Synthesis of tungsten complexes that contain hexaisopropylterphenyl- substituted triamidoamine ligands, and reactions relevant to the reduction of dinitrogen to ammonia

Article abstract of DOI:10.1139/v05-013

[HIPTN₃N]WCl (WCl) can be synthesized readily by adding H ₃[HIPTN₃N] to WCl₄(DME) followed by LiN(SiMe₃)₂ ([HIPTN₃N]^3- = [(HIPTNCH₂CH₂)₃N]^3- where HIPT = 3,5-(2,4,6-i-Pr₃C₆H₂)₂C ₆H₃ = HexalsoPropylTerphenyl). Reduction of WCl with KC₈ in benzene under N₂ yields WN=NK. WN=NK is readily oxidized in THF by ZnCl₂ to yield zinc metal and WN₂. Reduction of WN₂ to [WN₂]^- is reversible at -2.27 V vs. FeCp₂^+/0 in 0.1 mol/L [Bu₄N] [BAr′₄]/PhF electrolyte (Ar′ = 3,5-(CF₃) ₂C₆H₃), while oxidation of WN₂ to [WN₂]⁺ is also reversible at -0.66 V. Protonation of WN=NK by [Et₃NH][OTf] in benzene yields WN=NH essentially quantitatively. Protonation of WN=NH at N_β with [H(OEt)₂] [BAr′₄] in ether affords [W=NNH₂] [BAr′₄] quantitatively. Electrochemical reduction of [W=NNH₂][BAr′₄] in 0.1 mol/L [Bu₄N] [BAr′₄]/PhF is irreversible at scan rates of up to 1 V/s. Addition of NaBAr′₄ and NH₃ to WCl in PhF yields [W(NH₃)][BAr′₄]. Electrochemical reduction of [W(NH₃)][BAr′₄] in 0.1 mol/L [Bu₄N]- [BAr′₄]/PhF is irreversible at -2.06 V vs. FeCp ₂^+/0 at a scan rate of 0.5 V/s. Treatment of [W(NH ₃)][BAr′₄] with triethylamine and [FeCp ₂][PF₆] in C₆D₆, followed by LiN(SiMe₃)₂, yielded W≡N. Treatment of [W(NH ₃)][BAr′₄] with LiBHEt₃ (1 mol/L in THF) results in formation of WH, which is converted to WH₃ upon exposure to an atmosphere of H₂. Attempts to prepare WN=NH by treating WN ₂ with [2,6-LutH][BAr′₄] and CoCp₂ yielded only [W=NNH₂][BAr′₄]. [W=NNH ₂][BAr′₄] is reduced to W=NNH₂ by CoCp₂*, but this species disproportionates to yield WN=NH, W≡N, and ammonia. Reduction of [W(NH₃)][BAr′₄] with CoCp₂* does not yield any observable W(NH₃). Attempted catalytic reduction of dinitrogen using WN₂ as the catalyst under conditions identical or similar to those employed for catalytic reduction of dinitrogen by MoN₂ and related Mo complexes failed. Single crystal X-ray studies were carried out on W-N=NK, WN₂, W-N=NH, [W=NNH₂][BAr′₄], and [W(NH₃)] [BAr′₄].

Full text of DOI:10.1139/v05-013

341
Synthesis of tungsten complexes that contain
hexaisopropylterphenyl-substituted triamidoamine
ligands, and reactions relevant to the reduction of
dinitrogen to ammonia
Dmitry V. Yandulov and Richard R. Schrock
Abstract: [HIPTN3N]WCl (WCl) can be synthesized readily by adding H3[HIPTN3N] to WCl4(DME) followed by
LiN(SiMe3)2 ([HIPTN3N]3– = [(HIPTNCH2CH2)3N]3– where HIPT = 3,5-(2,4,6-i-Pr3C6H2)2C6H3 = HexaIsoPropylTer-
phenyl). Reduction of WCl with KC8 in benzene under N2 yields WN=NK. WN=NK is readily oxidized in THF by
ZnCl2 to yield zinc metal and WN2. Reduction of WN2 to [WN2]– is reversible at –2.27 V vs. FeCp2 in 0.1 mol/L
+/0
+
′
[Bu4N][BAr4]/PhF electrolyte (Ar′ = 3,5-(CF3)2C6H3), while oxidation of WN2 to [WN2] is also reversible at –0.66 V.
Protonation of WN=NK by [Et3NH][OTf] in benzene yields WN=NH essentially quantitatively. Protonation of WN=NH
′
′
at Nβ with [H(OEt)2][BAr4] in ether affords [W=NNH2][BAr4] quantitatively. Electrochemical reduction of
′
′
′
[W=NNH2][BAr4] in 0.1 mol/L [Bu4N][BAr4]/PhF is irreversible at scan rates of up to 1 V/s. Addition of NaBAr4 and
NH3 to WCl in PhF yields [W(NH3)][BAr4]. Electrochemical reduction of [W(NH3)][BAr4] in 0.1 mol/L [Bu4N]-
[BAr4]/PhF is irreversible at –2.06 V vs. FeCp2 at a scan rate of 0.5 V/s. Treatment of [W(NH3)][BAr4] with
triethylamine and [FeCp2][PF6] in C6D6, followed by LiN(SiMe3)2, yielded WϵN. Treatment of [W(NH3)][BAr4] with
′
′
+/0
′
′
′
LiBHEt3 (1 mol/L in THF) results in formation of WH, which is converted to WH3 upon exposure to an atmosphere
′
′
of H2. Attempts to prepare WN=NH by treating WN2 with [2,6-LutH][BAr4] and CoCp2 yielded only [W=NNH2][BAr4].
*
′
[W=NNH2][BAr4] is reduced to W=NNH2 by CoCp2, but this species disproportionates to yield WN=NH, WϵN, and
ammonia. Reduction of [W(NH3)][BAr4] with CoCp2 does not yield any observable W(NH3). Attempted catalytic re-
*
′
duction of dinitrogen using WN2 as the catalyst under conditions identical or similar to those employed for catalytic
reduction of dinitrogen by MoN2 and related Mo complexes failed. Single crystal X-ray studies were carried out on
′
′
W-N=NK, WN2, W-N=NH, [W=NNH2][BAr4], and [W(NH3)][BAr4].
Key words: dinitrogen, reduction, tungsten, ammonia.
Résumé : Le complexe [HIPTN3N]WCl (WCl) peut fac3il5e7ment être synthétisé par l’addition de H3[HIPTN3N] à du
WCl4(DME), suivie d’une additon de LiN(SiMe3)2 ([HIPTN3N]3– = [(HIPTNCH2CH2)3N]3– dans lequel HIPT = 3,5-
(2,4,6-i-Pr3C6H2)2C6H3 = HexaIsoPropylTerphényle). La réduction de WCl par du KC8, dans du benzène, sous atmos-
phère d’azote, conduit à la formation de WN=NK. Sous l’action du ZnCl2 dans du THF, le WN=NK s’oxyde faci-
′
lement pour conduire à la formation de zinc métallique et de WN2. Dans un électrolyte à 0,1 mol/L de [Bu4N][BAr4]/PhF
(Ar′ = 3,5-(CF3)2C6H3), la réduction du WN2 en [WN2]– est réversible à –2,27 V vs. FeCp2 alors que l’oxydation du
+/0
[WN2] en [WN2]+ est aussi réversible à –0,66 V. La protonation du WN=NK par le [Et3NH][OTf], dans le benzène,
conduit à la formation de WN=NH, avec un rendement pratiquement quantitatif. La protonation du WN=NH au niveau
′
′
du Nβ avec du [H(OEt)2][BAr4], dans l’éther, conduit à la formation de [W=NNH2][BAr4] avec un rendement quantita-
′
′
tif. La réduction électrochimique du [W=NNH2][BAr4] dans du [Bu4N][BAr4]/PhF à 0,1 mol/L est irréversible à des
vitesses de balayage allant jusqu’à 1 V/s. L’addition de NaBAr4 et de NH3 au WCl dans du PhF conduit à la forma-
tion de [W(NH3)][BAr4]. La réduction électrochimique du [W(NH3)][BAr4] dans du [Bu4N][BAr4]/PhF à 0,1 mol/L est
irréversible à –2,06 V vs. FeCp2 , à une vitesse de balayage de 0,5 V/s. Le traitement du [W(NH3)][BAr4] par de la
′
′
′
′
+/0
′
triéthylamine et du [FeCp2][PF6] dans du C6D6, suivi de LiN(SiMe3)2 conduit à la formation de WϵN. Le traitement
′
de [W(NH3)][BAr4] par du LiBHEt3 (1 mol/L dans du THF) conduit à la formation de WH qui peut être transformé
en WH3 par exposition à un atmosphère de H2. Des essais en vue de préparer le WN=NH par traitement du WN2 avec
*
′
′
du [2,6-LutH][BAr4] et du CoCp2 ne fournissent que du [W=NNH2][BAr4]. Sous l’influence du CoCp2, le [W=NNH2]-
′
[BAr4] est réduit en W=NNH2, mais cette espèce subit une réaction de dismutation conduisant à la formation de
WN=NH, de WϵN et d’ammoniac. La réduction du [W(NH3)][BAr4] par le CoCp2 ne fournit aucune trace de
*
′
W(NH3). Des essais en vue d’effectuer la réduction du diazote en utilisant du WN2 comme catalyseur, dans des
Received 11 August 2004. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 30 March 2005.
D.V. Yandulov1 and R.R. Schrock.2 Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139,
USA.
1Present address: Department of Chemistry, Stanford University, Stanford, CA 94305-5080, USA.
2Corresponding author (e-mail: rrs@mit.edu).
Can. J. Chem. 83: 341–357 (2005)
doi: 10.1139/V05-013
© 2005 NRC Canada

342
Can. J. Chem. Vol. 83, 2005
conditions identiques ou semblables à celles utilisées pour la réduction catalytique du diazote par le MoN2 et les com-
plexes apparentés du molybdène n’ont pas donné les résultats escomptés. Des études de diffraction des rayons X sur un
′
′
cristal unique ont été effectuées sur le W-N=NK, le WN2, le W-N=NH, le [W=NNH2][BAr4] et le [W(NH3)][BAr4].
Mots clés : diazote, réduction, tungstène, ammoniac.
[Traduit par la Rédaction] Yandulov and Schrock
Introduction
Fig. 1. A drawing of [HIPTN3N]Mo(N2) = MoN2.
Discovery of the first dinitrogen complex of a transition
metal, [Ru(NH3)5(N2)]2+, in 1965 (1) provided support for
the proposal that dinitrogen might be reduced to ammonia at
a molybdenum center in nitrogenase (only the FeMo nitro-
genase was known at the time (2–4)). As more and more
types of transition-metal-catalyzed reactions were discovered
in the following decades, scientists became optimistic that
catalysts would be discovered for the reduction of dinitrogen
to ammonia, or for the combination of dinitrogen with ele-
ments other than hydrogen (5–16). Unfortunately, although
hundreds of man years have been directed toward this goal
in the last 40 years, and although hundreds of dinitrogen
complexes of all transition metals in groups 4 through 10 (if
we include those detected at low temperature in matrices)
are now known, progress has been relatively slow.
Chatt, a pioneer, along with Hidai, in dinitrogen chemistry
involving W(0) and Mo(0) phosphine complexes (5–7, 12,
14) believed that dinitrogen could be reduced to ammonia at
a single metal center through end-on binding of dinitrogen,
and was the first to show that up to 2 equiv. of ammonia per
metal could be formed from M(0) dinitrogen complexes
(M = Mo or W) upon addition of protons, the six electrons
being provided by the metal (5). If dinitrogen could be re-
duced to ammonia catalytically through the addition of six
external protons and six external electrons, and if a distinct
intermediate were formed upon addition of each proton and
electron, then 12 intermediates could be imagined, or a total
of 14 if one includes a dinitrogen complex and a complex
that contains no nitrogen or nitrogen-derived ligand. Al-
though examples of almost all of the proposed intermediates
for reduction of dinitrogen at a single Mo or W center in a
“Chatt cycle” have been isolated, the ligand environment
(Lx) of the metal has varied from compound to compound,
and no catalytic reaction to give ammonia was ever achieved
(see, however, the synthesis of silylamines catalytically
(17)). Significant progress has been made toward utilizing
atmospheric dinitrogen to make organic molecules (6, 7, 18–
26). One catalytic reduction is known in which dinitrogen is
reduced to a mixture of hydrazine (largely) and ammonia (9,
27, 28). This system requires molybdenum, is catalytic with
respect to molybdenum, produces >90% hydrazine, and
takes place in methanol in the presence of Mg(OH)2 and a
strong reducing agent (e.g., Na amalgam). Unfortunately,
few details concerning the mechanism of this intriguing re-
action have been established. In this context, it should be
noted that ammonia can be formed from hydrazine via
disproportionation (to dinitrogen and ammonia) in the pres-
ence of transition metals. In fact, [Ru(NH3)5(N2)]2+ was first
prepared by treating Ru salts with hydrazine (1).
plore the chemistry of Mo complexes that contain a triami-
doamine ([(ArNCH2CH2)3N]3– = [ArN3N]3–, Ar = aryl)
ligand (30, 31). To prevent formation of what we believe to
be relatively stable and unreactive bimetallic [ArN3N]Mo-
N=N-Mo[ArN3N] complexes, maximize steric protection of
a metal coordination site in a monometallic species, and pro-
vide increased solubility in nonpolar solvents, we synthe-
sized species that contain a [HIPTN3N]3– ligand, where
HIPT = 3,5-(2,4,6-i-Pr3C6H2)2C6H3 (HexaIsoPropylTerphenyl,
see Fig. 1) (32, 33). Starting with MoCl (Mo = [HIPTN3N]Mo),
we showed that we could isolate and characterize many in-
termediates in
a hypothetical Chatt-like reduction of
dinitrogen, all of which contain the same [HIPTN3N]3– sup-
porting ligand (Fig. 2). These intermediates include para-
magnetic Mo(N2) (1), diamagnetic [Mo(N2)]– (2), diamagnetic
′
Mo-N=N-H (3), diamagnetic {Mo=N-NH2}{BAr4} (4; Ar′ =
3,5-(CF3)2C6H3), diamagnetic MoϵN (7), diamagnetic
′
′
{Mo=NH}{BAr4} (8), paramagnetic {Mo(NH3)}{BAr4} (12),
and paramagnetic Mo(NH3) (13). Extensive 15N labeling
studies, NMR studies, and X-ray studies (of 1, 2 (as two dif-
ferent Mg derivatives), 7, and 12 (33), and 3, 4, 8, and 13
(34)) all reveal a trigonal pocket in which N2 or its reduced
products are protected to a dramatic degree by three 3,5-
(2,4,6-i-Pr3C6H2)2C6H3 (HIPT) rings clustered around it.
Compounds 5, 6, 10, and 14 (Fig. 2) have not yet been ob-
served. The oxidation state of the metal varies between
Mo(III) and Mo(VI) in the species as they are drawn in
Fig. 2. No other unambiguous example of one of these com-
plexes (3) is known, presumably either because in other sys-
tems bimolecular processes that involve H radical or proton
transfer result in facile decomposition, or the synthetic meth-
ods are incompatible with the ligands that are present. Steric
protection of the nitrogen entities within the pocket provided
by the bulky HIPT groups are believed to be an important
reason why several of the most unusual species can be pre-
pared.
We have been interested in dinitrogen chemistry of early
metals in relatively high oxidation states for more than two
decades (29). Approximately 4 years ago, we began to ex-
© 2005 NRC Canada

Yandulov and Schrock
343
Fig. 2. Proposed intermediates in the reduction of dinitrogen at a
[HIPTN3N]Mo (Mo) center through the step-wise addition of
protons and electrons.
It has been known for some time that certain bacteria
grown in the presence of tungsten will produce a “FeW
nitrogenase”, but that enzyme will not reduce dinitrogen to
ammonia (2, 3, 38). Therefore, we became interested in pre-
paring some tungsten analogs of the [HIPTN3N]Mo species
shown in Fig. 2, exploring conversion of one to another, and
testing at least one of them for catalytic dinitrogen reduction
activity. We report this work here.
Results
Synthesis of WCl and diazenido species
The starting material for this chemistry ([HIPTN3N]WCl
(WCl)) can be readily synthesized by a two-step method
analogous to that used to prepare [HIPTN3N]MoCl (33) as
1
shown in eq. [1]. The H NMR spectrum of high-spin WCl
(d2) contains sharper resonances than does the spectrum of
MoCl; this difference, and the pattern of relative shifts of the
corresponding resonances between Mo and W complexes is
very similar to that observed previously for a series of
related [ArN3N]MCl (M = Mo, W) (30, 39, 40) and
[Me3SiN3N]MCl (41–43) complexes. Like MoCl, WCl is
exceedingly air- and moisture sensitive, so the utmost care is
required to ensure that solvents are pure, dry, and free of ox-
ygen.
Reduction of MoCl with magnesium in THF under an at-
mosphere of dinitrogen led to salts that contain the [Mo(N2)]–
anion (33). However, attempts to prepare [W(N2)]– salts in
this manner were not successful; WCl was slowly converted
into a complex mixture of unidentified products. Reduction
of WCl is markedly accelerated when potassium graphite
(KC8) is employed as the reductant in THF or DME, but a
complex mixture of products is still formed. However, when
WCl is treated with KC8 in benzene under dinitrogen, clean
and nearly quantitative conversion of WCl to diamagnetic
W-N=NK takes place over a period of 1 to 2 days at room
temperature (eq. [2]). No intermediates could be detected by
1H NMR when this reaction was performed in C6D6. Deep
red W-N=NK was isolated in good yield and was fully char-
acterized. Since W-N=NK can be dissolved in THF without
decomposition (vide infra), failure of the reductions in THF
and DME must be attributed to a some side reaction or reac-
tions prior to binding of dinitrogen to the metal, presumably
at the W(III) stage. WN=NK is readily soluble in aliphatic
We then showed that it is possible in fact to reduce dini-
trogen catalytically in heptane using {2,6-lutidinium}{BAr4}
as the proton source and decamethylchromocene as the elec-
tron source (35). The results of 16 runs using four different
Mo derivatives (1, 3, 7, or 12) revealed that ~8 equiv. of am-
monia are formed with an efficiency of 63%–66% based
′
on reducing equivalents available.
A run employing
Mo(15N15NH) under 15N2 yielded entirely 15N-labeled am-
monia. We assume at this stage that any equivalents of re-
ducing agent that are not consumed in making ammonia are
consumed to form hydrogen, and we now know that no
hydrazine is formed.3 The fact that compounds that contain
hexamethylterphenyl or hexa-tert-butylterphenyl variations
of the [HIPTN3N]3– ligand are relatively inefficient catalysts
for catalytic reduction of dinitrogen to ammonia (37) sug-
gests that the catalytic reaction is extremely finely balanced
and that perhaps most or all of the intermediates shown in
Fig. 2 are actually involved. All are likely to be equilibria
with exception of the actual N—N bond cleavage reaction
+
(Mo=N-NH3 + e → MoϵN + NH3).
3 W. Weare. Unpublished results.
© 2005 NRC Canada

344
Can. J. Chem. Vol. 83, 2005
Fig. 3. Thermal ellipsoid drawing of WN=NK (50% probability level, hydrogen atoms omitted).
solvents and can be recrystallized without change from
pentane.
different, with one HIPT group in contact with K being ro-
tated by only 10° out of the plane of the amide nitrogen
(N1), while the other two HIPT groups are rotated by an av-
erage of 28.3° outside of the plane of the amide nitrogen to
which they are attached. “Encapsulation” of the K ion be-
tween two aryl rings accounts for the high solubility of W-
N=NK in aliphatic solvents (binding of sodium or potassium
ions to aryl rings in high oxidation state amido complexes
has considerable precedent in the literature (44–47)). The
N—N (1.220(8) Å) and W—N (1.849(7) Å) bond distances
are consistent with a considerably greater degree of reduc-
tion of the N2 unit in W-N=NK relative to that in Mo-
N=NMgBr(THF)3 (1.156(8) and 1.863(7) Å, respectively)
(33). Greater backbonding from the tungsten center in W-
N=NK (vs. the molybdenum center in Mo-N=NMgCl(THF)3
(33)) is also evident from the relative values for νNN in the
two species (1745 cm–1 (W) and 1812 cm–1 (Mo), respec-
tively, in benzene). The W-Nα-Nβ angle is essentially linear
(177.3(6)°), as expected.
The structure of W-N=NK, as determined through a single
crystal X-ray study, is shown in Fig. 3 (see also Table 1 for
crystal data and Table 2 for selected M—N and N—N bond
distances and angles in W and Mo analogs, along with se-
lected IR and NMR data. A full set of X-ray data is available
as supporting information.4 The potassium ion is 2.536(7) Å
from N6, the β nitrogen in the unsubstituted diazenido ligand,
and is supported through six interactions with aryl carbon at-
oms in two Trip (2,4,6-i-Pr3C6H2) aromatic rings of two
HIPT substituents. The potassium ion resides 3.229 and
3.301 Å from the ring centroids, and all K···H contacts are
>2.92 Å. Interaction between the potassium and the aromatic
rings is not symmetric, with K···C distances spanning a
range of 0.52 Å for each ring. The presence of the potassium
ion has no noticeable effect on the C—C bond distances in
the two aryl rings with which it interacts. Understandably,
the orientations of the three HIPT groups are significantly
4 Supplementary data may be purchased from the Directory of Unpublished Data, Document Delivery, CISTI, National Research Council
Canada, Ottawa, ON K1A 0S2, Canada (http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml for information on ordering electronically).
CCDC 261864–261868 contain the crystallographic data for this manuscript. These data can be obtained, free of charge, via
ww.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
U.K.; fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
© 2005 NRC Canada

Yandulov and Schrock
345
Table 1. Crystal data and structure refinement for [HIPTN3N]W-N=NK, [HIPTN3N]WN2, [HIPTN3N]W-N=NH, {[HIPTN3N]W=N-
a
′
′
NH2}[BAr4], and {[HIPTN3N]W(NH3)}[BAr4].
′ ′
{W-N=NH2}[BAr4] {W(NH3)}[BAr4]
W-N=NK
WN2
W-N=NH
Empirical formula
C114H159KN6W
C114H159N6W
C114H160N6W
C146H173BF24N6W
C146H174BF24N5W
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
1 836.42
Monoclinic
P2(1)/n
18.297(4)
40.166(9)
19.528(4)
90
117.600(5)
90
12 718(5)
4
1 797.32
Monoclinic
Cc
16.0231(9)
40.125(2)
17.9933(10)
90
93.8110(10)
90
11 542.7(11)
4
1 798.33
Triclinic
P1
2 662.56
Triclinic
P1
2649.56
Triclinic
P1
18.2377(14)
20.1004(15)
37.763(3)
86.923(2)
85.346(2)
75.098(2)
13 326.0(17)
4
15.7795(10)
20.6647(14)
26.2549(17)
85.2740(10)
83.6730(10)
81.9220(10)
8405.1(10)
2
15.8411(9)
20.5894(11)
26.3012(14)
84.9740(10)
83.8560(10)
81.8820(10)
8420.7(8)
2
α (°)
β (°)
γ (°)
Volume (Å3)
Z
Density (calcd., Mg/m3)
Absorption coeff. (mm–1)
F(000)
0.959
0.981
3912
1.034
1.044
3836
0.896
0.904
3840
1.052
0.755
2772
1.045
0.754
2760
Crystal size (mm3)
θ range (°)
Index ranges
0.20 × 0.18 × 0.07 0.27 × 0.15 × 0.14 0.20 × 0.10 × 0.03 0.21 × 0.14 × 0.12
0.18 × 0.16 × 0.13
0.78–27.00
–20 ≤ h ≤ 20
–15 ≤ k ≤ 26
–33 ≤ l ≤ 33
57 597
36 583 (0.0817)
99.5
None
1.01–25.00
–14 ≤ h ≤ 21
–47 ≤ k ≤ 47
–21 ≤ l ≤ 23
62 953
22 389 (0.1916)
99.9
None
1.37–27.00
–20 ≤ h ≤ 20
–50 ≤ k ≤ 51
–10 ≤ l ≤ 22
35 587
16 642 (0.0509)
99.9
None
1.05–25.00
–21 ≤ h ≤ 18
–23 ≤ k ≤ 15
–44 ≤ l ≤ 44
69 284
46 258 (0.1000)
98.6
Empiricalb
0.78–27.00
–18 ≤ h ≤ 20
–26 ≤ k ≤ 25
–33 ≤ l ≤ 33
57 366
36 498 (0.0387)
99.4
Empiricalb
Reflections collected
Indep. reflections (R(int))
Compl. to θ max (%)
Absorption correction
Data/restraints/param.
Goodness-of-fit on F2
22 389/0/1 135
0.938
16 642/2/1 126
0.965
46 258/21/2 251
0.928
36 498/0/1 639
0.961
36 583/0/1 631
0.776
Final R indices (I > 2σ(I)) R1 = 0.0733
wR2 = 0.1517
R1 = 0.0434
wR2 = 0.0984
R1 = 0.0504
wR2 = 0.1004
1.765 and –1.530
R1 = 0.1026
wR2 = 0.2264
R1 = 0.2094
wR2 = 0.2632
1.295 and –1.619
R1 = 0.0611
wR2 = 0.1489
R1 = 0.0805
wR2 = 0.1577
2.172 and –1.566
R1 = 0.0612
wR2 = 0.1147
R1 = 0.1048
wR2 = 0.1235
1.244 and –1.090
R indices (all data)
R1 = 0.1362
wR2 = 0.1732
2.872 and –1.777
Peak and hole (e Å–3)
aIn all cases the temperature = 193(2) K, the wavelength = 0.710 73 Å, and the refinement method full-matrix least-squares on F2.
bEmpirical absorption correction was applied using the SADABS v2.10 program (36).
Table 2. A comparison of structural and spectroscopic properties of selected [HIPTN3N]W and [HIPTN3N]Mo complexes.a
+b
+b
Compound property
MoN2
WN2
WN=NK
MoN=NH
WN=NH
Mo=NNH2
W=NNH2
M-Nα (Å)
1.963(5)
1.061(7)
1990
1924
—
1.945(6)
1.132(8)
1888
1826
—
1.849(7)
1.220(8)
1745
1.810(7)
1.25(1)
1587
1.732(9)
1.29(1)
1542
1.737(3)
1.310(5)
—
1.745(3)
1.315(5)
—
Nα-Nβ (Å)
ν(14Nα14Nβ) (cm–1)
ν(15Nα15Nβ) (cm–1)
δ(15Nα15Nβ) (ppm)
1J(15Nβ15Nα, H) (Hz)
1688
1523
1499
—
—
357, 350
409, 233
394, 229
358, 140
344, 131
—
—
11.2
15.9, 53.6
15.4, 53.1
11.2, 90.5
10.1, 83.6
aIR and 15N NMR data in C6D6 at 22 °C, unless stated otherwise. The Mo compounds can be found in refs. 33 and 34.
bAnion = BAr4′–.
Doubly labeled W-15N=15NK reveals sharp doublet reso-
nances for 15Nα (at 357 ppm) and 15Nβ (at 350 ppm) in the
15N NMR spectrum in C6D6 at room temperature. The 15Nα
and 15Nβ resonances in W-15N=15NK are relatively close to-
gether, yet fall well within the range typical for compounds
of this general type (48, 49). The 1H NMR spectrum of
W-N=NK in C6D6 at room temperature features slightly
broadened resonances, yet is otherwise consistent with a
C3v-symmetric structure. Apparently the interactions be-
tween the potassium ion and the aryl rings are weak enough
so that intramolecular ligand fluxionality in W-N=NK, in-
cluding rotation of the HIPT rings around the N—Cipso
bonds, is not hindered significantly at room temperature.
Treatment of a C6D6 solution of W-N=NK with Bu4NCl
results in a rapid color change from maroon-red to dark
1
green. Both IR and H NMR spectra of the green solution
show complete conversion of W-N=NK to a single diamag-
netic product with a νNN frequency shifted to higher energy
© 2005 NRC Canada

346
Can. J. Chem. Vol. 83, 2005
Fig. 4. Thermal ellipsoid drawing of WN2 (50% probability level, hydrogen atoms omitted).
by 56 cm–1 from what it is in W-N=NK. These observations
are analogous to those made for related Mo diazenides (33)
and indicate formation of a species in which there is essen-
tially no interaction between the cation and the β nitrogen,
i.e., formation of [W-N=N]–. However, all attempts to crys-
tallize [W-N=N][Bu4N] (e.g., by precipitation with pentane)
have failed. This compound appears to decompose to yield
two major pentane-soluble diamagnetic tungsten complexes
and Bu3N. We speculate that part of the decomposition
consists of a reaction between [WN2]– and Bu4N+ to yield
W-N=NBu and Bu3N. In contrast, [Mo-N=N][Bu4N] can be
isolated and is stable at room temperature (33). A greater
nucleophilicity of [WN2]– compared to [MoN2]– at Nβ
would be consistent with the observed greater degree of
backbonding from tungsten to the N2 unit and would not be
surprising.
Fig. 5. Electrochemical behavior of MN2 (M = Mo or W) in
′
0.1 mol/L [Bu4N][BAr4]/PhF, recorded at a glassy carbon elec-
trode (3 mm diam.) at a scan rate of 500 mV/s. The potential
+/0
scale is referenced to FeCp2
.
Synthesis of a terminal dinitrogen complex of W(III)
Like various {Mo-N=N}– salts (33), WN=NK is readily
oxidized in THF by ZnCl2 to yield zinc metal and the neu-
tral dinitrogen complex, WN2, essentially quantitatively. The
WN2 complex was isolated as an orange solid in 53% yield
1
starting from WCl and has been fully characterized. The H
although one might expect the degree of this type of lib-
ration of a bound dinitrogen to decrease as backbonding to
dinitrogen increases. There is clearly a greater degree of
backbonding to N2 in WN2 than in MoN2 also according to
ν(N-N) frequencies, i.e., the value for ν(N-N) is nearly
100 cm–1 lower in WN2 (1888 cm–1) than in MoN2 (1990 cm–1).
All other structural details are normal for [HIPTN3N]3– com-
plexes (see supporting information).4
NMR spectrum of WN2 in C6D6 resembles that of the low-
spin d3 MoN2 analog (33), although the ligand backbone res-
onances of the W complex are broader and shifted with re-
spect to those for MoN2 (at 35, –11, and –23 ppm for WN2
and at 23 (NCH2), –5 (4,6-H), and –34 (NCH2) ppm in
MoN2). WN2 is the only known terminal dinitrogen complex
of tungsten in which a triamido/amine ligand is present and
the only one of W(III) to our knowledge.
A single crystal X-ray study of WN2 (Fig. 4) shows it
to have a structure analogous to that of MoN2 (33) with an
N—N bond distance of 1.132(8) Å (cf. 1.098 Å in free N2
and 1.061(7) Å in MoN2). The observed “shorter” N—N
bond length in MoN2 than in free dinitrogen is believed to
be a consequence of a slight “libration” of the bound dinitro-
gen about the center of the N—N bond, which would make
the N—N bond appear slightly shorter than it actually is
(34). The actual N—N bond distance in WN2 therefore actu-
ally also could be somewhat longer than what is observed,
Electrochemical reduction of WN2 to [WN2]– is reversible
+/0
and takes place at –2.27 V vs. FeCp2
in 0.1 mol/L
′
[Bu4N][BAr4]/PhF electrolyte (Fig. 5) (the advantages of
[Bu4N]+ salts of relatively unreactive and weakly coordinat-
ing anions as electrolytes in polar, but relatively unreactive
solvents such as fluorobenzene, were demonstrated more
than 10 years ago (50), and have been explored more sys-
tematically by Geiger and co-workers in the last several
0/–
years (51–55)). The WN2 potential is 260 mV more nega-
tive than the corresponding E°′(MoN20/–) value (34), consis-
tent with tungsten being more difficult to reduce than
© 2005 NRC Canada

Yandulov and Schrock
347
molybdenum, as expected. The tungsten dinitrogen complex
also undergoes a reversible electrochemical oxidation, but at
the same potential as the molybdenum dinitrogen complex
N=NH group. A greater degree of backbonding from W is
also evident from a lower value for ν(N-N) in WN=NH
(1542 cm–1) than in MoN=NH (1587 cm–1). Each molecule
of WN=NH is noticeably asymmetric within the
[HIPTN3N]3– ligand. One W—N(amide) distance is slightly
shorter than the other two in each molecule, and the HIPT
group attached to that amido nitrogen is rotated out of its
plane by an average of 68° in the direction opposite to the
average turn of 28° found for the other two HIPT groups in
each molecule. As a result, the three HIPT groups in each
molecule do not form a pseudo three-fold symmetric propel-
ler arrangement around the apical pocket similar to what is
found in 13 other [HIPTN3N]Mo and [HIPTN3N]W deriva-
tives. The hydrazide ligand is bent at both W (N5-W-N4 =
173.1(4)°) and N5 (W-N5-N6 = 173.6(9)°) in the same di-
rection, primarily toward the unique N(amide) (N3A in the
molecule shown in Fig. 6). The diazenide hydrogen was not
located, but was added and refined in the position shown. It
should be noted that Nα-Nβ-R angles in diazenido compounds
vary, e.g., from 132° in [(Me3SiNCH2CH2)2NCH2CH2NMe2)]-
Mo(CH3)(N2TMS) to 170° in {[(Me3SiNCH2CH2)2N-
CH2CH2NMe2]Mo-N=NSiMe3}OTf (56). Therefore, we
cannot say with any degree of certainty exactly where Hβ is
located. To our knowledge, MoN=NH (33, 34) and WN=NH
are the only structurally characterized examples of parent
diazenide complexes of any metal.
+/0
(–0.66 V, Fig. 5). The similarity in the MN2 potentials for
Mo and W can be ascribed to an effective “leveling” of the
HOMO energies in the two MN2 complexes as a conse-
quence of increased π donation from the metal to N2 in WN2
(∆ν(N-N) = 102 cm–1, vide supra). However, the HOMO en-
ergies in [MoN2]– and [WN2]– are not approximately the
same, and [WN2]– therefore remains a stronger one-electron
reductant than [MoN2]– by 260 mV under the conditions de-
scribed.
′
Synthesis of WN=NH and [W=NNH2][BAr4 ]
Protonation of WN=NK by [Et3NH][OTf] in benzene is
virtually instantaneous and yields WN=NH essentially quan-
titatively (eq. [3]). Diamagnetic WN=NH was isolated in
55% yield starting from WCl and has been fully character-
1
ized. The N=NH resonance appears at 8.43 ppm in the H
NMR spectrum in C6D6, flanked by 183W satellites with
3J(183W-H) = 17.4 Hz. The IR spectrum in C6D6 exhibits a
ν(N-N) absorption at 1542 cm–1, a frequency 45 cm–1 lower
than the corresponding value in MoN=NH (34). The 1H
NMR spectrum of doubly labeled W-15N=15NH features a
doublet of doublets for the 15N=15NH resonance (1J(15Nβ-
2
H) = 53.1 Hz, J(15Nα-H) = 6.7 Hz) and two doublets of
doublets at 394 (15Nα) and 229 (15Nβ) ppm (1J(15N-15N) =
15.4 Hz) in the 15N NMR spectrum. These NMR data are
similar to those observed for MoN=NH (Table 2). Unlike
MoN=NH, however, WN=NH is quite stable thermally, suf-
fering only minor decomposition after a solution in C6D6
had been heated to 65 °C for 11 days. In contrast, MoN=NH
decays in C6D6 with a half-life of ~7 h at 61 °C, forming
predominantly MoH, when prepared via a method analogous
to that shown in eq. [3]. This decomposition was shown to
be promoted by traces of acid (33) that are believed to re-
main after the Mo reaction analogous to that shown in eq. [3].
The X-ray crystal structure of WN=NH is shown in
Fig. 6. Owing to the presence of two independent molecules
in the unit cell and the difficulty of obtaining adequately
sized crystals of this compound, the quality of this structure
is mediocre (R1 = 10%). However, it was possible to refine
all non-hydrogen atoms anisotropically. The critical geomet-
rical parameters of the two crystallographically independent
molecules are statistically indistinguishable (within 3σ). The
average W—Nα (1.732(9) Å) and Nα—Nβ (1.286(11) Å)
distances are shorter and longer, respectively, than those in
MoN=NH (1.810(7) and 1.25(1) Å, Table 2), consistent with
a greater degree of backbonding from tungsten into the
Protonation of WN=NH at Nβ with [H(OEt)2][BAr4] in
′
′
ether affords the hydrazido(2–) species [W=NNH2][BAr4]
essentially quantitatively (eq. [4]). This W(VI) species can
be isolated in good yield as a pale-yellow solid and has been
+
fully characterized. The NNH2 resonance appears at
1
5.96 ppm in the H NMR spectrum in C6D6, flanked by
barely resolved 183W satellites (1J(183W-H) = 6 Hz). Labeled
15 15
′
[W= N NH2][BAr4] features a doublet of doublets for the
+
15N15NH2 resonance in the 1H NMR spectrum (1J(15Nβ-
H) = 83.6 Hz, J(15Nα-H) = 1.8 Hz) and a doublet at 344
2
+
(15Nα-15NβH2 ) and a triplet of doublets at 131 ppm (15Nα-
+
15NβH2 , J(15N-15N) = 10.1 Hz) in the 15N NMR spectrum.
′
While the spectra of [W=NNH2][BAr4] are generally similar
to those of [Mo=NNH2][BAr4], 15N NMR data for
′
[W=NNH2][BAr4] reveal a reduction of 1J(15Nβ-H) to
′
′
83.6 Hz in [W=NNH2][BAr4] from 90.5 Hz in [Mo=NNH2]-
′
[BAr4] (34), consistent with an increased p character in the
bonding at Nβ as a consequence of a greater degree of
′
backbonding to the NNH2 unit in [W=NNH2][BAr4].
The structure of [W=NNH2][BAr4] (Fig. 7) is similar to
other {[HIPTN3N]M}[BAr4] (M = Mo, W) derivatives that
have been characterized in an X-ray study. The Nα—Nβ
′
′
distance (1.315(5) Å) is essentially identical to that in
© 2005 NRC Canada

348
Can. J. Chem. Vol. 83, 2005
Fig. 6. Thermal ellipsoid drawing of W-N=NH (50% probability level, selected hydrogen atoms omitted).
′
difference therefore better reflects the difference in the
HOMO energy that results from substitution of W for Mo
alone. Consistent with this hypothesis, a large difference be-
tween Mo and W is also found for the M(NH3)+/0 potentials
(vide infra).
[Mo=NNH2][BAr4] (1.310(5) Å). Since the Nα—Nβ dis-
tance in [W=NNH2][BAr4] is similar to that in WN=NH
′
(1.29(1) Å), the amount of Nα—Nβ double bond character
must be significant, as one would expect in a M-N-N π sys-
tem of this type. The NNH2 hydrogens were not located,
+
but were refined in calculated positions. Regardless of the
particular orientation chosen for their representation, their
presence in the model had a slight, but constant, effect on
the refined N—N distance.
′
Synthesis of [W(NH3)][BAr4 ], its structure, and
reactivity
′
′
Electrochemical reduction of [W=NNH2][BAr4] in 0.1 mol/L
When WCl is treated with NaBAr4 and NH3 in PhF
[W(NH3)][BAr4] is formed nearly quantitatively (eq. [5]).
This reactivity mirrors that established for MoCl (33), ex-
cept that fluorobenzene had to be used as a solvent because
WCl was found to be incompatible with dichloromethane.
The ammonia adduct was isolated in good yield and fully
′
′
[Bu4N][BAr4]/PhF at a glassy carbon electrode is irrevers-
ible at scan rates of up to 1 V/s and takes place at Ipc
=
+/0
–2.2 V vs. FeCp2 (100 mV/s scan rate). The Mo analog,
[Mo=NNH2][BAr4], undergoes a quasi-reversible reduction
at –1.56 V under the same conditions (34), which allows us
to estimate the difference between the Mo=NNH2
W=NNH2 couples as being 400–500 mV. The magnitude
of this difference is considerably greater than that found for
[MN2]0/– (260 mV) and [MN2]+/0 (0 mV) potentials (M =
Mo or W, vide supra). Apparently π donation of electron
density from M into the [NNH2]2– system does not differ
′
+/0
1
and
characterized. Its H NMR spectrum in C6D6 features a pat-
+/0
tern of paramagnetically shifted and broadened resonances
characteristic of a high-spin d2 W environment in complexes
of this general type. The resonances in W(NH3)+ are
paramagnetically shifted to a greater extent than those of
WCl, a phenomenon that was also observed for Mo(NH3)+
vs. MoCl.
+/0
substantially for Mo and W, and the M=NNH2 potential
© 2005 NRC Canada

Yandulov and Schrock
Fig. 7. Thermal ellipsoid drawing of [W=NNH2][BAr4] (50% probability level, selected hydrogen atoms omitted).
349
′
′
′
The X-ray crystal structure of [W(NH3)][BAr4] (Fig. 8) is
essentially identical to that of [Mo(NH3)][BAr4] (33). The
of [Mo(NH3)][BAr4] is fully reversible both in PhF
(0.1 mol/L [Bu4N][BAr4]) at a glassy carbon electrode and
′
′
′
M—N(ammonia) distances are 2.210(4) Å in [W(NH3)][BAr4]
vs. 2.24(1) Å in [Mo(NH3)][BAr4] (Table 2). The ammonia
protons were not located, but are shown in calculated posi-
tions.
Electrochemical reduction of [W(NH3)][BAr4] in 0.1 mol/L
THF (0.4 mol/L [Bu4N][PF6]) at a platinum disk at all scan
rates and takes place at E°′(Mo(NH3)+/0) = –1.63 V vs.
′
+/0
′
FeCp2 in PhF (0.1 mol/L [Bu4N][BAr4]). The difference
in M(NH3)+/0 potentials (M = Mo or W) is thus ~430 mV,
+/0
similar to the difference between the Mo=NNH2
W=NNH2 potentials (vide supra). Chemical reductions of
both [W=NNH2][BAr4] and [W(NH3)][BAr4] are described
and
′
+/0
′
[Bu4N][BAr4]/PhF at a glassy carbon electrode reveals an ir-
′
′
reversible wave at a scan rate of 0.5 V/s (Fig. 9). The couple
appears to become more reversible at faster scan rates (1 and
2 V/s) at –2.06 V vs. FeCp2+/0, although it cannot be called
reversible even at 2 V/s. In contrast, one-electron reduction
below.
Deprotonation of [W(NH3)]+ with LiN(SiMe3)2/4TMEDA
in C6D6 yields a single paramagnetic product, which we pre-
© 2005 NRC Canada

350
Can. J. Chem. Vol. 83, 2005
′
Fig. 8. Thermal ellipsoid drawing of [W(NH3)][BAr4] (50% probability level, selected hydrogen atoms omitted).
sume to be (at least initially) W(NH2). However, attempts to
isolate the product in analytically pure crystalline form so
′
far have failed. Treatment of [W(NH3)][BAr4] with triethy-
lamine and [FeCp2][PF6] in C6D6, followed by LiN(SiMe3)2
yielded WϵN essentially quantitatively (eq. [6]). The nitride can
be prepared more conveniently via salt metathesis of WCl
with 3 equiv. of NaN3 in a mixture of C6H6 and MeCN over
a period of 16 h at room temperature (eq. [7]), the method
utilized originally in the synthesis of [Me3SiN3N]WϵN (42).
The IR spectra of WϵN and MoϵN in C6D6 are virtually
identical in the 1200–900 cm–1 region, with the exception
of an absorption at 1012 cm–1 in the MoϵN spectrum
(ν(Mo-N)) and at 1024 cm–1 in the WϵN spectrum. There-
fore, we assign the 1024 cm–1 absorption to the ν(W-N)
stretch. As in [Me3SiN3N]MϵN complexes, ν(M-N) is
slightly higher for WϵN than it is in MoϵN. The 15N NMR
spectrum of Wϵ15N shows a resonance for the nitride at
827 ppm (vs. 898 ppm in Moϵ15N).
′
Treatment of a C6D6 solution of [W(NH3)][BAr4] with
LiBHEt3 (1 mol/L in THF) results in a color change to
brown and formation of a largely one paramagnetic species,
which is tentatively assigned as the monohydride complex,
WH, on the basis of reactivity similar to that exhibited by
MoH (33). However, a small amount of a diamagnetic prod-
uct is present initially, which becomes the sole observable
species when the reaction mixture is exposed to an atmo-
sphere of H2 for ~10 min (eq. [8]); this diamagnetic species
(WH3) can be isolated in 42% yield. This compound was
identified on the basis of the similarity of its NMR and IR
spectra with those of the crystallographically characterized
© 2005 NRC Canada

Yandulov and Schrock
351
′
Fig. 9. Electrochemical behavior of [W(NH3)][BAr4] in
sults in a subtle color change, formation of a yellow precipi-
tate of [CoCp2][BAr4], complete consumption of the acid,
′
′
0.1 mol/L [Bu4N][BAr4]/PhF, recorded at a glassy carbon elec-
trode (3 mm diam.) at the scan rates shown in V/s. The potential
and conversion of approximately half of the starting WN2 to
+/0
′
scale is referenced to FeCp2
.
[W=NNH2][BAr4] in 3 h at 22 °C (eq. [9]). This mixture re-
mains unchanged over a period of 24 h. WN=NH is con-
′
verted quantitatively into [W=NNH2][BAr4] upon reaction
with 1 equiv. of [2,6-LutH][BAr4] in C6D6. Therefore, we
′
propose that WN=NH is formed in the reduction of WN2 in
the presence of [2,6-LutH][BAr4] and that WN=NH then
consumes proton relatively irreversibly to give
[W=NNH2][BAr4], thereby limiting consumption of WN2
when 1 equiv. of acid is present to ~50%. In contrast, the
analogous reaction between MoN2 and [2,6-LutH][BAr4]
and CoCp2 rapidly yields MoN=NH quantitatively (34).
[Mo=NNH2][BAr4] is considerably more acidic than
′
a
′
′
′
′
[W=NNH2][BAr4], as judged from the fact that treatment of
′
MoN=NH with 1 equiv. of [2,6-LutH][BAr4] in C6D6 gives
′
an equilibrium mixture of MoN=NH and [Mo=NNH2][BAr4]
(44:56); therefore, conversion of MoN2 to MoN=NH can be
complete when MoN2 is treated with only 1 equiv. of [2,6-
LutH][BAr4] and CoCp2. The most important point is that a
′
one-electron reduction–protonation of WN2 to yield
WN=NH is feasible, although this process appears to be
slower than conversion of MoN2 to MoN=NH under analo-
gous conditions.
complex, [Me3SiN3N]WH3 (57). Like [Me3SiN3N]WH3,
WH3 features a low-field resonance of intensity three in its
′
Isolated [W=NNH2][BAr4] reacts rapidly with 2 equiv. of
1
1H NMR spectrum in C6D6 at 11.99 ppm with J(183W-H) =
CoCp2 (E°′ = –2.01 V vs. FeCp2+/0 in PhF) in C6D6, as indi-
*
24 Hz (cf. 10.01 ppm and 23 Hz in [Me3SiN3N]WH3), in ad-
dition to characteristic ligand resonances. The IR spectrum
of WH3 in C6D6 shows two broad absorptions at 1958 and
1966 cm–1 consistent with ν(W-H) stretching modes (cf.
1903 and 1887 cm–1 in [Me3SiN3N]WH3 in Nujol). How-
ever, unlike [Me3SiN3N]WH3, WH3 can be induced to lose
H2, by exposure to a high vacuum at 70 °C to yield a mix-
ture that contains a considerable amount of WH after a few
hours. This difference in behavior can be attributed to less
electron donation by the [HIPTN3N]3– ligand compared to
the [Me3SiN3N]3– ligand. It should be noted that what is pro-
posed to be [HIPTN3N]MoH3 cannot be isolated since it
readily loses dihydrogen to form [HIPTN3N]MoH at room
temperature (34).
cated by the formation of
*
a yellow precipitate of
′
[CoCp2][BAr4]. However, instead of the expected one-
electron reduced (W(V)) product (W=NNH2), a proton
NMR spectrum of the reaction mixture recorded 20 min af-
*
′
ter mixing [W=NNH2][BAr4] and CoCp2 revealed that only
WN=NH and WϵN were present in a 52:48 ratio. After
17 h, the ratio of WN=NH to WϵN had not changed, but a
well-resolved triplet for free ammonia could be observed.
This ammonia was collected by vacuum transfer and shown
(using the indophenol analytical method) to amount to 0.43
′
equiv. of the original [W=NNH2][BAr4]. Therefore, one-
′
electron reduction of [W=NNH2][BAr4] predominantly re-
sults in the net reaction shown in eq. [10]. In fact, this reac-
15
15 15
′
tion was used to prepare Wϵ N from [W= N NH2][BAr4].
This stoichiometry is consistent with the disproportionation
of the initial reduction product, W=NNH2, a W(V) species,
into a W(IV) and a W(VI) species, WN=NH and WϵN, re-
spectively. A plausible sequence of elementary electron- and
Reduction of dinitrogen on W with protons and electrons
′
Treatment of WN2 with 1 equiv. of [2,6-LutH][BAr4]
(2,6-Lut = 2,6-lutidine) and 2 equiv. of CoCp2 in C6D6 re-
© 2005 NRC Canada

352
Can. J. Chem. Vol. 83, 2005
Scheme 1. A proposed sequence of reactions that yields the net
eq. [10] and (it is proposed) Scheme 1 (34). One of the rea-
sons to investigate the chemistry of [W=NNH2][BAr4] was
process shown in eq. [10]. Unobservable intermediates are shown
′
in boxes. The counterion (BAr4–) is excluded for clarity. PT =
′
the possibility that W=NNH2 would be more stable than
Mo=NNH2 and that W=NNH2, therefore, could at least be
observed. Instead, however, we find that W=NNH2 is even
less stable than Mo=NNH2. In each case, the high propen-
sity of M=NNH2 to disproportionate can be attributed to a
combination of a high Brønsted basicity of Nβ in combina-
tion with strong reducing properties. We attribute the strong
reducing properties to the presence of one of the five elec-
trons in the M-Nα-Nβ π system in a π orbital that has a node
at Nα; four electrons are in the two perpendicular lowest en-
ergy bonding molecular orbitals. This combination of prop-
erties makes M=NNH2 susceptible to both protonation by its
own oxidized form and oxidation by its own protonated
form, at least in a thermodynamic sense. If both steps are
kinetically feasible, then disproportionation proceeds readily,
perhaps in part because of the fact that the step in which am-
monia is formed (N—N bond cleavage) is almost certainly
irreversible. It is important to note that disproportionation of
M=NNH2 intermediates has no adverse consequences for
overall reduction of dinitrogen to ammonia, since it merely
leads to species that are themselves susceptible to further re-
actions that lead to the formation of ammonia. Dispropor-
tionations of d1 species and related intermolecular proton or
electron transfer reactions are likely to complicate the mech-
anism of ammonia formation depicted in Fig. 2, when all
species are in solution.
proton transfer, ET = electron transfer.
proton-transfer steps that comprises the overall process is
proposed in Scheme 1.
*
′
[10] [W=N-NH2]BAr 4 + 1.0CoCp2
C6 D6
⎯⎯⎯→ 0.5W-N=NH + 0.5W≡N
Attempted conversion of W(NH3)+ to WN2, and
catalytic reduction of N2
*
′
+ 0.5 NH3 + [CoCp2 ]BAr4
The results discussed in the previous section show that it
is possible to reduce WN2 to yield ammonia and WϵN in
the presence of acid and reductant. Since reduction of
MoϵN to Mo(NH3)+ is a relatively easy transformation in
the Mo dinitrogen reduction cycle (35), while reduction of
Mo(NH3)+ to Mo(NH3) and transformation of Mo(NH3) into
MoN2 can be observed (34), we examined the feasibility of
+
Initial one-electron reduction of W=NNH2 yields as yet
unobserved W=NNH2, possibly in a rapid equilibrium. A
small amount of W=NNH2 then protonates W=NNH2 to
+
+
yield the proposed hydrazidium(2–) species (W=NNH3 ),
+
while W=NNH2 is transformed into WN=NH. The
+
hydrazidium(2–) species (W=NNH3 ) then oxidizes
W=NNH2 to regenerate W=NNH2 , and in the process is
reduced to form WϵN and ammonia. Essentially, the dispro-
portionation of W=NNH2 is catalyzed by W=NNH2 .
It is possible that the disproportionation shown in
Scheme 1 requires some time to proceed to completion and
that the initial “W(V) = NNH2” exists in a fast electron-
transfer equilibrium with W=NNH2 and (or) CoCp2
which causes extensive broadening and (or) complete disap-
pearance of the resonances for W=NNH2 and CoCp2 in
the H NMR spectrum. A similar situation was encountered
in the reversible reduction of{[HTBTN3N]Mo(NH3)}[BAr4]
with CrCp2 in C6D6 (37). However, irreversibility of the
converting W(NH3)+ into WN2.
+
*
′
Reduction of [W(NH3)][BAr4] with 2 equiv. of CoCp2 in
+
C6D6 proceeds to some extent, as judged by the rapid forma-
*
′
tion of a yellow precipitate of [CoCp2][BAr4]. However, the
1H NMR spectrum of the resulting solution is unusually fea-
tureless, i.e., few resonances beyond isopropyl methyl reso-
nances and some aryl resonances can be observed. Since the
1H NMR spectrum of Mo(NH3) is similar to that of other d3
Mo complexes in this general family, we expected W(NH3)
to be observable and to have a very similar spectrum to that
of Mo(NH3).
+
+
*
,
+
+
*
1
′
*
′
The mixture of [W(NH3)][BAr4] and W(NH3) remained
′
electrochemical reduction of [W=NNH2][BAr4] suggests
unchanged for days and also did not change after being ex-
posed to 800 psi (1 psi = 6.894 757 kPa) of N2 for 48 h.
Thermolysis at 60 °C for a day resulted only in minor de-
composition and formation of WϵN and WH3. Treatment of
WN2 with ~4 equiv. of NH3 in C6D6 under argon also re-
sulted in no change after 15 days, again in sharp contrast to
the behavior in the analogous molybdenum system where an
equilibrium between Mo(NH3) into MoN2 is readily estab-
lished in a closed system over a period of several hours. Al-
though it has not been possible to transform W(NH3) into
WN2, W(NH3) appears to be relatively stable, at least as
that the sequence of reactions following the initial electron
transfer takes place rapidly at 22 °C. We are puzzled by the
apparently fast rate of net proton transfer in the absence of
conspicuous proton-transfer mediators. However, we note
that small amounts of the free ligand, H3[HIPTN3N], are
typically formed in a decomposition reaction, and could eas-
ily mediate the proton-transfer step. The actual mechanism
of the proton-transfer step remains to be clarified.
′
Reduction of [Mo=NNH2][BAr4] at room temperature
proceeds in a manner closely analogous to that shown in
© 2005 NRC Canada

Yandulov and Schrock
Table 3. Results of the attempted catalytic reduction of dinitrogen with WN2.a
353
d
Reductant
E°′(THF)b
E°′(PhF)c
Acid
[2,6-LutH][BAr4]
pKa (H2O)
Equiv. of NH3
*
′
CrCp2
–1.47
–1.84
–1.84
–1.63
–2.01
–2.01
6.68
6.68
10.67
1.31
1.51
0.62
*
′
[2,6-LutH][BAr4]
′
[Et3NH][BAr4]
CoCp2
*
CoCp2
aSingle runs of all experiments were carried out with 5.84 µmol of WN2, 36 equiv. of the reductant, and 48 equiv. of the acid,
under conditions otherwise identical to those previously described (35, 37).
+/0
bV vs. FeCp2+/0 in 0.4 mol/L [Bu4N][PF6].
cV vs. FeCp2 in 0.1 mol/L [Bu4N][BAr4′].
dThe maximum possible is 12 equiv. of ammonia (36 electrons available).
+
*
′
generated by reduction of [W(NH3)][BAr4] with CoCp2.
dinitrogen at various stages of reduction (WN=NH2 , WϵN,
and W(NH3)+). As expected, tungsten variants of known
molybdenum complexes are characterized by greater nucleo-
philicities (at Nβ) and lower reduction potentials. Interest-
ingly, however, the differences between Mo and W potentials
are considerably less pronounced for complexes that contain
However, this conclusion is difficult to reconcile with a lim-
ited degree of reversibility of the electrochemical reduction
′
′
of [W(NH3)][BAr4] ([Bu4N][BAr4]/PhF) (Fig. 9). There-
fore, the exact nature of the putative “W(NH3)” remains
obscure, and we cannot exclude the possibility that some re-
action involving oxidation of W(III) (e.g., intramolecular
NH addition) is facile. In any case, it seems likely that
W(NH3) is not strictly analogous to well-behaved Mo(NH3)
(34).
0/–
a dinitrogen ligand, e.g., 260 mV for MN2 and 0 mV for
MN2+/0. Structural and spectroscopic data (Table 2) signify
the more electron-rich nature of all other tungsten deriva-
tives without exception. We had hoped to observe and possi-
bly isolate W variants of several proposed Mo species that
are either exceedingly difficult to isolate, and in some cases
even observe in situ. However, many of the W complexes,
e.g., [WN2][Bu4N] and W(NH3), were in fact less stable
than the analogous Mo complexes. Also W=NNH2 dispro-
portionates into a mixture of WN=NH, WϵN, and ammonia
much faster than does Mo=NNH2. Although dinitrogen in
WN2 could be reduced to ammonia with acid and reductant
combinations similar to those used in the catalytic reduction
of dinitrogen with [HIPTN3N]Mo species, catalytic turnover
could not be achieved as a consequence of (inter alia) major
difficulties in converting W(NH3)+ into WN2. Complications
of this magnitude in a system where Mo analogs are inter-
mediates in the catalytic reduction of dinitrogen to ammonia
would appear to doom tungsten as a plausible site for dini-
trogen reduction. If MoFe and WFe nitrogenases are struc-
turally analogous, and if dinitrogen is reduced at Mo in the
MoFe nitrogenase, then one can imagine why reduction of
dinitrogen by the WFe nitrogenase might fail.
Treatment of the mixture containing partly reduced
W(NH3)+ in C6D6 with [Et3NH][BAr4] produced a new
′
paramagnetic species that gave rise to considerable quanti-
ties of both W(NH3)+ and WH3 after several days, but no
WN2 or products of its reduction. The net reaction of
W(NH3)+ with a reductant and an acid thus results primarily
in reduction of protons (presumably) and formation (ulti-
mately) of WH3 (eq. [11]), but dinitrogen is not bound or re-
duced.
*
′
′
[11] [W(NH3)][BAr4] + 4CoCp2 + 3[Et3NH][BAr4]
*
′
⎯⎯→ WH3 + NH3 + 4[CoCp2][BAr4] + 3Et3N
In view of the previous observations, it is perhaps not sur-
prising that attempted catalytic reduction of dinitrogen using
WN2 as the catalyst under conditions identical or similar to
those employed for catalytic reduction of dinitrogen by Mo
complexes (35) failed (Table 3). This can be attributed
chiefly to the inability to convert W(NH3)+ into WN2 to any
significant extent on the timescale of a typical catalytic ex-
periment (6 h), as was independently established in the ex-
periments previously described. Ammonia is formed, but
less than 2 equiv., i.e., at most 1.51 equiv. with CoCp2 as the
reductant and [2,6-LutH][BAr4] as the acid. Therefore,
while it is possible to cleave the N—N bond and form the
first equivalent of ammonia, and to form some WNHxy+ spe-
cies that yield additional ammonia upon workup, dinitrogen
is not bound and reduced further under the conditions em-
ployed.
Experimental section
*
General
All air- and moisture-sensitive compounds were manipu-
lated using standard Schlenk and glovebox techniques under
an atmosphere of nitrogen in flame- and oven-dried glass-
ware (including NMR tubes). Ether, pentane, and toluene
were purged with nitrogen, passed through activated alumina
columns (58), and freeze–pump–thaw degassed three times.
THF, DME, 1,4-dioxane, and benzene were distilled from
dark purple Na–benzophenone ketyl solutions. Dichloro-
methane and acetonitrile were distilled from CaH2, while
fluorobenzene was distilled from P2O5 under N2. All dried
and deoxygenated solvents were stored over molecular
sieves in a nitrogen-filled glovebox. Small quantities of hy-
drocarbon and ether solvents, as well as all such deuterated
NMR solvents, were purified a second time by vacuum
transfer from dark-purple solutions and (or) suspensions of
Na–benzophenone ketyl (or CaH2 for CH2Cl2, PhF, and
′
Conclusions
We have been able to synthesize a number of [HIPTN3N]W
complexes analogous to the [HIPTN3N]Mo species that are
proposed intermediates in the catalytic reduction of
dinitrogen to ammonia (35), among them the first terminal
dinitrogen complex of W(III) (WN2), the second structurally
characterized example of a parent diazenide of any metal
(WN=NH), and three other tungsten derivatives containing
© 2005 NRC Canada

354
Can. J. Chem. Vol. 83, 2005
CD2Cl2), degassed, and stored in gas-tight solvent bulbs inside
a glovebox. Molecular sieves (3 Å) and Celite® were acti-
vated in vacuo at 230 °C for several days. Cobaltocene (sub-
the residue with pentane (60 mL) afforded a dark orange
microcrystalline solid, which was collected by filtration on a
glass frit after standing the mixture at –35 °C overnight,
washed with cold pentane, and dried in vacuo at 85 °C for
*
*
(sublimed), [CoCp2][PF6] (Strem),
limed), CrCp2
1
(Me3Si)2NLi (sublimed), anhydr. ZnCl2, LiBHEt3 (1 mol/L
THF), and NaN3 (Aldrich) were used as received, unless in-
dicated otherwise. WCl4(DME) (59), H3[HIPTN3N] (33),
several hours. Yield: 11.28 g (6.25 mmol, 67%). H NMR
(C6D6, 20 °C) δ: 18.81 (s, 6H, 4′,6′-H), 7.49 (s, 12H,
3,5,3′′,5′′-H), 3.47 (br s, 12H, 2,6,2′′,6′′-CHMe2), 3.18 (sept,
JHH = 6.6 Hz, 6H, 4,4′′-CHMe2), 2.98 (s, 3H, 2′-H), 1.58 (d,
JHH = 6.6 Hz, 36H, 4,4′′-CH(CH3)2), 1.34 (s, 36H,
2,6,2′′,6′′-CH(CH3)2), 0.83 (br s, 36H, 2,6,2′′,6′′-CH(CH3)2),
–27.06 (s, 6H, NCH2), –50.1 (br s, 6H, NCH2). Anal. calcd.
for C114H159ClN4W (%): C 75.87, H 8.88, N 3.10, Cl 1.96;
found: C 75.97, H 8.78, N 2.97, Cl 2.10.
′
′
′
NaBAr4 (60), [H(OEt2)2][BAr4] (60), and [2,6-LutH][BAr4]
(35) were prepared according to the published procedures or
slight modifications thereof. Potassium graphite (KC8) was
prepared by stirring potassium metal, freshly cut in a nitro-
gen-filled glovebox, and graphite in 1:8 stoichiometry with a
glass-coated stirbar under 700 torr (1 torr = 133.322 Pa) of
*
Ar at 140 °C for an hour. CoCp2 was prepared from
*
[CoCp2][PF6] and a slight excess of KC8 in THF over 4 h
[HIPTN3N]WN=NK
′
and sublimed. [Et3NH][BAr4] was prepared from Et3NHCl
A mixture of WCl (500 mg, 0.277 mmol) and KC8
(93.6 mg, 0.692 mmol) was stirred in benzene (6 mL) under
1 atm of N2 (1 atm = 101.325 kPa) with a glass-coated mag-
netic stirbar for 42 h. The resulting dark maroon-red mixture
was taken to dryness in vacuo and the solid residue was ex-
tracted with pentane. The pentane extracts were filtered
through Celite® and concentrated to 5 mL; a black-red crys-
talline solid formed within in minutes. The mixture was kept
at –35 °C for several days and the product was then col-
lected on a frit, washed with cold pentane, and dried in
vacuo at 35 °C for 2 h. Yield: 310 mg (0.169 mmol, 61%).
′
and 1.05 equiv. of NaBAr4 in CH2Cl2. All metal complexes
were stored in the dark, under N2 at –35 °C. 1H, 19F, and 15
N
NMR spectra were recorded on a Varian Mercury 300 (1H
300 MHz, 19F 282 MHz), Varian Unity 300 (1H 300 MHz)
or a Varian Inova 500 (1H 500 MHz, 15N 50.7 MHz) spec-
trometers and referenced to the residual protio solvent peaks
(1H) or external neat PhF (19F, –113.15 ppm relative to
CFCl3) and neat MeCN (15N, +245.5 ppm relative to neat
NH3 at 303 K (61)). Attempted catalytic dinitrogen reduc-
tion runs and analyses of ammonia by the indophenol method
(62–65) utilized procedures, equipment, and reagents de-
scribed previously (35, 37). IR spectra were recorded on a
Nicolet Avatar 360 FT-IR spectrometer in 0.2 mm KBr solu-
tion cells; frequencies are given in cm–1. Elemental analyses
were performed by H. Kolbe Mikroanalytisches Laborator-
ium, Mülheim an der Ruhr, Germany.
1
IR (C6D6): 1745 (νN-N). IR (pentane): 1747 (νN-N). H NMR
(C6D6, 20 °C) δ: 7.54 (br s, 6H, 4′,6′-H), 7.16 (s, 12H,
3,5,3′′,5′′-H), 6.61 (s, 3H, 2′-H), 3.75 (br s, 6H, NCH2),
3.42 (sept, JHH = 6.8 Hz, 12H, 2,6,2′′,6′′-CHMe2), 2.84 (br
sept, JHH = 6 Hz, 6H, 4,4′′-CHMe2), 1.93 (br s, 6H, NCH2),
1.26 (d, JHH = 6.5 Hz, 36H, 4,4′′-CH(CH3)2), 1.24 (d, JHH
=
6.0 Hz, 36H, 2,6,2′′,6′′-CH(CH3)2), 1.2 (br sh, 36H,
2,6,2′′,6′′-CH(CH3)2). Anal. calcd. for C114H159KN6W (%):
C 74.56, H 8.73, N 4.58; found: C 74.43, H 8.59, N 4.45.
X-ray quality crystals were obtained from a supersatu-
rated heptane solution at room temperature.
Electrochemistry
Electrochemical measurements were carried out using a
′
BAS CV-50W potentiostat, 0.1 mol/L [Bu4N][BAr4] in PhF
electrolyte, and a standard three-electrode cell assembly with
glassy carbon (3.0 mm diam.) disk working electrode, plati-
num wire auxiliary electrode, and a AgCl-coated silver wire
reference electrode. All measurements were referenced inter-
nally with FeCp2, CoCp2, or MoN2 (35), as appropriate.
Compensation of internal solution resistance was used
to lower the peak separation of reversible couples to 100–
200 mV using the standard routines available with the BAS
CV-50W potentiostat and accompanying software. An expla-
nation of the technique can be found on the web at
http://www.epsilon-web.net/Ec/manual/Setup/ir_comp.html.
[HIPTN3N]W15N=15NK
A mixture of WCl (500 mg, 0.277 mmol) and KC8
(93.6 mg, 0.692 mmol) in benzene (6 mL), prepared under
regular N2 in a 25 mL flask fitted with a Teflon® valve, was
freeze–pump–thaw degassed five times, pressurized to 1 atm
(1 atm = 101.325 kPa) with 15N2 purified with solid Na–
Ph2CO and stirred with a glass-coated magnetic stirbar for
50 h. A work-up procedure analogous to that used in the
preparation of the unlabeled complex yielded 285 mg
(0.155 mmol, 56%) of a black-red solid, identified by spec-
troscopic data as the labeled compound enriched with 15N at
the Nα and Nβ diazenido sites to the extent of only 92% as a
consequence of some initial reaction under 14N2 to yield
[HIPTN3N]W14N=14NK. IR (C6D6): 1745 (ν14N–14N),
1688 (ν15N–15N). 15N NMR (C6D6, 20 °C) δ: 356.6 (d,
J(15Nα-15Nβ) = 11.2 Hz, 15Nα), 349.5 (d, J(15Nα-15Nβ) =
11.2 Hz, 15Nβ).
[HIPTN3N]WCl
A mixture of WCl4(DME) (3.897 g, 9.37 mmol) and
H3[HIPTN3N] (14.93 g, 9.40 mmol) was stirred vigorously
in benzene (100 mL) for 2 h to give a dark red-brown homo-
geneous solution. Solid (Me3Si)2NLi (4.868 g, 29.1 mmol)
was added in portions over a period of 10 min. The solution
was stirred for an additional 2 h and taken to dryness in
vacuo. The residue was dried at 70 °C overnight and ex-
tracted with benzene. The extracts were filtered through
Celite®, and the filtrates were concentrated and centrifuged
in portions to remove residual LiCl. The resulting homoge-
neous solution was then taken to dryness in vacuo and the
residue was heated at 85 °C for several hours. Trituration of
[HIPTN3N]WN2
A mixture of WCl (500 mg, 0.277 mmol) and KC8
(93.6 mg, 0.692 mmol) was stirred in benzene (6 mL) under
1 atm of N2 (1 atm = 101.325 kPa) with a glass-coated mag-
© 2005 NRC Canada

Yandulov and Schrock
355
[HIPTN3N]W15N=15NH
netic stirbar for 42 h. The resulting mixture was filtered
through Celite® and brought to dryness in vacuo. Solid
ZnCl2 (30.2 mg, 0.222 mmol) and THF (6 mL) were added
to the residue. After stirring the solution for a few minutes, a
dark orange-red solution and a precipitate of Zn metal
formed. The mixture was taken to dryness in vacuo and
heated to 75 °C for 2 h. The solid residue was extracted with
pentane and the pentane extracts were filtered through
Celite®. The extracts were taken to dryness in vacuo. Addi-
tion of Me4Si (7 mL) to the solid residue led to the forma-
tion of a microcrystalline orange solid after 1 h. The mixture
was kept at –35 °C for several days and the solid product
was collected on a frit, washed with cold Me4Si, and dried
in vacuo at 75 °C for 2 h. Yield: 265 mg (0.147 mmol,
W15N=15NK (40 mg) was treated with [Et3NH][OTf]
(1.05 equiv.) in C6D6 in an NMR tube to yield the title com-
pound, 92% 15N-enriched, essentially quantitatively. The so-
lution was taken to dryness in vacuo and the residue was
dissolved in C6D6 for spectroscopic analysis. IR (C6D6):
1
1499 (ν15 Ν−15 ΝΗ). H NMR (C6D6, 20 °C) δ: 8.42 (s, 0.08H,
1
2
14Nα=14NβH), 8.41 (dd, J(15Nβ-H) = 53.1 Hz, J(15Nα-H) =
6.7 Hz, J(183W-H) = 17.1 Hz, 0.92H, 15Nα=15NβH) (other
resonances are identical to those measured for the unlabeled
sample). 15N NMR (C6D6, 20 °C) δ: 393.5 (dd, J(15Nα-
2
15Nβ) = 15.8 Hz, J(15Nα-H) = 6.5 Hz, 15Nα=15NβH), 229.3
(dd, 1J(15Nβ-H) = 53.9 Hz, J(15Nα-15Nβ) = 14.9 Hz,
15Nα=15NβH).
1
53%). IR (C6D6): 1888 (νN-N). H NMR (C6D6, 20 °C) δ:
35.2 (br s, 6H, NCH2), 6.79 (s, 12H, 3,5,3′′,5′′-H), 2.83 (s,
6H, 4,4′′-CHMe2), 2.58 (br s, 36H, 2,6,2′′,6′′-CH(CH3)2),
1.29 (s, 36H, 4,4′′-CH(CH3)2), 0.66 (s, 36H, 2,6,2′′,6′′-
CH(CH3)2), –0.25 (s, 2H, 2′-H), –0.95 (s, 12H, 2,6,2′′,6′′-
CHMe2), –11.1 (br s, 6H, 4′,6′-H), –22.5 (br s, 6H, NCH2).
Anal. calcd. for C114H159N6W (%): C 76.18, H 8.92, N 4.68;
found: C 75.93, H 9.08, N 4.57.
′
{[HIPTN3N]W=NNH2}[BAr4 ]
An ether (4 mL) solution of WN=NH (100 mg,
55.6 µmol) was stirred and treated with solid
′
[H(OEt2)2][BAr4] (59.1 mg, 58.4 µmol) at –35 °C. The solu-
tion was stirred for 1 h while being allowed to warm up to
room temperature, and then taken to dryness in vacuo. The
resulting solid was extracted with pentane and the pentane
extracts were filtered through Celite® and concentrated to
~3 mL. Seeding this solution with a minute amount of
X-ray quality crystals were obtained from a supersatu-
rated heptane solution at room temperature.
′
{[HIPTN3N]Mo=NNH2}[BAr4] (33) led to a gradual forma-
tion of pale yellow microcrystals over a period of ~1 h. The
mixture stood at –35 °C for several days and the product
was isolated by filtration, washed with cold pentane, and
dried in vacuo at 60 °C. Yield: 106 mg (39.8 µmol, 72%).
1H NMR (C6D6, 20 °C) δ: 8.36 (br s, 8H, C6H3-3,5-(CF3)2-
2,6-H), 7.66 (br s, 4H, C6H3-3,5-(CF3)2-4-H), 7.13 (s, 12H,
3,5,3′′,5′′-H), 6.79 (br d, 6H, 4′,6′-H), 6.62 (br t, 3H, 2′-H),
5.96 (s, 2H, J(183W-H) ~ 6 Hz, N-NH2), 3.78 (br t, 6H,
NCH2), 2.88 (sept, JHH = 6.9 Hz, 6H, 4,4′′-CHMe2), 2.67
(sept, JHH = 6.8 Hz, 12H, 2,6,2′′,6′′-CHMe2), 2.32 (br t, 6H,
NCH2), 1.33 (d, JHH = 6.6 Hz, 36H, 4,4′′-CH(CH3)2), 1.11
[HIPTN3N]W(15N2)
A mixture of W15N=15NK (10 mg) and ZnCl2 (0.6 mg,
0.8 equiv.) in THF (0.6 mL) was stirred for 10 min. The
mixture was filtered through Celite® and the filtrate was
taken to dryness in vacuo. The solid residue was dissolved in
C6D6 for spectroscopic analysis. IR (C6D6): 1888 (ν14N–14N),
1826 (ν15N–15N).
[HIPTN3N]WN=NH
A mixture of WCl (500 mg, 0.277 mmol) and KC8
(93.6 mg, 0.692 mmol) was stirred in benzene (6 mL) with a
glass-coated magnetic stirbar for 42 h under 1 atm of N2
(1 atm = 101.325 kPa). The resulting dark maroon-red mix-
ture was treated with solid [Et3NH][OTf] (69.6 mg,
0.277 mmol), stirred for 30 min, and taken to dryness in
vacuo. The solid residue was extracted with pentane and the
pentane extracts were filtered through Celite®. Upon concen-
tration of this solution to ~5 mL, the product precipitated as
a cotton-like solid. Repeated warming and cooling of the
mixture to –35 °C induced crystallization of the product in
the form of a microcrystalline solid sufficiently dense to al-
low isolation by filtration. The resulting beige solid was
washed with cold pentane and dried in vacuo at 60 °C.
Yield: 273 mg (0.152 mmol, 55%). IR (C6D6): 1542 (νN-NH).
1H NMR (C6D6, 20 °C) δ: 8.43 (s, 1H, J(183W-H) = 17.4 Hz,
N=NH), 7.30 (br d, 6H, 4′,6′-H), 7.20 (s, 12H, 3,5,3′′,5′′-H),
6.64 (br t, 3H, 2′-H), 3.68 (br t, 6H, NCH2), 3.14 (sept,
(d, JHH = 6.8 Hz, 36H, 2,6,2′′,6′′-CH(CH3)2), 0.98 (d, JHH
=
6.8 Hz, 36H, 2,6,2′′,6′′-CH(CH3)2). 19F NMR (C6D6, 20 °C)
δ: –62.4 (s, CF3). Anal. calcd. for C146H173BF24N6W (%): C
65.86, H 6.55, N 3.16; found: C 65.74, H 6.46, N 3.08.
X-ray quality crystals were obtained from a supersatu-
rated heptane solution at room temperature.
15 15
′
{[HIPTN3N]W= N NH2}[BAr4 ]
W15N=15NH, generated in situ as described above, was
′
treated with 1 equiv. of [H(OEt2)2][BAr4] in C6D6 in an
NMR tube to yield the title compound, 92% 15N-enriched,
1
essentially quantitatively. H NMR (C6D6, 20 °C) δ: 5.99 (s,
0.08 H, 14Nα-14NβH2), 5.99 (dd, 1J(15Nβ-H) = 83.6 Hz,
2J(15Nα-H) = 1.8 Hz, 0.92 H, 15Nα-15NβH2) (other reso-
nances are identical to those measured for the unlabeled
sample). 15N NMR (C6D6, 20 °C) δ: 343.8 (d, J(15Nα-15Nβ) =
10.2 Hz, 1N, 15Nα-15NβH2), 131.1 (td, J(15Nβ-H) = 83.8 Hz,
1
J(15Nα-15Nβ) = 9.9 Hz, 1N, 15Nα-15NβH2).
JHH = 6.6 Hz, 12H, 2,6,2′′,6′′-CHMe2), 2.93 (sept, JHH
=
6.9 Hz, 6H, 4,4′′-CHMe2), 1.88 (br t, 6H, NCH2), 1.37 (d,
JHH = 6.6 Hz, 36H, 4,4′′-CH(CH3)2), 1.24 (d, JHH = 6.6 Hz,
36H, 2,6,2′′,6′′-CH(CH3)2), 1.14 (d, JHH = 6.6 Hz, 36H,
2,6,2′′,6′′-CH(CH3)2). Anal. calcd. for C114H160N6W (%): C
76.14, H 8.97, N 4.67; found: C 76.00, H 8.96, N 4.55.
X-ray quality crystals were obtained from a supersatu-
rated heptane solution at room temperature.
[HIPTN3N]WϵN
A mixture of WCl (500 mg, 0.277 mmol) and NaN3
(54.0 mg, 0.831 mmol) in a mixture of benzene and
acetonitrile (4:6 mL) was stirred vigorously for 16 h. The
mixture became lighter in color and was taken to dryness in
vacuo. The solid residue was dried at 80 °C in vacuo and ex-
© 2005 NRC Canada

356
Can. J. Chem. Vol. 83, 2005
tracted with benzene. The benzene extracts were filtered
through Celite® and the filtrates were taken to dryness in
vacuo. Addition of pentane (3 mL) led to the precipitation of
a pale beige solid after 1 h at room temperature. The mixture
was allowed to stand at –35 °C for several days, and the
product was then isolated by filtration, washed with cold
pentane, and dried at 70 °C in vacuo. Yield: 286 mg
(0.160 mmol, 58%). IR (C6D6): 1024 (νMo-N). 1H NMR
(C6D6, 20 °C) δ: 8.01 (br s, 6H, 4′,6′-H), 7.21 (s, 12H,
3,5,3′′,5′′-H), 6.59 (s, 3H, 2′-H), 3.66 (br t, 6H, NCH2), 3.10
(sept, JHH = 6.6 Hz, 12H, 2,6,2′′,6′′-CHMe2), 2.93 (sept,
JHH = 6.8 Hz, 6H, 4,4′′-CHMe2), 1.66 (br t, 6H, NCH2),
brown-red mixture was stirred for 30 min, freeze–pump–
thaw degassed, and exposed to 1 atm of H2 (1 atm =
101.325 kPa), which caused some lightening in color. The
mixture was stirred for 1 h and brought to dryness in vacuo.
The resulting solid was dried at 80 °C and extracted with
benzene. The benzene extracts were filtered through Celite®
and the filtrate was taken to dryness. Addition of pentane
(4 mL) to the residue led to a gradual formation of a light
brown solid. The mixture was left standing at –35 °C for
several days and the product was then isolated by filtration,
washed with cold pentane, and dried in vacuo at 70 °C. As
was explained in the text, WH3 loses hydrogen in the solid
state at elevated temperatures under vacuum, forming an ob-
servable quantity of a paramagnetic compound tentatively
assigned as the monohydride WH. Thus, solid WH3 isolated
as described previously was stored under an atmosphere of
H2 overnight, which nearly eliminated the WH impurity.
Yield: 208 mg (0.117 mmol, 44%). IR (C6D6): 1966, 1958
1.37 (d, JHH = 6.9 Hz, 36H, 4,4′′-CH(CH3)2), 1.24 (d, JHH
=
6.6 Hz, 36H, 2,6,2′′,6′′-CH(CH3)2), 1.10 (br d, JHH = 6.3 Hz,
36H, 2,6,2′′,6′′-CH(CH3)2). Anal. calcd. for C114H159N5W
(%): C 76.78, H 8.99, N 3.93; found: C 76.57, H 9.01, N
3.94.
1
[HIPTN3N]Wϵ15N
(both broad, νW-H). H NMR (C6D6, 20 °C) δ: 11.99 (s, 3H,
J(183W-H) = 24 Hz, W(H)3), 7.66 (s, 6H, 4′,6′-H), 7.21 (s,
12H, 3,5,3′′,5′′-H), 6.54 (s, 3H, 2′-H), 3.65 (s, 6H, NCH2),
3.10 (sept, JHH = 6.8 Hz, 12H, 2,6,2′′,6′′-CHMe2), 2.93
(sept, JHH = 6.9 Hz, 6H, 4,4′′-CHMe2), 2.01 (s, 6H, NCH2),
15 15
′
[W= N NH2][BAr4], generated in situ as described
*
above, was treated with 2 equiv. of CoCp2 in C6D6 in an
NMR tube to yield a mixture consisting predominantly of
W15N=15NH and the title compound. 15N NMR (C6D6,
20 °C) δ: 827.0 (s, Wϵ15N), 393.5 (dd, W15Nα=15NβH), 229.3
(dd, W15Nα=15NβH).
1.37 (d, JHH = 6.9 Hz, 36H, 4,4′′-CH(CH3)2), 1.24 (d, JHH
=
6.9 Hz, 36H, 2,6,2′′,6′′-CH(CH3)2), 1.12 (d, JHH = 6.6 Hz,
36H, 2,6,2′′,6′′-CH(CH3)2). Anal. calcd. for C114H162N4W
(%): C 77.25, H 9.21, N 3.16; found: C 77.08, H 9.28, N
3.06.
′
{[HIPTN3N]W(NH3)}[BAr4 ]
Ammonia (57 mL, 360 torr (1 torr = 133.322 Pa),
~1.11 mmol) was vacuum transferred from a bronze-colored
Na solution to a frozen mixture of WCl (500 mg,
Acknowledgements
′
0.277 mmol) and NaBAr4 (540.2 mg, 0.610 mmol) in
fluorobenzene (10 mL). The mixture was thawed and stirred
at room temperature for 2 h, at which time 1H NMR analysis
of an aliquot taken from the reaction mixture showed com-
plete consumption of WCl. The mixture was taken to dry-
ness in vacuo, and the solid residue was dried at 60 °C and
extracted with pentane. The pentane extracts were filtered
through Celite® and the filtrate was concentrated to 3 mL.
This solution was seeded with a minute quantity of crystal-
R.R.S. is grateful to the National Institutes of Health (GM
31978) for research support and both R.R.S. and D.V.Y. are
grateful to William E. Geiger for advice on electrochemistry
in weakly polar media.
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