The reactivity of a nucleophilic nickel acylate complex

Article abstract of DOI:10.1016/j.jorganchem.2008.02.028

The reactivity of a nucleophilic nickel acylate complex with a tungsten carbene complex, Fe(CO)₅, Cr(CO)₆, PPh₃, and CO was investigated. With the tungsten carbene complex, a methyl transfer occurred. With the metal carbonyl complexes, the acylate group on the nickel and a carbonyl on the iron or chromium traded places. With the PPh₃ and CO, the acylate anion was replaced by the phosphine or CO ligand.

Full text of DOI:10.1016/j.jorganchem.2008.02.028

Journal of Organometallic Chemistry 693 (2008) 2061–2064
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Note
The reactivity of a nucleophilic nickel acylate complex
*
James R. Hermanson, Timothy M. Figley, Anna L. Seibert, Allan R. Pinhas
Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, OH 45221-0172, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The reactivity of a nucleophilic nickel acylate complex with a tungsten carbene complex, Fe(CO)5,
Cr(CO)6, PPh3, and CO was investigated. With the tungsten carbene complex, a methyl transfer occurred.
With the metal carbonyl complexes, the acylate group on the nickel and a carbonyl on the iron or chro-
mium traded places. With the PPh3 and CO, the acylate anion was replaced by the phosphine or CO ligand.
Ó 2008 Elsevier B.V. All rights reserved.
Received 12 February 2008
Received in revised form 26 February 2008
Accepted 26 February 2008
Available online 4 March 2008
Keywords:
Acylate complex
Nucleophilicity
Carbene complexes
Metal carbonyls
Because chromium and iron acylate complexes are important
reactants in organometallic chemistry, there have been a large
number of studies of these complexes [1]. To further the use of
acylate complexes, we have been studying the reactivity of the
nickel acylate complex with a variety of electrophiles, including
electron-poor double bonds, alkynes, and alkyl halides [2]. In gen-
eral, when an acylate complex (1) reacts with a hard acid/electro-
phile, such as a trimethyl oxonium or a trimethylsilyl cation, the
oxygen is the nucleophilic center to give carbene complex 2. In
contrast, when 1 reacts with a soft acid/electrophile, such as allyl
bromide, a double, or a triple bond, the metal is the nucleophilic
center to give complex 3, which when oxidized to force a reduc-
tive-elimination reaction, generates ketone 4 [3,4] (see Scheme 1).
As part of our interest in the reactivity of nickel acylate com-
plexes, we wanted to compare their reactivity to that of the chro-
mium and iron complexes. Therefore, we investigated the reaction
of nickel complex 1c (R = alkyl) with (1) a carbene complex, (2)
metal carbonyl complexes, specifically Fe(CO)5 and Cr(CO)6, and
(3) triphenyl phosphine and carbon monoxide. In all cases, a ligand
exchange reaction occurs. We believe bimetallic complexes are
formed as intermediates in some of these exchange reactions.
generated. This is the same type of reaction found for a variety of
anionic complexes with carbene complex 2d [5]. Comparing these
results to those previously reported indicates that the reactivity of
À
complex 1c is about the same as Mn(CO)4PPh3À and Co(CO)3PPh3
,
and more nucleophilic than 1a or 1b.
2. Reaction of 1c with metal carbonyl complexes
As shown in Scheme 3, upon reaction of nickel acylate complex
1c (R = butyl) with Cr(CO)6 or Fe(CO)5, an acyl or alkyl anion-trans-
fer reaction occurred from the nickel complex to the chromium or
iron complex. This reaction is similar to the transmetallation of a
Fischer carbene complex [6].
It was found that these reactions are irreversible. The reaction
of either chromium acylate complex 1a or iron acylate complex
1b with nickel carbonyl generates none of nickel acylate complex
1c. In addition, chromium acylate complex 1a does not react with
iron carbonyl and iron acylate complex 1b does not react with
chromium carbonyl.
For many compounds, the pKa directly correlates with the
nucleophilicity of that compound’s conjugate base. Although the
pKa values of the conjugate acids of the three acylate complexes
are not known, nickel complex 1c must be a stronger base than
complexes 1a and 1b based on the observation that 1c is proton-
ated by water and 1a and 1b are stable in aqueous solution [4,7].
The basicities and the reactions shown in Scheme 3 indicate that
1c is a stronger nucleophile than 1a or 1b. This result is again con-
sistent with reactivity of complex 1c being about the same as
Mn(CO)4PPh3À and Co(CO)3PPh3À, which are stronger nucleophiles
than compounds 1a and 1b [5].
1. Reaction of 1c with a tungsten carbene complex
As a test of the nucleophilicity of nickel acylate complex 1c in
comparison to complexes 1a and 1b, the reaction of 1c (R = methyl
or butyl) with carbene complex 2d was run. As shown in Scheme 2,
nickel carbene complex 2c and tungsten acylate complex 1d were
* Corresponding author. Tel.: +1 513 556 9255; fax: +1 513 556 9239.
E-mail address: allan.pinhas@uc.edu (A.R. Pinhas).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.02.028

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J.R. Hermanson et al. / Journal of Organometallic Chemistry 693 (2008) 2061–2064
OCH3
(CH3)3O+
(CO)xM
(CO)xM
C
OLi
R
R
2
(CO)xM
C
Br
O
C
1
O
[O]
R
a M = Cr x = 5
b M = Fe x = 4
c M = Ni x = 3
d M = W x = 5
R
4
3
Scheme 1.
and thus, we believe that an acyl transfer occurs (Scheme 4). A
mechanism similar to this one has been proposed for an allenylid-
ene transfer reaction [10]. Note in Scheme 4, in contrast to Scheme
2, it is the nickel that is the nucleophilic center.
OLi
R
OCH3
+
(CO)5W
C
(CO)3Ni
C
Ph
2d
1c
3. Reaction of 1c with PPh3 and CO
In the reaction of iron acylate complex 1b (R = phenyl) with
PBu3, a ligand substitution of PBu3 for RCOÀ occurs [11]. Unfortu-
nately, the authors do not know what happens to the acyl anion
or the phenyl group. Therefore, it was decided to investigate the
reaction of complex 1c (R = methyl or butyl) with PPh3. As shown
in Scheme 5, the products from this reaction are a mixture of
(CO)3NiPPh3 and (CO)2Ni(PPh3)2, with the ratio depending on the
amount of triphenyl phosphine used. This reaction is slow, taking
about 3 days to go to completion. At the end of 3 days, in the IR
spectrum, the peaks for acylate complex 1c are no longer present;
only the peaks for (CO)3NiPPh3 and (CO)2Ni(PPh3)2 are observed.
After the 3-day period or even after a shorter time period, when
the reaction was quenched with water or CH3I, no organic com-
pound containing the original methyl or butyl group was detected
spectroscopically or chromatographically. In addition, when the
reaction was run in the presence of cyclohexanone, no cyclohexa-
none adduct was observed.
OLi
OCH3
+
(CO)5W
C
(CO)3Ni
C
Ph
R
1d
2c
Scheme 2.
In an effort to determine if this reaction goes by an acyl transfer,
as has been reported previously for the reaction of an iron anion
with a neutral manganese acyl complex [8], or an alkyl transfer,
as has been reported for an iridium complex [9], the reaction of
nickel acylate complex 1c with Fe(CO)5 was run in the presence
of cyclohexanone. It is known that nickel complex 1c does not re-
act with a simple ketone; however, an alkyllithium readily reacts
with a ketone [2,3]. Thus, if RLi were regenerated, it would react
with the cyclohexanone and a cyclohexanol derivative would be
generated. In contrast, if RLi were not generated, the cyclohexa-
none would remain unreacted. The latter result was observed,
To determine if this type of reaction is unique to a phosphine,
acylate complex 1c (R = methyl or butyl) was synthesized as usual
under an Ar atmosphere. When this dark maroon solution was
placed under a CO atmosphere and monitored by IR spectroscopy,
only the slow formation of Ni(CO)4 with simultaneous loss of com-
pound 1c was observed. As above, the reaction took about 3 days
OLi
(CO)3Ni
C
R
1c
Cr(CO)6
Fe(CO)5
Ni(CO)4
Ni(CO)4
OLi
OLi
R
Fe(CO)5
(CO)4Fe
C
(CO)5Cr
C
R
Cr(CO)6
1b
1a
Scheme 3.

J.R. Hermanson et al. / Journal of Organometallic Chemistry 693 (2008) 2061–2064
2063
O
Li+
OLi
R
O
C
M(CO)n
(CO)3Ni
O
M(CO)n
(CO)3Ni
C
1c
R
5
O
OLi
- Ni(CO)4
(CO)nM
1a or 1b
C
(CO)3Ni
M(CO)n
R
R
OLi
6
Scheme 4.
OLi
CO
PPh3
(CO)3NiPPh3 + (CO)2Ni(PPh3)2
(CO)3Ni
1c
C
Ni(CO)4
R
Scheme 5.
for total disappearance of complex 1c. If at some intermediate
time, the solution was again saturated with argon, both the gener-
ation of nickel carbonyl and the decomposition of 1c ceased.
Due to our fear that monitoring may be affecting the reaction in
some manner, as a control experiment, a solution of complex 1c
was stirred under an argon atmosphere for 3 days and monitored
by IR spectroscopy. For this reaction, at no time was any Ni(CO)4
generated. Therefore, the CO is required for the decomposition of
1 to occur, but as above, the fate of the alkyl group is not known.
5.2. Instrumentation
All infrared spectra were recorded on a Perkin–Elmer Model
1600 series FTIR spectrophotometer using KBr cells. All NMR spec-
tra were recorded on a Bruker 250 MHz spectrometer. 1H and 13C
spectra were referenced to tetramethylsilane at 0.00 ppm and 31P
spectra were referenced to PPh3 at -6.0 ppm.
5.3. Syntheses of acylate complexes
4. Conclusion
These were done in a similar manner to the published proce-
dure. [2] To a 3-neck round-bottom flask, under an inert atmo-
sphere and equipped with a glass stopper, a rubber septum, a stir
bar, and a gas inlet adapter, was added approximately 40 mL of de-
gassed THF. The solvent was cooled to À78 °C and 5.0 mmoles of
the appropriate metal carbonyl (0.60 mL of nickel tetracarbonyl,
0.66 mL of iron pentacarbonyl, or 1.1 g of chromium hexacarbonyl)
were added. To this was added 3.1 mL of 1.6 M butyllithium or
3.6 mL of 1.4 M methyllithium (5.0 mmoles) over a 15 min period.
(It has been found that near the end of the addition there is a sig-
nificant darkening of the color of the reaction mixture. Cleaner
reactions result when the addition is stopped at this point.) The
reaction was kept at À78 °C for an additional 15 min and then al-
lowed to warm to room temperature for 1 h.
Thus, nickel acylate complex 1c is a stronger nucleophile than
the analogous chromium (1a) or iron (1b) complexes, and is
approximately in the middle in comparison to other transition me-
tal anionic complexes.
5. Experimental
5.1. General
All reactions were carried out using glassware dried in a 110 °C
oven and cooled under an argon atmosphere or in a desiccator. All
reactions were run under an argon atmosphere. Tetrahydrofuran
was freshly distilled from potassium benzophenone ketyl. Nickel
tetracarbonyl was transferred from a one-pound lecture bottle
and filtered through celite into a 10 mL sidearm flask, maintaining
a strong argon flow, and stored under argon until used. Transfers
were made via syringe and excess Ni(CO)4 was quenched in an io-
dine/CHCl3 bath. (Ni(CO)4 is toxic and flammable when exposed to
air. All work should be conducted in a well-ventilated hood, main-
taining an argon atmosphere during all transfers and using non-
flammable solvents in the iodine bath to reduce the probability
of fire.) Iron pentacarbonyl and chromium hexacarbonyl were used
as received.
5.4. General procedure
After stirring at room temperature, the metal acylate complex
again was cooled to À78 °C. Then, depending on the reaction,
2.2 g (5.0 mmoles) of tungsten carbene complex 2d, 5.0 mmoles
of one of the other metal carbonyl complexes (0.60 mL of nickel
tetracarbonyl, 0.66 mL of iron pentacarbonyl, or 1.1 g of chromium
hexacarbonyl), 1.0 mL (10.0 mmoles) of cyclohexanone, and/or
1.3–2.6 g (5.0–10.0 mmoles) of PPh3 were added. In the CO
reaction, the solution was saturated with CO and left under a CO

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J.R. Hermanson et al. / Journal of Organometallic Chemistry 693 (2008) 2061–2064
(b) J.R. Hermanson, M.L. Gunther, J.L. Belletire, A.R. Pinhas, J. Org. Chem. 60
(1995) 1900;
(c) J.R. Hermanson, J.W. Hershberger, A.R. Pinhas, Organometallics 14 (1995)
5426;
(d) M.J.T. La Duca, J.L. Simunic, J.W. Hershberger, A.R. Pinhas, Inorg. Chim. Acta
222 (1994) 165.
atmosphere. Each reaction was kept at À78 °C for an additional
15 min and then allowed to warm to room temperature. Each reac-
tion was either monitored by IR spectroscopy, or worked-up in the
usual manner (using an I2 oxidation to destroy the metal com-
plex(es) and then extracting the resulting organic compound(s)
into diethyl ether). For those reactions that were worked-up, the
resulting products were analyzed by 1H, 13C, and where appropri-
ate 31P NMR spectroscopy and IR spectroscopy. The spectral data
for all compounds have been reported previously [2,5].
[3] (a) also see: S. Fukuoka, M. Ryang, S. Tsutsumi, J. Org. Chem. 36 (1971) 2721;
(b) E.J. Corey, L.S. Hegedus, J. Am. Chem. Soc. 91 (1969) 1233–4926.
[4] (a) See for example: M. Bergamo, T. Beringhelli, G. D’Alfonso, D. Maggioni, P.
Mercandelli, A. Sironi, Inorg. Chim. Acta 350 (2003) 475;
(b) J.E. Ellis, Organometallics 22 (2003) 3322;
(c) T.R. Hoye, K. Chen, J.R. Vyvyan, Organometallics 12 (1993) 2806;
(d) P. Laurent, G. Tanguy, H. des Abbayes, J. Chem. Soc., Chem. Commun.
(1986) 1754;
(e) H. Fischer, in: Transition Metal Carbene Complexes, Verlag Chemie,
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