Lithium, aluminum, and gallium complexes of the C6F 5-substituted β-diketiminate ligand [HC(CMe)2(NC 6F5)2]-

Article abstract of DOI:10.1002/zaac.200500221

The β-aminoimine [HC(CMe)₂(NC₆F ₅)₂]H (1) is readily converted into the lithium β-diketiminate [HC(CMe)₂(NC₆F₅) ₂]Li·OEt₂ (2) by treatment with MeLi. The al

Full text of DOI:10.1002/zaac.200500221

Lithium, Aluminum, and Gallium Complexes of the C6F5-Substituted
β-Diketiminate Ligand [HC(CMe)2(NC6F5)2]؊
Dragoslav Vidovic, Jamie N. Jones, Jennifer A. Moore, and Alan H. Cowley*
Austin, Texas/USA, Department of Chemistry and Biochemistry, The University of Texas at Austin
Received March 30, 2005.
Dedicated to Professor Herbert W. Roesky on the Occasion of his 70th Birthday
Abstract. The β-aminoimine [HC(CMe)2(NC6F5)2]H (1) is readily con-
verted into the lithium β-diketiminate [HC(CMe)2(NC6F5)2]Li·OEt2
(2) by treatment with MeLi. The aluminum and gallium derivatives
[{HC(CMe)2(NC6F5)2}MMe2] (M ϭ Al (3); Ga (4)) have been
prepared via the CH4 elimination reactions of 1 with MMe3. The
X-ray crystal structures of 1 Ϫ 4 have been determined and the
trends in metrical parameters are discussed.
Keywords: β-Diketiminates; Lithium; Aluminum; Gallium; Crystal
structure
Introduction
yield by treatment of [HC(CMe)2(NC6F5)2]H (1) with MeLi
in hexane/diethyl ether solution. The compound crystallizes
β-Diketiminates continue to attract considerable interest as
supporting ligands due to their range of coordination
modes and their propensity to stabilize low oxidation states
[1]. Illustrative of the latter point is the isolation and
characterization of novel aluminum(I) and gallium(I)
β-diketiminates by Roesky et al. [2] and Power et al. [3],
respectively. In the context of main group chemistry, the
β-diketiminate ligands that have been employed thus far
typically feature alkyl and/or aryl substituents [1]. Con-
siderably less information is available regarding β-diketim-
inate ligands bearing electron-withdrawing groups. How-
ever, one such ligand, [HC(CMe)2(NC6F5)2]Ϫ, has been re-
ported [4]. At the present time, this ligand has only been
used for the synthesis of two iron complexes [4]. To the best
of our knowledge, there is no record of the use of this li-
gand in main group chemistry. However, we have recently
employed the [HC(CMe)2(NC6F5)2]Ϫ ligand for the iso-
lation of an oxoborane (LBϭO) stabilized by complexation
to aluminum trichloride [5]. In the present paper, we de-
scribe the syntheses and X-ray crystal structures of lithium,
aluminum, and gallium complexes supported by the
[HC(CMe)2(NC6F5)2]Ϫ ligand. We also report the structure
of the corresponding β-aminoimine, [HC(CMe)2(NC6F5)2]H.
1
with a molecule of Et2O. The fact that the N-H H NMR
chemical shift of [HC(CMe)2(NC6F5)2]H is close to δ 12 [4]
implies that this proton should be sufficiently acidic to ren-
der protonolysis a viable synthetic route to β-diketiminate
complexes. Indeed, Power et al. [4] have employed 1 for the
protonolysis of metal amides in order to prepare the iron
β-diketiminates referred to earlier. We therefore adopted the
methane elimination approach outlined below for the syn-
thesis of the aluminum and gallium dimethyl derivatives, 3
and 4.
[HC(CMe)2(NC6F5)2]H ϩ MMe3
Ǟ
ϪCH4
[{HC(CMe)2(NC6F5)2}MMe2]
M ϭ Al(3), Ga(4)
X-ray Crystal Structures
To the best of our knowledge, 1 is only the third structurally
characterized β-aminoimine, the other two being
H(Ph)NC(Me)CHC(Me)N(Ph) (5) [6] and H(Ar)NC-
(Me)CH(Me)N(Ar) (Ar ϭ 2,6-i-Pr2C6H3) (6) [7]. Overall,
the structure of 1 (Figure 1 and Table 1) is very similar to
those of 5 and 6. The fact that the N(2)-C(4) bond distance
Results and Discussion
Syntheses
˚
˚
in 1 (1.300(3) A) is shorter than those in 5 (1.314(8) A) and
The lithium derivative [HC(CMe)2(NC6F5)2]Li·OEt2 (2), a
potentially valuable synthon, is readily prepared in high
˚
6 (1.313(4) A) might suggest that there is slightly more
double bond localization in the case of 1. However, note
˚
that the C(2)-C(3) bond distance in 1 (1.358(3) A) is the
˚
˚
same as those in 5 (1.352(8) A) and 6 (1.362(4) A) within
experimental error. A distinctive feature of the structure of
5 is that the nitrogen hydrogen atom is located equidistantly
between the two nitrogen atoms. However, for 1 and 6 this
hydrogen atom is much closer to one nitrogen (N(1) in
Fig. 1) than the other. This aspect has already been com-
mented on in the context of the structures of 5 and 6 [7].
* Prof. Dr. A. H. Cowley
The University of Texas at Austin
Department of Chemistry and Biochemistry
1 University Station A5300
Austin, TX 78712-0165 USA
Tel.: Int ϩ 1-512-471-7484
Fax: Int. ϩ 1-512-471-6822
e-mail: cowley@mail.utexas.edu
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DOI: 10.1002/zaac.200500221
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Lithium, Aluminum, and Gallium Complexes
˚
Table 1 Selected bond distances/A and angles/deg for 1 Ϫ 4.
1
2
3
4
N(1)-C(2)
N(1)-C(6)
C(2)-C(3)
C(3)-C(4)
1.361(3)
1.400(3)
1.358(3)
1.438(3)
1.300(3)
1.406(2)
126.87(17)
120.35(17)
125.79(18)
119.45(17)
122.18(17)
N(1)-Li
1.925(5)
1.336(3)
1.398(4)
1.405(3)
1.330(3)
1.925(5)
1.896(4)
97.4(2)
123.5(2)
122.6(2)
130.0(2)
122.2(2)
124.0(2)
N(1)-Al
1.926(2)
1.333(3)
1.395(3)
1.395(3)
1.343(3)
1.917(2)
1.957(3)
1.947(3)
93.99(8)
126.8(2)
122.2(2)
128.1(2)
122.6(2)
126.2(1)
118.9(1)
N(1)-Ga
1.987(3)
1.334(5)
1.381(6)
1.411(6)
1.330(5)
2.007(3)
1.956(5)
1.967(5)
92.5(1)
126.1(3)
123.1(3)
129.8(4)
122.3(4)
126.0(3)
123.9(2)
N(1)-C(2)
C(2)-C(3)
C(3)-C(4)
N(2)-C(4)
N(2)-Li
Li-O
N(1)-Li-N(2)
Li-N(1)-C(2)
N(1)-C(2)-C(3)
C(2)-C(3)-C(4)
C(3)-C(4)-N(2)
C(4)-N(2)-Li
N(1)-C(2)
C(2)-C(3)
C(3)-C(4)
N(2)-C(4)
N(2)-Al
C(18)-Al
C(19)-Al
N(1)-Al-N(2)
Al-N(1)-C(4)
N(1)-C(4)-C(3)
C(4)-C(3)-C(2)
C(3)-C(2)-N(2)
C(2)-N(2)-Al
N(1)-C(2)
C(2)-C(3)
C(3)-C(4)
N(2)-C(4)
N(2)-Ga
C(18)-Ga
C(19)-Ga
N(1)-Ga-N(2)
Ga-N(1)-C(2)
N(1)-C(2)-C(3)
C(2)-C(3)-C(4)
C(3)-C(4)-N(2)
C(4)-N(2)-Ga
C(18)-Ga-C(19)
N(2)-C(4)
N(2)-C(12)
C(2)-N(1)-C(6)
N(1)-C(2)-C(3)
C(2)-C(3)-C(4)
C(3)-C(4)-N(2)
C(4)-N(2)-C(12)
C(18)-Al-C(19)
Fig. 2 Molecular structure of [HC(CMe)2(NC6F5)2]Li·OEt2 (2)
showing the atom numbering scheme.Thermal ellipsoids are set to
30 % probability.
Fig. 1 Molecular structure of [HC(CMe)2(NC6F5)2]H (1) showing
the atom numbering scheme.Thermal ellipsoids are set to 30 % pro-
bability.
˚
The replacement of the nitrogen hydrogen atom of 1 by
a Li·OEt2 moiety results in 2 which, possesses a much more
symmetrical structure that is indicative of π-bonding in the
C3N2 fragment (Fig. 2 and Table 1). Thus, the two N-C and
the two C-C bond distances are the same within experimen-
tal error and the sum of bond angles within the C3N2Li
ring is 719.7(2)°. The Li atom, which is displaced from the
extended C3N2 plane by 0.018 A, possesses a distorted tri-
gonal planar geometry (sum of bond angles at Li ϭ
359.7(3)°). The average N-C and C-C bond distances in 2
(1.333(3) and 1.401(4) A, respectively) are very similar to those
˚
reported for the lithium monoetherate, [Dipp2nacnacLi-
˚
(OEt2)] (7) (1.324(3) and 1.402(3) A, respectively; Dipp2 ϭ
C6H3-2,6-Pr12) [7]. Likewise, the Li-N and Li-O bond dis-
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D. Vidovic, J. N. Jones, J. A. Moore, A. H. Cowley
˚
tances in 2 (1.925(5) and 1.896(4) A, respectively) are very
in 3 (93.99(8)°) is more acute than those in 8 (96.18(9)°) and
9 (94.72(14)°) yet the C3N2Al ring of 3 is close to planar.
similar to those reported for 7 [7] (1.915(4) (av.) and
1.911(4) A, respectively). The structure of 7 has also been
˚
˚
Moreover, the average Al-N bond distance in 3 (1.921(2) A)
˚
reported by Tolman et al. [8].
is almost the same as that for 8 (1.928(2) A) despite the
Previously reported, structurally characterized (β-diketi-
minate)AlMe2 compounds include [(Dipp2nacnac)AlMe2]
(8) [9, 10] (Dipp2 ϭ 2,6-i-Pr2C6H3 and [{HC(CMe)2-
(N-p-tolyl)2}AlMe2] (9) [10]. A characteristic feature of
both compounds is the pronounced puckering of the
C3N2Al ring. This puckering can be expressed as the dis-
tance of the metal atom from the averaged extended C3N2
plane or as the dihedral angle between the C3N2 and N2me-
tal planes. For 8 and 9, the displacement of the Al atom
fact that the Dipp substituent is appreciably more bulky
than C6F5. Likewise, [{(HC)(CPh)2N(p-tolyl)}AlMe2], with
an N-Al-N bond angle of 94.94(6)°, has an essentially
planar AlNCCN backbone [12]. On the other hand,
[{HC)(CPh)2N(SiMe3)2}AlMe2] adopts a boat confor-
mation [13]. Finally, we note that the metrical parameters
for the AlMe2 moieties of 3, 8, and 9 are very similar.
The molecular structure of 4 (Fig. 4 and Table 1) bears
a close resemblance to that of the analogous aluminum
compound, 3. Thus, in contrast to [(Dipp2nacnac)GaMe2]
(10), which possesses a puckered C3N2Ga ring [11], that of
4 is very close to planarity. In fact, within experimental
error, the Ga atom lies in the extended C3N2 plane. The
Ga-N, N-C and C-C bond distances for 4 are very similar
to those for 10 and indicative of π-delocalization in the
C3N2 fragment. The β-diketiminate [{(NC)C(CMe)2-
(NH)}GaMe2] also has a planar GaNCCCN backbone;
however, this may be a consequence of intermolecular
CN···HN hydrogen bonding [14].
˚
from the β-diketiminate plane amounts to 0.72 and 0.33 A,
respectively. In the case of 3 (Fig. 3 and Table 1), this dis-
˚
placement is considerably less (0.073 A) hence the C3N2Al
ring is close to planarity. The ring puckering has been attri-
buted to steric interactions between bulky groups such as
Dipp and the metal substituents [11]. Moreover, a corre-
lation has been observed between the N-Al-N angle and the
extent of ring puckering in the sense that the more acute
this angle, the greater the extent of ring puckering [11]. In
this context, it is interesting to note that the N-Al-N angle
Fig. 3 Molecular structure of [{HC(CMe)2(NC6F5)2}AlMe2] (3)
showing the atom numbering scheme.Thermal ellipsoids are set to
30 % probability.
Fig. 4 Molecular structure of [{HC(CMe)2(NC6F5)2}GaMe2] (4)
showing the atom numbering scheme.Thermal ellipsoids are set to
30 % probability.
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Lithium, Aluminum, and Gallium Complexes
Experimental Part
Single crystal X-ray crystallography
Synthesis
Crystals of 1Ϫ4 of suitable quality were collected directly from a
Schlenk-type flask under argon pressure, and covered immediately
with degassed perfluorinated polyether oil. The X-ray data were
collected on a Nonius Kappa CCD diffractometer. All four struc-
tures were solved by direct methods, and refined by full-matrix le-
ast-squares on F2 using the SHELXTL software package [15]. A
summary of crystallographic data is presented in Table 2. Note that
4 crystallizes as a racemic twin in the non-centrosymmetric space
group Pca21. The refined Flack Parameter is 0.582(14).
All manipulations and reactions were performed under a dry, oxy-
gen-free argon atmosphere or under vacuum using standard
Schlenk or drybox techniques; all glassware was oven-dried before
use. Toluene and pentane were distilled over sodium; both solvents
were degassed prior to use. Compound 1 was prepared according
to the literature method [4] and MeLi (1.6 M solution in Et2O),
AlMe3, and GaMe3 were procured commercially and used without
further purification.
2: 0.740 g (1.72 mmol) of 1 was dissolved in 30 mL of hexane in a
Schlenk flask. A solution of 1.08 mL of 1.6 M MeLi in Et2O was
added dropwise and the reaction mixture was allowed to warm to
25 °C. A crop of colorless crystals (mp 106-109 °C) formed upon
storage of the reaction mixture overnight at Ϫ30 °C. The yield of
2 was essentially quantitative.
1H NMR (C6D6) δ 4.96 (s, 1H, CH), 2.62 (q, 4H, CH2 (Et2O)), 1.74 (s, 6H,
CMe) 0.62 (t, 6H, CH3 (Et2O)). 19F NMR (C6D6) δ Ϫ153.1 (m, 2F, m-F),
Ϫ165.1 (m, 1F, p-F), Ϫ166.0 (m, 2F, o-F). MS (CIϩ, CH4): m/z 511 [M ϩ
H]ϩ. HRMS (CIϩ, CH4) Calc. for C21H18F10LiN2O: 511.1420. Found:
511.1417.
Crystallographic data for the structures 1Ϫ4 have been deposited
with the Cambridge Crystallographic Data Centre, CCDC 265977-
265980. Copies of the data can be obtained free of charge on
application to the Director, CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK (Fax:int.codeϩ(1223)336-033; e-mail for inquiry;
fileserv@ccdc.cam.ac.uk; e-mail for deposition: deposit@ccdc.
cam.ac.uk).
Spectroscopic measurements
3: 0.282 g (0.656 mmol) of 1 and 0.0472 g (0.656 mmol) of tri-
methylaluminum were allowed to mix in 50 mL of toluene at 25 °C
in a Schlenk flask. The resulting pale yellow reaction mixture was
stirred overnight at 25 °C. Removal of the solvent and volatiles
under reduced pressure afforded a 67 % yield of white solid 3 (mp
145-146 °C). Compound 3 was recrystallized from toluene.
1H NMR (C6D6): δ 4.74 (s, 1H, CH), 1.27 (s, 6H, CMe), Ϫ0.49 (s, 6H,
AlMe); 19F NMR (C6D6) δ Ϫ147 (m, 2F, m-F), Ϫ156 (t, 1F, p-F), Ϫ162 (m,
2F, o-F); 27Al NMR (C6D6): δ 212 (br, s). MS (CIϩ, CH4): m/z 487 (Mϩ).
HRMS (CIϩ, CH4) Calc. for C19H14AlF10N2 487.0813. Found: 487.0820.
CI mass spectra were measured on a Finnigan MAT TSQ-700 in-
strument. Solution-phase NMR spectra were recorded at 295 K on
a GE QE-300 spectrometer (1H, 300 MHz; 19F, 282 MHz; 27Al,
78.21 MHz) or a Varian Inova-500 spectrometer (1H, 500 MHz;
19F, 470 MHz; 27Al, 130 MHz). All NMR samples were flame-se-
aled or run immediately following removal from the drybox. Ben-
zene-d6 was dried over sodium-potassium alloy and distilled prior
1
to use. H NMR spectra are reported relative to tetramethylsilane
(δ 0.00) and are referenced to solvent. 19F chemical shifts are re-
ported relative to C6F6 (δ Ϫ162.9), and 27Al chemical shifts are
reported relative to [Al(D2O)6]3ϩ (δ 0.00). Melting points were ob-
tained in sealed capillaries under argon (1 atm) on a Fisher-Johns
apparatus and are uncorrected.
4: 0.181 g (0.158 mmol) of trimethylgallium was allowed to react
with 0.068 g (0.158 mmol) of 1 in 50 mL of toluene as described
above for the synthesis of 3. Removal of the solvent and volatiles
resulted in a 60 % yield of white, solid 4 (mp 140-142 °C). Com-
pound 4 was recrystallized from pentane.
1H NMR (C6D6) δ 4.70 (s, 1H, CH), 1.32 (s, 6H, CMe), Ϫ017 (s, 6H, GaMe).
19F NMR (C6D6) δ Ϫ145 (m, 2F, m-F), Ϫ151 (t, 1F, p-F), Ϫ164 (m, 2F,
o-F). MS (CIϩ, CH4): m/z 528 (Mϩ). HRMS (CIϩ, CH4) Calc. for
C19H14F10GaN2: 529.0253. Found: 529.0266.
Acknowledgement. We are grateful to the National Science Foun-
dation (CHE-024008) and the Robert A. Welch Foundation (Grant
F-135) for financial support.
Table 2 Crystallographic data for compounds 1 Ϫ 4.
1
2
3
4
formula
fw
C17H8F10N2
430.25
C21H17F10LiN2O
510.31
C19H13AlC10F10N2
486.29
C19H13F10GaN2
529.03
crystal system
space group
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
orthorhombic
Pca21
25.347(5)
9.330(5)
8.323(5)
90.0
1968.3(16)
4
1.500
˚
a /A
17.084(5)
10.808(5)
19.084(5)
110.095(5)
3309(2)
8
0.182
293(2)
0.0498
7.357(5)
19.517(5)
15.297(5)
94.541(5)
2189.5(17)
4
0.154
153(2)
0.0798
9.447(5)
24.498(5)
8.594(5)
95.396(5)
1980.1(16)
4
0.204
153(2)
0.0460
˚
b /A
˚
c /A
β /deg
3
˚
V /A
Z
µ (Mo-Kα) /mmϪ1
T/K
R1
153(2)
0.0392
wR2
0.1109
0.1138
0.0881
0.0785
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