A contribution to the chemistry of 2,2,6,6-tetramethylpiperidino aluminium compounds

Article abstract of DOI:10.1515/znb-2005-1002

tmpAlBr₂ (tmp = 2,2,6,6-tetramethylpiperidino) was prepared from AlBr₃ and tmp₂AlBr at 90°C in the absence of a solvent, but could not be crystallised from toluene or hexane because it reacted with the solvents to form tmpH·AlBr₃ in high yield. tmpH·AlMeCl₂, obtained from the components, decomposes at elevated temperatures but no tmpAlCl₂ could be isolated. Attempts to generate the cation [tmp-Al-tmp]⁺ from tmp₂AlBr or tmp₂AlCl by halide abstraction with B(C₆F ₅)₃, Ph₃C(SnCl₅) or SbCl₅ or from tmp₂AlR (R = Me, Ph) and B(C₆F₅) ₃ have failed. An unexpected reaction occurred on treatment of tmp-B=P(tBu)AlBr₃ with BH₃ in THF which led to the formation of [AlBr₂(thf)₄][AlBr₄]. The attempted synthesis of tBu₂Al(tmp) from tBu₂AlBr and Li(tmp) gave a product which, on exposure to CO₂ at dry ice temperature, yielded the salt [(tBuAl)₂(O₂C(tmp)) ₃][tBu₃ Al-Br-AltBu₃] in low yield. All isolated products were characterized by NMR spectroscopy and by X-ray determination of their molecular structures.

Full text of DOI:10.1515/znb-2005-1002

A Contribution to the Chemistry of 2,2,6,6-Tetramethylpiperidino
Aluminium Compounds
Klaus Knabel and Heinrich No¨th
Department of Chemistry, University of Munich, Butenandtstr. 5 – 13, House D, D-81377 Mu¨nchen,
Germany
Reprint requests to Prof. Dr. H. No¨th. E-mail: H.Noeth@lrz.uni-muenchen.de
Z. Naturforsch. 60b, 1027 – 1035 (2005); received June 7, 2005
Dedicated to Prof. Dr. Dr. h. c. mult. H. W. Roesky on the occasion of his 70th birthday
tmpAlBr2 (tmp = 2,2,6,6-tetramethylpiperidino) was prepared from AlBr3 and tmp2AlBr at 90 ◦C
in the absence of a solvent, but could not be crystallised from toluene or hexane because it re-
acted with the solvents to form tmpH·AlBr3 in high yield. tmpH·AlMeCl2, obtained from the
components, decomposes at elevated temperatures but no tmpAlCl2 could be isolated. Attempts
to generate the cation [tmp-Al-tmp]+ from tmp2AlBr or tmp2AlCl by halide abstraction with
B(C6F5)3, Ph3C(SnCl5) or SbCl5 or from tmp2AlR (R = Me, Ph) and B(C6F5)3 have failed. An
unexpected reaction occurred on treatment of tmp-B=P(tBu)AlBr3 with BH3 in THF which led
to the formation of [AlBr2(thf)4][AlBr4]. The attempted synthesis of tBu2Al(tmp) from tBu2AlBr
and Li(tmp) gave a product which, on exposure to CO2 at dry ice temperature, yielded the salt
[(tBuAl)2(O2C(tmp))3][tBu3Al-Br-AltBu3] in low yield. All isolated products were characterized
by NMR spectroscopy and by X-ray determination of their molecular structures.
Key words: Tetramethylpiperidino Alanes, Tetramethylpiperidino Aluminiumdihalides,
Dibromo-tetrakis(tetrahydrofuran)aluminium Tetrabromoaluminate,
Bis(tri-tert-butylaluminium)bromide Anion, X-Ray Structure
Introduction
Results
Tetramethylpiperidinoaluminium halides
2,2,6,6-Tetramethylpiperidino aluminium com-
pounds tmp2AlX (tmp = 2,2,6,6-tetramethylpiper-
idino) opened the door to a rich chemistry [1 – 5].
The compounds tmp2AlF and tmp2AlH [1, 2] are
dimeric via AlFAl or AlHAl bridges in contrast to
bis(dialkylamino)aluminium hydrides and halides [6 –
11] which dimerize via AlNAl bridges. However, most
tmp2AlX compounds (X = Me, Ph, OR. SR, SeR, TeR,
PR2, AsR2) are monomeric including the halides (X =
Cl, Br, I) [2, 3, 5]. While dialkylaminodihaloalanes
R2NAlX2 (R = Me, Pr, Ph, C5H10); X = Cl, Br)
are readily available [8, 11], tmpAlBr2 is difficult to
prepare (v. i.). The structures of the tmp2AlX com-
One might expect that the synthesis of the still un-
known tmpAlBr2 (1), could be easily achieved by melt-
ing together AlBr3 with tmp2AlBr at 90 ◦C in a 1 : 1 ra-
tio and in the absence of a solvent as shown in eq. (1).
This is indeed the case, because 1 dissolved in hex-
ane or toluene provided a single 27Al NMR signal at
δ = 108 ppm. These solutions, however, are unsta-
ble. After some hours at ambient temperature only a
signal at δ27Al = 90 ppm remains which is due to
tmpH·AlBr3 (2) [4]. Obviously a reaction with the sol-
vents occurs as proposed in eq. (2).
Therefore, we explored other procedures to pre-
pounds (X = Cl, Br, I) show fairly wide N-Al-N bond pare tmpAlX2 compounds. One possibility is shown in
angles suggesting that they may be used as precursors eq. (3). It was expected that the adduct 3 would read-
for the generation of linear dicoordinated Al cations ily be formed and loose methane at slightly elevated
[tmp-Al-tmp]+ by halide abstraction using a strong temperatures to produce 4. The first step (1) yields 3 in
halide acceptor. This goal has up to date not yet been good yield, however, on heating of compound 3 no gas
◦
achieved. New aspects related to tmpAl chemistry are evolution (methane) was noted up to 220 C. 3 turned
reported here.
into a brown material from which no pure compound
c
0932–0776 / 05 / 1000–1027 $ 06.00 ꢀ 2005 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
Unauthenticated
Download Date | 11/4/19 4:24 AM

1028
K. Knabel – H. No¨th · A Contribution to the Chemistry of 2,2,6,6-Tetramethylpiperidino Aluminium Compounds
could be isolated.
AlBr3 +tmp2AlBr
No reaction was observed when◦tmp2AlCl was treated
with SbCl5 in toluene at −50 C. At room tempera-
(1) ture, however, a reaction occurred as suggested by the
formation of a red solution and a black solid (Sb?). In
solution, however, the only Al species present was the
→
2tmpAlBr2
1
2tmpAlBr2 +RCH2R
AlCl4 anion (δ27Al = 103 ppm).
→ tmpH·AlBr3 +tmpAlBr(CHR2)
(2)
Using the method described by Jordan [16] we
treated tmp2AlR (R = Me, Ph) with B(C6F5)3.
No reactions were observed at low temperature by
2
tmpH+MeAlCl2
→
→
tmpH·AlCl2Me
11B NMR spectroscopy, but under ambient conditions
3
(3)
several 11B NMR signals were observed revealing
the presence of tetra- and tricoordinated boron com-
pounds in solution (see Experimantal Section). No
tmpAlCl2 +MeH
4
tmpSnMe3 +AlBr3 → tmpAlBr2 +Me3SnBr (4)
27Al NMR signals could be observed. Comparison of
the 11B chemical shifts of known species related with
those observed here suggests the following borates
and boranes to be present: MeB(C6F5)3−, B(C6F5)3,
tmp2BMe, MeB(C6F5)2, andMe2BC6F5.
Finally, we treated tmp2AlCl with [Ph3C]SnCl5 ex-
pecting a process according to eq. (7), but no reaction
was observed in toluene solution.
In another attempt to obtain tmpAlBr2 the well known
stannazane cleavage reaction was employed [12, 13]
as depicted in eq. (4). On◦warming the reaction mix-
◦
ture in toluene from −78 C to 20 C this reaction in-
deed occurs as shown by a dominant 27Al NMR sig-
nal at δ = 108 ppm which however lost its intensity
with time to make room for the 27Al NMR signal of
compound 2 (δ = 90 ppm; reported 88 ppm [4]). This
compound was isolated in 60% yield. The solution
showed an 119Sn resonance for Me3SnBr at δ119Sn =
128 ppm [14].
Two unexpected reactions
It has been shown that the amido-phosphinidene
borane adduct tmp-B=PtBu(AlBr3) [17] is a reac-
tive species. Its thermolysis might yield tmpAlBr2
but produced a mixture of compounds which could
not be separated. Addition of an electrophile EXn to
the P atom would be expected to lead to tmp(X)B-
P(EXn−1)tBu(AlBr3) complexes or to a transfer of
the tmp group to give tmpAlBr2. For this rea-
son compound tmp-B=PtBu(AlBr3) was treated with
BH3·THF in THF solution. It turned out that the
course of the reaction was determined by the THF
donor which removed its AlBr3 component from the
AlBr3 adduct in the form of the salt 5 according to
eq. (8).
In spite of many attempts we were unable to iso-
late and characterize the expected second aluminium
compound which must form during the reaction of
tmpAlBr2 with toluene. We had expected that this com-
pound might be tmpAlBr(CH2Ph) a compound that
should be soluble in toluene, but no 27Al NMR signal
could be detected [15].
Attempts to generate a bis(tetramethylpiperidino)alu-
minium cation
Bis(2,2,6,6-tetramethylpiperidino) alanes tmp2AlX
(X = Cl, Br, Me, Ph) are potential precursors for
the generation of compounds with a dicoordinated
bis(tetramethylpiperidino)-aluminium cation. For the
halides, a strong halide acceptor may react as shown
in eq. (5) while organyl anion acceptors could act as
depicted in eq. (6).
This reaction occurs only in the presence of
toluene as a cosolvent. Compound 5 is an isomer of
AlBr3(thf)2 [3]. Because it is well know that the BN
multiple bond of tmp-B=NtBu inserts readily into a
BH bond of BH3 with formation of a four-membered
B2N2 ring, we assume that the second product is the
tetra cyclic (tmp-BH-PtBu-BH2) adduct (ring closure
via a B-N bond) [18]. The conclusion is tentative be-
cause no 11B NMR data could be recorded.
tmp2AlX+EXn
(5)
−
→ [tmp−Al−tmp]+ +EXn+1
tmp2AlR+B(C6F5)3
(6)
→ [tmp−Al−tmp]+ +RB(C6F5)−
In connection with the synthesis and characteriza-
tion of di(tert.butyl)aluminium pseudohalides [19] we
tried to synthesize tBu2Altmp in hexane according to
tmp2AlCl+[Ph3C][SnCl5]
(7)
→ tmp2Al+ +SnCl5− +Ph3CCl
Unauthenticated
Download Date | 11/4/19 4:24 AM

K. Knabel – H. No¨th · A Contribution to the Chemistry of 2,2,6,6-Tetramethylpiperidino Aluminium Compounds
1029
eq. (9):
2tmp−B = PtBu(AlBr3)+4THF
→ (tmp−B = PtBu)2 +[AlBr2(thf)4][AlBr4] (8)
5
tBu2AlBr+Li(tmp)
→
tBu2Altmp+LiBr
(9)
6
4tBu2AlBr+3Li(tmp)+3CO2 →
[(tBuAl)2(O2Ctmp)3]+[tBu3AlBrAltBu3]−
+3LiBr
7
(10)
Fig. 2. Structure of the ions of the salt [AlBr2(thf)4]
In order to obtain crystals of 6, the solution was kept
at −78 ◦C in dry ice. After several weeks crystals had
formed but these proved not to be 6 but rather the salt 7,
which obviously was formed according to eq. (10) by
slow diffusion of CO2 into the reaction mixture.
[AlBr4] (5). Thermal ellipsoids are presented on a 25%
˚
probability level. Selected bond lengths (A) and bond
angles (◦): Al1-Br1 2.4366(8), Al1-O1 1.929(6), Al1-O2
1.942(7), O1-C1 1.48(1), O1-C4 1.48(1), Al2–Br2 2.290(2),
Al2-Br3 2.292(2). – O1-Al1-O1A 89.1(4), O1-Al1-O2
90.1(3), O1-Al1-O2A 179.2(3); O1-Al1-Br1 90.5(2),
O2-Al1-Br1 89.6(2), Br1-Al1-Br1A 178.9(2), Al1-O1-C1
125.0(5), Al1-O1-C4 125.9(5), Al1-O2-C5 125.7(5),
Al1-O2-C8 125.9(6), C1-O1-C4 109.1(7), C5-O2-C8
108.4(7), Br2-Al2-Br3 109.32(6), Br2B-Al1-Br3 109.79(5),
Br3-Al2-Br3B 110.9(2), Br2-Al2-Br2B 107.7(2).
Crystal structures
The new aluminium compounds have been char-
acterized not only by NMR methods and IR spectra
but particularly by their molecular structures as deter-
mined by X-ray diffraction. In the discussion the rele-
vant features will be compared.
The molecular structure of compound 2 has already
been described. The data now obtained agree well with
those reported [4].
there are strong deviations from a tetrahedral ar-
ray due to the presence of methyl group. Thus,
the “tetrahedral angles” range from 96.56(4)◦ for
N1-Al1-Cl2 to 119.90(6)◦ for C10-Al1-N1. The
twist angle H1-N1-Al1-Cl2 is −8.3◦. Intermolecu-
Crystals of 3 are monoclinic, space group P21/n
(see Fig. 1). For the tetracoordinated Al atom
˚
lar N-H···Cl hydrogen bonds (H1···Cl2 = 2.863 A;
N1-H1-Cl2 = 160.5◦) connect two molecules of 3 in
the unit cell.
The salt 5 crystallizes in the monoclinic space group
P2/c with Z = 2 (see Fig. 2). Both Al atoms are located
at a crystallographic twofold axis. The Al atom in the
cation [AlBr2(thf)4]+ has a tetragonal bipyramidal co-
ordination sphere with four O atoms in a slightly dis-
torted AlO4 plane as shown by two different Al-O bond
˚
lengths (1.926(6) and 1.942(7) A) and O1-Al-O1A and
O2-Al1-O2 bond angles of 89.1(4) and 90.1(3) ◦, re-
spectively. The two Br atoms are in trans position to
each other showing Br1-Al1-O bond angles of 90.5(2)
and 89.6(2)◦. As expected, the Al1-Br1 bond length
˚
is longer with 2.4366(8) A than the Al2-Br bonds
˚
(2.290(2), 2.292(2) A) at the almost perfectly tetrahe-
drally coordinated Al2 atom [20].
The molecular structure of the salt 7 is depicted
in Fig. 3. The compound crystallizes in the or-
thorhombic space group Pccn with Z = 4. Both
the cation and the anion show C2 symmetry. The
Fig. 1. Molecular structure of compound tmpH·AlMeCl2 (3).
Thermal ellipsoids are presented on a 25% probability level.
For selected bond lengths and bond angles see Table 1. Ad-
ditional bond angles (◦): C1-N1-C5 116.84(1), C1-N1-Al1
117.84(9), C5-N1-Al1 113.51(9), C10-Al1-Cl1 104.52(5),
C10-Al1-Cl2 113.23(6), Cl1-Al1-Cl2 105.94(3).
Unauthenticated
Download Date | 11/4/19 4:24 AM

1030
K. Knabel – H. No¨th · A Contribution to the Chemistry of 2,2,6,6-Tetramethylpiperidino Aluminium Compounds
Discussion
Substituent exchange between AlX3 and AlY3
species is in general a common route to mixed species
AlX3−mY3−n as depicted in eq. (11):
nAlX3 +mAlY3 → (n+m)AlX3−mY3−n (11)
This kind oft reaction either leads to pure com-
pounds AlX2Y or AlXY2 depending on stoichiome-
try, or to an equilibrium mixture of all components
depending on the nature of X and Y [18]. For in-
stance the reaction of Al2Br6 with [Al(NMe2)3]3 leads
to [BrAl(NMe2)2]2 (1 : 2 reaction) or to [Br2AlNMe2]2
(2 : 1 reaction) [7]. Since we had not been success-
Fig. 3. Molecular structure of the components of compound
[(AltBu)2(O2Ctmp)3]-[(tBu3Al)Br(AltBu3)] (7). Thermal
ellipsoids represent a 25 probability level. The Me groups
of tBu have been omitted for the sake of clarity. Selected
bond lengths (A) and bond angles ( ): Al1-C1 1.970(7),
Al1-O12 1.763(4), Al1-O21 1.757(5), Al1-O11A 1.770(4),
C11-N11 1.312(7), C11-O21 1.288(7), C11-O11 1.298(7),
N11-C61 1.534(8), C12-N12 1.305(9), N12-C22 1.534(6),
N12-C22B 1.534(6), C12-O12 1.289(5), Al2-C2 2.013(7),
ful in preparing tmpAlX2 halides (X = Cl, Br) by
allowing AlX3 to react with Li(tmp) we now ob-
tained tmpAlBr2 from the components AlBr3 and
◦
tmp2AlBr. Its 27Al NMR data (δ = 108.9 ppm, h1/2
=
˚
210 Hz) suggest that the compound is dimeric having a
tmp(Br)Al(µ-Br)2Al(Br)tmp structure. This argument
is based particularly on the line width as monomeric
tmp2AlBr shows a line width of 9100 Hz [3]. The sig-
nal of the Al atom of 2 is 10 ppm at higher field than
that of dimeric Me2NAlBr2 [19]. Also, the Al atom of 2
is better shielded by 22 ppm compared to tmp2AlBr
which is, however, not a strong additional criterion as
the shielding of the dimeric compounds Me2NAlX2
and (Me2N)2AlX (X = Cl, Br, I) are not strongly de-
pendent on the nature and number of halogen atoms.
The structure suggested for compound 2 with bro-
mide bridges is more likely than one with tmp bridges
also for steric reasons. So far all compounds carry-
Al2-C3 2.019(7), Al2-C4X 2.01(1).
– O12-Al1-O21
106.9(2), O12-Al1-O11B 107.9(2), O12-Al1-C1 112.8(2),
O21-Al1-O11B
O11B-Al1-C1
104.9(2),
111.2(3),
O21-Al1-C1
Al2-Br1-Al2A
112.7(3),
164.5(1),
Br1-Al2-C2 102.74(2), Br1-Al2-C3 98.6(2), Br1-Al2-C4
107.6(4), C2-Al2-C3 114.5(3), C2-Al2-C4 124.2(5),
C3-Al2-C4 105.9(5).
cation [(tBuAl)2(O2Ctmp)3]+ has a tricyclic struc-
ture containing tetracoordinated Al and trigonal planar
N atoms. The smallest angle at atom Al1 is 104.9(2)◦
for O21-Al1-O11B, the widest is found with 111.2(3)◦
for the angle O11A-Al1-C1. The angles at the car- ing a tmp group have no bridging tmp groups, excep-
baminate C and N atoms are close to 120◦ [119.3(5)
tions being species of the type tmp-BR-NtBu-AlX2
with strong Lewis acidic AlX2 groups and of small
steric requirements [15]. The ability of 2 to deprotonate
to 120.8(6)◦, and 119.1(5) to 121.5(5)◦, respectively].
˚
C=O bond lengths are on average 1.293(7) A for
C11-O11 and C11-O21, while the C11-N11 bond is toluene to give compound 3 can only be understood if
˚
only slightly longer with 1.312(7) A. These data are
the amino group is a strong base and this excludes a
tetracoordinated tmp N atom in a bridging position.
Compound 3 has been prepared from tmpH and
AlBr3 previously and its structure determined. The
typical for a carbaminate ligand which coordinates to
a metal centre by its two O atoms in accord with delo-
calized π-bonds [21].
Typical for the [tBu3Al-Br-AltBu3]− anion is adduct tmpH·AlMeCl2, 5, is a new member in the se-
the Al2-Br1-Al2A bond angle of 164.5(1)◦. The ries of tmpH adduct of aluminum halides [4]. This mer-
Al-C bonds in this anion are slightly longer than in its a short comparison of structural parameters which
˚
the cation [2.013(7), 2.01(1) and 2.019(7) A versus are summarized in Table 1.
1.970(7) A]. Steric interaction between the tBu groups
˚
All compounds crystallize in the monoclinic space
widens the C-Al2-C bond angles as shown for group P21/n except tmpH·AlH2Cl [4]. The iodide
C2-Al2-C3 = 114.5(3)◦, and C3-Al2-C4 = 123.9(5)◦ is exceptional in that it contains two independent
while sharp bond angles of 102.7(2)◦ and 98.6(2)◦ are molecules in the asymmetric unit. Typical for all
found for C2-Al2-Br1 and C3-Al2-Br1, respectively.
molecules are dihedral angles H-N-Al-X close to 0◦.
Unauthenticated
Download Date | 11/4/19 4:24 AM

K. Knabel – H. No¨th · A Contribution to the Chemistry of 2,2,6,6-Tetramethylpiperidino Aluminium Compounds
1031
Fig. 4. Hydrogen bonding and I···I contacts in tmpH AlX2Y compounds a) tmpH·AlCl3; b) tmpH·AlBr3; c) tmpH·AlI3 and
d) tmpH·AlCl2Me.
Table 1. Comparison of structural data of compounds
bond angles also differ considerably [105.94(4) and
113.23(5)◦].
tmpH·AlX3.
X = Cl
X = Br X = I
X = I
X = Me,Cl (3)
Finally, there are considerable differences in the in-
termolecular interactions between the adducts. Fig. 4
shows that tmpH·AlCl3 and tmpH·AlBr3 molecules are
associated into chains in the solid state via N-H-X
hydrogen bonds while no N-H-I hydrogen bonds are
present in tmpH·AlI3, I···I contacts are found instead.
On the other hand, the N-H-Cl hydrogen bridges in
compound 3 lead to dimeric units. The N-H···X hy-
drogen bonds induce a lengthening of the respective
Al-X bonds.
Al-N
2.014(2) 2.009(5) 2.038(9) 2.031(9) 2.039(1)
2.129(1) 2.280(2) 2.540(3) 2.532(3) 2.156(4)
2.136(1) 2.283(2) 2.535(3) 2.535(3) 2.169(5)
2.129(1) 2.310(2) 2.532(3) 2.523(3) 1.950(2) (Me)
Al-X1
Al-X2
Al-X3
N-Al-X1 99.86(7) 100.5(1) 99.00(3) 119.7(3) 114.10(4)
N-Al-X2 115.39(8) 116.0(1) 118.6(3) 98.3(3) 98.56(4)
N-Al-X3 116.72(3) 117.9(1) 119.2(3) 118.6(3) 119.90(6) C
H-N-Al-X 0 (Cl1)
2.3 (Br2) −1.8 (I2) 0.3 (I5) −8.3 (Cl2)
The Al–N bond lengths seem to increase in the order
Br < Cl < I and are longest for compound 3 but, they
do not differ much taking into account the 3σ criterion.
Although the Al-X bonds differ within each molecule
they fall in the range of those observed in other amine
adducts of aluminium trihalides [22]. Moreover, the
Al-Cl bond lengths in 3 are longer than in tmpH·AlCl3
and this is true also for the AlN bond lengths indicating
that MeAlCl2 is a weaker Lewis acid than AlCl3.
Although the chemistry of tmp-B≡P-tBu has been
studied in some detail [23] this holds not for its Lewis
acid adducts e.g. tmp-B=P(tBu)AlBr3 [19]. Several re-
actions can be envisaged for its behavior towards BH3
in THF solution: a) addition of BH3 at the B=P double
bond to generate tmp-BH-P(BH2)(tBu)AlBr3 with a
four-memberedring containing a tetracoordinated BH2
It is also worth noting that the N-Al-Cl2 bond angle group attached o the tmp N atom in analogy to tmp-
in 3 is quite acute with 98.56◦ while the N-Al-C bond BX-NtBu-EXn [18]; b) replacement of AlBr3 by BH3
angle of 119.90(6)◦ is wide. Consequently, the C-Al-Cl to give tmp-B=P(tBu)BH3 which in turn may form a
Unauthenticated
Download Date | 11/4/19 4:24 AM

1032
K. Knabel – H. No¨th · A Contribution to the Chemistry of 2,2,6,6-Tetramethylpiperidino Aluminium Compounds
four membered ring tmp-BH-P(tBu)-BH2 via a B-N tmp2AlCl were prepared as described in [3]. All other
reagents were obtained by literature methods or were of com-
mercial origin. Solvents were made anhydrous by standard
methods.
IR: Perkin Elmer FT 360, NMR: JEOL 400 with SiMe4
as internal standard (1H, 13C). and aqueous Al(NO3)3 as ex-
ternal standard (27Al). X-ray structures: Siemens P4 diffrac-
tometer equipped with an area detector and an LT2 low tem-
perature device, Mo-Kα radiation, graphite monochromator.
bond, and c) abstraction of AlBr3 by THF with gener-
ation of AlBr3·thf and (tmp-B=PtBu)2 [23]. This latter
reaction was observed, but the product did not prove to
be AlBr3·2thf [3] but the salt [AlBr2(thf)4][AlBr4] (5).
The adduct AlBr3·2thf has been obtained by an
ether displacement reaction from AlBr3·DME with
THF [3]. This reaction corresponds to the base dis-
placement from (Me2Si)2NH·AlCl3 with THF to give
AlCl3·2thf [24]. However, an AlBr3 solution in THF
leads to the formation of an ion pair, and the Fuoss-
Krauss plot [25] is in accord with the eq. (12)
2,2,6,6-Tetramethylpiperidino aluminium dibromide (1), and
tetramethylpiperidine aluminium tribromide (2)
a) A mixture of AlBr3 (7.38 g, 27.6 mmol) and tmp2AlBr
(10.7 g, 27.6 mmol) was heated in vacuo to 90 ◦C. A slightly
brownish melt formed which solidified at ambient temper-
ature to the raw product 1. Addition of toluene (100 ml)
dissolved much of the solid leaving a small residue (0.2 g).
2AlBr32thf ↔ [AlBr2(thf)4]+ +AlBr4
(12)
−
This result agrees with those found by Derroult et
al. for AlCl3 solutions in THF [26], and the forma-
tion of [AlCl2(thf)4]+[AlCl4]− has been ascertained The 27Al NNR spectrum of the solution showed a single sig-
nal at δ = 108.7 ppm (h1/2 = 100 Hz). The solution was
by Atwood et al. [27], in the reaction of AlCl3 in
toluene with THF. This corroborates with our observa-
tion that the bromide 5 can only be obtained from tmp-
B=P(tBu)AlBr3 and THF in the presence of toluene.
Both ionic compounds have rather similar structures.
It is well known that CO2 inserts like CS2 [28]
into the AlN bonds of aluminium amides to form
aluminium carbaminates. For instance, MeAl(tmp2)
adds CO2 to give [MeAl(O2Ctmp)2]2 with pentacoor-
dinated Al atoms and bridging carbaminate groups [5].
However, a rather unusual behavior is observed in
the attempt to prepare tBu2Altmp from tBu2AlBr and
Li(tmp). It is difficult to suggest a route to the forma-
tion of compound 7, but certainly a ligand exchange
which produces AltBu3 which adds to eliminated LiBr
with formation of the anion [tBu3AlBrAltBu3]− ex-
plains at least the formation of this anion. That alu-
minum trialkyls add to alkali metal halides to give
first reduced in volume in vacuo by 50% and then cooled
◦
to −30 C. After 24 h the crystals were collected (yield:
12.0 g, 54%) and identified as 2 as shown by its X-ray struc-
ture. The supernatant solution no longer showed the27Al sig-
nal at 108.7 ppm, but a new signal at 80 ppm (2) besides some
other weaker signals.
b) A solution of AlBr3 (0.62 g, 2.04 mmol) in hexane
(50 ml) was added to tmp2AlBr (0.62 g, 2.04 mmol) while
stirring. After the addition, the clear solution showed two sig-
nals in the 27Al NMR spectrum at δ = 108 and 80 (h1/2
=
30 Hz) in an intensity ratio of ∼ 1 : 4. From the concentrated
solution crystals separated on cooling to −30 ◦C. These
proved to be 2 by X-ray diffraction.
2,2,6,6-Tetramethylpiperidine-methyldichloroalane (3)
A solution of tmpH (1.70 ml, 10.09 mmol) in toluene
(10 ml) was cooled to −78 ◦C. While stirring a solution of
MeAlCl2 in toluene (10 ml, 1.0 M) was added. Then the re-
action mixture was allowed to attain room temperature, and
its volume was reduced to ∼ 5 ml. From this solution crystals
of 3 settled after cooling to −30 ◦C. Yield: 2.7 g (9.2 mmol,
92%), m.p. > 220 ◦C. – 1H NMR (C6D6): δ = 0.12 (s, 3H,
MeAl), 0.66 (m, 2H, CH2-3), 0.93 (m, 4H, CH2-2,4), 1.12 (s,
−
R3AlXAl3 anions is best known for X=F [29, 30]. In
these anions the Al-F-Al unit is linear. It is surprsing
that the Al-Br-Al bond angle in 7 is quite open (164.5◦)
as the correspondingAl-Br-Al bond angle in the anions
−
of type Al2Br7 are close to a tetrahedral bond angle
(about 108◦) [31]. However, to our knowledge the new
type of the cationic counterpart is, unique.
1
6H, N(CH3)2), 1.39 (s, 6H, N(CH3)2). – 13C{ H} NMR:
δ = 15.7 (C-3), 23.6 (C-2,4), 33.6 C(CH3)2, 40.3 [C(CH3)2],
57.8 (CMe2), AlC not observed. – 27Al NMR: δ = 104.4
(h1/2 = 80 Hz). – C10H22AlCl2 (254.18): calcd. C 47.25,
H 8.72, N 5.31; found C 46.39, H 8.49, N 5.37.
Obviously, the chemistry of tetramethylpiperidino-
alanes offers unique reaction pathways because the
bulkiness of its amino group rules out others.
Reaction of tmp2AlCl with antimony pentachloride
Experimental Section
All experiments were conducted by using the Schlenk
technique with Ar as a protecting gas. tmp2AlBr and (20 ml) was added at −50 C a solution of SbCl5 (0.72 g,
To a solution of tmp2AlCl (820 mg, 2.40 mmol) in toluene
◦
Unauthenticated
Download Date | 11/4/19 4:24 AM

K. Knabel – H. No¨th · A Contribution to the Chemistry of 2,2,6,6-Tetramethylpiperidino Aluminium Compounds
1033
2.40 mmol) in toluene (10 ml). NMR spectra showed no re- had disappeared. After reduction of the volume of the so-
action at this temperature. At room temperature the solution lution to 5 ml and cooling to −30 ◦C colorless prisms
turned orange, and after 3 d a dark red solution was present formed within 2 d (Yield of 4: 90 mg, 0.16 mmol, 80%);
containing a black precipitate (most likely Sb). A 27Al NMR m. p. 110 ◦C dec. – 27Al NMR (in C6D6): δ = 10.2 (broad),
spectrum revealed the presence of AlCl4− at δ = 103.0 ppm 54.2 (h1/2 = 230 Hz). – IR (nujol, hostraflon): ν = 2425 w,
(h1/2 = 25 Hz).
2489 w, 1629 w, 1509 w, 1480 w, 1470 m, 1450 st, 1430 w,
1419 w, 1370 w, 1358 st, 1326 m, 1301 m, 1251 st, 1209 w,
1195 st, 1175 st, 1140 w, 1043 st, 987 w, 952 st, 914 w,
863 st, 834 w, 694 m, 682 m, 599 m, 570 m, 433 st, 407 st,
326 w (cm−1).
Reaction of tmp2AlBr with tris(pentafluorophenyl)borane
To a solution of tmp2AlBr (30 mg, 0.08 mmol) in toluene
(1 ml) was added B(C6F5)3 (40 mg, 0.08 mmol) dissolved
in toluene (5 ml). The mixture was stirred for three weeks
and samples taken for NMR investigation once a weak. No
reaction was observed both by 11B and 27Al NMR. Addition
of some diethyl ether did not induce a reaction.
Reaction of di(tert.butyl)aluminiumbromide with lithium
(2,2,6,6-tetramethylpiperidide) and CO2
To tmpH (0.71 g, 5 mmol) was added dropwise and
with stirring a solution of LiBu in hexane (3.2 ml, 1.56 M,
5.0 mmol). After keeping the solution for 1 h at reflux the
yellow solution was diluted with hexane (20 ml) and cooled
Reaction of tmp2AlMe with tris(pentafluorophenyl)-borane
To tmp2AlMe (30 mg, 0.09 mmol) dissolved in toluene in dry ice. Then a solution of tBu2AlBr (1.10 g, 5 mmol) dis-
(1 ml) was added a solution of B(C6F5)3 (40 mg, 0.08 mmol)
in toluene (5 ml). After stirring for some days no reaction
solved in hexane (20 ml) was added and the mixture slowly
warmed to ambient temperature. After stirring over night the
was observed by 11B and 27Al NMR. The solution was then white solid that had formed (LiBr) was removed by filtration.
kept at reflux for 24 h. Subsequently several 11B NMR sig- Then ∼ 50% of the solvent were removed in vacuo and the
nals were observed (δ = −25.2 (?), −12.0 (MeB(C6F5)3−),
solution stored in dry ice. After several weeks 90 mg (5%)
41.6 (main signal, br, B(C6F5)3), 45.6 (tmp2BMe), of colorless crystals of 7 had formed, which were char-
71.0 (MeB(C6F5)2), 80.8 (Me2BC6F5) ppm), but no signal acterized by X-ray structure determination. – 1H NMR
in the 27Al NMR spectrum.
(CDCl3): δ = 0.95 (CMe3), 1.25 (m, CH2-2,4), 1.35 (m,
CH2-3), 1.51 (s, CCH3)2. – 13C{1H} NMR: δ = 17.7 (CH2-
4), 29,9 (C(CH3)3), 30.9 (C(CH3)2), 31.8 (C(CH3)2),
37.6 (CH2-2,4), 33.4 (CMe3), 51.6 (CMe2). – 27Al NMR:
δ = 53 (h1/2 = 3500 Hz), 154 (h1/2 = 3000 Hz).
Reaction of tmp2AlPh with tris(pentafluorophenyl)-borane
A mixture of of tmp2AlPh (50 mg) and B(C6F5)3
(70 mg,◦0.013 mmol) in toluene (20 ml) showed no reaction
at −50 C. After the suspension had attained room temper-
ature it was stirred for 24 h. Then its 11B NMR spectrum
showed the absence of B(C6F5)3. The solution was reduced
stepwise in its volume at room temperature. No crystals ap-
peared on cooling to −78 ◦C. After removal of all the solvent
a sticky oil remained. Neither a bulb to bulb microdistilla-
tion nor attempts to get crystals from hexane or diethyl ether
solution lead to a pure product.
NMR of the residue: 1H NMR (in d8-toluene): δ =
1.34 (m, CH2-2,4), 1.38 (s, C(CH3)2, 1.67 (m, CH2-3),
7.02 (m), 7.26 (m), 7.35 (t, 3J (H,H) = 4 Hz), 7.98 (d,
3J (H,H) = 4 Hz). – 11B NMR: δ = −14.4 (40%,
tmpB(C6F5)3−), −13.9 (5%, ?), −5.2 (55%, B(C6F5)4−). –
No 27Al NMR signal detectable.
X-ray structure determinations
Table 2 shows relevant crystallographic data and gives
information on data collection and structure solution. Se-
lected crystals were mounted on a glass fibre◦and placed on
the goniometer head. After cooling to −80 C five sets of
15 frames were recorded, and the unit cell determined from
the strongest reflections using the program SMART [32].
Data collection (1200 frames) was performed in the hemi-
sphere mode of the program SMART. The data were reduced
with the program SAINT [32] and the structures solved
with direct methods as implemented in SHELX97 [33].
Non hydrogen atoms were refined anisotropically, the hy-
drogen positions were placed in calculated positions except
NH hydrogens. Their positions were taken from the differ-
ence Fourier plot and refined isotropically. In compound 7
the Me groups of one tBu group in the anion were re-
fined in split positions. Additional data are deposited with
the Cambridge Crystallographic data centre and can be ob-
Reaction of 2,2,6,6-tetramethylpiperidino(tert.-butylphos-
phinidene)borane aluminium tribromide adduct with borane
in THF
To a yellow solution of tmp-B=(PtBu)AlBr3 (100 mg, tained via www.ccdc.cam.ac.uk/data request/cif or E-mail:
◦
0.2 mmol) in toluene (10 ml) was added at −30 C with deposit@ccdc.cam.ac.uk from the Director free of charge by
stirring a solution of BH3 in THF (0.20 ml of a 1 M solu- quoting the CCD numbers 273041 - 274043 and the literature
tion). On warming to ambient temperature the yellow color citation.
Unauthenticated
Download Date | 11/4/19 4:24 AM

1034
K. Knabel – H. No¨th · A Contribution to the Chemistry of 2,2,6,6-Tetramethylpiperidino Aluminium Compounds
Table 2. Relevant crystallo-
graphic data, and informa-
tion concerning data collection
and structure solution of com-
pounds 3, 5, and 7.
Compound
Chem. formula
Form wght.
3
5
7
C10H22AlCl2N
254.17
C16H32Al2Br6O4
488.98
0.30×0.30×0.40
monoclinic
P2/c
11.0956(9)
13.2397(1)
14.404(1)
90.00
107.060(1)
90.00
2022.8(3)
2
1.606
6.025
948
C62H126Al4BrN3O2
1197.49
0.35×0.28×0.19
orthorhombic
Pccn
14.2980(7)
19.634(1)
26.003(2)
90.00
90.00
90.00
7299.5(7)
4
1.090
Cryst. size [mm]
Cryst. system
Space group
0.20×0.20×0.22
monoclinic
P2(1)/n
8.2785(5)
14.2676(8)
11.5332(7)
90.00
92.424(1)
90.00
1361.0(1)
4
a
˚
w−1 = σ2F02 + (xP)2 + yP; P =
a [A]
(F02 +2F 2)/3.
˚
b [A]
c
˚
c [A]
α [◦]
β [◦]
γ [◦]
3
˚
V [A ]
Z
ρ(calcd.) [mg/m3]
1.240
0.509
544
µ [mm−1
F(000)
]
0.655
2616
Index range
−11 ≤ h ≤ 9
−17 ≤ k ≤ 18
−14 ≤ l ≤ 14
57.72
183
7729
2525
2175
0.0168
−14 ≤ h ≤ 11
−16 ≤ k ≤ 16
−17 ≤ l ≤ 17
57.90
193(2)
11267
3329
2211
0.0449
−16 ≤ h ≤ 16
−22 ≤ k ≤ 22
−29 ≤ l ≤ 29
47.96
200(2)
36118
5705
2833
0.0794
2θ [◦]
Temp. [K]
Refl. collected
Refl. unique
Refl. observed (4σ)
R(int)
No. variables
Weighting schemea
x/y
136
0.0143/0.064
191
0.0370/0.0330
341
0.1000/0.000
GOOF
1.044
1.096
1.172
Final R(4σ)
Final wR2
Larg. res. peak [e/A ]
0.0275
0.0685
0.232
0.0643
0.1682
0.940
0.0817
0.2087
1.152
3
˚
Acknowledgements
We thank Dr. D. Petrowski and Dr. T. Habereder for the
data collection, Mrs. E. Kiesewetter for recording the IR
spectra and Mr. P. Mayer for many NMR spectra.
[1] C. Klein, H. No¨th, M. Tacke, M. Thomann, Angew.
Chem. 105, 923 (1993); Angew. Chem. Int. Edit. Engl.
32, 886 (1993).
[2] I. Krossing, H. No¨th, H. Schwenk-Kircher. Chem. Ber.
Receuil 130, 1047 (1997).
[9] A. Ahmed, W. Schwarz, H. Hess, Z. Naturforsch. 33b,
43 (1978).
[10] J. K. Gilbert, J. D. Smith, J. Chem. Soc. (A)233 (1968).
[11] P. Wolfgardt, PhD Thesis, University of Munich
(1975).
[3] I. Krossing, H. No¨th, H. Schwenk-Kircher, Europ. J.
Inorg. Chem. 927 (1988).
[12] K. Jones, M. F. Lappert, Proc. Chem. Soc. 22
(1964).
[4] I. Krossing, H. No¨th, H. Schwenk-Kircher, T. Seifert,
C. Tacke, Europ. J. Inorg. Chem. 1925 (1998).
[5] K. Knabel, I. Krossing, H. No¨th, H. Schwenk-Kircher,
M. Schmidt-Amelunxen, T. Seifert, Europ. J. Inorg.
Chem. 1095 (1998).
[6] W. Marconi, A. Mazzei, F. Bonati, M. de Malde, Gazz.
Chim. Ital. 92, 1062 (1962).
[7] A. Mazzei, F. Bonati, W. Marconi, G. Manara,
G. Castelfranchi, Ital. Patent 684 (1967).
[13] H. No¨th, P. Otto, W. Storch, Chem. Ber. 118, 3020
(1985).
[14] T. A. Gorge, M. F. Lappert, Chem. Comm. 14, 463
(1966).
[15] Provided that this compound is monomeric one has to
expect a very broad signal. This could be the reason
why we failed to spot a second Al compound.
[16] C. E. Radzewich, I. A. Guzei, R. F. Jordan, J. Am.
Chem. Soc. 121, 8673 (1999).
[8] J. K. Ruff, J. Org. Chem. 83, 2835 (1961).
Unauthenticated
Download Date | 11/4/19 4:24 AM

K. Knabel – H. No¨th · A Contribution to the Chemistry of 2,2,6,6-Tetramethylpiperidino Aluminium Compounds
1035
[17] K. Knabel. T. M. Klapo¨tke, H. No¨th, R. T. Paine,
I. Schwab, Europ. J. Inorg. Chem. 1099 (2005).
[18] U. Braun, B. Bo¨ck, H. No¨th, I. Schwab, M. Schwartz,
S. Weber, U. Wietelmann, Eur. J. Inorg. Chem. 3612
(2004).
[25] I. Krossing, PhD thesis, University of Munich (1997).
[26] J. Derouault, M. T. G. Forel, Inorg. Chem. 16, 3207
(1977).
[27] N. C. Means, C. M. Means, S. G. F. Bott, J. L. Atwood,
Inorg. Chem. 26, 1466 (1986).
[19] K. Knabel. H. No¨th, Z. Naturforsch. 60b, 1 (2005).
[20] R. Staudigl, H. No¨th, Inorg. Chem. 21, 706 (1986).
[21] A. J. Downs (ed): The Chemistry of Aluminium, Gal-
[28] P. Konrad, H. No¨th, Chem. Ber. 116, 32 (1983).
[29] G. Allegra, G. Perego, Acta Crystallogr. 16, 185
(1963).
lium, Indium and Thallium, pp. 481 – 485, Chapmann [30] J. L. Atwood, W. R. Newberry III, J. Organomet.
and Hall, New York (1993).
Chem. 66, 185 (1974).
[22] J. L. Atwood, G. A. Koutsantonis, F. G. Lee, L. Raston,
Chem. Comm. 91 (1994) and lit. cited therein.
[23] P. Ko¨lle, G. Linti, H. No¨th, G. L. Wood, C. K. Narula,
R. T. Paine, Chem. Ber. 121, 871 (1988).
[31] E. Rytter, B. E. D. Rytter, J. Krogh Moe, Acta Crystal-
logr. 8, 1541 (1973).
[32] SMART, SAINT, Siemens Analytical Instruments,
Version 5.2, Madison (1995).
[24] A. H. Cowley, M. C. Cushner, R. E. Davies, P. Riely,
Inorg. Chem. 20, 1179 (1981)
[33] SHELX 97, G. W. Sheldrick, University of Go¨ttingen
(1997).
Unauthenticated
Download Date | 11/4/19 4:24 AM