New η5- and μ-(O)-Rh(I) phenoxide complexes: synthesis, characterisation and unconventional reactivity of η5-complexes towards carbon dioxide

Article abstract of DOI:10.1016/S0022-328X(00)00217-5

The synthesis, analytical and spectroscopic characterisation, and reactivity of three new Rh(I)-phenoxide complexes of formula Rh2(μ-OPh)2(C2H4)4 (1), Rh(C2H4)2(η⁵-OPh) (2) and Rh(diphos)(η⁵-OPh) (4) [diphos = 1,2-bis(diphenylphosphino)ethane] are reported. Both the η⁵- and μ-(O) modes of bonding have been found. The η⁵-complexes are quite air-sensitive. The reactivity towards carbon dioxide is described and the formation of an emicarbonate is reported with the O-bonded phenoxide. A ring carboxylation occurs with η-⁵-complexes.

Full text of DOI:10.1016/S0022-328X(00)00217-5

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 605 (2000) 143–150
New h5- and m-(O)-Rh(I) phenoxide complexes: synthesis,
characterisation and unconventional reactivity of h5-complexes
towards carbon dioxide
Michele Aresta a,b,*, Angela Dibenedetto a,b, Michelangelo Pascale a,b,
Eugenio Quaranta a,b, Immacolata Tommasi a,b
a Dipartimento di Chimica, Campus Uni6ersitario, Uni6ersita` di Bari, 70126 Bari, Italy
b Centro CNR-MISO, 6ia Amendola 173, 70126 Bari, Italy
Received 23 February 2000; received in revised form 16 April 2000
Abstract
The synthesis, analytical and spectroscopic characterisation, and reactivity of three new Rh(I)–phenoxide complexes of formula
Rh2(m-OPh)2(C2H4)4 (1), Rh(C2H4)2(h5-OPh) (2) and Rh(diphos)(h5-OPh) (4) [diphos=1,2-bis(diphenylphosphino)ethane] are
reported. Both the h5- and m-(O) modes of bonding have been found. The h5-complexes are quite air-sensitive. The reactivity
towards carbon dioxide is described and the formation of an emicarbonate is reported with the O-bonded phenoxide. A ring
carboxylation occurs with h5-complexes. © 2000 Published by Elsevier Science S.A. All rights reserved.
Keywords: Phenoxides; Phenoxo-complexes; Rhodium complexes; Carbon dioxide; Carboxylation
1. Introduction
phenyl-ring electron-system (p-bond). The former co-
ordination mode is expected for acid metal centres or
for metal centres bearing strong donor ligands. In this
case, the phenoxo-ligand can behave as a terminal- or a
bridging-ligand. Electron rich metal centres and
strongly unsaturated metal systems (10 or 12 e− sys-
tems) seem to prefer the p-coordination mode, usually
described as h5-OPh (see A), as it involves the ortho,
meta and para carbon atoms of the aromatic ring. The
ipso carbon lies far away from the metal atom. Accord-
ing to this mode of bonding, the planarity of the
phenoxo ring is practically lost with consequent reduc-
tion of its aromatic character and the p-bonded anion
can be more properly described as a h5-cyclohexa-
dienonyl ligand. Ru [11], Rh [11a,12] and rare examples
of Cr [13] and Mn [14] p-complexes have been reported
until now. Very interestingly, only one mode of bond-
ing has been found per each metal system.
Recently, the chemistry of alkoxo-[1] and aryloxo-
complexes [1a,e,2] of transition metals has been investi-
gated in detail with the aim of collecting information
on the reactivity of the metalꢀoxygen bond [3], consid-
ering the potential of these compounds in synthetic
chemistry [4–6] and in the catalytic conversion of alco-
hols [1d]. A particular interest lies with the utilisation of
these compounds in the synthesis of oxalates and or-
ganic carbonates, in alcohol oxidation, and in carbo–
alkoxylation-, carboxylation- and carbo-imidation
reactions [7]. As part of our studies on the formation of
CꢀC, CꢀO, CꢀN bonds via carbon dioxide fixation
[8,9], we have synthesised new Rh (this paper) and Mn
[10] complexes with the phenoxide anion as a ligand, in
order to investigate the correlation mode of bonding-
reactivity.
It is known that phenoxide ion can coordinate to a
metal centre through either the O-atom (s-bond) or the
* Corresponding author. Tel.: +39-080-5442084; fax: +39-080-
5442083.
E-mail address: csmima01@area.ba.cnr.it (M. Aresta).
0022-328X/00/$ - see front matter © 2000 Published by Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00217-5

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M. Aresta et al. / Journal of Organometallic Chemistry 605 (2000) 143–150
Moreover, the reaction of MꢀO bonds with carbon
the previous ones, low intensity signals at 4.88, 5.75 and
6.29 ppm are evident. These resonances are in the range
that is usual for p-bonded phenoxo-complexes
[11a,12a,b,e,13,23,24].
dioxide [15] has been studied for its relevance to the syn-
thesis of organic carbonates and transition metal emicar-
bonates are well documented. Conversely, the reactivity
of h5-phenoxo complexes towards CO2 has never been
described. We have found that the mode of bonding of
the phenoxo-ligand to the metal can drive the reaction
with carbon dioxide in the direction of insertion into the
MꢀO bond or towards the ring-carboxylation [10].
In this paper we present an interesting example of O-s
and ring-p bonding of the phenoxide anion to the
The 13C-NMR (50.3 MHz, 293 K) spectrum in CDCl3
clearly shows two sets of peaks of quite different inten-
sity. The first group is formed by the lines at 157.45 (s,
Cipso), 128.32 (s, Cmeta), 123.45 (s, Cortho), 120.77 (s,
Cpara) of the phenoxo-ligand and at 56.99 ppm (d,
1J(RhꢀC)=14.9 Hz, Cethylene) that can be attributed to
the O-bonded dimer form [2a–c,20,25]. The second
group of signals is formed by the resonances at 105.6 (d,
(C2H4)2Rh
moiety,
the
characterisation
of
Rh(diphos)(h5-OPh)
[diphos=1,2-bis(diphenylphos-
1J(RhꢀC)=3.6 Hz), 98.6 (d, J(RhꢀC)=2.4 Hz), 83.7
1
phino)ethane] via 1H- and 13C-NMR in various solvents
and the reaction with carbon dioxide of the Rh-com-
plexes so obtained. The h5-Rh complexes reported in
this paper are rare examples of isolated and fully charac-
terised in the solid state (elemental analysis, IR) and so-
lution (NMR) covalent compounds having the
unsubstituted phenoxo-ring coordinated to the metal
without further bonding to free phenol via H-bonding.
(d, J(RhꢀC)=2.2 Hz), assigned to the ortho, meta and
1
para carbon atoms of the phenoxo ligand, and at 49.3
ppm (d, 1J(RhꢀC)=12.3 Hz, Cethylene). The signal of the
ipso phenoxoꢀcarbon is not visible in the spectrum. The
existence of a coupling of the phenoxoꢀcarbon atoms
with Rh, that is not found in the first set of signals,
clearly indicates that the second group of resonances
cannot be related to an O-bonded species, but most
likely is due to a phenoxide anion coordinated to Rh
through the phenyl ring electrons: Rh(C2H4)2(h5-OPh)
(2). However, our NMR data are in good agreement
with the data available in the literature for such metal
systems [11c,d,12c,e].
2. Results and discussion
2.1. Synthesis and characterisation of the
Rh-complexes Rh2(v-OPh)2(C2H4)4 (1) and
Rh(C2H4)2(p5-OPh) (2)
These figures suggest that the material obtained by re-
action of Rh2(m-Cl)2(C2H4)4 with NaOPh is indeed a
mixture of two complexes, one bearing the phenoxide
anion O-bonded to Rh, the other with the phenoxo-
group p-bonded (Eq. (1)a and b). The two species having
similar solubility in several organic solvents were sepa-
rated after a long fractional crystallisation procedure.
The two quite pure compounds have each the set of the
spectroscopic properties discussed above. The molecular
mass (determined by crioscopy) for the dimer is 495. The
monomer shows a molecular mass of 240. This is the first
case for which it has been demonstrated the existence of
the two forms of bonding of phenoxide anion to the
same metal system. We have not observed any intercon-
version process of the pure compounds in solution.
Moreover, despite several trials, we were not able to set
preparative conditions selective for one of the two spe-
cies. The most abundant compound is always the O-
bonded complex (90–98%). The reaction solvent is not
selective neither, if non-coordinating and non-protic sol-
vents are used.
The reaction of Rh2(m-Cl)2(C2H4)4 with NaOPh in rig-
orously anhydrous benzene under a dinitrogen atmo-
sphere at 283 K affords an apparently orange solid, not
conducting in nitrobenzene, analysing correctly for
Rh(OPh)(C2H4)2. The determination of the molecular
mass (by crioscopy [16]) in benzene gives the value of
465, somewhat lower than the value of 504, calculated
for the dimer Rh2(m-OPh)2(C2H4)4 (1) in which phenox-
ide would act as O-bonded ligand bridging the two Rh-
atoms. This mode of bonding is also supported by the IR
spectrum that shows characteristic bands at 1228,
w(CꢀO), and 429 cm−1, w(RhꢀO). The band at 1228
cm−1 is in the range usually reported for the stretching
of the single bond CꢀO in phenoxo-anions O-bonded to
metal atoms [3c,d,11a,12a,17–20]. The presence of a
band of medium intensity at 1576 cm−1, that cannot be
simply attributed to the CꢁC double bond stretching,
coupled with the molecular mass determination data,
suggests that the compound is not the pure dimer. In
fact, the IR band is in the range where the w(CꢁO) of p-
bonded phenoxide is found [11a,12,13].
The 1H-NMR (200 MHz, 293 K) spectrum of the ma-
terial in CD2Cl2 shows, besides the very broad ethylene
resonance at 2.20, intense signals at 7.15 (m, Hmeta) and
6.84 ppm (m, Hortho and Hpara). The two last signals are
found in the usual range for the protons of a phenoxo-
anion s-bonded to a metal [2d,3d,12b,18–22]. Besides

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M. Aresta et al. / Journal of Organometallic Chemistry 605 (2000) 143–150
We have also attempted the synthesis of the phe-
trum of the NaOPh at 1321 and 1299 cm−1 and are
shifted upon coordination.
noxo–ethylene complex starting from Rh(C2H4)2-
(O3SCF3), produced in situ by reaction of Rh2(m-
Cl)2(C2H4)4 with silver triflate in THF (see Section 4).
Also in this case the product is essentially formed by 1
The NMR data unambiguously confirm the structure
of the complex in solution and support the coordina-
tion of phenoxide through the phenyl ring electrons.
The 31P{1H}-NMR (81 MHz, C6D6, 293 K) spectrum
1
(\98%), as demonstrated by the H- and 13C-NMR
1
analysis.
presents a single signal at 72.07 ppm, J(PꢀRh)=200.2
Hz, characteristic of Rh-complexes bearing one diphos
ligand coordinated to the metal centre (see Table 1)
[27,28]. The inspection of the data reported in Table 1
shows that both the position of the resonance and the
value of the PꢀRh coupling constant are useful diagnos-
tic tools for deciding if one or two chelating phospho-
rus ligands are bonded to Rh. Moreover, while the
position of the P-resonance depends strongly on the
magnitude (five or six) of the PꢀRhꢀP ring (see the l
value for the analogous diphos and dppp complexes in
2.2. Synthesis and characterisation of
Rh(diphos)(p5-OPh) (4)
The reaction of Rh(diphos)Cl [26], prepared in situ
from the Cramer’s complex and diphos (Eq. (2)), with
NaOPh in benzene, under
C H
6
Rh2(m-Cl)2(C2H4)4+diphos “62Rh(diphos)Cl+4C2H4
(2)
1
Table 1), the value of J(PꢀRh) is much more indepen-
pure dinitrogen, affords, after crystallisation from ben-
dent from this parameter and more closely related to
the number of chelating ligands coordinated to Rh and
the coordination number of Rh.
zene–pentane, two products: a brown material, 3 and
an orange solid,
4 that analyses correctly for
Rh(diphos)(h5-OPh) (Eq. (3)).
1
The H and 13C spectra of the phenoxo-group of 4
C H
6
Rh(diphos)Cl+NaOPh “6Rh(diphos)(h5-OPh)+NaCl
show inequivocally, the coordination through the
(3)
phenyl ring electron-system. This mode of bonding is
1
clearly supported by (a) the fact that the H and 13C
Compound 3 appears to be a mixture of compounds
and contains chlorinated species and the oxide of the
diphos ligand. Its formation is still under investigation,
as the operative conditions exclude in this case, as for
all other syntheses, the presence or entrance of air or
moisture in the reaction vessel. Compound 4 is a
monomeric complex (molecular mass determined by
crioscopy in benzene: Found 585, Calc. 594), extremely
sensitive to dioxygen and moisture in solution. Com-
plex 4 shows in the IR spectrum in Nujol a strong band
at 1588 cm−1 attributed to the stretching of the CꢀO
bond of the phenoxo-ligand. The position of this band
suggests p-coordination of phenoxide anion. Two
bands at 1264 and 1247 cm−1 are attributed to the
deformation of the group CꢀC(O)ꢀC that has a partial
ketone character. These bands are present in the spec-
signals of the p-phenoxo-ligand are upfield shifted with
respect to the positions where they are usually found in
systems containing a phenoxo group s-O-bonded to the
metal; (b) the splitting of the p-phenoxo-ligand 13C
resonances upon coupling with Rh and/or P. Such
couplings are not found when the phenoxo-ligands are
s-O-bonded to LnRh (L may be a P ligand) moieties.
However, their presence in the 13C spectrum results to
be a useful diagnostic tool for supporting the p-coordi-
nation of the phenoxo-ligand.
Complex 4 has been characterised by NMR spec-
troscopy in different solvents (Table 2). Interestingly,
the multiplicity of the p-phenoxo-bonded 13C reso-
nances seems to be affected by the nature of the sol-
vent. As a matter of fact, in C6D6 (unlike CD2Cl2), they
appear as singlets, likewise what reported, in the same
solvent (C6D6), for (Ph3P)2Rh(p-OPh)·2PhOH [12f],
known for a long time [11a] and structurally character-
ised recently in the solid state [12f].
Table 1
31P NMR data for RhI(LꢀL) and RhI(LꢀL)2 complexes
Complex
Solvent
lP (ppm) 1J(PꢀRh) Reference
Moreover, also the position of the resonances of the
p-phenoxide is dependent on the solvent used (Table 2).
In chlorinated solvents (CDCl3 and CD2Cl2), an up-
field shift of the proton resonances with respect to
benzene is observed. As from Table 2, the observed
shifts are: Zortho=1.08 (CDCl3) and 1.42 (CD2Cl2),
Zmeta=1.23 (CDCl3) and 0.70 (CD2Cl2), Zpara=2.02
ppm (CDCl3) and 1.55 (CD2Cl2). The most evident
difference is relevant to the para proton. We can ex-
clude that benzene coordinates to Rh or displaces the
phenate anion. In fact, no signals that can be assigned
(Hz)
Rh(diphos)(h5-OPh) C6D6
Rh(diphos)(Ph) CDCl3
72.07
74.6
71.8
200.2
199.1
198.4
176
This work
[27]
[27]
Rh2(m-Cl)2(diphos)2 CD2Cl2
[Rh(diphos)
THF
74.65
[27]
(CH3CN)2]Cl
Rh(diphos)(h6-BPh4) THF
74.28
59.4
206
133.5
184
[27]
[28]
[28]
[28]
[Rh(diphos)2]Cl
Rh2(m-Cl)2(dppp)2
[Rh(dppp)2]Cl a
CD2Cl2
a
(CH3)2CO 30.3
CD2Cl2
6.4
132
1
a dppp is 1,2-bis(diphenylphosphino)propane.
to free phenoxide can be observed in the H and 13C

146
M. Aresta et al. / Journal of Organometallic Chemistry 605 (2000) 143–150
Table 2
Complexes 2 and 4 are the first examples of covalent
Rh compounds bearing the unsubstituted phenoxide
anion p-coordinated to the metal without a further
binding, through hydrogen bonding, to a phenol
molecule. In fact, the two other classes of Rh-com-
plexes well characterised were either cationic complexes
bearing the unsubstituted phenoxide p-coordinated to
Rh [12e] or complexes of formula RhL2(OAr) where Ar
is the bulky (2,6-ter-Bu-4-Me-C6H2O) group that gives
a special stability to the p-bonded complex [12c].
Solvent dependence of 1H- and 13C-NMR phenoxide resonances (in
ppm) for Rh(diphos)(h5-OPh) a
Solvent
Hortho
Hmeta
Hpara
b
C6D6
7.13 dd
J=8.5 and 1
5.71 d
6.69 dd
J=8.5 and 7.1
5.99 t
6.45 tt
J=7.1 and 1
4.90 t
c
CD2Cl2
J=6.8
J=6.4
5.46 dd
J=7.2 and 5.6
J=6.5
4.43 t d
J$6
b
CDCl3
6.05 d d
J$7
Solvent
Cortho, Cmeta, Cpara
Cipso,OPh
2.3. Reaction of the phenoxo-complexes with carbon
dioxide
e
C6D6
115.87 s, 114.41 s, 100.19 s
115.74 d,102.80 dd, 96.32 m
168.43 s
162.0 s
e
CD2Cl2
We have studied the reactivity of complexes 1, 2, 4
with carbon dioxide, in order to understand if the
coordination of the phenoxide can induce a different
reactivity. In principle, the p-cordination stabilises a
keto-form with a delocalisation of a negative charge on
the aromatic ring. This could change the reactivity
towards CO2, as the nucleophilic centre is at one of the
phenyl-carbons, more than at the oxygen atom.
a J values are in Hz.
b At 200 MHz.
c At 500 MHz.
d Each term of the signal shows further poorly resolved minor
splittings.
e At 50.3 MHz.
spectra of 4 in C6D6. Moreover, the 13C spectrum of 4
in C6D6 does not show any resonance that can be
Complex 1 does not react with carbon dioxide (0.1
MPa) in the solid state, while, in CHCl3 solution, it
slowly (24 h) affords a brown powder, 5 whose IR
spectrum shows bands at 1531 and 1343 cm−1 charac-
teristic of organic emicarbonate moieties bonded to
metal centres [29]. Ethylene is not lost during the
reaction, as demonstrated by the IR bands at 1210, 991,
904, 829 and 731 cm−1 present in the final product.
These results infer that carbon dioxide interacts prefer-
ably with the RhꢀO bond to afford a carbonate com-
plex more than with the coordinated ethylene to afford
the formation of a metallacarboxylate [30,31] or acry-
late species [32]. Such situation is also supported by the
13C-NMR spectrum of the analogous complex 5a ob-
tained by reaction of 1 with 13CO2, that shows a
resonance at 165.47 ppm, not present in the spectrum
of the parent compound. This signal is a doublet
(J(CRh)=2.8 Hz) and is attributed to the carbonate
carbon of 5a [33,34]. The value is in the range usually
found for the metal carbonate complexes (155–170
ppm) [6,29a].
attributed to
Rh(diphos) moiety.
a
C6D6 ring p-coordinated to
a
A similar solvent dependence has been documented
previously by us for the proton resonances of the BPh4−
phenyl ring h6-coordinated to the Rh(diphos) moiety in
Rh(diphos)(h6-PhBPh3) [27]. This effect may be the
result of particular interactions between the metal com-
plex and the solvent molecules in the outer coordina-
tion sphere and, in the case of 4, could reflect a
different degree of distortion from planarity of the
p-coordinated phenoxo-ring in solution, depending on
the nature of solvent. The higher up-field proton shifts
observed in CDCl3 and CD2Cl2 with respect to C6D6
may suggest a greater distortion of the phenoxo-struc-
ture towards a cyclohexanedienone system (see B) in
chlorinated solvents and, consequently, a more marked
deviation from planarity than in benzene.
The reaction of 5 with HCl affords the starting
Rh2(m-Cl)2(C2H4)4 complex and emicarbonic acid
PhOC(O)OH that rapidly converts into CO2 and phe-
nol also in the presence BF3–MeOH [35].
Complexes 2 and 4, that bear a p-bonded phenoxide,
react with carbon dioxide in a different way. Indeed,
the emicarbonate is not produced and a new complex is
formed that bears a ring-functionalised phenoxide. As a
matter of facts, treatment of 6, obtained by reaction of
4 with CO2, with dry HCl produces 4-OH-benzoic acid
dosed by HPLC or, after treatment with CH3OH–BF3,
by GC–MS as the methyl ester. These data suggest that
the carboxylation takes place at the ring, as we have
Complex 4, in solution, is very air sensitive: the
phenoxide is readily converted into catechole. This is
the first evidence for such phenoxide–catecholate con-
version upon the oxidation of the p-coordinated phen-
oxide anion. The amount of phenoxide oxidised
depends on the reaction conditions. Studies are in
progress in order to shed light on this aspect.

M. Aresta et al. / Journal of Organometallic Chemistry 605 (2000) 143–150
147
4. Experimental
observed for analogous p-bonded-phenate Mn-com-
plexes [10]. This reaction is a unique example of direct
ring carboxylation of phenol at room temperature
(r.t.) and pressure. The delocalisation of the negative
charge from the oxygen atom, in free phenoxide, to
the phenyl ring, in p-coordinated phenoxide, makes
the latter enough nucleophilic to react with CO2 and
afford 4-OH-benzoic acid more than emicarbonates. It
is worth to emphasise that the carboxylation takes
place very selectively at the four-position (100% selec-
tivity). Salicilic acid is not observed. This reaction is
reminiscent of the benzene electro-carboxylation upon
coordination to Cr(0) and electron transfer to the co-
ordinated ring via electrochemical reduction [36]. Also
in this case, the enhanced nucleophilicity of the ring-
carbon atoms makes benzene enough reactive to af-
ford benzoic acid.
4.1. General procedures
Unless otherwise specified all manipulations were
made under dinitrogen using the vacuum-line tech-
nique. Solvents were purified by standard methods
[37]. Rh2(m-Cl)2(C2H4)4, diphos, silver triflate, phenol,
potassium and sodium were commercial products
(Aldrich). CO2 (99.995%) and 13CO2 (13C 99%) were
from Air Liquide and CIL, respectively.
IR spectra were obtained with a Perkin–Elmer 883
spectrophotometer. 1H-, 31P- and 13C-NMR spectra
were recorded with a Varian XL-200 or a Bruker AM
250 or 500. Proton and carbon chemical shifts are in
ppm versus TMS and have been referenced to the
solvent peak, while 31P resonances were calibrated
with respect to 85% H3PO4. Electrolytic conductivity
measurements were made on a WTW conductivity me-
ter apparatus. Molecular weights were determined by
cryoscopy using the apparatus described in Ref. [16].
GC–MS analyses were carried out with a HP 5890
gas-chromatograph linked to a HP 5970 selective mass
detector (capillary column: 30 m SE-30, 0.25 mm ID,
0.25 mm film thickness). Gas analysis was performed
with a Carlo Erba Fractovap Mod. C Instrument or a
DANI 86.10 GC with a TC detector, using a Chromo-
3. Conclusions
This study reports on the synthesis and characterisa-
tion of three novel covalent Rh(I)–phenoxide com-
plexes, 1, 2 and 4, and their reactivity towards carbon
dioxide.
Complexes 1 and 2 are formed simultaneously, but
with different yields, by reaction of Rh2(m-Cl)2(C2H4)4
with NaOPh in benzene. They represent the first docu-
mented example in which phenoxide anion exhibits
two different coordination modes towards the same
metal centre, Rh(C2H4)2.
Complex 4 is very air sensitive and has been ob-
tained by an analogous synthetic procedure, by react-
ing Rh(diphos)Cl with NaOPh in benzene and full
characterised by elemental analysis and NMR spec-
troscopy in several solvents.
Complexes 2 and 4 are rare examples of covalent
p-phenoxo–Rh-complexes in which the p-coordination
of the aryloxo ligand to the metal is not further sta-
bilised by hydrogen bonding with free phenol
molecules.
We have also shown that the coordination mode of
phenoxide to Rh affects the reactivity of the coordi-
nated ligand towards carbon dioxide and controls the
regioselectivity of the attack by the heterocumulene. In
fact, O-bonded phenoxide in complex 1 reacts with the
heterocumulene at the oxygen atom in a conventional
way to give the Rh–carbonate complex 5. Conversely,
in complexes 2 and 4, in which the negative charge is
displaced from the O-atom of phenoxide to the ring,
the electrophilic attack by CO2 is addressed onto the
p-bonded aromatic ring, resulting in the selective car-
boxylation of the p-phenoxide in para position to af-
ford 4-OH-benzoic acid.
,
sorb or 4 A molecular sieves packed column. HPLC
analyses were made using a Perkin–Elmer Series 4 LC
connected with a LC290 UV–vis spectrophotometric
detector.
4.2. Synthesis of sodium phenoxide and potassium
phenoxide
To a cold (273 K) solution of phenol (7.52 g, 79.9
mmol) in diethyl ether (150 ml) sodium (1.67 g, 76.2
mmol) powder was added. The reaction mixture was
stirred at r.t. (293 K) until complete reaction of the
metal (about 20 h). The white solid precipitated was
filtered out, washed with diethylether (30 ml) and
dried in vacuo. Yield: 7.24 g, 86% versus sodium.
Anal. Found: C, 62.00; H, 4.30. Calc. for C6H5ONa:
C, 62.07; H, 4.34%. IR (Nujol): 3061 m, 1584 vs (br),
1479 vs, 1321 s, 1299 s, 1166 s, 986 s, 871 s, 823 s, 764
s, 700 s cm−1. H-NMR (250 MHz, CD3CN, 293 K):
1
l 6.23 (tt, 3JH
H
=7.12 Hz, 4JH
=1 Hz,
ꢀH
ꢀH
para
para
meta
ortho
para), 6.40 (m, Hortho), 6.93 (m, Hmeta). 13C{1H}-
NMR (50.3 MHz, CD3CN, 293 K): l 111.1 (s, Cpara),
119.5 (s, Cortho), 129.2 (s, Cmeta), 170.3 (s, Cipso).
Potassium phenoxide was prepared in a similar way
and isolated in 90% yield. Anal. Found: C, 54.47; H,
3.85. Calc. for C6H5OK: C, 54.51; H, 3.81%. IR (Nu-
jol): 3057 m, 1584 s, 1546 m, 1474 s, 1314 s, 1304 s,
1250 m, 1161 m, 1145 m-w, 1064 m-w, 1014 m-w, 985
m, 980 m, 878 m, 820 m, 790 m, 782 m, 761 m, 718
m, 711 m, 700 m, 534 m, 450 m cm−1
.

148
M. Aresta et al. / Journal of Organometallic Chemistry 605 (2000) 143–150
4.2.1. Reaction of Rh2(v-Cl)2(C2H4)4 with NaOPh
NaOPh (0.207 g, 1.78 mmol) in THF (3 ml) was
added dropwise, under a dinitrogen stream, to a
filtered, cold (283 K) solution of Rh2(m-Cl)2(C2H4)4
(0.347 g, 0.892 mmol) in benzene (75 ml). The solution
colour changed from yellow–orange to pale yellow.
The reaction mixture was stirred at 283 K for 24 h. The
formed NaCl was filtered off and the mother solution
concentrated in vacuo to 10 ml. Pentane (40 ml) was
layered on the benzene solution and the system cooled
to 253 K. After 24 h a yellow–orange solid formed that
was filtered, washed with pentane and dried in vacuo.
The solid is soluble in benzene, toluene, CH2Cl2, CHCl3
and THF. Yield: 73% versus Rh. Spectroscopic analy-
ses show that the solid is a mixture of two products:
Rh2(m-OPh)2(C2H4)4 (1) and Rh(C2H4)2(h5-OPh) (2),
that have the same elemental analysis. Anal. Found: C,
47.86; H, 5.41. Calc. for C10H13ORh: C, 47.64; H,
5.20%. The two complexes were separated by repeated
crystallisation from benzene–pentane. Complex 1 is
slightly less soluble than complex 2.
to 253 K, a yellow–orange solid precipitated that was
isolated by filtration. The spectroscopic features (1H-,
13C-NMR) of this material show that the isolated
product is still a mixture of 1 (\98%) and 2.
4.3. Synthesis of Rh(diphos)(p5-OPh) (4)
To a filtered solution of Rh2(m-Cl)2(C2H4)4 (0.306 g,
0.787 mmol) in benzene (100 ml) the phosphine (0.628
g, 1.57 mmol) in 15 ml of benzene was added dropwise,
under dinitrogen and with vigorous stirring. A yellow
solid formed that redissolved under stirring. Evolved
ethylene was eliminated under vacuum. To the clear
solution of Rh(diphos)Cl, NaOPh (0.183 g, 1.57 mmol),
dissolved in THF (3 ml), was added dropwise. The
solution was stirred for 24 h and then filtered. The solid
(NaCl) was washed with benzene (3×5 ml) and dis-
charged. The mother solution was concentrated in
vacuo to 10 ml and 50 ml of pentane were added. By
slow diffusion of pentane a brown solid 3 was formed
that was filtered. By cooling the resulting solution to
253 K an orange solid 4 separated that was isolated by
filtration and dried in vacuo. Yield: 60%. Anal. Found:
C, 64.44; H, 4.87; P, 10.39. Calc. for C32H29P2ORh: C,
64.66; H, 4.92; P, 10.42%. Complex 4 is very air sensi-
tive, soluble in benzene, CHCl3, CH2Cl2. The formation
of 4 is very solvent dependent. If the reaction is carried
out in toluene or CH2Cl2, 4 was not formed.
4.2.2. Spectroscopic characterisation of
Rh(v-OPh)2(C2H4)4 (1)
Anal. Found: C, 47.71; H, 5.35. Calc. for
1
C10H13ORh: C, 47.64; H, 5.20%. H-NMR (200 MHz,
CD2Cl2, 293 K): l 2.20 (very broad, Hethylene), 6.78–
6.90 (m, Hortho and Hpara), 7.15 (m, Hmeta). 13C{1H}-
NMR (50.3 MHz, CDCl3, 293 K): l 56.99 (d,
1J(RhꢀC)=14.9 Hz, Cethylene), 120.77(s, Cpara), 123.45
(s, Cortho), 128.32 (s, Cmeta), 157.45 (s, Cipso).
Solid 3 is a mixture of compounds and has the
following elemental analysis. Found: C, 56.94; H, 4.51;
P, 8.68; Cl, 2.15%. The IR spectrum shows the presence
PꢁO groups. This is quite peculiar as air was totally
excluded during the reaction. This solid is still under
investigation.
4.2.3. Spectroscopic characterisation of
Rh(C2H4)2(p5-OPh) (2)
Anal. Found: C, 47.80; H, 5.40. Calc. for
1
C10H13ORh: C, 47.64; H, 5.20%. H-NMR (200 MHz,
4.3.1. Spectroscopic characterisation of
CD2Cl2, 293 K): l 2.20 (broad, Hethylene), 4.88, 5.75 and
6.29 (multiplets assigned to the aromatic protons).
13C{1H}-NMR: (50.3 MHz, CDCl3, 293 K): l 49.3 (d,
(diphos)Rh(p5-OPh) (4)
IR (Nujol): 3053 m, 1588 s w(CꢁO), 1478 vs, 1432 s
(sharp), 1264 s, 1247 s, 1157 m, 1100 m, 1067 m-w,
1020 m-w, 996 m, 878 m, 847 m, 817 m-s, 740 m-s, 696
s, 689 s, 534 s, 493 s, 425 m cm−1. 1H-NMR (200 MHz,
C6D6, 293 K): l 1.67 (d, J=17.28 Hz, CH2ꢀCH2), 6.45
1J(RhꢀC)=12.3 Hz, Cethylene), 83.7 (d, J(RhꢀC)=2.2
1
Hz), 98.6 (d, 1J(RhꢀC)=2.4 Hz), 105.6 (d,
1J(RhꢀC)=3.6 Hz) (ortho, meta and para carbon
atoms of p-OPh).
3
4
(tt, JH
=7.1 Hz, JH
=1 Hz, Hpara,OPh),
ꢀH
ꢀH
para
para
meta
6.69 (dd, 3JH
=8.5 Hz, orHthometa,OPh), 7.13 (dd,
ꢀH
meta
ortho
4.2.4. Reaction of Rh(C2H4)2(O3SCF3) with KOPh
Silver triflate (0.36595 g, 1.46 mmol) in THF (20 ml)
was added dropwise, under a dinitrogen stream, to a
filtered, cold (263 K) solution of Rh2(m-Cl)2(C2H4)4
(0.30415 g, 0.704 mmol) in THF (20 ml). After stirring
for 30 min, the reaction mixture was filtered out to
remove the AgCl precipitated. To the mother solution,
containing Rh(C2H4)2(O3SCF3) prepared in situ [38], a
solution of KOPh (0.20220 g, 1.53 mmol) in 15 ml of
THF was added. The reaction mixture was stirred at
273 K for 3 h, concentrated in vacuo to 15 ml and
filtered. Upon addition of pentane (30 ml) and cooling
Hortho,OPh), 6.9–7.1, 8.02–8.14 (multiplets assigned to
the aromatic protons of diphos). Assignments and eval-
uation of the coupling constants for the p-phenoxo
protons were supported by decoupling experiments.
1H-NMR (500 MHz, CD2Cl2, 293 K): l 2.15 (dd,
J=20 and 1.2 Hz, CH2ꢀCH2), 4.90 (t, J=6.5 Hz,
Hpara,OPh), 5.99 (t, J=6.4 Hz, Hmeta,OPh), 5.71 (d, J=
6.8 Hz, Hortho,OPh), 7.38–7.42, 7.60–7.65 (multiplets as-
signed to the aromatic protons of diphos).
13C{1H}-NMR (50.3 MHz, C6D6, 293 K): l 28.15 (m,
CH2ꢀCH2), 100.19 (s, COPh), 114.41 (s, COPh), 115.87 (s,
COPh), 128.04 (virtual t, J=4.6 Hz, Cmeta,diphos), 129.70

M. Aresta et al. / Journal of Organometallic Chemistry 605 (2000) 143–150
149
(s, Cpara,diphos), 133.95 (virtual t, J=5.6 Hz, Cortho,diphos),
136.56 (virtual t, J=19.1 Hz, Cipso,diphos), 168.43 (s,
C
l 29.2 (m, CH2ꢀCH2), 96.32 (m, COPh), 102.80 (dd,
J=2.3 and 3.8 Hz, COPh), 115.74 (d, J=3.3 Hz, COPh),
128.23 (virtual t, J=5.0 Hz, Cmeta,diphos), 129.79 (s,
of 1:1 diethyl ether–pentane (v/v). The analysis of the
organic phase by GC–MS using a SE-30 capillary
column showed the presence of 4-OH-benzoic acid
methyl ester.
The body of these results clearly demonstrates that
upon reaction of 4 with carbon dioxide, the p-bonded
phenoxo-ligand is selectively carboxylated in para
position.
ipso,OPh). 13C{1H}-NMR (50.3 MHz, CD2Cl2, 293 K):
C
para,diphos), 132.09 (virtual t, J=6.1 Hz, Cortho,diphos),
137.87 (virtual t, J=20.3 Hz, Cipso,diphos), 162.0 (s,
ipso,OPh). 31P{1H}-NMR (81 MHz, C6D6, 293 K): l
C
72.07 (d, JPꢀRh=200.2 Hz).
Acknowledgements
4.4. Reaction of Rh2(v-OPh)2(C2H4)4 (1) with CO2
The financial support of MURST (Ministero della
Universita` e della Ricerca Scientifica e Tecnologica),
Project no. 9803026360, is gratefully acknowledged.
Rh2(m-OPh)2(C2H4)4 (1) (0.150 g) was dissolved in 3
ml of CHCl3 under carbon dioxide and the solution was
stirred for 24 h. A brown solid 5 separates that was
filtered off and dried in vacuo. The brown solid is not
soluble in aromatic and halogenated solvents but is
soluble in DMSO. IR (Nujol) 1531 m w(CꢁO), 1343
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4.5. Reaction of Rh(diphos)(p5-OPh) (4) with CO2
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with the Rh-complex to afford a new species, 6 contain-
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compound could not be clarified in detail, as the at-
tempts to isolate it in a pure form were unsuccessful.
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para position, as demonstrated by the acidolysis experi-
ments with HCl.
The above solution was reacted with dry HCl and
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“ column: Erbasil C18/M, 25×0.46 cm;
“ mobile phase: acetonitrile (20%, v/v)/phosphate
buffer (80%, v/v; pH 3.5) containing 0.1% (v/v) of
butanol;
“ flow: 1 ml min−1
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the presence of 4-OH-benzoic acid.
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ml of BF3–OEt2. The mixture was heated at 333 K,
then treated with 1 ml of water, and extracted with 1 ml

150
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