Discrete Molecular Squares {[(en)M(CN)]4}4+ Derived from [(en)M(CN)2] (M = PtII, PdII)

Article abstract of DOI:10.1002/ejic.201001216

C_2h-symmetrical tetranuclear metallacycles {[M(en)(CN)] ₄}(NO₃)₄ with M = Pd^II (4) and Pt^II (5) have been prepared upon reacting M(en)(CN)₂ [M = Pd^II (1), Pt^II (2)] with [M(en)(H₂O) ₂](NO₃)₂. Replacement of the nitrate anions of 5 by terephthalate anions yields the corresponding salt 5a. The X-ray crystal structures of 1, 4, 5, and 5a have been determined. In the metallacycles 4, 5, and 5a the four metals form almost ideal squares with average M···M distances of ca. 5.05 A? (5, 5a) and 5.08 A? (4) along the sides. As shown by ¹H NMR spectroscopy, the Pt square 5 is stable in aqueous solution, whereas the Pd square 4 undergoes rearrangement reactions upon aging or the presence of other Pd species such as (bpy)Pd^II. Preliminary studies on the possibility of non-covalent interactions of 4 and 5 with model nucleobases in water reveal that only 5 is useful in this respect. According to the concentration-dependence ¹H NMR study, there is an interaction with the purine base 9-ethyladenine, molecular details of which are unclear at this stage, however. Compound 4 is substitutionally labile and is transformed into the coordination compound 8 with 1-methylcytosine. Two more side products, produced during the various reactions carried out, were characterized by X-ray crystallography: [Pt(en) ₂][Pt(CN)₄] (3) and [Pd(bpy)(en)](SO₄) ·3H₂O (7). Cationic molecular squares composed of (en)M (M = Pt^II, Pd^II) corners and cyanide bridges have been prepared and details of their formation and reactivity have been studied. Copyright

Full text of DOI:10.1002/ejic.201001216

FULL PAPER
DOI: 10.1002/ejic.201001216
Discrete Molecular Squares {[(en)M(CN)]4}4+ Derived from [(en)M(CN)2]
(M = PtII, PdII)
Anzhela Galstyan,[a] Pablo J. Sanz Miguel,[a,b] Jacqueline Wolf,[a] Eva Freisinger,[c] and
Bernhard Lippert*[a]
Keywords: Cyanides / Platinum / Palladium / Metallacycles / Model nucleobases
C2h-symmetrical tetranuclear metallacycles {[M(en)(CN)]4}-
(NO3)4 with M = PdII (4) and PtII (5) have been prepared upon
reacting M(en)(CN)2 [M = PdII (1), PtII (2)] with [M(en)(H2O)2]-
PdII. Preliminary studies on the possibility of non-covalent in-
teractions of 4 and 5 with model nucleobases in water reveal
that only 5 is useful in this respect. According to the concen-
tration-dependence 1H NMR study, there is an interaction
with the purine base 9-ethyladenine, molecular details of
which are unclear at this stage, however. Compound 4 is sub-
stitutionally labile and is transformed into the coordination
compound 8 with 1-methylcytosine. Two more side products,
produced during the various reactions carried out, were char-
acterized by X-ray crystallography: [Pt(en)2][Pt(CN)4] (3) and
[Pd(bpy)(en)](SO4)·3H2O (7).
(NO3)2. Replacement of the nitrate anions of
5 by
terephthalate anions yields the corresponding salt 5a. The X-
ray crystal structures of 1, 4, 5, and 5a have been determined.
In the metallacycles 4, 5, and 5a the four metals form almost
ideal squares with average M···M distances of ca. 5.05 Å (5,
5a) and 5.08 Å (4) along the sides. As shown by 1H NMR
spectroscopy, the Pt square 5 is stable in aqueous solution,
whereas the Pd square 4 undergoes rearrangement reactions
upon aging or the presence of other Pd species such as (bpy)-
stranded DNA (“quadruplex DNA”) which in turn is part
of the telomeres at the end of the chromosomes, and also
can be formed in the promoter region of the genes.[7] Tetra-
Introduction
Cyanide is an extremely versatile ligand in coordination
chemistry, binding to transition metal ions in numerous
ways such as monofunctional (via C or N), bifunctional
(via C and N or via C and π bond), or even trifunctional
(via C and two π bonds) manners.[1] It is widely applied
for the construction of coordination polymers,[2] but when
combined with capping ligands, discrete 3D- and 2D-coor-
dination compounds with bridging cyanide ligands can be
generated.[3,4] The interest in magnetic properties of large
clusters and the hope of generating novel molecule-based
magnets are major driving forces for research.
Our interest in the topic of μ-CN–-bridged complexes re-
lates to our ongoing efforts to generate discrete molecular
architectures from metal entities and heterocyclic ligands,
including nucleobases, in general,[5] and from our attempts
to produce (essentially) flat constructs,[6] which are kinet-
ically robust and have the potential of interacting with so
called guanine quartets (G4). The latter occur in tetra-
[a] Fakultät Chemie, Technische Universität Dortmund,
44221 Dortmund, Germany
Fax: +49-231-755-3769
E-mail: bernhard.lippert@tu-dortmund.de
[b] Departamento de Química Inorgánica, Instituto Universitario
de Catálisis Homogénea, Instituto de Ciencia de Materiales de
Aragón, Universidad de Zaragoza – C.S.I.C.
50009 Zaragoza, Spain
[c] Institute of Inorganic Chemistry, University of Zürich,
8057 Zürich, Switzerland
Figure 1. Schematic views of guanine quartet (G4) (top) and of
discrete molecular squares with cis-a2MII (bottom, left) and trans-
a2MII (with M = Pt or Pd) (bottom, right).
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201001216.
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A. Galstyan, P. J. Sanz Miguel, J. Wolf, E. Freisinger, B. Lippert
1
FULL PAPER
stranded DNA regions have become a major focus as a tures of compounds were obtained, according to H NMR
novel target in anticancer chemotherapeutic approaches in spectroscopy (see below). In order to reduce the number of
recent years.[8] In brief, any stabilization of quadruplex possible species, a “directed” approach[14] was undertaken
DNA, either in telomeres or in gene-regulatory elements, to obtain type-IV compounds by starting from (en)M-
for instance through π-π stacking by an organic ligand[9] or (CN)2 and reacting it subsequently with [(en)M(H2O)2]2+
a suitable metal compound,[10] appears to be advantageous (M = PtII and/or PdII). This strategy eventually led to Pt4
in terms of tumor suppression.
and Pd4 species. Previously reported examples of CN–-
The focus of our work is thus on complexes having a cis- bridged mixed Fe2Co2[3b] and mixed Fe2Cu2[3c] squares were
a2MII (M = Pt or Pd; a = NH3 or a2 = chelating diamine) synthesized applying the same approach.
1
in the corner of a square, or a corresponding trans-a2MII
Concerning the use of H NMR spectroscopy to study
(M = Pt or Pd; a = NH3 or monodentate amine) in the (en)M complexes, it was anticipated that binding of CN–
center of the side of a square (Figure 1). via C or binding via N should affect the CH2 protons of
Here we report on the synthesis and characterization of the en chelate ring differently. Consistent with this view, a
cyanido-bridged complexes of PtII and PdII, with the metals single CH2 resonance is to be expected for (en)M(CN)2,
residing in the corners of the squares.
whereas a mixed coordination (en)M(CN)(NC) or the si-
multaneous existence of (en)M(CN)2 and (en)M(NC)2
should give rise to two sets of CH2 resonances. It was un-
clear, however, whether the four possible cyclic structures
I–IV, could be differentiated by this method in case of four
identical metals (e.g., M = PtII or PdII), as there are four
M–C and four M–N bonds in each structure. For en ligands
Results and Discussion
General Aspects
Spontaneous self-assembly of cis-a2MII entity with CN–
to a molecular square can lead, in principle, to four dif-
ferent linkage isomers I–IV (Figure 2).
3
bonded to Pt, J coupling between methylene protons and
the 195Pt isotope was considered helpful. As another means
of differentiating terminal and bridging CN– ligands, IR
spectroscopy was applied.[15] This method relies on differ-
ences in the position of the ν(CN) stretching modes, which
occur at significantly higher wavenumbers when CN– acts
as bridge.
Formation of I feasibly could take place in a cyclization
reaction of four mononuclear cis-a2M(CN)+ cations, while
II and III could be formed upon condensation of a neutral
bis(cyanide) complex cis-a2M(CN)2 with two mononuclear
cis-a2M(CN)+ cations and a cis-a2M2+. Finally IV could
arrange from two neutral cis-a2M(CN)2 molecules and two
cis-a2M2+. In all cases it is assumed that initial coordination
of CN– to M is through the carbon atom. Moreover, this
view implies that no ligand scrambling occurs during syn-
thesis.
X-ray Crystal Structures of the Starting Complex, of a Side
Product and of Two Squares
Of the two building blocks with a cis geometry,
Pd(en)(CN)2 (1) and Pt(en)(CN)2 (2), only the Pd com-
pound was obtained as crystals suitable for X-ray diffrac-
tion (Figure 3). In 1 Pd adopts a square-planar geometry,
Although for the tetrameric [Me2Au(CN)]4 (Me =
methyl) a type-I isomer has been postulated, 1H NMR spec-
troscopy suggests the presence of disordered CN– li-
gands.[11] X-ray crystal structures of cyclic CN-bridged PtII
compounds are scarce and reveal both disordered[12a] and
ordered CN– bridges.[12b,12c] A further potential complica-
tion arises from the possibility of equilibria between
squares and triangles. Although relatively rare,[13] CN–-
bridged metallacycles have been reported and represent an
entropically favored alternative to molecular squares.
Attempts to prepare μ-CN compounds by reaction of
[(en)M(H2O)2]2+ with 1 equiv. of CN– failed in that mix- Figure 3. View of Pd(en)(CN)2 (1) with atom numbering scheme.
Figure 2. Four possible linkage isomers of cyclic, tetranuclear cis-{[a2M(CN)]4}4+ species and their respective symmetries.
1650 Eur. J. Inorg. Chem. 2011, 1649–1656
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Discrete Molecular Squares {[(en)M(CN)]4}4+
containing a chelating ethylendiamine ligand and two ter-
minal cyanido ligands.
Distances and angles about the Pd are normal, as are
the geometries of the en and cyanide ligands (Supporting
Information). The crystal packing of 1 is dominated by hy-
drogen bonding, involving nitrogen acceptor (CN–) and do-
nor (NH2) groups. No electrostatic or metal-metal interac-
tions are observed. An almost right dihedral angle (89.5°)
is formed between the PtC2N2 coordination planes of two
neighboring molecules of 1, which are twofold H-bonded:
N1···(H)N14, 3.038(3) Å; N2···(H)N14, 3.084(3) Å. Further
related twofold hydrogen bonds allow the formation of zig-
zag chains along the c axis. The distance between Pd atoms
within a chain is 5.3122(2) Å. Antiparallel chains are con-
nected by hydrogen bonds: N1···(H)N14, 2.991(3) Å;
N2···(H)N14, 3.101(3) Å.
Figure 5. View of the symmetry-generated cation of {[Pt(en)-
(CN)4]}(NO3)4 (5). The corresponding cation with Pd instead of Pt
(4) is very similar and not shown.
The analogous Pt complex, Pt(en)(CN)2 (2) was not ob-
tained in monocrystalline form, but its formation was con-
1
cluded from H NMR spectra (see below). However, a side
product which proved to be [Pt(en)2][Pt(CN)4] (3), was iso-
Salient structural features (given for 5 only, for 4 see Sup-
lated in low yield and characterized by X-ray analysis (Fig- porting Information) are as follows: two of the four Pt
ure 4). Compound 3 crystallizes in a way reminiscent of atoms are bonded to C atoms of the CN– bridges, while the
Magnus’ green salt,[16] hence has a chain structure with two others are bonded to the N atoms of CN–. Pt–C and
alternating [Pt(en)2]2+ cations and [Pt(CN)4]2– anions. How- Pt–N distances are Pt1–N1, 1.978(10) Å; Pt1–N2,
ever, Pt–Pt distances within the chain are markedly dif- 2.000(13) Å; Pt2–C1, 1.986(10) Å; Pt2–C2, 1.945(12) Å. Pt–
ferent, 3.3205(14) Å and 4.0106(14) Å, and therefore a more Pt distances along the sides of the molecular square are
appropriate description might be that of weakly interacting 5.0413(7) Å (Pt1–Pt2) and 5.0661(7) Å (Pt1–Pt2Ј) and the
cation-anion pairs. The polymeric chains are oriented along diagonals are 7.1008(7) Å (Pt1–Pt1Ј) and 7.1938(8) Å (Pt2–
the b axis and interconnected by hydrogen bonds between Pt2Ј). Although 4 and 5 are analogous, there are differences
NH2 groups of en ligands and N atoms of the terminal CN– in the solid state packing. Both complexes form stairs by
ligands [N4···N21, 2.99(4) Å; N6···N24, 2.99(4) Å]. Neigh- stacking of opposite corners of the squares with adjacent
boring chains also interact via H-bonding: N2···N11, squares (4 along the a axis, 5 along the b axis). Distances
2.97(4) Å; N2···N24, 3.01(3) Å; N8···N14, 2.90(4) Å.
between adjacent stacked squares (plane-centroid) are sig-
Reactions of 1 and 2, respectively, with one equivalent nificantly different, ca. 3.5 Å for 4, and 3.8 Å for 5 (Fig-
of [M(en)(H2O)2](NO3)2 produce the designed squares of ure 6). In addition, the geometrical projection of the atoms
composition{[Pd(en)(CN)]4}(NO3)4(4)and{[Pt(en)(CN)]4}- of a square in the plane of its neighbor also differs in 4 and
(NO3)4 (5). Both compounds were X-ray structurally char- 5. In the case of 4, two (en)Pd groups of the previous and
¯
acterized, and both crystallize in space group P1 with al- next squares of the stair are projected within the square.
most identical volumes of the unit cells. Both cations are The inside part of the square 5 includes the projection of a
therefore virtually analogous and only the cation of 5 is part (two carbon and one nitrogen atoms) of two en groups,
depicted in Figure 5. The refinements of both structures are resulting thus in a partially twisted stair. An inspection of
consistent with the presence of linkage isomer IV. Upon the packing patterns further reveals that stair formation in
sequentional inversion of C and N sites, larger R values and 4 and 5 is largely a consequence of hydrogen bonding be-
thermal parameters outside the range of average parameters tween the NH2 groups of the en ligands and nitrate anions
for the structure were obtained.
(roughly in the plane of the cations) as well as water mole-
Figure 4. Section of chain structure of alternating pairs of cations and anions in [Pt(en)2][Pt(CN)4] (3).
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A. Galstyan, P. J. Sanz Miguel, J. Wolf, E. Freisinger, B. Lippert
FULL PAPER
cules (both in the plane of the cations and between the cat-
ion sheets). Vertical Pd···Pd [3.5375(10) Å, 4] and Pt···Pt
[3.5699(11) Å, 5] interactions may add to the packing.
Figure 7. View of cation and anions of 5a.
tween stairs of 5a, interacting through hydrogen bonds with
water molecules and cations (Supporting Information).
IR and NMR Spectroscopy
Figure 6. Different crystal packing of cations 4 and 5.
As the wavenumbers of the CN stretching vibrations are
known to be indicative of the coordination mode of the
cyanide ligands, IR spectra were recorded. Wavenumbers of
the υ(CN) absorption bands are listed in Table 1. They are
close to expectations for terminal and bridging CN– li-
gands, with the latter usually occurring at significantly
higher wavenumbers.[15]
Anion Exchange with 5
In order to screen for any striking host-guest interactions
of cation 5 with selected anions, aqueous solutions of 5
were treated with increasing amounts of NaF, NaCl,
Na2SO4, and the disodium salt of terephthalate, and
changes in chemical shifts of the two methylene signals of
en were monitored by 1H NMR spectroscopy. For all
anions, smooth downfield shifts were observed, however,
they did not exceed Δδ values of 0.1 ppm even when present
in high excess (Ͼ 50 fold; [5] = 8ϫ10–3 m). The weak re-
sponse of both en resonances to the added anions suggests
Table 1. Vibrational frequencies of υ(CN) in the various com-
pounds.
1
3
4
5
6
υ(CN) [cm–1]
2145, 2135
2123
2190
2192
2190
As mentioned above, reactions of Pt(en)Cl2 with 1 equiv.
only weak anion binding and was therefore not further in- of AgNO3 and 1 equiv. of AgCN, hence with a Pt/CN– ratio
1
vestigated. From the NMR sample with terephthalate, crys- of 1:1, give a H NMR spectrum with multiple CH2 reso-
tals were obtained, which were studied by X-ray crystal- nances between 2.46 and 2.78 ppm. In contrast, both
lography and revealed a composition of {[Pt(en)(CN)]4}- Pt(en)(CN)2 (2) (δ = 2.65 ppm) and [Pt(en)2][Pt(CN)4] (3)
(C8H4O4)2·10H2O (5a). A view of 5a is given in Figure 7.
(δ = 2.78 ppm) display singlets with 195Pt satellites due to
Cation 5a shows in essence a very similar geometry as 4. 3J coupling (40 Hz in 2, 32 Hz in 3), whereas the cyclic tet-
Pt–C and Pt–N distances within the square are Pt1–N1, ramer 5 has two singlets of identical intensities at δ = 2.54
1.966(12) Å, Pt1–N2, 1.948(12) Å, Pt2–C1, 1.937(14) Å and and 2.71 ppm (Figure 8). Both resonances reveal 195Pt satel-
Pt2–C2, 1.923(15) Å. Likewise, the packing of cations 5a lites of ca. 36 Hz and 29 Hz, respectively. We tentatively as-
corresponds to that of 4, forming stairs (in this case along signment the two methylene proton signals to Pt(en)(NC)2
the a axis) with projections of the neighbor (en)Pt entities and Pt(en)(CN)2 on the basis of their different J(195Pt-1H)
4
within the square of 5a; the distance Pt2–Pt2 of two stacked coupling constants and expectations based on the trans-in-
cations is 3.4334(11) Å. Terephthalate anions are placed be- fluence concept.[17]
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Discrete Molecular Squares {[(en)M(CN)]4}4+
rated by our finding on formation of [Pt(en)2][Pt(CN)4] (3)
1
(see above). Furthermore, we note that the H NMR spec-
trum of [Pd(en)2]2+, prepared in situ by reaction of
[Pd(en)(D2O)2](NO3)2 with en·HCl in D2O is identical with
that given in Figure 9 (c). We rule out the possibility that 4
rearranges into any of the other metallacycles I–III (Fig-
ure 2), as for these species likewise more than a single en
resonance would be expected.
Lability of Ligands
Attempts to prepare Pd4 squares with two kinds of che-
lating amine ligands at the metals by combining
Pd(en)(CN)2 (1) with [Pd(bpy)(H2O)2](NO3)2 in water re-
sulted in formation of a gel. When MeOH was applied as
the solvent, a white material 6, insoluble in water and all
common organic solvents, was obtained. Compound 6 ana-
lyzes as [Pd4(bpy)2(en)2(CN)4](NO3)4·2H2O and has a
sharp IR band at 2190 cm–1, consistent with the presence
of bridging cyanide. The structure of this compound is un-
known at present. When this reaction was carried out in
Figure 8. CH2 resonances of en ligands in {[Pt(en)(CN)]4}(NO3)4
(5) with 195Pt satellites indicated.
1H NMR spectra (D2O) of Pd(en)(CN)2 (1) and
[Pd(en)(D2O)2]2+ show methylene resonances as singlets at
δ = 2.80 and 2.62 ppm, respectively. The 1H NMR spectrum
of a freshly prepared solution of the cyclic tetramer 4, ob-
tained by dissolving crystals of 4 in D2O and recording the
1H NMR spectrum immediately afterwards, shows two CH2
resonances in 1:1 ratio at δ = 2.63 and 2.75 ppm, similarly
to 5. In contrast to 5, which does not undergo changes
within 3 d at 80 °C, the spectrum of 4 changes with time
(Figure 9).
2–
more dilute aqueous solution, and with SO4 as anion,
there was likewise formation of a gel. However, at the same
time also a crystalline compound was obtained in 11%
yield, which proved to be [Pd(bpy)(en)](SO4)·3H2O (7). This
finding suggests that amine exchange reactions can occur
under the experimental conditions applied. A view of cation
7 is depicted in Figure 10 (a). Cation 7 is planar, except for
the carbon atoms of the en group, which are located slightly
above and below the plane. Distances and angles within the
cation are normal, with deviations of angles around the pal-
ladium atom from 90°: Pd1–N11, 2.011(7) Å; Pd1–N21,
2.018(6) Å; Pd1–N1, 2.021(7) Å; Pd1–N4, 2.027(6) Å; N11–
Pd1–N21, 81.3(3)°; N21–Pd1–N1, 97.9(3)°; N11–Pd1–N4
96.9(3)°; N1–Pd1–N4, 84.0(3)°. Cations of 7 display π–π
stacking of 2,2Ј-bipyridine rings along the b axis (3.3 Å),
and are alternatively disposed with the en ligands pointing
in opposite directions. Sulfate anions are inserted between
two en ligands via H-bonds (Figure 10, b) and water mole-
Figure 9. 1H NMR spectra (D2O, 300 MHz) of [Pd(en)(CN)]4-
(NO3)4 (4) a) immediately after dissolving the crystals, b) after 4 d
at room temp. and c) after 1 d at 60 °C.
An aged solution of 4 (3–4 d, r.t.) reveals a new reso-
nance at δ = 2.68 ppm (Figure 9, b), which becomes domi-
nant (and slightly downfield shifted) upon heating (Fig-
ure 9, c). Possibly a symmetrisation reaction as shown in
Figure 10. a) View of the cation of 7, and b) π-stacking of cations,
2–
Equation (1) could account for this observation, corrobo- with SO4 anions involved in hydrogen bonding.
(1)
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A. Galstyan, P. J. Sanz Miguel, J. Wolf, E. Freisinger, B. Lippert
The pyrimidine nucleobase 1-MeC is bonded to PdII
FULL PAPER
cules. Hydrogen bonding distances in the crystal packing of
7 are N1···O1w, 2.86(1) Å, N1···O11, 2.83(1) Å, N4···O12, through its N3 site, as is typical for the large majority of
2.878(9) Å, N4···O13, 2.913(10) Å, O11···O2w, 2.946(10) Å, metal complexes of N1 substituted cytosine ligands.[19] The
O13···O2w, 2.733(9) Å as well as O14···O1w, 2.669(10) Å.
geometry of the complex anion is normal as far as bond
lengths and angles are concerned (Supporting Information).
The 1-MeC ring is almost perpendicular to the Pd coordi-
nation plane (83.1°), very much as in the related trans-
PtI2(1-MeC) fragment.[20] The cation of 8 reveals the typical
geometry of hemiprotonated cytosine bases, which have
been crystallized with numerous other counter anions.[21]
Formation of the anion of 8 further confirms that the che-
lating en ligand in PdII complexes is labile and is prone to
substitution by the other ligands, here by I– and even 1-
MeC.
Interactions between Squares and Nucleobases
As a first test of possible interactions of the cationic
squares 4 and 5 with nucleobase aggregates (quartets, base
pairs) we decided to study the effects on selected model nu-
cleobases on these squares. For this purpose 1H NMR spec-
tra of mixtures of the Pt square 5 with increasing amounts
of 1-methylcytosine (1-MeC), 1-methyluracil (1-MeUH)
and 9-ethyladenine (9-EtA) were recorded in D2O. Other
bases (9-methylguanine, 1-methylthymine) were not consid-
ered because of low solubility. The experiments were carried
out at pH values, where no acid-base equilibria are relevant,
viz. pD 8.0Ϯ0.2 with 1-MeC, 6.8Ϯ0.4 with 1-MeUH, and
7.5Ϯ0.5 with 9-EtA. With all three nucleobases, increasing
concentrations (20–40 fold excess) of the base cause down-
field shifts of both CH2 resonances of the en ligands of 5,
but with 9-EtA this effect is almost three times as large
as with the two pyrimidine bases, namely between 0.2 and
0.3 ppm. On the other hand, 9-EtA proton resonances, and
There is no rapid reaction of the Pt square 5 with [1-
MeCH]I, but within days at room temperature there is for-
mation of a small amount of a poorly soluble precipitate
having an orange color. We believe that it is due to a Pt-
1
iodide species. Still, the H NMR spectrum is virtually un-
altered (no new H5, H6 doublets of 1-MeC detectable).
Conclusions
Two CN–-bridged molecular squares of (en)PdII (4) and
in particular those of the two aromatic protons H2 and H8, (en)PtII (5) have been obtained with the aim to further
undergo upfield shifts with increasing concentrations. In the study them with regard to their propensity to interact non-
concentration range 0.003–0.105 m these shifts are 0.30 ppm covalently with nucleobases, and preferably with guanine
for H2, 0.26 ppm for H8, and 0.20 ppm for CH2 of the ethyl quartets or nucleobase aggregates in general. The Pd square
group. CH3 of the ethyl group is virtually unaffected (Sup- proved not to be useful as it was unstable in solution, even
porting Information). This behavior is indicative of adenine in the absence of nucleobases. In contrast, the Pt square 5
stacking, as expected.[18] The marked difference in behavior is considerably more inert and consequently is more suitable
of the en resonances of 5 in the presence of 9-EtA strongly in terms of its potential to interact non-covalently with nu-
suggests that there is an interaction between the cation of 5 cleobases. With the model nucleobase 9-EtA a distinct ef-
and the adenine, the nature of which is unclear at present.
The above mentioned instability of the Pd square 4 was
also confirmed in an experiment aimed at cocrystallizing 4
with 1-methylcytosine and its hemiprotonated form, the
[1-MeC·1-MeCH]+ pair. Upon mixing 4 with [1-MeCH]I,
fect on 5 is seen, but details of this interaction are elusive.
Experimental Section
[22]
[23]
General: The complexes enPdCl2 and enPtCl2 were prepared
from K2[PdCl4] and K2[PtCl4] (Heraeus) by literature methods. The
model nucleobases 1-MeC,[24] 1-MeUH,[25] and 9-EtA[26] were like-
wise prepared as reported. All other chemicals were of commercial
origin and were used without further purification. 1H NMR spectra
were recorded in D2O on Varian Mercury 200 FT NMR and
Bruker DPX 300 spectrometers, with TSP used as an internal refer-
ence. pD values were determined by use of a glass electrode and
addition of 0.4 to the pH-meter reading. IR spectra were recorded
on an IR Fourier spectrometer IFS 28 Firma Bruker Optik GmbH
from KBr pellets. Elemental analyses were carried out on a Leco
CHNS-932 instrument.
rapid decomposition of
4 and formation of trans-
[PdI2(CN)(1-MeC)]– took place, forming a salt with the
hemiprotonated 1-MeC base pair, hence [1-MeC·1-MeCH]-
[PdI2(CN)(1-MeC)]·2H2O (8). The compound is depicted in
Figure 11.
enPd(CN)2 (1): A solution of enPdCl2 (237 mg, 1 mmol) and AgCN
(268 mg, 2 mmol) in water (40 mL), after 10 min ultrasonic treat-
ment, was stirred in dark at 40 °C for 18 h. AgCl was removed by
filtration and the volume of the solution was reduced to 5 mL.
Colorless cubic crystals were grown at 4 °C. The yield was 127 mg
(58%). C4H8N4Pd (218.55): calcd. C 21.98, H 3.69, N 25.6; found
1
C 22.0, H 3.7, N 25.6. IR (KBr): ν = 2145, 2135 [υ(CN)]cm–1. H
˜
NMR (300 MHz, D2O): δ = 2.80 (s, CH2) ppm.
enPt(CN)2 (2) and [Pt(en)2][Pt(CN)4] (3): A solution of enPtCl2
Figure 11. View of [1-MeC·1-MeCH][PdI2(CN)(1-MeC)]·2H2O (8).
(325 mg, 1 mmol) and AgCN (268 mg, 2 mmol) in water (40 mL)
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 1649–1656

Discrete Molecular Squares {[(en)M(CN)]4}4+
was stirred in dark at 40 °C for ca. 28 h. AgCl that had formed
was removed by filtration and the solution containing Pt(en)-
drogen atoms were refined anisotropically, whereas hydrogen atoms
were positioned geometrically and refined with isotropic displace-
ment parameters according to the riding model.
1
(CN)2 (2) was used directly for the synthesis of 5 (see below). H
NMR (300 MHz, D2O): δ = 2.65 (s, 3J = 40 Hz, 195Pt-1H, CH2)
ppm. When the solution was left at room temp. for 2–3 d, white
needles of [Pt(en)2][Pt(CN)4] (3) were isolated in ca. 10% yield.
CCDC-798819 (for 1), -798820 (for 3), -798821 (for 4), -798822 (for
5), -798823 (for 5a), -798824 (for 7), -798825 (for 8) contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
C8H16N8Pt2 · H2O (584.4): calcd. C 15.19, H 2.87, N 17.72; found
1
C 15.2, H 2.8, N 17.4. IR (KBr): ν = 2123 [υ(CN)]cm–1. H NMR
˜
(300 MHz, D2O): δ = 2.78 (s, J = 32 Hz, 195Pt-1 H, CH2) ppm.
3
Crystal Data for Pd(en)(CN)2 (1): [C4H8N4Pd], orthorhombic,
P212121, a = 6.8885(2) Å, b = 9.3234(3) Å, c = 10.9874(3) Å, Z =
4, fw = 218.54 gmol–1, V = 705.66(4) Å3, Dcalcd. = 2.057 Mgm–3, μ
{[Pd(en)(CN)]4}(NO3)4 (4): To a solution of [enPd(H2O)2](NO3)2,
obtained by stirring enPdCl2 (237 mg, 1 mmol) and AgNO3
(340 mg, 2 mmol) in water (30 mL) for 2 h at room temp. followed
by filtration of AgCl, was added freshly prepared enPd(CN)2
(1 mmol in 50 mL, water). The bright yellow color of the mixture
changed immediately to pale yellow. The volume of the solution
was then reduced to 5 mL. The product crystallized within several
hours at room temp. The yield was 550 mg (54%).
C12H32N16O12Pd4 (1018.2): calcd. C 14.16, H 3.17, N 22.01; found
= 2.545 mm–1, 10564 reflections collected, 1780 unique (Rint
=
0.0388), R1(Fo) = 0.0179 [IϾ2σ(I)], wR2 (Fo2) = 0.0362 (all data),
GOF = 1.011.
[Pt(en)2][Pt(CN)4] (3): [C8H16N8Pt2], monoclinic, P21/c,
a =
8.5969(6) Å, b = 13.2140(10) Å, c = 14.2944(10) Å, β = 123.782(5)°,
Z = 4, fw = 614.47 gmol–1, V = 1349.67(17) Å3, Dcalcd.
=
C 13.9, H 3.2, N 21.8. IR (KBr): ν = 2190 [υ(CN)] cm–1. 1H NMR
˜
3.024 Mgm–3, μ = 20.704 mm–1, 9736 reflections collected, 3004
unique (Rint = 0.0585), R1(Fo) = 0.0977 [IϾ2σ(I)], wR2 (Fo2) =
0.2434 (all data), GOF = 1.090.
(300 MHz, D2O): δ = 2.63 (s, CH2-Pd1), 2.75 (s, CH2-Pd2) ppm.
{[Pt(en)(CN)]4}(NO3)4 (5): To a solution of [enPt(H2O)2](NO3)2 ob-
tained by stirring enPtCl2 (340 mg, 1 mmol) and AgNO3 (340 mg,
2 mmol) in water (30 mL) for 2 h at 80 °C, followed by filtration of
AgCl, was added freshly prepared enPt(CN)2 (1 mmol in 50 mL
water). The mixture was stirred at room temp. for 24 h and the
volume of the solution was reduced to 5 mL. The product crys-
tallized within 2–3 weeks at room temp. The yield was 571 mg
(43%). C12H32N16O12Pt4 (1372.8): calcd. C 10.5, H 2.35, N 16.33;
¯
[Pd(en)(CN)]4(NO3)4·3H2O (4): [C12H38N16O15Pd4], triclinic, P1, a
= 7.0948(3) Å, b = 10.4874(6) Å, c = 11.9782(6) Å, α = 92.119(4)°,
β = 97.420(4)°, γ = 104.373(4)°, Z = 1, fw = 1072.18 gmol–1, V =
853.91(7) Å3, Dcalcd. = 2.085 Mgm–3, μ = 2.155 mm–1, 8078 reflec-
tions collected, 3551 unique (Rint = 0.0239), R1(Fo) = 0.0497
[IϾ2σ(I)], wR2 (Fo2) = 0.1471 (all data), GOF = 1.040.
found C 10.3, H 2.5, N 16.3. IR (KBr): ν = 2192 [υ(CN)] cm–1. 1H
˜
¯
[Pt(en)(CN)]4(NO3)4·4H2O (5): [C12H40N16O16Pt4], triclinic, P1, a
NMR (300 MHz, D2O): δ = 2.54 (s, CH2-Pt1), 2.71 (s, CH2-Pt2)
ppm.
= 7.9050(5) Å, b = 8.0482(5) Å, c = 13.7847(6) Å, α = 98.550(5)°,
β = 91.609(4)°, γ = 100.865(6)°, Z = 1, fw = 1444.96 gmol–1, V =
850.28(8) Å3, Dcalcd. = 2.822 Mgm–3, μ = 16.488 mm–1, 8904 reflec-
tions collected, 3796 unique (Rint = 0.0511), R1(Fo) = 0.0445
[IϾ2σ(I)], wR2 (Fo2) = 0.0838 (all data), GOF = 0.946.
[Pd4(bpy)2(en)2(CN)4](NO3)4·2H2O (6): For the preparation of 6,
two solutions, A and B, were prepared and subsequently mixed:
(A): Pd(bpy)Cl2 (167.5 mg, 0.5 mmol) was suspended in MeOH
(30 mL) and stirred at 45 °C for 1 d with AgNO3 (170 mg, 1 mmol).
AgCl was removed by filtration. (B): Pd(en)Cl2 (118.5 mg,
0.5 mmol) and AgNO3 (170 mg, 1 mmol) were stirred in MeOH
(30 mL) at room temp. overnight and then filtered from AgCl. The
two solutions (A) and (B) were then mixed and stirred at room
temp. for 1 d. The white precipitate was filtered and washed with
MeOH. The yield was: 63%. C14H16N8O6Pd6·2H2O: calcd. C
{[Pt(en)(CN)]4}(C8H4O4)·10H2O (5a): [C28H60N12O18Pt4], triclinic,
¯
P1, a = 7.0391(4) Å, b = 13.0244(8) Å, c = 13.1857(8) Å, α =
75.773(5)°, 79.050(5)°, 84.799(5)°, 1, fw
1633.24 gmol–1, V = 1149.21(12) Å3, Dcalcd. = 2.360 Mgm–3, μ =
β
=
γ
=
Z
=
=
12.215 mm–1, 9951 reflections collected, 5225 unique (Rint
=
0.0492), R1(Fo) = 0.0589 [IϾ2σ(I)], wR2 (Fo2) = 0.1393 (all data),
GOF = 0.940.
26.22, H 3.14, N 17.48; found C 26.3, H 3.1, N 17.5. IR (KBr): ν
˜
= 2190 [υ(CN)] cm–1.
[Pd(en)(2,2Ј-bipy)](SO4)·3H2O (7): [C12H22N4O7PdS], monoclinic,
P21/c, a = 12.7668(10) Å, b = 8.4206(8) Å, c = 19.7814(19) Å, β =
[Pd(bpy)(en)](SO4)·3H2O (7): 7 was obtained as described for 6
using as solvent H2O (80 mL for each step) instead of MeOH.
[Pd(bpy)(en)](SO4)·3H2O crystallized in form of colorless blocks in
11% yield. C12H16N4O4PdS·3H2O: calcd. C 30.48, H 4.69, N 11.85;
128.463(6)°, Z = 4, fw = 472.8 gmol–1, V = 1665.1(3) Å3, Dcalcd.
=
1.886 Mgm–3, μ = 1.285 mm–1, 7527 reflections collected, 3792
unique (Rint = 0.0443), R1(Fo) = 0.0792 [IϾ2σ(I)], wR2 (Fo2) =
0.1699 (all data), GOF = 1.120.
1
found C 30.0, H 4.7, N 11.9. H NMR (400 MHz, D2O): δ = 3.00
(s, CH2), 7.73 (ddd, 2 H, H5), 8.26–8.40 (m, 6 H, H3, H4 and H6)
ppm.
[1-MeC·1-MeCH][PdI2(CN)(1-MeC)]·2H2O (8): [C16H26I2N10O5Pd],
triclinic, P1, a = 7.4344(3) Å, b = 9.9380(4) Å, c = 18.7338(8) Å, α
¯
= 81.029(4) °, β = 86.096(4)°, γ = 69.151(4)°, Z = 2, fw =
[1-MeC·1-MeCH][PdI2(CN)(1-MeC)]·2H2O (8): 8 was obtained
when trying to cocrystallize [Pd(en)(CN)]4(NO3)4 (4) with
[1-MeC]I. The product was obtained in form of orange crystals
within 2 d at room temp. C16H22I2N10O3Pd·H2O: calcd. C 24.62,
H 3.10, N 17.94; found C 25.0, H 3.6, N 18.0.
798.67 gmol–1, V = 1277.54(9) Å3, Dcalcd. = 2.076 Mgm–3, μ =
3.190 mm–1, 12647 reflections collected, 5756 unique (Rint
=
0.0437), R1(Fo) = 0.0336 [IϾ2σ(I)], wR2 (Fo2) = 0.0457 (all data),
GOF = 0.959.
X-ray Crystal Structure Determination: Crystal structures 1, 3, 4,
5, 5a, 7, and 8 were determined with a Xcalibur diffractometer
[graphite mono-chromated Mo-Kα radiation (0.71073 Å)]. Data re-
duction was done with the CrysAlisPro software.[27] The structures
were solved by direct methods and full-matrix least-squares refined
on F2 using SHELXL-97 and WinGX software.[28,29] All non-hy-
Supporting Information (see footnote on the first page of this arti-
cle): Selected bond lengths and angles for 1, 3, 4, 5, 5a, 7, and 8;
crystal structure of 3 with atom numbering scheme, and crystal
packing of 3 indicating extensive intermolecular hydrogen bonding;
view of the cation of 4; packing arrangement of 5a in the crystal;
chemical shifts of 5 upon addition of 1-MeC, 1-MeUH and 9-EtA.
Eur. J. Inorg. Chem. 2011, 1649–1656
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1655

A. Galstyan, P. J. Sanz Miguel, J. Wolf, E. Freisinger, B. Lippert
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Published Online: February 15, 2011
1656
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 1649–1656