Synthesis of M2Rh2 Bis(μ3-carbon dioxide) complexes from the reaction between [Rh(OH) (η4-COD)]2 and cationic metal carbonyls

Article abstract of DOI:10.1021/om970443k

The M₂Rh₂ bis(μ₃-CO₂) complexes [Cp*(CO)(NO)Re(CO₂)Rh(η⁴-cod)]₂ (1) and [Cp*(CO)₂-MCO₂Rh(η⁴-cod)] ₂, M = Fe (2) and Ru (3), were synthesized in moderate to high yields from treating [Rh(OH)(η⁴-cod)]₂ (cod = 1,5-cyclooctadiene) with the carbonyl salts Cp*(L)(CO)₂M⁺ (L = CO, M = Fe, Ru; L = NO, M = Re) and a base. [An alternative synthesis of 1 and its crystallographic structure determination has been reported.] Although details on using several bases in the synthesis of 2 are reported, the use of volatile EtMe₂N in excess as the base is especially useful. IR and NMR spectral data are in accord with a M₂Rh₂ bis(μ3-CO₂) core for 2 and 3: they retain two μ₃-[η¹-C(M):η¹-O(Rh):η ¹-O′(Rh′)] carboxylate ligands that resemble those that were found for 1 and the Rh(I) carboxylates [(RCO₂)Rh(diene)]₂. Complexes 2 and 3 with their open-book structures are not fluxional at room temperature. Complex 2, its norbornadiene analogue, [Cp*(CO)₂FeCO₂Rh(η⁴-nbd)]₂ (4), and their ¹³C-labeled derivatives also are accessible from reactions of Cp*(CO)₂FeCO₂K [or Cp*(CO)₂-Fe¹³CO₂K] with [Rh(OSO₂CF₃)(η⁴-cod)_x. [Cp*(CO)₂Fe(¹³CO₂)Rh(η ⁴-diene)]₂ [diene = cod (2-¹³C); nbd (4-¹³C)] underwent carboxylate-carbonyl label shuttle to yield 1:1 mixtures of [Cp*(CO)(¹³CO)FeCO₂Rh(diene)]₂ (2a-C¹³) and 4a-C¹³. IR spectral assignments for the metallocarboxylate ν_OCO absorptions are also presented.

Full text of DOI:10.1021/om970443k

Organometallics 1998, 17, 1861-1867
1861
Syn th esis of M2Rh 2 Bis(µ3-ca r bon d ioxid e) Com p lexes
fr om th e Rea ction betw een [Rh (OH)(η4-COD)]2 a n d
Ca tion ic Meta l Ca r bon yls
Stephen M. Tetrick, Chongfu Xu, J ohn R. Pinkes, and Alan R. Cutler*
Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180
Received May 29, 1997
The M2Rh2 bis(µ3-CO2) complexes [Cp*(CO)(NO)Re(CO2)Rh(η4-cod)]2 (1) and [Cp*(CO)2-
MCO2Rh(η4-cod)]2, M ) Fe (2) and Ru (3), were synthesized in moderate to high yields from
treating [Rh(OH)(η4-cod)]2 (cod ) 1,5-cyclooctadiene) with the carbonyl salts Cp*(L)(CO)2M+
(L ) CO, M ) Fe, Ru; L ) NO, M ) Re) and a base. [An alternative synthesis of 1 and its
crystallographic structure determination has been reported.] Although details on using
several bases in the synthesis of 2 are reported, the use of volatile EtMe2N in excess as the
base is especially useful. IR and NMR spectral data are in accord with a M2Rh2 bis(µ3-CO2)
core for 2 and 3: they retain two µ3-[η1-C(M):η1-O(Rh):η1-O′(Rh′)] carboxylate ligands that
resemble those that were found for 1 and the Rh(I) carboxylates [(RCO2)Rh(diene)]2.
Complexes 2 and 3 with their “open-book” structures are not fluxional at room temperature.
Complex 2, its norbornadiene analogue, [Cp*(CO)2FeCO2Rh(η4-nbd)]2 (4), and their 13C-
labeled derivatives also are accessible from reactions of Cp*(CO)2FeCO2K [or Cp*(CO)2-
Fe13CO2K] with [Rh(OSO2CF3)(η4-cod)]x. [Cp*(CO)2Fe(13CO2)Rh(η4-diene)]2 [diene ) cod (2-
13C); nbd (4-13C)] underwent carboxylate-carbonyl label shuttle to yield 1:1 mixtures of
[Cp*(CO)(13CO)FeCO2Rh(diene)]2 (2a -C13) and 4a -C13. IR spectral assignments for the
metallocarboxylate νOCO absorptions are also presented.
In tr od u ction
(η4-cod)]x, or [Rh(OCH3)(η4-diene)]2, provided the prod-
ucts in moderate yields. These syntheses resemble the
procedures by which nearly all bi-, tri-, and tetrametallic
carbon dioxide complexes3 have been prepared: the
µ-CO2 ligand originates as a preformed monometal CO2
complex or metallocarboxylic acid derivative. Excep-
tions include several examples of incorporating CO2 into
a metal-metal bond4 or combining a metal oxo com-
pound with a metal carbonyl complex.5 We now report
a new synthetic procedure for carbon dioxide complexes
in which the µ-CO2 ligand is assembled from a metal
carbonyl complex plus a metal hydroxide in the presence
of base.6 Details are presented for combining [Rh(OH)-
We recently reported the synthesis and characteriza-
tion of a new series of carbon dioxide complexes that
retain a Re2Rh2(µ3-CO2)2 structural motif in which each
of the two rhenium metallocarboxylates bridge two
rhodium(I) η4-diene moieties.1 The presence of the two
µ3-[η1-C(Re):η1-O(Rh):η1-O′(Rh′)] carboxylate ligands was
established by an X-ray crystallographic structure de-
termination of [Cp*(CO)(NO)Re(CO2)Rh(η4-cod)]2 (1). Its
“open-book” structure, with a 85° angle between the two
µ-CO2Rh2 carboxylate bridges and a (nonbonding) Rh,
Rh separation of 3.24 Å, resembles the catalytically
active Rh(I) carboxylates [(RCO2)Rh(diene)]2.2
These Re2Rh2 bis(µ3-CO2) complexes were synthesized
by three synthetic routes starting with either a rhenium
metallocarboxylate or its metallocarboxylic acid in the
presence of a base. Treatment with the appropriate
rhodium(I) precursor, [RhCl(η4-cod)]2, [Rh(OSO2CF3)-
7
(η4-cod)]2 with cationic metal carbonyl complexes and
(3) (a) Gibson, D. H. Chem. Rev. 1996, 96, 2063. (b) J essop, P. G.;
Ikariya, T.; Noyori, R. Chem. Rev. 1995, 95, 259.
(4) (a) Kubiak, C. P.; Woodcock, C.; Eisenberg, R. Inorg. Chem. 1982,
21, 2119. (b) Pinkes, J . R.; Steffey, B. D.; Vites, J . C.; Cutler, A. R.
Organometallics 1994, 13, 21. (b) Hanna, T. A.; Baranger, A. M.;
Bergman, R. G. J . Am. Chem. Soc. 1995, 117, 11363.
(5) (a) Audett, J . D.; Collins, T. J .; Santarsiero, B. D.; Spies, G. H.
J . Am. Chem. Soc. 1982, 104, 1352. (b) Pilato, R. S.; Housmekerides,
C. E.; J ernakoff, P.; Rubin, D.; Geoffroy, G. L.; Rheingold, A. L.
Organometallics 1990, 9, 2333. (c) Housmekerides, C. E.; Ramage, D.
L.; Kretz, C. M.; Shontz, J . T.; Pilato, R. S.; Geoffroy, G. L.; Rheingold,
A. L.; Haggerty, B. S. Inorg. Chem. 1992, 31, 4453. (d) Most of these
reactions generate cationic bimetallic complexes in which the µ3-[η1-
C:η2-O,O′] CO2 ligand exhibits significant µ-(dioxocarbene) ligand
character, e.g., Cp2ModO + CpFe(CO)3+ f Cp(CO)2Fe+dC(O)2MoCp2.
(6) Bimetallic µ-CO2 complexes have been prepared by condensation
of a metal hydroxide or alkoxide1 and a metallocarboxylic acid: (a)
Beck, W.; Raab, K.; Nagel, V.; Steimann, M. Angew. Chem., Int. Ed.
Engl. 1982, 21, 526. (b) Bennett, M. A.; Robertson, G. B.; Rokicki, A.;
Wickramasinghe, W. A. J . Am. Chem. Soc. 1988, 110, 7098. (c)
Torresan, I.; Michelin, R. A.; Zanardo, A.; Pinna, F.; Strukul, G.
Organometallics 1991, 10, 623.
(1) (a) Tetrick, S. M.; Tham, F. S.; Cutler, A. R. J . Am. Chem. Soc.
1997, 119, 6193. (b) A related trimetallic CO2 compound, (PMe2Ph)3Os-
(µ-H)2(µ3-CO2)[Rh(COD)]2, with µ3-[η1-C(Os):η1-O(Rh):η1-O′(Rh′)] bound
CO2 was prepared by the addition of CO2 to (COD)RhH3Os(PMe2Ph)3.
An X-ray structure determination established that the Os carboxylate
bridges two RhI(COD) moieties in the presence of two Os-H-Rh
linkages: Lundquist, E. G.; Huffman, J . C.; Caulton, K. G. J . Am.
Chem. Soc. 1986, 108, 8309. Lundquist, E. G.; Huffman, J . C.; Folting,
K.; Mann, B. E.; Caulton, K. G. Inorg. Chem. 1990, 29, 128.
(2) Rhodium formate complexes have been implicated in Rh(I)-
catalyzed hydrogenation of CO2 to formic acid2b and in hydroformyla-
tion catalysis. (a) Leitner, W. Angew. Chem., Int. Ed. Engl. 1995, 34,
2207. Fornika, R.; Dinjus, E.; Go¨rls, H.; Leitner, W. J . Organomet.
Chem. 1996, 511, 145. (b) Suss-Fink, G.; Soulie, J .-M.; Rheinwald, G.;
Stoeckli-Evans, H.; Sasaki, Y. Organometallics 1996, 15, 3416. (c)
Mieczynska, E.; Trzeciak, A. M.; Zio´lkowski, J . J .; Lis, T. J . Chem.
Soc., Dalton Trans. 1995, 105.
(7) Uson, R.; Oro, L. A.; Cabeza, J . A. Inorg. Synth. 1985, 23, 123.
Selent, D.; Ramm, M. J . Organomet. Chem. 1995, 485, 135.
S0276-7333(97)00443-3 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 04/03/1998

1862 Organometallics, Vol. 17, No. 9, 1998
Tetrick et al.
(CO)2BF4 had been converted to 1. The solvent and residual
Me2EtN were evaporated (10-2 mm, 30 min), and the red-
brown solvent was solid was extracted with 3 × 5 mL of cold
(5 °C) benzene. The combined red extracts were evaporated
(10-2 mm, 1 h) [frozen immediately with an acetone-dry ice
bath], re-extracted with 3 × 5 mL of cold benzene, and
evaporated and dried on the vacuum line for 6 h. A brick red
solid (252 mg) was obtained that corresponded to spectroscopi-
cally pure 11 (99% yield).
a base in the efficient syntheses of 1 and [Cp*(CO)2MCO2-
Rh(η4-cod)]2, M ) Fe (2) and Ru (3). Complex 2, its
norbornadiene analogue, [Cp*(CO)2FeCO2Rh(η4-nbd)]2
(4), and their 13C-labeled derivatives also are accessed
from reactions of Cp*(CO)2FeCO2K [or Cp*(CO)2-
Fe13CO2K] with [Rh(OSO2CF3)(η4-cod)]x.
Exp er im en ta l Section
Syn th esis of [(η5-C5Me5)(CO)2F eCO2Rh (cod )]2 (1). Re-
a ct ion b et w een (η5-C5Me5)F e(CO)3P F 6 a n d [R h (OH )-
(cod )]2 in th e P r esen ce of Dim eth yleth yla m in e. A sus-
pension of Cp*Fe(CO)3PF6 (252 mg, 0.60 mmol) and [RhOH-
(cod)]2 (137 mg, 0.30 mmol) in 50 mL of THF (5 °C) was treated
with Me2EtN (0.30 mL, 2.8 mmol). An IR spectrum of the clear
orange solution after 10 min indicated that 85% of the starting
Cp*Fe(CO)3PF6 had converted to [Cp*(CO)2FeCO2Rh(cod)]2 (2),
Synthetic manipulations were performed in a nitrogen
atmosphere using a combination of standard Schlenk, glove-
box, and vacuum line procedures.8 Infrared spectra were
recorded with a Perkin-Elmer model 1600 spectrometer. 1H
and 13C{1H} NMR spectra were recorded in benzene-d6 or
toluene-d8 (both dried over 3A molecular sieves) using a Varian
Unity 500 or a Varian INOVA 300 spectrometer, and the data
were reported relative to residual C6D5H (1H, δ 7.15; 13C, δ
128.00) or C6D513CD2H (1H, δ 2.09; 13C, δ 20.4). Combustion
microanalyses were done by Quantitative Technologies, Inc.,
Whitehouse, NJ .
Tetrahydrofuran (THF), hexane, ether, and toluene were
distilled from sodium benzophenone ketyl; dichloromethane
was distilled from phosphorus pentoxide. Acetone and aceto-
nitrile were dried over 3A molecular sieves and deoxygenated
by sparging with nitrogen prior to use. Ethanol was deoxy-
genated by sparging with nitrogen prior to use. Matheson
“bone dry” carbon dioxide was dried further by passage
through a 1 cm × 1 m glass column that was packed with a
25% P2O5 powder dispersion on activated silica gel (70-200
mesh) containing 1% Congo Red indicator. Before using,
carbon dioxide and 99% 13C-labeled CO2 were frozen (-196
°C) and pumped on to remove noncondensable materials. All
other reagents were purchased from commercial sources and
used as received.
ν(CO) 1999, 1944 cm-1
. All subsequent operations were
carried out at or below 5 °C. The solvent and excess Me2EtN
were evaporated, and the red-brown solid was extracted with
3 × 5 mL of cold benzene. The combined filtrates were frozen
(-78 °C); then the benzene was evaporated at 5 °C. Another
benzene extraction (3 × 5 mL) and removal of solvent on the
vacuum line left 201 mg of a red-brown solid that was
identified as [Cp*(CO)2FeCO2Rh(cod)]2 (2) (67% yield): IR
(THF) νasym(OCO) 1470 (br, m) cm-1, νsym(OCO) 1262 (m), 1230
1
(m) cm-1; H NMR (500 MHz, C6D6) δ 4.40 (br m, 4H, dCH),
4.24 (br m, 4H, dCH), 2.79 (mult, 4H, exo-CHH), 2.38 (mult,
4H, exo-CHH), 1.67 (mult, 4H, endo-CHH), 1.62 (s, 30H,
C5Me5), 1.55 (br, 4H, endo-CHH); 13C{1H} NMR (C6D6) δ 217.39
(FeCO2), 214.67 (FeCO), 96.18 (C5Me5), 80.59, 75.80 (br, dC),
1
31.56, 31.00 (allylic C), 9.78 (C5Me5). The H NMR endo-CH2
spectral assignments were made on the basis of a COSY NMR
experiment. Anal. Calcd for C42H54Fe2O8Rh2: C, 50.23; H,
5.42. Found: C, 49.98; H, 5.30.
Silver triflate (AgOTf) was recrystallized from acetone and
dried under vacuum at 95 °C. [Cp*(CO)2Fe]2,9a,b Fp*COPF6,9b
Cp*Ru(CO)3PF6,9c Cp*Re(NO)(CO)2BF4,9d,e [Rh(cod)Cl]2, [Rh-
Rea ction of (η5-C5Me5)F e(CO)3P F 6 a n d [Rh OH(cod )]2
w ith 2,6-Lu tid in e. A stirred suspension of Cp*Fe(CO)3PF6
(84 mg, 0.20 mmol) and [Rh(OH)(cod)]2 (46 mg, 0.10 mmol) in
20 mL of THF (5 °C) was treated with 2,6-lutidine (0.022 g,
0.2 mmol). IR spectral monitoring of the clear red-brown
solution that resulted within 1 h indicated 70% conversion of
the reactants to [Cp*(CO)2FeCO2Rh(cod)]2 (2) along with <2%
[Cp*Fe(CO)2]2. The reaction was evaporated (5 °C), the red-
brown residue was extracted with 3 × 3 mL of cold benzene,
and the orange-brown filtrate was frozen (-78 °C). Removal
of solvent on a vacuum line (5 °C) left 62 mg of a gummy brown
11
(nbd)Cl]2,10 [Rh(cod)OH]2,7 and [Rh(µ-CH3CO2)(cod)]2 were
prepared by literature procedures and judged pure by IR and
1H NMR spectroscopy. IR spectra of Cp*(CO)2FeK,9b,12a gener-
ated by sonication of [Cp*Fe(CO)2]2 with potassium,13 routinely
exhibited the presence of only negligible concentrations of
hydride Cp*(CO)2FeH9a,12b,c [ν(CO) 1993, 1932 cm-1] or of
[Cp*Fe(CO)2]2 [ν(CO) 1923, 1754 cm-1]. The apparatus and
vacuum line procedure for treating THF solutions of Cp*-
(CO)2Fe- K+ with 1.0 equiv of CO2 or of 99% C13-labeled CO2
(-78 °C) have been described.14
1
solid, which by IR and H NMR spectral examination consisted
of 70% 1 and 30% [Cp*Fe(CO)2]2 [1H NMR (C6D6): δ1.60
(s,Cp*)], 2-5% lutidine, and residual THF.
P r ep a r a t ion of [(η5-C5Me5)(CO)(NO)R eCO2R h (cod )]2
(1). Cp*Re(NO)(CO)2BF4 (198 mg, 0.40 mmol) and [Rh(OH)-
(cod)]2 (91 mg, 0.20 mmol) were mixed in 25 mL of THF (5
°C), and Me2EtN (0.24 mL, 2.2 mmol) was injected. IR spectra
after 10 min indicated that all of the starting Cp*Re(NO)-
Rea ction of [(η5-C5Me5)(CO)2F eCO2Rh (cod )]2 (2) w ith
P h 3Sn Cl. A solution of 2 (40 mg, 0.040 mmol) in 20 mL of
THF (5 °C) that had been treated with Ph3SnCl (31 mg, 0.080
mmol) changed from light brown to orange over 30 min. IR
spectral monitoring indicated complete conversion of 2 to Cp*-
(CO)2FeCO2SnPh3, ν(CO) 2013, 1960 cm-1. The reaction was
evaporated, the residue was extracted with 10 × 5 mL hexane,
and the combined orange filtrates were evaporated. This left
42 mg of a brown solid that was identified by 1H and 13C NMR
spectroscopy as Cp*(CO)2FeCO2SnPh314b,15 (60%) admixed with
unidentified organic residues. The remaining solid residue
was extracted again with 3 × 3 mL of benzene, and the light
yellow filtrates were evaporated. A light yellow solid was
recovered (14 mg), which was identified as [RhCl(cod)]2 (71%
yield).
(8) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-Sensitive
Compounds, 2nd ed.; Wiley-Interscience: New York, 1986.
(9) (a) King, R. B. J . Organomet. Chem. 1967, 8, 287. (b) Catheline,
D.; Astruc, D. Organometallics 1984, 3, 1094. (c) Nelson, G. O.
Organometallics 1983, 2, 1474. (d) Graham, W. A. G.; Sweet, J . R. J .
Am. Chem. Soc. 1982, 104, 2811. (e) Patton, A. T.; Strouse, C. E.;
Knobler, C. B.; Gladysz, J . A. J . Am. Chem. Soc. 1983, 105, 5804.
(10) (a) Venanzi, L. M.; Chatt, J . J . Chem. Soc. 1957, 4735. Giordano,
G.; Crabtree, R. H. Inorg. Synth. 1979, 19, 218. (b) Abel, E. W.; Bennet,
M. A.; Wilkinson, G. J . Chem. Soc. 1959, 3178.
(11) (a) Chatt, J .; Venanzi, L. M. J . Chem. Soc. 1957, 4735. (a) Lahoz,
F. J .; Martin, A.; Esteruelas, M. A.; Sola, E.; Serrano, J . L.; Oro, L. A.
Organometallics 1991, 10, 1794.
(12) (a) Roger, C.; Tudoret, M.-J .; Guerchais, V.; Lapinte, C. J .
Organomet. Chem. 1989, 365, 347. (b) Lapinte, C.; Catheline, D.;
Astruc, D. Organometallics 1988, 7, 1683. (c) Bullock, R. M.; Samsel,
E. G. J . Am. Chem. Soc. 1990, 112, 6886.
Syn th esis of [(η5-C5Me5)(CO)2Ru CO2Rh (cod )]2 (3). To
a suspension of Cp*Ru(CO)3PF6 (186 mg, 0.40 mmol) and
[RhOH(cod)]2 (91 mg, 0.20 mmol) in 25 mL of THF (5 °C) was
added Me2EtN (0.24 mL, 2.2 mmol). After 10 min, IR spectral
(13) Pinkes, J . R.; Masi, C. J .; Chiulli, R.; Steffey, B. D.; Cutler, A.
R. Inorg. Chem. 1997, 36, 70.
(14) (a) Vites, J . C.; Steffey, B. D.; Giuseppetti-Dery, M. E.; Cutler,
A. R. Organometallics 1991, 10, 2827. (b) Pinkes, J . R.; Cutler, A. R.
Inorg. Chem. 1994, 33, 759.
(15) Gibson, D. H.; Ye, M.; Sleadd, B. A.; Mehta, J . M.; Mbadike, O.
P.; Richardson, J . F.; Mashuta, M. S. Organometallics 1995, 14, 1242.

M2Rh2 Bis(µ3-carbon dioxide) Complexes
Organometallics, Vol. 17, No. 9, 1998 1863
monitoring of the dark yellow solution indicated that all of
the starting Cp*Ru(CO)3PF6 had been consumed. Removal of
solvent and excess Me2EtN left a yellow-green solid, which was
extracted with 3 × 5 mL of benzene (5 °C). The combined
yellow-green filtrates were worked up as described for 2. The
resulting green solid (200 mg) was characterized as [(η5-C5-
Me5)(CO)2RuCO2Rh(cod)]2 (3), 91% yield: IR (THF) ν(CO)
2011, 1952 cm-1, νasym(OCO) 1479 sh, 1467 (m) cm-1, νsym(OCO)
1317 (w), 1256 (m), 1249 (sh) cm-1; 1H NMR (500 MHz, C6D6)
δ 4.39 (br m, 4H, dCH), 4.34 (br m, 4H, dCH), 2.80 (mult,
4H, exo-CHH), 2.43 (mult, 4H, exo-CHH), 1.70 (mult, 4H, endo-
CHH), 1.61 (s, 30H, C5Me5), 1.59 (br, 4H, endo-CHH); 13C{1H}
NMR (C6D6) δ 100.46 (s, C5 Me5), 79.37, 72.52 (dC-Rh, J ) 14.3
Hz, dC), 31.68, 30.90 (allylic C), 10.29 (s, C5Me5)). Anal. Calcd
for C42H54O8Rh2Ru2: C, 46.08; H, 4.97. Found: C, 45.57; H,
4.78.
Attempts at purifying 2 by chromatography and by crystal-
lization (hexane, acetonitrile, THF/hexane, toluene/hexane,
ether/hexane, and CH2Cl2/hexane) invariably enriched the
mixtures in [Cp*(CO)2Fe]2.
Syn th esis of [(η5-C5Me5)(CO)2F eCO2Rh (n bd )]2 (4) a n d
of [(η5- C5Me5)(CO)2F e13CO2R h (n bd )]2 (4-C13). [Rh(nbd)-
Cl]2 (115 mg, 0.25 mmol) in 10 mL of THF at -42 °C was
treated with AgOTf (128 mg, 0.50 mmol). After 20 min, the
yellow suspension of “Rh(nbd)(OTf)”16 was filtered and trans-
ferred to a THF solution of Cp*(CO)2FeCO2K (0.50 mmol, 10
mL) at -78° C. An infrared spectrum of the dark red solution
(30 min) showed the presence of [Cp*(CO)2FeCO2Rh(nbd)]2 (5)
[ν(CO) 1998, 1943 cm-1] as the only Cp*(CO)2Fe product. The
THF was evaporated, the dark red residue was extracted with
hexane (4 × 30 mL), and the combined extracts were evapo-
rated. A dark red powder (180 mg) remained that was
identified as 4 (75% yield) along with <5% Cp*2Fe2(CO)4.
[Cp*(CO)2FeCO2Rh(nbd)]2 (4): IR (THF) ν(CO) 1998, 1943
cm-1, νasym (OCO) 1466 (br, m), νsym (OCO) 1262 (m), 1229 (m)
Rea ction s of [(η5-C5Me5)(CO)2MCO2Rh (cod )]2 (M ) F e,
Ru ) w ith Ca r bon Mon oxid e. Carbon monoxide was bubbled
slowly through solutions of [Cp*(CO)2MCO2Rh(cod)]2 (2, M )
Fe; 3, M ) Ru) (0.02 mmol) in 600 mg of C6D6 in an NMR
tube (5 °C). Within 5 min, the solutions turned lighter brown.
NMR spectra were consistent with complete release of the cod
ligands leaving materials that were tentatively assigned as
[Cp*(CO)2FeCO2Rh(CO)2]2 (5) [1H NMR δ 1.66 (s, 30 H, C5Me5);
13C{1H} NMR δ 9.79 (C5Me5), 96.43 (C5Me5)] and [Cp*(CO)2-
RuCO2Rh(CO)2]2 (6) [1H NMR δ 1.54 (s, 30 H, C5Me5); 13C-
{1H} NMR δ 10.15 (C5Me5), 100.93 (C5Me5)]. When kept in
the original benzene solutions or in the presence of the CO
atmosphere, these materials were stable for 2-3 h. There-
after, these brown solutions degraded to unidentified materi-
als.
cm-1
C
;
1H NMR (500 MHz, C6D6) δ 3.95 (br m, 6H, dCH +
5(6)H), 1.08 (mult, 2H, CH2), 1.57 (s, 30H, C5Me5); 13C{1H}
NMR (C6D6) δ 217.29 (FeCO2), 212.82 (FeCO), 96.15 (C5 Me5),
59.46 (br, CH2), 50.59 (dCH + C5(6)H), 9.78 (C5Me5).
Substitution of 13CO2 in place of CO2 in the above procedure
afforded 170 mg of a dark red powder. An infrared spectrum
of the product in THF showed a mixture consisting of a
2:1:0.25:0.25 ratio of [Cp*(CO)2Fe13CO2Rh(nbd)]2 (4-C13),
[Cp*(CO)(13CO)FeCO2Rh(nbd)]2 (4a -13C), Cp*2Fe2(CO)4, and
Cp*2Fe2(CO)2(13CO)2. IR (THF): 4-C13, ν(CO) 1997, 1942 cm-1
νasym (OCO) 1417 (br, m), νsym (OCO) 1246 (m), 1213 (m) cm-1
,
;
4a -13C, ν(CO) 1983, 1912 cm-1
.
Into cold (-78 °C) THF solutions containing 2 and 3 (0.02
mmol in 10 mL) CO was slowly bubbled. IR spectral monitor-
ing indicated that 2 and 3 were transformed quantitatively
within 5 min to products tentatively formulated as [Cp*-
(CO)2FeCO2Rh(CO)2]2 (5) [IR ν(FeCO) 2008 (vs), 1956 (s),
ν(RhCO) 2078 (s), 2054 (m), 2008 (vs) cm-1, νasym(OCO) 1435
(br, m) cm-1, νsym(OCO) 1261 (m), 1229 (m) cm-1] and [Cp*(CO)2-
RuCO2Rh(CO)2]2 (6) [IR ν(RuCO) 2020 (s), 1964 (s), ν(RhCO)
2079 (s), 2055 (m), 2002 (vs) cm-1, νasym(OCO) 1436 (br, m)
cm-1, νsym(OCO) 1318 (w), 1259 (m), 1244 (m) cm-1]. After
being warmed above 0 °C, these solutions degraded to uni-
dentified materials.
Resu lts a n d Discu ssion
Treatment of the THF suspensions of the carbonyl
salts Cp*Re(NO)(CO)2BF4 or Cp*M(CO)3PF6 with [Rh-
(OH)(η4-cod)]2 and 5 equiv of EtMe2N in THF at 5 °C
produced the bis(µ3-CO2) complexes depicted in eq 1.
P r ep a r a tion of [(η5-C5Me5)(CO)2F e13CO2Rh (cod )]2 (2-
C13). A red solution of Cp*(CO)2FeK was generated by
sonication of a THF suspension (10 mL) of [Cp*Fe(CO)2]2 (140
mg, 0.283 mmol) and potassium metal (130 mg, 3.32 mmol)
in an ultrasonic cleaning bath (1 h). After centrifugation, the
solution was transferred to a 100-mL round-bottom flask and
treated with 13CO2 (99% labeled, 13.4 mL, 0.60 mmol) at -196
°C and then at -78 °C. The resulting dark red-brown Cp*-
(CO)2FeCO2K solution14 [IR 1974, 1904 cm-1 ν(CO)] was free
of starting Cp*(CO)2FeK and of contaminating Cp*(CO)2FeH
or [Cp*Fe(CO)2]2. To this flask was added a filtered solution
of “Rh(cod)(OTf)”,16 which was generated from a mixture of
[Rh(cod)Cl]2 (139 mg, 0.282 mmol) and AgOTf (144 mg, 0.562
mmol) in 10 mL of THF (-42 °C). An IR spectrum of the dark
red solution after stirring for 10 min (-78 °C) was consistent
with formation of a 2:1 mixture of [Cp*(CO)2Fe13CO2Rh(cod)]2
(2-C13) [1999, 1944 cm-1, ν(CO)] and [Cp*(CO)(13CO)FeCO2-
Rh(cod)]2 (2a -C13) [1983, 1913 cm-1, ν(CO)]. The THF was
evaporated, the remaining dark red residue was extracted with
hexane (3 × 5 mL), and the combined extracts were evapo-
rated. The resulting 60 mg of a dark red brown powder
contained a 1:1:0.25:0.25 mixture of 2-C13, 2a -C13, Cp*2-
Fe2(CO)2(13CO)2, and Cp*2Fe2(CO)4, as determined by IR and
1H NMR integration. IR spectral absorptions in the metallo-
carboxylate region appeared as broad envelopes 1475-1415
These red to red brown solids 1-3 were isolated after
evaporating excess amine and carrying out multiple
extractions using cold benzene. We quantitatively
isolated the previously reported1a Re2Rh2 bis(µ3-CO2)
complex 1 as a 1:1 mixture of diastereomers that was
1
characterized by H and 13C NMR spectroscopy. The
analytically pure Fe2Rh2 and Ru2Rh2 products 2 and 3
were isolated in 67% and 91% yields, respectively. The
reactions leading to and the workup procedures for 2
and 3 were conducted at 5 °C in order to minimize the
and 1255-1210 cm-1
.
(16) Aresta, M.; Quaranta, E.; Albinati, A. Organometallics 1993,
12, 2032.

1864 Organometallics, Vol. 17, No. 9, 1998
Ta ble 1. In fr a r ed Sp ectr a l Da ta for M2Rh 2 Bis(µ3-CO2) Com p lexes
Tetrick et al.
b
compounda
ν(CO) (cm-1
)
ν
OCO(asym) (cm-1
)
ν
OCO(sym) (cm-1
)
[Cp*(CO)(NO)ReCO2Rh(cod)]2c (1)
1960
1459 (br, m)
1304 (w), 1260 (m)
1694 (NO)
1999, 1944
1998, 1943
2011
[Cp*(CO)2FeCO2Rh(cod)]2 (2)
[Cp*(CO)2Fe13CO2Rh(cod)]2 (2-C13
[Cp*(CO)2RuCO2Rh(cod)]2 (3)
1470 (br, m)
1475-1415d
1479 (sh, m)
1467 (m)
1262 (w), 1230 (m)
)
1255-1210d
1317 (w), 1256 (m), 1249 (sh)
1952
[Cp*(CO)2FeCO2Rh(nbd)]2 (4)
1998, 1943
1997, 1942
2008, 1956
2078, 2054, 2008
2020 s, 1964 s
2079 s, 2055 m, 2002 vs
1969, 1707 (NO)
2078, 2054, 2003
1466 (br, m)
1417 (br, m)
1435 (br, m)
1262 (m), 1229 (m)
1246 (m), 1213 (m)
1261 (m), 1229 (m)
[Cp*(CO)2Fe13CO2Rh(nbd)]2e (4-C13
[Cp*(CO)2FeCO2Rh(CO)2]2 (5)
)
[Cp*(CO)2RuCO2Rh(CO)2]2 (6)
1436 (br, m)
1318 (w), 1259 (m), 1244 (m)
1260 (sh), 1254 (m)
c
[Cp*(CO)(NO)ReCO2Rh(CO)2]2
1454 (br, m)
1428 (br, m)
a
b
THF solutions. ν(CO) relative intensities: s, strong, for Re, Fe, and Ru-CO and s-m, medium-strong for Rh-CO absorptions. Other
relative intensities: m, medium; w, weak. c Reference 1. Broad envelope due to 1:1 mixture of 2-13C and [Cp*(CO)(13CO)FeCO2Rh(cod)]2
(2a -13C); ν(CO) bands 1983 and 1913 cm-1 e Isolated as 2:1 mixture of 4-C13 and [Cp*(CO)(13CO)FeCO2Rh(nbd)]2 (4a -13C); ν(CO) bands
1983 and 1913 cm-1
d
.
.
amount of dimeric [Cp*M(CO)2]2 that forms. Attempts
to further purify 2 or 3 by crystallization or chroma-
tography invariably enriched these mixtures in [Cp*M-
(CO)2]2.
correspond to “inside and outside” vinyl, exo-, and endo-
methylene protons, respectively. (Inside hydrogens on
each ligated cod are in the region between the two
rhodium centers; cf. eq 1.) Each endo-H couples to one
exo-H and a vinyl-H, each exo-H couples to a vinyl-H
and an endo-H, and the vinyl-H also couples to the other
vinyl-H. In the absence of definitive results of nOe
experiments, we could not assign these spins systems
as inside or outside, however. The same symmetry
constraintssinside and outside cod sitessare mani-
fested in the 13C{1H} NMR spectra of 2 and 3, which
show four cod resonances, two each for the vinyl and
allylic carbons.
NMR spectra for 2 and 3 are less complex than the
spectra for 1,1a which are complicated by the presence
of the stereogenic rhenium centers and the attendant
formation of a 1:1 diastereomeric mixture. The relative
simplicity of the present NMR spectra for 2 and 3
nevertheless are consistent with the illustrated folded
structures (crystallographically established for 1). The
number of observed NMR absorptions are inconsistent
with 2 and 3 in solution having either a planar structure
or with a chelated η2-O,O′ Cp*(CO)2FeCO2Rh(cod) struc-
ture.19 Either structure would have half the number
of NMR absorptions. In addition, any fluxional motion
for 2 and 3 must be extremely slow with respect to the
NMR time scale.20
Once isolated, [Cp*(CO)2FeCO2Rh(cod)]2 (2) has a
limited stability in benzene and THF solutions at room
temperature. A 0.02 M solution in THF decomposed
15% over 2 h to give a darker brown solution containing
[Cp*Fe(CO)2]2 as the IR detectable product; its t1/2 ) 6
h. 1H NMR spectral monitoring of C6D6 solutions was
consistent with 10% decomposition over 3 h to [Cp*Fe-
(CO)2]2 as the only Cp*Fe species and a t1/2 ) 16 h.
[Cp*(CO)2RuCO2Rh(cod)]2 (3) is somewhat less stable.
A 0.02 M solution of 2 in THF has a t1/2 of 3 h, whereas
in C6D6 15% of 2 decomposed over 3 h, t1/2 ) 9 h. In
both solvents, [Cp*Ru(CO)2]2 was the only Cp*Ru spe-
1
cies detected by IR and H NMR spectral monitoring,
respectively.17
Structures for the Fe2Rh2 and Ru2Rh2 products 2 and
3 were inferred from their IR, 1H, and 13C NMR spectra.
The IR spectra are dominated with a pair of intense
ν(CO) bands as well as medium-intensity metallocar-
boxylate νOCO absorptions, vide infra, between 1480 and
1230 cm-1 (Table 1). 1H and 13C NMR spectra for 2,
for example, exhibit single resonances for the CO, CO2,
and Cp* ligands. It was the NMR spectra of the cod
ligands, however, that indicated the presence of the
folded M2Rh2 structures illustrated.
The synthetic procedure outlined in eq 1 was opti-
mized for maximum isolated yields of 1-3 by working
at 5 °C and by using a large excess of a volatile tertiary
amine. We optimized the conversion of Cp*Fe(CO)3PF6
1H NMR spectra for 2 and 3 show the expected18
upfield progression of the COD vinyl, exo, and endo
methylene absorptions. The presence of two resonances
for each of the vinyl, exo-, and endo-CH2 hydrogens are
consistent with the low C2v symmetry of 2 and 3. This
(19) Monomeric bis(phosphine) rhodium(I) complexes bearing a
chelating (η2-O, O′) carboxylate ligand are known: Darensbourg, D.
J .; Grotsch, G.; Wiegreffe, P.; Rheingold, A. L. Inorg. Chem. 1987, 26,
3827. Werner, H.; Scha¨fer, M.; Nu¨rnberg, O.; Wolf, J . Chem. Ber. 1994,
127, 27. Scha¨fer, M.; Wolf, J .; Werner, H. J . Organomet. Chem. 1994,
476, 85. Kukla, F.; Werner, H. Inorg. Chim. Acta 1995, 235, 253.
(20) One or more pathways for stereochemical nonrigidity are
available for folded structures analogous to 1-3.18,21 The 1H NMR
spectrum of [CH3CO2Rh(cod)]2 at room temperature, for example,
exhibits three broadened absorptions at room temperature for the cod
ligand vinyl and exo- and endo-methylene hydrogens plus a sharp
methyl singlet. Cooling the toluene-d8 solution to 0 °C and recording
the NMR spectrum resolves each of the three broadened absorptions
into two sharp absorptions, while having no effect on the methyl
singlet. This temperature dependency can be accounted for by a tub-
tub ring flipping of the eight-member ring that interconverts inside
and outside sites but not the exo and endo hydrogens. The 1H NMR
spectrum of 2 in toluene-d8 solution is independent of temperature
between +25 and -65 °C.
1
doubling of H NMR resonances for ligated cod repre-
sents the presence of “inside and outside” cod protons
(inside hydrogens are juxtaposed in the region between
the two cod rings). The results of COSY NMR experi-
ments with 2 further allowed us to identify the two spin
systems, δ 4.40, 2.38, 1.62 and δ 4.24, 2.79, 1.67, which
(17) Solutions of [Cp*(CO)(NO)Re(CO2)Rh(η4-cod)]2 (1) are more
stable than those of 2 and 3 at 25 °C. In benzene, 1 degraded over 2 d
to Cp*(CO)(NO)ReC6H5 (37%) and low concentrations of unidentified
rhenium and rhodium residues. Solutions of 1 are more stable in ether
and THF.1
(18) (a) Rodman, G. S.; Mann, K. R. Inorg. Chem. 1988, 27, 3338;
J . Organomet. Chem. 1989, 378, 255. (b) Ciriano, M. A.; Perez-Torrente,
J . J .; Oro, L. A. J . Organomet. Chem. 1993, 445, 267.

M2Rh2 Bis(µ3-carbon dioxide) Complexes
Organometallics, Vol. 17, No. 9, 1998 1865
and [Rh(OH)(cod)]2 to 2 using a variety of bases and
reaction conditions. It is worth noting that, in the
absence of base, [Rh(OH)(cod)]2 converted up to 40% of
the Cp*Fe(CO)3PF6 to 2 within 20 min at room tem-
perature. Although this reaction consumed 60% of the
Cp*Fe(CO)3PF6 within 6 h, the concentration of 2 then
remained constant and the excess iron appeared as
[Cp*Fe(CO)2]2. Carrying out the same reaction but in
the presence of potassium hydride (1 equiv in THF at
-20 °C) immediately consumed the Cp*Fe(CO)3PF6 and
gave 2 (85% yield) contaminated with [Cp*Fe(CO)2]2.
Our best observation for the use of 1 equiv of an amine
base corresponded to 2,6-lutidine at 5 °C. Under these
conditions, lutidine converted 70% of the Cp*Fe-
(CO)3PF6 plus [Rh(OH)(η4-COD)]2 to 2 and less than 2%
[Cp*Fe(CO)2]2 after 1 h. Samples of 2 prepared by this
procedure tended to be thermally unstable at room
temperature. Attempts to purify this product by chro-
matography or by crystallization (hexane, acetonitrile,
THF/hexane, toluene/hexane, ether/hexane, and CH2-
Cl2/hexane) invariably enriched the mixtures in [Cp*-
(CO)2Fe]2. Moreover, excessive amounts of [Cp*Fe-
(CO)2]2 were obtained upon scaling up this reaction.
When this reaction with lutidine as the base was
performed at room temperature, IR spectral monitoring
also indicated that a maximum of 70% of the Cp*Fe-
(CO)3PF6 converted to 2 within 15 min. After 6 h, no
further Cp*Fe(CO)3PF6 was consumed, but 15% of the
2 had degraded to [Cp*Fe(CO)2]2. Substitution of 1
equiv of Me2EtN, Et3N, or iPr2EtN for the lutidine
converted at most 60% of the Cp*Fe(CO)3PF6 to 2 after
1 h.22 Thereafter, the remaining Cp*Fe(CO)3PF6 de-
creased by an additional 20% over 6 h as the of amount
of 2 dropped to 45% yield commensurate with building
up of [Cp*Fe(CO)2]2 (35% yield).
plus traces of [Cp*Fe(CO)2]2. The amine-promoted
equilibrium leading to 2 also can be reversed with the
presence of an excess of the amine salt conjugate acid.
A THF solution of purified 2 upon treatment with 4
equiv of Bu3NH+BF4- in THF solution at 5 °C afforded
a 60:40 mixture of 2 and Cp*Fe(CO)3BF4 within 1 h.
The presence of [RhOH(cod)]2 also was noted by its IR
ν(OH) absorption at 3506 cm-1
.
We also generated samples of 2 and its norbornadiene
analogue [Cp*(CO)2Fe(CO2)Rh(η4-nbd)]2 (4) by treating
the metallocarboxylate Cp*(CO)2FeCO2K with a rhod-
ium(I) η4-diene triflate, eq 3. The Cp*(CO)2FeCO2K was
produced from the metalate Cp*(CO)2FeK and CO2 in
THF at -78 °C,13,14b and the rhodium(I) triflates were
obtained in situ from [RhCl(η4-diene)]2 and silver tri-
flate.16 Rhodium(I) triflates were necessary since the
starting [RhCl(η4-diene)]2 complexes proved to be un-
reactive toward Cp*(CO)2FeCO2K (-78 °C)sonly [Cp*Fe-
(CO)2]2 was detected. In addition to evaluating an
alternative synthesis of 2, we wanted to obtain samples
of C13-labeled materials [Cp*(CO)2Fe(13CO2)Rh(η4-di-
ene)]2 [diene ) cod (2-13C) and nbd (4-13C)] in order to
help assign the carboxylate IR spectral absorptions.
Treatment of Cp*(CO)2FeCO2K with either of the
rhodium(I) η4-diene triflate intermediates (eq 3) quan-
titatively produced the Fe2Rh2 bis(µ3-CO2) complexes 2
and 4, as judged by IR spectral monitoring. Samples
of 2, although obtained in low overall yields after
benzene and hexane extraction, contained 90-95% pure
The use of amines in reaction 1 appears to involve
equilibria analogous to that in eq 2. An excess of 5 equiv
1
2 (IR, H, and 13C NMR spectroscopy), the remainder
being the ubiquitous [Cp*Fe(CO)2]2. These samples,
however, decomposed more readily in solution at room
temperature [t1/2 (THF) ) 76 min] than samples of 2
procured using the first route.
of lutidine transformed 85% of the Cp*Fe(CO)3PF6 to 2
in 15 min (IR spectral monitoring) while giving only
traces of [Cp*Fe(CO)2]2. Higher initial concentrations
of Me2EtN of up to 20 equiv at room temperature
quantitatively generated 2; slow evaporation of solvent
and excess amine (room temperature at 10-2 mm)
shifted the equilibrium to the left. Readdition of the
THF and recording of the IR spectrum then indicated
the presence of a 70:30 mixture of 2 and Cp*Fe(CO)3PF6
The nbd-containing 4 proved to be even less stable in
solution. All attempted workup procedures degraded
it to [Cp*Fe(CO)2]2 as the only IR detectable species.
The 75% isolated yield of 4 (with 5% [Cp*Fe(CO)2]2
contamination) reported in the Experimental Section
represents our best result. Although an elemental
analysis for 4 was not obtained, its IR and NMR spectral
data closely resemble that for 2.
(21) (a) Sielisch, T.; Cowie, M. Organometallics 1988, 7, 707. (c)
Tejel, C.; Villoro, J . M.; Ciriano, M. A.; Lo´pez, J . A.; Eguiza´bal, E.;
Lahoz, F. J .; Bakhumutov, V. I.; Oro, L. A. Organometallics 1996, 15,
2967. (d) Balley, J . A.; Grundy, S. L.; Stobart, S. R. Organometallics
1990, 9, 536. Bushnell, G. W.; Fjeldsted, D. O. K.; Stobart, S. R.; Wang,
J . Organometallics 1996, 15, 3785.
We repeated the preparations of 2 and 4 using 1.1
equiv of 99% labeled 13CO2. Under the conditions
reported, attempts to generate [Cp*(CO)2Fe13CO2Rh-
(cod)]2 (2-C13) instead provided 2:1 mixtures of 2-C13 and
[Cp*(CO)(13CO)FeCO2Rh(cod)]2 (2a -C13). Alternatively,
variable amounts of {Cp*(CO)(13CO)FeCO2[Rh(cod)]2O2-
13CFe(CO)2Cp*} if formed would have been indistin-
(22) Other bases tried included
1 equiv of (CH3)3COK or 1,8-
bis(dimethylamino)naphthalene. The reaction between Rh(OH)(η4-
cod)]2 and Cp*Fe(CO)3PF6 using the latter base was sluggish: After 2
h, 65% of the Cp*Fe(CO)3PF6 converted to 2. Thereafter further product
formation was offset by increasing production of [Cp*Fe(CO)2]2.
Potassium tert-butoxide, on the other hand, rapidly gave 2 (even at
-78 °C), but substantial concentrations of [Cp*Fe(CO)2]2 also formed.
The reaction at room temperature provided a 45:55 mixture of 2 and
[Cp*Fe(CO)2]2 within 15 min.
guishable from mixtures of 2-C13 and 2a -C13
.
IR
spectral monitoring of the cold reaction mixture never-
theless showed the expected 13C label shift for the

1866 Organometallics, Vol. 17, No. 9, 1998
Tetrick et al.
The carboxylate-carbonyl label shuttle observed in
generating 2a -C13 and 4a -C13 originated from 2-C13 and
4-C13, eq 4. An alternative prior equilibration of
Cp*(CO)2Fe13CO2K and Cp*(13CO)(CO)FeCO2K23a,24 fol-
lowed by trapping with the rhodium(I) triflate was
deemed less likely. The procedure reported here is
identical to that previously used for cleanly transform-
ing Cp*(CO)2Fe13CO2K and either tin chloride R3SnCl
(R ) CH3, Ph) to Cp*(CO)2Fe13CO2SnR3.14b The tri-
methyltin product subsequently underwent carboxy-
late-carbonyl label shuttle and competitive decarboxy-
lation to give a mixture of Cp*(13CO)(CO)FeCO2Sn-
(CH3)3 and Cp*(CO)2FeSn(CH3)3/Cp*(13CO)(CO)FeSn-
(CH3)3.
Table 1 collects solution IR spectral data for the M2-
Rh2 bis(µ3-CO2) complexes that were prepared in this
study. Assignment of the medium-intensity carboxylate
3a
vibrational modes νOCO(asym) and νOCO(sym) rests on
comparison of these spectra with appropriate standards,
typically [Cp2M(CO)4]2 and [RhCl(diene)]2, and on the
label shift for the carboxylate bands for 4 and 4-C13
.
Note that carboxylate IR spectral band assignments are
collected as νOCO(asym) and νOCO(sym) regions in this table
due to the existence of several bands that appear as two
distinct groups. It is important to note that the number
of these bands and their relative intensitiesstheir
overall appearancesis independent of the sample his-
tory, the temperature (-15 to +25 °C), and the method
of preparation. Moreover, the IR spectra in the car-
boxylate region of the iron and ruthenium complexes
2-6 resemble that of 1, which also exhibits the same
spectrum in the solid state as a KBr pellet.
F igu r e 1. Infrared spectra in THF: (A) [Cp*(CO)2FeCO2-
Rh(cod)]2 (2), isolated material from reaction between [Rh-
(OH)(η4-cod)]2 and Cp*Fe(CO)3PF6; (B) [Cp*(CO)2FeCO2-
Rh(nbd)]2 (4), generated in THF from Cp*(CO)2FeCO2K
and [Rh(OSO2CF3)(η4-cod)]x} (peaks at 1924 and 1756 cm-1
correspond to the [Cp*(CO)2Fe]2 impurity); (C) [Cp*-
(CO)2Fe13CO2Rh(nbd)]2 (4-C13) admixed with [Cp*(CO)-
(13CO)FeCO2Rh(nbd)]2 (4a -13C) (1983, 1912 cm-1).
The appearance of several bands in the carboxylate
region rather than one band each for the νOCO(asym) and
νOCO(sym) vibrational modes is unexpected. Thus far,
metallocarboxylates have been treated analogous to
organic carboxylate ligands25 in that their IR spectra
were expected to exhibit single νOCO(asym) and νOCO(sym)
bands, perhaps augmented by a δ bending vibrational
mode.26 This analogy clearly breaks down for the
present M2Rh2 bis(µ3-CO2) complexes. The rhodium(I)
Cp*(CO)(13CO)Fe ν(CO) absorptions.23 Further workup
of the reaction was necessary in order to remove soluble
triflate residues that absorbed in the carboxylate IR
region. This workupsbenzene or hexane extractionss
engendered further carboxylate-carbonyl label shut-
tle3,5a,23,24 that give a 1:1 mixture of 2-C13 and 2a -C13
along with labeled and unlabeled [Cp*Fe(CO)2]2. This
label shuttle is presumed to be an equilibrium process,
eq 4.
11
acetates [Rh(µ-CH3CO2)(cod)]2 and [Rh(µ-CH3CO2)-
(CO)2]2,27a,b in contrast, exhibit just two acetate car-
boxylate absorptions, 1580, 1426 cm-1 and 1570, 1436
cm-1, respectively, in THF. These rhodium acetates
(25) (a) Deacon, G. B.; Phillips, R. J . Coord. Chem. Rev. 1980, 33,
227. Deacon, G. B.; Huber, F.; Phillips, R. J . Inorg. Chim. Acta 1985,
104, 41. (b) Nakamoto, K. Infrared and Raman Spectra of Inorganic
and Coordination Compounds, 4th ed.; Wiley: New York, 1986; p 231.
(26) (a) J egat, C.; Fouassier, M.; Tranquille, M.; Massetti, J .;
Tommasi, I.; Aresta, M.; Ingold, F.; Dedieu, A. Inorg. Chem. 1993, 32,
1279. (b) J egat, C.; Fouassier, M.; Massetti, J . Inorg. Chem. 1991, 30,
1521. (c) J egat, C.; Fouassier, M.; Tranquille, M.; Massetti, J . Inorg.
Chem. 1991, 30, 1529. (d) Massetti, J .; Tranquille, M. J . Phys. Chem.
1988, 92, 2177.
(27) (a) Lawson, D. N.; Wilkinson, G. J . Chem. Soc. 1965, 1900. (b)
See also, Pruchnik, F.; Wajda, K. Inorg. Chim. Acta 1980, 40, 203.
Mieczynska, E.; Trzeciak, A. M.; Zio´lkowski, J . J . J . Mol. Catal. 1993,
80, 189. Bianchi, F.; Gallazzi, M. C.; Pori, L.; Diversi, P. J . Organomet.
Chem. 1980, 202, 99. (c) Esteruelas, M. A.; Lahuerta, O.; Modrego, J .;
Nurnberg, O.; Oro, L. A.; Rodriguez, L.; Sola, E.; Werner, H. Organo-
metallics 1993, 12, 266. Nagy-Magos, Z.; Vastag, S.; Heil, B.; Marko,
L. J . Organomet. Chem. 1979, 171, 97.
Similar attempts to generate [Cp*(CO)2Fe13CO2Rh-
(nbd)]2 (4-C13) reaction were more promising in that we
obtained a 2:1 mixture of 4-C13 and 4a -C13 after workup.
Figure 1 presents IR spectra for 4-C13 and 4a -C13 in
THF solution (scans B and C); scan A correponds to a
THF solution of 2 that was prepared by the first
procedure.
(28) (a) Reis, A. H., J r.; Willi, C.; Siegel, S.; Tani, B. Inorg. Chem.
1979, 18, 1859. (b) Sheldrick, W. S.; Gu¨nther, B. J . Organomet. Chem.
1989, 375, 233. (c) Mieczynska, E.; Trzeciak, A. M.; Zio´lkowski, J . J .
J . Mol. Catal. 1993, 80, 189. (d) Werner, H.; Poelsma, S.; Schneider,
M. E.; Windmu¨ller, B.; Barth, D. Chem. Ber. 1996, 129, 647. (e)
Fornika, R.; Dinjus, E.; Go¨rls, H.; Leitner, W. J . Organomet. Chem.
1996, 511, 145.
(23) For similar IR spectral ν(CO) assignments, see those for Cp-
(
13CO)(CO)MCO2ZrClCp2 [M ) Fe, Ru],4b,14a,23a Cp*(13CO)(CO)FeCO2-
SnMe3,14b and Cp(13CO)(CO)MCO2M′ [M ) Fe, Ru; M′ ) Na, K].13 (a)
Steffey, B. D.; Vites, J . C.; Cutler, A. R. Organometallics 1991, 10, 3432.
(24) (a) Lee, G. R.; Cooper, N. J . Organometallics 1985, 4, 1467. (b)
Lee, G. R.; Cooper, N. J . Organometallics 1985, 4, 794.

M2Rh2 Bis(µ3-carbon dioxide) Complexes
Organometallics, Vol. 17, No. 9, 1998 1867
should have structures28 similar to that of 1, which
presumably resembles those of 2-6.
analogous carbonylation product of [Rh(µ-CH3CO2)-
(cod)]2.27 Taken togeather, the spectral data for 5 and
6 are consistent with the illustrated “open-book” Rh2
structures. Solutions of 5 and 6 in benzene or THF were
unstable at room temperature, behavior reminiscent of
[Rh(µ-CH3CO2)(CO)2]2.11b
The presence of the carboxylate-carbonyl label shuttle
for 2-C13 and 4-C13 allowed us to assign the 13C NMR
resonances for the carbonyl and carboxylate ligands,
which appear as two singlets near δ 213-215 and δ 217.
Although 13C NMR spectra for these complexes exhib-
ited intense resonances in both regions due to the
presence of 2a -C13 and 4a -C13, the downfield singlet
initially was up to twice as intense. Hence, we assign
the downfield absorption to the carboxylate ligand.
These carbonyl and carboxylate chemical shift assign-
ments are consistent with those that we reported for
the related tin esters Cp*(CO)2Fe13CO2SnR3: R ) Ph,
δ 218 (CO2), 216 (CO); R ) Me, δ 216.5 (CO2), 212
(CO).14b
The rapid reaction with 1 equiv of Ph3SnCl (eq 6)
conveniently derivatized 2 as the known FeSn ester.14b,15
This FeSn ester retains the original CO2 ligand. IR
spectral monitoring of this reaction confirmed that it
occurred quantitatively within 0.5 h, and subsequent
workup afforded 60-70% isolated yields of the FeSn
ester and [Rh(Cl)(cod)]2.
We further characterized 2 and 3 by carbonylation
and by the reaction of 2 with Ph3SnCl. One atmosphere
of carbon monoxide promptly carbonylated the rhodium
centers, eq 5. IR spectra of the reaction solutions
Con clu sion s
In this article we document the synthesis of three M2-
Rh2 bis(µ3-CO2) complexes [Cp*(CO)(NO)Re(CO2)Rh(η4-
cod)]2 (1) and [Cp*(CO)2MCO2Rh(η4-cod)]2, M ) Fe (2)
and Ru (3), from the reaction between [Rh(OH)(η4-cod)]2
and a carbonyl salt Cp*(L)(CO)2M+ (L ) CO, M ) Fe,
Ru; L ) NO, M ) Re) and a base. The resulting µ3-
CO2 ligands thus originate from condensation of a metal
carbonyl plus a metal hydroxide with loss of a proton,
a reaction with little precedent.6 The more common
synthetic procedure3 of treating a metallocarboxylate
[Cp*(CO)2FeCO2K or Cp*(CO)2Fe13CO2K] with a metal
electrophile {[Rh(OSO2CF3)(η4-cod)]x} also was used to
prepare 2 and [Cp*(CO)2Fe13CO2Rh(diene)]2 [diene )
cod (2-13C) and nbd (4-13C)]. These 13C-labeled com-
pounds readily exchanged the 13C-label between the
carboxylate and carbonyl ligands. The IR and NMR
spectroscopy of these Re2Rh2 and Fe2Rh2 bis(µ3-CO2)
complexes are consonant with the presence of two µ3-
[η1-C(M):η1-O(Rh):η1-O′(Rh′)] carboxylate groups and an
overall structure that resembles the Rh(I) carboxylates
[(RCO2)Rh(diene)]2.
confirmed the presence of the iron or ruthenium met-
allocarboxylates and the rhodium dicarbonyl moiety;
NMR spectra confirmed the release of 1,5-cycloctadiene
and the presence of one set of Cp* resonances. The
three IR νCO bands for the rhodium, moreover, closely
resemble those observed for [Rh(µ-CH3CO2)(CO)2]2,11 the
Ack n ow led gm en t. Support from the National Sci-
ence Foundation, Grant CHE-9108591, is gratefully
acknowledged.
OM970443K