Bis-tropolonate complexes of tungsten: Scaffolds for selective side-on binding of nitriles, imines and ketones

Article abstract of DOI:10.1039/c4dt00734d

The chiral bis-tropolonate tungsten(ii) tricarbonyl compound, (trop) ₂W(CO)₃ (1), has been synthesized and structurally characterized. This seven-coordinate compound readily loses two carbonyl ligands to preferentially bind a series of π-bonding substrates to form six-coordinate complexes of the type (trop)₂W(CO)(L). Alkynes coordinate strongly to form (trop)₂W(CO)(η²-RCCR) (2) in which spectroscopic data is consistent with the alkyne serving as a 4-electron donor. Compound 1 will also preferentially coordinate organic nitriles in a side-on fashion through the CN triple bond. A dramatic shift in the nitrile carbon signals to greater than 210 ppm in the ¹³C NMR confirms the nitriles are coordinated in an η² 4-electron donating capacity. Aldehydes, ketones, and imines also react with 1 to form 4-electron donor η² adducts. The imine adduct (trop) ₂W(CO)(η²-MeNC(H)(tol)) (5) was characterized crystallographically and the short 1.91 A W-N bond distance supports the postulation of 4-electron donation from the imine through CN π-bonding and N lone pair donation. Side-on coordination of ligands of this type is rare and may provide a means towards asymmetric functionalization of these substrates. All of the tropolonate compounds are prone to oxidation in air and the alkyne compounds will oxidize to stable W^IV oxo alkyne species, (trop) ₂W(O)(η²-RCCR) (6). This causes a 90° rotation of the alkyne ligand and a reduction in alkyne donation to approximately 3 electrons, to maintain an optimal 18 electron configuration.

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Bis-tropolonate complexes of tungsten: scaffolds
for selective side-on binding of nitriles, imines and
ketones†
Cite this: Dalton Trans., 2014, 43,
8738
Joseph Becica,a Andrew B. Jackson,a William G. Dougherty,b W. Scott Kasselb and
Nathan M. West*a
The chiral bis-tropolonate tungsten(II) tricarbonyl compound, (trop)2W(CO)3 (1), has been synthesized and
structurally characterized. This seven-coordinate compound readily loses two carbonyl ligands to prefer-
entially bind a series of π-bonding substrates to form six-coordinate complexes of the type (trop)2W(CO)(L).
Alkynes coordinate strongly to form (trop)2W(CO)(η2-RCCR) (2) in which spectroscopic data is consistent
with the alkyne serving as a 4-electron donor. Compound 1 will also preferentially coordinate organic
nitriles in a side-on fashion through the CN triple bond. A dramatic shift in the nitrile carbon signals to
greater than 210 ppm in the 13C NMR confirms the nitriles are coordinated in an η2 4-electron donating
capacity. Aldehydes, ketones, and imines also react with 1 to form 4-electron donor η2 adducts. The
imine adduct (trop)2W(CO)(η2-MeNvC(H)(tol)) (5) was characterized crystallographically and the short
1.91 A W–N bond distance supports the postulation of 4-electron donation from the imine through CvN
π-bonding and N lone pair donation. Side-on coordination of ligands of this type is rare and may provide
a means towards asymmetric functionalization of these substrates. All of the tropolonate compounds are
prone to oxidation in air and the alkyne compounds will oxidize to stable WIV oxo alkyne species, (trop)2-
W(O)(η2-RCCR) (6). This causes a 90° rotation of the alkyne ligand and a reduction in alkyne donation to
approximately 3 electrons, to maintain an optimal 18 electron configuration.
Received 11th March 2014,
Accepted 16th April 2014
DOI: 10.1039/c4dt00734d
www.rsc.org/dalton
noting the greater π-acceptor capability of the nitrile due to the
electronegative N-atom.5 A variety of reactions at the η2-nitrile
Introduction
Coordination compounds containing four-electron donating moiety have been reported,6 including cleavage of the C–CN
alkynes are well known,1 however compounds featuring a four- bond,5,7 oxidation at the nitrile C-atom,4,8 and electrophilic
electron donor with a heteroatom are uncommon. The HOMO addition to the lone pair on the N-atom.2d,3a,b,9
of an organic nitrile is the nitrogen lone pair, and the vast
Heteroatom-containing unsaturated substrates, such as
majority of nitriles coordinate to a metal atom through this ketones and imines can also bond in an η2-fashion, but again
lone pair. However, there are limited examples of side-on this is an uncommon coordination mode.9c,10 Ketones and
coordination to transition metals,2 including some that have imines can also donate 4-electrons to electron deficient metal
been structurally characterized.3 Not all of these side-on centers, yet in a different fashion than that of the η2-nitrile
nitriles are 4-electron donors as that is not a requirement for and η2-alkyne. The filled CvX (X = O, NR) π orbital coordi-
this coordination mode, rather they serve as π-acid ligands sta- nates to the metal as would a traditional alkene. A lone pair on
bilizing electron-rich metal centers. The binding mode of the heteroatom is capable of donating two electrons into an
η2-nitriles has been compared to η2-alkynes,4 and the elec- empty dπ orbital of the metal atom. For this interaction to
tronic differences of these ligands have also been highlighted, occur, it is predicted that the CvX double bond is completely
reduced to a single bond to allow the rotation of the hetero-
atom lone pair to be pointed towards the metal center (Fig. 1).
aDepartment of Chemistry & Biochemistry, University of the Sciences, 600 S 43rd St,
Philadelphia, PA 19104, USA. E-mail: n.west@usciences.edu; Fax: +1-215-596-8543;
Tel: +1-215-596-7251
Studies by Jackson suggest a high degree of double bond char-
acter between the metal atom and the heteroatom of these
substrates when η2-bound.3c The fourteen-electron (acac)2-
WII(CO) (acac = acetylacetonate) scaffold has been proven to be
effective for coordinating a variety of nitriles, imines, and
ketones in an η2-fashion.3a–d,11
bDepartment of Chemistry, Villanova University, 800 E. Lancaster Avenue, Villanova,
PA 19085, USA
†CCDC 989124–989126 for 1, 2b, and 5. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4dt00734d
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fragment and alkynes, nitriles, imines, and ketones in an
η2-fashion. The parent complex, (trop)2WII(CO)3, releases
2 equivalents of CO upon reaction with an alkyne, nitrile, imine,
or ketone (L) and binds the unsaturated substrate in an η2
fashion to form complexes of the type (trop)2W(CO)(η2-L). The
complexes presented here are sensitive to air oxidation, and
the alkyne complexes readily form WIV oxides. We also report
mechanistic investigations on the exchange of η2-ketones and
η2-nitriles and discuss the implications of these studies on the
nature of the η2 4-electron interaction.
Fig. 1 Coordination of ketones and imines as 2 and 4 electron donors.
One of the drawbacks to the acac ligands that have been
used previously to stabilize WII 4-electron donor complexes is
the reactivity of the methyl groups, which are prone to deproto-
nation by strong bases. Additionally, the acac ligands are
nucleophilic at the β-carbon on the ligand backbone and can
undergo undesired reactions with electrophiles at this posi-
tion. These properties limit the types of reagents that can be
used to facilitate organometallic transformations. For example,
these chiral compounds could be used to reduce nitriles in an
enantiospecific fashion, but many of the reagents of choice
for forming new C–C bonds (alkyl lithiums and alkyl
magnesiums) will react with the acac ligand and lead to
decomposition.
Results and discussion
Formation of (trop)2W(CO)3
The parent tricarbonyl complex (trop)2W(CO)3, 1, forms by
reacting two equivalents of [HNEt3][trop] and [NEt4]-
[W(CO)4(I)3];11 the complex is dark red with a high absorption
coefficient. 1 can be isolated cleanly as a nearly black powder
by evaporation of the reaction solvent and extracting the
residue with diethyl ether. The compound is significantly less
stable than the acac analog and is extremely air- and tempera-
ture sensitive. The solution IR spectrum has CO stretches at
2020 and 1917 cm−1 which are about 7–8 cm−1 lower than
those for (acac)2W(CO)3 indicating that the tropolonate ligands
are slightly more donating than the acac ligands. The room
temperature 1H NMR spectrum is consistent with a C1 sym-
metric cis-bis-tropolonate complex. Each proton on the tropo-
lonate ligands is chemically inequivalent and has a complex
splitting pattern, making peak assignment challenging. At
room temperature, the 13C NMR spectrum is also consistent
with a cis-bis-tropolonate species, however, the three carbon
monoxide carbons are not observed and presumed to be
fluxional at this temperature.
By using the tropolonate ligand this problem is alleviated
as the ligand backbone consists entirely of planar conjugated
sp2 carbons and is less reactive towards nucleophiles or elec-
trophiles. This allows us to use a much larger toolbox of
reagents without
Dark red crystals were obtained from a benzene–hexane
mixture and the structure was determined by X-ray diffraction.
The X-ray structure confirms the compound to be (trop)2-
W(CO)3 as a distorted capped octahedron, similar to (acac)2-
W(CO)3 (Fig. 2). Compared to the acac complex the metal is
more exposed as the smaller tropolonate ligands do not
ð1Þ
side-reactions on the ligand backbone. The tropolonate ligand provide as much steric protection for the metal center. Each
features a 5-membered ring when bound to W, whereas acetyl- backbone of the tropolonate ligands is planar and both bite
acetonate forms a 6-membered ring. The change in geometry angles are 74°; the acac ligands have bite angles of 86° and 82°
of the complex may have significant impact on the reactivity showing that they are not only larger, but more flexible than
of the (LX)2W fragment; much work has been done with group the tropolonate ligands.
6 carbamate, thiocarbamate, and dithiocarbamate ligands
Reaction between 1 and phenylacetylene yields (trop)2-
(which form 4-membered rings) and yet none of those species W(CO)(PhCCH) (2a) in high yield. Compound 2a is dark
coordinate nitrile ligands in an η2 fashion. The tropolonate purple, thermally stable at room temperature, and can be puri-
ligand fills the gap between the 4 and 6 membered rings and fied via silica gel under an inert atmosphere. The solution IR
can perhaps combine advantages of both. The acac based CO stretch for 2a appears at 1889 cm−1, 6 cm−1 lower than the
nitrile/ketone complexes are quite stable and therefore not corresponding acac complex, again showing that the tropolo-
conducive to catalytic reactions of these ligands, but the nate ligands are slightly more donating than acac. The
smaller chelate ring may result in more lability and thus have increased donor ability of the tropolonate ligands is counter-
more catalytic potential.
intuitive based on its pKa of 6.9 versus 9.0 for acetylacetone;
This report details the formation of 4-electron interactions based on this data the σ-donor ability of tropolonate should be
between the monomeric d4 (trop)2WII(CO) (trop = tropolonate) less than that of acetylacetonate. The observed donor ability of
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Fig. 3 X-ray structure of (trop)2W(CO)(PhCCMe) (2b). Thermal ellip-
soids are shown at 50% probability and hydrogen atoms have been
omitted for clarity. Selected bond distances (Å) and angles (°): W(1)–
C(24) 1.929(3), W(1)–C(17) 2.015(3), W(1)–C(16) 2.056(3), W(1)–O(4)
2.1538(18), W(1)–O(1) 2.1613(18), W(1)–O(2) 2.0640(18), W(1)–O(3)
2.0408(18), C(16)–C(17) 1.316(4), C(24)–W(1)–C(17) 107.58(12), C(24)–
W(1)–C(16) 69.89(12), C(17)–W(1)–C(16) 37.71(11), O(3)–W(1)–O(4)
75.17(7), O(2)–W(1)–O(1) 73.15(7), C(24)–W(1)–O(4) 163.81(10), O(3)–
W(1)–O(2) 152.16(7).
Fig. 2 X-ray structure of (trop)2W(CO)3. Thermal ellipsoids are shown at
50% probability and hydrogen atoms have been omitted for clarity.
Selected bond distances (Å) and angles (°): W(1)–C(15) 1.9927(19), W(1)–
C(16) 2.000(2), W(1)–C(17) 1.998(2), W(1)–O(1) 2.0925(13), W(1)–O(2)
2.1313(13), W(1)–O(3) 2.1177(14), W(1)–O(4) 2.1188(13), C(15)–W(1)–C(17)
71.85(8), C(15)–W(1)–C(16) 101.61(8), C(17)–W(1)–C(16) 69.58(8), O(3)–
W(1)–O(4) 74.22(5), O(1)–W(1)–O(2) 74.49(5).
tropolonate could be due to increased π-donation from the tro-
polonate oxygens.
π-donation to the metal center. The room temperature 1H
NMR spectrum of 2b shows that the alkyne methyl protons
resonate as a singlet at 3.44 ppm. The alkyne carbons resonate
at 195.4 and 201.1 ppm and the peak for the carbon monoxide
carbon was identified at 240.7 ppm.
1
The room temperature H NMR spectrum of 2a shows the
distinctive signal for the acetylene proton as a broad singlet at
13.08 ppm indicating that the phenylacetylene ligand rotates
rapidly on the NMR timescale. The chemical shift of
13.08 ppm falls in the range of what is expected for a 4-elec-
tron donor alkyne and is similar to the value of 12.95 observed
for (acac)2W(CO)(PhCCH). The room temperature 13C NMR
spectrum of 2a shows that the carbonyl carbon resonates at
240.9 ppm, in accordance with the resonances of known analo-
gous W monocarbonyl species. The carbons of the η2-alkyne
were identified at 186.5 and 207.6 ppm.
The two isomers arising from PhCCH rotation were frozen
by cooling the NMR probe to 263 K. The ratio of isomers was
approximately 98 : 2. At this temperature the two singlets are
sharp and appear at 13.19 (major) and 12.49 (minor) ppm;
tungsten satellites are visible with coupling constants of 3 and
5 Hz, respectively.
Crystals of 2b were obtained from a mixture of CH2Cl2–
hexanes and its structure was determined by X-ray diffraction
(Fig. 3). The bite angles of the two chelates are 73.2° and 75.2°,
slightly deviated from the bite angles of the parent complex 1,
in which both bite angles were 74°. The average W–O bond dis-
tance is 2.10 Å, slightly shortened from the W–O bond dis-
tances in 1. In accordance with similar acac complexes,11 the
carbonyl and acetylene ligands are shown to be cis and paral-
lel. In the solid-state structure of 2b, the phenyl group of the
alkyne ligand orients distal to the carbonyl ligand and the
methyl group proximal. The small bite angle of the tropolonate
chelate results in a distorted octahedral geometry. The C–C
bond distance of the alkyne is 1.316 Å, indicative of a signifi-
cant loss of triple bond character, yet still shorter than what
would be expected for a C–C double bond in a metallocyclopro-
pene. The W–C bond distances of the alkyne are 2.02 Å and
2.06 Å, comparable to those of (acac)2W(CO)(PhCCH)
Similarly, reacting 1 with 1-phenyl-1-propyne forms
ð2Þ
ð3Þ
(trop)2W(CO)(η2-PhCCMe) (2b); the formation of a monocarbo-
nyl species was indicated by in situ solution IR spectroscopy
with a CO stretch at 1895 cm−1. The slightly higher stretching
frequency of 2b (Δν ≈ 6 cm−1) may suggest that the added
steric bulk from the methyl group leads to a small reduction in (∼2.04 Å).11
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nate-based compounds are consistently lower energy than
their acac analogues so a lack of electron density at the metal
to support a cationic species is not the source of the instability.
The small bite angle of the tropolonate ligands leaves the
metal and tropolonate oxygens relatively exposed and could
contribute to the inherent instability. The reason for lower
Fig. 4 Diastereotopic methylene protons of (trop)2W(CO)(η2-
NCCH2Ph).
Reaction of 1 with acetonitrile results in rapid conversion to
a dark red product containing one metal carbonyl stretch at
1881 cm−1 and a C–N stretch at 1664 cm−1. The acetonitrile
1
methyl protons appear at 3.8 ppm in the H NMR spectrum, a
ð4Þ
dramatic shift from free acetonitrile at 2.0 ppm. This shift is
consistent with what is expected for an η2 bound acetonitrile.3c
The 13C NMR spectrum further confirms this assignment as
the C–N carbon resonates at 212.6 ppm as indicated by corre-
lation to the acetonitrile methyl group by HMBC analysis. This
data strongly supports formation of (trop)2W(CO)(η2-NCCH3)
CO stretches in the trop complexes may be increased
pi-donation from the trop oxygens versus acac oxygens; if this
is the case the side-on coordination of nitriles may be
less necessary to reach an 18-electron configuration, resulting
in weaker binding and activation of the 4-electron donor
species.
(3a). Furthermore, addition of benzylnitrile to
1 forms
(trop)2W(CO)(η2-NCCH2Ph) (3b) as evidenced by the diastereo-
Compound 1 will also readily react with compounds such
as acetone and aryl aldehydes to form complexes in which the
carbonyl serves as a 4-electron donor; these complexes tend
to be dark purple in color. For example, reaction of 1 with
acetone results in the formation of (trop)2W(CO)(η2-acetone)
(4a). The metal carbonyl appears at 1851 cm−1 in CH2Cl2, and
the 1H NMR spectrum shows the two diastereotopic methyl
groups at 2.58 and 2.18 ppm. The 13C NMR data is consistent
with an η2-bound acetone ligand;3c,12 the acetone carbonyl
carbon resonates at 92.7 ppm; some 2-electron donor
η2-acetone complexes appear upfield of 90 ppm.13 Aryl alde-
hyde analogs of 4a were synthesized via a similar method and
in these cases the aldehyde proton provides a nice NMR
handle for the compounds. Reaction of 1 with 2,6-dichloroben-
zaldehyde or p-tolualdehyde results in the formation of η2-
aldehyde 4b or 4c. The formation of a monocarbonyl product
is evident by a CO stretches at 1878 and 1858 cm−1 in the solu-
tion IR, respectively. The 20 cm−1 difference in the CO stretch-
ing frequencies of 4b and 4c seems disproportionally large to
be caused by electronic differences in the two aldehydes and
probably indicates weaker binding to W because of steric inhi-
topic methylene group resonating as a pair of roofed doublets
with 16 Hz coupling at 5.25 and 5.33 ppm in the H NMR. In
1
addition to the ∼1.5 ppm downfield shift from free benzyl-
nitrile (Fig. 4), the splitting of the methylene protons is particu-
larly indicative of side-on binding mode as less differentiation
between the protons would be expected if the ligand were co-
ordinated through the nitrogen lone pair, rendering the
methylene group farther from the chiral metal center. The
complex with diphenylacetonitrile, (trop)2W(CO)(η2-NCCHPh2)
(3c), was formed as well. This nitrile again exhibits a dramatic
shift of the methane proton upon coordination from 5.12 to
6.64 ppm in the 1H NMR and the nitrile carbon resonates
at 217.1 in the 13C NMR consistent with an η2 binding
mode. Unfortunately, X-ray quality single crystals could not
be obtained; however, the spectroscopic data strongly
indicate side-on binding of the nitrile ligands in these
complexes.
Unlike the alkyne analogs, the nitrile complexes are rather
unstable and decompose in solution at room temperature in
hours; though they are somewhat more stable in the presence
of excess nitrile. These complexes also decompose rapidly if
they are loaded onto a silica gel column or exposed to air. This
instability stands in stark contrast to the acac-based nitrile
complexes, which can be purified by column chromatography
in air. Attempts to further functionalize the coordinated
nitriles were unsuccessful: addition of electrophiles (MeI,
MeOTf) and strong nucleophiles (LiHBEt3, MeLi, ZnMe2) all
resulted in decomposition. Side-on nitriles in the acac analogs
readily react with MeOTf to form isolable cationic iminoacyl
complexes.1a,3a The CO stretching frequencies of the tropolo-
1
bition. The room temperature H NMR spectrum of 4b shows
the aldehyde proton resonance at 8.12 ppm, while 4c displays
two isomers at in a 2.3 : 1 ratio 7.99 (major) and 7.93 ppm
(minor). The observation of only one major isomer in 4b
suggests that because of the steric bulk of the 2,6-dichlorophe-
nyl ring imparts a high diastereoselectivity to the complex.
This contrasts with the corresponding acac complex in which
two isomers were observed for this ligand, and in fact the spec-
troscopic properties of 4b, are quite similar to the properties
of the minor acac diastereomer suggesting that the diastereo-
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Table 1 NMR and IR data for selected W acac and trop complexes
Complex
IR: ν (cm−1
)
1H NMR: δ (ppm)
13C NMR: δ (ppm)
(acac)2W(CO)3
(trop)2W(CO)3 1
2028, 1924 (CuO)
2020, 1917 (CuO)
1903 (CuO)
231.0, 235.6, 257.8 (CuO).
Not Observed
185.6 (PhCuCH), 211.7 (PhCuCH)
186.5 (PhCuCH), 207.6 (PhCuCH)
208.8 (NuCCH3),
212.6 (NuCCH3)
97.2 (OvC)
92.7 (OvC)
89.0 (major), 84.9 (minor)
(OvCH)
(acac)2W(CO)(PhCuCH)
(trop)2W(CO)(PhCuCH) 2a
(acac)2W(CO)(CH3CuN)
(trop)2W(CO)(CH3CuN) 3a
(acac)2W(CO)((CH3)CvO)
(trop)2W(CO)((CH3)2CvO) 4a
(acac)2W(CO)(ArCOH)
12.96 (s, 1H, PhCuCH)
13.04 (s, 1H, PhCuCH)
3.71 (s, 3H, NuCCH3)
3.82 (s, 3H, NuCCH3)
2.53, 2.31 (s, 3H, (CH3)2CvO)
2.61, 2.21 (s, 3H, (CH3)2CvO)
8.03 (major), 8.26 (minor) (s, 1H,
ArC(vO)H)
1889 (CuO)
1895 (CuO), 1675 (CuN)
1881 (CuO), 1664 (CuN)
1883 (CuO)
1851 (CuO)
1866 (major), 1878 (minor)
(CuO)
(trop)2W(CO)(ArCOH) 4b
(acac)2W(CO)(MeNvC(H)(Ph)
1878 (CuO)
1904 (major), 1889 (minor)
(CuO)
8.12 (s, 1H, ArC(vO)H)
5.56 (major), 5.80 (minor) (s, 1H,
NvC(H)(Ph)
82.0 (OvCH)
60.8 (major), 57.2 (minor)
(NvCH)
(trop)2W(CO)(MeNvC(H)(tol) 5
1846 (CuO)
6.02 (s, 1H, NvC(H)(Ph)
59.8 (NvCH)
selectivity has switched by replacement of acac with trop
(Table 1). Imines will also bind to the W center in an η2 4e
donor fashion like the aldehydes. Addition of CH3NvC(H)-
(p-tolyl) to 1 and stirring for 2 hours resulted in the loss of 2
equivalents of CO and the formation of a dark blue solution of
(trop)2W(CO)(η2-CH3NvC(H)(p-tolyl)) (5). The formation of a
monocarbonyl species was confirmed by the single broad CO
stretch at 1846 cm−1 in the CH2Cl2 solution IR spectrum. The
imine C–H appears shifted upfield to 6.02 ppm in the 1H NMR
and the imine carbon appears at 59.8 ppm in the 13C NMR,
both indicating significant reduction of the CvN double
bond. Two diastereomers are observed for the imine
complex, and the major isomer is favored by a 5 : 1 ratio. Dark
blue/purple crystals were obtained by slow diffusion of
pentane into diethyl ether. X-ray crystallography confirmed the
structure of 5 as an η2-imine serving as a 4-electron donor
(Fig. 5).
Fig. 5 X-ray structure of (trop)2W(CO)(η2-CH3NvC(H)(p-tolyl)) (5).
Thermal ellipsoids are shown at 50% probability. One of two unique
molecules of 5 is displayed; an included Et2O molecule and hydrogen
atoms have been omitted for clarity. Selected bond distances (Å) and
angles (°): W–Navg 1.91; W–Cavg 2.25; N–Cavg 1.39; W1–O1 2.03; W1–O2
2.15; W1–O3 2.08; W1–O4 2.10; W1–C24 1.93; C24–O5 1.18; N1–W1–
O4 158.3; C1–W1–O4 150.9. Torsion Angles (°): C24–W1–N1–C1 5.4.
The imine is nearly aligned with the W–CO bond with a
torsion angle of only 5°. The tolyl group is located up and back
towards O1 on the tropolonate trans to CO and the N–Me is
located approximately trans to the tolyl group. The W–N bond
distance of 1.91 Å is consistent with partial multiple bond
character and the N–C bond distance has lengthened to 1.39 Å
indicating significant backbonding from W. The imine is
bound asymmetrically to the W with the N nearly in the
square plane of O4, O3, and O1; while the C1 carbon is well
above the square plane and has a long W–C1 distance of
2.25 Å showing the asymmetric binding of the imine to the
tungsten. The nitrogen is not totally planar, but the PhC–NMe
ð5Þ
torsion angle of 95° is drastically reduced from the planar free the NMR data suggests. The acetylene proton and the aceto-
imine (180° torsion) and is again consistent with partial W–N nitrile methyl groups both appear shifted downfield in the
multiple bond character.
trop complexes versus acac complexes. The imine (5), aceto-
Spectroscopic data for related (acac)2W and (trop)2W tung- nitrile (4a), and tolualdehyde (4c) complexes all have CO
sten complexes are compiled in Table 1. In general, the CO stretching frequencies that are especially low, ∼40 cm−1 below
stretches for the tropolonate complexes are lower in energy the corresponding acetylene and nitrile complexes and simi-
than their acac counterparts. Based on the carbonyl stretches larly below the analogous acac imine and aldehyde complexes.
we would expect the 4-electron donors in the tropolonate com- The crystal structure of 5 does not show any dramatic struc-
plexes to participate in a higher degree of backbonding than tural differences versus (acac)2W(CO)(MeNvCHPh), so the
the acac complexes, as
reason for the dramatic reduction in CO stretching frequency
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must be due to the smaller bite angle of the tropolonate than coordination of the side-on acetone. Likewise, addition of
ligands allowing more optimal π-donation from the oxygen phenylacetylene to 4a rapidly yields 2a. These results support
atoms.
an associative mechanism of ligand exchange as the rate of
exchange is highly dependent on the incoming ligand. The
acetonitrile ligand in 3a can be displaced by phenylacetylene
as well, but we have not observed any exchange of alkyne
ligands at room temperature. The triply bonded ligands thusly
appear to form more stable 4-electron donor complexes and
particularly the alkynes are resistant towards exchange. The
bulkier substrates, such as MeNvC(H)(tol) and Ph2HCCN are
more stable than the smaller analogs, most likely due to their
additional steric protection of the metal center from associat-
ive attack. The ability to readily exchange ligands on the
(trop)2W(CO) framework is important to developing a catalytic
cycle for asymmetric reduction of nitriles to imines or similar
transformations, because the product must be readily dis-
placed by the starting material in order to turn over the cycle.
The related acac complexes can support reduction of nitriles;
however displacement of the resulting imines with nitriles to
complete a stoichiometric cycle has not been reported, even
under forcing conditions.
Exchange of 4-electron donor ligands
It has been observed that the more unstable complexes
reported here, such as η2-nitriles and η2-ketone compounds,
are stabilized by the presence of excess ligand in solution. For
example, (trop)2W(CO)(η2-acetone) (4a) is indefinitely stable
when stored under N2 in acetone, but will decompose in a day
if stored in CH2Cl2. It is reasonable that the decomposition
pathway of the complex first involves the dissociation of the
η2-ligand. If the local availability of additional ligand is
sufficient, the complex can be regenerated. We have explored
the ligand exchange in complex 4a by monitoring the displace-
ment of η2-acetone by acetone-d6 via 1H NMR spectroscopy.
Heteroatom containing 4-electron donor ligands are rare and
thus their exchange processes have not been thoroughly
explored. Generally, an 18-electron octahedral complex would
be expected to undergo dissociative ligand exchange, in this
case dissociation of the 4-electron donor ligand would result
in a 14-electron intermediate which would likely be unstable
and lead to a high reaction barrier. One possibility here is that
the 4-electron donor can reorient such that it becomes a 2-elec-
tron donor before an incoming ligand coordinates and thus
undergo associative ligand exchange without formally exceed-
ing 18-electrons; this would be similar to ring slippage in a
cyclopentadienyl complex which is known to accommodate
ligand exchange in some systems.14
Oxidation with O2
The (trop)2WII complexes are highly sensitive to oxidation by
O2 in air. Formation of 1 does not occur without rigorous
exclusion of oxygen. Upon exposure to air, 1 and its nitrile,
ketone, and imine derivatives, 3, 4, and 5 oxidatively decom-
pose rapidly. Complex 2 is somewhat air stable and only com-
pletely oxidizes over several days in solution; however a CH2Cl2
solution of 2 will oxidize in ∼4 h when stirred with Na2SO4,
presumably with formation of Na2SO3. A stronger oxidant,
such as H2O2, will complete the reaction in less than 5 min.
The alkyne derivatives undergo loss of CO and 2-electron oxi-
dation to (trop)2WIV(O)(η2-alkyne) (6) (eqn (5)). The formation
of WIV oxides can be observed by the color change of the
strongly absorbing dark red or purple precursors to the pale
yellow of the oxides. IR spectroscopy confirms the dissociation
of the carbonyl ligand. The (acac)2WII(CO) complexes have
been oxidized by MCPBA, diazoalkanes, or azides, but the
compounds do not oxidize in air even on long timescales.3d,15
Oxidation of 2b results in the formation of (trop)2W(O)-
(η2-PhCuCMe) (6b). As consistent with the known tungsten-
oxo alkyne complexes, there are two isomers present. At room
The 1H NMR spectra of 4a in acetone-d6 over a 12 hour
period are shown in Fig. 6, the only signals that change over
time are the signals assigned to the acetone methyl groups,
indicating that there is clean conversion of 4a to 4a-d6 with no
side-products being formed. If acetonitrile is added to 4a, even
in neat acetone, rapid conversion to 3a is observed, indicating
that coordination of the side-on nitrile is much more favorable
1
temperature, the H-NMR spectrum shows singlets at 3.28 and
2.89 ppm representing the isomers of the alkyne methyl,
present in a 1.4 : 1 ratio, shifted upfield slightly from the
rapidly rotating propyne ligand of the parent carbonyl
complex; the methyl protons of which resonate as a sharp
singlet at 3.44 ppm. Oxidation of 2a in the same fashion
results in the formation of (trop)2W(O)(η2-PhCuCH) (6a). The
room temperature the 1H-NMR spectrum shows singlets at
11.25 and 11.16 ppm, also in a 1.4 : 1 ratio, the peaks have
tungsten satellites of 6.9 and 6.5 Hz. These signals represent
the acetylene proton on the two rotomers of 6a and are shifted
upfield from the acetylene proton of the parent carbonyl
complex, which appears at 13.04 ppm. The chemical shifts of
Fig. 6 Proton NMR spectra of 4a in acetone-d6 over a 12 hour period.
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Fig. 7 d-Orbital splitting diagrams for WIV oxo alkyne species.
the acetylene protons in the oxidized species are consistent aldehyde complexes there is significant metal heteroatom mul-
with other 3-electron donor W acetylene species.1d,9c,16 This tiple bond character due to strong π donation from the lone
supports the notion that the π-donating oxo ligand reduces the pair. No reaction occurs between compound 1 and ethylene,
electron donation required from the alkyne ligands to reach an further indicating the requirement of
a heteroatom for
18-electron configuration. binding. The small bite angle of the tropolonate ligands allows
In this d2 system, the oxo ligand can π-donate into the the 4-electron donor ligands to readily undergo ligand
empty dxz and dyz orbitals, allowing it to provide up to 6 elec- exchange, but it also leaves the metal exposed and makes the
trons to the metal between the σ and two π bonds. The alkyne compounds prone to air oxidation. The coordinated alkyne
can donate between 2 and 4 electrons depending on the complexes can be cleanly oxidized in air to form WIV oxo
amount of π-donation into the empty dxz orbital. Any combi- alkyne species. It may be possible to take advantage of this be-
nation of 8 total electrons from the oxo and alkyne ligands will havior to use similar complexes as oxidation catalysts with O2
give a stable 18 electron count. Both alkyne and oxo are com- as the terminal oxidant in the future. Attempts to functionalize
peting for donation into the dxz orbital and the spectroscopic η2 nitriles with either electrophilic or nucleophilic reagents
data suggests roughly equal contribution from both ligands, so have not resulted in stable products. Despite the significantly
this could be considered a 3-electron donor alkyne and 5 elec- lower pKa of tropolone versus acetylacetone, the tropolonate
tron donor oxo for an 18 electron complex.1d,9c,16 To accom- complexes have lower CO stretches than their acetylacetonate
plish this bonding scheme, the alkyne must rotate 90°, now analogs; this seems to indicate that the bite angle of the tropo-
perpendicular to the W–O bond. Because the 2 d-electrons are lonate ligands allows for more π donation from the oxygens to
localized in the dxy orbital due to oxo donation into the dxz the tungsten and this overcomes the reduced sigma donation.
and dyz orbitals leaving the alkyne ligand in the xy plane for Future use of 3,7 substituted tropolonates may provide the
backbonding from dxy into to the alkyne π* orbital (Fig. 7). advantages of the small bite angle while providing the metal
There is a with a high barrier to alkyne rotation so two dia- with more kinetic stability by steric protection.
stereomers are seen at room temperature.3d
Experimental
General procedures
Conclusions
We have synthesized a series of bis-tropolonate W compounds Reactions were performed under an atmosphere of dry nitro-
that coordinate a variety of unsaturated ligands in an η2 gen or argon using standard glove box and Schlenk tech-
fashion as 4-electron donors. These compounds bind preferen- niques. All glassware was oven dried or flame dried under
tially nitriles, imines, ketones and aldehyde compounds in an vacuum and cooled under a nitrogen atmosphere before
η2 fashion as 4-electron donors. In the imine, ketone, and use. Methylene chloride, hexanes, tetrahydrofuran, and diethyl
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monitored by in situ IR spectroscopy. The solvent volume was
reduced in vacuo, and then hexanes were added to precipitate
most of the ammonium salts. The solution was filtered and
the remaining solvent was then evaporated to yield a dark red
residue. The product was extracted with diethyl ether and evap-
orated to yield a dark red powder (97 mg, 0.1892 mmol, 11%).
Table 2 Crystal data, data collection and structural refinement para-
meters for complexes, 1, 2b, and 5
Complex
1
2b
5
Formula
Molecular
C
17H10O7W
C48.5H37ClO10
1182.93
W
C100H94N4O21W4
2423.19
510.09
weight
Crystal system
Space group
1
IR: (CH2Cl2) νCO = 2020, 1917 cm−1. H NMR (CD2Cl2, 298 K):
Triclinic
P1
Monoclinic
P2(1)/n
Monoclinic
C2/c
δ 7.55–7.02 (10H, C7H5O2). 13C NMR (CD2Cl2, 200 K): δ 118.1,
127.6, 129.8, 138.0, 139.6, 149.4, 161.1, 174.0, 181.9. Anal.
Calcd for C17H10O7W: C, 40.03; H, 1.98; N, 0.0. Found:
C, 41.29; H, 3.94; N, <0.02. Multiple attempts to achieve satis-
factory elemental analysis were unsuccessful.
ˉ
a/Å
b/Å
c/Å
α/°
7.0951(8)
7.2149(8)
15.5637(18)
83.208(2)
79.978(2)
87.293(2)
778.76(15)
Red blade
10.6825(6)
24.7828(12)
15.8496(9)
90.00
99.116(2)
90.00
36.265(7)
14.093(3)
19.302(4)
90
110.04(3)
90
9268(4)
Blue plate
0.17 × 0.17 ×
0.08
β/°
γ/°
V/Å3
4143.1
Purple blade
W(CO)(trop)2(η2-PhCuCH) (2a)
Color and habit
Crystal size/mm
0.26 × 0.10 × 0.16 × 0.10 ×
0.08
In
a 500 mL Schlenk flask, [NEt4][W(CO)5I] (600 mg,
0.03
1.03 mmol) was dissolved in CH2Cl2 (100 mL). Stoichiometric
addition of elemental iodine (261 mg, 1.03 mmol) resulted in
the immediate formation of the orange [NEt4][W(CO)4I3]
anion. In a separate flask, a solution of tropolone (H-trop)
(2 eq., 251 mg, 2.06 mmol) and triethylamine (211 mg,
2.06 mmol) in CH2Cl2 (10 mL) was prepared. This mixture was
combined with the solution containing [NEt4][W(CO)4I3] and
then stirred for 2 h to yield W(CO)3(trop)2, whose formation
was monitored by in situ IR spectroscopy. Excess phenyl-
acetylene (2 equiv., 226 μL, 2.06 mmol) was added, and the
solution slowly changed color from dark red to purple. The
reaction was stirred until IR spectroscopy showed a single CO
stretch. The solvent volume was reduced in vacuo, and then
hexanes were added to precipitate most of the ammonium
salts. The solution was filtered and the remaining solvent was
then evaporated to yield a dark purple residue. Purification
occurred on a silica column using 19 : 1 CH2Cl2–THF to elute a
dark purple band (257 mg, 0.462 mmol, 45%). IR: (CH2Cl2),
Z
2
4
4
Temperature/K
Radiation type
D(calc), g cm−3
μ/mm−1
Total reflections 16 242
Unique
reflections
Rint
Final R indices
[I > 2σ(I)]
R indices (all
data)
100(2)
MoKα
2.175
7.455
100(2)
MoKα
1.896
5.675
77 304
15 805
153(2)
MoKα
1.737
5.023
49 279
9492
5860
0.0181
R1 = 0.0165
wR2 = 0.0387 wR2 = 0.0526
R1 = 0.0182 R1 = 0.0536
wR2 = 0.0394 wR2 = 0.0579
1.050 1.007
0.0403
R1 = 0.0307
0.0970
R1 = 0.0371
wR2 = 0.0551
R1 = 0.0886
wR2 = 0.0679
0.995
GOF
ether were dried by passage through a column of activated
alumina under an argon atmosphere.17 NMR solvents CD2Cl2,
CDCl3, C6D6, and acetone-d6 were dried by passage through a
pipet containing activated alumina in the glove box. [NEt4]-
[W(CO)5I] was synthesized according to published pro-
cedures.11,18 All other reagents were used as received.
νCO 1889 cm−1 1H NMR (CD2Cl2, 298 K): δ 13.04 (s, 1H,
.
PhCuCH), 7.83–6.93 (m, 15H, C7H5O2, C6H5CuCH). 13C NMR
(CD2Cl2, 298 K): δ 126.4, 127.1, 127.4, 128.5, 129.6, 131.0,
132.0, 137.3, 137.7, 139.0, 139.2, 140.3, 140.4 (trop C–H,
C6H5CuCH), 171.9, 179.4 180.5, 184.5 (trop C–O), 186.5
(PhCuCH), 207.6 (PhCuCH), 240.9 (CuO). Elemental Analy-
sis C23H16O5W Theoretical: C 49.67, H 2.90, N 0.00; Found: C
49.81, H 3.09, N <0.02.
NMR spectra were recorded on 400 MHz Bruker Avance II
and Avance III spectrometers. IR spectra were recorded on a
Thermo Scientific Nicolet iS10 spectrometer. X-Ray structural
determinations were conducted using a Bruker D8 diffract-
ometer using a Mo source at 100 K for 1 and 2b and a Bruker
APEX II using a Mo source at 153 K for 5 (Table 2). Elemental
analysis was conducted by Robertson Microlit (Ledgewood,
NJ). High Resolution Mass Spectrometry was done using a
Thermo Scientific Orbitrap Exactive mass spectrometer using
the Matrix Assisted Inlet Ionization (MAII) method with 3-nitro-
benzonitrile as the matrix and an inlet temperature of 75 °C.19
W(CO)(trop)2(η2-PhCuCMe) (2b)
Same procedure as 2a, but using PhCCMe (2 equiv., 258 μL,
2.06 mmol) rather than PhCCH. A dark red/orange band was
eluted from a silica gel column to purify the complex (323 mg,
55%). Black crystals were obtained by layering CH2Cl2 with
W(CO)3(trop)2 (1)
In a 500 mL Schlenk flask, [NEt4][W(CO)5I] (1 g, 1.72 mmol) hexanes. IR: (CH2Cl2), νCO 1895 cm−1 1H NMR (CD2Cl2,
.
was dissolved in CH2Cl2 (150 mL). Stoichiometric addition of 298 K): δ 8.05 (d, 2H, C7H5O2), 7.30 (m, 3H, m-C6H5CuCMe,
elemental iodine (435 mg, 1.72 mmol) resulted in the immedi- p-C6H5CuCMe,), 7.04 (m, 2H, o-C6H5CuCMe,), 6.92 (d, 1H,
ate formation of the orange [NEt4][W(CO)4I3] anion. In a separ- C7H5O2), 6.78 (d, 1H, C7H5O2), 6.52 (t, 1H, C7H5O2), 6.43 (t,
ate flask, a solution of tropolone (H-trop) (2 eq., 419 mg, 2H, C7H5O2), 6.25 (t, 1H, C7H5O2), 6.15 (t, 1H, C7H5O2), 6.04 (t,
3.44 mmol) and triethylamine (351 mg, 3.44 mmol) in CH2Cl2 1H, C7H5O2), 3.44 (s, 3H, PhCuCCH3). 13C NMR (CD2Cl2,
(10 mL) was prepared. This mixture was combined with 298 K): δ 21.4 (PhCuCCH3), 125.4, 125.7, 128.0, 127.3, 128.3,
the solution containing [NEt4][W(CO)4I3] and then stirred for 128.4, 130.1, 130.9, 136.3, 138.0, 138.3, 138.9 (C6H5CuCCH3,
12 h at 10 °C to yield W(CO)3(trop)2, whose formation was trop C–H), 180.2, 180.9, 181.0, 185.0 (trop C–O), 195.4
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(PhCuCCH3), 201.1 (PhCuCCH3), 240.7 (CuO). Elemental then removed in vacuo to yield a dark purple residue. The
Analysis C24H18O5W Theoretical: C 50.55, H 3.18, N 0.00; residue was taken up into solution and used immediately as it
Found: C 50.08, H 2.94, N <0.02.
begins decomposing quickly; reaction proceeded to 100% con-
version by NMR. IR: (CH2Cl2), νCuO 1851 cm−1 1H NMR
(CD2Cl2, 298 K): δ 7.95 (d, 2H, C7H5O2), 7.79–7.65 (m, 2H,
.
W(CO)(trop)2(η2-CH3CuN) (3a)
A scintillation vial was charged with 20 mg (0.039 mmol) of 1, C7H5O2), 7.45–7.39 (m, 2H, C7H5O2), 7.33–7.25 (m, 2H,
and 10 mL of neat acetonitrile. The resulting solution was C7H5O2), 7.03 (d, 1H, C7H5O2), 6.89 (t, 1H, C7H5O2), 2.61, 2.21
stirred for 15 minutes until the solution turned dark red, indi- (each a s, 3H, CH3). 13C NMR (CD2Cl2, 298 K): δ 32.2, 32.6
cating the formation of the η2-acetonitrile complex. The (acetone CH3), 92.7 (acetone CvO), 127.1, 127.9, 128.0, 129.1,
solvent volume was then removed in vacuo to yield a dark red 132.0, 133.8, 137.7, 139.0, 139.2, 140.8 (trop C–H), 175.6,
residue. The residue was taken up into solution and used 176.3, 179.4, 184.8 (trop C–O), 229.8 (CuO).
immediately as it begins decomposing quickly; reaction pro-
W(CO)(trop)2(η2-ArC(vO)H) (Ar = 2,6-dichlorobenzene) (4b)
1881 cm−1, νCuN 1664 cm−1. H NMR (CD2Cl2, 298 K): δ 7.89 A scintillation vial was charged with 20 mg (0.039 mmol) of 1
ceeded to 100% conversion by NMR. IR: (CH2Cl2), νCuO
1
(d, 1H, C7H5O2), 7.75 (m, 1H, C7H5O2), 7.60 (m, 2H, C7H5O2), and 10 mL of CH2Cl2. Excess 2,6-dichlorobenzaldehyde (1.25
7.37 (m, 2H, C7H5O2), 7.23 (d, 1H, C7H5O2), 7.21 (d, 1H, eq., 8.72 mg, 0.049 mmol) was added, and the resulting solu-
C7H5O2), 7.14 (d, 1H, C7H5O2), 6.93 (t, 1H, C7H5O2), 3.82 (s, tion was stirred for 15 minutes until the solution turned
3H, NuCCH3). 13C NMR (CD2Cl2, 298 K): δ 21.7 (NuCCH3), purple, indicating the formation of the η2-aldehyde complex.
124.9, 127.3, 128.4, 129.5, 131.0, 133.6, 139.2, 140.8, 142.0 The solvent volume was then removed in vacuo to yield a dark
(trop C–H), 173.0, 178.9 (trop C–O), 212.6 (CH3CN), 241.8 purple residue. The residue was taken up into solution and
(CuO).
used immediately as it begins decomposing quickly; reaction
proceeded to 100% conversion by NMR. Only 1 diastereomer
W(CO)(trop)2(η2-PhCH2CuN) (3b)
observed. IR: (CH2Cl2), νCuO 1878 cm−1 1H NMR (CD2Cl2,
.
A scintillation vial was charged with 20 mg (0.039 mmol) of 1 298 K): δ 8.12 (s, 1H, OvCH), 7.96–6.87 (13H, C7H5O2,
and 10 mL of CH2Cl2. Excess (1.25 eq., 5.71 mg, 0.049 mmol) C6H3Cl2COH). 13C NMR (CD2Cl2, 298 K): δ 82.0 (OvCH),
benzylnitrile was then added to the solution. The solution was 127.8, 128.5, 128.9, 129.0, 129.6, 130.3, 130.4, 134.2, 134.3,
stirred for 15 minutes until the color changed to dark red and 134.4, 138.9, 139.2, 139.3, 139.9, 140.1, 141.6 (trop C–H,
IR spectroscopy revealed a single carbonyl stretch. The solvent C6H3Cl2COH), 177.3, 177.5, 179.7, 184.9 (trop C–O), 232.1
volume was then removed in vacuo to yield a dark red residue. (CuO).
Attempts to purify the complex from excess benzyl nitrile led
W(CO)(trop)2(η2-ArC(vO)H) (Ar = p-tolyl) (4c)
to decomposition, reaction proceeded to 100% conversion by
NMR. IR: (CH2Cl2), νCuO 1881 cm−1, νCuN 1682 cm−1. 1H NMR Same as 4b, but using p-tolualdehyde. The ratio of diastereo-
1
(CD2Cl2, 298 K): δ 5.25 (d, 1H, PhCHHCuN, JH–H = 16 Hz), mers is 2.3 : 1. IR: (CH2Cl2), νCuO 1858 cm−1 (only 1 broad
1
5.33 (d, 1H, PhCHHCuN, JH–H = 16 Hz), 7.96–6.91 (16H, aro- stretch observed for both isomers). 1H NMR (CD6H6, 298 K):
matic C7H5O2, C6H5CH2CuN). 13C NMR (CD2Cl2, 298 K): δ Major diastereomer: δ 7.99 (s, 1H, OvCH), 7.69–5.82 (14H,
43.4 (PhCH2CuN), 126.8, 127.0, 127.9, 129.3, 129.4, 130.4, C7H5O2, CH3C6H4COH), 2.21 (s, 3H, CH3C6H4COH). Minor dia-
130.5, 132.9, 137.8, 137.9, 138.2, 138.7, 139.3, 140.4 (trop C–H, stereomer: δ 7.93 (s, 1H, OvCH), 7.69–5.82 (14H, C7H5O2,
C6H5CH2CuN), 172.7, 179.5, 181.0, 186.1 (trop C–O), 215.5 CH3C6H4COH), 2.28 (s, 3H, CH3C6H4COH). 13C NMR (CDCl3,
(PhCH2CuN), 239.8 (CuO).
298 K): Major diastereomer: δ 21.2 (Ar–CH3), 84.8 (CvO),
176.2, 176.6, 179.5, 185.5 (trop C–O), 229.9 (CuO). Minor dia-
stereomer: δ 22.1 (Ar–CH3), 86.2 (CvO), 175.5, 177.2, 185.0
W(CO)(trop)2(η2-Ph2HCCuN) (3c)
Same as 3b, but using diphenylacetonitrile (1.25 eq., 9.46 mg, (trop C–O), 229.2 (CuO). Both diastereomers: δ 124.9, 125.9,
0.049 mmol), reaction proceeded to 100% conversion by NMR. 127.3, 137.5, 128.2, 128.2, 128.5, 128.9, 129.9, 130.1, 132.1,
IR: (CH2Cl2), νCuO 1886 cm−1, νCuN 1650 cm−1 1H NMR 132.3, 134.3, 134.4, 137.5, 137.8, 137.9, 138.0, 138.8, 139.0,
.
(CD2Cl2, 298 K):
δ 7.84–6.93 (22H, aromatic C7H5O2, 139.4, 140.7, 132.5, 143.2, 145.8 (trop C–H, CH3C6H4CHvO).
(C6H5)2CHCuN), 6.64 (s, 1H, Ph2CHCuN). 13C NMR (CD2Cl2,
298 K): δ 58.2 (Ph2CHCuN), 126.4, 126.9, 127.1, 127.5, 127.8,
W(CO)(trop)2(η2-CH3NvC(H)(p-tolyl)) (5)
127.9, 128.3, 128.6, 128.7, 131.5, 132.7, 137.9, 138.7, 139.2, 50 mg of 1 (0.098 mmol) was added to a Schlenk flask and dis-
140.6, 140.8, 141.5, (trop C–H, (C6H5)2CHCuN), 178.0, 179.1, solved in 50 mL of THF. 1.25 equivalents of CH3NvC(H)-
179.7, 184.6 (trop C–O), 217.1 (Ph2CHCuN), 238.3 (CuO).
(p-tolyl)·HCl (0.123 mmol, 20.7 mg) was added to a separate vial
and 10 mL of THF was added. The iminium was deprotonated
with potassium tert-butoxide (0.123 mmol, 13.8 mg). The solu-
W(CO)(trop)2(η2-(CH3)2CvO) (4a)
A scintillation vial was charged with 20 mg (0.039 mmol) of 1 tion containing the deprotonated imine was filtered into the
and 10 mL of neat acetone. The resulting solution was stirred solution containing 1 and the reaction mixture was stirred for
for 15 minutes until the solution turned purple, indicating the 2 h until the solution turned dark blue. The solvent volume
formation of the η2-acetone complex. The solvent volume was was removed in vacuo, and the resulting dark solid was dis-
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solved in Et2O and filtered. Dark, needle-like crystals precipi- 182.0 (trop C–O). Anal. Calcd for C23H18O5W·1/3CH2Cl2: C,
tated from the Et2O solution after storage at −33 °C for several 47.78; H, 3.21; N, 0.00. Found: C, 47.89; H, 3.30; N, <0.02.
weeks (46 mg, 82%). The ratio of diastereomers is 5 : 1. IR: HRMS M + H Calcd: 559.07. Found 559.0696.
(CH2Cl2), νCuO 1846 cm−1 (only 1 broad stretch observed for
both isomers). 1H NMR (CDCl3, 298 K): Major diastereomer:
δ 5.61 (s, 1H, HCvN), 3.95 (s, 3H, CvNCH3), 2.40 (s, 3H, Ar–
CH3). Minor diastereomer: δ 5.75 (s, 1H, HCvN), 3.88 (s, 3H,
Acknowledgements
CvNCH3), 2.42 (s, 3H, Ar–CH3). Both diastereomers: We thank Joseph L. Templeton for helpful discussions and the
δ 7.82–6.73 (m, 14H, C7H5O2, CH3C6H4CvN). 13C NMR (CDCl3, University of the Sciences for funding this research. The
298 K): Major diastereomer: δ 21.2 (Ar–CH3), 44.1 (CvNCH3), authors would also like to thank Nick Piro and Benny Chan for
58.6 (CvN), 126.4, 126.7, 126.8, 127.3, 127.8, 128.5, 129.3, their assistance with X-ray crystallography and Brian Koronkie-
129.6, 130.3, 131.5, 139.4, 137.6, 139.2, 139.3, 139.9, 144.6 wicz for assistance with synthesis.
(trop C–H, CH3C6H4CvN), 179.3, 179.5, 180.3, 185.2 (trop
C–O), 236.4 (CuO). Minor diastereomer: not observed.
Isomers present in a 5 : 1 ratio. Satisfactory elemental analysis
was not obtained despite multiple attempts.
Notes and references
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J. L. Templeton, Polyhedron, 2004, 23, 2831–2840;
W(O)(trop)2(η2-PhCuCH) (6a)
Methylene chloride (10 mL) was added to a vial containing
20 mg (0.0360 mmol) of 2a. 3–5 drops of a 30% by weight solu-
tion of H2O2 in H2O was added to the vial, and the resulting
solution turned yellow after stirring for 5 minutes indicating
the oxidation of 2a. MgSO4 was added and the solution was fil-
tered to yield 6a (17 mg, 0.0313 mmol, 83%). IR spectroscopy
confirmed the disappearance of the monocarbonyl stretch. 6a
can be purified on a silica column using CH2Cl2 to elute a
yellow band. The ratio of diastereomers is 1.4 : 1. 1H NMR
(CD2Cl2, 298 K): Major isomer: δ 11.16 (s, 1H, PhCuCH,
2JW–H = 6.5 Hz), 8.04–6.77 (m, 15H, C7H5O2, C6H5CuCH).
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2
Minor isomer: δ 11.28 (s, 1H, PhCuCH, JW–H = 6.9 Hz),
8.04–6.77 (m, 15H, C7H5O2, C6H5CuCH). Isomers present in
1.48 : 1 ratio. 13C NMR (CDCl3, 298 K): Major isomer: δ 157.3
(PhCuCH), 175.5 (PhCuCH), 180.1, 180.5, 181.4, 181.9 (trop
C–O). Minor isomer: δ 164.4 (PhCuCH), 169.5 (PhCuCH),
180.2, 181.0, 181.2, 181.5 (trop C–O). Both isomers: δ 126.8,
127.1, 127.56, 127.60, 127.7, 128.2, 128.4, 128.5, 128.6, 128.75,
128.80, 130.8, 131.0, 131.4, 132.1, 132.3, 132.6, 132.9, 136.4,
136.7, 138.9, 139.1, 139.57, 139.64, 139.7, 140.8, 140.9. (trop
C–H, C6H5CuCH). HRMS M + H Calcd: 545.06. Found
545.0541.
W(O)(trop)2(η2-PhCuCMe) (6b)
Same as 6a but using 20 mg of 2b (0.0351 mmol) and yielding
18 mg of 6b (0.0323 mmol, 88%). The ratio of diastereomers is
1.4 : 1. 1H NMR (CD2Cl2, 298 K): Major isomer: δ 7.94–6.81
(15H, C7H5O2, C6H5CuCH3), 3.28 (3H, s, PhCuCCH3). Minor
isomer: δ 7.94–6.81 (15H, C7H5O2, C6H5CuCH3), 2.87 (3H, s,
PhCuCCH3). Isomers present in 1.42 : 1 ratio. 13C NMR
(CDCl3, 298 K): Major isomer: δ 15.5 (PhCuCCH3), 167.2
(PhCuCCH3), 167.3 (PhCuCCH3). Minor isomer: δ 16.8
(PhCuCCH3), 160.3 (PhCuCCH3), 174.9 (PhCuCCH3). Both
isomers: 124.0, 125.5, 126.6, 126.7, 127.3, 127.35, 127.42,
127.7, 127.9, 128.12, 128.14, 128.19, 128.20, 128.4, 130.76,
130.79, 131.1, 131.3, 131.4, 132.1, 137.6, 138.3, 138.9, 139.2,
139.5, 139.6, 139.7, 140.1, 140.8, 140.9 (trop C–H,
C6H5CuCCH3), 180.1, 180.2, 180.6, 181.1, 181.1, 181.3, 181.4,
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