Stereoselective Assembly of Gigantic Chiral Molybdenum Blue Wheels Using Lanthanide Ions and Amino Acids

The synthesis of chiral polyoxometalates (POMs) is a challenge because of the difficulty to induce the formation of intrinsically chiral metal-oxo frameworks. Herein we report the stereoselective synthesis of a series of gigantic chiral Mo Blue (MB) POM clusters 1–5 that are formed by exploiting the synergy between coordinating lanthanides ions as symmetry breakers to produce MBs with chiral frameworks decorated with amino acids ligands; these promote the selective formation of enantiopure MBs. All the compounds share the same framework archetype, based on {Mo124Ce4}, which forms an intrinsically chiral Δ or Λ configurations, controlled by the configurations of functionalized chiral amino acids. The chirality and stability of 1–5 in solution are confirmed by circular dichroism, 1H NMR, and electrospray ion mobility–mass spectrometry studies. In addition, the framework of the {Mo124Ce4} MB not only behaves as a host able to trap a chiral {Mo8} cluster that is not accessible by traditional synthesis but also promotes the transformation of tryptophan to kynurenine in situ. This work demonstrates the potential and applicability of our synthetic strategy to produce gigantic chiral POM clusters capable of host–guest chemistry and selective synthetic transformations.


Structural analysis of 1-7
Although the wheel-type molybdenum blue architectures are very complex, the general approach to the structural analysis and formula determination is well documented. [4] The structural analysis requires the following lines of evidence / information to allow the assignment of formula and the structural details coupled with Single-crystal X-ray diffraction: S8 (i) Redox titration to help determine the number of reduced Mo V centres (Uv-vis-NIR spectroscopy also can help corroborate this data via the analysis of the extinction coefficient for the IVCT associated with the reduced Mo V centres. Each centre should contribute ca. 5 -6 x 10 3 L mol -1 ·cm -1 to ε).
(ii) Bond valence sum analysis to confirm the terminal oxo positions, reduced Mo V centres and the positions of the hydroxide ligands. [5] (iii) Elemental analysis of sodium, molybdenum, cerium and C, H, N analysis.
(iv) TGA to estimate the number of ligand and solvent water molecules.
Therefore, the analysis below both presents this data and demonstrates how the structural assignment is consistent with this data.

Redox titrations
Because of the rather poor solubility of compounds 1-7, it is impossible to perform redox tritration to determine the reduced eletrons on them.

Elemental analysis and C, H, N analysis
See Section 3. Synthetic procedure of 1-7

Uv-vis-NIR spectra
Because of the rather poor solubility of compounds 1-7, we could not prepare the related solution with accurate concentration. Therefore, the Uv-vis spectra of 1-7 were recorded in saturated aqueous solution S9 and ε was not caculated. All the Uv-vis spectra of 1-7 show the characteristic band of Mo Blue which is centered around 745 nm. Figure S1. Uv-vis spectra of Λ-1, Λ-2, Λ-3, Λ-4 and Λ-5.         (3)            Samples were prepared by dissolving pure crystals of 1 and 5 in HPLC grade water, at approx. 5 mg/ml; these solutions were filtered (some solid always remained) and analysed with no further purification.

Crystallographic data and crystal structures of 1-7
Spectra were acquired on a Waters Synapt G2 HDMS instrument in Sensitivity mode (except otherwise stated), with samples infused into the standard ESI source at 5 µl/min using a Harvard syringe pump. shown in the accompanying key; no filtering is applied to limit signals (eg. no filtering of signals < 5% in 2D map; that is, few other signals are visible in the raw data with no manipulation).
To determine drift times (t D) of species of interest in the IMS cell Arrival Time Distribution (ATD) data was extracted from Driftscope/Masslynx, and fit to Gaussian curves using Fityk v0.9.8 to determine a representative retention times peak centre.
Ion mobility calibration data was recorded for Equine Cytochrome C 4 , oligothymidine, 5 and another DNA strand (d[TTTAGGG]6), 6 and fit to their respective helium collision cross-sections (CCSHe) reported in the literature using the approach outlined in Reference 4; data was only omitted where peaks were very weak, fitting not possible/ambiguous, or conformations not observed. The resulting calibration curve (see ESI) was then used to estimate the CCSHe of ions of interest from observed drift times.

Calibration curve for CCSHe measurements
Calibration curve obtained using the method outlined in experimental section.
Note on drift time estimation: [Using drift time data acquired with N2 as a drift gas to estimate CCSHe, rather than CCSN2, may seem somewhat strange. Beyond the considerable usefulness of IMS-MS to resolve otherwise-overlapping (by m/z) ions (eg. monomers from oligomers), in many cases its primary use is to obtain quantitative CCS data is to compare with models. Since for a number of reasons (practical, historical, and economic) most commercially-available "Synapt" Travelling Wave instruments use N2 as a drift gas, but theoretical models (eg. Mobcal 7 + new Bowers models 8 ), most available calibration data and most published data with which results are likely to be compared, are for CCSHe. As a result, use of CCSHe is preferred and its validity (and potential pitfalls) becoming more thoroughly understood. 9