Novel tetraalkyltetraboranes of the type B4R4, B4H2R4 and B4H4R4

Article abstract of DOI:10.1016/s0020-1693(99)00058-4

The reduction of RBHal₂ (Hal=F, Cl, Br) with Na/K alloy gives the tetraboranes B₄R₄, B₄H₂R₄ or their mixtures, depending on temperature, reaction time and solvent. We describe here B₄^tBu₄, B₄(CH₂^tBu)₄, B₄H₂^tBu₄, B₄H₂(CH₂^tBu)₄ and B₄H₂(CMe₂Et)₄. B₄^tBu₂(CH₂^tBu) ₂ is formed by reducing the diborane Cl-B(^tBu)B(CH₂^tBu)-Cl with Li. The conversion of B₄H₂^tBu₄ into B₄H₄^tBu₄ proceeds in three steps: (i) reduction (→[B₄H₂^tBu₄]²);(ii) first protonation (→[B₄H₃^tBu₄]^-), and (iii) second protonation (→B₄H₄^tBu₄). The hypercloso-species B₄R₄ have tetrahedral structures. The two opposite edges of a B₄ tetrahedron of the closo-B₄H₂R₄ are bridged by H atoms (D_2d). The nido-species have a bicyclobutane-type B₄ skeleton; three of the four outside edges are bridged by H atoms; two terminal ligands are bound to one of the B atoms: H in exo- and^tBu in endo-position (C₁). Structural evidence is based on low-temperature NMR spectra, ab initio geometries, and IGLO(DZ) calculations of the¹¹B NMR shifts. Dynamical behavior of bridging H atoms is indicated by the NMR spectra at higher temperature.

Full text of DOI:10.1016/s0020-1693(99)00058-4

Inorganica Chimica Acta 289 (1999) 58–69
Novel tetraalkyltetraboranes of the type B4R4,
ꢀ
B4H2R4 and B4H4R4
Andreas Neu a, Theo Mennekes a, Peter Paetzold a,*, Ulli Englert a,
Matthias Hofmann b, Paul von Rague´ Schleyer b
a Institut fu¨r Anorganische Chemie, Technische Hochschule Aachen, D-52056 Aachen, Germany
b Center for Computational Chemistry, Uni6ersity of Georgia, Athens, GA 30602-2525, USA
Received 5 October 1998; received in revised form 20 January 1999
Abstract
The reduction of RBHal2 (Hal=F, Cl, Br) with Na/K alloy gives the tetraboranes B4R4, B4H2R4 or their mixtures, depending
on temperature, reaction time and solvent. We describe here B4tBu4, B4(CH2tBu)4, B4H2tBu4, B4H2(CH2tBu)4 and B4H2(CMe2Et)4.
B4tBu2(CH2tBu)2 is formed by reducing the diborane Cl–B(tBu)–B(CH2tBu)–Cl with Li. The conversion of B4H2tBu4 into B4H4tBu4
proceeds in three steps: (i) reduction (“[B4H2tBu4]2−); (ii) first protonation (“[B4H3tBu4]−), and (iii) second protonation
(“B4H4tBu4). The hypercloso-species B4R4 have tetrahedral structures. The two opposite edges of a B4 tetrahedron of the
closo-B4H2R4 are bridged by H atoms (D2d). The nido-species have a bicyclobutane-type B4 skeleton; three of the four outside
t
edges are bridged by H atoms; two terminal ligands are bound to one of the B atoms: H in exo- and Bu in endo-position (C1).
Structural evidence is based on low-temperature NMR spectra, ab initio geometries, and IGLO(DZ) calculations of the 11B NMR
shifts. Dynamical behavior of bridging H atoms is indicated by the NMR spectra at higher temperature. © 1999 Elsevier Science
S.A. All rights reserved.
Keywords: Tetraboranes; Tetraboratetrahedranes; Tetraborabicyclobutanes; Boron clusters
1. Introduction
1.1. B4H4 and deri6ati6es
Five parent tetraboranes can be distinguished by
cluster prefixes based on the formal electron count:
hypercloso-B4H4, closo-B4H6, nido-B4H8, arachno-
B4H10, and hypho-B4H12. Only one, B4H10, is known as
a storable substance [1]. (Evidence for B4H6 and B4H8
as reaction intermediates is discussed below.) Deriva-
tives of the parent molecules can be formed formally by
replacing H atoms with substituents X (F, OH, NH2,
CH3 etc.), by replacing two H atoms with a Lewis base
L, or by adding or removing a proton to give a cationic
or anionic species.
B4X4 (X=H, F, Cl, Br, I) molecules have been
computed ab initio at different levels (see [2–4] for a
selection of more recent work). B4H4 has been calcu-
lated by using full configuration interaction on the
MP2/6–31G* geometry [3]. The global minimum has a
planar bicyclobutane-type B4 skeleton with a bridging
H atom (Cs). This structure is 27 kJ mol−1 lower in
energy than the tetrahedral isomer (Td). A high-energy
non-planar cyclobutane-type B4 structure represents a
third shallow minimum (D2d).
ꢀ This paper is dedicated to Professor Peter Jutzi, on the occasion
of his 60th birthday.
* Corresponding author. Tel.: +49-241-80 4644; fax: +49-241-
8888 288.
E-mail address: peter.paetzold@ac.rwth-aachen.de (P. Paetzold)
The B4X4 (X"H) derivatives are predicted to have a
Td symmetry ground state. Four molecular orbitals,
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00058-4

A. Neu et al. / Inorganica Chimica Acta 289 (1999) 58–69
59
localized as (3c2e) bonds above the tetrahedral faces,
describe the bonding situation in good approximation
[2d,h]; electron localization functions (ELF) nicely il-
lustrate this situation [2d].
tetrahedral singlet is expected for the doubly deproto-
nated species [B4H4]2−, because only two electrons
are available to occupy the degenerate HOMO (e).
Consequently, a Jahn–Teller distortion from Td to
D2d and a large difference in the B–B bond distances
(152.6 versus 187.5 pm) [4] result. (Note that tetrahe-
dral symmetry is possible in B4H4 with empty and in
C4H4 with completely occupied e orbitals or in triplet
[B4H4]2− with single occupations.) The geometric situ-
ation is modified only slightly by replacing the termi-
Three derivatives had been isolated up to now:
B4Cl4, B4Br4 and B4tBu4. B4Cl4 is available from BCl3
or B2Cl4 by glow discharge [5a–d]. B4Br4 and B4tBu4
are obtained from B4Cl4 by exchanging Cl with BBr3
in the presence of AlBr3 [6] or with LitBu in pentane
t
[7], respectively. The reduction of BuBF2 by Na/K
gave crystalline B4tBu4 [8]. B4Cl4 and B4tBu4 exhibit
D2d and C2 symmetry, respectively, in the crystalline
state [8,9], but are tetrahedral in the gas phase, ac-
cording to ED measurements (B4Cl4: Td, 170.3(1.1)
pm (BB) [10]; B4tBu4: T, 170.4(4) pm (BB) [11]). The
HOMO/LUMO transitions (4t2“2e) [12] are found
in the UV spectra at 245 nm (X=Cl, log m=4.25
[13]) and 283 nm (X=tBu, log m=3.7 [8]). The 11B
NMR signals of B4Cl4 and B4tBu4 are shifted to ex-
tremely low field (85 [7] and 135.4 ppm [8]), in accord
with values calculated by the MC-IGLO [2e] and
GIAO-MP2 method [2f]. The photo electron spectrum
of B4Cl4 [14] is also well reproduced by theory [15].
The yellow crystals of both B4Cl4 and B4tBu4 are
stable up to 150°C. The chloroborane B4Cl4 under-
goes reactions with hydridic species [16]: B4H10 is ob-
tained with Me3SnH, a mixture of B5H9 and B6H10
with LiBH4, and a mixture of the boranes B6H6Cl4
and B10H14−nCln from B2H6.
nal
H
atoms by methyl groups in B4H2Me4,
[B4HMe4]− and in going from B4H2Me4 to
B4HMe4Li (C26) with a bridging Li [4].
Laser radiation of B5H9 gives the unstable
molecules B4H6 and BH3 as the primary fragments;
the final products include B10H14 [17]. The reaction of
B5H9 with alkynes RCCR in the presence of bases L,
leading to C2B4H8R2 and BH3L [18], proceeds possi-
bly via B4H6 as an intermediate.
We describe here the synthesis of stable derivatives
B4H2R4 of B4H6. One of these, B4H2tBu4, was the sub-
ject of a preliminary report [4]. We also reported
B4HtBu4[Li(thp)2] (thp=tetrahydropyrane) and its
crystal structure [4].
1.3. B4H8 and deri6ati6es
Theoretical studies have established the nature of
B4H8 [19]. Five structures correspond to minima in
energy; four of them represent B4 bicyclobutane-type
skeletons, differing in the distribution of the four
non-exo H atoms over endo and bridging positions,
and one structure represents a borylcyclotriborane
skeleton [19a].
We describe here the synthesis of further organo-
tetraboranes of the B4R4 type and also B4R2R%.
2
1.2. B4H6 and deri6ati6es
The ground-state structure of B4H6 is derived from
the tetrahedral B4H4 structure by bridging two oppo-
site edges with hydrogen. Remarkably, the two
bridged edges are shorter (162.3 pm) than the four
non-bridged ones (174.6 pm) (geometry optimized at
the MP2(fc)/6–31G* level). Three additional minima
of the bicyclobutane type were located, but are higher
in energy [4].
Only the low-energy structures of B4H6 and B4H8
allow all four B atoms to follow the octet rule ac-
cording to the localized molecular orbital (LMO) de-
scription. The prerequisites are fulfilled, i.e. the
balances ꢀo=3t+2y and ꢀe=2t+2y, where ꢀo
and ꢀe represent the sum of valence orbitals and
electrons and t and y the number of (3c2e) and (2c2e)
bonds, respectively. No (1c2e) lone pairs, (4c2e)
bonds or other multicenter bonds are assumed to be
involved. A t/y=4/5 value is found for B4H6 and
t/y=4/6 for B4H8; the B–Hexo bonds are included in
y.
The symmetry is reduced to C26 by removing one
of the bridging protons from an edge. This results in
[B4H5]− with a remarkably short BB distance oppo-
site to the bridged tetrahedral edge (153.2 pm). No

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A. Neu et al. / Inorganica Chimica Acta 289 (1999) 58–69
fluctional behavior, if present, has been analyzed. Ab
initio/IGLO/NMR studies have been carried out on
several of these species [24a,26,27].
The classical routes [23] to obtain B4H10 are the
protolysis of MgB2 by hydrochloric or phosphoric acid,
the thermolysis of B2H6 (at controlled pressure and
temperature conditions, particularly in a so-called hot–
cold reactor, also via silent or flow discharge), the
reductive coupling of B2H5I by sodium, or the protoly-
sis of [B3H8]− by HCl or H3PO4. More recent proce-
dures involve the reaction of NaB3H8 either with BX3
(X=F, Cl) [35] or with a mixture of NaBH4 and I2
(90% yield) [36].
Indirect experimental evidence for B4H8 comes from
the reaction of B4H10 with C2H2 [20] as well as from the
thermolyses of B4H10 and B4H8(CO) [21]. According to
kinetic studies the rate determining first step is the
elimination of H2 or CO to give B4H8 as a short-lived
intermediate. The experimental activation barrier for
the elimination of H2 (99.2 kJ mol−1) is not far from
that calculated (128.4 kJ mol−1 [22]). The H/D ex-
change with B4H10 can also be explained by assuming
elimination as the first step [21]. B4H8 is assumed to
react bimolecularly to give isolable B5H11 and short-
lived B3H5 intermediate.
The reactions of B4H8 with ammonia (“B4H8(NH3)),
ethyne (“2,4-B4H8(CHꢀCH)) and ethene (“2,4-
B4H8(CH2–CH2)) were studied by ab initio methods [22].
Earlier experimental indications of the formation of
B4H6X2 (X=NMe2) or B4H6L (L=NMe3, PPh3) [23]
species need further elucidation.
The triborate [B3H8]− also provides access to B4H10
derivatives: 2-B4H9Br (with HBr and BBr3 in two steps
[37]), 1,2:2,3-m-(NMe2)2B4H8 (with (Me2N)2BBr
[38,39]), B4H8Me2 (with Me2BCl [23]). Mono- and di-
methylated derivatives of B4H10 are available from
B4H10/HgMe2 or B3H7(thf)/B2H4Me2 or B2H4(PF3)2/
B2H4Me2 [23]. Among several routes to B4H8L deriva-
tives, the degradation of B5H11 by Lewis bases L seems
to be widely applicable, yielding a mixture of B4H8L
and BH3L [40].
Degradation by bases is the most characteristic reac-
tion of B4H10. Lewis bases L may give either an ionic
product [BH2L2]+[B3H8]− or two isomeric non-ionic
complexes, BH3L and B3H7L, depending on L and on
the temperature. In general, the ionic product is favored
at low temperature and the complexes at higher temper-
ature [23,41,42]. The situation is reminiscent of the well
known asymmetrical and symmetrical cleavage of dibo-
rane, which yields either [BH2L2]BH4 or BH3L. Anionic
bases may also cleave B4H10 into B1 and B3 fragments;
e.g. Br− gives [B3H7Br]− [43]. The triborate
[InMe2]+[B3H8]− is obtained along with B2H4Me2
from the reaction of B4H10 with InMe3, which acts as a
formal source of the base Me− [44]. The analogous
formation of [B3H8]− and BH3 would be expected with
the hydride anion; in actual fact, the reaction of B4H10
with hypho-B4H8L2 as the formal hydride source gives
[B4H7L2][B3H8] and BH3 [45]. The formation of
[B3H6L2]+[B3H8]− from B4H10 and the hypho-diborane
B2H4L2 formally involves the transfer of a BH2+ frag-
ment and thus, again implies a cleavage of B4H10 into
B1 and B3 fragments [46,47]. The ionic product
[B4H7L2]+[B3H8]−, storable at low temperature, re-
arranges into the neutral species B4H8L and B3H7L at
room temperature (r.t.), thus following the general
trend. Anionic hydrides, on the other hand, may also
deprotonate B4H10 to give [B4H9]− [33]. The addition
of bases to B4H8L or [B4H9]− gives hypho-species,
B4H8LL% (these can eliminate L to yield the nido-species
B4H8L%) [48,49] or [B4H9L]− [50], respectively. The
action of HCl on B4H8L in thf also results in a B1/B3
fragmentation to give BH3(thf) and B3H6ClL [51].
We describe here the synthesis of a B4H4R4 nido-
tetraborane and of the corresponding [B4H3R4]−
anion.
1.4. B4H10 and deri6ati6es
The well known bicyclobutane-type C26 structure of
arachno-B4H10 enjoys broad theoretical understanding
[19c,24]. The accurate hydrogen positions were first
established by the ab initio/IGLO/NMR method [24a],
and later confirmed by a gas phase electron diffraction
reinvestigation [24h]. Derivatives like 2,4-B4H8R2 (R2=
CH2–CH2 [25]) or 1-B4H8L (L=CO, PH3, PF3 [26,27])
have related structures. The dissociation energy of
B4H8(CO) into the B4H8+CO components is compara-
ble to that of BH3(CO) [27]. The endo orientation of
CO in B4H8(CO) is slightly lower in energy than the exo
[27].
Experimentally, the gas phase structures of B4H10
[24g,28], B4H8(CO) [29], B4H8(PF3) [30], and
B4H8(CH2CH2) [25] are well known from ED measure-
ments, that of B4H8(CO) by applying a 62/38 mixture
of the endo/exo isomers. A microwave study [24k] and
the crystal structure [24h,31,32] of B4H10 were also
reported. In C26 symmetry, B4H10 exhibits one shorter
(173.7(5) pm) and four longer (186.6(2) pm) B–B bonds
in the gas phase [24h]. The NMR spectra of B4H10 [23],
[B4H9−] [33], B4H8L (L=CO, PH3, PF3 [26], SMe2
[34]), B4H8(CH2CH2) [25] etc. had been recorded and

A. Neu et al. / Inorganica Chimica Acta 289 (1999) 58–69
61
The reaction of B4H10 with unsaturated hydrocar-
bons has been investigated widely. When alkenes are
added, products of the 2,4-B4H8(CHR–CHR%) type are
formed [52]. Alkynes, 1-ene-3-ynes, or allene lead to a
variety of nido- or arachno-, mono-, di-, and tricarba-
boranes [20,53].
H/X exchange is possible with certain protic agents;
e.g. thiols RSH give 1,2:1,4-m-(RS)2B4H8 [54], and
HSCH2CH2SH yields the corresponding tricyclic
product [55]. Deuterated species B4HxD10−x are
formed from B4H10 and D2 [21]. A pentaboryl group
becomes attached to B4H10, when B4H10 and B5H9 react
in the presence of PtBr2; 1:2%-B4H9-B5H9 is formed [56].
A solution of the product, (tBuCH2)BF2, in pentane
(30 cm3) boils spontaneously, when 5 g of Na/K alloy
are added. After 2 h stirring, the 11B NMR signal of the
starting material disappears. The solids are filtered and
washed with two portions of pentane (20 cm3). Volatiles
are removed from the filtrate in vacuo. A yellow liquid
is obtained by condensation (90°C, 0.001 mbar, 120
mg). Repeated crystallization from toluene at −78°C
t
gives 30 mg (2.2%; 0.33% with respect to BuCH2Br) of
1
yellow crystalline product. H NMR: l 1.09, 1.82 (2s,
9:2). 11B NMR: l 139.3. 13C NMR: l 32.4 (q), 50.1 (t).
(NMR data in C6D5CD3, Varian Unity 500.) The
filtered solid is extracted with three portions of thf (20
cm3). The solvent is removed in vacuo and the colorless
residue is washed twice with diethyl ether. The dried
material has the K[BF3(CH2tBu)] composition by C/H
1.5. B4H12 and deri6ati6es
1
elemental analysis. H NMR: l 0.90, 0.27 (2s, 9:2). 11B
Although a C26 B4H12 structure is computed to be a
metastable minimum on the potential energy surface
[57,58], isolation of this eight-membered ring with four
BHB bridges cannot be expected, since the dissociation
into B4H10 and H2 or into two B2H6 fragments is
computed to be exothermic by 91.2 and 110.0 kJ
mol−1, respectively [58]. The structures of hypho-spe-
cies of the type B4H8L2, [B4H7L3]+, and [B4H9L]− have
not yet been fully elucidated; these species had been
synthesized by the addition of L to B4H8L [48] or to
[B4H7L2]+ [59] or to [B4H9]− [50], respectively.
The B1/B3 fragmentation by Lewis bases is also ob-
served with hypho-tetraboranes: B4H8L2 yields BH3L/
B3H5L3 [60] and [B4H7L3]+ gives BH3L/[B3H4L4]+ [59].
A hypho/arachno transformation can be achieved by
hydride abstraction from B4H8L2 to give [B4H7L2]+;
hydride acceptors can be [CPh3]+ (“HCPh3),
B3H7(thf) (“[B3H8]−/thf) or B4H10 in thf (“BH3(thf)/
[B3H8]−) [45]. The corresponding transformation of
hypho-[B4H9L]− into arachno-B4H8L proceeds by ap-
plying BH3 as a hydride acceptor [50].
NMR: l 7.6 (s). 13C NMR: l 30.6 (t), 32.9 (q). (NMR
data in D8-thf, Varian Unity 500.)
t
t
2.2. B4 Bu2(CH2 Bu)2
t
The diborane(4), Bu(Me2N)B–B(NMe2)Cl [61] (5.36
g, 26.5 mmol), is added at 0°C to a solution of
LiCH2tBu in hexane (30 cm3); the lithium compound
had been freshly sublimed (120°C, 0.001 mbar). The
solution is stirred (2 h, 25°C), refluxed (3 h) and then
filtered over diatomaceous earth. Hexane is removed
from the filtrate in vacuo. A crude product (2.83 g) is
obtained from the residue by condensation. It contains
the diborane tBu(Me2N)B–B(NMe2)(CH2tBu) as the
main component in addition to the starting material
and by-products. The diborane gives characteristic
1
NMR signals. H NMR: l 1.11, 1.12, 2.60, 2.63, 2.65
(5s, 3:3:1:2:1; 10 Me). 11B NMR: l 51.5, 54.0.
The crude product is dissolved in ether (25 cm3).
Methanol (1 cm3) and then ethereal HCl (18 cm3, 1.32
mol l−1) are added at −78°C. After stirring at 0°C (2
h), ether is removed in vacuo, pentane (20 cm3) is
added, solids are filtered and washed with pentane (20
cm3), pentane is removed from the filtrate in vacuo at
2. Experimental
t
0°C, and the diborane, Bu(MeO)B–B(OMe)(CH2tBu),
t
2.1. B4(CH2 Bu)4
is obtained by condensation (1.51 g; about 10% of the
product is found to be B2tBu(OMe)3 by characteristic
A Grignard solution (180 cm3) is prepared from
tBuCH2Br (50 g, 0.33 mol) and Mg (8.5, 0.35 mol) in
diethyl ether. The solution is added to B(OMe)3 (51.5 g,
0.50 mol) in ether (300 cm3) at −78°C. After 12 h
stirring, HCl (100 cm3, 6 mol l−1) is added, and the
mixture is again stirred (1 h). The ethereal layer is
separated and ether is removed. Gaseous BF3 is intro-
duced into the remaining viscous liquid until the gas is
no longer dissolved. The mixture is heated up to 120°C.
The product, (tBuCH2)BF2, is finally distilled at 46°C
(5.9 g, 15%). 1H NMR: l 0.83, 0.67 (2s, 9:2). 11B NMR:
1
NMR data). H NMR: l 1.0, 1.1, 3.5 (3s, 3:3:2; 8 Me).
11B NMR: l 64.5.
All of the product is now dissolved in pentane (10
cm3) and BCl3 (2.5 g, 21.3 mmol) is added at −78°C.
After stirring the mixture at 0°C (2 h), the solvent is
removed at 0°C in vacuo. A white solid (1.28 g) is
sublimed (25°C, 0.001 mbar). This contains the dibo-
rane tBu(Cl)B–BCl(CH2tBu) in a \90% purity, accord-
ing to NMR data. The product must be stored at
1
−78°C. H NMR: l 0.93, 1.01, 1.57 (3s, 9:9:2; 6 Me,
CH2). 11B NMR: l 84.1.
1
l 28.0 (t, J=85 Hz). 13C NMR: l 29.0 (t), 31.6 (q).
This solid diborane is dissolved in thf (10 cm3) at
−78°C. Lithium (73 mg, 10.5 mmol) is added and the
(NMR data in C6D6, Varian Unity 500.)

62
A. Neu et al. / Inorganica Chimica Acta 289 (1999) 58–69
t
suspension is stirred at −25°C (12 h). The solvent is
removed in vacuo and pentane (10 cm3) is added. The
solution is filtered and the solvent removed again.
The yellow oily residue is washed with two portions
of MeCN (10 cm3) and dried in vacuo. The product
could not be crystallized, but the NMR data leave no
doubt that the product is pure B4tBu2(CH2tBu)2. The
yield is 70 mg, corresponding to an overall yield of
2.5. B4H2(CH2 Bu)4
Dichloro(neopentyl)borane can be synthesized in
1
analogy to tert-butyldichloroborane [63]. H NMR: l
0.91, 1.33 (2s, 9:2), 11B NMR: l 63.0. 13C NMR: l 31.7
(q), 40.2 (t).
The borane (tBuCH2)BCl2 (1.0 g, 6.5 mmol) in
pentane (4 cm3) was stirred with Na/K alloy (1.0 g) for
2 h. Larger amounts of the borane reduced the final
yield. This reaction was repeated twice. The three
portions obtained were united and filtered. The solid
was washed with two portions of pentane (10 cm3). The
solvent was removed from the filtrate in vacuo, and the
residue transported into a condenser (70°C, 0.001
mbar). A pale yellow product (30 mg, ca. 5.6%)
crystallized from toluene (1 cm3) at −78°C. According
to the NMR spectra, a 1:1 mixture of B4(CH2tBu)4 and
B4H2(CH2tBu)4 was present. The closo-borane in the
1
0.9% from the starting diborane(4). H NMR: l 1.08,
1.11, 1.82 (3s, 9:9:2). 11B NMR: l 134.5, 138.7 (1:1; a
cross-peak is observed in the 2D-11B/11B NMR
spectrum). 13C NMR: l 28.2, 31.4 (2q), 50.1 (t);
these three signals correspond to the 1.11, 1.08,
1
1.82 H NMR chemical shifts, respectively (according
to the 1H/13C-hmqc NMR spectrum). (NMR data
in C6D6, Varian Unity 500.) MS (Finnigan
MAT 95) Found for 11B412C18 H40 300.3502 (Calc.:
1
300.3502).
1
mixture was identified by NMR and MS. H NMR: l
1.05, 1.24 (2s, 9:2); a signal at l 1.4 is detected in the
1H/11B-hmqc NMR spectrum. 11B NMR: l 9.0. 13C
NMR: 31.7 (q), 34.2 (t). (NMR data in C6D5CD3,
2.3. Cl(tBu)B–BCl2(NHMe2)
An ethereal solution of HCl (38 cm3, 1.35 mol l−1
)
Varian Unity 500.) MS (Finnigan MAT 95) Found for
is dropped into
a
solution of (Me2N)tBuB–
11B4
C
1H46 330.397 (Calc.: 330.3972).
20
12
BCl(NMe2) [61] (3.5 g, 17 mmol) in ether (15 cm3) at
0°C. After filtering off the ammonium salts, the pure
colorless product crystallizes from the filtrate at ambi-
ent temperature and can be recrystallized from ether.
Anal. Found: C, 31.29; H, 7.36; N, 6.07. Calc. for
2.6. B4H2(CMe2Et)4
(1,1-Dimethylpropyl)difluoroborane [64] (7.80 g, 65.0
mmol) in pentane (40 cm3) was treated at −78°C with
Na/K alloy (6 g). When removed from the cooling
bath, the reaction mixture warmed and refluxed for 10
min. After refluxing for 2 h, the mixture was filtered
and the solids washed with pentane (3×20 cm3). Water
(30 cm3) and, after the evolution of gas, sodium
hydroxide (2 g) were added and the mixture stirred for
30 min. The pentane phase was separated, the solvent
removed in vacuo and the residue transported into a
condenser (70°C, 0.001 bar). The colorless product (50
mg, 1%) crystallizes from acetonitrile at −25°C. The
mother liquor contains another portion of the product
1
C6H16B2Cl3N: C, 31.31; H, 7.01; N, 6.09%. H NMR:
l 1.11 (s, tBu), 2.71 (d, 3J=6.0 Hz, NMe2), 4.7
(broad, NH). 11B NMR: l 4.3, 80.5 (2s giving a
cross-peak in the 2D-11B/11B NMR spectrum). 13C
NMR: l 26.7, 38.2 (2q), 29.6 (broad). The crystal
structure analysis gave the orthorhombic space group
Pna21 (No. 33) (a=1782.4(3); b=651.44(6); c=
1005.00(2) pm; Z=4; R=0.034; Rw=0.034).
2.4. Li[B4HtBu4]
The synthesis of Li(thp)2B4HtBu4 from B4H2tBu4
and Li in thp is described elsewhere [4]. We surmised
that gaseous H2 was also formed in this reaction. In
order to verify this assumption, the reaction was re-
peated in a closed system under argon. No volume
expansion is observed; the by-product was LiH in-
stead. This was shown by starting with larger
amounts of B4H2tBu4 (606 mg, 2.21 mmol; Li: 31 mg,
4.5 mmol). The yield of Li(thp)2B4HtBu4, crystallized
from hexane, increased from 42 [4] to 85%. The
residue was washed with ether (10 cm3) and then re-
acted on the porcelain filter with BEt3 (1.0 g, 10
mmol) in thf (10 cm3). The solvent was removed
from the filtered solution in vacuo. The residue con-
together
with
the
yellow
hypercloso-species
B4(CMe2Et)4 (11B NMR: l 137). Anal. Found: C, 73.60;
1
H, 14.88. Calc. for C20H46B4: C, 72.83; H, 14.06. H
NMR: l 0.99, 1.07, 1.43 (t, s, q; 3:6:2; J=7.5 Hz), 1.1
3
(1H/11B-hmqc NMR cross-peak). 11B NMR: l 14.3. 13C
NMR: l 9.75, 28.43 (2 q), 36.07 (t); BC peak not
detected. (NMR data in C6D6, Varian Unity 500.)
The solids from the reaction mixture in pentane were
extracted with thf (3×20 cm3). Removing the solvent,
washing the residue with Et2O (2×20 cm3) and drying
in vacuo gave K[B(CMe2Et)F3] (5.52 g, 71%, assuming
that this product contains all the fluorine from the
starting material). Anal. Found: C, 33.81; H, 6.52. Calc.
1
1
tains Li[BHEt3] (11B NMR: l −12.3, d, J=55 Hz
[62]).
for C5H11BF3K: C, 33.73; H, 6.23%. H NMR: l 0.59,
3
0.69, 1.10 (s, t, q; 6:3:2; J=8.0 Hz). 11B NMR: l 5.6.

A. Neu et al. / Inorganica Chimica Acta 289 (1999) 58–69
63
13C NMR: l 8.68, 23.22 (2q), 32.10 (t). (NMR data in
D8-thf, Varian VXR 300.)
B4H4tBu4 are identified in the product from its NMR
signals.
t
t
2.7. [K(thp)6][B4H3 Bu4]
2.10. closo-1,5-C2B3Me2 Bu3
The borane B4H2tBu4 (400 mg, 1.46 mmol) and 2-bu-
tyne (1.7 g, 32 mmol) are heated in a steel autoclave (3
days, 6 bar, 160°C). When excess butyne is removed, a
mixture of the starting borane and the carbaborane
product is obtained. The carbaborane is only slightly
more volatile than the borane. A first sublimation
fraction (23°C, 0.001 bar, 140 mg) contains 70%
carbaborane (ca. 26% yield) and 30% borane, accord-
ing to NMR data. The structure of the carbaborane in
the mixture can be identified by a comparison with the
well known data of the analogous carbaborane
A suspension of potassium sand (0.4 g, obtained in
hexane and washed with thp) and thp (2 cm3) is cooled
to 0°C. A solution of B4H2tBu4 [4] (540 mg, 1.97 mmol)
in thp (6 cm3) is added dropwise. Hexamethyldilazane
(0.5 cm3) is added to the reddish brown solution after
2 h. After stirring for 3 days, the reaction is complete.
Hexane (8 cm3) is added to the filtered solution. The
slightly yellow product (1.39 g, 85%) crystallized from
the thp layer after 1 week at −20°C. The product is
characterized by NMR spectra (see Section 3). A solid
is obtained by removing the solvent from the mother
liquor after crystallization. This contains the anions
[tBuBH3]− and [N(SiMe3)2]−, as identified by NMR.
11B NMR: l −22.0 (q, 1J=75 Hz). 1H NMR: l
−0.19 (SiMe3). 13C NMR: l 6.86 (SiMe3). (NMR data
in C6D6, Varian Unity 500.)
1
C3B3Me2Et3 [65]. H NMR: l 0.90, 1.95 (2s, 9:2). 11B
NMR: l 14.6 (s). 13C NMR: l 9.83, 28.18 (2q, Me,
tBu), 95 (skeletal C). (NMR data in CDCl3, Varian
Unity 500.)
If no (Me3Si)2NH is added to the reaction mixture,
a mixture of the anions [B4HtBu4]− and [B4H3tBu4]−
in a ratio of 7:3 can be identified after 3 days by
NMR.
3. Results and discussion
3.1. The series B4R4
t
2.8. B4H4 Bu4
We reduced several alkyldihaloboranes RBHal2 with
sodium–potassium alloy. As a general result, mixtures
of hypercloso-B4R4 and closo-B4H2R4 are formed. The
proportion of closo species becomes larger with longer
reaction times and at higher temperatures. In addition
to the two cluster products, K[RBF3] salts are pro-
duced when fluorine is the halogen (Eq. (1)). KCl or
KBr are the by-products with Hal=Cl, Br (Eq. (2)).
The two cluster H atoms of the closo species do not
originate from the solvent (shown by deuterated
toluene with R=tBu [4]).
Ethereal HCl (1.2 cm3, 0.48 mol l−1) is added at
0°C to a solution of [K(thp)6][B4H3tBu3] (500 mg, 0.60
mmol) in thf (3 cm3). The solvents are removed in
vacuo and the residue is extracted twice with hexane (3
cm3). The hexane solution is filtered and hexane is
removed in vacuo. The product is obtained first by
sublimation (40°C, 0.001 mbar) and then by recrystal-
lization from acetonitrile (140 mg, 85%). The colorless
needles are characterized by NMR spectra. Traces of
the starting material B4H2tBu4 are detected in the
product by NMR.
12RBF2+8K“B4R4+8K[RBF3]
(1)
4RBHal2+8K“B4R4+8KHal (Hal=Cl, Br)
(2)
t
2.9. Dehydridation of [B4H3 Bu4]−
t
The preparation of B4tBu4 from BuBF2 in pentane (10
t
BH3(thf) (0.15 cm3, 1 mol l−1) is added to a solu-
tion of [K(thp)6][B4H3tBu4] (100 mg, 0.12 mmol) in thf
(1 cm3). The spontaneously formed colorless solid is
separated from the solution by centrifugation. The bo-
rane B4H2tBu4 is identified in the resulting solution by
NMR data. The solid is soluble in D2O and contains
min reflux; 79%) and of B4H2tBu4 from BuBBr2 in
toluene (15 min reflux; 13%) are well known [4,8]. We
report here on the reduction with Na/K alloy of two
boranes RBF2, whose alkyl groups are either less
t
bulky (R=CH2tBu) or larger (R=CMe2Et) than Bu.
The reduction of (tBuCH2)BF2 in pentane gives hyper-
closo-B4(CH2tBu)4 (pentane, 2 h reflux; 2%), whereas
closo-B4H2(CMe2Et)4 is obtained from (EtMe2C)BF2
KBH4 (11B NMR: l −43, quint, J=80 Hz) in addi-
1
tion to traces of B(OD)3 (11B NMR: l 7, s).
Colorless needles of the same product B4H2tBu4 (80
mg, ca. 75%) are obtained when a solution of
[K(thp)6][B4H3tBu4] (322 mg, 0.39 mmol) and EtBr (45
mg, 4.3 mmol) in thf (5 cm3) is stirred at r.t. for 40 h
with subsequent filtration, removal of solvent and con-
densation of the product (40°C, 0.001 bar). Traces of
(pentane, 2 h reflux; 1%). However, hypercloso-
B4(CMe2Et)4 was identified in the reaction mixture by
its characteristic NMR signals. The chloroborane
(tBuCH2)BCl2 gives a mixture containing comparable
amounts of the hypercloso and the closo species (pen-
tane, 2 h r.t.; 5%).

64
A. Neu et al. / Inorganica Chimica Acta 289 (1999) 58–69
t
Starting from the known diborane(4) Bu(Me2N)B–
(1%) from the reduction of RBHal2 with Na/K alloy
(R=tBu: Hal=Br, 15 min reflux in toluene [4]; R=
CMe2Et: Hal=F, 2 h reflux in pentane). A solid 1:1
mixture of B4R4 and B4H2R4 is obtained from
tBuCH2BCl2 and Na/K alloy (2 h stirring in pentane at
22°C). B4H2tBu4 had also been synthesized by reducing
B4tBu4 with Na/K alloy in the presence of [18]crown-6
and subsequent protonation of the anionic intermediate
with HCl [4]. Without [18]crown-6, a radical anion
B(NMe2)Cl [61], the synthesis of a tetraborane like
B4R2R%, R/R%=tBu/CH2tBu, involves four steps: alkyla-
2
tion, replacement of Me2N by MeO, replacement of MeO
by Cl, and reductive coupling with Li. None of the three
intermediate products can be obtained in more than 90%
purity. The yellow product is pure, according to NMR
data, but cannot be crystallized. The overall yield is only
0.9%.
t
−
’
[B4Bu4]
is formed [70]. Indeed, a reversible one-elec-
tron reduction of B4tBu4 at −1.77 V was observed by
cyclicvoltammetry, but further reduction up to −3.2 V
did not take place.
The four BR units in the three B4H2R4 molecules are
1
equivalent, as shown from the H, 11B and 13C NMR
spectra. The 11B NMR chemical shifts of B4H2R4 (l 14.8,
9.0, 14.3 for R=tBu, CH2tBu, CMe2Et, respectively) are
distinctly different from those of the corresponding
hypercloso species and are in accord with the shift (l 13.9
for R=Me [4]) calculated by IGLO/DZ [71]. The NMR
signals of the two bridging protons are found as cross-
1
peaks in the H/11B-hmqc NMR spectra at rather char-
acteristic shift values (l 0.85, 1.4, 1.1). Obviously,
molecules are present with structures derived from the
tetrahedral B4R4 by bridging two opposite edges (D2d
symmetry). This agrees with the calculated ground state
structure of the parent molecule B4H6 [4]. The NMR
spectra do not exclude the possibility of molecules with
two rapidly fluctuating protons (pseudo-Td); however,
the signal pattern is not altered down to −80°C.
The B4H2R4 molecules are the first neutral derivatives
of a closo-borane in the BnHn+2 series. Anionic closo-bo-
rates of the [BnHn]2− type, on the other hand, are well
known for n=6–12. The larger cluster anions (n=10–
12) are very weak Brønsted bases, but smaller clusters
(n=6: pKB=7 [72]) are only weak bases. The [B4H4]2−
anion is expected to be a strong base since the negative
charge is concentrated on a relatively small cluster
surface. B4H6 and, hence, B4H2R4, vice versa, are cer-
tainly very weak acids.
The tetrahedral structure of the B4R4 tetraboranes can
easily be established from the characteristic low-field
shifted 11B NMR signals, e.g. at l 135.4 for the struc-
turally well characterized B4tBu4 [8]: l 139.3 (R=
CH2tBu), 137.0 (R=CMe2Et), 134.5/138.7 (R/R%=tBu/
CH2tBu); even the 2D-11B/11B NMR cross-peak in the
B4R2R%-type product was detected. The isolated yellow
2
solids B4(CH2tBu)4 and B4tBu2(CH2tBu)2 are storable at
r.t. in the absence of air.
Boranes of the BnXn type with electron donating
groups X are expected to exhibit classical ring rather than
deltahedral cluster structures; the six-membered ring
compound B6(NMe2)6 with electron donating amino
groups is an example [66]; four-membered rings of the
type B4(NR2)4 had also been claimed in the literature
[5b,67]. The question arises, whether
a molecule
B4tBu2(NMe)2 with only two donating ligands would
form a four-membered ring or a tetrahedrane. We tried
to synthesize the diborane(4) Cl(tBu)B–B(NMe2)Cl by
treating the known (Me2N)(tBu)B–B(NMe2)Cl with
HCl. The intention was to attempt alkali metal dechlo-
rination subsequently. Even though a stoichiometric
amount of HCl was employed, only the stable Cl(tBu)B–
BCl2 (NHMe2) adduct was obtained. This product could
be characterized by a crystal structure analysis. (We do
not report details, since crystal structures of amine-di-
borane(4)-type molecules are already known [68,69].)
3.2. The series B4H2R4
Fig. 1. Interatomic distances in the clusters B4HMe4Li (Calc.: D2d
)
and B4HtBu4[Li(thp)2] (X-ray diffraction; two independent molecules;
We recovered the colorless crystalline pure closo clus-
ters B4H2R4 with R=tBu (13%, [4]) and R=CMe2Et
C2) (pm) [4].

A. Neu et al. / Inorganica Chimica Acta 289 (1999) 58–69
65
Nevertheless, we could remove one of the bridging
protons in B4H2tBu4 by the reaction with lithium in
thp.
B4H2tBu4+2Li+2thp“B4HtBu4[Li(thp)2]+LiH
We had assumed in a preliminary report [4] that H2
was the by-product. Subsequent efforts to detect H2
failed. Instead, we found that LiH was produced since
LiBHEt3 formed when BEt3 was added. The structure
of B4HtBu4[Li(thp)2] in the crystal [4] fits quite well
with that computed for B4HMe4Li (Fig. 1). The
molecule exhibits fluxional behavior in solution at am-
bient temperature. At low temperature, however, the
NMR data are consistent with the existence of two
different species, B4HtBu4LiLx (L: solvent) with C26
skeletal symmetry and an ion pair [LiLx]+/
[B4HtBu4]−. The ratio of these products depends on
the temperature and on the polarity of the solvent. The
two 11B NMR signals computed for B4HMe4Li and
measured for B4HtBu4LiLx (in Et2O) are in excellent
agreement [4]. The lithiated species is easily protonated
to give neutral B4H2tBu4, even with Brønsted acids as
weak as (Me3Si)2NH.
1
Fig. 2. Experimental H and 11B NMR chemical shifts l of the anion
[B4H3R4]− (a) in D10-Et2O at −30°C (1H{11B}decoupled) and (b) in
D8-thf at 60°C (R=tBu; Varian Unity 500).
steps. The first was reversible at −2.36 V, the second
was irreversible at −2.87 V. In order to shift the ratio
of the two anions toward the nido-anion, we found a
Brønsted acid, the disilazane (Me3Si)2NH, which meets
three conditions: (1) rapid protonation of nido-
[B4H2tBu4]2− to give nido-[B4H3tBu4]−; (2) protonation
of closo-[B4HtBu4]− to give the starting material [4];
(3) metastable towards potassium in the absence of
metallic catalysts. Actually, the product [K(thp)6][B4H3t
Bu4] is produced in an 85% yield when B4H2tBu4 is
reduced by K in thp in the presence of (Me3Si)2NH.
The reactions of B4H2tBu4 need systematic further
investigation. Here we contribute one result. Heating
with excess 2-butyne gives the 1,5-dicarba-closo-penta-
borane C2B3Me2tBu3. Although this could not be sepa-
rated from unreacted closo-tetraborane, its structure
has been identified unambiguously by NMR methods
(see Section 2). A hypothetical RBH2 product, the
missing link stoichiometrically, is certainly not ex-
pected to be stable towards 2-butyne or under the
reaction conditions.
B4H2tBu4+2K+(Me3Si)2NH+6thp
“[K(thp)6][B4H3tBu4]+K[N(SiMe3)2]
The structure of the nido-anion [B4H3tBu4]− can be
deduced from the NMR spectra (Fig. 2). At low tem-
perature an unsymmetric structure with a bicyclobu-
tane-type B4 skeleton is found with four different B
t
atoms, four different Bu groups and two different m-H
atoms. As expected, 2D-11B/11B NMR correlations are
detected between the B1/B2 and B3/B5 pairs, but not
between the H-bridged B1/B3 and B2/B5 pairs. The
terminal H atom is coupled to the adjacent m-H atom
by 2J=6.5 Hz. The bridging positions are deduced
from the detection of three of the four expected 2D-
1H/11B correlations; we did not find the correlation of
l −1.32 with l 59.1 because of the line width of the
latter signal (ꢀ1/2=620 Hz). The terminal H atom is
bound to B5, as is established by the corresponding 2D
t
3.3. The nido-anion [B4H3 Bu4]−
When B4H2tBu4 in thp is reduced by potassium in-
stead of lithium, a 7:3 mixture of the [B4HtBu4]− and
[B4H3tBu4]− anions can be identified by NMR meth-
ods. We assume that two competing reduction reac-
tions take place, the first of which is analogous to the
known reaction with Li.
t
correlation. Neither the Bu groups at B1 and B5 nor
those at B2 and B3 can be distinguished, but B1/B5
can be differentiated from B2/B3 by raising the tem-
perature. The high temperature spectra (Fig. 2(b)) are
in accord with Cs symmetry: B2 and B3 as well as the
B4H2tBu4+2K“K[B4HtBu4]+KH
t
corresponding Bu groups are equivalent. We attribute
+HX
the equivalence of the bridging H atoms to a dynamic
process, whose transition state is represented in part
(b) of Fig. 2. The two bridging H atoms are distributed
over all four outer BB edges. Their bridging character
follows from the proton chemical shift, which is essen-
tially unaltered in the low and high temperature struc-
B4H2tBu4+2K“K2[B4H2tBu4]ꢁꢁꢁꢂ K[B4H3tBu4]+KX
The acid HX stands for the medium. The strongly
basic nido-dianion [B4H2tBu4]2− seems to be a plausible
intermediate, since the cyclovoltammetric reduction of
B4H2tBu4 in thf (Pt electrodes, [NBu4]PF6) gave two

66
A. Neu et al. / Inorganica Chimica Acta 289 (1999) 58–69
Fig. 3. Calculated minimum structures and relative energies (kJ mol−1
)
of [B4H7]− (RMP2(fc)/G-311+G*).
Fig. 4. Experimental 1H and 11B NMR chemical shifts l of the
tetraborane B4H4R4 in D8-thf (a) at −50°C (1H{11B}decoupled) and
(b) at 60°C (R=tBu; Varian Unity 500).
1
ture. The terminal H atom is coupled to B5 by J=100
Hz; this value is more typical for an exo- than an
endo-position. The product obtained is more likely,
give the neutral B4H4tBu4 nido-tetraborane. This is the
first storable derivative of B4H8. An unsymmetric bicy-
clobutane-type structure can be deduced from the NMR
spectra at −50°C (Fig. 4(a)): four different B atoms, four
t
therefore, to be the endo-isomer with respect to Bu in
position 5; this reduces the steric interactions among
the Bu groups to a minimum. The coalescence temper-
t
ature for the dynamic process is found to be at 45°C in
t
different Bu groups and three different bridging H
the 11B NMR spectrum (Dw=448.8 Hz, in D8-thf),
atoms. The 1H NMR sequence l 0.90/1.00/1.13/1.24 (4s;
tBu) can be correlated by the 2D-1H/13C hmqc technique
giving an activation barrier of about 59 kJ mol−1
.
The ground-state structure (a), deduced from NMR
data, is confirmed by ab initio calculations on [B4H7]−.
Four minima are found on the potential energy surface,
three with a cyclobutane-type (c, e, f) and one with a
‘tetrahedral’ structure (d, Fig. 3). Structure (c) with the
lowest energy clearly corresponds to structure (a) (Fig.
2). The calculated 11B NMR shifts (IGLO(DZ) [71]) of
structures (c) and (e) fit the experimental data best
(Table 1). [B4H3Me4]− gives calculated 11B NMR shifts
((g) and (h) in Table 1) that are even closer to those
found for [B4H3tBu4]−; the shifts of the endo-isomer
(with the terminal H atom at B5 exo) correspond
slightly better to the experimental values than those of
the exo-isomer.
t
to a 13C NMR sequence l 33.8/31.7/34.8/31.5 (4q; Bu).
The expected 2D-1H/11B correlations allowed the assign-
ment of the boron-bound H atoms, three as bridging and
one as a terminal H atom. The tBu groups at B2 and B3
cannot be distinguished. In analogy to the corresponding
anion, the distinction B1/B5 versus B2/B3 can be made
at higher temperature: two halves of the molecule become
equivalent in the NMR spectra (Fig. 4(b)). We explain
the pseudo-Cs symmetry by a dynamic process that
transforms structure (a) into its enantiomer via a transi-
tion state (b). The terminal and one of the bridging H
atoms are involved in this process, but not the two H
atoms that bridge B1 with B2 and B3. Due to the dynamic
process, the 11B NMR signal of B5 appears as a pseudo-
triplet (1J=76 Hz). Again, the non-planarity of the
Altogether the evidence is convincing that the
ground-state structure of the nido-anion [B4H3tBu4]− is
t
the bicyclobutane-type structure (a) with the Bu group
at B5 in the endo-position.
t
3.4. The nido-tetraborane B4H4 Bu4
Ethereal HCl,
a stronger Brønsted acid than
HN(SiMe3)2, easily protonates the [B4H3tBu4]− anion to
Table 1
11B NMR shifts l of [B4H3tBu4]− (structure (a), Fig. 2) from experiment
(−30°C), of [B4H7]− (structures (c)–(f), Fig. 3) and of [B4H3Me4]−
(exo-isomer (g) and endo-isomer (h)) from IGLO calculations
B1
59.1
45.6
−17.9
47.9
27.9
46.3
49.5
B2
10.3
4.2
−51.2
6.5
11.7
10.2
12.4
B3
B5
−9.7
−23.8
−51.2
−32.0
−24.6
−15.3
−10.6
(a)
(c)
(d)
(e)
(f)
6.1
4.2
−17.9
5.3
11.7
13.9
16.5
(g)
(h)
Fig. 5. Calculated minimum structures (c)–(f), transition states (g)–(i)
and relative energies (kJ mol−1) of B4H8 (RMP2(fc)/6–31G*).

A. Neu et al. / Inorganica Chimica Acta 289 (1999) 58–69
67
t
Table 2
corresponding B4H8 barrier since the Bu groups at B3
or B5 are less crowded sterically in the transition state.
Apparently, the tert-butyl group occupies an endo-
position at B5 in the ground state GSendo of B4H4tBu4
(Fig. 6). The enantiomerization GSendo ?GSe*ndo via the
transition state TSendo proceeds rapidly at 60°C, but
slowly at −50°C, with respect to the NMR time scale.
The transition state TSendo is related to the high energy
transition state (i) of B4H8 (Fig. 5). No indication was
found for the formation of the corresponding exo-iso-
mer of B4H4tBu4, whose ground state GSexo (Fig. 6)
should enantiomerize more rapidly than GSendo, since
the transition state TSexo is related to the low energy
transition state (h) of B4H8 (Fig. 5).
Calculated skeletal distances of B4H4Me4 (RMP2(fc)/6–31G*) and
B4H4tBu4 (RHF/6–31G*) in the endo-configuration (a) (see Fig. 4)
(pm); IGLO(DZ)-11B NMR shifts of B4H4Me4, calculated on the
optimized geometry of B4H4Me4 (Me I) and of B4H4tBu4 (Me II)
Me
tBu
Me I
Me II
Exp.
B1–B2
B1–B3
B5–B2
B5–B3
B2–B3
173.4
162.8
177.7
177.9
170.8
178.9
165.6
184.5
183.5
176.2
B1
B2
B3
B5
26.9
20.9
23.4
34.8
10.3
8.5
37.2
16.0
10.7
−8.8
−6.6
−4.2
B4-butterfly structure defines an exo/endo alternative
t
The well-known aza-nido-tetraborane NB3H2tBu4 [73]
is isoelectronic with B4H4tBu4 and exhibits the same
bicyclobutane-type skeleton. The steric situation is
comparable, but there are also differences. A racemate
for the ligands H and Bu at B5. The steric interaction
of this tBu group with those at B2 and B3 is expected to
be greater in exo- than in the endo-position; hence, the
terminal H atom should be exo. In this position, the
tBu groups at B1 and B5 should interact, consistent
with the observation of a corresponding NO effect.
GSendo/GS* of NB3H2tBu4 is present in the solid state,
endo
according to crystal structure analysis [73]. The
GSendo ?GSe*ndo enantiomerization in solution is so
slow, even at 40°C, that the ground state NMR signals
are observed, in contrast to GSendo of B4H4tBu4. A
second set of NMR signals is present, however, which
corresponds formally to a TSexo structure. These signals
could not be transformed into those expected for GSexo
by lowering the temperature to −50°C. Hence, we
1
Similar NOE correlations between the H NMR signals
t
of the Bu groups at B1 and at B2/B3 suggest exo-
placements for all these three groups and allow the l
1
t
1.24 H NMR signal to be assigned to the Bu at B1;
the l 0.90 signal must then be assigned to the tBu at B5.
The 10°C coalescence temperature for the dynamic
process related to the NMR signals of B2 and B3
corresponds to an activation barrier of ca. 50 kJ
assume a rapid enantiomerzation GSexo ?GS* via a
exo
mol−1
.
low lying transition state TSexo, to explain the pseudo-
Cs symmetry. Unlike B4H4tBu4, there is also an equi-
librium between GSendo and GSexo in NB3H2tBu4. This is
slow with respect to the NMR time scale at 40°C, but
rapid enough to be established immediately by chang-
ing the temperature and also by dissolving crystalline
GSendo in appropriate solvents (the term immediately is
used here with respect to the NMR measurements). As
deduced from the temperature dependence of the equi-
librium GSendo ?Gsexo, the enthalpy of GSendo is 3 kJ
mol−1 lower but its entropy is 1 J mol−1 less fa-
vorable. The mechanism of the exo/endo isomerization
Structure (a) of B4H4tBu4 (Fig. 4), concluded from
experiment, corresponds to the structure (c) of B4H8
(Fig. 5) which is the lowest minimum of four alterna-
tives, (c)–(f), calculated at RMP2(fc)/6–31G*. Earlier
theoretical work had located a ground state structure of
the same C1 symmetry [19] (see Section 1.3), but not the
higher energy minima. Structure (c) was the basis of the
starting geometry for optimizations of B4H4Me4 and
B4H4tBu4 in the endo-configuration. The bulky tBu
groups influence the skeletal distances remarkably. The
IGLO-calculated 11B NMR shifts of B4H4Me4 (Table 2)
do not fit well to the corresponding experimental values
of B4H4tBu4 (Fig. 4(a)), but the calculated values ap-
proach the experimental ones nicely when the 11B NMR
shifts of B4H4Me4 are calculated on the basis of the
optimized geometry of B4H4tBu4 (Table 2). This is fur-
ther evidence of the ground state structure of nido-
tetraborane.
Three transition states (g)–(i) of B4H8 that would
transform the ground state (c) into its enantiomer via
mirror planes through B2/B3 (g) or B1/B5 (h), (i) were
located (Fig. 5). The transition state with the highest
activation barrier (DE" =82 kJ mol−1) corresponds to
the transition state (b) (Fig. 4), assumed from experi-
mental evidence (DG" =50 kJ mol−1). We suppose the
activation barrier of B4H4tBu4 to be lower than the
Fig. 6. The four enantiomerizations GS ?GS* via the corresponding
transition states TS for B4H4R4 and for the isoelectronic NB3H2R4 in
the exo- and endo-configuration (R=tBu).

68
A. Neu et al. / Inorganica Chimica Acta 289 (1999) 58–69
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could be a ring folding process via a planar bicyclobu-
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In addition to NB3H2tBu4, a series of aza-nido-tetrab-
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exo
ture of the type TSexo is found for NB3H3tBu3 (R=H)
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Tetraboranes B4R4 and B4H2R4 with sterically de-
manding groups R are accessible from the reduction of
RBHal2. Both types of tetraboranes adopt tetrahedral
structures; the two extra hydrogen atoms of B4H2R4
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further reduced to give [B4H3R4]−, presumably via
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.
5. Supplementary material
Details on the crystal structure investigation may be
obtained from the Fachinformationszentrum Karlsruhe,
D-76344 Karlsruhe-Leopoldshafen, Germany, on quot-
ing the depository number CSD-410125.
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We gratefully acknowledge support from the Deutsche
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