Bis(acetylacetonate) tungsten(IV) complexes containing a π-basic diazoalkane or oxo ligand

Article abstract of DOI:10.1021/om201050j

Tungsten(IV) bis(acetylacetonate) (acetylacetonate = acac) complexes were synthesized by incorporating either a diazoalkane or an oxo ligand into the coordination sphere of a tungsten(II) reagent. The reaction of free diazoalkane (N₂CRR′) with

Full text of DOI:10.1021/om201050j

Article
pubs.acs.org/Organometallics
Bis(acetylacetonate) Tungsten(IV) Complexes Containing a π-Basic
Diazoalkane or Oxo Ligand
Chetna Khosla, Andrew B. Jackson, Peter S. White, and Joseph L. Templeton*
W.R. Kenan Laboratory, Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290,
United States
S
* Supporting Information
ABSTRACT: Tungsten(IV) bis(acetylacetonate) (acetylacetonate = acac) complexes were
synthesized by incorporating either a diazoalkane or an oxo ligand into the coordination
sphere of a tungsten(II) reagent. The reaction of free diazoalkane (N2CRR′) with
W(CO)3(acac)2 leads to loss of two carbon monoxide ligands and coordination of the
diazoalkane reagent through the terminal nitrogen to produce W(CO)(acac)2(N2CRR′). This
monomer is best formulated as a tungsten(IV) complex. A second example of converting a d4
tungsten carbonyl complex to a d2 product involves oxidation of W(CO)(acac)2(PhCN)
with m-chloroperoxybenzoic acid (MCPBA) to replace the CO ligand with an oxygen atom.
This increase in metal oxidation state causes rotation of the nitrile ligand by 90° relative to the
two bidentate acac ligands. Electrophilic addition at the nitrogen of the π-bound nitrile ligand
using methyl triflate (MeOTf) and subsequent nucleophilic addition at carbon with sodium
trimethoxyborohydride, Na[HB(OMe)3], reduces the CN bond stereoselectively and
produces the neutral imine complex W(O)(acac)2(PhHCNMe) with a diastereomeric ratio of 11:1.
complexes in our attempts to produce tungsten imido species.18
Additionally, sequential diastereoselective reduction of a
π-bound nitrile to an imine ligand in a tungsten(IV) oxo
coordination sphere is reported.
INTRODUCTION
The reduction of carbon−nitrogen multiple bonds is an
important topic spanning several branches of chemistry. In
■
organometallic chemistry, one example of such a reduction is
seen in the homogeneous hydrogenation of nitriles.1−6
RESULTS AND DISCUSSION
W(IV) Diazoalkane Synthesis. Three coordination modes
are possible for diazoalkane ligands in monomeric metal
■
Stepwise reduction of σ-bound nitriles to amines has been
achieved at a single metal site.7−9 Sequential addition of
nucleophiles and electrophiles to the carbon−nitrogen triple
complexes: side-on coordination through the N−N π-bond or,
more commonly, σ-coordination through either nitrogen or the
carbon.10−13 Many stable σ-coordinated diazoalkane complexes
have been isolated without extrusion of the N2 fragment,
although thermolysis often leads to N2 elimination to form
metallacarbene complexes or decomposition prod-
ucts.10−13,19,20 Previous success promoting η2-coordination of
ligands such as nitriles, ketones, and aldehydes to the
W(CO)(acac)2 moiety led us to ask whether η2-coordination
of diazoalkane ligands might be possible with this d4 fragment.
Addition of 1 equiv of (trimethylsilyl)diazomethane to a
CH2Cl2 solution of W(CO)3(acac)2 leads to a color change
from light brown to green-brown (eq 1). Monitoring the
reaction progress via in situ IR spectroscopy reveals a single CO
absorbance at 1915 cm−1 after 30 min, indicative of loss of 2
bond produces stepwise reduction of the acetonitrile ligand in
[Tp′W(CO)(RCCR′)(NCMe)][BF4].8,9 Reduction of the
carbon−nitrogen bond in diazoalkane ligands (MN−N
CR2) is another example of metal activation of ligand −CN−
moieties toward the addition of electrophiles and/or
nucleophiles.10−13
Reduction of a π-bound nitrile ligand to a π-bound iminium
ligand has been accomplished in a tungsten(II) bis-
(acetylacetonate) system by introducing a variable-electron-
donating alkyne ligand into the coordination sphere.14,15
Replacement of the carbon monoxide ligand in the W(II)
complex [W(CO)(η2-RNCR)(acac)2]+ with a variable
electron donor alkyne ligand relieved the iminoacyl ligand of
its responsibility to function as a four-electron donor to
tungsten. Following addition of a nucleophile to the carbon of
the reduced nitrile the nitrogen lone pair of the η2-imine ligand
is available to electrophilic reagents to yield a π-iminium ligand.
We have described a number of tungsten(II) bis-
(acetylacetonate) complexes with two π-acids in the coordina-
tion sphere.14−17 We now report oxidation of the metal to
generate tungsten(IV) bis(acetylacetonate) d2 complexes with
π-basic diazoalkane or oxo ligands in the coordination sphere.
We have previously reported the formation of tungsten(IV) oxo
1
equiv of carbon monoxide. H NMR spectroscopy indicates
formation of a bis(acac) tungsten species with the presence of a
trimethylsilyl signal as well as a signal for one proton at 8.00
ppm, compatible with formation of W(CO)-
(acac)2(N2CHSiMe3) (1-N2CHSiMe3). This result follows
Received: October 30, 2011
Published: January 17, 2012
© 2012 American Chemical Society
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N18−C19 bond length is 1.30 Å. The bond lengths are
consistent with six-electron donation from the terminal
nitrogen to the metal center via a multiple bond.10,11,13
Two different formalisms, both valid, exist for evaluating the
electron donation of the diazoalkane ligand.13 In the neutral
formalism the ligand bears no charge and donates four
electrons to the metal through two covalent bonds and one
dative bond. In the ionic formalism, the ligand is considered to
be reduced by two electrons to form a hydrazonido moiety with
a −2 charge. The hydrazonido ligand donates six electrons
to the d2 W(IV) metal center through one σ- and two
π-interactions. The diazoalkane complexes reported here are
described using the ionic formalism.
Oxidation of W(II) Alkyne Carbonyl Complexes to
W(IV) Alkyne Oxo Complexes. Preparation of W(IV)
bis(acetylacetonate) complexes with diazoalkane reagents led
us to search for other oxidants that could produce multiply
bound ligands in our tungsten bis(acac) system. For octahedral
group VI complexes, π-conflicts invariably exist between
carbonyl ligands and multiply bound, cylindrically symmetric
carbyne or oxo ligands.22−26 In the case of carbonyl−oxo
complexes, the carbonyl and oxo ligands prefer a cis geometry
to minimize sharing dπ orbitals in this mixed π-acid/π-base
system. In contrast, the use of a single-faced π-acid such as an
olefin or an alkyne allows selective stabilization of a single dπ
orbital to accommodate a π-base and a π-acid in the same
coordination sphere. Therefore, mixing oxo ligands with single-
faced π-acid ligands should be possible in the coordination
sphere of W(IV) d2 bis(acetylacetonate) complexes.
the pattern of reactivity reported by Hillhouse and Haymore in
which addition of diazoalkane to W(CO)3(S2CNR2)2 forms a
monocarbonyl complex with terminally bound diazoalkane.21
A
13C NMR signal at 168 ppm for the diazomethane carbon of
1-N2CHSiMe3 is in the range for end-on coordination of the
terminal nitrogen. The 13C NMR signal is broad due to the
influence of the adjacent quadrupolar nitrogen. 13C NMR
spectroscopy also reveals a lone metal carbonyl signal downfield
at 267 ppm.
Reacting diphenyldiazomethane with W(CO)3(acac)2 pro-
duces W(CO)(acac)2(N2CPh2) (1-N2CPh2), a diphenyl
analogue of 1-N2CHSiMe3 (eq 1), with an infrared absorbance
at 1910 cm−1 for the lone CO ligand. A 13C NMR spectrum of
W(CO)(acac)2(N2CPh2) exhibits a carbon monoxide signal at
269 ppm with tungsten satellites (1JC−W = 209 Hz). The
diazomethane NC carbon signal at 161 ppm also presents
with tungsten satellites (3JC−W = 7 Hz).
Oxidation of metal carbonyl species by a metal oxo transfer
oxidant, O2, or other oxidants is well-known.25−43 Of the group
VI metals, molybdenum−oxo linkages have received consid-
erable attention due to their presence in biological
enzymes.37−40 Tungsten−oxo complexes have been implicated
as both precursors and active species in olefin metathesis.43,44
Additionally, a number of tungstoenzymes have received
attention in recent years.39,40
1-N2CPh2 was concentrated in a solution of methylene
chloride and stored under N2 at −35 °C. Green-brown X-ray
quality crystals formed after four weeks. As anticipated from
13C NMR data, the solid-state structure shows end-on
coordination of the diphenyldiazomethane ligand (Figure 1).
The oxo ligand, O2−, can be described as a closed-shell anion
with a −2 oxidation state.26 Metal−oxo π-bonding can occur
when a metal possesses empty dπ orbitals, hence the prevalence
of oxo complexes with high-valent metal centers. When the oxo
ligand functions as a dianionic ligand and as a six-electron
donor, it can be drawn as a triple bond, as is usually seen for
mono-oxo metal complexes. In this work, our focus will be on
examples illustrating oxygen atom replacement of CO to
produce monomers reflecting simple replacement of CO with
O, a formal two-electron oxidation of the metal.
Previous work has explored similar oxidations in the tungsten
bisdithiocarbmate system in which an oxygen atom transfer
reagent reacts with W(CO)(S2CNR2)2(η2-RCCR) to form
W(O)(S2CNR2)2(η2-RCCR).31 Although pyridine N-oxide
has proven to be a convenient reagent in numerous group 6
oxidation reactions,25,41,42 our efforts to oxidize W(CO)-
(acac)2(η2-PhCCH) with pyridine N-oxide were unsuccess-
ful. However, treatment of W(CO)(acac)2(η2-PhCCH)
(1-PhCCH) with m-chloroperoxybenzoic acid (MCPBA)45,46
in methylene chloride at 0 °C leads to clean formation of the
neutral oxo complex W(O)(acac)2(η2-PhCCH) (2-PhC
CH) (eq 2). IR spectroscopy shows the disappearance of a CO
absorbance at 1900 cm−1 and the appearance of a WO
absorbance at 950 cm−1.
Figure 1. ORTEP diagram (50% ellipsoids) of W(CO)(Ph2CN2)-
(acac)2, 1-N2CPh2. Hydrogen atoms and one molecule of CH2Cl2
solvent are omitted. Selected interatomic distances (Å) and bond
angles (deg): W1−N17 = 1.796(3), N17−N18 = 1.310(4), N18−C19 =
1.302(4), W1−C1 = 2.003(3), W1−O3 = 2.087(2), W1−O7 =
2.065(2), W1−O10 = 2.122(2), W1−O14 = 2.090(2), W1−N17−
N18 = 177.2(2), N17−N18−C19 = 120.8(3), O3−W1−O7 =
84.10(9), O10−W1−O14 = 85.94(9).
The W1−N17−N18 linkage is nearly linear (177°), and the
N17−N18−C19 linkage is bent; the angle of 121° is in accord
with an sp2-hybridized nitrogen. The W1−N17 bond length is
1.80 Å, while the N17−N18 bond length is 1.31 Å and the
1
At room temperature, the H NMR spectrum of 2-PhC
CH shows a mixture of two isomers in a 1.2:1 ratio.
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The isomers stem from the 90° rotation of the terminal alkyne
ligand from the xz plane of the starting materials into the xy
plane upon coordination of the π-basic oxo ligand (the carbonyl
and oxo ligands are sited on the z-axis) to produce two
rotamers. The parent carbonyl complex 1-PhCCH displays a
broad singlet at ∼13.0 ppm at room temperature, indicative of
alkyne rotation on the NMR time scale. In contrast, the
phenylacetylene ligand in the oxo analogue 2-PhCCH does
not freely rotate on the NMR time scale. At room temperature
two sharp singlets are observed for the terminal alkyne proton
at 11.38 and 11.11 ppm, each accompanied by tungsten
satellites (2JW−H = 14 Hz), corresponding to the two distinct
spatial arrangements (along either the +y or the −y axis) of the
terminal acetylenic hydrogen.
The symmetric diphenylacetylene derivative W(O)-
(acac)2(η2-PhCCPh) (2-PhCCPh) was synthesized to
avoid the complications arising from two isomers (eq 2). This
complex is more stable than its phenylacetylene counterpart
with no decomposition observed after exposure to air for 1 h. In
the 13C NMR spectrum of 2-PhCCPh, the two alkyne
carbon signals resonate at 172 and 167 ppm, in the range for a
three-electron donor alkyne.47 As is often the case, the oxo
ligand can be drawn as either doubly or triply bonded to
indicate oxo-to-metal π-bonding. Complexes boasting an alkyne
cis to an oxo ligand have been investigated, and for metal
centers with d2 configurations it is clear that the alkyne π⊥
orbital and a p orbital on the oxo ligand compete for electron
donation into a vacant dπ metal orbital to generate a three-
center four-electron bond.27−34
Figure 2. ORTEP diagram (50% ellipsoids) of W(O)(acac)2(η2-
PhCCPh), 2-PhCCPh. Hydrogen atoms are omitted for clarity.
Selected interatomic distances (Å) and bond angles (deg): W1−O1 =
1.7145(12), W1−C2 = 2.0973(14), W1−C3 = 2.0939(14), W1−O16 =
2.1636(12), W1−O20 = 2.0712(11), W1−O23 = 2.1519(11), W1−
O27 = 2.0671(11), C2−C3 = 1.3075(19), O16−W1−O20 = 81.30(4),
O23−W1−O27 = 82.63(4), C2−W1−C3 = 36.35(5).
variable and complicates simple electron counting, as shown by
our depiction of the tungsten−oxo bond with a dashed bond.
Adding an alkyne ligand provides additional electrons, also
variable, but surely in accord with a full valence shell. Sitting on
the z axis, the π-basic oxo ligand, counted as O2−, donates into
the vacant dxz and dyz orbitals, raising the energies of those
orbitals as they dominate the antibonding molecular orbitals
(Figure 3). Coordination of the alkyne ligand along the x axis
The solid-state structure of 2-PhCCPh shows the alkyne
ligand arranged cis to the newly minted oxo ligand, as in the
analogous CO complex 1-PhCCH (Figure 2).17 Note that
the C−C bond of the alkyne is perpendicular to the W−O
bond of the oxo linkage. This contrasts with the similar
precursor alkyne−CO complex, W(CO)(acac)2(η2-PhC
CH), in which the C−C alkyne bond is oriented parallel to
the W−CO axis of the carbonyl ligand.17 Oxidation from a d4
to a d2 metal center causes rotation of the single-faced π-donor
alkyne ligand by 90°, as has been noted in similar carbonyl-to-
oxo transformations.27−34 The W1−C2 and W1−C3 metal−
alkyne bonds have lengthened to 2.10 and 2.09 Å from 2.04 Å
for the analogous bonds in the precursor complex, indicating
decreased bonding to the alkyne ligand. The C2−C3 bond
lengthens only slightly to 1.308 Å from 1.302 Å, showing
virtually no loss of triple-bond character. Given that the alkyne
provides both σ and π forward donation to the metal and
accepts metal dπ electron donation into π∥*, a decrease in π⊥
donation from the alkyne seems most likely to cause the
lengthening of the M−C bonds. The metal oxygen bond
distance is 1.71 Å, typical for mono-oxo tungsten com-
plexes.26,48
Figure 3. Molecular orbital diagrams for alkyne orientation.33.
stabilizes the two electrons in the dxy orbital via back-bonding
to the alkyne π*∥. Donation from π⊥ on the alkyne ligand
further raises the energy of the dxz orbital as it becomes the
most antibonding of a three-center−four-electron π-bonding
scheme.33 In contrast, if the alkyne ligand is aligned parallel to
the metal−oxo axis, it will act as a π-acceptor for the dxz orbital
and a π-donor for the dxy orbital, resulting in π-conflicts for
both of these dπ orbitals.
η2-Binding of Nitrile Ligand in W(O)(acac)2(η2-PhC
N). Successful oxidation of tungsten carbonyl alkyne complexes
with the W(acac)2(η2-PhCCR) moiety intact led us to
explore oxidation reactions of other W(CO)(acac)2 derivatives.
Treatment of the neutral nitrile−carbonyl complex W(CO)-
(acac)2(η2-PhCN) with MCPBA results in the formation of
W(O)(acac)2(η2-PhCN) (2-PhCN), providing another
example of a η2-nitrile complex (eq 3).49,50 This oxygen atom
Orbital Interactions in W(II) and W(IV) Octahedral
Complexes. Oxidation of the W(CO)(acac)2(η2-RCCR)
system to replace carbon monoxide with an oxo ligand
illustrates the interplay of d electron configuration with ligand
orientation.26−35 The W(O)(acac)2 fragment is unambiguously
a W(IV) d2 fragment. The extent of π-donation from O2− is
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Synthesis of W(IV) Oxo Imine, Aldehyde, and Ketone
Complexes. Oxidation of W(CO)(acac)2(η2-PhHCNPh)
to W(O)(acac)2(η2-PhHCNPh), 2-PhHCNPh, with
MCPBA produces four isomers in a 13:4:2:1 distribution, as
reflected in the 1H NMR spectrum (eq 4). The two
transfer contrasts with an earlier case in which a W(II) η2-nitrile
complex, Tp′WI(η2-NCMe)(CO), reacts with the oxygen atom
transfer reagent pyridine N-oxide to form the acetylimido
complex Tp′WI(NC(O)Me)(CO).51 The H NMR spectrum
1
of 2-PhCN shows two isomers in a 1.7:1 ratio. The 13C
NMR spectrum shows an upfield shift of the nitrile carbon
resonance from 211 ppm to 201 ppm in comparing the W(II)
reagent and the W(IV) product. Only small differences were
noted relative to the starting material and the product in both
diastereomers present in the imine-carbonyl precursor can
rotate clockwise or counterclockwise 90° into the xy plane of
the resultant oxo complex, producing four possible products
(Figure 5). In the 1H NMR spectrum, the imine proton
resonates at 4.49 (major), 4.31, 4.14, and 3.82 ppm in these
four isomers. These values are shifted well upfield from
1
the H and the 13C NMR spectra.
A sample of 2-PhCN was dissolved in CH2Cl2 and layered
with hexanes to produce yellow crystals suitable for X-ray
analysis. The solid-state structure shows the triple bond of the
nitrile ligand in 2-PhCN is arranged cis to the newly formed
oxo ligand as the C−N bond is perpendicular to the W−oxo
linkage (Figure 4). The W1−C3 and W1−N2 bonds have
1
chemical H NMR signals of 6.15 and 5.71 ppm in the parent
CO complex. In the 13C NMR spectrum, the imine carbon
resonances are located at 80.9, 80.5, 77.7 (major), and 76.6
ppm. These values are shifted downfield from the two peaks
observed at 57.3 (major) and 54.3 ppm in the carbonyl
precursor.
Slow evaporation of a solution of 2-PhHCNPh in
CH2Cl2/hexanes produced orange needles suitable for X-ray
diffraction. A solid-state structure of 2-PhHCNPh reveals
the W1−N2 distance is 1.98 Å, elongated from 1.93 Å in the
imine-carbonyl precursor and consistent with loss of double-
bond character of the W−N bond (Figure 6). The W1−C3
bond tightens significantly to 2.17 Å from 2.24 Å in the
precursor. The carbon−nitrogen bond distance is only slightly
shortened to 1.387(5) Å from 1.395(8) Å in the imine-carbonyl
complex. The shorter W−C bond indicates increased sp2
character of the carbon atom, as noted in the 13C NMR
spectrum.
Addition of MCPBA to W(CO)(acac)2(η2-PhHCO)
produces W(O)(acac)2(η2-PhHCO) (2-PhHCO) with
four isomers detected in a 9:6:4:1 ratio by 1H NMR
spectroscopy (eq 4). The aldehyde protons are located in the
4.7−5.7 ppm range, well upfield of the range of 6.9−8.2 ppm
found for the W(II) precursor complex. The aldehyde carbon is
found at 104−108 ppm in the 13C NMR spectrum, compared
to 86 and 89 ppm in the precursor. These spectral results
mirror those seen for the 2-PhHCNPh and suggest
increased sp2 character of the carbon atom of the aldehyde
ligand in the oxidized complex.
Figure 4. ORTEP diagram (50% ellipsoids) of W(O)(acac)2(η2-
PhCN), 2-PhCN. Hydrogen atoms are omitted for clarity.
Selected interatomic distances (Å) and bond angles (deg): W1−C3 =
2.067(8), W1−N2 = 2.103(6), W1−O1 = 1.719(5), W1−O10 =
2.070(5), W1−O14 = 2.074(5), W1−O17 = 2.046(5), W1−O21 =
2.142(5), N2−C3 = 1.217(10), O10−W1−O14 = 83.1(2), O17−
W1−O21 = 83.2(2), N2−W1−C3 = 33.9(3), N2−C3−C4 =
134.2(7).
Oxidation of the carbonyl-acetone complex W(CO)-
(acac)2(η2-Me2CO) produces W(O)(acac)2(η2-Me2CO)
in only two isomers in a 1.1:1 ratio due to the symmetry of the
ketone (eq 4). The acetone ketone carbon resonates at 115 and
116 ppm compared to 97 ppm in the precursor in the 13C
NMR spectrum.
lengthened to 2.07 and 2.10 Å, respectively, from 2.04 and 2.02
Å in the analogous precursor complex W(CO)(acac)2(η2-2,6-
Cl2H3C6CN), mirroring the decrease in π⊥ donation from
the alkyne demonstrated in 2-PhCCPh. The C3−N2 bond
shortens slightly to 1.22 Å from 1.27 Å, showing a small
increase in triple-bond character.
Synthesis of [W(O)(acac)2(η2-PhCNMe)]+. Oxidation
was also attempted with cationic W(II) complexes. Addition of
MCPBA to the iminoacyl complex [W(CO)(acac)2(η2-PhC
NMe)][BAr′4] leads to formation of two W(IV) isomers of [W(O)-
(acac)2(η2-PhCNMe)][BAr′4] ([2-PhCNMe][BAr′4]) in
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Figure 5. Four possible isomers of 2-PhHCNPh.
iminoacyl ligand and then an imine.14,15 This sequential
reduction shows little diastereoselectivity in the formation of
either the imine or the iminium ligand. Reduction of an
iminoacyl to an imine was accomplished with carbon monoxide,
isonitrile, or alkyne as a cis ligand in the coordination sphere.
Addition of methyl triflate (MeOTf) to a solution of 2-PhC
N produces [W(O)(acac)2(η2-PhCNMe)][OTf] (eq 6).
Figure 6. ORTEP diagram (50% ellipsoids) of W(O)(acac)2(η2-
PhHCNPh), 2-PhHCNPh. Nonessential hydrogen atoms are
omitted for clarity. Selected interatomic distances (Å) and bond angles
(deg): W1−N2 = 1.980(3), W1C3 = 2.166(4), W1−O1 = 1.711(3),
W1−O16 = 2.130(2), W1−O20 = 2.068(2), W1−O23 = 2.133(2),
W1−O27 = 2.047(2), N2−C3 = 1.387(5), O16−W1−O20 =
80.96(10), O23−W1−O27 = 82.33(10), C3−W1−N2 = 38.76(12).
This result is similar to the methylation of W(CO)(acac)2(η2-
PhCN).14 Clean products are formed in a 1:1.1 ratio, with
respect to the isomers seen for [2-PhCNMe][BAr′4]. In
contrast to the complex with the BAr′4 counterion, [2-PhC
NMe][OTf] is not sufficiently robust to withstand chromatog-
raphy on silica.
1
a 1.3:1 ratio (eq 5). The H NMR spectrum for [2-PhC
Addition of Na[HB(OMe)3] to [2-PhCNMe][OTf]
produced in situ from methylation of 2-PhCN leads to
formation of W(O)(acac)2(η2-PhHCNMe) (2-PhHC
NMe) (eq 6). The synthesis provides diastereoselective
formation of the imine complex in an 11:1 ratio of the two
major isomers as determined by 1H NMR spectroscopy.
Previous syntheses of the carbonyl analogue, W(CO)-
(acac)2(η2-PhHCNMe) (1-PhHCNMe), produced two
diastereomers in a 2:1 ratio.14 The diastereomeric selectivity
leading to 2-PhHCNMe is surprising given the presence
of two isomers of the cationic iminoacyl reagent coupled with
the low level of diastereoselectivity seen for formation of
1-PhHCNMe.
NMe][BAr′4] indicates similar chemical shifts to those seen in
the precursor W(II) molecule. In the 13C NMR spectrum, the
coordinated iminoacyl carbon has shifted upfield as a result of
oxidation, from 221 ppm to 202 and 207 ppm.
Oxidation of 1-PhHCNMe with MCPBA also produces
2-PhHCNMe. Again, significant diastereoselectivity is
observed with a ratio of 10:1 for the two major isomers. The
imine proton of 2-PhHCNMe resonates at 3.83 ppm, well
upfield of the precursor signal in the 1H NMR spectrum at 5.56
ppm. Additionally, one of the acac methine protons is found
shifted upfield to 4.76 ppm, compared to 5.72 and 5.48 ppm in
Reduction of [W(O)(acac)2(η2-PhCNMe)]+. Successful
isolation of a cationic iminoacyl−oxo complex prompted us to
further investigate addition reactions to nitrile-derived ligands.
Reduction of a π-bound nitrile ligand in 1-PhCN to a π-
bound iminium ligand, a stepwise process, passes through an
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1-PhHCNMe. The methyl bound to nitrogen is shifted
downfield to 4.30 ppm upon metal oxidation from 3.71 ppm in
the W(II) reagent. The imine carbon resonates at 85.1 ppm in
the 13C NMR spectrum, compared to 60.8 ppm in the parent
W(II) complex.
Crystals suitable for X-ray analysis were grown by
evaporation of a hexanes solution under an inert atmosphere
to provide the solid-state structure of neutral 2-PhHCNMe
(Figure 7). The geometric results of oxidation from 1-PhHC
on nitrogen is rotated out of the W−N−C plane and oriented
trans to the imine-phenyl group to minimize unfavorable steric
interactions. The H− nucleophile would presumably prefer to
approach the less sterically hindered face of the iminoacyl
ligand, thus adding from the oxo face in order to avoid the
bulkier acetylacetonate ligand. Finally, there appears to be a
π-interaction between the imine phenyl ring and the adjacent acac
ligand, most likely providing stabilization for the diastereomer
observed in the solid-state structure. This π-interaction places
the arene π-cloud near the acac central carbon and is likely the
cause of the upfield shift to 4.76 ppm of one acac methine
1
signal in the H NMR spectrum.
Similar arguments presumably apply to the orientation of the
imine ligand in 2-PhHCNPh. However, this complex shows
a lower diastereomeric preference. The two rotamers with the
phenyl groups on the imine oriented trans to each other would
presumably be sterically favored. The most abundant isomer is
likely the one observed in the solid-state structure with the
phenyl groups trans to each other and the phenyl group on
carbon participating in a π-interaction with the adjacent
acetylacetonate ligand. The two other isomers are populated
in much lower ratios.
SUMMARY
■
Figure 7. ORTEP diagram of W(O)(acac)2(η2-PhHCNMe) (2-
PhHCNMe), shown with 50% probability thermal ellipsoids.
Nonessential hydrogen atoms are omitted for clarity. Selected
interatomic distances (Å) and bond angles (deg): W1−C4 =
2.1771(19), W1−N3 = 2.0008(16), W1−O1 = 1.7099(13), W1−
O11 = 2.0809(13), W1−O15 = 2.1205(12), W1−O18 = 2.1118, W1−
O22 = 2.0584(13), C4−N3 = 1.384(2), O11−W1−O15 = 81.15(5),
O18−W1−O22 = 83.35(5), C4−W1−N3 = 38.04(7).
Addition of N2CR2 to W(CO)3(acac)2 produces W(CO)-
(acac)2(N2CR2) with the diazoalkane ligand acting as a six-
electron donor bound through the terminal nitrogen to the
tungsten(IV) metal center. Addition of MCPBA to W(CO)-
(acac)2(η2-PhCN) produces the oxygen-atom-transfer
tungsten(IV) product W(O)(acac)2(η2-PhCN). W(O)-
(acac)2(η2-PhCN) reacts with MeOTf to produce the
iminoacyl complex [W(O)(acac)2(η2-PhCNMe)][OTf], a
rare example of methylation of a π-bound nitrile ligand.14,35,52,53
Addition of a nucleophilic hydride reagent results in addition to
the iminoacyl carbon to produce W(O)(acac)2(η2-PhHC
NMe) with a diastereomeric ratio of 11:1, demonstrating
stepwise reduction of a η2-nitrile ligand to an η2-imine in a
stereoselective manner.
NMe are similar to those observed for oxidation of 1-PhHC
NPh. The W1−N3 bond length is 2.00 Å, an increase from 1.91
Å in the analogous 1-PhMeCNMe complex and close to the
value of 1.98 Å for 2-PhHCNPh. The W1−C4 bond
distance is 2.18 Å, decreased from 2.27 Å in 1-PhMeCNMe
and again similar to 2-PhHCNPh with a value of 2.17 Å. The
C4−N3 bond length exhibits virtually no change at 1.384 Å
from 1.383 Å in the similar precursor. These changes in bond
lengths are consistent with the previous examples of metal
oxidation with a heteroatomic single-face π-acid in the
coordination sphere.
The solid-state structure of 2-PhHCNMe provides some
insight into the high diastereomeric ratio observed by NMR
spectroscopy. Surprisingly, the methyl group on the three-
coordinate nitrogen atom is rotated approximately 40° out of
the W−N−C plane (Figure 7). In the analogous precursor
W(II) complex, 1-PhMeCNMe, the N-methyl carbon is
almost coplanar with the W−N−C triangle. This comparison
invites steric or electronic arguments for the deviation from
planarity observed for 2-PhHCNMe. With the imine group
now perpendicular to the W−O axis, the N-methyl group
would sterically conflict with the adjacent acac group if it were
in the W−N−C plane. Additionally, in 1-PhMeCNMe, the
sp2-hybridized nitrogen provides effective lone pair donation
from a nitrogen p orbital to the metal center, totaling donation
of four electrons from the neutral imine ligand. However, the
imine ligand in 2-PhHCNMe is liberated from this stringent
electronic requirement by the addition of the oxo ligand, which
is also capable of lone pair donation to complete the valence
shell of tungsten. Thus it seems logical that the methyl group
EXPERIMENTAL SECTION
■
General Information. All air- and moisture-sensitive materials
were handled using Schlenk or glovebox techniques under a dry
nitrogen or argon atmosphere, respectively. All glassware was oven-
dried before use. Methylene chloride, diethyl ether, and hexanes were
purified by passage through an activated alumina column under a dry
argon atmosphere.54 THF was distilled from a sodium ketal
suspension. Methylene chloride-d2 was dried over CaH2 and degassed.
W(CO)3(acac)2,17 W(CO)(L)(acac)2,16,17 [W(CO)(PhCNMe)-
(acac)2][BAr′4],14 NaBAr′4,55 and diphenyldiazomethane56 were
made in accordance with literature procedures. All other reagents
were purchased from commercial sources and used without further
purification.
NMR spectra were recorded on Bruker DRX 500, DRX400,
AMX400, or AMX300 spectrometers. Infrared spectra were recorded
on an ASI Applied Systems React IR 1000 FT-IR spectrometer.
Elemental analysis was performed by Robertson Microlit, Madison, NJ.
W(CO)(acac)2(N2CHSiMe3) (1-N2CHSiMe3). In a representative
synthesis, W(CO)3(acac)2 (0.808 g, 1.73 mmol) was dissolved in
CH2Cl2. A solution of (trimethylsilyl)diazomethane (2.0 M in hexanes,
0.87 mL, 1.74 mmol) was added, and the solution was stirred for 1.5 h.
As the reaction progressed, a color change from light brown to green
was noted, as well as a shift to a single IR absorbance at 1915 cm−1.
Trituration with hexanes led to isolation of a green powder. Yield:
0.730 g, 1.39 mmol, 80%. Chromatography on silica with CH2Cl2
produced pure material for analysis. IR (CH2Cl2): νCO = 1915 cm−1.
992
dx.doi.org/10.1021/om201050j | Organometallics 2012, 31, 987−994

Organometallics
Article
1H NMR (CD2Cl2, 298 K, δ): 8.00 (s, 1H, N2CHTMS), 5.81, 5.63 (s,
1H, acac CH), 2.62, 2.21, 2.18, 2.00 (s, 3H, acac CH3), 0.21 (s, 9H,
Si(CH3)3). 13C{1H} NMR (CD2Cl2, 298 K, δ): 267.45 (CO), 192.2,
190.5, 190.3, 187.9 (acac CO), 168.1 (N2CHTMS), 102.3, 101.0 (acac
CH), 27.5, 27.3, 26.9, 25.9 (acac CH3), −2.8 (Si(CH3)3). Anal. Calcd
for C15H24N2O5SiW (524.29): C, 34.36; H, 4.61; N, 5.34. Found: C,
34.26; H, 4.39; N, 5.19.
(579.29): C, 47.69; H, 4.35; N, 2.41. Found: C, 47.84; H, 4.54;
N, 2.30.
W(O)(acac)2(η2-PhHCO) (2-PhHCO). 2-PhHCO was
prepared in the same manner as above. Yield: 0.098 g, 0.194 mmol,
1
84%. Ratio of isomers: 9:6:4:1. H NMR (CD2Cl2, 298 K, δ, major
isomer): 6.8−8.1 (C6H5), 5.87, 4.92 (s, 1H, acac CH), 5.72 (s, 1H,
2
OCH, JW−H = 7 Hz), 2.39, 2.20, 2.12, 1.44 (s, 3H, acac CH3).
13C{1H} NMR (CD2Cl2, 298 K, δ, major isomer): 193.2, 190.0, 189.8,
189.3 (acac CO), 125.0−143.3 (C6H5), 104.6, 101.6 (acac CH), 104.1
(CO, 1JW−C = 22 Hz), 28.1, 27.4, 27.3, 25.8 (acac CH3). Anal. Calcd
for C17H20O6W (504.18): C, 40.50; H, 4.00. Found: C, 40.78; H, 3.94.
W(O)(acac)2(η2-Me2CO) (2-Me2CO). 2-Me2CO was pre-
pared in the same manner as above. Yield: 0.054 g, 0.118 mmol, 74%.
W(CO)(acac)2(N2CPh2) (1-N2CPh2). 1-N2CPh2 was prepared in
the same manner as 1-N2CHSiMe3. Yield: 0.148 g, 0.245 mmol, 76%.
Chromatography on silica with CH2Cl2 produced pure material for
analysis. The product was dissolved in a small amount of methylene
chloride and stored at −35 °C to produce green-brown crystals after
four weeks. IR (CH2Cl2): νCO = 1910 cm−1. 1H NMR (CD2Cl2, 298 K,
δ): 7.27−7.68 (m, 10H, C6H5), 5.80, 5.53 (s, 1H, acac CH), 2.66, 2.22,
2.17, 1.90 (s, 3H, acac CH3). 13C{1H} NMR (CD2Cl2, 298 K, δ):
1
Ratio of isomers: 1:1. H NMR (CD2Cl2, 298 K, δ, both isomers):
5.87, 5.78, 5.68, 5.62 (s, 1H, acac CH), 2.99, 2.98, 2.30, 2.03 (s, 3H,
(CH3)2CO) 2.41, 2.36, 2.33, 2.32, 2.13, 2.11, 1.79, 1.78 (s, 3H, acac
CH3). 13C{1H} NMR (CD2Cl2, 298 K, δ, both isomers): 192.9, 191.8,
191.3, 191.1, 190.9, 189.7, 189.6, 188.9 (acac CO), 115.6, 115.2 (C
O), 104.1, 103.9, 103.4, 102.6 (acac CH), 32.1, 31.1, 26.6 (O
C(CH3)2), 28.0, 27.6, 27.5, 27.4, 27.4, 27.1, 26.8, 26.6, 26.0 (acac CH3
and OC(CH3)2). Anal. Calcd for C13H20O6W (456.13): C, 34.23;
H, 4.42. Found: C, 34.50; H, 4.50.
1
268.7 (CO, JC−W = 209 Hz), 191.7, 190.3, 190.1, 187.7 (acac CO),
3
161.4 (N2CPh2, JC−W = 7.3 Hz), 128.0−141.5 (C6H5) 102.2, 100.8
(acac CH), 27.5, 27.4, 27.0, 25.9 (acac CH3). Anal. Calcd for
C24H24N2O5W·CH2Cl2 (689.23): C, 43.57; H, 3.80; N, 4.06. Found:
C, 43.62; H, 4.01; N, 3.84.
W(O)(acac)2(η2-PhCCH) (2-PhCCH). In a representative
synthesis, a Schlenk flask was charged with W(CO)(acac)2(η2-
PhCCH) (1-PhCCH) (0.150 g, 0.293 mmol) and 20 mL of
CH2Cl2. The amber-colored solution was cooled to 0 °C, and 1 equiv
of MCPBA (0.066 g, 0.294 mmol) was added under a backflow of N2.
The solution color changed to yellow, and in situ IR spectroscopy
indicated loss of the lone carbonyl absorbance. The product was
chromatographed on NEt3-treated silica with CH2Cl2 to elute an
orange-yellow band. Yield: 0.093 g, 0.186 mmol, 63%. Ratio of
[W(O)(acac)2(η2-PhCNMe)][OTf] ([2-PhCNMe][OTf]). In an
NMR tube 2-PhCN, CD2Cl2, and MeOTf were combined. The
solution turned green in color. 1H NMR spectroscopy shows complete
conversion to product. Ratio of isomers: 1.1:1. 1H NMR (CD2Cl2, 298
K, δ, major isomer): 7.83−8.40 (C6H5), 6.20, 5.99 (s, 1H, acac CH),
4.27 (s, 3H, N−CH3), 2.55, 2.52, 2.30, 1.90 (s, 3H, acac CH3).
[W(O)(acac)2(η2-PhCNMe)][BAr′4] ([2-PhCNMe][BAr′4]).
To a solution of 1-PhCN (0.100 g, 0.195 mmol) in CH2Cl2 was
added methyl triflate (0.033 mL, 0.292 mmol). The solution was
stirred for 2 h until a single IR absorbance was noted at 1985 cm−1.
NaBAr′4 was added under a backflow of nitrogen to generate 1-PhC
NMe[BAr′4] in situ after 2 h of stirring. The solution was cannula-
filtered to another Schlenk flask and cooled to 0 °C. MCPBA (0.045 g,
0.200 mmol) was added under a backflow of N2. The brown product
was chromatographed on NEt3-treated silica to elute a red-brown
fraction with CH2Cl2. Yield: 0.188 g, 0.136 mmol, 70%. Ratio of
1
isomers: 1.1:1. IR (KBr): νWO = 950 cm−1. H NMR (CD2Cl2, 298
2
K, δ, major isomer): 11.10 (s, 1H, PhCCH, JW−H = 14 Hz), 7.65
(d, 2H, o-C6H5), 7.46 (t, 2H, m-C6H5), 7.36 (t, 1H, p-C6H5), 5.85,
5.32 (s, 1H, acac CH), 2.35, 2.19, 2.14, 1.70 (s, 3H, acac CH3).
13C{1H} NMR (CD2Cl2, 298 K, δ, major isomer): 192.0, 190.2, 189.4,
1
188.3 (acac CO), 175.1 (PhCCH, JW−C = 29 Hz), 160.4 (PhC
1
2
CH, JW−C = 38 Hz), 137.4 (ipso-C6H5, JW−C = 7.2 Hz) 131.7 (o-
C6H5), 129.0 (p-C6H5), 128.6 (m-C6H5), 103.9, 102.5 (acac CH), 27.6,
27.4, 26.5 (acac CH3, 2:1:1). Anal. Calcd for C18H20O5W (500.19): C,
43.22; H, 4.03. Found: C, 42.97; H, 3.78.
1
isomers: 1.3:1. H NMR (CD2Cl2, 298 K, δ, major isomer): 8.14 (d,
2H, o-C6H5), 7.87 (t, 2H, m-C6H5), 7.82 (obscured by BAr′4, 1H, p-
C6H5), 6.22, 5.65 (s, 1H, acac CH), 4.33 (s, 3H, N−CH3), 2.51, 2.44,
2.39, 1.73 (s, 3H, acac CH3). 13C{1H} NMR (CD2Cl2, 298 K, δ, major
isomer): 206.6 (CN), 195.8, 194.1, 191.6, 191.3 (acac CO), 138.1
(ipso-C6H5), 134.0, 130.9 (o/m-C6H5), 125.6 (p-C6H5), 107.6, 105.8
(acac CH), 35.5 (N-CH3), 27.4, 27.1, 26.8, 26.3 (acac CH3).
W(O)(acac)2(η2-PhCCPh) (2-PhCCPh). 2-PhCCPh was
prepared in the same manner as 2-PhCCH. Yield: 0.076 g, 0.132
mmol, 61%. The product was dissolved in CH2Cl2 and layered with
hexanes to produce yellow crystals suitable for X-ray analysis. 1H NMR
(CD2Cl2, 298 K, δ): 7.28−8.10 (m, 10H, C6H5), 5.85, 5.34 (s, 1H,
acac CH), 2.25, 2.16, 2.14, 1.74 (s, 3H, acac CH3). 13C{1H} NMR
(CD2Cl2, 298 K, δ): 192.2, 190.1, 189.3, 187.9 (acac CO), 172.4, 166.9
(PhCCPh), 138.5, 138.0 (ipso-C6H5), 131.1, 130.9 (p-C6H5), 128.5,
128.5, 128.4 (o, m-C6H5), 103.9, 102.9 (acac CH), 27.6, 27.5, 27.1,
26.6 (acac CH3). Anal. Calcd for C24H24O5W (576.28): C, 50.02; H,
4.20. Found: C, 49.79; H, 4.11.
W(O)(acac)2(η2-PhHCNMe) (2-PhHCNMe). Method A. To a
yellow solution of 2-PhCN (0.075 g, 0.150 mmol) in CH2Cl2 was
added MeOTf (0.020 mL, 0.177 mmol). The solution turned pale
green after 5 min and was allowed to stir for 1 h. Solvent was removed
and THF was added. The solution was cooled to −78 °C, and a cooled
solution of Na[HB(OMe)3] was cannula-transferred to the flask. The
solution turned yellow in color and was warmed to room temperature
and stirred for 10 min. The product was chromatographed on silica
with CH2Cl2 to elute a yellow band. Yield: 0.034 g, 0.066 mmol, 44%.
Ratio of isomers: 11:1. Slow evaporation of a solution of 2-PhHC
NMe in hexanes under an inert atmosphere produces yellow crystals
for X-ray analysis. Method B. A red solution of 1-PhHCNMe in
CH2Cl2 was cooled to 0 °C. MCPBA was added under a backflow of
N2 to produce a yellow solution. The solution was warmed to room
temperature, and solvent was removed. The product was chromato-
graphed on NEt3-treated silica with CH2Cl2 to elute a pale yellow
W(O)(acac)2(η2-PhCN) (2-PhCN). 2-PhCN was prepared
in the same manner as above. Yield: 0.104 g, 0.208 mmol, 71%. Ratio
1
of isomers: 1.7:1. H NMR (CD2Cl2, 298 K, δ, major isomer): 7.46−
8.25 (5H, C6H5), 5.92, 5.73 (s, 1H, acac CH), 2.47, 2.38, 2.19, 1.70 (s,
3H, acac CH3). 13C{1H} NMR (CD2Cl2, 298 K, δ, major isomer):
200.5 (CN), 191.7, 190.6, 190.2, 189.4 (acac CO), 129.3−137.2
(C6H5), 104.5, 104.1 (acac CH), 27.6, 27.4, 26.9, 26.5 (acac CH3).
Anal. Calcd for C17H19NO5W (501.18): C, 40.74; H, 3.82; N, 2.79.
Found: C, 40.36; H, 3.76; N, 2.71.
W(O)(acac)2(η2-PhHCNPh) (2-PhHCNPh). 2-PhHCNPh
was prepared in the same manner as above. Yield: 0.072 g, 0.124
mmol, 76%. Ratio of isomers: 13:4:2:1. Slow evaporation of a solution
of 2-PhHCNPh in CH2Cl2/hexanes produced orange needles
suitable for X-ray diffraction. 1H NMR (CD2Cl2, 298 K, δ, major
isomer): 6.8−8.1 (C6H5) 5.85, 4.87 (s, 1H, acac CH), 4.49 (s, 1H,
NCHPh), 2.40, 2.16, 2.13, 1.49 (s, 3H, acac CH3). 13C{1H} NMR
(CD2Cl2, 298 K, δ, major isomer): 193.8, 190.1, 189.3, 187.7 (acac
CO), 121.4−156.7 (C6H5), 104.5, 101.1 (acac CH), 77.7 (CN),
30.0, 28.3, 27.4, 26.8 (acac CH3). Anal. Calcd for C23H25O5NW
1
band. Ratio of isomers: 10:1. H NMR (CD2Cl2, 298 K, δ, major
isomer): 7.16 (t, 2H, m-C6H5), 6.88 (d, 2H, o-C6H5), 6.82 (t, 1H, p-
C6H5), 5.81, 4.76 (s, 1H, acac CH), 4.30 (s, 3H, N−CH3), 3.83 (s, 1H,
PhHCN), 2.36, 2.09, 1.45 (s, 3:6:3H, acac CH3). 13C{1H} NMR
(CD2Cl2, 298 K, δ, major isomer): 193.4, 190.0, 189.3, 188.5 (acac
CO), 144.1, 126.8, 126.3, 126.2 (C6H5), 103.9, 100.4 (acac CH), 85.1
1
(CN, JW−C = 17 Hz), 49.2 (N-CH3), 28.2, 27.4, 27.3, 25.8 (acac
CH3). Anal. Calcd for C18H23NO5W (517.22): C, 41.80; H, 4.48, N,
2.71. Found: C, 42.76; H, 4.61; N, 2.48.
993
dx.doi.org/10.1021/om201050j | Organometallics 2012, 31, 987−994

Organometallics
Article
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ASSOCIATED CONTENT
■
S
* Supporting Information
Full details of the crystal structure analyses for 1-N2CPh2, 2-
PhCCPh, 2-PhCN, 2-PhHCNPh, and 2-PhHC
NMe. NMR spectra for [2-PhCNMe][BAr′4] and 2-
PhHCNMe. This material is available free of charge via the
Internet at http://pubs.acs.org.
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39, 1081.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: joetemp@unc.edu.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation (CHE-0717086)
for financial support.
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