The haloboration of tri-tert-butylazadiboriridine NB2R3: Ring opening versus NB3 cluster formation

Article abstract of DOI:10.1002/(sici)1521-3749(200006)626:6<1349::aid-zaac1349>3.0.co;2-q

The azadiboriridine [-BR-NR-BR-] (1; R = tBu) is bromoborated at the B-B bond by alkyldibromoboranes R′BBr₂ to give the products Br-BR-NR=BR-BR′-Br (8a-g: R′ = Me, Bu, iBu, Bzl, CH₂CHEt₂, CH₂Cy, CH₂(4

Full text of DOI:10.1002/(sici)1521-3749(200006)626:6<1349::aid-zaac1349>3.0.co;2-q

The Haloboration of Tri-tert-butylazadiboriridine NB2R3:
Ring Opening versus NB3 Cluster Formation
M. MuÈller, J. MuÈller and P. Paetzold*
Aachen, Institut fuÈr Anorganische Chemie der Technischen Hochschule
K. Radacki
Â
Â
Gdansk/Poland, Department of Chemistry of the Politechnika Gdanska
Received January 4th, 2000
Dedicated to Professor Heinrich Vahrenkamp on the Occasion of his 60th Birthday
Abstract. The azadiboriridine [±BR±NR±BR±] (1; R = tBu)
is bromoborated at the B±B bond by alkyldibromoboranes
R'BBr2 to give the products Br±BR±NR=BR±BR'±Br
(8 a±g: R' = Me, Bu, iBu, Bzl, CH2CHEt2, CH2Cy,
CH2(4-C6H4tBu)). Two isomers of each of the products 8 a±g
are formed and attributed to a cis/trans isomerism at
the BN double bond; the isomerization is followed thermo-
dynamically and kinetically by NMR methods with 8 a±d.
according to a preceding dismutation into BCl3 and BH2Cl.
The configuration at the B3 atom of the nido-clusters
NB3R3H2X (X = H, Cl) is discussed on the basis of the cor-
responding model molecules NB3Me3H2X, whose structure
and NMR signals are computed by the B3LYP method. The
boranes 8 b±g can be debrominated with Li in the presence
of tmen on applying ultrasound. The products are found to
be the B-borylated azadiboriridines [±BR±NR±B(BRR')±]
(9 b±g). The 2-borylazadiboriridines NB3H4 (9 h) and
NB3Me4 (9 i) were found as local minima on the energy
hyperface by the B3LYP method, but minima for structural
isomers with lower energy were also found; the tetrahedral
clusters NB3R4 give high-energy minima with triplet ground
states. Computations of the 11B NMR shifts of 9 h and 9 i
support the proposed structures of 9 b±g.
The analogous chloroboration of
1 with BCl3 yields
Cl±BR±NR=BR±BCl2 (8 h), which at ambient temperature
undergoes a degenerate exchange of the ligands Cl and BCl2
along the B±N±B skeleton. At room temperature, the isomer
Cl±BR±NR=BCl±BR±Cl (8 h') is slowly formed by an irre-
versible exchange of R and Cl along the B±B bond of 8 h.
Different from BCl3, the chloroborane BH2Cl is simply
added to the B±B bond of 1 under formation of the aza-
nido-tetraborane NB3R3H2Cl (2 b). The chloroborane BHCl2
gives a mixture of 8 h' and 2 b upon addition to 1, apparently
Keywords: Boron; Azadiboriridine; Haloboration; Azatetra-
boranes; exo/endo Configuration; Ab initio computations
Die Haloborierung von Tri-tert-butylazadiboriridin NB2R3:
Ringoffnung oder Bildung von NB -Clustern
È
3
InhaltsuÈbersicht. Das Azadiboriridin [±BR±NR±BR±] (1;
R = tBu) laÈût sich mit Alkyldibromboranen R'BBr2
an der B±B-Bindung unter Bildung der Produkte
Br±BR±NR=BR±BR'±Br (8 a±g: R' = Me, Bu, iBu, Bzl,
CH2CHEt2, CH2Cy, CH2(4-C6H4tBu)) bromoborieren. Zwei
Isomere werden fuÈr jedes der Produkte 8 a±g beobachtet
und einer cis/trans-Isomerie an der BN-Doppelbindung zu-
geschrieben; im Falle von 8 a±d wurde die Isomerisierung
mit NMR-Methoden kinetisch und thermodynamisch ver-
folgt. Die analoge Chloroborierung von 1 mit BCl3 ergibt
Cl±BR±NR=BR±BCl2 (8 h), das bei Raumtemperatur einem
entarteten Austausch der Liganden Cl und BCl2 entlang
dem B±N±B-GeruÈst unterliegt. Bei Raumtemperatur bildet
sich aus 8 h langsam das Isomere Cl±BR±NR=BCl±BR±Cl
(8 h') durch einen irreversiblen Austausch von R und Cl ent-
lang der B±B-Bindung von 8 h. Anders als BCl3 addiert sich
das Chlorboran BH2Cl lediglich an die B±B-Bindung von 1,
wobei das Aza-nido-tetraboran NB3R3H2Cl (2 b) gebildet
wird. Das Chlorboran BHCl2 ergibt bei der Addition an 1
eine Mischung von 8 h' und 2 b, der offenbar eine Dismutie-
rung von BHCl2 in BCl3 und BH2Cl vorausgeht. Die Kon-
figuration am Atom B3 der nido-Cluster NB3R3H2X (X =
H, Cl) wird auf der Grundlage von ModellmolekuÈlen
NB3Me3H2X diskutiert, deren Struktur und NMR-Signale
mit dem B3LYP-Verfahren berechnet wurden. Die Borane
8 b±g koÈnnen mit Li in der Gegenwart von tmen und unter
Anwendung von Ultraschall debromiert werden; dabei ent-
stehen die B-borylierten Azadiboriridine [±BR±NR±B(BRR')±
]
(9 b±g). Die 2-Borylazadiboriridine NB3H4 (9 h) und
NB3Me4 (9 i) wurden mittels der B3LYP-Methode als lokale
Minima auf der EnergiehyperflaÈche gefunden, allerdings
neben den Minima geringerer Energie von Isomeren anderer
Struktur; die tetraedrisch gebauten Cluster NB3R4 entspre-
chen Minima hoher Energie mit Triplett-Grundzustand. Die
fuÈr 9 h und 9 i berechneten NMR-Verschiebungen bestaÈtigen
die fuÈr 9 b±g angenommene Struktur.
* Prof. Dr. P. Paetzold,
Institut fuÈr Anorganische Chemie,
Technische Hochschule Aachen,
D-52056 Aachen, Germany
E-mail: peter.paetzold@ac.rwth-aachen.de
Z. Anorg. Allg. Chem. 2000, 626, 1349±1360 Ó WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 0044±2313/00/6261349±1360 $ 17.50+.50/0 1349

M. MuÈller, J. MuÈller, P. Paetzold, K. Radacki
Introduction
cycloaddition at the two BN double bonds to yield the
nido-cluster 6 in a sterospecific way [5±7].
The azadiboriridine NB2R3 (1; R = tBu) adds Lewis
acids A across its basic B±B bond under formation of
a bent bicyclic NB2A skeleton [1]. In terms of loca-
lized molecular orbitals, the B±B (2c2e) r-bond of 1 is
transformed into a (3c2e) BBA r-bond. The octet rule
is obeyed, when a (3c2e) BNB p-bond is present in
NB2R3 as well as in the adduct NB2R3A, but the p-
bonding in the adduct is mixed with some r-bonding
because of the bent NB2A skeleton.
Finally, a Lewis acid XBH2 with a stericly demanding
and p-electron donating group X might completely
open the B±B bond of 1 to yield a B±N±B±B±X open-
chain product of type 7; the degenerate rearrange-
ment (1) was observed by NMR measurements at
elevated temperature, which includes an exchange of
the groups H and BHX along the B±N±B chain
(X = NiPr2) [3].
In the case of boranes XBH2 as the Lewis acids A,
three types of adducts have been observed: the simple
adduct 2 (X = H: 2 a) [2], the H-bridged adduct 3
(X = Ph, sBu) [3], and the X-bridged adduct 4 is
formed, when a lone pair is available at X (X = NHPr,
NHtBu, NMe2, NEt2, SPr) [3, 4]. The 11B NMR sig-
nals had turned out to be characteristic for each of
the three types (Table 1). Note that the (3c2e) BNB p-
bond in 1 and 2 must be replaced by a (2c2e) BN p-
bond in 3 and 4 in order to meet the octet rule.
In the present paper, we first report on the reaction of
1 with haloboranes and discuss the products in terms
of the above mentioned products 2, 3, 4, and 7, using
NMR spectra as the source of structural information
(Table 1). We then discuss the configuration of the
atom B3 in type 2 and 3 products, making use of den-
sity functional calculations on model molecules. Final-
Table 1 11B NMR shifts of the atoms B1, B2, B3 in the
cluster molecules 2, 3, 4, 7 [2±4]
Equilibria between type 2 and 3 adducts were
detected in solution with X = alkyl R' (R' = Me, tBu,
CMe2CHMe2) [3] and also between a type 2 and 4
adduct with X = SPh [4]. The type 3 compound
NB3H2R4 (R = tBu) was crystallized from the mixture
at low temperature and structurally analyzed; upon
dissolving, the equilibrium mixture was promptly
formed again [3].
2
X
H
Me
tBu
CMe2CHMe2
SPh
B1, B2
B3
32.9
±14.1
29.0
±2.6
28.0
6.8
27.6
8.0
29.6
±3.4
3
X
Me
tBu
CMe2CHMe2
Ph
sBua)
B1
B2
B3
22.6
32.5
1.9
22.6
38.7
12.8
24.7
40.1
8.0
20.7
31.7
6.0
24.3/24.7
31.6/31.6
3.3/4.5
Azadiboriridines NB2R2R', in which at least one
group R = tBu is replaced by a stericly less demanding
group R', are unstable with respect to a dimerization.
The B±B bond of one molecule NB2R2R' is attacked
by the BR' moiety of a second molecule, which acts as
the Lewis acid, followed by the same reaction of the
two starting molecular units with reversed roles, giving
finally the tetracyclic molecule 5 (R' = CH2CMe3) [5].
With a series of ligands R' (e. g. R' = iPr), the product
5 may undergo an antarafacial intramolecular (2 + 2)
4
X
SPh
SPr
NHPr
NHtBu NMe2
NEt2
B1
B2
B3
2.6
35.4
±22.1
±0.9
34.5
±20.3
±0.8
37.4
±22.4
±7.7
38.9
±6.1
37.1
±5.3
36.7
±17.5
±27.4
±15.7
7
X
NiPr2 (65 °C)
NiPr2 (±10 °C)
30.4, 41.0b)
44.6
B1, B2
B3
34.4
45.1
a)
Diastereomers. b) Assignment B1/B2 uncertain.
1350
Z. Anorg. Allg. Chem. 2000, 626, 1349±1360

The Haloboration of Tri-tert-butylazadiboriridine NB2R3
ly, we report on molecules NB3R3R', formed from the
bromoboration products by debromination, and again
we discuss the structure by comparing to computed
models.
(ca. 0.25 mol/l) in the temperature range ±40 to 35 °C
(8 a), ±10 to 70 °C (8 b), and 24 to 90 °C (8 c, d); lower
temperatures than the mentioned ones make the iso-
merization too slow for reaching the equilibrium state
within reasonable time. Equilibrium constants can be
concluded from the intensity ratios at different tem-
peratures. The isomerization is endothermic and endo-
tropic when going from the majority to the minority
isomer. The corresponding DH/DS values are (in
kJ mol±1 and J mol±1, respectively): 4.7/7.7 (8 a), 3.5/
6.6 (8 b), 3.6/3.6 (8 c), and 2.9/8.1 (8 d).
Reaction of NB2R3 with Bromoboranes R'BBr2
Dibromoorganoboranes give a quantitative 1 : 1 addi-
tion to NB2R3 (1) under opening of the B±B bond
[Eq. (2)].
The structures of 8 a±g are deduced from the
NMR spectra. The NMR signals of the
Br(R)B±N(R)±B(R)±BBr± fragment, common to all
members of 8 a±g, are presented in Table 2, those of
the varying R' fragments in Tables 3 and 4. The assign-
ment of the R' NMR signals is straight forward, in
accordance with general experience. Noteworthy is
that the two methylene protons of iBu, Bzl and
CH2(C6H4tBu) (8 c, d, g; Bzl = Benzyl) are not equiva-
lent (1H NMR); the same is true for the methyl
groups of iBu (8 c), the ethyl groups of CH2CHEt2
(8 e), and the CH2 couples 2/6 and 3/5 of Cy (8 f;
Cy = Cyclohexyl) (13C NMR). The 11B NMR signals
are found at about d = 64, 42, and 88. These are
assigned to the atoms B1, B2, B3 in the chain
Br±B1(R)±NR=B2(R)±B3(R')±Br. The formulation of a
single bond B1±N implies that the bond planes around
B1 and N should be almost orthogonal to one another.
A comparable situation had been assumed for diboryl-
amines Hal±B1(R')±N(R@)±B2(R@')±Hal with stericly
demanding organic groups, exhibiting 11B NMR sig-
nals for B1 of about d = 64 [7]. The signal assigned
to B2 of 8 can be compared to the signal of B2 in
H±NR=B(2)(R)±B(3)(R)±Br (R = tBu) at d = 47.5
[8], whereas the signal assigned to B3 of 8 fits to the
signal d = 88.0 of Br±BR±BR±Br (R = tBu) [9]. More-
over, the atoms B2 and B3 are related by cross-peaks
in the 2 D-11B/11B NMR spectra, detected at 80 °C in
[D8]toluene with 8 a±d. (Half widths of 500±900 Hz of
the 11B NMR signals of 8 hinders the observation of
2 D peaks at room temperature.)
Upon mixing the components at ±78 °C, an inter-
mediate product of unknown structure is formed,
that exhibits a single broad 11B NMR signal at ca.
54 ppm. It is transformed into the products 8 after
hours at room temperature or, in the case of 8 c, e±g,
in refluxing pentane. The raw material is obtained
with ca. 95% yield in a purity of more than 90%, ac-
cording to the NMR spectra. The products 8 a, c, d
are pure after crystallization from hydrocarbons at
±78 °C, and the thermally most stable among the
products, 8 b, can be purified by high vaccuo conden-
sation at 120 °C. Long-term storing is only possible
at low temperature.
1
Two sets of H and 13C NMR signals are observed
with intensities that depend on temperature. This
points to an equilibrium between isomers. We ob-
served the 1H and 13C NMR spectra in [D8]toluene
Table 2 1H, 11B, and 13C NMR shifts of the Br(R)B±N(R)±B(R)±BBr± fragment of both isomers of 8 a±g at 24 °C (majority
isomer noted first)
8 a
8 b
8 c
8 d
8 e
8 f
8 g
1H(R)a)
1H(R)a)
1H(R)a)
11B(B1)
(B2)
1.14/1.16
1.09c)
1.18/1.19
64.7
42.7
1.15/1.17
1.12/1.13
1.20/1.22
64.6
42.5
1.13/1.16
1.11/1.12
1.21/1.22
64.2
42.3
0.77/0.87
1.11/1.05
1.16/1.12
63.9
40.9
1.13/1.12
1.15/1.18
1.22/1.24
63.8
42.0
1.12/1.11
1.13/1.16
1.22/1.25
64.0
42.2
1.10/1.16b)
1.17/1.21
1.25/1.26
63.9
40.6
(B3)
87.6
88.4
89.2
86.3
89.8
89.5
87.0
13C(Me)a)
13C(Me)a)
13C(Me)a)
(NC)d)
29.41/29.32
29.29/29.33
29.53/29.73
32.68/32.82
59.49/58.65
29.13/29.32
29.38/29.23
32.56/33.00
58.30/58.48
29.26/28.94
29.79/29.55
32.25/32.52
58.66/58.57
29.28/29.25
29.32/29.35
32.67/33.01
58.27/58.54
29.21/29.29
29.31/29.34
32.56/32.99
58.29/58.45
28.94/29.28b)
29.66/29.85
32.16/32.50
58.53/58.64
29.36c)
32.83/33.02
58.56/58.70
a)
No assignment of R to B1, B2, N possible. b) Because of the isomer ratio 1 : 1, the three signal couples for R in 8 g are noted in the order of increasing
shift values without assignment to isomers. c) Broad signals, close to coalescence. d) 13C NMR signals of BC not detected.
Z. Anorg. Allg. Chem. 2000, 626, 1349±1360
1351

M. MuÈller, J. MuÈller, P. Paetzold, K. Radacki
The opening of the B±B bond of 1 by bromobora-
tion, according to Eq. (2), is comparable to the men-
tioned formation of 7 by hydroboration of 1 with
H±BHNiPr2, but the B=N double bond seems to be in
a different position: B1 = N in 7, but N=B2 in 8, pre-
sumably caused by differences in the steric situation.
There are at least four reasonable possibilities how
the structures of the two isomers of 8 could be
described. An exchange of Br and BR'Br along the
B1±N±B2 chain of 8, corresponding to the H/BHX ex-
change of 7, can be excluded, because the isomeriza-
tion would then be degenerate with K = 1 at any tem-
perature. a) An exchange of Br and the amino group
NR(BRBr) along the B2±B3 skeleton cannot be ex-
cluded. b) Conformational isomers could be described
by the obvious assumption, that the two boryl groups
BRBr and BR'Br define bond planes orthogonal to
the central plane B1±N±B2±B3. The normal ligand
conformation in diborane(4) derivatives is staggered
and the same is true for one of the aminoborane frag-
ments in stericly crowded diborylamines, as men-
tioned above. The groups R (at B1) and R' in syn or
anti position of 8 then cause conformational isomer-
ism. c) An exchange of the boryl groups BRBr and
BR'Br along the N=B2 skeleton can also not be ex-
cluded. d) A fourth possibility, the most likely one, is
the cis/trans isomerism with respect to the B=N dou-
ble bond, well established in the aminoborane chemis-
try for a long time.
Table 3 1H NMR shifts of the groups R' of both isomers of
8 c, d, ga) (majority isomer noted first)
8 c
8 d
8 g
±CH2CHMe2
dd mc d
±CH2Ph
±CH2C6H4R
d
m
d
m
s
1.66/1.67 ddb)
1.14 (dd)e)
2.13 mc
2.81/2.73 dc)
3.04/3.04 dc)
6.95±7.22 m
2.74/2.85 dd)
3.06/3.08 dd)
7.09±7.30 m
0.77/0.91 s
1.02/1.03 d
a)
In addition 8 a: d = 0.98/1.00 (s); 8 b: d = 0.80±1.78; 8 e: d = 0.8±1.9; 8 f:
d = 0.8±2.1. b) 2J = 16.0/17.0 Hz, 3J = 9.5/9.5 Hz. c) 2J = 16.2/16.8 Hz. d) 2J =
16.5/16.5 Hz; the four doublets of 8 g are noted arbitrarity without assign-
ment to isomers (1 : 1 ratio). e) Hidden under tBu peak, but observable as
2 D-1H/1H cross-peak with d = 1.66, 1.67.
With respect to the NMR time scale, the isomeriza-
tion rate is rapid enough to cause an observable coa-
lescence of the corresponding NMR signals above
room temperature in the case of products 8 with the
less bulky ligands R' (8 a±d). We followed the kinetics
of the isomerization with four signals of 8 a and calcu-
lated the free activation energy in both directions
(Table 5) by an established method (Treatment of
Unequally Populated Doublets [10]). The resulting
average values, DG# = 70.9 and 68.5 kJ mol±1 in the
two directions, are in agreement with a cis/trans iso-
merization, which is found in normal aminoboranes in
a range of 70±100 kJ mol±1 [11, 12].
We tried to react the azadiboriridine 1 with boranes
R'BBr2, whose alkyl groups R' are either a-branched
or tertiary in b-position (R' = iPr, tBu, CH2tBu,
CH2SiMe3). No reaction took place at low tempera-
ture and decomposition into unknown products was
observed upon long-term heating.
Table 4 13C NMR shifts of the groups R' of both isomers of
8 b±g (majority isomer noted first)
8 ba)
8 c
8 da)
±CH2CH2CH2Me
±CH2CHMe2
±CH2C(CHCH)2CH
t
t
q
br
d
q
s
d
d
d
30.64/30.95 t
22.60/26.57 t
14.00/14.16 q
40.16 br
142.66/143.00 s
125.15/124.88 d
128.93/128.63 d
128.93/129.19 d
28.39/28.46 d
25.53/25.63 q
27.36/27.20 q
8 ea)
8 f
8 gb)
±CH2CH(CH2Me)2
±CH2CH(CH2CH2)2CH2 ±CH2C(CHCH)2CCMe3
br br s s q
38.47 br 25.09 br
d
t
q
d
t
t
t
s d d
40.40/40.94 d
28.53/28.74 t
28.83/29.03 t
11.10/11.20 q
11.31/11.49 q
37.98/38.02 d
26.55/26.55 t
26.75/26.92 t
27.05/27.23 t
36.35/36.38 t
37.73/37.70 t
34.25/34.28 s
139.67/139.93 s
147.38/147.70 s
125.53/125.90 d
128.56/128.92 d
31.55/31.55 q
a)
13C NMR signal of BC not detected. b) Notation of shift couples with
increasing values and without assignment (1 : 1 isomer ratio).
Reaction of NB2R3 with Chloroboranes
Trichloroborane BCl3 quantitatively chloroborates
NB2R3 (1) at low temperature according to Eq. (3),
comparable to the bromoboration (2). The raw mate-
rial contains the product 8 h in more than 95% purity,
according to the NMR spectra; the product decom-
poses on attempts of further purification. The
11B NMR signals at ±60 °C (d = 56.6, 49.6, 71.3 for B1,
B2, and B3) are not far away from those of the dibro-
mo species 8 (Table 2). There are no indications for
isomers like in the case of the dibromo species, but a
degenerate isomerization is concluded from the NMR
Table 5 Coalescence temperature Tc ( °C), shift difference
Dm (Hz) of isomers at ±80 °C, equilibrium constants K at Tc,
and free activation enthalpies DG# (kJ mol±1) in both direc-
tions of the isomerization of 8 a with respect to four selected
chemical shifts d (measured at Tc)
Tc
Dm
K
DG#
d(1H) = 1.18
d(1H) = 0.99
d(13C) = 68.63
d(13C) = 32.99
45
50
50
57
17.5
33.0
30.4
38.3
2.34
2.28
2.28
2.19
70.9/68.6
70.2/68.0
71.4/68.3
71.2/69.2
1352
Z. Anorg. Allg. Chem. 2000, 626, 1349±1360

The Haloboration of Tri-tert-butylazadiboriridine NB2R3
spectra at room temperature giving, e. g., one
11B NMR signal at d = 51.4 for B1 and B2. We suggest
that a BCl2/Cl exchange takes place [Eq. (4)], compar-
able to the BHX/H exchange in the case of 7
(X = NiPr2 [3]). A bicyclic NB3 cluster of type 2 would
be a reasonable intermediate of that exchange.
The reaction of 1 with BHCl2 (added as the SMe2
adduct) gives a mixture of NB3R3Cl3 (8 h') and
NB3R3H2Cl (2 b), showing that BCl2H undergoes a
dismutation into BCl3 and BH2Cl during this reaction.
The product 8 h' can be purified by crystallization
from hexane. A partial dismutation is also observed
during the reaction of BHClR'(SMe2) (R' =
CMe2CHMe2) with 1. A part of this borane gives the
type 3 adduct NB3R3R'HCl with H in a bridging posi-
tion; another part gives a dismutation into BH2R' and
BCl2R'. We could not detect BCl2R' among the pro-
ducts, but BH2R' adds to 1 in the known manner to
give a 13 : 1 equilibrium mixture of the corresponding
type 3 and type 2 adduct [3]. The type 3 product
NB3R3R'HCl could clearly be identified in a mixture
with those two BH2R' adducts by typical NMR shifts.
The three 11B NMR shifts of NB3R3R'HCl correspond
closely to NB3R3R'H2 of type 3 (Table 1). The
2 D-11B/1H cross-peaks between the signals of l-H
and both, B1 and B3, prove the bridging character of
this H atom.
The product 8 h is not stable at room temperature. It
rearranges to give 8 h' by an irreversible R/Cl ex-
change at the B2±B3 chain fragment [Eq. (5)]. The
molar ratio of 8 h/8 h' is 0.8 after 60 min at 24 °C, and
the rearrangement is complete after 60 min at 60 °C.
The 11B NMR signals (d = 64.3, 39.1, 80.5) agree with
those typical for 8, and the signals of B2 (d = 39.1) and
B3 (d = 80.5) are related by a 2 D-11B/11B cross-peak.
No indications for isomers like in the case of the di-
bromo species of type 8 were found.
Different from BCl3, the addition of BH2Cl (ap-
plied as the SMe2 adduct) to 1 gives the simple 1 : 1
adduct of type 2 (NB3R3H2X, X = Cl: 2 b), isolated as
a glassy colourless solid at ±75 °C, which undergoes
slow decomposition at room temperature. The purity
is ca. 80%. The structure can be deduced from the
NMR spectra: tBu groups and B atoms in the ratio
2 : 1 and an 11B NMR triplet for the BH2 group with a
coupling constant in the typical range of cluster endo-
H atoms (85 Hz) are observed. A 2 D-11B/1H cross-
peak is found for B3, but not for B1 and B2, indicat-
ing that the H atoms of BH2 are not in a bridging po-
sition. The two 11B NMR signals are related by a
cross-peak; they fit well to the corresponding signals
of the comparable type 2 product NB2R3H3 (Tables 1,
6).
Attempts to react BHClR(thf) with 1 yielded BCl2R
and the known adduct NB3R4H2 in the known type 2/
type 3 equilibrium [3] in the course of a complete
dismutation of BHClR into BH2R and BCl2R (R =
tBu).
The Configuration at B3 of the Clusters NB3R3H2X
The NB3 skeleton of the nido-clusters NB3R3H2X of
type 2 and 3 forms a bent bicyclobutane-type structure,
which is typical for four-vertex nido-species derived
from the corrresponding trigonal bipyramidal closo-
structure. Interplanar angles between the two skeletal
triangles of 137.4° (X = H, type 2) and 139.8° (X = R,
type 3) were found in the crystal [2, 3]. The bent struc-
ture defines an exo/endo alternative for the position
of X. An endo-structure of type 2 (Cs) includes two
exo-H atoms at one cluster vertex, which means an
unfamiliar situation in oligoborane chemistry. In the
case X = H, type 2, we define exo/endo with respect to
the H atom in the mirror plane, whereas the exo/endo
Z. Anorg. Allg. Chem. 2000, 626, 1349±1360
1353

M. MuÈller, J. MuÈller, P. Paetzold, K. Radacki
alternative remains undefined in the case of X = H,
type 3.
Fig. 1 Calculated potential energy surface (kJ mol±1) of
NB3H6 as depending from the interplanar angle (u) and the
ring angle B1±B2±B3 (s) relative to the energy minimum in
the type 3 structure (u = 145°, s = 58°).
thoroughly tried to find a type 2 endo-structure on the
energy hyperface in the case of the parent compound
2 c, but could not find any stationary point; this corre-
sponds to the mentioned general lack of finding two
exo-H atoms at one vertex of borane clusters. How do
we then explain the observed equilibria between
NB3R3H2R' of type 2 and 3 [3] (R = tBu; for alkyl
groups R' see X in Table 1)? The observed type 2 iso-
mers should actually be type 3 exo-isomers, which un-
dergo a rapid enantiomerization, as pointed out
[Eq. (8)]. The observed type 3 isomers, on the other
hand, are actually present as type 3 endo-isomers, es-
tablished in the crystal by X-ray analysis. Obviously,
they cannot undergo an enantiomerization, since an
adequate transition state of the type 2 endo-configura-
tion is not easily available. The question remains, how
the type 3 molecules NB3R3H2R' can switch from the
endo- into the exo-configuration and vice versa in an
equilibrium, observed on changing the temperature or
dissolving crystalline endo-isomer, slowly enough to
make both of the isomers observable by NMR meth-
ods. A reasonable isomerization path seems to be the
inversion of the interplanar angle u between the NB3
skeletal triangles via a planar structure. We therefore
calculated the change in energy, when the type 3
structure of the parent molecule NB3H6 is planarized,
starting from u = 145° at the minimum geometry. It
turned out that u = 180° is approached at about
48 kJ mol±1, but the reaction path becomes more
favourable, when simultaneously the ring angle
B1±B2±B3 (s; B1±B3 is the H-bridged edge) is wi-
dened, starting from the minimum value of 58°. The
We calculated the molecules NB3Me3H3 (2 c, 3 c) and
NB3Me3H2Cl (2 d, 3 d), the formal adducts of BH3
and BH2Cl to the azadiboriridine NB2Me3, by density
functional methods. Optimizations of geometry and
calculations of frequency were performed at the
B3LYP/6±31++G(d,p) level [13]. It turned out in both
cases that the hydrogen-bridged isomers 3 c and 3 d
represent minima on the potential energy hyperface,
with the Cl ligand in the exo-position of 3 d. The type
2 exo-structures, which had been deduced from NMR
data for NB3R3H2X (2 a: [2]; 2 b: preceding section),
were found as very low lying transition states, 3.5 (2 c)
and 5.6 kJ mol±1 (2 d) apart from the corresponding
ground states 3 c, d. (These ground states exhibit a
staggered conformation of N-CH3 with respect to
BH2X; the eclipsed conformation of these groups is
represented by a second order saddle point at 5.3 and
7.5 kJ mol±1, respectively.) Apparently, the type 2 exo-
transition states (Cs) allow the rapid enantiomeriza-
tion of the chiral type 3 exo-molecules [Eq. (8)], mak-
ing clear that the products formulated earlier as 2 a
and 2 b actually exhibit Cs pseudo-symmetry, even at
low temperature, and should better be designated 3 a
and 3 b in the free molecules, although the exo-2 a
structure is established in the crystal [2]. Interestingly,
a type 4 isomer of the compound NB3H5Cl with Cl in
a bridging position is also found as a minimum on the
energy hyperface, which is 35.4 kJ mol±1 apart from
the corresponding type 3 exo-ground state.
A type 3 endo-structure, undefined for 3 c, repre-
sents a minimum of potential energy in the case of
3 d, 23.2 kJ mol±1 apart from the exo-ground state. We
1354
Z. Anorg. Allg. Chem. 2000, 626, 1349±1360

The Haloboration of Tri-tert-butylazadiboriridine NB2R3
Fig. 2 Calculated structures of NB3Me3H2X in the exo-3 ground state and in the exo-2 transition state (X = H, 3 c, 2 c upper
1
values; X = Cl, 3 d, 2 d lower values): relative energies (kJ mol±1; bold); BN, BB and BX distances (pm; in rectangles); H and
11B NMR shift values (ppm; in circles; GIAO) [B3LYP/6±31++G(d,p) level for geometry and NMR data]
situation is illustrated in Fig. 1. The energy map was
obtained by a relaxed potential energy surface scan
Table 6 Comparison of NMR shift values observed for
NB3R3H2X (X = H: 2 a; X = Cl: 2 b) and calculated for
with constrained angles u and s, proceeding with steps
of 2°. A saddle point of ca. 30 kJ mol±1 is reached at
about u = 157° and s = 67° (P1 in Fig. 1). Though the
system could go down to an energy of 8.3 kJ mol±1 at
u = 180° and s = 97° (P2 in Fig. 1), the real pathway
from the saddle point to the planarized structure
(u = 180°) could be reached on a higher energy level
(s < 97°) at the temperature of experiment. Anyhow,
widening of s corresponds to opening of the B1±B3
bond. We conclude that the endo/exo isomerization of
the experimental molecules NB3R3H2R' is likely to
proceed through some B±B opening via a structure,
which is close to the products of type 8. The reaction
path from the minimum of type 3 via P1 to P2 was ad-
ditionally confirmed by the method of Intrinsic Reac-
tion Coordinates.
NB3Me3H2X (X = H: 3 c, 2 c; X = Cl: 3 d, 2 d; ground state 3,
transition state 2)
2 a
2.03
0.92
32.9
3 c
2.00
1.06
29.8
2 c
1.51
1.20 2.76
31.1 30.0
2 b
3 d
2 d
1H NMR, d(exo-H)
d(endo-H)
/
/
/
2.76
27.8
±5.8
2.86
30.2
±14.4
11B NMR, d(B1/B2)
d(B3)
±14.1 ±19.1 ±27.0 ±5.6
the observed and the calculated values are in good
concordance.
The products NB3R3H2X can be considered to be
derivatives of the parent nido-cluster NB3H6, which is
member of an isoelectronic series B4H8, CB3H7, and
NB3H6. Recently, we reported on tert-butyl derivatives
of B4H8 [15] and CB3H7 [16]. The configurational
situation of B4H8 is comparable to that of NB3H6, but
± different from NB3H6 and its derivatives ± a transi-
tion state analogous to the endo-2 type of NB3H6 is
detected for B4H8 by theory.
The optimized geometry data of exo-2 c, d and exo-
3 c, d were used as a basis for the calculation of the
NMR signals using the GIAO method [14]. Part of
the results are presented in Fig. 2.
In Table 6, we compare the observed data of
NB3tBu3H3 (exo-2 a [2], obviously better formulated
as the rapidly enantiomerizing isomer 3 a) to the data
calculated for NB3Me3H3 in the ground state (3 c, the
shift values of l-H/endo-H and B1/B2, Fig. 2, are aver-
aged) and transition state (exo-2 c), and we do the
same for NB3tBu3H2Cl (exo-2 b, see Exp. Sect.) and
NB3Me3H2Cl (exo-3 d, exo-2 d, Fig. 2). The calculated
11B NMR signals of B3 of 3 c,d/2 c,d are somewhat
high-field shifted as compared to the observed signals,
but the difference between the ligands Me and tBu
makes such shift differences reasonable. Altogether,
Formation of NB3R3R' by Debromination of 8 b±g
Upon applying ultrasound, the reduction of the bor-
anes Br±BR±NR=BR±BR'±Br (8 b±g) with lithium in
hexane in the presence of Me2NCH2CH2NMe2 (tmen)
gives products NB3R3R' (9 b±g), according to Eq. (9).
The application of ultrasound as well as of lithium
and tmen turned out to be essential. The oily products
are thermally labile, extremely sensitive to traces of
air, and are obtained in about 90% purity; the product
Z. Anorg. Allg. Chem. 2000, 626, 1349±1360
1355

M. MuÈller, J. MuÈller, P. Paetzold, K. Radacki
1
9 c (R' = iBu), however, can be purified by slow high
the H and 13C NMR signals of BR belong together
(line 2 to line 8 and line 3 to line 9 values in Table 7),
but an assignment of the two BR groups to B1 and
B3 is not possible on the basis of the data presently
available.
vacuo condensation at room temperature.
The room temperature NMR spectra do not give
evidence for non-equivalent methylene protons in the
corresponding groups R' or for non-equivalent Me
groups in the case of R' = iBu. We cannot explain this
by a bond plane around B3, which is coplanar with
the ring plane (Cs symmetry), having in mind that the
two bond planes around boron in diborane(4) deriva-
tives are mostly arranged more or less orthogonally.
We instead assume an orthogonal position of these
planes in the ground state (C1 symmetry) and a rapid
rotation around the B2±B3 bond, thus inducing Cs
pseudo-symmetry with respect to the NMR observa-
tion. In order to support this assumption, we cooled
9 c down to ±60 °C (in [D8]toluene) and found a split-
ting of the methylene proton signals into d = 1.57
(dd, 2J = 17.4 Hz, 3J = 9.5 Hz) and d = 1.27 (dd, 2J =
17.4 Hz, 3J = 4.5 Hz). The methyl proton signal re-
mained a broad pseudo-singlet on cooling, but the
methyl 13C NMR peak was transformed into two
peaks at d = 26.87 and 25.52 (±60 °C). Even the methyl
1H NMR pseudo-singlet could be resolved into two
shift values by 1H/13C HETCOR measurements at
±80 °C, giving the 1H/13C couples d = 25.50/1.07 and
d = 26.90/1.04. Coalescence of the two 13C NMR
methyl signals is observed at 0 °C (Dm = 176 Hz at
±80 °C) and, hence, an activation barrier of
DG# = 53 kJ mol±1 can be estimated [20].
A plausible explanation, by which mechanism the
debromination of type 8 products, Eq. (9), might pro-
ceed, starts from the exchange of Br and R' along the
B2±B3 bond of 8, which completely parallels the ex-
change of Cl and R, Eq. (5), in the case of the rearran-
gement of 8 h into 8 h'. The Wurtz-type debromination
would then be the plausible second step in the forma-
tion of the type 9 products. The products 9 b±g are deri-
vatives of a parent molecule NB3H4, formally, what on
naive consideration counts for a closo-cluster, presum-
ably with the C3v structure of a trigonally distorted tet-
rahedron. We have concluded quite a different structure
for 9 b±g from NMR-based arguments. So reasonable
these arguments might be, they do not finally prove the
structure. Since crystals are not available, we studied
the structure of the parent compound NB3H4 (9 h) and
of NB3Me4 (9 i) by ab initio calculations and derived
the 11B NMR shifts by the GIAO method.
The structures are concluded from the NMR spectra.
The NB3R3 fragment of NB3R3R', common to all
members in the series 9 b±g, exhibits 1H, 11B, and
13C NMR shifts, whose concordance shows that the
same type of structure is present (Table 7). The 1H
and 13C NMR signals of the R' fragment can be un-
equivocally assigned, according to general experience,
in the case of 9 c additionally by applying special
NMR techniques (see Exp. Sect.). It is evident from
the spectra, that the two fragments BR are not
equivalent and that there is a fragment NR. The
11B NMR signal at d = 99.1±99.7 points to a B atom,
that is coordinated by two alkyl and one boryl group,
comparable to the situation in diboranes R'2B±BR'2
(d ca. 105 [17, 18]). The B atoms at d(11B) = 49.8±50.3
and 54.1±55.1 seem to be similarly coordinated as
both of the B atoms of 1 (d = 51.9 [6]). There are two
cross-peaks in the 2 D-11B/11B NMR spectrum of 9 c
(65 °C), a weak one between the signals of B1 and B2
and a strong one between the signals of B2 and B3; a
weak cross-peak is expected for B atoms that are
bound together by a direct bond and additionally by a
bridging N atom [19]. All the given data are in accord
with a 2-borylazadiboriridine-type structure of 9 b±g.
The two 13C/1H-HETCOR correlations for the
13C/1H shift couples d = 31.14/1.18 (J = 145 Hz) and
56.19/1.18 (J = 5 Hz) of 9 c allow to distinguish the
1H and 13C NMR signals of NtBu from those of
BtBu, since the 13C NMR peak at d = 56.19 clearly
marks the C1 atom of NtBu, whereas the signals for
C1 of BtBu, as usual, could not be detected in the
spectra. The 13C/1H HETCOR couples d = 28.32/1.06
and 29.24/1.20, observed with 9 c, prove, which of
Table 7 1H, 11B, and 13C NMR shifts of 9 b±g (without
shifts of R')
9 b
9 c
9 d
9 e
9 f
9 g
1H (NR)
(BR)
1.19
1.05
1.20
50.3
55.1
1.18
1.06
1.20
50.1
54.9
1.08
0.94
1.12
49.8
54.3
1.19
1.07
1.19
50.2
54.9
1.19
1.07
1.21
50.2
54.9
1.17
1.11
1.22
49.9
54.1
(BR)
11B (B1)
(B2)
Theoretical Model of NB3H4 (9 h) and NB3Me4 (9 i)
(B3)
99.7
99.2
99.1
99.6
99.5
99.6
We found local minima on the potential energy hyper-
face for the six structures A±F of 9 h, i at the B3LYP/
6±31++G(d,p) level [13]. The calculated relative energies,
bond distances and NMR shifts are presented in Fig. 3.
13C (Me, NR)
(Me, BR)
(Me, BR)
(NC)
31.24
28.47
29.28
56.19
31.14
28.32
29.24
56.19
30.84
28.47
29.29
55.91
31.15
28.48
29.25
56.57
31.15
28.33
29.23
56.19
30.73
28.40
29.24
55.76
1356
Z. Anorg. Allg. Chem. 2000, 626, 1349±1360

The Haloboration of Tri-tert-butylazadiboriridine NB2R3
Fig. 3 Calculated minimum structures A±F of NB3R4 (R = H, 9 h: upper values; R = Me, 9 i: lower values): symmetry group
and relative energies (kJ mol±1; bold); BN and BB distances (pm; in rectangles); 11B NMR shift values (ppm; in circles; GIAO)
[B3LYP/6±31++G(d,p) level for geometry and NMR data]
The structure lowest in energy contains an open
chain with a very short BN bond (BN ªtriple bondº).
The structure by far worst in energy turns out to be
the trigonally distorted tetrahedron with a triplet
ground state. With the ligand set R,R,R,R' (9 b±g),
only the structures D and E fit the observed NMR
spectra, in particular the non-equivalence of all the
three B atoms and groups R. The calculated 11B NMR
shifts of D agree to the experimental values of 9 b±g,
whereas the 11B NMR shifts of E are far away from
the experimental values. The calculated values
strongly support the structure assumed for 9 b±g.
tively by assuming some overlap between the B1±B2
r-bond and the empty p-orbital of B3, which breaks
down, when this p-orbital is twisted out of the plane
of optimal overlap. In terms of localized molecular or-
bitals, the B3 atom obtains an electron octet by con-
structing an open B1±B2±B3 (3c2e) bond. This (3c2e)
bond and the BNB (3c2e) p-bond are the two (3c2e)
bonds, which follow necessarily from the orbital/elec-
tron balances of a molecule NB3H4, obeying the octet
rule. [Orbitals: Ro = 20 = 3 t + 2 y; electrons: Re =
18 = 2 t + 2 y; hence, the number of (3c2e) bonds t = 2,
the number of (2c2e) bonds y = 7.]
Presumably, the rotation of the BRR' group around
the B2±B3 axis goes through the coplanar arrange-
ment of the bond planes around B2 and B3 as the
transition state (Cs). Actually, such a coplanar struc-
ture is a transition state in the case of D, according
to our calculations, which is 46.6 kJ mol±1 apart from
D (R = H). The rotational barrier of the molecule
H2B±BH2, for comparison, had been calculated to be
52.7 kJ mol±1 [21]. The calculated geometry of that
transition state differs from D in three parameters
that are noteworthy: Going from D to the transition
state, the B1±B2 bond is shortened (163.9 ®
161.4 pm), the B2±B3 bond is lengthened (162.8 ®
171.0 pm), and the bond angle B1±B2±B3 is widened
(158.4 ® 169.2°). We explain these changes qualita-
Though the structure E looks unfamiliar, as com-
pared to conventional molecules, we considered it ser-
iously, having in mind the isoelectronic molecule
C2B2H4 and its derivatives, for which a bicyclobutane-
type unconventional structure, similar to E, had been
established by theory [22] and experiment [23, 24].
Experimental
General
All reactions were conducted under dry nitrogen. Starting
materials were commercially available or were synthesized
according to cited procedures. NMR spectra were re-
corded in solutions of C6D6 [B(CH2CHEt2)3, B(CH2Cy)3,
B[CH2(4-C6H4tBu)]3, BBr2(CH2CHEt2), BBr2Cy, 8 e±g,
Z. Anorg. Allg. Chem. 2000, 626, 1349±1360
1357

M. MuÈller, J. MuÈller, P. Paetzold, K. Radacki
NB3tBu3H2Cl, NB3tBu3HCl(CMe2CHMe2), 9 c, e±g] or
Dibromo(cyclohexylmethyl)borane: Starting from B(CH2Cy)3
(3.79 g, 12.5 mmol), BBr3 (6.25 g, 25.0 mmol) and BH3(SMe2)
(3 drops) in hexane (10 ml), the same procedure gives a
[D8]toluene
(ca.
0.25 mol/l;
8 a±d, h, h',
9 b, d)
at
499.843 MHz (1H; tms), 160.364 MHz (11B; BF3(OEt2)),
125.639 MHz (13C; tms). MS (70 eV): Finnigan MAT-95.
1
colourless liquid (5.7 g, 57%) by distillation (53 °C/0.1 Pa). H
NMR: d = 0.73, 0.98, 1.12, 1.51, 1.54, 1.69 [6 m, 2 : 2 : 2 : 2 : 2 : 1;
from 2 D-1H/13C NMR: H2/6, H4 (accidental degenerassy of
Hax and Heq), H3/5, H3/5, H2/6, H1 of Cy], 1.24 (d, J = 7 Hz,
BCH2). 11B NMR: d = 64.8. 13C NMR: d = 26.09, 26.51, 35.34,
(3 t, 1 : 2 : 2; C4, C3/5, C2/6 of Cy), 36.68 (d), 45.2 (br, BC).
Synthesis of trialkylboranes
Tris(2-ethylbutyl)borane: In analogy to a known procedure
[25], a solution of 1-bromo-2-ethylbutane (14.9 g, 90 mmol)
in Et2O (60 ml) is dropped into a mixture of Mg (4 g,
0.17 mol) and BF3(OEt2) (4.3 g, 30 mmol) in Et2O (30 ml) so
rapidly that reflux is observed. Further reflux (2 h), stirring
overnight at ambient temperature and filtration gives a solu-
tion, from which the product (5.3 g, 66%) is obtained by dis-
tillation (87 °C/0.1 Pa) as a colourless liquid. 1H NMR:
d = 0.89 (t, J = 7.5 Hz, 2 Me), 1.22 and 1.37 (2 ddq, J = 13.5,
7.0, 7.5 Hz, CHa and CHb of 2 CH2), 1.37 (d, 7.0 Hz, BCH2),
1.73 (mc, CH). 11B NMR: d = 88.1. 13C NMR: d = 11.64 (q),
29.52 (t), 34.6 (br, BC), 38.88 (d). The assignments are in
accord with 2 D-1H/1H and -1H/13C NMR spectra.
Dibromo[(4-tert-butylphenyl)methyl]borane: On the addition
of BBr3 (15.0 g, 60 mmol) to
a solution of B[CH2-
(4-C6H4tBu)]3 (10.0 g, 22 mmol) in hexane (10 ml), a red-
brown colour is observed, which disappears, when five drops
of BH3(SMe2) are added. After refluxing for 2 h, volatile
materials are removed in vacuo. Distillation (75 °C/0.1 Pa)
1
gives a colourless liquid (7.6 g, 36%). H NMR: d = 1.16, 2.71
(2 s, 9 : 2), 6.90, 7.16 (2 mc, 1 : 1). 11B NMR: d = 62.7.
13C NMR: d = 31.40 (q), 34.31 (s), 43.6 (br, BC), 125.9, 129.5
(2 d), 132.6, 148.9 (2 s).
Tris(cyclohexylmethyl)borane: This product is obtained by an
analogous procedure, starting from bromo(cyclohexyl)-
methane (25.0 g, 141 mmol), BF3(OEt2) (6.7 g, 47 mmol) and
Mg (6 g, 0.25 mol). A high vacuo condensation at a bath
temperature of 170 °C yields the colourless solid product
(8.0 g, 56%). 1H NMR: d = 0.6±1.6. 11B NMR: d = 88.
13C NMR: d = 26.86, 27.11, 36.98, 38.86 (4 t, 1 : 2 : 2 : 1),
35.66 (d).
Synthesis of [2-alkyl-2-bromo-1-tert-butyldiborane(4)yl]-
tert-butyl(tert-butylbromoboryl)amines 8 a±g
The bromoborane R'BBr2 is added to pentane or hexane
(10±20 ml) at ±78 °C (8 a±c) or vice versa (8 d±g). The solu-
tions are warmed to room temperature within 2 h and then
either stirred at this temperature (8 a, b: 2 h; 8 d: 12 h) or re-
fluxed (8 c, f, g: 8 h; 8 e: 24 h). The removal of volatile mate-
rials in vacuo gives oily products. These are dissolved in hex-
ane and filtered over kieselgur in the case of 3 e, f. The
amounts of starting materials, the yields of oily products and
the estimated purity (according to NMR spectra) are sum-
marized in Table 8. The products 8 a±d can be brought to
purity (as found by microanalysis of the elements C, H, N)
either by condensation in vacuo (8 b; oil bath temperature
120 °C) or by crystallization from hexane at ±80 °C (8 a: m. p.
25 °C; 8 c: m. p. 20 °C; 8 d: dec. at ca. 25 °C).
Tris[(4-tert-butylphenyl)methyl]borane: A similar procedure
is applied with bromo(4-tert-butylphenyl)methane (25.0 g,
107 mmol), BF3(OEt2) (5.1 g, 36 mmol) and Mg (5 g,
0.21 mol). Before filtering the magnesium salts, ether is re-
moved in vacuo and replaced by hexane (80 ml). Reversely,
hexane is then replaced by ether (70 ml). 1,2-Bis(4-tert-butyl-
phenyl)ethane crystallizes as a byproduct overnight. After
filtration and removal of the solvent, a colourless solid pro-
1
duct remains (10 g, 61%). H NMR: d = 1.25, 2.69 (2 s, 9 : 2),
6.87, 7.23 (2 mc, 1 : 1). 11B NMR: d = 81.7. 13C NMR: d 31.56
(q), 34.26 (s), 36.48 (br, BC), 125.45, 129.28 (2 d), 136.76,
147.47 (2 s).
Reaction of NB2R3 with Chloroboranes
Reaction of NB2R3 with BCl3: A solution (0.5 mol/l) of BCl3
in hexane (3.2 ml) is added to 1 (0.33 g, 1.59 mmol) in pen-
tane (15 ml) at ±78 °C. The solution is brought to ambient
temperature within 15 min. Volatile materials are removed
in high vacuo at ±78 °C. The remaining product 8 h can
Synthesis of alkyldibromoboranes
Benzyldibromoborane: Analogous to a known procedure
[26], a solution of BBr3 (55 g, 0.22 mol) and B(CH2Ph)3 [27]
(28.4 g, 10.0 mmol) in toluene is stirred for 2 d in the pre-
sence of 2 g 9-borabicyclo[3.3.1]nonane. The catalyst is then
deactived by the addition of 0.5 g 1-octene. Distillation
(106 °C/15 mbar) gives the pure product (60 g, 76%) as a
colourless liquid.
Table 8 Synthetic data of the products 8 a±e: mass and mo-
lar amount of the starting materials 1 (m1, n1) and R'BBr2
(m2, n2), yield of products (m3, n3/n1)a) and purity (according
to NMR spectra)a)
Dibromo(2-ethylbutyl)borane: A mixture of B(CH2CHEt2)3
(4.82 g, 18.1 mmol), BBr3 (9.07 g, 36.2 mmol), and
BH3(SMe2) (0.07 g) in pentane (15 ml) is refluxed (3 h). A
distillation (50 °C/4 mbar) yields a colourless liquid (8.0 g,
Product
8 a
8 b
0.79
3.82
0.87
3.82
1.51
8 c
8 d
8 e
8 f
8 g
m1 [g]
n1 [mmol]
m2 [g]
n2 [mmol]
m3 [g]
n3/n1 [%]
1.00
4.80
0.90
4.80
1.85
2.76
13.34
3.04
13.34
5.75
99
0.69
3.34
0.88
3.36
1.50
0.97
4.70
1.20
4.70
1.98
1.45
7.00
1.98
7.00
3.10
1.28
6.18
1.96
6.17
3.18
1
58%). H NMR: d = 0.70 (t, J = 7.5 Hz, 2 Me), 1.06 and 1.18
(2 ddq, 13.8, 6.5, 7.5 Hz, CHa and CHb of 2 CH2), 1.28 (d,
7.0 Hz, BCH2), 1.74 (mc, CH). 11B NMR: d = 64.8. 13C NMR:
d = 11.35 (q), 28.48 (t), 40.41 (d), 41.44 (br, BC). The assign-
ments are in accord with 2 D-1H/1H and -1H/13C NMR spec-
tra.
98
91
100
97
95
93
95
98
95
Purity [%] 95
95
85
a)
The yields in% are calculated under the assumption that the impurities
have the same molar mass as the products.
1358
Z. Anorg. Allg. Chem. 2000, 626, 1349±1360

The Haloboration of Tri-tert-butylazadiboriridine NB2R3
be characterized by NMR (purity > 90%). 1H NMR: d
(24 °C) = 1.05, 11.24 (2 s, 2 : 1); d (±60 °C) = 1.00, 1.04, 1.15
(3 s, 1 : 1 : 1). 11B NMR: d (24 °C) = 51.4 (x1/2 = 640 Hz, 2 B),
71.3 (x1/2 = 320 Hz, 1 B); d (±60 °C) = 49.6 (x1/2 > 1000 Hz,
1 B), 56.6 (x1/2 > 1000 Hz, 1 B), 71.4 (x1/2 > 100 Hz, 1 B).
13C NMR: d (24 °C) = 29.47, 33.59 (2 q, 2 : 1), 56.71 (s); d
(±60 °C) = 29.05, 29.37, 33.30 (3 q), 56.76 (s, NC). ± The pro-
duct 8 h can be stored at ±80 °C, but slowly rearranges into
8 h' at room temperature. The rearrangement is complete
the starting material BHCl(CMe2CHMe2), a mixture of
NB3H2R3(CMe2CHMe2) of type 3 and type 2 in the known
equilibrium ratio of 13 : 1 [3], and NB3R3HCl(CMe2CHMe2).
The oily mixture is dissolved in pentane. A 4 : 1 mixture
of NB3R3HCl(CMe2CHMe2) and NB3R3H2(CMe2CHMe2)
crystallizes in the course of 14 d at ±78 °C. We describe the
1
NMR data of NB3R3HCl(CMe2CHMe2). H NMR: d = 0.61,
3
1.33 (2 s, BCMe2), 0.92, 1.04 (2 d, J = 6.7 Hz, CCMe2), 1.21,
1.22, 1.23 (3 s, tBu), 2.28 (sept, 3J = 6.7 Hz, CH), 2.71
(x1/2 = 160 Hz, l-H). 11B NMR: d = 13.2 (br, B3) 24.0 (B1),
37.5 (B2).
1
after 60 min at 60 °C (purity > 90%). H NMR: d = 1.12, 1.13,
1.15 (3 s, 1 : 1 : 1). 11B NMR: d = 39.1, 64.3, 80.5 (3 s, 1 : 1 : 1).
MS: m/z = 266 (27%, M±Bu), 210 (20%, M±Bu±C4H8), 57
(100%, C4H9+) etc.
Reaction of NB2R3 with BHClR(thf): We first obtained a so-
lution of BHClR(thf) (R = tBu) by adding ethereal HCl
(2.5 ml, 0.5 mol/l) to Li[BH3R] in thf (1.2 ml, 0.96 mol/l) at
±78 °C. The solution is brought to room temperature
(20 min). Further stirring (3 h) gives NMR spectroscopically
pure BHClR(thf). 1H NMR: d = 0.74 (tBu), 3.10 (2 D-
11B/1H HMQC cross-peak). 11B NMR: d = 13.7 (d, J =
134 Hz). 13C NMR: d = 28.23 (CH3). On storing solutions of
BHClR(thf) for several hours, decomposition is observed,
presumably by the formation of RBCl2 and subsequent clea-
vage of thf. The dismutation of BHClR'L into (R'BH2)2 and
R'BCl2 had been observed as an equilibrium reaction with
different bases L in the case of R' = CMe2iPr [29]. An equi-
molar amount of 1 (1.15 mmol) in thf (1 ml) is added to the
solution of BHClR(thf), obtained as described, at ±78 °C.
After warming to ambient temperature (30 min) and further
stirring (2 h), the solvents are removed and [D8]thf is added.
Unreacted 1 and the type 2/type 3 equilibrium mixture of
NB3R4H2 are identified by their known NMR signals [3].
The signals of RBCl2 had been detected in the primary reac-
tion mixture; during the removal of the solvents the borane
RBCl2 is consumed by ether cleavage.
Reaction of NB2R3 with BH2Cl(SMe2): The commercially
available sulfane-borane contains 70% BH2Cl(SMe2) (11B
NMR: d ±6.7, t, J = 131 Hz), 15% BHCl2(SMe2) (11B NMR:
d = 2.2, d, J = 157 Hz), and 15% BH3(SMe2) (11B NMR:
d ±20.0, q, J = 104 Hz), as is known from the literature [28].
This sulfane-borane mixture (0.27 g, 2.5 mmol) was added
to a solution of 1 (0.51 g, 2.5 mmol) in pentane (15 ml) at
±78 °C. White flocs are precipitated. After 2 h stirring at
±78 °C, volatiles are removed in high vacuo. The product is
volatile in the high vacuo at an oil bath temperature of 70 °C
and is condensed at ±78 °C as a glassy solid, which becomes
oily at room temperature and undergoes slow decomposi-
tion. Besides NB3R3H2Cl as the main product, the product
mixture contains ca. 10% NB3R3H3 (formed from 1 and
BH3(SMe2) and identified by the known NMR signals [2])
and ca. 8% of an unknown product [11B NMR: d = 6.4, 15.4
(2 d, J = 90 and 42 Hz); 1H NMR = 3.27, 1.31 (related by
cross-peaks to the 11B NMR peaks)]. The main NMR peaks
(82%) belong to NB3R3H2Cl. 1H NMR: d = 1.06, 1.16 (2 s,
1 : 2, tBu), 2.76 (q, 1J = 85 Hz, 2 H). 11B NMR: d = ±5.6 (t,
1J = 88 Hz, 1 B), 30.0 (s, 2 B). 13C NMR: d = 30.10, 31.91 (2 q,
2 : 1), 53.38 (NC).
Debromination of the boryl(tert-butyl)-
Reaction of NB2R3 with BHCl2(SMe2): Commercially avail-
able BHCl2(SMe2) (0.36 g, 2.46 mmol) is added to 1 (0.51 g,
2.46 mmol) in hexane (8 ml) at ±78 °C. Stirring for 2 h gives
a clear solution, whose volume is reduced in vacuo (4 ml).
The product 8 h' crystallizes at ±78 °C within 2 d. The mother
liquor is removed with the aid of a syringe. The solid is re-
crystallized at ±78 °C, first from 10 ml, then from 20 ml hex-
ane. The product 8 h' is pure, according to NMR spectra
(0.20 g, 25%). Another portion of 8 h' (0.15 g 18%) can be
obtained from the united mother liquors by removing vola-
tile materials in vacuo and subliming 8 h' at an oil bath tem-
perature of 40 °C in the high vacuo. The product 8 h' is iden-
tified by its NMR spectra. It can be stored at ±80 °C, but
4 months storing at ±27 °C gives 40% decomposition.
The mother liquor contains a considerable amount of
NB3R3H2Cl identified by its NMR signals. It is removed
from 8 h' in vacuo together with the solvent.
diboranylamines 8
The products 8 b±g (ca. 5 mmol) and tmen (ca. 10 mmol) are
dissolved in hexane (10±20 ml). A colourless precipitate is
formed. A 15fold excess of lithium powder is added. Ultra
sound is applied to the mixture at room temperature as long
as starting material is present, which is followed by
11B NMR measurements. Filtration over kieselgur at ±30 °C
and subsequent removal of volatiles in high vacuo give the
oily products 9 b±g in ca. 90% purity, according to 11B NMR
signals. Attempts to crystallize the products are not success-
ful, and decomposition is observed on attempts to heat them
for distillation. The only product obtained in purity is 9 c,
which slowly condenses into a cooled receiver at 24 °C/
0.1 Pa (57% yield). The products 9 can be stored at ±80 °C
for days, but decompose at room temperature, if not dis-
solved in hydrocarbons. The NMR data of 9 b±g are pre-
sented in Table 6, with the exception of the data for R',
which are mentioned here. 1H NMR: d(9 b) = 0.89 (t, Me),
0.85±1.50 (C3H6); d(9 c) = 1.02 (d, Me), 1.46 (br, CH2), 1.80
(mc, CH); the signals d = 1.02/1.80 are related by a cross-
peak in the 2 D-1H/1H spectrum; irradiation at d =
1.80 makes 1H-TOCSY signals observable at d = 1.02 and
1.46, showing that the broad signal at d = 1.46 represents
both of the CH2 protons; d(9 d) = 2.29 (br, CH2), 6.90, 6.95
(2 mc, Ph); d(9 e) = 0.90 (t, Me), 1.0±1.9 (CH2CH(CH2)2);
d(9 f) = 0.9±2.0; d(9 g) = 0.92 (s, tBu), 2.85 (br, CH2), 6.98,
Reaction of NB2R3 with BHCl(CMe2CHMe2)(SMe2): This
sulfane-borane adduct is synthesized according to a known
procedure [29] (11B NMR in CD2Cl2: d = 7.8, 1J = 128 Hz;
1H NMR from 2 D-11B/1H: d = 3.1). The azadiboriridine 1
(0.43 g, 2.1 mmol) in hexane (8 ml) is added to the sulfane-
borane in CH2Cl2 (2.7 ml, 1 mol/l) at ±78 °C. Stirring at room
temperature (3 h) and subsequent removal of volatiles in
1
vacuo give an oily liquid, whose H and 11B NMR spectra in
CD2Cl2 reveal, that a mixture of four components is present:
Z. Anorg. Allg. Chem. 2000, 626, 1349±1360
1359

M. MuÈller, J. MuÈller, P. Paetzold, K. Radacki
7.13 (2 mc, C6H4). 13C NMR: d(9 b) = 14.35 (q), 26.96 (t),
31.24 (t); d(9 c) = 26.08 (br), 28.91 (d), 41.10 (br, BC);
2 D-13C/1H HETCOR peaks (J = 145 Hz) prove relations
within the 13C/1H couples d = 26.08/1.02 and d = 28.91/1.80,
[8] W. Rattay, Dissertation, UniversitaÈt MuÈnchen, 1987.
[9] W. Biffar, H. NoÈth, H. Pommerening, Angew. Chem.
1980, 92, 63; Angew. Chem. Int. Ed. Engl. 1980, 19, 56.
[10] J. SandstroÈm, Dynamic NMR Spectroscopy, Second
Printing, Acad. Press, London, 1984.
and
a
relation d = 41.10/1.46 is established by 2 D-
13C/1H HMQC, thus confirming all the assignments given for
iBu; d(9 d) = 37.40 (t), 124.37, 128.23, 128.55 (3 d), 144.67 (s);
d(9 e) = 11.93 (q), 29.38 (t), 35.53 (br, BC), 42.32 (d);
d(9 f) = 26.68, 27.19 (2 t), 36.85 (br, C2/6 of Cy), 38.93 (d),
39,34 (br, BC); d(9 g) = 31.55 (q, s; accidental degenerassy of
both of the tBu signals), 36.78 (br, BC), 125.10, 128.19 (2 d),
141.51, 146.72 (2 s). MS(9 c): 275.31270 (obs.), 275.31269
(calc. for M+). MS(9 d): 252.22665 (obs.), 252.22661 (calc. for
M±C4H9).
[11] C. Brown, R. H. Cragg, T. J. Miller, D. O. Smith, J. Or-
ganomet. Chem. 1983, 244, 209.
[12] A. J. Ashe III, W. Klein, R. Rousseau, J. Organomet.
Chem. 1994, 468, 21.
[13] Gaussian 98, Revision A.6, M. J. Frisch, G. W. Trucks,
H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Chee-
seman, V. G. Zakrzewski, J. A. Montgomery, Jr., R. E.
Stratmann, J. C. Burant, S. Dapprich, J. M. Millam,
A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas,
J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci,
C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A.
Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Ma-
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shenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W.
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[14] R. Ditchfield, Mol. Physics 1974, 27, 789.
[15] A. Neu, T. Mennekes, U. Englert, M. Hofmann, P. von
Rague Schleyer, Inorg. Chim. Acta 1999, 289, 58.
[16] A. Neu, K. Radacki, P. Paetzold, Angew. Chem. 1999,
111, 1358; Angew. Chem. Int. Ed. 1999, 38, 1281.
[17] H. NoÈth, B. Wrackmeyer in P. Diehl et al. (Eds.) NMR,
Basic Principles and Progress, Vol. 14, Springer, Berlin
1978.
Ab initio calculations
The GAUSSIAN 98 package [13], run on a cluster of work-
stations (Rechenzentrum der RWTH Aachen), was applied
for all calculations. The total energies Eh and ZPVE (in par-
entheses), all calculated on the B3LYP/6±31++G(d,p) level,
are as follows (in Hartrees): NB3H6: exo-2 ±132.953560
(0.071968), exo-3 ±132.954643 (0.073227), P2 (Fig. 1)
±132.951477 (0.071646), P1 (Fig. 1) ±132.943151 (0.072384);
NB3Me3H3: exo-2 ±250.933736 (0.155864), exo-3 ±250.935373
(0.157371); NB3H5Cl: exo-2 ±592.590250 (0.065081), exo-3
±592.592394 (0.066407), endo-4 ±592.578909 (0.065553), endo-3
±592.584792 (0.066734); NB3Me3H2Cl: exo-2 ±710.573843
(0.149039),
exo-3
±710.575969
(0.150579),
endo-3
±710.567150 (0.151032); NB3H4: A ±131.731454 (0.049558),
B
D
F
±131.727217 (0.050556),
±131.700598 (0.051704),
±131.623541 (0.051472); NB3Me4:
C
E
±131.708100 (0.053350),
±131.708037 (0.052683),
A
C
E
±289.076671
±289.027189
±289.012076
(0.162974),
(0.165577),
(0.166198),
B
D
±289.074689 (0.163562),
±289.019674 (0.165028),
[18] B. Wrackmeyer in G. A. Webb (Ed.) Annual Reports on
NMR Spectroscopy, Vol. 20, Academic Press, London
1988.
F
±288.942284 (0.164986). Calculated energies
are mentioned in the preceding sections without ZPVE.
[19] D. F. Gaines, G. M. Edvenson, T. G. Hill, B. R. Adams,
Inorg. Chem. 1987, 26, 1813.
We gratefully acknowledge the support of this work by
Deutsche Forschungsgemeinschaft and Fonds der Che-
mischen Industrie. We are also grateful to the Rechenzen-
trum of our university for generously placing computation
time at our disposal.
[20] M. Hesse, H. Meier, B. Zeeh, Spektroskopische Metho-
den in der organischen Chemie, 4th ed., Thieme, Stutt-
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