Alkene Epoxidation Catalysts [Ru(pdc)(tpy)] and [Ru(pdc)(pybox)] Revisited: Revealing a Unique Ru^IV═O Structure from a Dimethyl Sulfoxide Coordinating Complex

Abstract

The X-ray crystal structure of a dimethyl sulfoxide (DMSO) coordinating complex [Ru II (κ 2 -pdc)(tpy)(DMSO)] (H 2 pdc = 2,6-pyridyl dicarboxylic acid and tpy = 2,2′:6′,2″-terpyridine) led to the discovery of a unique Ru IV O configuration for the Ru-pybox (pybox = pyridinebis(oxazoline) ligands) epoxidation catalyst by theoretical calculations. On the basis of this structure, a detailed theoretical study was conducted on the alkene epoxidation reaction using ruthenium-based epoxidation catalysts. It was found that the process of H 2 O 2 coordination proceeded via an associative path in which one carboxylate detached. The following H 2 O-elimination step was found to be facilitated by the detached carboxylate group. The resulting Ru IV O rearranges to the species trans-2a-oxo, in which one carboxylate group is situated over the tpy ring; the trans-2a-oxo was found to have the lowest activation free energies toward alkene epoxidation. These results demonstrated the importance of the hemilabile properties of the pdc 2− ligand for the Ru-pdc alkene epoxidation catalysts.

Fate of the Molecular Ru–Phosphonate Water Oxidation Catalyst under Turnover Conditions

“…Then, the latter must be a further attack by another RuO group, which after successive reactions lead to the formation of a carboxylate group. A similar pathway has been proposed for the Ru complex [Ru(L)(pic) 2 ], 5 (L = 6,6′bis(2-hydroxyphenolate)-2,2′-bipyridine), 36,37 containing phenolates as coordinating groups. Upon oxidation, this complex furthers evolves to the corresponding bis-carboxylate complex [Ru(bda)(pic) 2 ], 6, where both phenolates are transformed into carboxylates.…”

Evidence for Oxidative Decay of a Ru-Bound Ligand during Catalyzed Water Oxidation

“…Therefore, 4 2+ and 4 aq 2+ should exist in equilibrium in the aqueous solution. The importance of the hemilabile carboxylate ligand that facilitates water association has been observed for other ruthenium complexes . It should be noted, however, that of 4 2+ and 4 aq 2+ only 4 aq 2+ is capable of being oxidized to the Ru IV state and the proposed active Ru V state via proton-coupled electron transfer (PCET) processes.…”

“…The importance of the hemilabile carboxylate ligand that facilitates water association has been observed for other ruthenium complexes. 30 It should be noted, however, that of 4 2+ and 4 aq 2+ only 4 aq 2+ is capable of being oxidized to the Ru IV state and the proposed active Ru V state via proton-coupled electron transfer (PCET) processes. A recent report demonstrating Fe-catalyzed water oxidation sheds light on an inner-sphere catalytic mechanism through the identification of an active Fe IV (O)(μ-O)Ce IV intermediate.…”

The Carboxylate Ligand as an Oxide Relay in Catalytic Water Oxidation

“…In a Ru epoxidation catalyst containing a pyridine-2,6-dicarboxylate ligand, the ligand not only stabilized the catalyst by making it coordinatively saturated, but was also capable of dissociating the carboxylate to allow for reactants to attach. 23 This hemilabile coordination yielded stable catalysts, yet with high reactivity. In Sun's Ru(bda)L 2 water oxidation catalysts (bda = 2,2′-bipyridine- 6,6′-dicarboxylate and, L = typically nitrogen-containing heterocyclic ligand), the carboxylates modulate the reduction potential, by stabilizing higher oxidation states.…”

“…Inspired by the structure of [FeFe] hydrogenases, Bullock and co-workers have developed a variety of metal complexes that contain an amine base in the second coordination sphere, adjacent to the metal center, with turnover frequencies exceeding 100 000 s –1 for H 2 production and high rates for H 2 oxidation. ,, In water oxidation, the carboxylate group in hangman corroles was suggested to act as a proton relay that speeds up the reaction by accepting protons from water and thereby facilitating the water nucleophilic attack (WNA). , While carboxylates certainly potentially function as proton relays, they have several other unique properties in the first and second coordination sphere. In a Ru epoxidation catalyst containing a pyridine-2,6-dicarboxylate ligand, the ligand not only stabilized the catalyst by making it coordinatively saturated, but was also capable of dissociating the carboxylate to allow for reactants to attach . This hemilabile coordination yielded stable catalysts, yet with high reactivity.…”