Synthesis and reactivity of the imidotungsten methyl cation [W(N2Npy)(NPh)Me]+: CO2 adds to the W=NPh bond and does not insert into the W-Me bond

Article abstract of DOI:10.1039/b208327b

The imidotungsten dimethyl compound [W(N₂N_py)(NPh)Me₂] 2 reacts with BAr^F₃ to form the cationic complex [W(N₂N_py)(NPh)Me]⁺ 3⁺ [anion = [Me-BAr^F

Full text of DOI:10.1039/b208327b

Synthesis and reactivity of the imidotungsten methyl cation
[W(N2Npy)(NPh)Me]+: CO2 adds to the WNNPh bond and does not insert
into the WUMe bond†
Benjamin D. Ward,a Eric Clot,b Stuart R. Dubberley,a Lutz H. Gadec and Philip Mountford*a
a Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, UK OX1 3QR.
E-mail: philip.mountford@chemistry.oxford.ac.uk
b LSDSMS (UMR 5636), cc 14, Université de Montpellier 2, 34095 Montpellier cedex 5, France
c
Laboratoire de Chimie Organométallique et de Catalyse, UMR 7513, Institut Le Bel, Université Louis
Pasteur, 4, rue Blaise Pascal, 67000 Strasbourg, France
Received (in Cambridge, UK) 28th August 2002, Accepted 20th September 2002
First published as an Advance Article on the web 16th October 2002
The
imidotungsten
dimethyl
compound
As part of an ongoing program in early transition metal imido
chemistry4a using the diamide-donor ligand4b,c MeC(2-
C5H4N)(CH2NSiMe3)2 (abbreviated as N2Npy) we prepared the
dimethyl complex [W(N2Npy)(NPh)Me2] 2 according to eqn.
(1) in 35% yield from the dichloride 1.† The di-13CH3 and di-
[W(N2Npy)(NPh)Me2] 2 reacts with BArF to form the
3
cationic complex [W(N2Npy)(NPh)Me]+ 3+ [anion = [Me-
BArF3]2;
ArF
=
C6F5;
N2Npy
=
MeC(2-
C5H4N)(CH2NSiMe3)2] which undergoes methyl group ex-
change with added 2, [Cp2ZrMe2] or ZnMe2; treatment of
cation 3+ with CO2 or isocyanates leads to cycloaddition
reactions at the WNNPh bond and not insertion into the
WUMe bond, despite the latter product being the most
thermodynamically favourable according to DFT calcula-
tions.
(1)
Broadly speaking, the molecular reactions of transition metal
imido complexes [M(NR)n(L)m] (R = alkyl or aryl; (L)m
=
CD3 isotopomers of 2 were similarly prepared and have been
used in parallel with 2 throughout this work to support the
proposed structures and reactions. The X-ray structure of 2 is
shown in Fig. 1 and establishes the connectivity and other
geometrical features.† Reaction of 2 with BArF3 (ArF = C6F5)
supporting ligand or ligand set) fall into two categories: (i) those
in which the MNNR linkage is the reactive site undergoing
transformations such as activation and coupling of saturated or
unsaturated organic substrates, and (ii) those where the imido
group(s) act as ‘ancillary’ or ‘spectator’ ligands with the
transformation(s) being studied occuring at some other part of
the molecule.1 One of the best known examples of this latter
category from Group 6 are the olefin metathesis catalysts
in
CD2Cl2
formed
the
five-coordinate
cation
[W(N2Npy)(NPh)Me]+ 3+ and the anion [MeBArF ]2 which,
3
according to the 1H and 19F NMR spectra, is non-coordinating
in this solvent.5 Cation 3+ decomposes only fairly slowly (half-
life ca. 2 h) at room temperature in CH2Cl2 or CD2Cl2 thereby
allowing ample opportunity to explore its reactivity. Although
too reactive to be obtained in the solid state, 3+ can be stabilised
and isolated as the acetonitrile adduct [W(N2Npy)(NPh)-
i
[M(NAr)(CHR)(ORA)2] (M = Mo or W, Ar = 2,6-C6H3 Pr2, RA
= bulky alkyl or aryl group) which undergo cycloaddition
reactions at the MNCHR alkylidene group.1d,e Another example
from Group 6 is provided by Gibson’s bis(imido)chromium(VI
)
t
compounds [Cr(NRA)2R2] (RA = Bu or aryl, R = Me or CH2Ph)
which provide very active and long-lived ethylene polymer-
isation catalysts.2 At first sight these appear to be relatively
simple cases where an active cationic species of the form
[Cr(NRA)2R]+ (R = growing polymer chain) would be antici-
pated, with propagation straightforwardly involving the incom-
ing ethylene weakly coordinating to the d0 metal and under-
going migratory insertion into the CrUR bond. However, very
recent calculations by Jensen and Børve3 have cast doubt on this
picture. For chemically realistic model systems of [Cr(NRA)2R]+
it appears that ethylene cycloaddition to the CrNNRA bond can
compete with insertion into the CrUR bond, in turn leading (after
further modification) to a reduced Cr(IV) species which may in
fact be the resting state for this catalyst system. In the light of
these results, and their serious implications for the design and
mechanistic interpretation of imido-base reagents and catalysts,
it is clearly important to harvest more information concerning
the relative reactivity aptitudes of competing metalUalkyl and
metalUimide functionalities towards unsaturated substrates.
Here we outline the synthesis and reactions of a cationic
imidotungsten(VI) methyl complex with heterocumulenes and
show how insertion of these substrates into a tungstenUalkyl
bond is kinetically disfavoured compared to cycloaddition at the
tungstenUimide bond.
(MeCN)Me]+ 4+ (anion = [MeBArF ]2). To the best of our
3
knowledge these cationic imido alkyl complexes are only the
second such examples from Group 6 to be reported, the other
† Electronic supplementary information (ESI) available: crystal data for 2,
DFT/computational notes, supporting characterising data and selected
experimental details for the new compounds. See http://www.rsc.org/
suppdata/cc/b2/b208327b/
Fig. 1 Displacement ellipsoid plot (20%) of [W(N2Npy)(NPh)Me2] 2 with H
atoms omitted for clarity. Selected distances (Å) and angles (°): W(1)UN(1)
1.745(2), W(1)UN(2) 1.997(2), W(1)UN(3) 2.042(2), W(1)UN(4) 2.336(2),
W(1)UC(7) 2.250(2), W(1)UC(8) 2.228(2), W(1)UN(1)UC(1) 164.8(2).
2618
CHEM. COMMUN., 2002, 2618–2619
This journal is © The Royal Society of Chemistry 2002

being the chromium bis(imido) species [Cr(NtBu)2(CH2Ph)]+
alluded to above and characterised in situ.2 Apart from its
reactivity with ethylene, no other reactions of this species were
reported (other than the formation of Lewis base adducts).
room temperature to form the N,O-bound carbamate product of
imide cycloaddition [W(N2Npy){N(Ph)C(O)O}Me]+ 6+ and not
the alternative insertion product [W(N2Npy)(NPh)(O2Me)]+ 7+
(Scheme 1) or one of its isomers. Support for formulating the
CO2 reaction product as 6+ and not 7+ comes from 13C labelling
studies (of both the methyl group in 3+ and in the CO2): no
Treatment of 2 with half of an equivalent of BArF gives
3
solutions containing the binuclear methyl-bridged cation
[W2(N2Npy)2(NPh)2Me2(m-Me)]+ 5+ which can also be formed
by treating methyl cation 3+ with one equivalent of dimethyl 2.
13
13CH3U CO2 coupling is observed in doubly-labelled 6+, and
the magnitude of the 183WUMe coupling constant (1JWUMe = 92
Hz) is comparable to those in 2, 3+ and 4+. Reaction of 3+ with
phenyl isocyanate gives an analogous N,NA-bound ureate, imide
cycloaddition product [W(N2Npy){N(Ph)C(O)N(Ph)}Me]+ 8+
and, again, not insertion into the WUMe bond. Hydrolysis of 8+
afforded the expected derivative diphenylurea as one of the
products thus confirming the ureate ligand’s presence in 8+.
The reactions of [W(N2Npy)(NPh)Me]+ 3+ described above,
which give exclusively products of cycloaddition at the WNNPh
bond, appear to be governed by kinetic factors. Thus, DFT
Labelling
studies
(treatment
of
either
[W(N2Npy)(NPh)(13CH3)2] or [W(N2Npy)(NPh)(CD3)2] with
one equivalent of [W(N2Npy)(NPh)Me][MeBArF ] 3) show that
3
the methyl groups in 5+ all scramble (but not with the methyl of
the anion [MeBArF ]2). Treatment of 5+ with an equivalent of
3
BArF3 gave complete conversion to [W(N2Npy)(NPh)Me]+. The
formation of a bridging methyl cation like 5+ is precedented for
main group, actinide and early transition metals.6 Interestingly,
[W(N2Npy)(NPh)Me]+ also undergoes intermolecular methyl
group exchange with ZnMe2 on the NMR timescale (confirmed
by a spin saturation transfer NMR experiment), and also slowly
with [Cp2ZrMe2] (as established by deuterium labelling experi-
ments with [W(N2Npy)(NPh)CD3]+ or [Cp2Zr(CD3)2]) pre-
sumably through methyl bridged intermediates analogous to 5+.
In these regards, therefore, the imidotungsten methyl species 3+
has ‘normal’ alkyl cation behaviour and hence was a good
candidate to compare its reactivity towards substrates that might
react at either the WUMe or the WNNPh bond.
(B3PW91)
calculations†
of
model
systems
for
[W(N2Npy){N(Ph)C(O)O}Me]+ 6+ and [W(N2Npy)(N-
Ph)(O2Me)]+ 7+ have shown that the WUC insertion product 7+
is in fact actually 50.6 kcal mol21 more stable than the observed
WNN cycloaddition product 6+. Detailed experimental and
computational studies of the reactions and products of
[W(N2Npy)(NPh)Me]+ 3+ and related compounds with a
number of unsaturated organic substrates are currently under-
way to identify the underlying mechanistic and other factors
governing the reactions of imido alkyl complexes. The question
of at which point ancillary imido ligands become reactive
ligands in chemical transformations will have clear implications
for the use of imido-based species in catalytic and stoichio-
metric chemistry.
Unfortunately [W(N2Npy)(NPh)Me]+ 3+ does not react at all
with ethylene so we could not make comparisons with the
computational predictions concerning the [Cr(NRA)2R]+ sys-
tems. However, insertion reactions of tungsten alkyl com-
pounds with CO2 (to form O-bound acetate derivatives) have
been carefully studied in detail;7 moreover, we ourselves8a,b
have recently been exploring the still quite rare CO2 activation
reactions of transition metal imides,8 which have not before
been identified in Group 6 imido chemistry at all. Reaction of 3+
with CO2 (Scheme 1) proceeds rapidly and quantitatively at
We thank the EPSRC and Royal Society (PM), British
Council (PM and LHG), the CNRS (LHG and EC), the Institut
Universitaire de France (LHG) and the Université de Mon-
tpellier 2 (EC) for support and Professor O. Eisenstein for
helpful discussions.
Notes and references
1 For recent reviews and leading references see: (a) W. A. Nugent and J. M.
Mayer, Metal-Ligand Multiple Bonds, Wiley-Interscience, New York,
1988; (b) D. E. Wigley, Prog. Inorg. Chem., 1994, 42, 239; (c) P. R.
Sharp, Dalton Trans., 2000, 2647; (d) R. R. Schrock, Acc. Chem. Res.,
1990, 23, 158; (e) V. C. Gibson, Adv. Materials, 1994, 6, 37; (f) Special
Theme Issue BOrganometallic Chemistry with N and O p-donor ligandsB,
J. Organomet. Chem., Guest Editor P. Mountford, 1999, 591, pp.
2U213.
2 M. P. Coles, C. I. Dalby, V. C. Gibson, I. R. Little, E. L. Marshall, M. H.
Ribeiro da Costa and S. Mastroianni, J. Organomet. Chem., 1999, 591,
78.
3 V. R. Jensen and K. J. Børve, Chem. Commun., 2002, 543; V. R. Jensen
and K. J. Børve, Organometallics, 2001, 20, 616.
4 (a) L. H. Gade and P. Mountford, Coord. Chem. Rev., 2001, 216-217, 65;
(b) S. Friedrich, M. Schubart, L. H. Gade, I. J. Scowen, A. J. Edwards and
M. McPartlin, Chem. Ber.-Recueil, 1997, 130, 1751; (c) L. H. Gade,
Chem. Commun., 2000, 173.
5 A. D. Horton, Organometallics, 1996, 15, 2675; B. E. Bosch, G. Erker,
R. Fröhlich and O. Meyer, Organometallics, 1997, 16, 5449.
6 Binuclear cations with bridging methyl groups: S. Zhang and W. E. Piers,
Organometallics, 2001, 20, 2088; P. G. Hayes, W. E. Piers and R.
McDonald, J. Am. Chem. Soc., 2002, 124, 2132; M. Bochmann and S. J.
Lancaster, Angew. Chem., Int. Ed. Engl., 1994, 33, 1634; X. Yang, C. L.
Stern and T. J. Marks, Organometallics, 1991, 10, 840; M. P. Coles and
R. F. Jordan, J. Am. Chem. Soc., 1997, 119, 8125.
7 M. M. Holl, G. L. Hillhouse, K. Folting and J. C. Huffman,
Organometallics, 1987, 6, 1522; D. J. Darensbourg, R. Kudaroski
Hanckel, C. G. Bauch, M. Pala, D. Simmons and J. N. White, J. Am.
Chem. Soc., 1985, 107, 7463; P. Kundel and H. Berke, J. Organomet.
Chem., 1988, 339, 297.
8 (a) A. E. Guiducci, A. R. Cowley, M. E. G. Skinner and P. Mountford, J.
Chem. Soc., Dalton Trans., 2001, 1392; (b) A. J. Blake, J. M. McInnes,
P. Mountford, G. I. Nikonov, D. Swallow and D. J. Watkin, J. Chem.
Soc., Dalton Trans., 1999, 379; (c) R. E. Blake, D. M. Antonelli, L. M.
Henling, W. P. Schaefer, K. I. Hardcastle and J. E. Bercaw, Organome-
tallics, 1998, 17, 718; (d) D. S. Glueck, J. Wu, F. J. Hollander and R. G.
Bergman, J. Am. Chem. Soc., 1991, 113, 2041.
Scheme 1 Synthesis and reactions of [W(N2Npy)(NPh)Me]+ 3+ (anion =
[MeBArF3]2).
CHEM. COMMUN., 2002, 2618–2619
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