Electronic properties of the trimethylenemethaneiron tricarbonyl group

Article abstract of DOI:10.1016/S0022-328X(00)00812-3

The substituent electronic effects of trimethylenemethaneiron tricarbonyl [TMMFe(CO)₃] have been studied, with σ values reported. Determination of these values was accomplished through three different techniques: a study of acidity relative to benzoic acid with TMMFe(CO)₃ acting as a substituent of benzoic acid; ¹⁹F-nuclear magnetic resonance (NMR) shielding analysis of m-fluorophenylTMMFe(CO)₃; and ¹³C-NMR shielding analysis of phenylTMMFe(CO)₃. The TMMFe(CO)₃ group behaves in much the same manner as a phenyl group (i.e. weakly electron releasing by resonance and weakly electron attracting by induction). The syntheses of the substituted phenyltrimethyleneiron tricarbonyl compounds are described.

Full text of DOI:10.1016/S0022-328X(00)00812-3

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 620 (2001) 308–312
Note
Electronic properties of the trimethylenemethaneiron tricarbonyl
group
Terry Moore, Chris Kiely, P.C. Reeves *
Department of Chemistry, Abilene Christian Uni6ersity, Abilene, TX 79699, USA
Received 15 August 2000; received in revised form 19 September 2000
Abstract
The substituent electronic effects of trimethylenemethaneiron tricarbonyl [TMMFe(CO)3] have been studied, with | values
reported. Determination of these values was accomplished through three different techniques: a study of acidity relative to benzoic
acid with TMMFe(CO)3 acting as a substituent of benzoic acid; 19F-nuclear magnetic resonance (NMR) shielding analysis of
m-fluorophenylTMMFe(CO)3; and 13C-NMR shielding analysis of phenylTMMFe(CO)3. The TMMFe(CO)3 group behaves in
much the same manner as a phenyl group (i.e. weakly electron releasing by resonance and weakly electron attracting by
induction). The syntheses of the substituted phenyltrimethyleneiron tricarbonyl compounds are described. © 2001 Elsevier Science
B.V. All rights reserved.
Keywords: Trimethylenemethane; Iron; Complexes; Spectroscopy
1. Introduction
a position adjacent to I, lead to the formation of
(cross-conjugated pentadienyl)iron tricarbonyl cations,
III [5,6]. These cross-conjugated metal-stabilized
cations have been employed in a variety of organic
syntheses [7,8].
As part of our continuing studies of highly reactive
C4-hydrocarbons bonded to metal carbonyl systems
[1,2], we have examined the electronic behavior of
trimethylenemethaneiron tricarbonyl (I, TMMFe(CO)3)
when it is present as a substituent on a benzene ring.
Some organometallic systems have been shown to be
very electron rich.
It has been suggested [9] that the diminished reactiv-
ity of I in Friedel–Crafts reactions may result because
it is too electron poor. On the basis of photoelectron
[10] and ESCA data [11] it has been shown that 1,3-bu-
tadiene, cyclobutadiene, and trimethylenemethane pos-
sess very similar ligand donor-acceptor properties.
However, it has been observed that the iron tricarbonyl
complexes of 1,3-butadiene and cyclobutadiene un-
dergo electrophilic substitution reactions readily. There-
fore, in an attempt to explore the electronic character
of I as a substitutent, we have examined its effect on the
acidity of benzoic acids, as well as its effects on the 19F
chemical shift in fluorobenzenes and the 13C chemical
shifts of carbon atoms in benzene.
For example, ferrocene undergoes electrophilic aro-
matic substitution reactions rapidly and stabilizes adja-
cent carbocations to a greater extent than does benzene
[3]. On the other hand, electrophilic aromatic substitu-
tion reactions with TMMFe(CO)3 are only marginally
successful [4] and attempts to generate a carbocation at
* Corresponding author. Fax: +1-915-674-6988.
E-mail address: reeves@chemistry.acu.edu (P.C. Reeves).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00812-3

T. Moore et al. / Journal of Organometallic Chemistry 620 (2001) 308–312
309
2. Results and discussion
The compounds required for this study were pre-
pared by a modification of the procedure Ehrlich and
Emerson employed to synthesize phenyltrimethylen-
emethaneiron tricarbonyl [4]. Overall yields for this
three-step process (Scheme 1) range from 20–35%.
Alkaline hydrolysis of the esters (VIId, e) produces the
corresponding benzoic acids (VIIf, g). Proton (Table 1)
and carbon NMR (Table 2) chemical shifts are similar
for all of these compounds.
The electronic character of organometallic systems is
often probed by measuring their ability to stabilize
adjacent carbocations. This technique depends on es-
tablishing an equilibrium between an alcohol and its
related carbocation in acidic solutions. Since the TMM-
Fe(CO)3-substituted alcohols rearrange to cross-conju-
gated pentadienyl cations in acid solution [5], we have
been unable to utilize this approach. However, a num-
ber of important relationships between substituent
groups and chemical properties have been established.
The Hammett equation is one of the most widely
applied of these relationships. When pKa values for
Scheme 1. Pathway for synthesis of substituted aryl trimethylen-
emethaneiron tricarbonyl complexes. a, H; b, m-F; c, p-F; d, m-
CO2CH3; e, p-CO2CH3.
Table 1
Proton chemical shifts in substituted trimethylenemethaneiron tricarbonyl complexes (CDCl3 solvent)
a
c
b
a
b
Compound
Substituent
Ha
Hb
Hc
Hd
He
Aryl
VIIa
VIIb
VIIc
VIIf
VIIg
H
m-F
p-F
m-COOH
p-COOH
1.90
1.83
1.89
1.90
1.91
1.92
1.82
1.91
1.88
1.88
2.37
2.27
2.36
2.33
2.31
2.91
2.80
2.83
2.82
2.82
4.22
4.12
4.28
4.21
4.18
7.1–7.4
6.7–7.2
6.8–7.3
7.3–7.9
7.2–7.9
a Doublet, Jad=4.2–4.5 Hz.
b Doublet, Jce=2.1–2.5 Hz.
c Singlet.
Table 2
Carbon chemical shifts in substituted trimethylenemethaneiron tricarbonyl complexes
Compound Substituent
C1
C2
C3
C4
Aryl
MetalꢀCO
COOH
a
VIIa
H
m-F
p-F
m-COOH
p-COOH
51.5 54.8 79.1 103.2 126.8, 128.5, 129.5, 139.0
51.7 54.7 77.2 103.3 113.7, 115.9, 125.1, 129.9, 130.1, 141.7
51.5 54.6 77.9 103.3 115.2, 115.5, 131.0, 134.5
52.7 55.3 78.8 104.2 128.5, 129.4, 130.8, 131.7, 134.3, 140.4
52.6 55.2 78.2 103.9 129.3, 129.5, 130.3, 145.2
212.2, 211.7, 210.0
211.7, 211.3, 210.0
211.9, 211.5, 210.0
212.6, 212.4, 211.1
212.2, 212.1, 210.7
–
–
–
VIIb a
VIIc a
VIIf b
VIIg b
167.7
167.4
a CDCl3 solution.
b d6-Acetone solution.

310
T. Moore et al. / Journal of Organometallic Chemistry 620 (2001) 308–312
Table 3
an improved method [15] has been developed based on
13C-NMR analysis of the para-carbon chemical shifts
of monosubstituted benzenes. Analysis of the 13C-spec-
trum of phenylTMMFe(CO)3 in CDCl3 solution pro-
Acidity constants in 50% aqueous ethanol
Compound
pKa values
p-Nitrobenzoic acid
p-Chlorobenzoic acid
VIIf
4.40
5.12
5.47
5.52
5.77
5.89
vides a chemical shift of 126.83 ppm for the
para-carbon atom of the complex compared to 128.39
ppm for the carbon atoms in benzene. Using this
method, a |R° value of −0.13 is calculated for the
TMMFe(CO)3 substituent; therefore, TMMFe(CO)3 is
shown to be weakly electron releasing by resonance.
An examination of various tables of constants reveals
that the TMMFe(CO)3 group most closely approxi-
mates the behavior of a phenyl substituent group in a
variety of chemical reactions. A comparison is shown in
Table 4. These results do not explain the observed
sluggish behavior of TMMFe(CO)3 complexes in elec-
trophilic aromatic substitution reactions. Instead they
suggest that the reaction intermediates may be too
unstable for the reaction to proceed normally.
VIIg
p-Methylbenzoic acid
p-Methoxybenzoic acid
Table 4
Comparison of substitutent constant values [16]
Substituent
|
|
|
|
meta
para
I
R°
TMMFe(CO)3
Phenyl
0.03
0.06
−0.01
−0.01
0.12
0.10
−0.13
−0.11
meta- and para-substituted benzoic acids (measured in
water) are plotted versus Hammett substituent con-
stants (|meta, |para), linear graphs are obtained. [12].
Since the TMMFe(CO)3-substituted benzoic acids are
water insoluble, the titrimetric determination of pKa
was conducted in 50% aqueous ethanol solution. Acid-
ity constants (i.e. pKa) for several substituted benzoic
acids, as well as VIIf, g, were determined in 50%
aqueous ethanol (Table 3) and plotted versus Hammett
substituent values. From this standard graph, the fol-
lowing | values were obtained for the TMMFe(CO)3
substituent: |meta=0.03, |para= −0.01. The |meta con-
stants are considered to be measures of the inductive
effect of a group; whereas, |para values are combina-
tions of inductive and resonance effects. Therefore, this
data indicates that the TMMFe(CO)3 group is slightly
electron withdrawing by induction.
3. Experimental
3.1. Analytical ser6ices
NMR spectra were obtained on Varian Gemini 200
MHz and Unity 300 MHz FT-NMR instruments utiliz-
ing either CDCl3, d6-acetone, or d6-benzene as solvents.
Tetramethylsilane (TMS) was employed as an internal
standard for the proton and carbon spectra while
CFCl3 was used as the standard for the fluorine spectra.
The IR spectra were obtained on a Perkin–Elmer 1600
FT-IR. Elemental analyses were performed by Chema-
lytics, Inc., Tempe, Arizona. Pertinent analytical and
infrared spectral data for all new compounds are in-
cluded in Table 5.
Another constant, |I, has been proposed by Taft [13]
as a measure of the polar (i.e. inductive) effect of a
substituent. It has been shown that fluorine NMR
shielding parameters for meta-substituted fluoroben-
zenes can be correlated with |I [14]. Thus the 19F
chemical shift of the m-fluorophenylTMMFe(CO)3
complex was determined in both CDCl3 and d6-benzene
solutions. In order to determine the fluorine chemical
shift difference between fluorobenzene and m-
fluorophenylTMMFe(CO)3 a spectrum was obtained of
a dilute solution of a mixture of VIIb and fluoroben-
zene in CDCl3. The difference was small (0.33 ppm)
with the signal of VIIb being further upfield from the
internal standard (CFCl3). Using Taft’s equations, the
|I value of the TMMFe(CO)3 group is calculated to be
+0.12. This supports the earlier conclusion that the
TMMFe(CO)3 group is weakly electron withdrawing by
induction.
3.2. Chemicals
The m- and p-carbomethoxybenzaldehydes were syn-
thesized according to literature procedures [17]. All
other reagents were purchased from Aldrich Chemical
Company and used as received.
3.3. pKa measurements
All pKa measurements were conducted in 50% (by
weight) aqueous ethanol solutions by titrimetric meth-
ods at 2091°C. Standardization was accomplished by
the use of an aqueous ethanol buffer [18]. During the
titration, equal weights of dilute base and ethanol were
added to maintain the percent composition of the solu-
tion. The pH values thus obtained were used to calcu-
late the pKa values by the method of Albert and
Serjeant [19].
Resonance effects, measured by the constant |R°, can
also be calculated from the fluorine spectra; however,

T. Moore et al. / Journal of Organometallic Chemistry 620 (2001) 308–312
311
3.4. Synthesis of the substituted
trimethylenemethaneiron tricarbonyl compounds
where N2 gas was bubbled through the solution for 10
min. Diiron nonacarbonyl (5.11 g, 0.014 mol) was
added, and the solution was allowed to stir at room
temperature for two days. At the end of this time, the
mixture was filtered through a celite pad. After the
solvent was removed, the product was purified by elu-
tion from an alumina column with a 25% ethyl ether/
75% hexanes solution. The product was obtained as a
pale yellow solid (1.19 g, 58% — based on amount of
alkene used), mp 39–41°C. Fluorine-NMR chemical
shifts for VIIb were measured as −113.18 ppm (d6-
benzene solvent) and −113.79 ppm (CDCl3 solvent)
utilizing CFCl3 as the internal standard.
The para-fluoro compound (VIIc) was prepared by
the same pathway. Fluorine-NMR chemical shifts were
measured as −114.95 ppm (d6-benzene solvent) and
−115.55 ppm (CDCl3 solvent) utilizing CFCl3 as the
internal standard.
The syntheses of the other compounds were virtually
identical except that the crude dibromo derivatives of
the 1-(carbomethoxyphenyl)-2-methylpropenes were re-
acted with diiron noncarbonyl in benzene solution in-
stead of hexanes.
3.4.1. Phenyltrimethylenemethaneiron tricarbonyl (VIIa)
This compound was prepared as described by Emer-
son [4].
3.4.2. Preparation of
(3-Fluorophenyl)trimethylenemethaneiron Tricarbonyl
(VIIb)
A dry 250 ml three-necked flask was equipped with a
stir bar and a condenser fitted with a nitrogen inlet/out-
let tube. After the flask was flushed with nitrogen gas
for 10 min, isopropyl triphenylphosphonium iodide
(10.38 g, 0.024 mol) and 60 ml of anhydrous ethyl ether
were added and the necks sealed with a glass stopper
and a rubber septum. Fifteen milliliters of a 1.6 M
solution of butyllithium in hexanes (0.024 mol) were
added by syringe over a 1 h period, and the mixture
was allowed to stir at room temperature for 4 h. A
solution of 2.23 g (0.018 mol) of 3-fluorobenzaldehyde
in 10 ml anhydrous tetrahydrofuran (THF) was added
over a period of 1 h, and the mixture stirred at room
temperature overnight. The mixture was filtered
through a celite pad, and the pad was washed with 50
ml of petroleum ether. The filtrate was washed with
water and dried over magnesium sulfate. After removal
of the solvent, 2.18 g (81%) of the crude product
remained as a yellow oil which was purified by distilla-
tion, bp 76–80°C, 28 mm Hg.
In a 250 ml round-bottom flask was placed 1.11 g
(0.0074 mol) of the purified alkene (from above), 2.90 g
(0.016 mol) N-bromosuccinimide and 30 ml of CCl4.
Nitrogen gas was bubbled through the solution for 15
min, 0.25 g of dibenzoyl peroxide was added and the
flask was fitted with a condenser topped with a N2
inlet/outlet tube. The mixture was refluxed for 4.5 h.
After cooling to room temperature, the succinimide was
removed by filtration and the solvent removed under
reduced pressure to yield the crude dibromo compound
as a yellow oil (2.19 g). The oil was dissolved in 100 ml
hexanes and transferred to a 250 ml round-bottom flask
3.4.3. Preparation of (3-carboxyphenyl)-
trimethylenemethaneiron tricarbonyl (VIIf )
In a 100 ml round-bottom flask was placed 30 ml
methanol which was purged with N2 gas for 10 min. To
the stirred methanol was added 0.88 g (2.7 mmol) of
(3-carbomethoxyphenyl)trimethylenemethaneiron tri-
carbonyl, 0.56 g (9.3 mmol) of potassium hydroxide
and 0.8 ml water. The mixture was stirred for two days
at room temperature under a N2 atmosphere. At the
end of this period, the mixture was poured into water
and extracted with diethyl ether. The aqueous solution
was acidified with concentrated hydrochloric acid;
whereupon a precipitate formed. The product was iso-
lated by filtration, washed with water and dried under
vacuum to yield 0.74 g (88%) of the desired product as
a pale yellow solid, mp 172–174°C. The para-substi-
tuted benzoic acid (VIIg) was obtained in a similar
manner.
Table 5
Physical properties and IR spectral data
Compound
Substituent
mp (°C)
Analysis, found (calcd.) (%)
IR metal carbonyl (cm−1
)
C
H
VIIb
VIIc
VIIf
VIIg
m-F
p-F
m-COOH
p-COOH
39–41
58–60
172–174
207–209
54.40 (54.21)
54.46 (54.21)
53.37 (53.54)
53.70 (53.54)
3.12 (3.15)
3.08 (3.15)
3.07 (3.21)
3.29 (3.21)
2039, 1990 a
2037, 1989 a
2060, 2002, 1972 b
2068, 2002, 1980 b
a CS2 solution.
b Nujol mull.

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T. Moore et al. / Journal of Organometallic Chemistry 620 (2001) 308–312
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