Ligand Modification Transforms a Catalase Mimic into a Water Oxidation Catalyst

Abstract

The catalytic reactivity of the high-spin Mn(II) pyridinophane complexes [(Py2NR2)Mn(H2O)2]2+ (R = H, Me, tBu) towards O2 formation is reported. With small macrocycle N-substituents (R = H, Me), the complexes catalytically disproportionate H2O2 in aqueous solution. While this catalysis is shut down with a bulky substituent (R = tBu), the complex becomes active for electrocatalytic H2O oxidation in aqueous solution. Control experiments are in support of a homogeneous molecular catalyst and preliminary mechanistic studies suggest the catalyst is mononuclear. This ligand-controlled switch in catalytic reactivity has implications for the design of new manganese-based water oxidation catalysts.

Quantum Chemical Modeling of Homogeneous Water Oxidation Catalysis

“…[92] Transition state TS1 prefers to be a quintet, featuring ah igh-spin Mn III interacting witho ne oxyl radicalf erromagnetically and the other oxyl radical antiferromagnetically.S pin crossingi st hus neededf rom the triplet to the quintet during oxo-oxo coupling.T his is followed by insertion of water,c oncomitant with proton transfer from the water molecule to the peroxide, which generates a [Mn III L(OH)(OOH)] + intermediate. The alternative of attack by water,a ss uggested by Smith and co-workers, [46] was also considered, in which one of the two oxygen atoms could function as ag eneral base to abstract ap roton during attack by water. Both pathways arrive at aM n III -peroxide intermediate, from which an oxygen molecule can be released after two oxidation steps with aquite low barrier.…”

Molecular complexes in water oxidation: Pre-catalysts or real catalysts

“…Another analytical technique used for the elemental analysis or chemical characterization of samples is EDX, which is usually combined with TEM or scanning electron microscope (SEM) for element analysis at the nanometer level [26][27][28].…”

“…CV of 36 showed two irreversible oxidation waves at 0.87 V and 1.41 V in NaPi buffer at pH 7.0, which were assigned to the oxidation of Ni(II) to Ni(III) and the further oxidation of Ni(III) to Ni(IV). Compared with other homogeneous transition metal-based WOCs (500-800 mV)[27,112,126,127], a relatively low overpotential was exhibited by 36. To verify that the electrocatalysis by 36 is a homogeneous reaction, a series of control experiments were performed: (1) Catalytic current decreased as a result of multiple CV scans and became constant finally, this is in contrast to the observed feature for heterogeneous Ni-based catalysts[122,124].…”

The biological water-oxidizing complex at the nano–bio interface

“…Several studies have also used various biomimetic systems to obtain a good water-oxidizing catalyst [13,[69][70][71][72][73][74][75][76][77]. The role of the Ca 2+ ion in artificial photosynthesis was examined through the synthesis of CaMn 2 O 4 .xH 2 O, which showed good catalytic activity toward water oxidation [78].…”