Synthesis and characterization of new alkyl-carbon dioxide compounds and the first neutral acyl-carbonyl niobocene complexes

Article abstract of DOI:10.1021/om9702643

The olefin-hydride complexes Cp′₂Nb(H)(η²-RHC=CH₂) (Cp′ = η⁵-C₅H₄SiMe₃; R = H (3), C₆H₅ (4, endo isomer)) were prepared by the alkylation reactions of [Cp′₂NbCl]₂ (1) with the appropriate Grignard reagents RMgX (R = CH₂CH₃, CH₂CH₂C₆H₅) followed by a stereoselective β-elimination from the intermediate alkyl complexes Cp′₂Nb(CH₂CH₂R). Complexes Cp′₂Nb(H)(η²-RHC=CH₂) (R = C₆H₄CH₃ (5), C₆H₄OCH₃ (6)) were prepared as a mixture of endo and exo isomers by reaction of Cp′₂NbH₃ (2) with the corresponding olefin. Furthermore, reactions of CO with 3 and 4, and the reactions of CO₂ with 3-6, afforded the alkylniobocene complexes Cp′₂Nb(CO)R (R = CH₂CH₃ (7), CH₂CH₂C₆H₅ (8)) Cp′₂Nb(η²-CO₂)(R) (R = CH₂-CH₃ (9)), CH₂CH₂C₆H₅ (10), CH₂CH₂C₆H₄CH₃ (11), CH₂CH₂C₆H₄OCH₃ (12)). The reactivity of 9 and 10 toward the strong Lewis acid B(C₆F₅)₃ was also studied; in a first step the adducts Cp′₂Nb(η²-CO₂-B(C₆F ₅)₃)(CH₂CH₂R) (R = H (13), C₆H₅ (14)) were formed and subsequently evolved to give the oxo - alkyl complexes Cp′₂Nb(O-B(C₆F₅)₃)(CH ₂CH₂R) (R = H (15), C₆H₅ (16)) with the loss of CO. Finally, reactions of 3 and 4 with CO under appropriate conditions gave the neutral acylniobocene complexes Cp′₂Nb(CO)(η¹-C(O)CH₂CH₂R) (R = H (17), C₆H₅ (18)), which were alternatively prepared from the reactions of 7 and 8 with CO. The different complexes were characterized by spectroscopic methods.

Full text of DOI:10.1021/om9702643

Organometallics 1997, 16, 4161-4166
4161
Syn th esis a n d Ch a r a cter iza tion of New Alk yl-Ca r bon
Dioxid e Com p ou n d s a n d th e F ir st Neu tr a l
Acyl-Ca r bon yl Niobocen e Com p lexes
Antonio Antin˜olo, Fernando Carrillo-Hermosilla, Isabel del Hierro, and
Antonio Otero*
Departamento de Quı´mica Inorga´nica, Orga´nica y Bioqu´ımica, Universidad de Castilla-La
Mancha, Campus Universitario, 13071-Ciudad Real, Spain
Mariano Fajardo
Departamento de Quı´mica Inorga´nica, Universidad de Alcala´, Campus Universitario,
28371-Alcala´ de Henares, Spain
Yves Mugnier
Laboratoire de Synthe`se et d’Electrosynthe`se Organome´talliques, associe´ au CNRS (URA
1685), Faculte´ des Sciences, 6 Boulevard Gabriel, 21000 Dijon, France
Received March 28, 1997X
The olefin-hydride complexes Cp′2Nb(H)(η2-RHCdCH2) (Cp′ ) η5-C5H4SiMe3; R ) H (3),
C6H5 (4, endo isomer)) were prepared by the alkylation reactions of [Cp′2NbCl]2 (1) with the
appropriate Grignard reagents RMgX (R ) CH2CH3, CH2CH2C6H5) followed by a stereose-
lective â-elimination from the intermediate alkyl complexes Cp′2Nb(CH2CH2R). Complexes
Cp′2Nb(H)(η2-RHCdCH2) (R ) C6H4CH3 (5), C6H4OCH3 (6)) were prepared as a mixture of
endo and exo isomers by reaction of Cp′2NbH3 (2) with the corresponding olefin. Furthermore,
reactions of CO with 3 and 4, and the reactions of CO2 with 3-6, afforded the alkylniobocene
complexes Cp′2Nb(CO)R (R ) CH2CH3 (7), CH2CH2C6H5 (8)) Cp′2Nb(η2-CO2)(R) (R ) CH2-
CH3 (9)), CH2CH2C6H5 (10), CH2CH2C6H4CH3 (11), CH2CH2C6H4OCH3 (12)). The reactivity
of 9 and 10 toward the strong Lewis acid B(C6F5)3 was also studied; in a first step the adducts
Cp′2Nb(η2-CO2-B(C6F5)3)(CH2CH2R) (R ) H (13), C6H5 (14)) were formed and subsequently
evolved to give the oxo-alkyl complexes Cp′2Nb(O-B(C6F5)3)(CH2CH2R) (R ) H (15), C6H5
(16)) with the loss of CO. Finally, reactions of 3 and 4 with CO under appropriate conditions
gave the neutral acylniobocene complexes Cp′2Nb(CO)(η1-C(O)CH2CH2R) (R ) H (17), C6H5
(18)), which were alternatively prepared from the reactions of 7 and 8 with CO. The different
complexes were characterized by spectroscopic methods.
In tr od u ction
of H2 followed by olefinic coordination of the Cp2MH
species. In addition, elegant and extensive work of
Bercaw, Green, and co-workers have described NMR
studies of the dynamic processes in the olefin-hydride
niobocene and tantalocene complexes.3d,e,g Further-
more, the metal-promoted transformation of carbon
dioxide into organic products of practical interest is an
attractive goal in the field of organometallic chemistry.4
In this context, there have been recent reports5 on the
Olefin-hydride metal systems, including those cor-
responding to group 5 metals, are of great interest in a
host of catalytic processes. Particularly, insertion of
olefins into metal-hydride bonds and its microscopic
reverse, â-elimination, are very common steps in indus-
trial catalytic reactions, such as olefin polymerization,
hydrogenation, and hydroformylation.1
After the preparation of the first olefin-hydride bis-
(cyclopentadienyl) complex of group 5 metals, Cp2Nb-
(H)(η2-C2H4) (Cp ) η5-C5H5), by Tebbe and Parshall,2
several olefin-hydride complexes of Nb and Ta were
synthesized by employing two alternative processes,
namely the reaction of Cp2MCl2 and Cp2MH3 (M ) Nb,
Ta) with various alkylmagnesium halides and olefins,
respectively.3 The first process takes place via a ste-
reospecific â-H elimination from the monoalkyl com-
plexes Cp2MR and the second by thermal elimination
(3) (a) Klazinga, A. H.; Teuben, J . H. J . Organomet. Chem. 1978,
157, 413. (b) Eichiner, M. E.; Att, H. G.; Rausch, M. D. J . Organomet.
Chem. 1984, 264, 309. (c) Klazinga, A. H.; Teuben, J . H. J . Organomet.
Chem. 1980, 194, 309. (d) Doherty, N. M.; Bercaw, J . E. J . Am. Chem.
Soc. 1985, 107, 2670. (e) Burger, B. J .; Santarsiero, B. D.; Trimmer,
M. S.; Bercaw, J . E. J . Am. Chem. Soc. 1988, 110, 3134. (f) Bercaw, J .
E.; Burger, B. J .; Green, M. L. H.; Santarsiero, B. D.; Sella, A.;
Trimmer, M. S.; Wong, L.-L. J . Chem. Soc., Chem. Commun. 1989,
734. (g) Green, M. L. H.; Sella, A.; Wong, L.-K. Organometallics 1992,
11, 2650. (h) Antin˜olo, A.; Carrillo, F.; Garcia-Yuste, S.; Otero, A.
Organometallics 1994, 13, 2761. (i) Antin˜olo, A.; Chaudret, B.; Com-
menges, G.; Fajardo, M.; J alo´n, F.; Morris, R. H.; Otero, A.; Schweitzer,
C. T. J . Chem. Soc., Chem. Commun. 1988, 1210.
(4) See for example the following reviews: (a) Behr, A. Angew.
Chem., Int. Ed. Engl. 1988, 27, 661. (b) Braunstein, P.; Matt, D.; Nobel,
D. Chem. Rev. 1988, 88, 747. (c) Walther, D. Coord. Chem. Rev. 1987,
79, 135.
(5) (a) Fu, P.-F.; Masood, A. K.; Nicholas, K. M. J . Am. Chem. Soc.
1992, 114, 6579. (b) Fu, P.; Khan, M. A.; Nicholas, K. M. J . Organomet.
Chem. 1996, 506, 49.
X Abstract published in Advance ACS Abstracts, August 15, 1997.
(1) (a) Parshall, G. A. Homogeneous Catalysis; Wiley: New York,
1980. (b) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry, 2nd
ed.; University Science Books: Mill Valley, CA, 1987.
(2) Tebbe, F. N.; Parshall, G. W. J . Am. Chem. Soc. 1971, 93, 3794.
S0276-7333(97)00264-1 CCC: $14.00 © 1997 American Chemical Society

4162 Organometallics, Vol. 16, No. 19, 1997
Antin˜olo et al.
isolation and reactivity of several alkyl-carbon dioxide
niobocene complexes obtained by oxidation of the cor-
responding alkyl-carbonyl species with molecular oxy-
gen and on the chemical and electrochemical reduction
of the niobocene dichlorides in the presence of carbon
dioxide.6 Following on from studies on olefin-hydride3h
and heterocumulene complexes,7 we report here the
outcome of recent studies on the preparation of new
olefin-hydride niobocene complexes by alkylation of the
[Cp′2NbCl]2 species and the subsequent â-elimination
process and by reaction of Cp′2NbH3 with the corre-
sponding olefin, the coordination of carbon dioxide on
alkylniobocene species to give the corresponding carbon
dioxide alkylniobocenes, and the first well-established
insertion process of carbon monoxide into a niobium-
carbon bond of a niobocene complex to give the corre-
sponding acyl derivatives.
Cp′2NbH3 + R′CHdCH2 f
Cp′2Nb(H)(CH2dCHR′) (endo + exo) (2)
R′ ) C6H5 (4), C6H4CH3 (5), C6H4OCH3 (6)
sponding alkylniobocene complexes Cp′2Nb(CO)(R) (R
) CH2CH3 (7), CH2CH2C6H5 (8)) and Cp′2Nb(η2-CO2)(R)
(R ) CH2CH3 (9), CH2CH2C6H5 (10), CH2CH2C6H4CH3
(11), CH2CH2C6H4OCH3 (12) (eq 3).
Cp′2Nb(H)(CH2dCHR′) + L f Cp′2Nb(L)(R) (3)
3-6
L ) CO, R′ ) H (3), C6H5 (4); R ) CH2CH3 (7),
CH2CH2C6H5 (8)
7-12
L ) CO2, R′ ) H (3), C6H5 (4), C6H4CH3 (5),
C6H4OCH3 (6); R ) CH2CH3 (9), CH2CH2C6H5 (10),
CH2CH2C6H4CH3 (11), CH2CH2C6H4OCH3 (12)
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion of Olefin - a n d
Alk yl-Con ta in in g Niobocen e Com p lexes. Aspects
of the reactivity of the chloroniobocene complex [Cp′2-
NbCl]2 (1)8 suggest it to be ideal for studying its
behavior with alkylating agents, such as Grignard
compounds, and thus extending the knowledge of the
alkylation processes in haloniobocene species.
The reaction with a mixture of endo and exo styrene-
hydride isomers gave complexes 8 and 10-12 only.
Transformation of the exo to the endo isomers to give
the corresponding alkyl complexes probably occurs, as
the primary alkyl-containing complexes 8 and 10-12
would be the expected kinetic products on the basis of
steric arguments. We have previously observed similar
results in the reaction of olefin-hydride niobocene
complexes with carbon disulfide.9 Alkyl-carbonyl niob-
ocene complexes were previously prepared by alkylation
of halo-carbonyl complexes,10 while Nicholas et al.,5a
in an elegant study, reported (as noted above) for the
first time the isolation of several alkyl-carbon dioxide
niobocene complexes. However, this is one of the first
reports on group 5 metallocene-promoted coordination
of CO2 and it is noteworthy that the process proceeds
in acceptable yield (ca. 60%) and under mild conditions
(3 and 4 atm of CO2 are required to isolate 9 and 10-
12, respectively; see Experimental Section). Lappert
and co-workers reported11 for the first time the prepara-
tion of Cp′2Nb(η2-CO2)(CH2SiMe3) (Cp′ ) η5-C5H4Me) by
the reaction of Cp′2Nb(Cl)(CH2SiMe3) with sodium
amalgam under an atmosphere of carbon dioxide, and
several related complexes were recently prepared by
Nicholas and co-workers5b in a similar way in modest
yield (ca. 20%).
In fact, the starting niobium complex [Cp′2NbCl]2 (1;8a
Cp′ ) η5-C5H4SiMe3) reacts with 1 equiv of RMgX (R )
Et, CH2CH2C6H5) to generate at room temperature in
high yields the corresponding olefin-hydride complexes
Cp′2Nb(H)(η2-RHCdCH2) (R ) H (3) C6H5 (4, endo
isomer)) by a highly stereoselective â-elimination from
the initially formed monoalkyl species Cp′2Nb(CH2-
CH2R), which have not been detected (eq 1).
[Cp′2NbCl]2 + RMgX f [Cp′2NbR] f
Cp′2Nb(H)(CH2dCHR′) (1)
3,4
R ) CH2CH3, CH2CH2C6H5
R′ ) H (3), C6H5 (4)
This synthetic method, which was initially described
several years ago by Teuben,3a,c has subsequently been
employed by others.3d,e,g,h In addition, styrene hydride
complexes, Cp′2Nb(H)(η2-RHCdCH2) (R ) C6H5 (4),3h
C6H4CH3 (5), C6H4OCH3 (6)) were also obtained, as a
mixture of endo and exo isomers, by reaction of the
Spectral characterizations of complexes 3-12 are as
follows. The IR spectra for 3 and 6 show a weak broad
band at ca. 1740 cm-1, assigned to ν(Nb-H), and for
complexes 7, 8 and 9-12, intense bands at ca. 1896 and
1697 cm-1 are seen, which correspond respectively to
ν(CO) of coordinated CO and CO2. The 1H NMR spectra
of 3-6 show the hydride resonance at δ ca. -3 ppm.
The 13C NMR spectra of 7-12 show diagnostic reso-
nances of coordinated CO and CO2 at δ 270 and ca. 200
ppm, respectively (see Experimental Section).
3i
niobocene trihydride complex Cp′2NbH3 (2) and the
corresponding olefin (eq 2).
Furthermore, the reactions of complexes 3 and 4 with
CO and of 3-6 with CO2 favored the insertion process
to give the alkyl tautomer with isolation of the corre-
(6) El Krami, S.; Mourad, Y.; Mugnier, Y.; Antin˜olo, A.; Del Hierro,
I.; Garc´ıa-Yuste, S.; Otero, A.; Fajardo, M.; Brunner, H.; Gehart, G.;
Wachter, J .; Amaudrut, J . J . Organomet. Chem. 1995, 498, 165.
(7) (a) Antin˜olo, A.; Fajardo, M.; Lo´pez-Mardomingo, C.; Otero, A.;
Mourad, Y.; Mugnier, Y.; Sanz-Aparicio, J .; Fonseca, I.; Florencio, F.
Organometallics 1990, 9, 164. (b) Antin˜olo, A.; Fajardo, M.; Garc´ıa-
Yuste, S.; Del Hierro, I.; Otero, A.; El Krami, S.; Mourad, Y.; Mugnier,
Y. J . Chem. Soc., Dalton Trans. 1995, 3409.
(8) See, for example: (a) Antin˜olo, A.; Garc´ıa-Lledo´, S.; Mart´ınez
de Ilarduya, J . M.; Otero, A. J . Organomet. Chem. 1987, 335, 85. (b)
Antin˜olo, A.; Espinosa, P.; Fajardo, M.; Go´mez-Sal, P.; Lo´pez-Mardo-
mingo, C.; Mart´ın-Alonso, A.; Otero, A. J . Chem. Soc., Dalton Trans.
1995, 1007.
Especially noteworthy is the 1H NMR spectral pattern
of the ethyl group in 9, as illustrated in Figure 1,
because it displays an ABM3 pattern, indicating the
(9) Antin˜olo, A.; del Hierro, I.; Fajardo, M.; Garcia-Yuste, S.; Otero,
A.; Blaque, O.; Kubicki, M. M.; Amaudrut, J . J . Organometallics 1996,
15, 1966.
(10) Antin˜olo, A.; Gomez-Sal, P.; Martinez de Ilarduya, J . M.; Otero,
A.; Royo, P. J . Chem. Soc., Dalton Trans. 1988, 2865.
(11) Bristow, G. S.; Hitchcock, P. B.; Lappert, M. F. J . Chem. Soc.,
Chem. Commun. 1981, 1145.

New Alkyl-Carbon Dioxide Compounds
Organometallics, Vol. 16, No. 19, 1997 4163
Sch em e 1
adducts Cp′2Nb(η2-CO2-B(C6F5)3)(CH2CH2R) (R ) H
(13), C6H5 (14)), which subsequently evolved to the oxo-
alkyl complexes Cp′2Nb(O-B(C6F5)3)(CH2CH2R) (R ) H
(15), C6H5 (16); see Scheme 1), with the loss of CO.
Nicholas and co-workers16 have studied the behavior
of an analogous complex, Cp′2Nb(η2-CO2)(CH2SiMe3)
(Cp′ ) η5-C5H4Me), toward different Lewis acids, namely
LiPF6, BF3‚OEt2, and ZnCl2, and they have found that
initial coordination of the Lewis acid to the niobocene
moiety, through the CO2 ligand, was followed by a
decarbonylation instead of a migratory insertion promo-
tion into the Nb-C bond. Complexes 13 and 14 are very
unstable toward the loss of CO and were isolated from
the reaction mixture by evaporation of the solvent after
a brief reaction time (see Experimental Section). The
formation of these adducts can be considered the result
of the interaction of the Lewis acid B(C6F5)3 with the
noncoordinated oxygen atom of the CO2 ligand. Their
IR spectra exhibited an absorption band at ca. 1570
cm-1 which corresponds to ν(CO2), which is significantly
lower than the value for ν(CO2) found in the starting
complexes, indicating an appreciable reduction of elec-
tron density in the CdO bond as a result of the
coordination of the Lewis acid. Complexes 13 and 14
were found to be unstable in solution, gradually decom-
posing to the new complexes 15 and 16 with loss of CO,
where the B(C6F5)3 molecule is coordinated to the oxo
ligand, through a Nb-O-B linkage. These compounds
can alternatively be prepared by the direct reaction of
the oxo complexes Cp′2Nb(O)(R) (R ) CH2CH3, CH2-
CH2C6H5) with B(C6F5)3. The IR spectra of the decar-
bonylated products 15 and 16 exhibit ν(NbdO) at ca.
839 cm-1, which is slightly shifted with regard to the
oxo complexes (ν(NbdO) at ca. 875 cm-1), indicating
that the electron density around the NbdO bond is
reduced through the coordination of the Lewis acid
B(C6F5)3. The formation of adducts such as 13 and 14,
F igu r e 1. 1H NMR spectra of [Cp′2Nb(η2-CO2)(CH2CH3)]
in the ethyl region: (a) experimental; (b) simulated.
presence of an unusually weak agostic interaction
between an R-hydrogen atom of the ethyl group and the
metal center (J MB ) 7.0 Hz, J MA ) 7.5 Hz, J AB ) 18
Hz).12 This proposal, however, should be considered
cautiously, because in the 13C NMR spectrum we have
1
not been able to distinguish the two J (13C-1H) con-
stants corresponding to the CH2 group.
Reactivity of η2-CO2 Com plexes with Lewis Acids.
In ser tion P r ocesses. Complexes 9 and 10 react
instantaneously with B(C6F5)3 to initially give the
(12) A similar weak R-interaction was described for the 18-electron
Cp2Nb(CH3CtCCH3)(CH2CH3) complex: Yasuda, H.; Yamamoto, H.;
Arai, T.; Nakamura, A.; Chen, J .; Kai, Y.; Kasai, N. Organometallics
1991, 10, 4058. Other examples of group 5 alkyl complexes with
13
R-agostic interactions are (C5tBu5)TaCl2(CH2tBu)(CH2CMe2CH2)2
(C5H5)Nb(N-2,6-C6H3 Pr2)(CH2tBu)2,14 and Tp′NbCl(CH2CH3)(RCtCR′).15
i
(13) van der Heidjen, H.; Gal, A. W.; Pasman, P.; Orpen, A. G.
Organometallics 1985, 4, 1847.
(14) Poole, A. D.; Williams, D. N.; Kenwright, A. M.; Gibson, V. C.;
Clegg, W.; Hockless, D. C. R.; O’Neil, P. A. Organometallics 1993, 12,
2549.
(16) Fu, P.-F.; Khan, M. A.; Nicholas, K. M. Organometallics 1992,
11, 2607.
(15) Etienne, M. Organometallics 1994, 13, 410.

4164 Organometallics, Vol. 16, No. 19, 1997
Antin˜olo et al.
where the CdO band of the coordinated CO2 is polarized
and the electron density around the metal center is
reduced, opens up new prospects for reactivity studies
on these molecules with, for example, unsaturated
compounds. 1H and 13C NMR spectra of 13 and 14 show
little differences from the corresponding spectra of 9 and
10, although all attempts to detect the resonances of
the coordinated CO2 were unsuccessful. 19F NMR
spectra for complexes 13-16 (see Experimental Section)
have confirmed the presence of the B(C6F5)3 group
bonded to the niobocene moieties.
Finally, in an effort to perform a more in-depth study
on the reactivity of the olefin-hydride complexes 3 and
4 toward carbon monoxide, toluene solutions of these
complexes were refluxed under pressure of CO and led
to the isolation of acylniobocene species Cp′2Nb(CO)-
(η1-C(O)CH2CH3) (17) and Cp′2Nb(CO)(η1-C(O)CH2-
CH2C6H5) (18) after appropriate workup (eq 4).
electron configuration for a Nb(III) ion.21 This repre-
sents an unusual situation, because η2-acyl coordination
is very commonly encountered among the early transi-
tion metals.22 In addition, the IR carbonyl CtO stretches
for complexes 17 and 18 appear at 1895 and 1897 cm-1
,
respectively, similar to the values found for 7 and 8.
The 13C NMR spectra provide additional information.
The acyl carbon resonates downfield at 278 and 277 ppm
for 17 and 18, thus appearing at higher field than in
the cationic acylniobocenes (ca. 285 ppm). Finally, the
carbon atoms of the carbonyl ligands resonate at 263
and 262 ppm, respectively (see Experimental Section).
In conclusion, we have described in this paper the
activation of carbon dioxide by alkylniobocenes to give
stable carbon dioxide complexes and established ex-
amples of migratory insertion of alkyl-carbonyl niob-
ocene complexes to give the first characterized neutral
acylniobocene complexes
Cp′2Nb(H)(CH2dCHR′) + CO 9
∆8
Exp er im en ta l Section
Cp′2Nb(CO)(η1-C(O)CH2CH2R′) (4)
17, 18
All reactions were performed using standard Schlenk-tube
techniques under an atmosphere of dry nitrogen. Different
styrenes, C6H5CH2CH2MgBr and CH3CH2MgCl, were pur-
chased from Aldrich and used as received. Solvents were
distilled from appropriate drying agents and degassed before
use. Microanalyses were carried out with a Perkin-Elmer 2400
CHN analyzer. Infrared spectra were obtained in the region
200-4000 cm-1 using a Perkin-Elmer 883 spectrophotometer.
1H, 13C, and 19F NMR spectra were recorded on Varian-Unity
FT-300 and Gemini FT-200 spectrometers, and the chemical
shifts were determined by reference to the residual deuterated
solvent peaks.
R′ ) H (17), C6H5 (18)
The same complexes were obtained from the reaction
of 7 and 8 with CO under similar experimental condi-
tions. Furthermore, reactions of 17 and 18 with CF3-
COOH in acetone as solvent gave the corresponding
aldehydes and the niobocene trifluoroacetato complex
Cp′2Nb(CO)(η1-(OC(O)CF3)),17 in agreement with eq 5.
P r ep a r a tion of [(C5H4SiMe3)2Nb(H)(CH2dCH2)] (3). To
an Et2O (40 mL) solution of [(C5H4SiMe3)2NbCl]2 (490 mg, 1.36
mmol) was added an equimolar quantity of CH3CH2MgCl (2
M, Et2O) at 0 °C. The solution was stirred and warmed to
room temperature for 3 h. The solvent was removed in vacuo,
and the resulting solid was extracted with cold pentane (20
mL). Complex 3 was obtained as a microcrystalline yellow
solid (387 mg, 0.98 mmol, 72%) after concentration and cooling
of the solution.
3: 1H NMR (C6D6) δ (ppm) -3.20 (s, 1H, NbH), 0.24 (s, 18H,
SiMe3), 0.7 and 1.05, (4H, CH2dCH2), 3.44 (2H), 3.71 (2H),
4.99 (4H) (m, C5H4SiMe3); 13C{1H} NMR (C6D6) δ (ppm) 0.64
(SiMe3), 10.34 and 14.73 (CH2dCH2), 92.15, 93.67, 93.77, 99.82
(C5H4SiMe3). Anal. Calcd for C18H31Si2Nb: C, 54.50; H, 7.80.
Found: C, 54.40; H, 7.90. For complex 4 see ref 3h.
Although cyclopentadienylacylniobium and -tantalum
species arising from migratory insertion of alkyl-
carbonyl complexes have been claimed to participate as
intermediates in different processes,18 no neutral acyl-
metallocenes have been isolated or characterized. It is
noteworthy that Bercaw and co-workers have previously
reported19 the formation of similar neutral acyl-carbo-
nyl niobocene complexes but no spectroscopic details
were presented. Furthermore, several cationic acyl
complexes, Cp′2Nb+Cl(R1R2HCCdO), formed by the
protonation of niobocene ketene complexes, have been
described.20 Spectroscopic data are consistent with the
proposed formulas for complexes 17 and 18. The IR
spectra contain an acyl CdO stretch at 1599 cm-1, and
although this value is lower that reported for the
foregoing cationic acylniobocenes which contain an η2-
acyl (ca. 1620 cm-1), an η1-acyl coordination must be
proposed in our complexes in order to attain an 18-
P r ep a r a tion of [(C5H4SiMe3)2Nb(H)(CH2dCHR)] (R )
C6H4CH3 (5), C6H4OCH3 (6) (En d o + Exo Mixtu r es)). To
a solution of (C5H4SiMe3)2Nb(H)3 (300 mg, 0.81 mmol) in 20
mL of toluene was added by syringe a 1.62 mmol portion of
the corresponding styrene. The mixture was warmed to 65
°C and stirred for 3 h. Alternatively, the reaction can be
carried out at room temperature over 48 h. The resulting
solution was filtered and evaporated to dryness for several
hours. The yellow-brown oily residue was extracted with
diethyl ether (20 mL). Complexes 5 and 6 were isolated on
cooling as yellow crystalline needles (356 mg, 0.73 mmol, 90%
yield for 5; 365 mg, 0.73 mmol, 90% yield for 6).
5-exo: 1H NMR (C6D6) δ (ppm) -3.20 (s, 1H, NbH), 0.09
(s, 9H, SiMe3), 0.15 (s, 9H, SiMe3), 0.90 (dd, J AB ) 2 Hz, J AM
) 5 Hz, 1H, CHHdCHR), 1.40 (J BA ) 2 Hz, J BM ) 5 Hz, 1H,
CHHdCHR), 2.20 (s, 3H, CH3), 3.30 (J MA ) 5 Hz, J MB ) 5 Hz,
1H, CH2dCHR), 3.51 (1H), 3.61 (1H), 3.72 (2H), 4.61 (1H), 4.81
(1H), 5.02 (2H) (m, C5H4SiMe3), 7.00-7.40 (m, 4H, C6H4).
(17) J alo´n, F. A.; Otero, A.; Manzano, B. R.; Villasen˜or, E.; Chaudret,
B. J . Am. Chem. Soc. 1995, 117, 10123.
(18) (a) Otto, E. E. H.; Brintzinger, H. H. J . Organomet. Chem. 1979,
170, 209. (b) Wood, C. D.; Schrock, R. R. J . Am. Chem. Soc. 1979, 101,
5421. (c) Arnold, J .; Tilley, T. D.; Rheingold, A. L.; Geib, S. J .; Arif, A.
M. J . Am. Chem. Soc. 1989, 111, 149.
(19) Bell, R. A.; Cohen, S. A.; Doherty, N. M.; Threikel; R. S.; Bercaw,
J . E. Organometallics 1986, 5, 972.
(20) (a) Antin˜olo, A.; Otero, A.; Fajardo, M.; Lo´pez-Mardomingo, C.;
Mugnier, Y.; Lanfranchi, M.; Pellinghelli, M. A. J . Organomet. Chem.
1992, 435, 55. (b) Bruno, J . W.; Fermin, M. C.; Halfon, S. E.; Schulte,
G. K. J . Am. Chem. Soc. 1989, 111, 8738.
(21) Lahuer, J . W.; Hoffmann, R. J . Am. Chem. Soc. 1976, 1729.
(22) Durfee, L. D.; Rothwell, I. P. Chem. Rev. 1988, 88, 1059.

New Alkyl-Carbon Dioxide Compounds
Organometallics, Vol. 16, No. 19, 1997 4165
1
5-en d o: H NMR (C6D6) δ (ppm) -2.60 (s, 1H, NbH), 0.10
13C{1H} NMR (C6D6) δ (ppm) -0.88 (SiMe3), 23.48 (CH2CH3),
19.53 (CH2CH3), 117.37 (Cipso), 99.81, 101.33, 107.09, 119.47
(C5H4SiMe3), 198.24 (CO2). Anal. Calcd for C19H31Si2NbO2:
C, 51.80; H, 7.09. Found: C, 51.50; H, 6.90.
P r ep a r a t ion s of [(C5H 4SiMe3)2Nb (η2-CO2)(CH 2CH 2-
C6H4R)] (R ) H (10), CH3 (11), OCH3 (12)). A yellow hexane
solution (30 mL) of [(C5H4SiMe3)2Nb(H)(C6H5CHdCH2)] (a
mixture of isomers, exo:endo ratio 2:1; 800 mg, 1.67 mmol)
under a CO2 atmosphere (4 atm in a Fisher-Porter bottle) was
stirred for 48 h at room temperature, and a white precipitate
was formed. The solid was isolated by filtration, washed with
cold hexane, and then recrystallized from toluene to obtain
white needles of 10 (515 mg, 1 mmol, 60% yield).
(s, 9H, SiMe3), 0.12 (s, 9H, SiMe3), 1.30 (J AB ) 2 Hz, J AM ) 5
Hz, 1H, CHHdCHR), 1.60 (J BA ) 2 Hz, J BM ) 5 Hz, 1H,
CHHdCHR), 2.30 (s, 3H, CH3), 2.60 (J MA ) 5 Hz, J MB ) 5 Hz,
1H, CH2dCHR), 3.31 (1H), 3.51 (1H), 3.80 (2H), 4.21 (1H), 4.81
(1H), 5.11 (1H), 5.52 (1H) (m, C5H4SiMe3), 7.00-7.40 (m, 4H,
C6H4). Anal. Calcd for C25H37Si2Nb: C, 61.70; H, 7.66.
Found: C, 61.50; H, 7.63.
6-exo: 1H NMR (C6D6) δ (ppm) -3.20 (s, 1H, NbH), 0.10 (s,
9H, SiMe3), 0.20 (s, 9H, SiMe3), 0.90 (J AB ) 2 Hz, J AM ) 5 Hz,
1H, CHHdCHR), 1.30 (J BA ) 2 Hz, J BM ) 5 Hz, 1H,
CHHdCHR), 3.20 (s, 3H, CH3), 3.30 (J MA ) 5 Hz, J MB ) 5 Hz,
1H, CH2dCHR), 3.51 (1H), 3.70 (1H), 3.82 (1H), 3.90 (1H), 4.61
(1H), 4.81 (1H), 5.09 (2H) (m, C5H4SiMe3), 7.00-7.40 (m, 4H,
C6H4).
Complexes 11 (529 mg, 1 mmol, 60% yield) and 12 (515 mg,
1 mmol, 60% yield) were prepared in a similar way.
1
10: IR (KBr) 1697 cm-1 (νCO); 1H NMR (CDCl3) δ (ppm) 0.18
(s, 18H, SiMe3), 1.63 (m, 2H, CH2CH2Ph), 2.95 (m, 2H,
CH2CH2Ph), 5.32 (2H), 5.72 (2H), 5.86 (2H), 6.07 (2H) (m,
C5H4SiMe3), 7.2-7.4 (C6H5); 13C{1H} NMR (C6D6) δ (ppm)
-1.01 (SiMe3), 32.43 (CH2CH2Ph), 40.38 (CH2CH2Ph), 116.57
(Cipso), 101.58, 101.67, 108.55, 117.64 (C5H4SiMe3), 147.12
(Cipso), 125.06, 127.56, 128.21 (C6H5), 200.94 (CO2). Anal.
Calcd for C25H35Si2O2Nb: C, 58.12; H, 6.80. Found: C, 58.30;
H, 6.70.
6-en d o: H NMR (C6D6) δ (ppm) -2.60 (s, 1H, NbH), 0.14
(s, 9H, SiMe3), 0.15 (s, 9H, SiMe3), 1.40 (J AB ) 2 Hz, J AM ) 5
Hz, 1H, CHHdCHR), 1.50 (J BA ) 2 Hz, J BM ) 5 Hz, 1H,
CHHdCHR), 3.40 (s, 3H, CH3), 2.60 (J MA ) 5 Hz, J MB ) 5 Hz,
1H, CH2dCHR), 3.31 (1H), 3.41 (1H), 3.50 (1H), 4.31 (2H), 5.01
(1H), 5.52 (2H) (m, C5H4SiMe3), 7.00-7.40 (m, 4H, C6H4).
Anal. Calcd for C25H37Si2ONb: C, 59.74; H, 7.42. Found: C,
59.65; H, 7.35.
P r ep a r a tion of [(C5H4SiMe3)2Nb(CO)(CH2CH3)] (7). A
yellow hexane solution (30 mL) of [(C5H4SiMe3)2Nb(H)-
(CH2dCH2)] (298 mg, 0.78 mmol) under a CO atmosphere was
stirred for 16 h at room temperature. The solution became
increasingly green, and finally a green oily residue was
obtained after removing the solvent. All attempts to crystallize
this oily material in several solvents, namely pentane, diethyl
ether, acetonitrile, dichloromethane, etc., were unsuccessful.
Nevertheless, the green oily product showed spectroscopic
purity by NMR (90% yield).
11: IR (KBr) 1710 cm-1 (νCO); 1H NMR (CDCl3) δ (ppm) 0.17
(s, 18H, SiMe3), 1.64 (m, 2H, CH2CH2C6H4CH3), 2.95 (m, 2H,
CH2CH2C6H4CH3), 2.30 (s, 3H, CH3), 5.29 (2H), 5.69 (2H), 5.84
(2H), 6.04 (2H) (m, C5H4SiMe3), 7.00-7.22 (C6H4); 13C{1H}
NMR (C6D6) δ (ppm) -1.12 (SiMe3), 20.74 (CH2CH2C6H4CH3),
32.55 (CH2CH2C6H4CH3), 39.84 (CH3), 116.44 (Cipso), 101.50,
101.66, 108.43, 117.52 (C5H4SiMe3), 144.00 (Cipso), 127.34,
128.79 (C6H4), 198.00 (CO2). Anal. Calcd for C26H37Si2O2Nb:
C, 58.85; H, 7.03. Found: C, 58.30; H, 6.80.
12: IR (KBr) 1720 cm-1 (νCO); 1H NMR (CDCl3) δ (ppm) 0.20
(s, 18H, SiMe3), 1.60 (m, 2H, CH2CH2C6H4OCH3), 2.92 (m, 2H,
CH2CH2C6H4OCH3), 3.81 (s, 3H, CH3), 5.33 (2H), 5.73 (2H),
5.87 (2H), 6.08 (2H) (m, C5H4SiMe3), 6.88-7.20 (C6H4); 13C-
{1H} NMR (C6D6) δ (ppm) -1.03 (SiMe3), 32.68 (CH2CH2C6H4-
OCH3), 39.42 (CH2CH2C6H4OCH3), 55.09 (CH3), 116.56 (Cipso),
101.55, 101.62, 108.55, 113.63 (C5H4SiMe3), 157.19 (Cipso),
The reaction can be speeded up at higher temperature (50
°C), taking 6 h instead of 16 h.
7: IR (KBr) 1895 cm-1 (νCO); 1H NMR(C6D6) δ (ppm) 0.09 (s,
18H, SiMe3), 0.5 (q, 2H, CH2), 1.36 (t, 3H, CH3, J ) 7.0 Hz),
4.39 (2H), 4.40 (2H), 4.67 (2H), 4.95 (2H) (m, C5H4SiMe3); 13C-
{1H} NMR (C6D6) δ 0.39 (SiMe3), 10.20 (CH2CH3), 23.53
(CH2CH3), 91.80 (Cipso), 90.20, 93.73, 98.29, 101.23 (C5H4-
SiMe3), 266 (CO).
128.40, 139.35 (C6H4), 200.96 (CO2). Anal. Calcd for C26H37
-
Si2O3Nb: C, 57.13; H, 6.82. Found: C, 57.10; H, 6.70.
P r ep a r a t ion of [(C5H 4SiMe3)2Nb (CO)(CH2CH 2C6H 5)]
(8). A hexane solution (30 mL) of [(C5H4SiMe3)2Nb(H)(C6H5-
CHdCH2)] (endo and exo isomers) (162 mg, 0.30 mmol) under
a CO atmosphere (3 atm in a Fisher-Porter bottle) was stirred
for 48 h at room temperature. The solution became increas-
ingly green, and finally a green oily residue was obtained after
removing solvent. All attempts to crystallize this oily material
in several solvents, namely pentane, diethyl ether, acetonitrile,
dichloromethane, etc., were unsuccessful. Nevertheless, the
green oily product showed spectroscopic purity by NMR (90%
yield).
P r ep a r a t ion s of [(C5H 4SiMe3)2Nb (η2-CO2-B(C6F 5)3)-
(CH2CH2R)] (R ) H (13), C6H5 (14)). To a solution of [(C5H4-
SiMe3)2Nb(η2-CO2)(CH2CH3)] (85 mg, 0.20 mmol) in 10 mL of
dichloromethane at -70 °C was added an equimolar amount
of B(C6F5)3 (99 mg, 0.20 mmol), dissolved in 10 mL of the same
cold solvent. The colorless solution became yellow immedi-
ately. The solution was warmed and the solvent removed
under vacuum as fast as possible to give a pale yellow solid of
complex 13 (181 mg, 0.19 mmol, 95% yield). Attempts to
crystallize this solid from toluene or dichloromethane solutions
were unsuccessful, because the liberation of CO was observed
and the complex 15 was isolated.
The reaction can be speeded up at higher temperature (50
°C), taking 12 h instead of 48 h.
1
8: IR (KBr) 1897 cm-1 (νCO); H NMR (C6D6) δ (ppm) 0.08
Complex 14 (195 mg, 0.19 mmol, 95% yield) was prepared
in a similar way.
Solutions of compounds 13 and 14 were monitored by 1H
NMR to observe their evolution to complexes 15 and 16,
respectively.
(s, 18H, SiMe3), 0.4 (m, 2H, CH2CH2Ph), 2.52 (m, 2H, CH2CH2-
Ph), 4.37 (2H), 4.49 (2H), 4.68 (2H), 4.97 (2H) (m, C5H4SiMe3),
7-7.25 (C6H5); 13C{1H} NMR (C6D6) δ 0.24 (SiMe3), 7.17 (CH2-
CH2Ph), 46.64 (CH2CH2Ph), 95.98 (Cipso), 90.20, 96.95, 98.68,
101.08 (C5H4SiMe3), 150.11 (Cipso), 124.96, 127.82, 128.47
(C6H5), 266 (CO).
13: IR (KBr) 1570 cm-1 (νCO), 1600, 1514, 978 cm-1 (νCF);
1H NMR (C6D6) δ (ppm) -0.11 (s, 18H, SiMe3), 1.04 (t, 3H,
CH3), 1.28 (q, 2H, CH2, J ) 7.0 Hz), 4.71 (2H), 5.21 (4H), 5.35
(2H) (m, C5H4SiMe3); 13C{1H} NMR (C6D6) δ (ppm) -0.64
(SiMe3), 17.60 (CH2CH3), 29.10 (CH2CH3), 108.71, 111.94,
111.91, 115.49 (C5H4SiMe3), 135.97, 139.37, 146.67, 149.98
(B(C6F5)3); 19F NMR (C6D6) -134.22 (F o, dd), -160.72 (F p, t),
-165.11 (F m, td). Anal. Calcd for C37H31F15BSi2NbO2: C,
46.66; H, 3.28. Found: C, 46.30; H, 3.21.
14: IR (KBr) 1575 cm-1 (νCO), 1610, 1513, 968 cm-1 (νCF); 1H
NMR (C6D6) δ (ppm) -0.09 (s, 18H, SiMe3), 1.62 (m, 2H, CH2-
CH2Ph), 2.34 (m, 2H, CH2CH2Ph), 4.84 (2H), 5.28 (2H), 5.30
(2H), 5.42 (2H, m, C5H4SiMe3), 7.00-7.20 (C6H5); 13C{1H}
P r ep a r a tion of [(C5H4SiMe3)2Nb(η2-CO2)(CH2CH3)] (9).
A yellow hexane solution (30 mL) of [(C5H4SiMe3)2Nb(H)-
(CH2dCH2)] (464 mg, 1.17 mmol) under a CO2 atmosphere (3
atm in a Fisher-Porter bottle) was stirred for 48 h at room
temperature, and a white precipitate was formed. The solid
was isolated by filtration, washed with cold hexane, and then
recrystallized from toluene to give white needles of 9 (384 mg,
0.90 mmol, 77% yield).
9: IR (KBr) 1697 cm-1 (νCO); 1H NMR (CDCl3) δ (ppm) 0.20
(s, 18H, SiMe3), 1.41 (q, 2H, CH2), 1.54 (t, 3H, CH3, J ) 7.0
Hz), 5.26 (2H), 5.64 (2H), 5.83 (2H), 6.02 (2H) (m, C5H4SiMe3);

4166 Organometallics, Vol. 16, No. 19, 1997
Antin˜olo et al.
NMR (C6D6) δ (ppm) -0.66 (SiMe3), 37.66 (CH2CH2Ph), 39.07
(CH2CH2Ph), 111.62 (Cipso), 108.57, 111.33, 112.27, 115.62
(C5H4SiMe3), 146.41 (Cipso), 135.98, 139.29, 147.05, 150.10
(B(C6F5)3); 19F NMR (C6D6) δ (ppm) -134.72 (F o, dd), -159.09
(F p, t), -163.12 (F m, td). Anal. Calcd for C43H35F15BSi2-
NbO2: C, 50.21; H, 3.43. Found: C, 50.40; H, 3.40.
P r ep a r a t ion s of [(C5H 4SiMe3)2Nb (O-B(C6F 5)3)(CH 2-
CH2R)] (R ) H (15), C6H5 (16)). To a solution of [(C5H4-
SiMe3)2Nb(η2-CO2)(CH2CH3)] (200 mg, 0.45 mmol) in 20 mL
of dichloromethane was added an equimolar amount of B(C6F5)3
(230 mg, 0.45 mmol) in 10 mL of dichloromethane, at room
temperature. The colorless solution became yellow immedi-
ately. The mixture was stirred for 12 h and the solvent
removed under vacuum to give a yellow solid, which was
recrystallized from CH2Cl2 to give rise to yellow crystals of 15
(380 mg, 0.40 mmol, 90% yield).
h. The yellow solution became red-brown. The solvent was
removed in vacuo, giving rise to a brown oily material (yield
almost quantitative by NMR). All attempts to crystallize this
product from several solvents, namely pentane, diethyl ether,
acetonitrile, dichloromethane, etc., were unsuccessful; there-
fore, complex 17 was isolated as a pure oily material.
17: IR (KBr) 1599, 1895 cm-1 (νCO); 1H NMR (C6D6) δ (ppm)
-0.07 (s, 18H, SiMe3), 0.99 (t, 3H, CH3), 2.68 (q, 2H, CH2, J
) 7.20 Hz), 4.51 (2H), 4.62 (2H), 4.86 (2H), 5.03 (2H) (m, C5H4-
SiMe3); 13C{1H} NMR (C6D6) δ (ppm) 0.04 (SiMe3), 9.54
(CH2CH3), 59.91 (CH2CH3), 91.81, 96.42, 98.03. 100.87 (C5H4-
SiMe3), 263 (CO), 277.97 (C(O)R).
P r ep a r a tion of [(C5H4SiMe3)2Nb(CO)(η1-C(O)CH2CH2-
C6H5)] (18). A yellow toluene solution (20 mL) of [(C5H4-
SiMe3)2Nb(H)(C6H5CHdCH2)] (300 mg, 0.63 mmol) under a CO
atmosphere (3 atm in a Fisher-Porter bottle) at 80 °C was
stirred for 48 h. The yellow solution became red-brown. The
solvent was removed in vacuo, giving a brown oily material
(yield almost quantitative by NMR). All attempts to crystallize
this product from several solvents, namely pentane, diethyl
ether, acetonitrile, dichloromethane, etc., were unsuccessful.
Complex 18 was isolated as a pure oily material.
18: IR (KBr) 1599, 1897 cm-1 (νCO); 1H NMR (C6D6) δ (ppm)
0.05 (s, 18H, SiMe3), 2.80 (m, 2H, CH2CH2Ph), 2.99 (m, 2H,
CH2CH2Ph), 4.45 (2H), 4.49 (2H), 4.81 (2H), 4.95 (2H) (m,
C5H4SiMe3), 7.00-7.20 (m, 5H, C6H5); 13C{1H} NMR (C6D6) δ
(ppm) 0.00 (SiMe3), 31.21 (CH2CH2Ph), 68.49 (CH2CH2Ph),
96.58 (Cipso), 91.75, 96.52, 98.32, 101.37 (C5H4SiMe3), 143.79
(Cipso), 125.58, 128.52, 129.19 (C5H5), 262.52 (CO), 277.14
(C(O)R).
Complex 15 can be also prepared by reaction of a dichlo-
romethane solution (20 mL) of (C5H4SiMe3)2Nb(O)(CH2CH3)
(200 mg, 0.50 mmol) with an equimolar amount of B(C6F5)3
(256 mg, 0.50 mmol) at room temperature. The solution was
stirred over 30 min, and after workup similar to that described
above, yellow crystals of 15 were obtained.
Complex 16 (410 mg, 0.40 mmol, 90% yield) was prepared
in a similar way.
15: IR (KBr) 897 cm-1 (νNbdO), 1640, 1514, 974 cm-1 (νCF);
1H NMR (C6D6) δ (ppm) -0.08 (s, 18H, SiMe3), 1.25 (t, 3H,
CH3), 2.01 (q, 2H, CH2, J ) 7.0 Hz), 5.44 (2H), 5.62 (2H), 5.84
(2H), 5.98 (2H) (m, C5H4SiMe3); 13C{1H} NMR (C6D6) δ (ppm)
-0.54 (SiMe3), 21.26 (CH2CH3), 42.03 (CH2CH3), 119.96 (Cipso),
110.51, 118.94, 120.73, 122,52 (C5H4SiMe3), 135.75, 139.06,
146.95, 150.13 (B(C6F5)3); 19F NMR(C6D6) δ (ppm) -132.04 (F o,
P r oton a tion of 17 a n d 18 w ith CF 3COOH. CF3COOH
was added to an acetone-d6 solution of 17 or 18 in a 5 mm
NMR tube, at room temperature, to give the corresponding
aldehydes and the niobocene trifluoroacetato complex [(C5H4-
SiMe3)2Nb(CO)(OC(O)CF3)].17
CH3CH2CHO: 1H NMR (acetone-d6) δ (ppm) 1.02 (CH3),
2.43 (CH2), 9.73 (CHO).
dd), -58.73 (F p, t), -164.71 (F m, td). Anal. Calcd for C36H31
15BSi2NbO: C, 46.77; H, 3.38. Found: C, 46.20; H, 3.80.
16: IR (KBr) 894 cm-1 (νNbdO), 1600, 1514, 980 cm-1 (νCF);
-
F
1H NMR (C6D6) δ (ppm) -0.07 (s, 18H, SiMe3), 2.29 (m, 2H,
CH2CH2Ph), 2.75 (m, 2H, CH2CH2Ph), 5.58 (2H), 5.61 (2H),
5.92 (2H), 6.10 (2H) (m, C5H4SiMe3), 7.00-7.20 (C6H5); 13C-
{1H} NMR (C6D6) δ (ppm) -0.50 (SiMe3), 42.20 (CH2CH2Ph),
49.34 (CH2CH2Ph), 119.80 (Cipso), 110.01, 119.15, 120.99,
123.48 (C5H4SiMe3), 135.97, 139.31, 147.08, 150.28 (B(C6F5)3);
19F NMR (C6D6) δ (ppm) -132.41 (F o, dd), -158.80 (F p, t),
-164.91 (F m, td). Anal. Calcd for C42H35F15BSi2NbO: C,
50.40; H, 3.53. Found: C, 50.01; H, 3.43.
PhCH2CH2CHO: 1H NMR (acetone-d6) δ (ppm) 2.60 (PhCH2),
2.80 (CH2CHO), 7.00-7.42 (C5H5), 9.78 (CHO).
Ack n ow led gm en t. We gratefully acknowledge fi-
nancial support from the Direccio´n General de Inves-
tigacio´n Cient´ıfica y Te´cnica of Spain (Grant No.
PB-92-0715).
P r ep a r a tion of [(C5H4SiMe3)2Nb(CO)(η1-C(O)CH2CH3)]
(17). A yellow toluene solution (20 mL) of [(C5H4SiMe3)2Nb-
(H)(CH2dCH2)] (210 mg, 0.52 mmol) under a CO atmosphere
(3 atm in a Fisher-Porter bottle) at 80 °C was stirred for 48
OM9702643