A Heteroaromatically Functionalized Hexamolybdate

A new heteroaromatic thiophene containing organoimido functionalized hexamolybdate has been synthesized and characterized in both solid form and solution. Structural analysis shows successful introduction of the organoimido ligand through replacement of one terminal oxo site on [Mo6O19]2− to yield the singly functionalized hexamolybdate. Spectroscopic and theoretical analysis indicates charge transfer between the inorganic and organic components, with a significantly red-shifted lowest lying transition of 399 nm vs. the parent Lindqvist ion of 325 nm. Additional characterization includes, thermal gravimetric analysis (TGA), infrared (IR), cyclic voltammetry (CV), nuclear magnetic resonance (NMR) and time-dependent density functional theory (TD-DFT) studies.


Introduction
Recent interest in the development of methodologies for the covalent grafting of organic molecules to the surface of polyoxometalates (POMs) has been driven by the utility of the resulting inorganic-organic hybrids in a range of applications [1][2][3][4][5][6].One such methodology can be used to yield organoimido functionalized Lindqvist structures of general formula [Mo6O(19−x)(NR)x] 2− where x = 1-6 and (NR) can be aromatic or aliphatic in nature [7].Since the first report of organoimido OPEN ACCESS functionalized hexamolybdates, several groups have shown interest in the synthesis of this structural class, and have subsequently reported a variety of modified synthetic approaches and reaction conditions to achieve this goal [8][9][10].That being said, drawing clear conclusions as to a rational synthetic approach from the literature is challenging when targeting multiple organoimido-substituted hexamolybdates.The use of aniline-based hydrochloride salts is a general synthetic route to the formation of mono-functionalized hexamolybdates and was first reported by Wei and Guo [11].Both the aniline and its HCl salt were used for the synthesis of mono-functionalized complexes containing electron-withdrawing groups.This route involves the reaction of the α isomer of octamolybdate [Mo8O26] 4− with the hydrochloride salt of the aromatic amine of choice in dry acetonitrile in the presence of the dehydrating and activating agent N,N'-Dicyclohexylcarbodiimide (DCC) [12].
Several key examples of post-synthetic Sonogashira and Heck couplings include the preparation of complexes bearing metal binding ligands [17] and the incorporation of the hexamolybdate in main chain polymers [16], as well as the preparation of polymers with POM containing pendant side chains [9].From a synthetic standpoint it is significant to note the varying stabilities of [Mo6O(19−x)(NR)x] 2− complexes to the post-synthetic conditions used in these reactions, in particular the tolerance of the base.The complex reported herein is currently under investigation regarding its utility for inclusion in materials prepared using these methodologies.

Scheme 1. Synthesis of the monofunctionalized organoimido hybrid (1-L4).
The molecular structure of (1-L4) (Figure 1) is characteristic of previously reported mono-imido hexamolybdates, with one of the terminal [Mo≡O] 4+ units being replaced by a more electron-rich [Mo≡NR] 4+ unit.The short Mo-N bond length (1.768(7) Å) and Mo-N-C bond angle (169.1(7)°) are in agreement with the assignment, and indicative of an Mo≡N triple bond character.Typical displacement of the central oxygen atom (Oc) within the hexamolybdate complex towards [Oc-Mo≡NR] (2.205(5) Å) vs. the average of the remaining non-substituted sites [Oc-Mo≡Ot] (2.34(1) Å) is also observed.Disorder of the thiophene portion of (1-L4) is observed due to free rotation around the single bond connecting the thiophene and aniline aromatic systems.The observed positions of carbon atoms C9, C12, and C13 were not affected by this disorder, whereas the positions of S1, C10, C11 the methyl substituent C14 and the bromine substituent Br1 were.Their positions were modeled using free variables, and by constraining the disordered atoms displacement parameters, atomic positions and bond lengths.The two observed positions refine well with occupancies of 58% and 42%.Furthermore, in the solid state the thiophene rings of two neighboring molecules are involved in pi-pi interactions resulting in the formation of a dimeric motif (Figure 1).The intermolecular separation was found to be 3.529(1) Å from ring center to ring center.Two charge balancing tetrabutyl-ammonium (TBA) cations are also required however only one could be successfully modeled, with the other showing significant disorder.A solvent mask was, therefore, implemented after the assignment of all locatable atoms from the difference map, with chemical and thermal analysis being consistent with the assignment of two TBA cations and the absence of any additional solvate in the bulk sample.
The electronic spectroscopy of (1-L4) (Figure A2) clearly shows a significant bathochromic shift and increase in intensity of the complexes lowest energy electronic absorption when compared to the parent plenary Linqvist polyanion [Mo6O19] 2− .Indeed this 72 nm shift to longer wavelengths (1-L4 399 nm vs. [Mo6O19] 2− 325 nm) is among the largest observed for any mono-functionalized organoimido hexamolybdate with an ε = 2.47 × 10 4 M −1 cm −1 .This absorption seems to be similar to other mono-functionalized compounds of extended conjugation presented in literature, such as that presented by Peng (382 nm, 392 nm, 403 nm) [15,17,18].This significant bathochromic shift is indicative of extended conjugation between the organic and inorganic components, with the presence of the electron withdrawing -Br attached on the thiophene ring probably aiding electronic mobility within the hybrid material.
The nature of the absorption band for 1-L4 is further analyzed with linear-response time-dependent density functional theory (TD-DFT) calculations.Calculations at the TDA-CAMB3LYP/ def2-TZVPP [19][20][21] //TPSS-D3/def2-TZVP [21][22][23][24] level of theory show qualitative agreement with experiment with a bright transition at 390 nm.The blue-shift of 9 nm compared to the experimental value is within the usual error margin for this level of theory [25] and expected for gas-phase calculations.Further analysis reveals that this bright electronic excitation is dominated by a transition from the highest occupied (HOMO) to the lowest unoccupied molecular orbital (LUMO).The HOMO is predominantly localized around the imido bridge and the adjacent six-membered ring, while the LUMO is additionally characterized by a strong contribution from the thiophene unit (Figure A9a,b).Inspection of the difference density (Figure A9c) further confirms that this excitation is of charge-transfer (CT) character with the imido bridge functioning as the electron donor and the thiophene as the electron acceptor; see also References [26] and [27] for related computational work.
The initial voltammetric response from solutions of (1-L4) reveal a reversible reduction with E°′ = −1.07V with an additional weak, reversible process with E°′ = −0.92V, due to contamination of the sample by [Mo6O19] 2− (Figure 2).The presence of [Mo6O19] 2− in the sample allows quantification of the shift in reduction potential with functionalization of the plenary Lindqvist polyanion.A scan to strongly reduce potentials lower than −1.65 V is accompanied by formation of daughter products which give rise to distinct anodic processes at potentials above 0.1 V (Figure A8), which are due to decomposition products.Similar anodic waves have previously been attributed to the oxidation of isopoly blues of unknown composition [28].Consistent with earlier reports, there is evidence for surface adsorption of the reduced polyoxo species with subsequent voltammograms featuring increasingly current with broad and sharp features consistent with a mixture of surface immobilized and solute species.
FTIR spectroscopy of (1-L4) (Figure A4) shows the presence of several bands associated with the polyanion such as ν(Mo-Ot) and ν(Mo-Ob-Mo) stretches at 950 and 794 cm −1 , respectively.The characteristic ν(Mo-N) is observed as a sharp shoulder band at 975 cm −1 .
The solution stability of (1-L4) has been confirmed by 1 H NMR studies (Figure A7), with all protons being unambiguously assigned versus that observed for the (L4) starting material.Aryl aniline protons give a singlet at 7.30 ppm 2H, meanwhile the thiophene proton also gives a singlet at 7.23 ppm 1H.These peaks integrate well with the two sets of aniline methyl protons 2.62 ppm 6H and thiophene methyl protons 2.43 ppm 3H.Signals at 0.97, 1.35, 1.60, and 3.08 ppm are attributed to the protons in the tetrabutylammonium counterions.Higher integrations of the cation protons indicate the possibility of some residual hexamolybdate that could not be separated via fractional crystallization or during the isolation process.(1-L4) was also studied by LC-MS showing isotopic cluster anions centered at m/z 579.16, and 1159.39.The signals are thus assigned as the parent cluster M 2− (M = [Mo6O18NC13SH12Br]), calculated 579.17 and M 2− + H + , calculated 1159.38 (Figure A1).

Conclusions
A new heteroaromatically derivatized hexamolybdate has been synthesized and extensively characterized.Theoretical TD-DFT calculations qualitatively agree with experimental observations, with the lowest lying excitation having charge transfer character whereby the HOMO is localized around the organoimido bridge and the LUMO on the thiophene.The electrochemical response of (1-L4) is similar to that of the parent Lindqvist polyanion with the potential of the first reversible reduction shifted cathodically by 150 mV as a result of derivitization.The reversibility of the process indicates that the reduced compound does not undergo dissociation of the ligand or fragmentation over the timeframe of the cyclic voltammetric experiment.Further reduction results in rapid decomposition of the complex.The molecule represents a rare example of a halogenated organoimido functionalized hexamolybdate with potential for further post-synthetic modification as a result, which is currently under investigation.

A1.2. Elemental Analysis
Chemical analysis was performed on Carlo Erba Elemental Analyser EA 1108, (The Campbell Microanalytical Laboratory, Department of Chemistry, University of Otago, Otago, New Zealand).

A1.9. Electrochemistry
Electrochemical experiments were conducted using a purpose built cell previously described [30].Experiments employed 3 mm diameter platinum working, silver pseudo-reference and platinum foil counter electrodes.Solutions for electrochemical analysis were prepared under strictly anaerobic conditions using a Vacuum Atmospheres glove box.The applied potential was controlled using a PAR model 362 potentiostat where waveforms were generated using EChem V1. 5.2 software in conjunction with a Powerlab 4/20 interface (ADInstruments, University of Melbourne, Melbourne, Australia).

A2. Experimental Section
Chemicals were used as purchased without further purification.Solvents were degassed and dried over 3 Å molecular sieves using standard laboratory procedures.Previously reported compounds (L1) and (L2) were synthesised as described in the original paper [31].

A3.4. Electrochemistry
Following the completion of voltammetric experiments a sample of ferrocene (Fc) was added to the solution and all potentials are referenced against the Fc + /Fc couple.
The results discussed in the manuscript were obtained with the range-separated CAMB3LYP density functional [20].Additional calculations with the range-separated wB97X-D3 [34] functional confirmed the charge-transfer character of the first bright excitation and were in qualitative agreement with the CAMB3LYP calculations (absorption at 379 nm).The resulting highest occupied and lowest unoccupied molecular orbitals obtained at the CAMB3LYP level are shown in Figure A9, along with the difference density, and they clearly indicate the charge-transfer character of the excitation.

A3.7. Crystallography
Table A2.X-ray Data Collection.Single crystal X-ray data was collected using an Agilent Technologies SuperNova Dual Wavelength single crystal X-ray diffractometer at 130 K using Mo-Kα radiation (λ = 0.71073 Å) for (L3) and Cu-Kα radiation (λ = 1.5418Å) for (1-L4) fitted with a mirror monochromator.Crystals were transferred directly from the mother liquor to the oil, to prevent solvent loss.The data was reduced using CrysAlisPro software (Version 1.171.36.28) (University of Melbourne, Melbourne, Australia) using a numerical absorption correction based on Gaussian integration over a multifaceted crystal model.Data was solved using direct methods by SHELXT and refined using a full-matrix least square procedure based upon F 2 .All ordered non-H atoms were refined anisotropically.

Figure 2 .
Figure 2. Cyclic voltammetry of a 2 mM solution of (1-L4) with (Bu4N)PF6 (0.1 M) in CH3CN.The scans were recorded from a freshly polished electrode using Pt working (3 mm diameter) and counter electrodes and an Ag pseudo-reference electrode.Potentials were corrected vs. ferrocene.

Figure A9 .
Figure A9.(a) Highest occupied molecular orbital obtained at the CAMB3LYP/def2-TZVPP level of theory displayed with an isovalue of 0.02 e − /Å 33 .(b) Lowest unoccupied molecular orbital obtained at the CAMB3LYP/def2-TZVPP level of theory displayed with an isovalue of 0.02 e − /Å 33 .(c) Difference density for the CT transition obtained at the TDA-CAMB3LYP/def2-TZVPP level of theory displayed with an isovalue of 0.0004 e − /Å 33 .Green lobes indicate an increase in electron density upon electronic excitation and blue indicates a decrease.All surface plots were generated with Gaussview 5.0 [34].