Reports: GB447864-GB4: Generation and Kinetic Studies of High-Valent Metal-Oxo Intermediates

Rui Zhang , Western Kentucky University

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 RuVI(TPFPP)O2.

Scheme 2.tif

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.

Fig 3.tif

Figure 1. UV-visible spectral change of RuIV(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 [RuVI(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 1.tif


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 lmax 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 logkrel and sp [krel = 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 [RuVI(Por)O2] (3) with organic sulfides a


Metal-oxo species


k2 (M-1s-1)























methyl phenyl sulfoxideb








methyl phenyl sulfoxideb

8.0 ± 0.4

3.0 ± 0.3

8.1 ± 0.5

(3.1 ± 0.4) — 10-4

48.0 ± 2.0

38.0 ± 3.0

59.6 ± 2.0

56.0 ± 1.0

42.4 ± 3.0

39.4 ± 2.0

38.0 ± 4.0

(2.6 ± 0.3) — 10-2

a Second-order rate constants in units of M-1s-1 for the reactions at 22 ± 2oC 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 2.tif

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



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.


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