When Identification of the Reduction Sites in Mixed Molybdenum/Tungsten Keggin-Type Polyoxometalate Hybrids Turns Out Tricky

The mixed molybdenum/tungsten Keggin-type polyoxometalate (POM) hybrid (TBA)4[PW9Mo2O39{Sn(C6H4I)}] (TBA = tert-butylammonium) has been prepared by the reaction between [α-PW9Mo2O39]7– and [Cl3Sn(C6H4I)] in dried acetonitrile, in the presence of tetra-n-butylammonium bromide. A further coupling reaction affords the ferrocenyl derivative (TBA)4[PW9Mo2O39{Sn(C6H4)C≡C(C6H4)Fc}]. The POM hybrids have been thoroughly characterized by NMR and IR spectroscopies. Electrochemical analysis confirms their ease of reduction compared to the all-W analogue, albeit with a second reduction process occurring at a lower potential than in the all-Mo species. It is noteworthy that the second reduction is accompanied by an unusual red shift of the electronic absorption spectrum. Whereas there is no doubt that the first reduction deals with Mo, the location of the second electron in the bireduced species, on the second Mo or on W, has thus been the subject of a cross-investigation by spectroelectrochemistry, electron spin resonance, and theoretical calculations. Finally, it came out that the second reduction is also Mo-centered with two unpaired and antiferromagnetically coupled extra electrons.


DFT calculations
Sn calculated with different density functionals. Figure S10.

Chemical reduction of (TBA) 4 [PW 9 Mo 2 O 39 {Sn(C 6 H 4 I)}] K W9Mo2 Sn and characterization of the 1e reduced [PW 9 Mo 2 O 39 {Sn(C 6 H 4 I)}] 5-(I)
A 0.37 M solution of sodium naphtalenide NaNID has been prepared by addition of metallic sodium to naphtalene, as previously described. 1 S-8 First, reduction of K W9Mo2 Sn (0) in solution in dry and degassed CD 3 CN has been monitored directly in a 31 P NMR tube equipped with a Young valve by adding successive aliquots of the NaNID solution. The characteristic peak at -9.87 ppm progressively decreased with simultaneous increase of a new peak at -7.67 ppm, that we ascribed to the 1e-reduced K W9Mo2 Sn (I). The deshielding of the signal of the 1e-reduced species parallels that observed for [PMo 12 O 40 ] 3-. 2 Curiously, no other signal could be detected upon further addition of one equivalent of NaNID. Then, several trials have been made to prepare the 2e-K W9Mo2 Sn (II) in a Schlenk tube by addition of 2 equivalents of the NaNID solution to a solution of K W9Mo2 Sn (0) in dry and degassed CH 3 CN. However, after evaporation to dryness, the 31 P NMR spectrum of the blue solid redissolved in dry and degassed CD 3 CN disclosed a mixture of 0 and I. At this stage we do not understand the reason of this apparent difficulty in getting II. A competitive reduction path involving the iodo-aryl function might be proposed. But the cyclo-votammograms of K W9Mo2 Sn and the UV-Vis spectra obtained by spectro-electrochemistry are characteristic of (multiply) reduced POMs and do not point out the reduction of an organic moiety, that we have never observed in similar compounds. The use of a reducing agent milder than NaNID is probably advisable.
The UV-Vis-NIR and ESR spectra of the mixture of 0 and I obtained by chemical reduction are presented below. They display features analogous to that described in the manuscript for solution of I in CH 3 CN (TBAPF 6 0.1 M), obtained by electrochemical reduction. The observed shift of the wavelength at the maximum absorption can probably be explained by the difference in the environment (presence of sodium cation throughout the chemical reduction and of the supporting electrolyte during the electrochemical reduction), which is known to affect IVCT processes. Figure S9. Left: UV-Vis-NIR spectra of a 6 10 -4 M solution of the as obtained 0 + I solid mixture redissolved in CH 3 CN; right: X-band ESR spectrum of a frozen (20 K) aliquot of the mother solution taken before evaporation to dryness.
S-9 Sn , II) The spectra of species I and II were simulated by means of time-dependent DFT using the HSE06 functional, which provides the least overestimated HOMO-LUMO gap, along with rather well-reproduced reduction potentials (see Table S1 and Table 2 in the main text). As shown in Figure S17 (left), the shape of the absorption spectrum of I matches rather well with the experimental one (Figure 3), although all bands are slightly blue-shifted, presumably due to the overestimated band gap typical of hybrid functionals. The spectrum shows two bands in the visible region (centered at 475 and 603 nm) that can be attributed to the experimental band at 560 nm and the broader shoulder around 720 nm; in addition to another band in the near infrared region, accounting for a SOMO→LUMO excitation (Mo→Mo). When moving to the spectrum of II ( Figure S17, right), the latter disappears as the low energy-lying d(Mo) orbitals are both occupied. Moreover, in agreement with the evolution of the experimental absorption spectrum upon reduction, the intensity of the bands in the visible region increases due to the contribution of two Mo(V) ions to excitations of Mo→W character. The appearance of an intense band of Mo→W character at 683 nm can be attributed to the band centered at 720 nm in the experimental spectrum, while the less intense band at 491 nm might correspond to the broad shoulder centered at ca. 560 nm (Figure 3).

S-10
Cartesian coordinates (Å) for the most representative structures optimized at the B3LYP-D3 level. [