A Heteroaromatically Functionalized Hexamolybdate

Abstract

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.

1. 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 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].

Post-synthetic reactions involving organoimido hexamolybdates containing terminal halogen or ethynyl groups has now been well documented [9,13,14,15,16,17]. Both Sonogashira and Heck Pd-catalyzed carbon-carbon coupling reactions involving organoimido hexamolybdate complexes have been reported, showing their promise as building blocks in the design of elaborate molecular and polymeric compounds. 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.

2. Results and Discussion

Here, we report a novel mono-functionalized organoimido hexamolybdate [Mo₆O₁₈(L4)]²⁻ (1-L4) where L4 = 4-(4-bromo-5-methylthiophen-2-yl)-2,6-dimethylaniline. L4 was synthesized via the palladium catalysed cross coupling of 3,5-dibromo-2-methylthiophene (L1) with {4-[(diphenylmethylidene)amino]-3,5-dimethylphenyl}boronic acid (L2) to yield N-[4-(4-bromo-5-methylthiophen-2-yl)-2,6-dimethylphenyl]-1,1-diphenylmethanimine (L3) that was then subsequently deprotected and used as the HCl salt, as shown in (Scheme 1). The synthesis of (1-L4) involved the preparation of a solution of (Bu₄N)₄[Mo₈O₂₆] (0.0561 mmol) in dry acetonitrile (2 mL) to which 1.34 equivalents of 4-(4-bromo-5-methylthiophen-2-yl)-2,6-dimethylaniline and 2 equivalents of DCC were added. This solution was then sealed in a 5 mL biotage microwave vial and reacted at 120 °C for 90 min at 4 bar in a biotage initiator microwave reactor. On cooling to room temperature, the reaction mixture was filtered and the solvent removed under vacuum. The resulting oil was washed with diethyl ether and subsequently recrystallized from acetonitrile via diethyl ether vapor diffusion. Fractional crystallization was then required, with the yellow tetrabutylammonium (TBA) salt of [Mo₆O₁₉]²⁻ crystalizing initially. The remaining solution was decanted, concentrated and subjected to further diethyl ether diffusion resulting in the desired compound, in moderate yield, over 5–10 days.

Scheme 1. Synthesis of the monofunctionalized organoimido hybrid (1-L4).

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 (O_c) within the hexamolybdate complex towards [O_c–Mo≡NR] (2.205(5) Å) vs. the average of the remaining non-substituted sites [O_c–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 × 10⁴ 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°′ = −1.07 V with an additional weak, reversible process with E°′ = −0.92 V, 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-O_t) and ν(Mo-O_b-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 ¹H 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 M²⁻ (M = [Mo₆O₁₈NC₁₃SH₁₂Br]), calculated 579.17 and M²⁻ + H⁺, calculated 1159.38 (Figure A1).

Figure 1. Graphical representation of (1-L4) (top) and space filling representation of the pi-stacked dimers observed in the solid state (bottom). Mo = Orange spheres; O = Red spheres; N = Blue sphere, C = Black spheres; S = Yellow sphere; Br = Teal sphere; H = White spheres [29].

Figure 1. Graphical representation of (1-L4) (top) and space filling representation of the pi-stacked dimers observed in the solid state (bottom). Mo = Orange spheres; O = Red spheres; N = Blue sphere, C = Black spheres; S = Yellow sphere; Br = Teal sphere; H = White spheres [29].

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 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.

3. 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.