Synthesis and electrochemistry of iron-pyrylium complexes

Article abstract of DOI:10.1021/om0343420

The syntheses and characterization of three cationic iron-pyrylium complexes (2⁺, [2-OMe-3-Fp-5,6-Ph₂(C₅HO)] ⁺; 3⁺, [2-OMe-3-Fp-6-Ph(C₅H₂O)] ⁺; 4⁺, [2-OMe-3-Fp′-6-Ph (C₅H ₂O)]⁺) that result from the sequential addition of methyl propiolate and PhCΞCR (R = Ph, H) to FpBF₄ (Fp = η⁵-C₅H₅(CO)₂Fe) or Fp′PF₆ (Fp′ = η⁵-(C₅H ₄Me)(CO)₂Fe) are presented. The initially formed cationic Fp-methyl propiolate complex is shown to convert into a complicated mixture that includes the corresponding vinylidene and alkynyl complexes. The iron-pyrylium complexes have been characterized by ¹H NMR and IR spectroscopy, elemental analysis, FAB-MS, electrochemical, and fiber-optic-IR and ESR spectroelectrochemical techniques. These metal-pyrylium complexes undergo electron transfer at the pyrylium ring rather than at the metal center. The subsequent chemistry of the neutral species 2·, 3·, and 4· is strongly influenced by the nature of the substituents on the ring. The sterically protected complex 2⁺ undergoes chemically and electrochemically reversible reductions. The complexes 3⁺ and 4 ⁺ undergo dimerization reactions after reduction presumably through the 4-position of the pyrylium ring.

Full text of DOI:10.1021/om0343420

2778
Organometallics 2004, 23, 2778-2783
Syn th esis a n d Electr och em istr y of Ir on -P yr yliu m
Com p lexes
Michael J . Shaw,* Seema J . Afridi, Susan L. Light, J aime N. Mertz, and
Shane E. Ripperda
Department of Chemistry, Box 1652, Southern Illinois University Edwardsville,
Edwardsville, Illinois 62026
Received December 2, 2003
The syntheses and characterization of three cationic iron-pyrylium complexes (2+, [2-OMe-
3-Fp-5,6-Ph2(C5HO)]+; 3+, [2-OMe-3-Fp-6-Ph(C5H2O)]+; 4+, [2-OMe-3-Fp′-6-Ph (C5H2O)]+) that
result from the sequential addition of methyl propiolate and PhCtCR (R ) Ph, H) to FpBF4
(Fp ) η5-C5H5(CO)2Fe) or Fp′PF6 (Fp′ ) η5-(C5H4Me)(CO)2Fe) are presented. The initially
formed cationic Fp-methyl propiolate complex is shown to convert into a complicated mixture
that includes the corresponding vinylidene and alkynyl complexes. The iron-pyrylium
1
complexes have been characterized by H NMR and IR spectroscopy, elemental analysis,
FAB-MS, electrochemical, and fiber-optic-IR and ESR spectroelectrochemical techniques.
These metal-pyrylium complexes undergo electron transfer at the pyrylium ring rather than
at the metal center. The subsequent chemistry of the neutral species 2•, 3•, and 4• is strongly
influenced by the nature of the substituents on the ring. The sterically protected complex
2+ undergoes chemically and electrochemically reversible reductions. The complexes 3+ and
4+ undergo dimerization reactions after reduction presumably through the 4-position of the
pyrylium ring.
In tr od u ction
Mn(CO)5-pyrylium σ-complex, 1+, and explored its
reactivity.5 The ligand turned out to be less electron-
rich than neutral hydrocarbyl ligands because of the
delocalization of the positive charge around the hetero-
cyclic ring. Consistent with this relative lack of electron
density,6 the Mn-pyrylium σ-bond resisted electrophilic
cleavage by H+ and did not undergo migratory insertion
reactions to coordinated CO.5 Reduction occurred at the
ligand, but the metal’s reactivity was increased,7 thereby
resulting in a rapid electrocatalytic ligand substitution
reaction involving the initial loss of CO. Ligand-based
reactivity in the form of dimerization reactions and
H-atom abstraction reactions were also observed. In this
contribution, we outline the preparation of several iron-
based pyrylium complexes and explore their redox
chemistry by using fiber-optic-based IR spectroelectro-
chemistry.8
Pyrylium salts are important intermediates in the
regiospecific synthesis of many organic compounds such
as nitrobenzenes, pyridines, thiopyryliums, phospha-
benzenes, and 2,4-pentadienones.1 Despite the large
number of pyrylium compounds known, few σ-bonded
organometallic pyrylium complexes have been prepared
and investigated.2,3 This oversight is surprising in light
of the utility of purely organic pyrylium ions in organic
synthesis and their applications as dyes, laser dyes,
photosensitizers, and corrosion-inhibiting additives in
paints.1 Pyrylium ions have a rich redox chemistry and
can undergo reduction to form stable radicals, dimers,
or pyran compounds, depending on steric factors and
on the reducing agent. The coupling of this redox
chemistry to the recognized properties of a transition
metal has the potential to yield systems with useful
physical and chemical properties.
Resu lts a n d Discu ssion
(1) Syn th esis of Dica r bon ylcyclop en ta d ien yl-3-
(2-m et h oxy-5,6-d ip h en ylp yr yliu m )ir on (II) Tet r a -
(2) Ohe, K.; Mike, K.; Yokoi, T.; Nishino, F.; Uemura, S. Organo-
metallics 2000, 19, 5525, and references therein.
(3) Garnovskii, A. D.; Sadimenko, A. P. Adv. Heterocycl. Chem. 1998,
72, 1.
(4) Legzdins, P.; McNeil, W. S.; Vessey, E. G. Organometallics 1992,
11, 2718.
(5) Shaw, M. J .; Mertz, J . Organometallics 2002, 21, 3434.
(6) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987, and references
contained therein.
Cationic hydrocarbyl groups such as pyridinium or
pyrylium bound to the metal via a M-C σ-bond are very
rare.2,4 Recently, we prepared an example of a cationic
* Corresponding author. E-mail: michsha@siue.edu.
(1) (a) Balaban, A. T.; Dinculescu, A.; Dorofeenko, G. N.; Fischer,
G.; Koblik, A. V.; Mezheritskii, V. V.; Schroth, W. Pyrylium Salts:
Syntheses Reactions and Physical Properties: Advances in Heterocyclic
Chemistry Supplement 2; Academic Press: New York, 1982. (b)
Farcasiu, D.; Balaban, A. T.; Bologa, U. L. Heterocycles 1994, 37, 1165.
(7) This effect has been observed in other systems: Weinberger, D.
A.; Higgins, T. B.; Mirkin, C. A.; Stern, C. L.; Liable-Sands, L. M.;
Rheingold, A. L. J . Am. Chem. Soc. 2001, 123, 2503.
(8) Shaw, M. J .; Henson, R. L.; Houk, S. E.; Westhoff, J . W.; Richter-
Addo, G. B.; J ones, M. W. J . Electroanal. Chem. 2002, 534, 47.
10.1021/om0343420 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 04/29/2004

Synthesis of Iron-Pyrylium Complexes
Organometallics, Vol. 23, No. 11, 2004 2779
transition metal complex such as Fp+, there are anti-
bonding interactions between the filled d orbitals on the
metal and the π orbital on the alkyne.15 This interac-
tion should result in more electron density being de-
localized to the ester CdO, thereby accounting for the
considerable shift in its IR band. The metal-bound
alkyne CtC band has not been observed, either because
it is under the carbonyl bands or because it is too broad.
In CD2Cl2 solution, FpBF4 also reacts with a 2-fold
excess of methylpropiolate to form the bright red 5+
complex. The alkyne complex is characterized by three
peaks in the expected 5:3:1 integration ratio. A C5H5
peak is observed at 5.73 ppm, a value consistent with a
cationic Fp complex. The bound alkyne’s OCH3 and H
groups display signals at 4.02 and 6.29 ppm, respec-
tively. These values are shifted considerably downfield
of the free ligand’s value of 3.77 and 2.98 ppm, respec-
tively. The large shift in the terminal alkyne hydrogen
signal is particularly indicative of side-on bonding of the
methylpropiolate ligand through the CtC triple bond.16,17
The methylpropiolate complex 5+ is unstable in solu-
tion with respect to vinylidene complex formation (eq
2).18 The vinylidene complex 6+ (2097, 2063 cm-1, Cp
at 5.22 ppm, OCH3 at 4.44 ppm) is acidic and is in
equilibrium with the corresponding deprotonated Fp-
alkynyl complex 7 (2028, 1984 cm-1, Cp at 5.09 ppm,
OCH3 at 3.90 ppm). Efforts to observe the vinylidene
terminal H signal were unsuccessful. Two other minor,
as-yet unidentified signals are also observed in the Cp
Sch em e 1: Syn th esis of Ca tion ic F p -P yr yliu m
Com p lexes
flu or obor ate (2+BF4), Dicar bon ylcyclopen tadien yl-
3-(2-m eth oxy-6-p h en ylp yr yliu m )ir on (II) Tetr a flu o-
r ob or a t e (3+BF 4), a n d Dica r b on yl(m et h ylcyclo-
pentadienyl)-3-(2-methoxy-6-phenylpyrylium)iron(II)-
Hexa flu or op h osp h a te (4+P F 6). The ability of metal
cations to condense organic compounds into cyclic
structures is well-documented.9 In particular, the ability
of Fp+ (Fp ) η5-C5H5(CO)2Fe) to condense acetylenic
esters with alkenes to form cyclic lactones10 prompted
Legzdins to verify that [CpM(NO)2]+ salts (Cp′ ) Cp,
Cp*; M ) Cr, Mo, W) also form corresponding lactones.11
With a second, more electron-rich alkyne instead of an
alkene, the nitrosyl salts mediated the formation of
pyrylium complexes. We have found that other electro-
philic transition metal cations, specifically [Mn(CO)5]+,
Fp+, and Fp′+ (Fp′ ) η5-(C5H4Me)(CO)2Fe), can also
mediate the condensation of methyl propiolate and
certain alkynes to form organometallic pyrylium com-
plexes as in Scheme 1.5,12,13
1
region of the H spectrum at 5.15 and 5.41 ppm. Their
corresponding IR bands are likely hidden by the bands
due to Fp+, 5+, 6+, and 7. The overall result is that
solutions of 5+ spontaneously develop into a mixture of
other species within a few minutes of preparation.
It seems reasonable to suggest that the first step of
the reaction involves η2-coordination of the methyl
propiolate to the metal center (eq 1), as originally
proposed by Rosenblum for the preparation of Fp-
lactone complexes.10
This alkyne complex 5+ has been identified for the
first time in CH2Cl2 by using in-situ fiber-optic IR
techniques and in CD2Cl2 by 1H NMR. Addition of 3
equiv of methyl propiolate to a 0.03 M solution of FpBF4
in CH2Cl2 results in the loss of IR intensity due to the
starting FpBF4 complex (2081, 2037 cm-1) and the
growth of bands at 2073, 2027, and 1631 cm-1, the latter
bands being more clearly revealed when the starting
materials’ bands are subtracted out. The two product
carbonyl bands are appropriate for a cationic Fp com-
plex,14 and the band at 1631 cm-1 represents the ester
ν(CO), which has been shifted considerably from the free
ligand’s value of 1718 cm-1. In a pseudo-octahedral d6-
Equivalent studies in CDCl3 were complicated be-
cause the equilibration appears to be more rapid in this
solvent and because of problems of solubility of the
various cationic species. However, within a few minutes
of mixing the CDCl3 solutions display a similar set of
five Cp peaks as the CD2Cl2 solutions display after a
day of equilibration.
Complex 7 was isolated by treating an equilibrated
solution of FpBF4 and methylpropiolate with solid
(15) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 3rd ed.; Wiley-Interscience: New York, 2001.
(16) Birdwhistell, K. R.; Tonker, T. L.; Templeton, J . L. J . Am. Chem.
Soc. 1987, 109, 1401.
(17) Bartlett, I. M.; Connelly, N. G.; Orpen, A. G.; Quayle, M. J .;
Rankin, J . C. Chem. Commun. 1996, 2583.
(9) (a) Lautens, M.; Klute, W.; Tam, W. Chem. Rev. 1996, 96, 49.
(b) Fru¨hauf, H.-W. Chem. Rev. 1997, 97, 523.
(10) Rosenbloom M.; Scheck, D. Organometallics 1982, 1, 397.
(11) Legzdins, P.; McNeil, W. S.; Vessey, E. G. Organometallics 1992,
11, 2718.
(12) Reger, D. L.; Coleman. J . Organomet. Chem. 1977, 131, 153.
(13) Chaffee, S. C.; Sutton, J . C.; Babbitt, C. S.; Mayer, J .; Guy, K.
A.; Pike, R. D.; Carpenter, G. B. Organometallics 1998, 17, 5586.
(14) Bullock, J . P.; Palazotto, M. C.; Mann, K. R Inorg. Chem. 1991,
30, 1284.
(18) Such interconversions are common: (a) Connelly, N. G.; Geiger,
W. E.; Lagunas, C.; Metz, B.; Rieger, A. L.; Rieger, P. H.; Shaw, M. J .
J . Am. Chem. Soc. 1995, 117, 12202. (b) Connelly, N. G.; Orpen, A. G.;
Rieger, A. L.; Rieger, P. H.; Scott, C. J .; Rosair, G. M. J . Chem. Soc.,
Chem. Commun. 1992, 1293.

2780 Organometallics, Vol. 23, No. 11, 2004
Shaw et al.
NaHCO3 overnight, followed by filtration and crystal-
lization from CH2Cl2/toluene. Addition of >10 equiv of
excess HBF4 to a CDCl3 solution of 7 resulted in an
This evidence suggests two conclusions. First, Lewis
acid binding to the ester group is not the initial step of
the pyrylium-formation sequence. Second, since carbo-
cations are presumably formed when H+ reacts with
phenylacetylene, it shows that binding of methyl pro-
piolate to the metal center enhances the nucleophilicity
of the carbonyl oxygen relative to the unreactive free
alkyne.
1
equilibrium mixture of 6+ and 7, as judged by the H
NMR and IR signals, which match those described
above. The fact that 7 resists full protonation, even in
the presence of excess strong acid, suggests that the
electron-withdrawing ester group on the terminal vi-
nylidene carbon atom increases the acidity of this site
in 6+.
(4) Only aromatic alkynes were observed to undergo
cyclization with FpBF4. No pyrylium-containing prod-
ucts were isolated or detected in NMR experiments
when 1-hexyne, 3-hexyne, trimethylsilylacetylene,
or bis(trimethylsilyl)acetylene was used in place of
phenylacetylene or diphenylacetylene. The C5H5 regions
of these NMR solutions show only the mixture of five
signals discussed above. Since aliphatic alkynes are less
likely to form resonance-stabilized carbonations (eq 4),
they are not attacked by the nucleophilic ester group of
5+. These alkynes are also less likely than aromatic
alkynes to coordinate to Fp+.22 Interestingly, the
[Cp′M(NO)2]+ (Cp′ ) Cp, Cp*; M ) Cr, Mo, W) systems
are known to cyclize both aliphatic and aromatic alkynes
with methylpropiolate,4 a capability that may indicate
a different mechanism than in the present case.
While the pyrylium complexes are the only products
observed when diphenylacetylene or phenylacetylene is
reacted with a metal cation and methyl propiolate after
several days under NMR conditions, isolation of samples
of compounds that are pure enough for electrochemical
characterization has only been accomplished in fairly
low yields (15-20%) to date, probably due to the
interference of 6+ and 7. It is noteworthy that the
preparation of the electrophilic metal cation may be
accomplished in situ by halide abstraction without ill
effects upon the overall reaction.
The proposed mechanism of the formation of the
pyrylium complex 2+ is shown in eqs 4 and 5.
Protonation of diphenylacetylene forms a resonance-
stabilized carbocation. The H+ comes from the vinyl-
idene/alkynyl equilibrium in eq 3. Attack by the nu-
cleophilic CdO of the ester functionality begins the ring
formation, which is completed after loss of H+ and
attack on the CH group of metal-bound alkyne as
shown. This mechanism is supported by the following
observations:
(1) Addition of both methyl propiolate and diphenyl-
acetylene at the same time to a solution that contains
FpBF419 results in a mixture of [Fp(η2-PhCtCPh)]BF4
20
and the pyrylium complex 2+. No [Fp(η2-PhCtCPh)] is
detected when diphenylacetylene is added 30 min after
the methyl propiolate, conditions that favor the complete
conversion of FpBF4 into 5+ but minimize the amount
of 6+ and 7 present. This evidence also suggests that
attack by 5+ on [Fp(η2-PhCtCPh)] is not a significant
pathway, otherwise the whole solution would be ex-
pected to be converted to 2+.
These iron complexes appear to be thermally sensi-
tive, which may be another reason for their low pre-
parative yields. While they persist for months when
stored under argon at -30 °C, they decompose overnight
when stored under argon at room temperature. Still,
formation of the pyrylium complexes is more selective
than the Fp+-mediated formation of the lactone com-
plexes, which form in refluxing THF as a mixture of five-
(2) FpOTf, when combined first with methyl pro-
piolate followed by diphenylacetylene, yields Fp(η2-
PhCtCPh)]OTf as the only detectable product. This
evidence suggests that methyl propiolate is not a strong
enough ligand to displace the triflate anion from the
metal's coordination sphere, thereby underscoring the
importance of the coordination of methyl propiolate to
the metal center in the overall cyclization reaction.
(3) H+, BF3, and Cp2Zr(BF4)2 were used as Lewis acids
in three separate NMR tube experiments. No detectable
pyrylium ions were formed when the Lewis acids were
combined with methyl propiolate and diphenylacetylene.
This result contrasts with recently reported intra-
molecular cyclizations of acetylenic carbonyl compounds
by H+ to form pyrylium compounds.21 Since these Lewis
acids lack d electrons, they could coordinate to the O
atom of methyl propiolate, but not to the triple bond.
10
and six-membered rings, possibly from both 5+ and
6+. As was the case with 1+, the physical properties of
2+ to 4+ are consistent with the positive charge being
distributed around the pyrylium ring. These compounds
have CO bands in their IR spectra that appear at
frequencies similar to those of neutral compounds. For
example, the complex 2+ exhibits ν(CO) frequencies at
2038 and 1989 cm-1, which are very similar to those
displayed by the neutral complexes Fp-Cl (2040, 1999
cm-1) and Fp-CtCPh (2040, 2000 cm-1).23 The 1H
NMR spectra reveal signals assigned to the H atom on
the 4-position of the pyrylium ring at 8.47 and 8.62 ppm
(d, J ) 7 Hz) in 2+ and 3+, respectively. In addition, 3+
displays a doublet at 7.37 ppm for the hydrogen at the
5-position, with a coupling constant consistent with H
atoms on two adjacent positions on the ring.24 Similar
features are observed for 4+. Such highly deshielded
(19) Although we used the method in ref 12 to prepare [Fp(THF)]-
BF4, the red substance we obtained did not show any trace of THF by
NMR.
(20) Munetaka, A.; Masako, T.; Shuji, O.; Yoshihiko, M. Organo-
metallics 1990, 9, 816.
(21) Tovar, J . D.; Timothy M. Swager, T. M. J . Org. Chem. 1999,
64, 6499.
(22) Akita, M.; Kakuta, S,; Sugimoto, S.; Terada, M.; Tanaka, M.;
Moro-Aka, Y. Organometallics 2001, 20, 2736.
(23) Green, M. L. H.; Mole, T. J . Organomet. Chem. 1968, 12, 404.
(24) Ashe, A. J ., III; Sharp, R. R.; Tolan, J . W. J . Am. Chem. Soc.
1976, 98, 5451.

Synthesis of Iron-Pyrylium Complexes
Organometallics, Vol. 23, No. 11, 2004 2781
F igu r e 1. CV of 2+ in THF/0.1 M NBu4PF6 at 800 mV/s.
F igu r e 2. Fiber-optic spectroelectrochemistry of 2+ in
CH2Cl2/0.1 M NBu4PF6. Each subsequent difference spec-
trum was recorded at 5 s intervals after application of
potential.
signals indicate that the positive charge resides on the
pyrylium ring, thereby leaving the metal center rela-
tively uncharged.
The pyrylium ligand in both complexes seems to be
the primary redox site. Since the only difference be-
tween the two pyrylium rings is the substituent metal
complex meta to the O atom, it is surprising that 2+ and
3+ display such different electrochemical properties
(vide infra).
Electr och em istr y of Com p lex 2+. Cyclic voltam-
mograms (CV) of complex 2+ measured in THF at 22
°C at scan rates larger than 400 mV/s reveal a chemi-
cally reversible reduction feature at -1.49 V versus
Cp2Fe, as shown in Figure 1. Pyrylium derivatives are
known to undergo reduction in the range -0.5 to -1.5
V versus Cp2Fe,1b so this derivative appears to be
electron-rich, which is probably a result of the electron-
donating ability of the methoxy group. In comparison,
Fp-CH3 is irreversibly reduced at considerably more
negative potentials (-2.3 V versus Cp2Fe).25 The dif-
ference in potentials suggests that the pyrylium ring is
reduced first. In-situ ESR spectroelectrochemistry re-
veals the presence of a sharp signal at g ) 2.018 after
reduction of the complex, but no hyperfine couplings
were observed. However, fiber-optic IR spectroelectro-
chemistry reveals that upon reduction, the bands for 2+
are consumed and replaced by bands at 2015 and 1956
cm-1, as shown in a series of difference spectra pre-
sented in Figure 2. The relatively small shifts in the IR
bands (23 and 33 cm-1) support the idea that the metal
center is not the main site of electron transfer since
otherwise a shift of about 100 cm-1 would be expected.26
The appearance of the reduction wave in CH2Cl2 is
equally reversible. Controlled potential coulometry in
this solvent confirms that this feature is due to the one-
electron reduction of 2+. Exhaustive electrolysis results
in the passage of ca. 1 Faraday equivalent of charge but
a solution devoid of any electrochemical features or
infrared bands in the carbonyl region. This result
indicates that on longer time scales the reduced complex
2• is unstable and decomposes.
F igu r e 3. CV of 4+ in CH2Cl2/0.1 M NBu4PF6 at 200
mV/s.
by bulk electrolysis. It is unlikely that the M-C bond
is cleaved under these conditions to form a free pyrylium
ion, since such a species would be redox active in the
potential range examined, and no new features were
observed. The reversibility of the reduction of 2+ is
independent of whether HSnPh3, alcohols, phenylacetic
acid, or water has been added to the solution. On the
CV time scale, these observations indicate that the
reduced neutral radical 2 is not subject to attack by H+
and does not participate in H-atom abstraction proc-
esses, in contrast to what was observed for the manga-
nese analogue 1+.5
Electr och em istr y of Com p lex 3+ a n d 4+. Cyclic
voltammetry reveals electrochemically irreversible fea-
tures for 3+ and 4+ in contrast to the reversible feature
observed for 2+. This observation is consistent with the
fact that the 4-position on the pyrylium ring is more
accessible on 3+ and 4+ than it is on 2+, due to the
absence of a second phenyl ring. For 3+ in THF, these
reductions are electrochemically irreversible at 3.2 V/s
even at temperatures as low at -45 °C. However, in
CH2Cl2 there are anodic features that manifest at -0.26
V versus Cp2Fe that are only observed after the reduc-
tion waves are scanned, as shown for 4+ in Figure 3.
For 1+ it was previously shown that a slow dimerization
reaction occurs after electron transfer, consistent with
known pyrylium chemistry.5 The dimer, 42, was ob-
served to give a similar electrochemical response. For
3+ and 4+, it is likely that a similar dimerization of the
neutral organic radicals through the 4-position of the
Addition of alcohols or water to solutions containing
2+ results in no change in the reversibility of the
reduction features, but does result in a slow diminish-
ment in size of the CV waves due to 2+ over the period
of an hour, which ultimately precludes meaningful study
(25) Miholova, D.; Vlcek, A. A. J . Organomet. Chem. 1982, 240, 413.
(26) Nakamoto, K. Infrared and Raman Spectra of Inorganic and
Coordination Compounds, 4th ed.; J ohn Wiley: New York, 1986; p 291.

2782 Organometallics, Vol. 23, No. 11, 2004
Shaw et al.
shaped current feature is not observed and the process
can be outpaced at faster scan rates. Similar chemistry
was observed with 1+.5
Con clu sion s
The synthesis of metal-pyrylium complexes is com-
plicated by various equilibria that result from the
addition of methyl propiolate to a cationic transition
metal complex. Initial coordination of methyl propiolate
to Fp+ results in an alkyne complex that isomerizes to
a vinylidene complex within minutes. The vinylidene
is strongly acidic and is in equilibrium with the corre-
sponding alkynyl species. Cyclization occurs only when
the second alkyne added to the reaction can form a
resonance-stabilized carbocation. The resulting metal-
pyrylium complexes undergo electron transfer at the
pyrylium ring rather than at the metal center, and they
display subsequent chemistry that is strongly influenced
by the nature of the substituents on the ring. The
sterically protected complex 2+ undergoes chemically
and electrochemically reversible reductions, even in the
presence of H-atom donors and fairly strong acids. The
complexes 3+ and 4+ undergo dimerization reactions
after reduction through the neutral radicals 3• and 4•.
F igu r e 4. Spectrum of 4• obtained by subtraction from
pulsed spectroelectrochemistry data.
pyrylium ring occurs. Accordingly, a fiber-optic spectro-
electrochemical investigation of 4+ where the difference
spectrum is recorded as the electrode is held at -1.5 V
versus Cp2Fe revealed the reductive conversion of the
IR bands due to 4+ to bands at 2008 and 1943 cm-1, a
change of 27 and 40 cm-1, respectively. These bands are
consistent with the formation of a neutral Fp′-alkyl
species, such as that expected when 4• dimerizes to form
42. This conversion was found to be chemically revers-
ible, in that reoxidation of 42 resulted in the clean
formation of 4+.
Exp er im en ta l Section
In an effort to provide more detail to this dimerization
reaction, a modification of the fiber-optic spectro-
electrochemical experiment was performed where the
potential on the electrode was synchronized with the
motion of the moving mirror in the IR spectrometer.
Typically, a scan of the moving mirror lasted 420 ms.
At the beginning of each scan, the potential was pulsed
for 100 ms to -1.5 V versus Cp2Fe. When the rest
potential was insufficient to reoxidize the dimer, the
aforementioned bands ascribed to the conversion of 4+
to its neutral dimer were observed with small shoulders
at higher frequencies. Subtraction of the data from an
experiment where the potential on the electrode was set
to -1.5 V for 90 s resulted in the clear observation of
another pair of bands at 2021 and 1969 cm-1, as shown
in Figure 4. These bands are intermediate between 4+
and those of 42 and are consistent with the bands
observed directly for 2•. We therefore ascribe this low-
concentration electrode product to be 4•. When the rest
potential was set to -0.05 V versus Cp2Fe, these bands
were seen more clearly since the spectral changes due
to the conversion of 4+ to 42 cancel out in the difference
spectrum. The fact that these bands are not observed
at longer electrolysis times is consistent with the speed
with which 4• dimerizes.
To probe the reactivity of the electrochemically gener-
ated neutral radical, the reduction of 3+ in THF was
studied in the presence of a hydrogen-atom donor. The
cyclic voltammogram of 3+ undergoes no change upon
the addition of up to 5 equiv of HSnPh3 at any scan rate.
However, when 2.5 equiv of phenylacetic acid is also
added, a 10% enhancement in the cathodic current is
observed at low scan rates (<200 mV/s). At higher scan
rates (>800 mV/s) the current is the same as in the
absence of phenylacetic acid. Thus, the current enhance-
ment is due to a relatively slow process that regenerates
the starting complex 3+ during the scan, since a plateau-
All reactions and subsequent manipulations involving or-
ganometallic reagents were performed under anaerobic and
anhydrous conditions in an atmosphere of prepurified di-
nitrogen or argon. Diethyl ether, hexanes, dichloromethane,
and chloroform-d were purified by distillation from CaH2 into
500 mL rotoflow vessels charged with additional CaH2. THF
was distilled from sodium-benzophenone ketyl into a 500 mL
rotoflow vessel charged with fresh benzophenone ketyl. All
solvents were vacuum transferred directly into reaction vessels
when needed. Methyl propiolate (Acros), diphenylacetylene
(Acros), tetrafluoroboric acid (Acros), and cyclopentadienyldi-
carbonyliron dimer (Aldrich) were used as received. FpBF4,12
and Fp′I27 were prepared as described in the literature. NMR
spectra were obtained with a 300 MHz Varian 300 Unity Plus
spectrometer. IR spectra were obtained using a Mattson
Research Series IR spectrophotometer.
General procedures used for electrochemistry5 and fiber-
optic IR spectroelectrochemistry8 in these laboratories have
been described previously. CV experiments were performed
at 0 °C. Electrochemical measurements were performed using
an EG&G PAR 263 potentiostat interfaced to a computer
running EG&G PAR Model 270 software. Pulsed-potential
spectroelectrochemistry experiments were performed by di-
recting the signal from the IR detector to the computer
interfaced to the potentiostat via a National Instruments
analogue input board. A program written in LabView was
designed to detect the centerburst from the IR detector, wait
until the start of the next IR scan, and then send a command
to the potentiostat to change the electrode potential. The
potential was held for typically 100 ms and then returned to
the resting value. The synchronization was checked and
adjusted empirically with the aid of an HP-7090 digital plotter.
Copies of this program are available on request. Pt-disk
working electrodes were obtained from BAS, and a vacuum
cell was custom-built by ChemGlass with metal-to-glass seals
obtained from Wilmad. Bulk electrolyses were performed using
(27) Fp′I was prepared in a fashion analogous to Fp-I. King, R. B.
Organometallic Synthesis; Academic Press: New York, 1965; Vol. 1, p
175.

Synthesis of Iron-Pyrylium Complexes
Organometallics, Vol. 23, No. 11, 2004 2783
a BAS bulk electrolysis cell contained within the vacuum cell.
Simulations were performed using DIGISIM28 software or ESP
2.4.29
The dark brown solution brightened to red, and a voluminous
white precipitate formed. The mixture was stirred for an hour,
whereupon methyl propiolate (200 µL, 2.2 mmol) was added
by syringe. The mixture was stirred for a further 30 min, and
phenylacetylene (200 µL, 1.8 mmol) was added. The solution
was stirred overnight and filtered through Celite supported
on a medium porous glass frit, toluene (2 mL) was added, and
then the mixture was concentrated under reduced pressure.
The solution was cooled to -30 °C overnight, and reddish-black
crystals of 4PF6 (0.15 g, 20%) were isolated by filter cannu-
lation followed by washing with toluene (2 × 5 mL). Anal.
Calcd for C20H17O4FePF6: C, 46.00; H, 3.28. Found: C, 46.24;
H, 3.36. 1H NMR (CDCl3) δ: 8.58 (d, 1H, 4-C5H2 (OMe)Ph,
3J H-H ) 7.3 Hz), 7.89-7.50, (m, 5H, Ph) 7.38 (d, 1H, 5-C5H2
Syn th esis of 2BF 4. FpBF4 (0.26 g, 1.0 mmol) was dissolved
in CH2Cl2 (20 mL), and methyl propiolate (200 µL g, 2.2 mmol)
was added by syringe. The mixture was stirred for 30 min,
and the diphenylacetylene (0.18 g, 1.0 mmol) was added. The
solution was stirred overnight and filtered through Celite
supported on a medium porous glass frit, and then the solvent
was removed under reduced pressure. THF (10 mL) was
vacuum transferred onto the residue and then concentrated
under reduced pressure until incipient precipitation had been
reached. The solution was cooled to -30 °C overnight, and
reddish-black crystals of 2BF4 (0.13 g, 19%) were isolated by
filter cannulation followed by washing with diethyl ether (2
× 5 mL). No further crops were obtained from this solution.
Anal. Calcd for C25H19O4FeBF4: C, 57.08; H, 3.64. Found: C,
3
(OMe)Ph, J H-H ) 7.3 Hz), 5.04 (d, 2H, C5H4Me), 4.95 (d, 2H,
C5H4Me), 4.57 (s, 3H, OCH3), 2.02 (s, 3H, C5H4Me). IR
(CH2Cl2): ν(CO) 2030, 1980 cm-1, ν(pyrylium CO) 1641 cm-1
.
1
57.28; H, 3.73. H NMR (CDCl3) δ: 8.47 (s, 1H, C5H(OCH3)-
ESI-MS (m/z): 377 (P+ - BF4), 321 (P+ - BF4 - 2CO).
Ph2), 7.7-7.2 (m, 10H, Ph), 5.18 (s, 5H, C5H5) 4.54 (s, 3H,
E
pc(CH2Cl2): -1.47 V vs Cp2Fe.
OCH3). IR: ν(CO) 2038, 1989 cm-1, ν(pyrylium CO) 1631 cm-1
.
Syn th esis of 7. FpBF4 (0.15 g, 0.56 mmol) was dissolved
ESI-MS (m/z): 439 (P+-BF4), 383 (P+-BF4 - 2CO). E°′(THF):
-1.49 V vs Cp2Fe. UV-vis (CH2Cl2) λmax: 278 nm (ꢀ ) 5700
M-1 cm-1), λmax: 381 nm (ꢀ ) 3800 M-1 cm-1).
in CH2Cl2 (40 mL), and methyl propiolate (300 µL, 3.5 mmol)
was added. The solution was monitored by in-situ IR spec-
troscopy and was stirred overnight. Excess NaHCO3 was added
(0.5 g), and the solution was stirred for a further 24 h. The
mixture was filtered through Celite supported on a medium
porous glass frit, toluene (2 mL) was added, and then the
mixture was concentrated under reduced pressure. The solu-
tion was cooled to -30 °C overnight, and reddish-black crystals
of 7 (0.04 g, 27%) were isolated by filter cannulation followed
by washing with toluene (2 × 5 mL) and drying in vacuo. Anal.
Calcd for C11H8O4Fe: C, 50.81; H, 3.10. Found: C, 51.01; H,
3.15. 1H NMR (CDCl3) δ: 5.09 (s, 5H, C5H5), 3.90 (s, 3H,
OCH3). IR (CH2Cl2): ν(CO), 2028, 1984 cm-1, ν(ester CO) 1720
cm-1. ESI-MS (m/z): 260 (P+).
Syn th esis of 3BF 4. FpBF4 (0.26 g, 1.0 mmol) was dissolved
in CH2Cl2 (20 mL), and methyl propiolate (200 µL, 2.2 mmol)
was added by syringe. The mixture was stirred for 30 min,
and the phenylacetylene (200 µL, 1.8 mmol) was added. The
solution was stirred overnight and filtered through Celite
supported on a medium porous glass frit, and then the solvent
was removed under reduced pressure. THF (10 mL) was
vacuum transferred onto the residue and then concentrated
under reduced pressure until incipient precipitation had been
reached. The solution was cooled to -30 °C overnight, and
reddish-black crystals of 3BF4 (0.10 g, 19%) were isolated by
filter cannulation followed by washing with diethyl ether (2
× 5 mL). No further crops were obtained from this solution.
Anal. Calcd for C19H15O4FeBF4: C, 50.71; H, 3.35. Found: C,
51.04; H, 3.45. 1H NMR (CDCl3) δ: 8.62 (d, 1H, 4-C5H2 (OMe)-
Ack n ow led gm en t. This research was supported by
a Cottrell College Science Award from Research Cor-
poration (CC5264) and a NSF grant to M.J .S. (CHE-
0213297). We also acknowledge the Department of
Chemistry, the College of Arts and Sciences, and the
Graduate School of Southern Illinois University Ed-
wardsville for their continuing financial support. FAB
mass spectrometry was provided by the Washington
University Mass Spectrometry Resource, an NIH Re-
search Resource (Grant No. P41RR00954), and ESI
mass spectra and some spectroelectrochemical data
were recorded at the University of Oklahoma courtesy
of Dr. George Richter-Addo.
3
Ph, J H-H ) 7.0 Hz), 7.89 (s, 1H, Ph para H) 7.55, (s, 2H, Ph
ortho H’s) 7.38 (d, 1H, 5-C5H2 (OMe)Ph, 3J H-H ) 7.0 Hz), 7.26
(s, 2H, Ph meta H’s), 5.15 (s, 5H, C5H5), 4.59 (s, 3H, OCH3).
IR (CH2Cl2): ν(CO) 2039, 1988 cm-1, ν(pyrylium CO) 1630
cm-1. ESI-MS (m/z): 363 (P+ - BF4), 307 (P+ - BF4 - 2CO).
E
pc(THF): -1.42 V vs Cp2Fe.
Syn th esis of 4P F 6. FpI (0.45 g, 1.4 mmol) was dissolved
in CH2Cl2 (15 mL), and AgPF6 (0.35 g, 1.4 mmol) was added.
(28) CV simulations were performed using Digisim 3.1 (Bioanalytical
Systems).
(29) SWV simulations were performed using ESP version 2.4
(written by Carlo Nervi), available at http://lem.ch.unito.it/
OM0343420