Synthesis and Structural Characterization of ... - Jacobs University

Chapter 3
Unpublished Results
This connectivity type of the acetate ligands to the lanthanide centres was first reported by our
group for the lanthanum-containing Wells-Dawson dimer [{La(CH 3 COO)(H 2 O) 2 (α 2 -
P 2 W 17 O 61 )} 2 ] 16− which consists of two monolacunary Wells-Dawson α 2 -P 2 W 17 fragments
connected by a lanthanum-acetate dimer, (La 2 (CH 3 COO)(H 2 O) 4 ) 4+ . Each La 3+ ion is ninecoordinated
in a monocapped, square-antiprismatic fashion. 5a Later Mialane et al. synthesized
the [{Ln(CH 3 COO)(H 2 O) 2 (α-SiW 11 O 39 )} 2 ] 12− (Ln = Gd 3+ and Yb 3+ ) type. 5b In analogy to 21–
23 the Ln atoms are also eight-coordinated and have a distorted square-antiprismatic
geometry. The average distance between the two lanthanide atoms in 21–23 is ca. 4.11 Å and
the Ln–O–Ln bridging angle is 115.76°. The average bond distances around the lanthanide
centers in 21–23 can be grouped as follows: Ln–O(W) = 2.24 – 2.31 Å, Ln–O (C) = 2.42 –
2.51 Å, La–O (C) = 2.41 – 2.51 Å, and La–OH 2 = 2.33 – 2.44 Å. The average bond lengths
and angles of the Keggin fragment are not unusual.
Interestingly synthesis of 21–23 was accomplished by reaction of Ln 3+ ions with the tri- or
monovacant Keggin precursor ([A--GeW 9 O 34 ] 10− , [-GeW 11 O 39 ] 8− ). Considering that our
reaction was carried out at pH 4.8 and 90°C it is not entirely unexpected that we obtained the
monolacunary Keggin fragment in our product. We were also able to synthesize other
lanthanide analogues of 21–23, namely [{Ln(-CH 3 COO)(H 2 O) 2 (-GeW 11 O 39 )} 2 ] 12− (Ln =
Nd, Sm, Dy, Yb), based on the FTIR studies.
Thermogravimetric analyses (TGA) were performed on the salts of 21–23 to determine
the respective degree of hydration and the thermal stability. The thermograms of all three
compounds showed the expected weight loss domain between 25 and 400 ºC corresponding to
dehydration (lattice water molecules), see Figure 3.39. The degree of hydration was calculated
for all compounds and gave 24 water molecules per formula unit for K-21 and K-22 while 18
for KCsNa-23.
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