Hydroxo-bridged dinuclear cupric complexes encapsulated in various mesoporous silicas to mimic the catalytic activity of catechol oxidases: Reactivity and selectivity study

We report the synthesis and characterization of two hydroxo-bridged dinuclear cupric complexes, HPC [((phen) ₂Cu-OH-Cu(phen) ₂) ³⁺, phen=1,10-phenanthroline] and HBC [((bpy) ₂Cu-OH-Cu(bpy) ₂) ³⁺, bpy=2,2'-bipyridine], encapsulated in porous materials for the oxidation of 3,5-di-tert-butylcatechol (DTBC) to the corresponding quinone, 3,5-di-tert-butylquinone (DTBQ), to mimic catechol oxidases (COs). The separations of the two Cu(II) centers are 2.9, 3.51, and 3.65 Å for CO, HPC, and HBC, respectively. The stability of dinuclear cupric complexes, turnover number (TON), and selectivity of DTBQ were examined in Na Y zeolite (pore size 0.74 nm) and the solid mesoporous silicas (MPSs) MCM-41 (2.4 nm), MCM-48 (2.5 nm), and MAS-9 (9.0 nm). The studies showed that the MCM-41 and MCM-48 provided a better stability against the irreversible dissociation of dinuclear cupric complexes for their matching size, while Na Y has too small and MAS-9 has too large pore size to stabilize these dinuclear copper complexes. The EPR studies showed that HBC immobilized in MPS solids yielded more mononuclear cupric complexes than HPC samples, which may come from the low stability of HBC undergoing the dissociation of OH bridge via the Lewis acid (aluminum sites in the solid support) catalytic activities under the ion-exchanging process. The catalytic pathways for the production of DTBQ and byproducts are proposed on the basis of spectroscopic characterizations and activity measurements. The main byproduct observed in Na Y supports was formed from a DTBC-mononuclear copper intermediate and followed the pathway of electron transfer, oxygen insertion, ring-opening, and oxidation reaction. Furthermore, the rigid and bulky structure of HPC molecule (planar phen ligands) has more confinement effect in MCM-41 and MCM-48 solids than the flexible HBC molecule (nonplanar bpy), which can prevent an excessive separation of the dinuclear cupric centers in the deoxy state and yield a higher stability and selectivity. The smaller separation of the two Cu(II) ions in HPC may also be responsible for the observed higher oxidation selectivity. However, the bulky structure of four phen ligands in HPC molecules exhibits greater steric hindrance and decreases the contact of the substrate and yields a lower TON. The nanochannels of aluminum-substituted MPS provide the needed confined spaces and surface charge and maintain the separation of the dinuclear cupric centers after removing the hydroxo bridge in the catalytic cycle.