Toward the understanding of radical reactions: Experimental and computational studies of titanium(III) diamine bis(phenolate) complexes

Article abstract of DOI:10.1021/ic401008y

Radical reactions of titanium(III) [Ti(^tBu2O ₂NN′)Cl(S)] (S = THF, 1; S = py, 2; ^tBu2O ₂NN′ = Me₂N(CH₂)₂N(CH ₂-2-O-3,5-^tBu₂C₆H₂) ₂) are described. Reactions with neutral electron acceptors led to metal oxidation to Ti(IV), [Ti(^tBu2O₂NN′)Cl(TEMPO)] (4) being formed with the TEMPO radical and [Ti(^tBu2O ₂NN′)Cl₂] (9) with PhN=NPh. [Ti( ^tBu2O₂NN′)Cl₂] was also formed when [Ti(^tBu2O₂NN′)Cl(S)] was oxidized by [Cp ₂Fe][BPh₄], but the [Cp₂Fe][PF₆] analogue yielded [Ti(^tBu2O₂NN′)ClF] (8). The reactions of [Ti(^tBu2O₂NN′)Cl(S)] with O₂ gave [Ti(^tBu2O₂NN′)Cl]₂(μ-O) (3). The DFT calculated Gibbs energy for the above reaction showed it to be exergonic (ΔG₂₉₈ =-123.6 kcal·mol^-1). [Ti( ^tBu2O₂NN′)(CH₂Ph)(S)] (S = THF, 5; py, 6) are not stable in solution for long periods and in diethyl ether gave 1:1 cocrystals of [Ti(^tBu2O₂NN′)(CH₂Ph) ₂] (7) and [Ti(^tBu2O₂NN′)Cl] ₂(μ-O) (3), most probably resulting from a disproportionation process of titanium(III) followed by oxygen abstraction by the resulting Ti(II) species. The oxidation of [Ti(^tBu2O₂NN′) (κ²-{CH₂-2-(NMe₂)-C₆H ₄})] (10), which is a Ti(III) benzyl stabilized by the intramolecular coordination of the NMe₂ moiety, led to a complex mixture. Recrystallization of this mixture under air led to a 1:1 cocrystal of two coordination isomers of the titanium oxo dimer (3). In one of these isomers, one metal is pentacoordinate and the dimethylamine moiety of the diamine bis(phenolate) ligand is not bonded to the metal, displaying a coordination mode of the ligand never observed before. The other titanium center is distorted octahedral with two cis-phenolate moieties. In the second unit, the coordination of the two ancillary ligands to the titanium centers reveals mutually cis-phenolate groups in one-half of the molecule and trans-coordinated in the other titanium center, keeping a distorted octahedral environment around each titanium.

Full text of DOI:10.1021/ic401008y

Article
pubs.acs.org/IC
Toward the Understanding of Radical Reactions: Experimental and
Computational Studies of Titanium(III) Diamine Bis(phenolate)
Complexes
Sonia Barroso,† Filipe Madeira,† Maria Jose Calhorda,‡ M. Joao Ferreira,† M. Teresa Duarte,†
́
́
̃
and Ana M. Martins*,†
†Centro de Química Estrutural, Instituto Superior Tec
́
nico, TU Lisbon, 1049-001 Lisboa, Portugal
cias, Universidade de Lisboa, Campo Grande, Ed. C8, 1749-016 Lisboa,
‡Departamento de Química e Bioquímica, Faculdade de Cien
̂
Portugal
S
* Supporting Information
ABSTRACT: Radical reactions of titanium(III) [Ti(tBu2O2NN′)Cl(S)]
(S = THF, 1; S = py, 2; tBu2O2NN′ = Me2N(CH2)2N(CH2-2-O-3,5-tBu2C6H2)2)
are described. Reactions with neutral electron acceptors led to metal
oxidation to Ti(IV), [Ti(tBu2O2NN′)Cl(TEMPO)] (4) being formed
with the TEMPO radical and [Ti(tBu2O2NN′)Cl2] (9) with PhNNPh.
[Ti(tBu2O2NN′)Cl2] was also formed when [Ti(tBu2O2NN′)Cl(S)] was
oxidized by [Cp2Fe][BPh4], but the [Cp2Fe][PF6] analogue yielded
[Ti(tBu2O2NN′)ClF] (8). The reactions of [Ti(tBu2O2NN′)Cl(S)] with
O2 gave [Ti(tBu2O2NN′)Cl]2(μ-O) (3). The DFT calculated Gibbs energy
for the above reaction showed it to be exergonic (ΔG298 = −123.6 kcal·mol−1).
[Ti(tBu2O2NN′)(CH2Ph)(S)] (S = THF, 5; py, 6) are not stable in
solution for long periods and in diethyl ether gave 1:1 cocrystals of
[Ti(tBu2O2NN′)(CH2Ph)2] (7) and [Ti(tBu2O2NN′)Cl]2(μ-O) (3), most
probably resulting from a disproportionation process of titanium(III) followed by oxygen abstraction by the resulting Ti(II)
species. The oxidation of [Ti(tBu2O2NN′)(κ2-{CH2-2-(NMe2)-C6H4})] (10), which is a Ti(III) benzyl stabilized by the
intramolecular coordination of the NMe2 moiety, led to a complex mixture. Recrystallization of this mixture under air led to a 1:1
cocrystal of two coordination isomers of the titanium oxo dimer (3). In one of these isomers, one metal is pentacoordinate and
the dimethylamine moiety of the diamine bis(phenolate) ligand is not bonded to the metal, displaying a coordination mode of
the ligand never observed before. The other titanium center is distorted octahedral with two cis-phenolate moieties. In the second
unit, the coordination of the two ancillary ligands to the titanium centers reveals mutually cis-phenolate groups in one-half of the
molecule and trans-coordinated in the other titanium center, keeping a distorted octahedral environment around each titanium.
achieved for the catalyst with an adamanthyl substituent).5 More-
INTRODUCTION
■
over, the very narrow molecular weight distributions are
in agreement with the living nature of the polymerization.
Ti(IV) and Zr(IV) complexes with dianionic and trianionic
amine-phenolate type ligands, including the tripodal ones,
have also shown satisfactory results in ROP of L-lactide.6
Again, the relatively narrow polydispersity index values ob-
tained for isotactic polylactide indicate living polymerization
systems.
We have recently explored the chemistry of tripodal diamine
(bis)phenolate complexes in oxidation catalysis and showed that
vanadium compounds are extremely active and selective in the
sulfoxidation of thioethers.7 Furthermore, we described M(III)
complexes of general formula [M(tBu2O2NN′)Cl(S)] (M = Ti, Y;
tBu2O2NN′ = Me2N(CH2)2N(CH2-2-OH-3,5-tBu2C6H2)2; S =
THF, DME) and reported preliminary studies of their reactivity.8
A variety of structural architectures based on the combination
of hard phenolate donors with neutral 2e donors were shown
to stabilize early transition metal and lanthanide complexes.1
The strong interactions established by the phenolate moieties
with those electrophilic metals, in combination with one or two
amine fragments incorporated in the ligand sets, give rise to well-
defined monomeric transition metal compounds when bulky
substituents occupy the ortho-phenolate positions.2 The
investigation of this class of compounds focused mainly on
medicinal and catalytic applications.3 Ti(IV) complexes of
tetradentate diamine bis(phenolate) ligands revealed higher
cytotoxic activity than that measured for cisplatin toward colon
and ovarian tumor cells.3b Ti(IV) and Zr(IV) complexes of this
type were also explored as catalysts of olefin polymerization and
ROP of cyclic esters.4 It was recently reported by Kol and co-
workers that several Ti(IV) salalen-type complexes bearing chiral
moieties exhibit high activity in polymerization of 1-propylene
with very high isotacticities (>99% of isotactic polypropylene
Received: April 23, 2013
Published: August 8, 2013
© 2013 American Chemical Society
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Scheme 1
As part of this program, we report now further reactions that
reveal new reactivity patterns of Ti(III) complexes supported by
the tripodal tBu2O2NN′ donor set.
yield (Scheme 1). [Ti(tBu2O2NN′)Cl]2(μ-O) is a very stable
complex, not susceptible to hydrolysis in the air. The 1H NMR
spectrum of 3 in C6D6 exhibits two doublets (4JHH = 2.2 Hz) for
the aromatic protons, two singlets for the tert-butyl groups, two
multiplets for the protons of the C2 chain linked to the tripodal
nitrogen, one singlet for the CH3 protons of the dimethylamine
fragment, and one AX system (at δ = 5.77 and 3.30 ppm)
assigned to the methylene protons of the NCH2tBu2PhO groups.
This pattern points out a rigid chelation on the NMR time scale,
consistent with C2v symmetry, with mutually trans-phenolate
groups. The molecular structure of 3 determined by single crystal
X-ray diffraction confirms this pattern. An ORTEP drawing of
the structure of 3 is shown in Figure 1, and relevant bond lengths
and angles are displayed in Table 1.
The structure features an oxygen-bridged dinuclear titanium-
(IV) complex with slightly distorted octahedral geometry around
each metal center. The equatorial planes are defined by O1, O2,
and N2 of the diamine bis(phenolate) ligands and the bridging
oxygen O3, while the axial positions are occupied by the tripodal
nitrogens N1 and the chloride ligands Cl1. The two sides of the
molecule are structurally related across a linear oxo bridge by a
rotation of 180°. In each titanium center, the bridging oxygen
and the chloride ligand occupy cis positions. The phenolate
groups present trans-coordination with a dihedral angle of
154.2(1)° between them. The bond distances and angles are
within the expected ranges for titanium(IV) complexes.13 The
linearity of the Ti−O−Ti fragment is in agreement with a Ti−O
multiple bond with a significant π contribution from the oxygen.
The Ti−(μ-O) distance and the Ti−(μ-O)−Ti angle compare
well with values reported for other linear oxo bridged titanium(IV)
complexes such as [Ti(tBu2O2NN′)(OEt)]2−(μ-O).14 The
Ti1−N2 bond length is longer than Ti1−N1 owing to the
trans influence of the μ-oxo group.
The significance of Ti(III) complexes in organic syntheses
is largely documented.9 Pinacol and McMurry reactions as well
as the opening of strained rings, with particular emphasis on
epoxides, are powerful methods for constructing organic
molecules that are often not accessible by other methods.10
These reactions often suffer from reproducibility problems
intrinsic to the substrates or the reduced titanium species that are
commonly generated in situ, by the addition of reducing agents to
titanium(IV) precursors.
Up to now, the stabilization of Ti(III) complexes was mainly
based on biscyclopentadienyl motifs,11 and beyond those, only
a few titanium(III) complexes with silox and triamidoamine
ancillary ligands were reported.12 Taking into account the
successful applications of diamine bis(phenolate) metal
complexes, it is surprising that paramagnetic transition metal
species of this family remain essentially unexplored. We report
here electron-transfer reactions mediated by [Ti(tBu2O2NN′)Cl(S)]
and DFT calculations that complement and clarify its reaction
with O2. This work is thus a contribution to the broadening and
improved understanding of the reactivity of paramagnetic
titanium compounds that is central for the development of
useful applications.
RESULTS AND DISCUSSION
■
Chemical Studies. As previously reported, [Ti(tBu2O2NN′)-
Cl(THF)] (1; tBu2O2NN′ = Me2N(CH2)2N(CH2-2-OH-3,5-
tBu2C6H2)2) is readily prepared by treatment of [TiCl3(THF)3]
with Na2(tBu2O2NN′) in THF. The addition of pyridine to a
solution of 1 leads to THF replacement and the formation of
[Ti(tBu2O2NN′)Cl(py)] (2).8 Reactions of 1 or 2 with oxygen,
carried out in THF solutions by allowing the slow diffusion of air
over a column of anhydrous CaCl2, gave rise to the formation of
the oxo-bridged dimer [Ti(tBu2O2NN′)Cl]2(μ-O) (3), in high
Compounds containing the Ti−O−Ti core are commonly
obtained by controlled hydrolysis of Ti−X bonds (X = halide,
alkyl) of Ti(IV) complexes, but in view of the high oxophilicity of
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is itself a diradical, are exceptionally favored; yet the isolation of
titanium peroxo species by binding of oxygen between two
titanium centers remains elusive.16 This type of intermediate,
which is significant in the context of dioxygen reduction,17 was
claimed to form on the TiO2 surface along the process of photo-
induced water oxidation18 and was characterized for copper
as trans-M−O−O−M by X-ray diffraction.19 On the other
hand, instead of the corresponding peroxo complexes, the
reactions of [Ti(acen)Cl(THF)] (acen = N,N′-ethylenebis-
(acetylacetoneiminate) dianion) or [TiH(ArO)3(PMe3)]
(ArO = 2,6-iPr2PhO) with O2 produce [Ti(acen)Cl]2(μ-O)
and [Ti(OAr)3]2(μ-O), respectively.20 The thermal instability
and great oxidation power of titanium peroxides21 may be
responsible for the restricted number of structurally charac-
terized titanium peroxides.22 The formation of the Ti−O−Ti
core from putative M−O−O−M species formed from reactions
of Ti(III) with O2 may be envisaged as the one electron oxidation
of Ti(III) by titanium peroxides as shown in Scheme 2. The
Figure 1. ORTEP-3 diagram of [Ti(tBu2O2NN′)Cl]2(μ-O) (3), using
40% probability level ellipsoids. The equivalent atoms labeled with # are
generated using the symmetry transformation −x + 1, −y + 1, −z.
Hydrogen atoms are omitted for clarity.
Scheme 2
Table 1. Selected Structural Parameters for 3, 4, 8, and 9
3
4
8
9
thermodynamics of the formation of [Ti(tBu2O2NN′)Cl]2(μ-O)
(3) from [Ti(tBu2O2NN′)Cl]2(μ-O2) was studied by means of
DFT calculations (see below).
distances (Å)
Ti1−O1
Ti1−O2
Ti1−X
Ti1−N1
Ti1−N2
Ti1−Cl1
N3−O3
Ti1-eq. plane
1.869(1)
1.861(1)
1.820(1)
2.246(2)
2.422(2)
2.322(1)
1.894(3)
1.887(3)
1.783(3)
2.228(4)
2.441(4)
2.345(1)
1.407(5)
0.154(2)
1.818(2)
1.832(2)
1.861(1)
2.243(2)
2.297(2)
2.289(1)
1.842(2)
1.833(2)
2.320(1)
2.254(2)
2.291(3)
2.304(1)
Treatment of [Ti(tBu2O2NN′)Cl(THF)] (1) with the nitroxyl
radical TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) pro-
ceeded with radical coupling between titanium and TEMPO
and resulted in the formal reduction of the latter and metal
oxidation to titanium(IV). [Ti(tBu2O2NN′)Cl(TEMPO)] (4)
was isolated as a red/orange crystalline solid in quantitative yield
(Scheme 1). The 1H and 13C NMR spectra of 4 in C6D6 revealed
only one resonance for the methyl groups of TEMPO, in
agreement with κ1-bonding to the titanium in a complex of Cs
symmetry.23 The pattern observed for the diamine bis-
a
b
0.184(1)
0.251(1)
0.191(1)
angles (deg)
O1−Ti1−O2
O1−Ti1−X
O1−Ti1−N1
O1−Ti1−N2
O1−Ti1−Cl1
O2−Ti1−X
O2−Ti1−N1
O2−Ti1−N2
O2−Ti1−Cl1
N1−Ti1−X
166.1(1)
165.6(1)
95.1(1)
84.9(1)
82.7(1)
93.8(1)
95.6(1)
84.5(1)
85.2(1)
93.5(1)
95.0(1)
170.7(1)
87.9(1)
75.8(1)
163.7(1)
101.3(1)
142.9(3)
142.3(3)
95.0(1)
91.1(1)
82.0(1)
158.8(1)
106.3(1)
167.5(1)
84.1(1)
90.9(1)
93.2(1)
86.0(1)
79.7(1)
93.6(1)
78.3(1)
171.5(1)
95.6(1)
135.4(2)
145.3(2)
167.8(1)
95.2(1)
84.0(1)
87.1(1)
92.3(1)
93.5(1)
84.9(1)
82.1(1)
96.0(1)
92.6(1)
167.8(1)
87.7(1)
75.7(1)
163.1(1)
104.1(0)
143.0(1)
144.0(1)
90.2(1)
84.5(1)
86.9(1)
97.5(1)
90.5(1)
83.2(1)
90.5(1)
94.4(1)
91.9(1)
170.4(1)
87.9(1)
78.7(1)
166.4(1)
101.5(0)
145.6(2)
146.0(2)
1
(phenolate) ligand in the H NMR spectrum is analogous to
the one described above for 3, and the NOESY spectrum shows
cross peaks between the tert-butyl groups in the ortho-phenolate
positions and the dimethylamine group of the ancillary ligand,
corroborating static κ4-O2NN′ coordination to the titanium on
the NMR time scale. Cross peaks between the ortho-phenolate
substituents and the methyl groups of TEMPO further attest the
bonding of the latter to titanium.
The structure of 4, depicted in Figure 2, was confirmed by
X-ray diffraction. Relevant bond lengths and angles are displayed
in Table 1. The titanium coordination geometry is once again
distorted octahedral and the trans-phenolate groups determine
the Cs symmetry observed in solution by NMR. The six-membered
ring of the TEMPO ligand is in the chair conformation. The
N1−O3 bond length of 1.407(5) Å confirms the κ1-binding
mode of TEMPO, consistent with an anionic ligand.23 The short
Ti1−O3 bond distance of 1.783(3) Å and large Ti1−O3−N3
angle of 174.1(3)° suggest a strong O−Ti pπ−dπ interaction,24
also in agreement with the reduction of the TEMPO radical by
titanium. This strong interaction is responsible for the elongation
of the Ti1−N2 bond trans to the TEMPO oxygen. Despite the
multiple character of the Ti−O bond, the NMR spectra do not
reveal constraints to the rotation around this bond in solution at
room temperature.
N2−Ti1−X
N2−Ti1−Cl1
N1−Ti1−N2
N1−Ti1−Cl1
X−Ti1−Cl1
Ti1−O1−C11
Ti1−O2−C21
Ti(1)−O3−Ti(1)# 180.0
Ti(1)−O3−N3
174.1(3)
c
θ
154.2(1)
153.4(2)
112.4(1)
176.8(2)
a
b
X = O3 (3, 4); F1 (8); Cl2 (9). The equatorial plane is defined by
atoms O1, O2, O3, and N2. θ is the dihedral angle between the planes
containing the phenolate rings.
c
titanium and the variety of oxygen bonding patterns, these
species may present an array of assemblies that depend on the
ancillary ligands.14,15 Reactions of Ti(III) species with O2, which
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supporting its formulation as a solvent stabilized hexacoordi-
nated species. On the other hand, the results of elemental
analysis of 5 systematically deviated from the theoretical values,
reflecting its extreme instability and decomposition under
analytical experimental conditions. The description of both
complexes as solvent adducts is further reinforced by the
structures of the parent complexes and also of [Ti(tBu2O2NN′)-
(κ2-CH2-2-NMe2-C6H4)], which is stabilized by the coordina-
tion of the dimethylamine substituent on the ortho carbon of
the benzyl ring.8
The EPR spectrum of 5 in toluene at 293 K exhibits two
symmetrical single lines at g1 = 1.951, corresponding to a major
species, and at g2 = 1.963, integrating to a minor compound. The
main signal is attributed to [Ti(tBu2O2NN′)(CH2Ph)(THF)],
and the less intense signal is tentatively assigned to a
pentacoordinated Ti(III) species resulting from THF dissocia-
tion. These data compare with those obtained for the parent
complex [Ti(tBu2O2NN′)Cl(THF)] that displays a similar EPR
spectrum with lines at g = 1.954 and g = 1.962.8 The EPR
spectrum of 6, also obtained in toluene at 293 K, discloses only
one symmetrical line with g = 1.952. As a whole, the results are in
line with the higher instability of 5 relative to 6 and the weak
bonding of THF in comparison with pyridine.
Figure 2. ORTEP-3 diagram of [Ti(tBu2O2NN′)Cl(TEMPO)] (4),
using 40% probability level ellipsoids. Hydrogen atoms are omitted for
clarity.
The Ti−O bonding energy in Ti(TEMPO) complexes, which
is a critical parameter in the homolytic cleavage of Ti−O bonds,
may be readily modulated by the donor ability of the ancillary
ligands.25 Increasing the electron density at the metal, by in-
creasing the number of cyclopentadienyl ligands along the series
TiCpnCl(3−n)(TEMPO) (n = 0, 1, 2), causes a significant decrease
of the Ti−O bonding energy. In consequence, the biscyclo-
pentadienyl complex behaves as a source of Ti(III) as it produces
TiCp2Cl and the nitroxyl radical upon thermal activation.23b In
the case of 4, the cleavage of the Ti−O bond does not lead to the
parent Ti(III) complex but to [Ti(tBu2O2NN′)Cl]2(μ-O) (3)
that forms readily in CCl4 or by standing for long periods in
toluene solutions. This result reflects the poor electron donor
properties of the diamine bis(phenolate) donor set in com-
parison to biscyclopentadienyl.24,26
Treatment of toluene solutions of 1 or 2 with one equivalent of
PhCH2MgCl in Et2O gave the corresponding benzyl derivatives
[Ti(tBu2O2NN′)(CH2Ph)(S)] (S = THF, 5; py, 6), as shown in
Scheme 1. The two compounds are identical with the exception
of the neutral coligand (THF or py), but 6 was reproducibly
isolated in higher yield than 5 (68% vs 44%) under the same
experimental conditions. Both compounds are extremely sen-
sitive, slowly decomposing in solution. Satisfactory elemental analysis
was, however, obtained for [Ti(tBu2O2NN′)(CH2Ph)(py)] (6),
Attempts to obtain crystals of either 5 or 6 were unfruitful.
Surprisingly, the analysis of the crystals obtained from an
Et2O solution of 6 by X-ray diffraction revealed the presence
of two Ti(IV) species, [Ti(tBu2O2NN′)(CH2Ph)2] (7) and
[Ti(tBu2O2NN′)Cl]2(μ-O) (3), which cocrystallized in a 1:1 ratio
(Figure 3). The cocrystal 3·7 displays half a molecule of 3 and
one molecule of 7 in the asymmetric unit. The titanium dibenzyl
complex 7 was previously synthesized by Kol and co-workers in a
one-pot reaction from sequential addition of the ligand pre-
cursor, titanium tetra(isopropoxide), TMSCl, and PhCH2MgCl.2a
The structural parameters obtained for [Ti(tBu2O2NN′)-
(CH2Ph)2] in the cocrystal 3·7 are in accordance with the data
reported by Kol and co-workers. The data obtained for the
molecule of 3 in the cocrystal are also in accordance with the data
obtained for the single crystal of 3 described above.
The formation of the dibenzyl derivative 7 from [Ti(tBu2O2NN′)-
(CH2Ph)(py)] (6) may be assumed as resulting from the transfer of
a benzyl radical between two molecules of 6. The overall outcome of
this exchange is the disproportionation of Ti(III) with formation
Figure 3. ORTEP-3 diagram of cocrystallized [Ti(tBu2O2NN′)(CH2Ph)2] and [Ti(tBu2O2NN′)Cl]2(μ-O) (3·7), using 40% probability level ellipsoids.
One half of 3 is generated using the symmetry transformation −x + 1, −y + 1, −z. Hydrogen atoms are omitted for clarity.
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of the Ti(IV)dibenzyl species 7 and a hypothetical (tBu2O2NN′)-
Ti(II) fragment that converts to 3 by abstracting oxygen and a
chloride ligand.27 The bridging oxo ligand may come from
vestigial amounts of moisture or O2, or the solvent. It is known
from the literature that the strength of TiX bonds is an
effective driving force for the reducing behavior displayed by
Ti(II) complexes and that the high oxophilicity of reduced
titanium complexes is responsible for particularly successful atom
transfer reactions when X = O.28 The source of the chloride
ligand is uncertain, but it may possibly come from traces of
MgCl2 that did not separate completely during the extraction of
[Ti(tBu2O2NN′)(CH2Ph)(py)] (6). It is known that removal of
MgCl2 upon alkylation reactions using Grignard reagents or
reduction reactions with Mg is in some cases difficult.29 Indeed,
though the low solubility of MgCl2 in organic solvents is a driving
force for chloride metathesis, its removal from solution is often
incomplete. The high affinity of magnesium for oxygen donors
may be responsible for the binding and solubilization of MgCl2,
which is particularly effective when O-donor chelating ligands are
present. Alternatively, it may be envisaged that a Ti(μ-Cl)MgCl
core is present as a contaminant. Such types of situations may
not be excluded and appear to be the most plausible explanations
for the origin of the chloride ligand during the crystalliza-
overlapping of most resonances was observed. VT NMR
experiments run between −80 and 100 °C did not disclose the
interconversion of the two species, which denotes the static
coordination of the dimethyamine fragment after the formation
of 8. DFT calculations (see below) showed that a model of 8a,
with tert-buthyl groups replaced by methyls, is 3.5 kcal·mol−1
more stable than a similar model of 8b. The small energy
difference is in agreement with the NMR results. The molecular
structure of 8a, depicted in Figure 4, was further confirmed by
Figure 4. ORTEP-3 diagram of [Ti(tBu2O2NN′)ClF] (8a), using 40%
probability level ellipsoids. Hydrogen and disordered carbon atoms are
omitted for clarity.
i
tion process. A related compound, presenting a bridging PrO
+
group between Ti(III) and one Na(THF)n cation, was pre-
viously reported and characterized by X-ray diffraction as
[Ti{Me2NC2H4N(CH2-3,5-tBu2C6H2O-2)2}(OiPr)2Na(THF)2].30
Other examples of well characterized compounds displaying
bridging chlorides between group 4 and representative metals
have been reported.31 Attempts to grow crystals from a solution
of [Ti(tBu2O2NN′)(CH2Ph)(THF)] (5) after complete removal
of MgCl2 with dioxane were unproductive. Although a greenish
microcrystalline solid formed from hexane solution after several
weeks at −20 °C, the crystals and the mother solution turned
orange at room temperature, and the proton NMR of this sample
was inconclusive.
X-ray diffraction. Relevant bond lengths and angles are displayed
in Table 1. The titanium coordination geometry is distorted
octahedral, but the coordination mode of the diamine bis-
(phenolate) ligand to the titanium is different from that observed
in the other complexes presented above. The two phenolate
moieties are mutually cis with a dihedral angle of 112.4(1)°
between the planes of the two rings. The Ti−F bond distance in 8
compares well with the values reported for titanium(IV)
complexes displaying terminal Ti−F bonds, namely [Cp2Ti-
(CF3)F] and [{Cp*TiF(μ-F)(μ-OSO2-p-C6H4CH3)}2] (Cp* =
η5-C5Me5).33 As expected, the Ti−F bond length in 8 is much
shorter than in [Ti3(NCtBu2)6(μ2-F)3((μ3-F)2][PF6] despite
the cationic nature of the latter complex.32b The high elec-
tronegativity of the fluoride ligand causes a more extensive
pπ−dπ interaction between Ti1 and O2, as attested by the
Ti1−O2−C21 angle that is 10° wider than Ti1−O1−C11.
In the reaction with [Cp2Fe][BPh4], the bulkiness of the
Reactions of [Ti(tBu2O2NN′)Cl(S)] (S = THF, 1; py, 2) with
[Cp2Fe][X] (X = PF6, BPh4) showed that the diamine
bis(phenolate) set is not efficient for the stabilization of the
corresponding Ti(IV) cations. Fluoride abstraction and for-
mation of [Ti(tBu2O2NN′)ClF] (8) takes place in the presence of
−
PF6 , while [Ti(tBu2O2NN′)Cl2] (9) is obtained in the presence
of the noncoordinating BPh4− anion (Scheme 1). These results
attest the extreme electrophilicity of the transient Ti(IV) cations
that result from the 1:1 reaction of 1 with ferrocenium. It is likely
that the abstraction of fluoride occurring in the presence of the
−
BPh4 anion and its weak coordinating capabilities hamper the
stabilization of [Ti(tBu2O2NN′)Cl]+ and give rise to the for-
mation of [Ti(tBu2O2NN′)Cl2] (9) and other nonidentified
decomposition products. Compound 9 was obtained as a crys-
talline material from Et2O solution after sublimation of the
ferrocene produced by the redox reaction. Hypothetically, the
formation of [Ti(tBu2O2NN′)Cl2] may involve the cleavage of a
dichloride bridged dication {[Ti(tBu2O2NN′)]2(μ-Cl)2}2+ that
would originate the loss of at least 50% of titanium and would be
responsible for the failure in the isolation of the envisaged
cations. Orange crystals of 9 suitable for X-ray diffraction were
grown from a C6D6 solution. The molecular structure of 9 is
depicted in Figure 5, and relevant bond lengths and angles are
displayed in Table 1. The titanium coordination geometry is
distorted octahedral, with the two phenolate moieties trans-
coordinated. The side arm fragment is coordinated to the metal,
and the tripodal ligand forces the two chloride ligands to adopt
mutually cis dispositions. The Ti−Cl distances compare well with
the values reported for other titanium(IV) systems supported
by O- and N-based ligands.34 The wide Ti1−O1−C11 and
−
PF6 anion is sustained by an inner sphere ion pair interaction
[Ti(tBu2O2NN′)Cl(FPF5)] that evolves to [Ti(tBu2O2NN′)ClF].
Similar processes involving acidic group 4 metal cations have
been reported.32 Contrary to all other complexes with R2O2NN′
4a,7,8
t
ancillary ligands (R = Me, Bu),
8 displays mutually cis-
phenolate moieties that originate C1-symmetry species. The
isomerization process may proceed through the formation of a
penta-coordinated trigonal bipyramidal intermediate, formed
upon dissociation of THF/pyridine or the dimethylamine frag-
ment. The origin of the isomerization process is likely
determined by the bulkiness of the tert-butyl groups in the
ortho positions of the phenolate groups that hinder the approach
of the PF6− anion in the equatorial plane bisecting the O−Ti−O
angle. Two isomers, 8a and 8b, are clearly detected in the 19F
NMR spectrum as two broad signals that integrate 6:1 at δ 122.5 ppm
and 96.4 ppm. The presence of a second minor species of C1
symmetry could also be detected by 1H and 13C NMR, although
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of [Ti(tBu2O2NN′)(κ2-{CH2-2-(NMe2)-C6H4})][PF6] suitable
for X-ray diffraction failed, but surprisingly, orange crystals grew
from a Et2O solution of the reaction mixture exposed to air. The
analysis of the crystals revealed the formation of two cocrystal-
lized coordination isomers (11 and 12) of [Ti(tBu2O2NN′)-
Cl]2(μ-O) (3), as shown in Scheme 3.
An ORTEP view of cocrystal 11·12 is depicted in Figure 6. In
11, the coordination geometry around the two titanium atoms is
distorted octahedral, but the phenolate rings occupy cis positions
in one metal center, while they are trans-coordinated in the
other. In 12, one of the titanium centers presents cis-phenolate
moieties, and the NMe2 fragment is coordinated to the metal
defining a distorted octahedral geometry around the metal.
In the second titanium center of 12, the NMe2 moiety is not
coordinated to the metal, leading to a highly distorted trigonal
bipyramidal geometry. Relevant bond lengths and angles for the
cocrystal 11·12 can be found in the Supporting Information.
The chloride ligands in 11 and 12 probably arise from the
lithium salt formed in the metathesis reaction that leads to
[Ti(tBu2O2NN′)(κ2-{CH2-2-(NMe2)-C6H4})]. The ubiquitous
appearance of chloride ligands in the reactions described indicate
that the alkaline metal salts are forming aggregates with the
reaction products and are not efficiently removed by solvent
extraction and filtration.29 The different configurations of the
phenolate groups found in 11·12 suggest that the formation of
the cationic species may involve an isomerization process as the
one observed in the formation of 8. The dissociation of the
dimethylamine fragment may originate a mixture of cis- and trans-
phenolate arrangements that are responsible for the complex 1H
and 13C NMR spectra already mentioned.
Figure 5. ORTEP-3 diagram of [Ti(tBu2O2NN′)Cl2] (9), using 40%
probability level ellipsoids. Hydrogen atoms are omitted for clarity.
Ti1−O2−C21 angles (145.6(2)° and 146.0(2)°, respectively)
reflect some oxygen to metal pπ−dπ donation.
Attempts to reduce azobenzene by 1 also led to the isolation of
a small amount of [Ti(tBu2O2NN′)Cl2]. Although in a first
instance this result might be surprising, it is in accordance with
the reaction of 1 with [Cp2Fe][BPh4] described above. Once
more, electron transfer should generate a Ti(IV) cation that is
not stabilized by the azobenzene radical anion.
The oxidation of previously described [Ti(tBu2O2NN′)(κ2-{CH2-
2-(NMe2)-C6H4})] (10), prepared from the reaction of 1 with
LiCH2-2-(NMe2)-C6H4 (Scheme 3),8 was also attempted
1
through reaction with [FeCp2][PF6]. The H and 13C NMR
spectra of the red crystalline solid isolated after sublimation of the
ferrocene formed are very complicated, revealing a mixture of
compounds, and did not allow an unequivocal characterization.
However, in C6D6, the 19F NMR spectrum shows a doublet
at δ = −70.69 ppm (1JFP = 711 Hz), and the 31P NMR spectrum
displays a septet at δ = −143.0 ppm (1JPF = 711 Hz), which
DFT Studies. Density functional theory calculations35
(Gaussian 03/PBE1PBE,36 see Computational Details) were
performed in order to understand the origin of complexes 3 and
8 described above. Complex 1, [Ti(tBu2O2NN′)Cl(THF)], loses
THF in toluene solution, as revealed by EPR results, although
[Ti(tBu2O2NN′)Cl] was not isolated.8 Therefore, the model
complex [Ti(Me2O2NN′)Cl] (1*) was used as the starting com-
−
attest to the presence of the PF6 anion and are consistent
with the formation of a cationic species. If the compound
[Ti(tBu2O2NN′)(κ2-{CH2-2-(NMe2)-C6H4})][PF6] would form
in this reaction, the complexity of the NMR spectra might be
related to the presence of isomers, the occurrence of fluxional
processes, or even decomposition. All attempts to grow crystals
t
pound for the calculations (Me was used instead of Bu in all
Scheme 3
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Figure 6. ORTEP-3 diagram of cocrystallized isomers of [Ti(tBu2O2NN′)Cl]2(μ-O) (11·12), using 30% probability level ellipsoids. Hydrogen atoms,
tert-butyl groups, and disordered carbon atoms are omitted for clarity.
−
calculations). The formation of 1* by dissociation of THF from 1
has been calculated as ΔG298 = −7.6 kcal·mol−1.
typical of O2 ), indicating that Ti(III) has been oxidized to
Ti(IV) and O2 reduced.
The Ti−O−Ti core of [Ti(tBu2O2NN′)Cl]2(μ-O) (3) formed
in the reaction of [Ti(tBu2O2NN′)Cl] (1*) with O2 is proposed
to result from the one electron oxidation of Ti(III) in 1 via a
titanium peroxide species Ti−O−O−Ti (intermediate B).
Therefore, it was considered that 1* (Scheme 4) would react
The overall reaction of four moles of Ti(III) complex 1* and
1 mol of O2 to yield two moles of 3* is favored by 123.6 kcal·mol−1
of O2 (ΔG298 in THF). This result is almost the same when
starting from compound 1 instead of 1*. The first step involves
the coordination of O2 to Ti(III), followed by electron transfer to
afford the radical intermediate A (Scheme 4). This associative
process is only marginally favorable (ΔG298 = −2.2 kcal·mol−1).
The reaction between A and the reactant 1* is more exergonic
(−33.3 kcal·mol−1). All together, the formation of a bridging
peroxide titanium dimer from Ti(III) and O2 is a favorable
process from the thermodynamic point of view, corresponding to
a global ΔG298 = −35.5 kcal·mol−1.
Scheme 4. Gibbs Energy for Possible Reactions Leading to the
Formation of [Ti(L)Cl]2(μ-O) (3*) from [Ti(L)Cl] (1*) and
a
O2 (L = Me2O2NN′; ΔG in THF, kcal·mol−1)
The peroxide dimer B may convert into [Ti(L)Cl]2(μ-O), 3*,
through the homolytic cleavage of the O−O bond of B, leading
to two titanium oxido radicals (C) as shown on the left side
of Scheme 4 (ΔG298 = −18.0 kcal·mol−1). This negative ΔG298
value shows that the Ti−O−O−Ti species B is unstable relative to
the Ti−O radicals resulting from O−O cleavage. The last step of the
mechanism corresponds to the formation of 3* from the reaction of
the titanium oxido radical C with another Ti(III) species 1*. The
coupling of these two radicals is favorable by −35.1 kcal·mol−1.
Alternatively, one may envisage that the cleavage of the
O−O bond in B is assisted by Ti(III) (1*), leading to 3* and
concomitantly to the Ti−O radical C (Scheme 4, center). The
latter reaction corresponds to the one-electron oxidation of
Ti(III) through oxygen atom transfer from B, a process that is
reminiscent of what is commonly accepted to occur in the
oxidation of organic substrates by metal peroxides. This trans-
formation is very favorable thermodynamically (ΔG298 = −53.0
kcal mol−1). The remaining Ti−O radical C couples with another
Ti(III) species 1* to form 3*, as described above.
a
1* and 3* are structural models of 1 and 3, respectively, where the
tert-butyl groups were replaced by methyls.
DFT calculations were also performed in order to explain the
formation of the two isomers of 8 detected in the NMR. The
structures of four models of complex 8, in which the tert-butyl
groups were replaced by methyl groups (8*), were fully op-
timized. The isomer identified by X-ray diffraction, 8a*, is
more stable than 8b*, the other one displaying cis-phenolates,
by −3.5 kcal mol−1 (Figure 8). 8b* must be the other isomer
with O2 to form a superoxide radical derivative (A), which reacts
with a second equivalent of 1* to give an intermediate B having
a bridging peroxide. The optimized geometries of the models
reproduce the experimental available ones (see Supporting
Information). The calculated geometries of the species A and B
are shown in Figure 7. The spin density of intermediate A, also
shown, is concentrated on the O2 ligand (O−O distance 1.289 Å,
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Figure 7. DFT optimized geometries of intermediates A (left) and B (right) with selected bond lengths (Å) and spin density of A (left, bottom).
Figure 8. DFT optimized structures of 8a (8a*) and its isomer 8b (8b*), with selected bond distances (Å).
identified in the NMR spectrum because of the low symmetry.
The two trans-phenolate isomers, 8c* (Cl trans to tripodal N)
and 8d* (F trans to tripodal N), shown in Scheme 5, are only 1.8
of a dichloride bridged dication {[Ti(tBu2O2NN′)]2(μ-Cl)2}2+,
the initial trans-phenolates disposition is maintained. Full
optimization of the two models of complex 9 (9a*, with trans-
phenolates, and 9b*, with cis-phenolates) showed that the trans
arrangement of 9a* is more stable, by 2.4 kcal·mol−1.
Scheme 5
Simultaneous presence of cis- and trans-phenolates was,
however, observed in the cocrystal 11·12. An isomerization
process similar to the one observed in the formation of 8 may
have occurred in the formation of this mixture of isomers, 11 and
12, obtained upon exposition of the product of the oxidation
of [Ti(tBu2O2NN′)(κ2-{CH2-2-(NMe2)-C6H4})] (10) with
[FeCp2][PF6] to air.
Models of dimers 11 and 12 (11* and 12*), which are co-
ordination isomers of [Ti(tBu2O2NN′)Cl]2(μ-O) (3) with
different dispositions and coordination modes of the diamine
bis(phenolate) ligand, were fully optimized. Both dimers 11*
and 12* are less stable than 3* by 4.8 and 13.2 kcal·mol−1, respec-
tively. These relatively small energy differences may easily be
overcome by crystal packing forces acting cooperatively.
and 0.3 kcal·mol−1 less stable than 8a*, respectively. The small
energy gaps suggest that the formation of the species displaying
cis-phenolates (8a* and 8b*) is rather kinetic than thermody-
−
namic. The approach of the PF6 anion to a {[Ti(tBu2O2NN′)-
Cl]}+ cationic species most likely occurs between one of the
phenolate groups and the dimethylamine moiety (with possible
decoordination) leading to a cis-phenolate disposition.
The Ti−F and particularly the Ti−Cl bond lengths change
significantly from 8a*, the structurally characterized isomer, to
8b*, detected in solution, while the Ti−O and Ti−N bond
lengths differ by much smaller amounts (Figure 8). The Ti−F
and Ti−Cl distances calculated for 8a* are in better agreement
with the experimental ones determined for 8. These results are
consistent with the experimental findings as they point out for
the preferential formation of one isomer, but the energy dif-
ference between them does not exclude the formation of a minor
species.
CONCLUDING REMARKS
■
The reactivity of titanium(III) complexes [Ti(tBu2O2NN′)Cl(S)]
(S = THF, py) supported by a tripodal diamine bis(phenolate)
ligand is described. Reactions with neutral electron acceptors as
the TEMPO radical led to the formation of [Ti(tBu2O2NN′)Cl-
(TEMPO)] that displays an anionic TEMPO ligand. A similar
reaction with PhNNPh did not allow the isolation of any
species displaying the azobenzene radical anion, but only a small
amount of [Ti(tBu2O2NN′)Cl2]. This result is consistent with the
occurrence of a one-electron transfer from Ti(III) to azobenzene,
but given the instability of a {Ti(PhNNPh)} radical, the resulting
In contrast, in the analogous complex [Ti(tBu2O2NN′)Cl2] (9)
with two chloride ligands, hypothetically formed from the cleavage
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Ti(IV) cation rearranges to give [Ti(tBu2O2NN′)Cl2]. Actually,
the latter complex was also formed when [Ti(tBu2O2NN′)Cl(S)]
was oxidized by [Cp2Fe][BPh4], corroborating the instability
of [Ti(tBu2O2NN′)Cl]+ and its tendency to convert to
[Ti(tBu2O2NN′)Cl2]. In line with this hypothesis is also the
formation of [Ti(tBu2O2NN′)ClF] by fluoride abstraction from
EXPERIMENTAL SECTION
■
General Procedures. All preparations and subsequent manipu-
lations of air/moisture sensitive compounds were performed under a
nitrogen atmosphere using standard Schlenk line and drybox
techniques. THF, toluene, and Et2O were dried by standard methods
and distilled prior to use. C6D6 was dried over Na and distilled under
reduced pressure. Toluene-d8 and CDCl3 were dried with 4 Å and
degassed with the freeze−pump−thaw method. Unless stated otherwise,
all reagents were purchased from commercial suppliers and used
as received. The diamine bis(phenolate) ligand,13f TiCl3(THF)3,37
and [Cp2Fe][BPh4]38 were prepared as described in the literature.
[Ti(tBu2O2NN′)Cl(THF)] (1), [Ti(tBu2O2NN′)Cl(py)] (2), and
[Ti(tBu2O2NN′)(κ2-{CH2-2-(NMe2)-C6H4})] (10) were prepared as
previously described.8 The 1D and 2D (COSY, NOESY, HSQC) NMR
spectra were recorded on Bruker Advance II+ 300 and 400 MHz
(UltraShield Magnet) instruments at ambient temperature, unless stated
−
PF6 , upon reaction of the Ti(III) complexes with [Cp2Fe]-
[PF6]. Contrary to all diamine bis(phenolate) compounds
previously described, [Ti(tBu2O2NN′)ClF] displays cis-phenolate
coordination. The isomer displaying the chloride ligand in the
axial position, trans to the tripodal nitrogen, was identified by
single crystal X-ray diffraction and shown by DFT calculations to
be 3.5 kcal·mol−1 more stable than the complex with the fluoride
ligand at the axial position.
The reactions of [Ti(tBu2O2NN′)Cl(S)] with O2 led to
[Ti(tBu2O2NN′)Cl]2(μ-O). The Gibbs free energy for the
reaction starting from the five coordinated intermediate
[Ti(tBu2O2NN′)Cl] was calculated (DFT) as −123.6 kcal per
mole of O2. The calculations also revealed that the formation of
Ti−O−Ti by reaction of the Ti−O−O−Ti core with two moles
of Ti(III) is thermodynamically favored.
1
otherwise. H and 13C chemical shifts (δ) are expressed in parts per
million relative to Me4Si, whereas 19F and 31P nuclei were referenced
to external reference capillaries with CF3COOH and H3PO4 (80%
solution), respectively. EPR experiments were run in a Bruker EMX EPR
Spectrometer and calibrated using a Perilene+•/H2SO4 solution as the
internal standard. Carbon, hydrogen, and nitrogen analyses were
performed in-house using an EA110 CE Instruments automatic
analyzer. The results presented are, in general, the average of two
independent determinations. Low contents of C were obtained for some
low oxidation state titanium and vanadium complexes due to their
extreme instability to air and moisture.
[Ti(tBu2O2NN′)(CH2Ph)(S)] (S = THF, py) were obtained
by chloride metathesis from [Ti(tBu2O2NN′)Cl(S)] and
PhCH2MgCl. The compounds are not stable in solution for
long periods and, in the case of S = THF, dissociation of THF
occurs readily in toluene solution. One-to-one cocrystals of
[Ti(tBu2O2NN′)(CH2Ph)2] and [Ti(tBu2O2NN′)Cl]2(μ-O) were
grown up from a solution of [Ti(tBu2O2NN′)(CH2Ph)(py)] in
diethyl ether. The formation of the compounds most likely
results from a disproportionation process of titanium(III) that
results from the transfer of one benzyl radical and the abstraction
of the oxygen from the solvent or traces of moisture, to generate
the Ti−O−Ti moiety. These reactions take place in the presence
of vestigial amounts of MgCl2 that remain in the samples of
[Ti(tBu2O2NN′)(CH2Ph)(S)], if dioxane is not used to ensure its
complete removal.
Synthetic Procedures. [Ti(tBu2O2NN′)Cl]2(μ-O), 3. A solution of 1
(1.40 g, 2.07 mmol) in THF was exposed to air, through a CaCl2 drying
tube, for 12 h. The red/orange solution obtained was evaporated to
dryness, and the residue was extracted with Et2O and filtered.
Evaporation of the Et2O solution to dryness led to an orange crystalline
solid. Yield: 1.27 g, 91%. Crystals of 3 suitable for X-ray diffraction were
grown from Et2O at −20 °C and also from a C6D6 solution at room
temperature. 1H NMR (300 MHz, CDCl3, ppm): δ 7.13 (d, 4H, 4JHH
=
2.2 Hz, p-CH-Ar), 6.90 (s, 4H, 4JHH = 2.1 Hz, o-CH-Ar), 5.41 (d, 4H,
2JHH = 13.8 Hz, NCH2Ar), 3.36 (d, 4H, 2JHH = 13.9 Hz, NCH2Ar), 2.75
(m, 4H, CH2), 2.13 (s, 12H, N(CH3)2), 1.64 (m, 4H, CH2), 1.26
1
(s, 36H, C(CH3)3), 0.98 (s, 36H, C(CH3)3). H NMR (300 MHz,
The oxidation of [Ti(tBu2O2NN′)(κ2-{CH2-2-(NMe2)-C6H4})],
which is a Ti(III) benzyl stabilized by the intramolecular
coordination of the NMe2 moiety, led to a complex mix-
ture. Recrystallization of this mixture under air led to a 1:1
cocrystal of two coordination isomers of the titanium oxo dimer
[Ti(tBu2O2NN′)Cl]2(μ-O) differing on the coordination mode of
the diamine bis(phenolate) ligands. Both molecules display C1
symmetry and, surprisingly, in one of these dimers one metal is
pentacoordinated and the other is hexacoordinated. This is the
first compound where the dimethylamine moiety of the diamine
bis(phenolate) ligand is not bonded to the metal. In the other
complex, the coordination of the two ancillary ligands to the
titanium centers reveals mutually cis-phenolate groups in one-
half of the molecule, while they are trans-coordinated in the other
titanium center.
This is the first systematic study on the reactivity of para-
magnetic titanium(III) complexes of a nonmetallocene sys-
tem. The results revealed that these species are very reactive,
presenting several reaction patterns that are extremely sensitive
to the experimental conditions. Cationic complexes of Ti(IV)
tend to balance its extreme acidity by abstracting X type ligands
or oxygen from the medium. Along with these rearrangements,
the ancillary ligand may display hemilabile behavior. Although
not identified in the many processes mediated by titanium
diamine bis(phenolate) complexes described to date, the results
reported here are relevant for the outlining of the applications of
these compounds.
C6D6, ppm): δ 7.47 (s, 4H, p-CH-Ar), 7.11 (s, 4H, o-CH-Ar), 5.77 (d,
4H, 2JHH = 13.7 Hz, NCH2Ar), 3.30 (d, 4H, 2JHH = 13.8 Hz, NCH2Ar),
2.25 (m, 4H, CH2), 1.90 (s, 12H, N(CH3)2), 1.46 (m, 4H, CH2), 1.44 (s,
36H, C(CH3)3), 1.36 (s, 36H, C(CH3)3). 13C-{1H} NMR (75 MHz,
C6D6, ppm): δ 158.0, 138.4, 132.1, and 124.4 (Cipso-Ar), 123.6 (p-CH−
Ar), 122.3 (o-CH−Ar), 64.1 (NCH2Ar), 57.6 (CH2), 51.4 (CH2), 48.4
(N(CH3)2), 33.1 and 32.3 (C(CH3)3), 30.0 and 29.5 (C(CH3)3). EA
calculated for C68Cl2H108N4O5Ti2·2(C4H10O): C, 66.31; H, 9.37; N,
4.07. Found: C, 66.77; H, 9.29; N, 3.66.
[Ti(tBu2O2NN′)Cl(TEMPO)], 4. A solution of 2,2,6,6-tetramethylpiper-
idine-1-oxyl (TEMPO) (0.156 g, 1.00 mmol) in toluene was added to a
solution of 1 (0.674 g, 1.00 mmol) in the same solvent at −30 °C. The
temperature was allowed to rise slowly to room temperature, and the
mixture was further stirred overnight. The red solution obtained was
filtered, and the solvent was removed under reduced pressure leading to
a red/orange crystalline solid. Yield: 0.754 g, 99%. Crystals of 4 suitable
for X-ray diffraction were obtained from toluene at −20 °C. 1H NMR
(400 MHz, C6D6, ppm): δ 7.62 (d, 2H, 4JHH = 2.4 Hz, p-CH-Ar), 7.03
(d, 2H, 4JHH = 2.4 Hz, o-CH-Ar), 4.44 (d, 2H, 2JHH = 13.0 Hz, NCH2Ar),
2.97 (d, 2H, 2JHH = 13.1 Hz, NCH2Ar), 2.23 (m, 2H, NCH2CH2NMe2),
2.12 (s, 6H, N(CH3)2), 1.79 (s, 18H, C(CH3)3), 1.59 (m, 2H,
NCH2CH2NMe2), 1.49(s, 12H, CH3-TEMPO), 1.37(s, 18H, C(CH3)3),
1.18 (m, 4H, CH2-TEMPO), 1.10 (m, 2H, CH2-TEMPO). 13C {1H}
NMR (101 MHz, C6D6, ppm): δ 160.2, 140.6, and 135.6 (Cipso-Ar),
126.5 (CH−Ar), 125.4 (Cipso-Ar), 124.7 (CH−Ar), 66.3 (NCH2Ar), 64.3
(Cipso-TEMPO), 60.3 (NCH2CH2NMe2), 52.6 (NCH2CH2NMe2), 51.1
(N(CH3)2), 41.0 (CH2-TEMPO), 36.2 and 34.7 (C(CH3)3), 32.8 and
32.3 (C(CH3)3), 28.0 (br, CH3-TEMPO), 17.0 (CH2-TEMPO). EA
calculated for C43H72ClN3O3Ti·0.5(C7H8O): C, 69.08; H, 9.47; N, 5.20.
Found: C, 68.80; H, 9.54; N, 4.31.
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[Ti(tBu2O2NN′)(CH2Ph)(THF)], 5. A 1 M solution of PhCH2MgCl in
Et2O (1 mL, 1 mmol) was added to a solution of 1 (677 mg, 1 mmol) in
toluene at −80 °C. The temperature was allowed to rise slowly to room
temperature, and the mixture was further stirred overnight. The red
solution obtained was evaporated to dryness, and the residue was
extracted with Et2O and filtered. Evaporation of the solvent to dryness
led to a red crystalline solid. Yield: 325 mg, 44%. EPR (3 × 10−3 M in
toluene, 293 K): g1 = 1.963 (minor species), g2 = 1.951 (major species).
[Ti(tBu2O2NN′)(CH2Ph)(py)], 6. A 1.64 M solution of PhCH2MgCl in
Et2O (0.37 mL, 0.60 mmol) was added to a solution of 2 (411.00 mg,
0.60 mmol) in toluene at −80 °C. The temperature was allowed to rise
slowly to room temperature, and the mixture was further stirred for 6 h.
The purple solution obtained was evaporated to dryness, and the residue
was extracted with Et2O and filtered. Evaporation of the Et2O solution to
dryness led to a purple crystalline solid. Yield: 300 mg, 68%. Crystals of
cocrystallized [Ti(tBu2O2NN′)Cl]2(μ-O) and [Ti(tBu2O2NN′)-
(CH2Ph)2] (3·7) suitable for X-ray diffraction were obtained from
an Et2O solution of [Ti(tBu2O2NN′)(CH2Ph)(py)] at −20 °C. EPR
(3 × 10−3 M in toluene, 293 K): g = 1.952. EA calculated for
C46H66N3O2Ti·(MgCl2): C, 68.06; H, 8.19; N, 5.18. Found: C, 68.32;
H, 8.97; N, 5.73.
product. Orange crystals of 9 suitable for X-ray diffraction were obtained
from a C6D6 solution at +4 °C. (b) Reaction of [Ti(tBu2O2NN′)Cl(THF)]
with PhNNPh: A solution of PhNNPh (0.083 g, 0.458 mmol) in toluene
was added to a solution of 1 (0.310 g, 0.458 mmol) in the same solvent
at −50 °C. The temperature was allowed to rise to room temperature,
and the mixture was stirred overnight. The red/brown solution obtained
was filtered and evaporated to dryness to give a brown powder. In
toluene solution at +4 °C, an orange solid (9) precipitates from the
brown solution. 1H NMR (300 MHz, C6D6, ppm): δ 7.59 (d, 2H, 4JHH
2.3 Hz, p-CH-Ar), 6.97 (d, 2H, 4JHH = 2.3 Hz, o-CH-Ar), 4.35 (d, 2JHH
=
=
13.8 Hz, 2H, NCH2Ar), 2.79 (d, 2JHH = 13.3 Hz, 2H, NCH2Ar), 2.14 (m,
2H, CH2), 1.94 (s, 6H, N(CH3)2), 1.80 (s, 18H, C(CH3)3), 1.61 (m, 2H,
CH2), 1.39 (s, 36H, C(CH3)3). 13C {1H} NMR (75 MHz, C6D6, ppm):
δ 160.2, 143.6, 136.7, and 127.6 (Cipso-Ar), 125.0 (p-CH−Ar), 124.4
(o-CH−Ar), 67.2 (NCH2Ar), 60.6 (CH2), 54.3 (CH2), 51.3 (N(CH3)2),
36.1 and 35.0 (C(CH3)3), 32.2 and 31.4 (C(CH3)3).
Oxidation of [Ti(tBu2O2NN′)(κ2-{CH2-2-(NMe2)-C6H4})] with [FeCp2]-
[PF6]. [FeCp2][PF6] (94 mg, 0.28 mmol) was added to a solution of
10 (200 mg, 0.28 mmol) in toluene at −70 °C. The temperature was
allowed to rise to room temperature, and the mixture was stirred for 6 h.
The red solution obtained was evaporated to dryness and the residue
extracted with Et2O. Evaporation of the Et2O solution to dryness led to a
red crystalline solid. The proposed product is [Ti(tBu2O2NN′)(η2-
CH2C6H4NMe2)][PF6]. Yield: 110 mg, 46%. 19F NMR (282 MHz,
C6D6, ppm): δ −70.69 (d, 1JFP = 711 Hz). 31P NMR (121 MHz, C6D6,
[Ti(tBu2O2NN′)ClF], 8. [FeCp2][PF6] (0.30 g, 0.90 mmol) was added
to a solution of 2 (0.62 g, 0.90 mmol) in toluene at −60 °C. The
temperature was allowed to rise to room temperature, and the mixture
was further stirred for 4 h. The red solution obtained was evaporated to
dryness and the residue extracted with Et2O. Evaporation of the Et2O
solution to dryness led to an orange crystalline solid. Compound 8 is
obtained pure after sublimation to remove the residual ferrocene. Yield:
0.66 g, 88%. Orange crystals of 8a suitable for X-ray diffraction were
obtained from an Et2O solution at −20 °C. The analysis of the NMR
data revealed the presence of a minor second species, 8b, in a relative
ratio of 1:6 (coordination isomer with F− ligand trans to the tripodal N).
8a: 1H NMR (300 MHz, toluene−d8, ppm): δ 7.57 (d, 1H, 4JHH = 2.4
1
ppm): δ −143.0 (sept, JPF = 711 Hz). Orange crystals of two
cocrystallized isomers of [Ti(tBu2O2NN′)Cl]2(μ-O) (11·12) were
grown from an Et2O solution of the red crystalline solid exposed to air.
General Procedures for X-Ray Crystallography. Crystals
suitable for single-crystal X-ray analysis were obtained for 3, 4, 3·7,
8a, and 9 as described in the synthetic procedures. Crystals of air/
moisture sensitive compounds were selected inside the glovebox,
covered with polyfluoroether oil, and mounted on a nylon loop. The
data were collected using graphite monochromated Mo Kα radiation
(λ = 0.71073 Å) on a Bruker AXS-KAPPA APEX II diffractometer
equipped with an Oxford Cryosystem open-flow nitrogen cryostat. Cell
parameters were retrieved using Bruker SMART software and refined
using Bruker SAINT on all observed reflections. Absorption corrections
were applied using SADABS.39 The structures were solved and refined
using direct methods with SIR97,40 SIR2004,41 or SHELXS-9742 using
the WINGX-Version 1.80.0143 SHELXL44 system of programs. All non-
hydrogen atoms were refined anisotropically, and the hydrogen atoms
were inserted in idealized positions and allowed to refine riding on the
parent carbon atom. The molecular diagrams were drawn with ORTEP-
3 for Windows45 or Mercury 1.4.2,46 included in the software package.
For crystallographic experimental data and structure refinement
parameters, see Table 2. Data for structures 3, 4, 3·7, 8a, and 9 were
deposited in CCDC under the deposit numbers CCDC 639419 and
926866−926870 and can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.uk/
data_request/cif.
Hz, p-CH-Ar), 7.41 (d, 1H, 4JHH = 2.3 Hz, p-CH-Ar), 7.03 (d, 1H, 4JHH
=
2.3 Hz, o-CH-Ar), 6.65 (d, 1H, 4JHH = 2.3 Hz, o-CH-Ar), 4.85 (d, 1H,
2JHH = 13.8 Hz, NCH2Ar), 3.98 (d, 1H, 2JHH = 14.0 Hz, NCH2Ar), 2.79
2
(d, 1H, JHH = 13.7 Hz, NCH2Ar), 2.71 (m, 1H, CH2), 2.60 (m, 1H,
CH2), 2.54 (d, 1H, 2JHH = 14.2 Hz, NCH2Ar), 2.49 (s, 3H, N(CH3)2),
1.93 (s, 3H, N(CH3)2), 1.83 (m, 1H, CH2), 1.71 (s, 9H, C(CH3)3), 1.53
(s, 9H, C(CH3)3), 1.40 (m, 1H, CH2), 1.39 (s, 9H, C(CH3)3), 1.28 (s,
9H, C(CH3)3). 13C {1H} NMR (75 MHz, toluene−d8, ppm): δ 158.7,
157.3, 143.3, 142.4, 137.3, 136.8, 125.0, and 124.9 (Cipso-Ar), 124.6,
124.5, 123.9, and 123.7 (CH−Ar), 64.6 and 61.5 (NCH2Ar), 58.5 and
55.2 (CH2), 52.1 and 48.3 (N(CH3)2), 36.1, 36.0, 35.0, and 34.8
(C(CH3)3), 32.3, 32.2, 30.9, and 30.5 (C(CH3)3). 19F NMR (282 MHz,
toluene-d8, ppm): δ 190.8 (br). 1H NMR (300 MHz, CDCl3, ppm): δ
7.26 (d, 1H, 4JHH = 1.7 Hz, p-CH-Ar), 7.19 (d, 1H, 4JHH = 1.6 Hz, p-CH-
Ar), 6.95 (s, 1H, o-CH-Ar), 6.88 (s, 1H, o-CH-Ar), 5.10 (d, 1H, 2JHH
=
13.7 Hz, NCH2Ar), 4.12 (d, 1H, 2JHH = 14.0 Hz, NCH2Ar), 3.38 (d, 1H,
2JHH = 13.8 Hz, NCH2Ar), 3.28 (m, 1H, CH2CH2NMe2), 3.23 (m, 1H,
2
CH2CH2NMe2), 3.20 (d, 1H, JHH = 14.2 Hz, NCH2Ar), 2.83 (s, 3H,
N(CH3)2), 2.41 (s, 3H, N(CH3)2), 2.34 (m, 1H, CH2CH2NMe2),
1.53 (s, 9H, C(CH3)3), 1.38 (s, 9H, C(CH3)3), 1.28 (m, 1H,
CH2CH2NMe2), 1.27 (s, 9H, C(CH3)3), 1.25 (s, 9H, C(CH3)3). 13C
{1H} NMR (75 MHz, CDCl3, ppm): δ 157.8, 156.4, 143.4, 142.6, 136.5,
135.7, 125.4, and 125.0 (Cipso-Ar), 124.3, 124.1, 123.5, and 123.2
(CH−Ar), 64.5 and 61.6 (NCH2Ar), 58.7 and 55.7 (CH2), 51.9 and 48.8
(N(CH3)2), 36.0 and 35.0 (C(CH3)3), 32.3, 32.2, 30.9, and 30.5
COMPUTATIONAL DETAILS
■
DFT35 calculations for 1*, 3*, 8(a-d)*, 9, 11, 12, A, B, and C (Me was
t
used instead of Bu) were performed using the Gaussian 03 software
package36 and the PBE1PBE functional, without symmetry constraints.
This functional uses a hybrid generalized gradient approximation
(GGA), including a 25% mixture of Hartree−Fock47 exchange with
DFT35 exchange correlation, given by the Perdew, Burke, and Ernzerhof
functional (PBE).48 The optimized geometries were obtained with a
standard 6-31G(d,p)49 basis set for all elements (basis b1). The elec-
tronic energies obtained were converted to standard free Gibbs energies
at 298.15 K by using zero point energy and thermal energy corrections
based on structural and vibrational frequency data calculated at the
PBE1PBE/b1 level of theory. Single point energy calculations were
performed using an improved basis set (basis b2) and the geometries
optimized at the PBE1BPE/b1 level. Basis b2 consisted of a standard
6-311++G(d,p).50 Solvent effects (THF) were considered in the
PBE1BPE/b2//PBE1BPE/b1 energy calculations using the polarizable
continuum model (PCM) initially devised by Tomasi and co-workers51
(C(CH3)3). 19F NMR (282 MHz, CDCl3, ppm): δ 122.5 (br). 8b: 19
F
NMR (282 MHz, toluene-d8, ppm): δ 164.2 (br). 19F NMR (282 MHz,
CDCl3, ppm): δ 96.4 (br). EA calculated for C34ClFH54N2O2Ti·0.5-
(C4H10O): C, 65.30; H, 8.98; N, 4.23. Found: C, 65.30; H, 9.91; N, 4.25.
[Ti(tBu2O2NN′)Cl2], 9. (a) Reaction of [Ti(tBu2O2NN′)Cl(THF)] with
[FeCp2][BPh4]: A solution of 1 (0.358 g, 0.529 mmol) in toluene was
added to a suspension of [FeCp2][BPh4] (0.267 g, 0.529 mmol) in
toluene at −60 °C. The temperature was allowed to rise to room tem-
perature, and the mixture was stirred overnight. The red/orange
solution obtained was evaporated to dryness and the residue extracted
with Et2O. Evaporation of the Et2O solution to dryness led to an orange
solid. After sublimation to remove the residual ferrocene, 9 is the major
9436
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as implemented in Gaussian 03.52 The molecular cavity was based on the
united atom topological model applied on UAHF radii, optimized for
the HF/6-31G(d) level. Three-dimensional structures were obtained
with ChemCraft53 and the spin density with Molekel.54
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ASSOCIATED CONTENT
* Supporting Information
■
́
C. Molecules 2012, 17, 14700−14732. (d) Ramon, D. J.; Yus, M. Chem.
S
Rev. 2006, 106, 2126−2208.
Tables with atomic coordinates of all the optimized molecules.
Table with selected structural parameters for cocrystal 11·12.
X-ray crystallographic data for 3, 4, 3·7, 8a, and 9 are available as
an electronic file in CIF format. 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: ana.martins@ist.utl.pt.
Notes
■
116, 986−997. (g) Gansauer, A.; Narayan, S. Adv. Synth. Catal. 2002,
̈
344, 465−475. (h) Gansauer, A.; Lauterbach, T.; Narayan, S. Angew.
̈
The authors declare no competing financial interest.
Chem., Int. Ed. 2003, 42, 5556−5573. (i) Justicia, J.; Cienfuegos, L. A.;
Campana, A. G.; Miguel, D.; Jakoby, V.; Gansauer, A.; Cuerva, J. M.
̃
̈
ACKNOWLEDGMENTS
The authors thank the Portuguese NMR Network (IST-UTL
Center) for providing access to the NMR facilities and Fundaca
Chem. Soc. Rev. 2011, 40, 3525−3537.
■
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para a Ciencia e a Tecnologia, Lisbon, Portugal, for funding
(SFRH/BD/28762/2006, SFRH/BPD/7394/2010, PTDC/
QUI/66187/2006, Pest-OE/QUI/UI0100/2011, and PEst-
OE/QUI/UI0612/2013).
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