Selective Incorporation of Primary Amines into a Trizirconium Imido System and Catalyic Cyclization of Aminoalkynes

Article abstract of DOI:10.1021/acs.inorgchem.7b01311

The trinuclear zirconium imido complex [{LZr(NMe₂)}₃] (2, L = C₅Me₄CH₂CH₂N) was synthesized by amine elimination between Zr(NMe₂)₄ and endo-olefinic isomers of (tetramethylcyclopentadienyl)ethanamine (LH₃) (1). To study the fundamental reactivity of the trizirconium system, reactions of 2 with primary amines were examined. Selective incorporations of the primary amines were observed, depending on steric and electronic natures of the amine substrates. The amine-incorporated complexes [(LZrNHR)(LHZrNHR)(LHZr)(μ-NHR)(μ₃-NR)] (3, R = Pr, Et), [(LZrNHR)₂(LHZr)(μ-NR)] (4, R = Pr, i-Bu), [(LZr)₂(LZrNMe₂)(μ-NR)] (5, R = neo-Pen), and [(LZr)(LZrNHAr)(LH₂Zr)(μ-NAr)₂] (6, Ar = Ph, C₆H₄-4-Br, C₆H₄-4-OMe) were structurally characterized by NMR and XRD analysis and showed several coordination modes of the substrate nitrogen ligands: i.e., terminal amides, bridging amides, and bridging imides but not terminal imides. Thermolysis of a mixture of 3 and 4 led to C-H bond activation, giving rise to the zirconaaziridines [{LZr(η²-NCHR)}(LZr)(LHZr)(μ-NHCH₂R)] (12, R = Et, Me). Complex 2 proved to be a competent precatalyst in the hydroamination of the aminoalkynes (H₂NCH₂CR¹₂CH₂C=CR²) (13, R¹ = H, R² = Bu, Ph, t-Bu; 14, R¹ = Me, R² = Et, Ph). Stoichiometric or semicatalytic reactions of 2 and the aminoalkynes were studied to explore the reactivity of in situ formed Zr₃ species.

Full text of DOI:10.1021/acs.inorgchem.7b01311

Article
pubs.acs.org/IC
Selective Incorporation of Primary Amines into a Trizirconium Imido
System and Catalyic Cyclization of Aminoalkynes
Masataka Oishi,*,† Yusuke Nakanishi,‡ and Hiroharu Suzuki‡
‡
†School of Materials and Chemical Technology and Graduate School of Science and Engineering, Tokyo Institute of Technology,
2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan
S
* Supporting Information
ABSTRACT: The trinuclear zirconium imido complex [{LZr(NMe2)}3] (2, L = C5Me4CH2CH2N) was synthesized by amine
elimination between Zr(NMe2)4 and endo-olefinic isomers of (tetramethylcyclopentadienyl)ethanamine (LH3) (1). To study the
fundamental reactivity of the trizirconium system, reactions of 2 with primary amines were examined. Selective incorporations of
the primary amines were observed, depending on steric and electronic natures of the amine substrates. The amine-incorporated
complexes [(LZrNHR)(LHZrNHR)(LHZr)(μ-NHR)(μ3-NR)] (3, R = Pr, Et), [(LZrNHR)2(LHZr)(μ-NR)] (4, R = Pr, i-Bu),
[(LZr)2(LZrNMe2)(μ-NR)] (5, R = neo-Pen), and [(LZr)(LZrNHAr)(LH2Zr)(μ-NAr)2] (6, Ar = Ph, C6H4-4-Br, C6H4-4-
OMe) were structurally characterized by NMR and XRD analysis and showed several coordination modes of the substrate
nitrogen ligands: i.e., terminal amides, bridging amides, and bridging imides but not terminal imides. Thermolysis of a mixture of
3 and 4 led to C−H bond activation, giving rise to the zirconaaziridines [{LZr(η2-NCHR)}(LZr)(LHZr)(μ-NHCH2R)] (12, R
= Et, Me). Complex 2 proved to be a competent precatalyst in the hydroamination of the aminoalkynes (H2NCH2CR1 CH2C
2
CR2) (13, R1 = H, R2 = Bu, Ph, t-Bu; 14, R1 = Me, R2 = Et, Ph). Stoichiometric or semicatalytic reactions of 2 and the
aminoalkynes were studied to explore the reactivity of in situ formed Zr3 species.
Chart 1. Zirconium Polyamidoimidonitrido Complexes9,10
INTRODUCTION
■
Zirconium nitride (ZrN) thin films have many applications
deriving from their unique properties such as hard protecting
coatings,1 diffusion barriers,2 gate materials,3 and Josephson
junctions.4 A variety of deposition techniques for ZrN thin
films have been developed using certain zirconium precursors
with NH3, H2/N2, or the forming gas plasma.5−7 Among the
precursors Zr(NMe2)4 is the most representative, enabling mild
and pinhole-free film growth. Thermal decomposition and
deposition mechanism of such homoleptic zirconium amido
precursors have been studied.8 Zirconium polyamidoimidoni-
tride structures may be formed as intermediates in the process,
and the relevant molecular complexes containing amidoimido-
nitride substructures were reported previously.9−12 Wolczanski
workers reported reactions of zirconocene dichlorides with
potassium in liquid ammonia, giving the pentanuclear
zirconium complex IIb (X = η5-C5H4CH3, Chart 1) with loss
of one cyclopentadienyl ligand.10a,b More recently, Xue and co-
studied ammonolysis of XZr(CH2Ph)3 (X = t-Bu3CO) with
NH3 (1.0−1.75 equiv) and obtained hexa- and pentanuclear
zirconium complexes, (XZr)6(μ6-N)(μ3-NH)6(μ-NH2)3 (I)
and (XZr)5(μ5-N)(μ3-NH)4(μ-NH2)4 (IIa) (X = OCt-Bu3,
Chart 1).9a Further ammonolysis of IIa led to the formation of
a polyamidonitrido complex. Independently, Roesky and co-
Received: May 22, 2017
Published: August 7, 2017
© 2017 American Chemical Society
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Scheme 1. Formation of Zr3 Imido Complex 2
workers investigated the reaction between M(NMe2)4 (M = Zr,
Hf) and dioxygen, resulting in the formation of the trinuclear
oxo aminoxido complex M3(NMe2)6(μ-NMe2)3(μ3-O)(μ3-
ONMe2) with liberation of Me2NNMe2.12 The DFT study
supports that insertion of dioxygen into M−N bonds and
cleavage of one of the O−NMe2 bonds lead to the formation of
a key Zr2 intermediate.
In spite of these structural studies of the multinuclear
zirconium complexes as the products, the reaction chemistry of
the nitrogen-rich systems has not been studied in detail and
may attract renewed attention to the chemistry of acid−base
complexes. We envisage that the coordination environment
created by a multimetallic motif could render multiple substrate
incorporation and/or multisite activation of a single substrate,
whereby bridging nitrogen ligands could serve as proton
acceptors toward protic substrates, with cooperation of the Zr
centers as Lewis acidic coordination sites. Accordingly, it is a
highly challenging task to identify reactive nitrogen-bridged
multizirconium systems under stoichiometric and catalytic
reaction conditions. Herein we report the synthesis and
structure of Zr3 imido complexes and reactions with primary
amines.
RESULTS AND DISCUSSION
Figure 1. Crystal structure of 2 (ellipsoids set at 40% probability,
hydrogen atoms omitted for clarity). Selected bond lengths (Å) and
angles (deg): Zr(1)−N(1) 2.064(5), Zr(2)−N(2) 2.022(7), Zr(3)−
N(3) 2.039(7), Zr(1)−N(2) 2.093(7), Zr(2)−N(3) 2.102(7), Zr(3)−
N(1) 2.083(5), Zr(1)−N(4) 2.080(5), Zr(2)−N(5) 2.079(7), Zr(3)−
N(6) 2.073(6), Zr(1)−Zr(2) 3.713, Zr(2)−Zr(3) 3.782, Zr(3)−Zr(1)
3.759; N(1)−Zr(1)−N(2) 108.(2), N(2)−Zr(2)−N(3) 109.5(2),
N(3)−Zr(3)−N(1) 106.6(2).
■
Synthesis and Structure of Zr3 Imido Complex 2.
Chelating cyclopentadienyl-amide ligands have been recognized
as excellent ancillary ligands for group 4 metal complexes in
many catalytic reactions and polymerizations.13−17 In contrast
to the dianionic ligand class, chelating cyclopentadienyl-imide
systems are far less prevalent. The synthesis and structure of
dinuclear niobium and titanium complexes bearing a cyclo-
pentadienyl-propylimide ligand were previously reported by
Green and Mountford.18 We decided to exploit the known
(2,3,4,5-tetramethylcyclopenta-1,4-dienyl)-1-ethanamine (1) as
a supporting ligand and initially attempted metalation of 1 with
Zr(NMe2)4. After several examinations, we found that the Zr3
imido complex 2 could be obtained from the endo-diene
isomers of 1 (including the exo isomer <5%) in a moderate
yield (Scheme 1).19 There are two possible stereoisomers for
complex 2: up−up−up and up−up−down orientations for the
three NMe2 groups over the six-membered Zr3N3 ring.
Solution- and solid-state structural characterizations revealed
specific formation of the C1-symmetric up−up−down structure
(Figure 1). In the Zr3 core containing three supporting imides,
the mean distance of the Cp,N-nonchelating Zr−N bonds
(2.093 Å) is comparable to that of the Cp,N-chelating bonds
(2.042 Å). This may imply robustness of the μ-imido linkages.
In fact, the Zr3 core displayed inertness toward some Lewis
bases: e.g., THF and 4-(dimethylamino)pyridine.20
Amine Incorporations. To find out the fundamental
reactivity of 2 toward nitrogen-containing substrates, reactions
of 2 with several primary amines were examined, and the results
are summarized in Scheme 2. An NMR-scale reaction of 2 with
ca. 9 equiv of n-PrNH2 was performed in C6D6 at 25 °C. Four
equivalents of n-PrNH2 was smoothly consumed, and a new
complex 3akinetic was initially observed. After 16 h, instead of the
initially observed structure (3akinetic), a more thermodynami-
cally stable structure (3a) emerged in the NMR spectrum (92%
NMR yield), still containing 5 equiv of free n-PrNH2 (Figure
S2 in the Supporting Information). The structure of 3a was
determined by NMR and XRD analysis, as shown in Scheme 2
(Figure S4 in the Supporting Information). Interestingly, use of
decreased amounts of n-PrNH2 or exposure of 3a to reduced
pressure gave a mixture of the two complexes 3a and 4a. In
practice, the ratio 3a/4a varied by addition or removal of n-
1
PrNH2. The structure of metastable 4a was determined by H
NMR and preliminary XRD analysis (Figure S5 in the
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Scheme 2. Reactivty of 2 toward Primary Amines
Figure 2. Crystal structures of 4c (left) and 6b (right). Thermal ellipsoids are set at 40% probability, and carbon atoms of the supporting ligands and
hydrogen atoms except NH protons are omitted for clarity.
Supporting Information). Similar behavior was observed by
NMR analysis when 2 was treated with a THF solution of
EtNH2, affording 3b via 3bkinetic. Treatment of 2 with an excess
amount of i-BuNH2 under similar conditions gave 4c
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Scheme 3. Formation of Monoaryloxide 7
Scheme 4. Reaction of 8 with neo-PenNH2
1
selectively, which was characterized by NMR and XRD analysis
to be a three-amine-incorporated complex (Figure 2, left). The
crystal structures of 4a,c have common structural features; one
of the supporting imide ligands is slipped to the Zr3 face, and
the three substrate nitrogen ligands coordinate to Zr centers in
the up−up−down orientation as one μ-imide and two terminal
amides, respectively. The solid-state structures of 4 are quite
consistent with the solution structures as assigned by NMR
analysis.
Upon addition of neo-PenNH2 to complex 2, only two Me2N
ligands were liberated, affording the μ-neopentylimido complex
5, in which three μ-imide supporting ligands remained intact
(Figure S6 in the Supporting Information). The rest of the
Me2N group in 5 could not be replaced with neo-PenNH2 even
at 90 °C. Reaction of 2 with the much bulkier t-BuNH2 did not
occur even under more forcing conditions (3 days at 90 °C),
resulting in recovery of 2. In the above reactions of 2 with the
primary amines more than two Me2N ligands are eliminated,
and we attempted to verify the most reactive Me2N group in
complex 2. 2,6-Di-tert-butylphenol was employed as a bulky
protic reagent for this purpose, and monoaryloxide 7 was
formed from 2 and the phenol as the sole product in high yield
(Scheme 3). XRD analysis of 7 unambiguously clarified the
regiochemistry (Figure S7 in the Supporting Information). The
reactivity difference between the two adjacent NMe2 groups
and the significant deviation of the N angles of neo-PenNZr2
moiety in 5 (Zr(1)−N(4)−Zr(3) = 95.08(14)°, C(34)−N(4)−
Zr(1) = 94.2(3)°, C(34)−N(4)−Zr(3) = 156.1(3)°) can be
ascribed to the steric effect of the peralkylated cyclopentadienyl
ligand.21
under identical conditions. H NMR spectra of 6a−c are quite
similar to each other with respect to resonance patterns for the
ethylene-chain protons of the supporting ligands. Complex 6b
was characterized crystallographically. As shown in Figure 2, the
structure of 6b, in contrast to 4a,c, reveals that three
bromoanilines lie on the same side of the Zr3 plane (up−
up−up orientation) and two of three bromoanilines coordinate
as μ-imido ligands and the third bromoaniline coordinates as a
terminal amide. Complex 6b has a quite long Zr−Nsupporting
distance (Zr(3)−N(3) = 2.463(7) Å), assigned as a terminal
amine ligand on the basis of these NMR and XRD results.22
In addition, 1H NMR spectra of 6 recorded at 298 K showed
restricted rotation of one of the two μ-N−Caromatic bonds,
nonequivalent NH2,support (6a, δ 2.00 and 0.78 ppm; 6b, δ 1.73
and 0.52 ppm; 6c, δ 2.02 and 0.89 ppm), and screened
resonances for one of three sets of the ethylene-chain protons
(6a, δ 1.75−1.30 ppm; 6b, δ 1.70−1.20 ppm; 6c, δ 1.75−1.44
ppm). A variable-temperature 1H NMR study of 6b was carried
out. The spectra in the aromatic region are shown in Figure S3
in the Supporting Information. Two signals for the m-Ar-H of
the μ-NAr group, restricted at ambient temperature (δ 7.47 and
7.31 ppm), coalesced at 333 K (marked with solid circles).
When the temperature was lowered to 233 K, two doublet
signals for the other μ-NAr′ group decoalesced into four broad
signals (open squares) while the terminal NH−CAr″ bond was
rotating even at 193 K (solid triangles). A NOESY spectrum
recorded at 273 K showed correlation between signals for o-Ar-
H and o-Ar″-H (δ 6.82−6.40 ppm), indicating that the NAr
ligand is located at a position cis to the NHAr″ ligand. These
observations suggest that the three aniline ligands in 6 are
located in significantly different environments over the
temperature range and do not undergo site exchange on the
Zr3 system.
The reaction of 2 with excess amounts of unsubstituted
aniline, 4-bromoaniline, or p-anisidine at 25 °C led to the
formation of a Zr3 complex (6a−c), incorporating three
molecules of aniline (Scheme 2), whereas no reaction was
observed with the sterically demanding 2,4,6-trimethylaniline
In the case of a related mononuclear bis(amido) complex,
Me2Si(C5Me4)(Nt-Bu)Zr(NMe2)2 (8), reaction with neo-
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Chart 2. Veff (Å3) for NMe2 Group(s) Estimated from X-ray
Structures of 2 and Reported Half-Zirconocene
Bis(amides)27,28
PenNH2 (excess) in a sealed NMR tube resulted in an
equilibrium mixture containing mono-, bis-, and tris-
(neopentylamido) complexes 9−11 and HNMe2 (Scheme 4).23
In this context, the selective reactions of 2 with the amines
are noteworthy. We deem that the number of amines
incorporated into the Zr3 core may be dependent upon two
factors: (i) NH acidity of the amines (pKa: anilines (3.9−5.3 in
H2O) vs aliphatic amines (10.2−10.7 in H2O))24 and (ii) the
size of alkyl groups on the N atom (t-Bu ≫ neo-Pen ≫ i-Bu >
n-Pr). Complex 4c was treated with 6 equiv of 4-bromoaniline
in C6D6, and the reaction was monitored by 1H NMR analysis.
After the reaction mixture was heated at 60 °C for 2 h, relatively
clean formation of 6b was observed with liberation of i-BuNH2.
This observation accounts for the order of the NH acidity:
namely, the pKa dependence. The origin of the latter size
selectivity of 2 toward amines, in particular aliphatic amines,
may be addressed in part by comparison of effective volumes
(Veff) of the actor NMe2 ligands in the crystal structure of 2
because of the consistency between the solution- and solid-state
structures. According to the definition by Ohashi and co-
workers,25,26 Veff values of the NMe2 groups in 2 and known
mononuclear zirconium bis(amido) complexes bearing a
permethylated cyclopentadienyl ligand, [Me2Si(C5Me4)(E)Zr-
(NMe2)2] (E = N-t-Bu,27a NCHMePh,27b NC6H4-4-CH
CH2,27c N(2-Py),27d C2B10H227e) and [(C5Me5)-
(salicyloxazoline)Zr(NMe2)2] (salicyloxazoline = OC6H2-2,4-
t-Bu2-6-(4,4-Me2-4,5-dihydrooxazol-2-yl), OC6H2-2,4-t-Bu2-6-
(4-t-Bu-4,5-dihydrooxazol-2-yl))28 were approximated by the
SV-Cavity (Figure 3, Figure S8 in the Supporting Information,
ratio of four complexes slightly changed to 3a:4a:12a:12a′ =
38:13:38:11 with a decreased amount of free n-PrNH2.
Repeated thermolysis/evaporation resulted in nearly complete
consumption of 3a and 4a, leaving complexes 12a and 12a′.
1
The uniform ratio of 12a and 12a′ (∼3.3:1) observed by H
NMR spectra and SST experiments (Figure S9 in the
Supporting Information) showed that 12a and 12a′ are in
equilibrium in solution. Although the minor product 12a′ could
not be characterized clearly, COSY, HSQC, and HMBC
experiments of the mixture suggested that the major product
12a contained not only μ-NHPr-n ligand (NH protons: δ 0.44
ppm) but also one zirconaaziridine substructure ((κN:η2-NCH-
Et): δ(13C/1H) 91.2/(2.15−2.09) ppm).29 Likewise, the
corresponding EtNH2-derived complexes 12b and 12b′ were
formed by thermolysis of 3b. We have not been able to obtain
crystal structures of 12 or 12′ and tentatively propose the most
plausible structure for 12a by a ROESY experiment (Figure S10
in the Supporting Information). Formation of multinuclear
group 4 metalaaziridines was reported by Schafer and co-
workers;30,31 a dinuclear titanaaziridine was isolated and
characterized by crystallography. In addition, generation of
the related dinuclear zirconaaziridines from primary amines in
intramolecular hydroaminoalkylation is proposed. Accordingly,
we summarize the proposed equilibrium of the Zr3 complexes
with such n-alkylamines in Scheme 5.
Catalytic Cyclization of Aminoalkynes. Hydroamination
has been intensely studied for the last several decades using a
variety of metal complexes.32 In particular, intramolecular
hydroamination is of synthetic importance, because it provides
an atom-economical route for the synthesis of nitrogen-
containing heterocycles. It would be an appropriate test
catalytic reaction using complex 2 as a potential precatalyst.
We undertook NMR-scale cyclization of five aminoalkynes (13
and 14) in C6D6 (Table 1). In the cyclization of 13a and 13b
heating at 90 °C was required to complete the reaction, and the
corresponding cyclic imines 15a and 15b were produced in
excellent yields (entries 1 and 2). Substrate 13c with a t-Bu
Figure 3. Representation of Veff for two adjacent NMe2 groups in 2.
and Chart 2). These data indicate that the Veff value in complex
2 is significantly small among the relevant chelating cyclo-
pentadienyl-amido complexes, even smaller than that for a
salicyloxazoline system with a four-legged piano-stool geome-
try. Therefore, the space available for incoming substrates in the
Zr3 system seems to be significantly limited by the three up−
up−down peralkylated cyclopentadienyl-imide ligands in 2.
Thermolysis of 3 and 4. The above equilibrium between 3
and 4 led us to study further elimination of the amines under
thermolytic conditions. A C6D6 solution of a mixture of 3a and
4a (57:43) was heated at 100 °C for 1 h, and then a mixture of
the new complexes 12a and 12a′ was obtained with free n-
PrNH2 and complexes 3a and 4a (3a:4a:12a:12a′ =
24:19:44:13). After the solution stood at 25 °C for 9 h, the
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a
Scheme 5. Proposed Equilibrium between Zr3 Species and n-Alkylamines
a
R = H, Me, etc. The most plausible structure is given for 12.
a
group on the alkyne carbon atom needed higher temperature
and longer reaction time (entry 3). On the other hand,
substrates 14 with gem-dimethyl substitution β to nitrogen
cyclized under much milder conditions than for 13 (entries 4
and 5).
Table 1. Catalytic Cyclization of 13 and 14
Group 4 terminal and bridged imido complexes have been
well characterized,21b,33 and the terminal imide structure is
recognized as the reactive species in catalytic hydroaminations:
in particular, alkyne and allene hydroaminations.34 On the
other hand, mechanisms for alkene hydroaminations by group 4
metal catalysts, more investigated than alkyne hydroaminations,
are currently argued to have three major mechanisms: (i) a
migratory insertion into the metal−nitrogen bond,15b,35 (ii) an
imido mechanism via [2π + 2π] cycloaddition,36 and (iii) a
proton-assisted mechanism via a six-centered, concerted
transition state for C−N and C−H bond formation (Scheme
6).37 According to Scheme 2, primary amines are incorporated
into the Zr3 core as bridging imides, amides, and terminal
amides, whereas no terminal imido structure is observed in the
b
entry substrate
R1, R2
conditions (°C, h) product yield (%)
1
2
3
4
5
13a
13b
13c
14a
14b
H, Bu
90, 6
15a
15b
15c
16a
16b
>95
>95
81
H, Ph
90, 6
H, t-Bu
Me, Et
Me, Ph
110, 120
40, 3
>95
>95
40, 2
a
The reaction was carried out in a J. Young NMR tube and monitored
by 1H NMR analysis using an internal standard: 2 (2.5−4.7 μmol), 13
b
or 14 (20 equiv), C6D6 solvent (0.45 mL). NMR yield determined on
the basis of (Me3Si)2CH2 as the internal standard.
Scheme 6. Three Simplified Mechanisms for Aminoalkene Cyclizations (e.g., M = Zr, Ti)15b,35−37
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system. Therefore, a mechanism for the hydroamination using
complex 2 might be incompatible with the well-recongnized
imido mechanism for many group 4 metal systems.
Accordingly, stoichiometric or semicatalytic cyclizations were
examined to explore the reaction behaviors of Zr3-based
species.
Four molecules of 13 were readily incorporated into the Zr3
1
complex at ambient temperature. In fact, H NMR spectra for
the catalytic cyclizations of these substrates in C6D6 recorded
before heating (0.5 h at 25 °C) showed characteristic signals at
low field for parts of ethylene protons of the supporting ligands
of 17kinetic (17akinetic, δ 4.77−4.68 (2H, m), 4.47 (1H, dd),
4.00−3.91 (1H, m) ppm; 17bkinetic, δ 4.78−4.70 (2H, m), 4.47
(1H, dd), 4.00−3.90 (1H, m) ppm; 17ckinetic, δ 4.74−4.65 (2H,
m), 4.35 (1H, dd), 3.96−3.88 (1H, m) ppm), signal patterns
which are quite similar to those of 3kinetic (Figure S11 in the
Supporting Information). For spectral simplicity, substrate 13c
was chosen for the stoichiometric study in this work. After a
mixture of complex 2 and 13c (1:7.5) in C6D6 stood at 25 °C
for 20 h, analogously to the formation of 3, the majority of the
initially observed 17ckinetic was replaced by the more
thermodynamically stable form 17c (eq 1 and Figures S12
proceeded at 25 °C significantly faster than the initial
stoichiometric amine incorporations, resulting in low con-
version of 2 and catalytic formation of 16. Owing to the lack of
information under these conditions, semicatalytic reactions of
14 using 15 mol % of 2 were undertaken, and the metal-based
products were monitored by 1H NMR spectroscopy. The NMR
spectra for the reaction of 14b (C6D6 solvent, 25 °C) are
1
shown in Figure 4. The H NMR spectrum at the initial stage
(e.g., t = 30 min) indicated the presence of almost a single new
species, 19b, on the basis of 1 set of 12 singlet signals for 3
C5Me4 ligands (δ 2.26−1.92 ppm). Because 19b disappeared at
the end of the semicatalytic reaction, the structure was
1
determined only by comparison of the H NMR spectrum
with those of 5 to be a mono-μ-imido complex (Figure S15 in
the Supporting Information) (characteristic resonances: NMe2
group, δ 2.80 ppm (solid circles in Figure 4); three CH2Nsupport
moieties, δ 5.07 (2H, m), 4.93 (1H, dd), 4.78 (2H, m), and
4.47 ppm (1H, dd)). After thorough consumption of substrate
14b (t ≈ 2 h), complex 19b was replaced by another new
species, 20b, which has two NMe2 groups (δ 2.89 and 2.74
ppm (1:1); open squares in Figure 4) (Scheme 7).
Complex 20b could not be isolated, and the structure was
tentatively determined by COSY, ROESY, HSQC, and HMBC
spectroscopic experiments to be a tris(amide) with a 2-
benzylidene-pyrrolidide ligand (characteristic signals for
benzylidene CCHPh moiety: δ(1H) 5.25 ppm and δ(13C)
95.5 ppm (Supporting Information and Figure S16)). Active
metal complexes were also observed in the semicatalytic
reaction of 14a. However, the NMR spectra became
complicated at the end of the reaction; therefore, no other
species were identified. The two possible intermediates Inta and
Intb between 19b and 20b are proposed in Chart 3. Inta is
directly associated with a migratory insertive mechanism of the
catalytic reaction, whereas Intb is a requisite intermediate for a
concerted proton-assisted cyclization, directly leading to 20b.
Although further studies are required to prove their existence
and to determine the mechanism of the catalytic reaction,
formation of the Me2NH adduct 20b indicates significant
reactivity of the bridging imide moiety in 19b.
and S13 in the Supporting Information), which we were not
able to isolate but whose 13C NMR spectrum clearly showed
that 17c contained 4 mol equiv of 13c. After it was heated in
C6D6 at 100 °C for 1 h, 17c disappeared, resulting in the
formation of the new complexes 18c and 18c′ (4.3:1) together
with the organic product 15c (eq 2). The major product, 18c,
was detected throughout the above NMR-scale catalytic
reaction. Although isolation of 18c was not successful, NMR
analysis showed that 18c was an analogue of 12 incorporating
two amine substrates, on the basis of the presence of a bridging
Nsub-H proton (δ 0.37 ppm) and a bridging N′sub-CH (κN:η2-
N′CHCH2CH2CCBu-t: δ(13C/1H) 87.9/(2.32−2.22) ppm)
(Figure S14 and the Supporting Information). An attempted
NMR-scale thermolytic cyclization of 18c and 18c′ (C6D6
solvent at 100 °C) resulted in no ligand cyclization. Therefore,
we currently surmise that zirconaaziridines 18 and 18′ are
resting states and incorporate more amine substrates for the
catalytic reaction.
CONCLUSIONS
■
We have synthesized and structurally characterized the
trinuclear zirconium complex 2 with three cyclopentadienyl-
imide ligands. The multimetallic motif allowed us to observe
Next, stoichiometric reactions of 2 with substrates 14 were
examined. For 14 the subsequent catalytic cyclizations
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Figure 4. Partial 1H NMR spectra for the semicatalytic reaction of 14b using 2 (15 mol %) (25 °C in C6D6). [14b]0 = 0.075 M. Open triangles, solid
circles, and open squares show Me2N signals for complexes 2, 19b, and 20b, respectively. The asterisk and double asterisk denote signals for
substrate 14b and product 15b, respectively.
Scheme 7. Formation of Me2NH Adduct 20b
selective incorporation of primary amines, depending upon
their size and electronic nature. For instance, n-PrNH2 as a
relatively small substrate displayed several types of incorpo-
ration into the Zr3 system (two to four amine molecule
incorporation); in sharp contrast, only a single molecule of neo-
PenNH2 can be bound as a bridging imide ligand. The selective
reactivity may be ascribed to both the flexibility and robustness
of the supporting bridging imide ligands, which can adequately
serve as proton acceptors toward incoming amine substrates
and be converted to terminal amine, bridging amides, and μ-
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Notes
Chart 3. Possible Intermediates between 19b and 20b
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof Shigeru Ohba from Keio University and Dr.
Hiroyasu Sato from Rigaku Corp. for assistance with X-ray
structure determinations and Dr. Takashi Nemoto from Kyoto
University for valuable discussions and assistance with SV-
Cavity software.
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Experimental procedures and spectroscopic and crystallo-
graphic data of new compounds (PDF)
Accession Codes
CCDC 1551050−1551055 and 1551396 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.O.: oishi.m.ac@m.titech.ac.jp.
■
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ORCID
Masataka Oishi: 0000-0002-5100-4066
Funding
This research was supported generally by Tokyo Institute of
Technology and partially by the Japan Society for the
Promotion of Science (Grant No. 16K05793).
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characteristic signal for a new complex at high field (δ −2.07 ppm)
were observed in a 1H NMR spectrum. Further characterization of this
complex was not performed.
1
were proposed by H NMR analysis of the reactions under slightly
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