Some reactions involving [W(N-2,6-Me2C6H 3)(OCMe2CF3)2]2, a symmetric d2/d2 dimer that contains no bridging ligands

Article abstract of DOI:10.1021/om800182g

(Me₂CF₃CO)₂(Ar′N)W=W(NAr′) (OCMe₂CF₃)₂ (Ar′ = 2,6-Me ₂C₆H₃) reacts immediately at room temperature with 1 equiv of CCl₄ to give the sparingly soluble [W(OCMe ₂CF₃)₂Cl]₂(μ-NAr′) ₂. [W(NAr′)(OCMe₂CF₃)₂] ₂ forms 1:1 adducts with simple donor ligands such as PMe ₃, PPhMe₂, and pyridine, in which the base is likely to be coordinated to a single W center. Addition of ethylene to [W(NAr′) (OCMe₂CF₃)₂]₂ yields [W(NAr′)(OCMe₂CF₃)₂]₂(M-C ₂H₄), in which ethylene is bound essentially to one W, although it also could be argued that a 1,2-dimetallacyclobutane ring is present. Heating a toluene solution of [W(NAr′)(OCMe₂CF ₃)₂]₂(w-C₂H₄) produces (OCMe₂CF₃)₂(Et)W(μ-C₂H ₄)(μ-N-2-CH₂-6-MeC₆H₃)W(OCMe ₂CF₃)₂, in which the proton required to form the ethyl ligand arises through CH activation in one of the methyl groups of the NAr′ ligand. Treatment of [W(NAr′)(OCMe₂CF ₃)₂]₂ with 2-butyne yields [W(NAr′) (OCMe₂CF₃)₂]₂(μ-MeCCMc). Addition of 4 equiv of CH₃CN to [W(NAr′)(OCMe₂CF ₃)₂]₂ in benzene gives a mixture of two products, A and B, which have the empirical formula W(NAr′)(OCMe ₂CF₃)₂(CH₃CN), according to elemental analyses. Compound B was found to contain the [μ-NC(Me)=C(Me)N] ^4- ligand in an unsymmetric dimer in which one nitrogen of the [μ-NC(Me)=C(Me)N]^4- ligand is bridging while the other is bound in a terminal fashion to one metal. Compound A is proposed to be a C _s-symmetric structural isomer of B in which the [N(Me)C=C(Me)N] ^4- ligand is symmetrically bridging the two metal centers. X-ray studies are reported for [W(OCMe₂CF₃)₂Cl] ₂(μ-NAr′)₂, [W(NAr′)(OCMe ₂CF₃)₂]₂(μ-MeCCMe), and B.

Full text of DOI:10.1021/om800182g

Organometallics 2008, 27, 3857–3865
3857
Some Reactions Involving [W(N-2,6-Me2C6H3)(OCMe2CF3)2]2, a
Symmetric d2/d2 Dimer that Contains No Bridging Ligands
Lourdes Pia H. Lopez, Richard R. Schrock,* and Peter Mu¨ller
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
ReceiVed February 26, 2008
(Me2CF3CO)2(Ar′N)WdW(NAr′)(OCMe2CF3)2 (Ar′ ) 2,6-Me2C6H3) reacts immediately at room
temperature with 1 equiv of CCl4 to give the sparingly soluble [W(OCMe2CF3)2Cl]2(µ-NAr′)2.
[W(NAr′)(OCMe2CF3)2]2 forms 1:1 adducts with simple donor ligands such as PMe3, PPhMe2, and
pyridine, in which the base is likely to be coordinated to a single W center. Addition of ethylene to
[W(NAr′)(OCMe2CF3)2]2 yields [W(NAr′)(OCMe2CF3)2]2(µ-C2H4), in which ethylene is bound essentially
to one W, although it also could be argued that a 1,2-dimetallacyclobutane ring is present. Heating a
toluene solution of [W(NAr′)(OCMe2CF3)2]2(µ-C2H4) produces (OCMe2CF3)2(Et)W(µ-NAr′)(µ-N-2-CH2-
6-MeC6H3)W(OCMe2CF3)2, in which the proton required to form the ethyl ligand arises through CH
activation in one of the methyl groups of the NAr′ ligand. Treatment of [W(NAr′)(OCMe2CF3)2]2 with
2-butyne yields [W(NAr′)(OCMe2CF3)2]2(µ-MeCCMe). Addition of
4 equiv of CH3CN to
[W(NAr′)(OCMe2CF3)2]2 in benzene gives a mixture of two products, A and B, which have the empirical
formula W(NAr′)(OCMe2CF3)2(CH3CN), according to elemental analyses. Compound B was found to
contain the [µ-NC(Me)dC(Me)N]4- ligand in an unsymmetric dimer in which one nitrogen of the
[µ-NC(Me)dC(Me)N]4- ligand is bridging while the other is bound in a terminal fashion to one metal.
Compound A is proposed to be a Cs-symmetric structural isomer of B in which the [N(Me)CdC(Me)N]4-
ligand is symmetrically bridging the two metal centers. X-ray studies are reported for [W(OCMe2CF3)2Cl]2(µ-
NAr′)2, [W(NAr′)(OCMe2CF3)2]2(µ-MeCCMe), and B.
bonds that are not bridged by imido or alkoxide ligands.3 All
species contain an unbridged W(IV)/W(IV) double bond with
a WdW bond distance of 2.4-2.5 Å, an approximate 90° angle
between the imido ligands and the WdW bond, and a transoid
relationship between the two imido ligands across the WdW
bond. There is no indication that WdW bonds are broken
spontaneously in any of these species: e.g., heterochiral and
homochiral [W(NR)(CH2-t-Bu)(OR′′)]2 species do not inter-
convert readily. Molybdenum imido alkylidene complexes also
decompose to yield dimeric species. Dimeric Mo compounds
that contain imido bridging ligands, as noted above for [Mo(µ-
NAr)(O-t-Bu)2]2,2 as well as compounds that contain unbridged
ModMo bonds analogous to WdW species have been isolated.
In one case, {Mo(N-1-adamantyl)[OCMe(CF3)2]2}2, both iso-
mers have been crystallographically characterized.4 Rhenium
complexes of the type Re(C-t-Bu)(CH-t-Bu)(OR′′)2,5 which are
isoelectronic with W(NAr)(CH-t-Bu)(OR′′)2 species, have been
shown to react with CH2dCHOEt to yield observable but
unstable intermediate Re(C-t-Bu)(CHOEt)(OR′′)2 species. These
species decompose to yield [Re(C-t-Bu)(OR′′)2]2 complexes that
also contain unbridged RedRe bonds, a staggered, ethane-like
geometry, and a transoid relationship between the two neopen-
tylidyne ligands across the RedRe bond. Upon irradiation with
360 nm light the [Re(C-t-Bu)(OR)2]2 complexes are transformed
into [Re(µ-C-t-Bu)(OR)2]2 species whose structures contain µ-C-
t-Bu groups, according to NMR studies and an X-ray study of
Introduction
In 1990 we reported the syntheses of a variety of tungsten
imido alkylidene bis(alkoxide) complexes of the type
W(NR)(CHR′)(OR′′)2, in which R ) 2,6-Me2C6H3 (Ar′), 2,6-
i-Pr2C6H3 (Ar).1 We found that compounds in which the
alkylidene is relatively small (CHR′ ) CHEt) decompose to
give dimeric species with the empirical formula W(NR)(OR′′)2
(according to NMR and elemental analyses) that we proposed
to be bimetallic species in which the two metals are bridged by
imido ligands: i.e., [W(µ-NR)(OR′′)2]2. (A bridging imido
formulation was proposed by analogy with the X-ray structure
obtained for [Mo(µ-NAr)(O-t-Bu)2]2, a compound obtained upon
decomposition of the unsubstituted molybdacyclobutane com-
plex Mo((CH2CH2CH2)(NAr)(O-t-Bu)2.2) For example, {W-
(NAr′)[OCMe(CF3)2]2}2 was isolated from the reaction between
W(NAr′)(CH-t-Bu)[OCMe(CF3)2]2 and 3-hexene to yield in-
termediate, unstable W(NAr′)(CHEt)[OCMe(CF3)2]2. An at-
tempted X-ray study of brown crystalline “W(NAr′)[OCMe-
(CF3)2]2” was not of sufficient quality to conclude whether
bridging imido ligands were present or not. Other propylidene
complexes also were found to be unstable with respect to
formation of dimers via bimolecular coupling of alkylidenes.
We have now shown that several of these [W(NR)(OR′′)2]2
compounds, including {W(NAr′)[OCMe(CF3)2]2}2, as well as
several compounds of the type [W(NR)(CH2-t-Bu)(OR′′)]2 (both
heterochiral and homochiral forms) actually contain WdW
(1) Schrock, R. R.; DePue, R. T.; Feldman, J.; Yap, K. B.; Yang, D. C.;
Davis, W. M.; Park, L. Y.; DiMare, M.; Schofield, M.; Anhaus, J.;
Walborsky, E.; Evitt, E.; Kru¨ger, C.; Betz, P. Organometallics 1990, 9,
2262.
(2) Robbins, J.; Bazan, G. C.; Murdzek, J. S.; O’Regan, M. B.; Schrock,
R. R. Organometallics 1991, 10, 2902.
(3) (a) Lopez, L. P. H.; Schrock, R. R. J. Am. Chem. Soc. 2004, 126,
9526. (b) Lopez, L. P. H.; Schrock, R. R.; Mu¨ller, P. Organometallics 2006,
25, 1978.
(4) Lopez, L. P. H. Ph.D. Thesis, Massachusetts Institute of Technology,
2005.
(5) Toreki, R.; Schrock, R. R. J. Am. Chem. Soc. 1992, 114, 3367.
10.1021/om800182g CCC: $40.75
2008 American Chemical Society
Publication on Web 07/11/2008

3858 Organometallics, Vol. 27, No. 15, 2008
Lopez et al.
a derivative.6 Therefore, it has been established that d2/d2 imido
species can be formed in which the MdM bond (M ) Mo, W,
Re) is not bridged by any potentially bridging ligand. It should
be noted that reduction of W(N-t-Bu)(Silox)2Cl2 yields pseudo-
trigonal-planar W(N-t-Bu)(Silox)2; presumably a WdW bond
cannot form for steric reasons under these circumstances.7
Dimeric species that contain MdM bonds are extremely rare.
The only compounds whose reactivities have been studied in
some depth are [Os2(CO)8],8 [W2(OCH2-t-Bu)8],9 and [(η5-
C5Me5)2Re2(CO)4].10 The d2/d2 WdW and ModMo species are
especially interesting, as a consequence of the fact that they
will metathesize certain olefins slowly,11 possibly as a conse-
quence of reacting with some olefins to produce
M(NR)(CHR′)(OR′′)2 species in low yield. How this might be
accomplished is not known. However, one attractive possibility
is that the manner in which MdM bonds are formed is reversible
to some degree in some circumstances: i.e., MdM bonds react
with certain CdC bonds to yield MdC species. This reaction
would be analogous to that between Mt M bonds and Ct C
bonds to yield Mt C bonds.12 There are several other possibili-
ties, among them rearrangement of an olefin to an alkylidene
in a manner described by Wolczanski in Nb and Ta chemistry13
and formation of an alkylidene through contraction of a
metallacyclopentane to a metallacyclobutane ring.14
to give a sparingly soluble, light red product in high yield (93%).
Proton and fluorine NMR spectra suggest that only one type of
alkoxide ligand is present in a highly symmetric species. An
X-ray structure of crystals grown from toluene at -30 °C
showed the compound to be [W(OCMe2CF3)2Cl]2(µ-NAr′)2 (eq
1). The product results from a net oxidation of each metal center
from WIV to WV. Only 1 equiv of carbon tetrachloride (per
dimer) is required, but the fate of the “CCl2” byproduct has not
been determined. Further oxidation of [W(OCMe2CF3)2Cl]2(µ-
NAr′)2 by CCl4 was not observed at room temperature in
solution.
The solid-state structure of [W(OCMe2CF3)2Cl]2(µ-NAr′)2
(Table 1) is shown in Figure 1, and the pertinent bond lengths
and angles are given in Table 2. The structure exhibits
approximate trigonal -bipyramidal geometry about each metal,
with equatorial alkoxide and bridging imido groups and an axial
Cl ligand. The structure is slightly folded about the W-W bond,
with a dihedral “butterfly” angle of 155.70(12)°. However, NMR
data at 22 °C suggest that the folded structure is not maintained
in solution: i.e., the two alkoxides are equivalent on the NMR
time scale. The W-W bond length of 2.6296(2) Å is consistent
with a (W-W)10+ core which contains a formal d1-d1 M-M
single bond,9a,c in agreement with the observed diamagnetism
of the compound. Lengthening of the W-N bonds by ∼0.2 Å
and the decrease in W-N-C bond angles (to ∼135°) relative
to those of the parent compound (WdN ) 1.753(3) Å and
WdN-C ) 172.1(3)°) are what one might expect from the
observed change in coordination mode of the imido ligands.
The W-Cl distances are not unusual,15 and other bond lengths
and angles are not much different from those in the parent
species, [W(NAr′)(OCMe2CF3)2]2.
In this paper we report some exploratory reactions of one
unbridged WdW species, [W(NAr′)(OCMe2CF3)2]2. This spe-
cies can be obtained on a multigram scale from W(NAr′)(CH-
t-Bu)(OCMe2CF3)2.1 We have avoided reactions involving
dimers of the type [W(NAr′)(CH2-t-Bu)(OR)]2 in this initial
study, in order to avoid complications arising from (inter alia)
formation of diastereomers and in order to eliminate possible
reactions involving the neopentyl ligand.
Results and Discussion
Oxidation of the WdW Bond. [W(NAr′)(OCMe2CF3)2]2
reacts immediately at room temperature with 1 equiv of CCl4
Attempts to further oxidize the dimer to give W(NAr′)-
(OCMe2CF3)2Cl2 consisted of treatment of a hydrocarbon
solution of [W(NAr′)(OCMe2CF3)2]2 with 2 equiv of C2Cl6 at
-30 °C. Only substantial decomposition was observed. Simi-
larly, the reaction between [W(NAr′)(OCMe2CF3)2]2 and 2 equiv
of I2 did not yield W(NAr′)(OCMe2CF3)2I2 but a complex
mixture that contained (inter alia) W(NAr′)(OCMe2CF3)3I and
mer-W(OCMe2CF3)3I3, the identities and connectivities of which
were determined through X-ray crystallographic techniques.16
Complete structural characterization of these W(VI) compounds
was not pursued. It is clear that ligand transfer from one metal
to another and W-W bond cleavage to yield monomeric species
are potentially facile under some conditions.
Reactions Involving Two-Electron Donors. Studies revealed
that [W(NAr′)(OCMe2CF3)2]2 forms 1:1 adducts with Lewis
bases such as PMe3, PPhMe2, and pyridine, even in the presence
of an excess of base (eq 2). The 1H NMR spectra of the
monoadducts are all characteristic of molecules that have no
symmetry. For example, the 1H NMR spectrum of
[W(NAr′)(OCMe2CF3)2]2(PMe3) reveals four inequivalent
OCMe2CF3 ligands with eight distinct singlets being observed
for the Me groups. All four CF3 groups likewise appear as well-
separated resonances in the 19F NMR spectrum. The 31P NMR
(6) Toreki, R.; Schrock, R. R.; Davis, W. M. J. Organomet. Chem. 1996,
520, 69.
(7) Eppley, D. F.; Wolczanski, P. T.; Van Duyne, G. D. Angew. Chem.,
Int. Ed. Engl. 1991, 30, 584.
(8) (a) Hembre, R. T.; Scott, C. P.; Norton, J. R. J. Am. Chem. Soc.
1987, 109, 3468. (b) Grevels, F. W.; Klotzbuecher, W. E.; Seils, F.;
Schaffner, K.; Takats, J. J. Am. Chem. Soc. 1990, 112, 1995. (c) Haynes,
A.; Poliakoff, M.; Turner, J. J.; Bender, B. R.; Norton, J. R. J. Organomet.
Chem. 1990, 383, 497.
(9) (a) Chisholm, M. H.; Streib, W. E.; Tiedtke, D. B.; Wu, D. D. Chem.
Eur. J. 1998, 4, 1470. (b) Chisholm, M. H.; Click, D. R.; Gallucci, J. C.;
Hadad, C. M.; Wilson, P. J. J. Am. Chem. Soc. 2002, 124, 14518. (c)
Chisholm, M. H.; Click, D. R.; Gallucci, J. C.; Hadad, C. M.; Wilson, P. J.
Organometallics 2003, 22, 4725. (d) Bollinger, J. C.; Chisholm, M. H.;
Click, D. R.; Folting, K.; Hadad, C. M.; Tiedtke, D. B.; Wilson, P. J.
J. Chem. Soc., Dalton Trans. 2001, 14, 2074.
(10) (a) Casey, C. P.; Sakaba, H.; Hazin, P. N.; Powell, D. R. J. Am.
Chem. Soc. 1991, 113, 8165. (b) Casey, C. P.; Carino, R. S.; Sakaba, H.;
Hayashi, R. K. Organometallics 1996, 15, 2640. (c) Casey, C. P.; Carino,
R. S.; Hayashi, R. K.; Schladetzky, K. D. J. Am. Chem. Soc. 1996, 118,
1617. (d) Casey, C. P.; Carino, R. S.; Sakaba, H. Organometallics 1997,
16, 419.
(11) Schrock, R. R.; Lopez, L. P. H.; Hafer, J.; Singh, R.; Sinha, A.;
Mu¨ller, P. Organometallics 2005, 24, 5211.
(12) (a) Schrock, R. R.; Listemann, M. L.; Sturgeoff, L. G. J. Am. Chem.
Soc. 1982, 104, 4291. (b) Listemann, M. L.; Schrock, R. R. Organometallics
1985, 4, 74. (c) Murdzek, J. S.; Schrock, R. R. In Carbyne Complexes;
VCH: New York, 1988.
(13) Hirsekorn, K. F.; Veige, A. S.; Marshak, M. P.; Koldobskaya, Y.;
Wolczanski, P. T.; Cundari, T. R.; Lobkovsky, E. B. J. Am. Chem. Soc.
2005, 127, 4809.
(14) (a) McLain, S. J.; Sancho, J.; Schrock, R. R. J. Am. Chem. Soc.
1979, 101, 5451. (b) Schrock, R. R.; McLain, S. J.; Sancho, J. Pure Appl.
Chem. 1980, 52, 729. (c) Schrock, R. R.; Duval-Lungulescu, M.; Tsang,
W. C. P.; Hoveyda, A. H. J. Am. Chem. Soc. 2004, 126, 1948.
(15) Cotton, F. A.; DeMarco, D.; Kolthammer, B. S.; Walton, R. A.
Inorg. Chem. 1981, 20, 3048.
(16) The connectivities of W(NAr′)(OCMe2CF3)3I and mer-
W(OCMe2CF3)3I3 were determined by Dr. A. S. Hock.

A Symmetric Tungsten d2/d2 Dimer
Organometallics, Vol. 27, No. 15, 2008 3859
Table 1. Crystal Data and Structure Refinement Details for [W(OCMe2CF3)2Cl]2(µ-NAr′)2 (05145), [W(NAr′)(OCMe2CF3)2]2(µ-η2:η2-MeCCMe)
(05058), and (OCMe2CF3)2W(µ-NAr′)[µ-N(Me)CdC(Me)Nd]W(NAr′)(OCMe2CF3)2 (05134)a
05145
05058
05134
empirical formula
formula wt
cryst syst
space group
unit cell dimens
a (Å)
C32H42Cl2F12N2O4W2
1185.28
orthorhombic
P212121
C
38.5H54F12N2O4W2
C36H48F12N4O4W2
1196.48
1204.54
monoclinic
P21/c
triclinic
j
P1
11.3299(3)
12.9753(14)
10.6803(9)
b (Å)
18.5670(7)
11.3961(13)
12.2820(9)
c (Å)
18.7721(7)
90
90
90
30.151(4)
90
98.649(4)
90
16.3289(12)
86.482(3)
82.954(3)
74.284(3)
2045.4(3)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
3948.9(2)
4407.6(9)
Z
4
4
2
calcd density (Mg/m3)
1.994
6.051
2280
1.815
5.306
2348
1.943
5.717
1160
abs coeff (mm-1
F(000)
)
cryst size (mm3)
0.03 × 0.03 × 0.03
0.15 × 0.10 × 0.10
0.11 × 0.04 × 0.02
θ range for data collecn (deg)
index ranges
1.54-29.57
1.91-29.57
1.72-28.28
-15 e h e 15, -25 e k e 25,
-25 e l e 26
88 550
-18 e h e 18, -15 e k e 15,
-41 e l e 41
80 658
-14 e h e 14, -16 e k e 16,
-21 e l e 21
423 211
no. of rflns collected
no. of indep rflns
completeness to θ ) 29.57° (%)
max, min transmissn
11 077 (R(int) ) 0.0490)
12 346 (R(int) ) 0.0537)
10 173 (R(int) ) 0.0408)
100.0
100.0
99.9
0.8393, 0.8393
11 077/0/499
1.048
R1 ) 0.0226, wR2 ) 0.0477
R1 ) 0.0246, wR2 ) 0.0485
1.771, -0.743
0.6190, 0.5033
12 346/34/566
1.100
R1 ) 0.0304, wR2 ) 0.0671
R1 ) 0.0378, wR2 ) 0.0701
2.275, -0.900
0.8942, 0.5720
10 173/0/537
1.023
R1 ) 0.0255, wR2 ) 0.0552
R1 ) 0.0354, wR2 ) 0.0588
1.369, -0.611
no. of data/restraints/params
goodness of fit on F2
final R indices (I > 2σ(I))
R indices (all data)
largest diff peak, hole (e Å-3
)
a In all cases the wavelength was 0.710 73 Å, the temperature 100(2) K, the absorption method semiempirical from equivalents, and the refinement
method full-matrix least squares on F2. For 05145 the absolute structure parameter is -0.014(5).
more labile. Attempts to isolate analytically pure samples of
the PPhMe2 and pyridine adducts were hampered by the loss
of base in vacuo. No X-ray-quality crystals of any of the three
adducts could be obtained. The structure of [W(NAr′)(OCMe2-
CF3)2]2(PMe3) shown in eq 2 is one in which the base has added
trans to one of the trifluoro-tert-butoxides to give a trigonal-
bipyramidal arrangement around the metal with the WdW and
WdN bonds lying in the equatorial plane. It is only one of
several possibilities that include one or more bridging imido or
alkoxide ligands.
Figure 1. Thermal ellipsoid plot of [W(OCMe2CF3)2Cl]2(µ-NAr′)2.
When ethylene was condensed into hydrocarbon solutions
of [W(NAr′)(OCMe2CF3)2]2, the green reaction mixture turned
yellow upon being warmed to room temperature. NMR spectra
of the golden yellow product are consistent with it being a
monoethylene adduct. Like other monoadducts described here,
NMR spectra suggest that the adduct has no symmetry in
solution. The molecule is stereochemically rigid on the NMR
time scale at room temperature, with distinct resonances for all
four ethylene protons at 4.31 (m, JCH ) 158 Hz), 3.11 (m, JCH
Table 2. Selected Intramolecular Distances (Å) and Bond Angles
(deg) in [W(OCMe2CF3)2Cl]2(µ-NAr′)2
W(1)-W(2)
W(1)-Cl(1)
W(2)-Cl(2)
W(1)-N(1)
W(1)-N(2)
W(2)-N(1)
W(2)-N(2)
2.6296(2)
2.4061(8)
2.4135(8)
1.931(3)
1.970(3)
1.971(3)
1.931(3)
W(1)-O(1)
W(1)-O(2)
W(2)-O(3)
W(2)-O(4)
N(1)-C(11)
N(2)-C(21)
1.911(2)
1.823(2)
1.913(3)
1.850(2)
1.436(4)
1.440(4)
) 150 Hz), 1.84 (m, JCH ) 150 Hz), and 1.62 ppm (m, JCH
153 Hz). The ethylene carbons were observed at 66.18 (JCW
)
)
W(1)-N(1)-W(2)
W(2)-N(2)-W(1)
N(2)-W(2)-N(1)
84.74(11)
84.76(11)
92.46(12)
N(2)-W(1)-N(1)
N(2)-W(1)-Cl(1)
N(1)-W(2)-Cl(2)
92.48(11)
166.31(8)
166.93(8)
60 Hz, 1JCC ) 26 Hz) and 33.97 ppm (JCW ) 22 Hz, 1JCC ) 26
Hz) in the 13C NMR spectrum (toluene-d8). One set of tungsten
satellites is evident on each carbon resonance of the bound
ethylene in the 13C NMR spectrum, implying a strong
metal-ethylene interaction at room temperature. However,
variable-temperature NMR studies revealed that the adduct loses
ethylene at elevated temperatures to re-form [W(NAr′)(OCMe2-
spectrum shows 183W coupling to the phosphorus atom of the
PMe3 ligand (1JPW ) 271 Hz). Note, however, that while bound
PMe3 does not exchange with free PMe3 at room temperature
on the 31P NMR time scale, PPhMe2 and pyridine are relatively

3860 Organometallics, Vol. 27, No. 15, 2008
Lopez et al.
is consistent with the asymmetric binding of ethylene to the
two metal centers, with the interaction between C(1E) and W(1)
approaching a nonbonding distance (2.445(7) Å) and a
W(2)-C(1E) distance (2.322(7) Å) close to being a W-C single
bond (typically ∼2.2 Å). Other metal-ligand bond lengths are
what might be expected: e.g., bridging W-OR bonds (2.106(4)
and 2.202(5) Å) are longer than terminal W-OR bonds (average
∼1.9 Å). The W(µ-OR) distances, however, are slightly
different, consistent with asymmetric bonding of the bridging
alkoxide ligand.
The W2C2 core of the ethylene complex can be compared
with the WC3 ring of known metallacyclobutane20 complexes.
For instance, the short W(2)-C(2E) bond (2.098(7) Å) is
comparable to the W-CR bonds in WC3 rings. Therefore, there
is some justification in proposing that [W(NAr′)(OCMe2CF3)2]2(µ-
C2H4) contains a 1,2-dimetallacyclobutane ring. The ethylene
adduct of [W(NAr′)(OCMe2CF3)2]2 is related to the ethylene
adducts of [Os2(CO)8]8b,c and [W2(OCH2-t-Bu)8].9a However,
the ethylene adduct of [Os2(CO)8] binds ethylene in a sym-
metrically bridging fashion to give a C2-symmetric species.21
All four ethylene protons are equivalent and were observed in
the 1H NMR spectrum as a single sharp resonance down to -90
°C. [W2(OCH2-t-Bu)8], on the other hand, reacts with 1 equiv
of ethylene to give a monoadduct in which ethylene was
proposed to bind to a single metal center.9a Unfortunately,
crystals of the monoethylene adduct of [W2(OCH2-t-Bu)8]
suitable for X-ray diffraction studies were not obtained.
Heating a toluene solution of [W(NAr′)(OCMe2CF3)2]2(µ-
C2H4) at 80 °C in a sealed glass vessel under ethylene (1 atm)
for several days produced a sparingly soluble brick red
compound. The 1H NMR spectrum of this unsymmetric species
suggested that an ethyl ligand was present as a consequence of
CH activation in an o-methyl group in the NAr′ ligand (eq 3).
Formulation of the compound was based on the appearance of
Figure 2. Thermal ellipsoid plot of [W(NAr′)(OCMe2CF3)2]2(µ-
C2H4).
Table 3. Selected Intramolecular Distances (Å) and Bond Angles
(deg) in [W(NAr′)(OCMe2CF3)2]2(µ-C2H4)
W(1)-W(2)
C(1E)-C(2E)
W(1)-C(1E)
W(2)-C(1E)
W(2)-C(2E)
W(1)-N(1)
2.6429(4)
1.493(9)
2.445(7)
2.322(7)
2.098(7)
1.759(5)
W(2)-N(2)
W(1)-O(1)
W(1)-O(2)
W(1)-O(4)
W(2)-O(4)
W(2)-O(3)
1.759(5)
1.923(4)
1.871(5)
2.202(5)
2.106(4)
1.925(5)
C(2E)-C(1E)-W(2)
C(2E)-C(1E)-W(1)
C(1E)-C(2E)-W(2)
W(2)-C(1E)-W(1)
62.3(3)
126.2(5)
78.6(4)
W(2)-O(4)-W(1)
C(11)-N(1)-W(1)
C(21)-N(2)-W(2)
75.65(14)
174.5(5)
174.2(5)
67.29(18)
CF3)2]2. The adduct also loses ethylene slowly in the solid state,
especially in vacuo, a fact that could account for the low carbon
content in all attempted elemental analyses.
An X-ray study of crystals grown from pentane at -30 °C
shows the structure to be a dimer with an alkoxide and an
ethylene ligand bridging the two metal centers, relatively
asymmetrically in the case of the ethylene ligand (Figure 2).
Bond distances and angles are given in Table 3, but the X-ray
data are not given in Table 1. (These data were deposited as
Supporting Information in a communication.11) All four protons
on the ethylene fragment were located, and there is no evidence
for any agostic interaction17 of an ethylene C-H bond with
tungsten. The C1E-C2E bond distance of 1.493(9) Å is notably
longer than that of free ethylene (cf. 1.337 Å);18 it is essentially
a C-C single bond. This is consistent with the low JCC value
(26 Hz), as well as with the loss of planarity of the ethylene
ligand, as manifested in the bending back of the hydrogen atoms
away from the metal. The parameter commonly used to describe
the bending back of the olefin substituents is the dihedral angle
formed by the two H-C-H planes,19 which is 180° for free
ethylene; in this case it is 103°. The W-W bond (2.6429(4) Å)
is significantly lengthened compared to that of the parent
compound (WdW ) 2.4905(3) Å) and is more consistent with
the view that a single bond is present. The 13C resonances
proton NMR resonances for only three of the four original NAr′
methyl groups, and a doublet at 4.45 ppm (JCH ) 18 Hz) for
one of the methylene protons in the CH activated NAr′ methyl
group, in addition to other ligand peaks.
The structure was determined in a single-crystal X-ray
structure study (Figure 3 and Table 4). (X-ray data for this
compound were deposited as part of the Supporting Information
in the communication.11) The molecular structure clearly shows
that ethylene indeed has been converted into an ethyl group
with the required proton arising through CH activation of one
of the imido methyl groups. Both imido nitrogens are sym-
metrically bridging the two metal centers and form the shared
edge of two pseudo-trigonal-bipyramidal fragments. No bonds
or angles (Table 4) in (OCMe2CF3)2(Et)W(µ-NAr′)(µ-N-2-CH2-
6-MeC6H3)W(OCMe2CF3)2 are unusual.
observed at 66.18 (JCW(2) ) 60 Hz) and 33.97 ppm (JCW(2)
)
22 Hz) are assigned to C(2E) and C(1E), respectively, on the
basis of the relatively short W(2)-C(2E) distance of 2.098(7)
Å combined with the larger JCW value. The fact that only one
coupling to W(2) can be observed for each ethylene carbon atom
(17) Brookhart, M.; Green, M. L. H.; Wong, L. Prog. Inorg. Chem.
1988, 1.
(18) Gordon, A. J.; Ford, R. A. The Chemist’s Companion: A Handbook
of Practical Data, Techniques, and References; Wiley: New York, 1972.
(19) Ittel, S. D.; Ibers, J. A. AdV. Organomet. Chem. 1976, 14, 33.
(20) Feldman, J.; Schrock, R. R. Prog. Inorg. Chem. 1991, 39, 1.
(21) (a) Motyl, K. M.; Norton, J. R.; Schauer, C. K.; Anderson, O. P.
J. Am. Chem. Soc. 1982, 104, 7325. (b) Anderson, O. P.; Bender, B. R.;
Norton, J. R. Organometallics 1991, 10, 3145.

A Symmetric Tungsten d2/d2 Dimer
Organometallics, Vol. 27, No. 15, 2008 3861
Figure 4. Thermal ellipsoid plot of [W(NAr′)(OCMe2CF3)](µ-
MeCCMe)(µ-NAr′)[W(OCMe2CF3)3].
Figure 3. Thermal ellipsoid plot of (OCMe2CF3)2(Et)W(µ-NAr′)(µ-
N-2-CH2-6-MeC6H3)W(OCMe2CF3)2.
Table 5. Selected Intramolecular Distances (Å) and Bond Angles
(deg) in [W(NAr′)(OCMe2CF3)2]2(µ-η2:η2-MeCCMe)
Table 4. Selected Intramolecular Distances (Å) and Bond Angles
(deg) of (OCMe2CF3)2(Et)W(µ-NAr′)(µ-N-2-CH2-6-MeC6H3)-
W(OCMe2CF3)2
W(1)-W(2)
C(1)-C(2)
W(1)-C(1)
W(1)-C(2)
W(2)-C(1)
2.7559(3)
1.389(5)
2.466(3)
1.972(3)
1.976(4)
W(2)-C(2)
W(1)-N(1)
W(2)-N(1)
W(2)-N(2)
2.518(4)
1.977(3)
1.962(3)
1.755(3)
W(1)-W(2)
C(1)-C(2)
W(1)-C(1)
W(2)-C(17)
W(1)-N(1)
W(1)-N(2)
2.61757(14) W(2)-N(1)
1.962(2)
1.966(2)
1.9622(18)
1.8967(17)
1.8461(18)
1.9545(18)
1.525(4)
2.145(3)
2.156(3)
1.964(2)
1.930(2)
W(2)-N(2)
W(1)-O(1)
W(1)-O(2)
W(2)-O(3)
W(2)-O(4)
C(2)-C(1)-C(3)
C(1)-C(2)-C(4)
C(3)-C(1)-W(1)
C(2)-C(1)-W(1)
C(1)-C(2)-W(1)
C(4)-C(2)-W(1)
C(4)-C(2)-W(2)
C(1)-C(2)-W(2)
127.2(3)
127.1(3)
141.0(2)
53.01(17)
92.8(2)
135.3(3)
144.1(2)
51.40(19)
C(2)-C(1)-W(2)
C(3)-C(1)-W(2)
W(2)-N(1)-W(1)
C(11)-N(1)-W(1)
C(11)-N(1)-W(2)
C(21)-N(2)-W(2)
O(1)-W(1)-N(1)
95.3(2)
134.2(3)
88.79(11)
137.4(2)
133.8(2)
164.5(3)
163.64(11)
C(2)-C(1)-W(1)
110.95(18)
C(17)-W(2)-W(1) 110.65(7)
C(11)-N(1)-W(1) 148.44(17)
C(11)-N(1)-W(2) 121.18(16)
C(21)-N(2)-W(1) 146.31(16)
C(21)-N(2)-W(2) 128.61(16)
W(2)-N(1)-W(1)
W(1)-N(2)-W(2)
83.63(8)
84.42(8)
It has been established by variable-temperature NMR studies
that the ethylene adduct loses ethylene at elevated tempera-
tures (vide supra). Formation of the ethyl complex from
[W(NAr′)(OCMe2CF3)2]2 in the presence of free ethylene
requires bridging of the NAr′ ligands, CH activation in one of
the NAr′ methyl groups, and insertion of ethylene into a W-H
bond. It is not known exactly how that is accomplished. In any
case, the fact that CH activation takes place in a methyl group
may compromise some of the chemistry of dimers that contain
NAr′ ligands and may involve formation of an isomer of
[W(NAr′)(OCMe2CF3)2]2 in which the NAr′ ligands are bridg-
ing.Althoughthereactionthatgivesriseto(OCMe2CF3)2(Et)W(µ-
NAr′)(µ-N-2-CH2-6-MeC6H3)W(OCMe2CF3)2 could be revers-
ible, or an R-hydrogen might be removed from the ethyl group
to form an ethylidene ligand, there is no indication that such
reactions take place to any observable extent under the condi-
tions explored so far.
Reaction Involving 2-Butyne. Treatment of a hydrocarbon
suspension of [W(NAr′)(OCMe2CF3)2]2 with 2-butyne at room
temperature resulted in immediate formation of a red-orange
solution from which bright orange crystals were isolated in 82%
yield. An X-ray study (Table 1, Figure 4) established that the
product is the unsymmetric 1:1 adduct [W(NAr′)(OCMe2CF3)](µ-
MeCCMe)(µ-NAr′)[W(OCMe2CF3)3]. Pertinent bond lengths
and angles are given in Table 5.
Chart 1
binding are 0 and 90°, respectively. Several intermediate
structures have been documented.23,24 Of particular interest to
us are the nonparallel, nonperpendicular bridging alkyne adducts
of the d2/d2 dimer [W2(OCH2-t-Bu)8], with C-C/W-W twist
angles of ∼65°.24 For the [W(NAr′)(OCMe2CF3)2]2(µ-MeC-
CMe) species reported here, the observed C-C/W-W twist
angle of 78° is closer to a perpendicular mode of addition,
suggestive of a µ-η2:η2 coordination mode. If the MeCCMe
moiety is viewed as occupying a single coordination site, the
geometry about W(1) may be viewed as pseudo trigonal
bipyramidal, with the bound 2-butyne and alkoxides (O(2) and
O(3)) occupying the equatorial plane. The local geometry about
W(2), on the other hand, is approximately tetrahedral. The two
tungsten centers share an edge formed by 2-butyne and the N(1)
atom of the bridging imido ligand. The W-W distance
(2.7559(3) Å) is notably longer than in [W2(OCH2-t-Bu)8](µ-
RCt CR) complexes (2.59-2.65 Å)13 but is otherwise compa-
rable to other W-W single bonds.15 The central C-C distance
of the coordinated alkyne (1.389(5) Å) is comparable to those
Dinuclear complexes bridged by acetylenes generally adopt
variations of a “parallel” or a “perpendicular” structure (Chart
1).22 The two can be distinguished by the angle between the
C-C and W-W bonds, which for parallel and perpendicular
(23) (a) Calhorda, M. J.; Hoffmann, R. Organometallics 1986, 5, 2181.
(b) Cotton, F. A.; Feng, X. Inorg. Chem. 1990, 29, 3187.
(24) (a) Chisholm, M. H.; Folting, K.; Lynn, M. A.; Streib, W. E.;
Tiedtke, D. B. Angew. Chem., Int. Ed. 1997, 36, 52. (b) Chisholm, M. H.;
Lynn, M. A. J. Organomet. Chem. 1998, 550, 141.
(22) Hoffman, D. M.; Hoffmann, R.; Fisel, C. R. J. Am. Chem. Soc.
1982, 104, 3858.

3862 Organometallics, Vol. 27, No. 15, 2008
Lopez et al.
reported for other bridging acetylenes and is consistent with a
CdC double bond.9a–c,22,24b Moreover, the bound 2-butyne
approaches the geometry of cis-2-butene with both the Ct C-C
angles ∼53° less than 180°. The nonlinear Ct C-C angles,
along with the relatively short W-C distances, clearly imply
extensive π back-bonding into the 2-butyne. The 2-butyne is
also twisted, with a C-Ct C-C torsion angle of 27.7° and
substantially different W(1)-C and W(2)-C distances: e.g.,
W(1)-C(2) ) 1.972(3) Å and W(2)-C(2) 2.518(4) Å. Other
bonds to the metal center reveal the expected trend. The µ-N-W
distances (1.977(3) and 1.962(3) Å), for instance, are typical
of bridging imido ligands.25
NMR spectra of [W(NAr′)(OCMe2CF3)2]2(µ-MeCCMe) are
not trivial. The CR atoms are observed at δ 223.19 ppm (JCW
)
C
114 and 12 Hz) and 197.18 ppm (JCW ) 134 and 12 Hz) in 13
NMR spectra in toluene-d8. The two 2-butyne carbons are
coupled strongly to one 183W nucleus but only weakly to the
other, consistent with the observed unsymmetric binding of
2-butyne to the two metal centers noted above. Spectra of
[W(NAr′)(OCMe2CF3)2]2(µ-MeCCMe) at low temperatures are
consistent with the observed solid-state structure. The methyl
resonances of the tri-tert-butoxide ligands are observed as eight
sharp singlets in the region between 0.6 and 2.0 ppm.
As the temperature of the sample is raised, rapid rotation of
one of the Ar′ rings on the 1H NMR time scale and a fluxional
process involving two of the alkoxide groups are observed.
Above 40 °C only five types of alkoxide methyl groups in the
ratio of 1:4:1:1:1 are observed. Since five-coordinate systems
are notorious for their fluxionality, it is likely that the observed
fluxional behavior involves two of the alkoxides in the distorted
pseudo-trigonal-bipyramidal tungsten center (W(1)). However,
since the nature of the participation of the other ligands (e.g.,
MeCt CMe) in the dynamic process was not elucidated, the
exact nature of the fluxional process cannot be stated with
certainty.
Figure 5. Thermal ellipsoid plot of the coupled acetonitrile complex
B.
Table 6. Selected Intramolecular Distances (Å) and Bond Angles
(deg) in (OCMe2CF3)2W(µ-NAr′)[µ-N(Me)CdC(Me)N]W(NAr′)-
(OCMe2CF3)2
W-W
3.2703(3)
1.377(5)
1.739(3)
2.000(3)
2.003(3)
2.026(3)
2.031(3)
W(2)-N(4)
N(3)-C(1)
N(4)-C(2)
C(1)-C(3)
C(2)-C(4)
N(1)-C(11)
N(2)-C(21)
1.817(3)
1.387(4)
1.367(4)
1.496(5)
1.501(5)
1.395(4)
1.430(4)
C(1)-C(2)
W(1)-N(1)
W(1)-N(2)
W(2)-N(2)
W(1)-N(3)
W(2)-N(3)
W(1)-N(2)-W(2)
W(1)-N(3)-W(2)
C(11)-N(1)-W(1)
C(2)-N(4)-W(2)
109.55(12)
107.41(12)
170.9(2)
C(1)-N(3)-W(1)
C(1)-N(3)-W(2)
C(21)-N(2)-W(1)
C(21)-N(2)-W(2)
134.6(2)
116.4(2)
133.5(2)
116.0(2)
126.8(2)
Chart 2
Reaction with Acetonitrile. Addition of 4 equiv of CH3CN
to a solution of [W(NAr′)(OCMe2CF3)2]2 in benzene over a
period of 1 h generates a red-brown reaction mixture. Removal
of the volatiles in vacuo and recrystallization of the residue from
pentane afforded red-brown microcrystals which were found
through NMR studies to be a mixture of two products, A and
B. The major product, A (∼90%), displays sharp singlets at
2.42 ppm for the equivalent methyl groups in the Ar′ group, at
1.48 and 1.45 ppm for the diastereotopic methyl groups in the
consists of two distorted-trigonal-bipyramidal tungsten fragments
joined along the edges by bridging NAr′ and “NC(Me)C(Me)N”
ligands. The “NC(Me)C(Me)N” unit is derived from the
reductive coupling of two acetonitrile molecules.
1
OCMe2CF3 groups, and at 1.85 ppm for “CH3CN” in the H
NMR spectrum (C6D6, 500 MHz). This spectrum remained
virtually unchanged at temperatures as low as -80 °C. Only
one type of CF3 group is present in A, giving rise to a singlet
at -82.89 ppm in the 19F NMR. An empirical formula of
“W(NAr”)(OCMe2CF3)2(CH3CN)” and a Cs-symmetric dimeric
structure are proposed for the major product A.
On the other hand, the minor product, B, has four inequivalent
CF3 groups in the 19F NMR spectrum, indicative of an
asymmetric structure in solution. Resonances typical of an
asymmetric compound are also observed in the 1H NMR
spectrum. Unfortunately, overlap of these resonances with those
of A hampers the full spectroscopic characterization of product
B.
Dimerization of acetonitrile ligands to yield “NC(Me)C-
(Me)N” ligands has been observed in a number of circum-
stances.26 Isolation and structural characterization of these
compounds have established two possible formulations for the
“NC(Me)C(Me)N” ligand. These are [butane-2,3-diiminato]2-
and [butene-2,3-diimido]4-, which result from two- and four-
electron reductive acetonitrile coupling, respectively (see Chart
2). These two forms are readily distinguished by their structural
parameters. The [butane-2,3-diiminato]2- linkage, where the
C-C bond is typically longer than the C-N bonds, has been
(26) (a) Finn, P. A.; King, M. S.; Kilty, P. A.; McCarley, R. E. J. Am.
Chem. Soc. 1975, 97, 220. (b) Cotton, F. A.; Hall, W. T. Inorg. Chem.
1978, 17, 3525. (c) De Boer, E. J. M.; Teuben, J. H. J. Organomet. Chem.
1978, 153, 53. (d) Cotton, F. A.; Hall, W. T. J. Am. Chem. Soc. 1979, 101,
5094. (e) Duchateau, R.; Williams, A. J.; Gambarotta, S.; Chiang, M. Y.
Inorg. Chem. 1991, 30, 4863. (f) Young, C. G.; Philipp, C. C.; White, P. S.;
Templeton, J. L. Inorg. Chem. 1995, 34, 6412. (g) Tsai, Y. C.; Stephens,
F. H.; Meyer, K.; Mendiratta, A.; Gherorghiu, M. D.; Cummins, C. C.
Organometallics 2003, 22, 2902. (h) Cross, J. L.; Garrett, A. D.; Crane,
T. W.; White, P. S.; Templeton, J. L. Polyhedron 2004, 23, 2831.
A single crystal picked from the mixture was found in an
X-ray study to be that of the minor (asymmetric) product, B.
The molecular structure is shown in Figure 5, and selected bond
lengths and angles are summarized in Table 6. The structure
(25) Chisholm, M. H.; Hoffman, D. M.; Huffman, J. C. J. Am. Chem.
Soc. 1984, 106, 6815.

A Symmetric Tungsten d2/d2 Dimer
Organometallics, Vol. 27, No. 15, 2008 3863
bonds to associative chemistry, probably initially at one metal
center. Reactions of [W(NAr′)(OCMe2CF3)2]2 with oxidizing
agents and nucleophiles resulted in addition of ligands to one
or both metal centers. Ligand addition typically is accompanied
by conversion of the WdW double bond into a W-W single
bond and bridging of imido and/or alkoxide ligands. In some
cases (e.g., reaction with I2), further addition of ligands resulted
in the cleavage of the W-W bond and oxidation of W(V) to
give monomeric W(VI) species. Structural characterization of
the various products obtained revealed the general tendency of
the dimer to undergo transfer of imido and/or alkoxide groups
from one metal to the other upon ligand binding. It therefore is
likely that the controlled cleavage of the MdM bond to give 2
equiv of W(NAr′)(OR)2(L)x may be the exception rather than
the rule. Reactions of the type described here are related to a
large number of reactions of M-M triple bonds that have been
described primarily by Chisholm.28
Some of the reactions noted here are relevant to the
observation that unsupported MdM double bonds will slowly
catalyze olefin metathesis reactions. Addition of ethylene to
[W(NAr′)(OCMe2CF3)2]2 yielded a monoethylene adduct, which
upon heating is transformed into an ethyl complex as a
consequence of the CH activation of one of the o-methyl groups
of the NAr′ ligand. This is analogous to the rearrangement of
(t-Bu3SiO)3Ta(olefin) to tantalum “tuck-in” complexes contain-
ing an alkyl ligand through CH activation in one of the Silox
ligands, which is the proposed intermediate in the olefin-to-
alkylidene transformation reported by Wolczanski.13 It seems
plausible that the metathetically active alkylidene formed when
[W(NAr′)(OCMe2CF3)2]2 is treated with alkenes11 is formed
through a similar sequence of reactions. However, the ethyl
complex described here was found to polymerize norbornene
much more slowly than the ethylene complex. Therefore, other
mechanism(s) of alkylidene formation may be more accessible
under the conditions employed, or the alkylidene is not formed
in a bimetallic species, etc. The low yield of presumed
monomeric W(NR)(CHR′)(OR′′)2 species from dimeric WdW
species upon reaction with an olefin therefore seems unlikely
to be a high-yield reaction in general. However, for the purpose
of observing some metathesis behavior in the form of polym-
erization of norbornene, a low yield of alkylidene is sufficient.
It remains to be seen whether some of the chemistry discovered
here is relevant to the formation of alkylidenes in “classical”
Mo or W systems.29
Figure 6. One possible structure of complex A.
observed in [µ-NC(Me)C(Me)N][W(CO)2{HB(Me2pz)3}]2,26f
where [HB(Me2pz)3]- ) hydrotris(3,5-dimethylpyrazol-1-yl)bo-
rate anion, and in [µ-NC(Me)C(Me)N]{Mo[N(t-Bu)Ar]3}2.26g
The formation of [butene-2,3-diimido]4- from four-electron
reductive coupling of acetonitrile has been observed more
often.26a–e The [µ-NC(Me)dC(Me)N]4- moiety is distinguished
by the double-bond character of the central C-C bond, as well
as by the MdN bonds formed between the nitrogen atoms and
the metal (MdN).
In the planar “N(Me)CC(Me)N” ligand in B the newly formed
C(1)-C(2) bond has a length of 1.377(5) Å, consistent with a
double bond (cf. C-C ) 1.54 Å, CdC ) 1.34 Å).18 The C-N
distances of 1.387(4) and 1.367(4) Å are longer than typical
CdN double bonds (cf. 1.27 Å)18 and are indicative of single-
bond character. Clearly, the distances and angles observed in
the “NC(Me)C(Me)N” ligand present in the compound reported
here are only consistent with a formal butene-2,3-diimido4-
electronic description. The W-N(4) bond (1.817(3) Å), although
slightly longer than the WdN(1) bond (1.739(3) Å), is nonethe-
less within the range observed for other terminal imido ligands.25
The similarity of the W-N bond lengths in µ-NAr′ to those of
the nitrile-derived N(3) atom likewise supports the tetraanionic
formulation of the coupled acetonitrile ligand. The W · · · W
distance of 3.2703(3) Å is consistent with the absence of any
significant M-M bonding interaction. The absence of a W-W
bond is in agreement with the four-electron reductive coupling
of acetonitrile to give the enediimido ligand, with each metal
behaving as a two-electron reducing agent. It is worth noting
that the observed binding mode of [N(Me)CdC(Me)N]4- in B
is atypical of [butene-2,3-diimido]4- ligands derived from
dimerization of acetonitrile. This type of binding was first
observed in the [Re2X3(µ-HN2C2Me2)(µ-dppm)2(NCMe)]PF6 (X
) Cl, Br, dppm ) Ph2PCH2PPh2) complexes reported by Walton
and co-workers.27 However, in these dirhenium compounds the
reductive coupling of acetonitrile ligands is accompanied by
protonation of one of the nitrogen atoms of the coupled ligand
to yield the “µ-HN2C2Me2” moiety. In enediimido complexes
the bridging [butene-2,3-diimido]4- ligand binds to two metal
centers.
Experimental Section
General Considerations. All air-sensitive work was carried out
in a Vacuum Atmospheres drybox under a dinitrogen atmosphere
or by standard Schlenk techniques. Commercially available chemi-
cals were obtained from Strem Chemicals, Inc., or Aldrich. Solid
reagents were used as received unless otherwise stated. Liquids
were distilled from CaH2 under dinitrogen. Pentane, toluene, and
benzene were sparged with dinitrogen and passed through columns
ofactivatedalumina.Alldeuteratedsolventswerefreeze-pump-thaw
degassed. All solvents and liquid reagents were stored over 4 Å
molecular sieves. 1H NMR spectra were obtained on an instrument
operating at 500 MHz unless otherwise stated. 13C NMR spectra
were obtained on an instrument operating at 125 MHz, while 19F
NMR spectra were obtained on a 282 MHz instrument. All spectra
The spectroscopic properties of the as yet unidentified product
A is consistent with a Cs-symmetric “[W(NAr′)(OCMe2CF3)2-
(CH3CN)]2”: i.e., A is likely to be a structural isomer of B
wherein the [N(Me)CdC(Me)N]4- ligand is symmetrically
bridging the two metal centers. One possible structure is shown
in Figure 6, although other possibilities, e.g. one in which two
NAr” ligands are bridging, cannot be discounted.
Summary and Conclusions
The results presented here demonstrate the susceptibility of
dimeric species that contain unbridged W(IV)/W(IV) double
(28) (a) Chisholm, M. H. Acc. Chem. Res. 1990, 23, 419. (b) Chisholm,
M. H.; Huffman, J. C.; Hoffman, D. M. Chem. Soc. ReV. 1985, 14, 69. (c)
Chisholm, M. H. J. Chem. Soc., Dalton Trans. 1996, 1781.
(29) Ivin, K. J.; Mol, J. C., Olefin Metathesis and Metathesis Polym-
erization; Academic Press: San Diego, CA, 1997.
(27) (a) Esjornson, D.; Fanwick, P. E.; Walton, R. A. Inorg. Chem. 1988,
27, 3066. (b) Esjornson, D.; Derringer, D. R.; Fanwick, P. E.; Walton, R. A.
Inorg. Chem. 1989, 28, 2821.

3864 Organometallics, Vol. 27, No. 15, 2008
Lopez et al.
were recorded at room temperature. 1H and 13C NMR data are listed
in parts per million downfield from tetramethylsilane and were
referenced using the residual protonated solvent resonance. 19F
NMR shifts are reported relative to C6H5F, while 31P NMR shifts
are reported relative to PPh3 used as external reference. Elemental
analyses were performed by Kolbe Microanalytical Laboratories
(Mu¨hlheim an der Ruhr, Germany). [W(N-2,6-Me2C6H3)(OCMe2-
CF3)2]2 was prepared as described in the literature.1
81.97 (q, JCF ) 29 Hz), 80.51 (q, JCF ) 29 Hz), 66.18 (JCW ) 60
Hz, JCC ) 26 Hz), 33.97 (JCW ) 22 Hz, JCC ) 26 Hz), 27.18,
25.43, 25.10, 24.95, 24.48, 24.47, 23.45, 23.28, 19.18; 19F NMR
(C6D6) δ -80.07 (s, 1), -80.90 (s, 1), -80.94 (s, 1), -81.44 (s,
1). Multiple attempts at elemental analyses gave low values for C
and H. The reason is proposed to be partial loss of of ethylene
before analysis.
Single crystals suitable for X-ray diffraction studies were grown
[W(OCMe2CF3)2Cl]2(µ-N-2,6-Me2C6H3)2. A solution of 150 mg
of [W(NAr′)(OCMe2CF3)2]2 (0.135 mmol) in 3 mL of benzene was
stirred, and 16 µL of carbon tetrachloride (0.16 mmol) was added.
The color immediately changed from dark yellow-brown to dark
red-brown. After 2 h all volatiles were removed in vacuo.
Recrystallization of the residue from toluene afforded analytically
pure product: yield 148 mg (93%); 1H NMR (C6D6) δ 7.01 (d, 2),
6.81 (t, 1), 1.88 (s, 6), 1.45 (s, 6), 1.23 (s, 6); 19F NMR (C6D6) δ
-81.46 (s). Anal. Calcd for C16H21F6NO2ClW: C, 32.43; H, 3.57;
N, 2.36. Found: C, 32.08; H, 3.44; N, 2.17.
from pentane solution at -30 °C.
Reaction between [W(N-2,6-Me2C6H3)(OCMe2CF3)2]2 and
Acetonitrile. Acetonitrile (46.8 µL, 0.897 mmol) was added via
syringe to a solution of [W(NAr′)(OCMe2CF3)2]2 (250 mg, 0.224
mmol) in 5 mL of benzene. The reaction was stirred for 1 h at
room temperature. The solvent was then removed in vacuo to afford
a brown residue. Recrystallization in pentane gave red-brown
microcrystals in 47% yield (127 mg). The 19F NMR showed there
to be a mixture of two products, the major one comprising 90% of
the mixture. Major product (A), “W(N-2,6-Me2C6H3)(OCMe2-
1
Single crystals suitable for X-ray diffraction studies were grown
from toluene solution at -30 °C.
CF3)2(CH3CN)”: H NMR (C6D6) δ 6.99 (d, 2), 6.77 (t, 1), 2.42
(s, 6), 1.85 (s, 3), 1.48 (s, 6), 1.45 (s, 6); 19F NMR (C6D6) δ -82.39
(s). Minor product (B), (OCMe2CF3)2W(µ-N-2,6-Me2C6H3)[µ-
N(Me)CdC(Me)N]W(OCMe2CF3)2: 19F NMR (C6D6) δ -80.73
(s, 1), -80.99 (s, 1), -81.19 (s, 1), -81.42 (s, 1). Anal. Calcd for
C18H24F6N2O2W: C, 36.14; H, 4.04; N, 4.68. Found: C, 35.88; H,
3.92; N, 4.60.
Reaction between [W(N-2,6-Me2C6H3)(OCMe2CF3)2]2 and
I2. To a -30 °C suspension of [W(NAr′)(OCMe2CF3)2]2 (100 mg,
0.0897 mmol) in 3 mL of pentane was added a cold solution of
iodine (45.7 mg, 0.180 mmol) in 3 mL of pentane. The reaction
mixture was stirred for 6 h at room temperature, during which time
the color slowly changed from dark yellow-brown to red to green.
The reaction mixture was then filtered through Celite and all
solvents removed from the filtrate in vacuo to yield a solid that
was determined by NMR to be a mixture of at least two products.
Addition of 3 mL of pentane to the residue gave a mixture of green
solution and insoluble brick red solids. The brick red solids were
collected on a frit, washed with 3 mL of pentane, and redissolved
in 1 mL of toluene. Crystals of W(NAr′)(OCMe2CF3)3I suitable
for X-ray diffraction studies were obtained by storing the solution
at -30 °C. Single crystals of W(OCMe2CF3)3I3, on the other hand,
were obtained from the green filtrate upon storage at -30 °C. Full
characterization of W(NAr′)(OCMe2CF3)3I and W(OCMe2CF3)3I3
and identification of any other products formed in the reaction were
not pursued.
Single crystals suitable for X-ray diffraction studies were grown
from pentane solution at -30 °C. A crystal of the minor product
(B) was selected (at random) for crystallographic studies.
{W(N-2,6-Me2C6H3)[OCMe2(CF3)]2}2(µ-η2,η2-MeCCMe). To
a rapidly stirred suspension of [W(NAr′)(OCMe2CF3)2]2 (500 mg,
0.449 mmol) in 5 mL of pentane was added 70 µL (0.90 mmol) of
2-butyne. The initially dark brown mixture turned red-orange within
2 min. The reaction mixture was then stirred at room temperature.
After 2 h the mixture was filtered through Celite and the filtrate
was concentrated to dryness in vacuo to give a red-orange residue.
Recrystallization of the residue from pentane gave 428 mg of
analytically pure sample in 82% yield: 1H NMR (toluene-d8) δ 6.99
(d, 1), 6.97 (d, 1), 6.82 (m, 3), 6.67 (t, 1), 3.20 (s, 6), 2.99 (s, 3),
2.96 (s, 3), 2.26 (br s, 6), 1.77 (br s, 6), 1.31 (s, 3), 1.17 (s, 3),
0.99 (s, 3), 0.86 (br s, 6), 0.63 (s, 3); 13C NMR (toluene-d8) δ
223.19 (JCW ) 114 and 12 Hz), 197.18 (JCW ) 134 and 12 Hz),
158.38, 154.07, 134.59, 132.99, 131.77, 130.46, 128.25, 126.40,
126.15, 83.46 (q, JCF ) 29 Hz), 82.36 (q, JCF ) 29 Hz), 25.45,
25.31, 25.25, 25.00, 24.46, 24.05, 23.43, 23.20, 22.97, 19.50; 19F
NMR (C6D6) δ -80.68 (br s, 2), -81.72 (s, 1), -82.71 (s, 1). Anal.
Calcd for C18H24F6NO2W: C, 37.01; H, 4.14; N, 2.40. Found: C,
36.82; H, 4.08; N, 2.38.
[W(N-2,6-Me2C6H3)(OCMe2CF3)2]2(PMe3). PMe3 (95 µL, 0.90
mmol, 10 equiv) was added via syringe to a rapidly stirred dark
brown suspension of [W(NAr′)(OCMe2CF3)2]2 (100 mg, 0.0897
mmol) in 5 mL of pentane. The reaction mixture immediately turned
dark green. After 1.5 h, all volatiles were removed in vacuo to
give 104 mg of product as a dark green powder in quantitative
1
yield: H NMR (C6D6) δ 7.11 (d, 1), 7.06 (s, 3), 6.96 (d, 1), 6.85
(t, 1), 3.31 (s, 3), 2.52 (s, 6), 2.13 (s, 3), 1.65 (s, 3), 1.59 (s, 3),
1.47 (s, 3), 1.32 (s, 3), 1.29 (s, 3), 1.27 (s, 3), 1.22 (s, 3), 1.15 (s,
(OCMe2CF3)2(Et)W(µ-N-2,6-Me2C6H3)(µ-N-2-CH2-6-
MeC6H3)W(OCMe2CF3)2. The ethylene adduct [W(NAr′)(OCMe2-
CF3)2]2(µ-C2H4) was prepared in situ by exposing a freeze-pump-
thaw degassed solution of {W(NAr′)[OCMe2(CF3)]2}2 (500 mg,
0.449 mmol) in 20 mL of benzene to an atmosphere of ethylene (7
equiv). The yellow-orange reaction mixture was heated at 90 °C
for 6 days. All volatiles were removed in vacuo to give a red-
orange residue, which was then triturated in 5 mL of cold pentane.
The solids were collected on a frit, washed with more pentane,
and dried to give 170 mg of brick red powder. An additional 134
mg of product was recovered from the filtrate to give a combined
yield of 304 g (60%): 1H NMR (toluene-d8) δ 7.06 (d, 1), 7.01 (t,
1), 6.95 (m, 3), 6.87 (d, 1), 4.45 (d, 1, JHH ) 18 Hz), 2.82 (s, 3),
1.88 (s, 3), 1.72 (s, 3), 1.64 (m, 1), 1.51 (s, 3), 1.50 (t, 3), 1.46 (d,
1, JHH ) 18 Hz), 1.38 (m, 1), 1.34 (s, 3), 1.25 (s, 3), 1.23 (s, 3),
1.13 (s, 3), 1.11 (s, 3), 1.02 (s, 3), 0.99 (s, 3); 13C NMR (toluene-
d8) δ 169.89, 163.86, 149.27, 132.16, 130.65, 129.91, 129.24,
128.77, 128.63, 128.11, 127.94, 127.30, 86.43 (q, JCF ) 31 Hz),
83.84 (q, JCF ) 29 Hz), 83.11 (q, JCF ) 28 Hz), 80.35 (q, JCF ) 28
Hz), 67.57 (JCW ) 97 Hz), 57.44 (JCW ) 100 Hz), 25.09, 24.55,
2
3), 1.04 (d, 9, JHP ) 9 Hz); 19F NMR (C6D6) δ -80.38 (s, 1),
-80.65 (s, 1), -80.88 (s, 1), -81.87 (s, 1); 31P NMR (C6D6) δ
-1 (s, JHW ) 271 Hz). Anal. Calcd for C35H51F12N2O4PW2: C,
35.31; H, 4.32; N, 2.35. Found: C, 35.39; H, 4.42; N, 2.28.
[W(N-2,6-Me2C6H3)(OCMe2CF3)2]2(µ-C2H4). To a 100 mL
Schlenk bomb containing a stir bar was added 400 mg of
[W(NAr′)(OCMe2CF3)2]2 (0.359 mmol) and 20 mL of pentane. The
reaction mixture was freeze-pump-thaw degassed three times
before being exposed to an atmosphere of ethylene (∼10 equiv).
The color changed from green to yellow-orange as the reaction
mixture was warmed to room temperature. The reaction was stirred
at room temperature for 4 h. The volatiles were then removed in
1
vacuo to give 383 mg of yellow powder (93% yield): H NMR
(toluene-d8) δ 6.71 (m, 5), 6.56 (t, 1), 4.31 (m, 1, JCH ) 158 Hz),
3.11 (m, 1, JCH ) 150 Hz), 2.31 (s, 6), 2.27 (s, 6), 1.87 (s, 3), 1.84
(m, 1, JCH ) 150 Hz), 1.82 (s, 3), 1.64 (s, 3), 1.62 (m, 1, JCH
)
153 Hz), 1.43 (s, 3), 1.35 (s, 3), 1.25 (s, 3), 1.18 (s, 3); 13C NMR
(toluene-d8) δ 154.09, 153.67, 136.62, 135.40, 127.86, 127.20,
126.94, 123.31, 84.51 (q, JCF ) 29 Hz), 82.14 (q, JCF ) 29 Hz),

A Symmetric Tungsten d2/d2 Dimer
Organometallics, Vol. 27, No. 15, 2008 3865
24.53, 23.76, 23.29, 22.98, 22.49, 22.27, 20.63, 19.12, 18.33, 18.32,
13.22; 19F NMR (C6D6) δ -79.22 (s, 1), -80.90 (s, 1), -82.03 (s,
1), -82.50 (s, 1). Anal. Calcd for C17H23F6NO2W: C, 35.75; H,
4.06; N, 2.45. Found: C, 35.56; H, 3.97; N, 2.36.
restraints for anisotropic displacement parameters. Details of the
data quality and a summary of the residual values of the three
structures are given in Table 1.
Single crystals suitable for X-ray diffraction studies were grown
from toluene solution at -30 °C.
Acknowledgment. We thank the National Science Foun-
dation (Grant No. CHE-0138495) for supporting this re-
search. We thank A. S. Hock for determining the connec-
tivities in W(NAr′)(OCMe2CF3)3I and mer-W(OCMe2CF3)3I3
through preliminary X-ray studies.
X-ray Crystallography. Low-temperature diffraction data were
collected on a Siemens Platform three-circle diffractometer coupled
to a Bruker-AXS Smart Apex CCD detector with graphite-
monochromated Mo KR radiation (λ ) 0.710 73 Å), performing
and ω scans. The structures were solved by direct methods using
SHELXS and refined against F2 on all data by full-matrix least
squares with SHELXL-97.30 All non-hydrogen atoms were refined
anisotropically. All hydrogen atoms were included into the model
at geometrically calculated positions and refined using a riding
model. The isotropic displacement parameters of all hydrogen atoms
were fixed to 1.2 times the U value of the atoms they are linked to
(1.5 times for methyl groups). A disordered pentane molecule in
the structure of [W(NAr′)(OCMe2CF3)2]2(µ-η2:η2-MeCCMe) (05058)
was refined with the help of similarity restraints on 1-2 and 1-3
distances and displacement parameters as well as rigid bond
Supporting Information Available: Tables and figures giving
experimental details and labeled thermal ellipsoid drawings, crystal
data and structure refinement details, atomic coordinates, bond
lengths and angles, and anisotropic displacement parameters for
[W(OCMe2CF3)2Cl]2(µ-NAr′)2 (05145), [W(NAr′)(OCMe2CF3)2]2(µ-
η2:η2-MeCCMe) (05058), and (OCMe2CF3)2W(µ-NAr′)[µ-
N(Me)CdC(Me)Nd]W(NAr′)(OCMe2CF3)2 (05134). This material
is available free of charge via the Internet at http://pubs.acs.org.
Data for the structures are also available to the public at
http://www.reciprocalnet.org/.
(30) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.
OM800182G