Formation of dimers that contain unbridged W(IV)/W(IV) double bonds

Article abstract of DOI:10.1021/ja0400988

A tungsten neopentylidene complex has been found to decompose to yield a heterochiral dimer that contains a W=W double bond and no bridging ligands. Decompositions of related bisalkoxide complexes also yield compounds that contain an unsupported W=W double bond, while a sample of [Mo(NAr)(CH₂-t-Bu)(OC₆F₅)]₂ has been found to be a homochiral species related to [W(NAr)(CH₂-t-Bu)(OC₆F₅)]₂. Copyright

Full text of DOI:10.1021/ja0400988

Published on Web 07/16/2004
Formation of Dimers That Contain Unbridged W(IV)/W(IV) Double Bonds
Lourdes Pia H. Lopez and Richard R. Schrock*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received April 5, 2004; E-mail: rrs@mit.edu
Certain molybdenum alkylidene complexes of the type Mo(NR)-
(CHR′)(OR′′)2 (or metalacyclobutane complexes derived from them)
have been shown to decompose to yield dimeric species, e.g. [Mo-
(µ-NAr)(O-t-Bu)2]2 (Ar ) 2,6-i-Pr2C6H3), which contains a sym-
metric, planar Mo2N2 core and a pseudotetrahedral arrangement
about each Mo.1 It has been assumed that tungsten complexes of
the same general type also contain bridging imido groups.2 In this
communication we show that new tungsten imido alkylidene
complexes of the type W(NR)(CH-t-Bu)(CH2-t-Bu)(OC6F5) de-
compose to yield dimeric species that contain unbridged WdW
double bonds. Such species are closely related to two compounds
of the type [Re(C-t-Bu)(OR)2]2 (OR ) O-t-Bu or OCMe(CF3)2),3
which also contain “unsupported” MdM bonds and are formed
upon decomposition of alkylidene complexes of the type Re(C-t-
Bu)(CHR′)(OR)2 (R′ ) OEt or OSiMe3).3
Recently, we showed that active olefin metathesis catalysts of
the type Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OR) (Ar ) 2,6-i-Pr2C6H3)
can be prepared by adding a variety of alcohols to Mo(NAr)(CH-
t-Bu)(CH2-t-Bu)2.4 Therefore we became interested in exploring
analogous reactions involving W(NAr)(CH-t-Bu)(CH2-t-Bu)2.2 A
three-step synthesis of W(NAr)(CH-t-Bu)(CH2-t-Bu)2 that is more
convenient than the reported2 six-step synthesis (from WCl6) is
shown in eq 1.5 Sterically crowded W(NAr)(CH2-t-Bu)4, shown
related neophyl and neophylidene species) and adamantanol,4
W(NAr)(CH-t-Bu)(CH2-t-Bu)(OAd) is proposed to arise as a
consequence of addition of adamantanol across a W-C bond.
Upon heating solutions of W(NAr)(CH2-t-Bu)3(OC6F5) or
W(NAr)(CH2-t-Bu)3[OC(CF3)3], neopentane is evolved, and
W(NAr)(CH-t-Bu)(CH2-t-Bu)(OC6F5) (δCH ) 9.29 ppm, JHW ) 15
Hz) and W(NAr)(CH-t-Bu)(CH2-t-Bu)[OC(CF3)3] (δCH ) 9.39 ppm,
JHW ) 15 Hz) are formed. Plots of log[W(NAr)(CH2-t-Bu)3(OR)]
versus time at 60 °C are linear. When OR ) OC6F5, k ) 7.0 ×
10-5 s-1, while when OR ) OC(CF3)3 k ) 1.7 × 10-4 s-1. Neither
W(NAr)(CH-t-Bu)(CH2-t-Bu)(OC6F5) nor W(NAr)(CH-t-Bu)(CH2-
t-Bu)[OC(CF3)3] has been isolated in crystalline form, the latter
because it has so far resisted all attempts to crystallize it, and the
former because it, unlike Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OC6F5),4
is unstable with respect to bimolecular decomposition (see below).
NMR studies suggest that all W(NAr)(CH-t-Bu)(CH2-t-Bu)(OR)
compounds (OR ) OC6F5, OC(CF3)3, or OAd) are syn species in
which the neopentylidene’s tert-butyl group points toward the imido
ligand;6,7 no alkylidene HR resonances for anti isomers have been
observed.
Although W(NAr)(CH-t-Bu)(CH2-t-Bu)[OC(CF3)3] and W(NAr)-
(CH-t-Bu)(CH2-t-Bu)(OAd) are relatively stable in solution at 80
°C, W(NAr)(CH-t-Bu)(CH2-t-Bu)(OC6F5) (formed by heating a 0.1
M solution of W(NAr)(CH2-t-Bu)3(OC6F5) to 80 °C in toluene-d8)
begins to decompose after ∼80% of it has formed from W(NAr)-
(CH2-t-Bu)3(OC6F5). The only observable products are t-BuCHd
CH-t-Bu (one isomer, presumably trans) and sparingly soluble
“W(NAr)(CH2-t-Bu)(OC6F5).” Two doublet resonances (JHH ) 14.5
Hz) are found for the neopentyl methylene protons at 2.47 and 3.09
ppm in the proton NMR spectrum of “W(NAr)(CH2-t-Bu)(OC6F5)”,
consistent with an absence of a plane that passes through the
neopentyl methylene carbon atom.
arbitrarily as a trigonal bipyramidal species, is believed to be the
intermediate that undergoes R hydrogen abstraction6,7 to produce
W(NAr)(CH-t-Bu)(CH2-t-Bu)2 (cf. W(NPh)(CHSiMe3)(CH2SiMe3)28).
W(NAr)(CH-t-Bu)(CH2-t-Bu)2, prepared as shown in eq 1, has been
obtained only as a red-brown oil. However, addition of three
equivalents of t-BuCH2MgCl to W(NAr)Cl4 yields W(NAr)(CH2-
t-Bu)3Cl cleanly (cf. W(NPh)(CH2-t-Bu)3Cl8), which can be isolated
and alkylated with LiCH2-t-Bu to yield W(NAr)(CH-t-Bu)(CH2-t-
Bu)2 virtually quantitatively as an orange solid.
An X-ray study reveals that sparingly soluble “W(NAr)(CH2-t-
Bu)(OC6F5)” is a centrosymmetric (heterochiral) dimer (Figure 1)
in which there are no bridging ligands and a double bond (2.4445-
(3) Å) between the two tungstens. Among the striking features of
the structure is a N-W-W angle close to 90° (90.38(10)°). The
W-N bond length (1.749(3) Å) and W-N-C angle (165.5(3)°)
are not unusual. The W-C(19)-C(20) angle (119.3(3)°) is typical
of a relatively “undistorted” neopentyl group, while the W-O(1)-
C(13) angle (146.7(3)°) suggests that a significant degree of π
bonding is present, which opens the W-O-C angle significantly.
This structure is similar to the structures of two related d2/d2 [Re-
(C-t-Bu)(OR)2]2 (OR ) O-t-Bu or OCMe(CF3)2) species3 in which
the RedRe bond lengths are 2.3836(8) and 2.396(1) Å (respec-
tively), the CtRedRe angles are 90.0(2)° and 89.5(1)° (respec-
tively), and the alkylidyne ligands are trans to one another. The
shapes of the W (and Re) compounds suggest that three mutually
perpendicular π bonds are formed from the dxz, dyz, and dxy orbitals,
with the four σ bonds being formed from the three p orbitals and
Addition of C6F5OH to W(NAr)(CH-t-Bu)(CH2-t-Bu)2 (∼0.1 M)
in C6D6 at 25 °C rapidly yields W(NAr)(CH2-t-Bu)3(OC6F5) (δCH
2
) 2.25 ppm), which can be isolated as a yellow powder.9 A similar
reaction between (CF3)3COH and W(NAr)(CH-t-Bu)(CH2-t-Bu)2
in pentane at 25 °C yields yellow crystalline W(NAr)(CH2-t-Bu)3-
[OC(CF3)3] (δCH ) 2.20 ppm in C6D6). Species of this type are
2
believed to have trigonal bipyramidal structures in which the
alkoxide is in an apical position. They should be compared with
known W(NPh)(CH2-t-Bu)3(O-t-Bu) (δCH ) 1.94 ppm).8
2
In contrast, addition of one equivalent of 1-adamantanol to
W(NAr)(CH-t-Bu)(CH2-t-Bu)2 in pentane yields W(NAr)(CH-t-Bu)-
(CH2-t-Bu)(OAd) as a yellow powder (in C6D6 δCH ) 8.86 ppm,
JHW ) 14.5 Hz; δCH ) 253.1 ppm, JCH ) 110 Hz). By analogy
with similar reactions between Mo(NAr)(CH-t-Bu)(CH2-t-Bu)2 (and
some combination of s and dz orbitals.10 The transoid arrangement
2
of the multiply bound alkylidyne ligands in the Re complexes was
9
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C O M M U N I C A T I O N S
methyl groups in the trifluoro-tert-butoxide are inequivalent (with
resonances at 1.27 and 1.92 ppm in toluene-d8). Therefore, we
suspect that {W(NAr′)[OCMe(CF3)2]2}2 and {W(NAr′)[OCMe2-
(CF3)]2}2 also contain unsupported WdW bonds. (No X-ray study
of either species has yet been successful.)15
Finally, we recently found that Mo(NAr)(CH-t-Bu)(CH2-t-Bu)-
(OC6F5),4 which is much more stable than W(NAr)(CH-t-Bu)-
(CH2-t-Bu)(OC6F5), will react with 10 equiv of trans-3-hexene at
25 °C to yield [Mo(NAr)(CH2-t-Bu)(OC6F5)]2, which was shown
in an X-ray study to be a homochiral molecule related to [W(NAr)-
(CH2-t-Bu)(OC6F5)]2 (Figure 1).14 Therefore unsupported MdM
bonds are not restricted to W and Re.
To the best of our knowledge, no Mo or W dimers are known
that contain “unsupported” MdM bonds in the presence of
potentially bridging ligands. (See also discussion in refs 3 and 10.)
These species are strikingly different from a compound such as
[W(OCH2-t-Bu)4]x, a polymeric species that contains bridging
neopentoxides.13 We are now in the process of expanding the library
of synthetically accessible MdM species and exploring their
fundamental reactions.
Figure 1. Chem3D drawing of the structure of centrosymmetric [W(NAr)-
(CH2-t-Bu)(OC6F5)]2. (W-N ) 1.749(3) Å, W-O ) 1.933(2) Å, W-C )
2.139(3) Å, W-W# ) 2.4445(3) Å, W-N(1)-C(1) ) 165.5(3)°, W-O(1)-
C(13) ) 146.7(3)°, W-C(19)-C(20) ) 119.3(3)°, N(1)-W-W# ) 90.38-
(10)°, O(1)-W-W# ) 113.17(9)°, C(19)-W-W# ) 98.08(10)°, N(1)-
W-O(1) ) 132.30(13)°, N(1)-W-C(19) ) 106.23(15)°, O(1)-W-C(19)
) 110.31(14)°).
Acknowledgment. We thank the National Science Foundation
(CHE-0138495) for supporting this research and D. Yandulov for
assistance in refining the crystal structure.
Supporting Information Available: Experimental details, labeled
thermal ellipsoid drawing, crystal data and structure refinement, atomic
coordinates, bond lengths and angles, and anisotropic displacement
parameters for [W(NAr)(CH2-t-Bu)(OC6F5)]2. X-ray crystallographic
data in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
found to be energetically preferred in theoretical studies of model
compounds.10
The reaction between W(NAr′)(CH-t-Bu)(dme)(triflate)2 (Ar′ )
2,6-Me2C6H3)11 and two equivalents of t-BuCH2MgCl yields
W(NAr′)(CH-t-Bu)(CH2-t-Bu)2 as an orange powder, which upon
treatment with pentafluorophenol yields W(NAr′)(CH2-t-Bu)3-
(OC6F5) quantitatively. Upon heating a toluene-d8 solution (0.10
M) of W(NAr′)(CH2-t-Bu)3(OC6F5) to 60 °C, W(NAr′)(CH-t-Bu)-
(CH2-t-Bu)(OC6F5) forms and decomposes to yield t-BuCHdCH-
t-Bu and sparingly soluble [W(NAr)(CH2-t-Bu)(OC6F5)]2. On the
basis of the inequivalent neopentyl methylene protons in the proton
NMR spectrum of [W(NAr)(CH2-t-Bu)(OC6F5)]2 in toluene-d8 we
propose that this compound also contains an unsupported WdW
bond. W(NAr′)(CH2-t-Bu)3(OC6F5) evolves neopentane intramo-
lecularly more slowly than W(NAr)(CH2-t-Bu)3(OC6F5), while
intermediate W(NAr′)(CH-t-Bu)(CH2-t-Bu)(OC6F5) decomposes
bimolecularly more quickly than intermediate W(NAr)(CH-t-Bu)-
(CH2-t-Bu)(OC6F5) at the concentrations employed. Both can be
ascribed to subtle steric differences between the NAr and NAr′
groups.
It has been reported that unstable W(CHEt)(NAr′)[OCMe-
(CF3)2]2, which is prepared by treating W(CH-t-Bu)(NAr′)[OCMe-
(CF3)2]2 with cis-3-hexene, decomposes to yield {W(NAr′)[OCMe-
(CF3)2]2}2.2 It was proposed that {W(NAr′)[OCMe(CF3)2]2}2 contains
bridging imido ligands, although the presence of two inequivalent
trifluoromethyl groups in the carbon NMR spectrum of {W(NAr′)-
[OCMe(CF3)2]2}2 is inconsistent with a structure analogous to that
of [Mo(µ-NAr)(O-t-Bu)2]2 (Ar ) 2,6-i-Pr2C6H3).1 We have shown
that W(CH-t-Bu)(NAr′)[OCMe2(CF3)]2 also reacts with cis-2-
pentene to yield {W(NAr′)[OCMe2(CF3)]2}2 in which the two
References
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(4) Sinha, A.; Schrock, R. R. Organometallics 2004, 23, 1643-1645.
(5) The feasibility of this approach was first demonstrated by A. Sinha with
t-BuCH2MgCl; this reaction mixture tends to contain a significant quantity
of W(NAr)(CH2-t-Bu)3Cl.
(6) Schrock, R. R., Ed. Reactions of Coordinated Ligands; Plenum: New
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(7) Feldman, J.; Schrock, R. R. Prog. Inorg. Chem. 1991, 39, 1-73.
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(9) Preparative scale reactions were carried out in pentane.
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Organomet. Chem. 1994, 478, 153-160.
(11) This species was prepared by a method analogous to that employed to
prepare W(NAr)(CH-t-Bu)(dme)(triflate)2; see Supporting Information for
details.
(12) Tsang, W. C. P.; Hultzsch, K. C.; Alexander, J. B.; Bonitatebus, P. J. J.;
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(14) A. Sinha; unpublished results to be reported in due course.
(15) Note added in proof: An X-ray structure of the latter has confirmed a
structure that is, overall, similar to that shown in Figure 1 and that contains
an unbridged WdW bond 2.4905(3) Å long.
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