Synthesis and reactivity of molybdenum and tungsten bis(dinitrogen) complexes supported by diphosphine chelates containing pendant amines

Article abstract of DOI:10.1039/c2dt12224c

Molybdenum and tungsten bis(dinitrogen) complexes of the formula M(N ₂)₂(PNP)₂ (M = Mo and W) and W(N ₂)₂(dppe)(PNP), supported by diphosphine ligands containing a pendant amine of the formula (CH₂PR₂) ₂NR′ = P^RN^R′P^R (R = Et, Ph; R′ = Me, Bn), have been prepared by Mg reduction of metal halides under an N₂ atmosphere. The complexes have been characterized by NMR and IR spectroscopy, X-ray crystallography, and cyclic voltammetry. Reactivity of the target Mo and W bis(dinitrogen) compounds with CO results in the formation of dicarbonyl complexes.

Full text of DOI:10.1039/c2dt12224c

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PAPER
Synthesis and reactivity of molybdenum and tungsten bis(dinitrogen)
complexes supported by diphosphine chelates containing pendant amines†
Charles J. Weiss,a Amy N. Groves,a Michael T. Mock,*a William G. Dougherty,b W. Scott Kassel,b
Monte L. Helm,‡a,c Daniel L. DuBoisa and R. Morris Bullock*a
Received 21st November 2011, Accepted 11th January 2012
DOI: 10.1039/c2dt12224c
Molybdenum and tungsten bis(dinitrogen) complexes of the formula M(N2)2(PNP)2 (M = Mo and W)
and W(N2)2(dppe)(PNP), supported by diphosphine ligands containing a pendant amine of the formula
(CH2PR2)2NR′ = PRNR′PR (R = Et, Ph; R′ = Me, Bn), have been prepared by Mg reduction of metal
halides under an N2 atmosphere. The complexes have been characterized by NMR and IR spectroscopy,
X-ray crystallography, and cyclic voltammetry. Reactivity of the target Mo and W bis(dinitrogen)
compounds with CO results in the formation of dicarbonyl complexes.
ligands, produced a stoichiometric quantity of ammonia. Since
Introduction
this pioneering work, only two examples of catalytic production
of ammonia have been reported. Schrock5b–e demonstrated NH3
The Haber–Bosch process is the industrial method for pro-
duction of ammonia using hydrogen and nitrogen feedstocks.1
could be produced at a single metal center by a Mo(III) catalyst
Nitrogenase enzymes convert N2 to NH3 under ambient con-
ditions.2 In contrast, the energy-intensive industrial synthesis of
NH3, which includes the production of H2 (by steam reforming
of methane) and the reduction of N2 at high temperatures and
pressures, is estimated to account for approximately 1% of the
global human energy consumption.1b,3 The coordination of di-
nitrogen to a transition-metal center has been known for almost
50 years.4 This discovery led to the development of coordination
complexes and homogeneous catalysts that can facilitate the
reduction of dinitrogen to ammonia under mild temperatures and
pressures.5
supported by a sterically demanding triamidoamine ligand,
[HIPTN3N]3− = {3,5-(2,4,6-iPr3C6H2)2C6H3NCH2CH2]3N}3−
Recently, a catalytic system was reported by the group of Nishi-
.
bayashi using a Mo(0) catalyst [Mo(L)(N2)2]2(μ-N2), (L = 2,6-
bis(di-tert-butyl-phosphinomethyl)pyridine) supported by
a
phosphine-based pincer ligand.5a These Mo complexes reduce
N2 catalytically upon addition of protons and electrons. The
protons come from the weak acids, [lutidinium][B(3,5-
(CF3)2C6H3)4 and 2,6-lutidinium triflate, while the electrons are
from a chemical reductant, decamethyl chromocene and cobalto-
cene, respectively.
Brønsted acid-mediated reduction of dinitrogen to ammonia at
a single transition-metal center has been pursued as a more
energy efficient route for ammonia production. The advantages
are both in the milder temperatures and pressures required for N2
reduction and bypassing the energy-intensive H2 production by
using a combination of protons and electrons. The pioneering
research by Chatt, Hidai and coworkers6 demonstrated that
addition of strong acids to molybdenum and tungsten dinitrogen
complexes, supported by monodentate or chelating phosphine
Our group is interested in the preparation of molecular electro-
catalysts that can be used to perform multi-proton, multi-electron
transformations.7 In our efforts, we implement proton relays,
functional groups that are intended to facilitate the delivery of
protons, into the second coordination sphere of the catalysts. For
example, nickel complexes containing amine proton relays in the
backbone of chelating diphosphine ligands have been shown to
produce highly active electrocatalysts for the reduction of
protons to H2.8 Our interest in designing electrocatalysts for
9
10
applications such as reduction of O2 and N2 builds upon this
concept.
aCenter for Molecular Electrocatalysis, Pacific Northwest National
Laboratory, P.O. Box 999, Richland, Washington 99352, USA.
E-mail: michael.mock@pnnl.gov, morris.bullock@pnnl.gov
bDepartment of Chemistry, Villanova University, Villanova,
Pennsylvania 19085, USA
The achievement of catalytic reduction of N2 of metal-bound
dinitrogen with the addition of protons and electrons described
above, inspired us to investigate if “Chatt-type” dinitrogen com-
plexes with group 6 metals, (M(N2)2(P–P)2, P–P = diphosphine
ligand, M = Mo and W), may benefit from adding proton relays,
where incorporation of a pendant amine in the phosphine ligand
backbone could assist with the delivery of protons to a metal-
bound dinitrogen ligand. In a related report, Tuczek et al.11
explored the coordination chemistry of Mo and W complexes
bearing diphosphine ligands containing proton relays. The
cDepartment of Chemistry, Fort Lewis College, Durango, Colorado
81301, USA
†Electronic supplementary information (ESI) available: X-ray crystallo-
graphic data and experimentals, NMR and IR electrochemical data.
CCDC reference numbers 854188–854202 and 863033. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c2dt12224c
‡Sabbatical visitor at PNNL, 2010–2011.
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addition of 3 equiv. HBF4·OEt2 to one such complex, trans-[Mo
(N2)2(dppe)(pyNP2)], (dppe 1,2-bis(diphenylphosphino)
for crystallographic metrics). Similar Mo dimers have been
reported by Walton18 and Cotton,19 such as [MoCl2(dppe)]2,19a
through the reaction of molybdenum halide salts (e.g.,
K4Mo2Cl8) and monodentate or chelating phosphines. Treatment
of [MoBr2(PEtNBnPEt)]2 with Mg powder does not yield a di-
nitrogen complex, suggesting it is not an intermediate in the
reductive synthesis of the Mo(N2) complexes (vide infra). In
contrast to the synthesis of 1 and 2, the preparation of dibromide
complexes containing phenylphosphine-substituted ligands,
MoBr2(PPhNMePPh)2 (3) and MoBr2(PPhNBnPPh)2 (4), proceeds
smoothly with yields of 78–80%. All MoBr2(PNP)2 complexes
are insoluble in ether or hexanes, and are sparingly soluble in
dichloromethane and THF.
=
ethane; pyNP2 = N,N-bis(diphenylphosphinomethyl)-2-amino-
pyridine) resulted in the isolation of trans-[MoF(NNH2)(dppe)
(pyN(H)P2)][BF4]2, where protonation occurred at the pyridine
nitrogen of the proton relay and at the dinitrogen ligand.12
Herein we report the synthesis and characterization of molyb-
denum bis(dinitrogen) complexes supported by two PNP diphos-
phine ligands, Mo(N2)2(PNP)2; PNP = [(R2PCH2)2N(R′), R =
Et, Ph; R′ = Me, Bn], and a series of tungsten bis(dinitrogen)
complexes containing one or two PNP ligands, W(N2)2(dppe)
(PNP) and W(N2)2(PNP)2, respectively, and the analogues
without pendant amines for comparison. These Mo– and W–N2
complexes, which contain flexible11,13 pendant amines in the
second coordination sphere, were characterized by NMR and IR
spectroscopy, cyclic voltammetry and structural characterization
by X-ray crystallography. In addition, reactivity of the complexes
with CO is described.
Synthesis and characterization of Mo(N2)2(PNP)2 complexes
To prepare the molybdenum bis(dinitrogen) complexes using the
direct reduction route, MoBr3(THF)3 is stirred with magnesium
powder in THF (Scheme 1) in the presence of two equivalents of
PNP ligand. Contrasting isomeric products are observed depend-
ing on the identity of the substituents on phosphorus. Reactions
monitored by 31P{1H} NMR and IR spectroscopy indicate that
when PEtNBnPEt ligand is used, the product cis-[Mo
(N2)2(PEtNBnPEt)2] (cis-5) is formed after 2.5 h with only a
minor amount of the trans isomer. The cis isomer exhibits
pseudo triplets of an AA′BB′ spin system in the 31P{1H} NMR
spectrum at 15.9 and 12.5 ppm and has νNN stretching bands at
1945 and 2006 in the IR. Similar results are observed with
PEtNMePEt to generate cis-[Mo(N2)2(PEtNMePEt)2] (cis-6) (see
Table 2 for spectral parameters). The cis isomers can be isolated
by concentration and cooling to −35 °C overnight, but are found
to decompose over a period of 10 days in the solid state at room
temperature. Upon stirring in THF for 3 days, the cis products
are >85% converted to the trans isomers. Trans-[Mo
(N2)2(PEtNBnPEt)2] (trans-5) and trans-[Mo(N2)2(PEtNMePEt)2]
(trans-6) isolated as diamagnetic yellow solids in 64–71% yield.
The 31P{1H} NMR spectrum exhibits singlet resonances at 12.4
and 12.0 ppm, respectively, as expected for a structure contain-
ing four equivalent phosphorus atoms, while the IR spectrum
contains an intense νNN band at 1937 cm−1 for both compounds.
In contrast to the direct reduction synthesis described above,
when PPhNMePPh and PPhNBnPPh ligands are used, better yields
are obtained through a stepwise approach. The synthons 3 and 4
are reduced with Mg powder in THF, affording trans-[Mo
(N2)2(PPhNMePPh)2] (7-Mo) and trans-[Mo(N2)2(PPhNBnPPh)2]
(8) as yellow solids in 15–40% yield. Using this synthetic route,
the cis isomers are not observed by 31P{1H} NMR, rather, 3 and
4 are reduced directly to the trans bis(dinitrogen) complexes.
7-Mo and 8 exhibit singlets in the 31P{1H} NMR spectrum at
25.3 and 27.9 ppm and νNN bands at 1955 and 1954 cm−1 in the
IR, respectively.
Results and discussion
Entry into PNP-supported molybdenum chemistry is sensitive to
the nature of the Mo starting material. Efforts to prepare the tar-
geted Mo(N2)2(PNP)2 complexes by reported reductive synthetic
routes14 using starting materials such as MoCl5, MoCl4(Et2O),
or MoCl3(THF)3 were unsuccessful.15 These results are surpris-
ing, considering these starting materials are routinely used
with non-pendant amine containing phosphines, such as the
reductive synthesis of trans-[Mo(N2)2(dppe)2]16 or [Mo
(N2)2(PMePh2)4].17 In our studies, MoBr3(THF)3 was used to
prepare the target Mo(N2)2(PNP)2 complexes by two methods:
a direct reduction of MoBr3(THF)3 to Mo(N2)2(PNP)2 or by the
stepwise reduction of isolated MoBr2(PNP)2, Scheme 1.
The dibromide complexes, MoBr2(PNP)2 are prepared in THF
by reducing MoBr3(THF)3 with Zn flakes in the presence of two
equivalents of the desired PNP ligand. Isolated yields of the
yellow or orange, paramagnetic MoBr2(PNP)2 complexes were
found to be dependent upon the identity of the phosphorus sub-
stituents on the PNP ligand. The yield of isolable dibromide
compounds, MoBr2(PEtNBnPEt)2 (1) and MoBr2(PEtNMePEt)2 (2),
is low (21–29%), which is in part due to the formation of a
blue–green molybdenum dimer, [MoBr2(PEtNRPEt)]2 (R = Bn
and Me), isolated in 29% yield by diethyl ether extraction, and
structurally characterized by X-ray diffraction, Fig. 1h (see ESI†
The phenylphosphine-substituted complexes exhibit νNN
bands at a higher frequency than the ethyl-substituted com-
pounds resulting from greater electron-donating ability of ethyl
groups on phosphorus. This increase in phosphine electron
density results in a more electron-rich metal center that can
better donate electron density into N2 π* orbitals, weakening the
NuN bond resulting in elongation (i.e., more activation).
Scheme 1
4518 | Dalton Trans., 2012, 41, 4517–4529
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Fig. 1 Molecular structures of 1 (a), 4 (b), trans-5 (c), 8 (d), trans-6 (e), 7-Mo (f), cis-6 (g), and MoBr2[(PEtNBnPEt)]2 (h) with partial atom number-
ing. Thermal ellipsoids are drawn at 30% probability. The hydrogen atoms are omitted for clarity, and all carbons on phenyl rings except ipso carbons
omitted in structures (b), (d) and (f).
Table 2 summarizes the IR and 31P{1H} NMR data for cis- and
trans-isomers of Mo(N2)2(PNP)2 complexes 5–8.
Crystals of 1, 4 and 7-Mo were grown by slow evaporation of
THF solution, cis-6 was grown in pentane at −35 °C, while
trans-5 and trans-6 were grown by slow evaporation of Et2O. 8
was grown by diffusion of ether into a THF solution. General
structural features of these octahedral trans-[MoBr2(PNP)2] and
trans-[Mo(N2)2(PNP)2] complexes include PNP ligand bite
angles between 82 and 86°. The six-membered chelate rings of
the coordinated PNP ligand are in the chair conformation and
the pendant amines positioned on opposite sides of the molecule
Structural studies of molybdenum complexes
X-ray diffraction studies were carried out on crystals of dibro-
mide complexes 1 and 4 and dinitrogen complexes, trans-5, cis-
6, trans-6, 7-Mo and 8. The molecular structures are shown in
Fig. 1 and selected metric parameters are listed in Table 1.
This journal is © The Royal Society of Chemistry 2012
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Table 1 Selected bond lengths (Å) and angles (°) of molybdenum bis
(dinitrogen) and dibromide complexes
(Fig. 1f) the Mo–P bonds are 2.4493(5) and 2.4758(5) Å, while
in 8 (Fig. 1d) they are 2.4549(6) and 2.4798(6) Å. Upon binding
to Mo, the dinitrogen bond length becomes longer compared to
free N2 at 1.0975 Å.4a,20 With an electron-rich metal center,
trans-5 and trans-6 the NuN bond distances are 1.118(1) and
(1.117(2), 1.114(2)) Å, respectively, indicating slight activation
of N2. The NuN bond length is slightly longer for cis-6 (com-
pared to trans-6) at 1.123(2) and 1.121(2) Å, due to the N2
ligands position trans to a phosphine donor.21 Elongation of the
NuN bond is expected for 7-Mo and 8, given the similar red
shifted νNN band in the IR, however, while the NuN bond
length of 8 is in the expected range at 1.116(2) Å, the NuN
bond length of 1.095(2) Å for 7-Mo is not elongated compared
to free N2.
trans-[MoBr2(PEtNBnPEt)2] (1)
Mo(1)–P(1)
Mo(1)–P(2)
Mo(1)–Br(1)
2.5000(3)
2.4981(3)
2.57773(12)
P(1)–Mo(1)–P(2)
P(2)–Mo(1)–P(1A)
P(1)–Mo(1)–P(1A)
Br(1)–Mo(1)–Br(1A)
P(2)–Mo(1)–Br(1)
P(1)–Mo(1)–Br(1)
93.513(9)
86.487(9)
180.0
180.0
86.189(7)
94.805(7)
trans-[MoBr2(PPhNBnPPh)2] (4)
Mo(1)–P(1)
Mo(1)–P(2)
Mo(1)–Br(1)
2.5566(3)
2.5164(3)
2.58297(12)
P(1)–Mo(1)–P(2)
P(2)–Mo(1)–P(1A)
P(2)–Mo(1)–P(2A)
P(2)–Mo(1)–Br(1)
P(1)–Mo(1)–Br(1)
84.528(10)
95.471(10)
180.0
93.471(8)
100.520(7)
trans-[Mo(N2)2(PEtNBnPEt)2] (trans-5)
Mo(1)–P(1)
Mo(1)–P(2)
Mo(1)–N(2)
2.4474(3)
2.4585(3)
2.0163(10)
P(1)–Mo(1)–P(2)
P(2)–Mo(1)–P(1A)
P(2)–Mo(1)–P(2A)
N(2)–Mo(1)–N(2A)
N(3)–N(2)–Mo(1)
N(2)–Mo(1)–P(2)
N(2)–Mo(1)–P(1)
82.744(11)
97.255(11)
180.0
180.0
Synthesis and characterization of tungsten complexes
trans-[W(N2)2(dppe)(PNP)] and trans-[W(N2)2(PNP)2]
N(2)–N(3)
1.1178(14)
178.71(10)
90.21(3)
89.74(3)
trans-[Mo(N2)2(PPhNBnPPh)2] (8)
The synthesis of tungsten–phosphine bis(dinitrogen) complexes
is well established, and many complexes have been prepared
through reduction of WCl6, WCl4(dppe) or WCl4(PPh3)2 by Mg
or Na–Hg.22,14a The latter two are utilized here and are syn-
thesized by zinc reduction of WCl6 with PPh3 to afford
WCl4(PPh3)2 (eqn (1)) followed by phosphine ligand exchange
at W(IV) to afford WCl4(dppe) (eqn (2)). Trans-[W(N2)2(dppe)
(PNP)] mixed complexes are prepared by reduction of
WCl4(dppe) with one equivalent of the desired PNP ligand and
excess Mg powder in THF under an atmosphere of N2 (eqn (3)).
Mo(1)–P(1)
Mo(1)–P(2)
Mo(1)–N(2)
2.4549(6)
2.4798(6)
2.0263(17)
P(1)–Mo(1)–P(2)
P(2)–Mo(1)–P(1A)
P(2)–Mo(1)–P(2A)
N(2)–Mo(1)–N(2A)
N(3)–N(2)–Mo(1)
N(2)–Mo(1)–P(2)
N(2)–Mo(1)–P(1)
95.153(17)
84.847(17)
180.0
180.0
N(2)–N(3)
1.116(2)
176.91(15)
96.37(5)
86.28(5)
trans-[Mo(N2)2(PEtNMePEt)2] (trans-6)
Mo(1)–P(1)
Mo(1)–P(2)
Mo(1)–P(3)
Mo(1)–P(4)
Mo(1)–N(3)
Mo(1)–N(5)
2.4470(7)
2.4469(6)
2.4406(7)
2.4491(6)
2.0220(14)
2.0215(13)
P(1)–Mo(1)–P(2)
P(3)–Mo(1)–P(4)
P(1)–Mo(1)–P(3)
P(2)–Mo(1)–P(4)
N(3)–Mo(1)–N(5)
N(4)–N(3)–Mo(1)
N(6)–N(5)–Mo(1)
N(3)–Mo(1)–P(2)
N(3)–Mo(1)–P(1)
82.83(2)
81.95(2)
178.800(13)
179.731(14)
178.48(5)
179.56(12)
177.88(11)
89.52(4)
ð1Þ
ð2Þ
N(3)–N(4)
N(5)–N(6)
1.1166(18)
1.1140(17)
89.12(4)
trans-[Mo(N2)2(PPhNMePPh)2] (7-Mo)
Mo(1)–P(1)
Mo(1)–P(2)
Mo(1)–N(2)
2.4758(5)
2.4493(5)
2.0338(19)
P(1)–Mo(1)–P(2)
P(2)–Mo(1)–P(1A)
P(2)–Mo(1)–P(2A)
N(2)–Mo(1)–N(2A)
N(3)–N(2)–Mo(1)
N(2)–Mo(1)–P(2)
N(2)–Mo(1)–P(1)
85.924(18)
94.075(18)
180.0
180.0
N(2)–N(3)
1.095(2)
177.07(17)
90.42(5)
98.63(5)
ð3Þ
cis-[Mo(N2)2(PEtNMePEt)2] (cis-6)
The complexes trans-[W(N2)2(dppe)(PEtNMePEt)] (9) and
trans-[W(N2)2(dppe)(PPhNMePPh)] (10), were obtained using this
procedure, the latter of which has been previously reported by
Tuczek and coworkers.11 The 31P{1H} NMR spectrum for 9
exhibits two AA′XX′ multiplet resonances at 45.6 and
−13.2 ppm with 183W satellites (JP–W = 319 and 302 Hz,
respectively). The IR spectrum for 9 contains a strong asym-
metric νNN band at 1921 cm−1 and a weak symmetric νNN band
at 1990 cm−1, similar to 10. Despite the electron-donating phos-
phine substituents, the νNN band for 9 is at 11 cm−1 higher fre-
quency than 10 (Table 2). The isotopically-labelled complex
trans-[W(15N2)2(dppe)(PEtNMePEt)] (9-15N2) was prepared using
15N2 gas, resulting in an asymmetric νNN band at 1859 cm−1, a
62 cm−1 shift versus the unlabelled complex.23
Mo(1)–P(1)
Mo(1)–P(2)
Mo(1)–P(3)
Mo(1)–P(4)
Mo(1)–N(3)
Mo(1)–N(5)
2.4650(4)
2.4617(4)
2.4635(4)
2.4500(4)
2.0287(12)
2.0143(13)
P(1)–Mo(1)–P(2)
P(3)–Mo(1)–P(4)
P(1)–Mo(1)–P(3)
P(2)–Mo(1)–P(4)
N(3)–Mo(1)–N(5)
N(4)–N(3)–Mo(1)
N(6)–N(5)–Mo(1)
N(3)–Mo(1)–P(2)
N(3)–Mo(1)–P(1)
85.378(12)
82.780(13)
95.501(13)
177.296(14)
85.79(5)
175.93(13)
177.54(12)
92.86(3)
N(3)–N(4)
N(5)–N(6)
1.1226(17)
1.1216(18)
173.82(4)
with amine substituents in the equatorial positions. For cis-6 the
pendant amines are positioned so the lone pair electrons are
facing the adjacent N2 ligand. The Mo–P bond lengths for 1
(Fig. 1a), are 2.5000(3) and 2.4981(3) Å, while those for 4
(Fig. 1b) are slightly longer at 2.5566(3) and 2.5164(3) Å. In the
structure of trans-5 (Fig. 1c), the Mo–P bond lengths are shorter
than those for the dibromide complexes at 2.4585(3), 2.4474(3)
Å. The average Mo–P bond length of 2.4460(3) Å was deter-
mined for trans-6, which is slightly shorter than the average
Mo–P bond length in cis-6 (Fig. 1g) at 2.4600(4) Å. In 7-Mo
Attempts to incorporate PNP ligand variants PEtNBnPEt,
PPhNPhPPh, PCyNMePCy and PCyNBnPCy into trans-[W(N2)2
(dppe)(PNP)] proved to be challenging. Using PEtNBnPEt,
trans-[W(N2)2(dppe)(PEtNBnPEt)] (11) was formed based on 31P
{1H} NMR (45.5 and −14.0 ppm with JP–W = 319 and 303 Hz
4520 | Dalton Trans., 2012, 41, 4517–4529
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Table 2 Summary of 31P{1H}, IR, crystallographic, and electrochemical data for W and Mo bis(dinitrogen) complexes
h
Compound
31P NMRa
νNNb (asym)
νNNb (sym)
M–Ne
N–Ne
E1/2
trans-[Mo(N2)2(PEtNBnPEt)2] (trans-5)
cis-[Mo(N2)2(PEtNBnPEt)2] (cis-5)
trans-[Mo(N2)2(PEtNMePEt)2] (trans-6)
cis-[Mo(N2)2(PEtNMePEt)2] (cis-6)
trans-[Mo(N2)2(PPhNMePPh)2] (7-Mo)
trans-[Mo(N2)2(PPhNBnPPh)2] (8)
trans-[W(N2)2(dppe)(PEtNMePEt)] (9)
trans-[W(N2)2(dppe)(PPhNMePPh)] (10)
trans-[W(N2)2(dppe)(PEtNBnPEt)] (11)
trans-[W(N2)2(PPhNMePPh)2] (7-W)
trans-[W(N2)2(PEtNMePEt)2] (trans-12)
cis-[W(N2)2(PEtNMePEt)2] (cis-12)
trans-[W(N2)2(dppe)(dppp)] (13)
trans-[W(N2)2(dppp)2] (14)
12.4
15.9; 12.5
12.0
15.2; 12.8
25.3
27.9
1937
1945
1937
1941
1955
1954
1921
1910c
1922
1925
1942d
1916
1917
1912
1900
1943
2006
2006
—
2007
—
2020
1990
2013c
1990
2027
—
1981
1995
—
2.016(1)
—
2.022(1)
2.014(1) 2.029(1)
2.034(2)
2.026(2)
2.020(2) 2.010(2)
2.033(1) 2.001(1)
—
2.021(2)
—
1.118(1)
—
1.114(2) 1.117(2)
1.123(2) 1.122(2)
1.095(2)
−1.06
—
−1.07
—
−0.82
−0.81
−0.96
−0.83
—
−0.80
—
—
1.116(2)
45.6; −13.2
41.4; 4.3c
45.5; −14.0
−3.1
1.129(2) 1.128(2)
1.130(2) 1.120(2)
—
1.103(3)
—
−15.2
−12.1; −13.7
44.2; 1.5
−3.3
—
—
—
—
−0.79
—
—
−0.82
1.996(2)
1.125(3)
trans-[W(N2)2(PPh2Me)4]
trans-[W(N2)2(dppe)2]
−9.4
46.5
1973
2008
1.991(1)f 2.001(1)
1.996(11) 1.985(10)g
1.134(2)f 1.132(2)
1.126(15) 1.141(16)g
a 31P{1H} NMR (ppm) spectra were collected in THF and referenced against H3PO4. b IR (cm−1) spectra were collected as KBr pellets. c Ref. 11.
e In hexanes. e Bond lengths reported in Å. f See ESI† for X-ray structure. g Ref. 25. h Cyclic voltammetry performed in THF–nBu4NB(C6F5)4 (0.2 M)
at 0.1 V s−1. The E1/2 values are for reversible MI/0 couples.
183W satellites, respectively) and IR (νNN at 1922(s) and 1990
(w) cm−1) data, however it was not successfully isolated in pure
form. Using PPhNPhPPh, a paramagnetic product was obtained.
The 31P{1H} NMR spectrum shows the liberation of PPh3 and
consumption of free PPhNPhPPh, indicating ligand exchange, but
no νNN bands were observed in the IR spectrum. The use of
P
CyNBnPCy and PCyNMePCy resulted in unidentified products as
determined by 31P{1H) NMR, and no νNN bands were observed
in the IR spectrum.
To prepare tungsten bis(dinitrogen) complexes containing two
PNP ligands, (i.e., trans-[W(N2)2(PNP)2]), two equivalents of
PRNMePR ligand (R = Ph, Et, Cy) were mixed with WCl4(PPh3)2
in THF with excess Mg powder and stirred under N2 for a
minimum of 18 h. An orange solid was collected using
PPhNMePPh and PCyNMePCy24 and a yellow oil was obtained for
PEtNMePEt. Trans-[W(N2)2(PPhNMePPh)2] (7-W) was recrystal-
lized from THF or toluene and obtained in a 9% crystalline yield
(eqn (4a)).
Fig. 2 Infrared spectra (KBr) of 15N2 labelled (dotted) and unlabelled
(black) trans-[W(N2)2(PPhNMePPh)2] (7-W). The asymmetric tungsten
N2 stretch is shifted by 58 cm−1 as a result of the isotopic labelling.
labelled trans-[W(15N2)2(PPhNMePPh)2] (7-W15N2) exhibits a
strong νNN band at 1867 and a weak band at 1959 cm−1 (Fig. 2).
After degassing and placing the sample under Ar, a 15N NMR
spectrum of 7-W15N2 was collected at 25 °C in THF. Two strong
resonances at −51 (doublet, JN–N = 5.3 Hz) and −62 (multiplet
with JN–W = 52 Hz 183W satellites) ppm were observed and
assigned to the distal and proximal tungsten-bound dinitrogen
atoms, respectively (referenced to CH315NO2, δ = 0). A small
amount of free 15N2 gas is observed at −73 ppm indicating liber-
ation of tungsten-bound 15N2 in THF (see ESI† for spectrum).26
In contrast, when the reduction is performed using PEtNMePEt
ligand, cis-[W(N2)2(PEtNMePEt)2] (cis-12, eqn (4b)) was obtained
based on 31P{1H} NMR, virtual triplets of an AA′BB′ pattern at
−12.1 and −13.7 ppm with JP–W = 296 and 304 Hz 183W sat-
ellites, respectively, as
a yellow oil with <5% trans-
[W(N2)2(PEtNMePEt)2] (trans-12). Trans-6 exhibits a singlet at
−15.2 ppm with JP–W = 303 Hz 183W satellites. However, due to
the high solubility of these products in organic solvents, cis-12
and trans-12 compounds could not be completely isolated from
the free ligand and were not obtained pure. Isomerization occurs
slowly, over a period of 4 months in pentane at −35 °C, after
this time trans-12 had increased to ∼25% by 31P{1H} NMR.
ð4Þ
The 31P{1H} NMR spectrum shows a singlet resonance at
−3.1 ppm with JP–W = 308 Hz 183W satellites while the IR spec-
trum contains a strong asymmetric νNN band at 1925 cm−1 and a
weak symmetric νNN band at 2027 cm−1. The isotopically-
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trans-[W(N2)2(dppe)(dppp)] and trans-[W(N2)2(dppp)2]
dinitrogen indicated by lengthening of the NuN bond compared
to free N2 (1.0975 Å).4a,20 The degree of N2 activation, deter-
mined by IR and X-ray crystallography, typically follows the
trend where complexes containing electron-donating substituents
exhibit νNN bands that occur at lower energy and exhibit slightly
longer N–N bond distances.4a The Mo- and W-N2 PNP com-
plexes reported here follow these trends with the exception of
the mixed phosphine complexes 9 and 10, where the NuN bond
lengths are nearly identical and the asymmetric νNN band for 10
(1910 cm−1) is 11 cm−1 lower in energy than 9 (1921 cm−1),
despite the electron withdrawing phenyl substituents on PNP
phosphorus atoms. We suggest that the reason the less electron-
rich metal center in 10 exhibits the more activated N–N bond
could be due to the greater steric requirement of the phenyl
groups of the PNP ligand versus ethyl groups. The bulky phenyl
groups cause the W–N–N bond angle to deviate from linearity,
as a result, more effective orbital overlap leads to increased
back-donation from the metal to π* orbitals of N2, weakening
the N–N bond.21b,29 A similar observation is observed for
complex 14 and 7-W where 14 exhibits a longer NuN bond
(1.125 Å) and is more red shifted νNN band (1912 cm−1) than
7-W (1.103 Å and 1925 cm−1) despite identical ligand ring size
and phenyl substituents on phosphorus atoms. The pendant
methylamine group in the ligand backbone of 7-W is not
expected to have a significant electronic effect. As the W–N–N
bond angles deviate from linearity to a slightly greater extent in
compound 14 (175.6(2)°) versus 7-W (177.9(2)°), this deviation
may again contribute to the weakening of the N–N bond as
observed in the IR. A comparison of M–N2 and N–N bond dis-
tances between analogous complexes 7-Mo and 7-W shows that
dinitrogen is more strongly activated in the tungsten complex.
The νNN bands are consistent with these crystallographic obser-
vations, with the 7-W complex exhibiting a 30 cm−1 lower fre-
quency (i.e., more activated N2) than 7-Mo. The greater N2
activation by 7-W can also be explained by the more electron
releasing nature of tungsten compared to molybdenum.21b,30
An X-ray crystallographic study performed on crystals grown
from the reaction intended to produce the mixture of cis-12 and
As a comparison for the structural and spectroscopic analysis of
the mixed trans-[W(N2)2(dppe)(PNP)] complexes 9 and 10, the
analogous tungsten bis(dinitrogen) complex, trans-[W(N2)2
(dppe)(dppp)] (13) (dppp = 1,3-bis(diphenylphosphino)propane)
without a pendant amine was prepared. WCl4(dppe) was stirred
in THF with Mg powder and 1 equiv. dppp, affording 13 as a
red–orange microcrystalline solid in 5% yield. The 31P{1H}
NMR spectrum contains AA′XX′ resonances at 44.2 and
1.5 ppm (JP–W = 311 and 314 Hz 183W satellites, respectively)
corresponding to the dppe and dppp ligands, respectively. The
product also contained ∼15% trans-[W(N2)2(dppe)2] (a singlet
at 46.5 ppm in THF in the 31P{1H} NMR spectrum). The separ-
ation of the two compounds has not been achieved. The IR spec-
trum of 13 contains an asymmetric νNN band at 1917 cm−1
which is slightly higher in energy than the asymmetric νNN band
for 10 at 1910 cm−1 11
Efforts to prepare trans-[W(N2)2(dppp)2]
,
.
(14) by the procedure used for trans-[W(N2)2(PNP)2] complexes
resulted in formation of the target complex only as a minor
product, exhibiting a singlet at −3.3 ppm in the 31P{1H} NMR
with JP–W = 307 Hz 183W satellites, among multiple unidentified
tungsten–phosphine products by 31P{1H} NMR resonances with
183W satellites.27 The IR spectrum contained a single asym-
metric νNN band at 1912 cm−1. Because 14 was a minor product
in this complex mixture, only a few small crystals could be iso-
lated, but a full spectroscopic analysis was performed. The diffi-
culty synthesizing 14 is surprising considering the prevalence of
trans-[W(N2)2(dppe)2] in the literature6d,28 and may be due to
the formation of a less favorable six-membered ring upon coordi-
nation of PNP ligand to the metal in the resulting complex.
Structural studies
X-ray crystallographic studies were performed for the trans-
[W(N2)2(dppe)(PNP)] complexes 9 and 10 as well as 7-Wand the
non-pendant amine containing complex 14. Crystals were grown
in THF at −35 °C or by slow evaporation at room temperature.
The molecular structures are shown in Fig. 3 and selected bond
lengths and angles are listed in Table 3. Each complex exhibits
octahedral geometry with the bidentate PNP, dppe and/or dppp
ligands in the equatorial plane and the dinitrogen ligands in the
axial positions. For the mixed ligand complexes 9 and 10, the
PNP or dppp ligand forms a six-membered ring upon coordi-
nation to the metal center; all of these six-membered rings adopt
a chair conformation. For 7-W and 14 the six-membered ring
also resides in the chair conformation with the pendant amines
or central methylene group from one ligand pointing in the oppo-
site direction as the other pendant amine or methylene group for
the second ligand. The bite angles for the PNP/dppp and dppe
ligands are in the range 83–85° and 79–80°, respectively. The
N–W–N bond angles of the dinitrogen ligands are 180° for the
bis(PNP) and bis(dppp) complexes 7-W, trans-12, and 14, due
to crystallographic symmetry. The N–W–N bond angles for the
heteroleptic complexes 9 and 10 deviate slightly from linearity at
173.83(7)° and 173.35(5)°. The W–P bond lengths fall between
2.43–2.48 Å.
trans-12 consisted of
a mixture of trans-12 and trans-
[WCl2(PEtNMePEt)2], Fig. 3f. From the X-ray data, the occu-
pancy of N2 to Cl was modelled at a 25 : 75 ratio.31 The N–N
bond distance was determined to be 1.16(2) Å; however, because
of the N2 and Cl ligand disorder, the accuracy of this bond dis-
tance is less reliable (see ESI† for crystallographic metrics). The
complex cis-[WCl2(PEtNMePEt)2], Fig. 3e, was also obtained
from this reaction. The six-coordinate cis-dichloride complex
exhibits a highly distorted octahedral geometry. To our knowl-
edge, the only other structural report of a tungsten cis-dichloride
complex is cis-[WCl2(depe)2] which exhibits a nearly identical
distorted octahedral structure.32
Electrochemical studies
Cyclic voltammetry experiments of the molybdenum and tung-
sten bis(dinitrogen) complexes were performed in THF using 0.2
M [nBu4N][B(C6F5)4] as the supporting electrolyte. All poten-
tials are reported versus the ferrocene–ferrocenium couple at 0
V. Table 4 summarizes the electrochemical data for all complexes
As observed in the Mo bis(dinitrogen) complexes, coordi-
nation of dinitrogen to tungsten results in weak activation of
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Fig. 3 Molecular structures of 9 (a), 10 (b), 7-W (c), 14 (d), cis-[WCl2(PEtNMePEt)2] (e), and trans-12. Thermal ellipsoids are drawn at 30% prob-
ability. Only ipso carbons on the dppe, dppp, and PPhNMePPh phosphine phenyls are shown. The hydrogen atoms are omitted for clarity. The structure
of trans-12 consists of trans-[WCl2(PEtNMePEt)2] and trans-[W(N2)2(PEtNMePEt)2] with the occupancy of N2 to Cl modelled at a 25 : 75 ratio.
examined in this study (see the ESI† for all cyclic voltammo-
grams). The oxidation potentials for the Mo and W MI/0 and
MII/I waves are related to the electron-donating ability of the sub-
stituents on phosphorus. Thus, the complexes that contain
electron-donating ethyl groups exhibit MI/0 waves at more nega-
tive potentials (−0.30 V) compared to those with phenyl groups
on phosphorus. The MI/0 couples fall between the range of
−0.80 to −1.07 V, and the peak potential for the irreversible
MII/I couple is between 0.01 to 0.18 V. No reduction of the com-
plexes was observed, even at potentials as low as −2.0 V.33
The cyclic voltammogram of 9, shown in Fig. 4, consists of a
reversible WI/0 couple, E1/2 = (−0.96 V, and an irreversible WII/I
wave with a peak potential, Epa = 0.04 V. For all complexes con-
taining PNP ligands in this study, an irreversible MII/I wave was
observed, with no change in irreversibility at scan rates up to 0.5
V s−1. However, with the exception of 9, when scanning through
both MI/0 and MII/I couples, loss of current is observed on the
return scan for the MI/0 couple, resulting in a quasireversible
M
I/0 wave. Scanning through the MI/0 couple before reaching the
MII/I wave, results in a MI/0 couple that exhibits no loss of
current. The loss of current for cathodic portion of the MI/0 wave
after reaching the potential of the MII/I wave is likely due to loss
of one or both bound dinitrogen ligands upon oxidation to M2+
state. In contrast to the tungsten bis(dinitrogen) PNP-containing
complexes, the W bis(dinitrogen) complexes without pendant
amines, e.g. 13, exhibit a quasireversible MII/I couple (Fig. 5).
The electrochemical reversibility of this wave is scan rate depen-
dant, becoming more reversible at faster scan rates, i.e. 0.5 V s−1
(see ESI† for details). Performing the cyclic voltammogram
under an Ar atmosphere does not change the quasi-reversible
nature of the MII/I couple at 0.1 V s−1, suggesting that the N2
ligands may not be liberated from the metal center as readily as
in the pendant amine containing complexes. We postulate that
pendant amine could promote N2 ligand loss by interacting with
the M2+ center. This type of interaction has been observed with a
Cr complex containing ligands with pendant amines.10
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Table 3 Selected bond lengths (Å) and angles (°) of tungsten bis
(dinitrogen) complexes
Table 4 Redox couples (V vs. Cp2Fe+/0) for Mo and W PNP
complexes performed in THF–nBu4NB(C6F5)4 (0.2 M) at 0.1 V s−1
trans-[W(N2)2(dppe)(PEtNMePEt)] (9)
Complexes
MI/0 (E1/2
)
MII/I (Epa)
W(1)–P(1)
W(1)–P(2)
W(1)–P(3)
W(1)–P(4)
W(1)–N(4)
W(1)–N(2)
N(2)–N(3)
N(4)–N(5)
2.4313(5)
2.4385(5)
2.4542(5)
2.4407(5)
2.0102(19)
2.020a0(18)
1.128(2)
P(1)–W(1)–P(2)
P(3)–W(1)–P(4)
P(1)–W(1)–P(3)
N(2)–W(1)–N(4)
N(3)–N(2)–W(1)
N(5)–N(4)–W(1)
N(2)–W(1)–P(1)
N(2)–W(1)–P(3)
80.250(18)
84.708(18)
175.149(19)
173.83(7)
177.58(17)
176.31(17)
86.56(5)
trans-[Mo(N2)2(PEtNMePEt)2] (trans-6)
trans-[Mo(N2)2(PEtNBnPEt)2] (trans-5)
trans-[Mo(N2)2(PPhNMePPh)2] (7-Mo)
trans-[Mo(N2)2(PPhNBnPPh)2] (8)
trans-[W(N2)2(PPhNMePPh)2] (7-W)
trans-[W(N2)2(dppe)(PEtNMePEt)] (9)
trans-[W(N2)2(dppe)(PPhNMePPh)] (10)
trans-[W(N2)2(dppe)(dppp)] (13)
trans-[W(N2)2(dppe)2]
−1.07
−1.06
−0.82
−0.81
−0.80
−0.96
−0.79
−0.83
−0.82
0.01
0.02
0.18
0.23
0.07
0.04
0.14
0.24a
0.22a
1.129(2)
88.67(5)
trans-[W(N2)2(dppe)(PPhNMePPh)] (10)
W(1)–P(1)
W(1)–P(2)
W(1)–P(3)
W(1)–P(4)
W(1)–N(4)
W(1)–N(2)
N(2)–N(3)
N(4)–N(5)
2.4305(4)
2.4333(4)
2.4552(4)
2.4553(4)
2.0326(13)
2.0011(13)
1.1301(18)
1.1195(19)
P(1)–W(1)–P(2)
P(3)–W(1)–P(4)
P(1)–W(1)–P(3)
N(2)–W(1)–N(4)
N(3)–N(2)–W(1)
N(5)–N(4)–W(1)
N(2)–W(1)–P(1)
N(2)–W(1)–P(3)
85.307(13)
79.444(13)
176.757(13)
173.35(5)
175.05(12)
178.42(14)
85.15(4)
a E1/2 for quasireversible MII/I couples.
97.07(4)
trans-[W(N2)2(PPhNMePPh)2] (7-W)
W(1)–P(1)
W(1)–P(2)
W(1)–P(1A)
W(1)–P(2A)
W(1)–N(2)
W(1)–N(2A)
2.4410(6)
2.4710(6)
2.4410(6)
2.4710(6)
2.021(2)
2.021(2)
P(1)–W(1)–P(2)
85.15(2)
85.15(2)
180.0
180.0
177.9(2)
P(1A)–W(1)–P(2A)
P(1)–W(1)–P(1A)
N(2)–W(1)–N(2A)
N(3)–N(2)–W(1)
N(2)–W(1)–P(1)
N(2)–W(1)–P(1A)
90.22(6)
89.78(6)
N(2)–N(3)
1.103(3)
trans-[W(N2)2(dppp)2] (14)
W(1)–P(1)
W(1)–P(2)
W(1)–P(1A)
W(1)–P(2A)
W(1)–N(1)
W(1)–N(1A)
2.4692(7)
2.4792(6)
2.4691(7)
2.4793(6)
1.996(2)
1.996(2)
P(1)–W(1)–P(2)
85.37(2)
85.37(2)
180.00(3)
180.0
P(1A)–W(1)–P(2A)
P(1)–W(1)–P(1A)
N(2)–W(1)–N(2A)
N(2)–N(1)–W(1)
Fig. 4 Cyclic voltammogram of trans-[W(N2)2(dppe)(PEtNMePEt)] in
THF–nBu4NB(C6F5)4 (0.2 M) at 0.1 V s−1 showing a reversible MI/0
couple at −0.96 V and an irreversible MII/I couple at 0.04 V. Referenced
to ferrocenium–ferrocene (0.0 V).
175.6(2)
N(1)–W(1)–P(1)
N(1)–W(1)–P(1A)
82.99(6)
97.01(6)
N(1)–N(2)
1.125(3)
Reactions of Mo and W complexes with CO
To examine the reactivity of the PNP-containing Mo and W bis
(dinitrogen) complexes, reactivity studies with CO were carried
out on selected complexes. In a typical experiment, a solution of
the Mo or W dinitrogen complex in an NMR tube was exposed
to 1 atm CO at room temperature. The treatment of a mixture of
cis-5 and trans-5 with CO indicates the cis isomer reacts quickly,
ejecting both equivalents of N2, fully converting to cis-
[Mo(CO)2(PEtNBnPEt)2], 15, in less than 5 min. 15 exhibits two
multiplets at 15.4 and 3.1 ppm in the 31P{1H} NMR spectrum.
Trans-5 requires 40 h at room temperature to proceed to >92%
conversion to 15, which exhibits νCO bands at 1837 and
1768 cm−1, similar to other Mo(CO)2 complexes.34 When the
reaction is performed with 13CO, the νCO bands shift to 1802
Fig. 5 Cyclic voltammogram of trans-[W(N2)2(dppe)(dppp)] in
THF–nBu4NB(C6F5)4 (0.2 M) at 0.1 V s−1 showing a reversible MI/0
couple at −0.83 Vand a quasireversible MII/I couple at 0.24 V.
and 1742 cm−1
.
In contrast to the isomerization of trans-5 to 15, when a sol-
ution of 7-Mo is exposed to a 13CO atmosphere, the initial 31P
{1H} NMR spectrum recorded after 3 h showed a ∼1 : 1 mixture
of 7-Mo and trans-[Mo(CO)2(PPhNMePPh)2] (trans-16), ident-
ified by a triplet resonance at 31.4 ppm (JP–C = 9.0 Hz) and a
single asymmetric ν13CO band in the IR at 1885 cm−1. After
24 h, cis-[Mo(13CO)2(PPhNMePPh)2] (cis-16) was identified in
∼2 : 1 cis–trans Mo(CO)2 ratio by 31P{1H} NMR, by two multi-
plets at 27.5 and 12.3 ppm. The IR spectrum contains ν13CO
bands of nearly equal intensity at 1834 and 1772 cm−1. Loss of
the PNP ligand from Mo was evident after this time by the pres-
ence of free ligand in the 31P{1H} NMR spectrum.
The addition of one atmosphere of CO to 9 in THF leads to
the formation of cis-[W(CO)2(dppe)(PEtNMePEt)] (17) over the
course of 72 h. The 31P{1H} NMR spectrum indicated the dis-
appearance of the multiplet resonances at 45.6 and −13.2 ppm
corresponding to 9 and the appearance of four ddd resonances at
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46.8 (dppe), 39.9 (dppe), −11.2 (PEtNMePEt), and −24.5
(PEtNMePEt) ppm with 183W satellites, (see ESI† for spectra) the
result of four inequivalent phosphorus environments expected
for 17. The IR spectrum confirmed the cis geometry by the pres-
ence of two νCO bands of nearly equal intensity at 1837 and
(diphenylphosphino)propane (dppp), and molybdenum hexacar-
bonyl were purchased from Sigma-Aldrich. Triethylamine was
purchased from Alfa-Aesar and distilled over CaH2 before use.
Ferrocene (Aldrich) was sublimed under vacuum before use.
MoBr3(THF)3,35 10,11 WCl4(PPh3)2,22b PPhNPhPPh,36 and tetra-
butylammonium tetrakis(pentafluorophenyl)borate37 were syn-
thesized according to literature preparations.
1773 cm−1
.
Conclusions
General procedures. NMR spectra were recorded on a Varian
500 MHz Inova or NMR S system unless otherwise indicated.
1H and 13C chemical shifts are referenced to residual protio
impurity in deuterated solvent. 31P chemical shifts are proton
decoupled unless otherwise noted and referenced to H3PO4 as an
external reference. 15N NMR chemical shifts were externally
referenced to NH4Cl (δ = − 341) relative to CH3NO2 (δ = 0).
Infrared spectra were recorded on a Thermo Scientific Nicolet
iS10 FT-IR spectrometer at ambient temperature and under a
purge stream of nitrogen gas. Solid-state FT-IR samples were
prepared as KBr pellets. Elemental analysis was performed by
Atlantic Microlabs, Norcross, GA. Cyclic voltammetry was per-
formed in a Vacuum Atmospheres Nexus II glovebox under an
N2 atmosphere using a CH Instruments model 620D or 660C
potentiostat in THF using 0.2 M tetrabutylammonium tetrakis
(pentafluorophenyl)borate as the supporting electrolyte.
Measurements were performed using standard three-electrode
cell containing a 1 mm PEEK-encased glassy carbon, Cypress
Systems EE040, a 3 mm glassy carbon rod (Alfa) as the counter
electrode, and a silver wire suspended in electrolyte solution and
separated from the analyte solution by a Vycor frit (CH Instru-
ments 112) as the pseudoreference electrode. Prior to the acqui-
sition of each voltammogram, the working electrode was
polished using 0.1 μm γ-alumina (BAS CF-1050), and rinsed
with THF. Ferrocene or decamethylferrocene (−0.53 V vs.
Cp2Fe+/0 in THF) was used as an internal reference. All poten-
tials are reported versus the ferrocenium–ferrocene couple at
0.0 V and all cyclic voltammograms are collected by first scan-
ning potential in the positive direction.
Molybdenum and tungsten bis(dinitrogen) complexes bearing
pendant amines have been synthesized and isolated in low to
moderate yields. Compared to the reductive synthesis of non-
pendant amine containing Mo(N2)2 dinitrogen complexes, the
preparation of Mo(N2)2(PNP)2 compounds is sensitive to the
nature of the Mo starting material. Cis and trans isomers of Mo
bis(dinitrogen) complexes were isolated with ethyl-substituted
PNP ligands. The synthesis of W(N2)2 complexes containing
one or two PNP ligands was accomplished by following known
synthetic procedures. These complexes have been characterized
by NMR and IR spectroscopy, X-ray crystallography and cyclic
voltammetry. Based on X-ray data, the complexes are found to
activate dinitrogen to a similar degree as analogous complexes
without pendant amines. A comparison of νNN IR stretches and
M–N and N–N bond distances between Mo and W PNP com-
plexes indicates stronger M–N2 coordination and more dinitro-
gen bond activation for tungsten complexes versus similar
molybdenum complexes, suggesting that tungsten complexes
may be better candidates for nitrogen reduction when comparing
the molecules examined in this report. In addition, cyclic voltam-
metry indicates that the incorporation of the pendant amines
affects the reversibility of the MII/I couple in the dinitrogen com-
plexes, likely from loss of N2 facilitated by the pendant amine
groups. The addition of CO to selected dinitrogen PNP com-
plexes affords Mo and W dicarbonyl complexes. The reactivity
of these dinitrogen complexes with acids is being examined to
understand the role of the pendant amine upon protonation and
how their presence will affect the intermediates formed during
the reduction of dinitrogen.
Typical procedure for synthesis of PRNMePR (R = Ph, Cy, and
Et) ligands. To a 50 mL Schlenk flask paraformaldehyde
(0.34 g, 11 mmol) and ethanol was loaded and degassed by
cycling under vacuum and N2. The phosphine (11 mmol) was
syringed in and the resulting mixture stirred at room temperature
for 0.5 h. Methylamine in ethanol (33 wt% in EtOH, 0.60 mL,
5.6 mmol) and triethylamine (1.0 mL) were syringed in and
stirred for an additional 3 h at room temperature (50 °C for R =
Cy) before removing solvent38 and volatiles under vacuum
to give clear oils or white solids in 70–84% yields for
PEtNMePEt,13b 81% yield for PPhNMePPh,11 and 97% yield
for PCyNMePCy.39
Experimental
General
Synthesis and materials. All synthetic procedures were per-
formed under an atmosphere of N2 using standard Schlenk or
glovebox techniques. Unless described otherwise, all reagents
were purchased from commercial sources and were used as
received. Solvents were dried by passage through activated
alumina in an Innovative Technology, Inc., PureSolv solvent
purification system. Isotopically labelled gases (13CO, 15N2)
were purchased and used as received from Cambridge Isotope
Laboratories. Deuterated THF, toluene, and benzene were pur-
chased from Cambridge Isotope Laboratories, dried over NaK
and vacuum transferred before use. Tungsten hexachloride, tri-
phenylphosphine, diphenylphosphine, diethylphosphine, di-
cyclohexylphosphine, and 1,2-bis(diphenylphosphino)ethane,
were purchased from Strem Chemicals, Inc. Magnesium powder,
zinc flakes, paraformaldehyde (95%), benzylamine (95%), meth-
PCyNBnPCy. To a 50 mL Schlenk flask, paraformaldehyde was
loaded (0.34 g, 11 mmol) and flushed with N2 before adding
20 mL anhydrous toluene. Dicylcohexylphosphine (2.2 mL,
11 mmol) was added followed by benzylamine (0.60 mL,
5.6 mmol). The reaction was stirred at 65 °C for 14 h before
removing solvent under vacuum to give a clear, viscous oil.
Yield (2.24 g, 4.2 mmol, 76%). 1H NMR (THF-d8, 500 MHz): δ
= 7.1–7.0 (m, 5H, NCH2C6H5); 3.63 (s, 2H, NCH2Ph); 2.61
(s, 4H, PCH2N); 1.7–0.9 (m, 44H, PC6H11). 13C{1H} NMR
ylamine
in
ethanol
(33%
wt),
bromine,
1,3-bis
This journal is © The Royal Society of Chemistry 2012
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(THF-d8, 125 MHz): δ = 140.9, 130.7, 128.7, 127.6, 62.4, 53.4,
33.8, 30.6, 27.9, 26.4. 31P{1H} NMR (THF): δ = − 16.97 (s).
C66H62Br2MoN2P4: %C, 62.77; %H, 4.95; %N, 2.22. Found: %
C, 64.77; %H, 5.13; %N, 2.20.40
PEtNBnPEt. A reaction flask was charged with 2 mL (1.6 g,
18 mmol) diethylphosphine, (0.34 g, 11 mmol) paraformalde-
hyde and (0.16 g, 5.5 mmol) benzylamine, all dissolved in
oxygen free ethanol. The reaction was heated at 70 °C for 2 h.
Volatile materials were removed under reduced pressure afford-
Synthesis of cis and trans-[Mo(N2)2(PNP)2]
complexes
One pot method
1
ing a colorless, viscous oil in 84% yield (2.3 g, 7.6 mmol). H
cis-[Mo(N2)2(PEtNBnPEt)2] (cis-5). MoBr3(THF)3 (0.225 g,
0.41 mmol), PEtNBnPEt ligand, (0.25 g, 0.82 mmol), and Mg
powder (0.58 g, 24 mmol) was stirred in 50 mL THF in a
100 mL round bottom flask. After 1.5 h, the yellow solution
changed to yellow–green, which slowly turned dark orange–
brown within 1 h. The reaction mixture was filtered through a
medium porosity frit with a Celite pad (0.25 inch), and then the
solution was concentrated in vacuo and placed in a −35 °C
freezer overnight to afford an orange solid in a 17 : 3 ratio of cis–
NMR (THF-d8): δ = 7.33–7.12 (m, 5H, Ph H): 3.79 (s, 2H
NCH2Ph); 2.74 (m, 4H, PCH2N), 1.40–1.29 (m, 8H, PCH2CH3),
1.01 (dt, 12H, Jt = 7.4, Jd = 13.9 Hz, PCH2CH3). 13C{1H} NMR
(C6D6, 125 MHz): δ = 140.3, 130.0 128.3, 127.5, 62.1, 57.3,
19.0, 10.5. 31P{1H} NMR (THF-d8): δ = − 33.0 (s).
PPhNBnPPh. A reaction flask was charged with (2.5 g,
11 mmol) diphenylhydroxymethylphosphine and (0.16 g,
5.5 mmol) benzylamine, dissolved in oxygen free acetonitrile.
The reaction was heated at 70 °C for 2 h. Volatile materials were
removed under reduced pressure affording a colorless, viscous
oil in 92% yield (2.6 g, 5.1 mmol). 1H NMR (THF-d8): δ =
7.38–7.04 (m, 25H, Ph-H): 4.01 (s, 2H NCH2Ph); 3.56 (d, 4H,
JH–P = 3.1 Hz, PCH2N). 13C{1H} NMR (C6D6, 125 MHz): δ =
139.1, 139.0, 134.0, 133.8, 130.1, 129.0, 128.9, 127.6, 61.4,
59.1. 31P{1H} NMR (THF-d8): δ = − 27.9 (s).
1
trans isomers. H NMR (THF-d8): δ 7.29 s, 4H, Ph H), 7.28 (d,
4H, J = 3 Hz, Ph H), 7.21 (m, 2H, Ph H), 3.54 (d, 2H, J = 12.5
Hz, PhCH2N), 3.46 (d, 2H, J = 12.5 Hz, PhCH2N), 3.24 (m, 2H,
PCH2N), 3.19 (m, 2H, PCH2N), 2.29 (m, 2H, PCH2N), 2.24 (m,
2H, PCH2N), 2.19 (m, 2H, PCH2CH3), 2.02 (m, 2H, PCH2CH3),
1.99 (m, 2H, PCH2CH3), 1.65 (m, 2H, PCH2CH3), 1.56 (m, 2H,
PCH2CH3), 1.46 (m, 4H, PCH2CH3), 1.35 (m, 2H, PCH2CH3),
1.01 (m, 12H, PCH2CH3), 0.92 (m, 12H, PCH2CH3). 31P{1H}
NMR (THF-d8): δ 15.9(t), 12.5(t): AA′BB′, splitting 24 Hz. IR
(KBr, νNN, cm−1): 2006 (w, sym), 1945 (w, asym).
Synthesis of molybdenum bromide complexes
MoBr3(THF)3. The preparation by Poli35 was modified by
reducing the temperature for the second bromine addition to
−10 °C (ice–salt bath). The reaction was stirred at −10 °C for
4 h or until visible gas evolution has ceased.
trans-[Mo(N2)2(PEtNBnPEt)2]
(trans-5). Yield
(0.23
g,
1
0.30 mmol, 64%) H NMR (THF-d8): δ 7.37–7.33 (m, 4H, Ph
H), 7.31–7.27 (m, 4H, Ph H), 7.24–7.19 (m, 2H, Ph H), 3.51 (s,
4H, CH2Ph), 2.80 (s, 8H, PCH2N), 1.94–1.82 (m, 8H,
PCH2CH3), 1.80–1.69 (m, 8H, PCH2CH3), 1.03–0.93 (m, 24H,
MoBr2(PEtNBnPEt)2 (1). Following the procedure from above
using 0.225 g of MoBr3(THF)3, the reaction turned bright green
and an orange precipitate was observed. Yield (0.075 g,
CH2CH3). 31P{1H} NMR (THF-d8): δ 12.4 (s). IR (KBr, νNN
,
cm−1): 2006 (w, sym), 1937 (s, asym). Anal. Calcd for
C34H62MoN6P4: %C, 52.71; %H, 8.07; %N, 10.85. Found: %C,
51.04; %H, 8.02; %N, 10.05.
1
0.085 mmol, 21%) H NMR (CD2Cl2): δ 9.63 (br), 8.66 (d, J =
7.1 Hz), 7.82 (m), 6.55 (s), −5.49 (br), −17.39 (br), −17.76 (br).
31P{1H} NMR: no signal in a range of −500 ppm to +500 ppm.
Anal. Calcd for C34H62MoBr2N2P4: %C, 46.48; %H, 7.11: %N,
3.19. Found: %C, 45.10; %H, 6.87; %N, 3.13.40
cis-[Mo(N2)2(PEtNMePEt)2] (cis-6). Following the procedure
from above, a 14 : 6 ratio of cis–trans isomers was obtained. H
1
NMR (THF-d8): δ 3.10 (m, 2H, PCH2N), 2.99 (m, 2H, PCH2N),
2.30 (s, 6H, NCH3), 2.26 (m, 2H, PCH2N), 2.23 (m, 2H,
PCH2CH3), 2.12 (m, 2H, PCH2N), 2.08 (m, 2H, PCH2CH3),
1.94 (m, 2H PCH2CH3), 1.82 (m, 4H, PCH2CH3), 1.45 (m, 4H,
PCH2CH3), 1.42 (m, 2H, PCH2CH3), 1.13 (m, 12H, PCH2CH3),
1.06 (m, 6H, PCH2CH3), 1.00 (m, 6H, PCH2CH3). 31P{1H}
NMR (THF-d8): δ 15.2(t), 12.8(t): AA′BB′, splitting 24 Hz. IR
(KBr, νNN, cm−1): 2007 (s, sym) 1940 (s, asym).
MoBr2(PPhNMePPh)2 (3). Following the procedure from above
using 0.25 g of MoBr3(THF)3. Yield (0.40 g, 0.36 mmol, 78%)
1H NMR (CD2Cl2): δ 13.40 (br), 11.39 (br), 10.74 (br), 10.45
(br), 6.84 (br), 4.69 (br), −4.87 (br), −29.57 (br). 31P{1H}
NMR: no signal in a range of −500 ppm to +500 ppm. Anal.
Calcd for C54H54Br2MoN2P4: %C, 58.39; %H, 4.90; %N, 2.52.
Found: %C, 57.92; %H, 4.72; %N, 2.43.
Note: the orange microcrystalline solid of cis-[Mo(N2)2
(PNP)2] complexes turns brown over a period of approximately
10 days, and no longer contain νNN bands by IR.
MoBr2(PPhNBnPPh
) (4). MoBr3(THF)3 (0.25 g, 0.46 mmol)
2
and PPhNBnPPh (0.46 g, 0.90 mmol) were dissolved in 40 mL of
THF. Zinc flakes (0.093 g, 1.4 mmol) were added and the reac-
tion was stirred for 19 h resulting in a color change to yellow–
brown with a yellow precipitate. The precipitate was isolated on
a medium porosity frit. An additional crop can be collected by
reducing the volume of the mother liquor (∼5 mL) and storing at
trans-[Mo(N2)2(PEtNMePEt)2] (trans-6). Following the pro-
cedure from above, the trans isomer is prepared by stirring the
reaction mixture for 22 h. The solvent was removed and the
trans isomer was extracted with diethyl ether to give 85% trans
1
1
−35 °C. Yield (0.46 g, 0.37 mmol, 80%) H NMR (CD2Cl2): δ
isomer. Yield (0.28 g, 0.45 mmol, 71%). H NMR (300 MHz,
13.28 (br), 11.40 (br), 10.76 (br), 8.01 (m), 7.54 (m), 7.05 (m),
6.76 (m), 4.68 (br), −4.38 (br), −30.63 (br). 31P{1H} NMR: no
signal in a range of −500 ppm to +500 ppm. Anal. Calcd for
THF-d8): δ 2.69 (s, 8H, PCH2N), 2.31 (s, 6H, NCH3), 1.79–1.87
(m, 8H, PCH2CH3), 1.87–1.75 (m, 8H, PCH2CH3), 1.19–1.07
(m, 24H, PCH2CH3). 31P{1H} NMR (THF-d8): δ 12.0 (s). IR
4526 | Dalton Trans., 2012, 41, 4517–4529
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cis-[Mo(13CO)2(PPhNMePPh)2]. 31P{1H} NMR (THF-d8): δ
27.5 (m), 12.3 (m). IR (THF, νCO, cm−1): 1834 (s, sym), 1772
(s, asym).
(KBr, νNN
,
cm−1): 1937 (s, asym). Anal. Calcd for
C22H54MoN6P4: %C, 42.44; %H, 8.74; %N, 13.50. Found: %C,
42.84; %H, 8.76; %N, 13.00.
Synthesis of tungsten complexes
Synthesis of trans-bis(dinitrogen) complex: stepwise method
trans-[W(N2)2(dppe)(PEtNMePEt)] (9). To a 100 mL round-
bottom flask, loaded PEtNMePEt (0.062 g, 0.26 mmol),
WCl4(dppe) (0.187 g, 0.26 mmol), Mg powder (0.317 g,
13.0 mmol), and 30 mL of THF and stirred vigorously under N2
for 64 h. The reaction mixture was filtered and dried in vacuo
affording and extracted with pentane to obtain a red–orange sol-
ution. The material was cooled to −35 °C in THF to obtain red-
trans-[Mo(N2)2(PPhNMePPh)2]
(7-Mo). MoBr2(PPhNMePPh)2
(0.175 g, 0.157 mmol) was suspended in 75 mL THF with Mg
powder (0.075 g, 2.25 mmol). The reaction was stirred for 24 h.
The reaction mixture was filtered through Celite and the mother
liquor concentrated in vacuo to ∼10 mL. The solution was stored
at −35 °C for 18 h affording a dark yellow microcrystalline solid
that was dried under vacuum. Yield (0.024 g, 0.023 mmol,
15%). 1H NMR (THF-d8): δ 7.13 (t, 8H, J = 7.2 Hz, Ph-H), 7.03
(m, 16H, Ph-H), 6.98 (t, 16H, J = 7.2 Hz, Ph-H), 3.20 (s, 8H,
PCH2N), 2.09 (s, 6H, NCH3). 31P{1H} NMR (THF-d8): δ 25.3
(s). IR (KBr, νNN, cm−1): 1955 (s, asym). Anal. Calcd for
C54H54MoN6P4: %C, 64.41; %H, 5.41; %N, 8.35. Found: %C,
64.15; %H, 5.30; %N, 7.99.
1
orange crystals. Yield (0.037 g, 0.042 mmol, 16%). H NMR
(THF-d8): δ 7.49 (t, 8H, J = 8.0 Hz, PhH), 7.30 (t, 8H, PhH),
7.24 (m, 4H, PhH), 2.81 (s, 4H, PCH2N), 2.26 (s, 3H, NCH3),
2.24–2.11 (m, 4H, PCH2CH2P), 1.88–1.78 (m, 8H, PCH2CH3),
0.75 (t, 6H, J = 7.7 Hz, PCH2CH3), 0.72 (t, 6H, J = 7.7 Hz,
PCH2CH3). 31P{1H} NMR (THF-d8): δ 45.6 (m with JP–W
319 Hz 183W satellites, 2P), −13.2 (m with JP–W = 302 Hz 183
=
W
satellites, 2P). IR (KBr, νNN, cm−1): 1990 (w, sym), 1921 (s,
asym). Anal. Calcd for C37H51N5P4W: %C, 50.87; %H, 5.88;
%N, 8.02. Found: %C, 51.32, %H, 5.85; %N, 7.79.
trans-[Mo(N2)2(PPhNBnPPh)2] (8). Following the procedure
from above using (0.20 g, 0.16) MoBr2(PPhNBnPPh)2. Yield
(0.084 g, 0.072 mmol, 40%). 1H NMR (THF-d8): δ 7.14 (m, 8H,
Ph H), 7.04–6.90 (m, 38H, Ph H), 6.67 (m, 4H, Ph H), 3.31 (s,
4H, NCH2Ph), 2.29 (s, 8H, NCH2Ph). 31P{1H} NMR (THF-d8):
δ 27.9 (s). IR (KBr, νNN, cm−1): 1954 (s, asym).
trans-[W(15N2)2(dppe)(PEtNMePEt)] (9-15N). Following the
procedure from trans-[W(N2)2(dppe)(PEtNMePEt)2] using labelled
15N2 gas. The reaction work-up and subsequent handling were
performed under Ar. 15N{1H} NMR (THF-d8): δ −46 (d, JN–N
=
2 Hz, W–NuN), −58 (m, JN–W = 51 Hz 183W satellites,
W–NuN). IR (KBr, νNN, cm−1): 1859 (s, asym).
Synthesis of cis and trans Mo(CO)2(PNP)2
Beginning with the complex 7-Mo, 0.05g was dissolved in THF-
d8 in a septum capped NMR tube, the N2 atmosphere was
purged by bubbling an atmosphere of CO through the reaction
mixture for ∼2 min, and then periodically mixed by a vortex
mixer over the course of 24 h, until all bis(dinitrogen) complex
had been consumed. The products were monitored by NMR,
showing initial formation of trans dicarbonyl. Over the course of
8 h the trans isomer converts to the cis isomer. In the case of the
13C labelled CO, a J. Young NMR tube with a Teflon valve was
used to evacuate the headspace of the tube (freeze, pump, thaw
three times), and finally dose in the isotopically labelled gas.
trans-[W(N2)2(dppe)(PEtNBnPEt)] (11). Following the pro-
cedure from trans-[W(N2)2(dppe)(PEtNMePEt)] using PEtNBnPEt
in place of PEtNMePEt, trans-[W(N2)2(dppe)(PEtNBnPEt)] was
synthesized. Due to impurities, full characterization is not
obtainable. 31P{1H} NMR (THF-d8): δ 45.5 (m with JP–W = 319
Hz 183W satellites, 2P), −14.0 (m with JP–W = 303 Hz 183W sat-
ellites, 2P). IR (KBr, νNN, cm−1): 1990 (w, sym), 1922 (s,
asym).
trans-[W(N2)2(dppe)(dppp)] (13). Dppp ligand (0.94 g,
2.33 mmol), WCl4,(dppe) (1.64 g, 2.27 mmol), Mg powder
(1.00 g, 82.3 mmol), and 50 mL of THF were combined and
stirred vigorously for 44 h under N2. The reaction was filtered
and concentrated in vacuo affording a red–orange solid. Yield
(0.121 g, 0.115 mmol, 5%) product containing 15% trans-
cis-[Mo(CO)2(PEtNBnPEt)2]. 31P{1H} NMR (THF-d8): δ 15.4
(t), 3.1(t): AA′BB′, splitting 30 Hz. IR (THF, νCO, cm−1): 1837
(s), 1768 (s).
1
[W(N2)2(dppe)2] contaminant. H NMR (THF-d8): δ 2.70 (bs,
cis-[Mo(13CO)2(PEtNBnPEt)2]. δ 15.4 (tt, Jt = 10.8 Hz, AA′BB′
splitting 30 Hz), 3.1 (td, Jd = 16.6 Hz, AA′BB′ splitting 30 Hz).
IR (THF, νCO, cm−1): 1802 (s, sym), 1742 (s, asym).
4H, PCH2), 2.01 (bs, 4H, PCH2), 1.89 (m, 2H, (PCH2)2CH2). IR
(KBr, νNN, cm−1): 1995 (w, sym), 1917 (s, asym). 31P{1H}
NMR (THF-d8): δ 44.2 (m with JP–W = 311 Hz 183W satellites),
1.5 (m with JP–W = 314 Hz 183W satellites).
trans-[Mo(CO)2(PPhNMePPh)2]. 31P{1H} NMR (THF-d8): δ
31.4 (s). IR (KBr, νCO, cm−1): 1805 (s, asym).
trans-[W(N2)2(PPhNMePPh)2] (7-W). PPhNMePPh (1.01 g,
2.37 mmol), WCl4,(PPh3)2 (1.02 g, 1.20 mmol), Mg powder
(2.91 g, 120 mmol), and 70 mL of THF were added and stirred
vigorously for 18 h under N2. The reaction mixture was filtered
and concentrated in vacuo and cooled to −35 °C crashing out a
light orange solid which was dissolved solid in minimal toluene
and cooled to −35 °C to obtain red–orange crystals. Yield
(0.12 g, 0.11 mmol, 9%) 1H NMR (THF-d8): δ 7.12 (t, 8H,
trans-[Mo(13CO)2(PPhNMePPh)2]. 31P{1H} NMR (THF-d8): δ
31.4 (t, J = 9.2 Hz). IR (THF, νCO, cm−1): 1885 (s, asym).
cis-[Mo(CO)2(PPhNMePPh)2]. 31P{1H} NMR (300 MHz, THF-
d8): δ 31P{1H} NMR (THF-d8): δ 27.5(t), 12.3(t): AA′BB′, split-
ting 12 Hz. IR (THF, νCO, cm−1): 1846 (s, sym), 1781 (s, asym).
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J = 7.3 Hz, Ph H), 7.06–6.95 (m, 32H, Ph H), 3.26 (s, 8H,
PCH2N), 2.06 (s, 6H, NCH3). 31P{1H} NMR (THF-d8): δ −3.1
(s with JP–W = 308 Hz 183W satellites). IR (KBr, νNN, cm−1):
2027 (w, sym), 1925 (s, asym). Anal. Calcd for
C54H54N6P4W·C4H8O: C, 59.70; H, 5.36; N, 7.20. Found: C,
60.22; H, 5.35; N, 7.07.
mounted using NVH immersion oil onto a nylon fiber and
cooled to the data collection temperature of 100(2) K. Data were
collected on a Brüker-AXS Kappa APEX II CCD diffractometer
with 0.71073 Å Mo-Kα radiation. Unit cell parameters were
obtained from 90 data frames, 0.3° Φ, from three different sec-
tions of the Ewald sphere. The data-set was treated with
SADABS absorption corrections based on redundant multi-scan
data. All non-hydrogen atoms were refined with anisotropic dis-
placement parameters and were treated as idealized contri-
butions. Details for solution and refinement are provided in the
ESI† section.
trans-[W(15N2)2(PPhNMePPh)2] (7-W15N2). Following the pro-
cedure from trans-[W(N2)2(PPhNMePPh)2] using labelled 15N2
gas. The reaction work-up and subsequent handling were per-
formed under Ar. 15N{1H} NMR (THF-d8): δ −51 (d, JN–N = 5
Hz, W–NuN), −62 (d with JN–W = 51 Hz 183W satellites, JN–N
= 5 Hz, W–NuN). IR (KBr, νNN, cm−1): 1959 (w, sym), 1867
(s, asym).
Acknowledgements
cis-[W(N2)2(PEtNMePEt)2] (cis-12). PEtNMePEt (0.051 g,
0.22 mmol) and WCl4(PPh3)2 (0.093 g, 0.11 mmol) were com-
bined in 30 mL THF and stirred for 3 min. Mg powder (0.771 g,
31.7 mmol) was added and the reaction was stirred for 23 h
before being filtered and dried in vacuo to afford a yellow oil.
Due to the high solubility in a variety of organic solvents, the
This material is based upon work supported as part of the Center
for Molecular Electrocatalysis, an Energy Frontier Research
Center funded by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences, under FWP 56073.
Pacific Northwest National Laboratory is operated by Battelle for
DOE.
1
complex was not obtained pure. H NMR (THF-d8): δ 3.18 (m,
2H, PCH2N), 3.12 (m, 2H, PCH2N), 2.36 (m, 2H, PCH2CH3),
2.28 (m, 2H, PCH2N), 2.26 (s, 6H, NCH3), 2.20 (m, 2H,
PCH2CH3), 2.14 (m, 2H, PCH2N), 2.10 (m, 2H PCH2CH3),
1.94 (m, 4H, PCH2CH3), 1.53 (m, 4H, PCH2CH3), 1.43 (m, 2H,
PCH2CH3), 1.12 (m, 12H, PCH2CH3), 1.04 (m, 6H, PCH2CH3),
0.98 (m, 6H, PCH2CH3). 31P{1H} NMR (THF-d8): δ −12.1 (t
with 183W satellites JP–W = 296 Hz, 2P), −13.7 (t with 183W sat-
ellites JP–W = 304 Hz, 2P): AA′BB′, splitting 15 Hz. IR (KBr,
νNN, cm−1): 1981 (s, sym), 1916 (s, asym).
Notes and references
1 (a) R. Schlögl, Angew. Chem., Int. Ed., 2003, 42, 2004; (b) V. Smil,
Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of
World Food Production, MIT Press, Cambridge, 2001.
2 (a) J. B. Howard and D. C. Rees, Chem. Rev., 1996, 96, 2965;
(b) B. K. Burgess and D. J. Lowe, Chem. Rev., 1996, 96, 2983.
3 B. E. Smith, Science, 2002, 297, 1654.
4 (a) M. D. Fryzuk and S. A. Johnson, Coord. Chem. Rev., 2000, 200–202,
379; (b) R. L. Richards, Coord. Chem. Rev., 1996, 154, 83;
(c) A. D. Allen and C. V. Senoff, J. Chem. Soc. Chem. Commun., 1965,
621.
5 (a) K. Arashiba, Y. Miyake and Y. Nishibayashi, Nat. Chem., 2010, 3,
120; (b) D. V. Yandulov and R. R. Schrock, Inorg. Chem., 2005, 44,
1103; (c) D. V. Yandulov and R. R. Schrock, Science, 2003, 301, 76;
(d) W. W. Weare, X. Dai, M. J. Byrnes, J. M. Chin, R. R. Schrock and
P. Müller, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 17099;
(e) R. R. Schrock, Acc. Chem. Res., 2005, 38, 955.
6 (a) J. Chatt, J. R. Dilworth and R. L. Richards, Chem. Rev., 1978, 78,
589; (b) G. J. Leigh, Acc. Chem. Res., 1992, 25, 177; (c) J. Chatt,
A. J. Pearman and R. L. Richards, J. Chem. Soc., Dalton Trans., 1976,
1520; (d) M. Hidai and Y. Mizobe, Chem. Rev., 1995, 95, 1115;
(e) T. Takahashi, Y. Mizobe, M. Sato, Y. Uchida and M. Hidai, J. Am.
Chem. Soc., 1980, 102, 7461; (f) J. Chatt, A. J. Pearman and R.
L. Richards, Nature, 1975, 253, 39.
7 (a) D. L. DuBois and R. M. Bullock, Eur. J. Inorg. Chem., 2011, 1017;
(b) M. Rakowski DuBois and D. L. DuBois, in Catalysis Without Pre-
cious Metals, ed. R. M. Bullock, Wiley-VCH, Weinheim, 2010, pp.
165–180; (c) M. Rakowski DuBois and D. L. DuBois, Acc. Chem. Res.,
2009, 42, 1974.
trans-[W(N2)2(PEtNMePEt)2] (trans-12). Following the pro-
cedure from cis-[W(N2)2(PEtNMePEt)2] (cis-12), trans-12 was
produced as a minor product. 31P{1H} NMR (THF-d8): δ −15.2
(s). IR (KBr, νNN, cm−1): 1942 (s, asym).
trans-[W(N2)2(dppp)2] (14). Following the procedure from
trans-[W(N2)2(PPhNMePPh)2] using dppp in place of PPhNMePPh,
trans-[W(N2)2(dppp)2] was synthesized. Due to the complex
product mixture and small relative quantity of trans-[W
(N2)2(dppp)2] to other products, full characterization is not
obtainable. See ESI† for the 31P NMR spectrum of the product
mixture. 1H NMR (C6D6): δ 7.38 (m, 16H, Ph H), 7.05 (m, 24H,
Ph H), 2.04 (m, 8H, PCH2), 1.66 (m, 4H, P(CH2)2CH2). 31P
{1H} NMR (C6D6): δ −3.5 (s with JP–W = 307 Hz 183W satel-
lites). IR (KBr, νNN, cm−1): 1912 (s, asym).
cis-[W(CO)2(dppe)(PEtNMePEt)2] (17). 31P{1H} NMR (THF-
8 M. L. Helm, M. Stewart, R. M. Bullock, M. Rakowski DuBois and D.
L. DuBois, Science, 2011, 333, 863.
d8): δ 46.8 (ddd, 1P, JP–P = 74.3, 18.5, 8.5), 39.9 (ddd, 1P, JP–P
=
18.5, 8.5, 18.5), −11.2 (ddd, 1P, JP–P = 74.3, 18.5, 27.9), −24.5
(ddd, 1P, JP–P = 18.5, 27.9, 18.5). IR (KBr, νCO, cm−1): 1837
(s, sym), 1773 (s, asym).
9 T. A. Tronic, M. Rakowski DuBois, W. Kaminsky, M. K. Coggins, T. Liu
and J. M. Mayer, Angew. Chem., Int. Ed., 2011, 50, 10936.
10 M. T. Mock, S. Chen, R. Rousseau, M. J. O’Hagan, W. G. Dougherty, W.
S. Kassel, D. L. DuBois and R. M. Bullock, Chem. Commun., 2011, 47,
12212.
11 G. Stephan, C. Näther, C. Sivasankar and F. Tuczek, Inorg. Chim. Acta,
2008, 361, 1008.
12 G. Stephan, C. Näther and F. Tuczek, Acta Crystallogr., Sect. E: Struct.
Rep. Online, 2008, E64, m629.
13 (a) K. D. Welch, W. G. Dougherty, W. S. Kassel, D. L. DuBois and R.
M. Bullock, Organometallics, 2010, 29, 4532; (b) C. J. Curtis,
A. Miedaner, R. F. Ciancanelli, W. W. Ellis, B. C. Noll, M. Rakowski
DuBois and D. L. DuBois, Inorg. Chem., 2003, 42, 216.
X-Ray diffraction studies. Single crystals of trans-
[W(N2)2(dppe)(PEtNMePEt)], trans-[W(N2)2(PEtNMePEt)2], trans-
[W(N2)2(dppe)(PPhNMePPh)], trans-[W(N2)2(PPhNMePPh)2], and
trans-[W(N2)2(PMePh2)4] and all of the trans isomers of trans-
[Mo(N2)2(PEtNBnPEt)2], trans-[Mo(N2)2(PPhNMePPh)2], trans-
[MoBr2(PEtNBnPEt)2],
trans-[MoBr2(PPhNBnPPh)2],
trans-
[Mo(N2)2(PEtNMePEt)2], and cis-[Mo(N2)2(PEtNMePEt)2] were
4528 | Dalton Trans., 2012, 41, 4517–4529
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14 (a) M. Yuki, Y. Miyake and Y. Nishibayashi, Organometallics, 2008, 27,
3947; (b) F. Stoffelbach, D. Saurenz and R. Poli, Eur. J. Inorg. Chem.,
2001, 2699.
15 The reason for the difference in reactivity between molybdenum chlorides
and bromides is not clear but we speculate that the molybdenum chlorides
are forming insoluble aggregating during the reduction with PNP ligands.
16 (a) T. A. George and M. E. Noble, Inorg. Chem., 1978, 17, 1678;
(b) J. R. Dilworth and R. L. Richards, Inorg. Synth., 1980, 20, 119.
17 N. J. Lazarowych, R. H. Morris and J. M. Ressner, Inorg. Chem., 1986,
25, 3926.
18 S. A. Best, T. J. Smith and R. A. Walton, Inorg. Chem., 1978, 17, 99.
19 (a) P. A. Agaskar and F. A. Cotton, Inorg. Chem., 1984, 23, 3383;
(b) P. A. Agaskar and F. A. Cotton, Inorg. Chem., 1986, 25, 15.
20 R. J. Butcher and W. J. Jones, J. Chem. Soc., Faraday Trans., 1974, 70,
560.
21 (a) T. A. George, J. R. D. DeBord, B. B. Kaul, C. J. Pickett and D.
J. Rose, Inorg. Chem., 1992, 31, 1295; (b) W. Hussain, G. J. Leigh, H.
M. Ali, C. J. Pickett and D. A. Rankin, J. Chem. Soc., Dalton Trans.,
1984, 1703.
26 Liquid ammonia is located at −380 ppm in a 15N NMR spectrum refer-
enced to nitromethane (0 ppm).
27 See ESI† for 31P{1H} NMR spectrum of product mixture.
28 B. A. MacKay and M. D. Fryzuk, Chem. Rev., 2004, 104, 385.
29 D. L. DuBois and R. Hoffmann, Nouv. J. Chem., 1977, 1, 479.
30 R. A. Henderson, J. Chem. Soc., Dalton Trans., 1982, 917.
31 Images of the structures of trans-12 and trans-[WCl2(PEtNMePEt)2] are
available in the ESI† (Fig. S8-9).
32 cis-[WCl2(depe)2] is reported as private communication by
J. Cugny, H.W. Schmalle, and H. Berke (CCDC 248972). The
bond distances and angles for cis-[WCl2(PEtNMePEt)2] are reported
in the ESI†.
33 Examination of potentials more negative than −2.0 V was limited by the
solvent window for THF stopping at −2.5 to −3.0 V.
34 (a) T. J. Chow, C.-Y. Wang, S.-C. Sheu and S.-M. Peng, J. Organomet.
Chem., 1986, 311, 339; (b) M. Hidai, K. Tominari and Y. Uchida, J. Am.
Chem. Soc., 1972, 94, 110.
35 B. E. Owens, R. Poli and A. L. Rheingold, Inorg. Chem., 1989, 28,
1456.
22 (a) R. Römer, C. Gradert, A. Bannwarth, G. Peters, C. Näther and
F. Tuczek, Dalton Trans., 2011, 40, 3229; (b) J. R. Dilworth and R.
L. Richards, Inorg. Synth., 2007, 28, 3343.
36 A. Balch, M. M. Olmstead and S. P. Rowley, Inorg. Chim. Acta, 1990,
168, 255.
37 R. J. LeSuer and W. E. Geiger, Angew. Chem., Int. Ed., 2000, 39, 248.
38 Add more methylamine and stir for an additional 3 h if R2PCH2OH inter-
mediate is not fully consumed. The R2PCH2OH intermediates exhibit
singlets in the 31P{1H} NMR at −17.2, −17.0, and −12.2 ppm in THF
for R = Et, Cy, and Ph, respectively.
39 O. Serindag, R. D. W. Kemmitt, J. Fawcett and D. R. Russell, Transition
Met. Chem., 1999, 24, 486.
40 While the carbon values are slightly off in the combustion analysis in
four separate attempts, hydrogen and nitrogen consistently gave accepta-
ble values.
23 The symmetric band for trans-[W(15N2)2(dppe)(PEtNMePEt)] is obscured
by bands due to trans-[W(14N2)2(dppe)(PEtNMePEt)] and thus could not
be accurately measured.
24 A νNN stretch in the IR spectrum at 1945 cm−1 was observed for a tung-
sten complex synthesized with PCyNMePCy: suggesting an N2 complex
was formed. No signals are observed in the 31P{1H} NMR spectrum,
indicating a paramagnetic species.
25 C. Hu, W. C. Hodgeman and D. W. Bennett, Inorg. Chem., 1996, 35,
1621.
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