Liquid-Phase Reaction of Monosubstituted and Disubstituted Alkynes with Tetrarhodium Dodecacarbonyl under CO and CO/H2 Mixtures. In-Situ IR Spectroscopic Characterization of 20 New Alkyne-Rhodium Complexes

Article abstract of DOI:10.1021/om9900869

It is well-known that the liquid-phase homogeneous unmodified rhodium-catalyzed hydroformylation of alkenes is irreversibly poisoned by the presence of trace quantities of alkynes. In the present contribution, we examined the reaction of four series of monosubstituted and disubstituted alkynes (20 compounds) with Rh₄(CO)₁₂ in n-hexane solvent at 293 K under both (A) 2.0 MPa CO and (B) 2.0 MPa CO and 2.0 MPa H₂. The analytic method used was in-situ high-pressure infrared spectroscopy. It was observed that (I) all alkynes used in this study reacted quantitative with Rh₄(CO)₁₂ in a matter of hours, (II) the final spectra were not influenced by the presence of hydrogen, (III) the monosubstituted alkynes consistently gave a final product involving six terminal ν_CO vibrations in the region 2036-2121 cm^-1 and two vibrations at ca. 1668 and 1689 cm^-1, and (IV) the disubstituted alkynes consistently gave a final spectrum consistent with the superposition of the spectra obtained in III plus a spectrum involving five terminal ν_CO vibrations in the region 2030-2100 cm^-1 and one vibration at ca. 1630 cm^-1. These results are consistent with the existence of two primary types of observable species in the final products. Due to the band positions and absorptivities, we tentatively propose that these species are substituted dirhodium carbonyl species, specifically Rh₂(CO)₆{μ-η¹-(CO-HC₂R)} for terminal alkynes and Rh₂(CO)₆{μ-η¹-(CO-R¹C ₂R²)} and Rh₂(CO)₆{μ-η²-(R¹C ₂R²)}in the case of disubstituted alkynes. These complexes are the rhodium analogues of well-known dicobalt carbonyl alkyne complexes. It appears that, in the case of terminal alkynes, the dirhodium-alkyne complexes undergo rapid CO insertion under 2.0 MPa CO. In the case of disubstituted alkynes, CO insertion seems more difficult to obtain, and an observable equilibrium is established between the bridged alkyne species and the insertion product. In both cases the final alkyne complexes are stable under CO even in the presence of molecular hydrogen. This is probably the primary reason that trace quantities of alkynes are able to poison the catalytic alkene hydroformylation reaction.

Full text of DOI:10.1021/om9900869

Organometallics 1999, 18, 3457-3467
3457
Liqu id -P h a se Rea ction of Mon osu bstitu ted a n d
Disu bstitu ted Alk yn es w ith Tetr a r h od iu m
Dod eca ca r bon yl u n d er CO a n d CO/H2 Mixtu r es. In -Situ
IR Sp ectr oscop ic Ch a r a cter iza tion of 20 New
Alk yn e-Rh od iu m Com p lexes†
Guowei Liu and Marc Garland*,‡
Laboratorium fu¨r Technische Chemie, Universita¨tstrasse 6,
ETH-Zu¨rich Eidgeno¨ssische Technische Hochschule, CH-8092 Zu¨rich, Switzerland
Received February 10, 1999
It is well-known that the liquid-phase homogeneous unmodified rhodium-catalyzed
hydroformylation of alkenes is irreversibly poisoned by the presence of trace quantities of
alkynes. In the present contribution, we examined the reaction of four series of monosub-
stituted and disubstituted alkynes (20 compounds) with Rh4(CO)12 in n-hexane solvent at
293 K under both (A) 2.0 MPa CO and (B) 2.0 MPa CO and 2.0 MPa H2. The analytic method
used was in-situ high-pressure infrared spectroscopy. It was observed that (I) all alkynes
used in this study reacted quantitative with Rh4(CO)12 in a matter of hours, (II) the final
spectra were not influenced by the presence of hydrogen, (III) the monosubstituted alkynes
consistently gave a final product involving six terminal νCO vibrations in the region 2036-
2121 cm-1 and two vibrations at ca. 1668 and 1689 cm-1, and (IV) the disubstituted alkynes
consistently gave a final spectrum consistent with the superposition of the spectra obtained
in III plus a spectrum involving five terminal νCO vibrations in the region 2030-2100 cm-1
and one vibration at ca. 1630 cm-1. These results are consistent with the existence of two
primary types of observable species in the final products. Due to the band positions and
absorptivities, we tentatively propose that these species are substituted dirhodium carbonyl
species, specifically Rh2(CO)6{µ-η1-(CO-HC2R)} for terminal alkynes and Rh2(CO)6{µ-η1-
(CO-R1C2R2)} and Rh2(CO)6{µ-η2-(R1C2R2) }in the case of disubstituted alkynes. These
complexes are the rhodium analogues of well-known dicobalt carbonyl alkyne complexes. It
appears that, in the case of terminal alkynes, the dirhodium-alkyne complexes undergo
rapid CO insertion under 2.0 MPa CO. In the case of disubstituted alkynes, CO insertion
seems more difficult to obtain, and an observable equilibrium is established between the
bridged alkyne species and the insertion product. In both cases the final alkyne complexes
are stable under CO even in the presence of molecular hydrogen. This is probably the primary
reason that trace quantities of alkynes are able to poison the catalytic alkene hydroformy-
lation reaction.
In tr od u ction
well as their sensitivity to trace impurities of dienes and
alkynes,3 is largely responsible for the infrequent use
of unmodified rhodium carbonyl complexes on an in-
dustrial scale today. In contrast, not only are many
rhodium phosphine complexes relatively insensitive to
the trace presence of dienes and alkynes, but such
complexes can actually hydroformylate such substrates.
Examples include the hydroformylation of 1-hexyne to
heptanal, 1-butyne to pentanal, and 2-butyne to 2-me-
thylbutanal using HRh(CO)(PPh3)3, and the hydro-
formylation of acetylene, 1-butyne, and 2-butyne using
mixtures of Rh2O3 and phosphines.4
Rhodium complexes have been used in the liquid-
phase homogeneous catalytic hydroformylation reaction
for approximately 50 years. Although the hydroformy-
lation of alkenes was initially performed with unmodi-
fied rhodium complexes,1 phosphine-modified rhodium
complexes are now responsible for much of the ca. 107
tonnes per annum of alkene hydroformylation products.
The low activity of the unmodified systems compared
to modified systems under mild reaction conditions,2 as
† Experimental research performed at ETH-Zurich, 1992-1995.
Analysis performed at NUS, 1998.
* To whom correspondence should be sent. E-mail: chemvg@
nus.edu.sg.
(2) (a) Marko, L. In Aspects of Homogeneous Catalysis; Ugo, R., Ed.;
Reidel: Dorrecht, 1974; Vol. II. (b) Dickson, R. S. Homogeneous
Catalysis with Compounds of Rhodium and Iridium; Reidel: Dor-
drecht, 1986. (c) Pino, P.; Piacenti, F.; Bianchi, M. In Organic Syntheses
via Metal Carbonyls; Wender, I., Pino, P., Eds.; Wiley: New York, 1977;
Vol. II.
‡ Present address: Department of Chemical and Environmental
Engineering, The National University of Singapore.
(1) (a) Schiller, G. (Chem. Verwertungsges. Oberhausen) Ger. Pat.
965.605, 1956; Chem Abstr. 1959, 53, 11226. (b) Hughes, V. L. (Esso
Res. Eng. Co) U.S. Pat. 2.880.241, 1959; Chem. Abst. 1959, 53, 14938.
(c) Hughes, V. L. (Esso Res. Eng. Co) Brit. Pat. 801.734, 1958; Chem.
Abst. 1959, 53, 7014.
(3) Cornils, B. In New Syntheses with Carbon Monoxide; Falbe, J .,
Ed.; Springer: Berlin, 1980.
10.1021/om9900869 CCC: $18.00 © 1999 American Chemical Society
Publication on Web 08/16/1999

3458 Organometallics, Vol. 18, No. 17, 1999
Liu and Garland
Exp er im en ta l Section
Over the past decade our group has focused on
detailed in-situ infrared spectroscopic and kinetic stud-
ies of the unmodified rhodium hydroformylation using
numerous alkenes as substrates. These studies have
concentrated on (A) the first identification of acyl
rhodium tetracarbonyl as the only observable interme-
diate in Rh4(CO)12 hydroformylations of 3,3-dimethyl-
but-1-ene to 4,4-dimethylpentanal at 293 K,5 (B) the
identification of acyl rhodium tetracarbonyl as the only
observable intermediate in CoRh(CO)7 and CoRh(CO)8
hydroformylations of 3,3-dimethylbut-1-ene to 4,4-dim-
ethylpentanal at 293 K, as well as the associated
kinetics of the induction period, product formation, and
deactivation,6 (C) the detailed kinetics of the hydro-
formylations of 3,3-dimethylbut-1-ene to 4,4-dimethyl-
pentanal using Rh4(CO)12 as precursor,7 (D) the detailed
kinetics of the hydroformylation of cyclohexene to cy-
clohexane carboxaldehyde using Rh4(CO)12 as precursor
where the phenomenon of equilibrium precursor conver-
sion was observed,8 (E) the kinetics of the hydroformy-
lation of 3,3-dimethylbut-1-ene to 4,4-dimethylpentanal
at 293 K using a variety of phosphine-free rhodium
complexes,9 (F) the experimental, numerical, and sta-
tistical aspects of evaluating hydroformylation turnover
frequencies,10 (G) the detailed regioselective kinetics of
the hydroformylation of styrene to (()-2-phenylpropanal
and 3-phenylpropanal using Rh4(CO)12 as precursor,11
(H) the search for a catalytic binuclear elimination
reaction mechanism at very high Rh4(CO)12 concentra-
tions during the hydroformylation of cyclohexene,12 and
(I) the characterization of ca. 20 new acyl rhodium
tetracarbonyls during hydroformylations with Rh4(CO)12
as precursor.13 However, until now, we have not inves-
tigated the chemistry of the deactivation phenomenon
initiated by either dienes nor alkynes.
Gen er a l In for m a tion . All solution preparations were
carried out under argon (99.999% Pan Gas AG, Luzern,
Switzerland) using standard Schlenk techniques.14 The argon
was further purified prior to use by passage through a column
containing 100 g of reduced BTS-catalyst (Fluka AG Buchs,
Switzerland) and 100 g of 4 Å molecular sieves to adsorb trace
oxygen and water, respectively. All reactions were carried out
under carbon monoxide (99.997% Messner Griesheim GmbH,
Germany) and hydrogen (99.999% Pan Gas AG, Luzern,
Switzerland) after further purification through deoxy and
zeolite columns.
The precious metal complex Rh4(CO)12, with stated purity
of 98% min, was obtained from Strem Chemicals SA (Bis-
chheim, France) and was used without further purification,
although trace quantities of the high-nuclearity cluster Rh
-
6
(CO)16 is virtually always present. The complex Rh4(CO)12 is
known to be oxygen, water, and light sensitive.15 The n-hexane
solvent (stated purity >99.6%, Fluka AG) was refluxed over
sodium potassium alloy under argon. Weights were measured
with a precision of (0.1 mg. Volumes were measured with an
precision of (0.045 mL. Further microanalytic techniques were
not employed.16 All alkynes used in this study were of the
highest quality commercially obtainable, usually 99.0+%
(Wiley Organic Chemicals or Lancaster Chemicals, U.K.).
Concerning further purification, the alkynes were simply
degassed before use.
Equ ip m en t. In-situ spectroscopic studies were performed
in a 1.5 L stainless steel (SS316) autoclave (Bu¨chi-Uster,
Switzerland), which was connected to a high-pressure infrared
cell. The autoclave (Pmax ) 22.5 MPa) was equipped with a
packed magnetic stirrer with six-bladed turbines in both the
gas and liquid phases (Autoclave Engineers, Erie, PA) and was
constructed with a heating/cooling mantle. A high-pressure
membrane pump (Model DMK 30, Orlita AG, Geissen, Ger-
many), with a maximum rating of 32.5 MPa and a 3 L/h flow
rate, was used to circulate the n-hexane solutions from the
autoclave to the high-pressure IR cell and back to the autoclave
via jacketed 1/8 in. (SS316) high-pressure tubing (Autoclave
Engineers). The entire system, autoclave, transfer lines, and
infrared cell, was cooled using a Lauda cryostat Model RX20
and could be maintained isothermal (∆T e 0.5 °C) at 298-
313 °C. Temperature measurements were made at the cry-
ostat, autoclave, and IR cell with PT-100 thermoresistors. The
necessary connections to vacuum and gases were made with
1/4 in. (SS316) high-pressure tubing (Autoclave Engineers),
and 1.0, 5.0, and 10.0 piezocrystals were used for pressure
measurements (Keller AG Winterthur, Switzerland). The
entire system was gastight under vacuum as well as at 20.0
MPa, the maximum operating pressure.
In the present contribution we investigate the reac-
tivity of four series of alkynes (20 compounds) toward
Rh4(CO)12 in n-hexane as solvent at 293 K in the
presence 2.0 MPa CO and 2.0 MPa CO/2.0 MPa H2. The
alkynes are conveniently classified as (a) a homologous
series of terminal unsymmetric alkynes from 1-pentyne
to 1-hexadecyne, (b) a homologous series of symmetric
alkynes from 2-butyne to 6-dodecyne, (c) a homologous
series of increasingly symmetric alkynes from 1-decyne
to 5-decyne, and (d) a miscellaneous collection of other
monosubstituted and disubstituted alkynes. The objec-
tive of the study was to identify the predominate
rhodium alkyne complex(es) present under hydroformy-
lation conditions, and hence the probable reason that
rhodium is unavailable for alkene hydroformylation in
alkene hydroformylations tainted by the presence of
trace alkynes.
The high-pressure infrared cell was constructed at the ETH-
Zu¨rich of SS316 steel and could be heated and cooled. The CaF2
single-crystal windows (Korth Monokristalle, Kiel, Germany)
had dimensions of 40 mm diameter by 15 mm thickness. Two
sets of Viton and silicone gaskets provided sealing, and Teflon
spacers were used between the windows. The construction of
the flow-through cell17 is a variation on a design due to Noack18
and differs in some respects from other high-pressure infrared
cells described in the literature (for a review, see Whyman19).
(4) (a) Brown, C. K.; Georgiou, D.; Wilkinson, G. J . Chem. Soc. (A)
1971, 3120. (b) Fell, B.; Beutler, M. Tetrahedron Lett. 1972, 33, 3455.
(c) Fell, B.; Beutler, M. Erdoel Kohle 1976, 29, 149.
(5) Garland, M.; Bor, G. Inorg. Chem. 1989, 28, 410.
(6) Garland, M. Dissertation 8585, ETH-Zurich, 1988.
(7) Garland, M.; Pino, P. Organometallics 1991, 10, 1693.
(8) Fyhr, Ch.; Garland, M. Organometallics 1993, 12, 1753.
(9) Garland, M. Organometallics 1993, 12, 535.
(14) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-
Sensitive Compounds; Wiley: New York, 1986.
(15) Chini, P.; Heaton, B. T. Tetranuclear Carbonyl Clusters. Top.
Curr. Chem. 1977, 71, 1.
(16) (a) Cheronis, N. D. Micro and Semimicro Methods; Inter-
science: New York, 1954. (b) Ma, T. S.; Horak, V. Microscale Manipu-
lations in Chemistry; Wiley: New York, 1976. (c) Kirk, P. L. Quanti-
tative Ultramicro Analysis; Wiley: New York, 1954.
(17) Dietler, U. K. Dissertation 5428, ETH-Zurich, 1974.
(18) Noack, K. Spectrochim. Acta 1968, 24A, 1917.
(10) Shirt, R.; Garland, M.; Rippin, D. W. T. Anal. Chim. Acta 1998,
374, 67.
(11) Feng, J .; Garland, M. Organometallics 1999, 18, 417.
(12) Feng, J .; Garland, M. Organometallics 1999, 18, 1542.
(13) Liu, G.; Volken, R.; Garland, M. Organometallics, in press.
(19) Whyman, R. In Laboratory Methods in Vibrational Spectros-
copy, 3rd ed.; Willis, H. A., van der Maas, J . H., Miller, R. G. J ., Eds.;
Wiley: New York, 1987; Chapter 12.

20 New Alkyne-Rhodium Complexes
Organometallics, Vol. 18, No. 17, 1999 3459
The high-pressure cell was situated in a Perkin-Elmer 983
infrared spectrometer. The resolution was set to 4 cm-1 for
all spectroscopic measurements. A schematic diagram of the
experimental setup can be found in ref 11.
In -Situ Sp ectr oscop ic Stu d ies. All experiments were
performed in a similar manner. First, single-beam background
spectra of the IR sample chamber were recorded. Then 1-10
mL of alkyne was dissolved in 150 mL of n-hexane, and this
solution was transferred under argon to the autoclave. Under
0.2 MPa CO pressure, infrared spectra of the alkyne solution
in the high-pressure cell were recorded. The total system
pressure was raised to 2.0 MPa CO, and the stirrer and high-
pressure membrane pump were started. After equilibration,
infrared spectra of the alkyne/n-hexane/CO solution in the
high-pressure cell were recorded. A solution of 100 mg of Rh4-
(CO)12 dissolved in 100 mL of n-hexane was prepared, trans-
ferred to the high-pressure reservoir under argon, pressured
with CO, and then added to the autoclave. Infrared spectra of
the alkyne/n-hexane/CO/Rh4(CO)12 solution in the high-pres-
sure cell were recorded. If this was a CO only experiment,
spectra were recorded at 30 min intervals in the range 1600-
2200 cm-1. For CO and H2 experiments, 2.0 MPa hydrogen
was further added. Spectra were recorded at 30 min intervals
in the range 1600-2200 cm-1
.
A considerable number of spectral subtractions were per-
formed on each reaction spectrum in order to subtract the
absorbance of the n-hexane solvent, dissolved CO, Rh4(CO)12,
and the alkynes.
The solubility of CO under these reaction conditions was
approximately 0.033 mole fraction, and the solubility of H2 was
approximately 0.018 mole fraction.9 Consequently, the moles
in each experiment were 1.51 mol of n-hexane, 0.05 mol of CO,
and 0.03 mol of H2. Since the amount of Rh4(CO)12 used in the
experiments was ca. 100 mg, and since the concentration of
alkynes was low, the corresponding in-situ measured initial
concentrations of Rh4(CO)12 in the experiments was ca. 8 ×
10-5 mole fraction.
F igu r e 1. Mid-infrared spectra of alkyne-rhodium com-
plexes after subtraction of hexane, CO, and free alkyne.
Homologous series of monosubstituted alkynes from 1-pen-
tyne to 1-hexadecyne.
Resu lts
Ter m in a l Alk yn es. Seven alkynes were used in the
series of experiments involving terminal alkynes. These
were 1-pentyne, 1-hexyne, 1-heptyne, 1-octyne, 1-de-
cyne, 1-dodececyne, and 1-hexadecyne. The spectra
collected under CO reaction conditions and under CO
and H2 reaction conditions were essentially identical.
After a few hours, quantitative conversion of Rh4(CO)12
was observed for all reactions. Figure 1 shows the
spectra of the seven CO reaction mixtures after subtrac-
tion of background, hexane, CO, and alkyne. As shown
by Figure 1, the resultant spectra are essentially
identical for all alkynes used.
Sym m etr ic Disu bstitu ted Alk yn es. Five alkynes
were used in the series of experiments involving sym-
metric disubstituted alkynes. These were 2-butyne,
3-hexyne, 4-octyne, 5-decyne, and 6-dodecyne. The
spectra collected under CO reaction conditions and
under CO and H2 reaction conditions were essentially
identical. After a few hours, quantitative conversion of
Rh4(CO)12 was observed for all reactions. Figure 2 shows
the spectra of the five CO reaction mixtures after
subtraction of background, hexane, CO, and alkyne. As
shown by Figure 2, the resultant spectra are not
identical. In fact, as the molecular weight of the alkynes
increases, more bands become visible.
The spectra are only a little similar in the positions
of the absorbance maxima as well as the relative
intensities of the terminal CO vibrations. However, clear
systematic trends are apparent. For example, the result
of the 2-butyne experiment is almost identical to the
terminal alkyne reactions. However, as the molecular
The weak vibration at 1820 cm-1 is due to the
bridging carbonyls on Rh6(CO)16. This weak absorbance
is seen in this set of experiments as well as experiments
with symmetric disubstituted alkynes, asymmetric di-
substituted decynes, and other alkynes. In addition,
there is a very weak vibration at ca. 2000 cm-1 in these
spectra as well as all other spectra. Due to the magni-
tude of this vibration, as well as its position ca. 35 cm-1
lower than the lowest terminal CO vibration, this band
is most likely due to the isotopomers, i.e., the 1.1%
natural abundance of 13CO.20
(20) (a) Bor, G. Inorg. Chim. Acta 1967, 1, 81. (b) Bor, G. Chem.
Commun. 1969, 641. (c) Bor, G.; J ung, C. Inorg. Chim. Acta 1969, 3,
69. (d) Braterman, P. S.; Harrill, R. W.; Kaesz, H. D. J . Am. Chem.
Soc. 1967, 89, 2851 (e) Cotton, F. A.; Musco, A.; Yagupsky, G. Inorg.
Chem. 1967, 6, 1357. (f) Harrill, R. W.; Kaesz, H. D. J . Am. Chem.
Soc. 1968, 90, 1449. (g) Kaesz, H. D.; Bau, R.; Hendrickson, D.; Smith,
J . M. J . Am. Chem. Soc. 1968, 90, 1449. (h) Lewis, J .; Manning, A. R.;
Miller, J . R. J . Chem. Soc. A 1966, 843. (i) Loutellier, A. Bigorgne, M.
Bull. Soc. Chim. Fr. 1965, 3186. (j) Noack, K. J . Organomet. Chem.
1968, 12, 181. (k) Poletti, A.; Catiotti, R.; Foffani, A. Inorg. Chim. Acta
1968, 2, 157.
The spectra obtained in the present series are similar
in both the positions of the absorbance maxima and the
relative intensities. Both are tabulated in Table 1. As
indicated by Table 1 there appears to be six primary
terminal CO vibrations and two clear vibrations at low
cm-1
.

3460 Organometallics, Vol. 18, No. 17, 1999
Ta ble 1. Rela tive In fr a r ed Absor ba n ce of Ter m in a l Alk yn e Com p lexes
Liu and Garland
ν
co (cm-1) above, relative intensity below
ν (cm-1
)
alkyne
1-pentyne
2122
0.41
2122
0.40
2121.6
0.40
2120
0.43
2122
0.42
2121.7
0.45
2121.7
0.44
2089.4
2082.4
2071
0.55
2070.6
0.54
2070.5
0.53
2070.5
0.53
2070.6
0.52
2070.6
0.56
2070.7
0.52
2043
0.32
2042.5
0.33
2043
0.32
2043
0.34
2042.5
0.31
2042.3
0.33
2042.6
0.32
2037
0.45
2036.4
0.45
2036.3
0.42
2036.3
0.45
2036.3
0.44
2036.4
0.46
2036.2
0.44
1689
0.20
1689
0.20
1689
0.19
1689
0.20
1689
0.19
1689
0.19
1689
0.19
1668
0.22
1669
0.18
1667.5
0.18
1667.5
0.19
1668
0.19
1667.6
0.19
1668
0.19
0.66
1
2082.2
1
1-hexyne
2089.2
0.64
2089
0.65
2089.2
0.67
2089.2
0.64
2089
0.68
2089
0.64
1-heptyne
1-octyne
2082
1
2082
1
2082
1
2082
1
1-decyne
1-dodecyne
1-hexadecyne
2082
1
The spectra are only a little similar in the positions
of the absorbance maxima as well as the relative
intensities of the terminal CO vibrations. However, a
clear systematic trend is apparent. For example, the
result of the 2-decyne experiment is almost identical to
the 1-decyne reaction. However, as the triple bond
becomes more internal, more terminal CO bands become
apparent (although the intensity of the new bands is
low) and the band at 1630 cm-1 appears. The results
are tabulated in Table 3.
Oth er Alk yn es. Five other alkynes were used in a
comparative series of experiments. These were cyclo-
hexylacetylene, 3,3-dimethylbut-1-yne, 1-phenyl-1-pro-
pyne, diphenylacetylene, and phenylacetylene. The
spectra collected under CO reaction conditions and
under CO and H2 reaction conditions were essentially
identical. After a few hours, quantitative conversion of
Rh4(CO)12 was observed for all reactions. Figure 4 shows
the spectra of the five CO reaction mixtures after
subtraction of background, hexane, CO, and alkyne. As
shown by Figure 4, the resultant spectra are not
identical.
The terminal alkynes, namely cyclohexylacetylene
and 3,3-dimethylbut-1-yne, produced spectra typical of
terminal alkynes. Phenylacetylene produced a spectrum
with only five terminal CO bands, where obvious
coalescence of two bands at 2089 cm-1 occurs. Further-
more, the disubstituted alkynes, namely diphenylacety-
lene and 1-phenyl-1-propyne, did not produce a spec-
trum with a visible a band at 1630 cm-1. The results
are tabulated in Table 4.
An a lysis a n d Decon volu tion of Mon osu bstitu ted
Alk yn e P r od u cts As already shown in Figure 1 and
Table 1, the terminal alkynes 1-pentyne to 1-hexadecyne
(and cyclohexylacetylene and 3,3-dimethylbut-1-yne)
lead to almost exactly the same infrared spectrum. The
reaction of 1-dodecyne was taken as a characteristic
result. A PEAKFIT solution for the terminal CO region
is shown in Figure 5. The PEAKFIT basis functions
were chosen as Gauss-Lorenz areas. The fit was very
good, and Figure 5 clearly shows six strong terminal
CO vibrations for a new complex, and one terminal CO
vibration at 2074.5 cm-1 for the ubiquitous Rh6(CO)16.
The characteristic vibrations are 2036.1, 2042.5, 2070.5,
2082, 2089.2, and 2121.7 cm-1. Their relative intensities
are 0.42, 0.32, 0.51, 1, 0.67, and 0.48. All the bands have
widths at half-height of ca. 5 ( 1 cm-1, indicative of
sharp terminal CO vibrations.
F igu r e 2. Mid-infrared spectra of alkyne-rhodium com-
plexes after subtraction of hexane, CO, and free alkyne.
Homologous series of symmetric disubstituted alkynes from
2-butyne to 6-dodecyne.
weight increases, more terminal bands become apparent
(although they are small) and a new band at 1630 cm-1
appears. The results are tabulated in Table 2.
Asym m etr ic Disu bstitu ted Decyn es Five alkynes
were used in the series of experiments involving asym-
metric disubstituted alkynes. These were 1-decyne,
2-decyne, 3-decyne, 4-decyne, and 5-decyne. The spectra
collected under CO reaction conditions and under CO
and H2 reaction conditions were essentially identical.
After a few hours, quantitative conversion of Rh4(CO)12
was observed for all reactions. Figure 3 shows the
spectra of the five CO reaction mixtures after subtrac-
tion of background, hexane, CO, and alkyne. As shown
by Figure 3, the resultant spectra are not identical.
A PEAKFIT solution for the low-cm-1 region is shown
in Figure 6. The PEAKFIT basis functions were chosen

20 New Alkyne-Rhodium Complexes
Organometallics, Vol. 18, No. 17, 1999 3461
Ta ble 2. Rela tive In fr a r ed Absor ba n ce of Sym m etr ic Disu bstitu ted Alk yn e Com p lexes
ν
co (cm-1) above, relative intensity below
ν (cm-1
)
alkyne
2-butyne 2121.1
0.41
3-hexyne 2120
0.42
4-octyne 2122.7
0.61
5-decyne 2123
0.61
6-dodecyne 2120
0.50
2088.9
0.55
2088
0.44
2090.5
1
2091
0.98
2087.2
0.64
2078.4
2070.6
0.48
2069.6
0.42
2048.3sh 2037.4
0.40 0.40
2047.6 2036.9
0.04 0.36
2047.5 2038.7
0.14 0.64
2047.3 2038.5
0.15 0.60
2047.8 2036.7
0.06 0.40
2033
0.44
2032.7
0.42
2032.8sh 1687sh 1663
0.22 0.20 0.28
2034sh 1678.3 1667.2
0.22 0.27 0.28
2031.9 1674sh 1663.5
0.38 0.14 0.20
1681
0.13
1676.7
0.13
1658.2
1631
0.01
1628
0.01
1635
0.06
1633
0.06
1632.8
0.03
1
2077.9
1
0.20
1665
0.24
2095.4
0.94
2077.3sh 2073.2
2055.1
0.20
2054.6
0.21
2054.4
0.04
0.61
0.86
2095.1
1
2095.1
2076.6sh 2072.9
0.51
2077
0.94
2069.5
0.47
0.18
1
shows a weaker pattern of five weak terminal CO
vibrations at 2046.8, 2055.0, 2084.9, 2090.0, and 2095.0
cm-1 with relative intensities of 0.20, 0.10, 0.69, 1, and
0.72, and one terminal CO vibration at 2073.4 for the
ubiquitous Rh6(CO)16. All the bands have widths at half-
height of ca. 5 ( 1 cm-1, indicative of terminal CO
vibrations.
A PEAKFIT solution for the low-cm-1 region is shown
in Figure 8. The PEAKFIT basis functions were chosen
as Gauss-Lorenz areas, and again a reasonably good
fit was obtained. The characteristic vibrations are
1670.2, 1661.4, and 1635.2 cm-1. However, the bands
are different with widths at half-height of ca. 18.8, 17.3,
and 11 cm-1, respectively. This suggests two things,
namely, broadening of the first two bands when larger
substituents are placed on the alkynes and at least two
and possibly three different types of moieties present.
P r elim in a r y Con clu sion s. The tables of spectro-
scopic characteristics as well as the analysis of the
deconvolutioned infrared spectra suggest that there are
only two different types of alkyne-rhodium complexes
present in this study.
For one pattern, the infrared spectrum is character-
ized by six terminal vibrations and two low cm-1
vibrations. The six terminal vibrations are almost
exactly the same, in both position and relative intensity,
for both monosubstituted and disubstituted alkyne
complexes. The low-cm-1 vibrations are well separated
at 1668 and 1689 cm-1 in the case of monosubstituted
alkyne complexes, but start to coalesce and become
broader for disubstituted alkyne complexes. This is
the only pattern for monosubstituted alkyne com-
plexes, and it is the predominant pattern (corresponding
to the major product) for disubstituted alkyne com-
plexes.
F igu r e 3. Mid-infrared spectra of alkyne-rhodium com-
plexes after subtraction of hexane, CO, and free alkyne.
Homologous series of alkynes from 1-decyne to 5-decyne.
as Gauss-Lorenz areas, and again a very good fit was
obtained. The characteristic vibrations are 1668.1 and
1688.8 cm-1. However, the bands are different, with
widths at half-height of ca. 13.6 and 9.9 cm-1, respec-
tively. This suggests that two different types of moieties
are present.
An a lysis a n d Decon volu tion of Disu bstitu ted
Alk yn e P r od u cts. As already shown in Figures 2-4
and Tables 2-4, the reactions of the disubstituted
alkynes lead to complex infrared spectra. We took the
reaction of 6-dodecyne as our characteristic result. A
PEAKFIT solution for the terminal CO region is shown
in Figure 7. The PEAKFIT basis functions were chosen
as Gauss-Lorenz areas. The fit was very good, and
Figure 7 clearly shows a strong pattern of six strong
terminal CO vibrations at 2031.6, 2037.2, 2069.1,
2077.2, 2087.4, and 2120.5 cm-1, with relative intensi-
ties of 0.37, 0.38, 0.46, 1, 0.51, and 0.52. Figure 7 also
For the second pattern, the infrared spectrum is
characterized by five relatively weak terminal vibrations
(corresponding to a minor product) and one weak low-
cm-1 vibration. The pattern only appears for disubsti-
tuted alkyne complexes. The low-cm-1 vibration at 1630
cm-1 is particularly interesting because it is the correct
wavenumber for a CdC vibration.
Discu ssion
The homogeneous catalytic chemistry of alkynes and
the stoichiometric organometallic chemistry of alkyne
complexes is quite extensive.21 Accordingly, the discus-
sion of the results is best approached by first mentioning
some of the catalytic aspects, followed by comparisons
(21) J eannin, Y. Transition Met. Chem. 1993, 18, 122.

3462 Organometallics, Vol. 18, No. 17, 1999
Liu and Garland
Ta ble 3. Rela tive In fr a r ed Absor ba n ce of Decyn e Com p lexes
co (cm-1) above, relative intensity below
ν
ν (cm-1
)
alkyne
1-decyne 2122
2089.2
0.64
2095.7sh 2089
0.16 0.65
2095.3 2089.3
0.57
2095
0.96
2095.1
2082
1
2078.1
1
2077.1
2070.6
0.52
2069.9
0.50
2073
0.79
2050.6
0.04
2056.2
0.04
2054.9
0.13
2054.6
0.19
2042.5
0.31
2048.1
0.06
2047.6
0.19
2036.3
0.44
2037.6
0.40
2037.9
0.57
2038.3
0.63
2038.5
0.60
1689
0.19
1674
0.14
1678.1sh 1666.4
0.17 0.29
1665
0.28
1667.2
0.28
1668
0.42
0.19
2-decyne 2121.1
0.48
3-decyne 2122
0.65
4-decyne 2122.8
0.65
5-decyne 2123
0.61
2033
0.39
2033sh
0.34
2031.7sh 1675.9
0.19 0.20
1678.3
0.27
1663
0.22
1635.2
0.01
1636.2
0.08
1632.3
0.05
1633
0.06
1
2090.1
1
0.78
2077.1sh 2073
0.60 0.88
2072.9 2054.6
0.94 0.21
2091
1
0.98
Various combinations of alkynes and rhodium com-
plexes have been observed to lead to catalytic trans-
formations. Such transformations include (I) the dimer-
ization of alkynes such as 1-octyne to give free 7-meth-
ylenepentadec-8-yne in the presence of RhCl(PPh3)3 at
343 K,22 (II) the cyclotrimerization of alkynes to give
substituted benzenes by rhodium carbonyl complexes
again in the absence of CO at 333 K (which is closely
related to the well-documented cobalt-catalyzed cyclo-
trimerization reaction),23 (III) the synthesis of furan-
2(5H)-ones by Rh4(CO)12 in the presence of alkynes and
100 atm CO at 373 K,24 (IV) the synthesis of substituted
cyclopentanones by Rh4(CO)12 in the presence of alkynes,
CO, and H2O,25 (V) the cross-hydrocarbonylation of
alkynes and ethylene in the presence of Rh4(CO)12, CO,
and H2 at 423 K to form R,â-unsaturated ethyl ke-
tones,26 and (VI) the reaction of alkynes and ethylene
in the presence of Rh4(CO)12 and CO to form 5-ethyl-
3,4-disubstituted-2(5H)-furanones.27
In con sisten t Alk yn e Com p lexes. Rh 4(CO)10
-
(R1C2R2). The reaction of tetranuclear cluster Rh4(CO)12
with alkynes in the absence of CO has been reported
on at least two occasions.28 Iwashita and Tamura
monitored the reaction of diphenylacetylene with Rh4-
(CO)12 in the infrared in n-hexane as solvent. They
observed the intermediate formation of a bridged car-
bonyl species which they tentatively assigned to be
Rh4(CO)10(PhC2Ph). Subsequently, Booth et al. per-
formed the same reaction and isolated Rh4(CO)10-
(PhC2Ph) with predominant spectral characteristics of
2087(s), 2068(vs), 2056(vs), 2043(vs), 2016(vs), 1845(s),
and 1825(s) cm-1, where the last two vibrations are due
to bridging carbonyls. A similar complex was also
prepared from CF3C2CF3. Our spectroscopic results
indicate that no bridging carbonyls are present in our
species.
F igu r e 4. Mid-infrared spectra of alkyne-rhodium com-
plexes after subtraction of hexane, CO, and free alkyne.
Assortment of miscellaneous monosubstituted and disub-
stituted alkynes.
to known alkyne complexes which are inconsistent with
our results, and finally by comparisons with known
alkyne complexes that appear consistent with the
spectroscopic results. Given the very broad area covered
by organometallic alkyne complexes, we try to restrict
discussion to pertinent cobalt and rhodium complexes
as far as possible.
Importantly, both groups reported that the reaction
of other hydrocarbon-substituted alkynes with Rh4-
(CO)12 in the absence of CO leads to a host of very labile
and nonisolatable complexes. If benzene is used as
solvent, catalytic cyclotrimerization of diphenylacetylene
to hexaphenylbenzene occurs.
R eleva n t H om ogen eou s Ca t a lyt ic Asp ect s of
Alk yn es. As already mentioned in the Introduction,
alkynes can be hydroformylated by phosphine-modified
rhodium complexes. Although no hydroformylation of
alkynes by Rh4(CO)12 was observed in this study when
using a mixture of CO and molecular hydrogen at 293
K and although no other catalytic transformations of
alkynes by Rh4(CO)12 were observed in the presence of
CO alone, the potential for catalytic activity cannot be
underestimated.
(22) Carlton, L.; Read, G. J . Chem. Soc., Perkin Trans. 1 1978, 1631.
(23) McAllister, D. R.; Bercaw, J . E.; Bergman, R. G. J . Am. Chem.
Soc. 1977, 99, 1666.
(24) Doyama, K.; J oh, T; Onitsuka, K.; Shiohara, T.; Takahashi, S.
J . Chem. Soc., Chem. Commun. 1987, 649.
(25) J oh, T.; Doyama, K.; Onitsuka, K.; Shiohara, T.; Takahashi, S.
Organometallics 1991, 10, 2493.
(26) Mise, T.; Hong, P.; Yamazaki, H. Chem. Lett. 1982, 401.
(27) Hong, P.; Mise, T.; Yamazaki, H. Chem. Lett. 1981, 989.
(28) (a) Iwashita, Y. J .; Tamura, F. Bull. Chem. Soc. J pn. 1970, 43,
1517. (b) Booth, B. L.; Else, M. J .; Fields, R.; Haszeldine, R. N. J .
Organomet. Chem. 1971, 27, 119.

20 New Alkyne-Rhodium Complexes
Organometallics, Vol. 18, No. 17, 1999 3463
Ta ble 4. Rela tive In fr a r ed Absor ba n ce of Miscella n eou s Alk yn e Com p lexes
ν
co (cm-1) above, relative intensity below
ν (cm-1
)
alkyne
phenylacetylene
2122.7
0.45
2122.2
0.77
2121.7
0.55
2121.7
0.45
2121.1
0.53
2088.5
1
2073.6
0.59
2072.9
0.66
2071.9
0.63
2070
0.46
2070.2
0.59
2048
0.21
2045.2
0.30
2042.6
0.29
2039
0.38
2041.3
0.32
2036.8
0.41
2033.2
0.40
2033.6
0.46
2035.2
0.42
2035.1
0.50
1692
0.13
1676.4
0.20
diphenyacetylene
1-phenyl-1-propyne
3,3-dimethylbutyne
cyclohexylacetylene
2090.9
2082
1678
0.20
1674.1
0.20
1667.3
0.26
1661.3sh
0.12
1668
0.26
1671
0.21
0.74
1
2082.5
1
2089.4
0.65
2089.2
0.58
2088.5
0.76
1680.9sh
0.17
2079
1
2080.7
1
1690
0.18
1690.2
0.18
F igu r e 5. Deconvolution of terminal CO region of 1-dode-
cyne rhodium complex spectrum using Gauss-Lorenz
areas.
F igu r e 6. Deconvolution of low-cm-1 region of 1-dodecyne
rhodium complex spectrum using Gauss-Lorenz areas.
σ-Alk en yl Com p lexes. Both monosubstituted and
disubstituted alkynes are known to react with metal
hydrides to yield σ-alkenyl complexes. Examples involv-
ing rhodium include the reaction of RhH(CO)(PPh3)3
with diphenylacetylene to yield (CO)Rh(PPh3)2(PhC2-
HPh)32 and the reaction of RhCl2H(PPh3)2 with acety-
lene to yield RhCl2(PPh3)2(C2H3).33 In such cases the Cd
C vibration appears at ca. 1567 cm-1. The existence of
σ-alkenyl complexes of arbitrary nuclearity in the
present study seems remote since the infrared spectra
can be generated in both the absence and presence of
molecular hydrogen. Furthermore, no other source for
the generation of metal hydrides in the system con-
sisting of rhodium carbonyl/alkyne/CO is readily ap-
parent.
Meta llocyclop en ta d ien e Com p lexes. Both mono-
substituted and disubstituted alkynes are known to
insert multiple times on a wide variety of coordinatively
unsaturated mononuclear complexes, dinuclear com-
plexes, etc., to form metallocyclopentadienes. Thus in
the case of rhodium, mononuclear rhodium cyclopenta-
diene complexes of the form RhClL2{C4(CF3)4} (L )
AsPh3, AsMePh2, AsMe2Ph, etc.) have been synthe-
sized,29 as well as RhClL2{C4(CO2CH3)4} (L ) AsPh3),30
and in the case of dinuclear complexes, (η-C5H5)2Rh2-
{µ-η2,η4-C4(CF3)2HR} has been made.31 Given the con-
jugated structure of metallocyclopentadienes, a complex
vibration in the range 1500-1400 is to be expected.
Such a vibration is not present in our spectra.
(29) Mague, J . T.; Nutt, M. O.; Gause, E. H. J . Chem. Soc., Dalton
Trans. 1973, 2578.
(30) Collman, J . P.; Kang, W. J .; William, F. L.; Sullivan, M. F. Inorg.
Chem. 1968, 7, 1299.
(32) (a) Booth, B. L.; Lloyd, A. D. J . Organomet.Chem. 1972, 35,
195. (b) Sanchez, R. A.; Wilkinson, G. J . Chem. Soc., Dalton Trans.
1977, 804.
(31) Dickson, R. S.; McLure, F. I.; Nesbit, R. J . J . Organomet. Chem.
1988, 349, 413.
(33) Baird, M. C.; Mague, J . T.; Osbourn, J . A.; Wilkinson, G. J .
Chem. Soc. (A) 1967, 1347.

3464 Organometallics, Vol. 18, No. 17, 1999
Liu and Garland
F igu r e 8. Deconvolution of low-cm-1 region of 6-dodecyne
rhodium complex spectrum using Gauss-Lorenz areas.
F igu r e 7. Deconvolution of terminal CO region of 6-dode-
cyne rhodium complex spectrum using Gauss-Lorenz
areas.
located at ca. 1700-1900 cm-1. For example, the
complex ClRh(PPh3)2 (π-PhC2Ph) has a characteristic
Multiple alkyne insertions into σ-alkenyl complexes
have also been noted. Thus RhH(CO)(PPh3)3 has been
shown to react with CF3C2CF3 to yield (CO)Rh(PPh3)2-
(CF3C2HCF3), which further reacts with MeCO2C2CO2-
Me to yield (CO)Rh(PPh3)2(MeCO2C2CO2MeF3C2HCF3).34
σ-Alk yn yl Com p lexes. Monosubstituted alkynes are
known to react with metal hydrides via the elimination
of molecular hydrogen to yield σ-alkynyl complexes.
Examples involving rhodium include the reaction of
RhH(CO)(PPh3)3 with propyne to yield (CO)Rh(PPh3)2-
(CtC-CH3)35 and the reaction RhH(CO)(PPh3)3 with
1-hexyne to yield (CO)Rh(PPh3)2(CtC-C4H9). In such
cases, a weak CtC vibration appears at ca. 2100-2130
cm-1. The existence of σ-alkynyl complexes of arbitrary
nuclearity in the present study can be ruled out entirely
since the 6-terminal-CO pattern in the infrared spectra
can be generated for both monosubstituted and disub-
disubstituted alkynes.
vibration at 1916 cm-1 37
There was no infrared spec-
.
troscopic evidence to suggest that π-bonded alkyne
complexes exist in the present study.
“La cton e” Com p lexes. Cobalt carbonyls react in the
presence of alkynes and CO to form “lactone” complexes
of the general form Co2(CO)7(R1R2C4O2), i.e., µ2-carbo-
nyl-µ2-spiro-[2,3-substituted-2-butenolide-4-ylidene]bis-
[tricarbonylcobalt][Co-Co] derivatives.38 The lactone
structure has a CdC conjugated to a CdO group. The
typical wavenumber for the carbonyl in the lactone
group is ca. 1775 cm-1. Our spectra are inconsistent
with a lactone structure.
Con sisten t Alk yn e Com p lexes. Co2(CO)6 {µ-η2-
(R1C2R2)}. Both monosubstituted and disubstituted
alkynes react with Co2(CO)8 to give the product Co2-
(CO)6 {µ-η2-(R1C2R2)}.39 This is one of the most exten-
sively documented types of organometallic alkyne com-
plexes.40 Crystallographic studies firmly establish the
geometry as Co2(CO)6 {µ-η2-(R1C2R2)}, at least in the
diphenylacetylene complex.41 Infrared analyses confirm
π-Alk yn e Com p lexes. Coordinately unsaturated
complexes can react with both monosubstituted and
disubstituted alkynes to give the corresponding π-bond-
ed alkyne complex, and numerous such complexes
involving Pt, Rh, Ir, and Ni are known and have been
characterized in detail.36 All have a CtC vibration
(37) Mague, J . T.; Wilkinson, G. J . Chem. Soc. (A) 1966, 1736.
(38) (a) Sternberg, H. W.; Shukys, J . G.; Donne, C. D.; Markby, R.;
Friedel, R. A.; Wender, I. J . Am. Chem. Soc. 1959, 81, 2339. (b) Mills,
O. S.; Robinson, G. Proc. Chem. Soc. 1959, 156. (c) Guthrie, D. J . S.;
Khand, I. U.; Knox, R. G.; Kollmeier, J .; Pauson, P. L.; Watts, W. E. J .
Organomet. Chem. 1975, 90, 93. (d) Palyi, G.; Varadi, G.; Vizi-Orosz,
A.; Marko, L. J . Organomet. Chem. 1975, 90, 85. (e) Varadi, G.;
Horvath, I. T.; Palyi, G.; Marko, L. J . Organomet. Chem. 1981, 206,
119.
(34) Eshtiagh-Hosseii, H.; Nixon, J . F.; Poland, J . S. J . Organomet.
Chem. 1979, 164, 107.
(35) Brown, C. K.; Georglou, D.; Wilkinson, G. J . Chem. Soc. (A)
1971, 3121.
(36) van Gaal, H. L. M.; Graef, M. W. M.; van der Ent, A. J .
Organomet.Chem. 1977, 131, 453.

20 New Alkyne-Rhodium Complexes
Organometallics, Vol. 18, No. 17, 1999 3465
that the alkyne vibration is more like a CdC vibration,
i.e., ca. 1630 cm-1, at least for the disubstituted com-
plexes with aliphatic groups.42 In addition, there are
usually only five terminal CO vibrations instead of six
for these complexes, despite their Cs or C1 symmetry.39b,e
For example, the alkyne complex formed from but-2-
yne has absorbance maxima at 2086(s), 2045(vs), 2023-
(vs), 2011(vs), 2006(sh), and 1978(w) cm-1, or approxi-
mate relative intensities of 0.75, 1.0, 1.0, 1.0, 0.1, and
0.25.
µ-η1 a n d µ-η2 Alk yn yl Dir h od iu m Com p lexes.
Dickson et al. prepared a number of dirhodium com-
plexes of the general formulas (η5-C5H5)2Rh2(CO){µ-η1-
(RC2R)} and (η5-C5H5)2Rh2(µ-CO){µ-η2-(RC2R)}, where
(R ) CH3, CF3, or Ph) in the absence of additional CO.43
It should be noted that the µ-η1 alkynyl complexes
assume a terminal arrangement for all the carbonyl
ligands, whereas the µ-η2 alkynyl complexes prefer a
bridged arrangement for at least one carbonyl ligand.
In the former case, the vibration for {µ-η1-(RC2R)}
appears at 1630 cm-1, and in the latter case the
vibration for {µ-η2-(RC2R)} appears at a lower wave-
number such as 1592 cm-1. Also, the µ-η1 alkynyl
complex Rh2Cl2{µ-η1-(CF3C2CF3)}has a prominent vi-
F igu r e 9. Structure of the proposed Rh2(CO)6{µ-η2-
(R1C2R2)} complexes.
and that only arises in the case of disubstituted alkynes
is treated first. This pattern strongly suggests a simple
alkyne-bridged dirhodium carbonyl complex, probably
of the formula Rh2(CO)6(R1C2R2) and quite possibly of
the geometry Rh2(CO)6{µ-η2-(R1C2R2)}, where no bridg-
ing CO ligands are evident. By analogy with the
complexes Co2(CO)6{µ-η2-(R1C2R2)}, such complexes have
only five of the six bands predicted from the Cs or C1
symmetry, even for the symmetric alkyne-substituted
complexes.39b,e Figure 9 shows the proposed structure
for these complexes.
bration at 1638 cm-1 44
.
A question naturally arises concerning the reason that
this complex exists in the case of the disubstituted
alkyne complexes, but not in the case of monosubsti-
tuted alkyne complexes. The most straightforward
explanation seems to be that CO insertion can take
place on the disubstituted alkyne complexes under 2.0
MPa CO; however, conversion is not complete. In other
words, CO insertion into the {Rh-CH} bond in the
monosubstituted alkyne complexes is much more facile,
and the equilibrium is shifted entirely toward the
insertion product under 2.0 MPa CO. The presence of
long alkyl and aryl substituents on the alkyne moiety
appears to considerably hinder CO insertion. Given the
above considerations, we tentatively assign the spectral
pattern consisting of six terminal CO vibrations and two
low cm-1 to the CO insertion product.
The reaction of the monosubstituted alkynes with
rhodium carbonyl gave an infrared spectrum consisting
of six terminal CO vibrations and two low-cm-1 vibra-
tions of almost equal absorptivity. This was true for the
entire homologous series from 1-pentyne to 1-hexade-
cyne and for 3,3-dimethylbutyne. Cyclohexylacetylene
gave a very similar spectral pattern, and phenylacety-
lene gave a pattern with only five terminal CO vibra-
tions, where the relative intensities clearly indicate
coalescence of two bands. The appearance of six termi-
nal CO vibrations is consistent with the further loss of
symmetry elements upon CO insertion into the {Rh-
CH} bond and the expected Cs or C1 symmetry. This
lowering of the symmetry would result in six terminal
CO vibrations in a complex with the formula Rh2(CO)6-
(CO-HC2R1).
CoRh (CO)6{µ-η2-(P h C2P h )}. Diphenylacetylene has
been shown to react with Co2Rh2(CO)12 at 293 K to give
CoRh(CO)6{µ-η2-(PhC2Ph)}.45 The infrared spectrum
showed terminal CO vibrations at 2095(m), 2060(vs),
2044(s), 2034.5(s), 2015(m), and 2009.5(m) cm-1 in
n-hexane. Unfortunately, no low cm-1 were reported,
but the NMR is consistent with the structural assign-
ment.
(µ-η1-CORC2R) Din u clea r Com p lexes.A few ex-
amples exist for the insertion of CO into a metal-alkyne
bond in dinuclear complexes leading to the formation
of (µ-η1-CORC2R) dinuclear complexes. These include
the complex (η5-C5H5)2Rh2(µ-CO){µ-η1-(HC2TMS)}{µ-η1-
(COCF3C2CF3),} for which spectroscopic evidence is
available.46 Furthermore, in the case of cobalt both the
complexes Co2(CO)6{µ-η1-(COHC2H)} and Co2(CO)6{µ-
η1-(COHC2HCO)} have been postulated due to the
formation of CH2dCHCOOEt and EtOOCHCHd
CHCHCOOEt upon treatment with EtOH.47
Th e Ten ta tive Assign m en t. Given the above ob-
servations, we propose a tentative assignment for our
complexes. The spectral pattern consisting of five ter-
minal CO vibrations and a vibration at ca. 1630 cm-1
(39) (a) Sly, W. G.; Breu, R. Chem. Ber. 1957, 90, 1270. (b) Bor, G.
Proc. Conf. Coord. Chem. Bratislava 1965, 362. (c) Sternberg, H. W.;
Greenfield, H.; Friedel, R. A.; Wotiz, J . H.; Markby, R.; Wender, I. J .
Am. Chem. Soc. 1954, 76, 1457. (d) Greenfield, H.; Sternberg, H. W.;
Friedel, R. A.; Wotiz, J . H.; Markby, R.; Wender, I. J . Am. Chem. Soc.
1956, 78, 120. (e) Bor, G. Chem. Ber. 1963, 96, 2644. (e) Tirpak, M. R.;
Wotiz, J . H.; Hollingsworth, C. A. J . Am. Chem. Soc. 1958, 80, 4265.
(40) Dickson, R. S.; Fraser, P. J . Adv. Organomet. Chem. 1974, 12,
323.
(41) Sly, W. G. J . Am. Chem. Soc. 1959, 81, 18.
(42) Iwashita, Y.; Ishikawa, A.; Kainosho, M. Spectrochim. Acta
1971, 27A, 271.
(43) Dickson, R. S.; Pain, G. N. J . Chem. Soc., Chem. Commun. 1979,
197, 277.
(44) Cowie, M.; Southern, T. G. Inorg. Chem. 1981, 20, 2682.
(45) Horvath, I. T.; Zsolnai, L.; Huttner, G. Organometallics, 1986,
5, 180.
(46) Dickson, R. S.; McLure, F. L.; Nesbit, R. J . J . Organomet. Chem.
1988, 349, 413.
(47) Iwashita, Y. J ; Tamura, F.; Wakamatsu, H. Bull. Chem. Soc.
J pn. 1970, 43, 1520.
Furthermore, we note again that the two low-cm-1
bands are not identical. The band at 1689 cm-1 has a
relatively narrow width at half-height and that band
at 1668 cm-1 is somewhat broader. The positions of
these bands, their widths at half-height, and their
approximately equal integral absorbance suggest that
the band at 1689 cm-1 belongs to a CdO moiety, the
band at 1668 cm-1 belongs to a CdC moiety, the two

3466 Organometallics, Vol. 18, No. 17, 1999
Liu and Garland
Com m en t on th e Mech a n ism of F or m a tion . The
formation of dirhodium carbonyl alkyne complexes from
tetrarhodium dodecacarbonyl and alkynes most likely
follows one of the following two mechanisms. Mech a -
n ism 1: It is well-known that the dinuclear complex
Rh2(CO)8 is readily formed from Rh4(CO)12 under CO,
although its concentration is very low at ambient
temperatures.50 Given the presence of equilibrated Rh2-
(CO)8 in solution and the frequent and facile dissociation
of carbonyl ligands, reaction of an alkyne with a
coordinatively unsaturated dirhodium carbonyl species
could easily explain the mechanism of formation for the
species under consideration. Mech a n ism 2: The de-
tailed studies of unmodified rhodium-catalyzed hydro-
formylations of alkenes starting with Rh4(CO)12, men-
tioned previously in the Introduction, have shown that
CO-assisted polyhedron opening seems to be the com-
mon initial step in the induction kinetics. This is then
followed by molecular hydrogen attack on the opened
cluster. In the present case dealing with the formation
of dirhodium alkyne complexes, it may be possible that
Rh4(CO)12 is opened by CO and then alkynes attack the
opened cluster directly, resulting in the dirhodium
alkyne complexes without formally going through dirhod-
ium carbonyl intermediates.
F igu r e 10. Structure of the proposed Rh2(CO)6{µ-η1-(CO-
R1C2R2)} complexes.
groups are conjugated, and the stoichiometry is 1:1 for
these functional groups. Given the precedence of the
complex
(η5-C5H5)2Rh2(µ-CO){µ-η1-(HC2TMS)}{µ-η1-
(COCF3C2CF3)}, it would seem that our species is most
probably Rh2(CO)6 {µ-η1-(CO-HC2R1)}. The conjugation
of CdO with CdC would exist in a rather electron-rich
environment, assuming the presence of a Rh-Rh bond
(which would be predicted by electron-counting rules).
Assuming an electron-rich environment, it is instructive
to compare the characteristic vibrations of 1689 and
1668 cm-1 seen in these complexes for CdO and CdC,
respectively, with the characteristic vibrations of 1679
and 1650 cm-1 seen in the extremely electron-rich
compound p-benzoquinone for the same groups.48
A few outstanding details should be addressed con-
cerning the low-cm-1 bands. First, the CdO vibration
occurs at a higher wavenumber in the monosubstituted
alkyne complexes than in the disubstituted alkyne
complexes. Second, in the case of the disubstituted
alkyne complexes, the low-cm-1 vibrations slowly coa-
lesce as the molecular weight of the substituents
increases. Third, there is definite signal broadening.
These characteristics are most readily seen in the
sequence of complexes 1-decyne to 5-decyne (Figure 2).
In particular, we note that the CdC vibration is es-
sentially stationary, whereas the CdO vibration only
undergoes a shift to lower cm-1 with increasing sub-
stituent length. To explain this result, we look at the
series of organic compounds 2-cyclohexen-1-one, 2-methyl-
2-cyclohexen-1-one, 3-methy-2-cyclohexen-1-one, and
2,3-dimethyl-2-cyclohexen-1-one. In this series, the Cd
O vibrations occur at 1691, 1681, 1681, and 1673 cm-1
and the CdC vibrations occur at 1618, 1641, (unknown),
and 1634 cm-1, respectively.49 Therefore substitution
leads to a systematic shift in the CdO vibration toward
lower cm-1. In addition, increased alkyl substituent
length should further the trend. Increasing alkyl sub-
stituent length should also lead to additional stearic
influences and a broadening of the bands, which is also
apparent in these complexes. Therefore, we tentatively
assign the probable structure Rh2(CO)6{µ-η1-(CO-
R1C2R2)} to the disubstituted alkyne complexes. Figure
10 shows the proposed structure for these complexes.
Finally, it would appear that high-pressure NMR
would be the most direct means of conclusively deter-
mining the structure of these complexes, since they
cannot be isolated.
Con clu sion s
The in-situ infrared spectroscopic results strongly
suggest that only two types of alkyne rhodium com-
plexes result from the reaction of alkynes with Rh4-
(CO)12 under CO and CO/H2 at 293 K in n-hexane
solvent, i.e., under reaction conditions resembling hy-
droformylation. Indeed, analysis of the deconvolution
results shows that only two repeating absorbance pat-
terns occur throughout the study. A large number of
possible species were discounted by simple inspection
and comparison of the spectral patterns; these included
metallocyclopentadienes, metal σ-alkenyl complexes,
metal σ-alkynyl complexes, metal “lactone” complexes,
etc.
The reaction of the monosubstituted alkynes consis-
tently gave a final product involving six terminal νCO
vibrations in the region 2030-2121 cm-1 and two strong
vibrations at ca. 1668 and 1686 cm-1, and the disub-
stituted alkynes consistently gave a final spectrum
consistent with the superposition of the aforementioned
spectrum, plus a spectrum involving five terminal νCO
vibrations in the region 2046-2095 cm-1 and one strong
vibrations at ca. 1630 cm-1. Due to the band positions
and absorptivities, we tentatively propose that these
species are substituted dirhodium carbonyl species,
specifically Rh2(CO)6{µ-η2-(R1C2R2)} for the spectral
pattern consisting of five terminal CO vibrations and
arising only in the case of disubstituted alkynes. The
spectral pattern consisting of six terminal CO vibrations
is tentatively assigned to Rh2(CO)6{µ-η1-(CO-HC2R1)}
and Rh2(CO)6{µ-η1-(CO-R1C2R2)} for the terminal
alkynes and disubstituted alkynes, respectively. In other
words, the present complexes appear to be the rhodium
(48) Padler, K. G. R.; Matlok, F.; Grunlich, H. U. Merck FTIR Atlas;
VCH: Weinheim, 1988.
(49) (a) Noack, K. Spectrosc. Chim. Acta 1962, 215. (b) Bellamy, L.
J .; Pace, R. J . Spectrosc. Chim. Acta. 1963, 1831. (c) Inayama, S. Chem.
Pharm. Bull. 1956, 4, 198. (d) Mecke, R.; Noack, K. Spectrosc. Chim.
Acta. 1960, 210. (e) Dolphin, D.; Wick, A. Tabulation of Infrared
Spectral Data; Wiley: New York, 1977.
(50) (a) Whyman, R. J . Chem. Soc., Dalton Trans. 1972, 1375. (b)
Vidal, J . L.; Walker, W. E. Inorg. Chem. 1981, 20, 249. (c) Oldani, F.;
Bor, G. J . Organomet. Chem. 1983, 246, 309. (d) Hanlan, L. A.; Ozin,
G. A. J . Am. Chem. Soc. 1974, 96, 6324.

20 New Alkyne-Rhodium Complexes
Organometallics, Vol. 18, No. 17, 1999 3467
analogues of the well-known dicobalt carbonyl alkyne
complexes.
The stability of the present alkyne complexes under
CO even in the presence of molecular hydrogen appears
to be the primary reason that the presence of trace
quantities of alkynes can poison alkene hydroformyla-
tions. The alkynes react with the rhodium in the system
and prevent the availability of rhodium for the alkene
hydroformylations.
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