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Chapter 4 119
(e.g. H2ip, H3BTC, etc.) inclusion happens in the case of D6 and D8. Thus, the highest
incorporation degree of ip in Cu-BTC should be around 25% (D2 or D5). Although utilizing
Cu(NO3)2·3H2O as metal-precursor, the samples with the same incorporation level of ip
could be obtained (D5, 23% ip), the usage of nitrate salt results solids with lower porosity.
Attempts to incorporate even more ip most probably would lead to the guest inclusion in
the pores of DEMOFs. Due to less nucleophilic and basic character of “inert” [BF4] - anion
in comparison with [NO3] - , employing Cu(BF4)2·6H2O probably facilitates the selfassembly
between the metal clusters and the linkers during synthesis and could avoid the
guest inclusion
4.3.3 Oxidation state(s) of metal-sites in prepared Cu-DEMOFs
In the earlier report from Hupp and co-workers, the generation of missing Cu 2+ -nodes was
proposed. [140] In order to gain a deeper understanding of the defects generated in Cu-
DEMOFs under different synthetic conditions, D1 and D2 samples, which are synthesized
from different Cu-salts (Cu(BF4)2·6H2O) and feature high porosity as well as distinct
incorporation level of ip, have been chosen as candidates for CO probing monitored by
UHV-IR spectroscopy. It is interesting to investigate if there is any presence of Cu + in the
obtained solids. Thus, the bands observed at 2178 cm -1 for D1 and D2 appear at the same
position as in the spectra of the parent Cu-BTC and correspond to CO bound to Cu 2+ -sites
in the PW through electrostatic and σ-donation interactions (Figure 4.50). [246] Besides,
additional intense bands at 2120 cm -1 due to C-O stretching vibrations that are associated
with Cu + - species are seen in spectra of both D1 and D2. The appearance of Cu + - species
in sample D1 and D2 can origin from several possibilities:
i) “intrinsic” defects in the framework of Cu-BTC and its analogs. It is known that intrinsic
defects can be present to some extent in the MOF solids. [137, 247-249] Especially, intrinsic
defects in HKUST-1 has been described. [249-250] . The activation procedure (i.e. thermal
treatment under vacuum) lead to some decarboxylation and “missing” COO-sites, [249]
therefore, the generation of Cu + was observed. It is possible that during the thermal
treatment of D1 and D2, such intrinsic defects are formed as well.
ii) “intentional” defects