Trapping of Cp*W(NO)(η2-PhC≡CH), the key intermediate in the C-H activation of alkanes by Cp*W(NO)(CH2SiMe3)(CPh≡CH2)

Article abstract of DOI:10.1021/om970118x

Thermolysis of the vinyl nitrosyl complex Cp*W(NO)(CH₂SiMe₃)(CPh=CH₂) (1) in ethyl acetate or acetonitrile affords the oxa- or azametallacycles Cp*W(NO)(η²-OC(Me)(OEt)CH=CPh) (2) or Cp*W(NO)(η³-HNC(Me)=NC(=CH₂)CH=CPh) (3) and Cp*W(NO)-(OH)(η²-CN=C(Me)CH=CPh) (4), respectively. Kinetic and mechanistic studies provide compelling evidence for a pathway involving initial rate-limiting unimolecular elimination of SiMe₄ from 1 with concomitant formation of Cp*W(NO)(η²-CPh≡CH) (A).

Full text of DOI:10.1021/om970118x

Organometallics 1997, 16, 1825-1827
1825
Tr a p p in g of Cp *W(NO)(η2-P h CtCH), th e Key
In ter m ed ia te in th e CsH Activa tion of Alk a n es by
Cp *W(NO)(CH2SiMe3)(CP h dCH2)
Peter Legzdins* and Sean A. Lumb
Department of Chemistry, The University of British Columbia,
Vancouver, British Columbia, Canada V6T 1Z1
Received February 18, 1997X
Sch em e 1
Summary: Thermolysis of the vinyl nitrosyl complex
Cp*W(NO)(CH2SiMe3)(CPhdCH2) (1) in ethyl acetate or
acetonitrile affords the oxa- or azametallacycles Cp*W-
(NO)(η2-OC(Me)(OEt)CHdCPh) (2) or Cp*W(NO)(η3-
HNC(Me)dNC(dCH2)CHdCPh) (3) and Cp*W(NO)-
(OH)(η2-HNdC(Me)CHdCPh) (4), respectively. Kinetic
and mechanistic studies provide compelling evidence for
a pathway involving initial rate-limiting unimolecular
elimination of SiMe4 from 1 with concomitant formation
of Cp*W(NO)(η2-CPhtCH) (A).
We recently reported the unique double CsH activa-
tion of alkanes under mild thermal conditions by the
tungsten vinyl complex Cp*W(NO)(CH2SiMe3)(CPhd
CH2) (1).1 The key intermediate invoked to account for
this chemistry is the coordinatively unsaturated η2-
phenylacetylene complex A, which results from 1 ini-
tially undergoing â-H reductive elimination as shown
in Scheme 1. Herein we report compelling evidence for
the existence of A by describing its quantitative trapping
by ethyl acetate and acetonitrile via reductive coupling
reactions (Scheme 1). Interestingly, the reaction with
ethyl acetate affords the expected oxametallacyclopen-
tene complex Cp*W(NO)(η2-OC(Me)(OEt)CHdCPh) (2),
but the reactions with acetonitrile produce two surpris-
ing compounds. Under rigorously anhydrous conditions,
2 equiv of CH3CN is incorporated during the formation
of the bicyclic complex Cp*W(NO)(η3-HNC(Me)dNC-
(dCH2)CHdCPh) (3). If water is present, only 1 equiv
of CH3CN is incorporated to form the hydroxy azametal-
lacyclopentadiene complex Cp*W(NO)(OH)(η2-HNdC-
(Me)CHdCPh) (4), whose solid-state molecular struc-
ture has been established by X-ray crystallography.2
Preliminary kinetic investigations of these reactions
indicate that their rates are independent of both the
incoming organic substrate and water, thereby implying
a mechanism having the unimolecular formation of A
as the rate-determining step followed by its rapid
trapping in subsequent steps to form the observed
products.
reductive coupling of ethyl acetate and phenylacetylene
results in the oxametallacyclopentene complex 2, which
is isolated as deep purple needles in >84% yield
(Scheme 1).4 The spectroscopic properties of 2 are
consistent with its formulation. Thus, the absence of
characteristic CdO and CtC absorptions in its Nujol-
mull IR spectrum implies that the ethyl acetate and
acetylene fragments are fused.5 In addition, a 1H
NOEDIFF NMR experiment reveals the close proximity
of the acetal methyl substituent to both the vinyl H and
the Cp* ring methyl groups, thereby allowing assign-
ment of the stereochemistry at the acetal carbon.
Finally, a doublet centered at about δ 152.2 (1J CH ) 159
Hz) in its 13C NMR spectrum in CDCl3 is attributable
to the vinylic carbon and is consistent with sp2 hybrid-
ization at that atom.
In rigorously dried acetonitrile A reacts with 2 equiv
of CH3CN to form the diazametallabicyclic complex 3,
which is isolable as an air-stable, deep red microcrys-
talline powder in 78% yield. Space-filling models con-
firm that the imine group is correctly disposed relative
to the NO ligand for dative-bond formation as depicted
in Scheme 1, thus rendering 3 electronically and coor-
At 45 °C in neat ethyl acetate 1 does not effect CsH
bond activation as it does in n-pentane and n-hexane
under the same experimental conditions.3 Instead,
X Abstract published in Advance ACS Abstracts, April 1, 1997.
(1) Debad, J . D.; Legzdins, P.; Lumb, S. A. J . Am. Chem. Soc. 1995,
117, 3288.
(2) Coupling reactions of acetylenes and unsaturated organic mol-
ecules in metal coordination spheres have been reported previously;
see: (a) Broene, R. D.; Buchwald, S. L. Science 1993, 261, 1696. (b)
Buchwald, S. L.; Neilsen, R. B. Chem. Rev. 1988, 88, 1047. (c) Maier,
M. E.; Oost, T. J . Organomet. Chem. 1995, 505, 95. (d) Peulecke, N.;
Ohff, A.; Baumann, W.; Kempe, R.; Burlakov, V. V.; Rosenthal, U.
Organometallics 1996, 15, 1340.
(4) The three reactions forming complexes 2-4 are quantitative, as
judged by 1H NMR spectroscopic monitoring.
(3) This contrasts with the activation of a CsH bond in the methoxy
group of MeOAc by [Cp*Ir(PMe3)(Me)(ClCH2Cl)]+; see: Arndtsen, B.
A.; Bergman, R. G. Science 1995, 270, 1970.
(5) Nakamoto, K. Infrared and Raman Spectra of Inorganic and
Coordination Compounds, 4th ed.; Wiley-Interscience: New York,
1986.
S0276-7333(97)00118-0 CCC: $14.00 © 1997 American Chemical Society

1826 Organometallics, Vol. 16, No. 9, 1997
Communications
Sch em e 2
F igu r e 1. Solid-state molecular structure of 4. Selected
interatomic distances (Å) and angles (deg) W(1)-N(1) )
2.150(3), W(1)-C(4) ) 2.227(4), W(1)-O(1) ) 2.064(2),
N(1)-C(1) ) 1.295(5), C(1)-C(3) ) 1.434(5), C(3)-C(4) )
1.366(5); W(1)-N(2)-O(2) ) 172.7(3), O(1)-W(1)-N(1) )
75.52(11), N(1)-W(1)-C(4) ) 72.45(12); N(2)-W(1)-N(1)-
C(1) ) 84.7(3), N(1)-C(1)-C(3)-C(4) ) 6.4(5), N(1)-W(1)-
C(4)-C(3) ) -3.1(3).
dinatively saturated at the metal center.6 A broad
1
signal at δ 4.73 in the H NMR spectrum of 3 in CDCl3
atom was not located during the structural analysis, the
W(1)-O(1) bond distance of 2.064(2) Å is most charac-
teristic of a terminal OH group.11 The NMR spectro-
scopic data for 4 indicate that it retains its solid-state
structure in solution. For example, a characteristic
four-bond coupling constant12 of 3.5 Hz is observed
between the azacyclopentadiene ring proton and the
imide proton.13 Interestingly, a carbenoid signal at
229.5 ppm in the 13C NMR spectrum of 4 in CDCl3 is
attributable to the carbon nucleus adjacent to the W
center and indicates a degree of aromaticity in the
ring.14
The metallacyclic nature of 4 corroborates the struc-
tures invoked for complexes 2 and 3 and implies the
formation of a common intermediate that results from
the reductive coupling of coordinated organic substrate
and phenylacetylene in the metal’s coordination sphere.
Consequently, the mechanism that we propose for the
formation of complexes 2-4 (Scheme 2) involves the
formation of the oxametallacyclopentene complex 3 or
the azametallacyclopentadiene complex C by reductive
coupling of the two unsaturated ligands in intermediate
B (L ) EtOAc, MeCN). Intermediate C is probably
electronically unsaturated, since the imide fragment
cannot adopt the required orientation relative to the
M-NO linkage in order to function as a three-electron
donor to the metal center15 and therefore is Lewis basic.
In acetonitrile containing trace water, protonation of the
integrated for one proton is attributable to the amine
proton, and a νNH band at 3258 cm-1 in the KBr IR
spectrum confirms its existence. In addition, an IR
band at 1613 cm-1 is attributable to the imide NdC
stretching mode.7 Furthermore, the results of 1D and
2D NMR experiments are consistent with the ring
1
connectivity depicted for 3. The H NMR spectrum of
3 displays signals at δ 6.83, 4.45 and 4.28, and 2.11
which are attributable to the endocyclic vinyl, exocyclic
vinyl, and exocyclic methyl environments, respectively,
while the results of a short-range 1H-13C HMQC
experiment permit the assignment of the carbon nuclei
to which these protons are bound.
A
1H NOEDIFF
NMR experiment yields the spatial relationships of
these proton environments, and a long-range 1H-13C
HMBC NMR experiment permits the assignment of the
quaternary carbon nuclei and hence the carbon back-
bone of the rings. That the second equivalent of
acetonitrile has inserted to form complex 3 and has not
simply coordinated to the metal center is indicated both
by the absence of characteristic IR5 and 13C NMR8
evidence and by a lack of reactivity of 3 with MeCN-d3
and PMe3.9
In wet acetonitrile A incorporates only one CH3CN
molecule during the formation of the hydroxy azametal-
lacyclopentadiene complex Cp*W(NO)(OH)(η2-HNdC-
(Me)CHdCPh) (4; Scheme 1), which is isolable as red
crystals in 80% yield.10 The solid-state molecular struc-
ture of 4 clearly reveals the nearly planar azacyclopen-
tadiene ring (Figure 1). Though the hydroxyl hydrogen
(11) The complex [Mo(O)(OH)(CN)4]3- has ModO ) 1.697(7) Å and
MosOH ) 2.077(7) Å; see: Robinson, P. R.; Schlemper, E. O.;
Murmann, R. K. Inorg. Chem. 1975, 14, 2035. Typical terminal W-oxo
bond distances lie in the range 1.68-1.72 Å; see: Nugent, W. A.; Mayer,
J . M. Metal Ligand Multiple Bonds; Wiley: New York, 1988.
(12) Christensen, N. J .; Hunter, A. D.; Legzdins, P. Organometallics
1989, 8, 930.
(13) An analogous coupling is observed in the iridapyrrole complex
recently reported by Carmona et al.; see: Alvarado, Y.; Daff, P. J .;
Pe´rez, P. J .; Poveda, M. L.; Sa´nchez-Delgado, R.; Carmona, E.
Organometallics 1996, 15, 2192.
(14) Aromaticity has been invoked for similar iridium aza- and
thiapentadienyl systems; see: (a) Reference 13. (b) Bleeke, J . R.;
Ontwerth, M. F.; Rohde, A. M. Organometallics 1995, 14, 2813.
(15) Ashby, M. T.; Enemark, J . H. J . Am. Chem. Soc. 1986, 108,
730.
(6) Debad, J . D.; Legzdins, P.; Lumb, S. A. Organometallics 1995,
14, 2543.
(7) Filippou, A. C.; Vo¨lkl, C.; Kiprof, P. J . Organomet. Chem. 1991,
415, 375.
(8) Yeh, W.-Y.; Ting, C.-S.; Chih, C.-F. J . Organomet. Chem. 1991,
427, 257.
(9) Legzdins, P.; Sayers, S. J . Am. Chem. Soc. 1994, 116, 12105.
(10) Terminal OH ligands in group 6 organometallic complexes are
rare, due to their propensity to form very stable bridging oxo ligands
by protonolysis of metal-ligand linkages; see: (a) Gilje, J . W.; Roesky,
H. W. Chem. Rev. 1994, 94, 895. (b) Bergman, R. G. Polyhedron 1995,
14, 3227.

Communications
Organometallics, Vol. 16, No. 9, 1997 1827
basic imide nitrogen in C shifts the D/C equilibrium
toward the irreversible formation of the azametallacy-
clopentadiene 4.16 In contrast, in dry acetonitrile an
insertion of coordinated CH3CN in D yields the strongly
basic amidine group in E. A subsequent proton tau-
tomerization such as that observed by Wigley17 leads
to the formation of complex 3. The extensive conjuga-
tion within the ligand backbone in 3 is probably a strong
driving force for this reaction.
The use of deuterated solvents provides additional
support for these mechanistic proposals. When dry CD3-
CN is employed, Cp*W(NO)(η3-DNC(CD3)dNC(dCD2)-
CHdCPh) (3-d 6) results, thereby indicating that the
amine proton originates from the methyl substituent
through a tautomerization process. In addition, no
conversion to 4 is detectable when pure 3 is placed in
wet acetonitrile under thermolysis conditions, thereby
indicating that both species are formed irreversibly.
When D2O in acetonitrile is employed, Cp*W(NO)(OD)-
(η2-DNdC(Me)CHdCPh) (4-d 2) is formed, thereby im-
plying that the amide H originates directly from water
since no label is incorporated elsewhere in the ring. In
other words, the observed product results from a direct
protonation by water similar to that observed by Car-
mona for the related iridium complex.14 Complex 4 also
exchanges with D2O to form 4-d 2 under the same
thermolysis conditions. Since the formation of 4 is
irreversible (vide supra), this observation implies that
the OH ligand is not labile but readily undergoes proton
exchange with water in solution.
nucleus of the substrate prior to elimination of SiMe4.
This would lead to a dependence of the rate on sub-
strate. However, the reactions of 1 with ethyl acetate
and dry acetonitrile at 45 °C exhibit first-order rate
constants of [2.93(10)] × 10-5 and [4.71(17)] × 10-5 s-1
,
respectively.18 The similarity of these two rate con-
stants despite the marked difference in the electrophi-
licity of these two substrates implies a rate-determining
step that is independent of the substrate. The slight
variance in the observed rates can be ascribed to the
differences in bulk properties of the solvents.19 Con-
sistently, monitoring the reactions of 1 in wet acetoni-
trile affords observed rate constants from 4.60 × 10-5
to 4.67 × 10-5 s-1 for water concentrations ranging from
0.006 to 0.12 M. The similarity of all these results
strongly implies an E1-type mechanism involving a rate-
determining loss of alkane from 1.
Currently we are extending the kinetic analysis to
encompass isotope effects and activation parameters.
Synthetically, we are expanding the scope of these
coupling reactions to include other esters, nitriles, and
related organic amides, as well as other protic sources
with pKa values comparable to that of water.
Ack n ow led gm en t. We are grateful to the Natural
Sciences and Engineering Research Council of Canada
for support of this work in the form of grants to P.L.
and a postgraduate scholarship to S.A.L. We thank Dr.
P. J . Daff and Professor E. Carmona for their stimulat-
ing discussions of the kinetic and mechanistic analyses.
We acknowledge Dr. V. G. Young and the X-ray Crys-
tallographic Laboratory at the University of Minnesota
for the structure determination of compound 4.
Further mechanistic insight has been gained through
the kinetic analysis of these three transformations. The
mechanism we originally proposed for the CsH activa-
tion of alkanes by 1 is an E1-type dissociative mecha-
nism independent of substrate that invokes an initial
rate-limiting elimination of SiMe4 to generate A. How-
ever, an alternative bimolecular SN2-type associative
mechanism could also be operative, and the products
could be formed by the initial nucleophilic attack of the
vinyl â-carbon at the electrophilic quaternary carbon
Su p p or tin g In for m a tion Ava ila ble: Text and tables
giving experimental procedures and characterization data for
complexes 2-4 and full details of the crystal structure
analysis, including associated tables, for 4 (11 pages). Order-
ing information is given on any masthead page.
OM970118X
(18) The formation of 2-4 was monitored by UV-vis spectroscopy
and is cleanly first order in 1. Monitoring was effected for not less
than 3.5 half-lives, and the temperature was controlled to 44.5 ( 0.25
°C.
(19) Connors, K. A. Chemical Kinetics: The Study of Reaction Rates
in Solution; VCH: New York, 1990; Chapter 8.
(16) For other examples of imide protonation, see: (a) Lorente, P.;
Carfangna, C.; Etienne, M.; Donnadieu, B. Organometallics 1996, 15,
1090. (b) Reference 13.
(17) Strickler, J . R.; Wigley, D. E. Organometallics 1990, 9, 1665.