4 research outputs found
Oxygen Reduction Mechanism of Monometallic Rhodium Hydride Complexes
No full textThe
reduction of O<sub>2</sub> to H<sub>2</sub>O mediated by a series
of electronically varied rhodium hydride complexes of the form <i>cis</i>,<i>trans</i>-Rh<sup>III</sup>Cl<sub>2</sub>H(CNAd)(P(4-X-C<sub>6</sub>H<sub>4</sub>)<sub>3</sub>)<sub>2</sub> (<b>2</b>) (CNAd = 1-adamantylisocyanide; X = F (<b>2a</b>), Cl (<b>2b</b>), Me (<b>2c</b>), OMe (<b>2d</b>)) was examined through synthetic and kinetic studies. Rhodium(III)
hydride <b>2</b> reacts with O<sub>2</sub> to afford H<sub>2</sub>O with concomitant generation of <i>trans</i>-Rh<sup>III</sup>Cl<sub>3</sub>(CNAd)(P(4-X-C<sub>6</sub>H<sub>4</sub>)<sub>3</sub>)<sub>2</sub> (<b>3</b>). Kinetic studies of the reaction of
the hydride complex <b>2</b> with O<sub>2</sub> in the presence
of HCl revealed a two-term rate law consistent with an HX reductive
elimination (HXRE) mechanism, where O<sub>2</sub> binds to a rhodium(I)
metal center and generates an η<sup>2</sup>-peroxo complex intermediate, <i>trans</i>-Rh<sup>III</sup>Cl(CNAd)(η<sup>2</sup>-O<sub>2</sub>)(P(4-X-C<sub>6</sub>H<sub>4</sub>)<sub>3</sub>)<sub>2</sub> (<b>4</b>), and a hydrogen-atom abstraction (HAA) mechanism,
which entails the direct reaction of O<sub>2</sub> with the hydride.
Experimental data reveal that the rate of reduction of O<sub>2</sub> to H<sub>2</sub>O is enhanced by electron-withdrawing phosphine
ligands. Complex <b>4</b> was independently prepared by the
addition of O<sub>2</sub> to <i>trans</i>-Rh<sup>I</sup>Cl(CNAd)(P(4-X-C<sub>6</sub>H<sub>4</sub>)<sub>3</sub>)<sub>2</sub> (<b>1)</b>. The reactivity of <b>4</b> toward HCl reveals
that such peroxo complexes are plausible intermediates in the reduction
of O<sub>2</sub> to H<sub>2</sub>O. These results show that the given
series of electronically varied rhodium(III) hydride complexes facilitate
the reduction of O<sub>2</sub> to H<sub>2</sub>O according to a two-term
rate law comprising HXRE and HAA pathways and that the relative rates
of these two pathways, which can occur simultaneously and competitively,
can be systematically modulated by variation of the electronic properties
of the ancillary ligand set
Photophysical Properties of β‑Substituted Free-Base Corroles
No full textCorroles
are an emergent class of fluorophores that are finding an application
and reaction chemistry to rival their porphyrin analogues. Despite
a growing interest in the synthesis, reactivity, and functionalization
of these macrocycles, their excited-state chemistry remains undeveloped.
A systematic study of the photophysical properties of β-substituted
corroles was performed on a series of free-base β-brominated
derivatives as well as a β-linked corrole dimer. The singlet
and triplet electronic states of these compounds were examined with
steady-state and time-resolved spectroscopic methods, which are complemented
with density functional theory (DFT) and time-dependent DFT calculations
to gain insight into the nature of the electronic structure. Selective
bromination of a single molecular edge manifests in a splitting of
the Soret band into <i>x</i> and <i>y</i> polarizations,
which is a consequence of asymmetry of the molecular axes. A pronounced
heavy atom effect is the primary determinant of the photophysical
properties of these free-base corroles; bromination decreases the
fluorescence quantum yield (from 15% to 0.47%) and lifetime (from
4 ns to 80 ps) by promoting enhanced intersystem crossing, as evidenced
by a dramatic increase in <i>k</i><sub>nr</sub> with bromine
substitution. The nonbrominated dimer exhibits absorption and emission
features comparable to those of the tetrabrominated derivative, suggesting
that oligomerization provides a means of red-shifting the spectral
properties akin to bromination but without decreasing the fluorescence
quantum yield
