Catalytic oxo-transfer reactivity of 11-14

In order to assess the oxo transfer behavior of the complexes with phenol containing ligands a series of reactions were carried out with 11-14. Herein we used two biological substrates, nitrate and dimethyl sulfoxide (DMSO), as oxidizer, and phosphines (PPh3) as reducing substrate.

First of all, the catalytic properties for the oxygen atom transfer reaction from NO3

to PPh3

were investigated for compounds 11-14 under the exact same conditions according to Scheme 12.

PR₃ + NO₃^- [MO₂L^X]

OPR₃ + NO₂ -Scheme 12.

Nitrate reduction by PPh3 in the presence of molybdenum complexes as catalysts is well known.^[167] In our investigation, the catalytic system involved complex (11-14), PPh3 and (Bu₄N)(NO₃) with a ratio 1:10:20 in CDCl₃ at room temperature. PPh₃ consumption and OPPh3 production were confirmed by ³¹P NMR spectroscopy. Without catalyst no nitrite is produced under these conditions. At the initial stage 0-5795 min (Figure 40 (a)) catalytic reactions were carried out without colour change for all solutions, which demonstrate that the catalytic cycles were performed corresponding to the reactions 1 and 2.

M^VIO₂L + PPh₃ M^IVOL + OPPh₃ (1)

0 2000 4000 6000 8000 10000

t / min

0 5 10 15 20 25 30

0 10000 20000 30000 40000 50000 60000 70000

t / min

conversion / %

Mo-S 11 Mo-Se 12 W-S 13 W-Se 14

(b)

Figure 40. Conversion of PPh₃ oxidized by NO₃^- with time for complexes 11-14. (a) the initial stage; (b) the whole process.

In this stage, the conversion of PPh3 to OPPh3 reached 15% for catalyst 11, while the conversions catalyzed by 12, 13 and 14 were lower and only reach 6% for 12, 4% for 13 and 14. At ca. 5795 min, the curves of 13 and 14 intersect. The conversions decreased according to this sequence: [MoO2L^S] (11) > [MoO2L^Se] (12) > [WO2L^S] (13) > [WO2L^Se] (14). This maybe is an indication that molybdenum compounds have much better catalytic activity than their tungsten analogues and sulfur containing compounds are better catalysts than their selenium containing analogues at least for this kind of compounds. Hereafter all reactions were accompanied by a gradual color change. The solution for 11 changes from dark purple finally to yellow-red, for 12 from dark purple to yellow and for both 13 and 14 from deep red to light red. The overall development of the conversion of PPh3 to OPPh3 with time is shown in Figure 40 (b). The increases of conversion of PPh₃ to OPPh₃ for the four compounds are very slow, especially for tungsten compounds 13 and 14 (lower than 10%), and almost tend to level. The developing trend combined with the color change is probably caused by the

formation of μ-oxo dimers in reaction 3.

M^VIO₂L + M^IVOL (3) M^V

O

O M^V O

L L

Due to the presence of an equilibrium in reaction 3, PPh3 will be oxidized slowly. When the equilibrium of the whole process is reached, the conversion of PPh3 will be maintained at a constant level.

As a further example of the reactivity complexes 11-14 were treated with 2.5-15 equiv PPh₃ in degassed, dry DMSO-d6 solution, respectively. The developments of the PPh3 to OPPh3 for catalysts 11-14 over time with different catalyst:PPh3 ratios are shown in Figure 41. In this case, compounds 11 and 13 catalyzed completely the reaction of PPh3 to OPPh3 considerably fast with different quantities of substrate except for the ratio of 11:PPh3 = 1:15 (Figure 41 (a) and (c)). They were all accomplished within 25 min and no colour changes were observed.

Exceptively, the catalytic reaction of 11 with 15 equiv PPh₃ exhibited different behavior. The solution started with a colour change from dark purple to brown at about 50% conversion (Figure 41 (b)), which can be likely attributed to the formation of a μ-oxo dimer. A conversion of 100% was reached for all catalyzed reactions by 14 in which PPh3 was completely oxidized with in 27 h (Figure 41 (d)). They show no colour change, either. However, both oxo-transfer reactions catalyzed by 12 with two different ratios of catalyst and PPh₃ exhibited the same behavior. First the conversion of OPPh3 developed relatively fast within 10% conversion.

After that the colour of solutions changed gradually from dark purple to red-yellow and the conversion increased very slowly, which can also be explained by the formation of a μ-oxo dimer as well.

(a) (b)

0 2000 4000 6000 8000 10000 12000

t / min

conversion / %

Mo-Se:PPh3=1:5 Mo-Se:PPh3=1:10

(e)

Figure 41. Conversion of PPh₃ oxidized by DMSO with time with different catalyst:PPh₃ ratios (a) and (b) for complex 11, (c) for complex 13, (d) for complex 14 and (e) for complex

0

0 1000 2000 3000 4000 5000 6000

t / min

12.

The kobs values determined from exponential fits ([OPPh₃]_t/[PPh₃]₀=1-exp(-kobs·t)) were calculated for all reactions catalyzed by 11-14 (for the reactions in which μ-oxo dimers were formed the kobs was calculated at the initial stage). In Figure 42, kobs values are represented vs.

PPh3 concentration. The trace in Figure 42 indicates that the reaction rate decreases as the PPh3 concentration increases, attaining a constant value. Further increases in the concentration of PPh₃ have no effect on the reaction rate.

Figure 42. Dependence of the kobs on the concentration of PPh₃ in DMSO for 11-14.

Through the comparison with the kobs values under the same ratio of catalyst and PPh3 in DMSO for 11-14, we can conclude that in this kind of catalytic reactions, tungsten compounds show better catalytic behavior than molybdenum analogues, and sulfur containing compounds have higher catalytic ability than selenium analogues.

Interestingly, the exchange of metals (Mo and W) causes a different influence on the catalytic abilities of compounds for NO3

and DMSO as the oxiding substrates. The replacement of sulfur by selenium in ligands makes the compounds have accordant catalytic behaviors for the

0

two substrates. The catalytic properties of molybdenum 11 and 12 for the oxygen atom transfer reaction from DMSO to PPh3 are reverse to those of complexes 7 and 8, which may be due to the influences of two phenolate groups in ligands on the coordinations of S or Se to molybdenum atom. In general, the four compounds catalyze the oxygen atom transfer reaction from DMSO to PPh3 much more effective than from NO3

to PPh3.