Ion pairing in Ti(IV) trisamidotriazacyclononane compounds

Article abstract of DOI:10.1021/ic0510258

[Ti{N(Ph)SiMe₂}₃-tacn]X complexes (X = Cl, 1; I, 2; PF₆, 3; BPh₄, 4) were studied by NMR and electron absorption and emission methods, which showed that these compounds exist in bromobenzene and dichloromethane solutions as ion pairs. The significant modifications observed in the proton resonances of tacn in C₆D ₅Br, which follow the sequence BPh₄^- ≥ PF₆^- ≥ I^- ≈ Cl^-, are a qualitative indication of the strength of the interactions that depend on the anion. The reaction of 2 with LiNMe₂ led to [Ti(NPh){NPh(SiMe ₂)}₂-tacn], 5, that forms upon attack of Me ₂N^- at one SiMe₂ group. The formation of 5 is discussed on the basis of the interactions identified in solution.

Full text of DOI:10.1021/ic0510258

Inorg. Chem. 2005, 44, 9017−9022
Ion Pairing in Ti(IV) Trisamidotriazacyclononane Compounds
Ana M. Martins,* Jose´ R. Ascenso,* S´ılvia M. B. Costa, Alberto R. Dias, Humberto Ferreira, and
Jose´ A. B. Ferreira
Centro de Qu´ımica Estrutural, Complexo Interdisciplinar, Instituto Superior Te´cnico,
AV. RoVisco Pais, 1, 1049-001 Lisboa, Portugal
Received June 22, 2005
[Ti{N(Ph)SiMe2}3-tacn]X complexes (X ) Cl, 1; I, 2; PF6, 3; BPh4, 4) were studied by NMR and electron absorption
and emission methods, which showed that these compounds exist in bromobenzene and dichloromethane solutions
as ion pairs. The significant modifications observed in the proton resonances of tacn in C6D5Br, which follow the
-
-
sequence BPh4
on the anion. The reaction of 2 with LiNMe2 led to [Ti(NPh)
Me2N- at one SiMe2 group. The formation of 5 is discussed on the basis of the interactions identified in solution.
g
PF6
g
I-
≈
Cl-, are a qualitative indication of the strength of the interactions that depend
NPh(SiMe2) 2-tacn], 5, that forms upon attack of
{
}
Introduction
counterion participation.11 Despite the success in reducing
or modifying the nature of ion-pair interactions, these are
intimately related to the driving force of key aspects of
chemical processes such as conversion and stereo-
chemistry.12-14
The relevance of ion pairing in determining the structures
and reactivity of transition metal complexes is now widely
recognized and sustained by a rising number of reports.1-4
Strongly related to the latest and most significant advances
in the understanding of ion pair interactions are NMR
methodologies, as NOE and PGSE, 5-8 which give informa-
tion about solution structural features and allow the com-
parison between solution and solid-state structures, where
packing forces may originate specific interactions, not
necessarily relevant to the reactivity in solution.
The interference of halide and formerly thought “non-
coordinating” anions in many stoichiometric and catalytic
reactions9,10 pushed the search for weakly coordinating
anions, intended to promote the dissolution of ionic com-
pounds in nonpolar solvents and also to explore the reactivity
of the cation in condensed phase without, or with minimal,
Considering transition metal hydrides, hydrogen bonding
revealed to be critical to many aspects of their chemistry
such as the equilibrium ratio between isomers, the pathways
of protonation reactions, the stability of dihydrogen com-
plexes, and the kinetic features of dihydrogen complexes
reactions.15,16
Studies on Diels-Alder reactions17 and homogeneous
polymerization18-21 also showed the dependence of the
activity and the selectivity on ion pair interactions.
In this work, we present NMR and UV-vis data for Ti(IV)
trisamido-tacn cations of general formula [Ti{N(Ph)SiMe2}3-
tacn]X and discuss ion pair interactions in those complexes.
These results are compared to the solid-state structures of
[Ti{N(Ph)SiMe2}3-tacn]X (X ) I, BPh4). The reaction of
* To whom correspondence should be addressed. Tel: +351-
218419284. Fax: +351-218464457. E-mail: ana.martins@ist.utl.pt.
(1) Macchioni, A. Chem. ReV. 2005, 105, 2039.
(2) Epstein, L. M.; Shubina, E. S. Coord. Chem. ReV. 2002, 231, 165.
(3) Braga, D.; Grepioni, F.; Desiraju, G. R. Chem. ReV. 1998, 98, 1375.
(4) Macchioni, A. Eur. J. Inorg. Chem 2003, 195.
(5) Beck, S.; Geyer, A.; Brintzinger, H. H. J. Chem. Soc., Chem. Commun.
1999, 2477.
(11) Krossing, I.; Raabe, I. Angew. Chem., Int. Ed. 2004, 43, 2066.
(12) Adam, W.; Roschmann, K. J.; Saha-Mo¨ller, C. R.; Seebach, D. J. Am.
Chem. Soc. 2002, 124, 5068.
(13) Zuccaccia, D.; Sabatini, S.; Bellachioma, G.; Cardaci, G.; Clot, E.;
Macchioni, A. Inorg. Chem. 2003, 42, 5465.
(6) Valentini, M.; Pregosin, P. S.; Ru¨egger, H. Organometallics 2000,
19, 2551.
(7) Binotti, B.; Macchioni, A.; Zuccaccia, C.; Zuccaccia, D. Comments
Inorg. Chem. 2002, 23, 417.
(8) Valentini, M.; Pregosin, P. S.; Ru¨egger, H. J. Chem. Soc., Dalton
Trans. 2000, 4507.
(14) Drago, D.; Pregosin, P. S.; Pflatz, A. Chem. Commun. 2002, 286.
(15) Fong, T. P.; Forde, C. E.; Lough, A. J.; Morris, R. H.; Rigo, P.;
Rocchini, E.; Stephan, T. J. Chem. Soc., Daton Trans. 1999, 4475.
(16) Basallote, M. G.; Besora, M.; Du´ran, J.; Fernandez-Trujillo, M. J.;
Lledo´s, A.; Man˜ez, M. A.; Maseras, F. J. Am. Chem. Soc. 2004, 126,
2320.
(9) Fagnou, K.; Lautens, M. Angew. Chem., Int. Ed. 2002, 41, 26.
(10) Beck, W.; Su¨nkel, K. Chem. ReV. 1988, 88, 1405.
(17) Kumar, P. G. A.; Pregosin, P. S.; Vallet, M.; Bernardinelli, G.; Jazzar,
R. F.; Viton, F.; Ku¨ndig, E. P. Organometallics 2004, 23, 5410.
10.1021/ic0510258 CCC: $30.25
Published on Web 10/27/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 24, 2005 9017

Martins et al.
Scheme 1
As shown in Figure 1 and Table 1, at 23 °C, the tacn
methylenic proton resonances and the dimethylsilyl proton
resonances of complexes [Ti{N(Ph)SiMe2}3-tacn]X are
strongly dependent on the solvent and counterions.
In d5-bromobenzene, all complexes display an AA′BB′
pattern for the methylenic Hsyn and Hanti resonances that is
maintained in CD2Cl2, except for 3 that leads to a deceptively
simple AA′BB′ spectrum. In CD3CN, the Hsyn and Hanti
resonances of 1 and 2 appear superimposed in an apparent
singlet whereas 3 and 4 give rise to a more complex AA′BB′
pattern. In d2-dichloromethane and d5-bromobenzene, Hsyn
resonances appear at higher field than Hanti for complexes
1, 2, and 3, although for complex 4, those resonances invert
their relative positions, with Hanti being observed at higher
field than Hsyn. In both solvents, Hsyn and Hanti resonances
are more shielded in complex 4 than in complexes 1, 2, and
3. The shifts to high field registered in all complexes are
[Ti{N(Ph)SiMe2}3-tacn]I with LiNMe2 is also reported and
discussed in the light of the interactions described.
Results and Discussion
Cationic [Ti{N(Ph)SiMe2}3-tacn]X (X ) Cl, 1; I, 2; PF6,
3; BPh4, 4; tacn ) triazacyclononane) have been recently
reported by some of us.22 The complexes, which are readily
prepared by oxidation of the neutral Ti(III) precursor, were
characterized by X-ray diffraction (2 and 4) and NMR that
confirmed the titanium coordination to six nitrogen atoms
in a distorted trigonal prismatic geometry.
more pronounced for Hanti than for Hsyn
.
The NOESY spectrum of complex 4 in CD2Cl2 (Figure
2) shows (i) strong interactions between the ortho- and meta-
phenyl anion protons (δ 7.34 and 7.06, respectively) and the
cation Hanti resonance (δ 3.08-2.96), (ii) weak interaction
between the ortho-Ph anion protons (δ 7.34) and the cation
Hsyn (δ 3.36-3.24), and (iii) medium interactions between
all BPh4- protons and the Si(CH3)2 protons (δ 0.41). These
In solution, a fluxional process consisting in the trigonal
twist of the amido and amine nitrogen planes is accompanied
by a change in the conformation of the five-member Ti-
NCH2CH2N metallocycles, in such a way that the overall
process corresponds to the intramolecular conversion of
enantiomers (∆(λλλ)/Λ(δδδ)) (Scheme 1). An analogous
equilibrium was identified for [Ti{N(Ph)SiMe2}3-tacu]I (tacu
) triazacycloundecane).23 In this case, due to the different
macrocycle chain lengths (C2 and C3), the global fluxional
process has a further component that is the boat/chair
conversion of the six-member metallocycles formed by the
bonding of the NC3N moieties to the titanium. Although a
static 1H spectrum was not obtained until -70 °C in CD2Cl2
and overlapping resonances did not allow the estimation of
activation parameters, variable-temperature NMR studies
allowed the differentiation of the three independent processes
and gave information about their limit temperatures. δ/λ
isomerization is the higher-activation-energy process, being
blocked at -40 °C, ∆/Λ equilibrium stops at -70 °C and
the chair/boat conversion is still operative at -70 °C.
In addition to the intramolecular processes mentioned, the
solutions of cations [Ti{N(Ph)SiMe2}3-tacn]X also disclose
ion pair formation that have a deep influence on the 1H NMR
and UV-vis absorption and emission spectra.
-
results show that the BPh4 anions are located at the
macrocycle side, close to Hanti and to the dimethylsilyl
groups, with which nonbonded interactions between atoms
of the two ionic moieties are the strongest exhibited (Scheme
-
2). The lack of NOE between the BPh4 and NPh protons
further confirms the anion preference to locate on the side
of the triazacyclononane ring.
Thus, the shifts to high field observed for the macrocyclic
-
protons are consistent with the BPh4 ring current effects.
The outcome is mainly noticed in the Hanti resonance because
the anion is closer to Hanti than to Hsyn protons. The shielding
of Si(CH3)2 signals, more pronounced in d5-PhBr than in
dichloromethane, also reflect the proximity of the anion and,
as expected due to the lower dielectric constant of bro-
mobenzene, ion pairing is more effective in this solvent than
in dichloromethane.
To evaluate the importance of ion pairing in the solutions
of complexes 2 and 3, absorption and emission spectra were
recorded in CH2Cl2 and CH3CN (Figures 3 and 4). In
CH3CN, the two electronic absorption spectra are identical,
with maximum, λa, at 285 nm for 2 and at 286 nm for 3.
Electronic excitation at higher and lower energies relative
to the maximum of the band centered at λa around 285 nm
produces similar emission spectra. Accordingly, lumines-
cence collected at higher and lower energies relative to the
emission maximum also gives rise to similar spectral
distributions corresponding well to the respective absorption
profiles. Thus, the 2 and 3 emitting states in CH3CN can be
ascribed to the directly excited (relaxed) electronic states of
the cation. In CH3CN, the emission spectra of 2 displays
more intense fluorescence than that of 3. In spite of the
(18) Song, F.; Lancaster, S. J.; Cannon, R. D.; Schormann, M.; Humphrey,
S. M.; Zuccaccia, C.; Macchioni, A.; Bochmann, M. Organometallics
2005, 24, 1315.
(19) Zuccaccia, C.; Stahl, N. G.; Macchioni, A.; Chen, M.-C.; Roberts, J.
A.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 1448.
(20) Babushkin, D. E.; Brintzinger, H. H. J. Am. Chem. Soc. 2002, 124,
12869.
(21) Chen, Y.-X. E.; Metz, M. V.; Li, L.; Stern, C. L.; Marks, T. J. J. Am.
Chem. Soc. 1998, 120, 6287.
(22) Dias, A. R.; Martins, A. M.; Ascenso, J. R.; Ferreira, H.; Duarte, M.
T.; Henriques, R. T. Inorg. Chem. 2003, 42, 2675.
(23) Martins, A. M.; Ascenso, J. R.; Azevedo, C. G.; Dias, A. R.; Duarte,
M. T.; Ferreira, H.; Ferreira, M. J.; Henriques, R. T.; Lemos, M. A.;
Li, L.; Ferreira da Silva, J. L. Eur. J. Inorg. Chem. 2005, 1689.
-
extremely high dielectric constant of the solvent, the PF6
anion may act as a more efficient quencher, thereby
9018 Inorganic Chemistry, Vol. 44, No. 24, 2005

Ion Pairing in Ti Trisamidotacn Complexes
1
Figure 1. Selected H methylenic resonances of [Ti{N(Ph)SiMe2}3-tacn]X in CH3CN, CD2Cl2, and d5-bromobenzene.
1
Table 1. Selected H NMR Data for [Ti{N(Ph)SiMe2}3-tacn]X (X ) Cl, 1; I, 2; PF6, 3; BPh4, 4)
δ Hanti
δ Hsyn
δ Si(CH3)2
CD3CN
3.59
3.57
3.62-3.44
3.61-3.41
CD2Cl2
C6D5Br
CD3CN
3.59
3.57
3.62-3.44
3.61-3.41
CD2Cl2
C6D5Br
CD3CN
CD2Cl2
C6D5Br
1
2
3
4
4.10-4.00
4.00-3.90
3.61
4.96-4.78
4.70-4.50
3.86-3.73
2.93-2.78
3.60-3.50
3.65-3.55
3.61
4.00-3.80
3.95-3.75
3.72-3.69
3.20-3.05
0.48
0.48
0.48
0.48
0.45
0.47
0.49
0.41
0.76
0.71
0.62
0.33
3.08-2.96
3.36-3.24
reducing the emission of 3. In CH2Cl2, both compounds
display very broad additional absorption bands with λa at
325 nm for 2 and at 320 nm for 3, consistent with stronger
anion-cation interactions and higher extinction coefficients
in the main absorption band. The shifts to higher wavelength
and broadening registered in CH2Cl2 vs CH3CN are indicative
of the existence of contact ion pairs in dichloromethane, and
the higher shift for 3 reflects a stronger anion-cation
interaction in this complex.24 This conclusion was supported
for 2 and 3 in CH2Cl2 by the lack of fluorescent emission
that is probably strongly quenched by charge-transfer
processes between the ions.25,26
of Hanti and Si(CH3)2 resonances. In CH2Cl2 and CD3CN,
the methylenic protons NMR patterns observed are not
symptomatic.
The differences observed in the proton NMR spectra of
complexes 1 and 2 in CD2Cl2 and C6D5Br are slight and
suggest that, regardless the solvent, Cl- and I- interact
similarly with the macrocyclic protons. However, a minor
effect that is reflected in both solvents by a small shielding
of Hanti in 2 may be noted (in CD2Cl2, δHanti is 4.25 in 1 and
3.95 in 2 and in C6D5Br, δHanti is 4.87 in 1 and 4.61 in 2,
whereas δHsyn is the same for 1 and 2: 3.60 in CD2Cl2 and
3.90 in C6D5Br). Although the significance of this difference
is questionable, it is consistent with the higher polarizability
The occurrence of ion pair association in C6D5Br solutions
of 3 is clearly expressed in the significant high-field shifts
(25) Lucia, L. A.; Abboud, K.; Schanze, K. S. Inorg. Chem. 1997, 36,
6224.
(24) Sotomayor, J.; Santos, H.; Pina, F. Can. J. Chem. 1991, 55, 1019.
(26) McCosar, B. H.; Schanze, K. S. Inorg. Chem. 1996, 35, 6800.
Inorganic Chemistry, Vol. 44, No. 24, 2005 9019

Martins et al.
1
Figure 2. Section of the H NOESY spectrum of [Ti{N(Ph)SiMe2}3-tacn]BPh4 showing the interaction between the anion and the cation.
Scheme 2
macrocyclic Hanti protons are 3.593 (I-H(5)) and 3.256 Å
(I-H(4)). Taking into account the difference between carbon
and iodine covalent radii, the two anions approach the cation
similarly, and given that stereochemical constraints are less
important in the case of I-, its proximity to Si(CH3)2 is most
probably the reflection of the electrophilicity of this fragment.
To test this assumption, we carried out reactions of 2 with
nucleophiles. Although we have not been able to isolate any
product from the reaction with LiCH3, the reaction of
[Ti{N(Ph)SiMe2}3-tacn]I, 2, with LiNMe2 led to the forma-
of I- versus Cl- and may thus reflect a preference of I- to
be closer to the cation.
tion of [Ti(NPh){NPh(SiMe2)}2-tacn], 5, which results from
the nucleophilic attack of dimethylamido on one SiMe2
pendant group (Scheme 3). The cleavage of the Si-NPh
bond, leading to the formation of an imido ligand, is a
consequence of the addition of NMe2. The intermediate
aminal formed decomposes, giving rise to a neutral secondary
amine function in the tacn ring. The origin of this reaction
is uncertain. Although accidental hydrolysis may not be ruled
out, the reproducibility and low yield (η ca. 30%) observed
for the reaction may suggest an intrinsic instability of the
aminal. Indeed, this result sustains our previous observations
on the lack of success in the synthesis of {NH(R)SiMe2}3-
tacn compounds with R groups other than Ph, suggesting
that the stabilization of the N-Si-N fragment requires the
presence of electron acceptor groups on the nitrogen.
The reaction sequence shown in Scheme 3 is straightfor-
wardly reasoned on the basis of the ion pair interactions
discussed for complexes [Ti{N(Ph)SiMe2}3-tacn]X.
The ion pair structure identified in solution for 4 is also
observed in the solid-state structure (Figure 4) that shows
the proximity of the anion phenyl rings and the macrocycle.22
The orientation of one of the phenyls is such that one Hanti
(H(1)) points to the aromatic π system, with short contact
distances to the meta and para phenyl carbons (C(75)) and
C(74)) and protons (H(75) and H(74)). This arrangement
approaches the ortho proton of the same phenyl ring (H(76))
to the methyl protons of the Si(CH3)2 group (H(8)). Short
contact distances are also observed between the ortho and
meta protons of other anion phenyl group (H(52) and H(53))
-
and the Si(CH3)2 protons H(8) (Table 2) and between BPh4
Hortho (H(42A)) and macrocyclic Hanti (2.812 Å) and Hsyn
(3.896 Å) generated by symmetry in the solid-state structure,
which allow for the strongest spatial interactions observed
in the NOESY spectrum.
In the solid-state structure of 2, iodine is located in a “hole”
defined by the atoms CH2-N(SiMe2)-CH2 (Figure 5).
The shortest distance between I- and the dimethylsilyl
protons is 3.179 Å (I-H(18)) and the distances to the
1
The H NMR data for 5 display two resonances for the
methyl groups of the SiMe2 fragments, six multiplets for the
methylenic protons, two sets of aromatic resonances integrat-
ing 2:1 for the phenyl rings, and one signal at 3.94 ppm
9020 Inorganic Chemistry, Vol. 44, No. 24, 2005

Ion Pairing in Ti Trisamidotacn Complexes
Figure 3. (1)[Ti{N(Ph)SiMe2}3-tacn]I, (2) [Ti{N(Ph)SiMe2}3-tacn]PF6: (a) electronic absorption, (b) emission, λexc) 280 nm and (c) excitation λem ) 350
nm spectra in acetonitrile, and (d) electronic absorption in dichloromethane. (a) and (d) right scale (A); (b) and (c) left scale (Ie).
Two possible isomers may be drawn for 5. One, presenting
Cs symmetry, would display the imido ligand and the tacn
NH fragment in the trans position, 5a (shown in Scheme 3),
the other, having a NSi tacn amine trans to the imido ligand,
is a chiral molecule, 5b (Scheme 4). In solution, however,
5b may be involved in a fluxional process that creates an
average Cs symmetry, as represented in Scheme 4. The
distinction between the two structures would be possible if
low-temperature NMR experiments stopped the fluxional
process, which did not happen until -90 °C. The assignment
of a definite structure to 5 is therefore not possible. (The
authors believe that, by analogy to other unpublished
compounds which structures have been characterized by
X-ray diffraction, 5b reflects the actual structure of the
complex.)
Figure 4. Section of the ORTEP representation of 4 showing the proximity
of two anion phenyl rings and the macrocycle (N(1), C(1), and C(6)) and
one pending arm (Si(1), C(7), and C(8)).22
Conclusion
NMR and UV-vis experiments have shown that com-
plexes [Ti{N(Ph)SiMe2}3-tacn]X (X ) Cl, 1; I, 2; PF6, 3;
BPh4, 4) exist in bromobenzene and dichloromethane solu-
-
tions as ion pairs. The proximity of the BPh4 anion to the
Hanti methylenic protons of tacn and to the bridging Si(CH3)2
protons was deduced from the NOESY spectrum of 4 in CD2-
-
Cl2. The interactions between the iodine and PF6 anions
and the cation were supported by electron absorption and
emission measurements in CH2Cl2 and CH3CN. The results
showed a stronger interaction in complex 3 solutions and
also pointed out the influence of the solvent dielectric
constant and polarizability. Ion pairing is responsible for the
differences observed in the proton resonances of tacn and
Si(CH3)2 in CD3CN, CD2Cl2 and C6D5Br. These differences
are particularly significant in C6D5Br, the less-polar solvent,
where the macrocyclic Hanti and the Si(CH3)2 proton reso-
Figure 5. Section of the ORTEP representation of 2 showing the location
of the anion between the macrocycle (N(3), C(4), and C(5)) and one pending
arm (Si(3), C(17), and C(18)).22
Table 2. Shortest Contact Distances (Å) for 4
H(anion)‚‚‚H(cation)
H(76)‚‚‚H(8)
H(75)‚‚‚H(1)anti
H(74)‚‚‚H(1)anti
H(52)‚‚‚H(8)
H(53)‚‚‚H(8)
C(anion)‚‚‚H(cation)
C(76)‚‚‚H(8)
C(75)‚‚‚H(1)anti
C(74)‚‚‚H(1)anti
C(52)‚‚‚H(8)
C(53)‚‚‚H(8)
3.111
2.666
2.647
2.899
3.057
3.198
2.841
2.839
2.763
2.885
-
nances are progressively shifted from 1 to 4 (BPh4- g PF6
g I- ≈ Cl-). The interactions with the macrocyclic protons
are consistent with the trend observed for other nitrogen-
based ligands such as diimine and (polypyrazolyl)-borate and
-methane, for which a preferential interaction with the protons
R to the nitrogen donors are observed.28,29 The results
corresponding to the NH proton of tacn. The 13C NMR
spectrum is consistent with the Cs symmetry observed in the
proton NMR spectrum. The ipso carbon of the imido ligand
appears at 161.0 ppm, in agreement with values for Tid
NPh complexes.27
(28) Binotti, B.; Bellachioma, G.; Cardaci, G.; Macchioni, A.; Zuccaccia,
C. Organometallics 2002, 21, 346.
(27) Blake, A. J.; Collier, P. E.; Dunn, S. C.; Li, W. S.; Mountford, P.;
(29) Bellachioma, G.; Cardaci, G.; D’Onofrio, F.; Macchioni, A.; Sabatini,
S.; Zuccaccia, C. Eur. J. Inorg. Chem. 2001, 1601.
Shishkin, O. V. J. Chem. Soc., Dalton Trans. 1997, 1549.
Inorganic Chemistry, Vol. 44, No. 24, 2005 9021

Martins et al.
Scheme 3
Scheme 4
sion and excitation spectra were recorded using a Perkin-Elmer
spectrofluorimeter (model LS-50B). Absorption and fluorescence
10-mm quartz cells were used. Temperature was kept within T )
296 ( 1 K using circulating water streams on both apparatus’ cell
holders. Samples (5 mL) were prepared from stock solutions of
each solvent from where aliquots were taken and diluted by addition
of pure solvent. The final concentration was kept at 30 µM (to
avoid aggregation and inner filter effects). Fresh samples were used
in all the measurements. Elemental analyses were obtained from
Laborato´rio de Ana´lises do IST, Lisbon, Portugal. [Ti{N(Ph)-
SiMe2}3-tacn]X (1-4) were prepared according to described
procedures.22
described also provide an interpretation for the unusual ring
opening of a tacn-derived amido ligand ([iPr2-tacn]-) reported
by Arnold et al., where an intramolecular acid-base reaction
between one benzyl ligand and one [iPr2-tacn]- proton is
responsible for the elimination of toluene and the formation
of a new acyclic diamino-amino pincer ligand.30 The
interactions with dimethylsilyl protons, on the other hand,
have not been previously reported, but we estimate that it
may be common on the basis of the reactivity displayed by
dimethylsilylsilylamido metal derivatives that are prone to
dimethylsilylsilyl C-H bond cleavage.31 The formation of
5 is thus a direct consequence of the steric protection of
titanium and the positive charge at silicon.
[Ti(NPh){NPh(SiMe2)}2-tacn], (5).
A THF solution of
[Ti{N(Ph)SiMe2}3-tacn]I (1.62 g, 2.16 mmol) cooled at -60 °C
was treated with a suspension of LiNMe2 (0.12 g, 2.27 mmol) in
10 mL of THF. The temperature was allowed to rise slowly to room
temperature, and the mixture was stirred during 24 h. The volatiles
were evaporated to dryness, and the residue was washed with a
small portion of cold hexane. The product was extracted in diethyl
ether, and a concentrate solution was cooled at -20 °C to afford
1
3
0.36 g of 5. Yield, 30%. H NMR (C6D6): δ 7.32 (t, 4H, JHo-Hm
)
3JHp-Hm ) 7.8 Hz, Hm, Me2SiNPh), 7.27 (t, 2H, JHo-Hm
)
3
3JHp-Hm ) 6.6 Hz, Hm, TiNPh), 7.11 (t, 2H, 3JHp-Hm ) 7.8 Hz, Hp,
3
Me2SiNPh), 6.87 (t, 1H, JHp-Hm ) 6.9 Hz, Hp, TiNPh), 6.82 (d,
3
3
4H, JHo-Hm ) 7.8 Hz, Ho, Me2SiNPh), 6.17 ((d, 2H, JHo-Hm
)
Experimental Section
7.8 Hz, Ho, TiNPh), 3.94 (s, 1H, NH), 3.05-2.80 (m, 4H, Hsyn
SiN(CH2)2NSi, Hsyn, SiNCH2CH2NH), 2.55-2.45 (m, 2H, Hsyn
SiNCH2CH2NH), 2.19-1.92 (m, 4H, Hanti, SiN(CH2)2NSi, Hanti
,
,
,
General Procedures and Starting Materials. All reactions were
conducted under a nitrogen atmosphere. Solvents were predried
using 4 Å molecular sieves and refluxed over sodium-benzophe-
none (diethyl ether, tetrahydrofuran, and toluene) or calcium hydride
(n-hexane) under a nitrogen atmosphere and distilled. Deuterated
solvents were dried with molecular sieves and freeze-pump-thaw
degassed prior to use. Proton (300 MHz) and carbon (75.419 MHz)
NMR spectra were recorded in a Varian Unity 300, at 298 K unless
stated otherwise, referenced internally to residual protio-solvent (1H)
or solvent (13C) resonances and reported relative to tetramethylsilane
(0 ppm). 10-2 M solutions of complexes 1-4 were used to run the
NMR spectra. Electronic absorption spectra were recorded using a
Jasco double-beam spectrophotometer (model Jasco V-560). Emis-
SiNCH2CH2NH), 1.75-1.62 (m, 2H, Hanti, SiNCH2CH2NH), 0.32
(s, 6H, SiCH3), 0.20 (s, 6H, SiCH3). 13C-{1H} NMR (C6D6): δ
161.0 (Cipso, TiNPh), 155.8 (Cipso, Me2SiNPh), 128.7 (Cm, Me2-
SiNPh), 128.6 (Cm, TiNPh), 123.3 (Co, TiNPh), 122.7 (Co, Me2-
SiNPh), 118.5 (Cp, Me2SiNPh), 117.9 (Cp, TiNPh), 48.2 (SiNCH2-
CH2NH), 48.0 (SiN(CH2)2NSi), 46.7 (SiNCH2CH2NH), 1.54 (SiCH3),
-1.62 (SiCH3). Anal. Calcd for C28H40N6Si2Ti: C, 59.55; H, 7.14;
N, 14.88. Found: C, 59.36; H, 7.04; N, 14.14.
Acknowledgment. The authors are thankful to Funda-
c¸a˜opara a Cieˆncia e a Tecnologia, Portugal that funded this
work (research project Praxis/C/QUI/39734/2001 and Ph.D.
grant Praxis XXI/ BD/16128/98).
(30) Giesbrecht, G. R.; Shafir, A.; Arnold, J. Chem. Commun. 2000, 2135.
(31) Yu, X.; Bi, S.; Guzei, I. A.; Lin, Z.; Xue, Z.-L. Inorg. Chem. 2004,
43, 7111.
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