Kinetic Studies of the Reduction of [Co(dmgH)₂(py)(Cl)] Revisited: Mechanisms, Products, and Implications

We report on a mechanistic investigation regarding the reduction of [Co^III(dmgH)₂­(py)­(Cl)] (dmg = dimethylglyoxime) by several complementary techniques. The reduction of [Co^III(dmgH)₂­(py)­(Cl)] was initiated by either electrochemical, photochemical, or pulse radiolytical techniques, and the corresponding products were analyzed by ESI mass spectrometry. In addition, all of the rate constants for each step were determined. We have found solid experimental as well as theoretical evidence for the appearance of a dinuclear complex [Co^IICo^III­(dmgH)₄­(py)₂­(H₂O)₂]⁺ to be the final product of reduction, implying the initially reduced form of [Co^III(dmgH)₂­(py)­(Cl)] undergoes a dimerization with the starting material in solution

Supramolecular Assembly of Multicomponent Photoactive Systems via Cooperatively Coupled Equilibria

Here, we show that the synergistic interplay between two binding equilibria, acting at different sites of a (Zn)­phthalocyanine-amidine molecule (<b>Pc1</b>), enables the dissociation of the photoinactive phthalocyanine dimer (<b>Pc1</b>)₂ into a three-component system, in which a sequence of light harvesting, charge separation, and charge shift is successfully proven. The aforementioned dimer is assembled by dual amidine-Zn­(II) coordination between neighboring <b>Pc1</b> molecules and gives rise to high association constants (<i>K</i>_D ≈ 10¹¹ M^–1). Such extraordinary stability hampers the individual binding of either carboxylic acid ligands through the amidine group or pyridine-type ligands through the Zn­(II) metal atom to (<b>Pc1</b>)₂. However, the combined addition of both ligands, which cooperatively bind to different sites of <b>Pc1</b> through distinct noncovalent interactions, efficiently shifts the overall equilibrium toward a photoactive tricomponent species. In particular, when a fullerene-carboxylic acid (<b>C</b>_<b>60</b><b>A</b>) and either a dimethylamino-pyridine (<b>DMAP</b>) or a phenothiazine-pyridine ligand (<b>PTZP)</b> are simultaneously present, the photoactivity is turned on and evidence is given for an electron transfer from photoexcited <b>Pc1</b> to the electron-accepting <b>C</b>_<b>60</b><b>A</b> that affords the <b>DMAP-Pc1</b>^•+-<b>C</b>_<b>60</b><b>A</b>^•– or <b>PTZP-Pc1</b>^•+-<b>C</b>_<b>60</b><b>A</b>^•– radical ion pair states. Only in the latter case does a cascade of photoinduced electron transfer processes afford the <b>PTZP</b>^•+<b>-Pc1-C</b>_<b>60</b><b>A</b>^•– radical ion pair state. The latter is formed via a thermodynamically driven charge shift evolving from <b>PTZP-Pc1</b>^•+-<b>C</b>_<b>60</b><b>A</b>^•– and exhibits lifetimes that are notably longer than those of <b>DMAP-Pc1</b>^•+-<b>C</b>_<b>60</b><b>A</b>^•–

Photoinduced Charge Transfer in Porphyrin–Cobaloxime and Corrole–Cobaloxime Hybrids

We report on the synthesis of hybrid molecules consisting of a porphyrin or corrole chromophore axially coordinated to a [Co^III(dmgH)₂(Cl)]^±0 (dmg = dimethylglyoxime) unit via a pyridine group as potential hydrogen forming entities in H₂O/THF medium. Photophysical, electrochemical, and pulse radiolysis studies on the hybrids and/or their separate components show that selective excitation of the porphyrin or corrole chromophore in its first singlet excited state leads to fast charge separation due to chromophore to cobalt electron transfer. However, this charge separation is followed by even faster charge recombination thereby preventing the accumulation of a reduced cobalt species which would lead to hydrogen production. It is important, nevertheless, that addition of a sacrificial electron donor slows the charge recombination down. In light of the latter it comes as hardly surprising that the photocatalysis experiments in the presence of a sacrificial electron donor (i.e., triethylamine) show modest rates of hydrogen production