Pyridonate-Supported Titanium(III). Benzylamine as an Easy-To-Use Reductant

Article abstract of DOI:10.1021/acs.organomet.5b00469

The reaction of bis(3-phenyl-2-pyridonate)Ti(NMe₂)₂ with excess benzylamine leads to an unexpected reduction of the metal center from Ti(IV) to Ti(III). The reduced titanium species was isolated and revealed as tris(3-phenyl-2-pyrido

Full text of DOI:10.1021/acs.organomet.5b00469

Article
pubs.acs.org/Organometallics
Pyridonate-Supported Titanium(III). Benzylamine as an Easy-To-Use
Reductant
Eugene Chong,† Wei Xue,† Tim Storr,‡ Pierre Kennepohl,† and Laurel L. Schafer*,†
†Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1
‡Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, Canada V5A 1S6
S
* Supporting Information
ABSTRACT: The reaction of bis(3-phenyl-2-pyridonate)Ti-
(NMe2)2 with excess benzylamine leads to an unexpected
reduction of the metal center from Ti(IV) to Ti(III). The
reduced titanium species was isolated and revealed as tris(3-
phenyl-2-pyridonate)Ti(NH2Bn)2. Ammonia and N-benzyl-1-
phenylmethanimine are released as byproducts of the reaction,
thereby confirming benzylamine as a mild reductant. This new
pyridonate-supported titanium(III) complex has been fully
characterized, and experimental data and theoretical calculations confirm a d1 metal center with no ligand-based radical
character. This Ti(III) complex does not react with aminoalkenes, suggesting that radical species are not viable intermediates for
the hydroaminoalkylation or hydroamination reaction.
INTRODUCTION
■
Reduced titanium species are versatile reagents and catalysts for
mediating various bond-forming reactions.1−14 Many of these
previous reports involve the use of titanocene complexes for
which titanium(III) complexes, such as Cp2TiCl, can be
generated from Cp2TiCl2 by the addition of stoichiometric
amounts of reducing metals.15−18 Indeed, strong reducing
agents, including alkali-metal and alkaline-earth-metal amal-
gams, naphthalenides, and metal−alkyl reagents are typically
used for such reductions, which could lead to undesirable over-
reduced metal impurities or salt byproducts.19 Alternative
This difference in reactivity results from an unexpected change
in mechanism when using amidate vs pyridonate catalysts.32,34
methods have been reported for the generation of titanium(III)
We have proposed pyridonate dinuclear species as key
intermediates for promoting hydroaminoalkylation preferen-
tially.32,34 Alternatively, such mechanistic variability could result
from accessing reduced metal species that would promote a
different reaction path.
species from titanium(IV) precursors under salt-free conditions.
Such approaches include the thermal homolysis of select Ti−
O20 or Ti−C bonds21 or the use of organosilicon reducing
agents.22 Recent reports have also shown that delocalized
radicals formed by the addition of alkyl radicals to arenes can
reduce Ti(IV) to Ti(III).23,24 To our knowledge, the use of a
simple amine has not been reported for the clean reduction of
Ti(IV) to Ti(III) species, although the use of tertiary amines,
such as NEt3, are known reductants for mid to late transition
metals.19,25−29 In this work, we report the reduction of Ti(IV)
pyridonate complexes to pyridonate-supported Ti(III) com-
pounds using benzylamine as the reducing agent.
Previous studies in our group have focused on bis(N,O)-
ligated titanium systems30−34 for applications in the catalytic
synthesis of amines using hydroamination35,36 and hydro-
aminoalkylation37,38 reactions. Recent reactivity investigations
of group 4 pyridonate complexes have revealed unique trends
that distinguish these systems from reported amidate and
ureate hydroamination precatalysts.32,34 Specifically, group 4
pyridonate complexes (e.g., 1) can be used to selectively
promote hydroaminoalkylation over hydroamination (eq 1).34
Benzylamine is a known challenging substrate for hydro-
amination, which is prone to promoting catalyst decomposition
and often gives mixtures of products in titanium-catalyzed
hydroamination reactions.39−42 The nature and the possible
identity of such decomposed species have not been reported
thus far. Herein, we provide insight for such species using our
N,O-ligated titanium systems with pyridonates as the
supporting ligand in stoichiometric reaction studies with
benzylamine. We report the synthesis and characterization of
a novel tris(2-pyridonate)titanium(III) complex using benzyl-
amine as an unexpected reductant. This newly obtained
titanium(III) species has also been evaluated for its viability
in aminoalkene hydroamination/hydroaminoalkylation.
Received: June 3, 2015
Published: September 28, 2015
© 2015 American Chemical Society
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Article
sign of bond elongation that would be consistent with a ligand-
centered radical (Table S1 in the Supporting Information).45
Notably, complex 2 is NMR silent due to its paramagnetic
character.
RESULTS AND DISCUSSION
■
The reaction of bis(2-pyridonate)-supported titanium complex
134 (Scheme 1) with benzylamine gave a deeply colored
Given that the tris(3-phenyl-2-pyridonate)titanium(III)
complex 2 was isolated from the reaction using the bis(3-
phenyl-2-pyridonate)titanium(IV) precursor 1 (Scheme 1), we
speculated that ligand redistribution of complex 1 preceded the
reduction of the metal center from Ti(IV) to Ti(III). The
hemilabile nature of the pyridonate ligands may facilitate such
ligand redistribution. Thus, tris(3-phenyl-2-pyridonate)Ti-
(NMe2) (3) was first prepared from the protonolysis reaction
of Ti(NMe2)4 and 3 equiv of 3-phenyl-2-pyridone (Scheme 2).
Scheme 1. Reaction of 1 with Benzylamine To Access a
Ti(III) Complex
solution that has a 1H NMR spectrum with broad signals of low
intensity, suggesting the formation of paramagnetic species.
Fortuitously, small quantities of brown crystals suitable for X-
ray crystallographic analysis were isolated from recrystallization
in a toluene/pentane mixture. The identity of the isolated
compound was revealed to be the tris(3-phenyl-2-pyidonate)-
titanium(III) complex 2 with two neutrally coordinated
benzylamines (Figure 1).
Scheme 2. Synthesis of Tris(3-phenyl-2-
pyridonate)Ti(NMe2)
In the solid state, complex 3 exhibits a Cs-symmetric structure
with pseudo-pentagonal-bipyramidal coordination geometry
(Figure 2). Similar to the case for 2, the pyridonate ligands
Figure 2. ORTEP representation of the solid-state molecular structure
of 3 plotted with 50% probability ellipsoids for non-hydrogen atoms.
The THF solvent molecule is omitted for clarity. Selected bond
lengths (Å) and angles (deg): Ti1−O1, 2.0889(11); Ti1−O2,
2.0572(10); Ti1−O3, 2.0432(9); Ti1−N1, 2.1864(12); Ti1−N2,
2.2101(12); Ti1−N3, 2.2053(12); Ti1−N4, 1.8799(12); O1−Ti1−
N1, 62.11(4); O2−Ti1−N2, 61.58(4); O3−Ti1−N3, 61.87(4); N4−
Ti1−O1, 94.54(4); N4−Ti1−N1, 156.43(4).
Figure 1. ORTEP representation of the solid-state molecular structure
of 2 plotted with 50% probability ellipsoids for non-hydrogen atoms.
Selected bond lengths (Å) and angles (deg): Ti1−O1, 1.9790(15);
Ti1−O2, 2.1156(15); Ti1−O3, 2.1091(15); Ti1−N2, 2.1998(18);
Ti1−N3, 2.2565(18); Ti1−N4, 2.204(2); Ti1−N5, 2.2128(19); N4−
C34, 1.481(3); N5−C41, 1.485(3); O2−Ti1−N2, 60.98(6); O3−
Ti1−N3, 60.59(6); N4−Ti1−O1, 94.11(7); N4−Ti1−N5 176.20(8);
Ti1−O1−C1, 136.56(14); C34−N4−Ti1, 124.10(14); C41−N5−Ti1,
114.83(14).
in 3 bind to titanium in an asymmetric κ2-N,O-chelating motif,
with the oxygen donor (Ti−Oavg 2.0631(10) Å) binding more
closely than the nitrogen donor (Ti−Navg 2.2006(12) Å) to the
metal center.
To determine if 3 is a suitable precursor to the reduced
complex 2, it was reacted with 3 equiv of benzylamine (Scheme
3). Using the alternative titanium(IV) precursor 3, the
Complex 2 adopts a C1-symmetric pentagonal-bipyramidal
coordination geometry with two pyridonate ligands exhibiting a
κ2-N,O-chelating motif with the oxygen donor (Ti−Oavg
2.1124(15) Å) binding closer to the titanium center than the
nitrogen donor (Ti−Navg 2.2282(18) Å) and the third
pyridonate with a κ1-O binding motif (with a much shorter
Ti−O bond, 1.9790(15) Å). This variable binding mode of the
pyridonate ligands illustrates the hemilabile nature of these
ligands. The coordination of two neutral benzylamine donors in
the axial positions is evidenced by a long Ti−Navg bond length
(2.208(2) Å). An examination of ligand bond lengths shows no
Scheme 3. Reduction of 3 to 2 with Benzylamine
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titanium(III) complex 2 can be isolated in reproducibly high
yields (83−93%). Consistent with the initial isolation
conditions, complex 2 can be recrystallized from a toluene/
pentane mixture, and X-ray crystallographic analysis confirmed
the formation of product 2 (Figure 1). Thus, the reliable
synthesis of the titanium(III) complex 2 from the tris(2-
pyridonate)titanium complex 3 suggests that bis(2-pyridonate)
titanium complex 1 is susceptible to ligand redistribution prior
to reduction of the metal center with easily handled
benzylamine.
The paramagnetic character of 2 has been further confirmed
by EPR spectroscopy (Figure 3). The spectrum is consistent
Figure 4. Spin density plot of 2.
quenched reaction mixture by GC-MS analysis shows the
presence of three compounds: benzylamine, 3-phenyl-2-
pyridone, and a signal with m/z 195 corresponding to N-
benzyl-1-phenylmethanimine.46 The broad signals observed in
1
the H NMR spectrum of the reaction mixture are due to this
imine byproduct, while complex 2 is NMR silent. The
formation of N-benzyl-1-phenylmethanimine is accompanied
by the release of ammonia. This byproduct can be trapped as
the NH4Cl salt47 and was identified by H NMR spectroscopy
1
in d6-DMSO, which revealed a diagnostic triplet of 14NH4Cl
centered at δ 7.12 with a coupling constant of 51 Hz
( J N− H).45 Thus, the release of N-benzyl-1-phenylmethani-
mine and ammonia as the only observed byproducts from the
reaction rationalizes simple benzylamine as a stoichiometric
reductant in this reaction.
With the isolation of 2 in high yield and identification of N-
benzyl-1-phenylmethanimine and ammonia as the only by-
products of the reaction, we propose the following process for
benzylamine-promoted reduction in Scheme 4. The pyridonate-
supported Ti(IV) complex 3 undergoes an amido exchange
with 1 equiv of benzylamine to give Ti(IV) intermediate A with
the release of dimethylamine. On the basis of the evidence that
titanium complexes are capable of α-C−H activation of amines
1
14
1
Figure 3. EPR spectrum (toluene, 77 K) of 2 overlaid with a
simulation.
with an axially elongated octahedral Ti(III) complex with gx =
1.948, gy = 1.958, and gz = 1.990. L/G = 0. Hyperfine splittings
due to the naturally occurring 47Ti and 49Ti isotopes, which
constitutes 12.83% natural abundance and have none-zero
nuclear spins, can also be seen in the spectrum. More
importantly, hyperfine interactions that could arise from radical
behavior in the 2-pyridonates, should a ligand-based radical be
formed, are not present. These observations suggest localization
of the unpaired electron on the titanium metal center.
Density functional theory (DFT) calculations have been used
to further investigate the geometric and electronic structure of
complex 2.45 The calculations reproduce the experimental
coordination sphere bond lengths to within 0.06 Å (Table S3
in the Supporting Information). The calculation also supports
the experimental assignment of a d1 Ti(III) metal center, with
the spin density based at the metal center (Figure 4). Thus, the
predicted electronic structure is in good agreement with the
EPR analysis, with insignificant 2-pyridonate ligand radical
behavior.
Scheme 4. Postulated Process for Reduction of Pyridonate-
Supported Ti(IV) to Ti(III) with Benzylamine
1
Careful analysis of the reaction mixture in d6-benzene by H
NMR spectroscopy and identification of organic byproducts by
GC-MS clarifies how benzylamine functions as a reductant (eq
1
2). Upon reaction completion the H NMR spectrum has
broad signals of weak intensity and signals for 3 and
benzylamine are no longer present, although a new singlet
appears at δ 4.60 in the reaction mixture.45 Examination of the
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diffractometer at the Department of Chemistry, University of British
Columbia, by Jacky C.-H. Yim or Scott Ryken.
to generate structurally characterized metallaziridine spe-
cies,32,48 and the known reactivity of group 4 pyridonates32,34
for hydroaminoalkylation reactions, we propose that the
benzylic C−H bond is cleaved by β-hydrogen abstraction by
a pyridonate ligand to form the titanaziridine intermediate B.
Group 4 metallaziridine species are known to have two
resonance structures,49,50 in which the other resonance form of
Ti(IV) species B is the formally Ti(II) species C that has the
metal bound by a η2-CN imine linkage. Considering the
literature precedent that the η2-imine can be displaced from the
metal center of a titanaziridine50 and that comproportionation
of Ti(II) and Ti(IV) can quantitatively yield Ti(III) in the
synthesis of (Cp2TiIIICl)2 from Cp2TiII(η2-C2(SiMe3)2) and
Cp2TiIVCl2,51 we postulate that A and C undergo a
comproportionation reaction to yield D. This Ti(III) species
D is then trapped by 2 equiv of benzylamine to give the isolable
complex 2. The 0.5 equiv of the released benzylamine and
phenylmethanimine then undergo a condensation reaction to
afford N-benzyl-1-phenylmethanimine and ammonia as by-
products of the reaction.
With the reduced Ti complex 2 in hand we tested this
complex as a potential intramolecular hydroaminoalkylation or
hydroamination precatalyst in eq 1. Attempts to convert 2,2-
diphenylaminoalkene with 20 mol % 2 in toluene solvent to
either hydroamination or hydroaminoalkylation cyclized
products resulted in no product formation. These results
suggest that Ti(III) species supported by pyridonate ligands are
not viable intermediates for these catalytic processes.
In summary, we have demonstrated that a tris(2-pyridonate)-
titanium(IV) complex can be cleanly reduced to titanium(III)
using benzylamine as an easy-to-use reducing agent. Such
reactivity may also rationalize why some amine substrates are
challenging substrates in catalytic reactions.39−42 The lack of
productive hydroamination or hydroaminoalkylation reactivity
using 2 suggests that such Ti(III) species do not promote
desirable reactivity but rather deleterious catalyst decomposi-
tion pathways. Work exploring the generality of benzylamine as
a useful reductant and the reactivity of such readily accessed
reduced Ti(III) species is underway.
EPR Data Acquisition and Process. EPR data were collected on
an Elexsys E500 series continuous wave EPR spectrometer (Bruker,
Billerica, MA, USA). The spectrometer was operated at a frequency of
9.40 GHZ (X-band) at 77 K, 100 kHz field modulation, 1 G
modulation amplitude, and 0.64 mW microwave power. Each
spectrum recorded was the averaged result over five scans. Frequency
calibration53 was independently verified using 2,2-diphenyl-1-picrylhy-
drazyl (DPPH, g = 2.0036) (Sigma-Aldrich, St Louis, MO, USA) as an
external standard. Spectra were simulated using EasySpin.
Synthesis of Tris(3-phenyl-2-pyridonate)Ti(NMe2) (3). A
Teflon-capped 20 mL vial, equipped with a magnetic stir bar, was
charged with 3-phenyl-2-pyridone (0.257 g, 1.50 mmol) and ∼3 mL of
benzene. In this vial, Ti(NMe2)4 (0.112 g, 0.500 mmol) dissolved in
∼2 mL of benzene was quantitatively added. The mixture was stirred
at room temperature for 4 h, forming a deep red-brown solution, and
the solvent was removed in vacuo. The product was dissolved in hot
toluene (∼10 mL), and storing at −35 °C overnight afforded a tan
brown microcrystalline solid (0.275 g, 91%). Single crystals suitable for
X-ray crystallography were obtained from a concentrated solution of
the complex in THF left at room temperature over a couple of days.
1H NMR (400 MHz, C6D6): δ 8.01 (d, 6H, 3JHH = 7.2 Hz, Ph-H), 7.77
3
4
3
(dd, 3H, JHH = 5.2, JHH = 1.8 Hz, Ar-H), 7.32 (dd, 3H, JHH = 7.4,
4JHH = 1.8 Hz, Ar-H), 7.26 (t, 6H, JHH = 7.7 Hz, Ph-H), 7.13 (t, 3H,
3
3JHH = 7.5 Hz, Ph-H), 6.06 (dd, 3H, 3JHH = 7.4, 5.2 Hz, Ar-H), 3.61 (s,
6H, − N(CH3)2). 13C NMR (100 MHz, C6D6): δ 171.5, 141.8, 139.4,
136.8, 129.1, 129.0, 127.9, 122.1, 113.2, 49.0. MS (EI): m/z 602 (M+),
558 (M+ − NMe2). Anal. Calcd for C35H30N4O3Ti: C, 69.77; H, 5.02;
N, 9.30. Found: C, 70.12; H, 5.14; N, 8.97.
Synthesis of Tris(3-phenyl-2-pyridonate)Ti(NH2Bn)2 (2). In a
small vial, complex 3 (30.1 mg, 0.0500 mmol) and benzylamine (16.4
μL, 0.150 mmol) were mixed in 1 mL of benzene. The resulting brown
solution was transferred to a J. Young NMR tube and heated to 65 °C
overnight, which gave a deep brown-red solution. The solution was
filtered through Celite and concentrated in vacuo. The crude residue
was recrystallized from a solution of the complex in toluene (∼2 mL)
at −35 °C, layered with pentane (∼4 mL), overnight to give the
product as a brown microcrystalline solid (36 mg, 93%). The reaction
was repeated twice using the same procedure at a larger scale over a
reaction time of 2 h, using 3 (60.3 mg, 0.100 mmol) and benzylamine
(32.8 μL, 0.300 mmol), to afford 64 mg (83%) and 66 mg (85%) of
the product. Single crystals suitable for X-ray crystallography were
obtained from slow diffusion of pentane into a solution of the complex
in toluene overnight. MS (EI): m/z 558 (M+ − 2BnNH2), 388 (M+ −
2BnNH2 − L), where L = 3-phenyl-2-pyridonate. Anal. Calcd for
C47H42N5O3Ti: C, 73.05; H, 5.48; N, 9.06. Found: C, 72.70; H, 5.82;
N, 9.37.
EXPERIMENTAL SECTION
■
General Considerations. All reactions were conducted in oven-
dried glassware under an atmosphere of dry dinitrogen using standard
Schlenk line and glovebox techniques. Benzene, toluene, and pentane
were passed over activated alumina columns and stored over activated
4 Å molecular sieves prior to use. d6-Benzene and d8-toluene were
dried over 4 Å molecular sieves, degassed by three freeze−pump−thaw
cycles, and stored in Teflon-sealed Schlenk flasks prior to use.
Ti(NMe2)4 was purchased from Sigma-Aldrich and used as received.
Benzylamine was distilled over CaH2 and degassed by three freeze−
pump−thaw cycles prior to use. Complex 1 and 3-phenyl-2-pyridone
were synthesized according to literature procedures.34 N-Benzyl-1-
phenylmethanimine was synthesized from benzylamine and benzalde-
hyde using a literature procedure,52 and 1H NMR spectral data match
reported literature values.46 1H and 13C NMR spectra were recorded
on a Bruker 400 MHz Avance spectrometer at ambient temperature,
and chemical shifts are given relative to the corresponding residual
protio solvent. Mass spectra were recorded on a Kratos MS-50
spectrometer using an electron impact (70 eV) source. Elemental
analyses were recorded on a Carlo Erba EA 1108 Elemental Analyzer.
GC/MS analyses were conducted on an Agilent 7890A GC equipped
with a 5975C inert XL CI mass detector using methane as the
chemical ionization agent. Single-crystal X-ray structure determina-
tions were performed on a Bruker X8 APEX II or Bruker APEX DUO
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00469.
Crystallographic data for 2 and 3 (CIF)
NMR spectra and calculations (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail for L.L.S.: schafer@chem.ubc.ca.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the NSERC for financial support of this work and a
postgraduate scholarship to E.C. We thank Jacky C.-H. Yim and
■
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Coupling Reactions and More; de Meijere, A., Brase, S., Oestreich, M.,
̈
Scott Ryken for assistance with X-ray crystallography. The
authors thank Westgrid for access to computational resources.
This work was undertaken, in part, thanks to funding from the
Canada Research Chairs program.
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