethyl ether as diffusion solvent gave green single crystals of {Mg 2 V 12 } in yields of 64% (based on {V 12 }, see Methods and Supplementary Section 2).When the reaction was performed in the dark under otherwise identical conditions, only the starting material {V 12 } was recovered as yellow crystals.Crystallographic analysis by single-crystal X-ray diffraction shows that {Mg 2 V 12 } crystallizes in the monoclinic space group P2 1 /c with cell axes a = 24.3414(9)Å, b = 16.7474(7)Å, c = 24.4623(9)Å and cell angles β = 94.6107(17)°,α = γ = 90°(for crystallographic details see Methods and Supplementary Section 2.9).Note that this crystal lattice is virtually identical to the previously reported di-functionalized species {Mn 2 V 12 } (nBu 4 N) 4 [(MnCl) 2 V 12 O 32 Cl] 34 .For full characterization of {Mg 2 V 12 } see Supplementary Section 2. The metal oxo framework of {Mg 2 V 12 } is isostructural to the di-metal-functionalized {V 12 } species reported earlier (i.e., {Mn 2 V 12 } 34 , {Ca 2 V 12 } 33 , {K 2 V 12 } 32 , {Sr 2 V 12 } 35 , {Ce 2 V 12 } 30 ), the two square-pyramidal Mg 2+ ions reside in the metal binding sites on top and bottom of the cluster and feature a terminal chloride ligand (Fig. 1a, b).
UV-Vis-NIR spectroscopy of {Mg 2 V 12 } confirms the mixed valent (V IV/V ) character of the species, as indicated by the characteristic, broad intervalence charge-transfer (IVCT) band between ~600-1200 nm 34 .Further support of the mixed-valent nature of {Mg 2 V 12 } is given by continuous wave electron paramagnetic resonance (EPR) spectroscopy which unambiguously shows the presence of one V IV species with S = ½ (Supplementary Fig. 3), while a pure V V cluster would be EPR silent.Furthermore, EPR-based spin counting is in good agreement with one V IV centre per {Mg 2 V 12 }.

Photochemical studies
Irradiation of a reaction mixture containing {V 12 } and Mg 2+ in MeCN with a broadband LED light source resulted in the emergence of the IVCT transitions characteristic for the formation of mixed-valence VI VI/V species (vide supra).Thus, the change of the UV-Vis-NIR signals over time can be used to monitor the rate of {Mg 2 V 12 } formation, see Fig. 1c,  d.This provides the ideal conditions to explore the fundamentals of the light-induced formation mechanism of metal-functionalized vanadates: to this end, we compared the photoreduction of {V 12 } in the presence and absence of Mg 2+ (Fig. 1d).Strikingly, {V 12 } reduction is only observed in the presence of Mg 2+ , while in the absence of Mg 2+ , no formation of V IV centres and no IVCT signal is detected.We hypothesized that this finding indicates that Mg 2+ interacts with {V 12 } in the reaction solution to give a photoactive reactive intermediate.Based on our understanding of the system, we suggested that this intermediate could be the mono-functionalized species {MgV 12 } (= {(DMA)[(MgCl) V V 12 O 32 Cl]} 3-).The formation of an intermediate species is indicated by UV-Vis-NIR spectroscopy, which shows distinct spectral changes in the region between 300 nm to 500 nm upon addition of Mg 2+ to the {V 12 } reaction solution (Fig. 2a).Also, 51 V NMR spectroscopy shows that immediately after addition of Mg 2+ to an acetonitrile solution of {V 12 }, a four-signal spectrum is observed which is characteristic for the monometal-functionalized {MV 12 } species (Fig. 2b).

Mechanistic analyses
Further evidence for the formation of {MgV 12 } is revealed by characteristic changes in the respective 1 H and 1 H DOSY spectra (Supplementary Section 3.1).Upon addition of Mg 2+ to a {V 12 } solution in acetonitrile, DMA cations are released from their original, {V 12 }-bound positions into solution, resulting in a dynamic equilibrium between cluster-bound and "free" DMA cations.This results in a characteristic low-field shift of the N-H proton resonances from δ ~6.3 ppm to δ ~7.0 ppm (Supplementary Fig. 12 and Fig. 13). 1 H DOSY NMR spectra were collected to further study the release of DMA cations upon Mg 2+ addition to {V 12 } solutions in acetonitrile.Specifically, we studied the characteristic changes of the respective diffusion coefficients D based on analysis of the DMA− 1 H resonances (methyl groups, δ ca.2.6 ppm; ammonium groups, δ ca.6.3 -8.6 ppm, (Supplementary Fig. 14) 47 .These analyses show the expected trend, i.e., the diffusion coefficients decrease with increasing size of the species studied in the order "free" DMA <{MgV 12 } <{V 12 }.
Note that the characteristic four-line 51 V NMR signal pattern assigned to {MgV 12 } was observed even under the dilute concentration conditions of the HR ESI MS experiments ([Mg 2+ ] = 0.21 mM, [{V 12 }] = 0.05 mM, see Supplementary Fig. 15).
Next, we explored the Mg 2+ functionalization further by performing a 51 V NMR spectroscopy titration, where increasing amounts of Mg 2+ were added to {V 12 } solutions in acetonitrile.To assess the formation of {MgV 12 }, the characteristic 51 V NMR signals were integrated, and the integral areas were plotted as a function of the Mg 2+ equivalents added.As shown in Fig. 3, integration of the three non-overlapping signals unambiguously indicates, that changes of the integrated area are only observed up to 1.0 equivalents Mg 2+ .Higher equivalents do not change the spectra observed.This strongly suggest the presence of a 1:1 molar species, which is in line with the formation of {MgV 12 }.These observations are supported by an identical 1 H NMR titration study which shows that upon Mg 2+ binding to {V 12 }, release of DMA cations (indicated by characteristic shifts of the DMA proton signals) is observed, see Supplementary Fig. 13.
The different photoactivities of {V 12 } and {MgV 12 } were probed experimentally by wavelength-selective irradiation: when the standard {V 12 }/Mg 2+ reaction mixture in acetonitrile was irradiated with a monochromatic 405 nm LED light source, the characteristic {MgV 12 } reduction and formation of the characteristic IVCT band was observed.In contrast, when the same experiment was performed for a pure {V 12 } solution (in acetonitrile, without added Mg 2+ ), no vanadate reduction was observed.Also, irradiation of the standard {V 12 }/Mg 2+ reaction mixture using a monochromatic 470 nm LED light source also did not lead to reduction of the vanadate cluster, see Supplementary Fig. 18.
Further insights into the electronic structure of {MgV 12 } and {Mg 2 V 12 } were obtained by theoretical computations using density functional theory (DFT) using the B3LYP functional 48,49 combined with the def2-SVP basis set 50 .Analysis of the HOMO-LUMO levels of {MgV 12 } and analysis of the calculated UV-Vis-NIR spectrum show an intense ligand-to-metal-charge-transfer (LMCT) transition at the UV-to-Vis border, which we attribute to the experimentally observed Vis photoactivity of {MgV 12 }.For {Mg 2 V 12 }, similar LMCT transitions are observed, and in addition, the broad characteristic IVCT transition in the Vis-NIR range are reproduced by the calculations.For details, see Supplementary Section 4.
Based on Le Chatelier's principle, we hypothesized that addition of DMA to the reaction solution should shift the equilibrium to the reagent side (see Fig. 4a), thus preventing the formation of the photoactive {MgV 12 }.This behaviour is indeed observed: when an excess of DMACl is added to the standard {V 12 } reaction solution and the sample is irradiated, no reduction is observed by UV-Vis-NIR spectroscopy (Supplementary Fig. 19).This suggests that the reactive intermediate which enables photoreduction is not present under these conditions and lends further support to {MgV 12 } being the photoactive intermediate.Also, when the {V 12 } photoreduction is performed in the presence of Mg 2+ and air, virtually no vanadate reduction is observed (as indicated by the absence of IVCT bands in the UV-Vis-NIR spectrum, see Supplementary Fig. 20).This provides further support that a light-induced electron transfer to the photoexcited {MgV 12 } is a key process in the formation of {Mg 2 V 12 } and suggests that interference between the photoexcited {MgV 12 } and O 2 (e.g., by triplet quenching 51 ) could prevent formation of the reduced vanadate species 52 .
In sum, these data suggest that reaction of {V 12 } and Mg 2+ results in formation of the mono-functionalized {MgV 12 } as photoactive, reactive intermediate, which can then be converted to the di-functionalized {Mg 2 V 12 } upon irradiation with visible light.To gain insights into the sacrificial electron donor, we used cyclic voltammetry to compare the redox potentials of the possible donors (DMA, nBu 4 N + , MeCN).Based on this data, DMA is the most likely sacrificial electron donor, as it is significantly easier to oxidize (E ox = 0.7 V vs Fc + /Fc) compared with the other possible electron donors, i.e., nBu 4 N + and MeCN (E ox > 1.6 V vs Fc + /Fc), see Supplementary Fig. 21.Based on these considerations, the following coupled reaction equilibria are proposed, see Fig. 4. Finally, to probe whether the observed light-induced reactivity is unique to Mg 2+ or can be extended to other metal functionalizations, we performed the {V 12 } metal functionalization experiments using Ca 2+ instead of Mg 2+ (experimental details see Supplementary Section 3.4).Reaction of CaCl 2 x 2H 2 O (21.1 mM) with {V 12 } (5.0 mM) in acetonitrile led to the observation of the characteristic four-line 51 V NMR signal pattern assigned to {CaV 12 } (Supplementary Fig. 22).Visible light-irradiation of the {V 12 } / Ca 2+ reaction solution resulted in the formation of the characteristic IVCT bands between ~600 and 1200 nm which is indicative of the formation of the reduced, dimetal-substituted vanadate species (see Supplementary Fig. 23).These findings suggest that the light-induced metal functionalization reported here is not unique to Mg 2+ and can be transferred to other metal cation species, and possibly also to other vanadate cluster architectures.

Discussion
We report the first example of a light-dependent, coupled set of solution-phase equilibria, enabling the controlled metalfunctionalization of molecular vanadium oxides.Light-independent reaction of {V 12 } with Mg 2+ results in a dynamic pre-equilibrium, where one DMA placeholder cation on {V 12 } is replaced with one Mg 2+ ion, resulting in formation of the Vis-photoactive intermediate {MgV 12 }.The formation of a 1:1 species was verified by 51 V NMR / 1 H NMR spectroscopy as well as HR ESI MS studies.Competitive binding studies using DMA and Mg 2+ show, that this pre-equilibrium is sensitive to the Mg 2+ / DMA molar ratio, essentially allowing an on/off switching of the metal functionalization.
Visible light-irradiation of {MgV 12 } solutions results in the oneelectron photoreduction of the cluster, release of the second DMA placeholder cation, and binding of a second Mg 2+ metal centre, yielding {Mg 2 V 12 }.The increased visible-light photoactivity is in line with recent literature reports which show that metal-incorporation in POMs leads to a lowering of the HOMO-LUMO gap and thus, increased photactivity 53 .The photoredox processes at {MgV 12 } only occur in the absence of water and oxygen, indicating possible interference of these species with the light-induced electron transfer to the cluster.Electrochemical studies suggest that DMA is the most likely electron donor based on analysis of the redox potentials of the reagents used.Mechanistic experimental and theoretical studies show the lightdependent nature of the assembly process and emphasize how supramolecular reaction control can be used to trigger or inhibit photoactivity.Finally, initial experiments show that a similar route can be followed to enable {V 12 } functionalization with Ca 2+ using lightinduced cluster assembly.Thus, the principles outlined in this report open new paths for designing multi-stimuli-responsive molecular materials.

Solution-phase NMR spectroscopic analyses
Mechanistic reactivity studies using 1 H and 51 V NMR spectroscopy were carried by dissolving the compound under study in the respective solvent.The reactions were performed under the given conditions as stated above and in the Supplementary Information.All solutions were prepared in an argon-filled glovebox unless stated otherwise.