Reactions of laser-ablated iron atoms with halomethanes: Infrared spectra, density functional calculations, and structures of simple iron insertion and methylidene complexes

Article abstract of DOI:10.1021/om8005189

Reactions of laser-ablated Fe atoms with halomethanes and ethane have been investigated through matrix infrared spectra and density functional calculations. Only insertion complexes are identified froth reactions of Fe with methyl fluoride and ethane, parallel to the previously observed insertion complex CH₃-FeH from the photochemical reaction with methane. However, Fe also forms methylidene complexes through α-X migration in the insertion products from Fe reactions with di-, tri-, and tetrahalomethanes. Calculated structures show no agostic distortion in these methylidene complexes. However, an interesting distortion and elongation of the C-Cl bond is observed in the Fe insertion products, such as H₂ClC-FeCl, while no such distortion of the C-H or C-F bond is found in analogous calculated structures for HaFC-FeCl. This distortion is likely due to a Cl lone pair to Fe bonding interaction that ultimately leads to formation of the lower energy CH ₂=FeCl₂ methylidene complex. This Cl atom transfer is reversible on visible-ultraviolet irradiation. There is no evidence for methylidyne complexes of the type recently observed for Ru, as such higher oxidation state structures become more stable on going down the family group.

Full text of DOI:10.1021/om8005189

Organometallics 2008, 27, 5241–5251
5241
Reactions of Laser-Ablated Iron Atoms with Halomethanes:
Infrared Spectra, Density Functional Calculations, and Structures of
Simple Iron Insertion and Methylidene Complexes
Han-Gook Cho,† Jonathan T. Lyon,‡ and Lester Andrews*,‡
Departments of Chemistry, UniVersity of Incheon, 177 Dohwa-dong, Nam-ku, Incheon 402-749,
South Korea, and UniVersity of Virginia, P.O. Box 400319, CharlottesVille, Virginia 22904-4319
ReceiVed June 5, 2008
Reactions of laser-ablated Fe atoms with halomethanes and ethane have been investigated through
matrix infrared spectra and density functional calculations. Only insertion complexes are identified from
reactions of Fe with methyl fluoride and ethane, parallel to the previously observed insertion complex
CH3-FeH from the photochemical reaction with methane. However, Fe also forms methylidene complexes
through R-X migration in the insertion products from Fe reactions with di-, tri-, and tetrahalomethanes.
Calculated structures show no agostic distortion in these methylidene complexes. However, an interesting
distortion and elongation of the C-Cl bond is observed in the Fe insertion products, such as H2ClC-FeCl,
while no such distortion of the C-H or C-F bond is found in analogous calculated structures for
H2FC-FeCl. This distortion is likely due to a Cl lone pair to Fe bonding interaction that ultimately leads
to formation of the lower energy CH2dFeCl2 methylidene complex. This Cl atom transfer is reversible
on visible-ultraviolet irradiation. There is no evidence for methylidyne complexes of the type recently
observed for Ru, as such higher oxidation state structures become more stable on going down the family
group.
provide highly stereoselective synthetic routes and reactions with
olefins to form asymmetric cyclopropanes through backside ring
closure.4a,b
Introduction
The discovery of high-oxidation-state transition-metal com-
plexes in the 1970s1,2 greatly expanded the horizon of coordina-
tion chemistry and the understanding of carbon-metal multiple
bonds. Their rich chemistry includes distinctive structures as
well as growing applications in various syntheses, particularly
C-H insertion and catalytic metathesis reactions.3 Among the
numerous high-oxidation-state complexes provided through a
variety of synthetic routes, relatively small numbers of Fe and
Ru derivatives have been produced,1,4-7 and therefore, their
structural and photochemical properties remain largely unknown.
Cyclopentadienyldicarbonyliron carbene complexes are used to
Oxidative C-H insertion of CH4 by Fe occurs during
photolysis after codeposition of the reactants, forming the
insertion complex (CH3-FeH), but further rearrangement to
higher oxidation state complexes is not observed.8 In addition,
both FeCH2 and HFeCH have been identified from reactions of
Fe with CH2N2 and subsequent photolysis.9 In addition, Ru
insertion or methylidene complexes are also produced in
reactions of Ru with halomethanes along with the methylidyne
complex, while only the insertion complex is identified from
reactions with methane.10 All three types of complexes were
observed in group 6 metal atom reactions, with the established
* To whom correspondence should be addressed. E-mail:
lsa@virginia.edu.
† University of Incheon.
(6) (a) Gallop, M. A.; Roper, W. R. AdV. Organomet. Chem. 1986, 25,
121. (b) Coalter, J. N.; Bollinger, J. C.; Eisenstein, O.; Caulton, K. G. New
J. Chem. 2000, 24, 925. (c) Sanford, M. S.; Henling, L. M.; Day, M. W.;
Grubbs, R. H. Angew. Chem., Int. Ed. 2000, 39, 3451. (d) Gonzales-Herrero,
P.; Weberndorfer, B.; Ilg, K.; Wolf, J.; Werner, H. Angew. Chem., Int. Ed.
2000, 39, 3266. (e) Gonzales-Herrero, P.; Weberndorfer, B.; Ilg, K.; Wolf,
J.; Werner, H. Organometallics 2001, 20, 3672. (f) Amoroso, D.; Snelgrove,
J. L.; Conrad, J. C.; Droulin, S. D.; Yap, G. P. A.; Fogg, D. E. AdV. Synth.
Catal. 2002, 344, 757 (Ru carbyne synthesis).
(7) (a) Fischer, H.; Hofmann, P.; Kreissl, F. R.; Schrock, R. R.; Schubert,
U.; Weiss, K. Carbyne Complexes; VCH: New York, 1988. (b) Gallop,
M. A.; Roper, W. R. AdV. Organomet. Chem. 1986, 25, 121. (c) Engel,
P. F.; Pfeffer, M. Chem. ReV. 1995, 95, 2281.
(8) (a) Billups, W. E.; Konarski, M. M.; Hauge, R. H.; Margrave, J. L.
J. Am. Chem. Soc. 1980, 102, 7393. (b) Ozin, G. A.; McCaffrey, J. G.
Inorg. Chem. 1983, 22, 1397. (c) Kafafi, Z. H.; Hauge, R. H.; Margrave,
J. L. J. Am. Chem. Soc. 1985, 107, 6134. (d) Yamada, Y.; Katsumata, K.;
Shimasaki, H.; Ono, Y.; Yamaguchi, K Bull. Chem. Soc. Jpn. 2002, 75,
277 (Fe + CH4).
(9) (a) Chang, S.-C.; Hauge, R. H.; Kafafi, Z. H.; Margrave, J. L. J. Am.
Chem. Soc. 1988, 110, 7975. (b) Chang, S.-C.; Kafafi, Z. H.; Hauge, R. H.;
Billups, W. E.; Margrave, J. L. J. Am. Chem. Soc. 1985, 107, 1447 (Fe +
CH2N2).
‡ University of Virginia.
(1) (a) Herndon, J. W. Coord. Chem. ReV. 2007, 251, 1158. (b) Herndon,
J. W. Coord. Chem. ReV. 2006, 250, 1889. (c) Herndon, J. W. Coord. Chem.
ReV. 2005, 249, 999. (d) Herndon, J. W. Coord. Chem. ReV. 2004, 248, 3.
(e) Schrock, R. R. Chem. ReV. 2002, 102, 145.
(2) (a) Fischer, E. O.; Kreis, G.; Kreiter, C. G.; Mu¨ller, J.; Huttner, G.;
Lorenz, H. Angew. Chem., Int. Ed. Engl. 1973, 12, 564. (b) McLain, S. J.;
Wood, C. D.; Messerle, L. W.; Schrock, R. R.; Hollander, F. J.; Youngs,
W. J.; Churchill, M. R. J. Am. Chem. Soc. 1978, 100, 5962.
(3) (a) Grubbs, R. H., Ed. Handbook of Metathesis; Wiley-VCH:
Weinheim, Germany, 2003; Vols. 1 and 3. (b) Bunz, U. H. F. Chem. ReV.
2000, 100, 1605. (c) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple
Bonds; Wiley: New York, 1988. (d) See article on iron as a catalyst in:
Chem. Eng. News 2008, July 28, 53.
(4) (a) Ishii, S.; Zhao, S.; Helquist, P. J. Am. Chem. Soc. 2000, 122,
5897, and references therein. (b) Wang, Q.; Fo¨rsterling, F. H.; Hossain,
M. M. J. Organomet. Chem. 2005, 690, 6238. (c) Zhang, S.; Xu, Q.; Sun,
J.; Chen, J. Chem. Eur. J. 2003, 9, 5111. (d) Brookhart, M.; Studabaker,
W. B. Chem. ReV. 1987, 87, 411. (e) Kremer, K. A. M.; Kuo, G.-H.;
O′Conner, E. J.; Helquist, P.; Kerber, R. C. J. Am. Chem. Soc. 1982, 104,
6119(Fe carbenes).
(5) (a) Fischer, E. O.; Schneider, J.; Neugebauer, D. Angew. Chem. 1984,
96, 814. (b) Grady, W. L.; Bursey, M. M. Int. J. Mass Spectrom. Ion
Processes 1984, 56, 161 (Fe+ carbyne intermediates).
(10) Cho, H.-G.; Andrews, L. Eur. J. Inorg. Chem. 2008, 2537 (Ru +
CH(X)4).
10.1021/om8005189 CCC: $40.75
2008 American Chemical Society
Publication on Web 09/26/2008

5242 Organometallics, Vol. 27, No. 20, 2008
Cho et al.
and 6-311++G(3df,3pd) basis sets for C, H, F, Cl, and Fe20 to
provide a consistent set of vibrational frequencies for the reaction
products. Geometries were fully relaxed during optimization, and
the optimized geometry was confirmed by vibrational analysis.
BPW9121 calculations were also done to complement the B3LYP
results. The vibrational frequencies were calculated analytically,
and the zero-point energy is included in the calculation of binding
energies. The computed S(S + 1) expectation values show that
the triplet and quintet state reaction products are mixed spin states;
thus, our DFT calculations are approximate and useful only as a
general guide to facilitate identification of product spectra. Previous
investigations have shown the DFT calculated harmonic frequencies
are usually slightly higher than observed frequencies, depending
on the mode anharmonicity,10-12,15,22,23 but this is not always the
case. The reader must appreciate that the calculation of vibrational
frequencies is not an exact science, and these calculations provide
useful predictions of infrared spectra of new molecules.
trend of increased stability for higher oxidation states going
down the family group.11 Osmium fits this premise, as the
exclusive formation of Os methylidynes is found in reactions
of Os with CH4, C2H6, and CH3X, showing high preference for
the carbon-osmium triple bond.12
The importance of C-H activation continues to increase in
synthetic and industrial chemistry.4,13,14 Recently a new breed
of simple high-oxidation-state complexes has been introduced
by activation of small alkanes and halomethanes with laser-
ablated transition-metal atoms, many of which show unique
structures and reversible photochemistry.15 Here, we report the
IR spectra of isotopic products from reactions of laser-ablated
Fe atoms with halomethanes and ethane. The insertion and
carbene products are identified depending on the system. DFT
computations reveal unique structures for the products, and the
structural effects of the chlorine lone electron pair on the metal
center are examined for these simple complexes.
Results and Discussion
Experimental and Computational Methods
Reactions of iron atoms with halomethane and ethane
isotopomers were carried out, and the matrix infrared spectra
of new products will be compared with frequencies for low-
energy product structures calculated by density functional theory.
Spectra from similar experiments with other laser-ablated metals
and these same precursors were examined to make sure that
the new absorptions considered here are unique to the iron and
precursor molecule reaction.10,11
Laser-ablated Fe (Johnson-Matthey) atoms were reacted with
CH3F (Matheson), CD3F (synthesized from CD3Br and HgF2),
13CH3F (Cambridge Isotopic Laboratories, 99%), C2H6 (Matheson),
C2D6 (MSD Isotopes), CH2F2, CD2F2,16 CH2FCl, CD2FCl,16
CH2Cl2, CD2Cl2, 13CH2Cl2, CHF3, CDF3,16 CHCl3, CDCl3,16
13CHCl3, CF3Cl, 13CF3Cl, CF2Cl2, CCl4 (Dupont), and 13CCl4 in
excess argon during condensation at 10 K using a closed-cycle
refrigerator (Air Products HC-2). These methods have been
described in detail elsewhere.17 Reagent gas mixtures ranged
0.5-1.0% in argon. After reaction, infrared spectra were recorded
at a resolution of 0.5 cm-1 using a Nicolet 550 spectrometer with
an MCT-B detector. Samples were later irradiated for 20 min
periods by a mercury arc street lamp (175 W) with the globe
removed and a combination of optical filters and subsequently
annealed to allow further reagent diffusion.17
Fe + CH3F. Shown in Figure 1 are the CH3F spectra in the
Fe-F stretching and CH3 rocking regions. The observed
frequencies of the product absorptions are given in Table 1 and
compared with the predicted values in Table S1 (Supporting
Information). Previous studies show that reactions of metal
atoms with small alkanes and halomethanes generate small metal
complexes (insertion, carbene, and carbyne products).15 The
product absorptions marked i (for insertion product) are
increased from their original intensities about 5 and 30% and
slightly more on visible (λ > 420 nm), UV (240 < λ < 380
nm), and full arc (λ > 220 nm) irradiations, respectively. The
sharp strong absorption at 662.3 cm-1, showing small D and
13C shifts of -7.2 and -1.2 cm-1, is attributed to the Fe-F
stretching mode of CH3-FeF. The group of absorptions at 581.0,
578.6, 577.1, and 574.5 cm-1 along with similarly split D and
In order to support the assignment of new experimental frequen-
cies, density functional theory (DFT) calculations were carried out
using the Gaussian 03 package,10-12,18 B3LYP density functional,19
(11) Lyon, J. T.; Cho, H.-G.; Andrews, L. Organometallics 2007, 26,
6373 (Cr, Mo, W + CH2X2, CHX3, CX4).
(12) (a) Cho, H.-G.; Andrews, L. Organometallics 2008, 27, 1786 (Os
+ CH4, C2H6, CH3X). (b) Cho, H.-G.; Andrews, L. Organometallics 2007,
26, 4098. (c) Cho, H.-G.; Andrews, L. Inorg. Chem. 2008, 47, 1653 (Re +
alkanes). (d) Lyon, J. T.; Cho, H.-G.; Andrews, L.; Hu, H.-S.; Li, J. Inorg.
Chem. 2007, 46, 8728 (Re + CHX3, CX4).
(13) D´ıaz-Requejo, M. M.; Belderrain, T. R.; Nicasio, M. C.; Pe´rez, P.
Dalton Trans. 2006, 5559.
(14) Campos, K. R. Chem. Soc. ReV. 2007, 36, 1069.
(15) Andrews, L.; Cho, H.-G. Organometallics 2006, 25, 4040, and
references therein (review article).
(16) Isotopic modifications synthesized: Andrews, L.; Willner, H.;
Prochaska, F. T. J. Fluorine Chem. 1979, 13, 273.
(17) (a) Andrews, L.; Citra, A. Chem. ReV. 2002, 102, 885, and
references therein. (b) Andrews, L. Chem. Soc. ReV. 2004, 33, 123, and
references therein.
(18) Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi,
J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y. ; Kitao, O.; Nakai, H.;
Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
Revision B.04; Gaussian, Inc., Pittsburgh, PA, 2003.
13C counterparts (D and 13C shifts of -135.3 and -3.6 cm-1
)
are assigned to the degenerate CH3 rocking modes of CH3-FeF
with C3V symmetry. The good agreement with the DFT values
in Table S1, as marked by the strong Fe-F stretching mode
falling between the B3LYP and BPW91 values, while the
predicted CH3 rocking frequencies are slightly lower, substanti-
ates formation of the insertion product. No other product
absorptions are observed, and formation of the insertion complex
is consistent with the previously reported observation of
CH3-FeH in photochemical reactions of Fe atoms and meth-
ane.8 DFT calculations also show that the insertion complex is
most stable among the plausible products: CH3-FeF (Q),
CH2dFeHF (T), and HCtFeH2F (S) are 50 and 20 kcal/mol
lower and 5 kcal/mol higher in energy than the reactants,
respectively. (Here, Q, T, and S denote quintet, triplet, and
(20) Raghavachari, K.; Trucks, G. W. J. Chem. Phys. 1989, 91, 1062.
(21) Burke, K.; Perdew, J. P.; Wang, Y. In Electronic Density Functional
Theory: Recent Progress and New Directions; Dobson, J. F., Vignale, G.,
Das, M. P., Eds.; Plenum: New York, 1998.
(22) Cho, H.-G.; Andrews, L. Organometallics 2005, 24, 5678 (Cr, Mo,
W + CH3F).
(23) (a) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502. (b)
Andersson, M. P.; Uvdal, P. L. J. Phys. Chem. A 2005, 109, 3937.
(19) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
Y.; Parr, R. G. Phys. ReV. B 1988, 37, 785.

Reactions of Laser-Ablated Iron Atoms
Organometallics, Vol. 27, No. 20, 2008 5243
Table 1. Frequencies of Product Absorptions (Group i) Observed from Reactions of Fe with Methyl Fluoride and Ethane Isotopomers in Excess
Argona
CH3F
CD3F
13CH3F
C2H6
C2D6
description
662.3
581.0, 578.6, 577.1, 574.5
655.1
445.7, 443.0, 441.3
661.1
577.4, 575.0, 573.5, 541.3
CH3 rock
Fe-X(H) str
C-Fe str
1662.8, 1660.5
482.9
1204.0, 1196.8
a All frequencies are in cm-1. The description gives the major coordinate, and i stands for insertion products.
Figure 2. IR spectra in selected regions of 1690-1640, 1240-1180,
and 500-460 cm-1 for laser-ablated Fe atoms codeposited with
C2H6 and C2D6 in excess argon at 10 K and their variation: (a) Fe
+ 1.0% C2H6 in Ar codeposited for 1 h; (b) as in (a) after photolysis
(λ > 420 nm); (c) as in (b) after photolysis (240 < λ < 380 nm);
(d) as in (c) after photolysis (λ > 420 nm); (e) as in (d) after
photolysis (240 < λ < 380 nm); (f) as in (e) after annealing to 28
K; (g) Fe + 1.0% C2D6 in Ar codeposited for 1 h; (h) as in (g)
after photolysis (λ > 420 nm); (i) as in (h) after photolysis (240 <
λ < 380 nm); (j) as in (i) after annealing to 28 K. i denotes the
product absorption, and c indicates bands common to this precursor
with different laser-ablated metals.
Figure 1. IR spectra in the regions of 680-550 and 460-420 cm-1
for laser-ablated Fe atoms codeposited with CH3F isotopomers in
excess argon at 10 K and their variation: (a) Fe + 0.5% CH3F in
Ar codeposited for 1 h; (b) as in (a) after photolysis (λ > 420 nm);
(c) as in (b) after photolysis (240 < λ > 380 nm); (d) as in (c)
after photolysis (λ > 220 nm); (e) as in (d) after annealing to 28
K; (f) Fe + 0.5% CD3F in Ar codeposited for 1 h; (g) as in (f)
after photolysis (240 < λ < 380 nm); (h) as in (g) after annealing
to 28 K; (i) Fe + 0.5% 13CH3F in Ar codeposited for 1 h; (j) as in
(i) after photolysis (240 < λ < 380 nm); (k) as in (j) after annealing
to 28 K. i denotes the product absorption.
than for the methyl-FeH counterpart,8 and the C-Fe stretch is
about 38 cm-1 lower.
Fe + CH2X2 (X ) F, Cl). Figure 3 shows the CH2F2 and
CH2FCl spectra in the C-F stretching and low-frequency
regions, and frequencies are given in Table 2. Unlike the cases
of CH3F and C2H6, two sets of product absorptions (i and m
for insertion and methylidene products) are observed in all
methylene halide spectra, showing that R-halogen migration is
favorable in the reaction with CH2X2. In the methylene fluoride
spectra, the i absorptions increase 30, 20, and another 20% (total
70%) on visible, UV, and full arc irradiations, respectively. The
m absorptions, on the other hand, slightly decrease on visible
and full arc photolysis but increase about 30% on UV photolysis.
The i absorption at 937.0 cm-1 with its deuterium counterpart
at 915.5 cm-1 (H/D ratio of 1.023) is assigned to the C-F
stretching mode of CH2F-FeF on the basis of its frequency
and small D shift. Another i absorption observed at 666.0 cm-1
is assigned to the Fe-F stretching mode, whereas the
CD2F-FeF counterpart (expected at 4 cm-1 lower) is evidently
covered by residual CO2 absorption. Table S3 (Supporting
Information) shows good correlation between the matrix infrared
and DFT calculated frequencies, especially for the unusually
low C-F stretching frequency.
singlet electronic states, respectively.) The last is not a meth-
ylidyne complex, as the C-Fe bond length is 1.923 Å. Finally,
the computed structure for the unobserved higher energy
CH2dFeHF (T) complex shows no evidence of agostic distor-
tion, in contrast to the analogous group 6 methylidenes,22 but
the H atom bonded to Fe is tilted back toward carbon.
Fe + C2H6. Figure 2 illustrates the ethane reaction product
spectra in the Fe-H stretching and C-Fe stretching regions.
The sharp Fe-H stretching i absorption at 1662.8 and 1660.5
cm-1 has the D counterpart at 1196.8 cm-1 (H/D ratio of 1.387).
They decrease slightly on visible irradiation but almost triple
in the following UV irradiation, and their frequencies are
compared with FeH2 and FeD2 absorptions of 1660.8 and 1204.2
cm-1 24
The Fe-H stretching absorption indicates formation
.
of an insertion complex (CH3CH2-FeH), parallel to the cases
of Fe + CH4 and Fe + CH3F. A weak i absorption observed at
482.9 cm-1 is assigned to the C-Fe stretching mode without
observation of the deuterium counterpart. The observed frequen-
cies match well with the DFT frequencies as shown in Table
S2 (Supporting Information). The broader 1672 and 1220 cm-1
bands increase on annealing and are believed due to ethane
precursor perturbations of the insertion complex. The Fe-H
stretching frequency for ethyl-FeH is about 10 cm-1 higher
The m absorptions are all observed below 700 cm-1
,
indicating the absence of a C-F bond in the product responsible
for the m absorptions. The strong m absorptions at 695.3 and
689.2 cm-1 along with the deuterium counterparts at 691.3 and
688.1 cm-1 are assigned to the FeF2 antisymmetric stretching
mode of CH2dFeF2 on the basis of the frequencies and small
shifts of deuterium substitution. The weaker absorptions at 594.0
(24) (a) Rubinovitz, R. L.; Nixon, E. R. J. Phys. Chem. 1986, 90, 1940.
(b) Chertihin, G. V.; Andrews, L. J. Phys. Chem. 1995, 99, 12131 (FeH2).

5244 Organometallics, Vol. 27, No. 20, 2008
Cho et al.
Table 2. Frequencies of Product Absorptions Observed from Reactions of Fe with Methylene Halide (CH2X2, X ) F, Cl) Isotopomers in Excess
Argona
CH2F2
CD2F2
CH2FCl
CD2FCl
CH2Cl2
CD2Cl2
1023.9
13CH2Cl2
description
Group i
1380.1
1376.1
CH2 scis
CH2 rock
C-Fe str
C-X str
Fe-X str
552.9
580.0
543.3
549.6
567.7
538.2
514.4
927.1
477.2
903.3
553.9
525.3
937.0
666.0
915.5
Group m
671.9
610.9
CH2 wag
506.1
532.2
615.3
678.5
599.0
594.1
CH2 rock
CdFe str
FeX2 asym str
FeX2 sym str
695.3, 689.2
594.0
691.3, 688.1
480.3, 477.5, 474.5
478.6, 475.7, 472.8
480.3, 477.4, 474.4
a All frequencies are in cm-1. The description gives the major coordinate, and i and m denote insertion and methylidene products, respectively.
ablated Fe atoms turns out to be an effective way to form very
simple iron carbenes via C-X insertion and subsequent
R-halogen migration.
The product absorptions in the CH2FCl spectra in Figure 3
are stronger, which suggests that C-Cl insertion may be faster
than C-F insertion, as the chlorine electron cloud cross-section
is larger than that for fluorine (the reactions with CH2FCl are
computed to be only 20% more exothermic than the analogous
CH2F2 reactions). The i absorptions remain unchanged on visible
photolysis but decrease 70% on the following UV photolysis
and later partly recover in the early stage of annealing. The m
absorptions, on the other hand, decrease 50% on visible
photolysis but recover on the following UV photolysis. The i
absorption at 927.1 cm-1 and its deuterium counterpart at 903.3
cm-1 are slightly lower than the C-F stretching frequencies of
CH2F-FeF, and they are assigned to the C-F stretching modes
of CH2F-FeCl and CD2F-FeCl. Another i absorption at 514.4
cm-1 is assigned to the C-Fe stretching mode of CH2F-FeCl
with the CD2F-FeCl counterpart at 477.2 cm-1
.
The relatively strong m absorption at 678.5 cm-1 is assigned
to the Fe-F stretching mode of CH2dFeFCl. The strong
coupling between the Fe-F stretching and CH2 wagging modes
results in a large variation in intensity of the bands upon
deuteration, as shown in Table S6 (Supporting Information).
The absorption at 671.9 cm-1 in the CD2FCl spectra is assigned
to the CH2 wagging mode, while the H counterpart expected at
about 705 cm-1 is believed to be covered by a precursor
absorption. The m absorptions at 615.3 and 532.2 cm-1 in the
CH2FCl spectra are assigned to the C-Fe stretching and CH2
rocking modes without observation of the D counterparts. The
insertion and carbene complexes are 58 and 54 kcal/mol more
stable than the reactants.
Figure 3. IR spectra in the regions of 950-900 and 700-480 cm-1
for laser-ablated Fe atoms codeposited with CH2F2 and CH2FCl
isotopomers in excess argon at 10 K and their variation: (a) Fe +
0.5% CH2F2 in Ar codeposited for 1 h; (b) as in (a) after photolysis
(λ > 420 nm); (c) as in (b) after photolysis (240 < λ < 380 nm);
(d) as in (c) after photolysis (λ > 220 nm); (e) as in (d) after
annealing to 28 K; (f) Fe + 0.5% CD2F2 in Ar codeposited for
1 h; (g) as in (f) after photolysis (λ < 420 nm); (h) as in (g) after
photolysis (240 < λ < 380 nm); (i) as in (h) after photolysis (λ >
220 nm); (j) as in (i) after annealing to 28 K; (k) Fe + 0.5% CH2FCl
in Ar codeposited for 1 h; (l) as in (k) after photolysis (λ > 420
nm); (m) as in (l) after photolysis (240 < λ < 380 nm); (n) as in
(m) after annealing to 28 K; (o) Fe + 0.5% CD2FCl in Ar
codeposited for 1 h; (p) as in (o) after photolysis (λ > 420 nm);
(q) as in (p) after photolysis (240 < λ < 380 nm); (r) as in (q)
after annealing to 28 K. i and m denote the product absorption
groups. CO2, c, and P indicate residual CO2, bands common to
this precursor and different metals, and precursor absorptions,
respectively.
and 506.1 cm-1 are designated to the symmetric FeF2 stretching
and CH2 rocking modes without observation of the deuterium
counterparts. Table S4 (Supporting Information) shows that all
other bands are much weaker and provides good correlation
for the observed and DFT values, supporting formation of the
higher oxidation state product, CH2dFeF2. Our B3LYP calcula-
tions also show that CH2F-FeF (Q) and CH2dFeF2 (T) are 49
and 45 kcal/mol more stable than the reactants, but CHtFeHF2
(S) is almost 90 kcal/mol higher energy than CH2dFeF2 (T).
Ligand-stabilized iron carbene complexes are rare, and many
of their structural properties remain to be explored.4 The present
result shows that the reaction of small haloalkanes with laser-
Figures S1 and 4 show the CH2Cl2 spectra in the CH2 bending
and Fe-Cl stretching regions. The i and m absorptions show
reversible intensity changes on irradiation. With visible light,
the i absorptions double their original intensities, but the m
absorptions almost disappear. However, on the following UV
irradiation the i absorptions decrease back to their original
intensities, but the m absorptions increase to 3 times as strong
as their original intensities. They repeat these variations in
intensity on the following cycle of visible and UV irradiation.
This photoreversibility suggests that the products responsible
for the i and m absorptions are interconvertible during the
process of irradiation.
The i absorption at 1380.1 cm-1 along with its D and 13C
counterparts at 1023.9 and 1376.1 cm-1 (H/D and 12C/13C ratios
of 1.348 and 1.003) is assigned to the CH2 scissoring mode of
CH2Cl-FeCl on the basis of the frequency and the large D and
small 13C shifts. The CH2 scissoring band of CH2dFeCl2 would

Reactions of Laser-Ablated Iron Atoms
Organometallics, Vol. 27, No. 20, 2008 5245
tion is only 13 cm-1 higher. Another m absorption at 599.0
cm-1 with a 13C shift of -4.9 cm-1 is appropriate for a CH2
motion, and the rock is computed at 519 cm-1 for the triplet
state but higher at 554 cm-1 for the quintet state. This agreement
is poor owing to mode mixing with the quintet state, as the
strongest quintet state modes fall in this region (Table S8
footnote). The deuterium counterpart of another mode observed
at 610.9 cm-1 in the CD2Cl2 spectra is assigned to the CD2
wagging mode, which is computed in this region (Table S8),
while the H and 13C counterparts are covered by the strong
precursor C-Cl stretching absorptions.
The correlation of the diagnostic FeCl2 stretching mode with
the BPW91 and B3LYP calculated values is good enough to
suggest identification of the interesting CH2dFeCl2 iron carbene
molecule, but differences between other frequencies computed
with these two density functionals and the 2.63 and 2.23 spin
expectation values for the triplet state using two functionals all
point to the multireference nature of this molecule and the
approximation provided by density functional theory. The
quintet state CH2sFeCl2 molecule is isoergic at the B3LYP level
and 7 kcal/mol higher than the triplet CH2dFeCl2 ground state
(BPW91), and the CsFe bond length is 0.13 Å (B3LYP) or
0.15 (BPW91) longer in the quintet state. The molecule we
observe in solid argon is probably a mixed configuration
dominated by these two low-energy states. The reversible
photochemical equilibrium established with the above insertion
product CH2Cl-FeCl also supports formation of the carbene
product.
The common set of absorptions at 493.2, 490.3, and 487.3
cm-1 with intensity ratio of about 9:6:1 in the methylene
chloride spectra remain the same in the process of irradiation
and are due to the FeCl2 molecule byproduct, on the basis of
agreement within 0.2 cm-1 to reported frequencies25b for this
stable triatomic molecule isolated in solid argon.25 The FeCl2
absorptions are not observed in the CH2FCl spectra but appear
in the spectra from iron reactions with CHCl3 and CCl4 to be
described below, indicating that FeCl2 is a byproduct of the
atomic Fe reaction with the precursor.
Figure 4. IR spectra in the 620-450 cm-1 region for laser-ablated
Fe atoms codeposited with methylene chloride isotopomers in
excess argon at 10 K and their variations: (a) Fe + 0.5% CH2Cl2
in Ar codeposited for 1 h; (b) as in (a) after photolysis (λ > 420
nm); (c) as in (b) after photolysis (240 < λ < 380 nm); (d) as in
(c) after photolysis (λ > 420 nm); (e) as in (d) after photolysis
(240 < λ < 380 nm); (f) as in (e) after annealing to 28 K; (g) Fe
+ 0.5% CD2Cl2 in Ar codeposited for 1 h; (h) as in (g) after
photolysis (λ > 420 nm); (i) as in (h) after photolysis (240 < λ <
380 nm); (j) as in (i) after photolysis (λ > 420 nm); (k) as in (j)
after photolysis (240 < λ < 380 nm); (l) as in (k) after annealing
to 28 K; (m) Fe + 0.5% 13CH2Cl2 in Ar codeposited for 1 h; (n)
as in (m) after photolysis (λ > 420 nm); (o) as in (n) after photolysis
(240 < λ < 380 nm); (p) as in (o) after photolysis (λ > 220 nm);
(q) as in (p) after annealing to 28 K. i and m denote the product
absorption groups.
appear in the region about 130 cm-1 lower (see Table S8 in
the Supporting Information). Another i absorption at 580.0 cm-1
with relatively small D and sizable 13C shifts of -26.1 and
-12.3 cm-1 (H/D and 12C/13C ratios of 1.047 and 1.022) are
characteristic of the C-Fe stretching mode. Another i absorption
at 552.9 cm-1 along with its 13C counterpart at 549.6 cm-1
(12C/13C ratio of 1.006) is assigned to the CH2 rocking mode,
while the deuterium counterpart is apparently beyond our
observation range. The i absorption at 543.3 cm-1 has its D
and 13C counterparts at 525.3 and 538.2 cm-1, which are
appropriate for the C-Cl stretching mode. The low C-Cl
stretching frequency suggests an unusually weak C-Cl bond,
which will be discussed in Fe Product Structures. Our reasonable
agreement of observed and DFT calculated frequencies is shown
in Table S7 (Supporting Information).
Again, the product energies relative to the Fe(5D) and CH2Cl2
reagents help to understand the yield of products. Figure 4 shows
a comparable yield of the first insertion product H2ClC-FeCl
and the methylidene CH2dFeCl2, which are respectively 68 and
60 kcal/mol lower in energy than the reagents. On the other
hand, triplet HCtFeHCl2 with no symmetry converged to the
methylidene, and calculation for the singlet HCtFeHCl2
methylidyne in Cs symmetry finds a transition state with 41 kcal/
mol higher energy than the reagents. Thus, the methylidyne
complex is not a stable species.
An interesting photochemical rearrangement is observed for
the two methylene chloride reaction products, which takes
advantage of the Cl lone-pair interaction with Fe in the insertion
product to assist the Cl transfer to Fe in the methylidene
complex. Ultraviolet irradiation initiates rearrangement to the
slightly higher energy methylidene, and visible light reverses
this process (eq 1). The observation of isolated FeCl2 absorptions
here is important as an insightful byproduct of the iron and
dichloromethane reaction.
The three strong absorptions with intensity ratio of about 9/6/1
at 480.3, 477.5, and 474.5 cm-1 have similarly split D and 13
C
counterparts at 478.6, 475.7, and 472.8 cm-1 and at 480.3,
477.4, and 474.4 cm-1. Such a 9/6/1 triplet pattern identifies
the motion of two equivalent chlorine atoms. This is an
antisymmetric FeCl2 stretching mode on the basis of the
frequencies, isotopic shifts, and statistical chlorine isotopic
weights, which provides strong evidence for formation of a
product with two equivalent Cl atoms in a FeCl2 group. The
differences in frequency of 2.8 and 3.0 cm-1 between the
neighboring absorptions also exactly match with the B3LYP
values of 2.8 and 3.0 cm-1 and the BPW91 values of 2.9 and
3.0 cm-1. Note below that the isolated FeCl2 molecule absorp-
Fe + CH2Cl2 f H2ClC-FeCl {UV} CH2dFeCl2
(1)
vis
Fe + CHX3. Figure 5 illustrates the CHF3 spectra, where
only one group of absorptions is observed, and values are given
in Table 3. The m absorptions at 1161.2 and 925.0 cm-1 in the
CDF3 spectra are assigned to the coupled C-F stretching and

5246 Organometallics, Vol. 27, No. 20, 2008
Cho et al.
Figure 6. IR spectra in the 560-420 cm-1 region for laser-ablated
Fe atoms codeposited with chloroform isotopomers in excess argon
at 10 K and their variations: (a) Fe + 0.5% HCCl3 in Ar codeposited
for 1 h; (b) as in (a) after photolysis (λ > 420 nm); (c) as in (b)
after photolysis (240 < λ < 380 nm); (d) as in (c) after photolysis
(λ > 420 nm); (e) as in (d) after photolysis (240 < λ < 380 nm);
(f) as in (e) after annealing to 28 K; (g) Fe + 0.5% DCCl3 in Ar
codeposited for 1 h; (h) as in (g) after photolysis (λ > 420 nm). (i)
as in (h) after photolysis (240 < λ < 380 nm); (j) as in (i) after
photolysis (λ > 420 nm); (k) as in (j) after photolysis (240 < λ <
380 nm); (l) as in (k) after annealing to 28 K; (m) Fe + 0.5%
H13CCl3 in Ar codeposited for 1 h; (n) as in (m) after photolysis
(λ > 420 nm); (o) as in (n) after photolysis (240 < λ < 380 nm);
(p) as in (o) after photolysis (λ > 420 nm); (q) as in (p) after
photolysis (240 < λ < 380 nm); (r) as in (q) after annealing to 28
K. i and m denote the product absorption groups.
Figure 5. IR spectra in the regions of 1210-1140, 960-900, and
700-580 cm-1 for laser-ablated Fe atoms codeposited with
fluoroform isotopomers in excess argon at 10 K and their variations.
(a) Fe + 0.5% HCF3 in Ar codeposited for 1 h; (b) as in (a) after
photolysis (λ < 290 nm); (c) as in (b) after photolysis (λ < 220
nm); (d) as in (c) after annealing to 30 K; (e) Fe + 0.5% DCF3 in
Ar codeposited for 1 h; (f) as in (e) after photolysis (λ < 290 nm);
(g) as in (f) after photolysis (λ < 220 nm); (h) as in (g) after
annealing to 30 K. m denotes the product absorption. P and c
designate the precursor and bands common to this precursor with
different laser-ablated metals.
Table 3. Frequencies of Product Absorptions Observed from
Reactions of Fe and with Haloform (CHX3, X ) F, Cl) Isotopomers
in Excess Argona
symmetric FeF2 stretching mode. Correlation of the observed
frequencies with the DFT values substantiate formation of the
simple methylidene complex (CHFdFeF2), particularly the
coupled C-F stretch, DCF bending pair of bands. It should be
pointed out here that the above frequencies cannot be assigned
to the corresponding i species CHF2-FeF, since the diagnostic
1161.2 and 925.0 cm-1 bands in the CDF3 spectra are not
compatible with the frequencies computed for CDF2-FeF
(Table S9, Supporting Information; the calculated value of the
strongest band is 88 cm-1 lower than our observed value), and
accordingly this insertion product is not observed in our
experiments.
Apparently, we did not trap the CHF2-FeF intermediate,
which is 42 kcal/mol lower than Fe + CHF3, although the final
CHFdFeF2 product is 37 kcal/mol lower in energy than the
reagents at the B3LYP level of theory. However, the chloroform
reaction is more exothermic and gives a higher yield, and both
products are observed.
Shown in Figures S2 (Supporting Information) and 6 are the
CHCl3 spectra, where the m absorptions are relatively stronger
than i absorptions. The i absorption intensities almost triple on
visible photolysis, while the m absorption intensities halve. The
i absorptions, however, nearly disappear after UV photolysis,
whereas the m absorptions dramatically increase to become 30%
stronger than their original intensity. Similar changes in intensity
repeat in the following cycle of visible and UV irradiations.
This photoreversibility, parallel to the case of Fe + CH2Cl2,
suggests interconversion between the insertion and carbene
products (eq 2). The i absorption at 1187.9 cm-1 has a D
CHF3 CDF3
CHCl3
CDCl3
13CHCl3
description
Group i
1187.9
510.5, 507.5 498.0
871.4
1185.9
501.9
C-H bend
C-X str
Group m
925.0 1083.4
540
1036.4
534
HCFe bend
CdFe str
1161.2 870.3, 865.6 804.7, 798.3 849.6, 845.5 C-X str
672.2 668.8 453.1
602.0
452.0
452.9
FeX2 asym str
FeX2 sym str
a All frequencies are in cm-1. The description gives the major
coordinate, and i and m stand for insertion and methylidene products,
respectively.
DCF bending modes of CDFdFeF2 on the basis of the frequency
positions and DFT calculations, while the H counterparts
expected at about 1138 and 1239 cm-1 are evidently covered
by precursor absorptions (Table S10, Supporting Information).
The strong m absorption at 672.2 cm-1 and its counterpart at
668.8 cm-1 in the deuterium-substituted species (H/D ratio of
1.005) are due to the FeF2 antisymmetric stretching mode on
the basis of their frequency and the small deuterium shift. The
weak m absorption at 602.0 cm-1 is tentatively assigned to the
(25) (a) Thompson, K. R.; Carlson, K. D. J. Chem. Phys. 1968, 49,
4379. (b) Jacox, M. E.; Milligan, D. E. J. Chem. Phys. 1969, 51, 4143. (c)
Hastie, J. W.; Hauge, R. H.; Margrave, J. L. High Temp. Sci. 1971, 3, 257.
(d) Green, D. W.; McDermott, D. P.; Bergman, A J. Mol. Spectrosc. 1983,
98, 111 (FeCl2).

Reactions of Laser-Ablated Iron Atoms
Organometallics, Vol. 27, No. 20, 2008 5247
counterpart at 871.4 cm-1 (needs amplification of the com-
pressed spectrum in Figure S2) and a 13C component at 1185.9
cm-1, and these bands are assigned to the C-H bending mode
of CHCl2-FeCl, which is 64 kcal/mol lower energy than Fe +
CHCl3, on the basis of the large D and small 13C shifts (Table
S11, Supporting Information). Another i absorption at 510.5
cm-1 with its D and 13C counterparts at 498.0 and 501.9 cm-1
(H/D and 12C/13C ratios of 1.025 and 1.017) is assigned to the
C-Fe stretching mode of the insertion complex (CHCl2-FeCl)
due to the frequency and sizable 13C shift. Unfortunately, other
i absorptions are covered, are too weak, or are beyond our
observation range on the basis of the prediction of calculations
(Table S11).
Fe + CHCl3 f HCl2C-FeCl {UV} CHCldFeCl2 (2)
vis
On the other hand, with higher product yield more and
stronger m absorptions are observed, and triplet CHCldFeCl2
is 72 kcal/mol lower in energy than the combined reagents. The
m absorption at 1083.4 cm-1 with its 13C counterpart at 1036.4
cm-1 is assigned to the HCFe bending mode of CHCldFeCl2
without observation of the D counterpart (Tables 3 and S12
(Supporting Information)). The strong m absorptions at 870.3
and 865.6 cm-1 with their D and 13C counterparts at 804.7 and
798.3 cm-1 and at 849.6 and 845.5 cm-1 (H/D and 12C/13C
ratios of 1.083 and 1.024) are assigned to the C-Cl stretching
mode on the basis of the frequencies and the sizable 13C shifts.
A broad m absorption and its 13C counterpart at 540 and 534
cm-1 (12C/13C ratio of 1.011) are tentatively assigned to the
C-Fe stretching modes. The strong m absorption at 453.1 cm-1
Figure 7. IR spectra in the regions of 1300-1180, 1040-970,
700-620, and 520-420 cm-1 for laser-ablated Fe atoms co-
deposited with CF3Cl isotopomers and CF2Cl2 in excess argon at
10 K and their variations: (a) Fe + 0.5% CF2Cl2 in Ar codeposited
for 1 h; (b) as in (a) after photolysis (λ < 290 nm); (c) as in (b)
after photolysis (λ < 220 nm); (d) as in (c) after annealing to 30
K; (e) Fe + 0.5% CF3Cl in Ar codeposited for 1 h; (f) as in (e)
after photolysis (λ > 420 nm); (g) as in (f) after photolysis (240 <
λ < 380 nm); (h) as in (g) after photolysis (λ > 220 nm); (i) as in
(h) after annealing to 28 K; (j) Fe + 0.5% 13CF3Cl in Ar
codeposited for 1 h; (k) as in (j) after photolysis (λ > 420 nm); (l)
as in (k) after photolysis (240 < λ < 380 nm); (m) as in (l) after
photolysis (λ > 220 nm); (n) as in (m) after annealing to 28 K. i
and m denote the product absorption groups, and P and c designate
the precursor and bands common to this precursor with different
laser-ablated metals, respectively.
has D and 13C counterparts peaking at 452.0 and 452.9 cm-1
,
and they are assigned to the FeCl2 antisymmetric stretching
mode of CHCldFeCl2. These m isotopic absorptions, unlike
the CH2dFeCl2 bands split into 9/6/1 triplets, are not resolved
in this larger molecule. Finally, the very good correlation
between observed and calculated frequencies includes four
intense IR bands that are within 4.4, 0.5, 3.7, and 2.4% of the
B3LYP calculated values and the correct isotopic shifts for the
C-Cl stretching mode in the CHCl functional group, including
interaction with the C-H(D) bending mode, supports this
assignment.
Fe + CX4. Figure 7 illustrates the CF3Cl, 13CF3Cl, and
CF2Cl2 spectra in the C-F stretching and low-frequency regions,
where the product absorptions marked i and m are much stronger
than the cases described above; values are given in Table 4.
The intensities of both i absorptions in the CF3Cl spectra
increase about 20% on visible irradiation, while the m absorp-
tions remain unchanged. The former, however, decrease to 30%
of the original intensity and almost disappear on UV and full
arc irradiations, and the latter increase about 40% on UV
irradiation and slightly more on full arc irradiation. The i
absorptions are believed to arise from CF3-FeCl. The broad i
absorption at 1014 cm-1 with its 13C counterpart at 992 cm-1
(12C/13C ratio of 1.022) is assigned to the nearly degenerate A′
and A′′ CF3 stretching modes. The Cs structure of CF3-FeCl
is close to a C3V structure, as shown in Figure 9. Another i
absorption at 450 cm-1 shows a negligible 13C shift and is
assigned to the Fe-Cl stretching mode.
counterparts at 1198.9 and 1196.2 cm-1 (12C/13C ratios of 1.026
for both) and are assigned to the antisymmetric CF2 stretching
mode. Another m absorption at 658.4 cm-1 with its 13C
counterpart at 658.1 cm-1 is assigned to the CF2 scissoring mode
on the basis of the frequencies, small 13C shift, and relatively
low intensity. The strong m absorption at 649.1 cm-1 shows a
negligible 13C shift and is assigned to the Fe-F stretching mode,
due to the high intensity and negligible isotopic shift. Again
the correlation with DFT values as shown in Table S14
(Supporting Information) substantiate formation of the higher
oxidation state complex (CF2dFeFCl).
Only m absorptions are observed in the CF2Cl2 spectra. The
strong m absorption pair split by the matrix at 1246.4 and 1244.3
cm-1, whose frequencies are slightly lower than the corre-
sponding bands in the CF3Cl spectra, is assigned to the CF2
symmetric stretching mode of CF2FeCl2. Another m absorption
on the red side at 1220.7 cm-1 is assigned to the antisymmetric
stretching mode. The group of strong m absorptions partially
resolved at 463.3, 460.3, and 457.3 cm-1 with the intensity ratio
The strong m absorptions at 1249.9 and 1249.2 cm-1 with
the 13C counterparts at 1218.0 and 1215.3 cm-1 (12C/13C ratios
of 1.025 and 1.028) are designated to the symmetric CF2
stretching mode of CF2dFeFCl on the basis of the frequencies,
substantial 13C shift, and agreement with DFT values. Another
m absorption pair at 1230.4 and 1227.7 cm-1 has their 13C

5248 Organometallics, Vol. 27, No. 20, 2008
Cho et al.
Table 4. Frequencies of Product Absorptions Observed from Reactions of Fe with Carbon Tetrahalide (CX4, X ) F, Cl) Isotopomers in Excess
Argona
CF3Cl
13CF3Cl
992.0
CF2Cl2
CCl4
13CCl4
618
description
Group i
1013.5
1010.2
449.7
637
A′′ CX3 str
A′ CX3 str
A′ Fe-X str
449.7
Group m
1249.9, 1249.2
1230.4, 1227.7
658.4
1218.0, 1215.3
1198.9, 1196.2
658.1
1246.4, 1244.3
1220.7
860.4
838.7
832.6
812.5
CX2 sym str
CX2 asym str
CX2 bend
649.1
649.0
463.3, 460.3, 457.3
456.5
473.0, 470.1
456.5
473.0, 470.1
FeX2 asym str
(pert.) FeCl2
a All frequencies are in cm-1. The description gives the major coordinate, and i and m stand for insertion and methylidene products, respectively.
of 9/6/1 in agreement with the statistical isotopic ratio for two
equivalent chlorine atoms is assigned to the antisymmetric FeCl2
stretching mode. It is notable that geometry optimization for
the insertion product ends up with carbene complexes in both
the quintet and triplet states (Figure 10). CF2-FeCl2 (Q) is 6
kcal/mol (B3LYP) lower in energy than CF2dFeCl2 (T), the
C-Fe bond length is 0.424 Å longer in the quintet (Figure 10),
and their vibrational characteristics are very similar and correlate
equally well with the observed spectrum (Tables S13 and S14,
Supporting Information). Further multiconfiguration investiga-
tion is needed to determine the ground electronic state of
CF2FeCl2. It is significant, though, that the above spectroscopic
evidence indicates transfer of Cl and not F to the iron center.
This is expected, as the CCl2dFeF2 isomer is 16 kcal/mol higher
in energy (B3LYP), and the computed frequencies for C-Cl
modes (876, 845 cm-1) and Fe-F modes (738, 607 cm-1) are
quite different from the observed spectrum.
Figure 8 illustrates the CCl4 and 13CCl4 reaction product
spectra in the C-Cl and Fe-Cl stretching regions. The 637
cm-1 i absorption intensity increases about 400% and an
additional 100% on visible and UV photolysis, whereas the m
absorptions halve on visible irradiation and recover some on
UV irradiation and at the early stage of annealing. The broad i
absorption at 637 cm-1 with its 13C counterpart at 618 cm-1
(12C/13C ratio of 1.031 compared with predicted ratio of 1.034)
is assigned to the antisymmetric CCl3 stretching modes of
CCl3-FeCl, which are predicted within 2 cm-1 (B3LYP; Table
S15, Supporting Information). The weaker Fe-Cl mode cannot
be observed in the low signal 440-420 cm-1 region. However,
Figure 8. IR spectra in the regions of 790-890, 670-600, and
Table S15 shows a considerable difference between the B3LYP
and BPW91 frequencies originating from the difference in the
520-420 cm-1 for laser-ablated Fe atoms codeposited with carbon
tetrachloride isotopomers in excess argon at 10 K and their
predicted structures. This will be discussed more in the Fe
Product Structures. The absorption pair at 473.0 and 470.1 cm-1
with no measurable 13C shift has the FeCl2 chlorine isotopic
profile and is just 20 cm-1 lower than isolated FeCl2: this band
increases on irradiation and is probably due to FeCl2 perturbed
by some other species. Very weak absorption at 442 cm-1 has
the chlorine isotopic profile and does not change on irradiation,
which could be due to FeCl3, but this is just a possibility for
consideration.
variations: (a) Fe + 0.5% CCl4 in Ar codeposited for 1 h; (b) as in
(a) after photolysis (λ > 420 nm); (c) as in (b) after photolysis
(240 < λ < 380 nm); (d) as in (c) after annealing to 28 K; (e) Fe
+ 0.5% 13CCl4 in Ar codeposited for 1 h; (f) as in (e) after
photolysis (λ > 420 nm); (g) as in (f) after photolysis (240 < λ <
380 nm); (h) as in (g) after annealing to 28 K. i and m denote the
product absorption groups. P and c designate the precursor and
bands common to this precursor with different laser-ablated metals,
respectively.
The strong m absorption at 860.4 cm-1 shows a 13C shift of
-27.8 cm-1 (12C/13C ratio of 1.0334) for a C-Cl stretching
mode and an unresolved chlorine isotopic contour for two
equivalent chlorine atoms, and the related satellite band at 838.7
cm-1 exhibits a -26.2 cm-1 13C shift (12C/13C ratio of 1.0322).
Another m absorption at 456.5 cm-1 with no measurable 13C
shift is assigned to the antisymmetric FeCl2 stretching mode.
Chlorine isotopic splittings are not resolved on the FeCl2
stretching absorption, although the band profile is appropriate,
probably because this CCl2FeCl2 molecule interacts more
strongly with the matrix cage, than for example CH2FeCl2,
where the isotopic splittings are resolved (Figure 4).
In addition, one Fe experiment with 0.05% CCl4 in excess
hydrogen as the matrix codeposited at 4 K gave a simpler
spectrum with better isotopic resolution in the upper region. A
9/6/1 triplet was observed at 858.7, 857.5, and 856.2 cm-1 and
the first two 9/6 components of the lower band at 840.2, 837.9
cm-1, which are diagnostic for two equivalent chlorine atoms
in the CCl2FeCl2 product. These absorptions and an associated
452 cm-1 band decreased on UV irradiation. Another broad

Reactions of Laser-Ablated Iron Atoms
Organometallics, Vol. 27, No. 20, 2008 5249
Figure 9. Optimized molecular structures of Fe insertion complexes in the ground quintet states. The structures were calculated with
B3LYP/6-311++G(3df,3pd), except for the CCl3-FeCl structure, which was calculated with BPW91/6-311++G(3df,3pd). CH3-FeF has
a C3V structure, whereas CHCl2-FeCl has a C1 structure. Others have Cs structures. The bond lengths and angles are in Å and deg, respectively.
The product energy relative to Fe and the parent molecule is given in kcal/mol.
634 cm-1 band increased on UV irradiation, which is the
insertion product. Interestingly, no band for FeCl2 or a perturbed
FeCl2 species was observed in the 470-500 cm-1 region, but
UV irradiation of the CCl2FeCl2 product in solid hydrogen also
gave an increased 1755 cm-1 absorption for HFeCl.26
Tables S16 and S17 (Supporting Information) compare the
three observed frequencies with those calculated for the lowest
triplet state of CCl2dFeCl2 with a 1.777 Å C-Fe bond length
and a 7 kcal/mol lower energy quintet state for CCl2sFeCl2
with a longer 1.990 Å C-Fe bond length. The B3LYP
frequencies bracket the observed values with the quintet
frequencies lower and the triplet results higher. Both calculations
reveal slightly larger 13C shifts and smaller 37Cl shifts for the
higher frequency mode, which is symmetric CCl2 stretching,
than the lower frequency mode, which is antisymmetric CCl2
stretching. In fact, the calculated 37Cl shifts for the two triplet
state modes are exactly twice that observed for the first band
separation, which is expected for the 35Cl,35Cl to 35Cl,37Cl
splitting. The calculated frequencies in Tables S16 and S17 show
convincingly that our three observed bands can be assigned to
the CCl2FeCl2 methylidene molecule and suggest that the
ground-state configuration is dominated by mixed triplet and
quintet spin states.
The Fe reaction with CCl4 proceeds through the insertion
complex CCl3-FeCl, which is 61 kcal/mol lower in energy than
the reactants, to the 24 kcal/mol lower energy methylidene
complex (B3LYP, quintet insertion to triplet methylidene),
which is 85 kcal/mol lower than the reagents (eq 3). However,
visible irradiation converts some methylidene back to the higher
energy insertion complex. Another R-Cl transfer to Fe to form
ClCtFeCl3 (T) is unfavorable in terms of energy, as this species
with real frequencies and a 1.627 Å C-Fe bond length is 21
kcal/mol higher than CCl2dFeCl2 (T) and the S counterpart with
a 1.590 Å C-Fe bond length is 40 kcal/mol higher in energy
than CCl2dFeCl2 (T). Finally, our search for this iron methyl-
idyne complex was not successful (the strong diagnostic C-Cl
stretching mode in the 1200 cm-1 region11 was not observed).
Fe + CCl4 f Cl3C-FeCl {
\
} CCl2dFeCl2
(3)
vis
The present results demonstrate that, unlike CH3F and C2H6,
the di-, tri-, and tetrahalomethanes form both insertion and
carbene complexes in reactions with Fe. Furthermore, the
relative yield of m to i increases with the number of halogens
in the precursor. Substitution of hydrogen with halogen ef-
ficiently stabilizes the higher oxidation state complex with
carbon-metal double bonds, due to the strong metal-halogen
bond, particularly for heavier members of the family group.11,12
(26) Parker, S. F.; Peden, C. H. F.; Barrett, P. H.; Pearson, R. G. J. Am.
Chem. Soc. 1984, 106, 1304 (HFeCl).

5250 Organometallics, Vol. 27, No. 20, 2008
Cho et al.
Figure 11. Molecular orbital for the CH2Cl-FeCl insertion complex
showing the chlorine lone pair-iron interaction, which leads to
R-Cl transfer and the CH2dFeCl2 methylidene complex, also
illustrated.
BPW91 frequencies. Therefore, the insertion complexes con-
taining a C-Cl bond in Figure 9 all show the chlorine distortion
toward the Fe atom.
An agostic structure is normally observed for a complex
containing a C-H bond in the proximity of a metal center:
coordination of the σ C-H bonding electrons to the electron-
deficient transition-metal atom causes inclination of the C-H
bond.15,27 It is interesting that similar distortion occurs in
complexes possessing a C-Cl bond next to a metal center, while
no such C-H or C-F distortion is observed from the Fe
insertion complexes. Moreover, no such distortion of the
C-H(X) bond has been observed from the insertion complexes
produced from reactions of transition metals with methane,
ethane, and halomethanes, whereas inclination of a C-H bond
to the metal center is commonly observed from the carbene
products of early transition metals.15 Clearly, coordination of
the electron-rich Cl atom to Fe is highly favored in the insertion
complexes, and this is probably due to coordination of chlorine
lone pair electrons to the Fe center. The chlorine lone pair
molecular orbital for H2ClC-FeCl illustrated in Figure 11
demonstrates this interaction with the 3d orbitals of the iron
center. A continuation of this interaction leads directly to the
methylidene complex. Lone-pair bonding effects have been
discussed by Clot and Eisenstein, who consider this as different
from the agostic interaction.27d
Figure 10. Optimized molecular structures of Fe carbene complexes
in the ground triplet states. The structures were calculated with
B3LYP/6-311++G(3df,3pd), and they have mutually perpendicular
CX2 and FeX2 subunits (slight deviations in dihedral angles for
CH2dFeFCl (92.6°), CH2dFeCl2 (86.6°), CHCl)FeCl2 (91.1°), and
CF2dFeFCl (89.4°)). The C-Fe bond lengths are considerably
shorter than those of the insertion complexes. However, the C-Fe
bond of CF2dFeCl2 in the quintet ground state is even longer than
those of the insertion complexes, indicating that the single
carbon-metal bond is further weakened by the chlorine atom tilted
toward the C-Fe bond. The bond lengths and angles are in Å and
deg, respectively. The product energy relative to Fe and the parent
molecule is given in kcal/mol.
However, no iron carbyne product is identified from the matrix
infrared spectra. B3LYP calculations also show that, while the
energies of the insertion and carbene products are comparable,
the carbyne products are significantly higher in energy: the
carbyne complexes are 20 kcal/mol or more higher in energy
than the carbene products.
The Fe carbenes investigated here, on the other hand, all have
allene-type structures with mutually perpendicular CX2 and FeX2
subgroups, as shown in Figure 10, in line with those of the
previously studied small Ru carbenes, which may be compared
with the planar or near-planar structures of the early-transition-
metal carbenes.10,15 The CF2FeCl2 structures in the quintet and
Fe Product Structures. Figures 9 and 10 illustrate the
structures of Fe insertion, and carbene products examined in
this study were calculated at the B3LYP level of theory (except
where otherwise noted). While the insertion complexes mostly
have a Cs structure in the quintet ground state, it is notable that
in CH2Cl-FeCl and CHCl2-FeCl one of the chlorine atoms is
inclined to the Fe atom, showing considerable C-Cl bond
distortion, resulting in an unusual C1 structure in the case of
CHCl2-FeCl. Similar structures were also observed in the
5
triplet states are also shown. The carbene in the A′ state is 6
kcal/mol more stable than in the triplet state, and the observed
vibrational characteristics are consistent with the calculated
frequencies for both states, which points to a multiconfiguration
description. The C-Fe bond in the quintet structure (2.193 Å)
is much longer than that in the triplet structure (1.769 Å) and
is considered a single bond. It is notable that the relatively long
Fe-Cl bond (2.199 Å) is inclined to the CF2 group. The Fe
BPW91 calculations ( ClCFe
) 82.7 and 87.9° for
CH2Cl-FeCl and CHCl2-FeCl). As a result, the C-Cl bonds
of CH2Cl-FeCl and CHCl2-FeCl (1.861 and 1.913 Å) are
relatively very long (cf. 1.803 Å for CCl3-FeCl in Figure 9),
which result in the relatively low C-Cl stretching frequency
of 543.3 cm-1 for CH2Cl-FeCl (that for CHCl2-FeCl is too
low to observe). The B3LYP structure of CCl3-FeCl is close
to C3V symmetry; however, the BPW91 structure clearly shows
a C-Cl inclination toward the Fe atom ( ClCFe ) 80.1°),
similar to those of CH2Cl-FeCl and CHCl2-FeCl. It is also
described above that the observed frequencies, particularly the
A′ CX3 symmetric stretching frequencies, fit better with the
(27) (a) Crabtree, R. H. Chem. ReV. 1995, 95, 987, and references therein.
(b) Wada, K.; Craig, B.; Pamplin, C. B.; Legzdins, P.; Patrick, B. O.; Tsyba,
I.; Bau, R. J. Am. Chem. Soc. 2003, 125, 7035. (c) Ujaque, G.; Cooper,
A. C.; Maseras, F.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1998,
120, 361. (d) Clot, E.; Eisenstein, O. In Computational Inorganic Chemistry;
(Eds.: Kaltzoyannis, N, McGrady, J. E., Eds.; Springer: Heidelberg,
Germany, 2004; Structure and Bonding, pp 1-36. (e) Roos, B. O.; Lindh,
R.; Cho, H.-G.; Andrews, L. J. Phys. Chem. A 2007, 111, 6420. (f) Scherer,
W.; McGrady, G. S. Angew. Chem., Int. Ed. 2004, 43, 1782.

Reactions of Laser-Ablated Iron Atoms
Organometallics, Vol. 27, No. 20, 2008 5251
carbene complex in the quintet state can be considered as an
extreme case of C-Cl lone pair distortion to the Fe center, in
line with the Fe insertion complexes containing a C-Cl group,
shown in Figure 9 (e.g., CH2Cl-FeCl and CHCl2-FeCl). The
present results illustrate a strong interaction between the C-Cl
group and the adjacent Fe atom.
and CCl3-FeCl (5A′), and the CF2-FeCl2 (5A′) state shows an
extreme case of this distortion, resulting in a carbene structure.
This unusual C-Cl distortion and bond elongation evidently
results from coordination of the electron-rich Cl atom to the Fe
center, as no such distortion was observed for C-H or C-F
bonds. While most insertion complexes have a Cs structure, the
carbene products all have allene-type structures with no evidence
of agostic distortion. Substitution of Cl for F generally increases
the reaction yield, suggesting that C-Cl insertion and the
following Cl migration are more favorable reactions. In contrast
to the group 6 analogues,22 computed structures for the
unobserved higher energy CH2dFeHF carbene shows no agostic
distortion, and the transferred H on Fe is tilted back toward
carbon. We hope these density functional investigations of iron
methylidene complexes will foster higher level theoretical
calculations, in order to understand these interesting interactions
and reactive species in more detail.
In addition, the analogous CCl2FeCl2 molecule also has
triplet-quintet multiconfigurational character.
Conclusions
Laser-ablated iron atoms were reacted with halomethanes and
ethane, and the products were identified in the matrix infrared
spectra on the basis of comparisons with product energies and
frequencies calculated with density functional theory. Iron forms
insertion complexes in reactions with methyl fluoride and ethane,
in line with the previously studied Fe + CH4 reactions.8
However, for di-, tri-, and tetrachloromethanes, the matrix
spectra contain distinct FeCl2 group absorptions with statistical
chlorine isotopic splittings for two equivalent chlorine atom
populations and demonstrate the formation of methylidene as
well as insertion complexes. Several of these product pairs show
interesting photochemical properties, including photoreversibil-
ity. The large yield of CH2dFeCl2 in methylene chloride
experiments underscores the favorable C-Cl insertion reaction
and R-Cl transfer in these systems. This methylidene species
and the CF2FeCl2 and CCl2FeCl2 analogues appear to contain
significant triplet-quintet multiconfigurational character.
Interestingly enough, C-Cl lone pair distortions are observed
in the DFT structures of CH2Cl--FeCl (5A′), CHCl2-FeCl (5A),
Acknowledgment. We gratefully acknowledge financial
support from the National Science Foundation (U.S.) by
Grant No. CHE 03-52487 to L.A. and support from the Korea
Institute of Science and Technology Information (KISTI) by
Grant No. KSC-2006-S00-2014.
Supporting Information Available: Tables S1-S17, giving
calculated product frequencies, and Figures S1 and S2, giving
product IR spectra. This material is available free of charge via
the Internet at http://pubs.acs.org.
OM8005189