Reactions of (H2AlMes*)2 (Mes* = 2,4,6-(t-Bu)3C6H2) with H2EAr (E = N, P, or As; Ar = aryl): Characterization of the ring compounds (Mes*AlNPh)2 and (Mes*AlEPh)3 (E = P or As)

Article abstract of DOI:10.1021/ja952021c

The reaction between sterically crowded, dimeric, arylalane (H₂AlMes*)₂ and some aryl amines, phosphines, or arsines are described. Treatment of (H₂AlMes*)₂ (Mes* = 2,4,6-(t-Bu)₃C₆H₂) with the amines H₂NPh and H₂NDipp (Dipp = 2,6-(i-Pr)₂C₆H₃) affords the monomeric bis(amino)alane products Mes*Al(NHPh)₂?Et₂O (1?Et₂O) and Mes*Al-(NHDipp)₂ (2) which feature three-coordinate aluminum and nitrogen centers and short Al-N bonds. Heating of a 1:1 mixture of H₂AlMes* and aniline under carefully controlled conditions affords the dimer (Mes*AlNPh)₂, 3. Reaction of (Mes*AlH₂)₂ with H₂PPh or H₂AsPh at ca. 160°C gives the trimeric, six-membered-ring compounds (Mes*AlPPh)₃?Et₂O (4?Et₂O) and (Mes*AlAsPh)₃?Et₂O (5?Et₂O) which are formal valence analogues of borazine. The structure of 3 features a planar Al₂N₂ core, which also extends to the ipso carbons of the aryl rings; the Al-N distance is 1.824(2) A?. The structures of 4 and 5 have six-membered rings of alternating aluminum and phosphorus or arsenic atoms arranged in a nonplanar boat conformation. Little or no delocalization of the phosphorus or arsenic lone pairs is evident since the average Al-P and Al-As distances are consistent with single bond lengths. Moreover, pyramidal coordination is observed at all the pnictide centers. The structures of 5 and 4 represent respectively the first and second examples of bonding between three-coordinate aluminum and arsenic or phosphorus, whereas 3 represents a unique instance of an Al₂N₂ ring with organic substituents. The compounds were characterized by ¹H, ¹³C, and ³¹P (4, only) NMR spectroscopy and by X-ray crystallography in the case of 1, 3, 4, and 5. Crystal data with Cu Kα (λ = 1.54178 A?) radiation at 130 K: 1, a = 10.255(3) A?, b = 16.733(5) A?, c = 19.267(8) A?, β = 95.09(3)°, V = 3293(2) A?³, Z = 4, space group P2₁/c, 3070 (I > 2σ(I)) data, R = 0.061; 3, a = 8.750(1) A?, b = 9.326(2) A?, c = 14.025(3) A?, α = 101.04(2)°, β = 92.90(1)°, γ = 113.11(1)°, V = 1088.7(4) A?³, Z = 1, space group P1, 2629 (I > 2σ(I)) data, R = 0.053; 4, a = 9.383(5) A?, b = 17.719(7) A?, c = 23.603(14) A?, α = 80.86(4)°, β = 82.50(5)°, γ = 75.61°(4), V = 3736(3) A?³, Z = 2, space group P1, 5626 (I > 2σ(I)) data, R = 0.084; 5, a = 9.404(7) A?, b = 17.79(2) A?, c = 23.643(10) A?, α = 81.01(6)°, β = 83.04(5)°, γ = 75.68(7)°, V = 3771(5) A?³, Z = 2, space group P1, 6363 (I > 2σ(I)) data, R = 0.063.

Full text of DOI:10.1021/ja952021c

J. Am. Chem. Soc. 1996, 118, 791-797
791
Reactions of (H2AlMes*)2 (Mes* ) 2,4,6-(t-Bu)3C6H2) with
H2EAr (E ) N, P, or As; Ar ) aryl): Characterization of the
Ring Compounds (Mes*AlNPh)2 and (Mes*AlEPh)3
(E ) P or As)
Rudolf J. Wehmschulte and Philip P. Power*
Contribution from the Department of Chemistry, UniVersity of California,
DaVis, California 95616
ReceiVed June 21, 1995. ReVised Manuscript ReceiVed September 22, 1995X
Abstract: The reaction between sterically crowded, dimeric, arylalane (H2AlMes*)2 and some aryl amines, phosphines,
or arsines are described. Treatment of (H2AlMes*)2 (Mes* ) 2,4,6-(t-Bu)3C6H2) with the amines H2NPh and H2NDipp
(Dipp ) 2,6-(i-Pr)2C6H3) affords the monomeric bis(amino)alane products Mes*Al(NHPh)2‚Et2O (1‚Et2O) and Mes*Al-
(NHDipp)2 (2) which feature three-coordinate aluminum and nitrogen centers and short Al-N bonds. Heating of a
1:1 mixture of H2AlMes* and aniline under carefully controlled conditions affords the dimer (Mes*AlNPh)2, 3.
Reaction of (Mes*AlH2)2 with H2PPh or H2AsPh at ca. 160 °C gives the trimeric, six-membered-ring compounds
(Mes*AlPPh)3‚Et2O (4‚Et2O) and (Mes*AlAsPh)3‚Et2O (5‚Et2O) which are formal valence analogues of borazine.
The structure of 3 features a planar Al2N2 core, which also extends to the ipso carbons of the aryl rings; the Al-N
distance is 1.824(2) Å. The structures of 4 and 5 have six-membered rings of alternating aluminum and phosphorus
or arsenic atoms arranged in a nonplanar boat conformation. Little or no delocalization of the phosphorus or arsenic
lone pairs is evident since the average Al-P and Al-As distances are consistent with single bond lengths. Moreover,
pyramidal coordination is observed at all the pnictide centers. The structures of 5 and 4 represent respectively the
first and second examples of bonding between three-coordinate aluminum and arsenic or phosphorus, whereas 3
represents a unique instance of an Al2N2 ring with organic substituents. The compounds were characterized by 1H,
13C, and 31P (4, only) NMR spectroscopy and by X-ray crystallography in the case of 1, 3, 4, and 5. Crystal data
with Cu KR (λ ) 1.54178 Å) radiation at 130 K: 1, a ) 10.255(3) Å, b ) 16.733(5) Å, c ) 19.267(8) Å, â )
95.09(3)°, V ) 3293(2) Å3, Z ) 4, space group P21/c, 3070 (I > 2σ(I)) data, R ) 0.061; 3, a ) 8.750(1) Å, b )
9.326(2) Å, c ) 14.025(3) Å, R ) 101.04(2)°, â ) 92.90(1)°, γ ) 113.11(1)°, V ) 1088.7(4) Å3, Z ) 1, space
group P1h, 2629 (I > 2σ(I)) data, R ) 0.053; 4, a ) 9.383(5) Å, b ) 17.719(7) Å, c ) 23.603(14) Å, R ) 80.86(4)°,
â ) 82.50(5)°, γ ) 75.61°(4), V ) 3736(3) Å3, Z ) 2, space group P1h, 5626 (I > 2σ(I)) data, R ) 0.084; 5, a )
9.404(7) Å, b ) 17.79(2) Å, c ) 23.643(10) Å, R ) 81.01(6)°, â ) 83.04(5)°, γ ) 75.68(7)°, V ) 3771(5) Å3, Z
) 2, space group P1h, 6363 (I > 2σ(I)) data, R ) 0.063.
Introduction
isoelectronic B-P and Al-N rings display planar geometries
that extend to the carbon atoms bound directly to the ring.
Recent calculations8,9 on the hypothetical molecules (HBPH)3
and (HAlNH)3 and related species have indicated that in the
ground state, at least, only the B-P system (which is not quite
planar but has flattened pyramidal phosphorus coordination) has
a significant resonance energy, which is slightly greater than
that found in borazine.8 In contrast, the six-membered Ga-P
The recent synthesis of well-characterized, sterically hindered,
uncomplexed, primary alanes or gallanes such as (Mes*MH2)n
(M ) Al1 (n ) 2) or Ga,2 (n ) 1)) and related species has
provided a new, potentially important, source of starting
materials from which new examples of main group III-V
compounds (including main group III-V ring species) may be
derived. Currently, inorganic main group III-V rings that are
analogous to benzene are limited to relatively few types of
molecules.3 The best known example is borazine which was
first reported in 1926.4 Other main group III-V analogues were
not reported until recently and the currently known range
includes only the B-P (e.g. (MesBPPh)3),5 Al-N (the unique
compound {MeAlN(2,6-(i-Pr)2C6H3)}36 ), and Ga-P (the unique
7
ring in {2,4,6-Ph3C6H2GaPC6H11}3 is not planar, instead it
assumes a twist-boat geometry with normal single Ga-P bond
lengths and pyramidal coordination at the phosphorus centers.
In addition to the six-membered borazine analogues there are
the related four-membered gallium-phosphorus ring species (t-
10
BuGaPMes*)2 (Ga-P ) 2.274(4) Å) and the compound
7
11
compound{2,4,6-Ph3C6H2GaPC6H11}3 ) ring systems. Both the
Cp*{(Me3Si)2N}AlN(µ-AlCp*)(µ-Al{N(SiMe3)2})NAlCp*2
(Cp* ) η1-C5Me5) that features a central Al2N2 ring with an
X Abstract published in AdVance ACS Abstracts, January 15, 1996.
(1) Wehmschulte, R. J.; Power, P. P. Inorg. Chem. 1994, 33, 5611.
(2) Cowley, A. H.; Gabbai, F.; Isom, H. S.; Carrano, C. J.; Bond, M. R.
Angew. Chem., Int. Ed. Engl. 1994, 33, 1253.
(7) Hope, H.; Pestana, D. C.; Power, P. P. Angew. Chem., Int. Ed. Engl.
1991, 30, 691.
(3) Power, P. P. J. Organomet. Chem. 1990, 400, 49.
(4) Stock, A.; Pohland, E. Dtsch. Chem. Ges. 1926, 59, 2215.
(5) Dias, H. V. R.; Power, P. P. Angew. Chem., Int. Ed. Engl. 1987, 26,
1270.
(6) Waggoner, K. M.; Hope, H.; Power, P. P. Angew. Chem., Int. Ed.
Engl. 1988, 27, 1699
(8) Fink, W. H.; Richards, J. C. J. Am. Chem. Soc. 1991, 113, 3385.
(9) Matsuanga, N.; Gordon, M. S. J. Am. Chem. Soc. 1994, 116, 11407.
(10) Atwood, D. A.; Cowley, A. H.; Jones, R. A.; Mardones, M. A. J.
Am. Chem. Soc. 1991, 113, 7050.
(11) Schultz, S.; Ha¨ming, L.; Herbst-Irmer, R.; Roesky, H. W.; Sheldrick,
G. Angew. Chem., Int. Ed. Engl. 1994, 33, 969.
0002-7863/96/1518-0791$12.00/0 © 1996 American Chemical Society

792 J. Am. Chem. Soc., Vol. 118, No. 4, 1996
Wehmschulte and Power
C42H65N2Al: C, 80.71; H, 10.48; N, 4.48. Found: C, 80.30; H, 10.63;
H, 4.69. 1H NMR (C6D6): 7.48 (s, m-H (Mes*), 2H), 7.08 (d, m-H
(Dipp), 4H, 3JHH ) 7.7 Hz), 6.97 (t, p-H (Dipp), 2H), 3.29 (sept, H-C
(CH3)2, 4H, 3JHH ) 6.9 Hz), 3.00 (s, NH, 2H), 1.59 (s, o-CH3 (Mes*),
18H), 1.30 (s, p-CH3 (Mes*), 9H), 1.08 (d, o-CH3 (Dipp), 24H). 13C
{1H} NMR (C6D6): 158.5 (o-C(Mes*)), 151.1 (p-C(Mes*)), 143.4 (i-C
(Dipp)), 140.6 (o-C (Dipp)), 123.5 (m-C (Dipp)), 121.9, 121.5 (m-C
(Mes*), p-C (Dipp)), 37.7 (o-C (CH3)3), 34.8 (p-C (CH3)3), 32.5 (o-
CH3 (Mes*)), 31.3 (p-CH3 (Mes*)), 28.8 (o-CH), 23.5 (o-CH3 (Dipp)).
average Al-N distance of 1.811(7) Å. There is no evidence
for significant delocalization in these “antiaromatic’ species.
The synthesis of further examples of unsaturated main group
III-V ring systems has been slowed by limitations of previously
known routes, which, to date, have proved inapplicable to other
rings. From a wider perspective, there are relatively few
compounds of any kind that have the low-coordination numbers
that are expected in the title novel ring systems. For example,
there are no well-characterized molecular compounds that
feature bonding between three-coordinate aluminum and arsenic.
Furthermore, there is only a single example of a structurally
characterized compound with bonding between three-coordinate
aluminum and phosphorus12 and three-coordinate aluminum-
nitrogen species are limited to a handful of compounds.13
In this paper the results of the interaction of the recently
synthesized primary arylalane (Mes*AlH2)2 with the primary
pnictide derivatives H2NPh, H2NDipp (Dipp ) 2,6-(i-Pr)2C6H3),
H2PPh, and H2AsPh are described. The most important result
of these investigations is that the new ring compounds
(Mes*AlNPh)2, (Mes*AlPPh)3, and (Mes*AlAsPh)3 (Mes* )
2,4,6-(t-Bu)3C6H2) can be relatively easily isolated and char-
acterized.
IR: νNH ) 3348, 3363 cm-1
.
(c) (Mes*AlNPh)2 (3). H2NPh (0.17 mL, 1.9 mmol), 0.177 g) was
added via syringe to a solution of 0.57 g (1.04 mmol) of (Mes*AlH2)2
in 20 mL of ethylbenzene at 0 °C. Gas evolution (H2) commenced
immediately and ceased after ca. 5 min. The colorless solution was
warmed to room temperature and stirred for 2 h. The flask was then
placed in a preheated (125 °C) oil bath, without stirring. After a few
minutes a smooth gas evolution occurred which was followed by the
gradual formation of colorless needle-shaped crystals. The gas evolu-
tion appeared to cease after about 1 h. The mixture was maintained at
125 °C for an additional hour and slowly cooled to room temperature.
The resultant crystals which reached lengths of up to 2 mm were of
X-ray quality. Isolation of the crystals, followed by washing twice
with 20 mL of n-pentane, resulted in 0.27 g of 3. Yield: 39.1%. Mp:
>300 °C. Anal. Calcd for C48H68Al2N2: C, 79.30; H, 9.43; N, 3.86.
Found: C, 79.15; H, 9.26; N, 3.78. 1H NMR (C7D8, 110 °C): 7.58 (s,
m-H, 4H), 6.76 (t, m-H(Ph), 4H, J ) 7.8 Hz), 6.54 (“t” o-H(Ph), 4H),
∆ν ) 6.0 Hz), 6.42 (t, p-H(Ph), 2H, J ) 7.1 Hz), 1.71 (s, o-CH3, 36H),
1.33 (s, p-CH3, 18H).
Experimental Section
General Procedures. All experiments were performed under a
nitrogen atmosphere either by using modified Schlenk techniques or
in a Vacuum Atmospheres HE43-2 drybox. Solvents were freshly
distilled from a sodium-potassium alloy and degassed twice prior to
(d) (Mes*AlPPh)3‚OEt2 (4‚Et2O). H2PPh (0.40 mL, 0.40 g, 3.6
mmol) was added via syringe to 0.50 g (0.9 mmol) of finely ground
(Mes*AlH2)2 at room temperature. The almost clear, slightly viscous,
mixture was then placed in a preheated oil bath at 160 °C. Intense
frothing commenced after ca. 1-2 min and continued for ca. 3 min
after which time the mixture solidified. After cooling to room
temperature, and removal of volatile fractions under reduced pressure
(3 h), the pale yellow solid was dissolved in Et2O (30 mL), filtered,
and cooled to -20 °C overnight to afford colorless X-ray quality
crystals, which easily lose solvent of crystallization. Yield: 0.38 g
(52.1%). Mp: softens at 193 °C, melts at 241-2 °C. 1H-NMR
(C6D6): 7.53 (s, m-H, 6H), 7.14 (s, broad, o-H, PhP, 6H), 6.61 (m,
1
use. 13C, H, and 31P NMR spectra were recorded in C6D6 or C7D8
solutions by using a General Electric QE-300 spectrometer. The
compounds (Mes*AlH2)2,1 H2PPh,14 and H2AsPh15 were synthesized
by literature methods. The amines H2NPh and H2NDipp were obtained
from commercial suppliers and dried by standard procedures.
Synthesis. (a) Mes*Al(NHPh)2 (1). A solution of 0.69 g (1.25
mmol) of (Mes*AlH2)2 in toluene (40 mL) was treated with H2NPh
(0.23 mL, 2.5 mmol) at room temperature. The H2 evolution com-
menced immediately and ceased after ca. 15 min. The mixture was
stirred for another 45 min, filtered, concentrated to ca. 5 mL, and cooled
in a -20 °C freezer for 24 h to yield 1 as a colorless solid.
Recrystallization from ca. 15 mL of Et2O in a -20 °C freezer for 1
week results in colorless, X-ray quality plates of Mes*Al(NHPh)2‚-
Et2O (1‚Et2O) which lose solvent relatively easy. Yield: 0.11 g, 17%.
Mp: softens at 85 °C, melts at 180-184 °C. 1H NMR (C6D6): 7.57
(s, m-H (Mes*), 2H), 6.94 (t, 4H, JHH ) 7.8 Hz), 6.64 (partially obscured
by signal at 6.61), 6.61 (d, 6H, JHH ) 7.8 Hz), 3.19 (s, NH, 2H), 1.55
(s, o-CH3, 18H), 1.28 (s, p-CH3, 9H). 13C{1H} NMR (C6D6): 158.6
(o-C (Mes*)), 151.9 (p-C (Mes*)), 150.1 (i-C (Ph)), 129.8 (m-C (Ph)),
122.0 (m-C (Mes*)), 117.4 (p-C (Ph)), 117.0 (o-C (Ph)), 37.9 (o-C
p-H, PhP, 3H), 6.56 (m, m-H, PhP, 6H), 3.25 (q, OCH2, trace, 3JHH
)
6.9 Hz), 1.79 (s, o-CH3, 54H), 1.32 (s, p-CH3, 27H), 1.10 (t, CH3, trace).
13C{1H} NMR (C6D6, 60 °C): 158.5 (o-C), 151.4 (p-C), 134.9 (o-C,
PhP), 128.2 (m-C, PhP), 124.9 (p-C, PhP), 122.3 (m-C), 39.0 (o-
C(CH3)3), 33.9 (o-CH3), 31.4 (p-CH3). 31P NMR (C6D6): -144.2 (s).
(e) (Mes*AlAsPh)3 (5). PhAsH2 (7.6 mmol, 0.86 mL, 1.17 g) was
added via syringe to finely ground (Mes*AlH2)2 (1.9 mmol, 1.04 g) at
room temperature. The Schlenk tube containing the resultant, slightly
viscous, slurry was then placed in a preheated oil bath at 140 °C. After
ca. 2 min the slurry became a clear liquid. The temperature was slowly
raised and at 145 °C slight gas evolution was observed. This intensified
when the temperature was raised to ca. 155-160 °C. After 2 min at
this temperature the frothing liquid became yellow and then solidified.
Gas evolution essentially ceased, indicating that the reaction process
was essentially complete. After being cooled to room temperature the
solid was placed under reduced pressure for ca 1 h, then for an
additional hour at 100 °C. The yellow solid was dissolved in toluene
(25 mL), and the small amount of insoluble material was filtered off.
The pale yellow filtrate was concentrated to ca. 15-20 mL and cooled
in a -20 °C freezer overnight to yield 0.58 g of fine colorless needles
which lose co-crystallized solvent (PhMe) upon isolation. A second
crop of ca. 0.1 g was isolated from the concentrated mother liquor
upon further cooling for 3 days at -30 °C. X-ray quality crystals of
5‚Et2O were grown from ether. Yield: 0.68 g (42%). Mp: turns
orange at 208 °C, melts with a color change to red at 212-216 °C.
The compound loses ether of crystallization when isolated. 1H-NMR
(C6D6, 60 °C): 7.53 (s, m-H, 6H), 7.15 (d, 6H, Ph, As, J ) 7.5 Hz),
6.72 (t, 3H, Ph, As, J ) 7.2 Hz), 6.61 (t, 6H, Ph, As, J )7.2 Hz), 3.33
(q, 2.5H, OCH2, J ) 6.9 Hz), 1.78 (s, o-CH3, 54H), 1.36 (s, p-CH3,
27H), 1.11 (t, CH3CH2, 3.7 H). 13C{1H}-NMR, (60 °C): 158.5 (o-C),
151.3 (p-C), 137.9 (i-C, Mes* or Ph, As), 136.2 (o- or m-C, Ph, As),
132.3 (i-C, Mes* or Ph, As), 128.5 (o-or m-C, Ph, As), 125.3 (p-C,
(CH3)3), 34.9 (p-C (CH3)3), 32.3 (o-CH3), 31.4 (p-CH3). IR: νNH
)
3361 cm-1
.
(b) Mes*Al(NHDipp)2 (2). H2NDipp (0.18 mL, 0.91 mmol, 0.16
g) was added via syringe to finely ground (Mes*AlH2)2 (0.25 g, 0.46
mmol) at room temperature. After a few minutes a slight gas evolution
was observed, whereupon the mixture became viscous. The flask was
placed in a 100 °C oil bath to give a more intense gas evolution and a
clear liquid. After 10 min the mixture had become viscous and gas
evolution had ceased. After being cooled to room temperature, the
glass-like solid was dissolved in n-hexane (20 mL), filtered, concen-
trated to ca. 3 mL, and cooled in a -20 °C freezer for 1 week to yield
0.26 g of a colorless microcrystalline solid which still contains some
Mes*H. Recrystallization from n-hexane (5 mL) at ca. 5 °C for 1 week
results in large colorless plates of Mes*Al(NHDipp)2. Yield: 0.08 g
(28%). Mp: softens at 80 °C, melts at 95-104 °C. Anal. Calcd for
(12) Wehmschulte, R.; Ruhlandt-Senge, K.; Power, P. P. Inorg. Chem.
1994, 33, 3205.
(13) Brothers, P. J.; Wehmschulte, R. J.; Olmstead, M. M.; Ruhlandt-
Senge, K.; Parkin, S. R.; Power, P. P. Organometallics 1994, 13, 2792 and
references therein.
(14) Horvat, R. J.; Furst, A. J. Am. Chem. Soc. 1952, 74, 562.
(15) Palmer, C. S.; Adams, R. J. Am. Chem. Soc. 1922, 44, 1356.

Reactions of (H2AlMes*)2 with H2EAr
J. Am. Chem. Soc., Vol. 118, No. 4, 1996 793
Table 1. Crystallographic Data for 1‚Et2O, 3, 4‚Et2O, and 5‚Et2Oa
compound
formula
fw
1‚Et2O
C34H51AlN2O
530.7
3
4‚Et2O
C76H112Al3OP3
1215.51
5‚Et2O
C76H112Al3As3O
1347.36
C48H68Al2N2
727.0
crystal description
crystal dimens, mm
a, Å
b, Å
c, Å
colorless block
0.28 × 0.30 × 0.42
10.255(3)
16.733(5)
19.267(8)
colorless plate
0.10 × 0.48 × 0.67
8.750(1)
9.326(2)
14.025(3)
101.04(2)
92.90(1)
103.11(1)
1088.7(4)
1
colorless needle
0.15 × 0.18 × 0.70
9.383(5)
17.719(7)
23.603(14)
80.86(4)
82.50(5)
75.61(4)
3736(3)
colorless needle
0.12 × 0.20 × 1.00
9.404(7)
17.79(2)
23.643(10)
81.01(6)
83.04(5)
75.68(7)
3771(5)
R, deg
â deg
95.09(3)
γ, deg
V, Å3
3293(2)
4
Z
2
2
space group
d(calc), g/cm3
linear abs coeff, mm-1
2θ range, deg
no. of obsd reflcns
no. of variables
R1, Rw
P21/c
1.070
0.724
0-114
3070 (I > 2σ(I))
343
P1h
P1h
P1h
1.109
1.081
1.186
0.840
0-114
1.366
0-114
2.214
0-110
2629 (I > 2σ(I))
244
5626 (I > 2σ(I))
744
6363 (I > 2σ(I))
779
0.061, 0.070
0.053, 0.144b
0.084, 0.185b
0.063, 0.134b
2
a The intensity data sets were collected at 130 K using a Syntex P21 diffractometer with Cu KR (λ ) 1.54178 Å) radiation. b Rw based on Fo
- Fc2.
Table 2. Atom Coordinates (×104) for Important Atoms in
1‚Et2O, 3, 4‚Et2O, and 5‚Et2O
Ph, As), 122.0 (m-C), 39.1 (o-C(CH3)3), 34.9 (p-C(CH3)3), 33.9 (o-
CH3), 31.4 (p-CH3).
x
y
z
X-ray Data Collection, the Solution and Refinement of
the Structures
1‚Et2O
Al(1)
O(1)
N(1)
N(2)
C(1)
C(19)
C(25)
1284(1)
4372(3)
1474(3)
2713(3)
-259(4)
829(4)
1702(1)
569(2)
1250(2)
1471(2)
2329(2)
1352(3)
1490(2)
6644(1)
7456(2)
7491(2)
6209(2)
6297(2)
8090(2)
5510(2)
Crystals of 1 and 3-5 were coated with a layer of hydrocarbon oil
upon removal from the Schlenk tube. Suitable crystals were selected,
attached to a glass fiber by silicon grease, and immediately placed in
the low-temperature N2 stream.16 X-ray data were collected with a
Syntex P21 diffractometer equipped with a graphite monochromator
and a locally modified LT apparatus. Calculations were carried out in
a MicroVax 3200 computer using the SHELXTL-Plus program system
(1) and on an IBM compatible 486 PC using the SHELXTL-94 program
system for 3, 4, and 5. Neutral atom scattering factors and the
correction for anomalous dispersion were those supplied by SHELXTL-
Plus or SHELXTL-94. Some details of the data collections and the
refinements are provided in Table 1. Further details are in the
supporting information. Structures of 1, 3, and 5 were solved by direct
methods. The structure of 4 was solved by using the data set of 5
without the hydrogen atoms. The compounds were refined by full-
matrix least-squares procedures. Hydrogen atoms were included by
the use of a riding model with C-H distances of 0.96 Å and fixed
isotropic thermal parameters. Important atom coordinates and bond
distances and angles are given in Tables 2 and 3.
2968(4)
3
Al(1)
N(1)
C(1)
C(2)
C(6)
C(19)
C(20)
C(24)
4594(1)
3855(2)
3973(3)
2912(3)
4510(3)
2602(3)
1379(3)
2480(3)
9669(1)
8944(2)
9056(3)
9719(3)
7902(3)
7803(3)
7096(3)
7297(3)
5828(1)
4546(1)
7043(2)
7596(2)
7389(2)
4056(2)
4547(2)
3038(2)
4‚Et2O
P(1)
Al(1)
P(2)
Al(2)
P(3)
Al(3)
C(1)
C(19)
C(25)
C(43)
C(49)
C(67)
4991(2)
4831(2)
7536(2)
6900(2)
5577(2)
5617(2)
4037(8)
5203(7)
7874(8)
7973(8)
4506(9)
4183(8)
1559(1)
1265(1)
2906(1)
2174(1)
2066(1)
3255(1)
409(4)
1676(1)
2675(1)
1922(1)
1299(1)
3211(1)
2637(1)
3166(3)
1370(3)
498(3)
Results
Synthesis. A major objective of the work described here
was the synthesis of the new ring compounds involving
aluminum. It was felt that the use of (Mes*AlH2)21 would prove
a suitable starting material owing to the fact that, in reactions
with primary amines, phosphines, or arsines, hydrogen would
be the sole byproduct of the reaction thereby simplifying
purification as depicted in eq 1.
640(4)
2161(4)
3782(4)
4267(4)
2159(4)
1474(3)
2900(3)
3838(3)
5‚Et2O
As(1)
Al(1)
As(2)
Al(2)
As(3)
Al(3)
C(1)
C(19)
C(25)
C(43)
C(49)
C(67)
4832(1)
4814(3)
7722(1)
6922(3)
5714(1)
5687(3)
4036(8)
5138(8)
7888(8)
8095(9)
4555(8)
4131(8)
1613(1)
1272(1)
2887(1)
2183(1)
2038(1)
3271(1)
420(4)
1641(1)
2678(1)
1945(1)
1285(1)
3261(1)
2657(1)
3166(3)
1350(3)
491(3)
(Mes*AlH2)2 + 2H2EPh f (Mes*AlEPh)n + 4H2 (1)
(E ) N, P, or As)
609(4)
For E ) P or As the reactions between (Mes*AlH2)2 and H2-
PPh or H2AsPh were relatively straightforward and 4 and 5 were
readily obtained. The reactions proceeded under milder condi-
2157(4)
3841(4)
4278(4)
2174(4)
1465(3)
2916(3)
3883(3)
6
tions (ca. 125 °C for 3, cf. 170 °C for (MeAlNDipp)3 ) than
(16) This method is described by: Hope, H. A Practicum in Synthesis
and Characterization. In Experimental Organometallic Chemistry; Wayda,
A. L., Darensbourg, M. Y., Eds.; ACS Symposium Series 375; American
Chemical Society: Washington, DC, 1987; Chapter 10.
similar eliminations involving alkanes, which makes the pos-
sibility of side reactions less likely (vide infra for details). No
attempts were made to isolate intermediates in the case of the

794 J. Am. Chem. Soc., Vol. 118, No. 4, 1996
Wehmschulte and Power
Table 3. Important Bond Distances (Å) and Angles (deg) in
1‚Et2O, 3, 4‚Et2O, and 5‚Et2O
1‚Et2O
Al(1)-N(1)
Al(1)-N(2)
Al(1)-C(1)
O(1)-C(31)
1.795(3)
1.794(4)
1.966(4)
1.432(6)
O(1)-C(33)
N(1)-C(19)
N(2)-C(25)
1.415(6)
1.391(5)
1.394(5)
N(1)-Al(1)-N(2)
N(1)-Al(1)-C(1)
N(2)-Al(1)-C(1)
107.9(2)
123.7(2)
128.5(2)
C(31)-O(1)-C(33)
Al(1)-N(1)-C(19)
Al(1)-N(2)-C(25)
113.2(3)
133.0(3)
132.7(3)
3
Al(1)-N(1)
Al(1)-N(1a)
Al(1)-C(1)
1.824(2)
1.824(2)
1.962(2)
Al(1)-Al(1a)
N(1)-C(19)
N(1)-Al(1a)
2.607(1)
1.386(3)
1.824(2)
Figure 1. Thermal ellipsoid plot (30%) of 1‚Et2O. H atoms (except
N-H’s) are not shown for clarity.
N(1)-Al(1)-N(1a)
N(1)-Al(1)-C(1)
N(1a)-Al(1)-C(1)
88.75(9)
133.5(1)
137.7(1)
C(19)-N(1)-Al(1) 134.2(2)
C(19)-N(1)-Al(1a) 134.6(2)
Al(1)-N(1)-Al(1a) 91.25(9)
C(1)-Al(1)-Al(1a) 176.91(8)
4‚Et2O
P(1)-C(19)
P(1)-Al(1)
P(1)-Al(2)
Al(1)-C(1)
Al(1)-P(3)
P(2)-C(43)
1.840(7)
2.325(3)
2.332(3)
1.994(6)
2.324(3)
1.838(7)
P(2)-Al(2)
P(2)-Al(3)
Al(2)-C(25)
P(3)-C(67)
P(3)-Al(3)
Al(3)-C(49)
2.328(3)
2.336(3)
1.990(7)
1.842(7)
2.323(3)
1.985(8)
C(19)-P(1)-Al(1)
C(19)-P(1)-Al(2)
Al(1)-P(1)-Al(2)
C(1)-Al(1)-P(3)
C(1)-Al(1)-P(1)
P(3)-Al(1)-P(1)
C(43)-P(2)-Al(2)
C(43)-P(2)-Al(3)
Al(2)-P(2)-Al(3)
108.1(2)
C(25)-Al(2)-P(2)
C(25)-Al(2)-P(1)
P(2)-Al(2)-P(1)
C(67)-P(3)-Al(3)
C(67)-P(3)-Al(1)
Al(3)-P(3)-Al(1)
C(49)-Al(3)-P(3)
C(49)-Al(3)-P(2)
P(3)-Al(3)-P(2)
120.3(2)
125.0(2)
114.6(1)
113.3(2)
104.1(2)
108.1(1)
121.0(2)
134.3(2)
103.1(1)
110.9(2)
113.4(1)
112.8(2)
128.2(2)
118.9(1)
106.4(2)
111.0(3)
111.6(1)
Figure 2. Thermal ellipsoid plot (30%) of 3. H atoms are not shown
for clarity.
hydrogens from o-t-Bu groups that involve the atoms H8c
(Al- - -H ) 2.37 Å) and H18a (Al- - -H ) 2.35 Å).
(b) (Mes*AlNPh)2 (3). Molecules of 3 consist of centrosym-
metric dimers that feature a central, almost perfectly square,
Al2N2 ring. The ipso carbon atoms of the Ph and Mes*
substituents are coplanar with the Al2N2 array. Thus, the
coordination at the aluminum and nitrogen centers is distorted
trigonal planar. The internal ring angles at Al(1) and N(1) are
88.75(9)° and 91.25(9)° with equivalent Al-N distances of
1.824(2) Å. The Al-C(1) distance is 1.962(2) Å and the
Al- - -Al separation is 2.607(1) Å. The shortest Al- - -H
contacts, which involve the C(10) and C(18) methyl groups,
are near 2.23 Å. The angle between perpendiculars to the Al2N2
and Mes* planes is 86.8° while the corresponding angle
involving the Ph plane is 13.2°.
5‚Et2O
As(1)-C(19)
As(1)-Al(1)
As(1)-Al(2)
Al(1)-C(1)
Al(1)-As(3)
As(2)-C(43)
1.957(8)
2.432(3)
2.433(3)
1.990(7)
2.433(3)
1.971(7)
As(2)-Al(3)
As(2)-Al(2)
Al(2)-C(25)
As(3)-C(67)
As(3)-Al(3)
Al(3)-C(49)
2.428(3)
2.435(3)
1.985(7)
1.959(7)
2.418(3)
1.990(8)
C(19)-As(1)-Al(1) 104.3(2) C(25)-Al(2)-As(1) 123.9(2)
C(19)-As(1)-Al(2) 108.7(2) C(25)-Al(2)-As(2) 119.9(2)
Al(1)-As(1)-Al(2)
C(1)-Al(1)-As(1)
C(1)-Al(1)-As(3)
109.4(1) As(1)-Al(2)-As(2) 116.1(1)
127.8(2) C(67)-As(3)-Al(3) 110.2(2)
111.1(2) C(67)-As(3)-Al(1) 101.0(2)
As(1)-Al(1)-As(3) 121.0(1) Al(3)-As(3)-Al(1)
104.5(1)
C(43)-As(2)-Al(3) 108.4(2) C(49)-Al(3)-As(3) 121.1(2)
C(43)-As(2)-Al(2) 104.4(2) C(49)-Al(3)-As(2) 135.4(2)
(c) (Mes*AlEPh)3‚Et2O (E ) P (4‚Et2O), As (5‚Et2O)).
The crystals of 4‚Et2O and 5‚Et2O are isomorphous and are
formed with a molecule of solvent ether in each asymmetric
unit. There appears to be no significant interactions between
the ether and the (Mes*AlEPh)3 species. The latter are
composed of a six-membered ring of alternating aluminum and
phosphorus or arsenic atoms arranged in a nonplanar boat
conformation. The aluminums have planar coordination whereas
the phosphorus and arsenic atoms are pyramidally coordinated.
The average Al-P and Al-As distances are 2.328(3) and
2.430(5) Å and the average sums of angles (Σ°) at phosphorus
and arsenic are 329.1(3.0) and 319.7(3.0)°. In addition, there
are relatively short Al- - -H contacts with some of the o-t-Bu
hydrogens as exemplified by Al(1)- - -H(17c) ) 2.21 Å,
Al(2)- - -H(2b) ) 2.06 Å, and Al(3)- - -H(57a) ) 1.95 Å in 4
and Al(1)- - -H(17a) ) 2.15 Å, Al(2)- - -H(32b) ) 2.14 Å, and
Al(3)- - -H(57d) ) 2.20 Å in 5. The Mes*C(ipso)- - -C(para)
vectors subtend angles of as much as 15° with the corresponding
Al-C bonds. Furthermore, the rings are oriented almost
perpendicularly to the coordination planes at the aluminums.
Further structural details are provided in Table 3.
Al(3)-As(2)-Al(2)
108.4(1) As(3)-Al(3)-As(2) 101.3(1)
reactions with these phosphines or arsines. In the case of H2NPh
or H2NDipp, however, the first products to be isolated were
the bis(amido)alane compounds Mes*Al(NHPh)2 (1) or Mes*Al-
(NHDipp)2 (2), suggesting that 3 is produced in a stepwise
manner through amido intermediates.
Structural Descriptions. (a) Mes*Al(NHPh)2‚Et2O (1‚Et2O).
The molecular structure of 1, which is illustrated in Figure 1,
contains monomeric Mes*Al(NHPh)2 and one ether molecule
in each of the asymmetric units. The closest interactions
between the two molecules involve the nitrogen hydrogens
and oxygen with the distances H(1)- - -O(1) ) 2.29 Å and
H(2)- - -O(1) ) 2.34 Å (N(1)- - -O(1) ) 3.188(5) Å, N(2)- - -
O(1) ) 3.196(5) Å) being observed. The aluminum has a planar
coordination that almost includes the phenyl groups at the
nitrogens. The torsion angles between the perpendiculars to
the coordination plane at aluminum and those at N(1) and N(2)
are 14.7 and 18.0°. The Al-N and Al-C distances are 1.795(4)
Å (av) and 1.966(4) Å and the N-Al-N angle is 107.9(2)°.
There are also close approaches between the aluminum and

Reactions of (H2AlMes*)2 with H2EAr
J. Am. Chem. Soc., Vol. 118, No. 4, 1996 795
some detail. An initial product can be obtained by the addition
of 1 equiv of aniline to a toluene solution of Mes*AlH2 at 0
°C, followed by concentration and crystallization at -20 °C.
Alternatively, addition of 1 equiv of H2NPh to an Et2O solution
of Mes*AlH2 at 0 °C followed by evaporation of volatile
materials under reduced pressure gives a spectroscopically
similar product. Its IR spectrum shows two absorptions in the
ν
N-H region (3365 (br) and 3300 cm-1 (sharp), relative intensity
) 1:1) and two bands in the νAl-H region (1866 and 1836 cm-1
relative intensity ) 1:1.3). No absorptions indicating Al-H-
Al bridges are observed in the range of 1400-1000 cm-1
,
.
Comparison with the IR spectra of 1 and (Mes*AlH2)2 shows
the presence of traces of 1 but no (Mes*AlH2)2.
The 1H NMR spectrum of the initial product at room
temperature in the aromatic region shows the presence of 1 (ca.
24%), Mes*H (ca. 8%), and a broad signal at 7.49 ppm (∆ν1/2
) 15 Hz; ca. 68%). Heating of this sample to 85 °C causes a
slight broadening of the m-H signal of 1 and a more pronounced
broadening of the o-t-Bu and p-t-Bu signals of 1 which appear
to coalesce with the signals of the remaining “68% fraction”.
A similar observation is made when a 1:1 mixture of Mes*AlH2
and 1 in C6D6 is heated to 80 °C. This leads to the conclusion
that an exchange of hydride and amide ligands occurs between
these two species with 1 being either the most stable or the
species that crystallizes first. This conclusion is supported by
the isolation of [{2,6-Mes2C6H3AlH(µ-NHPh)}2]19 and [2,6-
Mes2C6H3Al(H)(µ-NHPh)2(NHPh)AlC6H3-2,6-Mes2]19 from the
reaction of [2,6-Mes2C6H3AlH2]2 with H2NPh (Al:N ) 1:1)
(Mes ) 2,4,6-Me3C6H2). Further heating of the initial product
to ca. 120-130 °C results in a melt which shows a smooth gas
evolution for a few minutes and then solidifies to a colorless
solid.
Figure 3. Thermal ellipsoid plot (30%) of 4‚Et2O. Et2O and H atoms
are not shown for clarity.
This solid proved to be insoluble in refluxing Et2O, THF, or
benzene. Only small amounts of 1 and Mes*H could be found
in the supernatant liquid. The IR spectrum of this solid does
not contain bands attributable to N-H or Al-H vibrations but
shows the presence of the Mes* and N-Ph groups. Thus, this
solid is tentatively identified as (Mes*AlNPh)2. Heating of this
solid to 200-210 °C under reduced pressure causes it to partially
melt and colorless crystals of Mes*H sublime. Crystals of
(Mes*AlNPh)2 (3) were eventually obtained in ca. 40% yield
by treatment of (Mes*AlH2)2 with 2 equiv of H2NPh in hot
(ca. 125 °C) ethylbenzene. The employment of the Mes* in
the synthesis and stabilization of the Al2N2 species may be
contrasted with the reaction of Mes*NH2 and AlMe3 which
Figure 4. Thermal ellipsoid plot (30%) of 5‚Et2O. H atoms are not
shown for clarity.
Discussion
affords the ortho-metallated species (MeAlN(H)C6H3-2,4-(t-
The synthesis of the new ring compounds 3, 4, and 5 proceeds
through the reaction of 1 equiv of Mes*AlH2 with H2NPh,
H2PPh, or H2AsPh with concomitant elimination of dihydrogen.
This route is a new one for unsaturated, quasiaromatic, main
group III-V rings as the previous routes involved phosphine,5
methane,6 or salt eliminations.7 Hydrogen elimination reactions
between phosphines, arsines, and alanes are quite rare and
calculations have shown that the model species H3AlPH3,
representing the initial step of the interaction between phos-
phines and alanes, is energetically less favored than dialane
H2Al(µ-H)2AlH2 and PH3.17 Reactions between AlH3 and
amines, however, have been used to synthesize higher poly-
(imino)alanes of formula (HAlNR)n (n g 4).18
Bu)2-6-CMe2CH2)2.20 As already mentioned the hydrogen
elimination route to 3 proceeds under sufficiently mild condi-
tions that the ortho-metallation and C-H activation found with
some alkane eliminations20a,b are not observed.
The bisamide compounds 1 and 2 are rare instances of
unassociated aluminum bisamides. The only previously struc-
turally characterized example for aluminum is MesAl{N-
(SiMe3)2}213 which has an average Al-N distance of 1.807(3)
Å with rather large torsion angles (ca. 45-50 °) between the
aluminum and nitrogen planes. The average Al-N distance in
1 is 1.795(4) Å and the torsion angles, 14.7 and 18.0°, are much
smaller. It could be argued that these parameters are evidence
for Al-N π-bonding. However, it is probable that such
differences are mainly a result of a change in the steric crowding
The mechanism of the reaction that results in the isolation
of the diamides 1 or 2 is not fully understood at present although
the Mes*AlH2/H2NPh reaction system has been examined in
(19) Wehmschulte, R. J.; Power, P. P. Unpublished results. Spectroscopic
and structural details of these two species will be provided upon request.
(20) (a) Hitchcock, P. B.; Jasim, H. A.; Lappert, M. F.; Williams, H. D.
Polyhedron 1990, 9, 245; (b) Waggoner, K. M.; Olmstead, M. M.; Power,
P. P. Polyhedron 1990, 9, 257.
(17) Bennett, F. R.; Elms, F. M.; Gardiner, M. G.; Koutsantonis, G. A.;
Raston, C. L.; Roberts, N. K. Organometallics 1992, 11, 1457.
(18) Cucinella, S.; Salvatori, T.; Busetto, C.; Mazzei, A. J. Organomet.
Chem. 1974, 78, 185.

796 J. Am. Chem. Soc., Vol. 118, No. 4, 1996
Wehmschulte and Power
and the electronic properties of the amide ligands. The structure
of 1 may also be compared with that of the recently reported
gallium analogue Mes*Ga(NHPh)213 which was synthesized in
a different manner from 1 by the reaction of Mes*GaCl2 with
2 equiv of LiNHPh. In this compound the Ga-N distance is
1.837(8) Å with a torsion angle of 7.3° between the coordination
planes at gallium and nitrogen. The longer metal-nitrogen
distance in the case of the gallium species can be traced in part
to the lower ionic character of the Ga-N bond. However, it is
also possible that the presence of hydrogen-bonded ether in the
unit cell of 1 plays a role. The weak O- - -H interaction in 1
may increase the electron density in the amido nitrogens thereby
increasing the ionic component in the Al-N bond which should
result in a shorter bond length being observed.
Al species23 but it does not necessarily indicate the presence of
any significant Al-Al bonding. Finally, it should be mentioned
that the planar geometry of the nitrogens in 3 or (MeAlNDipp)3
can be accounted for in terms of the low or zero inversion barrier
at nitrogen caused by the electropositive Al substituents.24
The compounds 4 and 5 crystallize as trimers from ether
solution with one non-interactive Et2O molecule included in each
asymmetric unit. The higher aggregation in 4 and 5 compared
to 3 is presumably due to the relaxed steric requirements as a
consequence of the larger size of P or As in comparison to N.
The central, isostructural, six-membered rings are composed of
alternating Al and P or As atoms. The phosphorus and arsenic
centers are quite pyramidal with average sums of angles at P
and As of 329.1(3.0)° and 319.7(3.0)°. The average Al-P
distance, 2.328(3) Å, is very close to that (2.342(2) Å) observed
in (2,4,6-(i-Pr)3C6H2)2AlP(1-adamantanyl)SiPh3 (Σ°P ) 330.0°)
which is the only previously known structure of a molecule with
bonding between three-coordinate Al and P.12 In this molecule
there is thought to be essentially no Al-P π-interaction.
The structure (Mes*AlNPh)2 (3) may be compared with the
closely related molecules (MeAlNDipp)36 and Cp*{(Me3Si)2N}-
11
AlN(µ-AlCp*)(µ-Al{N(SiMe3)2})NAlCp*2 (6). The latter
compound also features a planar Al2N2 core which has an
average Al-N distance of 1.811(7) Å. However, the ring
nitrogens are substituted by the three-coordinate aluminum
moieties Cp*{(Me3Si)2N}Al- or Cp*2Al-, and one of the ring
aluminums is substituted by an amido (-N(SiMe3)2) ligand. This
makes possible Al-N π-interactions within the Al2N2 ring in 6
more difficult to assess owing to roughly equally probable Al-N
π-interactions exo to the ring. In 3, however, all the ligands
are aryl groups which display no evidence of π-interaction with
the Al2N2 core.
Although there are no molecular structures with bonding
between three-coordinate Al and As the average Al-As bond
length, 2.430(5) Å, is consistent with single bonding, being
almost identical to the sum of the covalent radii of Al and As
when corrected for ionic effects.25 This, together with the
observation of pyramidal geometry at each arsenic, is consistent
with negligible π-delocalization in the Al3As3 array. The major
reasons for the lack of delocalization in either 4 or 5 are the
high inversion barriers24 at P or As and the relatively large sizes
of Al, P, or As which diminish side-on p-p π-overlap. This
view of the structures is in harmony with recent calculations9
which indicated essentially no π-delocalization and which
favored a chair over a boat configuration in such molecules by
The Al-N distance observed in 3, 1.824(2) Å, is slightly
longer than the 1.811(7) Å observed in 6. However, both values
are well within the currently known range (1.78(2)-1.879(4)
Å)13 for compounds that involve bonding between three-
coordinate nitrogen and aluminum. It is also significantly longer
than the Al-N distance of 1.782(4) Å observed in the
quasiaromatic trimer (MeAlNDipp)3.6 The shorter Al-N bond
lengths observed in (MeAlNDipp)3 tended to suggest greater
delocalization and Al-N π-bonding in the Al3N3 array.
Furthermore, the longer Al-N bonds in 3 are consistent with
antiaromatic character for the Al2N2 array.8,9 Calculations have
shown, however, that the amount of delocalization in the six-
membered Al3N3 ring is minimal and that any differences
between the two structures are more than likely due to steric
factors or to changes in σ-hybridization. With regard to the
latter, it is notable that in 3 the Al-N bonds are approximately
at right angles to each other indicating that the Al-N bonds
have almost pure p-character while the Al-C(Mes*) bond has
increased s-character. In (MeAlNDipp)3 the aluminum has
approximately trigonal planar coordination with interligand
angles at aluminum all within 5° of 120°. Thus, Al-N
σ-bonding in (MeAlNDipp)3 involves approximate sp2 hybrid-
ization at aluminum whereas in 3 the aluminum orbitals used
in σ-bonding have almost pure p-character. This suggests that
the Al-N bonds in 3 should be longer than those in (MeAlN-
Dipp)3 which is what is observed. It is also notable that the
Al-C bond length in 3, 1.962(2) Å, is nominally shorter than
the 1.978(15) Å Al-C distance in (MeAlNDipp)3. This is in
spite of the large size of the Mes* substituent and is consistent
with increased s-character for the Al-C bond in 3. An
interesting aspect of the structure of 3 is that the Al- - -Al
separation across the Al2N2 ring is just 2.6074(14) Å. This
distance is significantly shorter than the Al-Al single bond
length in R2Al-AlR2 (R ) -CH(SiMe3)2 (2.660(1) Å),21
-C6H2-2,4,6-(i-Pr)3 (2.647(3) Å)22 ) compounds and other Al-
3.3-6.8 kcal mol-1
.
The Al-As distance in 5 is ca. 0.1 Å shorter than the average
lengths of 2.535 and 2.567 Å found in the dimers {Et2AlAs(t-
26
Bu)2}2 and {Et2AlAs(SiMe3)2}2.27 The longer distances in
the latter species are presumably a consequence of the four-
coordinate nature of the Al and As centers. However, an Al-
As distance of 2.463(2) Å has been reported for the adduct
28
(Me3Si)3AsAlCl3 and this may be a result of the enhanced
Lewis acidity for the aluminum center owing to halide substitu-
tion. The average Al-As length in 5 may also be contrasted
with the value of 2.342(5) Å found for the terminal Al-As bond
in the Zintl species [Al2As4]6- which is thought to involve
significant Al-As multiple bonding.29
The 31P NMR spectrum of 4 displays a single resonance at
-144.2 ppm. The relatively high upfield value is consistent
with the fact that two of the substituents at phosphorus are
electropositive aluminum centered groups. The shift is similar
to that observed for (2,4,6-(i-Pr)3C6H2)2AlPMes(SiPh3) (-157.1
ppm)12 or PhP(SiMe3)2 (-132.8 ppm).30 The high value of the
shift of 4 signifies considerable electron density at phosphorus
(23) Uhl, W. Angew. Chem. Int., Ed. Engl. 1993, 32, 1386.
(24) Lambert, J. B. In Topics in Stereochemistry; Allinger, N. L., Eliel,
E. L., Eds.; Wiley; New York, 1971; Vol. 6, p 19.
(25) Schomaker, W.; Stevenson, D. P. J. Am. Chem. Soc. 1941, 63, 37.
Covalent radii of 1.3 Å for Al and 1.21 Å for As were used as well as
Allred-Rochow electronegativities.
(26) Heaton, D. E.; Jones, R. A.; Kidd, K. B.; Cowley, A. H.; Nunn, C.
M. Polyhedron 1988, 7, 1901.
(27) Wells, R. L.; McPhail, A. T.; Speer, T. M. Organometallics 1992,
11, 960.
(28) Wells, R. L.; Pitt, C. G.; McPhail, A. T.; Purdy, A. P.; Shafieezad,
R. B.; Hallock, R. B. Mater. Res. Soc. Sym. Proc. 1989, 131, 45.
(29) Somer, M.; Thiery, D.; Peters, K.; Walz, L.; Hartweg, M.; Popp,
T.; von Schnering, H. G. Z. Naturforsch. 1991, 46b, 789.
(30) Appel, R.; Geisler, K. J. Organomet. Chem. 1976, 112, 61.
(21) Uhl, W. Z. Naturforsch. 1988, 43b, 1113.
(22) Wehmschulte, R. J.; Ruhlandt-Senge, K.; Olmstead, M. M.; Hope,
H.; Sturgeon, B. E.; Power, P. P. Inorg. Chem. 1993, 32, 2983.

Reactions of (H2AlMes*)2 with H2EAr
J. Am. Chem. Soc., Vol. 118, No. 4, 1996 797
and is consistent with the absence of delocalization in the Al3P3
ring.
Acknowledgment. We thank the National Science Founda-
tion and the donors of the Petroleum Research Fund, adminis-
tered by the American Chemical Society, for financial support.
Conclusion
Supporting Information Available: Tables giving full data
collection parameters and further details of refinement, atomic
coordinates and equivalent isotropic thermal parameters, bond
distances and angles, hydrogen coordinates, and anisotropic
thermal parameters (45 pages). This material is contained in
many libraries on microfiche, immediately follows this article
in the microfilm version of the journal, can be ordered from
the ACS, and can be downloaded from the Internet; see any
current masthead page for ordering information and Internet
access instructions.
Use of the sterically encumbered alane (Mes*AlH2)2 allows
the ready isolation of previously unknown dimeric aluminum
imide and trimeric phosphinidine and arsinidine derivatives. The
reactions proceed at temperatures that are lower than alkane
eliminations involving comparably crowded group III-V pre-
cursors. The absence of a significant tendency to activate
substituent C-H bonds in the presence of low-coordinate
aluminum at H2 elimination temperatures suggests that this
method may be used to generate other interesting main group
III-V compounds.
JA952021C