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Cleavage of the N2 Triple Bond by the Ti Dimer A Route to Molecular Materials for Dinitrogen Activation.

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Angewandte
Chemie
N2 Activation
DOI: 10.1002/anie.200503709
Cleavage of the N2 Triple Bond by the Ti Dimer:
A Route to Molecular Materials for Dinitrogen
Activation?**
Hans-Jrg Himmel,* Olaf Hbner, Wim Klopper, and
Laurent Manceron
Dinitrogen activation is still a very lively field of research that
has witnessed some major advances in recent years.[1] One
important direction of this research is the search for new
transition- and rare-earth-metal complexes that coordinate N2
and allow its reduction or oxidation; for example, an Mo
complex has been reported as being capable of reducing N2 to
NH3 under mild conditions.[2] The associated catalytic cycle
was investigated with the aid of quantum chemical calculations and compared with catalysis by biological systems
involving nitrogenase.[3] This cycle includes a sequence of
steps in which the formal oxidation state of the Mo atom
varies between iii and vi.
We and other research groups have generated and
characterized the Ti dimer with the aid of the matrix isolation
technique. On the basis of resonance Raman measurements,[4]
the TiTi bond energy has been estimated to be approximately 120 kJ mol1, although quantum chemical calculations[5] suggest a somewhat higher value of 150 kJ mol1. We
have also characterized several electronically excited states of
this interesting molecule by using absorption spectroscopy
and quantum chemical calculations. Such excited states are
very important in relation to the unusual reactivity of metal
Figure 1. Infrared spectrum of Ti2N2 isolated in solid neon at 3 K in the
fundamental and combination regions. Left (top and bottom): spectra
observed with 14N2 or 15N2 precursors; insert: the dotted line presents a
simulation for two bands centered at 1315.2 and 1311.1 cm1 for the
expected Ti2 natural isotopic distribution. The asterisks designate CH4 and
N2O impurity bands. Center and right: spectrum observed with Ti214N2
and a comparison with a stick spectrum (g) representing the expected
Ti2 natural isotopic ACHTUNGREdistribution.
[*] Dr. H.-J. Himmel
Inorganic Chemistry Laboratory
University of Oxford
South Parks Road, Oxford OX1 3QR (UK)
Fax: (+ 44) 1865-272-690
E-mail: hans-jorg.himmel@chem.ox.ac.uk
Dr. O. HAbner
Institut fAr Nanotechnologie
Forschungszentrum Karlsruhe
Postfach 3640, 76021 Karlsruhe (Germany)
Prof. Dr. W. Klopper
Institut fAr Physikalische Chemie
UniversitEt Karlsruhe (TH)
76128 Karlsruhe (Germany)
Dr. L. Manceron
LADIR-Laboratoire de Dynamique
Interactions et RIactivitI, UMR 7075
UniversitI Pierre et Marie Curie-CNRS
case 49, 4, Place Jussieu, 75252 Cedex, Paris (France)
[**] H.-J.H. gratefully acknowledges financial support by the Deutsche
Forschungsgemeinschaft (DFG) and the German Academic
Exchange Service (DAAD PROCOPE grant). W.K. acknowledges
support by the DFG through the Center for Functional Nanostructures (CFN, Project No. C3.3). L.M. acknowledges support by
the Centre National de la Recherche Scientifique (PICS3032) and
MinistJre des Affaires EtrangJres (PROCOPE09643SG).
Angew. Chem. Int. Ed. 2006, 45, 2799 ?2802
dimers such as Ti2 as well as small clusters.[6] The unusually
high reactivity of Ti2 has already been observed in the
complex it forms with Xe.[4] Herein, we report the reaction
with N2, which proceeds in a single step without a significant
activation barrier to result in complete cleavage of the strong
NN triple bond and the formation of a cyclic TiACHTUNGRE(m-N)2Ti
molecule. By contrast, a Ti atom in its electronic ground state
does not react with N2 ; matrix-isolation experiments give no
spectroscopic evidence to suggest the formation of even a
loosely bound complex. The behavior of Ti2 thus provides a
textbook example of the remarkable differences in reactivity
between metal atoms, dimers, and small clusters. Our results
pave the way to new catalytically active, Ti-nanoparticlecontaining materials for dinitrogen activation.
Figure 1 shows the IR spectrum of an Ne matrix containing Ti2 and 0.05 % 14N2.[7] The IR spectrum is characterized by
intense bands which can be assigned to a single product of the
spontaneous reaction between Ti2 and N2. The most intense
band occurs at 774.77 cm1, and a second feature that also
exhibits a marked Ti isotopic pattern characteristic of the
presence of two equivalent Ti atoms is observed at slightly
higher energy (Table 1). This second band has its central
component at 782.25 cm1 and exhibits fine structure at a
resolution of 0.02 cm1. Since the relative intensities change
when the reaction conditions are altered, a matrix effect is
judged to be responsible for this fine structure. In the lowwavenumber region, there is a third, less intense band of the
same product at 149.72 cm1. Three additional bands showing
isotopic patterns characteristic of two equivalent Ti atoms are
also observed with central components at 1315.2, 1311.1, and
1246.15 cm1, and a fourth band is observed at 1238.7 cm1.
Subsequent analysis shows that these bands are associated not
with fundamental wavenumbers, but rather with combination
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2799
Communications
Table 1: Observed wavenumbers ([cm1], relative intensities in parentheses) of the bands of different isotopomers of Ti2N2 in solid neon.
Ti214N2
Ti215N2
Ti214N15N
1315.2
1311.1
1246.15
1238.7
1281.6
1277.2
1223.7
1216.4
1298.5
782.25 (0.53)
774.77 (1.00)
149.72 (0.18)
762.20
754.77
145.85
1235.7
801.2 (0.08)
772.52 (0.50)
761.54 (1.00)
147.78 (0.2)
modes. Experiments in which the concentration of titanium
and dinitrogen were altered, as well as annealing experiments,
confirm that all these bands belong to a single product.
The experiments were repeated with 15N2, equimolar
mixtures of 14N2 and 15N2, as well as 1:2:1 mixtures of 14N2/
14 15
N N/15N2. In the experiments with 15N2, all the bands were
shifted, as anticipated, to lower wavenumbers. The spectrum
obtained in the experiments using equimolar mixtures of 14N2
and 15N2 is essentially a superposition of the spectra recorded
with 14N2 and 15N2 separately. This result indicates that the
product contains no more than two N atoms and that the
reaction proceeds by cleavage of a single N2 molecule. In
diffusion experiments at 9 K in the dark, the Ti2 absorption
bands are observed to decrease concomitantly with the
growth of Ti2N2 absorption bands. It appears therefore that
the reaction of Ti2 with N2 under matrix conditions leads
without activation energy to a molecule with the overall
formula Ti2N2. Additional bands are observed in the experiments with 1:2:1 mixtures of 14N2/14N15N/15N2 that are not
present in the experiments with 14N2 or 15N2 alone, which
results in a triplet pattern for each of the absorption bands.
The strongest band now appears at 761.54 cm1, which is not
quite midway between the corresponding bands in the 14N2
(774.77 cm1) and 15N2 experiments (754.77 cm1). The patterns imply that the two N atoms in the Ti2N2 product are
equivalent. The spectra from the experiments with 14N15N
show a fourth fundamental band, located at 801.2 cm1. The
fact that this absorption is observed only if the symmetry is
reduced by the presence of 14N and 15N in the product
molecule argues for a point group with a center of inversion.
Most fitting with these observations is the inference that
Ti2N2 exhibits D2h symmetry, and that the symmetry is
reduced to C2v in the Ti214N15N isotopomer. Only three of
the six fundamental vibrations are IR active in the D2h point
group. The band at low wavenumber (149.72 cm1 for Ti214N2)
can thus be assigned to the out-of-plane deformation [n6ACHTUNGRE(b2u)],
and the bands at 782.25 and 774.77 cm1 for Ti214N2 can be
assigned to the stretching modes n4ACHTUNGRE(b1u) and n5ACHTUNGRE(b3u), respectively. Finally, the band at 801.2 cm1 observed for the
Ti214N15N isotopomer corresponds to the n1(a1) mode of the
C2v-symmetric molecule; the corresponding stretching mode
in D2h symmetry has ag symmetry and is therefore IR silent. If
the symmetry is reduced from D2h to C2v, the b3u mode (n5)
transforms to the a1 representation. This mode appears at
761.54 cm1 for Ti214N15N. As a result of coupling with the
other a1 mode at 801.2 cm1, the wavenumber of this mode is
2800
www.angewandte.org
red-shifted with respect to the mean of the wavenumbers
observed for the n5ACHTUNGRE(b3u) mode of Ti214N2 and Ti215N2. The
bands at 1315.2, 1311.1, 1246.15, and 1238.7 cm1 of Ti214N2
can be assigned to the combinations n3ACHTUNGRE(b2g) + n4ACHTUNGRE(b1u), n3ACHTUNGRE(b2g) +
n5ACHTUNGRE(b3u), n2(ag) + n4ACHTUNGRE(b1u), and n2(ag) + n5ACHTUNGRE(b3u). Hence the wavenumbers of the gerade modes can be estimated, and thus a
relatively complete vibrational analysis of the molecule in its
electronic ground state can be carried out. Unfortunately it
was not possible to obtain Raman spectra of this species.
In summary, the experimental results show that Ti2 reacts
with N2 without any significant reaction barrier to give the
D2h-symmetric Ti2N2 molecule [Eq. (1)]. This molecule has a
planar, cyclic structure with alternating Ti and N atoms. A
normal-coordinate analysis yields an N-Ti-N bond angle of
(90 5)8 and a TiN stretching force constant of 322 N m1.
The TiN bond length can be estimated by using an empirical
correlation[8] to be (175 3) pm. The NиииN separation is
estimated to be approximately 247 pm, which indicates that
there is no significant direct interaction between the two
N atoms. The strong NN bond is thus completely cleaved in
an exothermic reaction.
Quantum-chemical calculations were carried out to obtain
more information about the reaction product and the
thermodynamics of this unusual change. In contrast to an
earlier calculation, which predicted a 3B1u electronic ground
state,[9] the present calculations with the multireference
configuration interaction (MRCI) method based on multiconfigurational self-consistent field calculations with natural
atomic orbital basis sets (Ti: [7s 6p 4d 3f 2g]; N: [5s 4p 3d 2f])[10]
yield a 1Ag electronic ground state, whereas the 3B1u state is
22.4 kJ mol1 higher in energy.
Figure 2 shows isodensity surfaces of the molecular
valence orbitals of the 1Ag state. The two lowest orbitals
comprise essentially the nitrogen lone pairs. Following these
are six N(p)?Ti(d) bonding orbitals (about 1.95 electrons per
orbital), all of which are polarized towards the nitrogen
atoms. The two highest orbitals (1.5 and 0.5 electrons) are the
bonding and antibonding combinations of Ti d orbitals. A
bond order of 1.5 per TiN bond is obtained from the six
approximately doubly occupied TiN bonding orbitals. The
NиииN and TiиииTi separations in the ground state are calculated
according to MRCI to be 251.0 and 255.4 pm, respectively,
without Davidson corrections, and 251.9 and 256.1 pm,
respectively, with Davidson corrections. These values are
pleasingly close to the experimental estimates. For computational efficiency, we rely on density functional calculations
(BP86/def2-TZVP)[11] for the vibrational frequencies. Table 2
compares the experimentally observed vibrational wavenumbers with the calculated values. The agreement is highly
satisfactory and lends strong support to our assignment.
We found that Ti atoms under similar conditions do not
react with N2. Thus, the Ti dimer, although featuring a
relatively weak TiTi bond, shows a reactivity completely
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2799 ?2802
Angewandte
Chemie
the DHf8 of Si(g) (+ 450 kJ mol1),[9] the DHf8 of Si2N2 is
calculated to be about 467 kJ mol1. The DHf8 of Ti2N2(g) can
thus be estimated to be approximately 437 kJ mol1
(DHf8 values for TiCl4 and SiCl4 : 763.2 and
662.8 kJ mol1, respectively[13]). In view of the larger
DHf8 value for two single N atoms (945.4 kJ mol1), this
value for the DHf8 of Ti2N2(g) demonstrates the high affinity
of titanium for nitrogen.[14]
This value also implies that the reaction between Ti2(g) (for
which DHf8 is ca. 833 kJ mol1) and N2 is highly exothermic
(DHf8 estimated to be ca. 396 kJ mol1).[15] The enthalpy
change for the reaction 2 Ti(s) + N2 !2 TiN(s) (double the
enthalpy of formation) amounts to 675.4 kJ mol1. These
considerations, together with the fact that crystalline titanium
is inert toward nitrogen up to elevated temperatures[16] (the
nitride is formed by direct reaction above 1000?1400 8C), are
in line with the view that the barrier to the formation of solid
TiN from solid Ti and N2 is caused by the thermal energy
required to form Ti2 or other small clusters from solid Ti.
According to these calculations, Ti2N2 could very well be an
intermediate in the formation of solid titanium nitride
(Figure 3). We are presently not able to give a detailed
Figure 2. Isodensity surfaces of the valence orbitals of the 1Ag ground
state of Ti2N2.
Table 2: Comparison of the wavenumbers [cm1] observed and calculated for 48TiACHTUNGRE(m-14N)248Ti.
Obsd
[b]
811
464 5[c]
538 5[c]
782.25
774.77
149.72
Calcd[a]
Assignment
836.9
471.2
551.0
829.0
780.7
212.2
n1(ag)
n2(ag)
n3ACHTUNGRE(b2g)
n4ACHTUNGRE(b1u)
n5ACHTUNGRE(b3u)
n6ACHTUNGRE(b2u)
[a] Harmonic frequencies calculated by using the BP86 functional and
polarized triple-zeta valence basis sets (def2-TZVP) for the 1Ag ground
state in the broken-symmetry approximation. RN-N = 249.9 pm, RTi-Ti =
256.0 pm. Alignment of the Ti atoms along the z-axis and N atoms along
the x-axis. [b] Estimated on the basis of a normal coordinate analysis,
considering also the wavenumber observed for this mode for Ti214N15N.
[c] Estimated from the wavenumbers observed for the combination
bands.
different from Ti atoms. Although the ground state of Ti2
shows one dominant configuration, the contribution of
excited valence configurations is not negligible. Therefore
we do not have much confidence in the density functional
calculations of the reaction enthalpy. We decided to estimate
the standard enthalpy for the formation (DHf8) of Ti2N2
through isodesmic reactions based on the hypothetical
reaction 2 TiCl4 + Si2N2 !2 SiCl4 + Ti2N2. This reaction
enthalpy was calculated (BP86/def2-TZVP)[11] to be
170 kJ mol1, and thus the reaction is endothermic. Si2N2 has
already been characterized in matrix-isolation experiments.[12]
To calculate its enthalpy of formation, we first estimated the
enthalpy change for the reaction 2 Si(g) + N2(g)!Si2N2(g)
(433 kJ mol1) by quantum chemical calculations. From
Angew. Chem. Int. Ed. 2006, 45, 2799 ?2802
Figure 3. Thermodynamics of the reactions leading to Ti2N2 and solid
TiN.
mechanism for the spontaneous reaction of Ti2 with N2, but it
is probable that the relatively large number of low-lying
electronic states of Ti2 facilitates the reaction. Our quantum
chemical calculations on Ti2[5] reveal at least 15 different
electronic terms with energies up to 1 eV, the lowest at about
0.2 eV. In contrast, the Ti atom has only two excited terms
below 1 eV (5F, 1D), the lower at 0.8 eV.
We are currently extending our work to host systems other
than noble-gas matrices, namely microporous materials,
which allow the stabilization of Ti2 or Ti nanoparticles at
ambient temperature. These Ti-loaded materials may be able
to be used for industrial processes involving the reduction or
oxidation of N2. Our current research also highlights the
difference in reactivity between a metal atom M and an
aggregate Mn.
Received: October 19, 2005
Published online: March 21, 2006
.
Keywords: cluster compounds и matrix isolation и
NN activation и nitrides и reaction mechanisms
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2801
Communications
[1] See, for example: a) R. R. Schrock, Philos. Trans. R. Soc.
London Ser. A 2005, 363, 959; b) D. C. Rees, F. A. Tezcan,
C. A. Haynes, M. Y. Walton, S. Andrade, O. Einsle, J. B. Howard,
Philos. Trans. R. Soc. London Ser. A 2005, 363, 971, and
references therein.
[2] D. V. Yandulov, R. R. Schrock, Science 2003, 301, 76.
[3] a) F. Studt, F. Tuczek, Angew. Chem. 2005, 117, 5783; Angew.
Chem. Int. Ed. 2005, 44, 5639; b) J. KJstner, P. E. BlKchl,
ChemPhysChem 2005, 6, 1724.
[4] H.-J. Himmel, A. Bihlmeier, Chem. Eur. J. 2003, 9, 627.
[5] O. HLbner, H.-J. Himmel, L. Manceron, W. Klopper, J. Chem.
Phys. 2004, 121, 7195.
[6] a) H.-J. Himmel in Inorganic Chemistry in Focus II, Wiley-VCH,
Weinheim, 2005, pp. 1 ? 13; b) A. KKhn, B. Gaertner, H.-J.
Himmel, Chem. Eur. J. 2003, 9, 3909.
[7] Titanium vapor was generated by heating a Ti?Mo filament
(Godfellow, 85 %Ti, 15 % Mo) to 1400?1600 8C. The metal
deposition rate was monitored with a quartz crystal microbalance and varied from 1.5 to 20 nmol min1. Ne and N2 were
obtained from Air Liquide, France (99.9995 % and 99.998 %
purities, respectively), and 15N2 was obtained from Isotec, USA
(99.0 % 15N). 14N15N was generated from an equimolar mixture
of 14N2 and 15N2 in a high-voltage, 100-mA DC discharge. See
reference [5] for details of the matrix apparatus.
[8] H.-J. Himmel, O. HLbner, W. Klopper, L. Manceron, Phys.
Chem. Chem. Phys., submitted.
[9] G. P. Kushto, P. F. Souter, G. V. Chertihin, L. Andrews, J. Chem.
Phys. 1999, 110, 9020.
[10] a) MOLPRO, a package of ab initio programs designed by H.-J.
Werner and P. J. Knowles, Version 2002.6, R. D. Amos, A.
Bernhardsson, A. Berning, P. Celani, D. L. Cooper, M. J. O.
Deegan, A. J. Dobbyn, F. Eckert, C. Hampel, G. Hetzer, P. J.
Knowles, T. Korona, R. Lindh, A. W. Lloyd, S. J. McNicholas,
F. R. Manby, W. Meyer, M. E. Mura, A. Nicklass, P. Palmieri, R.
Pitzer, G. Rauhut, M. SchLtz, U. Schumann, H. Stoll, A. J. Stone,
R. Tarroni, T. Thorsteinsson, and H.-J. Werner; b) P.-O. Widmark, P.-O. Malmqvist, B. O. Roos, Theor. Chim. Acta 1990, 77,
291; c) P. AmPrigo, M. MerchQn, I. Nebot-Gil, P.-O. Widmark,
B. O. Roos, Theor. Chim. Acta 1995, 92, 149.
[11] a) R. Ahlrichs, M. BJr, M. HJser, H. Horn, C. KKlmel, Chem.
Phys. Lett. 1989, 162, 165; b) O. Treutler, R. Ahlrichs, J. Chem.
Phys. 1995, 102, 346; c) F. Weigend, R. Ahlrichs, Phys. Chem.
Chem. Phys. 2005, 7, 3297.
[12] G. Maier, H. P. Reisenauer, J. Glatthaar, Organometallics 2000,
19, 4775. Si2N2 exhibits a butterfly structure (C2v symmetry).
[13] M. W. Chase, Jr., NIST-JANAF Thermochemical Tables, 4th
Edition, J. Phys. Chem. Ref. Data, Monograph 9, ACS and AIP,
Washington DC, 1998.
[14] Ti sponge heated to 1000 8C can be used to free Ar gas from
traces of N2.
[15] The compound [{VACHTUNGRE(Me3SiN{CH2CH2NSiMe3}2)ACHTUNGRE(m-N)}2] is
another example of a bis(m-nitrido)-bridged complex (G. K. B.
Clentsmith, V. M. E. Bates, P. B. Hitchcock, F. G. N. Colke, J.
Am. Chem. Soc. 1999, 121, 10 444). The reaction between two
moieties of the model complex [HNACHTUNGRE(CH2CH2NH)2V] and N2
was also calculated to be highly exothermic (V. M. E. Bates,
G. K. B. Clentsmith, F. G. N. Cloke, J. C. Greene, H. D. Ll.
Jenkin, Chem. Commun. 2000, 927).
[16] Comprehensive Inorganic Chemistry, Vol. 3 (Eds.: J. C. Bailar,
H. J. Emelens, R. Nyholm, A. F. Trotman-Dickinson), Pergamon, Oxford, 1973, p. 381.
2802
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