Merging of the photocatalysis and copper catalysis in metal–organic frameworks for oxidative C–C bond formation

A new approach to merge Cu-catalysis/Ru-photocatalysis within one single MOF was achieved by incorporating [SiW11O39Ru(H2O)]5– into Cu–BPY MOFs.


Introduction
The direct formation of new C-C bonds through oxidative coupling reactions from the lower active sp 3 C-H bonds using oxygen as oxidant is an important area in sustainable chemistry. 1 Among the reported promising examples, the oxidative activation of the C-H bonds adjacent to nitrogen atom in tertiary amines represents a powerful strategy, giving valuable, highly reactive iminium ion intermediates for further functionalization. 2 Recent investigations also revealed that the visible light photoredox catalysis was a promising approach to such reaction sequences 3 with respect to the development of new sustainable and green synthetic methods. It was also postulated that the combination of the photocatalysis and the metal catalysis within a dual catalytic transformation is attractive to circumvent the potential side reactions relative to the highly active intermediates that exist in the photocatalysis. 4 The hurdles that need to be overcome include the careful adaptation and the ne tuning of the reaction rates of the two catalytic cycles, 5 beside the appropriate choice of the metal catalysis and photocatalysis. 6 Metal-organic frameworks are hybrid solids with innite network structures built from organic bridging ligands and inorganic connecting nodes. Besides the potential applications in many diverse areas, 7 MOFs are ideally suited for catalytic conversions, since they can impose size and shape selective restriction through readily ne-tuned channels or pores, 8 providing precise knowledge about the pore structure, the nature and distribution of catalytically active sites. 9 In comparison to the heterogeneous catalytic systems that have been examined earlier, the design exibility and framework tunability resulting from the huge variations of metal nodes and organic linkers allow the introduction of more than two independent catalyses in one single MOF. 10 The combination of photocatalysis with the metal ions or organocatalysis was expected to be a promising approach to create synergistic catalysts. 11 By incorporating a ruthenium(III) substituted polyoxometalate [SiW 11 O 39 Ru(H 2 O)] 5À within the pores of copper(II)-bipyridine MOFs, herein, we reported a new approach to merge the visible light photocatalytic aerobic oxidation and copper(II) catalytic coupling reaction within one MOF (Scheme 1). We envisioned that the ruthenium-containing fragments possibly worked as oxidative photocatalyst to generate the iminium ion from N-phenyl-tetrahydroisoquinolines, 12 whereas the Cu-based MOF potentially activated the nucleophiles, as it was shown in the oxidative C-C bond coupling. 13 a State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian, 116024, P. R. China. E-mail: cyduan@dlut.edu.cn

Synthesis and characterizations of CR-BPY1
Solvothermal reaction of 4,4 0 -bipyridine (BPY), Cu(NO 3 ) 2 $3H 2 O and K 5 [SiW 11 O 39 Ru(H 2 O)]$10H 2 O gave CR-BPY1 in a yield of 52%. Elemental analyses and powder X-ray analysis indicated the pure phase of its bulk sample. Single-crystal structural analysis revealed that CR-BPY1 crystallized in a space group P42 1 m. Two crystallographically independent copper(II) ions are connected by BPY ligands and m 2 -water bridges alternatively to produce 2D wavy-like Cu-BPY sheets (Fig. S5, ESI †). The Cu(2) atom adopted a six-coordinate octahedral geometry with four nitrogen atoms from four BPY ligands positioned in the equatorial plane and two water molecules occupied the axial positions. The Cu(1) atom displayed a ve-coordinate square pyramidal geometry with two m 2 -water groups and two nitrogen atoms of BPY ligands positioned in the basal plane, and a terminal oxygen atom of the depronated [SiW 11 O 39 Ru(H 2 O)] 5À polyoxoanion occupied the vertex position. The ruthenium atom disordered in the twelve equivalent positions within a depronated [SiW 11 O 39 Ru(H 2 O)] 5À . 14 The availability of vacant dorbitals on the metal atoms adjacent to the heteroatom allows the polyoxometalate matrix to function as a p-acceptor ligand. 15 While these copper atoms were connected by the BPY ligands to form two-dimension square grid at rst, adjacent sheets were connected together using the deprotonated [SiW 11 O 39 Ru(H 2 O)] 5À polyoxoanion by Cu II -O-W(Ru) bridges to generate a 3D framework. Two symmetric-related frameworks further interpenetrated each other perpendicularly to consolidate the robust structure ( Fig. 1), in which the opening of the pores was reduced to 10.0Å Â 5.3Å. To the best of our knowledge, CR-BPY1 represents the rst example of MOFs which are comprised of ruthenium substituted polyoxometalate [SiW 11 O 39 Ru(H 2 O)] 5À . As the noble metal substituted polyoxometalates exhibited excellent photoreactivity in various catalytic oxidation processes of organic substrates, 16 such kinds of MOFs potentially allow the combination of photocatalysis and MOF-based heterogeneous catalysis to achieve synthetically useful organic transformations. Moreover, the directly bridging of the copper and ruthenium by Cu II -O-W(Ru) provided a promising way to achieve the synergistic catalysis between photocatalyst and metal catalyst.
Confocal uorescence microscopy has attracted much attention in biological imaging. It may provide a way to analyse relatively thick porous materials, because it offers the advantage of increased penetration depth (>500 mm). 17 The assessment of guest-accessible volume in MOFs can be reliably done by using confocal uorescence microscopy with a tool-box of dyes with a wide range of sizes. It would be applicable to any porous materials, whose single-crystal structures are not available, or non-crystalline materials. 18 Dye uptake investigation was carried out by soaking CR-BPY1 in a methanol solution of 2 0 ,7 0 -dichlorouorescein. It gave the quantum uptake equivalent to 5% of the MOF weight (Fig. S11, ESI †). 19 The confocal laser scanning microscopy exhibited strong green uorescence (l ex ¼ 488 nm) assignable to the emission of the uorescein dye ( Fig. 2), conrming the successful uptake of the dye molecules inside the crystals of the MOF. 20 Furthermore, the rather uniform distribution of the dye molecules throughout the crystal suggested that the dyes penetrated deeply into the crystal rather than staying on the external surface. Without guest water molecules, the effective free volume of CR-BPY1 was estimated to be 29.0% by PLATON soware. 21 CR-BPY1 exhibited an absorption band centered at 398 nm in the solid state UV-vis absorption spectrum (Fig. S1, ESI †), assignable to the transitions of [SiW 11 O 39 Ru(H 2 O)] 5À . 22 Upon excitation at this band, CR-BPY1 did not exhibit any obvious emission, however, progressive addition of the N-phenyl-tetrahydroisoquinoline into the dichloromethane suspension of  CR-BPY1 up to 0.50 mM caused the appearance of the Ru IIrelative emission band at about 422 nm (Fig. 3a). 23 The results suggested that CR-BPY1 oxidized N-phenyl-tetrahydroiso-quinoline to form the Ru II species and the iminium intermediate. 24 Electrospray ionization mass spectrometry of the CH 2 Cl 2 suspension containing N-phenyl-tetrahydroisoquinoline and CR-BPY1 aer 3 hours light irradiation exhibited an intense peak at m/z ¼ 208. This peak was assignable to the relative imine ion, conrming that CR-BPY1 oxidized N-phenyl-tetrahydroisoquinoline to form the Ru II species and the iminium intermediate ( Fig. S13, ESI †). The electron paramagnetic resonance (EPR) of CR-BPY1 exhibited the characteristic signal of Cu II with g ¼ 2.14 ( Fig. 3c). Solid state electrochemical measurements (Fig. 3d) exhibited a broad redox band centred at À186 mV (vs. SCE) relative to the overlap of the Cu II /Cu I and Ru III /Ru II redox couples. The potentials were comparable to these Cu II and Ru III -containing catalysts, 25 and enabled CR-BPY1 to prompt the oxidative coupling of N-phenyl-tetrahydroisoquinoline with nucleophiles under light. 26 It seems that CR-BPY1 adsorbed the N-phenyl-tetrahydroisoquinoline in its pores and activated the substrate to form the iminium intermediate.

Catalysis details of CR-BPY1
The catalysis was examined initially using N-phenyl-tetrahydroisoquinoline and nitromethane as the coupling partners, along with a common uorescent lamp (18 W) as the light source. The resulting reaction gave a yield of 90% aer 24 hours irradiation. The removal of CR-BPY1 by ltration aer 18 hours shut down the reaction, and the ltrate afforded only 12% additional conversion for another 18 hours at the same reaction conditions. The observation suggested that CR-BPY1 was a true heterogeneous catalyst. 27 Solids of CR-BPY1 could be isolated from the reaction suspension by simple ltration alone and reused at least three times with moderate loss of activity (from 90% to 82% of yield aer three cycles). The index of XRD patterns of CR-BPY1 ltrated off from the reaction mixture suggested the maintenance of the crystallinity (Fig. S14, ESI †). With the size of the microcrystals reduced to 2 mm by grinding CR-BPY1 crystals for 20 min, the time of the reaction giving the same conversion to that of the as-synthesized materials was reduced by about 10% (Fig. S15, ESI †). It seems that the MOFbased particles having well-dened size were really helpful for the catalytic reactions, but the size of the crystals did not dominate the catalysis directly.
Control experiments for the C-C coupling reaction of N-phenyl-tetrahydroisoquinoline and nitromethane were carried out and summarized in Table 1. Almost no conversion was observed when the reaction was conducted in the dark (entry 7), while a very slow background reaction was observed in the absence of catalyst (entry 6), which demonstrated that both the light and the photocatalyst are required for efficient conversion to the coupling products. In addition, using the same equiv. of copper(II) salts or/and K 5 [SiW 11 O 39 Ru(H 2 O)] as catalysts, respectively gives conversions of 39%, 25% and 42% in homogeneous fashion (entry 3-5). These results suggested that the direct connection of copper(II) ions to [SiW 11 O 40 Ru] 7À anions not only enabled the dual catalysts to individually activate N-phenyl-tetrahydroisoquinoline and nitromethane, but also enforced the proximity between the potential intermediates i.e. the iminium ion and nucleophile, avoiding the unwanted side reactions or reverse reactions. 28 Although several examples of photocatalysts and metal copper catalysts have been reported to prompt the oxidative coupling C-C bond formation, CR-BPY1 represents a new example of a heterogeneous bimetal catalyst that merges the copper catalyst and the ruthenium(III) substituted polyoxometalate catalyst within one single material. The high  (b). Three images at the end of axes in (e) exemplify the X, Y and Z-axis projections of the soaked dye. Fig. 3 (a) Family of emission spectra of CR-BPY1 (0.1% in weight) in CH 2 Cl 2 suspension upon addition of N-phenyl-tetrahydroisoquinoline up to 0.50 mM, excitation at 362 nm. (b) The luminescence intensities of CR-BPY1 and CR-BPY2 upon addition of the same amount of Nphenyl-tetrahydroisoquinoline up to 0.50 mM at room temperature in CH 2 Cl 2 ; the catalytic activities of CR-BPY1 and CR-BPY2 using N-phenyl-tetrahydroisoquinoline and nitromethane as the coupling partners. (c) EPR spectra of CR-BPY1 (g ¼ 2.1438) and CR-BPY2 (g ¼ 2.1320) in solid state at 77 K, respectively. (d) Solid state cyclic voltammetry of CR-BPY1 and CR-BPY2, respectively, scan rate: 50 mV s À1 . modularity of the systems allows an easy adaptation of the concept of dual catalysis, and represents an example of combining dual catalysis to achieve synthetically useful organic transformation. In this case, our catalytic systems were extended with other pronucleophiles, i.e. substituted acetophenones. As shown in Fig. 4, 1 H NMR of desolvated CR-BPY1 solids immersed in a dichloromethane solution of acetophenone exhibited that CR-BPY1 could adsorb about 2 equiv. of acetophenone per copper(II) moiety. IR spectrum of CR-BPY1 impregnated with a dichloromethane solution of acetophenone revealed a C]O stretching vibration at 1679 cm À1 . The red shi from 1685 cm À1 (free acetophenone) suggested the adsorption and the activation of the acetophenone in the channels of the MOFs. It is hypothesized that the interactions between the copper ions in CR-BPY1 and the C]O groups of acetophenone possibly gave an active nucleophile. 29 The reactions were carried out in the presence of a common used secondary amine, L-proline, as an organic co-catalyst to activate the ketones. 30 In the case of the acetophenone as reactant with a uorescent lamp (18 W) as the light source; the catalytic reaction gave a yield of 72%. Control experiments demonstrated that the use of K 5 [SiW 11 O 39 Ru(H 2 O)] or copper(II) salts as catalysts, only gave less than 25% of the conversions, respectively. The results indicated the signicant contribution of cooperative effects of the individual parts within one single MOF. From the mechanistic point of view, the ruthenium(III) of the polyoxometalate [SiW 11 O 39 Ru(H 2 O)] 5À interacted with Nphenyl-tetrahydroisoquinoline to form iminium ions, whereas the copper atoms coordinated to the acetophenones weakly to form the enol intermediate that worked as active nucleophile for the oxidative coupling C-C bond formation. At the same time, the presence of copper ions could enhance the activation of N-phenyl-tetrahydroisoquinoline, beneting the synergistic catalysis between photocatalyst and metal catalyst. Importantly, in contrast to the smooth reactions of substrates 1-3, the C-C coupling reaction in the presence of bulky ketone (1-(3 0 ,5 0 -ditert-butyl [1,1 0 -biphenyl]-4-yl)-ethanone) 4, gave less than 10% conversion under the same reaction conditions (Table 2, entry 4). The negligible adsorption by immersing CR-BPY1 into a dioxane solution of substrate 4, coupled with the fact that the size of substrate 4 was larger than that of the channels, 31 revealed that 4 was too large to be adsorbed in the channels. Furthermore, it is suggested that the synergistic catalytic coupling reaction indeed occurred in the channels of the MOF, not on the external surface.

Synthesis and catalytic characterizations of CR-BPY2
To further investigate the synergistic interactions between the inorganic copper and [SiW 11 O 39 Ru(H 2 O)] 5À anion, a reference compound CR-BPY2 was assembled using the same starting components but different synthetic conditions (hierarchical diffusion). CR-BPY2 was synthesized by a diffusion method in a test tube by laying a solution of 4,4 0 -bipyridine in acetonitrile onto the solution of K 5 [SiW 11 O 39 Ru(H 2 O)]$10H 2 O and Cu(NO 3 ) 2 $3H 2 O in water for several days in a yield of 59%. Elemental analyses and powder X-ray analysis indicated the pure phase of its bulk sample. Single-crystal structural analysis revealed that CR-BPY2 crystallized in the orthorhombic lattice with a space group Pccn. Two crystallographically independent copper(II) ions connected four BPY bridges alternatively to produce a 2D sheet (Fig. 5), which were further stacked paralleled along the crystallographic a axis to form the 3D structure with embedded [SiW 11 O 39 Ru(H 2 O)] 5À (Fig. S8, ESI †). The copper(II) ions resided in an octahedral geometries with the equatorial plane which was dened by four nitrogen atoms of BPY ligands, and the axial positions were occupied by two water molecules (Fig. S7, ESI †). Without guest water molecules, the effective free volume of CR-BPY2 was also estimated to be 33.9% by PLATON soware, which is quite larger than that of CR-BPY1. These results suggested that the pore of CR-BPY2 is larger enough to adsorb the substrates. Since [SiW 11 O 39 Ru(H 2 O)] 5À polyoxoanions were embedded in the channels, it is thus an excellent reference for investigating the catalytic activity on the same coupling reaction.  CR-BPY2 also exhibited an absorption band centered at 398 nm in the solid state UV-vis absorption spectrum. Upon excitation at this band, CR-BPY2 did not exhibit obvious emission, however, progressive addition of the N-phenyl-tetrahydroisoquinoline into the dichloromethane suspension of CR-BPY2 up to 0.50 mM caused the appearance of the Ru II -relative emission band at about 422 nm, suggesting that CR-BPY2 oxidized N-phenyl-tetrahydroisoquinoline to form the Ru II species. The EPR of CR-BPY2 exhibited the characteristic signal of Cu II (g ¼ 2.13). 32 The sharper peak shape compared to that of CR-BPY1 might be one of the indicator of isolated Cu II ions in CR-BPY2. No metal-metal interactions were found corresponding to the Cu II ions in CR-BPY2. Solid state electrochemical measurements exhibited two redox peaks corresponding to the Cu II /Cu I and Ru III /Ru II redox couples, with the redox potential calculated at 75 mV and 84 mV (vs. SCE). The potentials enabled CR-BPY2 to prompt the oxidative coupling of N-phenyl-tetrahydroisoquinoline with nucleophiles under light. However, the separated redox peaks also suggested that these Cu I and Ru III ions did not interacted directly. It seems that CR-BPY2 adsorbed the N-phenyl-tetrahydroisoquinoline in its pores and was a convincing reference to investigate the synergistic action between Cu II -O-W(Ru) bimetal of CR-BPY1.
The catalytic activities of CR-BPY2 in the C-C coupling reactions were examined under the same conditions using nitromethane and N-phenyl-tetrahydroisoquinoline as the reactants. About 1 mol% loading amount of the catalyst gave rise to a 46% conversion, which was superior to the case when copper(II) salts and the K 5 [SiW 11 O 39 Ru(H 2 O)] were employed as catalysts, indicating the signicance of the two constitute parts for CR-BPY2 as a photocatalyst. However, the catalytic activities of CR-BPY2 were signicantly weaker than that of CR-BPY1 (Fig. 3b). It should be concluded that the direct bridging of the copper and ruthenium by Cu II -O-W(Ru) provided a promising way to achieve the synergistic catalysis between photocatalyst and metal catalyst, and the high reaction efficiency in the reactions was dominated by the spacious environment of the channels, like those of CR-BPY1.

Conclusions
In a summary, we reported the new example of copper MOFs containing the ruthenium substituted polyoxometalate with the aim of merging the synergistic Cu-catalysis/Ru-photocatalysis in a single MOF. CR-BPY1 exhibited perpendicularly inter-penetrated structure and the catalytic sites positioned in the robust pores of MOFs. Luminescence titration and IR spectra of the MOF-based material revealed the adsorbance and activation of N-phenyl-tetrahydroisoquinoline and acetophenone, by the ruthenium center and copper ions, respectively. The direct connection of copper(II) ions to [SiW 11 O 40 Ru(H 2 O)] 5À not only provided the possibility of the dual catalysts to individually activate the substrates, but also enforced the proximity between the intermediates, avoiding the unwanted side reactions or reverse reactions. CR-BPY1 exhibited high activity for the photocatalytic oxidative coupling C-C bond formation with excellent size-selectivity, suggesting the catalytic reactions occurred in the channels of the MOF, and not on the external surface.

General methods and materials
All chemicals were of reagent grade quality obtained from commercial sources and used without further purication. 1 H NMR was measured on a Varian INOVA-400 spectrometer with chemical shis reported as ppm (in DMSO-d 6 or CDCl 3 , TMS as internal standard). The elemental analyses (EA) of C, H and N were performed on a Vario EL III elemental analyzer. Inductively coupled plasma (ICP) analyses were performed on a NexION 300D spectrometer. The powder XRD diffractograms were obtained on a Riguku D/MAX-2400 X-ray diffractometer with Cu sealed tube (l ¼ 1.54178Å). IR spectra were recorded as KBr pellets on a NEXUS instrument. Solid UV-vis spectra were recorded on a U-4100 spectrometer. Liquid UV-vis spectra were performed on a TU-1900 spectrophotometer. Solid uorescent spectra were measured on a JASCO FP-6500 instrument. The excitation and emission slits were both 3 nm wide. Solid state cyclic voltammograms were measured on a Zahner PP211 instrument by using a carbon paste working electrode. Thermogravimetric analyses (TGA) were carried out at a ramp rate of 5 C min À1 in nitrogen ow with a SDTQ600 instrument. Electron paramagnetic resonance (EPR) spectra were recorded on a EMX-10/12 spectrometer. Scanning electron microscopy (SEM) images were taken with a NOVA NanoSEM 450 microscope. All confocal laser scanning microscopy (CLSM) micrographs were collected by Olympus Fluoview FV1000. Products were puried by ash column chromatography on 200-300 mesh silica gel SiO 2 .
Synthesis of CR-BPY1

X-ray crystallography
Data of CR-BPY1 and CR-BPY2 were collected on a Bruker Smart APEX CCD diffractometer with graphite-monochromated Mo-Ka (l ¼ 0.71073Å) using the SMART and SAINT programs. 34 Their structures were determined and the heavy atoms were found by direct methods using the SHELXTL-97 program package. 35 Crystallographic data for them are summarized in Table 3. Except some partly occupied solvent water molecules, the other non-hydrogen atoms were rened anisotropically. Hydrogen atoms within the ligand backbones were xed geometrically at their positions and allowed to ride on the parent atoms. In both of the two structures, the ruthenium atoms were disordered in the equivalent positions of tungsten atoms. For CR-BPY2, several bond distances constraints were used to help the renement on the BPY moiety, and thermal parameters on adjacent oxygen atoms of the polyoxometalate anion were restrained to be similar.