Dioxouranium(VI) complexes with 2,6-acetylpyridinebenzoylhydrazones and -semicarbazones

Article abstract of DOI:10.1016/j.ica.2004.07.011

Symmetric and asymmetric benzoylhydrazones and semicarbazones derived from 2,6-diacetylpyridine react with common dioxouranium(VI) compounds to form air-stable complexes. The neutral compounds contain doubly deprotonated ligands and possess uranium atoms with distorted pentagonal-bipyramidal coordination spheres. The equatorial coordination spheres of the metal atoms can be extended by the addition of a monodentate ligand such as pyridine or DMSO. Benzoylhydrazones and semicarbazones derived from 2,6-diacetylpyridine react with common dioxouranium(VI) compounds such as uranyl nitrate or [NBu ₄]₂[UO₂Cl₄] to form air-stable complexes. 2,6-Diacetylpyridinebis(benzoylhydrazone) (H₂L ¹), 2,6-diacetylpyridinebis(N4-phenylsemicarbazone) (H ₂L²) and the asymmetric proligand 2,6- diacetylpyridine(benzoylhydrazone)(N4-phenylsemicarbazone) (H₂L ³) give yellow products of the composition [UO₂(L)]. The neutral compounds contain doubly deprotonated ligands and possess uranium atoms with distorted pentagonal-bipyramidal coordination spheres. The equatorial coordination spheres of the metal atoms can be extended by the addition of a monodentate ligand such as pyridine or DMSO. The uranium atoms in the resulting complexes have hexagonal-bipyramidal coordination environments with the oxo ligands in axial positions. X-ray diffraction studies on [UO₂(L ¹)(DMSO)], [UO₂(L²)], [UO₂(L ²)(DMSO)] and [UO₂(L³)] show relatively short U-O bonds to the benzoylic oxygen atoms between 2.273(6) and 2.368(5) ?. This suggests a preference of these donor sites of the ligands over their imino and amine functionalities (U-N bond lengths: 2.502(7)-2.671(7) ?). The addition of a sixth ligand to the equatorial coordination sphere results in a lengthening of the metal-pyridine bonds.

Full text of DOI:10.1016/j.ica.2004.07.011

Inorganica Chimica Acta 357 (2004) 4405–4412
www.elsevier.com/locate/ica
Dioxouranium(VI) complexes with
,6-acetylpyridinebenzoylhydrazones and -semicarbazones
2
a
C.C. Gatto , E. Schulz Lang , A. Jagst , U. Abram
a
b
b,
*
a
Departamento de Quimica, Universidade Federal de Santa Maria, 97.111 Santa Maria-RS, Brazil
b
Freie Universit a¨ t Berlin, Institute of Chemistry, Fabeckstr. 34-36, D-14195 Berlin, Germany
Received 13 February 2004; accepted 10 July 2004
Available online 26 August 2004
Abstract
Benzoylhydrazones and semicarbazones derived from 2,6-diacetylpyridine react with common dioxouranium(VI) compounds
1
such as uranyl nitrate or [NBu
4
]
2
[UO
2
Cl
4
] to form air-stable complexes. 2,6-Diacetylpyridinebis(benzoylhydrazone) (H
2
L ), 2,
L ) and the asymmetric proligand 2,6-diacetylpyridine(benzoylhydrazone)-
N4-phenylsemicarbazone) (H L ) give yellow products of the composition [UO (L)]. The neutral compounds contain doubly
2
6
-diacetylpyridinebis(N4-phenylsemicarbazone) (H
2
3
(
2
2
deprotonated ligands and possess uranium atoms with distorted pentagonal-bipyramidal coordination spheres. The equatorial
coordination spheres of the metal atoms can be extended by the addition of a monodentate ligand such as pyridine or DMSO.
The uranium atoms in the resulting complexes have hexagonal-bipyramidal coordination environments with the oxo ligands in axial
positions.
1
2
2
3
X-ray diffraction studies on [UO
to the benzoylic oxygen atoms between 2.273(6) and 2.368(5) A. This suggests a preference of these donor sites of the ligands over
2
(L )(DMSO)], [UO
2
(L )], [UO
˚
2
(L )(DMSO)] and [UO
2
(L )] show relatively short U–O bonds
˚
their imino and amine functionalities (U–N bond lengths: 2.502(7)–2.671(7) A). The addition of a sixth ligand to the equatorial coor-
dination sphere results in a lengthening of the metal–pyridine bonds.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Uranium; Benzoylhydrazones; Semicarbazones; Actinides; Structure analysis
1
. Introduction
to establish a bond between the relatively ꢀhardꢁ metal
2+
ion UO2 and ꢀsoftꢁ donor sites such as the thione
sulfur atoms in thiosemicarbazones. More stable com-
plexes are expected with ꢀharderꢁ donor atoms such as
oxygen, as are provided by semicarbazones or hydra-
zones. A number of studies have been undertaken
with both ligand systems and the formation of che-
lates with various compositions has been suggested
on the basis of spectroscopic data [2]. Some of the
compounds have been proposed for the spectrophoto-
As a part of our systematic studies of the coordina-
tion chemistry of the early actinide elements, we re-
cently published a report on the structural chemistry
of thiosemicarbazonato complexes of uranium(VI)
[
1]. The formation of complexes with moderate stabil-
ity was observed with acetylpyridine thiosemicarba-
zone. The presence of the pyridine nitrogen atom
was mandatory for the formation of chelate com-
plexes. This additional donor site is obviously essential
2
+
metric determination of metal ions including UO2
3], but less is known about the composition of the
[
coordination environment of the metal. First struc-
tural reports of complexes with 2,6-diacetylpyridine-
bis(2 -pyridyl)hydrazones [4], benzoylhydrazones [5]
*
Corresponding author. Tel.: +49 30 83854002; fax: +49 30
8
38542676.
E-mail address: abram@chemie.fu-berlin.de (U. Abram).
0
0
020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.07.011

4
406
C.C. Gatto et al. / Inorganica Chimica Acta 357 (2004) 4405–4412
3
and semicarbazones [6] describe uranyl complexes hav-
ing pentagonal bipyramidal or hexagonal bipyramidal
coordination spheres. The reasons for the respective
composition of the equatorial coordination spheres
and the influence of substituents are not yet clear.
In the present paper, we describe the reactions of
uranyl nitrate and [NBu ] [UO Cl ] with 2,6-diacetyl-
2.2. H L
2
3
a
A suspension of HL (340 mg, 1.2 mmol) and ben-
zoylhydrazine (160 mg, 1.2 mmol) in 30 ml of iso-propa-
nol was heated at reflux for 36 h. The hot reaction
mixture was filtered and the bright-yellow precipitate
was washed with 5 ml of ethanol. Yield: 0.33 g (66%).
Anal. Calc. for C H N O : C, 66.7; H, 5.4; N, 20.3.
4
2
2
4
1
a
pyridinebis(benzoylhydrazone) (H L ), 2,6-diacetyl-
2
2
pyridinebis(N4-phenylsemicarbazone) (H L ), and
23 22
6
2
ꢀ1
Found: C, 66.5; H, 5.6; N, 19.9%. IR (mmax/cm ):
2
1
2
carbazone) (HL ), and the solid state structures of the
3
products. H L represents a prototype of new of
,6-diacetylpyridine(benzoylhydrazone)(N4-phenylsemi-
3
3389, 3201 (NH), 1689, 1653 (C@O). H NMR (400
MHz, dmso-d ): d (ppm): CH 2.42 (s, 3H,); 2.53 (s,
6
3
3H); phenyl 7.04–8.43 (m, 13H); NH 8.99 (s, 1H), 9.96
+
(s, 1H), 10.89 (s, 1H). FAB -MS: m/z = 415 (3%,
2
asymmetric ligands, which combine three donor func-
tionalities (aromatic amine, benzoylhydrazone and
semicarbazone) in one molecule.
+
+
[M ꢀ H] ); 105 (12%, [PhAC@O] ).
H C
CH3
NH
H C
CH3
NH
3
3
N
H C
CH3
NH
N
3
N
N
O
N
O
N
O
N
O
HN
N
O
N
O
HN
HN
N
H
N
H
N
H
R1
R1
1
a
1
1b
1
2
3
H L : R = H
H L : R = OMe
H L
2
H L
2
2
2
1
a
2.3. [UO (L )]
2
. Experimental
2
1
HL and HL were prepared by refluxing equimolar
2
1a
HL (440 mg, 1.1 mmol) was partially dissolved in 5
ml of methanol and added to a solution of UO2-
(NO ) Æ 6 H O (502 mg, 1 mmol) in 3 ml of methanol.
amounts of 2,6-diacetylpyridine (Aldrich) and ben-
zoylhydrazine (ACROS) or 4-phenylsemicarbazide (AC-
ROS) in ethanol and recrystallizing from ethanol/water
3
2
2
A clear yellow solution was formed while heating the
mixture at reflux, and a bright yellow powder began to
precipitate after about 15 min. After an overall refluxing
time of 2 h, the mixture was cooled to room temperature
and the yellow solid was filtered off.
The reaction rate can be increased by the addition of
two drops of triethylamine. Then, the precipitation of
the solid is complete after 30 min. The solid is almost
insoluble in alcohols. Yield: 633 mg (95%). Anal. Calc.
[
6,7]. [NBu ] [UO Cl ] was prepared from uranyl ni-
4 2 2 4
trate, aqueous HCl and tetrabutylammonium chloride
following a standard procedure. Infrared spectra have
been recorded for KBr pellets on a Shimadzu FT IR
ꢀ
1
spectrometer in the range between 4000 and 400 cm
.
2
HL
.1. 2,6-Diacetylpyridinemono(4N-phenylsemicarbazone),
3
a
for C H N O U: C, 41.4; H, 2.8; N, 10.5. Found: C,
5
23 19
4
ꢀ
1
4
9
1.2; H, 2.7; N, 10.5%. IR (mmax/cm ): 1590 (C@N),
00 (O@U@O).
The same procedure can be applied when the reaction
2,6-Diacetylpyridine (1.63 g, 10.0 mmol) was dis-
solved in 50 ml of a hot 1:1 mixture of ethanol and
water. A solution of 4-phenylsemicarbazide (1.51 g,
is performed with [NBu ] [UO Cl ] instead of uranyl
2
4 2
4
nitrate.
1
0.0 mmol) in 75 ml of water was slowly added to the
reaction mixture. After heating at reflux for 1 h, the col-
ourless precipitate was filtered off and washed with eth-
anol. Yield: 2.75 g (93%). Anal. Calc. for C H N O :
1
2.4. [UO (L )(DMSO)]
a
2
1
6
16
4
2
1
[UO (L )] (333 mg, 0.5 mmol) was dissolved in
a
C, 64.9; H, 5.4; N, 18.9. Found: C, 64.6; H, 5.5; N,
1
2
ꢀ
1
(
9.1%. IR (mmax/cm ): 3375, 3194 (NH), 1697,1680
about 1 ml of hot DMSO. Large red blocks deposited
upon slow cooling of the solvent. More product could
be obtained by diffusion of water into the DMSO solu-
tion. Yield: 224 mg (60%). Anal. Calc. for C H N O -
1
C@O). H NMR (400 MHz, dmso-d ): d (ppm): CH
6
3
2
NH 9.01, 10.03 (s, 2H). FAB -MS: m/z = 297
.20 (s, 3H), 2.67 (s, 3H); phenyl 7.03–8.65 (m, 8H);
+
2
5
25
5
5
+
+
(
[M ꢀ H] ); 204 (58%, [M ꢀ NH ꢀ Ph] ).
SU: C, 41.2; H, 3.4; N, 9.6; S, 4.4. Found: C, 41.2; H,

C.C. Gatto et al. / Inorganica Chimica Acta 357 (2004) 4405–4412
4407
ꢀ
1
3
(
.3; N, 9.7; S, 4.5%. IR (mmax/cm ): 1585 (C@N), 914
3. Results and discussion
UO (NO ) Æ 6H O as well as [NBu ] [UO Cl ] react
with the benzoylhydrazone or semicarbazone ligands
O@U@O).
Crystals suitable for X-ray diffraction were obtained
directly from the reaction mixture.
2
3 2
2
4 2
2
4
1
3
H L –HL in boiling methanol under complete substitu-
2
2
.5. [UO (L )]
tion of the equatorial coordination sphere of the metal.
Under these conditions, complete deprotonation of both
ligand types is observed independent of the uranium
precursor used, and neutral dioxouranium complexes
are formed. No evidence has been found for the forma-
tion of complexes with non-deprotonated or semi-
deprotonated organic ligands, as has been observed for
related semicarbazones of lanthanides and chro-
mium(III) [10], and other highly charged metal ions
2
2
The compound was prepared from UO (NO ) Æ
6H O or [NBu ] [UO Cl ] and H L following the pro-
2 4 2 2 4 2
cedure given for [UO (L )]. Yellow powder. Yield:
2
3 2
2
1
a
2
6
0%. Anal. Calc. for C H N O U: C, 39.6; H, 3.0;
23 21 7 4
N, 14.1. Found: C, 40.0; H, 3.1; N, 14.0%. IR (mmax/
ꢀ
1
cm ): 1570 (C@N), 905 (O@U@O).
[
6]. A complicated coordination pattern has also been
2
.6. Attempts to prepare [UO (L )(DMSO)]
2
described for products obtained from reactions of ura-
nyl salts and 2,6-diacetylpyridine bis(p-methoxybenzoyl-
hydrazone), where different structures have been
assigned on the basis of spectroscopic data depending
on the solvents and the uranium starting materials used
[5b]. The only compound in this report which has been
characterized by X-ray crystallography, however, is a
neutral dioxouranium(VI) complex with a doubly
deprotonated organic ligand in which the uranium atom
is seven-coordinate.
2
2
UO (L )] (348 mg, 0.5 mmol) was dissolved in about
[
ml of hot DMSO. Orange-red crystals deposited upon
2
3
diffusion of water into the DMSO solution. The crystals
that deposit from such solutions have a composition of
2
2
[
suitable for X-ray diffraction. Yield: 176 mg (45%).
UO (L )] Æ [UO (L )(DMSO)] Æ DMSO Æ H O and were
2 2 2
Anal. Calc. for C H N O SU: C, 41.2; H, 3.4; N,
5
2
5
25
5
9
.6; S, 4.4. Found: C, 41.2; H, 3.3; N, 9.7; S, 4.5%. IR
ꢀ1
(
mmax/cm ): 1572 (C@N), 908 (O@U@O).
Similar complexes are obtained when the ligands
2
1
a
3
H L , H L or H L are heated with UO (NO ) Æ 6H O
2
2
2
2
3 2
2
3
.7. [UO (L )]
or [NBu ] [UO Cl ] in boiling methanol. The reaction is
4 2 2 4
2
2
faster when a supporting base such as triethylamine is
added, but this is not mandatory. Yellow precipitates
are formed upon cooling, which have been isolated and
characterized by elemental analysis, IR and (partially)
NMR measurements showing that the products formed
from the different uranium precursors were identical.
The compounds are poorly soluble in common solvents
and, thus, do not allow a detailed NMR analysis, and
in coordinating solvents such as DMSO or pyridine a
conversion of the complexes is observed.
The compound was prepared from UO (NO ) Æ
2
3 2
3
6
cedure given for [UO (L )]. Yellow crystals suitable
H O or [NBu ] [UO Cl ] and H L following the pro-
2 4 2 2 4 2
1
a
2
for X-ray diffraction deposited directly from the reaction
mixture. Yield: 40%. Anal. Calc. for C H N O U: C,
2
3
20
6
4
4
1
(
0.5; H, 2.9; N, 12.3. Found: C, 40.1; H, 3.4; N,
ꢀ
1
1.7%. IR (mmax/cm ): 1591, 1554 (C@N), 908
O@U@O).
The U@O double bonds show characteristic IR
ꢀ
1
2
.8. X-ray structure determinations
bands between 900 and 908 cm . This value is in accord
with the neutral uranyl complexes in [5b], whereas ani-
onic derivatives absorb in the range between 920 and
The X-ray diffraction data were collected on an
ꢀ
1
automated single crystal diffractometer of the type
CAD4 (Enraf-Nonius, Delft) with Mo Ka radiation.
The structures were solved by direct methods using
SHELXS-97 [8]. Subsequent Fourier-difference map anal-
yses yielded the positions of the non-hydrogen atoms.
Refinement was performed using SHELXL-97 [9]. The
positions of the hydrogen atoms were calculated for
idealized positions and treated with the ꢀriding modelꢁ
option of SHELXL-97. Crystal data and more details
of the data collections and refinements are contained
in Table 1. Additional information on the structure
determinations have been deposited at the Cambridge
Crystallographic Data Centre. See information for
Authors, Issue No. 1.
950 cm . The proton NMR spectra prove the absence
of NH protons of the organic ligands. No evidence
was found for the coordination of methanol or water.
This allows the conclusion that complexes of the compo-
sition [UO (L)] have been formed, despite the fact that
4
the crystal structure of [UO
bis(2,6-diacetylpyridinebis(semicarbazone))) has been
reported in a previous paper [6]. No evidence could be
found for anionic uranyl complexes with non-deproto-
nated or monodeprotonated ligands. This strongly sug-
gests that the hydrazonato and semicarbazonato
complexes of the present study possess the same coordi-
nation sphere as the uranyl complex with 2,6-diacetylpy-
2
4
(L )(H O)] (H L =
2 2
2
1
b
ridine bis(methoxybenzoylhydrazone), H L , which
2

4408
C.C. Gatto et al. / Inorganica Chimica Acta 357 (2004) 4405–4412
Table 1
X-ray structure data collection and refinement parameters
1
a
2
2
3
[
UO
2
(L )(DMSO)]
[UO
2
(L )] Æ [UO
2
(L )(DMSO)] Æ DMSO Æ H
2
O
2
[UO (L )]
Formula
M (g mol
C
25
H
25
745.60
N
5
O
5
SU
C
50
H
54
N
14
O S
11 2
U
2
23 20 6 4
C H N O U
ꢀ
1
)
Crystal system
1567.25
triclinic
682.48
monoclinic
P2 /n
orthorhombic
Pnma
ꢀ
Space group
Unit cell dimensions
P1
1
a
b
c
18.130(3)
17.514(1)
8.221(3)
90
8.284(1)
8.141(2)
10.985(2)
29.120(2)
90
15.283(2)
23.700(3)
75.52(1)
88.92(1)
78.30(1)
2843.1(7)
2
a
b
c
90
90
93.71(9)
90
˚
3
V (A )
Z
2610.5(6)
4
2598.7(8)
4
ꢀ
3
D
c
(g cm
)
1.897
6.343
1.831
5.832
1.744
6.284
ꢀ
1
)
Linear absorption coefficient (mm
Absorption correction
W scans
0.1339/0.4831
6963
W scans
0.7094/0.9783
14998
DELABS [13]
0.230/0.391
3849
T
min/Tmax
Measured reflections
Independent reflections/Rint
Refined parameters
2928/0.055
178
12389/0.033
716
3048/0.049
307
2
a
R
Goodness-of-fit
1
(F)/wR
2
(F ) (I > 2r(I))
0.0459/0.0976
1.162
0.0459/0.0922
0.997
0.0677/0.1535
1.034
CCDC Deposit Nos.
230958
230959
230960
a
R1 ¼ jF o ꢀ F cj=jF oj; wR2 ¼ ½W ðF o ꢀ F c Þ =ðwF o2Þꢁꢀ1=2
2
2
2
.
shows a distorted pentagonal-bipyramidal coordination
sphere of the metal atom [b]. The bond angles between
neighbouring atoms inside the equatorial coordination
sphere are approximately 63° (4 times) and 107° (be-
tween the oxygen atoms of the benzoyl groups) [5b]
rized in Table 2. The coordination environment of the
uranium atom can be described as a slightly distorted
hexagonal bipyramid. Whereas the five donor atoms of
the hydrazonato ligand and the uranium atom are al-
most perfectly coplanar (mean deviations from the mean
˚
(
see also Table 2), which suggests the possibility of the
least-square plane: 0.0816 A, r.m.s. 0.0670), O30 devi-
˚
ated from this plane by 0.619(9) A. The phenyl groups
extension of the coordination number of the uranium
atom by the addition of an appropriate monodentate
ligand. This has been tested by heating the [UO (L)]
of the ligands are coplanar with the equatorial coordina-
tion sphere within 5°. The X–U–Y angles (X and Y are
neighbouring equatorial donor atoms) range between
59.7(2)° and 61.6(1)°. The extension of the coordination
number of the uranium atom causes lengthening of the
uranium–nitrogen bonds to the chelating ligand as can
be seen from a comparison with the corresponding data
2
complexes in a strongly coordinating solvent such as
DMSO. The yellow solids slowly dissolve in these sol-
vents and the colour changes. Whereas a dark red solu-
1
a
tion was obtained with [UO (L )], an orange-red colour
2
2
was obtained with [UO (L )]. This behaviour suggests
2
1
b
different reaction patterns for the hydrazonato and sem-
icarbazonato complexes.
A reaction sequence according to Scheme 1 can be de-
in the seven-coordinate complex [UO (L )] [5b]. Se-
2
lected bond lengths and angles of this compound are in-
cluded in Table 1; the atomic numbering has been
adopted to fit the labelling scheme applied in the present
paper. No significant change can be derived for the
bonding situation inside the hydrazonato ligand, which
is doubly deprotonated in both compounds. Whereas
the double bond between C7 and N2 is almost localized,
a delocalization of the electron density of the C17–O18
double bond can be concluded. This results in a signifi-
cant shortening of the C17–N3 bond as a consequence
of complex formation and can best be described by the
bonding scheme represented in I, irrespective of the
coordination number of the uranium atoms and the lig-
and type, and has also been found in the semicarbazo-
1
a
rived for HL . It results in the formation of a neutral
1
a
[
UO (L )(DMSO)] complex, which can be isolated in
2
crystalline form by the addition of a small amount of
water to the DMSO solution and slow cooling. The
U@O stretching vibration is slightly shifted to higher
frequencies. Similar changes in the position of the uranyl
bands in dependence on the coordination number of the
metal atom have been observed previously [5b].
1
a
Single crystals of [UO (L )(DMSO)] have been stud-
2
ied by X-ray crystallography, showing that the complex
possesses an eight-coordinate uranium atom. An ellip-
soid representation of the molecular structure is shown
in Fig. 1. Selected bond lengths and angles are summa-
2
2
nato complexes [UO (L )] and [UO (L )(DMSO)].
2
2

C.C. Gatto et al. / Inorganica Chimica Acta 357 (2004) 4405–4412
4409
Table 2
Selected bond lengths (A) and angles (°) in [UO
˚
1a
(L )(DMSO)], [UO
1b
(L )] , [UO
a
2
(L )(DMSO)] and [UO
2
(L )]
2
2
2
2
1
a
1b
(L )]
a
2
(L )(DMSO)]
2
(L )]
[UO
2
(L )(DMSO)]
[UO
2
[UO
2
[UO
2
U–O10/O11
U–O20/O12
1.771(7)
1.764(6)
2.359(5)
1.77
1.77
2.32
2.30
2.47
2.49
2.52
1.752(5)
1.763(5)
2.360(5)
2.368(5)
2.559(6)
2.583(6)
2.610(6)
2.673(6)
1.278(8)
1.259(9)
1.313(9)
1.34(1)
1.748(6)
1.762(6)
2.341(5)
2.273(6)
2.505(7)
2.503(7)
2.502(7)
U–O18/O58
U–O28/O68
b
U–N2/N42
U–N4/N44
2.617(5)
b
U–N1/N41
U–O30
2.671(7)
2.542(6)
1.265(8)
O18/O58–C17/C57
O28/O68–C27/C67
C17/C57–N3/N43
C27/C67–N5/N45
N3/N43–N2/N42
N4/N44–N5/N45
N2/N42–C7/C47
N4/N44–C9/C49
C7/C47–C6/C46
C9/C49–C2/C42
1.26
1.30
1.33
1.30
1.41
1.40
1.31
1.31
1.50
1.48
1.27(1)
1.29(1)
1.33(1)
1.32(1)
1.381(9)
1.38(1)
1.29(1)
1.28(1)
1.46(1)
1.48(1)
b
1.306(9)
b
1.380(8)
b
1.373(8)
1.385(8)
1.290(9)
1.278(9)
1.45(1)
1.303(8)
1.44(1)
b
1.47(1)
O18/O58–U–N2/N42
O28/O68–U–N4/N44
N2/N42–U–N1/N41
N4/N44–U–N1/N41
O18/–U–O30
59.7(2)
63.5
61.5(2)
61.3(2)
61.6(2)
61.1(2)
61.7(2)
63.6(2)
63.6(2)
63.8(2)
63.5(2)
b
59.7(2)
59.9(1)
63.3
63.3
63.3
b
59.9(1)
61.6(1)
120.6(2)
2.64
0
O18–U–O18 /O28
O58–U–O68
106.8
113.7(2)
105.5(2)
N1ꢂ ꢂ ꢂN2/N41ꢂ ꢂ ꢂN42
N1ꢂ ꢂ ꢂN4/N41ꢂ ꢂ ꢂN44
N2ꢂ ꢂ ꢂO18/N21ꢂ ꢂ ꢂO58
N4ꢂ ꢂ ꢂO28/N44ꢂ ꢂ ꢂO68
a
2.62
2.65
2.64
2.52
2.53
2.64
2.64
2.56
2.52
b
2.64
2.49
2.63
2.52
2.52
b
2.49
Values taken from [5b] and adopted to the atomic labelling schemes given in Figs. 1 and 2.
Symmetry related.
b
CH3
CH3
O N
H3C
HN
CH3
NH
N
O N
N
N
N
N
+
DMSO
N
O
N
O
in MeOH
3
H C
H3C
+ UO2(NO3)2
O
N
N
U
O
O
N
N
U
O
-
2 HNO3
O
S
O
O
H3C
CH3
Scheme 1.
different coordination environments around uranium
in the same crystals is somewhat surprising in light of
the fact that the extension of the coordination number
H C
CH3
1
3
N
U
proceeds readily at [UO
vent molecules (H O and DMSO) are present. The
2
(L )] and that additional sol-
N
N
N
2
I
N
structures of the complex molecules are illustrated in
Fig. 2. The sulfur atom of the DMSO ligand in
R
O
O
R
R = Ph, NHPh
2
UO (L )(DMSO)] is disordered over two positions.
[
The overall bonding pattern in [UO (L )(DMSO)] is
2
2
2
1
similar to that in [UO (L )(DMSO)] and [UO (L )-
a
4
2
2
2
UO (L )] and [UO (L )(DMSO)] co-crystallize as
2
[
large orange-red plates when [UO (L )] is recrystallized
(H O)], with the uranium atom in an almost regular hex-
2
2
2
2
agonal-bipyramidal coordination environment. The
pentadentate semicarbazonato ligands are almost
2
from hot DMSO. The presence of two complexes with

4410
C.C. Gatto et al. / Inorganica Chimica Acta 357 (2004) 4405–4412
C4
perfectly planar in both complexes. There are maximum
deviations from the mean least-squares planes which in-
C5
C6
2
clude all atoms of the L2 ligands, with the exception of
ꢀ
C8
˚
the phenylrings of the semicarbazone units, of 0.18 A
O20
C7
N2
N1
2
r.m.s.: 0.0747) in [UO (L )] and 0.081 A (r.m.s.:
˚
(
0.0351) in [UO (L )(DMSO)]. The phenyl rings are
2
2
U
N3
2
slightly twisted against these planes with angles between
.1° and 11.8°. The oxygen atom of the DMSO ligand in
C17
C16
2
[
O10
O30
2
UO (L )(DMSO)] has a distance of 1.26 A to the plane
O18 C11
C12
˚
2
formed by the donor atoms of the pentadentate ligand.
It is interesting to note that the increase of the coordina-
tion number and the observed lengthening of the ura-
nium–nitrogen bonds are not related to an increase of
the bite angles of the binding sites, which can be esti-
mated by the intramolecular distances between the do-
nor atoms of the five-membered chelate rings. The
corresponding values are also listed in Table 2. A length-
C15
C14
C13
S30
C30
1
a
Fig. 1. Ellipsoid presentation [14] of [UO
ellipsoids represent 50% probability.
2
(L )(DMSO)]. Thermal
(
a)
C44
C43
C45
C46
˚
ening of the U–N1 bond by 0.18 A is the result of the
C48
2
coordination of a DMSO ligand to [UO (L )], despite
the fact that the resulting long U–O30 bond length
C42
C49
2
C50
C47
N42
O12
N41
˚
of 2.673(6) A reflects an only weak interaction (cf.
N44
˚
uranium–DMSO bond lengths of 2.542(6) A in [UO -
N43
2
U2
N45
C67
1a
(L )(DMSO)] or a mean value of 2.38 A in [UO (DM-
˚
C57
2
SO) ][ClO ] [11]).
5
4 2
N46
C51
O58 O11
The U–N bond lengths are longer in the eight-coordi-
nate [UO (L)(solvent)] complexes irrespective of the
O68
N47
2
C61
semicarbazonato or benzoylhydrazonato side arms. This
is another indication that the observed bond lengthening
is due to steric overcrowding of the equatorial coordina-
tion spheres in the compounds with coordination num-
(
b)
C4
ber eight.
C3
C5
C6
3
The proligand H L contains one benzoylhydrazone
2
C2
C10
and one semicarbazone side arm and allows the coordi-
nation of metal atoms in an asymmetric manner. The
3
synthesis of H L succeeded by a stepwise condensation
C9
C8
C7
N2
N1
O20
N4
2
N5
of 2,6-diacetylpyridine with N4-phenylsemicarbazide
and benzoylhydrazine according to Scheme 2. This se-
quence is mandatory, since the reaction of 2,6-diacetyl-
pyridine with semicarbazide always yields significant
amounts of the bis-condensation product irrespective
of the amounts of reactants applied. The general route
of Scheme 2 can also be applied for the synthesis of
other asymmetric ligands carrying Schiff base or thio-
semicarbazone side arms. It gives approach to ꢀtailor-
madeꢁ ligand systems in terms of ꢀhardꢁ and ꢀsoftꢁ
C27
N3
C17
U1
C11
C21 N7
O28
O10
O30
O18
N6
2
(L )] and (b)
Fig. 2. Ellipsoid presentations [14] of (a) [UO
2
UO (L )(DMSO)]. Thermal ellipsoids represent 50% probability.
2
[
2
H C
CH3
H C
CH3
3
3
N
N
PhNHC(O)NHNH2
(1:1)
PhC(O)NHNH2
1:1)
N
O
N
O
N
O
O
H3C
CH3
HN
(
HN
NH
N
EtOH/H O
i-PropOH
N
H
2
N
H
O
O
HL3a
H2L3
Scheme 2.

C.C. Gatto et al. / Inorganica Chimica Acta 357 (2004) 4405–4412
4411
donor atoms and/or their protonation/deprotonation
behaviour.
duction of disproportional steric strain. This has been
3
proven by heating [UO (L )] in DMSO, which results
2
3
Uranyl nitrate and [NBu ] [UO Cl ] react with H L
2
in the formation of a deep red solution and the forma-
tion of the DMSO adduct is suggested with respect to
4
2
2
4
in methanol to yield a yellow complex of the composi-
3
tion [UO (L )]. Expectedly, both side arms of the lig-
1
the similar behaviour of [UO (L )] and partially
2
2
2
[UO (L )]. The red crystals, however, decompose rap-
and are deprotonated and a neutral dioxouranium
compound is formed. The U@O frequency is found
2
idly when they are removed from the solvent to give
an yellow-orange powder, the analysis of which did
not give satisfactory evidence for an intact DMSO ad-
duct. A mixed composition of the material containing
ꢀ
1
at 908 cm . The molecular structure of this com-
pound, which is the first example of a mixed benzoyl-
hydrazone/semicarbazone complex, is shown in Fig.
3
3
3
. Selected bond lengths and angles are summarized
[UO (L )] and [UO (L )(DMSO)] is strongly suggested,
2 2
in Table 3. The bonding situation inside the chelate
rings is similar to those in the symmetric benzoylhydra-
zone and semicarbazone complexes. As has been ob-
served previously, the U–N bond lengths are shorter
in the seven-coordinate complexes compared with their
analogues with coordination number eight, and almost
localized C–N double bonds between the hydrazine
nitrogen atoms and their neighbours can be stated.
but we were not yet able to grow single crystals suita-
ble for an X-ray structure analysis.
Fig. 4 shows a comparison of the coordination
1
a
2
2
spheres of [UO (L )(DMSO)], [UO (L )] [UO (L )
2
2
2
3
(DMSO)] and [UO (L )] containing uranium with the
2
coordination numbers seven and eight. Particularly,
the side views indicate the steric hindrance inside the
equatorial coordination spheres of the metal atoms.
The views along the uranyl units, however, show
almost regular hexagons for the equatorial coordina-
The O18–U–O28 angle of 109.9(6)° is larger than the
2
corresponding value in [UO (L )] and should allow
the extension of the coordination sphere without intro-
2
1
a
2
tion spheres of [UO (L )(DMSO)] and [UO (L )
2 2
(DMSO)], and the space between the oxygen donor
atoms of the complexes with coordination number
seven.
C4
C3
C5
The present study shows that acetylpyridine benzoyl-
hydrazones and semicarbazones are well suited to form
stable complexes with dioxouranium(VI) units. All
hydrazone and semicarbazone units of the ligands are
deprotonated in the uranium complexes studied in this
paper. This is in contrast to the behaviour in some lan-
thanide complexes where no or only partial deprotona-
C2
C6
C7
C8
C9
N1
C10
O20
N2
N4
N3
C17
N5
N7
U
C27
O10 O18
O28
C11
tion of the NH groups was observed [12]. The
3
successful synthesis of the asymmetric ligand H L and
C21
2
its uranium complex may provide a valuable tool for
developing ligand systems, which can perfectly fit the
coordination requirements of lanthanide and actinide
ions.
3
(L )]. Thermal ellipsoids
Fig. 3. Ellipsoid presentation [14] of [UO
represent 50% probability.
2
Table 3
Selected bond lengths (A) and angles (°) in [UO
˚
3
(L )]
2
U–O10
U–O20
U–O18
U–O28
U–N1
1.726(1)
1.70(2)
2.30(1)
2.33(1)
2.54(2)
2.53(2)
2.55(2)
1.32(2)
C17–N3
N2-N3
1.31(2)
1.39(2)
1.28(2)
1.44(3)
1.36(2)
1.25(3)
1.43(2)
1.36(3)
N2–C7
C7–C6
C27–O28
C27–N5
N4–N5
C27–N7
U–N2
U–N4
C17–O18
N1–U–N2
N1–U–N4
O18–U–28
61.2(5)
62.4(6)
109.9(6)
N2–U–O18
N4–U–O28
62.7(5)
63.7(6)
N1ꢂ ꢂ ꢂN2
N1ꢂ ꢂ ꢂN4
2.58
2.64
N2ꢂ ꢂ ꢂO18
N4ꢂ ꢂ ꢂO28
2.52
2.59

4412
C.C. Gatto et al. / Inorganica Chimica Acta 357 (2004) 4405–4412
1
a
2
2
3
2 2 2 2
Fig. 4. Representation [15] of the coordination spheres of (a) [UO (L )(DMSO)], (b) [UO (L )], (c) [UO (L )(DMSO)] and (d) [UO (L )] in top and
side views with respect to the O–U–O axis illustrating the distortions of the equatorial coordination spheres of the complexes in dependence on the
chelating ligands and the coordination number of the metal atoms.
Acknowledgements
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