Iron complexes of dioxoborolanylbutadienes

Article abstract of DOI:10.1016/S0020-1693(99)00136-X

The first [2,3-dialkyl-1,4-bis(1,3-dioxo-4,4,5,5-tetramethyl-2-borolanyl]-1,3-butadiene] Fe(CO)₃ complexes (2, alkyl=n-butyl; 3, alkyl=cyclopentyl; 4, 'alkyl'=phenyl; 5, alkyl=3-chloropropyl) have been prepared from the reaction of bis(cis-cycl

Full text of DOI:10.1016/S0020-1693(99)00136-X

www.elsevier.nl/locate/ica
Inorganica Chimica Acta 290 (1999) 159–166
Iron complexes of dioxoborolanylbutadienes
Ralf Widmaier a,1, John J. Alexander a,*, Jeanette A. Krause Bauer a, Morris Srebnik b,2,
Guillaume Desurmont b,3, Craig A. Homrighausen a
a Department of Chemistry, Uni6ersity of Cincinnati, Cincinnati, OH 45221, USA
b Department of Chemistry, Uni6ersity of Toledo, Toledo, OH 43606, USA
Received 2 July 1998; accepted 5 March 1999
Abstract
The first [2,3-dialkyl-1,4-bis(1,3-dioxo-4,4,5,5-tetramethyl-2-borolanyl]-1,3-butadiene]Fe(CO)3 complexes (2, alkyl=n-butyl; 3,
alkyl=cyclopentyl; 4, ‘alkyl’=phenyl; 5, alkyl=3-chloropropyl) have been prepared from the reaction of bis(cis-cyclooctadi-
ene)Fe(CO)3 (1) with the borolanylbutadienes. The X-ray crystal structures of 2 and 3 are reported. The geometries of the
coordinated ligands are compared with those of the free ligands. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Borolanylbutadiene complexes; Butadiene complexes; Iron complexes
1. Introduction
The X-ray crystal structure of 2,3-di-n-butyl-1,4-
bis(1,3 - dioxo - 4,4,5,5 - tetramethyl - 2 - borolanyl] - 1,3-
butadiene]Fe(CO)3 [3a] showed that the molecule has
high planarity and linearity. These structural features
make it a good candidate for evaluation as a liquid crystal
[4]. Moreover, metal complexes of S-trans borolanylbu-
tadienes might also make good liquid crystals. Since no
metal complexes of borolanylbutadienes had been previ-
ously reported, we explored reactions of these species to
Fe(CO)3 groups to show that complexation could occur.
These complexes would be expected to have (and did
indeed show) an S-cis geometry for the coordinated
butadienes. The results of these investigations are re-
ported here.
Reihlen et al. prepared the first metal complexes of
butadienes in 1930 [1]. Many examples of metal–butadi-
ene complexes are now known [2]. In 1996, Srebnik et
al. [3] prepared and characterized a series of [2,3-dialkyl-
1,4-bis(1,3-dioxo-4,4,5,5-tetramethyl-2-borolanyl]-1,3-
butadiene]Fe(CO)3 complexes (2, alkyl=n-butyl; 3,
alkyl=cyclopentyl; 4, ‘alkyl’=phenyl; 5, alkyl=3-
chloropropyl). Zirconocene-induced coupling on a series
of 1-alkenyl pinacolboronates resulted in a mix-
ture of two regioisomers: 1,3-(1,3-dioxo-2-borolanyl)-
butadienes and 1,4-(1,3-dioxo-2-borolanyl)-butadienes
both as S-trans isomers. No S-cis isomers were obtained.
The S-trans-1,4-(1,3-dioxo-2-borolanyl)butadienes could
be separated from the crude reaction mixture by crystal-
lization at −20°C. These highly crystalline borolanylbu-
tadienes are very stable to air, water and heat. Prolonged
heating at 150°C or more under an inert atmosphere did
not cause any noticeable decomposition or isomeriza-
tion.
2. Experimental
All reactions were carried out under an Ar atmo-
sphere using Schlenk techniques. Solvents were ob-
tained from ACROS or Fischer Scientific (ACS reagent
grade) and were dried before use. Benzene and
methylene chloride were refluxed over phosphorus pen-
toxide and distilled. Tetrahydrofuran (THF), pentane,
hexane and toluene were dried over potassium ben-
zophenone ketyl and distilled. Anhydrous diethyl ether
was used as received.
* Corresponding author. Tel.: +1-513-556 9200; fax: +1-513-556
9239.
1 Present address: Technical University of Berlin, Berlin, Germany.
2 Present address: Department of Natural Products, Hebrew Uni-
versity in Jerusalem, PO Box 1172, Jerusalem, 91010, Israel.
3 Present address: Hiroshima University, Higashi-hiroshima 739,
Japan.
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00136-X

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R. Widmaier et al. / Inorganica Chimica Acta 290 (1999) 159–166
Low temperature photolysis was carried out in a
able. 1 was added and the resulting yellow solution
stirred and allowed to warm to 0°C over a period of
1–2 h. An IR spectrum was then taken; if no bands due
to 1 were present, the solution was re-cooled and a
second portion of 1 added in order to consume the
borolanylbutadiene completely. When the IR spectrum
showed bands attributable to the desired product, 1 and
Fe(CO)4(CCO), the reaction was stopped. The solution
was concentrated and filtered through a fine glass frit to
yield a dark green solution; by cooling to −10°C it was
possible to crystallize Fe3(CO)12 which could then be
filtered off. The pale green solution was chro-
matographed. Two bands were eluted with hexane: the
first yellow band contained Fe(CO)4(CCO) and the
second green band contained more Fe3(CO)12. The
third yellow band contained the desired product and
was eluted with hexane/THF. Solvent was removed
from this fraction in a stream of Ar. The resulting
yellow oil was dissolved in pentane and the solvent
again removed with Ar. This procedure was repeated
twice to yield yellow crystals which were recrystallized
from pentane at −5°C and dried in vacuo at room
temperature. Solvent evaporation in vacuo results in
decomposition; only crystals pre-dried in a stream of Ar
could be dried further in vacuo without complications.
The products are stable in solution at room tempera-
ture for periods up to 4 days.
custom-made immersion well using a large Dewar as a
low-temperature bath. Acetonitrile/dry ice (about
−42°C) and n-decane/dry ice(about −30°C) were used
as cooling baths. Storage tubes made of Pyrex glass
were used as reaction vessels.
Chromatography was accomplished on alumina (ba-
sic, grade I) or on silica gel.
The borolanylbutadienes were prepared by literature
methods [3].
NMR Solvents were purchased from ACROS (C6D6,
99.5% D, without internal standard) Cambridge Iso-
tope Laboratories (C2D6CO, 99.9%, without internal
standard) and Aldrich (CDCl3, 99.8%D, with TMS as
internal standard). The following abbreviations are
used: singlet (s), doublet (d), triplet (t), multiplet (m),
coupling constant (J). Chemical shifts for H and C are
reported in parts per million downfield from TMS. For
NMR spectra in C6D6 the solvent signal was used as
the standard (l=7.15 ppm). 11B chemical shifts are
reported in parts per million downfield relative to
BF.3OEt2 as an external standard. NMR spectra were
recorded on a Bruker AC 250 MHZ instrument and the
IR spectra on a Perkin–Elmer FT-IR 1600. X-ray data
were collected on a Siemens P3/PC diffractometer.
Mass spectra were obtained on a VG-30-250 quadru-
pole mass spectrometer. Elemental analysis was per-
formed by M-H-W Laboratories, Phoenix, AZ.
2: Scale 1.00 mmol; total reaction time 1.5 h; chro-
matography on Grade I basic alumina; product eluted
with 5% THF in hexane; yield 30%; Anal. Calc. For
C27H44B2FeO7: C, 58.11; H, 7.95. Found: C, 58.00; H,
7.80%. M.p. 68–72°C; IR (n-pentane): w(CO) 2052 (s),
1991 (s), 1977 (s) cm−1. l(1H, in C6D6)=3.02 [m, 2H,
CHH), 2.75 (m, 2H, CHH), 1.64 (m, 4H, CH2), 1.47
(m, 4H, CH2), 1.08 (s, 24H, CH3), 0.96 (t, 6H, J=7 Hz,
CH2CH3), −0.12 (s, 2H, CꢀCH–B); l(13C{1H}, in
C6D6)=111.1, 82.8, 36.5, 31.5, 24.9, 24.3, 23.5, 14.3);
l(13CO{1H}, in C3D6O, 211.4 at 25°C; 207.5, 213.8 at
−65°C; l(11B)=30.4. Mass spectrum: m/e 559
(MH+), 558 (M+), 531, 503, 475, 460, 416.
3: Scale 0.005 mmol; total reaction time 1.5 h; chro-
matography on silica gel; product eluted with 1:1 THF/
hexane; yield 95%; M.p. 65–66°C; IR (n-pentane):
w(CO) 2050 (s), 1989 (s), 1976 (s) cm−1. l(1H)=3.51
[m, 2H, J=9.1 Hz, ꢀC–CH), 2.33–1.99 and 1.92–1.75
(m, 12H), 1.71–1.60 (m, 4H), CH(CH2)4, 1.09 (s, 24H,
CH3), −0.09 (s, 2H, C ꢀCH–B); l(13C{1H})=114.8,
83.0, 42.6, 36.0, 34.8, 27.8, 26.8, 24.8, 24.4). Mass
spectrum: m/e 583.3 (MH+), 582 (M+), 567, 554, 526,
498, 483, 370.
2.1. Preparation of Fe(CO)3(cis-cyclo-C8H14)2 (1)
This compound was prepared by a literature proce-
dure [5]; however, we were unable to dry the crystals in
vacuo below 0°C without their decomposing. Hence,
the workup was modified as follows: the reaction solu-
tion was cooled to −78°C, let settle for a day and the
solvent removed via syringe. Pre-cooled hexane was
added; the solution allowed to warm to −45°C, stirred
and cooled again. By repeating this procedure twice,
cis-cyclooctene (CCO) was removed almost completely.
The crystals could be stored covered by hexane over
dry ice under argon for months. The product was used
as a suspension in hexane.
2.2. Preparation of [2,3-dialkyl-1,4-bis(1,3-dioxo-
4,4,5,5-tetramethyl-2-borolanyl]-1,3-butadiene]Fe(CO)3
complexes (2, alkyl=n-butyl; 3, alkyl=cyclopentyl; 4,
‘alkyl’=phenyl; 5, alkyl=3-chloropropyl)
A typical reaction for the synthesis is given followed
by more detail for each individual compound. A solu-
tion containing a weighed amount of diborolanylbuta-
diene in hexane was cooled below −60°C under an Ar
atmosphere. The scale on which reactions were run was
dictated by the quantities of borolanylbutadienes avail-
4: Scale 0.0065 mmol; total reaction time 3 h; chro-
matography on silica gel; product eluted with THF;
yield 30%. Anal. Calc. For C31H38B2FeO7: C, 62.29; H,
6.07. Found: C, 62.86; H, 6.22%. M.p. 91–93°C; IR
(n-pentane): w(CO) 2056 (s), 1996 (s), 1985 (s) cm−1
.

R. Widmaier et al. / Inorganica Chimica Acta 290 (1999) 159–166
161
l(1H)=7.40/7.37 (d, 5H, phenyl), 6.93/6.90 (d, 5H,
phenyl), 0.94 (s, 12H, CH3), 0.87 (s, 12H, CH3), 0.36 (s,
2H, CꢀCH–B); l(13C{1H})=213.8, 207.5, 138.2, 132.7,
128.5, 127.1, 127.0, 114.6, 82.9, 30.4, 24.8, 24.0. Mass
spectrum: m/e 599 (MH+), 571, 543, 515, 416.
of 10–30°. Intensity data in the 2q range of 3.5–55° (2:
05h512, −225k522, −185l518; 3: 05h512,
05k522, −225l522) were collected using variable
speeds (3.0–29.0° min−1 with respect to ꢀ). The data
for 2 and 3 were corrected for decay, Lorentz and
polarization effects. An empirical absorption correction
was applied to the data based on measured c-scans for
3; no correction was applied to the data for 2.
5: Scale 0.0065 mmol; total reaction time 2 h; chro-
matography on silica gel; product eluted with hexane;
yield 30%; Anal. Calc. For C25H38B2Cl2FeO7: C, 50.16;
H, 6.40. Found: C, 50.46; H, 6.12%. M.p. 101–104°C;
The structures were solved by a combination of
direct methods for 2 and Patterson methods for 3 using
SHELXTL 5.03 and the difference Fourier technique
and refined by full-matrix least squares on F2. All
unique reflections were used in the refinement of 2. In
the case or 3, 12 reflections were omitted from the
refinement due to very negative F2 values or because
they suffered from systematic errors. All non-hydrogen
atoms were refined with anisotropic displacement
IR (n-pentane): w(CO) 2055 (s), 1996 (s), 1980 (s) cm−1
.
l(1H)=3.33 [t, 4H, J=6.6 Hz, CH2Cl), 2.96 (m, 2H,
ꢀC–CH2–CH2), 2.72 (m, 2H, ꢀC–CH2–CH2), 1.86 (m,
4H, –CH2–CH2–CH2Cl), 1.06 (s, 24H, CH3), −0.03
(s, 2H, CꢀCH–B); l(13C{1H})=109.7, 83.0, 44.7, 36.8,
29.2, 24.9, 24.3). Mass spectrum: m/e 599 (MH+), 514.
2.3. Crystal structure analysis
parameters. Weights were assigned as ꢀ−1=|2(Fo )+
2
(aP)2+bP, where P=0.33333Fo +0.66667Fc and
a=0.0494, b=0.0114 for 2 and a=0.0476, b=1.7539
for 3. Hydrogen atom positions were either located
directly from the electron density map or calculated
based on a geometric criterion and all were allowed to
ride on their respective atoms. Hydrogen atom isotropic
temperature factors were defined as U(C)*a=U(H),
where a=1.5 for methyl hydrogens and a=1.2 for the
remaining hydrogens. Large thermal motion indicative
of disorder is associated with a number of the methyl
groups and with O2 and O3 for 2; however a reason-
able disorder model could not be sorted out. The final
difference Fourier maps for both structures appeared
featureless with the largest residual electron density of
0.216 and 0.358 eD−3 for 2 and 3, respectively. The
refinements for 2 and 3 converged with crystallographic
agreement factors indicated in Table 1.
2
2
Crystal data and numerical details of the structure
determinations of 2 and 3 are given in Table 1. Suitable
crystals of 2 (0.30×0.30×0.10 mm) and 3 (0.85×
0.60×0.60 mm) were grown from pentane, coated with
a light film of epoxy resin and mounted on a glass fiber.
Intensity data were collected at 24°C on a Siemens
P3/PC diffractometer with graphite- monochromated
,
Mo Ka radiation (u=0.71073 A). Lattice parameters
were obtained by least squares refinement of the angu-
lar settings from 25 to 30 reflections lying in a 2q range
Table 1
Crystallographic data for 2 and 3
Compound
2
3
Empirical formula
Formula weight
Crystal system
Space group
C27H44B2FeO7 C29H44B2FeO7
558.09
monoclinic
P21/n
582.11
monoclinic
P21/c
3. Results and discussion
,
a (A)
13.187(6)
17.071(7)
14.122(4)
96.81(3)
3157(2)
4
1192
1.174
0.516
0.9410/1.0089 0.9657/1.0064
0.2117/0.2555
6274
(Rint=0.1124) (Rint=0.0529)
6274/334
9.887(7)
17.527(13)
17.471(7)
94.91(5)
3016(3)
4
1240
1.282
0.543
We tried several methods which had been successful
with other butadienes for preparing diborolanylbutadi-
ene iron tricarbonyl complexes. In particular, irradia-
tion of Fe(CO)5 and a diborolanylbutadiene in benzene
at 50°C for 2.5 days gave no product. Treatment of
Fe(CO)5 with trimethylamine N-oxide in the presence
of borolanylbutadienes in hexane, benzene, THF and
diethyl ether at temperatures from 0 to 65°C gave either
no product or only traces of product detectable by IR.
Only displacement of cis-cyclooctene (CCO) from pre-
viously prepared Fe(CO)3(CCO)2 at low temperatures
resulted in isolation of the desired complexes. As shown
in Eq. (1), byproducts of the synthesis are Fe3(CO)12
and Fe(CO)4(CCO), both of which presumably result
from decomposition of (excess) Fe(CO)3(CCO)2. Evi-
dently, the diborolanylbutadienes can displace only
,
b (A)
,
c (A)
i (°)
V (A )
3
,
Z
F(000)
Density (calc.) (Mg m−3
)
)
Absorption coeff. (mm−1
Decay correction (min./max.)
Transmission factors (min./max.)
Independent reflections
6954
Data/parameters
6942/352
1.043
R1=4.79,
wR2=10.61
R1=8.80,
wR2=17.22
Goodness-of-fit on F2
Final R indices [I\2|(I)] (%)
0.998
R1=6.75,
wR2=11.26
R1=20.25,
wR2=15.90
R indices (all data) (%)

162
R. Widmaier et al. / Inorganica Chimica Acta 290 (1999) 159–166
Table 2
consists of two singlets, indicating that the energy bar-
rier for rotation of the dioxaborolanyl ring around the
bond must be higher than that in the other derivatives
due to interactions of the five-membered ring with the
phenyl rings which are in close proximity. This also
very likely explains the appearance of one more aro-
matic peak than expected in the 13C NMR. A variable
temperature NMR study of 4 between −10 and +
45°C showed no change in the NMR spectrum. Simi-
larly, no changes were detected in the spectrum of 3
over this temperature range. Lowering the temperature
of a solution of 3 in CDCl3 to −58°C resulted in
broadening, but no splitting of the methyl signal. Cy-
clopentyl groups are apparently less effective at slowing
the dioxaborolanyl ring rotation. This may be due to
possibilities for ring folding which are not present for
phenyl rings (vide infra).
Also interesting is the chemical shift for the H at-
tached to the butadiene double bond. The chemical
shifts of these protons occur at about 6 ppm in the free
ligands and are seen at slightly negative values in
complexes 2, 3 and 5. Again, 4 shows a major deviation
with l= +0.36 ppm. Possibly responsible for this ob-
servation are the two phenyl rings which could delocal-
ize a significant amount of electron density from the
butadiene system, making the ligand a less effective
donor and the iron less negative; consequently, the
signal would be shifted less far upfield than in cases
where R is saturated. In the literature, chemical shifts
for hydrogens attached to the terminal carbons of
butadiene complexes have been reported to range from
l= +0.3 to −0.3 ppm [6].
Selected 1H NMR data for 2,3-dialkyl-1,4-bis(1,3-dioxo-4,4,5,5-te-
tramethyl-2-borolanyl)-1,3-butadiene]Fe(CO)3 complexes
Complex
R
l(s, CꢀCH, 2H) l(CH3, 24H)
(ppm)
(ppm)
2
3
4
5
n-butyl
cyclopentyl
phenyl
−0.12
−0.09
+0.36
−0.03
1.08 singlet
1.09 singlet
0.94/0.87 2 singlets
1.06 singlet
3-chloropropyl
very weakly coordinating ligands such as CCO from
Fe(CO)3, perhaps for steric reasons. Because we were
unable to dry 1 without decomposition, the reactions
were conducted using weighed amounts of the di-
oxaborolanylbutadienes to which portions of 1 were
added until the dioxaborolanylbutadiene was consumed
as indicated by the presence of unreacted 1 detected in
the IR spectrum of the reaction mixture. Thus, the
yields are based on the masses of diborolanylbutadienes
employed in the reactions.
Table 3 shows selected 13C NMR data. The only
values of some surprise are the ones for the C’s substi-
tuted with two methyl groups in the dioxaborolanyl
ring; one would expect the chemical shift to be in the
range of 60–85 ppm. In the free ligands, the shifts are
at 8291 ppm. The upfield shifts seen here may indicate
that electron delocalization in the p-system extends to
the O atoms in the dioxaborolanyl rings.
Chemical shifts for the double bond carbons in the
free ligands are in the range of 115–125 ppm for C4
and 160–172 ppm for C5 (labels from Figs. 2 and 3).
The upfield shift observed in our complexes can be
explained by the redistribution of electron density to-
ward the iron center on coordination of the ligands. In
(1)
Complexes 2–5 display IR spectra consisting of three
carbonyl stretching bands consistent with the overall Cs
symmetry of the complexes. NMR spectra were
recorded in C6D6 or C2D6O since much better resolu-
tion was obtained in these solvents than in CDCl3
where decomposition was apparent after 1–2 h. Table 2
1
reports selected H NMR data for 2–5.
As expected, 2, 3 and 5 show a singlet for the methyl
groups of the dioxaborolanyl rings indicating that some
dynamic process renders the two sets of methyl protons
equivalent. Presumably free rotation about the C–B
bond is possible. Unexpectedly, the methyl signal of 4
Table 3
Selected 13C{1H} NMR data for 2,3-dialkyl-1,4-bis(1,3-dioxo-4,4,5,5-tetramethyl-2-borolanyl)-1,3-butadiene]Fe(CO)3 complexes
Complex
R
l(C–CH3)
l(C–CH3)
l(CꢀC–B)
l(CꢀC–B)
2
3
4
5
n-butyl
cyclopentyl
phenyl
24.3
24.4
24.0/24.8
24.3
31.5
34.8
30.4
29.2
82.8
83.0
82.9
83.0
111.1
114.8
114.6
109.7
3-chloropropyl

R. Widmaier et al. / Inorganica Chimica Acta 290 (1999) 159–166
163
iron [54Fe (5.8%), 56Fe (91.7%) and 57Fe (2.2%)] and C
[12C (98.9%) and 13C (1.1%)]. Table 4 presents a com-
parison of the calculated and measured data for this
peak. Only 16O (99.763% abundance) was assumed to be
present in the calculations. For example, the relative
abundance of a peak containing 16O7 and one contain-
ing 16O618O is 1: 2×10−3. Likewise, the natural abun-
dance of deuterium has been ignored since the relative
Table 4
Mass spectrometry data for parent peak of 5
Peak mass (Da) Calculated abundance
Measured abundance
594
595
596
597
598
599
600
601
602
603
604
605
0.29
2.52
0.32
2.6
11.0
44.7
100.0
55.0
65.8
22.3
12.8
3.0
10.54
42.51
100.00
54.75
67.54
23.31
13.67
3.49
1
abundance of a peak containing H44 and one contain-
1
2
ing H43H1 is 1: 1.6×10−4. The 12 calculated peaks of
highest abundance given here account for 99.91% of the
total abundance. The measured and observed parent
isotopic peaks are presented in Table 4 and displayed in
Fig. 1.
0.65
0.08
0.42
(butadiene)Fe(CO)3 the chemical shifts for C4 and C5
are seen at 40.53 and 85.49 ppm, respectively. In none
of the 13C{H} NMR spectra at room temperature in
C6D6 could peaks corresponding to CO groups be
observed, although the pulse width of the spectrometer
was increased to 7 ms and the relaxation time to 25 s.
However, the spectrum of 2 in C3D6O showed a very
weak signal at 210.8. On cooling to −65°C, two signals
were visible at 207.5 and 213.8 ppm in a 2:1 ratio.
Similar behavior has been observed for other butadiene
complexes containing the Fe(CO)3 group [7].
3.1. X-ray crystal structures of 2 and 3
The single crystal X-ray structures of 2 and 3 both
reveal h4-bonded diborolylbutadienes in the S-cis ge-
ometry on Fe(CO)3 units. Figs. 2 and 3 show ORTEP
drawings of 2 and 3, respectively. Selected bond dis-
tances are given in Table 5 and bond angles in Table 6.
The structure of (h4-butadiene)Fe(CO)3 has been de-
termined both by X-ray diffraction [8] and by gas-phase
electron diffraction [9]. The overall features of this
compound are very similar to those found for all (h4-bu-
tadiene)Fe(CO)3 complexes [10–12] including those pre-
pared by us. The geometry for (h4-butadiene)Fe(CO)3
complexes is derived from a square pyramid with axial
CO. This CO is parallel to the butadiene plane and
directed to the open side of the butadiene. The four
butadiene carbon atoms are coplanar with a distance of
about 161 pm from the Fe to the plane. The double
bonds of the organic ligand occupy two basal positions
with the other two occupied by CO’s.
In all FAB+ MS, peaks corresponding to the molec-
ular ion M+ (for 2 and 3) or MH+ for (4 and 5) could
be observed. Peaks corresponding to the loss of three
CO groups were also present in all spectra.
The general fragmentation pattern is successive loss
of CO groups, sometimes with loss of a methyl group
before the loss of the next CO. Loss of the Fe(CO)3
groups and the dioxaborolane rings also occurred.
As a check on the composition of the complexes, the
isotope pattern in the molecular ion peak of 5 was
investigated. The isotopic pattern arises from the rela-
tive abundances of the isotopes of chlorine [35Cl (75.5%)
and 37Cl (24.5%)], boron [10B (19.8%) and 11B (80.2%)],
In 2 the four butadiene carbons are planar within 0.62
pm and in 3 within 0.13 pm. Fe lies about 161 pm above
the center of the planes with nearly equal distances from
Fe to C4 and C7 and to C5 and C6. In 2 the
Fig. 1. Calculated and observed abundances for the parent peak of 5.

164
R. Widmaier et al. / Inorganica Chimica Acta 290 (1999) 159–166
Fig. 2. ORTEP view of (2,3-di-n-butyl-1,4-bis(1,3-dioxo-4,4,5,5-tetramethyl-2-borolanyl)-1,3-butadiene)Fe(CO)3 (2).
Fig. 3. ORTEP view of (2,3-dicyclopentyl-1,4-bis(1,3-dioxo-4,4,5,5-tetramethyl-2-borolanyl)-1,3-butadiene)Fe(CO)3 (3).
Fe–Couter bonds are nearly equal and at the average
of the range observed for symmetrically substituted
(butadiene)Fe(CO)3 complexes (Table 7). The Fe–
Cinner bonds differ by 2.4 pm and both are shorter by
4.55 pm than the Fe–Couter bonds. In contrast the
Fe–Couter bonds in 3 differ by 1.1 pm and the Fe–
Cinner bonds by about half that (0.5 pm).
The angle Cbasal–Fe–Cbasal for 2 and 3 is smaller
than the usual range seen in (diene)Fe(CO)3 com-
plexes (Table 7). In 2 the Fe(CO)3 unit is almost
symmetric and close to the average structure. 3 ex-
hibits a fairly distorted Fe(CO)3 unit with one axial–
basal angle close to the average, one at the lower
limit and the basal–basal angle below the lower limit.
In 2,3-di-n-butyl-1,4-bis(1,3-dioxo-4,5,5-tetramethyl-
2-borolanyl)buta-1,3-diene [3a] the heterocycle exhibits
a slight envelope conformation with one carbon 36
pm out of the plane defined by the remaining four
atoms. The dioxaboloranyl rings in 2 have geometries
similar to those in the free ligand, however the com-
plexes do tend to be less planar. C9 lies 38.5 pm
below the plane defined by the other four atoms
while C23 lies 47.0 pm below the corresponding plane
in the second ring. The dihedral angles between the
butadiene and the nearest dioxaborolanyl rings are
20.0 and 18.1° in contrast to the 4.48° observed for
the free ligand. All the non-H substituents on the
butadiene are displaced toward the Fe atom (20 pm

R. Widmaier et al. / Inorganica Chimica Acta 290 (1999) 159–166
165
for B1, 31 pm for B2, 8 pm for C14 and 4 pm for
C18).
configuration with C19 twisted out of the plane defined
by C20–C21–C22–C23; the other is twisted with C17
and C18 above and below the plane defined by C14–
C15–C16. The dioxaboloranyl rings in 3 have ge-
ometries similar to those found in 2, C9 extends out of
the plane by 40.2 while C25 lies 40.8 pm out of the
corresponding plane. The dihedral angles between the
butadiene and the nearest dioxaborolanyl rings are 22.2
and 48.7°, the larger angle corresponding to the ring
nearer the twisted cyclopentyl group. Note that in the
free ligand the corresponding angle is 94.4°, a configu-
ration which is almost orthogonal but more transoid
than cisoid. Three of the butadiene substituents in 3 are
displaced toward Fe: B1 by 45.7 pm, C14 by 4.5 pm
and B2 by 34.3 pm. C19 is displaced 14.0 pm away
from Fe.
The crystal structure of 2,3-dicyclopentyl-bis-1,4-(1,3-
dioxo-4,5,5-tetramethyl-2-borolanyl)-1,3-butadiene [3b]
shows a twisted conformation for the pinacol borate
ring. The cyclopentyl rings adopt a slight envelope
conformation with C1 deviating by 53.7 pm. In con-
trast, the coordinated 2,3-dicyclopentyl-1,4-bis(2,3-
dioxo-4,4,5,5-tetramethyl-2-borolanyl)-1,3-butadiene in
3 exhibits different conformations for the cyclopentyl
groups. One cyclopentyl group adopts an envelope
Table 5
,
Selected bond distances for 2 and 3 (A)
2 (R=n-butyl)
3 (R=cyclopentyl)
Fe–C1
Fe–C2
Fe–C3
C1–O1
C2–O2
C3–O3
Fe–C4
Fe–C5
Fe–C6
Fe–C7
C4–C5
C5–C6
C6–C7
1.787(7)
1.779(7)
1.785(6)
1.144(6)
1.143(6)
1.138(5)
2.123(5)
2.067(5)
2.091(5)
2.126(5)
1.440(6)
1.421(6)
1.442(6)
Fe–C1
Fe–C2
Fe–C3
C1–O1
C2–O2
C3–O3
Fe–C4
Fe–C5
Fe–C6
Fe–C7
C4–C5
C5–C6
C6–C7
1.790(3)
1.783(3)
1.800(3)
1.147(3)
1.138(3)
1.141(3)
2.115(3)
2.086(3)
2.091(3)
2.104(3)
1.447(3)
1.438(3)
1.449(3)
4. Supplementary material
Crystallographic data (excluding structure factors)
for structures 2 and 3 reported in this paper have been
deposited with the Cambridge Crystallographic Data
Centre as supplementary publication no. CCDC-103316
and CCDC-103317, respectively. Copies of the data can
be obtained free of charge on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (fax: int. code
+ 44(1223)336-033; e-mail: deposit@ccdc.cam.ac.uk).
Structure factors are available upon request from au-
thor J. K-B.
Table 6
Selected bond angles for 2 and 3 (°)
2 (R=n–butyl)
3 (R=cyclopentyl)
Acknowledgements
C1–Fe–C2
C1–Fe–C3
C2–Fe–C3
100.9(3)
101.2(3)
88.4(3)
C1–Fe–C2
C1–Fe–C3
C2–Fe–C3
102.86(13)
86.94(13)
99.85(13)
Dr Elwood Brooks obtained the 11B NMR spectra.
Mass spectra were recorded by James Carlson. R.W.
thanks the German Academic Exchange Service
(DAAD) for financial support and the University of
Cincinnati for a scholarship. We are grateful to a
referee for useful suggestions.
Table 7
Selected structural parameters for symmetrically substituted (butadi-
ene)Fe(CO)3 complexes
Bond lengths
Range (pm)
Mean (pm)
References
Fe–C(inner)
Fe–C(outer)
C–C(inner)
C–C (outer)
203–208
210–216
138.6–142.6
139.4–143.4
205
213
140.9
141.9
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Range (°)
Mean (°)
Cbasal–Fe–Cbasal
Cbasal–Fe–Caxial
89–94
95–103
92
100
Displacements of substituents out of the butadiene plane (pm)
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C–1,C–4 syn
C–1, C–4 anti
C–2, C–3
−15 to +20
+50 to +65
−5 to −15

166
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