Synthesis and characterization of a β-diketiminate-supported aluminum dication

Article abstract of DOI:10.1016/j.jorganchem.2007.10.003

A base-stabilized mononuclear aluminum dication has been prepared by extrusion of two triflate anions from a precursor β-diketiminate-supported aluminum bis(triflate) using the tris(2-aminoethyl)amine (tren) ligand.

Full text of DOI:10.1016/j.jorganchem.2007.10.003

Available online at www.sciencedirect.com
Journal of Organometallic Chemistry 692 (2007) 5683–5686
www.elsevier.com/locate/jorganchem
Communication
Synthesis and characterization of a b-diketiminate-supported
aluminum dication
*
Dragoslav Vidovic, Michael Findlater, Gregor Reeske, Alan H. Cowley
Department of Chemistry and Biochemistry, University of Texas at Austin, 1 University Station A5300, Austin, TX 78712, USA
Received 13 September 2007; received in revised form 2 October 2007; accepted 2 October 2007
Available online 9 October 2007
Abstract
A base-stabilized mononuclear aluminum dication has been prepared by extrusion of two triflate anions from a precursor b-diketi-
minate-supported aluminum bis(triflate) using the tris(2-aminoethyl)amine (tren) ligand.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Aluminum; Dication; b-Diketiminate; Triflate; X-ray structure
1. Introduction
copy. The reaction of 1 with an equimolar quantity of
2,20-bipyridine (bipy) in CH2Cl2 solution at ambient tem-
perature resulted in the formation of the 1:1 complex 2 in
71% yield (Scheme 1). The X-ray crystal structure of 2
(Fig. 1) shows that it is a neutral six-coordinate complex
of 1 with bipy and that both triflate groups remain coordi-
nated. Interestingly, however, the Al(1)–O(1) distance is
Interest in aluminum cations has been stimulated by
their use as catalysts and Lewis acid promoters. Several
types of aluminum cations have now been recognized [1].
However, all reported examples are monocationic and, to
the best of our knowledge, mononuclear aluminum dica-
tions [2] have not been prepared. We describe the first
example of such a dication. At the outset, it was clear that
the selection of the supporting ligand would be important.
Since b-diketiminate ligands have been used effectively for
the support of other potentially redox active aluminum
entities such as Al(1) [3,4], we opted to employ this ligand
class in the present case.
˚
ꢀ0.04 A longer than the Al(1)–O(4) distance, possibly
implying incipient ionization of one of the triflate ligands.
One of the consequences of the coordination of the bipy
ligand is that the C3N2Al ring is significantly distorted
and adopts a boat conformation. The dihedral angles of
the N(1)–Al(1)–N(2) and C(1)–C(2)–C(3) planes with
respect to the N(1)–C(1)–C(3)–N(2) plane are 12.21ꢁ and
20.21ꢁ, respectively. A further consequence of the steric
crowding in 2 is that both C6F5 substituents are bent down
with respect to the N(1)–C(1)–C(3)–N(2) plane. The two
triflate ligands maintain their non-equivalence in solution
as evidenced by the observation of distinct 19F NMR
chemical shifts (d ꢁ79.73 and ꢁ80.56 ppm).
2. Results and discussion
Treatment of [HC(CMe)2(NC6F5)2]AlCl2 [5] with two
equivalents of AgOTf in CH3CN solution at room temper-
ature resulted, after workup, in an 82% yield of pale yellow
[HC(CMe)2(NC6F5)2]Al(OTf)2 (1). The bis(triflate) 1 was
characterized by HRMS and multinuclear NMR spectros-
The ambient temperature reaction of 2,20:60,200-terpyri-
dine (terpy) with 1 also resulted in the formation of a com-
plex with 1:1 stoichiometry. However, examination of the
crystalline product by X-ray diffraction revealed that one
of the triflate ligands had been extruded and the aluminum
monocation 3 is formed (Fig. 2). The closest anion–cation
*
Corresponding author. Tel.: +1 5124717484; fax: +1 5124716822.
E-mail address: cowley@mail.utexas.edu (A.H. Cowley).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.10.003

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D. Vidovic et al. / Journal of Organometallic Chemistry 692 (2007) 5683–5686
R
OTf
OTf
Me
Me
N
Al
N
R
1
+ terpy
2+
R
N
N
R
N
N
R
N
N
Me
Me
Me
Me
Me
Me
N
OTf
N
[OTf ]
2
N
Al
Al
Al
N
N
N
R
OTf
OTf
N
N
R
N
R
OTf
2
3
4
R = C6F5
Scheme 1.
Fig. 2. ORTEP view of the [{HC(CMe)2(NC6F5)2}Al(terpy)(OTf)]+
cation (3+) showing the atom numbering scheme. Selected bond distances
Fig. 1. ORTEP view of {HC(CMe)2(NC6F5)2}Al(bipy)(OTf)2 (2) showing
the atom numbering scheme. Selected bond distances (A) and angles (ꢁ):
˚
(A) and angles (ꢁ): Al(1)–O(1) 1.933(3), Al(1)–N(3) 2.0563(3), Al(1)–N(4)
˚
1.991(3), Al(1)–N(5) 2.060(3), Al(1)–N(1) 1.974(4), N(1)–C(1) 1.328(5),
C(1)–C(2) 1.400(6), C(2)–C(3) 1.380(6), C(3)–N(2) 1.357(5), N(2)–Al(1)
1.967(4), N(3)–Al(1)–N(5) 155.70(15), N(3)–Al(1)–N(4) 78.10(14), N(4)–
Al(1)–N(5) 77.67(14), O(1)–Al(1)–N(4) 85.37(13), N(1)–Al(1)–N(2)
92.19(15), Al(1)–N(1)–C(1) 126.2(3), N(1)–C(1)–C(2) 122.1(4), C(1)–
C(2)–C(3) 128.5(4), C(2)–C(3)–N(2) 123.4(4), C(3)–N(2)–Al(1) 124.6(3).
Al(1)–O(1) 1.941(2), Al(1)–O(4) 1.9025(18), Al(1)–N(3) 2.053(2), Al(1)–
N(4) 2.047(2), Al(1)–O(4) 1.9025(18), Al(1)–N(3) 2.053(2), Al(1)–N(4)
2.047(2), Al(1)–N(1) 1.989(2), N(1)–C(1) 1.341(3), C(1)–C(2) 1.389(4),
C(2)–C(3) 1.389(4), C(3)–N(2) 1.348(3), N(2)–Al(1) 1.975(2), O(1)–Al(1)–
O(4) 86.87(8), N(1)–Al(1)–N(2) 90.24(10), Al(1)–N(1)–C(1) 124.37(18),
N(1)–C(1)–C(2) 122.6(3), C(1)–C(2)–C(3) 127.3(3), C(2)–C(3)–N(2)
122.3(3), C(3)–N(2)–Al(1) 125.47(18).
tion would necessitate the use of a stabilizing tetradentate
ligand. Accordingly, 1 was treated with an equimolar quan-
tity of tris(2-aminoethyl)amine (tren) in CH3CN solution
at ambient temperature. The 19F NMR spectrum of the
product (4) revealed the presence of only one resonance
for the triflate groups (d ꢁ79.65 ppm) thus implying that
both triflate anions had been extruded. This conclusion
was confirmed by the X-ray crystal structure of 4
(Fig. 3). The two triflate anions are well separated from
the aluminum dication. The closest anion–cation contact
˚
˚
contacts are 8.572 A between Al(1) and O(4), and 6.804 A
between Al(1) and F(16). The Al–O distance for the bound
˚
triflate (1.933(3) A) falls between those found for the bipy
complex, 2 (1.9025(18) and 1.941(2) A). Overall, the terpy
˚
complex 3 is less sterically strained than the bipy complex
2. This is evident from the fact that e.g. the sum of bond
angles for the C3N2Al ring of 3 (717.0(4)ꢁ) is greater than
that for 2 (712.3(3)ꢁ).
At this point it was clear that the removal of both triflate
anions and generation of the targeted base-stabilized dica-
˚
is 4.302 A between Al(1) and O(1). The b-diketiminate ring

D. Vidovic et al. / Journal of Organometallic Chemistry 692 (2007) 5683–5686
5685
in 0.820 g (82%) of pale yellow solid 1. This solid was used
in subsequent reactions without further purification. MS
(CI+, CH4): m/z 755 [M+H]+. HRMS (CI+, CH4). Calc.
for C19H8AlF16N2O6S2, m/z 754.9384. Found: 754.9389.
1H NMR (CD3CN): d 2.26 (s, 6H, CH3), 5.54 (s, 1H,
c-CH). 19F NMR (CD3CN): d ꢁ79.45 (s, 6F, OTf),
ꢁ145.68 to ꢁ164.50 (m, 10F, C6F5). 27Al NMR (CD3CN):
dꢁ5.40.
3.2. Preparation of
[CH(CMe)2(NC6F5)2]Al(bipy)(OTf)2 (2)
Fig. 3. ORTEP view of the [{HC(CMe)2(NC6F5)2}Al(tren)]2+ cation (4+)
showing the atom numbering scheme. Selected bond distances (A) and
˚
Solid 2,20-bipyridine (0.021 g, 0.134 mmol) was added to
10 mL of a methylene chloride solution containing 0.100 g
(0.132 mmol) of 1. The reaction mixture was allowed to stir
overnight, after which it was filtered. Hexane (10 mL) was
added to the filtrate resulting in the formation of 0.852 g
(71%) of pale yellow crystalline 2 after 2 d. MS (CI+,
CH4): m/z 911 [M+H]+. HRMS (CI+, CH4). Calc. for
angles (ꢁ): Al(1)–N(1) 1.962(5), Al(1)–N(2) 2.052(4), Al(1)–N(3) 2.093(5),
Al(1)–N(4) 2.053(5), Al(1)–N(5) 2.086(4), Al(1)–N(6) 2.037(5), N(1)–C(1)
1.365(7), C(1)–C(2) 1.372(7), C(2)–C(3) 1.405(8), C(3)–N(2) 1.354(7),
N(2)–Al(1) 2.052(4), N(4)–Al(1)–N(6) 164.4(2), N(1)–Al(1)–N(5)
87.65(18), N(2)–Al(1)–N(3) 102.00(18), N(1)–Al(1)–N(2) 88.18(18),
Al(1)–N(1)–C(1) 120.1(3), N(1)–C(1)–C(2) 121.7(5), C(1)–C(2)–C(3)
125.2(5), C(2)–C(3)–N(2) 122.8(5), C(3)–N(2)–Al(1) 118.1(4).
1
C19H16AlF16N4O6S2, m/z 911.0071. Found: 911.0064. H
of 4 is significantly more distorted toward the boat confor-
mation than those of the bipy and terpy complexes. Thus,
the dihedral angles between the C(1)–N(1)–C(3)–N(2)
plane and the N(1)–Al(1)–N(2) and C(1)–C(2)–C(3) planes
are 36.87ꢁ and 20.59ꢁ, respectively, and the sum of bond
angles for the C3N2 Al ring is 694.2(5)ꢁ. Significant distor-
tions are also evident in the roughly octahedral geometry at
the six-coordinate aluminum center. For example, the axial
bond angle, N(4)–Al(1)–N(6), is 164.4(2)ꢁ and the equato-
rial bond angles range from 81.31(19)ꢁ to 102.00(18)ꢁ.
To minimize the steric interactions with the coordinated
tren ligand, both C6F5 groups are bent away from the
C(1)–N(1)–C(3)–N(2) plane such that the C6F5 ring cen-
NMR (CD3CN): d 1.28 (s, 6H, CH3), 5.50 (s, 1H, c-CH),
7.90 (m, 2H, 5,50-CH), 8.45 (m, 2H, 4,40-CH), 8.52 (m,
2H, 3,30-CH), 8.90 (m, br, 2H, 6,60-CH). 19F NMR
(CD3CN): d ꢁ79.73 and ꢁ80.56 (s, 6F, OTf), ꢁ143.98 to
ꢁ151.13 (m, o-F, 4F), ꢁ158.29 to ꢁ161.04 (m, p-F, 2F),
ꢁ164.26 to ꢁ164.78 (m, m-F, 4F). 27Al NMR (CD3CN):
d 13.09 (broad, s).
3.3. Preparation of
[(CH(CMe)2(NC6F5)2)Al(terpy)(OTf)][OTf] (3)
This compound was prepared in a similar fashion to 2
using 1 (0.100 g, 0.132 mmol) and 2,20:60,20 0-terpyridine
(0.031 g, 0.133 mmol). Yield of pale yellow crystalline
3 = 0.097 g (74%). MS (CI+, CH4): m/z 838 [MꢁOTf]+.
HRMS (CI+, CH4). Calc. for C33H18AlF13N5O3S, m/z
838.0738. Found: 838.0732. 1H NMR (CD3CN): d 1.65
and 1.67 (s, 3H, CH3), 2.25 and 2.26 (s, 3H, CH3), 5.74
(s, 1H, c-CH), 8.05 (m, 2H, 2,40-6,40 0-CH), 8.42 (m, 2H,
2,50-6,50 0-CH), 8.45 (m, 1H, 4-CH), 8.50 (m, 2H, 3-5-
CH), 8.52 (m, 2H, 2,60-6,60 0-CH), 8.99 (m, 2H, 2,30-6,30 0-
CH). 19F NMR (CD3CN): d ꢁ79.53 and ꢁ80.35 (s, 6F,
˚
troids deviate from said plane by an average of 1.348 A.
The corresponding distances for 2 and 3 are 0.801 and
˚
0.316 A, respectively.
3. Experimental
All reactions were performed under a dry, oxygen-free
atmosphere utilizing Schlenk manifold techniques or a
drybox. All solvents were dried and distilled under nitrogen
prior to use. NMR spectra were recorded at 295 K on a
Varian Unity +300 instrument (1H 300 MHz; 13C{1H}
75 MHz; 19F 282 MHz; 27Al{1H} 78 MHz). Low- and
high-resolution mass spectra were measured on Finnigan
MAT TSQ-700 and VG Analytical ZAB-VE sector instru-
ments, respectively.
3
OTf), ꢁ144.64 and ꢁ146.67 (d, o-F, 4F, JFF = 16.9 Hz),
ꢁ154.74 and ꢁ158.40 (m, p-F, 2F), ꢁ161.74 and ꢁ163.04
(m, m-F, 4F). 27Al NMR (CD3CN): d 17.72 (broad, s).
3.4. Preparation of
[(CH(CMe)2(NC6F5)2)Al(tren)][2 Æ OTf] (4)
3.1. Preparation of [CH(CMe)2(NC6F5)2]Al(OTf)2 (1)
This compound was prepared in a similar fashion to 2
using 0.100 g (0.132 mmol) of
1 and tris(2-amino-
Solid AgOTf (0.680 g, 2.656 mmol) was added to 40 mL
of an acetonitrile solution containing 0.700 g (1.328 mmol)
of [(HC(CMe)2(NC6F5)2]AlCl2 at ambient temperature in
the absence of light. The reaction mixture was left to stir
for 2 d, after which it was filtered through Celiteꢂ and
the solvent was removed under reduced pressure resulting
ethyl)amine (0.020 g, 0.137 mmol). Yield of pale green
crystalline 4 = 0.092 g (77%). MS (ESI+): m/z 601
[M2+ꢁH+]+. HRMS (ESI+). Calc. for C23H24AlF10N6,
m/z 601.1718. Found: 601.1726. 1H NMR (CD3CN,
[M] @ 0.040): d 1.99 (s, 6H, CH3), 2.74 (t, 6H, N–CH2,
3
3JHH = 5.4 Hz), 2.97 (t, 6H, H2N–CH2, JHH = 5.7 Hz),

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D. Vidovic et al. / Journal of Organometallic Chemistry 692 (2007) 5683–5686
5.34 (s, 1H, c-CH), 5.38 (s, br, 6H, NH2). 13C NMR
(CD3CN, saturated solution): 20.47 (s, CH3), 38.52 (s,
CH2–NH2), 52.25 (s, N–CH2), 98.74 (s, H–C(c)), 120.90
153(2) K. The structures were solved by direct methods
(SIR-97) [6] and refined by full-matrix, least-squares tech-
niques on F2 using SHELX-97 [7].
1
(q, CF3, JCF = 319 Hz), 137.04, 140.23 and 143.50
(C6F5), 165.53 (s, C–N). 19F NMR (CD3CN, saturated
solution): d ꢁ79.65 (s, 6F, OTf), ꢁ151.72 (d, o-F, 4F,
4. Supplementary material
3
3JFF = 16.1 Hz), ꢁ163.64 (t, p-F, 2F, JFF = 20.3 Hz),
CCDC 645182, 645180 and 645181 contain the supple-
mentary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crys-
tallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
ꢁ166.02 (m, m-F, 4F). 27Al NMR (CD3CN): no signal
detected.
3.5. Crystal data
For 2: C29H15AlF16N4O6S2, Mr = 910.57; a = 34.635(5),
Acknowledgements
3
˚
b = 12.688(5), c = 17.564(5) A ; a = 90ꢁ, b = 103.128(5)ꢁ,
3
˚
c = 90ꢁ; V = 7517(4) A ; Z = 8; space group C2/c (mono-
We are grateful to the Robert A. Welch Foundation
(F-0003) for financial support of this work.
clinic); T = 153(2) K; k = 0.71073 A; l = 0.291 mmꢁ1
;
˚
q
calc = 1.609 g cmꢁ3
;
2hmax = 55.00ꢁ;
14331 measured reflections, 8618 independent (Rint
F(000) = 3632;
=
References
0.0364); R1 = 0.0530, wR2 = 0.1362; largest difference peak
ꢁ3
˚
and hole 0.421/ꢁ0.398 e A
.
For 3 Æ (CH2Cl2)3:
[1] (a) D.A. Atwood, Coord. Chem. Rev. 176 (1998) 407;
(b) D.A. Atwood, Main Group Chem. News 5 (1997) 30;
For recent articles, see e.g.(c) A. Mitra, L.J. DePue, S. Parkin, D.A.
Atwood, J. Am. Chem. Soc. 128 (2006) 1147;
C37H24AlCl6F16N5O6S2, Mr = 124.41; a = 19.758(5), b =
˚
13.678(5), c = 19.512(5) A; a = 90ꢁ, b = 113.289(5)ꢁ,
3
˚
c = 90ꢁ; V = 4843(2) A ; Z = 4; space group P21/c (mono-
clinic); T = 153(2) K; k = 0.71073 A; l = 0.571 mmꢁ1
;
˚
´
´
(d) P.J. Aragon Saenz, S.H. Oakley, M.P. Coles, P.B. Hitchcock,
Chem. Commun. (2007) 816.
q
calc = 1.704 g cmꢁ3
;
2hmax = 55.00ꢁ;
19427 measured reflections, 10976 independent (Rint
F(000) = 2480;
[2] Dinuclear aluminum dications are known, however. See [1a].
[3] C. Cui, H.W. Roesky, H.-G. Schmidt, M. Noltemeyer, H. Hao, F.
Cimpoesu, Angew. Chem., Int. Ed. Engl. 39 (2000) 4274;
H.W. Roesky, S.S. Kumar, Chem. Commun. (2005) 4027.
[4] For a theoretical study of one-electron reduction of the model
b-diketiminate [HC(MeC)2(NMe)2]Al, see I. McKenzie, P.W.
Percival, J.A.C. Clyburne, Chem. Commun. (2005) 1134.
[5] D. Vidovic, J.A. Moore, J.N. Jones, A.H. Cowley, J. Am. Chem. Soc.
127 (2005) 4566.
[6] A. Altomare, M.C. Burla, M. Camalli, G.L. Cascarano, C. Giaco-
vazzo, C. Guagliardi, A.G.G. Polidori, R. Spagna, J. Appl. Cryst. 115
(1999) 32.
[7] G.M. Sheldrick, SHELX-97, Program for Crystal Structure Refinement,
University of Go¨ttingen, Go¨ttingen, Germany, 1997.
=
0.0720); R1 = 0.0670, wR2 = 0.1692; largest difference peak
and hole 0.797/ꢁ0.816 e Aꢁ3. For 4 Æ (CH2Cl2)2: C27H23
˚
AlCl4 F16N6O6S2, Mr = 1064.41; a = 11.195(2), b =
˚
21.287(4), c = 17.378(4) A; a = 90ꢁ, b = 100.34(3)ꢁ,
3
˚
c = 90ꢁ; V = 4073.9(14) A ; Z = 4; space group P21/c
˚
(monoclinic); T = 153(2) K; k = 0.71073 A; l = 0.537
mmꢁ1
; q ; 2hmax = 54.00ꢁ; F(000) =
calc = 1.745 g cmꢁ3
2152; 14891 measured reflections, 8853 independent
(Rint = 0.0696); R1 = 0.0763, wR2 = 0.1971; largest differ-
ence peak and hole 0.965/ꢁ0.933 e Aꢁ3. The crystal data
˚
were collected on a Nonius Kappa CCD diffractometer at