6-Coordinate tungsten(vi) tris-n-isopropylanilide complexes: Products of terminal oxo and nitrido transformations effected by main group electrophiles

Article abstract of DOI:10.1039/b801037d

The nitridotungsten(vi) complex NW(N[i-Pr]Ar)₃ (1-N, Ar = 3,5-Me₂C₆H₃) reacts with (CF ₃C(O))₂O followed by ClSiMe₃ to give the isolable trifluoroacetylimido-chloride complex 1-(

Full text of DOI:10.1039/b801037d

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PAPER
www.rsc.org/dalton | Dalton Transactions
6-Coordinate tungsten(VI) tris-n-isopropylanilide complexes: products of
terminal oxo and nitrido transformations effected by main group
electrophiles†‡
Christopher R. Clough, Peter Mu¨ller and Christopher C. Cummins*
Received 21st January 2008, Accepted 3rd March 2008
First published as an Advance Article on the web 1st July 2008
DOI: 10.1039/b801037d
The nitridotungsten(VI) complex NW(N[i-Pr]Ar)3 (1-N, Ar = 3,5-Me2C6H3) reacts with (CF3C(O))2O
followed by ClSiMe3 to give the isolable trifluoroacetylimido-chloride complex 1-(NC(O)CF3)Cl, with
oxalyl chloride to give cyanate-dichloride 1-(OCN)(Cl)2, and with PCl5 to give
trichlorophosphinimide-dichloride 1-(NPCl3)(Cl)2. The oxo-chloride complex 1-(O)Cl, obtained from
1-N upon treatment with pivaloyl chloride, reacts with PCl5 to give trichloride 1-(Cl)3. Synthetic and
structural details are reported for the new tungsten trisanilide derivatives.
Introduction
Results and discussion
Recently we showed that the nitridotungsten(VI) complex NW(N[i-
Pr]Ar)3 (1-N; Ar = 3,5-Me2C6H3) can be used as a reagent for the
transformation of acid chlorides into organic nitrides according
to: 1-N + R1C(O)Cl → 1-(O)Cl + R1CN (see Scheme 1).1 For
R1 = t-Bu or 1-Ad, the latter reaction proceeds quantitatively at
25 ◦C in less than 1 h and is an intriguing example of an isovalent
N for (O)Cl exchange process. Acylimido-chloride complexes 1-
(NC(O)R1)Cl are observable during the reaction as monitored by
1H or 13C NMR spectroscopy, and they are kinetically competent
to be intermediates.
The tungsten trifluoroacetylimido trifluoroacetate complex 1-
(NC(O)CF3)(O2CCF3), was obtained previously by treatment
of 1-N with trifluoroacetic anhydride (TFAA), and was the
subject of an X-ray diffraction study.1 One possible explanation
for the failure of 1-(NC(O)CF3)(O2CCF3) to thermally extrude
CF3CN was that bidentate trifluoroacetate coordination to W
served to inhibit the formation of a metallacyclic acylimido
complex similar to the proposed structure 2 (Scheme 1). This
idea was rendered implausible by the observed structure of 1-
(NC(O)CF3)(O2CCF3),1 in which monodentate trifluoroacetate
was observed and the coordination geometry at tungsten (includ-
ing ancillary N[i-Pr]Ar substituent conformation) was essentially
identical to that found for oxo-chloride 1-(O)Cl. It began to seem,
therefore, that the electron-withdrawing nature of the CF3 group
in trifluoroacetylimido 1-(NC(O)CF3)(O2CCF3) was principally
responsible for the failure of this complex to mediate nitrile
formation.
Probing the stability of 1-(NC(O)CF3)Cl
To investigate the effect of electron withdrawing groups in
the conversion of 1-(NC(O)CF3)Cl to 1-(O)Cl, we treated 1-
(NC(O)CF3)(O2CCF3) with excess ClSiMe3 to provide trifluo-
roacetylimido chloride 1-(NC(O)CF3)Cl. As was the case for
its synthetic precursor we find 1-(NC(O)CF3)Cl to be thermally
stable, and now report an X-ray structural study of this complex
(Fig. 1). Like oxo-chloride 1-(O)Cl, and like its precursor 1-
(NC(O)CF3)(O2CCF3), the trifluoroacetylimido chloride complex
1-(NC(O)CF3)(Cl) incorporates trigonal-bipyramidal coordina-
tion at W, with an equatorial metal–ligand multiple bond.
If the mechanism of nitrile formation as effected by 1-N indeed
involves metallacycles such as 2 in Scheme 1, then either the
specific choice of R1 = CF3 obviates metallacycle formation, or
else it renders the reaction thermodynamically uphill.2 Either way,
CF3CN is a nitrile not available when using the 1-N reagent.
Since our utilization of sterically demanding ancillary anilide
ligands (here, N[i-Pr]Ar) is motivated by a desire to foster low coor-
dination number and low nuclearity, it was with some trepidation
that we proposed the intermediacy of 6-coordinate metallacycles
2. For this reason, we were fascinated to find examples of bona
fide 6-coordinate, octahedral complexes supported by platform 1.
This occurred in the course of surveying the reactivity of 1-N with
a variety of acid chlorides.
Isolation of 6-coordinate complexes
Treatment of 1-N with 0.5 equiv of oxalyl chloride was intended
to provide cyanogen. Instead, only 0.5 equiv of the initial 1-
N was consumed, indicating that a 1 : 1 reaction was preferred.
Accordingly, treatment of 1-N with 1.0 equiv of oxalyl chloride was
found to provide, with effervescence attributed to CO liberation,
the cyanato-dichloride complex 1-(OCN)(Cl)2. The latter has in-
teresting NMR spectroscopic properties. It is C1 chiral as indicated
by (i) the presence of three distinct N[i-Pr]Ar ligand environments
in a 1 : 1 : 1 ratio, and (ii) the diastereotopic nature of each of
the three N[i-Pr]Ar ligand environments. From this combination
Room 6-435, Department of Chemistry, Massachusetts Institute of Technol-
ogy, 77 Massachusetts Avenue, Cambridge, MA, 02139-4307, United States
of America
† Based on the presentation given at Dalton Discussion No. 11, 23–25 June
2008, University of California, Berkeley, USA.
‡ Electronic supplementary information (ESI) available: Details of crystal
structures of all prepared complexes. CCDC reference numbers 675710–
675713. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/b801037d
4458 | Dalton Trans., 2008, 4458–4463
This journal is
The Royal Society of Chemistry 2008
©

Fig.
Selected interatomic distances (A) and angles (◦) for
˚
1
1-(NC(O)CF3)Cl: W1–N4, 1.791(3); N4–C41, 1.329(5); O1–C41, 1.199(5);
C41–N4–W1, 156.8(3).
Scheme 1
of facts, we can infer that the combination of a fac and Cs
arrangement of the (OCN)(Cl)2 substituents, combined with a fac
and frozen-out C3 three-bladed propellor of N-isopropylanilide
residues, leads to a chiral metal environment with overall C1
symmetry. This is consistent with an X-ray structural study of 1-
(OCN)(Cl)2 (Fig. 2). It is from the X-ray study that we tentatively
assign 1-(OCN)(Cl)2 as equipped with an O-coordinated cyanate
ligand, the IR data (mNCO = 2200 cm−1, vs) being insufficient
information to make the distinction.3
Normally, molecules with three N(i-Pr)Ar ligands, e.g. 1-N or
1-(O)Cl, are not at 25 ◦C frozen out into a static C3 configuration.
Such complexes typically evince a single, non-diastereotopic
ligand environment. That 1-(OCN)(Cl)2 is frozen out at room
temperature signifies substantial steric crowding.
Another example of a C1-symmetric derivative with an oc-
tahedral coordination environment at tungsten is that obtained
by reaction of 1-N with PCl5. It is known that PCl5 has a
propensity to react with N-containing compounds to give products
with 4-coordinate P and P–N multiple bonding.4 As in the case
of the oxalyl chloride reaction, tungsten accepts two chloride
Fig. 2 View of 1-(OCN)(Cl)2 normal to the 001 plane (space group
R3). The OCN and two Cl ligands are positionally disordered about the
crystallographic C3 axis passing through W1.
¯
ligands while expanding to 6-coordination. The product molecule,
=
1-(N PCl3)(Cl)2, incorporating a rare trichlorophosphinimide
2
ligand (31P NMR: d = −49.8 ppm, JWP = 85 Hz), is an orange-
red compound soluble in THF or benzene, but of limited ether
or pentane solubility. An X-ray structural study of the complex
revealed a moderately bent phosphinimide nitrogen, together with
overall conformational attributes very much reminiscent of 1-
(OCN)(Cl)2 (Fig. 3). The related trichlorophosphinimide complex
=
Cl5W(N PCl3) has been prepared by treatment of WCl6 with
5,6
=
=
Cl3P NSiMe3. Also, Ph4P[Cl5Mo(N PCl3)] has been prepared
by treatment of nitride Ph4P[Cl4MoN] with PCl3/PCl5, in what
appears to be the closest precedent for our synthesis of 1-
7
=
(N PCl3)(Cl)2.
Since oxo-chloride 1-(O)Cl is the ultimate product in the
reaction of 1-N with acid chlorides, we are interested in methods
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 4458–4463 | 4459
©

Fig. 4 Space filling model of 1-(Cl)3.
chloride and t-BuC(O)Cl were purchased from Aldrich and
distilled under N2. PCl5 was purchased from Aldrich and used
as received. Diethyl ether, n-pentane, and toluene were dried and
deoxygenated by the method of Grubbs.11 THF was distilled from
purple Na/benzophenone and collected under nitrogen. C6D6 was
Fig.
3
Selected interatomic distances (A) and angles (◦) for
˚
1-(NPCl3)(Cl)2: W1–N4, 2.047(4); P1–N4, 1.449(4); P1–N4–W1, 155.9(2).
for the recycling of 1-(O)Cl back to the nitride reagent 1-N. This
objective is similar to another realized recently, namely the
activation of terminal oxo product ONb(N[Np]Ar)3 by reaction
with triflic anhydride to give bistriflate (TfO)2Nb(N[Np]Ar)3; the
latter is then reduced to its P4-activating niobaziridine-hydride
form in our synthesis of phosphaalkyne (RC P) molecules.
Activation of the oxo in 1-(O)Cl with triflic anhydride was not
successful, this reaction giving oxo triflate 1-(O)(OTf) instead. On
the other hand, we find that PCl5 serves smoothly to transform
oxo chloride 1-(O)Cl into trichloride 1-(Cl)3, with POCl3 as the
sole byproduct.9 We had expected that a fac arrangement of
three chloride ligands together with a fac and C3 frozen out
arrangement of three N[i-Pr]Ar ligands would provide trichloride
˚
degassed and dried over 4 A molecular sieves. Other chemicals
were purified and dried by standard procedures or were used as
R
received. Celiteꢀ 545, alumina and 4 A molecu◦lar sieves were dried
˚
in vacuo overnight at a temperature above 200 C. 1H, 13C, 19F, and
31P NMR spectra were recorded on Varian Mercury-300, Varian
8
≡
INOVA-500, or Bruker AVANCE-400 spectrometers. 1H and 13
C
chemical shifts are reported with respect to internal solvent (C6D6,
7.16 and 128.39 ppm, respectively). 19F and 31P chemical shifts are
reported with respect to external reference (CFCl3, 0.0 ppm and
85% H3PO4, 0.0 ppm, respectively). Infrared spectra were recorded
on a Bio-Rad 135 Series FTIR spectrometer.
Crystallography
1
1-(Cl)3 with a single yet diasterotopic set of N[i-Pr]Ar ligand H
NMR resonances at 25 ◦C. That expectation was borne out in
X-ray data collections were carried out on a Siemens Platform
three-circle diffractometer equipped with a Bruker-AXS Apex
CCD detector and an Oxford Cryosystems CryoStream 700 low-
temperature device. Graphite-monochromated Mo-Ka radiation
1
full: the H NMR spectrum of 1-(Cl)3 has a pair of aryl methyl
resonances, three aryl proton signals in a 1 : 1 : 1 ratio, and a
pair of isopropyl methyl doublets. The other example of a fac
trisamide tungsten trichloride complex that is potentially C3 with
diastereotopic ligand environments (based on its crystal structure)
is WCl3(NEt2)3.10 This molecule shows a single ethyl group triplet
˚
(k = 0.71073 A) was used in all cases. All software for diffraction
data processing and crystal-structure solution and refinement are
contained in the SHELXTL (v6.14) program suite (G. Sheldrick,
Bruker AXS, Madison, WI).12 Details of crystallographic data and
refinement are given in Table 1 and in the ESI.‡
1
and quartet in its H NMR spectrum, indicative of free rotation
about its W–N linkages.
An X-ray structural study of trichloride 1-(Cl)3 (space-filling
diagram in Fig. 4) validates our formulation of this molecule while
illustrating the severe inter-ligand steric interactions present in 6-
coordinate tungsten systems based on the trisanilide platform 1.
Such interactions are expected to be a destabilizing influence on
proposed metallacycles 2.
Syntheses
Synthesis of (Ar[i-Pr]N)3W(NC(O)CF3)Cl (1-(NC(O)CF3)Cl).
1-(C(O)CF3)(O2CCF3) (525 mg, 0.767 mmol) was dissolved in
minimal Me3SiCl (∼5 mL) and the resulting red solution was
R
stirred for ∼5 min, filtered through a bed of Celiteꢀ 545 and
the filtrate cooled to −35 ◦C overnight. 1-(NC(O)CF3)(Cl) was
obtained as a red precipitate which was washed with cold pentane,
and dried in vacuo (160 mg, 0.196 mmol, 25.5%). X-Ray quality
crystals can also be grown by following the same procedure using
smaller amounts of 1-(C(O)CF3)(O2CCF3) (ca. 100 mg) in more
dilute solutions of Me3SiCl. 1H NMR (500 MHz, C6D6): d = 6.57
(s, 3H, para), 6.52 (s, 6H, ortho), 5.33 (septet, 3H, i-Pr methine),
2.05 (s, 18H, ArCH3), 1.14 (d, 18H, i-Pr methyl) ppm. 13C NMR
(100 MHz, C6D6): d = 149.3 (ipso), 138.2 (meta), 129.1 (para),
126.3 (ortho), 65.6 (i-Pr methine), 23.0 (methyl), 21.6 (methyl)
Experimental
General
Unless stated otherwise, all operations were performed in a
Vacuum Atmospheres drybox under an atmosphere of purified
nitrogen or using Schlenk techniques under an argon atmosphere.
≡
N W(N[i-Pr]Ar)3 (1-N, Ar = 3,5-Me2C6H3), (Ar[i-Pr]N)3W(O)Cl
(1-(O)Cl), and (Ar[i-Pr]N)3W(NC(O)CF3)(O2CCF3) (1-(NC(O)-
CF3)(O2CCF3)) were prepared as previously published.1 Oxalyl
4460 | Dalton Trans., 2008, 4458–4463
This journal is The Royal Society of Chemistry 2008
©

Table 1 Crystallographic data for 1-(NC(O)CF3)Cl, 1-(OCN)(Cl)2, 1-(NPCl3)(Cl)2 and 1-(Cl)3
1-(NC(O)CF3)Cl
1-(OCN)(Cl)2
1-(NPCl3)(Cl)2
1-(Cl)3
Empirical formula
Formula weight
Color
Morphology
Crystal size/mm
T/K
C35H48ClF3N4OW
817.07
Dark red
C42H64Cl2N4O3Wa
927.72
Yellow
C37H56Cl5N4OPWb
964.93
Orange
C33.50H49Cl4N3Wc
819.41
Yellow
Plate
Plate
Shard
Shard
0.14 × 0.08 × 0.02
0.27 × 0.23 × 0.04
0.10 × 0.09 × 0.06
0.13 × 0.10 × 0.09
100(2)
100(2)
100(2)
100(2)
Crystal system
Space group
Monoclinic
P21/n
Rhombohedral
¯
R3
Monoclinic
P21/c
Triclinic
¯
P1
Unit cell dimensions:
˚
a/A
18.1298(6)
10.4918(3)
19.4084(5)
90
107.0410(10)
90
3529.67(18)
4
1.538
3.397
1648
1.83 to 28.28
73315
13.3600(12)
13.3600(12)
39.556(8)
90
90
120
6114.5(14)
6
1.512
3.008
2856
15.9389(6)
13.7357(5)
18.9527(6)
90
97.4620(10)
90
4114.2(3)
4
1.558
3.206
1952
1.84 to 27.88
81238
10.7751(3)
13.3987(3)
13.5032(4)
88.8520(10)
71.9870(10)
83.6570(10)
1842.38(9)
2
1.477
3.451
826
1.53 to 28.28
38272
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
Density calc./Mg m−3
Absorption coefficient/mm−1
F(000)
Theta range for data collection/◦
Reflections collected
1.54 to 25.14
10803
Independent reflections, Rint
Completeness to theta (%)
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
8736 (0.0907)
100.0
0.9352 and 0.6477
8736/0/412
1.075
R1 = 0.0359
wR2 = 0.0727
R1 = 0.0522
wR2 =0.0791
1.302 and −1.122
2442 (0.0634)
100.0
0.8891 and 0.4972
2442/345/288
1.152
R1 = 0.0450
wR2 = 0.1157
R1 = 0.0650
wR2 = 0.1394
2.017 and −1.728
9806 (0.0533)
100.0
0.8309 and 0.7399
9806/74/452
1.070
R1 = 0.0332
wR2 = 0.0839
R1 = 0.0436
wR2 = 0.0895
2.006 and −0.720
9143 (0.0321)
99.9
0.7465 and 0.6626
9143/85/406
1.103
R1 = 0.0293
wR2 = 0.0826
R1 = 0.0328
wR2 = 0.0847
2.061 and −0.416
Final R indices [I > 2r(I)]
R indices (all data)
−3
˚
Largest diff. peak and hole/e A
a Two heavily disordered molecules of tetrahydrofuran are present in the asymmetric unit. b A disordered molecule of tetrahydrofuran is present in the
asymmetric unit. c One half of a heavily disordered molecule of methylene chloride is present in the asymmetric unit.
ppm. 19F NMR (376 MHz, C6D6): d = −72.7 (s, 3F, CF3) ppm.
Anal. calcd for C35H48ClF3N4OW: C, 51.45; H, 5.92; N, 6.86.
Found: C, 50.95; H, 6.16; N, 6.73.
1.52 (d, 3H, i-Pr methyl), 1.39 (d, 3H, i-Pr methyl), −0.27 (d,
3H, i-Pr methyl), −0.30 (d, 3H, i-Pr methyl), −0.31 (d, 3H, i-
Pr methyl) ppm. 13C NMR (125 MHz, C6D6): d = 152.1 (ipso),
151.9 (ipso), 151.7 (ipso), 138.51 (meta), 138.50 (meta), 138.4
(meta), 137.3 (meta), 137.1 (meta), 137.0 (meta), 136.8 (OCN),
129.04 (ortho), 129.00 (ortho), 128.8 (ortho), 124.2 (ortho), 124.01
(ortho), 123.92 (para), 123.85 (para), 123.82 (ortho), 123.6 (para),
68.0 (i-Pr methine), 67.7 (i-Pr methine), 67.1 (i-Pr methine), 23.4
(i-Pr methyl), 23.3 (i-Pr methyl), 23.2 (i-Pr methyl), 23.0 (i-Pr
methyl), 22.5 (i-Pr methyl), 22.08 (2C, ArCH3), 22.06 (ArCH3),
21.74 (ArCH3), 21.72 (ArCH3), 21.70 (ArCH3), 21.5 (i-Pr methyl)
ppm. FTIR (C6D6, KBr): mNCO = 2200 cm−1 (vs). Anal. calcd for
C34H48Cl2N4OW: C, 52.12; H, 6.17; N, 7.15. Found: C, 51.75; H,
6.18; N, 6.93.
Synthesis of (Ar[i-Pr]N)3W(OCN)(Cl)2 (1-(OCN)(Cl)2). Oxa-
lyl chloride (39.0 lL, 0.447 mmol) was added to a colorless solution
of 1-N (304 mg, 0.444 mmol) in Et2O (10 mL) using a microliter
syringe. The reaction mixture turned blood-red upon addition and
shortly changed color to brown. The reaction mixture was stirred
for 20 min at which point the volatiles were removed in vacuo giving
a brownish-yellow solid. The crude material was scraped onto a
fritted glass filter and washed with Et2O revealing a bright yellow
solid. The bright yellow solid was dissolved in minimal THF, the
R
solution filtered through a plug of Celiteꢀ 545 and the filtrate
cooled to −35 ◦C overnight. From the THF solution, small yellow
crystals were harvested, washed with pentane and dried in vacuo
(87 mg, 0.11 mmol, 25%). Recrystallized samples of 1-(OCN)(Cl)2
contain ∼0.5 equiv of THF that persists even after drying in vacuo
=
=
Synthesis of (Ar[i-Pr]N)3W(N PCl3)(Cl)2 (1-(N PCl3)(Cl)2).
A thawing, colorless solution of 1-N (505 mg, 0.738 mmol) in
Et2O (5 mL) was added to a thawing suspension of PCl5 (154 mg,
0.740 mmol) in Et2O (5 mL) resulting in an orange-red reaction
mixture upon addition. The reaction mixture was allowed to warm
to room temperature and stirred for 1 h after which time the
volatiles were removed in vacuo leaving an orange-yellow solid. The
solid was collected on a fritted glass filter, washed with pentane,
and dried under vacuum (505 mg, 0.566 mmol, 76.6%). The sample
used to collect the NMR spectra was recrystallized from THF
layered with Et2O at −35 ◦C. The sample contains ∼1 equiv of
1
as evidenced by the H and 13C NMR spectra. X-Ray quality
crystals of 1-(OCN)(Cl)2 can be grown from a saturated THF
◦
1
solution layered with pentane and stored at −35 C. H NMR
(500 MHz, C6D6): d = 7.65 (s, 1H, para), 7.61 (s, 1H, para), 7.16 (s,
1H, para), 6.56 (s, 3H, ortho), 6.43 (septet, 1H, i-Pr methine), 6.36
(s, 3H, ortho), 6.26 (septet, 1H, i-Pr methine), 6.01 (septet, 1H,
i-Pr methine), 2.15 (s, 9H, ArCH3), 2.054 (s, 3H, ArCH3), 2.049
(s, 3H, ArCH3), 2.02 (s, 3H, ArCH3), 1.56 (d, 3H, i-Pr methyl),
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 4458–4463 | 4461
©

THF of co-crystallization as seen in the 1H and 13C NMR spectra.
The THF remains in the sample even after prolonged periods of
drying in vacuo. 1H NMR (400 MHz, C6D6): d = 7.74 (s, 1H, para),
7.72 (s, 1H, para), 7.32 (s, 1H, para), 6.61 to 6.47 (7H, ortho and
i-Pr methine), 6.19 (septet, 1H, i-Pr methine), 5.88 (septet, 1H,
i-Pr methine), 2.20 (s, 9H, ArCH3), 2.16 (s, 3H, ArCH3), 2.09 (s,
3H, ArCH3), 2.08 (s, 3H, ArCH3), 1.65 (d, 3H, i-Pr methyl), 1.55
(d, 3H, i-Pr methyl), 1.44 (d, 3H, i-Pr methyl), −0.22 to −0.25
(9H, i-Pr methyl) ppm. 13C NMR (100 MHz, C6D6): d = 153.0
(ipso), 152.5 (ipso), 151.9 (ipso), 138.1 (meta), 137.9 (meta), 137.8
(meta), 137.1 (meta), 136.6 (meta), 136.4 (meta), 125.4 (2C, ortho),
125.2 (2C, ortho), 124.5 (2C, ortho), 124.3 (para), 124.2 (para),
124.1 (para), 68.9 (i-Pr methine), 68.1 (i-Pr methine), 66.1 (i-Pr
methine), 23.7 (methyl), 23.4 (methyl), 23.2 (methyl), 23.1 (methyl),
23.0 (methyl), 22.7 (methyl), 22.02 (methyl), 21.95 (methyl), 21.70
(methyl), 21.66 (methyl) ppm. 31P NMR (162 MHz, C6D6): d =
−49.8 (2JWP = 85 Hz) ppm. Anal. calcd for C33H48Cl5N4PW: C,
44.39; H, 5.42; N, 6.27. Calcd for C37H56Cl5N4OPW (1.0 equiv
THF—as seen in the crystal structure): C, 46.05; H, 5.85; N, 5.81.
Found: C, 45.95; H, 5.84; N, 5.72.
Table 2 Total bonding energies for molecules involved in the nitrile
elimination reactions
Molecule
Total energy/kcal mol−1a
CH3CN
CF3CN
CH3C(O)NW(NH2)3Cl
CF3C(O)NW(NH2)3Cl
OW(NH2)3Cl
−837.37
−851.07
−2417.45
−2442.01
−1579.16
a With respect to spherical atomic fragments.
Parallel Quantum Solutions (http://www.pqs-chem.com). All
results reported are with reference to fully optimized geometries
with no imaginary frequencies.20,21
From the above total bonding energies (Table 2) we can compute
DHrxn for the two nitrile elimination reactions as follows:
Synthesis of (Ar[i-Pr]N)3W(Cl)3 (1-(Cl)3). A thawing, red
solution of 1-(O)Cl (794 mg, 1.10 mmol) in Et2O (5 mL) was added
to a thawing suspension of PCl5 (228 mg, 1.10 mmol) in Et2O
(3 mL) resulting in an orange reaction mixture upon addition.
The reaction mixture was allowed to warm to room temperature
and was stirred for 0.5 h. A canary yellow solid precipitated
out of solution and was collected on a fritted glass filter. The
solids were washed with pentane (3 × 20 mL) and dried under
vacuum (595 mg, 0.766 mmol, 69.6%). Samples contain ∼1 equiv
of Et2O as evinced by 1H and 13C NMR. The Et2O remains even
after prolonged periods of drying in vacuo. X-Ray quality crystals
of 1-(Cl)3 can be grown from a saturated methylene chloride
solution layered with diethyl ether and stored at −35 ◦C. 1H NMR
(300 MHz, C6D6): d = 7.77 (s, 3H, para), 6.55 (s, 3H, ortho), 6.44 (s
coincident with septet, 6H, ortho and i-Pr methine, respectively),
2.15 (s, 9H, ArCH3), 2.05 (s, 9H, ArCH3), 1.16 (d, 9H, i-Pr methyl),
−0.27 (d, 9H, i-Pr methyl). 13C NMR (100 MHz, C6D6): d = 152.3
(ipso), 138.2 (meta), 136.8 (meta), 124.0 (2C, ortho), 123.9 (para),
68.5 (i-Pr methine), 23.4 (methyl), 22.1 (methyl), 22.0 (methyl),
21.6 (methyl) ppm. Anal. calcd for C33H48Cl3N3W: C, 51.01; H,
6.23; N, 5.41. Calcd for C33.5H49Cl4N3W (0.5 equiv methylene
chloride—as seen in the crystal structure): C, 49.10; H, 6.04; N,
5.13. Found: C, 48.58; H, 6.38; N, 4.32.
Based on calculations with the above model complexes, ex-
trusion of acetonitrile from CH3C(O)NW(NH2)3 is essentially
thermoneutral while formation of trifluoroacetonitrile from the
analogous tungsten complex is significantly uphill.
Conclusions
Synthesis and characterization of 6-coordinate tungsten com-
=
plexes 1-(N PCl3)(Cl)2, 1-(OCN)(Cl)2 and 1-(Cl)3 lends credence
to the proposed intermediate 2. Similar metallacycles have been
proposed both by us22 and others,23 but there had been some doubt
as to whether the tungsten trisanilide platform 1 could adopt a
pseudo-octahedral structure. Additionally, we have discovered a
new mode of reactivity for 1-N and 1-(O)Cl that awaits further
exploitation.
Acknowledgements
For stimulating discussions the authors thank Richard R. Schrock.
We gratefully acknowledge for funding this work the U. S. National
Science Foundation (CHE-0316823).
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