Dimers that contain unbridged W(IV)/W(IV) double bonds

Article abstract of DOI:10.1021/om050961s

Compounds that contain an unbridged W=W bond can be prepared through bimolecular decomposition of various imido alkylidene complexes. One type of W= W species is [W(NR)(CH₂-t-Bu)(OR′)]₂ (R = 2,6-dimethylphenyl (Ar′) or 2,6-diisopropylphenyl (Ar); R′ = pentafluorophenyl or hexafluoroisopropyl), which is found in both heterochiral and homochiral forms. The other type is [W(NR)(OR′)2]2 (R′ = trifluoro-tert-butyl or hexafluoro-tert-butyl). All species contain an unbridged W(IV)/W(IV) double bond with a W=W bond distance of 2.4-2.5 A, a 90° angle between the imido ligands and the W=W bond, and an anti relationship of the imido ligands. There is no indication that W=W bonds are broken spontaneously; for example, heterochiral and homochiral [W(NR)(CH ₂-t-Bu)(OR′)]₂ species do not interconvert.

Full text of DOI:10.1021/om050961s

1978
Organometallics 2006, 25, 1978-1986
Dimers that Contain Unbridged W(IV)/W(IV) Double Bonds
Lourdes Pia H. Lopez, Richard R. Schrock,* and Peter Mu¨ller
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
ReceiVed NoVember 9, 2005
Compounds that contain an unbridged WdW bond can be prepared through bimolecular decomposition
of various imido alkylidene complexes. One type of WdW species is [W(NR)(CH2-t-Bu)(OR′)]2 (R )
2,6-dimethylphenyl (Ar′) or 2,6-diisopropylphenyl (Ar); R′ ) pentafluorophenyl or hexafluoroisopropyl),
which is found in both heterochiral and homochiral forms. The other type is [W(NR)(OR′)2]2 (R′ )
trifluoro-tert-butyl or hexafluoro-tert-butyl). All species contain an unbridged W(IV)/W(IV) double bond
with a WdW bond distance of 2.4-2.5 Å, a 90° angle between the imido ligands and the WdW bond,
and an anti relationship of the imido ligands. There is no indication that WdW bonds are broken
spontaneously; for example, heterochiral and homochiral [W(NR)(CH2-t-Bu)(OR′)]2 species do not
interconvert.
∼112°. Nevertheless, proton NMR spectra show no evidence
Introduction
of inequivalent tert-butoxide ligands at room temperature. Since
High oxidation state imido alkylidene complexes of tungsten
and molybdenum, especially those of the type M(NR)(CHR′)-
(OR′′)2, comprise a large class of well-defined olefin metathesis
catalysts.1 Although there is little doubt concerning many details
of olefin metathesis reactions, relatively little is known about
how catalytic intermediates decompose and the reverse of those
reactions, i.e., if and how alkylidenes might be regenerated.
Evidence suggests that there are two main deactivation path-
ways. One is rearrangement of intermediate metalacyclobutane
complexes.2 A second is bimolecular coupling of alkylidenes
to give olefins.3 The possibility that a third, formation of a
cyclopropane, is operative in certain circumstances cannot be
excluded. Bimolecular decomposition of alkylidenes is slowest
for neopentylidene or neophylidene complexes and most rapid
for methylene complexes. M(NR)(OR′′)2(olefin) complexes have
been observed,2 and in one case an ethylene complex has been
isolated and crystallographically characterized.4
no propylene was observed as a product of this reaction,
[Mo(µ-NAr)(O-t-Bu)2]2 was proposed to form via bimolec-
ular decomposition of intermediate Mo(NAr)(CH2)(O-t-Bu)2
species (eq 1). Evidently neither an ethylene complex nor a
molybdacyclopentane complex is stable with respect to forma-
tion of [Mo(µ-NAr)(O-t-Bu)2]2, and any back reaction be-
tween [Mo(µ-NAr)(O-t-Bu)2]2 and ethylene is slow. A com-
pound with the empirical formula W(NAr)[OCMe(CF3)2]2
(according to NMR and elemental analyses) was proposed to
be a bimetallic complex bridged by imido ligands, i.e., {W(µ-
NAr)[OCMe(CF3)2]2}2.5 This compound, which was isolated
from the reaction between W(NAr)(CH-t-Bu)[OCMe(CF3)2]2
and 3-hexene, is formed upon decomposition of observable, but
unstable, orange W(NAr)(CHEt)[OCMe(CF3)2]2, via bimolecu-
lar coupling of propylidenes. Other propylidene complexes,
W(NAr)(CHEt)(OR)2 (OR ) O-t-Bu, OCMe2CF3), were also
found to be unstable with respect to formation of dimers via
bimolecular decomposition.5 An attempted X-ray study of brown
crystalline “W(NAr)[OCMe(CF3)2]2” was of sufficient quality
to conclude that it was a dimeric species. However, severe
disorder prevented a solution of this structure and even a
conclusion as to whether bridging imido ligands were present
or not. The 13C NMR spectrum of “W(NAr)[OCMe(CF3)2]2”
revealed two CF3 carbon resonances, which would be consistent
with a structure in which no plane of symmetry runs through
the W2O4 core.
Alkylidenes were believed to decompose to give bimetallic
species that contain bridging imido ligands.2 The only crystal-
lographically characterized species in the literature is [Mo(µ-
NAr)(O-t-Bu)2]2 (Ar ) 2,6-i-Pr2C6H3; Mo-Mo ) 2.654(1) Å),
which is formed in a reaction between Mo(NAr)(CH-t-Bu)-
(O-t-Bu)2 and ethylene (eq 1).2 [Mo(µ-NAr)(O-t-Bu)2]2 has
Rhenium complexes of the type Re(C-t-Bu)(CH-t-Bu)(OR)2,6
which are isoelectronic with W(NAr)(CH-t-Bu)(OR)2 species,
have been shown to be viable olefin metathesis catalysts when
OR is sufficiently electron withdrawing (e.g., OCMe(CF3)2 )
ORF6).7 Complexes in which OR is O-t-Bu or ORF6 react with
CH2dCHOEt to yield observable but unstable intermediate
Re(C-t-Bu)(CHOEt)(OR)2 species that decompose to yield the
[Re(C-t-Bu)(OR)2]2 complexes (eq 2). X-ray studies showed that
approximately a tetrahedral arrangement of ligands about each
metal. However, the Mo2(O-t-Bu)4 core is not flat; the O-M-
M-O dihedral angle is ∼15°, and the O-Mo-O angle is
(1) Schrock, R. R.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2003, 42,
4592.
(2) Robbins, J.; Bazan, G. C.; Murdzek, J. S.; O’Regan, M. B.; Schrock,
R. R. Organometallics 1991, 10, 2902.
(3) Schrock, R. R. In Reactions of Coordinated Ligands; Braterman, P.
R., Ed.; Plenum: New York, 1986.
(4) Tsang, W. C. P.; Jamieson, J. Y.; Aeilts, S. A.; Hultzsch, K. C.;
Schrock, R. R.; Hoveyda, A. H. Organometallics 2004, 23, 1997.
(5) 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.
(6) Toreki, R.; Schrock, R. R. J. Am. Chem. Soc. 1992, 114, 3367.
(7) Toreki, R.; Vaughan, G. A.; Schrock, R. R.; Davis, W. M. J. Am.
Chem. Soc. 1993, 115, 127.
10.1021/om050961s CCC: $33.50 © 2006 American Chemical Society
Publication on Web 03/14/2006

Dimers that Contain Unbridged WdW Bonds
Organometallics, Vol. 25, No. 8, 2006 1979
ligand, and two resonances are observed for the isopropyl methyl
groups, consistent with rapidly rotating Ar rings through which
a mirror plane does not pass. The two doublets observed for
the neopentyl methylene protons (3.09 and 2.47 ppm, JHH
)
14.5 Hz) are assigned to diastereotopic neopentyl methylene
protons. The 19F spectrum shows three distinct resonances for
the five fluorine atoms of the OC6F5 ligand. All NMR data are
consistent with a dimeric species in which no mirror plane passes
through any ligand.
An X-ray diffraction study (Tables 1 and 2; Figures 1a and
1b) revealed that the compound is [W(NAr)(CH2-t-Bu)-
(OC6F5)]2 in which the WdW bond is not bridged. The struc-
ture adopts a staggered ethane-like geometry in which the two
terminal imido ligands are anti with respect to one another and
the two ends of the molecule are related by an inversion center.
Therefore it is a heterochiral dimer.
these decomposition products are MdM dimers that contain
an unbridged metal-metal double bond (2.3836(8) Å when OR
) ORF6 and 2.396(1) Å when OR ) O-t-Bu) and a staggered,
ethane-like geometry. These compounds reportedly do not react
readily with various phosphines, olefins, or acetylenes, although
their reactivity was not explored in great detail. 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 species8 whose structures
appear to contain µ-C-t-Bu groups, according to NMR studies
and an X-ray study of a derivative.
The most important feature of the structure of [W(NAr)(CH2-
t-Bu)(OC6F5)]2 is the presence of the unbridged W(IV)/W(IV)
double bond, even though imido and alkoxide ligands are present
that could, and often do, bridge metal-metal bonds. Table 2
lists selected bond lengths and angles of hetero-[W(NAr)(CH2-
t-Bu)(OC6F5)]2. The WdW bond distance of 2.4443(2) Å is
well within the range of WdW bond lengths found in other
compounds reported here and others found in the literature (see
Discussion section). The W-N bond length and W-N-C angle
(1.745(2) Å and 165.37(19)°) are not unusual, while the
W-C(19)-C(20) angle (119.25(19)°) is typical of a neopentyl
group that is relatively undistorted by steric pressure. The
relatively large W-O(1)-C(13) angle (146.57(18)°), on the
other hand, must be ascribed to a significant degree of π
bonding, possibly in combination with some steric pressure that
opens up the W-O(1)-C(13) angle. Finally, a striking feature
of the molecule is the virtual 90° angle (90.38(7)°) between
the imido ligands and the WdW bond vector. (The anti
orientation of neopentylidyne ligands in isoelectronic and
isostructural [Re(C-t-Bu)(OR)2]2 (OR ) O-t-Bu or OCMe-
(CF3)2) complexes is what one might expect on the basis of
steric repulsions between alkylidyne ligands, although an analy-
sis of the bonding and energetics of model compounds sug-
gested that this arrangement is also preferred electronically.12)
Since an electronic barrier to rotation about the RedRe bond
in [Re(C-t-Bu)(OR)2]2 species was predicted, we expect there
to be a barrier to rotation about the WdW bond in hetero-
[W(NAr)(CH2-t-Bu)(OC6F5)]2.
When W(NAr)(CH-t-Bu)(CH2-t-Bu)(OC6F5) is treated with
2-pentene at room temperature, two products are formed, one
of them being hetero-[W(NAr)(CH2-t-Bu)(OC6F5)]2. This reac-
tion is believed to involve formation of W(NAr)(CHR)(CH2-
t-Bu)(OC6F5) (R ) Me or Et) species, which decompose more
rapidly in a bimolecular fashion than does W(NAr)(CH-t-
Bu)(CH2-t-Bu)(OC6F5). The same mixture is obtained when
W(NAr)(CH-t-Bu)(CH2-t-Bu)(OC6F5) is added to a solution of
2-pentene at room temperature. In the more soluble product the
CHMe2 resonances are broad, consistent with a rate of rotation
of the Ar rings in that species on the order of the NMR time
scale. The C6F5 rings, however, are freely rotating on the 19F
NMR time scale, as they are in hetero-[W(NAr)(CH2-t-Bu)-
(OC6F5)]2.
Our interest in the possibility of synthesizing M(NR)(CHR′)-
(OR′′)2 species in situ (M ) Mo or W) led us to explore
reactions between Mo(NAr)(CH-t-Bu)(CH2-t-Bu)29 or W(NAr)-
(CH-t-Bu)(CH2-t-Bu)210 and alcohols. We found that only one
neopentyl group was protonated to yield species of the type
M(NAr)(CH-t-Bu)(CH2-t-Bu)(OR); in some cases M(NAr)(CH2-
t-Bu)3(OR) was formed instead and then could be transformed
into M(NAr)(CH-t-Bu)(CH2-t-Bu)(OR) upon heating. As we
began to explore the chemistry of M(NAr)(CH-t-Bu)(CH2-t-
Bu)(OR) and related complexes, we found that M(NAr)(CH-
t-Bu)(CH2-t-Bu)(OC6F5) complexes decompose bimolecularly
to give dimers that do not contain bridging imido ligands, i.e.,
species analogous to the Re species shown in eq 2. We then
revisited tungsten bisalkoxide complexes that we had assumed
to contain bridging imido ligands and found that they too do
not contain bridging ligands. The results of these investigations
are reported in full here.
Results
Syntheses and Structures of [W(NAr)(CH2-t-Bu)(OR)]2
Complexes (OR ) OC6F5 or OCH(CF3)2). W(NAr)(CH2-t-
Bu)3(OC6F5) is formed upon addition of C6F5OH to W(NAr)-
(CH-t-Bu)(CH2-t-Bu)2.10 Although W(NAr)(CH2-t-Bu)3(OC6F5)
is stable in solution at room temperature for hours, it decomposes
in a first-order manner upon heating to 60 °C (k ) 10 × 10-5
s-1 in C6D6) to give neopentane and W(NAr)(CH-t-Bu)(CH2-
t-Bu)(OC6F5). A preparative reaction is more conveniently
carried out at 80 °C. W(NAr)(CH-t-Bu)(CH2-t-Bu)(OC6F5) can
be isolated only when the decomposition is carried out under
relatively dilute conditions. We propose that W(NAr)(CH-t-Bu)-
(CH2-t-Bu)(OC6F5)10 is a dimer in the solid state analogous to
its molybdenum analogue, [Mo(NAr)(CH-t-Bu)(CH2-t-Bu)-
(OC6F5)]2, which contains bridging pentafluorophenoxides that
behave as donors trans to the neopentylidene ligand.11 When
relatively concentrated benzene or toluene solutions of W(NAr)-
(CH2-t-Bu)3(OC6F5) are heated at 80 °C for several hours,
W(NAr)(CH-t-Bu)(CH2-t-Bu)(OC6F5) is formed but then de-
composes to give di-tert-butylethylene and a sparingly soluble
1
red compound. The H NMR spectrum of this species shows
only imido and neopentyl ligand resonances. A single septet is
observed for the CHMe2 protons in the NAr (N-2,6-i-Pr2C6H3)
The second, more soluble product was found to be a C2-
symmetric analogue of hetero-[W(NAr)(CH2-t-Bu)(OC6F5)]2,
i.e., homochiral [W(NAr)(CH2-t-Bu)(OC6F5)]2 (Tables 1 and 2;
Figures 2a and 2b). Despite the inherent differences in the
(8) Toreki, R.; Schrock, R. R.; Davis, W. M. J. Organomet. Chem. 1996,
520, 69.
(9) Sinha, A.; Schrock, R. R. Organometallics 2004, 23, 1643.
(10) Lopez, L. P. H.; Schrock, R. R. J. Am. Chem. Soc. 2004, 126, 9526.
(11) Sinha, A.; Lopez, L. P. H.; Schrock, R. R.; Hock, A.; Mueller, P.
Organometallics, in press.
(12) Barckholtz, T. A.; Bursten, B. E.; Niccolai, G. P.; Casey, C. P. J.
Organomet. Chem. 1994, 478, 153.

1980 Organometallics, Vol. 25, No. 8, 2006
Lopez et al.
Table 1. Crystal Data and Structure Refinement for Hetero-[W(NAr)(CH2-t-Bu)(OC6F5)]2, Homo-[W(NAr)(CH2-t-Bu)(OC6F5)]2,
and Homo-{W(NAr)(CH2-t-Bu)[OCH(CF3)2]}2
empirical formula
fw
C46H56F10N2O2W2
1226.63
C53H64F10N2O2W2
1318.76
C40H58F12N2O2W2
1194.58
temp (K)
cryst syst
space group
unit cell dimens
a, Å
b, Å
c, Å
193(2)
monoclinic
P2(1)/c
194(2) K
monoclinic
P2/n
194(2)
orthorhombic
Pbca
10.5289(6)
9.9479(6)
22.5331(14)
90
94.8380(10)
90
2351.7(2)
13.1211(15)
11.1659(13)
18.596(2)
18.3215(7)
19.8284(7)
26.2556(10
90
90
90
9538.3(6)
8
1.664
4.900
R, deg
90
â, deg
100.116(3)
90
2682.2(5)
γ, deg
volume (Å3)
Z
2
2
density (calcd; Mg/m3)
1.732
4.965
1200
0.45 × 0.41 × 0.30
1.81 to 27.88
-12 e h e 13
-12 e k e 13
-29 e l e 22
14 637
1.633
4.360
1300
0.20 × 0.12 × 0.07
1.82 to 26.02
-16 e h e 14
-13 e k e 13
-22 e l e 15
16 997
abs coeff (mm-1
F(000)
)
4672
cryst size (mm3)
θ range (deg)
index ranges
0.95 × 0.90 × 0.75
1.70 to 28.29
-24 e h e 13
-26 e k e 24
-31 e l e 35
65 656
no. of reflns collected
no. of indep reflns [R(int)]
completeness to θ ) 29.13°
abs corr
max. and min. transmn
no. of data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]
5558 [R(int) ) 0.0236]
99.2%
5181 [R(int) ) 0.0302]
98%
11 836 [R(int) ) 0.0508]
99.9%
semiempirical
0.3174 and 0.2135
5558/81/312
1.028
R1 ) 0.0200
wR2 ) 0.0470
R1 ) 0.0246
wR2 ) 0.0484
0.756 and -0.774
semiempirical
0.7500 and 0.4759
5181/343/418
1.026
R1 ) 0.0225
wR2 ) 0.0504
R1 ) 0.0291
wR2 ) 0.0531
0.902 and -0.493
semiempirical
0.1203 and 0.0898
11 836/17/535
1.064
R1 ) 0.0443
wR2 ) 0.1054
R1 ) 0.0637
wR2 ) 0.1162
2.920 and -0.898
R indices (all data)
largest diff peak and hole (e Å-3
)
a In all cases the wavelength was 0.71073 Å and the refinement method was full-matrix least-squares on F2.
Table 2. Selected Bond Distances (Å) and Angles (deg) in
Heterochiral and Homochiral [W(NAr)(CH2-t-Bu)(OC6F5)]2
and Homochiral {W(NAr)(CH2-t-Bu)[OCH(CF3)2]}2
upon heating solutions of one or the other. Apparently the Wd
W bonds do not cleave spontaneously into pseudo-trigonal
W(NAr)(CH2-t-Bu)(OC6F5) fragments analogous to structurally
characterized W(N-t-Bu)[OSi(t-Bu)3]2.13 Most likely the large
silox ligands in W(N-t-Bu)[OSi(t-Bu)3]2 prevent formation of
{W(N-t-Bu)[OSi(t-Bu)3]2}2, while the double bonds in hetero-
[W(NAr)(CH2-t-Bu)(OC6F5)]2 and homo-[W(NAr)(CH2-t-Bu)-
(OC6F5)]2 are simply too strong to cleave homolytically under
relatively mild conditions.
hetero-OC6F5 homo-OC6F5
homo-OCH(CF3)2
WdW
2.4443(2)
1.745(2)
1.9339(18)
2.135(3)
2.4492(3)
1.747(3)
1.924(2)
2.134(3)
91.06(8)
136.23(11)
102.46(12)
109.23(12)
109.94(7)
102.72(9)
171.4(2)
2.4537(3)
W-N
1.757(5)
1.908(4)
2.123(6)
91.55(14)
131.0(2)
105.9(2)
109.3(2)
112.89(14) 111.68(17)
101.86(17) 101.98(18)
166.4(4)
146.7(4)
117.8(4)
1.747(5)
1.915(5)
2.123(7)
92.67(16)
129.5(2)
106.5(2)
110.3(3)
W-O
W-C
N-WdW
N-W-O
N-W-C
O-W-C
O-W-W
C-W-W
C-N-W
C-O-W
C-C-W
90.38(7)
132.40(9)
106.28(11)
110.23(10)
113.15(6)
97.99(7)
165.37(19)
146.57(18)
119.25(19)
Although W(NAr)(CH-t-Bu)(CH2-t-Bu)[OCH(CF3)2] is rela-
tively stable thermally,10 upon treating a concentrated solution
of W(NAr)(CH-t-Bu)(CH2-t-Bu)[OCH(CF3)2] in pentane with
2-pentene, the initially yellow reaction mixture turned red within
minutes and red crystals were deposited over a period of days
at -30 °C. Two products were observed whose solubilities and
NMR properties are analogous to those for hetero-[W(NAr)-
(CH2-t-Bu)(OC6F5)]2 and homo-[W(NAr)(CH2-t-Bu)(OC6F5)]2.
Therefore we propose that these species are hetero-{W(NAr)-
(CH2-t-Bu)[OCH(CF3)2]}2 and homo-{W(NAr)(CH2-t-Bu)-
[OCH(CF3)2]}2; the more soluble isomer shows slower rotation
of the NAr rings on the NMR time scale, as found for
homochiral [W(NAr)(CH2-t-Bu)(OC6F5)]2.
168.0(4)
148.4(5)
119.3(5)
156.4(2)
115.9(2)
molecular symmetry (C2 vs Ci) and spectroscopic behavior of
homo-[W(NAr)(CH2-t-Bu)(OC6F5)]2 versus hetero-[W(NAr)-
(CH2-t-Bu)(OC6F5)]2 (vide supra), the bond lengths and angles
in the coordination sphere of homo-[W(NAr)(CH2-t-Bu)-
(OC6F5)]2 are virtually indistinguishable from those in hetero-
[W(NAr)(CH2-t-Bu)(OC6F5)]2 (Table 2). However, the C-O-
W-W dihedral angles in the two structures are significantly
different. In hetero-[W(NAr)(CH2-t-Bu)(OC6F5)]2 the C-O-
W-W dihedral angle is -118°, while in homo-[W(NAr)(CH2-
t-Bu)(OC6F5)]2 it is 41°. As a consequence, the C6F5 ring in
the heterochiral dimer is directed away from the imido ligand,
while in the homochiral dimer it is pointed toward the imido
group. This structural difference might lead to slight differences
in steric congestion in the two isomers and, therefore, to the
observed slower rate of rotation of the i-Pr2C6H3 rings in homo-
[W(NAr)(CH2-t-Bu)(OC6F5)]2.
An X-ray structural study of the more soluble product con-
firms that it is homochiral {W(NAr)(CH2-t-Bu)[OCH(CF3)2]}2
(Tables 1 and 2, Figure 3). This compound shares the same
general features as homo-[W(NAr)(CH2-t-Bu)(OC6F5)]2 shown
in Figure 2, e.g., a staggered ethane-like geometry, anti imido
groups, close to 90° NdWdW angles (91.55(14) and 92.67-
(16)°), and a W(IV)/W(IV) double bond (2.4537(3) Å). The
C-O-W-W dihedral angle in homo-{W(NAr)(CH2-t-Bu)-
So far we have seen no interconversion of hetero-[W(NAr)-
(CH2-t-Bu)(OC6F5)]2 and homo-[W(NAr)(CH2-t-Bu)(OC6F5)]2
(13) Eppley, D. F.; Wolczanski, P. T.; Van Duyne, G. D. Angew. Chem.,
Int. Ed. Engl. 1991, 30, 584.

Dimers that Contain Unbridged WdW Bonds
Organometallics, Vol. 25, No. 8, 2006 1981
Figure 1. (a) Thermal ellipsoid drawing of the molecular structure
of heterochiral [W(NAr)(CH2-t-Bu)(OC6F5)]2. (b) Chem 3D view
from the end of heterochiral [W(NAr)(CH2-t-Bu)(OC6F5)]2.
[OCH(CF3)2]}2 is -9°, while it is 41° in homo-[W(NAr)(CH2-
t-Bu)(OC6F5)]2.
Synthesis of [W(NAr′)(CH2-t-Bu)(OC6F5)]2 (Ar′ ) 2,6-
Me2C6H3). Bisalkoxide complexes of the type W(NAr′)(CH-
t-Bu)(OR)2 have been prepared from W(NAr′)(CH-t-Bu)Cl2-
(dme), which was prepared through base-catalyzed proton
transfer in W(NHAr′)(C-t-Bu)Cl2(dme).5 W(NAr′)(CH-t-Bu)-
(CH2-t-Bu)2 has not been reported. It can be prepared from
W(NAr′)(CH-t-Bu)(OTf)2(dme), which was synthesized in a
manner analogous to that for W(NAr)(CH-t-Bu)(OTf)2(dme),
i.e., by treating W(NAr′)2(CH-t-Bu)2 with 2 equiv of triflic acid.
Figure 2. (a) Thermal ellipsoid drawing of the molecular structure
of homochiral [W(NAr)(CH2-t-Bu)(OC6F5)]2. (b) Chem 3D view
from the end of homochiral [W(NAr)(CH2-t-Bu)(OC6F5)]2.
to that obtained upon addition of pentafluorophenol to W(NAr)-
(CH-t-Bu)(CH2-t-Bu)2.10 Heating a toluene-d8 solution (0.10 M)
of W(NAr′)(CH2-t-Bu)3(OC6F5) to 60 °C yields what we propose
to be W(NAr′)(CH-t-Bu)(CH2-t-Bu)(OC6F5) (δHR ) 9.36 ppm),
although all efforts to isolate this species proved unsuccessful
as a consequence of its facile decomposition to yield t-BuCHd
CH-t-Bu and [W(NAr)(CH2-t-Bu)(OC6F5)]2 (eq 3). The basic
1
The H NMR spectrum of yellow-orange W(NAr′)(CH-t-Bu)-
(CH2-t-Bu)2 shows a singlet at 6.87 ppm (JHW ) 16 Hz) for
the neopentylidene proton. This is comparable to HR of 6.74
ppm observed for W(NAr)(CH-t-Bu)(CH2-t-Bu)25 and δ HR )
6.61 ppm reported for W(NPh)(CH-t-Bu)(CH2-t-Bu)2,14 which
was obtained as a red oil upon addition of LiCH2-t-Bu to
W(NPh)(CH2-t-Bu)3Cl. Attempts to synthesize W(NAr′)(CH-
t-Bu)(CH2-t-Bu)2 via alkylation of W(NAr′)Cl4(THF) with 3
equiv of t-BuCH2MgCl at -78 °C consistently gave a mixture
of W(NAr′)(CH2-t-Bu)3Cl and W(NAr′)(CH-t-Bu)(CH2-t-Bu)2,
according to NMR spectra, and possibly other (unidentified)
products, from which W(NAr′)(CH-t-Bu)(CH2-t-Bu)2 could not
be isolated in pure form.
Addition of pentafluorophenol to W(NAr′)(CH-t-Bu)(CH2-
t-Bu)2 yields W(NAr′)(CH2-t-Bu)3(OC6F5), a product analogous
(14) Pedersen, S. F.; Schrock, R. R. J. Am. Chem. Soc. 1982, 104, 7483.

1982 Organometallics, Vol. 25, No. 8, 2006
Lopez et al.
Figure 4. Thermal ellipsoid drawing of the molecular structure of
{W(NAr′)[OCMe(CF3)2]2}2 (one molecule of two).
Figure 3. Thermal ellipsoid drawing of the molecular structure of
homochiral [W(NAr)(CH2-t-Bu)[OCH(CF3)2]}2.
identical to {W(NAr′)[OCMe(CF3)2]2}2 (Tables 3 and 4, Fig-
ures 5 and 6). In proton NMR spectra of both compounds, the
two methyl groups of the OCMe2CF3 ligands are inequiva-
lent, consistent with the Ci-symmetric structure. [W(NAr)-
(OCMe2CF3)2]2 is the species reported here that is obtained in
the lowest yield and not reproducibly. Clearly formation of
the WdW bond is very sensitive to the size of the covalently
bound ligands and conditions in a manner that is not yet
quantifiable.
problem appears to be that W(NAr′)(CH2-t-Bu)3(OC6F5) is less
sterically crowded than W(NAr)(CH2-t-Bu)3(OC6F5) and so
decomposes to yield a neopentylidene complex more slowly
than W(NAr)(CH2-t-Bu)3(OC6F5), while W(NAr′)(CH-t-Bu)-
(CH2-t-Bu)(OC6F5), again for steric reasons, is more prone to
decompose bimolecularly than is W(NAr)(CH-t-Bu)(CH2-t-Bu)-
(OC6F5). Therefore only a relatively small quantity of W(NAr′)-
(CH-t-Bu)(CH2-t-Bu)(OC6F5) (a maximum of ∼10%) has been
observed. As expected, [W(NAr′)(CH2-t-Bu)(OC6F5)]2 contains
inequivalent neopentyl methylene protons, characteristic of
dimeric species of this general type, and therefore almost
certainly also contains an unsupported W(IV)/W(IV) double
bond. We believe [W(NAr′)(CH2-t-Bu)(OC6F5)]2 to be a het-
erochiral dimer on the basis of its formation via coupling of
neopentylidene complexes and the equivalence of all 2,6-
1
Me2C6H3N groups on the H NMR time scale. However, it is
possible that the hindered rotation of Ar rings in the homochiral
dimers discussed above is a consequence of bulky isopropyl
groups being present and not an intrinsic property of all
homochiral dimers regardless of the nature of the imido group.
Therefore we cannot say with certainty that [W(NAr′)(CH2-t-
Bu)(OC6F5)]2 is in fact heterochiral.
Syntheses and Structures of {W(NAr′)[OCMe(CF3)2]2}2,
[W(NAr′)(OCMe2CF3)2]2 (Ar′ ) 2,6-Me2C6F3), and [W(NAr)-
(OCMe2CF3)2]2. Upon discovery of the [W(NR)(CH2-t-Bu)-
(OR′)]2 complexes described above, we considered the possi-
bility that only monoalkoxide species of this general type had
unbridged MdM bonds. However, some inconsistensies in the
literature concerning the products of decomposition of W(NAr)-
(CHR)(OR′)2 complexes, as described in the Introduction,
suggested that this may not be the case.
Unstable W(NAr′)(CHEt)[OCMe(CF3)2]2 decomposes to yield
“W(NAr′)[OCMe(CF3)2]2”, which was proposed to be {W(µ-
NAr′)[OCMe(CF3)2]2}2.5 Crystals of this compound suitable for
X-ray diffraction studies were obtained in the same manner as
described in the literature.5 The X-ray study was marred by a
complex disorder (see Experimental Section). However, the
disorder was solved satisfactorily and this species was found
to be {W(NAr′)[OCMe(CF3)2]2}2, in which the WdW bond is
unsupported by bridging ligands (2.4751(4) Å; Tables 3 and 4
and Figure 4). Structural features of this dimer are analogus to
those for the monoalkoxide dimers described above.
Discussion
There are relatively few examples of MdM bonds in the lit-
erature, and most of those that are known contain bridging lig-
ands. For example, Casey reported (η5-C5Me5)(CO)2RedRe(CO)2-
(η5-C5Me5), which contains a Re-Re double bond (2.723(1)
Å) in which two of the carbonyl ligands are “semibridging”.15,16
Bridged W(IV)/W(IV) double bonds are found in [K(18-crown-
6)1.5][W2(µ-H)(µ-O)(OC-t-Bu)6] (2.445(1) Å),17 [W2(κ-O2C-t-
Bu)4(µ-MeCCMe)2] (2.4888(2) Å),18 [W2(κ-O2C-t-Bu)4(µ-
PhCCMe)2] (2.4925(2) Å),18 and W2(OCH2-t-Bu)8.19 Examples
of complexes that contain a metal-metal double bond and
no potentially bridging ligands include macrocycle complexes
such as [M(octaethylporphyrin)]2 (M ) Ru, Os)20 and [Ru-
(C22H22N4)]2.21 The compound (i-PrO)4WW(DMPE)2(CO)22 (no
bridging ligands) could be said to be a d2/d6 dimer in which a
(15) Casey, C. P.; Sakaba, H.; Hazin, P. N.; Powell, D. R. J. Am. Chem.
Soc. 1992, 113, 8165.
(16) Casey, C. P. Science 1993, 259, 1552.
(17) Budzichowski, T. A.; Chisholm, M. H.; Folting, K.; Huffman, J.
C.; Streib, W. E. J. Am. Chem. Soc. 1995, 117, 7428.
(18) Byrnes, M. J.; Chisholm, M. H.; Gallucci, J.; Wilson, P. J.
Organometallics 2002, 21, 2240.
(19) Chisholm, M. H.; Streib, W. E.; Tiedtke, D. B.; Wu, D. D. Chem.
Eur. J. 1998, 4, 1470.
(20) Collman, J. P.; Barnes, C. E.; Swepston, P. N.; Ibers, J. A. J. Am.
Chem. Soc. 1984, 106, 3500.
(21) Warren, L. F.; Goedken, V. L. J. Chem. Soc., Chem. Commun. 1978,
909.
(22) Chisholm, M. H.; Folting, K.; Kramer, K. S.; Streib, W. E. Inorg.
Chem. 1998, 37, 1549.
[W(NAr)(OCMe2CF3)]2 and [W(NAr′)(OCMe2CF3)]2 were
prepared in a manner analogous to that employed to prepare
{W(NAr′)[OCMe(CF3)2]2}2, as shown in eq 4. X-ray diffraction
studies showed that both are unbridged dimers that are virtually

Dimers that Contain Unbridged WdW Bonds
Organometallics, Vol. 25, No. 8, 2006 1983
Table 3. Crystal Data and Structure Refinement for [W(NAr′)(ORF6)2]2, [W(NAr′)(ORF3)2]2, and [W(NAr)(ORF3)2]2
[W(NAr′)(ORF6)2]2
[W(NAr′)(ORF3)2]2
[W(NAr)(ORF3)2]2
empirical formula
fw
C32H30F24N2O4W2
1330.28
C32H42F12N2O4W2
1114.38
C40H58F12N2O4W2
1226.58
temp (K)
cryst syst
space group
unit cell dimens
a, Å
100(2)
monoclinic
P2(1)/n
193(2)
monoclinic
P2(1)/n
193(2) K
monoclinic
P2(1)/c
17.1623(5)
13.2753(4)
36.7743(11)
90
96.4960(10)
90
8324.7(4)
8
2.123
5.669
10.4324(7)
13.2926(9)
14.0726(10)
90
97.1920(10)
90
1936.1(2)
2
1.911
6.031
19.7134(8)
11.1767(4)
21.4229(8)
90
90.0250(10)
90
4720.1(3)
4
1.726
4.956
b, Å
c, Å
R, deg
â, deg
γ, deg
volume (Å3)
Z
density (calcd; Mg/m3)
abs coeff (mm-1
F(000)
)
5056
1072
2400
cryst size (mm3)
θ range (deg)
index ranges
0.15 × 0.15 × 0.05
1.26 to 29.57
-23 e h e 23
0 e k e 18
0 e l e 51
331 577
0.11 × 0.06 × 0.05
2.12 to 28.27
-13 e h e 12
-13 e k e 17
-18 e l e 18
13 428
0.30 × 0.17 × 0.15
1.90 to 28.28
-19 e h e 26
-14 e k e 14
-22 e l e 28
32 828
no. of reflns collected
no. of indep reflns [R(int)]
completeness to θ ) 29.13°
abs corr
max. and min. transmn
no. of data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]
23 350 [R(int) ) 0.0594]
100.0%
4790 [R(int) ) 0.0185]
99.9%
11 707 [R(int) ) 0.0327]
100.0%
semiempirical
0.7647 and 0.4835
23 352/3692/1403
1.275
R1 ) 0.0622
wR2 ) 0.1385
R1 ) 0.0727
wR2 ) 0.1438
3.175 and -1.503
semiempirical
0.7525 and 0.5567
4790/0/241
1.029
R1 ) 0.0218
wR2 ) 0.0563
R1 ) 0.0249
wR2 ) 0.0588
1.813 and -0.946
semiempirical
0.5235 and 0.3179
11 707/562/671
1.087
R1 ) 0.0404
wR2 ) 0.0976
R1 ) 0.0572
wR2 ) 0.1045
1.625 and -0.549
R indices (all data)
largest diff peak and hole (e Å-3
)
a In all cases the wavelength was 0.71073 Å and the refinement method was full-matrix least-squares on F2.
Table 4. Selected Bond Lengths (Å) and Angles (deg) in [W(NAr′)(ORF6)2]2, [W(NAr′)(ORF3)2]2, and [W(NAr)(ORF3)2]2
Complexes
[W(NAr′)(ORF6)2]2
[W(NAr′)(ORF3)2]2
[W(NAr)(ORF3)2]2
WdW
2.4751(4)
2.4909(2)
1.751(2)
1.9239(18)
1.860(2)
2.4925(3)
W-N
1.755(7), 1.737(6)
1.902(6), 1.892(5)
1.915(5), 1.909(5)
92.4(2), 93.8(2)
114.5(3), 111.8(3)
117.2(3), 120.2(3)
112.0(3), 112.6(3)
104.88(18), 102.48(18)
113.7(2), 113.10(18)
170.3(7), 169.5(6)
135.4(5), 132.9(5)
151.4(6), 163.3(5)
1.749(5), 1.754(5)
1.914(4), 1.919(4)
1.878(4), 1.876(4)
94.92(16), 93.75(15)
114.86(18), 119.2(2)
116.9(2), 117.0(2)
109.80(19), 108.8(2)
104.72(14), 104.10(14)
112.41(14), 113.86(15)
169.6(4), 166.7(4)
137.5(4), 134.1(4)
152.0(4), 159.9(4)
W-O
W-O
N-WdW
N-W-O
N-W-O
O-W-O
O-WdW
O-WdW
C-N-W
C-O-W
C-O-W
92.59(7)
114.90(9)
119.55(9)
110.19(9)
104.29(6)
113.53(6)
172.42(9)
132.61(17)
153.09(18)
WdW bond is present (2.52429(9) Å), although alternative
explanations are possible. We believe that unbridged MdM
complexes of the type [Re(C-t-Bu)(OR)2]2,23 and the WdW
species reported here, are the only symmetric MdM complexes
that are stable at room temperature and in which potentially
bridging ligands are present. They also appear to contain the
shortest WdW bonds known. The fact that they also have
MdCHR relatives that are olefin metathesis catalysts is also
a noteworthy feature of such species. Therefore one could
view the relationship between the unbridged MdM bonds
described here and MdC bonds in their relatives as analogous
to the relationship between M≡M bonds and MtC bonds24 in
alkyne metathesis catalysts; for example, alkynes can react with
MtM bonds to yield alkylidynes25,26 reversibly.27 The recent
finding that MdM species can initiate olefin metathesis reac-
tions28 raises the possibility that MdM species can react with
CdC bonds to yield MdC species, the reverse of the reaction
in which they are formed. However, there are several alternative
possibilities, so whether this is true or not is unknown.
The WdW bonded species reported here are characterized
by a 90° W-W-N angle and a 180° N-W-W-N dihedral
angle. The 90° W-W-N angle suggests that the three π bonds
around the metal center are formed from the three mutually
perpendicular, essentially pure dxy, dyz, and dxz orbitals, while
the four σ bonds are formed from the three p orbitals and some
2
combination of s and dz orbitals. In models of [Re(C-t-Bu)-
(25) Schrock, R. R.; Listemann, M. L.; Sturgeoff, L. G. J. Am. Chem.
Soc. 1982, 104, 4291.
(26) Listemann, M. L.; Schrock, R. R. Organometallics 1985, 4, 74.
(27) Chisholm, M. H.; Hoffman, D. M.; Northius, J. M.; Huffman, J. C.
Polyhedron 1997, 16, 839.
(28) Schrock, R. R.; Lopez, L. P. H.; Hafer, J.; Singh, R.; Sinha, A.;
Mueller, P. Organometallics 2005, 24, 5211.
(23) Toreki, R.; Schrock, R. R.; Vale, M. G. J. Am. Chem. Soc. 1991,
113, 3610.
(24) Murdzek, J. S.; Schrock, R. R. In Carbyne Complexes; VCH: New
York, 1988.

1984 Organometallics, Vol. 25, No. 8, 2006
Lopez et al.
1,2-dimetalacyclic species from which olefin can be lost readily.
In fact, a 1,2-dimetallacyclic species that contains one bridging
alkoxide has been observed to form reversibly when ethylene
is added to (CF3Me2CO)2(Ar′N)WdW(NAr′)(OCMe2CF3)2 (eq
6).28 Moreover, (CF3Me2CO)2(Ar′N)WdW(NAr′)(OCMe2CF3)2
will metathesize certain olefins slowly.28 Therefore there is some
possibility that monomeric alkylidenes are formed from the 1,2-
dimetalacyclobutane. That is not yet known. In any case it seems
clear that MdM bonds form from alkylidenes in small steps as
the olefin is formed and ejected. Whether a MdM bond actually
forms is likely to depend on a number of factors, among them
the overall steric bulk of the covalently bound ligands involved
(imido, alkoxide, alkyl) and what ligands (e.g., olefins) bind to
the metal in a monomer or dimer and thereby prevent formation
of the MdM species.
Figure 5. Thermal ellipsoid drawing of the molecular structure of
[W(NAr′)(OCMe2CF3)2]2.
Reduction of W(N-t-Bu)(Silox)2Cl2 (Silox ) OSi(t-Bu)3) has
produced pseudo-trigonal planar W(N-t-Bu)(Silox)2,13 instead
of hypothetical (Silox)2(t-BuN)WdW(N-t-Bu)(Silox)2, presum-
ably because dimerization is prevented by the bulk of the Silox
groups. Recently, reduction of W(NAr′)(OCMe2CF3)2Cl2 has
been shown to yield (CF3Me2CO)2(Ar′N)WdW(NAr′)(OCMe2-
CF3)2,28 while sodium amalgam reduction of Mo(N-t-Bu)[OSi-
(t-Bu)3]2Cl2 has been reported to yield {Mo(N-t-Bu)[OSi-
(t-Bu)3]2}2(µ-Hg).30 The spectroscopy of {Mo(N-t-Bu)[OSi-
(t-Bu)3]2}2(µ-Hg), along with calculations, allowed the strength
of the π component of the ModMo bond in [Mo(NAr)(CH2-
Figure 6. Thermal ellipsoid drawing of the molecular structure of
[W(NAr)(OCMe2CF3)2]2.
9
t-Bu)(OC6F5)]2 to be estimated as ∼27 kcal/mol. If the π
component is worth 27 kcal/mol, we might guess that the entire
double bond (including the σ portion) will be worth ∼75 kcal/
mol and that WdW bonds probably would be stronger than
ModMo bonds. The findings in this paper are consistent with
WdW bonds being relatively strong and the failure to see
scrambling between heterochiral and homochiral dimers. Future
studies will be concerned with reactions that involve WdW
bonds along the lines that have appeared in a preliminary
study.28
(OR)2]2 (OR ) O-t-Bu or OCMe(CF3)2) complexes23 Bursten
found12 that the anti arrangement of the multiply bound
alkylidyne ligands was energetically preferred. A staggered
structure is sensible on the basis of steric arguments alone.
We can imagine that MdM bonds might be formed through
coupling of alkylidenes as shown in eq 5. One (δ+)MdC(δ-)
Experimental Details
General Procedures. All air-sensitive work was carried out in
a Vacuum Atmospheres drybox under a dinitrogen atmosphere or
by standard Schlenk techniques. All solvents, except methylene
chloride and dimethoxyethane, were sparged with nitrogen and
passed through a column of activated alumina. Methylene chloride
was distilled from CaH2, while dimethoxyethane was distilled
from benzophenone ketyl before being degassed (freeze-pump-
thaw) three times. All deuterated solvents, as well as the alco-
hols and phenols used, were degassed before being brought into
the box. All solvents were stored over 4 Å molecular sieves. All
lithium alkoxides were prepared through addition of n-butyllithium
to the alcohol. W(NAr)Cl4(Et2O),5 W(NAr′)(CH-t-Bu)(OTf)2-
(dme),10 W(NAr)(CH-t-Bu)(OCMe2CF3)2,31 W(NAr)(CH-t-Bu)-
bond effectively reacts in a manner analogous to a (polarized)
CdC bond with the other (δ+)MdC(δ-) bond to form a 1,3-
dimetallacyclobutane intermediate. What amounts to a com-
pound of this type recently has been observed through NMR
studies.29 The compound is [W(NAr)(13CH2)(Biphen)]2 where
Biphen2- ) 3,3′-di-tert-butyl-5,5′,6,6′-tetramethyl-1,1′-biphenyl-
2,2′-diolate. A key feature of its 13C{1H} NMR spectrum is a
singlet resonance at 185.9 ppm flanked with two sets of tungsten
satellites (1JWC ) 78.9, 36.7 Hz), characteristic of the methylene
carbon being coupled to two different 183W nuclei. The proton-
coupled spectrum reveals that the methylene carbon is coupled
to two inequivalent protons (1JCH ) 148, 131 Hz) located at
8.20 and 7.37 ppm in the proton NMR spectrum. The fact that
this species can be observed and that no carbon-carbon bond
has yet formed suggests that it may have to rearrange first to a
5
(CH2-t-Bu)[OCH(CF3)2],10 and {W(NAr′)[OCMe(CF3)2]2}2 were
prepared as described in the literature.
(30) Rosenfeld, D. C.; Wolczanski, P. T.; Barakat, K. A.; Buda, C.;
Cundari, T. R. J. Am. Chem. Soc. 2005, 127, 8262.
(29) Tsang, W. C. P.; Hultzsch, K. C.; Alexander, J. B.; Bonitatebus, P.
J. J.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2003, 125, 2652.
(31) Schrock, R. R.; Murdzek, J. S.; Bazan, G. C.; Robbins, J.; DiMare,
M.; O’Regan, M. J. Am. Chem. Soc. 1990, 112, 3875.

Dimers that Contain Unbridged WdW Bonds
Organometallics, Vol. 25, No. 8, 2006 1985
1
Reported H and 13C NMR chemical shifts are listed in parts
solution was stirred for 1 h, after which the volatiles were removed
in vacuo. The residue was collected on a frit and washed with cold
pentane to give a yellow powder; yield 420 mg (62%): 1H NMR
(C6D6) δ 6.91 (d, 2), 6.78 (t, 1), 4.15 (m,2), 2.61 (s, 6), 2.21 (s, 6,
JHW ) 9.6 Hz), 1.10 (s, 27); 19F NMR (C6D6) δ -156.9 (d, 2),
-165.2 (t, 2), -170.9 (t, 1). Anal. Calcd for WNOF5C29H42: C,
49.8; H, 6.05; N, 2.00. Found: C, 49.75, H, 6.11, N, 1.95.
per million referenced to either the residual protons or 13C atoms
of the deuterated solvents. 19F NMR shifts are reported relative to
C6F6 used as an external reference. 1H NMR spectra were obtained
on an instrument operating at 300 MHz (unless otherwise noted),
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 were recorded near 22 °C.
Observation of W(NAr′)(CH2-t-Bu)3[OCH(CF3)2]. Hexafluor-
oisopropyl alcohol (6.7 µL, 1.1 equiv) was added via syringe to a
solution of W(NAr′)(CH-t-Bu)(CH2-t-Bu)2 (30 mg, 0.058 mmol)
in 0.80 mL of C6D6 in an NMR (JY) tube. A proton NMR spectrum
showed that W(NAr′)(CH2-t-Bu)3[OCH(CF3)2] had formed quan-
Homo- and Hetero-{W(NAr)(CH2-t-Bu)[OCH(CF3)2]}2. To a
rapidly stirred solution of W(N-2,6-i-Pr2C6H3)(CH-t-Bu)(CH2-t-Bu)-
[OCH(CF3)2] (470 mg, 0.704 mmol) in a minimum amount of
pentane at room temperature was added dropwise 1.52 mL (14.1
mmol) of cis-2-pentene. The mixture was left to stir overnight. The
volatiles were removed in vacuo to give a red oil. Minimum amount
of pentane was added and the solution kept at -30 °C. Dark red
crystals, formed over a period of two weeks, were collected on a
frit and washed with 3 × 1 mL of cold pentane to give 60 mg of
the product as a mixture of homo- and heterochiral isomers in
approximately a 7:1 ratio (11% yield). Homo-{W(NAr)(CH2-t-Bu)-
[OCH(CF3)2]}2: 1H NMR (C6D6, 500 MHz) δ 7.10 (t, 2), 7.03 (d,
4), 4.89 (m, 2), 4.44 (m, 2), 3.07 (m, 2), 2.37 (d, 2, JHW ) 15 Hz),
2.04 (d, 2, JHW ) 15 Hz), 1.61 (d, 6), 1.32 (d, 6), 1.25 (d, 6), 1.02
(s, 18), 1.01 (d, 6). Hetero-{W(NAr)(CH2-t-Bu)[OCH(CF3)2]}2: 1H
NMR (C6D6, 500 MHz) δ 7.09 (t, 2), 7.01 (d, 4), 5.22 (m, 2), 3.85
(m, 4), 2.59 (d, 2, JHH ) 14 Hz), 2.42 (d, 2, JHH ) 15 Hz), 1.30 (d,
12), 1.23 (d, 12), 0.99 (s, 18). Anal. Calcd for C20H29NOF6W: C,
40.22; H, 4.89; N, 2.35. Found: C, 40.36; H, 5.01; N, 2.32.
1
titatively: H NMR (C6D6) δ 6.90 (d, 2), 6.78 (t, 1), 5.19 (m, 1),
2.57 (s, 6), 2.01 (s, 6), 1.07 (s, 27); 19F NMR (C6D6) δ -71.33,
-71.35.
W(NAr)(CH2-t-Bu)3[OCH(CF3)2]. To a solution of 300 mg
(0.493 mmol) of W(NAr)(CH2-t-Bu)3(Cl) in 5 mL of benzene was
added as a solid 1 equiv of LiOCH(CF3)2. The reaction mixture
was heated at 60 °C overnight as it was being stirred, after which
it was filtered through a bed of Celite, and the filtrate was
concentrated to dryness to give the desired product as a red oil in
virtually quantitative yield (320 mg): 1H NMR (C6D6) δ 8.96 (s,
1, JHW ) 15 Hz), 7.05 (m, 3), 4.50 (m, 1), 3.70 (m, 2), 2.58 (d, 1,
JHH ) 15 Hz), 2.13 (d, 1, JHH ) 15 Hz), 1.28 (d, 6), 1.26 (d, 6),
1.14 (s, 9), 1.12 (s, 9); 19F NMR (C6D6) δ -74.0 (q, 3) and -74.3
(q, 3).
[W(NAr′)(CH2-t-Bu)(OC6F5)]2. A solution of W(NAr′)(CH2-
t-Bu)3(OC6F5) prepared as described above from 1.03 g (1.472
mmol) of W(NAr′)(CH-t-Bu)(CH2-t-Bu)2 in a minimum amount
of benzene was heated at 60 °C for 48 h. The resulting red solution
was cooled to -30 °C. The red solid that crystallized out of the
solution was collected on a frit and washed with pentane. The filtrate
was then concentrated to dryness, and the residue collected on a
frit and washed with pentane to give a red powder. Combined
yield: 427 mg (52%); 1H NMR (toluene-d8, 500 MHz) δ 6.98 (d,
4), 6.94 (t, 2), 3.08 (d, 2, JHH ) 14.5 Hz), 2.46 (s, 12), 2.36 (d, 2,
JHH ) 14.5 Hz), 0.91 (s, 18); 19F NMR (C6D6) δ -158.8 (d, 4),
-164.2 (t, 4), -167.80 (t, 2). Anal. Calcd for C19H20NOF5W: C,
40.96; H, 3.62; N, 2.51. Found: C, 40.84; H, 3.68; N, 2.57.
W(NAr′)(CH-t-Bu)(OCMe2CF3)2. W(NAr′)(CH-t-Bu)(OTf)2-
(dme) (1.00 g, 1.31 mmol) was dissolved in 30 mL of THF. The
solution was cooled to -30 °C, and 352 mg (2.627 mmol) of
LiOCMe2CF3 was added. The mixture was allowed to warm to
room temperature as it was being stirred. After 12 h, the reaction
mixture was concentrated to dryness in vacuo. The residue was
then extracted into pentane, and the mixture was filtered. Removal
of volatiles in vacuo afforded 670 mg of a yellow powder, which
was pure by NMR (81% yield): 1H NMR (C6D6) δ 8.48 ppm (s,
1), 6.93 (d, 2), 6.86 (t, 1), 2.28 (s, 6), 1.31 (s, 6), 1.25 (s, 6), 1.13
(s, 9); 19F NMR (C6D6) δ -82.1 ppm (s). According to the NMR
data and properties, this compound is identical to that published.5
[W(NAr′)(OCMe2CF3)2]2. A solution of W(N-2,6-Me2C6H3)(CH-
t-Bu)(OCMe2CF3)2 (4.70 g, 7.49 mmol) in 20 mL of pentane was
treated with 10 equiv of 2-pentene (8.1 mL, 75 mmol) at room
temperature. The yellow-orange solution immediately turned red.
The mixture was stirred overnight, during which time yellowish-
brown microcrystals were formed. The solids were collected on a
frit, washed with liberal amounts of pentane, and dried in vacuo to
give 3.48 g of product (83% yield): 1H NMR (toluene-d8) δ 6.99
(m, 6), 2.54 (s, 12), 1.26 (s, 12), 1.19 (s, 12); 19F NMR (toluene-
d8) δ -82.72 (s). Anal. Calcd for C32H42F12N2O4W2: C, 34.49; H,
3.80; N, 2.51. Found: C, 34.28; H, 3.74; N, 2.49.
Hetero-[W(NAr)(CH2CMe3)(OC6F5)]2. A sample of W(NAr)-
(CH2CMe3)3(OC6F5) (1.010 g) was dissolved in a minimum amount
of toluene, and the solution was heated at 80 °C for 48 h. A red
solid crystallized out of solution, was collected on a frit, and was
washed liberally with pentane; yield 585 mg (70%): 1H NMR
(C6D6, 500 MHz) δ 7.17 (d, 4), 7.08 (t, 2), 3.86 (m, 4), 3.09 (d, 2,
JHH ) 14.5 Hz), 2.47 (d, 2, JHH ) 14.5 Hz), 1.30 (d, 12), 1.01 (s,
18), 0.99 (d, 12); 19F NMR (C6D6) δ -159.29 (d, 4), -164.80 (t,
4), -167.80 (t, 2). Anal. Calcd for C23H28NOF5W: C, 45.04; H,
4.60; N, 2.28. Found: C, 44.88, H, 4.54, N, 2.33.
Homo-[W(NAr)(CH2-t-Bu)(OC6F5)]2. A solution of W(NAr)-
(CH-t-Bu)(CH2-t-Bu)(OC6F5) (700 mg, 1.024 mmol) in 10 mL of
pentane was added dropwise to a rapidly stirred solution of 2.21
mL (20.85 mmol) of cis-2-pentene in 10 mL of pentane. Solids
formed within 20 min. The dark red reaction mixture was stirred
overnight. The insoluble product was collected on a frit, washed
with liberal amounts of pentane (25 °C), and dried to give 40 mg
of red-orange hetero-[W(NAr)(CH2-t-Bu)(OC6F5)]2, which was
identical to the compound prepared via the preceding method. The
filtrate was collected and concentrated to dryness. The residue was
collected on a frit, washed with cold pentane, and dried to give
250 mg of red homo-[W(NAr)(CH2-t-Bu)(OC6F5)]2; yield 40%: 1H
NMR (C6D6, 500 MHz) δ 6.86 (br, 6), 4.49 (br, 2), 3.06 (br, 2),
2.60 (d, 2, JHW ) 15 Hz), 2.19 (d, 2, JHW ) 15 Hz), 1.72 (br, 6),
1.36 (br, 6), 1.25 (br, 6), 1.12 (s, 18), 0.88 (br, 6); 19F NMR (C6D6)
δ -158.6 (d, 4), -164.3 (t, 4), -167.1 (t, 2).
W(NAr′)(CH-t-Bu)(CH2-t-Bu)2. Neopentylmagnesium chloride
(3.41 mL, 1.35 M) was added dropwise to a rapidly stirred solution
of W(N-2,6-Me2C6H3)(CH-t-Bu)(OTf)2(dme) (1.75 g, 2.298 mmol)
in 20 mL of ether at -30 °C. After 8 h, all volatiles were removed
in vacuo. The product was taken up in 20 mL of pentane, the
solution was filtered through a bed of Celite, and the filtrate was
concentrated to dryness to give a yellow-orange solid (1.13 g) in
90% yield: 1H NMR (C6D6) δ 7.00 (2, 3), 6.87 (t, 1), 6.78 (s,1,
JHW ) 15.5 Hz), 2.48 (s, 6), 2.37 (d, 2, JHH ) 14 Hz), 1.18 (s, 9),
1.16 (s, 18), 0.78 (d, 2, JHH ) 14 Hz). Anal. Calcd for WNC23H41:
C, 53.6; H, 8.02; N, 2.72. Found: C, 53.48, H, 8.11, N, 2.71.
Single crystals suitable for X-ray diffraction studies were grown
in toluene solution at -30 °C.
W(NAr′)(CH2-t-Bu)3(OC6F5). W(N-2,6-Me2C6H3)(CH2-t-Bu)-
(CH-t-Bu)2 (500 mg, 0.970 mmol) was dissolved in pentane and
was treated with 1 equiv of HOC6F5 (179 mg). The resulting yellow
[W(NAr)(OCMe2CF3)2]2. cis-2-Pentene (1.58 mL, 14.6 mmol)
was added to a rapidly stirred solution of W(N-2,6-i-Pr2C6H3)(CH-
t-Bu)(OCMe2CF3)2 (500 mg, 0.732 mmol) in 5 mL of pentane at

1986 Organometallics, Vol. 25, No. 8, 2006
Lopez et al.
room temperature. The resulting red solution was stirred for 16 h.
Removal of volatiles in vacuo gave a red oil, into which was added
1 mL of pentane. The solution was kept at -30 °C. Red-brown
crystals suitable for X-ray diffraction studies were formed over a
period of two weeks. Several crystals were manually isolated and
subjected to crystallographic studies. The remaining product was
collected on a frit, washed with 2 × 1 mL of cold pentane, and
dried to give 18 mg of the product (5% yield): 1H NMR (C6D6) δ
7.16 (m, 6), 4.02 (m, 4) 1.34 (s, 12) 1.31 (br s, 24), 1.25 (s, 12);
19F NMR (C6D6) δ -82.2 (s).
carbon atoms of the phenyl rings in both crystallographically
independent components to be perfectly hexagonal.
The structures of [W(NAr′)(ORF6)2]2 and [W(NAr)(ORF3)2]2 both
show whole-molecule disorder. In the structure of [W(NAr′)-
(ORF6)2]2, both crystallographically independent molecules show
at least 3-fold (if not 4-fold) whole-molecule disorder. The major
components have occupancies of 87.4% and 86.3%, respectively;
the occupancies of the minor components refined to 8.8% and 3.9%
for one molecule and 9.8% and 3.9% for the other. The whole-
molecule disorder of [W(NAr)(ORF3)2]2 is only 2-fold, and the ratio
converged at 95:5. In both structures, only some of the lighter atoms
of the minor components can be found and the refinement of those
positions was not stable. Therefore, only the tungsten positions were
modeled as involved in the whole-molecule disorder and the
occupancies of the lighter atoms were left at 100%. This leads to
somewhat inflated looking thermal ellipsoids for those lighter atoms.
Furthermore, in the structures of both [W(NAr′)(ORF6)2]2 and
[W(NAr)(ORF3)2]2 several CF3 and CCH3(CF3)2 groups are disor-
dered within the main component of the whole-molecule disorder.
These disorders were refined with the help of similarity restraints
on 1-2 and 1-3 distances and displacement parameters as well as
rigid bond restraints for anisotropic displacement parameters. The
ratios were refined freely, while constraining the total occupancy
of both components to unity.
X-ray Diffraction Studies. Low-temperature data were collected
on a Siemens Platform three-circle diffractometer coupled to a
Bruker-AXS Smart Apex CCD detector with graphite-monochro-
mated Mo KR radiation (λ ) 0.71073 Å), performing æ- and
ω-scans. All structures were solved by direct methods using
SHELXS32 and refined against F2 on all data by full-matrix least
squares with SHELXL-97.33 All non-hydrogen atoms were refined
anisotropically. All hydrogen atoms (except hydrogen atoms on
carbon that binds directly to tungsten) were included in the model
at geometrically calculated positions and refined using a riding
model. All hydrogen atoms bound to carbon atoms directly linked
to tungsten have been taken from the difference Fourier synthesis
and refined semi-freely with the help of distance restraints. 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). Details of the data quality and a summary of
the residual values of the refinements are listed in Tables 1 and 3.
Both the homo- and hetero-[W(NAr)(CH2-t-Bu)(OC6F5)]2 crys-
tallize in monoclinic space groups with half a molecule in the
asymmetric unit, and in both structures the one crystallographically
independent t-Bu group is disordered. These disorders were refined
with the help of similarity restraints on 1-2 and 1-3 distances
and displacement parameters as well as rigid bond restraints for
anisotropic displacement parameters. The ratios were refined freely,
while constraining the total occupancy of both components to unity.
In addition to the half molecule of homo-[W(NAr)(CH2-t-Bu)-
(OC6F5)]2, there is half a toluene molecule in the asymmetric unit.
This solvent molecule is disordered over four positions, involving
the crystallographic 2-fold. It was necessary to constrain the six
Acknowledgment. We thank the National Science Founda-
tion (CHE-9988766 and CHE-0138495) for supporting this
research.
Supporting Information Available: Crystal data and structure
refinement, atomic coordinates and equivalent isotropic displace-
ment parameters, bond lengths and angles, anisotropic displacement
parameters, and hydrogen coordinates for hetero-[W(NAr)(CH2-t-
Bu)(OC6F5)]2 (03265A), homo-[W(NAr)(CH2-t-Bu)(OC6F5)]2 (04148),
homo-{W(NAr)(CH2-t-Bu)[OCH(CF3)2]}2 (04097), {W(NAr′)[OC-
Me(CF3)2]}2 (04231), [W(NAr′)(OCMe2CF3)2]2 (04102), and
[W(NAr)(OCMe2CF3)2]2 (04107). This information is available free
of charge via the Internet at http://pubs.acs.org. Data for the four
structures are also available to the public at http://www.recipro-
calnet.org/. The number in parentheses above identifies each
structure at this site.
(32) Sheldrick, G. M. Acta Crystallogr. Sect. A 1990, 46, 467.
(33) Sheldrick, G. M. SHELXL 97; Universita¨t Go¨ttingen: Go¨ttingen,
Germany, 1997.
OM050961S