Synthesis and spectroscopic (1H NMR, ESR) characterization of new aryloxy-Mn(II) complexes: Steric control over O- vs. phenyl-π-coordination of ArO- ligands (ArO- = C6H5O-, 4-methyl-C6H4O-, 3,5-dimethyl-C6H3O-

Article abstract of DOI:10.1139/v01-013

The coordination chemistry of phenoxide ligands, such as C₆H₅O^-, 4-(CH₃)-C₆H₄O^-, 3,5-(CH₃)₂-C₆H₃O^-, 2,6-(tert-butyl)₂-C₆H₃O^-, 2,6-(CH₃)₂-C₆H₃O^-, to Mn(II) has been investigated because of the possible implication of Mn(II)-phenoxide complexes as intermediates in the phenylphosphate carboxylase enzyme, a protein which catalyses the selective carboxylation of phenylphosphate to 4-OH-benzoic acid using CO₂. We report here the synthesis and characterization of [CpMn(μ-OAr)(THF)]₂ (ArO = C₆H₅O^-, 4-(CH₃)-C₆H₄O^-, 3,5-(CH₃)₂-C₆H₃O^-, 2,6-(CH₃)₂-C₆H₃O^-) and [CpMn(η⁵-ArO)] (ArO = 2,6-(tert-butyl)₂-C₆H₃O^- and 2,6-(CH₃)₂-C₆H₃O^-) complexes, the first examples of mixed-sandwich complexes with Cp and phenate as π-ligands. The latter bear the 2,6-substituted phenoxide π-coordinated to the [Mn(Cp)]⁺ moiety. The different mode of bonding of the phenoxide ligands to Mn(II), substantiated by ¹H NMR and electron spin resonance (ESR) spectroscopy, is controlled by the steric hindrance of substituents at the 2- and 6- position. The reactivity of the π-bonded ligand towards CO₂ is also reported as a quite rare example of nucleophilic attack at the cumulene by the ring-carbon of the phenoxide, which is driven by electron density localization at the 4-position generated upon π-coordination to Mn(II).

Full text of DOI:10.1139/v01-013

570
Synthesis and spectroscopic (1H NMR, ESR)
characterization of new aryloxy–Mn(II) complexes:
steric control over O– vs. phenyl–π-coordination of
ArO– ligands (ArO– = C6H5O–, 4-methyl-C6H4O–, 3,5-
dimethyl-C6H3O–, 2,6-di-tert-butyl-C6H3O–, 2,6-
dimethyl-C6H3O–) to the “Mn(II)Cp” moiety, and
their reactivity with carbon dioxide
M. Aresta, I. Tommasi, C. Dileo, A. Dibenedetto, M. Narracci, J. Ziolkowski, and
A. Jezierski
Abstract: The coordination chemistry of phenoxide ligands, such as C6H5O–, 4-(CH3)-C6H4O–, 3,5-(CH3)2-C6H3O–,
2,6-(tert-butyl)2-C6H3O–, 2,6-(CH3)2-C6H3O–, to Mn(II) has been investigated because of the possible implication of
Mn(II)–phenoxide complexes as intermediates in the phenylphosphate carboxylase enzyme, a protein which catalyses
the selective carboxylation of phenylphosphate to 4-OH-benzoic acid using CO2. We report here the synthesis and char-
acterization of [CpMn(µ-OAr)(THF)]2 (ArO = C6H5O–, 4-(CH3)-C6H4O–, 3,5-(CH3)2-C6H3O–, 2,6-(CH3)2-C6H3O–) and
[CpMn(η5-ArO)] (ArO = 2,6-(tert-butyl)2-C6H3O– and 2,6-(CH3)2-C6H3O–) complexes, the first examples of mixed-
sandwich complexes with Cp and phenate as π-ligands. The latter bear the 2,6-substituted phenoxide π-coordinated to
1
the [Mn(Cp)]+ moiety. The different mode of bonding of the phenoxide ligands to Mn(II), substantiated by H NMR
and electron spin resonance (ESR) spectroscopy, is controlled by the steric hindrance of substituents at the 2- and 6-
position. The reactivity of the π-bonded ligand towards CO2 is also reported as a quite rare example of nucleophilic at-
tack at the cumulene by the ring-carbon of the phenoxide, which is driven by electron density localization at the 4-
position generated upon π-coordination to Mn(II).
1
Key words: Mn(II)-complexes, phenoxide ligands, H NMR spectroscopy, ESR spectroscopy, reaction with carbon diox-
ide.
Résumé : On a étudié la chimie de coordination des ligands phénolates, tels que C6H5O–, 4(CH3)-C6H4O–, 3,5-(CH3)2-
C6H3O–, 2,6-(tert-butyl)2-C6H3O– et 2,6-(CH3)2-C6H3O–, avec le Mn(II) en raison de l’implication possible des com-
plexes Mn(II)-phénolates comme intermédiaires dans l’enzyme carboxylase du phosphate de phényle, une protéine qui
catalyse la carboxylation sélective du phosphate de phényle en 4-HO-C6H4-C(O)OH à l’aide de CO2. On a synthétisé et
caractérisé les complexes [CpMn(µ-OAr)(THF)]2 (ArO = C6H5O–, 4(CH3)-C6H4O–, 3,5-(CH3)2-C6H3O– et 2,6-(CH3)2-
C6H3O–) et [CpMn(η5-ArO)] (ArO = 2,6-(tert-butyl)2-C6H3O– et 2,6-(CH3)2-C6H3O–), le premier exemple d’un
complexe sandwiche mixte comportant un Cp et un phénate comme ligands π. Dans le dernier, le phénolate 2,6-
disubstitué est fixée par une coordination π à la partie [Mn(Cp)]+. Les différents modes de fixation des ligands phéno-
1
lates au Mn(II), tels que confirmés par RMN du H et par spectroscopie RPE, sont contrôlés par l’encombrement sté-
rique des substituants en positions 2 et 6. On a aussi déterminé la réactivité des ligands fixés d’une façon π vis à vis
du CO2; il s’agit d’un rare exemple d’attaque nucléophile au niveau du cumulène par l’atome de carbone du cycle du
phénolate alors que celle-ci est amenée par la localisation de la densité électronique en position 4 sous l’influence
d’une coordination π avec le Mn(II).
1
Mots clés : complexes du Mn(II), ligands phénolates, spectroscopie RMN du H, spectroscopie RPE, réaction avec le
bioxyde de carbone.
[Traduit par la Rédaction] Aresta et al. 577
Received August 8, 2000. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on May 31, 2001.
Dedicated to Professor Brian James on the occasion of his 65th birthday.
M. Aresta1, I. Tommasi, C. Dileo, M. Narracci, and A. Dibenedetto. Department of Chemistry and CNR-MISO, University of
Bari, Campus Universitario, 70126 Bari, Italy.
J. Ziolkowski and A. Jezierski. Institute of Chemistry, University of Wroclaw, F. Joliot-Curie 14, 50383, Wroclaw, Poland.
1Corresponding author (telephone: +39 080 544-2430; fax: +39 080 544-2429; e-mail: aresta@meteo.uniba.it).
Can. J. Chem. 79: 570–577 (2001)
DOI: 10.1139/cjc-79-5/6-570
© 2001 NRC Canada

Aresta et al.
571
Fig. 1. ESR sectrum of [Cp(THF)Mn(µ-O-C6H5)]2 (in THF, T=
77 K, mod amplitude: 2.299 G, frequency: 9.4247866 GHz,
power: 2.00e + 01 mW).
Introduction
Phenol and its derivatives have been shown to act as lig-
ands to transition metal systems (1) through either the O-
atom or the phenyl π-electron system. Bulky substituents in
both 2- and 6-position of the phenyl ring, as well as an un-
saturated coordination of the metal, may force π-phenyl
bonding to the metal center (2). Such a situation has been
structurally documented for diamagnetic Rh(I) complexes
(3). Although both diamagnetic and paramagnetic
phenoxide-complexes are extensively described in the litera-
ture, no examples of phenoxide π-coordination to a para-
magnetic center have been reported to date.
Iron(III) complexes bearing phenol or substituted phenols,
which are exclusively η1–O-bonded, have been extensively
studied as model compounds of catechol oxidase (4). The
importance of Mn(II) systems in several biosystems (5) and
in material science has prompted investigatations of com-
plexes or clusters bearing the η1–O-bonded phenoxide anion
(6). Only a single case of π-bonded phenate to Mn(I) is re-
ported in the literature (7). Despite this interest practically
no reports are available in the literature that describe both
modes of bonding of phenol and (or) its substituted forms to
manganese(II).
very clean and can be followed by GC that allowed the reac-
tion stoichiometry to be determined for eq. [1].
[1]
2Cp2Mn + 2C6H5OH
THF → [Cp(THF)Mn(µ-O-C6H5)]2 + 2C5H6
Very recently we have extracted a phenol carboxylase en-
zyme from the anaerobic bacteria Thauera aromatica (8).
The extracted enzyme was supported onto an organic matrix
and shown to catalyse very selectively the conversion of
phenylphosphate into 4-OH-benzoic acid in mesophilic con-
ditions (9) with significant turnover frequency (TOF) and
turnover number (TON). While information about the struc-
ture of the active site is still not available, it is known that
the enzymatic activity is Mn(II) dependent (8) and that if the
enzyme is incubated with phenol this activity is strongly re-
tained. This finding prompted us to investigate the coordina-
tion of phenol and its derivatives to Mn(II) in order to gain
information about the relevance of the mode of bonding to
reactivity. Therefore, we decided to investigate the reaction
of Mn(Cp)2 with C6H5OH, 4-(CH3)-C6H4OH, 3,5-(CH3)2-
C6H3OH, 2,6-(tert-butyl)2-C6H3OH, and 2,6-(CH3)2-C6H3OH.
In this paper, we report the synthesis of complexes of the
formula [CpMn(ArO)(THF)] (ArO = C6H5O–, 4-(CH3)-
C6H4O–, 3,5-(CH3)2-C6H3O–, 2,6-(tert-butyl)2-C6H3O–, and
2,6-(CH3)2-C6H3O–), which are quite rare examples of para-
magnetic complexes bearing one Cp and one phenoxide
ligand. The different mode of bonding of phenoxide anion to
298K
GC analysis of the reaction solution after precipitation of
the solid, shows that one mol of C5H6 per mol of Cp2Mn is
released after addition of phenol. Residual phenol is not
found in solution.
Complex 1 is very oxygen- and moisture-sensitive. The
white solid is pyrophoric and could not be correctly charac-
terized by conventional elemental analysis techniques. In
spite of this, it was possible to ascertain its composition by
acidolysis of the compound and quantitative determination,
via GC, of the ligands released (see Experimental). Analysis
of 1, by this methodology, ascertained that Cp and PhO– are
coordinated to the metal in a 1:1 ratio and that one mol of
solvent (THF) per mol of Cp and PhO– is also present in the
metal coordination sphere. Quantitative determination of Mn
was achieved by atomic absorption after acidolysis of the
compound in aqueous solution and was determined to be
present in a 1:1 ratio with the other ligands (PhOH, Cp,
THF). When 1 was suspended in pentane under continuous
stirring for approximately two days, the analysis of the re-
covered compound, after acidolysis, still gave a 1:1:1:1 ratio
for Cp, THF, phenol, and Mn, respectively, indicating that
the THF ligand remains quite strongly bound to the metal
when the solid is suspended in non-coordinating solvents.
Complex 1 has been also characterized by spectroscopic
1
Mn(II) is substantiated by H NMR, electron spin resonance
(ESR), and IR spectroscopy, and by the reactivity of Mn–
phenoxides with CO2. A unique example of ring carboxyl-
ation of phenol derivatives to substituted 4-OH-benzoic acid
under ambient conditions, which is fully mimetic of the
above mentioned biological carboxylation of phenol, is also
reported.
1
techniques (ESR, IR, H NMR) and by magnetic measure-
ments. The IR spectrum (Nujol™ suspension) shows bands
at 1587 (s, (C=C)) and 1260 (m, (C-O)) cm–1. The ESR
spectrum of 1 (Fig. 1), measured at 77 K in frozen THF,
shows several not well resolved absorptions in the range
0–5000 G.
Results and discussion
The two transitions in the range 1800–2800 G show an
hyperfine structure consisting of 11 lines with a hyperfine
coupling constant of 42 ± 2 G, in which the intensities fol-
low quite well the 1:2:3:4:5:6:5:4:3:2:1 ratio. The observed
value (42 G) is approximately half the value (85–90 G)
found for monomeric complexes (10). Preliminary theoreti-
Synthesis and characterization of the [Cp(THF)Mn(µ-
O-C6H5)]2 complex (1)
Cp2Mn reacts with PhOH (in THF) in a 1:1 molar ratio at
room temp to afford a white solid (1), which separates quan-
titatively from the reaction mixture (eq. [1]). This reaction is
© 2001 NRC Canada

572
Can. J. Chem. Vol. 79, 2001
1
Fig. 2. Proposed structure of dimeric Mn-complexes.
Fig. 3. H NMR spectrum of [Cp(THF)Mn(µ-O-C6H5)]2 in THF-
d8, T = 298 K.
1
documented in the literature, the H NMR signals of CH3-
protons attached to aromatic systems in paramagnetic com-
plexes are narrower with respect to the signals of protons di-
rectly attached to the carbon atoms of the phenyl ring and,
thus, they can be more easily recognized in the spectrum.
(ii) If strong spin density polarisation is taking place through
the aromatic ring, the substitution of H with a CH3 group is
expected to significantly shift the proton resonance. This
feature is often very helpful in proton assignment.
In fact, a comparative analysis of the variation of the CH3-
proton shifts with the position of the substituents in the ring
made possible the assignment of almost all resonances for
the complexes with the different ligands listed above; the re-
sults are reported in Table 1.
On the basis of the total spectral assignments for the com-
plexes bearing coordinated phenoxide and its substituted forms,
we can, for complex 1, attribute the signal at 34 ppm to
phenolate-meta-protons and the signals at –18 and –11 ppm
to the phenolate ortho- and para-protons, respectively. The
signal at –23 ppm is assigned to the coordinated Cp protons.
This value is in agreement with the data reported in the liter-
ature for half-sandwich complexes with facial coordination
of the Cp ligand (13). The band at 24 ppm is assigned to the
coordinated THF molecule. When the spectrum was regis-
tered in THF-d8 this signal was not observed, because of the
exchange with the deuterated solvent. By adding small
amounts of THF directly to the solution in the NMR tube,
the resonance at 24 ppm did finally appear.
cal calculations2 also support the presence of a dimeric sys-
tem. The above reported spectral features indicate the
presence of two 55Mn centers (I = 5/2) with strong electron
spin exchange interaction through the bridging O-bonded
phenoxide ligands. Such features are common to other
Mn(II)-complexes bearing O-bridging ligands (11). The pro-
posed structure for compound 1 is shown in Fig. 2 and ac-
counts for a 17 electron-system.
Complex 1 has a magnetic moment of µeff = 5.35 µB per
Mn atom measured at room temp and 4.54 µB at 80 K. The
product (χm· θ) is almost constant over the entire range of
temperature. This value may indicate the presence of high
spin antiferromagnetically coupled Mn(II) centers (12).
Unfortunately, it was not possible to obtain the 13C NMR
1
spectrum of 1. The H NMR spectrum of 1 (Fig. 3), how-
ever, shows a very complex pattern with broad signals (half-
widths of ca. 4 kHz) at 34, 24, –11, –18, and –23 ppm. The
1H NMR spectra reported throughout this paper are quite
rare examples of proton NMR spectra of Mn(II) complexes,
for which very often it is reported that it was not possible to
1
obtain the H NMR spectrum due to sensivity reasons.
Reaction of Cp2Mn with 4-CH3-C6H4OH
1
The observed pattern in the H NMR spectra strongly in-
The reaction of Cp2Mn with 4-(CH3)-C6H4OH in a 1:1
molar ratio in THF affords compound 2, which is soluble in
THF and can be separated from a very concentrated solution.
dicates the dominance of the spin density polarisation effect
through the aromatic ring (5). As the integral and the chemi-
cal shifts were not helpful for the assignment of the reso-
nances for the phenoxide, THF, and Cp protons, a systematic
study of the 1H NMR spectra of half-sandwich Mn(II)
complexes with ortho-, meta-, and para-substituted phenols
(4-(CH3)-C6H4OH, 3,5-(CH3)2-C6H3OH, 2,6-(tert-butyl)2-
C6H3OH), and 2,6-(CH3)2-C6H3OH) was undertaken. This
study was reputed useful for the following reasons. (i) As
[2]
2Cp2Mn + 2(4-CH3-C6H4OH)
THF → [Cp(THF)Mn(µ-O-4-(CH3)-C6H4)]2
298 K
+ 2C5H6
2 V. Minin. Kurnakov Institute of General and Inorg. Chem. of the Russian Academy of Sciences, Moscow, Russia (personal communica-
tion).
© 2001 NRC Canada

Aresta et al.
573
1
Table 1. H NMR data for Mn-complexes.
H-ortho
H-meta
H-para
Cp
[Cp(THF)Mn(µ -C6H5)]2 (1)
–18
n.a.
–2
10 (t-Bu)
28 (CH3)
–24 (CH3)
34
–11
31 (CH3)
–18
–22
–18
–23
–26
–26
3
–29
2
[Cp(THF)Mn(µ -O-4-(CH3)-C6H4)]2 (2)
[Cp(THF)Mn(µ -O-3,5-(CH3)2-C6H3)]2 (3)
[CpMn(η5–2,6-(tert-butyl)2-C6H3O)] (4)
[Cp(THF)Mn(µ -O-2,6-(CH3)2-C6H3)]2 (5)
[CpMn(η5–2,6-(CH3)2-C6H3O)] (6)
26a
–5 (CH3)
31
35
25
–18
Note: n.a. = not assigned.
aTentative assignment.
In this case, GC analysis of the reaction solution is not
helpful for monitoring the amount of Cp released upon the
reaction of Cp2Mn with 4-(CH3)-C6H4OH. In fact, because
the product is soluble in the reaction solvent, the GC
technique cannot distinguish the free Cp in solution from
that generated upon decomposition of the complex in the in-
jector. It was, however, possible to follow the reaction by
monitoring the IR spectrum of the reaction solution. In fact,
just minutes after the addition of 4-CH3-(C6H4)OH, the band
at 3600 cm–1 due to the free phenol ν(OH) completely disap-
pears, allowing one to conclude that the added substituted
phenol has completely reacted with Mn(II). Complex 2 was
analyzed by GC and atomic absorption analysis after
acidolysis of the solution (see Experimental) and shown to
bear Mn in a 1:1 ratio to Cp, THF, and 4-(CH3)-C6H4OH.
The analytical data do not change if complex 2 is suspended
in pentane (even for 2 days), indicating that THF is always
strongly bound to the metal. The ESR spectrum of 2 mea-
sured at 77 K, as a frozen THF solution, is very similar to
that of 1, including the presence of the 11 line hyperfine
structure for bands in the range 1800–2800 G with hyperfine
coupling of 42 ± 2 G. These features suggest a dimeric
structure for complex 2, with µ-O-bonded phenoxide lig-
The ESR spectrum of 3 in frozen THF solution at 77 K
consists of bands in the range 1800–2800 G, with a
hyperfine structure comprised of 11 lines, and hyperfine cou-
pling of 42 ± 2 G. A dimeric structure is also assigned to
complex 3 (Fig. 2).
1
The H NMR spectrum of compound 3 in THF-d8 shows
that the CH3-meta-protons (–5 ppm) and the Cp protons
(–26 ppm) almost overlap the signals found at –2.5 and
–18 ppm, which are assigned to the para- and ortho-
phenoxide protons, respectively, in the dimeric µ-O-bonded
phenoxide complex (compound 3, Fig. 2). This is in agree-
ment with a negative spin density delocalization on the CH3-
meta-protons of the µ-O-coordinated phenoxide anion and
confirms the spectral analogies between Mn(II)– and Fe(III)–
phenoxide complexes (5). In fact, the pattern of the isotropic
shifts of CH3-meta- or para-protons in compound 3 or 2,
respectively, is very similar to what observed for
(salen)Fe(III) – methyl-substituted-phenoxide complexes,
and demonstrates the presence of a strong spin polarization
through the aromatic ring. On this basis, a positive spin den-
sity delocalization is expected and revealed for protons of
CH3-group ortho- or para- in the phenoxide ligand, as found
also in relevant (salen)Fe(III) complexes.
1
ands. The H NMR spectrum of 2 shows only two intense
signals at 31 and –26 ppm, assigned to the CH3-para-pro-
tons of the phenoxide ligand and the cyclopentadienyl pro-
tons, respectively. The shift towards lower fields of the CH3-
para-phenoxide protons is in agreement to what is observed
for CH3-para-protons of 4-(CH3)-C6H4O– coordinated to
Fe(III)–salen complexes (5), and the observed value of
–26 ppm, is in the correct range for Cp protons. The signals
of the ortho- and meta-protons of the phenoxide ligand are
probably obscured by the two large signals reported above.
The structure for compound 2 is shown in Fig. 2.
Reaction of Cp2Mn with 2,6-(tert-butyl)2-C6H3OH
When Cp2Mn is reacted in anhydrous THF with 2,6-(tert-
butyl)2-C6H3OH in a 1:1 molar ratio a white solid product
(4) is isolated. (eq. [4])
[4]
Cp2Mn + 2,6-(tert-butyl)2-C6H3OH
THF → [CpMn(η5–2,6-(tert-butyl)2-C6H3O)]
298 K
+ C5H6
Again, the reaction cannot be followed by GC analysis be-
cause of the solubility of the reaction product, but can be
isolated and analyzed by GC after acidolysis. The ligands
(Cp, THF, and 2,6-(tert-butyl)2-C6H3OH) are determined to
be in a 1:1:1 reciprocal ratio and each in 1:1 ratio to Mn. If
4 is suspended in pentane for 12 h, analysis of the solid
shows that THF has been released almost quantitatively (less
than 5% of product still contains coordinated THF, as dem-
onstrated by GC analyses).
The ESR spectrum of 4 measured at 77 K, in a frozen
THF solution, shows only one large signal at g = 2.003 G
due to a low spin Mn(II) with a six line hyperfine structure.
The coupling constant (A = 85 ± 3 G) and the six lines
clearly indicate the presence of a monomeric species (10).
When 2,6-(tert-butyl)2-C6H3OH is added in a 1:1 ratio to a
Reaction of Cp2Mn with 3,5-(CH3)2-C6H3OH
The reaction of Cp2Mn with 3,5-dimethylphenol (1:1 mo-
lar ratio) in THF closely matches the reaction reported for 2.
A white powder is isolated from a very concentrated solu-
tion, which was analyzed to be [Cp(THF)Mn(µ-O-3,5-
(CH3)2-C6H3)]2 (3).
[3]
2Cp2Mn + 2(3,5-(CH3)2-C6H3OH)
THF → [Cp(THF)Mn(µ-O-3,5(CH3)2-C6H3)]2
298 K
+ 2C5H6
THF is strongly bound as shown by the stability of 3 in
pentane even for a long period of time (2 days).
© 2001 NRC Canada

574
Can. J. Chem. Vol. 79, 2001
Fig. 4. Proposed structure of monomeric [CpMn(η5–2,6-(tert-
butyl)2-C6H3O)].
Reaction of Cp2Mn with 2,6-(CH3)2-C6H3OH
The reaction of Cp2Mn with 2,6-(CH3)2-C6H3OH in a 1:1
molar ratio in THF affords a mixture of solid products.
Analysis shows the ligands and Mn to all be in a 1:1 ratio.
However, the suspension of the white solid in pentane for
12 h allows for isolation of a powder showing, after
acidolysis, a Cp:THF:phenoxide ratio of 1:0.5:1, indicating
that THF is released from the coordination sphere even in
non-coordinating solvents. Such finding must be related to a
possible change of the coordination mode of the ligand.
Spectroscopic data of the white solid clearly show the
presence of two products (5 and 6) (see discussion below)
formed according to eq. [5]:
[5]
3Cp2Mn + 3(2,6-(CH3)2-C6H3OH)
solution of Cp2Mn and the ESR spectrum of the reaction
mixture is measured at low temperature (80 K) with time,
several changes are observed. Immediately after the addition
of 2,6-(tert-butyl)2-C6H3OH, a complex pattern of signals
appears, similar to that observed for compounds 1–3. If the
solution is allowed to warm to room temp, and after 30 min
the spectrum is repeated at low temperature (80 K), one sin-
gle signal is observed as specified above. These features in-
dicate the initial formation of a high spin species (most
probably, the O-bonded phenoxide complex is the kinetic
product), which in a short time converts into a low spin spe-
cies. The complex bearing the phenol moiety π-bonded to
the metal is the thermodynamic product.
THF → [Cp(THF)Mn(µ-O-2,6-(CH3)2-C6H3)]2
298 K
+ [CpMn(η5-2,6-(CH3)2-C6H3O)] + 3C5H6
Such data can be rationalized considering the features of
complexes 1–4 described above. The ESR spectrum of the
reaction product measured at 77 K in a frozen THF solution
shows a quite complex pattern of signals, only in part similar
to the ESR spectrum of compound 1. The most important
differences are related to: (i) the absence of the hyperfine
structures in the bands in the range 1800–2800 G; and
(ii) the presence of hyperfine splitting in the region of 500
G. In this region a six-line pattern is observed. Such a
hyperfine structure suggests the presence of a monomeric
Since, Cp2Mn and substituted cyclopentadienyl
manganocenes show a spin cross-over effect at temperature
close to room temp that depends on the matrix that contains
the compound, a detailed analysis of the dependence of the
1
species, as found for complex 4. The analysis of the H
NMR spectrum (in THF-d8) shows two broad and partially
overlapping signals at 35 and 28 ppm. One very broad
strong signal appears at –28 ppm with two less intense sig-
nals in the high field region at –18 and –24 ppm. The signals
at 28 and –24 ppm are easily recognized as CH3-protons on
the basis of their half-width of ca. 1 kHz. The signal at
28 ppm is assigned to CH3-protons of a dimeric compound
(compound 5, Fig. 1). The difference in the absolute value of
the isotropic shift for the tert-butyl-protons of compound 4
compared with the CH3-protons of compound 6 is due to the
absence of the strong hyperconjugation effect operative in
the case of the CH3-protons. While the presence of the CH3-
proton signal at 28 ppm is in agreement with the above dis-
cussed pattern, the signal at –24 ppm deviates strongly if
a µ-O-structure is considered. The latter signal is assigned to
a species bearing the O-2,6-(CH3)2-C6H3 moiety π-coordi-
nated to the Mn(II) center. The signals at 35 and –18 ppm
are assigned to the meta- and para-protons, respectively, of
compound 5. The Cp ligand in this complex is found at
–29 ppm. In 6 the Cp signal is shifted to lower field (2 ppm),
and partially overlaps with the solvent signal.
high spin W low spin equilibrium for 4 is in progress.
Complex 4 is soluble enough in benzene at room temp
for preparing a 1 × 10–2 M solution, which was not possi-
ble for dimeric compounds described above, to allow for
molecular mass determination of 4 by cryoscopy (14).
The molecular mass of 4 was found to be 355 g mol–1 (cal-
culated 325 g mol–1), thus, confirming the monomeric struc-
1
ture of the complex. The H NMR of 4 in THF-d8 shows a
signal at 10 ppm for the tert-butyl-protons. As the tert-butyl-
protons are one C—C bond further away from the aromatic
ring with respect to the CH3-protons, the slight shift to lower
fields confirms that in this complex the same mechanism of
spin density delocalization is operative as described for the
other complexes in this paper. The signal due to Cp protons
overlaps with that of the solvent and is found around 3 ppm.
1
The H NMR spectrum of 4 in C6D6 shows the same fea-
tures of that measured in THF-d8, demonstrating that both
benzene and THF do not coordinate to the metal center and,
thus, validates the cryoscopic measurement of the molecular
mass made in C6H6. The monomeric structure of 4, as shown
by ESR and cryoscopic measurements, can be explained by
considering a π-coordination of the aromatic ring to the
metal center (Fig. 4).
A single O-bonded phenoxide to the CpMn moiety
should, in fact, give a coordinatively unsaturated 13e– spe-
cies. While in the literature, one example of a dimeric Mn–
phenoxide complex (15) which formally accounts for 11e–,
is reported, all known momeric cyclopentadienyl complexes
account for 17–19e–.
Both the ESR and 1H NMR spectra and the analytical data
support the conclusion that two products (5 and 6) are
formed as shown in eq. [5]. The experimental evidence that
half of the coordinated THF is released after suspension of
the solid in pentane, indicates that approximately half of the
solid is a species in which THF is not strongly bound to the
metal and can be removed by the action of a non-
coordinating solvent. By simply taking into account elec-
tron-counting, compound 5 can reach a 17e– configuration
by being a dimer with a molecule of THF in the coordination
© 2001 NRC Canada

Aresta et al.
575
sphere. The monomeric species (6), which on the basis of
the analytical data is composed of Mn, Cp, and phenoxide in
1:1:1 ratio, does not need THF to reach the 17e– configura-
tion as it bears the π-coordinated phenoxide ligand. In com-
pound 6, the presence of two methyl groups in the 2-
and 6-positions, forces the η5-coordination of the substituted
phenoxide, but the steric hindrance is not such to afford the
π-coordination as the only form, as observed in the case of
4. Attempts to isolate compounds 5 and 6 as pure solids
have been done. By crystallization from benzene or toluene
it is possible to obtain a compound richer in 6, but not to-
contact with air, was put in a Schlenck tube cooled to 0°C
with an ice water bath and, under vigorous stirring, was
added dry toluene (2 mL) previously saturated with dry, gas-
eous HCl. A brown solid immediately separated. After depo-
sition of the solid, the liquid fraction was analysed by GC
using a temperature program, which allowed for the separa-
tion of cyclopentadiene (1.07 min), THF (1.72 min), toluene
(7.8 min), and phenols (C6H5OH, 10.40 min; 4-CH3-
C6H4OH, 12.38 min; 3,5-(CH3)2-C6H3OH, 14.5 min; 2,6-
(tert-butyl)2-C6H3OH, 15.3 min; 2,6-(CH3)2-C6H3OH,
12.87 min). (ii) Alternatively, the solid sample was treated
with a large (>100 molar ratio) excess of propionic acid. The
solution was analysed by the same GC procedure (propionic
acid retention time is 5.5 min in the reaction conditions and
does not interfere with other species).
tally free of 5. The interconversion (5 W 6) is not operating
at room temp or lower.
Reaction of 4 and 6 with CO2
For the quantitative determination of Mn, the following
procedure was applied: to 15–20 mg of carefully weighted
compound was put in a Schlenck tube under N2 and was
added, under vigorous stirring, distilled water (2 mL) and
concentrated H2SO4 (analytical grade) (1 mL). The light pink
solution obtained was quantitatively transferred into a 50 mL
analytical flask and diluted with distilled water. The solution
was then analysed by atomic absorption spectroscopy.
When complex 4 (or 6) is exposed to CO2, 4-OH-3,5-di-
tert-butyl-benzoic acid (or 4-OH-3,5-dimethyl-benzoic acid)
is isolated (yield ca. 10%). When Mn-complexes bearing
O-bonded phenols are used, ring-carboxylation is not ob-
served. The behaviour of µ-O- and π-bonded phenols can be
explained by taking into account that the π-coordination
forces electron-dislocation in the 4-position to facilitate
nucleophilic attack on CO2. This produces the ring carboxyl-
ation, as also observed for similar Rh-complexes (4a), and
benzene electro-carboxylation, via electron-transfer, to the
ring coordinated Cr(0)-system upon electrochemical reduc-
tion of the metal center (16). Conversely, the µ-O-bonded
phenols undergo CO2 insertion into the Mn—O bond. Sub-
sequent acidolysis produces the semi-carbonate (ROC(O)OH),
which promptly decomposes into ROH and CO2.
Synthesis of [Cp(THF)Mn(µ-O-C6H5)]2 (1)
To a solution of Cp2Mn (400 mg, 2.1 mmol) in anhydrous
THF (30 mL) under a N2 atmosphere was added, with vigor-
ous stirring, phenol (200 mg, 2.1 mmol) dissolved in THF
(10 mL). A white solid immediately precipitated. After stir-
ring for 1 h, the mixture was filtered and the white solid
washed with THF (10 mL), pentane (20 mL), and finally
dried in vacuo. The compound was isolated in 95% yield.
The GC and atomic absorption analysis, carried out after
acidolysis of 1 (as previously described), revealed the pres-
ence of Cp, THF, phenol, and Mn in a 1:1:1:1 ratio.
Compound 1 was suspended in anhydrous pentane
(30 mL) under a N2 atmosphere with vigorous stirring for 2
days. The solvent was eliminated and the complex washed
and dried in vacuo. Analysis, after acidolysis, of the isolated
product showed the same composition as reported above.
FT-IR (Nujol/KBr) (cm–1): (PhO-ligand) 1587 (vs) (C=C),
1260 (m) (C-O), 753 (s) (γ C-H), 693 (s) (γ C-H); (cyclo-
pentadienyl ligand) 1025 (vs) (δC-H), 1006 (s) (δC-H), 760
(s) (γ C-H). 1H NMR (THF-d8) δ: 34 (H-meta), –11 (H-
para), –18 (H-ortho), –23 (Cp).
Experimental
General procedure
Unless otherwise stated, all reactions and manipulations
were conducted under a nitrogen (N2) atmosphere by using
vacuum-line techniques. Samples for ESR studies were pre-
pared using a Brown glove-box under N2 atmosphere. All
solvents were dried as described in the literature (17) and
stored under nitrogen. FT-IR spectra were recorded using a
Bruker 113V Fourier transform interferometer. Frequencies
are accurate to ±1 cm–1. Solid samples were studied as
Nujol™ mulls (Nujol™ was previously dried on sodium
wires and bubbled with argon). GC analyses were performed
with a Dani 3800 gas chromatograph equipped with a FID
detector and a packed SE30 column. Quantitative Mn deter-
mination was performed with an PerkinElmer 3110 atomic
absorption spectrometer equipped with Mn lamp.
Synthesis of [Cp(THF)Mn(µ-O-4-(CH3)-C6H4)]2 (2) and
[Cp(THF)Mn(µ-O-3,5-(CH3)2-C6H3)]2 (3)
NMR spectra were recorded using a AM 500 Bruker spec-
trometer. ESR spectra were recorded at 9.4247866 GHz
(power = 2.00 × 101 mW) on a Bruker ESP300E spectrome-
ter, which has a 100 kHz magnetic field modulation and is
equipped with a Bruker NMR gaussmeter (ER 035M) and a
Hewlett–Packard microwave frequency counter (HP 5350B).
(i) A solution of 4-CH3-C6H4OH (292 mg, 2.7 mmol) dis-
solved in anhydrous THF (15 mL) was added to Cp2Mn
(500 mg, 2.7 mmol) in THF (35 mL) (under N2 atmosphere)
with stirring. The resulting mixture was stirred for 4 h and
then concentrated in vacuo to 5–7 mL. A light-brown solid
separated from the solution and the organic phase was
removed. The solid was washed anhydrous THF (2 × 5 mL),
anhydrous pentane (10 mL), and dried in vacuo. The com-
pound, isolated as a grey powder in 90% yield, was deter-
mined to be [Cp(THF)Mn(µ-O-4-(CH3)-C6H4)]2 by GC
analysis after acidolysis (as previously described).
Acidolysis, GC, and atomic absorption analysis of
compounds
All described compounds were pyrophoric and could not
be analysed by conventional analytical techniques. The fol-
lowing methodology allowed for compositional analysis:
(i) 20–30 mg of the compound, carefully weighted out of the
Compound 2 was suspended in anhydrous pentane follow-
ing the same procedure as described for compound 1. Analysis,
© 2001 NRC Canada

576
Can. J. Chem. Vol. 79, 2001
showed that no change had occurred. FT-IR (Nujol/KBr)
(cm–1): (4-CH3-C6H4O– ligand) 1589 (s) (C=C), 1260 (vs)
(C-O), 750 (s) (γC-H), 700 (s) (γC-H); (cyclopentadienyl
ligand) 1025 (vs) (δC-H), 1006 (s) (δC-H), 770 (s) (γ C-H).
1H NMR (THF-d8) δ: 26 (H-meta), 31 (CH3-para), –26 (Cp).
Reaction of 4 and 6 with CO2
Complex 4 (180 mg, 0.5 mmol) (180 mg (0.7 mmol) for
6) was dissolved in THF and the solution exposed to
0.1 MPa of CO2 at room temp. After 6 h stirring, the solu-
tion was dried in vacuo and the residual solid treated with
BF3–CH3OH. Water was added to eliminate the excess of
BF3 and the mixture extracted with diethyl ether. The GC
analysis of the resulting ether solution showed the formation
of the methyl ester of 4-OH-3,5-(tert-butyl)-benzoic acid if
starting from 4 (or of 4-OH-3,5-(CH3)2-benzoic acid if start-
ing from complex 6).
(ii)
A
solution of 3,5-(CH3)2-C6H3OH (278 mg,
2.28 mmol) in anhydrous THF (10 mL) was added, under N2
flow and vigorous stirring, at room temp to a flask contain-
ing Cp2Mn (420 mg, 2.28 mmol) dissolved in THF (30 mL).
The mixture was worked up as described above and
[Cp(THF)Mn(µ-O-3,5-(CH3)2-C6H3)]2 was isolated in 90%
yield.
Compound 3 in anhydrous pentane did not change its
composition after two days. FT-IR (Nujol/KBr) (cm–1): (3,5-
(CH3)2-C6H3O– ligand) 1585 (s) (C=C), 1155 (vs) (C-O),
750 (s) (γ C-H), 690 (s) (γ C-H); (cyclopentadienyl ligand)
1025 (vs) (δ C-H), 1006 (s) (δC-H). H NMR (THF-d8) δ:
–5 (CH3-meta), –2 (H-ortho), –18 (H-para), –26 (Cp).
Conclusions
The results reported above demonstrate that bulky sub-
stituents at the 2,6-positions of the phenyl ring may force the
η5-coordination to Mn(II) of substituted phenoxides. Cou-
1
1
pling ESR and H NMR spectroscopy proved to be a useful
tool for structure determination in solution. The η5-coordina-
tion of the phenol prompts the phenyl-ring to a nucleophilic
attack of CO2, which produces the ring carboxylation.
Synthesis of [CpMn(η5-2,6-(tert-butyl)2-C6H3O)] (4)
A solution of Cp2Mn (300 mg, 1.62 mmol) dissolved in
anhydrous THF (20 mL) under a N2 atmosphere was added
to 2,6-(tert-butyl)2-C6H3OH (334 mg, 1.62 mmol) dissolved
in THF (10 mL). The mixture was stirred for 4 h and then
worked up as for 2. A white solid compound was isolated in
90% yield which analysis, after acidolysis, showed Cp, THF,
2,6-(tert-butyl)2-C6H3OH, and Mn to be in a 1:1:1:1 ratio.
Compound 6 was suspended in anhydrous pentane (30 mL)
and stirred for 12 h. The solvent was eliminated and the
product obtained was dried in vacuo. The analysis after
acidolysis of the obtained complex showed Cp, THF,
phenoxide, and Mn to be in a 1:0.05:1:1 ratio. FT-IR
(Nujol/KBr) (cm–1): (2,6-(tert-butyl)2-C6H3O– ligand) 1575
(s) (C=O and C=C), 1181 (vs) (C-O), 840 (m) (γ C-H), 610
(m) (γ C-H); (cyclopentadienyl ligand) 1000 (s) (δC-H), 775
(s) (γ C-H). 1H NMR (THF-d8) δ: 31 (H-meta), –22 (H-
para), 10 (t-Bu-ortho), 3 (Cp).
Acknowledgements
This work was supported by MURST, project no.
9 803 026 360. A partial financial support to J. Ziolkowski
and A. Jezierski from the TEMPUS Programme (JEP 8164)
is also acknowledged. Professor Claudio Luchinat is grate-
fully thanked for helpful discussions. Mr. Giuseppe Cosmai
is thanked for skillful technical assistance in hardware con-
struction for the manipulation of compounds.
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