Syntheses and structures of diplatinum hexatriynediyl complexes, in which the sp carbon chains are shielded by sp3 carbon chains

Article abstract of DOI:10.1021/om0493558

When trans,trans-(C₆F₅)(p-tol₃P) ₂Pt(C≡C)₃Pt-(P-p-tol₃)₂(C ₆F₅) and Ar₂P(CH₂) _mPAr₂ (m l Ar = 8 / p-tol, 10 / Ph, 11 / Ph, 12 / Ph, 14 / p-tol) are reacted, p-tol₃P is displaced to give products in which the diphosphines span the platinum end groups. One crystal structure (m = 14) shows a double-helical conformation in which the sp³ carbon chains twist around the sp chain, and others (m = 10, 11) show nonhelical conformations with lateral shielding of the sp chain.

Full text of DOI:10.1021/om0493558

Organometallics 2004, 23, 5889-5892
5889
Syntheses and Structures of Diplatinum Hexatriynediyl
Complexes, in Which the sp Carbon Chains Are Shielded
by sp3 Carbon Chains
Gareth R. Owen, Ju¨rgen Stahl, Frank Hampel, and J. A. Gladysz*
Institut fu¨r Organische Chemie, Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg,
Henkestraâe 42, 91054 Erlangen, Germany
Received August 19, 2004
Summary: When trans,trans-(C6F5)(p-tol3P)2Pt(CtC)3Pt-
(P-p-tol3)2(C6F5) and Ar2P(CH2)mPAr2 (m/Ar ) 8/p-tol,
10/Ph, 11/Ph, 12/Ph, 14/p-tol) are reacted, p-tol3P is
displaced to give products in which the diphosphines
span the platinum end groups. One crystal structure (m
) 14) shows a double-helical conformation in which the
sp3 carbon chains twist around the sp chain, and others
(m ) 10, 11) show nonhelical conformations with lateral
shielding of the sp chain.
Over the past decade, molecules in which sp carbon
chains span two transition metals have attracted great
attention, from both the standpoint of fundamental
properties and possible applications in molecular de-
vices.1 The Cx linkages more efficiently delocalize the
odd electrons of radical ions between the metal end
groups, as compared to most other types of unsaturated
bridging ligands.1b We have described several series of
complexes with Pt(CtC)nPt units (n ) 3, 4, 6, 8).2-6 For
the octatetraynediyl complex trans,trans-(C6F5)(p-tol3P)2-
Pt(CtC)4Pt(P-p-tol3)2(C6F5) (PtC8Pt), reactions with
diphosphines Ar2P(CH2)mPAr2 (m ) 10-14) yield sub-
stitution products of the formula trans,trans-(C6F5)-
the barrier for converting one enantiomer of Ia to the
other, a process that requires a conformation with
coplanar end groups such as Ib and is fast on the NMR
time scale for all complexes of the type PtC8Pt-14/Ar.
In this communication, we report (1) an improved
synthesis of the hexatriynediyl complex trans,trans-
(C6F5)(p-tol3P)2Pt(CtC)3Pt(P-p-tol3)2(C6F5) (PtC6Pt)3 that
makes multigram quantities available, (2) substitutions
with longer-chain diphosphines leading to assemblies
of the type Ia, (3) substitutions with medium-chain
diphosphines leading to assemblies of the type Ib, and
(4) attendant crystallographic, VT NMR, and cyclic
voltammetry data. In the following communication,
reactions involving short-chain diphosphines that afford
tetraplatinum complexes with “bundled” polyyne chains
are described.8
As shown in Scheme 1, PtC6Pt was prepared from
the same two precursors, PtC6SiEt3 and PtCl, as
reported earlier.3 In the first step, PtC6SiEt3 and the
fluoride salt n-Bu4N+F- were combined in protic media
to generate the labile complex PtC6H. Interestingly,
when the subsequent CuCl-catalyzed cross-coupling was
conducted in the presence of stoichiometric amounts of
KPF6 and t-BuOK, the yield of PtC6Pt increased from
34% to 94%.9 Quantities exceeding 2 g were easily
prepared.
(Ar2P(CH2)mPAr2)Pt(CtC)4Pt(Ar2P(CH2)mPAr2)(C6F5)
(PtC8Pt-m/Ar). These feature sterically shielded sp
carbon chains. In the case with m ) 14, the sp3 chains
wrap around the sp chain in a striking chiral double-
helical conformation in the solid state (Ia; eq i). The
corresponding radical cations become longer lived, invit-
ing analogies to “insulated molecular wires”.
We sought to extend this chemistry to lower homo-
logues with shorter (CtC)n bridges. It was thought that
end group-end group steric interactions might lead to
intrinsically dissymmetric structures, analogous to the
twisted ground states of biaryls.7 This might increase
(1) (a) Bruce, M. I.; Low, P. J. Adv. Organomet. Chem. 2004, 50,
179. (b) 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. (c) Szafert,
S.; Gladysz, J. A. Chem. Rev. 2003, 103, 4175.
(2) Peters, T. B.; Bohling, J. C.; Arif, A. M.; Gladysz, J. A. Organo-
metallics 1999, 18, 3261.
(3) Mohr, W.; Stahl, J.; Hampel, F.; Gladysz, J. A. Chem. Eur. J.
2003, 9, 3324.
(4) Computational study: Zhuravlev, F.; Gladysz, J. A. Chem. Eur.
J., in press.
(5) 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.
(6) For other complexes with Pt(CtC)3Pt and Pt(CtC)4Pt linkages,
see: (a) Wong, W.-Y.; Wong, C.-K.; Lu, G.-L.; Cheah, K.-W.; Shi, J.-X.;
Lin, Z. Dalton 2002, 4587. (b) Yam, V. W.-W.; Wong, K. M.-C.; Zhu,
N. Angew. Chem., Int. Ed. 2003, 42, 1400; Angew. Chem. 2003, 115,
1438.
(7) (a) Arulmozhiraja, S.; Fujii, T. J. Chem. Phys. 2001, 115, 10589
and references therein. (b) 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.
(8) Owen, G. R.; Hampel, F.; Gladysz, J. A. Organometallics 2004,
23, 5893.
10.1021/om0493558 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 11/10/2004

5890 Organometallics, Vol. 23, No. 25, 2004
Scheme 1. Syntheses of Diplatinum Hexatriynediyl Complexes
Communications
The crystal structure of a solvate was determined as
described in the Supporting Information. As shown in
Figure 1A,B, it exhibited an inversion center at the
chain midpoint and the C6H4R/C6F5/C6H4R π-stacking
interaction commonly seen in this series of com-
pounds.3,5,10,11 To our surprise, the end groups were
coplanar, with no van der Waals contacts (shortest
hydrogen-hydrogen distance 2.71 Å). DFT calculations
indicate no electronic conformational preference.4 Since
this motif is observed with other platinum substituents
and longer sp chain lengths,2,3,12 it must reflect a crystal-
packing effect of considerable generality. Substitutions
with diphosphines were nonetheless investigated, with
the hope that steric interactions might still ensue
between the modified end groups.
tol, 16/Ph).13 In each case, the desired bis(diphosphine)
complexes trans,trans-(C6F5)(Ar2P(CH2)mPAr2)Pt(Ct
C)3Pt(Ar2P(CH2)mPAr2)(C6F5) (PtC6Pt-m/Ar) formed.
For the first five reactions, workups gave PtC6Pt-m/
Ar in 57-82% yields.14 In the last case, a 31P NMR
spectrum of the reaction mixture suggested complete
conversion to PtC6Pt-16/Ph. However, the powder
isolated (64%) could not again be dissolved. This is
characteristic of oligomers that sometimes form upon
sample concentration. Similar behavior is seen when
PtC8Pt and PtC12Pt are combined with longer-chain
diphosphines.15 The mass spectra of PtC6Pt-m/Ar
exhibited strong molecular ions, and IR spectra showed
a single weak νCtC band at 2104-2123 cm-1. The PtCt
CC 13C NMR signals were essentially identical with
those of PtC6Pt (PtCtC, 95.7-95.2 ppm; PtCtCC,
61.7-60.3 ppm; in CDCl3).
THF solutions of PtC6Pt were treated with Ar2P-
(CH2)mPAr2 (m/Ar ) 8/p-tol, 10/Ph, 11/Ph, 12/Ph, 14/p-
(9) (a) Many Fe(CtC)nH and FeCl complexes undergo efficient cross-
coupling with treated with KPF6/t-BuOK: Coat, F.; Guillemot, M.;
Paul, F.; Lapinte, C. J. Organomet. Chem. 1999, 578, 76 and references
therein. (b) No PtC6Pt forms in the absence of CuCl.
(10) (a) Bauer, E. B.; Szafert, S.; Hampel, F.; Gladysz, J. A.
Organometallics 2003, 22, 2184. (b) Shima, T.; Bauer, E. B.; Hampel,
F.; Gladysz, J. A. Dalton 2004, 1012.
The crystal structures of PtC6Pt-10/Ph, PtC6Pt-11/
Ph, and PtC6Pt-14/p-tol, or solvates thereof, were
determined as described in the Supporting Information.
(11) (a) The bond lengths and angles for all compounds are typical
and are given in the Supporting Information. (b) The least-squares
planes [(PA-PtA-PA) + PtB] and [PtA + (PB-PtB-PB)] are used to
calculate the angles defined by the end groups. The inclusion of a
platinum from each terminus minimizes nonidealities arising from
chain curvature. (c) Average C6H4R-C6F5 centroid-centroid distances
(Å): PtC6Pt, 3.665; PtC6Pt-10/Ph, 3.631; PtC6Pt-11/Ph, 3.705;
PtC6Pt-14/p-tol, 4.197. In the last compound, the nonstacked aryl
groups nest close to the sp3 chains.
(13) Mohr, W.; Horn, C. R.; Stahl, J.; Gladysz, J. A. Synthesis 2003,
1279. Preparations for the diphosphines not found in this paper are
given in the Supporting Information.
(14) All new compounds have been characterized by microanalysis,
IR and NMR (1H, 13C, 31P) spectroscopy, and mass spectrometry, as
described in the Supporting Information.
(15) (a) Mohr, W. Doctoral Thesis, Universita¨t Erlangen-Nu¨rnberg,
2002. (b) The general structures of these oligomers can be proved by
adding an excess of the more basic phosphine PEt3. Substitution occurs
to give the independently synthesized diplatinum complexes
trans,trans-(C6F5)(Et3P)2Pt(CtC)nPt(PEt3)2(C6F5).3
(12) Zheng, Q.; Hampel, F.; Gladysz, J. A. Organometallics 2004,
23, 5896.

Communications
Organometallics, Vol. 23, No. 25, 2004 5891
Figure 1. Space-filling and ORTEP representations of the molecular structures of (A, B) PtC6Pt, (C-F) PtC6Pt-10/Ph,
(G) PtC6Pt-11/Ph, and (H-J) PtC6Pt-14/p-tol.
The molecular structure of PtC6Pt-10/Ph, which fea-
tures an inversion center at the chain midpoint and
coplanar end groups, is depicted in Figures 1C-E. The
methylene chain is not long enough to twist by 180° as
in Ia. Rather, the central (CH2)6 segments run roughly
parallel to the sp chain in an “all-anti” conformation,
laterally shielding two “sides”. The average distance
from the methylene carbons to the Pt-Pt vector is 4.86
Å. The van der Waals radii of sp carbon, sp3 carbon,
and hydrogen atoms are 1.78, 1.70, and 1.20 Å, respec-
tively.16
The structure of PtC6Pt-11/Ph, which also features
an inversion center at the chain midpoint and coplanar
end groups, is depicted in Figure 1G. The extra meth-
ylene group leads to a sp3 chain that is more “curved”,
or richer in gauche conformational units, and that
nearly crosses from the “top” to “bottom” half of the
molecule at the midpoint. Nonetheless, each sp3 chain
returns to its hemisphere of origin, avoiding a helical
conformation. Interestingly, the average distance from
(16) Bondi, A. J. Phys. Chem. 1964, 68, 441.

5892 Organometallics, Vol. 23, No. 25, 2004
Communications
the methylene carbons to the Pt-Pt vector decreases
to 4.03 Å, the lowest of all complexes in this study.
The structure of PtC6Pt-14/p-tol contains two dis-
ordered methylene groups, and the dominant conformer
is depicted in Figures 1H-J. The longer methylene
chains are now able to wrap around the sp chain in a
double-helical motif. The end groups define an angle of
189.3°smore than a half twist.11b Both enantiomers are
present in the unit cell. The sp chain is highly shielded,
visible only through the bowl-like cavity highlighted in
Figure 1J. However, the C6H4R/C6F5/C6H4R stacking
interaction is weaker than in the other complexes,11c and
the sp and sp3 chains are not in van der Waals contact.
The average distance from the methylene carbons to the
Pt-Pt vector is 4.42 Å. In the three previously reported
double-helical complexes of the formula PtC8Pt-14/Ar,
the average distances are only 3.93-3.97 Å, indicating
tighter winding. Given the lower van der Waals surface
of the C6 vs C8 bridge, it is not surprising that the
methylene groups in PtC6Pt-14/p-tol spiral with a
greater average radius.
We sought to probe the structure of PtC6Pt-14/p-tol
in solution. The aryl groups and PCH2 protons are
diastereotopic in chiral helical conformations as in
Figures 1H-J. However, when NMR spectra were
recorded in CDClF2 at -135 °C, separate aryl 13C or
PCH2 1H signals could not be observed. This indicates
that the helical conformation, if maintained in solution,
rapidly untwists and retwists to the opposite enanti-
omer via a formally achiral conformation such as Ib.
Analogous results were obtained earlier with the higher
homologues PtC8Pt-14/Ar.5 Hence, there is no dramatic
increase in the racemization barrier for complexes with
shorter C6 chains.
and 0.71) intermediate between those of PtC4Pt and
PtC8Pt (n-Bu4N+BF4-/CH2Cl2, 100 mV/s).3 The steri-
cally shielded complexes PtC6Pt-10/Ph and PtC6Pt-
12/Ph gave somewhat more reversible couples (∆E )
85, 75 mV; ic/ia ) 0.82, 0.84). Unexpectedly, PtC6Pt-
14/p-tol showed a much less reversible couple (∆E )
117 mV; ic/ia ) 0.45). This is the first time such a trend
has been observed for a complex of the type PtCxPt-
m/Ar that is capable of a double-helical conformation.
The basis for the attenuated stability of the correspond-
ing radical cation remains under investigation.
In summary, we have been able to extend the novel
structural motifs Ia and Ib to diplatinum hexatriynediyl
complexes using methodology developed earlier for the
higher octatetraynediyl homologues.5 However, al-
though PtC6Pt-14/p-tol exhibits a double-helical con-
formation in the solid state, this does not translate into
an enhanced barrier to racemization in solution or
longer lived radical cations. Future reports will describe
lower butadiynediyl homologues, in which end group-
end group interactions cannot be avoided. Dissymmetric
structures are indeed realized, but additional unusual
conformational phenomena involving the sp3 carbon
chains are encountered.17 Further types of novel struc-
tures based upon polyynediyl building blocks are de-
scribed in the following communications.8,12
Acknowledgment. We thank the Deutsche For-
schungsgemeinschaft (DFG, SFB Grant 583), the Hum-
boldt Foundation (Fellowship to G.R.O.), and Johnson
Matthey PMC (platinum loans) for support.
Supporting Information Available: Text and tables
giving experimental procedures, characterization data14 for all
new compounds, and crystallographic data; crystallographic
data are also available as CIF files. This material is available
free of charge via the Internet at http://pubs.acs.org.
Finally, cyclic voltammograms of representative com-
plexes were recorded. In each case, partially reversible
one-electron oxidations were observed. The unshielded
complex PtC6Pt exhibited, as expected, Ep,a, Ep,c, E°,
∆E, and ic/ia values (1.156, 1.066, and 1.111 V, 90 mV,
OM0493558
(17) Stahl, J. Doctoral Thesis, Universita¨t Erlangen-Nu¨rnberg, 2003.