Ring-opening reaction of phosphorus-bridged [1]ferrocenophane via ring slippage from η5- to η1-Cp

Article abstract of DOI:10.1021/ja029297m

A reaction mechanism was investigated for a ring-opening reaction of RP(E)-bridged [1]ferrocenophane, where RP(E) = PhP(S) (3a), PhP (3b), and MesP (3c) (Mes = 2,4,6-trimethylphenyl). Irradiation of UV-vis light in the presence of an excess amount of P(OMe)₃ transformed 3a to [Fe{PhP(S)(η⁵-C₅H₄)(η¹-C₅H₄)}{P(OMe)₃}₂] (4a), in which one of the two cyclopentadienyl (Cp) rings of 3a changed its coordination mode from η⁵ to η¹ and vacant coordination sites thus formed on the iron center were occupied by two P(OMe)₃ ligands. The molecular structure of 4a was determined by X-ray analysis, in which η¹-Cp adopted a 1-Fe-2-P-1,3-cyclopentadiene structure. Under the same reaction conditions, 3b and 3c also gave similar ring-slipped products 4b and 4c, respectively. Photolysis of 3a using more strongly coordinating PMe₃ in place of P(OMe)₃ led to complete dissociation of a Cp ligand from the iron center to form [Fe{PhP(S)(η⁵-C₅H₄)(C₅H₄)}(PMe₃)₃] (5). The formation of the ring-slipped and -dissociated products on the photolysis of 3 strongly supports the view that photolytic ring-opening polymerization of 3 proceeds via an unprecedented Fe-Cp bond cleavage mechanism. Copyright

Full text of DOI:10.1021/ja029297m

Published on Web 01/31/2003
Ring-Opening Reaction of Phosphorus-Bridged [1]Ferrocenophane via Ring
Slippage from η5- to η1-Cp
Tsutomu Mizuta, Yuki Imamura, and Katsuhiko Miyoshi*
Department of Chemistry, Graduate School of Science, Hiroshima UniVersity, Kagamiyama 1-3-1,
Higashi-Hiroshima, Hiroshima 739-8526, Japan
Received November 11, 2002; E-mail: kmiyoshi@sci.hiroshima-u.ac.jp
Ferrocene has been generally recognized as a thermally stable
molecule because of its large Fe-Cp dissociation energy of ca.
380 kJ/mol.1 However, some ferrocene compounds such as acyl-
substituted ferrocenes2,3 and boron-bridged [1]ferrocenophane,4 have
been reported to undergo an Fe-Cp cleavage upon irradiation with
light. Since ferrocene compounds are widely used in contemporary
chemistry not only as a reagent in synthesis and catalysis but also
as an advanced material,5 elucidation of a mechanism for such
photolyses, especially isolation of intermediates during the Fe-
Cp cleavage, is of general importance. On the basis of a ring-
slippage chemistry of transition-metal Cp complexes,6 it is highly
plausible that the photolytic Fe-Cp cleavage proceeds via a
haptotropic shift of the Cp ligand. To the best of our knowledge,
however, an intermediate of such a reaction has not been isolated
regarding ferrocene compounds.
Recently, our group reported that phosphorus-bridged [1]ferro-
cenophanes polymerized on irradiation of UV-vis light.7 In analogy
with the photolysis mentioned above, our photolytic polymerization
may proceed also via the Fe-Cp cleavage (route ii in Scheme 1).
On the other hand, thermal, living-anionic, and metal-catalyzed
polymerizations of ERn-bridged [1]ferrocenophanes (E ) B, Si,
Ge, Sn, and P) were investigated extensively,8 and it is reported
that some of them proceed via the cleavage of a ferrocene-ERn
bond (route i).9-13 Here, we wish to report the unprecedented
identification of the intermediates supporting the route ii mechanism
for the photolytic polymerization of phosphorus-bridged [1]ferro-
cenophanes.
A THF solution of 3a was irradiated in the presence of a large
excess of P(OMe)3 (eq 1). After 10 min irradiation, a 31P{1H} NMR
spectrum of the resultant solution showed no signals attributable
to polymeric products, but new signals were observed at 33.9, 186.1,
and 194.4 ppm in addition to that at 141.0 ppm due to free P(OMe)3.
Since the signals at 186.1 and 194.4 ppm are mutually coupled
with a large coupling constant (2JPP ) 151 Hz), they are assigned
to the two P(OMe)3 ligands coordinating to the iron center, and
the remaining signal at 33.9 ppm is to the bridging phosphorus.
Finally, a molecular structure of this product was determined by
X-ray analysis as shown in Figure 1. One Cp ring coordinates to
the iron center with a usual η5-mode, whereas the other has changed
its coordination mode from η5 to η1 with itself connected to the
η5-Cp ring through an SdPPh group. Although several reports
described the ring slippage of ferrocene derivatives, to our best
knowledge, this is the first example of the η5-to-η1 haptotropic shift
characterized by X-ray analysis. The geometry of the η1-Cp ring
is interesting owing to its tautomerism. Summations of the bond
angles around C6 and C7 are 359.7(9)° and 360.1(9)°, respectively,
and bond lengths for C6-C7 and C9-C10 are 1.372(8) Å and
1.34(1) Å, respectively, which are significantly smaller than those
for the other three bonds of the five-membered ring. These structural
characteristics are consistent with a right-hand tautomeric structure
shown in Figure 1.
Scheme 1. Two Routes for Ring-opening Reaction of 1
As previously reported, photoirradiation of PhP(S)-bridged [1]-
ferrocenophane 3a in THF gives polymeric products in an almost
quantitative yield. The formation of the polymer can be readily
confirmed by the appearance of 31P{1H} NMR signals at 37.3 and
30.9 ppm with an intensity ratio of 94:6.10c If this polymerization
proceeds via the route ii, the dissociation of the Cp ring from the
iron center should leave vacant coordination sites on the iron center,
which are expected to be temporarily occupied by THF molecules
before intermolecular recombination leading to polymerization.
Therefore, if such a recombination can be suppressed by replace-
ment of coordinating solvents with strongly coordinating ligands
such as a phosphorus ligand, the reaction intermediate may be stable
enough to be isolated and characterized. With this expectation in
mind, we carried out the photolysis of 3 in the presence of P(OMe)3
or PMe3.
Figure 1. ORTEP drawing of 4a at 50% probability. Hydrogen atoms are
depicted only for an η1-Cp ring.
The two remaining coordination sites on the iron center are
occupied by the two P(OMe)3 ligands as expected. They were
observed as an AB quartet in 31P{1H} NMR, since these phosphite
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C O M M U N I C A T I O N S
ligands are at diastereotopic positions due to the presence of the
asymmetric bridging phosphorus atom.
plausible mechanisms for the propagation step of the polymeriza-
tion. Further study on this point is still under way.
For PhP- and MesP-bridged [1]ferrocenophanes, 3b and 3c,
respectively, a photolysis under similar conditions gave the products
having AB quartets around 190 ppm in 31P{1H} NMR, indicating
that the two P(OMe)3 ligands coordinate to the iron center after a
ring-slippage similar to that observed for 4a. In these cases,
however, 31P{1H} NMR data of the reaction mixture given in the
Supporting Information indicate the presence of two isomers,
probably arising from the tautomerism of the η1-Cp.
Here, to confirm that the isolated 4a possesses structural
characteristics requisite for the polymerization, a thermal polym-
erization of 4a was carried out. After reflux of 4a in THF for 24
h, a 31P{1H} NMR spectrum of the reaction mixture showed signals
around 37 ppm as well as at 141.0 ppm due to free P(OMe)3. The
former signals are identical to those of the polymer obtained by
the photoreaction of 3a. A yield of the product was 43% after
treatment described in the Supporting Information. GPC analysis
showed that the product obtained by the thermolysis is an oligomer
having a molecular weight in a range of 300-10000 (Mn ) 730
and Mw/Mn ) 2.23 relative to polystylene standards), which was
considerably lower than that of the product formed by the direct
photolysis of 3a (Mn ) 2.9 × 104 and Mw/Mn ) 1.63). It is probable
that strongly coordinating P(OMe)3 ligands depress smooth propa-
gation of a polymer chain.
Finally, it is an important point whether the η1-Cp ring dissociates
completely from the iron center, because such a step is indispensable
for the polymerization reaction. To clarify this point, a similar
photolysis of 3a was carried out using more strongly coordinating
PMe3 in place of P(OMe)3. In a 31P{1H} NMR spectrum, the
bridging phosphorus was observed at 26.0 ppm, and PMe3 was
observed at 23.4 ppm not as an AB quartet but a sharp siglet having
a much higher intensity than that of the bridge. In 1H NMR, PMe3
was observed at 1.45 ppm with 27H of intensity. These NMR data
regarding PMe3 indicate that three PMe3 ligands coordinate to the
iron center to form a product 5 (eq 2). With respect to the
Acknowledgment. This work was supported by a Grant-in-Aid
for Scientific Research (No. 13640560) from the Ministry of
Education, Science, Sports, and Culture, Japan.
Supporting Information Available: Experimental procedures for
the syntheses of 4a, 4b, 4c, and 5 and their NMR data (PDF) and
crystallographic data for 4a (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
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