Photoreaction of silyliron(II) complex Cp*Fe(CO)2SiMe3 (Cp* = η5C5Me5) in the presence of p-tolylgermane

Article abstract of DOI:10.1021/om960504z

Photolysis of Cp*Fe(CO)₂SiMe₃ (3) in the presence of p-TolGeH₃ (4) gave a mixture of cis and trans mono(germylene)-bridged diiron complexes, Cp*₂Fe₂(CO)₂(μ-CO)(μ-Gep-TolH) (cis-5, trans-5), a mixture of two kinds of trans bis(germylene)-bridged diiron complexes, Cp*₂Fe₂(CO)₂(μ-Gep-TolH)₂ (syn-trans-6, anti-trans-6), and a complex with two Fe?H?Ge 3-center 2-electron bonds, Cp*₂Fe₂(CO)₂(μ;-Gep-TolH ₂)₂ (7). A cis isomer of complex 6 (syn-cis(H)-6) was formed as the sole product by the thermolysis of complex 7. Photolysis of 7 or syn-cis(H)-6 resulted in the conversion to a mixture of syn-trans-6 and anti-trans-6. Reversible thermal and photochemical interconversion between cis and trans isomers of complex 5 was observed. The molar ratio of the cis and trans isomers of complex 5 in C₆D₆ was 7:93 at thermal equilibrium at 55°C and 84:16 at the photostationary state at 6°C.

Full text of DOI:10.1021/om960504z

4954
Organometallics 1996, 15, 4954-4958
P h otor ea ction of Silylir on (II) Com p lex Cp *F e(CO)2SiMe3
(Cp * ) η5-C5Me5) in th e P r esen ce of p-Tolylger m a n e
Amr El-Maradny, Hiromi Tobita,* and Hiroshi Ogino*
Department of Chemistry, Graduate School of Science, Tohoku University,
Sendai 980-77, J apan
Received J une 21, 1996X
Photolysis of Cp*Fe(CO)2SiMe3 (3) in the presence of p-TolGeH3 (4) gave a mixture of cis
and trans mono(germylene)-bridged diiron complexes, Cp*2Fe2(CO)2(µ-CO)(µ-Gep-TolH) (cis-
5, trans-5), a mixture of two kinds of trans bis(germylene)-bridged diiron complexes,
Cp*2Fe2(CO)2(µ-Gep-TolH)2 (syn-trans-6, anti-trans-6), and a complex with two Fe‚‚‚H‚‚‚Ge
3-center 2-electron bonds, Cp*2Fe2(CO)2(µ-Gep-TolH2)2 (7). A cis isomer of complex 6 (syn-
cis(H)-6) was formed as the sole product by the thermolysis of complex 7. Photolysis of 7 or
syn-cis(H)-6 resulted in the conversion to a mixture of syn-trans-6 and anti-trans-6.
Reversible thermal and photochemical interconversion between cis and trans isomers of
complex 5 was observed. The molar ratio of the cis and trans isomers of complex 5 in C6D6
was 7:93 at thermal equilibrium at 55 °C and 84:16 at the photostationary state at 6 °C.
In tr od u ction
The oxidative addition of Si-H bonds to low-valent
transition metal centers is one of the most important
reactions in the formation of transition metal to silicon
bonds. Many silyl(transition metal) complexes1 and
several complexes with M‚‚‚H‚‚‚Si 3-center 2-electron
bonds2 have been characterized. On the other hand,
very little is known about analogous complexes possess-
ing a M‚‚‚H‚‚‚Ge 3-center 2-electron bond.3 Recently,
we prepared the first fully characterized complex with
two Fe‚‚‚H‚‚‚Ge 3-center 2-electron bonds, Cp*2Fe2(CO)2-
(µ-Get-BuH2)2 (1),4 which may be an intermediate in the
formation of the bis(germylene)-bridged diiron complex
via H2 expulsion.4 These complexes (bis(germylene)-
bridged diiron) can adopt five different geometries (two
trans and three cis, Figure 1).
F igu r e 1. Five possible geometric isomers of Cp*2Fe2-
(CO)2(µ-GeRH)2 (6).
of this 3-center 2-electron bond containing complex, we
isolated three isomers of a bis(germylene)-bridged diiron
complex among the five possible isomers of this complex.
Also, thermal and photochemical interconversion be-
tween the cis and trans isomers of a mono(germylene)-
bridged diiron complex was studied.
Recently, we reported the photochemical interconver-
sion between the cis and trans isomers of Cp*2Fe(CO)2-
(µ-Gep-Tol2)2.5 The isomers of complexes of type Cp2M2-
(CO)2(µ-CO)(µ-ER2) (E ) group 14 elements) can undergo
either thermal or photochemical interconversion. The
interconversion between these isomers has been re-
ported for Cp2M2(CO)2(µ-CO)(µ-CR2) (M ) Fe, Ru; R )
H, alkyl, alkenyl, Ph, SR, etc.),6a-i Cp2M2(CO)2(µ-CO)-
(µ-CdCHR) (M ) Fe, Ru; R ) H, alkyl, Ph),6g-i
Cp2Fe2(CO)2(µ-CO)(µ-SiHMe),7 Cp2Fe2(CO)2(µ-CO)(µ-
SiXt-Bu) (X ) Cl, Br, I, Me),8 Cp2Fe2(CO)2(µ-CO)(µ-
GeMe2),9 and Cp*2Fe2(CO)2(µ-CO)(µ-Sip-TolH) (2).10
Exp er im en ta l Section
All manipulations were performed under an inert atmo-
sphere using standard Schlenk techniques. Reagent-grade
benzene, diethyl ether, hexane, pentane, and tetrahydrofuran
(6) (a) Korswagen, R.; Alt, R.; Speth, D.; Ziegler, M. L. Angew.
Chem., Int. Ed. Engl. 1981, 20, 1049. (b) Altbach, M. I.; Muedas, C.
A.; Korswagen, R. P. J . Organomet. Chem. 1986, 306, 375. (c) Dyke,
A. F.; Knox, S. A. R.; Morris, M. J .; Naish, P. J . J . Chem. Soc., Dalton
Trans. 1983, 1417. (d) Davies, D. L.; Knox, S. A. R.; Mead, K. A.;
Morris, M. J .; Woodward, P. J . Chem. Soc., Dalton Trans. 1984, 2293.
(e) Gracey, B. P.; Knox, S. A. R.; Macpherson, K. A.; Orpen, A. G. J .
Chem. Soc., Dalton Trans. 1985, 1935. (f) Colborn, R. E.; Dyke, A. F.;
Knox, S. S. A. R.; Mead, K. A.; Woodward, P. J . Chem. Soc., Dalton
Trans. 1983, 2099. (g) Colborn, R. E.; Davies, D. L.; Dyke, A. F.;
Endesfelder, A.; Knox, S. A. R. J . Chem. Soc., Dalton Trans. 1983,
2661. (h) Schoeder, N. C.; Funchess, R.; J acobson, R. A.; Angelici, R.
J . Organometallics 1989, 8, 521. (i) Casey, C. P.; Austin, E. A. J . Am.
Chem. Soc. 1988, 110, 7106.
Here we report the synthesis of the second diiron
complex with two Fe‚‚‚H‚‚‚Ge 3-center 2-electron bonds
by photolysis of Cp*Fe(CO)2SiMe3 (3) in the presence
of p-TolGeH3 (4). Through photolysis and thermolysis
X Abstract published in Advance ACS Abstracts, October 1, 1996.
(1) Aylett, B. J . Adv. Inorg. Radiochem. 1982, 25, 1.
(2) (a) Hart-Davis, A. J .; Graham, W. A. G. J . Am. Chem. Soc. 1971,
94, 4388. (b) Schubert, U.; Scholz, G.; Mu¨ller, J .; Ackermann, K; Worle,
B. J . Organomet. Chem. 1986, 306, 303. (c) Schubert, U.; Mu¨ller, J .;
Alt, H. G. Organometallics 1987, 6, 469. (d) Luo, X.-L.; Crabtree, R.
H. J . Am. Chem. Soc. 1987, 111, 2527.
(7) Malisch, W.; Ries, W. Angew. Chem., Int. Ed. Engl. 1978, 17,
120.
(3) Carre, F.; Colomer, E.; Corriu, R. J . P.; Vioux, A. Organometallics
1984, 3, 1272.
(4) Submitted for publication.
(8) Tobita, H.; Kawano,Y.; Ogino, H. Chem. Lett. 1989, 2155.
(9) J ob, R. C.; Curtis, M. D. Inorg. Chem. 1973, 12, 2514.
(10) Kawano, Y.; Tobita, H.; Ogino, H. J . Organomet. Chem. 1992,
428, 125.
(5) El-Maradny, A.; Tobita, H.; Ogino, H. Chem. Lett. 1996, 83.
S0276-7333(96)00504-3 CCC: $12.00 © 1996 American Chemical Society

Photoreaction of Cp*Fe(CO)2SiMe3
Organometallics, Vol. 15, No. 23, 1996 4955
(THF) were distilled from sodium-benzophenone ketyl im-
TolH)2 (6) (224 mg, 0.29 mmol, 21%, syn-trans:anti-trans )
61:39), and (3) an orange complex Cp*2Fe2(CO)2(µ-Gep-TolH2)2
(7) (243 mg, 0.320 mmol, 23%). 5: MS m/z 631 (3, M+), 603
(23, M+ - CO), 540 (55, M+- p-Tol). Anal. Found: C, 57.48;
H, 6.51. Calcd for C30H38O3Fe2Ge: C, 57.11; H, 6.07. cis-5:
1H NMR (C6D6) δ 1.60 (30H, s, 2Cp*), 2.12 (3H, s, C6H4CH3),
7.24, 8.36 (2H × 2, d, J HH ) 7.74 Hz, C6H4CH3), 8.62 (H, s,-
GeH); IR (KBr) 1994 (w) (νGeH), 1942 (vs), 1903 (m) (νCOterm),
1731 (s) (νCObrid) cm-1. trans-5: 1H NMR (C6D6) δ 1.61, 1.67
(30H, 2s, 2Cp*), 2.20 (3H, s, C6H4CH3), 7.24, 8.20 (2H × 2, d,
J HH ) 7.59 Hz, C6H4CH3), 8.42 (H, s, GeH); IR (KBr) 1992 (w)
(νGeH), 1932 (m), 1901 (vs) (νCOterm), 1747 (s) (νCObrid) cm-1. 6:
MS m/z 766 (43, M+), 738 (16, M+ - CO), 618 (10, M+ - 2CO
- p-Tol). Anal. Found: C, 56.05; H, 5.84. Calcd for
mediately before use. Deuterated NMR solvents were stored
11
over molecular sieves 4A under N2. Cp*Fe(CO)2SiMe3 was
prepared according to the published procedure. p-Chloro-
toluene was distilled over P2O5 immediately prior to use.
Germanium tetrachloride was stored under N2 in a refrigera-
tor. Other chemicals were used without further purification.
Photolysis was performed with an Ushio UM-452 450 W
medium-pressure Hg lamp placed in a water-cooled, quartz
jacket. Sample solutions were irradiated in Pyrex tubes.
1H NMR and 13C NMR spectra were recorded on a Bruker
AC-300 spectrometer. Variable-temperature 1H NMR spectra
were recorded also on the Bruker AC-300 spectrometer. IR
spectra were obtained on a Horita FT-200 spectrometer. Mass
spectra were taken on a Hitachi M-52 or J EOL J MS-HX 110
mass spectrometer, and high-resolution mass spectra, on a
J EOL J MS-HX 110 mass spectrometer. UV-visible spectra
were recorded on a Shimadzu UV-260 UV-visible recording
spectrophotometer connected to a TCC-260 temperature-
controlled cell holder.
P r ep a r a tion of p-Tolylger m a n e. (p-Tolyl)magnesium
chloride was prepared by the addition of a solution of p-
chlorotoluene (50.6 g, 0.400 mol) dissolved in THF (100 mL)
into a flask containing Mg turnings (10 g, 0.42 mol) in THF
(10 mL) over 70 min (Note: Mg was stirred and heated at 80
°C for 1-2 days, under a gentle flow of N2, in order to activate
it). The mixture in the flask was stirred well during the
addition and then for an additional 2 h at 45 °C. To a
vigorously stirred solution of GeCl4 (64.8 g, 0.300 mol) dis-
solved in Et2O (100 mL) was added the Grignard reagent over
90 min. The mixture was then gently refluxed for 2 h, MgCl2
was filtered off over Celite, and the mixture was washed twice
with 100 mL Et2O. All solvents were removed to yield a yellow
solution. A fractional distillation of the latter solution afforded
trichloro-p-tolylgermane, p-TolGeCl3, as a colorless liquid (27
g, 0.10 mol) in 33% yield (bp 60 °C, 0.6 mmHg) (lit. bp 129-
131 °C, 24 mmHg12 ). 1H NMR (C6D6): δ 1.85 (3H, s,
C6H4CH3), 6.72, 7.26 (2H × 2, d, J HH ) 7.81 Hz, C6H4CH3).
13C NMR (CDCl3): δ 21.7 (C6H4CH3), 130.1, 131.2, 131.7, 143.9
(C6H4CH3). MS: m/z 270 (41, M+). Dichlorodi-p-tolylger-
mane13 was also obtained in 28% yield.
C
36H44O2Fe2Ge2: C, 56.48; H, 5.79. syn-trans-6: 1H NMR
((CD3)2CO) δ 1.57, 1.62 (30H, 2s, 2Cp*), 2.41 (6H, s, 2C6H4CH3),
7.32, 7.93 (4H × 2, d, J HH ) 7.52 Hz, 2C6H4CH3; see Results
and Discussion), 8.04 (2H, s, GeH); IR (KBr) 1930 (m), 1889
(s) (νCOterm) cm-1
.
anti-trans-6: 1H NMR ((CD3)2CO) δ 1.55
(30H, s, 2Cp*), 2.38 (6H, s, 2C6H4CH3), 7.28, 7.87 (4H × 2, d,
2C6H4CH3; see Results and Discussion), 7.95 (2H, s, GeH); IR
(KBr) 1876 (vs) (νCOterm) cm-1
.
7: 1H NMR (C6D6) δ -12.74
(2H, s, 2FeH), 1.51 (30H, s, 2Cp*), 2.18 (6H, s, 2C6H4CH3),
3.81 (2H, s, 2GeH), 7.13, 8.02 (4H × 2, d, J HH ) 7.78 Hz, 2C6H4-
CH3); 13C NMR (C6D6) δ 10.1 (CH3, Cp*), 21.4 (C6H4CH3), 90.9
(Cp*, ring C), 128.6, 135.6, 137.2, 141.6 (C6H4CH3), 222.9 (CO);
IR (KBr) 1998 (w) (νGeH), 1930 (vs), 1921 (vs) (νCOterm), 1898
(m) (νGe-H-Fe) cm-1; MS m/z 768 (10, M+), 697 (13, M+ - Me -
2CO). Anal. Found: C, 55.98; H, 6.50. Calcd for
C
36H48O2Fe2Ge2: C, 56.33; H, 6.04.
Th er m olysis of Cp *2F e2(CO)2(µ-Gep-TolH2)2 (7). A solu-
tion of 7 (85 mg, 0.11 mmol) in benzene (30 mL) was refluxed
at 70 °C for 3 h. The color of the solution changed gradually
from orange to red during thermolysis. Removal of volatile
materials from the reaction mixture followed by flash chro-
matographic separation of the residue in the dark (silica gel,
eluent hexane:benzene ) 3:1) gave two different bands. The
first yellow band was a decomposed material. From the second
red band, a syn-cis(H)-6 isomer of bis(germylene)-bridged
diiron complex Cp*2Fe2(CO)2(µ-Gep-TolH2)2 (6) (20 mg, 26
mmol, 25%) was obtained as a red powder. Further purifica-
tion for this complex was unsuccessful because of the light
sensitivity. 1H NMR (C6D6): δ 1.58 (30H, s, 2Cp*), 2.15 (6H,
s, 2C6H4CH3) 7.29, 8.48 (4H × 2, d, J HH ) 7.66 Hz, 2C6H4-
CH3), 8.67 (2H, s, GeH). 13C NMR (C6D6): δ 11.0 (CH3, Cp*),
21.5 (C6H4CH3), 91.6 (Cp*, ring C), 129.0, 136.1, 138.0, 149.4
To a well-stirred suspension of LiAlH4 (3.2 g, 88 mmol) in
Et2O (50 mL) was added a solution of the above trichloro-p-
tolylgermane (27 g) in Et2O (30 mL) over 50 min. The mixture
was then gently refluxed for 4 h. Excess LiAlH4 was destroyed
by the slow addition of 10% H2SO4. The reaction mixture was
extracted with Et2O. The organic layer was dried over
anhydrous MgSO4. Fractional distillation afforded p-tolyl-
germane, p-TolGeH3 (4), as a colorless liquid (12.6 g, 0.075 mol)
in 75% yield (bp 40 °C, 0.6 mmHg). 1H NMR (C6D6): δ 2.05
(3H, s, C6H4CH3), 4.26 (3H, s, GeH), 6.94, 7.27 (2H × 2, d,
J HH ) 7.69, C6H4CH3). 13C NMR (CDCl3): δ 22.0 (C6H4CH3),
128.0, 129.7, 136.0, 139.4 (C6H4CH3). IR (benzene solution):
(C6H4CH3), 215.6 (CO). IR (KBr): 1921 (s), 1894 (m) (νCOterm
)
cm-1. MS: m/z 766 (9, M+), 738 (14, M+ - CO). Exact mass:
found: 768.0473; calcd for C36H44O2Fe2Ge2, 768.0464.
P h otolysis of Cp *2F e2(CO)2(µ-Gep-TolH2)2 (7). Photoly-
sis of a benzene-d6 solution of 7 in an NMR tube at 6 °C for 10
min was monitored by 1H NMR spectroscopy. A trans mixture
of 6 (molar ratio of syn-trans-6 to anti-trans-6 was 61:39) was
observed with some decomposed materials and H2.
2071 (s) (νGeH) cm-1
.
MS: m/z 168 (21, M+). Anal. Found:
C, 50.61; H, 5.78. Calcd for C7H10Ge: C, 50.42; H, 6.04.
P h otolysis of Cp *F e(CO)2SiMe3 (3) in th e P r esen ce of
p-TolGeH3 (4). Cp*Fe(CO)2SiMe3 (3) (670 mg, 2.09 mmol)
and p-TolGeH3 (4) in a small excess (550 mg, 3.30 mmol) in
pentane (30 mL) were irradiated at 6 °C for 100 min.
Evolution of carbon monoxide was observed, and the color of
the solution changed from pale orange to dark red during the
photolysis. Evaporation of volatile materials and subsequent
flash chromatographic separation of the residue (silica gel,
eluent hexane:benzene ) 3:1) gave (1) a red mixture of cis and
trans mono(germylene)-bridged diiron complexes Cp*2Fe2-
(CO)2(µ-CO)(µ-Gep-TolH) (5) (281 mg, 0.450 mmol, 32%, cis:
trans ) 21:79), (2) a violet mixture of two kinds of trans
bis(germylene)-bridged diiron complexes Cp*2Fe2(CO)2(µ-Gep-
P h otolysis of Cp *2F e2(CO)2(µ-Gep-TolH)2 (syn -cis(H)-
6). The monitoring of the photolysis of a benzene-d6 solution
of syn-cis(H)-6 in an NMR tube at 6 °C for 1 min by H NMR
spectroscopy showed a quantitative conversion from syn-
cis(H)-6 to the mixture of syn- and anti-trans-6 with molar
ratio 61:39.
1
Kin etic Stu d y of th e In ter con ver sion of cis a n d tr a n s
Isom er s of Com p lex 5. The change of the UV-visible
spectrum of a mixture of cis and trans isomers of complex 5
with molar ratio 84:16 in benzene was monitored in a quartz
cell, at five different temperatures between 25.0 and 55.0 °C.
Before the reaction, the mixture showed an absorption peak
at 544 nm, which over time shifted to slightly longer wave-
length, and an isosbestic point was observed at 505 nm. The
values of equilibrium constants at different temperatures were
determined by the integration of 1H NMR spectra of Cp*
groups. Plots of ln(A∞ - At/A∞ - Ao) vs time were linear (A∞ )
(11) Randolph, C. L.; Wrighton, M. S. Organometallics 1987, 6, 365.
(12) Seyferth, D.; Hetflejs, J . J . Organomet. Chem. 1968, 11, 253.
(13) Mochida, K.; Matsushige, N.; Hamashima, M. Bull. Chem. Soc.
J pn. 1985, 55, 1443.
absorption at equilibrium; At ) absorption at time t; Ao
)

4956 Organometallics, Vol. 15, No. 23, 1996
El-Maradny et al.
initial absorption), and least-squares analyses gave correlation
coefficients of >0.98 (with a minimum of eight data points).
The activation parameters were determined from five data
points using the Kaleida Graph, and least-squares analyses
gave correlation coefficients of >0.98.
Resu lts a n d Discu ssion
P h otolysis of Cp *F e(CO)2SiMe3 (3) in th e P r es-
en ce of p-TolGeH3 (4). Irradiation of Cp*Fe(CO)2-
SiMe3 (3) in the presence of p-TolGeH3 (4) in pentane
for 100 min afforded mono- and bis(germylene)-bridged
diiron complexes and a complex with two Fe‚‚‚H‚‚‚Ge
3-center 2-electron bonds (eq 1). The major product was
the mono(germylene)-bridged diiron complex Cp*2Fe2-
(CO)2(µ-CO)(µ-Gep-TolH) (5) that was isolated as two
fractions by flash chromatography with the total yield
of 32%. The cis and trans isomers were separated in
1
the column. However the H NMR spectra showed the
cis isomer fraction contained 16% of the trans isomer,
and the trans isomer fraction contained 5% of the cis
isomer. This is attributable to the thermal isomeriza-
tion between cis and trans isomers even at room
temperature. The 1H NMR spectrum of the trans
isomer showed two Cp* signals, while that of the cis
isomer showed only one. Both isomers showed one
methyl signal of the p-tolyl group, two doublet signals
in the aromatic region, and a singlet due to the Ge-H
proton. In the IR spectrum of the trans isomer, the
νCOasym band is stronger than νCOsym, while in the cis
isomer this relationship is reversed. These data were
found to be very similar to those of the analogous
silylene complex Cp*2Fe2(CO)2(µ-CO)(µ-Sip-TolH) (2)10
except for a small shift of the GeH protons (8.62 ppm
for cis and 8.42 ppm for trans in C6D6) toward low field
relative to the SiH protons (7.44 ppm for cis and 7.01
ppm for trans in cyclohexane-d12) in the 1H NMR spectra
and the difference in the molar ratio between cis and
trans isomers. The thermal and photochemical inter-
conversion between cis and trans isomers of com-
plex 5 will be discussed in detail in a later section of
this paper.
1
Figu r e 2. Temperature-dependent H NMR spectra (CDCl3)
of the syn- and anti-trans-6 mixture in the aromatic signal
region.
4
(CO)2(µ-Get-BuH)2 exclusively adopted the syn-cis(H)
configuration. This time, using a moderately sterically
hindered p-tolyl group as a substituent on germanium,
a violet mixture of syn- and anti-trans isomers of
complex 6 was isolated in 21% yield with the ratio of
syn-trans-6:anti-trans-6 ) 61:39. The characterization
of these two isomers was based on the H NMR and IR
spectra. In the H NMR spectrum, syn-trans-6 showed
two singlet signals in the area of Cp*. Among the five
possible geometric isomers of 6, the only isomer that
1
1
1
can show two signals in the H NMR spectrum for Cp*
The second product was the bis(germylene)-bridged
diiron complex Cp*2Fe(CO)2(µ-Gep-TolH)2 (6). Five
geometric isomers are possible for complex 6 as shown
in Figure 1. We have previously found that the bulki-
ness of the germanium substituents plays an important
role in determining the geometry of the isomer.4,14
is the syn-trans one. Another piece of evidence for that
structure is the IR spectrum that showed a medium
band assigned to a symmetric νCO at 1930 cm-1, while
the asymmetric stretch at 1889 cm-1 was found to be a
strong band. The 1H NMR spectrum of the other isomer
in this mixture showed only one signal for Cp*, one
methyl signal of p-tolyl groups, two doublet signals in
the aromatic region, and one signal for GeH; i.e., syn-
cis(H), anti-trans, and syn-cis(p-Tol) isomers were can-
didates for the other isomer in this mixture because all
14
Thus, the analogous complex Cp2Fe(CO)2(µ-Get-BuH)2
was isolated as an anti-trans isomer, while Cp*2Fe2-
(14) Kawano, Y.; Sugawara, K.;. Tobita, H.; Ogino, H. Chem. Lett.
1994, 293.

Photoreaction of Cp*Fe(CO)2SiMe3
Organometallics, Vol. 15, No. 23, 1996 4957
Ta ble 1. Va lu es of th e Equ ilibr iu m Ra tio,
Equ ilibr iu m Con sta n t (K), a n d Ra te Con sta n ts (k1,
k-1) of cis f tr a n s Isom er iza tion of Com p lex 5 a t
298.2, 308.2, 313.2, 318.2, a n d 328.2 K
temp/K
cis/trans
K
k1/s-1
k-1/s-1
298.2
308.2
313.2
318.2
328.2
5.4:94.6
5.6:94.4
5.8:94.2
6.1:93.9
6.7:93.3
17.5
16.9
16.2
15.4
13.9
8.74 × 10-5
1.49 × 10-4
2.23 × 10-4
3.26 × 10-4
7.90 × 10-4
4.99 × 10-6
8.84 × 10-6
1.37 × 10-5
2.12 × 10-5
5.68 × 10-5
exhibits two weak bands at 1998 and 1898 cm-1 that
can be assigned to νGeH and νGe-H-Fe, respectively,
besides two intense bands of νCO at 1930 and 1921 cm-1
.
1
Both the H NMR and IR spectrum of complex 7 were
found to be very similar to those of the analogous
complex 1,4 which was characterized by X-ray crystal-
lography.
These results differ markedly from the analogous
reaction of Cp*Fe(CO)2SiMe3 (3) in the presence of
p-TolSiH3 that afforded only the mono(silylene)-bridged
diiron complex Cp*2Fe2(CO)2(µ-CO)(µ-Sip-TolH) (2) as
a cis-trans mixture.10
Th er m olysis a n d P h otolysis of Com p lex 7. Un-
like complex 1 that shows the same behavior during
either thermolysis or photolysis, affording only one
isomer of Cp*2Fe2(CO)2(µ-Get-BuH)2 (syn-cis(H)) with
evolution of H2 gas, the behavior of complex 7 in
thermolysis is different from the photolysis reaction. As
expected, the photolysis of complex 7 afforded a mixture
of syn-trans-6 and anti-trans-6 complexes (eq 2) with
F igu r e 3. 1H NMR spectral changes in the Cp* signal
region in C6D6 on thermolysis of complex 5 at 55 °C: (∆)
cis-5, (0) trans-5.
of them can show this 1H NMR spectrum. In the IR
spectrum, syn-cis(H) and syn-cis(p-Tol) isomers must
show both symmetric and asymmetric modes, while the
anti-trans isomer must show only the asymmetric mode
of the terminal carbonyl. Experimentally only one
strong band for νCO was observed at 1876 cm-1. Thus
we assigned the other complex in this mixture as anti-
trans-6.
the same molar ratio as in the original reaction shown
in eq 1. In contrast, thermolysis of complex 7 afforded
the third isomer of complex 6 which adopted the syn-
cis(H)-6 configuration (eq 3). So far, the mechanism of
1
The H NMR signals of the aromatic protons of syn-
trans-6 and anti-trans-6 complexes were found to be
temperature-dependent, as shown in Figure 2. The
spectrum at 218 K (in CDCl3) appeared to show all the
eight doublet resonances of the aromatic protons of
p-tolyl groups for the two isomers. By a gradual
increasing of the temperature, the eight doublet reso-
nances converted to four doublet resonances. The
missing of one doublet in both cases is attributable to
the overlapping with the solvent peak, which also did
not allow us to assign each signal to which com-
plex, especially at low temperature. Such behavior
apparently results from the hindered rotation of p-
tolyl groups in the presence of the adjacent bulky Cp*
groups.
this dehydrogenation reaction is not clear. But if the
thermal reaction is an intramolecular concerted process,
the syn-cis(H) configuration of the product is consistent.
The photolysis of syn-cis(H)-6 isomer afforded a mixture
of syn- and anti-trans-6 with the molar ratio of 61:39
(eq 4). This isomerization was found to proceed rela-
tively rapidly even under the usual fluorescent light.
The third product was Cp*2Fe2(CO)2(µ-Gep-TolH2)2 (7)
with two Fe‚‚‚H‚‚‚Ge 3-center 2-electron bonds that was
1
isolated in 23% yield. In the H NMR spectrum of 7 in
C6D6, the signals for bridging hydrides and terminal
Ge-H’s appear as two singlets with equal intensity at
-12.74 and 3.81 ppm, respectively. The unusual high-
field shift of the latter signal is attributable to the ring
current effect of Cp* ligands. The IR spectrum of 7
Rever sible Th er m a l a n d P h otoch em ica l In ter -
con ver sion betw een cis a n d tr a n s Isom er s of th e
Ger m ylen e-Br id ged Diir on Com p lex Cp *2Fe2(CO)2-
(µ-CO)(µ-Gep-TolH) (5). The cis and trans isomers of
complex 5 were found to undergo interconversion either

4958 Organometallics, Vol. 15, No. 23, 1996
El-Maradny et al.
Ta ble 2. Activa tion P a r a m eter s ∆Hq, ∆Sq, a n d ∆Gq for th e Isom er iza tion of Com p lexes 5 a n d 2
cis-5 f trans-5
trans-5 f cis-5
cis-2 f trans-2a
trans-2 f cis-2a
∆Hq/kJ mol-1
55.5 ( 5.8
-137.2 ( 18.6
96.4 ( 12.2
62.3 ( 6.4
-138.4 ( 20.6
103.6 ( 13.5
71.1 ( 6.4
-90.3 ( 20.5
98.0 ( 12.5
80.6 ( 9.1
-90.0 ( 30.7
107.4 ( 18.2
∆Sq/J K-1 mol-1
∆Gq298/kJ mol-1
a
Data are taken from ref 15.
k-1) are summarized in Table 1. A benzene solution of
complex 5 (with molar ratio of cis and trans isomers of
84:16) was used to study the thermal isomerization by
monitoring the electronic spectral changes of this mix-
ture at 25.0, 35.0, 40.0, 45.0, and 55.0 °C. Table 2 shows
the parameters of activation of both cis f trans and
trans f cis isomerization. These results indicate that
the trans isomer is thermodynamically more stable than
the cis isomer, apparently due to the large steric
repulsion between the two Cp* rings with the p-tolyl
group in the latter as in the case of the isostructural
silylene complex Cp*2Fe2(CO)2(µ-CO)(µ-Sip-TolH) (2).15
The large negative entropies of activation imply par-
ticipation of solvent which has been also proposed for
the cis f trans isomerization of Cp2Fe2(CO)2(µ-CO)(µ-
CH2)6a,b and 2.15 In comparison with the analogous
silylene complex 2, the ∆Hq values of the isomerization
of complex 5 are lower. The proportion of the cis isomer
in equilibrium is larger in 5 (5%) than in the silicon
analogue 2 (2%), apparently due to the longer Fe-Ge
bond compared to the Fe-Si bond.10,14 It must be also
noted that a benzene-d6 solution of complex 5 (the molar
ratio of cis:trans equals 5:95) reached the photostation-
ary state (cis:trans ) 84:16) only after 1 min of irradia-
tion. This is significantly faster than the corresponding
process of 2, which takes ca. 20 min.15
thermally or photochemically (eq 5). This behavior is
closely analogous to that of the silicon analogue of 5
which we reported previously,15 but the rate of the
interconversion in 5 apparently is faster. The complexes
with formula Cp2Fe2(CO)2(µ-CO)(µ-ER2) (E ) C, Si, or
Ge; R ) H, aliphatic, or aromatic groups) are known to
exist usually as a mixture of cis and trans isomers in
solution as in the cases of Cp2Fe2(CO)2(µ-CO)(µ-CMeH),6c
Cp2Fe2(CO)2(µ-CO)(µ-SiMeH),7 and Cp2Fe2(CO)2(µ-CO)-
(µ-Ge(p-Tol)2),5 and it is rare to find only one isomer in
solution as in the case of Cp2Fe2(CO)2(µ-CO)(µ-Sit-
1
BuH).10 Figure 3 shows the H NMR spectral change
in the area of the Cp* signals accompanying the thermal
isomerization of complex 5 at 55 °C. Starting from the
photostationary state mixture with the cis:trans ratio
of 84:16, the cis:trans ratio changed gradually with time
during thermolysis, where the signals of the trans
isomer slowly increased in parallel with the decrease
in the signals of the cis isomer, and the final (equilib-
rium) ratio of cis:trans became 7:93. At room temper-
ature, a benzene-d6 solution of complex 5 reached
equilibrium after 24 h. The equilibrium ratios, the
equilibrium constants (K), and the rate constants (k1,
Ack n ow led gm en t. This work was supported by a
Grant-in-Aid for Scientific Research on Priority Area of
Reactive Organometallics No. 07216206 from the Min-
istry of Education, Science, and Culture of J apan and a
Kurata Research Grant. We thank J apan Electronic
Metals Co., Ltd., for the gift of tetrachlorogermane.
(15) Kawano, Y.; Tobita, H.; Ogino, H. Organometallics 1992, 11,
499.
OM960504Z