Infrared spectra of CH3-MH, CH3-M, and CH 3-MH- prepared via methane activation by laser-ablated Au, Ag, and Cu atoms

Article abstract of DOI:10.1039/c0dt01827a

Methane activation by laser-ablated, excited Group 11 metal atoms has been carried out, leading to generation of CH₃-MH, CH₃-M, and CH₃-MH^-, which are identified in the product infrared spectra on the basis of isotopic shifts and correlation with DFT calculated frequencies. The products reveal that C-H insertion by excited Au, Ag, and Cu readily occurs, and subsequent hydride-detachment or electron addition also follows. Each type of product has similar photochemical properties regardless of the metal. DFT computed energies reveal facile hydride dissociation and high electron affinities for the insertion complexes. The methyl metal species have the shortest C-M bonds, consistent with their highest calculated effective bond order, and the CH₃-MH complexes have higher electron affinities than the metal atoms.

Full text of DOI:10.1039/c0dt01827a

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PAPER
Infrared spectra of CH3–MH, CH3–M, and CH3–MH- prepared via methane
activation by laser-ablated Au, Ag, and Cu atoms†
Han-Gook Choa and Lester Andrews*b
Received 24th December 2010, Accepted 24th February 2011
DOI: 10.1039/c0dt01827a
Methane activation by laser-ablated, excited Group 11 metal atoms has been carried out, leading to
generation of CH3–MH, CH3–M, and CH3–MH-, which are identified in the product infrared spectra
on the basis of isotopic shifts and correlation with DFT calculated frequencies. The products reveal
that C–H insertion by excited Au, Ag, and Cu readily occurs, and subsequent hydride-detachment or
electron addition also follows. Each type of product has similar photochemical properties regardless of
the metal. DFT computed energies reveal facile hydride dissociation and high electron affinities for the
insertion complexes. The methyl metal species have the shortest C–M bonds, consistent with their
highest calculated effective bond order, and the CH3–MH complexes have higher electron affinities than
the metal atoms.
AgH- and CH3–CuH-) and dialkyl metals in the fragmentation
Introduction
of [RCO2AgO2CR]- by means of mass spectrometry.8
Group 11 metals have played distinctive roles for making coinage,
Extensive recent studies have shown that C–H activation of
jewelry, and art since the beginning of human history. They are at
alkanes by transition-metal atoms is in fact a general phenomenon,
the borderline between the main group elements and transition-
leading to small insertion and high oxidation-state complexes
metals due to their d10 electronic configuration.1–3 The coinage
metals often behave as Lewis acids with their relatively large
electron affinities. In particular, gold has a small atomic radius
and the highest electronegativity (2.54) among transition-metals
along with its strong relativistic contraction, and its chemistry has
been investigated extensively.1 Recently there has been an upsurge
of research activity in homo- and heterogeneous catalytic reactions
by the coinage metals, which provide detailed information on their
complexes, particularly with p systems.1–3
(carbenes and carbynes).9–13 Their small sizes are amenable to high-
level computations,14 and many of them show interesting structural
distortions and photochemical reactions. In this study, we report
the results of Group 11 metal atom reactions with methane in
excess argon. The products are identified on the basis of isotopic
shifts and correlation with DFT calculations. Thus, coinage metal
atoms are also effective methane activation agents.
Coinage metal complexes with carbon–metal bonds are rare.
Alkyl silver was first prepared by reaction of tetra-alkyl lead and
silver nitrate at liquid nitrogen temperature,4 and alkyl copper
by reaction of alkyl lithium and cuprous chloride below 0◦ C.5
Grotjahn et al. characterized CH3–Cu with gas phase rotational
spectra.5a Parnis et al. reported the formation of CuH, CH3–CuH,
and CH3–Cu in a CH4 matrix after UV irradiation, and provided
clear evidence that the reaction proceeds with Cu excited to the
2P state,6a which is in line with the later finding of the Weisshaar
group that ground state copper atoms are not reactive with alkanes
in the gas phase.6c Chang et al. observed CuCH2 in the matrix IR
spectra from the reaction of Cu with CH2N2.7 Rijs and O’Hair
have observed anionic Ag and Cu insertion complexes (CH3–
Experimental and computational methods
Laser ablated Au, Ag, and Cu atoms were reacted with CH4 (Math-
eson, UHP grade), 13CH4, CD4, CH2D2 (Cambridge Isotopic
Laboratories) in excess argon during condensation at 8 K using
a closed-cycle refrigerator (Air Products Displex). Reagent gas
mixtures ranged 0.5–5.0% (typically 2%) in argon. A small amount
of CCl4 (Fisher) was added to the sample (~0.02%) as an electron
scavenger in selected experiments to gain information for the iden-
tification of ionic species as described in previous publications.9,15
The Nd : YAG laser fundamental radiation (1064 nm, 10 Hz
repetition rate, 10 ns pulse width) was focused on rotating Au, Ag,
and Cu targets using 5–10 mJ pulse-1, and the ablated material
was co-deposited with the argon–methane samples as illustrated.9
After initial reaction, infrared spectra were recorded at 0.5 cm-1
resolution using a Nicolet 550 spectrometer with a Hg–Cd–Te
range B detector. Then samples were irradiated for 20 min periods
by a mercury arc street lamp (175 W) with the globe removed using
a combination of optical filters and were annealed to allow further
reagent diffusion.
aDepartment of Chemistry, University of Incheon, 12-1 Songdo-dong, Yonsu-
gu, Incheon, 406-772, South Korea
bDepartment of Chemistry, University of Virginia, P. O. Box 400319,
Charlottesville, Virginia, 22904-4319. E-mail: lsa@virginia.edu
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c0dt01827a
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 11115–11124 | 11115
©

To provide support for the assignment of new experimental fre-
quencies and to correlate with related work,9–13 density functional
theory (DFT) calculations were performed using the Gaussian
09 program system,16 the B3LYP density functional,17 the 6-
311++G(3df,3pd) basis sets for H, C, and Cu18 and SDD core
potential and basis sets for Au and Ag19 to provide vibrational
frequencies for the reaction products. Geometries were fully
relaxed during optimization, and the optimized geometry was
confirmed by vibrational analysis. The BPW9120 functional and
MP221 method were also employed to complement the B3LYP
results. The vibrational frequencies are calculated analytically, and
zero-point energy is included in the calculation of binding and
reaction energies. Previous investigations have shown that DFT
calculated harmonic frequencies are usually slightly higher than
observed frequencies,9–13,22,23 and they provide useful predictions
for the infrared spectra of new molecules.
methane isotopomers were investigated, and DFT frequency
calculations of the products (Tables S1–S9†) and their structures
(Fig. 7–9) will be presented in turn.
Au + CH4
Infrared spectra are shown in Fig. 1 for the reaction products of
laser ablated Au atoms co-deposited with CH4 and with CH4 +
CCl4 in excess argon during condensation at 8 K. Fig. 2 and
S1† show the corresponding absorptions from the deuterated and
half deuterated products. Three groups of product absorptions
are observed (marked “i, a, and i-” for insertion, methyl auride,
and insertion product anion). The absorption groups are based
on the intensity variations upon photolysis and annealing. The
i absorptions vanish on visible (l > 420 nm) irradiation, but
reappear on UV (240 < l < 380 nm) irradiation and further
increase on full arc (l > 220 nm) irradiation and annealing to
~50% of the original intensity. The a absorptions increase by ~5,
~25, and ~10% on visible, UV, and full arc photolysis (~40%
increase in total) and decrease markedly on annealing. The i-
absorptions remain almost unchanged on visible irradiation, but
decrease ~30% each on UV and full arc irradiation.
Results and discussion
The matrix infrared spectra (Fig. 1–6 and S1–3†, and Table 1–
3) from reactions of laser-ablated Au, Ag, and Cu atoms with
Table 1 Observed frequencies and assignments for the product absorptions from Au + methane isotopomer reactionsa
Groupb
CH4
CD4
13CH4
CH2D2
Descriptionc
i
1853.5
1044.9
CH3–AuH, Au–H stretch
CH3–AuH, CH3 deform
CH3–AuH, CH3 rock
CH3Au, CH3 deform
790.3
1040.2
977.0, 898.4
644.0
1123.6, 983.2
a
1188.3,
1181.7
794.5
925.7,
920.5
584.0
1341.5
858.0
553.4
1178.4,
1173.3
790.6
784.4, 599.7, 697.7
1866.6, 1345.7
CH3Au, CH3 rock
i-
1868.7
1868.7
CH3–AuH-, Au–H stretch
CH3–AuH-, CH3 deform
CH3–AuH-, CH3 rock
726.6
722.8
a Observed in an argon matrix. Frequencies are in cm-1. b Product absorption group on the basis of intensity variation in the process of photolysis and
annealing. c Designated product and vibrational mode.
Table 2 Observed frequencies and assignments for the product absorptions from Ag + methane isotopomer reactionsa
Groupb
CH4
CD4
13CH4
CH2D2
Descriptionc
a
1076.4
666.1
1562.0
824.1
499.1
1124.2
1069.3
663.6
1562.0
917.1
586.7
1560.9
CH3–Ag, CH3 deform
CH3–Ag, CH3 rock
i-
CH3–AgH-, Ag–H stretch
a Observed in an argon matrix. Frequencies are in cm-1. b Product absorption group on the basis of intensity variation in the process of photolysis and
annealing. c Designated product and vibrational mode.
Table 3 Observed frequencies and assignments for the product absorptions from Cu + methane isotopomer reactionsa
Groupb
CH4
CD4
13CH4
CH2D2
Descriptionc
CuH
i
1878.5 (1850)d
1718.8 (1697)d
1012.1 (1011)d
1203.0 (1196)d
648.3
1353.9 (1333)
1243.7 (1229)
786.0 (786)
920.0 (916)
494.2
1879.1 (1850)
1717.7 (1697)
1003.6 (1003)
1195.1 (1189)
645
1878.7, 1353.9
1718.9, covered
861.0
CuH, Cu–H stretch
CH3–CuH, Cu–H stretch
CH3–CuH, CH3 deform
CH3–Cu, CH3 deform
CH3–Cu, CH3 rock
a
637.2, 594.4, 563.7, 498.1
1611.9, 1166.0
i-
1612.9
1165.0
1612.9
CH3–CuH-, Cu–H stretching
a Observed in an argon matrix. Frequencies are in cm-1. b Product absorption group on the basis of intensity variation in the process of photolysis and
annealing. c Designated product and vibrational mode. d Observed in solid methane matrix, ref. 6a.
11116 | Dalton Trans., 2011, 40, 11115–11124
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The Royal Society of Chemistry 2011
©

Fig. 1 Infrared spectra in the 1970–1770, 1250–1000, and 850–650 cm-1 regions for the reaction products of the laser-ablated gold atom with CH4 and
with CH4 + CCl4 in excess argon at 8 K. (a) Au and CH4 + CCl4 (2.0 and 0.02% in argon) co-deposited for 1 h, (b) as (a) after visible (l > 420 nm)
irradiation, and (c) as (b) after UV (240–380 nm) irradiation. (d) Au and CH4 reagent (2.0% in argon) co-deposited for 1 h, (e)–(g) as (d) after visible, UV,
and full arc (l > 220 nm) irradiation, and (h) as (g) after annealing to 26 K. a, i, and i- stand for product absorption groups. Common products from
CCl4, which was added to serve as an electron scavenger in the matrix sample, are labelled.
with the B3LYP values of 1089.7, 264.6, and 5.5 cm-1 for the
strongest absorption. The observed product frequencies in the
CH2D2 spectra are also in line with the B3LYP values of 1018.4
and 922.7 cm-1 for CH2D–AuD and CHD2–AuH.
The weak i absorptions observed at 1853.5 and 644.0 cm-1 in the
CH4 and CH2D2 spectra are assigned to the Au–H stretching and
CH3 rocking modes as shown in Table 1. The excellent agreement
between the observed and computed frequencies and isotopic
shifts substantiates formation of the gold insertion complex (CH3–
AuH) in the doublet ground state (Table S1†). The present results
show that the ordinarily unreactive gold atom, upon excitation,
undergoes C–H insertion with CH4, producing a simple insertion
complex like other transition-metals (Groups 3–10), lanthanides,
and actinides.9–13 This reaffirms that C–H insertion by an excited
transition-metal is a general chemical phenomenon.
The a absorption at 1181.7 cm-1 shows D and 13C counterparts at
Fig. 2 Infrared spectra in the 1450–1250 and 1000–500 cm-1 regions for
the reaction products of the laser-ablated gold atom with CD4 in excess
920.5 and 1173.3 cm-1 (H : D and 12 : 13 ratios of 1.284 and 1.007,
respectively). We assign this absorption to the CH3 deformation
argon at 8 K. (a) Au and CD4 (2.0% in argon) co-deposited for 1 h, (b)–(d)
mode of methyl auride (CH3–Au) on the basis of the good
as (a) after visible, UV, and full arc irradiation, and (e) as (d) after annealing
agreement with the predicted values (e.g. B3LYP values of 1199.5,
to 26 K. a, i, and i- stand for product absorption groups while P and c
922.2, and 1192.2 cm-1 for CH3–Au, CD3–Au, and 13CH3–Au
stand for the precursor and common absorptions from methane.
shown in Table S2†). The observed frequencies of 1123.6 and
A strong i absorption observed at 1044.9 cm-1 has its D and
13C counterparts at 790.3 and 1040.2 cm-1 (H : D and 12 : 13 ratios
of 1.322 and 1.005). Absorptions from partly deuterated products
are also observed at 977.0 and 898.4 cm-1 in the CH2D2 spectra
in Fig. S1.† The strong absorption showing large D and small
C shifts is assigned to the CH3 deformation mode of the most
probable insertion complex (CH3–AuH). The observed frequency
and D and 13C isotopic shifts (254.6 and 4.7 cm-1) are compared
983.2 cm-1 in the CH2D2 spectra are also consistent with the
B3LYP values of 1125.6 and 999.0 cm-1 for CH2D–Au and CHD2–
Au.
The stronger a absorption at 794.5 cm-1 carries D and 13C
counterparts at 584.0 and 790.6 cm-1, (compared with the B3LYP
values of 803.4, 594.0, and 799.9 cm-1) and it is assigned to the
CH3 rocking mode of methyl auride. The observed frequencies
of 784.4, 697.7, and 599.7 cm-1 in the CH2D2 spectra are in
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 11115–11124 | 11117
©

line with the predicted values of 790.3 cm-1 for CH2D–Au and
697.5 and 596.4 cm-1 for CHD2–Au. The observed frequencies,
isotopic shifts, and good agreement with the predicted values all
provide strong evidence for formation of another primary product
of methane activation by gold, CH3–Au.
Because the auride anion lies between bromide and iodide in
size,1g it may be interesting to compare some properties of CH3I
and CH3Au. The B3LYP Mulliken charges for CH3Au (-0.54,
0.07 ¥ 3 and 0.32) are similar to those of CH3I (-0.44, 0.10 ¥
3 and 0.14), but the dipole moment 0.20 D is smaller than that
of CH3I, 1.75 D. Likewise the methyl deformation frequencies
are also similar 1181.7 and 794.5 cm-1 for CH3Au, and 1245 and
882 cm-1 for CH3I in solid argon.
periodic table with the exception of the halogens.1g,25 Various
anionic gold complexes have been reported,26 and more recently
anionic Group 11 metal hydrides are also identified in the matrix
spectra from reactions of laser ablated Au and electrons with H2.24
The formation of an anionic species in the reaction of Au with
CH4 is, therefore, not surprising.
CH3–AuH, CH3–Au, and CH3–AuH- are 80, -211, and
26 kJ mol-1 higher in energy than Au(2S) + CH4, Au(2S) + CH3,
and Au-(1S) + CH4, respectively. However, the insertion reaction
probably proceeds via Au(2P), which is 446 kJ mol-1 higher in
energy than Au(2S),27 so the insertion product is 367 kJ mol-1
lower in energy than Au(2P) + CH4 reagents. Nevertheless, the
identified products are the most stable among the plausible
products. For example, the unidentified CH2–AuH2, CH2–AuH,
The strongest product absorption in the original deposition
spectra in Fig. 1 is observed at 1868.7 cm-1 (marked “i-”), its
D counterpart is observed at 1341.5 cm-1 (H : D ratio of 1.393),
and it reveals a negligible 13C shift, as shown in Table 1 and S3.†
The frequency is also compared with the previously reported
Au–H stretching frequencies of 2082, 1991.7, 1676.4, 1661.5,
and CH2–AuH2 are 157, 5.6, and 340 kJ mol-1 higher than
-
the corresponding reference energies (Au(2S) + CH4, Au(2S) +
CH3, and Au-(1S) + CH4). The high energy of CH3–AuH is
in line with the disappearance of the i absorptions on visible
irradiation, whereas during UV and full arc photolysis the i and a
and 1636.0 cm-1 for AuAuH, AuAuH-, AuH4 , (H2)AuH3, and
absorptions increase owing to 2P ‹ S excitation of Au,27 while the
-
2
AuH2 ,24 which are not observed in this study. This strong i-
i- absorptions decrease as shown in Fig. 1, 2, and S1,† raising the
possibility that UV irradiation causes detachment of an electron
from CH3–AuH-.
-
absorption not only decreases on UV and full arc (240 < l <
380 nm and l > 220 nm) irradiation, but also disappears from
the product spectrum when trace CCl4 is added to the reagent
mixture as shown in Fig. 1. Extensive work in this laboratory
has demonstrated that CCl4 is an effective electron scavenger
in matrix samples,15,24 and this chemical diagnostic facilitates
the identification of anion product absorptions. Isolated anions
have been observed in previous laser ablation matrix isolation
experiments owing to the production of electrons in the ablation
process.15,24 We assign the i- absorption to the Au–H stretching
mode of the methyl gold hydride anion (CH3–AuH-), the most
probable anionic insertion complex.
Its frequency and high absorption intensity are also well
reproduced by electronic structure computations for the singlet
ground state; however, the DFT and MP2 results differ somewhat
as the latter tends to over estimate frequencies. For example, the
B3LYP and MP2 Au–H stretching frequencies are 1843.7 and
1992.2 cm-1. In addition the MP2 computation gives a C3v structure
similar to the previously studied Ag and Cu analogues8 while the
DFT methods give a near C3v structure. The distinctively very
strong Au–H stretching absorption of CH3–AuH- is in contrast to
the weak counterpart for CH3–AuH. A weak possible i- absorption
at 726.6 cm-1 shifts to 553.4 and 722.8 cm-1 on deuteration and
13C substitution. The frequency, large D and small 13C shifts, and
correlation with computational results allow tentative assignment
of this as the CH3 rocking mode (Table 1 and S3†). The triplet
state is 241 kJ mol-1 higher in energy, and the strong Au–H
stretching mode predicted near 1530 cm-1 is not observed. The
observed frequencies and isotopic shifts and the agreement with
the predicted values for the singlet ground state all support
formation of the anionic gold insertion complex with a very intense
infrared absorbing Au–H stretching mode.
CH3 is also produced by the laser plume radiation during co-
deposition of Au and CH4, leaving the broad CH3 absorption at
620–600 cm-1, parallel to the previously studied reactions of laser
ablated transition-metal atoms with methane.9–13 Milligan and
Jacox showed that 121.6 nm radiation is effective for dissociation
of the C–H bond during deposition of CH4 to generate the
CH3 radical.28 However, the concentration of CH3 is still very
low relative to that of CH4, and in our previously studied
reactions of transition-metals with methane, methyl metal species
have not been identified.9–13 The ablation plume also contains
electrons, providing opportunity for anionic species to form, but
anionic species have only been observed in methane activation
by transition-metals in the case of the Nb and Ta methylidyne
anions.9–13
Ag + CH4
The product absorptions marked “a” and “i-” from Ag reactions
with CH4 and CH4 + CCl4 in Fig. 3 show intensity variations
similar to those of the Au counterparts on photolysis and
annealing. They remain unchanged on visible irradiation, but the
a absorptions increase on UV and full arc photolysis while the i-
absorptions decrease. They are weaker than in the Au + CH4 case
and no i absorptions are observed.
The a absorption at 1076.4 cm-1 is accompanied by D and 13
counterparts at 824.1 and 1069.3 cm-1 (H/D and 12/13 ratios of
1.306 and 1.007) (Fig. 4), and the frequency, large D and small 13
C
C
shifts, and consistency with the Au case lead to their assignment to
the CH3 deformation mode of methyl silver (CH3–Ag), which is the
second strongest band of the product. The observed frequencies
of the CH3–Ag isotopomers are in good correlation with the
B3LYP values of 1097.7, 845.4, and 1090.6 cm-1 (Table S5†). The
a absorption observed at 917.1 cm-1 in the CH2D2 spectra is in line
with the calculated value of 931.0 cm-1 for CHD2–Ag. Another a
absorption is observed at 666.1 cm-1 with D and 13C counterparts
at 499.1 and 663.6 cm-1 (H : D and 12 : 13 ratios of 1.335 and
Other plausible anionic species with an Au–H bond, such
as CH2–AuH-, were also examined computationally, but the
predicted vibrational characteristics (strong absorptions at 1863
and 602 cm-1 for CH2–AuH- and at 1765 cm-1 for CH–AuH-)
are not consistent with the observed values. The high electron
affinity of gold is profound, the largest (-223 kJ mol-1) in the
11118 | Dalton Trans., 2011, 40, 11115–11124
This journal is
The Royal Society of Chemistry 2011
©

Fig. 3 Infrared spectra in the 1650–1450, 1180–980, and 740–540 cm-1 regions for the reaction products of the laser-ablated silver atom with CH4 and
CH4 + CCl4 in excess argon at 8 K. (a) Ag and CH4 + CCl4 (2.0 and 0.02% in argon) co-deposited for 1 h, (b) and (c) as (a) after visible and UV irradiation.
(d) Ag and CH4 reagent (2.0% in argon) co-deposited for 1 h, (e)–(g) as (d) after visible, UV, and full arc irradiation, (h) as (g) after annealing to 26 K. a
and i- stand for product absorption groups while w designates water residue absorptions.
stretching mode predicted near 1280 cm-1 is not observed. The
observed frequencies and isotopic shifts and the agreement with
the predicted values for the singlet ground state all support this
assignment.
Rijs and O’Hair have shown that CH3–AgH- and CH3–CuH-
are among the primary products in gas phase fragmentation
reactions of [RCO2MO2CR]-.8 They examined the reaction paths
by DFT calculations and collision-induced dissociation and
deuterium labeling experiments, and showed that a diverse set
of pathways including bond homolysis, bond heterolysis, and b-
hydride elimination were apparently involved. The present results
reveal that a combination of laser ablation and direct reaction
Fig. 4 Infrared spectra in the 1200–800 and 650–450 cm-1 regions for the
reaction products of the laser-ablated silver atom with CD4 in excess argon
with alkanes is an effective method to provide the anionic coinage
metal insertion complexes.
at 8 K. (a) Ag and CD4 (2.0% in argon) co-deposited for 1 h, (b)–(d) as (a)
As in the Au case, CH3–AgH, CH3–Ag, and CH3–AgH- are
after visible, UV, and full arc irradiation, and (e) as (d) after annealing to
161, -154, and 43 kJ mol-1 higher than Ag(2S) + CH4, Ag(2S) +
26 K. a and i- stand for product absorption groups while P and c stand for
CH3, and Ag-(1S) + CH4, respectively. The higher energies of
the precursor and common absorptions.
the products relative to those in the Au case are consistent with
the weaker product absorptions. In particular, the endothermicity
of 161 kJ mol-1 for CH3–AgH is much higher than the value
of 80 kJ mol-1 for CH3–AuH, providing some rationale for the
absence of its absorptions. However, the anion is most likely
produced by electron capture of this neutral molecule, but note
that the infrared intensity of the anion Ag–H vibration is on the
order of 40 times larger than for the neutral analogue. Using the
Ag(2P) reagent, the reaction to form CH3–AgH is 192 kJ mol-1
exothermic. Other plausible products are still energetically even
1.004), and we assign it to the strongest mode, CH3 rocking. The
observed frequencies are again in an excellent agreement with the
DFT values (e.g. B3LYP values of 693.6, 512.7, and 690.7 cm-1),
and the observed frequency of 586.7 cm-1 in the CH2D2 spectra
correlates with the predicted value of 604.6 cm-1 for CHD2–Ag,
substantiating formation of CH3–Ag.
The i- absorption at 1562.0 cm-1 has its D counterpart at
1124.2 cm-1 (H : D ratio of 1.389), but shows no 13C shift. This Ag–
H stretching frequency is compared with those at 1746.5, 1717.0,
1691.9, and 1427.5 cm-1 found for (H2)AgH, AgH, AgAgH, and
-
higher. For example, CH2–AgH2, CH2–AgH, and CH2–AgH2
are 235, 125, and 373 kJ mol-1 higher than the corresponding
references.
AgH2 ,24 which are not observed in this study. The i- Ag–H
-
stretching absorption also disappears on addition of CCl4 and
it is accordingly assigned to CH3–AgH-. Its frequency and high
absorption intensity are in reasonable agreement with the B3LYP
and BPW91 computed frequencies of 1554.6 and 1575.5 cm-1
and large intensities of 615 and 587 km mol-1 (Table S6†). The
triplet state is 223 kJ mol-1 higher in energy, and the strong Ag–H
Cu + CH4
Fig. 5 shows the reaction product spectra from Cu atoms co-
deposited with CH4 and with CH4 + CCl4 in excess argon during
condensation at 8 K. Fig. 6 and S3† show the corresponding
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Dalton Trans., 2011, 40, 11115–11124 | 11119
©

Fig. 5 Infrared spectra in the 1950–1550, 1250–900, and 750–550 cm-1 regions for the reaction products of the laser-ablated copper atom with CH4
and CH4 + CCl4 in excess argon at 8 K. (a) Cu and CH4 + CCl4 (2.0 and 0.02% in argon) co-deposited for 1 h, (b) and (c) as (a) after visible and UV
irradiation. (d) Cu and CH4 reagent (2.0% in argon) co-deposited for 1 h, (e)–(g) as (d) after visible, UV, and full arc irradiation, (h) as (g) after annealing
to 26 K. a, i, and i- stand for product absorption groups while w designates water residue absorptions.
Fig. 6 Infrared spectra in the 1400–1100 and 950–450 cm-1 regions for the reaction products of the laser-ablated copper atom with CD4 in excess argon
at 8 K. (a) Cu and CD4 (2.0% in argon) co-deposited for 1 h, (b)–(d) as (a) after visible, UV, and full arc irradiation, and (e) as (d) after annealing to 26 K.
a, i, and i- stand for product absorption groups.
absorptions from the deuterated and half deuterated products.
In a parallel manner to Au, three groups of product absorptions
are marked “i, a, and i-” depending on the intensity variation in
the process of photolysis and annealing. The absorption intensity
variation of the i, a, and i- absorptions are similar to those in the
Au and Ag experiments.
Fig. 5 and 6 also show strong CuH and CuD absorptions at
1878.5 and 1353.9 cm-1 (H : D ratio of 1.387),24 which increase
slightly on visible irradiation, but more than triple on UV
irradiation and slightly decrease on full arc photolysis. The strong
CuH absorption contrasts with the absence of the AuH and AgH
absorptions. Parnis et al. reported the dramatic increase of the
CuH absorption at 1850 cm-1 along with the emergence of CH3Cu
and CH3CuH absorptions in the methane matrix on UV (305–
325 nm) irradiation.6a
Ag, and Cu, which were assigned to the CH3MH species.6b
Unfortunately, these assignments are not correct: the highest
frequencies reported, after calibration, are probably due to the
diatomic MH molecules in solid methane.
The i absorption for Cu at 1718.8 cm-1 shifts to 1243.7 cm-1
on deuteration (H : D ratio = 1.382) and shows a negligible 13C
shift. This Cu–H stretching frequency is compared with 1862.5
-
and 1497.2 cm-1 for (H2)CuH and CuH2 ,24 which are observed
in this study. The new Cu–H stretching absorption is assigned to
CH3–CuH, following the Au case. The observed Cu–H and Cu–D
stretching frequencies and relatively low intensities are reproduced
by DFT computations (e.g. B3LYP frequency and intensity of
1707.5 cm-1 and 18 km mol-1, Table S7†). A stronger i absorption
is observed at 1012.1 cm-1, and its D and 13C counterparts at
786.0 and 1003.6 cm-1 (H : D and 12 : 13 ratios of 1.288 and
1.008), and it is assigned to the CH3 deformation mode. Again
the observed frequencies are in line with the DFT frequencies (e.g.
Billups et al. reported the results of transition metal atom
reactions in solid methane after UV irradiation, including Au,
11120 | Dalton Trans., 2011, 40, 11115–11124
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the B3LYP values of 1050.3, 810.8, and 1003.6 cm-1). The observed
i absorptions correspond with the weak absorptions emerging at
1697 and 1011 cm-1 on UV irradiation in a solid methane matrix.6a
Two a absorptions are observed in Fig. 5. The band at
1203.0 cm-1 shifts to 920.0 cm-1 on deuteration (H : D ratio of
1.308), whereas its 13C counterpart is at 1195.1 cm-1. The other
absorption at 648.3 cm-1 is much stronger and its D and 13C
counterparts are at 494.2 and 645 cm-1 (H : D and 12 : 13 ratios
of 1.312 and 1.005). They are assigned to the CH3 deformation
and rocking modes of CH3–Cu (the two strongest bands for methyl
copper) on the basis of the frequencies, large D and small 13C shifts,
and correlation with the DFT values (Table S8†). For example the
observed methyl rocking frequencies are compared with slightly
higher B3LYP isotopic values of 662.7, 494.0, and 659.5 cm-1. In
contrast the methyl deformation frequencies are somewhat higher
than the computed values. This may be due to overestimation of
An i- absorption, which is the strongest product absorption in
the original deposition spectrum, is also observed at 1612.9 cm-1.
Deuteration shifts it to 1165.0 cm-1 (H : D ratio of 1.384), and it
shows no 13C shift. Parallel to the Au and Ag cases, this absorption
disappears on addition of CCl4 (Fig. 5). We, therefore, assign
it to the Cu–H stretching mode of the methyl copper hydride
anion, CH3–CuH-. The observed Cu–H stretching frequency
and distinctively strong intensity are also reproduced by DFT
computations (the B3LYP and BPW91 frequencies of 1606.1 and
1629.7 cm-1) (Table S9†) for the singlet ground state anion. The
triplet state is 212 kJ mol-1 higher in energy, and the strong Cu–H
stretching mode predicted near 1470 cm-1 is not observed.
The CH3–CuH, CH3–Cu, and CH3–CuH- products are
85 kJ mol-1 higher, 203 kJ mol-1 lower, and 23 kJ mol-1 lower
than the Cu(2S) + CH4, Cu(2S) + CH3, and Cu-(1S) + CH4
reagents, respectively. Since the insertion reaction proceeds with
Cu(2P),6a which is 365 kJ mol-1 higher in energy than Cu(2S),27
CH3–CuH is 279 kJ mol-1 lower in energy than Cu(2P) +
CH4 reagents. The observed products are again the most stable
among the corresponding plausible products. For example, the
unidentified CH2–CuH2, CH2–CuH, and CH2–CuH2- are 158, 35,
and 270 kJ mol-1 higher than the corresponding reagents.
˚
the C–Cu bond length (1.918 A) in our calculation, as compared to
5a
˚
the experimental measurement (1.8841 A), with corresponding
underestimation of the H-to-Cu repulsion, which affects the
methyl deformation frequency.
Parnis et al. have reported eight absorption frequencies for
CH3–Cu with their D and 13C counterparts,6a which are more
than the number of vibrational modes of methyl copper (6 modes
and three of them are very weak). Their 1196 cm-1 band and its
isotopic counterparts correspond to our 1203.0 cm-1 absorption.
In addition, these workers observed a band near 640 cm-1 labelled
CH3, which tracks the 1196 cm-1 band on irradiation, and we
believe that this band should be reassigned to CH3Cu in solid
methane. The other observed frequencies do not agree with the
frequencies predicted for the methyl copper (Table S8†), and thus,
they probably originate from other unknown species. Recently
Grotjahn et al. produced CH3–Cu, the most studied coinage alkyl
metal and determined the rotational constants in the ground (v =
0) and lowest excited state of the C–Cu stretch (v3 = 1).5a They
Structures of small Au, Ag, and Cu complexes
The structures of the Group 11 metal complexes investigated in this
study are illustrated in Fig. 7–9. The C–Au, C–Ag, and C–Cu bond
lengths of the small Group 11 complexes are comparable to those
29
30
˚
˚
of the previously introduced Au (1.9–2.3 A), Ag (2.1–2.5 A),
31
˚
and Cu (1.9–2.3 A) complexes, many of which are p-complexes.
The computed C–Cu bond length of 1.918 A for CH3–Cu is slightly
˚
longer than that determined from the rotational spectrum (1.8841
5a
A). While CH3–MH has a Cs structure, CH3–M and CH3–MH-
˚
have C3v structures.8,32
˚
determined the C–Cu bond length to be 1.8841 A and the C–
The CH3–M molecules contain the shortest C–M bond, con-
sistent with the high natural effective bond obtained from NBO
analysis (Table S10†),33 and the bonded C and M have mostly sp3
Cu stretching mode to be near 586 cm-1, slightly higher than our
calculated value.
Fig. 7 The B3LYP structures of the identified Au products with the 6-311++G(3df,3pd) basis set for H, C, and Au. Bond distances and angles are
-
˚
in A and deg. The methyl auride and anionic insertion complex (CH3–Au and CH3–AuH ) have C3v structures whereas the neutral insertion complex
(CH3–AuH) possesses a Cs structure.
˚
Fig. 8 The B3LYP structures of the Ag products with the 6-311++G(3df,3pd) basis set for H, C, and Ag. Bond distances and angles are in A and deg.
The methyl silver and anionic insertion complexes are identified in the matrix spectra (see text). The methyl silver and anionic insertion complex (CH3–Ag
and CH3–AgH-) have C3v structures whereas the neutral insertion complex (CH3–AgH) possesses a Cs structure.
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Fig. 9 The B3LYP structures of the identified Cu products with the 6-311++G(3df,3pd) basis set for H, C, and Cu. Bond distances and angles are
-
˚
in A and deg. The methyl copper and anionic insertion complex (CH3–Cu and CH3–CuH ) have C3v structures whereas the neutral insertion complex
(CH3–CuH) possesses a Cs structure.
and sd hybridizations. For example, the C–Au bond in CH3–Au
consists of s(14.5%) and p(85.4%) for C and s(80.4%) and d(19.5%)
for Au, indicating that the C–Au bond formation apparently does
not require electron promotion from the electronic configuration
of gold [Xe]4f145d106s1. On the other hand, the slightly longer C–
Au bonds of the insertion complexes CH3–AuH and CH3–AuH-
carry significant p contributions for Au. For example, Au has
s(41.8%), p(24.1%), and d(34.0%) for the C–Au bond of CH3–AuH
and s(42.3%), p(47.0%), and d(10.4%) for that of CH3–AuH-.
(2a)
(2b)
CH3–MH* → CH3–M + H
CH3 + M → CH3–M
On the other hand, addition of an electron produced in the
ablation process to the insertion complex is very exothermic.
CH3–AuH- is 268 kJ mol-1 lower in energy than CH3–AuH,
which is in fact larger than the electron affinity of the Au atom,
223 kJ mol-1.1g,25 In parallel, electron attachments to CH3–AgH
and CH3–CuH lower the energies by 252 and 226 kJ mol-1, which
are considerably larger than the electron affinities of the Ag and Cu
atoms themselves, at 126 and 119 kJ mol-1.25 The higher electron
affinities of the insertion complexes relative to those of the metal
atoms are probably due to stabilization of negative charge by the
methyl and hydride substituents: natural charges (Table S10†) on
the methyl group (3 ¥ 0.167–1.108 = -0.607) and hydride (- 0.413)
for CH3–AuH- substantiate this rationale.
Reactions
The reaction medium contains M atoms, CH4, CH3, and electrons,
and thus, combinations of them can provide sources of the
primary products (CH3–MH, CH3–M, and CH3–MH- from M +
CH4, M + CH3, and M + e- + CH4, respectively). However, the
concentrations of CH3 and e- in the reaction medium are low,
and the alkyl metal and anionic insertion complexes have not
been identified in the reactions of other transition-metal reactions
while similar amounts of CH3 and electrons are believed to exist
in the matrix samples.9–13 Therefore, generation of the methyl
metal and anionic insertion complexes results from the unique
properties of the coinage metals. The i absorptions in the original
deposition spectra suggest that CH3–MH is first produced via C–H
insertion of CH4 by M* (excited), similar to the previously studied
transition-metal systems,9–13 and the subsequent transformations
to CH3–M and CH3–MH- probably follow. The earlier copper–
methane experiments demonstrated that 2P excited Cu atoms were
required for the reaction,6a and excitation in the laser ablation
process and through subsequent UV irradiation provides this
required energy.
CH3–MH + e- → CH3–MH-
(3)
The higher electron affinities of the insertion complexes also
suggest that the insertion complex is probably formed first, and
subsequent electron attachment produces the anionic insertion
complex (CH3–MH-), which is observed here owing to the very
high intensity of the metal–hydride vibration. This should be more
efficient than the C–H insertion reaction by the electron-rich metal
anion to form the product. The electron affinities of the coinage
metal insertion complexes are also much higher than those of other
transition-metal analogues, for example, 83, 165, and 94 kJ mol-1
for CH3–ZrH, CH3–NiH, and CH3–PtH, and the electron affinities
of Zr, Ni, and Pt are 41, 112, and 205 kJ mol-1.25 Finally, the
corresponding methylene or methylidene complexes (CH2–MH2)
are not observed probably due to their high energies.
Fig. 1–6 and S1–S3† also show that the i- absorptions diminish
upon UV and full arc irradiation while the i and a absorptions
increase, suggesting that the anionic insertion complex photode-
taches an electron to form the insertion and alkyl metal complexes
during sample irradiation. The UV photon would provide CH3–
MH- with the energy to detach an electron to generate the
CH3–MH and the excess reaction energy would in turn lead to
dissociation of H to produce CH3–M, reaction (4).
M*(2P) + CH4 → CH3–MH* → CH3–MH
(1)
Release of H from CH3–MH requires extra energy, which is
consistent with the weak a absorptions in the original spectra.
The excess energy from the insertion reaction or plume radiation
would provide the energies for H-detachment from the Au, Ag, and
Cu insertion complexes (134, 110 and 137 kJ mol-1), generating the
alkyl metal during co-deposition. The alkyl metals are relatively
stable due to the single valence electrons (s1) of the Group 11
metals,34 leading to the much smaller H-detachment energies for
the insertion complexes than those for other transition-metal
analogues. For example, H-detachment energies of 290, 213, and
327 kJ mol-1 are estimated for CH3–ZrH, CH3–NiH and CH3–
PtH. In addition, the CH3–M molecules can be prepared by direct
combination reactions (2b) as the methyl radical is produced and
trapped in these experiments from laser plume photodissociation
of the methane precursor.
CH3–MH- + UV → CH3–MH* + e- → CH3–M + H + e-
(4)
A 300 nm photon corresponds to 400 kJ mol-1, which is roughly
equal to the transformation energy from CH3–AuH- to CH3–M
via CH3–MH; those for the Ag and Cu analogues are 361 and
278 kJ mol-1.
The Au and Cu spectra also show that the i absorptions almost
disappear on visible irradiation while the a band (and CuH
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absorption in the Cu spectra), and the CH3 absorption remain
essentially unchanged, suggesting that a visible photon dissociates
the insertion complex mostly to M + CH4 in the matrix rather
than to CH3–M + H or CH3 + MH.
6 (a) J. M. Parnis, S. A. Mitchell, J. Garcia-Prieto and G. A. Ozin, J. Am.
Chem. Soc., 1985, 107, 8169; see also; (b) W. E. Billups, M. M. Konarski,
R. H. Hauge and J. L. Margrave, J. Am. Chem. Soc., 1980, 102, 7393;
(c) D. Ritter, J. J. Carroll and J. C. Weisshaar, J. Phys. Chem., 1992, 96,
10636.
7 S.-C. Chang, Z. H. Kafafi, R. H. Hauge, W. E. Billups and J. L.
Margrave, J. Am. Chem. Soc., 1987, 109, 4508.
8 (a) N. J. Rijs and R. A. J. O’Hair, Organometallics, 2010, 29, 2282;
(b) N. J. Rijs and R. A. O’Hair, Organometallics, 2009, 28, 2684. (CH3–
AgH- and CH3–CuH-).
9 L. Andrews and H.-G. Cho, Organometallics, 2006, 25, 4040, and
references therein (review article, Groups 4–6).
10 (a) H.-G. Cho and L. Andrews, Organometallics, 2007, 26, 633; (b) H.-
G. Cho and L. Andrews, J. Phys. Chem. A, 2008, 112, 1519; (c) H.-G.
Cho and L. Andrews, J. Phys. Chem. A, 2006, 110, 3886. (Groups 3, 4
and 5).
Conclusions
Reactions of laser-ablated Group 11 metal atoms with methane
isotopomers have been carried out, and the products are identified
from the matrix IR spectra on the basis of frequencies, isotopic
shifts, and correlation with two DFT frequency calculations.
The insertion complexes (CH3–MH) are identified along with
methyl metal and anionic insertion complexes (CH3–M, and
CH3–MH-), showing that excited Group 11 metal atoms also
readily undergo C–H insertion and subsequent reactions follow.
The present results reconfirm that the C–H activation of alkanes
by excited transition-metal atoms is a common phenomenon.9–13
Facile hydride dissociation and high electron affinity of the coinage
metal insertion complexes lead to generation of the unique methyl
metal and anionic insertion products from the initially formed
insertion complex.
DFT computations for the primary products reproduce not only
the observed vibrational characteristics but their stabilities over
other plausible products as well. The calculated C–M bond lengths
for the Au, Ag, and Cu complexes are appropriate and in line with
those of the previously investigated Group 11 metal complexes.
The methyl metal species have the shortest computed C–M bond
lengths, which are consistent with their highest effective bond
orders, and the CH3–MH complexes have higher electron affinities
than the metal atoms. The NBO33 analyses also show that in CH3–
MH and CH3–MH-, the metal atom has spd hybridization for the
C–M bond, whereas in CH3–M, the metal contributes mostly s
character to the carbon–metal bond.
11 (a) H.-G. Cho and L. Andrews, Inorg. Chem., 2008, 47, 1653; (b) H.-G.
Cho and L. Andrews, Organometallics, 2007, 26, 4098; (c) H.-G. Cho
and L. Andrews, Organometallics, 2008, 27, 1786. (Re and Os).
12 (a) H.-G. Cho and L. Andrews, J. Phys. Chem. A, 2008, 112, 12293;
(b) H.-G. Cho and L. Andrews, Organometallics, 2009, 28, 1358. (Pt).
13 (a) L. Andrews and H.-G. Cho, J. Phys. Chem. A, 2005, 109, 6796;
˚
(b) J. T. Lyon, L. Andrews, P.-A Malmqvist, B. O. Roos, T. Yang and
B. E. Bursten, Inorg. Chem., 2007, 46, 4917. (actinides); (c) H.-G. Cho,
L. Andrews, unpublished work (lanthanides).
14 (a) G. von Frantzius, R. Streubel, K. Brandhorst and J. Grunenberg,
Organometallics, 2006, 25, 118, and references therein; (b) N. Berkaine,
P. Reinhardt and M. E. Alikhani, Chem. Phys., 2008, 343, 241; (c) G.
Chung and M. S. Gordon, Organometallics, 2003, 22, 42; (d) G. T. de
Jong and F. M. Bickelhaupt, J. Chem. Theory Comput., 2007, 3, 514;
(e) B. O. Roos, R. Lindh, H.-G. Cho and L. Andrews, J. Phys. Chem.
A, 2007, 111, 6420.
15 (a) L. Andrews and A. Citra, Chem. Rev., 2002, 102, 885, and references
therein; (b) L. Andrews, Chem. Soc. Rev., 2004, 33, 123, and references
therein.
16 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C.
Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci,
M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M.
Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E.
Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P.
Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D.
Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K.
Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S.
Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,
I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y.
Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. G. Johnson,
W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN 03
(Revision C.02), Gaussian, Inc., Wallingford, CT, 2004.
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R. G. Parr, Phys. Rev. B, 1988, 37, 785.
Acknowledgements
We gratefully acknowledge financial support from National
Science Foundation (U. S.) Grant CHE 03-52487 to L. A., and
support from the Korea Research Foundation (KRF) grant funded
by the Korean government (MEST) (No 2010-0016527).
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