Unprecedented reactivity initiated by insertion of 2,6-xylylisonitrile into the w-alkyl linkages of Cp*W(NO)(n-alkyl)(η3-CH 2CHCHMe) complexes

Article abstract of DOI:10.1021/om900739k

Treatment of the compounds Cp*W(NO)(R)(η³-CH ₂CHCHMe) (R = n-C₅H₁₁, n-C₇H ₁₅, n-C₈H₁₇) with 2,6-xylylisonitrile first produces the expected complexes bearing η²

Full text of DOI:10.1021/om900739k

Organometallics 2009, 28, 6139–6141 6139
DOI: 10.1021/om900739k
Unprecedented Reactivity Initiated by Insertion of 2,6-Xylylisonitrile into the
W-Alkyl Linkages of Cp*W(NO)(n-alkyl)(η3-CH2CHCHMe) Complexes
Scott P. Semproni and Peter Legzdins*
Department of Chemistry, The University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
Received August 24, 2009
Summary: Treatment of the compounds Cp*W(NO)(R)(η3-
CH2CHCHMe) (R = n-C5H11, n-C7H15, n-C8H17) with 2,6-
xylylisonitrile first produces the expected complexes bearing
η2-iminoacyl ligands arising from migratory insertion of the
isonitrile into the tungsten-alkyl linkages, processes that
result in the allyl ligands undergoing concomitant η3 f η1
haptotropic shifts. These η1-allyl complexes then undergo sub-
sequent intramolecular nucleophilic attacks by their allyl groups
on the nitrogen atoms of the η2-iminoacyl ligands to form novel
metallacyclic compounds that contain R-aminocarbene ligands.
The migratory insertion of organic isonitriles into metal-
alkyl bonds is an important organometallic reaction that has
received a fair amount of attention from various researchers
over the years.1,2 Such insertions have been reported for most
of the early transition metals as well as some lanthanides and
actinides, and they lead to the formation of η2-iminoacyl
ligands. Hence, it did not surprise us that during our
recent investigation3 of the characteristic chemistry of the
two Cp*W(NO)(R)(η3-CH2CHCHMe) complexes having
R = CH2SiMe3, CH2C6H5 we found that they react with
ability thermal ellipsoids shown. Selected interatomic distances
Figure 1. Solid-state molecular structure of A with 50% prob-
˚
(A) and angles (deg): W(2)-C(43) = 2.214(3), W(2)-N(4) =
2.170(2), W(2)-C(34) = 2.101(3), C(34)-N(4) = 1.267(4),
W(2)-N(3) = 1.761(3); W(2)-N(4)-C(34) = 69.78(17),
W(2)-C(34)-N(4) = 75.77(17).
Scheme 1
2,6-xylylisonitrile by inserting the isonitrile into the tung-
sten-allyl linkages (Scheme 1).4
During an extension of these original studies we have now
discovered that those members of this family of tungsten
compounds that have R =n-alkyl exhibit an entirely different
mode of reactivity involving their alkyl ligands when treated
with 2,6-xylylisonitrile. It is these novel chemical transforma-
tions that we now wish to communicate in some detail.
As summarized in Scheme 2, treatment of Cp*W(NO)-
(n-C5H11)(η3-CH2CHCHMe) with the isonitrile first pro-
duces the expected compound A, containing an η2-iminoacyl
ligand arising from migratory insertion of the isonitrile into
Scheme 2
(4) Klei, E.; Teuben, J. H.; de Liefde Meijer, H. J.; Kwak, E. J.; Bruins,
A. P. J. Organomet. Chem. 1982, 224, 327–339 and references cited therein.
(5) For additional examples of η1-allyl ligands functioning as nucleo-
philes, see: Rosenblum, M. Acc. Chem. Res. 1974, 7, 122–128.
˚
(6) Crystal data for A þ B: triclinic, space group P1, a=7.884(5) A,
˚
˚
b=18.274(5) A, c=19.322(5) A, R=102.850(5)°, β=100.902(5)°, γ=
3
˚
93.820(5)°, V = 2648(2) A , Z = 2, R1 = 0.0409, wR2 = 0.0477, and
GOF(F2) = 1.020 for 12 416 observed reflections and 700 variable
parameters.
*To whom correspondence should be addressed. E-mail: legzdins@
chem.ubc.ca.
(1) Durfee, L. D.; Rothwell, I. P. Chem. Rev. 1988, 88, 1059–1079.
(2) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 4th ed.; Wiley: New York, 2005; Chapter 7.
(7) The molecular structure of B exhibits some disorder of the pentyl
ligand. Only one of the rotational isomers is shown in Figure 2.
(8) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1–S19.
ꢀ
(9) Kreiter, C. G.; Formacek, V. Angew. Chem., Int. Ed. Engl. 1972,
(3) Semproni, S. P.; Graham, P. M.; Buschhaus, M. S. A.; Patrick,
B. O.; Legzdins, P. Organometallics 2009, 28, 4480–4490.
11, 141–142.
(10) See the Supporting Information for details.
r
2009 American Chemical Society
Published on Web 10/13/2009
pubs.acs.org/Organometallics

6140 Organometallics, Vol. 28, No. 21, 2009
Semproni and Legzdins
the tungsten-alkyl linkage,4 a process that results in the allyl
ligand undergoing a concomitant η3 f η1 haptotropic shift.
Compound A then undergoes a subsequent intramolecular
nucleophilic attack by the η1-allyl group5 on the nitrogen
atom of the η2-iminoacyl ligand to form the unusual metal-
lacyclic compound B, which contains an R-aminocarbene
ligand. Interestingly, the conversion of A to B does not
proceed to completion but rather continues only until an
equilibrium is established between the two isomeric com-
plexes (K = 1.15 at 20 °C) after approximately 24 h.
The solid-state molecular structures of A and B have been
confirmed by an X-ray crystallographic analysis of a single
crystal containing both complexes,6 and they are shown in
Figures 1 and 2, respectively.
to the tungsten center that thus attains the favored 18e
configuration. The four electrons are provided via a CfW
dative interaction involving C(6) and synergic η2-(CdC)-W
bonding involving C(17) and C(18). Consistent with this
view are the following facts.
(1) The C(6)-N(2) and C(17)-C(18) bond lengths are
1.317(4) and 1.438(5) A, respectively, and they are shorter
˚
than typical C-N and C-C single-bond distances of 1.469
and 1.514 A, respectively.
8
˚
(2) The coordination geometries at C(6) and N(2) are
essentially planar, consistent with these atoms being sp2-
hybridized.
(3) The 13C NMR spectrum of B in C6D6 exhibits a
downfield resonance attributable to C(6) at δ 258.9. For
comparison, the carbene carbon signal for W{C(NHMe)-
Me}(CO)5 in C6D6 occurs at 255.6 ppm.9
While the intramolecular metric parameters of A are as
expected for a molecule of this type,4 those of B constitute a
more fascinating story.7 The newly formed R-aminocarbene
ligand is best viewed as functioning as a four-electron donor
The other spectroscopic properties of A and B indicate that
they retain their solid-state molecular structures in solutions.10
The conversion depicted in Scheme 2 for the n-C5H11
reactant also occurs for the Cp*W(NO)(R)(η3-CH2CH-
CHMe) complexes having R = n-C7H15 and n-C8H17. The
organometallic products in each case (i.e., A0 and B0, and A00
and B00, respectively, in Scheme 2) have been characterized by
conventional spectroscopic and analytical methods.10 Just as
for the R=n-C5H11 case (vide supra), both reactions attain
equilibrium in about 24 h, and from the variation of ln K vs
1/T ΔH and ΔS can be estimated for these transformations.
In the case of the n-pentyl reactant in C6D6 with an A/B
mixture concentration of ca. 0.05 M, such a plot (Figure 3)
affords reasonable values for ΔH and ΔS of -19.70 ( 1.5 kJ/
mol and -65.4 ( 4.7 J/mol, respectively, which reflect both
the stronger bonds and the more ordered nature of B.
Finally, we have also established that the relatively pure
metallacyclic complex B does not react with electrophiles
such as B-chlorocatecholborane, whereas the comparably
pure η1-allyl complex A undergoes electrophilic attack at the
W-C bond while leaving the isonitrile-inserted unit intact
(Scheme 3).
Figure 2. Solid-state molecular structure of B with 50% prob-
ability thermal ellipsoids shown. Selected interatomic distances
˚
In summary, we have discovered that the allyl ligand
can become involved in subsequent reactivity after Cp*W-
(NO)(R)(η3-CH2CHCHMe) (R = n-alkyl) complexes have
undergone insertion of 2,6-xylylisonitrile into their W-R
linkages. Specifically, the initially formed η2-iminoacyl ligands
(A) and angles (deg): W(1)-C(18) = 2.182(3), W(1)-C(17) =
2.163(3), W(1)-C(6) = 2.134(3), C(6)-N(2) = 1.317(4), N(2)-
C(16) = 1.511(4), C(17)-C(18) = 1.438(5); C(16)-N(2)-
C(6) = 116.7(2), W(1)-C(6)-N(2) = 118.4(2), C(17)-C(16)-
N(2) = 106.2(2).
Figure 3. Plot of ln K vs 1/T for the equilibrium between A and B.

Communication
Organometallics, Vol. 28, No. 21, 2009 6141
Scheme 3
similar modes of reactivity are exhibited by other Cp*W(NO)-
(alkyl)(η3-allyl) complexes with other isocyanides. Studies de-
signed to provide these answers and to develop more fully the
chemistry of the novel metallacyclic complexes are currently in
progress.
Acknowledgment. We are grateful to Dr. Brian O.
Patrick for his assistance in sorting out the disorder extant
in the solid-state molecular structures of A and B, and we
thank Mr. Rhett A. Baillie for technical assistance.
Financial support from the NSERC of Canada is also
acknowledged.
then undergo intramolecular nucleophilic attack by the allyl
ligand that has undergone a concomitant η3 f η1 haptotropic
shift to form azametallacycles that involve olefin and carbene
links from R-aminocarbene ligands to the tungsten centers. To
the best of our knowledge, these transformations involving the
methylallyl ligand are without precedent. However, given the
central role that migratory insertion reactions play in transition-
metal-catalyzed processes,2 these results are of fundamental
significance. Nevertheless. it remains to be ascertained whether
Supporting Information Available: Text and tables providing
full details of experimental procedures and characterization
data for all new complexes and a CIF file containing details
of the crystallographic analysis of complexes A and B. This
material is available free of charge via the Internet at http://
pubs.acs.org.