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30 Chapter 3
formula of [Ru3 II,III (BTC)2Cl1.5]n was initially assigned and obtained either from RuCl3 or
[Ru2(OOCCH3)4Cl]n. The powder X-ray diffraction (PXRD) data support an analogous
structure as [Cu3 II,II (BTC)2]n. Element-analytical, magnetic, X-ray photoelectron
spectroscopy (XPS) and probe molecule adsorption combined with IR spectroscopy
pointed to a mixed-valence Ru2 II,III PW units as the framework’s nodes. The Cl - counterions
are most likely strongly coordinated to the Ru-centers to balance the overall charge
of the framework. This material shows both good chemical and thermal stability;
however, the Ru-centers are partly occupied and coordinated Cl cannot be removed by
standard activation protocols. Hence, the number of active sites for catalysis and gas
sorption is obviously reduced. Later, another analog described as [Ru3(BTC)2][BTC]0.5 was
reported by Dincă et al. [83] The authors employed [Ru2(OOCC(CH3)3)4(H2O)Cl] as
ruthenium source. The absence of Cl - in the samples of [Ru3(BTC)2][BTC]0.5 was postulated
based on elemental analysis data only, and extra framework BTC serving as counter-ion
was suggested. This solid showed better Brunauer-Emmett-Teller (BET) surface area
values compared to the [Ru3 II,III (BTC)2Y1.5]n prepared first in our group. However, the
presence of guest molecules such as trimethyl-acetate or acetate (arising from CSA and/or
synthetic conditions) in the structure still could not be unambiguously determined or
ruled out. Thus, modification of the starting Ru-sources used for the synthesis seems to
be interesting for tuning the metal sites (i.e., oxidation state and CUS), composition and
stability of the HKUST-1 analogous Ru-MOFs, which in turn can have a profound impact
on other properties such as porosity and catalysis, for instance.
Previously [Ru3 II,III (BTC)2Y1.5]n has been studied where the mixed-valence Ru II,III state was
characterized in particular by CO and CO2 adsorption monitored by the FT-IR
spectroscopy at low temperature under ultra high vacuum (UHV) conditions. [198] Also, the
co-assembly of BTC with pyridine-3,5-dicarboxylate (pydc) to yield mixed-component
(linker-based) solid solution type samples of the formula [Ru3(BTC)2-x(pydc)xYy]n·G g was
achieved. These so-called “defect engineered” Ru-MOFs showed interesting new
properties (e.g. dissociative chemisorption of CO2). [138] Motivated by these promising
results, it would be of great interest to investigate the synthesis of the parent Ru-MOF in
more systematic fashion, dependence on Ru-precursors featured with diruthenium PW
units (e.g. applying CSA), variation of the reaction conditions, work-up and activation in
order to optimize the sample quality.