sp carbon chains surrounded by sp3 carbon double helices: Coordination-driven self-assembly of wirelike Pt(C≡C)nPt moieties that are spanned by two P(CH2)mP linkages

Article abstract of DOI:10.1021/ja0716103

Reactions of trans, trans-(C₆F₅)(p-tol ₃P)₂Pt(C≡C)₄Pt(Pp-tol₃) ₂(C₆F₅) and diphosphines Ar ₂P(CH₂)_m-PAr₂ yield trans,trans-(C₆F₅)(Ar₂P(CH₂) _mPAr₂)Pt(C≡C)₄Pt(Ar₂P(CH ₂)_mPAr₂)(C₆F₅), in which the platinum atoms are spanned via an sp and two sp³ carbon chains (Ar/m = 3, Ph/14, 87%; 4, p-tol/14, 91%; 5, p-C₆H₄-t-Bu/ 14, 77%; 7, Ph/10, 80%; 8, Ph/11, 80%; 9, Ph/12, 36%; only oligomers form for m > 14). Crystal structures of 3-5 show that the sp³ chains adopt chiral double-helical conformations that shield the sp chain at approximately the van der Waals distance, with both enantiomers in the unit cell. The platinum square planes define angles of 196.6°-189.9° or more than a half twist. Crystal structures of 7-9, which have shorter sp³ chains, exhibit nonhelical conformations. Reaction of the corresponding Pt-(C≡C) ₆Pt complex and Ph₂P(CH₂)₁₈PPh ₂ gives an analogous adduct (27%). The crystal structure shows two independent molecules, one helical and the other not. Low-temperature NMR data suggest that the enantiomeric helical conformations of 3-5 rapidly interconvert in solution. Cyclic voltammograms of 3-5 show more reversible oxidations than model compounds lacking bridging sp³ chains. These are the only double-helical molecules that do not feature bonding interactions between the helix strands, or covalent bonds to templates dispersed throughout the strands, or any type of encoding. The driving force for helix formation is analyzed.

Full text of DOI:10.1021/ja0716103

Published on Web 06/13/2007
sp Carbon Chains Surrounded by sp3 Carbon Double
Helices: Coordination-Driven Self-Assembly of Wirelike
Pt(CtC)nPt Moieties That Are Spanned by Two P(CH2)mP
Linkages
Ju¨rgen Stahl,† Wolfgang Mohr,† Laura de Quadras,† Thomas B. Peters,‡
James C. Bohling,‡ Jose´ Miguel Mart´ın-Alvarez,‡ Gareth R. Owen,†
Frank Hampel,† and John A. Gladysz*,†
Contribution from the Institut fu¨r Organische Chemie and Interdisciplinary Center for Molecular
Materials, Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Henkestraâe 42,
91054 Erlangen, Germany, and Department of Chemistry, UniVersity of Utah,
Salt Lake City, Utah 84112
Received March 13, 2007; E-mail: gladysz@chemie.uni-erlangen.de
Abstract: Reactions of trans,trans-(C6F5)(p-tol3P)2Pt(CtC)4Pt(Pp-tol3)2(C6F5) and diphosphines Ar2P(CH2)m-
PAr2 yield trans,trans-(C6F5)(Ar2P(CH2)mPAr2)Pt(CtC)4Pt(Ar2P(CH2)mPAr2)(C6F5), in which the platinum
atoms are spanned via an sp and two sp3 carbon chains (Ar/m ) 3, Ph/14, 87%; 4, p-tol/14, 91%; 5,
p-C6H4-t-Bu/14, 77%; 7, Ph/10, 80%; 8, Ph/11, 80%; 9, Ph/12, 36%; only oligomers form for m > 14).
Crystal structures of 3-5 show that the sp3 chains adopt chiral double-helical conformations that shield
the sp chain at approximately the van der Waals distance, with both enantiomers in the unit cell. The
platinum square planes define angles of 196.6°-189.9° or more than a half twist. Crystal structures of
7-9, which have shorter sp3 chains, exhibit nonhelical conformations. Reaction of the corresponding Pt-
(CtC)6Pt complex and Ph2P(CH2)18PPh2 gives an analogous adduct (27%). The crystal structure shows
two independent molecules, one helical and the other not. Low-temperature NMR data suggest that the
enantiomeric helical conformations of 3-5 rapidly interconvert in solution. Cyclic voltammograms of 3-5
show more reversible oxidations than model compounds lacking bridging sp3 chains. These are the only
double-helical molecules that do not feature bonding interactions between the helix strands, or covalent
bonds to templates dispersed throughout the strands, or any type of encoding. The driving force for helix
formation is analyzed.
Introduction
been immortalized in books and plays. Since this structural motif
intrinsically shields interior functional domains, it represents
Of the many types of helical molecules,1,2 double helices have
captured the greatest attention.2-8 The structure elucidation of
the first compound recognized as a double helix, DNA,3 has
an astute evolutionary choice for carrying the genetic code.
However, we are not aware of any efforts to employ double
helices as “protecting groups” for abiological functional arrays.
To our knowledge, all double-helical molecules feature either
(1) bonding interactions between the helix strands, such as
hydrogen bonds, or (2) covalent bonds to metal templates
dispersed throughout the strands. Factors that promote or enforce
complementarity, e.g., the programming9 or “encoding of
specific structural and conformational information”,4b are thought
to be required. We wondered whether it would be possible to
design double-helical molecules that lack these conventional
driving forces. In the extreme, might certain substances appear
to yield double helices because “they felt like it”?
† Universita¨t Erlangen-Nu¨rnberg.
‡ University of Utah.
(1) Meurer, K. P.; Vo¨gtle, F. Top. Curr. Chem. 1985, 127, 1.
(2) (a) Piguet, C.; Bernardinelli, G.; Hopfgartner, G. Chem. ReV. 1997, 97,
2005. (b) Albrecht, M. Chem. ReV. 2001, 101, 3457. (c) Albrecht, M.
Angew. Chem., Int. Ed. 2005, 44, 6448; Angew. Chem. 2005, 117, 6606.
(3) Watson, J. D.; Crick, F. H. C. Nature 1953, 171, 737 and 964.
(4) (a) Lehn, J.-M.; Rigault, A.; Siegel, J.; Harrowfield, J.; Chevrier, B.; Moras,
D. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 2565. (b) Ohkita, M.; Lehn,
J.-M.; Baum, G.; Fenske, D. Chem.sEur. J. 1999, 5, 3471. (c) Funeriu,
D. P.; Lehn, J.-M.; Fromm, K. M.; Fenske, D. Chem.sEur. J. 2000, 6,
2103. (d) Berl, V.; Huc, I.; Khoury, R. G.; Lehn, J.-M. Chem.sEur. J.
2001, 7, 2810.
(5) Mamula, O.; von Zelewsky, A.; Bark, T.; Bernardinelli, G. Angew. Chem.,
Int. Ed. 1999, 38, 2945; Angew. Chem. 1999, 111, 3129.
(6) Casnati, A.; Liantonio, R.; Metrangolo, P.; Resnati, G.; Ungaro, R.;
Ugozzoli, F. Angew. Chem., Int. Ed. 2006, 45, 1915; Angew. Chem. 2006,
118, 1949.
(7) Goto, H.; Katagiri, H.; Furusho, Y.; Yashima, E. J. Am. Chem. Soc. 2006,
128, 7176.
(8) For other types of double helices based upon platinum phosphine or alkynyl
complexes, see: (a) Peng, H.-Y.; Lam, C.-K.; Mak, T. C. W.; Cai, Z.; Ma,
W.-T.; Li, Y.-X.; Wong, H. N. C. J. Am. Chem. Soc. 2005, 127, 9603. (b)
Furusho, Y.; Tanaka, Y.; Yashima, E. Org. Lett. 2006, 8, 2583.
Toward this end, we sought rodlike building blocks, repre-
sented by A in Scheme 1. Such moieties are seeing extensive
use in the development of molecular-scale devices.10 The most
(9) Rowan, A. E.; Nolte, R. J. M. Angew. Chem., Int. Ed. 1998, 37, 63; Angew.
Chem. 1998, 110, 65.
(10) (a) Schwab, P. F. H.; Levin, M. D.; Michl, J. Chem. ReV. 1999, 99, 1863.
(b) Schwab, P. F. H.; Smith, J. R.; Michl, J. Chem. ReV. 2005, 105, 1197.
9
8282
J. AM. CHEM. SOC. 2007, 129, 8282-8295
10.1021/ja0716103 CCC: $37.00 © 2007 American Chemical Society

sp Carbon Chains within sp3 Carbon Double Helices
A R T I C L E S
Scheme 1. Design of Possible Double-Helical Molecules (C) and
Some Alternative Conformations (D, E)
enhanced kinetic stabilities. New length regimes would likely
be accessible.
Also, molecules in which unsaturated ligands span two
transition metals can often be generated in more than one
oxidation state, each of which can undergo interesting charge-
transfer phenomena.15 However, the stabilities of oxidized or
reduced species derived from metal-capped polyynes dramati-
cally decrease with sp chain length.16 No examples with more
than eight carbon atoms have been isolated,17 hampering certain
applications of these wirelike species. The flexible chains in
C-E should again serve a stabilizing function, which for odd-
electron systems would be conceptually analogous to the
insulation about household electrical wire. Others have reported
complementary approaches to shielding unsaturated assemblies
that connect two electroactive groups.18-21
Hence, we set out to investigate whether Pt(CtC)nPt moieties
could serve as core units for double-helical assemblies C. One
of several underlying considerations involved the well-
established redox chemistry of monoplatinum(II) complexes.
In most but not all cases, electrochemical oxidations to platinum-
(III) are only partially reversible.22 Thus, it would be simple to
qualitatively detect a stabilizing effect of the flexible chains
upon the corresponding radical cations.
As summarized in Scheme 2, two synthetic strategies were
considered. The first involved reactions of preformed Pt(Ct
C)nPt species (F) with bis(R,ω-diarylphosphino)alkanes, in
which the phosphorus atoms are separated by flexible poly-
methylene chains. This can be viewed as a coordination-driven
“self-assembly” process with potential for operationally simple
nanodevice synthesis. In terms of precedent, Su¨nkel had shown
that when the chloride-substituted diplatinum ethynediyl com-
plex Cl(Ph3P)2PtCtCPt(PPh3)2Cl was treated with excess PEt3,
all of the PPh3 ligands could be replaced.23
The second strategy involved the synthesis of platinum
chloride complexes with ω-functionalized n-alkyl monophos-
phine ligands (G). The sp carbon chain would then be
introduced, and the phosphine ligands on opposite platinum
termini coupled, generating bridging diphosphines within the
metal coordination sphere. In both approaches, there would be
the possibility of byproducts in which the diphosphine ligands
span trans positions of the same platinum atom, as shown in
H′.
fundamental and radially compact would be a chain of sp
hybridized carbons.11 An atom that supports at least two
additional bonds would then be grafted onto each terminus.
Transition metals are obvious candidates,12 as exemplified by
the square planar endgroups B in Scheme 1. These could anchor
two long flexible chains, containing for example polymethylene
((CH2)m) or polydifluoromethylene ((CF2)m) domains.
Conformation is then an issue. Will the flexible chains twist
about the rod to give double helices as in the case of C (Scheme
1)? Alternatively, will they preferentially adopt conformations
with “holes” (D) or “fingers” (E)? Since monocyclic crown
ethers and related macrocycles seldom exhibit conformations
with holes,13 we discounted possibility D. However, E was
viewed as a plausible alternative. Regardless, all of these
conformations sterically shield the rodlike segment to varying
degrees.
In this paper, we report successful syntheses of the target
molecules C by the first strategy. This simple route allows rapid
access to a number of complexes but also has limitations as
described below. In the following paper, we report successful
Such assemblies would have potential applications. For
example, platinum(II)-capped polyynes Pt(CtC)nPt (polyynediyls)
with as many as 28 sp carbon atoms have been isolated,14
corresponding to a metal-metal separation of 3.9 nm. However,
all polyynes become increasingly more labile with sp carbon
chain length. Since the decomposition pathways are believed
to be bimolecular, systems of the type C-E should exhibit
(15) Paul, F.; Lapinte, C. In Unusual Structures and Physical Properties in
Organometallic Chemistry; Gielen, M., Willem, R., Wrackmeyer, B., Eds.;
Wiley: New York, 2002; pp 220-291.
(16) (a) Dembinski, R.; Bartik, T.; Bartik, B.; Jaeger, M.; Gladysz, J. A. J. Am.
Chem. Soc. 2000, 122, 810. (b) Meyer, W. E.; Amoroso, A. J.; Horn, C.
R.; Jaeger, M.; Gladysz, J. A. Organometallics 2001, 20, 1115.
(17) Coat, F.; Lapinte, C. Organometallics 1996, 15, 477.
(18) See: Frampton, M. J.; Anderson, H. L. Angew. Chem., Int. Ed. 2007, 46,
1028; Angew. Chem. 2007, 119, 1046; and earlier work of this group
reviewed therein.
(19) Schenning, A. P. H. J.; Arndt, J.-D.; Ito, M.; Stoddart, A.; Schreiber, M.;
Siemsen, P.; Martin, R. E.; Boudon, C.; Gisselbrecht, J.-P.; Gross, M.;
Gramlich, V.; Diederich, F. HelV. Chim. Acta 2001, 84, 296.
(20) (a) Lee, D.; Swager, T. M. Synlett 2004, 149. (b) Kwan, P. H.; Swager, T.
M. Chem. Commun. 2005, 5211.
(21) Li, C.; Numata, M.; Bae, A.-H.; Sakurai, K.; Shinkai, S. J. Am. Chem.
Soc. 2005, 127, 4548.
(11) Szafert, S.; Gladysz, J. A. Chem. ReV. 2003, 103, 4175; 2006, 106, PR1-
PR33.
(12) For a review of complexes of the formula LmMCxMLm, and related species,
see: Bruce, M. I.; Low, P. J. AdV. Organomet. Chem. 2004, 50, 179.
(13) Gokel, G. W. Crown Ethers and Cryptands; Royal Society of Chemistry:
Cambridge, U.K., 1991; Chapter 4.
(14) (a) Mohr, W.; Stahl, J.; Hampel, F.; Gladysz, J. A. Chem.sEur. J. 2003,
9, 3324. (b) Zheng, Q.; Bohling, J. C.; Peters, T. B.; Frisch, A. C.; Hampel,
F.; Gladysz, J. A. Chem.sEur. J. 2006, 12, 6486.
(22) (a) Klein, A.; Kaim, W. Organometallics 1995, 14, 1176. (b) Klein, A.;
Hasenzahl, S.; Kaim, W.; Fiedler, J. Organometallics 1998, 17, 3532.
(23) Su¨nkel, K.; Birk, U.; Robl, C. Organometallics 1994, 13, 1679.
9
J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007 8283

A R T I C L E S
Stahl et al.
Scheme 3. First Attempt to Synthesize Target Complexes
Scheme 2. Synthetic Planning: Routes to Target Complexes
(CtC)nPt-derived materials below. Monoplatinum complexes
of other types of long-chain chelating diphosphines have also
been observed to oligomerize.28b
In an attempt to avoid the generation of oligomers, two
strategy modifications were implemented: (a) platinum end-
groups with enhanced Lewis acidities were targeted, with the
idea that the dissociation of one terminus of the diphosphine
ligand in H (Scheme 2) would be retarded - a logical initial
event in oligomerization; (b) since this might in turn render
substitution reactions of F (Scheme 2) more difficult, approaches
involving functionalized phosphines (G) were investigated. In
this way, no platinum-phosphorus bond-making or -breaking
reactions are required after the initial substitution. As described
in the following paper,24 this sequence proved successful with
pentafluorophenyl-substituted platinum endgroups and provided
the first syntheses of the target molecules. However, an obvious
question remained: would modification (a) alone have sufficed?
applications of the second strategy, using alkene metathesis as
the means of ligand coupling.24 This longer method is more
reliable, but the overall yields are somewhat lower. Portions of
this work have been communicated,25 and additional details are
described elsewhere.26
Results
1. Syntheses of Target Molecules: 8 sp and 14 sp3
Carbons. Reactions of the p-tolyl-substituted diplatinum oc-
tatetraynediyl complex trans,trans-(p-tol)(Ph3P)2Pt(CtC)4Pt-
(PPh3)2(p-tol) (1; Scheme 3)14b were investigated first. An NMR
tube was charged with 1 and the diphosphine Ph2P(CH2)12-
PPh2,27,28 which features 12 sp3 carbon atoms. Displacement
of the PPh3 ligands proceeded rapidly below room temperature,
generating one platinum-containing product as assayed by 31P
NMR and TLC. However, when solutions were concentrated,
insoluble, presumably oligomeric or polymeric materials formed.
Only small portions could be induced to redissolve.
As shown in Scheme 4, the answer is in many cases yes. A
CH2Cl2 solution of the diphosphine Ph2P(CH2)14PPh2 (2.6
equiv),27 which features 14 sp3 carbon atoms, was added to a
moderately dilute CH2Cl2 solution of the pentafluorophenyl-
substituted diplatinum complex trans,trans-(C6F5)(p-tol3P)2-
Pt(CtC)4Pt(Pp-tol3)2(C6F5) (2; 0.0025 M).14a Workup gave
the target complex trans,trans-(C6F5)(Ph2P(CH2)14PPh2)Pt-
(CtC)4Pt(Ph2P(CH2)14PPh2)(C6F5) (3) as an analytically pure
Comparable results were obtained with the longer-chain
diphosphine Ph2P(CH2)16PPh2.28 Extensive attempts to crystal-
lize the major products from dilute solutions were unsuccessful.
Oligomeric structures are rigorously established for similar Pt-
yellow powder in 87% yield. The structure was verified
crystallographically, as described below. Analogous reactions
with similar phenyl-substituted diphosphines, (p-RC6H4)2P-
(CH2)14P(p-C6H4R)2 (R ) CH3, t-Bu),30 gave the corresponding
complexes 4 and 5 in 77-91% yields.
Complexes 3-5, and all other compounds isolated below,
were stable in air as solids for extended periods. They were
characterized by IR, NMR (1H, 13C, 31P), and UV-visible
(24) de Quadras, L.; Bauer, E. B.; Mohr, W.; Bohling, J. C.; Peters, T. B.; Mart´ın-
Alvarez, J.; Hampel, F.; Gladysz, J. A. J. Am. Chem. Soc. 2007, 129, 8296.
(25) (a) Stahl, J.; Bohling, J. C.; Bauer, E. B.; Peters, T. B.; Mohr, W.; Mart´ın-
Alvarez, J. M.; Hampel, F.; Gladysz, J. A. Angew. Chem., Int. Ed. 2002,
41, 1871; Angew. Chem. 2002, 114, 1951. (b) See also: Owen, G. R.;
Stahl, J.; Hampel, F.; Gladysz, J. A. Organometallics 2004, 23, 5889.
(26) (a) Mohr, W. Doctoral Dissertation, Universita¨t Erlangen-Nu¨rnberg, 2002.
(b) Stahl, J. Doctoral Dissertation, Universita¨t Erlangen-Nu¨rnberg, 2003.
(27) Mohr, W.; Horn, C. R.; Stahl, J.; Gladysz, J. A. Synthesis 2003, 1279.
(28) (a) Hill, W. E.; McAuliffe, C. A.; Niven, I. E.; Parish, R. V. Inorg. Chim.
Acta 1980, 38, 273. (b) Hill, W. E.; Minahan, D. M. A.; Taylor, J. G.;
McAuliffe, C. A. J. Am. Chem. Soc. 1982, 104, 6001.
(29) A similar reaction of the Pt(CtC)8Pt homologue14a of 2 and 10 with
Ph2P(CH2)28PPh2 gave exclusively oligomeric products.
(30) de Quadras, L.; Stahl, J.; Zhuravlev, F.; Gladysz, J. A. J. Organomet. Chem.
2007, 692, 1859.
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sp Carbon Chains within sp3 Carbon Double Helices
A R T I C L E S
Scheme 4. Reactions of the Pt(CtC)4Pt Complex 2 with
spectroscopy, and mass spectrometry, as summarized in the
Experimental Section. The IR and UV-visible spectra were
quite similar to those of the precursor 214a and essentially
identical with those of the model compound 6.30 The latter is a
Bis(R,ω-diarylphosphino)alkanes
better electronic model for 3, lacking only bridging sp3 carbon
chains. The PtCtC 13C NMR signals of 3-5 (98.6/94.2, 99.2/
94.1, 99.1/94.4 ppm) differed slightly from those of 2 (100.6/
96.7) but were nearly identical with those of 6 (100.0/94.2 ppm).
The PCH2CH2CH2 13C NMR signals were also very close to
those of 6 (28.1-28.3/25.5-25.7/30.6-30.7 vs 28.2/25.5/31.3
ppm). The mass spectra exhibited intense molecular ions
(100%). DSC data for 3 and 4 showed endotherms at 244 and
213 °C (Te; presumably phase transitions) with exotherms
beginning at higher temperatures (271 and 260 °C, Ti) that were
accompanied by mass loss (TGA data).
2. Crystallography: 8 sp and 14 sp3 Carbons. Complexes
3, 4, and 5 readily crystallized. The first was obtained as both
benzene and toluene sesquisolvates (3‚(benzene)1.5, 3‚(tolu-
ene)1.5), and the last, as two different toluene solvates (5‚
(toluene)5.5 and 5‚(toluene)2) or pseudopolymorphs.33 The crystal
structures were solved as summarized in the Supporting
Information. Those of 3‚(benzene)1.5 and 3‚(toluene)1.5 were very
similar. Key metrical parameters are given in Tables 1-3.
The structures of 3‚(toluene)1.5, 4, and 5‚(toluene)5.5 are
compared in Figure 1 (I-K). In all three compounds, the sp3
carbon chains exhibit a striking double-helical conformation.
Helices are intrinsically chiral, and both enantiomers are present
in the crystal lattice. With 3‚(benzene)1.5 and 3‚(toluene)1.5, the
twisting is less “uniform” than that in the other complexes, a
feature that can be analyzed via torsion angle patterns (Table
3) as described below. The idealized structures (i.e., as
represented in Scheme 4) belong to the point group D2.
The bond lengths, bond angles, and slight sp chain curvatures
in 3‚(toluene)1.5, 4, and 5‚(toluene)5.5 are unexceptional. They
are very close to those of 2,14a which lacks bridging diphos-
phines, as well as higher and lower homologues.14a,25b Consider-
ably greater sp chain curvature has been observed,11 a normal
phenomenon that has various measures, such as the average
bond angles in Table 1 or the difference between the platinum-
platinum distances and the sum of the intervening bond lengths.
These parameters and others have been analyzed for all Pt(Ct
C)nPt complexes in detail elsewhere.11
3‚(toluene)1.5, 4, and 5‚(toluene)5.5 range from 189.8° to 196.6°,
more than a half twist. This feature is emphasized in the partial
structure of 3‚(toluene)1.5 shown in Figure 2 (L). Helical objects
are also characterized by a degree of helicity and a pitch (Table
1).1,31 The former is derived by dividing the preceding angles
by 360°, and the latter represents the hypothetical distance at
which a full turn would be reached.
The crystal structure of precursor 2 has been previously
reported,14a and two views with all atoms at van der Waals radii
are illustrated in Figure 3 (M, N; parallel and perpendicular to
the planes of the C6F5 rings). The sp carbon chain is clearly
accessible to external reagents. Figure 3 also gives analogous
representations of 3‚(toluene)1.5 (Q, R), as well as structures
from which the phenyl rings and hydrogen atoms are omitted
(O, P). For reference, comparable views of 4 are depicted (S,
T); 5‚(toluene)5.5 is very similar. These structures nicely illustrate
the extensive shielding of the sp chains in 3-5, although R
and T reveal that a line-of-sight through a protecting rim of
peripheral groups remains.
Figure 3 shows that the sp and sp3 carbon chains of
3‚(toluene)1.5 and 4 entwine with a spacing slightly greater than
their van der Waals radii (1.78 and 1.70 Å).32 A cylindrical
helix is characterized by a radius, which can be approximated
by calculating the average distances of the sp3 carbon atoms
from the platinum-platinum vectors. As summarized in Table
1, these span the narrow range from 3.977 to 4.053 Å. A higher
value indicates a “looser” helix. Accordingly, there is an inverse
correlation with the degree of helicity. The packing diagrams
show essentially parallel sp chains, the spacing between which
The angles defined by the square planes of the platinum
termini are of particular interest. Of the measures summarized
in Table 1, we favor the planes defined by the P-Pt-P linkage
on one endgroup and the platinum on the other. These emphasize
the twist defined by the P-Pt-P linkages and are less effected
by various geometric nonidealities. The values in 3‚(benzene)1.5,
(31) Gray, A. The Helix and its Generalizations. In Modern Differential
Geometry of CurVes and Surfaces with Mathematica, 2nd ed.; CRC Press:
Boca Raton, FL, 1997; pp 198-200.
(32) Bondi, A. J. Phys. Chem. 1964, 68, 441.
(33) Threlfall, T. L. Analyst 1995, 120, 2435.
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A R T I C L E S
Stahl et al.
Table 1. Key Crystallographic Distances [Å] and Angles [deg] for Helical Complexes
a
b
complex
3
‚
(benzene)1.5
3‚
(toluene)1.5
4
5
‚
(toluene)5.5
11‚(CH2Cl2)0.67
Pt•••Pt
sum of bond lengths,
Pt1 to Pt2
12.775(8)
12.956
12.7653(14)
12.914
12.6984(5)
12.890
12.8867(3)
12.896
17.9897(8)
18.023
Pt1-C1
2.003(3)
1.207(5)
1.368(5)
1.207(5)
1.354(5)
1.216(5)
1.368(5)
1.209(5)
-
1.987(4)
1.215(7)
1.365(7)
1.214(7)
1.360(7)
1.207(7)
1.360(7)
1.212(6)
-
1.991(8)
1.211(9)
1.345(10)
1.201(9)
1.363(10)
1.198(9)
1.372(10)
1.222(9)
-
1.988(4)
1.205(6)
1.369(6)
1.206(6)
1.360(6)
1.209(6)
1.356(6)
1.220(6)
-
1.962(13)
1.207(19)
1.38(2)
1.23(2)
1.37(2)
1.16(2)
1.36(2)
1.22(2)
1.34(2)
1.21(2)
1.41(2)
1.176(19)
1.998(15)
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
C6-C7
C7-C8
C8-C9
C9-C10
C10-C11
C11-C12
C8-Pt2 or C12-Pt2
-
-
-
-
-
-
-
-
-
-
-
-
1.994(3)
1.994(4)
1.987(7)
1.983(4)
Pt1-C1-C2
C1-C2-C3
C2-C3-C4
C3-C4-C5
C4-C5-C6
C5-C6-C7
C6-C7-C8
C7-C8-C9
C8-C9-C10
C9-C10-C11
C7-C8-Pt2 or
C11-C12-Pt2
178.0(3)
177.5(4)
177.3(4)
176.9(5)
178.5(5)
176.3(4)
174.1(4)
-
175.9(5)
178.9(6)
178.4(6)
178.2(6)
177.0(6)
177.3(6)
172.8(5)
-
178.8(6)
176.4(8)
176.0(8)
175.3(8)
176.0(8)
177.9(8)
178.2(8)
-
179.3(4)
178.3(5)
179.9(7)
178.5(6)
177.9(6)
178.5(5)
179.1(5)
-
179.0(14)
174.7(18)
178(2)
177(2)
174(2)
174(2)
178(2)
177(3)
180(3)
-
-
-
-
-
-
-
-
176.9(18)
177.8(15)
171.7(3)
171.6(4)
171.5(6)
179.3(4)
av Pt-Csp-Csp
av Csp-Csp-Csp
174.9
176.8
173.8
177.1
175.2
176.6
179.3
178.7
178.4
176.6
av sp/sp3 distancec
4.053
3.896
3.977
3.755
4.042
3.772
3.988
3.704
3.868
3.907
av π stackingd
P-Pt-P/Pt anglee
196.6
198.1
196.5
200.8
189.9
188.6
193.3
193.5
163.2
163.9
Pt+P+P+Ci+C1 anglee
helix pitchf
23.72
23.66
24.45
24.02
39.76
degree of helicity
54.6%
54.6%
52.7%
53.7%
45.3%
g
Csp-Csp
8.071
0.55
7.974
0.38
9.339
0.49
11.959
0.44
9.028
0.44
fractional offseth
a In order to better compare the two similar noncentrosymmetric solvates of 3, the numbering of the PtC8Pt segment in 3‚(benzene)1.5 has been reversed
in this table. b Data for the nonhelical molecule in the unit cell is given in Table 2. c Average distance from every CH2 group to the Pt-Pt vector. d Distance
between midpoints of the C6F5 and C6H4R rings. e Angle between planes defined by these atoms on each endgroup. f The hypothetical distance at which
a full helix turn is reached. g The shortest carbon-carbon distance between parallel sp chains in the crystal lattice. h Of parallel sp chains: see ref 11.
increases from 7.97 to 11.96 Å as the sizes of the p-aryl
substituents increases (Table 1).
nearly all (C6F5)Pt adducts of aryl phosphines,14a,25b,34,35 and
average distances are summarized in Tables 1 and 2. The origin
of this phenomenon, which is general for highly fluorinated
arenes, is well understood.36 Although it plays a role in crystal
packing, it is not believed to be a determinant of sp3 chain
conformation in the title molecules. Note that the shortest
average distance is found in nonhelical 5‚(toluene)2.
In contrast to the preceding four crystal structures, that of
the pseudopolymorph 5‚(toluene)2 exhibited a nonhelical con-
formation. As shown in Figure 4, the sp3 carbon chains define
loops with respect to the sp chain. However, when atoms are
viewed at van der Waals radii, there are no “holes” as in the
case of D (Scheme 1). The average distance of the sp3 carbon
atoms from the platinum-platinum vector (Table 2) is somewhat
greater than that in helical 5‚(toluene)5.5 (4.064 vs 3.988 Å).
Interestingly, the crystal density of 5‚(toluene)2, a property
sometimes used to gauge the relative stabilities of polymorphs,33
is higher than that of 5‚(toluene)5.5 (1.333 vs 1.256 Mg/m3); in
contrast to the other structures, the lattice features two nonparal-
lel sets of parallel sp chains. In any event, V and W (Figure 4)
show that the sp chain in 5‚(toluene)2 is only slightly less
shielded than those in the helical complexes. Importantly, the
structure requires that nonhelical conformations of 5 be popu-
lated in solution.
3. Syntheses of Target Molecules: Other sp/sp3 Carbon
Chain Lengths. Several questions follow from the preceding
results: (a) How short an sp3 carbon chain can be introduced?
(b) How long an sp3 carbon chain can be introduced? (c) Can
analogous chemistry be effected with longer sp carbon chains?
When the sp3/sp carbon ratio becomes too small, helical
conformations should no longer be possible. However, such
species would still possess sp carbon chains that are to various
degrees sterically protected. Furthermore, when the sp3 and sp
chains have similar numbers of atoms, they are constrained over
(34) Bauer, E. B.; Hampel, F.; Gladysz, J. A. Organometallics 2003, 22, 5567.
(35) Shima, T.; Bauer, E. B.; Hampel, F.; Gladysz, J. A. Dalton Trans. 2004,
1012.
In all of the preceding structures, including that of 2, aryl/
C6F5/aryl stacking interactions are evident. These are found for
(36) Reichenba¨cher, K.; Su¨ss, H. I.; Hullinger, J. Chem. Soc. ReV. 2005, 34,
20.
9
8286 J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007

sp Carbon Chains within sp3 Carbon Double Helices
A R T I C L E S
Table 2. Key Crystallographic Distances [Å] and Angles [deg] for Nonhelical Complexes
a
a,b
complex
5
‚
(toluene)2
7‚
(toluene)4
7‚
(CHCl3)2
9‚
(CH2Cl2)2.5
11‚(CH2Cl2)0.67
Pt•••Pt
sum of bond lengths,
Pt1 to Pt2
12.8820(21)
12.907
12.9120(3)
12.933
12.6417(31)
12.953
12.9580(2)
12.980
17.6087(13)
18.06
Pt1-C1
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
C6-C7
C7-C8
C8-Pt2
1.989(7)
1.220(9)
1.356(10)
1.202(10)
2.014(6)
1.192(8)
1.366(8)
1.208(8)
1.361(9)
1.215(8)
1.368(8)
1.210(8)
1.999(6)
1.998(5)
1.219(6)
1.359(7)
1.224(7)
1.362(6)
1.212(6)
1.369(6)
1.209(6)
2.001(4)
2.009(4)
1.205(6)
1.379(6)
1.214(7)
1.365(7)
1.222(7)
1.365(6)
1.212(6)
2.009(4)
1.960(16)
1.23(2)
1.36(2)
1.19(2)
1.38(3)
1.24(2)
1.34(4)
-
1.377(15)
-
-
-
-
-
Pt1-C1-C2
C1-C2-C3
C2-C3-C4
C3-C4-C5
C4-C5-C6
C5-C6-C7
C6-C7-C8
C7-C8-Pt2
175.7(7)
175.6(8)
179.4(10)
178.8(12)
-
-
-
-
174.0(6)
178.2(8)
178.3(7)
178.5(9)
179.6(9)
178.5(8)
177.5(7)
176.7(6)
171.1(4)
176.4(6)
177.4(5)
177.4(5)
177.0(5)
175.0(5)
173.5(5)
169.4(4)
176.0(5)
178.5(6)
178.9(6)
179.6(6)
179.2(6)
178.6(5)
178.7(5)
175.1(4)
166.7(14)
170.8(18)
174.5(18)
176(2)
178(2)
178(2)
-
-
av Pt-Csp-Csp
av Csp-Csp-Csp
175.7
177.9
175.4
178.4
170.3
176.1
175.6
178.9
166.7
175.5
av sp/sp3 distancec
4.064
3.633
3.932
3.803
3.989
3.756
4.090
3.766
4.157
3.979
av π stackingd
P-Pt-P/Pt anglee
0.0
0.0
0.2
2.7
5.1
40.4
12.9
18.3
0.0
0.0
Pt+P+P+Ci+C1 anglee
f
Csp-Csp
11.433
0.88
7.296
0.41
7.780
0.10
9.738
0.32
9.028
-
fractional offsetg
a This molecule exhibits an inversion center. bData for the helical molecule in the unit cell is given in Table 1. cAverage distance from every CH2 group
e
f
to the Pt-Pt vector. dDistance between midpoints of the C6F5 and C6H4R rings. Angle between planes defined by these atoms on each endgroup. The
g
shortest carbon-carbon distance between parallel sp chains in the crystal lattice. Of parallel sp chains: see ref 11.
Table 3. Torsion Angles [deg] Involving sp3 Carbon Atoms for Ar2P(CH2)14PAr2-Bridged Complexes
a
a
a
complex
3
‚(benzene)1.5
3‚
(toluene)1.5
4a
5‚
(toluene)5.5
5‚(toluene)2
P-C1-C2-C3
-158.8/-80.8
66.5/-71.9
171.4/173.3
-174.9/176.9
-174.8/166.5
175.8/-172.6
66.6/173.6
173.4/171.1
173.8/68.5
173.2/166.0
58.0/172.0
-161.2/85.3
64.6/-173.8
173.2/166.5
179.0/-176.4
-176.3/170.1
-178.6/173.1
67.8/61.9
171.5/172.1
174.1/172.4
174.2/177.9
57.0/165.3
175.2/170.2
54.9/64.3
62.1/62.1
178.3/-179.4
66.8/64.9
63.9/64.4
177.9/-170.6
173.5/-68.8
C1-C2-C3-C4
C2-C3-C4-C5
C3-C4-C5-C6
C4-C5-C6-C7
C5-C6-C7-C8
C6-C7-C8-C9
C7-C8-C9-C10
C8-C9-C10-C11
C9-C10-C11-C12
C10-C11-C12-C13
C11-C12-C13-C14
C12-C13-C14-P′
69.2/-89.8b
174.8/179.5
-172.1/179.5
175.2/-177.0
172.3/178.2
-175.8/176.2
176.2/-176.4
-164.4/179.0
68.3/61.9
169.9/174.9
173.1/179.2
-178.5/-178.7
170.2/170.7
179.6/178.7
178.7/177.7
178.0/173.2
65.9/62.7
176.8/-160.4b
-179.3/-82.1b
-160.9b/148.0b
-84.6b/-84.6b
148.0b/-160.9b
-82.1b/-179.3
-160.4b/176.8
-89.8b/69.2
57.1/74.9
177.7/-158.6
57.1/69.2
-179.4/-150.9
-176.1/55.3
169.4/171.6
63.3/63.4
-179.8/175.4
-68.8/173.5
-170.6/177.9
b
a The sp3 carbon chains in these structures are not equivalent. The torsion angles for both are presented starting from the same platinum atom. There is
considerable vibrational disorder associated with the atoms from which these values are derived.
their entire lengths to be in close proximity, providing a valuable
reference state.
Analogous reactions of longer-chain diphosphines Ph2P-
(CH2)mPPh2 (m ) 16, 18, 32)27,28 with highly dilute CH2Cl2
solutions of 2 (0.0001, 0.0001, 0.000 075 M) gave only insoluble
oligomers (85%, 94%, 61%, respectively).26a The IR spectra of
these materials were very similar. Mass spectra did not exhibit
any ions for diplatinum complexes.
As shown in Scheme 4 (bottom), CH2Cl2 solutions of the
shorter-chain diphosphines Ph2P(CH2)mPPh2 (m ) 10, 11,
12)27,28 were added to CH2Cl2 solutions of 2 that were more
dilute than those employed above. Workups gave the substitution
products 7, 8, and 9 in 80%, 80%, and 36% yields, respectively.
In the last reaction, which is closely related to the unsuccessful
synthesis in Scheme 3, oligomeric byproducts were evident, and
a higher dilution was necessary ([2] ) 0.0005 M). The IR,
NMR, UV-visible, MS, and DSC/TGA properties of 7-9 were
very similar to those of 3-5, including the 13C NMR chemical
shifts of the sp and sp3 carbon atoms.
As shown in Scheme 5 (top), a CH2Cl2 solution of the
diphosphine Ph2P(CH2)18PPh2 was added to an extremely dilute
CH2Cl2 solution of the diplatinum dodecahexaynediyl or Pt-
(CtC)6Pt complex 10 (0.000 03 M).14a Workup and crystal-
lization gave the substitution product 11, which has sp and sp3
chains of 12 and 18 carbon atoms, in 27% yield. Oligomeric
material was also isolated in 64% yield. As with 3-9, the IR
9
J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007 8287

A R T I C L E S
Stahl et al.
Figure 3. Space filling representations of selected Pt(CtC)4Pt com-
plexes: M, N, views of 2‚(toluene) parallel and perpendicular to the planes
of the C6F5 ligands; O, P, analogous views of 3‚(toluene)1.5 with phenyl
rings, hydrogen atoms, and solvate molecules omitted; Q, R, analogous
views of 3‚(toluene)1.5 but with phenyl rings and hydrogen atoms; S, T,
analogous views of 4.
Figure 1. Thermal ellipsoid plots (50% probability level) of the double-
helical Pt(CtC)4Pt complexes with hydrogen atoms and solvate molecules
omitted: I, 3‚(toluene)1.5; J, 4; K, 5‚(toluene)5.5
.
PPh2 and Ph2P(CH2)20PPh2, mass spectra of the reaction
mixtures showed ions for diplatinum complexes, and trace
amounts could be extracted from the oligomers.
Experiments were conducted to confirm the oligomeric nature
of these products. First, the material derived from 10 and
Ph2P(CH2)20PPh2 gave an appropriate microanalysis. Second,
it was treated with an excess of the monophosphine PEt3.
Trialkylphosphines are commonly more basic than diarylalkyl-
phosphines. As shown in Scheme 5, substitution occurred to
give the known complex trans,trans-(C6F5)(Et3P)2Pt(CtC)6Pt-
(PEt3)2(C6F5) (12), which was isolated in 56% yield.14a Analo-
gous additions of PEt3 to oligomers derived from 2 and
Ph2P(CH2)mPPh2 (m ) 16, 18, 32) similarly gave trans,trans-
(C6F5)(Et3P)2Pt(CtC)4Pt(PEt3)2(C6F5).14a
4. Crystallography: Other sp/sp3 Carbon Chain Lengths.
Complexes 7, 9, and 11 could also be crystallized. In the case
of 7, two pseudopolymorphs, 7‚(toluene)4 and 7‚(CHCl3)2, were
obtained. Their crystal structures, and those of 9‚(CH2Cl2)2.5
and 11‚(CH2Cl2)0.67, were determined as summarized in the
Supporting Information. Portions of the sp3 carbon chains of
7‚(toluene)4 and 9‚(CH2Cl2)2.5 were disordered, but this could
Figure 2. View of 3‚(toluene)1.5 along the platinum-platinum axis with
non-ipso aryl carbon atoms, hydrogen atoms, and solvate molecules omitted.
and UV-visible properties, and 13C NMR chemical shifts of
the sp carbon atoms, were similar to those of the unbridged
precursor. However, as summarized in Scheme 5 (bottom),
reactions of 10 and longer-chain diphosphines gave almost
exclusively oligomeric products.29 In the cases of Ph2P(CH2)19-
9
8288 J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007

sp Carbon Chains within sp3 Carbon Double Helices
A R T I C L E S
Figure 4. Structure of the nonhelical Pt(CtC)4Pt complex 5‚(toluene)2 with solvate molecules omitted: U, thermal ellipsoid plot (50% probability level)
with hydrogen atoms omitted; V, W, views parallel and perpendicular to the planes of the C6F5 ligands with atoms at van der Waals radii.
Scheme 5. Reactions of the Pt(CtC)6Pt Complex 10 with
Bis(R,ω-diphenylphosphino)alkanes
Figure 5. Thermal ellipsoid plots (50% probability level) of other nonhelical
Pt(CtC)4Pt complexes with hydrogen atoms and solvate molecules
omitted: X, 7‚(toluene)4 (dominant conformation); Y, 7‚(CHCl3)2; Z, 9‚
(CH2Cl2)2.5 (dominant conformation).
be resolved and only data for the dominant conformations are
presented. Key metrical parameters are listed in Table 2. The
shown in Figure 5. In all three cases, the angles defined by the
structures of 7‚(toluene)4, 7‚(CHCl3)2, and 9‚(CH2Cl2)2.5 are
P-Pt-P/Pt planes were near 0° (0.2°-12.9°).
9
J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007 8289

A R T I C L E S
Stahl et al.
Figure 6. Structures of the two independent Pt(CtC)12Pt complexes in crystalline 11‚(CH2Cl2)0.67 with solvate molecules omitted: AA, thermal ellipsoid
plot (50% probability level) of the double-helical complex with hydrogen atoms omitted; BB, CC, views parallel and perpendicular to the planes of the C6F5
ligands with atoms at van der Waals radii; DD, EE, FF, analogous views of the nonhelical complex.
As would be expected from the shorter sp3 carbon chains,
7‚(toluene)4 and 7‚(CHCl3)2 crystallized in nonhelical conforma-
tions. The sp3 chains of the former laterally shielded the sp chain,
whereas those of the latter bowed in similar directions, exposing
more of the sp chain. The average sp/sp3 distance in the former
(3.932 Å) was shorter than that in the double-helical complexes;
that in the latter (3.989 Å) was comparable to the double-helical
complexes. However, 7‚(CHCl3)2 was more dense (1.616 vs
1.468 Mg/m3), likely due in part to the heavier solvate molecule.
In 9‚(CH2Cl2)2.5, the longer sp3 chains also bowed in similar
directions, with a somewhat greater average sp/sp3 distance
(4.090 Å).
The dodecahexaynediyl complex 11‚(CH2Cl2)0.67 exhibited
two independent molecules in the unit cell (2:1 ratio). As shown
in Figure 6, the dominant form showed a double-helical
conformation (AA-CC). However, it was less twisted than the
others, with an angle of 163.2° between the P-Pt-P/Pt planes
and a helix pitch of 39.76 Å. Hence, the sp carbon chain is
much less shielded than with the octatetraynediyl complexes
in Figures 1-3. The second form was nonhelical (DD-FF),
with an average sp/sp3 distance much greater than that of the
helical form (4.157 vs 3.868 Å). Metrical data for the first are
given in Table 1, and those for the second are presented in Table
2. From these results, we propose that an sp3/sp carbon ratio of
at least 1.50 is required for a helical conformation.
5. Dynamic and Redox Properties. For each of the above
molecules that could reasonably be expected to adopt a double-
helical conformation, a confirmatory crystal structure has been
obtained. Thus, even though 5 and 11 can also crystallize in
nonhelical conformations, we thought it likely that helical motifs
would be favored in solution and not solely imposed by crystal
lattices. Helical molecules often contain diastereotopic groups,
such as the PAr2 substituents in 3-5. In solution, separate NMR
signals are frequently observed.4b However, when the helix can
9
8290 J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007

sp Carbon Chains within sp3 Carbon Double Helices
A R T I C L E S
Table 4. Cyclic Voltammetry Data
Ep,a
[V]
Ep,c
[V]
E
°
∆E
[mV]
complexa
[V]
ic
/a
2b
3
4
5
7
1.261
1.306
1.259
1.249
1.315
1.326
1.467b
1.465
1.301
1.143
1.224
1.168
1.156
1.179
1.241
1.306
1.372
1.214
1.202
1.265
1.214
1.203
1.247
1.284
1.387
1.418
1.258
118
82
91
93
136
85
161
93
87
0.48
0.78
0.71
0.76
0.10
0.36
-
8
10b
11
6
0.35
0.68
a Conditions: (7-9) × 10-4 M, n-Bu4N+ BF4-/CH2Cl2 at 22.5 ( 1 °C;
Pt working and counter electrodes, potential vs Ag wire pseudoreference;
b
scan rate 100 mV s-1; ferrocene ) 0.46 V. Data from ref 14a.
“untwist” and rewind to an enantiomer, net exchange occurs.
When this is rapid on the NMR time scale, only a single signal
is observed.
Room-temperature NMR spectra of 3-5 showed only a single
signal for all potentially diastereotopic atoms or groups. Thus,
numerous low-temperature spectra were recorded, some of
which are depicted elsewhere.26b However, even at the lowest
feasible temperatures, separate signals could not be observed
(3, CD2Cl2 (-40 °C; solubility limit), toluene-d8 (-90 °C; 13
C
signals i/p to P); 4, THF-d8 (-120 °C; 13C signals i/p to P;
CH3 1H and 13C signals); 5, THF-d8 (-120 °C; 13C signals i/p
to P; C(CH3)3 1H and 13C signals). As analyzed further below,
we interpret this as indicating a low barrier, e12 kcal/mol, for
interconverting enantiomeric double helices.
In order to characterize the redox properties of the preceding
complexes, cyclic voltammograms were recorded in CH2Cl2.
The conditions were identical to those used previously for
diplatinum complexes such as 2 and 10.14a Data are summarized
in Table 4, and typical traces are compared in Figure 7.
Oxidations, presumably to the corresponding mixed-valent
radical cations,22 were always observed, but no reductions took
place prior to the solvent-induced limit.37 Although the oxida-
tions were not chemically or electrochemically reversible, the
degree of reversibility, as reflected by the ic/a and ∆E values,
was greater for the helical complexes. This is readily seen in
the pairs of traces in Figure 7. Although the anodic currents for
3 and 4 are less than or equal to those of the nonhelical model
compounds 6 and 2, their cathodic currents are significantly
greater, indicative of longer-lived radical cations.
Figure 7. Comparison of the cyclic voltammograms of double-helical
complexes 3 and 4 with those of model compounds 6 and 2.
route in Scheme 2, using alkene metathesis/hydrogenation
sequences. The resulting samples show no tendency to oligo-
merize.
It is also not obvious why systems with p-tolyl-substituted
platinum endgroups (1, Scheme 3) are more disposed toward
oligomerization than those with pentafluorophenyl-substituted
end groups (2, 10; Schemes 4, 5). This may reflect subtle
differences in the relative rate constants for the initial phosphine
substitution step and the rate-determining step for oligomeriza-
tion. However, deeper insight will require mechanistic investiga-
tions beyond the scope of this study. The use of Pp-tol3 ligands
in 2 and 10 as opposed to the PPh3 ligands in 1 gives somewhat
more soluble educts, in which the phosphines remain weakly
basic enough to be displaced by the monophosphine PPh2(n-
octyl) (leading to 6)30 and the electronically similar diphosphines
Ph2P(CH2)mPPh2.
In principle, the reactions in Schemes 3-5 could also give
isomers of the type H′ (Scheme 2), which contain trans-spanning
diphosphine ligands. However, attempts to generate similar
monoplatinum species via substitution reactions proceed in very
low yields and give much oligomer.34 Authentic samples have
been synthesized by another route24 and do not readily
equilibrate with the title molecules.
Discussion
1. Syntheses. Our route to the title molecules (Schemes 4,
5) can be viewed as a one-dimensional analogue of the
coordination-driven self-assembly processes used by Stang and
Fujita to access two- and three-dimensional polypalladium and
polyplatinum complexes with other types of rigid spacers.38
However, as noted above, success is capricious. In some cases,
oligomerization appears to be competitive; in other cases, the
target molecules appear to form but oligomerize upon attempted
workup. As described in the following paper,24 many complexes
that cannot be accessed are easily prepared by the alternative
2. Spectroscopic and Redox Properties. The IR and UV-
visible spectra of diplatinum polyynediyl complexes trans,trans-
(Ar′)(Ar3P)2Pt(CtC)nPt(PAr3)2(Ar′) have been extensively ana-
lyzed in previous papers.14 As noted above, the termini-spanning
diphosphine ligands in 3-9 and 11 have no detectable effect.
Given the magnetic anisotropy and deshielding effects that
(37) Reduction of the Pt(CtC)8Pt homologue of 2 and 10 can be observed in
THF, which has a wider cathodic window. However, species with shorter
sp carbon chains exhibit no reduction.
(38) (a) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100, 853. (b)
Fujita, M.; Tominaga, M.; Hori, A.; Therrien, B. Acc. Chem. Res. 2005,
38, 369.
9
J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007 8291

A R T I C L E S
Stahl et al.
Scheme 6. Conformational Changes Required for the
Interconversion of Enantiomers of the Double-Helical Complexes
of chiral conformational moieties in the title molecules, the
helical chirality is derived from these gauche segments. In order
for the enantiomers to interconvert, each gauche segment must
invert. One would expect the lowest energy pathway to involve
“untwisting” to an anti conformation and then continued twisting
to a gauche conformation of the opposite chirality. Only in
complexes with longer sp3 chains would “belt-tightening” syn
transition states be conceivable.
One question is whether there are any “conserved regions”
in the Pt(CtC)4Pt complexes with Ar2P(CH2)14PAr2 bridges.
As noted above and reflected by the torsion angles in Table 3,
the structures of 3‚(benzene)1.5 and 3‚(toluene)1.5 are very
similar. One sp3 chain exhibits an identical pattern of four
gauche and nine anti segments (angles given on left). The other
exhibits three mismatches, with three and four gauche segments,
respectively. In neither molecule are the gauche/anti patterns
of the two sp3 chains identical or symmetrical about the
midpoint. In contrast, the sp3 chains of 5‚(toluene)5.5 exhibit
identical patterns (four gauche, nine anti) that are symmetrical
about the midpoint. The sp3 chains in 4 are very similar, with
just one anti/gauche mismatch.
characterize CtC linkages,39 we had thought that NMR
chemical shifts of the sp3 carbon chains might reflect average
distances from the sp carbon chain. However, DFT calculations
show that when an sp3 carbon atom is moved from 3.5 Å to 5.5
Å away from the midpoint of an octatetrayne in a perpendicular
plane, the 13C NMR chemical shift varies negligibly.40
The thermal stabilities of 3-9 and 11 are also comparable
to analogues without termini-spanning diphosphine ligands.
However, the redox properties show several interpretable
differences. Oxidations of other diplatinum(II) complexes with
bridging unsaturated ligands afford mixed valent platinum(II)/
platinum(III) radical cations.22 Furthermore, there is good
evidence that platinum(III) species often decompose via solvent
interaction with the vacant axial coordination site. Stabilities
are enhanced when the axial position is sterically shielded.22
Importantly, the sp3 carbon chains in the title molecules
markedly increase axial shielding (compare Q and S vs M in
Figure 3).
All of the CH2CH2CH2PAr2 segments in these four molecules
exhibit anti conformations, which avoid 1,5-synperiplanar
interactions42 with PAr2 groups. With a single exception, the
immediately adjacent CH2CH2CH2CH2 segments exhibit gauche
conformations, representing the onset of helicity. In 4 and 5‚
(toluene)5.5, the anti CH2CH2CH2CH2 segments are grouped in
the middle of the chain, leading to the motif evident in S in
Figure 3. In all of the sp3 chains except one of 3‚(benzene)1.5
,
Consider the cyclic voltammetry data for 3 in Table 4. It
undergoes the most reversible oxidation of the Pt(CtC)4Pt
complexes, as reflected by the ic/a and ∆E values. Complexes
4 and 5 are comparable; their E° values are slightly less positive,
indicating thermodynamically more favorable oxidations, con-
sistent with the electron-releasing p-alkyl PAr2 substituents. In
going from 3 to 6, the sp3 chains are replaced by n-octyl groups,
making the sp carbon chains and axial positions on platinum
more accessible. The E° values of 3 and 6 are quite close (1.265
vs 1.258 V), but the ∆E and ic/a values (82 vs 87 mV; 0.78 vs
0.68) indicate a loss of reversibility, as highlighted in the
overlaid traces in Figure 7 (top).
In going from 4 to 2, the sp3 chains are replaced by p-tolyl
substituents. The ∆E and ic/a values (91 vs 118 mV; 0.71 vs
0.48) now indicate a substantial loss of reversibility (see Figure
7, bottom). In 7 and 8, the shorter sp3 chains do not shield the
Pt(CtC)4Pt moiety as extensively. Also, some ring strain may
be introduced. Accordingly, reversibilities drop dramatically (ic/a
0.10 and 0.36). In accord with MO analyses,41 complexes with
longer sp chains are thermodynamically more difficult to
oxidize14,16 and furthermore show much poorer reversibilities.
Both trends are evident with the Pt(CtC)6Pt complexes 10 and
11. However, the sterically shielded complex again exhibits
superior reversibility.
the gauche torsion angles have the same sign, indicating the
same helical chirality.
In the nonhelical structure 5‚(toluene)2, the sp3 chain must
connect two phosphorus atoms that are syn as opposed to anti.
Hence, the distance that must be spanned is shorter. Accordingly,
the sp3 chains in 5‚(toluene)2 are more kinked, with five gauche
segments each. Since this structure has a center of symmetry,
the chains are identical, but the gauche segments are asym-
metrically distributed. In the nonhelical complexes with shorter
sp3 chains (Figure 5), the distances that must be spanned per
methylene group are greater. All exhibit fewer gauche segments;
in the nondisordered structure 7‚(CHCl3), there is only one in
each chain (Table s2, Supporting Information).
In both the helical and nonhelical molecules in 11‚(CH2-
Cl2)0.67, the CH2CH2CH2PAr2 and immediately adjacent CH2-
CH2CH2CH2 segments exhibit anti and gauche conformations,
respectively. Each sp3 chain features 10-12 anti and 5-7
gauche conformations, the latter of both signs, as tabulated in
the Supporting Information. In one sp3 chain of the helical form,
the pattern is symmetrical about the midpoint; the other contains
two mismatches. There is no discernible pattern in the nonhelical
form, in which the chains are symmetry equivalent. Although
the angle defined by the P-Pt-P/Pt planes in the helical form
(163.2°) is only ca. 15% less than that with 3-5, the much
greater platinum-platinum distance results in a much longer
helix pitch (39.76 vs 23.66-24.45 Å).
3. Crystal Structures. Of the structural features described
above, the conformations of the CH2CH2CH2CH2 and CH2CH2-
CH2PAr2 moieties are particularly deserving of further analysis.
As illustrated in Scheme 6, gauche and anti geometries are
possible; the former are chiral. Although there are other types
4. Solution Structures and Dynamic Properties. The
crystallographic data for the Pt(CtC)4Pt complexes with
Ar2P(CH2)14PAr2 bridges suggest that they exist as equilibrium
(39) Wannere, C. S.; Schleyer, P. v. R. Org. Lett. 2003, 5, 605.
(40) Jiao, H. Unpublished results, Universita¨t Erlangen-Nu¨rnberg.
(41) Zhuravlev, F.; Gladysz, J. A. Chem.sEur. J. 2004, 10, 6510.
(42) Hoffmann, R. W. Angew. Chem., Int. Ed. 2000, 39, 2054; Angew. Chem.
2000, 112, 2134.
9
8292 J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007

sp Carbon Chains within sp3 Carbon Double Helices
A R T I C L E S
mixtures of at least several chiral double-helical and formally
achiral nonhelical conformations in solution. As analyzed above
and below, we believe that the former should dominate.
However, we have been unable to establish a chiral ground state,
which would feature diastereotopic aryl groups, by low-
temperature NMR. This in turn raises several questions.
First, might the double-helical conformations in Figures 1-3
reflect some deep-seated lattice packing preference? For ex-
ample, diplatinum polyynediyl complexes trans,trans-(X)(R3P)2-
Pt(CtC)nPt(PR3)2(X) (n g 3; R ) alkyl, Ar; X ) Ar′, Cl)
always crystallize with essentially coplanar endgroups,14,25b,43
even though DFT calculations show no electronic driving
force.41 Since a packing driving force would be difficult to
prove, we have focused on structural modifications that might
raise the barrier for interconverting enantiomeric helical ground
states.
interactions or complementarity between the two helical chains.
The sp carbon chain provides a mechanical support, without
which the helical conformation would not be energetically
competitive in solution or the solid state. Although the
comparison is unflattering in the nanotechnology era, our
complexes can be viewed as molecular analogues of beanpoles,
implements used by the most ancient agrarian societies to
template upwardly spiraling vines.
Comparable molecules are challenging to identify. Distant
relatives would include biphenyls in which the o/o and o′/o′
positions are bridged by sp3 carbon chains. Systems with as
many as four methylene groups have been described.47 However,
the helix strands are splayed outward from the biphenyl axis.
Future studies of “carbo-mer” or (CtC)-expanded analogues48
would be worthwhile and may yield structures resembling those
in Figures 1-3.
Several components to this barrier can be identified. First,
as noted above, there are torsional motions involving the CH2-
CH2CH2PAr2 and CH2CH2CH2CH2 segments and other portions
of the molecule. These would occur both sequentially and in
concert. The activation energy for the conversion of gauche- to
anti-butane (see Scheme 6) is ca. 2.7 kcal/mol. If the rate-
determining step featured one such energy maximum in each
sp3 chain, the conformation of which would logically be derived
from a local minimum higher than the ground state, the barrier
should significantly surpass 5.4 kcal/mol. Second, there should
be attractive van der Waals and possibly CH/π interactions
between the sp and sp3 chains. These should be maximized in
helical conformations and diminished in the transition state for
the rate-determining step.
Thus, higher barriers would be expected for complexes with
longer sp and sp3 chains, which should have greater van der
Waals contact surfaces. This possibility is explored in the
following paper.24 Another approach would be to decrease the
sp chain length, such that endgroup-endgroup interactions
destabilize the achiral planar ground state, analogous to the
situation in ortho-substituted biaryls. Efforts in this direction
have also been initiated.25b Alternatively, functional groups could
be introduced on the sp3 chain, which might enhance attractive
interactions with the sp chain. Recently, similar complexes with
ethereal diphosphines have been reported.44
There is an extensive literature of single helical molecules,1
some of which are particularly relevant to the title molecules.
Rebek has demonstrated that when guests with longer (CH2)m
segments bind within cavitands or container molecules of
appropriate diameters and/or lengths, helical conformations are
induced.49 This can be viewed as an “external” templating, in
contrast to our “internal” beanpole template. Both we and Hirsch
have prepared polyynes in which the termini are joined by a
single (CH2)m tether.50,51 However, no structural data are so far
available.
Although we could not obtain barriers for the interconversion
of the enantiomers of the helical molecules in Figures 1-3, the
values would represent a measure of the sp carbon chain
protection, analogous to the way insulation for electrical wire
is graded. A correlation to the extra activation energy required
for attack upon the sp carbon chains, versus model compounds
such as 6, would be expected. However, from the standpoint of
engineering functional molecular devices, a portfolio of flexible
bridges with a gradation of barriers would be optimal.
6. Conclusion
This study has demonstrated the accessibility of an unprec-
edented new class of double-helical molecules in which two
sp3 carbon chains wrap around an sp carbon chain via a one-
step coordination-driven self-assembly process. In contrast to
other double-helical molecules, there is an absence of bonding
interactions or complementarity between the strands. There are,
however, synergies between the chains. The sp chain provides
a mechanical support, without which helical conformations
would not be energetically competitive. The sp3 chain in turn
serves as a protecting group, enhancing the stabilities of mixed-
valent radical cations. There is an obvious analogy to household
insulated wire. Alternative routes to this class of molecules, and
a variety of isomers and derivatives, will be detailed in upcoming
full papers.24,44b
The gain that might be associated with the first strategy is
illustrated by data of Moore. He measured the binding energies
of rodlike guests in the inner cavities of single-helical foldamer
hosts.45 These increased with increasing lengths of the hosts,
up to the point where that of the guest molecule was exceeded.
Nonetheless, none of these approaches or others46 have yet led
to a complex for which a chiral ground state can be demon-
strated. Hence, the dominant conformation of the title molecules
in solution remains an unsettled question.
5. Further Analyses of Helicity. The preceding issues aside,
the title molecules are unique in the absence of bonding
(43) Zheng, Q.; Hampel, F.; Gladysz, J. A. Organometallics 2004, 23, 5896.
(44) (a) de Quadras, L.; Hampel, F.; Gladysz, J. A. Dalton Trans. 2006, 2929.
(b) de Quadras, L.; Bauer, E. B.; Stahl, J.; Zhuravlev, F.; Hampel, F.;
Gladysz, J. A. New J. Chem. 2007. Manuscript submitted for publication.
(45) (a) Tanatani, A.; Mio, M. J.; Moore, J. S. J. Am. Chem. Soc. 2001, 123,
1792. (b) Stone, M. T.; Heemstra, J. M.; Moore, J. S. Acc. Chem. Res.
2006, 39, 11.
(46) Helical structures might have greater inversion barriers in more polar
solvents. However, these tend to have higher freezing points, hampering
low temperature measurements. Also, since a positive ∆Vq value would be
expected, rates should slow under pressure.
(47) Mu¨llen, K.; Heinz, W.; Kla¨rner, F.-G.; Roth, W. R.; Kindermann, I.;
Adamczak, O.; Wette, M.; Lex, J. Chem. Ber. 1990, 123, 2349.
(48) Maraval, V.; Chauvin, R. Chem. ReV. 2006, 106, 5317.
(49) (a) Trembleau, L.; Rebek, J., Jr. Science 2003, 301, 1219. (b) Scarso, A.;
Trembleau, L.; Rebek, J., Jr. Angew. Chem., Int. Ed. 2003, 42, 5499; Angew.
Chem. 2003, 115, 5657.
(50) (a) Horn, C. R.; Mart´ın-Alvarez, J. M.; Gladysz, J. A. Organometallics
2002, 21, 5386. (b) Horn, C. R.; Gladysz, J. A. Eur. J. Inorg. Chem. 2003,
9, 2211.
(51) Klinger, C.; Vostrowsky, O.; Hirsch, A. Eur. J. Org. Chem. 2006, 1508.
9
J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007 8293

A R T I C L E S
Stahl et al.
PtCtC), 63.5 (s, PtCtCC), 57.8 (s, PtCtCCtC), 30.7 (virtual t, 3JCP
) 7.8 Hz, PCH2CH2CH2), 30.0 (s, CH2), 29.6 (s, CH2), 29.4 (s, CH2),
28.3 (virtual t, 1JCP ) 18.4 Hz, PCH2), 28.0 (s, CH2), 25.5 (s, PCH2CH2),
Experimental Section
trans,trans-(C6F5)(Ph2P(CH2)14PPh2)Pt(CtC)4Pt(Ph2P(CH2)14PPh2)-
1
21.2 (s, CH3); 31P{1H} 13.5 (s, JPPt ) 2562 Hz).55 IR (cm-1, powder
(C6F5) (3). A Schlenk flask was charged with trans,trans-(C6F5)(p-
tol3P)2Pt(CtC)4Pt(Pp-tol3)2(C6F5) (2;14a 0.204 g, 0.100 mmol) and
CH2Cl2 (40 mL). A solution of Ph2P(CH2)14PPh2 (0.150 g, 0.265
mmol)27 in CH2Cl2 (20 mL) was passed through a 2 cm silica gel pad
directly into the Schlenk flask with stirring. After 2 h, the solvent was
removed by oil pump vacuum, and ethanol (30 mL) was added. The
yellow solid was collected by filtration, washed with ethanol (2 × 10
mL), and dried by oil pump vacuum to give 3 (0.170 g, 0.087 mmol,
87%), mp >245 °C dec. Calcd for C96H96F10P4Pt2: C, 59.01; H, 4.95.
Found: C, 59.48; H, 5.27. DSC: endotherm with Ti, 181.6 °C; Te,
244.0 °C; Tp, 263.7 °C; Tc, 270.3 °C; Tf, 270.2 °C; exotherm with Ti,
271.0 °C.52 TGA: onset of mass loss (Ti), 286.0 °C.
film): νCtC 2146 (m), 2003 (w). MS:57 2066 (4+, 100%), 1973 ([4-
C6F5-H2]+, 2%), 1897 ([4-2C6F5-H2]+, 4%), no other significant
peaks >850.
trans,trans-(C6F5)((p-t-BuC6H4)2P(CH2)14P(p-C6H4-t-Bu)2)Pt(Ct
C)4Pt((p-t-BuC6H4)2P(CH2)14P(p-C6H4-t-Bu)2)(C6F5) (5). Complex 2
(0.153 g, 0.075 mmol), CH2Cl2 (50 mL), and a solution of (p-t-
BuC6H4)2P(CH2)14P(p-C6H4-t-Bu)2 (0.149 g, 0.188 mmol)30 in CH2Cl2
(94 mL) were combined in a procedure analogous to that for 3. An
identical workup gave 5 as a yellow solid (0.138 g, 0.057 mmol, 77%),
mp >210 °C dec. Calcd for C128H160F10P4Pt2: C, 63.99; H, 6.71.
Found: C, 64.72; H, 6.92.
1
NMR (δ, CDCl3) H 7.40 (m, 16H of 8 Ph), 7.28 (m, 8H of 8 Ph),
7.19 (m, 16H of 8 Ph), 2.68 (m, 8H, PCH2), 2.15 (m, 8H, PCH2CH2),
NMR (δ, CDCl3) 1H 7.30 (m, 16H, o to P), 7.17 (m, 16H, m to P),
2.61 (m, 8H, PCH2), 2.20 (m, 8H, PCH2CH2), 1.63-1.49 (m, 8H, PCH2-
CH2CH2), 1.46-1.25 (m, 32H, remaining CH2), 1.23 (s, 72H, CH3);
13C{1H}53,54 153.3 (s, p to P), 145.5 (dd, 1JCF ) 224 Hz, 2JCF ) 24 Hz,
1.63-1.47 (m, 8H, PCH2CH2CH2), 1.46-1.22 (m, 32H, remaining
CH2); 13C{1H}53,54 145.6 (dd, JCF ) 223 Hz, JCF ) 22 Hz, o to Pt),
136.3 (dm, 1JCF ) 240 Hz, m/p to Pt), 132.8 (virtual t, 2JCP ) 5.7 Hz,
o to P), 132.0 (virtual t, 1JCP ) 27.9 Hz, i to P), 130.1 (s, p to P), 127.8
1
2
o to Pt), 136.0 (dm, 1JCF ) 248 Hz, m/p to Pt), 132.6 (virtual t, 2JCP
)
3
1
(virtual t, JCP ) 5.0 Hz, m to P), 98.6 (s, JCPt ) 960 Hz,55 PtCt),
5.8 Hz, o to P), 128.8 (virtual t, 1JCP ) 28.7 Hz, i to P), 124.7 (virtual
2
94.2 (s, JCPt ) 266 Hz,55 PtCtC), 63.4 (s, PtCtCC), 57.9 (s, PtCt
3
t, JCP ) 5.1 Hz, m to P), 99.1 (s, PtCt), 94.4 (s, PtCtC), 63.7 (s,
3
CCtC), 30.6 (virtual t, JCP ) 8.0 Hz, PCH2CH2CH2), 30.0 (s, CH2),
PtCtCC), 57.5 (s, PtCtCCtC), 34.7 (s, C(CH3)3), 31.1 (s, CH3), 30.6
1
29.5 (s, CH2), 29.3 (s, CH2), 28.1 (virtual t, JCP ) 18.3 Hz, PCH2),
(virtual t, 3JCP ) 7.9 Hz, PCH2CH2CH2), 30.4 (s, CH2), 29.9 (s, CH2),
28.0 (s, CH2), 25.6 (s, PCH2CH2); 31P{1H} 14.9 (s, 1JPPt ) 2582 Hz).55
IR (cm-1, powder film): νCtC 2148 (m), 2007 (w). UV-vis:56 265
(72 800), 291 (86 000), 318 (109 000), 353 (7200), 379 (4800), 412
(2400). MS:57 1954 (3+, 100%), 1784 ([3-C6F5-H2]+, 2%), no other
significant peaks >1300.
1
29.5 (s, CH2), 28.3 (virtual t, JCP ) 18.3 Hz, PCH2), 27.9 (s, CH2),
1
25.7 (s, PCH2CH2); 31P{1H} 12.7 (s, JPPt ) 2561 Hz).55 IR (cm-1
,
powder film): νCtC 2146 (m), 2003 (w). UV-vis:56 292 (114 000),
318 (154 000), 351 (8000), 379 (8000), 410 (4000). MS:57 2402 (5+,
100%), no other significant peaks >975.
trans,trans-(C6F5)(p-tol2P(CH2)14Pp-tol2)Pt(CtC)4Pt(p-tol2P-
(CH2)14Pp-tol2)(C6F5) (4). Complex 2 (0.153 g, 0.075 mmol), CH2Cl2
trans,trans-(C6F5)(Ph2P(CH2)10PPh2)Pt(CtC)4Pt(Ph2P(CH2)10PPh2)-
(C6F5) (7). Complex 2 (0.153 g, 0.075 mmol), CH2Cl2 (75 mL), and a
solution of Ph2P(CH2)10PPh2 (0.093 g, 0.180 mmol)27,28 in CH2Cl2 (180
mL) were combined in a procedure analogous to that for 3. An identical
workup gave 7 as a yellow solid (0.111 g, 0.060 mmol, 80%), mp
>200 °C dec. DSC: endotherm with Ti, 222.9 °C; Te, 258.1 °C; Tp,
271.5 °C; Tc, 279.1 °C; Tf, 278.9 °C; exotherm with Ti, 279.1 °C.52
TGA: onset of mass loss (Ti), 264 °C.
(40 mL), and a solution of p-tol2P(CH2)14Pp-tol2 (0.120 g, 0.120 mmol)30
in CH2Cl2 (20 mL) were combined in a procedure analogous to that
for 3. An identical workup gave 4 as a yellow solid (0.140 g, 0.068
mmol, 91%), mp >210 °C dec. Calcd for C104H112F10P4Pt2: C, 60.46;
H, 5.46. Found: C, 60.42; H, 5.60. DSC: endotherm with Ti, 196.2
°C; Te, 213.2 °C; Tp, 242.2 °C; Tc, 255.6 °C; Tf, 259.3 °C; exotherm
with Ti, 260.0 °C.52 TGA: onset of mass loss (Ti), 260.3 °C.
1
NMR (δ, CDCl3) H 7.38 (m, 16H of 8 Ph), 7.29 (m, 8H of 8 Ph),
NMR (δ, CDCl3) 1H 7.27 (m, 16H, o to P), 6.99 (m, 16H, m to P),
2.63 (m, 8H, PCH2), 2.29 (s, 24H, CH3), 2.10 (m, 8H, PCH2CH2), 1.60-
1.47 (m, 8H, PCH2CH2CH2), 1.46-1.21 (m, 32H, remaining CH2);
7.21 (m, 16H of 8 Ph), 2.71 (m, 8H, PCH2), 2.01 (m, 8H, PCH2CH2),
1.58-1.47 (m, 8H, PCH2CH2CH2), 1.40-1.25 (m, 16H, remaining
1
1
CH2); 13C{1H}53,54 145.8 (d, JCF ) 227 Hz, o to Pt), 136.4 (dm, JCF
1
2
13C{1H}53,54 145.7 (dd, JCF ) 222 Hz, JCF ) 22 Hz, o to Pt), 140.3
(s, p to P), 136.3 (dm, 1JCF ) 250 Hz, m/p to Pt), 132.8 (virtual t, 2JCP
) 6.0 Hz, o to P), 128.7 (virtual t, 1JCP ) 29.4 Hz, i to P), 128.5 (virtual
2
) 239 Hz, m/p to Pt), 132.8 (virtual t, JCP ) 5.8 Hz, o to P), 131.9
1
(virtual t, JCP ) 27.5 Hz, i to P), 130.3 (s, p to P), 127.9 (virtual t,
3JCP ) 5.1 Hz, m to P), 101.6 (s, PtCt), 94.0 (s, PtCtC), 64.4 (s,
PtCtCC), 59.3 (s, PtCtCCtC), 32.2 (virtual t, 3JCP ) 7.6 Hz, PCH2-
CH2CH2), 30.7 (s, CH2), 29.9 (s, CH2), 29.2 (virtual t, 1JCP ) 18.3 Hz,
3
2
t, JCP ) 5.5 Hz, m to P), 99.2 (s, PtCt), 94.1 (s, JCPt ) 269 Hz,
PCH2), 26.4 (s, PCH2CH2); 31P{1H} 14.4 (s, JPPt ) 2572 Hz).55 IR
(52) (a) DSC and TGA data were treated as recommended by Cammenga, H.
K.; Epple, M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1171; Angew. Chem.
1995, 107, 1284. The Te values best represent the temperature of the phase
transition or exotherm. (b) Except in cases of desolvation, DSC measure-
ments were not continued beyond the onset of mass loss (TGA).
(53) (a) In most 13C NMR spectra, the i-C6F5 signal was not observed. (b) For
virtual triplets (Hersh, W. H. J. Chem. Educ. 1997, 74, 1485), the J values
represent the apparent couplings between adjacent peaks. (c) The PtCt
CCtC signals were assigned according to trends established earlier.14
(54) Complexes with PtPCH2CH2CH2 linkages exhibit a characteristic pattern
of 13C NMR signals. A 1H,1H COSY spectrum of 4 was used to make
definitive assignments of the corresponding 1H signals, and a 1H,13C COSY
spectrum in turn gave definitive assignments of the 13C signals. Analogous
experiments have been conducted with closely related compounds.34
(55) This coupling represents a satellite (d; 195Pt ) 33.8%) and is not reflected
in the peak multiplicity given.
1
(cm-1, powder film): νCtC 2138 (m), 1996 (w). MS:57 1841 (7+, 100%),
1763 ([7-C6H5-H]+, 2%), 1673 ([7-C6F5-H]+, 4%), no other
significant peaks >900.
trans,trans-(C6F5)(Ph2P(CH2)11PPh2)Pt(CtC)4Pt(Ph2P(CH2)11PPh2)-
(C6F5) (8). Complex 2 (0.153 g, 0.075 mmol), CH2Cl2 (75 mL), and a
solution of Ph2P(CH2)11PPh2 (0.094 g, 0.180 mmol)27,28 in CH2Cl2 (180
mL) were combined in a procedure analogous to that for 3. An identical
workup gave 8 as a yellow solid (0.113 g, 0.060 mmol, 80%), mp
>220 °C dec. DSC: endotherm with Ti, 188.5 °C; Te, 225.1 °C; Tp,
244.1 °C; Tc, 251.8 °C; Tf, 254.1 °C; exotherm with Ti, 256 °C.52
TGA: onset of mass loss (Ti) 267 °C.
(56) UV-visible spectra were recorded in CH2Cl2 (1.25 × 10-6 M). Absorptions
are in nm (ꢀ, M-1 cm-1).
(57) FAB (3-nitrobenzyl alcohol matrix); m/z for the most intense peak of the
isotope envelope.
1
NMR (δ, CDCl3) H 7.39 (m, 16H of 8 Ph), 7.29 (m, 8H of 8 Ph),
7.21 (m, 16H of 8 Ph), 2.71 (m, 8H, PCH2), 2.09 (m, 8H, PCH2CH2),
(58) Additional data are available from the Cambridge Crystallographic Data
Centre via the following CCDC numbers: 171482 (3‚(toluene)1.5), 176209
(3‚(benzene)1.5), 171483 (4), 171486 (5‚(toluene)5.5), 606935 (5‚(toluene)2),
171485 (7‚(toluene)4), 171484 (7‚(CHCl3)2), 606936 (9‚(CH2Cl2)2.5), 171481
(11‚(CH2Cl2)0.67).
1.56-1.23 (m, 28H, remaining CH2); 13C{1H}53,54 145.8 (d, 1JCF ) 228
1
Hz, o to Pt), 136.4 (dm, JCF ) 247 Hz, m/p to Pt), 132.9 (virtual t,
1
2JCP ) 5.6 Hz, o to P), 131.9 (virtual t, JCP ) 27.7 Hz, i to P), 130.3
9
8294 J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007

sp Carbon Chains within sp3 Carbon Double Helices
A R T I C L E S
3
(s, p to P), 127.9 (virtual t, JCP ) 5.1 Hz, m to P), 99.4 (s, PtCt),
with ethanol. The filtrate was stored at -18 °C. Yellow needles formed
overnight and were collected by filtration, washed with ethanol (5 mL),
and dried by oil pump vacuum to give 11‚(CH2Cl2)0.67 (0.029 g, 0.014
mmol, 27%), mp >230 °C dec. Calcd for C108H112F10P4Pt2‚(CH2-
Cl2)0.67: C, 60.13; H, 5.26. Found: C, 59.40; H, 5.25. DSC: desolvation
endotherm with Ti, 45.3 °C; Te, 71.4 °C; Tp, 99.5 °C; Tc, 112.9 °C; Tf,
131.2 °C; exotherm with Ti, 172.9 °C; Te, 190.9 °C; Tp, 203.1 °C; Tc,
211.8 °C; Tf, 215.2 °C; endotherm with Ti, 215.3 °C; Te, 227.9 °C; Tp,
241.1 °C; Tc, 246.1 °C; Tf, 246.1 °C.52 TGA: mass loss 1.9% between
60 °C and 102 °C (calculated for (CH2Cl2)0.67, 1.7%); onset of further
mass loss (Te), 236.3 °C.
93.7 (s, PtCtC), 63.4 (s, PtCtCC), 58.5 (s, PtCtCCtC), 31.3 (virtual
t, 3JCP ) 7.9 Hz, PCH2CH2CH2), 30.8 (s, CH2), 30.4 (s, CH2), 29.4 (s,
1
CH2), 28.7 (virtual t, JCP ) 18.0 Hz, PCH2), 26.3 (s, PCH2CH2);
31P{1H} 14.6 (s, JPPt ) 2576 Hz).55 IR (cm-1, powder film): νCtC
1
2146 (m), 2003 (w). UV-vis:56 275 (76 000), 291 (118 400), 316
(148 800), 352 (9600), 379 (8800), 410 (4800). MS:57 1869 (8+, 100%),
1791 ([8-C6F5]+, 2%), 1701 ([8-2C6F5]+, 6%), no other significant
peaks >825.
trans,trans-(C6F5)(Ph2P(CH2)12PPh2)Pt(CtC)4Pt(Ph2P(CH2)12PPh2)-
(C6F5) (9). Complex 2 (0.103 g, 0.050 mmol), CH2Cl2 (100 mL), and
a solution of Ph2P(CH2)12PPh2 (0.065 g, 0.12 mmol)27,28 in CH2Cl2 (120
mL) were combined in a procedure analogous to that for 3. After 14 h,
the solvent was removed by oil pump vacuum. The residue was
chromatographed (25 cm silica gel column, 30:70 v/v CH2Cl2/hexanes).
The solvent was removed from the product-containing fraction by rotary
evaporation and oil pump vacuum to give 9 as a yellow solid (0.034 g,
0.018 mmol, 36%), mp >210 °C dec.
1
NMR (δ, CDCl3) H 7.38 (m, 16H of 8 Ph), 7.29 (m, 8H of 8 Ph),
7.21 (m, 16H of 8 Ph), 5.28 (s, CH2Cl2), 2.69 (m, 8H, PCH2), 2.08 (m,
8H, PCH2CH2), 1.52 (m, 8H, PCH2CH2CH2), 1.36 (m, 8H, PCH2CH2-
CH2CH2), 1.21 (m, 40H, remaining CH2); 13C{1H}53,54 143.8 (dd, 1JCF
2
1
) 225 Hz, JCF ) 30 Hz, o to Pt), 136.5 (dm, JCF ) 242 Hz, m/p to
2
1
Pt), 132.8 (virtual t, JCP ) 6.0 Hz, o to P), 131.6 (virtual t, JCP
)
28.2 Hz, i to P), 130.4 (s, p to P), 127.9 (virtual t, 3JCP ) 5.0 Hz, m to
P), 103.8 (s, PtCt), 93.6 (s, PtCtC), 65.0, 62.7, 61.0, 57.3 (4 s, PtCt
1
NMR (δ, CDCl3) H 7.37 (m, 16H of 8 Ph), 7.28 (m, 8H of 8 Ph),
7.19 (m, 16H of 8 Ph), 2.68 (m, 8H, PCH2), 2.04 (m, 8H, PCH2CH2),
3
CCtCCtC), 53.4 (s, CH2Cl2), 30.9 (virtual t, JCP ) 7.9 Hz, PCH2-
CH2CH2), 30.2 (s, CH2), 30.1 (s, CH2), 30.0 (s, CH2), 30.0 (s, CH2),
1.56-1.46 (m, 8H, PCH2CH2CH2), 1.45-1.23 (m, 24H, remaining
28.4 (virtual t, JCP ) 18.2 Hz, PCH2), 25.7 (s, PCH2CH2); 31P{1H}
1
1
2
CH2); 13C{1H}53,54 145.7 (dd, JCF ) 225 Hz, JCF ) 22 Hz, o to Pt),
136.3 (dm, 1JCF ) 240 Hz, m/p to Pt), 132.8 (virtual t, 2JCP ) 6.0 Hz,
o to P), 131.9 (virtual t, 1JCP ) 28.0 Hz, i to P), 130.2 (s, p to P), 127.9
(virtual t, 3JCP ) 5.2 Hz, m to P), 98.8 (s, PtCt), 94.8 (s, 2JCPt ) 272
Hz,55 PtCtC), 63.4 (s, PtCtCC), 58.4 (s, PtCtCCtC), 31.2 (virtual
14.5 (s, JPPt ) 2554 Hz).55 IR (cm-1, powder film): νCtC 2131 (m),
1
2092 (s), 1996 (m). UV-vis:56 312 (79 200), 331 (233 600), 354
(360 800). MS:57 2113 ([11-H]+, 100%), 1945 ([11-C6F5]+, 8%), no
other significant peaks >825.
3
t, JCP ) 7.2 Hz, PCH2CH2CH2), 30.1 (s, 2CH2), 28.5 (s, CH2), 28.4
Acknowledgment. We thank the Deutsche Forschungs-
gemeinschaft (SFB 583), the US NSF (CHE-9732605), Johnson
Matthey (platinum loan), the Ministerio de Educacio´n y Ciencia,
and the Fulbright Commission (Fellowships, J.M.M.-A.) for
support, and Dr. A. M. Arif for one crystal structure.
(virtual t, 1JCP ) 18.4 Hz, PCH2), 25.5 (s, PCH2CH2); 31P{1H} 13.7 (s,
1JPPt ) 2573 Hz).55 IR (cm-1, powder film): νCtC 2142 (m), 1999 (w).
MS:57 1897 (9+, 100%), 1729 ([9-C6F5]+, 6%).
trans,trans-(C6F5)(Ph2P(CH2)18PPh2)Pt(CtC)6Pt(Ph2P(CH2)18PPh2)-
(C6F5) (11). A Schlenk flask was charged with trans,trans-(C6F5)(p-
tol3P)2Pt(CtC)6Pt(Pp-tol3)2(C6F5) (10;13a 0.105 g, 0.050 mmol) and
CH2Cl2 (2.0 L). A solution of Ph2P(CH2)18PPh2 (0.060 g, 0.106 mmol)27
in CH2Cl2 (10 mL) was added over 5 h. The solution was concentrated
by rotary evaporation (100 mL), and ethanol (20 mL) was added. The
solution was further concentrated until a yellow powder precipitated
(oligomers). The powder was removed by filtration (G3 frit) and washed
Supporting Information Available: Details of general meth-
ods including cyclic voltammetry, crystallographic data collec-
tion and refinement,58 and associated tables. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA0716103
9
J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007 8295