Titration of Zr3(μ‐OH) Hydroxy Groups at the Cornerstones of Bulk MOF UiO‐67, [Zr6O4(OH)4(biphenyldicarboxylate)6], and Their Reaction with [AuMe(PMe3)

Abstract

AbstractThe known MOF UiO‐67 of general formula [Zr6O4(OH)4(bpdc)6] [bpdc = biphenyldicarboxylate, O2C(C6H4)2CO2] has been synthesized on a 3 g scale and characterized by BET, TGA, XRD, IR and 13C NMR spectroscopy, and elemental analyses. The chemical accessibility of the hydroxy ligand Zr3(μ‐OH) was assessed by the addition of D2O: The expected isotopic shift of ν(OH) = 3673 cm–1 to ν(OD) = 2709 cm–1 in the IR spectrum was observed. The OH content in bulk UiO‐67, previously mildly activated at 120 °C under vacuum (10–5 Torr) overnight, was quantitatively determined by three methods: (1) By integration of the IR ν(OH) region and comparison with calibrated spectra of MCM‐41 previously dehydroxylated at 500 °C, which gave a spectroscopically measured OH content in bulk UiO‐67 of 2.2 mmol/g (37 mg OH/g); (2) by extrapolation of the OH content from the measured weight loss between 250 and 400 °C in TGA, which corresponded to 1.6 mmol OH/g (27 mg OH/g); and (3) by chemical titration of UiO‐67 with CH3MgBr and GC determination of the evolved methane, which gave 1.8 mmol OH/g (31 mg OH/g). The three methods, and in particular the latter chemical titration, are in very good to excellent agreement with the nominal OH content based on the molecular formula [Zr6O4(OH)4(bpdc)6] (expected: 1.9 mmol OH/g, 32 mg OH/g; experimental/calculated OH content = 110, 85, and 95 %, respectively, for the three methods). The weak acidity of the OH moiety in UiO‐67 was assessed by IR and 31P NMR monitoring of the physisorption of PMe3 in the UiO‐67 cavities. Inclusion of the organometallic AuI complex [AuMe(PMe3)] in a 1:1 molar ratio with respect to [Zr6O4(OH)4(bpdc)6] was achieved. Some chemisorption at 20 % of the cornerstone hydroxy sites also occurred to yield [Zr6O4(OH)3(bpdc)6(OAuPMe3)].