Side-on dinitrogen complexes of titanocenes with disubstituted cyclopentadienyl ligands: Synthesis, structure, and spectroscopic characterization

Article abstract of DOI:10.1021/om300156z

Reduction of the 1,3-disubstituted titanocene complexes, (η⁵-C₅H₃-1,3-ⁱPr ₂)₂TiI or rac, meso-(η⁵-C₅H ₃-1-ⁱPr-3-Me)₂TiI, with excess 0

Full text of DOI:10.1021/om300156z

Article
pubs.acs.org/Organometallics
Side-on Dinitrogen Complexes of Titanocenes with Disubstituted
Cyclopentadienyl Ligands: Synthesis, Structure, and Spectroscopic
Characterization
Scott P. Semproni,† Carsten Milsmann,† and Paul J. Chirik*,†
†Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
S
* Supporting Information
ABSTRACT: Reduction of the 1,3-disubstituted titanocene
complexes, (η5-C5H3-1,3-iPr2)2TiI or rac, meso-(η5-C5H3-1-iPr-
3-Me)2TiI, with excess 0.5% sodium amalgam under an N2
atmosphere furnished the corresponding titanocene dinitrogen
compounds, [(η5-C5H3-1,3-iPr2)2Ti]2(μ2,η2,η2-N2) and [(η5-
C5H3-1-iPr-3-Me)Ti]2(μ2,η2,η2-N2). Crystallographic studies
on both molecules revealed side-on bound, [N2]2‑ ligands
with N−N distances of 1.226(5) and 1.216(5) Å, respectively.
Variable temperature magnetic susceptibility studies estab-
lished population of a triplet ground state at ambient temperature that is slightly higher in energy than the singlet. Reducing the
size of the 1,3-cyclopentadienyl substituents to methyl groups, [(η5-C5H3-1,3-Me2)2Ti], resulted in crystallization of a trimetallic
titanium dinitrogen complex with an activated μ3,η2,η1,η1-N2 ligand with an N−N distance of 1.320(3) Å. Hydrogenation of the
isomeric titanocene dimethyl complex, (η5-C5H3-1,2-Me2)2TiMe2, in the presence of dinitrogen did not result in N2 coordination
but rather furnished the bimetallic titanium compound, (η5-C5H3-1,2-Me2)2Ti(μ2-H)Ti(η5-C5H3-1,2-Me2)(η5,η1-C5H2-1,2-Me2),
resulting from C−H activation of a cyclopentadienyl ring position. Addition of PhCCPh furnished (η5-C5H3-1,2-Me2)2Ti(η2-
PhCCPh), demonstrating that the C−H bond activation event was reversible. By contrast, a bridging formyl complex was
obtained following addition of five equivalents of CO, highlighting the availability of hydride insertion chemistry.
bound dinitrogen ligands. An early example is Pez and co-
workers’ crystallographic characterization of a tetrametallic
titanocene cluster with a μ3,η2,η1,η1-N2 ligand.19 The N−N
bond distance of 1.301(12) Å remains one of the most
activated reported in titanocene dinitrogen chemistry. More
typically, titanocene dinitrogen complexes or related examples
with fulvalenes20 are dimeric and exhibit weak activation of the
μ2,η1,η1-dinitrogen ligand.12,21−25 Noncyclopentadienyl tita-
nium complexes have been prepared and contain modestly
activated end-on dinitrogen ligands with N−N bond distances
between 1.255(7)26 and 1.286(4)27 Å, consistent with two
electron reduction and formation of [N2]2‑.28,29 By comparison,
complexes with side-on, μ2,η2,η2-N2 ligands are less common.
Fryzuk and co-workers have invoked this hapticity during
formation of N−P bonds from phosphine ligands following
dinitrogen cleavage, although no intermediates were observed
or characterized.26 Gambarotta and co-workers have reported
the synthesis and crystallographic characterization of
[[((Me3Si)2N)2Ti]2(μ2,η2,η2-N2)2][(TMEDA)2Li] (TMEDA
= N,N,N′,N′-tetramethylethylenediamine), the first isolable
titanium compound with a side-on bound dinitrogen ligand.30
Our laboratory has a long-standing interest in using
cyclopentadienyl substituent effects to manipulate N2 hapticity
and ultimately reactivity.31,32 In both zirconocene5 and
INTRODUCTION
■
Zirconocene and hafnocene complexes with side-on bound
dinitrogen ligands1−3 exhibit strong activation of the N−N
bond and offer a wealth of functionalization chemistry including
N2 hydrogenation,4−7 carboxylation,8 silylation,9 and CO-
induced N2 cleavage.10 Extending this chemistry to include
titanium is of interest given that some of the attractive
transformations for a potential catalytic cycle such as alkali-
metal free routes to reduced dinitrogen complexes11 and release
of the functionalized product from the metal coordination
sphere are likely more feasible with a first row transition metal.
Titanium complexes also have a long-standing history in
nitrogen fixation.12 In 1966, Vol’pin and Shur reported that
hydrolysis of mixtures of (η5-C5H5)2TiCl2 and various Grignard
reagents under an N2 atmosphere yielded free ammonia.13
Subsequently, Sobota and workers reported isocyanate
formation from carbonylation of N2 in the presence of a (η5-
C5H5)2TiCl2/Mg mixture,14 while Mori and co-workers have
evolved this and other related titanium-alkali metal combina-
tions to elaborate molecular nitrogen into more complex
organic molecules.15 Gambarotta has also reported the partial
hydrogenation, silylation, and cleavage of N2 with titanium
pyrollide complexes,16 and in matrix isolation experiments,
cleavage of N2 has been observed with bare titanium atoms and
dimers to form titanium nitrides.17,18
Considerable effort has been devoted to preparing well-
defined, molecular titanium compounds with activated, side-on
Received: February 26, 2012
Published: April 12, 2012
© 2012 American Chemical Society
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hafnocene6c chemistry, we have demonstrated that while [(η5-
C5Me5)2Zr(N2)]2(μ2,η1,η1-N2) contains weakly activated end-
on dinitrogen ligands, removal of a methyl group from each
ring furnishes [(η5-C5Me4H)2Zr]2(μ2,η2,η2-N2), a side-on
bound dinitrogen complex that offers rich functionalization
chemistry. For the smaller first row metal, titanium, Teuben
and co-workers established that [(η5-C5Me4H)2Ti]2(μ2,η1,η1-
N2) is a weakly activated, end-on dinitrogen complex,23 while
our group later synthesized [(η5-C5H2-1,2,4-Me3)2Ti]2(μ2,η2,η2-
N2)33 and demonstrated that side-on coordination was once
again feasible by reducing the steric protection of the metal
center by systematic removal of methyl groups (Figure 1).
products. Repeating the reduction of (η5-C5H3-1,3-Me2)2TiI in
pentane with excess KC8 for three days followed by filtration
and recrystallization from pentane at −35 °C furnished a small
quantity of blue-green crystals identified as the trimetallic side-
on, end-on dinitrogen compound, 1 (eq 1). Unfortunately the
synthesis of 1 proved unreliable and time sensitive. Attempts to
improve the yield and reproducibility of the reaction by stirring
the reduction of (η5-C5H3-1,3-Me2)2TiI for longer time periods
were unsuccessful and resulted in isolation of K[(η5-C5H3-1,3-
Me2)].
The solid-state structure of 1 was determined by X-ray
diffraction (Figure 2) and establishes a trimetallic titanium
Figure 1. End-on versus side-on dinitrogen coordination in bis-
(cyclopentadienyl)titanium complexes.
Crystallographic characterization of [(η5-C5H2-1,2,4-
Me3)2Ti]2(μ2,η2,η2-N2) revealed an N−N bond distance of
1.216(3) Å, more consistent with an [N2]2‑ ligand rather than
the four-electron reduction to [N2]4‑ observed with zirconium
and hafnium complexes with side-on dinitrogen ligands.1,34,35
Computational studies have posited that the preference for the
less reducing titanium, the +3 oxidation state is preferred to +4.
The resulting dinitrogen compound adopts a triplet rather than
singlet ground state and results in decreased activation and
accounts for the lack of reactivity with dihydrogen.36
Because of the prevalence of [N2]2‑ in lieu of [N2]4‑, we
sought to explore even less sterically protected bis-
(cyclopentadienyl)titanium dinitrogen complexes with the
goal of increasing overlap between the metal and side-on N2
molecular orbitals. Such a strategy has recently proved
successful in hafnocene chemistry where [(η5-C5H2-1,2,4-
Me3)2Hf]2(μ2,η2,η2-N2) exhibits a more activated side-on
dinitrogen ligand than [(η5-C5Me4H)2Hf]2(μ2,η2,η2-N2) and
undergoes N2 silylation, whereas the latter compound does
not.9b Here we report the results of a related study with
titanium supported by a family of disubstituted bis-
(cyclopentadienyl) ligands.
Figure 2. Representation of the solid-state structure of 1 at 30%
probability ellipsoids. Hydrogen atoms omitted for clarity.
dinitrogen complex with an activated μ3,η2,η1,η1-N2 ligand and
exhibits an N−N distance of 1.320(3) Å. The coordination
environment of the N2 ligand is reminiscent of Pez and co-
workers’ structure of the tetranuclear titanium derivative where
an N−N distance of 1.301(12) Å was reported.19 In trimetallic
1, the titanocene unit bonded η2 to the N2 ligand is intact, while
the two others exhibiting η1-N2 coordination are coupled
through the 5-positions of the cyclopentadienyl ligands with a
C−C distance of 1.458(3) Å, typical of fulvalene-type ligands in
this structural motif.12 To our knowledge, this is the longest
observed N−N bond in a titanocene dinitrogen complex and is
consistent with either an [N2]3‑ or [N2]4‑ ligand. While solid-
state magnetic measurements on this compound were not
obtained due to its irreproducible synthesis, a solution magnetic
moment of 2.2(3) μB was determined at 20 °C by the method
of Evans.37 While conclusive assignment of titanium oxidation
states in 1 is hampered by the lack of solid-state magnetic data,
the extent of reduction of the dinitrogen fragment suggests that
RESULTS AND DISCUSSION
■
Reduction of Titanocene Complexes with 1,3-Disub-
stituted Cyclopentadienyl Ligands: (η5-C5H3-1,3-R2)2TiI
(R = Me, iPr). Our studies commenced with the reduction of a
family of 1,3-substituted bis(cyclopentadienyl)titanium(III)
iodide compounds with the goal of synthesizing complexes
with strongly activated, side-on bound dinitrogen ligands. The
titanocene(III) iodide compounds, (η5-C5H3-1,3-R2)2TiI (R =
i
Me, Pr), were chosen due to the ease of their reduction and
were readily synthesized by treatment of the corresponding
chloride derivatives with Me3SiI as described previously.33
Stirring (η5-C5H3-1,3-Me2)2TiI with excess, typically four
equivalents, of 0.5% sodium amalgam in toluene under a
dinitrogen atmosphere did not yield the desired titanocene
dinitrogen compound and produced an unidentified mixture of
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each titanocene is best described as Ti(III). The low solution
magnetic moment may arise from antiferromagnetic coupling of
the spin carriers through the N2 unit, though an in depth
computational analysis and variable temperature magnetic
analysis has not been undertaken.
To protect against cyclopentadienyl C−H activation and
coupling, isopropyl substituents were introduced onto the rings.
Reduction of either (η5-C5H3-1-iPr-3-Me)2TiI or (η5-C5H3-
1,3-iPr2)2TiI with excess of 0.5% sodium amalgam in toluene
under a dinitrogen atmosphere furnished [(η5-C5H3-1-iPr-3-
Me)2Ti]2(μ2,η2,η2-N2) and [(η5-C5H3-1,3-iPr2)2Ti]2(μ2,η2,η2-
N2) as dark blue crystals in 33 and 66% yields, respectively
(eq 2). The diasteromeric purity of the starting monohalide,
diffraction, and representations of the molecular structures are
presented in Figure 3. For [(η5-C5H3-1-iPr-3-Me)2Ti]2(μ2,η2,η2-
N2), only the meso isomer was present in the lattice. There is an
inversion center at the midpoint of the dinitrogen fragment,
which results in one-half of the dimer being generated by
symmetry. The unit cell of [(η5-C5H3-1,3-iPr2)2Ti]2(μ2,η2,η2-
N2) contains one full dimeric molecule and two half molecules
which possess inversion centers similar to that of [(η5-C5H3-
1-iPr-3-Me)2Ti]2(μ2,η2,η2-N2). All molecules in the asymmetric
unit have their cyclopentadienyl rings geared to minimize steric
repulsion between adjacent isopropyl groups.
The electronic ground state of [(η5 -C5 H3 -
1,3-iPr2)2Ti]2(η2,η2-N2) was also investigated by SQUID
magnetometry (Figure 4). The effective magnetic moment,
(η5-C5H3-1-iPr-3-Me)2TiI, was determined by oxidation with I2
and 1H NMR analysis of the resulting titanocene diiodide. This
procedure established formation of a 2.7:1 ratio of diaster-
eomers of (η5-C5H3-1-iPr-3-Me)2TiI2 with the major being the
C1 symmetric meso isomer.
Upon reduction of the mixture of diastereomers of (η5-C5H3-
1-iPr-3-Me)2TiI, only the meso diastereomer of the dinitrogen
complex was isolated and crystallographically characterized
(vide infra). The diasteromeric purity of bulk samples of
paramagnetic [(η5-C5H3-1-iPr-3-Me)2Ti]2(μ2,η2,η2-N2) was as-
sayed by treatment with excess CO to form the diamagnetic
titanocene dicarbonyl complex, (η5-C5H3-1-iPr-3-Me)2Ti-
Figure 4. Variable temperature magnetic data for [(η5-C5H3-
1,3-iPr2)2Ti]2(η2,η2-N2). Fit data: xp = 1.0% (monomeric S = 1/2
Ti(III) impurity), J = −55.0 cm−1, and xTIP = 6.47 × 10−5 cm3/mol.
1
(CO)2. Analysis of the resulting yellow-brown solid by H
μeff, of 2.2 μB per dimer at 300 K is in good agreement with the
solution phase data obtained by the Evans method (μeff =
2.3(2) μB, 295 K) in benzene-d6. The temperature dependence
of μeff is shown in Figure 4 and is indicative of moderate
antiferromagnetic coupling between the two Ti(III) ions
resulting in a diamagnetic ground state. At higher temperatures,
population of the triplet state leads to an increase of μeff
and 13C NMR spectroscopy in benzene-d6 established
formation of only meso-(η5-C5H3-1-iPr-3-Me)2Ti(CO)2, con-
sistent with the diastereomeric purity of the starting titanocene
dinitrogen complex.
Both [(η5-C5H3-1-iPr-3-Me)2Ti]2(μ2,η2,η2-N2) and [(η5-
C5H3-1,3-iPr2)2Ti]2(μ2,η2,η2-N2) were characterized by X-ray
Figure 3. Representations of the solid state structures of meso, meso-[(η5-C5H3-1-iPr-3-Me)2Ti]2(μ2,η2,η2-N2) (left) and [(η5-C5H3-
1,3-iPr2)2Ti]2(μ2,η2,η2-N2) at 30% probability ellipsoids. The former compound contained a minor monohalide impurity (3%). Disordered
orientations of isopropyl groups and hydrogen atoms omitted for clarity.
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approaching the spin-only value for two uncoupled S = 1/2
centers (μeff = 2.45 μB) at 300 K. The data were corrected for
temperature independent paramagnetism, xTIP = 6.47 × 10−5
cm3/mol, and readily fit using the usual exchange Hamiltonian
Hex = −2JS1S2 with S1 = S2 = 1/2 and a coupling constant J =
−55 cm−1. The fit required a small contribution (1.0%) from a
second paramagnetic species (S = 1/2) which is most likely due
to a monomeric Ti(III) impurity. The value of the coupling
parameter is smaller than that found for [(η5-C5H3-1,3-
tBu)2Ti]2(N2) (J = 105 cm−1) or [(η5-C5H3-1,3-(SiMe3)2)-
Ti]2(N2) (J = 100 cm−1).24,38 The difference in the size of the
singlet−triplet gap (−2J) of the two sets of compounds may
arise from the end-on versus side-on binding mode of the
dinitrogen fragment and the resulting different coupling
pathways.
Vibrational Data for Group 4 Metallocene Complexes
with Side-On Dinitrogen Ligands. Solution Raman spectra
were collected on [(η5-C5H3-1-iPr-3-Me)2Ti]2(η2,η2-N2) and
[(η5-C5H3-1,3-iPr2)2 Ti]2(η2,η2-N2), to determine the N−N
stretching frequencies of titanocene dinitrogen complexes with
side-on bound N2 ligands. Such data are often valuable for
assigning the redox level and bond order of the N2 ligand and
hence understanding the overall electronic structure of metal-
dinitrogen compounds.39,40 A representative spectrum is
presented in Figure 5, and the N−N stretching frequencies
TiCl]2(η1,η1-N2) (dNN = 1.289(9) Å). The N−N stretching
frequency measured for [(η5-C5H3-1,3-iPr2)2Ti]2(η2,η2-N2) is
comparable to the value of 1736 cm−1 reported by Evans and
co-workers for the lutetium dinitrogen compound with an η2,η2
dinitrogen ligand, [(η5-C5Me5)(η5-C5Me4H)Lu]2(η2,η2-N2),42
again supporting like values within a given N2 hapticity.
The electronic structure of [(η5-C5H3-1-iPr-3-
Me)2Ti]2(η2,η2-N2) was investigated by DFT calculations
using the BP86 functional. The experimentally determined
singlet ground state of the molecule was computed using the
broken symmetry (BS) formalism. Importantly, this approach
allows the spin-up and spin-down electrons to occupy different
portions of the molecule, allowing magnetic coupling without
forced pairing. The robustness of the BS solution was verified
by a complementary calculation using a spin-restricted
approach, which yielded a higher energy state. The structural
parameters obtained from the BS(1,1) geometry optimization
are in excellent agreement with the data from X-ray
crystallography. A plot of the spin-density obtained from
Mulliken population analysis is shown in Figure 7 and reveals
that the spin density is almost exclusively centered on the Ti
ions, corroborating the Ti(III) assignment with a singly
occupied 1a1 orbital on each titanocene fragment.46 Small
contributions are observed from the bridging N2 ligand and
account for the antiferromagnetic coupling observed exper-
imentally.
The triplet state of [(η5-C5H3-1-iPr-3-Me)2Ti]2(η2,η2-N2)
was obtained computationally via a simple spin-unrestricted
calculation. The optimized geometry shows no significant
differences from the structure obtained from the BS(1,1)
calculation and is in good agreement with the experimental
data. Consistent with the magnetic data, the triplet state is only
0.2 kcal/mol higher in energy than the diamagnetic BS(1,1)
state. Using the formalism developed by Yamaguchi et al.47,48
a
coupling constant of J = −77.5 cm−1 was calculated, which is in
excellent agreement with the experimental value of −55 cm−1
measured by SQUID magnetometry and further validates the
computational output.
The oxidation of [(η5-C5H3-1,3-iPr2)2Ti]2(η2,η2-N2) was also
explored. Bouwkamp and co-workers have previously reported
that oxidation of the weakly activated, end-on dinitrogen
complex, [(η5-C5Me5)(η6-C5H4C10H14)Ti]2(η1,η1-N2) with
[Cp2Fe][BPh4] in THF resulted in N2 loss and formation of
the corresponding Ti(III) compound, [(η5-C5Me5)(η6-C5H4
C10H14)Ti(THF)][BPh4].20c To determine if N2 coordination
could be preserved upon oxidation with a more activated side-
on dinitrogen compound, a THF solution of [(η5-C5H3-
1,3-iPr2)2Ti]2(η2,η2-N2) was treated with AgBPh4, which
resulted in loss of dinitrogen and isolation of the cationic
Ti(III) complex, [(η5-C5H3-1,3-iPr2)2Ti(THF)][BPh4] as dark
green crystals in 74% yield (eq 3).
Figure 5. Raman spectra of [(η5-C5H3-1,3-iPr2)2Ti]2(η2,η2-N2) (blue)
and [(η5-C5H3-1,3-iPr2)2Ti]2(η2,η2−15N2) (green) recorded in pentane
solution.
for this and related end-on and side-on dinitrogen compounds
are presented in Table 1. Data new to this study include the
zirconocene and hafnocene side-on dinitrogen complexes, [(η5-
C5Me4H)2Zr]2(η2,η2-N2) and [(η5-C5H2-1,2,4-Me)2Hf]2(η2,η2-
N2), representative spectra of which are reported in the
Supporting Information.
As reported previously,40 there is an essentially linear
correlation of the N−N stretching frequency with the bond
distance, in accordance with Badger’s rule.41 An updated
correlation incorporating the data from this study is presented
in Figure 6. The N−N stretching frequency of 1747 cm−1 for
[(η5-C5H3-1,3-iPr2)2Ti]2(η2,η2-N2) is comparable to the values
of 1749 and 1719 cm−1 reported for the fulvalene and
titanocene dinitrogen compounds, [(η5-C5Me5)(η6-C5H4CR2)-
Ti]2(η1,η1-N2) (R = tolyl)20b and [(η5-C5Me4Et)2Ti-
(N2)]2(η1,η1-N2),25 respectively. A change in hapticity of the
N2 ligand results in more dramatic changes of the N−N
stretching frequency as evidenced by the much lower stretching
frequency of 1284 cm−1 for [(Me2Si)2N(TMEDA)-
As expected for a titanium(III), d1 compound, [(η5-C5H3-
1,3-iPr2)2Ti(THF)][BPh4] is paramagnetic with a solution
magnetic moment (benzene-d6, 23 °C) of 1.7(1) μB. The solid-
state structure was determined by X-ray diffraction and
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Table 1. Dinitrogen Stretching Frequencies (cm−1) and Bond Distances (Å) of Compounds with End-on and Side-on
Dinitrogen Ligands As Determined by Raman and Infrared Spectroscopy
entry
compound
ν(N−N) (cm‑1)
d(N−N) (Å)
reference
20b
1
2
[(η5-C5Me5)(η6-C5H4C(C6H4-4-Me)2)Ti]2(η1,η1-N2)
[(η5-C5H3-1,3-iPr2)2Ti]2(η2,η2-N2)
1749
1.160(3)
1.226(5)
1747 (14N)
1690 (15N)
1742
this work
3
4
[(η5-C5H3-1-iPr-3-Me)2Ti]2(η2,η2-N2)
[(η5-C5Me5)(η5-C5Me4H)Lu]2(η2,η2-N2)
1.216(5)
1.275(3)
this work
42
1736 (14N)
1678 (15N)
1719
5
6
7
8
[(η5-C5Me4Et)2Ti(N2)](η1,η1-N2)
[(η5-C5Me5)2Zr(N2)]2(η1,η1-N2)
[(Me2Si)2N(TMEDA)TiCl]2 (η1,η1-N2)
{[(Me3Si)2N]2(THF)Y}2(μ3-η2,η2,η2-N2)K
1.150(2)
1.182(5)
1.289(9)
1.405(3)
25
1578
43
1284
20b, 30
44
989 (14N)
956 (15N)
922 (14N)
893 (15N)
818 (14N)
782 (15N)
775 (14N)
753 (15N)
9
[(η5-C5Me4H)2Zr]2(η2,η2-N2)
[(η5-C5H2-1,2,4-Me)2Hf]2(η2,η2-N2)
[(P2N2)Zr]2(η2,η2-N2)
1.377(3)
1.457(5)
1.43(1)
this work, 5
this work, 9b
45
10
11
Figure 6. N−N stretching frequencies of selected group 4 and
lanthanide dinitrogen compounds of different bond orders and
hapticities as a power function of their bond length.
Figure 8. Representation of the solid state structure of [(η5-C5H3-
1,3-iPr2)2Ti(THF)][BPh4] at 30% probability ellipsoids. Hydrogen
atoms and disordered isopropyl groups omitted for clarity.
Chemistry of Reduced 1,2-Dimethyl-Substituted Tita-
nocene Compounds. Alkali metal reduction of (η5-C5H3-1,2-
Me)2TiI was explored with the goal of preparing a titanocene
compound with an activated, side-on bound dinitrogen ligand.
Stirring the titanocene iodide with excess 0.5% sodium
amalgam resulted in an unidentified mixture of products with
no evidence for formation of a dinitrogen compound. An
alternative route to titanocene dinitrogen compounds is via
reductive elimination of dihydrogen from a titanocene hydride
complex in the presence of N2. For example, Teuben and co-
workers have reported that exposure of (η5-C5Me4H)2TiH to
an N2 atmosphere yields the weakly activated, end-on
dinitrogen compound, [(η5-C5Me4H)2Ti]2(η1,η1-N2).23 Our
laboratory extended this method to the preparation of the
side-on bound titanocene dinitrogen complex, [(η5-C5H2-1,2,4-
Me3)2Ti]2(η1,η1-N2).33
establishes the identity of the molecule (Figure 8). The THF is
coordinated in the central position of the metallocene wedge,
and there are no close contacts between the titanocene cation
and the [BPh4]− anion.
Addition of two equivalents of methyl lithium to a diethyl
ether solution of (η5-C5H3-1,2-Me)2TiCl2 at −35 °C followed
by stirring at ambient temperature for 3 h and subsequent
purification furnished (η5-C5H3-1,2-Me)2TiMe2 as diamagnetic,
bright yellow crystals in 77% yield (Scheme 1). Exposure of a
toluene solution of (η5-C5H3-1,2-Me)2TiMe2 to 4 atm of a
90:10 mixture of H2:N2 resulted in isolation of a diamagnetic
Figure 7. Spin density plot for [(η5-C5H3-1-iPr-3-Me)2Ti]2(η2,η2-N2)
obtained from a Mulliken population analysis. Positive spin density is
shown in red and negative spin density in yellow.
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Scheme 1. Synthesis of (η5-C5H3-1,2-Me)2TiMe2 and 2
dark-blue green crystalline solid in 93% yield (Scheme 1).
Repeating the reaction using only H2 gas produced the same
product in similar yield. X-ray diffraction (Figure 9) established
(see the Supporting Information), but the peaks did not
sharpen to show coupling or allow assignment. Solution
magnetic measurements (Evans method) in benzene-d6 at 20
°C were consistent with the solid-state data and a diamagnetic
compound.
The unusual NMR behavior of 2 prompted us to examine the
electronic structure of this compound by DFT calculations
using the same approach as described for [(η5-C5H3-1-iPr-3-
Me)Ti]2(η2,η2-N2). A BS(1,1) solution successfully reproduced
the structural parameters obtained by X-ray crystallography and
was found to be significantly lower in energy than the
corresponding spin-restricted alternative. The triplet state for
2 was only 1.5 kcal/mol higher in energy than the BS solution
resulting in a calculated coupling constant of J = −537.5 cm−1.
The magnitude of this exchange coupling suggests that the
triplet state is not significantly populated at room temperature
in agreement with the experimental magnetic data. However,
the chemical shifts obtained from NMR spectroscopy are much
more sensitive to small contributions from close-lying excited
paramagnetic states,53,54 which explains the unusual shifts for 2
at room temperature. The experimental and computational
evidence for a low lying excited triplet state led to a more
careful analysis of the Ti−Ti interaction in 2. The Mayer
population analysis for the BS(1,1) ground state yielded a bond
order of only 0.52. The relatively low bond order for the Ti−Ti
interaction can be further understood by an inspection of the
magnetic orbitals55 of the complex shown in Figure 10. Each
Figure 9. Solid state structure of [(η5-C5H3-1,2-Me2)2Ti(μ2-H)Ti(η5-
C5H3-1,2-Me2)(η5,η1-C5H2-1,2-Me2) (2) at 30% probability ellipsoids.
Hydrogen atoms, except those attached to titanium, omitted for clarity.
the identity of the compound as the μ-hydrido bis(titanium)
derivative, (η5-C5H3-1,2-Me2)2Ti(μ2-H)Ti(η5-C5H3-1,2-Me2)-
(η5,η1-C5H2-1,2-Me2) (2), arising from C−H activation of a
cyclopentadienyl position, rather than the desired dinitrogen
complex. The two titanium centers are inequivalent, one being
coordinated by two intact [(η5-C5H3-1,2-Me2)] ligands and also
η1- bound to a bridging cyclopentadienyl ligand from the
adjacent metallocene. A hydrogen atom bridging the two
titanium centers was also located. The Ti(1)−Ti(2) distance of
2.9785(4) Å is consistent with a close contact between the two
metals but is outside the range necessary for a metal−metal
bond.
Pez has reported a similar structural motif following isolation
of a crystalline solid from low temperature potassium
naphthalenide reduction of (η5-C5H5)2TiCl2 in THF.49 The
original report established a bimetallic titanium complex with a
bridging η5,η1-cyclopentadienyl ligand. One of the titanium
centers also was coordinated by a molecule of THF. However, a
bridging hydride was not identified in the original report. In
2002, Harrod and co-workers reported the structural revision of
this compound and correctly identified it as [(η5-C5H5)2Ti(μ2-
H)Ti(η5-C5H5)(η5,η1-C5H4)(THF) following treatment of (η5-
C5H5)2TiMe2 with PhMeSiH2 in the presence of lactones.50
The Ti−Ti distance of 3.329(5) Å is outside the range typically
associated with a Ti−Ti bond and is longer than the value
found in 2. The Pez and Harrod compound differs from the
structure of “titanocene”, initially elucidated by Brintzinger and
Bercaw51 and later crystallographically verified by Mach.52
Figure 10. Magnetic orbitals of 2 obtained from an unrestricted
corresponding orbital transformation.56 The HOMO of the spin-up
manifold is shown on the left, and the HOMO of the spin-down
manifold is shown on the right.
Ti(III) ion carries one unpaired electron in an orbital of 1a1
symmetry with respect to the titanocene fragment. These
orbitals possess only small coefficients along the Ti−Ti vector
resulting in a small spatial overlap of 0.40 and a weakly bonding
interaction.
To probe whether C−H activation of the cyclopentadienyl
ring was reversible and to determine whether 2 could serve as a
source of [(η5-C5H3-1,2-Me)2Ti] or contained a reactive
titanium-hydride, reactivity studies were conducted. Addition
of PhCCPh to a toluene solution of 2 resulted in rapid
formation of a brown solution from which brown crystals
identified as (η5-C5H3-1,2-Me)2Ti(η2-PhCCPh) were isolated
in 60% yield (Scheme 2). Diamagnetic (η5-C5H3-1,2-Me)2Ti-
1
The toluene-d8 H NMR spectrum of 2 contains a number of
broad peaks at 20 °C. Cooling the sample to −60 °C resulted in
a contraction of the resonances toward the diamagnetic region
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Scheme 2. Reactivity of 2 with PhCCPh and Carbon Monoxide
(η2-PhCCPh) was independently synthesized in 75% yield by
CONCLUDING REMARKS
■
reduction of (η5-C5H3-1,2-Me)2TiCl2 with excess magnesium in
the presence of PhCCPh. Thus, the addition of an internal
alkyne established the reversibility of the C−H cyclometalation
reaction.
The dinitrogen chemistry of titanocene complexes bearing
disubstituted cyclopentadienyl ligands has been explored. For
the 1,3-dimethyl substituted titanocene, a trimetallic compound
with a μ3,η2,η1,η1-N2 ligand and an N−N distance of 1.320(3) Å
was, albeit irreproducibly, isolated and represents one of the
most activated dinitrogen ligands observed in bis-
(cyclopentadienyl) titanium chemistry. Increasing the size of
the ring substituents to include an isopropyl group furnished
the dimeric titanocene dinitrogen complexes, [(η5-C5H3-
1,3-iPr2)2Ti]2(μ2,η2,η2-N2) and [(η5-C5H3-1-iPr-3-
Me)2Ti]2(μ2,η2,η2-N2) with modestly activated, side-on
bound, [N2]2‑ ligands with N−N distances of 1.226(5) and
1.216(5) Å, respectively. Magnetic studies establish the
accessibility of both singlet and triplet states. Solution Raman
data establish that the N−N stretching frequency is more
sensitive to N2 hapticity rather than formal oxidation state. For
the 1,2-dimethyl substituted titanocene, hydrogenation of the
dimethyl derivative, (η5-C5H3-1,2-Me2)TiMe2, furnished the
bimetallic titanium compound, [(η5-C5H3-1,2-Me2)2Ti(μ2-H)-
Ti(η5-C5H3-1,2-Me2)(η5,η1-C5H2-1,2-Me2), which exhibited a
bridging hydride that reduced CO to a μ-formyl ligand.
Carbonylation of 2 was of interest because carbon monoxide
and dinitrogen are isoelectronic. Exposure of a benzene
solution of 2 to 5 equivalents of CO followed by removal of
the volatiles and recrystallization from diethyl ether at −35 °C
furnished 3 (Scheme 2). Concomitant with the formation of 3,
a small amount of (η5-C5H3-1,2-Me)2Ti(CO)2 was also
1
observed by H NMR spectroscopy. Continued carbonylation
of the mixture produced (η5-C5H3-1,2-Me)2Ti(CO)2 as soon as
the solutions were thawed; the fate of the formyl ligand in this
transformation is unknown. The ratio of the two products was
dependent on the pressure of added CO; (η5-C5H3-1,2-
Me)2Ti(CO)2 was formed exclusively when benzene solutions
of 2 were treated with 1 atm of CO.
The benzene-d6 1H and 13C NMR spectra of 3 established a
C1 symmetric molecule suggesting the structural features of 2
were maintained. Repeating the carbonylation in benzene-d6
with 13CO produced a doublet (1JC−H = 141.5 ppm) centered at
1
7.91 ppm in the H NMR spectrum consistent with a bridging
EXPERIMENTAL SECTION
■
formyl compound. The formyl resonance was located at 221.3
General Considerations. All air- and moisture-sensitive manip-
ulations were carried out using standard high vacuum line, Schlenk, or
cannula techniques or in an M. Braun inert atmosphere drybox
containing an atmosphere of purified nitrogen. The M. Braun drybox
was equipped with a cold well designed for freezing samples in liquid
nitrogen. Solvents for air- and moisture-sensitive manipulations were
dried and deoxygenated using literature procedures.61 Deuterated
solvents for NMR spectroscopy were distilled from sodium metal
under an atmosphere of argon and stored over 4 Å molecular sieves.
Nitrogen and hydrogen gas were purchased from Airgas Incorporated
and passed through a column containing manganese oxide on
vermiculite and 4 Å molecular sieves before admission to the high
vacuum line. Carbon monoxide gas was passed through a column
containing 4 Å molecular sieves before use. The titanium dinitrogen
compound [(C5H3-1-iPr-3-Me)2Ti]2(η2,η2-N2) was prepared by a
slight modification of the literature procedure.62 The titanocene
1
ppm in the 13C NMR spectrum. The observation of sharp H
and 13C NMR resonances in the diamagnetic region of the
spectrum establishes an electronic structure for 3 that is unique
from 2.
Schrock and co-workers have reported similar observations in
tantalum hydride chemistry,56 and the resulting formyl
compound has been structurally characterized.57 Addition of
CO to [(η5-C5Me4Et)Cl2TaH]2 resulted in formation of a
[μ2,η2,η2-CHO] ligand. The low temperature 1H NMR
spectrum exhibits a peak at 7.5 ppm assigned as the formyl
proton, similar to the value observed for 3. The 13C resonance
for the tantalum compound is located at 168 ppm (1JC−H = 168
Hz), upfield shifted from the titanium compound. Insertion of
CO into a terminal thorocene hydride has also been reported
by Marks and co-workers to yield enediolates,58 chemistry also
known with group 4 metallocenes.59 Fryzuk and co-workers
have also reported the synthesis of a zirconium formyl complex
by insertion of CO into a zirconium-hydride.60 Phosphine
coordination and accompanying P−C bond formation also
stabilizes the formyl unit.
63
halides, (C5H3-1,2-Me2)2TiCl2 and (C5H3-1,2-Me2)2TiCl64 were
prepared as reported. All other reagents were used as received.
1H NMR spectra were recorded on a Varian Inova 400
spectrometer operating at 399.860 MHz. All chemical shifts are
1
reported relative to SiMe4 using H (residual) chemical shifts of the
solvent as a secondary standard. 13C NMR spectra were recorded on a
Bruker 500 spectrometer operating at 125.71 MHz. 13C chemical shifts
are reported relative to SiMe4 using chemical shifts of the solvent as a
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secondary standard where applicable. For paramagnetic molecules, the
1H NMR data are reported with the chemical shift followed by the
peak width at half height in Hertz or multiplicity, followed by
integration value and where possible, peak assignment. UV−visible
spectra were recorded on a Shimadzu UV-2450 spectrophotometer
equipped with a diode array detector from 200 to 1100 nm in pentane
solution. Raman spectra were collected on a Thermo-Fisher DXR
Raman with a 720 nm excitation wavelength. Elemental analyses were
performed at Robertson Microlit Laboratories, Inc., in Ledgewood, NJ.
Single crystals suitable for X-ray diffraction were coated with
polyisobutylene oil in a drybox, transferred to a nylon loop, and then
quickly transferred to the goniometer head of a Bruker X8 APEX2
diffractometer equipped with molybdenum and copper X-ray tubes (λ
= 0.71073 and 1.54184 Å respectively). Preliminary data revealed the
crystal system. The collection routine was optimized using the Bruker
COSMO software suite. The space group was identified, and the data
were processed using the Bruker SAINT+ program and corrected for
absorption using SADABS. The structures were solved using direct
methods (SHELXS) completed by subsequent Fourier synthesis and
refined by full-matrix least-squares procedures.
Solution magnetic moments were determined by the method of
Evans at 22 °C using a ferrocene standard unless otherwise noted.37
Magnetic susceptibility balance measurements were performed with a
Johnson Matthey instrument that was calibrated with HgCo(SCN)4.
SQUID magnetization data of crystalline powdered samples were
recorded with a SQUID magnetometer (Quantum Design) at 10 kOe
between 5 and 300 K for all samples. Values of the magnetic
susceptibility were corrected for the underlying diamagnetic increment
by using tabulated Pascal constants and the effect of the blank sample
holders (gelatin capsule/straw). Samples used for magnetization
measurement were recrystallized multiple times and checked for
chemical composition by elemental analysis. The program julX written
by E. Bill was used for (elements of) the simulation and analysis of
magnetic susceptibility data.65
reaction mixture was observed. The solution was filtered through a pad
of Celite, the blue-green solution was collected, and the volatiles were
removed in vacuo. Recrystallization of the residue from pentane
afforded a small quantity of dark-blue crystals of (η5-C5H3-1,3-
Me2)2Ti(μ3,μ1,η2,η1,N2)(η5,η5-C10H4-1,3-Me4)[(C5H3-1,3-Me2)Ti]2
suitable for X-ray diffraction (27 mg, 20% yield based on Ti). Anal.
Calcd for C42H52N2Ti3: C, 69.25; H, 7.19; N, 3.85. Found: C, 68.86;
H, 7.19; N, 3.85. Magnetic Susceptibility (benzene-d6): μeff = 2.2(3)
μB.
Preparation of (η5-C5H3-1,3-iPr2)2TiI. This molecule was
prepared in an identical manner to (η5-C5H3-1,3-Me2)2TiI, with 4.7
g (0.030 mol) of Li[C5H3-1,3-iPr2] and 5.6 g (0.015 mol) of
TiCl3(THF)3 to generate (η5-C5H3-1,3-iPr2)2TiCl in situ. After
filtration through Celite and treatment with 3.38 g (0.017 mol) of
trimethylsilyl iodide, 3.35 g (47% based on Ti) of (η5-C5H3-
1,3-iPr2)2TiI was obtained as a green solid following recrystallization
from pentane. Anal. Calcd for C22H34ITi: C, 55.83; H, 7.24. Found: C,
1
56.02; H, 7.00. H NMR (benzene-d6,23 °C): δ 1.99 (Δν1/2 = 105
Hz), 3.40 (Δν1/2 = 185 Hz), 6.90 (Δν1/2 = 5 Hz), unable to assign
resonances based on integration. Magnetic Susceptibility: (benzene-
d6) μeff = 1.9(2) μB.
Preparation of (η5-C5H3-1-iPr-3-Me)2TiCl. A thick walled glass
vessel was charged with a stirbar, 4.04 g (0.032 mol) of Li[C5H3-1-iPr-
3-Me], 5.84 g (0.016 mol) of TiCl3(THF)3 and 75 mL of
tetrahydrofuran. The vessel was degassed and heated to reflux for 3
d. The solvent was removed from the brown solution in vacuo, and the
resulting brown residue was dissolved in pentane and filtered through
Celite to remove LiCl. The residue was recrystallized from diethyl
ether at −35 °C to afford (η5-C5H3-1-iPr-3-Me)2TiCl as a greenish-
brown solid in 53% yield. Anal. Calcd for C18H26ClTi: C, 66.37; H,
8.05. Found: C, 66.82; H, 7.90. 1H NMR (benzene-d6, 23 °C): δ 1.93
(br m, overlapping), 2.29 (br m, overlapping), unable to assign
resonances based on integration. Magnetic Susceptibility: (benzene-
d6) μeff = 1.8(1) μB.
Preparation of (η5-C5H3-1-iPr-3-Me)2TiI. A 100 mL round-
bottom flask was charged with 2.00 g (6.15 mmol) of (η5-C5H3-1-iPr-
3-Me)2TiCl, 40 mL of toluene, and a stirbar. To this rapidly stirring
solution was added 3.71 g (18.5 mmol) of trimethylsilyliodide. The
resulting brown solution was stirred at room temperature for 3 d, after
which time the volatiles were removed in vacuo. The resulting brown
residue was recrystallized from diethyl ether in multiple crops to yield
(η5-C5H3-1-iPr-3-Me)2TiI as a brown solid (1.20 g, 47% yield). Anal.
Quantum-Chemical Calculations. All DFT calculations were
performed with the ORCA program package.66 The geometry
optimizations of the complexes and single-point calculations on the
optimized geometries were carried out at the BP86 level67−69 of DFT.
The all-electron Gaussian basis sets were those developed by the
Ahlrichs group.70−72 Triple-ζ quality basis sets def2-TZVP with one
set of polarization functions were used for all atoms. Auxiliary basis
sets to expand the electron density in the resolution-of-the-identity
(RI) approach were chosen to match the orbital basis.73−75
1
Calcd for C18H26ITi: C, 51.82; H, 6.28. Found: C, 52.13; H, 6.16. H
NMR (benzene-d6, 23 °C): δ 3.69 (br m, overlapping), 4.63 (br m,
overlapping), unable to assign resonances based on integration.
Magnetic Susceptibility: (benzene-d6) μeff = 1.3(3) μB.
Throughout this paper we describe our computational results by
using the broken-symmetry (BS) approach by Ginsberg76 and
Noodleman.77 Because several broken symmetry solutions to the
spin-unrestricted Kohn−Sham equations may be obtained, the general
notation BS(m,n)78 has been adopted, where m (n) denotes the
number of spin-up (spin-down) electrons at the two interacting
fragments. Canonical and corresponding56 orbitals as well as spin
density plots were generated with the program Molekel.79
Spectroscopic Characterization of (η5-C5H3-1-iPr-3-Me)2TiI2.
In the glovebox, a 20 mL scintillation vial was charged with 40 mg
(0.096 mmol) of (η5-C5H3-1-iPr-3-Me)2TiI and 5 mL of toluene. The
solution was stirred rapidly, and a 1 mL toluene solution containing 12
mg (0.047 mmol) of iodine was added slowly. The solution was stirred
for 18 h at room temperature, changing color from dark brown to dark
red over the course of 30 min. The solvent was removed in vacuo, and
the resulting oily red residue was analyzed by 1H and 13C NMR
spectroscopy to reveal two isomers present in a 2.7:1 ratio of meso:rac
Preparation of (η5-C5H3-1,3-Me2)2TiI. A 100 mL round-bottom
flask was charged with 0.774 g (2.88 mmol) of (η5-C5H3-1,3-
Me2)2TiCl, 1.74 g (8.66 mmol) of trimethylsilyl iodide, and
approximately 30 mL of toluene. The resulting dark reaction mixture
was stirred for 3 days, after which time the solvent and volatiles were
removed in vacuo. The resulting red residue was recrystallized from
diethyl ether at −35 °C to yield 0.685 g (66%) of (η5-C5H3-1,3-
Me2)2TiI as dark red crystals. Anal. Calcd for C14H18ITi: C, 46.57; H,
diastereomers. Meso: 1H NMR (benzene-d6, 23 °C): δ 0.88 (d, 3JHH
=
7.0, 3H, C5H3-1,3-Me-iPr), 0.94 (d, 3JHH = 7.0, 3H, C53H3-1,3-Me-iPr),
3
1.13 (d, JHH = 7.0, 3H, C5H3-1,3-Me-iPr), 1.17 (d, JHH = 7.0, 3H,
C5H3-1,3-Me-iPr), 2.03 (s, 3H, C5H3-1,3-Me-iPr), 2.06 (s, 3H, C5H3-
3
1
1,3-Me-iPr), 3.08 (septet, JHH = 7.0, 1H, C5H3-1,3-Me-iPr), 3.12
5.02. Found: C, 46.21; H, 4.85. H NMR (benzene-d6, 23 °C): δ 28.3
(septet, 3JHH = 7.0, 1H, C5H3-1,3-Me-iPr), 5.55 (m, 1H, C5H3-1,3-Me-
iPr), 5.63 (m, 1H, C5H3-1,3-Me-iPr), 5.67 (m, 1H, C5H3-1,3-Me-iPr),
5.72 (m, 1H, C5H3-1,3-Me-iPr), 7.06 (m, 1H, C5H3-1,3-Me-iPr), 7.09
(m, 1H, C5H3-1,3-Me-iPr). {1H}13C NMR (benzene-d6, 23 °C): δ
18.7, 18.8 (C5H3-1,3-Me-iPr), 21.4, 22.2, 22.2, 24.1 (C5H3-1,3-Me-iPr),
31.3, 31.4 (C5H3-1,3-Me-iPr), 113.4, 114.2, 114.7, 116.8, 117.2, 117.8,
125.9, 126.0, 129.4, 142.3 (C5H3-1,3-Me-iPr). Rac: 1H NMR
(Δν1/2 = 1995 Hz, C5H3-1,3-Me2), 37.8 (Δν1/2 = 8190 Hz, C5H3-1,3-
Me2). Magnetic Susceptibility (benzene-d6): μeff = 2.5(2) μB.
Preparation of (η5-C5H3-1,3-Me2)2Ti(μ3,μ1,η2,η1,N2)(η5,η5-
C10H4-1,3-Me4)[η5-(C5H3-1,3-Me2)Ti]2 (1). A 20 mL scintillation
vial was charged with 0.200 g (0.55 mmol) of (η5-C5H3-1,3-Me2)2TiI
and approximately 10 mL of pentane. The solution was stirred rapidly,
and a suspension containing 0.313 g (2.2 mmol) of potassium graphite
in approximately 5 mL of pentane was prepared. The suspension was
added in small portions, and the resulting dark-red brown solution was
stirred at room temperature for 3 days over which time a green-blue
3
(benzene-d6, 23 °C): δ 1.04 (d, JHH = 7.0, 6H, C5H3-1,3-Me-iPr),
3
1.05 (d, JHH = 7.0, 6H, C5H3-1,3-Me-iPr), 1.77 (s, 6H, C5H3-1,3-Me-
3
iPr), 2.85 (septet, JHH = 7.0, 2H, C5H3-1,3-Me-iPr), 6.32 (m, 2H,
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C5H3-1,3-Me-iPr), 6.41 (m, 2H, C5H3-1,3-Me-iPr), 6.55 (m, 2H,
C5H3-1,3-Me-iPr). {1H}13C NMR (benzene-d6, 23 °C): δ 20.6 (C5H3-
1,3-Me-iPr), 24.3, 24.4 (C5H3-1,3-Me-iPr), 32.8 (C5H3-1,3-Me-iPr),
119.5, 120.1, 121.5, 141.8, 153.6 (C5H3-1,3-Me-iPr).
N, <0.02. Magnetic Susceptibility: (Magnetic Susceptibility balance)
μeff = 1.7(1) μB.
Preparation of (η5-C5H3-1,2-Me2)2TiMe2. A 20 mL scintillation
vial was charged with 0.600 g (1.97 mmol) of (η5-C5H3-1,2-
Me2)2TiCl2 and approximately 10 mL of diethyl ether. The resulting
solution was chilled to −35 °C, and 2.6 mL (5.60 mmol) of 1.6 M
methyl lithium solution in diethyl ether was added. The resulting
reaction mixture was stirred for 3 h at room temperature. The solvent
was removed in vacuo, and the residue was extracted with pentane,
filtered through Celite, and recrystallized from pentane to afford 0.400
g (77%) of (η5-C5H3-1,2-Me2)2TiMe2 as bright yellow needles. Anal.
Preparation of [(η5-C5H3-1,3-iPr)2Ti]2(η2,η2-N2). A 100 mL
round-bottom flask was charged with 19.4 g of 0.5% sodium amalgam
and approximately 10 mL of toluene. With vigorous stirring of the
amalgam, a slurry containing 0.500 g (1.05 mmol) of (η5-C5H3-1,3-
iPr2)2TiI in approximately 20 mL of toluene was added. The resulting
dark blue-green reaction mixture was stirred at ambient temperature
for 3 days under a dinitrogen atmosphere. After one day a color
change to royal blue was observed. Following filtration through Celite,
the solvent was removed in vacuo, and the resulting dark residue was
recrystallized from pentane at −35 °C yielding 0.240 g (66%) of
analytically pure dark blue crystals over the course of 1 day. Anal.
Calcd for C44H68N2Ti2: C, 73.32; H, 9.51; N, 3.89. Found: C, 72.96;
H, 9.52; N, 3.56. 1H NMR (benzene-d6, 23 °C): δ 1.41(br s
overlapped), 1.75 (br s overlapped), 1.99 (br s overlapped), 21.1
(Δν1/2 = 1180 Hz), unable to assign resonances based on integration.
Magnetic Susceptibility: (benzene-d6) μeff = 2.3(2) μB. UV−vis
(pentane): 306 nm (11 954 M−1 cm−1), 344 nm (9914 M−1 cm−1),
1
Calcd for C16H24Ti: C, 72.73; H, 9.16. Found: C, 72.55; H, 8.88. H
NMR (benzene-d6, 23 °C): δ −0.24 (s, 6H, TiMe2), 1.93 (s, 12H,
C5H3-1,2-Me2), 5.02 (t, 3JHH = 3.1, 2H, C5H3-1,2-Me2), 5.32 (d, 3JHH
=
3.1, 4H, C5H3-1,2-Me2). {1H}13C NMR (benzene-d6, 23 °C): δ 13.7
(C5H3-1,2-Me2), 44.3 (TiMe2), 106.3, 113.2, 123.4 (C5H3-1,2-Me2).
Preparation of [(η5-C5H3-1,2-Me2)2Ti(μ2-H)Ti(η5-C5H3-1,2-
Me2)(η5,η1-C5H2-1,2-Me2). A thick walled glass vessel was charged
with 0.130 g (0.53 mmol) of (η5-C5H3-1,2-Me2)2TiMe2 and
approximately 10 mL of toluene. The vessel was transferred to a
high vacuum line, and the contents of the vessel cooled to 77 K and
degassed. At this temperature, one atmosphere of hydrogen gas was
admitted. The dark yellow solution was warmed to room temperature
and shaken vigorously forming a dark green solution after 15 s. The
solution was stirred for 1 h at room temperature before the solvent was
removed in vacuo. Recrystallization of the dark green residue from
pentane yielded 0.230 g (93%) of dark blue-green crystals. Similar
results were obtained when a mixture of hydrogen and nitrogen gas
was used. Anal. Calcd for C28H36Ti2: C, 71.81; H, 7.75. Found: C,
−1
570 nm (13 815 M−1 cm−1). Raman (pentane): νNN = 1747 cm
(14N2), 1688 cm−1 (15N2).
Preparation of [(η5-C5H3-1-iPr-3-Me)2Ti]2(η2,η2-N2). This com-
pound was prepared in
a
similar manner to (η5-C5H3-
1,3-iPr)2Ti]2(η2,η2-N2), using 0.500 g (1.20 mmol) of (η5-C5H3-1,3-
Me-iPr)2TiI and 22.1 g of 0.5% Na/Hg. Following filtration of the
reaction mixture through Celite, the solvent was removed in vacuo,
and the resulting dark residue was recrystallized from pentane at −35
°C to yield analytically pure dark blue crystals suitable for X-ray
diffraction (125 mg, 33% yield). Anal. Calcd for C36H52N2Ti2: C,
1
71.71; H, 7.42. H NMR (benzene-d6, 23 °C): δ −1.55 (Δν1/2 = 49
Hz), −0.79 (Δν1/2 = 56 Hz), 11.4 (Δν1/2 = 383 Hz), 13.2 (Δν1/2
=
1
1
317 Hz). H NMR (toluene-d8, 19 °C): δ −1.49 (Δν1/2 = 46 Hz),
−0.71 (Δν1/2 = 46 Hz), 7.03 (Δν1/2 = 214 Hz), 11.4 (Δν1/2 = 225
Hz), 12.6 (Δν1/2 = 208 Hz), 13.9 (Δν1/2 = 216 Hz). 1H NMR
(toluene-d8, −42 °C): δ 0.03 (Δν1/2 = 45 Hz), 0.97 (Δν1/2 = 79 Hz),
3.89 (Δν1/2 = 710 Hz), 7.66 (Δν1/2 = 418 Hz), 9.61 (Δν1/2 = 196 Hz).
UV−vis (pentane): 404 nm (1157 M−1 cm−1), 615 nm (228 M−1
cm−1).
71.05; H, 8.61; N, 4.60. Found: C, 70.75; H, 8.19; N, 4.38. H NMR
(benzene-d6, 23 °C): δ 2.51, 2.77, 2.92, unable to assign resonances
based on integration. Magnetic Susceptibility: (benzene-d6) μeff
=
2.4(2) μB. UV−vis (pentane): 300 nm (11 801 M−1 cm−1), 360 nm
(13 215 M−1 cm−1), 560 nm (3306 M−1 cm−1). Raman (toluene): νNN
−1
= 1742 cm (14N2).
Spectroscopic Characterization of (η5-C5H3-1-iPr-3-Me)2Ti-
(CO)2. In the glovebox was dissolved 10 mg (0.016 mmol) of [(η5-
C5H3-1-iPr-3-Me)2Ti]2(η2,η2-N2) in benzene-d6 (0.5 mL), and a dark
blue solution formed. The tube was frozen in liquid nitrogen and
degassed, and 1 atm of CO was admitted to the vessel. The contents of
the tube were thawed and then shaken vigorously. A color change
from blue to yellow-brown was observed. 1H and 13C NMR
spectroscopic analysis revealed the presence of only the meso
Preparation of (η5-C5H3-1,2-Me2)2Ti(η2-PhCCPh). Method 1: A
20 mL scintillation vial was charged with a stirbar, 0.036 g (0.08
mmol) of 2, and approximately 5 mL of toluene. The dark-green
solution was stirred, and 0.050 g (0.28 mmol) of diphenylacetylene
was added as a solution in approximately 2 mL of toluene. The
solution was stirred at room temperature overnight before the solvent
was removed in vacuo, and the yellow residue was recrystallized from
diethyl ether:pentane to yield 60% of brown crystals identified as
(C5H3-1,2-Me2)2Ti(η2-PhCCPh). Method 2: A 20 mL scintillation
vial was charged with a stirbar, 0.050 g (0.19 mmol) of (C5H3-1,2-
Me2)2TiCl, 0.036 g (0.20 mmol) of diphenylacetylene, and 0.044 g
(1.90 mmol) of magnesium powder. The brown solution was stirred
overnight before being filtered through Celite and recrystallized from
diethyl ether to afford (C5H3-1,2-Me2)2Ti(η2-PhCCPh) in 75% yield.
Anal. Calcd for C42H46Ti: C, 81.55; H, 6.84. Found: C, 81.58; H, 6.65.
1H NMR (benzene-d6, 23 °C): δ 1.80 (s, 12H, C5H3-1,2-Me2), 5.37 (t,
1
3
diastereoisomer. H NMR (benzene-d6, 23 °C): δ 1.00 (d, JHH
=
3
6.5, 3H, C5H3-1,3-Me-iPr), 1.02 (d overlapped, JHH = 6.5, 6H, C5H3-
1,3-Me-iPr), 1.05 (d, JHH = 6.5, 3H, C5H3-1,3-Me-iPr), 1.76 (s, 3H,
3
C5H3-1,3-Me-iPr), 1.78 (s, 3H, C5H3-1,3-Me-iPr), 2.29 (m overlapped,
1H, C5H3-1,3-Me-iPr), 2.32 m overlapped, 1H, C5H3-1,3-Me-iPr), 4.37
(m overlapped, 1H, C5H3-1,3-Me-iPr), 4.39 (m overlapped, 2H, C5H3-
1,3-Me-iPr), 4.42 (m overlapped, 1H, C5H3-1,3-Me-iPr), 4.43 (m
overlapped, 1H, C5H3-1,3-Me-iPr), 4.44 (m overlapped, 1H, C5H3-1,3-
Me-iPr). {1H}13C NMR (benzene-d6, 23 °C): δ 15.8, 15.9 (C5H3-1,3-
Me-iPr), 24.3, 24.3, 25.2, 25.3 (C5H3-1,3-Me-iPr), 29.2, 30.6, (C5H3-
1,3-Me-iPr), 91.0, 93.4, 93.7, 94.1, 94.3, 106.8, 106.9, 121.1, 121.2,
129.7 (C5H3-1,3-Me-iPr), 263.7, 263.8 (Ti-CO). IR(benzene-d6): νCO
= 1960, 1882, 1850 cm−1.
3
3JHH = 3.0, 2H, C5H3-1,2-Me2), 5.91 (d, JHH = 3.0, C5H3-1,2-Me2),
6.70 (m, 2H, Ar−CH), 6.95 (m, 4H, Ar−CH), 7.06 (m, 4H, Ar−CH).
{1H}13C NMR (benzene-d6, 23 °C): δ 14.4 (C5H3-1,2-Me2), 109.9
(C5H3-1,2-Me2), 113.5 (C5H3-1,2-Me2), 125.9 (aryl CH), 127.5
(C5H3-1,2-Me2), 128.4 (aryl CH), 132.2 (aryl CH), 142.2 (aryl C),
198.2 (Ti-C).
Preparation of [(η5-C5H3-1,3-iPr)2Ti(THF)][BPh4]. A 20 mL
scintillation vial was charged with 0.200 g (0.278 mmol) of [(η5-
C5H3-1,3-iPr)2Ti]2(η2,η2-N2), a stirbar, and 5 mL of tetrahydrofuran.
To this dark purple solution was added a suspension containing 0.260
g (0.620 mmol) of AgBPh4 in 5 mL of tetrahydrofuran. The resulting
dark purple solution was stirred at ambient temperature for 18 h. The
volatiles were removed in vacuo, and the brown residue was dissolved
in approximately 10 mL of toluene and filtered through Celite. The
filtrate was removed, and the resultant brown oil was recrystallized
from fluorobenzene at −35 °C to afford 0.300 g (74%) of dark green
needles identified as [(η5-C5H3-1,3-iPr)2Ti(THF)][BPh4]. Anal. Calcd
for C50H61OBTi: C, 81.41; H, 8.47; N, 0.00. Found: C, 81.38; H, 8.33;
Spectroscopic Characterization of [(C5H3-1,2-Me2)2Ti(μ2-C-
(O)H)Ti(C5H3-1,2-Me2)(η5,η1-C5H2-1,2-Me2) (3). A thick walled glass
vessel was charged with 0.045 g (0.10 mmol) of [(C5H3-1,2-
Me2)2Ti(μ2-H)Ti(C5H3-1,2-Me2)(η5,η1-C5H2-1,2-Me2) and approxi-
mately 5 mL of benzene. The dark green solution was degassed at the
vacuum line, and CO gas was added via calibrated gas bulb (71 Torr
from a 100.1 mL gas bulb, 0.40 mmol). The dark green solution was
shaken vigorously, and the color changed to dark brown upon mixing.
1H and 13C NMR analysis of the mixture revealed the presence of 3
and a small amount of (η5-C5H3-1,2-Me2)2Ti(CO)2. 1H NMR
3680
dx.doi.org/10.1021/om300156z | Organometallics 2012, 31, 3672−3682

Organometallics
Article
(benzene-d6, 23 °C): δ 1.64 (s, 3H, C5H3-1,2-Me2), 1.70 (s, 3H, C5H3-
1,2-Me2), 1.81 (s, 6H, C5H3-1,2-Me2), 1.86 (s, 6H, C5H3-1,2-Me2),
1.90 (s, 3H, C5H3-1,2-Me2), 2.22 (s, 3H, C5H3-1,2-Me2), 4.87 (m, 1H,
C5H3-1,2-Me2), 5.10 (m, 1H, C5H3-1,2-Me2), 5.29 (m, 1H, C5H3-1,2-
Me2), 5.32 (m, 1H, C5H3-1,2-Me2), 5.39 (m, 1H, C5H3-1,2-Me2), 5.71
(m, 1H, C5H3-1,2-Me2), 5.81 (m, 1H, C5H3-1,2-Me2), 6.00 (m, 1H,
C5H3-1,2-Me2), 6.17 (m, 1H, C5H3-1,2-Me2), 6.61 (m, 1H, C5H3-1,2-
(d) Hanna, T. E.; Keresztes, I.; Lobkovsky, E.; Chirik, P. J. Inorg.
Chem. 2007, 46, 1675.
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dinitrogen complex with alkali metal coordination see: Pun, D.;
Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2008, 130, 6047.
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1
Me2), 7.43 (m, 1H, C5H3-1,2-Me2), 7.91 (d, JCH = 141.5, 1H, C(O)-
H). {1H}13C NMR (benzene-d6, 23 °C): δ 12.6, 13.4, 13.6, 13.6, 14.0,
14.2, 14.6, 14.6, 14.6 (C5H3-1,2-Me2), 89.4, 94.5, 98.9, 106.8, 107.1,
111.3, 112.9, 114.2, 115.0, 115.6, 115.7, 115.8, 119.2, 121.1, 122.6,
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Preparation of (η5-C5H3-1,2-Me2)2Ti(CO)2. A thick walled glass
vessel was charged with a stirbar, 0.040 g (0.15 mmol) of (C5H3-1,2-
Me2)2TiCl, and 0.020 g (0.75 mmol) of magnesium powder. The
vessel was removed from the glovebox, brought to the high vacuum
line, and evacuated. Approximately 10 mL of diethyl ether was added
by vacuum transfer, and 1 atm of CO gas was added at 77 K. The
green-brown solution was stirred for 18 h at room temperature. The
vessel was degassed and brought into the box, and the solution was
filtered through Celite to remove excess magnesium. The residue was
recrystallized from diethyl ether at −35 °C to afford (C5H3-1,2-
Me2)2Ti (CO)2 as an analytically pure brown powder in 80% yield.
Anal. Calcd for C16H18O2Ti: C, 66.22; H, 6.25. Found: C, 65.88; H,
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5.99. H NMR (benzene-d6, 23 °C): δ 1.66 (s, 12H, C5H3-1,2-Me2),
4.41 (t, 3JHH = 3.0, 2H, C5H3-1,2-Me2), 4.56 (d, 3JHH = 3.0, 4H, C5H3-
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ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic details for 1, [(η5-C5H3-1,3-iPr2)2Ti]2(η2,η2-
N2), [(η5-C5H3-1-iPr-3-Me)2Ti]2(η2,η2-N2), [(η5-C5H3-
1,3-iPr)2Ti(THF)][BPh4], and [(η5-C5H3-1,2-Me2)2Ti(μ2-H)-
Ti(η5-C5H3-1,2-Me2)(η5,η1-C5H2-1,2-Me2) in cif format and
additional spectroscopic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
̈
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AUTHOR INFORMATION
Corresponding Author
*E-mail: pchirik@princeton.edu.
Notes
■
The authors declare no competing financial interest.
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Chirik, P. J. Organometallics 2004, 23, 3446.
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ACKNOWLEDGMENTS
■
We thank the Director of the Office of Basic Energy Sciences,
Chemical Sciences Division, U.S. Department of Energy (DE-
FG-02-05ER15659) for financial support. S.P.S. thanks the
Natural Sciences and Engineering Research Council of Canada
for a predoctoral fellowship (PGS-D). C.M. acknowledges the
Alexander von Humboldt Foundation for a Feodor Lynen
postdoctoral fellowship and ACS-Frasch foundation for
financial support.
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