Activation of CO2 by laser-ablated group 6 metal atoms

Article abstract of DOI:10.1039/a700360i

The primary reaction products of laser-ablated group 6 atoms with CO₂ prove to be the insertion products OMCO and O₂M(CO)₂ (M = Cr, Mo, W) which are isolated in argon matrices and identified by the effects of isotopic subs

Full text of DOI:10.1039/a700360i

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Activation of CO2 by laser-ablated group 6 metal atoms
Philip F. Souter and Lester Andrews*
Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901, USA
10
The primary reaction products of laser-ablated group 6
atoms with CO2 prove to be the insertion products OMCO
and O2M(CO)2 (M = Cr, Mo, W) which are isolated in argon
matrices and identified by the effects of isotopic substitution
on their IR spectra.
investigated both experimentally in solid CO2 and theoret-
ically.11 In this laboratory, the reactions of B, U, Ti and Be with
CO2 have been investigated and all four were found to insert
into CO2 to give OMCO (M = B, U, Ti, Be) as the primary
reaction product.12–15 Much attention has been devoted to the
organometallic chemistry of CO2 and a comprehensive review
has recently been published.4 The main problem with the
chemistry of CO2 lies in its inherent thermodynamic stability
and methods of ‘activating’ CO2 are consequently actively
being sought. We have studied the reactions between laser-
ablated group 6 metal atoms and CO2–Ar mixtures and have
trapped the reaction products, including OMCO and
O2M(CO)2, in argon matrices and identified them by the effects
of isotopic substitution.
Carbon dioxide is a naturally abundant carbon source that has
been implicated as a contributor to the predicted global
warming often referred to as the ‘greenhouse effect’. The
interaction between CO2 and transition-metal centres is receiv-
ing increased attention1–4 as the possibilities of recycling CO2
generated in industrial emissions5 and also of replacing
petroleum with CO2 as the starting material for the synthesis of
fine chemicals6 both represent exciting and important goals.
Previous studies of the interaction of metal atoms with CO2 in
low-temperature matrices focussed on the interaction of Li,7 Cs8
and Al9 with CO2. Additionally the interaction of several of the
first-row transition metals, including Cr, with CO2 has been
The technique used for matrix investigation of the reactions
of pulsed laser-ablated metal atoms has been detailed previ-
ously.12–16 FTIR spectra were recorded on a Nicolet 550 at 0.5
cm21 resolution. Typically mixtures of between 0.5 and 2%
O2W(CO)2
1.4
5.0
O2W(CO)2
OWCO
O2WCO
CO
OWCO
O2W(CO)2
O2W(CO)2
13CO
(d)
(c)
(b)
(a)
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1.2
(d)
OWCO
O2W(CO)2
OWCO
O2W(CO)2
O2W(CO)2
C18
O2WCO
1.0
0.8
0.6
0.4
0.2
0.0
O2W(CO)2
CO
O
(c)
(b)
OWCO
O2W(CO)2
O2W(CO)2
O2W(CO)2
O2W(CO)2
O2WCO
OWCO
CO
CO
O2W(CO)2
O2W(CO)2
O2W(CO)2
O2WCO
OWCO
OWCO
O2W(CO)2
(a)
960
940
920
900
–1
880
860
840
2100
2050
2000
ν / cm
1950
–1
1900
1850
~
~
ν / cm
Fig. 1 IR spectra in the region 2150–1800 cm21 for samples from the
reactions of W atoms with CO2 mixtures in argon during condensation at
6–7 K; (a) 2% CO2, (b) 0.5% CO2, (c) 0.5% C16O2–C16O18O–C18O2 and (d)
0.5% 12CO2–13CO2 after annealing to 20 K followed by broad-band UV
photolysis and further annealing to 30 K
Fig. 2 IR spectra in the region 980–830 cm21 for samples from the reactions
of W atoms with CO2 mixtures in argon during condensation at 6–7 K; (a)
2% CO2, (b) 0.5% CO2, (c) 0.5% C16O2–C16O18O–C18O2 and (d) 0.5%
12CO2–13CO2 after annealing to 20 K followed by broad-band UV
photolysis and further annealing to 30 K
Table 1 Observed IR absorptions (cm21) of the dominant products in the reaction of laser-ablated group 6 atoms with CO2 trapped in an Ar matrix at ca.
6–7 K
Molecule
OCrCOa
OMoCOb
OWCO
CO2
13CO2
12CO2–13CO2
C18O2
C16O2–C16OC18O–C18O2 Assignment
2014.4
866.3
1847.0
951.8
1879.0
969.6
2123.2
2059.7
981.4
1969.7
866.3
1804.3
952.4
1837.0
969.6
2074.8
2013.8
981.4
2014.4, 1969.7
866.3
1847.0, 1804.3
951.8, 952.4
1879.0, 1837.0
969.6
2123.2, 2074.8
2059.7, 2013.8
981.4
1967.3
829.5
1806.1
905.3
1838.8
918.9
2075.3
2011.6
945.1
2014.4, 1967.3
866.3, 829.5
1847.0, 1806.1
951.8, 905.3
1879.0, 1838.8
969.6, 918.9
2123.2, 2107.2, 2075.3
2058.7, 2027.6, 2011.6
981.4, 975.4, 945.1
941.6, 903.0, 842.2
2101.9, 2084.6, 2054.7
2020.1, 1990.0, 1972.7
885.6, 856.7, 846.1
2091.1, 2073.4, 2045.1
1998.2, 1969.7, 1952.1
961.6, 944.3, 909.4
901.1, 870.4, 856.1
n(C–O)
n(Cr–O)
n(C–O)
n(Mo–O)
n(C–O)
n(C–O)
a
O2Cr(CO)2
n(C–O)
n(C–O)
n(Cr–O)c
n(Cr–O)d
n(C–O)
941.6
941.6
941.6
892.2
b
O2Mo(CO)2
O2W(CO)2
2101.9
2020.1
885.6
2091.1
1998.2
961.6
2053.9
1975.4
885.6
2042.9
1953.6
961.6
2101.9, 2084.0, 2053.8
2020.1, 1991.4, 1975.4
885.6
2091.1, 2072.7, 2042.9
1998.2, 1970.4, 1953.6
961.6
2054.7
1972.7
846.1
2045.1
1952.1
909.4
n(C–O)
n(Mo–O)c
n(C–O)
n(C–O)
n(W–O)d
n(W–O)c
901.1
901.1
901.1
856.1
a Refers to 52Cr. b Refers to 98Mo. c Antisymmetric M–O stretch. d Symmetric stretch.
Chem. Commun., 1997
777

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CO2 in Ar were deposited at a rate of ca. 3 mmol h21 for 1–2 h
onto a CsI window held at 6–7 K while the metals were ablated
using 35–50 mJ pulse21 of the YAG 1064 nm fundamental.
Figs. 1 and 2 show the effects of isotopic substitution upon
the IR spectra of matrix-isolated deposits containing the
products from the reaction of W with CO2 in Ar. The
frequencies of the observed bands from the reactions of CO2
with Cr, Mo and W and their proposed assignments are
presented in Table 1.
Other bands: the previously characterised metal monoxides,
MO,21–23 were observed although this represents the first
reported isolation of CrO (846.5 cm21) in a matrix. In the case
of the reaction with tungsten, a bridged species OW(m-
CO)2WO, the dimer of OWCO, is formed. Only the antisym-
metric CO stretching mode is observed in a region typical for
bridging CO groups. The observed band at 1713.4 cm21 from
the reaction of W with CO2 shifts to 1674.5 cm21 on reaction of
W with 13CO2 and 1675.9 cm21 on reaction of W with C18O2.
It shows a 1:2:1 triplet at 1713.4, 1692.8 and 1675.9 cm21
upon reaction with either C16O2–C18O2 or C16O2– C16O18O–
C18O2, and a 1:2:1 triplet at 1713.4, 1691.9 and 1674.5 cm21
upon reaction with a 12CO2–13CO2 mixture, indicating two
equivalent CO groups. The product is not observed in reactions
involving CO and its yield is suppressed by using higher
concentrations (2%) of CO2, presumably due to further reaction
of OWCO with CO2. The lack of an analagous product for Cr or
Mo probably reflects the greater strength of the metal–metal
bonding, present in such a complex, of tungsten. DFT
calculations confirm the presence of metal–metal bonding in
this molecule. Other products observed include the molecules
OMCO: the dominant product from the reactions of group 6
metal atoms with 0.5% CO2–Ar mixtures is the direct insertion
product OMCO. Full details are given in Table 1 for all three
metals which behave in an analogous fashion. In the case of
tungsten, absorptions at 1879.0 and 969.6 cm21 correspond to
the CO and WO stretching fundamentals of the OWCO
molecule. In experiments run with 13CO2 and C18O2, these
bands shift to 1837.0 and 969.6 cm21 and 1838.8 and 918.9
cm21, respectively, confirming that this molecule contains a CO
and a WO group, based upon comparison of observed isotopic
frequency ratios with calculated harmonic diatomic ratios.
Experiments run with 12CO2–13CO2, C16O2–C18O2 and C16O2–
C16O18O–C18O2 all revealed isotopic doublets in the CO
stretching region and the latter two revealed doublets in the WO
stretching region. This isotopic pattern confirms the presence of
exactly one CO group and exactly one WO group allowing
definitive spectral assignment of these bands to the OWCO
molecule. For both Cr and Mo isotopic splittings due to the
natural abundances of the metal isotopes were observed for the
MO stretching fundamental and the observed statistical dis-
tribution confirmed the presence of exactly one metal atom.
These findings are in contrast to a previous study of the reaction
of thermally generated chromium atoms with neat CO2 in which
there was no evidence for the insertion reaction.10†
1
O2MCO, OCr(CO)2 and the CO2 complex Cr(h -OCO) and
these results will be discussed in a full paper.
Footnotes
* E-mail: lsa@virginia.edu
† The band observed in ref. 10 at 960 cm21, tentatively assigned to a CO2
reaction product, occurs at almost the same energy as the antisymmetric
stretch of CrO2, an almost inevitable impurity in a reaction involving
thermally generated Cr atoms. With no isotopic data and no associated
carbonyl stretches this band is most likely due to CrO2 in a CO2 matrix.
References
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proved to arise from the insertion of the metal atoms into two
CO2 molecules. As might be anticipated, by raising the
concentration of CO2 from 0.5 to 2% the relative yield of
O2M(CO)2 to OMCO increases. In the CO stretching region for
all three metals, the symmetric and antisymmetric CO stretch-
ing fundamentals were both observed. In the case of the reaction
of W with CO2, the bands at 2091.1 and 1998.2 cm21 can be
assigned to the symmetric and antisymmetric CO stretching
modes of O2W(CO)2 while the bands at 961.6 and 901.1 cm21
can be assigned to the symmetric and antisymmetric WO
stretching modes of the O2W(CO)2 molecule. Reaction with
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13CO2 gave bands at 2042.9, 1953.6, 961.6 and 901.1 cm21
,
whereas reaction with C18O2 gave bands at 2045.1, 1952.1,
909.4 and 856.1 cm21. Reaction with either C16O2–C18O2 or
C16O2–C16O18O–C18O2 gave rise to 1:2:1 triplets for all four
bands. This is indicative of the presence of precisely two
equivalent CO groups and two equivalent oxygen atoms and
confirms that the two oxygens and CO groups bound to the
tungsten must come from two different CO2 molecules.
Reaction with a 12CO2–13CO2 mixture revealed a 1:2:1
isotopic triplet for both CO stretching fundamentals further
confirming the presence of two equivalent CO groups. In the
cases of both Cr and Mo, the presence of exactly one metal atom
is clearly demonstrated by the statistical distribution of the
metal isotopic pattern for the M–O antisymmetric stretching
fundamental. There have been numerous previous studies
involving the photolysis of M(CO)6 with O218–21 and bands in
these spectra were assigned to the O2M(CO)2 molecules. The
CO stretching region in these experiments was extremely
congested but the bands that are observed here for O2Mo(CO)2
and O2W(CO)2 are essentially the same as those observed by
Almond et al. for their species ‘C’ which was assigned to
O2M(CO)x, not those observed for their species ‘D’ which was
assigned to O2M(CO)2.20 Our results for O2Cr(CO)2 do
however confirm the previous correct identification of this
molecule.18,20
Received in Columbia, MO, USA, 15th January 1997; Com.
7/00360I
778
Chem. Commun., 1997