Synthesis and reactivity of a terminal scandium imido complex

Article abstract of DOI:10.1021/om300913d

Preparation of a terminal scandium imido complex, 2·DMAP, was accomplished through thermolysis of an arylamido methyl complex, 1, stabilized by a bulky β-diketiminato ligand in the presence of 4-N,N- dimethylaminopyridine (DMAP). Mechanistic studies revealed that the reaction proceeds by initial metalation of 1, followed by rapid DMAP-promoted alkane elimination to generate the scandium imido complex. Kinetic studies of the reaction between separately synthesized metalate 3 and DMAP under pseudo-first-order conditions yielded activation parameters of ΔH ^a = 73.5(2) kJ mol^-1 and ΔS ^a = -70.4(5) J K^-1 mol^-1. The reaction of 2·DMAP with tert-butyl amine or phenylacetylene resulted in addition of the N-H or C-H bond across the scandium imide linkage, respectively, to furnish complexes endo-/exo-4 and endo-5. These compounds were fully characterized, including via structural analysis, providing further evidence for the terminal scandium imido derivative 2·DMAP.

Full text of DOI:10.1021/om300913d

Article
pubs.acs.org/Organometallics
Synthesis and Reactivity of a Terminal Scandium Imido Complex
Terry Chu, Warren E. Piers,* Jason L. Dutton,† and Masood Parvez
Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4
S
* Supporting Information
ABSTRACT: Preparation of a terminal scandium imido complex,
2·DMAP, was accomplished through thermolysis of an arylamido methyl
complex, 1, stabilized by a bulky β-diketiminato ligand in the presence of
4-N,N-dimethylaminopyridine (DMAP). Mechanistic studies revealed
that the reaction proceeds by initial metalation of 1, followed by rapid
DMAP-promoted alkane elimination to generate the scandium imido
complex. Kinetic studies of the reaction between separately synthesized
metalate 3 and DMAP under pseudo-first-order conditions yielded
activation parameters of ΔH⧧ = 73.5(2) kJ mol−1 and ΔS⧧ = −70.4(5) J
K−1 mol−1. The reaction of 2·DMAP with tert-butyl amine or
phenylacetylene resulted in addition of the N−H or C−H bond across
the scandium imide linkage, respectively, to furnish complexes endo-/
exo-4 and endo-5. These compounds were fully characterized, including
via structural analysis, providing further evidence for the terminal scandium imido derivative 2·DMAP.
an α-C−H bond of coordinated pyridine could not be
prevented, and so an amido complex was the isolated product.
Our own attempts to generate β-diketiminato (“nacnac”)
supported terminal imido scandium complexes using high-
temperature-induced alkane elimination methodologies (pio-
neered by Bergman14 and Wolczanski15 for the group 4 metal
systems) with the scandium nacnac amido alkyl derivative I met
with failure.26 Very recently, however, the group of Chen solved
this problem by incorporating a pendant amine donor into the
nacnac ligand framework and induced alkane elimination by use
of an external 4-N,N-dimethylaminopyridine (DMAP) donor.
They thus were able to report the first crystallographically
characterized terminal scandium imido complex, the five-
coordinate derivative II.27 Subsequent papers from this group
disclosed a second five-coordinate derivative (III) and reported
on the reactivity of these novel species.28,29 Essentially, the use
INTRODUCTION
■
Terminal imido ligands play a central role in various aspects of
early-transition-metal chemistry. As potential 2σ−4π, six-
electron donors,1 they are isolobal with the ubiquitous
cyclopentadienide anion and have been used as a Cp substitute
in an ancillary role in the organometallic chemistry of metals
from groups 4,2−4 5, and 6.5−7 On the other hand, in part
because of the variable bond order possible in the MN bond,
they can be highly reactive ligands and serve as intermediates in
important catalytic reactions such as imine metathesis8 and
early-transition-metal-catalyzed hydroamination cycles.9−13
Furthermore, as discovered in seminal papers by Bergman14
and Wolczanski,15 terminal early-metal imides can activate the
C−H bonds of hydrocarbons, including methane.16−18 Thus,
the terminal imido ligand is a versatile chemical entity in the
chemistry of the group 4−6 metals.
Less is known about their role in group 3 metal or lanthanide
chemistry.19 Indeed, several groups have attempted to prepare
terminal imido compounds of these more electropositive metals
and map out their chemistry, but while inroads have been
made, such compounds remain rare and elusive. One issue is
the prevention of dimerization through use of a ligand
environment of suitable bulk. For example, although Hessen
et al. postulated the intermediacy of a terminal imido scandium
species supported by a cyclopentadienyl amino ligand,20 the
compound was isolated and characterized as a dimer. This issue
is exacerbated in larger group 3 metals and lanthanides.21−24 A
second factor concerns the potentially high reactivity of the
LnNR bond, documented by Mindiola and co-workers in a
P−N−P pincer ligand supported scandium complex.25 Here,
convincing evidence for the intermediacy of a terminal
scandium imido derivative was presented, but the addition of
of internal or external donors was key to inducing alkane
elimination from the amido alkyl precursors at low enough
temperatures that the product imido complex was thermally
stable. The downside of this approach is that the resulting
Special Issue: Recent Advances in Organo-f-Element Chemistry
Received: September 26, 2012
Published: November 2, 2012
© 2012 American Chemical Society
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terminal imido complexes are more coordinatively saturated
and therefore less reactive. We thus thought to revisit our
earlier chemistry involving nacnac derivatives of I by adopting
the Chen strategy of using an external base to induce alkane
elimination, as opposed to thermolysis, and generate lower
coordinate scandium terminal imido derivatives; these results
are described herein.
RESULTS AND DISCUSSION
The arylamido alkyl nacnac complex 1 (Scheme 1) was
reported by us in 2004 as a potential precursor to a scandium
■
Scheme 1
1
Figure 1. Partial 400 MHz H NMR spectrum of a mixture of 1 and
DMAP heated at 50 °C in C6D6 showing the loss of the Sc−CH3
resonance and the appearance of CH4.
the Nimido−Caryl bond, rendering the isopropyl groups
diastereotopic. Although numerous attempts were made to
grow crystals of 2·DMAP, samples suitable for X-ray analysis
eluded us. Nevertheless, elemental analysis data are consistent
with this formulation and reactivity studies of this compound as
described below confirm its identity as assigned.
The mechanism by which 2·DMAP is formed from 1 can
occur by two plausible pathways, paths A and B, as shown in
Scheme 1. In path A, metalation occurs first, followed by
(relatively) rapid, DMAP-prompted alkane elimination from 3.
In path B, DMAP coordinates to 1, inducing methane
elimination to give the imido derivative 2·DMAP. Several
lines of experimental evidence indicate that the former path
(path A) is operative. First, heating is required for this reaction
to proceed, the optimal temperature being 50 °C to prevent
product degradation. Higher temperatures would presumably
favor the left-hand side of the equilibrium at the top of Scheme
1 but would promote metalation to produce 3. Second, when 1
is mixed with DMAP, even at low temperature, no evidence for
imido complex via thermally induced methane elimination.26
Although methane elimination was observed upon heating
solutions of the complex to 90 °C, the observed product was
the metalated species 3, which was fully characterized (vide
infra). Furthermore, deuterium labeling experiments showed
that metalates closely related to 3 are formed directly, without
the intermediacy of the anticipated imido species.26,30
1
formation of the adduct 1·DMAP is apparent in the H NMR
In contrast, when 1 was heated at 50 °C in the presence of
spectrum; only signals for 1 and free DMAP are observed.
Thus, the equilibrium constant is very small (<10−2 L mol−1)
for this complexation. Third, and most telling, when d-1
(selectively labeled at the arylamido position) was heated in the
presence of DMAP, only CH4 was observed (i.e., no CH3D was
DMAP, no evidence for metalation product 3 was observed in
1
the H NMR spectrum. Rather, smooth conversion over the
course of 4−5 days to a different product, assigned as the
DMAP-stabilized terminal imido complex 2·DMAP, was
observed. During the reaction, the resonance at 0.03 ppm for
the Sc-Me group in 1 gradually disappeared and that for CH4
grew in (Figure 1), concurrent with the appearance of signals
for 2·DMAP. Instead of the complex set of ligand resonances
characteristic of the highly unsymmetrical metalated derivative
3,26,31 2·DMAP exhibited nacnac ligand resonance patterns
seen in Cs-symmetric derivatives with two different ligands
bonded to the (nacnac)Sc fragment.31,32 Thus, one singlet for
2
produced), and in the H NMR spectrum, signals at 0.74 and
1.30 ppm coinciding with two of the methyl resonances in the
isopropyl substituents on the N-aryl groups of 2·DMAP were
observed. These observations are completely consistent with
rate-limiting metalation and elimination of methane followed
by rapid reaction between 3 and DMAP to give 2·DMAP.
This postulate suggests that separately synthesized 3 should
react rapidly with DMAP to furnish the product 2·DMAP.
Accordingly, metalate 3 was synthesized on a preparative scale
by thermolysis of 1 in the absence of DMAP. It was isolated as
a yellow solid in 53% after workup and exhibited a characteristic
pattern of resonances in the 1H NMR spectrum for this
desymmetrized structure.26 A resonance at 4.17 ppm for the
retained NH proton and an AB quartet centered at 0.48 ppm
for the diastereotopic ScCH2 protons of the metalated
isopropyl methylene group are particularly diagnostic. An X-
ray analysis on crystals obtained from a concentrated hexanes
solution was not of high enough quality to obtain metrical data
of meaningful accuracy but certainly established the con-
nectivity within the molecule (Figure S1, Supporting
t
the Bu protons was accompanied by four doublets and two
septets for the β-diketiminato N-aryl isopropyl methyl and
methine protons, respectively. Upfield-shifted resonances at
6.37, 5.77, and 2.14 ppm for the coordinated DMAP ligand
were distinct from those observed for free DMAP (cf. 8.36,
6.08, and 2.29 ppm). The backbone CH proton is observed at
5.92 ppm, while the characteristic NH resonance for the
arylamido ligand in 1 was absent from the spectrum. Finally,
two doublets and two septets were observed for the isopropyl
groups of the imido N-aryl substituent, suggesting that this ring
is oriented in the plane of symmetry of the molecule as
depicted in Scheme 1 and that there is restricted rotation about
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Scheme 2
Information), which is similar to that found for a previously
characterized derivative in which the group on the amido
nitrogen is Bu rather than the 2,6-diisopropylphenyl group in
ΔS⧧ = −70.4(5) J K−1 mol−1 (−16.8 cal K−1 mol−1) (Figure
2b). This is a somewhat smaller enthalpic barrier than that
recorded for alkane elimination in a comparable zirconium
system (25.9 kcal mol−1)15 but a higher entropic barrier; the
value of ΔS⧧ in the Zr system was −7 eu. Here, the role of the
external base DMAP serves to lower the enthalpic barrier but
may require extra ordering in the transition state as a result.
The necessary intermediacy of 3 to increase the facility of
alkane elimination to yield a scandium imido group is
interesting. Direct methane elimination from 1 appears to be
precluded because the metal center is too hindered to allow for
coordination of DMAP but not crowded enough to stimulate
methane elimination across the Sc−N bond. Loss of methane
via metalation to produce 3 creates more open space about the
metal, allowing for DMAP coordination but also perhaps
provides strain release energy as a driving force for the
subsequent alkane elimination step leading to ScN bond
formation. Other examples where a premetalation step aids in
overcoming energetically difficult steps34,35 or leads to facile
metalate ring opening36 have appeared in the literature.
t
3.26,33 Reaction of isolated, pure metalate 3 with 1 equiv of
DMAP results in a rapid color change from yellow to red
followed by gradual lightening of the solution over the course
of a few hours at room temperature. Monitoring the reaction by
1H NMR spectroscopy shows that the red color is likely due to
a DMAP adduct of 3; signals for free DMAP and 3 are
supplanted by those for coordinated DMAP and a second
unsymmetrical species (3·DMAP). Loss of CH4 is accompanied
by the growing in of signals for 2·DMAP. Thus, it appears that
rapid DMAP coordination is followed by rate-limiting loss of
methane to give the imido product (Scheme 2).
Kinetic studies are completely consistent with this postulate.
When 3 is reacted with 10, 15, 20, and 25 equiv of DMAP
(pseudo-first-order conditions) at 295 K, while there is a steady
increase in the observed pseudo-first-order rate constant, the
effect is slight and not indicative of first-order behavior in
[DMAP] (Figure 2a). Instead, this effect is likely reflective of
Our inability to obtain X-ray structural data on 2·DMAP
raises a kernel of doubt concerning its assignment as a
monomeric scandium terminal imide. While the observation of
only one signal in the 1H NMR spectrum for the nacnac ligand
backbone suggests that dimers are not present,37 it is possible
that only one of the three possible dimeric isomers is
energetically feasible.38 Unfortunately, mass spectroscopic
analysis was precluded by the high air and water sensitivity of
the compound and IR spectroscopy was inconclusive; therefore,
we explored derivatization reactions as a means for providing
further evidence for our structural assignment of 2·DMAP.
No reaction of 2·DMAP with tert-butylamine or phenyl-
acetylene was observed at room temperature, but upon heating
to 70 °C in the presence of these reagents, clean conversion to
new products was observed to occur over the course of 72 h
(Scheme 3). These products were identified as the diaster-
eoisomers of the mixed bis-amido complex 4 (the endo and exo
designations here refer to the position of the newly introduced
tert-butylamido group) and the phenylacetylido arylamido
complex endo-5 (the endo designation here refers to the
acetylido group). The identity of these products was confirmed
through their separate synthesis from a suitable amido chloride
precursor26 and either LiNHAr or LiCCPh, as shown in the
bottom right corner of Scheme 3. The products arise from
Figure 2. (A) Plot of the observed pseudo-first-order rate constant for
the formation of 2·DMAP as a function of [DMAP]. (B) Eyring plot
for the formation of 2·DMAP (15 equiv of added DMAP).
t
addition of an N−H bond in BuNH2 or the acetylenic C−H
bond in PhCCH across the ScN bond in 2·DMAP; as for 1,
these products do not coordinate the free DMAP that is
released in this reaction, which is washed away in workup.
Consistent with this proposal, use of deuterated reagents
(tBuND2 or PhCCD) resulted in d2-endo-/exo-4 or d-endo-5,
respectively, labeled exclusively at the amido ND positions. In
the case of 4, the two Cs-symmetric isomers, present in a 2:1
the shifting of the adduct-forming equilibrium toward 3·DMAP
as the DMAP concentration is increased. An Eyring plot
obtained by measuring the pseudo-first-order rate constant for
the reaction of 3 with 15 equiv of DMAP at various
temperatures ranging from 282 to 323 K gives activation
parameters of ΔH⧧ = 73.5(2) kJ mol−1 (17.6 kcal mol−1) and
1
equilibrium ratio, are in slow exchange on the H NMR time
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Scheme 3
scale; therefore, the spectrum is complex. We were unable to
conclusively assess which isomer was the major one and which
was the minor. Typically, for electronically similar ligands, the
sterically more bulky ligand is favored in the exo position;31 the
isomer endo-4 was that present in the single crystal that was
analyzed by X-ray diffraction (vide infra), but this does not
necessarily mean that this is the major isomer in solution. In the
case of endo-5, only one isomer is observed in solution. Since
we have shown that stronger π donors are also favored in the
exo position,26 it is likely this isomer is the one with the
acetylide ligand in the endo position; this was confirmed by X-
ray diffraction. The exclusive observation of C−H activation in
the reaction with phenylacetylene is interesting, since metalla-
cycle formation via 2 + 2 addition is also a possible reaction
channel.39
Figure 3 shows the molecular structure of endo-4, along with
selected metrical data, and Figure 4 does the same for endo-5.
Figure 4. Thermal ellipsoid diagram (50% probability) of endo-5.
Isopropyl methyl groups on the Ar substituents have been removed for
clarity. Selected bond distances (Å): Sc−N(1), 2.072(2); Sc−N(2),
2.163(2); Sc−N(3), 2.023(3); Sc−C(12), 2.194(3). Selected bond
angles (deg): N(1)-Sc−N(2), 91.05(9); N(3)−Sc−C(12),
122.14(12); Sc−N(3)−C(44), 145.7(2); Sc−C(12)−C(13), 165.6(3).
istics; the primary amido ligands have short Sc−N distances
ranging from 2.004(2) to 2.039(2) Å, consistent with strong π
bonding from nitrogen to scandium. Consequently, the Sc−
Namido−C bond angles are rather large, ranging from 145.7(2)
to 149.17(16)°. In endo-5, the Sc−C(12) bond length of
2.194(3) Å is slightly shorter than the typical distance of ∼2.22
Å for Sc−C(sp3) bonds, as might be expected due to the sp
hybridization at C(12). The Sc−Cacetylide distances in a six-
coordinate complex with two phenylacetylide ligands are
2.262(3) and 2.296(4) Å.40
The full characterization of these two products strongly
suggests that 2·DMAP is a terminal imido species in which the
DMAP ligand stabilizes a highly reactive three-coordinate imido
complex: i.e., 2 (Scheme 4). Despite the fact that tert-
butylamine and phenylacetylene are chemically very different
reagents, the rates of their reaction with 2·DMAP are
(qualitatively at least) very similar, occurring slowly at 70 °C
over the course of a few days. This observation suggests that
Figure 3. Thermal ellipsoid diagram (50% probability) of endo-4.
Isopropyl methyl groups on the Ar substituents have been removed for
clarity. Selected bond distances (Å): Sc−N(1), 2.005(2); Sc−N(2),
2.040(2); Sc−N(3), 2.126(2); Sc−N(4), 2.196(2). Selected bond
angles (deg): N(1)−Sc−N(2), 110.88(9); N(3)−Sc−-N(4), 91.04(8);
Sc−N(1)−C(11), 149.26(18); Sc−N(2)−C(61), 146.84(17).
As in the several other structures for compounds in this family,
the scandium centers lie out of the plane defined by the C3N2
ligand atoms; the metrical data for the nacnac ligand framework
are similar to those found in other complexes and analyzed in
detail elsewhere.31 The other ligands bound to these four-
coordinate scandium centers also exhibit standard character-
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obtained by the Instrumentation Facility of the Department of
Chemistry. Details on the X-ray analyses can be found in the deposited
CIF files (Supporting Information) or via the Cambridge Crystallo-
graphic Data Centre (CCDC 902751 and 902752).
Scheme 4
Synthesis of 1. Compound 1 was prepared by a modified literature
procedure.26 A 50 mL flask was charged with A (0.500 g, 0.809 mmol)
and methyllithium (0.071, 3.230 mmol). Toluene (25 mL) was
condensed into the evacuated flask at −78 °C. The mixture was
warmed to room temperature and stirred for 2 h. The reaction mixture
was filtered, and NH2Dipp (0.150 mL, 0.809 mmol) was added via
syringe and then stirred at room temperature for 12 h. The toluene
was removed in vacuo to afford an orange solid. Hexanes (25 mL) was
condensed into the flask and the assemblage sonicated for 20 min; the
hexanes was then removed to give a yellow solid (0.490 g, 0.664 mmol,
82%). The spectroscopic data are consistent with previously published
data.26
Generation of 1-d. An NMR tube was charged with C (0.015 g,
0.026 mmol) and D2NDipp (5.00 μL, 0.026 mmol) in 0.6 mL of d8-
toluene. The tube was shaken and left at room temperature for 12 h.
these reactions proceed via rate-limiting dissociation of DMAP
to produce a highly reactive scandium imide that rapidly adds
the E−H bond of the reagent to form the products (Scheme 4).
Attempts to observe exchange between free and coordinated
1
The H NMR spectrum matched that of 1, except that no resonance
was observed for the amido proton. 2H NMR (C7D8): δ 5.44 (s, ND).
Synthesis of 2·DMAP. Method A. 1 (0.150 g, 0.203 mmol) and
DMAP (0.025 g, 0.203 mmol) were charged in a 50 mL flask dissolved
in 10 mL of toluene. The solution was heated to 50 °C under argon
for 5 days. After the heating was completed, toluene was removed in
vacuo and hexanes (10 mL) was condensed into the flask and
sonicated for 20 min. Evaporation of hexanes yielded 0.109 g of a fluffy
yellow powder (0.129 mmol, 64%).
1
DMAP by high-temperature H NMR spectroscopy or EXSY
spectroscopy experiments were not successful, suggesting that
this process is slow on the NMR time scale. Thus, although
quantitative kinetic experiments have not been performed, it is
likely that dissociation of DMAP from 2·DMAP is required in
order for these reactions to proceed.
Method B. Toluene (15 mL) was condensed into an evacuated flask
containing 1 (0.200 g, 0.271 mmol) at −78 °C. The mixture was
heated with stirring at 90 °C for 12 h. In a separate vial, DMAP (0.035
g, 0.271 mmol) was dissolved in toluene (5 mL) and added to the flask
via syringe. The reaction mixture was stirred for 12 h at room
temperature. Solvent was removed to yield an orange oil, and hexanes
(10 mL) was condensed into the flask and sonicated for 20 min.
Removal of the solvent in vacuo afforded 0.158 g of a fine yellow
In summary, we have utilized the Lewis base promoted
alkane elimination reaction to access a four-coordinate
scandium imide. Mechanistic studies show that the formation
of 2·DMAP proceeds indirectly via a metalated product rather
than directly from the methyl arylamido starting material.
Although not structurally characterized, spectroscopic data and
its reactivity toward an N−H bond in tert-butylamine and the
acetylenic C−H bond of phenylacetylene are strong evidence
for the assignment of 2·DMAP as a terminal imido derivative.
1
powder (0.187 mmol, 69%). H NMR (C7D8): δ 7.20−6.96 and 6.78
3
(m, 9H, ArH), 6.65 (d, 2H, o-H py-NMe2, JH−H = 6.2 Hz), 5.92 (s,
1H, CH), 5.76 (dd, 2H, m-H py-NMe2, 3JH−H = 6.2 Hz), 3.82 (sp, 2H,
3
EXPERIMENTAL SECTION
CHMe2, JH−H = 6.8), 3.44 (sp, 1H, NH-2,6-(CHMe2)2C6H3, 3JH−H
=
■
6.8 Hz), 3.11 (sp, 1H, NH-2,6-(CHMe2)2C6H3, 3JH−H = 6.8 Hz), 2.89
(sp, 2H, CHMe2, 3JH−H = 6.8 Hz), 2.14 (br s, 6H, py-NMe2), 1.4, (ov
d, 12H, CHMe2 and NH-2,6-(CHMe2)2C6H3), 1.30 (d, 6H, CHMe2,
3JH−H = 6.8 Hz), 1.27 (br s, 18H, NCCMe3), 1.23 (d, 6H, CHMe2,
General Procedures and Equipment. An argon atmosphere
MBraun glovebox was employed for manipulation and storage of all
oxygen and moisture-sensitive compounds. Reactions were performed
on a double-manifold high-vacuum line using standard techniques.
Toluene and hexane were dried and purified using the Grubbs/Dow
purification system41 and stored in evacuated glass vessels over
sodium/benzophenone ketyl. Pentane was predried and distilled from
3 Å molecular sieves and then stored over sodium/benzophenone in
evacuated glass vessels. d6-Benzene and d8-toluene were predried and
distilled from sodium/benzophenone and stored in a glass vessel in the
glovebox. NMR spectra were obtained on Bruker DRX400, AVANCE
400 MHz, and AVANCE III 400 MHz spectrometers. 1H and 13C{1H}
chemical shifts were referenced to residual proton and naturally
abundant 13C resonances of the deuterated solvent, respectively,
relative to tetramethylsilane. NMR spectra were processed and
analyzed with MestReNova software (v6.2.1−7569). 4-N,N-dimethyl-
aminopyridine (DMAP), 2,6-di-iso-propylaniline (NH2Dipp), tert-
3JH−H = 6.8 Hz), 0.90 (d, 6H, NH-2,6-(CHMe2)2C6H3, JH−H = 6.8
3
Hz), 0.74 (d, 6H, CHMe2, 3JH−H = 6.8 Hz). 13C[1H} NMR (C7D8): δ
175.6 (NCCMe3), 153.0 (p-C py-NMe2), 151.8, 146.8, 144.0, 142.7,
142.1, 137.3, 132.0, 124.4, 124.2, 123.8, 122.4, and 116.7 (ArC) 110.0
(o-C py-NMe2), 107.5 (m-C py-NMe2), 95.3 (CH), 44.9 (NCCMe3),
38.72 (py-NMe2), 33.5 (NCCMe3), 29.9 and 28.9 (CHMe2), 29.3 and
28.9 (NH-2,6-(CHMe2)2C6H3), 27.6, 25.8, 24.8, and 24.7 (CHMe2),
25.8 and 25.1 (NH-2,6-(CHMe2)2C6H3). Anal. Calcd for C54H80N5Sc:
C, 76.83; H, 9.55; N, 8.30. Found: C, 77.03; H, 9.88; N, 8.05.
Synthesis of 3. In an evacuated flask at −78 °C containing 1
(0.200 g, 0.271 mmol) toluene (10 mL) was condensed. The solution
was heated at 90 °C for 12 h and solvent removed under vacuum,
yielding an orange oil. Hexanes (10 mL) was added, and the mixture
was sonicated for 15 min. The hexane was removed in vacuo, affording
an orange powder. Hexamethyldisiloxane (7 mL) was transferred onto
the powder and cooled to −30 °C for 1 h. The mixture was filtered
and solvent removed to afford a yellow powder (0.102 g, 0.141 mmol,
53%). X-ray-quality crystals were grown from concentrated pentane
solutions at −35 °C. 1H NMR (C7D8): δ 7.25−6.80 and 6.66 (m, 9H,
ArH), 5.82 (s, 1H CH), 4.17 (br s, 1H, NH), 3.41 (sp, 1H, CHMe2,
3JH−H = 6.8 Hz), 3.25 (m, 1H, CH2CHMe), 3.13 (m, 2H, CHMe2),
t
butylamine (NH2 Bu), and phenylacetylene were purchased from
Aldrich Chemicals and purified using standard methods. Amines and
phenylacetylene were predried and distilled from 3 Å molecular sieves
and then stored over fresh sieves in the glovebox. Lithium amides were
n
prepared by lithiation with 1 equiv of BuLi to produce LiNHR (R =
42
t
Dipp, Bu) salts. Lithium phenylacetylide was isolated from the
reaction of phenylacetylene and 1 equiv of nBuLi.43 For the deuterium
labeling studies, D2NDipp and D2NtBu were prepared according to the
literature procedure.44 Quenching lithium phenylacetylide with excess
D2O followed by distillation under an argon atmosphere allowed for
the preparation of d-phenylacetylene.35 Compounds LScCl2 (A),31
LScClNHAr (B),26 LScMe2 (C),31 and LScClNHtBu (D)26 were
prepared according to literature procedures. Elemental analyses were
3
2.07 (sp, 2H, NH-2,6-(CHMe2)2C6H3, JH−H = 6.4 Hz), 1.47 (d, 3H,
3
CHMe2, JH−H = 6.7 Hz), 1.29−1.22 (m, 15H, CHMe2), 1.18 (d, 3H,
CH2CHMe, 3JH−H = 6.8 Hz), 1.11 (br s, 9H, NCCMe3), 1.08 (m, 10H,
NCCMe3 and CH2CHMe), 0.90 (d, 12H, NH-2,6-(CHMe2)2C6H3,
3JH−H = 6.6 Hz) and 0.48 (ov dd, 1H, CH2CHMe). 13C{1H} NMR
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(C7D8): δ 175.9 and 175.5 (NCCMe2), 149.9, 148.9, 144.6, 142.9,
142.0, 141.5, 133.8, 126.9, 124.2, 122.8, 122.6, 117.0 (ArC), 99.7
(CH), 55.3 (ScCH2), 43.7 and 42.8 (NCCMe3), 38.9 (CH2CHCH3),
32.8 and 31.6 (NCCMe3), 29.7 (NH-2,6-(CHMe2)2C6H3), 28.8, 28.6,
and 28.5 (CHMe2), 26.0, 25.2, 25.1, 25.0, 24.5, 23.2, 22.1
(CH(CH3)2), 24.7 (NH-2,6-(CHMe2)2C6H3). Anal. Calcd for
C47H70N3Sc: C, 78.18; H, 9.77; N, 5.82. Found: C, 78.34; H, 9.85;
N, 6.04.
Generation of d-3. d-1 (0.019 g, 0.026 mmol) was dissolved in 0.6
mL of d8-toluene. The solution was mixed thoroughly and heated to
90 °C for 12 h. The 1H NMR spectrum matched that of 3, except that
no resonances were observed for the amido proton. 2H NMR (C7D8):
δ 4.30 (br s, ND).
filtered to afford a yellow solution. Removal of the solvent in vacuo
afforded a tacky orange solid. Crystals for X-ray diffraction were grown
from concentrated pentane solution at −35 °C (0.140 g, 0.170 mmol,
1
3
86%). H NMR (C7D8): δ 7.46 (d, 2H, o-H Ph, JH−H = 7.2 Hz),
7.12−6.82 and 6.65 (m, 12H, ArH and m,p-H Ph), 6.06 (s, 1H, CH),
5.92 (br s, 1H, NH), 3.92 (sp, 2H, CHMe2, 3JH−H = 6.6 Hz), 3.11 (sp,
3
2H, CHMe2 JH−H = 6.7 Hz), 2.06 (sp, 2H, NH-2,6-(CHMe2)2C6H3,
3
3JH−H = 6.8 Hz), 1.55 (d, 6H, CHMe2, JH−H = 6.6 Hz), 1.36 (d, 6H,
3
3
CHMe2, JH−H = 6.6 Hz), 1.19 (d, 6H, CHMe2, JH−H = 6.7 Hz), 1.16
(br s, 18H, NCCMe3) 1.09 (d, 6H, CHMe2, 3JH−H = 6.7 Hz), 1.03 (d,
12H, NH-2,6-(CHMe2)2C6H3, JH−H = 6.8 Hz). 13C{1H} NMR
3
(C7D8): δ 176.46 (NCCMe3), 149.15, 143.63, 142.90, 133.92, 126.86,
126.70, 124.35, 122.91, 118.88, and 117.85 (ArC), 141.70 (ScCC-Ph),
132.08 (o-C Ph), 128.25 (m-C Ph), 126.36 (p-C Ph), 100.83
(ScCCPh), 96.45 (CH), 44.41 (NCCMe3), 32.29 (NCCMe3), 30.73
(NH-2,6-(CHMe2)2C6H3), 29.23 and 28.76 (CHMe2), 27.24, 25.29,
24.58, and 24.47 (CHMe2), 22.75 (NH-2,6-(CHMe2)2C6H3). Anal.
Calcd for C60H88N3Sc (including 1 C5H12 of solvation): C, 80.40; H,
9.90; N, 4.69. Found: C, 79.81; H, 10.57; N, 4.82.
Synthesis of endo-/exo-4. Method A. An NMR tube was charged
t
with 2·DMAP (0.008 g, 9.483 × 10−3 mmol) and NH2 Bu (1.00 μL,
9.483 × 10−3 mmol) in 0.6 mL of d8-toluene. The tube was shaken and
heated at 70 °C for 3 days.
Method B. A 50 mL flask was charged with D (0.215 g, 0.329
mmol) and solid LiNHDipp (0.066 g, 0.361 mmol). Toluene (25 mL)
was condensed into the evacuated flask at −78 °C. The mixture was
heated to 70 °C and stirred for 12 h. The toluene was removed in
vacuo to afford an orange solid. Hexanes (25 mL) was condensed into
the flask and the assemblage sonicated for 20 min. The reaction
mixture was filtered and hexanes removed to afford a yellow solid
(0.243 g, 0.306 mmol, 93%). X-ray-quality crystals were grown from a
Generation of d-endo-5. 2 (0.015 g, 0.018 mmol) and d-
phenylacetylene (2.00 μL, 0.018 mmol) were dissolved in 0.6 mL of
d8-toluene. The solution was mixed thoroughly and heated to 70 °C
1
for 72 h. The H NMR spectrum matched that of endo-5, except that
no resonances were observed for the amido proton. 2H NMR (C7D8):
δ 6.02 (br s, ND).
1
concentrated hexanes solution at room temperature. H NMR for the
major isomer (C7D8): δ 7.12−6.84 and 6.64 (m, 9H, ArH), 5.83 (s,
1H, CH), 5.29 (s, 1H, NH-2,6-(CHMe2)2C6H3), 3.89 (sp, 2H,
Concentration Dependence of DMAP in the Production of
2·DMAP. In a typical experiment, 3 (0.004 g, 0.005 mmol) and the
desired quantity of DMAP were dissolved in 0.5 mL of d8-toluene at
room temperature. The sample was shaken and quickly inserted into
the NMR probe. The progress of the reaction was monitored by
integration of the amido proton in 3 and the p-H of coordinated
DMAP in 2·DMAP in the 1H NMR spectrum. The reaction was
followed until 95% completion.
Activation Parameters in the Production of 2·DMAP. In a
typical experiment, 3 (0.004 g, 0.005 mmol) and DMAP (0.009 g,
0.074 mmol) were dissolved in 0.5 mL of d8-toluene at room
temperature. The sample was shaken and quickly inserted into the
NMR probe, at which time it was given 5 min to equilibrate to the
specified temperature. The progress of the reaction was monitored by
integration of the amido proton in 3 and the p-H of coordinated
DMAP in 2·DMAP in the 1H NMR spectrum. The reaction was
followed until 95% completion.
3
3
CHMe2, JH−H = 6.7 Hz), 3.06 (ov sp, 2H, CHMe2, JH−H = 6.8 Hz),
3.40 (s, 1H, NH-tBu), 2.16 (sp, 2H, NH-2,6-(CHMe2)2C6H3, 3JH−H
=
3
6.4 Hz), 1.42 (d, 6H, CHMe2, JH−H = 6.7 Hz), 1.36 (d, 6H, CHMe2,
3JH−H = 6.6 Hz), 1.21 (d, 6H, CHMe2, JH−H = 6.7 Hz), 1.17 (s, 9H,
3
NH-tBu), 1.14 (s, 18H, NCCMe3), 1.10 (d, 6H, CHMe2, JH−H = 6.7
3
3
Hz), 0.92 (d, 12H, NH-2,6-(CHMe2)2C6H3, JH−H = 6.6 Hz).
Complete assignment of the H NMR for the minor isomer was not
possible, due to signal overlap with the other diastereomer. H NMR
1
1
for the minor isomer (C7D8): δ 7.12−6.84 and 6.64 (m, 9H, ArH),
5.82 (s, 1H, CH), 5.04 (s, 1H, NH-2,6-(CHMe2)2C6H3), 4.15 (s, 1H,
3
NH-tBu), 3.42 (ov sp, 2H, NH-2,6-(CHMe2)2C6H3, JH−H = 6.8 Hz),
3
3.06 (ov sp, 2H, CHMe2, JH−H = 6.8 Hz), 1.30 (s, 18H, NCCMe3),
1.27 (m, 12H, CHMe2), 0.72 (s, 9H, NH-tBu). Complete assignment
of the 13C{1H} NMR spectrum was not possible, and such data are
reported by carbon type: δ 150.5, 149.5, 144.7, 144.1, 142.8, 142.7,
142.0, 141.0, 134.0, 126.4, 126.0, 125.2, 124.5, 124.4, 124.0, 123.0,
117.4, and 117.2 (ArC), 27.88, 26.29, 26.03, 24.99, 24.92, and 24.84
(CHMe2). All other carbon resonances were attributable to a specific
diastereomer. 13C{1H} NMR for exoNHtBu (C7D8): δ 176.0 (NCCMe3),
94.83 (CH), 54.48 (NHCMe3), 44.56 (NCCMe3), 35.01 (NHCMe3),
32.64 (NCCMe3), 29.22 (NH-2,6-(CHMe2)2C6H3), 28.46 and 27.15
(CHMe2) 24.69 (NH-2,6-(CHMe2)2C6H3). 13C{1H} NMR for
endoNHtBu (C7D8): δ 175.6 (NCCMe3), 93.01 (CH), 53.88
(NHCMe3), 44.68 (NCCMe3), 34.07 (NHCMe3), 32.87 (NCCMe3),
29.77 (NH-2,6-(CHMe2)2C6H3), 28.25 and 26.69 (CHMe2), 24.80
(NH-2,6-(CHMe2)2C6H3). Anal. Calcd for C51H81N4Sc: C, 77.03; H,
10.27; N, 7.05. Found: C, 76.98; H, 10.26; N, 6.90.
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files giving crystallographic data for endo-4 and endo-5 and
Figures S1−S6, giving the X-ray structure of 3, additional NMR
spectra, and pseudo-first-order plots. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Generation of d2-endo-/exo-4. An NMR tube was charged with
2·DMAP (0.015 g, 0.018 mmol) and D2NtBu (2.00 μL, 0.018 mmol)
in 0.6 mL of d8-toluene. The tube was shaken and heated at 70 °C for
Corresponding Author
*E-mail: wpiers@ucalgary.ca.
1
3 days. The H NMR spectrum matched that of endo-/exo-4, except
2
Present Address
that no resonances were observed for the amido protons. H NMR
†Department of Chemistry, School of Molecular Sciences, La
Trobe University, Melbourne, Victoria, Australia.
(C7D8): δ 5.10 (br s, ND-2,6-(CHMe2)2C6H3), 1.08 (br s, ND-tBu).
Synthesis of endo-5. Method A. 2·DMAP (0.007 g, 9.106 × 10−3
mmol) and phenylacetylene (1.00 μL, 9.106 × 10−3 mmol) were
dissolved in 0.6 mL of d8-toluene. The solution was mixed thoroughly
and heated to 70 °C for 72 h.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Method B. Toluene (20 mL) was condensed into an evacuated flask
containing B (0.150 g, 0.198 mmol) and lithium phenylacetylide
(0.024 g, 0.218 mmol) at −78 °C. The solution was stirred at 80 °C
for 12 h, upon which the solvent was removed under vacuum. The
orange oil was sonicated in hexanes (20 mL) for 20 min and then
Notes
The authors declare no competing financial interest.
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Article
(37) Knight, L. K.; Piers, W. E.; McDonald, R. Chem. Eur. J. 2000, 6,
4322.
ACKNOWLEDGMENTS
■
W.E.P. acknowledges the NSERC of Canada for a Discovery
Grant and the Canada Council of the Arts for a Killam
Research Fellowship (2012−2014).
(38) The two coordination sites on scandium are diastereotopic in
these complexes, and we have referred to the site “underneath” the
C3N2 ligand plane as the endo site and that pointing away from the
ligand the exo position. These sites are exchangable via a C2v structure
in which the Sc atom passes through the ligand plane, but this process
is generally slow on the NMR time scale. In dimers, there are three
possible isomers depending on the relative orientations of the two
scandium centers, and in the slow exchange regime, this is most
apparent by observing the region of the spectrum where the nacnac
backbone methine hydrogen is found.
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