Electronic Consequences of Ligand Substitution at Heterometal Centers in Polyoxovanadium Clusters: Controlling the Redox Properties through Heterometal Coordination Number

Abstract The rational control of the electrochemical properties of polyoxovanadate‐alkoxide clusters is dependent on understanding the influence of various synthetic modifications on the overall redox processes of these systems. In this work, the electronic consequences of ligand substitution at the heteroion in a heterometal‐functionalized cluster was examined. The redox properties of [V5O6(OCH3)12FeCl] (1‐[V5FeCl]) and [V5O6(OCH3)12Fe]X (2‐[V5Fe]X; X=ClO4, OTf) were compared in order to assess the effects of changing the coordination environment around the iron center on the electrochemical properties of the cluster. Coordination of a chloride anion to iron leads to an anodic shift in redox events. Theoretical modelling of the electronic structure of these heterometal‐functionalized clusters reveals that differences in the redox profiles of 1‐[V5FeCl] and 2‐[V5Fe]X arise from changes in the number of ligands surrounding the iron center (e.g., 6‐coordinate vs. 5‐coordinate). Specifically, binding of the chloride to the sixth coordination site appears to change the orbital interaction between the iron and the delocalized electronic structure of the mixed‐valent polyoxovanadate core. Tuning the heterometal coordination environment can therefore be used to modulate the redox properties of the whole cluster.


Introduction
Polyoxometalates (POMs) are molecular metal oxide clusters that have shown great utility in many areas of research, including medicinal chemistry, [1] materials cience, [2] molecular magnetism, [3] catalysis, [4] and energy storage. [5] The rich electrochemicalp roperties and stability exhibited by these polynuclear assemblies make them good candidates for av ariety of electrochemical applications.F or example, POMs have displayed great promise as electrocatalysts for the reduction of protons and oxoanions (e.g.,n itrite, chlorate, and bromate) and the oxidation of alkanes, alkenes,a lcohols, sulfoxides,a nd phosphines. [6] Recent work has also demonstrated their competency as chargecarriers in redox flow batteries. [7] Apopularmethodfor modifying the electrochemical properties of polyoxometalates is to incorporate other elements into the cluster framework. For example, in the case of the Keggin polyoxomolybdate cluster [XMo 12 O 40 ] nÀ (X = P V ,S i IV ,e tc.), replacemento ft he central phosphorusa tom with silicon shifts the redox eventstoward more reducing potentials. [8] While much is known of strategies to modify the redox properties of molybdate and tungstate clusters, considerably less is understood relating to strategies for modulating the electrochemical profiles of vanadium-oxide derivatives. This is often attributed to the fact that the structuralf lexibilityo fv anadiumm akes precise and controlled modification to polyoxovanadate frameworks challenging. [9] Af ew examples of functionalized vanadium oxide clusters in the literature illustrate that, like their molybdate and tungstate congeners, the magnetic, photochemical, electrochemical, and catalytic properties of these assemblies can be tuned. [10] However,t he development of ag eneral theoretical framework linking systematic molecular modificationst ot he resulting redox behaviord isplayedb yp olyoxovanadates has been outside the scope of these studies.
To ward buildingalibrary of polyoxovanadate clusters with tunable physicochemical properties, part of our research team has been studying the synthesis and characterization of Lindqvist polyoxovanadate-alkoxide( POV-alkoxide) clusters, [V 6 O 7 (OR) 12 ]( R= CH 3 ,C 2 H 5 ). [11] We have demonstrated that the solubility and electrochemical properties of these clusters can be varied through an umber of modifications to the cluster core, rendering this family of molecular vanadates useful motifs for probing electrochemical consequences of these structuralp erturbations (Figure1). In particular, integration of transition metal or metalloid ions within the clusterc ore ([V 5 O 6 (OCH 3 ) 12 M],M = Ti 4 + ,Z r 4 + ,H f 4 + ,F e 3 + ,G a 3 + )h as been shown to serve as an effective method for tuningt he redox properties of POV-alkoxides, [12] similar to previous studies on heterometallic tungstates and molybdates.
While we have provided extensive evidencet hat the aforementioned variations of heteroion and bridging alkoxide ligand identity can modify the physicochemical properties of the assembly,t he effects of modifying the ligand environment surrounding ah eteroion embedded within aP OV cluster have not been reported. This gap of knowledge is strikingw hen considering the importance of ligand field considerations in dictating the electronic structure of monometallic, transition metal complexes. Herein, we report the electrochemical consequences of halide coordination to the heterometal of an ironfunctionalized POV-alkoxidec luster,[ V 5 O 6 (OCH 3 ) 12 FeCl],a nd comparing its redox properties to that of previouslyr eported, iron-installed POV-alkoxide clusters ( Figure 1). Ac ombination of experimental and theoretical investigations is used to develop am ore complete understanding of the comparative electrochemical effects of incorporating coordinating (e.g.,c hloride) versus non-coordinating( e.g.,p erchlorate) ligands at the site-differentiated metal center.

Results and Discussion
Electrochemical characterizationof1- [V 5 FeCl] The isolation of the chloride-functionalized analogue of the iron-containing species, [V 5 O 6 (OCH 3 ) 12 FeCl] (1-[V 5 FeCl]), was previously reported by someo fu s, using an adapted synthetic procedure to that reportedf or the gallium-functionalized variant, [V 5 O 6 (OCH 3 ) 12 GaCl]. [12d] Briefly,s elf-assembly of the hetero-metallicL indqvist cluster occurs following heatingo fatetrahydrofurans olution of [VO(OCH 3 ) 3 ]( 5equiv.) andF eCl 3 (2 equiv.) in the presence of NaBH 4 (1 equiv.). Spectroscopic investiga-Carsten Streb is Director of the Institute of Inorganic Chemistry Ia tU lm University and groupl eader at the Helmholtz Institute Ulm. His love for POMss tarted during his PhD, workingw ith Lee Cronin at the Universityo f Glasgow. His current research is focused on designing POM-based functional materials and composites to address global chemical challenges including energy conversion/storage, water purification and public health. In particular, he enjoys exploring the supramolecular chemistry and metal-functionalization of molecular vanadium oxide clusters.
Ellen Matson received her PhD from Purdue University.F ollowing ap ostdoctoral appointment at theU niversity of Illinois at Urbana-Champaign, Matson began her independent careera tt he University of Rochester (2015), wheres he is currently the Wilmot Assistant Professor of Chemistry. The Matson Laboratory studiest he synthesis and reactivity of reducedv anadium oxide clusters. Ellen Matson has earned severala wards recognizing early success in research and teaching, including a SloanF ellowship, aC ottrell Scholar Award, a Camille DreyfusT eaching-Scholar Award and the Edith Flanigen Award. Figure 1. Previously reported syntheticmodifications for POV-alkoxidec lusters. [7,13] tions andc harge balancing revealed the oxidation state distribution for complex 1- [V 5 FeCl] to be V IV 3 V V 2 Fe III .W hile the electronic structureo ft he neutral species was rigorously characterized, the electrochemical profile of 1- [V 5 FeCl] wasn ot explored.T hus, we set out to understand the consequences of halide-coordination to the heteroion in 1- [V 5 FeCl] on its redox chemistry and electronic structure.
The cyclic voltammogram (CV) of 1- [V 5 FeCl],c ollected in acetonitrile, displayed four,e venly spacedr edox events (E 1/2 = À0.68, À0.21, + 0.31, + 0.83 Vv s. Fc 0/ + ;F igure 2);C Ve xperiment with expanded electrochemical window can be found in Figure S1 in the Supporting Information. These eventsa re anodically shiftedc ompared to the hexavanadate cluster by approximately 0.1 V. Thus, the replacement of a" V=O" unit for a"FeÀCl" effectively modulates the potentials of the redox processes without compromising the rich electrochemical properties. This modulation has also been observed in other heterometal-functionalized POV-alkoxides. [12c, d] Interestingly,c omparing the redoxp rofile of 1- [V 5 FeCl] to other iron-functionalized POV-alkoxide clusters reportedb yo ur group, namely [V 5 O 6 (OCH 3 ) 12 Fe]X (X = ClO 4 , 2-[V 5 Fe]ClO 4 ;X = OTf, 2-[V 5 Fe]OTf), shows that the series of four electrochemical events of the halogenated species is shiftedb ya pproximately 1V .This substantial anodic shift upon coordination of the chloride is unexpected,a st he additional negative charge supplied by the anionic ligand should shift the redox profile to more reducing potentials. As such, am ore in-depth chemical analysis of the electronicc haracteristics of 1-[V 5 FeCl] is required to explain this result.

Isolation of redox isomersof1 -[V 5 FeCl]
The striking differences in the electrochemical profiles of the two FePOV-alkoxide clusters, 1-[V 5 FeCl] and 2-[V 5 Fe]X (X = ClO 4 ,O Tf), prompted ad etailed investigation to understand the changes in the electronic structure of 1-[V 5 FeCl] acrossa ll charge states. We hypothesized that the change in coordina-tion number of the iron center might be influencing the energetics of electron storagea nd release in these heterometallic assemblies. Indeed, the neutral complex 1- [V 5 FeCl] has been shown to retain a6 -coordinate iron center (pseudo O h geometry) in the solid-state and in solution. [12d] In contrast, we have previously reported that the coordination environment of iron in complex 2-[V 5 Fe]X (X = OTf, ClO 4 ) is best described as squarep yramidal (5-coordinate), as the weakly-coordinating natureo fO Tf À and ClO 4 À counter ions resultsi nt heir dissociation from the clusterc ore. [12a,b] These conclusions are supported by Mçssbauer analysiso fs olid-state samples, revealing a mixture of the 5-coordinate (62 %) and 6-coordinate (38 %) species in complex 2-[V 5 Fe]X.F ollowing clusterr eduction, the mixture collapses to as ingle set of resonances consistent with a5 -coordinate Fe III .E lectrospray ionization-mass spectrometry (ESI-MS) data also suggestst hat, in solution,t he counter ion (X = OTf, ClO 4 )i sf ully dissociated from the iron-functionalized POV-alkoxide core.
To establish whether the cathodic shift in the redox profile of 1-[V 5 FeCl] relative to 2-[V 5 Fe]X is ag eneral phenomenon for iron-functionalized POV-alkoxidec lusters containing 6-cooridnate iron centers, we obtained the CV of the previously reported complex [V 5 O 6 (OCH 3 ) 12 Fe(OCN)], 1-[V 5 FeOCN] in acetonitrile ( Figure S2, Table S1, CV experiment with expandede lectrochemical window in Figure S3). The cyanate derivative was selected because the cyanate anion has been shown to strongly bind to the iron center, and thus is ag ood example of a6coordinate species. [12b] The CV of the cyanate-functionalized cluster is identicalt ot hat of 1-[V 5 FeCl], providing further support for our hypothesist hat the observed ligand-dependent shifts in redox potentials result from changes in coordination number of the iron center,rather than ligand identity.
In particular, we becamei nterested in understanding how the coordination of ac hloride ion to the heterometal (iron) affects the storage and release of electron density from the cluster.S ynthetic isolation of the various oxidations tates of 2-[V 5 Fe]ClO 4 and [V 6 O 7 (OR) 12 ]( R= CH 3 ,C 2 H 5 )h as been shown to be an effective methoda te lucidating the electrochemical properties of theseh eterometallic systems. [11b, c, 12b] As such, we soughttoperform as imilar analysis with 1- [V 5 FeCl].
In our originalr eport of the synthesis of 1-[V 5 FeCl],w ep roposed that the FePOV-alkoxide clusterbearsamixed-valento xidation state distribution of metal ions (V V 2 V IV 3 Fe III ). [12d] The open circuit potentialo ft he neutral cluster (À0.05 Vv s. Fc 0/ + ) is located between sets of two reduction and oxidation waves. This suggestst hat the parent cluster 1-[V 5 FeCl] could be twice oxidizeda nd twicer educed, affording af amily of five redox isomerso ft he FePOV-alkoxide cluster, each differing by a single electron.
The CV of complex 1-[V 5 FeCl] suggests as econd, more-oxidized product should be accessible via the two-electron oxidation of the parent cluster.H owever,a ddition of two equivalents of NOPF 6 to 1- [V 5 FeCl] in dichloromethane (NOPF 6 in dichloromethane, E 0 = 1.00 Vv s. Fc 0/ + ) [13] resulted in the formation of the mono-cationic species, [V 5 O 6 (OCH 3 ) 12 FeCl]PF 6 (confirmed by 1 HNMR and infrared spectroscopic analyses, Figure S7). Attempts to electrochemically access the di-cationic species, via bulk oxidation in acetonitrile, resulted in decomposition ( Figure S8). Indeed, the poor chemical reversibility of this second oxidation event is illustrated when the redox process is isolated at varyings can rates from 10 to 1000 mV s À1 in acetonitrile ( Figure S9). As the scan rate is increased, the peak-topeak separation between the oxidative and reductive features increases and the redox event becomes ill defined. These results suggest that the most oxidizing event is actually an irreversible process. The inability to synthetically or electrochemically access the di-cationic species of 1-[V 5 FeCl] is not wholly surprising, as we have previously reported the oxidativei nstability of the homometallic POV-alkoxide cluster, [V 6 O 7 (OCH 3 ) 12 ]. [7d, 11a] In principle, the E 1/2 value of the most oxidizing event of complex 1- [V 5 FeCl] occurs at ap otential at which chloride oxidation is possible. To assess whether the irreversibility of this redox event was due to chloride dissociation and subsequent oxidation, we titrated as olution of (nBu 4 N)Cl into aC Vc ell containing 1- [V 5 FeCl] (see supporting information for experimental details, Figure S10). Heterogeneous chloride oxidation resultsi nadistinct electrochemical event at ap otentialo f 0.79 V( vs. Fc 0/ + ), as indicated by an increased current response proportional to increased concentrationso f( nBu 4 N)Cl in the first scan rate of the sample. Third scan analysis, allowing for the sample to reach chemical equilibrium,r eveals chloride consumption,a nd retention of the four redox processes native to complex 1-[V 5 FeCl] (Figure S10), indicatingt hat Cl 2 production does not correlate with clusterd ecomposition. As such, we hypothesize that the observed lack of reversibility is indicative of complete cluster degradation,a nd not dissociation/oxidation of the chloride ligand.

Electronic structure of the redox isomers of 1-[V 5 FeCl]
With the oxidized and reduced clusters in hand, we turned to elucidating the electronic characteristics of these compounds. In particular, we were interested in understandingt he participation of transition metal ions in the reduction and oxidation events observed in the CV of complex 1-[V 5 FeCl].F ouriertransform infrared (FT-IR) ande lectronic absorption spectroscopies are complementary techniques that have been used to give valuable insighti nto the oxidation state distribution within POV-alkoxide clusters. [11b, c, 12b] Thus, we applied these spectroscopic methods to analyze the electronic structures of

)b and in the FT-IR spectrao f POV-alkoxide clusters is characteristic of the Robin and Day
Class II delocalized electronic structure displayed by these assemblies. [11b, c] Therefore, the FT-IR spectra of the 1-[V 5 FeCl] redox series show that both the Lindqvist motif and the electronic delocalization are retained as electrons are added and removed.
As the cluster is oxidized from the di-anionic to the neutral species, the v(V = O t )b and shifts to highere nergy.T his is consistent with the strengthening of the terminal VÀOb ond as electron density is removed from the cluster core. In contrast, the v(O b -CH 3 )b and shifts to lower energy upon oxidation as a result of the decrease in the partial negative chargeo nt he bridging oxygen atoms. The FT-IR spectrum of 3-[V 5 FeCl]SbCl 6 only displayso ne broad band at 984 cm À1 ,w hich is due to the v(V = O t )a nd v(O b -CH 3 )v ibrations shifting upon oxidation such that they overlap. Similar shifts in the v(V = O t )a nd v(O b -OCH 3 ) bands upon oxidation have been observed in other iron-functionalized, POV-alkoxide clusters. [12b] For these systems, the redox activitywas shown to be localized on the vanadiumcenters, while the Fe III centerd id not show any redox-activity acrosst he redox series. The chloride-functionalized derivatives reported here appear to behaveinasimilarmanner.
Next, electronic absorption spectra of the redox isomerso f the halide-functionalized clustersw ere collected in acetonitrile (Figure 3b,T able S4). Complexes 1-[V 5 FeCl], 3-[V 5 FeCl]SbCl 6 , and 4-K[V 5 FeCl] all have absorbance features at 382 and 990 nm, diagnostic of mixed valent (V IV /V V ), POV-alkoxide clusters. The higher energy feature corresponds to the V IV (d xy ) ! V V (d x2-y2 )i ntervalence charget ransfer (IVCT), while the lower energy is assigned to aV IV (d xy ) ! V V (d xy )I VCT event. [11c, 12a, d] The intensity of the electronic transitions between V IV and V V centers vary with the number of V IV and V V within the clusters, thus giving insight into the oxidation state distribution of the metal centers. For example, if the reduction of 1-[V 5 FeCl] to 4-K[V 5 FeCl] is vanadium-based, the molar absorptivity of the two IVCT bandss hould be halved,a st he number of V V ions decreases from two to one;w enote that this prediction is observed. Furthermore, the absorbances of the 382 and the 990 nm bands of the 3-[V 5 FeCl]SbCl 6 are similar to those of 1- [V 5 FeCl].T his is consistent with V IV 3 V V 2 Fe III and V IV 2 V V 3 Fe III assignmentsf or 1-[V 5 FeCl] and 3-[V 5 FeCl]SbCl 6 ,r espectively,a s there are two pairs of V IV and V V ions for charget ransfer to occur in both clusters.
In the case of complex 5-(CoCp 2 ) 2 [V 5 FeCl],t he absence of the characteristicV IV ! V V IVCT in the electronic absorption spectrum indicates an isovalent oxidation state distribution of vanadium ions within the cluster, that is, [V IV 5 Fe III ]. In line with this oxidations tate assignment, al ow intensity absorption feature at 598 nm (235 cm À1 m À1 )i so bserved. Similar absorption features have been observedi no ther hexavanadate and heterometallic POV-alkoxides, where all the vanadium ions are in the tetravalents tate, and has been assigned as at ransition localized on asingle V IV center. [11b, c, 12b, c] An additional intense feature between 294 and 316 nm is observedf or all redox isomers. In our previousw ork with heterometal-functionalized POV-alkoxide clusters bearing ac hloride ligand (1-[V 5 FeCl] and[V 5 O 6 (OCH 3 ) 12 TiCl]), this has been assigned as aC l:!Ml igand-to-metal charget ransfer (LMCT) event. [12d, 14] The presence of this feature in each redox isomer confirmst hat the chloride ion remains boundt ot he cluster in solution.N otably,a st he halide-functionalized FePOV clusters are reduced, ah ypsochromic shift in the Cl:!Fe III transition occurs. This implies that while the redox activity of 1- [V 5 FeCl] is localized to the vanadium ions, the surroundingP OV-alkoxide framework affects the electronic structure of the Fe-Cl moiety.S pecifically,a st he metalloligand becomes more electron rich, the ability of the Fe III center to accept electron density from the chloride anion decreases. While oxygen-to-vanadium LMCT bands might alsoo ccur in these high-energyr egimes,the absence of these bands in the spectra of POV-alkoxide clusters lacking halide ligandss uggests that this feature indeed derivesf rom an electronic transition from Cl to the heteroion ( Figure S20).
Ta ken together,t he spectroscopic data of complexes 1-[V 5 FeCl], 3-[V 5 FeCl]SbCl 6 , 4-K[V 5 FeCl],a nd 5-(CoCp 2 ) 2 [V 5 FeCl], confirm that the redoxa ctivity of the cluster is localized within the vanadate cluster core. This is consistent with our previous reports investigating the redox chemistry of heterometal-functionalized POV-alkoxide clusters. [12b-d] We can thus assign the oxidation states distributions of transition metal ions in the chloride-functionalized, iron-functionalized POV clusters as follows: With the oxidation state distributionsf or the redox isomerso f 1- [V 5 FeCl] identified, we became interested in gaining af undamental understandingo fh ow the coordination environment aroundt he iron center imparts the different electrochemical behaviors observed in the CV of 1-[V 5 FeCl] and 2-[V 5 Fe]ClO 4 . As such, theoretical calculations were employed. First, the redox transitions of 1-[V 5 FeCl] (CN = 6) and 2-[V 5 Fe]ClO 4 (CN = 5) were obtained by DFT-level calculations using the B3LYP functional via the Jaguar program suite (see SI for details). [15] As shown in Ta ble 1, the theoretically calculated reduction potentials for processes 1', 2',a nd 3' for 1- [V 5 FeCl] are in excellent agreement with the experimentally observed values, with maximum deviations < 0.1 V. However,f or process 4',t he experimental data (À0.68 V) differs significantly from our calculated value (À0.37 V). This is possibly relatedt os olvation effects, where the acetonitrile solvation shell surrounding 1-[V 5 FeCl] is bound rather strongly as the negative charge is increased. [8] For 2-[V 5 Fe]ClO 4 ,w here we expect dissociation of the perchlorate ion in solution,t he calculations were performed for the 5coordinate iron center. The reduction potentials for the experimentally observed processes 2', 3',a nd 4' are in excellent agreement with the calculated values, with maximum deviations < 0.1 V. However, for process 5',w en ote as ignificant differencei nc alculated and experimental reduction potentials (DE ca. 1.7 V). This deviation is currently not understood and still under investigation. Weh ypothesize that it might be related to changes of the coordinatione nvironment aroundt he iron centerupon accessing the di-anionic species.
Frontier molecular orbital analysiso ft he highest occupied molecular orbitals (HOMO) of all five possible redoxi somers observed in the CV of 1-[V 5 FeCl] (Figure 4) indicate that all reductionp rocesses 1', 2', 3',a nd 4' lead to the reduction of the vanadium centers in 1- [V 5 FeCl].T his results in ar etention of the ferric center throughout the series of redox isomers. Similar observations were made upon analysiso fc omplex 2-[V 5 Fe]ClO 4 ( Figure S21), where the calculation showedt he redox activity to be localized to the POV scaffold. Furthermore, the atomics pin densities from Mulliken analysis for the complete set of redox-isomers for both clusters shows that iron centerr emains as high-spini ron(III) ion S Fe = 5 / 2 (Table S5, S6). These assignments are consistentwith the experimental results described above.
With the validity of these calculations established, the individual redox profiles of 1-[V 5 FeCl] and 2-[V 5 Fe]ClO 4 were analyzed theoretically by DFT-level computations to gain insights Table 1. Experimental and calculated reduction potentials for 1-[V 5 FeCl] and 2-[V 5 Fe]ClO 4 .All potentials referenced against Fc 0/ + . into the different behavioro bserved upon oxidation from the [Fe III V IV 3 V V 2 ]f orm. These oxidativep rocesses were selected to be interrogated since calculations were shown to model the E 1/2 values for these processes better than their reductive counterparts. In particular,w ec alculatedt he HOMO energies of the clusters upon oxidation starting from the parent[ V IV 3 V V 2 Fe III ] state. As shown in Figure 5, we observe that for 1- [V 5 FeCl] the first and second oxidations are energetically more accessible when compared to the first and second oxidations of 2-[V 5 Fe]ClO 4 .T his suggests that within the accessible solvent window,t he first and second oxidationo f1-[V 5 FeCl] are energetically possible. In contrast, for 2-[V 5 Fe]ClO 4 ,o nly the first oxidation is observed, while the second oxidation-while thermodynamically possible-isl ocated outside the solvents tability window.These resultsare consistentwith the increased negative charge provided by the chloride (or cyanate) anion stabilizing the oxidative processes.

Rationalizings hifts in half-wave potentials in transition metal functionalized POV-alkoxide clusters
Comparing the CV of 1- [V 5 FeCl] to other previously reported heterometal-functionalized POV-alkoxide clusters ( Figure S22, Ta ble S7), the E 1/2 values 1- [V 5 FeCl] and the gallium derivative are essentially identical. This is notable, as Ga III has been used as ar edox innocent surrogate for Fe III in bioinorganics tudies due to their similar charge andi onic radii. [16] The redox profiles of these clusters are anodically shifted from the parent, hexavanadate cluster, [ V 6 O 7 (OCH 3 )  In an efforttodevelop amodel that rationalizes and predicts the observed shifts in the CV of heterometallic POV-alkoxide clusters,w el ooked for correlationsb etween quantitative parameters describing physicalp roperties of the heteroions and the E 1/2 values of the POV-alkoxide compounds. Lewis acidity has been shown to be ag ood predictor for shifts in the electrochemical profiles of bimetallic [17] and multi-metallic [18] systems. Specifically,t he aqueous pK a ,p K a (H 2 O), of the installed heteroion has been used as am easureo ft he Lewis acidityi n order to quantify these effects. Ta king inspiration from these studies, the electrochemical potentials of the [V IV 5 M]/[V IV 4 V V M] redox event for [V 5 O 6 (OCH 3 ) 12 MX] (M = Hf 4 + ,Z r 4 + ,T i 4 + ,X = OCH 3 À ;M = Fe 3 + ,G a 3 + ,X = Cl À ;M = V 4 + ,X = O 2À )c lusters, in acetonitrile, were plottedagainst the pK a (M(OH 2 ) n )ofthe installed heteroion ( Figure 6, Ta ble S7). Similarp lots comparing E 1/2 vs. pK a (M(OH 2 ) n )f or the other redox events were analyzed and are reported in the supporting information file ( Figure S23, Ta ble S7). Good correlations betweent he positions of the vanadium-based redox eventsa nd the Lewisa cidity of the heteroion were observed (R 2 = 0.79-0.90). The E 1/2 values decreased by an average of À150 AE 9mVp er pK a unit, which is consistent with our previous analysis of redox dependence on Lewis acidity for these Lindqvist POV systems. [12d] Interestingly,t he steep-  couple. This suggests that the Lewis acidity of the heteroion has ag reater influence on the redox properties of the more oxidized specieso ft hese clusters.F urthermore, these slopes are significantly steeper than those reported previouslyf or heterometallic systems, specifically bimetallic, Schiff base cobalt( ca. À50 mV/pK a ) [17a] and nickel (ca. À70 mV/ pK a ) [17c] complexes and tetranuclear iron oxide (ca. À70 mV/pK a ) [18d] and manganese oxide (ca. À100 mV/ pK a ) [18b, c] clusters. Thus, for the heterometal-functionalized POV-alkoxidec lusters,t he electrochemistry of the surrounding vanadium centers appears to be particularly sensitive to the Lewisa cceptor capability of the incorporated heteroion.
Notably,i ron-functionalized POV-alkoxides with weakly coordinating anions,n amely 2-[V 5 Fe]ClO 4 and 2-[V 5 Fe]OTf,d eviate substantially from the linear relationship between the Lewis acidity of the installed heteroion established for all other heterometallic, POV-alkoxide clusters reported to date ( Figure S24, Ta ble S7). [12d] For example, 2-[V 5 Fe]ClO 4 exhibits E 1/2 values at significantly more reducing potentials (shiftedb yc a. À0.17 V) comparedt ot hat of 1-[V 5 FeCl].T his is likely duet ot he fact that the aqua speciesu sed for thesep K a determinationsa re six-coordinate iron centers, thus rendering thesev alues poor representations of the Lewis acidity of the 5-coordinate iron center.
The differences in the electronic profiles of the 5-coordinate and 6-coordinate iron-functionalized clusters run deeper than the overall shift in the redox profiles of the two clusters.  19 CoX] (X = OC 2 H 5 -,F -,C l -, Br -,a nd I -), the ligand field of cobalt centerh as as ignificant effect on the optical band gap of these clusters. [19] Similarly, ligand field effects could also be significant fort he open-shell Fe III centeri nt he iron-functionalized, POV-alkoxides reported herein.W estill do not fully understand the cause for the differences in the electrochemistry of 1-[V 5 FeCl] and 2-[V 5 Fe]ClO 4 , which is the subjecto fo ngoing investigations by our research team.

Conclusions
Herein,c ombined experimental andt heoretical analyses of the electrochemical features of two iron-polyoxovanadate-alkoxide clusters have been studied, focusingont he changes in electrochemicalb ehavior between five-coordinate and six-coordinate iron sites. Coordination of ac hloride to the iron(III) center led to an anodics hift in the redox events and stabilization of an additional oxidative feature in the CV.B oth experimental and theoretical studies showed that storage and releaseo fe lectron density is localized within the vanadatec lusterc ore for 1-[V 5 FeCl],w hich was also seen in previously reported 2-[V 5 Fe]ClO 4 .F urthermore, changing the geometry of the ferric centerf rom square pyramidal to octahedral upon coordination of the chloride ligand appears to tune these vanadium-based redox eventsb ya djusting the orbitalo verlap between the iron and the POV metalloligand. Therefore, both the type and the ligand environment of the installed heteroion will be important in selectively designing polyoxovanadate clusters with specific electrochemical properties.

Experimental Section
General Considerations:A ll manipulations were carried out in the absence of water and oxygen in aU niLab MBraun inert atmosphere glovebox under an atmosphere of dinitrogen. Glassware was oven dried for am inimum of 4hours and cooled in an evacuated antechamber prior to use in the drybox. Celite 545 (J. T. Baker) was dried in aS chlenk flask for at least 14 hours at 150 8Cu nder vacuum prior to use. 3 molecular sieves (Fisher Scientific) were activated using the same drying method. All solvents were dried and deoxygenated on aG lass Contour System (Pure Process Te chnology,L LC) and stored over activated 3 molecular sieves. Nitrosonium hexafluorophosphate (NOPF 6 ,9 6%)w as purchased from Alfa Aesar and used as received. Tris(4-bromophenyl)ammoniumyl hexachloroantimonate ((N(C 6 H 4 Br-4) 3 )SbCl 6 ,t echnical grade) and bis(cyclopentadienyl)cobalt(II) (CoCp 2 ,9 8%)w ere purchased from Sigma-Aldrich and used as received. Te trabutylammonium hexafluorophosphate ((nBu 4 N)PF 6 ,9 8%)w as also purchased from Sigma-Aldrich, recrystallized three times from hot ethanol, and stored under dynamic vacuum in the glovebox prior to use. Potassium graphite (KC 8