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Chapter 3 29<br />
between gas molecules and such coordinatively unsaturated sites (CUS) center often<br />
enhance the general adsorption properties of the MOF materials. [191-192]<br />
Figure 3.2. Modification of the Cu-sites in Cu-HKUST-1 after activation (and i.e., formation of Cu-<br />
CUS) and substrate coordination. Green, red, gray and violet balls represent Cu, O, C atoms and<br />
substrate (either gas or liquid molecules), respectively.<br />
Given the presence of metal CUSs and the permanent porosity, a number of isostructural<br />
[M3(BTC)2]n frameworks with various 3d and 4d metal-ions (M = Mo, [81, 83] Cr, [78, 83] Fe, [80]<br />
Ni, [79, 83] Ru [82, 138] and Zn [77] ) were reported later. Among them, [Cr3(BTC)2]n featuring Cr II<br />
open metal sites exhibited reversible and highly-selective binding of O2. More<br />
interestingly, among this formally homologous series as suggested by the (simplified)<br />
general formula [M3(BTC)2]n, the Fe and Ru analogs (abbreviated hereafter as Fe-MOF and<br />
Ru-MOF) were found to possess mixed-valence M II,III PW units, which might be of great<br />
use due to their potential photo and redox properties. [193] However, Fe-MOF was reported<br />
with no measurable porosity while Ru-MOF presented quite high porosity as well as good<br />
thermal and chemical stability. In fact, as a consequence of the mixed-valence state, the<br />
actual chemical composition and microstructure of these (Fe, Ru)-MOFs turned out to be<br />
more sophisticated. Counter-ions (X) and strongly bound guest molecules (G) seem to be<br />
notoriously present even after activation. The incorporation of these components as a<br />
function of synthetic conditions must be taken into account. Thus, the empirical formula<br />
of these materials should be more precisely written as [M3(BTC)2Yy]n·G g.<br />
3.1.2 Background and the state-of-the-art in research on Ru-analogs of<br />
[M3(BTC)2]n<br />
Looking through all the reports, the parent material [Cu3(BTC)2]n (HKUST-1) can be<br />
obtained through solvothermal, [35] electrochemical [194] , microwave syntheses [195] and<br />
others [196-197] . However, the synthesis of the Ru-analog is far less investigated. [82, 193] The
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