Impact of substituents on molecular properties and catalytic activities of trinuclear Ru macrocycles in water oxidation

Abstract

Conformational changes induced by ligand substituents in macrocyclic Ru complexes strongly affect their chemical and photocatalytic efficiencies in water oxidation.

A Calix[4]arene‐Based Cyclic Dinuclear Ruthenium Complex for Light‐Driven Catalytic Water Oxidation

“…These studies reveal an outstanding performance of our novel dinuclear cyclic WOC dimer 1 in photocatalytic water oxidation with aT OF of 15.5 s À1 which is indeeds of ar unprecedented for dinuclear homogenous ruthenium WOCs and even superior to our previously reported highly active trinuclear macrocyclic Ru catalysts [43] (see Ta bleS3f or ac omparison of the TOF and TON values of selected mono-, di-and trinuclear Ru WOCs). Usually,s ignificantly higher catalytic values are observed for chemical water oxidation compared with photocatalytic oxidationd ue to the lower stability and general complexity of the photocatalytic system, [75,79] whichi se xplained based on the limited stability of the photosensitizer.…”

“…Upon increasing the potentialf rom 500 mV to approximately 800 mV,t he MLCT bands at 350 nm and 450-500 nm are bleached with concomitant appearance of an ew band at 700 nm which is characteristic for the Ru 2 III state. [41,43] Af urther increaseo ft he potential to approximately 960 mV results in ad ecrease of the transition at 700 nm andabroad absorptiona ta round5 20 nm arises for the formation of the Ru 2 IV state (Figures1 and S10). [43] No fur-ther spectral changes were observed upon increasing the potentialb ecause as trongc atalytic current is associated with the formation of Ru V and subsequent oxidation of water.…”

“…[41,43] Af urther increaseo ft he potential to approximately 960 mV results in ad ecrease of the transition at 700 nm andabroad absorptiona ta round5 20 nm arises for the formation of the Ru 2 IV state (Figures1 and S10). [43] No fur-ther spectral changes were observed upon increasing the potentialb ecause as trongc atalytic current is associated with the formation of Ru V and subsequent oxidation of water. [39,41] Chemical water oxidation under acidic conditions…”

“…[22] The highlyv ersatile Ru(bda) fragment facilitated by axial ligand exchanget he isolation of multinuclearR uc omplexes including dimers [23][24][25] and trimers, [26] whichs howed increasedc atalytic activities compared to mononuclear complexes due to covalent linkageo ft he catalytic centers accelerating the underlying bimolecular I2M (interaction of two M-O units)m echanism. [27] In recent years, many examples reported in the literature have emphasized the role of secondc oordination spheree ffects, [13] that is, p-p interactions, [22,[28][29][30][31][32][33] accessible pendant bases, [34][35][36][37][38] hydrogen bonding, [39][40][41][42] and steric effects [39,41,43] on the overall water oxidation mechanism. In this regard, several supramolecular approaches have mainly focused on accelerating the underlying bimolecular I2M mechanism by self-assembly of two catalytic subunitst hrough p-p interactions, [22,[28][29][30][31][32][33] whereast he integration of accessible pendant bases like carboxylates in the second coordination sphere leads to am echanistic change to the water nucleophilica ttack (WNA)p athway due to their ability to act as ai ntramolecularp rotona cceptor in the rate-determining step of oxygen bond formation.…”

“…In recent years, many examples reported in the literature have emphasized the role of second coordination sphere effects, [13] that is, π–π interactions, [22, 28–33] accessible pendant bases, [34–38] hydrogen bonding, [39–42] and steric effects [39, 41, 43] on the overall water oxidation mechanism. In this regard, several supramolecular approaches have mainly focused on accelerating the underlying bimolecular I2M mechanism by self‐assembly of two catalytic subunits through π–π interactions, [22, 28–33] whereas the integration of accessible pendant bases like carboxylates in the second coordination sphere leads to a mechanistic change to the water nucleophilic attack (WNA) pathway due to their ability to act as a intramolecular proton acceptor in the rate‐determining step of oxygen bond formation [34–38] .…”