† Electronic supplementary informationcharacterization data, as well as additioand 1443256. For ESI and crystallograpformat see DOI: 10.1039/c6ra07100g
Cite this: RSC Adv., 2016, 6, 51936
Received 17th March 2016Accepted 16th May 2016
DOI: 10.1039/c6ra07100g
www.rsc.org/advances
51936 | RSC Adv., 2016, 6, 51936–5194
: direct crystallographicobservation of the site-selective C(sp3)–Hfunctionalization†
A 3D breathing MOF was synthesized by hierarchical methods for
light-driven acceleration of the site-selective C(sp3)–H activation. The
1D channels of the assembled MOF can encapsulate the reaction
substrates via a breathing process that was observed by single-crystal
X-ray analyses. The weak interactions between the encapsulated
substrate and the photoactive center facilitated the in situ generation
of the active alkyl radicals.
Transition-metal-catalyzed direct functionalization of C–Hbonds constitutes a faster and more atom-economical approachto shortening synthetic schemes in organic chemistry.1 Amongthe various examples reported thus far, the visible-light-prompted site-selective functionalization of unactivatedaliphatic C–H bonds represents a shining approach to suchreaction sequences; it is particularly promising toward thedevelopment of sustainable and green synthetic methods thatcan tackle the intrinsic inertness of C(sp3)–H bonds and thedifficulty of regioselectivity.2 Visible-light photocatalysis withpolyoxometalates (POMs) for the direct functionalization ofC–H bonds under mild conditions constantly attract lots ofattention.3 New synthetic routes that operate under mildtemperature and pressure using light as a renewable energysource are the goal for sustainable chemistry.4
On the other hand, metal–organic frameworks (MOFs) arehybrid solids with innite network structures built from organicbridging ligands and inorganic connecting nodes, which arehighly desirable for numerous applications, including hetero-geneous catalysis, gas storage, and drug delivery.5 FlexibleMOFs possess both the highly ordered network and structuraltransformability, which can reversibly change the size andshape of their pores in response to the penetration of guest
alian University of Technology, Dalian
edu.cn
(ESI) available: Experimental details,nal tables and gures. CCDC 1443254hic data in CIF or other electronic
0
molecules.6 Moreover, the dynamic guest accommodation ofthese so structures would enable the uniform arrangementand orientation of guest molecules.7 Although publicationsabout MOFs have increased exponentially, reports on exibleMOFs associated breathing properties are surprisingly scarce.8
The majority of these papers are focused on rigid MOFs and theassociated adsorption properties of small gas molecules; veryfew deal with hydrocarbons in heterogeneous photocatalysis.9
Herein, we report the synthesis of a exible decatungstate-based MOF [Cu2(DPDO)4.5(H2O)W10O32]$3CH3CN$2H2O (deno-ted as DT–DPDO; DPDO ¼ 4,40-bipyridine-N,N0-dioxide), thatexhibits the breathing capability due to the framework expan-sion and contraction upon uptake and release. DT–DPDO is anexcellent heterogeneous photocatalyst for the site-selectiveC(sp3)–H alkylation of aliphatic nitriles (Scheme 1). When thereactants were put inside the pores of DT–DPDO, they sponta-neously underwent the photocatalytic reactions at roomtemperature and atmospheric pressure. This reaction was
Scheme 1 Synthetic procedure of the flexible MOF, showing the 1Dchannels encapsulated the reaction substrates via a breathing processand the site-selective C(sp3)–H alkylation of aliphatic nitriles.
Fig. 2 Single-crystal structure of the catalytic site distribution in DT–DPDO absorbed valeronitrile, showing the absorbency and activationof the substrate by hydrogen-bonding interactions in the porous of theMOF.
Table 1 Crystallographic data structure refinement for DT–DPDO andVN@DT–DPDO
DT–DPDO VN@DT–DPDO
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directly observed by X-ray crystallography because the hostcrystallinity was maintained throughout. To the best of ourknowledge, this is the rst example of the single-crystal tosingle-crystal observation of the site-selective C(sp3)–H activa-tion inside the pores of a exible MOF.
The exible MOF of DT–DPDO (CCDC no. 1443254) wassynthesized via a diffusion method by laying an acetonitrilesolution of [(n-C4H9)4N]4[W10O32] onto the aqueous solution ofCu(ClO4)2$6H2O and DPDO in a test tube for several days,producing a yield of 43%. Elemental analyses and powder X-rayanalysis indicated the pure phase of its bulk sample. Single-crystal structural analysis revealed the inclusion of free deca-tungstate anions embedded in the pores of the copper-basedframework. It crystallized in the triclinic lattice with a spacegroup P�1. As shown in Fig. 1a and b, two crystallographicallyindependent copper(II) ions adopted the same ve-coordinatesquare-pyramidal geometry. Four oxygen atoms of DPDOligands positioned in the basal plane and a water molecule forCu(1) or an oxygen atom of DPDO for Cu(2) occupied the vertexposition. These Cu(1) and Cu(2) ions were alternatively con-nected by the basal plane DPDO ligands to form 2D squaregrids; adjacent sheets were further connected together via thevertex oxygen atom from DPDO ligands to form a 3D Cu–DPDOframework. The photoactive [W10O32]
4� polyoxoanions wereembedded in the pores of Cu–DPDO via electrostatic attractionto generate the DT–DPDO POM@MOF with the 1D channel of12.0 �A � 3.6 �A (Fig. 1c and d). The electrostatic attractionbetween POMs and MOFs was oen discussed in the [email protected] This is the rst example of exibledecatungstate-based MOF, as the high sensitivity of decatung-state to the temperature and acidity make the incorporation andstabilization quite difficult.
Without guest water molecules and acetonitrile molecules,the effective free volume of DT–DPDO was estimated to be18.9% by PLATON soware.11 Since DPDO has enormous
Fig. 1 The 3D flexible MOF of DT–DPDO, which is generated by theCu–DPDO framework (drawn as a ball-and-stick model) and[W10O32]
4� embedded (drawn as purple polyhedra) with the 1Dchannel of 12.0 �A � 3.6 �A. Symmetry operation: A: �x, 1 � y, 3 � z.
exibility as a supramolecular linker, it can be involved not onlyin coordination and hydrogen-bonds via its N,N0-dioxide oxygencentres, but also the pyridine-N-oxide rings can form aromaticp–p stacking interactions.12 Thus, the exible DT–DPDO couldreversibly change the size and shape of the pores in response tothe penetration of guest molecules by a change in unit cellvolume (DV s 0). When single crystals of DT–DPDO wereimpregnated in valeronitrile at room temperature for 2 days, thevaleronitrile impregnated crystals (VN@DT–DPDO, CCDC no.1443256) were formed without a loss in crystallinity. During thisprocess, the guest water molecules and acetonitrile molecules
Empirical formula C51H51N12O44W10Cu2 C50H51N10O44W10Cu2M, g mol�1 3501.62 3461.59Crystal system Triclinic TriclinicSpace group P�1 P�1a, �A 12.8423(8) 13.0242(7)b, �A 12.9467(8) 13.7795(8)c, �A 23.9678(14) 23.9552(14)a, deg 81.4810(10) 81.259(3)b, deg 86.3570(10) 86.490(3)g, deg 70.6300(10) 70.046(2)V, �A3 3717.5(4) 3993.9(4)Z 2 2Dcalcd, g cm�3 3.128 2.878T, K 180(2) 150(2)Re. collected/unique 107 967/17 030,
Rint ¼ 0.094681 164/13 389,Rint ¼ 0.1714
m, mm�1 16.067 14.952GOOF 1.075 1.049R1
a (I > 2s(I)) 0.0762 0.0746wR2
b (I > 2s(I)) 0.1252 0.2015R1
a (all data) 0.1221 0.1052wR2
b (all data) 0.1363 0.2181Diff peak and hole,e$�A�3
in the channels were replaced by valeronitrile molecules (Fig. 2).With the breathing behavior of the exible framework, thecharacteristic volume of the MOF unit cell was enlarged (DV ¼276 �A3) in the phase of VN@DT–DPDO (Table 1).
From the crystallographic investigation, valeronitrile mole-cules were positioned near the decatungstate anions and thecopper nodes of the Cu–DPDO framework. Hydrogen-bondinginteractions between the nitrogen atoms of valeronitrile mole-cules and the coordinated water molecules (N/O separation of2.882�A) xed the valeronitrile in the pores of DT–DPDO. Weak
Fig. 3 (a) Solid-state IR spectra of DT–DPDO (i), and of DT–DPDOimpregnated in valeronitrile (ii) (the blue triangle represented the signalof valeronitrile); (b) solid-state IR spectra of DT–DPDO (i), and of DT–DPDO impregnated in acrylonitrile (ii) (the red triangle represented thesignal of acrylonitrile).
Table 2 Selective C–H alkylation of aliphatic nitriles with DT–DPDO as
Entry Nitrile Alkene
1
2
3
4
5
a Reaction conditions: aliphatic nitrile (2.5 mmol), acrylonitrile (0.5 mmola 500 W Xe lamp at room temperature for 48 h. b Isolated yields aer as
51938 | RSC Adv., 2016, 6, 51936–51940
C–H/O hydrogen bonds (2.619�A) were also observed betweenthe valeronitrile and the decatungstate anions in VN@DT–DPDO.
To better understand the possible mechanism for substrateactivation, the IR spectrum of VN@DT–DPDO showed a C^Nstretching frequency of valeronitrile at 2235 cm�1, which wasred-shied by about 13 cm�1 compared with free valeronitrile(Fig. 3). DT–DPDO exhibited an absorption band at about 400nm in the solid state UV-vis absorption spectrum (Fig. S3a,ESI†), which was assigned to the oxygen-to-tungsten chargetransfer transition.13 Upon excitation at this band, the uores-cence emission spectrum of free DT–DPDO exhibited an intenseluminescence band at approximately 455 nm (Fig. S3b, ESI†).Noticeable, the progressive addition of valeronitrile to theCH3CN suspension of DT–DPDO caused substantial lumines-cence quenching. This quenching process conrmed theoccurrence of direct photoinduced electron transfer (PET) fromthe substrate to the excited state of the decatungstate moie-ties.14 These results suggested that the possibility of the reactiveexcited state of [DT–DPDO]* oxidized valeronitrile for thegeneration of active alkyl radicals, which is further used for thefunctionalization of nitrile substrates.
To probe the catalytic activity of DT–DPDO in the site-selective C(sp3)–H alkylation of aliphatic nitriles, the reactionof isovaleronitrile and acrylonitrile was initially investigatedwith a Xe lamp (500 W) as light source (Table 2). The 1 mol%
Fig. 4 Color changes of the reaction system for DT–DPDO indicatingthe blue color that is characteristic of H+[DT–DPDO]� [(a) reactiontime for 0 h, (b) reaction time for 0–48 h, (c) after the reaction, themixture was stirred under air atmosphere].
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photocatalyst loading gave b-substituted isovaleronitrile ina yield of 86% aer 48 h irradiation. This result demonstratedthe successful execution of our exible POM@MOF design,showing a high site-selective efficiency. Other aliphatic nitriles(e.g., valeronitrile and isocapronitrile) were also found to beprone to react with acrylonitrile under the same conditions.With the breathing behavior of the framework, DT–DPDOrepresents the successful example of a exible POM-basedheterogeneous photocatalyst, which could reversibly changethe size and shape of the pores in response to the penetration ofaliphatic nitriles.
A series of control experiments revealed the heterogeneousand photocatalytic natures of the reactions. When the reactionwas conducted in the dark or in the absence of DT–DPDOcatalyst, almost no conversion was observed, which suggest thatboth the light and the photocatalyst are indispensable for effi-cient conversion.15 Notably, solid DT–DPDO could be easilyisolated from the reaction suspension by ltration and wassubsequently reused at least three times, with only slightdecrease in its reactivity and selectivity (Table S1, ESI†). TheXRD patterns of DT–DPDO collected from the reaction mixturedemonstrated the intact crystallinity (Fig. S5, ESI†). Interest-ingly, the efficiency of the heterogeneous DT–DPDO is compa-rable to that of the homogeneous decatungstate catalyst (TableS1, entry 6, ESI†). The recyclability of DT–DPDO renders oursynthetic approach superior to other relevant heterogenizingmethods.
However, in contrast to the smooth reactions (Table 2,entries 1–3), the site-selective reaction in the presence of bulkypropenamide or/and bulky nitrile (Table 2, entries 4 and 5) gaveless than 5% conversion under the same conditions. As the sizeof bulky propenamide and bulky nitrile are much larger thanthe size of the pores of DT–DPDO, the quite lower the catalyticactivity towards the alkylation reaction provides additionalproofs of that the catalysis occurred within the pores of theMOFs.
From a mechanistic viewpoint (Scheme 2), the active excitedstate [DT–DPDO]*, which is characterized by a hole in an O-based wO, efficiently abstracts a hydrogen from the b- or g-C–H bond of aliphatic nitriles to form alkyl radicals.16 The alkylradicals then combine with acrylonitrile to form the adductradical. Back-hydrogen atom transfer from the reduced formH+[DT–DPDO]� gives the alkylated product and regenerates the
Scheme 2 Proposed reaction mechanism for the DT–DPDO photo-catalyzed functionalization of unactivated C(sp3)–H bonds.
starting DT–DPDO.17 Obviously, the occurrence of the reactionis indicated by the shi to the blue color which is characteristicof H+[DT–DPDO]� (Fig. 4).18
In conclusion, a new exible decatungstate-based MOF wassynthesized by hierarchical methods, which was used asa heterogeneous photocatalyst for the site-selective C(sp3)–Halkylation of aliphatic nitriles. The 1D channels of the breathingMOF can provide appropriate size and shape to encapsulate thereaction substrates that were observed by single-crystal X-rayanalyses. This study may have applications in observing manyother biologically and chemically important reactions insidethe pores of crystalline hosts and could facilitate investigationsof their mechanisms.
Acknowledgements
We are grateful for the National Natural Science Foundation ofChina (21531001 and 21421005).
Notes and references
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