Coordination-driven face-directed self-assembly of trigonal prisms. Face-based conformational chirality

Article abstract of DOI:10.1021/ja710715e

The coordination-driven self-assembly of four different trigonal prisms from 3 equiv of one of four different tetrapyridyl star connectors and 6 equiv of a platinum linker dication in nitromethane is presented. This face-directed approach affords high yields without template assistance. The prisms have been characterized by multinuclear and DOSY NMR and dual ESI-FT-ICR mass spectrometry. The use of a conformationally chiral star connector leads to a conformationally chiral prism when connector arm ends attached to a vertex have a strongly correlated twist sense and chirality is communicated across polyhedral faces, edges, and vertices. Molecular mechanics results suggest that in the smallest prism 3d collective effects dominate and the all-P and all-M conformers are strongly favored. NMR data prove that the two edges of the pyridine rings in the triflate salts of 3a-3d are distinct. An Eyring plot of rates obtained from line-shape analysis and 1-D EXCHSY NMR yields an activation enthalpy ΔH^? of ～12 kcal/mol and activation entropy ΔS^? of ～-15 cal/mol·K for the edge interconversion process, compatible with pyridine rotation around the Pt-N bond. For 3c, this behavior is observed only up to ～318 K. At higher temperatures, the Eyring plot is again linear but follows a very different straight line, with a ΔH^? of ～35 kcal/mol and ΔS^? of ～60 cal/mol·K. This highly unusual result is further investigated and discussed in the following companion paper.

Full text of DOI:10.1021/ja710715e

Published on Web 05/21/2008
Coordination-Driven Face-Directed Self-Assembly of Trigonal
Prisms. Face-Based Conformational Chirality
Douglas C. Caskey,§ Takuya Yamamoto,† Chris Addicott,† Richard K. Shoemaker,§
Jaroslav Vacek,§,| Adam M. Hawkridge,‡ David C. Muddiman,‡ Gregg S. Kottas,§
Josef Michl,*,§,| and Peter J. Stang*,†
Department of Chemistry, UniVersity of Utah, 315 South 1400 East, Room 2020,
Salt Lake City, Utah 84112, W. M. Keck FT-ICR Mass Spectrometry Laboratory, Department of
Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695, Department of
Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309-0215, and
Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic,
166 10 Prague 6, Czech Republic
Received November 29, 2007; E-mail: stang@chem.utah.edu; michl@eefus.colorado.edu
Abstract: The coordination-driven self-assembly of four different trigonal prisms from 3 equiv of one of
four different tetrapyridyl star connectors and 6 equiv of a platinum linker dication in nitromethane is
presented. This face-directed approach affords high yields without template assistance. The prisms have
been characterized by multinuclear and DOSY NMR and dual ESI-FT-ICR mass spectrometry. The use of
a conformationally chiral star connector leads to a conformationally chiral prism when connector arm ends
attached to a vertex have a strongly correlated twist sense and chirality is communicated across polyhedral
faces, edges, and vertices. Molecular mechanics results suggest that in the smallest prism 3d collective
effects dominate and the all-P and all-M conformers are strongly favored. NMR data prove that the two
edges of the pyridine rings in the triflate salts of 3a-3d are distinct. An Eyring plot of rates obtained from
line-shape analysis and 1-D EXCHSY NMR yields an activation enthalpy ∆H‡ of ∼12 kcal/mol and activation
entropy ∆S‡ of ∼-15 cal/mol ·K for the edge interconversion process, compatible with pyridine rotation
around the Pt-N bond. For 3c, this behavior is observed only up to ∼318 K. At higher temperatures, the
Eyring plot is again linear but follows a very different straight line, with a ∆H‡ of ∼35 kcal/mol and ∆S‡ of
∼60 cal/mol ·K. This highly unusual result is further investigated and discussed in the following companion
paper.
Introduction
building blocks that define some or all of their edges. Lower
symmetry assemblies, for example, trigonal21–24 and tetrago-
Coordination-driven self-assembly via the directional bonding
approach is a useful strategy for the preparation of 2- and 3-D
supramolecules.1–14 Some of the 3-D examples include cubes,15,16
adamantanoids,17 double squares,18 cuboctahedra,19 and dodeca-
hedra.20 These are highly symmetrical species composed of
nal22,23 prisms, are less common and are more easily approached
using face-directed techniques in which component linkers span
the faces of the desired aggregate. Varying the component
dimensions then alters the size and shape of the final assembly.
This methodology allows the creation of 3-D supramolecular
† University of Utah.
‡ North Carolina State University.
(15) Johannessen, S. C.; Brisbois, R. G.; Fischer, J. P.; Grieco, P. A.;
Counterman, A. E.; Clemmer, D. E. J. Am. Chem. Soc. 2001, 123,
3818.
§ University of Colorado.
| Academy of Sciences of the Czech Republic.
(1) Seidel, S. R.; Stang, P. J. Acc. Chem. Res. 2002, 35, 972.
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(6) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100, 853.
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1185.
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Commun. 1998, 1681.
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2, 1255.
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chi, K. Angew. Chem., Int. Ed. 1998, 37, 2082.
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1999, 398, 796.
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S.; Yamaguchi, K. J. Am. Chem. Soc. 2001, 123, 980.
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Commun. 2005, 70, 1970.
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7620 J. AM. CHEM. SOC. 2008, 130, 7620–7628
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2008 American Chemical Society

Face-Based Conformational Chirality
A R T I C L E S
Scheme 1. Self-Assembly of Tetrapyridyl Ligands 1 with
cis-(Me3P)2(OTf)2 2 Leads to Trigonal Prisms
Figure 1. Reaction of tetratopic ligand 1 with linker 2 in a 1:2 ratio may
lead to coordination prisms 3–6.
hosts of relatively low symmetry which ultimately should show
enhanced guest selectivity.
Planar tetratopic ligands 1 have been of interest to us in other
contexts,25 and several are readily available. Their reaction with
an approximate26 90° linker 2 in a 1:2 ratio may lead to trigonal
(3), tetragonal (4), pentagonal (5), hexagonal (6), and possibly
even larger coordination prisms, in which the tetratopic ligands
span the side panels and the linkers occupy the vertices (Figure
1). The use of face-defining star-shaped connectors in general,
and of tetratopic tetrapyridyl ligands in particular, offers some
interesting opportunities for the production of chiral prisms.
Although prisms whose edges are mere lines are achiral, prisms
whose edges are constructed from molecules and whose vertices
are metal atoms are potentially chiral, and this has already been
exploited by two groups of authors. Chiral ligands at square
planar vertex metal atoms,27 later also using edges of C2h
symmetry,28,29 were utilized by Stang and co-workers, first with
molecular squares, and then with polyhedra.30 Optically active
three-dimensional cages with vertex metal atoms carrying achiral
ligands in a chiral octahedral arrangement have been explored
by Raymond and co-workers.7,31,32
an isolated star connector such as 1, the P-M interconversion
by rotation around single bonds is ordinarily very facile, and
the enantiomers cannot be separated and stored at room
temperature. In a polyhedron, which consists of a collection of
faces, two limiting cases are conceivable. In the “independent
face limit”, the inversion of the chiral sense of a star connector
in any face occurs independently of the chirality adopted by
any other face. In the “coupled face limit”, star connectors
located in different faces affect each other strongly through
linkers located at shared edges or vertices. Collective effects
result, and a P-M interconversion at one face occurs only with
simultaneous interconversion at some or all other faces.
We find that the self-assembly of four trigonal prismatic cages
3 from four distinct star connectors 1 and the 90° platinum linker
2 proceeds well without template assistance (Scheme 1). We
argue below that the tetrapyridyl ligands 1 vary in the expected
degree of communication of chirality from face to face.
Molecular modeling results strongly suggest that the smallest
cage 3d is in the coupled face limit and will exhibit collective
We now introduce the concept of face-based cage chirality,
which has a different origin, as it is defined by the cage faces
and not vertices or edges. Star connectors used to define cage
faces are frequently propeller-shaped, with all arms attached to
the central moiety twisted in the same sense. Then, they impart
conformational P or M helical chirality33 to each cage face. In
(25) Michl, J.; Magnera, T. F. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4788.
(26) Yamamoto, T.; Arif, A. M.; Stang, P. J. J. Am. Chem. Soc. 2003,
125, 12309.
(27) Stang, P. J.; Olenyuk, B.; Arif, A. M. Organometallics 1995, 14, 5281.
(28) Stang, P. J.; Olenyuk, B. Angew. Chem., Int. Ed. Engl. 1996, 35, 732.
(29) Olenyuk, B.; Whiteford, J. A.; Stang, P. J. J. Am. Chem. Soc. 1996,
118, 8221.
(30) Stang, P. J.; Olenyuk, B.; Muddiman, D. C.; Smith, R. D. Organo-
metallics 1997, 16, 3094.
(31) Seeber, G.; Tiedmann, B. E. F.; Raymond, K. N. Top. Curr. Chem.
2006, 265, 147.
(32) Davis, A. V.; Fiedler, D.; Ziegler, M.; Terpin, A.; Raymond, K. N.
J. Am. Chem. Soc. 2007, 129, 15354.
(33) Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds;
Wiley: New York, 1994; p 1120.
9
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A R T I C L E S
Caskey et al.
behavior and, thus, sets the stage for a future experimental
testing of the notion of face-based cage chirality.
Each prism 3 is composed of three molecules of the star
connector 1 and six units of the linker 2. The structures have
been characterized by multinuclear and DOSY NMR and dual
electrospray ionization Fourier transform ion cyclotron reso-
nance (dualESI-FT-ICR) mass spectrometry.34 In the case of
3d, evidence for the formation of a larger dimeric aggregate
under ESI conditions is also presented.
The rotational degree of freedom of the pyridine rings in the
arms of the cross connectors is essential for the understanding
of the conformational mobility and face-based chirality of cages
such as 3. The activation parameters are unexceptional in all
cases except 3c, whose automerization rate exhibits a bilinear
Eyring plot with strikingly high ∆H‡ and ∆S‡ values in the high-
temperature regime. This most unusual result is addressed in
the following companion paper.35
Results
Stirring 1 equiv of each of 1a-1d with 2 equiv of 2 in
CD3NO2 for 18 h at room temperature results in the quantitative
(by NMR) formation of molecular cages 3a-3d, respectively.
Their structure was examined by (i) NMR spectroscopy, (ii)
mass spectrometry, and (iii) diffusion constant measurement.
In other solvents (CH2Cl2, CHCl3), the products are stable, but
they do not form as readily and cleanly.
Figure 2. Calculated structures of (A) 3c with CpCo residues inside and
(B) 3c with CpCo residues outside the cage.
NMR Spectroscopy. In all cases the 31P and 1H NMR spectra
are consistent with the formation of a single discrete product.
The 31P NMR spectra displayed a sharp peak near -28 ppm
with concomitant 195Pt satellites. This upfield shift of ap-
proximately 8 ppm relative to 2 is consistent with back-donation
from the platinum atoms upon coordination. The 1H NMR
spectra showed two sets of signals for the 12 equivalent pyridyl
rings. Although these protons would have magnetically different
environments even if the pyridine rings were located in the
planes of the prism faces, the preferences of the platinum linkers
discussed below make it very likely that these rings do not lie
in the prism faces but are partially or orthogonally twisted. The
two magnetic environments then correspond to protons located
on the inside and the outside of the prism. There is ample
precedent for this behavior and for restricted ring rotational
equilibration.21,36,37
The pyridyl proton resonances of 3 are shifted slightly to
lower field as expected due to the loss of electron density that
occurs upon coordination. The only exception is the resonance
of one HR nucleus of 3d at 8.20 ppm, which shifts to higher
field relative to the starting ligand 1d (8.51 ppm). Platinum back-
donation causes the resonance of the phosphine methyl groups
to move approximately 0.1 ppm to higher field. Interestingly,
the resonances of the vinylic protons of 3b experience quite
large low field shifts (0.2–0.4 ppm).
Figure 3. PPP and MMM enantiomers of 3d.
effects ascribable to the proximity of other rings. This strongly
suggests that the Cp rings are located quite far apart, hence
outside the prisms.
Molecular Mechanics. Molecular mechanics modeling
(UFF,38,39 using the computer program TINK40) was used for
two purposes. The first was to aid with the consideration of the
issue discussed just above, the location of the CpCo moieties
in 3c and 3d. We found that in the larger prism 3c there is
enough internal space for both locations to be possible (Figure
2). The CpCo-inside isomer is calculated to be the less stable
of the two by ∼1 kcal/mol, but this small difference could be
changed by differential solvation effects and in any event is
within the error of the method. While we cannot rule out the
CpCo-inside isomer with absolute certainty in the absence of
X-ray data that we have been unable to obtain, we consider
this structure rather unlikely for the reasons discussed above.
The only way these residues can be located in the smaller prism
3d is on the outside (Figure 3).
The three CpCo moieties in both 3c and 3d are all equivalent
and appear to be either all located inside or all outside of the
square faces of the self-assembled cage. The proton chemical
shifts are entirely normal and show no indications of ring current
(34) Nepomuceno, A. I.; Muddiman, D. C.; Bergen, H. R.; Craighead, J. R.;
Burke, M. J.; Caskey, P. E.; Allan, J. A. Anal. Chem. 2003, 75, 3411.
(35) Vacek, J.; Caskey, D. C.; Horinek, D.; Shoemaker, R. K.; Stang, P. J.;
Michl, J. J. Am. Chem. Soc. 2008, 130, 7629.
(36) Chi, K.-W.; Addicott, C.; Kryschenko, Y. K.; Stang, P. J. J. Org. Chem.
2004, 69, 964.
(38) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A., III;
Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024.
(37) Kuehl, C. J.; Huang, S. D.; Stang, P. J. J. Am. Chem. Soc. 2001, 123,
9634.
(39) Rappe, A. K.; Goddard, W. A., III. J. Phys. Chem. 1991, 95, 3358.
(40) Vacek, J.; Michl, J. New J. Chem. 1997, 21, 1259.
9
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Face-Based Conformational Chirality
A R T I C L E S
The second issue to which molecular mechanics was applied
is the face-based chirality of the prism 3d. Geometry optimiza-
tion yielded a single pair of enantiomeric conformers, PPP and
MMM, where the labels P and M refer to the three tetrapy-
ridylcyclobutadiene propellers located in the individual prism
faces (Figure 3). The four pyridine rings in a face are not equally
twisted out of the plane of the central cyclobutadiene ring. Two
located at opposed vertices of the ring are twisted by 45° (Type
I), and the other two are twisted only by 15° (Type II), such
that the average twist is still similar to that in the free connector
1d. Each platinum atom is in a nearly perfectly square planar
environment and is coordinated to one pyridine ring of each
type. The Type I pyridine makes a dihedral angle of 82° with
the NPtN plane, whereas the Type II pyridine makes an angle
of 65° and both thus deviate from the optimal angle of 90°. If
the cyclopentadiene rings were circular, the prism 3d would
have D3 symmetry. We made no attempt to calculate the reaction
barrier for the interconversion of Type I and Type II rings, but
such averaging must be fast on the NMR scale to make all
pyridines appear equivalent.
All attempts to find PPM and MMP conformers failed, and
we strongly suspect that structures in which the absolute chirality
of one of the three faces differs from the other two are not local
minima on the potential energy surface. All local minima other
than PPP/MMM that were found were higher by at least 20
kcal/mol and contained one or two faces in which the four
pyridine rings were not twisted in the same sense and no longer
resembled a propeller.
Mass Spectrometry. Evidence for the trimeric nature of the
products was obtained with dualESI-FT-ICR mass spectrometry.
This gentle ionization technique allowed the intact prisms (minus
several triflate counterions) to be observed. The high resolving
power afforded by FT-ICR mass spectrometry allowed un-
equivocal charge state determination. Supramolecules of this
size (approximately 5000 Da) display larger isotopic shifts
(defined as the difference between average and monoisotopic
masses) relative to proteins of similar molecular mass, due to
the presence of numerous platinum atoms. Thus, in this situation
the most abundant isotope is considered for the determination
of molecular mass, rather than the monoisotopic mass. This
could have an error of (1 amu or 200 ppm for molecules with
molecular masses on the order of 5000 amu.
The internally calibrated dualESI-FT-ICR mass spectrum of
3a is shown in Figure 4a. The inset shows an expansion of an
isotopically resolved species (top) carrying a 4+ charge centered
at m/z ) 1180.8486, assigned to [3a-4OTf-]4+. It is in excellent
agreement with the theoretical isotope distribution (bottom)
predicted for this species. A mass measurement accuracy after
internal calibration (MMAint) of -0.3 ppm for the most abundant
isotope is also consistent with the proposed structure. Extensive
fragmentation at lower m/z values is evident, suggesting that
3a may still dissociate even under the softer ionization condi-
tions employed.
In contrast, the assembly 3b displayed a simpler mass
spectrum (Figure 4b). Due to the poor signal-to-noise ratio for
this structure, the hexapole accumulation was significantly
increased, precluding internal calibration. Two charge states
assignable to 3b were detected, namely [3b-3OTf-]3+ (m/z )
1632.2074) and the more abundant [3b-4OTf-]4+ (m/z )
1186.8201). Both correlate well with their theoretical distribu-
tions and have good MMA values for the most abundant isotope
despite the absence of internal calibrants.
Figure 4. DualESI-FT-ICR mass spectra of cages 3 with experimental and
theoretical isotopic distributions (inset). Internal calibrant abbreviations are
designated AngI (Angiotensin I), G (Glucagon), and Ins (Insulin). MMAint
is the mass measurement accuracy using internal calibration, MMAext is
the mass measurement accuracy using external calibration, * is the most
abundant isotope, and • is electronic noise.
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A R T I C L E S
Caskey et al.
Table 1. Radii of Prisms 3 and 6 As Deduced from Diffusion
Coefficients of Triflate Salts and As Predicted by Molecular
Mechanics
predicted R from UFFe
a,b
NO
2
compd
DCHD
Da,c
R from Dd
trimer 3
hexamer 6
2
3a
3b
3c
3d
20.9 ( 0.8
19.7 ( 0.8
19.5 ( 0.8
21.4 ( 0.8
4.0 ( 0.6
2.6 ( 0.6
1.9 ( 0.6
3.8 ( 0.6
8.8
13.5
18.3
9.2
10.8
10.6
14.8
9.2
20.4
20.2
26.1
15.1
a Diffusion coefficient in units of 10-10 m2 s-1, obtained from DOSY
NMR measurements in CD3NO2 at 298 ( 0.25 K. b The diffusion
coefficient of the solvent molecules in the solution measured. The value
in pure solvent is (22.2 ( 0.8) × 10-10 m2 s-1, and the slight variation
is attributed to differences in solution viscosity. c The diffusion
coefficient of the solutes 3. d Radius in Å, calculated from the Stokes–
Einstein equation, assuming viscosity of the pure solvent. These values
are only approximate. The error is believed to be dominated by the
unknown uncertainties associated with the use of this equation (see
text). e Average distance in Å from the prism center to each platinum
nucleus of the UFF38–40 optimized geometry.
Figure 5. Self-assembled cubes 7a and 7b.
The mass spectrum of the largest cage 3c is displayed in
Figure 4c. A small peak with a 4+ charge, centered near m/z
1482.4063, was assigned to [3c-4OTf-]4+ (MMAext -3 ppm).
We were unable to detect any of the larger prisms 4-6 in the
mass spectra of 3a–3c under a variety of electrospray and
instrumental conditions.
electrospray process due to the lack of competition with the
solvent. Such dimerization, observed previously in 2-D sys-
tems,41 is a consequence of mild ionization conditions.
Diffusion Constant Determination. In an attempt to obtain
additional confirmation of their trimeric stoichiometry, en-
sembles 3 were subjected to DOSY NMR measurements.42,43
Numerous chemical and biological systems23,24,44–48 have been
studied with this method, including our nanoscopic dodecahe-
dra.20 The present results are summarized in Table 1. They were
obtained with the DOSY Bipolar Pulse Pair Stimulated Echo
(Dbppste) pulse sequence with carefully calibrated gradients and
temperature control (298 K). Viscosity variation among the
various solutions measured had a maximum effect of (5% on
the observed D values, as judged by the values obtained for
the residual CHD2NO2 solvent proton resonance. The accuracy
of the NMR diffusion experiments and the sources of error are
described in more detail in the Experimental Section.
Finally, the mass spectrum of 3d is shown in Figure 4d.
Similar to 3b, minimal fragmentation of the intact structure was
observed. An isotopically resolved cluster at m/z ) 1626.0660
displaying a 3+ charge is assigned to [3d-3OTf-]3+. The
unavoidable ion abundance mismatch between the experimental
and theoretical distributions is due to ion counting statistics and/
or artifacts involving Coulombic interactions in the mass
spectrometer. Consequently, the experimentally second most
abundant isotope (m/z ) 1625.7377) has the best MMAint (-0.7
ppm) with the theoretically most abundant isotope. Interestingly,
another peak centered at m/z ) 1981.0706 with a 5+ charge
was observed. This is uncharacteristic of the discrete prism 3d.
The isotopic pattern is consistent with an ionic formula derived
from six molecules of 1d and twelve molecules of 2 minus five
triflate ions (the internally calibrated MMA of the most abundant
isotope was -3 ppm).
There are at least three structures that fit this stoichiometry.
The first is a cube-like structure 7a, feasible due to the square
panel nature of 1d. Indeed, Grieco and collaborators15 have
reported that the reaction of 1d with Pd(en)(NO3)2 in a 1:2 ratio
affords the cubic supramolecule 7b which they characterized
with NMR and nested ion mobility/time-of-flight mass spec-
trometry (Figure 5).
The second possibility is a hexagonal prism of type 6. This
is more compatible with our NMR signals than 7b, which shows
only one set of signals for the pyridyl rings, apparently due to
a low rotation barrier around the Pd-N bond. However, as we
shall describe below, prism 6 is hard to reconcile with the DOSY
NMR results. Moreover, based on entropy considerations, there
is no apparent reason why a prism of this size would be formed
instead of or in conjunction with the smaller tetragonal (4) and
pentagonal (5) prisms, for which no mass spectral evidence was
found. Finally, the formation of 6 or 7 from 3d would require
complex rearrangements not likely to occur under electrospray
conditions.
Each predicted radius R was estimated from a UFF38–40
optimized geometry of the prisms 3 and 6. Each experimental
radius R was calculated from the diffusion coefficient D using
the Stokes–Einstein equation approximation and, therefore,
contains possible contributions from firmly associated counter-
ions. The values of R are only approximate, because the
Stokes–Einstein equation was derived for a neutral spherical
macroscopic object, whereas our prisms are charged, only very
approximately spherical, and of molecular dimensions. In view
of this intrinsic inaccuracy in the conversion of D to R, the
inaccuracy of the value of D obtained in the DOSY experiments
and the uncertainties associated with the imperfect knowledge
of temperature and viscosity (see Experimental Section) are
considered negligible.
In general the prisms are quite symmetrical and possess large
cavities. The distance from each prismatic center to the platinum
nuclei is approximately equal to the radius of the smallest sphere
required to enclose each supramolecule. For 3a this is estimated
(41) Schalley, C. A.; Müller, T.; Linnartz, P.; Witt, M.; Schäfer, M.; Lützen,
A. Chem. Eur. J. 2002, 8, 3538.
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A third and most likely explanation for the unexpected peak
is the dimeric aggregate [3d2-5OTf-]5+ held together by weak
electrostatic forces between the prisms and counterions. These
forces become more important as the solution evaporates in the
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Face-Based Conformational Chirality
A R T I C L E S
to be 10.8 Å, while for 6a it is 20.4 Å. The experimental radius
of 8.8 Å indicates that 3a is the most likely structure. In fact,
the predicted radii of all trimeric prisms 3 are in much better
agreement with the experimental values than those of hexamers
6. This reinforces the conclusions based on our mass spectral
data.
Intracage Dynamics by Variable Temperature NMR. In
addition to providing suitable substrates for future studies of
the concept of face-based chirality, facile access to prisms 3
also offered an opportunity to examine the internal mobility of
species self-assembled through metal coordination chemistry.
Only a few such studies have been reported, despite continued
interest in the area.49–54
Since the 1H NMR AB multiplets due to the R and ꢀ proton
pairs of the pyridyl moiety are inequivalent for prisms 3a-3d,
we were able to obtain the enthalpy (∆H‡) and entropy (∆S‡)
of activation for pyridyl edge interchange in each assembly in
CD3NO2 with a DNMR study. NMR peak assignments of the
exchanging protons were verified by 2D ROESY experiments,
relying on the opposite phase of the NOE and the chemical
exchange correlations. Since the exchanging proton pairs
appeared as non-first-order AB multiplets, computer simulation
was used to determine their chemical shifts and J coupling
constant before DNMR line shape simulation. The rates were
deduced from spectra obtained at a series of temperatures
spanning the range from the low-temperature to the high-
temperature limit. They were cross-checked at selected tem-
peratures by repeating the measurements at three different
magnetic field strengths. These yielded different line shapes but
identical rate constants.
The rates were independently verified and extended to lower
temperatures by transient exchange (1-D EXCHSY) measure-
ments, using an array of mixing times for chemical exchange.
In these experiments, the exchange rate constant is measured
directly by fitting an appropriate exponential function55 to the
rise of the ratio of the resonance peak due to exchange to a
selectively inverted resonance peak. They permit the measure-
ment of exchange rates that are well below the limit accessible
by line shape simulation.
Figure 6. Eyring plots for pyridine edge interconversion rate constants in
CD3NO2 for 3a/12 TfO- ([), 3b/12 TfO- (9), 3c/12 TfO- (0), and 3d/
12 TfO- (b).
Table 2. Eyring Activation Parameters for Interconversion of
Pyridine Edges in 3a-3d Triflates in CD3NO2
∆H‡, kcal/mol
∆S‡, cal/mol·K
R2
3a 12 TfO-
3b 12 TfO-
3c 12 TfO-
12.4 ((2.1)
11.6 ((2.0)
11.1 ((1.9)
35.1 ((2.5)
13.2 ((2.2)
-16.1 ((5.5)
-14.4 ((4.9)
-17.5 ((6.0)
58.1 ((8.1)
0.995
0.994
0.993
1.000
0.973
<∼318 K
>∼318 K
3d 12 TfO-
-13.2 ((4.5)
in CD3NO2 and pyridyl proton exchange rates were remeasured.
They were unaffected and lay within 5% of previously measured
rates.
Discussion
Cage Structure. The combined evidence provided by NMR,
MS, and diffusion coefficient values, in analogy to prior work,
leaves no doubt in our minds that the self-assembled products
have the trigonal prismatic structure 3. The efficiency with which
they form in the absence of a template is remarkable.
The results can be compared with those of a related study
by Fujita and co-workers,22 in which the tetrapyridyl unit,
3,3′,5,5′-tetrakis(3-pyridyl)biphenyl, and their well developed
Pd(en)(NO3)2 linker were self-assembled into coordination
boxes. A simple combination of these two components in a 1:2
ratio yielded a tetragonal prism as the major product. When
biphenyl was included in the reaction, a trigonal prism was
formed quantitatively instead and two molecules of biphenyl
were encapsulated in the cavity. These assemblies, together with
uncharacterized minor products, were all in dynamic equilib-
rium.22 In contrast, our trigonal prisms are formed without
template assistance and are not in detectable equilibrium with
other structures. The stronger coordination bonding of the Pt-N
versus the Pd-N systems is a likely explanation for the
differences.15,22 The work presented herein illustrates the subtle
deviations that can occur in abiological self-assembly between
seemingly similar systems and shows why it is imperative to
examine a variety of reaction substrates and conditions.
Pyridine Ligand Rotation. Moving from considerations of
the favored prism structure to a discussion of its internal
mobility, we note that rotation of the pyridine ligands leads to
the exchange of their distinct edges and is relevant for the
racemization process discussed below. Because each prism
contains 12 pyridine rings whose rotations need not be
independent, a full kinetic analysis requires far more experi-
The Eyring plots are shown in Figure 6, and the activation
parameters are collected in Table 2. Linear fits (R2 g 0.97) were
obtained for the triflate salts of all prisms in CD3NO2, except
3c (R2 ) 0.91) for which a bilinear fit with a crossing point at
∼318 K is clearly superior (R2 g 0.99 in both regions).
In order to investigate the possibility that the nonlinear Eyring
behavior arises from partial decomposition into some platinum
colloid which then promotes a heterogeneous process, a drop
of mercury, known56 to suppress such processes through
physisorption or amalgamation, was added to a solution of 3c
(49) Park, S.-J.; Shin, D. M.; Sakamoto, S.; Yamaguchi, K.; Chung, Y. K.;
Lah, M. S.; Hong, J.-I. Chem. Eur. J. 2005, 11, 235.
(50) Megyes, T.; Jude, H.; Grósz, T.; Bakó, I.; Radnai, T.; Tárkányi, G.;
Pálinkás, G.; Stang, P. J. J. Am. Chem. Soc. 2005, 127, 10731.
(51) Tiede, D. M.; Zhang, R.; Chen, L. X.; Yu, L.; Lindsey, J. S. J. Am.
Chem. Soc. 2004, 126, 14054.
(52) Fuss, M.; Siehl, H.-U.; Olenyuk, B.; Stang, P. J. Organometallics 1999,
18, 758.
(53) Fan, J.; Whiteford, J. A.; Olenyuk, B.; Levin, M. D.; Stang, P. J.;
Fleischer, E. B. J. Am. Chem. Soc. 1999, 121, 2741.
(54) Tárkányi, G.; Jude, H.; Pálinkás, G.; Stang, P. J. Org. Lett. 2005, 7,
4971.
(55) Clare, B.; Sarker, N.; Shoemaker, R.; Hagadorn, J. R. Inorg. Chem.
2004, 43, 1159.
(56) Widegrin, J. A.; Finke, R. G. J. Mol. Catal. A: Chem. 2003, 198,
317.
9
J. AM. CHEM. SOC. VOL. 130, NO. 24, 2008 7625

A R T I C L E S
Caskey et al.
mental information than is available presently and represents a
formidable task. At present, we are forced to assume that each
pyridine rotates independently and shall thus deal only with their
average behavior. As will be argued below, this assumption is
undoubtedly poor in the case of 3d, and we shall deemphasize
this prism in what follows.
whatever degree conjugation through the triple bond favors
coplanarity, the terminal pyridine rings of 1c will be twisted
similarly. However, it will take little energy to twist a pyridine
ring out of coplanarity with its phenyl partner, and it should
still be quite easy to adapt the orientation of the pyridine
moieties to the requirements of the linker 2.
The activation parameters for the pyridine edge exchange
reaction in the triflate salts of 3a, 3b, and 3d, whose Eyring
plots can be fitted with a single straight line, are all identical
within our experimental accuracy (Table 2) and lie within the
error bar of those found for the low-temperature process in the
triflate of 3c, 11.1 kcal/mol and -17.5 cal/mol ·K, respectively.
To our knowledge, there is only one previous report of ∆H‡
and ∆S‡ values for a platinum-bipyridyl based assembly, where
Tárkányi et al.54 found ∆H‡ values of 12.5 and 14.1 kcal/mol
and ∆S‡ values of -13.9 cal/mol·K and -17.2 cal/mol·K by
selective inversion–recovery (SIR) NMR experiments for hexaflu-
orophosphate salts of a self-assembled rectangle and triangle,
respectively. These values are essentially the same as those
found in this study. They have been assumed to correspond to
a simple reaction path in which the pyridine edge exchange by
rotation around an intact Pt-N bond, and this interpretation
appears perfectly natural to us. This simplicity is however only
apparent, and we have found that the rates of pyridine rotation
are significantly affected by the counterions present, implying
a more complex process. This will be considered in the
following companion paper,35 along with the unusual bilinear
nature of the Eyring plot for 3c, which is of particular interest.
There is a hint of bilinearity also in the Eyring plot for 3d, but
it is much less pronounced.
Only the connector 1d is expected to act differently. Here
again, judging by known X-ray structures,57 the rings attached
to the cyclobutadiene square are twisted by about 30°–40°, all
in the same sense, P or M, and according to NMR evidence
conformational racemization must be very facile. However, since
their four nitrogen atoms will be directly attached to the Pt atoms
of the four linkers 2 at the vertices, the sense of twist that defines
their chirality can be expected to be transferred through the four
vertices to adjacent faces quite forcefully.
Chirality in the Prisms 3. If the edge or vertex linkers that
tie the faces together into a cage discern their chirality, there
will be an energy difference between a PP (or MM) and a PM
face combination at a shared edge, and also among the various
face combinations at a shared vertex. A collective behavior will
result, and definite patterns of P and M faces on the cage will
be energetically favored to different degrees. Cage racemization,
which calls for a P-M interconversion at all the faces, could
then require a significantly higher activation energy than the
facile P-M interconversion of an isolated star connector. If the
P-M interconversion is attempted at a single face at a time,
energy penalties will have to be paid at each mismatched edge
or vertex, and if it is attempted at two or more faces at the
same time to avoid these mismatches, the activation energies
of the interconversions will add up.
An additional aspect of our results appears worthy of
discussion and future pursuit, namely the chirality of the prisms,
which is of a type different from those recognized up to now.31
We consider first the chirality of the isolated star connectors
1a-1d and then the chirality of the assembled prisms.
Chirality of the Star Connectors 1. The chirality-inducing
twist of the four pyridyl substituents out of the plane of the
central ring of these star connectors is ultimately due to steric
crowding. This is absent in 1a, in which one can expect the
four pyridine rings to be coplanar with the benzene ring at
the achiral equilibrium geometry, with the barrier to twisting
the pyridine rings being low, and dictated only by the weak
pyridine-benzene conjugation through the triple bond. In cages
built with 1a, the pyridine rings will be nearly free to adopt the
twist angles favored by the linker 2, and there should be little
if any communication of the sense of twist from any of the
four arms to the others through the central benzene ring. A
similar situation can be expected with 1b, although the four
double bonds will most likely not be entirely coplanar and there
may be a small degree of correlation between the twists of the
four pyridine rings.
The linker 2 has no chirality on its own. Even after it is
attached to two pyridine ligands, it keeps a plane of symmetry,
as the pyridine rings prefer to orient perpendicular to the
P-Pt-P plane.26,52,59 However, according to molecular models,
if one of these rings is twisted about its N-Pt bond, steric
constraints cause the other ring to twist in the same sense. In a
related complex 8, it costs about 14 kcal/mol to rotate one of
the rings relative to its N-Pt axis, keeping the other intact.52
In our trigonal prismatic cages 3 we would expect the pyridine
rings to be oriented at angles that reflect a compromise between
the preferences of the star connectors 1 and the linkers 2.
In contrast, based on the crystal structures of tetraphenylcy-
clobutadienecyclopentadienylcobalt and similar sandwich com-
plexes,57 there is no doubt that the four phenyl rings attached
to the central ring in 1c will be twisted by close to 30°–40°, all
in the same sense of twist, making the favored conformations
of this star connector distinctly chiral. However, judging by
variable-temperature NMR spectra of this class of compounds,58
the interconversion of enantiomers is extremely facile. To
In 3a-3c this compromise should be easy to reach without
serious energetic consequences. The pyridine rings should be
close to orthogonal to the prism face since the preferences of
the connectors 1a-1c are not strong, and they should not
introduce any particular ground-state strain. The barriers for
rotation about the Pt-N bonds should be perhaps even a little
lower than the 14 kcal/mol found for 8, due to the additional
π-electron conjugation gained when the pyridine is turned
(57) Harrison, R. M.; Brotin, T.; Noll, B.; Michl, J. Organometallics 1997,
16, 3401.
(58) Zheng, X.; Mulcahy, M. E.; Horinek, D.; Galeotti, F.; Magnera, T. F.;
Michl, J. J. Am. Chem. Soc. 2004, 126, 4540.
(59) Chi, K.-W.; Addicott, C.; Arif, A. M.; Das, N.; Stang, P. J. J. Org.
Chem. 2003, 68, 9798.
9
7626 J. AM. CHEM. SOC. VOL. 130, NO. 24, 2008

Face-Based Conformational Chirality
A R T I C L E S
parallel to the face of the prism. In these three prisms the
rotations at the individual vertices should not be strongly
correlated and should occur nearly independently (independent
face limit).
The availability of prism 3c, whose pyridine edge interchange
displays a highly unusual kinetic behavior, offers unique
opportunities for additional insight into the properties of self-
assembled polyhedra, characterized and discussed in the fol-
lowing companion paper.35 We propose that they pertain to the
first steps of cage unraveling, and thus the last steps in the
mechanism of its self-assembly, and to the structure of
the solvent around and/or inside the cage.
In 3d, in the earlier reported cubic cage 7, and in other
polyhedra built from 1d, molecular modeling suggests that the
situation is different. The preference of the star connector 1d
for a 30°–40° twist out of the prism face is quite distinct, and
in prisms whose faces are represented by this connector a P or
M chirality on one face should definitely favor a like chirality
at all adjacent faces. This is indeed the result obtained in our
molecular mechanics calculations for 3d (Figure 3). It is
interesting to see how the molecule of 3d copes with the
conflicting stereochemical preferences of the square connectors
(propeller shape) and the platinum atoms (pyridine plane
perpendicular to the NPtN plane). The connector compromises
by twisting two of the pyridines more (Type I) and two very
little (Type II), and the platinum atom compromises by not
insisting on having both pyridines perpendicular to the NPtN
plane, twisting one (Type I) very little and the other one (Type
II) significantly. The mechanism of chirality transfer from face
to face is clearly apparent.
We expect the favored stereochemistry of all cages built from
1d to be all-P or all-M. Their ground states should be
destabilized since preferences of the star connectors 1 and the
metal linkers 2 clash and a serious compromise is necessary.
However, since pyridine rotation is now expected to require
destruction of the propeller-like arrangement of the four pyridine
rings at the center of one or more of the square connectors, or
collective motion of the pyridine ligands, the transition state
for racemization by rotation should be destabilized as well, and
in the end the activation parameters for the interchange of the
inner and outer pyridine protons in 3d by pyridine rotation could
be similar to those observed for the other prisms. This indeed
appears to be the case in Table 2.
Experimental Section
General. Ligands 1b,61 1d,57 and the linker cis-(Me3P)2Pt(OTf)2
2
62 were prepared according to known procedures. IR spectra were
recorded on a Mattson GL-3020 FTIR spectrometer. The dualESI-
FT-ICR mass spectrometry instrumental setup has been described
previously.34,63 NMR spectra were recorded on a Varian Unity 300
spectrometer operating at 300 MHz for 1H and 121.4 MHz for 31
P
and an Inova 400 spectrometer operating at 161.9 MHz for 31P.
The 1H chemical shifts are reported relative to the residual protons
of CD3NO2 (δ 4.33 ppm). An external, unlocked sample of H3PO4
was used to reference 31P spectra (δ 0.0 ppm). DOSY NMR
experiments were performed on an Inova 500 MHz spectrometer.
NMR samples were not degassed.
Temperature-dependent 1H NMR spectra for line-shape analysis
were recorded with 300, 400, and 600 MHz instruments, over a
range spanning the low- to the high-temperature limit. Temperatures
were calibrated by standard methods using methanol below the
ambient and glycerol above and were known to better than 0.25
K. Dynamic NMR simulations were performed using the DNMR3
utilities in the Spinworks 2.4 software.64 Complete assignment of
the exchanging proton spin systems was accomplished by measuring
the low temperature ROESY spectrum in CD3NO2, relying on the
opposite phase of the NOE correlations and the chemical exchange
correlations. The chemical shifts and J coupling constants were
accurately determined with the NUMARIT spin-simulation package
in Spinworks 2.4, and this was followed by DNMR line shape
simulation of the AB spin systems (see Supporting Information).
Exchange rate constants k were determined at each temperature
using the DNMR3 simulation algorithm, and the results were
independent of magnetic field strength.
Transient exchange (1D-EXCHSY) experiments were performed
using the DPFGSE sequence,65 which uses shaped, selective
excitation pulses to selectively invert the magnetization correspond-
ing to ½ of a pair of exchanging protons, and monitoring the buildup
of the inverted magnetization of the exchanging partner. Experi-
ments were performed using a minimum of six mixing times over
a range of 0.05 to 0.5 s. Selective inversion pulses were calculated
using Varian’s “PBOX” software, implementing sech180 shaped
pulses centered on the desired resonance of duration adequate to
effect selective inversion of the desired resonance, while not
affecting proton magnetization associated with nearby peaks in the
spectrum.
At the moment, the anticipated favored all-P and all-M
chirality of 3d and similar polyhedra is merely a result of
modeling and calculations, and it would be interesting to
stabilize and detect it experimentally, perhaps after inclusion
of a small chiral molecule inside the prism cavity.
Conclusions
Trigonal prisms 3 are easily accessible in high yield from
tetrapyridyl star connectors 1 and the platinum acceptor 2 via
template-free self-assembly and have been characterized by
multinuclear and DOSY NMR spectroscopy and dualESI-FT-
ICR mass spectrometry. The products 3 are expected to be
entropically and enthalpically favored over larger coordination
prisms 4-6 and open-chain oligomeric systems, respectively.
The presence of a positive cooperative effect influencing their
formation is unlikely.60
An examination of the prisms 3 revealed two points of
particular interest. They permit the study of endo–exo pyridine
edge interchange, and they introduce the previously unappreci-
ated face-based conformational cage chirality. Such conforma-
tional chirality should be a general phenomenon in polyhedra
built from star-shaped connectors and edge or vertex based
linkers. For 1d and similar star connectors with a strongly
correlated twist sense on the individual arms, all-P and all-M
faced polyhedra 3d are expected to be favored.
DOSY NMR measurements of the diffusion coefficient D were
performed using the DOSY Bipolar Pulse Pair Stimulated Echo
pulse sequence (Dbppste) sequence and a Nalorac Triple-Inverse
probe with a Z-axis PFG coil. No gradient shaping was applied, as
it had led to no improvement in previous calibrations with this probe
and to no detectable problems due to gradient eddy-current effects.
PFG bipolar pulse pairs of 2.0 ms each were applied using gradient
(61) Amoroso, A. J.; Cargill Thompson, A. M. W.; Maher, J. P.;
McCleverty, J. A.; Ward, M. D. Inorg. Chem. 1995, 34, 4828.
(62) Stang, P. J.; Cao, D. H.; Saito, S.; Arif, A. M. J. Am. Chem. Soc.
1995, 117, 6273.
(63) Kryschenko, Y. K.; Seidel, S. R.; Muddiman, D. C.; Nepomuceno,
A. I.; Stang, P. J. J. Am. Chem. Soc. 2003, 125, 9647.
(64) Kirk Marat, Spinworks 2.4 Software, University of Manitoba. Available
from: <http//www.umanitoba.ca/chemistry/nmr/spinworks>.
(65) Stott, K.; Keeler, J.; Van, Q. N.; Shaka, A. J. J. Magn. Reson. 1997,
125, 302.
(60) Ercolani, G. J. Am. Chem. Soc. 2003, 125, 16097.
9
J. AM. CHEM. SOC. VOL. 130, NO. 24, 2008 7627

A R T I C L E S
Caskey et al.
strengths ranging from 9.03 to 44.25 gauss/cm, where careful
calibration showed linearity better than 99.8%; this probe and
gradient-pulse amplifier are capable of gradient strengths up to 68
gauss/cm. Gradient stability tests with this probe and amplifier
yielded a reproducibility >99.9% over 20 repetitions. A constant
diffusion delay ∆ of 0.050 s was used in all experiments. The
sample temperature was set to 298 K using ethylene glycol and
standard calibration methods. The use of bipolar gradient-pulse pairs
eliminates many sources of error, and gradient strengths were
calibrated using the width (in Hz) of a sample of known length
along the NMR-tube (Z) axis. Calibrations were further checked
988, 846, 816, 750, 662, 630, 588, 546, 522. Anal. Calcd for
C61H21N4Co · 2(CH3OH): C, 79.74; H, 4.78; N, 5.90. Found: C,
80.08; H, 5.01; N, 5.96.
General Procedure for the Preparation of Trigonal Prisms
3. The appropriate pyridyl linker 1 (1 equiv) and cis-(Me3P)2-
Pt(OTf)2 (2 equiv) were placed in a 1-dram vial. CD3NO2 (2 mL)
was added, and the mixture stirred for 18 h at room temperature.
The solution was transferred to an NMR tube for analysis. For 3a
and 3b the solvent was then evaporated, while 3c and 3d were
precipitated with diethyl ether. In all cases the product was washed
with diethyl ether and then dried under reduced pressure.
1
by multiple, redundant measurements using the residual H NMR
1
3
3a. Yield 91%. H NMR (CD3NO2) δ 9.28 (br d, J ) 6.6 Hz,
12H, HR), 8.94 (br d, 3J ) 6.5 Hz, 12H, HR), 7.99 (s, 6H, Hphenyl),
7.77 (m, 24H, Hꢀ), 1.78 (d, 2JH-P ) 11.3 Hz, 108H, PCH3); 31P{1H}
signal from 99.9% D2O (D ) 18.72 × 10-10 m2/s,66 also reported
as 19 × 10-10 m2/s67). Repeated measurements at temperatures of
297, 298, and 299 K yielded temperature-corrected values of D )
18.5 ( 0.6 m2/s (ranging from 17.9 × 10-10 to 19.1 × 10-10).
The values of D determined by the DOSY experiment were used
to calculate the hydrodynamic radii (R) from the Stokes–Einstein
equation: D ) kBT/6πRη, where kB is the Boltzmann constant, T is
the absolute temperature, and η is the published68 viscosity of
nitromethane (0.0062 g s-1cm-1). The effect of variable viscosity
from sample to sample was examined using the D value observed
for the residual protons in the solvent resonance, which ranged from
19.5 × 10-10 to 21.4 × 10-10 m2/s, with an average value of 20.4
× 10-10 m2/s. This results in a limiting error due to variable
viscosity of (5%.
1
NMR (CD3NO2) δ -27.7 (s, 195Pt satellites, JPt-P ) 3128 Hz).
IR (KBr, cm-1): 3101 w, 3005 w, 2923 w, 2216 w, 1612 s, 1510 w,
1429 m, 1262 s, 1226 s, 1162 s, 1063 w, 1032 s, 975 m, 953 s,
861 w, 838 w, 638 s, 574 w, 551 w, 518 m. Anal. Calcd for
C
150H162F36N12O36P12Pt6S12 ·4H2O: C, 33.41; H, 3.18; N, 3.12.
Found: C, 33.07; H, 3.26; N, 3.16.
1
3b. Yield 97%. H NMR (CD3NO2) δ 8.84 (m, 24H, HR), 8.27
(d, 3J ) 4.9 Hz, 12H, Hꢀ), 8.20 (s, 6H, Hphenyl), 8.12 (d, 3J ) 15.8
3
3
Hz, 12H, C)CH), 7.61 (d, J ) 4.9 Hz, 12H, Hꢀ), 7.32 (d, J )
2
15.8 Hz, 12H, C)CH), 1.72 (d, JH-P ) 11.2 Hz, 108H, PCH3).
31P{1H} NMR (CD3NO2) δ -28.0 (s, 195Pt satellites, 1JPt-P ) 3125
Hz). IR (KBr, cm-1): 2981 w, 2926 w, 1611 s, 1434 w, 1384 w,
1260 s, 1226 m, 1165 m, 1067 w, 1031 s, 974 m, 954 m, 638 m,
520 w. Anal. Calcd. for C150H186F36N12O36P12Pt6S12•2CH3NO2•
2(CH3CH2)2O: C, 34.23; H, 3.81; N, 3.49. Found: C, 34.00; H,
4.12; N, 3.46.
Calculations. Geometries were minimized with the TINK40
program using the UFF force field38 with Powell and truncated
Newton optimization methods. Some structures were subjected to
short dynamics runs in an effort to relax them and find additional
minima. Charges for each species were calculated by charge
equilibration39 and were kept throughout.
Synthesis of 1,2,4,5-Tetrakis(4-pyridylethynyl)benzene 1a. The
reported conditions61 were difficult to reproduce. Instead, 1,2,4,5-
tetraiodobenzene69 (0.40 g, 0.69 mmol), 4-ethynylpyridine (0.36
g, 3.49 mmol), Pd(PPh3)4 (0.17 g, 0.15 mmol), and CuI (0.03 g,
0.16 mmol) were placed in an oven-dried Schlenk flask under N2.
THF (8 mL) and triethylamine (2.4 mL) were added. The reaction
mixture was heated in an oil bath at 60 °C for 24 h. The precipitated
product was filtered, taken up in chloroform, treated with activated
carbon, and filtered again. Concentration of the filtrate and addition
of hexane yielded 1a (0.08 g, 24%) of sufficient purity (1H NMR)
for further reaction.
1
3
3c. Yield 93%. H NMR (CD3NO2): δ 8.87 (d, J ) 3.7 Hz,
3
3
24H, HR), 7.73 (d, J ) 6.3 Hz, 12H, Hꢀ), 7.69 (d, J ) 5.8 Hz,
12H, Hꢀ), 7.50–7.37 (m, 48H, phenyleneH), 4.82 (s, 15H, CpH),
1.74 (d, 2JH-P ) 11.2 Hz, 108H, PCH3). 31P{1H} NMR (CD3NO2)
δ -27.8 (s, 195Pt satellites, JP-Pt ) 3150 Hz). IR (KBr, cm-1):
1
2929 w, 2216 m, 2171 w, 1612 m, 1597 m, 1515 m, 1431 w, 1259 s,
1225 m, 1160 m, 1063 w, 1030 s, 974 m, 952 m, 844 w, 752 w,
681 w, 638 m, 553 w. Anal. Calcd for C231H219Co3F36N12-
O36P12Pt6S12 ·5CH3NO2 ·11H2O: C, 40.32; H, 3.67; N, 3.39. Found:
C, 40.20; H, 4.00; N, 2.99.
1
3d. Yield 97%. H NMR (CD3NO2) δ 8.80 (d, 3J ) 5.3 Hz,
12H, HR), 8.29 (d, 3J ) 4.8 Hz, 12H, Hꢀ′ or HR′), 8.20 (d, 3J ) 4.8
Hz, 12H, Hꢀ′ or HR′), 7.57 (d, 3J ) 5.3 Hz, 12H, Hꢀ), 4.50 (s, 15H,
Synthesis of Tetrakis[4-(4-pyridylethynyl)phenyl]cyclobuta-
dienecyclopentadienylcobalt(I) 1c. To a flame-dried 50 mL three-
neck flask with a condenser were added tetrakis(4-iodophenyl)cy-
clobutadienecyclopentadienylcobalt(I) (100 mg, 0.10 mmol),57
4-ethynylpyridine hydrochloride (63 mg, 0.44 mmol), (PPh3)2PdCl2
(7 mg, 0.01 mmol), and CuI (4 mg, 0.02 mmol). The flask was
evacuated for 10 min and purged with argon. The solids were
dissolved in 10 mL of THF, and then 10 mL of triethylamine was
added. The solution was refluxed for 8 h, and the reaction progress
was monitored by NMR. Upon completion, the flask was cooled
and all solvents were removed under reduced pressure. The resulting
dark solid was chromatographed on silica gel with 9/1 chloroform/
methanol. The first band off the column contained triphenylphos-
phine salts, followed by the product as an orange solid. The solid
was crystallized by the slow evaporation of CH2Cl2 from an ∼3:1
MeOH/CH2Cl2 solution to yield 60 mg (68%) of 1d as violet
2
CpH), 1.73 (d, JH-P ) 10.8 Hz, 108H, PCH3). 31P{1H} NMR
(CD3NO2) δ -28.1 (s, 195Pt satellites, 1JPt-P ) 3132 Hz). IR (KBr,
cm-1): 3105 w, 2997 w, 2926 w, 1610 s, 1508 w, 1432 w, 1385 w,
1265 s, 1226 m, 1162 m, 1063 w, 1032 s, 974 m, 953 m, 861 w,
840 w, 638 s, 592 w, 577 w, 518 w. Anal. Calcd for C135H171
-
Co3F36N12O36P12Pt6S12 ·(CH3CH2)2O: C, 30.92; H, 3.38; N, 3.11.
Found: C, 30.96; H, 3.73; N, 3.49.
Acknowledgment. Dedicated to Prof. John D. Roberts on the
occasion of his 90th birthday. We are grateful to Prof. R. G.
Bergman for suggesting the mercury drop test. Financial support
from the National Science Foundation (CHE-0446688, and OISE
0532040), National Institutes of Health (5R01GM57052), the W. M.
Keck Foundation, the USARO (DAAD19-01-1-0521), DoD High
Performance Modernization Office (Challenge Project C1R and
ARONC014), the European Commission (STRP NMP4-013880 and
MCRTN-CT-2005-019481), the GAAV (IAA400550616), and
Ministry of Education of the Czech Republic (KONTAKT ME-
857) is gratefully acknowledged.
1
crystals. Mp: 310 °C (dec). H NMR (CDCl3, 500 MHz): δ 8.60
3
(dd, J ) 5.2 Hz, 8H, HR), 7.37-7.42 (m, 24H, Ar and Hꢀ), 4.64
(s, 5H, Cp). 13C{1H} NMR (CDCl3, 400 MHz): δ 150.02, 137.53,
131.96, 131.62, 128.89, 125.67, 120.28, 94.30, 87.62, 83.77, 74.80.
IR (KBr, cm-1): 3034, 2215, 1712, 1590, 1513, 1405, 1213, 1108,
Supporting Information Available: NMR and infrared spectra
of 3a-d. This material is available free of charge via the Internet
at http://pubs.acs.org.
(66) Bruker Instruments, personal communication.
(67) Longsworth, L. G. J. Phys. Chem. 1960, 64, 1914.
(68) CRC Handbook of Chemistry and Physics, 67th ed.; West, R. C., Ed.;
CRC Press: Boca Raton, FL, 1986; p F-40.
(69) Mattern, D. L. J. Org. Chem. 1983, 48, 4772.
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