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)]

The 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(mu-OH) was assessed by the addition of D2O: The expected isotopic shift of ?(OH) = 3673 cm1 to ?(OD) = 2709 cm1 in the IR spectrum was observed. The OH content in bulk UiO-67, previously mildly activated at 120 degrees C under vacuum (105 Torr) overnight, was quantitatively determined by three methods: (1) By integration of the IR ?(OH) region and comparison with calibrated spectra of MCM-41 previously dehydroxylated at 500 degrees 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 degrees 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)].