2 research outputs found
Effect of Substituents on the Water Oxidation Activity of [Ru<sup>II</sup>(terpy)(phen)Cl]<sup>+</sup> Procatalysts
No full textA series of [Ru<sup>II</sup>(terpy-R)(phen-X)Cl]PF<sub>6</sub> complexes
was designed where terpy-R
is the tridentate 4′-(4-methylmercaptophenyl)-2,2′:6′2″-terpyridine
ligand MeMPTP and phen-X is a substituted phenanthroline with hydro
(<b>1</b>), 5-nitro (<b>2</b>), 5,6-dimethyl (<b>3</b>), and 3,4,7,8-tetramethyl (<b>4</b>). This series allows us
to compare the reactivity of phenanthroline-containing procatalysts
with that of its well-established bipyridine counterparts as well
as to study the effects of electron-withdrawing and -donating substituents
on water oxidation. These species were thoroughly characterized by
spectroscopic and spectrometric methods, and the structures of <b>1</b>, <b>3</b>, and <b>4</b> were determined by single-crystal
X-ray diffraction. The procatalysts <b>1</b>–<b>4</b> show opposite trends compared to known terpyridine/bipyridine species;
the unsubstituted procatalyst <b>1</b> yields a turnover number
(TON) of 410 followed by 250 and 150 for complexes <b>3</b> and <b>4</b> with electron-donating substituents. Species <b>2</b>, with electron-withdrawing properties, yields the lowest TON of
60. Although the TONs decrease upon substitution, the presence of
electron-donating methyl substituents enhances the rate of O<sub>2</sub> evolution during an early stage of catalysis. Interestingly, no
evidence of conversion from chlorido-containing procatalysts into
expected aqua-containing catalysts was observed for <b>1</b>–<b>4</b> by NMR and UV–visible spectroscopy
during the induction period. This observation, along with reactivity
toward (NH<sub>4</sub>)<sub>2</sub>[Ce<sup>IV</sup>(NO<sub>3</sub>)<sub>6</sub>], suggests that water nucleophilic attack happens to
a high-valent ruthenium species rather than while at the Ru<sup>II</sup> oxidation state. Reactivity follows a trend similar to the rate
of O<sub>2</sub> evolution in all complexes. Furthermore, the electrospray
ionization mass spectrometry and <sup>1</sup>H NMR analyses of <b>1</b>, as recovered after catalysis, indicate the presence of
a chlorido ligand
