Metal tetrahydroborates and tetrahydroborato metallates, 30 [1]. Alkoxo-substituted alkali metal tetrahydroborates: Studies in solution and structures in the solid state

Article abstract of DOI:10.1515/znb-2006-0906

Reactions of MBH₄ (M = Li, Na, K) with tBuOH, Ph₃COH, PhOH, F₅C₆OH, and 2,4-tBu₂C₆H ₃OH in THF in a 1:1 ratio were followed by ¹¹B NMR spectroscopy. No M[H₂B(OR)₂] species could be detected, but minor amounts of M[H₃BOR] and larger amounts of M[HB(OR) ₃]. In the reaction of LiBH₄ with 2,4-tBu ₂C₆H₃OH also a fair proportion of (RO) ₂BH was generated. The perfluorophenolato borane (F₅C ₆O)₂BH·THF was prepared from the phenol and BH ₃·THF in THF solution. It is unstable to disproportionation. Compound (C₆F₅O)₃B·THF was isolated and its crystal structure determined. Reaction of LiBH₄ with F ₅C₆OH in hexane generated a solid that proved to be Li[H₂B(OC₆F₅)₂]. It is unstable in THF. On the other hand, 2,2′-dihydroxydiphenyl in the presence of secondary amines reacts to give Li[C₁₂H₈O ₂B(NR₂)₂] (3-5). Li[B(O₂C ₁₂H₈)₂], 2, is formed when HN(tBu)Ph is used as a secondary amine. The unstable phthalatoborane H{C₆H ₄[C(O)O]₂}BH·THF (7), is stabilized as its pyridine adduct (phth)BH·py (8). 7 reacts with 3 equivalents of LitBu to give [Li(HBtBu)₃] (11), isolated as its tris(THF) solvate. Analogously, 7 reacts with LiNMePh to produce compound Li[HB(NMePh)₃] (10). Similarly, 7 and NaOtBu (molar ratio 1:3) give access to Na[HB(OtBu) ₃] (9). In attempts to grow single crystals, specimens resulting from a hexane solution showed that partial hydrolysis has occurred to give Na[HB(OtBu)₃]·Na[(tBuO)₂BO]·Na[tBuOB(O)H], which crystallizes as a centrosymmetric dimer. While catecholborane when treated with LitBu in THF and DME gave access to (dme)₂Li[catB(tBu) ₂], 12 (dme)₂, several compounds were observed when Li piperidide was used as nucleophile. Amongst these, the most interesting one was (dme)(THF)Li₂(cat)(catBH), 13 (dme)THF, the crystal structure of which was determined. In all cases where the borate species carried OR groups the O atoms of the RO or PhO group coordinate with the alkali metal cation. DFT calculations for the series of anions H_4-nBX_n^- showed that HBX₃^- is the most stable species for X = F, OH, NH₂. This confirms experimental results.

Full text of DOI:10.1515/znb-2006-0906

Metal Tetrahydroborates and Tetrahydroborato Metallates, 30 [1].
Alkoxo-Substituted Alkali Metal Tetrahydroborates:
Studies in Solution and Structures in the Solid State
Jo¨rg Knizek, Heinrich No¨th, and Markus Warchhold
Department of Chemistry, University of Munich, Butenandtstr. 5 – 13, D-81377 Mu¨nchen, Germany
Reprint requests to H. No¨th. E-mail: H.Noeth@lrz.uni-muenchen.de
Z. Naturforsch. 61b, 1079 – 1093 (2006); received February 26, 2006
Dedicated to Professor Kurt Dehnicke on the occasion of his 70th birthday
Reactions of MBH4 (M = Li, Na, K) with tBuOH, Ph3COH, PhOH, F5C6OH, and 2,4-
tBu2C6H3OH in THF in a 1 : 1 ratio were followed by 11B NMR spectroscopy. No M[H2B(OR)2]
species could be detected, but minor amounts of M[H3BOR] and larger amounts of M[HB(OR)3]. In
the reaction of LiBH4 with 2,4-tBu2C6H3OH also a fair proportion of (RO)2BH was generated. The
perfluorophenolato borane (F5C6O)2BH·THF was prepared from the phenol and BH3 · THF in THF
solution. It is unstable to disproportionation. Compound (C6F5O)3B·THF was isolated and its crystal
structure determined. Reaction of LiBH4 with F5C6OH in hexane generated a solid that proved to be
Li[H2B(OC6F5)2]. It is unstable in THF. On the other hand, 2,2’-dihydroxydiphenyl in the presence
of secondary amines reacts to give Li[C12H8O2B(NR2)2] (3 – 5). Li[B(O2C12H8)2], 2, is formed
when HN(tBu)Ph is used as a secondary amine.
The unstable phthalatoborane H{C6H4[C(O)O]2}BH·THF (7), is stabilized as its pyridine adduct
(phth)BH·py (8). 7 reacts with 3 equivalents of LitBu to give [Li(HBtBu)3] (11), isolated as its
tris(THF) solvate. Analogously, 7 reacts with LiNMePh to produce compound Li[HB(NMePh)3]
(10). Similarly, 7 and NaOtBu (molar ratio 1 : 3) give access to Na[HB(OtBu)3] (9). In attempts
to grow single crystals, specimens resulting from a hexane solution showed that partial hydrolysis
has occurred to give Na[HB(OtBu)3]·Na[(tBuO)2BO]·Na[tBuOB(O)H], which crystallizes as a cen-
trosymmetric dimer.
While catecholborane when treated with LitBu in THF and DME gave access to (dme)2Li[catB-
(tBu)2], 12 (dme)2, several compounds were observed when Li piperidide was used as nucleophile.
Amongst these, the most interesting one was (dme)(THF)Li2(cat)(catBH), 13 (dme)THF, the crystal
structure of which was determined. In all cases where the borate species carried OR groups the O
atoms of the RO or PhO group coordinate with the alkali metal cation.
DFT calculations for the series of anions H4−nBXn− showed that HBX3− is the most stable species
for X = F, OH, NH2. This confirms experimental results.
Key words: Hydridoborates, Catecholatohydridoborates, Aminohydridoborates, Alkali Metals,
DFT Calculations
Introduction
metal salts in the solid state are known, as for exam-
ple LiBH3CN [8], (HO(CH2)2N(CH2)2]2Na[BH3CN]
It is well known that the reducing ability of al- [9], or [(HO(CH2)2]3N · Na[H3BCN] [10], but little is
known about their structures in solution [11].
Many reductions of organic functional groups with
NaBH4 are performed either in aqueous or alcoholic
solutions, although NaBH4 reacts with both solvents
slowly with hydrogen evolution. In general, reactions
in alcohols are preferred to reductions in other media
[3, 4, 12]. It was suggested that this may be due to the
formation of alkoxohydridoborates [BH4−n(OR)n]−
[2 – 4, 13].
kali metal tetrahydroborates [2 – 4] depends on a num-
ber of factors, the most important one being the ionic
character which increases as one moves from LiBH4
to CsBH4 while the reducing power decreases. The
−
substitution of hydrogen atoms of the BH4 unit by
CN, OR, RC(O)O leads generally to higher selectivity.
Compounds such as NaBH3CN [5], Na[HB(OtBu)3]
[6] or Na[HB(OC(O)Me)3] [7] are commercially avail-
able. Nevertheless, only few structures of the alkali
c
0932–0776 / 06 / 0900–1079 $ 06.00 ꢀ 2006 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
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J. Knizek et al. · Metal Tetrahydroborates and Tetrahydroborato Metallates, 30
Santaniello et al. showed that NaBH4 reacts with they contained 19% and 39% of these components, re-
polyethyleneglycol (peg) with formation of the anion spectively. The main product was actually NaBH4 [21].
[H2B(peg)]− [14, 15] as deduced from 11B NMR stud- This is also true for the reaction with KOtBu. Potas-
ies. Kinetic studies of aqueous solutions of NaBH4 sium tetrahydridoborate precipitated from the solution.
showed that the anion [H2B(OH)2]− plays an im- The main product in solution was K[HB(OtBu)3], and
portant role [16]. However, this species could not it is interesting that B(OtBu)3 is formed in a signifi-
be detected in solution by 11B NMR in contrast to cant amount (23%) in case of LiOtBu, but not in the
[BH3OH]− and BH(OH)3]− [16]. Moreover, there is reactions with NaOtBu or KOtBu.
evidence that [BH2(OR)2]− species are, in general, not
very stable in solution [4, 13, 17 – 19].
51% of LiOCPh3 is converted into LiBH4, and al-
most the same amount into Li[HB(OCPh3)3] (47%).
Li[H3BOCPh3] is a minor product (2%). In con-
trast, the rather bulky 2,4,6-tri-tert-butylphenolate
gave only 4% of Li[HB(OC6H2tBu3)3] besides 41%
of LiBH4 and, rather unexpected 55% of the borane
HB(OC6H2tBu3)2. These results show i) that the re-
actions do not stop at the adduct stage M[H3BOR],
ii) that the hydride/OR exchange – by what mechanism
whatsoever – has a driving force that favours MBH4
and M[HB(OR)3]. Moreover, the stoichiometry of the
reaction products requires the formation of MH, in case
of the reaction with LiOC6H2tBu3 about 55% of LiH.
We assume that it is formed in a reaction according to
eq. (4), but alternatives are also feasible. Since in most
reactions the solutions did not turn turbid, one has to
assume that the alkali metal hydride is complexed by
the alkoxohydridoborate molecules.
Results
Reactions of alkalimetal alkoxides with borane-tetra-
hydrofuran
There are several routes to alkoxohydridoborates,
the most convenient ones being reactions of alcohols
with MBH4, the addition of an alkali metal hydride
MH to BH3−n(OR)n, or the addition of BH3·THF to al-
kali metal alkoxides. In the latter case and using homo-
geneous conditions one can expect reactions according
to eq. (1) by using equivalent amounts of the reagents.
However, if there is a local excess of BH3·THF hy-
dride abstraction from M(H3BOR) as shown in eq. (2)
can be expected as well as substituent exchange reac-
tions as depicted by eq. (3). Continuous substituent ex-
change leads finally to MBH4 and M[B(OR)4]. This
latter process is technically achieved in the synthesis
of NaBH4 from NaH and B(OMe)3 which proceeds at
elevated temperatures via Na[HB(OMe)3] [20]. The re-
action according to eq. (2) can be suppressed to a cer-
tain extent by using high dilution and/or low tempera-
tures [17].
Lithium bis(pentafluorophenoxo)dihydridoborate
Brown et al. [22] have shown that the reaction
of phenol with LiBH4 in THF produces a mixture
of Li[HB(OPh)3] and Li[B(OPh)4], We can confirm
this. Neither Li[H3BOPh] nor Li[H2B(OPh)2] could
be detected by 11B NMR. However, we expected that
the synthesis of a Li[H2B(OAr)2] species might be
achieved by employing pentafluorophenolate as a less
nucleophilic reagent [13]. In the hope to avoid substi-
tutent exchange reactions we first studied the reaction
of HB(OC6F5)2 with LiH, expecting a reaction accord-
ing to eq. (5). Compound HB(OC6F5)2 was prepared
as shown in eq. (6)
M(OR)+H3B·THF → M(H3BOR)+THF (1)
M(H3BOR)+H3B·THF
→ M(BH4)+H2BOR·THF
(2)
2M(H3BOR) → M(BH4)+M[H2B(OR)2] (3)
2MOR+H3B·THF → HB(OR)2 +2MH+THF (4)
HB(O6F5)2 +LiH → Li[H2B(OC6F5)2]
(5)
(6)
Reactions between BH3·THF and MOR can be fol-
lowed by 11B NMR spectroscopy. The results listed
in Table 1 were obtained with solutions of M(OtBu)
(M = Li, Na, K), LiOCPh3 and LiOC6H2(tBu)3 in THF
in a 1 : 1 ratio. In none of the reactions studied could
species of the type M[H2B(OR)2] and M[B(OR)4] be
detected. We were also unable to isolate Na[H3B(OR)]
or Na[HB(OR)3] (R = tBu) from the solutions although
2HOC6F5 +H3B·THF
→ HB(OC6F5)2 ·THF+2H2
3HB(OR)2 +THF → 2B(OR)3 +H3B·THF (7)
LiBH4 +2HOC6F5 → Li[H2B(OC6F5)2]+2H2 (8)
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J. Knizek et al. · Metal Tetrahydroborates and Tetrahydroborato Metallates, 30
1081
3Li[H2B(OC6F5)2]+2THF
triplet structure. Mass spectrometry led to signals at
m/e = 386/385 corresponding to (C6F5O)2BH. How-
ever, the elemental analysis of the solid fitted well with
Li[H2B(OC6F5)2]. The compound is highly sensitive
to moisture. Obviously, hydrolysis had occurred during
the recording of its IR spectrum as the main band in the
νBH region was at 2483 cm−1 with two weak bands at
2308 and 2275 cm−1. The former would be compatible
with HB(OC6F5)2, the latter with [H2B(OC6F5)2]−.
Attempted dissolution of the solid in THF resulted
in a slurry. The supernatant solution showed a dou-
blet at δ11B = 6.1, 1J(11B1H) = 135 Hz, correspond-
ing to (C6F5O)2BH·THF. This result demonstrates that
Li[H2B(OC6F5)2] is unstable in polar solvents, but sta-
ble as a solid.
→ 2(F5C6O)3B·THF+LiBH4 +2LiH
(9)
1
Lithium bis(catecholato)borates and related species
Although it is well known that catechol re-
acts with LiBH4 in polar solvents to give lithium
bis(catecholato)borate irrespective of the stoichiom-
etry employed, we hoped that the introduction of
bulky tbutyl−groups into the catechol, as in 4,6-
tBu2H2C6O2 (cat’) might retard or even prohibit the
formation of a bis(catecholato)borate. If this could be
achieved than a reaction according to eq. (10) might be
realized.
Fig. 1. Molecular structure of (F5C6O)3B·THF (1). Ther-
mal ellipsoids are depicted on a 25% probability level. Se-
◦
˚
lected bond lengths (in A) and bond angles (in ): B1–O1
1.431(4), B1–O4 1.558(9), O1–C1 1.348(5), O4–C8 1.47(2),
O1–B1–O1A 113.8(3), O1–B1–O4 104.7(3), C8–O4–B1
121.1(6), B1–O1–C1 123.8(3), O1–C1–C6 123.6(4), O1–
C1–C2 120.4(4). – Interplanar angles: C1 to C6/C1A to 6A
74.3◦; C1O1B1/C1 to C6 103.6◦.
1
An 11B NMR signal at δ = 7.4 [d, J(11B1H) =
142 Hz] has shown that the bis(pentafluoro-phenolato)
borane was present in solution. Compared with δ val-
ues for other HB(OR)2 species [23] the boron nucleus
is better shielded by about 20 ppm, and this indicates to
the formation of a THF adduct. On the other hand, the
coupling constant fits better to a tri-coordinated boron
atom. In addition, a singlet at δ = 3.1 (∼ 16%) has
shown clearly the formation of B(OC6F5)3·THF, 1. On
standing over night the reaction mixture turned into a
gel (THF polymerization) in which crystals were em-
bedded. These were isolated and proved to be 1 by an
X-ray structure determination (see Fig. 1). 1 is most
likely generated by a substituent exchange as shown in
eq. (7). Because (C6F5O)2BH·THF could not be ob-
tained in a pure state, we turned to the reaction of
LiBH4 with C6F5OH. This reaction in a polar solvent
provided only Li[HB(OC6F5)3] [13]. In our case we
Using pyridine as a solvent we observed the
formation of H3B·py by 11B NMR. On heating
the solution only Li[B(O2C6H2tBu2)2] and LiBH4
could be detected. This suggests that in a first
step the di-tertbutylcatechol was monometallated by
LiBH4 with formation of hydrogen, H3B·py and
Li[OC6H2(tBu)2OH]. These last two components then
reacted at higher temperature with formation of
Li[Bcat’2] and LiBH4 probably via Li[H2Bcat’].
A similar behaviour was observed for the reaction of
avoided the presence of a polar solvent and treated LiBH4 with 2,2’-dihydroxydiphenyl.In the presence of
a suspension of LiBH4 in benzene with the phenol pyridine the formation of H3B·py was noted. On heat-
according to eq. (8) in hexane. Hydrogen was read- ing, the final products were Li[B(O2R’)2ꢀ] (2) (R’ =
ily evolved. The solid was only sparingly soluble in C6H4-C6H4) and H3B·py (see eq. 11). However, when
CDCl3, and exhibited an 11B NMR signal at δ = 1.7. the reactions were performed in the presence of piperi-
This signal was rather broad and showed no clear dine, pyrollidine, and morpholine then the Li salts of
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J. Knizek et al. · Metal Tetrahydroborates and Tetrahydroborato Metallates, 30
Table 1. Reaction products (in % of B concentration) formed
from MOR and BH3 ·THF (ratio 1 : 1) in THF solution.
MOR
M[BH4] M[H3BOR] M[HB(OR)3 B(OR)3 HB(OR)2
LiOtBu
NaOtBu
KOtBu
LiOCPh3
26
43
7
19
44
38
65
47
4
23
–
–
–
–
–
–
–
–
15 calcd. 35
51
2
LiOC6H2tBu3 41
–
55
the diphenylenedioxa-di(amino)borates 3 – 5 as shown
in eq. (12) were formed. Due to their insolubility, these
salts can be readily isolated. However, in the presence
of NHtBuPh only 2 results but no 6. Formation of these
anions is an indication that during the course of the re-
action of LiBH4 with 2,2’-dihydroxydiphenyl the an-
ion [H2BO2R’]− is an intermediate.
11 could not be prepared from BtBu3 and LiH in THF
[24, 25]. It is the previously missing link in the series
of compounds Li[H4−nB(tBu)n] [26, 27].
Compound Li[HB(NMePh)3] (10), was the main
product in the reaction of LiNMePh with 7. However,
10 is unstable in solution decomposing slowly with
elimination of LiH and formation of B(NMePh)3. The
structure of this tris(amino)borane is already known
[28], and we characterized it by comparing the cell pa-
rameters of the crystal.
Reactions of phthalatoborane with nucleophiles
Because the reaction of phthalic acid with LiBH4
led always to a mixture of products we synthesised
the phthalatoborane [C6H4(C(O)O]2BH from phthalic
acid and borane in THF as shown in eq. (13). 11B NMR
spectroscopy showed that this compound was present
in solution only in minute amounts (∼ 2%). The main
product was the THF adduct 7. However, 7 is not a sta-
ble compound, it decomposes in solution. Therefore,
its reactions were studied by using only freshly pre-
pared material. In contrast, the pyridine adduct 8 is per-
fectly stable.
1 : 1-Reactions of 7 with sodium tbutylate, lithium
amides or lithium tbutyl resulted in the formation of
various compounds. We found it essential that the so-
lution of 7 is added to the solid alkali metal compounds
in a 3 : 1 molar ratio in order to get reproducible results.
Under these conditions reactions according to eq. (14)
occurred.
Also NaOtBu reacted with 7 according to eq. (14).
The main product (∼ 80%) was Na[HB(OtBu)3], 9,
characterized by its 11B NMR spectrum which showed
1
a doublet at δ11B = 0.4 ppm, J(11B1H) = 121 Hz.
It was accompanied by a second product (∼ 20%)
with δ11B = 2.3 ppm. This chemi−cal shift corresponds
−
with B(OtBu)4 [e. g. B(OMe)4 shows a signal at
δ
11B = 2.7 ppm] [23]. Attempts to obtain single crys-
tals of Na[HB(OtBu)3] from the mixture of the two
products by first extracting it with hexane followed by
diethyl ether showed that most of the Na[B(OtBu)4]
went int◦o the ether phase. Storing the hexane solution
at −80 C and removing the solvent partially in sev-
eral steps resulted in the formation of colorless crys-
tals which deteriorated rapidly in a dry stream of N2.
Therefore, single crysta◦ls for a structure determination
When 7 was treated with three equivalents of LitBu were selected at −30 C. These proved to have the
compound (THF)3Li[HB(tBu)3] (11), could be iso- rather complex composition Na[HB(OtBu)3]·Na[OB-
lated from the solution as colorless crystals. Its crys- (OtBu)2]·Na[OB(OtBu)OH0.64H0.36]·. Obviously hy-
tal structure was determined which is shown in Fig. 4. drolysis of Na[HB(OtBu)3] had occurred to a certain
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J. Knizek et al. · Metal Tetrahydroborates and Tetrahydroborato Metallates, 30
1083
◦
Table 2. Reaction products observed by 11B NMR of a 1 : 1
reaction of catecholatoborane with tBuLi in ether/hexane at
−78 ◦C. Coupling constants 1J(11B1H) in parentheses.
−78 C. Table 2 shows that several compounds were
formed, the main product being a dihydroborate, in this
case the known [BH2(tBu)2]− [26]. Addition of DME
to the mixture led to the crystallization of one of the by
products as (dme)2Li[(C6H4O2)B(tBu)2] (13) whose
structure could be determined (see Fig. 4).
11B Chemical
shift (ppm)
Percentage
proportion
CatBtBu
36.1 s
14.6 br
7.8 s
−4.5 t (69 Hz)
−20.3 quart. (79 Hz)
−40.8 quint. (81 Hz)
15
10
3
50
17
5
Li[CatB(tBu2)]
Li(BCat2)
Li(H2BtBu2)
Li(H3BtBu)
LiBH4
A totally different behavior was observed when
lithium piperidide was used as the nucleophile. Once
again a mixture of compounds was generated. In this
case no dihydroborate was formed, but Li[H3Bpip]
appeared in small amounts (6%). The main compo-
nents of the mixture were HB(pip)2 (34%) and a
catecholatoborate whose composition turned out to
be (dme)Li[catB(H)O-C6H4OLi(THF)] (13) (38%). It
was characterized by its crystal structure. Table 3
presents the result of this reaction.
Table 3. Reaction products observed by 11B NMR of
a 1 : 1 reaction of c◦atecholatoborane with LiNC5H10 in
ether/hexane at −78 C. Coupling constants 1J(11B1H) in
parentheses.
11B Chemical
shift (ppm)
25.8 d (124 Hz)
8.4 s
6.5 br
−16.3 br
Percentage
proportion
34
22
38
6
HB(NC5H10
Li(BCat2)
Li(CatBH-OC6H4OLi)
Li(H3BNC5H10) [29]
)
2
Crystal structures
Although NMR data, in particular 11B NMR data
allow the assignment of most of the species by their
chemical shifts and 1J(11B1H) coupling constants,
they give, however, no information as to the degree of
association in solution which is expected particularly
for the O and N containing products which may inter-
act with the alkali metal ions [11]. Since a number of
hitherto unknown borates were formed, X-ray structure
determinations were essential.
degree. The compounds crystallized with 1 mol of
hexane.
Reactions of catecholborane with nucleophiles
In analogy to the reactions of 7 with nucleophiles
we also studied similar reactions with catecholatobo-
rane. There is a high driving force for the formation of
the alkali metal bis(catecholato)borate. Therefore, one
may expect reactions according to eq. (15, 16).
Fig. 1 shows the structure of (C6F5O)3B·THF. The
¯
The reaction of catecholborane with LitBu was per- compound crystallized in space group R3, and the
formed in a mixture of diethyl ether and hexane at boron atom and the oxygen atom of THF molecule oc-
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J. Knizek et al. · Metal Tetrahydroborates and Tetrahydroborato Metallates, 30
Also, Li-O bond lengths differ considerably: from
˚
1.917(4) to 2.510(4) A. The longest bond is to the oxy-
gen atom O1. The surrounding about the boron cen-
ter is also rather asymmetric. B-O bonds range from
˚
1.429(3) to 1.464(3) A, and O-B-O bond angles from
100.7(2)◦ to 112.7(2)◦. While oxygen atoms of the
2,’2-dioxodiphenyl unit are bonded to the boron atom
only two of them coordinate also to the Li1 atom.
It is well known that the two phenyl rings of
diphenyl are twisted against each other. In 2 the inter-
planar angles are 135.6 and 142.5◦, respectively. More-
over, the central LiO2 B ring is not planar as shown by
an interplanar angle O1Li1O4/O1B1O4 of 128.2◦. The
point group symmetry of the molecule is C1.
Fig. 3 (top) depicts the rather unusual struc-
ture of a dimeric sodium hydrido-tris(tert-butoxo)
borate
{Na[HB(OtBu)3]·Na[OB(OtBu)2]·Na[OB-
Fig. 2. Molecular structure of (dme·THF)Li[B(O2C12H8)2],
2·THF·dme. Thermal ellipsoids are depicted on a 25% (OtBu)OH0.64H0.36]}2. For the sake of clarity the
˚
probability level. Selected bond lengths (in A) and bond
Me groups have been omitted. The molecule is
centrosymmetric and contains a Na6O6 core. All of
its oxygen atoms except O5 and O5A coordinate with
sodium ions. The Na6O6 core can be viewed as two
face sharing cubes with two opposite corners missing.
However, the two missing sites could be assumed as
filled in by atoms O3/O3A but the Na1–O3 distance
angles (in ◦): Li1–O1 2.510(4), Li1–O4 1.917(4), Li1–
O5 1.931(4), Li1–O6 2.036(4), Li1–O7 1.920(4), B1–
O1 1.454(3), B1–O2 1.438(3), B1–O3 1.429(3), B1–
O4 1.464(3), O1–C1 1.356(2), O2–C12 1.345(2), C6–
C7 1.459(3), O2–C12 1.345(2), O3–C13 1.353(2), O4–
C24 1.360(2), C18–C19 1.458(3). – O1–Li1–O4 59.2(1),
O1–Li1–O5 93.9(2), O1–Li1–O6 160.0(2), O1–Li1–O7
102.3(2), O4–Li1–O5 124.6(2), O4–Li1–O6 108.2(2),
O4–Li1–O7 111.7(2), O5–Li1–O6 180.6(2), O5–Li1–O7
˚
is 3.272 A, and this excludes a sufficiently strong
121.5(2), O6–Li1–O7 96.9(2), O1–B1–O2 112.5(2), O1– interaction.
B1–O3 113.8(2), O1–B1–O4 100.7(2), O2–B1–O3 105.3(2),
While Fig. 3 (middle) shows the asymmetric unit
O2–B1–O4 112.7(2), O3–B1–O4 112.2(2), Li1–O1–B1
86.5(2), Li1–O1–C1 154.5(2), Li1–O4–B1 112.5(2), Li1–
O4–C24 121.2(2). – Interplanar angles (in ◦): C1–C6/C7–
C12 135.6, C13–C18/C19–C24 142.5, O1B1O4/O3B1O2
84.3, O1Li1O4/O1B1O4 128.2.
of the molecule including half of the hexane solvent
molecule, one can readily note the Na[HB(OtBu)3]
building block in Fig. 3 (bottom). The Na[HB(OtBu)3
part of the molecule coordinates with its three O atoms
to three different Na atoms. The sum of the O–B–O
bond angles is 317.4◦, about 10◦ less than for an ideal
tetrahedral arrangement. The average B–O bond length
cupy a special position on the threefold axis. There-
fore, the positions of the C atoms of the THF molecule
are threefold disordered. Fig. 1 shows only one of these
three orientations. The B–O bond lengths of the phe-
˚
is 1.495(5) A. This lies well in the range of B–O bond
˚
lengths found for alkoxoborates (1.42 to 1.59 A).
˚
nolic oxygen [1.431(4) A] falls into the category ob-
In addition to this building block there are two dif-
ferent anionic groups containing tri-coordinated boron
atoms: [(tBuO)2BO]− and [tBuOB(OH,H)O]−. The
second of these showed an unusually high thermal
parameter for its O9 atom suggesting that close to
its position an other lighter atom was present. There-
served for tetraorganyloxoborates. However the B–O
˚
bond to THF is longer, 1.557(9) A. The C–O–B bond
angles to the C6F5 group is quite open with 123.8(6)◦,
similar to the C–O–B bond angle of 124.7(7)◦ for the
THF oxygen atom.
Fig. 2 shows the molecular structure of compound fore, we placed a hydrogen atom closer to the boron
2·THF·DME. It crystallizes in the monoclinic space atom and refined the site occupation factors for H and
group P21/n with Z = 4. Its Li atom is pentacoor- OH. A reasonable solution resulted for the composi-
dinated. The Li1-O bonds show that the coordina- tion [tBuOB(OH)0.64H0.36O]−. The unique O atoms in
tion polyhedron around the Li center is a strongly these two anions coordinate differently with Na atoms.
distorted trigonal bipyramide with O-Li-O bond an- Atom O8 of the anion [tBuOB(OH)0.64H0.36O]− co-
gles spanning the range from 59.2(1)◦ to 160.0(1)◦. ordinates to four Na atoms while atom O6 of the
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1085
←− Fig. 3. Top: Molecular structure of {Na[HB(OtBu)3]·-
[Na[(tBuO)2BO]·[Na(tBuOB(O)(OH)0.64H0.36)]}2. Only
one C atom for each tert-butoxy group shown. Ther-
mal ellipsoids are depicted on a 25% probability level.
Selected bond lengths (in A) and bond angles (in ◦):
˚
Na1–O1 2.286(3), Na1–O8 2.319(3), Na1–O8A 2.397(3),
Na1–O6A 2.233(3), Na2–O2 2.249(3), Na2–O6 2-223(3),
Na2–O7A 2.297(3), Na2–O8A 2.434(3), Na3–O4 2.278(3),
Na3–O3 2.292(3), Na3–O8 2.491(2), Na3–O9 2.513(5),
Na3–O2 2.866(3), B1–O1 1.498(6), Na–O2 1.510(5),
B1–O3 1.476(5), B2–O4 1.406(5), B2–O5 1.392(5),
B2–O6 1.319(5), B3–O7 1.389(6), B3–O8 1.269(5), B3–O7
1.389(5), B1–H1 1.11(1). – O6A–Na1–O1 144.8(1), O6A–
Na1–O8 92.1(1), O1–Na1–O8 121.7(1), O6A–Na1–O8A
88.6(1), O6–Na2–O2 103.4(1),O6–Na2–O7A 118.4(1),
O2–Na2–O7A 138.3(1), O6–Na2–O8A 89.3(1), O2–Na2–
O8A 127.2(1), O7A–NA2–O8A 567.0(1), O4–Na3–O3
143.3(1), O4–Na3–O6 58.7(1), O3–Na3–O6 131.2(1),
O4–Na3–O8 119.3(1), O3–Na3–O8 97.4(1), O6–Na3–O8
85.8(1), O4–Na3–O9 116.2(1), O8–Na3–O9 102.7(1),
O4–Na3–O2 106.2(1), O1–B1–O2 105.3(4), O1–B1–O3
107.6(3), O2–B1–O3 104.5(3), O5–B2–O6 128.3(5), O4–
B2–O6 117.5(4), O4–B2–O5 114.2(4), O8–B3–O9 123.9(5),
B1–O1–Na1 121.6(2), B1–O2–Na2 117.4(2), B1–O2–Na3
85.3(2), B2–O4–Na3 94.8(2), B2–O6–Na2 136.3(3), B3–
O8–Na1 134.1(2), B3–O8–Na2A 91.3(3), B3–O8–Na1
134.4(3). Torsion angles (in ◦): Na1O1B1O2 −86.6,
Na3O3B1O2 150.9, Na2O2B1O1 21.4. Interplanar angles
(in ◦): Na1AO6Na2/O6B2O4 49.4, Na1AO8Na1/O8B3O2
82.7, Na3O6Na2/O6B2O4 136.5.
Middle: The Na[HB(OtBu)3] part of the structure shown
above.
Bottom: The Na[HB(OtBu)3] building block of the struc-
ture shown above. C25 – C27 shows one half of the hexane
solvent.
ordinates to the solvated Li ion via its hydrogen atom.
The Li1–H1–B1 bond angle is 153(1)◦. Steric repul-
sion between the tBu groups opens the C–B–C bond
angles on average to 113◦. However, this type of inter-
action becomes more significant when looking at the
B–C bond lengths. Within the series Li[H3BtBu]<Li-
[H2BtBu2]<Li[HBtBu3] there is a constant increase
of the B–C bond lengths from 1.625(3) via 1.643(3)
˚
to 1.71(1) A [26, 27]. The B–H bond length is on the
long side of those known in organylhydridoborates,
but the Li–H distance falls well within those found in
other lithium hydridoborates.
Fig.
5 depicts the structure of compound
(dme)2Li[catB(tBu)2] (12). Its structure is rather
simple compared to those shown in Fig. 3a but
nevertheless provided a surprise, because one of the
two DME ligands is monodentate while the other is
bidentate. This is due to the fact that one oxygen atom
[(tBuO)2BO]− group interacts with only three Na
atoms.
Fig.
4
shows the molecular structure of of the catecholato unit binds to the Li center resulting
(THF)3Li[HB(tBu)3]. As expected the anion co- in the shortest of the four Li–O bonds. The longest
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J. Knizek et al. · Metal Tetrahydroborates and Tetrahydroborato Metallates, 30
Fig. 4. Molecular structure of (THF)3Li[HB(tBu)3],
11·3THF. Thermal ellipsoids are depicted on a 25%
˚
probability level. Selected bond lengths (in A) and bond
angles (in ◦): Li1–O1 1.97(1), Li1–O2 1.94(2), Li1–O3 1.96,
B1–C1 1.71(1), B1–C5 1.73(1), B1–C9 1.69(1), Li1–H1
Fig. 6a. The asymmetric unit of 13.
1.81(2).
– C1–B1–C5 113.2(6), C1–B1–C9 113.8(7),
C5–B1–C9 112.7(7), C1–B1–O4 1.464(3).
Fig. 6b. Molecular structure of {(thf·Li)(dme·Li)[(catBH)-
(cat)]}2, (13)2. Thermal ellipsoids are depicted on a 25%
˚
probability level. Selected bond lengths (in A) and bond
angles (in ◦): Li1–O4 1.887(4), Li1–O5 2.022(4), Li1–O6
2.010(4), Li1–O1A 1.953(5), Li2–O4 1.940(4), Li2–O7
1.910(4), Li2–O7 1.910(4), Li2–O3A 1.944(4), B1–O1
1.519(3), B1–O2 1.466(4), B1–O3 1.471(4). – O1–B1–O2
Fig. 5. Molecular structure of (dme)2Li[tBu2Bcat] (12). 104.1(2), O1–B1–O3 113.0(1), O2–B1–O3 111.6(2),
Thermal ellipsoids are depicted on a 25% probabil- H1–B1–O2 112(1), O4–Li1–O5 118.6(2), O4–Li1–O6
˚
ity level. Selected bond lengths (in A) and bond an- 112.0(2), O4–Li1–O1A 112.6/(2), O4–Li2–O7 120.1(2),
gles (in ◦): Li1–O2 1.936(5), Li1–O3 2.013(6), Li1–O4 O4–Li2–O4A 99.5(2), O4–Li2–O3A 106.5(2), O3A–Li2
2.010(6), Li1–O5 1.966(6), B1–O1 1.531(4), B1–O2 –O4A 85.2(2), O7–Li2–O4A 124.5(2), O7–Li2–O3a
1.589(4), B1–C7 1.646(5), B1–C11 1.644(5), O2–C1 114.9(2), O3A–Li2–O4A 85.2(2). Interplanar angles (in ◦):
1.361(3), O1–C6 1.350(3), O3–C16 1.382(4), O4–C17 Li2O4Li2AO2A/O4Li2AO3 38.8, O4Li2AO3/BaO1O2
1.385(4), O4–C18 1.415(4), O5–C19 1.428(4), O4–C20 69.7, B1O1O2/C1-C6 2.4, O4O3Li2A/O4ALi1AO1 71.3.
1.445(4), O6–C21 1.415(5), O6–C22 1.411(4). – O4–Li1–
O5 117.1(3), O4–Li1–O2 116.2(3), O4–Li1–O3 81.2(2),
Li–O bonds are those to the chelated DME molecule.
O2–Li1–O3 131.8(3), O2–Li1–O5 108-8(3), O3–Li1–O5
As expected, there are two different B–O bond lengths
99.8(2), O4–Li1–O5 117.1(3), Li1–O2–B1 133.8(2), Li1–
˚
for the catecholato unit: 1.531(4) and 1.589(4) A.
O2–C1 118.0(2), B1–O1–C6 108.9(2), B1–O2–C1 107.1(2),
O1–B1–C7 121.8(3), O1–B1–C11 108.8(2), C7–B1–C11
121.8(3). – Interplanar angles (in ◦) O3L1O4/O5Li1O2 75.9,
O1B1O2/B1C7C11 88.4, O1B1O2/C1-C6 2.9.
Coordination to the Li atom lengthens the B–O bond.
The two B–C bonds are of equal lengths. They are
˚
shorter by 0.06 A than in compound 11. Although the
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J. Knizek et al. · Metal Tetrahydroborates and Tetrahydroborato Metallates, 30
1087
O–B–C bond angles are close to 110◦ the C–B–C bond
angle is quite open and corresponds with 121.8(3)◦ to
an sp2 type boron atom.
Table 4. Point group symmetry for a series of substituted hy-
dridoborates.
F, Cl
C3v
C2v
C3v
Td
OMe
C1
C2
Cs
D2d
CN
C3v
C2v
C3v
Td
NCS
C1
C2v
C3v
D2d
Me
C3v
C2v
C3v
T
NH2
C1
C1
C1
C1
H3BX−
An interesting structure is also provided by com-
pound 13. Fig. 6a shows the asymmetric unit. The
low coordination of the Li atoms suggest that it is
associated. One of the two Li atoms interacts with a
chelating DME molecule while the other one binds
one THF molecule. Indeed, the molecule is dimeric
in the solid sate. The association to a centrosym-
metric dimer is represented in Fig. 6b. Now the Li
atoms are tetra-coordinated. The structure of the dimer
reveals that one catecholate unit is doubly depro-
tonated, one Li atom forms an almost planar five-
membered LiO2C2 ring while the chatecholatoborane
−
−
H2BX2
HBHX3
−
BX4
Table 5. Calculated reaction energies (in kJ/mol for 1 mole
of H4−nBXn−) for the disproportionation of substituted hy-
dridoborate anions.
X =
F
Cl OMe CN NCS Me NH2
4 H3BX− → BX4− +3BH4 −43 +25 −38 +53 +49 −9 −31
−
2 H2BX2− → BX4− +BH4 −44 +18 −26 +68 +56 −4 −32
−
4 HBX3− → 3BX4− +BH4 −19 +29 −13 +49 +37 −5 −21
−
The data show that there are two types of sub-
unit coordinates with one of its O atoms to a Li atom stituents X. Substituents F,−OMe and NH2 favor the
(O1A-Li1). The molecule can be looked at as a lithium disproportionation to BH4 and BX4−, while sub-
tri(organyloxo)hydridoborate, because its boron atom stituents Cl, CN and NCS favor the mixed hydridobo-
binds to three oxygen atoms and one hydrogen atom.
rate species. The data for X = Me sh−ow that the energy
differences in the series BMe4−nHn are rather small.
This could indicate that disproportionation equilibria
are established. However, since all members of the se-
ries could be isolated and structurally characterized, it
is most likely that their stabilities are kinetically con-
trolled. Because examples are known for M = Li, Na,
and K, the cation has no influence on the stability. The
calculated data for X = F, OMe and NH2 are in ac-
Stability of substituted hydridoborates
Previous work [6 – 8, 12– 14, 26, 27, 29– 32] has
shown that substituted hydridoborates have a tendency
to substituent exchange where the species [HBX3]−
seems to be the most stable for X = F, OR and NR2. In
contrast, all members of the alkali metal alkyl substi-
tuted hydridoborates with different R groups are read-
ily accessible and for some of them the crystal struc-
tures have been determined [26, 32 – 38]. In this study
the missing member in the series Li[H4−nB(tBu)n], the
compound with n = 3 was prepared, and structurally
characterized. As far as we are aware no theoretical
study has been performed which could give evidence
that the cited ten−dencies of substitutent exchange to
−
cord with the preferred formation of HBX3 species,
−
while for X = CN and NCS it is the species H2BX2
.
The substituents F, OMe and NH2 are π-donors. Ob-
viously th−is property is one factor that destabilizes the
H4−nBXn (n = 1,2) species. There is no correlation
with electronegativity.
The type and number of substituents should influ-
ence the B–H and B–X bond lengths. Calculated bond
−
give finally BH4 and BX4 is thermodynamically or
−
kinetically controlled. Therefore, we performed DFT
calculations at the B3LYP/6-311++G(d,p) level for a
series of substituted hydridoborate species. Solvent
and cation effects were not taken into account. Table 4
lists the point group symmetry for these species. Cal-
culations for these geometries showed no imaginary
frequencies, i. e. the calculations refer to minima on the
local hypersurface.
lengths for the H4−nBXn anions as a function of n
and X are listed in Tables 6 and 7. In most cases the
B–H distance decreases with increasing numbers of X.
The differences are small for X = OMe, Me and NH2,
but significant for X = F, Cl and CN. In case of X = Me
the B–H bond length increases as the number of Me
groups increases. Amongst the X groups studied, the
low electronegativity of the Me group with its electron
The calculated ∆Hf energies for the species allow donating function results obviously in bond lengthen-
an estimation of the reaction energies for the isodesmic ing. For X = F, Cl, OMe, NH2, and NCS the B–X bond
reactions summarized in Table 5. Because the number lengths decrease with increasing n while the B–C bond
of bonds remain the same in the ligand exchange re- lengths of the cyanohydridoborate and the methylhy-
actions, systematic errors are compensated [38]. The dridoborate remain almost constant. So, only strong
energies quoted in Table 5 refer to one mole of the sub- electronegative groups have a significant influence on
stituted hydridoborate.
the bond lengths.
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J. Knizek et al. · Metal Tetrahydroborates and Tetrahydroborato Metallates, 30
−
˚
Table 6. Calculated B–H bond lengths (in A) for H4−nBXn
this holds also for the Li[HB(OtBu)3] system. In the
series Li < Na < K the formation of M[H3B(OtBu)]
compounds is favored, showing that substituent ex-
change is retarded with increasing ionic character of
the compounds. Also noteworthy is the observation
that lithium tris(2,4-di[tert-butyl)phenolato]-hydrido
borate is converted to a large extent not only into
LiBH4 but also into HB[OC6H3(tBu)2]2. This latter
compound may result from a reaction according to
eq. (17)
as a function of n and X.
X =
F
Cl
OMe
CN
NCS
Me
NH2
H3BX−
1.240 1.215 1.242 1.224 1.222 1.243 1.245a
1.238 1.200 1.238 1.215 1.213 1.245 1.246b
1.228 1.191 1.242 1.210 1.208 1.249 1.241
b
−
H2BX2
−
HBX3
a
˚
Average value of 1.239, 1.239 and 1.256 A;
average value of
˚
1.241, 1.251 A.
−
˚
Table 7. Calculated B–X bond lengths (in A) for H4−nBXn
as a function of n and X.
X =
F
Cl
OMe
CN
NCS
Me
NH2
H3BX−
1.483 2.012 1.506 1.590 1.548 1.657 1.577
1.456 1.941 1.499 1.589 1.535 1.653 1.570a
1.433 1.901 0.484 1,t591 1.525 1,654 1.566
1.417 1.882 1.479 1.596 1.518 1.658 1.565
BH3 ·THF+Li[H2B(OR)2]
(17)
→ HB(OR)2 +LiBH4 +THF
−
H2BX2
−
HBX3
BX4
−
a
˚
Average value of 1.556 and 1.583 A.
While Singaram et al. [13] observed that LiBH4
reacted with C6F5OH in THF with formation of
Li[HB(OC6F5)3] only, we obtained good evidence
that in non polar solvents the reaction provides
Li[H2B(OC6F5)2] which decomposes in THF to
Li[HB(OC6F5)3]. On the other hand, the reaction
of HB(OC6F5)2·THF with LiH in THF gave only
(C6F5O)3B·THF as an identifiable product. It is one
of the rare examples that a borane (RO)3B forms
an adduct with THF. There is only one compound
known which is comparable, the dimeric [N(1,3-
O2C6H3)3]B·THF [45].
Two,−respectively three, different bond lengths for
H2BX2 and H3BX− result from the different orien-
tations of the lone pair of electrons of the NH2 group
relative to the H or X atoms.
Discussion
It is well known that organyl hydroborates
M[H4−nBRn] are less prone to a substituent exchange
than alkoxohydroborates M[H4−nB(OR)n] [39].
Amongst the latter the tris(organyloxo)hydroborates
M[HB(OR)3] seem to be the most stable ones in
this series. However, their stability against ligand
exchange depends on the size of the R group and
the metal atom: the stability increases in the series
Li < Na < K while it decreases with increasing size
of the organyl group [26, 39]. Brown et al. detected
that substituent exchange of the alkoxohydrido borates
K[HB(OR)3] could be prevented by an excess of KH
in THF solution [8, 29, 32]. On the other hand, alkox-
ohydridoborates M[H3BOR] and M[H2(OR)2] are th−e
2,2’-Dihydroxydiphenyl reacts with LiBH4 in THF
to give the lithium borate 2. However in the presence
of secondary amines, borates of the type Li[(OC6H4-
C6H4O)B(NR2)2] are readily accessible, and this is an
indication that Li[(OC6H4-C6H4O)BH2] is an interme-
diate. Normally, secondary and primary amines, un-
der ambient conditions, form only adducts with alkali
metal tetrahydridoborates [46].
The formation and stability of the phthalatoborane
7 is similar to the diacylboranes described by H.
C. Brown derived from dichloroacetic acid, benzoic
acid and p-chlorobenzoic acid. These also form THF
adducts [47]. Compound 7 reacts easily with nucle-
ophiles to give hydridoborates of type M[HB(Nu)3].
Thus the reaction with LitBu generated Li[HB-
(tBu)3]·3 THF, which could so far not be observed by
other routes [48]. With NaOtBu we obtained in THF
least stable species in the series of [H3−nB(OR)n+1
]
anions (n = 1,2,3). An alkali metal salt of the
catecholatodihydroborate anion [H2B(O2C6H4] is
unknown, but it can be stabilized by coordination to a
transition metal fragment. Coordination involves two
MHB bonds [40– 43].
Our study supports previous investigations in this solution Na[HB(OtBu)3]. Upon attempts to isolate it
field [12, 13, 16]. The formation of B(OtBu)3 in the re- in the form of single crystals we obtained Na[HB-
action of BH3·THF with LiOtBu shows that an elim- (OtBu)3]·Na[B(OtBu)2O]·Na[(H,OH)B(OtBu)O]2 as
ination of LiH occurs during the reaction, and re- an unexpected hydrolysis product, which has an un-
garding the results by Brown et al. [44] in the stabi- precedented Na6O6 core structure. The O atoms of the
lization of K[HB(OiPr)3] by KH it may well be that HB(OtBu)3 anion bind to three different Na atoms.
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J. Knizek et al. · Metal Tetrahydroborates and Tetrahydroborato Metallates, 30
1089
The structure of the comparable centrosymmetric Bis(pentafluorophenoxy)borane tetrahydrofuran and
tris(pentafluorophenoxy)borane tetrahydrofuran (1)
phenolate {Na[HB(OPh)3](THF)2}2 is a dimer. Its
phenoxy oxygen atoms coordinate to the Na atoms,
To 8.03 ml of a stirred solution of BH3 in THF
(21.7 mmol) was added slowly a solution of pentafluorophen-
ol (7.98 g, 43.4 mmol) in THF (20 ml). After 2 h the hydro-
gen gas evolution had ceased. The 11B NMR spectrum of the
solution showed signals at δ = 3.1 [s, 16%, 1] and 7.4 ppm
[d, 1J(11B1H) = 142 Hz), 84%, (C6F5O)2BH·THF]. A gel
two forming B–O–Na bridges and one a B–O(Na)2
˚
bridge [49]. The B–O bond of this unit is 0.3 A longer
than the other two, and it may well be that these inter-
actions favor the substituent exchange of the organy-
loxo hydrido borates. Alkali metal alkoxoborates offer
a fair number of structural motifs. These range from formed overnight in which crystals of octahedral shape were
present. They proved to be 1. These were isolated, but not all
of the gel could be properly removed.
solvent separated ion pairs [5] via dimers [50] to catena
type structures [50].
C22H8BF15O4 (632.10): calcd. C 41.80, H 1.27; found
C 40.54, H 1.72.
Although a number of interesting new compounds
could be characterized there remain still a number of
questions to be answered in order to better understand
the governing principles of these reactions. The calcu-
lational results give a hint to the thermodynamics in-
volved, but it appears that kinetic control is at least as
essential.
1H NMR (270 MHz, C6D6, SiMe4): δ = 1.33, 1.67
1
(m, CH2-CH2), 3.30, 3.52 (m, O-CH2). – 13C{ H} NMR
(C6D6): δ = 25.42 (OCH2CH2), 26.97 (OCH2CH2), 68.07
(OCH2), 70.78 (OCH2). 13C NMR signals for the aryl group
could not be observed. – 19F NMR (C6D6, CFCl3): δ =
−176.0 (unresolved triplet, p-F), 170.6 (t, 3J(19F19F) =
20 Hz, m-F), −168.8 (d, 3J(19F19F) = 20 Hz). – 11B NMR
(THF, BF3·OEt2) = 3.1 (s).
Experimental Section
1 : 1 Reaction of LiBH4 with phenol: 67 mg (3.1 mmol) of
LiBH4 was dissolved in THF (40 ml). To the stirred solution
was added a solution of phenol (275 mg, 2.92 mmol) in THF
(20 ml). A rapid gas evolution ceased within 15 min. Stirring
was continued for 1 h. Then the 11B NMR spectrum of the
solution showed only the presence of Li[HB(OPh)3] [δ11B =
2.6 (d, 1J(11B1H) = 115 Hz, 33%] and LiBH4 [δ11B = −41,
quint., 1J(11B1H) = 81 Hz, 67%].
All reactions were carried out under dry nitrogen gas us-
ing the Schlenk technique. Stock solutions were prepared by
dissolving diborane generated in the reaction of NaBH4 with
BF3 in diglyme. LiBH4, NaBH4, and LitBu were obtained
from Chemetall GmbH. The metallated alcohols, phenols
and amides were prepared by standard methods. All other
reagents were commercially available.
NMR spectra were measured with Jeol GSX 270 or Jeol
EX 400 instruments. The chemical shifts are referenced to
the usual standards either internal or external (11B, 7Li, 19F).
Data sets for the crystal structure determinations were col-
lected with a Siemens P4 diffractometer equipped with an
area detector and a low temperature device LT2. Elemental
analyses were performed in the Department’s microanalyti-
cal laboratory.
Lithium (2,2’-dioxodiphenylene)bis(piperidino)borate (3)
53 mg of LiBH4 (2.4 mmol) were dissolved in piperidine
(15 ml). To the stirred solution was added solid 2,2’-dihydr-
oxy-diphenyl (459 mg, 2.46 mmol). After gas evolution had
ceased the turbid solution was heated for 12 h at reflux. The
solid that had formed was removed by filtration and dried.
Only a single 11B NMR signal could then be detected in the
solution at δ11B = 26.9 [d, 1J(11B1H) = 118 Hz] which is
due to HBpip2. Yield of 3: 430 mg (48%), m. p. ∼ 150 ◦C.
1H NMR (400 MHz, d6-DMSO): δ = 1.37 (m, 8H), 1.45
(m, 4H), 2.62 (m, 8H), 6.88 [d, 2H, 3J(1H1H) = 7.9 Hz],
Reactions of H3B·THF with alkali metal alkoholates and
phenolates: (see also Table 1)
To 50 ml of a 0.064 M solution of BH3·THF was added at
−78 ◦C 25 ml of a THF solution containing LiOtBu (283 mg,
3.54 mmol). While stirring, this mixture was allowed to
warm to r. t. A sample was investigated by 11B NMR which
showed signals at −41.2 (26%), −15.0 (7%), −0.6, 1.2 and
3.5 (together 44%) and 16.1 ppm (23%). Signals in the re-
gion −0.6 to 3.5 ppm overlapped; insufficient resolution did
not allow the determination of the number of H atoms at
these boron atoms. The same result was obtained when the
reagents were mixed at ambient temperature. Table 1 lists
the results of analogous reactions with alkali metal alkoho-
lates and phenolates.
3
3
6.96 [t, 2H, J(1H1H) = 7.5 Hz], 7.23 [t, 2H, J(1H1H) =
7.9 Hz], 7.33 [d, 2H, 3J(1H1H) = 7.5 Hz]. – 13C NMR
(101 MHz, d6-DMSO): δ = 25.1, 26.8, 46.9, 120.1, 122.3,
128.0, 128.7, 132.1, 157.4. – 11B NMR (97 MHz. d6-
DMSO): δ = 7.3.
C
22H28O2N2Li (369.99): calcd. C 71.43, H 7.63, N 7.52;
found C 71.46, H 7.73, N 6.85.
Lithium-(2,2’-dioxodiphenyl)bis(pyrrolidino)borate (4)
124 mg of LiBH4 (5.69 mmol) were dissolved in pyrro-
lidine (25 ml). After addition of 2,2’-dihydroxydiphenyl
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J. Knizek et al. · Metal Tetrahydroborates and Tetrahydroborato Metallates, 30
(1.06 g, 5.69 mmol) the mixture was stirred for 1 h. No gas Lithium bis(pentafluorophenolato)dihydridoborate
evolution was noted, but commenced on heating to reflux.
To a stirred suspension of LiBH4 (240 mg, 11.2 mmol) in
A solid started separating after about 30 min. Refluxing was
benzene (25 ml) was added a solution of C6F5OH (4.12 g,
continued for another 3.5 h. Then the supernatant solution
22.4 mmol) in benzene (50 ml). Hydrogen gas was gener-
showed an 11B NMR signal at δ = −17.3 (quart.) due to
3
ated slowly. After 19 h the H2 evolution had ceased. /4 of
H3B·HNC4H8. The solid was isolated and washed with hex-
the benzene was removed from the suspension and the solid
ane to give 4 as a white powder. Yield: 660 mg (34%), m. p.
isolated. Yield: 3.4 g (79%), m. p. 219 – 220 ◦C.
180 ◦C dec.
NMR: 13C and 1H NMR signals could not be detected in
1H NMR (270 MHz, d6-DMSO): δ = 1.54 (m, 8H),
d6-DMSO solutions. – 11B NMR (87 MHz, CDCl3) = 1.7
2.62 (m, 8H), 6.88 [d, 3J(1H1H) = 7.6 Hz, 2H], 6.96 [t,
(br, 11B1H coupling could not be observed); (in THF): δ =
3J(1H1H) = 7.6 Hz, 2H], 7.23 [t, 3J(1H1H) = 7.6 Hz, 2H],
6.1 [d, 1J(11B1H) = 135 Hz] assigned to (C6F5O)2BH·THF.
7.32 [d, 3J(1H1H) = 7.6 Hz, 2H]. – 13C NMR [68 MHz, d6-
C12H2O2BF10Li (385.89): calcd. C 37.35, H 0.52; found
DMSO): δ = 25.8, 47.1, 120.4, 122.6, 128.2, 128.7, 132.3,
C 35.76, H 0.64.
157.7. – 11B NMR (87 MHz, d6-DMSO): δ = 7.5 (s). – 7Li
NMR (105 MHz, LiCl in D2O): δ = −1.1.
Reaction of LiBH4 with 3,5-di(tert-butyl)catechol in pyridine
C20H24O2N2BLi (342.21): calcd. C 71.41, H 7.80,
61 mg of LiBH4 were dissolved in pyridine (5 ml).
To the stirred solution was added 612 mg of 3,5-di(tert-
N 8.19; found C 71.41, H 7.60, N 8.07.
butyl)catechol. A rapid gas evolution started. After 10 h
Lithium (2,2’-dioxodiphenyl)bis(morpholino)borate (5)
the suspension showed only 11B NMR signals for BH3.py:
δ
11B = −12.0 [quart., 1J(11B1H) = 93 Hz, 80%] and
387 mg of LiBH4 (17.8 mmol) were dissolved in mor-
pholine (50 ml). After addition of 2,2’dihydroxydiphenyl
(3.297 g, 17.71 mmol) gas evolutions set in. Stirring was con-
tinued for 1 h and the resulting suspension was then heated
to reflux for 4 h. The solid 5 was isolated and washed twice
with hexane (50 ml) and dried. Yield: 3.93 g (59%), m. p.
170 ◦C dec.
LiBH4 [δ11B = −39.1 ppm (quint., 1J(11B1H) = 81 Hz,
20%]. The suspension was heated to reflux for 20 h. Then
the supernatant solution showed the presence of BH3.py
[δ11B = −12.0 ppm (quart., 1J(11B1H) = 93 Hz, 20%]
and Li[Bcat(tBu2)2] [δ11B = −14.2 ppm (s, 70%)] besides
some minor other signals.
1H NMR (270 MHz, d6-DMSO): δ = 2.67 (m, 8H),
3.51 (m, 8H), 6.93 [d, 3J(1H1H) = 7.8 Hz, 2H], 6.99 [d,
3J(1H1H) = 7.6 Hz, 2H], 7.26 [t, 3J(1H1H) = 7.6 Hz, 2H],
7.36 [d, 3J(1H1H) = 7.8 Hz, 2H]. – 13C NMR (68 MHz; d6-
DMSO): δ = 46.7, 68.0, 120.4, 122.6, 128.2, 128.7, 132.4,
157.7. – 11B NMR (97 MHz, d6-DMSO): δ = 7.5 (s). – 7Li
NMR (105 MHz, d6-DMSO): δ = −0.9.
Phthalatohydroborane
To a 2.12 M solution of BH3 in THF were added 9 ml of
THF. Stirring was then started and a solution of phthalic acid
(0.30 g, 1.8 mmol) dissolved in 15 ml of THF was added.
Gas evolution set in rapidly. After 15 min gas evolution had
ceased. The 11B NMR spectrum of the solution showed a
doublet at δ11B = 4.1 [d, 1J(11B1H) = 143 Hz] assigned
to 7. Two more signals at δ11B = 18.0 [s, 18%, a B(OR)3
species] and at δ11B = 27.1 (d, 163 Hz, 2%, most likely
phthalBH) were also noted.
Addition of pyridine to the solution led to a single
11B NMR signal at δ11B = 3.0 [d, J(11B1H) = 133 Hz)],
which corresponds to the pyridine adduct 8 of the phthalato-
hydroborane.
C20H24O4N2BLi (374.20): calcd. C 64.20, H 6.47,
N 7.49; found C 64.87, H 7.07, N 7.43.
Lithium bis(2,2’-dioxodiphenyl)borate (2)
1
To a stirred solution of LiBH4 (44 mg, 2.02 mmol) in
benzyl-tert-butylamine (20 ml) was added 2,2’-dihydroxy-
diphenyl (2.02 mmol). No gas evolution set in. On heating to
reflux for 8 h a voluminous colorless precipitate was present.
It was isolated by filtration and washed with hexane. Yield:
480 mg of 2 (62%), m. p.>200 ◦C.
While 7 is stable in solution for several hours, removal
of the solvent produced a white powder which was hardly
soluble in deuterobenzene, and this solution showed two
11B NMR signals as singlets at δ11B = 18.0 and 3.7 in a
1 : 1 ratio.
1H NMR (270 MHz, d6-DMSO): δ = 6.87 [d,
3J(1H1H) = 7.5 Hz. 4H], 6.96 [t, 3J(1H1H) = 7.5 Hz, 4H],
7.23 [t, 3J(1H1H) = 7.5 Hz, 4H], 7.32 [3J(1H1H) = 6.6 Hz,
4H]. – 13C NMR (101 MHz, d6-DMSO): δ = 120.3, 122.6,
128.2, 128.7, 132.2, 157.7. – 11B NMR (64 MHz, d6-
DMSO): δ = 9.0 (s). – 7Li NMR (105 MHz, d6-DMSO):
δ = −1.1.
Reaction of sodium tert-butylate with phthalatohydroborane
in THF
NaOtBu (0.534 g, 5.56 mmol) was placed in a 50 ml
bulb flushed with N2. While stirring a solution of phtha-
C24H16O2N2BLi (386.13): calcd. C 74.65, H 6.25; found latohydroborane (1.8 mmol) in 25 ml of THF was added.
C 74.24, H 6.13. The suspension turned immediately yellow. Checking the
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1091
Table 8. Crystallographic data and parameters for data collection and structure solutions.
Compound
Chem. formula
Form. wght.
1
2
9
11
C27H62.36B3Na3O8.64 C24H52BLiO3
626.33 406.41
12
13
C22H8BF15O4
632.09
C32H34BLiO7
548.34
C22H42BLiO6
420.31
C20H27BLi2O7
404.11
Cryst. size [mm]
Cryst. system
Space group
0.15×0.32×0.45 0.10×0.10×1.00 0.10×0.10×0.30
0.05×0.20×0.20 0.05×0.10×0.20 0.20×0.20×0.20
hexagonal
R-3
monoclinic
P2(1)/c
15.551(9)
8.955(5)
19.576(1)
90.00
triclinic
P-1
orthorhombic
Pna2(1)
17.281(3)
10.1769(18)
14.989(3)
90.00
monoclinic
P2(1)/c
9.8850(9)
12.707(1)
20.742(2)
90.00
triclinic
P-1
˚
a [A]
13.40(1)
13.40(1)
21.82(2)
90.00
12.707(1)
13.678(2)
13.806(2)
66.858(2)
63.218(2)
69.529(2)
1925.0(4)
2
9.5059(9)
10.643(1)
11.967(1)
70.656(2)
72.608(2)
82.998(1)
1089.76(18)
2
˚
b [A]
˚
c [A]
α [◦]
β [◦]
γ [◦]
90.00
94.261(9)
90.00
2718(3)
4
90.00
90.00
2636.1(9)
4
100.3580(1)
90.00
2562.9(4)
4
120.00
3395(4)
6
3
˚
V [A ]
Z
ρ (calcd.), [mg/m3] 1.855
1.340
1.081
1.024
1.089
1.232
µ [mm−1
F(000)
]
0.206
0.092
0.104
0.063
0.075
0.089
1872
1160
682
912
920
428
Index range
−7 < h < 13
−13 < k < 4
−26 < l < 7
54.80
193
1067
−20 < h < 19
−11 < k < 10
−25 < l < 25
58.80
193(2)
15649
−13 < h < 16
−16 < k < 18
−17 < l < 17
58.02
193(2)
11215
−17 < h < 17
−10 < k < 10
−15 < l < 15
42.52
183(2)
9048
−10 < h < 10
−14 < k < 14
−22 < l < 20
46.50
193(2)
11149
−11 < h < 12
−13 < k < 13
−15 < l < 13
57.64
193(2)
6234
2 θ [◦]
Temp, [K]
Refl. collected
Refl. unique
993
5546
5907
2913
3516
3276
Refl. observed (4σ) 931
2801
0.0615
2668
0.0751
1288
0.1910
2008
0.0636
2267
0.0216
R (int.)
0.0123
No. variables
Weight.a x/y
GOOF
143
382
0.2401/0.000
0.896
400
0.0731/0.000
0.938
272
0.050/0.000
0.944
281
0.0799/0.000
0.985
277
0.0809/0.2540
1.032
0.1744/12.3827
1.132
Final R (4σ)
Final wR2
0.0921
0.2699
0.487
0.0496
0.0972
0.193
0.0612
0.1275
0.207
0.0741
0.1254
0.176
0.0561
0.1300
0.196
0.0509
0.1339
0.187
Larg. res. peak
3
˚
[e/A ]
a
w
−1 = s2Fo2 +(xP)2 +yP; P = (Fo2 +2F 2)/3.
c
resulting solution by 11B NMR showed the formation of tion was slowly added a solution of phthalatoborane in THF
NaHB(OtBu)3 [δ11B = 0.4 ppm, d, 1J(11B1H) = 121 Hz] (25 ml, 5.3 mmol) diluted with diethyl ether (50 ml). The so-
and a “contamination” [δ11B = 2.3, s; NaB(OtBu)4] in the lution turned immediately red. After addition was completed,
ratio of ∼ 4 : 2. After removal of the solvent the slightly stirring was continued at −78 ◦C for 30 min. An 11B NMR
yellow powder was first treated with some hexane and the spectrum of the solution showed 11B NMR signals at 5.6 (d),
solid extracted with diethyl ether. The diethyl ether con- −0.4 (d), −4.5 (t) and 83 ppm (s). Most of the solvents were
tained most of the NaB(OtBu)4. The hexane phase was stripped off in vacuo. The residue was dissolved in 5 ml of
◦
stored at −80 C. After two weeks crystals had separated, hexane and then kept at −30◦C. After several days crystals of
which became opaque in a stream of dinitrogen. There- (THF)3LiHB(tBu3) (11) had separated. The filtrate showed
fore, crystals for a structure determination were selected an 11B NMR signal at 83.0 ppm for BtBu3.
◦
at −30 C. According to the X-ray structure determination
they had the composition Na[HB(OtBu)3]-[NaOB(OtBu)- Lithium tris(methylphenylamino)hydridoborate and
(OH)0.64(H0.36)]·0.5 C6H14. The 11B NMR spectrum of the tris(methylphenylamino)borane
supernatant hexane solution showed two signals in a 1 : 1 ra-
◦
At −78 C 700 mg of LiNMePh (6.19 mmol) was dis-
1
tio at δ11B = 25.5 [d, J(11B1H) = 163 Hz], and a singlet
solved in THF (20 ml). To the solution was added 25 ml of
at δ11B = 16.0 indicating the presence of HB(OtBu)2 and
a THF solution of phthalatoborane (18 mmol). After several
B(OtBu)3, respectively.
hours the solvents were removed in vacuo from the slightly
Lithium tris(tert-butyl)hydroborate tris(tetrahydrofuran)
yellow solution. The yellow residue was washed with di-
ethyl ether and then dissolved in THF. The 11B NMR spec-
(11)·(THF)3◦
At −78 C a solution of LitBu in pentane (2.0 mmol, trum showed the presence of Li[HB(NMePh)3)] [doublet at
2.65 M) was diluted with hexane (20 ml). To the stirred solu- −2.1 ppm, 1J(11B1H) = 111 Hz] besides an 11B NMR sig-
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J. Knizek et al. · Metal Tetrahydroborates and Tetrahydroborato Metallates, 30
nal at 30 ppm for B(NMePh)3. The solution was stored at shift for 13 was 7.2 ppm (br). BH coupling could not be re-
−20 ◦C. The crystal that separated proved to be B(NMePh)3 solved.
as shown by the determination of its unit cell [28].
Crystal structure determination
Lithium di(tert-butyl)catecholatoborate bis(dimethoxy-
ethane) (10)·(dme)2
Crystals were placed in perfluoroether oil and suitable
specimens selected and placed on the tip of a glass fiber.
◦
1.8 ml of a pentane solution of LitBu (1.65 M, 3.0 mmol)
was diluted with hexane (10 ml). Drop wise addition of a cat-
echolato borane solution (0.30 g, 2.8 mmol) in diethyl ether
(10 ml) resulted in the formation of a yellow precipitate.
The solution showed the following 11B NMR signals: 36.1
(s, 15%), 14.6 (s, 10%), 7.8 (s, 3%), −4.5 [t, 69 Hz, 50%,
Li(H2BtBu2)], −20.3 (quart., 79 Hz, 17%), −40.8 (quint.,
81 Hz, 5%). The solid (predominantly LiBcat2 was isolated
by filtration. DME (5 ml) was added to the filtrate. Within
several hours colorless needles of (dme)2Li[catB(tBu)2],
crystallized. Yield: 78 mg (7%). It was characterized by its
crystal structure. Compound Li(H2BtBu2) could not be iso-
lated.
1H NMR (400 MHz, C6D6): δ = 1.34 (12 H, CMe3), 2.96
(12 H, OMe), 3.00 (8 H CH2O), 6.8 (m, 4H). – 13C NMR
(101 MHz, C6D6): δ = 30.3 CMe3, 58.7 (OCH3), 70.8
(OCH2), 108.0, 117.3. – 11B NMR (87 MHz, C6D6): δ =
14.6.
They were cooled to −80 C. The reflections on a total of
60 frames were used to determine the dimension of the unit
cell. Data were recorded on a total of 1200 frames with a
speed of 5 s/frame. Data reduction was performed with the
program SAINT [53], and the SHELX93 [54] programs were
used for structure solution and refinement. All non-hydrogen
atoms were refined with anisotropic parameters and hydro-
gen atoms bound to carbon atoms were placed in calculated
positions. Positions of BH hydrogen atoms were found in
difference Fourier maps. Relevant crystallographic data are
summarized in Table 8.
In compound 1 the boron atom and the oxygen atom of
THF occupied special position at the threefold axis. Con-
sequently this thf molecule was triply disordered, and no
H atoms could be found for its C atoms. In compound 8
the oxygen atom of the assumed OH group had an unusually
large thermal parameter suggesting that a H atom was posi-
tioned close to it. Therefore, we refined the site occupation
factors which improved the R value, but the B–O distance
turned out to be too short.
Dilithium di(catecholato)hydroborate (13)·THF·dme
Crystallographic data for the structure determinations
have been deposited with the Cambridge Crystallographic
Data Centre, CCDC 612051- 612056. Copies of the data can
be obtained free of charge on application to The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: int.
code +(1223)336 033; e-mail: fileserv@ccdc.cam.ac.uk).
0.169 g lithium piperidide (1.86 mmol) was suspended at
◦
−78 C in THF (15 ml). This suspension was treated with
a solution containing 0.2 ml (1.9 mmol) of catecholato bo-
rane in THF (10 ml). The 11B NMR spectrum of the so-
lution showed at r. t. 11B NMR signals at 25.8 (d, 124 Hz,
34%), 12.4 (s, 22%), 7.2 (br, 38%), and −16.3 ppm (quart.
95 Hz, 6%). After adding DME (2 ml) to the solution a color-
less precipitate formed which dissolved on addition of THF
(20 ml). Within 12 h crystals of 13·THF·dme together with
a powder had separated. One of the clear crystals was used
Acknowledgements
We gratefully acknowledge the generous support of
Chemetall GmbH and Fonds der Chemischen Industrie. Mr.
for structure determination. A quantitative separation from P. Mayer, Mrs. D. Ewald helped us in getting NMR and IR
spectra. And we thank Dr. T. Habereder for X-ray crystallo-
graphic data sets.
byproducts by extraction with benzene or hexane was not
achieved. Only THF dissolved the solid. The 11B chemical
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J. Knizek et al. · Metal Tetrahydroborates and Tetrahydroborato Metallates, 30
1093
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