Bis(acetylacetonato)tricarbonyl tungsten(II): A convenient precursor to chiral bis(acac) tungsten(II) complexes

Article abstract of DOI:10.1021/ic0600329

Two equivalents of acetylacetonate (acac) have been successfully introduced into a monomeric tungsten(II) coordination sphere. With the tetracarbonyltriiodotungsten(II) anion as a precursor, the formation of a tungsten(II) bis(acac) tricarbonyl complex, W(CO)₃(acac)₂, 1, has been accomplished. The addition of PMe₃ or PMe₂Ph to tricarbonyl complex 1 formed tungsten(II)bis(acac)dicarbonylphosphine complexes 2a and 2b, respectively. Single-crystal X-ray diffraction studies of the parent tricarbonyl complex, 1, and dicarbonyl trimethylphosphine complex 2a confirmed seven-coordinate geometries for both complexes. Variable-temperature ¹H and ¹³C{¹H} NMR spectroscopy revealed fluxional behavior for these seven-coordinate molecules: rapid exchange of the three carbon monoxide ligands in 1 was observed, and movement of the phosphine ligand through a mirror plane in a C_S intermediate species was observed for both 2a and 2b. Tricarbonyl complex 1 reacted readily with alkyne reagents to form bis(acac)monocarbonylmonoalkynetungsten(II) complexes 3a (PhC≡CH) and 3b (MeC≡CMe). Variable-temperature ¹H NMR spectroscopy was used to probe rotation of the alkyne ligand in 3a and 3b. The introduction of two alkyne ligands was accomplished thermally using excess PhC≡CPh to form bis(alkyne) complex 4 which was characterized crystallographically, as well as by ¹H and ¹³C NMR spectroscopy. The availability of W(CO)₃(acac)₂ as a source of the W(acac)₂ d⁴ moiety lies at the heart of the chemistry reported here.

Full text of DOI:10.1021/ic0600329

Inorg. Chem. 2006, 45, 6205−6213
Bis(acetylacetonato)tricarbonyl Tungsten(II): A Convenient Precursor to
Chiral Bis(acac) Tungsten(II) Complexes
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
Received January 6, 2006
Two equivalents of acetylacetonate (acac) have been successfully introduced into a monomeric tungsten(II)
coordination sphere. With the tetracarbonyltriiodotungsten(II) anion as a precursor, the formation of a tungsten(II)
bis(acac) tricarbonyl complex, W(CO)3(acac)2, 1, has been accomplished. The addition of PMe3 or PMe2Ph to
tricarbonyl complex 1 formed tungsten(II)bis(acac)dicarbonylphosphine complexes 2a and 2b, respectively. Single-
crystal X-ray diffraction studies of the parent tricarbonyl complex, 1, and dicarbonyl trimethylphosphine complex 2a
confirmed seven-coordinate geometries for both complexes. Variable-temperature 1H and 13 1H
C{ } NMR spectroscopy
revealed fluxional behavior for these seven-coordinate molecules: rapid exchange of the three carbon monoxide
ligands in 1 was observed, and movement of the phosphine ligand through a mirror plane in a CS intermediate
species was observed for both 2a and 2b. Tricarbonyl complex 1 reacted readily with alkyne reagents to form
bis(acac)monocarbonylmonoalkynetungsten(II) complexes 3a (PhC
tCH) and 3b (MeCtCMe). Variable-temperature
1H NMR spectroscopy was used to probe rotation of the alkyne ligand in 3a and 3b. The introduction of two alkyne
ligands was accomplished thermally using excess PhC
t
CPh to form bis(alkyne) complex 4 which was characterized
crystallographically, as well as by H and 13C NMR spectroscopy. The availability of W(CO)3(acac)2 as a source of
the W(acac)2 d4 moiety lies at the heart of the chemistry reported here.
1
I. Introduction
iridium(III) bis(acac) fragment. To date, few complexes of
Group VI metals with two acac ligands have been reported.
Monomeric Group VI d4 complexes containing anionic
bidentate S-donor dialkyldithiocarbamate (dtc) ligands have
been thoroughly investigated, but examples with two anionic
bidentate O-donor ligands bound to Group VI metals are
scarce. Higher oxidation states of molybdenum, such as IV14
and VI,15 readily bind two equivalents of acetylacetonate.
The venerable Mo(O)2(acac)2 complex has been used as a
catalyst for the epoxidation of olefins16 and as a catalyst in
dehydrative cyclizations.17 Dimeric d4 paddlewheel com-
plexes of tungsten(II) and molybdenum(II) that boast biden-
tate anionic oxygen chelates, including acac, have multiple
bonding between metal centers.18 Chromium(II) bis(acac)19
Widespread use of acetylacetonato (acac) derivatives as
ligands over the past four decades1-5 and the recent renais-
sance of NacNac chemistry6-10 provide a wealth of literature
knowledge for launching Group VI d4 (acac)2MLn chemistry.
Periana’s developments in C-H activation11-13 used an
* To whom correspondence should be addressed. E-mail:
joetemp@unc.edu.
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10.1021/ic0600329 CCC: $33.50
Published on Web 07/13/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 16, 2006 6205

Jackson et al.
is one of few Group VI d4 complexes in the literature with
two coordinated acac chelates. No low-valent monomeric
Group VI complex has been available as a convenient source
of the metal bis(acac) fragment for developing additional
chemistry.
The [NEt4][W(CO)5I] reagent was made following a literature
procedure.36 It was important to purify this reagent prior to use via
chromatography on an alumina column with acetone as the eluent.
W(CO)3(acac)2 (1). In a 200 mL Schlenk flask, [NEt4][W(CO)5I]
(590 mg, 1.0 mmol) was dissolved in CH2Cl2 (20 mL). Stoichio-
metric addition of elemental iodine (265 mg, 1.0 mmol) resulted
in the immediate formation of the orange [NEt4][W(CO)4I3] anion.
In a separate flask, a solution of 2,4-pentanedione (H-acac) (0.220
mL, 2.05 mmol) in CH2Cl2 (10 mL) was deprotonated by triethy-
lamine (0.285 mL, 2.05 mmol) to yield [acac]- in situ. This solution
was combined with the solution containing the tetracarbonyl
triiodide complex and then stirred for 2 h to yield W(CO)3(acac)2,
whose formation was monitored by in situ IR spectroscopy. The
solvent volume was reduced in vacuo, and then hexanes were added.
The remainder of the CH2Cl2 solvent was removed, and the dark
red residual liquid was cannula filtered into another flask and stored
at -30 °C for crystallization (90 mg, 20%). IR: (hexanes) νCO
One motivation for using oxygen donor ligands to build a
chiral bis(L- -X)2M d4moiety comparable to (dtc)2W(CO)3
was to use a more robust fragment as an auxiliary. The rich
chemistry of (dtc)2WL3 includes sulfur redox chemistry,
evident in unwanted dtc C-S cleavage reactions.20-27 We
chose to investigate acac to better control both electronic
and steric factors around the metal center and to avoid the
undesirable reactivity of the sulfur donor chelates. Acetyl-
acetonate was an attractive choice because of its ease of
preparation and storied history. Previous attempts to coor-
dinate acac to tungsten(II) carbonyl complexes generated a
series of mono(acac) complexes;28-33 only one W(II) bis-
(acac) complex has been reported.34 Coordination of two
formato ligands to a Group VI d4 metal has been reported,
but attempts to incorporate acetylacetonate into that system
yielded only mono(acac) derivatives.35 We report the coor-
dination of two acetylacetonate ligands to tungsten(II) centers
to form d4 W(II) bis(acac) derivatives.
1
2028, 1924; (KBr) ν 2026, 1908, 1895 cm-1. H NMR (CD2Cl2,
298 K): δ 5.67 (s, 2H, acac CH), 2.08 (s, 12H, acac CH3). 13C-
{1H} NMR (CD2Cl2, 298 K): δ 27.4 (acac CH3), 102.8 (acac CH),
190.1 (acac CO), 242.5 (CtO). 13C{1H} NMR (CDCl2F, 153 K):
δ 231.0 (CtO), 235.6 (CtO), 257.8 (CtO). Anal. Calcd for
C13H14O7W: C, 33.50; H, 3.03. Found: C, 33.10; H, 3.19.
W(CO)2(PMe3)(acac)2 (2a). A flask was charged with 195 mg
of W(CO)3(acac)2 1, which was dissolved in 10 mL of CH2Cl2. In
a separate flask, trimethylphosphine (0.040 mL) was added to
hexanes (7 mL). The phosphine solution was cannula filtered into
the flask containing 1, and the reaction mixture was stirred for 1 h.
The reaction progress was monitored via IR spectroscopy, and the
appearance of two new absorptions suggested formation of a
dicarbonyl product. The solvent was removed in vacuo to yield a
II. Experimental
General Information. Reactions were performed under a dry
nitrogen atmosphere using standard Schlenk techniques. Methylene
chloride, diethyl ether, hexanes, toluene, and pentane were purified
by passage through an activated alumina column under a dry argon
atmosphere. Methylene chloride-d2 was dried over CaH2 and
degassed. All other reagents were purchased from commercial
sources and were used without further purification.
NMR spectra were recorded on a Bruker DRX400, AMX400,
or AMX300 spectrometer. Infrared spectra were recorded on an
ASI Applied Systems ReactIR 1000 FT-IR spectrometer. Elemental
analysis was performed by Atlantic Microlab, Norcross, GA.
dark red solid (110 mg, 54%). IR: (hexanes) νCO 1928, 1831 cm-1
.
1H NMR (CD2Cl2, 298 K): δ 5.75, 5.37 (each a s, each 1H, acac
CH), 2.15 (br s, 6H, acac CH3), 1.93 (s, 6H, acac CH3), 1.52 (d,
P-CH3, 2JP-H ) 10 Hz). 13C{1H} NMR (CD2Cl2, 298 K): δ 14.6
1
(d, PMe3, J
) 33 Hz), 27.3, 27.4 (acac CH3), 101.1, 102.2
C-P
(acac CH), 188.6, 188.9, 189.7 (acac CO). 13C{1H} NMR (CD2-
1
2
Cl2, 193 K): δ 246.7 (CtO, JW-C ) 166 Hz, J
) 8 Hz),
C-P
276.6 (CtO, JW-C ) 144 Hz, JP-C ) 41 Hz). 31P{1H} NMR
1
2
1
(CD2Cl2, 298 K): δ 11.21 (s, JW-P ) 217 Hz). Anal. Calcd for
(19) Cotton, F. A.; Rice, C. E.; Rice, G. W. Inorg. Chim. Acta 1977, 24,
231-234.
(20) Herrick, R. S.; Nieter-Burgmayer, S. J.; Templeton, J. L. J. Am. Chem.
Soc. 1983, 105, 2599.
(21) Goh, L. Y.; Weng, Z.; Hor, A. T. S.; Leong, W. K. Organometallics
2002, 21, 4408.
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1992, 31, 4008.
(23) Ricard, L.; Estienne, J.; Weiss, R. Inorg. Chem. 1973, 12, 2182.
(24) Morrow, J. R.; Tonker, T. L.; Templeton, J. L. Organometallics 1985,
4, 745.
(25) Morrow, J. R.; Templeton, J. L.; Bandy, J. A.; Bannister, C.; Prout,
C. K. Inorg. Chem. 1986, 25, 1923.
(26) Brower, D. C.; Tonker, T. L.; Morrow, J. R.; Rivers, D. S.; Templeton,
J. L. Organometallics 1986, 5, 1093.
(27) Coucouvanis, D. Prog. Inorg. Chem. 1979, 26, 301.
(28) Baker, P. K.; Kendrick, D. A. Polyhedron 1991, 10, 433.
(29) Baker, P. K.; Flower, K. R. J. Organomet. Chem. 1993, 447, 67.
(30) Baker, P. K.; Drew, M. G. B.; Evans, D. S.; Johans, A. W.; Meehan,
M. M. J. Chem. Soc., Dalton Trans. 1999, 2541.
(31) Baker, P. K.; Clark, A. I.; Drew, M. G. B.; Durrant, M. C.; Richards,
R. L. J. Organomet. Chem. 1997, 549, 193.
(32) Baker, P. K.; Clark, A. I.; Coles, S. J.; Drew, M. G. B.; Durrant, M.
C.; Hursthouse, M. B.; Richards, R. L. J. Chem. Soc., Dalton Trans.
1998, 1281.
(33) Armstrong, E. A.; Baker, P. K.; Drew, M. G. B. Organometallics 1988,
7, 319.
(34) Armstrong, E. A.; Baker, P. K. Inorg. Chim. Acta 1988, 143, 13.
(35) Brower, D. C.; Winston, P. B.; Tonker, T. L.; Templeton, J. L. Inorg.
Chem. 1986, 25, 2883.
C15H23O6PW: C, 35.45; H, 4.57. Found: C, 34.76; H, 4.57.
W(CO)2(PMe2Ph)(acac)2 (2b). A flask was charged with 120
mg of W(CO)3(acac)2 1, which was dissolved in 10 mL of CH2-
Cl2. In a separate flask, dimethylphenylphosphine (0.035 mL) was
added to hexanes (7 mL). The phosphine solution was cannula
transferred into the flask with containing 1, and the reaction mixture
was monitored via IR spectroscopy. Two new CO absorptions
appeared within 30 min, at which point the solvent was removed
in vacuo to yield a dark red solid (50 mg, 31%). IR: (hexanes)
1
νCO 1930, 1835 cm-1. H NMR (CD2Cl2, 298 K): δ 7.50-7.51
(m, 2H, P-C6H5), 7.40-7.43 (m, 3H, P-C6H5), 5.76, 5.40 (each
a s, each 1H, acac CH), 2.16, 2.12 (each a s, each 3H, acac CH3),
1.94 (s, 6H, acac CH3), 1.76 (d, 6H, PMe2Ph, 2JP-H ) 10 Hz). 1H
NMR (CD2Cl2, 193 K): δ 7.43 (s, 5H, P-C6H5), 5.85, 5.44 (each
a s, each 1H, acac CH), 2.19, 2.16, 1.96, 1.91 (each a s, each 3H,
2
acac CH3), 1.73, 1.66 (each a d, each 3H, P-CH3, JP-H ) 10
Hz). 13C{1H} NMR (CD2Cl2, 298 K): δ 13.7 (d, PMe2Ph, 1JP-C
)
33 Hz), 27.3, 27.5 (acac CH3), 101.1, 102.3 (acac CH), 128.5 (d,
o-C6H5, 2JP-C ) 9 Hz), 130.3 (p-C6H5), 131.2 (d, m-C6H5, 3JP-C
8 Hz), 135.6 (d, ipso- C6H5, JP-C ) 49 Hz), 188.6, 188.9, 189.9
)
1
(36) Abel, E. W.; Butler, I. S.; Reid, J. G. J. Chem. Soc. 1963, 2068.
6206 Inorganic Chemistry, Vol. 45, No. 16, 2006

Bis(acetylacetonato)tricarbonyl Tungsten(II)
Scheme 1. Synthetic Route to Heptacoordinate W(CO)3(acac)2 (1)
(acac CO). 13C{1H} NMR (CD2Cl2, 193 K): δ 245.9 (d, CtO,
(each a s, each 6H, acac CH3). 13C{1H} NMR (CD2Cl2, 298 K): δ
26.6, 28.1 (acac CH3), 102.0 (acac CH), 127.7, 128.0, 128.2, 128.7,
128.8, 129.7, 130.9, 129.9, 141.1 (H5C6CtCC6H5), 185.4, 190.4
(acac CO), 194.3, 201.4 (PhC≡CPh). Anal. Calcd for C38H34O4W:
C, 61.79; H, 4.65; O, 8.66. Found: C, 61.93; H, 4.57; O, 8.61.
2
2JP-C ) 8 Hz), 277.3 (d, CtO, JP-C ) 43 Hz). 31P{1H} NMR
1
(CD2Cl2): δ 18.7 (s, JP-W ) 225 Hz).
W(CO)(η2-PhCtCH)(acac)2 (3a). A flask was charged with
75 mg of W(CO)3(acac)2 1, which was dissolved in 10 mL of CH2-
Cl2. Excess phenylacetylene (2 equiv) was added, and the solution
changed color from dark red to brown. The reaction was stirred
until IR spectroscopy showed a single CO stretch, and the solvent
was removed in vacuo to yield a brown solid. Purification occurred
on a silica column using CH2Cl2 and hexanes to elute a dark orange/
brown band. Black crystals were obtained by layering CH2Cl2 with
hexanes (56 mg, 68%). IR: (hexanes) νCO 1920; (KBr) νCO 1903
III. Results and Discussion
Synthesis of W(CO)3(acac)2 (1). Stoichiometric amounts
of [NEt4][W(CO)5I] and elemental iodine were stirred in
methylene chloride at room temperature. Oxidation to
tungsten(II) resulted in formation of the seven-coordinate
anionic species, [NEt4][W(CO)4I3],37 which was subjected
to 2 equiv of deprotonated 2,4-pentanedione. Coordination
of two chelates results in liberation of one carbon monoxide
ligand and three iodide ligands, ultimately forming the bis-
(acetylacetonato)tricarbonyl tungsten(II) complex, 1 (Scheme
1).
X-ray Structure of W(CO)3(acac)2 (1). Crystals for X-ray
diffraction studies were generated from a concentrated
solution of 1 in hexanes at -30 °C. The red needlelike
crystals were air and solvent sensitive, decomposing after a
few hours. The crystal structure confirmed the formulation
as a heptacoordinate tungsten(II) complex containing three
metal carbonyl ligands, all cis relative to one another, and
two acetylacetonate ligands, both chelated to the metal
(Figure 1). Coordination of two bidentate chelates in this
1
cm-1. H NMR (CD2Cl2, 298 K): δ 12.96 (br s, 1H, PhCtCH),
7.58-7.60 (m, 2H, C6H5 ortho), 7.44-7.48 (m, 2H, C6H5 meta),
7.28-7.32 (m, 1H, C6H5 para), 5.58, 5.51 (each a s, each 1H, acac
CH), 2.20, 2.16, 2.14, 1.89 (each a s, each 3H, acac CH3). 1H NMR
(CD2Cl2, 193 K): δ 13.00 (s, PhCtCH, major rotamer, 98%), 12.49
(s, PhCtCH, minor rotamer, 2%). 13C{1H} NMR (CD2Cl2, 298
K): δ 26.1, 26.6, 26.7, 28.3 (acac CH3), 100.2, 102.1 (acac CH).
13C{1H} NMR (CD2Cl2, 193 K): δ 128.3 (C6H5-ortho), 129.5
(C6H5-para), 130.7 (C6H5-meta), 133.9 (C6H5-ipso) 185.4, 187.8,
188.2, 194.9 (acac CO), 185.6 (PhCtCH), 211.7 (PhC≡CH), 240.9
(CtO). Anal. Calcd for C19H20O5W: C, 44.56; H, 3.94; O, 15.62.
Found: C, 44.71; H, 3.93; O, 15.51.
W(CO)(η2-MeCtCMe)(acac)2 (3b). A flask was charged with
140 mg of W(CO)3(acac)2 1, which was dissolved in 10 mL of
CH2Cl2. Excess 2-butyne (2 equiv) was added, and the reaction
mixture was stirred until the IR spectrum displayed a single CO
absorption. The solvent was removed in vacuo, and the brown
product was triturated with hexanes. Purification occurred on a silica
column using CH2Cl2 and hexanes to elute a dark orange/brown
band (85 mg, 61%). IR: (hexanes) νCO 1906 cm-1. 1H NMR (CD2-
Cl2, 298 K): δ 5.53, 5.49 (each a s, each 1H, acac CH), 3.14 (br
s, 6H, H3C-CtC-CH3), 2.18, 2.16, 2.09, 1.86 (each a s, each
3H, acac CH3). 1H NMR (CD2Cl2, 250 K): δ 3.18, 3.13 (each a s,
each 3H, H3C-CtC-CH3). 13C{1H} NMR (CD2Cl2, 298 K): δ
26.2, 26.7, 26.8, 28.4 (acac CH3), 100.2, 101.7 (acac CH), 186.1,
187.6, 188.1, 194.4 (acac CO). 13C{1H} NMR (CD2Cl2, 193 K):
δ 16.2, 20.1 (H3C-CtC-CH3), 188.4 (H3C-C≡C-CH3), 209.2
(H3C-CtC-CH3), 242.4 (CtO). Anal. Calcd for C15H20O5W: C,
38.81; H, 4.35; O, 17.23. Found: C, 38.51; H, 4.46; O, 16.83.
W(η2-PhCtCPh)2(acac)2 (4). Complex 1 was generated in situ
from 590 mg of [NEt4][W(CO)5I], and then it was extracted into
toluene, which was cannula filtered to a separate flask. Excess
diphenylacetylene (∼5 equiv) was added, and the solution was
refluxed until IR spectroscopy indicated an absence of the metal
carbonyl stretches. The solvent was removed in vacuo, and the crude
material was chromatographed on alumina using toluene as an
eluent, yielding a dark yellow band. The toluene was removed on
a rotary evaporator, and the yellow product was washed with
pentane to remove excess alkyne. Bright yellow crystals were grown
by slow evaporation of a solution of 4 in CH2Cl2/hexanes (150
mg, 20%). IR: (KBr) νCtC 1742 cm-1. 1H NMR (CD2Cl2, 298 K):
δ 7.15-7.85 (m, 20H, C6H5), 5.44 (s, 2H, acac CH), 2.17, 1.82
Figure 1. ORTEP diagram of W(CO)3(acac)2 (1).
fashion generates a chiral metal center; the cis orientation
of the carbon monoxide ligands maximizes their ability to
accept π-electron density from the d4 tungsten(II) center.
The bite angles adopted by the two acac chelates in 1 are
86.2 and 82.4°. Note that coordination of the two acac
chelates to the smaller chromium metal in planar Cr(acac)2
yielded bite angles of approximately 90°.19 The related Mo-
(VI) d0 complex, Mo(acac)2(O)2, contained acac chelate bite
(37) Burgmayer, S. J. N.; Templeton, J. L. Inorg. Chem. 1985, 24, 2224.
Inorganic Chemistry, Vol. 45, No. 16, 2006 6207

Jackson et al.
Scheme 2. Synthetic Route to Heptacoordinate W(II)
Dicarbonylphosphine Complexes
Table 1. Selected Bond Lengths (Å) in W(CO)3(acac)2 (1)
W(1)-C(25)
W(1)-C(21)
W(1)-C(23)
W(1)-O(8)
W(1)-O(12)
W(1)-O(1)
W(1)-O(5)
1.960(12)
1.994(15)
1.997(11)
2.083(7)
2.092(7)
2.106(7)
2.115(7)
Table 2. Selected Bond Angles (deg) in W(CO)3(acac)2 (1)
C(25)-W(1)-C(21)
C(25)-W(1)-C(23)
C(21)-W(1)-C(23)
O(8)-W(1)-O(12)
O(1)-W(1)-O(5)
69.7(5)
104.5(4)
67.9(4)
86.2(3)
82.4(3)
ature to 153 K resolved these signals into separate reso-
nances. The room-temperature carbon monoxide peak was
resolved into three distinct resonances (231, 236, and 258
ppm), compatible with the C1 symmetry of the solid-state
structure. Resolution of these three metal carbonyl carbon
peaks at low temperature confirmed the fluxionality of this
seven-coordinate molecule.
angles of 81 and 84°,38 respectively, close to the values
observed here for the acac ligands.
The tungsten-oxygen bond lengths to the four acac
oxygen atoms ranged from 2.08 to 2.12 Å for W(CO)3(acac)2,
comparable to the other Group VI bis(acac) distances. The
molybdenum(VI) dioxo complex showed a wider range of
metal-oxygen bond lengths (1.97-2.21 Å),38 reflecting the
trans effect of the oxo ligands. Both the C-C and C-O bond
lengths within each chelate ring reflected delocalization of
π-electron density. Each carbon-carbon and carbon-oxygen
bond was shorter than a typical single bond, yet longer than
a typical double bond. The same geometric description of
the chelate ring holds true for other structures determined
and discussed herein.
The three carbon monoxide ligands exhibited typical
tungsten-carbon bond lengths for tungsten carbonyl com-
plexes: 1.96, 1.99, and 2.00 Å.39 Ligand-metal-ligand bond
angles among the three carbon monoxide ligands with a
common tungsten central atom in W(CO)3(acac)2 (67.9, 69.7,
and 104.5°) were slightly smaller than but surprisingly
comparable to the OC-W-CO angles in W(CO)3(dtc)2
(71.7, 74.4, and 107.8°).40
NMR Data for W(CO)3(acac)2 (1). The room temperature
1H NMR spectrum of W(CO)3(acac)2 showed two singlets
in a 6:1 ratio: one for the twelve acac methyl protons (2.08
ppm) and one for the two acac methine protons (5.67 ppm).
As a result of the chiral tungsten(II) center and an absence
of any rotation axis, all four acac methyl groups and both
acac methine protons are positioned in unique chemical
environments in the solid state, so the room temperature 1H
NMR spectrum is clearly incompatible with a static structure
in solution. The simplicity of the spectrum, combined with
the C1 symmetry of the solid-state structure, suggested that
a fluxional process equilibrates all four methyl environments
and both methine environments on the NMR time scale.
At ambient temperature, the 13C NMR spectrum also
reflected fluxionality. A single peak was observed for all
four acac methyl carbons (27.4 ppm), one for the two acac
methine carbons (102.8 ppm), one for the four acac carbonyl
carbons (190.1 ppm), and one for the three metal carbonyl
carbons (242.5 ppm). Lowering of the acquisition temper-
Synthesis of W(CO)2(L)(acac)2 (2) [L ) PMe3 (2a) or
PMe2Ph (2b)]. The addition of one equivalent of the
appropriate phosphine to a solution of tricarbonyl complex
1 in methylene chloride led to replacement of one carbonyl
ligand by the added phosphine, forming a dicarbonylbis-
(acac)phosphine tungsten(II) complex (Scheme 2).
X-ray Structure of W(CO)2(PMe3)(acac)2 (2a). Dark red
crystals were grown from a solution of 2a in methylene
chloride layered with hexanes, which was cooled to -30
°C for 10 days. The X-ray structure of 2a confirmed the
identity of this heptacoordinate tungsten(II) complex contain-
ing two acetylacetonate chelates, two carbon monoxide
ligands, and one trimethylphosphine ligand in the coordina-
tion sphere (Figure 2). The two carbonyl ligands were cis to
one another, consistent with maximization of π-back-bonding
from the two filled dπ orbitals on the d4 metal center to CO
π* orbitals. The two tungsten-carbon bond lengths (1.96
and 1.97 Å) were nearly equal, and their values are typical
of W-C distances for tungsten-carbonyl complexes.39
Bond angles among the two carbon monoxide ligands and
the phosphine ligand with tungsten as the vertex can be
definitively mapped to comparable angles among the three
carbonyl ligands in 1. Two acute angles and a third obtuse
angle, which is ∼35° larger, characterize the ML3 fragment
geometries of both 1 and 2a. The phosphorus atom consti-
tutes one leg of the obtuse angle in the bis(acac) phosphine
(38) Krasochka, O. N.; Sokolova, Y. A.; Atovmyan, L. O. J. Struct. Chem.
1975, 16, 696.
(39) Drew, M. G. B.; Rix, C. J. J. Organomet. Chem. 1975, 102, 467.
(40) Templeton, J. L.; Ward, B. C. Inorg. Chem. 1980, 19, 1753.
Figure 2. ORTEP diagram of W(CO)2(PMe3)(acac)2 (2a).
6208 Inorganic Chemistry, Vol. 45, No. 16, 2006

Bis(acetylacetonato)tricarbonyl Tungsten(II)
Scheme 3. Proposed Movement of Phosphine Liganda
Table 3. Selected Bond Lengths (Å) in W(CO)2(PMe3)(acac)2 (2a)
W(1)-C(3)
W(1)-C(1)
W(1)-O(12)
W(1)-O(15)
W(1)-O(8)
W(1)-O(19)
W(1)-P(1)
1.962(7)
1.971(8)
2.118(5)
2.128(5)
2.141(5)
2.183(5)
2.496(2)
a Viewed parallel to the averaged mirror plane which is vertical and
perpendicular to the paper.
Table 4. Selected Bond Angles (deg) in W(CO)2(PMe3)(acac)2 (2a)
C(3)-W(1)-C(1)
C(1)-W(1)-P(1)
C(3)-W(1)-P(1)
O(15)-W(1)-O(19)
O(12)-W(1)-O(8)
73.2(3)
71.4(2)
107.6(2)
83.5(2)
82.6(2)
at 193 K to allow observation of all four unique acac methyl
peaks (2.16, 2.12, 1.96, and 1.91 ppm). More informative
was the presence of two doublets from the methyl groups of
2
the PMe2Ph ligand (1.73 and 1.66 ppm, JP-H ) 10 Hz).
These two methyl groups are diastereotopic at low temper-
ature, so the phosphine ligand is indeed bound to a chiral
tungsten center. Evidence of intramolecular fluxionality was
provided by the 31P{1H} NMR spectrum at room temperature
complexes. This thermodynamically preferred isomer is no
doubt easily accessible on the soft seven-coordinate energy
surface.
Tungsten-oxygen bond lengths for the two acetylaceto-
nate ligands range from 2.12 to 2.18 Å and are, on average,
slightly longer for the dicarbonyl phosphine complex 2a than
for the parent tricarbonyl complex 1. Increased σ-donation
from the phosphine relative to the displaced carbon monoxide
provides more electron density to the tungsten(II) center,
perhaps lengthening the tungsten-oxygen bonds. More
likely, the replacement of one small carbonyl ligand by a
bulky phosphine ligand crowds the coordination sphere and
lengthens the tungsten-oxygen bonds.
In accordance with the steric hindrance created by tri-
methylphosphine, the bite angles of the acac chelates in the
phosphine complex were, on average, slightly smaller than
those in the parent tricarbonyl complex. The loss of carbon
monoxide and coordination of a bulkier phosphine ligand
perhaps forces the two acac chelates to adopt a smaller bite
angle in a crowded seven-coordinate tungsten coordination
sphere.
The tungsten-phosphorus bond length in the bis(acac)
trimethylphosphine complex, 2a, was 2.49 Å. This value is
slightly shorter than bond lengths reported for other tungsten-
(II)-carbonyl-phosphine complexes.41,42 In 2a, the chelates
adopt a slightly different conformation than in precursor
complex 1 to accommodate the incoming phosphine ligand.
NMR Data for W(CO)2(L)(acac)2. The room temperature
1H NMR spectrum of dimethylphenylphosphine complex 2b
showed a distinct peak for each of the two methine protons
(5.76 and 5.40 ppm), indicating two inequivalent acac
environments. In the room-temperature 1H NMR spectrum,
the PMe2Ph ligand displays a lone doublet (1.77 ppm, 2JP-H
) 10 Hz) in the upfield methyl region. Note that the solid
state structure should lead to two doublets as a result of
phosphorus coupling to two distinct diastereotopic methyl
groups. Only three acac methyl peaks were observed, with
one integrating for 6H and the other two appearing as much
broader signals and integrating for 3H.
in which the lone singlet displayed tungsten satellites (183
W
(14.28%), I ) 1/2). Tungsten satellites would not be present
if the phosphine ligand were rapidly dissociating and
exchanging with other metal centers. These data indicated
that an intramolecular rearrangement process is rapidly
exchanging both the acac sites and the two carbonyl ligand
sites at room temperature, as well as equilibrating the two
phosphine methyl groups.
Additional experiments were conducted using variable-
temperature NMR to probe activation energies for the
fluxional process. The activation barrier, ∆Gq, was calculated
from the Eyring equation after the Gutowsky-Holm equation
was used to calculate kex at the coalescence temperature.43
1
At 210 K, in the H NMR spectrum, the two phosphine
methyl signals coalesced into a broad singlet, prior to
emerging into the doublet seen at ambient temperature. The
coalescence temperature and chemical shift difference led
to a calculated activation energy of 13.0 kcal/mol. The acac
methyl groups remained as distinguishable singlets until
coalescence occurred at 213 K, which, combined with the
low-temperature chemical shift difference, yielded an activa-
tion energy of 13.2 kcal/mol. Both the phosphine methyl and
the acac methyl exchange processes are attributed to a single
fluxional process with an activation barrier near 13 kcal
mol-1.
The low-temperature 1H NMR data for 2b was consistent
with the static C1 tungsten(II) bis(acac) phosphine structure
observed in the solid state for 2a. At room temperature, the
coalescence of the two diastereotopic phosphine methyl
signals and convergence of only one pair of acac methyl
signals suggested the presence of an effective mirror plane
(as a result of the intramolecular fluxional process) contain-
ing one acac metallacycle and bisecting the other. One can
imagine that the phosphine ligand swings through the mirror
plane, thus converting the diastereotopic methyl groups to
enantiomers of one another as it passes through the CS
intermediate structure.
A rapid rearrangement on the NMR time scale seemed
likely, and indeed, the fluxional process was sufficiently slow
The room-temperature 13C NMR spectrum provided further
insight into the fluxional behavior displayed by the phosphine
(41) Drew, M. G. B.; Tomkins, I. B.; Colton, R. Aust. J. Chem. 1970, 23,
2517.
(42) Churchill, M. R.; Youngs, W. J. Inorg. Chem. 1979, 18, 2454.
(43) Gutowsky, H. S.; Holm, C. H. J. Chem. Phys. 1956, 25, 1228.
Inorganic Chemistry, Vol. 45, No. 16, 2006 6209

Jackson et al.
Scheme 4. Synthetic Route to W(II) Mono(carbonyl) Mono(alkyne)
Complexes
complex, 2b. One key feature was the presence of two acac
R-carbon peaks, indicating inequivalent chelate environments
even in the fast-exchange limit. Three resonances appeared
for the acac ketone carbons, consistent with the presence of
an effective mirror plane at room temperature. Two of these
keto carbons are unique and lie in the mirror plane, and the
remaining two are equivalent to one another and lie on
opposite sides of it. However, rather than three acac methyl
carbon peaks, the spectrum showed only two. Given the fact
that three acac methyl carbons should be observed on the
Figure 3. ORTEP diagram of W(CO)(η2-PhCtCH)(acac)2 (3a).
Table 5. Selected Bond Lengths (Å) in W(CO)(η2-PhCtCH)(acac)2
(3a)
W(1)-C(1)
W(1)-C(3)
W(1)-C(4)
W(1)-O(11)
W(1)-O(15)
W(1)-O(18)
W(1)-O(22)
C(3)-C(4)
1.946(6)
2.036(5)
2.037(5)
2.058(4)
2.117(3)
2.145(3)
2.070(3)
1.301(8)
1
basis of the analogy to the H NMR spectrum, the data
suggested that one purely coincidental chemical shift overlap
occurred. A methyl doublet was observed upfield at 13.7
ppm (1J P-C ) 33 Hz) for the PMe2Ph ligand. The presence
of only one doublet demonstrated that the mirror plane also
relates the two phosphine methyl carbons to one another.
The two metal carbonyl peaks appeared as a broad singlet
in the 13C NMR spectrum at room temperature; this signal
decoalesced at low temperature. At 193 K, the metal carbonyl
carbons appeared as widely separated doublets at 245.9 (2JP-C
) 8 Hz) and 277.3 ppm (2JP-C ) 43 Hz). On the basis of an
Table 6. Selected Bond Angles (deg) in W(CO)(η2-PhCtCH)(acac)2
(3a)
C(1)-W(1)-C(3)
C(1)-W(1)-C(4)
C(3)-W(1)-C(4)
O(11)-W(1)-O(15)
O(18)-W(1)-O(22)
72.0(2)
109.2(2)
37.3(2)
82.4(1)
85.4(1)
bite angles of 82.4 and 85.4°, similar to those present in the
parent tricarbonyl complex 1. The average W-O bond
distance was 2.10 Å, also comparable to the W-O distances
in 1.
The solid-state structure showed that the alkyne and
carbonyl ligands are cis with respect to each other and that
they adopt a parallel configuration in terms of the CtC triple
bond axis and the W-CtO axis. The phenyl ring on the
alkyne is oriented anti to the carbonyl ligand.
The triple-bond carbons (C3 and C4) in the alkyne ligand
were separated by 1.30 Å, indicating a loss of some triple-
bond character upon coordination to the tungsten(II) center.
This bond length is actually closer to that of a carbon-carbon
double bond, and thus, it is compatible with a metallacy-
clopropene resonance description. Formally, 3a can be
classified as either a six- or seven-coordinate molecule, and
the metal-ligand bond angles reflect this ambiguity. It is
easy to map the geometry of 3a onto a distorted octahedral
skeleton with the alkyne ligand occupying a single site. An
angle of 161.5° was reported for O11-W1-O22, and one
of 163.5° was reported for O18-W1-C1. These values
reflect significant deviation from an ideal octahedral geom-
etry, as the atoms constituting these angles have shifted to
accommodate the incoming alkyne ligand.
2
empirical correlation of the JP-C coupling constant with
angle, the larger coupling constant suggests that carbon lies
at a more obtuse angle to the phosphine ligand than the
carbon with the smaller coupling constant.44,45 (Since these
complexes are seven-coordinate, not octahedral, the distinc-
tion is not between cis and trans but rather between acute
and obtuse.) The crystallographic data showed that a larger
angle exists between P1 and C3, so presumably this C3
carbon resonates at low field and displays the larger coupling
constant.
Synthesis of W(CO)(η2-R1C≡CR2)(acac)2 [R1 ) Ph, R2
) H (3a), R1 ) R2 ) CH3 (3b)]. The addition of 1 equiv of
alkyne to tricarbonyl complex 1 at ambient temperature
displaced two carbon monoxide ligands resulting in η2-
coordination of the incoming alkyne ligand and yielding a
monocarbonylmonoalkynebis(acac)tungsten(II)complex(Scheme
4).
X-ray Structure of W(CO)(η2-PhC≡CH)(acac)2 (3a).
Black crystals of 3a were obtained via evaporation from a
solution of methylene chloride and hexanes following column
chromatography. The X-ray crystal structure of 3a showed
a tungsten atom surrounded by two acetylacetonate ligands,
one carbon monoxide ligand, and one phenylacetylene ligand
bound in an η2-manner (Figure 3). The two chelates exhibited
NMR Data for W(CO)(η2-RC≡CR′)(acac)2. The room
temperature 1H NMR spectrum of 3a displayed four unique
methyl signals and two unique methine signals, reflecting
the inequivalent environments of the two â-diketonate
(44) Templeton, J. L.; Ward, B. C. J. Am. Chem. Soc. 1981, 103, 3743.
(45) Day, R. O.; Batschelet, W. H.; Archer, R. D. Inorg. Chem. 1980, 19,
2113.
6210 Inorganic Chemistry, Vol. 45, No. 16, 2006

Bis(acetylacetonato)tricarbonyl Tungsten(II)
Table 7. Selected Bond Lengths (Å) in W(η2-PhCtCPh)2(acac)2 (4)
Scheme 5. Rotation of the Alkyne Ligand along the Metal-Alkyne
Centroid
W(1)-C(16)
W(1)-C(30)
W(1)-C(15)
W(1)-C(29)
W(1)-O(1)
W(1)-O(8)
W(1)-O(5)
W(1)-O(12)
C(15)-C(16)
C(29)-C(30)
2.062(1)
2.067(1)
2.068(1)
2.073(1)
2.116(1)
2.130(1)
2.137(1)
2.144(1)
1.314(2)
1.311(2)
Scheme 6. Synthetic Route to W(η2-PhCtCPh)2(acac)2 (4)
Table 8. Selected Bond Angles (deg) in W(η2-PhCtCPh)2(acac)2 (4)
C(16)-W(1)-C(15)
C(30)-W(1)-C(29)
O(1)-W(1)-O(5)
O(8)-W(1)-O(12)
37.11(5)
36.92(5)
82.45(4)
82.81(4)
ligands. The terminal alkyne proton, resonating at 12.95 ppm,
appeared as a broad singlet at room temperature. On the basis
of the empirical correlation between terminal alkyne proton
chemical shifts of the coordinated acetylenic proton and
electron donation, the alkyne ligand donates four electrons
to the tungsten atom.46-49 The broadness of the acetylenic
proton peak can be attributed to the rotation about the formal
metal-alkyne centroid interconverting two isomers.46,47 The
observation of both alkyne rotamers was accomplished by
low-temperature NMR. At 193 K, the broad acetylenic proton
signal separated into two sharp singlets; the major isomer
(∼98%) resonated at 12.98 ppm, and the minor isomer
(∼2%) resonated at 12.49 ppm. The phenyl ring does not
downfield nearly 80-100 ppm from the two-electron donor
alkyne ligands.46,50,51 These 13C chemical shifts suggest that
the alkyne ligand in these tungsten(II) bis(acac) complexes
acts as a four-electron donor to the metal center, resulting
in a formal eighteen-electron complex.
Trapping of the tricarbonyl complex 1 with a symmetric
alkyne, in this case 2-butyne to yield 3b, allowed extraction
of the rotational barrier for the alkyne ligand. The rotational
activation barrier, ∆Gq, was calculated from the Eyring
equation after the Gutowsky-Holm equation was used to
calculate kex at the coalescence temperature. Variable-
temperature NMR indicated a coalescence temperature of
286 K for the rotating 2-butyne ligand, and the barrier to
rotation for this symmetric alkyne derivative was found to
be 17.8 kcal mol-1 (Scheme 5).
The room-temperature 13C NMR spectrum of 3b also
reflected helical coordination of the acac ligands. As for the
phenylacetylene derivative 3a, 10 distinct peaks were
observed for the 10 unique carbons that constitute the two
anionic chelates in 3b. Two broad peaks representing the
two methyl carbons on the alkyne ligand were resolved into
sharp singlets in the low-temperature spectrum. The char-
acterization data for 3b here differs from the data reported
earlier for this same complex, which was synthesized by a
different route. The discrepancy lies in the 1H and 13C NMR
data: two methine proton peaks and two acac methyl peaks
were reported34 in contrast to the C1 symmetry reflected in
two methine peaks and four acac methyl groups observed
in this work. Additionally, the signals reported for the metal
carbonyl carbon and the triple-bond carbon are assigned
differently here. The NMR data presented here for both
alkyne derivatives is compatible with the observed solid state
conformation of 3a.
1
reflect the fluxionality of the alkyne ligand in the H NMR
spectrum: the ortho, meta, and para protons remained sharp
and distinguishable at all temperatures. Presumably the
chemical shift variations for the aromatic protons are much
less than the roughly one-half ppm of the terminal alkyne
proton, so exchange with the minor isomer does not
noticeably broaden the aromatic peaks. The acetylacetonate
ligands and carbon monoxide ligand remained static as the
alkyne ligand rotates, and presumably the chemical shift
differences between the isomers are small so that no evidence
of fluxionality was observed in the proton signals for these
ligands by NMR spectroscopy.
At room temperature, the 13C NMR spectrum of 3a also
reflected the helical coordination mode of the two acac
ligands. Four distinct signals were present, both for the acac
methyl carbons and for the acac ketone carbons, as well as
two signals for the central acac R-carbons, compatible with
C1 symmetry for this complex. The metal carbonyl carbon
and the carbon atoms of the alkyne ligand were detected
only at low temperature. Rotation of the alkyne ligand at
room-temperature broadened the carbon signals to the point
that these 13C peaks were not observable. At 193 K it was
possible to assign resonances for the triple-bond carbons, as
well as for the carbon atoms in the phenyl ring. Chemical
shift values for the alkyne carbons in both 3a (211.7 and
185.6 ppm) and 3b (209.2 and 188.4 ppm) were shifted
Synthesis of W(η2-PhCtCPh)2(acac)2 (4). Tricarbonyl
reagent 1 was refluxed with excess diphenylacetylene which
led to replacement of all three carbon monoxide ligands by
two alkyne ligands, resulting in tungsten(II) bis(acac) bis-
(diphenylacetylene), complex 4 (Scheme 6).
X-ray Structure of W(η2-PhC≡CPh)2(acac)2 (4). Bright
yellow crystals of complex 4 were grown by evaporating a
solution of 4 in methylene chloride and hexanes. X-ray
(46) Ward, B. C.; Templeton, J. L. J. Am. Chem. Soc. 1980, 102, 1532.
(47) Templeton, J. L. AdV. Organomet. Chem. 1989, 29, 1.
(48) McDonald, J. W.; Corbin, J. L.; Newton, W. E. J. Am. Chem. Soc.
1975, 97, 1970.
(50) Thomas, J. L. Inorg. Chem. 1978, 17, 1507.
(49) King, R. B. Inorg. Chem. 1968, 7, 1044.
(51) Templeton, J. L.; Ward, B. C. J. Am. Chem. Soc. 1980, 102, 3288.
Inorganic Chemistry, Vol. 45, No. 16, 2006 6211

Jackson et al.
Table 9. Crystal and Data Collection Parameters for W(CO)3(acac)2 (1), W(CO)2(PMe3)(acac)2 (2a), W(CO)(η2-PhCtCH)(acac)2 (3a),
W(η2-PhCtCPh)2(acac)2 (4)
empirical formula
complex
C13H14O7W
1
C15H23O6PW
2a
C19H20O5W
3a
C38H34O4W
4
fw
466.09
514.15
512.24
738.50
color
temp
red
100 K
dark red
100 K
black
100 K
bright yellow
100 K
wavelength
cryst syst
space group
unit cell dimensions
0.71703 Å
monoclinic
C2/c
0.71703 Å
monoclinic
P21/c
0.71703 Å
monoclinic
P21/n
0.71703 Å
triclinic
P1h
a ) 9.545 Å
b ) 9.897 Å
c ) 16.929 Å
R ) 98.517°
â ) 91.869°
γ ) 91.631°
1580Å3
a ) 15.261 Å
b ) 15.416 Å
c ) 12.883 Å
R ) 90°
a ) 12.891 Å
b ) 9.4512 Å
c ) 15.714 Å
R ) 90°
a ) 8.439 Å
b ) 20.224 Å
c ) 11.192 Å
R ) 90°
â ) 94.87°
γ ) 90°
â ) 105.093°
γ ) 90°
â ) 106.649°
γ ) 90°
vol
3020 Å3
1848 Å3
1830 Å3
Z
8
4
4
2
density (calcd)
abs coeff
F(000)
2.050 Mg/m3
7.68 mm-1
1776
1.848 Mg/m3
6.36 mm-1
1000
1.866 Mg/m3
6.35 mm-1
998
1.552 Mg/m3
6.70 mm-1
736
cryst size
θ range
index ranges
0.30 × 0.05 × 0.05 mm3
1.88-24.07°
-17 e h e 17
-17 e k e 17
-14 e l e 12
13 400
0.25 × 0.20 × 0.10 mm3
2.71-25.00°
-14 e h e 15
-11 e k e 10
-18 e l e 18
10 605
0.20 × 0.20 × 0.10 mm3
5.00-56.00°
-11 e h e 10
0 e k e 26
0 e l e 14
17 909
0.35 × 0.20 × 0.15 mm3
2.08-30.00°
-13 e h e 13
-13 e k e 13
-23 e l e 23
44 704
reflns collected
independent reflns
2347
3237
4433
9174
Rint ) 0.0694
2347/0/195
1.083
Rint ) 0.0313
3237/0/216
1.177
Rint ) 0.042
4433/0/227
1.313
Rint ) 0.0218
9174/0/392
1.031
data/restraints/params
GOF on F2
final R indices
[I > 2σ(I)]
R1 ) 0.0379
wR2 ) 0.0709
R1 ) 0.0689
wR2 ) 0.0837
1.273 and -1.355 e Å-3
R1 ) 0.0338
wR2 ) 0.0742
R1 ) 0.0420
wR2 ) 0.0769
1.859 and -1.382 e Å-3
R1 ) 0.032
wR2 ) 0.039
R1 ) 0.044
wR2 ) 0.141
2.000 and -1.430 e Å-3
R1 ) 0.0139
wR2 ) 0.0341
R1 ) 0.0145
wR2 ) 0.0343
0.907 and -0.652 e Å-3
R indices
(all data)
largest diff.
peak and hole
NMR Data for W(η2-PhC≡CPh)2(acac)2 (4). The room-
1
temperature H NMR spectrum for 4 showed two acac
methyl resonances (2.17 and 1.82 ppm) and one acac methine
resonance (5.44 ppm) in addition to indistinguishable mul-
tiplets in the aromatic region (7.15-7.85 ppm), consistent
with formation of a bis(acac)bis(alkyne) complex. For
symmetric alkyne ligands, such as diphenylacetylene, these
molecules contain a C2 axis. The two acac ligands are
equivalent to one another because of the C2 axis, but within
each acac ligand, one of the methyl proton reporter groups
is cis to an alkyne ligand and one is trans, thus producing
1
two methyl signals in the H NMR.
Figure 4. ORTEP diagram of W(η2-PhCtCPh)2(acac)2 (4).
The room-temperature 13C NMR data for 4 confirmed the
presence of a 2-fold axis generated by the coordination of
two coparallel alkynes to the tungsten(II) bis(acac) fragment.
diffraction studies of 4 verify coordination of two acetylac-
etonate ligands and two diphenylacetylene ligands in a cis
fashion to a tungsten center (Figure 4). Both alkyne ligands
are parallel to one another; the triple bonds are aligned. This
mode of coordination has been observed in other group VI
bis(alkyne) complexes with two anionic bidentate che-
lates.52,53 The tungsten-oxygen bond lengths (2.12-2.14 Å)
are slightly longer, on average, than those seen in complexes
1 and 3a, but they are comparable to those of the phosphine
adduct, complex 2a. The bite angles of the oxygen donor
chelates are nearly equal in 4 (82.5 and 82.8°), reflecting
the C2 symmetry generated by coordination of two symmetric
alkynes.
1
The spectrum mimicked the behavior displayed in the H
NMR spectrum: two resonances were present for the four
acac methyl carbons (26.6 and 28.1 ppm), and one resonance
appeared for the two acac R-carbons, (102.0 ppm). Likewise,
only two peaks were present for the four acac ketone carbons
(185.4 and 190.4 ppm).
Previous studies have shown that the number of electrons
donated by η2-bound alkyne ligands roughly correlates with
the chemical shift of the alkyne carbons.51 The alkyne
carbons of 4 were slightly broadened in the room-temperature
13C NMRspectrum, suggesting that the rate of rotation of
the alkyne ligands was approaching the NMR time scale.
The chemical shifts of these two alkyne carbons appeared
at δ ) 194.3 and 201.4 ppm (av ) 197.9 ppm). This
(52) Herrick, R. S.; Nieter-Burgmayer, S. J.; Templeton, J. L. Inorg. Chem.
1983, 22, 3275.
(53) Herrick, R. S.; Templeton, J. L. Organometallics 1982, 1, 842.
6212 Inorganic Chemistry, Vol. 45, No. 16, 2006

Bis(acetylacetonato)tricarbonyl Tungsten(II)
averaged 13C chemical shift is farther downfield than
comparable values for other complexes containing two alkyne
ligands that formally donate three electrons each to the metal
center.51-53 Perhaps this stems from the electronegativity of
the oxygen-donor chelates. Surrounded by four oxygen
atoms, the tungsten center is more electron deficient than in
the analogous dithiocarbamate complexes, and as a result,
the two alkyne ligands provide greater π-electron donation
to counteract this effect. Of course, formal electron counting
is merely a method of bookkeeping and is not sophisticated
enough to track the continuum of bonding options available
to alkyne ligands.
plex, W(CO)3(acac)2 (1). The addition of phosphine nucleo-
philes (L ) PMe3 (2a), PMe2Ph (2b)) to 1 generated seven-
coordinate complexes of the type W(CO)2(L)(acac)2. The
parent complex, 1, also reacted readily with alkynes (PhCt
CH (3a) and MeCtCMe (3b)) to form complexes of the
type W(CO)(η2-R1CtCR2)(acac)2. Refluxing 1 with excess
diphenylacetylene led to coordination of two alkyne ligands
and yielded the bis(alkyne) complex, W(PhCtCPh)2(acac)2
(4). X-ray crystallography provided structural assignments
for a representative molecule from each class of these
complexes, and variable-temperature 1H and 13C NMR was
used to probe the fluxional processes within this series of
bis(acetylacetonate) tungsten d4 complexes.
IV. Summary
Acknowledgment. We thank the National Science Foun-
dation (CHE-0414726) for financial support.
Chiral, neutral, monomeric complexes containing the
W(II)(acac)2 (acac ) acetylacetonate) moiety have been
synthesized from a convenient starting material, the pentac-
arbonyl iodotungsten(0) anion. Oxidation of the tungsten(0)
reagent with elemental iodine and the subsequent addition
of two equivalents of the acac anion resulted in the formation
of a reactive seven-coordinate 18-electron tricarbonyl com-
Supporting Information Available: Crystallographic data in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC0600329
Inorganic Chemistry, Vol. 45, No. 16, 2006 6213