Synthesis and structural study of tetravalent (Zr4+, Hf4+, Ce4+, Th4+, U4+) metal complexes with cyclic hydroxamic acids

Article abstract of DOI:10.1002/ejic.201403206

Six- and seven-membered cyclic hydroxamic acids, such as 1-hydroxypiperidine-2-one (1H, 1,2-PIPOH) and 1-hydroxyazepan-2-one (2H), have recently been identified in some mixed siderophores as one of their three chelating subunits. Compared to their ubiquitous noncyclic counterparts, cyclic hydroxamates are preorganized for metal binding. Surprisingly, the coordination chemistry of these bidentate, monoanionic ligands remains virtually unknown, even in the case of iron(III). We report herein the first structural study of the complexes of 1^- and of 6^-, an unsaturated seven-membered ring analog of 2^-, with tetravalent cations of transition metals (zirconium and hafnium), lanthanide (cerium), and actinides (thorium and uranium). Structural characterization by means of X-ray crystallography of the corresponding ML₄ complexes evidenced distorted square antiprismatic coordination geometries with the exception of U⁴⁺, which favors a dodecahedral arrangement. 1-Hydroxypiperidine-2-one (1H, 1,2-PIPOH) is a cyclic hydroxamic acid that occurs in several siderophores as one of the three Fe^III binding units. It can form neutral homoleptic complexes with tetravalent transition-metal (Zr and Hf), lanthanide (Ce), and actinide (Th and U) cations with a diversity of coordination stereochemistries ranging from the square antiprism to the dodecahedron.

Full text of DOI:10.1002/ejic.201403206

FULL PAPER
DOI:10.1002/ejic.201403206
Synthesis and Structural Study of Tetravalent (Zr4+, Hf4+,
Ce4+, Th4+, U4+) Metal Complexes with Cyclic
Hydroxamic Acids
Pawel Jewula,[a] Jean-Claude Berthet,[b] Jean-Claude Chambron,*[a]
Yoann Rousselin,[a] Pierre Thuéry,[b] and Michel Meyer*[a]
Keywords: Zirconium / Hafnium / Thorium / Uranium / O ligands / Siderophores
Six- and seven-membered cyclic hydroxamic acids, such as
1-hydroxypiperidine-2-one (1H, 1,2-PIPOH) and 1-hy- bered ring analog of 2–, with tetravalent cations of transition
the complexes of 1– and of 6–, an unsaturated seven-mem-
droxyazepan-2-one (2H), have recently been identified in
some mixed siderophores as one of their three chelating sub-
units. Compared to their ubiquitous noncyclic counterparts,
cyclic hydroxamates are preorganized for metal binding. Sur-
prisingly, the coordination chemistry of these bidentate,
monoanionic ligands remains virtually unknown, even in the
case of iron(III). We report herein the first structural study of
metals (zirconium and hafnium), lanthanide (cerium), and ac-
tinides (thorium and uranium). Structural characterization by
means of X-ray crystallography of the corresponding ML4
complexes evidenced distorted square antiprismatic coordi-
nation geometries with the exception of U4+, which favors a
dodecahedral arrangement.
homolog 2H through attachment of an amino function at
position 3, as shown by optically active 3H and 4H (Fig-
ure 2), which were used as building blocks in the total syn-
thesis of natural siderophores.[2b,4b]
Introduction
Siderophores are natural iron(III) chelators that are se-
creted by mycobacteria and fungi when the bioavailability
of this essential nutrient and growth factor is low.[1] As Fe3+
is six-coordinate, most siderophores carry or incorporate
three chelating units that usually derive from catechol or
belong to the family of hydroxamic acids [–N(OH)C(=O)–].
Often linear, this latter motif shows restricted rotation
around the central C–N bond and thus prevails in solution
in the trans form, thereby minimizing the electrostatic repul-
sion between both oxygen donors that occurs in the less
stable cis conformation. However, in several instances this
chelating moiety can be incorporated into a six- or seven-
membered ring structure that makes it preorganized for
metal chelation. Indeed, siderophores such as exochelin
(Figure 1),[2] amamistatin,[3] mycobactin,[4] fluorescent
pseudobactin,[5] and tsukubachelin,[6] just to mention a few
of them, feature three different chelating subunits. Of these
subunits, a terminal one derives from either 1-hydroxypiper-
idine-2-one (1H or 1,2-PIPOH) or its seven-membered ring
Figure 1. Structural formula of exochelin MN. The coordinating
atoms of the three bidentate chelating units are highlighted in red.
[a] Institut de Chimie Moléculaire de l’Université de Bourgogne,
ICMUB, UMR CNRS 6302,
9 avenue Alain Savary, BP 47870,
21078 Dijon Cedex, France
E-mail: jean-claude.chambron@u-bourgogne.fr
michel.meyer@u-bourgogne.fr
www.icmub.fr
[b] CEA Saclay, DSM-IRAMIS-NIMBE, LCMCE, UMR CNRS
3685,
Figure 2. Structural formulae of selected cyclic hydroxamic acids
discussed herein. The atom-labeling scheme used for the assign-
ment of NMR spectroscopic resonance signals is also indicated.
91191 Gif-sur-Yvette, France
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201403206.
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Among these simple cyclic hydroxamic acids, 1H or 1,2- meric species of 2:2 stoichiometry upon coordinating to
PIPOH represents the saturated version of the aromatic 1- Zr4+ or Hf4+ rather than to form a mononuclear 1:1 com-
hydroxy-2(1H)-pyridinone (5H or 1,2-HOPO), a heterocy- plex. Intrigued by this unexpected behavior, we undertook
clic analog of catechol rarely found in natural siderophores, a detailed crystallographic analysis of a series of neutral
with the exception of cepabactin.[7] In spite of the rarer complexes that incorporates tetravalent 4d and 5d transi-
natural occurrence of siderophores based on 5H, the coor- tion metals (Zr4+ and Hf4+), as well as 4f (Ce4+) and 5f
dination chemistry of this ligand – or multidentate systems (U4+ and Th4+) cations with the prototypical six-membered
that incorporate it – with transition-metal,[8] lanthanide,[9] cyclic hydroxamate 1–. We show, in particular, that the coor-
and actinide[10] cations has been studied in detail, whereas dination geometry of U4+ is significantly different from that
that of biologically relevant 1H and 2H, and associated sid- of zirconium and hafnium in the tetraleptic [M(1)4] com-
erophores, seems to have been ignored with very few excep- plexes, which is an important piece of information when
tions.[11]
considering the design of specific chelators for actinides.
We have recently been engaged in a research program Moreover, we also present herein the synthesis and charac-
aimed at the design and study of chelators specific for tetra- terization of an olefinic seven-membered analog (6H) that
valent transition metals such as zirconium(IV) or haf- is reminiscent of 2H (Figure 2) together with the corre-
nium(IV), which are classically considered as “cold” surro- sponding [Zr(6)4] complex.
gates for radioactive plutonium(IV) owing to their strong
Lewis acid character. The binding of these 8-/9-coordinate
Results and Discussion
tetravalent cations by catecholate or hydroxamate ligands
was found to follow that of six-coordinate high-spin
iron(III).[12] Indeed, Pu4+ and Fe3+ are chemical analogs
with similar charge over ionic radius ratios (z/ri) and Lewis
acidities, both form highly insoluble hydroxides, and they
are taken up by microorganisms or stored in mammals by
similar biological mechanisms. Moreover, the positron-
emitting isotope 89Zr4+ has emerged as one of the most
promising radionuclides for antibody-based positron emis-
sion tomography (PET) imaging. The success of 89Zr-la-
beled radioimmunoconjugates rests on the relatively low en-
ergy of the β+-emitted particles (E = 395.5 keV) and on the
half-life of 78.4 h, which is commensurate with the biodis-
tribution kinetics of IgG antibodies. Thanks to these unique
properties, high-resolution PET images of tumors can be
collected several days after injection once binding of the
biological material to the target tissues has reached its opti-
mum while clearance of the unbound fraction from the
blood stream has occurred. Currently, radiopharmaceuti-
cals that incorporate desferrioxamine B (DFB) as zirco-
nium chelator are under clinical trials. DFB is the emblem-
Synthesis, Structures, and Properties of Cyclic Hydroxamic
Acids 1H, 2H, and 6H
Despite its simple structure, there is no high-yielding syn-
thetic procedure known today for preparing the cyclic hy-
droxamic acid 1H, whereas 2H is even less accessible. Com-
pound 1H was prepared according to Scheme 1 in 20%
yield by treating cyclopentanone with Piloty’s acid [PhS-
(O)2NHOH].[14]
Scheme 1. Synthesis of compound 1H from cyclopentanone and
Piloty’s acid.[14]
Subjecting cyclohexanone to a similar ring-expansion re-
atic hexadentate open-chain trishydroxamic siderophore, action did not allow us to isolate 2H in appreciable
excreted by Streptomyces strains and produced at the indus- amounts, in agreement with the literature.[14b] Therefore, to
trial level as a decorporating drug in the treatment of prepare compound 2H, we decided to adapt the multistep
iron(III) or aluminum(III) overloads.[1] Albeit nontoxic and route developed for making the amino-functionalized op-
showing only renal clearance, the in vivo stability of tically active analog 4H in the context of the synthesis of
[Zr(DFO)]+ bioconjugates is not sufficient to avoid partial (–)-cobactin T.[15] Accordingly, 5-pentenoic acid 7 was
dissociation and subsequent mineralization of the osteo- treated first with trityl-protected hydroxylamine 8 to pro-
philic 89Zr4+ cation in bones. As a matter of fact, the search duce 9 in 75% yield (Scheme 2). Interestingly, isobutyl chlo-
for more efficient ligands is currently actively pursued.[8c,13] roformate (IBCF) used as coupling agent[16] gave better re-
In that respect, we envisioned that it would be interesting sults than 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide/
to explore the potential of chelators based on 1H or 2H, 1-hydroxybenzotriazole (EDC/HOBt). Next, Pd-catalyzed
which have been ignored so far, for the coordination of tet- allylation of 9 with methyl allyl carbonate afforded diolefin
ravalent metals, including actinides. As a first step towards 10 in 80% yield. Ring-closing metathesis of 10 using the
that goal, an octadentate tetrahydroxamic calix[4]arene po- Grubbs II ruthenium catalyst produced the trityl-protected
dand that bears four terminal binding motifs derived from cyclic hydroxamic acid 11 in 73% yield. ORTEP views and
3H has been prepared and structurally characterized.[13a] crystallographic details of the intermediates 9, 10, and 11
This receptor, which can be considered to be the first ever can be found in the Supporting Information. Finally, hydro-
abiotic ligand that possesses only cyclic six-membered hy- lytic deprotection of 11 by means of BF3·Et2O in CH2Cl2/
droxamate groups, was shown to self-assemble into a di- MeOH released cyclic hydroxamic acid 6H in 44% yield.
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Scheme 2. Synthesis of compound 6H and attempted formation of 2H by simultaneous hydrogenolysis of the trityl protecting group and
hydrogenation of the double bond. The atom-labeling scheme used for the assignment of NMR spectroscopic resonances is also indicated.
Attempts to obtain 2H in pure form by one-pot hydrogen- almost planar arrangement of the hydroxamic fragment en-
ation/hydrogenolysis of 11 using Pd/C as catalyst failed, as forced by sp2-hybridized atoms N1 and C1 is evidenced by
the corresponding amide 12 was produced alongside with the two torsion angles of 1.7(2) and –8.2(2)° for C5–N1–
2H in a 1:2 ratio. Moreover, 12 could not be separated on C1–C2 and O2–N1–C1–O1, respectively. Ring-puckering
a preparative scale from 2H. Nevertheless, slow evaporation parameters (Figure 3)[18] and the endocyclic torsion angles
of a solution obtained by dissolving a sublimed sample of are close to the ideal values expected for the half-chair con-
that mixture in CH2Cl2/heptane afforded crystals of 2H that formation of cyclohexane, with C3 being the most out-of-
were suitable for X-ray diffraction studies. In turn, X-ray- plane atom (Table S1 in the Supporting Information).[19a]
quality crystals were directly obtained by sublimation (1H
Comparison of the twist angles and Cremer–Pople ring-
and 6H) or by slow evaporation of aqueous solutions (1H). puckering coordinates obtained for 2H and 6H (Figure 3)
Figure 3 shows ORTEP views[17] of the molecular struc- with those calculated for model cycloheptane[19b,19c] and cy-
tures of cyclic hydroxamic acids 1H, 2H, and 6H together cloheptene[20] (Tables S2 and S3 in the Supporting Infor-
with the atom-labeling scheme. Relevant distances, which mation) indicates that 2H adopts a slightly distorted chair
all fall within the same ranges [C1–O1 1.245(3) Å, N1–O2 conformation along the line that passes through the C4
1.397(3) Å, C1–N1 1.332(2) Å], are provided in the Sup- atom and bisects the N1–C1 bond, whereas 6H assumes
porting Information. The crystallographic structure of the a biplanar boat-sofa (also denoted as hinge) conformation
anhydrous form of 1H has been solved by Banerjee and owing to the adjacent trigonal–planar C4 and C5 olefinic
King in the orthorhombic Pbca space group with two carbon atoms. Intermolecular hydrogen bonds were found
1
hydrogen-bonded molecules of 1H in the asymmetric in both cases [O2···O1(1 – x, /2 + y, 2 – z) 2.601(3) Å, O2–
unit.[14b] We provide herein a more detailed structural de- H2···O1 173.2° for 2H; O2···O1(1 – x, y, 1/2 – z) 2.651(1) Å,
scription for the monohydrated form. Recrystallization in O2–H2···O1 162.3° for 6H].
water after sublimation of chromatographically purified
The acid–base properties of the cyclic hydroxamic acids
compound afforded colorless X-ray-quality crystals of 1H and 6H have been assessed in aqueous solution [I =
1H·H2O. The structure was solved in the monoclinic P21/c 0.1 m KNO3, T = 285.2(2) K] by means of glass-electrode
space group with four molecular units per unit cell. The potentiometry. Both compounds behave as weak mono-
co-crystallized water molecule is hydrogen bonded to three protic acids,[21] with pKa values of 8.85(2) and 9.02(8),
neighboring molecules of 1H. The strongest involve the N- respectively. By comparison, the pKa measured under iden-
hydroxy group and the water oxygen atom O3 [O2···O3 tical conditions for the open-chain analog N-methyl aceto-
2.585(1) Å, O2–H2···O3 175.0°], whereas the two others in- hydroxamic acid (NMAH) is 8.68(3),[21d] which is in excel-
volve the water protons and the carbonyl oxygen atom O1 lent agreement with literature data.[22] Owing to the cyclic
that belong to two distinct molecules of 1H [O3···O1(x, structure of 1– and 6–, both oxygen atoms are enforced to
3/2 – y, z – 1/2) 2.798(1) Å, O3–H3C···O1 156.6°; O3···O1(2 – adopt a cis arrangement, whereas the open-chain NMA–
1
3
x, y + /2, /2 – z) 2.738(1) Å, O3–H3D···O1 171.5°]. The anion is present as a mixture of cis and trans isomers in
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[(Me3Si)2N]2An[κ2-(N,C)-CH2Si(Me)2N(SiMe3)]2 (denoted
hereafter [AnN*2(N,C)]) with An = U[25] and Th,[26] respec-
tively. Four equivalents of 1H and one equivalent of
[AnN*2(N,C)] were combined in pyridine using Schlenk
techniques. The complexes [Th(1)4] and [U(1)4] were iso-
lated in 78 and 90% yields as crystalline compounds by
slow diffusion of pentane into the pyridine solution.
1
The H and 13C NMR spectra of the diamagnetic zirco-
nium, hafnium, cerium, and thorium complexes have been
1
interpreted using a combination of H–1H COSY, 13C–1H
HSQC, and HMBC correlations. They show only one set
of signals, supporting the presence of either a single isomer
or of a fluxional molecule. A comparison of the NMR spec-
troscopic data of ligand 1H and its complexes with Zr4+
,
Hf4+, and Ce4+ (Table 1) evidences only slight upfield shifts
induced by metal binding for all CH2 proton and C=O car-
bon atom resonances, the largest effect being observed for
the hydrogen atoms located in para (H-3) and meta (H-4)
positions with respect to the nitrogen atom, respectively.
Conversely, Th4+ produces overall much more pronounced
upfield shifts among the series of diamagnetic cations with
Δδ values of up to 0.56 ppm in the case of H-3. As a matter
of fact, the relative magnitude of Δδ increases in the order
Zr ≈ Ce Ͻ Hf ϽϽ Th, irrespective of the considered signal.
The electron-richer system 6H behaves in a similar way to
Figure 3. ORTEP views[17] of the six- and seven-membered cyclic
hydroxamic acids 1H, 2H, and 6H showing the atom-labeling
scheme. The thermal ellipsoids are drawn at the 50% probability
level. According to ring-puckering analysis,[18] 1H adopts a half-
chair conformation with a total puckering amplitude QT of 0.500
(θ = 131.4°, φ = 22.7°), whereas 2H and 6H assume a slightly dis-
torted chair (QT = 0.786, q2 = 0.445 Å, φ2 = 301.1°, q3 = 0.648 Å,
φ3 = 279.8°) and boat-sofa (QT = 0.843, q2 = 0.776 Å, φ2 = 316.8°,
q3 = 0.332 Å, φ3 = 267.8°) conformation, respectively.
1
1H upon formation of [Zr(6)4]. The H NMR spectrum of
solution.[21d,22b,23] Since the cis form gives rise to a greater
electrostatic repulsion energy, the slightly higher basicity of
the cyclic versus linear binding motifs (ΔΔG ≈ –1.0 and
–1.9 kJmol–1 for 1– and 6–, respectively) can be ascribed to
the higher strain relief that occurs upon protonating the N–
O– group located in the vicinity of the carbonyl oxygen
atom. It is, however, interesting to note that both |ΔΔG|
values are of the same order of magnitude as the unfavor-
able Gibbs free energy associated with the cisǞtrans isom-
erization of NMAH (ΔGcis/trans = 2.55 kJmol–1, which cor-
responds to Kcis/trans = 2.8 in D2O).[23]
the paramagnetic [U(1)4] complex spans a wider chemical-
shift range than the others; it extends from δ = –10.2 to
17.8 ppm, as expected for a 5f2 electronic configuration.
1
Table 1. Selected H NMR,[a] 13C NMR,[a] and IR spectroscopy[b]
data for 1H, 6H, and their zirconium, hafnium, cerium, thorium,
and uranium tetraleptic complexes.
δH-2 δH–3 δH–4 δH–5 δH–6 δCO νC=O νN–O
1H[c]
2.45 1.81 1.95 3.63
2.44 1.71 1.83 3.58
–
–
–
–
–
–
164.5 1604
161.9 1608
161.9 1612
926
931
933
925
[Zr(1)4][c]
[Hf(1)4][c] 2.42 1.70 1.82 3.57
[Ce(1)4][c] 2.44 1.72 1.80 3.51
163.6 1587
[U(1)4][d]
1597 925
[e]
[e]
[e]
[e]
[e]
Preparation and Spectral Characterization (NMR, IR) of
the Complexes
[Th(1)4][d] 2.14 1.25 1.34 3.52
162.4 1607
925
897
901
6H[c]
2.68 2.38 5.84 5.71 4.29 169.6 1655
2.62 2.27 5.73 5.58 4.31 165.9 1609
[Zr(6)4][c]
Zirconium(IV), hafnium(IV), and cerium(IV) complexes
of either ligand 1H or 6H were formed by using identical
methodologies. Four equivalents of ligand and one equiva-
lent of the corresponding acetylacetonate (acac–) precursor
complex [M(acac)4] were combined in methanol at room
[a] Units: ppm versus TMS. [b] Units: cm–1. [c] CDCl3. [d] [D5]pyr-
idine. [e] See Experimental Section and Supporting Information.
As shown in Table 1, neither the C=O nor the N–O vi-
temperature.[24] The three complexes [Zr(1)4], [Hf(1)4], and bration frequencies of compound 1H seem to be signifi-
[Ce(1)4] were isolated in crystalline form by slow evapora- cantly affected upon complexation as revealed by FTIR
tion of CH2Cl2/heptane solutions in 57, 59, and 50% yields, spectroscopy, whereas deprotonation by NaOH lowers the
respectively. [Zr(6)4] was obtained similarly in 59% yield. energy of the corresponding stretching modes by approxi-
Tetraleptic thorium(IV) and uranium(IV) complexes were mately 10 cm–1. This result is quite surprising in light of the
prepared in non-hydrolytic and anaerobic conditions. This changes in bond lengths observed by X-ray crystallography
is a particularly critical issue for the former, as the [U(L)4] (see below) and earlier observations,[8a,27] but it might re-
complexes that involve hydroxamate ligands are known to flect intermolecular hydrogen-bonding effects in 1H. In
readily oxidize in O2-containing solutions and to afford contrast, the νC=O band that appears at 1655 cm–1 for 6H
uranium(VI) [UO2(L)2] species, with the concomitant re- shifts to lower energies by 46 cm–1 upon zirconium uptake,
lease of 2 equiv. of the amide derivative of L.[20] The as might be anticipated when the carbonyl oxygen atom
actinide source was the organometallic amide precursor takes part in a chelate cycle as found in [Zr(6)4] (see below).
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X-ray Crystal-Structure Analyses of the Tetravalent
Zirconium, Hafnium, and Uranium Complexes
Crystals collected for four complexes, namely, [Zr(1)4]·
2CH2Cl2, [Hf(1)4]·2CH2Cl2, [U(1)4]·4C5H5N, and [Zr(6)4]·
2CH2Cl2 turned out to be suitable for X-ray diffraction
analysis. Crystallographic parameters, data collection, and
refinement information in tabular form are available in the
Supporting Information. Within the [M(1)4] series, at least
one hydroxamate chelate (rings A and B in [Zr(1)4], ring A
in [Hf(1)4], and ring C in [U(1)4]) exhibits two or three
slightly disordered methylenic carbon atoms spread over
two positions (see the Supporting Information). Through-
out, labels of the disordered atoms and hydroxamate rings
associated with the lowest site-occupation factors (sof) are
starred. ORTEP views[17] of the major conformers found
in the unit cell are displayed in Figure 4, whereas relevant
geometrical parameters (bond lengths and bite angles) are
collected in Table 2. Likewise, cycle C in the structure of
[Zr(6)4] was also found to be disordered over two positions,
denoted C and C* in Figure 5 for the major [Zr(6)4]-A and
minor [Zr(6)4]-B species, respectively. However, the differ-
ence resides in that case in the relative orientation of the
entire chelating unit C and C* with respect to the three
other ones. For [Zr(6)4]-A, the carbonyl groups that belong
to rings A, B, and C point towards the same direction,
which is opposite to that of ring D, whereas in the case of
[Zr(6)4]-B the C=O bonds assume an alternating up-and-
down orientation.
Coordination of ligand 1– increases on average the C1–
O1 carbonyl bond length by 0.080 (Zr4+), 0.085 (Hf4+), and
0.065 Å (U4+), and decreases both the N1–O2 and C1–N1
distances by 0.070 and 0.044 Å for Zr4+, 0.074 and 0.038 Å
for Hf4+, 0.060 and 0.033 Å for U4+. Likewise, the corre-
sponding bond length changes upon formation of [Zr(6)4]-
A are 0.058, –0.046, and –0.036 Å [ca. 0.05, –0.06, and
–0.02 Å for [Zr(6)4]-B], respectively. This is in agreement
with the recognized electronic delocalization along the O1–
C1–N1–O2 bond sequence in the metal complexes.[27] As
expected, the average Zr–ON distances are overall shorter
than the Zr–OC ones, although the difference (Δd) depends
both on the metal ion and ligand sizes. Whereas the six-
membered hydroxamate group 1– gives rise to a rather sym-
metrical chelate cycle when bound to Zr4+ or Hf4+ (Δd ≈
0.016 Å), the mean bond-length difference is about twice as
much for [U(1)4] (ca. 0.03 Å) and for both [Zr(6)4] conform-
ers (ca. 0.04 Å). It is worth noting that the evolution of
the M–OC and M–ON bond lengths perfectly reflects the
variation in ionic radii of these eight-coordinate M4+ ions
(Table 2), which shows a linear dependence within the
[M(1)4] series (Figure S23 in the Supporting Infor-
mation).[28] The same trend was also observed for 1,2-
HOPO– complexes with 4f and 5f elements, such as in
Figure 4. ORTEP views[17] showing the atom-labeling scheme of
the major conformer found in the crystal structures of (a) [Zr(1)4]·
2CH2Cl2, (b) [Hf(1)4]·2CH2Cl2, and (c) [U(1)4]·2CH2Cl2. The ther-
mal ellipsoids are drawn at the 50% probability level, whereas sol-
vent molecules, hydrogen, and disordered carbon atoms are omit-
ted for clarity. For [Zr(1)4] and [Hf(1)4], equivalent atoms are lab-
eled with a superscript and are generated using the following sym-
1
metry code: 1 – x, y, /2 – z.
Table 2. Average values of selected bond lengths and bite angles
derived from the X-ray crystal structures of ligands 1H and 6H,
and their zirconium, hafnium, and uranium tetraleptic complexes.[a]
M–OC M–ON
C=O
[Å]
N–O–
[Å]
OC–M–ON
[°]
[Å]
[Å]
1H
–
–
1.247(2) 1.397(1)
1.327(7) 1.327(4) 70.2(4)
1.332(6) 1.323(6) 70.6(4)
–
[Zr(1)4]
[Hf(1)4]
[U(1)4]
6H
2.209(3) 2.19(1)
2.195(4) 2.18(1)
2.39(1) 2.36(2)
1.31(2)
1.34(1)
65.1(3)
–
–
–
1.245(2) 1.394(1)
[Zr(6)4]-A 2.216(6) 2.180(9) 1.303(6) 1.348(9) 70.4(7)
[Zr(6)4]-B 2.22(1) 2.18(1) 1.30(2) 1.34(2) 70.3(6)
[a] Ionic radii of M4+ (CN = 8) are 0.84 Å for Zr4+, 0.83 Å for
[M(5)4] (M4+ = Ce4+, Pu4+ [10e]
)
and in the nine-coordinate
Hf4+, and 1.00 Å for U4+ [28]
.
[Th(5)4(H2O)] complex.[10a,10b]
As for the free ligand, the conformations of the six-mem-
bered rings of the bound ligand 1– in the [M(1)4] complexes the endocyclic torsion angles with those of cyclohexane[19a]
(M4+ = Zr4+, Hf4+, U4+) were assessed by comparison of and by ring-puckering analysis (Table S1 in the Supporting
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In the four complexes studied, the metal cation is bound
to eight oxygen atoms. The corresponding coordination
polyhedra are generally described in terms of three ideal-
ized geometries with equal distances between the center and
all vertices (i.e., uniform M–O distances): the hard-sphere
square antiprism (SAP), the dodecahedron (DD) with non-
regular trigonal faces, and the bicapped trigonal prism
(BTP). However, other remarkable spatial arrangements of
the eight donor atoms can be considered. Among the most
likely ones to be observed, the so-called Johnson versions
of the bicapped trigonal prism (J-BTP) and of the dodeca-
hedron, also known as the snub disphenoid (SD), which
possesses equilateral triangular faces, should be men-
tioned.[29a] These two polyhedra are characterized by un-
equal M–O distances and thus cannot be inscribed into a
sphere. Clearly, assigning a distorted polyhedron to one of
these ideal arrangements is not a straightforward task with-
out the help of adequate structural descriptors. As a general
approach, shape analysis of coordination polyhedra pro-
vides a quantitative means for evaluating the departure of
an experimental (Q) from an ideal archetypal stereochemis-
try (P). The continuous shape measure (CShM) criterion
SQ(P) [Equation (1)] introduced by Avnir and co-workers
reflects the minimal normalized square deviation between
the Cartesian coordinates of each vertex that belongs to the
Figure 5. ORTEP views[17] showing the atom-labeling scheme of
the neutral tetraleptic complexes found in the crystal structure of
[Zr(6)4]·4C5H5N. The thermal ellipsoids are drawn at the 50%
probability level, whereas pyridine molecules and hydrogen atoms actual coordination polyhedron Q and a perfect reference
geometry P.[29] Accordingly, SQ(P) ranges between 0 and
are omitted for clarity. The disordered part of the minor conformer
[Zr(6)4]-B found in the unit cell is represented with dashed bonds
100, a closer structural match that gives a lower SQ(P)
and nonshaded atoms (the atomic coordinates of the corresponding
value. Casanova and co-workers reported shape maps and
hydroxamate ring, labeled C*, have been refined isotropically).
reference polyhedra for eight-vertex structures.[29a]
Information). For [Zr(1)4], both crystallographically dis-
tinct rings are partially disordered over two positions. Cy-
cles A and B for the major conformer and A* for the less
abundant one adopt all three a nearly half-chair conforma-
tion as in the case of the free ligand, whereas ring B* is
close to an envelope arrangement with atom C3B* protrud-
(1)
ing out of the mean plane. In the case of [Hf(1)4], rings
A and B display a nearly perfect half-chair conformation,
The three lowest values of SQ(P), as determined by con-
whereas ring A* is intermediate between a half-chair and tinuous shape measure calculations using the SHAPE 2.1
an envelope. For [U(1)4], ligands A, B, C, C*, and D adopt program (Table 3),[30] correspond systematically to the three
all an approximately half-chair structure. Overall, compari- ideal geometries SAP, DD, and BTP, whereas much larger
son of the conformations of ligand 1– in the protonated and deviations are found for the Johnson J-BTP and SD polyhe-
complexed forms confirms that 1– is highly preorganized dra in spite of the intrinsic dissymmetry of the hydroxamate
for metal chelation, as anticipated from the cis orientation binding units and the slightly longer M–OC versus M–ON
of the oxygen coordination atoms. The same conclusion can distances. According to Table 3, the metal centers in [Zr-
be drawn when comparing the approximate boat-sofa con- (1)4], [Hf(1)4], and [Zr(6)4]-B are found in an approximate
formation taken up by the seven-membered hydroxamate square-antiprismatic environment, whereas the coordina-
groups in 6H and [Zr(6)4]. Owing to the increased electronic tion sphere of the uranium complex [U(1)4] can be best de-
delocalization over the O2, N1, C1, and O1 atoms when the scribed as pseudododecahedral. The situation is somewhat
oxygen atoms are interacting with a metal cation, this set less clear in the case of [Zr(6)4]-A, the coordination polyhe-
of atoms remains nearly planar while the N1–C1 bonds dron of which is only slightly closer to a bicapped trigonal
shorten. Hence, the four five-membered chelate cycles prism than to a dodecahedron but not too far away from a
formed by 1– or 6– upon binding to Zr4+ or Hf4+ adopt an square antiprism. However, this apparent intermediate
envelope conformation in which the metal ion lies out of character might be due in some part to the structural disor-
the mean plane by 0.36–0.48 Å, whereas they become essen- der that affects ligand C and the associated uncertainty in
tially planar in [U(1)4] as a consequence of the lengthening the coordinates of both pairs of oxygen atoms O1C/O2C*
of the M–O distances (the largest twist angle reaches 4.2°). and O2C/O1C* (Figure 5).
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Table 3. Continuous shape measure values SQ(P) calculated for the
coordination polyhedra found in the crystal structures of com-
plexes [Zr(1)4]·2CH2Cl2, [Hf(1)4]·2CH2Cl2, [U(1)4]·4C5H5N, and
[Zr(6)4]·2CH2Cl2. Limiting values corresponding to the ideal refer-
ence geometries are also included.[a]
Polyhedron (Q) SQ(SAP)
SQ(DD)
SQ(BTP)
Geometry
[Zr(1)4]
0.636
0.586
3.949
1.278
0.711
0
1.797
1.745
2.034
1.165
1.127
2.848
0
1.859
1.831
2.727
1.099
1.538
2.095
2.546
0
SAP
SAP
DD
BTP
SAP
[Hf(1)4]
[U(1)4]
[Zr(6)4]-A
[Zr(6)4]-B
Ideal SAP
Ideal DD
Ideal BTP
2.848
2.095
2.546
[a] Bold-faced numbers correspond to the lowest SQ(P) values cal-
culated according to Equation (1) by the SHAPE 2.1 program. The
considered ideal reference geometries (P) are the square antiprism
(SAP), the trigonal-faced dodecahedron (DD), and the spherical
bicapped trigonal prism (BTP).
Figure 6. Shape map in the SAP–DD space that shows the extent
to which the investigated coordination geometries deviate from
both ideal shapes. The minimal distortion interconversion path is
drawn as a red line. The positions of the ideal bicapped trigonal
prism (BTP) and of the Johnson bicapped trigonal prism (J-BTP)
are also shown as open circles. Blue triangles correspond to com-
plexes incorporating open-chain monohydroxamates [R–N(O–)-
C(=O)–RЈ] for which the atomic coordinates were retrieved from
the Cambridge Structural Database: [Zr(LMe,Me)4] (R = RЈ = Me),
A visual description of the distortion level encountered
by coordination spheres with respect to two ideal structures
is conveniently provided by a shape map. On the basis of
the CShM values collected in Table 3, such a map has been CSD code TEQSAL, ref.[22b]; [Hf(LPh,Ph)4] (R = RЈ = Ph), CSD
code BPHXHF01, ref.[32a]; [Ce(LPh,tBu)4] (R = Ph, RЈ = tBu), CSD
constructed in the relevant SAP–DD space (Figure 6), to-
gether with the limiting line that corresponds to the mini-
mal distortion path for a series of intermediate stereochem-
code YILGUX, ref.[32b]; [Th(LiPr,neo-Pn)4] (R = iPr, RЈ = neo-pentyl),
CSD code IPHTHB, ref.[27]
istries obtained by continuous interconversion of a D4d
square antiprism towards a D2d dodecahedron and vice sured by the BAAB torsion angle τ, and the dihedral angle
versa.[31] Clearly, the steric constrain imposed by the cyclic χ between both trapezoids can be used for evaluating the
hydroxamate binding units 1– and 6– results in significantly distortion level of an experimental coordination sphere in-
larger distortions than expected for intermediate SAP–DD termediate between a D4d square antiprism (τ = –38.41°, χ
structures. The same conclusion can be reached in the case = 79.33°) and a D2d dodecahedron (τ = 0°, χ = 90°).
of [U(1)4] if the DD–BTP path is considered.
A detailed analysis of nearly square-antiprismatic coordi-
To gain a deeper insight into the structural properties of nation geometries first requires the identification of both
the complexes characterized herein, a more detailed analy- pseudosquare faces. For [Zr(1)4], least-squares calculations
sis of the coordination polyhedra needs to be undertaken. show that the sets of four atoms that deviate less from a
For an ideal square antiprism, the spatial arrangement of mean plane are O1A (–0.121 Å), O2A (0.120 Å), O1B1
the eight donor atoms around the metal center can be de- (–0.120 Å), and O2B1 (0.121 Å); the second mean plane
scribed by several stereochemical parameters (Figure S24 in that comprises the symmetry-related atoms is generated by
the Supporting Information), such as the normalized bite the 21 axis passing through the metal center. Distortion
[b = d(O···O)/d(M···O)], the tetragonal twist angle formed from a regular SAP is reflected by a nonzero dihedral angle
by both tetragonal faces rotated along the C4 axis (φ = 45°), (δ = 14.3°) between the (O1A, O1B1, O2A) and (O1A,
the tilt angle between both tetragonal faces (ω = 0°), the O1B1, O2B1) triangular faces. Nevertheless, both mean pla-
interplanar or centroid–centroid (Ct···Ct) distance, the edge nes are almost parallel; they are tilted by no more than 0.8°
lengths, and dihedral angles between adjacent faces (δ). The and separated by 2.335 Å. Hence, the zirconium atom is
exact metal position is given by the lateral displacement located exactly halfway between them (dver = 0 Å) but it
with respect to the C4 axis (dlat = 0 Å), which corresponds is not strictly aligned with both centroids (dlat = 0.01 Å,
ideally to the line that joins the centroids of both tetragonal Ct···Zr···Ct 179.0°). Looking along the Ct1···Ct2 axis, both
faces, and the vertical deviation from the Ct···Ct midpoint the top and bottom idealized square faces are rotated by
(dver = 0 Å).[31] In turn, for an ideal dodecahedron of D2d 48.4(1)° relative to one another. In the crystal structure of
point-group symmetry, the eight vertices divide into two [Hf(1)4]·2CH2Cl2, the four oxygen atoms found in the asym-
sets of four sites each, conventionally labeled A and B, metric unit are almost coplanar [O1A (0.116 Å), O2A
which correspond to the intersection of either four or five (–0.117 Å), O1B (0.118 Å), and O2B (–0.116 Å) in which
edges, respectively (Figure S25a). Hence, this polyhedron the numbers in parentheses correspond to the atomic dis-
can be described as two interpenetrating BAAB trapezoids tances to the mean plane]. The symmetry-related plane is
that lie in mutually perpendicular mirror planes, which in- obtained by a C2 axis that runs through the hafnium cation
tersect along the S4 axis (Figure S25b). The degree of pla- located 1.164 Å above and below both essentially parallel
narity of the four corners that define the trapezoids, as mea- mean planes (ω = 0.56°). Considering the very close S(SAP)
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shape measures found for [Hf(1)4] and [Zr(1)4] as well as
the similar ionic radii of both metal centers, the geometric
parameters that describe both coordination polyhedra are,
unsurprisingly, virtually identical (Table 4).
Table 4. Parameters describing the coordination geometries of com-
plexes [Zr(1)4], [Hf(1)4], [U(1)4], and [Zr(6)4].
Parameter[a]
[Zr(1)4] [Hf(1) [Zr(6)4]- [Zr(6)4]-B [U(1)4]
]
4
A
b
1.150(7) 1.156(8) 1.153(9) 1.152(9) 1.077(9)
δ [°]
14.3
14.0
11.1,
27.9
2.17
11.1,
18.2
1.90
67.8,
67.6
ω [°]
φ [°]
Ct1···Ct2 [Å]
dlat [Å]
dver [Å]
0.80
0.56
n.r.[b]
n.r.[b]
3.911
0.061
0.029
48.4(1) 48.0(2) 45(1)[c]
43(1)[d]
2.340
0.019
0.019
2.335
0.010
0
2.327
0.011
0
2.347
0.045
0.023
[a] See text for the definition of each parameter. [b] Not relevant.
[c] Individual values are 44.0, 46.3, 44.2, 45.9°. [d] Individual values
are 41.6, 43.5, 41.7, 43.8°.
The structural results obtained thus far for the six-mem-
bered 1,2-PIPO– ligand can be understood within the elec-
trostatic repulsion model proposed by Kepert.[33] Accord-
ingly, the normalized bite b is the key parameter to consider
for predicting the stereochemical outcome upon [M(L)4]
metal complex formation. Bidentate ligands that give rise
to b values larger than 1.25 are expected to span four out of
the eight lateral edges of a SAP, thereby affording a helical
Figure 7. (a,c) Schematized and (b,d) crystallographic views of the
coordination polyhedra of (a,b) [Hf(1)4] and (c,d) [U(1)4] with
identification of vertices according to their type (A when inter-
secting four edges, B for five edges). Edges of the same color belong
to the same mean trapezoidal plane and serve as an eye guide for
a direct comparison between the MO8 frameworks of [Hf(1)4] and
[U(1)4].
structure of Λ or Δ chirality of maximal D4 symmetry that O2B, O1B, O1D, O2D (τ = 9.52°) for the other. The dihe-
resembles a four-bladed propeller. Clearly, neither the six- dral angle between the corresponding mean planes (χ =
or seven-membered cyclic hydroxamates, nor their open 89.79°) comes very close to the ideal value of 90°. The
chain analogs (Figure 6), fulfill that requirement for any of pseudo-S4 axis can be defined as the line that passes
the investigated tetravalent cations. The reason for this is through the centroids Ct1 and Ct2 that correspond to the
the angular opening of the bidentate ligand that is too O1A···O1C and O1B···O1D edges, respectively. As for the
small, as reflected by the ON···OC distance that is too short. previous structures, the U4+ cation does not perfectly sit at
The largest normalized bites are observed for Zr4+ and Hf4+ the center of the polyhedron; it is laterally displaced from
(Table 4) with values in the range of 1.15–1.16. These are the Ct1···Ct2 axis by 0.061 Å (Ct1···U···Ct2 176.45°) and
typically found for polyhedra intermediates between SAP lies 0.029 Å away from the midpoint of the centroid in the
and DD, in which both ligand pairs are arranged along the direction of Ct2. The four hydroxamate groups span the so-
opposite top and bottom square edges, a binding mode de- called m edges according to the nomenclature of Hoard and
noted hereafter as facial. Figure 7 (a,b) depicts the coordi- Silverton,[34] which lie on the mirror planes in an ideal D2d
nation polyhedra of [Hf(1)4], thus highlighting the spatial trigonal-faced dodecahedron (m edges connect B- and A-
arrangements of the oriented ligands. The vertices are iden- type vertices within both interlocked trapezoids as shown
tified according to the number of intersecting edges (A for in Figure 7, c). Interestingly, a similar spatial arrangement
four, B for five).
of the aromatic 1-oxy-2-pyridonate (5–) anions has been re-
Octacoordinated metals that possess larger ionic radii ported by Raymond and co-workers for the dodecahedral
than those exhibited by d elements, typically 4f and 5f ele- [Ce(5)4] complex, although the polyhedron is strongly dis-
ments, clearly give rise to longer M–O bonds and thus torted towards a SAP.[10e] However, the same authors found
smaller normalized bite values. For the latter, Kepert’s a diversity of coordination stereochemistries when the same
model predicts a smooth switch from SAP towards dis- ligand was crystallized with either thorium or plutonium.
torted DD structures that reach at highest an ideal D2d sym- Indeed, the former complexes are nine-coordinate with a
metry. In practice, this situation is almost systematically en- [Th(5)4(L)] formula and resembling a D3h tricapped trigonal
countered for b values up to about 1.11,[33] which is clearly prism for L = H2O[10a] or a distorted monocapped square
the case of [U(1)4] (b = 1.077). The salient structural fea- antiprism for L = MeOH.[10b] In turn, [Pu(5)4] adopts an
tures of the coordination sphere [Table 4 and Figure 7 (c,d)] intermediate DD–BTP geometry.[10e] More recently, this
indeed confirm the prediction. As expected for a dodecahe- group also described the crystal structure of a strongly dis-
dron, two interpenetrating trapezoids can be identified that torted dodecahedral Th4+ complex with an octadentate bi-
correspond to the following sequences of nearly planar brachial macrocyclic ligand that incorporates two bidentate
atoms: O2A, O1A, O1C, O2C (τ = 2.94°) for the one and terephthalamide moieties in the ring structure and two
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others in the appended arms. This receptor wraps around monohydroxamates, which are appended with either alkyl
the cation with both dangling units located on the same or phenyl groups with a general formula of R–N(O–)-
side of the macrocycle, thus allowing each binding motif to C(=O)–RЈ [(LR,RЈ)–]. The corresponding CShM criteria are
span the m edges of the DD.[35]
included in the SQ(DD)/SQ(SAP) shape map of Figure 6.
With its larger ring size expanded by one carbon atom, Among them, the tetraleptic thorium complex of ligand
the cyclic ligand 6– gives rise to normalized bites almost (LiPr,tBu)– (R = isopropyl, RЈ = tBu) evidences a rarely ob-
identical to those of 1– when bound to Zr4+. This is not served distorted coordination polyhedron intermediate be-
unexpected considering the sp2 character of the hydroxamic tween a cube [S(cube) = 2.604] and a dodecahedron [S(DD)
C1 and N1 atoms. In spite of that, 6– is more rigid owing = 2.746] [with S(SAP) = 5.739; not shown in Figure 6]. Ex-
to the steric constrain imposed by the double bond. This is changing the tBu for a neo-pentyl C substituent affords an
probably the reason why less regular and thus less predicta- approximately bicapped trigonal prismatic arrangement of
ble coordination polyhedra are observed, at least in the case the oxygen donor atoms around the thorium center [S(BTP)
of zirconium. In comparison with [Zr(1)4] [S(SAP) = 0.636], = 2.926]. However, as outlined by the authors, the smallest
the slightly higher shape measure found for [Zr(6)4]-B dihedral angle between two adjacent trigonal faces is
[S(SAP) = 0.711] is in line with a more distorted SAP. The 35.5°.[27] On the basis of that consideration, they described
flattest basal mean plane, defined by atoms O1A (0.093 Å), the polyhedron as a DD in spite of a higher shape measure
O2A (–0.092 Å), O2B (0.092 Å), and O1B (–0.093 Å), is value [S(DD) = 3.187]. Interestingly, the zirconium and haf-
folded along the O1B···O2A direction with a dihedral angle nium complexes of ligands (LMe,Me)– and (LPh,Ph)– have co-
of 11.1° between both triangular faces. The pair of oppos- ordination polyhedra that closely resemble a dodecahedron
ing faces is slightly more bent (δ = 18.2°) along the with S(DD) values of 1.099 and 0.744, respectively,[22b,32a]
O1C*···O2D edge, since the four oxygen atoms O1C*, whereas square antiprismatic structures are favored for the
O2C*, O2D, and O1D deviate more from their mean plane cyclic hydroxamates 1– and 6– with the same cations. Like-
(0.154, –0.157, 0.157, –0.154 Å, respectively). Overall, the wise, binding of (LPh,tBu)– to the larger Ce4+ ion generates
higher polyhedral distortion is also reflected by a larger tilt a square antiprismatic [S(SAP) = 1.269] rather than a do-
angle between both O4 planes (ω = 1.90°), more dispersed decahedral [S(DD) = 2.301] geometry.[32b] However, Fig-
values for the four twist angles (φ = 41.6–43.8°), as well as ure 6 clearly evidences that the data points corresponding
the decentering of the zirconium atom in both lateral (dlat to the four complexes formed with open-chain hydroxamato
= 0.019 Å) and vertical directions (dver = 0.019 Å, Zr···Ct1 ligands lie further away from the minimal distortion path
1.151 Å, Zr···Ct2 1.189 Å). The coordination polyhedron of in the shape map than those pertaining to the [Zr(1)4],
the second conformer [Zr(6)4]-A, co-crystallized in the [Hf(1)4], and [Zr(6)4] complexes. The steric hindrance pro-
asymmetric unit, evidences even higher distortions with re- vided by bulky N and C substituents cannot fully explain
spect to a SAP (Table 4). Shape analysis reveals in fact an this rather counterintuitive result if one considers the posi-
intermediate structure (Figure S26 in the Supporting Infor- tion of the less-encumbered [Zr(LMe,Me)4] complex in the
mation), which can be best viewed as a bicapped trigonal DD–SAP shape map. As several subtle contributions, in-
prism (Table 3) despite the nonparallel (ω = 11.8°) and cluding crystal packing effects, might impact the final lay-
slightly twisted arrangement (φ = 8.2, 5.9, 4.1°) of both tri- out of the coordination sphere, it appears difficult to estab-
gonal faces (O1A, O2A, O2D) and (O1B, O2B, O2C). Both lish rules of thumb with any predictive value at the current
respective centroids lie 2.940 Å apart from each other and stage of knowledge. To that end, more structures have to be
form an angle with the zirconium cation of 157.5°, instead collected.
of 180° for a regular BTP. The metal is displaced both verti-
cally from the central position towards the (O1B, O2B,
O2C) plane (dver = 0.028 Å, Zr···Ct2 1.472 Å, Zr···Ct1
1.526 Å) and horizontally (dlat = 0.292 Å from the Ct1···Ct2
Conclusion
axis) towards the O1C and O1D atoms capping both tetrag-
This work has established that deprotonated 1-hydroxy-
onal faces (O1C···Zr···O1D 128.8° instead of the ideal value piperidine-2-one (1,2-PIPO–), a bidentate chelating motif
of 120°). Accordingly, the hydroxamate binding units lab- present in several siderophores, forms tetraleptic [M(L)4]
eled A and B in Figure 5 span both short edges of the un- complexes without additional ligands with a range of d
capped quadrangular face. The O–C–N–O mean plane of transition-metal (Zr, Hf), 4f (Ce), and 5f (Th, U) tetravalent
each chelating unit is bent by 71.0° (ligand A) and 67.2° cations, and that the metal center can be found in distinct
(ligand B) with respect to the corresponding triangular coordinating environments. This result is quite significant
faces (O1A, O2A, O2D) and (O1B, O2B, O2C).
with regard to the lack of information on this subject, and
A comparison with crystal structures reported for tetra- highly useful with respect to the design of specific chelators
kis(hydroxamato) complexes revealed several interesting for tetravalent metals, with applications in medical imaging
points. Firstly, literature data are surprisingly scarce, as (89Zr4+) and sequestration of highly toxic actinides such as
atomic coordinates for only five complexes could be re- Pu4+. Considering our previous work on a C4-symmetric
trieved from the Cambridge Structural Database.[22b,27,32] podand (L) that bears four terminal 1,2-PIPO– chelating
All of them are homoleptic species and incorporate a tetra- units appended to a calix[4]arene scaffold,[13a] the facial in-
valent cation (Zr4+, Hf4+, Ce4+, Th4+) bound to open-chain stead of lateral binding mode observed herein for the nearly
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1-Hydroxypiperidine-2-one (1H): A 2 m aqueous solution of sodium
hydroxide (120 mL) was added to a chilled solution of cyclopenta-
none (2.90 mL, 0.32 mol) in ethanol (60 mL). Subsequently, a solu-
tion of Piloty’s acid (11.00 g, 0.64 mol) in ethanol (60 mL) was
added dropwise over 45 min at 0 °C to the reaction mixture, which
was stirred for 20 h at room temperature. Ethanol was removed
under reduced pressure, and the aqueous layer was extracted with
diethyl ether (2ϫ20 mL). After adjusting its pH to 4.5 by the ad-
dition of 2 m HCl, the aqueous solution was extracted with CHCl3
(8ϫ50 mL). The organic phase was dried with MgSO4, filtered,
square-antiprismatic [Zr(1)4] and [Zr(6)4] complexes pro-
vides a straightforward explanation for the formation of a
[M2(L)2]-type supramolecular assembly rather than the de-
sired [M(L)] stoichiometry. It now becomes clear that a tet-
rapodal ligand design does not appear to be well suited for
creating useful 89Zr4+ sequestering agents. For the larger
actinides, dodecahedral coordination geometries are fa-
vored, whereby the ligands span the so-called m-edges that
lie on the mirror planes of the ideal structure. As a matter
of fact, when ligands that incorporate four bidentate units and concentrated on a rotary evaporator to give a sticky orange
residue, which was purified by flash chromatography (silica gel,
CH2Cl2/MeOH, 96:4 v/v). Pure colorless crystals were obtained by
sublimation (40 °C, 2ϫ10–4 mbar), yield 0.751 g (20%), m.p. 61.4–
are conceived for coordinating tetravalent cations, either
linear or macrocyclic topologies able to wrap around the
metal should be favored. Investigations are currently un-
derway to exemplify these guidelines, and further work will
concern the thermodynamic and kinetic study of the inter-
action in solution of these ligands with M4+ ions, as well as
their coordinating behavior with M3+ ions such as Fe3+ and
its diamagnetic Ga3+ surrogate, in relation to siderophore
chemistry.
63.0 °C; ref.[14a] 55–57 °C. IR (ATR): ν = 3422 (br., m), 3239 (br.,
˜
m), 2963 (m), 2771 (br., m), 1604 (s, νC=O), 1349 (m), 1165 (m),
1
1093 (m), 926 (m) cm–1. H NMR (600 MHz, CDCl3, 298 K): δ =
3
1.81 (m, 2 H, 3-H), 1.95 (m, 2 H, 4-H), 2.45 (t, J = 6.0 Hz, 2 H,
3
2-H), 3.63 (t, J = 6.0 Hz, 2 H, 5-H), 8.91 (br. s, 1 H, OH) ppm.
13C NMR (150 MHz, CDCl3, 298 K): δ = 21.0 (C-3), 23.2 (C-4),
30.9 (C-2), 48.9 (C-5), 164.5 (C-1) ppm. HRMS (ESI): m/z calcd.
+
for C5H10NO2 116.0703 [M + H]+; found 116.0706; calcd. for
C5H9NO2Na+ 138.0521 [M + Na]+; found 138.0526. C5H9NO2
(115.13): calcd. C 52.16, H 7.88, N 12.17; found C 51.92, H 8.11,
N 12.22.
Experimental Section
N-(Trityloxy)pent-4-enamide (9): Isobutyl chloroformate (2.5 mL,
2.595 g, 19.0 mmol) was added to a solution of 4-pentenoic acid
(2.20 mL, 2.002 g, 20.0 mmol) and N-methylmorpholine (2.0 mL,
1.871 g, 18.5 mmol) in anhydrous THF (200 mL) at –10 °C, and
the reaction mixture was stirred for 15 min. Afterwards, O-tritylhy-
droxylamine (5.275 g, 19.2 mmol) and an additional amount of N-
methylmorpholine (4.1 mL, 3.743 g, 37.0 mmol) were added. After
further stirring at –10 °C for 1 h, the reaction mixture was stirred
at room temperature for 4 h. Next, it was poured into 2 m HCl
(350 mL). The resulting mixture was extracted with ethyl acetate
(3ϫ100 mL), and the organic layer washed cautiously with water
(3ϫ100 mL), 5% aqueous sodium hydrogen carbonate
(3ϫ100 mL), brine (100 mL), water (100 mL), and then dried with
MgSO4. The crude material obtained after solvent removal was
crystallized from heptane (150 mL) to give 5.22 g of colorless crys-
tals in 75% yield. An analytical amount was further purified by
flash column chromatography (silica gel, CH2Cl2/MeOH, 99:1 v/
v). X-ray-quality crystals were obtained by slow evaporation of a
CH2Cl2/MeOH (99:1 v/v) solution, m.p. 149–151 °C. 1H NMR
(300 MHz, CDCl3, 298 K): δ = 1.68 (br. t, 2 H, 3-H), 1.99 (br. s, 2
H, 2-H), 4.87 (br. d, 1 H, 5-Ht), 4.88 (dd, J = 1.3, J = 10.0 Hz, 1
H, 5-Hc), 5.59 (br. m, 1 H, 4-H), 7.28–7.55 (m, 15 H, H–Ar), 7.76
(br. s, 1 H, NH) ppm. 13C NMR (75 MHz, CDCl3, 298 K): δ =
27.7, 30.6, 93.6, 115.0, 128.1, 128.3, 129.2, 137.3, 141.2, 176.5 ppm.
LRMS (ESI): m/z calcd. for C24H23NO2Na+ 380.43 [M + Na]+;
found 380.18. C24H23NO2 (357.44): calcd. C 80.64, H 6.49, N 3.92;
found C 80.29, H 6.42, N 3.66.
General Information: All experiments were carried out under argon
(Ͻ5 ppm oxygen or water) in anhydrous solvents using a Schlenk
vessel and vacuum-line techniques or in a glovebox. Glassware was
dried overnight at 100 °C before use. Unless otherwise noted, start-
ing materials were obtained from commercial suppliers and used
without further purification. [Ce(acac)4] was synthesized from
[Ce(acac)3·2H2O].[24] Both precursors, [(Me3Si)2N]2An[κ2-(N,C)-
CH2Si(Me)2N(SiMe3)]2 (An =U,[25] Th[26]), hereafter denoted
[AnN*2(N,C)], were prepared as described previously. Toluene,
pentane, and tetrahydrofuran (THF) were dried with a mixture of
sodium benzophenone, whereas pyridine was dried with potassium.
All solvents were distilled immediately before use. Deuterated pyr-
idine (Eurisotop) was distilled from potassium and stored over 3 Å
molecular sieves in a glovebox. Methanol was heated under reflux
conditions over magnesium turnings (10% w/v) and iodine (1% w/
v). Fourier-transform infrared spectra (FTIR) were recorded either
with a Bruker ALPHA-P DTGS ATR spectrophotometer or, in the
case of [U(1)4] and [Th(1)4], in transmission mode as Nujol mulls
between KBr round-cell windows with a PerkinElmer FTIR 1725X
instrument. The 1H and 13C{1H} NMR spectra were recorded
either with a 200 MHz Bruker DPX 200, a 300 MHz Bruker Av-
ance III, or a 600 MHz Bruker Avance II spectrometer and refer-
enced internally using the residual protio solvent resonances rela-
tive to tetramethylsilane (δ = 0 ppm).[36] Electrospray ionization
mass spectra were obtained either with a LTQ Orbitrap XL (high-
resolution ESI-MS) or an Amazon SL (low-resolution ESI-MS)
instrument. Elemental analyses of uranium and thorium complexes
were performed by Analytische Laboratorien at Lindlar (Germany)
or Medac Ltd. at Chobham Surrey (UK); others were performed
locally with a CHNS/O Thermo Electron Flash EA 1112 Series
N-Allyl-N-(trityloxy)pent-4-enamide (10): Allyl methyl carbonate
(2.15 mL, 18.52 mmol) and [Pd(PPh3)4] (0.215 g, 0.185 mmol) were
added sequentially to a solution of 9 (3.31 g, 9.26 mmol) in dry
instrument. Uncorrected melting points were determined with a CH3CN (40 mL) at ambient temperature under nitrogen. After stir-
Büchi Melting Point B-545 apparatus. Caution: Depleted uranium ring for 2 h, the reaction mixture was poured into ethyl acetate
(primary isotope 238U) provided by CEA Saclay is a weak α-emitter (80 mL) and brine (80 mL). The organic layer was washed cau-
(4.197 MeV) with a half-life of 4.47ϫ109 years. Natural thorium
(primary isotope 232Th) is a weak α-emitter (4.012 MeV) with a
half-life of 1.41ϫ1010 years. All manipulations and reactions
should be carried out in monitored fume hoods or gloveboxes in a
laboratory equipped with α- and β-counting equipment.
tiously with water (80 mL) and dried with MgSO4. The solvent
was removed under reduced pressure and a pale yellow solid was
obtained, which was purified by flash column chromatography (sil-
ica gel, CH2Cl2), yield 2.75 g (75%). Colorless X-ray-quality crys-
1
tals were obtained by slow evaporation of a CH2Cl2 solution. H
Eur. J. Inorg. Chem. 2015, 1529–1541
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NMR (300 MHz, CDCl3, 298 K): δ = 1.98 (m, 4 H, 2-H and 3-H), C20H32N4O8NaZr+ 569.1159 [ZrL4 + Na]+; found 569.1282; calcd.
3
+
4.03 (br. d, J = 4.8 Hz, 2 H, 6-H), 4.84 (dq, 1 H, 5-Ht), 4.89 (br. for C40H64N8O16NaZr2 1115.2427 [Zr2L8
+
Na]+; found
s, 1 H, 5-Hc), 5.12 (dq, 1 H, 8-Ht), 5.19 (dq, 1 H, 8-Hc), 5.58 (m,
1 H, 4-H), 5.77 (m, 1 H, 7-H), 7.33 (br. s, 15 H, H–Ar) ppm. 13C
NMR (75 MHz, CDCl3, 298 K): δ = 28.5, 32.1, 50.6, 93.6, 114.8,
117.5, 127.8, 128.2, 129.9, 132.6, 137.6, 142.3, 176.5 ppm. HRMS
(ESI): m/z calcd. for C27H27NO2Na+ 420.1934 [M + Na]+; found
420.2055.
1115.2422. C20H32N4O8Zr·0.75H2O (561.26): calcd. C 42.80, H
6.02, N 9.98; found C 43.06, H 6.48, N 9.18.
Tetrakis(1-oxypiperidine-2-one)hafnium(IV) [Hf(1)4]: A solution of
[Hf(acac)4] (0.117 g, 0.203 mmol) in MeOH (50 mL) was added to
a solution of 1H (0.094 g, 0.816 mmol) in MeOH (50 mL). After
stirring the reaction mixture overnight at room temperature, the
solvent was removed under reduced pressure. The crude solid mate-
rial was crystallized from CH2Cl2/heptane (1:1, v/v) to afford
[Hf(1)4] as a colorless solid in 59% yield (0.077 g), m.p. 264 °C
1-(Trityloxy)-3,4-dihydro-1H-azepin-2(7H)-one (11): A mixture of
10 (2.75 g, 6.93 mmol) and Grubbs II catalyst (0.315 g,
0.371 mmol) was dissolved in CH2Cl2 (230 mL) and heated under
reflux conditions for 12 h. The dark red solid material obtained
upon solvent removal was purified by flash column chromatog-
raphy (silica gel, pentane/EtOAc, 1:0 to 3:2 v/v) to afford a reddish
solid product (1.877 g) in 73% yield. A small portion was recrys-
tallized (pentane/EtOAc, 3:2 v/v) to give X-ray-quality crystals,
(decomp.). IR (ATR): ν = 3389 (br., w), 2962 (w), 1612 (s, νC=O),
˜
1481 (s), 1161 (w), 933 (m), 727 (m), 526 (s) cm–1. 1H NMR
(600 MHz, CDCl3, 300 K): δ = 1.70 (m, 2 H, 3-H), 1.82 (m, 2 H,
3
3
4-H), 2.42 (t, J = 6.0 Hz, 2 H, 2-H), 3.57 (t, J = 6.0 Hz, 2 H, 5-
H) ppm. 13C NMR (150 MHz, CDCl3, 300 K): δ = 20.3 (C-3), 23.2
(C-4), 28.8 (C-2), 50.9 (C-5), 161.9 (C-1) ppm. HRMS (ESI): m/z
calcd. for C15H24N3O6Hf+ 522.1125 [HfL3]+; found 522.1131;
calcd. for C20H32N4O8HfNa+ 659.1581 [HfL4 + Na]+; found
m.p. 145.6–146.8 °C. IR (ATR): ν = 3063 (br., w), 1678 (m, νC=O),
˜
1444 (m), 1232 (m), 774 (m), 755 (s), 703 (s), 635 (m) cm–1. 1H
NMR (300 MHz, CDCl3, 298 K): δ = 2.06 (m, 2 H, 3-H), 2.32 (q,
2 H, 2-H), 4.04 (m, 2 H, 6-H), 5.44 (m, 1 H, 5-H), 5.64 (m, 1 H,
4-H), 7.27–7.33 (m, 9 H, 12-H, 13-H), 7.44–7.50 (m, 6 H, 11-H)
ppm. 13C NMR (75 MHz, CDCl3, 298 K): δ = 24.5, 33.0, 51.8,
93.9, 122.8, 127.4, 127.8, 130.2, 131.3, 142.9, 173.3 ppm. HRMS
(ESI): m/z calcd. for C25H23NO2Na+ 392.1621 [M + Na]+; found
392.1619. C25H23NO2 (369.46): calcd. C 81.27, H 6.27, N 3.79;
found C 81.01, H 6.80, N 3.96.
659.1585; calcd. for C40H64N8O16Hf2Na+ 1293.3264 [Hf2L8
+
Na]+; found 1293.3259. C20H32HfN4O8 (634.98): calcd. C 37.83, H
5.08, N 8.82; found C 38.46, H 5.54, N 8.24.
Tetrakis(1-oxypiperidine-2-one)cerium(IV) [Ce(1)4]: A solution of
[Ce(acac)4] (0.100 g, 0.186 mmol) in MeOH (50 mL) was added to
a solution of 1H (0.079 g, 0.686 mmol) in MeOH (50 mL). After
stirring the reaction mixture overnight at room temperature, the
solvent was removed under reduced pressure. The crude solid mate-
rial was crystallized from CH2Cl2/heptane (1:1, v/v) to afford
[Ce(1)4] as a reddish solid in 50% yield (0.055 g), m.p. 189 °C (de-
1-Hydroxy-3,4-dihydro-1H-azepin-2(7H)-one (6H). Method A: A
solution of 11 (1.370 g, 3.71 mmol) in CH2Cl2/MeOH (3:1 v/v;
40 mL) was stirred with BF3·OEt2 (1 mL, 7.42 mmol) for 2 h at
room temperature. After removing the solvent by rotary evapora-
tion, the crude product was purified by flash column chromatog-
raphy (silica gel, CH2Cl2/MeOH, 96:4 v/v) to give 0.210 g of an
orange sticky solid in 44% yield. Method B: A solution of 11
(0.893 g, 2.42 mmol) in CH2Cl2/MeOH (3:2 v/v; 50 mL) was treated
with trifluoroacetic acid (TFA; 0.20 mL, 2.66 mmol), and the reac-
tion mixture was stirred for 6 h at room temperature. Purification
by chromatography as described above afforded 6H in 24% yield
(0.073 g). Colorless crystals of 6H (0.045 g, 10% yield) were ob-
tained by sublimation (45 °C, 2ϫ10–4 mbar), m.p. 184–185 °C. IR
comp.). IR (ATR): ν = 3550 (br., w), 2945 (w), 1587 (s, νC=O), 1471
˜
(s), 1347 (m), 1106 (m), 925 (m), 875 (m), 823 (m), 720 (m), 495 (s)
cm–1. 1H NMR (600 MHz, CDCl3, 300 K): δ = 1.72 (m, 2 H, 3-
3
3
H), 1.80 (m, 2 H, 4-H), 2.44 (t, J = 6.0 Hz, 2 H, 2-H), 3.51 (t, J
= 6.0 Hz, 2 H, 5-H) ppm. 13C NMR (150 MHz, CDCl3, 300 K): δ
= 20.4 (C-3), 23.2 (C-4), 29.6 (C-2), 51.7 (C-5), 163.6 (C-1) ppm.
HRMS (ESI): m/z calcd. for C15H24N4O8Ce+ 482.0719 [CeL3]+;
found 482.0706; calcd. for C20H32N4O8CeNa+ 619.1167 [CeL4
+
Na]+; found 619.1156. C20H32CeN4O8·0.4CH2Cl2 (630.93): calcd.
C 38.07, H 5.24, N 8.88; found C 38.32, H 5.41, N 7.77.
(ATR): ν = 3197 (br., w), 2887 (w), 1655 (br. s, νC=O), 1447 (s),
˜
Tetrakis(1-oxypiperidine-2-one)thorium(IV) [Th(1)4]:
A Schlenk
1162 (s), 897 (m), 784 (s), 645 (br., s), 511 (s), 435 (m) cm–1. 1H
NMR (600 MHz, CDCl3, 300 K): δ = 2.38 (m, 2 H, 3-H), 2.68 (t,
2 H, 2-H), 4.29 (m, 2 H, 6-H), 5.71 (m, 1 H, 5-H), 5.84 (m, 1 H,
4-H) ppm. 13C NMR (150 MHz, CDCl3, 300 K): δ = 24.1 (C-3),
31.6 (C-2), 48.8 (C-6), 122.3 (C-5), 131.0 (C-4), 169.6 (C-1) ppm.
HRMS (ESI): m/z calcd. for C6H10NO2+ 128.0701 [M + H]+; found
128.07061; calcd. for C6H9NO2Na+ 150.0526 [M + Na]+; found
150.0520.
tube was charged with [ThN*2(N,C)] (20 mg, 0.028 mmol), 1H
(12.9 mg, 0.112 mmol), and pyridine (ca. 1 mL). After 2 h at 90 °C,
the colorless solution was concentrated to 0.5 mL, and pentane
(2 mL) was condensed into the solution, thus leading to a white
deposit that was filtered off and dried under vacuum for 1 h, yield
15.1 mg (78%). Colorless crystals of [Th(1)4] were formed upon
slow diffusion of pentane into a solution of pyridine. IR (Nujol
mull): ν = 1607 (vs, νC=O), 925, 722 cm–1. 1H NMR (200 MHz, [D5]
˜
pyridine, 296 K): δ = 1.25 (s, 2 H, 3-H), 1.34 (s, 2 H, 4-H), 2.14 (s, 2
H, 2-H), 3.52 (s, 2 H, 5-H) ppm. 13C NMR (50 MHz, [D5]pyridine,
296 K): δ = 21.5 (C-3), 24.2 (C-4), 31.5 (C-2), 52.8 (C-5), 162.4 (C-
1) ppm.
Tetrakis(1-oxypiperidine-2-one)zirconium(IV) [Zr(1)4]: A solution of
[Zr(acac)4] (0.419 g, 0.859 mmol) in MeOH (50 mL) was added
with a cannula to a solution that contained 4 equiv. of 1H (0.398 g,
3.46 mmol) in MeOH (150 mL). After stirring overnight, the sol-
vent was removed under reduced pressure to afford a crude mate-
Tetrakis(1-oxypiperidine-2-one)uranium(IV) [U(1)4]: An NMR spec-
rial that was recrystallized from CH2Cl2/heptane (1:1, v/v). Pure troscopy tube was charged with [UN*2(N,C)] (20 mg, 0.028 mmol),
[Zr(1)4] was isolated as a colorless solid in 57% yield (0.269 g), m.p. 1H (12.8 mg, 0.111 mmol), and pyridine (ca. 1 mL). The solution
194–195 °C. IR (ATR): ν = 3400 (br., w), 2958 (w), 1608 (s, νC=O), became blue-gray immediately and the mixture was heated at 90 °C
˜
1480 (s), 931 (w), 727 (m), 520 (s) cm–1. 1H NMR (600 MHz, for 30 min. Et2O (2 mL) was then condensed into the solution to
CDCl3, 300 K): δ = 1.71 (m, 2 H, 3-H), 1.83 (m, 2 H, 4-H), 2.44 give a blue-green deposit of [U(1)4], which was filtered off, washed
(t, 3J = 6 Hz, 2 H, 2-H), 3.58 (t, 3J = 6 Hz, 2 H, 5-H) ppm. 13C
with Et2O (2 mL), and then dried under vacuum for 3 h, yield
NMR (150 MHz, CDCl3, 300 K): δ = 20.2 (C-3), 23.1 (C-4), 28.6 17.4 mg (90%). Large pale green crystals of [U(1)4], the color of
(C-2), 50.7 (C-5), 161.9 (C-1) ppm. HRMS (ESI): m/z calcd. for which also appeared purple depending on the orientation of the
C15H24N3O6Zr+ 432.0707 [ZrL3]+; found 432.0759; calcd. for
crystals and lighting, were formed upon slow diffusion of pentane
Eur. J. Inorg. Chem. 2015, 1529–1541
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FULL PAPER
into a solution of pyridine. IR (Nujol mull): ν = 1597 (vs, νC=O),
Potentiometric Titrations: The pKa values of the hydroxamic acids
˜
925, 722 cm–1. 1H NMR (200 MHz, [D5]pyridine, 296 K): δ = 1H and 6H were determined by glass-electrode potentiometry un-
3
–10.42 (br. t, J = 6 Hz, 2 H), 5.33 (s, 2 H), 8.39 (s, 2 H), 18.06 (s,
der an argon atmosphere at a fixed ionic strength (I = 0.1 m KNO3)
2 H) ppm. 13C NMR (50 MHz, [D5]pyridine, 296 K): δ = 20.5, 34.2, and temperature [T = 285.2(2) K]. The experimental setup and con-
65.8, 103.4 ppm (C=O not seen). C20H32N4O8U (694.52): calcd. C ditions were identical to those previously described in detail else-
34.59, H 4.64, N 8.07; found C 35.13, H 4.51, N 8.48.
where.[42] The electrochemical cell was calibrated in concentration
units (p[H] = –log[H3O+]) by titrating standardized 0.1 m HNO3
by 0.1 m KOH solutions in the presence of supporting electrolyte.
All measured electromagnetic force (emf) values were corrected for
liquid-junction potential effects that occurred in acidic or basic re-
gions. Potentiometric data were refined by the weighted nonlinear
least-squares refinement program Hyperquad 2006.[43]
Tetrakis(1-oxy-3,4-dihydro-1H-azepin-2(7H)-one)zirconium(IV)
[Zr(6)4]: A solution of [Zr(acac)4] (0.044 g, 0.090 mmol) in MeOH
(50 mL) was added to a solution of 6H (0.045 g, 0.354 mmol) in
MeOH (50 mL). After stirring the reaction mixture overnight at
room temperature, the solvent was removed under reduced pres-
sure. Fractional crystallization of the yellow solid residue dissolved
in CH2Cl2/heptane (1:1, v/v) afforded [Zr(6)4] as colorless crystals
in 55% yield (0.029 g), m.p. 249.9–251.9 °C (decomp.). IR (ATR):
Supporting Information (see footnote on the first page of this arti-
1
cle): Definition of the stereochemical parameters, copies of the H
and 13C NMR spectra for all the compounds discussed in this
study, tables of X-ray crystallographic data.
ν = 3403 (br., w), 1609 (s, νC=O), 1487 (s), 1080 (s), 901 (m), 847
˜
(m), 810 (s), 639 (m), 507 (s) cm–1. 1H NMR (600 MHz, CDCl3,
300 K): δ = 2.27 (m, 2 H, 3-H), 2.62 (t, 3J = 6.0 Hz, 2 H, 2-H),
4.31 (m, 2 H, 6-H), 5.58 (m, 1 H, 5-H), 5.73 (m, 1 H, 4-H) ppm.
13C NMR (150 MHz, CDCl3, 300 K): δ = 23.7 (C-3), 29.1 (C-2),
52.0 (C-6), 122.7 (C-5), 130.2 (C-4), 165.9 (C-1) ppm. HRMS
(ESI): m/z calcd. for C18H24N3O6Zr+ 468.0701 [ZrL3]+; found
468.0694; calcd. for C24H32N4O8NaZr+ 617.1159 [ZrL4 + Na]+;
found 617.1145.
Acknowledgments
This work was financially supported by the Centre National de la
Recherche Scientifique (CNRS), the Conseil Régional de Bour-
gogne (programs PARI IME SMT8 and PARI II CDEA), and by
the CNRS-co-funded program NEEDS Environment (ACTISOL
project). P. J. thanks the CNRS and the Conseil Régional de Bour-
X-ray Crystallography: Diffraction data were collected with a Non-
ius Kappa Apex II diffractometer equipped with a nitrogen jet
stream low-temperature system (Oxford Cryosystems). The X-ray
gogne for
a
PhD fellowship (grant number 2009-
9201AAO037S03746).
source was graphite-monochromated Mo-Kα radiation (λ
=
0.71073 Å) from a sealed tube. The lattice parameters were ob-
tained by a least-squares fit to the optimized setting angles of the
entire set of collected reflections. No significant temperature drift
was observed during the data collection. Data were reduced by
using Denzo and Scalepack software without applying absorption
corrections.[37] The missing absorption corrections were partially
compensated by the data scaling procedure in the data reduction,
except for compound [Zr(1)4], for which multiscan absorption cor-
rection[38] was applied. Structures were solved by direct methods
using the SIR92[39] or SIR97[40] program. Refinements were carried
out by full-matrix least-squares on F2 using the SHELXL-97[41]
program and the complete set of reflections. Anisotropic thermal
parameters were used for non-hydrogen atoms except in the minor
disordered part of [Zr(6)4]. Hydrogen atoms from hydroxy groups
and water molecules were found in difference Fourier maps. Those
attached to carbon atoms were placed at calculated positions using
a riding model with C–H = 0.99 Å (methylene), O–H = 0.84 Å,
Uiso(H) = 1.2Ueq(CH), Uiso(H) = 1.2Ueq(CH2), or Uiso(H) =
1.5Ueq(OH). Some geometric parameters of disordered compo-
nents 65(1)/35(1)% in 10 were restrained by using DFIX[41] re-
straints. Similar Uij constraints were applied within the disordered
parts of [Zr(1)4], [Hf(1)4], and [Zr(6)4] to maintain a reasonable
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CCDC-90612 (for 1H), -90613 (for 2H), -90614 (for 6H), -90615
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Received: December 18, 2014
Published Online: February 16, 2015
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© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim