Sterically-directed consecutive and size-selective self-assembly of palladium diphosphane complexes with an Ar-BIAN ligand: Unexpected formation of pentameric and hexameric aggregates

Article abstract of DOI:10.1002/chem.200901126

The coordination properties of N,N'-bis[4-(4-pyridyl)phenyl] acenaphthenequinonediimine (L¹) and N,N'-bis[4-(2-pyridyl)phenyl] acenaphthenequinonediimine (L²) were investigated in self-assembly with palladium diphosphane complexes [P

Full text of DOI:10.1002/chem.200901126

DOI: 10.1002/chem.200901126
Sterically-Directed Consecutive and Size-Selective Self-Assembly of
Palladium Diphosphane Complexes with an Ar-BIAN Ligand: Unexpected
Formation of Pentameric and Hexameric Aggregates**
Eszter Hollꢀ-Sitkei,[a] Gꢁbor Szalontai,[b] Isabella Lois,[a] ꢂgnes Gçmçry,[a]
Ferenc Pollreisz,[a] Lꢁszlꢀ Pꢁrkꢁnyi,[a] Hershel Jude,[c, d] and Gꢁbor Besenyei*[a]
Abstract: The coordination properties
also observed. In all of these species,
which correspond to the general for-
mass-spectrometric and DOSY NMR
spectroscopic analysis. The sequential
of
N,N’-bis[4-(4-pyridyl)phenyl]ace-
naphthenequinonediimine (L1) and
N,N’-bis[4-(2-pyridyl)phenyl]acenaph-
thenequinonediimine (L2) were investi-
gated in self-assembly with palladium
mula [Pd
N
N
coordination of the [PdACTHUNGETRNNUG
(dppp)]2+ ion is
attributed to the dissimilar steric prop-
erties of the two coordination sites. In
the self-assembled species formed in a
diphosphane complexes [Pd
N
equivalent of the [Pd
1:1:1 mixture of [Pd
(dppe)]2+/L1, the sterically more de-
manding [Pd
(dppp)]2+ tectons are at-
(dppp)]2+/[Pd-
ACHTUNGTRENNUNG
A
U
results in coordination to the internal,
sterically more encumbered chelating
site and induces enhancement of the
higher nuclearity components. The
presence of higher-order aggregates
(n=5, 6), which were unexpected for
the interaction of cis-protected palladi-
um corners with linear ditopic bridging
ligands, has been demonstrated both by
ACHTUNGTRENNUNG
various analytical techniques, including
multinuclear (1H, 15N, and 31P) NMR
spectroscopy and mass spectrometry
(P^P=dppp, dppf, dppe; dppp=bis(di-
ACHTUNGTRENNUNG
tached selectively to the pyridyl
groups, whereas the more hindered
imino nitrogen atoms coordinate the
less bulky dppe complexes, thus result-
ing in a sterically directed, size-selec-
tive sorting of the metal tectons. The
propensity of the new ligands to incor-
porate hydrogen-bonded solvent mole-
cules at the chelating site was con-
firmed by X-ray diffraction studies.
phenylphosphanyl)propane,
dppf=
bis(diphenylphosphanyl)ferrocene, and
dppe=bis(diphenylphosphanyl)-
ethane). Beside the expected trimeric
and tetrameric species, the interaction
of an equimolar mixture of [Pd-
Keywords: aggregation
· bridging
ACHTUNGTRENNUNG
(dppp)]2+ ions and L1 also generates
ligands · phosphane ligands · self-
assembly · steric hindrance
pentameric aggregates. Due to the E/Z
isomerism of L1, a dimeric product was
Introduction
chemical properties of coordination compounds, and, in the
case of homogeneous catalytic transformations, they influ-
ence rates and selectivities.[1] Steric effects manifest them-
selves in a variety of ways in self-assembled metallamacro-
Steric interactions appear in many different forms in coordi-
nation chemistry. They affect the structural, physical, and
[a] E. Hollꢀ-Sitkei, Dr. I. Lois, Dr. ꢁ. Gçmçry, Dr. F. Pollreisz,
Dr. L. Pꢂrkꢂnyi, Dr. G. Besenyei
Chemical Research Center
[c] Dr. H. Jude
University of Utah
Department of Chemistry
Salt Lake City, Utah 84112 (USA)
Hungarian Academy of Sciences
1525 Budapest, POB 17 (Hungary)
Fax : (+36)1-438-1143
[d] Dr. H. Jude
Present address: Kalsec Inc., Kalamazoo, MI (USA)
E-mail: besenyei@chemres.hu
[**] Ar-BIAN= bisACTHNUTRGNEUNG(aryl)acenaphthenequinonediimine.
[b] Dr. G. Szalontai
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901126.
Pannon University, NMR Laboratory
8201 Veszprꢃm, POB 158 (Hungary)
10620
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Chem. Eur. J. 2009, 15, 10620 – 10633

FULL PAPER
cycles. A group of systematic studies that focused on the po-
tential effects of noncovalent interactions in self-assembly
exploited the complementary arrangements of ligands that
possess dissimilar steric demands. It has been shown that di-
topic ligands in which two 6-methyl-2,2’-bipyridine units are
connected by a (CH2)4 linker through C5 or C5’ (A and B,
respectively; Scheme 1) form heterotopic dinuclear palladi-
um complexes, whereas homo-complexation gives a mixture
of oligomeric products.[2] The enhanced steric demands of
2,2’,6,6’-tetramethyl-4,4’-bipyridine (4,4’-bpy*) relative to
unsubstituted 4,4’-bipyridine (4,4’-bpy) resulted in an alter-
nate arrangement of the bridging ligands in the tetrameric
8+
ion [Pd2(en)2ACHTUNGTRENNUNG(4,4’-bpy)(4,4’-bpy*)]2 (bpy=bipyridine, en=
ethylenediamine).[3] Similarly, the steric difference between
4,4’-bpy* and 4,4’-bis(4-pyridyl)biphenyl (4,4’-bpyPh) result-
ed in
a
spontaneous formation of the rectangular
Scheme 1. Studied ligands with various steric demands.
8+
[Pd2(en)2(4,4’-bpyPh)(4,4’-bpy*)]2
ion. Without steric di-
rection, homotopic complexes would have formed, as shown
by the size-selective reactions of ditopic organoplatinum
complexes with dipyridyl linkers of different lengths.[4] The
complementary arrangement of building blocks of dissimilar
steric demands has been employed as a successful strategy
for the construction of coordination cages as well.[3,5,6]
The assembly pathways of the building blocks can also be
regulated by the position and steric requirements of the sub-
stituents, which has been exemplified by the reaction of the
[Pd(en)]2+ ion with the triangular tetradentate ligands C
and D. The product of the former ligand mimics a tetrahe-
dron, whereas the latter is assembled into an open-cone
structure due to the steric restrictions of the side chains.[7]
Several studies have demonstrated that intramolecular re-
pulsions that emerge in self-assembled metallamacrocycles
may affect the nuclearity of the aggregates. Although there
are numerous reports on the formation of equilibrium mix-
tures that involve constituents with different numbers of
metal centers, we have strived to present examples in which
changing the nuclearity can be ascribed with certainty to
steric effects rather than to the flexibility of the bridging
ligand or the incorporation of guest molecules. In this re-
spect, a notable study with N,N’-diphenylamidinates has
clearly demonstrated that ligands without ortho substituents
on the aryl moieties establish a dimer/tetramer equilibrium
in solution with copper(I) ions, whereas replacement of the
ortho-hydrogen atoms by methyl groups produces only a di-
meric assembly.[8] The size of the metallamacrocycle has
been shown to depend on the substituent (R) of the pyrazol-
onate ligand in the pyrazolonate rhenium(I) complexes that
appeared as tetramers (R=H) or trimers (R=Me, Ph).[9] In-
Abstract in Hungarian: Tanulmꢀnyoztuk az N,N’-bisz(4-(4-
piridil)-fenil)-acenaftꢁnkinondiimin, L1, ꢁs az N,N’-bisz(4-(2-
piridil)-fenil)-acenaftꢁnkinondiimin, L2, [Pd
N
ACHTUGNRTNE(NUGN H2O)2]-
ACHTUNGTRENNUNG
˝
hatꢀsꢀt (P^P=dppp, dppf, dppe). Az çnszervezodꢁssel
˝
1
eloꢀllꢂ elegyek jellemzꢁsꢁre multinukleꢀris ( H, 15N ꢁs 31P)
NMR-spektroszkꢂpiai, DOSY-, ꢁs tçmegspektrometriai vizs-
(dppp)]2+ komplex kation ꢁs L1
ACHTUNGTRENNUNGgꢀlatokat vꢁgeztꢃnk. A [PdHCATUNGTRENNUGN
˝
1:1 mꢂlarꢀnyffl elegyꢁben fotermꢁkkꢁnt trimer, tetramer ꢁs
˝
pentamer komplexeket figyeltꢃnk meg, melyek kꢁpzodꢁsꢁt a
ligandum E/Z izomerizꢀciꢂja kçvetkeztꢁben dimer mellꢁkter-
mꢁk keletkezꢁse kísꢁri. E termꢁkek esetꢁben kizꢀrꢂlag a ter-
minꢀlis piridilcsoportok koordinꢀciꢂjꢀt tapasztaltuk. Egy
˝
ffljabb ekvivalensnyi Pd-tekton a kelꢀthelyzetu, sztꢁrikusan
gꢀtoltabb iminonitrogꢁnek koordinꢀciꢂjꢀt eredmꢁnyezi, mely
a nagyobb aggregꢀciꢂjffl komponensek relatív sffllyꢀnak nçve-
kedꢁsꢁhez vezet. Tçmegspektrometriai ꢁs DOSY-vizsgꢀla-
tainkkal a cisz-helyzetben vꢁdett síknꢁgyzetes pallꢀdium-
komplexek ꢁs lineꢀris, kꢁtfogffl ligandumok kçlcsçnhatꢀsꢀban
nem vꢀrt 5- ꢁs 6-tagffl makrociklusok lꢁtꢁt igazoltuk. A pallꢀ-
diumtekton konszekutív koordinꢀciꢂjꢀt a kꢁt koordinꢀciꢂs
terestingly, the phenoxy groups attached to the diazadiben-
2n+
zoperylene ligands (L) in [Pd
G
(n=3, 4;
dppp=bis(diphenylphosphanyl)propane) complexes induce
steric congestion between L on the edges rather than in-
creasing repulsion with the chelating dppp units. Conse-
quently, a preferential formation of the tetrameric assembly
could be achieved by increasing the size of the phenoxy
group.[10] However, predicting nuclearity is not always possi-
˝
hely eltꢁro sztꢁrikus sajꢀtsꢀgꢀval ꢁrtelmeztꢃk.
A [Pd-
A
ACHTUNGTRENNUNG
˝
aggregꢀtumok ꢀllnak elo, melyekben a terjedelmesebb dppp-
komplex szelektíven a piridilcsoportokhoz, míg a kisebb tꢁri-
˝
gꢁnyu dppe-komplex a sztꢁrikusan ꢀrnyꢁkoltabb iminonitro-
ble, as demonstrated by the unforeseen generation of trimer-
gꢁnekhez kapcsolꢂdik, megvalꢂsítva ezꢀltal a pallꢀdiumtekto-
nok sztꢁrikusan irꢀnyított, mꢁret szerinti szꢁtvꢀlogatꢀsꢀt. Az
fflj ligandumok hidrogꢁnkçtꢁs kialakítꢀsꢀra mutatott fokozott
hajlamꢀt oldꢂszeradduktumok rçntgendiffrakciꢂs szerkezet-
vizsgꢀlatꢀval igazoltuk.
6+ [12a]
ic species [Rh
G
(PMe3)
N
,
and
[12b]
[Ru(dmso)2A(pyr)Cl2]3
A
U
dmso=dimethyl sulfoxide, ox=oxalate, pyr=pyrazine) in
which the rigid bridging units (i.e., ox and pyr) were expect-
ed to construct tetrameric assemblies. Deviations from the
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G. Besenyei et al.
sterically predicted behavior were also reported in studies
with [M
(P^P)]2+ complexes (M=Pd, Pt; P^P=dppp, dppf,
The coordination properties of the new ligands were probed
G
by using palladium diphosphane complexes [Pd
(H2O)2](OTf)2 (1; P^P=dppp (1a), dppf (1b), dppe (1c);
OTf=triflate).
ACHTUNGTRENNUNG(P^P)-
and depe; dppf=bis(diphenylphosphanyl)ferrocene; depe=
A
ACHTUNGTRENNUNG
bis(diethylphosphanyl)ethane).[13] On the contrary, we have
shown with a series of sterically characterized [Pd
complexes that the distribution of the tectons between the
(N^N)]2+
ACHTUNGTRENNUNG
2n+
[Pd
G
G
trimeric and tetrameric products
Results and Discussion
(n=3, 4) follows a trend concluded from crystallographic
data (N^N=chelating diamine).[14]
Synthesis and structural characterization of ligands L1 and
L2: Ligands L1 and L2 were prepared by using a modified
procedure of Ragaini and co-workers (Scheme 3).[16b,c] The
Interested in how these steric effects are manifested, we
planned and synthesized new bis(arylimino)acenaphthenes
(Ar-BIANs). These compounds have been known for more
than four decades, mostly involving their application in ho-
mogeneous catalysis,[15] structural and physicochemical stud-
ies,[16] and preparation and structural characterization of
complexes with main-group elements.[17] To our knowledge,
they have not been utilized for the self-assembly of metalla-
macrocycles.
Depending on the size and position of the R substituents,
the aryl groups of bis(arylimino)acenaphthenes E exert
steric hindrance on the chelating imino nitrogen atoms to a
varying extent (Scheme 2).[16a,d] Our attempts to observe
Scheme 3. Preparation of L1 and L2. py=2- or 4-pyridyl.
treatment of the zinc complex of N,N’-bisACTHNUTRGNEUNG[bis(3,5-trifluoro-
methyl)phenyl]acenaphthenequinonediimine in methanol
with an amine results in exchange of the imine moieties.
The free ligands L1 and L2 can then be isolated from their
zinc complexes by suspending them in CH2Cl2 and shaking
them with a solution of potassium oxalate in water. A stan-
dard workup of the organic phase yields the target com-
pounds, from which minor contaminations can be effectively
removed by precipitation from CHCl3 with ethanol or dieth-
yl ether.
The protons of L1 and L2 show up separately in a narrow
1
region between d=6.8 and 9.0 ppm in the H NMR spectra,
1
but the H NMR spectra of the metal complexes are much
more complicated due to overlapping peaks. The 2D NMR
spectra (COSY) of the free ligands reveal three isolated
spin systems, which makes the assignment of the phenyl,
pyridyl, and naphthyl protons straightforward. Beside the
resonances attributable to the structures shown in Scheme 2,
we observed peaks of low intensity in the aromatic region
(Figure 1). These peaks did not change upon repeated re-
crystallization of the compounds, thus suggesting that the
second set of resonances of low intensity may be due to
structural (E/Z) isomerism around one of the C=N double
bonds rather than contamination. We confirmed this conclu-
sion by collecting the NMR spectroscopic data in several
solvents (CDCl3 and CD2Cl2). Figure 1a,b shows that the
ratio of the major and minor constituents depends on the
solvent. Further, the enhanced proportion of the minor com-
ponent in CD2Cl2 relative to that in CDCl3 could be con-
trolled by adding 2,2,2-trifluoroethanol to the sample, which
suggests that the structural properties of L1 in solution can
be influenced by the presence of hydrogen-bond donors. We
note that earlier studies on free aryl-BIAN ligands did not
report the presence of E/Z isomers except for those carrying
Scheme 2. Bis(arylimino)acenaphthenes E, L1, L2, and L3.
self-assembled macrocycles in the reaction of 4,4’-bpy with
the palladium complexes of bis(2,4,6-trimethylphenyl)BIAN
and
bis(4-methoxyphenyl)BIAN
(BIAN=1,2-bis-
ACHTUNGTRENNUNG
congestion around the coordinating nitrogen atoms of these
ligands. Therefore, we introduced terminal pyridyl groups,
thus generating the extended bipyridyls L1 and L3 (see
Scheme 2) as bridging tectons and leaving the internal bis-
ACHTUNGTRENNUNG
and L2. The steric features of the former ligand can control
1) the sequence of coordination of large metal centers in
complexes to its sterically different donor groups and 2) can
sort the interacting complexes by size between donor sites.
The emergence of unexpected pentameric and hexameric
complexes formed with this ligand will also be documented.
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Steric Effects in the Self-Assembly of Palladium Diphosphane Complexes
FULL PAPER
tence of weak hydrogen bonds between the chelating coor-
dination site and the solvate molecule. No interaction was
observed between the pyridyl nitrogen atoms and the etha-
nol solvent.
More importantly, crystallographic analysis of a single
crystal isolated from CHCl3 revealed two CHCl3 molecules
with hydrogen atoms pointed toward the chelating position
(Figure 2b). Although the Hsolvate···N8 distances (i.e., 2.61
À
and 2.47 ꢅ) are at the upper limit of the D H···A distances
that may be considered as hydrogen bonds, structures (a)
and (b) show the proneness of the chelating imino nitrogen
atoms to interact with positively charged or polarized parti-
cles. In line with the less acidic nature of dichloromethane,
no hydrogen bonds were detected if the crystals were grown
in CH2Cl2, even though one CH2Cl2 molecule for each L2
ligand was observed in the crystal structure. This observa-
tion coincides well with the solution behavior of the ligands,
namely, larger amounts of the minor constituent are detect-
ed in samples dissolved in less acidic CD2Cl2. It seems worth
mentioning that a search of the Cambridge Crystallographic
Database (CCDB)[18] for Ar-BIAN-type ligands has not
yielded structures involving hydrogen-bonded solvent mole-
cules at the chelating coordination site. However, the struc-
tural data, such as bond distances, bond angles, and tilting of
the phenyl groups with respect to the acenaphthene moiet-
ies, do not show any unusual values (see the material depos-
ited at the CCDC).
Figure 1. 1H NMR spectra of L1 in a) CD2Cl2, b) CDCl3, and c) CD2Cl2/
CF3CH2OH (50:3; 530 mL). d) 1H NMR spectrum of L2 in CD2Cl2 (aster-
isks indicate the resonances of the minor isomer).
a bis(3,5-trifluoromethyl)phenyl group at least at one of the
imino nitrogen atoms.[15e,16a–c]
The remarkable affinity of the ligands to acidic hydrogen
atoms (even very weak acids) has been demonstrated by X-
ray diffraction studies on L2. The molecular structure in Fig-
ure 2a shows that the product isolated from a CH2Cl2/EtOH
mixture is a 1:1 adduct of L2 and ethanol. The H(O) hydro-
gen atom is located 2.32 and 2.26 ꢅ from the N8 and N8a
imino nitrogen atoms, respectively, which supports the exis-
1
In summary, the major resonances in the H NMR spectra
of L1 and L2 have been attributed to the E/E isomers of the
ligands as shown by the diagrams in Scheme 2, whereas the
peaks of low intensity have been ascribed to molecules in
which one of the C=N double bonds is isomerized. Both the
coexistence of E/E and E/Z isomers in solution and the pro-
nounced tendency of the chelating coordination sites to in-
teract with hydrogen-bond donors, even with CHCl3, are sig-
nificant in the self-assembly reactions.
Self-assembly of L1 with palladium diphosphane complexes
1a–c: Ligand L1 possesses two types of coordination site:
terminal pyridyl nitrogen atoms and chelating imino nitro-
gen atoms. One may speculate that these coordination sites
are both energetically and sterically different. The position
of the imino nitrogen atoms enables them to bind simultane-
ously to form chelate complexes, thus stabilizing the metal–
nitrogen coordinate bond. As shown by a survey of CCDB,
Ar-BIANs are coordinated as chelates in the majority of
their complexes. However, one can expect considerable
steric congestion for L1 in the vicinity of the imino nitrogen
atoms due to the relatively bulky phenyl groups attached to
these atoms.[16d] On the contrary, the terminal pyridyl groups
are not sterically constrained but their coordination is not
enhanced by the chelate effect either. Therefore, L1 may
support a structurally induced selectivity in the spontaneous
assembly of metallamacrocycles.
Self-assembly of a 1:1 molar ratio of Pd/L1: The self-assem-
Figure 2. Molecular diagrams of a) L2·CH3CH2OH and b) L2·2CHCl3.
(OTf)2 with L1 was studied in
bly of [PdACHUTNGTREU(NGNN dppp)AHCTUNRTGEG(NNNU H2O)2]ACHTGUNTRENNUGN
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G. Besenyei et al.
Table 1. 15N NMR data of ligands L1 and L2 and their complexes with
CD2Cl2 and CDCl3 in the concentration range 4–48 mm. The
1H NMR spectrum of a 1:1 mixture of L1 and [Pd
(dppp)-
(H2O)2](OTf)2 is shown in Figure 3b. A comparison with the
[Pd
ACHUTGTNNERNUG(dppp)]ACHTUNTGERN(NUNG OTf)2.
AHCTUNGTRENNUNG
d (15N) [ppm]
G
ACHTUNGTRENNUNG
N(py)
304.5
N
(C=N)
solvent
L1
336.7
335.0
340.6
260.0
339.1
260.3
CDCl3
CDCl3
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
L1 +[Pd
L1 +[Pd
N
G
241.3,[a] 242.5[b]
239.6,[a] 241.5[b]
243.3,[a] 236.5[b]
306.6
L1 +2[Pd
R
L2
L2 +[Pd
303.0
[a] Tetrameric and higher aggregates. [b] Trimeric species.
The pyridine ortho-hydrogen signals occur in an area of
1
the H NMR spectra that is free of other overlapping peaks
and provide information on the number of components
formed in the self-assembly process. The two multiplets of
comparable intensity around d=9.0 ppm in the 1H NMR
spectrum suggest the presence of two major, highly symmet-
ric constituents in the reaction mixture and can be tentative-
ly assigned to tetrameric (square) and trimeric (triangular)
species, on the basis of the abundant structural data accumu-
lated on the reaction of cis-protected [Pd
ACHUTNGTREN(NUG P^P)ACHTUNGTRENNUNG(H2O)2]-
AHCTUNGTRENNUNG
coexistence of two major constituents is strongly supported
by Figure 3b and the concentration dependence of the equi-
1
librium composition in CD2Cl2 studied by H NMR spectro-
scopic analysis (see Figure S1 in the Supporting Informa-
tion). As shown by a series of spectra recorded at various
concentrations, the intensity ratios of the two major peaks
around d=9 ppm are inverted when the tecton concentra-
tion goes from 4 to 48 mm. This feature reflects a pro-
nounced entropic control over the equilibrium and suggests
1
that the H NMR resonances at d=9.02 and 8.95 ppm are
due to the larger and smaller species, respectively.
Figure 3. 1H NMR spectra of L1 (a) and mixtures of [Pd
(OTf)2/L1 in molar ratios of 1:1 (b) and 2:1 (c) in CD2Cl2 ([L1]=16 mm).
The triangles and squares label peaks assigned to trimeric and tetrameric
aggregates, respectively.
ACHUTGTNERN(NUG dppp)ACHTUGNTRENN(GUN H2O)2]-
Although the formation of two major constituents seems
to be a reasonable interpretation of the NMR spectra, close
inspection of Figure 3b and Figure S1 (see the Supporting
Information) reveals that an attempt to describe these reac-
tion mixtures as two-component systems is an undue simpli-
fication. A series of resonances of low intensity (labeled by
asterisks) clearly indicate the formation of a third constitu-
ent, although it is present in a much smaller proportion than
the major components at concentrations of ꢁ8 mm. Further-
more, the ortho-hydrogen atoms of the acenaphthene
moiety could be expected to reflect the formation of the as-
sumed trimeric and tetrameric aggregates, as do the pyridine
ortho protons. Both the number and the relative intensities
of the resonances between d=6.72 and 6.86 ppm suggest
that the formation of species other than trimers and tetram-
ers should be considered (see the inset of Figure 3b and Fig-
ure S1 in the Supporting Information). All these observa-
tions will be discussed in an ongoing section.
ACHTUNGTRENNUNG
1H NMR spectrum of L1 (Figure 3a) shows characteristic
shifts of the resonances attributed to protons at positions 3
and 7 (see the numbering scheme; Figure 3). The multiplets
assignable to the ortho-hydrogen atoms of the pyridyl
groups at dꢀ9 ppm indicate that the terminal donor atoms,
rather than the internal imino nitrogen atoms, are involved
in the interaction. The preferential coordination of the pyr-
idyl groups over the chelating imino nitrogen atoms was fur-
ther confirmed by 15N NMR spectroscopic investigations.
The data presented in Table 1 show that coordination of the
[PdACHTUNGTRENNUNG
(dppp)]2+ ion to L1 only induces a negligible decrease in
the chemical shift attributed to the imino nitrogen atoms.
On the other hand, the sign and magnitude of the observed
coordination shift (a difference of DdꢀÀ62 ppm was detect-
ed for the pyridyl nitrogen atoms) is indicative of the in-
volvement of the terminal pyridyl groups in coordination
and is in full agreement with the general trends.[19]
Interaction of L2 with [Pd (OTf)2 at Pd/L2 =
ACHTUNGTRNE(NGUN dppp)ACHUTNRTGEG(NNNU H2O)2]ACHTGUNTRENNUGN
1:1: Before we turned our attention to the self-assembly of
L1 with palladium diphosphane complexes at Pd/L1 =2:1, we
studied the interaction of N,N’-bis[4-(2-pyridyl)phenyl]ace-
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Steric Effects in the Self-Assembly of Palladium Diphosphane Complexes
FULL PAPER
naphthenequinonediimine (L2) with 1a. It was anticipated
that the position of the pyridyl nitrogen atoms in L2 was not
favorable for the formation of stable complexes with the
fore, ligand L2 is an excellent model compound to identify
the spectral changes associated with the incorporation of a
palladium tecton at the chelating site.
bulky [Pd
A
Further, the 15N NMR data in Table 1 confirm the utiliza-
tion of the imino nitrogen atoms as donor atoms in the in-
with diphosphane complexes may offer insight into the
structural changes induced by selective coordination at the
chelating imino nitrogen atoms, thus facilitating the assign-
ment of the spectra recorded at Pd/L1 =2:1 molar ratio.
teraction of the [PdACTHNUTRGNEUNG
(dppp)]2+ ion with L2. Finally, the sharp
singlet in the 31P NMR spectrum at d=15.2 ppm indicates a
plane of symmetry in the complex and suggests that both
dppp and L2 act as bidentate ligands (Figure 4b).
1
A comparison of the H NMR spectra in Figure 4 shows
that the interaction of L2 with 1a leads to the formation of
monomeric [Pd
G
N
Self-assembly at Pd/L1 =2:1: The 1H NMR spectrum that
proceeded the self-assembly of the [PdACTHNUTRGNEUNG
(dppp)]2+ ion with L1
in a 2:1 molar ratio ([L1]=16 mm) is presented in Figure 3c.
Several apparent differencies are exhibited compared to the
spectrum of a solution of a 1:1 molar ratio at the same con-
centration of L1 (Figure 3b). The most striking feature of
this spectrum is that new multiplets emerge in the region
d=5.6–5.9 ppm, which was free of signals in samples of
equimolar tectons. These resonances are attributed to the
ortho protons of the acenaphthene moiety and are a conse-
quence of binding a second palladium complex at the chelat-
ing site of L1. This chelating coordination is further con-
1
firmed by looking at the H NMR resonances attributed to
the aliphatic protons (Figure 3b,c). Furthermore, the intro-
duction of a second equivalent of the palladium tecton indu-
ces a significant shift of the resonances attributed to the che-
lating nitrogen atoms, whereas the pyridine nitrogen atoms
are only moderately affected (Table 1). Finally, the ratio of
the resonances at around d=9 ppm indicates the accumula-
tion of the larger aggregate upon addition of a second palla-
dium complex. Thus, anchoring the palladium complexes at
the internal coordination sites has the same effect on the nu-
clearity as the inclusion of guest molecules in self-assembled
species.[22]
Figure 4. a) 1H NMR spectrum of L2 and b) 1H and 31P NMR spectra of
the [Pd (OTf)2 (7a) in CD2Cl2.
(dppp)(L2)]
N
ACHTUNGTRENNUNG
Comparing the spectral portions between d=1.5–3.5 and
5.5–7.0 ppm in Figure 3b,c, we can draw the conclusion that
um center coordinates to the imino rather than to the pyrid-
yl nitrogen atoms. This outcome is well illustrated by the fol-
lowing observations: 1) the position of the pyridine ortho-
hydrogen atoms is not affected by the coordination and 2)
the ortho-hydrogen atoms of the acenaphthene moiety is
shifted upfield by almost Dd=1 ppm. This latter feature can
be ascribed to the more pronounced shielding by the neigh-
boring phenyl groups and is certainly attributable to the en-
hanced steric congestion around the imino nitrogen atoms
upon coordination of L2 to the bulky palladium diphosphane
complex.
the complexation of the first and second [PdACTHNUTRGNEUNG
(dppp)]2+ tec-
tons takes place in a strictly consecutive manner. No 2:1
complex is formed until all the terminal pyridyl groups are
consumed. However, a survey of the sections at d=8.8–9.1
and 5.5–6.0 ppm in Figure 3c leaves us in uncertainty re-
garding the number of species of different nuclearity, a case
that has been observed before with 1:1 mixtures. Thus, two
major issues have been left unanswered regarding the 1:1
and 2:1 reaction mixtures: 1) why is seemingly contradictory
information inferred from the pyridine ortho hydrogen
atoms and the acenaphthene ortho protons regarding the
number of species and 2) how many tectons are involved in
the observable species. To make an unambiguous assign-
ment of the major resonances in the NMR spectra of the 1:1
and 2:1 reaction mixtures, mass-spectrometric studies were
carried out with electrospray ionization (ESI). As demon-
strated previously, mass-spectrometric analysis has been
fruitfully utilized for characterizing self-assembled sys-
tems.[23–26] However, it is often hard to ionize weakly bound
complexes without completely fragmenting them, even with
soft ionization techniques, such as ESI. Mass-spectrometric
Thus, the remarkable shift of H3 (see the numbering
scheme) provides evidence that the imino nitrogen atoms
are involved in the coordination. A comparison of the reso-
nances attributable to the dppp propylene chains in Figur-
es 3b and 4b reveals further differences associated with the
coordination mode of the [Pd
the CH2A(PPh2) and CH2A(CH2) protons of the [Pd-
(dppp)L1]n
ACHTUNGTRENNUNG aggregates resonate at d=3.26 and 2.32 ppm
(dppp)]2+ tecton. Although
ACHTUNGTRENNUNG
C
CHTUNGTRENNUNG
2n+
in the former spectrum, the same hydrogen atoms show up
at lower frequencies (i.e., d=2.13 and 1.81 ppm) in the
ACTHNUTRGNEUNG
1H NMR spectrum of the [Pd(dppp)(L2)]2+ complex. There-
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10625

G. Besenyei et al.
parameters were carefully adjusted for detection of intact
complexes. In the ESI-MS spectrum of the Pd/L1 =1:1 com-
plex, the base peak is [L1 +H]+, whereas the most intense
es S1 and S2 in the Supporting Information) allows us to
assign the resonances to species of various nuclearity. The
interaction of linear ditopic ligands with cis-protected palla-
dium tectons preferentially leads to the formation of trimer-
ic and tetrameric species (with the predominance of the
former at low concentrations). The results of our mass-spec-
trometric and DOSY NMR spectroscopic experiments (see
below) confirm that the resonances at d=6.74 and 8.95 ppm
belong to H3 and H7 (see Figure 3) of the macrocyclic tri-
meric aggregate 3a, whereas the same hydrogen atoms of
the tetrameric species 4a were identified as doublets at d=
6.78 and 9.02 ppm. The intensity of the resonance at d=
6.83 ppm increases monotonously as the tecton concentra-
tion varies from 4 to 48 mm and was attributed to a pen-
tameric product 5a. Comparison of the integral values of
the two major doublets at d=8.95 and 9.02 ppm with those
of the relevant acenaphthane ortho protons shows that the
signals of the pyridine ortho-hydrogen atoms of the tetra-
meric and pentameric assemblies overlap.
peaks belong to the following ions: [Pd
(dppp)Cl]+, [Pd (OTf)]+, [Pd(dppp)L1
(dppp)
[Pd
(dppp)L1Cl]+.
ACHTUNGTRENNUNG
A
E
E
N
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
The region m/z 1500–2500 of the ESI-MS spectrum re-
corded of a Pd/L1 =1:1 reaction mixture is presented in
Figure 5. In the spectrum, the expected trimeric and tetra-
meric species can be readily identified. The isotope cluster
centered at m/z 1588.82 (see the inset of Figure 5) corre-
sponds to the intact square that carries five triflate anions,
that is, [Pd
the doubly charged [Pd
2457.66) overlaps with a less intense peak of the [Pd2-
(dppp)2L1 (OTf)3]+ ion (note that the instrument resolution
2A
4ACHTUNGTRENNUNG 4ACHTUGNTRENNNUG
(dppp)4L1
(OTf)5]3+. The isotope distribution of
A
A
4
4
A
CHTUNGTRENNUNG
permits separation of the two species). The tetrameric spe-
cies can also be observed as a minor constituent at m/z
2401.13 as the [Pd4ACHTUNGTRENNUNG 4HCATUNGTRENNGUN
(dppp)4L1 (OTf)5Cl]2+ ion. This outcome
1
shows that the triflate anion can be exchanged in part with
chloride. Trimeric species have been detected as [Pd3-
To complete the interpretation of the H NMR spectrum
of the 1:1 adducts, the presence of a minor product labeled
with asterisk in Figure 3b needs to be addressed. A summa-
ry of otherwise irrelevant observations sheds light on the
nature of this species. The coordination of the second [Pd-
ACHTUNGTRENNUNG
(dppp)3L13(OTf)4]2+ and [Pd3(dppp)3L13(OTf)3Cl]2+ ions (m/z
1806.50 and 1749.52, respectively).
An important observation is that a pentameric complex
can also be identified beside the expected trimeric and tetra-
meric species. The molecular masses and isotopic distribu-
AHCTUNGTRENNUNG
(dppp)]2+ tecton to L1 results in the fading of resonances,
which is attributed to the unknown component (Figure 3).
Furthermore, the concentration of the minor product was
noticeably greater in CD2Cl2 than in CDCl3, thus suggesting
that its formation can be influenced by the nature of the sol-
vent. This feature is similar to the behavior of the free
ligand, for which the concentrations of the E/E and E/Z iso-
mers depended on the solvent. The elimination of a series of
tions at m/z 1986.23 and 2023.57 (i.e., ([Pd
(dppp)5L1 -
5
A
R
CHTUNGTRENNUNG
clearly show the presence of these species, although at a rel-
atively small intensity.
A combination of these observations with the concentra-
1
tion dependence of the H and 31P NMR spectra (see Figur-
(OTf)2/L1 in acetone (c=0.1 mm). The range at m/z 1900–2500 is magnified threefold.
Figure 5. ESI-MS spectrum of a 1:1 mixture of [PdACHTNUGTREN(NNUG dppp)ACHTUNRTGEN(GUNN H2O)2]ACHTGUNTRENNUGN
(Pd=[PdACHTUNGTRENNUNG
(dppp)]2+ ion).
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Steric Effects in the Self-Assembly of Palladium Diphosphane Complexes
FULL PAPER
1
resonances of low intensities in the H NMR spectrum of L2
upon coordinating a palladium complex to the chelating ni-
trogen atoms seems to be a related phenomenon (Figure 4).
All these observations suggest that the formation of the
minor product at Pd/L1 =1:1 is attributable to the E/Z iso-
meric form of ligand L1. We may reasonably assume that
assemblies. On the other hand, a DOSY NMR spectrum ob-
tained of a sample at [tecton]=32 mm clearly shows that the
species that prevail at high concentrations certainly has a
nuclearity of n>4 and has been identified as a pentameric
complex (Figure 6b).
Interestingly, the reaction of cis-[PtACTHUNTRGENN(UG Et3P)2]ACHTUGNTNER(NUGN OTf)2 with
two molecules of the bent isomer with two [Pd
tons can be combined into a dimeric complex, 2a.
Spectroscopic evidence for the formation of dimeric spe-
cies 2a is observed in the 31P NMR spectra of 1:1 mixtures
in which resonances at d=6.73, 6.51, 6.14, and 5.92 ppm can
be clearly recognized as an AB doublet (dA =6.05 ppm, dB =
A
structurally related dipyridyl ligands that have an embedded
N,N’-ethylenebis(salicylimine) (salen) backbone followed a
different pathway in constructing self-assembled products.[27]
The exclusive formation of dimeric complexes was reported
when the reaction was carried out with free bases, which
could be rearranged to form squares by introducing a metal
cation into the chelating site. It is assumed that a similar
process occurs with the dimeric species reported herein, in
which the equilibrium of the E/E and E/Z isomers is influ-
enced by the incorporation of the second palladium tecton.
Multiple assemblies were also observed in the reaction
2
6.60 ppm, JPP =35 Hz; see Figure S3 in the Supporting In-
formation). The splitting pattern is in line with the nonequi-
valence of the phosphorus nuclei in the dimeric species.
Further evidence for the relative sizes of the self-assem-
bled species of 1:1 mixtures has been extracted from 1H
DOSY NMR spectroscopic experiments. As the proportions
of the components vary strongly with concentration (see
Figure S1 in the Supporting Information), a spectrum re-
corded at [tecton]=4 mm allowed us to identify the tetra-
meric (0.29ꢆ10À9 m2 sÀ1), trimeric (0.34ꢆ10À9 m2 sÀ1), and
minor dimeric (0.40ꢆ10À9 m2 sÀ1) components (Figure 6a).
mixtures of [Pd
ratio by using ESI-MS (Figure 7). A compilation of signals
identified as [Pd
(dppp)L1]n backbones (n=3–6) with varying
amounts of additional [Pd
(dppp)]2+ tectons and triflate ions
is given in Table S1 (see the Supporting Information). As
the [Pd
(dppp)]2+ tecton preferentially coordinates to the
terminal pyridyl group, deviations from the ideal [{Pd-
(dppp)}2nL1 (OTf)m](4nÀm)+ formulae for 2:1 complexes (n=
nA
(OTf)2 and L1 at a 2:1 molar
ACHTUNGTRNE(UNGN dppp)ACHUTNRTGEG(NNNU H2O)2]ACHTGUNTRENNUGN
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
The calculated hydrodynamic radii (rtetramer =18.3 ꢅ, rtrimer
15.7 ꢅ, rdimer =13.3 ꢅ) support the conclusion that the [Pd-
=
A
CHTUNGTRENNUNG
2n+
N
complexes (n=2, 3, 4) are all large molecular
3–6, m<4n) can be attributed to the less advantageous ener-
getics to bind palladium complexes at the sterically more
1
congested chelating site. In the H NMR spectra, however,
the integrals of the [PdACHTNUTGRNEUNG
(dppp)]2+ complex ions anchored at
the corners and on the edges are equal (see the aliphatic
region of the spectrum in Figure 3c), thus indicating that
both the terminal and central coordination sites of L1 are
fully occupied in solution. The presence of these intact spe-
cies has been confirmed by ESI-MS. The spectra show ions
that correspond, for example, to the following structures:
[Pd
(dppp)10L1
3032.08, 2678.53, 2502.02, and 2395.69, respectively).
G
E
,
[Pd
N
R
, [Pd10-
G
(OTf)16]4+, and [Pd
(dppp)12L1
CHTUNGTRENNUNG
At a Pd/L1 =2:1 molar ratio, no dimeric species can be
formed and the constituents present at low concentrations
can be attributed to the trimeric and tetrameric species.
Therefore, resonances at d=5.64 and 9.00 ppm have been
ascribed to H3 and 7 (see Figure 3c and Figure S4 in the
Supporting Information) of the macrocyclic trimeric aggre-
gate 3aa, whereas hydrogen atoms of the tetrameric species
4aa in the same positions have been detected as doublets at
d=5.84 and 9.06 ppm. Increasing the concentration does not
generate new resonances in the 1H NMR spectrum at
around d=9 ppm, but results in only a slight broadening of
the outermost doublet. The relative intensities of two over-
lapping doublets at d=5.82 and 5.80 ppm changes with in-
creasing concentration (see the insets of Figure S4 in the
Supporting Information). The resonance at d=5.82 ppm has
been ascribed to pentamers 5aa, whereas the latter domi-
nates at high concentration (24 mm) and has been attributed
to a hexameric product 6aa. Assignment of the 31P NMR
resonances (see Figure S5 in the Supporting Information)
Figure 6. Sections of 1H DOSY NMR spectra of a 1:1 mixture of Pd/L1
recorded at tecton concentrations of a) 4 and b) 32 mm (CD2Cl2, 298 K).
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G. Besenyei et al.
Figure 7. ESI-MS spectrum of a 2:1 mixture of [PdACTHUNGTREN(UNNG dppp)ACHTUTNGRENNUG(H2O)2]ACHTUNGERTNNUNG ACHTNGUTRENNGUN
(OTf)2/L1 in acetone ([L1]=1 mm; Pd=[Pd(dppp)]2+ ion).
has been accomplished accordingly. It is important to em-
phasize that compounds 5aa and 6aa also possess pyridine
ortho-hydrogen atoms, although the resonances do not sepa-
rate from those of the tetrameric species. To obtain the
same ratio of integrals that was calculated for the resonan-
ces of the pyridine ortho-hydrogen atoms, it was necessary
to compare the integral of the well-separated resonance at
d=5.64 (trimer 3aa) with the sum of the signals between
d=5.77 and 5.89 ppm (4aa–6aa).
An identical assignment can be made on the basis of the
1H DOSY NMR spectra recorded at L1 concentrations of 4
and 24 mm (see Figure 8a,b). The resonances at d=5.64 and
5.84 ppm, separated by the diffusion coefficient, were attrib-
uted to 3aa and 4aa, respectively, whereas the species that
emerge at high concentration can be characterized by larger
hydrodynamic radii; although the separation of the pen-
1
tameric and hexameric species in the H DOSY NMR spec-
trum could not be achieved due to their overlapping signals
at 9.38 T.
In a detailed mass-spectrometric study on related palladi-
um bispyridyl systems, Schalley et al. observed hexameric
and octameric species that were ascribed to the formation of
sandwich-like aggregations of trimers or tetramers in the
ESI process.[23] This conclusion is in agreement with all spec-
troscopic observations as there are no NMR spectroscopic
indications that self-assembled species other than trimers
and tetramers exist in those systems. For the system present-
ed herein, the emergence of a pentameric product in the 2:1
mixture cannot be explained by layered aggregation of a di-
meric and trimeric species because at this molar ratio mac-
rocyclic dimers (which would require the bent L1 ligand) do
1
Figure 8. Sections of the H NMR DOSY spectra of a 2:1 mixture of Pd/
L1 recorded at L1 concentrations of a) 4 and b) 24 mm (CD2Cl2, 298 K).
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Steric Effects in the Self-Assembly of Palladium Diphosphane Complexes
FULL PAPER
not exist. It is also important that hexamers could be detect-
ed but higher-order assemblies were not observed, although
they may also be formed by this mechanism. Most impor-
tantly, increasing the concentration not only changes the
trimer/tetramer ratio but also results in the emergence of
new resonances of slowly diffusing constituents, as demon-
strated by the 1D NMR and DOSY spectra. Therefore, the
aggregates with nuclearities higher than four are considered
to be existing components of the [PdACTHNUTRGNEUNG
(dppp)]/L1 system in
the solution phase. Although pentameric and hexameric
complexes have been constructed from appropriately choos-
en linear and angular tectons[28] and the intermediacy of a
pentameric aggregate with the sterically less demanding
[Pd(en)]2+ tecton has also been noted,[29] we are not aware
of any reports on the formation of high-nuclearity species
(n=5, 6) from cis-protected square planar diphosphane
complexes and linear ditopic donor building blocks. It seems
reasonable to attribute the unique ability of L1 to form un-
usually large metallacycles to its size (ca. 20 ꢅ) and flexibili-
ty, although a firm structure/reactivity relationship cannot
be described at the current time. Further accumulation of
data on the coordination properties of L1 and similar ligands
(e.g., L3) are needed to provide a more definite answer to
this question. The observations presented herein suggest
that species of high nuclearity (n=5, 6) may be present in
other related systems, although their concentration may be
too low to be detected. However, their involvement in the
interconversion of the observable constituents may be im-
portant; therefore, the direct observation of these aggre-
gates in the present study may be of use in mechanistic con-
siderations related to self-assembled systems.
Scheme 4. Notes: 1) Charges and anions are not shown; 2) the drawings
illustrate the connectivity and not the true spatial orientation of the tec-
tons.
Self-assembly of L1 with [Pd
complex ions: The interaction of L1 with the more bulky
[Pd
(dppf)]2+ cation (f=968) leads to the same type of com-
(dppf)]2+ and [Pd(dppe)]2+
ACHUTGTNRENNUG CAHTUNGTRENNUGN
AHCTUNGTRENNUNG
plexes that were observed with dppp as the ancillary ligand;
therefore, only representative spectra are presented and dis-
cussed. At an equimolar ratio, the multiplets of the pyridyl
ortho-hydrogen atoms indicate the presence of two major
constituents that have been identified as trimer 3b and
1
tetramer 4b on the basis of H DOSY NMR spectra and the
characteristic changes of the spectra upon increased tecton
concentration (Figure 9a). A trace amount of species that
presumably possess a nuclearity larger than four appears as
a shoulder at d=6.80 ppm. The cyclopentadienyl protons of
the trimeric and tetrameric species overlap.
To summarize the steric properties of L1, we have shown
(OTf)2 and L1
that a 1:1 molar mixture of [PdACTHNUGTREN(NNGU dppp)AHCUTNRTGENNG(NU H2O)2]ACHTGUNTRENNUGN
generates trimeric, tetrameric, and pentameric aggregates,
depending on the concentration, as major constituents with
the simultaneous formation of a minor amount of dimeric
species. Whereas the former three products are constructed
from the linear E/E isomer of L1, the dinuclear constituent
is attributed to the presence of traces of the E/Z form. The
experimental data show that only the terminal pyridyl
groups are coordinated in all these species. In other words,
the primary coordination site of L1 toward the sterically de-
manding [PdACHTUNGTRENNUNG
(dppp)]2+ complex is the terminal pyridyl
group, despite the beneficial effect that the chelating posi-
tion of donor atoms could exert in the stabilization of coor-
dinate bonds. The internal, more-hindered imino nitrogen
atoms interact with the palladium tecton only when the ster-
ically less-hindered positions are occupied. Therefore, due
to the sterically different environments of its donor atoms,
ligand L1 establishes a sterically induced sequence of coordi-
nation. No dimeric species can be detected at a 2:1 molar
ratio of Pd/L1, at which the coordination of a palladium
complex at the chelating donor atoms hampers the forma-
tion of the bent isomer. Scheme 4 summarizes our experi-
mental observations and gives a pictorial description of the
Figure 9. 1H and 31P NMR spectra of self-assembled species formed in
interaction of the [PdACTHNUTRGNEUNG
(dppp)]2+ complex ion with L1.
the interaction of [Pd
(OTf)2 with L1 in molar ratios of
ACHTUNGTRNE(UNGN dppf)ACHUTNRTGEG(NNNU H2O)2]ACHTGUNTRENNUGN
a) 1:1 and b) 2:1 (CD2Cl2, 258C, [L1]=30 mm).
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10629

G. Besenyei et al.
The introduction of a second [PdACHTUNGTRNEUNG
(dppf)]2+ tecton results
is the enhancement of higher-order assemblies, which is
demonstrated by the reversed ratio of peak intensities at d=
1
9 ppm in the H NMR spectrum and around d=33 ppm in
ACTHNUTRGNEUNG
the 31P NMR spectrum. Although the [Pd(dppf)]2+/L1
system produced much less intensive peaks attributable to
self-assembled species in the ESI-MS spectra, the signals at
m/z 1616.06 ([Pd4L
2417.53 ([Pd6L4A
(OTf)9]3+), 2736.86 ([Pd7L
2899.28 ([Pd7L5A
(OTf)11]3+) strongly indicate that trimeric,
(OTf)5]3+), 1935.76 ([Pd5L (OTf)7]3+),
3ACHUTNGTRENNUG CHUTNGTRENNUNG
3A
C
4ACHTUNGTRENNUNG
CHTUNGTRENNUNG
tetrameric, and pentameric aggregates are present in the
equilibrium mixture. Based on the spectral changes ob-
served in the NMR spectra upon increasing the tecton con-
centration or switching the Pd/L1 ratio from 1:1 to 2:1, reso-
nances at d=6.38 and 6.64 ppm have been ascribed to the
trimeric 3bb and tetrameric 4bb species. The formation of a
third component was evidenced by a growing signal at d=
6.58 ppm as the L1 concentration increased. This species has
Figure 11. 31P NMR spectra of the self-assembled complexes formed in
the interaction of the [PdACHTNUGRTNEUNG
(dppe)]2+ ion with L1 at molar ratios of a) 1:1
1
and b) 2:1 (CD2Cl2, 303 K, [L1]=15 mm).
the largest hydrodynamic radius, as shown by the H DOSY
NMR spectroscopic experiment (Figure 10) and has been
ces at d=64.68 and 65.47 ppm have been ascribed to palla-
dium tectons located on the tips of the squares (4cc) and tri-
angles (3cc), respectively, whereas those at d=68.95 and
68.51 ppm have been attributed to phosphorus atoms of
edge-bound palladium complexes. At a 1:1 molar ratio, how-
ever, the [PdACTHNUTRGNEUNG
(dppe)]2+ tecton coordinates not only to the
terminal pyridyl groups but to the internal imino nitrogen
atoms as well, as shown by in Figure 11a.
Self-assembly becomes fully controlled sterically, however,
when a metal/ligand ratio of 2:1 was established by using a
1:1:1 mixture of [PdACTHNUGRTENNUG ACHUTNGTRENNGUN
(dppp)]2+/[Pd(dppe)]2+/L1. The
31PNMR resonances of the dppp and dppe complexes
appear in significantly different regions of the NMR spectra.
A comparison of the data collected in Table 2 with the
Figure 10. Section of the 1H DOSY spectrum of a 2:1 mixture of 1b/L1
recorded at [L1]=30 mm (CD2Cl2, 298 K).
Table 2. 31P NMR chemical shifts of complexes formed at Pd/L1 =2:1.
d (31P) [ppm]
identified as pentameric species. As in the [PdACTHNUTRGNEUNG
(dppp)]/L1
tetrameric and higher aggregates
trimeric species
P^P
corner
edge
corner
edge
system, the resonances of the pyridyl ortho-hydrogen atoms
15.34,[a] 15.59
68.95
67.87
6.85
65.47
6.94
13.82
68.51
67.23
of the tetramers and pentamers overlap. Unlike the cyclo-
dppp
dppe
dppp+dppe
6.35
64.68
pentadienyl protons (H(Cp)) of the [Pd
ACHTUNGTRENNUNG
6.33, 6.37[a]
which reside at the corners, the edge-bound [PdACHTUNGTRENNUNG
[a] Overlapping resonances.
eties embedded in the trimeric and the nꢁ4 adducts of the
2:1 mixtures are clearly distinguishable (1H DOSY NMR
spectroscopy). The broad resonance in the 31P NMR spec-
trum at d=43.2 ppm is attributed to the edge-bound phos-
phane complexes.
31P NMR spectrum in Figure 12 reveals that the resonances
of 3ac and 4ac can be assigned to corner-bound dppp com-
These observations support the previous conclusion that
the preferred coordination sites of L1 are the terminal pyrid-
yl groups when the partners are bulky complexes. The for-
mation of self-assembled aggregates with a nuclearity higher
than four has also been confirmed.
plexes and edge-positioned [PdACTHNGUTERNNUG
(dppe)]2+ tectons, whereas
those ascribable to the opposite cases are missing from the
spectrum. In other words, the sterically more encumbered
[PdACHTNUTGRNEUNG
(dppp)]2+ tectons selectively coordinate to the less-hin-
dered pyridyl nitrogen atoms, whereas the sterically less-de-
manding dppe complexes occupy the more-hindered chelat-
ing positions. Thus, the steric properties of L1 arrange the
1a and 1c tectons in a complementary manner to achieve
the energetically most favorable conditions.
It is not surprising that the reaction of L1 with the sterical-
ly less demanding [Pd
(dppe)]2+ complex is less selective.
ACHTUNGTRENNUNG
The 31P NMR spectrum of a 2:1 reaction mixture is similar
to that of the [Pd
10630
ACHTUNGTRENNUNG
(dppp)]/L1 system (Figure 11b). Resonan-
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Chem. Eur. J. 2009, 15, 10620 – 10633

Steric Effects in the Self-Assembly of Palladium Diphosphane Complexes
FULL PAPER
Experimental Section
Instrumentation: NMR spectra were recorded in CD2Cl2 or CDCl3 on a
Bruker Avance 400 spectrometer (1H=400 MHz, 31P=161.97 MHz, 15N=
40.55 MHz (gradient-promoted, proton-detected HMBC experiments),
19F=376.498 MHz). A 5-mm z-gradient broadband observe (BBO) probe
was used in all the experiments. Chemical shifts are referenced in ppm
to trimethylsilane (TMS; X=100.000000 MHz) for 1H or to external
standards (31P: 85% H3PO4, X=40.480742 MHz; 15N: liquid NH3,
X=10.132912 MHz; 19F: CCl3F, X=94.094011 MHz). The software used
was TOPSPIN 2.1.3. For 15N NMR spectroscopy, HMQC spectra were
recorded with 15N–1H coupling values between 3 and 8 Hz. Diffusion-or-
dered spectroscopy (DOSY)[30] used a standard BPP-LED sequence, a
diffusion time of D=50 ms, a gradient ramp adjusted to between 2 and
95% over 16 equidistant steps, and a gradient strength calibrated for the
CDHCl2 signal (Dt =3.17ꢆ10À9 m2 sÀ1). The DOSY spectra were pro-
cessed using a monoexponential fitting procedure, and the experiments
were carried out in standard 5-mm sample tubes without temperature
regulation and heat-convection compensation. Abbreviations used in the
description of the NMR spectra are s=singlet, d=doublet, t=triplet,
m=multiplet, br=broad, o=overlapping, sh=shoulder. The IR spectra
were measured on a Thermo-Nicolet 320 FTIR spectrometer in KBr pel-
lets. The ESI mass spectra of organic compounds were obtained on a
Perkin–Elmer Sciex API 2000 triple quadrupole mass spectrometer fitted
with an ESI source. The samples were dissolved in acetonitrile and intro-
duced by flow-injection analysis. The mass-spectrometric experiments on
the self-assembled complexes were performed on a high-resolution Q-
TOF Premier mass spectrometer (Waters, Manchester) equipped with an
electrospray ion source. The complexes were dissolved in acetone at dif-
ferent concentrations (i.e., 10 mm, 0.1 mm, 1 mm). These solutions were di-
rectly transferred into the ion source with a syringe pump at a flow rate
of 5 mLminÀ1. A positive ionization mode was applied using the following
parameters: capillary voltage: 3 kV, cone: 0 eV, source temperature:
308C, desolvation temperature: 508C, desolvation gas: 800 LhÀ1, cone
gas: 20 LhÀ1, collision energy: 0–10 eV. MassLynx 4.1 software was used
for data acquisition and evaluation.
Figure 12. 31P NMR spectra of the self-assembled complexes formed at a
1:1:1 mixture of [Pd
(dppp)]2+/[Pd(dppe)]2+/L1 (CD2Cl2, 298 K, [L1]=
32 mm).
G
ACHTUNGTRENNUNG
Conclusions
The coordination properties of a novel Ar-BIAN ligand
have been studied in the formation of self-assembled aggre-
gates with cis-coordinated palladium complexes. We have
shown that the dissimilar steric properties of the coordina-
tion sites, terminal pyridyl groups, and internal chelating
imino nitrogen atoms can be exploited in two different ways
to direct self-assemblies. With complexes of bulky diphos-
phanes, such as dppp and dppf, the steric differences can
induce consecutive coordination of the metal complexes
when the sterically more-hindered, chelating imino nitrogen
atoms are utilized as donor sites only after the terminal pyr-
idyl groups have been complexed. On the other hand, with
an equimolar mixture of two metal complexes of dissimilar
steric demands, a sterically-induced, size-selective coordina-
tion takes place and the components are arranged in a com-
plementary manner. It should be possible to utilize this in-
formation to construct multinuclear, self-assembled catalysts
with a frame that is held together by appropriately bulky
corner tectons, whereas sterically less demanding metal
complexes can act as catalytic centers at the more protected
internal coordination positions. Furthermore, the observed
proneness of the imino nitrogen atoms to establish hydrogen
bonds may be a good starting point to design aggregates
that are able to form host/guest complexes with multiple hy-
drogen-bond donors.
X-ray diffraction studies: Intensity data were collected on a Rigaku R-
axis Rapid diffractometer with MoKa radiation (l=0.7107 ꢅ). The data
were corrected for absorption. The structures were solved by direct meth-
ods and refined by anisotropic least-squares against F2 for the non-hydro-
gen atoms. Experimental temperature, final R and wR2 values, number of
reflections are as follows: 295 K, 0.0511, 0.1267, 6355 (L2·CH2Cl2); 151 K,
0.0428, 0.1020, 4843 (L2·EtOH); 295 K, 0.0622, 0.1100, 2960 (L2·2CHCl3).
CCDC-711119 (L2·EtOH), CCDC-711120 (L2·2CHCl3), and CCDC-
711350 (L2·CH2Cl2) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
Syntheses: All moisture-sensitive experiments were carried out under ni-
trogen using standard Schlenk techniques. Dichloromethane was freshly
distilled from calcium hydride, methanol from magnesium methylate, and
diethyl ether from sodium benzophenone ketyl. The starting materials
for the ligand syntheses were purchased from commercial sources and
used without purification. Palladium diphosphane complexes 1a,[31] 1b,[32]
and 1c;[33] 4-(4-pyridyl)aniline;[34] 4-(2-pyridyl)nitrobenzene;[35] and 4-(2-
pyridyl)aniline[36] were prepared by using previously reported methods.
The intermediate [(N,N’-bisACTHNUTRGNE{NUG 3,5-bis(trifluoromethyl)phenyl}bian)ZnCl2]
In the course of our studies to identify the self-assembled
species, mass-spectrometric and DOSY NMR spectroscopic
investigations have led to the unexpected observation of
pentameric and hexameric aggregates. A joint evaluation of
the results of our NMR and MS studies has shown that
these high-nuclearity species should be considered as con-
stituents of the reaction mixtures rather than artifacts of the
ESI process. Structural studies on self-assembled aggregates
of related Ar-BIAN ligands are in progress.
complex was synthesized by a modified procedure of Ragaini et al.[16b,c]
Details of the preparations to yield L1 and L2 are described in the Sup-
porting Information.[37]
Synthesis of metallacycles: The self-assembly of molecular aggregates
from L1 and the diphosphane complexes was accomplished both “in situ”
in NMR tubes and on a preparative scale. To minimize the risk of hydrol-
ysis of the imino functionality, which may be catalyzed by traces of acids,
the NMR solvents were stored over CaH2 and transferred into the NMR
tubes by bulb-to-bulb distillation. The NMR spectroscopic data of species
observable in solution are collected in Tables 1, S2, S3, and S4 (see the
Supporting Information). Representative procedures are desribed below.
Chem. Eur. J. 2009, 15, 10620 – 10633
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10631

G. Besenyei et al.
Preparation of assemblies 3a–5a: [Pd
(dppp)
E
E
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0.210 mmol) and L1 (102.0 mg, 0.210 mmol) were stirred in dry CH2Cl2
(18 mL) in a Schlenk tube under N2 for 90 min. The solvent was reduced
to 5 mL under vacuum and freshly distilled diethyl ether (15 mL) was
added slowly. The orange–yellow precipitate was collected on a Schlenk
filter, washed with diethyl ether, and dried (252 mg, 92%).
Preparation of assemblies 3aa–6aa: [PdACHTUNGTRENN(UNG dppp)ACHTUNRTEGG(NNUN H2O)2]ACHTNGUTNER(NUGN OTf)2 (1a)
(170.6 mg, 0.200 mmol) and L1 (48.7 mg, 0.100 mmol) were placed in a
Schlenk tube and dry CH2Cl2 (20 mL) was added. The solution was
stirred at room temperature for 1.5 h under N2. The yellow solution was
concentrated to approximately 1/3 of the original volume and the product
was precipitated by adding dry diethyl ether (10 mL). The lemon-yellow
compound was isolated by filtration and dried (180 mg, 85%). Reactions
carried out analogously with 1b resulted in a light-brown complex in
nearly quantitative yield.
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The reagents were weighed on a semimicro balance for the “in situ”,
NMR-scale experiments. A representative experiment was performed as
follows: 1b (16.42 mg, 0.0165 mmol) was charged into an NMR tube and
placed in a Schlenk tube. The metal complex was evacuated and heated
at 1008C for 1.5 h. L1 (8.03 mg, 0.0165 mmol) was placed into an oven-
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[PdACHTUNGTRENNUNG
(dppf)L1] building block. Self-assemblies with 1a and 1c tectons were
carried out in a similar way to prepare Pd/L1 =1:1 and 2:1 complexes for
NMR spectroscopic studies.
Preparation of the monomer 7a: [PdACHTUNGTREN(NNGU dppp)ACHUTNRTEG(GNNUN H2O)2]ACHTNGUTRENN(UGN OTf)2 (12.4 mg,
0.0144 mmol) was dried in an NMR tube as described above. CD2Cl2
(0.6 mL) was distilled from a vial containing a suspension of CD2Cl2/
CaH2 into an NMR tube charged with L2 (7.1 mg, 0.0144 mmol). The
ligand was added to the metal complex under nitrogen to quantitatively
1
yield 7a (as shown by H and 31P NMR spectroscopic analysis).
Acknowledgements
This work was supported by the Hungarian Research Fund
(OTKAT047321). H.J. thanks the Chemical Research Center, Hungarian
Academy of Sciences for supporting his stay at CRC. The authors are in-
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Dr. K. Vꢃkey (Chemical Research Center, Budapest) for helpful discus-
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Chem. Eur. J. 2009, 15, 10620 – 10633

Steric Effects in the Self-Assembly of Palladium Diphosphane Complexes
FULL PAPER
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Received: April 28, 2009
Published online: September 11, 2009
Chem. Eur. J. 2009, 15, 10620 – 10633
ꢄ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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10633