Strain-controlled, photochemically, or thermally promoted haptotropic shifts of cyclopentadienyl ligands in group 8 metallocenophanes

Article abstract of DOI:10.1021/ja077334+

Cyclopentadienyl (Cp) ligands in moderately strained [1]- and [2]ferrocenophanes [Fe{(η⁵-C₅H₄) ₂-(ER_x)_y}. Fe{(η⁵-C ₅H₄)₂SiMe₂} (1), Fe{(η⁵-C₅H₄)CH₂}2 (10)] and highly strained [2]ruthenocenophanes [Ru{(η⁵-C₅H ₄)CR₂}₂ {R = H (15), Me (16)}] are susceptible to partial substitution by P donors and form mixed-hapticity metallocycles - [M(L₂){(η⁵-C₅H₄)(ER _x)_y(η¹-C₅H₄)}]: [Fe(dppe){(η⁵-C₅H₄)SiMe₂(η ¹-C₅H₄)}] (5), [Fe(dmpe){(η⁵- C₅H₄)SiMe₂(η¹-C ₅H₄)}] (6), [Fe(dmpe){(η⁵-C ₅H₄)(CH₂)₂(η¹-C ₅H₄)}] (11), [Ru(dmpe){(η⁵-C ₅H₄)(CH₂)₂(η¹-C ₅H₄)}] (17), [Ru(dmpe){(η⁵-C ₅H₄)(CMe₂)₂(η¹-C ₅H₄)}] (18), and [Ru(PMe₃)₂{(η ⁵-C₅H₄)(CH₂)₂(η ¹-C₅H₄)}] (19) - through haptotropic reduction of one η⁵-, π-bound Cp to η¹, σ-coordination. These reactions are strain-controlled, as highly ring-tilted [2]ruthenocenophanes 15 and 16 [tilt angles (α) ≈ 29-31°] react without irradiation to form thermodynamically stable products, while moderately strained [n]ferrocenophanes 1 and 10 (α ≈ 19-22°) require photoactivation. The iron-containing photoproducts 5 and 11 are metastable and thermally retroconvert to their strained precursors and free phosphines at 70°C. In contrast, the unprecedented ring-opening polymerization (ROP) of the essentially ring-strain-free adduct 6 to afford poly(ferrocenyldimethylsilane) [Fe(η⁵-C₅H ₄)₂SiMe₂]_n (M_w ≈ 5000 Da) was initiated by the thermal liberation of small amounts of P donor. Unlike reactions with bidentate analogues, monodentate phosphines promoted photolytic ROP of ferrocenophanes 1 and 10. MALDI-TOF analysis suggested a cyclic structure for the soluble poly(ferrocenyldimethylsilane), 8-cyclic, produced from 1 in this manner. While the polymer likewise produced from 10 was insoluble, the initiation step in the ROP process was modeled by isolation of a tris(phosphine)-substituted ring-opened ferrocenophane [Fe(PMe₃) ₃{(η⁵-C₅H₄)(CH₂) ₂(C₅H₅)}][OCH₂CH₃] (13[OCH₂CH₃]) generated by irradiation of 10 and PMe ₃ in a protic solvent (EtOH). Studies of the cation 13 revealed that the Fe center reacts with a Cp^- anion with loss of the phosphines to form [Fe(η⁵-C₅H₅){(η⁵-C ₅H₄)(CH₂)₂(C₅H ₅)}] (14) under conditions identical to those of the ROP experiments, confirming the likelihood of back-biting reactions to yield cyclic structures or macrocondensation to produce longer chains.

Full text of DOI:10.1021/ja077334+

Published on Web 03/04/2008
Strain-Controlled, Photochemically, or Thermally Promoted
Haptotropic Shifts of Cyclopentadienyl Ligands in Group 8
Metallocenophanes
David E. Herbert, Makoto Tanabe,† Sara C. Bourke, Alan J. Lough,‡
and Ian Manners*
School of Chemistry, UniVersity of Bristol, Bristol, England, BS8 1TS
Received October 19, 2007; E-mail: ian.manners@bristol.ac.uk
Abstract: Cyclopentadienyl (Cp) ligands in moderately strained [1]- and [2]ferrocenophanes [Fe{(η5-C5H4)2-
(ERx)y}: Fe{(η5-C5H4)2SiMe2} (1), Fe{(η5-C5H4)CH2}2 (10)] and highly strained [2]ruthenocenophanes [Ru-
{(η5-C5H4)CR2}2 {R ) H (15), Me (16)}] are susceptible to partial substitution by P donors and form mixed-
hapticity metallocycless[M(L2){(η5-C5H4)(ERx)y(η1-C5H4)}]: [Fe(dppe){(η5-C5H4)SiMe2(η1-C5H4)}] (5),
[Fe(dmpe){(η5-C5H4)SiMe2(η1-C5H4)}] (6), [Fe(dmpe){(η5-C5H4)(CH2)2(η1-C5H4)}] (11), [Ru(dmpe){(η5-C5H4)-
(CH2)2(η1-C5H4)}] (17), [Ru(dmpe){(η5-C5H4)(CMe2)2(η1-C5H4)}] (18), and [Ru(PMe3)2{(η5-C5H4)(CH2)2(η1-
C5H4)}] (19)sthrough haptotropic reduction of one η5-, π-bound Cp to η1, σ-coordination. These reactions
are strain-controlled, as highly ring-tilted [2]ruthenocenophanes 15 and 16 [tilt angles (R) ≈ 29-31°] react
without irradiation to form thermodynamically stable products, while moderately strained [n]ferrocenophanes
1 and 10 (R ≈ 19-22°) require photoactivation. The iron-containing photoproducts 5 and 11 are metastable
and thermally retroconvert to their strained precursors and free phosphines at 70 °C. In contrast, the
unprecedented ring-opening polymerization (ROP) of the essentially ring-strain-free adduct 6 to afford poly-
(ferrocenyldimethylsilane) [Fe(η5-C5H4)2SiMe2]n (Mw ≈ 5000 Da) was initiated by the thermal liberation of
small amounts of P donor. Unlike reactions with bidentate analogues, monodentate phosphines promoted
photolytic ROP of ferrocenophanes 1 and 10. MALDI-TOF analysis suggested a cyclic structure for the
soluble poly(ferrocenyldimethylsilane), 8-cyclic, produced from 1 in this manner. While the polymer likewise
produced from 10 was insoluble, the initiation step in the ROP process was modeled by isolation of a
tris(phosphine)-substituted ring-opened ferrocenophane [Fe(PMe3)3{(η5-C5H4)(CH2)2(C5H5)}][OCH2CH3]
(13[OCH2CH3]) generated by irradiation of 10 and PMe3 in a protic solvent (EtOH). Studies of the cation
13 revealed that the Fe center reacts with a Cp- anion with loss of the phosphines to form [Fe(η5-C5H5)-
{(η5-C5H4)(CH2)2(C5H5)}] (14) under conditions identical to those of the ROP experiments, confirming the
likelihood of “back-biting” reactions to yield cyclic structures or macrocondensation to produce longer chains.
Introduction
The cyclopentadienyl anion and its derivatives (C5H5-nRn;
“Cp”) are widely used cyclic polyenyl π-ligands in organome-
tallic complexes.1 This popularity owes much to stable metal-
Cp bonding and ancillary occupation of multiple coordination
sites that allow selective chemistry to proceed in the remaining
vacant coordination space at the metal center. Additionally,
isomerization between geometrical bonding arrangements (il-
lustrated in Figure 1), haptotropic shifts, can produce coordi-
native unsaturation and thereby enhance the reactivity of a metal
complex.2
Figure 1. Representative bonding modes of the cyclopentadienide (Cp-)
anion.
The tendency of complexes with metal-Cp bonds to engage
haptotropic flexibility during associative substitution reactions
is known to be amplified by electron-withdrawing substituents,3,4
and the most well-studied example, the “indenyl effect”,
highlights how a π system contiguous to one or more of the
Cp rings assists the ligand to seamlessly adjust to the electronic
requirements of a metal center.5 Group 6 metallocene derivatives
have also been shown to undergo well-characterized haptotropic
shifts of Cp ligands.6
† Present address: Chemical Resources Laboratory, Tokyo Institute of
Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan.
‡ X-Ray Crystallography, Department of Chemistry, University of
Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S 3H6.
(1) A recent publication cited that an estimated 80% of reported organometallic
compounds contain cyclopentadienyl or derivative ligands. See: (a) Tamm,
M.; Kunst, A.; Bannenberg, T.; Randoll, S.; Jones, P. G. Organometallics
2007, 26, 417 and references therein. Metallocenes containing heteroatom-
substituted Cp analogues (e.g., 1,2-diaza,3,5-diborolyl ligands) have also
been reported. See, for example: (b) Ly, H. V.; Forster, T. D.; Parvez, M.;
McDonald, R.; Roesler, R. Organometallics 2007, 26, 3516.
In dx (x > 4) [n]metallocenophanes (or ansa-metallocenes),
ring-strain increases the favorability of Cp ligand substitution
9
4166
J. AM. CHEM. SOC. 2008, 130, 4166-4176
10.1021/ja077334+ CCC: $40.75 © 2008 American Chemical Society

Haptotropic Shifts of Cp Ligands in Metallocenophanes
A R T I C L E S
Figure 2. Definition of geometric parameters R, â, δ, θ, and τ in [1]- and
[2]-bridged metallocenophanes. These angles become meaningful as qualita-
tive measures of ring strain for dx (x > 4) [n]metallocenophanes with short
(n e 2) bridges.13
In contrast, relatively few examples have been reported in
which strain is relieved through breaking a metal-Cp bond.
Highly strained, boron-bridged [1]ferrocenophanes [e.g., 2
(ERx)y ) BN(iPr)2, R ) 31°] undergo unexpected ring-opening
chemistry involving an Fe-Cp bond on reaction with metal
carbonyls, for example, Fe2(CO)9, to yield bimetallic species
3.14 Miyoshi and co-workers have shown that UV irradiation
of phosphorus-bridged [1]ferrocenophanes 2 [(ERx)y ) P(S)-
Ph] in the presence of phosphite ligands (e.g., P(OMe)3) yields
the ring-slipped product 4.15 Notably, mild photolytic ROP
procedures for [1]ferrocenophanes initiated by substitution of
one Cp ligand by moderate neutral15,16 or anionic17 bases have
recently been described. The increasing importance of Group 8
and 9 [1]- and [2]metallocenophanes as precursors to high
molecular weight functional metallopolymers through ROP10
has prompted our continued interest in their ring-opening
chemistry.18
reactions as ring tilting significantly raises the free energy and
weakens metal-Cp bonding.7 Nevertheless, in the wide range
of strained metallocenophanes with different bridging elements,8
Cp ligands retain a strong preference for η5-Cp coordination,
even with tilt angles (R, Figure 2) greater than 30° and â angles
of up to 43°.9 Accordingly, the majority of the reactivity
observed for these species involves polarized bonds between
the ipso carbon and the bridging element. For example, ring-
opening polymerization (ROP)10 (eq 1) of sila[1]ferrocenophanes
(R ≈ 19-22°, â ≈ 37-41°) initiated by anionic reagents such
as nBuLi11 or transition-metal catalysts12 has been shown to
proceed via Cipso-Si bond cleavage.
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In this article, we report our detailed comparative studies19
of the photolytic and thermal reactivity of several [1]- and
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Int. Ed. 2007, 46, 5082. For strained rings based on Cp-M moieties where
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3321. (j) Schachner, J. A.; Tockner, S.; Lund, C. L.; Quail, J. W.; Rehahn,
M.; Mu¨ller, J. Organometallics 2007, 26, 4658. For work on strained
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Kunst, A.; Bannenberg, T.; Herdtweck, E.; Sirsch, P.; Elsevier, C. J.;
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1326.
(6) For example, CrCp2 reacts with nitric oxide to yield Cr(η5-C5H5)(η1-C5H5)-
(NO)2 at 0 °C in solution, while reduction of W(Cp)2Cl2 under a high
pressure of CO yielded the unique mixed hapticity complex, W(η5-C5H5)-
(η3-C5H5)(CO)2, the first crystallographically characterized example of an
η3-Cp ligand in a M(Cp)2L2 complex. See: Cr (a) Hames, B. W.; Legzdins,
P.; Martin, D. T. Inorg. Chem. 1978, 17, 3644. W (b) Wong, K. L. T.;
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(9) Two examples can be found to the contrary. (1) In the ethene-bridged [2]-
ferrocenophane, ansa-(vinylene)ferrocene, the ipso carbons are located
slightly out of the plane formed by the remaining ring carbons with a fold
angle of 3.4°. See: (a) Aggarwal, V. K.; Jones, D.; Turner, M. L.; Adams,
H. J. Organomet. Chem. 1996, 524, 263 and (b) Buretea, M. A.; Tilley, T.
D. Organometallics 1997, 16, 1507. (2) In metallocenophanes formed
from heterocyclic η5-germacyclopentadienyl analogues, buckling of the
constituent atoms in the Cp heterocycle reduces the overall strain otherwise
expected from introducing a short ansa bridge. See: (c) Freeman, W. P.;
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1734.
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263. Unstrained, early transition-metal ansa-metallocene complexes are of
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9
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A R T I C L E S
Herbert et al.
[2]metallocenophanes, in which we demonstrate that either
reVersible or irreVersible haptotropic rearrangements from η5-
to η1-Cp are observed depending on the degree of strain present.
Results
1. Irradiation of the Sila[1]ferrocenophane 1 in the
Presence of P Donors. Our entry point was the moderately
strained sila[1]ferrocenophane 1 [R ) 20.8(5)°],20 a well-studied
monomer known to form high molecular weight poly(ferroce-
nylsilanes) through ROP.21 Irradiation of a slight excess of 1 in
the presence of 1,2-bis(diphenylphosphino)ethane (dppe) in THF
at 5 °C led to a haptotropic shift of a Cp ligand from η5 to η1
and coordination of the bidentate phosphine ligand to afford
the blood red solid [Fe(dppe){(η5-C5H4)SiMe2(η1-C5H4)}] (5)
in 90% yield (eq 2). The UV-vis spectroscopic profile of the
deeply colored solid is dominated by a high-energy transition
at 326 nm (ꢀ ) 3400 M-1 cm-1) that tails across the visible
region of the spectrum with a slight shoulder appearing around
440 nm. In contrast, 1 is a red crystalline solid with a well-
defined λmax at 478 nm (ꢀ ) 240 M-1 cm-1).22
Figure 3. ORTEP of 5 with displacement ellipsoids shown at the 30%
probability level. Hydrogen atoms are depicted only for the η1-Cp ring.
Selected bond distances (Å) and angles (°): Fe(1)-Cpcentroid 1.736(2), Fe-
(1)-P(1) 2.1543(6), Fe(1)-P(2) 2.1544(6), Fe(1)-C(6) 2.015(2), Si(1)-
C(1) 1.862(2), Si(1)-C(7) 1.884(2), C(6)-C(7) 1.524(3), C(6)-C(10)
1.367(3), C(7)-C(8) 1.483(3), C(8)-C(9) 1.343(4), C(9)-C(10) 1.464(3),
Cpcentroid-Fe(1)-C(6) 119.7(9), Cpcentroid-Fe(1)-P(1) 129.8(9), Cpcentroid
-
Fe(1)-P(2) 131.5(9), C(1)-Si(1)-C(7) 99.42(10), Fe(1)-C(6)-C(7) 118.01-
(16), Si(1)-C(7)-C(6) 100.07(14).
The 31P{1H} NMR spectrum of 5 consisted of slightly
broadened AB doublets at δ ) 109.4 and 108.5 ppm23 with the
same coupling constant [2J(P,P) ) 27.2 Hz] assigned to the
diastereotopic phosphorus atoms coordinated to the iron center.
The 29Si{1H} NMR spectrum of 5 consisted of a single broad
resonance at δ ) -8.7 ppm, shifted slightly upfield compared
to that of 1 (δ ) -4.6 ppm).
Figure 3 shows the molecular structure of 5 determined by
X-ray diffraction. The Fe center in photoproduct 5 possesses
piano-stool geometry. The C(1)-Si(1)-C(7) angle [99.42(10)°]
is larger than that of 1 [95.7(4)°], and the angle â of 5 [14.0-
(3)°], between the C(1)-Si(1) bond and its projection onto the
plane formed by cyclopentadienyl carbons C(1-5), has de-
creased significantly [from 37.0(6)° for 1], consistent with
reduced ring strain in 5 compared with 1.20 The bond lengths
for C(6)-C(10) [1.367(3) Å] and C(8)-C(9) [1.343(4) Å] in
the σ-bonded Cp ring are shorter than other bond lengths
[average 1.490(3) Å], indicating that the double bonds are
located in these positions in the solid state.
Figure 4. 1H NMR spectra of 5 in C6D5CD3 upon being heated to 70 °C.
Asterisks mark peaks from residual toluene (NMR solvent: C6D5CD3) and
∆ notes residual THF from sample preparation.
In solution, inequivalent proton and carbon signals for the
1
differently coordinated Cp rings were assigned by H-1H and
1H-13C COSY NMR spectroscopy at -40 °C. Interestingly,
one 1H nucleus in each Cp ring was observed at higher field (δ
) 2.82 ppm for η1-Cp; δ ) 3.03 ppm for η5-Cp). Upon raising
1
the temperature of the solution to 25 °C, all signals in the H
and 31P{1H} NMR spectra appeared broadened, suggesting
fluxional behavior around the Fe center. However, at 25 °C,
the two ligands do not interchange quickly on the time scale of
(17) (a) Tanabe, M.; Manners, I. J. Am. Chem. Soc. 2004, 126, 11434. (b)
Tanabe, M.; Vandermeulen, G. W. M.; Chan, W. Y.; Cyr, P. W.; Vanderark,
L.; Rider, D. A.; Manners, I. Nat. Mater. 2006, 5, 467. (c) Chan, W. Y.;
Lough, A. J.; Manners, I. Organometallics 2007, 26, 1217.
(18) Ieong, N. S.; Chan, W. Y.; Lough, A. J.; Haddow, M. F.; Manners, I.
Chem.-Eur. J. 2008, 14, 1253.
1
the H and 13C NMR spectroscopy experiments, as distinct
chemical environments are visible for two Cp rings.
(19) For a preliminary communication of some of the initial results of this work,
see: Tanabe, M.; Bourke, S. C.; Herbert, D. E.; Lough, A. J.; Manners, I.
Angew. Chem., Int. Ed. 2005, 44, 5886.
Remarkably, on heating at 70 °C in C6D5CD3 for 3 h,
photoproduct 5 underwent dissociation of the dppe ligand and
quantitative retroconversion into 1 (eq 2). The simplification
(20) Finckh, W.; Tang, B. Z.; Foucher, D. A.; Zamble, D. B.; Ziembinski, R.;
Lough, A.; Manners, I. Organometallics 1993, 12, 823.
(21) Manners, I. Chem. Commun. 1999, 857.
1
of the 13C and H NMR spectra (Figure 4) is accompanied by
(22) Fischer, A. B.; Kinney, J. B.; Staley, R. H.; Wrighton, M. S. J. Am. Chem.
Soc. 1979, 101, 6501.
the reappearance of the characteristically upfield-shifted signal
assigned to the ipso carbon atoms (33.2 ppm) and a downfield
shift of the single resonance observed by 29Si NMR spectroscopy
(-4.6 ppm), which are retained upon cooling to 20 °C. It appears
(23) These chemical shifts are similar to those of the dppe-coordinated Fe
complex [{(η5-Cp)Fe(dppe)}2(µ-CHdCHCHdCH)]. See: Chung, M.-C.;
Gu, X.; Etzenhouser, B. A.; Spuches, A. M.; Rye, P. T.; Seetharaman, S.
K.; Rose, D. J.; Zubieta, J.; Sponsler, M. B. Organometallics 2003, 22,
3485.
9
4168 J. AM. CHEM. SOC. VOL. 130, NO. 12, 2008

Haptotropic Shifts of Cp Ligands in Metallocenophanes
A R T I C L E S
that 5 is metastable at room temperature and that its formation
relies on the photoactivation of 1. At elevated temperatures,
the greater entropy of the species on the left side of eq 2 likely
dominates the enthalpic penalty incurred by re-forming the
moderately strained 1.24
Compound 5 slowly dissociates in solution even at room
temperature (23 °C). A 31P NMR resonance corresponding to
dissociated dppe (-12 ppm) appeared (5/dppe ≈ 30:1) after a
sample of 5 was left in C6D6 solution for 1 week. The addition
of a donor solvent (THF, 100 equiv) accelerated this process,
and a similar ratio was observed between the integrals of the
31P signals of 5 and dppe after 1 h. Interestingly, under these
1
conditions, H NMR spectroscopic analysis revealed signals
corresponding to 5 and the strained metallocenophane 1 in a
ratio of 6:1 after 18 h. The ratio of 5/dppe, however, was 3:1,
which suggested that the dissociation of a molecule of dppe
from 5 does not immediately lead to the regeneration of a
molecule of 1, though no solvated intermediates were detected
(vide infra). To probe the reactivity of 5 in the presence of excess
dppe, 31P-31P EXSY experiments were conducted for a variety
of mixing times (50, 100, and 250 ms) at 22 °C. No cross-
peaks were observed between coordinated and free phosphine,
demonstrating that no significant exchange is occurring on the
NMR time scale at this temperature. The broadening of the
phosphorus resonances in the 31P NMR spectrum of 5 at room
temperature is therefore not likely due to phosphine exchange.
The reactivity of 5 in the presence of a better σ-donating
phosphine, 1,2-bis(dimethylphosphino)ethane (dmpe), is dis-
cussed below.
After similar irradiation of 1 with dmpe, a photoinduced
haptotropic shift of a Cp ligand led to the formation of [Fe-
(dmpe){(η5-C5H4)SiMe2(η1-C5H4)}] (6), which was isolated as
deep purple crystals (eq 3). Mass spectrometry (EI) and
elemental analysis supported the identification of the crystalline
solid as the adduct 6. A tailing similar to the case of 5 across
the visible region was observed, and shoulders at 350 and 470
nm were detected in the UV-vis spectrum.
Figure 5. ORTEP of 6 with displacement ellipsoids shown at the 30%
probability level. Hydrogen atoms are omitted except for the η1-Cp ring.
Selected bond distances (Å) and angles (°): Fe(1)-Cpcentroid 1.717(10), Fe-
(1)-P(1) 2.176(3), Fe(1)-P(2) 2.179(3), Fe(1)-C(14) 2.249(9), Si(1)-
C(1) 1.867(9), Si(1)-C(18) 1.833(10), C(18)-C(17) 1.371(13), C(17)-
C(16) 1.429(14), C(16)-C(15) 1.375(14), C(15)-C(14) 1.453(14), C(14)-
C(18), 1.468(13), Cpcentroid-Fe(1)-C(14) 120.3(4), Cpcentroid-Fe(1)-P(1)
125.1(3), Cpcentroid-Fe(1)-P(2) 127.3(3), C(1)-Si(1)-C(18) 100.9(4), Fe-
(1)-C(14)-C(18) 105.1(6), Si(1)-C(18)-C(14) 120.2(7).
Table 1. Effect of Temperature on the Proposed Equilibrium
between 6a and 6b
T (K)
integral ratio 6b/6aa
∆
Geq (kJ/mol)b
273
253
233
213
193
5.4:1
6.3:1
8.1:1
11.3:1
16.5:1
-3.8
-3.9
-4.1
-4.3
-4.5
a Determined from 31P{1H} NMR spectroscopy. b Calculated from ∆G
) -RT ln Keq.
2
sharpened into two doublets (δ ) 88.2 and 85.4 ppm, J(P,P)
) 38 Hz) while the broad resonance (δ ) 78 ppm) narrowed
to a singlet (δ ) 80 ppm, -80 °C). These two independent
spin systems could arise from two possible structural isomers:
one with diastereotopic phosphorus environments (e.g., 6a) and
one without (e.g., 6b). The apparent equilibrium between the
two isomers slightly favors the symmetric isomer, increasingly
so at lower temperatures (eq 4, Table 1).
Single crystals of 6 were also characterized by X-ray
diffraction. The structure of 6 in the solid state (Figure 5) is
analogous to that of 5: the C(1)-Si(1)-C(18) angle [100.9-
(4)°] is larger than that in 1 [95.7(4)°] and the angle â (16.5°)
has again decreased significantly. Notably, at 2.249(9) Å, the
Fe-C(14) distance between the metal center and the σ-bound
Cp ring is significantly longer (0.23 Å) than the Fe-C(η1-Cp)
bond distance in 5 [2.015(2) Å].
The molecular structure of 6 in solution is more ambiguous
than that for 5. At 20 °C, the 31P{1H} NMR spectrum of an
analytically pure sample of 6 showed a single broad signal
centered at δ ) 78 ppm. At 0 °C, two broad signals appeared
downfield from the single resonance, and at -20 °C, these
Interestingly, unlike the case of 5, heating a C6D6 solution
of 6 at 55 °C for 4 h failed to regenerate the sila[1]-
ferrocenophane 1 as an identifiable product. The 1H NMR
spectra of a C6D6 solution of 6 did not indicate any retrocon-
version when monitored in 10° increments up to 75 °C over 1
h. However, a small proportion of uncoordinated dmpe [δ(31P-
{1H}) ) -48 ppm; ratio of 6/dmpe ≈ 200:1] was observed on
reaching 55 °C. After further heating over an additional 12 h,
100% conversion of the 31P NMR signal for 6 [δ(31P{1H}) )
77.6 ppm] into a new AMX spin system [δ(31P{1H}) ) 75.2
(doublet, 2J(P,P) ) 57 Hz), 34.9 (doublet of triplets, 2J(P,P) )
(24) The AB doublets in the 31P NMR spectra of 5 appear to collapse
asymmetrically approaching 70 °C, possibly because of the initial release
of a single end of the bis(phosphine) ligand. See Supporting Information.
3
3
57 Hz, J(P,P) ) 22 Hz), -47.3 ppm (doublet, J(P,P) ) 22
Hz)] and free dmpe (-48 ppm) was observed. 1H NMR
9
J. AM. CHEM. SOC. VOL. 130, NO. 12, 2008 4169

A R T I C L E S
Herbert et al.
Table 2. Thermal Ring-Opening Polymerizationa of 6
tentatively assigned in situ by the appearance of an AMX spin
system in 31P{1H} NMR spectra identical to that observed
following the initial heating of 6 for an extended period (12 h).
The proposed structure of 7 was supported by 1H NMR
spectroscopic characterization and electrospray mass spectrom-
etry (ESI-MS) analysis of a freshly prepared sample (Experi-
mental Section in Supporting Information); however, repeated
attempts at the further isolation of 7 in the solid state led only
to insoluble decomposition products.
[6] (M)
T (°
C)a
yield (%)b
Mw
PDI
0.01
0.10
0.24
0.51
0.98
60
60
55
55
55
<5
29
37
40
42
3410
5230
5070
4104
1.35
1.37
1.35
1.35
a Reactions were all performed in THF and heated for 12 h. No unreacted
6 was observed after this time. b Isolated yield of polymer after multiple
precipitations (2-3) into MeOH.
To elaborate on the proposed propagation step in the thermal
polymerization of 6, we also investigated the reactivity of this
species toward Cp anions. Na[C5H5] and 6 were stirred in THF
in the absence of light. After 1 h at 25 °C, dmpe was substituted
from 6 (observed by 31P{1H} NMR spectroscopy) and the color
of the solution changed from black to bright yellow (eq 5). The
previously reported ring-opened species 9 was subsequently
Scheme 1. Proposed Mechanism for the Thermal Polymerization
of 6
1
identified by H NMR spectroscopy.17a While 1 is unreactive
toward Cp anions at 25 °C in THF in the absence of light,17a
[C5H5]- was found to initiate the ROP of 1 in polar solvents
(THF, pyridine) at elevated temperatures (55 °C). Therefore, if
the process to generate the pendant Cp- propagating anion
begins with the thermal dissociation of dmpe from a small
amount of 6 re-forming 1, the regenerated metallocenophane
could be consumed as additional monomer by a propagating
polymer chain at 55 °C.
While 5 was found to be unreactive in the presence of excess
dppe, in the presence of a better σ-donor phosphine (dmpe) the
ring-slipped compound 5 was completely consumed over 2 h.
The addition of dmpe in THF, however, did not lead to
substitution of the η1-Cp ring at the Fe center, rather quantitative
phosphine exchange produced 6 and uncoordinated dppe
(Scheme 2). As the coordinated dppe ligand in 5 was observed
to dissociate in the presence of THF (vide supra), the addition
of dmpe likely intercepts a solvated transient (η5, η1) ferro-
cenophane (which may possess a monodentate dppe ligand)
before it can retroconvert to 1.
spectroscopic analysis suggested that the thermal treatment of
6 over 12 h had formed poly(ferrocenyldimethylsilane) 8 with
an end group consisting of one bidentate and one monodentate
dmpe ligand (seen in the AMX spin system observed by 31P-
{1H} NMR spectroscopy) and free dmpe. Repeat trials at various
initial molar concentrations of 6 produced identical results, and
the molecular weights of the polymers were determined by gel
permeation chromatography (GPC, Table 2).
A plausible mechanism for the thermal ring-opening polym-
erization of 6, outlined in Scheme 1, begins with thermal
dissociation of dmpe from a small fraction of 6, which then
can substitute the η1-Cp ring from a second equivalent to form
the zwitterionic species 7. The latter could then function as an
ROP initiator, assuming the rate of propagation of the growing
polymer chain exceeds the rate of addition of a second
equivalent of dmpe to 6. Thus, the pendant Cp anion of 7 could
initiate the thermal ring-opening polymerization of the remaining
ring-slipped species 6 with extrusion of dmpe, or alternatively
via the ROP of 1. As noted previously, Miyoshi et al. have
reported polymerization of phosphite adducts of phospha[1]-
ferrocenophanes (e.g., 4) in refluxing THF16 and Cp anions
have been shown to induce the ROP of 1 upon irradiation at
5-25 °C.17
Scheme 2. Possible Mechanism for Phosphine Exchange To
Produce 6 from 5
Irradiation of a THF solution of 1 and 1,2-bis(di-tert-
butylphosphino)ethane (dtbpe) produced a yellow-orange reac-
tion mixture. No evidence of coordination of 1 equiv of dtbpe
to trap a mixed hapticity metallocenophane was observed.
To examine the likelihood for the formation of the proposed
initiator (7), two additional equivalents of dmpe were added to
a solution of 6 at 25 °C. After 2 days, the zwitterionic 7 was
9
4170 J. AM. CHEM. SOC. VOL. 130, NO. 12, 2008

Haptotropic Shifts of Cp Ligands in Metallocenophanes
A R T I C L E S
Scheme 3. Photocontrolled Polymerization Behavior of 1 with
Table 3. Photolytic ROP of 1 Initiated by P Donors
Phosphine Initiators
initiator
b
b
run
(M/I)a
solvent
conditions
Mn
Mw
PDI
MALDIc
1
2
3
4
5
6
7
8
PMe3 (1:2) C6D6
PMe3 (1:3) THF
PPh3 (1:1) THF
PPh3 (1:5) THF
PPh3 (1:3) THFd
PPh3 (1:3) Me3SiCl
PPh3 (1:3) Me3SiCl
dtbpe (1:1) THF
5 h
5 h
3 h
3 h
2 h
5 h
3 h
3 h
25 °C 13000 18850 1.4
5 °C 2450 3140 1.3 4114 (17)
5 °C 27420 146470 5.3 9446 (39)
5 °C 24510 66650 2.7
5 °C 20370 46850 2.3 8470 (35)
5 °C 5590
5 °C 4950
5 °C 4450
7120 1.3
7790 1.6 1695 (7)
4980 1.1 5564 (23)
24880 39280 1.6
a Ratio of monomer to initiator. b Determined by conventional GPC.
c Highest MW peak observed in MALDI-TOF MS (linear mode) with degree
of polymerization (DPn) in the adjacent parentheses. Under our experimental
conditions, only lower molecular weight fractions (<10 000 Da) ionized.
d Solvent: THF + 1 equiv of Me3SiCl.
Instead, as suggested by the color of the reaction mixture, ROP
of 1 to form polyferrocenylsilane [Fe(η5-C5H4)2SiMe2]n was
confirmed by 1H NMR spectroscopy. GPC analysis showed the
product possessed a bimodal molecular weight distribution (Mn
) 4450, PDI ) 1.1 and Mn ) 24 880, PDI ) 1.6). However,
no evidence of incorporation of dtbpe end groups into the
1
polymer chain could be detected by H or 31P NMR spectros-
Figure 6. MALDI-TOF MS (linear (+) mode with dithranol as matrix) of
dtbpe-initiated 8-cyclic (Table 3, run 8). The most intense peak (4600 m/z)
corresponds to a chain length of 19 repeat units.
copy despite the low molecular weights.25a MALDI-TOF MS
analysis of the low molecular weight fraction of the polymer
suggested that back-biting by the propagating Cp anion to
replace the bulky P donor ligand had occurred, resulting in a
mass spectrum corresponding to exact integer values of the
monomer 1 consistent with the cyclic structure of 8-cyclic
(Scheme 3, Figure 6).25b The higher molecular weight fraction
of the bimodal distribution may have resulted from macrocon-
densation of two or more polymer chains. However, the end
group structure of the higher molecular weight fraction could
not be confirmed as this fraction did not ionize under the
experimental conditions employed.
Monodentate phosphines (PR3: R ) Me, Ph) were found to
promote photolytic ROP of 1 at 5-25 °C regardless of reaction
stoichiometry (Scheme 3, Table 3). Irradiation of 1 with excess
PMe3 in C6D6 at room temperature for 5 h afforded the ring-
opened polymer [Fe(η5-C5H4)2SiMe2]n with 100% conversion
and in 35% isolated yield (Mn ) 1.3 × 104, PDI ) 1.4),26
supporting the assertion that the rate of polymer propagation is
faster than the rate of formation of ring-slipped adducts.
Surprisingly, the photocontrolled polymerization induced by
PPh3 even occurred in the presence of Me3SiCl (Table 3, runs
5-7), which has been previously used as a reagent for anionic
chain termination. A few drops of a protic solvent (e.g., water)
are typically added to polymer solutions to quench the “living”
chain end produced by anionic ROP of [n]metallocenophanes.11
In place of protic quenching, chloro(triorgano)silanes have been
used to assist in the detection of polymer end groups.25 The
formation of polymer in neat Me3SiCl (Table 3, runs 6 and 7)
indicates that chain propagation is faster than the substitution
reaction of the propagating Cp- anion with the chlorosilane.27
While monomer conversions were typically high (∼80%),
isolated polymer yields were low (20-40%). Shorter oligomers,
which did not precipitate during the polymer workup, and ring-
opened monomer were identified as the other major products
by 1H and 31P NMR spectroscopy. For example, while a broad
singlet in the 31P{1H} NMR spectrum (at 25 ppm) suggested
coordinated P donors for run 2, no end groups were observed
for any of the isolated polymers by multinuclear NMR
spectroscopy, consistent with the cyclic structures suggested by
MALDI-TOF analysis (Scheme 3, Table 3).
2. Irradiation of the [2]Ferrocenophane 10 in the Presence
of P Donors. To explore the generality of the surprising
reversible haptotropic shifts and other unusual [1]ferrocenophane
reactivity of 1 described above, we investigated the analogous
chemistry of the ethane-bridged [2]ferrocenophane 10 [R )
21.6(4)°],28 which has a tilt angle similar to that of 1, suggesting
a similar degree of ring strain. No reaction was observed upon
irradiation of 10 in the presence of dppe in THF. However,
another ring-slipped Fe complex, [Fe(dmpe){(η5-C5H4)(CH2)2-
(25) (a) Multinuclear NMR spectroscopy has been used to identify Me3Si end
groups in oligoferrocenylsilanes with molar masses up to 2000 Da. See:
ref 11 and Rulkens, R.; Lough, A. J.; Manners, I.; Lovelace, S. R.; Grant,
C.; Geiger, W. E. J. Am. Chem. Soc. 1996, 118, 12683. (b) For a recent
report on the production and characterization of metallomacrocycles and
cyclic polymers, see: Chan, W. Y.; Lough, A. J.; Manners, I. Angew. Chem.,
Int. Ed. 2007, 46, 9069.
(27) The lower molecular weights obtained from P donor promoted ROP in
neat Me3SiCl are likely due to a solvent polarity effect (compare runs 4-7,
Table 3).
(28) (a) Lentzner, H. L.; Watts, W. E. Tetrahedron 1971, 27, 4343. (b) Nelson,
J. M.; Rengel, H.; Manners, I. J. Am. Chem. Soc. 1993, 115, 7035. (c)
Nelson, J. M.; Nguyen, P.; Petersen, R.; Rengel, H.; Macdonald, P. M.;
Lough, A. J.; Manners, I.; Raju, N. P.; Greedan, J. E.; Barlow, S.; O’Hare,
D. Chem.-Eur. J. 1997, 3, 573.
(26) Foucher, D. A.; Tang, B. Z.; Manners, I. J. Am. Chem. Soc. 1992, 114,
6246.
9
J. AM. CHEM. SOC. VOL. 130, NO. 12, 2008 4171

A R T I C L E S
Herbert et al.
scale, while the η5-Cp ring signals display no coalescence
because of more rapid ring rotation. The dynamic nature of the
NMR spectrum of 11 suggests fluxional behavior, which may
involve fast successive 1,2-shifts of the η1-Cp ring. Slow
coordination mode exchange between the two Cp ligands can
be ruled out by virtue of the four inequivalent resonances, two
arising from inequivalent quaternary carbon nuclei and two
attributed to the distinct bridging ethylene carbons, observed at
room temperature by 13C NMR spectroscopy. As with 5, heating
11 in C6D5CD3 for 3 h at 70 °C led to a quantitative reverse
haptotropic rearrangement with dissociation of the dmpe ligand
and the regeneration of moderately strained 10 (eq 6).
Also analogous to the behavior of the [1]ferrocenophane 1,
monodentate P donors promoted ROP of 10. Irradiation of a
solution of 10 with 3 equiv of PMe3 for 3 h in C6D6 led to the
yellow insoluble polymer 12 in 66% isolated yield (eq 7).28b,c
To directly observe the role of the P donor in promoting ROP,
we reinvestigated this reaction with the aim of trapping the
zwitterionic intermediate generated in the initiation step.
However, as the quenching reaction of pendant Cp- anions
generated from ring-opened metallocenophanes with chloro-
(triorgano)silane capping agents was found to be inefficient for
1 (section 1, Table 3), we sought a more reactive quenching
source and employed a protic solvent (ethanol).30
Irradiation of 10 and 3 equiv of trimethylphosphine in ethanol
(eq 8) produced a yellow reaction mixture that showed a new
31P{1H} resonance at 25 ppm attributed to a ring-opened
product. The product 13[OCH2CH3] was isolated in 80% yield
as a mustard-yellow powder that was further analyzed by ESI-
MS. Exchanging ethoxide for a larger tetraphenylborate anion
with Na[BPh4] allowed for the isolation of an air-stable yellow
powder. In solution, two structural isomers that differ only in
the position of the fifth proton on the pendant cyclopentadienyl
ring (cf. 6, eq 4) were observed in a 2:1 ratio (13a/13b) by 1H
NMR spectroscopy. Dark orange single crystals of the minor
isomer, 13b[BPh4], suitable for X-ray diffraction were grown
from diethylether-rich solutions of THF and diethylether.
Figure 7. ORTEP of 11 with displacement ellipsoids shown at the 30%
probability level. Hydrogen atoms are depicted only for the η1-Cp ring.
Selected bond distances (Å) and angles (°): Fe(1)-Cpcentroid 1.717(5), Fe-
(1)-P(1) 2.167(2), Fe(1)-P(2) 2.178(1), Fe(1)-C(12) 2.201(5), C(1)-C(6)
1.491(8), C(6)-C(7) 1.516(9), C(7)-C(8) 1.512(7), C(8)-C(9) 1.352(8),
C(8)-C(12) 1.476(7), C(9)-C(10) 1.428(8), C(10)-C(11) 1.350(8), C(11)-
C(12) 1.465(7). Cpcentroid-Fe(1)-C(12) 120.1(2), Cpcentroid-Fe(1)-P(1)
125.1(2), Cpcentroid-Fe(1)-P(2) 128.0(2), C(1)-C(6)-C(7) 111.6(5), C(6)-
C(7)-C(8) 113.5(5), C(7)-C(8)-C(12) 124.6(5), Fe(1)-C(12)-C(8)
110.6(3).
(η1-C5H4)}] (11), was obtained from irradiation of 10 in the
presence of dmpe in THF at 25 °C (eq 6). Recrystallization of
the crude product from toluene/hexane at -55 °C afforded 11
as dark red crystals in 83% yield.
Complex 11 was fully characterized by one- and two-
dimensional NMR spectroscopic methods at low temperature,
and the molecular structure was determined by X-ray diffraction
(Figure 7). The piano-stool Fe center is again capped by an
η5-Cp ligand, while the chelating dmpe ligand and σ-bound Cp
ring are arranged as a tripod base. The C(1)-C(6) bond was
found to lie almost in the plane of the η5-Cp ligand [â ) 1.9-
(3)°], indicating that the structure contains no significant ring
strain. The two double bonds in the η1-Cp ring were located at
C(8)-C(9) [1.352(8) Å] and C(10)-C(11) [1.350(8) Å], sup-
ported by the conformation about C(12), which is unsuited to
an sp2 carbon center.
Variable-temperature 31P{1H} NMR spectra of 11 exhibited
two doublets at δ ) 81.0 and 78.9 ppm (2J(P,P) ) 39 Hz) at
-40 °C that coalesced into a single signal at 79.0 ppm upon
warming to 20 °C. At -60 °C, the 1H NMR spectrum contained
eight signals attributed to the two inequivalent Cp rings in the
range δ ) 1.70-6.64 ppm. When the temperature was raised
to -20 °C, all signals broadened and coalescence was observed
around 0 °C (Supporting Information).
The fluxional behavior of σ-bound Cp rings such as those in
[Fe(η1-Cp)(η5-Cp)(CO)2] has been well studied.29 The η1-Cp
ligand of this species is involved in a fluxional process that
consists of successive 1,2-shifts, observable on the NMR time
The solid-state structure of 13b[BPh4] (Figure 8) contains a
pendant, protonated cyclopentadienyl group directed away from
(29) (a) Bennett, M. J., Jr.; Cotton, F. A.; Davison, A.; Faller, J. W.; Lippard,
S. J.; Morehouse, S. M. J. Am. Chem. Soc. 1966, 88, 4371. (b) Cotton, F.
A. Acc. Chem. Res. 1968, 1, 257. (c) Wright, M. E.; Nelson, G. O.; Glass,
R. S. Organometallics 1985, 4, 245.
(30) Protic solvents react with sila[1]ferrocenophanes via ring-opening proto-
nolysis at the Cipso-Si bond, and thus this reaction could not be explored
for species 1.
9
4172 J. AM. CHEM. SOC. VOL. 130, NO. 12, 2008

Haptotropic Shifts of Cp Ligands in Metallocenophanes
A R T I C L E S
Scheme 4. Reactivity of 13 with Na[C5H5]
Figure 9. ORTEP of 14 with displacement ellipsoids shown at the 30%
probability level. The position of the fifth hydrogen in the pendant
cyclopentadienyl ring is disordered between C(15a) (14a) and C(17a) (14b)
with equal occupancy. Only one isomer (14a) is shown for clarity, with
hydrogens omitted except for the cyclopentadienyl ring. Selected bond
distances (Å) and angles (°): C(13)-C(14) 1.352(6), C(14)-C(15a) 1.465-
(7), C(15a)-C(16) 1.382(6), C(16)-C(17a) 1.430(6), C(13)-C(17a) 1.430-
(6), C(13)-C(14)-C(15a) 109.2(4), C(14)-C(15a)-C(16) 107.3, C(15a)-
C(16)-C(17a) 109.0(4), C(16)-C(17a)-C(13) 106.5(4), C(14)-C(13)-
C(17a) 108.1(4).
Figure 8. ORTEP of the cation of 13b [BPh4] with displacement ellipsoids
shown at the 30% probability level. The anion and all hydrogens except
for the pendant cyclopentadienyl ring are omitted for clarity. Selected bond
distances (Å) and angles (°): Fe(1a)-P(1a) 2.202(1), Fe(1a)-P(2a) 2.215-
(1), Fe(1a)-P(3a) 2.222(1), Fe(1a)-Cpcentroid 1.734(5), C(1a)-C(6a) 1.518-
(7), C(6a)-C(7a) 1.551(8), C(7a)-C(8a) 1.497(8), C(8a)-C(12a) 1.342(8),
C(8a)-C(9a) 1.479(9), C(9a)-C(10a) 1.413(10), C(10a)-C(11a) 1.365-
(10), C(11a)-C(12a) 1.494(9), Cpcentroid-Fe(1a)-P(1a) 120.2(2), Cpcentroid
-
Fe(1a)-P(2a) 123.7(2), Cpcentroid-Fe(1a)-P(3a) 119.1(2), Cpcentroid-C(1a)-
C(6a) 6.7(5), C(1a)-C(6a)-C(7a) 112.3(4), C(6a)-C(7a)-C(8a) 113.3(5),
C(8a)-C(9a)-C(10a) 105.8(5), C(9a)-C(10a)-C(11a) 110.7(6), C(10a)-
C(11a)-C(12a) 106.6(6), C(11a)-C(12a)-C(8a) 108.4(6), C(12a)-C(8a)-
C(9a) 108.6(6).
Studies of the reactivity of 13 toward Cp anions allowed us
to investigate the “back-biting” step proposed to occur to
produce cyclic poly(ferrocenyldimethylsilane) 8-cyclic from the
photocontrolled, P donor promoted ROP of 1 (Scheme 3). No
change in the 31P{1H} NMR spectrum of 13[BPh4] was observed
after being stirred in THF with more than 1.5 equiv of Na-
[C5H5] for 1.5 h under ambient light. However, mimicking the
conditions of the P donor promoted polymerizations quan-
titatively liberated PMe3 after 1.3 h of irradiation at 5 °C
(Scheme 4). Thus, remarkably, the cyclization or macrocon-
densation side reactions involving phosphine substitution (Scheme
3) therefore likely operate under the photocontrolled ROP condi-
tions examined. The molecular structure of 14 is shown in Figure
9.
the piano-stool PMe3-substituted iron center. The bond angles
in cation 13b are not significantly altered from values typical
for similar organometallic complexes. For one of the two cations
located in the unit cell, the bond distances C(8)-C(12) [1.342-
(8) Å] and C(11)-C(10) [1.365(10) Å] indicate that the double
bonds of the terminal Cp ring are located between these carbons
and the fifth hydrogen is on C(9). However, for the second
cation, the C(11)-C(10) distance is slightly longer [1.438(14)
Å] than the C(9)-C(10) separation. Additionally, the small
internal ring angles about C(11) [106.6(6), 101.7(8)°], short
distances between C(9) and C(10) [1.413(10), 1.401(12) Å]
relative to sp3-sp2 distances in cyclopentadiene (computational
1.499 Å; microwave 1.505 Å),31 and large displacement
ellipsoids for some of the ring atoms suggest structural disorder
with a small proportion of C(11) sp3 hybridized (13a).
3. Reactions of [2]Ruthenocenophanes 15 and 16 with P
Donors. As the reactivity of [n]ferrocenophanes toward P donors
varies subtly with the electronic and steric properties of the
incoming Lewis base, we decided to investigate the effect of
increased ring strain in [n]metallocenophanes by exploring the
analogous reactivity of ethane-bridged [2]ruthenocenophanes
1532 and 16,33 which possess larger tilt angles of 29.6(5)° (15)
and 31.1° (16) by virtue of the larger covalent radius of Ru
relative to that of Fe.34 When reacted with excess dmpe without
irradiation, the new complexes [Ru(dmpe){(η5-C5H4)(CR2)2(η1-
(31) (a) Damiani, D.; Ferretti, L.; Gallinella, E. Chem. Phys. Lett. 1976,
37, 265. (b) Jiao, H.; von Rague´ Schleyer, P. Faraday Trans. 1994, 90,
1559.
9
J. AM. CHEM. SOC. VOL. 130, NO. 12, 2008 4173

A R T I C L E S
Herbert et al.
Figure 10. Molecular structures of 17 (left) and 18 (right) (one of two molecules in the asymmetric unit) with displacement ellipsoids shown at the 30%
probability level. Hydrogen atoms are depicted only for the η1-Cp rings. Selected bond distances (Å) and angles (°) for 17: Ru(1)-Cpcentroid 1.870(3),
Ru(1)-P(1) 2.2501(7), Ru(1)-P(2) 2.2544(8), Ru(1)-C(12) 2.266(3), C(5)-C(6) 1.494(5), C(6)-C(7) 1.520(5), C(7)-C(8) 1.507(5), C(8)-C(9) 1.363(5),
C(8)-C(12) 1.482(5), C(9)-C(10) 1.430(5), C(10)-C(11) 1.351(5), C(11)-C(12) 1.463(5), Cpcentroid-Ru(1)-C(12) 117.6(1), Cpcentroid-Ru(1)-P(1) 128.5-
(9), Cpcentroid-Ru(1)-P(2) 128.7(9), C(5)-C(6)-C(7) 112.7(3), C(6)-C(7)-C(8) 114.3(3), C(7)-C(8)-C(12) 126.2(3), Ru(1)-C(12)-C(8) 107.8(2). Selected
bond distances (Å) and angles (°) for 18: Ru(1)-Cpcentroid 1.878(5), Ru(1)-P(1) 2.2557(13), Ru(1)-P(2) 2.2513(13), Ru(1)-C(6) 2.276(5), C(1)-C(11)
1.517(6), C(11)-C(12) 1.595(7), C(12)-C(7) 1.523(6), C(7)-C(8) 1.374(6), C(6)-C(7) 1.471(6), C(8)-C(9) 1.427(7), C(6)-C(10) 1.449(6), C(10)-C(9)
1.352(7), Cpcentroid-Ru(1)-C(6) 120.0(2), Cpcentroid-Ru(1)-P(1) 130.5(1), Cpcentroid-Ru(1)-P(2) 125.1(1), C(1)-C(11)-C(12) 113.0(4), C(11)-C(12)-
C(7) 111.0(4), C(12)-C(7)-C(6) 125.5(4), Ru(1)-C(6)-C(7) 107.7(3).
C5H4)}] (17 R ) H, 18 R ) Me) formed in quantitative yields
(eq 9).
ring strain energy of ruthenocenophanes 15 and 16. In fact,
monitoring a sample of 17 in C6D5CD3 heated extensively at
110 °C by H NMR spectroscopy did not reveal detectable
1
retroconversion to 15.
Surprisingly, irreversible formation of a ring-slipped η1-Cp
complex [Ru(PMe3)2{(η5-C5H4)(CH2)2(η1-C5H4)}] (19) also
occurred upon mixing 15 with an excess of the monodentate
phosphine PMe3 (eq 10, Figure 11). No ROP of the highly
strained [2]ruthenocenophanes was observed in the presence of
excess PMe3 at 55 °C, a consequence of more stable metal-
(η1-Cp) bonding (vide infra). Neither were the PMe3 ligands
displaced by chelating P donors when 19 was heated to 55 °C
in the presence of dmpe or dtbpe (2.5 h).
The solid-state structures of 17 and 18 were investigated by
single-crystal X-ray diffraction. The molecular structures (Figure
10) and the η1-Cp ring geometry are comparable to that observed
for complex 11. The Cpcentroid-Ru distances (1.87 Å, average
17 and 18) are longer than the Cpcentroid-Fe distance in 11 (1.72
Å) because of the larger size of the ruthenium atom.
As expected from the reaction in the absence of UV
irradiation, the ring-slipped Ru complexes 17 and 18 are more
thermodynamically stable compounds compared to 15 and 16,
relative to the analogous Fe systems, likely because of the high
(32) Nelson, J. M.; Lough, A. J.; Manners, I. Angew. Chem., Int. Ed. 1994, 33,
989.
(33) Reactant 16 was prepared via an alternative synthesis to that previously
reported by Herberhold et al. in ref 10h. The tilt angle cited in the literature
was confirmed by an independent structure determination (Supporting
Information).
Figure 11. ORTEP of 19 with displacement ellipsoids shown at the 30%
probability level. Hydrogen atoms are depicted only for the η1-Cp ring.
Selected bond distances (Å) and angles (°): Ru(1)-Cpcentroid 1.863(3), Ru-
(1)-P(1) 2.2710(6), Ru(1)-P(2) 2.2719(6), Ru(1)-C(12) 2.275(2), C(1)-
C(6) 1.505(4), C(6)-C(7) 1.496(5), C(7)-C(8) 1.508(4), C(8)-C(12)
1.474(3), C(8)-C(9) 1.355(4), C(8)-C(12) 1.474(3), C(9)-C(10) 1.425-
(5), C(10)-C(11) 1.360(4), C(11)-C(12) 1.464(4), Cpcentroid-Ru(1)-C(12)
117.3(9), Cpcentroid-Ru(1)-P(1) 124.4(9), Cpcentroid-Ru(1)-P(2) 123.9(9),
C(1)-C(6)-C(7) 112.8(3), C(6)-C(7)-C(8) 114.4(3), C(7)-C(8)-C(12)
126.0(3), Ru(1)-C(12)-C(8) 108.23(16).
(34) As described in ref 13a, Green has calculated the degree of strain imposed
by tilting Cp ligands in [n]metallocenophanes for ferrocene and confirmed
that an R angle of 0° is the most stable arrangement for ruthenocene.
However, quantitative comparisons of the strain enthalpies of 1, 10, 15,
and 16 including the effect of the bridging atoms have not been reported,
and thus it must be noted that the relationship between ring strain and ring
tilt is accordingly a qualitatiVe one. A lower temperature reported for the
thermal ROP of the [2]ruthenocenophane 15 [R ) 29.6(5)°, Tonset
)
220 °C]32 compared to that of the [2]ferrocenophane 10 [R ) 21.6(4)°,
Tonset ) 300 °C]28b reflects this comparison.
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4174 J. AM. CHEM. SOC. VOL. 130, NO. 12, 2008

Haptotropic Shifts of Cp Ligands in Metallocenophanes
A R T I C L E S
Discussion
is hampered by the fact that the metal-bound ipso carbon in 5
is sp2 hybridized, while those in 6 and 11 are sp3 hybridized
(Figures 3, 5, and 7). However, the Fe-Cipso bond distance
connecting the metal center and η1-Cp ring in 6 is notably long
[5, 2.015(2) Å; 6, 2.249(9) Å; 11, 2.201(5) Å], consistent with
an easily dissociable η1-Cp ligand.38
The differences in reactivity observed with [n]metallo-
cenophanes in the presence of P donors appear to be a
consequence of the intricate combination of ring strain, metal-
Cp bond strength, and metal-phosphine bond strength occurring
within each system. Moderately strained [1]- and [2]ferro-
cenophanes [R ) 20.8(5)°, 1 and 21.6(4)°, 10] are inert toward
P donors under ambient conditions but react with irradiation to
give mixed hapticity bis(cyclopentadienyl) metallocycles, ring-
opened monomeric zwitterions, or polyferrocenes. The photo-
controlled reactivity of sila[1]ferrocenophanes has been previ-
ously exploited to provide mild routes to metallopolymers with
narrow molecular weight distributions.17 Highly tilted and
strained [2]ruthenocenophanes [R ) 29.6(5)°, 15 and R ) 31.1°,
16] react with P donors without irradiation to yield ring-slipped
products 17-19.
The difference in strain energies associated with the larger
tilt angles in [n]ruthenocenophanes compared to [n]ferro-
cenophanes with the same ansa bridge is a consequence of the
larger covalent radius of ruthenium. Despite lower ring strain,
[n]ferrocenophanes readily generate a pendant Cp anion to
initiate photocontrolled ROP. The reactivity of [n]rutheno-
cenophanes with P donors, however, is arrested after slippage
to η1-coordination of a Cp ligand, which suggests more stable
M-η1-Cp σ bonds.3d,35
When ring strain is not an issue, as in the heteroleptic complex
13[BPh4] or a zwitterionic polymer chain (e.g., 8), iron-bound
PR3 ligands were found to be readily substituted by a Cp anion
with irradiation. Though rare, several examples of phosphine
photolabilization have been previously reported.36 This result
provides an interesting parallel to the thermal retroconversion
of slipped Cp ligands back to η5-coordination with displacement
of a chelating phosphine, which occurs despite the enthalpic
penalty of re-forming a strained ring. Both cases suggest an
increased stability for homoleptic bonding arrangements over
mixed sets of ligands.37
The steric and electronic characteristics of P donors exert a
subtle influence on the photochemical reactions with 1 or 10.
With 1, partial displacement of a π-bound Cp ligand to produce
a ring-slipped product is observed with both dmpe and the more
sterically imposing dppe. The phenyl substituents restrict
fluxional behavior in 5 to a greater extent than in 6 or 11, as is
apparent in the multinuclear NMR spectra. Interestingly, the
weakly bound dppe does not remain coordinated to the iron
center in the presence of dmpe, while 6, with a more strongly
coordinated dmpe ligand, reacts with one donor site of a second
dmpe ligand with the loss of the σ-bound Cp ring to yield the
zwitterion 7. The dissociative mechanism proposed to operate
when 5 exchanges a dppe ligand for dmpe to form 6 is consistent
with the relatively stable (η5, η1) coordination of the two Cp
ligands in the 18-electron complex, 5. Notably, while the dppe
ligand in 5 readily dissociates in donor solvents and in the
presence of dmpe, the stability of dmpe coordination in 6
provides a convenient entry into the mechanistic manifold for
thermal ROP (Scheme 1). Interestingly, ROP in this case appears
to be entropy-driven, kinetically facilitated by the lability of a
weakly bound Cp ring, and not by release of significant ring-
strain. To date, few examples of ROP involving strain-free
monomers have been reported.39
Concluding Remarks
A systematic study into the influence of ring-strain on the
haptotropic flexibility of Cp ligands in [1]- and [2]metallo-
cenophanes has been performed. Reactions with bidentate P
donors revealed thermally reversible, photocontrolled haptotro-
pic shifts of a Cp ligand from η5- to η1-coordination in
moderately strained [1]- and [2]ferrocenophanes 1 and 10. The
highly strained [2]ruthenocenophanes 15 and 16 react without
photoinitiation to produce similar ring-slipped products. Interest-
ingly, though the [2]ruthenocenophanes react with P donors
under milder conditions, the reactivity of the Cp ligands is
arrested at the σ-bound complexes, whereas a Cp ligand can be
fully substituted in 1 and 10 by P donors, promoting ROP.
Despite the anticipated affinity of the iron center in [1]- and
[2]ferrocenophanes for monodentate phosphines, only macro-
cycles and cyclic polymers were observed from photolytic ROP
initiated by PMe3 or PPh3, which suggests that pendant Cp
anions can react with phosphine-substituted metal centers at the
other end of a polymer chain. Controlled stoichiometric
substitution of the three iron-bound PMe3 ligands of 13[BPh4]
by a Cp- anion was found to occur only with irradiation.
The thermal reversibility of Cp haptotropic shifts with loss
of the chelating dppe ligand from the SiMe2-bridged 5 is
attributed to the loosely bound P donor, which (unlike that in 6
or 11) slowly dissociates in donor solvents at 25 °C. In contrast,
in 11 the electron-donating ethane bridge promotes the strong
coordination of the σ-bound Cp ligand relative to the case of 6,
creating a more robust metallocycle. The difference in the
electronic features of the Cp ligand explains why, unlike 1, the
ethane-bridged [2]ferrocenophane 10 does not react with dppe:
because of a less electrophilic metal center as a result of more
electron-donating Cp ligands than those in 1. Thus, unlike in
silyl-bridged 6, the η1-Cp ring of 11 resists dissociation from
the metal center and contributes to the reversibility of eq 6.
Direct comparison of Fe-Cipso bond lengths in the solid state
(35) For a recent computational analysis of M-C bonds (M ) Fe or Ru) see:
(a) Krapp, A.; Pandey, K. K.; Frenking, G. J. Am. Chem. Soc. 2007, 129,
7596. (b) Ehlers, A. W.; Frenking, G. Organometallics 1995, 14, 423.
(36) (a) Davies, S. G.; Metzler, M. R.; Watkins, W. C.; Compton, R. G.; Booth,
J.; Eklund, J. C. Perkin Trans. 1993, 1005. (b) Davies, S. G.; Metzler, M.
R.; Yanada, K.; Yanada, R. Chem. Commun. 1993, 658. (c) Aplin, R. T.;
Booth, J.; Compton, R. G.; Davies, S. G.; Jones, S.; McNally, J. P.; Metzler,
M. R.; Watkins, W. C. Perkin Trans. 1999, 913.
(38) At 2.249(9) Å, the Fe-Cipso (η1-Cp) distance in 6 is particularly long for
a localized Fe-C(sp3) bond (cf. all Fe-C bond distances greater than 2.2
Å found in the Cambridge Structural Database involved allylic moieties or
3
2
bridging carbonyl groups). Typical Fe-Csp or Fe-Csp single bonds have
been reported in the range of 2.0-2.1 Å. See: (a) Johnson, M. D. In
ComprehensiVe Organometallic Chemistry; Wilkinson, G., Stone, F. G.
A., Abel, E. W., Eds.; Pergamon Press: Oxford, U.K., 1982; p 331. It also
should be noted that recent attempts at the ROP of the spirocyclic 1-sila-
3-ferracyclobutane (dppe)Fe(η5-C5H4SiMe2CH2) were unsuccessful. In this
compound, the σ-bound CH2-Fe distance was found to be 2.134(4) Å.
See: (b) Kumar, M.; Cervantes-Lee, F.; Sharma, H. K.; Pannell, K. H.
Organometallics 2007, 26, 3005.
(37) (a) [CpFeP3]+ cations have been reported. See: Edwards, P. G.; Malik, K.
M. A.; Ooi, L.; Price, A. J. Dalton Trans. 2006, 433. (b) For a recent report
discussing the stability of heteroleptic (Cp, N-donor) Fe complexes, see:
Chan, W. Y.; Lough, A. J.; Manners, I. Chem.-Eur. J. 2007, 13, 8867
and ref 25b.
(39) Cho, Y.; Baek, H.; Sohn, Y. S. Macromolecules 1999, 32, 2167.
9
J. AM. CHEM. SOC. VOL. 130, NO. 12, 2008 4175

A R T I C L E S
Herbert et al.
The new chemistry reported here may be of broad generality.
Interesting potential extensions can be envisaged to analogous
species with different transition metals, bridging groups, and
π-hydrocarbon rings, of which many new examples have been
recently reported.8b,10g-r Additionally, the ROP of the strain-
free complex 6 offers a potential route for preparing organo-
metallic polymers from unstrained monomers. The results
outlined may also allow for new catalytic processes by photo-
induced haptotropic shifts of Cp ligands.
stone for collecting GPC data. I.M. thanks the European Union
for a Marie Curie Chair and the Royal Society for a Wolfson
Fellowship. D.E.H. thanks the Natural Sciences and Engineering
Research Council of Canada for postgraduate scholar-
ships.
Supporting Information Available: Detailed experimental
section including syntheses and characterization of the com-
pounds presented, selected 31P{1H} and 1H NMR spectra, X-ray
crystallography experimental procedures, and CIF files for X-ray
structures of compounds 5, 6, 11, 13b[BPh4], 14, and 16-19.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. We thank Dr. George R. Whittell for
helpful discussions, Dr. Anne Staubitz, Dr. Craig P. Butts, and
Dr. Tim Burrow for assistance with 2D NMR spectroscopy,
Laurent Chabanne for MALDI-TOF MS, and Vivienne Black-
JA077334+
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4176 J. AM. CHEM. SOC. VOL. 130, NO. 12, 2008