Tribenzylamine C-H activation and intermolecular hydrogen transfer promoted by WCl6

Article abstract of DOI:10.1021/ic5001683

The 1:1 molar reaction of WCl₆ with tribenzylamine (tba), in dichloromethane, selectively afforded the iminium salt [(PhCH₂) ₂N=CHPh][WCl₆], 1, and the ammonium one [tbaH][WCl ₆], 2, in equimolar amou

Full text of DOI:10.1021/ic5001683

Article
pubs.acs.org/IC
Tribenzylamine C−H Activation and Intermolecular Hydrogen
Transfer Promoted by WCl6
Marco Bortoluzzi,† Fabio Marchetti,*,‡ Guido Pampaloni,‡ and Stefano Zacchini§
†Dipartimento di Scienze Molecolari e Nanosistemi, University of Venice, Dorsoduro 2137, I-30123 Venezia
‡Dipartimento di Chimica e Chimica Industriale, University of Pisa, Via Risorgimento 35, I-56126 Pisa
§Dipartimento di Chimica Industriale “Toso Montanari”, University of Bologna, Viale Risorgimento 4, I-40136 Bologna
S
* Supporting Information
ABSTRACT: The 1:1 molar reaction of WCl6 with tribenzylamine (tba), in
dichloromethane, selectively afforded the iminium salt [(PhCH2)2N
CHPh][WCl6], 1, and the ammonium one [tbaH][WCl6], 2, in equimolar
amounts. The products were fully characterized by means of spectroscopic
methods, analytical methods, and X-ray diffractometry. Density functional
theory calculations were carried out with the aim of comprehending the
mechanistic aspects.
crystallographic characterization was not provided. Efforts were
made in the direction of shedding light into the mechanism of
the interaction between TiCl4 and trialkylamines, since this
system works as an effective catalyst of diverse organic trans-
formations such as stereoselective aldol condensations,8j,16 C−C
coupling of esters,17 synthesis of 2,5-diarylpyrroles,18 and
synthesis of α,β-unsaturated carbonyl compounds.19 The unusual
formation of iminium intermediates by reaction of the amine
with the Lewis acidic titanium center was hypothesized; however,
this observation could not be supported by clear proof.19
Similarly, previous study of the interaction of tungsten
hexachloride, WCl6, with tertiary amines revealed the occurrence
of metal reduction and led to the hypothesis that the amine C−
H bond located α to the nitrogen atom was activated.11 Also in
this case no experimental evidence could be furnished.
In the framework of our interest in the reactivity of high-
valent early transition metal halides,20 we report herein on the
straightforward reaction of WCl6 with tribenzylamine (tba),
and crystallographic evidence is reported for the generation of
iminium species via amine C−H activation. This result rep-
resents a rare example of well-defined interaction of a tertiary
amine with a high-valent metal halide.
INTRODUCTION
■
The C−H bond activation of aliphatic amines is a cornerstone
of modern organometallic chemistry,1 and the use of transition
metal compounds aimed toward amine C−H bond cleavage in
organic synthesis has been established as an important field
of research.1d,2 In this context, low-valent transition metal
centers have been employed to promote the C−H activation of
aliphatic amines via hydride loss; examples include ruthenium,3
osmium,4 palladium,5 and mercury6 species and the homoleptic
halides of Ni(II) and Co(II).7
On the other hand, high-valent, early transition metal halides
usually manifest their strongly acidic nature upon reaction with
primary and secondary aliphatic amines, yielding aminolysis prod-
ucts. This feature is common to group 4 tetrahalides,8 Nb and Ta
pentahalides,8d,9 MoCl5,10 and WCl6;11 however, amido-derivatives
have been more commonly prepared by reaction of these chlorides
with lithium amides.12 Such synthetic approaches to metal-amido
compounds may be accompanied by metal reduction processes,
which often complicate the comprehension of the reaction pic-
ture.9e,11,12e,13 Instead, mixed amido-fluoride complexes of
Ti(IV) and M(V) (M = Nb, Ta) have been obtained with
specific procedures consisting of metathesis14 or treatment of
the parent homoleptic fluorides with trimethylsilylamides.15
Only sparse information has appeared in the literature about
the interaction of tertiary amines with the high-valent metal
halides of groups 4−6 of the periodic table, which remains
poorly understood. The formation of simple coordination
adducts was hypothesized with MoCl5,10a NbX5 (X = Cl, Br),9d
and TaCl5;9e nevertheless, the identification of the products
relied on limited spectroscopic data, and unambiguous
EXPERIMENTAL SECTION
■
General Considerations. Warning! The metal products reported in
this Paper are highly moisture-sensitive; thus, rigorously anhydrous
conditions were required for the reaction, crystallization, and separation
procedures. The reaction vessels were oven-dried at 150 °C prior to use,
Received: January 22, 2014
Published: March 13, 2014
© 2014 American Chemical Society
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Table 1. Crystal Data and Details of the Structure Refinement for 1 and 2.
complex
1
2
formula
Fw
C21H20Cl6NW
682.93
C21H22Cl6NW
684.95
T, K
λ, Å
100(2)
100(2)
0.710 73
triclinic
0.710 73
orthorhombic
Pna21
crystal system
space group
a, Å
P1
̅
10.288(4)
10.937(3)
11.342(3)
69.140(3)
85.436(3)
85.915(3)
1187.5(5)
2
14.7496(16)
15.1195(17)
10.8580(12)
90
b, Å
c, Å
α, deg
β, deg
90
γ, deg
cell volume, Å3
90
2421.4(5)
4
Z
Dc, g cm−3
1.910
1.879
μ, mm−1
5.548
5.442
F(000)
658
1324
crystal size, mm
θ limits, deg
reflections collected
independent reflections
data/restraints/parameters
goodness of fit on F2
R1 (I > 2σ(I))
wR2 (all data)
largest diff. peak and hole, e Å−3
0.19 × 0.13 × 0.11
1.92−25.67
11628
0.26 × 0.24 × 0.21
1.93−26.00
16322
4478 [Rint = 0.0526]
4478/1/265
1.012
4738 [Rint = 0.385]
4738/2/266
1.050
0.0406
0.0263
0.0948
0.0627
1.369/−1.388
1.897/−0.947
Scheme 1. The Reaction of WCl6 with Tribenzylamine (tba)
evacuated (10−2 mm Hg), and then filled with argon. WCl6 (99.9%)
was purchased from Strem and was stored under argon atmosphere as
received. Tribenzylamine (tba) was a commercial product (Sigma
Aldrich) of the highest purity available, dried over P4O10, and stored
under argon atmosphere. Solvents (Sigma Aldrich) were distilled from
P4O10 under argon atmosphere before use. Infrared (IR) spectra were
recorded at 298 K on a Perkin-Elmer Fourier transform (FT) IR
spectrometer, equipped with a universal attenuated total reflectance
sampling accessory. Magnetic susceptibility (reported per W atom)
was measured at 298 K on a solid sample (1:1 crystalline mixture of 1
and 2) with a Magway MSB Mk1 magnetic susceptibility balance
(Sherwood Scientific Ltd.). Diamagnetic corrections were introduced
Figure 1. Molecular structure of [(PhCH2)2NCHPh][WCl6], 1,
with key atoms labeled. Displacement ellipsoids are at the 50%
probability level.
The mixture was stirred at room temperature for 3 h more. Then the
volatile materials were removed, and the resulting yellow-brown
residue was washed with pentane (2 × 15 mL). A mixture of 1 and 2
according to Konig.21 NMR spectra were recorded at 298 K on a
was obtained based on IR and H NMR spectroscopy. Yield: 0.584 g,
1
̈
Bruker Avance DRX400 instrument equipped with a broad band
87%. IR (solid state): 3064w, 3033w, 2958w-m, 2924w-m, 2859w,
1633s, 1595s, 1497m, 1454s, 1439m-s, 1360w, 1302w, 1254w, 1201m,
1158w, 1077m, 1028m, 1002w, 964m, 905w-m, 871w, 844w, 808m-s,
744vs, 728s, 693vs cm−1. Magnetic measurement: χMcorr = 2.78 × 10−4
cgsu, μeff = 0.80 μB The solid was dissolved in CH2Cl2; by slow
diffusion of hexane into the solution, comparable amounts of yellow
and red crystals formed at −30 °C. The crystals were mechanically
separated and identified as [(PhCH2)2NCHPh][WCl6], 1, and
[tbaH][WCl6], 2.
Compound 1 (yellow solid). Anal. Calcd for C21H20Cl6NW: C,
36.93; H, 2.95; N, 2.05; Cl, 31.15. Found: C, 36.99; H, 2.91; N, 2.14;
Cl, 30.90%. IR (solid state): 1633s (CN) cm−1. 1H NMR (CD2Cl2):
δ = 9.00 (s, 1 H, HCN); 8.03, 7.94, 7.78, 7.48 (15 H, Ph); 5.38, 5.12
(s, 4 H, CH2) ppm. 13C NMR{1H} (CD2Cl2): δ = 180.7 (HCN);
138.7, 135.5, 133.4, 131.5−130.7 (Ph); 129.7, 129.2 (ipso-PhCH2);
70.8, 61.9 (CH2) ppm.
1
fluorine observation probe. The chemical shifts for H and 13C were
1
referenced to the nondeuterated aliquot of the solvent. The H and
13C NMR spectra were assigned with assistance of 1H, 13C correlation
measured through gradient-selected (gs) heteronuclear single-
quantum correlation and gs heteronuclear multiple-bond correlation
experiments.22 Conductivity measurements were carried out on
CH2Cl2 solutions ca. 0.03 M with a Eutech Con 700 Instrument
(cell constant = 1.0 cm−1).23 Carbon, hydrogen, and nitrogen analyses
were performed on a Carlo Erba model 1106 instrument. The chloride
content was determined by the Mohr method24 on solutions prepared
by dissolution of the solid in aqueous KOH at boiling temperature,
followed by cooling to room temperature and addition of HNO3 to
neutralization.
A. Synthesis and Characterization of [(PhCH2)2NCHPh]-
[WCl6], 1, and [tbaH][WCl6], 2. A mixture of WCl6 (0.390 g, 0.983
mmol) and CH2Cl2 (15 mL) was treated with tribenzylamine (0.285 g,
0.992 mmol). Quick formation of a dark-yellow solution occurred.
Compound 2 (red solid). Anal. Calcd for C21H22Cl6NW: C, 36.82;
H, 3.24; N, 2.04; Cl, 31.06. Found: C, 36.91; H, 3.16; N, 2.01; Cl,
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Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1
W(1)−Cl(1)
W(1)−Cl(3)
W(1)−Cl(5)
N(1)−C(1)
2.3078(19)
2.3410(19)
2.3191(19)
1.513(9)
W(1)−Cl(2)
W(1)−Cl(4)
W(1)−Cl(6)
N(1)−C(8)
2.3199(19)
2.3404(19)
2.3399(19)
1.283(9)
N(1)−C(15)
C(8)−C(9)
1.482(9)
C(1)−C(2)
1.496(10)
1.491(10)
177.35(7)
118.7(6)
1.445(10)
178.72(7)
176.93(7)
116.9(6)
C(15)−C(16)
Cl(2)−W(1)−Cl(6)
C(1)−N(1)−C(8)
C(8)−N(1)−C(15)
N(1)−C(8)−C(9)
Cl(1)−W(1)−Cl(4)
Cl(3)−W(1)−Cl(5)
C(1)−N(1)−C(15)
N(1)−C(1)−C(2)
N(1)−C(15)−C(16)
124.4(6)
113.7(6)
129.1(7)
112.8(6)
times at ca. 20 °C: ΛM = 8.6 (1 min), 9.9 (40 min), and 9.4 (4 d)
S × cm2 × mol−1. A CH2Cl2 solution (25 mL) of WOCl4(OCMe2),
obtained by treatment of WCl6 (0.355 g, 0.894 mmol) with acetone
(0.132 mL, 1.80 mmol),20b showed ΛM = 0.2 S × cm2 × mol−1.
D. X-ray Crystallographic Study. Crystal data and collection
details for 1 and 2 are listed in Table 1. The diffraction experiments
were carried out on a Bruker APEX II diffractometer equipped with a
CCD detector and using Mo Kα radiation (λ = 0.71073 Å). Data were
corrected for Lorentz polarization and absorption effects (empirical
absorption correction SADABS).25 The structures were solved by
direct methods and refined by full-matrix least-squares based on all
data using F2.26 All non-hydrogen atoms were refined with anisotropic
displacement parameters. All hydrogen atoms were fixed at calculated
positions and refined by a riding model, except H(8) in 1 and H(1)
in 2, which were located in the Fourier map and refined isotropically
using the 1.2-fold Uiso value of the parent atom. The N(1)−H(1)
distance of 2 was restrained to 0.89 Å (standard units 0.02), whereas
the C(8)−H(8) distance of 1 was restrained to 0.95 Å (standard units
0.02). Crystals of 2 are racemically twinned, with a refined Flack
parameter of 0.036(7).27
E. Computational Studies. The computational geometry
optimization of the complexes was carried out without symmetry
constraints, using the hybrid density functional theory (DFT) EDF2
functional28 in combination with the LACVP** basis set. The latter
is a combination of the 6-31G(d,p) basis set with the LANL2DZ effec-
tive core basis set.29 Further geometry optimization was performed
using the pure GGA functional PW9130 in combination with an atom-
centered doubly numerical polarized basis set and DFT semicore
pseudopots.31 Dispersion correction was added through the approach
proposed by Ortmann, Bechstedt, and Schmidt (the OBS cor-
rection),32 and the conductor-like screening model (COSMO) implicit
solvation model for dichloromethane was added.33 The “unrestricted”
formalism was applied for systems with unpaired electrons, and the
absence of meaningful spin contamination was verified by comparing
the computed ⟨S2⟩ values with the theoretical ones. In all of the cases,
the stationary points were characterized by IR simulations, by which
zero-point vibrational energies and thermodynamic parameters
were obtained.34 DFT-simulated IR data, obtained with harmonic
approximation, assisted the interpretation of experimental IR spectra.
Figure 2. Molecular structure of [tbaH][WCl6], 2, with key atoms
labeled. Displacement ellipsoids are at the 50% probability level.
30.87%. IR (solid state): 1595s (N−H) cm−1. 1H NMR (CD2Cl2): δ =
7.58, 7.33 (br, 15 H, Ph); 5.51 (br, 1 H, NH); 4.34 (s, 6 H, CH2) ppm.
13C NMR{1H} (CD2Cl2): δ = 132.3, 128.4, 127.8, 126.4 (Ph); 63.3
(CH2) ppm.
In a different experiment, a solution of WCl6 (0.250 g, 0.630 mmol)
in hexane (20 mL) was treated with tribenzylamine (0.182 g, 0.633
mmol). The mixture was stirred for 18 h at room temperature. The
obtained light-green precipitate was washed with toluene (20 mL) and
then dried in vacuo. A 1:1 mixture (1H NMR, IR) of compounds 1 and
2 was isolated in 82% yield.
B. NMR Studies. WCl6 (0.180 g, 0.454 mmol), CD2Cl2 (0.50 mL),
CHCl3 (0.450 mmol), and tba (0.130 g, 0.452 mmol) were introduced
into an NMR tube in the order given. The tube was sealed, shaken to
homogenize the content, and stored at room temperature. NMR
spectra recorded after 30 min evidenced the selective formation of
1 and 2 (1/2/CHCl3 ratio = 9:9:10). The 1H resonance related to the
NH proton (δ = 5.5 ppm) was clearly detected.
C. Conductivity Measurements. A stirred mixture of WCl6
(0.440 g, 1.11 mmol) and CH2Cl2 (20 mL) was treated with tba
(0.319 g, 1.11 mmol). Molar conductivities were measured at variable
Table 3. Selected Bond Lengths (Å) and Angles (deg) for 2
W(1)−Cl(1)
W(1)−Cl(3)
W(1)−Cl(5)
N(1)−C(1)
N(1)−C(15)
C(8)−C(9)
Cl(1)−W(1)−Cl(3)
Cl(4)−W(1)−Cl(6)
C(1)−N(1)−C(15)
N(1)−C(1)−C(2)
N(1)−C(15)−C(16)
2.3165(14)
2.3506(14)
2.3853(15)
1.509(6)
W(1)−Cl(2)
W(1)−Cl(4)
W(1)−Cl(6)
N(1)−C(8)
2.2921(16)
2.2988(11)
2.2916(12)
1.529(6)
1.506(7)
1.514(7)
177.09(7)
114.1(4)
112.7(4)
114.3(4)
1.513(6)
C(1)−C(2)
1.502(7)
C(15)−C(16)
Cl(2)−W(1)−Cl(5)
C(1)−N(1)−C(8)
C(8)−N(1)−C(15)
N(1)−C(8)−C(9)
177.43(5)
174.08(7)
110.7(4)
111.2(5)
112.4(4)
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DFT EDF2 calculations were carried out using Spartan 08,35 while
36
Dmol3 was used for PW91/OBS/COSMO calculations. Energy
variations (ΔG) cited in the present work refer to 298 K and contain
the implicit solvation contributions.
RESULTS AND DISCUSSION
■
The reaction of WCl6 with tribenzylamine, tba, was conducted
in dichloromethane and yielded a mixture of the iminium
[(PhCH2)2NCHPh][WCl6] (1) and the ammonium [tbaH]-
1
[WCl6] (2) salts (Scheme 1). According to H NMR spec-
troscopy, 1 and 2 formed in nearly equimolar amounts. A
mixture of yellow crystals of 1 and red crystals of 2 could be
obtained by crystallization from a dichloromethane solution
layered with hexane. The crystals of the two distinct com-
pounds could be mechanically separated and then characterized
by spectroscopic, analytical, and X-ray diffractometry techni-
ques. The X-ray structure and relevant geometric parameters of
1 are shown in Figure 1 and Table 2, respectively, while those
of 2 are given in Figure 2 and Table 3.
The solid-state structures of 1 and 2 consist in ionic packings
of octahedral [WCl6]− anions and [(PhCH2)2NCHPh]+ or
[tbaH]+ cations, respectively. The W(V) anions are rather
similar to the ones previously described in miscellaneous salts,
being based on an octahedral W(V) center coordinated to six
chloride ligands.20b,37 The W−Cl bonds [average 2.328(5) and
2.322(3) Å for 1 and 2, respectively] are considerably longer than
in neutral WCl6 [2.24−2.26 Å],38 as a consequence of the
different oxidation states of W, that is, +5 in the former and +6 in
the latter.
The crystallographic characterization of the N,N-dibenzyl-
benzylidenaminium cation [(PhCH2)2NCHPh]+ is unprece-
dented. The N(1)−C(8) interaction [1.283(9) Å] is substan-
tially shorter than N(1)−C(1) [1.509(6) Å] and N(1)−C(15)
[1.513(6) Å], in view of the double nature of the former.39 The
trigonal planar geometry of N(1) [sum angles 360.0(10)°]
and C(8) [sum angles 360(6)°] is in agreement with an sp2
hybridization of the two atoms.
The [tbaH]+ cation was previously characterized by X-ray
diffractometry in miscellaneous salts.40 The N(1)−C(1)
[1.509(6) Å], N(1)−C(8) [1.529(6) Å], and N(1)−C(15)
[1.513(6) Å] distances in 2 are almost identical and are typical
of a N−C single bond. The N(1) atom shows a pyramidal
structure [sum angles excluding H(1) = 337.5(7)°], in agree-
ment with sp3 hybridization. Weak H-bonds are present within
the crystals of 2, involving the N(1)−H(1) group of the cation
as a donor and Cl(5) of the anion as an acceptor [N(1)−H(1)
0.885(19) Å; H(1)···Cl(5) 2.43(3) Å; N(1)···Cl(5) 3.285(5) Å;
<N(1)H(1)Cl(5) 162(5)°].
The IR spectra of compounds 1 and 2 display two strong ab-
sorptions at 1633 and 1595 cm−1, respectively. Such absorp-
tions do not have any correspondence in the IR spectrum of the
precursor tba41 and have been assigned to the CN stretching
vibration (1) and the N−H bending vibration (2) with
assistance from simulated IR spectra.
Figure 3. DFT PW91 calculated structures of (a) [(PhCH2)2N
CHPh][WCl6], 1, and (b) [tbaH][WCl6], 2, with implicit solvation
(CH2Cl2) and OBS correction for dispersion. Selected hydrogen
atoms have been omitted for clarity.
spectra of 1 and 2 did not show any significant broadening of
the resonances.43
The NMR spectra of 1 exhibit the resonances related to
the nonequivalent methylene groups at 5.38 and 5.12 ppm
(1H) and at 70.8 and 61.9 ppm (13C). On the other hand, the
three equivalent methylenes belonging to the ammonium
cation in 2 resonate at 4.34 (1H) and 63.3 (13C) ppm.42 The N-
bound proton in 2 was seen at 5.51 ppm, while the 13C
resonance due to the iminium carbon in 1 falls at 180.7 ppm.
Despite the paramagnetic nature of the WCl6− anion, the NMR
To gain insight into the mechanism of the reaction leading
to 1 and 2, we performed a computational study. The cal-
culated structures of 1 and 2 are drawn in Figure 3; the relevant
bonding parameters are supplied as Supporting Information
(Tables S1 and S2), showing substantial agreement with the
corresponding experimental (X-ray) data.
According to DFT outcomes, the initial interaction be-
tween WCl6 and tba probably consists in the formation
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Figure 4. Calculated pathway for the synthesis of 1 and 2.
(ΔG = −12.9 kcal mol−1) of the transient adduct WCl6···tba, 3A
(Figure 4 and Figure S3A, given as Supporting Information). The
latter appears to be 36.5 kcal mol−1 more stable than the alternative
ionic species [WCl5(tba)][Cl] (singlet state),44 3B, see Supporting
Information, Figure S3B, resulting from tba/Cl− substitution at the
metal center. The theoretical formation of [WCl5(tba)][WCl7] does
not appear to be viable, due to the instability of the WCl7− anion
toward the decomposition into WCl6 and Cl−.
A list of calculated bonding parameters for 3A is given as
Supporting Information (Table S3). The intermolecular
interaction in 3A involves three W-bound chlorides and six
methylene hydrogens of tba, with the corresponding W−Cl and
C−H bonds undergoing slight elongation.
An NMR experiment on a CD2Cl2 reaction mixture of
WCl6/tba confirmed the selective formation of 1 and 2 (see
Experimental Section); the NH resonance was observed at ca.
5.5 ppm, thus indicating that the protic solvent is not directly
involved as hydrogen source.
We performed a DFT study aimed toward giving insight into
the atom interchange process leading to products 1 and 2. The
initially generated radical cation [tba]+ (Figure 4) may react
with still-unreacted tba, in view of the Bronsted acidic and basic
̈
nature of the former and the latter, respectively (eq 2).46 Then
the resulting α-amino radical (PhCH2)2NCHPh, 5 (Supporting
Information, Figure S5, Table S5), could reduce another
molecule of WCl6 (eq 3).
The formation of 3A is reasonably followed by fast, mono-
electron amine-to-W transfer to give the radical cation salt
[tba][WCl6], 4. The oxidative power of WCl6 has been well-
established, and it was previously documented with reference to
a tertiary aromatic amine, namely, tris(4-bromophenyl)amine.45
As a matter of fact, conductivity measurements have indicated
that ionic species exist in the reaction mixture immediately after
the mixing of the reactants WCl6 and tba (see Experimental
Section); in fact conductivity data on the WCl6/tba system re-
semble those previously reported for MCl6− salts (M = Nb, Ta).20c
The DFT structure of 4 is shown in Supporting Information,
Figure S4, with relevant bonding parameters in Table S4
(Supporting Information). In spite of the slightly disadvanta-
geous energy variation (ΔG = +5.9 kcal mol−1, see Figure 4),
the electron transfer step (3A → 4) is presumably driven by the
highly exothermic subsequent process (see eq 1 and Figure 4).
The formation of 1 and 2, in equimolar amounts, represents
the formal result of atom interchange between two [tba][WCl6]
(4) units, one hydrogen atom migrating from a methylene
moiety to the nitrogen of another radical species (eq 1). This
process is thermodynamically very favorable.
[tba][WCl6] + tba → (PhCH2)2NCHPh + [tbaH][WCl6]
× ΔG = −10.5 kcal mol−1
(2)
(PhCH2)2NCHPh + WCl6 → [(PhCH2)2NCHPh][WCl6]
× ΔG = −45.2 kcal mol−1
(3)
Mechanisms alternative to the intermediate formation of 5
do not appear likely. First, the direct interaction of two [tba]+
radical cations should be ruled out on considering the elec-
trostatic repulsion. Otherwise the possible involvement of the
protic solvent (CH2Cl2) as hydrogen carrier is not convincing,
since we could reproduce the high yield synthesis of 1 and 2
from WCl6/tba by using hexane as reaction medium (see
Experimental Section). Even the hypothetical, direct involve-
ment of traces of H2O does not seem viable.47
CONCLUSIONS
■
The chemistry of high-valent, early transition metal halides
with primary and secondary amines is well-documented and is
dominated by the strongly acidic nature of the metal center.
Otherwise sparse information have appeared on the interaction
of such halides with tertiary amines: the activation of amine
C−H bonds forming iminium cationsthat is, a process typically
promoted by low-valent, late transition metal compounds or
main group specieshas been hypothesized in some cases but
never unambiguously demonstrated. In the present Paper, we
1
[tba][WCl6] → [(PhCH2)2NCHPh][WCl6](1)
2
1
2
+
[tbaH][WCl6](2) ΔG = −24.4 kcal mol−1
(1)
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reported, with the support of X-ray and DFT analyses, that
tungsten hexachloride is able to promote selective C−H bond
activation of tribenzylamine, followed by H intermolecular
transfer to afford a 1:1 mixture of the relevant iminium and
ammonium ions. This reaction represents a rare example of
well-defined interaction between a high-valent metal halide and
a tertiary amine. The reactivity of WCl6 with tertiary amines
different from tba will be the subject of future reports.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures S1−S5 show the calculated structures of 1, 2, 3A, 3B, 4,
and 5. Tables S1−S5 contain the related bonding parameters.
Cartesian coordinates of the DFT-optimized geometries are
included, along with structures in CIF files. This material is
available free of charge via the Internet at http://pubs.acs.org.
CCDC reference numbers 980210 (1) and 980211 (2) contain
the supplementary crystallographic data for the X-ray studies
reported in this Paper. These data can be obtained free of
charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from
the Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge CB2 1EZ, U.K.; fax: (international) +44−1223/
336−033; e-mail: deposit@ccdc.cam.ac.uk].
AUTHOR INFORMATION
Corresponding Author
*E-mail: fabmar@dcci.unipi.it. Tel: +39 050 2219 245. Fax: +39
050 2219 246. Webpage: http://www.dcci.unipi.it/∼fabmar/.
■
(10) (a) Edwards, D. A.; Fowles, G. W. A. J. Chem. Soc. 1961, 24−28.
(b) Edwards, D. A.; Fowles, G. W. J. Less-Common Met. 1961, 3, 181−
187.
(11) Brisdon, B. J.; Fowles, G. W. A.; Osborne, B. P. J. Chem. Soc.
1962, 1330−1334.
Notes
(12) (a) Sharma, B.; Chen, S.-J.; Abbott, J. K. C.; Chen, X.-T.; Xue,
Z.-L. Inorg. Chem. 2012, 51, 25−27. (b) Zhang, X.-H.; Chen, S.-J.; Cai,
H.; Im, H.-J.; Chen, T.; Yu, X.; Chen, X.; Lin, Z.; Wu, Y.-D.; Xue, Z.-L.
Organometallics 2008, 27, 1338−1341. (c) Waters, T.; Wedd, A. G.;
Ziolek, M.; Nowak, I. Niobium and Tantalum. In Comprehensive
Coordination Chemistry II; J. A. McCleverty, I. A.; Meyer, T. J., Eds.;
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
CINECA is gratefully acknowledged for the availability of high-
performance computing resources and support.
■
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