Two types of intramolecular addition of an Al-N multiple-bonded monomer LAlNAr′ arising from the reaction of LAl with N3Ar′ (L = HC[(CMe)(NAr)]2, Ar′ = 2,6-Ar2C6H 3, Ar = 2,6-iPr2C6H3)

Article abstract of DOI:10.1021/ja0475712

The reaction of β-diketiminated aluminum(I) monomer LAl with a large bulky azide N₃Ar′ (L = HC(CMeNAr)₂, Ar′ = 2,6-Ar₂C₆H₃, Ar = 2,6-iPr₂C₆H₃) in the temperature range from -78 °C to room temperature affords two different isomers 2 and 3, which have been characterized by spectroscopic and X-ray structural analyses, as well as elemental analysis. The variable-temperature ¹H NMR kinetic studies of this reaction indicate the existence of the monomer LAlNAr′ (1) at low temperature and the thermal stability of the compounds increases in the order of 1 < 2 < 3. Copyright

Full text of DOI:10.1021/ja0475712

Published on Web 07/16/2004
Two Types of Intramolecular Addition of an Al-N Multiple-Bonded Monomer
LAlNAr′ Arising from the Reaction of LAl with N3Ar′ (L ) HC[(CMe)(NAr)]2, Ar′
) 2,6-Ar2C6H3, Ar ) 2,6-iPr2C6H3)
Hongping Zhu,† Jianfang Chai,† Vadapalli Chandrasekhar,† Herbert W. Roesky,*,† Jo¨rg Magull,†
Denis Vidovic,† Hans-Georg Schmidt,† Mathias Noltemeyer,† Philip P. Power,‡ and William A. Merrill‡
Institut fu¨r Anorganische Chemie der UniVersita¨t Go¨ttingen, Tammannstrasse 4, D-37077 Go¨ttingen, Germany, and
Department of Chemistry, UniVersity of California - DaVis, One Shields AVenue, DaVis, California 95616
Received April 27, 2004; E-mail: hroesky@gwdg.de
Scheme 1. Proposed Formation of 2 and 3 from the Reaction of
LAl with N3Ar′
Considerable attention has been attracted to the heavier group
13 imides, since they can be used as single-source precursors for
metal nitrides, which have interesting electronic properties and
therefore important applications for technological materials.1 In
recent years, studies of the syntheses and characterization of such
compounds have shown that they have a strong tendency to asso-
ciate, even to oligomerize.2 However, the modification of the sub-
stituents on both metal and nitrogen of the (MN)n core have a great
effect on their aggregation. The preparation of di- and tricoordinate
imido monomers Ar′MNL′′ (M ) Ga, In; Ar′ ) 2,6-Ar2C6H3, Ar
) 2,6-iPr2C6H3, L′′ ) 2,6-(4-tBu-Xyl)2C6H3) and LMNL′ (M )
Al, Ga; L ) HC[(CMe)(NAr)]2, L′ ) 2,6-Trip2C6H3, Trip ) 2,4,6-
iPr3C6H2), which have the lowest degree of association and the
unique M-N multiple bond, demonstrates the stabilization of Al
(Ga or In) imido monomers by the large bulky ligands.3 Nonethe-
less, the syntheses of compounds containing a Ga2N2 or an AlN4
ring,2e,4a which are proposed to proceed through the monomeric
Al(Ga) imide, in the absence of steric protection, suggests the high
reactivity of such monomers. Herein we show that the latent
reactivity of a monomeric Al-N multiple-bonded species initiates
further intramolecular addition. The reaction of the â-diketiminated
LAl4b with the bulky N3Ar′ can afford two different isomers 2 and
3 derived from their parent imide LAlNAr′ (1, Scheme 1). Also of
great interest is the thermal conversion of 2 into 3. This indicates
the difference of the thermal stability between 2 and 3.
the molecular ion [M+ - 1] at m/z 855. They have been charac-
terized by spectroscopic, analytical, and X-ray crystal measure-
ments.
The structure of 25a (Figure 1) exhibits a [2 + 2] cycloaddition
product. The four-membered AlNC2 ring is quasi-planar (∆ )
0.0680 Å) with Al-N bond length (1.876(2) Å) longer than those
in the AlN4 ring complex (1.851(2), 1.8152(15) Å)4 and Al-C
distance (1.998(3) Å) similar to that in the AlC3O ring compound
(1.9852(16) Å).6 The phenyl ring involved in the [2 + 2] cyclo-
addition becomes nonplanar, where C(50), C(51), C(52), and C(53)
are in-plane (∆ ) 0.0430 Å) and C(48) and C(49) are away from
this plane by 0.1813 and -0.2827 Å. The C-C bond lengths for
C(50)-C(51) and C(52)-C(53) are respectively 1.348(4) and 1.336-
(4) Å and are typical of CdC double bonds,7 while the remaining
four C-C distances are in the range (1.469(4)-1.536(4) Å) and
are indicative of C-C single bonds.7 The structural features of this
phenyl ring are in good agreement with the proton NMR spectrum.
The resonances at δ 6.08 (dd), 5.41 (d), and 2.43 (d) ppm are
assigned to He, Hf, and Hd, respectively, reflecting the extent of
(un)saturation at the carbons where these protons are attached.
The X-ray crystal structure of 3 is shown in Figure 2. Compound
3 is monomeric5b and contains a tetracoordinate Al (3N, 1C) center.
The Al atom is involved as part of two fused six-membered rings
(AlN2C3 and AlNC4). The Al(1)-C(222) (1.961(3) Å) and
C(222)-C(221) (1.556(3) Å) bond lengths are consistent with
experimental values observed for Al-C8 and C-C7 single bonds.
This suggests the absence of strain within the AlNC4 ring formed
as a result of Al-C ligation. The Al-NH(Ar′) bond length (1.858-
(2) Å) falls within the range (1.75-1.85 Å) of terminal Al-N single
bonds8 and is a little shorter than that of Al(1)-N(3) in 2. The NH
and the CH2 protons (Ha and Hb) resonate at δ 3.41 (s), -0.28
(dd), and -1.75 (t), respectively. The characteristic absorption at
3298 cm-1 is assignable to νNH in the IR spectrum of 3.
The reaction of LAl with N3Ar′ was carried out at -78 °C and
allowed to warm to room temperature. Partial removal of toluene
and addition of n-hexane led to crystallization of compounds 3
(colorless crystals in 24% yield) and 2 (yellowish crystals in 42%
yield), one after the other. Crystals of 2 are of X-ray quality, while
single crystals of 3 were obtained when 2 was treated by dissolving
in a hot toluene/n-hexane mixture and then kept at 4 °C to allow
crystallization. This suggests that a thermal conversion of 2 to 3 is
occurring.
The LAl reacting with N3Ar′ may proceed through an intermedi-
ate LAlNAr′ (1) with elimination of N2. A similar reaction of LM
(M ) Al, Ga) with N3L′ has been reported to afford LMNL′
1
containing a MdN multiple bond.3b The H NMR kinetic study
also suggests the existence of 1 at low temperature. Compound 2
is formed as a result of a [2 + 2] cycloaddition of a phenyl ring of
the Ar′ substituent on nitrogen (route b, Scheme 1), while the
formation of 3 might occur by an intramolecular C-H activated
addition involving the methyl group of the isopropyl substituent
on the â-diketiminato ligand (route a).
Compounds 2 and 3 are thermally stable. They change with color
at ∼320 °C and finally melt at 381-382 °C. Both of them show
Now we gain further insight into the reaction process between
LAl and N3Ar′ carried out by a variable-temperature 1H NMR study
† Institut fu¨r Anorganische Chemie der Universita¨t Go¨ttingen.
‡ University of California - Davis.
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J. AM. CHEM. SOC. 2004, 126, 9472-9473
10.1021/ja0475712 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. Molecular structure of 2. Selected bond lengths (Å) and angles
(deg): Al(1)-N(1) 1.915(2), Al(1)-N(2) 1.926(2), Al(1)-N(3) 1.876(2),
Al(1)-C(48) 1.998(3), C(48)-C(49) 1.536(4), C(49)-C(50) 1.520(4),
C(50)-C(51) 1.348(4), C(51)-C(52) 1.469(4), C(52)-C(53) 1.336(4),
C(53)-C(48) 1.475(4), N(3)-C(49) 1.537(3), N(1)-Al(1)-N(2)
96.92(10), N(3)-Al(1)-N(1) 127.76(10), N(3)-Al(1)-N(2) 117.99(10),
N(3)-Al(1)-C(48) 78.53(10), Al(1)-N(3)-C(30) 138.71(17).
Figure 3. Variable-temperature 1H NMR kinetic studies of the reaction of
LAl with N3Ar′ in [D8]toluene. (I) records the resonances of γ-CH proton
(δ 5.05-4.60 ppm) which correlate with changes of LAl moiety, and (II)
shows those of 2 (Hf and Hg, δ 6.20-5.20 ppm) and 1 (Ha and Hb, δ
0.00-2.00 ppm). 50* means the available 1H NMR data by keeping the
sample at 50 °C for 24 h.
Acknowledgment. We thank Mr. Wolfgang Zolke for the
variable-temperature 1H NMR study. We are grateful for the
financial support by the Deutsche Forschungsgemeinschaft and the
Go¨ttinger Akademie der Wissenschaften.
Supporting Information Available: The Experimental Section
including the detailed synthetic procedures, analytical and spectral
characterization data; CIF data for 2 and 3. This material is available
free of charge via the Internet http//pubs.acs.org.
Figure 2. Molecular structure of 3. Selected bond lengths (Å) and angles
(deg): Al(1)-N(1) 1.940(2), Al(1)-N(2) 1.913(2), Al(1)-N(3) 1.858(2),
Al(1)-C(222) 1.961(3), C(222)-C(221) 1.556(3), N(1)-Al(1)-N(2)
95.69(10), N(3)-Al(1)-N(1) 115.78(9), N(3)-Al(1)-N(2) 103.44(10),
N(3)-Al(1)-C(222) 122.34(10), Al(1)-N(3)-C(51) 138.72(16).
References
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(5) (a) Crystallographic data for 2 with Mo KR (λ ) 0.01073 Å) radiation at
133(2) K. a ) 12.3496(6) Å, b ) 26.2644(15) Å, c ) 16.0109(7) Å, â )
104.051(4)°, monoclinic, space group P2(1)/n, Z ) 4, R1 ) 0.0633, wR2
) 0.1360 for 5709 (I > 2σ (I)) data, and R1 ) 0.1059, wR2 ) 0.1551 for
all (8626) data. (b) Crystallographic data for 3 with Mo KR (λ ) 0.01073
Å) radiation at 133(2) K. a ) 11.806(8) Å, b ) 12.765(5) Å, c ) 19.718-
(8) Å, R ) 76.04(3)°, â ) 74.54(4)°, γ ) 64.63(4)°, triclinic, space group
P-1, Z ) 2, R1 ) 0.0595, wR2 ) 0.1415 for 6992 (I > 2σ (I)) data, and
R1 ) 0.0751, wR2 ) 0.1496 for all (8758) data.
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(Figure 3). The γ-CH of the starting material LAl resonates at ∼5.10
ppm,9 and its change is a good indicator for the progress of the
reaction. Thus in I, from -50 to -10 °C, the occurrence of one
singlet may indicate the formation of 1. From -10 to 50 °C, this
singlet was gradually transformed into two other singlets, suggesting
the further reaction of 1 and the concomitant formation of
compounds 2 and 3. This is further evidenced by the corresponding
presence of the characteristic proton resonances in II indicative for
2 and 3, respectively. The almost same integral intensity of 2 and
3 shows the equiponderance of the intramolecular addition of 1
via routes a and b. Keeping this reaction at 50 °C for 24 h, the
final spectrum shows the disappearance of the characteristic
resonances for 2 and the growing in of those for 3. This experiment
confirms the result of recrystallization of 2 from a hot solution to
yield 3.
The strained structure of 2 (torsional AlC2N and C6 rings)
compared to that of 3 may energetically favor this thermal
rearrangement, while the deliberate 1H NMR kinetic studies of this
reaction unambiguously reveal the thermal stability of these three
isomers in the order of 1 < 2 < 3.
In summary, we have shown an unprecedented reaction, which
occurs by an intramolecular addition to an Al-N multiple bonded
species LAlNAr′ and furthermore by rearrangement of 2 to 3
without changing the monomeric nature of the products. Presently,
we are involved in the low-temperature synthesis of 1 and especially
its X-ray structural analysis.
(8) Brothers, P. J.; Power, P. P. AdV. Organomet. Chem. 1996, 39, 1.
(9) The low-temperature 1H NMR spectrum of the starting material LAl was
measured as a reference. The resonances of γ-CH at δ 5.11, 5.13, 5.14,
and 5.15 ppm respectively correspond to -50, -30, -10, 25 °C.
JA0475712
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