Reactions of benzopinacol with group 13 alkyl metals:. Crystal structure of Me6In4[OC(C6H5)2C(C6H5)2O]2(OCH3)2, a new alkylalkoxo indium diolate

Article abstract of DOI:10.1016/j.poly.2006.11.032

Reactions of Me₅Al₃[OC(C₆H₅)₂C(C₆H₅)₂O]₂ (1) with alcohols ROH (R = Me, Et, ^tBu) in a 1:1 molar ratio afforded the compound Me₂Al₂[OC(C₆H₅)₂C(C₆H₅)₂O]₂(C₄H₈O) (2) and a mixture of methylaluminum alkoxides. The alcohols acted as the factor formally eliminating a molecule of Me₃Al (as a methylaluminum alkoxide) from compound 1. ^tBu₃Al reacted with an equimolar amount of benzopinacol to form the monomeric complex ^tBuAl[OC(C₆H₅)₂C(C₆H₅)₂O](C₄H₈O) (3). Reactions of Me₃Ga and Me₃In with benzopinacol yielded trinuclear complexes Me₅M₃[OC(C₆H₅)₂C(C₆H₅)₂O]₂ (4 (M = Ga), 5 (M = In)), isostructural to compound 1. In the presence of water and alcohols, compounds 4 and 5 underwent a decomposition reaction to benzopinacol and a mixture of metalloxanes and alkoxides. An unusual methylmethoxo indium benzopinacolate Me₆In₄[OC(C₆H₅)₂C(C₆H₅)₂O]₂(OCH₃)₂ (6) was obtained in the reaction of benzopinacol with Me₃In and Me₂InOMe in a 1:1:1 molar ratio. Molecular structures of the compounds 3, 4 and 6 were determined by X-ray crystallography.

Full text of DOI:10.1016/j.poly.2006.11.032

Polyhedron 26 (2007) 1436–1444
www.elsevier.com/locate/poly
Reactions of benzopinacol with group 13 alkyl metals:
Crystal structure of Me6In4[OC(C6H5)2C(C6H5)2O]2(OCH3)2,
a new alkylalkoxo indium diolate
a,
a
a
a
Wanda Ziemkowska *, Anna Kubiak , Szymon Kucharski , Robert Woz´niak ,
b
Romana Anulewicz-Ostrowska
a
Warsaw University of Technology, Faculty of Chemistry, Koszykowa 75, 00-662 Warsaw, Poland
b
Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
Received 28 August 2006; accepted 14 November 2006
Available online 25 November 2006
Abstract
t
Reactions of Me5Al3[OC(C6H5)2C(C6H5)2O]2 (1) with alcohols ROH (R = Me, Et, Bu) in a 1:1 molar ratio afforded the compound
Me2Al2[OC(C6H5)2C(C6H5)2O]2(C4H8O) (2) and a mixture of methylaluminum alkoxides. The alcohols acted as the factor formally elim-
t
inating a molecule of Me3Al (as a methylaluminum alkoxide) from compound 1. Bu3Al reacted with an equimolar amount of benzo-
t
pinacol to form the monomeric complex BuAl[OC(C6H5)2C(C6H5)2O](C4H8O) (3). Reactions of Me3Ga and Me3In with benzopinacol
yielded trinuclear complexes Me5M3[OC(C6H5)2C(C6H5)2O]2 (4 (M = Ga), 5 (M = In)), isostructural to compound 1. In the presence of
water and alcohols, compounds 4 and 5 underwent a decomposition reaction to benzopinacol and a mixture of metalloxanes and alk-
oxides. An unusual methylmethoxo indium benzopinacolate Me6In4[OC(C6H5)2C(C6H5)2O]2(OCH3)2 (6) was obtained in the reaction of
benzopinacol with Me3In and Me2InOMe in a 1:1:1 molar ratio. Molecular structures of the compounds 3, 4 and 6 were determined by
X-ray crystallography.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Aluminum; Gallium; Indium; Benzopinacol; Diol; Alkoxides
1. Introduction
undergo a selective partial hydrolysis reaction to yield
unique dinuclear complexes R2Al2[OC(C6H5)2C(C6H5)2O]2-
(THF) (for structures of methylaluminum derivatives 1 and
2 see Scheme 1). The dinuclear aluminum benzopinacolates
are the first alane aliphatic diolates, which demonstrate
ROP of cyclic esters [4]. It is noteworthy that aluminum
benzopinacolates contain ‘‘magic’’ diphenylhydroxymethyl
groups, the ability of which to enhance selectivity in asym-
metric synthesis and enantioselective catalysis is now well
documented [5]. Substituted ferrocenes with diphenylhydr-
oxymethyl groups were reported as effective catalysts for
the asymmetric aryl transfer reaction to aldehydes leading
to products with up to 99% ee [6].
Among metal alkoxides, aluminum alkoxides are espe-
cially interesting as catalysts in ring-opening polymerisa-
tion (ROP) due to their high Lewis acidity, low toxicity
and ability to initiate the polymerisation in a ‘‘living’’
and ‘‘immortal’’ fashion [1]. Aluminum alkoxides have
been also reported to be precursors to nanosized aluminum
oxides and Al2O3 films [2].
Recently, we have described numerous reactions of diols
with aluminum trialkyls leading to the formation of tri-
nuclear complexes R5Al3[diol-(2H)]2 (R = alkyl) [3]. We
found, that alkylaluminum 1,1,2,2-tetraphenylethane-1,2-
diolates (benzopinacolates) R5Al3[OC(C6H5)2C(C6H5)2O]2
Complexes of group 13 metals with ligands containing
diphenylhydroxymethyl groups are rare and unexplored
as catalysts in organic syntheses and polymerisation. There
are several examples of aluminum complexes with ‘‘magic’’
*
Corresponding author. Tel.: +48 22 6607316; fax: + 48 22 6605462.
E-mail address: ziemk@ch.pw.edu.pl (W. Ziemkowska).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.11.032

W. Ziemkowska et al. / Polyhedron 26 (2007) 1436–1444
1437
groups. An ionic [MeAl(OR)2AlMe2]+(MeAlCl3)ꢀ complex
Me
Me
Me
Me
and monomeric organoaluminum complexes R0Al(OR)2
and R02AlOR (where OR = (S)-a,a-diphenyl-2-pyrrolidi-
nyl-methoxide as a ligand possessing a ‘‘magic’’ group,
Al
Al
Al
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
O
O
O
THF
O
O
O
O
Ph
Ph
Ph
Ph
Al Me
+ ROH
O
- Me2AlOR
- RH
t
R0 = Me, Et, 1-Nor, Bu) were obtained in reactions of
Ph
Al
Ph
THF
(S)-a,a-diphenyl-2-pyrrolidinyl-methanol with Me2AlCl
and R03Al, respectively [7]. The reaction of a cyclopropene
analog of aluminum LAl[g2-C2(SiMe3)2] (L = HC[(CMe)-
(NAr)]2, Ar = 2,6-i-Pr2C6H3) with Ph2CO yielded the com-
plex LAl[OC(Ph)2C2(SiMe3)2] possessing the ‘‘magic’’
group. The complex was the result of an insertion of the car-
bonyl into an Al–C (vinyl) bond [8]. Treatment of the bridg-
ing hydride complex (l,g1:g1-Ph2pz)(AliBu2)2(l-H) with
diphenylmethanol or benzophenone afforded the diphenyl-
methoxide compound (l,g1:g1-Ph2pz)(AliBu2)2(l-OCHPh2)
[9]. Aluminum macrocyclic structures with the Ph2CO group
were synthesised in reactions of diphenylglycolic acid with
Me3Al [10]. Among the diphenylmethoxo aluminum
complexes, the four-coordinated monomeric complex
(EDBP)Al(OCHPh2)(THF) (EDBP-H2 = 2,20-ethylidene-
bis(4,6-di-tert-butylphenol)) is the only example to possess
efficient catalytic activity in the ROP of lactones [11].
Me
Me
1
2
R = Me, Et, tBu
Scheme 1.
partial hydrolysis product [4]. Similarly to water, the alco-
hols played the role of the factor formally eliminating a
Me3Al molecule (in a form of a methylaluminum alkoxide)
from the trinuclear compound 1.
2.2. Reactions of benzopinacol with tBu3M (M = Al, Ga, In)
tBu3Al reacted with an equimolar amount of benzopin-
acol to form the complex BuAl[OC(C6H5)2C(C6H5)2O]-
t
(C4H8O) (3) (Scheme 2).
In contrast to the trinuclear methyl- and ethylaluminum
benzopinacolates R5Al3[OC(C6H5)2C(C6H5)2O]2 (R = Me
(1), Et) [4], the tert-butyl compound 3 was monomeric.
The molecular structure of 3 has been determined by X-
ray crystallography and it is shown in Fig. 1. Data collec-
tion and structure analysis are presented in Table 1. The
Considering the ability of the Ph2CO group to enhance
the catalytic activities of organometallic complexes and
our last results involving the catalytic activity of dinuclear
alkylaluminum benzopinacolates in the ROP of e-caprolac-
tone, we found it imperative to study group 13 complexes
with diphenylmethoxo ligands. In this work we present
t
molecule of 3 consists of a diolate unit, Bu group bonded
to the aluminum atom and a THF molecule coordinated to
the Al atom. The aluminum atom is in a four coordinate
highly distorted tetrahedral environment. The Al–
t
the reactions of Bu3M (M = Al, Ga, In), Me3Ga and
Me3In with benzopinacol. We report the unusual structure
of a methylmethoxo indium benzopinacolate and the results
of alcoholysis reactions of group 13 benzopinacolates.
˚
O(3)THF bond [1.889(2) A] is much longer than the
˚
˚
Al–O(2) [1.745(2) A] and Al–O(1) [1.724(2) A] bonds. Fun-
damental studies have recently produced a number of tri-
metallic diolates [3], however compound 3 is the first
mononuclear product of the reaction of aliphatic diols with
alkylalanes. Several monomeric alane aromatic diolates
were prepared by Lin [12] and Okuda [13]. Recently
2. Results and discussion
2.1. Reactions of Me5Al3[OC(C6H5)2C(C6H5)2O]2 (1)
with alcohols
´
Lewinski [14] has reported a monomeric methylalumi-
num(bisphenoxide)-e-caprolactone complex.
As described in Section 1, the partial hydrolysis reaction
of compound 1 led to the formation of the dinuclear com-
plex Me2Al2[OC(C6H5)2C(C6H5)2O]2(C4H8O) (2). In this
study, we showed that the reaction of the compound 1 with
1 equiv. of alcohol ROH (R = Me, Et, Bu) resulted with
the formation of the same product 2 in 80–90% yield
(Scheme 1).1
In contrast to the dimeric aluminum benzopinacolates
R2Al2[OC(C6H5)2C(C6H5)2O]2(C4H8O) [R = Me (2), Et]
[4], the monomeric compound 3 demonstrated very low
activity in the polymerisation of e-caprolactone. An
t
t
attempt to activate 3 by replacing the Bu with an alkoxyl
group failed. Reactions of compound 3 with iso-propyl-
and benzyl alcohols resulted in the formation of a mixture
of tert-butylaluminum alkoxides and benzopinacol. In the
presence of dioxygen, compound 3 underwent transforma-
tion into a complicated mixture of products instead of the
expected tert-butoxoaluminum benzopinacolate.
Compound 2 was isolated from the post reaction mix-
ture as a precipitate after addition of n-hexane. The pres-
ence of signals at ꢀ1.85 (AlCH3), ꢀ1.06 (AlCH3), 1.45
(THF) and 3.52 (THF) ppm and aromatic proton signals
1
in the H NMR spectrum indicated that the structure of
compound 2 was the same as that of the earlier reported
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
tBu
O
O
OH
OH
THF
+ tBu3Al
M
1
Alcohols are more soluble in organic solvents than water. Therefore,
- 2 C4H10
the cost of the synthesis of complexes {R2Al2[OC(C6H5)2C(C6H5)2O]2(B)}
(R = alkyl group, B = Lewis base) in reactions of compounds
{R5Al3[OC(C6H5)2C(C6H5)2O]2} with alcohols, is lower due to the small
amount of solvents needed for the process.
THF
3
Scheme 2.

1438
W. Ziemkowska et al. / Polyhedron 26 (2007) 1436–1444
Reactions of benzopinacol with tBu3Ga and tBu3In were
C6
carried over 3 h upon refluxing in toluene. We found that
t
benzopinacol did not react with Bu3Ga, which was in
C7
O3
agreement with our earlier observation of less reactivity
C5
C3
t
t
t
of Bu3Ga and Bu3In in comparison with Bu3Al [15,16].
The 1H NMR spectrum of the post reaction mixture
revealed the signals of unreacted benzopinacol and tBu3Ga
C31
O1
t
C1
only. On the other hand, the reaction of Bu3In with ben-
Al1
C4
zopinacol resulted in the precipitation of indium and oxy-
genation of benzopinacol to benzophenone. The presence
of benzophenone in the post reaction mixture was con-
firmed on the basis of the CO carbon signal at 77.21 ppm
in the 13C NMR spectrum.
O2
C2
C33
C32
2.3. Methylgallium- and methylindium benzopinacolates
Independently of the molar ratio of the reagents,
reactions of Me3M (M = Ga, In) with benzopinacol affor-
ded trinuclear complexes Me5M3[OC(C6H5)2C(C6H5)2O]2
[4 (M = Ga), 5 (M = In)], containing two diolate units
(Scheme 3).
Compound 4 was crystallised from a post reaction mix-
ture in 90% yield. The molecular structure of 4 was deter-
mined on the basis of a X-ray diffraction study and it is
shown in Fig. 2. Data collection and structure analysis
t
Fig. 1. Molecular structure of BuAl[OC(C6H5)2C(C6H5)2O](C4H8O) (3).
Thermal ellipsoids are shown at the 30% level. Hydrogen atoms are
omitted for clarity. Selected bond lengths (A) and bond angles (ꢁ): Al–O(2)
1.745(2), Al–O(1) 1.724(2), Al–O(3) 1.889(2), Al–C(3) 1.961(2), O(1)–C(1)
1.431(2), O(2)–C(2) 1.423(3), C(1)–C(2) 1.645(3); O(2)–Al–O(1) 93.9(1),
O(2)–Al–O(3) 100.9(1), O(1)–Al–O(3) 111.6(1), O(2)–Al–C(3) 130.2(1),
O(1)–Al–C(3) 115.7(1), O(3)–Al–C(3) 103.6(1).
˚
Table 1
Crystal data and data collection parameters for 3, 4 Æ 2CH2Cl2 and 6 Æ 2CH2Cl2
3
4 Æ 2CH2Cl2
6 Æ 2CH2Cl2
Empirical formula
Formula weight
Temperature (K)
C34H37AlO3
520.62
293(2)
C57H55Ga3O4 Æ 2CH2Cl2
1183.02
100(2)
C60H64In4O6 Æ 2CH2Cl2
1510.24
293(2)
˚
Wavelength A
0.71073
0.71073
0.71073
Crystal system
Space group
a (A)
monoclinic
P21/c
12.843(1)
16.768(1)
14.916(1)
90
115.33(1)
90
2903.4(3)
4
1.191
0.102
monoclinic
P21/c
20.994(2)
10.807(1)
23.905(2)
90
106.046(8)
90
5212.4(9)
4
triclinic
ꢀ
P1
˚
10.956(2)
11.856(2)
12.862(3)
100.35(2)
97.15(2)
110.21(2)
1510.7(5)
1
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
Dcalc (g cmꢀ3
)
1.508
1.792
2424
1.660
1.733
752
Absorption coefficient (mmꢀ1
F(000)
)
1112
Crystal size (mm)
H Range for data collection (ꢁ)
0.20 · 0.20 · 0.15
2.77–25.00
0.20 · 0.15 · 0.15
3.06–25.00
ꢀ24 6 h 6 24, ꢀ12 6k 6 12,
ꢀ28 6l 6 28
36778
0.20 · 0.20 · 0.15
3.29–25.00
ꢀ13 6 h 6 13, ꢀ14 6 k 6 14,
ꢀ15 6 l 6 15
21863
Index ranges
ꢀ15 6 h 6 15, ꢀ19 6 k 6 19,
ꢀ17 6 l 6 17
Reflections collected
22089
Independent reflections [Rint
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
]
5088 [0.0881]
9144 [0.0809]
5300 [0.0682]
full-matrix least-squares on F2
5088/0/346
9144/0/693
1.022
R1 = 0.0552, wR2 = 0.1374
R1 = 0.0765, wR2 = 0.1504
0.961 and ꢀ1.047
5300/0/348
1.083
R1 = 0.0440, wR2 = 0.1113
R1 = 0.0547, wR2 = 0.1198
0.778 and ꢀ0.919
0.834
R1 = 0.0438, wR2 = 0.0917
R1 = 0.1109, wR2 = 0.1079
0.204 and ꢀ0.188
Maximum/minimum of residual
electron density

W. Ziemkowska et al. / Polyhedron 26 (2007) 1436–1444
1439
Me
with the measured structure. The signal of CO carbons at
91.29 ppm in the 13C NMR spectrum was situated in the
region of the CO signals (ppm) of alkylaluminum benzopin-
acolates {1 (93.31); Et5Al3[OC(C6H5)2C(C6H5)2O]2 (93.30);
Me
M
M
M
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
OH
OH
O
O
O
Me
Ph
Ph
2
+ 3 Me3M
O
- 4 MeH
2
(92.81, 87.73); Et2Al2[OC(C6H5)2C(C6H5)2O]2(THF)
Ph
Ph
(92.62, 87.78)} [4]. In comparison with CO signals of
alkylalane-2 and alkygallane aliphatic diolates (ppm), tBu4-
Ga2(OCH2C6H4CH2OH)2 (67.49); tBu5Ga3(OCH2C6H4-
Me
Me
4 (M = Ga) 5 (M = In)
t
CH2O)2 (68.30); Bu4MeGa3(OCH2C6H4CH2O)2 (67.58);
Scheme 3.
tBu2Me3Ga3(OCH2C6H4CH2O)2 (67.44, 65.63) [15]; Me5-
Ga3[OC(CH3)2CH2C(CH3)2O]2 (74.86) [21]; tBu5Ga3-
[OCH2C(CH3)2CH2O]2 (78.4) [20], the CO carbon signal
of 4 is dramatically shifted downfield, which is caused by
the electron withdrawing effect of the aromatic rings.
The methylindium benzopinacolate 5 was not isolated as
a pure compound. The 1H NMR spectrum of a white solid
crystallised in the post reaction mixture, besides signals of
compound 5, exhibited several small singlets in the region
from 0.0 to ꢀ1.0 ppm. It indicated the presence of indium
methyls, which crystallised together with 5. Attempts to
isolate the pure compound 5 failed, because it underwent
easy decomposition and became dark. The elemental anal-
ysis, molecular weight and melting point were not mea-
sured due to the impurity and the instability of
compound 5. The structure of 5 was proposed on the basis
C55
Ga3
C54
O2
C2
C1
O3
C3
Ga1
O4
C4
C51
Ga2
C53
O1
C52
1
of H and 13C NMR spectra. Similarly to the spectra of
compounds 1 [4] and 4, the 1H NMR spectrum of 5
revealed three singlets at ꢀ0.21, ꢀ0.47 and ꢀ0.72 ppm with
a 2:2:1 integration ratio, assigned to the protons of five
methyl groups bonded to indium atoms. The signal of
CO carbons (at 89.94 ppm) in the 13C NMR spectrum
was situated in the same region as those of aluminum-
and gallium benzopinacolates. The presence of one signal
of CO carbons in the spectrum indicated the equivalence
of the CO groups, which was in agreement with the pro-
posed structure of 5. The analogy of the spectra of com-
pounds 1, 4 and 5 showed that 5 was isostructural to the
compounds 1 and 4.
Fig. 2. Molecular structure of Me5Ga3[OC(C6H5)2C(C6H5)2O]2 Æ 2CH2Cl2
(4 Æ 2CH2Cl2). Thermal ellipsoids are shown at the 20% level. Hydrogen
atoms and molecules of CH2Cl2 are omitted for clarity. Selected bond
˚
lengths (A) and bond angles (ꢁ): Ga(1)–O(4) 1.924(3), Ga(1)–O(2) 1.932(3),
Ga(1)–C(51) 1.951(5), Ga(1)–O(3) 1.986(3), Ga(1)–O(1) 2.013(3), Ga(2)–
O(1) 1.964(3), Ga(2)–O(4) 1.992(3), Ga(3)–O(3) 1.978(3), Ga(3)–O(2)
1.994(3); O(4)–Ga(1)–O(2) 122.0(1), O(4)–Ga(1)–O(3) 79.6(1), O(2)–
Ga(1)–O(3) 79.4(1), O(4)–Ga(1)–O(1) 80.6(1), O(2)–Ga(1)–O(1) 79.5(1),
O(3)–Ga(1)–O(1) 137.2(1), O(4)–Ga(1)–C(51) 113.6(2), O(2)–Ga(1)–C(51)
124.2(2), C(51)–Ga(1)–O(3) 116.4(2), C(51)–Ga(1)–O(1) 106.3(2).
are presented in Table 1. Two molecules of CH2Cl2 are
present in the formula unit of 4. The molecule of 4 consists
of a tetracyclic structure formed from two Ga2O2 four-
membered rings and two GaO2C2 five-membered rings. A
central Ga(1) atom is five-coordinate, residing in the dis-
torted square pyramidal geometry with the basal plane
consisting of four oxygen atoms of the diol units and the
methyl group residing in an apical position. The coordina-
tion sphere geometry of the central metal atom is closer to
Attempts to synthesise the dinuclear complexes Me2M2-
[OC(C6H5)2C(C6H5)2O]2(C4H8O) (M = Ga, In), isostruc-
tural to the aluminum derivative 2, failed. In contrast to
the partial hydrolysis and alcoholysis reactions of the alu-
minum benzopinacolate 1, reactions of the methylgallium-
and methylindium benzopinacolates (4 and 5) with water
and alcohols resulted in the decomposition of the com-
pounds to benzopinacol and metalloxanes or a mixture of
a
square pyramidal structure [i.e., O(3)–Ga(1)–O(1)
1
137.2(1)ꢁ, O(4)–Ga(1)–O(2) 122.0(1)ꢁ] than to a trigonal
bipyramidal geometry. The same geometry around the cen-
tral metal atom was observed for related trinuclear alumi-
num complexes [4,17–20]. The sums of the angles about the
alkoxides, respectively. The H NMR spectra of the post
reaction mixtures revealed a singlet of OH group protons
at 3.04 ppm and a multiplet of aromatic protons (7.37–
7.21 ppm) characteristic for benzopinacol. The reaction
carried out under milder reaction conditions (at 0 ꢁC in a
diluted solution) resulted in the same products. In our
opinion, the digallium complex Me2Ga2[OC(C6H5)2-
C(C6H5)2O]2(C4H8O) was formed as a very unstable inter-
P
P
P
oxygen atoms { [O(1)] 350.3ꢁ; [O(2)] 358.9ꢁ; [O(3)]
P
358.8ꢁ; [O(4)] 359.9ꢁ} were slightly less than 360ꢁ, which
indicates a small steric strain in the molecule.
1
The H NMR spectrum of 4 revealed three singlets of
CH3Ga group protons (at ꢀ0.15, ꢀ0.67 and ꢀ0.71 ppm)
and multiplet of aromatic protons at 7.33–7.01 ppm with
the integration ratio of 6:6:3:40, which were in agreement
2
For chemical shift of CO signals of alkylalane aliphatic diolates see
Ref. [4], p. 2933.

1440
W. Ziemkowska et al. / Polyhedron 26 (2007) 1436–1444
mediate product, undergoing decomposition. In the reac-
tion of the indium compound 5 with water and alcohols,
benzopinacol underwent an oxidation to benzophenone,
whereas organoindium compounds were reduced to indium
(for details see Section 3.5). Two singlets of CH3M protons
characteristic for the expected Me2M2[OC(C6H5)2C-
(C6-H5)2O]2(C4H8O) (M = Ga, In) products were not
observed in the 1H NMR spectra, which indicated that
these products were not formed in the reactions. For com-
parison, the 1H NMR spectrum of the aluminum derivative
2 revealed the signals of CH3Al protons as two singlets at
ꢀ1.06 and ꢀ1.85 ppm [4].
C6
C5
C3
O3
In2
O1
C1
C4
In1
C2
O2
2.4. Methylmethoxo indium benzopinacolate
The reaction of benzopinacol with Me3In and Me2In-
OMe in
a 1:1:1 molar ratio afforded an unusual
methylmethoxo indium benzopinacolate Me6In4[OC-
(C6H5)2C(C6H5)2O]2(OCH3)2 (6) (Scheme 4). The pure
compound 6 was isolated by crystallisation from the post
reaction mixture in 35% yield. It has been spectroscopically
and crystallographically characterised. Data collection and
structure analysis are presented in Table 1. The molecular
structure of 6 is shown in Fig. 3 (top).
O3
H12
C12
C26
O1
H26
H112
C112
The centrosymmetric tetranuclear molecule of 6 consists
of a pentacyclic structure formed from three In2O2 four-
membered rings and two InO2C2 five-membered rings.
The tetranuclear compound is formed by the alkoxide
termini of two diolate ligands and two methoxyl groups
bridging two Me2In units and two central MeIn units. The
five-coordinate In(1) atoms reside in a distorted square
pyramidal geometry with the basal plane consisting of three
oxygen atoms of the diolate units and one atom of the OMe
group. The Me group is situated in an apical position. The
angles O(2)#1–In(1)–O(1) 135.4(1)ꢁ and O(2)–In(1)–O(3)
121.1(1)ꢁ indicate that the coordination geometry of the
In(1) atoms is closer to a square pyramidal structure than
to a trigonal bipyramidal geometry.
Fig. 3. Molecular structure of Me6In4[OC(C6H5)2C(C6H5)2O]2(OCH3)2 Æ
2CH2Cl2 (6 Æ 2CH2Cl2). (Top) Thermal ellipsoids are shown at the 40%
level. Hydrogen atoms and molecules of CH2Cl2 are omitted for clarity.
˚
Selected bond lengths (A) and bond angles (ꢁ): In(1)–O(1) 2.217(3), In(1)–
O(2) 2.156(3), In(1)–O(3) 2.176(3), In(1)–O(2)#1 2.182(3), C(4)–In(1)
2.138(5), C(5)–In(2) 2.131(6), C(6)–In(2) 2.148(6), In(2)–O(1) 2.157(3),
In(2)–O(3) 2.193(3); O(2)#1–In(1)–O(1) 135.4(1), O(2)–In(1)–O(3)
121.1(1), C(4)–In(1)–O(2)#1 116.3(2), C(4)–In(1)–O(1) 106.8(2), C(4)–
In(1)–O(2) 125.8(2), C(4)–In(1)–O(3) 110.5(2), C(5)–In(2)–C(6) 134.8(2),
C(5)–In(2)–O(1) 116.5(2), C(6)–In(2)–O(1) 99.2(2), C(5)–In(2)–O(3)
112.7(2), C(6)–In(2)–O(3) 101.5(2), O(1)–In(2)–O(3) 76.3(1). (Bottom) A
view of the intramolecular hydrogen bonds in the molecule of 6 Æ 2CH2Cl2.
Hydrogen atoms are omitted excluding those involved in the H-bond
formation. The dashed lines represent C–Hꢁ ꢁ ꢁO hydrogen bonds.
It is worthy of note, that the two terminal In2O2 rings
deviate highly from a plane tetragon {sums of the angles
P
P
about the oxygen atoms
342.8ꢁ; torsion angle O(3)–In(1)–O(1)–In(2): ꢀ16.8(1)ꢁ}.
[O(1)]: 349.8ꢁ and
[O(3)]:
Ph
OH
Ph
A detailed examination of the structural data showed that
very probably the deviation is the result of intramolecular
interactions of ortho-positioned hydrogens with O(1) and
2
+ 2 Me3In + 2 Me2InOMe
Ph
- 4 MeH
OH
Ph
˚
Ph
Ph
O(3) oxygen atoms [(O(1)–H(12): 2.40 A, C(12)–H(12)ꢁ ꢁ ꢁ
Ph
O
Ph
O
Me
O
˚
O(1): 102ꢁ, O(3)–H(26): 2.55 A, C(26)–H(26)ꢁ ꢁ ꢁO(3):
Me
In
Me
Me
Me
Me
˚
174ꢁ, O(3)–H(112): 2.64 A, C(112)–H(112)ꢁ ꢁ ꢁO(3): 138ꢁ)
In
In
In
(Fig. 3 bottom). The hydrogen atoms were placed in calcu-
lated positions, however short distances between the con-
firmed ortho-positioned carbons and O(1) and O(3)
O
O
O
Me
Me
Ph
Ph
Ph Ph
˚
oxygen atoms [C(26)–O(3): 3.499(6) A, C(112)–O(3):
6
˚
˚
3.406(7) A, C(12)–O(1): 2.760(5) A] indicated the possibil-
ity of involving the hydrogen atoms H(12), H(26) and
Scheme 4.

W. Ziemkowska et al. / Polyhedron 26 (2007) 1436–1444
1441
H(112) in hydrogen bond interactions with the oxygen
atoms O(1) and O(3). Intermolecular hydrogen interactions
in the crystal structure were not found. Moreover long-dis-
tances donor–acceptor interactions were absent in the supra-
molecular structure. Recently Lewinski has presented the
substantial role of non-covalent interactions (C–Harylꢁ ꢁ ꢁO,
C–Haliphꢁ ꢁ ꢁO and C–Hꢁ ꢁ ꢁp hydrogen bonds) in molecular
assembly and the structure of metal complexes [22].
Compound 6 was more stable than the compound 5. It
demonstrated very low solubility in solvents. The 1H
NMR spectrum of 6 revealed one singlet at 2.17 ppm of
OCH3 protons and three singlets (at ꢀ0.14, ꢀ0.81 and
ꢀ1.05 ppm) of InCH3 protons with a 1:1:1:1 integration
ratio, which is in agreement with the measured structure
of the compound. The presence of two signals of CO car-
bons (at 87.22 and 82.98 ppm) in the 13C NMR spectrum
indicate the inequivalence of the CO groups. In compari-
son with the CO signals of group 13 metal benzopinaco-
lates: 1 (93.31 ppm), 2 (92.81 and 87.73 ppm), 3 (88.85
ppm), 4 (91.29 ppm) and 5 (89.94 ppm), the CO signal of
6 at 82.98 ppm is shifted upfield. It is the result of weaken-
ing of the electron withdrawing effect of the aromatic rings
by a donating effect of the methoxyl groups.
ies on the syntheses and characteristics of alkylalkoxo
group 13 metal diolates will be continued.
3. Experimental
All manipulations were carried out using standard
Schlenk techniques under an inert gas atmosphere. The
solvents were distilled over a blue benzophenone-K com-
plex. Prof. K.B. Starowieyski (Warsaw University of
Technology) kindly supported us with Me3Ga and Me3In.
t
Compound 1, benzopinacol and Bu3Al were synthesised
as described in the literature [4,24,25]. 1H and 13C
NMR spectra were run on Mercury-400BB spectrometer.
1H NMR spectra were recorded at 400.09 MHz. Chemical
shifts were referenced to the residual proton signals of
CDCl3 (7.26 ppm). 13C NMR spectra were run at
100.60 MHz (standard: chloroform 13CDCl3, 77.20 ppm).
Elemental analysis of 6 was obtained on a Perkin–Elmer
2400 analyser.
3.1. Reactions of Me5Al3[OC(C6H5)2C(C6H5)2O]2 Æ
2CH2Cl2 (1 Æ 2CH2Cl2) with alcohols
Group 13 alkylalkoxides incorporating diolate ligands
are rare. Earlier Chisholm and Lin [23] reported one
dimeric methylalkoxo aluminum biphenolate (l-bipheno-
late)AlMe(l)–OCH(IPr)2AlMe2, prepared from the reac-
tion of biphenolate-H2 with 2 mol equiv. of Me3Al in the
presence of 1 mol equiv. of 2,4-dimethyl-3-pentanol. The
compound demonstrates a catalytic activity in ROP of
rac-lactide.
In conclusion, trinuclear alkylaluminum benzopinaco-
lates react with alcohols to yield binuclear complexes
R2Al2[OC(C6H5)2C(C6H5)2O]2(C4H8O). Alcohols, as well
as water, play the role of a factor formally eliminating a
R3Al molecule (as an alkylaluminum alkoxide) from the
trinuclear benzopinacolates. The compounds 4 and 5 were
studied as starting materials for the syntheses of dinuclear
complexes Me2M2[OC(C6H5)2C(C6H5)2O]2(C4H8O) (M =
Ga, In), a potential ROP of cyclic esters. Unfortunately,
in the presence of water and alcohols compounds 4 and 5
underwent a decomposition reaction to benzopinacol and
mixtures of metalloxanes and alkoxides. Contrary to the
stable aluminum compound 2, presumably the gallium
and indium derivatives Me2M2[OC(C6H5)2C(C6H5)2O]2-
(C4H8O) were formed as unstable intermediate products.
Further attempts with substituted benzopinacols will be
undertaken to obtain stable dinuclear complexes.
A solution of MeOH (0.016 g, 0.5 mmol) in THF
(4 cm3) was added to solid compound 1 Æ 2CH2Cl2
(0.528 g, 0.5 mmol). The mixture was set aside for ca.
24 h. Volatiles were removed in vacuo. A resulting glassy
solid consisting of 2 and Me2AlOMe was washed with n-
C6H14 (5 cm3) two times, then dried in vacuo furnishing a
1
white solid of 2 Æ THF (0.421 g, 88%). The H NMR spec-
trum of products washed away from 2 Æ THF and soluble in
n-C6H14 revealed signals of Me2AlOMe. 1H NMR
(CDCl3): d 3.63 [3H, s, (CH3)2AlOCH3], ꢀ0.95 [6H, s,
(CH3)2AlOCH3] ppm. The spectrum of Me2AlOMe
obtained in the reaction of 1 Æ 2CH2Cl2 with MeOH was
similar to that published elsewhere [26].
Similarly, from of EtOH (0.023 g, 0.5 mmol) and
1 Æ 2CH2Cl2 (0.528 g, 0.5 mmol) the white solid of 2 Æ THF
1
(0.392 g, 82%) was obtained. The H NMR spectrum of
products washed away from 2 Æ THF and soluble in n-
1
C6H14 revealed signals of Me2AlOEt. H NMR (CDCl3):
d 3.94 [2H, q, (CH3)2AlOCH2CH3], 1.39 [3H, t, (CH3)2A-
lOCH2CH3], ꢀ0.86 [6H, s, (CH3)2AlOCH2CH3] ppm.
The spectrum of Me2AlOEt obtained in the reaction of
1 Æ 2CH2Cl2 with EtOH was similar to that published else-
where [27].
The reaction of tBuOH (0.037 g, 0.5 mmol) with
1 Æ 2CH2Cl2 (0.528 g, 0.5 mmol) afforded 2 Æ THF (0.430 g,
1
We found that the reaction of benzopinacol with Me3In
and Me2InOMe in a 1:1:1 molar ratio afforded the unusual
tetranuclear methylmethoxo indium benzopinacolate 6.
The reaction of diols with a mixture of group 13 trialkyl
metals and dialkyl metal alkoxides can be regarded as the
general method of synthesis of alkylalkoxo group 13 metal
diolates. Diolate alkylmetal complexes supported by addi-
tional alkoxide ligands are potential useful reagents in
organic transformations and polymerisation. Further stud-
90%). The H NMR spectrum of products washed away
from 2 Æ THF and soluble in n-C6H14 revealed signals of
1
Me2AlOtBu. H NMR (CDCl3): d 1.27 [9H, s, (CH3)2A-
lOC(CH3)3], ꢀ0.51 [6H, s, (CH3)2AlOC(CH3)3] ppm. The
spectrum of Me2AlOtBu obtained in the reaction of
1 Æ 2CH2Cl2 with tBuOH was similar to that published else-
where [28].
The 1H NMR spectrum of 2 obtained in the above reac-
tions was the same as described earlier [4].

1442
W. Ziemkowska et al. / Polyhedron 26 (2007) 1436–1444
t
3.2. Synthesis of BuAl[OC(C6H5)2C(C6H5)2O](C4H8O)
(3)
127.17, 126.80, 126.32 (C aromat), 89.94 (CO), 0.98, 0.49
(InCH3) ppm.
t
A solution of Bu3Al Æ OEt2 (0.544 g, 2 mmol) in THF
3.5. Reactions of compounds 4 and 5 with water and alcohols
(10 cm3) was added to a stirred solution of benzopinacol
(0.732 g, 2.0 mmol) in THF (30 cm3) at 0 ꢁC. The reaction
mixture was allowed to warm to room temperature and
refluxed (1 h). Volatiles were removed in vacuo, then a
white colourless solid of 3 (0.718 g, 69%) was sublimed
off in vacuo. A solution of 3 in n-C6H14 upon cooling at
ꢀ25 ꢁC furnished X-ray quality crystals. M.p.: 171 ꢁC.
1H NMR (CDCl3): d 7.64–7.00 (20H, m, H aromat),
3.71 [broadened, 4H, m, CH2O (THF)], 1.15 (9H, s,
(CH3)3C), 1.76 [broadened, 4H, m, CH2 (THF)] ppm. 13C
NMR (CDCl3): d 150.31, 148.22, 129.37, 129.33, 126.78,
126.00, 125.49, 125.21 (C aromat), 88.85 (CO), 65.80
[CH2O (THF)], 30.41 [(CH3)3C], 24.94 [CH2 (THF)],
15.25 [(CH3)3C] ppm.
The reaction of compound 4 with water was carried out
as described in Section 3.1. using 0.592 g (0.5 mmol) of
4 Æ 2CH2Cl2 and 0.0045 g (0.25 mmol) of water. After
removal of THF from the post reaction mixture, n-C6H14
(10 cm3) was added to the reaction products. A white solid
precipitated. The solution was filtered to separate the solid.
The solvent was removed from the filtrate under reduced
pressure and products, soluble in n-C6H14, were obtained.
The products soluble in n-C6H14 and the solid were studied
1
by NMR spectroscopy. H NMR (reaction product of 4
with water, soluble in n-C6H14) (CDCl3): d 7.5–6.9 (H aro-
mat, m), 3.04 (OH, s, benzopinacol), 0.0 to ꢀ1.0 (CH3, m,
broad, galloxanes) ppm. 1H NMR (white solid insoluble in
n-C6H14) (CDCl3): d 7.37–7.21 (20H, m, H aromat, ben-
zopinacol), 3.04 (2H, s, OH, benzopinacol) ppm.
Molecular weight: found: 504, calc.: 520. Elemental
t
Anal. Calc.: Al, 5.19; Bu, 10.96 wt%. Found: Al, 5.30;
hydrolysable tert-butyl groups, 10.62.
Reactions of compound 5 with water and workup of the
products were carried out as described above for 4, using
0.574 g (0.5 mmol) of 5 and 0.0045 g (0.25 mmol) of water
at 0 and ꢀ40 ꢁC. The reaction mixtures were allowed to
warm to room temperature. After addition of n-C6H14 to
the post reaction mixture, a grey solid precipitated. 1H
NMR (reaction product of 5 with water, soluble in n-
C6H14) (CDCl3): d 7.45–7.82 (H aromat, m, benzophenone,
benzopinacol) 3.04 (OH, s, benzopinacol) ppm. 13C NMR
(reaction product of 5 with water, soluble in n-C6H14)
(CDCl3): d 77.21 (C@O, benzophenone) ppm. The grey
solid insoluble in n-C6H14, probably containing indium,
was a complicated mixture of products. The products of
the reactions carried out at 0 and ꢀ40 ꢁC were the same.
Reactions of 4 with alcohols were carried out as
described in Section 3.1 using 0.592 g (0.5 mmol) of
4 Æ 2CH2Cl2 and 0.5 mmol of alcohol (0.023 g of EtOH,
0.016 g of MeOH). After addition of n-C6H14 to the post
reaction mixture, a white solid appeared. The solution
was filtered to separate the solid. The solvent was removed
from the filtrate under vacuum and products soluble in n-
C6H14 were obtained. The products soluble in n-C6H14
and the solid were studied by NMR spectroscopy.
3.3. Synthesis of Me5Ga3[OC(C6H5)2C(C6H5)2O]2 Æ
2CH2Cl2 (4 Æ 2CH2Cl2)
A solution of benzopinacol (1.098 g, 3 mmol) in CH2Cl2
(30 cm3) was added to a stirred solution of Me3Ga (0.541 g,
4.7 mmol) in Et2O (20 cm3) at ꢀ78 ꢁC. The reaction mix-
ture was allowed to warm to room temperature (2 h).
The post reaction mixture was stored at ꢀ5 ꢁC, yielding
colourless crystals of 4 Æ 2CH2Cl2 (1.860 g, 1.35 mmol,
90%). m.p.: 186–189 ꢁC.
1H NMR (CDCl3): d 7.33–7.01 (40H, m, H aromat),
ꢀ0.15 (6H, s, GaCH3), ꢀ0.67 (6H, s, GaCH3), ꢀ0.71
(3H, s, GaCH3). 13C NMR (CDCl3): d 145.28, 144.92,
130.63, 130.59, 128.56, 127.28, 126.93, 126.52,
126.46,126.40 (C aromat) 91.29 (CO), ꢀ0.14, ꢀ0.31
(GaCH3) ppm. Anal. Calc. for C59H59Ga3O4Cl4: C,
59.85; H, 4.99%. C, 59.40; H, 5.30%.
3.4. Synthesis of Me5In3[OC(C6H5)2C(C6H5)2O]2 (5)
A solution of benzopinacol (0.732 g, 2 mmol) in CH2Cl2
(20 cm3) was added to a stirred solution of Me3In (0.525 g,
3.3 mmol) in Et2O (20 cm3) at ꢀ78 ꢁC. The reaction mix-
ture was allowed to warm to room temperature (2 h).
The post reaction mixture was stored at ꢀ25 ꢁC (one
week), yielding a white solid (0.340 g) sensitive to light
and higher temperature. The solid undergoes a slow
decomposition and becomes dark. NMR spectra revealed
the signals of 5 (main product) and a mixture of methylin-
dium compounds. An attempt to isolate the pure com-
pound 5 by recrystallisation failed.
1H NMR (reaction product of 4 with EtOH, soluble in
n-C6H14) (CDCl3): d 7.17–7.81 (H aromat, m), 3.87
(OCH2CH3, m), 1.26 (OCH2CH3, m), ꢀ0.2 to ꢀ0.5
(CH3Ga, broad, m, oligomeric methylgallium compounds),
3.04 (OH, s, benzopinacol), ꢀ0.15, ꢀ0.67 and ꢀ0.71
1
(CH3Ga, three singlets, unreacted compound 4) ppm. H
NMR (white solid, insoluble in n-C6H14) (CDCl3): d 7.37–
7.21 (20H, m, H aromat, benzopinocol), 3.04 (2H, s, OH,
benzopinacol) ppm. Similarly, from 4 and MeOH, products
1
soluble in n-C6H14 and a white solid were obtained. H
NMR (reaction product of 4 with MeOH, soluble in n-
C6H14) (CDCl3): d 7.02–7.32 (H aromat, m), 3.58 (OCH3,
m), ꢀ0.2 to ꢀ0.6 (CH3Ga, broad, m, oligomeric methylgal-
lium compounds), 3.04 (OH, s, benzopinacol), ꢀ0.15, ꢀ0.67
1H NMR (5) (CDCl3): d 7.85–7.04 (40H, m, H aromat),
ꢀ0.21 (6H, s, InCH3), ꢀ0.47 (6H, s, GInCH3), ꢀ0.72 (3H,
s, InCH3). 13C NMR (5) (CDCl3): d 147.78, 130.61, 130.52,

W. Ziemkowska et al. / Polyhedron 26 (2007) 1436–1444
1443
and ꢀ0.71 (CH3Ga, three singlets, unreacted compound 4)
ppm. 1H NMR (white solid, insoluble in n-C6H14) (CDCl3):
d 7.37–7.21 (20H, m, H aromat, benzopinacol), 3.04 (2H, s,
OH, benzopinacol) ppm.
The spectra indicated that the post reaction mixtures of
4 with EtOH and MeOH consisted of benzopinacol, meth-
ylgallium alkoxides and unreacted compound 4.
The reaction of compound 5 with MeOH was carried
out as described above using 0.574 g (0.5 mmol) of 5 and
0.016 g (0.5 mmol) of MeOH. A grey solid of indium pre-
cipitated from the post reaction mixture of 5 with MeOH
in THF. After filtration the grey solid was removed. THF
was distilled off from the filtrate and reaction products
were studied by 1H NMR spectroscopy. 1H NMR
(CDCl3): d 7.48–7.83 (H aromat, m, benzophenone,
benzopinacol), 3.69, 3.51 (OCH3, m, broad), ꢀ0.07, ꢀ0.35
(InCH3, m, broad, methylindium methoxides), ꢀ0.21,
ꢀ0.47 ꢀ0.72 (InCH3, s, unreacted compound 5) ppm. Sim-
ilarly to the partial hydrolysis products of 5, the post reac-
1.2ꢁ intervals with a counting time of 25 s. All of the data
were corrected for Lorentz and polarisation effects. No
absorption correction was applied. Data reduction and
analysis were carried out using the KUMA Diffraction
(Wrocław) programs. The structures of the investigated
crystals were solved by direct methods [29] and refined
using the SHELXS/SHELXL computer programs [30]. All
hydrogen atoms were placed in calculated positions and
their thermal parameters were refined isotropically. Scat-
tering factors were taken from the literature [31, Tables
6.1.1.4 and 4.2.4.2].
Absorption corrections were not made for compounds 4
and 6 because, following our experience, for such values of
absorption coefficients, the corrections do not lead to a sig-
nificant improvement of the final results. According to [32],
for 0 < l < 25 cmꢀ1 (0 < l < 2.5 mmꢀ1) an absorption cor-
rection is not usually necessary unless a crystal with a very
unfavorable shape is used, since if applied the absorption
corrections will not significantly improve in accuracy of
the results. In our case the crystals were rather small with
no big differences in the three dimensions.
tion mixture of
5 with MeOH contained indium,
benzophenone, benzopinacol and unreacted 5.
The X-ray structures were measured in the Crystallogra-
phy Unit of the Physical Chemistry Laboratory at the
Chemistry Department of the University of Warsaw.
3.6. Synthesis of Me6In4[OC(C6H5)2C(C6H5)2O]2-
(OCH3)2 Æ 2CH2Cl2 (6 Æ 2CH2Cl2)
A solution of methanol (0.096 g, 3 mmol) in Et2O
(2 cm3) was added to a solution of Me3In (0.960 g, 6 mmol)
in Et2O (10 cm3) at ꢀ78 ꢁC. A reaction mixture was
allowed to warm to room temperature (1 h), then cooled
to ꢀ78 ꢁC. A solution of benzopinacol (1.098 g, 3 mmol)
in CH2Cl2 (20 cm3) was added dropwise to the mixture.
A reaction mixture was allowed to warm to room temper-
ature (2 h). Colourless, X-ray quality crystals of
6 Æ 2CH2Cl2 (0.790 g, 0.53 mmol) were obtained after crys-
tallisation from the post reaction mixture at 5 ꢁC (yield
35%). The compound undergoes decomposition at 163 ꢁC
without melting.
Acknowledgement
The scientific work was financed by the Polish State
Committee for Scientific Research as Research Project 3
T08E 026 27.
Appendix A. Supplementary material
CCDC 602048, 602049 and 602050 contain the supple-
mentary crystallographic data for 3, 4 and 6. These data
can be obtained free of charge via http://www.ccdc.cam.ac.
uk/conts/retrieving.html, or from the Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@
ccdc.cam.ac.uk. Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.poly.2006.11.032.
1H NMR (CDCl3): d 7.00–7.40 (m, H aromat), 2.17 (6H,
s, OCH3), ꢀ0.14 (6H, s, InCH3), ꢀ0.81 (6H, s, InCH3),
ꢀ1.05 (6H, s, InCH3). 13C NMR (CDCl3): d 144.11,
130.84, 128.58, 127.29 (C aromat), 87.22, 82.98 (CO),
52.80 (OCH3), ꢀ4.26, ꢀ4.51 (InCH3) ppm. Anal. Calc.
for C62H68In4O6Cl4: C, 49.77; H, 4.50. Found: C, 49.02;
H, 4.76%.
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