Wire-like PtC≡CC≡CC≡CC≡CPt moieties surrounded by double-helical "insulation": New motifs featuring P(CH 2)20P and P(CH2)4O(CH 2)2O(CH2)4P linkages

Article abstract of DOI:10.1039/b604465b

Ring-closing alkene metatheses of trans,trans-(C₆F ₅)(Ph₂P-Z-CHCH₂)₂Pt(CC) ₄Pt(Ph₂P-Z-CHCH₂)₂(C ₆F₅) (Z = (CH₂)₉

Full text of DOI:10.1039/b604465b

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Wire-like PtC≡CC≡CC≡CC≡CPt moieties surrounded by double-helical
“insulation”: new motifs featuring P(CH2)20P and
P(CH2)4O(CH2)2O(CH2)4P linkages†
Laura de Quadras, Frank Hampel and J. A. Gladysz*
Received 27th March 2006, Accepted 28th April 2006
First published as an Advance Article on the web 22nd May 2006
DOI: 10.1039/b604465b
P(CH2)mPAr2) Pt(C≡C)nPt(Ar2P(CH2)mPAr2)(C6F5) (III).9 In all
Ring-closing alkene metatheses of trans,trans-(C6F5)(Ph2P–
=
=
Z–CH CH2)2Pt(C≡C)4Pt(Ph2P–Z–CH CH2)2(C6F5) (Z
=
cases, the termini-spanning diphosphine ligands sterically shield
the sp carbon chains. Crystal structures show that the flexible sp3
chains can, when sufficiently long (m = 14), wrap around the sp
chains in a striking chiral double-helical motif (IIIb). In all four
examples characterized to date, the endgroups define angles of
189.3–196.5◦, slightly more than a half twist.13
(CH2)9, (CH2)4O(CH2)2), followed by hydrogenation, give the
title compounds; the former exhibits an exceptionally twisted
conformation, and the latter establishes that functional
groups can be incorporated into the flexible sp3 chain.
We sought to further define the accessibility of such “insulated”
bimetallic systems with respect to various parameters. First, can
the sp3 chains be lengthened, and if so do more highly twisted
helices result? Second, can functional groups be incorporated
into the sp3 chains? These might provide control elements for
self-assembly processes. Third, can isomeric species with trans-
spanning diphosphine ligands also form (IV, Scheme 1), and if
so to what degree do they shield the sp chain? Unfortunately,
reactions of diplatinum complexes I and diphosphines II with m ≥
16 afforded only oligomers.9a,14 Hence, we turned to an alternative
synthesis involving alkene metathesis described in our original
study.9a Although this route is somewhat longer, it does not require
phosphine substitution and delivers the target molecules reliably.
An octatetraynediyl complex analogous to I but with the alkene-
There are many types of unsaturated ligands that can link two
metal or other redox-active centers and mediate various charge-
and electron-transfer processes.1 In the macroscopic world, elec-
tron transporting materials such as household electrical wire are
commonly insulated from their environments. Accordingly, several
research groups have endeavoured to “insulate” such molecular
assemblies, and a variety of clever strategies have been applied.2–5
In previous reports, we have described numerous diplatinum
polyynediyl or Pt(C≡C)nPt complexes (n = 3–14).6–11 These
feature the most fundamental type of unsaturated bridging
ligand, a wire-like sp carbon chain.12 As shown in Scheme 1,
the hexatriynediyl and octatetraynediyl complexes trans,trans-
(C6F5)(p-tol3P)2Pt(C≡C)nPt(Pp-tol3)2(C6F5) (I; n = 3, 4) and
the a,x-diphosphines Ar2P(CH2)mPAr2 (II; m = 8, 10–12, 14)
react to give the substitution products trans,trans-(C6F5)(Ar2
=
containing phosphine Ph2P(CH2)9CH CH2 (a) was prepared
similarly to a lower homolog9a as summarized in Scheme 2 and
detailed in the ESI†. First, a diplatinum di(tetrahydrothiophene)
Institut fu¨r Organische Chemie, Friedrich-Alexander-Universita¨t Erlangen-
Nu¨rnberg, Henkestraße 42, 91054 Erlangen, Germany.
† Electronic supplementary information (ESI) available: Synthesis and
complex and
a were reacted to give the monoplatinum
bis(phosphine) chloride complex 1a (95%).15 This was treated
with butadiyne/HNEt2 in the presence of a catalytic amount of
characterization of 1–6. See DOI: 10.1039/b604465b
Scheme 1 Limiting structures for complexes derived from C6F5Pt(C≡C)nPtC6F5 units and diphosphines Ar2P(CH2)mPAr (II).
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Scheme 2 Syntheses of precursors to double helical complexes. Conditions: (a) CH2Cl2; (b) H(C≡C)2H, cat. CuCl, HNEt2; (c) O2, cat. CuCl/TMEDA,
acetone.
CuI to yield the butadiynyl complex 2a (80%). Subsequent Hay
oxidative coupling afforded the desired diplatinum complex 3a
(77%). All complexes were characterized by microanalysis, mass
spectrometry, and IR and NMR (1H, 13C, 31P) spectroscopy, as
summarized in the ESI†.
As shown in Scheme 3 (top), 3a was treated with Grubbs’
catalyst. Subsequent hydrogenation and column chromatography
gave three fractions. The first contained the title complex 4
(17%), and the third the other possible intramolecular metathesis
product 5 (15%). These correspond to III and IV with n/m/Ar =
4/20/C6H5. An intermediate fraction contained a 2 : 1 4/5 mixture
(31%). The spectroscopic properties of 4 and 5 were rather similar.
However, the most intense ion in the mass spectrum of 4 was
the molecular ion, whereas 5 gave extensive fragmentation to
monoplatinum ions.
The crystal structures of 4 and 5 were determined as described
in the ESI† (including details of how nine disordered sp3 carbon
atoms in the former were modeled).16 As depicted in Fig. 1, 4
exhibited a markedly helical conformation, with the endgroups
defining an angle of 294.8◦—more than three-quarters of a twist.
As illustrated by the space-filling representations C and D, the sp
carbon chain is nearly completely shielded. Curiously, all other
Pt(C≡C)nPt complexes with n > 2 crystallize with the endgroups
within 18.4◦ of coplanarity.17 DFT calculations show that
Fig. 1 ORTEP (A, B, with thermal ellipsoids drawn at the 50% probability level and hydrogen atoms omitted for clarity) and space-filling (C, D)
representations of 4.
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Grubbs’ second generation catalyst was much more effective.
As shown in Scheme 3, subsequent hydrogenation gave the title
complex 6 in 27% overall yield after chromatography. Later
fractions contained only small amounts of non-oligomeric
products. Although no complex analogous to 5 was noted, a yield
of <5% would not have been detected.
The crystal structure of 6 was determined.16 As depicted in
Fig. 3, a double-helical conformation was again found. The
number of atoms in each flexible sp3 chain (sixteen) is intermediate
between those of IIIb (fourteen) and 4 (twenty). However, the
endgroups define an angle of 164.1◦—somewhat less than a half
twist, and less than the range of values for IIIb (189.3–196.5◦).
Thus, the degree of twisting is not a simple function of the
number of atoms in the flexible chains. View L shows that a
region of the sp chain remains exposed (the opposite side is more
shielded).
A cylindrical helix is characterized by a radius, which for 6, 4,
and IIIb can be approximated by calculating the average distance
from every atom in the sp3 chain to the Pt–Pt vector. The value for 6
˚
(4.28 A) is greater than those of other octatetraynediyl complexes
˚
of the type IIIb (3.76–4.04 A). This logically follows from the
combination of (a) the lower degree of twisting, and (b) the two
extra atoms in each sp3 chain. Perhaps there is an underlying
electronic effect of the oxygen atoms, the lone pairs of which point
conspicuously away from the sp chain. The value for the more
˚
highly twisted complex 4 (4.13 A) is intermediate. Note that as the
sp3 chains become longer and longer, the contact surface of the
sp chain will eventually be saturated, requiring helices of greater
radii, and/or alternative conformations.
Finally, cyclic voltammograms of 4–6 were recorded. In
each case, partially reversible oxidations were observed (Ep,a
,
Ep,c, E◦, DE, ic/a: 1.300/1.305/1.304 V, 1.236/1.208/1.231 V,
1.268/1.256/1.268 V, 64/97/73 mV, 0.82/0.53/0.75), which are
presumed to represent one-electron processes.19 These data can be
compared to those for the unshielded complexes 3a and 3b, and an
20
analogous species with the saturated phosphine Ph2P(CH2)7CH3
(1.408/1.310/1.301 V, 1.226/1.239/1.214 V, 1.317/1.274/1.258 V,
182/71/87 mV, 0.15/0.72/0.68). The DE and ic/a values show that
the double helical complex 4 (but not 5) undergoes somewhat more
reversible oxidations than the unshielded complexes, consistent
with a longer lived radical cation; however, oxygen-containing 6 is
not significantly better than 3b.
Scheme 3 Syntheses of double helical complexes. Conditions: (a) 7 mol%
=
Ru( CHPh)(PCy3)2(Cl)2; (b) 10 mol% Pd/C, 1 atm, 14 d; (c) 6 mol%
=
Ru( CHPh)(PCy3)(H2IMes)(Cl)2.
Complexes 4 and 6 add considerable diversity to the growing
number of artificial double-stranded helices.21 As noted earlier,9a
species of the type IIIb are distinguished by a lack of bonding
interactions or complementarity between the two flexible strands.
Thus, they should be more predisposed to dynamic behavior than
classical helicates. Accordingly, the NMR properties of 4 and 6 in-
dicate that the enantiomers are rapidly interconverting in solution,
even at low temperatures, presumably by an untwisting/retwisting
process. Naturally, other conformations must be populated, and
experiments to probe this point are in progress. In summary,
this study has established that alkene metathesis can be used to
access complexes IIIb with sp3 chains that are considerably longer
than those hitherto available, as well as functionalized. Further
extensions of the methodology in Scheme 3 will be reported in due
course.
endgroup rotation is essentially barrierless, with no electronically
preferred conformation.8 Like all similar double helical diplatinum
complexes,9 both enantiomers are present in the unit cell.
As depicted in Fig. 2, the isomer 5 crystallizes in a “double
half-clamshell” conformation, with the macrocycles directed on
opposite sides of the P2Pt–PtP2 plane and an inversion center
between C4 and C4a. Views G and H show that the sp chain is still
extensively shielded, although not to the extent as in 4. Two distinct
gaps remain above and below the P2Pt–PtP2 plane. Homologs of 5
with shorter sp3 chains crystallize in analogous conformations.18
Next,
the
oxygenated
alkene-containing
phosphine
=
Ph2P(CH2)4O(CH2)2CH CH2 (b) was synthesized (ESI†).
As illustrated in Scheme 2, it was converted to the corresponding
diplatinum octatetraynediyl complex 3b by a sequence parallel
to 3a. Complex 3b reacted sluggishly with Grubbs’ catalyst, but
We thank the Deutsche Forschungsgemeinschaft (DFG, SFB
583) and Johnson Matthey PMC (platinum loans) for support.
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Fig. 2 ORTEP (E, F with thermal ellipsoids drawn at the 50% probability level and hydrogen atoms omitted for clarity) and space-filling (G, H)
representations of 5.
Fig. 3 ORTEP (I, J with thermal ellipsoids drawn at the 50% probability level and hydrogen atoms omitted for clarity) and space-filling (K, L)
representations of 6.
P. N. Taylor, J. S. Wilson, F. Cacialli and H. L. Anderson, Chem.–Eur. J.,
Notes and References
2003, 9, 6167; (f) J. Terao, A. Tang, J. J. Michels, A. Krivokapic and
1 (a) J.-P. Launay and C. Coudret, in Electron Transfer in Chemistry,
ed. V. Balzani, Wiley/VCH, Weinheim, 2001, vol. 5, ch. 1, and other
chapters in this series; (b) H. Meier, Angew. Chem., Int. Ed., 2005, 44,
2482; H. Meier, Angew. Chem., 2005, 117, 2536.
2 (a) S. Anderson, R. T. Aplin, T. D. W. Claridge, T. Goodson, III, A. C.
Maciel, G. Rumbles, J. F. Ryan and H. L. Anderson, J. Chem. Soc.,
Perkin Trans. 1, 1998, 2383; (b) P. N. Taylor, M. J. O’Connell, L. A.
McNeill, M. J. Hall, R. T. Aplin and H. L. Anderson, Angew. Chem.,
Int. Ed., 2000, 39, 456; P. N. Taylor, M. J. O’Connell, L. A. McNeill,
M. J. Hall, R. T. Aplin and H. L. Anderson, Angew. Chem., 2000,
112, 3598; (c) J. E. H. Buston, F. Marken and H. L. Anderson, Chem.
Commun., 2001, 1046; (d) P. N. Taylor, A. J. Hagan and H. L. Anderson,
Org. Biomol. Chem., 2003, 1, 3851; (e) J. J. Michels, M. J. O’Connell,
H. L. Anderson, Chem. Commun., 2004, 56.
3 A. P. H. J. Schenning, J.-D. Arndt, M. Ito, A. Stoddart, M. Schreiber,
P. Siemsen, R. E. Martin, C. Boudon, J.-P. Gisselbrecht, M. Gross, V.
Gramlich and F. Diederich, Helv. Chim. Acta, 2001, 84, 296.
4 (a) D. Lee and T. M. Swager, Synlett, 2004, 149; (b) P. H. Kwan and
T. M. Swager, Chem. Commun., 2005, 5211.
5 C. Li, M. Numata, A.-H. Bae, K. Sakurai and S. Shinkai, J. Am. Chem.
Soc., 2005, 127, 4548.
6 (a) T. B. Peters, J. C. Bohling, A. M. Arif and J. A. Gladysz,
Organometallics, 1999, 18, 3261; (b) Q. Zheng and J. A. Gladysz, J. Am.
Chem. Soc., 2005, 127, 10508.
7 W. Mohr, J. Stahl, F. Hampel and J. A. Gladysz, Chem.–Eur. J., 2003,
9, 3324.
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8 F. Zhuravlev and J. A. Gladysz, Chem.–Eur. J., 2004, 10,
6510.
give the independently synthesized diplatinum complexes trans,trans-
(C6F5)(Et3P)2Pt(C≡C)nPt(PEt3)2(C6F5)3.
9 (a) J. Stahl, J. C. Bohling, E. B. Bauer, T. B. Peters, W. Mohr, J. M.
Mart´ın-Alvarez, F. Hampel and J. A. Gladysz, Angew. Chem., Int. Ed.,
2002, 41, 1871; J. Stahl, J. C. Bohling, E. B. Bauer, T. B. Peters, W.
Mohr, J. M. Mart´ın-Alvarez, F. Hampel and J. A. Gladysz, Angew.
Chem., 2002, 114, 1951; (b) G. R. Owen, J. Stahl, F. Hampel and J. A.
Gladysz, Organometallics, 2004, 23, 5889.
10 (a) Q. Zheng, F. Hampel and J. A. Gladysz, Organometallics, 2004, 23,
5896; (b) G. R. Owen, F. Hampel and J. A. Gladysz, Organometallics,
2004, 23, 5893.
11 For other complexes with Pt(C≡C)3Pt and Pt(C≡C)4Pt linkages, see:
(a) W.-Y. Wong, C.-K. Wong, G.-L. Lu, K.-W. Cheah, J.-X. Shi and
Z. Lin, J. Chem. Soc., Dalton Trans., 2002, 4587; (b) V. W.-W. Yam,
K. M.-C. Wong and N. Zhu, Angew. Chem., Int. Ed., 2003, 42, 1400;
V. W.-W. Yam, K. M.-C. Wong and N. Zhu, Angew. Chem., 2003, 115,
1438.
12 For reviews of such complexes, see: (a) M. I. Bruce and P. J. Low, Adv.
Organomet. Chem., 2004, 50, 179; (b) F. Paul and C. Lapinte, in Unusual
Structures and Physical Properties in Organometallic Chemistry, ed.
M. Gielen, R. Willem and B. Wrackmeyer, Wiley, New York, 2002,
pp. 220–291.
15 This compound has been previously reported: E. B. Bauer, F. Hampel
and J. A. Gladysz, Organometallics, 2003, 22, 5567.
16 Crystal data for 4/5/6·MeOH: formula
C108H120F10P4Pt/
C
108H120F10P4Pt2/C97H100F10O5P4Pt2, monoclinic/triclinic/monoclinic,
a = 39.9137(6)/11.4000(2)/21.4114(3), b = 11.1431(2)/14.3770(3)/
˚
17.9695(3),
c
=
22.6389(4)/15.1140(3)/24.2342(3) A,
a
=
90/
83.968(1)/90, b = 102.57(3)/80.358(1)/101.855(1), c = 90/80.128(1)/
◦
3
˚
90 , V = 9827.5(3)/2398.55(8)/9125.3(2) A , T = 173(2)/173(2)/
173(2) K, space groups P21/c, P-1, P21/n, Z = 4/1/4, l(Mo-Ka) =
2.974/3.047/3.204 mm−1, 37122/20633/39762 reflections measured,
21370/10992/20905 unique (Rint = 0.0490/0.0250/0.0497), which
were used in calculations. Final
0.0421/0.0303/0.0421; wR2 (all data)
R
values: R1 [I
=
>
2r(I)]
=
0.1123/0.0748/0.0979.
CCDC reference numbers 295839–295841. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b604465b.
Additional data are provided in the ESI† .
17 Sixteen cases have been reported,6a,7,9,10a,11b and there are numerous
unpublished examples. Additional analyses: S. Szafert and J. A.
Gladysz, Chem. Rev., 2003, 103, 4175.
18 E. B. Bauer and L. de Quadras, Doctoral Dissertations, Universita¨t
Erlangen-Nu¨rnberg, 2003 and 2006..
13 The least-squares planes [(PA– PtA – PA) + PtB] and [PtA + (PB
–
PtB – PB)] are used to calculate the angles defined by the endgroups. The
inclusion of a platinum from each terminus minimizes non-idealities
arising from chain curvature.
19 (a) A. Klein and W. Kaim, Organometallics, 1995, 14, 1176; (b) A.
Klein, S. Hasenzahl, W. Kaim and J. Fiedler, Organometallics, 1998,
17, 3532.
20 J. Stahl, Doctoral Dissertation, Universita¨t Erlangen-Nu¨rnberg, 2003.
21 M. Albrecht, Angew. Chem., Int. Ed., 2005, 44, 6448; M. Albrecht,
Angew. Chem., 2005, 117, 6606.
14 (a) W. Mohr, Doctoral Dissertation, Universita¨t Erlangen-Nu¨rnberg,
2002; (b) The oligomeric formulation can be proved by adding an
excess of the more basic phosphine PEt3. Substitution occurs to
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The Royal Society of Chemistry 2006
Dalton Trans., 2006, 2929–2933 | 2933
©