Organo-Functionalized Metal-Oxide Clusters: Synthesis and Characterization of the Reduced Cationic Species

A new diethanolamine functionalized oxovanadium cationic cluster was synthesized and characterized; electrochemical and UV-vis absorption properties are consistent with two distinct MLCT processes.


Introduction
Polyoxometalates (POMs) are polynuclear, metal-oxide anionic clusters formed mainly by the early transition metals V, Mo, and W, usually in their highest (d 0 ) oxidation state. 1,2 OMs have (idealized) symmetric structures composed of metal-oxygen polyhedra joined through cornerand edge-sharing of oxygen atoms.A number of prototypical POM structures are common, for example the Lindqvist (M 6 O 19 ), Anderson (XM 6 O 24 ), Keggin (XM 12 O 40 ), and Dawson (X 2 M 18 O 62 ) structures. 1,2 ouble-bonded, non-bridging oxygen atoms (M=O t ) terminate at the anion surface and offer a more-or-less facile dπ-pπ electron transfer path to the metal atoms, which may be reduced thereby to obtain mixed valence (d 0 /d 1 ) POM anions.A general redox capacity results from this interaction and is the origin of the compelling electronic and photophysical properties that are generally exhibited by POM anions.Broadly considered, these are the ability to accept multiple electrons in a chemically reversible manner, 2 and the presence

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of characteristic near-UV oxygen-to-metal (M=O t ) charge-transfer bands. 2 Practical application is found, for example, in oxidative chemical [3][4][5] and photochemical [6][7][8] catalysis.Reduced POMs support unpaired electrons and are thus attractive as potential magnetic centers. 9,10 OMs may be functionalized by replacement of some portion of the metal-oxygen framework by another atomic or molecular species, modifying their chemical properties and facilitating incorporation into extended structures. 11,12 [15] Of the prototypical POM structures, not all are observed for even the most common POMforming elements (V, Mo, and W).For example, there is no vanadium analogue of the Lindqvist and Anderson 16 structures formed of molybdenum.POM are derived from mono-and polynuclear metal oxide precursors by bench top and solvothermal methods.Covalent POM functionalization is often realized by self-assembly through addition of organic ligands to POM precursors. 11,12 he ligands are integrated into the metal oxide framework through (usually) a terminal oxygen atom (e.g., the hydroxyl oxygen of an alkoxide).Covalently functionalized polyoxovanadates began to appear apace around 25 years ago, and a number of early contributions 17 demonstrated the accessibility of functionalized polyoxovanadates containing the Lindqvist (e.g., 18 and Anderson (e.g., [(VO) 6 (CO 3 ) 4 (OH) 9 ] 5-) 19 structures.
As part of a continuing effort to synthesize new functionalized polyoxovanadate species with interesting electronic and magnetic properties, we prepared a series of new organo-functionalized heteropolyoxovanadium(IV) POMs,

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Although the structure of C2 was poorly resolved, it was clear that it contained the cationic cluster [NaV  1b.The functionalization of the oxometallic {NaV 6 O 6 } core by diethanolamine ligands in C2 indicates that some of the ligand L has undergone bond cleavage of the {CH 2 CH 2 NH 2 } group with concomitant protonation of nitrogen during the formation of C2.Interestingly, some of the uncleaved L is also present as a free molecule in the crystals of C2.The diethanolamine-functionalized cationic core of C2 offers the compelling prospect of versatile chemical activity at the NH groups; conceivably leading to a diversity of secondary functionalization.We therefore undertook to synthesize a compound containing the cationic core of C2 by a direct method using diethanolamine (CH

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Fig. 1 Ball-and-stick views of the cationic cores of (a The C1 core is identical to the C3 core except that the six ethanolic pendant arms of C1 are replaced by hydrogen atoms (not shown in figure) in C3.Color key: sodium(orange), vanadium(green), oxygen(red), nitrogen(blue), carbon(grey).Hydrogen atoms are omitted for clarity.

Experimental Materials and Methods
Commercially available reagent-grade chemicals were obtained from Sigma-Aldrich and used without additional purification.Ammonium decavanadate {(NH 4 )  21 All CV experiments were carried out using a BASi Epsilon potentiostat in a threeelectrode cell using a 3 mm diameter glassy carbon working electrode, a Ag/AgCl reference electrode, and a platinum wire counter electrode.The electrolyte was 0.01 M NaCl.The glassy carbon working electrode was polished (1 µm) prior to obtaining each CV and corresponding background.Potentials are reported versus the normal hydrogen electrode.FTIR spectra were obtained (4000-400 cm -1 , KBr method) using a Thermo Nicolet NEXUS 470 FT-IR spectrometer and processed using Thermo Fisher Scientific OMNIC v8.3 software.TGA curves were obtained at 3°C/min in argon (100 SCCM) using a Mettler Toledo TGA/SDTA 851e and processed using STARe v10.00 software.UV-vis spectra (190-800 nm, quartz cuvette, 1 cm path length) of 1x10 - 4 M solutions of C1 and C3 were obtained using a Beckman Coulter DU 800 UV/vis spectrophotometer and processed suing Beckman Coulter DU 800 System and Applications software v2.1.25.65; N, 7.5; H, 5.4; Cl, 1.6%).The calculated values are based on the X-ray results, with the addition of disordered fractional Cl - and OH -ions to conform to the elemental analysis and to provide charge balance.

Single Crystal X-ray Structure Analysis
An X-ray quality crystal of C3 (blue plate, 0.25 x 0.22 x 0.10 mm 3 ) was mounted on a thin glass fiber with hydrocarbon oil.A full sphere of data was collected at -185° C on a Bruker SMART APEX II diffractometer equipped with graphite monochromatized Mo Kα radiation.The data were processed with SAINT software 23 and corrected for absorption with SADABS. 24alculations were performed by use of the SHELXTL package. 25Hydrogen atoms, except for those on a disordered water oxygen atom, were located and refined by use of the riding model.At this point, the difference map was featureless and the unit cell had large void areas with an unrealistic volume of 26.5 Å 3 per nonhydrogen atom.Also, the calculated value of 1.18 g/cm 3 for the density was inconsistent with the observed density of 1.67 g/cm 3 .The PLATON/SQUEEZE program 26 was used to model the electron density in the void areas.The color diagrams were produced with Mercury and POV-Ray.Further details are included in the deposited .ciffile.The Crystallographic information file (CIF) is available from the Cambridge Crystallographic Data Center (CCDC 994749). 27

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We obtained the title compound, [NaV 6 O 6 {(OCH 2 CH 2 ) 2 NH} 6 ]•(OH) 0.5 Cl 0.5 •3(H 2 O) (C3), directly from the solvothermal reaction of diethanolamine, sodium chloride and ammonium decavanadate without using a formal solvent.While C1 was obtained in the presence of a mixed acetonitrile-ethanol solvent, we could not obtain C3 in either acetonitrile or ethanol, or an acetonitrile-ethanol mixture.This suggests that diethanolamine, unlike triethanolamine, interacts sufficiently with acetonitrile and ethanol to inhibit its reaction with the vanadium oxide intermediates present in the reaction vessel under solvothermal conditions.TGA indicated that the as-synthesized material contained about 4.5 weight percent water which is removed at low temperature (T < 100 °C).No weight loss associated with organic material was observed until T > 200 °C (under Ar, see ESI), indicating that the organo-functionalized core has good thermal stability.

Crystal Structure
Compound C3 is a new organo-functionalized heteropolyoxovanadium(IV) compound.Its structure, revealed by single crystal x-ray structure analysis, contains novel cation cluster species [NaV IV 6 O 6 {(OCH 2 CH 2 ) 2 NH} 6 ] + (Fig. 1b).Its structure consists of a fully reduced cyclic {NaV 6 N 6 O 18 } framework incorporating six diethanolamine ligands; chloride and hydroxide anions balance charge.The {NaV 6 N 6 O 18 } core adopts an Anderson-like structure 16 comprised of six edge-sharing distorted {VO 5 N} octahedra linked to the central {NaO 6 } unit.The octahedral geometry around each vanadium atom of the cyclic core is defined by five atoms from the diethanolamine ligands and one terminal oxygen atom (V=O t ).The {V 6 O 12 } ring in C3 may be regarded as a hexadentate ligand to the metal Na, forming a {NaV 6 O 12 } chelate.Like C1, the hexametalate ring of C3 contains six d 1 (V IV ) ions.
The two crystallographically independent vanadium atoms have virtually identical distorted octahedral configurations with bond distances in the expected ranges (mean values include: Full details of the structure analysis, including tables of distances and angles, are included in the deposited cif file.

FTIR Spectroscopy
The characteristic polyoxoanion FTIR spectrum contains a strong band associated with a M=O t vibrational mode and a number of lower energy bands associated with the vibrational modes of bridging oxygen atoms.The prototypical Anderson structure 16 is mainly observed as a polyoxomolybdate species {(XMo 6 O 24 )} and we could find no polyoxovanadate examples in the literature.A variety of different salts of this species have been characterized using FTIR and

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9][30][31] The Anderson structure contains two equivalent terminal molybdenum-oxygen bonds (Mo=O t ) which typically show strong IR absorption bands around 950 (ν sym Mo=O t ) and 890 cm -1 (ν as Mo=O t ).The structures of C1 and C3, while conceptually derived from the Anderson structure, are expected to exhibit significant deviations from the prototypical Anderson FTIR spectrum due to the organo-functionalization, where one terminal oxygen per vanadium addendum atom has been replaced by the nitrogen atom of the triethanolamine (C1) and diethanolamine (C3) ligands, and all remaining non-terminal surface oxygen atoms are bound to the organo carbon atoms.The terminal oxygen atoms (V=O t ) remain as a basic structural similarity between the prototypical Anderson structure and the structures of both C1 and C3.Therefore we might expect some commonality in the FTIR spectra of each species respecting the terminal oxygen (M=O t ).And indeed, each of the C1 and C3 spectra contains a feature centered around 950 cm -1 that appears to be a composite of two bands (Fig. 2a).We assign one of these bands to the (ν sym V=O t ) mode of C1 and C3 by comparison to the prototypical (ν sym Mo=O t ) band, and consider the possibility that the remaining band is associated with a V-N mode (e.g., ν sym V-N).The proximity of these features in the C1 and C3 spectra implies that the asymmetric stretch associated with the terminal oxygen atom (ν as V=O t ) should also be closely located in each spectra, and indeed a common feature is observed in both spectra at ~881 cm -1 .The remaining bands below 950 cm -1 are not inconsistent with contributions due to the ethanolamine functionalization 32 and various expected V-O-V vibrational modes.The multiple bands in the ~ 1500-1000 cm -1 region of the C1 and C3 FTIR spectra are also consistent with the respective organic ligands. 32The C-H stretching vibrations of normal alkanes appear in 3000-2840 cm -1 region (Fig. 2b).Four peaks, associated with the asymmetric and symmetric C-H stretching modes of the methyl and methylene groups are usually observed.For example, in dodecane, ν as CH 3 = 2962, ν s CH 3 = 2872,  as CH 2 = 2926, and ν s CH 2 = 2853 cm -1 . 33We attribute the multiple features observed in the 3000 -2840 cm -1 region of the C1 and C3 FTIR spectra to non-equivalent methylene groups contained in the organic ligands of C1 and C3.A broad O-H stretching band appears in the 3700-3000 cm -1 region due to the presence of surface water and water of crystallization contained in the polycrystalline powder samples of both C1 and C3.The C3 spectrum contains an additional band superimposed on the broad O-H stretching band and centered at 3235 cm -1 .Primary amines display asymmetrical and symmetrical N-H stretching modes around 3500 and 3400 cm -1 , respectively.Secondary amines display a single band around 3330 cm -1 (e.g., diethanolamine, 3288).No bands are present above the aliphatic C-H stretching region (i.e., > 3000 cm -1 ) in tertiary amines (e.g., triethanolamine). 33We therefore attribute the 3235 cm -1 band of the FTIR spectrum of C3 to the N-H stretching mode associated with its diethanolamine functionalization.

Cyclic Voltammetry
We investigated the aqueous phase electrochemical redox properties of the title compound and C1 by use of cyclic voltammetry (CV).Unfunctionalized POMs exhibit CV reduction waves in accord with the high oxidation state of the metal atoms-usually d 0 or a mixed valence combination of d 0 and d 1 .Electrochemical reduction is accompanied by electron transfer from terminal (non-bridging) oxygen surface atoms to the d 0 polyhedral metal atoms through the M=O t dπ-pπ interaction. 1 The functionalized POMs C1 and C3 could not be electrochemically reduced, but each exhibited a chemically irreversible oxidation wave (Fig. 3).This result is in accord with the fully reduced (d 1 ) state of each compound, which energetically disfavors further reduction.Electrochemical oxidation is accompanied by electron transfer from the d 1 vanadium (V IV ) to the terminal oxygen surface atom.The irreversible nature of the CV profile suggests decomposition of the cationic core upon oxidation.This behavior may be contrasted with, for example, the fully oxidized Lindqvist hexavanadate, [V V 6 O 13 {O 2 NC(CH 2 O) 3 } 2 ] 2-, which displays a reversible one-electron reduction. 34Peak positions indicate that C3 is more easily oxidized than C1, a result that may be rationalized on the basis of an effectively larger interfacial charge transfer distance expected for C1 due to the presence of the pendant ethanolic {-CH 2 -CH 2 -OH} arms. 35A leading shoulder is present in the CV profile of C3.Scan-rate dependent CV excluded adsorption of the oxidized species as the cause of the leading shoulder. 35We suspect that this shoulder originates in a V-N orbital interaction that permits electron transfer from the d 1 metal to the nitrogen atom, analogous to the dπ-pπ interaction centered on the V=O t double bond.Thus we suggest that C3 can be oxidized at nitrogen with an accompanying vanadium-tonitrogen electron transfer.The absence of a corresponding shoulder in the CV of C1 (Fig. 3) may be rationalized by insufficient overlap of electrodic and molecular orbitals (centered on nitrogen) due to the presence of the pendant ethanolic arms of C1 (Fig. 1).

UV-vis Spectroscopy
Aqueous phase UV-vis spectra of C1 and C3 indicate a strong UV charge transfer band peaking around 200 nm (Fig. 4).Such bands are typical of unfunctionalized POMs and are associated with the same oxygen-to-metal electron transfer that accompanies electrochemical reduction. 2he fully reduced state (d 1 ) of the functionalized POM cations of C1 and C3, along with the oxidation wave indicated in the CV, imply that the CT bands of Fig. 4 are associated with oxidative vanadium-to-oxygen charge transfer.A leading, albeit weak, near-UV shoulder is present in both spectra (Fig. 4 inset), indicating that an additional charge transfer process is present, in support of the proposed V-N CT orbital interaction.6 O 6 {(OCH 2 CH 2 ) 2 NH} 6 ] + .The cluster cation is composed of a fully reduced cyclic {NaV 6 N 6 O 18 } framework which adopts an Anderson-like structure.C3 can be obtained in a direct solvothermal reaction of diethanolamine and ammonium decavanadate without any formal solvent present.Electrochemical and UV-vis absorption properties of C3 are consistent with two distinct MLCT processes; the characteristic V=O t dπ-pπ interaction, and a second process occurring through the hydrogen-terminated nitrogen atoms (V-N-H) of the octahedra forming the cyclic {NaV 6 N 6 O 18 } core.This view of the V-N bond as an

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oxidation route suggests possible chemical activity of the title compound at the hydrogenterminated nitrogen atoms, and is under further investigation in our laboratory.

Fig. 4
Fig.4UV-vis absorption spectra of aqueous C1 and C3.Both species exhibit a strong charge transfer band (ε max ~ 200 nm) having a weak leading shoulder (inset).