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DOI: 10.1039/C5DT01682G
Two chiral porous metal-organic frameworks containing μ-oxo-bis[Ti(salan)] dimers are constructed from enantiopure dipyridylfunctionalized Ti(salan) bridging ligand. The frameworks are shown to be efficient and recyclable heterogeneous catalysts for asymmetric cyanation of aldehydes with markedly enhanced enantioselectivity with respect to the homogeneous counterpart.
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Chiral Porous Metal-Organic Frameworks Containing μ-oxobis[Ti(salan)] Units for Asymmetric Cyanation of Aldehydes† Received 00th January 20xx, Accepted 00th January 20xx
Xia Du,
a,b
a
a
b
a,c
Zijian Li, Yan Liu,* Shiping Yang and Yong Cui*
DOI: 10.1039/x0xx00000x www.rsc.org/
Two chiral porous Ti(salan)-based metal-organic frameworks are constructed and are shown to be efficient and recyclable heterogeneous catalysts for asymmetric cyanation reaction of aldehydes, displaying significantly higher enantioselectivity than the homogeneous counterpart. Emerging as one unique type of porous material, metal-organic frameworks (MOFs) have received intense scrutiny in the past decade owing to their intriguing potential applications in various fields along with impressive architectures and topologies.1,2 As the surface areas, internal pore metrics, and high density of diverse functional active sites within the open structures can be readily created/controlled, MOFs especially chiral species may offer great advantages in the field of catalysis.3,4 Chiral MOF catalysts are commonly assembled by pre- or post-synthetic modification in terms of supporting privileged homogeneous catalysts or encapsulating small organocatalytic moieties within the open structures.4,5 Within the confined frameworks, the reactants might be effective preorganized in a fashion highly favorable for transfer, meanwhile, the cooperative effect or isolation effect should favor a subsequent increase in selectivity relative to homogeneous cases.3a,3b,3e Despite some pioneering works on catalytic enantioselective reactions promoted by chiral MOFs, it is still challenging to build chiral MOF catalysts with superior performances.4,5 Chiral M(salan) complexes, structural more flexible than M(salen) compounds, have been established as another versatile family of catalysts enabling unprecedented asymmetric reactions.6 We have recently reported that Ti(salan)-based MOF materials are effective for asymmetric sulfoxidation, which show improved or comparable enantioselectivity relative to the homogeneous analogues,7a,7b however, efficient heterogeneous asymmetric catalytic systems derived from M(salan) are still limited.7 Herein we report two chiral porous MOFs assembled from dipyridyl-functionalized Ti(salan) a.
School of Chemistry and Chemical Technology and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China. Email: [email protected], [email protected]; Fax: (+86) 21-5474-1297 b. Department of Chemistry,College of Life and Environmental Science, Shanghai Normal University, Shanghai 200234, China. c. Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China † Electronic Supplementary Information (ESI) available: Experimental details, crystallographic data and spectroscopic data. CCDC 1058025 and 1058026. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b000000x/
Dalton Transactions Accepted Manuscript
DOI: 10.1039/C5DT01682G
ligand and demonstrate their abilities to carry out asymmetric cyanation reactions with much higher enantioselectivity than the homogeneous analogue.
Scheme 1. Synthesis of MOFs 1 and 2.
Solvothermal reactions between TiL(OBu)2 and Cd(OAc)2·2H2O (1:4 molar ratio) or TiL(OBu)2, Cd(OAc)2·2H2O and CdBr2·4H2O (1:2:2 molar ratio) in THF/MeOH afforded [Cd2(O2CCH3)4(TiL)2O (OMe)2]·2H2O (1) and [Cd2Br2(O2CCH3)2(TiL)2O(OMe)2]·2H2O (2) as yellow crystals. The compounds are stable in air as well as in various solvents and were formulated on the basis of single-crystal X-ray diffraction (XRD), thermogravimetry, elemental analysis and IR spectroscopy. Single-crystal X-ray diffraction revealed that 1 crystallizes in chiral orthorhombic space group P212121, comprising one whole formula unit in the asymmetric unit. The basic building unit is a dimeric (TiL)2(μ2-O) cluster built of TiL monomers, in which two six-coordinated Ti centers embedded in N2O2 pockets are bridged by a μ2-O atom, binding one MeO- anion each. The Ti-O bonds (1.8221.851Å) are almost identical to those of known μ-oxo titanium complexes. The ligand L employs a twisted cis-β coordination conformation. Of the two crystallographically distinct Cd atoms, Cd1 is octahedrally coordinated by four oxygen atoms from one chelate, one bidentate and one tridentate carboxylate groups of OAcanions and two pyridyl groups of two (TiL)2(μ2-O) dimers, whereas Cd2 adopts a distorted pentagonal bipyramidal geometry binding to two pyridyl groups of two (TiL)2(μ2-O) and five carboxylate oxygen atoms from one chelate, one bidentate and one tridentate OAccarboxylate groups. Cd1 and Cd2 are bridged by one μ2-η1,η1 and one μ2-η1,η2 carboxylate groups and each of them is also coordinated
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by one chelating carboxylate group to form a Cd 2(CO2)4 cluster. Each Cd2 cluster is surrounded by three (TiL)2(μ2-O) units and each (TiL)2(μ2-O) dimer acts as a tetradentate ligand connecting to adjacent three Cd2 clusters to form a 3D network, and three of such independent networks interpenetrated with each other furnishing a 3D network structure with 1D channels of 6.8 × 6.8 Å2 along the aaxis.
Published on 24 June 2015. Downloaded by Freie Universitaet Berlin on 25/06/2015 07:25:59.
(a)
(b)
(c)
Fig 1. Crystal structure of (TiL)2(μ2-O) dimer and the coordination environment of the dimer in 1 (a) and 2 (b) (H atoms are omitted for clarity). (c) A view of 3D framework of 1 along the a-axis [Cd bright green, Ti purple, N blue, O red, C light gray, Br Turquiose; the Cd centers are drawn as polyhedra].
2 is a 3-fold interpenetrated framework too, which View crystallizes in Article Online DOI: 10.1039/C5DT01682G the chiral tetragonal space group P41212 with a half formula unit in the asymmetric unit. Similar to 1, a pair of TiL units are linked by an μ-oxo atom to form a (TiL)2(μ2-O) basic building unit. Each Cd atom exhibits a distorted octahedral geometry completed by two pyridyl nitrogen atoms from two (TiL)2(μ2-O) dimers, two oxygen atoms from a chelating carboxyl group of OAc- and two Br- ions. Two Cd atoms are bridged by two bridging Br- ions to form a Cd2Br2 cluster. Each Cd2Br2 cluster connects with three (TiL)2(μ2-O) dimers and each dimer acts as a tetradentate ligand to connect with adjacent three Cd2Br2 clusters to form a 3D framework. The final 3-fold interpenetrated network was achieved via interpenetration of three independent networks. The resulting interpenetration framework exhibited 6.4 × 6.4 Å2 channels along the c-axis. Powder XRD patterns of bulk samples of 1 and 2 matched their simulated data well, thus confirming the phase purity of the materials. The chiral nature of complexes 1 and 2 was established by solid-state CD measurement, in which the spectra of two enantiomers formed from (R)- and (S)-TiL(OBu)2 are almost mirror images of each other. Thermogravimetric analysis (TGA) of 1 and 2 revealed that the frameworks are stable up to about 350 oC. Notably, both 1 and 2 retained their frameworks integrity even after guest removal, as evidenced by the PXRD patterns. PLATON showed that 1 and 2 have 21.8 % and 20.1 % of the total volume available for guest inclusion, respectively.8 The titanium ion in 1 and 2 is in +4 oxidation state, demonstrated by the Ti 2p3/2 and 2p1/2 peaks around 458.6 and 464.7eV in X-ray photoelectron spectroscopy (XPS) spectra.9 The permanent porosity of 1 and 2 was investigated by N2 adsorption measurements at 77K, which showed a type I sorption behavior with Brunauer-Emmett-Teller (BET) surface area of 107.5 m2 g-1 for 1. But for 2, it gave only surface adsorption result. 1 and 2 could adsorb ~0.67 and 0.61 methyl orange (MO, ~14.7 × 5.3 × 5.3 Å3 in size) per formula unit in solution, respectively, indicating the porous nature and structural integrity in solution. The stable porous structure combined with abundant Ti(salan) active centers exposed to the inner spheres prompted us to explore their utilization as heterogeneous catalysts for asymmetric cyanation of aldehydes. Initial optimization studies performed with the cyanation of p-methoxybenzaldehyde revealed that 1 (5 mol%) could give corresponding cyanohydrin silyl ether with the best 82% conversion and 90% enantioselectivity in the presence of 1.5 equivalents of Ph3PO in CH2Br2 at -10oC for 20 h. The product was obtained in 47% and 63% conversions after 4 and 10 h, respectively, and if the reaction was further continued for 20 h, 82% conversion could be obtained; while it was possible to achieve 73% enantioselectivity after 4 h, which remained almost constant (8990%) from 12 h to 20 h. Control experiment showed that the reaction could not take place without the MOF catalyst, and moreover, the absence of additive Ph3PO led to a significant decrease in enantioselectivity (67%). Investigation of the scope of the cyanation was conducted under the optimized conditions. As shown in Table 1, the cyanation of benzaldehyde and aromatic aldehydes bearing the electron-donating groups proceeded with good conversions and relative higher enantioselectivities (60-91% ee, entries 1, 4, 6 and 8), whereas enantioselectivities decreased when electron-withdrawing substituent was introduced (36% ee, entry 12). With the bulkier aldehydes such as 1- and 2-naphthaldehyde, moderate reactivity and enantioselectivities were obtained (34% and 59% ee, entries 15 and 17). Interestingly, for the aromatic aldehydes with substituents at the meta- and ortho- positions, e.g., m-methoxybenzaldehyde and 1naphthaldehyde, the enantioselectivies were lower than the para-
2 | J. Name., 2012, 00, 1-3
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Table 1. Asymmetric cyanation of aldehydes catalyzed by 1, 2 and TiL(OBu)2a
Catalyst (R)-1 (R)-2 (R)-TiL(OBu)2 (R)-1 (R)-2 (R)-1 (R)-2 (R)-1 (S)-1 (R)-2 (R)-TiL(OBu)2 (R)-1 (R)-2 (R)-TiL(OBu)2 (R)-1 (R)-2 (R)-1 (R)-TiL(OBu)2 (R)-1 (R)-2 (R)-1 (R)-TiL(OBu)2
Conv.(%)b 78 77 99 89 78 75 74 82 81 80 96(99)d 85 73 99 78 77 81 99 77 81
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This article can be cited before page numbers have been issued, to do this please use: X. Du, Z. Li, Y. Liu, S. Yang and Y. Cui, Dalton Trans., 2015, DOI: 10.1039/C5DT01682G.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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DOI: 10.1039/C5DT01682G
Two chiral porous metal-organic frameworks containing μ-oxo-bis[Ti(salan)] dimers are constructed from enantiopure dipyridylfunctionalized Ti(salan) bridging ligand. The frameworks are shown to be efficient and recyclable heterogeneous catalysts for asymmetric cyanation of aldehydes with markedly enhanced enantioselectivity with respect to the homogeneous counterpart.
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Chiral Porous Metal-Organic Frameworks Containing μ-oxobis[Ti(salan)] Units for Asymmetric Cyanation of Aldehydes† Received 00th January 20xx, Accepted 00th January 20xx
Xia Du,
a,b
a
a
b
a,c
Zijian Li, Yan Liu,* Shiping Yang and Yong Cui*
DOI: 10.1039/x0xx00000x www.rsc.org/
Two chiral porous Ti(salan)-based metal-organic frameworks are constructed and are shown to be efficient and recyclable heterogeneous catalysts for asymmetric cyanation reaction of aldehydes, displaying significantly higher enantioselectivity than the homogeneous counterpart. Emerging as one unique type of porous material, metal-organic frameworks (MOFs) have received intense scrutiny in the past decade owing to their intriguing potential applications in various fields along with impressive architectures and topologies.1,2 As the surface areas, internal pore metrics, and high density of diverse functional active sites within the open structures can be readily created/controlled, MOFs especially chiral species may offer great advantages in the field of catalysis.3,4 Chiral MOF catalysts are commonly assembled by pre- or post-synthetic modification in terms of supporting privileged homogeneous catalysts or encapsulating small organocatalytic moieties within the open structures.4,5 Within the confined frameworks, the reactants might be effective preorganized in a fashion highly favorable for transfer, meanwhile, the cooperative effect or isolation effect should favor a subsequent increase in selectivity relative to homogeneous cases.3a,3b,3e Despite some pioneering works on catalytic enantioselective reactions promoted by chiral MOFs, it is still challenging to build chiral MOF catalysts with superior performances.4,5 Chiral M(salan) complexes, structural more flexible than M(salen) compounds, have been established as another versatile family of catalysts enabling unprecedented asymmetric reactions.6 We have recently reported that Ti(salan)-based MOF materials are effective for asymmetric sulfoxidation, which show improved or comparable enantioselectivity relative to the homogeneous analogues,7a,7b however, efficient heterogeneous asymmetric catalytic systems derived from M(salan) are still limited.7 Herein we report two chiral porous MOFs assembled from dipyridyl-functionalized Ti(salan) a.
School of Chemistry and Chemical Technology and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China. Email: [email protected], [email protected]; Fax: (+86) 21-5474-1297 b. Department of Chemistry,College of Life and Environmental Science, Shanghai Normal University, Shanghai 200234, China. c. Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China † Electronic Supplementary Information (ESI) available: Experimental details, crystallographic data and spectroscopic data. CCDC 1058025 and 1058026. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b000000x/
Dalton Transactions Accepted Manuscript
DOI: 10.1039/C5DT01682G
ligand and demonstrate their abilities to carry out asymmetric cyanation reactions with much higher enantioselectivity than the homogeneous analogue.
Scheme 1. Synthesis of MOFs 1 and 2.
Solvothermal reactions between TiL(OBu)2 and Cd(OAc)2·2H2O (1:4 molar ratio) or TiL(OBu)2, Cd(OAc)2·2H2O and CdBr2·4H2O (1:2:2 molar ratio) in THF/MeOH afforded [Cd2(O2CCH3)4(TiL)2O (OMe)2]·2H2O (1) and [Cd2Br2(O2CCH3)2(TiL)2O(OMe)2]·2H2O (2) as yellow crystals. The compounds are stable in air as well as in various solvents and were formulated on the basis of single-crystal X-ray diffraction (XRD), thermogravimetry, elemental analysis and IR spectroscopy. Single-crystal X-ray diffraction revealed that 1 crystallizes in chiral orthorhombic space group P212121, comprising one whole formula unit in the asymmetric unit. The basic building unit is a dimeric (TiL)2(μ2-O) cluster built of TiL monomers, in which two six-coordinated Ti centers embedded in N2O2 pockets are bridged by a μ2-O atom, binding one MeO- anion each. The Ti-O bonds (1.8221.851Å) are almost identical to those of known μ-oxo titanium complexes. The ligand L employs a twisted cis-β coordination conformation. Of the two crystallographically distinct Cd atoms, Cd1 is octahedrally coordinated by four oxygen atoms from one chelate, one bidentate and one tridentate carboxylate groups of OAcanions and two pyridyl groups of two (TiL)2(μ2-O) dimers, whereas Cd2 adopts a distorted pentagonal bipyramidal geometry binding to two pyridyl groups of two (TiL)2(μ2-O) and five carboxylate oxygen atoms from one chelate, one bidentate and one tridentate OAccarboxylate groups. Cd1 and Cd2 are bridged by one μ2-η1,η1 and one μ2-η1,η2 carboxylate groups and each of them is also coordinated
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by one chelating carboxylate group to form a Cd 2(CO2)4 cluster. Each Cd2 cluster is surrounded by three (TiL)2(μ2-O) units and each (TiL)2(μ2-O) dimer acts as a tetradentate ligand connecting to adjacent three Cd2 clusters to form a 3D network, and three of such independent networks interpenetrated with each other furnishing a 3D network structure with 1D channels of 6.8 × 6.8 Å2 along the aaxis.
Published on 24 June 2015. Downloaded by Freie Universitaet Berlin on 25/06/2015 07:25:59.
(a)
(b)
(c)
Fig 1. Crystal structure of (TiL)2(μ2-O) dimer and the coordination environment of the dimer in 1 (a) and 2 (b) (H atoms are omitted for clarity). (c) A view of 3D framework of 1 along the a-axis [Cd bright green, Ti purple, N blue, O red, C light gray, Br Turquiose; the Cd centers are drawn as polyhedra].
2 is a 3-fold interpenetrated framework too, which View crystallizes in Article Online DOI: 10.1039/C5DT01682G the chiral tetragonal space group P41212 with a half formula unit in the asymmetric unit. Similar to 1, a pair of TiL units are linked by an μ-oxo atom to form a (TiL)2(μ2-O) basic building unit. Each Cd atom exhibits a distorted octahedral geometry completed by two pyridyl nitrogen atoms from two (TiL)2(μ2-O) dimers, two oxygen atoms from a chelating carboxyl group of OAc- and two Br- ions. Two Cd atoms are bridged by two bridging Br- ions to form a Cd2Br2 cluster. Each Cd2Br2 cluster connects with three (TiL)2(μ2-O) dimers and each dimer acts as a tetradentate ligand to connect with adjacent three Cd2Br2 clusters to form a 3D framework. The final 3-fold interpenetrated network was achieved via interpenetration of three independent networks. The resulting interpenetration framework exhibited 6.4 × 6.4 Å2 channels along the c-axis. Powder XRD patterns of bulk samples of 1 and 2 matched their simulated data well, thus confirming the phase purity of the materials. The chiral nature of complexes 1 and 2 was established by solid-state CD measurement, in which the spectra of two enantiomers formed from (R)- and (S)-TiL(OBu)2 are almost mirror images of each other. Thermogravimetric analysis (TGA) of 1 and 2 revealed that the frameworks are stable up to about 350 oC. Notably, both 1 and 2 retained their frameworks integrity even after guest removal, as evidenced by the PXRD patterns. PLATON showed that 1 and 2 have 21.8 % and 20.1 % of the total volume available for guest inclusion, respectively.8 The titanium ion in 1 and 2 is in +4 oxidation state, demonstrated by the Ti 2p3/2 and 2p1/2 peaks around 458.6 and 464.7eV in X-ray photoelectron spectroscopy (XPS) spectra.9 The permanent porosity of 1 and 2 was investigated by N2 adsorption measurements at 77K, which showed a type I sorption behavior with Brunauer-Emmett-Teller (BET) surface area of 107.5 m2 g-1 for 1. But for 2, it gave only surface adsorption result. 1 and 2 could adsorb ~0.67 and 0.61 methyl orange (MO, ~14.7 × 5.3 × 5.3 Å3 in size) per formula unit in solution, respectively, indicating the porous nature and structural integrity in solution. The stable porous structure combined with abundant Ti(salan) active centers exposed to the inner spheres prompted us to explore their utilization as heterogeneous catalysts for asymmetric cyanation of aldehydes. Initial optimization studies performed with the cyanation of p-methoxybenzaldehyde revealed that 1 (5 mol%) could give corresponding cyanohydrin silyl ether with the best 82% conversion and 90% enantioselectivity in the presence of 1.5 equivalents of Ph3PO in CH2Br2 at -10oC for 20 h. The product was obtained in 47% and 63% conversions after 4 and 10 h, respectively, and if the reaction was further continued for 20 h, 82% conversion could be obtained; while it was possible to achieve 73% enantioselectivity after 4 h, which remained almost constant (8990%) from 12 h to 20 h. Control experiment showed that the reaction could not take place without the MOF catalyst, and moreover, the absence of additive Ph3PO led to a significant decrease in enantioselectivity (67%). Investigation of the scope of the cyanation was conducted under the optimized conditions. As shown in Table 1, the cyanation of benzaldehyde and aromatic aldehydes bearing the electron-donating groups proceeded with good conversions and relative higher enantioselectivities (60-91% ee, entries 1, 4, 6 and 8), whereas enantioselectivities decreased when electron-withdrawing substituent was introduced (36% ee, entry 12). With the bulkier aldehydes such as 1- and 2-naphthaldehyde, moderate reactivity and enantioselectivities were obtained (34% and 59% ee, entries 15 and 17). Interestingly, for the aromatic aldehydes with substituents at the meta- and ortho- positions, e.g., m-methoxybenzaldehyde and 1naphthaldehyde, the enantioselectivies were lower than the para-
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Table 1. Asymmetric cyanation of aldehydes catalyzed by 1, 2 and TiL(OBu)2a
Catalyst (R)-1 (R)-2 (R)-TiL(OBu)2 (R)-1 (R)-2 (R)-1 (R)-2 (R)-1 (S)-1 (R)-2 (R)-TiL(OBu)2 (R)-1 (R)-2 (R)-TiL(OBu)2 (R)-1 (R)-2 (R)-1 (R)-TiL(OBu)2 (R)-1 (R)-2 (R)-1 (R)-TiL(OBu)2
Conv.(%)b 78 77 99 89 78 75 74 82 81 80 96(99)d 85 73 99 78 77 81 99 77 81
