Influence of alkoxy ligands on the Cp-Al bonding mode in [Cp2Al-μ-OR]2 from X-ray crystallographic and 27 Al-NMR spectroscopic solution studies

Article abstract of DOI:10.1016/S0022-328X(02)02218-0

The dicyclopentadienylaluminium alkoxides of general formula [Cp₂Al-μ-OR]₂ where R = Me, Et, n-Bu, i-Bu, CH₂t-Bu, s-Bu, CH₂Ph, C₆H₄-4-t-Bu (1-8) were prepared by reacting CpNa with ROAlCl

Full text of DOI:10.1016/S0022-328X(02)02218-0

Journal of Organometallic Chemistry 669 (2003) 64ꢀ71
/
www.elsevier.com/locate/jorganchem
Influence of alkoxy ligands on the CpꢀAl bonding mode in [Cp2Al-m-
/
27
OR]2 from X-ray crystallographic and Al-NMR spectroscopic
solution studies
S lawomir Szumacher a, Izabela Madura a, Janusz Zachara a, Antoni Ryszard Kunicki a,*,
Artur Cebulski a, Lucjan Jerzykiewicz b
a Chemistry Department, Warsaw University of Technology, ul. Noakowskiego 3, 00-664 Warsaw, Poland
b Faculty of Chemistry, University of Wroclaw, ul. F. Joliot-Curie 14, 50-383 Wroclaw, Poland
Received 9 October 2002; received in revised form 9 December 2002; accepted 9 December 2002
Abstract
The dicyclopentadienylaluminium alkoxides of general formula [Cp2Al-m-OR]2 where Rꢁ
8) or from the
reaction of Cp3Al with alcohol (1). The compounds were characterised by multinuclear NMR (1H, 13C, 27Al) method. The molecular
/
Me, Et, n-Bu, i-Bu, CH2t-Bu, s-Bu,
CH2Ph, C6H4-4-t-Bu (1ꢀ8) were prepared by reacting CpNa with ROAlCl2 at the molar ratio 2:1, respectively (2
/
/
ꢀ
structures of compounds 2, 5 and 6 were determined by X-ray crystallography. The Cpꢀ
/
Al bonding mode in dicyclopentadie-
nylaluminium alkoxides regarding the steric demands of alkoxy groups is discussed based on 27Al-NMR chemical shifts and
structural data obtained. The meaningful correlation between Cp ring-slippage and 27Al-NMR chemical shifts was observed.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Alkoxy ligands; X-ray crystallography; CpꢀAl bond mode
/
1. Introduction
e.g. Cp2AlOiPr, were found to be dimeric with alkoxy
oxygen bridging atom and Cp ring h1 bonded to
aluminium [14,15]. Until now there are no data on
dicyclopentadienylaluminium derivatives of tertiary al-
cohols. In the reaction of t-BuOAlCl2 with excess of
cyclopentadienylsodium one of chlorine atoms only is
substituted by cyclopentadienyl ligand due to the steric
hindrance of tert-butoxy group [12]. Attempts to pre-
pare t-BuOAlCp2 from Cp3Al and tert-butyl alcohol
were unsuccessful as well [16].
Cyclopentadienylaluminium compounds have been
investigated for over 40 years [1]. Their structures and
bonding are of considerable interest because of the
ability of cyclopentadienyl ring to form derivatives with
different bonding mode to aluminium atom resulting
from low energy barrier to exchange h1l
[2]. The spectroscopic investigations in solution can not
give clear explanation of CpꢀAl bondage [3ꢀ10] thus
/
h2l
/
h3lh5
/
/
/
the proper way to examine this area are X-ray measure-
ments or gas phase diffraction studies.
Several cyclopentadienylaluminium alkoxides or
Generally, the structures of alkyl and arylaluminium
alkoxides or phenoxides have been investigated widely
for years both in solution and solid state [17]. The
studies have been undertaken to obtain information
concerning the influence of substituents bonded to
aluminium atom on valence-angle strain, degree of
phenoxides have been described so far [11ꢀ15]. Some
/
of them reveal monomeric structure that results from
steric demands of hindered 2,6-di-tert-butyl-4-methyl-
phenoxy (BHT) group with cyclopentadienyl rings h5
bonded to aluminium atom [13]. While the other ones,
aggregation, stability of the aggregates, Alꢀ
strength, etc.
In this paper we present the analysis of Cpꢀ
/O bond
/
Al
bonding mode in dicyclopentadienylaluminium alkox-
ides regarding the steric demands of alkoxy groups
based on 27Al-NMR chemical shifts and X-ray crystal-
* Corresponding author. Tel.: ꢂ
5462.
E-mail address: kunicki@ch.pw.edu.pl (A.R. Kunicki).
/
48-22-660-5581; fax: ꢂ48-22-660-
/
0022-328X/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0022-328X(02)02218-0

S. Szumacher et al. / Journal of Organometallic Chemistry 669 (2003) 64ꢀ
/71
65
lographic data. The eight new dicyclopentadienylalumi-
nium alkoxides were synthesised and characterised by
1H- and 13C-NMR spectroscopy. The crystal structures
NMR (C6D6): d 50.64 (CH3), 111,71 (C5H5). 27Al-
NMR (C6D6): dꢁ77.81 (v1/2 1421 Hz).
/
ꢁ
/
of [Cp2Al-m-OR]2 where Rꢁ
/
Et [18], CH2t-Bu, s-Bu
2.4. Synthesis of [Cp2Al-m-OEt]2 (2)
were determined by X-ray diffraction studies.
EtOH (1 ml, 17 mmol) in hexane (50 ml) was added
dropwise to a stirred solution of MeAlCl2 (17 ml, 17
mmol) in hexane (75 ml) cooled to 0 8C. During the
reaction, methane evolved. The reaction mixture was
allowed to warm-up to r.t. and stirred overnight. The
formed yellow solution was added to solid CpNa (3.79
g, 43 mmol). The reaction mixture was stirred overnight.
Hexane solution was decanted and residue twice ex-
2. Experimental
2.1. General procedures
All manipulations were performed using vacuum and
Schlenk techniques. All solvents were distilled under
argon over potassium benzophenone ketyl (toluene,
hexane, benzene and THF). The NMR solvent (ben-
tracted with hexane (50 mlꢂ
hexane solution (ca. 180 ml) was concentrated to 70
ml in vacuum and cooled to ꢃ15 8C. The product
/50 ml). The collected
˚
zene-d6) was dried over activated 4-A molecular sieves
/
and deoxygenated in vacuum. Methylaluminium
dichloride, purchased as a 1.0-m solution in hexane
from Aldrich, was used without purification. Cyclopen-
tadienylsodium was obtained in the reaction of sodium
hydride in THF or sodium in xylene with freshly
distilled cyclopentadiene. Alcohols were distilled under
argon over sodium.
precipitated as light-yellow crystalline solid. Yield 1.65
g, 48%. Anal. Calc. for C12H15AlO (202.23): C, 71.27;
H, 7.48; Al 13.34. Found: C, 71.15; H, 7.41; Al, 13.39%;
1
Mwꢁ
(C6D6): dꢁ
7.14 Hz, 2H, CH2), 6.08 (s, 10H, C5H5). 13C-NMR
(C6D6): dꢁ16.81 (CH3), 61.13 (CH2), 112.12 (C5H5).
27Al (C6D6): dꢁ
84.69 (v1/2 1497 Hz).
/
391 (cryoscopic in benzene), nꢁ/1.94. H-NMR
/
0.86 (t, Jꢁ
/
7.14 Hz, 3H, CH3), 3.13 (q, Jꢁ
/
/
NMR spectra were recorded on Varian VXR-300
1
/
ꢁ
/
(300.13 MHz H, 74.43 MHz 13C, 78.206 MHz 27Al),
1
Varian VXR-200 (199.971 MHz H, 50.283 MHz 13C,
2.5. Synthesis of [Cp2Al-m-OnBu]2 (3)
52.106 MHz 27Al) and Varian VXR-400 (400.09 MHz
1H, 100.60 MHz 13C, 104.25 MHz 27Al). Chemical shifts
are reported relative to internal solvent resonances (13C,
1H) and external [Al(H2O)6]3ꢂ (27Al).
A solution of n-BuOH (1 ml, 11 mmol) in hexane (50
ml) was added dropwise to a stirred solution of MeAlCl2
(11 ml, 11 mmol) in hexane (50 ml) cooled to 0 8C. A
reaction workup similar to the one described above for 2
afforded 3 as light-yellow solid. Yield 1.44 g, 57%. Anal.
Calc. for C14H19AlO (230.28): C, 73.02; H, 8.32; Al,
2.2. Synthesis of Cp3Al
A suspension of CpNa (15.84 g, 0.18 mol) in toluene
(50 ml) was added to a stirred toluene (50 ml) suspension
11.72. Found: C, 73.15; H, 8.39; Al, 12.28%; Mwꢁ
(cryoscopic in benzene), nꢁ
1.98. 1H-NMR (C6D6): dꢁ
0.81 (t, Jꢁ7.35 Hz, 3H, CH3), 1.04 (m, Jꢁ7.35 Hz, 2H,
CH3CH2CH2CH2O), 1.49 (m, Jꢁ7.80 Hz 2H,
CH3CH2CH2CH2O), 3.20 (t, Jꢁ8.04 Hz 2H,
CH3CH2CH2CH2O), 6.14 (s, 10H, C5H5). 13C-NMR
(C6D6): dꢁ13.81 (CH3), 18.87 (CH3CH2CH2CH2O),
33.72 (CH3CH2CH2CH2O), 65.95 (CH3CH2CH2-
CH2O), 112.52 (C5H5). 27Al (C6D6): dꢁ
85.49 (v1/2
/
455
/
/
of AlCl3 (7.87 g, 0.059 mol) and cooled to ꢃ
/
50 8C. The
/
/
reaction mixture was warmed up to room temperature
(r.t.), then heated up to 80 8C and stirred for 24 h. After
decantation of toluene solution and evaporation of
/
/
solvent in vacuum the product was isolated as the
/
1
yellow oil with 80% yield. H-NMR (C6D6): dꢁ
/
5.92
112.12 (C5H5).
79.62. The spectroscopic data is
(s, 15H, C5H5). 13C-NMR (C6D6): dꢁ
27Al-NMR (C6D6): dꢁ
/
/
ꢁ
/
/
3421 Hz).
consistent with ones given before by Shapiro et al. [19].
2.6. Synthesis of [Cp2Al-m-OiBu]2 (4)
2.3. Synthesis of [Cp2Al-m-OMe]2 (1)
A solution of i-BuOH (1 ml, 10.8 mmol) in hexane (50
ml) was added dropwise to a stirred solution of MeAlCl2
(10.8 ml, 10.8 mmol) in hexane (50 ml) cooled to 0 8C. A
reaction workup similar to the one described for 2
afforded 4 as light-yellow solid. The compound decom-
posed during X-ray measurements. Yield 1.54 g, 62%.
Anal. Calc. for C14H19AlO (230.28): C, 73.02; H, 8.32;
MeOH (0.13 ml, 0.0032 mol) in toluene (50 ml) was
added dropwise to a stirred solution of Cp3Al in toluene
(50 ml) (0.71 g, 0.0032 mol) at r.t.. The reaction mixture
was heated up to 80 8C and stirred for 3 h. The product
was isolated by crystallisation as white solid. Yield 0.48
g, 70%. Anal. Calc. for C11H13AlO (188.20): C, 70.20;
H, 6.96; Al, 14.34. Found: C, 70.43; H, 6.89; Al, 14.05%;
1
2.01. H-NMR
Al, 11.72. Found: C, 72.96; H, 8.20; Al, 11.85%; Mwꢁ
436 (cryoscopic in benzene), nꢁ
1.90. 1H-NMR (C6D6):
dꢁ0.73 (d, Jꢁ6.80 Hz, 6H, CH3), 1.71 (m, Jꢁ6.80
/
Mwꢁ
/
380 (cryoscopic in benzene), nꢁ
/
/
(C6D6): dꢁ
/
2.65 (s, 3H, CH3), 6.04 (s, 10H, C5H5). 13C-
/
/
/

66
S. Szumacher et al. / Journal of Organometallic Chemistry 669 (2003) 64ꢀ71
/
Hz, 1H, CH), 3.13 (d, Jꢁ
10H, C5H5). 13C-NMR (C6D6): dꢁ
(CH), 73.12 (CH2), 112.79 (C5H5). 27Al (C6D6): dꢁ
90.44 (v1/2 3440 Hz).
/
6.80 Hz, 2H, CH2), 6.15 (s,
NMR (C6D6): dꢁ
/
4.16 (s, 2H, CH2), 5.97 (s, 10H,
/19.50 (CH3), 30.60
C5H5), 7.08ꢀ7.33 (m, 5H, C6H5).
/
/
ꢁ
/
2.10. Synthesis of [Cp2Al-m-OC6H4-4-tBu]2 (8)
2.7. Synthesis of [Cp2Al-m-OCH2tBu]2 (5)
A solution of 4-t-BuC6H4OH (2.25 g, 15 mmol) in
toluene (50 ml) was added dropwise to a stirred solution
of MeAlCl2 (15 ml, 15 mmol) in hexane (50 ml) cooled
to 5 8C. During the reaction methane evolved. The
reaction mixture was allowed to warm to r.t. and stirred
overnight. The obtained light-yellow solution was added
to solid CpNa (3.19 g, 36.3 mmol) and the reaction
A solution of t-BuCH2OH (1.36 g, 15.4 mmol) in
hexane (50 ml) was added dropwise to a stirred solution
of MeAlCl2 (15.4 ml, 15.4 mmol) in hexane (50 ml)
cooled to 0 8C. A reaction workup similar to the one
described for 2 afforded 5 as white solid. Yield 2.71 g,
72%. Anal. Calc. for C15H21AlO (244.31): C, 73.74; H,
mixture was stirred overnight. Tolueneꢀ
was decanted and residue twice extracted with toluene
(50 mlꢂ50 ml). The collected tolueneꢀhexane solution
(c.a. 150 ml) was concentrated to 20 ml in vacuum and
cooled to ꢃ15 8C. The product precipitated as white
solid. Yield 1.61g, 35%. H-NMR (C6D6): dꢁ
9H, CH3), 6.18 (s, 10H, C5H5), 7.01 (d, Jꢁ8.64 Hz, 2H,
m-C6H4), 7.20 (d, Jꢁ
8.64 Hz, 2H, o-C6H4). 13C-NMR
(C6D6): dꢁ31.47 (CH3), 34.06 (C), 112.93 (C5H5),
121.07, 126.74 (ArꢀC, o, m), 140.74, 148.23 (ArꢀC,
p; Arꢀ
CO). 27Al (C6D6): signal is extremely broad and
/
hexane solution
8.66; Al, 11.04. Found: C, 73.59; H, 8.75; Al, 10.87%;
1
Mwꢁ
(C6D6): dꢁ
10H, C5H5). 13C-NMR (C6D6): dꢁ
(C), 76,29 (CH2), 113.76 (C5H5). 27Al (C6D6): dꢁ
(v1/2 4221 Hz).
/
480 (cryoscopic in benzene), nꢁ/1.96. H-NMR
/
/
/
0.79 (s, 9H, CH3), 3.09 (s, 2H, CH2), 6.21 (s,
26.79 (CH3), 32.62
91.34
/
/
1
/
/1.22 (s,
ꢁ
/
/
/
2.8. Synthesis of [Cp2Al-m-OsBu]2 (6)
/
/
/
A solution of s-BuOH (1 ml, 10.9 mmol) in hexane
(50 ml) was added dropwise to a stirred solution of
MeAlCl2 (10.9 ml, 10.9 mmol) in hexane (50 ml) cooled
to 0 8C. A reaction workup similar to the one described
for 2 afforded 6 as light-yellow solid. Yield 1.60 g, 64%.
Anal. Calc. for C14H19AlO (230.28): C, 73.02; H, 8.32;
/
overlapping with probe signal. The product forms
clathrates with aromatic solvents.
2.11. X-ray structure determination
Al, 11.72. Found: C, 72.91; H, 8.41; Al, 11.38%; Mwꢁ
469 (cryoscopic in benzene), nꢁ
2.04. 1H-NMR (C6D6):
dꢁ0.53 (t, Jꢁ7.44 Hz, 3H, CH3CH2CHCH3), 0.95 (d,
Jꢁ5.79 Hz, 3H, CH3CH2CHCH3), 1.21 (m, 1H, HCH),
1.58 (m, 1H, HCH), 3.50 (m, 1H, CH), 6.12 (s, 10H,
C5H5). 13C-NMR (C6D6): dꢁ
10.43 (CH3CH2CHCH3),
21.26 (CH3CH2CHCH3), 31.98 (CH2), 75.61 (CH),
113.77 (C5H5). 27Al (C6D6): dꢁ
104.09 (v1/2 3741
Hz).
/
Single crystals of 2, 5 and 6 suitable for X-ray
diffraction studies were placed in a thin-walled capil-
laries (Lindemann glass) in an inert atmosphere. The
crystallographic data, the summary of data collection
and the refinement procedure are presented in Table 1.
The measurements for crystals of 2, 5 were performed
on four-circle diffractometer Kuma KM4 and for 6 on
Siemens P3 diffractometer.
/
/
/
/
/
/
ꢁ
/
Data were collected by the v ꢀ2u technique at
/
ambient temperature (in case of 2 the study was
performed at 100 K). The measured intensities were
processed with Lorentz-polarisation effects and crystal
decomposition (absorption corrections were not neces-
sary). The structures were solved by direct methods
using SHELXS-86 program [20], completed by subsequent
difference Fourier syntheses, and refined by full-matrix
least-squares method against F2 (SHELXL-97 [21]). All
non-hydrogen atoms were anisotropically refined. The
positional and isotropic thermal parameters for hydro-
gen atoms in 2 were refined. The Cp1 ring in 5 was
disordered over two rotational positions with refined
final s.o.f. equal to 0.59(2) and 0.41(2). The hydrogen
atoms in 5 were included in calculated positions and
refined isotropically except H(1) and H(6) where posi-
tions were also refined. As we found in the refinement
process of 6, the sec-butyl group was disordered over
two opposite sites that almost mirrored against a plane
defined by the central Al2O2 ring. The disorder was
2.9. Synthesis of [Cp2Al-m-OCH2Ph]2 (7)
A solution of PhCH2OH (1 ml, 9.7 mmol) in toluene
(50 ml) was added dropwise to a stirred solution of
MeAlCl2 (9.7 ml, 9.7 mmol) in toluene (50 ml) cooled to
ꢃ78 8C. During the reaction methane evolved. The
/
reaction mixture was allowed to warm up to r.t., stirred
half an hour and then was added to solid CpNa (2.21 g,
25.1 mmol). The mixture was stirred overnight. Toluene
solution was decanted and residue twice extracted with
toluene (50 mlꢂ
(ca. 150 ml) was concentrated to 80 ml in vacuum and
cooled to ꢃ15 8C. The product precipitated as light-
/50 ml). The collected toluene solution
/
yellow solid. Due to the low solubility of the product in
deuterated benzene or trichloromethane 13C- and 27Al-
NMR spectra could not be recorded. Yield 0.56 g, 22%.
Anal. Calc. for C17H17AlO (264.30): C, 77.25; H, 6.48;
1
Al, 10.21. Found: C, 77.52; H, 6.61; Al, 10.34%. H-

S. Szumacher et al. / Journal of Organometallic Chemistry 669 (2003) 64ꢀ
/71
67
Table 1
Summary of crystal data for compounds 2, 5 and 6
2
5
6
Empirical formula
Formula weight
Temperature (K)
Radiation
C24H30Al2O2
404.44
100.0(3)
C30H42Al2O2
488.60
293(2)
C28H38Al2O2
460.54
293(3)
˚
0.71073 A), graphite-
˚
0.71073 A), graphite-
˚
Moꢀ
/
Ka (lꢁ
/
Moꢀ
/
Ka (lꢁ
/
Moꢀ
/
Ka (lꢁ0.71073 A), graphite-
/
monochromated
Monoclinic
P21/n (no. 14)
2
monochromated
Triclinic
¯
P1 (no. 2)
monochromated
Monoclinic
P21/n (no. 14)
2
Crystal system
Space group
Z
1
Unit cell dimensions
˚
a (A)
˚
9.052(2)
9.682(2)
12.285(2)
90
8.646(2)
9.337(2)
9.387(2)
70.81(3)
88.05(3)
70.57(3)
672.5(3)
1.206
8.945(1)
9.393(1)
12.399(2)
90
b (A)
˚
c (A)
a (8)
b (8)
g (8)
93.71(3)
90
103.33(1)
90
3
˚
V (A )
1074.4(4)
1.250
0.152
1340.8(3)
1.141
0.130
Dcalc (Mg mꢃ3
Absorption coefficient
(mmꢃ1
)
0.133
)
Number of reflections col-
lected
2031
3403
5464
Number of unique reflections 1896 (Rint
Data/restraints/parameters
ꢁ
/
0.0176)
3213 (Rint
3213/30/226
1.044
ꢁ
/
0.0369)
2367 (Rint
ꢁ0.0286)
/
1896/0/187
1.027
1399
2367/9/185
1.008
1483
Goodness-of-fit on F2 a
Reflections with Iꢀ
/
2s(I)
2s(I))
2366
b
Final R indices (Iꢀ
/
R1ꢁ
R1ꢁ
0.263 and ꢃ
/
0.0286, wR2ꢁ
0.0576, wR2ꢁ
0.196
/
0.0762
R1ꢁ
R1ꢁ
0.289 and ꢃ
/
0.0428, wR2ꢁ
0.0678, wR2ꢁ
0.282
/
0.1173
R1ꢁ
R1ꢁ
0.166 and ꢃ
/
0.0445, wR2ꢁ
0.0808, wR2ꢁ
0.121
/
0.1060
b
R indices (all data)
/
/
0.0859
/
/
0.1309
/
/
0.1179
Largest difference peak and
ꢃ3
ꢂ/
/
ꢂ/
/
ꢂ/
/
˚
hole (e A
)
a
Goodness-of-fit Sꢁ
/
{[w(F2oꢃ
/
F2c)2]/(nꢃ
/
p)}1/2 where n is the number of reflections and p is the total number of parameters refined.
F2c)2]/S[w(F2o)2]}1/2
R1ꢁ
/
SjjFojꢃ
/
jFcjj/SjFcj, wR2ꢁ
/
{S[w(F2oꢃ
/
.
b
modelled in terms of two sets of s-Bu groups with
refined occupancy factors 0.62(1) and 0.38(1). In case of
6 H-atoms were placed in calculated positions with
The reaction products were not isolated and used
directly in the syntheses of dicyclopentadienylaluminium
derivatives 2ꢀ8.
/
assigned thermal parameters [U(H)ꢁ
[U(H)ꢁ1.5ꢄUeq(C)] for methyl and disordered
groups.
/
1.2ꢄ
/
Ueq(C)] and
[Cp2Al-m-OMe]2 (1) was obtained from the reaction of
Cp3Al and methyl alcohol in toluene at 80 8C according
to Eq. (3).
/
/
2Cp3Alꢂ2MeOH 0[Cp2Al-m-OMe]2 ꢂ2CpH
(3)
3. Results and discussion
3.1. Synthesis
3.2. Properties and NMR analysis
The dicyclopentadienylaluminium alkoxides [Cp2Al-
The dicyclopentadienylaluminium alkoxides (1ꢀ
were isolated as white or light-yellow solids by crystal-
lisation from hexane, toluene or a tolueneꢀhexane
mixture. The compounds are air and moisture sensitive.
In presence of a slight amount of oxygen brown solids
/
8)
m-OR]2 (where Rꢁ/Et (2), n-Bu (3), i-Bu (4), CH2t-Bu
(5), s-Bu (6), CH2Ph (7), Ph-p-t-Bu (8)) were obtained
by the method described previously [14] according to
Eq. (1) from the reaction of ROAlCl2 and CpNa in
/
hexane or tolueneꢀ/hexane mixture at ambient tempera-
are formed. Compounds 2ꢀ/6 are soluble in common
ture.
hydrocarbon solvents*hexane, toluene. Derivative 7 is
/
2ROAlCl2 ꢂ4CpNa 0[Cp2Al-m-OR]2 ꢂ4NaCl
(1)
practically insoluble in hexane, hardly soluble in tolu-
ene. Compounds 1 and 8 are insoluble in hexane, soluble
in toluene. All obtained compounds are soluble in THF.
The dichloroaluminium alkoxides were obtained in
the reaction of MeAlCl2 with alcohol (Eq. (2)).
Compounds 1ꢀ
/
8 do not redistribute in presence of THF
and do not form stable electron-donor complexes with
MeAlCl2 ꢂROH 0 ROAlCl2 ꢂMeH
(2)

68
S. Szumacher et al. / Journal of Organometallic Chemistry 669 (2003) 64ꢀ71
/
this solvent. After dissolving in THF and removal of the
1
solvent in vacuum the H-NMR spectra of 1ꢀ8 showed
/
no THF signals. Compound 8 was found to form
clathrates with aromatic solvents (benzene, toluene,
and mesitylene). The NMR spectra of the CDCl3
solution of crystals 8 showed characteristic patterns of
the aromatic solvents. In benzene solution the com-
pounds 1ꢀ6 were found to be dimers what was
/
determined by cryoscopic molecular weight investiga-
tions. We could not measure the molecular weight of 7
and 8 due to low solubility of 7 and inability to get 8
without the aromatic solvent.
The 1H-NMR of 1ꢀ
in deuterated benzene showed signal patterns character-
istic for alkoxy groups (1ꢀ7) or tert-butyl group on
/
8 and 13C-NMR spectra of 1ꢀ
6, 8
/
Fig. 1. An ORTEP [24] diagram of 2 showing 50% probability of
thermal ellipsoids. Hydrogen atoms are omitted for clarity.
/
phenyl ring (8) in high field region and one signal of the
cyclopentadienyl ligands in low one. In the low field
region the characteristic signal pattern of para substi-
tuted phenyl ring (8) was detected as well. The spectro-
scopic data are consistent with the proposed chemical
formulae of the obtained compounds. We were not able,
however, to determine the nature of Cp bonding to
aluminium atom in solution from NMR spectra. Rapid
rotation of the cyclopentadienyl rings gives a single
resonance in 1H- and 13C-NMR spectrum for the
cyclopentadienyl hydrogens and carbons. This beha-
viour is consistent with the calculated barrier of less
than 3 kcal molꢃ1 for 1,2-migration of the metal centre
about a cyclopentadienyl ring [2]. Similar dynamic
behaviour was reported for other cyclopentadienyl
compounds [11,12,14].
27Al-NMR chemical shifts of 1ꢀ
78ꢀ104 ppm and can be regarded as corresponding to
/
6 vary within range
Fig. 2. An ORTEP [24] diagram of 5 showing 30% probability of
thermal ellipsoids. Hydrogen atoms are omitted for clarity.
/
four-coordinate aluminium centre [14]. The spectra of 7
and 8 could not be measured due to poor solubility (7)
or overlapping with probe signal (8).
3.3. Molecular and crystal structure
The X-ray structure analysis of 2, 5 and 6 revealed the
presence of alkoxy-bridged centrosymmetric dimers in
the solid state (Figs. 1ꢀ3). Some selected geometrical
/
data are given in Table 2. The structural data for
[Cp2Al-m-Oi-Pr]2 (9) [14] are included in Table 2 for
comparison. The Al2(m-O)2 central ring is planar with
almost identical bond lengths and bond angles values in
all structures (Table 2).
In each reported dimer one of the AlꢀO bond is
/
˚
shorter of about 0.02 A than the second one. (This
differentiation could not be noticed in 5 due to
substantial disorder observed). The reason of this could
be the presence of the weak secondary, agostic type
interactions between metal and the hydrogen of alkoxy
a-carbon atom [22,23]. The mean Alꢀ ꢀ ꢀH(11) contact is
Fig. 3. An ORTEP [24] diagram of 6 showing 30% probability of
thermal ellipsoids. Hydrogen atoms are omitted for clarity.
The spatial arrangement of Cp rings with respect to
Al2(m-O)2 plane is presented for compared structures in
˚
2.87 A.

S. Szumacher et al. / Journal of Organometallic Chemistry 669 (2003) 64ꢀ
/71
69
Table 2
˚
Selected bond lengths (A) and bond angles (8) compounds 2, 5, 6 and
a
9
b
2
5
6
9
[14]
Bond lengths
Alꢀ
Alꢀ
Alꢀ
Alꢀ
/
O
1.822(1) 1.821(1)
1.841(1) 1.816(1)
2.006(2) 1.985(2)
2.041(2) 1.98(1)
2.468(2) 2.37(1)
1.448(2) 1.431(2)
1.278(4) 1.384(4)
1.821(2)
1.837(2)
2.006(3)
2.010(3)
2.722(3)
1.475(3)
1.811(5)
1.821(1)
1.837(1)
2.003(2)
2.022(2)
2.709(2)
1.473(2)
1.747(5)
/O?
/C(6)
/C(1)
Alꢀ ꢀ ꢀC(2)
OꢀC(11)
/
c
Cp1 ring slippage
Bond angles
Oꢀ
C(6)ꢀ
OꢀAlꢀ
O?ꢀAlꢀ
OꢀAlꢀC(1)
O?ꢀAlꢀC(1)
AlꢀOꢀAl?
Cg1ꢀC(1)ꢀ
Cg2ꢀC(6)ꢀ
T1 (Cg1ꢀ
Al?)
/
Alꢀ
Alꢀ
C(6)
C(6)
/
O?
/
80.48(6) 81.27(6)
124.79(7) 121.6(3)
113.37(7) 112.53(7)
113.07(7) 111.64(7)
110.65(7) 106.4(4)
105.49(7) 116.0(5)
99.52(6) 98.73(6)
80.79(7)
80.85(5)
/
C(1)
119.3(2)
112.0(1)
116.2(1)
110.0(2)
112.1(1)
99.21(7)
108.7(1)
114.6(1)
165.3(2)
118.66(9)
115.20(8)
114.50(9)
111.96(8)
109.63(8)
99.15(5)
106.7(1)
113.8(1)
167.4(1)
/
/
/
/
/
/
/
/
/
/
c
/
/Al
92.1(1)
95.8(11)
/
/Al
112.5(1) 118.8(1)
179.7(1) 177.1(8)
c
/
C(1)ꢀ
/
Alꢀ
/
ꢃ/
ꢃ/
T2 (Cg2ꢀ
/
C(6)ꢀ
/
Alꢀ
/
47.0(1)
51.6(2)
61.6(2)
50.2(2)
Al?)
Fig. 4. The Cp rings spatial arrangement towards Al2(m-O)2 plane in
four-coordinate dicyclopentadienylaluminium alkoxides. Top: side
a
Atoms labelled with prime belong to the centrosymmetric counter-
parts of the dimeric unit.
view perpendicular to the AlꢀAl vector; bottom: view on the molecular
/
b
Al2O2 mean plane. The positions of atoms in the central ring are
superimposed. The dark grey colours correspond to primary alcohols
derivatives 2 and 5, the lighter to compound 6 and 9. The alkyl groups
of all substituents bonded to the oxygen atoms are omitted for clarity.
The literature values are presented according to the atom
numbering scheme used in this work.
c
The mean value for two disordered positions of the Cp1 ring.
Fig. 4. As shown, Cp1 rings are in anti while Cp2 in syn
position. The relevant orientation of Cp rings towards
the central Al2(m-O)2 core depends on the nature of
alkoxy ligand. The different displacement of Cp1 and
Cp2 rings regarding the type of substituents on a-carbon
atom of the alkoxy ligand is observed. This displace-
their alteration consistent with h1 hapticity. For exam-
ple in case of 2 (the structure was determined at 100 K)
there are two short distances C(2)ꢀ
/
C(3) and C(4)ꢀ
/
C(5)
˚
of 1.375(3) and 1.378(3) A, two long bonds C(1)ꢀ
/
C(2)
˚
C(5) 1.434(3) and 1.432(3) A, respectively,
and C(1)ꢀ
/
˚
C(4) of 1.394(3) A.
and the remaining C(3)ꢀ
/
ment is described by torsion angles Cg1ꢀ
/
C(1)ꢀ
/
AlꢀAl?
/
The analysis of angles between ring centroids Cg1,
Cg2, carbons C(1), C(6) and aluminium atom revealed
the different nature of h1 bonding in studied com-
(T1) and Cg2ꢀC(6)ꢀAlꢀAl? (T2), where Cg1 and Cg2
/
/
/
denote Cp-ring centroids. The deviation of the Cp1 ring
from the parallel orientation to the central plane is
negligible for primary alcohol’s derivatives (T1 close to
1808 for 2 and 5) and significant for secondary ones (T1
pounds. The angles Cg1ꢀ
/
C(1)ꢀAl in 2 and 5 are close to
/
908 what points h1(p) type interactions. The remaining
angles values (see Table 2) are above 1058 what shows
equal approximately ꢃ1668). It is noteworthy that
/
h1(s) Cpꢀ
Due to various Cpꢀ
/
Al bond character [26].
although Cp2 rings are in syn position to the Al2O2
core they are located on the side of the bulkiest
substituent. As a result Cp2 rings are considerable
/
Al h1(s) or h1(p) character and
diversified Cp rings orientation versus central Al2O2
core there are significant differences in ring-slippage of
Cp1 and Cp2 [27]. Although Cp1 ring is in anti position
it is closer to aluminium atom than Cp2 ring. For all
compounds the Cp1 ring-slippage values range from
swung from the central position (T2ꢁ08) and the
/
values of T2 varies from 47.0(1)8 in 2 to 61.6(2)8 in 6.
The cyclopentadienyl groups are bonded to Al atom
in the h1 manner. In all cases there is one short
˚
1.28 to 1.82 A and they are much less than ones of Cp2
aluminium atom ꢀ
from 1.985(2) to 2.066(2) A. The Alꢀ
2.468(2) and 2.37(1) A for compounds 2 and 5,
respectively, are falling in the bonding distance range
for cyclopentadienylaluminium compounds [25]. The
/
ring carbon atom distance range
˚
(above 2.0 A). The shorter Cp1 ring-slippage values
˚
/
C(2) contacts of
˚
corresponds to [Cp2Al-m-OR]2 derivatives of primary
alcohols (Table 2).
We found the 27Al-NMR chemical shift of aluminium
atom in the studied aluminium compounds also depends
analysis of ring CꢀC bond lengths revealed however
/

70
S. Szumacher et al. / Journal of Organometallic Chemistry 669 (2003) 64ꢀ71
/
ligand on the Cp bonding mode. In the derivatives of
secondary alcohols both Cp rings are h1(s) bonded to
aluminium atom while in ones of primary alcohols
exhibits h1(s) and h1(p) Cpꢀ
/
Al bonding mode. For
[Cp2Al-m-OMe]2 we postulate that the one of cyclopen-
tadienyl ring is h1.5 or h2 and the second one h1 bonded
to the aluminium atom.
5. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC nos. 178972ꢀ178974 for com-
/
pounds 2, 5 and 6, respectively. Copies of this informa-
tion may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
Fig. 5. Plot of the 27Al chemical shift vs. the Cp1 cyclopentadienyl ring
slippage in four-coordinate dicyclopentadienylaluminium alkoxides.
(Fax:
ac.uk or www: http://www.ccdc.cam.ac.uk).
ꢂ44-1223-336033; e-mail: deposit@ccdc.cam.
/
on alcohol’s order. The observed 27Al-NMR spectra for
[Cp2Al-m-OR]2 derivatives of primary alcohols 2ꢀ
showed signals at the range 84ꢀ91 ppm. For secondary
ones 6, 9 the signals of aluminium atom are shifted
downfield (102ꢀ104 ppm). It shows that in [Cp2Al-m-
/
5
/
Acknowledgements
/
OR]2 compounds derived from primary alcohols the
cyclopentadienyl rings cause higher electron density
around aluminium atom than in secondary ones. The
more alkyl substituents are located on a-carbon atom of
alkoxy ligand the weaker aluminium atom shielding of
Cp ring is observed.
We found that the chemical shift of 27Al-NMR signals
correlates with X-ray structural parameter Cp1 ring-
slippage values (Fig. 5). The correlation factor is equal
to 0.98. According to this relation it is possible to
The authors thank The State Committee for Scientific
Research for financial support of this work (grant no.
PZB-KBN 15/09/T09/99/01i).
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/
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