Synthesis and structure analysis of tricarbonyl[tetrakis(1′-methylcyclopropyl)cyclobutadiene]iron(0)

Article abstract of DOI:10.1016/S0022-328X(00)00917-7

Heating of bis(1′-methylcyclopropyl)ethyne with Fe₃(CO)₁₂ yielded yellow crystals of tricarbonyl[tetrakis(1′-methylcyclopropyl)cyclobutadiene]iron (7), which were characterized by X-ray structure analysis. The complex 7 constitutes t

Full text of DOI:10.1016/S0022-328X(00)00917-7

Journal of Organometallic Chemistry 624 (2001) 110–113
www.elsevier.nl/locate/jorganchem
Synthesis and structure analysis of
tricarbonyl[tetrakis(1%-methylcyclopropyl)cyclobutadiene]iron(0)
Ingo Emme a, Thomas Labahn b, Armin de Meijere a,*
a Institut fu¨r Organische Chemie, Georg-August-Uni6ersita¨t Go¨ttingen, Tammannstrasse 2, D-37077 Go¨ttingen, Germany
b Institut fu¨r Anorganische Chemie, Georg-August-Uni6ersita¨t Go¨ttingen, Tammannstrasse 2, D-37077 Go¨ttingen, Germany
Received 18 September 2000; accepted 27 November 2000
Abstract
Heating of bis(1%-methylcyclopropyl)ethyne with Fe3(CO)12 yielded yellow crystals of tricarbonyl[tetrakis(1%-methylcyclo-
propyl)cyclobutadiene]iron (7), which were characterized by X-ray structure analysis. The complex 7 constitutes the bulkiest
metal-complexed peralkylated cyclobutadiene ever obtained. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Cyclobutadienes; Iron complexes; Alkynes; Cycloaddition; X-ray structure
1. Introduction
mixture of products from which, after flash column
chromatography on silica gel, the previously described
tricarbonyl(tetracyclopropylcyclopentadienone)iron (2)
(48%) [5], hexacyclopropylbenzene (3) (18%) [5], tetra-
cyclopropyl-p-benzoquinone (4) (3%) [6], and in addi-
The first cyclobutadienemetal complexes were re-
ported more than forty years ago by Criegee et al. and
six years later by Pettit et al. [1]. In the meantime, a
variety of methods for the synthesis of an increasing
number of such complexes has been established [2].
With sufficiently bulky substituents, the uncomplexed
cyclobutadiene can be kinetically stabilized, as demon-
strated by Maier et al. with the first preparation of the
non-complexed tetra-tert-butylcyclobutadiene [3]. How-
ever, with increasing steric demand of the substituents,
tion
the
unknown
tricarbonyl(tetracyclopropyl-
cyclobutadiene)iron (5) (1%) were isolated (Scheme 1).
Under the same conditions, the reaction of bis(1%-
methylcyclopropyl)ethyne (6) which was prepared by
twofold methylation of dicyclopropylethyne, with
Fe3(CO)12 yielded neither any of the peralkylated cy-
clopentadienone complex nor the corresponding ben-
zene or benzoquinone derivatives. The sole low
complexation of
a cyclobutadiene becomes more
difficult, thus no metal-complexed cyclobutadiene with
four quaternary alkyl substituents has as yet been de-
scribed. We now report the first formation and X-ray
crystal structure analysis of tricarbonyl[tetrakis(1%-
methylcyclopropyl)cyclobutadiene]iron(0) (7), the bulki-
est metal-complexed persubstituted cyclobutadiene
known so far.
2. Results and discussion
The reaction of dicyclopropylethyne (1) [4] with do-
decacarbonyltriiron in a sealed tube at 180°C yielded a
* Corresponding author.
E-mail address: armin.demeijere@chemie-uni-goettingen.de (A. de
Meijere).
Scheme 1.
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00917-7

I. Emme et al. / Journal of Organometallic Chemistry 624 (2001) 110–113
111
Scheme 2.
molecular weight product isolated as a crystalline yel-
low material (1.7%), showed in its H-NMR spectrum
3. Experimental
1
3.1. Reaction of dicyclopropylethyne (1) with Fe3(CO)12
two multiplets at l 0.45 and 0.97 (each 8 H) and one
singlet at l 1.18 (12 H). The 13C-NMR spectrum dis-
played signals at l 12.3 (Cquat), 16.5 (CH2), 26.8 (CH3),
93.6 (Cquat) and 216.3 (Cquat). The mass spectrum (EI,
70 eV) exhibited major peaks at m/z 408 (M+, 100%),
380 (M+−CO, 19%), 352 (M+−2CO, 39%), 324 (M+
−3CO, 61%), and 268 (M+−Fe(CO)3, 98%). Both the
NMR and the mass spectral data were consistent with
the structure 7 of a tetrasubstituted tricarbonylcyclobu-
tadieneiron complex. The IR spectrum showed promi-
nent bands at 3004, 2961, 2928 (C–H) and 2023 and
1948 cm−1 (CꢀO) which are also in accord with 7
(Scheme 2). The structure was eventually confirmed by
an X-ray analysis (Fig. 1).
In the literature [5,6] the compounds 2 and 4 were
not fully characterized and the experimental sections
The asymmetric unit cell contained two independent
molecules which do not differ significantly. The central
four-membered ring is planar (all four carbons are
,
within 0.0054 A of the plane) and, within the limits of
errors, quadratic with an average bond length of
,
1.465(3) A. In fact, the bond lengths in the central ring
Fig.
1.
Structure
of
tricarbonyl[tetrakis(1%-methylcyclo-
do differ slightly (Table 1) which agrees with theoretical
results of Chinn and Hall who predicted that those
bonds which are eclipsed with carbonyl groups should
be longer [7]. The bond lengths in the cyclobutadiene
ring of 7 are virtually the same as those reported for
tricarbonyl-1,2,3,4-tetraphenylcyclobutadieneiron com-
propyl)cyclobutadiene]iron (7) in the crystal.
Table 1
,
Selected bond distances (A) and bond angles (°) for tricarbonyl[te-
trakis(1%-methylcyclopropyl)cyclobutadiene]iron (7)
C1–C2
C3–C4
Fe–C1
Fe–C3
Fe–C5
Fe–C7
O12–C5
C4–C11
C11–C24
C11–C31
C12–C25
C12–C26
Fe–centroid
1.460(3)
1.476(3)
2.062(2)
2.076(2)
1.778(3)
1.778(3)
1.148(4)
1.482(4)
1.497(4)
1.513(4)
1.495(4)
1.512(4)
1.787(3)
C2–C3
C4–C1
Fe–C2
Fe–C4
1.450(3)
1.473(3)
2.079(3)
2.044(2)
1.784(3)
1.149(3)
1.147(4)
1.481(4)
1.505(4)
1.526(5)
1.504(4)
1.522(5)
,
plex (average bond length 1.459 A) [8].
The size of an unsubstituted cyclopropyl group is
slightly larger than that of an ethyl group. Thus a
1-methylcyclopropyl substitutent is larger than an iso-
propyl but smaller than a tert-butyl group. The pairs of
1-methylcyclopropyl substituents opposing each other
on the central ring take up the same conformation.
Two are closer to a bisected (27.4 and 31.7°) and the
other two are in a perpendicular orientation (89.6 and
89.5°) [9]. This must be due to crystal-packing forces,
since in the 1H- and 13C-NMR spectra the chemical
shifts of all methyl and all cyclopropylic methylene
protons are equal. All four substituents are bent out of
the plane of the cyclobutadiene ring away from the
tricarbonyliron group. The two groups which are closer
to a bisected orientation are bent by about 10° and the
other two by about 15° out of the plane. This feature
has also been observed in the crystal structure of tricar-
bonyl(tetraphenylcyclobutadiene)iron(0) [8].
Fe–C6
O13–C7
O11–C6
C1–C12
C11–C23
C24–C23
C12–C34
C26–C25
C1–C4–C3
C2–C1–C4
88.93(19)
90.18(19)
133.1(2)
115.4(3)
115.7(2)
60.16(19)
134.0(2)
114.7(2)
59.70(19)
C3–C2–C1
C2–C3–C4
90.46(19)
90.42(19)
135.0(2)
60.8(2)
59.05(19)
61.1(2)
133.4(2)
116.1(2)
59.19(19)
C2–C1–C12
C25–C12–C34
C34–C12–C26
C12–C25–C26
C1–C4–C11
C24–C11–C31
C11–C24–C23
C4–C1–C12
C25–C12–C26
C12–C26–C25
C24–C11–C23
C3–C4–C11
C23–C11–C31
C11–C23–C24

112
I. Emme et al. / Journal of Organometallic Chemistry 624 (2001) 110–113
are incomplete. A mixture of Fe3(CO)12 (300 mg, 0.596
mmol) and dicyclopropylethyne (1) (1.60 g, 15.1 mmol)
was heated for 2 h at 180°C in a 10 ml sealed tube.
After cooling to room temperature, Fe3(CO)12 (300 mg,
0.596 mmol) was added, and the mixture was heated for
an additional 2 h. The mixture was taken up in
dichloromethane and filtered through 10 g of Celite.
After evaporation of the solvent in vacuo and chro-
matography on silica gel (column 3.0×40 cm, pen-
tane–diethyl ether=10:1 to 2:1), gave a mixture of 3
(288 mg, 18%, based on 1) and 5 (18 mg, 1%, based on
1/3Fe3(CO)12), which could be separated by sublima-
tion under reduced pressure, 4 (62 mg, 3%, based on 1)
and 2 (656 mg, 48%, based on 1/3Fe3(CO)12). The
adding of the Fe3(CO)12 in two portions turned out to
be necessary to prevent the glass tube from bursting.
Selected data for 2. IR (KBr, cm−1): w=3012, 2049,
cooling to 0°C, the solution was treated with Me2SO4
(15.1 g, 120 mmol) and was stirred for an additional 1
h at room temperature. To the mixture was added an
ammonia solution (30% in water, 100 ml), and the
aqueous phase was extracted with Et2O (3×50 ml).
The combined organic layers were washed with 2 M
HCl (20 ml), NaHCO3 solution (5%, 20 ml) and water
(20 ml), and were dried over MgSO4. Trap-to-trap
distillation (53–55°C/16 mbar) yielded the monoalky-
lated alkyne (4.85 g, 86%) as a colorless liquid. This
alkyne (4.36 g, 36.3 mmol) and N,N,N%,N%-te-
tramethylethylenediamine (6.32 g, 54.4 mmol) were dis-
solved in 100 ml of dry hexane, and the solution was
treated dropwise with tBuLi in pentane (37.0 ml, 1.47
M, 54.4 mmol). After the solution had been stirred for
an additional 2 h at ambient temperature, Me2SO4
(8.83 g, 70.0 mmol) was added. After complete conver-
sion (1 h, room temperature) the solution was washed
with an ammonia solution (30% in water, 100 ml). The
layers were separated, the aqueous layer was extracted
with Et2O (3×50 ml), and the combined organic layers
were washed with 2 M HCl (50 ml), NaHCO3 solution
(5%, 20 ml) and water (3×20 ml) and dried over
MgSO4. Distillation (45°C/8 mbar) yielded 6 (4.11 g,
84%, 72% from 1 overall) as a colorless liquid. IR (neat,
cm−1): w=3086, 3007, 2964, 2933, 2904, 2870, 1464,
1424, 1379, 1357, 1090, 1019, 992, 717. 1H-NMR
(CDCl3): l=0.49 (AA% part of an AA%BB% system,
1
1979, 1635, 1437, 1027, 623, 592. H-NMR (250 MHz,
C6D6): l=0.62 (m, 10 H, cPr), 0.81 (m, 2 H, cPr), 1.03
(m, 2 H, cPr), 1.36 (m, 2 H, cPr), 1.59 (m, 2 H, cPr),
1.92 (m, 2 H, cPr). 13C-NMR (62.9 MHz, C6D6,
DEPT): l=6.81 (CH2, cPr), 7.17 (CH2, cPr), 7.60
(CH2, cPr), 8.08 (CH, cPr), 8.58 (CH2, cPr), 9.40 (CH,
cPr), 85.68 [Cquat, C-2(5)], 104.09 [Cquat, C-3(4)], 170.81
(Cquat, C-1), 210.27 [Cquat, CO]. MS (EI, 70 eV): m/z
(Irel): 380 (16, M+), 352 (52, M+−CO), 324 (19,
M+−2CO), 296 (64, M+−3CO), 240 [100, M+−
Fe(CO)3], 56 (12, Fe+). Anal. Calc. for C20H20FeO4
(380.2): C 63.18, H 5.30. Found: C 63.42, H 5.32%.
Calc. 380.0710 (correct HRMS).
3
3J=6.3, J=3.9 Hz, 4 H, CH2), 0.78 (BB% part of an
3
3
AA%BB% system, J=6.3, J=3.9 Hz, 4 H, CH2), 1.21
Selected data for 4. IR (KBr, cm−1): w=3084, 3007,
(s, 6 H, CH3). 13C-NMR (CDCl3, DEPT): l=6.58
1
1646, 1576, 1382, 1253, 1059, 928, 667. H-NMR (250
(Cquat, cPr), 16.22 (CH2, cPr), 24.72 (CH3), 81.45 (Cquat,
MHz, C6D6): l=0.77 (m, 8 H, cPr), 1.00 (m, 8 H,
cPr), 1.58 (m, 4 H, cPr). 13C-NMR (62.9 MHz, C6D6,
DEPT): l=8.79 (CH2, cPr), 11.15 (CH, cPr), 145.60
[Cquat, C-2(3,5,6)], 187.39 [Cquat, C-1(4)]. MS (EI, 70
eV): m/z (Irel): 268 (35, M+), 213 (48), 206 (65), 138
(100). Anal. Calc. for C18H20O2 (268.4): C 80.56, H
7.51. Found: C 80.50, H 7.51%. Calc. 268.1463 (correct
HRMS).
CꢁC). MS (EI, 70 eV): m/z (Irel): 134 (61, M+), 119 (59)
(M+−CH3), 91 (100, M+−CH3–C2H4). Anal. Calc.
for C10H14 (134.2): C 89.49, H 10.51. Found: C 89.49,
H 10.63%.
3.3. Reaction of 6 with Fe3(CO)12
Selected data for 5. IR (KBr, cm−1): w=3004, 2022,
1948, 1425, 1025, 922, 629, 597, 570. 1H-NMR (250
MHz, CDCl3): l=0.53 (m, 16 H, cPr), 0.97 (m, 4 H,
cPr). 13C-NMR (62.9 MHz, CDCl3, DEPT): l=7.00
(CH, cPr), 7.94 (CH2, cPr), 87.65 [Cquat, C-1(2,3,4)],
215.96 [Cquat, CO]. MS (EI, 70 eV): m/z (Irel): 352 (40,
M+), 324 (46, M+−CO), 296 (4, M+−2CO), 268 (46,
M+−3CO), 212 (100, M+−Fe(CO)3], 56 (24, Fe+).
C19H20FeO3 (352.2): Calc. 352.0761 (correct HRMS).
A mixture of Fe3(CO)12 (300 mg, 0.596 mmol) and 6
(1.70 g, 12.7 mmol) was heated for 2 h at 180°C in a 10
ml sealed tube. After cooling down, Fe3(CO)12 (300 mg,
0.596 mmol) was added and the mixture was heated for
an additional 2 h. The mixture was dissolved in
dichloromethane (100 ml), the solution was filtered
through 10 g of Celite. After evaporation of the solvent,
the residue was subjected to chromatography on silica
gel (column 3.0×40 cm, pentane–diethyl ether=10:1
to 2:1) to yield 7 (25 mg, 2%, based on 1/3Fe3(CO)12) as
yellow crystals. IR (KBr, cm−1): w=3004, 2961, 2928,
3.2. Preparation of 6
1
2023, 1948, 1667, 1455, 1380, 1021, 638, 602. H-NMR
To a stirred solution of 1 [4] (5.00 g, 47.1 mmol) in
Et2O (100 ml) was added dropwise at 0°C nBuLi in
hexane (42.4 ml, 2.36 M, 100 mmol), and the mixture
was stirred for 16 h at ambient temperature. After
(250 MHz, C6D6): l=0.45 (m, 8 H, cPr), 0.97 (m, 8 H,
cPr), 1.18 (s, 12 H, CH3). 13C-NMR (62.9 MHz, C6D6,
DEPT): l=12.25 (Cquat, cPr), 16.49 (CH2, cPr), 26.82
(CH3), 93.59 [Cquat, C-1(2,3,4)], 216.29 [Cquat, Fe(CO)3].

I. Emme et al. / Journal of Organometallic Chemistry 624 (2001) 110–113
113
MS (EI, 70 eV): m/z (Irel): 408 (100, M+), 380 (19,
M+−CO), 352 (39, M+−2CO), 324 (61, M+−3CO),
268 [98, M+−Fe(CO)3], 250 (35), 240 (54), 211 (34),
197 (33), 183 (50), 165 (51), 153 (32), 141 (36), 128 (36),
115 (31), 91 (30), 56 (56, Fe+). C23H28FeO3 (408.3):
Calc. 408.1387 (correct HRMS).
bridge Crystallographic Data Centre, CCDC no.
149148. Copies and information may be obtained free
of charge from The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (Fax: +44-1223-336033;
e-mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cm.ac.uk).
3.4. X-ray structure analysis of complex 7
Acknowledgements
A
suitable specimen of
7
was grown from
dichloromethane by slow evaporation of the solvent.
One of the yellow crystals (crystal size 0.50×0.40×
0.30 mm3) was mounted on a Stoe-Siemens-Huber dif-
fractometer, and the data were collected at 133 K, using
Mo–Ka radiation (graphite monochromator). Crystal
data and structure solution of 7: 2×C23H28FeO3, Mr=
408.30, monoclinic, P21/c, a=929.80(19), b=
1825.3(4), c=2503.0(5) pm, h=90, i=99.78(3),
k=90°, u(Mo–Ka)=71.073 pm, V=4186.3(15) nm3,
Z=4, dcalc=1.296 g cm−3, F(000)=1728, v=0.739
mm−1, ꢀ–2q scans (1.99BqB23.26). Out of 45117
This work was supported by the Fonds der Chemis-
chen Industrie. The authors are grateful to Dr B.
Knieriem for his careful proofreading of the
manuscript.
References
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collected reflections, 6010 were independent (Rint
0.0271). The structure was solved by direct methods
SHELXS-97) and refined by full-matrix least-square cal-
=
(
culations using all measured F2 data. All non-hydrogen
atoms were refined anisotropically. The refinement with
6010 reflections and 495 parameters converged to the
final values R1=0.0382 for I\2|(I) and R1=0.0407
for all data, wR2=0.0879 for I\2|(I) and wR2=
0.0894 for all data. The maximum and minimum resid-
ual electron densities were 708 and −292 e nm−3. The
molecular structure is shown in Fig. 1. Selected bond
lengths and angles are listed in Table 1.
[6] R. Victor, V. Usieli, S. Sarel, J. Organomet. Chem. 129 (1977)
387.
[7] (a) J.W. Chinn, Jr., M.B. Hall, Inorg. Chem. 22 (1983) 2759. (b)
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[8] R.P. Dodge, V. Schomaker, Acta Crystallogr. 18 (1965) 614.
[9] The interplanar (dihedral) angles in each case were calculated for
the following four points: [center of the cyclobutadiene ring]–[cy-
clobutadienylic carbon]–[quaternary cyclopropylic carbon]–[cal-
culated middle of the cyclopropylic CH2–CH2 bond].
4. Supplementary material
Atomic coordinates, bond lengths and angles, and
thermal parameters have been deposited at the Cam-
.