Cyclodimerization of the tropylium ring by reduction of [(η7-C7H7)Mo(CO)3]+ to give [{Mo(CO)3}2(μ-η5:η 5-C7H7-C7H7</su

Article abstract of DOI:10.1021/om9606909

Reductive activation of the tropylium ring in [(η⁷-C₇H₇)M(CO)₃]BF₄ (M = Cr, Mo, W) leads to cyclodimerization and formation of [{M(CO)₃}₂(μ-η⁵:η⁵-C ₇

Full text of DOI:10.1021/om9606909

Organometallics 1997, 16, 139-141
139
Notes
Cyclod im er iza tion of th e Tr op yliu m Rin g by Red u ction
7
+
of [(η -C H )Mo(CO) ] To Give
7
7
3
5
5
[
{Mo(CO) } (µ-η :η -C H -C H )][P P N]
3
2
7
7
7
7
2
†
David A. Brown,* J ohn C. Burns, Cordula Mock-Knoblauch, and
William K. Glass
Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland
Received August 13, 1996X
Summary: Reductive activation of the tropylium ring in
VI, M ) W) possibly formed by radical coupling.14
7
[
(η -C7H7)M(CO)3]BF4 (M ) Cr, Mo, W) leads to cy-
Electrochemical studies of I were originally interpreted
5
5
15
clodimerization and formation of [{M(CO)3}2(µ-η :η -
C7H7-C7H7)][PPN]2, which was isolated and character-
ized for M ) Mo and may also be formed by reduction
in terms of a two-electron reduction process, although
very recently a one-electron-transfer mechanism has
16
been suggested. Reduction by sodium amalgam of II
6
6
of the neutral dimer [{Mo(CO)3}2(η :η -C7H7-C7H7)]. The
chromium and tungsten analogues were identified spec-
troscopically.
was reported to give a green solution with infrared
carbonyl bands, indicating formation of an anionic
17
materia, but only IV was isolated. In view of the above
conflicting interpretations of the electrochemical reduc-
tion of I and the suggestion of the formation of an
anionic species from II, we decided to reinvestigate the
reduction of I-III using potassium naphthalenide as
reducing agent.
Electrochemical reduction of organometallic com-
pounds and its synthetic applications constitute an
important area of research.1
,2
More recently, the en-
hanced reactivity of 19e species to undergo rapid
electron transfer catalyzed carbonyl substitution has
been reported.3 In the case of complexes of π-acids such
as cyclopentadienyl or arenes, reduction may result in
Resu lts a n d Discu ssion
I
dimerization and C-C bond formation (e.g. [CpFe
+
(
arene)] ). However, permethylation of the Cp ring,
In the present study, dropwise addition (2:l molar
7
which decreases the spin density on the arene ligand,
increases the rate of dimerization so that spin density
is not the dominant factor in this case; instead, the 19e
ratio) of potassium naphthalenide to a slurry of [(η -
C7H7)Mo(CO)3]BF4 (II) in THF at -78 °C gave a dark
green solution with infrared carbonyl absorptions at
I
-1,
species [CpFe (arene)] is considered to be in equilibrium
1983, 1917, 1892, 1798, and 1744 cm
corresponding
II
5
with the 18e species [CpFe (η -arene)] with the electron
to a mixture of the neutral dimer V and the dianion
3
4,5
5
5
2-
localized on an arene C sp orbital.
Although there
[{Mo(CO)3}2(µ-η :η -C7H7C7H7)] (VIIIa). After 2 h, the
solution turned yellow and the infrared spectrum showed
only the carbonyl absorptions at 1893, 1798, and 1744
have been extensive studies of the electrochemical
reduction of the [(arene)Cr(CO)3] series,6
-13
relatively
-
1
few have been made of the closely related tropylium
cm due to VIIIa. However, direct workup gave only
the neutral dimer V, but if [PPN]Br was added after
the solution turned yellow, subsequent workup gave the
7
tricarbonyl group VI metal complexes [(η -C7H7)M(CO)3]-
BF4 (I, M ) Cr; II, M ) Mo; III, M ) W). Reduction of
I-III with a variety of reagents, including zinc dust,
sodamide, and phenyllithium, gives the neutral dimers
5
5
yellow complex VIII, [{Mo(CO)3}2(µ-η :η -C7H7-C7H7)]-
[PPN]2, which is stable in air for several minutes and
for 3-4 days under N2 at -20 °C. Satisfactory mi-
croanalysis was obtained for VIII. Exposure to air
6
6
[
{M(CO)3}2η : η -C7H7-C7H7)] (IV, M ) Cr; V, M ) Mo;
†
1
Erasmus Student, University of Wurzburg, Wurzburg, Germany.
Abstract published in Advance ACS Abstracts, December 15, 1996.
results in rapid oxidation of VIII to V. The H NMR
X
(
270 MHz) spectrum of VIII measured at -60 °C in
(
(
(
1) Morris, M. D. Electroanal. Chem. 1974, 7, 79.
2) Dessy, R. E.; Bares, L. A. Acc. Chem. Res. 1972, 5, 415.
3) Huang, Y.; Neto, C. C.; Pevear, K. A.; Banaszak Holl, M. M.;
1
acetone-d6 shows a triplet at δ 5.00 assigned to H4 (H4 )
for numbering see Scheme 1) with J 34 ) 6.23 Hz, a
(
Sweigart, D. A.; Chung, Y. K. Inorg. Chim. Acta 1994, 226, 53.
4) Ruiz, J .; Lacoste, M.; Astruc, D. J . Am. Chem. Soc. 1990, 112,
471.
double doublet at δ 4.83 (H3,5) with coupling to H4 and
the equivalent H2 and H6 (J 23 ) 10.2 Hz), and a doublet
at δ 3.46 (H2, J 23 ) 10.2 Hz). The broad singlet at δ
(
5
(
(
5) Astruc, D. Acc. Chem. Res. 1991, 24, 36.
6) Rieke, R. D.; Arney, J . S.; Rich, W. E.; Willeford, B. R.; Poliner,
1
.89 is assigned to H1 and H7, which do not couple
B. S. J . Am. Chem. Soc. 1975, 97, 5951.
(
(
7) Milligan, S. N.; Rieke, R. D. Organometallics 1983, 2, 171.
8) Dessy, R. E.; Stary, F. E.; King, R. B.; Waldrop, M. J . Am. Chem.
because of the estimated dihedral angle of ca. 90°
Soc. 1966, 88, 471.
9) Khandkarova, V. S.; Gubin, S. P. J . Organomet. Chem. 1970,
2, 149.
(
(14) Munro, J . D.; Pauson, P. L. J . Chem. Soc. 1961, 3484.
(15) Romanin, A. M.; Venzo, A.; Ceccon, A. J . Electroanal. Chem.
Interfacial Electrochem. 1980, 112, 147.
(16) Behrens, U.; Brossard, H.; Hagenau, U.; Heck, J .; Hendrickx,
E.; K o¨ rnich, J .; Van der Linden, J . G. M.; Persoons, A.; Spek, A. L.;
Veldman, N.; Voss, B.; Wong, H. Angew. Chem. 1996, 35, 98.
(17) Adams, H.; Bailey, N. A.; Willet, D. G.; Winter, M. J . J .
Organomet. Chem. 1987, 333, 61.
2
(
(
10) Gubin, S. P. Pure Appl. Chem. 1970, 23, 463.
11) Ceccon, A.; Gambaro, A.; Romanin, A. M. J . Organomet. Chem.
1
983, 254, 207.
12) Ceccon, A.; Corjaba, C.; Giacometti, G.; Venzo, A. J . Chem. Soc.,
Perkin Trans. 2 1978, 2, 283.
13) Henry, W. P.; Rieke, R. D. J . Am. Chem. Soc. 1983, 105, 6314.
(
(
S0276-7333(96)00690-5 CCC: $14.00 © 1997 American Chemical Society

1
40 Organometallics, Vol. 16, No. 1, 1997
Notes
Sch em e 1a
in THF at -78 °C, which again, after 30 min, gave a
yellow solution that on addition of [PPN]Br and workup
produced a pure sample of VIII.
If the above reduction of II is carried out at -78 °C
in the presence of the radical inhibitor CBr4 and the
reaction monitored by means of a low-temperature
infrared cell, three strong bands are observed at 1991,
-
1
1
925, and 1896 cm and attributed to the bromo ring
6
addition product [(η -C7H7-exo-Br)Mo(CO)3] (XI) by anal-
ogy with the infrared carbonyl bands of the correspond-
1
8
ing phosphine adducts.
The two weaker bands ob-
-
1
served at 2011 and 1967 cm are due to the normal
bromide [(η -C7H7)Mo(CO)2Br] (XIV). The intensities
7
of the ring adduct XI decreased on standing, while those
of XIV increased. Similar results were obtained for the
W complex III; however, in the case of the Cr complex
6
I, the peaks due to the ring adduct [(η -C7H7-exo-Br)-
Cr(CO)3] (X) which occurred at 1985, 1921, and 1896
-1
cm predominated and persisted up to -20 °C, at which
point they were replaced by those of the starter cation
-
1
Ic and two peaks at 2036 and 1990 cm . By analogy
with peaks for the corresponding Mo and W bromides
(
XIV and XV), these are assigned to the hitherto
7
unreported Cr bromide [(η -C7H7)Cr(CO)2Br] (XIII),
although subsequent workup gave only [(η -C7H7)Cr-
7
(CO)3]Br.
Probable mechanisms for these reactions are shown
in Scheme 1. Since molecular orbital calculations of the
complexes I-III show the LUMO to be ring-centered in
1
9
all three cases, it is suggested that initial one-electron
reduction occurs with potassium naphthalenide to give
the radicals Ir-IIIr, with a SOMO which is ring-
centered. In the absence of CBr4, these radicals dimer-
ize to give the corresponding neutral dimers IV-VI,
which undergo two-electron reduction to form the
diradical intermediates IVd-VId (M ) Cr, Mo, W), these
in turn undergo cyclodimerization to form the dianions
5
5
2-
[
{M(CO)3}2(µ-η :η -C7H7-C7H7)] (VIIa-IXa; M ) Cr,
Mo, W). Attempts to isolate the PPN derivatives of VIIa
and IXa were unsuccessful, although the infrared car-
bonyl bands were consistent with their formation;
however, workup in the Cr case gave mainly the neutral
dimer IV and in the W case mainly W(CO)6. Only in
the Mo case could we isolate and characterize the PPN
salt of VIIIa.
In the presence of CBr4, IIr combines with a bromine
6
atom to give the unstable ring adduct [(η -C7H7-exo-Br)-
Mo(CO)3] (XI), which decomposes heterolytically on
raising the temperature to re-form the starting com-
pound II and bromide ion, which then form the normal
a
7
Legend: (1) potassium naphthalenide/THF, -78 °C, one-
carbonyl substitution product [(η -C7H7)Mo(CO)2Br]
electron reduction; (2) dimerization; (3) CBr
reduction; (5) cyclodimerization.
4
; (4) two-electron
(
XIV).
Previous studies of the photochemical dimerization
of cycloheptatriene have yielded only the stable non-
between the two carbon-hydrogen bonds in the dimer.
In the case of a monomeric product such as [(η -C7H7)-
Mo(CO)3] , coupling between H1 and H7 would be
2
0
planar bicyclo[3.2.0]heptadiene.
The cyclodimeriza-
5
tion reported here is analogous to that observed recently
-
6
+ 21
for benzene in [(η -C6H6)Mn(CO)3] .
Protonation of
expected. The upfield shifts of the proton spectra of VIII
relative to those of V are also consistent with formation
of an anionic species. The phenyl protons of PPN occur
as a multiplet at δ 7.5-7.8 ppm. Unfortunately, we
were not able to obtain crystals of VIII suitable for X-ray
crystallography.
Further support for the dimeric nature of VIII is
provided by its formation from the neutral dimer V on
treatment of the latter with potassium naphthalenide
VIII in THF at -78 °C with glacial acetic acid gave an
(
18) Brown, D. A.; Burns, J . C.; Glass, W. K.; Cunningham, D.;
Higgins, T.; McArdle, P.; Salama, M. M. Organometallics 1994, 13,
662.
19) Brown, D. A.; Burns, J . C.; Conlon, P. C.; Deignan, J . P.;
Fitzpatrick, N. J .; Glass, W. K.; O’Byrne, P. J . Organometallics 1996,
5, 3147.
2
(
1
(
(
20) Van Tamelan, Pappas, S. P. J . Am. Chem. Soc. 1967, 89, 2979.
21) Thompson, R. L.; Gelb, S. J .; Cooper, N. J . J . Am. Chem. Soc.
1991, 113, 8961.

Notes
Organometallics, Vol. 16, No. 1, 1997 141
immediate color change from yellow to reddish orange
and infrared carbonyl peaks corresponding to those of
the neutral dimer V; however, workup gave only [(η -
[PPN]Br (0.8 g, 1.28 mmol) was added as a slurry in THF
precooled to -78 °C. This mixture was stirred for 90 min and
filtered cold and the volume reduced to 15 mL. On storage
6
5
5
overnight at -20 °C, yellow crystals of [{Mo(CO)
-C )][PPN] (IV) formed and were dried under vacuum.
Anal. Calcd for C46 Mo: C, 68.2; H, 4.60; N, 1.73.
3 2
} (µ-η :η -
C7H8)Mo(CO)3], arising from C-C bond cleavage of V.
Formation of this monomeric species on protonation of
the dimer VIII suggests that caution is necessary in
inferring that formation of a monomeric species such
C
7
H
7
7
H
7
2
H
3 2
37NO P
Found for VIII: C, 68.7; H, 4.93; N, 1.43.
7
5
Red u ction of [(η -C
7
H
7
)M(CO)
3
]BF
4
(M ) Cr , Mo, W)
as [(η -C6H7)Mn(CO)3] necessarily occurs by protonation
4
- 22
4
u sin g P ota ssiu m Na p h th a len id e in th e P r esen ce of CBr .
of another monomeric species, [(η -C6H6)Mn(CO)3] .
7
To a slurry of [(η -C
7 7 3 4
H )Mo(CO) ]BF (0.23 g, 0.64 mmol) and
CBr (0.21 g, 0.64 mmol) in THF (50 mL) at -78 °C was added
4
Exp er im en ta l Section
an equimolar amount of 0.2 M potassium naphthalenide (3.2
mL, 0.64 mmol). Samples were removed by syringe every 5
min for IR analysis using the low-temperature cell. After it
was stirred for 1 h or until no further change was observed in
the IR spectrum, the solution was filtered and concentrated
to 10 mL under vacuum. Recrystallization from acetone/ether
All solvents were freshly dried by standard methods. All
reactions and workup were carried out under a nitrogen
atmosphere, using standard Schlenk techniques. Infrared
spectra were measured using a 0.1 mm CaF cell on a Perkin-
2
Elmer 1720 Fourier Transform Spectrometer linked to a 3700
data station. Low-temperature spectra were obtained with a
Specac variable-temperature cell P/N 21,500. H NMR spectra
were recorded on a J EOL GX 270 spectrometer. Analyses were
performed by the Microanalytical Laboratory of the Chemical
7
yielded the dark green monocarbonyl substitution product [(η -
1
7
C
7
H
7
)Mo(CO)
2
Br] (62 mg, 32%). The reduction of [(η -C
7
H
7
)W-
(
CO)
3
]BF in the presence of CBr
4
4
using the identical procedure
7 2
H )W(CO) Br] (71 mg, 28%), while reduction of
7
yielded [(η -C
7
Services Unit of University College Dublin. The complexes
7
[(η -C
7
H
7
)Cr(CO)
3
]BF
7
4
in the presence of CBr
4
yielded only the
7
[
(η -C
7
H
7
)M(CO)
3
]BF
4
(Cr, Mo, W) were synthesized by previ-
starting cation [(η -C
7
H
7
)Cr(CO)
3
]Br.
2
3
ously well-established methods. Potassium naphthalenide
5
5
Rea ction of [{Mo(CO)
3
}
2
µ-η :η -C
7
H
7
-C
7
H
7
)][P P N]
2
w ith
)]-
was standardized by titration with standard 0.1 M HCl.
5
5
7
Gla cia l Acetic a cid . To [{Mo(CO)
3
}
2
(µ-η :η -C
7
H
7
-C
7
H
7
Syn th esis of VIII. To a slurry of [(η - C
7
H
7
)Mo(CO)
3
]BF
4
[
PPN]
2
(2.8 mmol) in THF at -78 °C was added glacial acetic
(
0.23 g, 0.64 mmol) in THF (50 mL) precooled to -78 °C was
added K[C10 ] (6.5 mL, 1.3 mmol) under argon. After 2 h
the solution turned from dark green to yellow, whereupon
acid (160 mL, 2.8 mmol) at -78 °C, upon which an immediate
color change occurred from yellow to red. The solution was
H
8
filtered, concentrated to 10 mL, and stored overnight at -20
6
(
22) Thompson, R. L.; Lee, S.; Rheingold, A. L.; Cooper, N. J .
Organometallics 1991, 10, 1657.
23) Dauben, H. J .; Hannen, L. R. J . Am. Chem. Soc. 1958, 80, 5570.
°C to yield [(η -C
7
H
8
)Mo(CO)
3
].
(
OM9606909