Treatment of tetramethylpyrazole and 3,5-diphenylpyrazole with dialkylaluminum hydrides - Hydroalumination versus deprotonation

Article abstract of DOI:10.1515/znb-2010-0604

Treatment of tetramethylpyrazole, N₂C₃Me₄, with equimolar quantities of di(tert-butyl)aluminum hydride leads to the addition of an Al-H bond to one of the C=N double bonds. The dimeric product (1) contains a central six-membered Al₂N₄ ring in which two ^tBu₂Al⁺ units are bridging two N ₂C₃ heterocycles. In the zwitterionic, non-centrosymmetric compound one aluminum atom is coordinated by two imino nitrogen atoms, while the second one is bonded to two amide nitrogen atoms. No double hydroalumination occurs upon treatment of tetramethylpyrazole with two equivalents of the hydride. Instead, an adduct (2) of the monomeric hydroalumination product with di(tert-butyl)aluminum hydride was isolated in which the two aluminum atoms are connected by a 3c-2e Al-H-Al bond. A unique trinuclear compound (3) is obtained upon reaction of tetramethylpyrazole with an excess of the sterically less shielded diethylaluminum hydride. It contains two different N₂C ₃ heterocycles: One still contains a C=N double bond similar to that in compounds 1 and 2, while the second one is completely reduced by double hydroalumination to give a saturated heterocycle. The two rings are bridged by three AlEt₂ groups. Deprotonation results upon treatment of 3,5-diphenylpyrazole, N₂C₃H₂(C ₆H₅)₂, with di(tert-butyl)aluminum hydride.

Full text of DOI:10.1515/znb-2010-0604

Treatment of Tetramethylpyrazole and 3,5-Diphenylpyrazole with
Dialkylaluminum Hydrides - Hydroalumination versus Deprotonation
Werner Uhl and Andreas Vogelpohl
Institut fu¨r Anorganische und Analytische Chemie der Universita¨t Mu¨nster, Corrensstraße 30,
48149 Mu¨nster, Germany
Reprint requests to Prof. Werner Uhl. E-mail: uhlw@uni-muenster.de
Z. Naturforsch. 2010, 65b, 687 – 694; received March 1, 2010
Treatment of tetramethylpyrazole, N2C3Me4, with equimolar quantities of di(tert-butyl)aluminum
hydride leads to the addition of an Al–H bond to one of the C=N double bonds. The dimeric product
(1) contains a central six-membered Al2N4 ring in which two tBu2Al+ units are bridging two N2C3
heterocycles. In the zwitterionic, non-centrosymmetric compound one aluminum atom is coordinated
by two imino nitrogen atoms, while the second one is bonded to two amide nitrogen atoms. No double
hydroalumination occurs upon treatment of tetramethylpyrazole with two equivalents of the hydride.
Instead, an adduct (2) of the monomeric hydroalumination product with di(tert-butyl)aluminum hy-
dride was isolated in which the two aluminum atoms are connected by a 3c-2e Al–H–Al bond. A
unique trinuclear compound (3) is obtained upon reaction of tetramethylpyrazole with an excess of
the sterically less shielded diethylaluminum hydride. It contains two different N2C3 heterocycles:
One still contains a C=N double bond similar to that in compounds 1 and 2, while the second one
is completely reduced by double hydroalumination to give a saturated heterocycle. The two rings
are bridged by three AlEt2 groups. Deprotonation results upon treatment of 3,5-diphenylpyrazole,
N2C3H2(C6H5)2, with di(tert-butyl)aluminum hydride.
Key words: Organoelement Compounds, Aluminum, Hydrazides, Heterocycles, Hydroalumination
Introduction
most quantitative formation of the corresponding hy-
drazides takes place spontaneously below room tem-
perature or upon slight warming. The products con-
tain a monoanionic hydrazido ligand and are dimeric
with four-, five- or six-membered heterocycles [1, 6 –
8]. Further reactions gave very interesting oligocyclic
or cage-like compounds which have dianionic hy-
drazinediido groups [1, 5, 7, 9]. The latter were also ac-
cessible by twofold hydroalumination of tetramethyl-
2,3-diazabutadiene. However, complete reduction to
yield a hydrazinediide occurred only with AlH3 [10],
while even an excess of dialkylaluminum hydrides or
gallium hydrides gave hydrazone derivatives with an
intact C=N double bond [11]. In continuation of these
investigations we also applied cyclic diaza compounds
and treated pyrazole derivatives with dialkylaluminum
hydrides.
Organoaluminum, -gallium or -indium hydrazides
[1] show a fascinating coordination chemistry similar
to that of hydroxylamides [2] or peroxides [3] which
results from the singular arrangement of two adjacent
donor atoms in the ligands. The synthesis of these hy-
drazides was accomplished via several different and ef-
ficient routes such as salt elimination, release of hy-
drogen or alkanes [1]. The latter method comprises
the treatment of the corresponding trialkylaluminum
or -gallium compounds with hydrazines having at least
one N-H function. We preferred this procedure in our
recent investigations, because the starting compounds
are easily available, and volatile alkanes are formed
as the only by-products in usually very selective re-
actions. Adducts are formed in the first step which
could be isolated and characterized in several cases [1,
4 – 6]. Steric interactions are determining their struc-
tures, and in most cases the less shielded nitrogen
atoms coordinate to the metal atoms. Only one com-
pound, Me3Ga←NH(Me)-NH2 [5], has the more ba-
sic alkylated nitrogen atom attached to the metal atom.
Results and Discussion
Reactions of tetramethylpyrazole with (Me3C)2Al-H
Tetramethylpyrazole was dissolved in toluene
The release of alkanes from these adducts with the al- and treated with an equimolar quantity of di(tert-
c
0932–0776 / 10 / 0600–0687 $ 06.00 ꢀ 2010 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
Unauthenticated
Download Date | 11/18/19 1:17 AM

688
W. Uhl – A. Vogelpohl · Hydroalumination versus Deprotonation
most ideally in a common plane, while C2, C3, C5
and C6 have a distorted tetrahedral surrounding. The
N–N bond length (140.0 pm) is in the lower range of
N–N single bonds of simple hydrazines (141 – 149 pm)
[1, 12]. The Al2N4 ring deviates considerably from
planarity and adopts a twist-conformation [13] (tor-
sion angles: +55.2, −25.8, −26.7, +54.4, −28.6, and
−30.7◦; clockwise, starting with Al1–N1–N2–Al2).
(1)
Caused by this arrangement the normals of the five-
membered rings enclose an angle of 58.7◦. Interest-
ingly, the molecular symmetry is not centrosymmet-
ric. One aluminum atom is coordinated by two imino
and the second one by two amido nitrogen atoms.
This bonding situation results in different Al–N dis-
tances (Al2–N 198.8 and Al1–N 190.3 pm, respec-
tively) which are in accordance with previous observa-
tions [1, 4 – 9]. The Al–C distances (203.0 pm on aver-
age) are not influenced by the different coordination of
the metal atoms. However, the smaller C–Al–C angle
at Al1 (114.1 versus 123.5◦) may indicate a stronger
steric stress at this position. The N–Al–N angles are
similar at both aluminum atoms (100.6 versus 101.5◦),
but the Al–N–N angles are different with the larger
ones at the negatively charged amido nitrogen atoms
(124.3 versus 118.6◦).
The NMR data of 1 are in accordance with the
molecular structure found in the solid state. We ob-
serve two resonances of tert-butyl groups and four
signals for the chemically different methyl groups at-
tached to the C3N2 heterocycle. One of the latter sig-
nals is split into a doublet by coupling with the ring
hydrogen atom (3JH-H = 6.6 Hz) which also leads to a
quartet at δ = 3.86. These NMR data verify the exis-
tence of dimeric formula units also in solution. Ow-
ing to the occurrence of two chiral carbon atoms in
the dimeric molecules of 1 four different stereoiso-
Fig. 1. Molecular structure of 1. The ellipsoids are drawn
at the 40 % probability level; hydrogen atoms and methyl
groups of CMe3 are omitted. Selected bond lengths (pm)
and angles (deg): Al1–N1 190.2(2), Al1–N4 190.4(2),
Al2–N2 198.9(2), Al2–N3 198.6(2), N1–N2 140.1(2), N3–
N4 139.9(2), N1–C3 149.2(3), N2–C1 130.2(3), N3–C4
130.0(3), N4–C6 149.1(3); N1–Al1–N4 100.56(8), N2–Al2–
N3 101.46(8), Al1–N1–N2 124.4(1), Al1–N4–N3 124.2(1),
Al2–N2–N1 118.3(1), Al2–N3–N4 118.9(1).
butyl)aluminum hydride at room temperature. The mers may be formulated. Two of these have a head-
color of the solution changed from yellow to orange. to-head (symmetry C2 according to the structure in the
Concentration of the reaction mixture and cooling to solid state or Cs), the other ones a head-to-tail arrange-
◦
+3 C afforded colorless crystals of the product (1, ment (C2 or Ci) of the heterocycles. The Cs structure
Eq. 1) in 34 % yield. Crystal structure determination should give NMR resonances of four different tert-
(Fig. 1) revealed a dimeric formula unit in the solid butyl groups and can clearly be ruled out. The NMR
state in which two aluminum atoms bridge two C3N2 data do not allow to distinguish between the three re-
heterocycles to give a central six-membered Al2N4 maining structures, however, a strong rearrangement
ring. Only one C=N double bond of each pyrazole unit compared to the situation in the solid state is necessary
was reduced by the addition of an Al–H bond which to form the head-to-tail isomers, hence, their forma-
results in markedly different endocyclic C–N bond tion seems to be relatively implausible. An equilibrium
lengths of 130.1 (C=N, average value) and 149.2 pm between two possible forms in solution was detected
(average C–N single bond). The carbon atoms C1 for dimeric 2-dimethylaluminumpyridine [14]. In this
and C4 are sp2-hybridized with the adjacent atoms al- case warming of the solution gave rearrangement and
Unauthenticated
Download Date | 11/18/19 1:17 AM

W. Uhl – A. Vogelpohl · Hydroalumination versus Deprotonation
689
ferent couplings the ring hydrogen atom gives a quar-
tet of quartets with an expected chemical shift of δ =
3.43. A broad resonance at δ = 2.97 is indicative of
a hydrogen atom bridging the two aluminum atoms.
The occurrence of a C=N double bond is in accor-
dance with a resonance at δ = 155.9 in the 13C NMR
spectrum (δ = 66.6 for the sp3-hybridized carbon atom
bonded to the nitrogen atom). We were not able to de-
termine the structure of 2 by X-ray diffraction in suf-
ficient accuracy because all crystals obtained from dif-
ferent solvents (n-pentane, cyclopentane, toluene, 1,2-
difluorobenzene, diethyl ether etc.) were of very poor
quality. The positions of the carbon, nitrogen and alu-
minum atoms could be found. They have verified the
structure of 2 given schematically in Eq. 2, but the pa-
rameters could not be refined satisfactorily.
(2)
Reaction of tetramethylpyrazole with Et2Al-H
Hydroalumination of both C=N double bonds of
the heterocycle could not be achieved in the reactions
of excess di(tert-butyl)aluminum hydride with tetra-
methylpyrazole. A reason may be the steric shielding
by the bulky tert-butyl groups which prevents the sec-
ond addition reaction. Therefore we reduced the steric
demand of the substituents attached to aluminum and
treated the pyrazole with two equivalents of diethyl-
aluminum hydride in toluene/n-pentane at r. t. (Eq. 3).
The color of the mixture changed from yellow to
dark red. Concentration and cooling of the solution
complete formation of the centrosymmetric form. We
do not have any indication of such an equilibrium for
compound 1, and heating of 1 in boiling toluene did
not result in any alteration of the NMR spectra.
The hydroalumination of both C=N double bonds
in each heterocycle could not be achieved even when
an excess of the hydride was reacted with tetramethyl-
pyrazole. Reactions in the ratio 2 : 1 gave an adduct
in 72 % yield in which a monomeric fragment of
1 is coordinated by an intact molecule of di(tert-
butyl)aluminum hydride via a 3c-2e Al–H–Al bond
and an interaction of the metal atom with one of the
nitrogen atoms of the C3N2 heterocycle (2, Eq. 2).
Longer reaction times or warming did not result in any
secondary process. The constitution of the yellow com-
pound 2 could unambiguously be assigned from the 1H
NMR data. We observed four different resonances of
tert-butyl groups which are caused by the different co-
ordination of the aluminum atoms to imino or amido
nitrogen atoms and the chirality of the carbon atom
bearing a hydrogen atom and a methyl group. Further,
we detected four resonances of methyl groups attached
to the C3N2 ring. Two of the latter resonances are split
into doublets. One coupling constant (6.7 Hz) is char-
◦
to +3 C afforded yellow crystals of compound 3 in
67 % yield. A crystal structure determination revealed
an interesting and unprecedented molecular structure
(Fig. 2) which contains two different five-membered
(3)
3
acteristic of a JH-H coupling to the hydrogen atom
added to the pyrazole ring by hydroalumination. The
resonance belongs to the methyl group bonded to the
C-H fragment of the ring. The occurrence of a second
doublet reflects a long-range coupling (5JH-H = 1.0 Hz)
between the ring hydrogen atom and the methyl group
attached to the C=N double bond. Caused by these dif-
Unauthenticated
Download Date | 11/18/19 1:17 AM

690
W. Uhl – A. Vogelpohl · Hydroalumination versus Deprotonation
mination of the C=N bonds, are on the same side of
the heterocyclic ring system and opposite to the AlEt2
group bridging the central Al2N4 ring.
The results of the NMR spectroscopic character-
ization are in complete agreement with the solid-
state molecular structure and the different degrees
of hydroalumination of the pyrazole rings. However,
very complicated spectra result in accordance with
the molecular symmetry. The ethyl substituents gave
1
broad multiplets in the H NMR spectrum between
δ = −0.14 and +0.58 for their CH2 and between δ =
1.36 and 1.46 for their CH3 groups which could not
be resolved and assigned to particular positions in the
molecule. However, in the 13C NMR spectrum the ex-
pected six resonances of the ethyl CH3 groups were ob-
served in the narrow range between δ = 10.4 and 10.9.
Fig. 2. Molecular structure of 3. The ellipsoids are drawn at
the 40 % probability level; methyl groups of CH2CH3 and
hydrogen atoms with the exception of C-H of the hetero-
cycle are omitted. Selected bond lengths (pm) and angles
(deg): Al1–N1 200.3(2), Al1–N4 196.0(2), Al2–N2 200.0(2),
Al2–N4 193.5(2), Al3–N1 201.8(2), Al3–N3 185.7(2), N1–
N2 145.3(3), N3–N4 150.9(3), N1–C1 150.5(4), N2–C3
128.6(3), N3–C4 145.1(3), N4–C6 151.3(3); N1–Al1–N4
86.79(8), N2–Al2–N4 89.10(9), N1–Al3–N3 89.80(9).
1
Three doublets were identified in the H NMR spec-
trum for the methyl groups attached to the C(H) atoms
of the C3N2 heterocycles (C1, C4 and C6, according
to Fig. 2). Four singlets resulted for the chemically dif-
C3N2 heterocycles. One has still a C=N double bond ferent methyl groups at the carbon atoms C2 and C5,
similar to the situation in compounds 1 and 2 as gen- and three quartets with equal intensity are characteris-
tic of the hydrogen atoms attached to C1, C4 and C6.
erated in a single hydroalumination step. The second
ring is completely saturated and results from twofold
A resonance at δ = 175.8 (C3) in the 13C NMR spec-
hydroalumination as originally intended. The first het- trum is indicative of the remaining N=C double bond,
erocycle is planar and has quite different C–N bond while the sp3-hybridized carbon atoms in the C3N2
rings which were generated by hydroalumination (C1,
responding to the different bond orders, while the sec- C4, C6) have resonances at δ = 63.4, 64.2 and 64.7.
lengths of 128.6 (N2–C3) and 150.5 pm (N1–C1) cor-
ond one adopts a twist conformation with both C–N
bond lengths in the range characteristic of single bonds
(N3–C4, 145.1 pm; N4–C6, 151.3 pm). The small dif-
Compound 3 is formally formed by the reaction
of the starting compounds in a molar ratio of 3 : 2
(aluminum hydride versus pyrazole). However, when
ference is caused by the different coordination num- the reaction is conducted with exactly this stoichio-
bers of three and four at the nitrogen atoms N4 and metric ratio at r. t. the resonances of 3 could not
be detected in the NMR spectra. Heating in toluene
N3, respectively. These heterocycles are bridged by
three diethylaluminum groups. Two metal atoms (Al2 for several days is required to form 3 under these
and Al3) bridge the C3N2 rings in a manner similar conditions. Complete hydroalumination of all C=N
double bonds was not achieved when the solutions
to that in compound 1 to give a six-membered Al2N4
ring in a twist-conformation. The third aluminum atom
were stirred for longer periods or heated under re-
◦
(Al1) bridges this ring to give a bicyclic, norbornane- flux (111 C). Even a large excess of the hydride (up
like molecular structure. All aluminum atoms have a to a ratio of 9 : 1) yielded compound 3 as the only
product.
coordination number of four. The Al–N distances dif-
fer markedly. The shortest distance (185.7 pm) occurs
between Al3 and the negatively charged, tricoordinated
nitrogen atom N3 (sum of the angles 359.2◦). The dis-
tance between the sp2-hybridized imino nitrogen atom
N2 (sum of the angles 359.6◦), and Al2 (200.0 pm) is
considerably longer. All other Al–N bond lengths be-
tween tetracoordinated aluminum and nitrogen atoms
are in the range of 193.5 to 201.8 pm. The ring hy-
drogen atoms, which have resulted from the hydroalu-
Reaction of 3,5-diphenylpyrazole with (Me3C)2Al-H
Hydrogen atoms bonded to a pyrazole ring can be
redistributed to produce a tautomeric form which con-
tains an N-H group and a C=C beside a C=N dou-
ble bond. The N-H function is relatively acidic, and
treatment with dialkylaluminum hydride therefore may
not result in hydroalumination as described before,
Unauthenticated
Download Date | 11/18/19 1:17 AM

W. Uhl – A. Vogelpohl · Hydroalumination versus Deprotonation
691
distances of 195.1(2) and 195.3(2) pm. The complete
delocalization of the π electron density and the aro-
matic character of the pyrazole ring is evident from the
quite similar C–N [C1–N1 and C3–N2 135.1(3) pm],
N–N [138.3(2) pm] and C–C bond lengths [C1–C2
138.6(3) and C2–C3 138.3(3) pm] [15]. The Al2N2H
ring is almost ideally planar with a maximum devia-
tion of an atom from the average plane of only 6 pm.
The normals of both heterocycles enclose an angle
of 17.4◦. The benzene rings are tilted with respect to
the pyrazole plane by 41.1 and 48.4◦. All NMR data
are in complete agreement with the solid-state molec-
ular structure, and no detailed discussion is required.
A similar compound has been obtained previously by
the corresponding reaction with diisobutylaluminum
hydride [16]. Its constitution analogous to that of 4
was not established by crystal structure determination,
but by NMR spectroscopy and structures of secondary
products.
(4)
Experimental Section
All procedures were carried out under purified argon
in dried solvents (n-pentane and n-hexane over LiAlH4,
toluene over Na/benzophenone). Tetramethylpyrazole [17],
3,5-diphenylpyrazole [18], di(tert-butyl)- [19] and diethyl-
aluminum hydride [20] were obtained according to literature
procedures.
Fig. 3. Molecular structure of 4. The ellipsoids are drawn
at the 40 % probability level; methyl groups and phenyl hy-
drogen atoms are omitted. Selected bond lengths (pm) and
angles (deg): Al1–N1 195.1(2), Al2–N2 195.3(2), Al1–H1
177(2), Al2–H1 169(2), N1–N2 138.3(2), N1–C1 135.1(3),
N2–C3 135.1(3), C1–C2 138.6(3), C2–C3 138.3(3); Al1–
N1–N2 117.5(1), Al2–N2–N1 116.2(1).
Reaction of tetramethylpyrazole with di(tert-butyl)aluminum
hydride in a molar ratio of 1 : 1; synthesis of 1
(Me3C)2AlH (0.200 g, 1.41 mmol), dissolved in 20 mL
of n-pentane was added to a solution of tetramethylpyrazole
(0.175 g, 1.41 mmol) in 20 mL of toluene at r. t. The color
changed from yellow to orange red. The solution was stirred
at r. t. for 18 h and concentrated. Colorless crystals of 1 pre-
but lead to deprotonation and release of hydrogen.
We observed such a reaction when we treated 3,5-
diphenylpyrazole with di(tert-butyl)aluminum hydride
(Eq. 4). Complete consumption of the starting com-
pounds was only observed for a molar ratio of 2 : 1 (hy-
dride versus pyrazole). Colorless crystals of the prod-
uct (4) were isolated after concentration of the reac-
tion mixture and cooling to −15 ◦C in 60 % yield. The
molecular structure of compound 4 (Fig. 3) consists
of two aluminum atoms which have two terminal tert-
butyl substituents and are bridged by a pyrazole ring
and a hydrogen atom via a 3c-2e Al–H–Al bridge. 4
is clearly formed by hydrogen release and coordina-
tion of a second molecule of the starting hydride. Each
nitrogen atom of the pyrazole ring is coordinated to
◦
cipitated upon cooling to +3 C. Yield: 0.126 g (34 %). –
M. p. (under argon, sealed capillary): 220 ◦C (dec.). – IR
(paraffin; CsBr plates): ν = 1580 m, 1560 m ν(C=N); 1462
vs, 1377 vs (paraffin); 1314 m, 1287 w δ(CH3); 1163 m,
1059 s, 1042 s, 1016 m, 997 m, 949 s, 928 m, 806 s ν(CC),
ν(CN); 723 s (paraffin); 687 m, 629 w, 611 m, 581 m,
563 m, 542 m, 476 cm−1 s ν(AlC), ν(AlN), δ(CC). – 1H
3
NMR (400 MHz, C6D6): d = 3.86 (q, JH-H = 6.6 Hz, 2H,
NCH), 1.70 (s, 6H, N=C-CH3), 1.40 and 1.11 (each s, 18H,
CMe3), 1.09 (d, 3JH-H = 6.6 Hz, 6H, N-CH-CH3), 1.03 and
0.73 (each s, 6H, CMe2). – 13C NMR (100 MHz, C6D6):
δ = 159.0 (N=C), 65.2 (N-C), 49.3 (CMe2), 33.0 and 32.0
(CMe3), 25.2 and 19.5 (CMe2), 19.1 (N-CH-CH3), 17.2 and
15.7 (CMe3), 13.5 (N=C-CH3). – MS (EI, 20 eV, 70 ◦C): m/z
an aluminum atom with almost indistinguishable Al–N (%) = 475 (100), 476 (30) [M–tBu–butene]+.
Unauthenticated
Download Date | 11/18/19 1:17 AM

692
W. Uhl – A. Vogelpohl · Hydroalumination versus Deprotonation
Table 1. Crystal data, data collection, and structure refine-
ment.
Reaction of tetramethylpyrazole with di(tert-butyl)aluminum
hydride in a molar ratio of 1 : 2; synthesis of 2
1·toluene
3
4
(Me3C)2AlH (0.444 g, 3.13 mmol), dissolved in 20 mL
of n-pentane, was added to a solution of tetramethylpyrazole
(0.194 g, 1.56 mmol) in 20 mL of toluene at r. t. The color
changed from yellow to deep red. The solution was stirred
Crystal data
Empirical formula
Mr
C37H70Al2N4 C26H57Al3N4 C31H48Al2N2
624.93
506.70
502.67
Crystal system
Space group [22]
a, pm
b, pm
c, pm
— monoclinic —
◦
P21/n
P21/n
P21/c
at +50 C for 12 h. Yellow crystals of 2 precipitated after
1206.32(5)
2231.1(1)
1456.69(6)
95.112(3)
3905.0(3)
1.06
1006.67(3)
1748.18(6)
1764.62(6)
95.600(1)
3090.6(2)
1.09
902.00(6)
4565.8(3)
819.76(4)
113.228(4)
3102.4(3)
1.08
concentration and cooling to +3 ◦C. Yield: 0.457 g (72 %). –
◦
M. p. (under argon, sealed capillary): 143 C. – IR (paraf-
fin; CsBr plates): ν = 1585 m ν(C=N); 1458 vs, 1377 vs
(paraffin); 1323 m, 1310 m, 1271 w δ(CH3); 1169 s, 1144
m, 1103 m, 1076 s, 1061 s, 1030 m, 1016 m, 1001 s, 951 s,
935 s, 841 w, 812 vs, 779 w ν(CC), ν(CN); 723 vs (paraffin);
708 vs, 615 m, 575 w, 540 w, 495 w, 457 cm−1 s ν(AlC),
ν(AlN), δ(CC). – 1H NMR (400 MHz, C6D6): δ = 3.43
β, deg
V, ×10−30 m3
ρ
calc, g cm−3
Z
4
4
4
F(000), e
1384
0.9 (CuKα )
1120
1096
µ, mm−1
0.1 (MoKα ) 1.0 (CuKα )
Data collection
T, K
Radiation
3
5
(qq, JH-H = 6.7 Hz, JH-H = 1.0 Hz, 1H, NCH), 2.97 (s,
153(2)
CuKα
7345
153(2)
MoKα
7079
153(2)
CuKα
5841
5
br., 1H, AlH), 1.49 (d, JH-H = 1.0 Hz, 3H, N=C-CH3),
1.28, 1.26, 1.183 and 1.177 (each s, 9H, CMe3), 0.98 (d,
3JH-H = 6.7 Hz, 3H, N-CH-CH3), 0.63 and 0.48 (each s, 3H,
CMe2). – 13C NMR (100 MHz, C6D6): δ = 155.9 (N=C),
66.6 (N-C), 50.0 (CMe2), 32.0, 31.7, 31.3 and 31.2 (CMe3),
22.9 and 15.7 (CMe2), 17.3, 16.3 and 15.6 (two resonances
coincide) (CMe3), 12.7 (N-CH-CH3), 12.4 (N=C-CH3). –
Unique reflections
Reflections I ≥ 2σ(I) 5743
5479
4701
Refinement
Refined parameters
411
332
0.064
0.181
332
0.056
0.144
Final R [I ≥ 2σ(I)]a 0.062
Final wR2b (all data) 0.176
∆ρfin (max / min),
0.92 / −0.34 1.08 / −0.65 0.46 / −0.26
◦
−3
˚
MS (EI, 20 eV, 30 C): m/z (%) = 351 (100), 352 (22) [M–
e A
tBu]+.
a
2
R1 = ΣꢀFo| − |Fcꢀ/Σ|Fo|, wR2 = [Σw(Fo − Fc2)2/Σw(Fo2)2]1/2
,
w = [σ2(Fo2)+(AP)2 +BP]−1, where P = (Max(Fo2,0)+2Fc2)/3
and A and B are constants adjusted by the program.
Reaction of tetramethylpyrazole with diethylaluminum
hydride in a molar ratio of 1 : 2; synthesis of 3
(CMe2 of the unsaturated ring), 46.2 (CMe2 of the saturated
ring), 25.0 and 18.5 (CMe2 of the unsaturated ring), 23.8 and
14.8 (CMe2 of the saturated ring), 14.9 and 14.8 (N-CH-CH3
of the saturated ring), 14.6 (N-CH-CH3 of the unsaturated
ring), 12.8 (N=C-CH3), 10.92, 10.88, 10.63, 10.53, 10.40
and 10.39 (CH3 of ethyl), 3.6, 2.7, 1.9 and 0.2 (broad reso-
nances of AlCH2). – MS (EI, 20 eV, 60 ◦C): m/z (%) = 506
(3) [M]+, 477 (67), 478 (16) [M–Et]+, 391 (43) [M–EtH–
AlEt2]+.
Et2AlH (0.5 mL, 0.400 g, 4.65 mmol), dissolved in 10 mL
of n-pentane, was added to a solution of tetramethylpyrazole
(0.285 g, 2.30 mmol) in 20 mL of toluene at r. t. The color
changed from yellow to deep red. The solution was stirred
at r. t. for 12 h. Yellow crystal◦s of 2 precipitated after con-
centration and cooling to +3 C. Yield: 0.387 g (67 %). –
◦
M. p. (under argon, sealed capillary): 186 C. – IR (paraf-
fin; CsBr plates): ν = 1556 m ν(C=N); 1460 vs, 1377 vs
(paraffin); 1304 m δ(CH3); 1192 m, 1109 m, 1088 w, 1026
w, 986 m, 955 m, 918 m, 891 m, 773 m ν(CC), ν(CN); 721
s (paraffin); 689 m, 642 s, 573 w, 548 w cm−1 s ν(AlC),
ν(AlN), δ(CC). – 1H NMR (400 MHz, C6D6): δ = 3.36 (q,
3JH-3H = 6.8 Hz, 1H, NCH of the unsaturated ring), 3.150
Reaction of 3,5-diphenylpyrazole with di(tert-butyl)-
aluminum hydride; synthesis of 4
(Me3C)2AlH (0.482 g, 3.39 mmol), dissolved in 10 mL of
(q, JH-H = 6.6 Hz, 1H, NCH of the saturated ring), 3.146 n-hexane, was added to a suspension of 3,5-diphenylpyrazole
(q, 3JH-H = 6.4 Hz, 1H, NCH of the saturated ring), 1.53 (s, (0.373 g, 1.69 mmol) in 20 mL of toluene at −30 C. The
◦
3H, N=C-CH3), 1.46 to 1.36 (m, 18H, CH3 of ethyl), 1.18 solid dissolved readily, and a clear, yellow solution resulted.
3
(d, JH-H = 6.4 Hz, 3H, N-CH-CH3 of the saturated ring), The solution was warmed to r. t. over a period of 12 h. Color-
3
1.14 (d, JH-H = 6.6 Hz, 3H, N-CH-CH3 of the saturated less crystals ◦of 4 precipitated after concentration and cool-
ring), 1.06 (d, 3JH-H = 6.8 Hz, 3H, N-CH-CH3 of the unsatu- ing to −15 C. Yield: 0.513 g (60 %). – M. p. (under ar-
◦
rated ring), 0.77 and 0.71 (each s, 3H, CMe2 of the saturated gon, sealed capillary): 134 C. – IR (paraffin; CsBr plates):
ring), 0.55 and 0.50 (each s, 3H, CMe2 of the unsaturated ν = 1558 w, 1543 m phenyl, pyrazole; 1462 vs, 1377 s
ring), 0.58 to −0.14 (m, 12H, CH2 of ethyl). – 13C NMR (paraffin); 1300 w, 1275 w δ(CH3); 1182 w, 1157 w, 1098
(100 MHz, C6D6): δ = 175.8 (N=C), 64.2 and 64.7 (N-C of m, 1069 m, 1003 m, 972 w, 934 w, 916 w, 812 m, 758 s
the saturated ring), 63.4 (N-C of the unsaturated ring), 53.5 ν(CC), ν(CN); 723 w (paraffin); 698 s phenyl; 671 w, 579
Unauthenticated
Download Date | 11/18/19 1:17 AM

W. Uhl – A. Vogelpohl · Hydroalumination versus Deprotonation
693
w, 556 w, 451 cm−1 m ν(AlC), ν(AlN), δ(CC). – 1H NMR matrix least-squares calculations based on F2 [21]. Hydro-
(400 MHz, C6D6): δ = 7.48 (m, 4H, ortho-H of phenyl), gen atoms with the exception of Al-H were calculated on
7.16 (m, 4H, meta-H of phenyl), 7.09 (m, 2H, para-H of ideal positions and refined by the riding model. Crystal data,
phenyl), 6.35 (s, 1H, pyrazole), 3.41 (s, br., 1H, AlHAl), data collection parameters and details of the structure re-
1.17 (s, 36H, CMe3). – 13C NMR (100 MHz, C6D6): δ = finement are given in Table 1. The crystals of 1 enclosed a
156.1 (NC of pyrazole), 131.4 (ipso-C of phenyl), 130.1 molecule of toluene per formula unit of the tricyclic com-
(para-C of phenyl), 129.6 (meta-C of phenyl), 127.8 (or- pound. The methyl group of an ethyl substitutent (C60) of 3
tho-C of phenyl), 106.9 (CH of pyrazole), 31.2 (CMe3), 16.8 was disordered over three positions; the atoms were refined
(CMe3). – MS (EI, 20 eV, 30 ◦C): m/z (%) = 445 (100), 446 with occupation factors of 0.33.
(29) [M–tBu]+.
CCDC 767773 (1), 767772 (3) and 767774 (4) contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk
/data request/cif.
Crystal structure determinations
Single crystals were obtained upon cooling of the reac-
tion mixtures as described above. Data collections were per-
formed on Bruker SMART APEX-II (MoKα radiation) and
Bruker SMART 6000 (CuKα radiation) diffractometers. The
structures were solved by Direct Methods and refined by full-
Acknowledgement
We are grateful to the Deutsche Forschungsgemeinschaft
for generous financial support.
[1] W. Uhl, in Group 13 Chemistry III – Industrial Appli-
cations, (Eds.: H. W. Roesky, D. A. Atwood), Structure
and Bonding, Springer Verlag, 2003, p. 41.
[2] C. Lustig, N. W. Mitzel, Angew. Chem. 2001, 113,
4521; Angew. Chem. Int. Ed. 2001, 40, 4390; N. W.
Mitzel, C. Lustig, M. Woski, Dalton Trans. 2004, 397;
P. Bo¨ sing, A. Willner, T. Pape, A. Hepp, N. W. Mitzel,
Dalton Trans. 2008, 2549.
[3] A. R. Barron, Chem. Soc. Rev. 1993, 22, 93; W. Uhl,
M. R. Halvagar, Angew. Chem. 2008, 120, 1981;
Angew. Chem. Int. Ed. 2008, 47, 1955; W. Uhl,
M. R. Halvagar, F. Rogel, W. Massa, Eur. J. Inorg.
Chem. 2009, 489; W. Uhl, M. R. Halvagar, M. Layh,
Chem. Commun. 2009, 4269; W. Uhl, M. R. Halvagar,
M. Claesener, Chem. Eur. J. 2009, 15, 11298; W. Uhl,
B. Jana, Chem. Eur. J. 2008, 14, 3067.
[4] W. Uhl, T. Abel, J. Ko¨sters, F. Rogel, Z. Naturforsch.
2008, 63b, 117; W. Uhl, C. H. Emden, G. Geiseler,
K. Harms, Z. Anorg. Allg. Chem. 2003, 629, 2157.
[5] W. Uhl, T. Abel, A. Hepp, S. Grimme, M. Steinmetz,
Eur. J. Inorg. Chem. 2008, 543.
[11] W. Uhl, J. Molter, B. Neumu¨ller, F. Schmock, Z. An-
org. Allg. Chem. 2001, 627, 909; W. Uhl, J. Molter,
B. Neumu¨ller, J. Organomet. Chem. 2001, 634, 193.
[12] N2H4: Y. Morino, T. Iijima, Y. Murata, Bull. Chem.
Soc. Jpn. 1960, 33, 46; N2(SiH3)4: C. Glidewell,
D. W. H. Rankin, A. G. Robiette, G. M. Sheldrick, J.
Chem. Soc. A 1970, 318; N2F4: M. M. Gilbert, G. Gun-
dersen, K. Hedberg, J. Chem. Phys. 1972, 56, 1691;
J. R. Durig, Z. Shen, J. Chem. Phys. A 1997, 101,
5010; N2Me4: V. A. Naumov, O. A. Litvinov, H. J.
Geise, J. Dillen, J. Mol. Struct. 1983, 99, 303; review:
K. Bode, U. Klingebiel, Adv. Organomet. Chem. 1996,
40, 1; S. Dielkus, C. Drost, R. Herbst-Irmer, U. Klinge-
biel, Angew. Chem. 1993, 105, 1689; Angew. Chem.,
Int. Ed. Engl. 1993, 32, 1625.
[13] R. Boucourt, in Topics in Stereochemistry, (Eds.: E. L.
Eliel, N. L. Allinger), Vol. 8, Wiley, New York, 1974,
p. 159.
[14] F. Garcia, A. D. Hopkins, R. A. Kowenicki, M. Mc-
Partlin, J. S. Silvia, J. M. Rawson, M. C. Rogers, D. S.
Wright, Chem. Commun. 2007, 586.
[6] W. Uhl, T. Abel, J. Ko¨sters, B. Rezaeirad, Z. Anorg.
Allg. Chem. 2009, 635, 1014.
[15] Lithium, magnesium and aluminum pyrazolates:
S. Beaini, G. B. Deacon, A. P. Erven, P. C. Junk, D. R.
Turner, Chem. Asian J. 2007, 2, 539; J. Hitzbleck,
G. B. Deacon, K. Ruhlandt-Senge, Eur. J. Inorg.
Chem. 2007, 582; S. A. Cortes-Liamas, M. A. Munoz-
Hernandez, Organometallics 2007, 26, 6844.
[16] Z. Yu, J. M. Wittbrodt, A. Xia, M. J. Heeg, H. B.
Schlegel, C. H. Winter, Organometallics 2001, 20,
4301.
[7] W. Uhl, T. Abel, A. Hepp, T. Nilges, F. Rogel, E.-U.
Wu¨rthwein, N. Ghavtadse, Inorg. Chem. 2009, 48, 752.
[8] W. Uhl, A. Vogelpohl, Z. Naturforsch. 2008, 63b, 1149;
W. Uhl, A. Vogelpohl, J. Ko¨sters, Z. Naturforsch. 2006,
61b, 854; W. Uhl, C. H. Emden, W. Massa, J. Or-
ganomet. Chem. 2006, 691, 1382.
[9] W. Uhl, A. Vogelpohl, Eur. J. Inorg. Chem. 2009, 93.
[10] W. Uhl, J. Molter, B. Neumu¨ller, Chem. Eur. J. 2001,
7, 1510; see also: R. J. Wehmschulte, P. P. Power, Inorg.
Chem. 1996, 35, 2717.
[17] K. Beck, A. Ho¨hn, S. Hu¨nig, F. Prokschy, Chem. Ber.
1984, 117, 517.
Unauthenticated
Download Date | 11/18/19 1:17 AM

694
W. Uhl – A. Vogelpohl · Hydroalumination versus Deprotonation
[18] J. Elguero, E. Gonzales, R. Jacquier, Bull. Chem. Soc.
Fr. 1968, 707.
Wisconsin (USA) 1990; G. M. Sheldrick, SHELXL-97,
Program for the Refinement of Crystal Structures,
University of Go¨ttingen, Go¨ttingen (Germany) 1997.
See also: G. M. Sheldrick, Acta Crystallogr. 2008,
A64, 112.
[19] W. Uhl, L. Cuypers, R. Graupner, J. Molter, A. Vester,
B. Neumu¨ller, Z. Anorg. Allg. Chem. 2001, 627, 607.
[20] H. Lehmkuhl, K. Ziegler, Methoden Org. Chem.
(Houben-Weyl), 4th ed., Vol. XIII/4, Thieme, Stuttgart,
1952, p. 58.
[22] T. Hahn (Ed.), International Tables for Crystal-
lography, Space-Group Symmetry, Vol. A, Kluwer
Academic Publishers, Dordrecht-Boston-London,
1989.
[21] G. M. Sheldrick, SHELXTL-PLUS (version 4.1),
Siemens Analytical X-Ray Instruments Inc., Madison,
Unauthenticated
Download Date | 11/18/19 1:17 AM