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Hydroxo-Bridged Dinuclear Cupric Complexes Encapsulated in Various Mesoporous Silicas to Mimic the Catalytic Activity of Catechol Oxidases: Reactivity and Selectivity Study Chia-Hung Lee,† Han-Chou Lin,† Shih-Hsun Cheng,† Tien-Sung Lin,*,‡ and Chung-Yuan Mou*,† Department of Chemistry, National Taiwan UniVersity, Taipei, Taiwan 106, and Department of Chemistry, Washington UniVersity, St. Louis, Missouri 63130 ReceiVed: January 2, 2009; ReVised Manuscript ReceiVed: May 5, 2009
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 NaY 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 NaY 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 NaY 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. Introduction The excellent activity and selectivity of natural metalloenzymes are usually obtained under milder conditions, as enzymes will lose stability under extreme conditions, such as high temperature or organic solvent. To overcome the stability issues, an alternative approach is to synthesize model compounds with metal-containing active centers.1-4 However, only a limited number of efficient biomimetic catalysts are known. Often, the difficulty in maintaining enzymelike high catalytic activity with a first-row transition metals mimic compound is its stability and accessibility. On the one hand, in the reaction stage the complex needs to have an open coordination site for the substrate. On the other hand, the coordinative unsaturation often leads to an unstable structure. Biological systems resolve this dilemma by site isolation of the active metals in a protein matrix, which allows the metal sites to maintain coordinative unsaturation. Reported biomimetic analogs rely often on inclusion of sterically * Corresponding authors. TSL: fax, 314-935-4481; e-mail,
[email protected]. CYM: fax, 886-2-2366-0954; e-mail,
[email protected]. † National Taiwan University. ‡ Washington University.
demanding groups on the periphery of the ligands to maintain its stability, but such bulky ligands usually reduce their activity. Many studies reported that immobilizing metal complexes in the solid supports can closely mimic the natural occurring enzymes, where the solid support could provide proper geometry and distance for catalytic purpose and increase the stability of catalytic centers.5-8 The incorporation of bioinspired metal complexes onto different silica supports by covalent bonding or ionic attraction has been reported to demonstrate their potential for heterogeneous catalysts.9-11 Site isolation of biomimetic complexes through encapsulation in porous solid materials would allow the use of less sterically demanding ligands while structural stability is retained. New materials that combine the advantages of both heterogeneous and homogeneous oxidation catalysis are thus much desired. Microporous zeolitic materials have been employed to encapsulate metal ions for mimicking the biochemistry of metalloenzymes.12 However, the pore size is usually too small ( MCM41 ∼ MAS-9 > MCM48. On the basis of the chromatograms and spectroscopic studies of the reaction mixtures, the possible pathways involving byproduct in the catalytic oxidation of DTBC are displayed in Scheme 2. First, the oxygen atoms in DTBC may be coordinated to the Cu(II) center (complex B), and another hydroxyl group in DTBC may replace the OH bridge of dicupric complex to produce a mononuclear Cu(II)-DTBC intermediate (complex C), which can be converted to a Cu(II)-semiquinonato intermediate (complex D)38 via the charge transfer of catecholate to Cu(II) and thus display an EPR signal of the solid samples at 3470 G (see Figure 8a,c).
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Figure 10. EPR spectra of the solution phase after the catalytic oxidation of DTBC in the presence of TEA at 300 K. Solid catalysts were removed by centrifuging before every EPR measurements. (a) NaY-HPC, (b) Al-MCM-41-HPC, (c) NaY-HBC, and (d) Al-MCM41-HBC as the catalyst. The signal at g ) 2.0044 (3470 G) is assigned to a stable o-semiquinone radical from mononuclear cupric complexes.
Figure 9. Chromatograms of the solution phase in the reaction mixtures after the catalytic oxidation of DTBC by encapsulated (a) HPC and (b) HBC in (i) NaY, (ii) Al-MCM-41, (iii) Al-MCM-48, and (iv) MAS-9 materials.
The presence of the o-semiquinone radical in the catalytic mixture has also been reported in the catalytic oxidation of DTBC.29,39 To further identify the byproduct at tR ) 7.51 min, we isolated and characterized the byproduct by LC-MS. The mass spectrum of electron impact ionization (EI) showed that the mass of the byproduct is 270 (Figure S2, Supporting Information). We further confirmed the identities of the major byproduct by NMR analysis (Figure S3, Supporting Information). A possible structure of this byproduct is 3,5-di-tert-butyl2-hydroxyhexa-2,4-dienedioic acid, which was preceded by the addition of dioxygen molecule to the double bonds of DTBC-monocupric complex (Scheme 2, complex C), and then the six-membered ring of the reactive substrate in complex E was converted to a seven-membered ring of complex F by the insertion of an oxygen atom. The ring-
opening step at the complex F gave compound X and then yielded the byproduct, compound Y, upon further oxidation. In fact, similar products have been reported previously for the catalytic oxidation of DTBC by various mononuclear metal complexes.10,40,41 Thus, the mononuclear copper complex can induce a parallel oxidation of DTBC to other products via a different mechanism with high efficiency. The relative stability of the dinuclear cupric complexes in different solid supports was clearly reflected in the different intensity of o-semiquinone radical leaching into the reaction medium (Figure 10) and the amounts of byproduct formation observed in the chromatograms (Figure 9). Previous studies reported that DTBC can be oxygenated via the insertion of molecular oxygen to give intra- or extradiol oxygenation products in the presence of a high valence mononuclear transition metal complexes as the catalysts.40-44 Since the byproduct produced in NaY supports was mostly from mononuclear cupric complexes, we examine the possible sources of mononuclear complexes: (1) The coordination of the hydroxyl groups of DTBC to a single cupric center will replace the OH2 bridge and decompose the dicupric complex (Scheme 2, complex B) into the two monocupric complexes (complex C). (2) NaY zeolite could not hold the deoxy state in a proper distance in the catalytic cycle. (3) The Lewis acid (aluminum sites of the solid supports) may catalyze the dissociation of the OH bridge of dinuclear cupric complexes into two mononuclear complexes in the immobilizing process. We note that the HPC complexes encapsulated in MCM41 and MCM-48 show more stablity and less dissociation of the dinuclear center than that in NaY, which yield only trace amounts of byproduct in the reaction mixtures (Figures 9aii and 9a-iii). On the other hand, HBC complexes encapsu-
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SCHEME 2: Mechanism for the Production of Byproduct by the Catalytic Cycle of Mononuclear Cupric Complexes, Especially for NaY as the Support
SCHEME 3: Upper: The Smaller Pore Size of Y Zeolites Allow Dinuclear Cupric Complexes Adsorbed Only on the Outer Surface or Cupric Ions Adsorbed on the Inner Cage Surface. Lower: Dinuclear Cupric Complexes Encapsulated in the Nanochannels of Al-MPS Provide the Stability by Space Confinement and Surface Charges To Prevent Excessive Separation of the Dinuclear Cupric Centers after Removal of the Hydroxo Bridge in the Catalytic Process
lated in MPS’s channels have longer Cu-Cu distance and less confinement effect and lead to low stability and more byproduct produced at tR ) 7.51, especially when HBC was encapsulated in NaY and MAS-9 supports. We attributed the greater amount of byproduct obtained in the above two supports to the existence of more mononuclear cupric complexes inside the nanochannels. The small pore size of
Y zeolite could only adsorb the mononuclear cupric complexes on the internal channels or adsorb dinuclear cupric complexes on the external surface (Scheme 3). In addition, the large pore size of MAS-9 solids provides less confinement for HBC samples and led to the poor protection against the dissociation of the deoxy state in the catalytic cycle. These observations imply that most likely copper complexes in the
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NaY cage channels are mononuclear complexes, even though the starting materials were dinuclear cupric complexes. Conclusions We have demonstrated that the nanochannels of Al-MPS solids provide confined spaces and surface charge to encapsulate and stabilize the dinuclear cupric complexes (HBC and HPC). We summarize schematically the structural and mechanistic differences among dinuclear cupric complexes immobilized in the smallest pore of NaY and other larger pore MPS solids in Scheme 3. The two copper ions in the deoxy state are held at a fixed distance to the confined nanospaces, even when the hydroxo bridge of the complex is removed in the catalytic cycle and the complex undergoes the reversible transformation between bridged and nonbridged states. Thus, high turnover number and reusability of catalysts can be achieved. We anticipate that other metal complex catalysts immobilized in the nanochannels of MPS could be utilized as viable systems for broad ranges of activities mimicking natural enzymes by influencing the chemoselectivity, regioselectivity, and shape selectivity of the catalytic reaction. For a given MPS solid, we observed the turnover number of HBC is higher than that of HPC, but the selectivity of HPC is higher than that of HBC. These differences in catalytic behaviors may arise from their structural differences: (1) the Cu · · · Cu distance of HPC is shorter than that of HBC and (2) phen ligands in HPC are more bulky, rigid (planar), and bpy ligands in HBC are less bulky, flexible (nonplanar). Note that it is the close proximity of two Cu(II) centers in CO enzyme and HPC and HBC complexes that gives rise to the selective oxidation. The bulky structure of HPC complexes encapsulated in the nanochannels of MPS seems to increase the confinement effect against the dinuclear cupric center from the overseparation and to give a higher selectivity. However, the high steric hindrance of four phen ligands in HPC molecule could decrease the coordination of the DTBC to the dinuclear cupric center and decrease the catalytic activity. This was supported by the EPR spectra, which shows that the majority of copper complexes in MPS-HPC samples are in the dinuclear form and retain their integrity even after cycles of catalytic oxidation of DTBC, where two proximate cupric sites with a hydroxo bridge can be restored easily in the catalytic cycle. On the other hand, the adsorbed dinuclear cupric complexes in NaY showed irreversible dissociation, which produced the mononuclear cupric complexes outside the cages surface of NaY. Moreover, we observed the use of MCM-48 as the support gave both the highest turnover number and highest selectivity, which may be attributed to the 3-D cagelike structure increasing mass transport and diffusion. We further elucidated the role of mononuclear cupric complexes in the production of byproducts, especially for NaY zeolite. The EPR signal of the Cu(II)-semiquinonato intermediate was formed via the electron transfer of DTBC to the mononuclear cupric center and thus oxygenated DTBC by the insertion of oxygen to give intra- or extradiol oxygenation products in further oxidation. Most recently, mononuclear cupric complexes with bipridine and phenanthroline ligands have also been shown to induce selective oxidation of tetralin with relatively high yield.45 Note Added in Proof. Most recently, in a review, we further discussed the importance of physical confinement of enzymes and biomimetic complexes in the nanospaces of mesoporous materials to improve the stability and to enhance catalytic reactivity.46
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