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Bull. Chem. Soc. Jpn. Vol. 85, No. 4 (2012) BCSJ AWARD ARTICLE
magnetic resonance (NMR) data were collected using a Jeol
JNM-EX 400 FT-NMR spectrometer with a Jeol EX-400 NMR
data processing system. X-ray powder diffraction (XRPD) was
carried out on a MacScience M18XHF material analysis and
characterization instrument using Cu K¡ radiation. Nitrogen
adsorption isotherms were measured using a Micromeritics
ASAP2010 volumetric adsorption apparatus. The samples of
RhCu-DMF and RhZn-DMF were immersed in THF for
several days to exchange all of the included nonvolatile solv-
ates (DMF and H2O), then evacuated at rt for 5 h. Only the
RhCu-EtOH sample was evacuated at rt for 24 h. Inductively
coupled plasma atomic emission spectroscopy (ICP-AES) was
conducted on a PII NT SPS3500. Samples (3-5 mg) were
digested in concd HNO3 and heated at 120 °C until the solu-
tion became almost clear. This concentrated acid solution
was diluted with deionized H2O. However, the compositional
amount of Rh, Cu, and Zn in the solid could not be quan-
titatively determined by ICP-AES due to their poor solubility;
therefore, only the ratios of Rh:Cu/Zn were determined.
Synthesis of [Rh2(2-TMS-ethyl-1,4-bdc)4] (2). Hydrogen
2-(trimethylsilyl)ethyl 1,4-benzenedicarboxylate (1) (328 mg,
1.23 mmol) and [Rh2(OAc)4] (0.118 mg, 0.267 mmol) were
dissolved in chlorobenzene (30 mL) in a Schlenk tube with
an argon atmosphere. The mixture was heated at 150 °C for
1 day and the solvent was then removed in vacuo. The resulting
material was purified by flash chromatography on silica gel with
MeOH/CH2Cl2 = 1/19 (v/v) as the eluent to yield compound
2 as a light greenish blue solid (320 mg, 0.253 mmol, 95%).
Rf = 0.72 (MeOH/CH2Cl2 = 1/19 (v/v)). 1H NMR (CDCl3,
400 MHz): ¤ 8.17 (d, J = 8.4 Hz, 8H), 8.13 (d, J = 8.4 Hz, 8H),
4.46 (t, J = 8.4 Hz, 8H), 1.16 (t, J = 8.4 Hz, 8H), 0.10 (s, 36H,
TMS). Anal. Calcd for C52H68O16Rh2Si4 or [Rh2(2-TMS-ethyl-
1,4-bdc)2] (Mr = 1267.24): C, 49.28; H, 5.41%. Found: C,
49.50; H, 5.63%. Selected IR (KBr): ¯ (cm¹1) = 2955(m),
2898(w), 1721(s), 1685(m), 1605(m), 1561(m), 1508(w),
1400(vs), 1276(vs), 1175(w), 1143(w), 1104(m), 1018(m),
930(w), 839(s), 737(s), 566(m).
Rh2
Rh2
Rh2
Rh2
Rh2
Conventional
route
Rh2
Rh2
Rh2
Rh2
Rh2
New route
M2
Rh2
M2
M2
Rh2
Rh2
M2
M2
Rh2
Rh2
M2
M2
Rh2
= Paddle-wheel nodes
= Terephthalate
Figure 1. Schematic illustration of the proposed method.
discrete paddle-wheel dirhodium(II) tetra(1,4-benzenedicarbox-
ylate) preconstructed by the reaction of dirhodium(II) tetra-
acetate with one of the two carboxy groups of 1,4-benzenedi-
carboxylic acid, combined with labile substitution of metal
ions such as Cu2+ or Zn2+ to construct “hetero bi-paddle-
wheel” MOFs (Figure 1). Using this method, we have suc-
cessfully synthesized three rhodium-containing MOFs with 1:1
abundance ratios of paddle-wheel units of Rh2 and Cu2/Zn2,
i.e., [Rh2Cu2(1,4-bdc)4]¢4DMF (RhCu-DMF), [Rh2Cu2(1,4-
bdc)4]¢2H2O¢EtOH (RhCu-EtOH), and [Rh2Zn2(1,4-bdc)4]¢
3.8DMF (RhZn-DMF). These MOFs exhibit high crystallinity
and are without metallic rhodium by-products, despite includ-
ing the paddle-wheel Rh2 unit. These complexes exhibit
permanent porosity, which is rare for mixed-metallic MOFs,
because the hetero bi-paddle-wheel MOFs are isostructural with
the original Cu-BDC MOFs, already known to have permanent
porosity. In this paper, we report the detailed synthesis,
structures, and gas adsorption properties of the hetero bi-
paddle-wheel MOFs.
Synthesis of (TBA)[Rh2(1,4-Hbdc)3(1,4-bdc)(H2O)2] (3).
¹1
Tetrabutylammonium fluoride (1 mol L
in 1.8 mL THF,
1.8 mmol) was added to a solution of [Rh2(2-TMS-ethyl-1,4-
bdc)4] (2) (320 mg, 0.253 mmol) in THF (20 mL) and the
resulting solution was stirred at rt for 1 h. Saturated NH4Cl(ag)
(20 mL) was added to this solution and only THF was evapo-
rated. The resulting precipitate was separated by filtration and
washed with H2O to give the complex (130 mg, 0.144 mmol,
57%) as a greenish blue powder. Partial deprotonation of
carboxy group occurred, i.e., one of four carboxy groups
in BDC exists as carboxylate anion, and requires a counter
cation such as tetrabutylammonium (TBA) to balance charge.
Therefore, the accurate formula is (TBA)[Rh2(1,4-Hbdc)3(1,4-
bdc)(H2O)2]. This deprotonation does not inhibit the sub-
sequent reactions. 1H NMR (DMSO-d6, 400 MHz): ¤ 7.85
(s, 16H), 3.17 (t, J = 8.4 Hz, 8H), 1.57 (m, 8H), 1.31 (m, 8H),
0.94 (t, J = 7.3 Hz, 12H). Anal. Calcd for C48H59NO18Rh2
or (TBA)[Rh2(1,4-Hbdc)3(1,4-bdc)(H2O)2] (Mr = 1143.79): C,
50.40; H, 5.20; N, 1.22%. Found: C, 50.39; H, 5.21; N, 1.18%.
Selected IR (KBr): ¯ (cm¹1) = 2959(m), 1717(s), 1600(s),
1561(m), 1507(m), 1396(vs), 1275(s), 1144(m), 1105(m),
1018(m), 877(m), 841(m), 786(m), 739(m), 565(m).
Experimental
General Methods.
All reagents were obtained from
commercial vendors and were used without further purification.
Water was distilled and deionized prior to use. Hydrogen
2-(trimethylsilyl)ethyl 1,4-benzenedicarboxylate (1) was pre-
pared according to the literature.11
CHN elemental analyses were carried out using a Perkin-
Elmer 2400 CHNS Elemental Analyzer II. Room temperature
infrared (IR) spectra were recorded with a Jasco 4100 FT-IR
spectrometer using a KBr disk. Thermogravimetric/differential
thermal analyses (TG/DTA) were acquired using a Rigaku
Thermo Plus 2 series TG/DTA TG 8120 instrument. Nuclear