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. 2021 Mar 12;2021(10):914-922.
doi: 10.1002/ejic.202001155. Epub 2021 Jan 18.

The Impact of Ligand Oxidation State and Fold Angle on the Charge Transfer Processes of MoIVO-Dithione Complexes

Sara A Dille  1 , Kyle J Colston  1 , Benjamin Mogesa  1 , Joseph Cassell  2 , Eranda Perera  1 , Matthias Zeller  2 , Partha Basu  1
Affiliations

The Impact of Ligand Oxidation State and Fold Angle on the Charge Transfer Processes of MoIVO-Dithione Complexes

Sara A Dille et al. Eur J Inorg Chem. .

Abstract

We report a series of mononuclear monooxo Mo(IV) complexes possessing either one or two fully oxidized dithiolene ligands; [MoOCl(R2Dt0)2][X], (1 and 2), and MoO(p-SC6H4Y)2(R2Dt0), (3 and 4), (R=Me, i Pr; X= PF6, SbF6, BF4; Y= H, Cl, F, CF3, Me, t Bu, OMe). Either four or two quasi-reversible ligand-based redox couples are detected depending upon the number of fully oxidized dithiolene ligands present. The molecular structure of 3 and 4 exhibit a large (47° to 70°) fold angle along the S•••S vector of the dithione ligand. The UV-Vis spectra of 3 and 4 exhibit a low energy charge transfer band at ~540 nm that are red-shifted ~200 nm compared to the spectra of 1 and 2. Density Functional Theory (DFT) calculations show that the low energy charge transfer band of 3 and 4 is heavily influenced by ligand fold angle. Reducing the fold angle decreases the charge transfer energy, and the transition becomes more ligand-based.

Keywords: Charge Transfer; Dithiolene; Donor-Acceptor; Fold Angle; Molybdenum.

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Figures

Figure 1.
Figure 1.
Oxidation states of dithiolene ligands.
Figure 2.
Figure 2.
The general structure of the molybdenum cofactor present in molybdopterin enzymes.
Figure 3.
Figure 3.
Molecular structure (30% probability) of complexes with the general formula MoO(p-SC6H4Y)2R2Dt0; hydrogen atoms are omitted for clarity.
Figure 4.
Figure 4.
Schematic representation of the fold angle between the dithiolene moiety and the plane formed by Mo and S donor atoms
Figure 5.
Figure 5.
Cyclic voltammograms of complex 1c (left) and complex 3a (right) . Scan rate, 100 mV/sec; temperature 25 °C, Pt-disk Ι MeCN (Et4NBF4) I Pt-wire, Ag+/Ag reference electrode.; supporting electrolyte Et4NBF4 (pink/black) or Et4NPF6 (blue). Potentials are referenced internally to Fc+/Fc couple.
Figure 6.
Figure 6.
Electronic spectra of complexes 3b (black) and 4b (red) in panel A, and 3e (orange) and 4e (blue) in panel B.The difference in energy for each band position is highlighted with the corresponding change in energy. All spectra are recorded in acetonitrile and their absorbances are normalized to the maximal value of the lowest energy LL’CT band.
Figure 7.
Figure 7.
Changes in atomic orbital contribution for 4a with changing ligand fold angle. Mo dxy (Δ), SPh (□), and Dt0 (○) contributions are relative to those reported for the gas phase optimized structure (Table 4). The fold angle for each optimized structure and solvent the optimization was generated in is listed in Table S2.
Figure 8.
Figure 8.
Relationship between fold angle (°) and energy (cm−1) from the band position of the lowest energy LL’CT calculated for 4a in acetonitrile. Spectra are superimposed in Figure S1. Linear regression produces a reasonable fit for the data (y = 6.0 x 10−3 – 37.758, R = 0.988).
Figure 9.
Figure 9.
1H NMR (CD3CN) spectrum of the reaction between 3a and TMAO in acetonitrile-d3 at room temperature. The unreacted starting material is marked with ●, uncoordinated dithione ligand is marked with ★, uncoordinated SPh ligand is marked with ■, and TMA is marked with ♦.
Figure 10.
Figure 10.
Superimposed low energy charge transfer bands of [2]+ (red) and 4a (blue). Included for each complex is an EDDM to better visualize the charge transfer for both. Light green orbitals are the charge accepting orbitals, whereas the donating orbitals correspond to the color of their spectra.
Scheme 1.
Scheme 1.
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