Structures and its dipole moments of half-sandwich type metal-benzene (1:1) complexes determined by 2-m long electrostatic hexapole

Article abstract of DOI:10.1016/j.chemphys.2004.02.016

The transition metal-benzene (1:1) complexes are synthesized by laser evaporation method followed by the size and structure selection with 2-m long electrostatic hexapole field. Electric dipole moments of half-sandwich type binary complexes for a series of the transition metals, i.e. Ti, V, Co, and Ni, with benzene molecule were determined by measuring the focusing curves, namely the dependence of the focused beam intensity upon the hexapole field strength. It is found that the dipole moments of Ti-C₆H₆ and V-C₆H₆ are larger than the ones of Co-C₆H ₆ and Ni-C₆H₆. Such large dipole moments of Ti-C₆H₆ and V-C₆H₆ can be ascribed to the strong orbital interaction due to charge transfer between the metal atom and benzene molecule.

Full text of DOI:10.1016/j.chemphys.2004.02.016

Chemical Physics 301 (2004) 183–187
www.elsevier.com/locate/chemphys
Structures and its dipole moments of half-sandwich type
metal–benzene (1:1) complexes determined by 2-m long
electrostatic hexapole
a,b
Kohei Imura , Hiroshi Ohoyama , Toshio Kasai
a
a,b,
*
a
Department of Chemistry, Graduate School Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
b
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
Received 8 October 2003; accepted 26 February 2004
Available online 28 March 2004
Abstract
The transition metal–benzene (1:1) complexes are synthesized by laser evaporation method followed by the size and structure
selection with 2-m long electrostatic hexapole field. Electric dipole moments of half-sandwich type binary complexes for a series of
the transition metals, i.e. Ti, V, Co, and Ni, with benzene molecule were determined by measuring the focusing curves, namely the
dependence of the focused beam intensity upon the hexapole field strength. It is found that the dipole moments of Ti–C6H6 and V–
C6H6 are larger than the ones of Co–C6H6 and Ni–C6H6. Such large dipole moments of Ti–C6H6 and V–C6H6 can be ascribed to
the strong orbital interaction due to charge transfer between the metal atom and benzene molecule.
ꢀ
2004 Elsevier B.V. All rights reserved.
1
. Introduction
also advantageous for small-size clusters for theoretical
as well as experimental approach can be done at the
same time, namely, direct comparison between exper-
iment data and theoretical results could be done in
many cases [10–12].
Recent development of laser evaporation method
makes it possible to synthesize and study organome-
tallic compounds in vacuum [1–9]. Most of the studies
related to organometallic complexes have focused their
attention on large-size clusters for investigating tran-
sient properties of molecules and atoms from in gas
phase to condensed phase, relevant to phase transition,
and singular behaviors in the course of clusterization,
for instance the presence of magic numbers. On the
other hand, it is also important to investigate small-
size clusters for investigating initial step of clustering
effect which is manifested particularly in the smallest
cluster, i.e. heterogeneous binary cluster such as metal–
molecule (1:1) complex. Using such (1:1) complexes,
one can study metal–ligand interaction in detail. It is
We have recently studied the structures and their
bonding characters of Al–C H and so forth [13]. For
Al–C H (1:1) complex, we find that two structural
6
6
6
6
isomers exist. One is asymmetric 1,2-complex and the
other one is nearly C6v symmetric 1,4-complex, and the
former one is more stable than the latter. The theoretical
calculation shows that 1,2-Al–C6H6 complex becomes
more stable due to the interaction between the 3px
atomic orbital of Al atom, which contains an unpaired
electron, and one of the C@C double bond units of
benzene through the 2p component of the benzene
lowest unoccupied molecular orbital.
In the present study, we generate a series of transition
metal–benzene (1:1) complexes, M–C6H6 (M ¼ Ti, V,
Co, Ni) and we experimentally determine their electric
dipole moments and structures using the beam focusing
technique with a 2-m long electrostatic hexapole field for
investigating metal–ligand interaction in a direct way.
*
Corresponding author. Tel.: +81-6-6850-5400; fax: +81-6-6850-
403.
E-mail address: tkasai@chem.sci.osaka-u.ac.jp (T. Kasai).
5
0
301-0104/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemphys.2004.02.016

184
K. Imura et al. / Chemical Physics 301 (2004) 183–187
Nozzle
Buffer
Hexapole field
Detection
C1
C2
C3
L0
L3
L2
L1
LHEX
TOF-MS
QMS
SK
A
B
Fig. 1. Schematic drawing of the experimental set-up. The spacing between the hexapole field center and each rod tip is 7.0 mm. Cn (n ¼ 1–3) show
the locations of the collimating apertures, and SK is a skimmer. The lengths with the arrows are: L0 ¼ 55, L1 ¼ 290, L2 ¼ 371, L3 ¼ 2906,
LHEX ¼ 2109, and the collimator diameters are C1 ¼ 0:8, C2 ¼ 8:0, C3 ¼ 1:5 in the unit of mm.
2
. Experimental
the calculation has been published elsewhere [14]. In
the trajectory simulation, angular divergence as well as
velocity distribution of the molecular beam have been
taken into account. Stream velocity was determined by
the delay time of an ion arrival between the observa-
tion point A in the buffer and B in the reaction
chamber. The velocity distribution was measured by
scanning the delay time of the ionization laser. We
tried to generate M–C H binary complexes in differ-
The experimental set-up is depicted schematically in
Fig. 1. The important lengths between the parts of the
apparatus are given. These lengths were employed in the
trajectory simulations to fit the observed focusing curves
later. The transition metal–benzene binary complexes,
M–C6H6 were synthesized using the laser evaporation,
where 532-nm second harmonic output from a pulsed
Nd:YAG laser (5–10 mJ/pulse) was focused on a rotat-
ing metal rod 4-mm in diameter to evaporate metal and
produce atoms. Evaporized atoms were cooled down by
collisions with He carrier gas and they move to a reac-
tion channel (10 mm in length, 2 mm £) where C6H6
vapor was mixed together. The beam of the gas mixture
is skimmed by a 0.8-mm £ skimmer and collimated by a
6
6
ent seeding gas condition, for instance, Ne and Ar
gases. The stream velocities for these molecular beams
were found to be smaller than that seeded in He, but
their production efficiencies became low. Moreover,
their velocity distributions are quite diverse. This in-
dicates that the rotational distribution with higher
temperatures. On the other hand, He seeding is an
effective cooling carrier, in fact, the velocity distribu-
tion of Ti–C H is measured to be V ¼ 1540 ms 1,
8
-mm £ collimator, then it is state-selected and focused
ꢀ
by a 2-m long electrostatic hexapole field. This electro-
static field selectively transmits and focuses a specific
complex by adjusting the hexapole rod voltage under the
requirement of symmetric-top structure and the exis-
tence of permanent dipole moment. The focused beam
of metal–benzene complexes are ionized by gently fo-
cused ArF excimer laser light, and time-of-flight (TOF)
mass analysis is carried out. Many TOF profiles were
averaged on each experimental point by a digital oscil-
loscope and the mean values are plotted as the function
of the hexapole rod voltage, for obtaining focusing
curves.
In the present study, the experimental set-up (geo-
metrical factors in the trajectory simulation) is not
optimized to carry out single rotational state selection,
to avoid the beam intensity loss, for typical beam in-
tensity of the produced complexes is not intense. This
measurement at low resolution give us structure-less
smooth rise in the focusing behavior. We carried out
the trajectory simulation, in which the first and the
second order Stark effects were taken into account, for
fitting the experimentally observed focusing curves to
determine the permanent dipole moment. The detail of
6
6
s
ꢀ
1
a ¼ 110 ms . In the trajectory calculation, we as-
s
sumed the complexes were relaxed in electronically
ground state, and rotational temperature was equiva-
lent to translational temperature Ttrans ꢁ 90 K which
1=2
was directly related to as, i.e. as ¼ ð2kTtrans=mÞ
.
Rotational states up to J ¼ 15, K ¼ 15 were calcu-
lated. As the result of acceptable simplification, the
dipole moment of the complex is left as the only one
adjustable parameter in this trajectory simulation. As
for the present experimental set-up, it is found that the
rotational temperature of the beam is not so sensitive
in determining the dipole moments. We have also
performed a test experiment by using sample reagents,
CF3H and CH3I to calibrate the above-mentioned
experimental procedure for confirming reliability of the
present dipole moment.
3. Results and discussion
The pressure dependence of the complex formation
on benzene pressure was measured in order to find out

K. Imura et al. / Chemical Physics 301 (2004) 183–187
185
optimum formation conditions for half-sandwich type
metal–benzene complexes. It was found that one to one
are not stable from the structural point of view, or
ionization potentials of these complexes would be
higher than the photon energy of ArF laser light, i.e.
6.42 eV.
(
1:1) complex formation was dominant at lower stag-
nation pressures of benzene, and (1:2) sandwich type
complex formation became exclusively dominant at
higher pressures of benzene for all metals we treated. It
was also found that for the early transition metals such
as V and Ti, the formation ratio of (1:1) to (1:2) com-
plexes was very sensitive to benzene pressure. The for-
mation of (1:2) sandwich type complexes of the late
transition metals were about 20–50% less than that of
the early transition metals such as Ni and Co. On the
other hand, the formation ratio was found to be not so
sensitive to benzene pressure for the late transition
metals. These results could be rationalized by the well-
known ‘‘18-electrons rule’’. Although Ti–(C H ) or V–
Fig. 2 shows the focusing curves of Ti–C6H6 and V–
C H obtained at 700 Torr stagnation pressure of He
carrier gas and 70 Torr of benzene. The direct beam
6
6
intensity which is defined as the beam intensity at V ¼ 0
0
kV at the hexapole electric field, is subtracted from the
signal intensity on each measured V0 point. Thus the
enhancement of the beam intensity at 12 kV measured
corresponds to the value Ið12 kVÞ=Ið0 kVÞ ꢁ 1:6. Al-
though the nominal enhancement due to Stark effect is
small with respect to a stable molecule, this result par-
tially causes from the heavy mass of the (1:1) complex
and its relatively fast stream velocity of the cluster beam.
However, the focusing effect is easily recognized at
higher voltages, indicating that the complex satisfies the
focusing condition for symmetric top structure with
non-zero permanent dipole moment. By the parameter
optimization in the trajectory simulation, we could de-
6
6 2
(
C6H6)2 sandwich type complex lacks one or two elec-
trons of the 18 electrons, respectively, they can be sta-
bilized through the back-donation from metal to ligand
(
the valence electrons of the metal occupy bonding or-
bitals.) Meanwhile, the sandwich type complex of the
late transition metal can occupy more than 18 electrons.
As a result, the total stabilization energy is expected to
become smaller, because additional electrons from 18
electrons have to occupy an anti-bonding orbital. It was
not possible to generate complexes of either Fe or Cu
with benzene. It could be a reason that these complexes
termine their dipole moments to be 2.4 D for Ti–C H6
6
and 1.0 D for V–C H within the experimental error of
6
6
±0.3 D. The similar experimental procedure was applied
to determine dipole moments for the other complexes,
though the observed beam enhancements were very
small (not shown in the figure). The enhancements at 12
Fig. 2. The focusing curves of the half-sandwich type (1:1) clusters, or complexes: (a) Ti–C6H6; (b) V–C6H6. The filled points with the error bars show
the experimental data. The solid lines are obtained by the trajectory simulation with the optimized values of dipole moments of 2.4 and 1.0 D (b) for
comparison, the calculated results by the same simulation are also presented, where the dotted and dashed lines stands for the simulated results with
1.1 and 0.9 D, respectively.

186
K. Imura et al. / Chemical Physics 301 (2004) 183–187
Table 1
The experimental dipole moments of the metal–benzene half-sandwich
complexes
M
lexp :=D
Ti
V
2.4 ± 0.3
1.0 ± 0.3
<0.4
Co
Ni
<0.6
kV were found to be ꢁ7% for both Co–C H and Ni–
6
6
C H . For these complexes, both experimentally
6
6
observed enhancement and transmission efficiency cal-
culated by the simulation were compared with the ones
for Ti–C6H6, and we estimated these dipole moments.
The results are summarized in Table 1.
Fig. 3. The schematic orbital interaction diagram of a half-sandwich
type complex of transition metal with benzene. The electronic config-
uration of the low spin state of Ti–C6H6 complex is shown as an
example.
It is noticed here that the complexes may be formed
different configuration as asymmetric structure in the
molecular beam. Stark effect for only slightly asym-
metric structure, which possibly generated in the beam
due to for example, Jahn–Teller distortion and other
electron correlations, can be described by quasi-first
order Stark effect. Thus, it leads to only minor cor-
rection. On the other hand, beam deflection from
highly asymmetric complexes can not be detected in
the present set-up. It should only generate small de-
flection such as two orders of magnitude smaller
compared with the corresponding symmetric complex
with a same dipole moment, since asymmetric com-
plexes would be deflected only by the second order
Stark effect.
to donate the electron into the Lb orbital. It is easily
1
recognized that electrons of the M–C H complex of
6
6
ꢂ
early transition metal do not have to occupy e anti-
1
bonding orbital in the electronic ground state. On the
other hand, electrons of corresponding complexes of late
ꢂ
transition metals have to occupy e even in the electronic
ground state.
1
For the M–C H complex, Le fi de donation as
6
6
1
1
well as de fi Le back-donation play a central role to
2
2
stabilize the complex. For the early transition metal
complexes, both Le fi de and de fi Le orbital inter-
1
1
2
2
actions contribute, but only de fi Le back-donation is
2
2
expected to stabilize the late transition metal complexes,
ꢂ
It is generally expected that a molecule in the con-
densed phase are stabilized by surrounding solvent and
it stays in electronic ground state. Thus we may expect
that the M–C H complexes formed under the experi-
on the other hand. Since the e orbital is already occu-
pied by electrons of the late transition metal, the do-
1
nation from Le MO is not likely. It should be also
1
6
6
mention that the 3d orbital energy of the investigated
metals increases from Ti to Ni. Therefore, the 3d–Le2
orbital interaction between metal and benzene becomes
weak from Ti to Ni. This should be the main reason that
the dipole moments of the complexes for the early
transition metals are larger than the ones for the late
transition metal complexes. Weak interaction for the
late transition metals would distort their structure of
symmetric top, and thus this small enhancement of the
focusing curve as well as the small dipole moment were
observed in the experiment.
mental conditions as described in the previous section
would be relaxed in the electronic ground state, namely
complexes generated in the reaction channel are easily
de-excited by collisions with He carrier gas.
Fig. 3 shows the orbital interaction diagram of rep-
resentative M–C6H6 complex, where M indicates a
transition metal. The 3d degenerated orbitals of the
1
2
metal atom are split into one 3da (dz ), two de (dxz
1
2
2
2
and dyz), and two de (dxy and dx –y ) orbitals when
molecular z axis was defined to be coincide with the C6
axis of benzene [15]. L on the right hand side of the
diagram indicates molecular orbitals of benzene. Thus,
the electrons of the complex tends to occupy the de2
lowest orbitals because this occupation maximizes at-
In order to confirm this structure deformation of the
complexes, we performed the optimization of the in-
vestigated complexes using the B3LYP/6-31G* [16]. It
was found that most of the optimized complexes were
symmetric and similar to the one found previously for
the Al–C H complex. We also found in the present
1
tractive interaction with Le2 orbitals. A 3da atomic
orbital of transition metal does not provide strong in-
6
6
teraction with the La orbital of benzene molecule, for
1
study that the optimized structures were strongly de-
pendent upon the initial starting geometry of the com-
plexes. Thus, it is difficult at this moment to compare the
experimental results with the calculated ones, unless
further improvement of the theoretical treatment is
performed in the future. However, it should be men-
its orbital is directed to the ‘‘hole’’ at the center of
benzene and therefore this only gives negligibly small
interaction with the La1 orbital. As a result, the 3da1
orbital remains as the 3da1 non-bonding orbital in M–
C6H6. Due to symmetry requirement, it is not possible

K. Imura et al. / Chemical Physics 301 (2004) 183–187
187
tioned that whole optimized structures found were not
seriously deformed from symmetric configuration,
namely, these were slightly symmetric top structure.
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(
We have synthesized metal–benzene half-sandwich
type complexes in the beam and size-selected by the 2-m
long electrostatic hexapole field. The observed focusing
curves show the clear enhancement of the focused beam,
indicating that the complexes are selectively focused due
to Stark effect in the hexapole field. We determined the
permanent dipole moments of the half-sandwich type
complexes of Ti, V, Co and Ni with benzene molecule by
simulating the observed focusing curves. The propensity
for producing large dipole moment for the early tran-
sition metals such as Ti and V indicates that the charge
transfer orbital interaction between metal atom to
benzene molecule plays a dominant role and produces
the large dipole moments of these metal–benzene
complexes.
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This work was performed as a project research ‘‘re-
search and development of system technologies for re-
source recycling and minimum energy requirement’’
financially supported by CREST of JST, and was par-
tially supported by a Grant-in-Aid from the Ministry of
Education, Culture, Sports, Science and Technology.