Transition Metal Formyl Complexes
A R T I C L E S
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extracted into THF (4 mL), filtered with two small washes (2 mL),
and cooled to -25 °C. Hexane (45 mL) was added until clouding
occurred. On sitting overnight, orange crystals formed. The product
was collected by filtration and washed with hexanes. On drying, 302
mg (68% yield) of CpRe(PMe3)(NO)(CHO) was obtained as an orange
solid. The material contained 1-2% of the starting [CpRe(PMe3)(NO)-
(CO)](BF4). Due to this fact, and the its low thermal stability, it was
plexes. This relationship should allow us to predict ∆G°H values
for new rhenium formyls by making a simple electrochemical
measurement.
Also, for the first time, the C-H bond dissociation free
energies (∆G°H ) of a series of rhenium formyls has been
•
measured. These values were found to be fairly low (∼50 kcal/
mol).35 The overall poor stability of these and other formyls is
1
not submitted for elemental analysis. H NMR data (300 MHz, CD3-
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likely due to low ∆G°H values, as has long been suspected.
CN): δ 16.60 (s, 1 H, C(O)H), 5.49 (d, J ) 0.6 Hz, 5 H, C5H5), 1.60
(d, J ) 10.8 Hz, 9 H, P(CH3)3). 31P NMR (121 MHz, CD3CN): δ
-24.27 (s).
Equilibration of [CpRe(PMe3)(NO)(CO)](PF6) with [Pt(dmpe)2H]-
(PF6). [CpRe(PMe3)(NO)(CO)](PF6) (10-15 mg, 0.02-0.03 mmol) and
1 equiv of [Pt(dmpe)2H](PF6) were added to an NMR tube in the
glovebox. The solids were dissolved in CD3CN (0.6 mL). The reaction
was monitored by 1H and 31P NMR for several hours. After ap-
proximately 5 h, the reaction had reached equilibrium. This experiment
was performed two times.
And finally, by studying the thermodynamics of hydride
addition to carbonyl complexes, we have been able to design
two systems which hydrogenate rhenium carbonyls to give
formyl complexes. As such, this serves as a useful model for
the catalytic cycle proposed for the hydrogenation of CO to
CH3OH shown in Scheme 1, by demonstrating both the
activation of H2 as well as a hydride transfer to give a formyl
complex.
The reverse reaction was performed as follows. The formyl 2 (12
mg, 0.03 mmol) and [Pt(dmpe)2](PF6)2 (26 mg, 0.03 mmol) were
dissolved in CD3CN (0.6 mL). After reacting for several hours,
Experimental Section
Materials and Methods. NMR spectra were recorded on a Varian
Unity 300 spectrometer. Proton chemical shifts are reported relative to
residual protons in CD3CN (1.93 ppm). 31P chemical shifts are reported
relative to an unlocked, external sample of H3PO4. All electrochemical
measurements were carried out under an atmosphere of N2 in 0.3 M
[Et4N][BF4] in acetonitrile with a Cypress Systems computer-aided
electrolysis system. The working electrode was a glassy-carbon disk
(2 mm diameter), and the counter electrode was a glassy-carbon rod.
A platinum wire immersed in a permethylferrocene/permethylferroce-
nium solution was used as a pseudo-reference electrode to fix the
potential. Ferrocene was used as an internal standard, and all potentials
are referenced to the ferrocene/ferrocenium couple.
Solvents were reagent grade and were purchased from Aldrich.
Hexanes and methylene chloride were degassed by using several
freeze-pump-thaw cycles prior to use. Tetrahydrofuran was distilled
from Na/benzophenone, and likewise degassed before use. CD3CN was
vacuum transferred from CaH2 and stored in a glovebox.
The nickel and platinum complexes used in these studies were
prepared by published methods. These include [Ni(dmpe)2][BF4]2,1 [Ni-
(dmpe)2H][PF6],2 [Pt(dmpe)2][PF6]2,1,36 [Pt(dmpe)2H][PF6],1 [Pt(depe)2]-
[PF6]2,37-39 and [Pt(dmpp)2][PF6]2.1 The metal carbonyl complexes were
also prepared by literature methods. These include [CpRe(NO)(CO)2]-
[BF4],6-8 [Cp*Re(NO)(CO)2][BF4],8 [CpRe(PMe3)(NO)(CO)][BF4],40
[CpRe(PEt3)(NO)(CO)][PF6],2 [CpRe(PPh3)(NO)(CO)][BF4],7 [Cp*Ru-
(CO)3][BF4],16 [(bipy)2Ru(CO)2][PF6]2,41 and [(PPh3)Mn(CO)5][BF4].19
The following formyls were also prepared according to literature
methods: CpRe(PPh3)(NO)(CHO)7 and [(bipy)2Ru(CO)(CHO)][PF6].12,13
Representative procedures used for the equilibration reactions of formyls
follow in this section.
CpRe(PMe3)(NO)(CHO) (2). The following was performed under
an atmosphere of N2 on a Schlenck line. Sodium borohydride (0.435
g, 11.5 mmol) was added to a 0 °C solution of [CpRe(PMe3)(NO)-
(CO)](BF4) (0.542 g, 1.15 mmol) in 1/1 THF/H2O (70 mL). The yellow
reaction mixture slowly became cloudy. After 1 h at 0 °C, the reaction
mixture was extracted with three portions of CH2Cl2 (25, 15, and 10
mL). The combined extracts were cooled to 0 °C and dried over MgSO4
for a few minutes. Upon filtration, and a small CH2Cl2 wash, the
solution was concentrated in vacuo to give a yellow oil. The oil was
1
equilibrium was established and concentrations were measured by H
and 31P NMR. The free energy of the reaction was calculated from the
equation ∆G° ) -RT lnKeq. The equilibrium constant, Keq, was
calculated according to eq 10.
PtH+ + Re(CO)+ h Pt2+ + Re(CHO)
(10)
Keq ) [Pt2+][Re(CHO)]/[PtH+][Re(CO)+]
Equilibration of [CpRe(PMe3)(NO)(CO)](PF6) with CpRe(PPh3)-
(NO)(CHO). [CpRe(PMe3)(NO)(CO)](PF6) (13 mg, 0.02 mmol) and
CpRe(PPh3)(NO)(CHO) (5 mg, 0.01 mmol) were dissolved in CD3CN
(0.6 mL) in an NMR tube under inert atmosphere. The reaction was
followed by 1H and 31P NMR over 2 h, during which time equilibrium
was reached. The reverse reaction, between formyl 3 and [CpRe(PMe3)-
(NO)(CO)](PF6) was performed two times.
Equilibration of [CpRe(NO)(CO)2](BF4) with Cp*Re(NO)(CO)-
(CHO). Formyl 2, CpRe(PMe3)(NO)(CHO) (11 mg, 0.03 mmol), and
[Cp*Re(NO)(CO)2](BF4) (16 mg, 0.03 mmol) were dissolved in CD3-
CN (0.6 mL). Quantitative hydride transfer to form Cp*Re(NO)(CO)-
(CHO) had occurred after 10 min, as seen in the 1H NMR spectrum of
the solution. Next, [CpRe(NO)(CO)2](BF4) (13 mg, 0.03 mmol) was
1
added, and the reaction was followed by H NMR. The reaction was
followed for 2 h, and had achieved equilibrium after 15 min. The
reaction was repeated one more time in the forward direction. By
reacting [CpRe(NO)(CO)2](BF4) with formyl 3 first, and subsequently
adding [Cp*Re(NO)(CO)2](BF4), the reaction was run one time in the
reverse direction.
Other equilibria. pKa of Proton Sponge. To three separate NMR
tubes was added Proton Sponge (10-20 mg, 0.05-0.1 mmol) and [Et3-
NH][BF4] (15-20 mg, 0.08-0.1 mmol). The solids were dissolved in
CD3CN (0.6 mL), and the reactions were monitored by 1H NMR for at
least an hour. Equilibration was rapid in all cases. The equilibrium
constant for the protonation of Proton Sponge was found to be 0.536
( 0.002, giving a pKa value of 18.19 for the conjugate acid.
Hydrogenation of [Cp*Re(NO)(CO)2](BF4). A mixture of [Cp*Re-
(NO)(CO)2](BF4) (24 mg; 0.05 mmol), [Pt(dmpp)2](PF6) (21 mg; 0.03
mmol), and Proton Sponge (25 mg; 0.12 mmol) was dissolved in CD3-
CN (0.6 mL) in an NMR tube and sealed with a rubber septum. The
septum was secured with a wire, and 5 mL of H2 was added to the
tube by syringe. The tube was mixed vigorously on a shaker when it
was not being monitored by NMR. After 1 h, the formyl 4 could be
observed in small conversion. After 6 h, it was present in 9% yield, as
well as numerous decomposition products.
(35) Griller, D.; Martinho Simoes, J. A.; Mulder, P.; Sim, B. A.; Wayner, D.
D. M. J. Am. Chem. Soc. 1989, 111, 7872.
(36) von Kozelka, J.; Ludwig, W. HelV. Chim. Acta 1983, 66, 902.
(37) Ittel, S. D. Inorg. Synth. 1990, 28, 98.
(38) Ittel, S. D. Inorg. Synth. 1977, 17, 117.
(39) Tolman, C. A.; Seidel, W. C.; Gosser, L. W. J. Am. Chem. Soc. 1974, 96,
53.
(40) Weiner, W. P.; Hollander, F. J.; Bergman, R. G. J. Am. Chem. Soc. 1984,
106, 7462.
(41) Kelly, J. M.; O’Connell, C. M. J. Chem. Soc., Dalton Trans. 1986, 253.
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J. AM. CHEM. SOC. VOL. 124, NO. 9, 2002 1931