Molecular fluorinated alumoxanes: One step towards well-defined fluorinated alumina

Article abstract of DOI:10.1016/j.inoche.2010.01.033

The unprecedented molecular alumoxane [{LAl(F)}₂(μ-O)] (2) is easily accessible from the reaction between [{LAl(H)}₂(μ-O)] (1) and the mild fluorinating agent Me₃SnF. Furthermore, the controlled hydrolysis of 2 led to the preparation of the binuclear aluminum hydroxyfluoride [{LAl(F)(μ-OH)}₂] (3) in a straightforward manner. The compounds were characterized by spectroscopic techniques (IR, EI-MS, solid state MAS NMR (¹³C and ²⁷Al), solution NMR (¹H, ¹³C, ¹⁹F and ²⁷Al)) and in the case of 3, through X-ray diffraction studies. Compound 3 represents the first example of a molecular aluminum hydroxyfluoride fully structurally characterized.

Full text of DOI:10.1016/j.inoche.2010.01.033

Inorganic Chemistry Communications 13 (2010) 543–545
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Molecular fluorinated alumoxanes: One step towards well-defined
fluorinated alumina
Sandra González-Gallardo b,2, Vojtech Jancik a,1, Ma. de las Nieves Zavala-Segovia a,1
,
Mónica Moya-Cabrera a, ,1
*
a Centro Conjunto de Investigación en Química Sustentable UAEM-UNAM, Carr. Toluca-Atlacomulco Km 14.5, Unidad San Cayetano C.P. 50200, Toluca Edo. de México, Mexico
b Instituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior, Ciudad Universitaria, México D.F. 04510, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
The unprecedented molecular alumoxane [{LAl(F)}2(l-O)] (2) is easily accessible from the reaction
Received 2 January 2010
Accepted 31 January 2010
Available online 6 February 2010
between [{LAl(H)}2(
l
-O)] (1) and the mild fluorinating agent Me3SnF. Furthermore, the controlled hydro-
lysis of 2 led to the preparation of the binuclear aluminum hydroxyfluoride [{LAl(F)(
l
-OH)}2] (3) in a
straightforward manner. The compounds were characterized by spectroscopic techniques (IR, EI–MS,
solid state MAS NMR (13C and 27Al), solution NMR (1H, 13C, 19F and 27Al)) and in the case of 3, through
X-ray diffraction studies. Compound 3 represents the first example of a molecular aluminum hydroxyflu-
oride fully structurally characterized.
Keywords:
Aluminum
Alumoxanes
Fluorinated alumina
b-Diketiminate ligands
Ó 2010 Elsevier B.V. All rights reserved.
Aluminum fluorides and hydroxyfluorides have been used as
catalysts for a wide range of important industrial reactions, such
as aromatic alkylation, Friedel-crafts acylation, and halogen ex-
change in CFCs and HFCs [1]. Therefore, they have been extensively
studied in order to understand the relationship between their
structural features and catalytic activity. It is known that the num-
ber and strength of the Lewis (Al3+) and Brönsted (–OH) acidic cen-
ters on the surface, as well as its structure, are factors that directly
influence the catalytic activity of the material [2]. Thus, the intro-
duction of fluoride groups to alumina increases its acidity and
modifies the nature of the hydroxy groups. A wide variety of syn-
thetic methods to obtain fluorinated alumina have been developed
[3]. However, the degree of fluorination largely depends on the
method used and the materials obtained contain aluminum sites
in various coordination environments randomly distributed [3a].
The octahedral sites can be generally described as AlF6ꢀx(OH)x
(x = 0ꢀ6). Furthermore, these inorganic compounds are insoluble
in organic solvents and in some cases, they occur in multi-phase
mixtures or only highly disordered X-ray amorphous materials
can be obtained [3c]. Adding to these disadvantages are the syn-
thetic methods employed, involving in some cases the use of fluo-
rine [3d], hydrofluoric acid or even anhydrous HF [1a,3c,e,f], which
is certainly convenient to avoid. Therefore, it is highly desirable to
develop a synthetic route to obtain analogous molecular, well-de-
fined species under mild conditions. These discrete molecules
could be fully characterized by common spectroscopic techniques
and thus serve as models to aid the understanding of the struc-
ture-activity relationship of the widely used fluorinated aluminas.
Following on our study on the reactivity of molecular alumox-
anes stabilized by the b-diketiminate ligand ðHC½ðCMeÞNð2; 4; 6-
Me3C6H2ÞꢁꢀÞ (L) [4], herein we report on the use of the alumoxane
2
hydride [{LAl(H)}2(l-O)] (1) for the synthesis of the first soluble
fluorinated alumoxane [{LAl(F)}2(l-O)] (2) under mild conditions
and its transformation into the corresponding unprecedented
molecular hydroxyfluoride [{LAl(F)(l-OH)}2] (3) by direct hydroly-
sis (Scheme 1).
Compound 2 was obtained as a white solid in good yields by
fluorination of the alumoxane hydride with Me3SnF [5]
(Scheme 1). Compound [{LAl(F)}2( -O)] (2) represents the first
1
l
example of a molecular fluorinated alumoxane. The presence of
the fluorine atoms greatly increases the Lewis acidity of the alumi-
num centers, which causes 2 to be extremely reactive towards H2O.
Furthermore, 2 is soluble in most common organic solvents albeit
extremely sensitive to moisture and oxygen. Compound 2 was
characterized by common spectroscopic methods: IR, NMR (1H,
13C, 19F, 27Al) and EI–MS. The IR and 1H RMN spectra of 2 both ex-
hibit the presence of the deprotonated b-diketiminate ligand.
Moreover, the 19F NMR spectrum of 2 displays a single signal at
d = –163.9 ppm corresponding to the fluorine atom in the Al–F
* Corresponding author. Fax: +52 722 1806217.
bond [6], while a peak at d 118 ppm (x1/2 = 560 Hz) owing to a tet-
E-mail address: monica.moya@correo.unam.mx (M. Moya-Cabrera).
Academic staff from the Instituto de Química, Universidad Nacional Autónoma de
racoordinate aluminum atom is observed in its 27Al NMR spectrum.
In addition, the MS–EI spectrum of 2 exhibits a peak at m/z 774
corresponding to [M+–H].
1
México.
2
Fax: +52 55 56162217.
1387-7003/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.01.033

544
S. González-Gallardo et al. / Inorganic Chemistry Communications 13 (2010) 543–545
Scheme 1.
Compound 2 reacts readily with one equivalent of water yield-
ing the corresponding hydroxyfluoride [{LAl(F)( -OH)}2] (3) as a
l
white solid in high yield. Indeed, 3 can also be obtained as colorless
crystals upon allowing the slow diffusion of a solution of water in
THF either to a toluene or CH2Cl2 solution of 2 at ambient temper-
ature [7]. Compound 3 is insoluble in organic solvents and mildly
sensitive to moisture and oxygen. Therefore it was characterized
by EI mass spectrometry, IR and MAS NMR (13C and 27Al) spectros-
copy and X-ray diffraction studies. Furthermore, [{LAl(F)(l-OH)}2]
(3) represents the first example of a structurally characterized alu-
minum hydroxyfluoride.
The IR spectrum of 3 shows the characteristic pattern for the
¼ 3677 cmꢀ1, assigned
~
deprotonated ligand as well as a band at
v
to the AlO–H vibration, while the mass spectrum exhibits a peak at
m/z 775 owing to the fragment [M+–OH]. The 13C MAS NMR spec-
trum of 3 shows the characteristic fragments of the deprotonated
ligand, whereas its 27Al MAS NMR spectrum exhibits a single signal
at d = 30 ppm (x1/2 = 810 Hz), suggesting the presence of chemi-
cally equivalent pentacoordinate aluminum centers [8]. This is in
agreement with the crystal structure determined by X-ray diffrac-
tion studies (vide infra).
Due to its insolubility, crystals suitable for X-ray diffraction
studies of two pseudo-polymorphs of 3 were obtained by slow dif-
fusion of water into concentrated solutions of 2 in toluene (3a) and
dichloromethane (3b). They both crystallize in the triclinic P1
space group with one half of a molecule for 3a and one half of a
molecule and a CH2Cl2 molecule for 3b in the asymmetric unit,
respectively [9]. No significant geometrical differences were found
for the pseudo-polymorphs and thus only the molecular structure
of 3a is presented in Fig. 1. Selected bond distances and angles for
3a and 3b are listed in Table 1.
Fig. 1. Molecular structure of compound 3a with thermal ellipsoids at 50% level of
probability. Hydrogen atoms except those from the hydroxy groups have been
omitted for clarity.
Table 1
Selected bond distances (Å) and angles (°) for 3a and 3b.
[{LAl(F)(l-OH)}2] (3a) [{LAl(F)(l-OH)}2]ꢂ2CH2Cl2 (3b)
Al(1)–N(1)
Al(1)–N(2)
Al(1)–F(1)
Al(1)–O(1)
1.956(2)
1.952(2)
1.684(1)
1.847(2)
1.850(2)
2.963(1)
106.6(1)
73.4(1)
90.8(1)
1.945(3)
1.966(3)
1.688(2)
1.863(3)
1.829(3)
2.949(2)
106.0(1)
74.0(1)
91.2(1)
In both cases, the aluminum atoms are pentacoordinated exhib-
iting a square-pyramidal geometry, (3a,
s = 0.04; 3b, s = 0.22) [10].
This geometry has also been observed for the related compound
Al(1)–O(1A)
[{iPrLAl(Cl)(
l
-OH)}2] ðiPrL ¼ HC½ðCMeÞNð2; 6 ꢀ 2C6H2ÞꢁꢀÞ [11]. The
iPr
Al(1)ꢂꢂꢂAl(1A)
Al(1)–O(1)–Al(1A)
O(1)–Al(1A)–O(1A)
N(1)–Al(1)–N(2)
O(1)–Al(1)–F(1)
2
four-membered Al(l-OH)2Al ring is located on an inversion center
thus is essentially planar while the fluorine atoms are found in an
anti conformation. The Al–O–Al angles in 3a (106.6(1)°) and 3b
(106.0(1)°) are identical and comparable to the corresponding an-
104.7(1)
101.1(1)
gle in [{iPrLAl(Cl)(
l-OH)}2] (107.6(1)°). Due to the higher coordina-
tion number in compound 3, the Al–F bonds (3a, 1.684(1) Å; 3b,
1.688(2) Å) are slightly longer than those observed in the mononu-
clear fluorides (CH2)3[N(2,6-iPr2C6H3)]2AlF(NMe3) (1.678(1) Å)
[12a] and [iPrLAlF2] (1.650(1), 1.655(1) Å) [12b], which are tetraco-
ordinated. Furthermore, the Al–OH and Al–N bonds in 3a and 3b
(Table 1) are shorter than those found in [{iPrLAl(Cl)(
l-OH)}2]

S. González-Gallardo et al. / Inorganic Chemistry Communications 13 (2010) 543–545
545
was stirred for 24 h and filtered. All volatiles were removed under vacuum to
leave a white residue, which was treated with hexane (10 mL). After filtration
and drying under vacuum, 2 was obtained as a white powder. Yield 0.45 g
(85%); m.p. 220 °C; 1H NMR (300 MHz, C6D6, 25 °C): d = 1.46 (s, 12H, CH3), 2.05
(s, 12H, p–Ar–CH3), 2.17 (s, 12H, o–Ar–CH3), 2.30 (s, 12H, o–Ar–CH3), 4.87
(Al–OH, 1.875(1), 1.886(1) Å; Al–N, 1.977(1), 1.984(1) Å). The dis-
tance between the two aluminum atoms (3a, 2.963(1) Å; 3b,
2.949(2) Å) is smaller than that observed for [{iPrLAl(Cl)(
l-OH)}2]
(3.034(1) Å), all of which are larger than the sum of the covalent
radii (RcovAl, Al = 2.698 Å) [13] with no evidence suggesting any
AlꢂꢂꢂAl interaction.
(s, 2H, c
–CH), 6.76–6.79 ppm (m, 8H, m–Ar–H); 13C NMR (75.58 MHz, CDCl3,
25 °C): d = 17.9 (CH3), 20.9 (p–Ar–CH3), 22.3 (o–Ar–CH3), 96.3 (c–CH), 128.9,
133.6, 134.7, 139.9 (Ar–C), 169.6 ppm (C@N); 19F NMR (282.73 MHz, CDCl3,
25 °C) = –163.9 ppm; 27Al RMN (78.16 MHz, CDCl3, 25 °C): d = 118 ppm
It is noteworthy to mention that due to the experimental diffi-
culty in the syntheses and characterization of fluorinated aluminas,
theoretical methods have been applied to study the nature of their
active sites. In this regard, recent reports have suggested that Al3+
centers with coordination numbers lower than six are present in
b-AlF3 and are assumed to act as strong Lewis acid sites, responsi-
ble for the catalytic activity of the material [14]. Thus, it is reason-
able to suggest that the five-fold coordinated aluminum centers in
3 should exhibit a stronger Lewis acidity than the six-fold coordi-
nated Al3+ sites found in most aluminum hydroxyfluorides.
–1
~
v
(tetracoordinate Al); IR (KBr):
= 1536 cm (s, C@N); IE–EM (70 eV): m/z
(%): 774(37) [M+–H]. Due to its extreme moisture sensitivity elemental
analyses could not be performed on 2.
[6] S.D. Waezsada, F.-Q. Liu, E.F. Murphy, H.W. Roesky, M. Teichert, I. Uson, H.-G.
Schmidt, T. Albers, E. Parisini, M. Noltemeyer, Organometallics 16 (1997) 1260.
[7] Compound 3 can be obtained by two different methods: (a) To a solution of
[{LAl(F)}2(l-O)] (2) (0.45 g, 0.62 mmol) in toluene (5 mL) was added a solution
of H2O (0.62 mmol) in THF (5 mL) at ambient temperature. The reaction
mixture was stirred for 24 h upon which an abundant precipitate was formed.
The slurry was then filtered and the white solid was washed with toluene
(3 mL) and dried under vacuum to give 3 as a microcrystalline white powder
(0.47 g, 96%); (b) [LAl(H)]2(l-O) (1) (0.50 g, 0.68 mmol) and Me3SnF (0.27 g,
0.75 mmol) were weighed together in a Schlenk flask and toluene (20 mL) was
added. The reaction mixture was stirred for 24 h and filtered to remove the
insoluble material. A Schlenk flask containing the filtrate was then connected
through a glass tube to another Schlenk flask containing a solution of H2O
(0.68 mmol) in THF (6 mL). After one week, colorless crystals were obtained
which were filtered and dried under vacuum to give 3 (0.48 g, 80%). M.p. 175–
180 °C (decomp.); 13C MAS RMN (75.58 MHz, 25 °C): d = 20.2 (CH3), 21.2
Conclusions
In this work we have shown the feasibility of obtaining a struc-
turally well-defined aluminum hydroxyfluoride, using a soluble,
molecular fluorinated alumoxane as a starting material under mild
conditions. Further work on the study of the acidic properties of
this material is currently in progress.
(p–Ar–CH3), 23.1 (o–Ar–CH3), 99.9 (c–CH), 130.3, 133.2, 134.5, 146.8 (Ar–C),
169.5 ppm (C@N); 27Al MAS RMN (78.16 MHz, 25 °C): d = 30 ppm
–1
~
v
(pentacoordinate Al); IR (KBr):
= 3677 (m, AlO–H), 1540 cm (s, C@N); IE–
EM (70 eV): m/z (%): 775(10) [M+–OH]; Elemental analysis (%) calc. for
C46H60Al2F2N4O2 (792.95): C 69.7, H 7.6, N 7.1; found: C 69.5; H, 7.5; N, 7.1.
[8] J.J. Delpuech, in: P. Laszlo (Ed.), NMR of Newly Accessible Nuclei, Academic,
New York, 1983, pp. 153–195.
Acknowledgements
[9] (a) Crystal data for 3a: C46H60Al2F2N4O2 (792.94), triclinic, space group P1,
This work was supported by the CONACyT (Grant 058484). We
also thank H. Rios for his technical assistance.
a = 8.274(2) Å, b = 10.289(2) Å, c = 13.542(2) Å;
a
= 71.82(2)°, b = 77.99(2)°,
calcd = 1.251 g cmꢀ3
F(0 0 0) = 424,
. Of the 11,011 measured reflections, 3709 were
c
l
= 76.17(1)°; V = 1052.3(4) Å3, Z = 1,
(MoK
) = 0.121 mmꢀ1
q
,
a
independent (Rint = 0.0420). The final refinements converged at R1 = 0.0457
for I > 2r
(I), wR2 = 0.1192 for all data. The final difference Fourier synthesis
Appendix A. Supplementary material
gave a min/max residual electron density of 0.299/ꢀ0.251 e Å3. (b) Crystal data
for 3b: C46H60Al2F2N4O2ꢂ2CH2Cl2 (962.79), triclinic, space group P1,
CCDC 758533 and 758534 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.inoche.2010.01.033.
a = 8.861(2) Å, b = 12.478(2) Å, c = 13.223(2) Å;
a
= 63.92(3)°, b = 71.26(2)°,
F(0 0 0) = 508,
. Of the 12,505 measured reflections, 4502 were
c
l
= 76.16(2)°; V = 1235(1) Å3, Z = 1,
(MoK
) = 0.324 mmꢀ1
q ,
calcd = 1.294 g cmꢀ3
a
independent (Rint = 0.0886). The final refinements converged at R1 = 0.0669
for I > 2r
(I), wR2 = 0.1482 for all data. The final difference Fourier synthesis
gave a min/max residual electron density of 0.297/ꢀ0.269 e Å3. Crystals were
mounted on nylon loops and rapidly placed in a stream of cold nitrogen.
Diffraction data were collected on a Bruker-APEX three-circle diffractometer
using MoKa radiation (k = 0.71073 Å) at 173 K. The structures were solved by
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