One-pot synthesis of Mo0 dinitrogen complexes possessing monodentate and multidentate phosphine ligands

Article abstract of DOI:10.1021/ic202430m

Mo⁰ dinitrogen complexes bearing electron-rich mono- and bidentate phosphines can be synthesized in good yields from inexpensive and readily accessible MoCl₅ via a one-step mild reduction with Mg metal. trans-[(N₂)₂Mo(PMePh₂)(PPh(CH ₂CH₂PPh₂)₂)] can also be obtained via this strategy. However, in the presence of tri- and tetradentate ligands that are sterically restrictive, the analogous reduction leads to either (η⁶-arene) formation or [Mo(multidentate phosphine) _m]_n oligomer complexes that have no dinitrogen ligands. One such η⁶-arene complex, where the Mo⁰ center is ligated by 1,1,1-tris(diphenylphosphinomethyl)ethane, was isolated and characterized via X-ray crystallography.

Full text of DOI:10.1021/ic202430m

Article
pubs.acs.org/IC
One-Pot Synthesis of Mo0 Dinitrogen Complexes Possessing
Monodentate and Multidentate Phosphine Ligands
Yalan Ning,† Amy A. Sarjeant,† Charlotte L. Stern,† Thomas H. Peterson,*,‡ and SonBinh T. Nguyen*,†
†Department of Chemistry, Northwestern University, 2145 Sheridan Rd., Evanston, Illinois 60208-3113, United States
‡The Dow Chemical Company, 1776 Building, Midland, Michigan 48674, United States
S
* Supporting Information
ABSTRACT: Mo0 dinitrogen complexes bearing electron-rich mono- and
bidentate phosphines can be synthesized in good yields from inexpensive
and readily accessible MoCl5 via a one-step mild reduction with Mg metal.
trans-[(N2)2Mo(PMePh2)(PPh(CH2CH2PPh2)2)] can also be obtained via
this strategy. However, in the presence of tri- and tetradentate ligands that
are sterically restrictive, the analogous reduction leads to either (η6-arene)
formation or [Mo(multidentate phosphine)m]n oligomer complexes that
have no dinitrogen ligands. One such η6-arene complex, where the Mo0
center is ligated by 1,1,1-tris(diphenylphosphinomethyl)ethane, was isolated and characterized via X-ray crystallography.
INTRODUCTION
RESULTS AND DISCUSSION
■
■
Mo0 complexes with phosphine ligands have attracted a
considerable amount of attention in inorganic, organometallic,
and catalytic chemistry. Because of the electron-rich nature of
the multiphosphine ligand environments, subsets of these
compounds, such as those containing dinitrogen or (η6-C6H5)
ligands, have been explored as potential catalysts for dinitrogen
fixation1,2 and other catalytic processes.3−8 However, these
complexes were often synthesized using multistep sequences
that involve the preparation of intermediate MoII/III/IV
complexes from high oxidation state MoV/VI salts followed by
further reduction to Mo0 (see Scheme 1 and further discussion
below). A variety of reducing agents has been employed in
these syntheses but with significant drawbacks such as high
flammability (Na,9,10 AlEt3,11 Al(iPr)311), toxicity (Na/
Hg12−22), long reaction times (AlEt3, Al(iPr)3),11 and/or low
yields (AlEt3, Al(iPr)3).11 Herein, we report a study on how
Mo0 dinitrogen complexes possessing a wide range of
monodentate to multidentate phosphine ligands can be
prepared directly from MoCl5 using Mg0 as a safe and mild
reducing agent. While this process is generally efficient for
electron-rich mono- and bidentate phosphines, extending it to
tri- and tetradentate ligands requires a careful consideration of
the spatial arrangement of the chelating motif. Multidentate
ligands with sterically restricted chelating motifs often lead to
either (η6-C6H5) complexes that lack dinitrogen ligands6 or
unidentifiable precipitates. The former observation can be
explained by the labile nature of the Mo0−N2 moiety, which
cannot compete with the thermodynamically favored Mo-arene
bond-formation. The precipitates in the latter case are most
likely [Mo(multidentate phosphine)m]n oligomers where the
multidentate phosphines are bridging across multiple metal
centers.
The majority of reported synthetic routes to phosphine-
containing Mo0 dinitrogen complexes are labor-intensive,
multistep procedures. Typical procedures start with the
preparation of a commercially unavailable precursor, such as
MoIII(acetylacetonate)3,11 MoCl4(PR3)2,17,23 MoCl3(THF)3
(THF = tetrahydrofuran),9,10,12,14,15,17,19 MoCl2(dmpe)2
(dmpe = 1,2-bis(dimethylphosphino)ethane),20 MoCl3(triphos
I) (triphos
phenylphosphine),14,18,21,22,24−26 and MoCl3(trident) (trident
( P h 2 P C H 2 C H 2 ) 2 O , ( P h 2 P C H 2 C H 2 ) 2 N R ,
I = bis(diphenylphosphinoethyl)-
=
(Ph2PCH2CH2CH2)2S, and (Ph2PCH2CH2CH2)2PPh))13 fol-
lowed by reduction to the desired phosphine-containing Mo0
species. Thus, direct reductive syntheses of Mo0 dinitrogen
compounds from MoCl5,27 one of the most affordable and
commercially available Mo sources, would be highly attractive.
However, to date there exist only a few one-pot synthe-
ses15,28−30 that utilize MoCl5 as the starting material, and most
of these are limited to specific mono and bidentate ligands
(Scheme 2). We hypothesize that such a strategy can be
generalized to a wider range of phosphine ligands in the
presence of Mg metal as a safe reductant.31
Mo0 Dinitrogen Complexes of the Type Mo(N2)2(L)4 (L
= Monodentate Phosphine Ligands). Mo0 bis(dinitrogen)
complexes of PMePh2 and PMe3 ligands have been isolated as
12,14,15,17,23,28
pure trans-Mo(N2)2(PMePh2)4
and cis-Mo-
(N2)2(PMe3)4, respectively.9 On the other hand, cis-Mo-
(N2)2(PMePh2)4 and trans-Mo(N2)2(PMe3)4 are unknown,
the former presumably because of steric issues and the latter
presumably because of electronic reasons.12 As such, we chose
trans-Mo(N2)2(PMePh2)4 and cis-Mo(N2)2(PMePh2)4 to test
Received: November 12, 2011
Published: February 22, 2012
© 2012 American Chemical Society
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Scheme 1. Traditional Multi-Steps Synthesis of Mo0 Dinitrogen Complexes with Phosphine Ligands
Scheme 2. Previously Reported One-Pot Syntheses of Mo0
Complexes with Mono and Bidentate Phosphine
Ligands15,28,29
yielded some trans-Mo(N2)2(PBu3)4, as indicated by 31P NMR
spectroscopy, along with an unidentified species but no cis-
product. Attempts to isolate the trans-Mo0(N2)2 isomer failed
to afford any solid, presumably because of the high solubility
afforded by the PBu3 ligand. Reduction in the presence of PCy3
and P(OMe)3 did not occur readily and left a lot of starting
materials, presumably because of the bulky nature of PCy3 and
the insufficient electron-donating properties of P(OMe)3. We
note that Mo(P(OMe)3)6 has been successfully obtained from
the direct reduction of MoCl5 but with K or K/Hg as the
reducing agent,33 suggesting the need to balance low ligand
nucleophilicity with a good reducing agent in these types of
one-pot reductions. These observations indicate that, for a
moderate reducing agent such as Mg0, the one-pot synthesis is
best suited to phosphines of the type RnPPh3−n having
moderate steric and nucleophilic properties.
the feasibility of our proposed one-pot Mg0-reduction of MoCl5
(eq 1).
Mo0 Dinitrogen Complexes of the Type Mo(N2)2(L2)2
(L = Bidentate Phosphine Ligands). Numerous Mo0
phosphine dinitrogen complexes of the type Mo(N2)2(L2)2
(L2 = dppe,11,17 dmpe,20 dppm (1,2-bis(diphenylphosphino)-
methane),11 dppp (1,2-bis(diphenylphosphino)propane),11
dippe (1,2-bis(diisopropylphosphino)ethane),10 diars (o-
phenylenebis(dimethylarsine),17 dpae (1,2-bis(diphenylarsino)-
ethane),17 arphos (1-diphenylarsino-2-diphenylphosphino-
ethane)15,17) have been reported. As mentioned in the
introduction, the syntheses of these compounds generally
employ commercially unavailable precursors such as
MoCl4(dppe),17 MoIII(acetylacetonate)3,11 MoCl3(THF)3,15,17
or MoCl2(dmpe)2.20 We were curious to see if the Mg0-
reduction of MoCl5 could be extended to include the bidentate
phosphine ligands dppm, dppe, and dppp (eq 2), with
increasing ligand bite angles. The Mo(N2)2(L2)2 complexes in
this series have all been made from the Na/Hg-reduction of
MoIII(acetylacetonate)3;11 however, these reductions were slow
and the yields were negligible (5% for dppm, 13% for dppe, and
4% for dppp).
cis-Mo(N2)2(PMe3)4 has been previously synthesized from
32
Mo(THF)3Cl3 and Mo(PMe3)3Cl3.9 However, Na was
employed as the reducing agent, and the described
experimental protocols were quite tedious. In our hands,
treatment of MoCl5 with an excess amount of magnesium in
the presence of 4.4 equiv of PMe3 in THF under a dinitrogen
atmosphere for 17 h gave cis-Mo(N2)2(PMe3)4 (1) in 58% in
one step. Comparing to the overall 35% yield previously
obtained from MoCl4(THF)2 in three steps,9,32 this is a good
improvement.
trans-Mo(N2)2(PMePh2)4 (2) has been made by various
routes,12,14,15,17,23,28 one of which involved recrystallizing of the
product from a large-scale (8.3 mmol) reduction of MoCl5 by
Mg0.28 In our hands, this reduction could be carried out on a
twenty-times-smaller scale with a 60% yield after recrystalliza-
tion, suggesting that this one-pot strategy could be easily scaled
down in the modern academic laboratory without a great
sacrifice in yield. Thus, we also attempted to extend it to
prepare Mo0(N2)2 complexes with other monodentate tertiary
phosphines. As the reported syntheses of Mo(N2)2(PMe2Ph)4
invariably give a mixture of cis- and trans- isomers,12,14,17,19,23
and attempts to synthesize Mo(N2)2(PPh3)4 via direct
reduction of MoIII(acetylacetonate)3 have only yielded arene
dimer,8,11 we did not pursue these compounds. Unfortunately,
our remaining attempts have proven to be unsuccessful.
Reduction in the presence of PBu3 proceeded readily and
In our hands, treatment of MoCl5 with an excess of
magnesium under an N2 atmosphere and in the presence of
2.1 equiv of dppm or dppe in THF gave trans-Mo-
(N2)2(dppm)2 (3) or trans-Mo(N2)2(dppe)2 (4) in good to
moderate yields (43% for dppm or 29% for dppe) after 17 h at
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room temperature. However, the analogous synthesis of trans-
Mo(N2)2(dppp)2 (5) required an additional period of over-
night heating at 80 °C to afford the product in 9% isolated
yield. Without this additional heating, the solution remained
light in color throughout the room-temperature period of
reaction and only yielded a greenish-yellow precipitate, which
we suspected to be a mixture of [Mo(N2)2(dppp)2]n and
phosphines such as Mo(N2)(triphos)(L2) (L2 = dppe,13,21
diars,18 dppm,13,22 dmpm (1,2-bis(dimethylphosphino)-
methane),18 depe (1,2-bis(diethylphosphino)ethane),21
dppp21); and other tridentate phosphine complexes of the
type Mo(N2)2(L3)(PR3)13 (L3 = (Ph2PCH2CH2)2O,
(Ph2 PCH2 CH2 )2 NR, (Ph2 PCH2 CH2 CH2 )2 S, and
(Ph2PCH2CH2CH2)2PPh). As in the cases for Mo(N2)2(L)4
and Mo(N2)2(L2)2 discussed above, most of these syntheses
start with presynthesized MoIII compounds such as
MoCl3(triphos)14,18,25,26 or MoCl3(L3)13 where the L3
chelating ligand has been precoordinated. Ligand exchange
between triphos I and monodentate complexes such as
Mo(N2)2(PMePh2)4 was observed to be quite slow on the
NMR scale,14 presumably because of the slow dissociation of
the monodentate ligand; thus, ligand exchange has not been
attempted as a viable route to Mo0 dinitrogen complexes
possessing triphos I, to the best of our knowledge. In addition,
the strong chelating effect of triphos I and II (1,1,1-
tris(diphenylphosphinomethyl)ethane), we speculated that a
one-pot direct reduction of MoCl5 in the combined presence of
PMePh2 and a tridentate ligand would afford Mo0 complexes of
the type Mo(N2)x(L3)(PMePh2)(3−x). Given the high prefer-
ence of triphos II to act as a tripodal ligand, disfavoring any
equatorial arrangement of the phosphine moieties,37 would
34
[Mo(dppp)m]n oligomers. The heating caused the reaction
solution, as well as the surface of the precipitate, to become
progressively more orange-red in color, presumably by breaking
apart the aforementioned oligomers and forming the desired
monomeric product. Unfortunately, heating longer (24 h) did
not improve the yield, presumably because of the low thermal
sensitivity of 5. Curiously, heating the reaction immediately
from the beginning did not lead to the darkening of the
reaction solution and gave no observable products. This
suggests a delicate balance between the reduction of MoV
versus the formation of the N2-containing Mo0 product 5,
which should be formed most easily from the [Mo-
(N2)2(dppp)2]n oligomer. However, this oligomer is more
thermally sensitive than [Mo(dppp)m]n and does not form in
large amounts if the reaction is heated too early.
While the one-pot Mg0-reduction-of-MoCl5 strategy can
clearly be extended to the synthesis of Mo(N2)2(bidentate
phosphine)2 complexes, the yields of the isolable monomeric
products (dppm > dppe > dppp) are inversely proportional to
the ligand bite angle (dppm < dppe < dppp).16,29,35,36 This
observation can be explained by the increased formation of
insoluble oligomeric byproducts as the bite angle of the
bidentate phosphine increases. When the chelate bite angle
becomes as large as that in dppp, it makes sense that
phosphine-bridging oligomers such as [Mo(N2)2(dppp)2]n
and [Mo(dppp)m]n, where the bidentate phosphines span
more than one metal center, can become major kinetic side
products. Thus, additional thermal treatment is needed to break
such oligomers up into the desired monomeric products, as the
synthetic conditions that we discussed (see preceding para-
graph) seem to indicate. Unfortunately, the insolubility of these
oligomers prevented us from obtaining spectroscopic evidence
for their formation. We note that the competition by oligomer
formation (eq 3) may explain why the reported one-step
favor cis-Mo(N2)x(triphos II)(PMePh2)(3−x)
.
Under a dinitrogen atmosphere, treatment of MoCl5 with an
excess amount of magnesium in the presence of 1 equiv of
triphos I and a slight excess of PMePh2 in THF for 17 h gave a
soluble reaction mixture that primarily contained unreacted
PMePh2, trans-Mo(N2)2(triphos I)(PMePh2) (6), and (η6-
C6H5PMePh)Mo(triphos I) (7) (top reaction in Scheme 3), as
Scheme 3. One-Pot Synthesis of Mo0 Complexes 6 and 7
with a Tridentate Phosphine Ligand
synthesis of Mo(N2 )2 (depf)2 (depf
= 1,1′-bis-
(diethylphosphino)ferrocene) from MoCl5 with Mg0 as the
reducing agent only resulted in low isolated yield (13%).29
1
analyzed by H and 31P NMR spectroscopies (see Supporting
Information). In contrast to the aforementioned synthesis of
trans-Mo(N2)2(dppp)2, solid polymeric precipitates were not
observed, presumably because of the preference of triphos I to
function as a monometallic chelating ligand rather than
bridging across multiple metal centers.38,39 Because of the
similar solubilities of 6 and 7, isolation of 6 by recrystallization
from a wide variety of solvent mixtures (toluene/hexane, THF/
methanol, toluene/methanol, and THF/hexane) was not
successful. Repeated recrystallization from THF/MeOH
eventually afforded pure 6 in low isolated yield (4%).
Mo0 Complexes of the Type Mo(N2)x(L3)L and Mo(η6-
Arene)(L3)L (L3 = Tridentate Phosphine Ligands). To date,
many Mo0 dinitrogen complexes bearing tridentate phosphine
ligands have been reported. These include complexes of triphos
I and monodentate phosphines, such as Mo(N2)x(triphos
I)(PR3)(3−x) (PR3 = P(OMe)3,24 PMe3,24 PMe2Ph,18,25,26
PPh3,14,25 PMePh2,14,25); complexes of triphos I and bidentate
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Suspecting that 6 is unstable during the in situ reduction and
can be converted into 7 via N2 elimination, we repeated the
aforementioned synthesis with an additional heating step (at 60
ligand and three for the coordinating phosphines from triphos I
(δ 106.3, 86.3 and 83.5) (Supporting Information, Figure S7).
1
°C for 24 h, Scheme 3). H and 31P NMR analyses of the
resulting reaction mixture (Supporting Information, Figures S8
and S9) revealed the complete conversion to unreacted
PMePh2 and (η6-C6H5PMePh)Mo (triphos I) (7), which was
isolated in 48% yield. Thus, the low yield of 6 at room
temperature is a consequence of its facile conversion into 7 via
elimination of the N2 ligand. Additional support for this
hypothesis is obtained by heating 6 to 60 °C to produce 7 in
57% yield after 17 h (bottom reaction in Scheme 3).5
Consistent with our results, η6-arene complex analogue of 7,
[(η6-4-CH3OC6H4)P(4-CH3OC6H4)2]Mo(triphos I), has been
obtained by Davies and George from the reduction of
MoCl3(triphos I) under Ar.6 We note that arene complexes
have been observed as side products in the reductive syntheses
of Mo0 dinitrogen complexes bearing monodentate phos-
phines,34,40 suggesting that these are low-energy side products
in the reduction of molybdenum halides.
The reduction of MoCl5 in the presence of triphos II, yielded
only (η6-C6H5PMePh)Mo(triphos II) (8) as a single product in
64% yield (eq 4). The molecular structure of 8 was
unambiguously established by X-ray crystallography (Figure
2) where the Mo center in 8 exhibits a distorted octahedral
The molecular structure of 6 was determined using single-
crystal X-ray diffraction (Figure 1). The structure features a six-
Figure 2. ORTEP depiction of the crystal structure of 8. Thermal
ellipsoids are drawn at 50% probability. Hydrogens are omitted for
clarity.
coordination environment, with the σ-bonded triphos II ligand
on one face and the η6-bonded phenyl ring on the other face.
The structure also features Mo−C(arene) (2.271(3)−2.357(3)
Å) distances that are similar to those reported for analogous
arene complexes.40 Numerous modifications of reaction
conditions (temperature and reaction time) were attempted
to effect the formation of cis-Mo(N2)2(triphos II)(PMePh2)
but were unsuccessful. Attempts to carry out the reduction of
MoCl5 in the presence of triphos II and a nonaromatic
phosphine such as PMe3 resulted also in unidentifiable
precipitates. In retrospect, these results are not surprising
given the strong η6-arene bond to Mo0 and the facially
restricted chelating environment of triphos II.
Figure 1. ORTEP depiction of the crystal structure of 6. Thermal
ellipsoids are drawn at 50% probability. Hydrogens are omitted for
clarity.
coordinate molybdenum center in a slightly distorted
octahedral geometry, with the two dinitrogen ligands occupying
axial positions (N−Mo−N = 175.46(10)°) and the four
phosphorus atoms occupying equatorial positions around the
Mo center (the sums of the P−Mo−P angles are 359.77°). This
1
structure of 7 was assigned based on a combination of H, 31P,
and 13C NMR spectroscopic data that are similar to those for
analogous complexes prepared via the thermal reaction of
triphos with Mo(η6-arene)2 complexes.3,4,7 The η6-C6H5 group
exhibits five distinct resonances between δ 4.4 to 3.5 in the 1H
NMR spectrum and six different signals (δ 85.2, 84.9, 80.5,
77.6, 73.2, and 71.3) in the 13C NMR spectrum (Supporting
Information, Figure S7). Additionally, 31P NMR spectrum
shows four equal intensity signals: one at δ −26.6 for the
noncoordination phosphine moiety in the (η6-C6H5PMePh)
Attempts to Synthesize Mo0 Complexes of the Type
Mo(N2)2(L4). Attempts to reduce MoCl5 with Mg0 in the
presence of the tetradentate phosphine ligands (1R,2R)-N,N′-
bis[2-(diphenylphosphino)benzyl]cyclohexane-1,2-diamine
(PNNP) and tris[2-(diphenylphosphino)ethyl]phosphine
(P3P) did not produce any identifiable products. Reduction
of MoCl5 in the presence of the PNNP ligand, either at room
temperature or on heating to 100 °C, produced large amounts
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of precipitates that we attributed to oligomer formation;34
however, we could not rule out the possibility of decomposition
of an Mo(N2)2(L4) intermediate via loss of N2 ligands or one
coordination arm in the tetradentate ligand.16 The reaction
involving the P3P ligand resulted in a dark brown suspension
that we interpreted as evidence of oligomer formation.
However, the 31P NMR spectrum of the filtered mother liquor
of the reaction mixture still shows a significant amount of
soluble species, with a signal at δ −8.4 that is very close to the
chemical shift of uncoordinated P3P and five signals at δ 112.4,
88.0, 85.3, 67.8, 65.4 that are similar to those in 6, 7, and 8.
These data are consistent with species where there is an
incomplete coordination of the tetradentate phosphine ligand
P3P to a single Mo0 center, as has been reported for complex
Mo(P3P)2,41 and this is a potential pathway for oligomer
formation.
Taken together with the results presented in the previous
sections, the aforementioned observations reinforce the
importance of having a stable monometallic phosphine-
coordination environment during the in situ reduction of
MoCl5 with Mg0. For this reaction to be successful in producing
an isolable, soluble product from a multidentate ligand, such a
ligand must be able to coordinate solely to a single metal center
instead of undergoing bridging. While this can be enforced via
either ligand geometry (as in the case of dppm) or via reversible
depolymerization (as in the case of dppp), a careful balance
between ligand coordination environment and steric demand
must be maintained or facile rearrangement into more stable
complex (as in the case of triphos I and II) will be more
favored. Indeed, Tuczek and co-workers have recently reported
a successful synthesis of both trans and cis-[(N2)2Mo0(prP4)]
complexes from the Mg-reduction of MoCl5 in the presence of
N2 and the linear tetradentate phosphine ligand prP4 (prP4 =
Ph2P(CH2CH2)PPh(CH2CH2CH2)PPh(CH2CH2)PPh2).30
Presumably, the linear arrangement of the coordinating
phosphine moieties in this ligand as well as its flexible ethyl
and propyl spacers allows it to completely enfold the Mo center
and stabilize it well enough during the reduction so that the
desired product can eventually be formed.
EXPERIMENTAL SECTION
■
Materials, Reagents, Methods, and Instrumentation. All
syntheses and manipulations of air- and moisture-sensitive materials
were carried out in oven-dried Schlenk-type glassware, either on a
dual-manifold Schlenk line or in a nitrogen-filled glovebox.
Hexanes, toluene, tetrahydrofuran (THF), and methanol are
obtained by passing HPLC-grade solvents (VWR Scientific) over a
column of activated neutral alumina in a Dow-Grubbs solvent system.
Benzene-d6 (Cambridge Isotope Laboratories) was dried over sodium/
benzophonone, vacuum-transferred into a Strauss flask, and stored
over activated (heated at 210 °C under reduced pressure for 20 h)
Davison 4 Å molecular sieves before use. Methylene chloride-d2
(Cambridge Isotope Laboratories) was dried and distilled over calcium
hydride and stored over activated Davison 4 Å molecular sieves before
use. Deionized water was obtained from a laboratory source provided
by Northwestern University.
Molybdenum(V) chloride, methyldiphenylphosphine, trimethyl-
phosphine, 1,2-bis(diphenylphosphino)methane (dppm), 1,2-bis-
(diphenylphosphino)ethane (dppe), 1,2-bis(diphenylphosphino)-
propane (dppp), 1,1,1-tris(diphenylphosphinomethyl)ethane, and
(1R,2R)-N,N-bis[2-(diphenylphosphino)benzyl]cyclohexane-1,2-dia-
mine (PNNP) were purchased from Strem Chemicals. Benzophonone,
tris[2-(diphenylphosphino)ethyl]phosphine (P3P), and calcium hy-
dride were purchased from VWR Scientific. Bis(diphenyl-
phosphinoethyl)phenylphosphine, magnesium, and iodine were
purchased from Aldrich. Magnesium was activated by iodine vapor
under static vacuum overnight at 50 °C. All other chemicals were used
as received.
NMR spectra were recorded on either a Bruker Avance III 500
NMR spectrometer (500 MHz for 1H, 125 MHz for 13C ) or a Varian
1
Mercury 400 FT-NMR spectrometer (400.6 MHz for H, 121.6 MHz
1
for 31P). Chemical shifts for H and 13C spectra were referenced to
internal solvent resonances and were reported as parts per million
relative to SiMe4, whereas 31P NMR spectra were referenced to
external H3PO4 (85%).
Inductively coupled plasma optical emission spectroscopy (ICP-
OES) was conducted on a Varian Vista-MDX (Varian, Walnut Creek,
CA) ICP-OES spectrometer equipped with simultaneous CCD to
cover the 175−785 nm spectral range. The instrument was controlled
by ICP expert software (version 4.1.0) and calibrated using deionized
water and mixtures containing Mo (2, 8, 15, and 25 ppm) and P (3,
15, 25, and 40 ppm) that were prepared from commercially available
ICP standard solutions.
In a typical ICP-OES experiment, sample (around 10 mg) of a
molybdenum complex was weighed into a VWR microwave vial (2−5
mL, Cat No 89079-404) inside a nitrogen-filled glovebox. After the vial
was taken outside, a magnetic stirbar was added and an aliquot (2 mL)
of a freshly prepared mixture of conc. H2SO4:H2O2 (30 wt % in H2O)
(3:1 v/v) was added. The resulting orange sample vial was crimped
before being placed into a Biotage (Uppsala, Sweden) SPX microwave
reactor (software version 2.3, build 6250). Digestion was carried out at
180 °C (the reactor tunes the microwave power to obtain desired
temperature and keeps it stable during course of the reaction
depending on the headspace available in the vial) while the content
of the vial was magnetically stirred during irradiation using the built-in
magnetic stirrer. When the solution became clear and colorless and no
further vapor was produced (1/2 h or more), the irradiation was
stopped and the vial was allowed to cool down. This acidic solution
was diluted to either 100 or 250 mL by the addition of deionized H2O
and analyzed for Mo (202.032, 203.846, and 204.598 nm) and P
(185.827, 185.878, and 214.914 nm) content using standard solutions.
We note that the aforementioned digestion protocol does not work for
trans-Mo(N2)2(dppm)2 (3) and trans-Mo(N2)2(dppp)2 (5), both of
which left behind insoluble residues.
CONCLUSION
■
In conclusion, we have mapped out the reaction scope for the
one-pot synthesis of Mo0 dinitrogen complexes possessing a
range of monodentate to multidentate phosphine ligands using
Mg metal as a reductant. In the cases using monodentate and
bidentate ligands where this strategy was most successful, it
enables the safe and facile production of the desired product
from inexpensive, commercially accessible starting materials.
The use of Mg0 as a reductant avoided a number of drawbacks
that other strategies had, not only with safety, but also with
relative reaction kinetics and low nitrogen solubility in organic
media. With a kinetically slower reducing agent such as Mg
metal, overpressures of nitrogen12,13 are not required because
nitrogen has time to be replenished in solution during reaction.
In the cases of tridentate and tetradentate phosphines,
successful syntheses of the desired dinitrogen Mo0 species
can be achieved if the ligand is flexible enough to stabilize the
Mo center during the reduction both sterically and spatially.30 If
the ligand chelating motif is too rigid or spatially too restrictive,
facile rearrangement into more stable η6-arene complexes and/
or oligomerization will become competitive.
Synthesis of cis-Mo(N2)2(PMe3)4 (1). Under a N2 atmosphere, a
100-mL Schlenk flask containing MoCl5 (1.0 g, 3.64 mmol, 1 equiv),
and a Teflon-coated magnetic stir bar were cooled in an acetone/dry
ice bath. A precooled (−78 °C) solution of PMe3 (1.24 g, 16.1 mmol,
4.42 equiv) in THF (40 mL) and activated magnesium (3.0 g, excess)
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were added sequentially. The bath was removed after 30 min, and the
resulting dark orange reaction mixture was allowed to stir overnight.
The next day, the reaction mixture was evaporated to dryness under
reduced pressure. The remaining brown solid was taken into the
glovebox, extracted with hexanes (30 mL), and vacuum-filtered
through a plug of Celite. The filtrate was evaporated to dryness under
reduced pressure to remove the solvents and PMe3. The remaining
dark-yellow solid was redissolved in hexanes (15 mL) and filtered
through a syringe filter (PFTE, 0.45 μm). The collected solution was
concentrated to 10 mL and kept at −30 °C overnight to crystallize out
a light yellow powder, which was isolated by filtration, washed with
cold methanol (2−3 mL), and dried under reduced pressure. This
recrystallization protocol was repeated two times to afford the pure
product as a light yellow powder (0.96 g, 58%). 1H NMR (400.6 MHz,
C6D6): δ 1.32 (t, 18H, J = 20.0 Hz, PMe3), 1.08 (t, 18H, J = 20.0 Hz,
PMe3). 31P NMR (121.6 MHz, C6D6): δ −6.5 (t, 2P, J = 18.0 Hz,
PMe3), −4.4 (t, 2P, J = 18.0 Hz, PMe3). ICP-OES: calc. = 25.24 wt %
Mo, found =26.30 wt %; calc. = 32.52 wt % P, found =31.03 wt %.
Synthesis of trans-Mo(N2)2(PMePh2)4 (2). trans-Mo-
(N2)2(PMePh2)4 was synthesized using a modification of the reported
procedure by Lazarowich et al.28 Under a N2 atmosphere, a 100-mL
Schlenk flask containing MoCl5 (0.1 g, 0.37 mmol, 1 equiv), and a
Teflon-coated magnetic stir bar were cooled in an ice bath. A
precooled (0 °C) solution of PMePh2 (0.32 g, 1.58 mmol, 4.33 equiv)
in THF (50 mL) and activated magnesium (1.5 g, excess) were added
sequentially. The reaction mixture was allowed to stir for 100 min at 0
°C, and the resulting golden yellow solution was cannula-transferred
into a 100-mL Schlenk flask and evaporated to dryness under reduced
pressure. The remaining yellow solid was taken into the glovebox,
extracted with toluene (30 mL), and filtered through Celite. The
collected solution was evaporated to dryness under reduced pressure
to afford an orange solid residue. This solid was redissolved in toluene
(8 mL) and kept at −30 °C overnight to crystallize out the product,
which was isolated by filtration, washed with methanol (3 mL), and
dried under reduced pressure. Further recrystallization of the mother
liquor afforded a second crop that is combined with the first to yield
of Celite. The collected filtrate was evaporated to dryness under
reduced pressure, redissolved in THF (10 mL), layered with methanol
(8 mL), and kept at −30 °C for several days. The precipitated product
was isolated by filtration, washed with methanol (5 mL), and dried
under reduced pressure to afford an orange powder (0.16 g, 29%). 1H
NMR (400.6 MHz, C6D6): δ 7.21 (br s, 16 H, C6H5), 7.01 (br s, 24 H,
C6H5), 2.28 (t, 8H, J = 8.5 Hz, CH2). 31P{1H} NMR (121.6 MHz,
C6D6): δ 66.1 ppm. ICP-OES: calc. = 10.11 wt %, found = 9.95 wt %
Mo; calc. = 13.03 wt % P, found = 13.52 wt %.
Synthesis of trans-Mo(N2)2(dppp)2 (5). Under a N2 atmosphere,
a 250-mL Schlenk flask containing dppp (0.40 g, 0.95 mmol, 2.1
equiv), MoCl5 (0.124 g, 0.45 mmol, 1 equiv), and a Teflon-coated
magnetic stir bar were cooled in an acetone/dry ice bath. Precooled
(−78 °C) THF (80 mL) and activated magnesium (1.5 g, excess) were
added sequentially. The bath was removed after 30 min, and the
reaction mixture was allowed to stir for 24 h before a water-cooled
reflux condenser was attached to the flask. The resulting brown
reaction mixture was heated up to 80 °C and kept at that temperature
for 17 h. The next day, the reaction mixture was evaporated to dryness
under reduced pressure. The remaining dark-yellow solid was taken
into the glovebox, extracted with with toluene (40 mL), and filtered
through a plug of Celite. The collected solution was evaporated to
dryness under reduced pressure, redissolved in THF (8 mL), layered
with methanol (5 mL) and kept at −30 °C for several days. The
precipitated product was isolated by filtration, washed with methanol
(5 mL), and dried under reduced pressure to afford an orange powder
(0.04 g, 9%). 1H NMR (400.6 MHz, C6D6): δ 7.17 (br s, 16 H, C6H5),
6.99 (br s, 24 H, C6H5), 2.41 (t, 8H, J = 5.6 Hz, CH2), 1.72 (q, 4H, J =
5.6 Hz, CH2). 31P{1H} NMR (121.6 MHz, C6D6): δ 25.5 ppm. ICP-
OES data could not be obtained for this compound because of our
inability to digest it.
Synthesis of trans-Mo(N2)2(triphos I)(PMePh2) (6). Under a N2
atmosphere, a 100-mL Schlenk flask containing bis(diphenyl-
phosphinoethyl)phenylphosphine (0.87 g, 1.58 mmol, 1.01 equiv),
MoCl5 (0.43 g, 1,57 mmol, 1 equiv), and a Teflon-coated magnetic stir
bar were cooled in an acetone/dry ice bath. A precooled (−78 °C)
solution of PMePh2 (0.34 g, 1.68 mmol, 1.07 equiv) in THF (70 mL)
and activated magnesium (1.5 g, excess) were added sequentially. The
bath was removed after 30 min, and the reaction mixture was allowed
to stir for 17 h. The next day, the resulting dark-orange reaction
mixture was evaporated to dryness under reduced pressure. The
remaining dark-orange solid was taken into the glovebox, extracted
with toluene (50 mL), and filtered through a plug of Celite. The
collected solution was evaporated to dryness under reduced pressure,
redissolved in THF (8 mL), layered with methanol (8 mL), and kept
at −30 °C for 2 days.
1
the final product as an orange powder (0.21 g, 61% yield). H NMR
(400.6 MHz, C6D6): δ 7.26 (br s, 16 H, C6H5), 6.87 (br s, 24 H,
C6H5), 1.85 (s, 12H, CH3). 31P{1H} NMR (121.6 MHz, C6D6): δ 19.9
ppm. ICP-OES: calc. = 10.07 wt % Mo, found = 9.54 wt %; calc. =
12.98 wt % P, found =12.97 wt %.
Synthesis of trans-Mo(N2)2(dppm)2 (3). Under a N2 atmos-
phere, a 250-mL Schlenk flask containing MoCl5 (0.13 g, 0.48 mmol, 1
equiv), dppm (0.4 g, 1.0 mmol, 2.08 equiv), and a Teflon-coated
magnetic stir bar were cooled in an acetone/dry ice bath. Precooled
(−78 °C) THF (70 mL) and activated magnesium (1.5 g, excess) were
added sequentially. The bath was removed after 30 min, and the
reaction mixture was allowed to stir overnight. The next day, the
reaction mixture was evaporated to dryness under reduced pressure.
The resulting brown solid was taken into the glovebox, extracted with
toluene (40 mL), and filtered through a plug of Celite. The collected
solution was evaporated to dryness under reduced pressure. The
remaining dark red solid was redissolved in THF (10 mL), layered
with methanol (10 mL), and kept at −30 °C for several days. The
precipitated product was isolated by filtration, washed with methanol
(5 mL), and dried under reduced pressure to afford a dark red powder
(0.19 g, 43%). 1H NMR (400.6 MHz, C6D6): δ 7.51 (br s, 8 H, C6H5),
6.97−6.67 (m, 32 H, C6H5), 5.01 (br s, 4H, CH2). 31P{1H} NMR
(121.6 MHz, C6D6): δ 16.7 ppm. ICP-OES data could not be obtained
for this compound because of our inability to digest it.
Synthesis of trans-Mo(N2)2(dppe)2 (4). Under a N2 atmosphere,
a 250-mL Schlenk flask containing dppe (0.50 g, 1.24 mmol, 2.1
equiv), MoCl5 (0.16 g, 0.58 mmol, 1 equiv), and a Teflon-coated
magnetic stir bar were cooled in an acetone/dry ice bath. Precooled
(−78 °C) THF (100 mL) and activated magnesium (1.5 g, excess)
were added sequentially. The bath was removed after 30 min, and the
reaction mixture was allowed to stir for 17 h . The next day, the
resulting dark-orange suspension was evaporated to dryness under
reduced pressure. The remaining dark-orange solid was taken into the
glovebox, extracted with toluene (70 mL), and filtered through a plug
The precipitated product was again redissolved in THF (15 mL),
layered with methanol (8 mL), and kept at −30 °C for 3 days. The
precipitated product was isolated by filtration to afford purified 6 as a
yellow-orange powder (50 mg, 4%) after being washed with methanol
1
(2 mL) and dried under reduced pressure. H NMR (400.6 MHz,
C6D6): δ 7.26 (br s, 14 H, C6H5), 7.06 (br s, 14 H, C6H5), 6.97 (br s, 7
H, C6H5), 2.85 (br d, 2H, CH2), 2.37 (br s, 2H, CH2), 2.10 (br s, 2H,
CH2), 1.78 (s, 3H, PMePh2), 1.67 (br s, 2H, CH2). 31P{1H} NMR
(121.6 MHz, C6D6): δ 103.9 (d, 1P, J = 101.7 Hz, PPh2PPhPPh2),
65.6 (d, 2P, J = 13.8 Hz, PPhPPh2), 23.8 (dt, 1P, J1= 101.7 Hz, J2=
13.8 Hz, PMePh2). 13C{1H} NMR (125 MHz, C6D6): δ 142.6, 142.4,
141.1 (td, 1C, J1 = 9.7 Hz, J2 = 3.1 Hz), 140.7 (t, 1C, J = 10.9 Hz),
138.0, 137.8, 133.2 (t, J = 5.4 Hz), 133.0 (t, J = 5.4 Hz), 132.3, 132.2,
132.0, 131.9, 129.4, 128.8, 128.5, 128.3 (Ph carbons), 142.1 (pentet,
1C, JP−C = 8.9 Hz, =CH), 141.8 (pentet, 1C, JP−C = 7.4 Hz, =CPh2),
36.3 (m, 2C, CH2), 27.0 (t, 1C, JP−C = 8.6 Hz, CH2), 26.8 (t, 1C, JP−C
= 8.6 Hz, CH2), 17.8 (t, 2C, JP−C = 18.9 Hz, PCH3).
Single crystals suitable for X-ray diffraction analysis were grown by
dissolving purified 6 (40 mg) in THF (3 mL) and layering the
resulting solution with methanol (3 mL) at −30 °C inside the
glovebox freezer. Yellow crystals were formed after a few days and
structurally characterized by X-ray diffraction analysis.
Synthesis of (η6-C6H5PMePh)Mo(triphos I) (7). Under a N2
atmosphere, a 100-mL Schlenk flask containing bis(diphenyl-
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phosphinoethyl)phenylphosphine (0.5 g, 0.907 mmol, 1 equiv),
MoCl5 (0.249 g, 0.907 mmol, 1 equiv), and a Teflon-coated magnetic
stir bar were cooled in an acetone/dry ice bath. A precooled (−78 °C)
solution of PMePh2 (0.185 g, 0.915 mmol, 1.01 equiv) in THF (50
mL) and activated magnesium (1.5 g, excess) were added sequentially.
The bath was removed after 30 min, and the reaction mixture was
allowed to stir for 17 h before a water-cooled reflux condenser was
attached to the flask. The resulting dark-orange suspension was then
heated to 60 °C and kept at this temperature for 24 h. The next day,
the reaction mixture was evaporated to dryness under reduced
pressure. The remaining dark-orange solid was taken into the
glovebox, extracted with toluene (40 mL), and filtered through a
plug of Celite. The collected solution was evaporated to dryness under
reduced pressure, and the resulting yellow-orange solid was stirred in
methanol (30 mL) for 1 h before being filtered over a fine-fritted
funnel. This washing and filtration process was repeated two more
times with the collected solid to remove all methanol-soluble
impurities. The purified product was a yellow powder (0.36 g, 48%)
after being dried under reduced pressure. 1H NMR (400.6 MHz,
C6D6): δ 7.83 (t, 2 H, J = 8.3 Hz, C6H5), 7.55 (t, 2 H, J = 7.6 Hz,
C6H5), 7.49 (t, 2 H, J = 6.9 Hz, C6H5), 7.35 (m, 5 H, C6H5), 7.26−
6.84 (m, 24 H, C6H5), 4.42 (br d, 1H, C6H5), 4.31, 3.80, 3.71, 3.67 (br
s, 4H, C6H5), 2.36−1.70 (br m, 6H, CH2), 1.26 (br d, 3H, PMePh2),
0.91, 0.73 (br s, 2H, CH2). 31P{1H} NMR (121.6 MHz, C6D6): δ
106.25 (td, 1P, J1 = 15.1 Hz, J2 = 4.0 Hz, PPh2PPhPPh2), 86.3 (t, 1P, J
= 16.4 Hz, PPhPPh2), 83.5 (t, 1P, J1= 16.5 Hz, PPhPPh2), −26.6 (d, P,
J = 3.8 Hz, PMePh2). 13C{1H} NMR (125 MHz, C6D6): δ 134.2,
134.1, 133.75, 133.71, 133.67, 133.62, 133.5, 133.3, 133.8, 132.7,
132.1, 132.0, 131.8, 131.7, 131.5, 131.4 (Ph carbons), 85.2, 84.9, 80.5,
77.6, 73.2 (d, J = 10.0 Hz), 71.3 (arene-Ph carbons), 34.1 (t, J = 21.5
Hz, CH2), 32.3 (m, 2C, CH2), 31.5 (t, J = 20.5 Hz, CH2), 13.1 (d, J =
14.5 Hz, PMePh2). ICP-OES: calc. = 11.07 wt % Mo, found = 10.23 wt
%; calc. = 14.27 wt % P, found = 12.98 wt %.
kept at room temperature inside the glovebox for a few days, single
crystals were formed and structurally characterized by X-ray diffraction
analysis.
X-ray Crystallographic Analyses of 6 and 8. Single crystals of
complexes 6 and 8 were mounted using oil (Infineum V8512) on a
glass fiber. Single X-ray crystallography data were collected on a
Bruker APEX-II CCD diffractometer (Bruker AXS Inc., Madison, WI)
equipped with an Apex II CCD detector using graphite mono-
chromated MoKα radiation at a temperature of 100(2) K with a θ
range of 1.95 to 29.15° for 6 or 1.31 to 31.60° for 8 and in 0.5°
oscillations with 20 s exposures. The crystal-to-detector distance was
60.00 mm.
The structure was solved by direct methods and expanded using
Fourier techniques.42 The non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were included but not refined. The
disordered methanol solvent molecules were found in the crystal
lattice of 8 and refined with rigid bond restraints (esd 0.01) on the
displacement parameters as well as restraints on similar amplitudes
(esd 0.05) separated by less than 1.7 Å. The carbon atoms on the
solvent molecules of 8 were restrained esd (0.01) that its Uij
components approximate to isotropic.
ASSOCIATED CONTENT
* Supporting Information
■
S
Digital images of NMR spectra and X-ray crystallography
analysis of 6 and 8. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: THPeterson@dow.com (T.H.P.), stn@northwestern.
edu (S.T.N.).
■
Synthesis of (η6-C6H5PMePh)Mo(triphos II) (8). Under a N2
atmosphere, a 100-mL Schlenk flask containing 1,1,1-tris-
(diphenylphosphinomethyl)ethane (1.55 g, 2.41 mmol, 1 equiv),
MoCl5 (0.66 g, 2.41 mmol, 1 equiv), and a Teflon-coated magnetic stir
bar under N2 were cooled in an acetone/dry ice bath. A precooled
(−78 °C) solution of PMePh2 (0.51 g, 2.52 mmol, 1.05 equiv) in THF
(70 mL) and activated magnesium (1.5 g, excess) were added
sequentially. The resulting dark red reaction mixture was allowed to
stir for 17 h, over which time it slowly warmed up to room
temperature. The next day, the reaction mixture was evaporated to
dryness under reduced pressure. The resulting dark-red solid was taken
into the glovebox, extracted with toluene (35 mL), and filtered
through a plug of Celite. The collected solution was evaporated to
dryness under reduced pressure. The remaining solid was resuspended
in THF (30 mL) and filtered over a fine-fritted funnel. The remaining
solid residue was washed on-frit with THF (1−2 mL) and methanol
(20 mL) before being dried under reduced pressure to give the first
crop of product as an orange-red powder. The combined filtrates were
concentrated to ∼10 mL, layered with methanol (5 mL), and kept at
−30 °C for 2 days to produce the second crop, which was isolated
using a similar protocol as described above for the first crop.
Combining both crops afforded purified 8 as an orange-red powder
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the Dow
Chemicals company. We thank Drs. Robert D. J. Froese and
Francis J. Timmers for helpful discussion. Y.N. thanks Drs.
Michael Zhuravel, Debashis Adhikari, and Tulay Atesin for help
with the proofreading of this manuscript.
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