Communications
doi.org/10.1002/ejic.202001118
interaction between the terminal amino moiety with a co-
coordination. The fingerprint region shows several stretching
bands of ν(C=N), ν(C=S) and deformation vibrations of δ(HNCS)
(see Figures S18–21). Despite significant shifts in the IR spectra,
1
2
3
4
5
6
7
8
9
crystallized THF at distance of 2.17 Å. Complexes 2 (CCDC
Nr. 2043820) and 3 (CCDC Nr. 2043821) crystallized without
further THF (see Figures S4–17).
The U1À S1 and U1À N2 bond lengths of 1 of 2.829 Å and
2.497 Å compare well to other structurally similar complexes
(see below). The formation of the four membered ring leads to
1
no shifts are observed in H-NMR spectra (Figures S39–44) and
no resonances are observed in the paramagnetic region
between +100 and À 100 ppm.
Cyclic and square-wave voltammetry allowed to detect one
oxidation wave between +0.42 and +0.71 V and two reduction
waves for the ligands L1 to L3 (Table 1) at around À 2.6 V, all of
them were irreversible (see SI). A very similar behaviour was
observed for the uranium(IV) complexes, but with markedly
reduced oxidation potentials for the complexes 1 and 2, while
complex 3 retained the potential compared with L3. SEC-UV/Vis
(SEC=spectroelectrochemistry)[32] studies showed the decrease
of the main absorption at 320 nm while a new band increased
at around 360 nm for the ligands, while for the complexes only
a subtle decrease in intensity of the 320 nm band was observed
(see Figures S26–38). Upon reduction, bands at around 400 nm
were observed for both ligands and complexes. The findings
indicate a reduction essentially centred at the ligand backbone,
probably including the benzylic moiety. The cathodic shift of
the oxidation potentials for the complexes 1 and 2 compared
with their ligands, suggests a destabilisation of the highest
occupied molecular orbital in the complexes. As the differences
are not very large, we tentatively assign this shift to the
difference between the thione-like structures of the uncoordi-
nated ligands (protonated) and the thiolate character (de-
protonated) of the ligands in the complexes. For complex 3 this
shift was not observed suggesting that the effect of the double
methylation at the N3 atom and the corresponding change
from a four- (1 and 2) to a five-membered coordination (3)
compensates for this effect and the different structures for 1
and 2 compared with 3 (Figure 1) are retained in solution.
The thermogravimetric analyses show a very similar behav-
iour for complexes 1, 2 and 3 (Figures S22–24). At the first step
°
a bond angle of 57.8 for the S1À U1À N2 coordination. This also
applies for the crystal structure of 2 with bond distances of
U1À S1, U1À N1 of 2.832 Å and 2.463 Å, respectively. The
coordination environment is also a distorted dodecahedron and
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
°
the bond angle of 57.5 is similar compared to 1. These
complexes compare to only a few literature-known uranium(IV)
compounds such as [U(SBT)4(thf)]
I
(SBT=2-mercapto
benzothiazolate),[24] [Cp*U(SBT)3] II, and [(Cp*)2U(SBT)2] III,[25]
[U(Spy)4(thf)] IV (Spy=mercapto pyridine)[26] and [(Cp*)2UMe
(Spy)] V.[27] In those complexes the ligands are coordinated in a
similar fashion. However, the ligand environment is much more
rigid due to the connection to a benzothiazole ring in I–III and
a mercaptopyridine in IV and V. The bond distances and angles
are of similar values for I–V as in our complexes (UÀ S: 2.71–
°
2.87 Å, UÀ N: 2.45–2.58 Å, SÀ UÀ N: 57–61 ). There are also uranyl
(VI) complexes with this four-membered coordination
motif,[28–30] however, due to their different oxidation state these
are not directly comparable.
The largest contrast is seen when the terminal NH2 group is
fully methylated. In complex 3 the coordination environment
changes from a four membered ring to a five membered UÀ N-
NÀ C-S ring, quite drastically changing the SÀ UÀ N bond angles
°
°
to 67.0 (S1À U1À N1) and 67.7 (S2À U1À N4), respectively. This
also results in a change of color of the crystals from yellow (1
and 2) to red (3). Additionally, the bond distances change with
a shortening of the UÀ S bond to 2.739 Å (U1À S1)/2.752 Å
(U1À S2) and an elongation of the UÀ N bond to 2.535 Å
(U1À N1)/2.569 Å (U1À N4). Only five other thiosemicarbazone
complexes with uranyl(VI) are known with this five-membered
coordination motif.[21,22,31] Thus, this is the first uranium(IV)
compound showing this five membered ring and a specific
change of the coordination motif by just the methylation of an
NH2 group.
°
the complexes lose remaining solvent starting at ca. 50 C
without a DSC signal. The complexes start to decompose at ca.
°
160 C and have a large mass loss with three moderately
separated steps and three endothermic DSC signals (1: 74.0%;
2: 72.4%; 3: 75.0%). On a closer look on the first derivative of
the TGA it seems that at the beginning of the combustion of 1
and 2 the phenyl group and the two nitrogen atoms split,
followed by the release of the amine, while in 3 this order is
While L1 and L2 show absorptions at 3412 cmÀ 1 and
3373 cmÀ 1 for the terminal NH2À or NH(CH3)-group respectively,
these absorptions are shifted in complex 1 and 2 towards
3339 cmÀ 1 and 3364 cmÀ 1, respectively. L3 shows no absorption
in this region, due to the full methylation in the amino group.
But the absorption at 3170 cmÀ 1, which is assigned to the
carbazone-NH ν(NH), is no longer observed due to complex-
ation to uranium. The ν(CH)Ph vibrations of the phenyl ring
appear between 3015 cmÀ 1 and 3007 cmÀ 1 and the ν(=CH)
between 2948 cmÀ 1 and 2928 cmÀ 1. The corresponding com-
plexes show significant shifts to lower wavenumbers in these
regions, indicating a reduced bond strength due to the
complexation. Additionally, the methylated derivatives L2 and
L3 show ν(CH)Me stretching bands at 2863 cmÀ 1 and 2810 cmÀ 1,
which are slightly shifted to higher wavenumbers in complex 2
(2860 cmÀ 1) and shifted to lower wavenumbers in complex 3
(2810 cmÀ 1), which can be attributed to the different N-
°
reversed. In the next step at 350 C sulfur evaporates. The
resulting masses fit very well to uranium and some carbon. For
1 the remaining material was found to be 24.8% (U calcd.:
24.7%), for 2 it was 23.7% (U calcd.: 22.8%) and for 3 it was
23.1% (U calcd.: 21.9%). To get a detailed overview see
Table 1. Redox potentials of L1, L2, L3, 1, 2, and 3.[a]
Redox Process
L1
1
L2
2
L3
3
Oxidation
First Reduction
0.67
À 2.57
0.61
À 2.58
0.71
À 2.74
0.53
À 2.62
0.42
À 2.62
0.43
À 2.67
[a] From cyclic or square-wave voltammetry measured in 0.1 m nBu4NPF6/
THF solution at a scan rate of 100 mV/s. All processes are irreversible and
given as peak potentials.
Eur. J. Inorg. Chem. 2021, 1137–1139
1138
© 2021 The Authors. European Journal of Inorganic Chemistry published
by Wiley-VCH GmbH