Tripodal amido boron and aluminum complexes

Article abstract of DOI:10.1021/ic0620889

The synthesis and structural elucidations of novel boron and aluminum complexes incorporating the tripodal triamido [N₃]^3- ligand framework that is hypothesized to promote the preorganized pyramidal geometry for high Lewis acidity are reported. Salt metathesis between the in situ-generated trianionic lithium complexes of the tripodal amido ligands with BCI₃ leads to boranes HC[SiMe₂N(4-MeC₆H ₄)]₃B (1) and MeSi[SiMe₂N(4-MeC ₆H₄)]₃B (2); however, substitution of the N-Ar group with the bulky ^tBu affords the unexpected non-boron-containing LiCl adduct {[HC(SiMe₂N^tBu)₂(SiMeN ^tBu)]Li₃(Et₂O)Cl}₂ (3) via apparent elimination of MeBCl₂. The products derived from the salt metathesis reaction with AlCl₃ are determined by the reaction medium: while the reaction in a hexanes-ether mixture or toluene affords solvated salt adduct HC[SiMe₂N(4-MeC₆H₄)]₃Al· ClLi(Et₂O)₂ (4) or salt adduct HC[SiMe₂N(4- MeC₆H₄)]₃Al·ClLi (5), respectively; the addition of a small amount of THF produces a mixture of complexes HC[SiMe ₂N(4-MeC₆H₄)]₃Al·(THF) (6, major) and HC[SiMe₂N(4-MeC₆H₄)] ₃Al(OCH=CH₂)·Li(THF)₂ (7, minor). The desired complex 6 can be exclusively formed using HC[SiMe₂N(4- MeC₆H₄)]₃Li₃·(THF) ₃ and the hexanes-ether mixture solvent. The molecular structures of complexes 1, 3, 5, 6, and 7 have been elucidated by X-ray diffraction studies. The structure of 1 shows an approximately trigonal pyramidal geometry at B with no significant N-B p-p π-interactions. The strong salt adduct and solvate formation of the tripodal amido Al complex, as well as its similarity to the strong Lewis acid Al(C₆F₅)₃ in the THF adduct and enolaluminate formation and structure, indicate the desired core structure [N₃]Al is indeed highly Lewis acidic.

Full text of DOI:10.1021/ic0620889

Inorg. Chem. 2007, 46, 1481−1487
Tripodal Amido Boron and Aluminum Complexes
Hongping Zhu and Eugene Y.-X. Chen*
Department of Chemistry, Colorado State UniVersity, Fort Collins, Colorado 80523-1872
Received November 1, 2006
The synthesis and structural elucidations of novel boron and aluminum complexes incorporating the tripodal triamido
-
[N3]3 ligand framework that is hypothesized to promote the preorganized pyramidal geometry for high Lewis acidity
are reported. Salt metathesis between the in situ-generated trianionic lithium complexes of the tripodal amido
ligands with BCl3 leads to boranes HC[SiMe2N(4-MeC6H4)]3B (1) and MeSi[SiMe2N(4-MeC6H4)]3B (2); however,
t
substitution of the N-Ar group with the bulky Bu affords the unexpected non-boron-containing LiCl adduct
{
[HC-
(SiMe2NtBu)2(SiMeNtBu)]Li3(Et2O)Cl
} (3) via apparent elimination of MeBCl2. The products derived from the salt
2
metathesis reaction with AlCl3 are determined by the reaction medium: while the reaction in a hexanes
mixture or toluene affords solvated salt adduct HC[SiMe2N(4-MeC6H4)]3Al ClLi(Et2O)2 (4) or salt adduct HC[SiMe2N-
(4-MeC6H4)]3Al ClLi (5), respectively; the addition of a small amount of THF produces a mixture of complexes
HC[SiMe2N(4-MeC6H4)]3Al (THF) (6, major) and HC[SiMe2N(4-MeC6H4)]3Al(OCH CH2) Li(THF)2 (7, minor). The
desired complex 6 can be exclusively formed using HC[SiMe2N(4-MeC6H4)]3Li3 (THF)3 and the hexanes ether
mixture solvent. The molecular structures of complexes 1, 3, 5, 6, and 7 have been elucidated by X-ray diffraction
studies. The structure of 1 shows an approximately trigonal pyramidal geometry at B with no significant N B p
-interactions. The strong salt adduct and solvate formation of the tripodal amido Al complex, as well as its similarity
−ether
‚
‚
‚
d
‚
‚
−
−
−p
π
to the strong Lewis acid Al(C6F5)3 in the THF adduct and enolaluminate formation and structure, indicate the
desired core structure [N3]Al is indeed highly Lewis acidic.
Introduction
strong electron-withdrawing perfluoroaryl ligands in the case
of E(C6F5)3 or the still not fully understood cluster structures
in the case of MAO that must be used in large excess to be
effective in catalysis. Because of the need of the expensive
perfluoroaryl ligands in E(C6F5)3 or use in large excess of
MAO, such reagents contribute significantly to the overall
cost of a given catalytic system.
In efforts toward the design and synthesis of alternative
strong Lewis acidic boron and aluminum structures that
neither contain strong electron-withdrawing ligands such as
C6F5 nor require their use in excess, we reasoned that, for
three-coordinate B and Al complexes to be effective LA
catalysts, especially cocatalysts for metal-mediated olefin
polymerization catalysis, they require (a) a preorganized
pyramidal geometry which not only provides a vacant sp3
orbital disposed ideally to accept a fourth donor ligand but
also needs to pay no (or minimal) energy penalty for
geometry reorganization (e.g., going from trigonal planar LAs
to pyramidal activated species) during LA-mediated chemical
processes, and (b) an adequate steric protection of the E
center against potential ligand redistribution reactions with
a substrate or within the activated species (Chart 1). A
Organo-boron and -aluminum Lewis acids (LAs) are
important reagents widely employed in organic synthesis,1
olefin polymerization catalysis,2 and polymerization of polar
monomers.3 In particular, strong organo-LAs E(C6F5)3 (E )
B, Al) and methylalumoxanes (MAO) are being extensively
used in academic and industrial laboratories as critical
components, catalysts, cocatalysts, stabilizers, scavengers,
etc., for various types of catalytic processes.2 Their high
Lewis acidity is rendered by either the inclusion of the three
* To whom correspondence should be addressed. E-mail: eychen@
lamar.colostate.edu.
(1) (a) Ishihara, K. In Lewis Acids in Organic Synthesis, Vol. 1;
Yamamoto, H., Ed.; Wiley-VCH: Weinheim, Germany, 2000; pp 89-
190. (b) Ooi, T.; Maruoka, K. In Lewis Acids in Organic Synthesis,
Vol. 1; Yamamoto, H., Ed.; Wiley-VCH: Weinheim, Germany, 2000;
pp 191-282. (c) Wulff, W. D. In Lewis Acids in Organic Synthesis,
Vol. 1; Yamamoto, H., Ed.; Wiley-VCH: Weinheim, Germany, 2000;
pp 283-354.
(2) (a) Piers, W. E. AdV. Organomet. Chem. 2005, 52, 1-76. (b) Erker,
G. Dalton Trans. 2005, 1883-1890. (c) Pedeutour, J.-N.; Radhakrish-
nan, K.; Cramail, H.; Deffieux, A. Macromol. Rapid Commun. 2001,
22, 1095-1123. (d) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000,
100, 1391-1434.
(3) Kuran, W. Prog. Polym. Sci. 1998, 23, 919-992.
10.1021/ic0620889 CCC: $37.00
Published on Web 01/20/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 4, 2007 1481

Zhu and Chen
Chart 1
to external CFCl3 and 11B spectra to BF3‚Et2O. Elemental analyses
were performed by Desert Analytics, Tucson, AZ. Commercial
reagents were purchased from Aldrich Chemical Co., and literature
procedures were employed for the preparation of the following
neutral tripodal amido ligands and their corresponding lithium salts
(unsolvated or solvated with 2Et2O): HC[SiMe2NH(Ar)]3 (Ar )
4-MeC6H4, HC[N3]H3) and HC[N3]Li3,10 MeSi[N3]H3 and MeSi-
[N3]Li3,11 as well as HC[SiMe2NH(tBu)]3 and HC[SiMe2N-
(tBu)]3Li3.12
potentially suitable ligand platform for supporting such
sterically protected pyramidal B and Al centers is Gade’s
tripodal triamido [N3]3- ligands having neopentane and
trisilylmethane (or trisilylsilane) backbones.4 These and
analogous tripodal triamido ligands provide adjustable bind-
ing pockets for different metal ions, while variation in the
peripheral N-substituents adds additional tools for the fine-
tuning of sterics and electronics, as well as introduction of
chirality at the typical tetravalent metal centers including
those of group 4 metals.5 However, to the best of our
knowledge, there were no prior reports on such tripodal
triamido complexes of trivalent boron and aluminum,6
although main-group (e.g., Verkade’s boron and aluminum
azatranes7) and transition-metal complexes bearing the related
tetradentate triamido-amine type ligand have been exten-
sively studied by Verkade8 and Schrock,9 respectively.
Reported here are the first syntheses and structural elucida-
tions of a series of boron and aluminum complexes incor-
porating tridentate tripodal amido ligands.
Synthesis of HC[SiMe2N(4-MeC6H4)]3B (1). nBuLi (5.63 mL,
1.6 M in hexanes, 9.00 mmol) was added to a stirred solution of
HC[N3]H3 (1.52 g, 3.00 mmol) in a solvent mixture (60 mL of
hexanes and 10 mL of ether) precooled to -50 °C. The mixture
was warmed gradually to ambient temperature and stirred for an
additional 5 h. The resulting HC[N3]Li3‚(Et2O)2 solution was cooled
to -78 °C, and BCl3 (3.00 mL, 1.0 M in hexanes, 3.0 mmol) was
quickly added. This mixture was warmed gradually to ambient
temperature while stirring overnight, after which the resulting
suspension was filtered. The filtrate was concentrated to ∼10 mL
and kept at -30 °C. Colorless crystals were obtained after 3 d,
which were collected and dried under vacuum affording 0.58 g (38%
yield) of the pure complex HC[N3]B (1). Concentration of the
mother liquor followed by crystallization gave 0.20 g (10% recovery
yield) of the unreacted [N3]Li3‚(Et2O)2 as colorless crystals. When
the same reaction was carried out in toluene, complex 1 was also
produced but in a much lower yield of only 11%. 1H NMR (C6D6,
23 °C): δ 7.14 (d,3JHH ) 8.4 Hz, 6 H), 6.89 (d,3JHH ) 8.4 Hz, 6
H) (4-MeC6H4), 2.09 (s, 9 H, 4-MeC6H4), 0.33 (s, 18 H, SiMe2),
-0.23 (s, 1 H, HC). 13C NMR (C6D6, 23 °C): δ 145.4, 130.7, 129.5,
120.4 (4-MeC6H4), 21.0 (4-MeC6H4), 13.9 (HC), 5.0 (SiMe2). 11B
NMR (C6D6, 23 °C): δ 39.11 (br). Anal. Calcd for C28H40BN3Si3:
C, 65.47; H, 7.85; N, 8.18. Found: C, 65.18; H, 7.83; N, 7.96.
Experimental Section
Materials and Methods. All syntheses and manipulations of
air- and moisture-sensitive materials were carried out in flamed
Schlenk-type glassware on a dual-manifold Schlenk line, on a high-
vacuum line, or in an argon- or nitrogen-filled glovebox. HPLC
grade organic solvents were sparged extensively with nitrogen
during filling of the solvent reservoir and then dried by passage
through activated alumina (for THF, Et2O, and CH2Cl2), followed
by passage through Q-5-supported copper-catalyst (for toluene and
hexanes) stainless steel columns. Benzene-d6 and toluene-d8 were
degassed, dried over sodium/potassium alloy, and filtered before
use, whereas CDCl3 and CD2Cl2 were degassed and dried over
activated Davison 4 Å molecular sieves. NMR spectra were
Synthesis of MeSi[SiMe2N(4-MeC6H4)]3B (2). Complex 2 was
synthesized using similar procedures to those for 1. MeSi[N3]H3
(0.86 g, 1.6 mmol), a solvent mixture (20 mL of hexanes and 10
mL of ether), nBuLi (3.0 mL, 1.6 M hexanes solution, 4.8 mmol),
and BCl3 (1.6 mL, 1.0 M hexanes solution, 1.6 mmol) were used.
The concentrated filtrate (∼15 mL) was kept at -30 °C for 2 d to
give 0.25 g (22% recovery yield) of the unreacted MeSi[N3]Li3‚
(Et2O)2. The filtrate was further concentrated to ∼3 mL and kept
at -30 °C for 2 weeks to afford 0.08 g (9% yield) of the pure
1
recorded on a Varian Inova 300 (FT 300 MHz, H; 96 MHz, 11B;
75 MHz, 13C; 282 MHz, 19F). Chemical shifts for 1H and 13C spectra
were referenced to internal solvent resonances and reported as parts
per million relative to SiMe4, whereas 19F spectra were referenced
(4) For reviews, see: (a) Gade, L. H. Acc. Chem. Res. 2002, 35, 575-
582. (b) Gade, L. H. Chem. Commun. 2000, 173-181. (c) Gade, L.
H. Angew. Chem., Int. Ed. 2000, 39, 2658-2678.
1
complex MeSi[N3]B (2) as colorless crystals. H NMR (C6D6, 23
°C): δ 7.24 (d,3JHH ) 8.4 Hz, 6 H), 6.89 (d,3JHH ) 8.4 Hz, 6 H)
(4-MeC6H4), 2.07 (s, 9 H, 4-MeC6H4), 0.51 (s, 18 H, SiMe2), 0.16
(s, 3 H, MeSi). Anal. Calcd for C28H42BN3Si4: C, 61.84; H, 7.78;
N, 7.73. Found: C, 61.85; H, 8.07; N, 7.75.
(5) (a) Turculet, L.; Tilley, T. D. Organometallics 2004, 23, 1542-1553.
(b) Turculet, L.; Tilley, T. D. Organometallics 2002, 21, 3961-3972.
(c) Jia, L.; Zhao, J.; Ding, E.; Brennessel, W. W. J. Chem. Soc., Dalton
Trans. 2002, 2608-2615. (d) Jia, L.; Ding, E.; Rheingold, A. L.;
Rhatigan, B. Organometallics 2000, 19, 963-965.
(6) 1-Boraadamantane is known, see: Mikhailov, B. M.; Baryshnikova,
T. K.; Kiselev, V. G.; Shashkov, A. S. IzV. Akad. Nauk SSSR, Ser.
Khim. 1979, 2544-2551.
(10) Memmler, H.; Gade, L. H.; Lauber, J. W. Inorg. Chem. 1994, 33,
3064-3071.
(7) Verkade, J. G. Acc. Chem. Res. 1993, 26, 483-489.
(8) Pinkas, J.; Gaul, B.; Verkade, J. G. J. Am. Chem. Soc. 1993, 115,
3925-3931.
(11) Schubart, M.; Findeis, B.; Gade, L. H.; Li, W.-S.; McPartlin, M. Chem.
Ber. 1995, 128, 329-334.
(9) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9-16.
(12) Gade, L. H.; Becker, C. Inorg. Chem. 1993, 32, 2308-2314.
1482 Inorganic Chemistry, Vol. 46, No. 4, 2007

Tripodal Amido B and Al Complexes
Synthesis of {[HC(SiMe2NtBu)2(SiMeNtBu)]Li3(Et2O)Cl}2 (3).
BCl3 (0.97 mL, 1.0 M hexanes solution, 0.97 mmol) was quickly
added to a stirred solution of HC[SiMe2N(tBu)]3Li3 (0.41 g, 0.97
mmol) in a solvent mixture (30 mL of toluene and 5 mL of Et2O)
precooled to -78 °C. This mixture was warmed gradually to
ambient temperature while stirring overnight, after which the
resulting suspension was evaporated to dryness in vacuo. The
residue was extracted with hexanes (40 mL), and the hexanes extract
was concentrated to ∼5 mL and then kept at -30 °C for 2 d,
affording 0.15 g (29% yield) of the pure complex 3 as colorless
crystals after filtration and drying under vacuum. 1H NMR (C6D6,
23 °C): δ 3.39 (q,3JHH ) 6.9 Hz, 4 H, OCH2CH3), 1.49 (s, 18 H),
1.28 (s, 9 H, tBu), 1.03 (t,3JHH ) 6.9 Hz, 6 H, OCH2CH3), 0.71 (s,
3 H), 0.57 (s, 6 H), 0.40 (s, 3 H), 0.32 (s, 3 H) (SiMe, SiMe2),
-0.17 (s, 1 H, HC). Anal. Calcd for [C22H53Li3N3OSi3Cl]2: C,
51.19; H, 10.35; N, 8.14. Found: C, 50.41; H, 10.29; N, 8.26.
Synthesis of HC[SiMe2N(4-MeC6H4)]3Al‚ClLi(Et2O)2 (4) and
HC[SiMe2N(4-MeC6H4)]3Al‚ClLi (5). The synthesis of complex
4 was similar to that of 1, employing the following reagents: HC-
[N3]H3 (1.62 g, 3.2 mmol), a solvent mixture (20 mL of hexanes
H, OCH2CH2), 2.21 (s, 9 H, 4-MeC6H4), 0.55 (s, 18 H, SiMe2),
0.42 (t,3JHH ) 6.6 Hz, 4 H, OCH2CH2), -0.26 (s, 1 H, HC). 13C
NMR (C6D6, 23 °C): δ 150.6, 130.3, 128.8, 126.3 (4-MeC6H4),
74.9 (OCH2CH2), 24.5 (OCH2CH2), 21.2 (4-MeC6H4), 9.2 (HC),
5.7 (SiMe2). Anal. Calcd for C32H48AlN3OSi3: C, 63.85; H, 8.04;
N, 6.98. Found: C, 63.41; H, 8.09; N, 6.86.
1H NMR for 7 (C6D6, 23 °C): δ 7.09 (d,3JHH ) 8.4 Hz, 6 H),
6.95 (d,3JHH ) 8.4 Hz, 6 H) (4-MeC6H4), 6.69 (dd,3JHH ) 6.0, 14.1
Hz, 1 H, OCH)CH2), 3.60 (qAB,3JHH ) 6.0, 14.1,2JHH ) 0.9 Hz,
2 H, OCH)CH2), 3.04 (t,3JHH ) 6.6 Hz, 8 H, OCH2CH2), 2.19 (s,
9 H, 4-MeC6H4), 1.17 (t,3JHH ) 6.6 Hz, 8 H, OCH2CH2), 0.61 (s,
18 H, SiMe2), -0.09 (s, 1 H, HC). Anal. Calcd for C38H59AlLiN3O3-
Si3: C, 63.04; H, 8.21; N, 5.80. Found: C, 63.08; H, 7.90; N, 6.11.
X-ray Crystallographic Analyses of 1, 3, 5, 6, and 7. Single
crystals of all complexes suitable for X-ray diffraction were grown
from hexanes at -30 °C inside the freezer of a glovebox. The
crystals were quickly covered with a layer of Paratone-N oil (Exxon,
dried and degassed at 120 °C/10-6 Torr for 24 h) after the mother
liquors were decanted, mounted on thin glass fibers, and transferred
into the cold nitrogen stream of a Bruker SMART CCD diffrac-
tometer. The structures were solved by direct methods and refined
using the Bruker SHELXTL program library by full-matrix least-
squares on F2 for all reflections.13 Unless otherwise indicated, all
non-hydrogen atoms were refined with anisotropic displacement
parameters, whereas hydrogen atoms were included in the structure
factor calculations at idealized positions. In 3, the hexane solvent
carbon atoms were located by difference Fourier synthesis but were
refined isotropically. One whole hexane solvent is disordered
viewing a cyclic structure when symmetrically operated. Thus, the
occupation of carbon atoms C(52), C(54), C(56), and C(57) was
set as 0.75, and that of C(51), C(53), C(55) and C(58) was set as
0.375. This gives the rational Ueq of these atoms; however, the
corresponding hydrogen atoms were geometrically included only
to 12. In another half hexane solvent molecule, although the
occupation of the whole molecule was set as 0.25, the Ueq of some
atoms was still large because of the thermal vibration of this free
solvent molecule in the crystal lattice. In 5, the hexane solvent atoms
were located by difference Fourier synthesis, with the whole
molecule disordered into two positions (the respective occupation
of 0.308 and 0.192), and refined isotropically. The subsequent
symmetric operation generated the adjacent hexane molecules, and
this did not allow the inclusion of the hydrogen atoms in the hexane.
In 6, two independent molecules were found, of which the two
coordinated THF [O(1)C(41)C(42)C(43)C(44) 0.70], [O(1A)C-
(41A)C(42A)C(43A)C(44A) 0.30], [O(2)C(91)C(92)C(93)C(94)
0.85], [O(2A)C(91A) C(92A)C(93A)C(94A) 0.15], and one p-tolyl
group [C(31)C(32)C(33)C(34)C(35)C(36)C(37) 0.85], [C(31A)C-
(32A)C(33A)C(34A)C(35A)C(36A)C(37A) 0.15] were disordered
and treated in part, respectively. Thus, average metric parameters
were given as follows: Al(1)-OTHF ) 1.8612(45) Å (av), Al(2)-
OTHF ) 1.8564(54) Å (av), OTHF-Al(1)-N(1) ) 109.03(16)° (av),
OTHF-Al(1)-N(2) ) 105.88(16)° (av), OTHF-Al(1)-N(3) )
n
and 20 mL of ether), BuLi (9.6 mmol), and a solution of AlCl3
(0.43 g, 3.2 mmol) in 20 mL of Et2O. The pure complex HC[N3]-
Al‚ClLi(Et2O)2 (4) (0.82 g, 70% yield) was obtained as a colorless
1
crystalline solid. H NMR (C6D6, 23 °C): δ 7.24 (d,3JHH ) 8.4
Hz, 6 H), 7.01 (d,3JHH ) 8.4 Hz, 6 H) (4-MeC6H4), 3.03 (q,3JHH
)
)
6.9 Hz, 8 H, OCH2CH3), 2.18 (s, 9 H, 4-MeC6H4), 0.83 (t,3JHH
6.9 Hz, 12 H, OCH2CH3), 0.45 (s, 18 H, SiMe2), -0.46 (s, 1 H,
HC). 13C NMR (C6D6, 23 °C): δ 149.8, 130.1, 129.1, 126.4 (4-
MeC6H4), 65.9 (OCH2CH3), 21.0 (4-MeC6H4), 14.9 (OCH2CH3),
8.5 (HC), 5.9 (SiMe2). The ether-free complex HC[N3]Al‚ClLi (5)
was prepared in a 91% yield by heating 4 at 165 °C under vacuum
(0.5 Torr) for 2 h. Alternatively, complex 5 was prepared in toluene
starting from the neutral ligand, affording an overall yield of 51%.
1H NMR (C6D6, 23 °C): δ 6.96 (qAB,3JHH ) 6.0 Hz, 12 H,
4-MeC6H4), 2.13 (s, 9 H, 4-MeC6H4), 0.34 (s, 18 H, SiMe2), -0.54
(s, 1 H, HC). Anal. Calcd for [C28H40AlLiN3Si3Cl]2: C, 58.77; H,
7.05; N, 7.34. Found: C, 58.37; H, 7.58; N, 6.60.
Synthesis of HC[SiMe2N(4-MeC6H4)]3Al‚(THF) (6) and HC-
n
[SiMe2N(4-MeC6H4)]3Al(OCHdCH2)‚Li(THF)2 (7). BuLi (4.8
mmol) was added to a stirred solution of HC[N3]H3 (0.81 g, 1.6
mmol) in a solvent mixture (10 mL of hexanes and 20 mL of ether)
precooled to -50 °C. The mixture was warmed gradually to ambient
temperature and stirred for an additional 5 h. The resulting HC-
[N3]Li3‚(Et2O)2 solution was cooled to -78 °C, and AlCl3 (0.21 g,
1.6 mmol) in a solvent mixture (20 mL of Et2O and 2 mL of THF)
was added. This mixture was warmed gradually to ambient
temperature while stirring overnight, after which the resulting
mixture was evaporated to dryness under vacuum. The residue was
extracted with Et2O (40 mL) and filtered to remove LiCl salt; the
ether solution was evaporated to dryness, and the residue was further
extracted with hexanes (20 mL). The hexanes extract was concen-
trated to ∼8 mL and kept at -30 °C for 3 d. Colorless crystals
obtained by recrystallization from hexanes were manually separated,
depending on the size and shape of the crystals, into complexes
HC[N3]Al‚(THF) (6) (large block, 0.18 g, 19% yield) and HC[N3]-
Al(OCHdCH2)‚Li(THF)2 (7) (small rhombic, 0.02 g, 2% yield).
When more THF was used in the reaction, the yield of 6 was
lowered. On the other hand, when HC[N]3Li3‚(THF)3 (0.83, 1.12
mmol) and AlCl3 (0.15 g, 1.12 mmol) were used in a solvent
mixture of hexanes (10 mL) and ether (25 mL), only complex 6
was formed and isolated in high yield (0.58 g, 86%).
112.00(13)° (av), OTHF-Al(2)-N(6) ) 106.39(21)° (av), OTHF
-
Al(2)-N(7) ) 114.01(18)° (av), OTHF-Al(2)-N(8) ) 106.20(19)°
(av). In 7, one carbon atom in the disordered 4-MeC6H4 group and
four carbon atoms in the disordered THF molecule were located
and treated in part into two positions [C(11)C(12)C(13)C(14)-
C(15)C(16)C(17) 0.54], [C(11A)C(12A)C(13A)C(14A)C(15A)
C(16A)C(17A) 0.46], [O(3)C(55)C(56)C(57)C(58) 0.50], [O(3A)-
C(55A)C(56A)C(57A)C(58A) 0.50], and the related carbon atoms
1H NMR for 6 (C6D6, 23 °C): δ 7.15 (d,3JHH ) 8.4 Hz, 6 H),
(13) SHELXTL, version 6.12; Bruker Analytical X-ray Solutions: Madison,
7.01 (d,3JHH ) 8.4 Hz, 6 H) (4-MeC6H4), 3.41 (t,3JHH ) 6.6 Hz, 4
WI, 2001.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1483

Zhu and Chen
Table 1. Crystal Data and Structure Refinements for 1, 3‚2C6H14, 5‚C6H14, 6, and 7a
1
3·2C6H14
5·C6H14
6
7
formula
fw
C28H40BN3Si3
513.71
C56H131Cl2Li6N6O2Si6
1201.75
C62H94Al2Cl2Li2N6Si6
1230.71
C32H48AlN3OSi3
601.98
C38H59AlLiN3O3Si3
724.07
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
monoclinic
P2(1)/n
7.6940(4)
26.1454(13)
14.5059(7)
monoclinic
P2(1)/n
10.5785(3)
21.6592(7)
17.0059(5)
monoclinic
C2/c
28.8457(11)
13.0329(6)
23.9200(9)
triclinic
P1h
monoclinic
P2(1)/c
11.9203(7)
26.6447(14)
13.5389(7)
14.7157(6)
14.7551(6)
18.0703(8)
90.799(2)
101.923(2)
118.783(2)
3335.1(2)
4b
94.895(2)
91.549(2)
126.888(1)
99.636(3)
2907.4(3)
4
1.174
0.185
1104
0.49 × 0.34 × 0.15
2.77-32.59
-11 e h e 11
-39 e k e 39
-21 e l e 21
95 108
3895.0(2)
2
1.025
0.212
1322
0.58 × 0.39 × 0.13
1.52-33.24
-15 e h e 16
-33 e k e 33
-26 e l e 26
66 125
7192.3(5)
4
1.137
0.254
2632
0.26 × 0.26 × 0.16
3.08-30.62
-41 e h e 41
-18 e k e 18
-34 e l e 34
87 853
4239.9(4)
4
1.134
0.169
1560
0.44 × 0.27 × 0.21
1.53-33.19
-15 e h e 18
-40 e k e 40
-20 e l e 20
78 983
F
calcd (g cm-3
)
1.199
0.198
1296
0.37 × 0.33 × 0.28
1.16-35.04
-22 e h e 23
-23 e k e 23
-29 e l e 28
127 598
µ (mm-1
F(000)
)
cryst size (mm3)
θ range (deg)
index ranges
collected data
unique data
10 551
14 904
11 031
29 278
16 105
(Rint ) 0.0365)
(Rint ) 0.0617)
(Rint ) 0.0370)
(Rint ) 0.0315)
(Rint ) 0.0514)
completeness to θ
data/restraints/params
GOF on F2
99.6%
99.4%
99.5%
99.4%
99.2%
10 551/0/325
1.073
14 904/13/369
1.033
11 031/13/392
1.066
29 278/414/887
1.002
16 105/0/512
1.018
final R indices
[I >2σ(I)]
R indices (all data)
R1 ) 0.0398
wR2 ) 0.0972
R1 ) 0.0596
wR2 ) 0.1094
R1 ) 0.0535
wR2 ) 0.1422
R1 ) 0.0858
wR2 ) 0.1663
R1 ) 0.0466
wR2 ) 0.1300
R1 ) 0.0558
wR2 ) 0.1386
R1 ) 0.0421
wR2 ) 0.1083
R1 ) 0.0631
wR2 ) 0.1216
R1 ) 0.0568
wR2 ) 0.1462
R1 ) 0.0921
wR2 ) 0.1683
a All data were collected at 173(2) K using Mo KR (λ ) 0.71073 Å) radiation. b Two independent molecules.
[C(17), C(17A), C(55), C(56), C(57), C(58), C(55A), C(56A),
C(57A), C(58A)] were refined isotropically.
Selected crystal data and structural refinement parameters are
collected in Table 1.
Results and Discussion
Synthesis and Structural Studies of Tripodal Amido
Boron Complexes. The metathesis reaction of the in situ-
generated trianionic lithium salt HC[N3]Li3‚(Et2O)2 (or in
its unsolvated form when generated in toluene) with BCl3
in a hexanes/ether solvent mixture (or in toluene) produces
the desired tripodal triamido borane HC[N3]B (1), which was
isolated as colorless crystals in a 38% yield (or 11% yield).
The donor solvent promotes higher yield, but in both cases
significant quantities (10%) of the unreacted HC[N3]Li3 (with
or without 2Et2O) was recovered during the workup, even
when an excess of BCl3 was employed. Noteworthy in the
1H NMR spectra on going from the neutral ligand to 1 is a
significant upfield shift by 1.1 ppm for the apical CH upon
complex formation of the time-average C3V-symmetric borane
1. The molecular structure of 1 (Figure 1) reveals that the B
atom is located above the N3-basal plane by 0.1785 Å, and
the sum of the angles about B is 355.7°. The geometry at
each of the three N atoms deviates somewhat from planarity,
with the sums of the angles around them ranging from 352.8
to 355.0°. There appears to be no significant N-B p-p
π-interactions as evidenced by the N-B bond lengths (from
1.459(2) to 1.469(2) Å) for typical N-B single bonds and
large twist angles (67.3-76.9°) between the N3-basal plane
and the B-N-Si planes. The absence of any significant
N-Ar p-π interactions is also implicated by the same
Figure 1. X-ray crystal structure of 1 with thermal ellipsoids drawn at
the 50% probability level. Selected bond lengths (Å) and angles (deg):
B-N(1) ) 1.459(2), B-N(2) ) 1.466(2), B-N(3) ) 1.469(2), Si(1)-
N(1) ) 1.786(1), Si(2)-N(2) ) 1.778(1), Si(3)-N(3) ) 1.784(1), Si(1)-
C(1) ) 1.899(1), Si(2)-C(1) ) 1.902(1), Si(3)-C(1) ) 1.901(1); N(1)-
B-N(2) ) 118.5(1), N(1)-B-N(3) ) 119.0(1), N(2)-B-N(3) ) 118.2(1).
N-C(ipso) bond length (within 2σ; av 1.421(2) Å) and large
torsion angles (46.1-52.2°) of Ar relative to the plane
defined by Si, N, and C(ipso). As a whole, all crystal-
lographic evidence is consistent with a trigonal pyramidal
geometry at B in the tripodal borane structure 1, although
the degree of pyramidalization is not significant in this
complex.
Subsequently, we set out to prepare two different classes
of tripodal amido borane derivatives: backbone (HC[N3] f
t
MeSi[N3]) and N-substitution (4-MeC6H4 f Bu). Specifi-
cally, the tripodal amido borane bearing the all-silicon (i.e.,
trisilylsilane) backbone, MeSi[N3]B (2) was successfully
synthesized via the salt metathesis route employing the
corresponding trianionic lithium salt MeSi[N3]Li3‚(Et2O)2;
1484 Inorganic Chemistry, Vol. 46, No. 4, 2007

Tripodal Amido B and Al Complexes
Scheme 1
however, it was isolated in low yield (9%), and a significant
amount (22%) of the unreacted MeSi[N3]Li3‚(Et2O)2 was
recovered after the reaction and workup. On the other hand,
t
when the N-Ar group was substituted with the bulky Bu,
the reaction of the lithium salt HC[Si(Me2)N(tBu)]Li3 with
BCl3 did not yield the desired tripodal triamido borane;
instead, the reaction afforded non-boron-containing LiCl
adduct {[HC(SiMe2NtBu)2(SiMeNtBu)]Li3(Et2O)Cl}2 (3) via
apparent elimination of MeBCl2, with the chloride deriving
from BCl3 and the Me from a Me2Si< group in the tripodal
ligand giving rise to the new Me(tBuN)Si< moiety (Scheme
1).
Figure 2. X-ray crystal structure of 3 with thermal ellipsoids drawn at
the 50% probability level. Selected bond lengths (Å) and angles (deg):
C(1)-Si(1) ) 1.867(2), C(1)-Si(2) ) 1.877(2), C(1)-Si(3) ) 1.906(2),
Si(1)-N(1) ) 1.760(1), Si(2)-N(2) ) 1.704(1), Si(3)-N(3) ) 1.683(1),
Si(3)-N(1) ) 1.797(1), Cl(1)-Li(1) ) 2.378(3), Cl(1)-Li(2) ) 2.477(3),
Cl(1)-Li(3) ) 2.439(3), Cl(1)-Li(2A) ) 2.391(3); Li(2)-Cl(1)-Li(2A)
) 88.16(9), Cl(1)-Li(2)-Cl(1A) ) 91.84(9).
Scheme 2
1
The solution H NMR spectrum of 3 shows two types of
t
the Bu groups (18H at 1.49 ppm and 9H at 1.28 ppm) and
four types of the SiMe groups at 0.71 (3H), 0.57 (6H), 0.40
(3H), and 0.32 (3H) ppm, respectively, and its molecular
structure was confirmed by X-ray diffraction (Figure 2),
featuring a centrosymmetric dimeric structure. The two
derived tripodal ligand moieties after the MeBCl2 elimination
are connected by tetracoordinate Li and Cl centers forming
a parallelogram linkage with alternating short and long Li-
Cl bonds that differ by only 0.086 Å; each ligand moiety
comprises 4 unique fused four-membered rings involving all
the mainframe atoms, except for Si(2), with 2 of them being
planar [ring 1, C(1)-Si(1)-N(1)-Si(3), ∆ ) 0.0128 Å; ring
2, N(1)-Si(3)-Li(2)-N(3), ∆ ) 0.0293 Å] and the other
two being quasiplanar [ring 3, Li(2)-N(3)-Cl(1)-Li(3), ∆
) 0.0662 Å; ring 4, Cl(1)-Li(3)-N(2)-Li(1), ∆ ) 0.0726
Å]. The dihedral angles are 113.6° (rings 1 and 2), 68.8°
(rings 2 and 3), and 135.1° (rings 3 and 4). All Li-N (amido)
bonds have comparable lengths [Li(1)-N(2) ) 1.951(3) Å,
Li(2)-N(3) ) 1.991(3) Å, Li(3)-N(2) ) 1.976(3) Å, Li-
(3)-N(3) ) 1.982(3) Å], except for the Li-N (amino) bond
which is significantly longer [Li(2)-N(1) ) 2.393(3) Å].
The structure clearly shows the new connection of the SiMe
group to another adjacent NtBu nitrogen atom through Si,
thus making that nitrogen a neutral amino N donor which
resets the whole ligand in divalent state.
(4) in a 70% yield, likely existing as a dimer (Scheme 2). In
an effort to remove the coordinated LiCl and its solvated
Et2O, complex 4 was subjected to heat (165 °C) under
vacuum (0.5 Torr) or to treatment with AgBPh4, affording
the Et2O-free complex HC[N3]Al‚ClLi (5) that can also be
produced directly from the reaction of HC[N3]Li3 with AlCl3
in a hexanes/toluene solvent mixture. In solution, complex
5 has a time-average C3V symmetry on the NMR time scale
at room temperature; however, the interaction between Li
and Ar is shown by an AB spin splitting pattern of the Ar
protons in 5 versus the typical AX(M) spin patterns for those
without such coordination (e.g., 1, HC[N3]Li3, and 4).
The molecular structure of tripodal aluminum complex 5
derived from X-ray diffraction (Figure 3) reveals a unique
centrosymmetric dimeric structure in which each tetracoor-
dinate Li center is bound to two N-aryl rings from two
separate [N3] ligand moieties, with two three-coordinate Cl
atoms completing its coordination sphere. One of the aryl
rings is coordinated to Li in a best-described η2-fasion [Li-
(1)-C(15) ) 2.387(3) Å, Li(1)-C(16) ) 2.345(3) Å; the
other Li(1)-C(Ar) distances are in the range of 2.698(3)-
3.052 Å with a Li(1)-C(Ar-ring centroid) distance of 2.339
Å], whereas the other is in a best-described η1-fashion [Li-
(1)-C(22A) ) 2.397(3) Å; the other Li(1)-C(Ar) distances
are in the range of 2.802-4.005 Å with the Li(1)-C(Ar-
ring centroid) distance of 2.997 Å]. Such N-Ar‚‚‚Li interac-
tions noticeably lengthen the distances of Al to such amido
nitrogens; thus, the two same Al-N bonds (within 2σ; av
1.8466(14) Å) exhibiting the Ar‚‚‚Li interactions are longer
Synthesis and Structural Studies of Tripodal Amido
Aluminum Complexes. The ionic radius of Al3+ is ap-
proximately double that of its cogeneric B3+, which should
render more pyramidalized geometry for the similar tripodal
triamido Al complexes. In this context, the HC[N3]H3 ligand
was selected for the synthesis of its tripodal aluminum
complexes using the salt metathesis approach, but they were
isolated either in its solvated form or as a LiCl adduct.
Specifically, the reaction of HC[N3]Li3‚(Et2O)2 with AlCl3
in a hexanes/ether solvent mixture led to formation of the
tripodal triamido aluminum complex HC[N3]Al‚ClLi(Et2O)2
Inorganic Chemistry, Vol. 46, No. 4, 2007 1485

Zhu and Chen
Figure 4. X-ray crystal structure of 6 with thermal ellipsoids drawn at
the 50% probability level. Selected bond lengths (Å) and angles (deg): Al-
(1)-N(1) ) 1.8458(9), Al(1)-N(2) ) 1.8300(9), Al(1)-N(3) ) 1.835(1),
Si(1)-N(1) ) 1.7457(9), Si(2)-N(2) ) 1.735(1), Si(3)-N(3) ) 1.734(1),
Al(1)-O(1) ) 1.861(5) (av for the disordered THF); N(1)-Al(1)-N(2) )
110.15(4), N(1)-Al(1)-N(3) ) 110.38(4), N(2)-Al(1)-N(3) ) 109.30-
(4).
Figure 3. X-ray crystal structure of 5 with thermal ellipsoids drawn at
the 50% probability level. Selected bond lengths (Å) and angles (deg): Al-
(1)-N(1) ) 1.847(1), Al(1)-N(2) ) 1.846(1), Al(1)-N(3) ) 1.829(1),
Al(1)-Cl(1) ) 2.2272(5), Cl(1)-Li(1A) ) 2.390(3), Cl(1)-Li(1) ) 2.527-
(3); N(1)-Al(1)-N(2) ) 109.00(6), N(1)-Al(1)-N(3) ) 111.50(6), N(2)-
Al(1)-N(3) ) 108.79(6), Cl(1)-Li(1)-Cl(1A) ) 90.9(1), Li(1)-Cl(1)-
Li(1A) ) 89.2(1).
Chart 2
than the third Al-N bond, absent of such interactions, by
∼0.02 Å. The two tripodal ligand moieties are further
connected by two bridging Cl atoms that form a nearly
rectangular linkage with two Li atoms exhibiting alternating
short and long Li-Cl bonds that differ by 0.137 Å. The
geometry at Al is that of a slightly distorted tetrahedron with
the sum of the N-Al-N angles being 329.3°, while the
adduct HC[N3]Al‚ClLi nature of complex 5 is recognized
by the observed relatively long Al-Cl bond length of 2.2272-
(5) Å and the normal Cl-Li bond length of 2.390(3) Å.
When a small amount of THF was added to the same salt
metathesis reaction in a hexanes/ether mixture, two products,
neutral alane-THF adduct HC[N3]Al‚(THF) (6, 19% yield)
and ionic lithium aluminate complex HC[N3]Al(OCHdCH2)‚
Li(THF)2 (7, 2% yield), were isolated (Chart 2), both of
which were structurally characterized by X-ray diffraction
(Figures 4 and 5). Depending on the acceptor metal (Al vs
Li), the coordinated THF molecules exhibit distinctly dif-
ferent chemical shifts in 1H NMR: 3.41 (t) and 0.42 (t) when
bound to Al and 3.04 (t) and 1.17 (t) when bound to Li. The
coordinated THF in 6 was not removed by vacuum or heat
treatment, indicative of the strongly Lewis acidic nature of
the transient tripodal amido alane species, “[N3]Al”. The
solid-state structure of the complex 6 (Figure 4) consists of
two independent molecules with similar metric parameters,
only one of which is shown and discussed here. The Al center
adopts a slightly distorted tetrahedral geometry with the sum
of the N-Al-N angles being 329.8°, while the Al atom sits
Figure 5. X-ray crystal structure of 7 with thermal ellipsoids drawn at
the 50% probability level. Selected bond lengths (Å) and angles (deg): Al-
(1)-N(1) ) 1.867(1), Al(1)-N(2) ) 1.844(1), Al(1)-N(3) ) 1.851(1),
Al(1)-O(1) ) 1.790(1), Li(1)-O(1) ) 1.924(4), Li(1)-O(2) ) 1.910(4),
Li(1)-O(3) ) 1.90(1) (av for the disordered THF), Li(1)-C(16) ) 2.466-
(4), C(41)-C(42) ) 1.318(3); N(1)-Al(1)-N(2) ) 107.69(6), N(1)-Al-
(1)-N(3) ) 105.90(6), N(2)-Al(1)-N(3) ) 108.15(6).
above the N3-basal plane by 0.5979 Å. The three Al-N
single-bond lengths slightly differ from one another with an
average Al-N length of 1.837(1) Å, which is longer than
the average Al-N (amido) distances of 1.809(6) and 1.802-
(2) Å observed for the tetracoordinate triamido aluminum
complexes N[CH2CH2(Me3Si)N]3Al14 and [Al(NMe2)3]215 for
the equatorial Al-N and terminal Al-N bonds, respectively.
The amido nitrogens are nearly planar with the average of
the sum of angles around them being 359.3°, which can be
attributed to partial π-bonding between long pairs of N and
d orbitals of the neighboring Si.14 The Al-O bond length
of 1.861(5) Å in 6 is identical to that observed in (C6F5)3-
Al‚(THF) (1.860(2) Å),16 further indicating high Lewis
acidity of the tripodal amido alane species [N3]Al.
(14) Pinkas, J.; Wang, T.; Jacobson, R. A.; Verkade, J. G. Inorg. Chem.
1994, 33, 4202-4210.
(15) Waggoner, K. M.; Olmstead, M. M.; Power, P. P. Polyhedron 1990,
9, 257-263.
(16) Belgardt, T.; Storre, J.; Roesky, H. W.; Noltemeyer, M.; Schmidt,
H.-G. Inorg. Chem. 1995, 34, 3821-3822.
1486 Inorganic Chemistry, Vol. 46, No. 4, 2007

Tripodal Amido B and Al Complexes
The details about the formation of the unanticipated
complex 7 are currently unknown, although it is clear that
its formation is directly related to the presence of free THF
in solution, suggesting a plausible pathway through the ring-
opening of THF followed by the C-C bond cleavage.
Accordingly, this side product can be either substantially
suppressed using a minimal amount of THF in the reaction
solvent mixture or completely avoided using HC[N3]Li3‚
(THF)3 for its reaction with AlCl3 in a hexanes-ether solvent
mixture. By the latter approach, the pure complex 6 was
isolated in an 85% yield. The enolate moiety (OCHdCH2)
protection provided by a class of tripodal triamido ligands
should be strong LAs, we have synthesized several such
novel B and Al tripodal triamido complexes, the structures
of five of which have been elucidated by X-ray diffraction
studies. The trigonal pyramidal geometry at B in the tripodal
triamido borane structure 1, as a consequence of the lack of
significant N-B p-p π-interactions, suggests significant
Lewis acidity imposed by the geometry within the tripodal
triamido binding pocket.18 Furthermore, the strong adduct
and solvate formations of the tripodal amido Al complexes,
as shown in 4, 5, and 6, as well as their similarity to the
exceptionally strong Lewis acid Al(C6F5)3 in the THF adduct
and enolaluminate formation and structure, as shown in 6
and 7, indicate the desired core structure [N3]Al is indeed
highly Lewis acidic. Our ongoing efforts are directed at the
synthesis of highly pyramidalized tripodal boranes and salt-
or donor-solvent-free tripodal amido alanes, as well as
catalytic applications of these preorganized pyramidal B and
Al complexes as LA catalysts or cocatalysts.
1
within complex 7 is readily characterized by its H NMR
resonances at 6.69 (dd, 1H) and 3.60 ppm (qAB, 2H), as well
as by the X-ray structure showing the C(41)-C(42) bond
length of 1.318(3) Å (Figure 5) for a CdC double bond.
The tetracoordinate Li is bound to three O atoms (1 from
the enolate and 2 from the two coordinated THF) and an
aryl C(ortho) in a best-described η1-fasion [Li(1)-C(16) )
2.466(4) Å; the other Li(1)-C(Ar) distances are in the range
of 2.927 to 4.247 Å with the Li(1)-C(Ar-ring centroid)
distance of 3.181 Å]. Because of such N-Ar‚‚‚Li interac-
tions, the bond length of Al to this amido N [Al(1)-N(1) )
1.867(1) Å] is noticeably longer than the other two Al-N
bonds that are essentially the same (within 2σ; av 1.847(1)
Å). The geometry around the four-coordinate Al center is
that of a distorted tetrahedron with the sum of the N-Al-N
angles being 321.7°; for the same reason described above,
the N(2) and N(3) adopt a nearly planar geometry with the
average of the sum of angles around them being 359.6°. The
overall structure of 7 somewhat resembles those of lithium
ester enolaluminate complexes Li[Me2C)C(OiPr)OAlMe-
(BHT)2] and Li(THF)2[Me2C)C(OiPr)OAl(C6F5)3].17
Acknowledgment. This work was supported by an
unrestricted research fund provided by Sumitomo Chemical
Co. Ltd. We thank Masayuki Fujita, Tatsuya Miyatake, and
Kazuo Takaoki for helpful discussions, as well as Susie
Miller for the X-ray diffraction data collections.
Supporting Information Available: Crystallographic data for
complexes 1, 3, 5, 6, and 7 (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org. Crystallographic data
for the five structures reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre as supplementary
numbers CCDC-624044 (1), 624045 (3‚2C6H14), 624046 (5‚C6H14),
624047 (6), and 624048 (7). These data can be obtained free of
charge from CCDC, 12 Union Road, Cambridge CB21EZ, U.K.
(fax (+44) 1223-336-033; e-mail deposit@ccdc.cam.ac.uk).
Conclusions
In summary, motivated by the hypothesis that preorganized
trigonal pyramidal boranes and alanes with an adequate steric
IC0620889
(17) Rodriguez-Delgado, A.; Chen, E. Y.-X. J. Am. Chem. Soc. 2005, 127,
961-974.
(18) Preliminary experiments showed facile reaction between 1 and a
metallocene complex, Cp2ZrMe2 or rac-C2H4(Ind)2ZrMe2.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1487