Oxidation of molybdenum(0) and tungsten(0) carbonyl complexes with silver triflate

Article abstract of DOI:10.1021/om970202c

Benzyltriethylammonium chloropentacarbonylmolybdate (1) and chloropentacarbonyl-tungstate (2) complexes react with CF₃SO₃Ag in DME via a combination of chloride exchange and a redox process, as revealed by cyclic voltammetry and ESR spectrometry. The resulting intermediate M(I) species disproportionate to M(0) and M(II) so that 3 equiv of TfOAg are required for the quantitative conversion into the M(II) complex. Analogous PPN complexes 3 and 4 only undergo the redox process (in DME); in this instance, the chloride exchange is precluded, presumably due to strong pairing of the complex anion with the counterion.

Full text of DOI:10.1021/om970202c

3690
Organometallics 1997, 16, 3690-3695
Oxid a tion of Molybd en u m (0) a n d Tu n gsten (0) Ca r bon yl
Com p lexes w ith Silver Tr ifla te
Andrew P. Abbott,*,† Andrei V. Malkov,† Nicole Zimmermann,†,‡
J . Barrie Raynor,† Ghafoor Ahmed,† J ohn Steele,§ and Pavel Kocˇovsky´*,†
Department of Chemistry, University of Leicester, Leicester LE1 7RH, U.K., and Discovery
Chemistry Department, Pfizer Central Research, Sandwich, Kent CT13 9NJ , U.K.
Received March 11, 1997X
Benzyltriethylammonium chloropentacarbonylmolybdate (1) and chloropentacarbonyl-
tungstate (2) complexes react with CF3SO3Ag in DME via a combination of chloride exchange
and a redox process, as revealed by cyclic voltammetry and ESR spectrometry. The resulting
intermediate M(I) species disproportionate to M(0) and M(II) so that 3 equiv of TfOAg are
required for the quantitative conversion into the M(II) complex. Analogous PPN complexes
3 and 4 only undergo the redox process (in DME); in this instance, the chloride exchange is
precluded, presumably due to strong pairing of the complex anion with the counterion.
In tr od u ction
implementing one or two weakly coordinating ligands
(in place of the CO group(s)) that could be more readily
replaced by the reactants.7-9 In combination with the
ability of Mo and W to form heptacoordinated com-
plexes,1,10 this ligand effect can be expected to result in
an enhanced or even novel catalytic activity.
Herein, we report on the attempted replacement of
the strongly coordinating chloride in the benzyltriethyl-
ammonium and bis(triphenylphosphine)ammonium
(PPN) complexes of molybdenum and tungsten of the
type [M(CO)6Cl]- (M ) W or Mo) by a weakly coordinat-
ing trifluoromethanesulfonate anion (TfO-) and on the
associated redox processes. Also reported is the effect
of the counterion and the solvent polarity on the
formation and properties of these new complexes.
The late transition metals owe their success in
catalytic chemistry to the readiness with which they
change their coordination number and exist as 18-,
16-, and 14-electron species.1 Analogous changes in
the coordination sphere of the group 6 metals are
usually more difficult to achieve. As a result, relatively
few catalytic processes employing the latter metals (in
lower oxidation states) have been developed to date.1,2
One successful example is the Mo(0)- and W(0)-catalyzed
allylic substitution.3,4 However, its variant employing
Pd(0) as the catalyst5,6 proceeds much more readily,
often at room temperature, in contrast to refluxing in
higher boiling solvents for several hours typically
required by Mo-carbonyl and W-carbonyl complexes.3,4
This striking difference can be attributed, in part, to
the ease of ligand dissociation in the case of palladium
catalysts,1 i.e., (Ph3P)4Pd a (Ph3P)3Pd + Ph3P, as
opposed to the relative stability of Mo(CO)6, W(CO)6,
and related complexes.
Resu lts a n d Discu ssion
In the search for new group 6 metal catalysts, we
endeavored to prepare Lewis-acidic, carbonyl com-
plexes of Mo and W with a weakly coordinating anion,
such as triflate (TfO-), in their coordination sphere.7-9
We envisaged a viable route to such complexes from the
halopentacarbonylmetalate(0) salts [M(CO)5X]- whose
halogen atom (X) could be replaced by the desired anion
on reaction with the corresponding silver(I) salt.7,8
The required chloropentacarbonylmolybdate and chlo-
ropentacarbonyltungstate complexes 1 and 2 were
prepared using a known procedure,11 via heating the
corresponding metal carbonyls with benzyltriethylam-
monium chloride in diglyme at 120 °C (Scheme 1). The
reaction can also be carried out in refluxing DME (at
80 °C),11 but we have now found that both the am-
monium salt and the solvent have to be rigorously dry.
We reasoned that the inherent coordination rigidity
of the group 6 metal complexes could be offset by
† University of Leicester.
‡ Exchange student from the Department of Chemistry, University
of Hamburg, D-20146 Hamburg, Germany.
§ Pfizer Central Research.
X Abstract published in Advance ACS Abstracts, J une 15, 1997.
(1) (a) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987. (b) Hegedus, L. S.
Transition Metals in the Synthesis of Complex Organic Molecules;
University Science Books: Mill Valley, CA, 1994.
(2) Grubbs, R. H.; Miller, S. J .; Fu, G. C. Acc. Chem. Res. 1995, 28,
446.
(3) Mo: (a) Trost, B. M.; Lautens, M. J . Am. Chem. Soc. 1982, 104,
5543. (b) Trost, B. M.; Lautens, M. J . Am. Chem. Soc. 1983, 105, 3343.
(c) Trost, B. M.; Lautens, M. Organometallics 1983, 2, 1687. (d) Trost,
B. M.; Lautens, M.; Peterson, B. Tetrahedron Lett. 1983, 24, 4525. (e)
Trost, B. M.; Lautens, M. J . Am. Chem. Soc. 1987, 109, 1469. (f) Trost,
B. M.; Lautens, M. Tetrahedron 1987, 43, 4817. (g) Trost, B. M.;
Merlic, C. A. J . Am. Chem. Soc. 1990, 112, 9590. W: Trost, B. M.;
Hung, M.-H. J . Am. Chem. Soc. 1983, 105, 7757.
(7) For weakly coordinating anions, see ref 8 and: (a) Marks, T. J .
Acc. Chem. Res. 1992, 25, 57. (b) Strauss, S. H. Chem. Rev. 1993, 93,
927.
(4) Dvorˇa´k, D.; Stary´, I.; Kocˇovsky´, P. J . Am. Chem. Soc. 1995, 117,
6130.
(5) For recent reviews, see ref 6b and: (a) Frost, C. G.; Howarth,
J .; Williams, J . M. J . Tetrahedron: Asymmetry 1992, 3, 1089. (b) Trost,
B. M. Pure Appl. Chem. 1994, 66, 2007. (c) Trost, B. M. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 259. (d) Frost, C. G.; Williams, J . M. J .
Contemp. Org. Synth. 1995, 2, 65.
(8) Van Seggen, D. M.; Hurlburt, P. K.; Anderson, O. P.; Strauss,
S. H. Inorg. Chem. 1995, 34, 3453 and references cited therein.
(9) Dvorˇa´kova´, H.; Dvorˇa´k, D.; Sˇrogl, J .; Kocˇovsky´, P. Tetrahedron
Lett. 1995, 35, 6351.
(10) Durrant, M. C.; Hughes, D. L.; Richards, R. L.; Baker, P. K.;
Harris, S. D. J . Chem. Soc., Dalton Trans. 1992, 3399.
(11) (a) Abel, E. W.; Butler, I. S.; Red, J . G. J . Chem. Soc. 1963,
2068. (b) Ganorkar, M. C.; Stiddard, M. H. B. J . Chem. Soc. 1965,
3494. (c) Sˇrogl, J .; Kocˇovsky´, P. Tetrahedron Lett. 1992, 33, 5991.
(6) (a) Stary´, I.; Kocˇovsky´, P. J . Am. Chem. Soc. 1989, 111, 4981.
(b) Stary´, I.; Zaj´ıcˇek, J .; Kocˇovsky´, P. Tetrahedron 1992, 48, 7229.
S0276-7333(97)00202-1 CCC: $14.00 © 1997 American Chemical Society

Oxidation of Mo(0) and W(0) Carbonyl Complexes
Organometallics, Vol. 16, No. 16, 1997 3691
Sch em e 1a
a
Bn ) PhCH2; PNP+ ) [Ph3PdNdPPh3]+; M ) Mo or W.
The analogous [Ph3PdNdPPh3]+ salts (PPN)12 3 and 4
were prepared in a similar manner. The structure of
these complexes was corroborated by comparison of
their data with those published by other authors for
analogous complexes. Thus, for instance, in accordance
with the literature,11 the Mo complex 1 showed four
carbonyl vibrations in the IR spectrum (at 1853 (s), 1926
(vs), 1982 (w), and 2061 (w) cm-1; in CH2Cl2). The
complexes 2-4, prepared in an analogous way, exhibited
similar characteristics.13
Rea ction of Com p lexes 1-4 with Silver (I). Treat-
ment of the halopentacarbonylmetalate halides
R4N+[M(CO)5X]- with silver(I) or thallium(I) carboxy-
lates has been reported to yield the expected carboxylate
complexes R4N+[M(CO)5(O2CR)]- and precipitated AgCl
(M ) Mo, W);12 silver tosylate is known to react in the
same way.12 In contrast to these reports, we have now
found that silver trifluoromethanesulfonate (TfOAg)
behaves differently: while the addition of 1 equiv still
gives mainly AgCl (in DME), the addition of an excess
of TfOAg results in the formation of a grey precipitate
of Ag(0) upon reaction with 1. Apparently, TfOAg is a
somewhat stronger oxidant than the corresponding
carboxylates.17
F igu r e 1. Elucidation of the reaction of complex 2 with
TfOAg in DME by cyclic voltammetry. (a) Voltammogram
of the parent complex 2. (b) Voltammogram of 2 after
addition of the first equivalent of TfOAg (;) and 80 s later
(- - -). (c) Voltammogram after addition of the second
equivalent of TfOAg. (d) Voltammogram after addition of
the third equivalent of TfOAg.
The cyclic voltammograms for the starting Mo and
W halopentacarbonyl complexes 1-4 gave similar re-
(12) (a) Cotton, F. A.; Darensbourg, D. J .; Kolthammer, B. W. S. J .
Am. Chem. Soc. 1981, 103, 398. (b) Doyle, G. J . Organomet. Chem.
1975, 84, 323. (c) Connor, J . A.; J ones, E. M.; McEwen, G. K. J .
Organomet. Chem. 1972, 43, 357.
(13) By contrast, the reaction of Mo(CO)6 with wet BnEt3N+Cl- took
a different course when carried out in DME (but not in diglyme!):
instead of the formation of the soluble complex 1, we have obtained a
sparingly soluble, presumably polymeric material, which exhibited
distinct carbonyl vibrations at 1723 (s), 1741 (s), 1782 (w), and 1887
(w) cm-1 in the IR spectrum (in KBr), the first three bands being
suggestive of the presence of bridged carbonyl group(s). The 1H NMR
spectrum (in DMSO-d6) showed a signal for water (singlet at 3.49 ppm
exchangeable with D2O), which was in ca. 1:1 ratio to the PhCH2 signal.
The presence of water is further evidenced by a broad band at 3260-
3500 cm-1 in the IR spectrum (in KBr or Nujol), in accord with the
behavior of related aqua complexes.14 Elemental analysis (C, H, N,
Mo) was compatible with the formula {(BnEt3N)4[Mo3(CO)9Cl4]‚3H2O}n.
The tungsten complex, synthesized in the same way (from wet
BnEt3N+Cl- in DME), exhibited analogous spectral characteristics.15,16
(14) (a) Kubas, G. J .; Burns, C. J .; Khalsa, G. R. K.; Van Der Sluys,
S. L.; Kiss, G.; Hoff, C. D. Organometallics 1992, 11, 3390. (b) Cotton,
F. A.; Ku¨hn, F. E. J . Am. Chem. Soc. 1996, 118, 5826.
(15) The striking difference between the reaction course in diglyme
and DME is difficult to understand. Presumably, diglyme, being a
tridentate ligand, can better coordinate water molecules, preventing
their reaction with the metal complex.
(16) For the effect of water contained in tetraalkylammonium salts
on the Heck-type reactions, see: (a) J effery, T. Tetrahedron Lett. 1994,
35, 3051. For other, recent examples of the effect of small amounts of
water on reativity, see: (b) Almeida, M. L. S.; Kocˇovsky´, P.; Ba¨ckvall,
J .-E. J . Org. Chem. 1996, 61, 6587. (c) Almeida, M. L. S.; Beller, M.;
Wang, G.-Z.; Ba¨ckvall, J .-E. Chem. Eur. J . 1996, 2, 1533. (d) Posner,
G. H.; Dai, H. Y.; Bull, P. S.; Lee, J . K.; Eydoux, F.; Ishihara, Y.; Welsh,
W.; Pryor, N.; Peter, S. J . Org. Chem. 1996, 61, 671. (e) Terada, M.;
Matsumoto, Y.; Nakamura, Y.; Mikami, K. J . Chem. Soc., Chem.
Commun. 1997, 281.
sults to those reported by Bond18 who used Bu4N+-
[M(CO)5X]-: all samples showed an irreversible, one-
electron transfer process at about +0.7 V.19 Elucidation
of the reaction between TfOAg and each of the com-
plexes 1-4 by cyclic voltammetry is described below.
The reactions have been found to be remarkably de-
pendent on the type of the quaternary ammonium
counterion and on the polarity of the solvent in which
the reaction was carried out.
Rea ction of [W(CO)5Cl]- w ith TfOAg in Dim eth -
oxyeth a n e. The starting benzyltriethylammonium
complex BnEt3N+[W(CO)5Cl]- (2) shows three oxidation
processes on a sweep from 0 to +2 V (Figure 1a), which
can be assumed to correspond to the oxidation of 2 to
the WI, WII, and WIII states, respectively. The first two
processes are irreversible and can be attributed to the
(17) Cyclic voltammetry of CF3CO2Ag and TfOAg showed a 0.10 V
difference in the redox potential of the Ag+ ion, which corresponds to
a difference of approximately 10 kJ ‚mol-1 in the Gibbs energy of the
two processes.
(18) Bond, G.; Bowden, J . A.; Colton, R. Inorg. Chem. 1974, 13, 605.
(19) The slight differences between the potentials reported in this
work and those in ref 18 result from the different reference electrodes
used.

3692 Organometallics, Vol. 16, No. 16, 1997
Abbott et al.
electrochemical reactions described by Bond;18 the third
process is quasi reversible.
Addition of 1 equiv of TfOAg to a solution of 2 in DME
resulted in an instantaneous formation of a predomi-
nantly white precipitate and changed the voltammo-
gram of the solution, as shown in Figure 1b. It should
be noted that no signal was observed for Ag+ ions,
showing that they were removed from the solution in
the reaction with 2. Moreover, the signal corresponding
to the oxidation of W0 to WI did not remain constant,
indicating that some of the complex had been oxidized.
This observation can be rationalized by assuming that
two reactions occur simultaneously (eqs 1 and 2).
[W0(CO)5Cl]- + TfOAg f [W0(CO)5OTf]- + AgCl V
(1)
[W0(CO)5Cl]- + TfOAg f WI(CO)5Cl + TfO- + Ag0 V
(2)
[W0(CO)5OTf]- + Ag+ f WI(CO)5OTf + Ag0 V (3)
Reaction 1 (apparently the dominant process) can also
be followed by the corresponding oxidation of the triflate
complex (eq 3),20 provided the latter process occurs with
a comparable or higher rate.
The current for WI f WII has been found to decrease
with time while that for W0 f WI gradually increased
(Figure 1b). This behavior is clearly indicative of a
disproportionation18 process that can be proposed to
occur as shown in eq 4. The kinetics of this dispropor-
F igu r e 2. ESR spectra obtained in frozen DME solutions
at 77 K for (a ) WI(CO)5(OTf) and (b) WI(CO)5Cl.
2[WI(CO)5(OTf)] f
excess. In consonance with this analysis, addition of a
third equivalent of TfOAg has been found to result in
the characteristic deposition and stripping peaks in the
voltammogram (Figure 1d). The latter features are
associated with the electrochemical reduction of AgI to
Ag0 (eq 5), showing that there is an excess of Ag+ in
[WII(CO)5(OTf)]+ + [W0(CO)5(OTf)]- (4)
tionation correspond to an expected second-order process
with the rate constant 1.87 mol-1 dm3 s-1 at 20 °C.
Hence, the disproportionation is relatively slow (τ/2 ≈ 3
min), which means that at least 5% of this species
should still be present after 30 min at room temperature
so that it could easily be investigated by ESR (vide
infra).
Ag+ + e- h Ag0
(5)
the solution and that the reaction has gone to comple-
tion.
The addition of 2 equiv of TfOAg led to the deposition
of a grey precipitate; suppression of the peaks corre-
sponding to W0 f WI and WI f WII transitions was
observed in the cyclic voltammogram (Figure 1c). These
results suggest that reaction 1 dominates when the first
equivalent is added, and reaction 3 occurs on addition
of the second equivalent. The second equivalent of
TfOAg would also replace Cl- in WI(CO)5Cl (formed in
the reaction 2) by TfO-. If we assume that each step
proceeds quantitatively and no other undetected side
reactions occur, 1 equiv of 2 would generate 0.5 equiv
of [WII(CO)5(OTf)]+ and 0.5 equiv of [W0(CO)5(OTf)]-,
consuming 2 equiv of TfOAg. For the oxidation of the
latter species, another equivalent of TfOAg would have
to be used. Hence, a total of 3 equiv of TfOAg will be
required for the quantitiative conversion of W0 to WII.
However, if the individual steps did not proceed quan-
titatively, 3 equiv would actually represent a slight
The ESR spectrum obtained at 77 K for the DME
solution of the complex generated from 2 and 1 equiv
of TfOAg showed a broad, weak feature with g| ) 1.801
and g ) 1.764 (species A). With 2 equiv of TfOAg, the
spectrum (Figure 2a) became much stronger and better
resolved but with the central feature shifted to g| )
1.963 and g ) 1.932 (species B). Weak satellite lines,
arising from the 183W isotope that has a nuclear spin of
1/2 and natural abundance of 14.28%, were observed.
These hyperfine couplings were A| ) 188 G and A )
125 G.21 A trace of the signal from species A was still
present. Species B can be assigned the structure
WI(CO)5OTf (eq 3), whereas species A may arise via a
minor reaction of the original complex with the first
equivalent of Ag+. The ESR spectrum obtained after
(21) Few other related systems have been investigated by ESR
spectroscopy. Electrochemical oxidation of [W(CO)2(CNR)2(PR3)2] has
yielded ESR spectra with similar parameters: g| ) 1.903 and g
)
(20) Both TfOAg and the precipitated AgCl may, a priori, serve as
oxidants. For an analogous oxidation of Cu(I) to Cu(II) by means of
AgCl, see: Smrcˇina, M.; Pola´kova´, J .; Vyskocˇil, Sˇ.; Kocˇovsky´, P. J . Org.
Chem. 1993, 58, 4534. However, AgCl should react more slowly in
view of its low solubility.
2.212. The Mo analogues had g| ) 1.97-1.99, g ) 2.00-2.02, and A|
) 50 G.22
(22) (a) Conner, K. A.; Walton, R. A. Inorg. Chem. 1986, 25, 4422.
See also: (b) Lang, R. F.; J u, T. D.; Kiss, G.; Hoff, H. D.; Bryan, J . C.;
Kubas, G. J . J . Am. Chem. Soc. 1994, 116, 7917.

Oxidation of Mo(0) and W(0) Carbonyl Complexes
Organometallics, Vol. 16, No. 16, 1997 3693
metric signal for Ag+ reduction was detected (Figure 3),
showing that the reaction had gone to completion.
These observations suggest that reaction 2 dominates
and reaction 1 is precluded.
Elucidating the latter reaction by ESR spectroscopy
further supported these conclusions: when 1 equiv of
TfOAg was added to 1 equiv of 4, the solution showed
an ESR spectrum having clear axial symmetry with g|
) 1.909 and g ) 1.930 (Figure 2b). Satellite lines
arising from the 183W isotope were insufficiently re-
solved to allow reliable measurement. The observation
of an ESR spectrum indicates immediate oxidation of
the tungsten without the initial substitution of the Cl-
by TfO-. This constrasts with the case of the BnEt3N+
salt 2 where adding 1 equiv of TfOAg resulted mainly
in substitution of Cl- by TfO- without oxidation. It is
pertinent to note that g| is sensitive to the axial ligand
field, and since Cl- has a weaker ligand field than TfO-,
the energy separation upon which g| depends, i.e.,
2
2
∆E(dx -y - dxy), will be smaller, thus making g| deviate
from that of WIC(O)5(OTf), as found.
It can be speculated that the difference in the reactiv-
ity of these two complexes toward TfOAg is due to the
association of ions in DME (a medium of relatively low
polarity23). It has been recently shown26 that small
anions can penetrate into the interstices of bulky
quaternary ammonium anions forming a “penetrating
ion pair”. The (PPN)+ ion has a cavity between the
phenyl groups into which the [W(CO)5Cl]- ion could fit.
The strong interaction between these two ions would
prevent the exchange of Cl- by TfO- ligands. Hence,
the role of TfOAg is only to oxidize the [W(CO)5Cl]-
anion, as in reaction 2. The latter oxidation strengthens
the W-Cl bond, thereby further preventing the ligand
exchange. By contrast, the BnEt3N+ cations, which does
not have such a cavity, allows the ligand exchange to
occur readily. Further oxidation of the latter W(I)
complex leads to the expected W(II) species, as evi-
denced by the IR spectrum, which is almost identical
to that of the triflate.
Rea ction of [Mo(CO)5Cl]- w ith TfOAg in Dim eth -
oxyeth a n e. The response of the Mo(0) complexes 1 and
3 was similar to that for their W(0) analogues 2 and 4,
respectively,27 which demonstrates that the behavior of
these species is independent of the metal center. One
slight exception was that the BnEt3N+[Mo(CO)5Cl]-
complex exhibited only two signals, one for the oxidation
Mo0 f MI (at +1.45 V) and one combined for MoI f MoII
f MoIII (at +1.40 V). These two signals are again
resolved following the addition of 1 equiv of TfOAg.
In analogy with the tungsten complex, the ESR
spectrum of the solution prepared by adding 1 equiv of
TfOAg to 1 exhibited a broad weak feature with g| )
1.911 and g ) 1.881 flanked by very weak satellite
lines. With 2 equiv of TfOAg, the spectrum (Figure 4)
was much stronger and better resolved with the same
central feature and satellite lines arising from 95,97Mo
F igu r e 3. Elucidation of the reaction of complex 4 with
TfOAg in DME by cyclic voltammetry: (;) voltammogram
of the parent complex 4; (- - -) voltammogram of 4 after
addition of the first equivalent of TfOAg; (•••) voltammo-
gram after addition of the second equivalent of TfOAg.
addition of 3 equiv of TfOAg showed a very broad, weak,
and featureless signal arising from a third species of
low abundance, with g ) 2.08. No satellite lines arising
from the 183W isotope could be resolved. Presumably,
this signal originates from an impurity, which implies
that most of the metal has been oxidized to W(II), which
should give no signal. Furthermore, the absence of the
species B after the addition of the first equivalent of
Ag+ rules out reaction 3 as a process competing with
reactions 1 and 2.
In the IR spectrum of the final species [i.e., (CO)5W-
(OTf)2] measured in CH2Cl2, peaks at 1975 (vs), 2010
(sh), and 2095 (w) cm-1 were identified, which is in
agreement with the pattern published for the analogous
isonitrile complex (t-BuNC)5Mo(OAc)2.23,24
The stoichiometry of the whole cascade would, theo-
retically, require exactly 3 equiv of Ag+. However, the
above experiments show that, in fact, slightly less than
3 equiv is sufficient to drive the process to completion,
which can be attributed to a nonquantitative conversion
in some of the transformation(s), presumably due to
undetected side reaction(s).
Replacement of BnEt3N+ by (PPN)+ as the counterion
had a significant effect on the reaction of the complex
with TfOAg. The cyclic voltammogram for a solution
containing 4 prior to the addition of TfOAg was similar
to that obtained for the BnEt3N+ complex 2, although
no signal was observed for the oxidation of WII to WIII
(Figure 3). The addition of 1 equiv of TfOAg led to the
formation of a grey precipitate and to a dramatic
decrease of the current corresponding to oxidation of W0
to WI (Figure 3); little or no disproportionation was
subsequently observed. After the addition of a second
equivalent of TfOAg to this solution, a large voltam-
(26) Abbott, A. P.; Schiffrin, D. J . J . Chem. Soc., Faraday Trans.
1989, 86, 1453. (b) Boche, G. Angew. Chem., Int. Ed. Engl. 1992, 31,
731.
(27) Oxidation of other molybdenum complexes by means of Ag(I)
has previously been reported to afford Mo(II) species.28 Thus, for
-
(23) The IR spectrum of (t-BuNC)Mo(OAc)2 exhibits the following
instance, treatment of [Mo(CO)2(bipy)2]+BF4 with AgBF4 in MeCN
features: 2115 (s), 2170 (sh) cm-1
.
gives [Mo(CO)2(bipy)2(MeCN)]2+(BF4-)2. The same product has also
been obtained from Mo(CO)2(bipy)2 on reaction with 2 equiv of AgBF4.28
(28) Connor, J . A.; J ames, E. J .; Overton, C.; El Murr, N. J . Chem.
Soc., Dalton Trans. 1984, 255.
24
(24) Girolami, G. S.; Andersen, R. A. Inorg. Chem. 1981, 20, 2040.
(25) Riddick, J . A.; Bunger, W. B.; Sakano, T. K. Organic Solvents,
4th ed.; J . Wiley and Sons: New York, 1986.

3694 Organometallics, Vol. 16, No. 16, 1997
Abbott et al.
state complexes 1-4 with TfOAg revealed that complex
redox processes occur, as well as halogen replacement.
The M(0) complex is first oxidized to M(I), which then
disproportionates to give the more stable M(II) species.
In the case of the tetraalkylammonium complexes 1 and
2, concomitant replacement of Cl- by TfO- has been
observed. By contrast, the latter reaction is precluded
for the PPN complexes 3 and 4, presumably due to
strong association to the counterion. However, this
association can be suppressed by using MeCN instead
of DME as the solvent, which enables the Cl- replace-
ment to occur. Some of the M(II) complexes exhibit
catalytic activity in allylic substitution9 and have the
promise of becoming a novel, wide-ranging class of
Lewis-acidic catalysts in organic chemistry.
F igu r e 4. ESR spectrum of MoI(CO)5(OTf) in DME.
5
isotopes (having a nuclear spin of /2 and total natural
abundance of 25.35%). The hyperfine couplings to
95,97Mo were A| ) 84 G and A ) 40 G. This spectrum
can be attributed to the MoI(CO)5OTf species. The ESR
spectrum obtained after addition of 3 equiv of TfOAg
showed a weak, poorly resolved signal arising from two
species, one with g| ) 2.063 and g ) 2.095 and the
other with g1 ) 1.954, g2 ) 1.931, and g3 ) 1.911. No
satellite lines arising from the Mo isotopes could be
resolved. Due to the low intensity of the signals, it was
not possible to characterize these species; the signals
are apparently associated with very minor byproducts.
The IR spectrum of the final species exhibited a similar
pattern to that of the tungsten analogue (vide su-
pra),23,24 namely peaks at 1985 (vs), 2005 (sh), and 2080
Exp er im en ta l Section
Meth od s. The reactions between the metal complexes and
TfOAg were monitored using cyclic voltammetry on a 1 mm
diameter Pt disc electrode. An EG & G model 173 potentiostat
and a model 175 wave form generator were used in these
studies. Potentials are given with respect to a saturated
calomel electrode (SCE). Tetrabutylammonium tetrafluorobo-
rate (Fluka, electrochemistry grade) [TBABF4] (0.1 mol dm-3
)
was used as an electrolyte in all electrochemical experiments.
Acetonitrile (BDH, HPLC grade) and 1,2-dimethoxyethane
(Lancaster, 99+%) were used as received. The ESR spectra
of frozen solutions of the complexes in DME were recorded at
77 K; g-values were calculated using Mn2+ in MgO as an
internal standard. The IR spectra were recorded in CH2Cl2.
The NMR spectra were recorded for CDCl3 solutions at 25 °C
on 250 or 300 MHz instruments. The coupling constants were
obtained by first-order analysis. The mass spectra were
measured using direct inlet and the lowest temperature
enabling evaporation or in a thermospray mode; chemical
ionization was used in certain cases (with NH4). GC analysis
was carried out using capillary columns (BP10 25 m × 2.65
µm). All reactions were carried out under nitrogen.
(w) cm-1
.
Rea ction of [W(CO)5Cl]- a n d [Mo(CO)5Cl]- w ith
TfOAg in Aceton itr ile. We reasoned that increasing
the polarity of the solvent would decrease the degree of
association between the complex anion and (PPN)+ (vide
supra). Therefore, acetonitrile, which has a higher
relative permittivity (ꢀ ) 35.9) than DME (ꢀ ) 7.2),25
was selected to test this hypothesis.
In MeCN, all of our complexes (1-4) gave the same
electrochemical response, irrespective of the metal
center or the quaternary ammonium counterion. In
each case, a white precipitate of AgCl was formed on
the addition of 1 equiv of TfOAg; a grey precipitate was
then produced on addition of the second equivalent. This
behavior shows that reaction 1 occurs in preference to
2. The lack of any disporportionation signal suggests
that the chemical reaction, which MI(CO)5OTf under-
goes to form the new complex, is much faster in MeCN
than in DME.
Ca ta lytic Activity of th e M(II) Com p lexes. In a
preliminary communication, we have recently demon-
strated that the M(II) species generated on TfOAg
oxidation of the tetraalkylammonium complexes are
capable of catalyzing allylic substitution.9 The full scope
of this reactivity as well as the catalysis in other
reactions, such as carbonyl-ene and Prins reaction and
Diels-Alder addition will be reported in due course.29
By contrast, the M(II) species generated from the PPN
complexes (in DME) proved catalytically inert, which
shows that the exchange of the Cl- ligand by TfO- is
essential for catalytic activity.
Ben zylt r iet h yla m m on iu m
Ch lor op en t a ca r b on yl-
m olybd a te (1). A mixture of molybdenum hexacarbonyl (1.32
g, 5 mmol) and benzyltriethylammonium chloride (1.14 g, 5
mmol) in dry diglyme (50 mL)30 was heated at 120 °C for 2 h.
After the reaction mixture was cooled, the solution was filtered,
and to the filtrate was added petroleum ether (50 mL). The
precipitate was isolated by filtration, washed with petroleum
ether, and dried in vacuum to give 1 (1.68 g, 72%) as a yellow
solid: IR (KBr) ν(CtO) 1821 (s), 1916 (vs), 2067 (w) cm-1; IR
(CH2Cl2) ν(CtO) 1853 (s), 1926 (vs), 1982 (w), 2061 (w) cm-1
;
MS (negative FAB) m/ z 273 [Mo(CO)5Cl-], 242 [Mo(CO)4Cl-],
217 [Mo(CO)3Cl-]. Anal. Calcd for C18H22ClMoNO5: C, 46.62;
H, 4.78; N, 3.02; Cl, 7.64. Found: C, 46.14; H, 5.08; N, 3.23;
Cl, 7.66.
Ben zyltr ieth yla m m on iu m Ch lor op en ta ca r bon yltu n g-
sta te (2). A mixture of tungsten hexacarbonyl (1.76 g, 5 mmol)
and benzyltriethylammonium chloride (1.14 g, 5 mmol) in dry
diglyme (50 mL) was heated at 120 °C for 2 h. After the
reaction mixture was cooled, the solution was filtered, and to
the filtrate was added petroleum ether (50 mL). The precipi-
tate was isolated by filtration, washed with petroleum ether,
and dried in vacuum to give 2 (2.06 g, 74%) as a yellow solid:
IR (KBr) ν(CtO) 1820 (s), 1906 (vs), 2066 (w) cm-1; IR (CH2-
Cl2) ν(CtO) 1848 (s), 1918 (vs), 1959 (sh), 2060 (w) cm-1. Anal.
Calcd for C18H22ClNO5W: C, 39.19; H, 4.02; N, 2.54; Cl, 6.43.
Found: C, 39.08; H, 4.10; N, 2.65; Cl, 6.40.
Con clu sion
Bis(tr ip h en ylp h osp h or a n ylid en e)a m m on iu m Ch lor o-
p en ta ca r bon ylm olybd a te (3). A mixture of molybdenum
Electrochemical elucidation of the reaction of chloro-
pentacarbonylmolybdate and chloropentacarbonyltung-
(30) In diglyme, the commercially available (i.e., wet) BnEt3N+Cl-
is suitable. However, if DME is used as the solvent, BnEt3N+Cl- has
to be rigorously dried (in vacuo over P2O5 at room temperature for 2
weeks).
(29) Malkov, A. V.; Ahmed, G.; Steele, J .; Kocˇovsky´, P. To be
published.

Oxidation of Mo(0) and W(0) Carbonyl Complexes
Organometallics, Vol. 16, No. 16, 1997 3695
hexacarbonyl (1.32 g, 5 mmol) and bis(triphenylphosphoran-
ylidene)ammonium chloride (2.87 g, 5 mmol) in dry DME (50
mL) was refluxed for 4 h. After the reaction mixture was
cooled, the solution was filtered, and to the filtrate was added
hexane (50 mL) to give rise to a brown-yellow oil. The solvent
was decanted, and a new portion of hexane was added. This
procedure was repeated several times until the oil began to
crystallize. The yellow solid was then isolated by filtration,
washed with hexane, and dried in vacuum to give 3 (2.83 g,
70%): IR (KBr) ν(CtO) 1850 (s), 1919 (vs), 1977 (w), 2061 (w)
cm-1; IR (CH2Cl2) ν(CtO) 1850 (s), 1925 (vs), 1975 (w), 2062
(w) cm-1, in accordance with the literature.12
black, and a precipate formed. After 30 min of stirring, the
mixture was filtered through a pad of Celite. The filtrate was
subjected to IR measurements: IR ν(CtO) 1975 (vs), 2010 (sh),
2095 (w) cm-1. ESR spectra are described in the Results and
Discussion. Attempted removal of the solvent and redissolving
(e.g., in CH2Cl2) led to a considerable decomposition and
reduction of the intensity of the IR signals.
Oxid a tion of P NP Ch lor op en ta ca r bon yltu n gsta te (4)
w ith Silver (I) Tr iflu or om eth a n esu lfon a te. The oxidation
of TfOAg (2 equiv only) was carried out in the same way as
that of 2. The product showed IR ν(CtO) 1975 (vs), 2010 (sh),
2100 (w) cm-1
.
Bis(tr ip h en ylp h osp h or a n ylid en e)a m m on iu m Ch lor o-
p en ta ca r bon yltu n gsta te (4). A mixture of tungsten hexa-
carbonyl (1.76 g, 5 mmol) and bis(triphenylphosphoranylidene)
ammonium chloride (2.87 g, 5 mmol) in dry DME (50 mL) was
refluxed for 4 h. After the reaction mixture was cooled, the
solution was filtered, and to the filtrate was added hexane (50
mL) to give rise to a brown-yellow oil. The solvent was
decanted, and a new portion of hexane was added. This
procedure was repeated several times until the oil began to
crystallize. The yellow solid was then isolated by filtration,
washed with hexane, and dried in vacuum to give 4 (3.19 g,
71%): IR (KBr) ν(CtO) 1855 (s), 1903 (vs), 2050 (w) cm-1; IR
Oxid a tion of Ben zyltr ieth yla m m on iu m Ch lor op en -
tacar bon ylm olybdate (1) with Silver (I) Tr iflu or om eth an e-
su lfon a te. This oxidation was carried out as described above
for the tungsten analogue. IR ν(CtO) 1985 (vs), 2005 (sh),
2080 (w) cm-1. ESR spectra are described in the Results and
Discussion. Attempted isolation of a solid material was
unsuccessful.
Oxid a tion of P NP Ch lor op en ta ca r bon ylm olybd a te (3)
w ith Silver (I) Tr iflu or om eth a n esu lfon a te. The oxidation
with TfOAg (2 equiv only) was carried out in the same way as
that of 1. The product showed IR ν(CtO) 1985 (vs), 2010 (sh)
cm-1
.
(CH2Cl2) ν(CtO) 1840 (s), 1909 (vs), 1968 (w), 2060 (w) cm-1
,
in accordance with the literature.12
Ack n ow led gm en t. We thank Dr. R. D. W. Kemmitt
for stimulating discussions. We are grateful EPSRC for
Grant No. GR/K07140 and EPSRC and Pfizer Ltd. for
a CASE award to G.A.
Oxid a tion of Ben zyltr ieth yla m m on iu m Ch lor op en -
tacar bon yltu n gstate (2) with Silver (I) Tr iflu or om eth an e-
su lfon a te. To a solution of benzyltriethylammonium chloro-
pentacarbonyltungstate (50 mg, 0.09 mmol) in DME or CH2Cl2
(1 mL)31 was added at room temperature silver(I) triflate (70
mg, 0.27 mmol) in one portion. The reaction mixture turned
Su p p or tin g In for m a tion Ava ila ble: Text giving the
experimental details for the reaction of Mo(CO)6 with wet
PhCH2(Et)3N+Cl- (1 page). Ordering information is given on
any current masthead page.
(31) The solvent (i.e., DME or CH2Cl2) does not seem to have an
effect on the result. Thus, the same ESR spectra were obtained,
regardless of whether the species was generated in DME or CH2Cl2.
For the measurement of the IR spectra, only the complex prepared in
CH2Cl2 was used.
OM970202C