Bis(alkylcyclopentadienyl)cadmium complexes and alkylcyclopentadienyl(mesityl)cadmium: Synthesis and Structure

Article abstract of DOI:10.1021/om0510021

Cadmium diiodide tetrahydrofuran adduct was prepared from the elements in tetrahydrofuran and converted to bis(alkylcyclopentadienyl)cadmium complexes with sodium tetraisopropylcyclopentadienide or sodium 1,2,4-tri(terf-butyl) cyclopentadienide. Both prod

Full text of DOI:10.1021/om0510021

Organometallics 2006, 25, 3175-3178
3175
Bis(alkylcyclopentadienyl)cadmium Complexes and
Alkylcyclopentadienyl(mesityl)cadmium: Synthesis and Structure
Dirk Bentz, Gotthelf Wolmersha¨user, and Helmut Sitzmann*
Fachbereich Chemie der TU Kaiserslautern, Erwin-Schro¨dinger-Strasse, 67663 Kaiserslautern, Germany
ReceiVed NoVember 21, 2005
Cadmium diiodide tetrahydrofuran adduct was prepared from the elements in tetrahydrofuran and
converted to bis(alkylcyclopentadienyl)cadmium complexes with sodium tetraisopropylcyclopentadienide
or sodium 1,2,4-tri(tert-butyl)cyclopentadienide. Both products are fluxional in solution, and for bis{tri-
(tert-butyl)cyclopentadienyl}cadmium η1,η1-bonding has been observed in the solid state. With 1 equiv
each of sodium 1,2,4-tri(tert-butyl)cyclopentadienide and mesitylmagnesium bromide the aryl complex
mesityl{tri(tert-butyl)cyclopentadienyl}cadmium could be obtained from cadmium diiodide in moderate
yield.
Introduction
Due to its position in the periodic table cadmium is a member
of the d-block elements, but is normally not regarded as a
transition metal, because it always retains a d10 electron
configuration in its compounds.1 Therefore, several textbooks
treat cadmium organyls within chapters on organometallic
compounds of main group elements (e.g., refs 2, 3). While some
bis(cyclopentadienyl)cadmium complexes with additional donor
ligands have been structurally characterized,4,5 there is no
structural information available on base-free bis(cyclopentadi-
enyl)cadmium compounds. The parent compound [Cd(C5H5)2]
is insoluble in unpolar solvents6 and has an unknown, probably
polymeric structure. Its permethylated derivative [Cd(C5Me5)2]
is pentane-soluble and light-sensitive and could not be obtained
as single crystals.7 The lack of experimental data as well as the
perspective of using alkylated cyclopentadienylcadmium com-
plexes as mild and easily available alkylcyclopentadienyl
transfer agents stimulated our interest in bis(alkylcyclopenta-
dienyl)cadmium compounds.
Figure 1. Line drawing of bis(tetraisopropylcyclopentadienyl)-
cadmium (1). The central atom shown corresponds to one of two
Cd positions very close to each other and related by a center of
symmetry. The second Cd position has been omitted for clarity.
found to decompose within a few hours upon sublimation in
an oil pump vacuum around 100 °C, which provided only a
low yield of microcrystalline sublimate. Crystals of 1 grown
from hexane solution at room temperature were twinned, and
X-ray diffraction gave only unsatisfactory results (see Experi-
mental Section; for a line drawing of 1 see Figure 1).
The Cd central atom is disordered between two positions
related by a center of symmetry and is coordinated to one of
the two five-membered rings in an η2-bonding mode (Cd-C
2.24(2) Å, 2.35(2) Å) and to the other in an η1-bonding mode
(Cd-C 2.27(2) Å). The other Cd‚‚‚C distances are all more
than 2.67 Å, in contrast to the η1/η5 situation found for bis-
(tetraisopropylcyclopentadienyl)zinc.8
NMR spectra of 1 at room temperature are similar to the
spectra of octaisopropylferrocene.9 Upon cooling of an octa-
deuterotoluene solution, however, signal coalescence was
observed at temperatures as low as 215 K for the two isopropyl
methyne protons and 210 K for the methyl groups. Cooling to
195 and finally 185 K produced still only one ring proton signal
for the two equivalent rings and was not enough for decoales-
cence to occur, while solvent mixtures of CD2Cl2 with CDCl3
Results and Discussion
When tetraisopropylcyclopentadienylsodium was reacted with
the tetrahydrofuran adduct of cadmium diiodide, light yellow,
air- and moisture-sensitive bis(tetraisopropylcyclopentadienyl)-
cadmium (1) was obtained in 75% yield (see Experimental
Section). 1 is decomposed by direct sunlight within 1-2 days,
but shows no signs of photolysis under normal fluorescent
laboratory illumination. 1 melts at 145 °C, but was repeatedly
* To whom correspondence should be addressed. Fax: (+)49-631/205-
4676. E-mail: sitzmann@chemie.uni-kl.de.
(1) Riedel, E. Anorganische Chemie, 3rd ed.; Walter de Gruyter: Berlin,
1994; p 731.
(2) Shriver, D. F.; Atkins, P. W.; Langford, C. H. Anorganische Chemie,
VCH: Weinheim, 1992; p 345.
(3) Elschenbroich, Ch.; Salzer, A. Organometallchemie, 3rd ed.; Teub-
ner: Stuttgart, 1990; p 22 ff.
(4) [(η1-C5H5)2Cd(py)2]: Smeets, W. J. J.; Spek, A. L.; Fischer, B.; van
Mier, G. P. M.; Boersma, J. Acta Crystallogr. 1987, C43, 893.
(5) [(η1-C5H5)2Cd(pmdta)] and [(η2-C5H5)2Cd(tmeda)]: Barr, D.; Ed-
wards, A. J.; Raithby, P. R.; Rennie, M.-A.; Verhorevoort, K. L.; Wright,
D. S. J. Organomet. Chem. 1995, 493, 175.
(6) Lorberth, J. J. Organomet. Chem. 1969, 19, 189.
(7) Cummins, C. C.; Schrock, R. R.; Davis, W. M. Organometallics
1991, 10, 3781.
(8) Burkey, D. J.; Hanusa, T. P. J. Organomet. Chem. 1996, 512, 165.
(9) Sitzmann, H. J. Organomet. Chem. 1988, 354, 203.
10.1021/om0510021 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 05/19/2006

3176 Organometallics, Vol. 25, No. 13, 2006
Bentz et al.
as [(C5HR4)AsCl2] (4, R ) CHMe2),11 whose structure closely
resembles 2 not only because of the η1 coordination but also
because the C-As bond is sticking out of the ring plane
approximately at a right angle like the C-Cd bond in 2 (92.5°
for 2 vs 86.9° for 4) and because of the localized intra-ring
single and double bonds (cf. caption of Figure 2). The Cd-C
bonds of 2 (2.201(2) Å) are significantly shorter than those
observed in donor complexes of bis(cyclopentadienyl)cadmium
(>2.30 Å)4,5 but still longer than the Hg-C bonds of 3 (2.153-
(3) Å), a consequence of the lanthanide contraction.
1
Because of sigmatropic shifts of the metal in solution, H
NMR spectra of 2 show a 4:18:36 signal pattern with the three
singlets corresponding to two symmetry-equivalent rings, each
one featuring two ring protons and two sets of tert-butyl groups
in a 1:2 ratio. In the 13C NMR spectra three signals are observed
for ring carbon atoms and four signals for two sets of tert-butyl
groups. The coupling constants 1JCd,C are similar for C2/C4 (the
two ring-CH units) and C3 (the CtBu unit between the two
ring-CH moieties) with 12.6 and 14.9 Hz, whereas C1/C5 (the
two vicinal CtBu groups of the five-membered ring) exhibit a
95.2 Hz coupling constant. This observation identifies C1 and
C5 (the latter is magnetically equivalent in solution as a
consequence of cadmium migration) as the most favorable
bonding partners for the metal atom, which is due to release of
steric strain because it allows one of the bulky tert-butyl
neighbors in the 1- and 2-position to bend out of the ring plane.12
This could be a reason why 2 does not adopt the η3 coordination
predicted for the bis(cyclopentadienyl)cadmium parent com-
pound.13 The cadmium satellites observed in the 1H NMR
spectra of 1 and 2 according to their intensity and to the shape
of some of the satellite signals appear to be superpositions of
satellites due to proton coupling with 111Cd and 113Cd nuclei (I
) 1/2), which are of almost equal abundance (12.75% vs
12.26%).
Figure 2. Crystal structure of the bis{tri(tert-butyl)cyclopentadi-
enyl} complex 2. Selected distances (Å) and angles (deg): Cd1-
C1 2.201(2), Cd1-C2 2.659(2), Cd1-C5 2.740(2), C1-C2 1.477-
(3), C2-C3 1.367(3), C3-C4 1.426(3), C4-C5 1.380(3), C5-C1
1.499(3), Cd1-C1-C10 116.27(15), angles between Cp plane and
lines C1-Cd1 92.8(2), C1-C10 30.3(3) (50% probability el-
lipsoids).
Scheme 1. Synthesis of Compounds 1-3
When cadmium diiodide was reacted with 1 equiv each of
tri(tert-butyl)cylopentadienylsodium and mesitylmagnesium bro-
mide, colorless, extremely air- and moisture-sensitive tri(tert-
butyl)cyclopentadienylmesitylcadmium (3) was obtained in 43%
yield. 3 melts at 105 °C, is very light-sensitive, and shows the
first signs of photolysis under fluorescent laboratory illumination
after 3 h. A few single crystals could be obtained by sublimation
for 6 h at 90 °C in a glass tube sealed under vacuum. X-ray
diffraction revealed a Cd central atom coordinated to the five-
membered ring in an η2-bonding mode and to the mesityl
fragment in a σ-bonding mode, but the quality of the data set is
unsatisfactory and a detailed discussion of structural parameters
of 3 is not warranted. NMR spectra of 3 show three singlets
corresponding to the five-membered ring, similar to 2, and three
signals for the mesityl fragment. 13C{1H} NMR spectra show
the typical resonances for the tri(tert-butyl)cyclopentadienyl
ligand except for one substituted ring carbon atom, whose signal
is hidden under the solvent peak. The 1JCd,C and 3JCd,C coupling
suitable for lower temperatures produced a colorless precipitate
with decomposition of 1.
A similar procedure gave light yellow bis{tri(tert-butyl)-
cyclopentadienyl}cadmium (2) in 79% yield as a hexane-soluble,
crystalline material, whose thermal stability is higher than that
of 1. 2 melts without decomposition at 143 °C and could be
obtained as single crystals suitable for X-ray diffraction by
sublimation at 100 °C during 12 h in a glass tube sealed under
vacuum.
The crystal structure of 2 (Figure 2) is very similar to bis-
{tri(tert-butyl)cyclopentadienyl}mercury (3).10
Both complexes exhibit an η1 coordination of both five-
membered rings to the central atom and a linear carbon-metal-
carbon arrangement (the Cd atom of 2 is located on a
crystallographic center of symmetry). Bonding may include a
weak ring-Cd π donation, tilting the ring toward the metal as
discussed and illustrated by Schrock et al.,7 but the Cd-C
distances are 21% and 24% longer for C2 and C5 than for C1
(cf. caption of Figure 2). The observed metal-ring geometry
parallels that of main group cyclopentadienyl complexes such
(11) Ehleiter, Y.; Wolmersha¨user, G.; Sitzmann, H.; Boese, R. Z. Anorg.
Allg. Chem. 1996, 622, 923.
(12) The direct migration of Cd from C1 to C5 would require the tert-
butyl group bound to C1 to come back into the ring plane, while the alkyl
group attached to C5 is tilted out of the ring plane. The activation barrier
t
for such a passage of vicinal Bu groups past each other should be rather
high and may likely result in an oscillation with reversal of the migration
direction at C1 and C5, which divides this activation energy in two separate
parts. A similar example has been observed for [(C5HR4)AsCl2] (R )
CHMe2),11 where NMR spectra indicate an oscillation of the AsCl2
substituent between C1 and C4 via C5 and not via C2/C3.
(10) Sitzmann, H.; Wolmersha¨user, G. Z. Anorg. Allg. Chem. 1995, 621,
109.
(13) Budzelaar, P. H. M.; Engelberts, J. J.; van Lenthe, J. H.
Organometallics 2003, 22, 1562.

Bis(alkylcyclopentadienyl)cadmium Complexes
Organometallics, Vol. 25, No. 13, 2006 3177
satellites, 4C, ring-CCHMe2, 1JCd,C ) 16.6 Hz), 132.7 (s, 4C, ring-
CCHMe2), 74,7 (s with satellites, 2C, ring-CH, 1JCd,C ) 150.0 Hz),
29.0 (s, 4C, CHMe2), 27.5 (s, 4C, CHMe2), 27.4 (s with satellites,
3
3
4C, CH3, JCd,C ) 9.3 Hz), 24.4 (s with satellites, 4C, CH3, JCd,C
3
) 13.9 Hz), 24.2 (s with satellites, 4C, CH3, JCd,C ) 11.1 Hz),
23.9 (s with satellites, 4C, CH3, JCd,C ) 14.8 Hz).
3
Bis{tri(tert-butyl)cyclopentadienyl}cadmium (2). To a solution
of cadmium diiodide tetrahydrofuran adduct (438 mg, 1 mmol) in
20 mL of tetrahydrofuran was added 2 equiv (512 mg, 2 mmol) of
sodium tri(tert-butyl)cyclopentadienide. The solution turned yellow
immediately and the solvent was evaporated to dryness. Extraction
with hexane (25 mL), filtration, and evaporation gave a light yellow,
microcrystalline powder (456 mg, 0.79 mmol, 79%), which melted
at 143 °C under argon and sublimed slowly above 100 °C in an
evacuated and sealed capillary. Anal. Calcd for C34H58Cd
1
(579.24): C, 70.50; H, 10.09. Found: C, 70.2; H, 10.1. H NMR
Figure 3. Line drawing of mesityl(tetraisopropylcyclopentadienyl)-
cadmium (3).
(400 MHz, 298 K, C6D6): δ 6.39 (s with satellites, 4H, ring-CH,
2JCd,H ) 9.2 Hz), 1.344 (s, 36H, CH3), 1.341 (s, 18H, CH3). 13C-
{1H} NMR (100 MHz, 298 K, C6D6): δ 139.0 (s with satellites,
constants are comparable with those observed for 2. The mesityl
fragment produces six resonances as expected.
2C, CCMe3, 1JCd,H ) 14.9 Hz), 125.2 (s with satellites, 4C, CCMe3,
1JCd,C ) 95.2 Hz), 113.3 (s with satellites, 4C, ring-CH, JCd,C
)
1
12.6 Hz), 34.3 (s with satellites, 12C, CH3, 3JCd,C ) 13.8 Hz),34.2
Conclusions
3
(s, CMe3), 32.2 (s, CMe3), 32.1 (s with satellites, 6C, CH3, JCd,C
) 9.2 Hz).
It has been demonstrated that cyclopentadienylcadmium
compounds with bulky alkyl substituents are suitable for normal
handling, storage, and even purification and crystal growth by
sublimation.
(Mesityl){tri(tert-butyl)cyclopentadienyl}cadmium (3). To a
suspension of cadmium diiodide tetrahydrofuran adduct (438 mg,
1 mmol) in 20 mL of diethyl ether were successively added 1 equiv
(256 mg, 1 mmol) of sodium tri(tert-butyl)cyclopentadienide and
1 equiv of mesitylmagnesium bromide diethyl ether adduct. The
mixture was stirred for 3 min at room temperature and the solvent
was evaporated to dryness. Extraction with hexane (25 mL),
filtration, and evaporation gave a colorless, microcrystalline powder
(199 mg, 0.43 mmol, 43%), which melted at 105 °C under argon
and sublimed with considerable decomposition above 90 °C in an
evacuated and sealed capillary. Anal. Calcd for C26H40Cd
(465.01): H, 8.67. Found: H, 8.37. Due to the extreme air and
moisture sensitivity of 3, we have been unable to obtain satisfactory
The different thermal stability of the CdC34H58 isomers
[{C5H(CHMe2)4}2Cd] (1) and [{C5H2(CMe3)3}2Cd] (2) may be
attributable to differences in the steric peoperties of the two
C17H29 ligands employed in this work. The η1,η1-bonded
structure of 2 illustrates the close relationship between cadmium
and mercury and contrasts with expectations based on theoretical
calculations, which predict an η3,η3-bonded structure for the
parent compound.
The formation of mesityl{tri(tert-butyl)cyclopentadienyl}-
cadmium by sequential reaction of cadmium diiodide with
sodium 1,2,4-tri(tert-butyl)cyclopentadienide and mesitylmag-
nesium bromide is likely to proceed via a mono(ring)cadmium
iodide intermediate.
1
carbon values in this case. H NMR (400,13 MHz, 298 K, C6D6):
δ 6.82 (s, 2H, ring-CH), 6.50 (s with satellites, 2H, ring-CH, 2JCd,H
) 14.3 Hz), 2.39 (s, 6H, ring-Me), 2.17 (s, 3H, ring-Me),1.49 (s,
18H, CMe3), 1.32 (s, 9H, CMe3). 13C{1H} NMR (100 MHz, 298
K, C6D6): δ 160.1 (s, 1C, ipso-C, mesityl, intensity too low for
observation of Cd satellites), 153.8 (s, 2C, mesityl-ring ortho-CMe),
Experimental Section
1
144.5 (s with satellites, 2C, C-CMe3, JCd,C ) 14.6 Hz), 132,7 (s,
Cadmium Diiodide Tetrahydrofuran Adduct. Iodine (8.00 g,
31.5 mmol) and freshly cut pieces of cadmium metal (4.00 g, 35.6
mmol) were stirred with refluxing tetrahydrofuran overnight. The
warm solution was filtered and allowed to cool to room temperature.
The product crystallized as colorless needles, which were isolated
from the mother liquor and dried in an oil pump vacuum to yield
12.3 g (28 mmol, 89%) of white powder. Anal. (%) Calcd for C4H8-
CdI2O (438.33 g/mol): C 10.96, H 1.84. Found: C 11.1, H 2.0.
Bis(tetraisopropylcyclopentadienyl)cadmium (1). To a solution
of cadmium diiodide tetrahydrofuran adduct (438 mg, 1 mmol) in
20 mL of tetrahydrofuran was added 2 equiv (512 mg, 2 mmol) of
sodium tri(tert-butyl)cyclopentadienide. The solution turned yellow
immediately and the solvent was evaporated to dryness. Extraction
with hexane (25 mL), filtration, and evaporation gave a light yellow,
microcrystalline powder (432 mg, 0.75 mmol, 75%), which melted
at 145 °C under argon and sublimed slowly with concomitant and
predominant decomposition above 100 °C in an evacuated and
sealed capillary. Anal. Calcd for C34H58Cd (579.24): C, 70.50; H,
10.09. Found: C, 69.9; H, 10.0. 1H NMR (400 MHz, 298 K,
1C, mesityl-ring-para-CMe), (1C, C-CMe3 and mesityl-ring-CH,
both hidden under the solvent signal), 120.0 (s with satellites, 2C,
1
Cp-ring-CH, JCd,C ) 13.2 Hz), 35.1 (s with satellites, 6C, CH3,
1JCd,C ) 15.9 Hz), 34.5 (s, 2C, CMe3), 33.2 (s, 2C, mesityl-ring-
ortho-CH3), 30.6 (s, 1C, mesityl-ring-para-CH3), 30.4 (s with
1
satellites, 3C, CH3, JCd,C ) 9.7 Hz), 29.7 (s, 1C, CMe3).
X-ray Crystallography. Bis(tetraisopropylcyclopentadienyl)-
cadmium (1). Attempted crystal structure determination on a
twinned crystal of the cadmium complex C34H58Cd (1): IPDS
diffractometer with imaging system (Stoe), Mo KR radiation, λ )
71.073 pm, 2θmax ) 51.34°, T ) 193(2) K, crystal dimensions 0.21
× 0.12 × 0.07 mm3, triclinic, space group P1h (No. 2), lattice
parameters (193 K): a ) 8.461(2) Å, b ) 9.543(2) Å, c ) 11.470-
(2) Å, R ) 109.24(2)°, â ) 109.48(3)°, γ ) 97.19(3)°, V ) 795.2-
(3) Å3, Z ) 1, Fcalc ) 1.210 g cm-1, µ(Mo KR) ) 7.05 cm-1, no
absorption corrections, transmission factors 0.79654 to 0.94494,
structure solution: direct methods (SHELXS-97), full-matrix least-
2
squares refinement based on Fo (SHELXL-97), 5121 reflections,
2
2044 unique reflections, 172 parameters, R1(2821 Fo > 2σ(Fo)) )
0.0647, overlapping reflections due to twinning were omitted for
refinement, wR2(all) ) 0.1737, disorder of the Cd atom.
Bis{tri(tert-butyl)cyclopentadienyl}cadmium (2). Crystal struc-
ture determination of the cadmium complex C34H58Cd (2): IPDS
C6D6): δ 4.67 (s with satellites, 2H, ring H, JCd,H ) 86.9 Hz),
3
3.18 (m, 4H, CHMe2), 3.05 (m, 4H, CHMe2), 1.39 (d, J(HH) )
7.34 Hz, 12 H, CH3), 1.37 (d, 3J(HH) ) 7.33 Hz, 12 H, CH3), 1.16
(d, 3J(HH) ) 6.75 Hz, 12 H, CH3), 1.04 (d, 3J(HH) ) 6.75 Hz, 12
H, CH3). 13C{1H} NMR (100 MHz, C6D6): δ 134.4 (s with

3178 Organometallics, Vol. 25, No. 13, 2006
Bentz et al.
diffractometer with imaging system (Stoe), Mo KR radiation, λ )
71.073 pm, 2θmax ) 53.56°, 276 images with 0° e φ e 250.2° and
∆φ ) 0.9°, T ) 193(2) K, crystal dimensions 0.25 × 0.25 × 0.16
mm3, monoclinic, space group P21/n (No. 14), lattice parameters
(193 K): a ) 8.9989(5) Å, b ) 11.6066(9) Å, c ) 15.7313(10) Å,
free of charge via www.ccdc.cam.ac.uk/data_request/cif, by e-
mailing data_request@ccdc.cam.ac.uk or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Rd., Cambridge CB2
1EZ, UK; fax +44 1223 336033 (CCDC xxxxxxxxxxx).
â ) 98.804(7)°, V ) 1673.72(19) Å3, Z ) 2, Fcalc ) 1.185 gcm-1
,
Acknowledgment. The authors thank Professor O. J. Scherer
for his friendly support.
µ(Mo KR) ) 6.91 cm-1, no absorption corrections, transmission
factors 0.8463 to 0.8975, structure solution: direct methods (SIR97),
2
full-matrix least-squares refinement based on Fo (SHELXL-97),
Supporting Information Available: Crystallographic data,
labeling diagrams, tables giving atomic positions and anisotropic
thermal parameters, bond distances, and angles are available free
of charge via the Internet at http://pubs.acs.org. Structure factor
tables are available from the authors.
17 136 reflections, 3450 unique reflections, 169 parameters, R1-
(2821 Fo > 2σ(Fo)) ) 0.0284, wR2(all) ) 0.0678, residual electron
density +0.345 to -0.500 e/Å3, no disorder, CH and CH3 groups
have been refined as rigid groups with variable torsion angles (C-H
) 0.96 Å, C-C-H and H-C-H ) 109.5°).
Crystallographic data have been deposited with the Cambridge
Crystallographic Data Centre; copy of the data can be obtained
OM0510021