Formation of CH3TiX, CH2=TiHX, and (CH 3)2TiX2 by reaction of methyl chloride and bromide with laser-ablated titanium atoms: Photoreversible α-hydrogen migration

Article abstract of DOI:10.1021/ic048615a

The simple methylidene (CH₂=TiHX) and Grignard-type (CH ₃TiX) complexes are produced by reaction of methyl chloride and bromide with laser-ablated Ti atoms and isolated in a solid Ar matrix, and they form a persistent photoreversible system via α-hydrogen migration between the carbon and titanium atoms. The Grignard-type product is transformed to the methylidene complex upon UV (240 nm < λ < 380 nm) irradiation and vice versa with visible (λ > 530 nm) irradiation. More stable dimethyl dihalide complexes [(CH₃)₂TiX₂] are also identified, whose relative concentration increases upon annealing and at high methyl halide concentration. The reaction products are identified with three different groups of absorptions on the basis of the behaviors upon broadband photolysis and annealing, and the vibrational characteristics are in a good agreement with DFT computation results.

Full text of DOI:10.1021/ic048615a

Inorg. Chem. 2005, 44, 979−988
Formation of CH3TiX, CH2dTiHX, and (CH3)2TiX2 by Reaction of Methyl
Chloride and Bromide with Laser-Ablated Titanium Atoms:
Photoreversible -Hydrogen Migration
r
Han-Gook Cho† and Lester Andrews*
Department of Chemistry, UniVersity of Virginia, P.O. Box 400319,
CharlottesVille, Virginia 22904-4319
Received October 5, 2004
The simple methylidene (CH2dTiHX) and Grignard-type (CH3TiX) complexes are produced by reaction of methyl
chloride and bromide with laser-ablated Ti atoms and isolated in a solid Ar matrix, and they form a persistent
photoreversible system via
is transformed to the methylidene complex upon UV (240 nm <
> 530 nm) irradiation. More stable dimethyl dihalide complexes [(CH3)2TiX2] are also identified, whose relative
R-hydrogen migration between the carbon and titanium atoms. The Grignard-type product
λ
< 380 nm) irradiation and vice versa with visible
(
λ
concentration increases upon annealing and at high methyl halide concentration. The reaction products are identified
with three different groups of absorptions on the basis of the behaviors upon broadband photolysis and annealing,
and the vibrational characteristics are in a good agreement with DFT computation results.
Introduction
ecules exhibit CdM stretching frequencies in the range from
513.7 (Zn) to 696.2 (Ni) cm-1.
High-oxidation-state alkylidene (MdCR1R2) and alkyli-
dyne (M≡CR) complexes have been the subject of numerous
studies since the first discovery of the high-oxidation-state
transition-metal complexes containing a multiple metal-
carbon bond in 1970s.1 Not only have these investigations
provided a wealth of information on the nature of metal
coordination chemistry, but these compounds are industrially
important as metathesis catalysts for alkenes, alkynes, and
cyclic compounds.1-4 The transition-metal methylidene
derived from a simple halomethane has been the subject of
various theoretical studies for molecular structure and
reactivity.5-7
Recently, early-transition-metal methylidenes have been
observed in reactions of laser-ablated transition-metal atoms
and methyl fluorides in excess argon during condensation.9-12
More recently, a methylidene complex was formed in the
reaction of methane and laser-ablated atomic zirconium.13,14
Interestingly enough, the agostic interaction15-17 involving
the carbon and metal atoms and one of the R-hydrogen atoms
is observed in these simple methylidene hydride complexes
experimentally13,14 as well as theoretically.9-14 Moreover,
persistent photoreversibility has been observed from the
systems containing CH2dZrH2,14 CH2dZrHF,11 and CH2d
TiHF.9
In the zirconium systems, the photoreversibility is believed
to involve the zirconium center and two different argon
The very simple CH2dM species have been prepared in
an elegant series of late transition metal Cr through Zn
reactions with diazomethane in excess argon.8 These mol-
(8) Billups, W. E.; Chang, S.-C.; Margrave, J. L.; Hauge, R. H.
Organometallics 1999, 18, 3551 and references therein to previous
work in the late transition metal series.
* To whom correspondence should be addressed. E-mail: lsa@virginia.edu.
† Current address: Department of Chemistry, University of Incheon, 177
Dohwa-dong Nam-ku, Incheon, 402-749 South Korea.
(1) Schrock, R. R. Chem. ReV. 2002, 102, 145.
(2) Buchmeiser, M. R. Chem. ReV. 2000, 100, 1565.
(3) Legzdins, P.; Tran, E. J. Am. Chem. Soc. 1997, 119, 5071.
(4) Choi, S.-H.; Lin, Z. Organometallics 1999, 18, 5488.
(5) Franci, M. M.; Pietro, W. J.; Hout, R. F., Jr.; Hehre, W. J.
Organometallics 1983, 2, 281; Organometallics 1983, 2, 815.
(6) Cundari, T. R.; Gordon, M. S. J. Am. Chem. Soc. 1992, 114, 539.
(7) Siegbahn, P. E. M.; Blomberg, M. R. A. Organometallics 1994, 13,
354.
(9) Cho, H.-G.; Andrews, L. J. Phys. Chem. A 2004, 108, 6294.
(10) Cho, H.-G.; Andrews, L. Inorg. Chem. 2004, 43, 5253.
(11) Cho, H.-G.; Andrews, L. J. Am. Chem. Soc. 2004, 126, 10485.
(12) Cho, H.-G.; Andrews, L. Organometallics 2004, 23, 4357.
(13) Andrews, L.; Cho, H.-G.; Wang, X. Angew. Chem. 2005, 117, 115.
(14) Cho, H.-G.; Wang, X.; Andrews, L. J. Am. Chem. Soc. 2005, 127,
465.
(15) Wada, K.; Craig; B. Pamplin, C. B.; Legzdins, P.; Patrick, B. O.; Tsyba,
I.; Bau, R. J. Am. Chem. Soc. 2003, 125, 7035.
(16) Ujaque, G.; Cooper, A. C.; Maseras, F.; Eisenstein, O.; Caulton, K.
G. J. Am. Chem. Soc. 1998, 120, 361.
(17) Scherer, W.; McGrady, G. S. Angew. Chem., Int. Ed. 2004, 43, 1782.
10.1021/ic048615a CCC: $30.25
Published on Web 01/15/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 4, 2005 979

Cho and Andrews
matrix configurations.11 The conversion is most likely
initiated by transition between the singlet and triplet CH2d
ZrH2 or CH2dZrHF states, giving two different matrix cage
structures. On the other hand, the photoreversibility between
CH2dTiHF and CH3TiF occurs via R-hydrogen migration.9
The concentration of CH2dTiHF is relatively low initially
after co-deposition of the laser-ablated metal atoms and
methyl fluoride, but the concentration increases significantly
upon UV irradiation of CH3TiF. The dimethyl dihalide
complex [(CH3)2TiF2] has also been identified,10 and the
relative concentration increases dramatically upon annealing
and at higher concentration of the methyl halide while those
of the smaller complexes decrease.
It is therefore necessary to examine whether other methyl
halides also form similar methylidene complexes and even
photoreversible systems. Previous studies show that methyl
fluoride is more reactive with vaporized metal atoms than
methane.11,14 The higher reactivity is considered to originate
from the lone electron pairs on the halogen atom, which
attract the electron-deficient metal atom. In this regard,
methyl chloride and bromide would be even more reactive
with vaporized transition metals. Previous studies also show
that the molecular distortion due to the agostic interaction15-17
between the metal and R-hydrogen atoms decreases in the
order of Ti > Zr > Hf,12 and therefore, another interesting
question is how the agostic interaction varies with the
halogen substituent on the metal center.
In this study, reactions of laser-ablated Ti atoms with
methyl chloride and bromide diluted in argon were carried
out, and the products isolated in a solid argon matrix were
investigated by means of infrared spectroscopy. Results
indicate that there are at least three different groups of
absorptions on the basis of the behaviors upon photolysis
and annealing, and interestingly enough, two of them form
a persistent photoreversible system in the matrix. Another
group of absorptions arises from a larger complex, the
dimethyl titanium dihalide. The vibrational characteristics
of the product absorptions are confirmed by isotopic
substitution and agreement with DFT-calculated frequencies.
Figure 1. IR spectra in the regions of 1580-1640 and 500-800 cm-1
for laser-ablated Ti atoms co-deposited with CH3Cl diluted in Ar at 7 K.
(a) Ti + 0.2% CH3Cl in Ar co-deposited for 1 h. (b) After broadband
photolysis with a filter (λ > 530 nm) for 20 min. (c) After broadband
photolysis with a UV-transmitting filter (240 nm < λ < 380 nm) for 20
min. (d) After broadband photolysis with a filter (λ > 530 nm) for 20 min.
(e) After broadband photolysis with a UV-transmitting filter (240 nm < λ
< 380 nm) for 20 min. (f) After broadband photolysis with a filter (λ >
530 nm) for 20 min. (g) After annealing to 32 K. I, II, and III stand for the
product band groups, and P indicates the strong C-Cl stretching absorption
of CH3Cl. The absorptions of water impurity are marked w, and unidentified
absorptions are marked with /.
products. Geometries were fully relaxed during optimization, and
the optimized geometry was confirmed via vibrational analysis. All
vibrational frequencies were calculated analytically, and the zero-
point energy is included in calculation of the binding energies.
Results and Discussion
Figure 1 shows the IR spectra in the regions of 1580-
1640 and 500-800 cm-1 for laser-ablated Ti atoms co-
deposited with 0.2% CH3Cl in argon at 7 K and their
variation upon photolysis and annealing. In the region of
1580-1640 cm-1, the product absorption at 1618.4 cm-1 is
clearly distinguished from the absorptions of water residue.
Photolysis with a broadband Hg lamp and filter (λ > 530
nm), after the co-deposition, decreases the absorption.
Whereas photolysis with shorter wavelength (380 < λ < 530
nm) does not produce a noticeable change in the absorption
at 1618.4 cm-1, UV photolysis (240 nm < λ < 380 nm)
causes a dramatic increase in the intensity. The absorption
later weakens in the subsequent photolysis with visible light
(λ > 530 nm) but increases again in the next UV photolysis.
An analogous absorption is observed at 1619.0 cm-1 in
the spectrum of Ti + CH3Br (Figure 2). Initially, it is weaker
Experimental and Computational Methods
Laser-ablated titanium atoms (Johnson-Matthey) were reacted
with CH3Cl, CH3Br, CD3Br (Cambridge Isotope Laboratories, 99%),
and CD3Cl (synthesized from HgCl2 and CD3Br) in excess argon
during condensation at 7 K using a closed-cycle He refrigerator
(Air Products HC-2). The methods are described in detail else-
where.18 Concentrations of gas mixtures range between 0.2% and
0.5% in argon. After reaction, infrared spectra were recorded at a
resolution of 0.5 cm-1 using a Nicolet 550 spectrometer with a
HgCdTe detector. Samples were later irradiated by a combination
of optical filters and a mercury arc lamp (175 W, globe removed)
and annealed, and more spectra were recorded.
(19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli C.,; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Rega, N.; Salvador, P.; DanneTierg, J. J.; Malick, D.
K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski,
J.; J. V. Ortiz,; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko,
A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; 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.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.;
Pople, J. A. Gaussian 98, revision A.11.4; Gaussian, Inc.: Pittsburgh,
PA, 2002.
Complementary density functional theory (DFT) calculations
were carried out using the Gaussian 98 package,19 B3LYP density
functional, and 6-311++G(2d,p) basis sets for C, H, F, and Ti to
provide a consistent set of vibrational frequencies for the reaction
(18) (a) Chertihin, G. V.; Andrews, L. J. Phys. Chem. 1995, 99, 6356. (b)
Andrews, L.; Citra, A. Chem. ReV. 2002, 102, 885 and references
therein.
980 Inorganic Chemistry, Vol. 44, No. 4, 2005

PhotoreWersible r-Hydrogen Migration
Figure 2. IR spectra in the regions of 1580-1640 and 480-800 cm-1
for laser-ablated Ti atoms co-deposited with Ar/CH3Br at 7 K. (a) Ti +
0.5% CH3Br in Ar co-deposited for 1 h. (b) After broadband photolysis
with a filter (λ > 530 nm) for 20 min. (c) After broadband photolysis with
a UV-transmitting filter (240 nm < λ < 380 nm) for 20 min. (d) After
broadband photolysis with a filter (λ > 530 nm) for 20 min. (e) After
broadband photolysis with a UV-transmitting filter (240 nm < λ < 380
nm) for 20 min. (f) After annealing to 32 K. I, II, and III stand for the
product band groups, and P indicates the strong C-Br stretching absorption
of CH3Br. The absorptions of water impurity marked with w, and
unidentified absorptions are marked with /.
Figure 3. IR spectra in the regions of 1140-1190 and 440-540 cm-1
for laser-ablated Ti atoms co-deposited with CD3Cl and CD3Br diluted in
Ar at 7 K. (a) Ti + 0.5% CD3Br in Ar co-deposited for 1 h. (b) After
broadband photolysis with a UV-transmitting filter (240 nm < λ < 380
nm) for 20 min. (c) After broadband photolysis with a filter (λ > 530 nm)
for 20 min. (d) After broadband photolysis with a UV-transmitting filter
(240 nm < λ < 380 nm) for 20 min. (e) After annealing to 32 K. (f) Ti +
0.5% CD3Cl in Ar co-deposited for 1 h. (g) After broadband photolysis
with a UV-transmitting filter (240 nm < λ < 380 nm) for 20 min. (h)
After broadband photolysis with a filter (λ > 530 nm) for 20 min. (i) After
broadband photolysis with a UV-transmitting filter (240 nm < λ < 380
nm) for 20 min. (j) After annealing to 32 K. I, II, and III indicate the product
band groups. The P and / labels indicate precursor and unidentified product
absorptions, respectively.
after co-deposition, but it grows more significantly (more
than 5-fold) upon UV photolysis. The absorptions at 1618.4
and 1619.0 cm-1 in Figures 1 and 2 show similar large
isotopic shifts of -449.9 and -450.5 cm-1 upon deuteration
(H/D isotopic ratios of 1.385 and 1.386) and are attributed
to the Ti-H stretching mode of the reaction products of Ti
+ CH3Cl and Ti + CH3Br. Earlier studies also show that
the hydrogen stretching absorptions of titanium hydrides
appear in the same frequency region.20,21 This indicates that
C-H insertion by the metal atom readily occurs in the
reaction of methyl halide with laser-ablated Ti atoms and
even more in UV photolysis afterward.9
Similar alternating variations in absorption intensities on
photolysis are also observed below 800 cm-1, accompanied
by the variations in the hydrogen stretching region, as shown
in Figures 1-3. It is notable that the strong absorption at
512.5 cm-1 in Figure 1 decreases on UV photolysis, whereas
the neighboring absorptions at 631.0, 643.8, and 648.3 cm-1
along with the broad absorption at 774 cm-1 increase
dramatically (about 3-fold) similarly to the Ti-H stretching
absorption in Figure 1. Subsequent photolysis with visible
light (λ > 530 nm) leads to a reversal. The same variations
in absorption intensities are observed repeatedly in the
subsequent cycles of irradiation with UV and visible light
without any noticeable decrease in the absorption intensities.
Similar spectral variations are also observed in Figures 2
and 3.
changes in the course of photolysis, the absorptions at 552.6,
555.2, and 566.5 cm-1 in Figure 1 increase only gradually
regardless of irradiation wavelength and later increase
dramatically upon annealing. Absorptions showing similar
behaviors are also observed in Figures 2 and 3.
Reactions of both methyl chloride and bromide yield
similar product absorptions, and their variations upon pho-
tolysis, annealing, change in concentration, and deuteration
are also comparable, as shown in Figures 1-3. Their
vibrational characteristics and variations are, in turn, con-
sistent with those in the previous study (Ti + CH3F).9,10
Moreover, both CH3Cl and CH3Br are apparently more
reactive with vaporized Ti atoms than CH3F. Under the same
experimental conditiond, the product absorptions from CH3-
Cl and CH3Br are generally 2-3 times stronger than those
from CH3F.
On the basis of the above behavior, observed product
absorptions are sorted into three groups as listed in Table 1,
and the frequencies are compared with the calculated values
in Tables 2-5. Group I absorptions are relatively weak in
the original spectrum after co-depositon of methyl halide and
Ti atoms, but increase dramatically (at least 3-fold) upon
UV irradiation. The Ti-H and Ti-D stretching absorptions
in the regions of 1580-1640 and 1140-1190 cm-1 shown
in Figures 1-3 belong to this group. They increase and
Apparently, UV irradiation is more effective than visible
irradiation, despite the lower intensity in the short-wavelength
region. About 3 times longer visible irradiation is necessary
to counteract the variation in absorption intensity caused by
UV (240 nm < λ < 380 nm) irradiation. It is also notable
that, whereas many product absorptions show dramatic
(20) Chertihin, G. V.; Andrews, L. J. Am. Chem. Soc. 1994, 116, 8322.
(21) Lee, Y. K.; Manceron, L.; Papai, I. J. Phys. Chem. A 1997, 101, 9650.
Inorganic Chemistry, Vol. 44, No. 4, 2005 981

Cho and Andrews
Table 1. Frequencies of Observed Product Absorptionsa
Group I. The Ti-H stretching absorptions described
above belong to group I. The observation of a metal-
hydrogen stretching absorption indicates that C-H insertion
by Ti occurs, and among the plausible products in the
reaction of vaporized Ti atoms and methyl halides, the
absorption most likely arises from the methylidene complex
CH2dTiHX.9 The absorptions at 648.3 and 648.4 cm-1 in
Figures 1 and 2 show isotopic shifts of -131.1 and -132.6
cm-1, respectively, upon deuteration (H/D isotopic ratios of
1.253 and 1.257), and the strong absorptions at 631.0 and
628.6 cm-1 in Figures 1 and 2 show similar isotope shifts
of -126.9 and -131.1 cm-1, respectively, upon D substitu-
tion (H/D isotopic ratios of 1.252 and 1.264). The frequencies
and isotope shifts show that the absorptions probably arise
from hydrogen bending modes. The calculation results, given
in Table 2, indicate that the absorptions probably arise from
the CTiH in-plane bending and CH2 wagging modes,
respectively.
group
I
CH3Cl
1618.4
CD3Cl
1168.3
CH3Br
1619.0
CD3Br
1168.5
773.9, 766.5
643.8, 648.3
631.0
1112.8
512.5
774.7
648.4
628.6
517.2
504.1
880.2
465.5
515.8, 510.0
503.4, 497.5
II
1112.7, 1108.9 875.3
508.8
457.3
412
III
2887.4
1373.4, 1372.3 1004.9
1365.7
1351.3
876.9
2886.5
1370.4
1367.9
2092.6
1003.4
909.3
903.1
869.4
529.9, 527.9
566.5
555.2
552.6
423
527.7
547.8
545.9
432.2
494.3
489.6
497.1
a All frequencies are in cm-1. Stronger absorptions are in bold.
decrease repeatedly upon photolysis with UV (240 nm < λ
< 380 nm) and visible (λ > 530 nm) light, respectively,
and weaken gradually in the process of annealing.
The Ti-Cl and Ti-Br stretching absorptions of CH2d
TiHX, unlike the case of methyl fluoride,9-12 are not
observed in this study, because of the low frequencies as
shown in Table 2. On the other hand, new absorptions are
observed at 766.5 and 774.7 cm-1 in Figures 1 and 2,
respectively. The frequencies are 8.7 and 16.9 cm-1 higher
than the corresponding one in the previous Ti + CH3F study.
Parallel to the previous studies, the absorptions are assigned
to the predominately CdTi stretching mode of the products.
Cartesian coordinate displacements reveal large C and Ti
movements but contribution from methylene hydrogen as
well. In the CH2dTiHF case, this mode at 757.8 cm-1 shifted
to 748.8 cm-1 with 13CH3F and to 644.9 cm-1 with CD3F,
indicating mixing with a CH2 motion. Because the CH2
modes (ν3 and ν7, Table 2) are nearly the same for CH2d
TiHCl and CH2dTiHBr, we expect comparable mode
mixing, and we take the increase in the “predominately Cd
Ti” stretching mode to suggest that the CdTi bonds of CH2d
TiHCl and CH2dTiHBr are, in fact, stronger than that of
CH2dTiHF. This is consistent with the computed decrease
in CdTi bond lengths in the series. It is appropriate to note
that the predominately CdTi stretching mode observed for
the CH2dTiHX species (757.8, 766.5, and 774.7 cm-1) are
higher than the CdM stretching modes assigned for CH2d
M species (M ) Cr-Zn, i.e., 567.0, 521.9, 623.9, 696.2,
Group II absorptions decrease by about 20% in intensity
upon UV irradiation. They decay and grow repeatedly upon
irradiation with UV and visible light, respectively, opposite
to group I. They decrease rapidly upon annealing to higher
than 20 K. No Ti-H stretching absorptions are included in
this group. The opposite behaviors of groups I and II upon
photolysis suggest that the two groups originate from
different reaction products and that the reaction products are
interconvertible in such a manner that an increase in
concentration of one product leads to a decrease in concen-
tration of the other product.
Group III absorptions are relatively weak in the original
spectrum after co-deposition of methyl halide and laser-
ablated Ti atoms. They grow gradually during photolysis,
regardless of the wavelength, and once they grow, they do
not decrease in the subsequent cycles of photolysis. In
annealing, they grow dramatically up to 32 K. Figure 4 shows
the variation in absorption intensity of the relatively strong
group III absorptions of Ti + CH3Cl upon annealing and at
higher concentration: the intensities increase about 5 times
in the process of annealing. Their relative intensities also
increase considerably at high concentration of methyl halide
in argon, as shown Figure 4.
Table 2. Observed and Calculated Fundamental Frequencies of CH2dTiHCl and CH2dTiHBr in the Ground Electronic State (1A)a
CH2dTiHCl
CD2dTiDCl
CH2dTiHBr
CD2dTiDBr
description
exp
calc
int
exp
calc
int
exp
calc
int
exp
calc
int
ν1 A′ C-H2 stretch
ν2 A′ C-H1 stretch
ν3 A′ Ti-H3 stretch
ν4 A′ CH2 scissor
ν5 A′ C-Ti stretch
ν6 A′ CTiH bend
ν7 A′ CH2 rock
3190.9
2795.9
1680.2
1340.6
822.7
693.2
442.4
409.7
147.4
684.4
487.9
65.0
2
2362.9
2034.2
1201.8
1051.5
57
548.4
330.5
389.1
133.5
539.0
345.7
49.7
4
2
211
28
774.7
58
4
42
3
118
12
82
3190.4
2785.5
1619.0
3
2
2362.6
2066.6
1202.0
1051.8
5
2
400
24
88
45
11
59
4
167
28
2
211
28
1618.4
1168.3
1680.6
1343.2
85
693.7
452.9
309.3
131.0
683.9
479.3
65.5
402
23
729.4
29
10
37
4
161
30
1168.5
766.5
648.3
727.4
517.2
823.8
648.4
55
515.8
539.2
335.9
297.2
118.6
537.9
339.3
49.1
62
5
31
3
82
14
72
ν8 A′ Ti-X stretch
ν9 A′ CTiX bend
ν
ν
ν
10 A′′ CH2 wag
11 A′′ CH2 twist
12 A′′ TiH OOP bend
631.0
504.1
628.6
497.5
138
122
a Frequencies and infrared intensities are in cm-1 and km/mol, respectively. Intensities are all calculated values.
982 Inorganic Chemistry, Vol. 44, No. 4, 2005

PhotoreWersible r-Hydrogen Migration
Table 3. Observed and Calculated Fundamental Frequencies of CH3TiCl and CH3TiBr in the Ground Electronic States (3A)a
CH3-Ti-Cl
CD3-Ti-Cl
CH3-Ti-Br
CD3-Ti-Br
description
exp
calc
int
exp
calc
int
exp
calc
int
exp
calc
int
ν1 A′ CH3 stretch
ν2 A′ CH3 stretch
ν3 A′ CH3 scissor
ν4 A′ CH3 deform.
ν5 A′ C-Ti stretch
ν6 A′ Ti-X stretch
ν7 A′ CH3 rock
3084.0
2960.9
1414.2
1146.0
531.3
432.3
362.8
96.3
3019.7
1427.2
389.1
99.1
4
7
1
2276.9
2124.7
1026.4
901.8
477.8
405.1
298.2
88.8
2230.1
1035.3
291.4
73.8
1
1
2
22
80
81
5
4
2
3
12
1
3084.0
2961.9
1412.8
1146.1
529.6
399.5
287.7
84.6
3020.9
1427.4
385.4 1
89.1
4
8
2
2276.8
2125.4
1025.5
901.8
472.3
332.5
267.0
77.2
2230.9
1035.4
288.9 1
66.6
1
1
2
21
63
72
3
3
3
3
2
1112.8
512.5
412
8
880.2
465.5
1112.7
508.8
7
875.3
457.3
66
120
4
4
7
3
18
0
61
77
23
3
9
3
ν8 A′ CTiX bend
ν9 A′′ CH3 stretch
ν
ν
ν
10 A′′ CH3 scissor
11 A′′ CH3 rock
12 A′′ CH3 distort.
7
0
0
a Frequencies and intensities are in cm-1 and km/mol, respectively. Intensities are all calculated values.
Table 4. Observed and Calculated Fundamental Frequencies of
(CH3)2TiCl2
Table 5. Observed and Calculated Fundamental Frequencies of
(CH3)2TiBr2
a
a
(CH3)2TiCl2
(CD3)2TiCl2
calc
(CH3)2TiBr2
(CD3)2TiBr2
description
exp
calc
int
exp
int
description
exp
calc
int
exp
calc
int
ν1 A1 CH3 stretch (a)
ν2 A1 CH3 stretch (s)
ν3 A1 CH3 scissor
ν4 A1 CH3 deform.
ν5 A1 TiC2 stretch (s)
ν6 A1 CH3 rock
ν7 A1 TiCl2 stretch (s)
ν8 A1 TiC2 IP bend
ν9 A1 TiCl2 bend
3094.0
3003.5
1365.7 1413.9
1160.2
3
0
9
4
2288.5
2148.9
1027.0
1
1
6
12
78
4
15
0
2
0
0
0
ν1 A1 CH3 stretch (a)
ν2 A1 CH3 stretch (s)
ν3 A1 CH3 scissor
ν4 A1 CH3 deform.
ν5 A1 TiC2 stretch (s)
ν6 A1 CH3 rock
ν7 A1 TiBr2 stretch (s)
ν8 A1 TiC2 IP bend
ν9 A1 TiBr2 bend
3094.6
3003.0
2
0
2288.9
2148.4
1026.2
912.8 12
501.1 60
377.5
274.2 13
131.9
71.9
2288.9
1022.2
377.1
108.5
67.0
0
1
6
b
1367.9 1412.7 10
1157.1
555.1 72 494.3
484.3
282.6 17
b
876.9
497.1
914.8
509.9
365.1
404.5
136.6
103.7
2289.9
1023.3
387.3
117.9
62.8
3
909.3
552.6
561.2 83
490.9
396.2 27
155.9
105.1
3093.8
1408.6
509.4
132.1
88.1
545.9
0
0
2
0
2
0
0
0
0
0
8
150.5
72.7
0
0
0
0
0
0
0
8
0
0
0
0
0
0
0
3
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
10 A2 CH3 stretch
11 A2 CH3 scissor
12 A2 CH3 rock
13 A2 TiCl2 twist
14 A2 CH3 distort.
15 B1 CH3 stretch
16 B1 CH3 scissor
17 B1 CH3 rock
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
10 A2 CH3 stretch
11 A2 CH3 scissor
12 A2 CH3 rock
13 A2 TiBr2 twist
14 A2 CH3 distort.
15 B1 CH3 stretch
16 B1 CH3 scissor
17 B1 CH3 rock
3092.8
1406.8
497.7
123.8
94.0
0
0
3
10
3097.1
2291.9
3096.1
2232
2291.1
1373.4 1415.3 17 1004.9 1027.3
1370.4 1413.5 19 1003.4 1026.2 11
566.5
423
579.2 94
438.5 67
151.5
114.5
529.9
526.8 142
527.7
543.5 37 432.2
364.7 71
134.0
121.6
459.8 73
324.1 35
120.8
86.3
18 B1 TiCl2 stretch (a)
19 B1 TiCl2 rock
366.5
137.3
81.4
2289.8
2147.4
1020.9
912.4
504.6
357.9
122.3
16
0
0
0
2
18 B1 TiBr2 stretch (a)
19 B1 TiBr2 rock
0
0
1
1
7
4
2
0
0
2
6
5
1
0
0
3
3
20 B1 CH3 distort.
21 B2 CH3 stretch (a)
22 B2 CH3 stretch (s)
23 B2 CH3 scissor
24 B2 CH3 deform.
25 B2 TiC2 stretch (a)
26 B2 CH3 rock
20 B1 CH3 distort.
21 B2 CH3 stretch (a)
22 B2 CH3 stretch (s)
23 B2 CH3 scissor
24 B2 CH3 deform.
25 B2 TiC2 stretch (a)
26 B2 CH3 rock
3094.3
3094.8
2290.2
2092.6 2146.9
1019.8
2887.4 3001.6
1351.3 1405.6
1147.8
2886.5 3001.1
1360.0 1404.0
1145.0
4
869.4
489.6
19
67
0
903.1
910.7 20
500.6 58
354.0
106.7
555.2
560.6 82
473.0
128.9
547.8
556.4 73 494.3
468.7
113.3
3
3
3
1
0
1
27 B2 TiCl2 OOP bend
2
27 B2 TiBr2 OOP bend
a Frequencies and intensities are in cm-1 and km/mol, respectively.
Calculations were done at the B3LYP/6-311++G(2d,p) level.
a Frequencies and intensities are in cm-1 and km/mol, respectively.
Calculations were done at the B3LYP/6-311++G(2d,p) level. b Overlapped.
614.0, and 513.7 cm-1, respectively),8 but the latter modes
are higher in CdM character according to the observed 13Cd
M shifts.
As with formation of simple methylidene complexes in
reactions of CH3F with vaporized group IV transition
metals,9,11-14 the present results show that methylidene
complexes can also be generated from other methyl halides
with vaporized transition-metal atoms or UV photolysis.
Moreover, CH3Cl and CH3Br are clearly more reactive than
CH3F.
Group II. Figures 1-3 show that the compound respon-
sible for group II is formed originally in a relatively large
amount in reaction of methyl halide and Ti atoms, in
comparison with the methylidene complex. The strongest
absorptions of group II are observed at 512.5 and 508.8 cm-1
in the spectra of Ti + CH3Cl and Ti + CH3Br as shown in
Figures 1 and 2, and they exhibit relatively small isotopic
shifts of -46.5 and -51.5 cm-1, respectively (H/D isotopic
ratios of 1.101 and 1.113), upon deuteration. The absorptions
are assigned to the C-Ti stretching mode. The absorptions
at 1112.8 and 1112.7 cm-1 show isotopic shifts of -232.6
and -237.4 cm-1, respectively, upon D substitution (H/D
isotopic ratios of 1.264 and 1.271), and on the basis of the
frequencies and the isotopic shifts, they are attributed to a
hydrogen deformation mode of the products. The absorption
at 412 cm-1 in the spectrum of Ti + CH3Cl is assigned to
the Ti-Cl stretching mode of the product, which is close to
the Ti-Cl stretching frequency of the TiCl radical (404.33
cm-1) in the ground electronic state (X4Φ).22
(22) Imajo, T.; Wang, D.; Tanaka, K.; Tanaka, T. J. Mol. Spectrosc. 2000,
203, 216.
Inorganic Chemistry, Vol. 44, No. 4, 2005 983

Cho and Andrews
Table 6. Calculated Geometrical Parameters and Physical Constants of
a
CH2dTiHCl, CH3sTiCl, and (CH3)2TiCl2
parameter
CH2dTiHCl
CH3sTiCl
(CH3)2TiCl2
r(C-H1)
r(C-H2)
r(C-Ti)
r(Ti-H3)
r(Ti-Cl)
r(Ti‚‚‚H1)
H1CH2
H2CH3
CTiCl
1.123
1.085
1.801
1.733
2.244
2.070
114.4
1.093
1.100
2.107
1.094
1.094
2.053
2.260
2.753
108.1
107.6
130.0
2.213
2.622
109.6
110.1
108.3
122.6
111.1
126.3
86.9
158.7
0.0
180.0
Cs
-0.44
0.13
CTiH3
H3TiCl
H1CTi
115.0
108.9
121.4
0.0
109.0
109.2
119.8
64.7
C2V
-0.39
0.15
0.15
0.15
0.17
H2CTi
Φ(H1CTiH3)
Φ(H1CTiCl)
mol symm
q(C)b
Cs
-0.59
0.12
0.10
0.10
0.62
-0.35
3.27
3A′′
q(H1)b
Figure 4. IR spectra in the regions of 1360-1400 and 540-600 cm-1
for laser-ablated Ti atoms co-deposited with CH3Cl diluted in Ar at 7 K.
(a) Ti + 0.5% CH3Cl in Ar co-deposited for 1 h. (b) Ti + 0.2% CH3Cl in
Ar co-deposited for 1 h. (c-e) After annealing to 20, 26, and 32 K,
respectively, following co-deposition of Ti + 0.2% CH3F in Ar for 1 h.
q(H2)b
0.15
-0.18
0.60
q(H3)b
q(Ti)b
q(Cl)b
-0.26
1.43
-0.14
µc
2.04
stated
1A′
1A1
∆Ee,f
102.2g
124.2h
203.0
It should be remembered at this point that the most stable
compound among the plausible products in the reaction of
methyl halide with vaporized Ti atoms is CH3-TiX in its
triplet ground state.9 Calculations show that the approach of
a Ti atom (3F2) to methyl halide on the side of halogen atom
eventually leads to the structure of CH3TiX (T) on the triplet
potential energy surface. Table 3 shows that the group II
absorptions in Figures 1 and 2 match with the predicted
strong absorptions of CH3TiCl and CH3TiBr. The present
results indicate that, parallel to the case of Ti + CH3F, CH3-
TiCl and CH3TiBr are produced mostly in the reaction of
vaporized Ti atoms with methyl chloride and bromide.
Therefore, CH3MX is evidently first formed in reaction
of methyl halide with laser-ablated transition-metal atoms
and then transformed to the methylidene complex or larger
complexes, even during co-deposition. Formation of CH3-
MX requires activation energy, laser ablation provides
excited transition-metal atoms,17 and the excited intermediate
thus formed can be relaxed by the matrix or undergo
R-hydrogen transfer.23
a Bond lengths and angles are in angstroms and degrees, respectively.
b Mulliken atomic charge. c Molecular dipole moments are in debye.
d Electronic state. e Binding energies in kcal/mol. f Transition state binding
energies: T, 86.0 kcal/mol, and S, 87.1 kcal/mol. g Lowest triplet state
binding energy is 90.0 kcal/mol. h Lowest singlet state binding energy is
99.1 kcal/mol.
Table 7. Calculated Geometrical Parameters and Physical Constants of
a
CH2dTiHBr, CH3sTiBr, and (CH3)2TiBr2
parameter
CH2dTiHBr
CH3sTiBr
(CH3)2TiBr2
r(C-H1)
r(C-H2)
r(C-Ti)
r(Ti-H3)
r(Ti-Br)
r(Ti‚‚‚H1)
H1CH2
H2CH3
CTiBr
1.125
1.085
1.798
1.731
2.403
2.055
114.4
1.093
1.100
2.107
1.094
1.094
2.052
2.421
2.759
108.0
107.7
131.3
2.372
2.621
109.7
110.1
108.5
122.8
110.6
126.6
86.1
CTiH3
H3TiBr
H1CTi
115.4
108.8
121.5
0.0
109.0
109.2
119.8
64.7
C2V
-0.60
0.12
0.14
0.14
1.37
H2CTi
159.5
Φ(H1CTiH3)
Φ(H1CTiBr)
mol symm
q(C)b
CH3X + M* f [CH3MX]* f CH2dMHX
(1)
Cs
-0.44
0.14
0.16
-0.17
0.67
Cs
-0.63
0.13
0.10
0.10
0.77
-0.47
3.35
3A′′
q(H1)b
Apparently, the conversion of the Grignard-type product to
the methylidene complex takes place most efficiently in a
system where the energy of the methylidene complex is
comparable to that of the Grignard-type complex.
In the titanium systems, the Grignard-type products are
most stable, relative to the corresponding methylidene
complex, among the Grignard-type complexes derived from
group IV transition metals (CH3TiX, CH3ZrX, and CH3HfX).
The energy differences are more than 20 kcal/mol at the
current level of theory (Tables 6 and 7 here and Table 6 in
ref 9). Studies show that CH3TiX is mostly produced in the
reaction of vaporized Ti atoms and methyl halide, and CH2d
q(H2)b
q(H3)b
q(Ti)b
q(Br)b
-0.37
1.48
-0.48
µc
1.99
stated
1A′
1A1
∆Ee,f
100.4g
123.1h
198.4
a Bond lengths and angles are in angstroms and degrees, respectively.
b Mulliken atomic charge. c Molecular dipole moments are in debye.
d Electronic state. e Binding energies in kcal/mol. f Transition state binding
energies: T, 84.6 kcal/mol and S, 86.4 kcal/mol. g Lowest triplet state
binding energy is 90.2 kcal/mol. h Lowest singlet state binding energy is
97.3 kcal/mol.
TiHX is formed initially only in a relatively small amount.9,10
The methylidene complex is increased several fold upon
subsequent UV photolysis to promote R-H transfer.
(23) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals; Wiley and Sons: New York, 2001; p 190.
984 Inorganic Chemistry, Vol. 44, No. 4, 2005

PhotoreWersible r-Hydrogen Migration
On the other hand, in the zirconium systems, the energy
difference between CH3ZrX and CH2dZrHX is 10-20 kcal/
mol, and both the Grignard-type and methylidene complexes
are produced in reaction of methyl fluoride with vaporized
Zr atoms.11 In the Hf system, the Grignard-type and
methylidene complexes are comparable in energy, and CH3-
HfF is not identified in the spectrum of Hf + CH3F, but
strong absorptions from the methylidene complex are
observed.12
For justification of the relative yield of the methylidene
complex, the further stabilization of the reaction products
in the matrix should also be considered. The interaction
between the metal atom and Ar matrix is expected to be
stronger for the methylidene complex, because of the higher
electron deficiency caused by the higher oxidation state of
the metal atom.1 Calculations show that an Ar atom can be
easily coordinated to the metal atom of the methylidene
complex, about 2.74 Å apart above or below the molecular
plane. The binding energy of a single Ar atom is about 2
kcal/mol. Therefore, in the case of the Hf system, the actual
energy of the methylidene complex could well be lower than
that of the Grignard-type product.
Group III. The group III absorptions in Figures 1-3
increase dramatically upon annealing. The variation is shown
in Figure 4; the group III absorptions in the regions of 540-
600 and 1360-1400 cm-1 for Ti + CH3Cl increase sub-
stantially at high concentration of CH3Cl as well as on
annealing. This indicates that group III absorptions arise from
a larger complex. It was previously reported that (CH3)2-
TiF2 is produced in the reaction of vaporized Ti atoms with
CH3F, whose absorptions grow significantly upon annealing
and at high CH3F concentration.10 The product absorptions
including Ti-C2 antisymmetric stretching and Ti-F2 sym-
metric and antisymmetric stretching absorptions, along with
the satellite absorptions caused by the Ti isotopes, are
observed, and the calculation results with the C2V structure
show excellent agreement with the observed values.
Martinsky and Minot theoretically studied the structures
of titanium chloride complexes including dinuclear and
dihalide complexes.24 Kaupp predicted the C2V structure of
dimethyl titanium dichloride and argued on the basis of NBO/
NLMO analyses that the influence of ligand-to-metal π-bond
should be very significant on the bond angles.25 More
recently, however, it was shown by computation that the
bond angles of (CH3)2TiF2 are very close to those of (CH3)2-
TiCl2.10 McGrady et al. prepared (CH3)2TiCl2 in the reaction
Me2Zn + TiCl4 f Me2TiCl2 + ZnCl2 under high vacuum
and rigorously moistureless conditions and observed the
spectra in the gas phase and inert matrixes (Ar and N2).26
The vibrational characteristics of group III absorptions of
Ti + CH3Cl are basically consistent with the previous results
for (CH3)2TiCl2 obtained by McGrady et al.26 Most Ar matrix
frequencies (measured at an optimum resolution of 1.7 cm-1)
in the previous study match within 3 cm-1 with the group
III frequencies. The group III frequencies are also compared
with the calculated values in Table 4, showing good
agreement, and they turned out to be the relatively strong
absorptions observed in the previous study and also predicted
in calculations. The present results, along with previous ones,
indicate that (CH3)2TiCl2 is indeed produced in the reaction
of CH3Cl with vaporized Ti atoms and isolated in a solid
Ar matrix.
The vibrational characteristics of group III absorptions in
the Ti + CH3Br spectra and their variation upon annealing
and concentration change follow those of Ti + CH3Cl, and
the observed frequencies are compared with the values
calculated for (CH3)2TiBr2 with a C2V structure in Table 5,
showing good agreement. This indicates that (CH3)2TiBr2 is
formed for the first time, and reaction of vaporized Ti atoms
with alkyl halide is a very efficient method to synthesize
dialkyl titanium dihalide compounds.
The general formation of (CH3)2TiX2 in the reaction of
vaporized Ti atoms and methyl halides and the remarkable
increase of the concentration most probably result from the
fact that (CH3)2TiX2 is much more stable than CH3TiX and
CH2dTiHX. The binding energies of (CH3)2TiCl2 and
(CH3)2TiBr2 are compared with those of smaller reaction
products (CH3TiX and CH2dTiHX) in Tables 6 and 7. The
stability of dimethyl titanium dichloride has also been
demonstrated experimentally.26 Our experiments show that,
once formed in the matrix, (CH3)2TiX2 does not decompose
in the process of photolysis and annealing until the Ar matrix
evaporates.
It is not clear at this point whether (CH3)2TiX2 is formed
from CH3TiX or CH2dTiHX. The CH3TiX molecule has a
triplet ground state, which is expected to be reactive with
CH3X located nearby in the matrix. Insertion into the C-X
bond of CH3X by CH3TiX should readily occur to form
(CH3)2TiX2, similar to the C-X insertion of CH3X by the
triplet Ti atom (3F2) to produce CH3TiX.
CH3TiCl (T) + CH3Cl (S) f (CH3)2TiCl2 (S)
∆E ) -79 kcal/mol (2)
CH3TiBr (T) + CH3Br (S) f (CH3)2TiBr2 (S)
∆E ) -75 kcal/mol (3)
On the other hand, alkylidene complexes are well-known
C-H activation agents,1-4,15,16 and therefore, C-H activation
of CH3X by CH2dTiHX and subsequent rearrangement
might result in formation of (CH3)2TiX2. The C-H activation
by an alkylidene complex appears similar to a typical addition
reaction to a double bond.
CH2dTiHCl + H-CH2Cl f CH3-TiHCl-CH2Cl f
(CH3)2TiCl2
∆E ) -101 kcal/mol (4)
CH2dTiHBr + H-CH2Br f CH3-TiHBr-CH2Br f
(CH3)2TiBr2 ∆E ) -98 kcal/mol (5)
(24) Martinsky, C.; Minot, C. Surface Sci. 2000, 467, 152.
(25) Kaupp, M. Chem. Eur. J. 1999, 5, 3631.
(26) (a) McGrady, G. S.; Downs, A. J.; Bednall, N. C.; McKean, D. C.;
Thiel, W.; Jonas, V.; Frenking, G.; Scherer, W. J. Phys. Chem. A 1997,
101, 1951. (b) McGrady, G. S.; Downs, A. J.; McKean, D. C.; Haaland,
A.; Scherer, W.; Verne, H. P.; Volden, H. V. Inorg. Chem. 1996, 35,
4713.
It is also possible that both CH3TiX and CH2dTiHX react
with CH3X nearby in the matrix to form (CH3)2TiX2. Further
Inorganic Chemistry, Vol. 44, No. 4, 2005 985

Cho and Andrews
7. The methylidene and Grignard-type complexes have (Cs)
structures. It is worth mentioning that both MP2 and DFT
methods with LanL or SDD effective core potential and basis
set for Ti lead to C1 structures for the methylidene complex
with Ti and C atoms serving as trigonal pyramidal apexes,
as found for CH2dZrH2.13,14 On the other hand, the all-
electron basis for Ti yields a planar methylidene.
The C-Ti bond length of the methylidene complex is
much shorter than that of the Grignard-type complex,
reflecting the higher bond order. The (CH3)2TiCl2 and (CH3)2-
TiBr2 molecules (Figure 6) and the previously10 studied
(CH3)2TiF2 all have almost identical C2V structures and
geometrical parameters including the X-Ti-X angle
( XTiX), other than the Ti-X bond lengths. The geometrical
parameters of (CH3)2TiCl2 in Table 6 are consistent with
theoretical and experimental results.25,26 This suggests that
the ligand-to-metal π bond does not affect the bond angles
substantially.
In the structures of the methylidene complexes shown in
Figure 5, the distorted CH2 group due to strong agostic
interaction between the metal atom and one of the R-hydro-
gen atoms is evident, along with the elongated C-H bond.
H1CTi is less than 90.0°, and the distance between one of
the methylene hydrogen atoms and the Ti atom is only 2.070
and 2.055 Å for CH2dTiHCl and CH2dTiHBr, respectively.
Agostic interactions have been found to be quite common,
provided a metal has a low-lying empty valence orbital and
a C-H bond in reasonable proximity.15-17 It is implicit that
this interaction involves attraction between the electron-
deficient metal center and the C-H bond acting as a Lewis
base, which forms at the expense of a significant distortion
within the ligand including bending at CH2 and lengthening
of the C-H bond in order to stabilize the CdTi bond.
Figure 5. Optimized molecular structures for Ti-methylidene and
Grignard-type complexes. The bond lengths and angles are in Å and degrees,
respectively, and the numbers in parentheses are the Mulliken charges of
the carbon, titanium, and halogen atoms. The methylene group is noticeably
distorted, and one of the methylene hydrogen atoms is very close to the Ti
atom, indicating that there is strong agostic interaction between the atoms.
All-electron calculations carried out using B3LYP and the 6-311++G-
(2d,p) basis set.
For comparison, the same all-electron basis calculation has
been performed for CH2dTiH2 and CH2dTiHF, and the
structures are also shown in Figure 5. Early calculations of
CH2dTiH2 failed to find the agostic interaction5,6 owing to
the lack of polarization functions in the basis set. More recent
work has noted the importance of polarization functions to
characterize the agostic interaction.16
Figure 6. Optimized structures of the (CH3)2TiCl2 and (CH3)2TiBr2
complexes. The bond lengths and angles are in angstroms and degrees,
respectively. All-electron calculations carried out using B3LYP and the
6-311++G(2d,p) basis set.
study will be necessary to determine the mechanism of
formation of the dialkyl titanium dihalide complex.
In a methylidene complex, the simplest alkylidene com-
plex, the strength of the agostic interaction can be roughly
estimated by the H1-C-M angle ( H1CM), the CdTi bond
length, and the distance between the interacting R-hydrogen
and metal atoms. Among the titanium methylidene com-
plexes, the agostic interaction increases in the order of CH2d
TiH2, CH2dTiHF, CH2dTiHCl, and CH2dTiHBr as deter-
mined by calculated values using all-electron basis sets
( H1CTi ) 91.9, 91.6, 86.9, and 86.1°); CdTi bond lengths
CH2dTiH2 (1.814 Å), CH2dTiHF (1.814 Å), CH2dTiHCl
(1.801 Å), and CH2dTiHBr (1.798 Å); and r(Ti‚‚‚H1) )
2.161, 2.156, 2.070, and 2.055 Å, respectively. The variation
is also consistent with the observed predominately CdTi
stretching frequencies of CH2dTiHF (757.8 cm-1), CH2d
TiHCl (766.5 cm-1), and CH2dTiHBr (774.7 cm-1).
Several other absorptions are common to different metal
experiments. Weak absorptions at 825.9, 820.1 cm-1 and
788.1, 781.5 cm-1 are due to the CH2Cl and CD2Cl free
radicals with resolved 35Cl, 37Cl isotopic absorptions, and
similar weak 692.8 and 655.0 cm-1 bands are due to the
CH2Br and CD2Br free radicals.27,28 Recall that the CH2F
and CD2F free radicals were observed in similar methyl
fluoride experiments.9 Radiation from the laser ablation
plume is capable of precursor photodissociation.
Molecular Structures. The optimized geometries of the
major products from reaction of CH3Cl and CH3Br with
laser-ablated Ti atoms are shown in Figures 5 and 6, and
the geometrical parameters are summarized in Tables 6 and
The Photoreversible System. Parallel to the case of Ti
+ CH3F,9 the methylidene and Grignard-type complexes
(27) Andrews, L.; Smith, D. W. J. Chem. Phys. 1970, 53, 2956.
(28) Smith, D. W.; Andrews, L. J. Chem. Phys. 1971, 55, 5295.
986 Inorganic Chemistry, Vol. 44, No. 4, 2005

PhotoreWersible r-Hydrogen Migration
Figure 7. The photoreversible system identified in this study. CH3TiCl is converted to CH2dTiHCl by UV (240 nm < λ < 380 nm) irradiation and vice
versa by visible (λ > 530 nm) irradiation. Intersystem crossings inevitably accompany the conversions (see text). Structures and energies are from all-
electron B3LYP calculations. The transition state (TS) is the optimized triplet with C1 symmetry; the singlet TS has a slightly different structure and nearly
the same energy.
form persistent photoreversible systems between CH3TiX and
CH2dTiHX via R-hydrogen migration. The photoreversible
Ti + CH3Cl system is illustrated in Figure 7, and that for Ti
+ CH3Br is essentially the same. In the reaction of methyl
halide and laser-ablated Ti atoms, CH3TiX is initially
generated mostly, perhaps because of the relative stability
of the Grignard-type product. UV photolysis (240 nm < λ
< 380 nm) transforms CH3TiX to CH2dTiHX, whereas
photolysis with visible light (λ > 530 nm) reverses the effect.
As listed in Tables 6 and 7, the methylidene and Grignard-
type complexes have singlet and triplet ground states,
respectively.9 Therefore, interconversions between the com-
plexes must involve intersystem crossings. The lowest triplet
and singlet states of CH2dTiHCl and CH3TiCl are computed
to be 10.2 and 25.1 kcal/mol above the singlet and triplet
ground states, respectively, and those of CH2dTiHBr and
CH3TiBr are 10.2 and 25.8 kcal/mol above the corresponding
ground states, respectively. Starting with triplet ground state
CH3TiCl, UV excitation to the triplet transition state 38.2
kcal/mol higher leads to the triplet CH2dTiHCl, which
intersystem crosses to the more stable singlet CH2dTiHCl
ground state. In the reverse process, visible excitation of
singlet CH2dTiHCl to the singlet transition state 15.1 kcal/
mol higher gives singlet CH3TiCl, which intersystem crosses
to the triplet CH3TiCl ground state. The photochemistry of
triplet CH3TiBr and singlet CH2dTiHBr proceeds in like
fashion.
of methylidene complexes produced from transition-metal
vapor and methyl halide or methane.9,11,12,14 On the other
hand, CH3TiX has a Cs structure similar to that in the ground
triplet state, where the halide, titanium, and carbon atoms
and one of the hydrogen atoms are in the same plane.
This persistent photoreversible system terminates by
further reaction of the products with methyl halide available
in the matrix to a larger complex upon annealing. This and
previous studies show that only a small amount of (CH3)2-
TiX2 is formed initially in reaction of methyl halide and
vaporized Ti; however, upon annealing, further reaction with
available methyl halide increases the concentration dramati-
cally.9 The (CH3)2TiX2 molecules are quite stable. Once the
complex is formed, it does not dissociate back to the smaller
precursors upon repeated photolysis and further annealing.
Conclusions
Reactions of laser-ablated Ti atoms with methyl halides
(CH3Cl and CH3Br) in excess argon have been carried out
during condensation at 7 K, and we find that CH3Cl and
CH3Br are more reactive with the metal atoms than CH4 and
CH3F. On the basis of the behaviors occurring upon
photolysis and annealing, the product absorptions are sorted
into three groups. Group I is relatively weak after co-
deposition of methyl halide and the metal atoms but grows
significantly upon UV irradiation and then repeatedly
decreases and increases following visible (λ > 530 nm) and
UV (240 nm < λ < 380 nm) irradiations, and group II shows
the reverse trend. Spectroscopic evidence and DFT calcula-
tions, along with previous results, indicate that groups I and
II arise from CH2dTiHX and CH3-TiX, and they define a
persistent photoreversible system via R-hydrogen migration.
Although the methylidene complex also has a planar
structure in the lowest triplet state, our calculations show
that the CH2 group is not distorted in contrast to the ground
singlet state. Apparently, the agostic interaction/distortion
is observed only in the ground singlet state with the shorter
CdTi bond, which is consistent with the previous studies
Inorganic Chemistry, Vol. 44, No. 4, 2005 987

Cho and Andrews
The molecular structures of CH2dTiHCl and CH2dTiHBr
show evidence of agostic interaction between the metal atom
and one of the R-hydrogen atoms in the ground singlet state.
The strength of this agostic interaction increases in the order
of CH2dTiH2, CH2dTiHF, CH2dTiHCl, and CH2dTiHBr
on the basis of the magnitude of CH2 distortion, the distance
between the R-hydrogen and titanium atoms, and the CdTi
bond stabilization. Ongoing investigation in this laboratory
finds an even stronger agostic interaction in the correspond-
ing iodide CH2dTiHI. Group III grows only slightly in the
cycles of photolysis but increases dramatically upon anneal-
ing, whereas groups I and II decrease substantially. Group
III is identified as (CH3)2TiX2, which is substantially more
stable than the smaller complexes. The dimethyl titanium
dihalides (CH3)2TiF2, (CH3)2TiCl2, and (CH3)2TiBr2 all have
very similar C2V structures.
Acknowledgment. We gratefully acknowledge financial
support for this work from NSF Grant CHE 03-52487 and
sabbatical leave support (H.-G.C.) from the Korea Research
Foundation (KRF-2003-013-C00044).
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988 Inorganic Chemistry, Vol. 44, No. 4, 2005