Third-order nonlinear optical properties of some electron-rich iron mono- And trinuclear alkynyl complexes

Article abstract of DOI:10.1021/om050030g

The syntheses of [1,3,5-{(n²-dppe)(n⁵-C ₅Me₅)FeC≡C-4-C₆H₄C} ₃C₆H₃] (1) and [(n²-dppe)(n ⁵-C₅Me₅)Fe(C≡C-1,4-C₆H ₄C=CPh)] (2) are reported, together with an X-ray diffraction study of 2. The linear optical spectra of these compounds reveal characteristic low-energy transitions at 430 and 436 nm, respectively, significantly red shifted in comparison to those recorded for [1,3,5-{(n²-dppe)(n ⁵-C₅Me₅)FeC=C}₃C₆H ₃] (3) and [(n²-dppe)(n⁵-C₅Me ₅)FeC≡CPh] (4), respectively. Cubic nonlinear optical response 'data for 1, 2, and 4 are reported. Cubic molecular nonlinearities by Z-scan at 695 nm reveal an increase in nonlinearities upon introduction of the ligated metal unit and progression from linear monometallic complex to octupolar trimetallic complex. Oxidation of 1 to 1³⁺ results in a change of sign and magnitude of the imaginary (absorptive) part of the third-order nonlinearity; that is, the molecule can be electrochemically cycled between two-photon absorber and saturable absorber states.

Full text of DOI:10.1021/om050030g

4280
Organometallics 2005, 24, 4280-4288
Third-Order Nonlinear Optical Properties of Some
Electron-Rich Iron Mono- and Trinuclear Alkynyl
Complexes
Marie P. Cifuentes,† Mark G. Humphrey,*,† Joseph P. Morrall,†,‡ Marek Samoc,‡
Fre´de´ric Paul,*,§ Claude Lapinte,§ and Thierry Roisnel
Department of Chemistry, Australian National University, Canberra, ACT 0200, Australia,
Laser Physics Centre, Research School of Physical Sciences and Engineering, Australian
National University, Canberra, ACT 0200, Australia, Organome´talliques et Catalyse: Chimie
et Electrochimie Mole´culaire, UMR CNRS 6509, Institut de Chimie de Rennes, Universite´ de
Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France, and Laboratoire de Chimie du
Solide et Inorganique Mole´culaire, UMR CNRS 6511, Institut de Chimie de Rennes, Universite´
de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France
Received January 13, 2005
The syntheses of [1,3,5-{(η2-dppe)(η5-C5Me5)FeCtC-4-C6H4CtC}3C6H3] (1) and [(η2-dppe)-
(η5-C5Me5)Fe(CtC-1,4-C6H4CtCPh)] (2) are reported, together with an X-ray diffraction
study of 2. The linear optical spectra of these compounds reveal characteristic low-energy
transitions at 430 and 436 nm, respectively, significantly red shifted in comparison to those
recorded for [1,3,5-{(η2-dppe)(η5-C5Me5)FeCtC}3C6H3] (3) and [(η2-dppe)(η5-C5Me5)FeCtCPh]
(4), respectively. Cubic nonlinear optical response data for 1, 2, and 4 are reported. Cubic
molecular nonlinearities by Z-scan at 695 nm reveal an increase in nonlinearities upon
introduction of the ligated metal unit and progression from linear monometallic complex to
octupolar trimetallic complex. Oxidation of 1 to 13+ results in a change of sign and magnitude
of the imaginary (absorptive) part of the third-order nonlinearity; that is, the molecule can
be electrochemically cycled between two-photon absorber and saturable absorber states.
Introduction
of redox-based switching or modulation of the NLO
properties.7,9-12 Electrochromic switching of third-order
NLO properties has recently been realized with mono-
and trinuclear ruthenium alkynyl complexes by some
of us.1,2,13
Iron alkynyl complexes of similar structures may
possess more readily accessible oxidation potentials
than their ruthenium analogues14 and comparable NLO
properties.7,10,15,16 Investigation of the potential of sev-
eral iron alkynyl complexes for electrochromic switching
of the third-order NLO activity was therefore very
appealing. We report herein the synthesis and charac-
terization of the trinuclear iron alkynyl complex 1 and
of the mononuclear model complex 2 of related structure
(Chart 1), as well as an assessment of the potential of
the former for electrochemically induced third-order
Materials with optimized nonlinear optical (NLO)
properties are required for the control and processing
of signal-carrying light beams in the emerging photo-
nics-based technologies.1,2 Organometallic alkynyl com-
plexes have considerable potential in this respect;
electron-rich ligated metal centers used as powerful
donor sites assembled around a central core by employ-
ing extended unsaturated carbon-rich linkers can afford
complexes with significant second- and third-order
nonlinearities.3-8 In addition, when the metal centers
are redox active, such assemblies afford the possibility
† Department of Chemistry, Australian National University.
‡ Laser Physics Centre.
§ Organome´talliques et Catalyse: Chimie et Electrochimie Mole´cu-
laire.
Laboratoire de Chimie du Solide et Inorganique Mole´culaire.
(1) Cifuentes, M. P.; Powell, C. E.; Humphrey, M. G.; Heath, G. A.;
Samoc, M.; Luther-Davies, B. J. Phys. Chem. A 2001, 105, 9625-9627.
(2) Samoc, M.; Luther-Davies, B.; Humphrey, M. G.; Cifuentes, M.
P.; McDonagh, A. M.; Powell, C. E.; Heath, G. A. SPIE Proc., Int. Soc.
Opt. Eng. 2001, 4461, 65-77.
(9) (a) Coe, B. J.; Houbrechts, S.; Asselberghs, I.; Persoons, A. Angew.
Chem., Int. Ed. 1999, 38, 366-369. (b) Coe, B. J. Chem. Eur. J. 1999,
5, 2464-2471.
(10) Paul, F.; Costuas, K.; Ledoux, I.; Deveau, S.; Zyss, J.; Halet,
J.-F.; Lapinte, C. Organometallics 2002, 21, 5229-5235.
(11) Malaun, M.; Reeves, Z. R.; Paul, R. L.; Jeffery, J. C.; McCleverty,
J. A.; Ward, M. D.; Asselberghs, I.; Clays, K.; Persoons, A. Chem.
Commun. 2001, 49-50.
(3) Whittall, I. R.; McDonagh, A. M.; Humphrey, M. G.; Samoc, M.
Adv. Organomet. Chem. 1998, 42, 291-362.
(4) Whittall, I. R.; McDonagh, A. M.; Humphrey, M. G.; Samoc, M.
Adv. Organomet. Chem. 1999, 43, 349-405.
(12) Asselberghs, I.; Clays, K.; Persoons, A.; Ward, M. D.; McClev-
erty, J. A. J. Mater. Chem. 2004, 14, 2831-2839.
(13) Powell, C. E.; Humphrey, M. G.; Cifuentes, M. P.; Morrall, J.
P.; Samoc, M.; Luther-Davies, B. J. Phys. Chem. A 2003, 107, 11264-
11266.
(5) McDonagh, A. M.; Humphrey, M. G.; Samoc, M.; Luther-Davies,
B. Organometallics 1999, 18, 5195-5197.
(6) McDonagh, A. M.; Humphrey, M. G.; Samoc, M.; Luther-Davies,
B.; Houbrechts, S.; Wada, T.; Sasabe, H.; Persoons, A. J. Am. Chem.
Soc. 1999, 121, 1405-1406.
(14) Paul, F.; Lapinte, C. Coord. Chem. Rev. 1998, 178/180, 427-
505.
(7) Weyland, T.; Ledoux, I.; Brasselet, S.; Zyss, J.; Lapinte, C.
Organometallics 2000, 19, 5235-5237.
(15) Weyland, T.; Costuas, K.; Mari, A.; Halet, J.-F.; Lapinte, C.
Organometallics 1998, 17, 5569-5579.
(8) Hurst, S. K.; Cifuentes, M. P.; McDonagh, A. M.; Humphrey, M.
G.; Samoc, M.; Luther-Davies, B.; Asselberghs, I.; Persoons, A.
J. Organomet. Chem. 2002, 642, 259-267.
(16) Weyland, T.; Costuas, K.; Toupet, L.; Halet, J.-F.; Lapinte, C.
Organometallics 2000, 19, 4228-4239.
10.1021/om050030g CCC: $30.25 © 2005 American Chemical Society
Publication on Web 07/12/2005

Iron Mono- and Trinuclear Alkynyl Complexes
Organometallics, Vol. 24, No. 17, 2005 4281
1H NMR spectroscopies. However, both alkynyl com-
plexes 1 and 2 were fully characterized by LSIMS, cyclic
voltammetry, and the usual spectroscopic techniques
(Tables 1-3). The identity of complex 2 was confirmed
by a single-crystal X-ray structural study (Tables 4 and
5).
Chart 1. Molecular Structures of 1-4
The 1H and 31P NMR spectra are not unusual for such
compounds and confirmed the purity of the isolated
samples.7,15,16,18 Thus, the 13C NMR spectra exhibit the
characteristic signatures of the alkynyl group at ca.
145 ppm for R-carbon atoms and at ca. 110 ppm for
â-carbon atoms. The former are easily recognizable
triplets due to their coupling to the phosphorus atoms
of the 1,2-bis(diphenylphosphino)ethane (dppe) ligand.
Two absorptions are also observed in the infrared
spectra νCtC region for each of these compounds, one
very strong in the usual range for iron alkynyls and a
second, weaker, around 2200 cm-1 (Table 1). Although
rather weak in 1 and barely detectable in 2, a corre-
sponding very weak absorption is also observed at quite
similar wavenumbers in the infrared spectra of the
organic alkyne precursors used to synthesize the com-
plexes. Raman spectroscopy further suggests that these
absorptions in 1 and 2 correspond to a fundamental
mode rather than a combination mode or harmonic
mode.19
Vibronic selection rules suggest that C3v symmetric
complexes such as 1 or 3 should possess two stretching
modes (A1 and E) in the infrared and Raman spectra
for each set of equivalent alkyne bonds,20 the A1 mode
being only weakly infrared-active.21 Thus, four alkynyl
stretching modes should be observed for 1, two intense
ones (E) and two weaker ones (A1). We believe, however,
that these vibronic fundamentals are not all distinct in
the vibronic spectra and that the observed modes
actually correspond to the two chemically nonequivalent
alkyne bonds present in 1. In accordance with this
hypothesis, the two stretching fundamentals (A1 and E)
expected in the alkynyl region for the trinuclear ana-
logue of 1 belonging to the same symmetry group and
possessing only one alkynyl spacer (i.e., 3) were previ-
ously not differentiated by infrared spectroscopy.15
Raman spectroscopy reveals that only one absorption
around 2047 cm-1 is present in the vibronic spectrum
of 3 (a shoulder at 2051 cm-1 can nevertheless be
detected which corresponds, perhaps, to the second
expected mode).
This hypothesis also implies that the two absorptions
detected in the νCtC region for 2 correspond to the
chemically nonequivalent alkyne groups. In accordance
with this, two clearly distinct absorptions at 2140 (vw)
and 2040 (s) cm-1 were detected for the meta analogue
(5; Chart 2) of 2 (Table 1) and also for the mononuclear
para-substituted complexes [(η2-dppe)(η5-C5Me5)Fe(Ct
C-1,4-C6H4CtCSiR3)] 6a (R ) Me; 2146, 2038 cm-1) and
6b (R ) iPr; 2145, 2042 cm-1), possessing two chemically
nonequivalent alkynes (Chart 2).22
NLO switching. Relevant data on the previously re-
ported trinuclear organoiron complex 3 and its mono-
nuclear counterpart 416-18 (Chart 1) are also included.
This series of complexes affords the possibility of as-
sessing the contribution of each branch of the C3v
trinuclear structure (when comparing 1 and 2) and the
influence of extending the organic bridge between the
metal sites (when comparing 1 with 3, or 2 with 4).
Results and Discussion
Synthesis and Characterization of Alkynyl Com-
plexes 1 and 2. The desired complexes were isolated
from reactions between the precursor iron chloro com-
plex and the organic alkynes. Activation of the terminal
alkyne groups yielded the corresponding vinylidene
complexes, and deprotonation of the latter formed the
alkynyl complexes (Scheme 1). In each case, the inter-
mediate vinylidene complexes 1′ and 2′ were isolated
and purified before deprotonation. Recrystallization of
the products 1 and 2 was also needed to isolate these
compounds in a pure state because minute amounts of
unidentified complexes were still present in the crude
reaction mixture. The intermediate vinylidene com-
plexes were not extensively characterized, their identity
and purity being confirmed by infrared and 31P and
(19) Paul, F.; Mevellec, J.-Y.; Lapinte, C. J. Chem. Soc., Dalton
Trans. 2002, 1783-1790.
(20) Coat, F.; Paul, F.; Lapinte, C.; Toupet, L.; Costuas, K.; Halet,
J.-F. J. Organomet. Chem. 2003, 683, 368-378.
(17) Connelly, N. G.; Gamasa, M. P.; Gimeno, J.; Lapinte, C.; Lastra,
E.; Maher, J. P.; Le Narvor, N.; Rieger, A. L.; Rieger, P. H. J. Chem.
Soc., Dalton Trans. 1993, 2575-2578.
(18) Denis, R.; Toupet, L.; Paul, F.; Lapinte, C. Organometallics
2000, 19, 4240-4251.
(21) If spherically symmetric metallic end-groups are considered,20
the symmetry group is now D3h and the νCtC fundamentals belong to
the A1′ and E′ representations with A1′ modes inactive in the infrared.
(22) Courmarcel, J.; Le Gland, G.; Toupet, L.; Paul, F.; Lapinte, C.
J. Organomet. Chem. 2003, 670, 108-122.

4282 Organometallics, Vol. 24, No. 17, 2005
Scheme 1. Syntheses of 1 and 2
Cifuentes et al.
Table 1. Infrared Data for Selected Complexes in CH2Cl2 Solutiona (cm-1
)
Fe(II)
νCtC
Fe(III)
νCtC
b
compound
∆νCtC
ref
[{[Fe]CtC-4-C6H4CtC}3C6H3] (1)
2202
2046
2209
2044
2050
2053
not obsd
1995
this work
-51
+4
-57
-40
-49c
[[Fe]CtC-4-C6H4CtCPh] (2)
2213
this work
1987
[{[Fe]CtC}3C6H3] (3)
[[Fe]CtCPh] (4)
2010
15
18
2021
1988
[[Fe]CtC-3-C6H4CtCPh] (5)
2134
2046
not obsd
22
a [Fe] ≡ (η2-dppe)(η5-C5Me5)Fe. Solid state νCtC values obtained in KBr for isolated complexes are given in the Experimental Section.
b Neutral vs oxidized νCtC difference. c Mean difference: two bands were observed for the Fe(III) parent, presumably due to Fermi coupling.19
Table 2. Electrochemical Data for Selected
Complexesa
potentials. The mononuclear complex 2 is slightly more
difficult to oxidize than the unsubstituted mononuclear
complex 4. Its oxidation potential is similar to that
previously reported for the meta analogue 5,22 consistent
with the electron-withdrawing character of the 4-phenyl-
alkynyl substituent.23 Compound 2 is oxidized at a
slightly more positive potential than 1. The latter shows
only one oxidation wave at -0.15 V vs SCE for the three
iron-based oxidations, whereas 3 exhibits three distinct
reversible redox processes. The redox wave of 1 is quite
large (∆Ep ) 0.11 V) in comparison to the strictly
monoelectronic process recorded for 2 (∆Ep ) 0.09 V)
or for 3 (∆Ep ) 0.07 V), suggesting that the three
oxidations of 1 do not actually take place at exactly the
same potential. From a thermodynamic point of view,
the data would indicate that the mixed-valence states
1+ and 12+ have a slight extra stability. However, the
cyclic voltammetry plainly shows that 1 cannot be
unambiguously categorized as a class II mixed-valence
complex in the classification of Robin and Day,14,24,25 in
contrast to 3.16
compound
∆Ep (V) E0 (V) b ic/ia
ref
[{[Fe]CtC-4-C6H4CtC}3C6H3]
(1)
[[Fe]CtC-4-C6H4CtCPh] (2)
[{[Fe]CtC}3C6H3] (3)
0.11
-0.15
1
this work
0.09
0.07
0.06
0.07
-0.13
-0.25
-0.13
0.00
1
1
1
1
this work
15
+0.53
-0.15
-0.13
[[Fe]CtCPh] (4)
[[Fe]CtC-3-C6H4CtCPh] (5)
0.08
0.08
1
1
18
22
a [Fe] ≡ (η2-dppe)(η5-C5Me5)Fe. All E values in V vs SCE.
Conditions: CH2Cl2 solvent, 0.1 M (NnBu4)(PF6) supporting
electrolyte, 20 °C, Pt electrode, sweep rate 0.100 V s-1. The
ferrocene/ferrocenium (Fc/Fc+) complex was used as an internal
reference for potential measurements. b E0 values corrected for
33
Fc/Fc+ at 0.460 V vs SCE in CH2Cl2, ∆Ep ) 0.06 V.
The absorption at ca. 2200 cm-1 in the spectra of 1
and 2 is rather energetic for an organic aryl-alkyne
νCtC band, especially so when compared to the corre-
sponding νCtC in 5 or in 6a,b. These modes certainly
benefit from strong vibronic coupling to other vibrators
belonging to the same symmetry representation, pos-
sibly C-C stretching modes involving the nearby aro-
matic core and perhaps also (but to a smaller extent)
CtC stretching modes of the nearby iron-acetylide
linkage. The iron-acetylide νCtC wavenumbers are not
significantly different from alkynyl stretching modes
usually observed in related mononuclear iron(II) alkynyl
complexes.19
The relatively low energy of the acetylide νCtC in the
infrared spectrum of 2 (and also in 1) contrasts with
the high energy of the other νCtC corresponding to the
organic alkyne (see above). If the existence of vibronic
coupling is not considered, the observed wavenumber
(23) Costuas, K.; Paul, F.; Toupet, L.; Halet, J.-F.; Lapinte, C.
Organometallics 2004, 23, 2053-2068.
(24) Robin, M. B.; Day, P. Adv. Inorg. Chem. Radiochem. 1967, 247-
422.
As revealed by cyclic voltammetry (Table 2), com-
plexes 1 and 2 undergo a reversible oxidation at similar
(25) Astruc, D. Electron Transfer and Radical Processes in Transi-
tion-Metal Chemistry; VCH Publishers: New York, 1995.

Iron Mono- and Trinuclear Alkynyl Complexes
Organometallics, Vol. 24, No. 17, 2005 4283
a
Table 3. UV-Vis Data for Selected Complexes in CH2Cl2
compound
absorptions in nm (10-3ꢀ in M-1 cm-1
)
ref
[{[Fe]CtC-4-C6H4CtC}3C6H3] (1)
[[Fe]CtC-4-C6H4CtCPh] (2)
[{[Fe]CtC}3C6H3] (3)
314 (91.5); 394 (sh, 40.4); 460 (62.2)
394 (sh, 31.5); 298 (37.1); 436 (22.9)
283 (53.0); 351 (35.0)
this work
this work
7
[[Fe]CtCPh] (4)
277 (sh, 14.5); 350 (13.6)
this work
a [Fe] ≡ (η2-dppe)(η5-C5Me5)Fe.
Table 4. Crystal Data, Data Collection, and
Refinement Parameters for 2
Table 5. Selected Bond Lengths (Å) and Angles
(deg) for 2
2‚C7H8
2‚C7H8
formula
fw
C52H48P2Fe1‚C7H8
820.72
Selected Bond Lengths
Fe-(Cp*)centroid
Fe-P1
1.738
temp (K)
cryst syst
space group
a (Å)
293(2)
2.1906(10)
2.1805(10)
1.893(4)
1.218(3)
1.434(3)
1.396(4)
1.376(4)
1.384(4)
1.392(4)
1.384(4)
1.441(3)
1.186(4)
1.431(4)
triclinic
P1h
Fe-P2
Fe-C37
12.077(5)
12.333(5)
17.469(5)
83.013(5)
79.834(5)
61.504(5)
2248.7(15)
2
C37-C38
C38-C39
C39-C40
C40-C41
C41-C42
C42-C43
C43-C44
C42-C45
C45-C46
C46-C47
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V(Å3)
Z
Dcalcd (g cm-3
)
1.212
cryst size (mm)
F(000)
0.45 × 0.42 × 0.03
862
Selected Bond Angles
diffractometer
radiation
KappaCCD (Nonius)
Mo KR
P1-Fe-P2
86.26(4)
86.99(7)
82.23(7)
178.7(2)
173.9(3)
121.0(3)
121.5(3)
120.1(3)
177.4(3)
179.7(3)
110.2
P1-Fe-C37
abs coeff (mm-1
)
0.442
P2-Fe-C37
data collection: θmax (deg), frames,
Ω rotation (deg), seconds/frame
θ range
27.5, 152, 2.0, 120
Fe-C37-C38
C37-C38-C39
C39-C40-C41
C40-C41-C42
C41-C42-C43
C42-C45-C46
C45-C46-C47
1.88-27.45
-15/15
-15/15
-22/22
30 565
h k l range
no. of total reflns
no. of unique reflns
10 268
Fe-(Cp*)centroid/C39-C40a
no. of obsd reflns [I > 2σ(I)]
no. of restraints/params
w ) 1/[σ2(Fo)2 + (aP)2+bP]
7686
C42-C41/C47-C48a
3.5
0/541
a Dihedral angle (Cp* ) pentamethylcyclopentadienyl ligand).
a ) 0.0673
(where P ) [Fo2 + Fc ]/3)
2
b ) 0.7862
0.0477
0.1245
0.0704
0.1409
1.017
final R
Chart 2. Molecular Structures of 5 and 6a,b
Rw
R indices (all data)
Rw (all data)
goodness of fit/F2 (Sw)
∆Fmax (e Å-3
)
)
0.314
∆Fmin (e Å-3
-0.466
would indicate a strong electron-withdrawing effect of
the para “CtC-Ar” substituent on the (η2-dppe)(η5-C5-
Me5)FeCtC-1,4-C6H4- fragment, especially when com-
pared to that previously observed for the mononuclear
complex 4. Indeed, the value is close to that observed
for a CtN substituent.19 However, the electrochemical
data tend to indicate a more moderate electron-
withdrawing influence of the “CtC-1,4-(C6H4)-CtC-
Ar” ligand, suggesting also the involvement of specific
vibronic coupling in the above-mentioned acetylide
νCtC. Within experimental accuracy, the redox potential
found for 2 is similar to that previously reported for the
meta analogue 5,22 indicating a mostly inductive elec-
tronic effect of the CtCPh substituent. The trinuclear
complex 1 is oxidized at a potential 0.02 V more negative
than 2, affording evidence that the electron-withdrawing
effect exerted by the CtC-1,3-C6H3CtC- spacer is
compensated by the presence of the electron-rich metal
centers capping each branch of 1. Not surprisingly, the
extended carbon-rich bridge in 1 transmits the electron-
releasing character of each metal center less efficiently
than in the related complex 3, which possesses a simple
alkynyl bridge between the capping metal atoms and
the central aryl ring. Thus, in 3, the oxidation of the
first metallic end-group occurs at a much more negative
potential (-0.25 V vs SCE) and in a stepwise fashion.
In the visible range, the electronic spectra of 1 and 2
present one distinct low-energy band (Table 3), which
corresponds to a MLCT transition, analogous to 3 and
4, which possess shorter alkynyl ligands. As expected,
this electronic transition is bathochromically shifted by
109 or 86 nm in proceeding from 3 to 1 or 4 to 2,
respectively, consistent with the extension of conjuga-
tion. While progression from mononuclear to trinuclear
structure has apparently no influence on the MLCT in
3 and 4, this change results in a slight bathochromic
shift of ca. 25 nm in proceeding from 2 to 1 (Figure 1).
The extinction coefficient for this absorption in 1 is
roughly three times that observed for 2, in accordance
with a compound possessing three iron-based chromo-
phores.
X-ray structure of [(η2-dppe)(η5-C5Me5)Fe(CtC-
1,4-C6H4CtCPh)] (2). The mononuclear complex 2
crystallizes in the triclinic space group. The crystal-

4284 Organometallics, Vol. 24, No. 17, 2005
Cifuentes et al.
domain, since resonant absorption of the laser beam
employed for the NLO studies described below could
typically take place in this frequency range.1,2,27
The oxidized complexes were generated in situ from
the neutral precursors 1 and 2 in the IR, ESR, and NIR
spectroscopic cells, using ferricinium hexafluorophos-
phate (Scheme 2). Oxidation is evidenced by a charac-
teristic color change from orange to green (1) or brown
(2). Completion of the reaction was confirmed by infra-
red spectroscopy, as described below. Ferricinium
hexafluorophosphate, as well as the coproduced fer-
rocene, are silent in the various frequency domains
explored. Because of the weak thermodynamic stability
of the mixed-valent states 1+ and 12+ revealed by cyclic
voltammetry (see above), these compounds cannot be
generated in situ in a pure state (i.e., the compropor-
tionation constants are small). These compounds were
therefore studied only in the NIR domain in equilibrium
with various amounts of 1 and 13+. In contrast, the fully
oxidized complexes 13+ and 2+ can be quantitatively
generated in situ using an excess of ferricinium salt.
Upon addition of the ferricinium oxidant, the char-
acteristic νCtC of the Fe(II) alkynyl group disappeared
and a new absorption appeared at lower wavenumbers
corresponding to the νCtC of the Fe(III) complex cations
(Table 1).18,19 The acetylide νCtC bands of 13+ and 2+
reveal a slightly more pronounced decrease in bond
order than is observed between 3 and 33+ or 4 and 4+,
suggestive of a stronger delocalization of the positive
charge in the CtC-1,4-C6H4CtC- bridges relative to
single CtC bridges. Given the possible existence of large
vibronic coupling involving the inner organic alkyne in
1 and 2 (see above), comparison of the infrared data with
the values obtained for 3/33+ and 4/4+ should be made
cautiously. The formation of radicals is also indicated
by characteristic ESR signals in frozen dichloromethane/
1,2-dichloroethane glasses at 80 K.17,28 These ESR
signals are only apparent at low temperatures, consis-
tent with their organometallic nature (Table 6). Thus,
for 13+, as expected for an electron-rich polyradical
species, a very broad signal is obtained at g ) 2.10,
Figure 1. UV-vis spectra of 1 and 2.
Figure 2. Thermal ellipsoid plot of 2. Thermal ellipsoids
are at the 50% probability level. Hydrogen atoms have been
omitted for clarity.
Figure 3. NIR spectra of 2+ (a), 13+ (b), and a mixture of
1, 1+, 12+, and 13+ (c).
similar to the one obtained previously for 33+ 15
Like-
.
wise, for 2+, an anisotropic ESR signal exhibiting three
g tensors reveals the presence of an iron-centered
radical.28
lographic data (Tables 4 and 5) are not unusual in terms
of bond lengths, conformation, and angles for such
mononuclear piano-stool iron compounds.18,23 The Fe-C
bond (1.89 ( 0.01 Å) and the CtC bond (1.22 ( 0.01 Å)
resemble those observed for the [(η2-dppe)(η5-C5Me5)-
Fe(CtCPh)] complex 4 (1.89 ( 0.01 and 1.20 ( 0.01 Å,
respectively). A slight bending along the Fe-CtC-aryl
axis, usually observed with complexes possessing elec-
tron-withdrawing substituents on the aryl ring,18 is also
observed here, although the overall bending/conforma-
tion of the carbon-rich ligand is presumably induced by
packing forces.26
No absorptions were detected for the pure Fe(II)
alkynyl complexes 1 and 2 in the NIR domain, but very
weak absorptions appeared upon addition of the ferri-
cinium oxidant (Table 7 and Figure 3). Thus, a weak
NIR absorption was observed near 1866 nm for 2 in the
presence of (excess) ferricinium, this absorption most
probably corresponding to a forbidden d-d transition
in the Fe(III) cationic radical species 2+.28,29 Progressive
addition of ferricinium oxidant was also performed on
1 (Figure 3). Under such conditions, the mixed-valent
complexes 1+ and 12+ are transiently generated upon
partial oxidation. However, as mentioned previously,
these mixed-valent complexes afford no significant extra
stability, and as a consequence, a near-statistical mix-
Brief Spectroscopic Characterization of the
Oxidized States 13+ and 2+. The stability and elec-
tronic structure of the oxidized complexes 1+, 12+, 13+
,
and 2+ were briefly investigated by infrared, electron
spin resonance (ESR), near-infrared (NIR), and UV-
vis-NIR spectroscopies; particular attention was paid
to the absorptions of these redox states in the NIR
(27) Powell, C. E.; Cifuentes, M. P.; Morrall, J. P.; Stranger, R.;
Humphrey, M. G.; Samoc, M.; Luther-Davies, B.; Heath, G. A. J. Am.
Chem. Soc. 2003, 125, 602-610.
(28) Paul, F.; Lapinte, C. Work in progress.
(29) Le Stang, S.; Paul, F.; Lapinte, C. Inorg. Chim. Acta 1999, 291,
403-425.
(26) Szafert, S.; Gladysz, J. A. Chem. Rev. 2003, 103, 4175-4250.

Iron Mono- and Trinuclear Alkynyl Complexes
Organometallics, Vol. 24, No. 17, 2005 4285
Scheme 2. Synthesis of 1m+ and 2+
Table 6. ESR Spectral Data for Compounds in Frozen CH2Cl2/1,2-C2H4Cl2 Solutions at 80 Ka
compound
g1
g2
g3
∆g
ref
[{[Fe]CtC-4-C6H4CtC}3C6H3]3+ (13+
)
2.10 (∆Hpp ca. 380 G)
2.030
this work
this work
15
[[Fe]CtC-4-C6H4CtCPh]+ (2+)
2.469
2.460
1.977
1.975
0.492
0.485
[{[Fe]CtC}3C6H3]3+ (33+
[[Fe]CtCPh]+ (4+)
)
2.13b (∆Hpp ) 330 G)
2.033
28, 45
a [Fe] ≡ (η2-dppe)(η5-C5Me5)Fe. At 77 or 80 K in CH2Cl2-C2H4Cl2 (1:1) glass. b Broad single signal with small signals at g ) 4.46 and
7.97.
Table 7. NIR Spectral Data for 13+, 2+, 33+, and 4+
in CH2Cl2 Solutionsa
νmax in nm/cm-1
compound
(ꢀ in M-1 cm-1
)
ref
[{[Fe]CtC-4-C6H4CtC}3C6H3]3+ 1864/5364 (398)
this work
(13+
)
[[Fe]CtC-4-C6H4CtCPh]+ (2+) 1866/5359 (145)
this work
[{[Fe]CtC}3C6H3]3+ (33+
[[Fe]CtCPh]+ (4+)
)
1851/5400 (ca. 150)
1846/5417 (94)
7
7, 28
a [Fe] ≡ (η2-dppe)(η5-C5Me5)Fe.
ture of the various redox isomers 1, 1+, 12+, and 13+
results when less than a 3-fold excess of ferricinium is
present. The initial solution turned from orange to
brown and finally to green. Again, an absorption cen-
tered at 1864 nm (i.e., very close to that recorded for
2+) was detected and grew continuously. We also at-
tribute this absorption to an Fe(III)-centered forbidden
d-d transition. Its energy is apparently not very dif-
ferent in 1+, 12+, and 13+, since its maximum does not
appear to shift with increasing amounts of ferricinium.
In accordance with our assignment of this band as
arising from a metal-centered transition is the fact that
the extinction coefficient for 13+ is roughly 3-fold that
recorded for 2+, as previously observed with the MLCT
band in 1 and 2. Importantly, no additional absorption
corresponding to intervalence charge transfer (IVCT)
transitions in the mixed-valent states 1+ and 12+ was
observed when less than 3 equiv of oxidant was present
(trace c; Figure 3). The low comproportionation con-
stants for 1+ and 12+ and the fact that the corresponding
IVCT bands might possibly be very weak render their
detection particularly difficult under such conditions.
Thus, we cannot at present exclude the possibility that
weak IVCT bands are hidden by the band at 1864 nm.
This experiment confirms, nevertheless, that 1+ and 12+
are at best weakly coupled mixed-valent complexes.
The electrochromic behavior of 1 and 2 (Figure 4,
Table 8) is broadly similar, consistent with their similar
coordination spheres. Both 1 and 2 are transparent at
frequencies below 17000 cm-1, whereas oxidation to
afford 13+ and 2+ results in the appearance of strong
absorption bands centered at ca. 13 700 cm-1; oxidation
therefore effectively “switches on” a transition at a
wavelength accessible to our laser. Because a change
in linear optical properties can be accompanied by a
Figure 4. UV-vis-NIR spectra of CH2Cl2 solutions of
(a) 1 and (b) 2 on application of 0.2 V (Fc/Fc+ ) 0.56 V), in
an OTTLE cell, path length 0.5 mm, during oxidation at
248 K.
change in nonlinear properties, we examined 1/13+ for
the possibility of electrochemical switching of cubic
nonlinearity (see below).
Third-Order NLO Studies. Third-order nonlineari-
ties of complexes 1, 2, and 4 were determined by Z-scan
measurements at 695 nm and are listed in Table 9
(complex 3 was insufficiently soluble to measure its
nonlinearity). The cubic nonlinearity at 800 nm of the
precursor acetylene HCtC-1,4-C6H4CtCPh has been
reported previously (γreal ) (25 ( 10) × 10-36 esu, γimag

4286 Organometallics, Vol. 24, No. 17, 2005
Cifuentes et al.
Table 8. Spectroelectrochemical Data for 1 and 2
ing the switching cycles. It is seen that the electro-
chemical changes are reversible. The calculated non-
linear properties of 13+ are given in Table 9.
1
13+
λmax, cm-1
ꢀ 104 M-1 cm-1
λmax, cm-1
ꢀ 104 M-1 cm-1
21 470
5.4
3.8
8.3
13 790
16 290 sh
20 640
27 960
31 870
34 960
1.9
1.2
25 450 sh
31 580
Conclusion
2.0
9.8
10.8
11.0
New iron(II) alkynyl complexes 1 and 2 have been
synthesized and characterized; their spectroscopic prop-
erties, in the light of other data collected for electron-
rich iron alkynyl complexes,23 have afforded some
insight into their electronic structures. A brief spectro-
scopic investigation revealed that the oxidized Fe(III)
species 13+ and 2+ can be cleanly generated in situ by
chemical oxidation and that these species are fairly
stable radical cations, as expected from previous inves-
tigations on related species.15,17,18
Cubic nonlinearities for 1, 2, and 4 have been deter-
mined at 695 nm and are similar in magnitude to
previously reported trans-[(η2-dppe)2Ru(CtCPh)]-con-
taining analogues. The structure-NLO property cor-
relations in the ruthenium system are also seen in the
present iron series: introduction of the ligated metal
center increases nonlinearity, and there is a further
significant increase in proceeding from linear mono-
metallic complex to octupolar trimetallic complex. Spec-
troelectrochemical studies in the UV-vis-NIR domain
revealed that oxidation “switches on” an absorption
band with appreciable intensity, in a spectroscopic
region that is transparent for the complexes in the
resting state. The reversible FeII/III couples in 1 have
been exploited in an examination of NLO properties in
the resting and oxidized forms. Similar to the ruthenium
system, oxidation of 1 results in a change in sign and
magnitude of γimag and σ2, corresponding to cycling be-
tween a two-photon absorber and a saturable absorber.
However, in contrast to our observations with the
related ruthenium-containing complex, the redox cycling
of 1 does not result in a change in sign of γreal, a negative
sign of the refractive nonlinearity being maintained.
2
2+
λmax, cm-1
ꢀ 104 M-1 cm-1
λmax, cm-1
ꢀ 104 M-1 cm-1
22 480
26 190
32 460
2.4
1.4
3.9
13 720
16 380
21 030
24 030 sh
29 360
30 728
32 620
34 860
0.59
0.32
0.55
1.1
3.5
3.6
4.3
4.6
< 6 × 10-36 esu);30 introduction of ligated iron centers
in proceeding to 2 results in a 2-orders-of-magnitude
increase in nonlinearity. The cubic nonlinearity of 4 had
not been determined previously; it is low, but introduc-
tion of a phenylethynyl “spacer” in proceeding from 4
to 2 results in a dramatic increase in nonlinearity.
Progression from the linear complex 2 to the octupolar
complex 1 results in a further increase in nonlinearity,
mirroring results obtained with the same alkynyl
ligands in the bis{bis(diphenylphosphino)ethane}phenyl-
ethynylruthenium system.6 Table 9 also includes two-
photon absorption (TPA) cross-sections, σ2 [TPA is an
NLO process related to the imaginary component of the
third-order nonlinearity; TPA materials have attracted
significant recent interest for applications in multi-
photon microscopy, optical limiting, and optical data
storage]. We have previously noted significant TPA
cross-sections in alkynylmetal complexes of octupolar
composition,1,5,31,32 and this is also true for 1.
The cyclic voltammetry studies revealed that these
complexes undergo reversible oxidation in solution, and
UV-vis-NIR spectroelectrochemistry studies showed
that these oxidation processes “switched on” optical
transitions at long wavelength. Linear electrochromic
effects should be accompanied by modifications in the
NLO properties, an effect we have exploited to demon-
strate NLO switching with related alkynylruthenium
complexes.1,13,27 In the present studies, we examined 1
in an OTTLE cell with switching potentials of 0.2 and
-0.5 V (vs Ag/AgCl). Figure 5 shows closed-aperture
Z-scans for the resting and oxidized forms of 1. The as-
prepared solution is a strong two-photon absorber,
whereas the oxidized form loses this property and
becomes a saturable absorber. At the same time, in all
cases the refractive part of the hyperpolarizability is
negative. Figure 6 illustrates the changes of linear
transmission and nonlinear transmission occurring dur-
Experimental Section
General Data. All manipulations were carried out under
an inert atmosphere. Solvents or reagents were used as
follows: Et2O and n-pentane, distilled from Na/benzophenone;
CH2Cl2, distilled from CaH2 and purged with argon; NHiPr2,
distilled from KOH and purged with argon; aryl bromides
(Acros, >99%), opened/stored under Ar. [Fe(η5-C5H5)2][PF6] was
prepared by previously published procedures.33 X-Band ESR
spectra were recorded on a Bruker EMX-8/2.7 spectrometer.
Transmittance-FTIR spectra were recorded using a Bruker
IFS28 spectrometer (400-4000 cm-1). Raman spectra of the
solid samples were obtained by diffuse scattering on the same
apparatus and recorded in the 100-3300 cm-1 range (Stokes
emission) with a laser excitation source at 1064 nm, using a
1.05-1.06 µm laser source (15 mW) and a quartz separator
with a FRA 106 detector. Liquid NIR spectra were recorded
on a Cary 5 spectrometer. UV-visible spectra were recorded
on an UVIKON XL spectrometer. Spectroelectrochemical data
were recorded on a Cary 5 spectrophotometer (45 000-4000
(30) Hurst, S. K.; Cifuentes, M. P.; Morrall, J. P. L.; Lucas, N. T.;
Whittall, I. R.; Humphrey, M. G.; Asselberghs, I.; Persoons, A.; Samoc,
M.; Luther-Davies, B.; Willis, A. C. Organometallics 2001, 20, 4664-
4675.
(31) Hurst, S. K.; Humphrey, M. G.; Isoshima, T.; Wostyn, K.;
Asselberghs, I.; Clays, K.; Persoons, A.; Samoc, M.; Luther-Davies, B.
Organometallics 2002, 21, 2024-2026.
(32) Hurst, S. K.; Lucas, N. T.; Humphrey, M. G.; Isoshima, T.;
Wostyn, K.; Asselberghs, I.; Clays, K.; Persoons, A.; Samoc, M.; Luther-
Davies, B. Inorg. Chim. Acta 2003, 350, 62-76.
cm-1
)
in
CH2Cl2. Solution spectra of the oxidized species at 253 K were
obtained by electrogeneration (Thompson 401E potentiostat)
at a Pt gauze working electrode within a cryostatted optically
transparent thin-layer electrochemical (OTTLE) cell, path
(33) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877-910.

Iron Mono- and Trinuclear Alkynyl Complexes
Organometallics, Vol. 24, No. 17, 2005 4287
Table 9. Experimental Linear and Cubic Nonlinear Optical Response Parameters
d
d
d
e
compound
λmaxb (ꢀ)c
γreal
γimag
|γ|
σ2
1
460 (62 200)
436 (22 900)
351 (35 000)
350 (13 600)
1864 (398)
-3300 ( 800
-1500 ( 1200
f
110 ( 100
-2000 ( 1000
2800 ( 700
200 ( 40
4300 ( 1100
1500 ( 1200
920 ( 250
66 ( 15
2
3
4
17 ( 10
-3300 ( 1000
110 ( 100
3900 ( 1400
5.6 ( 3
-1100 ( 330
13+
a Conditions: measurements were carried out in thf; all complexes are optically transparent at 695 nm. γ values are referenced to the
nonlinear refractive index of silica n2 ) 3 × 10-16 cm2 W-1
. .
b nm. c M-1 cm-1 d 10-36 esu. e 10-50 cm4 s. f Insufficiently soluble.
(0.460 V), unless otherwise noted.33 LSI/ESI-MS analyses were
effected at the “Centre Regional de Mesures Physiques de
l’Ouest” (Rennes; CRMPO) on a high-resolution MS/MS Zab-
Spec TOF Micromass spectrometer (8 kV). Elemental analyses
were performed at the Center for Microanalyses of the CNRS
at Lyon-Solaise or the CRMPO. Single-crystal X-ray data
collection was performed with a Nonius Kappa-CCD diffracto-
meter (Centre de Diffractome´trie, Universite´ de Rennes).
4-Ethynylphenylethynylbenzene and (HCtC-4-C6H4Ct
C)3C6H3 were synthesized as previously reported.6
[{(η2-dppe)(η5-C5Me5)FeCtC-1,4-C6H4CtC}3C6H3] (1).
{HCtC-4-C6H4CtC}3C6H3 (0.165 g, 0.369 mmol) was stirred
in a mixture of 5 mL of thf and 15 mL of methanol with
[(η2-dppe)(η5-C5Me5)FeCl]‚Et2O (0.772 g, 1.100 mmol) and
NH4PF6 (0.245 g, 1.500 mmol). An orange precipitate formed
in the initially green reaction medium. This suspension was
stirred 5 days at 25 °C. After evaporation of the reaction
mixture to dryness the resultant brown solid was extracted
with 10 mL of dichloromethane. Concentration of the extract
to dryness, washing with n-pentane (2 × 10 mL), and drying
in vacuo afforded a mixture containing predominantly the
corresponding vinylidene complex [{(η2-dppe)(η5-C5Me5)FeCd
CH-1,4-C6H4CtC}3C6H3](PF6)3 (1′) as an air-sensitive brown
solid (ca. 0.860 g, ca. 85%). Selected data for [{(η2-dppe)
(η5-C5Me5)FeCdCH-1,4-C6H4CtC}3C6H3](PF6)3: FT-IR (ν, KBr,
cm-1): 2039 (w, CtC), 1619 (s, FedCdC). 31P{1H} NMR
(δ, (CD3)2CO, 81 MHz): 87.0 (s, dppe); -143.0 (septuplet,
1JPF ) 707 Hz, PF6-). 1H NMR (δ, CDCl3, 200 MHz) 7.70-
Figure 5. Closed aperture Z-scans for 1 (diamonds) and
13+ (triangles). The full line is the theoretical fit for a
closed-aperture Z-scan dominated by two-photon absorption
in the solute, and the dotted line is the fit for the closed-
aperture scan dominated by absorption saturation.
7.50 (m, 63H, HAr/dppe); 7.31 (m, 12HAr); 7.16 (d,3JHH ) 8.0 Hz,
4
6H, HAr); 6.38 (d,3JHH ) 8.0 Hz, 6H, HAr); 5.18 (t, JHP
)
4.4 Hz, 3H, CdC(Ar)H); 3.26 (m, 6H, CH2dppe); 2.69 (m, 6H,
CH2dppe); 1.68 (s, 45H, C5(CH3)5). After recrystallization from
dichloromethane/diethyl ether mixtures, the vinylidene salt
(800 mg, 0.302 mmol) was then stirred 2 h in thf in the
presence of excess potassium tert-butoxide (0.183 g,
1.620 mmol). After evacuation of the solvent, extraction with
toluene, concentration of the extract to dryness, and subse-
quent washings with n-pentane, a crude orange solid was
isolated containing the desired complex [1,3,5-{(η2-dppe)-
(η5-C5Me5)FeCtC-1,4-C6H4CtC}3C6H3] (1). Recrystallization
from toluene/pentane mixtures afforded the compound in a
pure state (0.540 g, 0.244 mmol, 80%). Total yield 68%.
Figure 6. Changes of the linear transmission and the
nonlinear transmission as determined from the relative
depth (triangles: negative effect; nonlinear absorption) or
height (squares: positive effect; nonlinear transmission)
of the open-aperture Z-scan curve measured in-situ in an
OTTLE cell when the electrochemical potential was cycled
between the values of 0.2 and -0.5 V.
Color: brown. Anal. Calcd for C144H132P6Fe3: C, 78.05; H,
6.00. Found: C, 78.39; H, 5.83. MS (positive ETOF, CH2Cl2):
m/z 2215 (1+, 6); 589 ([(dppe)(C5Me5)Fe]+, 100). IR (ν, KBr/
CH2Cl2, cm-1): 2199/2202 (w, CtC); 2043/2046 (vs, FeCtC);
Raman (ν, neat, cm-1) 2203 (s, CtC); 2046 (s, Fe-CtC).
length 0.5 mm, mounted within the spectrophotometer. The
electrogeneration potential was ca. 300 mV beyond E1/2 for each
couple, to ensure complete electrolysis. The efficiency and
reversibility of each step was tested by applying a sufficiently
positive potential to reoxidize the product; stable isosbestic
points were observed in the spectral progressions for all the
transformations reported herein. 1H and 31P NMR spectra were
obtained on Bru¨ker AVANCE 200 (200 MHz) and AVANCE
500 (500 MHz) spectrometers. 1H and 31P NMR chemical shifts
are reported in units of parts per million (ppm) relative to
residual protiated solvent and H3PO4, respectively. Cyclic
voltammograms were recorded using a PAR 263 in CH2Cl2
(0.1 M [NnBu4]PF6) at 25 °C at a platinum electrode, using a
SCE reference electrode and ferrocene as internal calibrant
1
31P{1H} NMR (δ, CDCl3, 81 MHz, ppm): 100.9 (s). H NMR
(δ, CDCl3, 200 MHz): 7.90 (m, 12H, HAr); 7.60-6.80 (m, 69H,
3
HAr); 6.85 (d, JHH ) 7.2 Hz, 6H, HAr); 2.67 (m, 6H, CH2dppe);
2.02 (m, 6H, CH2dppe), 1.48 (s, 45H, C5(CH3)5). 13C NMR
2
(δ, CDCl3, 50 MHz, ppm): 147.0 (t, JCP ) 39 Hz, Fe-CtC);
139.7-127.0 (m, 8CAr/dppe + 2 Cquat./Ar + 4 C-HAr); 124.9 (s,
Cquat./Ar); 121.3 (br s, Fe-CtC); 116.3 (t, 2JCH ) 8 Hz, Cquat./Ar);
92.2 (br s, CtC); 88.6 (br s, CtC); 88.3 (br s, C5(CH3)5); 31.1
1
(m, CH2dppe); 10.5 (q, JCH ) 126 Hz, C5(CH3)5). E0 (∆Ep, ipa/
ipc): -0.15 V (0.11, 1) V vs SCE. UV-vis (CH2Cl2): λmax(ꢀ/
103 M-1 cm-1) 314 (91.5); 394 (sh, 40.4); 460 (62.2).

4288 Organometallics, Vol. 24, No. 17, 2005
Cifuentes et al.
[(η2-dppe)(η5-C5Me5)Fe(CtC-1,4-C6H4CtCPh)](2).4-Ethy-
nylphenylethynylbenzene (0.165 g, 0.817 mmol) was stirred
in a mixture of 3 mL of thf and 7 mL of methanol with
[(η2-dppe)(η5-C5Me5)FeCl]‚Et2O (0.475 g, 0.761 mmol) and NH4-
PF6 (0.165 g, 1.020 mmol). The initially green suspension was
stirred 12 h at 25 °C, after which time an orange precipitate
had formed in the reaction medium. After evaporation of the
reaction mixture to dryness the resultant orange solid was
extracted with 15 mL of dichloromethane. Concentration of
the extract to dryness, washing with n-pentane (2 × 10 mL),
and drying in vacuo quantitatively afforded the corresponding
vinylidene complex [(η2-dppe)(η5-C5Me5)Fe(CdCH-1,4-C6H4Ct
CPh)](PF6) (2′) as an air-sensitive orange solid after recrys-
tallization from dichloromethane/diethyl ether mixtures (ca.
0.650 g, 92%). Selected data for [(η2-dppe)(η5-C5Me5)Fe(CdCH-
1,4-C6H4CtCPh)](PF6): FT-IR (ν, KBr, cm-1): 1626 (s, Fed
CdC, CtC not observed). 31P{1H} NMR (δ, CDCl3, 81 MHz):
program suite.37 Structure determination was performed with
SIR97,38 revealing all the non-hydrogen atoms. The SHELXL
program39 was used to refine the structure. Atomic scattering
factors were taken from the literature.40 ORTEP views of 2
were realized with PLATON98.41
NLO Measurements. Third-order nonlinear optical mea-
surements of 1, 2, and 4 were performed with an amplified
femtosecond laser system consisting of a Clark-MXR CPA-2001
Ti-sapphire regenerative amplifier pumping a Light Conver-
sion TOPAS optical parametric amplifier providing tunable
femtosecond pulses of light. All experiments described here
were performed with the doubled signal from the OPA at
695 nm. The default system repetition rate of 1 kHz was
reduced to 250 Hz to minimize possible problems with cumula-
tive photochemical and thermal effects.42,43 Z-scans were
recorded with a laser beam passed through a set of pinholes
to generate a central lobe of an Airy pattern as described in
ref 44. The focused spot size was about 35 µm. The peak light
intensity used in the measurements was kept below 100 GW
cm-2. A series of Z-scans was recorded for two or three different
concentrations of each of 1, 2, and 4 in thf. These measure-
ments were used to determine the real and imaginary parts
of the cubic polarizabilities γreal and γimag. The solution
nonlinear parameters were obtained by numerical fitting of
the closed and open aperture Z-scans and calibration against
the nonlinear refractive index of fused silica taken to be
n2 ) 3 × 10-16 cm2 W-1. The solute hyperpolarizabilities were
then obtained by assuming linear dependences of solution non-
linearity on the solute concentration.
1
87.7 (s, dppe); -143.1 (septuplet, JPF ) 713 Hz, PF6-).
1H NMR (δ, CDCl3, 200 MHz): 7.70-7.05 (m, 26H, HAr/dppe);
4
6.27 (d,3JHH ) 8.2 Hz, 2H, HAr); 5.07 (t, JHP ) 4.6 Hz, 1H,
CdC(Ar)H); 3.14 (m, 2H, CH2dppe); 2.52 (m, 2H, CH2dppe); 1.60
(s, 15H, C5(CH3)5). This vinylidene salt (0.650 g, 0.680 mmol)
was then stirred 2 h in thf in the presence of excess potassium
tert-butoxide (0.115 g, 1.020 mmol). After evacuation of the
solvent and extraction with toluene, concentration of the
extract to dryness, and subsequent washings with n-pentane,
the crude orange complex [(η2-dppe)(η5-C5Me5)Fe(CtC-1,
4-C6H4CtCPh)] (2) was isolated (0.460 g, 0.583 mmol, 86%).
The complex was purified by recrystallization from toluene/
pentane mixtures. Total yield: 81%.
Electrochemical switching of the nonlinearity of 13+ was
investigated in an optically transparent thin-layer electro-
chemical (OTTLE) cell27 where the platinum working electrode
contained a ca. 1 mm diameter hole through which the Z-scan
measurements could be performed, in situ, during the applica-
tion of suitable electrochemical potentials.
Color: brown. Anal. Calcd for C52H48P2Fe1: C, 78.99; H,
6.12. Found: C, 78.46; H, 6.39. MS (positive ETOF, CH2Cl2):
m/z 790 ([(dppe)(C5Me5)Fe(CtCC6H4CtCPh)]+, 100); 589
([(dppe)(C5Me5)Fe]+, 5). IR (ν, KBr/CH2Cl2, cm-1): 2208/2209
(w, CtC); 2046/2044 (vs, FeCtC); Raman (ν, neat, cm-1
)
2209 (s, CtC); 2043 (s, FeCtC). 31P{1H} NMR (δ, CDCl3, 81
1
MHz, ppm): 100.9 (s). H NMR (δ, CDCl3, 200 MHz): 7.92-
Acknowledgment. The CNRS and Australian Re-
search Council (ARC) are thanked for financial support.
This project is proudly supported by the International
Science Linkages Programme established under the
Australian Government’s innovation statement, Back-
ing Australia’s Ability. M.G.H. is an ARC Australian
Professorial Fellow, and M.P.C. is an ARC Australian
Research Fellow. We thank J.-Y. The´pot for experimen-
tal assistance with the ESR measurements.
7.20 (m, 26H, HAr); 6.86 (m, 2H, HAr); 2.68 (m, 2H, CH2dppe);
2.04 (m, 2H, CH2dppe); 1.50 (s, 15H, C5(CH3)5). 13C NMR
(δ, CDCl3, 125 MHz, ppm): 146.5 (t, 2JCP ) 39 Hz, Fe-CtC);
139.5-127.0 (m, 8CAr/dppe + 2 Cquat./Ar + 4 C-HAr); 124.5 (m, 2JCH
) 7 Hz, Cquat./Ar); 121.4 (br s, Fe-CtC); 118.5 (t,2JCH ) 9 Hz,
3
3
Cquat./Ar); 91.4 (t, JCH ) 5 Hz, CtC); 89.7 (t, JCH ) 5 Hz, Ct
1
C); 88.3 (s, C5(CH3)5); 31.3 (m, CH2dppe); 10.5 (q, JCH
)
126 Hz, C5(CH3)5). E0 (∆Ep, ipa/ipc): -0.13 V (0.09, 1) V vs SCE.
UV-vis (CH2Cl2): λmax(ꢀ/103 M-1 cm-1) 298 (37.1); 394 (sh,
31.5); 436 (22.9).
Supporting Information Available: Full details of the
X-ray structure of 2 including tables of atomic positional
parameters, bond distances and angles, and anisotropic and
isotropic thermal displacement parameters. This material is
available free of charge via the Internet at http://pubs.acs.org.
Crystallography. Crystals of 2 were obtained by slow
diffusion of n-pentane into a solution of the complex in toluene.
Single-crystal data collection was performed at room temper-
ature, with Mo KR radiation (λ ) 0.71073 Å). A crystal-to-
detector distance of 25.0 mm was used, and data collection
(determination and optimization of the detector and gonio-
meter positions) was performed with the help of the COLLECT
program34 to measure Bragg reflections of the asymmetric
triclinic basal unit cell until θ ) 27.5°. A total of 152 frames
were recorded, using ∆ω ) 2.0° rotation scans to fill the
asymmetric unit cell (exposure time ) 120 s deg-1). A total of
30 573 reflections were indexed, Lorentz-polarization cor-
rected, and then integrated in the triclinic symmetry (P1h point
group) by the DENZO program of the Kappa-CCD software
package.35 Numerical absorption corrections were performed
using the multiscan procedure36 included in the WinGX
OM050030G
(37) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837-838.
(38) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R.
J. Appl. Crystallogr. 1999, 32, 115-119.
(39) Sheldrick, G. M. SHELX97-2. Program for the refinement of
crystal structures; University of Go¨ttingen: Germany, 1997.
(40) International Tables for X-ray Crystallography; Kluwer Aca-
demic Publishers: Dordrecht, 1992; Vol. C.
(41) Spek, A. L. PLATON. A Multipurpose Crystallographic Tool;
Utrecht University: Utrecht, The Netherlands, 1998.
(42) Kamada, K.; Matsunaga, K.; Yoshino, A.; Ohta, K. J Opt. Soc.
Am. B-Opt. Phys. 2003, 20, 529-537.
(34) Kappa CCD Software; B. V. Nonius: Delft, The Netherlands,
1999.
(43) Samoc, M.; Samoc, A.; Luther-Davies, B.; Humphrey, M. G.;
Wong, M.-S. Opt. Mater. 2003, 21, 485-488.
(35) Otwinowski, Z.; Minor, W. In Methods in Enzymology; Carter,
C. W., Sweet, R. M., Eds.; Academic Press: London, 1997; Vol. 276,
pp 307-326.
(44) Rhee, B. K.; Byun, J. S.; van Stryland, E. W. J. Opt. Soc. Am.
B 1996, 13, 2720-2723.
(45) Le Stang, S.; Paul, F.; Lapinte, C. Organometallics 2000, 19,
1035-1043.
(36) Blessing, R. H. Acta Crystallogr. 1995, A51, 33-38.