Synthesis, characterization, and reactivities of molybdenum and tungsten PONOP pincer complexes

Article abstract of DOI:10.1021/acs.organomet.6b00461

A new series of molybdenum and tungsten tricarbonyl pincer complexes, bearing pyridine-based PONOP-type pincer ligands, have been synthesized and fully characterized. Addition of HBF₄·Et₂O to these tricarbonyl complexes generated seven-coordinate molybdenum and tungsten hydride complexes, and these compounds have been isolated in good yields. These metal hydrides show fluxional behavior in solution. The hydride ligands on these metal complexes are acidic in nature and are readily deprotonated by bases. The molybdenum hydride complex is shown to catalyze isomerization of 1-hexene to internal isomers under mild conditions.

Full text of DOI:10.1021/acs.organomet.6b00461

Article
pubs.acs.org/Organometallics
Synthesis, Characterization, and Reactivities of Molybdenum and
Tungsten PONOP Pincer Complexes
Ruth Castro-Rodrigo,‡ Sumit Chakraborty,‡ Lloyd Munjanja,‡ William W. Brennessel,
and William D. Jones*
Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
S
* Supporting Information
ABSTRACT: A new series of molybdenum and tungsten
tricarbonyl pincer complexes, bearing pyridine-based PONOP-
type pincer ligands, have been synthesized and fully charac-
terized. Addition of HBF4·Et2O to these tricarbonyl complexes
generated seven-coordinate molybdenum and tungsten hydride
complexes, and these compounds have been isolated in good
yields. These metal hydrides show fluxional behavior in solution.
The hydride ligands on these metal complexes are acidic in
nature and are readily deprotonated by bases. The molybdenum
hydride complex is shown to catalyze isomerization of 1-hexene
to internal isomers under mild conditions.
i
t
(RPONOP = 2,6-(R2PO)2(C5H3N); R = Pr, Bu) could be
synthesized analogously to Kirchner’s compounds and if the
synthesis of the molybdenum and especially the tungsten
tricarbonyl complexes could be synthesized directly from the
M(CO)6 carbonyls. Herein, we report the synthesis and
characterization of molybdenum and tungsten tricarbonyl and
hydridotricarbonyl complexes supported by pyridine-based
PONOP-type pincer ligands. Additionally, we have applied
molybdenum hydridotricarbonyl complexes in the catalytic
isomerization of terminal olefins to internal isomers.
INTRODUCTION
■
Transition-metal complexes containing tridentate pincer-type
ligands have attracted much attention recently.1 Part of the
reason is that pincer ligands, in general, form robust metal
complexes due to the chelating effect of the ligand. The steric,
electronic, and stereoelectronic properties of the resulting metal
complexes can be readily tuned by varying the substituents on
the donor atoms or introducing functionalities in the pincer
backbone. Among varieties of reported pincer ligands, PNpyP-
type ligands that have a central pyridine nitrogen atom and
disubstituted phosphorus-based donor sites linked with CH2
groups (i.e., −CH2PR2) have been widely used in organo-
metallic chemistry including catalysis.1 Numerous examples of
PNP pincer complexes are reported in the literature.2 Milstein
and co-workers have demonstrated that the PNpyP ligand
exhibits metal−ligand cooperativity through dearomatization/
aromatization of the pyridine ring.3 This unique behavior of the
ligand when linked with a transition-metal center gives rise to
interesting stoichiometric and catalytic reactivity.
Of particular interest is the PNP pincer ligand-supported
molybdenum and tungsten tricarbonyl and hydrido-tricarbonyl
derivatives reported by Kirchner and co-workers.4 Although the
molybdenum tricarbonyl complexes were synthesized directly
from the corresponding PNP ligand and Mo(CO)3(MeCN)3
precursor, analogous tungsten tricarbonyl complexes were
prepared in a multistep process by reduction of the
bromocarbonyl tungsten species [(PNP)W(CO)3Br]Br with
NaHg. The cationic bromocarbonyl compound was synthesized
from the dinuclear tungsten precursor [W(CO)4(μ-Br)Br]2.
Recently, they have also reported that the products can be
obtained directly from the carbonyl precursors in a few hours
using a solvothermal method in a sealed high-pressure
microwave vial.5 We were curious if the related PONOP ligand
RESULTS AND DISCUSSION
■
Synthesis of the Molybdenum and Tungsten PONOP
6
Pincer Complexes. Treatment of Mo(CO)3(CH3CN)3 and
W(CO)3(CH3CN)3 precursors with the RPONOP ligands8
7
(1a: R = tBu, 1b: R = iPr) at room temperature resulted in the
formation of the corresponding molybdenum and tungsten
tricarbonyl complexes 2a−d (Scheme 1). These complexes
were isolated as yellow/orange solids (74−86%) and have been
completely characterized by a combination of 1H, 31P{1H}, and
13C{1H} NMR spectroscopy, IR spectroscopy, and elemental
analysis. Noticeable characteristic spectroscopic features of 2a−
d include, in the 13C{1H} NMR spectrum, one low-field triplet
(2JP−C = 6.1−10.1 Hz) and one low-field broad singlet
resonance within the ranges δ 210−223 and 217−229
assignable to the carbonyl carbon atoms trans and cis to the
pyridine nitrogen atom, respectively. In general, 13C resonances
for the carbonyl carbon atoms present in the tungsten
complexes appeared slightly upfield in comparison to the
analogous molybdenum complexes. In the 31P{1H} NMR
Received: June 8, 2016
© XXXX American Chemical Society
A
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complexes, the PNP pincer ligand adopts a meridional
geometry.
Scheme 1. Synthesis of the (PONOP)Mo and -W tricarbonyl
complexes
The solid-state structures of 2a−d were determined by
single-crystal X-ray crystallography. X-ray structures are shown
in Figure 1, with selected bond distances and angles in Table S-
2. Molybdenum and tungsten centers adopt a distorted
octahedral geometry with P1−M−P2 and trans-CCO−M−CCO
bond angles deviating significantly from linearity. For example,
for the molybdenum complexes 2a and 2b, these angles were
found to be 150.023(16)° and 152.93(4)°, respectively, and for
the tungsten complexes 2c and 2d, 150.113(14)° and
152.62(2)° were observed. Irrespective of the metal centers,
the trans-C−M−C bond angle seems to vary more (∼4°) than
the P1−M−P2 bond angle (∼2°) on changing the substituents
on the phosphorus atoms. Molybdenum and tungsten metals
reside in the same square plane defined by the P1−N−P2−
C(CON‑trans) atoms. The M−CON‑cis bond distances were found
to be slightly longer than the M−CON‑trans distances possibly
due to the greater trans influence of the CO ligand in
comparison to the pyridine nitrogen atom.
spectra, singlet resonances were observed between δ 210 and
246 for 2a−d, which is indicative of C2-symmetric structures.
Tungsten complexes 2c and 2d each exhibit a single resonance
1
at δ 221−210 with JW−P coupling constants (183W: 14%
Protonation Reactions of the (PONOP)Mo and -W
Tricarbonyl Complexes. As part of our interest in generating
the seven-coordinate molybdenum and tungsten hydride
complexes, we began to investigate the reactivity of 2a−d
with a strong acid, HBF4·Et2O (pKa ≈ −0.4 in H2O9). Addition
of 1 equiv of HBF4·Et2O to a CH2Cl2 solution of [(RPONOP)-
Mo(CO)3] (2a and 2b) or [(RPONOP)W(CO)3] (2c and 2d)
abundance, I = 1/2) of 326 Hz. Solid-state IR spectra of these
complexes show three absorption bands in the range 1800−
2000 cm−1, consistent with a C2v symmetry of the molecule and
a meridional CO arrangement. These bands are assignable to
one strong asymmetric and one weaker plus one stronger
symmetric νCO stretching mode. In all of these Mo and W
Figure 1. X-ray crystal structures of 2a, 2b, 2c, and 2d. Ellipsoids are at the 50% probability level.
B
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resulted in an instant color change from bright yellow to pale
yellow. This observation is consistent with the protonation of
the Mo(0) and W(0) metal centers in 2a−d to generate Mo(II)
and W(II) hydride species [(RPONOP)Mo(CO)3H]BF4 (3a:
R = tBu; 3b: R = iPr) and [(RPONOP)W(CO)3H]BF4 (3c: R =
the reported molybdenum hydrides are acidic, we carried out
stoichiometric reactions with organic and inorganic bases.
Treatment of 3b with 1 equiv of triethylamine generated 2b
quantitatively and the corresponding triethylammonium salt
(Scheme 2). Interestingly, the hydride ligand on 3b is
i
tBu; 3d: R = Pr), respectively (eq 1). Similar experimental
Scheme 2. Deprotonation of 3b with Organic and Inorganic
Bases
observation was reported for the analogous PNP-based
molybdenum and tungsten complexes by Kirchner et al.4
Seven-coordinate metal complexes often show fluxional
behavior in solution,10 and such dynamic behavior is also
observed for the molybdenum hydride 3b in the variable-
temperature 1H NMR spectroscopy (Figure 2). For example, in
sufficiently acidic to be deprotonated by tetrahydrofuran (as
solvent) to afford the corresponding oxonium salt in nearly
40% yield. Similarly, a stoichiometric reaction of 3b with a basic
transition-metal hydroxide, [Tp′Ni(μ-OH)]2 (Tp′: tris(3,5-
dimethylpyrazolyl)borate), instantaneously formed 2b and
presumably the corresponding paramagnetic nickel-aqua
species. These reactions suggest that the hydride moieties
present in these molybdenum complexes are indeed acidic in
nature.
Catalytic Isomerization of 1-Hexene with 3b. Catalytic
isomerization of terminal alkenes to internal isomers is an
atom-economical organic reaction that has several applications
in industry.15 Although this is a thermodynamically favorable
process, catalytic reactions often produce a mixture of internal
alkenes with E- and Z-isomers.16 Currently, there are a variety
of homogeneous catalysts reported that can carry out this
process with E-selectivity16c,e,f,17 and a few with Z-selectivity.18
Since the hydride moiety present in 3b is quite acidic, we
hypothesized that the molybdenum hydride species might
catalyze olefin isomerization reactions. We observed that
indeed 3b (10 mol %) catalyzed the isomerization of 1-hexene
to 2-hexenes (both E- and Z-isomers) and 3-hexene (only E-
isomer) within 9 h at 65 °C in CD2Cl2 (eq 2).19 Isomer Z-3-
1
Figure 2. Variable-temperature (°C) H NMR spectra of 3b.
1
the H NMR spectra of 3b, the hydride ligand appeared as a
triplet (A2X spin system, where A = P and X = H) at δ −5.17 at
room temperature. Upon cooling the NMR solution to −57 °C,
the triplet Mo−H resonance collapsed to a doublet of doublets
(AMX spin system) with one small and one large coupling
constant of 22 and 60 Hz, respectively.11 Similarly, in the
31P{1H} NMR spectrum of 3b, there are two broad singlets
observed at room temperature. However, upon lowering the
temperature to −80 °C, a sharp pair of doublet resonances is
observed at δ 221.12 and 194.55 with 2JP−P = 84 Hz. From the
variable-temperature 1H NMR data, the barrier for the fluxional
process can be estimated as ΔG⧧ = 14.0(1.2) kcal mol−1 at 14
°C.12
Complexes 3a−d have been isolated in good yields as pale
yellow powders and characterized by H, 13C{1H}, 31P{1H}
1
NMR spectroscopy, IR spectroscopy, and elemental analysis
(see Supporting Information). Despite several attempts to grow
single crystals of complexes 3a−d, we were unable to obtain
crystalline material suitable for X-ray crystallographic studies.
Deprotonation of the Mo−H in 3b with Organic/
Inorganic Bases. It is well precedented in the literature that
group VI metal hydrides are acidic in nature,13 and this often
gives rise to unique reactivity.14 In order to determine whether
1
hexene was not detected in the reaction mixture by H NMR
spectroscopy. The relative ratio of E-3-hexene and 2-hexenes
was found to be approximately 1:11 after 9 h with 98%
consumption of 1-hexene.
Monitoring the reaction over time (Figure 3) showed
comparable formation of E-2-hexene and Z-2-hexene within the
C
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inantly formed from the isomerization of Z-2-hexene. In
addition, the data show that the thermodynamic ratio of Z/E-2-
hexene of 2:7 (0.28) is easily accessible under the catalytic
conditions, thus indicating that steric bulk of the iPrPONOP
ligand on 3b does not hinder the E/Z-2-hexene isomerization
equilibrium. No isomerization activity was observed when 10
mol % of 3b in CD2Cl2 was reacted with E-3-hexene for 9 h (eq
5). The thermodynamic ratio of E/Z-3-hexene isomers is never
reached since catalyst 3b starts degrading beyond 24 h.
1H NMR spectra recorded at different time intervals during
catalysis reveal that 3b is the resting state of the catalyst.
Catalysis could also be performed with 0.7 mol % of 3b,
affording a 98% conversion of 1-hexene in 9 h. Remarkably, the
molybdenum hydride complex remains intact after the reaction
and catalysis resumes as soon as a new batch of substrate is
introduced, as demonstrated three times with 0.7 mol % of 3b.
i
Figure 3. Monitoring the isomerization of 1-hexene over time.
Conditions: [3b] = 32 mM, [1-hexene] = 328 mM, [internal standard,
1,4-dibromobenzene] = 328 mM, 0.5 mL of CD2Cl2. The lines are a
simulated fit for equilibrating isomers.
Next we compared the reactivity of the Pr-substituted
t
molybdenum hydride (3b) with the bulkier Bu-substituted
version 3a in the isomerization of 1-hexene to understand the
effect of steric crowding on this reaction. Compound 3a
catalyzed the isomerization of 1-hexene sluggishly under the
same reaction conditions since only 68% of 1-hexene was
converted in 9 h to afford Z-2-hexene and E-2-hexene with a
relative ratio of 15:53 (0.28) (eq 6). Unlike the isomerization
first hour, with E-3-hexene being the minor isomer. After the
initial hour the formation of Z-2-hexene remains almost
constant, while E-2-hexene and E-3-hexene increase. These
results indicate that 3b isomerizes a terminal alkene at a much
faster rate than an internal olefin, and therefore internal 2-
hexene isomers build up in higher concentrations during the
initial stage of catalysis. In addition, the data denote that all four
isomers are in a reversible equilibrium. Beyond 8 h of reaction,
the amounts of Z-2-hexene and E-2-hexene decrease slightly at
the same rate, giving the ratio of 2:7 (0.28) at 16 h, consistent
with their thermodynamic ratio.20 The E/Z-2-hexene isomer-
ization equilibrium is achieved within 3 h of reaction, whereas
E/Z-3-hexene isomerization equilibrium was not achieved
under the catalytic conditions. Catalyst 3b is selective for the
2-alkenes over 3-alkenes in the early reaction times. During the
course of the reaction the concentration of E-3-hexene
increases with no formation of Z-3-hexene, consistent with an
inaccessible transition state for Z-3-hexene. The reverse
reaction for the formation of 1-hexene is less favorable.
with 3b, no formation of 3-hexene isomers is observed in this
reaction, suggesting 3a is presumably an inefficient catalyst for
the isomerization of internal alkenes in 9 h, although E/Z
equilibration occurs without double-bond migration. To test
this hypothesis, a catalytic reaction at 65 °C in CD2Cl2 was
performed with 10 mol % of 3a in the presence of Z-2-hexene
(eq 7). After 9 h the ratio of Z-2-hexene and E-2-hexene is
15:53 (0.28). In addition minimal amounts of 1-hexene (<1%)
can be observed in the 1H NMR spectrum. No 3-hexenes were
observed.
To understand further the isomerization process, a catalytic
reaction of Z-2-hexene with 3b was performed in a separate
experiment. Z-2-Hexene isomerized to yield 1-hexene, E-2-
hexene, and E-3-hexene in the ratio of 1:59:25 after 9 h with
85% conversion of Z-2-hexene (eq 3). On the other hand, the
No isomerization of 1-hexene was observed by molybdenum
tricarbonyl complex (2a or 2b), strongly implicating the
participation of the molybdenum hydride (3a and 3b) in the
isomerization.
Other examples of olefin isomerization were also examined.
Heating 3b with 1-octene in dichloromethane also results in
formation of a mixture of internal isomers (eq 8). Similarly,
heating 3b with allylbenzene results in formation of the more
stable β-methylstyrenes (eq 9). See Supporting Information for
NMR spectra, kinetics, and GC data.
Probing the Isomerization Mechanism. The mecha-
nism(s) of transition-metal-catalyzed olefin isomerization
reaction involving M−H bonds is generally well studied in
the literature.21 The two most common pathways are alkyl- and
π-allyl-type mechanisms. In the alkyl pathway, [2,1]-insertion of
reaction of E-2-hexene with 10 mol % of 3b resulted in a
mixture containing unconverted E-2-hexene (78%), Z-2-hexene
(21%), and 1-hexene (1%) after 9 h (eq 4). These results
suggest that the E-3-hexene isomerization product is predom-
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was performed in the presence of excess CO (1 atm). At 9 h,
conversion of 1-hexene is only 4.4% under 1 atm of CO
compared to 98% without CO. As a result, isomerization of 1-
hexene catalyzed by 3b is likely to occur via a CO elimination
pathway.
a terminal alkene into a M−H bond generates a metal alkyl
species, followed by a β-hydride elimination, which isomerizes
the CC double bond (Scheme 3a).21 In contrast, a π-allyl
We also probed the participation of the hydride in the
isomerization from the synthesis of molybdenum deuteride 3b-
d1. In a separate experiment a catalytic reaction with 10 mol %
of 86% deuterated [(iPrPONOP)Mo(CO)3D]BF4 (3b-d1) and
Scheme 3. Alternative Pathways for Alkene Isomerization
1
1-hexene was carried out over 9 h in CD2Cl2. Using H NMR
spectroscopy a decrease in the deuteration of complex 3b-d1
from 86% to 28% is observed during the isomerization of 1-
hexene, with 98% consumption of 1-hexene after 9 h. No
deuteration is observed in the PONOP ligand. On the basis of
this observation, we conclude that some, but not all, of the
Mo−H participates in the isomerization of 1-hexene.
The above kinetic and labeling experiments are consistent
with an alkyl mechanism for isomerization of 1-hexene
(Scheme 3a). Reversible coordination and insertion of the
olefin leads to isomerization, and formation of 3-hexene
isomers would be anticipated to be disfavored kinetically as
the alkyl derivatives required would be sterically the most
crowded (3-hexyl vs 2-hexyl). The ethyl and propyl groups on
the 3-hexyl intermediate prior to the formation of either Z-3-
hexene or E-3-hexene are likely to be in severe steric conflict
with the isopropyl moieties on the molybdenum PONOP
ligand on 3b, thus disfavoring formation of 3-alkenes.25 A π-
allyl pathway (Scheme 3b) would require a second vacant site
at the metal (one to coordinate hexene, a second for the
hydride migration) and is therefore disfavored. The electro-
philic mechanism (Scheme 3c) would not require that the
metal hydride be involved, and MoII is not particularly
electrophilic. An acid-catalyzed mechanism for olefin isomer-
mechanism involves the reversible π-binding of an olefin
followed by oxidative addition of an allylic C−H bond that can
result in an overall [1,3]-H shift (Scheme 3b).21,22 Another
variation of the [1,3]-H shift mechanism is the cationic
mechanism commonly observed for electrophilic transition-
metal catalysts. Examples include [Pd(CH3CN)4][BF4] and
AgOTf (silver trifluoromethanesulfonate).23,24 The mechanism
involves olefin coordination to form β-cationic alkyl metal
intermediates, which then undergo γ-elimination to give an
isomerized allyl group. Protolytic cleavage of the metal−allyl
bond gives an isomerized olefin product (Scheme 3c).
In all of these pathways, alkene coordination to the metal
center is generally proposed. It is reasonable to hypothesize
that in order to bind 1-hexene, the seven-coordinate
molybdenum complex 3b must create a vacant coordination
site by eliminating one or more CO ligands. To test this
possibility, isotopic exchange studies were performed using
13CO gas. Interestingly, a stoichiometric reaction of 3b and 1-
hexene with 13CO (1 atm) incorporated isotopically labeled
CO into the complex after heating the solution at 65 °C in
CD2Cl2 for 9 h, as indicated by 13C{1H} NMR spectroscopy.
13CO incorporation was observed in both the Mo−CON‑cis (δ
211.20) and the Mo−CON‑trans (δ 204.67) triplet resonances
(eq 10), indicative of CO exchange in 3b.
ization with 3b acting as a Bronsted acid would be expected to
̈
produce a mixture of all possible linear olefins and can be
discounted also.26 This catalyst is rather slow for olefin
isomerization. It stands in contrast to HCo3(CO)9, which
isomerizes olefins at a rate of over 5000 s−1 at 0 °C!27
CONCLUSIONS
■
In conclusion, we have synthesized and fully characterized a
series of molybdenum and tungsten tricarbonyl complexes
supported by PONOP-type pincer ligands. Protonation of these
tricarbonyl complexes produced the respective seven-coordi-
nate molybdenum or tungsten hydrides, which have also been
characterized by multinuclear NMR spectroscopy, IR spectros-
copy, and elemental analysis. The molybdenum hydride 3b
showed fluxional behavior in solution and reacts with organic
and inorganic bases to regenerate the tricarbonyl complex 2a.
Complex 3b has also been shown to catalyze isomerization of
1-hexene, selectively forming 2-alkenes over 3-alkenes. A
bulkier molybdenum hydride derivative 3a also catalyzes the
same reaction sluggishly. Isotope exchange studies with 13CO
indicate that the isomerization mechanism involves a CO-
elimination pathway, while deuterium labeling studies show
participation of the hydride in the isomerization.
This notion of CO lability to create a vacant site for alkene
coordination was further reinforced when we observed a
significant decrease in reaction rate when the catalytic reaction
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2
MHz, C6D6): δ 220.96 (broad triplet, W−CON‑cis), 218.89 (t, JP−C
=
EXPERIMENTAL SECTION
■
3
7.0 Hz, W−CON‑trans), 164.64 (t, JP−C = 4.0 Hz, ArC), 140.35 (s,
ArC), 101.72 (s, ArC), 43.41 (s, JP−C = 5.5 Hz, PC(CH3)3), 29.09 (t,
JP−C = 3.6 Hz, PC(CH3)3). 31P{1H} NMR (162 MHz, C6D6): δ 227.52
(1JW−P = 326 Hz). Anal. Calcd for C24H39NO5P2W: C, 43.19; H, 5.89;
N, 2.10. Found: C, 43.09; H, 5.90; N, 2.07. IR (ATR, cm−1): 1934
(νCO), 1813 (νCO, bands overlapped).
General Experimental Information. Unless otherwise noted, all
the organometallic compounds were prepared and handled under a
nitrogen atmosphere using standard Schlenk and glovebox techniques.
Dry and oxygen-free solvents (THF, ether, CH2Cl2, and pentane)
were collected from an Innovative Technology PS-MD-6 solvent
purification system and used throughout the experiments. C6D6 was
purchased from Cambridge Isotope Laboratories, Inc., and distilled
from a dark purple solution of Na and benzophenone under a nitrogen
atmosphere prior to use. CH3CN was dried using CaH2 and stored
over 4 Å molecular sieves. CD2Cl2 was dried over molecular sieves.
Substrates 1-hexene, cis-2-hexene, trans-2-hexene, cis-3-hexene, trans-3-
hexene, and allylbenzene were obtained from Sigma-Aldrich. PONOP
ligands were prepared as previously described.8 Mo(CO)3(CH3CN)3
and W(CO)3(CH3CN)3 precursors were prepared as described in the
literature.28 1H, 13C, and 31P NMR spectra were recorded on Bruker
Avance 400 and Avance 500 NMR spectrometers. Chemical shift
Synthesis of [iPrPONOP]W(CO)3 (2d). This compound was
prepared from 1b (88 mg, 0.256 mmol) and W(CO)3(CH3CN)3
(100 mg, 0.256 mmol) with 86% yield (134 mg). X-ray-quality crystals
were grown by allowing a layer of pentane to slowly diffuse into a
saturated THF solution of 2d at −30 °C. 1H NMR (400 MHz, C6D6):
3
3
δ 6.65 (t, JH−H = 8.0 Hz, 1H, ArH), 6.12 (d, JH−H = 8.0 Hz, 2H,
ArH), 2.48−2.38 (m, 4H, PCH(CH3)2), 1.22−1.15 (m, 24H,
PCH(CH3)2). 13C{1H} NMR (101 MHz, C6D6): δ 217.89 (broad
triplet, W−CON‑cis), 210.18 (t, 2JP−C = 7.0 Hz, W−CON‑trans), 164.52 (t,
3JP−C = 4.0 Hz, ArC), 139.56 (s, ArC), 101.51 (s, ArC), 34.88 (t, JP−C
=
10.1 Hz, PCH(CH3)2), 18.18 (s, PCH(CH3)2), 17.85 (broad triplet,
1
values in H and 13C NMR spectra were referenced internally to the
PCH(CH3)2). 31P{1H} NMR (162 MHz, C6D6): δ 209.80 (1JW−P
=
residual solvent resonances, whereas 31P NMR spectra were referenced
externally to 85% H3PO4 (0 ppm). Infrared spectra were recorded in
the solid state on a Thermo Scientific Nicolet 4700 FT-IR
spectrometer equipped with a Smart Orbit Diamond attenuated
total reflectance (ATR) accessory. Elemental analysis was performed
by the University of Rochester using a PerkinElmer 2400 Series II
elemental analyzer in CHN mode.
327 Hz). Anal. Calcd for C24H31NO5P2W: C, 39.30; H, 5.11; N, 2.29.
Found: C, 39.51; H, 5.11; N, 2.29. IR (ATR, cm−1): 1950 (νCO), 1839
(νCO), 1820 (νCO).
General Procedure for the Synthesis of the PONOP
Molybdenum and Tungsten Hydrido Tricarbonyl Complexes
(3a−d). Under a nitrogen atmosphere, to a 5 mL CH2Cl2 solution of
2 (0.172 mmol) was added HBF4·Et2O (44 μL, 0.174 mmol, 54% w/w
in diethyl ether). The resulting light yellow solution was stirred at
room temperature for 1 h and then dried under vacuum to afford an
oily residue. Upon washing with diethyl ether (∼10 mL), a solid
material was obtained, which was further dried in vacuo to afford the
title compound.
General Procedure for the Synthesis of the PONOP
Molybdenum and Tungsten Tricarbonyl Complexes (2a−d).
Under a nitrogen atmosphere, 10 mL of THF was added to a mixture
of 1 (0.330 or 0.256 mmol) and M(CO)3(CH3CN)3 (0.330 or 0.256
mmol, where M = Mo and W). The resulting clear yellow solution was
stirred at room temperature for a day and then dried under vacuum to
afford a yellow powder. The crude material was washed with ca. 20 mL
of cold pentane (−30 °C) and further dried in vacuo to yield the title
compound as a yellow solid.
Synthesis of [(tBuPONOP)Mo(CO)3H]BF4 (3a). This compound
was prepared by a reaction of 2a and HBF4·Et2O with 68% isolated
1
3
yield (78 mg). H NMR (400 MHz, CD2Cl2): δ 8.01 (t, JH−H = 8.2
Hz, 1H, ArH), 7.03 (broad singlet, 2H, ArH), 1.70−1.04 (m, 36H,
PC(CH3)3), −4.74 (m, 1H, Mo−H). 31P{1H} NMR (162 MHz,
Synthesis of [tBuPONOP]Mo(CO)3 (2a). This compound was
prepared from 1a (131 mg, 0.330 mmol) and Mo(CO)3(CH3CN)3
(100 mg, 0.330 mmol) with 75% yield (143 mg). X-ray quality crystals
were grown by allowing a layer of pentane to slowly diffuse into a
saturated THF solution of 2a at −30 °C. 1H NMR (400 MHz, C6D6):
2
2
CD2Cl2): δ 197.37 (d, JP−P = 83.0 Hz), 176.51 (d, JP−P = 82.5 Hz).
13C{1H} NMR (126 MHz, CD2Cl2): δ 203.26−203.04 (m, Mo−
2
CON‑cis), 196.79 (t, JP−C = 7.4 Hz, Mo−CON‑trans), 161.98 (s, ArC),
146.73 (s, ArC), 106.40 (d, 2J P−C = 5.1 Hz, ArC), 34.37 (d, 2JP−C = 7.0
3
3
3
3
δ 6.67 (t, JH−H = 8.0 Hz, 1H, ArH), 6.06 (d, JH−H = 8.0 Hz, 2H,
Hz, PC(CH3)3), 18.03 (d, JP−C = 10.6 Hz, PC(CH3)3), 17.48 (d, J
= 5.2 Hz, PC(CH3)3). Anal. Calcd for C24H40NO5P2MoBF4: C,
43.20; H, 6.04; N, 2.10. Found: C, 42.97; H, 6.01; N, 2.32. IR (ATR,
ArH), 1.36 (virtual triplet, 36H, PC(CH3)3). 13C{1H} NMR (101
P−C
MHz, C6D6): δ 229.80 (broad triplet, Mo−CON‑cis), 223.81 (t, 2JP−C
=
3
cm−1): 1938 (νCO), 1922 (νCO), 1906 (νCO).
9.7 Hz, Mo−CON‑trans), 163.83 (t, JP−C = 4.0 Hz, ArC), 140.57 (s,
ArC), 102.17 (s, ArC), 42.26 (broad triplet, PC(CH3)3), 28.96 (t, 3JP−C
= 3.7 Hz, PC(CH3)3). 31P{1H} NMR (162 MHz, C6D6): δ 241.67.
Anal. Calcd for C24H39NO5P2Mo: C, 49.75; H, 6.78; N, 2.42. Found:
Synthesis of [(iPrPONOP)Mo(CO)3H]BF4 (3b). This compound
was prepared by a reaction of 2b and HBF4·Et2O with 82% yield (86
1
3
mg). H NMR (400 MHz, CD2Cl2): δ 7.99 (t, JH−H = 8.1 Hz, 1H),
6.99 (d, 3JH−H = 8.1 Hz, 2H), 3.07−2.75 (m, 4H, PCH(CH3)2), 1.56−
1.19 (m, 24H, PCH(CH3)2), −5.18 (m, 1H, Mo−H). 13C{1H} NMR
(126 MHz, CD2Cl2): δ 211.30−211.10 (m, Mo−CON‑cis), 204.67 (t,
2JP−C = 9.6 Hz, Mo−CON‑trans), 161.65 (s, ArC), 146.50 (s, ArC),
C, 49.77; H, 6.84; N, 2.50. IR (ATR, cm−1): 1940 (νCO), 1820 (νCO
,
bands overlapped).
Synthesis of [iPrPONOP]Mo(CO)3 (2b). This compound was
prepared from 1b (113 mg, 0.330 mmol) and Mo(CO)3(CH3CN)3
(100 mg, 0.330 mmol) with 79% yield (136 mg). X-ray quality crystals
were grown from a saturated THF solution of 2b at −30 °C. 1H NMR
(400 MHz, C6D6): δ 6.64 (t, 3JH−H = 8.0 Hz, 1H, ArH), 6.10 (d, 3JH−H
= 8.0 Hz, 2H, ArH), 2.46−2.36 (m, 4H, PCH(CH3)2), 1.25−1.15 (m,
24H, PCH(CH3)2). 13C{1H} NMR (101 MHz, C6D6): δ 226.59
2
106.46 (d, JP−C = 3.9 Hz, ArC), 33.64 (broad singlet, PCH(CH3)2),
17.57 (broad singlet, PCH(CH3)2). 31P{1H} NMR (202 MHz,
2
2
CD2Cl2, − 80 °C): δ 221.12 (d, JP−P = 84.4 Hz), 194.55 (d, JP−P
= 84.4 Hz). Anal. Calcd for C20H32NO5P2MoBF4: C, 39.30; H, 5.28;
N, 2.29. Found: C, 39.21; H, 4.98; N, 2.12. IR (ATR, cm−1): 1948
(νCO), 1898 (νCO), 1848 (νCO).
2
(broad triplet, Mo−CON‑cis), 216.49 (t, JP−C = 10.3 Hz, Mo−
CON‑trans), 163.62 (t, 3JP−C = 4.5 Hz, ArC), 139.73 (s, ArC), 101.87 (s,
ArC), 34.10 (t, JP−C = 7.4 Hz, PCH(CH3)2), 17.91 (s, PCH(CH3)2),
Synthesis of [(tBuPONOP)W(CO)3H]BF4 (3c). This compound
was prepared by a reaction of 2c and HBF4·Et2O with 50% yield (64
2
17.53 (t, JP−C = 3.6 Hz, PCH(CH3)2). 31P{1H} NMR (162 MHz,
1
3
mg). H NMR (400 MHz, CD2Cl2): δ 8.05 (t, JH−H = 8.1 Hz, 1H,
3
3
C6D6): δ 228.94. Anal. Calcd for C24H31NO5P2Mo: C, 45.90; H, 5.97;
N, 2.68. Found: C, 46.13; H, 6.01; N, 2.51. IR (ATR, cm−1): 1957
(νCO), 1861 (νCO), 1833 (νCO).
ArH), 7.07 (d, JH−H = 8.1 Hz, 2H, ArH), 1.53 (d, JP−H = 15.7 Hz,
36H, PC(CH3)3), −3.69 (t, JP−H = 40.0 Hz, 1H, W−H). 13C{1H}
2
NMR (125 MHz, CD2Cl2): δ 205.48 (broad triplet, W−CON‑cis),
201.27 (broad triplet, W−CON‑trans), 162.34 (s, ArC), 147.13 (s, ArC),
106.84 (s, ArC), 44.90 (d, JP−C = 12.5 Hz, PC(CH3)3), 29.12 (s,
PCH(CH3)3). 31P{1H} NMR (162 MHz, CD2Cl2): δ 216.18 (broad
doublet), 199.12 (broad doublet). Anal. Calcd for
C24H40NO5P2WBF4: C, 38.17; H, 5.34; N, 1.85. Found: C, 37.93;
H, 5.21; N, 2.10. IR (ATR, cm−1): 1900 (νCO), 1884 (νCO), 1871
(νCO).
Synthesis of [tBuPONOP]W(CO)3 (2c). This compound was
prepared from 1a (102 mg, 0.256 mmol) and W(CO)3(CH3CN)3
(100 mg, 0.256 mmol) with 74% yield (126 mg). X-ray quality crystals
were grown by allowing a layer of pentane to slowly diffuse into a
saturated THF solution of 2c at −30 °C. 1H NMR (400 MHz, C6D6):
3
3
δ 6.65 (t, JH−H = 8.0 Hz, 1H, ArH), 6.08 (d, JH−H = 8.0 Hz, 2H,
ArH), 1.37 (virtual triplet, 36H, PC(CH3)3). 13C{1H} NMR (101
F
DOI: 10.1021/acs.organomet.6b00461
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Synthesis of [(iPrPONOP)W(CO)3H]BF4 (3d). This compound
was prepared by a reaction of 2d and HBF4·Et2O with 87% yield (104
ACKNOWLEDGMENTS
■
We thank the National Science Foundation (NSF Grant No.
1360985) for their support of this work. R.C.R. thanks Spanish
1
3
mg). H NMR (400 MHz, CD2Cl2): δ 8.00 (t, JH−H = 8.2 Hz, 1H,
3
ArH), 7.02 (d, JH−H = 7.2 Hz, 2H, ArH), 3.08−2.81 (m, 4H,
́
Ministerio de Educacion for a postdoctoral fellowship grant.
PCH(CH3)2), 1.53−1.28 (m, 24H, PCH(CH3)2), −4.79 (m, 1H, W−
H). 13C{1H} NMR (125 MHz, CD2Cl2): δ 196.77 (broad triplet, W−
CON‑cis), 162.06 (broad triplet, W−CON‑trans), 146.70 (s, ArC), 106.39
REFERENCES
■
2
(d, JP−C = 7.5 Hz, ArC), 85.20 (s, ArC), 34.40 (broad singlet,
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C. M., Eds.; Elsevier Science: Amsterdam, 2007.
2
PCH(CH3)2), 18.04 (d, JP−C = 10 Hz, PCH(CH3)2), 17.51 (broad
singlet, PCH(CH3)2). 31P{1H} NMR (162 MHz, CD2Cl2): δ 202.11
(2) (a) Kawatsura, M.; Hartwig, J. F. Organometallics 2001, 20, 1960.
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2
(d, JP−P = 82.4 Hz), 181.21 (d, JP−P = 82.7 Hz). Anal. Calcd for
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4.69; N, 2.12. IR (ATR, cm−1): 1940 (νCO), 1919 (νCO), 1908 (νCO).
Stoichiometric Reactions of 3b with Bases. A J. Young NMR
tube was charged with 3b (15 mg, 25 μmol), an organic/inorganic
base, triethylamine/Tp′Ni(μ-OH)2 (Tp′
= tris(3,5-
dimethylpyrazolyl)borate) (12.5−25 μmol), and ∼0.6 mL of
CD2Cl2. The resulting mixture was stirred at room temperature, and
1H and 31P{1H} NMR spectra were recorded periodically to monitor
the progress of the reaction. A cloudy suspension resulted after 1 h
because of the formation of ionic salts during these reactions.
General Procedure for the Catalytic Isomerization Reac-
tions. In a J. Young NMR tube was loaded Mo−H (3a or 3b) (0.016
mmol) (10 mol %), 10 equiv of alkene isomer (0.16 mmol), internal
standard 1,4-dibromobenzene (39 mg, 0.16 mmol), and 0.5 mL of
CD2Cl2. The resulting solution was heated at 65 °C in an oil bath, and
1H NMR spectra were recorded periodically to monitor the progress
of the reaction. GC traces were also recorded and compared to
authentic samples.
(3) (a) Ben-Ari, E.; Leitus, G.; Shimon, L. J. M.; Milstein, D. J. Am.
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Angew. Chem., Int. Ed. 2015, 54, 12236.
Synthesis of [(iPrPONOP)Mo(CO)3-d1]BF4, 3b-d1. Under a
nitrogen atmosphere, to a 3 mL CH2Cl2 solution of 2b (123 mg,
0.235 mmol) were added HBF4·Et2O (60 μL, 0.235 mmol, 50−55%
w/w in diethyl ether) and 0.2 mL of D2O. The resulting light yellow
solution was stirred at room temperature for 24 h and then dried
under vacuum to afford a light yellow residue. Upon washing with
diethyl ether (∼10 mL), a solid material was obtained, which was
further dried in vacuo to afford 50 mg of 3b-d1 (86% D). 1H, 13C{1H},
and 31P{1H} spectra are similar to those of 3b.
̈
̈
(4) Oztopcu, O.; Holzhacker, C.; Puchberger, M.; Weil, M.; Mereiter,
K.; Veiros, L. F.; Kirchner, K. Organometallics 2013, 32, 3042.
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Kirchner, K. Organometallics 2016, 35, 229.
(6) Mo(CO)3(CH3CN)3 was prepared by refluxing a solution of
Mo(CO)6 in CH3CN for 16 h.
(7) W(CO)3(CH3CN)3 was prepared by refluxing a solution of
W(CO)6 in CH3CN for 72 h.
̈
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metallics 2009, 28, 4791.
ASSOCIATED CONTENT
■
S
* Supporting Information
(9) Bessiere, J. Bull. Soc. Chem. Fr. 1969, 9, 3356.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00461.
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Organomet. Chem. 2002, 663, 45.
NMR, IR, and GC characterization data for compounds
and reactions; VT NMR data and simulations; listing of
crystallographic information including data collection
parameters, bond lengths, fractional atomic coordinates,
and anisotropic thermal parameters for complexes 2a−d
(PDF)
Optimized coordinates (MOL)
Crystallographic data (CCDC deposition nos. 1481786−
1481789) (CIF)
(11) Complex 3d hydride shows similar fluxional behavior in the 1H
NMR.
(12) ΔG⧧ can be estimated at the coalescence temperature using k =
πΔν/21/2 and the Eyring equation. See the Supporting Information for
detailed simulations (Figures S-32, S-33).
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Dordrecht, 2004.
AUTHOR INFORMATION
■
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Corresponding Author
*E-mail: william.jones@rochester.edu.
Author Contributions
‡R. Castro-Rodrigo, S. Chakraborty, and L. Munjanja
contributed equally.
Notes
The authors declare no competing financial interest.
G
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Organometallics
Article
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H
DOI: 10.1021/acs.organomet.6b00461
Organometallics XXXX, XXX, XXX−XXX