Reactivity of the Metallocarboxylates Cp(NO)(PPh3)ReCO2-M+ toward Excess Carbon Dioxide: Degradation to a Bimetallic μ-[η1-C(Re):η1-O,O′(Re′)] Carbon Dioxide Complex Cp(NO)(PPh3)ReCO2Re(NO)(CO)(PPh3)(η 1-C5H5)

Article abstract of DOI:10.1021/om980642j

The rhenium η¹C carboxylate Cp(PPh₃)(NO)ReCO₂^-(1Li⁺ or 1K⁺) undergoes an unusual reductive disproportionation sequence in the presence of excess CO₂ that generates a new Re₂(μ₂-η³-CO₂) complex Cp(PPh₃)(NO)ReCO₂Re(CO)(NO)(PPh₃)(η ¹-Cp) (2) plus CO₃^2-. The overall stoichiometry entails 3 equiv of CO₂ plus 2 equiv of Cp(PPh₃)(NO)ReLi transforming to ligated CO₂ and CO (i.e., 2) plus CO₃^2-. Three procedures were used to generate 2: treating (a) Cp(PPh₃)(NO)ReLi with excess CO₂, (b) 1K⁺ with excess CO₂, and (c) 1K⁺ with Cp(PPh₃)-(NO)Re(CO)BF₄. The results of this study are consistent with the intermediacy of an undetected metalloanhydride Cp(PPh₃XNO)ReC(O)OC(O)Re(NO)(PPh₃)Cp (8), which upon a subsequent η⁵→η¹ Cp ring shift on one rhenium center affords 2. This product undergoes different solvolysis reactions in the presence of methanol and triethylsilanol. The former gives 2 equiv of the methyl ester Cp(NO)(PPh₃)ReCO₂CH₃ (6), whereas the latter replaces the η¹-Cp group with a silanolate to give the previously characterized Cp(PPh₃)(NO)ReCO₂-Re(CO)(NO)(PPh₃)(OSiEt ₃) (5a).

Full text of DOI:10.1021/om980642j

Organometallics 1999, 18, 1741-1746
1741
Rea ctivity of th e Meta lloca r boxyla tes
-
Cp (NO)(P P h 3)ReCO2 M+ tow a r d Excess Ca r bon Dioxid e:
Degr a d a tion to a Bim eta llic µ-[η1-C(Re):η1-O,O′(Re′)]
Ca r bon Dioxid e Com p lex
Cp (NO)(P P h 3)ReCO2Re(NO)(CO)(P P h 3)(η1-C5H5)
Stephen M. Tetrick and Alan R. Cutler*
Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180
Received J uly 27, 1998
The rhenium η1-C carboxylate Cp(PPh3)(NO)ReCO2- (1Li+ or 1K+) undergoes an unusual
reductive disproportionation sequence in the presence of excess CO2 that generates a new
Re2(µ2-η3-CO2) complex Cp(PPh3)(NO)ReCO2Re(CO)(NO)(PPh3)(η1-Cp) (2) plus CO32-. The
overall stoichiometry entails 3 equiv of CO2 plus 2 equiv of Cp(PPh3)(NO)ReLi transforming
to ligated CO2 and CO (i.e., 2) plus CO32-. Three procedures were used to generate 2: treating
(a) Cp(PPh3)(NO)ReLi with excess CO2, (b) 1K+ with excess CO2, and (c) 1K+ with Cp(PPh3)-
(NO)Re(CO)BF4. The results of this study are consistent with the intermediacy of an
undetected metalloanhydride Cp(PPh3)(NO)ReC(O)OC(O)Re(NO)(PPh3)Cp (8), which upon
a subsequent η5fη1 Cp ring shift on one rhenium center affords 2. This product undergoes
different solvolysis reactions in the presence of methanol and triethylsilanol. The former
gives 2 equiv of the methyl ester Cp(NO)(PPh3)ReCO2CH3 (6), whereas the latter replaces
the η1-Cp group with a silanolate to give the previously characterized Cp(PPh3)(NO)ReCO2-
Re(CO)(NO)(PPh3)(OSiEt3) (5a ).
In tr od u ction
reductive disproportionation sequence for the rhenium
η1-C carboxylate Cp(PPh3)(NO)ReCO2- (1Li+ or 1K+)9,10
that generates a new Re2(µ2-η3-CO2) complex Cp-
(PPh3)(NO)ReCO2Re(CO)(NO)(PPh3)(η1-Cp) (2) plus free
The synthesis of carbon dioxide complexes1 in which
the CO2 binds η1-C or η2-C,O to a metal center is limited
by the tendency of these complexes to promote reductive
disproportionation2 of the ligated CO2, Scheme 1.3 This
disproportionation occurs when an electron-rich transi-
tion-metal CO2 complex incorporates a second equiva-
lent of CO2 and forms a C2 ligand that subsequently
fragments to carbonate plus carbon monoxide.4 These
C1 products have been observed as (a) ligated carbonate
and CO at the same2a,c,5 or different2c,6 metal centers,
(b) ligated CO and free carbonate,2b,7 and (c) ligated
carbonate and free CO.8 We now document an unusual
(4) (a) An example of a chelating C2O4 intermediate as an addend
of ligated CO2 and CO has been characterized: Herskovitz, T.;
Guggenberger, L. J . J . Am. Chem. Soc. 1976, 98, 1615. (b) The reverse
reaction, addition of free carbonate to a carbonyl ligand to generate a
metallocarboxylate plus free CO, recently has been established.
Nakajima, H.; Tsuge, K.; Tanaka, K. Chem. Lett. 1997, 485. (c) Air
oxidation of a carbonyl ligand to coordinated CO2 also has been
reported: Fu, P.-F.; Khan, M. A.; Nicholas, K. M. J . Am. Chem. Soc.
1992, 114, 6579. El Krami, S.; Mourad, Y.; Mugnier, Y.; Antin˜olo, A.;
Del-Hierro, I.; Garcia-Yuste, S.; Otero, A.; Fajardo, M.; Brunner, H.;
Gehart, G.; Wachter, J .; Amaudrut, J . J . Organomet. Chem. 1995, 498,
165.
(5) (a) Karsch, H. H. Chem. Ber. 1977, 110, 2213. (b) Hossain, S. F.;
Nicholas, K. M.; Teas, C. L.; Davis, R. E. J . Chem. Soc., Chem.
Commun. 1981, 268. (c) Alvarez, R.; Carmona, E.; Marin, J . M.; Poveda,
M. L.; Gutierrez-Puebla, E.; Monge, A. J . Am. Chem. Soc. 1986, 108,
2286. Alvarez, R.; Atwood, J . L.; Carmona, E.; Pe´rez, P. J .; Poveda, M.
L.; Rogers, R. D. Inorg. Chem. 1991, 30, 1493.
(1) Gibson, D. H. Chem. Rev. 1996, 96, 2063.
(2) (a) Chatt, J .; Kubota, M.; Leigh, G. J .; March, F. C.; Mason, R.;
Yarrow, D. J . J . Chem. Soc., Chem. Commun. 1974, 1033. (b) Maher,
J . M.; Cooper, N. J . J . Am. Chem. Soc. 1980, 102, 7604. Lee, G. R.;
Maher, J . M.; Cooper, N. J . J . Am. Chem. Soc. 1987, 109, 2956. (c)
Carmona, E.; Gonzalez, F.; Poveda, M. L.; Marin, J . M. J . Am. Chem.
Soc. 1983, 105, 3365. (d) Reinking, M. K.; Ni, J .; Fanwick, P. E.;
Kubiak, C. P. J . Am. Chem. Soc. 1989, 111, 6459.
(6) (a) Belmore, K. A.; Vanderpool, R. A.; Tsai, J .-C.; Khan, M. A.;
Nicholas, K. M. J . Am. Chem. Soc. 1988, 110, 2004. (b) Burlakov, V.
V.; Dolgushin, F. M.; Yanovsky, A. I.; Struchkov, Y. T.; Shur, V. B.;
Rosenthal, U.; Thewalt, U. J . Organomet. Chem. 1996, 522, 241.
(7) (a) Nakajima, H.; Kushi, Y.; Nagao, H.; Tanaka, K. Organome-
tallics 1995, 14, 5093. (b) An early report of release of carbonate from
(3) Other available degradation/reaction pathways for η1-C or η2-
C,O transition-metal CO2 complexes include deoxygenation (O atom
transfer to a substrate,3a including CO22) and decarbonylation leaving
a metal oxo or oxide compound.3b,7a (a) Deoxygenation: Bianchini, C.;
Meli, A. J . Am. Chem. Soc. 1984, 106, 2698. Lee, G. R.; Cooper, N. J .
Organometallics 1985, 4, 1467. This degradation is commonly associ-
ated with electrophilic attack on the coordinated CO2: Hirano, M.;
Akita, M.; Tani, K.; Kumagai, K.; Kasuga, N. C.; Fukuoka, A.; Komiya,
S. Organometallics 1997, 16, 4206, and references therein. (b) Decar-
bonylation: Alt, H. G.; Schwind, K. H.; Rausch, M. D. J . Organomet.
Chem. 1987, 321, C9. Bryan, J . C.; Mayer, J . M. J . Am. Chem. Soc.
1990, 112, 2298. Hall, K. A.; Mayer, J . M. J . Am. Chem. Soc. 1992,
114, 10402. Fu, P.; Khan, M. A.; Nicholas, K. M. Organometallics 1991,
10, 382. Fu, P.-F.; Khan, M. A.; Nicholas, K. M. Organometallics 1992,
11, 2607; J . Organomet. Chem. 1996, 506, 49. Antin˜olo, A.; Carrillo-
Hermosilla, F.; del Hierro, I.; Otero, A.; Fajardo, M.; Mugnier, Y.
Organometallics 1997, 16, 4161.
-
Cp(CO)2FeCO2 and excess CO2 has been questioned: Evans, G. O.;
Walter, W. F.; Mills, D. R.; Streit, C. A. J . Organomet. Chem. 1978,
144, C34. Lee, G. R.; Cooper, N. J . Organometallics 1985, 4, 794.
(8) (a) Fachinetti, G.; Floriani, C.; Chiesi-Villa, A.; Guastini, G. J .
Am. Chem. Soc. 1979, 101, 1768. (b) Bottomley, F.; Lin, I. J . B.;
Mukaida, M. J . Am. Chem. Soc. 1980, 102, 5238. (c) Breimair, J .; Robl,
C.; Beck, W. J . Organomet. Chem. 1991, 411, 395.
(9) Senn, D. R.; Emerson, K.; Larsen, R. D.; Gladysz, J . A. Inorg.
Chem. 1987, 26, 2737.
(10) Analogous rhenium η1-C carboxylates: (a) Cp(NO)(CO)ReCO2
,
-
Sweet, J . R.; Graham, W. A. G. Organometallics 1982, 1, 982. (b)
Cp(N2Ar)(CO)ReCO2-, Barrientos-Penna, C. F.; Gilchrist, A. B.; Klahn-
Oliva, A. H.; Hanlan, A. J . L.; Sutton, D. Organometallics 1985, 4,
478. (c) Cp*(NO)(CO)ReCO2-, DiBiase-Cavanaugh, M.; Tetrick, S. M.;
Masi, C. J .; Cutler, A. R. J . Organomet. Chem. 1997, 538, 41.
10.1021/om980642j CCC: $18.00 © 1999 American Chemical Society
Publication on Web 04/09/1999

1742 Organometallics, Vol. 18, No. 9, 1999
Tetrick and Cutler
Sch em e 1
carbonate (eq 1).
lographic study for the phenyldimethylsilanolate 5b
(SiR3 ) SiMe2Ph)12 and moreover is comparable to the
structure that we now are reporting for 2. In this study
we demonstrate that the reductive disproportionation
of ligated CO2 on 1 plus exogenous CO2 to yield 2, a net
degradation of a metallocarboxylate to a bimetallic CO2
complex, resembles the rearrangement of 4 to 5.
Exp er im en ta l Section
Synthetic manipulations were performed using a combina-
tion of standard Schlenk line, glovebox, and vacuum line
procedures.14 Infrared spectra of THF solutions were recorded
on a Perkin-Elmer FT spectrophotometer, Model No. 1600,
over the carbonyl ν(CO) frequency range (2200-1600 cm-1).
NMR spectral data were obtained in C6D6 and were reported
as δ values relative to residual C6D5H (1H: 7.15 ppm) and C6D6
(13C: 128.00 ppm) using Varian 300 MHz INOVA and Unity
500 spectrometers. Experimental details for working with
purified CO2 have been reported.15 Combustion microanalyses
were done by Quantitative Technologies, Bound Brook, NJ .
Organic and inorganic reagents were obtained commercially
and used as received; silanes and C6D6 were stored in a
glovebox under nitrogen. THF and benzene were distilled from
sodium benzophenone ketyl. Trialkylsilanols, Et3SiOH, and
PhMe2SiOH were prepared as previously reported.10c The
The starting rhenium CO2 complex 1 originally was
synthesized by Gladysz and co-workers by deprotonation
of Cp(PPh3)(NO)ReCO2H.9 Once generated, 1Li+ and
1K+ are stable in solution. Both metallocarboxylates and
their Sn, Ge, and Pb(IV) derivatives Cp(PPh3)(NO)ReCO2-
MPh3 were characterized by IR and 1H, 13C, and 31P
NMR spectroscopy, as well as by an X-ray structure
determination of the tin ester (M ) Sn). In a more recent
study, we used 1 as precursers to a Re2Rh2(µ3-CO2)2
complex [Cp(PPh3)(NO)Re(CO2)Rh(η4-COD)]2 (3) in which
each µ3-[η1-C(Re):η1-O(Rh):η1-O′(Rh′)] rhenium carboxy-
late ligand bridges two rhodium(I) (η4-COD) moieties.11
Treatment of 3 with PhMe2SiH or Et3SiH initially
produced the silyl esters Cp(PPh3)(NO)ReCO2SiR3 (4)
(eq 2),12 but these promptly degraded in solution to Re2-
rhenium compounds Cp(PPh3)(NO)ReCO2H,16 Cp(PPh3)(NO)-
ReH,17 and Cp(NO)(PPh3)ReCO+BF4
were prepared by
- 18
literature procedures and judged pure by IR and 1H NMR
spectroscopy.
Ca r boxyla tion of [Cp (NO)(P P h 3)Re]-Li+. To a precooled
(-20 °C) suspension of Cp(NO)(PPh3)ReH (194 mg, 0.356
mmol) in 10 mL of THF was added 0.28 mL of n-BuLi solution
(1.6 M in hexane, 0.45 equiv). Within 30 min a solution color
change from orange to red was observed; IR spectral monitor-
ing was consistent with conversion to Cp(NO)(PPh3)ReLi [ν-
(NO) 1460, 1458, 1433, and 1396 cm-1].17 This red solution
was frozen by immersing the reaction flask (which remained
attached to a vacuum line) in liquid nitrogen. After three
freeze-pump-thaw cycles of the reaction mixture, excess CO2
(1 mmol) was condensed onto the frozen solution. An immedi-
(11) Tetrick, S. M.; Tham, F. S.; Cutler, A. R. J . Am. Chem. Soc.
1997, 119, 6193.
(12) Tetrick, S. M.; DiBiase-Cavanaugh, M.; Tham, F. S.; Cutler,
A. R. Organometallics 1998, 17, 7, 1925.
(13) This rearrangement and degradative chemistry of 4 was not
observed for the relatively stable silyl esters Cp*(NO)(CO)ReCO2-
SiR3.10c
(14) Shriver, D. F.; Drezdon, M. A. The Manipulation of Air-Sensitive
Compounds, 2nd ed.; Wiley-Interscience: New York, 1986.
(15) (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.
(16) Tam, W.; Lin, G.-Y.; Wong, W.-K.; Kiel, W. A.; Wong, V. K.;
Gladysz, J . A. J . Am. Chem. Soc. 1982, 104, 141.
(17) Crocco, G. L.; Gladysz, J . A. J . Am. Chem. Soc. 1988, 110, 6110.
(µ2-η3 CO2) silanolate compounds 5.13 The depicted
structure of 5 conforms to the result of a X-ray crystal-

Reactivity of Metallocarboxylates toward CO2
Organometallics, Vol. 18, No. 9, 1999 1743
-
ate reaction was evident as the red reaction mixture turned
orange and melted at -78 °C; after warming to room temper-
ature over 30 min, the reaction was judged complete by IR
spectral monitoring. The IR spectrum was dominated by one
ν(CO) band at 1972 cm-1 and two ν(NO) bands at 1718 and
1684 cm-1 that indicate the presence of Cp(NO)(PPh3)ReCO2-
Re(η1-Cp)(CO)(NO)(PPh3) (2). (Identical results were obtained
in the presence of 1 equiv of freshly distilled N,N′-tetrameth-
ylethylenediamine.)
The reaction solution was evaporated, and the orange
residue was extracted with benzene (5 × 5 mL). These extracts
were filtered (medium-porosity sintered glass), combined, and
evaporated. The resulting orange residue was dissolved in 10
mL of THF and treated with 1.0 mL of methanol (25 mmol).
IR spectra of the yellow solution were consistent with quan-
titative conversion to Cp(NO)(PPh3)ReCO2Me (6),18a ν(NO)
1678, ν(CdO) 1592 cm-1. The product was isolated as a yellow
solid (112 mg, 57%) after crystallization at -20 °C from 11
mL of 1:10 CH2Cl2/pentane; it was established as spectroscopi-
cally pure 6 by multinuclear NMR spectroscopy.
precooled (-78 °C) slurry of Cp(NO)(PPh3)ReCO+BF4 (122
mg, 0.184 mmol) in 5 mL of THF. Another 2 mL of THF was
used to wash the frit, and the resulting orange suspension was
stirred at -78 °C for 10 min before the reaction mixture was
warmed to and stirrred at room temperature for 1 h.
The resulting orange solution was evaporated in vacuo, the
residue was extracted with benzene (3 × 3 mL), and the
combined extracts were filtered through Celite and evaporated.
An orange residue remained that was redissolved over 10 min
in methylene chloride (10 mL); pentane (20 mL) was added
slowly to form a cloudy orange solution. Upon cooling at -20
°C overnight, the resulting orange crystals were filtered,
washed with pentane (3 × 5 mL), and dried in vacuo overnight.
This material was identified as Cp(NO)(PPh3)ReCO2Re(CO)-
(NO)(PPh3)(η1-C5H5)‚1.0CH2Cl2 (2), yield 0.159 g (75%). The
relative amount of the methylene chloride solvate was con-
1
firmed in the H NMR spectrum of this complex (4.26 ppm in
C6D6). Anal. Calcd for C49H42Cl2N2O5P2Re2: C, 47.31; H, 3.40.
Found: C, 47.11; H, 3.61. IR (KBr): 1970 (CO), 1707, 1676
(NO), 1482, 1436, (PPh3), 1362 (vw) 1288, 1263, 1236, 1217,
1187 (OCO region) cm-1. IR (CH2Cl2): 1970 (CO), 1716 1679
A portion of the remaining white benzene-insoluble residue
(49 mg) was immediately dissolved by 6 M HCl with vigorous
effervescence of presumed CO2. The remaining solid was
extracted with deionized water (3 × 1 mL) and treated with
0.5 mL of a 0.1 M Pb(NO3)2 solution. The resulting white
precipitate was centrifuged, washed with water, recentrifuged,
and dried in a vacuum desiccator over P2O5. An IR spectrum
of this solid as a KBr pellet was identical to one run of an
(NO), 1482, 1435 (PPh3), 1358 (m), 1300-1200 (v br), cm-1
.
IR (THF): 1971 (CO), 1718, 1684 (NO), 1482, 1436 (PPh3),
1
1358 (m), 1300-1200 (v br), 1218 (m) (OCO region) cm-1. H
3
NMR (C6D6): δ 7.65 (mult., 2H, o-H, PPh3), 7.61 (dd, J H,P
)
3
10.2, J (H(o),H(m) ) 7.8 Hz, 4H, o-H, PPh3), 7.55 (br mult, 6H,
o-H, PPh3), 6.9-7.14 (mult, 18H, m- and p-H, PPh3), 6.32 and
6.28 (br s, ∆ν1/2 ) 8 Hz, 48/29 (by area), 5H, η1-C5H5), 4.59
and 4.53 (s, 5H, ∆ν1/2 ) 2 Hz, 25/51 (by area), η5-Cp). 13C{1H}
authentic sample of PbCO3: a broad band at 1410 cm-1 and a
2
2
19
weaker absorption at 840 cm-1
.
NMR: δ 134.62 (d, J C,P ) 11.5 Hz, o-C), 134.54 (d, J C,P
)
2
2
Rea ction of Cp (NO)(P P h 3)ReCO2-K+ (1K+) w ith Ex-
cess CO2. To a mixture of KH (0.199 g, 4.96 mmol) and Cp-
(NO)(PPh3)ReCO2H (0.115 g, 0.195 mmol) was added THF (20
mL), and the yellow suspension was stirred at room temper-
ature for 1 h. This gave an orange suspension, which was
filtered. An IR spectrum of the filtrate was consistent with
1K+: 1634, 1622, (NO), 1440 (v br), 1240 (br) (OCO region)
cm-1. After a CO2 atmosphere (1 atm) was maintained over
this vigorously stirred solution for 5 min the IR spectrum of
the red-orange solution was dominated by the diagnostic ν-
(CO) and ν(NO) bands for 5, although weak, unassigned ν-
(NO) bands at 1657, 1636, and 1608 cm-1 also were present.
The solvent was removed in vacuo; the residue was extracted
with benzene (2 × 5 mL) and filtered, and the benzene was
evaporated prior to redissolving the orange residue in dichlo-
romethane (5 mL). Addition of pentane (10 mL) afforded a
cloudy solution, which upon maintaining overnight at -20 °C
produced salmon-colored crystals. These were filtered, washed
with hexane, and dried in vacuo: yield 0.052 g of 5 (46%).
11.4 Hz, o-C), 134.12 (d, J C,P ) 8.4 Hz, o-C), 134.04 (d, J C,P
1
) 9.1 Hz, o-C), 133.20 (d, J C,P ) 40.5 Hz, ipso-C), 132.71 (d,
1J C,P ) 41.2 Hz, ipso-C), 130.76 (p-C), 130.53 (p-C), 130.46 (p-
3
3
C), 128.86 (d, J C,P ) 9.9 Hz, m-C), 128.82 (d, J C,P ) 9.2 Hz,
3
3
m-C), 128.66 (d, J C,P ) 10.7 Hz, m-C), 128.63 (d, J C,P ) 10.9
Hz, m-C), 128.29 (m-C), 128.11 (p-C), 127.88 (m-C), 127.69 (m-
C), 127.50 (m-C), 117.17 (br, ∆ν1/2 ) 75 Hz, η1-C5H5), 92.72
and 92.61 (s, 1/2 (by area), η5-Cp), 53.24 (CH2Cl2). 31P{1H}
NMR: δ 18.75, 15.38 (major); 17.99, 14.22 (minor).
Rea ction of Cp (NO)(P P h 3)ReCO2Re(η1-Cp )(CO)(NO)-
(P P h 3) (2) w ith Et3SiOH. An orange solution containing 5
(0.0179 mmol) in 600 mg of C6D6 was quantified by H NMR
1
spectroscopy via an anisole internal standard. Then Et3SiOH
(3.0 µL, 19.6 µmol) was added with a 10 µL syringe, and an
1H NMR spectrum was recorded after standing for 18 h.
Prominent absorptions were assigned to Cp(NO)(PPh3)ReCO2-
Re(CO)(NO)(PPh3)(OSiEt3) (5b), 53% yield as a 33:24 mixture
of the major and minor diastereoisomers12 (Cp, δ 5.13 and 5.10,
respectively). The identity of 5b was confirmed by 13C and 31P
NMR spectroscopy. Free cyclopentadiene was evident (yield,
49%) from the signals at δ 6.45, 6.28, and 2.67. In addition to
(Et3Si)2O (1.00, 0.57), unreacted silanol (δ 0.94, 0.49), unre-
acted starting material (10%, δ 4.59 and 4.53 resonances, 4:7),
and Cp(NO)(PPh3)ReCO2SiEt3 (4)12 were found (19%), δ 4.85
(Cp), 1.13 (dd, J ) 8.5, 7.3 SiCH2CH3), 0.81 (dq, J ) 15.1, 7.3,
SiCHRHâ), 0.72 (dq, J ) 15.1, 8.5, SiCHRHâ). After 41 h, all of
the starting 2 and 4 had been replaced by free cyclopentadiene
(63%) and 5b (74%) as a 51/28 ratio of diastereoisomers. IR
(THF): 1971 (CO), 1694, 1685 (NO), 1290, 1249 (OCO). 1H
NMR (C6D6): δ 7.70-6.80 (m, Ph), 5.13 (s, Cp) (major diast),
1.22 (t, J ) 7.8, CH3), 0.84 (q, J ) 7.8, CH2), 5.10 (s, Cp) (minor
diast), 1.21 (t, J ) 7.8, CH3), 0.86 (q, J ) 7.8, CH2). 13C{1H}
NMR: δ 136.20-130.42 (m, Ph), 93.29 (C5H5) (major diast),
8.25 (CH3), 8.03 (CH2), 93.09 (C5H5) (minor diast), 8.37 (CH3),
8.15 (CH2). 31P{1H} NMR: δ 22.10, 15.06 (major); 20.41, 17.47
(minor). 29Si{1H} NMR: δ 7.60.
R ea ct ion of Cp (NO)(P P h 3)R eCO+BF 4 a n d Cp (NO)-
-
(P P h 3)ReCO2-K+ (1b). To a yellow suspension of Cp(NO)-
(PPh3)ReCO2H16 (105 mg, 0.179 mmol) in 10 mL of THF was
transferred by cannula a white slurry of KH (100 mg, 2.49
mmol) in 3 mL of THF. Within 30 min of stirring at room
temperature, this slurry afforded a yellow-orange supernatant
solution of 1bK+ (identified by IR spectroscopy: ν(NO) 1622
(br) cm-1, ν(OCO) 1474 (br), 1240 cm-1)9 with noticeable gas
evolution (presumably hydrogen). This suspension was trans-
ferred by a cannula to a Schlenk filter (the frit was covered
with Celite) and was filtered with nitrogen pressure into a
(18) In addition to deprotonating metallocarboxylic acids, the other
common synthetic route to anionic metallocarboxylates entails car-
boxylation of metallocarboxylates.1 Thus, treatment of Cp(CO)2FeM
(M ) Li, Na, K) [which is isolobal to Cp(PPh3)(NO)ReM] with CO2
under comparable conditions cleanly generates the extensively studied
η1-C CO2 adduct Cp(CO)2FeCO2-. Pinkes, J . R.; Masi, C. J .; Chiulli,
R.; Steffey, B. D.; Cutler, A. R. Inorg. Chem. 1997, 36, 70, and
references therein.
(19) (a) Merrifield, J . H.; Strouse, C. E.; Gladysz, J . A. Organome-
tallics 1982, 1, 1204. (b) Agbossou, F.; O’Connor, E. J .; Garner, C. M.;
Me´ndez, N. Q.; Ferna´ndez, J . M.; Patton, A. T.; Ramsden, J . A.;
Gladysz, J . A. Inorg. Synth. 1992, 29, 210.
Rea ction of Cp (NO)(P P h 3)ReCO2Re(η1-Cp )(CO)(NO)-
(P P h 3) (2) w ith CH3OH. To a C6D6 solution (600 mg)
containing 2 (3.7 µmol, quantified vs an anisole internal
standard) was added anhydrous and deoxygenated MeOH (0.4
µL, 9.9 µmol). NMR spectra of the yellow solution after 8 h

1744 Organometallics, Vol. 18, No. 9, 1999
Tetrick and Cutler
were consistent with the presence of 6% remaining 2 and an
82% yield of Cp(NO)(PPh3)ReCO2CH3 (6):18a 1H NMR δ 4.85
(Cp), 3.42 (Me); 13C{1H} NMR δ 91.89 (Cp), 49.15 (OMe).
Resu lts a n d Discu ssion
-
The rhenium η1-C CO2 adduct Cp(PPh3)(NO)ReCO2
(1Li+ or 1K+), originally prepared by deprotonating Cp-
(PPh3)(NO)ReCO2H with LiH or KH in THF at 25 °C,9
is among the more stable and easily handled metallo-
carboxylates.1 It was thus surprising that attempts to
generate 1Li+ by treating the well-known metalate Cp-
(PPh3)(NO)ReLi17 with excess dry CO2 between -78 °C
and room temperature failed.18 An immediate reaction
occurred at -78 °C as a red solution of Cp(PPh3)(NO)-
ReLi plus CO2 turned orange; no further changes in the
IR spectra were noted as the cold solutions warmed to
room temperature. We never detected 1Li+ in these
reaction mixtures, although a single, THF- and benzene-
soluble product 2 appeared to form that had three
dominant and equally intense absorptions: a ν(CO)
absorption at 1972 cm-1and two ν(NO) bands at 1718
These major and minor diastereomers of 2 varied from
2:1 to 1.7:1 mixtures (as judged from 1H, 13C, and 31P
NMR spectra), which proved to be invariant with time
and handling. The 31P NMR spectra of 2 accordingly
indicate the presence of two different PPh3 ligands: two
pairs of absorptions are evident, two signals for each
diastereomer.
1
Both the H and 13C NMR spectra of 2 are dominated
by the presence of absorptions for both η1- and η5-C5H5
ligands. Two narrow η5-C5H5 resonances come at 4.55
and 4.53 ppm (ca. 1:2 relative intensities) and the two
broadened η1-C5H5 singlets are at 6.32 and 6.28 ppm
(1:2). The 13C NMR spectra likewise exhibit two sets of
Cp resonances: two singlets at 92.72 and 92.61 ppm (1:2
intensities) and one broad absorption at 117 ppm. We
observed no major changes in the 1H NMR spectrum
(toluene-d8) upon cooling to -75 °C, which is consistent
with similar rapid fluxional rearrangement of the η1-
C5H5 group that recently has been reported for related
rhenium complexes Re(CO)3(PR3)2(η1-C5H5).21g Further
support for the presence of a η1-C5H5 ligand on 2
originates with the observation that its solvolysis with
a silanol (vide infra) also releases 1 equiv of free
and 1684 cm-1
.
Two other salient observations helped to further this
characterize this unusual reaction in which CO2 gets
incorporated as a neutral rhenium carbonyl (eq 3). First,
1
cyclopentadiene (identified by H and 13C NMR spec-
troscopy).
Since the µ-CO2 ligand on 2 was not detected in the
13C NMR spectrum, we resorted to nonspectroscopic
procedures for establishing its presence. Treatment
of 2 with Et3SiOH accordingly provided the Cp(PPh3)-
(NO)ReCO2Re(CO)(NO)(PPh3)(OSiEt3) (5a ) in 74%
NMR spectral yield (along with C5H6, 63%), eq 5. This
treating the benzene extract of the reaction mixture
with methanol in C6D6 quantitatively afforded the
known19 Cp(NO)(PPh3)ReCO2CH3 (6), as deduced from
its 1H NMR spectrum. This material was isolated in 65-
82% yields. Second, an insoluble white material that
also formed during the carboxylation of 1Li+ dissolved
in water and reprecipitated upon treatment with aque-
ous Pb(NO3)2. An IR spectrum of the resulting white
solid as a KBr pellet matched that of an authentic
sample of PbCO3.20
Two other reactions also gave 2 as the only organ-
orhenium product (eq 4). Brief exposure of 1K+ to CO2
(1 atm) generated 2, as did the reaction between 1K+
rhenium silanolate previously had been characterized;
an X-ray crystallographic structure determination of its
phenyldimethylsilanolate congener 5b established the
presence of a µ-[η1-C(Re1):η2-O,O′(Re2)] bridging car-
and the rhenium carbonyl Cp(NO)(PPh3)ReCO+BF4
.
-
From these reactions 2 was isolated as a stable light
orange solid after crystallization from CH2Cl2/pentane
at -20 °C. Isolated yields ranged from 46 to 75% for a
material that was characterized as a mono-CH2Cl2
solvate. Its NMR and IR spectral data are consistent
with the presence of two diastereomers, each having two
rhenium centers that retain a total of two Cp, two PPh3,
two NO, one terminal CO, and one µ2-η3 CO2 as ligands.
(21) (a) O’Connor, J . M.; Casey, C. P. Chem. Rev. 1987, 87, 307. (b)
Casey, C. P.; O’Connor, J . M.; J ones, W. D.; Haller, K. J . Organome-
tallics 1982, 2, 535. (c) Casey, C. P.; O’Connor, J . M.; Haller, K. J . J .
Am. Chem. Soc. 1985, 107, 1241. (d) Young, K. M.; Miller, T. M.;
Wrighton, M. S. J . Am. Chem. Soc. 1990, 112, 1529. (e) Hubbard, J .
L.; Kimball, K. L.; Burns, R. M.; Sum, V. Inorg. Chem. 1992, 31, 4224.
(f) Casey, C. P.; Widenhoefer, R. A.; O’Connor, J . M. J . Organomet.
Chem. 1992, 428, 99. (g) Dahlenburg, L.; Hillman, G.; Markus, E.; Moll,
M.; Knoch, F. J . Organomet. Chem. 1996, 525, 115.
(20) Miller, F. A.; Wilkins, C. H. Anal. Chem. 1952, 24, 1253.

Reactivity of Metallocarboxylates toward CO2
Organometallics, Vol. 18, No. 9, 1999 1745
Sch em e 2
Sch em e 3
Sch em e 4
boxylate,12 as well as the fac-stereochemistry of the
O-ligands depicted for the carboxylate-chelated rhenium
center.
tively) as candidates for the νOCO(asym) assignment, since
absorptions with identical energies and relative band
shapes are found for Cp(PPh3)(NO)ReH and Cp(PPh3)-
(NO)ReCH3. Previously, we had assigned the medium
to strong intensity νOCO(sym) and νOCO(asym) bands for 5a
(in THF) at 1290 and 1249 cm-1 and those for 5b at
1287 and 1248 cm-1; both molecules likewise exist as
1:1 to 1:2 diastereomeric mixtures.12 It therefore seems
plausible that the νOCO bands for 2 fall within the 1300-
1200 cm-1 region. Although such an assignment would
lead to a ∆ν ) {[νOCO(asym)] - [νOCO(sym)]} < 100 cm-1, a
corresponding value of 80 cm-1 was measured for
Cp(CO)2RuCO2ZrClCp2 (νOCO(asym) 1349, νOCO(sym) 1292
cm-1), and these assignments followed from the results
of a labeling study after incorporating 13CO2.15a,22
The IR spectra of 2 in THF, C6H6, and CH2Cl2
solutions or as a KBr pressed pellet exhibit comparable
ν(CO) and ν(NO) absorptions, but differ somewhat in
the appearance of the ν(OCO) region. Although we find
what appears to be the νOCO(sym) band at 1218 cm-1 in
THF (or C6H6 and CH2Cl2) solution, this peak corre-
sponds to a local maximum on a broad envelope encom-
passing 1300-1200 cm-1. The IR spectrum as a KBr
pellet clearly shows five medium to strong intensity
bands in this region: 1288, 1263, 1236, 1217, and 1187
cm-1. We rule out either of the 1482 and 1436 cm-1
bands (weak and medium relative intensities, respec-

1746 Organometallics, Vol. 18, No. 9, 1999
Tetrick and Cutler
A plausible pathway for transforming the rhenium
volysis of the η1-Cp. Prior ionization of the Cp- ligand,21c
however, is unlikely since methanolysis of 2 under these
conditions also would have given cyclopentadiene. In
Scheme 4 we speculate on a mechanism in which both
Et3SiOH and methanol initially solvolyze the carboxy-
late ligand on 2 and provide their respective rhenium
esters 4 or 6 plus the hypothesized η1-Cp hydroxyrhe-
nium intermediate 9. This latter intermediate rear-
ranges via a an η1 f η5 ring shift to give the rhenium
acid, which either undergoes methanolysis to yield the
second equivalent of 6 or (with silanol) reacts with 4 to
generate 5. Both reactions of the rhenium acid have
been established independently.12
-
(η1-C) carboxylate Cp(PPh3)(NO)ReCO2 (1Li+ or 1K+)
plus excess CO2 to 2 appears in Scheme 2. The pre-
sumed C2O4 adduct 74 incorporates a second equivalent
of CO2 needed for the observed reductive disproportion-
ation that releases carbonate. Although the anticipated
Cp(NO)(PPh3)ReCO+ byproduct was not detected, we
independently established that treatment of it (as the
BF4- salt, eq 4) with 1K+ provided 2. The transience of
another postulated intermediate, the metalloanhydride
8, followed by an η5-η1 Cp ring shift21 commensurate
with O,O′-chelation of the rhenium carboxylate and
deinsertion of the rhenium carbonyl group accounts for
2. Irreversible Cp ligand slippage21a has been docu-
mented for other Re(I) compounds as a means of opening
up coordination sites.21b-g
Con clu sion s
Reductive disproportionation of ligated CO2 is re-
garded as a deleterious process that irreversibly de-
stroys the CO2 complexation in favor of forming CO and
carbonate, either of which may remain coordinated.1,2
In the present study the otherwise stable rhenium η1-C
carboxylate Cp(PPh3)(NO)ReCO2- (1Li+ or 1K+)9 readily
incorporated additional CO2, but its ensuing reductive
disproportionation generated a new CO2 complex in
addition to CO32-. This new CO2 complex, Cp(PPh3)-
(NO)ReCO2Re(CO)(NO)(PPh3)(η1-Cp) (2), retains a Re2-
(µ2-η3 CO2) motif and ligated CO. It evidently originates
via an initial reductive disproportionation of Cp(PPh3)-
The metalloanhydride 8 represents a viable interme-
diate in the three procedures that we used to generate
2, even though it was not detected by IR spectral
monitoring. Transience of other η2-C,C′ metalloanhy-
dride intermediates involving a single metal center Lx-
MC(O)OC(O)23 (as opposed to a bridging metalloan-
hydride),23c although precedented, apparently is not
germane to the present work and to the analogous
degradation of the silyl esters Cp(PPh3)(NO)ReCO2-
SiR3 (4).
(NO)ReCO2 (1) plus CO2 to Cp(PPh3)(NO)ReCO+ and
-
In a previous study we had postulated the interme-
diacy of both the metalloanhydride 8 and 2 during the
degradation of the silyl esters 4 to 5 (Scheme 3).12
Hydrolysis of 4 evidently yields the rhenium acid Cp-
(PPh3)(NO)ReCO2H, which then interacts with ad-
ditional 4 to give 8 and releases another equivalent of
silanol. Rearrangement of 8 to 2 followed by silanolysis
of the Re-η1-Cp yields the observed products. Indeed,
treatment of Cp(NO)(PPh3)ReCO+BF4- with 1 equiv of
1K+ followed by addition of silanol R3SiOH conveniently
gave 5 (even though 2 had not been detected).12 In the
present study, we found that treating the isolated and
fully characterized 2 with Et3SiOH delivered 5a (eq 5).
free carbonate (Scheme 2); the rhenium carbonyl, how-
ever, subsequently intercepts 1 to generate the stable
bimetallic CO2 complex 2. The overall stoichiometry (eq
3) requires 3 equiv of CO2 plus 2 equiv of the electron-
rich metalate [i.e., Cp(PPh3)(NO)ReLi], transforming to
-
ligated CO2 and CO (i.e., 2) plus CO3
.
Three procedures were used to generate 2: treating
(a) Cp(PPh3)(NO)ReLi with excess CO2, (b) 1K+ with
excess CO2, and (c) 1K+ with Cp(PPh3)(NO)Re(CO)BF4.
Results of studying these reactions are consistent with
the intermediacy of an undetected metalloanhydride Cp-
(PPh3)(NO)ReC(O)OC(O)Re(NO)(PPh3)Cp (8), which upon
subsequent η5-η1 Cp ring shift on one rhenium center
affords 2. This product undergoes different solvolysis
reactions in the presence of methanol and triethylsil-
anol. The former gives 2 equiv of the methylester Cp-
(NO)(PPh3)ReCO2CH3 (6), whereas the latter replaces
the η1-Cp group with a silanolate, to give Cp(PPh3)(NO)-
ReCO2Re(CO)(NO)(PPh3)(OSiEt3) (5a ).
The intriguing solvolysis reactivity of 2 toward metha-
nol and silanol (eq 5) warrants comment. Methanolysis
of 2 surprisingly retains both Cp ligands and generates
2 equiv of 6, whereas Et3SiOH transforms 2 to the
dirhenium silanolate 5 plus free cyclopentadiene. Since
silanols are more acidic than alcohols,24 Et3SiOH could
preferentially abstract cyclopentadiene from 2 via sol-
Ack n ow led gm en t. Support from the Department
of Energy, Office of Basic Energy Science, and from the
National Science Foundation, Grant CHE 9412837, is
gratefully acknowledged.
(22) (a) Steffey, B. D.; Vites, J . C.; Cutler, A. R. Organometallics
1991, 10, 3432. (b) Analogous assignments for Cp(CO)2FeCO2ZrClCp2
[νOCO(asym) 1363, νOCO(sym) 1283 cm-1], ref 15b.
(23) Considerably more evidence is available for transience of η2-
C,C′ metalloanhydride intermediates involving a single metal center,
LxMC(O)OC(O),18,23a,b as opposed to those involving a bridging metal-
loanhydride ligand.23c (a) Lee, G. R.; Cooper, N. J . Organometallics
1985, 4, 794. (b) Cutler, A. R.; Hanna, P. K.; Vites, J . C. Chem. Rev.
1988, 88, 1363. (c) Lee, G. R.; Cooper, N. J . Organometallics 1985, 4,
1467.
OM980642J
(24) (a) Bassindale, A. R.; Taylor, P. G. In The Chemistry of
Organosilicon Compounds; Patai, S., Rappoport, Eds.; Wiley: New
York, 1989; Vol. 1, Chapter 12, p 809. (b) Lickiss, P. D. Adv. Inorg.
Chem. 1995, 42, 147.