Chiral Metal–Organic Frameworks
FULL PAPER
ther collected by filtration under argon and dried in vacuum at room
temperature. Yield: 64 mg (71% referred to the amount of H3ChirBTB-
2). IR: n˜ =656 (w), 700 (m), 729 (m), 756 (m), 796 (m), 825 (m), 864 (w),
1001 (w), 1038 (m), 1082 (m), 1115 (m), 1144 (s), 1240 (m), 1265 (m),
1311 (m), 1400 (s), 1435 (s), 1456 (s), 1498 (m), 1552 (m), 1614 (s), 1657
(s), 1734 (s), 1763 (s), 2912 (m), 2978 (w), 3028 (w), 3062 (w), 3086 (w),
2700–3700 cmÀ1 (br); elemental analysis calcd (%) for Zn3(ChirBTB-2)2-
space in the channels found in the crystal structure of 7 is
not sufficient for the adsorption of 10, probably because
these channels are occupied by a third of the oxazolidinone
moieties. Both MOFs exhibit metal sites accessible for po-
tential substrates, since they are occupied by solvent mole-
cules. These metal sites are surrounded by the chiral oxazo-
lidinone subunits fixed at the BTB linker (see Figures S1
and S2 in the Supporting Information). In the Mukaiyama
aldol reactions catalyzed by 6 and 7, enantiomeric excesses
of up to 40% were obtained. Thus the preparation of chiral
MOF catalysts by the incorporation of auxiliaries known
from homogeneous catalysis into linkers not yet established
in MOF synthesis has been proven to be successful. Even
though the selectivity in these few test reactions was low,
the enantioselectivity is a proof of the validity of this new
concept. Further development and understanding of linker–
substrate interactions is required for enhancing enantiose-
lectivity and yield.
In summary, we have presented the synthesis of the two
new chiral linkers H3ChirBTB-1 (5a) and H3ChirBTB-2
(5b) and their successful use for the synthesis of chiral
MOF catalysts for asymmetric catalysis. Although com-
pound 6 has the same topology as HKUST-1, slight changes
in the oxazolidinone substituent lead to a completely differ-
ent topology in 7. The pore systems of both materials are ac-
cessible to even large molecules. Both MOFs are active
Lewis acid catalysts and show enantioselectivity in the Mu-
kaiyama aldol reaction.
(H2O)3:[31] C 62.7, H 4.75, N 4.72, O 19.5, Zn 8.26; found: C
ACHUTNRGEN(NUG DEF)2ACHTUNGTRENNUGN
(62.6Æ0.5), H (4.97Æ0.05), N (4.82Æ0.08), O (19.2Æ0.2), Zn (7.92Æ
0.04).
Crystal data: Zn3(C57H42O12N3)2·2ACTHNUTRGNEUNG
(C5H11NO); Mr =2320.38 gmolÀ1; tet-
ragonal, P43212 (no. 96); a=28.3927(4), c=20.3756(3) ꢀ; V=
16425.7(4) ꢀ3; Z=4; 1calcd =0.920 gcmÀ3; synchrotron l=0.88561 ꢀ; T=
208C; qmax =28.418; reflections collected/unique 38101/10603; Rint
=
0.0561, R1 =0.0963, wR2 =0.2580; Flack parameter x=À0.09(2); largest
diff. peak and hole: 0.085 and À0.011 eꢀÀ3
.
CCDC-735821 (6) and 735822 (7) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
ac.uk/data_request/cif.
Dye adsorption from solution: Freshly prepared samples of 6 and 7, re-
spectively, were transferred to concentrated solutions of fluorescein (8)
in DEF/CH2Cl2, merocyanine 9 in DEF/MeOH, and Reichardtꢄs dye (10)
in CH2Cl2. After several days, the crystals were washed with fresh DEF
to remove dye molecules not soaked into the MOF.
To determine the amount of adsorbed 10, sample 6 was placed in a solu-
tion of dye in CH2Cl2 for 6 d and washed thoroughly with CH2Cl2 (see
Figure S7 in the Supporting Information). The resulting dye@MOF was
subsequently hydrolyzed with aqueous HCl. The 1H NMR of the hydro-
lyzed MOF shows a ratio of 2:1 (linker/dye) (see Figures S13 and S14).
Performance of Mukaiyama aldol reactions: The Mukaiyama aldol reac-
tions were performed by using 6 and 7 as catalysts, as well as by applying
MIL-101 (17), MOF-177 (16), and [ZnACHTNURTGENNG(U NO3)2ACHUTNTGREN(NUGN H2O)4] (15) as reference
catalysts. The amount of catalyst employed was calculated in a way that
15 mol% accessible metal centers were present in reference to the
amount of benzaldehyde (11a) or 1-naphthaldehyde (11b). Since 6 and 7
cannot be generated in a solvent-free manner, these materials were treat-
ed in the following way prior to use. Freshly prepared samples of 6 and 7
were washed with fresh DEF and treated with absolute ethanol to ex-
change the occluded high-boiling solvent DEF with a solvent miscible
with n-heptane. The mass of the MOF samples was determined using a
pycnometer and the ethanol was replaced by dry n-heptane three times.
The other catalysts were treated as described in the supplementary part.
After transferring the respective catalyst to the reaction vessel, dry n-
heptane (10 mL) was introduced. Under stirring at room temperature,
benzaldehyde (319 mg, 11a, 3 mmol) or 1-naphthaldehyde (469 mg, 11b,
3 mmol), respectively (each freshly distilled), and silyl enol ether
(1.046 g, 12, 6 mmol) were added. The reaction was monitored by GC-
MS analysis using n-octane as standard. For the determination of the iso-
lated yields of 13a and 13b, the reaction was conducted in dichlorome-
thane and the reaction mixture was filtered through a short pad of silica
(pretreated with NEt3), washed with EtOAc, and concentrated under re-
duced pressure. The crude product was purified by flash chromatography
(silica pretreated with NEt3). The results of the catalytic test reactions
are summarized in Table 2.
Experimental Section
Synthesis of H3ChirBTB-1 and -2: A detailed description of the synthesis
route is given in the Supporting Information.
Synthesis and crystal structure of [Zn3(ChirBTB-1)2]·3DEF (6):
H3ChirBTB-1 (5a, 85.0 mg 0.104 mmol) and zinc nitrate tetrahydrate
(82.0 mg, 0.314 mmol, Merck 98.5%) were dissolved in DEF (2.5 mL).
The solution was heated in a Pyrex tube at 1008C for 20 h. The resulting
yellowish crystals were washed with fresh DEF and subsequently ex-
changed with dichloromethane within three days. The product was fur-
ther collected by filtration under argon and dried in vacuum at room
temperature. Yield: 58 mg (52% referred to the amount of H3ChirBTB-
1). IR: n˜ =658 (w), 725 (w), 769 (w), 798 (m), 825 (w), 850 (w), 877 (w),
976 (w), 1014 (w), 1053 (w), 1082 (w), 1119 (m), 1149 (m), 1236 (m), 1263
(w), 1402 (s), 1433 (s), 1643 (s), 1732 (s), 1761 (s), 2875 (w), 2931 (m),
2960 (m), 2700–3700 cmÀ1 (br); elemental analysis calcd (%) for
Zn3(ChirBTB-1)2ACHTUNGTRENNUNG(DEF)3ACHTUNGTRENNUNG
(H2O)5:[28] C 56.7, H 5.76, N 5.67, O 23.0, Zn
8.82; found: C (56.2Æ0.2), H (5.98Æ0.09), N (5.72Æ0.07), O (23.5Æ0.4),
Zn (8.35Æ0.03).
For the filtration test, the addition of the enol ether 12 to 11a with 6 was
run again in the presence of mesitylene as an internal standard and the
MOF was removed by filtration after 18 h. The ratio of product (13a) to
standard was measured directly after the filtration, after 30 h, 54 h and
7 d by means of GC-MS. No more product was formed after filtration.
Crystal data: Zn3(C45H42O12N3)2·3C5H11NO; Mr =2133.27 gmolÀ1; cubic,
F432 (no. 209); a=47.515(2) ꢀ; V=107271(9) ꢀ3; Z=16; 1calcd
=
0.527 gcmÀ3; synchrotron l=0.88561 ꢀ; T=208C; qmax =32.308; reflec-
tions collected/unique 71285/8014; int =0.0534, R1 =0.0686, wR2 =
R
0.2214; Flack parameter x=À0.13(2); largest diff. peak and hole: 0.397
and À0.895 eꢀÀ3
.
Synthesis and crystal structure of [Zn3(ChirBTB-2)2]·2DEF (7):
H3ChirBTB-2 (5b, 75.3 mg, 0.078 mmol) and zinc nitrate tetrahydrate
(61.5 mg, 0.236 mmol, Merck 98.5%) were dissolved in DEF (2.1 mL).
The solution was heated in a Pyrex tube at 1008C for 20 h. The resulting
yellowish crystals were washed with fresh DEF and subsequently ex-
changed with dichloromethane within three days. The product was fur-
Acknowledgements
The authors thank Dr. G. Auffermann (Max Planck Institute for Chemi-
cal Physics of Solids, Dresden) for the elemental analyses and P. Woll-
Chem. Eur. J. 2011, 17, 2099 – 2106
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2105