Reaction of the Primary Alane (2,4,6-t-Bu3H2C6AlH2)2 with Nitriles, Isonitriles, and Primary Amines

Article abstract of DOI:10.1021/ic9810992

The reactions of the sterically encumbered primary alane (Mes*AlH₂)₂ (Mes= C₆H₂-2,4,6-t-Bu₃) with the nitriles t-BuCN, MesCN (Mes = C₆H₂-2,4,6-Me₃) or MeCN lead eventu

Full text of DOI:10.1021/ic9810992

6906
Inorg. Chem. 1998, 37, 6906-6911
Reaction of the Primary Alane (2,4,6-t-Bu3H2C6AlH2)2 with Nitriles, Isonitriles, and Primary
Amines
Rudolf J. Wehmschulte† and Philip P. Power*
Department of Chemistry, University of California, Davis, California 95616
ReceiVed September 11, 1998
The reactions of the sterically encumbered primary alane (Mes*AlH2)2 (Mes* ) C6H2-2,4,6-t-Bu3) with the nitriles
t-BuCN, MesCN (Mes ) C6H2-2,4,6-Me3) or MeCN lead eventually to dimeric amido alane products in which
one of the ortho t-Bu groups of the Mes* ligand is metalated and the nitrile is reduced to the amide ligand
N(H)CH2R (R ) t-Bu, Mes, or Me). The compounds [AlC6H2-2,4-t-Bu2-6-CMe2CH2{µ2-N(H)CH2R}]2 (R )
t-Bu, 2 (cis), 3 (trans); Mes, 4 (cis), 5 (trans); Me, 6 (cis)) have been isolated and characterized spectroscopically
and also by X-ray crystallography in the cases of 4 and 5. The intermediate, dimeric iminato complex [Mes*Al-
(H){µ2-NC(H)t-Bu}]2 (1), can also be isolated under carefully controlled, mild conditions. Reaction of (Mes*AlH2)2
with the isonitrile t-BuNC affords the cyclic species {Mes*AlN(t-Bu)CH2}2 (7) featuring a six-membered (AlNC)2
ring which, when heated, affords the cyclometalated species [AlC6H2-2,4-t-Bu2-6-CMe2CH2{N(t-Bu)H}(CH2)]2
(8). Recognition that the products 2-6 and 8 were derivatives of primary amides led to an investigation of synthetic
approaches to these product types via the direct reaction of (Mes*AlH2)2 with some primary amines. Treatment
of (Mes*AlH2)2 with H2NCH2Mes affords the dimeric amido alane [Mes*(H)Al{µ2-N(H)CH2Mes}]2 as a mixture
of trans (9) and cis (10) isomers. Further heating of 9 and 10 affords the ortho-metalated compounds 4 and 5. The
reaction of (Mes*AlH2)2 with H2NSiPh3 furnishes the bis amido aluminum compound Mes*Al{N(H)SiPh3}2 (11)
and [Mes*(H)Al{N(H)SiPh3}]2. The latter yields the dimeric imide {Mes*AlNSiPh3}2 (12) at elevated temperature.
Introduction
and characterization of several reaction products from the
treatment of (Mes*AlH2)2 with the nitriles RCN (R ) t-Bu,
Mes, or Me), the isonitrile t-BuNC, and the amines H2NCH2-
Mes and H2NSiPh3 are described. These reactions are sum-
marized in Scheme 1.
(Mes*AlH2)2,1 a bulky primary alane, has been shown to react
smoothly with PhEH2 (E ) N, P, As) and (Me2SiO)3 to form
the novel, formally unsaturated ring systems (Mes*AlEPh)n2 (E
) N, n ) 2; E ) P, As, n ) 3) and (Mes*AlO)4.3 In addition,
reaction with azobenzene (i.e., Ph-NdN-Ph) affords an
Experimental Section
General Procedures. All work was performed under anaerobic and
anhydrous conditions by using either modified Schlenk techniques or
a Vacuum Atmospheres HE43-2 drybox. MeCN, t-BuCN, t-BuNC, and
MesCN were obtained from commercial suppliers and dried by standard
procedures. (Mes*AlH2)21 and H2NSiPh37 were synthesized by literature
methods, and H2NCH2Mes (which was originally synthesized by the
reaction of MesCH2Cl with potassium amide8) was obtained by the
reduction of MesCN with LiAlH4. Solvents were freshly distilled under
N2 from Na/K alloy and degassed twice prior to use. NMR spectra
were recorded on a General Electric GE-300 spectrometer, and IR
spectra were obtained on a Perkin-Elmer PE-1430 spectrometer.
[Mes*Al(H){µ2-NdC(H)-t-Bu}]2 (1). A solution of (Mes*AlH2)2
(0.59 g, 1.1 mmol) in n-pentane (60 mL) was treated with t-BuCN
(0.24 mL, 0.18 g, 2.15 mmol) at room temperature. After ca. 1 min, a
fine, colorless precipitate formed. The solution was stirred for 1 h, after
which the solid was collected on a sintered glass frit and dried under
reduced pressure. Yield: 0.53 g, 60%. Mp: 265-270 °C. Calcd for
C23H40NAl: C, 77.26; H, 11.28; N, 3.92. Found: C, 77.33; H, 11.02;
N, 3.84. 1H NMR (C6D6): 8.52 (s, NdCH, 2H), 7.54, 7.48 (AB system,
isoelectronic analogue of a cyclopentadienide anion, [Mes*Al-
(µ2-PhNNPh)Al(Mes*)(µ2-NPh)].4 The reactivity of (Mes*AlH2)2
toward the polar, formally triply or doubly bonded nitrile or
isonitrile groups has not been investigated, however. Reduction
of nitriles by using aluminum hydrides was first reported in
19575 and it is a well-known procedure in organic synthesis. It
was hoped that the bulky Mes* group would allow the isolation
and characterization of intermediates of this hydride reduction.
It seems that only one such intermediate, the compound [(i-
Bu)2Al{µ2-NdC(H)(C6H3-2,6-Me2)}]2, has been characterized
by X-ray crystallography.6 This species was obtained from the
reaction of 2,6-Me2H3C6CN with HAl(i-Bu)2. In addition, it was
possible that reduction of RCN with a bulky alane such as
(Mes*AlH2)2 would be an alternative, nonprotic approach to
the synthesis of aluminum imides. In this paper the synthesis
† Present address: Room 208, Department of Chemistry & Biochemistry,
620 Parrington Oval, Norman, OK 73019.
4
m-H, 4H), JHH ) 1.5 Hz, 5.1 (s, broad, Al-H, 2H), 1.80, 1.52, 1.39
(1) Wehmschulte, R. J.; Power, P. P. Inorg. Chem. 1994, 33, 5611.
(2) Wehmschulte, R. J.; Power, P. P. J. Am. Chem. Soc. 1996, 118, 791.
(3) Wehmschulte, R. J.; Power, P. P. J. Am. Chem. Soc. 1997, 119, 8387.
(4) Wehmschulte, R. J.; Power, P. P. Inorg. Chem. 1996, 35, 2717.
(5) (a) Zakharkin, L. I.; Khorlina, I. M. Proc. Acad. Sci. USSR 1957,
116, 879. (b) Zakharkin, L. I.; Khorlina, I. M. Bull. Acad. Sci. USSR
(Engl. Transl.) 1959, 523. (c) Andreoli, P.; Billi, L.; Carnelli, G.;
Panunzio, M.; Martelli, G.; Spunta, G. J. Org. Chem. 1990, 55, 4199.
(6) Jensen, J. A. J. Organomet. Chem. 1993, 456, 161.
(s, o-, p-Me, 18H each), 0.67 (s, t-Bu, 18H). 13C{1H} NMR (C6D6):
190.6 (NdC), 162.3, 160.5 (o-C), 149.5 (p-C), 121.3, 120.7 (m-C),
41.2, 38.9, 38.6, 34.9 (C(CH3)3), 34.6, 33.4, 31.5 (o-, p-CH3), 24.3
(t-Bu). IR: 1828 (st, νAl-H), 1668 (sh), 1642 (st, νNdC).
(7) Kraus, C. A.; Rosen, R. J. Am. Chem. Soc. 1925, 47, 2739.
(8) Hauser, C. R.; Brasen, W. R.; Skell, P. S.; Kantor, S. W.; Brodhag,
A. E. J. Am. Chem. Soc. 1956, 78, 1653.
10.1021/ic9810992 CCC: $15.00 © 1998 American Chemical Society
Published on Web 12/09/1998

Reactions of a Primary Alane
Inorganic Chemistry, Vol. 37, No. 26, 1998 6907
(dd, N-CH2, 2H, 3), 2JHH ) 11.1 Hz, 3JHH ) 11.1 Hz, 2.75 (d, N-CH2,
4H, 2), 3JHH ) 6.3 Hz, 2.55 (d, N-CH2, 2H, 3), 1.90 (s, C(CH3)2, 6H,
3), 1.75 (s, C(CH3)2, 12H, 2), 1.63, 1.60 (s, o-C(CH3)3, 18H ea), 1.58
(s, C(CH3)2, 6H, 3), 1.35, 1.33 (s, p-C(CH3)3, 18H ea), 0.82 (d, Al-
Scheme 1. Summary of the Reactions Leading to 1-12a
2
CH2, 2H, 3), JHH ) 14.7 Hz, 0.66 (s, Al-CH2, 4H, 2), 0.60 (d, Al-
CH2, 2H, 3), 0.55 (s, CH2-C(CH3)3, 36H). 13C{1H} NMR (2, 3)
(C6D6): 168.1, 167.0 (6-C), 157.4, 156.7 (2-C), 151.3 (p-C), 126.5,
120.0, 119.7, 119.4 (m-C), 56.7, 56.5 (N-CH2), 41.5, 41.3 (C-Me2),
38.9 (CMe2, 3), 38.0 (CMe2, 2), 36.6 (CMe2, 3), 34.1 (o-C(CH3)3, 31.7
(p-C(CH3)3), 27.3, 27.4 (N-C-C(CH3)3), 26.0 (broad, Al-CH2), 20.1
(broad, Al-CH2). IR: 3295 (w, νN-H).
cis- and trans-[AlC6H2-2,4-t-Bu2-6-CMe2CH2{µ2-N(H)CH2Mes}]2
(cis, 4; trans, 5). A solution of MesCN (0.20 g, 1.38 mmol) in hexane
(10 mL) was added to a rapidly stirred solution of (Mes*AlH2)2 (0.38
g, 0.69 mmol) in hexane (30 mL) at room temperature. The mixture
was warmed to reflux temperature (ca. 70 °C), at which point stirring
was discontinued and the mixture was maintained at 70 °C for 3 days.
Upon cooling of the mixture to room temperature, a small amount of
white insoluble solid precipitated. This was removed by filtration, and
the resulting clear colorless solution was concentrated to ca. 25 mL
and cooled in a -20 °C freezer overnight to give 0.14 g of the trans
isomer 5 as a fine colorless powder. Yield: 24%. Mp: 245-255 °C,
melts partially with color change to orange, does not melt completely
1
4
under 300 °C. H NMR (C6D6): 7.68 (d, m-H, 2H), JHH ) 1.5 Hz,
7.57 (d, m-H, 2H), 6.50 (s, m-H (Mes), 4H), 4.12 (t, NH, 2H), 3JHH
)
2
4.0 Hz, 4.04 (d, N-CH2, 2H), JHH ) 12.3 Hz, 3.96 (d, N-CH2, 2H)
(ABX system), 1.94 (s, p-CH3(Mes), 6H), 1.84 (s, o-CH3 (Mes), 12H),
1.71, 1.54 (2s, C(CH3)2, 12H), 1.46 (s, o-C(CH3)3, 18H), 1.41 (s,
p-C(CH3)3, 18H), 0.91 (d, Al-CH2, 2H) 2JHH ) 15.3 Hz, 0.43 (d, Al-
CH2, 2H). 13C NMR (C6D6): 166.9 (6-C), 157.1 (2-C), 151.5 (p-C),
137.2, 135.2 (i-, p-C(Mes)), 136.3 (o-C(Mes)), 129.7 (m-C(Mes)), 120.5,
119.8 (m-C), 42.2 (N-CH2), 41.4 (CMe2), 38.4, 36.9 (CMe2), 35.0 (p-
C(CH3)3), 33.9 (o-C(CH3)3), 31.7 (p-C(CH3)3), 25.1 (broad, Al-CH2),
20.7 (p-CH3(Mes)), 19.9 (o-CH3(Mes)). IR: νNH ) 3305 cm-1 (m).
The colorless supernatant liquor from 5 was concentrated to ca. 3 mL
and cooled in a -20 °C freezer for 1 week to give 0.15 g of the cis
isomer 4 as the adduct 4‚hexane in the form of large colorless plates.
Yield: 23%. Mp: 222-225 °C, turns opaque at 50 °C, softens at 210
1
4
°C. H NMR (C7D8, 20 °C): 7.71 (d, 5-H, 2H), JHH ) 1.5 Hz, 7.50
3
(d, 3-H, 2H), 6.42 (s, m-H(Mes), 4H), 4.01 (d, N-CH2, 4H), JHH
)
4.5 Hz, 1.91 (s, p-CH3(Mes)), 6H, 1.85 (s, o-CH3(Mes), 12H), 1.84 (s,
C(CH3)2, 12H) or vice versa, 1.40 (s, o-C(CH3)3, 18H), 1.20 (s,
p-C(CH3)3, 18H), 0.87 (s, Al-CH2, 4H). 13C NMR (C6D6, 20 °C): 167.7
(6-C), 156.9 (2-C), 151.4 (4-C) 137.0, 134.8 (i-, p-C(Mes)), 136.2 (o-
C(Mes)), 129.5 (m-C(Mes)), 119.5 (m-C), 41.5 (CMe2), 41.3 (N-CH2),
38.0 (CMe3), 35.0 (p-CMe3), 33.5 (o-CMe3), 20.7 (p-CH3(Mes)), 20.2
(Al-CH2; broad), 20.0 (o-CH3(Mes)). IR: νNH ) 3295 cm-1 (m).
Crystals of 4‚toluene and 5‚3benzene suitable for X-ray diffraction
studies were grown from concentrated toluene (in a -20 °C freezer)
and benzene (at ca. +6 °C) solutions over a period of several days.
cis-[AlC6H2-2,4-t-Bu2-6-CMe2CH2{µ2-N(H)CH2CH3}]2 (6). MeCN
(0.079 mL, 0.061 g, 1.5 mmol) was added to a solution of (Mes*AlH2)2
(0.41 g, 0.75 mmol) in n-hexane (30 mL) at room temperature. The
MeCN phase disappeared after 3-4 min, and the resulting clear,
colorless solution turned pale yellow after ca. 15 min. After stirring
for ca. 3 h, the solution was concentrated to ca. 10 mL and cooled in
a -20 °C freezer overnight. As no crystals were formed, all the volatile
materials were removed under reduced pressure. The solid was then
redissolved in n-hexane, and the solution was heated to reflux for 18
h. Small amounts of a fine colorless solid were precipitated and removed
by filtration. Concentration of the filtrate to ca. 20 mL and cooling in
a -20 °C freezer for 1 week afforded 0.09 g of 6 as a colorless,
microcrystalline solid. Yield: 19%. Mp: softens at 264 °C, melts at
266-269 °C. Calcd for C20H34NAl: C, 76.14; H, 10.86; N, 4.44.
a The formula for t-BuNC is given as the doubly bonded canonical
form although it is also possible to write it with an NC triple bond.
cis- and trans-[AlC6H2-2,4-t-Bu2-6-CMe2CH2{µ2-N(H)CH2(t-
Bu)}]2 (2 and 3). A solution of 1 in benzene (15 mL) was heated to
80 °C for 40 h. Removal of the solvent followed by crystallization
from n-hexane (15 mL) at -20 °C for 6 days afforded 0.9 g of a 1:1
mixture of 2 and 3. Yield: 75%. Mp: 303-310 °C. Calcd for C23H40-
NAl: C, 77.26; H, 11.28; N, 3.92. Found: C, 77.81, H, 11.41; N, 3.76.
1H NMR (2, 3) (C6D6): 7.70, 7.68, 7.61, 7.58 (s, m-H, 2H ea), 2.87
1
Found: C, 76.01, H, 10.59; N, 4.31. H NMR (C6D6): 7.70 (d, m-H,
4
3
2H), JHH ) 1.5 Hz, 7.58 (d, m-H, 2H), 2.68 (dq, N-CH2, 4H), JHH
) 7.2H (d and q), 1.70 (s, C(CH3)2, 12H), 1.49 (s, o-C(CH3)3, 18H),
1.41 (s, p-C(CH3)3, 18H), 0.62 (t, N-C-CH3, 6H), 0.56 (s, Al-CH2,
4H). 13C{1H} NMR (C6D6, 60 °C): 168.2 (6-C), 157.1 (2-C), 151.3
(p-C), 119.5, 119.1 (m-C), 41.5 (N-CH2), 38.8, 38.0, 35.1 (quarternary

6908 Inorganic Chemistry, Vol. 37, No. 26, 1998
Wehmschulte and Power
carbons), 37.6 (C(CH3)2), 34.1 (o-C(CH3)3), 31.8 (p-C(CH3)3), 19.4 (N-
C-CH3). IR: 3290 cm-1 (w, νN-H).
Table 1. Crystallographic Data for 4‚toluene, 5‚3benzene, 7,
and 12
{Mes*AlN(t-Bu)CH2}2 (7). A slurry of (Mes*AlH2)2 (0.43 g, 0.75
mmol) in n-pentane (20 mL) was treated with t-BuNC (0.17 mL, 0.12
g, 1.5 mmol) at 0 °C to give, almost immediately, a clear solution.
After ca. 5 min the solution was cooled slowly in a ca. -20 °C freezer
for 5 days to give 0.19 g of colorless, microcrystalline 7. A second
crop of 7 (0.12 g) was obtained by concentration of the mother liquor
to ca. 2-3 mL followed by cooling to ca. -20 °C for 1 week. Crystals
of 7 suitable for X-ray crystallographic studies were grown from a
saturated pentane solution at ca. -20 °C. Yield: 58%. Mp: disente-
grates into a powder at 113 °C; becomes yellow orange at 265 °C,
4‚toluene 5‚3benzene
7
12
formula
C63H92-
Al2N2
931.35
C79H102
Al2N2
1073.54
P1h
C46H80
Al2N2
715.08
P21/c
11.317(2) 10.110(2)
9.668(2)
20.651(4) 13.937(3)
108.61(2)
C72H88
Al2N2Si2
1091.58
fw
space group
a, Å
b, Å
c, Å
R, Å
â, Å
γ, Å
V, Å3
Z
P1h
P1h
11.896(4)
14.315(4)
18.354(4)
78.53(2)
71.60(2)
84.69(2)
10.927(2)
11.160(1)
14.231(2)
89.228(8)
89.546(10) 90.17(2)
72.469(9)
11.988(3)
95.16(2)
104.36(2)
1
melts with gas evolution at 286-287 °C. H NMR (C6D6): 7.52 (s,
2905.2(14) 1654.6(3)
2259.4(7) 1524.3(6)
m-H, 4H), 2.46 (s, Al-CH2, 4H), 1.74 (s, o-C(CH3)3, 36H), 1.34 (s,
p-C(CH3)3, 18H), 1.16 (s, N-C(CH3)3, 18H). 13C{1H} NMR (C6D6):
156.4 (o-C), 149.6 (p-C), 135.7 (i-C), 122.0 (m-C), 54.1 (N-C(CH3)3),
38.4 (Al-CH2), 36.6 (o-C(CH3)3), 34.7 (p-C(CH3)3), 33.4 (o-C(CH3)3),
31.5 (p-C(CH3)3), 30.0 (N-C(CH3)3).
2
130
1
130
2
130
1
123
T, K
λ, Å
1.541 78
1.065
0.724
0.071
1.541 78
1.077
0.695
0.042
1.541 78
1.051
0.794
0.0559
1.541 78
1.189
1.132
0.0606
d
calc, g cm-3
µ, mm-1
R1 (for I >
2σ(I) data)a
wR2a
[AlC6H2-2,4-t-Bu2-6-CMe2CH2{N(t-Bu)H}(CH2)]2 (8). 7 (0.14 g,
0.2 mmol) in benzene (25 mL) was heated to 80 °C for 1 h. Standing
at room temperature overnight afforded a microcrystalline solid, which
was recrystallized from 10 mL of benzene at ca. 6 °C for 1 week.
Yield: 0.02 g (14%). Mp: turns orange and melts with slight gas
evolution at 284-285 °C. Calcd for C23H40NAl: C, 77.26; H, 11.28;
N, 3.92. Found: C, 77.93; H, 11.46; N, 3.68. 1H NMR (C6D6, 80 °C):
0.144
0.102
0.1566
0.1645
a R1 ) ∑||Fo - Fc||/∑|Fo|; wR2 ) [∑[w(Fo2 - Fc2)2]/∑[w(Fo )2]]1/2
.
2
(o-C(CH3)3), 31.5 (p-C(CH3)3). IR: νNH ) 3350 (w), 3297 (w), rel
intensity ≈ 1:2.
4
7.68 (d, m-H, 2H), JHH ) 1.7 Hz, 7.59 (d, m-H, 2H), 2.80 (d, N-H,
(Mes*AlNSiPh3)2 (12). The synthesis was identical to that of 11
except that the reaction mixture was heated to 135 °C for 17.5 h.
Concentration to ca. 15 mL followed by dissolution of microcrystalline
solid at reflux temperature and slow cooling to room temperature
afforded 0.12 g of 12 as colorless crystals of sufficient quality for X-ray
crystallography. Yield: 15%. Mp: >300 °C. 1H NMR (C6D6, 80 °C):
7.57 (dd, SiPh, 12H), JHH ) 8.1 Hz, JHH ) 1.5 Hz, 7.41 (s, m-H(Mes*),
4H), 6.97 (m, SiPh, 18H), 1.56 (s, o-C(CH3)3, 36H), 1.35 (s, p-C(CH3)3,
18H). The low solubility of 12 did not allow the recording of a 13C
NMR spectrum. Removal of the solvent from the mother liquor and
recrystallization of the sticky residue from n-pentane (5 mL, -20 °C)
for 1 week afforded 0.25 g of 11.
3
2
2H), JHH ) 12 Hz, 2.20 (dd, N-CH, 2H), JHH ) 13.8 Hz, 1.99 (d,
N-CH, 2H), 1.95 (s, C(CH3)2, 6H), 1.53 (s, o-C(CH3)3, 18H), 1.50 (s,
C(CH3)2, 6H), 1.41 (s, p-C(CH3)3, 18H), 0.82 (s, N-C(CH3)3, 18H),
0.65, 0.57 (AB system, Al-CH2, 4H), 2JHH ) 14.4 Hz. 13C{1H} NMR
(C6D6, 80 °C): 121.0, 119.0 (m-C), 57.8, 41.8, 40.1, 37.6, 36.4, 35.0;
33.4 (o-C(CH3)3), 31.8 (p-C(CH3)3), 28.3 (N-C(CH3)3). IR: 3225 cm-1
(m, νN-H).
cis- and trans-[Mes*Al(H){µ2-N(H)CH2Mes}]2 (9 and 10). A
solution of (Mes*AlH2)2 (0.30 g, 0.54 mmol) in n-pentane (20 mL)
was treated dropwise with a solution of MesCH2NH2 (0.16 g, 1.07
mmol) in n-pentane (10 mL) at 0 °C. After the initial formation of a
clear colorless solution, a fine colorless precipitate began to form at
the end of the addition. The mixture was brought to room temperature
and stirred for another 14 h. The colorless precipitate was collected on
a sintered glass frit and dried under reduced pressure. Yield: 86%.
Mp: melts with gas evolution at 161-162 °C. Calcd for C28H44NAl:
X-ray Crystallographic Studies. X-ray quality crystals of 4‚PhMe,
5‚3C6H6, 7, or 12 were removed from the Schlenk tube and immediately
covered with a layer of hydrocarbon oil. A suitable crystal was selected,
attached to a glass fiber, and immediately placed in a low-temperature
N2 stream as previously described.9 All data were collected at 130 K
on a Syntex P21 or Siemens P4/RA diffractometer equipped with a
low-temperature device and a graphite (Syntex P21) or nickel foil
(Siemens P4/RA) monochromator. Calculations were carried out with
SHELXTL-PLUS programs.10a Scattering factors and the correction for
anomalous scattering were taken from common sources.10b The
structures were solved by direct methods and refined by full-matrix
least squares refinement. An absorption correction was applied by using
the program XABS2.11 Crystal data for 4, 5, 7, and 12 are given in
Table 1. Selected bond distances and angles are provided in Table 2.
1
C, 79.76; H, 10.52; N, 3.32. Found: C, 80.08; H, 10.96; N, 3.19. H
NMR (C6D6): 7.51 (s, m-H (10), 2H), 7.31 (s, m-H (9), 4H), 7.19 (s,
m-H (10), 2H), 6.63 (s, m-H(Mes), 4H), 6.61 (s, m-H(Mes), 4H), 4.95
(s, broad, Al-H, 2H), 4.73 (s, broad, Al-H, 2H), 4.46 (d, N-CH2
(9), 4H), 3JHH ) 6.6 Hz, 4.12 (dd, N-CH2 (10), 2H), 2JHH ) 12.9 Hz,
3JHH ) 10.5 Hz, 3.77 (d, N-CH2 (9), 2H), 2.23 (s, o-CH3(Mes), 12H),
2.14 (s, o- and p-CH3(Mes), 18H), 2.10 (s, p-CH3(Mes), 6H), 1.77 (s,
o-C(CH3)3 (10), 18H), 1.43 (s, C(CH3)3, 18H), 1.36 (s, C(CH3)3, 18H),
1.35 (s, C(CH3)3, 18H), 1.28 (s, o-C(CH3)3 (9), 36H). IR: 2297 cm-1
(w, νN-H), 1841 cm-1 (st, νAl-H).
Mes*Al{N(H)SiPh3}2 (11). A solution of H2NSiPh3 (0.41 g, 1.5
mmol) in ethylbenzene (20 mL) was added dropwise to a solution of
(Mes*AlH2)2 (0.41 g, 0.75 mmol) in ethylbenzene (20 mL) at room
temperature, which resulted in vigorous gas evolution, which stopped
a few minutes after the addition was complete. After 2 h at room
temperature the clear colorless solution was placed in a preheated oil
bath at 125-130 °C and maintained at that temperature for 4 h. After
cooling to room temperature, a small amount of colorless solid was
removed and the solution was cooled to -20 °C for 4 days. As no
crystals were obtained, all volatile materials were removed under
reduced pressure and the resulting glass was recrystallized in a -20
°C freezer from n-hexane (ca. 2 mL) to afford 0.21 g of a colorless,
microcrystalline solid in spherical aggregates. Yield: 34% (based on
H2NSiPh3). 1H NMR (C6D6): 7.58 (m, SiPh, 12H), 7.35 (s, m-H(Mes*),
2H), 7.12 (m, SiPh, 18H), 1.44 (s, o-C(CH3)3, 18H), 1.32 (s, p-C(CH3)3,
9H), 0.75 (s, N-H, 2H). 13C{1H} NMR (C6D6): 158.2 (o-C(Mes*)),
150.5 (p-C(Mes*)), 138.4 (i-C), 135.9, 129.4, 127.9 (tertiary carbons,
SiPh3), 121.3 (m-C(Mes*)), 37.8 (o-C(CH3)3), 34.8 (p-C(CH3)3), 33.3
Discussion
The reaction between (Mes*AlH2)2 and t-BuCN in pentane
at room temperature affords in the first instance the dimeric
species [Mes*Al(H){µ2-NdC(H)-t-Bu}]2 (1). Unfortunately,
crystals of 1 have not yet been obtained that have proven suitable
for X-ray crystallography. The H and 13C{1H} NMR spectra
1
(9) This method is described by Hope: Hope, H. A Practicum in Synthesis
and Characterization. In Experimental Organometallic Chemistry;
Wayda, A. L., Darensbourg, M. Y., Eds.; ACS Symposium Series
357; American Chemical Society: Washington, DC, 1987; Chapter
10.
(10) (a) SHELXTL, A Program for Crystal Structure Determinations,
version 5.03; Siemens Analytical Instruments: Madison, WI, 1994.
(b) International Tables for X-ray Crystallography, D. Reidel Publish-
ing Co.; Dordrecht, The Netherlands, 1993; Vol. C.
(11) Parkin, S. R.; Moezzi, B.; Hope, H. J. Appl. Crystallogr. 1995, 28,
53.

Reactions of a Primary Alane
Inorganic Chemistry, Vol. 37, No. 26, 1998 6909
Table 2. Selected Bond Distances (Å) Angles (deg) for 4, 5, 7,
and 12
5,
4, cis isomer
trans isomer
Al-N
1.941(4)
1.970(4)
1.970(4)
1.950(5)
1.509(6)
2.763(2)
85.7(2)
1.945(4)
1.969(4)
1.959(4)
1.952(5)
1.512(6)
1.950(2)
1.963(2)
1.971(2)
1.954(2)
1.511(2)
2.8172(11)
87.92(7)
93.2(4)
131.52(7)
117.69(7)
112.26(8)
116.31(9)
93.18(8)
Al-C(Ar)
Al-C(CH2)
N-C
Al- -Al
N-Al-N
Al-N-Al
N-Al-C(Ar)
85.6(2)
89.9(2)
138.8(2)
114.4(2)
112.5(2)
112.2(2)
95.5(2)
89.8(2)
137.0(2)
115.4(2)
112.9(2)
111.2(2)
95.1(2)
N-Al-C(CH2)
C-Al-C
7
12
Figure 1. Thermal ellipsoid plot (30%) of 4 with hydrogen atoms
omitted for clarity.
Al(1)-N(1)
1.796(2)
1.986(2)
1.988(2)
1.472(3)
1.490(3)
Al(1)-N(1)
Al(1)-N(1A)
N(1)-Si(1)
1.842(2)
Al(1)-C(1)
1.841(2)
1.705(2)
1.975(3)
2.528(2)
Scheme 2. Possible Initial Sequence for the Interaction of
(Mes*AlH2)2 with t-BuCtN
Al(1)-C(19)
N(1)-C(19A)
N(1)-C(20)
N(1)-Al(1)-C(1)
N(1)-Al(1)-C(19)
C(1)-Al(1)-C(19)
Al(1)-N(1)-C(19A) 110.9(1)
Al(1)-N(1)-C(20) 132.6(1)
C(19A)-N(1)-C(20) 114.5(2)
C(2)-C(1)-C(6) 117.0(2)
Al(1)-C(1)
Al(1)- -Al(1A)
127.26(9) N(1)-Al(1)-N(1A) 93.3(1)
119.7(1)
113.0(1)
Al(1)-N(1)-Al(1A) 86.7(1)
C(1)-Al(1)-N(1) 133.3(1)
C(1)-Al(1)-N(1A) 133.5(1)
Si(1)-N(1)-Al(1) 135.7(1)
Si(1)-N(1)-Al(1A) 136.4(1)
of 1 display two sets of signals for the ortho t-Bu groups, meta
hydrogens, and the ortho and meta carbons indicating hindered
rotation around the Al-C bonds. The 1H and 13C NMR chemical
shifts of the imide hydrogen and carbon (NdCH; 8.52 and 190.6
ppm) are in the expected region. In addition, the IR spectrum
displays a strong Al-H stretching absorption at 1828 cm-1 and
a strong NdC stretch at 1642 cm-1. The most likely structure
of 1 is probably a dimer featuring bridging iminato groups and
terminal hydrogens as given by
2 and 3 as depicted in Scheme 1. This also results in the addition
of two hydrogens to each bridging iminato group to afford
primary amido ligands N(H)CH2(t-Bu) which bridge the alu-
minums. It is possible that this reaction occurs via an aluminum
iminato intermediate of formula {Mes*AlNCH2(t-Bu)}n which,
under the reaction conditions, proceeds further to give the
aluminum amide complexes 2 and 3.
The reaction of (Mes*AlH2)2 with 2 equiv of MesCN in
refluxing hexane yields the ortho-metalated species 4 and 5 in
moderate yield. Monitoring of the reaction by 1H NMR shows
that signals at 8.9 and 9.3 ppm are detectable after mixing of
the reactants. These signals may be assigned to the NdC(H)-
Mes moiety of the iminato complex (analagous to 1) which is
initially produced, but which could not be isolated. Reaction of
(Mes*AlH2)2 with 4 equiv of MesCN at room temperature in
an attempt to synthesize the bisiminato complex Mes*Al{Nd
C(H)Mes}2 led only to 4 and 5 and unreacted MesCN.
Apparently, ortho metalation is preferred to reaction with more
than 1 equiv of MesCN by each aluminum center. The structures
of 4 and 5 are depicted in Figures 1 and 2. The cis isomer 4
has no crystallographically imposed symmetry restrictions, and
it can be seen that both the nitrogen CH2Mes substituents and
the ortho-metalated “t-Bu” groups are mutually cis to each other.
In the trans isomer 5 there is an inversion center in the center
of the Al2N2 ring. The data in Table 2 show that the structural
parameters for 4 and 5 differ only slightly. Moreover, the bond
distances are quite close to those previously observed for
bonding involving aluminum amide dimers.12 Apart from their
different isomeric forms, the most obvious structural difference
between the two compounds is the fact that the Al2N2 core in
4 is folded whereas in 5 it is strictly planar. This bending results
This proposal is consistent with the spectroscopic data, and
addition of the hydrogen to the nitrilic carbon is consistent with
X-ray data on related systems such as 4 and 5 (see below) and
the structure of the complex [(i-Bu)2Al{µ2-NdC(H)(C6H3-2,6-
6
Me2)}]2 featuring bridging iminato groups.
The exact mechanism of the reduction of nitriles to imines
by aluminum hydrides is unknown. However, it is probable that
the first step involves coordination of nitrile nitrogen to an
aluminum center whereupon the electron density removed from
nitrogen induces a corresponding drift of electrons toward
nitrogen from the nitrilic carbon, which is then attacked by a
bridging hydrogen to generate an intermediate in which the
aluminums are bridged by hydrogen and an iminato group as
shown in Scheme 2.
(12) (a) Hess, A.; Hinderer, A.; Steinhauser, S. Z. Z. Anorg. Allg. Chem.
1970, 377, 1. (b) Gosling, K.; McLaughlin, G. M.; Sim, G. A.; Smith,
J. D. J. Chem. Soc., Dalton Trans. 1972, 2719. (c) Lappert, M. F.;
Power, P. P.; Sanger, A. R.; Srwastava, R. C. Metal and Metaloid
Amides; Ellis Horwood-Wiley: Chichester, 1979.
Heating of 1 at ca. 80 °C for 40 h leads to the activation of
an ortho t-Bu group on each Mes* ring to afford the products

6910 Inorganic Chemistry, Vol. 37, No. 26, 1998
Wehmschulte and Power
Scheme 3. Possible Initial Sequence for the Interaction of
(Mes*AlH2)2 with t-BuNdC
Molecules of 7 are characterized by a crystallographically
required center of symmetry in the middle of the six-membered
(AlCN)2 ring. The ring has a chairlike structure with a planar
N(1) C(19) N(1A) C(19A) array. The aluminums are ca. 0.4 Å
above or below this plane such that there is a 25.1° angle
between the plane defined by C(1) C(19) Al(1) N(1) and the
averaged N(1) C(19) N(1A) C(19A) plane. The Mes* ring plane
subtends an angle of 86.4° with respect to the averaged
coordination plane at aluminum. The aluminum has almost
trigonal planar coordination, ∑°Al(1) ) 358.1(1)°, and the N(1)
coordination geometry also shows a slight deviation from
planarity, ∑°N(1) ) 357.99(16)°, with N(1) ca. 0.12 Å out of
the Al(1) C(20) C(19A) plane. The ring may be regarded as a
formal analogue of 1,4-cyclohexadiene if π-delocalization of
the nitrogen lone pair onto aluminum is assumed. Indeed, the
“short” Al-N bond length of 1.796(2) Å is consistent with this
view. Nonetheless, the preponderance of theoretical and ex-
perimental data show that the shortening in Al-N bonds of this
type is mostly due to the low aluminum coordination number
and ionic factors rather than delocalization.13 Refluxing 7 in
benzene for 1 h results in the ortho-metalated product 8.
Unfortunately, no single crystals large enough for X-ray
crystallography could be obtained. 1H NMR spectroscopy
indicates that 8 probably has an all-trans structure owing to the
AMX pattern for the Al-CH2-N-H moiety and an AB pattern
seen for the Al-CH2-C(Me2) hydrogens. The IR spectrum
Figure 2. Thermal ellipsoid plot (30%) of 5 with hydrogen atoms
omitted for clarity.
Figure 3. Thermal ellipsoid plot (30%) of 7 with hydrogen atoms
omitted for clarity.
in a slightly closer approach of the aluminums in 4. The Al-N
distances in the cores are essentially equivalent with an average
value near 1.96 Å.
Treatment of (Mes*AlH2)2 with 2 equiv of acetonitrile and
refluxing in hexane solution results in the compound 6.
1
Monitoring of this reaction by H NMR showed that ortho
metalation had already begun after ca. 15 min at room
temperature. Refluxing in hexane completed this process in a
few hours. The dimeric 6 was the only isolable product from
the reaction. Strong positive NOE interactions of the Al-CH2
and N-CH2 moieties suggest that 6 exists in the cis form with
a structure similar to that of 4.
displays an N-H absorption at 3225 cm-1
.
Isolation of the amide products 2-6 and 8 led to the
investigation of the direct reaction of some primary amines with
(Mes*AlH2)2. Thus, the treatment of (Mes*AlH2)2 with 2 equiv
of H2NCH2Mes led to the dimeric primary amide [Mes*Al(H)-
{µ2-N(H)CH2Mes}]2 as a 50:50 mixture of cis (9) and trans
(10) isomers. Heating of this mixture of 9 and 10 to 150-200
°C for 30 min does not lead to the iminato complex {Mes*Al-
(µ2-NCH2Mes)}n but to a 50:50 mixture of the ortho-metalated
derivatives 4 and 5. This transformation can also be effected in
solution, but it requires ca. 3 days to complete at a temperature
of ca. 70 °C. Reaction of (Mes*AlH2)2 with H2NSiPh3 at room
temperature affords a mixture of the bisamide Mes*Al{N(H)-
SiPh3}2 (11) and amide/hydride [Mes*(H)Al{N(H)SiPh3}]2 as
well as some unreacted (Mes*AlH2)2. This result is very similar
to that obtained in the corresponding reaction of (Mes*AlH2)2
with H2NPh, which affords the analogous bisamide Mes*Al-
{N(H)Ph}2, whose structure has been determined.2 At higher
temperatures (ca. 125-130 °C), H2 is eliminated from the
The addition of the isonitrile t-BuNC to (Mes*AlH2)2 resulted
in a vigorous reaction to afford 7 in good yield. In fact, care
has to be taken so that the reaction temperature does not exceed
ca. 10 °C since ortho-metalated side products are rapidly
produced above this temperature. Refluxing 7 in hexane leads
to the ortho-metalated product 8, which can be isolated in low
1
yield as a colorless microcrystalline solid. H and 13C NMR
studies indicate that it exists in the all-trans isomer as shown in
Scheme 1. The structure of 7 is illustrated in Figure 3. It may
be noted that it is an isomer of 1. Consistent with the
spectroscopic data, two hydrogens have been added to the
carbon of the isonitrile to afford units of formula Mes*AlN(t-
Bu)CH2 which are dimerized in the solid. The mechanism
whereby this process occurs is currently unknown, but it
probably involves initial coordination of isonitrile carbon to
aluminum (or its insertion into an Al-(µ-H) bond) followed
by hydrogen transfer and rearrangement as proposed in Scheme
3.
(13) (a) Brothers, P. J.; Power, P. P. AdV. Organomet. Chem. 1996, 39, 1.
(b) Knabel, K.; Krossing, I.; No¨th, H.; Schwenk-Kircher, H.; Schmidt-
Amelunxen, M.; Seifert, T. Eur. J. Inorg. Chem. 1998, 1095.

Reactions of a Primary Alane
Inorganic Chemistry, Vol. 37, No. 26, 1998 6911
distances in 12 are marginally (ca. 0.015 Å) longer than those
observed in (Mes*AlNPh)2. In addition, the internal ring angle
at aluminum in 12 is 4.5° wider than it is in (Mes*AlNPh)2,
whereas the internal angle at nitrogen is a corresponding amount
narrower. It is probable that these structural differences are due
to the increased size of the nitrogen substituent.
Conclusion. The reaction of (Mes*AlH2)2 with nitriles or an
isonitrile results in hydrogen addition to the unsaturated carbon.
The reaction appears to proceed in a stepwise manner with
addition of the first hydrogen producing an iminato species as
an intermediate. A second hydrogen addition to carbon readily
takes place in the reaction with the isonitrile t-BuNC to generate
the compound 7. However, with nitriles the addition of the
second hydrogen to carbon proceeds only at elevated temper-
atures, which also results in ortho metalation to give the amide
products 2-6. These species are also accessible through the
direct reaction of (Mes*AlH2)2 with the appropriate amines.
However, direct reaction of (Mes*AlH2)2 with H2NSiPh3 or (as
previously reported) H2NPh results in the diamides Mes*Al-
{N(H)R}2 (R ) Ph or SiPh3) and the monoamides {Mes*(H)-
AlN(H)R}2, which upon further heating afford the dimeric
imides (Mes*AlNR)2 with no ortho metalation. The different
susceptibility to ortho metalation of the compounds at high
temperature is probably connected to the different types of
precursors (i.e., a bisamide species such as 11 versus an iminato
species such as 1) involved although a more detailed explanation
is not possible at present.
Figure 4. Thermal ellipsoid plot (30%) of 12 with hydrogen atoms
omitted for clarity.
monoamide coproduct [Mes*(H)Al{N(H)SiPh3}]2 and the dimer-
ic imide (Mes*AlNSiPh3)2 12 is obtained. This compound has
the centrosymmetric, dimeric structure depicted in Figure 4,
which is similar to that obtained for (Mes*AlNPh)2.
The planar Al2N2 core has essentially equal Al-N distances
(cf. Table 2) with a slightly wider internal ring angle at
aluminum (93.27(1)°) than at nitrogen (86.73(10)°). The coor-
dination at aluminum is planar, but the nitrogen geometry is
slightly pyramidal, ∑°N ) 358.83(13)°. The Al- -Al separation
is only 2.528(2) Å, which is shorter than that in many
compounds known to have aluminum-aluminum bonds.14 The
plane of the Mes* ring is oriented almost perpendicular to the
Al2N2 core. Compound 12 is the newest member of a very small
group of dimeric imidoalanes. Known examples include Cp*-
{(Me3Si)2N}AlN(µ2-AlCp*)(µ2-(Al{N(SiMe3)2})AlCp*,15 (Cp*-
Acknowledgment. We thank the donors of the Petroleum
Research Fund, administered by the American Chemical Society,
for financial support.
Supporting Information Available: Tables giving full data col-
lection parameters and further details of refinement, atomic coordinates
and equivalent isotropic thermal parameters, bond distances and angles,
hydrogen coordinates, and isotropic thermal parameters (44 pages).
Ordering information is given on any current masthead page.
16
AlNC6H3-2,6-i-Pr2)2 (Cp* ) C5Me5), and the previously
2
mentioned (Mes*AlNPh)2 species. The Al-N and Al-C
(14) (a) Uhl, W. Angew. Chem., Int. Ed. Engl. 1993, 32, 1386. (b)
Dohmeier, C.; Loos, D.; Schno¨ckel, H. Angew. Chem., Int. Ed. Engl.
1996, 35, 124. (c) Power, P. P. J. Chem. Soc., Dalton Trans. 1998,
2939.
IC9810992
(15) Schultz, S.; Ha¨ming, L.; Herbst-Irmer, R.; Roesky, H. W.; Sheldrick,
G. Angew. Chem., Int. Ed. Engl. 1994, 33, 969.
(16) Fisher, J. D.; Shapiro, P. J.; Yap, G. P. A.; Rheingold, A. L. Inorg.
Chem. 1996, 35, 271.