www.acsprf.org

Scientific and Technical Description of the Results

Photochemical generation of trans-dioxoruthenium(IV) porphyrin complexes

First, we report our full findings on the photochemical generation of trans-dioxoruthenium(VI) complexes (3) by irradiation of porphyrin-ruthenium(IV) dichlorate or dibromate complexes (2) that result in homolytic cleavage of the O-X bond in both axial ligands (Scheme 1). We have demonstrated that this photochemical method can be used to generate trans-dioxoruthenium(VI) species in six porphyrin systems under similar condition, in particular the very electron-demanding Ru^VI(TPFPP)O₂.

Scheme 1. Photochemical synthesis of trans-dioxoruthenium(VI) porphyrins

We have also discovered that visible light irradiation of the photo-labile dibromate species gave trans-dioxoruthenium(VI) porphyrin (3a) with well-anchored isosbestic points (Figure 1). The photochemical reaction that gave homolytic cleavage of the O-Br bonds is directly the same as that of the ruthenium(IV) dichlorates.

Figure 1. UV-visible spectral change of Ru^IV(TMP)(BrO₃)₂ (8 — 10^-6 M) with 5-fold excess of AgBrO₃ in anaerobic CH₃CN solution upon irradiation with visible light at 22 ^oC over 80 min.

Kinetic studies of sulfoxidation reactions by trans-dioxoruthenium(VI) porphyrins In view of limited studies thus far on oxidation of compounds of hereoatoms, particularly sulfides by these well characterized metal-oxo intermediates, we have conducted kinetic studies of sulfide oxidation reactions by trans-dioxoruthenium(VI) complexes with tunable structural and electronic prosperities. The complexes 3 are competent oxidants for sulfide oxidations at ambient conditions. In stoichiometric reactions, sulfoxides were produced in > 90% yields for 3a and 3f, and 78% for 3c. The current consensus mechanism is that [Ru^VI(Por)O₂] functions as a two-electron oxidant in the oxygen atom transfer process with formation of a ruthenium(IV)-oxo porphyrin complex (Scheme 2).

Scheme 2. Stoichiometric oxidation of thioanisole by trans-dioxoruthenium(VI) porphyrins

In kinetic studies, solutions containing the trans-dioxoruthenium(VI) oxidant were mixed with solutions containing large excesses of sulfide substrate, and pseudo-first-order rate constants for decay of the ruthenium-oxo species were measured spectroscopically. The dioxo 3 decayed rapidly in the presence of the thioanisoles, reacting as fast as 30 seconds.  For all oxo species, we monitored decay of the Soret-band l_max at 422 nm (3a), 418 nm (3c) and 412 nm (3f). Figure 2 shows typical kinetic results for reactions of TPFPP oxo (3f).

Figure 2. Typical reactions of 3f in CHCl₃ at 23 ± 2 ^oC. (A) Time resolved spectrum over 60 s for reaction of 3f with 0.25 mM thioanisole. (B) Kinetic traces at 412 nm for reactions of 2c with thianisole at different concentrations; (C) Plots of the observed pseudo-first-order rate constants versus the concentration of thioanisole and para-substituted thioanisoles.

The second-order rate constants for reactions of 3 with a variety of organic sulfides were summarized in Table 1. For a given oxo species, various thioanisoles react in a relatively narrow kinetic range; typically the second-order rate constants for the oxidation of para substituted thioanisoles with 3f showed little variation (Table 1, entries 7-12).  Furthermore, there is no linear correlation between the logk_rel and s_p [k_rel = k₂(substituted thioanisole)/k₂(thioanisole)], implying that no appreciable charge developed on the sulfur during the oxidation process by dioxoruthenium(VI) species.

Table 1. Rate constants (k₂) for the reactions of [Ru^VI(Por)O₂] (3) with organic sulfides ^a

^a Second-order rate constants in units of M^-1s^-1 for the reactions at 22 ± 2^oC in CHCl₃. The data reported here are averages of 2-3 runs with a standard deviation of 2s.

^b Higher concentrations (0.2 to 1.0 M) were used for kinetic studies.

We propose the reactions of trans-dioxoruthenium(VI) compounds with sulfides proceed via a concerted mechanism where a direct oxygen transfer occurs from ruthenium to sulfides without the participation of intermediates (Scheme 3, pathway A).  However, the smaller k₂ values (entries 10 and 11, Table 1) in the more electron-rich substrates (p-methyl and p-methoxythioanisole) is indicative of the relatively facile electron transfer from the thioanisoles without significant S-O bond formation. The contribution of an alternative pathway, i.e. electron-transfer followed by oxygen transfer (Scheme 3, pathway B), may be operative in the oxidation of electron-rich substrates.

Scheme 3. Proposed mechanisms for oxygen atom transfer processes from 3 to thioanisole

Conclusions

In summary, we report a new photochemical preparation of trans-dioxoruthenium(VI) porphyrin complexes in six porphyrin systems. We have also conducted the kinetic studies of sulfoxidation reactions with these well characterized metal-oxo species 3. The kinetic results obtained in this study indicate a concerted oxygen atom transfer and/or electron transfer followed by oxygen transfer mechanism from oxidant to sulfide.