Hemilability of the 1,2-Bis(dimethylphosphino)ethane (dmpe) Ligand in Cp?Mo(NO)(κ2-dmpe)

Article abstract of DOI:10.1021/acs.inorgchem.7b01733

Reaction of Cp?Mo(NO)Cl₂ with 1 equiv of 1,2-bis(dimethylphosphino)ethane (dmpe) in THF at ambient temperature forms [Cp?Mo(NO)(Cl)(κ²-dmpe)]Cl (1), which is isolable as an analytically pure yellow powder in 65% yield. Further addition of 2 equiv of Cp₂Co to 1 in CH₂Cl₂ affords dark red Cp?Mo(NO)(κ²-dmpe) (2), which was isolated in 36% yield by recrystallization from Et₂O at -30 °C. Reaction of a benzene solution of 2 with an equimolar amount of elemental sulfur results in the immediate production of dark blue (μ-S)[Cp?Mo(NO)(κ¹-dmpeS)]₂ (3), which is a rare example of a bimetallic transition-metal complex bridged by only a single sulfur atom and involving Mo=S=Mo bonding. In contrast, reaction of 2 with an excess of sulfur in benzene results in the formation of Cp?Mo(NO)(η²-S₂)(κ¹-dmpeS) (4). Complex 4 can also be formed by the addition of elemental sulfur to 3, thereby indicating that 3 is a precursor to 4. Cp?Mo(NO)(κ²-dmpe) (2) also undergoes interesting transformations when treated with organic bromides. For instance, reaction of 2 with 5 equiv benzyl bromide in THF produces the bimetallic complex (μ-dmpe)[ Cp?Mo(NO)Br₂]₂ (5) and bibenzyl after 4 d at 70 °C probably via radical intermediates. In contrast to its reaction with benzyl bromide, complex 2 forms [Mo(NO)Br₂(κ²-dmpe)]₂ (6), olefin, alkane, and Cp?H when treated with 5 equiv of 1-bromopropane or 1-bromooctane in THF at 70 °C for 72 h. Interestingly, complex 2 does not display any reactivity with bromobenzene or 1-bromoadamantane even after being heated for several days at 70 °C. All new complexes were characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them were established by single-crystal X-ray crystallographic analyses.

Full text of DOI:10.1021/acs.inorgchem.7b01733

Article
pubs.acs.org/IC
Hemilability of the 1,2-Bis(dimethylphosphino)ethane (dmpe) Ligand
in Cp*Mo(NO)(κ2‑dmpe)
Aaron S. Holmes, Brian O. Patrick, Taleah M. Levesque, and Peter Legzdins*
Department of Chemistry, The University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada
S
* Supporting Information
ABSTRACT: Reaction of Cp*Mo(NO)Cl2 with 1 equiv of 1,2-bis(dimethylphosphino)ethane (dmpe) in THF at ambient
temperature forms [Cp*Mo(NO)(Cl)(κ2-dmpe)]Cl (1), which is isolable as an analytically pure yellow powder in 65% yield.
Further addition of 2 equiv of Cp2Co to 1 in CH2Cl2 affords dark red Cp*Mo(NO)(κ2-dmpe) (2), which was isolated in 36%
yield by recrystallization from Et2O at −30 °C. Reaction of a benzene solution of 2 with an equimolar amount of elemental sulfur
results in the immediate production of dark blue (μ-S)[Cp*Mo(NO)(κ1-dmpeS)]2 (3), which is a rare example of a bimetallic
transition-metal complex bridged by only a single sulfur atom and involving MoSMo bonding. In contrast, reaction of 2
with an excess of sulfur in benzene results in the formation of Cp*Mo(NO)(η2-S2)(κ1-dmpeS) (4). Complex 4 can also be
formed by the addition of elemental sulfur to 3, thereby indicating that 3 is a precursor to 4. Cp*Mo(NO)(κ2-dmpe) (2) also
undergoes interesting transformations when treated with organic bromides. For instance, reaction of 2 with 5 equiv benzyl
bromide in THF produces the bimetallic complex (μ-dmpe)[Cp*Mo(NO)Br2]2 (5) and bibenzyl after 4 d at 70 °C probably via
radical intermediates. In contrast to its reaction with benzyl bromide, complex 2 forms [Mo(NO)Br2(κ2-dmpe)]2 (6), olefin,
alkane, and Cp*H when treated with 5 equiv of 1-bromopropane or 1-bromooctane in THF at 70 °C for 72 h. Interestingly,
complex 2 does not display any reactivity with bromobenzene or 1-bromoadamantane even after being heated for several days at
70 °C. All new complexes were characterized by conventional spectroscopic and analytical methods, and the solid-state molecular
structures of most of them were established by single-crystal X-ray crystallographic analyses.
Scheme 1. Lewis-Acid Induced Intramolecular C−H
INTRODUCTION
■
Activation
We previously developed Cp*M(NO)-containing complexes
(Cp* = η5-C5Me5; M = Mo or W) as reagents for the selective
activation and functionalization of unreactive hydrocarbon C−
H bonds.1,2 We recently extended our investigations to
compounds that possess electron-rich metal centers such as
Cp*M(NO)(κ2-dppe) (dppe = Ph2PCH2CH2PPh2) that
should, in principle, facilitate the oxidative addition of
hydrocarbon C−H bonds to them. Interestingly, the Cp*M-
(NO)(κ2-dppe) compounds are rendered prone to effecting
C−H activation reactions not by thermolyses but rather by
treatment with equimolar amounts of appropriate Lewis acids
(LA) which form adducts with the basic O-termini of their
nitrosyl ligands (Scheme 1).3
Regrettably, these systems only promote intramolecular C−
H activations, presumably because steric factors in the metal’s
coordination sphere bring one of the phenyl substituents of the
dppe ligand into a position in which it can readily undergo
ortho-metalation. To overcome this mode of reactivity, we then
focused our attention on the related but less sterically
encumbered Cp*M(NO)(κ2-dmpe) (dmpe = 1,2-bis-
(dimethylphosphino)ethane, Me2PCH2CH2PMe2) complexes.
Regrettably, treatment of the dmpe compounds with Lewis
acids does not result in the desired intermolecular C−H
activations.4 Consequently, we extended our investigations to
Received: July 7, 2017
© XXXX American Chemical Society
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Scheme 2. Two-Step Preparation of Cp*Mo(NO)(κ2-dmpe)
encompass other reactants to establish the characteristic
chemical properties of the Cp*M(NO)(κ2-dmpe) complexes.
In this article, we report on the novel reactions of one of them,
namely Cp*Mo(NO)(κ2-dmpe), with elemental sulfur and
organic halides. These reactants were chosen for initial
examination because they are known to exhibit a variety of
different reaction modes with transition-metal complexes.
exhibited by related species.6 The spectroscopic properties of
1 are fully consistent with the [Cp*Mo(NO)(κ2-dmpe)Cl]+
1
cation retaining this structure in solution. Thus, the H NMR
spectrum of 1 in CD2Cl2 (Figure S1) exhibits signals due to
four inequivalent methyl and methylene groups and a singlet at
δ 2.00 ppm corresponding to the Cp* ligand. Consistently, its
31P{1H} NMR spectrum in the same solvent (Figure S3) shows
2
two doublets at δ 37.4 and 44.9 ppm with JPP = 45.6 Hz.
Step 2: Reduction of [Cp*Mo(NO)(κ2-dmpe)Cl]Cl (1)
with 2 equiv of Cp2Co affords dark red Cp*Mo(NO)(κ2-
dmpe) (2) which was isolated as an analytically pure solid in
36% overall yield from Cp*Mo(NO)Cl2 by recrystallization
RESULTS AND DISCUSSION
■
Two-Step Preparation of Cp*Mo(NO)(κ2-dmpe). Step
1: As summarized in Scheme 2, treatment of a green THF
solution of Cp*Mo(NO)Cl25 with 1 equiv of dmpe at ambient
temperatures results in a ligand-substitution reaction and the
production of [Cp*Mo(NO)(κ2-dmpe)Cl]Cl (1), which can
be isolated as a yellow powder in 65% yield. Recrystallization of
this powder from CH2Cl2/hexanes at room temperature affords
single crystals of the salt as a dihydrate that were subjected to
an X-ray diffraction analysis. The crystal structure of 1·2H2O
consists of the packing of discrete cations and anions along with
the two water molecules with the solid-state molecular structure
of the cation (shown in Figure 1) being that of a four-legged
piano-stool having metrical parameters similar to those
1
from Et2O at −30 °C. The H, 13C{1H}, and 31P{1H} NMR
spectra of 2 in C6D6 solutions confirm the three-legged piano-
stool molecular structure shown for the complex in Scheme 2.
For instance, its 1H NMR spectrum in C6D6 (Figure S5) shows
two sets of doublets that belong to two of the P(CH3) groups
that are directed toward and two away from the Cp* ring. As
expected, the 31P{1H} NMR spectrum (Figure S7) shows a
singlet at δ 57.1 ppm. The IR spectrum of 2 exhibits a strong
ν(NO) absorption at 1535 cm−1, which is a manifestation of
the significant electron density extant at the Mo(0) center that
results in turn in considerable Mo → NO backbonding. The
solid-state molecular structure of 2 was confirmed by a single-
crystal X-ray diffraction analysis and is shown in Figure 2. As
expected, its metrical parameters are similar to those exhibited
by Cp*Mo(NO)(κ2-dppe).3
The presence of the electron-rich metal center in Cp*Mo-
(NO)(κ2-dmpe) led us to investigate its reactivity with typical
oxidants such as elemental sulfur and alkyl bromides to
delineate its characteristic chemical properties. The reactions
with sulfur are described first.
Reaction of Cp*Mo(NO)(κ2-dmpe) with 1/8 S8:
Preparation of (μ-S)[Cp*Mo(NO)(κ1-dmpeS)]2 (3). As
summarized in Scheme 3, reaction of a benzene solution of
Cp*Mo(NO)(κ2-dmpe) with an equimolar amount of
elemental sulfur results in an oxidation reaction and the
immediate production of the Mo(II) complex, (μ-S)[Cp*Mo-
(NO)(κ1-dmpeS)]2 (3), which was isolated as a dark blue solid
in 48% yield following column chromatography on basic
alumina. Approximately one-half of the initial reactant 2 can be
recovered from the final reaction mixture, thereby indicating
that the formation of 3, a 3:2 S:Mo bimetallic complex, is
favored over that of other molybdenum complexes incorporat-
ing one or two sulfur atoms per metal center. The solid-state
molecular structure of 3 is shown in Figure 3, and it confirms
that one phosphorus end of the dmpe ligand in 2 was displaced
from the molybdenum’s coordination sphere and converted
into a phosphine sulfide. In addition, the resulting 16e
Cp*Mo(NO)(κ1-dmpeS) fragment is linked to another of its
kind by a MoSMo bridge that allows each metal center to
attain the favored 18e configuration.7 The molybdenum−sulfur
bond length of 2.2965(4) Å is intermediate between those
reported for MoS (∼2.10−2.20 Å) and Mo−S (∼2.35−2.45
Figure 1. Solid-state molecular structure of [Cp*Mo(NO)(κ2-
dmpe)Cl]Cl (1) as its dihydrate with 50% probability thermal
ellipsoids shown. Most hydrogen atoms were omitted for the sake of
clarity. Selected bond lengths (Å) and angles (deg): Mo(1)−P(1) =
2.5447(5), Mo(1)−P(2) 2.5537(5), Mo(1)−Cl(1) = 2.4785(5),
Mo(1)−N(1) = 1.7889(18), N(1)−O(1) = 1.201(2), Mo(1)−
N(1)−O(1) = 171.82(16), P(1)−Mo(1)−P(2) = 74.840(17),
P(2)−Mo(1)−Cl(1) = 72.186(16), Cl(1)−Mo(1)−N(1) =
91.76(6), N(1)−Mo(1)−P(1) = 80.89(5).
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Figure 3. Solid-state molecular structure of (μ-S)[Cp*Mo(NO)(κ1-
dmpeS)]2 (3) with 50% probability thermal ellipsoids shown.
Hydrogen atoms were omitted for the sake of clarity. Only one-half
of the bimetallic molecule was fully labeled for the sake of clarity
because the two halves are symmetry related. Selected bond lengths
(Å) and angles (deg): Mo(1)−S(2) = 2.2965(4), Mo(1)−P(1) =
2.4550(8), Mo(1)−N(1) = 1.781(3), N(1)−O(1) = 1.226(3), P(1)−
C(11) = 1.820(3), P(1)−C(12) = 1.827(4), P(1)−C(15) = 1.832(3),
P(2)−C(13) = 1.793(4), P(2)−C(14) = 1.798(4), P(2)−C(16) =
1.816(3), P(2)−S(1) = 1.9647(12), C(15)−C(16) = 1.531(4),
Mo(1)−N(1)−O(1) = 172.6(3), S(2)−Mo(1)−P(1) = 91.57(2),
P(1)−Mo(1)−N(1) = 87.98(8), N(1)−Mo(1)−S(2) = 105.68(9),
Mo(1)−S(2)−Mo(1′) = 151.93(5).
Figure 2. Solid-state molecular structure of Cp*Mo(NO)(κ2-dmpe)
(2) with 50% probability thermal ellipsoids shown. Hydrogen atoms
were omitted for the sake of clarity. Selected bond lengths (Å) and
angles (deg): Mo(1)−P(1) = 2.3884(4), Mo(1)−P(2) = 2.3866(4),
Mo(1)−N(1) = 1.7864(11), N(1)−O(1) = 1.2243(14), P(1)−C(11)
= 1.8284(14), P(1)−C(12) = 1.8270(14), P(1)−C(13) = 1.8439(14),
P(2)−C(14) = 1.8556(13), P(2)−C(15) = 1.8217(14), P(2)−C(16)
= 1.8287(14), C(13)−C(14) = 1.5288(19), Mo(1)−N(1)−O(1) =
177.33(11), N(1)−Mo(1)−P(1) = 87.45(4), N(1)−Mo(1)−P(2) =
87.31(4), P(1)−Mo(1)−P(2) = 79.813(13).
Scheme 3. Reaction of Cp*Mo(NO)(κ2-dmpe) with an
Equimolar Amount of S8
an excess of elemental sulfur affords the novel Mo(II) complex
Cp*Mo(NO)(η2-S2)(κ1-dmpeS) (4), which can be isolated in
reasonable yields as a red solid following column chromatog-
raphy on basic alumina (Scheme 4). The 1H and 31P{1H} NMR
spectroscopic properties of C6D6 solutions of 4 (Figures S12
and S14) are fully consistent with it possessing the monomeric
structure shown in Scheme 4.
Scheme 4. Reaction of Cp*Mo(NO)(κ2-dmpe) with an
Excess of S8
Å) linkages,8−11 and there is no Mo−Mo bond because the
separation of 4.4559(10) Å precludes a significant interaction
between the metal centers.11,12 In addition, the PS distance
of 1.9647(12) Å is as expected for such a bond.13 In the solid
state, the two halves of 3 are crystallographically equivalent, and
the molecule belongs to the C2 point group (Figure 3).
The spectroscopic properties of 3 in solutions are completely
consistent with the molecular structure depicted in Figure 3.
Small red crystals of 4 suitable for an X-ray diffraction
analysis were obtained by vapor diffusion of Et2O into a C6H6
solution of the complex over 3 d in an inert atmosphere at
room temperature. The solid-state molecular structure of 4 is
that of a four-legged piano stool (Figure 4). The Mo(1)−S(1)
and Mo(1)−S(2) linkages are both single bonds, as indicated
by their bond lengths of 2.450(5) and 2.459(4) Å, respectively,
and the S(1)−S(2) linkage in the η2-S2 ligand has a bond length
of 2.059(4) Å, as expected for a S−S single bond.14,15 Finally,
complex 4 has a Mo(1)−P(1) bond length of 2.4696(13) Å,
1
Thus, its H and 31P{1H} NMR spectra in C6D6 (Figures S8
and S10, respectively) exhibit signals resulting from the two
different PMe2 and CH2 environments. In addition, the
nitrosyl-stretching frequency of 1562 cm−1 in its IR spectrum
again indicates the presence of significant electron density at
the two molybdenum centers.
Reaction of Cp*Mo(NO)(κ2-dmpe) with an Excess of
S8: Preparation of Cp*Mo(NO)(η2-S2)(κ1-dmpeS) (4).
Exposure of a benzene solution of Cp*Mo(NO)(κ2-dmpe) to
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rather incorporates another sulfur atom on the way to forming
the MoSMo bridge in (μ-S)[Cp*Mo(NO)(κ1-dmpeS)]2
(3). The last step to form Cp*Mo(NO)(η2-S2)(κ1-dmpeS) (4)
involves cleavage of the MoSMo bridge by additional
sulfur atoms with concomitant formation of the η2-S2 ligand at
each molybdenum so that the favored 18e configuration can be
maintained.
Reaction of Cp*Mo(NO)(κ2-dmpe) with an Excess of
O2. Exposure of an Et2O solution of 2 to an oxygen atmosphere
results in the formation of a yellow precipitate which has only
been partially characterized because all attempts to purify it by
chromatography or crystallization have been unsuccessful to
date. Nevertheless, NMR and MS data are consistent with the
yellow solid being formed by the uptake of either 1 or 2 equiv
of oxygen by the initial reactant. For example, the 31P{1H}
NMR spectrum of the yellow solid in C6D6 (Figure S17)
exhibits two doublets at chemical shifts comparable to those in
the analogous spectra of 3 and 4, thereby indicating the
formation of an organometallic complex in which the two
phosphorus atoms in the possibly modified dmpe ligand are no
longer chemically equivalent.
Reaction of Cp*Mo(NO)(κ2-dmpe) with Benzyl Bro-
mide. Oxidation of 2 with 5 equiv of benzyl bromide in THF
results in the formation of the bimetallic Mo(II) complex (μ-
dmpe)[Cp*Mo(NO)Br2]2 (5) and bibenzyl after 4 d at 70 °C
(Scheme 5). During the course of the reaction, the initial
orange solution darkens and deposits a molybdenum-
containing beige precipitate whose identity remains unknown.
The bibenzyl product can be isolated in 71% yield by flash silica
column chromatography with hexanes as the eluant. A small
amount of X-ray quality orange crystals of 5 was obtained on
only one occasion by slow cooling of the reaction mixture, but
other attempts to purify 5 from the THF supernatant solution
by column chromatography or crystallization were unsuccessful
to date. For instance, addition of pentane to the THF
supernatant solution results in the immediate deposition of
the beige precipitate that is initially formed during the reaction.
It thus appears that 5 is a thermally unstable intermediate
complex that does not persist in solution but slowly converts to
the as yet uncharacterized organometallic precipitate.
The formation of bibenzyl suggests that a possible
mechanism for the reaction shown in Scheme 5 involves
radical species. Many examples of reactions involving the
homolytic cleavage of alkyl−halide bonds and atom-transfer
processes resulting in the formation of alkyl radicals and metal−
halides are described in the literature.17−22 For instance, the
anionic pincer complex, [PCP-Pt0]− [PCP = C6H3(CH2Pt-
Bu2)2], reacts with benzyl chloride in THF and forms PCP-
Figure 4. Solid-state molecular structure of Cp*Mo(NO)(η2-S2)(κ1-
dmpeS) (4) with 50% probability thermal ellipsoids shown. Hydrogen
atoms were omitted for the sake of clarity. Selected bond lengths (Å)
and angles (deg): Mo(1)−S(1) = 2.450(5), Mo(1)−S(2) = 2.459(4),
Mo(1)−P(1) = 2.4696(13), Mo(1)−N(1) = 1.825(5), N(1)−O(1) =
1.172(6), S(1)−S(2) = 2.059(4), P(1)−C(11) = 1.843(8), P(1)−
C(12) = 1.797(7), P(1)−C(13) = 1.834(5), C(13)−C(14) =
1.523(7), P(2)−S(3) = 1.962(2), P(2)−C(14) = 1.807(5), P(2)−
C(15) = 1.817(6), P(2)−C(16) = 1.792(5), Mo(1)−N(1)−O(1) =
170.1(5), S(1)−Mo(1)−S(2) = 49.61(9), S(2)−Mo(1)−P(1) =
76.00(8), P(1)−Mo(1)−N(1) = 82.20(14), N(1)−Mo(1)−S(1) =
95.55(19), S(1)−S(2)−Mo(1) = 64.98(16), S(2)−S(1)−Mo(1) =
65.41(18).
which is similar to that extant in 3 and an essentially linear
Mo(1)−N(1)−O(1) linkage of 170.1(5)°.
Because treatment of 3 with additional sulfur also results in
the production of 4, it is likely that the oxidation of 2 by
elemental sulfur proceeds in a stepwise manner. In the first
step, the sulfur effects the oxidation of one end of the dmpe
ligand to transiently form [Cp*Mo(NO)(κ2-dmpeS)], which
contains the hemilabile dmpeS ligand.16 Dissociation of the
sulfur donor site from the molybdenum’s coordination sphere
then generates the 16e [Cp*Mo(NO)(κ1-dmpeS)] fragment.
Interestingly, this fragment does not activate the benzene
solvent to form 18e Cp*Mo(NO)(κ1-dmpeS)(H)(Ph) but
Scheme 5. Reaction of Cp*Mo(NO)(κ2-dmpe) with Benzyl Bromide
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PtCl and bibenzyl.23 Nevertheless, the detailed mechanism for
the conversion shown in Scheme 5 remains to be ascertained.
The solid-state molecular structure of 5 is shown in Figure 5.
The two halves of the bimetallic complex are related by a center
precipitate and the formation of a pale yellow supernatant
solution after 72 h at 70 °C. The crystalline product can be
isolated in 69% yield by decanting away the supernatant
solution and washing the crystals with THF. The crystals are of
sufficient quality for a single-crystal X-ray diffraction analysis
that has revealed them to be the Mo(II) complex [Mo(NO)-
Br2(κ2-dmpe)]2 (6). The oxidation reaction shown in Scheme 6
involves the loss of the Cp* ligand and the incorporation of two
bromine atoms per metal center from the 1-bromopropane
reactant. A molybdenum−molybdenum single bond is invoked
in 6 to allow both metal centers to attain the favored 18e
configuration
All spectroscopic evidence is consistent with 6 having the
structure depicted in Scheme 6. The bimetallic complex
exhibits a ν(NO) band in its IR spectrum at 1619 cm−1 with
a shoulder at 1577 cm−1 attributable to the asymmetric stretch
of the nitrosyl ligands. The complex is completely insoluble in
most NMR solvents and is only sparingly soluble in CD2Cl2.
1
The H, 13C{1H}, and 31P{1H} NMR spectra of 6 in CD2Cl2
are as expected and indicate that the nuclei in the two κ2-dmpe
1
ligands are in chemically equivalent environments. The H
NMR spectrum (Figure S18) consists of four resonances
attributable to the two groups of methyl protons and the two
groups of methylene protons that are either proximal or distal
to the nitrosyl ligands. Consistently, the 31P{1H} NMR
spectrum (Figure S20) displays a lone singlet at δ 35.4 ppm.
1H NMR spectroscopic and GCMS analyses of the
supernatant solution indicate that the organic portions of the
reactants in the reaction involving 1-bromopropane are
converted into Cp*H, propene, and propane. Regrettably, the
amounts of propane and propene cannot be quantified by
GCMS because they coelute from the GC column and overlap
in the chromatogram. Furthermore, due to both the volatility of
propane and propene as well as the presence of unreacted 1-
Figure 5. Major disordered fragment of the solid-state molecular
structure of 5 with 50% probability thermal ellipsoids shown.
Hydrogen atoms were omitted, and only one-half of the molecule
was fully labeled for the sake of clarity. Selected bond lengths (Å) and
angles (deg): Mo(1)−Br(1) = 2.6068(6), Mo(1)−Br(2) = 2.6014(8),
Mo(1)−P(1) = 2.492(7), Mo(1)−N(1) = 1.806(8), N(1)−O(1) =
1.144(9), P(1)−C(1) = 1.810(9), P(1)−C(2) = 1.841(8), P(1)−C(3)
= 1.837(7), C(3)−C(3)′ = 1.532(15), Br(1)−Mo(1)−Br(2) =
80.79(3), Br(2)−Mo(1)−P(1) = 74.39(18), P(1)−Mo(1)−N(1) =
82.8(3), N(1)−Mo(1)−Br(1) = 82.08(19), Mo(1)−N(1)−O(1) =
165.5(7).
1
bromopropane that dominates certain regions in the H NMR
spectrum of the final reaction mixture, NMR analysis is also
unable to quantify the relative amounts of the organic products.
A notable feature of the transformation depicted in Scheme 6
is the loss of the Cp* ligand from 2. The Cp* ligand is
ubiquitous in transition-metal organometallic chemistry, and
when coordinated to a transition-metal center, it often remains
as a spectator ligand. On rare occasions it can be a participant in
the chemistry exhibited by its molecular scaffolds while
remaining coordinated to the metal center. For instance,
under a variety of experimental conditions, the C−H bond
activation of a methyl group in coordinated Cp* ligands was
shown to result in the formation of either η2:η4 or η1:η5
tetramethylfulvene-like ligands via H atom abstraction.24
Recently, it has also been reported that treatment of 18e
Cp*Rh(bpy) [bpy = κ2-2,2′-bipyridine] with a protonic acid
initially affords the 16e pentamethylcyclopentadiene-containing
cation, [(η4-Cp*H)Rh(bpy)]+, which is believed to result from
the C−H bond-forming reductive elimination from the
transient rhodium hydrido complex, [Cp*Rh(bpy)H]+.25,26
of inversion, and both molybdenum centers are in four-legged
piano-stool geometries capped by Cp* rings. A single dmpe
ligand forms the bridge between the two metal centers. The
nitrosyl ligands are essentially linear with Mo−N−O bond
angles of 165.5(7)°, and the Mo−P bond lengths of 2.492(7) Å
are comparable to those found in related complexes (vide
supra).
Reaction of Cp*Mo(NO)(κ2-dmpe) with 1-Bromopro-
pane. A THF solution of 2 containing 5 equiv of 1-
bromopropane results in the deposition of a red crystalline
Scheme 6. Balanced Reactions of Cp*Mo(NO)(κ2-dmpe) with 1-Bromopropane and 1-Bromooctane
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Figure 6. Major component of the solid-state molecular structure of 6 with 50% probability thermal ellipsoids shown. Hydrogen atoms were omitted,
and only one-half of the molecule was fully labeled for the sake of clarity. Selected bond lengths (Å) and angles (deg): Mo(1)−Mo(1)′ =
2.9819(10), Mo(1)−Br(1) = 2.6315(19), Mo(1)−Br(2) = 2.5847(9), Mo(1)−Br(2)′ = 2.5871(9), Mo(1)−P(1) = 2.4790(17), Mo(1)−P(2) =
2.4825(17), Mo(1)−N(1) = 1.776(13), N(1)−O(1) = 1.35(2), P(1)−C(1) = 1.811(7), P(1)−C(2) = 1.801(7), P(1)−C(3) = 1.824(7), P(2)−
C(4) = 1.834(7), P(2)−C(5) = 1.804(7), P(2)−C(6) = 1.821(7), C(3)−C(4) = 1.525(10), Mo(1)−N(1)−O(1) = 175.2(13), Mo(1)−Br(2)−
Mo(1)′ = 70.42(3), Mo(1)′−Mo(1)−N(1) = 94.0(3), Mo(1)′−Mo(1)−Br(1) = 98.64(4), P(1)−Mo(1)−P(2) = 80.53(6), Br(2)−Mo(1)−Br(2)′
= 109.58(3), P(1)−Mo(1)−Br(2) = 85.58(4), P(2)−Mo(1)−Br(2)′ = 84.35(4).
Figure 7. Expansion of the 1H NMR spectrum (400 MHz, C6D6) from δ 2.14 to 6.44 ppm showing the resonances attributable to the two methine
protons of trans-2-octene and the single nonmethyl proton of pentamethylcyclopentadiene and their relative integrations.
Even rarer are the cases in which Cp*H is liberated as the free
liberation of Cp*H (i.e., whether it involves the reductive
elimination of a hydrido and the Cp* ligand from the transition
metal’s coordination sphere or radical processes wherein
hydrogen radicals couple with the Cp* rings, or some other
mechanism entirely) remains to be ascertained.
The solid-state molecular structure of complex 6 was
established by a single-crystal X-ray diffraction analysis, and it
is shown in Figure 6. The structure is that of a bimetallic
complex in which the two halves of the molecule are related by
an inversion center, and the two molybdenum centers are in a
pseudo-octahedral coordination geometry. Two bromine atoms
form bridging linkages between the molybdenum centers with
Mo−Br bond lengths that are essentially the same. A metal−
diene, having been previously reported for the solvent-induced
27
elimination of Cp*H from [Cp*Ir(H)3(PPh3)]+ and the
phosphine-induced liberation from Cp*Rh(H)2(PMe3).28
During the conversion shown in Scheme 6, the Cp* ligand is
not only a participant in the characteristic reactivity of 2 but is
subsequently liberated from the transition metal’s coordination
sphere as free Cp*H. This aspect is unique for Cp*Mo(NO)-
(κ2-dmpe) (2) because, during the chemistry exhibited by other
Cp*M(NO)-containing complexes (M = Mo or W), the 14e
fragment remains intact throughout all the chemical trans-
formations.1 The exact mechanism of the reaction of 2 with
bromoalkanes such as 1-bromopropane that results in the
F
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Article
metal single bond exists in the structure with the Mo−Mo bond
length of 2.9819(10) Å, being comparable to other such
linkages described in the literature.12,29,30 Finally, the nitrosyl
ligand is part of a disordered system and shares space with the
much larger Br atom, a fact that results in a distortion of the
N−O bond length.
CONCLUSION
■
This study demonstrated that hemilability of the symmetrically
bound dmpe ligand in Cp*Mo(NO)(κ2-dmpe) (2) can be
induced by exposing the electron-rich complex to oxidizing
conditions. Thus, reactions of a benzene solution of 2 with
either an equimolar amount or an excess of elemental sulfur
afford (μ-S)[Cp*Mo(NO)(κ1-dmpeS)]2 (3) and Cp*Mo-
(NO)(η2-S2)(κ1-dmpeS) (4), respectively. In both 3 and 4,
one phosphorus end of the dmpe ligand in 2 was displaced
from the molybdenum’s coordination sphere and converted
into a phosphine sulfide linkage. In a related reaction, exposure
of 2 to 5 equiv benzyl bromide in THF produces the bimetallic
complex (μ-dmpe)[Cp*Mo(NO)Br2]2 (5), in which the dmpe
ligand links the two metal centers. In contrast, complex 2 forms
[Mo(NO)Br2(κ2-dmpe)]2 (6), olefin, alkane, and Cp*H when
treated with 5 equiv of 1-bromopropane, 1-bromooctane, or
1,5-dibromopentane in THF at 70 °C. The mechanisms of
these latter transformations as well as other aspects of the
characteristic chemical properties of Cp*Mo(NO)(κ2-dmpe)
remain to be investigated.
Reaction of Cp*Mo(NO)(κ2-dmpe) with 1-Bromooc-
tane. 1-Bromooctane was used as the alkyl halide reactant
instead of 1-bromopropane to produce less volatile organic
products that are easier to quantify as compared to those
resulting from the reaction with 1-bromopropane. As expected,
reaction of 5 equiv of 1-bromooctane with 2 in a THF solution
at 70 °C affords a pale yellow supernatant solution and red
crystals of [Mo(NO)Br2(κ2-dmpe)]2 (6) after 16 h (Scheme 6).
1
GCMS and H NMR analyses of the supernatant solution
indicate the principal organic products of the reaction to be
Cp*H, octane and trans-2-octene with trace amounts of 1-
octene and cis-2-octene. The 1H NMR spectrum of the
supernatant solution (Figure 7) also shows that trans-2-octene
and Cp*H are present in a 3:2 ratio, respectively. As depicted
in Scheme 6, the experimental data thus indicates that the
reaction involves two equiv of 2 and four equiv of 1-
bromooctane being transformed into one equiv of the dimeric
complex 6 accompanied by three equiv of trans-2-octene and 1
equiv of octane. The mechanism of this remarkable conversion
remains to be established.
EXPERIMENTAL SECTION
■
General Methods. All reactions and subsequent manipulations
involving organometallic reagents were performed under anhydrous
and anaerobic conditions except where noted. High-vacuum and inert-
atmosphere techniques were performed using double-manifold
Schlenk lines or in Innovative Technologies LabMaster 100 and
MS-130 BG dual-station gloveboxes equipped with freezers main-
tained at −30 °C. Preparative scale reactions were performed with
Schlenk or round-bottom flasks; reactions were performed in thick-
walled glass reaction flasks (larger scale) or J. Young NMR tubes
(smaller scale), both of which were typically sealed with Kontes
greaseless stopcocks. Tetrahydrofuran (THF) and diethyl ether
(Et2O) were dried over sodium/benzophenone ketyl and freshly
distilled prior to use; n-pentane was dried over calcium hydride and
freshly distilled prior to use; all other solvents were dried according to
standard procedures.31 Cp*Mo(NO)Cl2 was prepared according to
the published procedure.5 Pentamethylcyclopentadiene was obtained
from the Boulder Scientific Co. All other chemicals and reagents were
ordered from commercial suppliers and used as received.
Reaction of Cp*Mo(NO)(κ2-dmpe) with 1,5-Dibromo-
pentane. Given the chemistry exhibited by complex 2 with
monobrominated alkanes, investigations into its reactions with
terminal dibromoalkanes were initiated. Treatment of Cp*Mo-
(NO)(κ2-dmpe) with an excess of 1,5-dibromopentane under
the same experimental conditions as previous reactions gives
the expected outcome, namely the loss of the Cp* ligand as
Cp*H, formation of the dimeric complex 6, 1-bromopentane,
and a bromoalkene. It appears that when a large excess of 1,5-
dibromopentane is employed, one brominated terminus of the
starting alkane does not undergo any reactivity, and therefore
the reaction proceeds in the same manner as the mono-
brominated alkanes (Scheme 6). The exact identity of the
alkene formed is not known with certainty because its analyses
by GCMS and NMR spectroscopy have not given consistent
results.
In an attempt to induce cyclization of 1,5-dibromopentane,
its reaction with 2 was repeated but with a stoichiometric
amount of the dibromoalkane. Unfortunately, the reaction is
not straightforward, and multiple reaction pathways appear to
be occurring. Thus, in addition to the pathway resulting in loss
of Cp*H and formation of 6 and its associated organic products
(cf. Scheme 6), the reaction pathway described in the reaction
of 2 with benzyl bromide (Scheme 5) also occurs, and
decomposition products are formed. In addition, GCMS and
1H NMR data suggest the presence of 2-pentene and
cyclopentene in the final reaction mixture.
Unless otherwise specified, all IR samples were prepared as Nujol
mulls sandwiched between NaCl plates, and their spectra were
recorded on a Thermo Nicolet model 4700 FT-IR spectrometer.
Except where noted, all NMR spectra were recorded at room
temperature on a Bruker AV-400 instrument (direct and indirect
probes), and all chemical shifts are reported in ppm and coupling
1
constants in Hz. H NMR spectra were referenced to the residual
protio isotopomer present in C6D6 (7.16 ppm) or CD2Cl2 (5.32 ppm).
13C NMR spectra were referenced to C6D6 (128.39 ppm) or CD2Cl2
(53.84 ppm). 31P NMR spectra were externally referenced to 85%
H3PO4. For the characterization of most complexes, two-dimensional
NMR experiments, {1H−1H} COSY, {1H−13C} HSQC, {1H−31P}
HMBC, and {1H−13C} HMBC were performed to correlate and assign
1H, 13C, and 31P NMR signals and establish atom connectivity. GC
analyses were performed on an Agilent Technologies 7890B GC
equipped with a HP-5MS (30 m × 0.250 mm × 0.25 μm) capillary
column coupled to an Agilent Technologies 5977A MSD analyzer.
Low- and high-resolution mass spectra (EI, 70 eV) and MALDI-TOF
spectra were recorded by M. Lapawa of the UBC mass spectrometry
facility using a Kratos MS-50 spectrometer and a Bruker Autoflex
spectrometer, respectively. M. Yung recorded ESI mass spectra on a
Bruker HCT spectrometer. Elemental analyses were performed by D.
Smith, and GC analyses were performed by Dr. Y. Ling and M.
Lapawa in the UBC microanalytical facility. X-ray crystallographic data
collection, solution, and refinement were performed at the UBC X-ray
crystallography facility.
Attempted Reactions of Cp*Mo(NO)(κ2-dmpe) with
Other Hydrocarbyl Halides. Interestingly, complex 2 does
not display any reactivity with bromobenzene or 1-
bromoadamantane even after heating for several days at 70
°C. Similar experiments of 2 with 1-chloroalkanes also produce
no detectable changes, whereas 1-iodoalkanes under similar
reaction conditions produce intractable mixtures of products.
G
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Article
Preparation of [Cp*Mo(NO)(κ2-dmpe)Cl]Cl (1). In a glovebox, a
Schlenk flask was loaded with Cp*Mo(NO)Cl2 (0.200 g, 0.602 mmol)
and a stir bar. Also in the glovebox, another Schlenk flask was charged
with dmpe (0.10 mL, 0.60 mmol). On a double manifold, THF (ca. 50
mL) was cannulated into both flasks, affording a bright green solution
and a colorless solution, respectively. The dmpe solution was then
transferred by cannula into the flask containing the stirred Cp*Mo-
(NO)Cl2 solution, whereupon the color of the reaction mixture
changed from green to yellow, and a yellow precipitate formed. After
10 min, the solvent was decanted, and the remaining solid was washed
with THF (2 × 20 mL) before being taken to dryness in vacuo to
obtain [Cp*Mo(NO)(Cl)(κ2-dmpe)]Cl (1) as a fine yellow powder
(0.189 g, 0.392 mmol, 65% yield). Recrystallization of 1 from 1:1
CH2Cl2/hexanes in an aerobic environment afforded small yellow
crystals of [Cp*Mo(NO)(Cl)(κ2-dmpe)]Cl·2H2O that were subjected
to an X-ray diffraction analysis.
Preparation of (μ-S)[Cp*Mo(NO)(κ1-dmpeS)]2 (3). In a glove-
box, a thick-walled flask was charged with Cp*Mo(NO)(κ2-dmpe)
(0.084 g, 0.205 mmol), C6H6 (ca. 20 mL), and a magnetic stir bar. On
a double manifold, elemental sulfur (0.007 g, 0.218 mmol) was added
to the vessel, whereupon the stirred orange solution immediately
developed a brown coloration, and a precipitate formed. The solvent
was removed in vacuo to obtain a blue power that was purified by
column chromatography over basic alumina (1 × 4 cm) made up in
Et2O. Elution of the column with 1:1 THF:Et2O developed a dark blue
band that was eluted and collected. Removal of solvents from the
eluate under reduced pressure afforded (μ-S)[Cp*Mo(NO)(κ1-
dmpeS)]2 (3) as a dark blue solid (0.032 g, 0.035 mmol, 48%
yield). Recrystallization of 3 from a 1:10 pentane:benzene solution left
at ambient temperatures for 15 d afforded small blue crystals of
sufficient quality for a single-crystal X-ray diffraction analysis.
Characterization data for [Cp*Mo(NO)(Cl)(κ2-dmpe)]Cl (1): IR
(cm−1): 1661 (s, ν(NO)). MALDI-TOF (dtcb/CDCl3, m/z): 448.2
for C16H31ClNOP2Mo ([M − Cl]•+, 98Mo). HRMS-ESI(+) (40 V, m/
z): ([M − Cl]•+, 92Mo) calcd for C16H31ClNOP292Mo, 442.0638;
Characterization data for (μ-S)[Cp*Mo(NO)(κ1-dmpeS)]2 (3): IR
(cm−1): 1562 (s, νNO). ESI(+)-MS (40 V); m/z 445.1 for
C16H31NOP2SMo, (M+, 98Mo). HRMS-ESI(+) (40 V, m/z): ([M +
H]+, 92Mo) calcd for C16H32NOP2S92Mo, 440.0748; found, 440.0748.
1H NMR (400 MHz, C6D6): δ 1.07 (d, 2JHP = 7.9, 3H, PAMe), 1.36 (d,
1
2
found, 442.0633. H NMR (400 MHz, CD2Cl2): δ 1.75 (d, JHP
=
2
2
10.4, 3H, PMe), 1.81 (d, JHP = 10.8, 3H, PMe), 1.84 (d, JHP = 10.6,
3H, PMe), 2.00 (s, 15H, C5Me5), 2.03−2.11 (overlapped m, 1H,
2
PCH2CH2P), 2.19 (d, JHP = 10.0, 3H, PMe), 2.40−2.49 (m, 1H,
PCH2CH2P), 2.81−2.94 (overlapped m, 2H, PCH2CH2P). 13C APT
2JHP = 12.9, 3H, PBMe), 1.37 (d, 2JHP = 8.1, 3H, PAMe), 1.39 (d, 2JHP
=
1
NMR (100 MHz, CD2Cl2): δ 10.6 (d, JC1P = 28.8, PMe), 11.4
1
12.8, 3H, PBMe), 1.87 (s, 15H, C5Me5), 1.88 (overlapped m, 1H,
PACH2), 2.02 (m, 1H, PBCH2), 2.19 (m, 1H, PACH2), 2.37 (m, 1H,
PBCH2). 13C APT NMR (100 MHz, C6D6): δ 11.6 (C5Me5), 14.4 (d,
(C5Me5), 12.9 (d, JCP = 28.0, PMe), 15.4 (d, JCP = 21.5, PMe), 17.6
1
1
2
(d, JCP = 36.8, PMe), 25.4 (dd, JCP = 27.6, JCP = 8.4, PCH2CH2P),
1
2
33.7 (dd, JCP = 36.4, JCP = 14.6, PCH2CH2P), 117.2 (C5Me5).
1
1
1JCP = 27.9, PAMe), 15.8 (d, JCP = 23.9, PAMe), 20.4 (d, JCP = 53.3,
31P{1H} NMR (162 MHz, CD2Cl2): δ 37.4 (d, 2JPP = 45.6, MoP), 44.9
PBMe), 21.6 (d, 1JCP = 55.5, PBMe), 25.4 (d, 1JCP = 26.0, PACH2), 30.0
2
(d, JPP = 45.6, MoP). Anal. Calcd for C16H31Cl2NOP2Mo: C, 39.85;
(d, JCP = 51.5, PBCH2), 107.9 (C5Me5). 31P{1H} NMR (162 MHz,
1
H, 6.48; N, 2.90. Found: C, 39.77; H, 6.58; N, 2.62.
3
3
Preparation of Cp*Mo(NO)(κ2-dmpe) (2). In a glovebox, a
Schlenk flask was charged with Cp*Mo(NO)Cl2 (1.002 g, 3.017
mmol) and a stir bar, a second Schlenk flask with dmpe (0.51 mL, 3.0
mmol), and a third Schlenk flask with Cp2Co (1.142 g, 6.038 mmol).
The Cp*Mo(NO)Cl2 and dmpe were reacted in the manner described
in the preceding section to obtain [Cp*Mo(NO)(Cl)(dmpe)]Cl (1)
as a yellow powder. CH2Cl2 (ca. 100 mL) was then cannulated into
the flask to obtain a clear yellow solution. The Cp2Co was dissolved in
CH2Cl2 (ca. 30 mL), and this solution was then added dropwise to the
yellow solution, whereupon it became black. The reaction mixture was
stirred for 30 min to obtain a clear, dark red supernatant solution that
was removed by filter cannulation. Removal of the solvent from the
filtered solution in vacuo and recrystallization of the residue from Et2O
afforded Cp*Mo(NO)(κ2-dmpe) (2) as dark red crystals (0.449 g,
1.092 mmol, 36% yield).
C6D6): δ 22.7 (d, JPP = 47.6, MoPA), 39.2 (d, JPP = 47.6, SPB).
Preparation of Cp*Mo(NO)(η2-S2)(κ1-dmpeS) (4). In a glove-
box, a thick-walled flask was charged with Cp*Mo(NO)(κ2-dmpe)
(0.164 g, 0.400 mmol), C6H6 (ca. 30 mL), and a magnetic stir bar to
obtain a bright orange solution. To this solution was added an excess
of elemental sulfur (0.064 g, 2.00 mmol) on a double manifold
whereupon the reaction mixture became a dark red-brown in color.
The solvent was removed in vacuo, and the remaining brown residue
was purified by column chromatography on a basic alumina column (1
× 4 cm) made up in Et2O. Elution of the column with 1:1 THF:Et2O
developed a single red band that was eluted and collected. Removal of
solvents from the eluate under reduced pressure afforded Cp*Mo-
(NO)(η2-S2)(κ1-dmpeS) (4) as a red solid (0.083 g, 0.163 mmol, 41%
yield). Small red crystals of 4 suitable for an X-ray diffraction analysis
were obtained by vapor diffusion of Et2O into a C6H6 solution of the
complex over 3 d at ambient temperatures.
Characterization data for Cp*Mo(NO)(κ2-dmpe) (2). IR (cm−1):
1535 (s, νNO). MS (LREI m/z, probe temperature 150 °C) 413 [M+,
98Mo]. MS (HREI, m/z, 92Mo): calcd 407.09449, found 407.09490.
1H NMR (400 MHz, C6D6): δ 0.92 (d, 2JHP = 6.5, 6H, PMe), 1.07 (m,
Characterization data for Cp*Mo(NO)(η2-S2)(κ1-dmpeS) (4): IR
(cm−1): 1593 (s, νNO). MS (LREI, m/z, probe temperature 150 °C):
509 [M+, 98Mo], 445 [M+ − S2, 98Mo]. MS (HREI, m/z, 92Mo): calcd
2
2H, CH), 1.29 (m, 2H, CH), 1.39 (d, JHP = 8.4, 6H, PMe), 1.95 (s,
15H, C5Me5). 13C APT NMR (100 MHz, C6D6): δ 12.2 (C5Me5), 16.7
(m, PMe), 19.2 (m, PMe), 31.9 (m, PCH2), 101.2 (C5Me5). 31P{1H}
NMR (162 MHz, C6D6): δ 57.1 (s, MoP). Anal. Calcd for
C16H31MoNOP2: C, 46.72; H, 7.60; N, 3.41. Found: C, 46.80; H,
7.37; N, 3.19.
1
503.01112, found 503.01123. H NMR (400 MHz, C6D6): δ 1.03 (d,
2JHP = 8.8, 3H, PAMe), 1.07 (d, 2JHP = 8.8, 3H, PAMe), 1.12 (d, 2JHP
=
2
12.7, 3H, PBMe), 1.17 (d, JHP = 12.7, 3H, PBMe), 1.62 (s, 15H,
H
DOI: 10.1021/acs.inorgchem.7b01733
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Article
C5Me5), 1.71 (m, 1H, PBCH2), 1.82 (m, 1H, PBCH2), 2.27 (m, 1H,
PACH2), 2.35 (m, 1H, PACH2). 13C APT NMR (100 MHz, C6D6): δ
10.7 (C5Me5), 12.5 (d, 1JCP = 26.0, PAMe), 13.4 (d, 1JCP = 28.7, PAMe),
20.4 (d, 1JCP = 54.0, PBMe), 21.2 (d, 1JCP = 54.9, PBMe), 28.5 (dd, 1JCP
reaction vessel were of suitable quality for a single-crystal X-ray
diffraction analysis.
2
1
2
= 35.6, JCP = 3.0, PACH2), 29.5 (dd, JCP = 50.8, JCP = 0.9, PB3CH2),
110.8 (C5Me5). 31P{1H} NMR (162 MHz, C6D6): δ 21.8 (d, JPP
48.5, MoPA), 37.9 (d, JPP = 49.5, SPB).
=
3
Characterization data for [Mo(NO)(Br)2(dmpe)]2. IR (cm−1):
1619 (w, ν(NO)), 1577 (sh, w, ν(NO)). MALDI-TOF (LDI, m/z):
843.7 for C12H32Br4Mo2N2O2P4 ([M − NO]•+), 792.7 for
C12H32Br3Mo2N2O2P4 ([M − Br] +). HRMS-ESI(+) m/z: ([M −
Br]+, 92Mo, 81Br) calcd for C12H32Br3N2O2P492Mo2, 780.7106; found,
Reaction of Cp*Mo(NO)(κ2-dmpe) (2) with Excess O2. In a
glovebox, a thick-walled flask was charged with Cp*Mo(NO)(κ2-
dmpe) (0.191 g, 0.464 mmol), Et2O (ca. 20 mL), and a magnetic stir
bar to form a bright orange solution. Immediately upon the
introduction of 15 psig of O2 into this flask outside the glovebox,
the solution became brown and a yellow precipitate deposited. The
solvent was removed from the final mixture under reduced pressure,
and the remaining solid was washed with Et2O (6 × 10 mL) before
being taken to dryness in vacuo to obtain 0.123 g of a yellow solid.
Further purification of this solid by column chromatography or
recrystallization from mixtures of Et2O, n-pentane, THF, and C6H6 has
not been successful to date.
1
2
780.7101. H NMR (400 MHz, CD2Cl2): δ 1.84 (d, JHP = 9.6, 6H,
2
PMe), 2.06 (d, JHP = 10.8, 6H, PMe), 2.09−2.25 (m, 4H, PCH2),
2.34−2.56 (m, 4H, PCH2). 13C{1H} HMBC NMR (100 MHz,
CD2Cl2): δ 12.8 (m, PMe3), 17.0 (m, PMe3), 27.2 (m, PCH2CH2P).
31P{1H} NMR (162 MHz, CD2Cl2): δ 35.4 (s, MoP). Anal. Calcd for
C12H32Br4N2O2P4Mo2: C, 16.53; H, 3.70; N, 3.21. Found: C, 18.75;
H, 4.08; N, 3.04. A more satisfactory elemental analysis of the complex
has not yet been obtained.
Partial characterization data for the yellow solid: ESI(+)-MS (40 V,
m/z): 445.1 for C16H31MoNO3P2 ([Cp*Mo(NO)(dmpe) + O2]•+,
98Mo), 429.2 for C16H31MoNO2P2 ([Cp*Mo(NO)(dmpe) + O]•+,
98Mo). 1H NMR (400 MHz, C6D6): δ 1.04 (d, 2JHP = 12.6, 3H, PMe),
1.06 (d, 2JHP = 13.5, 3H, PMe), 1.11 (d, 2JHP = 9.8, 3H, PMe), 1.21 (d,
2JHP = 9.4, 3H, PMe), 1.72 (s, 15H, C5Me5). 31P{1H} NMR (162 MHz,
C6D6): δ 24.3 (d, JPP = 44.6, MoP), 37.8 (d, JPP = 44.6, MoP).
Reaction of Cp*Mo(NO)(κ2-dmpe) (2) with Benzyl Bromide.
In a glovebox, a flask was charged with 2 (0.076 g, 0.185 mmol) and
THF (ca. 10 mL), resulting in a clear bright orange solution. On a
double manifold, benzyl bromide (0.110 mL, 0.924 mmol) was added
to the reaction flask via a micropipette. No immediate color change
occurred. The flask and its contents were maintained at 70 °C for 4 d,
after which time a beige precipitate had deposited, and the supernatant
solution had become dark orange in color. Slow cooling of the
supernatant solution to ambient temperatures resulted in the
formation of red crystals on the sides of the flask. Analysis of these
red crystals determined their identity to be (μ-dmpe)[Cp*Mo(NO)-
(Br)2]2 (5). Unfortunately, characterization of the complex was not
possible because multiple attempts at repeating the reaction did not
result in crystallization of the complex, and purification methods
attempted on the supernatant solution resulted in the deposition of
more of the beige precipitate that formed initially in the reaction
vessel. The isolation of bibenzyl was carried out from a separate
reaction wherein complex 2 (0.076 g, 0.185 mmol) and benzyl
bromide (0.110 mL, 0.924 mmol) in THF were reacted in a manner
identical to that described above. The supernatant solution was
decanted into a separate flask, and the THF solvent was removed in
vacuo. Column chromatography on flash silica of the remaining
residue with hexanes as eluate afforded bibenzyl as a white crystalline
solid (0.024 g, 0.132 mmol, 71% yield).
Partial characterization data for Cp*H: GC−MS m/z (% relative
intensity, ion): 136 (95, M+), 121 (100, [M − CH3]+), 105 (88, [M −
1
C2H7]+), 93 (71, [M − C3H7]+), 79 (79, [M − C4H9]+). H NMR
3
(400 MHz, C6D6): δ 0.99 (d, JHH3 = 7.63, 3H, CH3), 1.74 (s, 6H,
CH3), 1.79 (s, 6H, CH3), 2.42 (q, JHH = 7.63, 1H, CH). These data
match spectroscopic data obtained from an authentic sample of pure
Cp*H in C6D6.
Reaction of Cp*Mo(NO)(κ2-dmpe) (2) with 1-Bromooctane.
In a glovebox, a thick-walled flask was charged with complex 2 (0.101
g, 0.246 mmol) and THF (ca. 10 mL), resulting in a clear bright
orange solution. On a double-manifold Schlenk line, 1-bromooctane
(0.212 mL, 1.227 mmol) was added into the reaction flask via a
micropipette. No color change occurred. The flask was then heated to
70 °C, and the temperature was maintained for 16 h, after which time
large dark red crystals had deposited around the sides of the flask, and
the supernatant solution had become pale yellow in color. The
supernatant solution was decanted from the flask, and the red crystals
remaining were washed with THF (3 × 1 mL) before being dried in
vacuo. [Cp*Mo(NO)Br2(κ2-dmpe)]2 was collected as a red crystalline
solid (0.062 g, 0.071 mmol, 58% yield).
Reaction of Cp*Mo(NO)(κ2-dmpe) (2) with 5 equiv 1,5-
Dibromopentane. In a glovebox, a flask was charged with
Cp*Mo(NO)(κ2-dmpe) (0.102 g, 0.248 mmol) and THF (ca. 8
mL) to obtain a clear, bright orange solution. On a double manifold,
1,5-dibromopentane (0.169 mL, 1.24 mmol) was added to the flask via
a micropipette. The stirred contents of the flask were heated at 70 °C
for 3 d, whereupon a red, shard-like precipitate deposited on the walls
of the flask and the solution lightened to a pale yellow color. The
solution was decanted from the flask, and the precipitate was washed
with THF until the washes became colorless (5 × 2 mL). The
precipitate was isolated by the removal of solvent in vacuo to obtain
complex 6 as a fine red powder (0.081 g, 0.093 mmol, 75% yield).
GCMS and 1H NMR spectroscopic analyses of the supernatant
solution indicated the presence of Cp*H, 1-bromopentane, and a
bromopentene.
Partial characterization data for bibenzyl: MS (LREI, m/z, probe
1
temperature 150 °C): 182 [M+], 91 [M+ − CH2Ph]. H NMR (400
MHz, CDCl3): δ 2.91 (s, 4H, CH2), 7.17−7.29 (m, 10H, aryl H).
These data match previously reported spectroscopic data.32
Reaction of Cp*Mo(NO)(κ2-dmpe) (2) with 1-Bromopro-
pane. In a glovebox, a thick-walled flask was charged with complex 2
(0.101 g, 0.246 mmol) and THF (ca. 10 mL), resulting in a clear dark
orange solution. On a double-manifold Schlenk line, 1-bromopropane
(0.112 mL, 1.23 mmol) was added to the reaction flask using a
micropipette. No color change occurred. The flask was heated to 70
°C, and the temperature was maintained for 4 d, after which time large
dark red crystals had deposited from the now pale yellow solution. The
supernatant solution was decanted from the flask, and the red crystals
remaining were washed with THF (3 × 1 mL) before being dried in
vacuo. [Mo(NO)(Br)2(dmpe)]2 (6) was collected as a red crystalline
solid (0.074 g, 0.085 mmol, 69% yield). The red crystals from the
Reaction of Cp*Mo(NO)(κ2-dmpe) (2) with 1 Equiv of 1,5-
Dibromopentane. In a glovebox, a flask was charged with
Cp*Mo(NO)(κ2-dmpe) (0.162 g, 0.393 mmol) and THF (ca. 10
mL) to obtain a clear, bright orange solution. On a double manifold,
1,5-dibromopentane (0.054 mL, 0.39 mmol) was added to the flask via
a micropipette. The stirred contents of the flask were heated at 70 °C
for 3 d, whereupon the solution turned an opaque brown, and a fine
red precipitate of 6 deposited on the walls of the flask. GCMS and 1H
NMR spectroscopic analyses of the supernatant solution indicated the
presence of 2-pentene and cyclopentene in addition to the products
I
DOI: 10.1021/acs.inorgchem.7b01733
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
identified from the reaction of 2 with 5 equiv of 1,5-dibromopentane
described in the preceding paragraph.
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diffraction analyses are presented in the Supporting Information.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01733.
■
S
1H NMR, 1H{31P}, and 31P{1H} NMR spectra of
representative complexes and full details of the single-
crystal X-ray diffraction analyses of the reported
complexes (PDF)
(9) Kawaguchi, H.; Yamada, K.; Lang, J.-P.; Tatsumi, K. A New Entry
into Molybdenum/Tungsten Sulfur Chemistry: Synthesis and
Reactions of Mononuclear Sulfido Complexes of Pentamethylcyclo-
pentadienyl-Molybdenum(VI) and -Tungsten(VI). J. Am. Chem. Soc.
1997, 119, 10346−10358.
Accession Codes
CCDC 1559147−1559152 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
(10) Ito, J.; Ohki, Y.; Iwata, M.; Tatsumi, K. Trithio-Chloro
Molybdate [MoClS3]−: A Versatile Precursor for Molybdenum
Trisulfido Complexes. Inorg. Chem. 2008, 47, 3763−3771.
(11) Adams, H.; Bancroft, M. N.; Morris, M. J.; Riddiough, A. E.
Sequential Construction of One, Two, or Three Dithiolene Ligands
from Alkynes and Sulfur in Dinuclear Cyclopentadienyl Molybdenum
Complexes. Inorg. Chem. 2009, 48, 9557−9566.
AUTHOR INFORMATION
Corresponding Author
*E-mail: legzdins@chem.ubc.ca.
■
(12) Lewis, J. Metal-Metal Interaction in Transition-Metal
Complexes. Pure Appl. Chem. 1965, 10, 11−36.
(13) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. Tables of Bond Lengths Determined by X-Ray and
Neutron Diffraction. Part 1. Bond Lengths in Organic Compounds. J.
Chem. Soc., Perkin Trans. 2 1987, S1−S19.
ORCID
Peter Legzdins: 0000-0002-3914-3768
Notes
The authors declare no competing financial interest.
(14) Steudel, R. Properties of Sulfur-Sulfur Bonds. Angew. Chem., Int.
Ed. Engl. 1975, 14, 655−664.
(15) Muller, A.; Jaegermann, W.; Enemark, J. H. Disulfur Complexes.
̈
ACKNOWLEDGMENTS
Coord. Chem. Rev. 1982, 46, 245−280 and references cited therein.
(16) This mode of reactivity parallels that recently reported for bis-
phosphine palladium precatalysts for which mono-oxidation of the bis-
phosphine ligand is critical for the formation of the active catalyst that
contains a hemilabile κ2-P-O ligand. See: Ji, Y.; Plata, R. E.; Regens, C.
S.; Hay, M.; Schmidt, M.; Razler, T.; Qiu, Y.; Geng, P.; Hsiao, Y.;
Rosner, T.; Eastgate, M. D.; Blackmond, D. G. Mono-Oxidation of
Bidentate Bis-phosphines in Catalyst Activation: Kinetic and
Mechanistic Studies of a Pd/Xantphos-Catalyzed C-H Functionaliza-
tion. J. Am. Chem. Soc. 2015, 137, 13272−13281.
■
We are grateful to The Dow Chemical Company for financial
support of our work and to our Dow colleague Devon
Rosenfeld for assistance and helpful comments. We also thank
Russell Wakeham, Rex Handford, and Monica Shree for their
contributions during the early stages of these investigations and
Judy Wrinskelle for her invaluable technical assistance. We are
also grateful to the reviewers for their helpful suggestions and
comments.
(17) Kochi, J. K.; Powers, J. W. Mechanism of Reduction of Alkyl
Halides by Chromium(II) Complexes. Alkylchromium Species as
Intermediates. J. Am. Chem. Soc. 1970, 92, 137−146.
(18) Dykerman, B. A.; Smith, J. J.; McCarvill, E. M.; Gallant, A. J.;
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M. Synthesis of chromium(III) bis(benzamidinate) complexes via
single electron oxidation. J. Organomet. Chem. 2007, 692, 3183−3190.
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reactivity and radical polymerization processes. Angew. Chem., Int. Ed.
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