A synthetic breakthrough into an unanticipated stability regime: Readily isolable complexes in which C16-C28 polyynediyl chains span two platinum atoms

Article abstract of DOI:10.1021/ja0534598

The oxidative cross-coupling of trans-(p-tol)(p-tol₃P)₂Pt(C_≡C)₂H (PtC₄H) and excess H(C_≡C)₂SiEt₃ (O₂, cat. CuCl, TMEDA, acetone; Hay conditions) gives PtC₈Si (29%), PtC₁₂Si (30%), and PtC₁₆Si (1%; Si = SiEt₃). The less stable PtC₆H is generated in situ from PtC₆Si and n-Bu₄N⁺F^-; following the addition of ClSiMe₃ (F^- scavenger) and excess H(C_≡C)₂SiEt₃, the Hay conditions afford PtC₁₀Si (59%) and PtC₁₄Si (7%). Analogous sequences can be conducted with PtC₈Si and PtC₁₀Si. When PtC₆Si, PtC₈Si, and PtC₁₀Si are similarly reacted in the absence of H(C_≡C)₂SiEt₃, homocouplings to PtC₁₂Pt (88%), PtC₁₆Pt (70%), and PtC₂₀Pt (72%) occur. However, analogous reactions of PtC₁₂Si and PtC₁₄Si fail, presumably due to the rapid decomposition of PtC₁₂H and PtC₁₄H. Bronsted acidity trends suggest that (C_≡C)_n moieties should become better leaving groups with increasing chain length. Thus, when PtC₁₂Si and PtC₁₄Si are subjected to the Hay conditions alone, PtC₂₄Pt (36%) and PtC₂₈Pt (51%) are isolated, with substantial recovery of starting material from the former reaction. Presumably adventitious water and/or other nucleophiles effect desilylation (as also seen in the cross-couplings), after which homocoupling is rapid due to the simultaneous availability of an oxidizing agent. PtC₂₄Pt and PtC₂₈Pt are thermally stable to ≥140 °C, whereas all other known dodecaynes and tetradecaynes rapidly decompose at room temperature. There is every reason to believe that this series can be further extended. UV-visible spectra show progressively red-shifted and more intense bands with ε > 400000 M-1 cm^-1. Copyright

Full text of DOI:10.1021/ja0534598

Published on Web 07/12/2005
A Synthetic Breakthrough into an Unanticipated Stability Regime: Readily Isolable
Complexes in which C16-C28 Polyynediyl Chains Span Two Platinum Atoms
Qinglin Zheng and J. A. Gladysz*
Institut fu¨r Organische Chemie, Friedrich-Alexander UniVersita¨t Erlangen-Nu¨rnberg, Henkestrasse 42,
91054 Erlangen, Germany
Received May 27, 2005; E-mail: gladysz@chemie.uni-erlangen.de
Scheme 1. Syntheses of Diplatinum Complexesa
Polyynediyl moieties, (CtC)n, are seeing increasing use as
connectors in sophisticated molecular assemblies.1 There are many
applications for which very long segments would be desirable.
However, available data suggest certain stability thresholds. For
example, tert-butyl-capped polyynes Me3C(CtC)nCMe3 with n )
4-8, 10, and 12 have been isolated, but that with n ) 12
decomposed within 8 min at room temperature to an insoluble black
material.2 In the triethylsilyl series, Et3Si(CtC)nSiEt3, species with
n ) 6 and 8 were isolable, but decomposed at room temperature
or below; higher homologues (n ) 10, 12, 16) were only generated
in solution.3 In more recent work, Tykwinski has shown analogues
with bulkier tri(isopropyl)silyl endgroups (n ) 8, 10) to be stable
to g105 °C.4 Hirsch has described two polyyne series with 3,5-
disubstituted aryl endgroups, the highest members of which (n )
8-10) were stable solids at room temperature.5 We have studied
polyyne series with transition metal endgroups.6,7 With the rhenium
fragment (η5-C5Me5)Re(NO)(PPh3), the complex with n ) 10 was
isolable, but noticeably more labile than lower homologues.6 With
the pentafluorophenyl platinum fragment (C6F5)(Pp-tol3)2Pt, only
complexes with n e 8 could be synthesized in quantity. However,
traces of higher homologues with appreciable lifetimes at room
temperature were detected by HPLC.7
a Conditions: (a) wet n-Bu4N+F-, acetone; (b) ClSiMe3; (c) O2, cat.
CuCl, TMEDA, acetone.
Given the earlier studies sketched above, there has been a long-
standing consensus that polyynes with n g 12 should not be stable
as solids at room temperature. However, some of the more recent
data have provided grounds for cautious optimism, provided that
various synthetic problems with carbon chain extension could be
surmounted. In this communication, we report new methodology
that enables previous chain length “records” for stable polyynes to
be shattered. The resulting air-stable, p-tolyl-substituted diplatinum
complexes provide the closest models yet for the elusive one-
dimensional carbon allotrope, carbyne.8 Furthermore, there is every
indication that the stability regime extends considerably beyond
the title compounds.
Scheme 2. Four-Carbon sp Chain Extension Reaction
In earlier work, we described the oxidative cross-coupling of
trans-(p-tol)(p-tol3P)2Pt(CtC)2H (PtC4H) and excess HCtCSiEt3
(O2, cat. CuCl, TMEDA, acetone) to give PtC6Si (Si ) SiEt3).9
Reaction with wet n-Bu4N+F- gave the labile but isolable hexatri-
ynyl complex PtC6H, which could be similarly cross-coupled to
PtC8Si. When PtC4H and PtC6H were analogously reacted in the
absence of HCtCSiEt3, homocoupling to PtC8Pt (74%, yellow)
and PtC12Pt (83%, orange) occurred (Scheme 1, route A). However,
efforts to extend these sequences were unsuccessful. Problems
included (a) the dramatically decreasing stability of PtCxH with
chain length, and (b) an increasing tendency for homocoupling of
PtCxH at the expense of cross-coupling. The same difficulties were
encountered with related pentafluorophenyl platinum complexes.7
The homocoupling rates show an empirical correlation with the
Brønsted acidities, which increase with chain length for terminal
polyynes.10 We therefore wondered whether higher homologues of
HCtCSiEt3, which would be more acidic, would give better cross-
coupling results.
As shown in Scheme 2 (top), an analogous cross-coupling of
PtC4H and the butadiyne H(CtC)2SiEt3 (18 equiv)6,11 was at-
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C O M M U N I C A T I O N S
tempted. Chromatography gave, in inverse order of elution, the
target complex PtC8Si (29%), PtC12Si (30%; orange), and a small
amount of PtC16Si (1%; deep red).12 The simplest rationale for the
formation of the higher homologues would involve the competing
desilylation of PtCxSi under the reaction conditions, a subject
treated further below. Next, PtC6Si and n-Bu4N+F- were combined
(Scheme 2, bottom) to generate PtC6H, which was treated in situ
13
with ClSiMe3 and then H(CtC)2SiEt3 under cross-coupling
conditions. Chromatography gave PtC10Si (59%, orange) and PtC14-
Si (7%, red). Similar smaller-scale sequences with PtC8Si and
PtC10Si gave PtC12Si (42%) and PtC14Si (20%). To our knowledge,
PtC14Si and PtC16Si are the longest polyynes with a single transition
metal endgroup.
Homocouplings of PtCxH were again investigated, but now with
samples that had been generated from PtCxSi and n-Bu4N+F- in
situ and treated with ClSiMe3.13 As shown in Scheme 1 (route B),
reactions of PtC6Si, PtC8Si, and PtC10Si gave PtC12Pt (88%) and
the new complexes PtC16Pt (70%, red) and PtC20Pt (72%, red).12
Note the progressively lower temperatures required (RT, 0 °C, -25
°C). However, similar sequences with PtC12Si (-25 or -45 °C)
gave no tractable products. The rate of decomposition of the
intermediate PtC12H was presumed to be too rapid.
Thus, a modified protocol was investigated. Qualitatively, the
rates of desilylation of PtCxSi appeared to increase with chain
length, in accord with the Cx leaving group abilities expected from
the Brønsted acidity trends noted above.10 Also, desilylation occurs
under the conditions of Scheme 2 (top), in which fluoride ion is
absent. We therefore wondered whether PtC12Si might desilylate
to PtC12H under the homocoupling conditions, promoted by
adventitious water or other nucleophiles.14 Since PtC12H would
be generated in the presence of an oxidizing agent, homocoupling
might better compete with decomposition (similar factors likely
facilitate multiple heterocouplings in Scheme 2). As shown in
Scheme 1 (route C), PtC12Si was so reacted at 10 °C (7 h).
Chromatography gave PtC24Pt (36%, deep red), along with
recovered PtC12Si (38%). An analogous reaction of PtC14Si at 0
°C (58 h) gave PtC28Pt (51%, deep red).
The PtCxPt complexes were obtained as air-stable, analytically
pure powders. PtC20Pt, PtC24Pt, and PtC28Pt showed no significant
decomposition after several days, although they slowly decomposed
in solution. Lower homologues were stable for months. Thermolyses
were monitored by DSC and TGA (Supporting Information). The
decomposition points of PtC12Pt and PtC16Pt were ca. 200 °C,
and those of PtC20Pt, PtC24Pt, and PtC28Pt were 150-140 °C. In
no case did any mass loss occur below 200 °C. From the crystal
structure of PtC12Pt,9 the platinum-platinum separation in PtC28-
Pt can be estimated as 38.8 Å.
Figure 1. UV-visible spectra of PtCxPt (1.25 × 10-6 in CH2Cl2).
represents a breakthrough into an unanticipated stability regime,
which we expect can be extended to other polyynes with bulky
and/or electropositive endgroups. Although product mixtures are
obtained in Scheme 2, we anticipate that this can be avoided with
other silicon endgroups; however, for many purposes, easily
separated mixtures of oligomers are advantageous. The elaboration
of the compounds reported herein to diplatinum complexes with
odd numbers of triple bonds and still longer Cx chains, as well as
more complex assemblies16,17 will be described in future publica-
tions.
Acknowledgment. We thank the Deutsche Forschungsgemein-
schaft (DFG, GL 300/1-3) for support.
Supporting Information Available: Experimental procedures and
characterization12 for all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
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As noted above, the colors of PtCxPt progressively shifted from
yellow to deep red, consistent with the UV-visible spectra in Figure
1. As usual for series of polyynes,2-7 increasing numbers of
progressively more intense and red-shifted bands were observed.
In the case of PtC28Pt, the molar extinction coefficient ꢀ remained
above 185 000 M-1 cm-1 from 360 through 515 nm (402 000 M-1
cm-1 at λmax (489 nm)), tailing to less than 10 000 M-1 cm-1 only
at 560 nm. The nature of the transitions has been analyzed in detail
elsewhere.15 The NMR and IR properties were similar to those of
other metal-capped polyynes6,7 and will be discussed in our full
paper.
In summary, diplatinum adducts of polyynediyls consisting of
as many as 28 carbon atoms have been synthesized by generating
the labile PtCxH complexes in the presence of a suitable oxidizing
agent (Scheme 1, route C). There is no indication that a feasibility
limit has been reached with the highest homologue PtC28Pt. This
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(11) Eastmond, R.; Johnson, T. R.; Walton, D. R. M. Tetrahedron 1972, 28,
4601.
(12) All new compounds were characterized by IR, NMR (1H, 13C, 31P), and
UV-visible spectroscopy, mass spectrometry, microanalysis, and DSC/
TGA, as summarized in the Supporting Information.
(13) This reagent, which is necessary for the success of the couplings, is
believed to serve as a F- ion scavenger.7
(14) For the in situ desilylation and homocoupling of R(CtC)nSiMe3 species
(K2CO3, Cu(OAc)2‚H2O, pyridine), see: Haley, M. M.; Bell, M. L.; Brand,
S. C.; Kimball, D. B.; Pak, J. J.; Wan, W. B. Tetrahedron Lett. 1997, 38,
7483.
(15) Zhuravlev, F.; Gladysz, J. A. Chem.sEur. J. 2004, 10, 6510.
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Alvarez, J. M.; Hampel, F.; Gladysz, J. A. Angew. Chem., Int. Ed. 2002,
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Hampel, F.; Gladysz, J. A. Organometallics 2004, 23, 5889.
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23, 5896.
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