Metal–organic framework composites with luminescent pincer platinum(ii) complexes: 3MMLCT emission and photoinduced dehydrogenation catalysis

Metal–organic framework materials are introduced to provide a “solid solution” environment for easy access to metal–metal-to-ligand charge transfer excited states of pincer platinum(ii) complexes and act as photocatalysts.


Introduction
Platinum(II) complexes are known to exhibit weak intramolecular and intermolecular interactions that lead to triplet metal-metal-to-ligand charge transfer ( 3 MMLCT) and/or 3 [5ds* / 6ps] excited states. 1,2 The diradical character of such excited states with enhanced metal-metal bonding interactions renders these complexes capable of performing photocatalytic C-X bond cleaving reactions. 3,4 The classic example, [Pt 2 (m-P 2 O 5 H 2 ) 4 ] 4À , is a highly active catalyst for the photoinduced dehydrogenation of alcohols to aldehydes/ketones in the absence of a sacricial electron acceptor via its long-lived 3 [5ds*6ps] excited state. 1a Due to the uniqueness of the mpyrophosphito ligand, extending the photochemistry of [Pt 2 (m-P 2 O 5 H 2 ) 4 ] 4À to other platinum(II) complexes is a non-trivial task. In this regard, pincer Pt II complexes are appealing alternatives because their structures can be readily modied to elicit intermolecular Pt/Pt interactions and hence emissive 3 MMLCT excited states in concentrated solutions or in the solid state. 5 However, the photophysical and photochemical properties associated with the 3 MMLCT excited states of Pt II complexes oen vanish in dilute solutions.
Metal-organic frameworks (MOFs) have emerged as a new class of highly promising porous materials. 6,7 In particular, the porous environment in MOFs provides a unique platform to conne and stabilize guest species, and as a result, novel properties of the incorporated guest may emerge. 8 In the literature, reports on the incorporation of Pt II complexes into MOFs are sparse; 9 these previously reported Pt II -MOF composites were formed by the coordination of Pt II Cl 2 (ref. 9a-d) or Pt II L 2 (L 2 ¼ 2,2 0 -bipyridine, (OPPh 3 ) 2 or (PPh 3 ) 2 ) 9e moieties to the bipyridine units of MOFs and in some cases they were studied as photocatalysts for hydrogen production from water. 9a,c,d We envisage that incorporating luminescent pincer Pt II complexes into the pores of MOFs by, for example, a cation exchange method, can be a strategy to develop the 3 MMLCT photochemistry of platinum(II) complexes. This method has been shown to be effective in placing phosphorescent d 6 and d 8 metal complexes inside MOFs. 10,11 In this work, Pt II complexes with a pincer C^N^C (where C is an N-heterocyclic carbene) ligand, [Pt(C^N^C)(C^CC 6 H 5 )] + (Pt1) and [Pt(C^N^C)(C^N)] + (Pt2; both Pt1 and Pt2 have PF 6 À as a counteranion) were synthesized and used as guest species for three MOFs with different porous structures. These Pt II @MOF composites were found to display matrix-dependent emission properties with emission peak maxima (l max ) ranging from 450 to 625 nm in air and also to catalyze photoinduced dehydrogenation reactions of various organic compounds with activities higher than those of the corresponding Pt II complexes in homogeneous solutions by an order of magnitude.

Results and discussion
Syntheses and characterization of Pt II @MOFs Pt1 and Pt2 ( Fig. 1a and  O, respectively (TATAT ¼ 5,5 0 ,5 00 -(1,3,5triazine-2,4,6-triyl)tris(azanediyl)triisophthalate; BTB ¼ 4,4 0 ,4 00benzene-1,3,5-triyl-tribenzoate). Single-crystal X-ray structure determination revealed that ZJU-28 is a framework of parallel interwoven corrugated 6 3 nets containing two types of 1D channel (Fig. 1c) with a maximal pore size of 14.7 Â 9.8Å 2 . 13 MOF1 shows a 3D chiral framework featuring an alternating arrangement of hexagonal and trigonal prismatic cages ( Fig. 1d) with a maximal window size of 14.3 Â 11.5Å 2 , 14 and MOF2 has a chiral framework with metal-organic nanotubes formed by heterometallic helical rods (Fig. 1e) and a channel size of 17.0 Â 23.0Å 2 . 15 Pt II @MOFs with different loadings of Pt II complexes, namely, Pt1@ZJU-28 (1a-1e), Pt1@MOF1 (2a-2e), Pt1@MOF2 (3a-3e) and Pt2@ZJU-28 (4a-4e), were obtained as yellow or pale yellow solids (Fig. S1, ESI †) by immersing MOF crystals in DMF or MeCN solutions of the Pt II complexes at different concentrations ((0.25-10) Â 10 À4 M). Pt2 was found to be unstable upon incorporation into MOF1 or MOF2, which precluded studies on the Pt2@MOF1 and Pt2@MOF2 composites. The powder X-ray diffraction (PXRD) data of these composites were nearly identical to those of their matrix MOFs, indicating that the ion exchange process does not affect the crystallinity of the host materials ( Fig. S2, ESI †). Inductively coupled plasma (ICP) mass spectrometric (MS) measurements (Table 1) showed that the loadings of the Pt II complexes ranged from 0.08 to 8.68 wt%. Distribution of the Pt II complex cation in a MOF was examined by optical microscopy, scanning electron microscope (SEM) imaging, energy dispersive X-ray (EDX) elemental mapping and N 2 sorption experiments. Analysis of a cross section of a crystal of 2e under an optical microscope showed that all of the surfaces of the split crystal emitted yellow light under light irradiation at 365 nm, indicating uniform distribution of Pt1 throughout the crystal (Fig. S3, ESI †). SEM imaging and EDX elemental mapping in a cross section of a split crystal of Pt II @MOFs showed the Pt element to have random distribution in the inner space of the Pt II @MOFs ( Fig. S4-S7, ESI †). Simulation by DFT calculation for the Pt II @MOFs, taking Pt1@MOF1 (2) as an example, revealed that the Pt II complex resides close to the ligand of MOF and the distance between the pyridine ring in the C^N^C ligand of the Pt II complex and the ligand of the MOF is $3.3Å (Fig. S8, ESI †), with the adsorption energy being 8.270 eV. N 2 sorption measurements revealed that the Brunauer-Emmett-Teller (BET) surface area decreased by >26% on going from MOF1 to 2e (1112 / 818 m 2 g À1 , Fig. S9, ESI †), supporting the connement of Pt1 in the pores/channels of MOF1.

Spectroscopy and photophysical measurements
The electronic absorption spectra of composites 1, 2 and 3 ( Fig. 2, Table 2) with low complex concentrations showed intense absorption bands at 325, 327 and 330 nm, and moderately intense bands at 390, 405 and 400 nm, respectively. The high energy absorption bands are assigned to the absorption of the matrix MOFs and intraligand ( 1 IL) p / p* transitions of the -C^CC 6 H 5 and C^N^C pincer ligands, whereas the lower-energy bands are assigned to the mixed singlet metal-toligand charge transfer ( 1 MLCT) [dp(Pt) / p*(C^N^C)] and the alkynyl-to-C^N^C ligand-to-ligand charge transfer (LLCT) [p(-C^CC 6 H 5 ) / p*(C^N^C)] transitions, both of which are characteristic absorptions of monomeric pincer Pt II complexes in solution. 12 Notably, a redshi of the low energy absorption band was observed in composites with a higher loading of the Pt II complex. For example, composite 4a ([Pt II ] ¼ 0.19%; Fig. 2d) showed absorption at only 300-400 nm. For composites 4d and 4e with higher [Pt II ] loadings of 1.32 and 2.41 wt%, respectively, there was a new, broad absorption band at 400-500 nm attributable to a 1 MMLCT transition of aggregated species of Pt2 (ref.  12) inside the MOF (Fig. 2d). For comparison, increasing the concentration of Pt2 in solution from 5 Â 10 À5 M to 1 Â 10 À3 M did not result in a notable shi in the absorption peak maxima or the formation of a new absorption band (Fig. 2f).
The emission properties of 1-4 were investigated. As depicted in Fig. 3, 1a displays broad emission with l max at 530 nm (monomer emission). As the loading of Pt1 increased (1e), another emission band at 620 nm gradually developed (aggregate emission). For the composites of 2, 2a displayed broad emission with l max at 510 nm. When the loading of Pt1 increased (2d and 2e), there was a gradual redshi of the monomer l max accompanied by the appearance of aggregate emission as a shoulder at 590 nm. The redshi of the monomer emission is ascribed to the intermolecular interactions of Pt II complexes in the ground state. 16 A similar redshi of the high energy emission band by $20 nm was also found for 3, the framework of which contains the same organic ligand as 2. A more distinct aggregate emission was observed for 3e, presumably because of the higher concentration of Pt1, which enhances the formation of aggregate species. Similarly, Pt1 in dilute solutions ((1-5) Â 10 À5 M, Fig. S11, ESI †) displayed intense, unstructured emission with l max at 530 nm and an additional emission band at 615 nm in concentrated solutions (10 À4 -10 À3 M). For the composites of 4, a vibronic structured blue emission band 17 with l max at 450 and 470 nm was found for 4a, whereas an additional broad emission band at 600 nm became apparent in 4d, which has a much higher Pt2 loading. For 4e, which has the highest Pt2 loading (2.41 wt%), only aggregate emission was observed and the emission quantum yield of 4e was 81%. For comparison, Pt2 in dilute solution (Fig. S11b, ESI †) ((1-5) Â 10 À5 M) showed vibronic structured emission with l max at 449 and 470 nm, whereas a broad emission peak at 593 nm was observed in concentrated solutions (10 À4 -10 À3 M). In contrast to composite 4e, the monomer emission of Pt2 could still be observed in solution, even at a concentration of 1 Â 10 À3 M.
Nanosecond time-resolved emission spectra of both Pt1 (5 Â 10 À4 M) and Pt2 (1 Â 10 À3 M) in MeCN (Fig. 4) exhibited gradually developing emission at $600 nm in addition to prompt phosphorescence at 530 nm for Pt1 and 449-470 nm for Pt2 (Fig. 4e). The growth and subsequent decay of this low   energy ($600 nm) emission is similar to the excimeric emission formed between the excited state and ground state of the Pt II complex. 1,18 However, different kinetic behavior for the low energy emission was observed in the Pt II @MOF composites. The aggregate emissions of 1e, 2e, 3e and 4d were instantaneously generated aer laser pulse excitation, similar to their corresponding monomer emission. In 4e (Fig. S12, ESI †), only one phosphorescence band with l max at 600 nm was detected, which was consistent with the result from the steady-state measurement. The nearly simultaneous decay in the beginning of the low and high energy emission bands is indicative of the absence of excimeric emission in Pt II @MOFs, and therefore, the aggregate emissions of Pt II @MOFs are proposed to originate from ground state aggregate species. Comparing the electronic absorption spectra of 1-4 with their corresponding excitation spectra (Fig. S13, ESI †), which showed vastly different excitation proles for emission at 450-530 nm and 600-620 nm, led to the attribution of their aggregate emissions to a 3 MMLCT excited state of ground state aggregates of the Pt II complexes 19 within the pores/channels of the MOFs. The origin of the difference in emission for the aggregated emissions of Pt II in solution and in the MOFs suggests that the photophysical properties of Pt II complexes could be altered by employing MOFs as host materials.

Photo-catalysis
Although several Pt II complexes with high energy 3 IL or 3 MLCT excited states (>2.5 eV) are active catalysts for photooxidation and photoinduced aerobic C-C bond formation reactions, there have been few reports on employing complexes with low energy 3 MMLCT excited states for such reactions. 3d,20 Composite 4e, which displays a predominant 3 MMLCT excited state, was examined as a catalyst for the photoinduced a-cyanation of tertiary amines and reductive cyclization of alkyl iodides. These two reactions were performed in MeCN at room temperature (RT) under light irradiation (l > 370 nm, Fig. 5). For the acyanation reaction, a product turnover number (TON) of $680 was achieved over 8 hours of irradiation (turnover frequency (TOF): 90.6, Table S2, ESI †) with 100% substrate conversion and 88% product yield. The catalyst could be recycled by washing with MeCN. Aer ve cycles, the yield still reached a good value of $63%. No leaching of Pt2 was observed aer the photochemical reaction, using ICP-MS analysis of the recovered 4e. When using Pt2 (5 Â 10 À4 M) as a catalyst, the product TON was found to be $30% of that of 4e under the same conditions. With ZJU-28 alone as a catalyst, the product TON was <10. For the photoinduced cyclization reaction, a TON of $155 for the desired product was achieved with 4e over 10 hours (TOF: 15.5) with 99% yield; this TON was 5-fold higher than that found with Pt2 in the corresponding homogeneous reaction. We envisage that the 3 MMLCT excited states of the Pt II @-MOF composites are highly reactive and can be harnessed for photoinduced C-H dehydrogenation reactions, similar to [Pt 2 (P 2 O 5 H 2 ) 4 ] 4À . 1a This type of reaction proceeds via innersphere atom abstraction by triplet excited species with vacant coordination site(s). 21 The photocatalytic activity of Pt II @MOFs towards the conversion of 1-phenylethanol, benzyl alcohol, isopropanol (IPA) and cyclohexene to the corresponding ketone, aldehyde and cyclohexane was evaluated using 1 and 4 as the catalysts (Fig. 6 and Table 3) and MeCN as the solvent under a N 2 atmosphere and at RT.
Irradiation (l > 370 nm) of an MeCN solution of 1-phenylethanol for 6 hours with 1d or 1e as the catalyst provided acetophenone with TONs of 363 and 216 (TOF: 60.5 and 36), respectively (Fig. 6, the TON for hydrogen was not determined  due to its possible adsorption on the inner surface of the MOF materials). Similar photochemical reactions with Pt1 at concentrations of 0.5-5 Â 10 À4 M afforded trace amounts of the product. Similarly, composites 4d and 4e showed superior performance compared to Pt2 in the same photoinduced reaction. The leaching of the Pt II complex from the composites was not observed aer photolysis according to ICP-MS analysis (Table  S1, ESI †). Aer catalysis, no obvious changes in the PXRD patterns were detected ( Fig. S14 and S15, ESI †). The control experiment using pure ZJU-28 as the catalyst did not show obvious product formation. As composites 1d, 1e, 4d and 4e show a predominant 3 MMLCT excited state upon photoexcitation, the photo-catalysis results suggest that the 3 MMLCT excited states in Pt II @MOF materials are responsible for the observed photocatalytic C-H bond dehydrogenation reactions. When 1phenylethanol was replaced with benzyl alcohol, a TON of 47.5 for benzaldehyde was produced using 1d, which is $5-fold higher than that obtained in Pt1 solution at 5 Â 10 À4 M (Table 3). A negligible amount of product was detected when a low concentration (5 Â 10 À5 M) of Pt1 was used. This divergence in reactivity was also observed between 4d and Pt2 in solution. Pt II @MOFs also reacted with IPA to furnish TONs of 6.1-16.6 of acetone upon light irradiation for 12 hours (Table 3). However, an MeCN solution of Pt1 or Pt2 did not show obvious acetone formation under similar conditions. Furthermore, aer a mixture of IPA and cyclohexene was irradiated in the presence of 1d or 1e for 6 hours, cyclohexane was furnished with TONs of 23.6 and 31.2 (Table 3), respectively. The homocoupling product of the asformed cyclohexenyl radicals (P2) and their partially hydrogenated derivative (P3) were also detected in the reaction mixture. The formation of cyclohexane is proposed to originate from the hydrogenation of cyclohexene by an in situ-generated Pt-H species, which might be formed from the abstraction of the allylic C-H atom of cyclohexene or from the reaction with IPA. 1a To elucidate the origin of the hydrogen atoms, deuterated (d 8 ) IPA was used in the reaction. Signals with a m/z of 84, 162 and 164, which correspond to non-deuterated P1, P2 and P3, respectively, could still be detected as the sole products by GC-MS, thereby excluding the possibility of IPA serving as the H-atom source. Notably, when the same reaction was conducted with Pt1 or Pt2 as the catalyst in homogeneous solution, cyclohexane was not detected. Pt II @MOFs can also catalyze the photoinduced dehydrogenation of indoline and 1,2,3,4-tetrahydroquinoline (Table  4) For detailed reaction conditions, please refer to the ESI. b The amount of acetone formed cannot be determined because of the overlap of its GC signal with that of cyclohexene.
respectively, which were approximately 6-fold and 17-fold higher than those obtained using Pt1 or Pt2 at a concentration of 5 Â 10 À4 M as the photocatalyst. When the concentration of Pt1 or Pt2 in the homogeneous reaction was reduced to 5 Â 10 À5 M, only a trace amount of 1H-indole was detected. For the dehydrogenation reaction of 1,2,3,4-tetrahydroquinoline, TONs of 25-27.2 of 3,4-dihydroquinoline (P5) and TONs of 8.3-12.3 of quinoline (P6) were produced using 1d and 4d (Table 4). Pt II @MOFs also catalyzed the photoinduced dehydrogenative cyclization of o-aminobenzamide with benzyl alcohol at RT. 2-Phenylquinazolin-4(3H)-one (P7) was obtained with TONs of 14.9 and 9.4 using 1d and 4d as photocatalysts, respectively (Table 5). In contrast to the heterogeneous catalyst, Pt1 and Pt2 at a concentration of 5 Â 10 À4 M in MeCN did not show catalytic activity in this reaction.
The improved performances of Pt II @MOF catalysts such as 1d and 4d over that of Pt1 and Pt2, respectively, in the photocatalysis described above reveals the benecial effects of encapsulation of the Pt II complexes in the pores of the MOF hosts. For the photo-induced catalytic a-cyanation of tertiary amines (reaction I) and reductive cyclization of alkyl iodide (reaction II), which are proposed to proceed via singlet oxygen 11,22 and outer-sphere electron transfer 23 pathways, respectively, the higher activity of Pt II @MOF catalysts relative to Pt II complexes is reminiscent of the better catalytic activity of Au III @MOF than that of Au III complexes for the two reactions. 11 In the cases of the other photo-induced catalytic reactions studied in this work, that is, photo-induced dehydrogenation reactions, the enhancement of the catalytic activity upon formation of Pt II @MOFs could be ascribed to the aggregation of the Pt II complexes in the pores of the MOF hosts resulting in the 3 MMLCT excited state. Based on the mechanism proposed for the [Pt 2 (m-P 2 O 5 H 2 ) 4 ] 4À system, 1a and considering the observed hydrogenated by-products in the cyclohexene reaction, a mechanism involving light irradiation generating the 3 MMLCT excited state species (Pt-Pt)* which abstracts a hydrogen atom from, for example, the a-C-H or allylic C-H bond of the alcohol, indoline, 1,2,3,4-tetrahydroquinoline or cyclohexene substrate to form a H-(Pt-Pt) species, is proposed for the Pt II @MOF system; the H-(Pt-Pt) species may further abstract a hydrogen atom, forming H 2 (Pt-Pt) and the dehydrogenation product, and H 2 is eliminated from the H 2 (Pt-Pt) species to regenerate (Pt-Pt). The formation of a reactive H-(Pt-Pt) species could be inferred from the formation of cyclohexane from cyclohexene. For the dehydrogenative coupling reaction (Table 5), the photo-induced dehydrogenation of benzyl alcohol catalyzed by Pt II @MOF would generate benzaldehyde, which undergoes a condensation reaction with o-aminobenzamide, followed by an intramolecular nucleophilic attack on the carbon of the C]N imine bond by NH 2 of the amide group, to give a 2-phenyl-2,3-dihydroquinazolin-4(1H)-one intermediate; dehydrogenation of this intermediate by Pt II @MOF gives the nal product.

Conclusions
A series of Pt II @MOFs composites displaying strong matrixdependent phosphorescence were prepared via a cation exchange method. The cages and nanotubes of the MOFs function as concentrators for the Pt II complexes and induce aggregation inside the MOFs, leading to 3 MMLCT emission. With the diradical character of the 3 MMLCT excited state, the Pt II @MOFs showed superior performance in photoinduced C-C bond formation, dehydrogenation and dehydrogenative cyclization reactions compared to the corresponding Pt II complexes in solution. This simple approach for preparing highly photocatalytically active MOF composites offers a new entryway to new classes of phosphorescent and heterogeneous photofunctional materials with useful applications.