Rationalizing the formation and versatility of multinuclear metal complexes of bis(1-methyluracil-5-yl)methane as hybrids between classical calix[n]arenes and metallacalixaromatics

Article abstract of DOI:10.1016/j.ica.2014.01.020

Metallacalix[n]arenes are a distinct class of metallacyclic compounds consisting of heteroaromatic rings L and square-planar cis-a₂M ^II entities (M = Pt or Pd; a = NH₃ or amine; or a ₂ = chelating diamine) instead of methylene bridges as in the classic calix[n]arenes. Here a series of hybrid compounds is described, which simultaneously have bridging -CH₂- as well as cis-a₂M ^II units, and uracil containing ligands L. Specifically, L = bis(1-methyluracil-5-yl)methane (1) and in one case a derivative of it, bis(1-methyluracil-5-yl)methylbenzene (2), have been reacted with cis-[a ₂M(H₂O)₂]²⁺ (with a = NH₃ or a₂ = 2,2′-bipyridine or bis(pyrazloyl-1-yl)propane) in water and products were isolated. Altogether X-ray crystal structures of eight metallacycles (complexes 3-6, 8-11) of M₂L₂, M ₄L₂, and M₆L₆ stoichiometries have been determined as well as a second modification of 1. In all closed metallacycles the 1-methyluracil entities are deprotonated with metals coordinating via N3 positions, and without exception the uracil rings adopt 1,3-alternate conformations. A special feature of ligand 1 in its twofold deprotonated form is its propensity to bind additional metal ions through its exocyclic oxygen functionalities. While O4 sites appear to be favored as secondary metal binding sites, linkage isomerism and involvement of O2 is likewise possible (compounds 4, 9, 10). On the basis of the X-ray crystal structures, a reaction scheme is proposed which accounts for the different stoichiometries observed.

Full text of DOI:10.1016/j.ica.2014.01.020

Inorganica Chimica Acta xxx (2014) xxx–xxx
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Inorganica Chimica Acta
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Rationalizing the formation and versatility of multinuclear metal
complexes of bis(1-methyluracil-5-yl)methane as hybrids between
classical calix[n]arenes and metallacalixaromatics
a,
Anupam Khutia a, Wei-Zheng Shen a, Neeladri Das a,1, Pablo J. Sanz Miguel b, , Bernhard Lippert
⇑
⇑
a Fakultät Chemie und Chemische Biologie, Technische Universität Dortmund, 44221 Dortmund, Germany
b Departamento de Química Inorgánica, Instituto de SíntesisQuímica y Catálisis Homogéna (ISQCH), Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online xxxx
Metallacalix[n]arenes are a distinct class of metallacyclic compounds consisting of heteroaromatic rings L
and square-planar cis-a2MII entities (M = Pt or Pd; a = NH3 or amine; or a2 = chelating diamine) instead of
methylene bridges as in the classic calix[n]arenes. Here a series of hybrid compounds is described, which
simultaneously have bridging –CH2– as well as cis-a2MII units, and uracil containing ligands L. Specifi-
cally, L = bis(1-methyluracil-5-yl)methane (1) and in one case a derivative of it, bis(1-methyluracil-5-
yl)methylbenzene (2), have been reacted with cis-[a2M(H2O)2]2+ (with a = NH3 or a2 = 2,20-bipyridine or
bis(pyrazloyl-1-yl)propane) in water and products were isolated. Altogether X-ray crystal structures of
eight metallacycles (complexes 3–6, 8–11) of M2L2, M4L2, and M6L6 stoichiometries have been deter-
mined as well as a second modification of 1. In all closed metallacycles the 1-methyluracil entities are
deprotonated with metals coordinating via N3 positions, and without exception the uracil rings adopt
1,3-alternate conformations. A special feature of ligand 1 in its twofold deprotonated form is its propen-
sity to bind additional metal ions through its exocyclic oxygen functionalities. While O4 sites appear to be
favored as secondary metal binding sites, linkage isomerism and involvement of O2 is likewise possible
(compounds 4, 9, 10). On the basis of the X-ray crystal structures, a reaction scheme is proposed which
accounts for the different stoichiometries observed.
Keywords:
Supramolecular chemistry
1-Methyluracil
Metallacalix[n]arenes
Platinum
Palladium
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
purely organic cousins, hence have the potential to occur as differ-
ent conformers. Furthermore, they are to be differentiated from
In classical organic calix[n]arenes, n methylene groups bridge n
para-substituted phenols, yielding cyclic compounds [1]. Depend-
ing on the rotational states of the phenol entities with respect to
each other, calix[n]arenes can exist in a variety of conformers.
Replacement of the CH2 groups by numerous other groups or het-
eroatoms (e.g., O, NH, NR, S, SO2, etc.) and of the phenols by other
heterocyclic rings produces numerous types of ‘‘heterocalix[n]aro-
matics’’ [2]. Taking a further step in modification of organic
calix[n]arenes, cis-L2MII entities with M = Pt or Pd and L being a
variable donor ligand can likewise be utilized to substitute the
bridging methylene groups, thereby generating metallacalix[n]are-
nes [3–6], a distinct subgroup of metallamacrocyclic compounds
[7]. They display the characteristic conformational flexibility of their
those metal complexes of calix[n]arenes, where metal coordination
takes place through donor atoms within the rim and/or the bridg-
ing groups, or via dangling groups attached to the rims of the
calix[n]arenes [8–10]. Applying substituted pyrimidines as hetero-
aromatic ligands, including the parent nucleobases uracil and cyto-
sine, we and others have prepared and characterized a number of
such compounds with n = 4 [3,4a–f], 6 [4f,5c], and 8 [5a], occasion-
ally using also two different nucleobases and two different metal
ions simultaneously. Our interest in metallacalix[n]arenes contain-
ing pyrimidine ligand not only relates to non-covalent interactions
with DNA – some of these compounds display surprising effects on
the structure of double-stranded DNA [11] – but also to host–guest
chemistry with anions [4a,4f,12] and, surprisingly, even with cat-
ions [4c,13]. In fact, the propensity of these positively charged,
hence cationic metallacycles to coordinatively bind additional me-
tal ions via their exocyclic oxygen and/or deprotonated amino
groups, is astonishing, although it reflects the behavior of numer-
ous small metal complexes of heavy metal ions with pyrimidine
⇑
Corresponding authors.
E-mail addresses: pablo.sanz@unizar.es (P.J. Sanz Miguel), bernhard.lippert@
tu-dortmund.de (B. Lippert).
1
Present address: Department of Chemistry, Indian Institute of Technology Patna,
Patna 800013, Bihar, India.
http://dx.doi.org/10.1016/j.ica.2014.01.020
0020-1693/Ó 2014 Elsevier B.V. All rights reserved.
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2
A. Khutia et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx
nucleobases as well as related amidate ligands [14]. As a result,
multinuclear metal complexes are formed which frequently fea-
ture short intermetallic distances.
1, both uracil rings were tilted 77.82(9)° in an intermediate dispo-
sition between cisoid and transoid, namely, with the C6 site of one
uracil pointing roughly toward the centroid of the other (Fig. 1).
Supramolecular arrangement of 1a is dominated by ribbons in a
zig–zag fashion, which are assembled by twofold hydrogen bonded
(N3aꢀ ꢀ ꢀO2b0, 2.8421(16) Å; O2aꢀ ꢀ ꢀN3b0, 2.8045(16) Å) units of the
molecule along the c axis. Water molecules of crystallization
(O1w) join parallel ribbons (O1wꢀ ꢀ ꢀO4a, 2.8109(17) Å; O1wꢀ ꢀ ꢀO4b0,
2.9144(17) Å), thus forming a 3D polymeric network.
In an earlier communication [15], we have reported the design
and synthesis of novel hybrids between classical calix[4]arene and
metallacalix[4]arenes (Chart 1). The term ‘‘hybrid’’ has been coined
since these metallacycles, based on uracil moieties, contained both
methylene groups and square-planar cis-X2PtII (X = PPh3 or
X2 = 2,20-bpy) units. They were synthesized by reacting the ditopic
ligand bis(1-methyluracil-5-yl)methane C11H12N4O4 (1) with the
corresponding cis-X2PtII entities. In continuation of our efforts to
exploit the coordination properties of 1, we herewith report the
synthesis and characterization (1H NMR and single crystal X-ray
crystallography) of several representative examples of this new
class of hybrid metallacycles. A derivative of 1 (bis(1-methylura-
cil-5-yl)methylbenzene, ligand 2) is also being dealt with in this
article. 2 extends the list of related polytopic ligands containing
1-methyluracil-5-yl entities recently reported by us [16].
At an early stage of our studies it became evident that 1 gives
rise to products of different stoichiometries and nuclearities, in
part depending on the nature and steric requirements of the
co-ligands L of the square-planar PtII ion. By further varying these
co-ligands (L = NH3; L2 = bis(pyrazol-1-yl)propane, bipzp) and also
by substituting in part Pt by Pd, the attempt was made to generate
a unifying reaction scheme which explains the different stoichiom-
etries and nuclearities without explicitly differentiating between
the heterocyclic ligand (1 or 2), the metal entities (PtII or PdII),
and the co-ligands L2.
2.2. Synthesis of bis(1-methyluracil-5-yl)methyl-benzene (2) and
comparison of its pKa value with those of 1
The ligand bis(1-methyluracil-5-yl)methyl-benzene (2) was
synthesized by the acid-catalyzed condensation of benzaldehyde
with 1-methyluracil (1-MeUH) (Scheme 1). The product was ob-
tained as a white solid in 94% yield. The 1H NMR spectrum of 2 (re-
corded in Me2SO-d6) is consistent with its composition (Supporting
Information). The connectivity between the two 1-MeUH units and
the benzylic carbon is clearly indicated by the presence of a 1H sin-
glet at d 5.09 and a 2H singlet d 7.03 due to the hydrogen at ben-
zylic position and the hydrogen attached to C6 of each uracil
unit, respectively. The pKa values of 2 were determined by use of
pD dependent 1H NMR spectroscopy. A single pKa of 10.05 0.04
(converted from D2O to H2O; average of H6 and N1–CH3 reso-
nances) was obtained for two overlapping deprotonation steps, vir-
tually identical with that for 1 (10.02 0.02) [15]. Individual pKa
values of 2 are therefore in the range of ca. 9.75 to ca. 10.35
(Supporting Information). As compared to 1-methyluracil
(pKa = 9.63 0.05) [18], the 1-methyluracil-5-yl entities in 2, like
in 1, are thus somewhat less acidic than the parent ligand 1-MeUH
itself. This is reminiscent of the situation with 1-methylthymine,
which likewise has a lower acidity (pKa = 10.3) [19] than 1-methyl-
uracil, as it carries a methyl group at the C5 position. Unlike with 1
(1a), attempts to obtain X-ray-quality single crystals of 2, failed.
2. Results and discussion
2.1. Second polymorph of bis(1-methyluracil-5-yl)methane (1a)
Reaction of 1-methyluracil (1-MeUH) with paraformaldehyde
under HCl-acidic conditions and recrystallization of the crude
product from H2O/EtOH yields a monohydrate of 1, which crystal-
2.3. Synthesis and characterization of [Pd(bipzp)(10)]2ꢀ13H2O (3)
ꢀ
lizes in the triclinic crystal system and space group P1 [15,17]. We
have now isolated 1 as a second polymorph 1a (monoclinic, space
group P21/c; monohydrate) during an unsuccessful attempt to syn-
thesize a silver complex by reacting AgNO3 with 1 in a 1:1-mixture
of H2O and MeOH. The colorless crystals recovered after cooling
the solution proved to be 1a. The two modifications 1 and 1a differ
essentially in the mutual disposition of the uracil entities, which is
transoid in the case of 1a, forming a dihedral angle of 83.51(3)°. In
Reaction of [Pd(bipzp)(H2O)2](NO3)2 (bipzp = bis(pyrazol-1-
yl)propane), obtained in situ by treatment of PdCl2(bipzp) [20] with
AgNO3, and 1 in a 1:1 ratio in water, pH 9.0, yielded the neutral
metallacalixarene [Pd(bipzp)(10)]2ꢀ13H2O (3) (with 10 = dianion of
1, deprotonated at the two N3 positions of the uracil rings)
(Scheme 2). The 1H NMR spectrum of 3 indicated the formation
of a single and finite macrocyclic species, as evident from the pres-
ence of sharp peaks corresponding to PdII(bipzp) and 10. Reso-
nances of the three protons H30, H40, and H50 of bis(pyrazol-1-
yl)propane are observed at 8.12 (d), 6.45 (t) and 7.63 (d) ppm,
respectively, and of the CH3 protons of bipzp as a singlet at
Chart 1. Schematic views of (a) classical calix[4]arene, (b) metallacalix[4]arene,
Fig. 1. View and atom numbering of bis(1-methyluracil-5-yl)methane (1a) (mono-
and (c) hybrid between (a) and (b).
hydrate, second polymorph).
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A. Khutia et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx
3
Scheme 1. Formation of ligand 2.
2.99 ppm. The singlet at 7.14 ppm corresponds to C6H of the uracil
units in 10.
The structure of 3 was determined by X-ray crystallography.
Light yellow, prism shaped X-ray quality single crystals were ob-
tained upon slow evaporation of an aqueous solution of the com-
plex. A view of compound 3 is shown in Fig. 2. It reveals that 3 is
a metallacalixarene in which two PdII(bipzp) units are bridged by
two dianions of 1, hence 10, via the N3 deprotonated sites in a cyclic
fashion. Both bridging ligands 10 are transoid as far as the two ura-
cil rings are concerned, and the pairs of coordinating uracil entities
are mutually head–tail at each metal center. Hence the four uracils
adopt a 1,3-alternate conformation according to the metallaca-
lix[4]arene nomenclature. The cation, which represents a chiral
double helicate and its mirror image are present in a 1:1 mixture
Fig. 2. Two views and atom numbering scheme of neutral complex 3.
N3a angle of 175.03(9)°. The distance between O4 sites of opposite
uracil rings are 6.334(3) Å (ring a–d), and 6.473(3) Å (ring b–c)
(wide rim), while the N1-methyl groups of opposite uracil rings
are 3.821(4) Å (ring a–d) and 3.666(4) Å (ring b–c) apart (narrow
rim). The ‘‘height’’ of the calixarene, as defined in this case by the
average distances between N-methyl groups, is ca. 9.0 Å.
ꢀ
in the crystal, as required by space group P1. The overall shape
of this Pd2 metallacycle 3 is thus similar to the Pt2 compound that
we have reported in our previous communication [15], although
less regular than the former. The Pt2 and Pd2 metallacycles differ
from similar bis(pyridine-4-yl)methane [21] and bis(pyridine-4-
yl)dimethylsilane [22] metallacycles in that the latter are achiral
and do not provide the potential of calixarenes to adopt different
rotamer states of its heterocyclic pyridine components. The four
atoms comprised of two Pd atoms and two C atoms of the methy-
lene bridges in 3 are located nearly in a plane, producing a rhom-
boid shape. Intermolecular dimensions are 8.9436(4) Å (Pdꢀ ꢀ ꢀPd)
and 6.424(3) Å (CH2ꢀ ꢀ ꢀCH2). The tilt angles between the uracil rings
and this plane are 81.4(6)°, 80.86(5)°, 72.04(5)°, and 69.71(5)°, for
rings a, b, c, and d, respectively. Considering these angles, it can be
concluded that 3 exists in a pinched 1,3-alternate conformation. The
average distances between two alternate uracil rings range from
4.36 Å at the lower rim to 5.72 Å at the upper rim. The average dis-
tance between the centroid of two alternate uracil rings is 5.01 Å.
Other structural features of 3 are as follows: The geometry around
each Pd is close to square-planar, with a N11–Pd1–N21 angle of
89.22(8)°, a N3a–Pd1–N3c angle of 87.58(8)°, and a N21–Pd1–
The most outstanding aspect of the crystal packing of 3 is the
water cluster assembled therein. It is rarely seen a crystal structure
of about 150 atoms including two PdII ions and 13 fully occupied
and crystallographically different water molecules with such a
quality of the data. Furthermore, we could find an adequate orien-
tation of the hydrogen atoms within the H-bonding pattern. The
nature of this water cluster is two-dimensional and extends along
the a axis. Dimensions are comparable to those of a water cluster
nanotube [23]. However, the water molecules of 3 are disposed fol-
lowing a central backbone of fused squares and hexagons (Fig. 3).
Some additional hexagons and pentagons are also present in the
skeleton. Concretely, the central hexagon ‘‘H1’’ {O1w–O4w–
O12w–O7w–O11w–O5w} shares their alternate edges with three
different squares ‘‘S1’’ {O4w–O12w–O3w–O10w}, ‘‘S2’’ {O7w–
O11w–O7w0–O11w0}, and ‘‘S3’’ {O1w–O5w–O1w0–O5w0}. An addi-
tional hexagon ‘‘H2’’ {O6w–O2w–O9w–O7w–O11w–O5w} results
from the fusion of ‘‘H1’’, ‘‘S2’’, and the pentagon ‘‘P1’’ {O1w–
O5w–O11w–O13w–O8w}, which is also merged with ‘‘H1’’ and
CH3
N
CH3
N
O
O
O
O
N
N
a
a
O
O
O
O
HN
NH
2 Pd(bipzp)(NO3)2
2
Pd
Pd
60 ºC, pD 9
4 NO3
N
N
O
O
a
a
CH3
CH3
N
N
1
O
O
N
N
CH3
CH3
bipzp = bis(pyrazol-1yl)propane
3
Scheme 2. Synthesis of Pd2 complex 3. Deprotonation of 1 is achieved by means of NaOH.
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4
A. Khutia et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx
Fig. 3. Packing of the water cluster present in 3.
‘‘S3’’. Hydrogen bonding distances of closed water cluster are listed
in Table 1. Growing of the two dimensional water cluster follows a
120° zig-zag degree along the {H1 ? S2 ? H1 ? S3 ? }n direction.
Analogously, the definition of an alternative growing along the
{S3 ? P1 ? H2 ? S2 ? H2 ? P1 ? }n direction would be also va-
lid. This intriguing water organization is anchored to complex 3
via the carbonyl O2 and O4 groups: O2aꢀ ꢀ ꢀO8w, 2.796(3) Å;
O4aꢀ ꢀ ꢀO9w, 2.736(3) Å; O2bꢀ ꢀ ꢀO3w, 2.658(2) Å; O4bꢀ ꢀ ꢀO6w,
2.748(3) Å; O2cꢀ ꢀ ꢀO2w, 2.701(3) Å; O4cꢀ ꢀ ꢀO10w, 2.841(2) Å;
O2dꢀ ꢀ ꢀO2w, 2.716(3) Å; O4dꢀ ꢀ ꢀO3w, 2.869(3) Å.
From model building it appears perfectly reasonable to assume
that a cone conformer of 3 is likewise feasible, in which the two
bis(1-methyluracilate-5-yl)methane ligands are oriented cisoid.
In this case O2 sites are pairwise adjacent at close distances, as
are O4 sites. Either proton binding, hence the presence of a mono-
anionic species of 1, metal crosslinking through pairs of adjacent
oxygen atoms, or the presence of a templating agent favoring a
cone conformer structure, would appear helpful in stabilizing such
a structure.
Fig. 4. Section of polymeric structure of [{Pd(bipzp)(10-N3)}2(Ag–O2,O4)]n(NO3)n-
ꢀ15H2O (4).
Table 2
Hydrogen bonding distances [Å] within the crystal packing of 4.
4
2.4. Synthesis and characterization of [{Pd(bipzp)(10-N3)}2(Ag-
O2,O4)]n(NO3)nꢀ15H2O (4)
O1wꢀ ꢀ ꢀO4b
2.784(15)
2.780(17)
2.778(17)
2.67(3)
2.727(19)
2.839(15)
2.711(17)
2.83(2)
O5wꢀ ꢀ ꢀO4c
2.813(18)
2.78(2)
2.56(3)
2.83(2)
2.31(3)
2.63(3)
2.81(4)
2.84(3)
2.85(3)
2.92(4)
2.83(4)
3.61(5)
O1wꢀ ꢀ ꢀO2w[a]
O1wꢀ ꢀ ꢀO8w
O1wꢀ ꢀ ꢀO11w
O2wꢀ ꢀ ꢀO5w[b]
O3wꢀ ꢀ ꢀO2d[b]
O3wꢀ ꢀ ꢀO4w[a]
O3wꢀ ꢀ ꢀO5w[a]
O3wꢀ ꢀ ꢀO8w[c]
O4wꢀ ꢀ ꢀO2a
O6wꢀ ꢀ ꢀO9w
O6wꢀ ꢀ ꢀO13w
O7wꢀ ꢀ ꢀO2b[e]
O7wꢀ ꢀ ꢀO11
It has been previously reported by us that the cyclic cation
[Pt(en)(UH-N1,N3)]44+ is markedly stabilized in the 1,3-alternate
conformation (in solid state) due to the formation of intramolecu-
lar hydrogen bonds between exocyclic hydroxyl and exocyclic car-
bonyl groups of the rare uracil anion (UH) tautomer [3]. In solution
and following deprotonation of the UH monoanion to the U dianion
at basic pH, the cation exists in an equilibrium mixture of cone and
O7wꢀ ꢀ ꢀO14w
O9wꢀ ꢀ ꢀO9w[a]
O9wꢀ ꢀ ꢀO11w
O10wꢀ ꢀ ꢀO11w
O10wꢀ ꢀ ꢀO12w
O12wꢀ ꢀ ꢀO11
O12wꢀ ꢀ ꢀO13
2.74(2)
2.754(18)
2.87(3)
O4wꢀ ꢀ ꢀO15w[d]
O5wꢀ ꢀ ꢀO2a
3.053(14)
1,3-alternate conformations.
Furthermore,
it
has
been
Symmetry codes: [a] 1 ꢁ x, 1 ꢁ y, ꢁz; [b] x, y ꢁ 1, z; [c] ꢁx, 1 ꢁ y, ꢁz; [d] 2 ꢁ x, 1 ꢁ y,
ꢁz; [e] ꢁx, 1 ꢁ y, 1 ꢁ z.
Table 1
Hydrogen bonding distances [Å] including hydrogen atoms involved in the water
cluster of 3.
demonstrated that Ag+ ions can spontaneously deprotonate UH li-
gands and are able to convert the 1,3-alternate conformer to the
cone conformer in acidic medium, thereby producing the octanu-
clear species [Pt(en)(UH-N1,N3,O2)Ag]84+ [13a]. As has been men-
tioned previously, the oxo-surface formed by the exocyclic atoms
of uracil nucleobases provides a platform for binding four Ag+ ions.
The four uracil moieties in metallacalixarene 3 provide altogether
eight uncoordinated exocyclic oxygen atoms which could act as
anchor sites for additional metal ions. In order to test its likelihood,
an aqueous solution of 3 was mixed with silver nitrate, anticipating
3
O1w(H1w)ꢀ ꢀ ꢀO5w
O1w(H2w)ꢀ ꢀ ꢀO5w[a]
O2w(H3w)ꢀ ꢀ ꢀO2c[b]
O2w(H4w)ꢀ ꢀ ꢀO2d
2.786(2)
2.823(3)
2.701(3)
2.716(3)
2.869(3)
2.658(2)
2.852(3)
2.790(3)
2.748(2)
2.804(3)
2.806(3)
2.748(3)
2.717(3)
O7w(H14w)ꢀ ꢀ ꢀO9w
2.713(2)
2.796(3)
2.903(3)
2.808(3)
2.736(3)
2.841(3)
2.776(3)
2.784(3)
2.793(3)
2.877(3)
2.801(3)
2.771(3)
2.825(3)
O8w(H15w)ꢀ ꢀ ꢀO2a[e]
O8w(H16w)ꢀ ꢀ ꢀO1w
O9w(H17w)ꢀ ꢀ ꢀO2w
O3w(H5w)ꢀ ꢀ ꢀO4d[c]
O3w(H6w)ꢀ ꢀ ꢀO2b[c]
O4w(H7w)ꢀ ꢀ ꢀO1w
O4w(H8w)ꢀ ꢀ ꢀO10w
O5w(H9w)ꢀ ꢀ ꢀO6w
O5w(H10w)ꢀ ꢀ ꢀO11w[a]
O6w(H11w)ꢀ ꢀ ꢀO2w
O6w(H12w)ꢀ ꢀ ꢀO4b
O7w(H13w)ꢀ ꢀ ꢀO12w[d]
O9w(H18w)ꢀ ꢀ ꢀO4a[b]
O10w(H19w)ꢀ ꢀ ꢀO4c[e]
O10w(H20w)ꢀ ꢀ ꢀO3w[f]
O11w(H21w)ꢀ ꢀ ꢀO7w[g]
O11w(H22w)ꢀ ꢀ ꢀO7w[a]
O12w(H23w)ꢀ ꢀ ꢀO7w[f]
O12w(H24w)ꢀ ꢀ ꢀO4w
O13w(H25w)ꢀ ꢀ ꢀO8w[e]
O13w(H26w)ꢀ ꢀ ꢀO11w[e]
a similar coordination behavior as with [Pt(en)(UH-N1,N3)]44+
.
However, the product obtained proved to be a coordination poly-
mer rather than a discrete multinuclear entity, in which the two
10 ligands retain their transoid orientations as well as the head–tail
arrangement with respect to the two Pd centers, and hence the 1,
Symmetry codes: [a] ꢁx + 1, ꢁy, ꢁz + 1; [b] x ꢁ 1, y, z; [c] ꢁx + 1, ꢁy + 1, ꢁz; [d] ꢁx,
ꢁy, ꢁz + 1; [e] ꢁx + 1, ꢁy + 1, ꢁz + 1; [f] ꢁx, ꢁy + 1, ꢁz + 1; [g] x + 1, y, z.
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A. Khutia et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx
5
Fig. 5. Schematic view of the extended structure of 4 along the [110] direction (green arrow) displaying the alignment of the silver atoms. Hydrogen atoms and bipzp ligands
attached to the PdII ions are omitted for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
O
CH3
N
O
N
a
O
O
N
a
Pd
a
N
Pd
O
a
HN
NH
4 Pd(2,2'-bpy)(NO3)2
2
O
O
a
O
pD 9
8 NO3
Pd
N
N
O
O
N
a
a
Pd
CH3
CH3
N
a
N
N
CH3
O
1
CH3
O
a2 = 2,2'-bpy
5
Scheme 3. Synthesis of Pd4 complexes 5.
3-alternate arrangement of the four uracil residues. According to
the results of the crystallographic analysis of 4, silver ions bridge
adjacent metallacalixarene units, thereby generating the mixed-
metal complex [{Pd(bipzp)(10-N3)}2(Ag–O2,O4}]nꢀ15H2O. A seg-
ment of the coordination polymer 4 is depicted in Fig. 4. Each
Ag+ ion is coordinated by four exocyclic oxygen atoms of the urac-
ilato ligands, via two O2 and two O4 atoms. This mixed O2,O4
coordination by two uracilate ligands, although seen before [24],
appears to be less common than a sequential coordination of O4
sites first, and O2 sites only second.
The geometry around the silver ion is square-planar, which is
the least common geometry as far as coordination chemistry of
Ag(I) is concerned [25], even though it has some precedence in a
Pt,Ag,1-MeU complex [26]. There is distortion within the square-
planar coordination geometry of the Ag+ ion in that two trans-
Ag–O bonds are longer than the other two: The Ag1–O2c bond
(2.380(10) Å) is longer than the cis-Ag1–O4a bond (2.307(8) Å).
Similarly, the Ag2–O2b bond (2.463(8) Å) is longer than the cis-
Ag2–O4d bond (2.247(10) Å). The average Ag–O bond length of
2.35 Å suggests relatively strong coordination bonds. Crystallo-
graphically, Ag+ ions are sitting on a center of inversion as the coor-
dinating ligands are the same. For each Ag+ ion, two PdII moieties
are residing in axial positions, with intermetallic distances of
2.9655(11) Å (Pd1–Ag1) and 3.0503(10) Å (Pd2–Ag2). Surprisingly
these distances are longer than those in mixed Pt, Ag complexes
with tetrahedral Ag geometries and 1-MeU ligands [27] and thus
suggest at most very weak Pd–Ag interactions in 4 as well as neg-
ligible tetragonal distortion about the square planar environment
of both Ag and Pd centers. Clearly, these intermetallic distances
are significantly longer than those observed in heteronuclear Pt,
Ag complexes with dative Pt ? Ag bonds [28]. As far as the struc-
tural details of the individual Pd2-macrocycle 4 are concerned,
there are only minimal deviations from 3.
Silver atoms within the 1D coordination polymer are aligned
along the [110] direction, displaying intermetallic distances of
8.9969(9) Å (Ag1–Ag2). As in the case of compound 3, the crystal
packing retains a high amount of crystallized water, namely 15
water molecules per cation of 4. They form an intriguing two
dimensional hydrogen bonding pattern connecting the cations of
4 and the nitrate anions. Hydrogen bond distances are listed in
Table 2 (Fig. 5).
2.5. Synthesis and characterization of ht,ht-{[Pd(2,20-
bpy)]2(10)}2(H3O)(NO3)5ꢀ8H2O (5)
Reaction of [Pd(2,20-bpy)(H2O)2](NO3)2 in water with 1 in 1:
1-ratio yielded
a
Pd4 ‘‘open molecular box’’ ht,ht-{[Pd(2,20-
bpy)]2(10)}2(H3O)(NO3)5ꢀ8H2O (5) (Scheme 3). 5 was characterized
by 1H NMR spectroscopy, elemental analysis, and eventually X-ray
crystallography. The formation of a single and finite macrocyclic
species was indicated by the 1H NMR spectrum of 5 due to the
presence of sharp peaks corresponding to PdII(2,20-bpy) and 10. Rel-
ative intensities of the resonances of 10 and 2,20-bpy are consistent
with the above composition. The chemical shifts in D2O (d, 7.60,
7.23) of the two different H5 resonances of the 2,20-bpy ligand
(present in a 1:1 ratio, Supporting Information) clearly prove a
head–tail orientation within two stacked PdII(2,20-bpy) entities
[29].
This conclusion was confirmed by the results of the X-ray crys-
tal structure determination of 5. A view of the cation of 5 is de-
picted in Fig. 6. It is closely isostructural to the corresponding Pt
complex reported earlier by us [15] and its other modification (6,
see below). Each Pd is simultaneously bonded to a deprotonated
N3 position of 1 and to an O4 site, hence the 10 ligands in each
dinuclear Pd2 unit are head–tail. As observed for the other com-
plexes (3 and 4), uracil moieties retain their transoid orientations
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6
A. Khutia et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx
7.79 ppm in 5; second sets buried under signals between 8.0 and
8.2 ppm). Differences in overlap of the stacked bpy rings may ac-
count for it.
2.7. h,t-{[(2,20-bpy)Pt]2[1-MeU-N3,O4-CH(Ph)-1-MeUH]2}2+ (7)
Reaction of [Pt(2,20-bpy)(H2O)2](NO3)2 with 2 in water yielded
the dinuclear metal complex 7 (Scheme 4), which was identified
by a preliminary X-ray analysis.2 It establishes a head–tail orienta-
tion of the two coordinating uracil entities. We mention its existence
in the context of discussions regarding possible primary products
during reactions of cis-a2MII with bis(uracil-5-yl) type ligands (see
Section 2.9).
2.8. cis-Diammineplatinum(II) complexes of 1: multiple products
Fig. 6. View of the crystal structure of cation 5.Water molecules of crystallization
have been omitted for clarity.
Reactions carried out between cis-(NH3)2PtII and 1 on a 1H NMR
scale in D2O indicated the formation of multiple products side by
side, depending somewhat on reaction conditions (pH, concentra-
tion, ratio between metal and 1). A typical example is given in
the Supporting Information. It reveals (at least) nine uracil-H6
singlets, none of which is due to free 1. Five of these singlets cor-
respond to complexes described below; the remaining four reso-
nances are presently unassigned. Altogether four distinct
compounds (8–11) were crystallized and X-ray structurally charac-
terized, as outlined below. Although it proved difficult to selec-
tively isolate all four of the complexes in good yields, we report
on our findings as these provide insight into the complexity of met-
allacalix[n]arene systems containing pyrimidine nucleobases, and
allow for justified speculations regarding the existence of addi-
tional species.
of the N1–CH3 groups and the overall 1,3-alternate conformation
of the uracil rings. The cation thus represents a chiral double heli-
cate with two dipalladium stacks instead of two single metal ions,
as in 3. Unlike 3 with its rhomboid shape, cation 5 looks more like
an open box with comparable distances between the CH2 groups
(7.00(4) Å) and the Pd ions across the square (Pd1–Pd3,
7.293(3) Å; Pd2–Pd4, 7.186(3) Å. As compared to 3, the ‘‘height’’
of metallacalixarene 5 (distances between N-methyl groups) is
higher by ca. 0.4 Å. The distance between two Pd centers within
the stacked Pd(2,20-bpy) entities are 2.863(3) Å (Pd1–Pd2) and
2.844(3) Å (Pd3–Pd4), comparable to other stacked Pd(2,20-bpy)
entities [5b]. The 2,20-bpy planes are rotated slightly, by 6.8°
(average).
The crystal packing of 5 is dominated by
tions between the 2,20-bpy rings of the cations (Pd1,Pd2 rings,
3.5 Å), which form infinite arrays. Combination of further p–p
p–p stacking interac-
2.8.1. ht,ht-{[cis-Pt(NH3)2]2(10)}2(NO3)4ꢀ11H2O (8)
Complex 8 is a structural analogue of the tetranuclear com-
plexes 5 and 6. The cation consists of four cis-(NH3)2PtII units coor-
dinated to two 10 ligands, in which 10 adopts a transoid structure,
with the Pt ions bonded pairwise to N3 and O4 sites of the uracil
rings and mutually arranged in head–tail fashion. The cation is chi-
ral as a consequence of the head–tail arrangement of the uracilate
rings. Both enantiomers are present in the lattice of 8. Fig. 7 gives a
view of the cation of 8.
stacking involving the (Pd3,Pd4)-2,20-bpy rings builds a two-
dimensional stacking network which extends along the bc plane.
An additional feature of the crystal packing is the formation of a
hexagonal water cluster, to which the nitrate ligands and HNO3
are hydrogen bonded (Supporting Information): O1wꢀ ꢀ ꢀO4w,
2.48(4) Å; O4wꢀ ꢀ ꢀO3w, 2.65(4) Å; O3wꢀ ꢀ ꢀO6w, 2.69(4) Å;
O6wꢀ ꢀ ꢀO2w, 2.83(4) Å; O2wꢀ ꢀ ꢀO5w, 2.72(3) Å; O5wꢀ ꢀ ꢀO1w,
2.73(4) Å;
O1wꢀ ꢀ ꢀO11,
2.86(4) Å;
O4wꢀ ꢀ ꢀO53,
2.50(4) Å;
Salient structural features of 8 are as follows: The uracil rings in
each ligand 10 are almost perpendicular to each other, with dihe-
dral angles of 89.0(6)° and 87.5(6)°. As compared to 5 and 6, cation
8 is more extended as far as intramolecular distances between the
methyl groups of 10 (C1a–C1c, 10.33(3) Å; C1b–C1d, 10.29(4) Å)
and the methylene bridges (C51–C52, 6.70(2) Å) are concerned.
Distances between the Pt centers across the calix core are
7.2450(11) Å (Pt1–Pt4) and 7.3261(11) Å (Pt2–Pt3), hence longer
than in the cases of 5 and 6. On the other hand, Pt–Pt distances
within the dimetal cores are also significantly longer
2.9667(12) Å (Pt1–Pt2) and 2.9659(13) Å (Pt3–Pt4) in 8, probably
a consequence of the replacement of the well-stacking 2,20-bpy li-
gands by ammonia ligands. This difference in metal–metal contacts
is accompanied by slightly larger torsional angles of adjacent Pt
coordination planes in 8 (7.5(6)° and 8.7(8)°). Additional bond dis-
tances and angles are provided in the Supporting Information. As in
the previous cases, a complicated hydrogen bonding pattern
assembled by the cation of 8, counter anions and numerous water
O3wꢀ ꢀ ꢀO33, 2.73(5) Å; O6wꢀ ꢀ ꢀO13, 2.84(5) Å; O6wꢀ ꢀ ꢀO43,
2.73(4) Å; O2wꢀ ꢀ ꢀO22, 2.74(4) Å; O5wꢀ ꢀ ꢀO2b, 2.74(4) Å. Concern-
ing the location of the HNO3 proton, we could deduce, from the
short O1w–O4w and O4w–O53 distances the existence of a
H5O+2NOꢁ3 unit which in principle could stabilize this proton.
2.6. Second polymorph of ht,ht-{[Pt(2,20-bpy)]2(10)}2(NO3)4ꢀ12H2O (6)
The reaction of [Pt(2,20-bpy)(H2O)2](NO3)2 with 1 in a 2:1-ratio
yielded a second polymorph of the Pt4 ‘‘open molecular box’’
{[(2,20-bpy)Pt]2(10)}2(NO3)4ꢀ12H2O (6), which has been previously
been reported by us (compound 3 in Ref. [15]). The cation is de-
picted in the Supporting Information, with some selected bond dis-
tances and angles given. There are no major differences in bond
lengths and bond angles as compared to the reported structure.
In the crystal lattice of 6, cations align in infinite piles of
p–p
stacked 2,20-bpy ligands.
The 1H NMR resonances of 6 (Supporting Information) have
been previously assigned [15]. A comparison with the 1H NMR
spectrum of 5 (Supporting Information) reveals virtual identical
shifts of 7.93 ppm for the uracil H6 signals in the two complexes,
but some differences in bpy resonances, and in particular of one
of the two non-equivalent bpy-H6 doublets (8.35 ppm in 6;
2
Although of poor quality, and therefore not shown here, X-ray analysis of 7 is
consistent with a head–tail structure of the cation (Pt–Pt, 2.8597(4) Å) and a cisoid
orientation of the coordinated uracilate and the terminal uncomplexed uracil
(dihedral angle 84.78(14)°). The stereogenic centers at the two phenyl bonded C
atoms are R and S configurated, hence the cation represents a meso form.
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A. Khutia et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx
7
2+
CH3
N
CH3
Ph
N
O
O
Ph
O
O
a
N
N
Pt
O
O
a
HN
NH
2 Pt(2,2'-bpy)(NO3)2
2
a
O
O
pD 9
4 NO3
Pt
N
N
O
O
N
N
CH3
CH3
a
O
O
a2 = 2,2'-bpy
1
N
N
Ph
CH3
CH3
7
Scheme 4. Formation of complex 7.
Fig. 8. Sections of 1H NMR spectrum (D2O) of compound 9 with an admixture of 8
and an unknown impurity (⁄). Four sets of uracil-H6 and -CH3 signals are observed.
Fig. 7. Side view (top) and space filling representation (bottom) of cation of 8.
molecules of crystallization (11 in this case) is present in the crys-
tal packing.
As expected, the 1H NMR spectrum of 8 is very simple. In D2O
only three singlet resonances in the proper relative intensities
are observed, at 7.45 ppm (H6), 3.54 ppm (N1–CH3), and
3.08 ppm (CH2).
2.8.2. hh,ht-{[cis-Pt(NH3)2]2(10)}2(NO3)4ꢀ9.5H2O (9)
Fig. 9. View of molecular cation 9.
In the course of our studies, also a crystalline product was iso-
lated whose 1H NMR spectrum revealed the presence of four sets of
1-MeU resonances in a 1:1:1:1- ratio (Fig. 8).
From model building it is evident that this slippage causes rel-
atively minor structural changes as compared to 8. The main differ-
ence is that at the site of slippage, one Pt is now coordinated to two
N3 sites, while the other Pt is bonded to O4 and O2 sites of two dif-
ferent uracil rings. We call this bonding situation, somewhat sim-
plified, head–head, although we are aware that in the strict sense
this term should be reserved for situations in which one metal is
coordinated to two N3 sites, whereas the other one is coordinated
to identical sites, hence either two O4 or two O2 donor atoms.
X-ray crystallography (Fig. 9) subsequently proved the origin of
this spectral feature by revealing that the two ligands 10 displayed
different coordination modes: While one of the two bridging li-
gands in 9 exhibits the metal binding patterns seen in 5, 6, and
8, the second ligand uses N3 and O4 as coordination sites on one
edge, and N3 and O2 at the other edge. As the transoid orientations
of the two uracil rings are maintained in both cases, a simple slip-
page movement has occurred at one of the two edges (Scheme 5).
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8
A. Khutia et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx
Scheme 5. Structural relationship between Pt2 edge in 8 and 9: Slippage of uracil ring from N3,O4 bridge to O2,N3 bridge.
Fig. 10. Top and side views of the highly symmetrical cation 11.
Structural details of 9 are as follows: The unit cell contains two
The two different cations in 10 do not differ much with regard to
structural details from their individual components, as observed
in a preliminary X-ray structure (Supporting Information).
crystallographically different cations of 9, which present an almost
identical geometry. The high number of water molecules of crystal-
lization – 21 crystallographically different occupied sites in this
case – impedes a good quality of the collected data. Tilt angles val-
ues between the uracil rings in each ligand 10 are slightly distant
from perpendicularity in comparison with 5, 6, and 8 (88.1(4)°,
83.8 (5)°, 84.9(4)°, 80.0(6)°). Due to the coordination slippage, the
cations of 9 display marked differences between C1–C1 distances:
8.52(3) Å (C1d–C1e) and 8.56(3) Å (C1e–C1g) in the short edge,
and 10.59(3) Å (C1a–C1c) and 10.62(3) Å (C1f–C1h) in the long
edge. Distances between adjacent platinum atoms are similar to
the found in the previous cases (Pt1–Pt2, 2.9152(11) Å; Pt3–Pt4,
2.9242(12) Å). Distances between the platinum ions, and distances
between the CH2 bridges across the calix cores are in this case sig-
nificantly longer, also a consequence of the different geometry:
2.8.4. Hexanuclear complex ht,ht,ht-{[cis-Pt(NH3)2(10)]3(NO3)6ꢀ9H2O
(11)
As a final product isolated from reaction mixtures of cis-(NH3)2
PtII and bis(1-methyluracil-5-yl)methane (1) we wish to report on
a cyclic complex comprised of three Pt2 stacks and three dianions
of 1. ht,ht,ht-{[cis-Pt(NH3)2(10)]3(NO3)6ꢀ9H2O (11) forms in the
presence of an excess of Pt over 1 (2:1) at pH 5. Needle-shaped,
light green crystals of 11 were isolated from a reaction mixture
that had turned dark blue, presumably due to partial oxidation of
Pt, similar to the situation with ‘‘platinum pyrimidine blues’’
[30]. The concentration of the solution from which 11 was ob-
tained, was five times higher than that from which the tetranuclear
complexes 8, 9, and 10 were isolated. Fig. 10 provides two views of
the hexanuclear cation 11. As can be seen, all three ligands 10 are
transoid and all three Pt2 pairs are coordinated in head–tail fashion
through N3 and O4 sites of the anionic uracil rings. While the top
view of 11 gives the impression of a twisted triangular prism with
sides of 8.8336(12) Å (as defined by the distances between plati-
num atoms), a side view reveals it to have more of a bowl shape,
with access from two sides (see Fig. 10, left). The interior cavity
is not empty, but rather filled with a water molecule (Fig. 11).
Pt–Pt distances within the dinuclear edges are 2.9544(14) Å, and
distances between centroids of opposite uracil rings are ca.
7.74 Å. Additional structural details are given in the Supporting
Information.
Pt1–Pt3,
7.9569(11) Å;
Pt2–Pt4,
7.7411(11) Å;
Pt5–Pt7,
8.0097(12) Å; Pt6–Pt8, 7.9327(11) Å; C51–C52, 6.82(2) Å; C53–
C54, 6.75(2) Å. The crystal packing presents a high density of hydro-
gen bonding between cations, nitrate anions and water molecules.
Slippage could occur, in principle, also at the other edge of the
metallacycle. The mixed Pd,Ag complex 4 provides an excellent
example to support this option, in that substitution of the Ag+ ions
by square-planar cis-a2M (M = PtII or PdII) entities would generate
exactly such a product with twofold N3 binding of two of the metals,
and mixed O2,O4 binding of the two others, hence head–head orien-
tations at both edges of the metallacycle (in the restricted definition,
see above). In the 1H NMR, this coordination scheme would result
again in a simplification, namely to a single set of resonances of 10.
2.8.3. A 2:1-adduct of 8 and 9: Compound 10
2.9. Interrelationship between complexes described: A minimal
formation scheme
The X-ray analysis of yet another species isolated from a reac-
tion mixture of cis-(NH3)2PtII and 1 upon slow evaporation proved
to be a 2:1-adduct of compounds 8 and 9, namely 2[ht,ht-{[cis-
Pt(NH3)2]2(10)}2]ꢀ[hh,ht-{[cis-Pt(NH3)2]2(10)}2](NO3)12ꢀ16H2O (10).
In this paper we report on eight metal complexes containing the
(twofold) deprotonated ligand 1 and the monodeprotonated ligand
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A. Khutia et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx
9
considered logical primary products to be formed between cis-
a2MII and L, subject to reaction conditions (ratio of reactants, con-
centration, . . .) and differences in kinetics (PdII reacting much fas-
ter than PtII). These primary products are expected to be of ML2 (I),
ML (II), and M2L (III) stoichiometries. It is assumed that the first
metal coordination takes place at N3 sites of the uracil entities,
with the possible exception of Pt2L, if favorable stacking interac-
tions of the a2 co-ligands (e.g., 2,20-bpy) might favor the second
M to bind to an exocyclic oxygen, most likely O4 (IIIb) rather than
N3 at the other side of the ligand (IIIa). Starting out from these
three (four) species, the formation of any of the various types of
complexes reported here, can be rationalized in straightforward
ways (Scheme 6, middle part). These are (i) the metallacycle 3 of
M2L2 stoichiometry, (ii) the dimer 7 of likewise M2L2 stoichiome-
try, (iii) the tetranuclear complexes 5, 6, and 8 of M4L2 stoichiom-
etries, (iv) the tetranuclear complex
9 of likewise M4L2
stoichiometry, and (v) the hexanuclear complex 11 of M6L3 stoichi-
ometry. Complex 10 needs not to be discussed in this context, and
we shall return to 4 as a model for additional possibilities of com-
plex composition (see below). Possible rearrangement (isomeriza-
tion) reactions between individual types of compounds shall not be
further discussed, although they should not be excluded, in partic-
ular in the case of M = Pd, but even with M = Pt [31]. The possibil-
Fig. 11. Space filling representation of cation 11 with the inserted water molecule
(yellow). (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
2, respectively. In all cases reactions with cis-a2MII (M = Pt or Pd) in
water lead to coordination of uracil-N3 sites, which become depro-
tonated. Despite relatively high pKa values of the uracil moieties in
1 and 2, metal coordination does not require high pH, as the cis-
[a2M(H2O)2]2+ diaqua species occur as mixed hydroxo, aqua spe-
cies at neutral or weakly acidic pH, and are thus capable of deprot-
onating the uracil rings with their hydroxo groups.
Ignoring differences in the nature of the metal M (Pt or Pd), the
nature of the a2 ligands (2,20-bpy; bipzp; (NH3)2), and also in the
two bis(uracil) ligands L (1 and 2), the similarities in structural fea-
tures of the complexes characterized allow for a combined view on
the products formed and permit a minimal reaction scheme for
their formation to be put together. On the top of Scheme 6 simple
complexes are drawn which, although not isolated, can be
ity, that
9 could be the result of a minor rearrangement
(‘‘slippage’’) within 8 has been briefly mentioned. Alternatively, 9
could form in a stepwise reaction of ML2 (I) upon successive coor-
dination of three more metal entities M.
Complex 4 deserves some more comments (see also Scheme 6,
bottom). In it, Ag+ ions adopt unusual square-planar coordination
geometries, involving O2 and O4 sites in their binding sites to urac-
ilate anions. Instead of forming a coordination polymer, as realized
in 4, square-planar cis-a2MII entities could bind in the very same
fashion to 3, thereby producing yet another isomer of M4L2 stoichi-
ometry, different from the linkage pattern realized in 5, 6, or 8 on
one hand, and in 9 on the other. In fact, 9 with its mixed O2,O4
Scheme 6. Reaction products feasible and/or formed between cis-a2MII (M = Pt or Pd) and bis(1-methyluracil-5-yl)methane 1 (or related ligand 2): Postulated initial products
I–III (top), isolated species 3, 5–9, 11 (middle), and analogues derived on the basis of structure 4 (bottom). Additional possible isomers derived from ligand 1 in its cisoid
conformation are not shown.
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A. Khutia et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx
coordination sphere of one of the four Pt ions, can be considered an
intermediate between the structures seen in 5, 6, 8 (with all-ht
binding modes through N3 and O4) and the putative M4L2 ana-
logue of 4 with mixed O2,O4 binding of two of the four metals. It
is obvious, that with a larger excess of M, even products of M5L2
as well as M6L2 stoichiometries are feasible.
With the exception of 7, all structurally characterized com-
plexes have the bridging ligands 10 in transoid orientations. As
briefly pointed out above (Section 2.3.) there is no compelling rea-
son why the bis(1-methyluracil-5-yl)methane ligand in its depro-
tonated form (10) should not be able to also adopt a cisoid
arrangement and to form cone calixarene structures. If realized,
yet additional linkage isomers of M4L2 stoichiometry would be
possible, and in addition species of composition M3L2, M5L2, and
M6L2. In the latter case, all eight exocyclic oxygen atoms of the four
uracil rings would be occupied by metal ions, and in addition all
four N3 sites.
obtained was dissolved in 40 mL dichloromethane. The bis(pyra-
zol-1-yl)propane (282 mg, 1.60 mmol) was added and the resulting
solution was stirred at room temperature for 12 h. The yellow solid
was filtered off, washed with CH2Cl2 and dried at 40 °C. Yield:
500 mg (88%). Anal. Calc. for C9H12N4Cl2Pd: C, 30.6; H, 3.4; N,
15.9. Found: C, 30.4; H, 3.5; N, 16.8%. 1H NMR (CDCl3, 200 MHz,
d/ppm): 8.32 (H3, d, 3J = 3.0 Hz), 7.74 (H5, d, 3J = 3.0 Hz), 6.46
(H4, t, 3J = 3.0 Hz), 2.78 (CH3, s).
4.3. Second polymorph of bis(1-methyluracil-5-yl)methane
monohydrate (1a)
AgNO3 (85 mg, 0.5 mmol) was added to a solution of 1 (52.8 mg,
0.2 mmol) in a 1:1-mixture of MeOH and water (2 mL), the mixture
stirred for 2 h, and then left in a refrigerator at 4 °C for crystalliza-
tion. Two days later, colorless crystals of 1a were collected (30 mg,
57%). X-ray crystallography showed the crystals not to be a Ag
complex of 1 but rather to be a second polymorph of the previously
studied 1 [15].
3. Summary
4.4. Bis(1-methyluracil-5-yl)methyl-benzene (2)
In this manuscript we describe a series of metal complexes de-
rived from cis-a2MII and bis(1-metyluracil-5-yl)methane ligands.
With a single exception (complex 7) all compounds are metallacy-
cles, hence can be termed metallacalix[n]arenes. It strikes that all
compounds except 3 contain stacked dimetal units. This feature
is attributed to the pronounced basicity of the exocyclic oxygen
atoms of uracil anions, as numerously demonstrated by us, and
has some precedence in cyclic complexes of uracil and cytosine
[3,5] as well as larger imidato ligands [32] or the cyanurato ligand
with its three imide functionalities [33]. The complexity of the here
discussed metallacalix[4]arenes is, among others, evident from the
fact that species of M4L2 stoichiometry, for example, can conceiv-
ably exist in numerous isomeric variants, and that all species hav-
ing head–tail disposed dimetal entities are inherently chiral.
Structurally related cages (e.g., triangle [34], square [35], octahe-
dron [36]) have also been synthesized by applying dimetal units
with metal–metal bonds and tetradentate bridging ligands such
as amidinates, oxalate, and dicarboxylates, but in these complexes
the dimetal units do not represent stereogenic elements. In related
work we have uncovered that metallacalix[4]arenes derived from
the parent nucleobases uracil and cytosine can occur in a large
number of isomeric forms [35], and have proposed that this feature
may be one reason to account for the difficulty in fully character-
izing a class of potent antitumor agents called ‘‘platinum pyrimi-
dine blues’’ [37]. It appears that the here described bis(1-
methyluracil-5-yl)methane ligand is similar as far as the complex-
ity in terms of metal coordination by cis-a2PtII is concerned.
This compound was synthesized in 90% yield according to a
slightly modified version from that reported in the literature
[15,17]. Specifically, 1-methyluracil was reacted with benzalde-
hyde in conc. HCl. 1H NMR (DMSO-d6, 298 K, d, ppm): 11.32 (2H,
s, N3–H), 7.03 (2H, s, C6–H), 5.09 (1H, s, CHPh), 3.19 (6H, s, CH3),
7.17 (2H, d, Ph), 7.24 (1H, dd, Ph), 7.32 (2H, dd, Ph).
4.5. [Pd(bipzp)(10)]2. in [Pd(bipzp)(10)]2ꢀ13H2O (3)
A
mixture of PdCl2(bipzp) (353 mg, 1 mmol) and AgNO3
(340 mg, 2 mmol) was stirred in water (50 mL) at 60 °C for 2.5 h
in dark. The suspension obtained was cooled to 0 °C and the precip-
itated AgCl was subsequently removed by filtration. To the filtrate
thus obtained, 1 (264 mg, 1 mmol) was added and pH of the solu-
tion adjusted to ca 9.0 by addition of aqueous 1 M NaOH solution.
The reaction mixture was stirred overnight at 60 °C. The resulting
solution was concentrated to 5 mL, centrifuged and left in a beaker
at 4 °C. Prism shaped, light yellow crystals were obtained after two
days (150.6 mg, 12%). Anal. Calc. for C40H62N16O17Pd2 (9-hydrate):
C, 38.4; H, 5.0; N, 17.9. Found: C, 38.4; H, 5.1; N, 18.0%. 1H NMR
3
(D2O, pD 5.8, 298 K, d, ppm): 8.120 (4H, d, H30, JH–H = 2.5 Hz),
7.632 (4H, s, H50), 7.416 (4H, s, H6), 6.451 (4H, s, H40), 3.356
(12H, s, CH3), 3.076 (4H, s, CH2), and 2.991 (12H, s, CH03).
4.6. [{Pd(bipzp)(10-N3)}2(Ag–O2,O4)]n in [{Pd(bipzp)(10-N3)}2(Ag–
O2,O4)]nꢀ15H2O (4)
4. Experimental
To an aqueous solution (3 mL) of 3 (65 mg, 0.05 mmol), 0.5 mL
aqueous solution of AgNO3 (42.5 mg, 0.25 mmol) was added with
stirring. The formation of a colorless precipitate was observed.
The reaction mixture was cooled in an ice bath for 2 h and the pre-
cipitate formed in due course was subsequently collected by filtra-
tion. Recrystallization from water (4 mL) yielded colorless single
crystals (10.6 mg crystals, 10%). Anal. Calc. for C40H64N17O21Pd2Ag
(5-hydrate): C, 35.6; H, 4.0; N, 17.6. Found: C, 35.5; H, 3.9; N,
17.6%. The 1H NMR spectrum of 4 in D2O is virtually identical with
that of compound 3, as can be expected.
4.1. Starting materials and preparations
2,20-bipyridine (2,20-bpy), uracil, paraformaldehyde, benzalde-
hyde, HCl, K2PtCl4, K2PdCl4, PdCl2, and AgNO3 were of commercial
origin and used as received without further purification. 1-Methyl-
uracil [38], bis(pyrazol-1-yl)propane [39] bis(1-methyluracil-5-
yl)methane monohydrate (1) [15] PtCl2(2,20-bpy) [40], and
PdCl2(2,20-bpy) [41] were prepared as described in the literature.
4.2. Preparation of PdCl2(bipzp)
4.7. ht,ht-{[Pd(2,20-bpy)]2(10)}2(H3O)(NO3)5ꢀ8H2O (5)
This compound was prepared in a slightly modified version of
that reported in Ref. [20]. PdCl2 (285 mg, 1.60 mmol) was dissolved
in refluxing acetonitrile (15 mL) and then cooled to room temper-
ature. The solution was evaporated to dryness and the solid
Pd(2,20-bpy)Cl2 (67 mg, 0.2 mmol) was stirred with AgNO3
(68 mg, 0.4 mmol) in water (10 mL) with light excluded at 40 °C
for 12 h. The suspension obtained was cooled to 0 °C in ice bath
for 30 min. Then AgCl was removed from the solution by filtration.
Please cite this article in press as: A. Khutia et al., Inorg. Chim. Acta (2014), http://dx.doi.org/10.1016/j.ica.2014.01.020

A. Khutia et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx
11
1 (52.8 mg, 0.2 mmol) was added to the clear filtrate and pH was
measured and found to be 2.0. Then the mixture was stirred at
40 °C for 2 h. The resulting light yellow solution was concentrated
to a volume of 5 mL at 40 °C on a rotary evaporator. Yellow cubes
were obtained after one day. The yield was 85 mg (60%). 1H NMR
(D2O, pD 2.0, 298 K, d, ppm): 10, 3.69 (12 H, s, CH3), 3.60 (4H, s,
(b) The same reaction was carried out under identical condi-
tions as procedure (a), yet at higher dilution (20 mL). Here
also colorless crystals were obtained after six days, and
one crystal was measured by X-ray crystallography, which
proved to be complex 10, a 2:1 adduct of 8 and 9.
3
CH2), 7.93 (4H, s, H6); 2,20-bpy, 7.23 (4H, dd, H5, JH–H = 6.0 Hz,
The 1H NMR spectra of the isolated solids suggest that proce-
dures (a) and (b) lead to qualitatively similar mixtures of 8 and
9, with 9 representing the major species. The crystals of complex
8 and complex 9 have practically identical shapes and cannot be
separated under a microscope.
3
4JH–H = 3.0 Hz), 7.60 (4H, t, H50, JH–H = 7.2 Hz), 7.79 (4H, dd, H6,
3JH–H = 6.0 Hz), 7.94–8.18 (20H, m, H3,H30,H4,H40,H60).
4.8. Second poymorph of ht,ht-{[Pt(2,20-bpy)]2(10)}2(NO3)4ꢀ12H2O (6)
This compound was prepared in analogy to the earlier reported
one (8-hydrate) [15], except that the Pt:1-ratio had been altered,
from 1:1 originally to 2:1 in the present case. X-ray crystallography
of a suitable crystal proved it to be a second polymorph of the pre-
viously described compound.
4.12. ht,ht,ht-{[cis-Pt(ND3)2(10)]3(NO3)6ꢀ9D2O (11)
cis-Pt(NH3)2Cl2 (60 mg, 0.2 mmol) was mixed with AgNO3
(68 mg, 0.4 mmol,) in D2O (2 mL) with light excluded and stirred
at 80 °C for 2 h. The suspension obtained was cooled to 0 °C in an
ice bath for 30 min. Then AgCl was removed from the solution by
centrifugation. 1 (26.4 mg, 0.1 mmol) was added to the filtrate
and the pD was adjusted to 5.0 by adding NaOD. Then the mixture
was warmed at 40 °C for 7 d. After centrifugation of some precipi-
tate, which was not further characterized, the resulting solution
was kept in a refrigerator at 4 °C for several days. During the time
the color of the solution turned dark blue. Needle shaped, light
green crystals were obtained after 1 week. The isolated yield was
ca. 5 mg, 6%. A crystal with an appearance different from that of
either 8 or 9 was picked for X-ray analysis and showed it to be
compound 11. The 1H NMR spectrum of the isolated material
(D2O, pD 5.2) was practically identical with that of complex 8,
however, suggesting that 11 was a very minor component only
or that it had formed from 8 during the final crystallization step.
Monitoring the reaction on an NMR scale likewise did not reveal
any strong indication for the existence of another species besides
8 or 9 in solution. We rule out that 8 and 11 could have identical
1H NMR spectra, despite identical N3, O4 binding patterns.
4.9. h,t-{[(2,20-bpy)Pt]2[1-MeU-N3,O4-CH(Ph)-1-MeUH]2}2+ (7)
An aqueous suspension (30 mL) of PtCl2(2,20-bpy) (84.4 mg,
0.2 mmol) and AgNO3 (68 mg, 0.4 mmol) was stirred for 24 h at
60 °C. The solution was cooled in ice for 1 h and the resultant AgCl
precipitate was filtered off. 2 (68 mg, 0.2 mmol) was added and the
solution was stirred for 3 days at 65 °C. The resulting solution was
concentrated to a volume of 6 mL in a rotary evaporator, centri-
fuged, and filtered. The clear filtrate was kept in an open beaker
at room temperature. After 4 d, 167 mg of dark red crystals were
harvested. The compound was not obtained in analytically pure
form, but a preliminary X-ray crystal structure of a crystal con-
1
firmed its dinuclear head–tail structure (see Ref. and Supporting
Information).
4.10. ht,ht-{[cis-Pt(NH3)2]2(10)}2(NO3)4ꢀ11H2O (8)
An aqueous suspension (20 mL) of cis-[Pt(NH3)2Cl2] (120 mg,
0.4 mmol) and AgNO3 (136 mg, 0.8 mmol) was stirred at 60 °C for
12 h. The solution was cooled in ice and the AgCl precipitate was
filtered off. 1 (52.8 mg, 0.2 mmol) was added to the solution and
the mixture was then stirred at 40 °C. The pH of the solution was
repeatedly adjusted to 5 using 0.1 M NaOH solution. After one
week the solution was concentrated to a volume of 3 mL, filtered
and kept at 4 °C. After 4 days, colorless crystals were obtained.
The yield was 131.7 mg (78%). Anal. Calc. for C22H44N20O20Pt4: C,
15.7, H, 2.6, N, 16.6. Found: C, 15.4, H, 2.9, N, 16.3%. 1H NMR
(D2O, pD 2.0, 298 K, d, ppm): 3.08 (4H, s, CH2), 3.54 (12H, s, CH3),
7.45 (4H, s, H6). According to 1H NMR spectroscopy, the compound
isolated this way consisted exclusively of 8, whereas slight modifi-
cations of the synthetic procedure yielded in addition other prod-
ucts (see below).
4.13. Instrumentation
1H NMR spectra were recorded on Bruker AC 300 and Bruker
DRX 400 NMR instruments with 3-(trimethylsilyl)propanesulfo-
nate (TSP, d = 0 ppm) as internal reference in D2O and the solvent
residual peak (d = 2.50 ppm) as reference for DMSO-d6. Elemental
(C, H, N) analysis data were obtained on a Leco CHNS-932 instru-
ment. In the large majority of cases elemental analysis data gave
values which agreed with significantly lower water contents than
established by X-ray analysis
4.14. Determination of pKa values
The pKa values of 2 were determined by pD-dependent 1H NMR
spectroscopy. The changes in chemical shift of the C6–H and N1–
CH3 (no change for CH(Ph) proton) protons were recorded at the
different pD stages. pD values of NMR samples were measured
by use of a glass electrode and addition of 0.4 units to the uncor-
rected pH meter reading (pH⁄). pD values were varied by addition
of NaOD and DNO3 to the respective samples. The graphs (chemical
shift versus pD) were evaluated with a nonlinear least-squares fit
according to the Newton–Gauss method [42] The obtained acidity
constants (for D2O) were transformed to values valid for H2O [43].
4.11. hh,ht-{[cis-Pt(NH3)2(10)}2(NO3)4ꢀ9.5H2O (9) and 2[ht,ht-{[cis-
Pt(NH3)2]2(10)}2]ꢀ[hh,ht-{[cis-Pt(NH3)2]2(10)}2](NO3)12ꢀ16H2O (10)
(a) An aqueous suspension (3 mL) of cis-Pt(NH3)2Cl2 (120 mg,
0.4 mmol) and AgNO3 (136 mg, 0.8 mmol) was stirred at
60 °C for 12 h. The solution was then cooled in ice and the
AgCl precipitate was filtered off. 1 (52.8 mg, 0.2 mmol) was
added to the solution and the pH of the solution was
adjusted to 5 using 0.1 M NaOH solution (one time). The
mixture was then stirred at 40 °C for one week. The solution
was subsequently centrifuged from some unidentified pre-
cipitate and the filtrate was kept at 4 °C. It adopted a light
blue color within two days. Colorless crystals were har-
vested after five days. One crystal was picked from solution
for X-ray analysis and proved to be 9.
4.15. X-ray crystallography
X-ray crystal data collections for 1a, 3, 4, 5, 6, 8, 9, and 11 were
recorded at 150 K on an Xcalibur Sapphire3 diffractometer
equipped with an area detector and graphite monochromated Mo
Ka radiation (0.71073 Å). Data reduction was performed using
Please cite this article in press as: A. Khutia et al., Inorg. Chim. Acta (2014), http://dx.doi.org/10.1016/j.ica.2014.01.020

12
A. Khutia et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx
the CRYSALISPRO software package [44]. The structures were solved by
direct methods and refined by full-matrix least-squares methods
based on F2 using the SHELXL-97 programs [45]. All non-hydrogen
atoms were refined anisotropically with thermal and distance
restraints applied when necessary. Hydrogen atoms were posi-
tioned geometrically and refined with isotropic displacement
parameters according to the riding model. All calculations were
carried out with the SHELXL-97 and WINGX programs [45,46].
K
a
) = 0.71073 Å, T = 150 K,
collected, 12005 observed (Rint = 0.0812), R1(Fo)
[I > 2 0.2019 (all data), Goodness-of-fit
(I)], wR2(F2o)
l
= 10.257 mmꢁ1, 27200 reflections
0.0778
=
r
=
(GOF) = 0.970. CCDC 974347.
4.15.8. Crystal data for 9
[C44H164N40O59Pt8], monoclinic, P21/c, a = 20.1556(16) Å,
b = 29.455(2) Å,
c = 21.743(3) Å,
V = 11022.9(19) Å3, Dcalc. = 2.265 g cmꢁ3
a) = 0.71073 Å, T = 150 K, l
= 10.223 mmꢁ1, 53944 reflec-
b = 121.358(8)°,
Z = 4,
Mr = 3758.87 g molꢁ1
,
,
4.15.1. Crystal data for 1a
k(Mo K
tions collected, 25506 observed (Rint = 0.0871), R1(Fo) = 0.0598
[I > 2 0.1251 (all data), Goodness-of-fit
(I)], wR2(F2o)
[C11H14N4O5], triclinic, P21/c, a = 8.2374(3) Å, b = 8.7690(4) Å,
c = 17.0166(7) Å,
V = 1223.42(9) Å3, Dcalc. = 1.532 g cmꢁ3, k(Mo K
T = 150 K,
= 0.123 mmꢁ1, 23787 reflections collected, 3061 ob-
served (Rint = 0.0511), R1(Fo) = 0.0381 [I > 2
(I)], wR2(F2o) = 0.0742
b = 95.547(4)°,
Z = 4,
Mr = 282.26 g molꢁ1
,
r
=
a) = 0.71073 Å,
(GOF) = 0.973. CCDC 974348.
l
r
4.15.9. Crystal data for 11
(all data), Goodness-of-fit (GOF) = 0.990. CCDC 974350.
[C33H81N30O40Pt6],
c = 18.0896(9) Å, Z = 2, Mr = 2708.82 g molꢁ1
Dcalc. = 1.998 g cmꢁ3
k(Mo
= 9.377 mmꢁ1
41046 reflections collected, 3831 observed
(Rint = 0.0540), R1(Fo) = 0.0785 [I > 2
(I)], wR2(F2o) = 0.2271 (all data),
trigonal,
P–31c,
a = 16.9534(9) Å,
,
V = 4502.7(4) Å3,
4.15.2. Crystal data for 3
,
Ka)
=
0.71073 Å, T = 150 K,
ꢀ
[C40H70N16O21Pd2],
b = 14.7605(5) Å, c = 15.7897(6) Å,
= 80.895(3)°, Z = 2, Mr = 1323.92 g molꢁ1
Dcalc. = 1.591 g cmꢁ3
k(Mo 0.71073 Å, T = 150 K,
= 0.740 mmꢁ1
23780 reflections collected, 12452 observed
(Rint = 0.0332), R1(Fo) = 0.0315 [I > 2
(I)], wR2(F2o) = 0.0561 (all
triclinic,
P1,
a = 12.3877(5) Å,
l
,
a
= 86.764(3)°, b = 75.852(3)°,
r
c
,
V = 2763.80(18) Å3,
Goodness-of-fit (GOF) = 1.076. CCDC 974349.
,
Ka
)
=
l
,
Acknowledgements
r
data), Goodness-of-fit (GOF) = 0.839. CCDC 974351.
This work was supported by Deutsche Forschungsgemeinschaft
(DFG), the ‘‘International Max Planck Research School in Chemical
Biology’’ in Dortmund (for a fellowship to A.K.) and DAAD (for a
‘‘Forschungsaufenthalt’’ award to N.D.). P.J.S.M. thanks funding
4.15.3. Crystal data for 4
ꢀ
[C40H74AgN17O26Pd2],
triclinic,
P1,
a = 13.4345(10) Å,
= 71.703(8)°,
Mr = 1529.83 g molꢁ1
k(Mo 0.71073 Å,
= 0.979 mmꢁ1, 25456 reflections collected, 13735 ob-
served (Rint = 0.0772), R1(Fo) = 0.1092 [I > 2
(I)], wR2(F2o) = 0.3038
b = 15.2333(13) Å,
c = 17.3649(15) Å,
= 77.475(7)°, Z = 2,
V = 3099.1(4) Å3, Dcalc. = 1.639 g cmꢁ3
T = 150 K,
a
from the Spanish Ministerio de Economía
y Competitividad
b = 67.541(8)°,
c
,
(‘‘Ramón y Cajal’’ program and CTQ2011-27593). We thank Ms
Stefanie Seidel for experimental assistance.
,
Ka) =
l
r
Appendix A. Supplementary material
(all data), Goodness-of-fit (GOF) = 1.054. CCDC 974352.
NMR spectra and pH dependence of compound 2. CCDC
974347–974354 contain the supplementary crystallographic data
(1a, 3, 4, 5, 6, 8, 9, and 11). These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.ica.2014.01.020.
4.15.4. Crystal data for 5
[C62H71N21O32Pd4],
b = 47.844(3) Å, c = 15.4389(13) Å,
Mr = 2048.00 g molꢁ1 V = 7496.5(10) Å3,
k(Mo K ) = 0.71073 Å, T = 150 K,
= 1.048 mmꢁ1, 38845 reflec-
tions collected, 16431 observed (Rint = 0.1765), R1(Fo) = 0.2173
[I > 2 0.4871 (all data), Goodness-of-fit
(I)], wR2(F2o)
monoclinic,
P21/c,
b = 104.141(8)°,
Dcalc. = 1.815 g cmꢁ3
a = 10.4660(9) Å,
Z = 4,
,
,
a
l
r
=
(GOF) = 1.005. CCDC 974353.
References
4.15.5. Crystal data for 6
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[2] (a) M.-X. Wang, Chem. Commun. (2008) 4541. and refs. cited;
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[C62H76N20O32Pt4],
triclinic,
P1,
= 99.061(3)°, b = 94.446(3)°,
V = 7725.3(5) Å3,
0.71073 Å, T = 150 K,
63581 reflections collected, 34457 observed
(Rint = 0.0777), R1(Fo) = 0.0927 [I > 2
(I)], wR2(F2o) = 0.2338 (all data),
a = 14.4817(5) Å,
b = 22.4096(9) Å, c = 24.7764(9) Å,
a
c
= 101.702(3)°, Z = 4, Mr = 2393.79 g molꢁ1
,
Dcalc. = 2.058 g cmꢁ3
= 7.321 mmꢁ1
,
k(Mo
Ka)
=
l
,
r
Goodness-of-fit (GOF) = 1.001. CCDC 974354.
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(c) J.A.R. Navarro, E. Freisinger, B. Lippert, Inorg. Chem. 39 (2000) 2301;
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4.15.6. Crystal data for 7
[C54H98N14O40Pt2],
monoclinic,
C2/c,
b = 91.934(3)°,
a = 23.5996(7) Å,
Z = 4,
b = 26.8002(10) Å,
c = 14.5305(5) Å,
Mr = 1973.64 g molꢁ1, V = 9184.9(5) Å3, Dcalc. = 1.427 g cmꢁ3, k(Mo
[5] (a) E.G. Bardají, E. Freisinger, B. Costisella, C.A. Schalley, W. Brüning, M. Sabat,
B. Lippert, Chem. Eur. J. 13 (2007) 6019;
Ka) = 0.71073 Å, T = 150 K, l
= 3.129 mmꢁ1, 29426 reflections col-
lected, 10627 observed (Rint = 0.0385), R1(Fo) = 0.0337 [I > 2r(I)],
(b) A. Khutia, P.J. Sanz, Inorg. Chem. 49 (2010) 7635;
(c) A. Khutia, P.J. Sanz, Chem. Eur. J. 17 (2011) 4195;
(d) A. Khutia, P.J. Sanz, Chem. Eur. J. 17 (2011) 4205.
wR2(F2o) = 0.0900 (all data), Goodness-of-fit (GOF) = 0.988. CCDC
974355.
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[C22H55N20O31Pt4],
monoclinic,
P21/n,
b = 104.296(5)°,
a = 10.9959(5) Å,
Z = 4,
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Mr = 1887.31 g molꢁ1, V = 5494.8(4) Å3, Dcalc. = 2.281 g cmꢁ3, k(Mo
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