Tungsten(II) monocarbonyl bis(acetylacetonate): A fourteen-electron docking site for η2 four-electron donor ligands

Article abstract of DOI:10.1021/ja0728270

Tungsten(II) tricarbonyl bis(acetylacetonate) readily loses two equivalents of carbon monoxide upon exposure to nitrile reagents. Nitriles bind to tungsten as four-electron donors via their π-bonds, as previously reported with alkyne ligands. Imines, aldehydes, and ketones also bind to the tricarbonyl reagent as four-electron donors with two electrons originating from the π-bond and the final two electrons stemming from heteroatom lone pair donation. Experimental data suggest a high degree of double-bond character between tungsten and the heteroatom of the imine, aldehyde, or ketone. These findings are supported by DFT calculations on model complexes. Copyright

Full text of DOI:10.1021/ja0728270

Published on Web 08/16/2007
Tungsten(II) Monocarbonyl Bis(acetylacetonate): A Fourteen-Electron
Docking Site for η2 Four-Electron Donor Ligands
Andrew B. Jackson, Cynthia K. Schauer, Peter S. White, and Joseph L. Templeton*
Department of Chemistry, UniVersity of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290
Received April 23, 2007; E-mail: joetemp@unc.edu
Organometallic complexes of d4 tungsten and molybdenum often
promote formal four-electron bonding of alkynes,1-3 and nitriles4-9
from the triple bond. Molecules containing a carbon-heteroatom
double bond such as imines,10-13 and aldehydes and ketones,14-18
typically bind to these metals as formal two-electron donors whether
bound as η1 or η2 ligands.
We recently reported the reactivity of W(CO)3(acac)2, 1, (acac
) acetylacetonate) with four-electron donor alkynes and two-
electron donor phosphines.19 Coordination of alkynes as formal four-
Figure 1. (Left) ORTEP diagram of W(CO)(acac)2(η2-2,6-dichloroben-
zonitrile) 2c; (right) ORTEP diagram of W(CO)(acac)2(η2-PhNdCHPh) 3.
Hydrogen atoms are omitted for clarity.
electron donor ligands via the CtC triple bond π-electrons19
prompted us to explore isoelectronic nitrile reagents as possible
four-electron donor ligands to this tungsten(II) center. Indeed, when
W(CO)3(acac)2 1 is exposed to nitriles, two equivalents of carbon
monoxide are released to accommodate coordination of an η2-nitrile
ligand. Monitoring reaction progress via IR spectroscopy revealed
the growth of a single CO stretch consistent with the formation of
W(CO)(acac)2(η2-NtCR) [R ) Me (2a), Ph (2b)] (Scheme 1).
The 1H NMR spectrum of the acetonitrile adduct, W(CO)(acac)2-
(η2-NtCCH3), 2a, exhibits two singlets for the acac methine
protons and four singlets for the acac methyl groups. The methyl
group of the bound acetonitrile resonates downfield at 3.71 ppm,
suggestive of a π-bound nitrile ligand. More diagnostic than proton
NMR data is the 13C NMR chemical shift for the nitrile carbon at
208.7 ppm, consistent with data reported for other four-electron
donor nitriles.4-9
Scheme 1
The X-ray structure of the 2,6-dichlorobenzonitrile derivative
2c shows two acac chelates and one carbon monoxide ligand bound
to tungsten with a π-bound nitrile ligand occupying the sixth site
of a primitive octahedron. The η2-nitrile ligand is located cis to
carbon monoxide, and the CtN triple bond is approximately
parallel to the W-CtO axis (Figure 1). The carbon atom in the
nitrile ligand is located proximal to carbon monoxide and the
nitrogen atom is located distal to CO. Bond distances to the nitrile
ligand are nearly equal: W(1)-C(4) ) 2.04 Å and W(1)-N(3) )
2.02 Å. The carbon-nitrogen triple bond distance in the coordinated
nitrile ligand is 1.27 Å.
Selective coordination of nitrile reagents through the π-system
reflects the propensity of the W(CO)(acac)2 fragment to bind a
ligand that will provide four electrons to the metal. Could other
small molecules containing heteroatom π-bonds serve as formal
four-electron donors to the 14-electron W(CO)(acac)2 fragment?
Surprisingly, treatment of 1 with N-benzylideneaniline (PhNd
CHPh) in CH2Cl2 at room temperature generates an η2-imine
complex of the formula W(CO)(acac)2(η2-PhNdCHPh), 3 (Scheme
1). Coordination of the imine to tungsten creates a new stereocenter
at the imine carbon, with tungsten as a second stereocenter.
Formation of two diastereomers in a 2:1 ratio is indicated in the
1H NMR spectrum by an upfield shift in the imine proton to 6.15
(major) and 5.71 (minor) ppm. An even more dramatic upfield shift
is observed for the imine carbon which resonates at 57.3 (major)
and 54.3 (minor) ppm in the 13C NMR spectrum. Although η2-
imine complexes are well-known,10-13 simple substitution of two
carbon monoxide ligands by an imine reagent at room temperature
is unprecedented.
Similar results were obtained upon addition of benzaldehyde to
1
tricarbonyl reagent 1. The H NMR spectrum for W(CO)(acac)2-
(η2-OdCHPh), 4a, shows two diastereomers in a 5:4 ratio.
Coordination to the tungsten(II) center shifts the aldehyde proton
upfield to 7.73 (major) and 7.66 (minor) ppm. 13C NMR data shows
the coordinated aldehyde carbon shifted upfield to 88.9 (major)
and 86.7 (minor) ppm.
To check ketone reactivity, 1 was stirred in neat acetone. As
with the imine and aldehyde substrates, IR spectroscopy suggested
a tungsten product retaining one CO ligand. Note that W(CO)-
(acac)2[η2-OdC(CH3)2], 5, lacks a second stereocenter, so only one
product was observed in the 1H NMR spectrum. The diastereotopic
methyl groups of coordinated acetone resonate at 2.53 and 2.31
1
ppm in the H NMR spectrum, while the ketone carbon resonates
upfield at 97.2 ppm in the 13C NMR spectrum.
X-ray structures of complexes with imine 3 (Figure 1), aldehyde
4b, and ketone 5 (Figure 2) ligands show η2
-coordination of the
organic substrate with the CdX (X ) N, O) unit aligned nearly
parallel to the W-CtO axis. This system exhibits a definitive
preference for positioning the carbon atom proximal to CO and
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10.1021/ja0728270 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
eters of the optimized DFT structures are summarized in Table 2.
As in the structurally characterized aldehyde complex, 4b, the
lowest-energy rotamer of the model formaldehyde complex 6
positions the oxygen atom distal to CO, with the proximal rotamer
lying 11.5 kcal mol-1 higher in energy. The difference in the M-C
and M-O distances in 4b (0.26 Å) is reproduced well in the
calculated structure for 6 (0.24 Å). Calculated structures for both
of the d6 complexes show a significantly longer M-O bond, which
gives rise to a smaller difference between the M-O and M-C
distances (0.06 Å). These calculated structural features are consistent
with the absence of lone-pair donation from oxygen in these η2-
aldehyde adducts. Additionally, the lowest energy rotamer of the
d6 complexes aligns the CdO unit perpendicular to CO, allowing
backbonding to the η2-aldehyde from dxy without competition from
CO.
Figure 2. (Left) ORTEP diagram of W(CO)(acac)2(η2-2,6-dichloroben-
zaldehyde) 4b; (right) ORTEP diagram of W(CO)(acac)2(η2-acetone) 5.
In summary, we report a tungsten(II) d4 center that seeks four-
electrons from η2-nitriles, imines, aldehydes, and ketones. Nitrile
ligands donate four electrons from the two orthogonal π-orbitals
of the triple bond. Imines, aldehydes, and ketones also provide four
electrons to the d4 metal center, but two originate in the CdX
π-bond and the other two arise from a lone pair on the heteroatom.
Figure 3. Orbital interactions of tungsten and π-bound substrate.
Table 1. Selected Bond Lengths (in Å) for Complexes 2c, 3, 4b, 5
2c
3
4b
5
Acknowledgment. We wish to thank the National Science
Foundation (Grant 0717086) for funding, and Chetna Khosla for
experimental assistance.
W-C
W-Xa
Xa-C
2.038(5)
2.018(5)
1.270(7)
2.237(7)
1.934(6)
1.403(8)
2.210(3)
1.954(4)
1.377(4)
2.211(8)
1.934(5)
1.380(10)
Supporting Information Available: Experimental procedures and
characterization of complexes 2a-5, X-ray data for 2c, 3, 4b, and 5
(CIF), and DFT computational details of 6-8. This material is available
free of charge via the Internet at http://pubs.acs.org.
a X ) N (2c, 3), O (4b, 5).
Table 2. Selected Bond Lengths (in Å) from DFT Studies of
W(acac)2(CO)(CH2O) (6), W(acac)2(CO)(CH2O)2- (7), and
Os(acac)2(CO)(CH2O) (8)
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