Metal tetrahydridoborates and tetrahydridoborato metalates. 23.1 amine solvates of lithium and sodium tetrahydridoborate

Article abstract of DOI:10.1021/ic990205c

A series of amine solvates of LiBH₄ and NaBH₄ have been prepared and characterized by IR and NMR spectroscopy as well as by X-ray single-crystal structure determinations. LiBH₄ crystallizes from pyridine as LiBH₄·3(py), 1, in which the BH₄ anion acts as a bidentate ligand. However, in the structure of LiBH₄·3py*, 2 (py* = p-benzylpyridine), a tridentate But group is observed. In contrast, LiBH₄·2(coll), 3 (coll = 2,4,6-trimethylpyridine, collidine), possesses only a bidentate tetrahydridoborate group, while a tridentate BH) group is present in monomeric LiBH₄·PMDTA, 4 (PMDTA = pentamethyldiethylenetriamine). In contrast, NaBH₄·PMDTA, 6, is dimeric in the solid state: three of the four H atoms of each BH₄ group coordinate to the Na atoms; two form a double bridge to two Na atoms while the third one is bonded only to one Na center. LiBH₄·TMTA, 5 (TMTA = trimethylhexahydrotriazine), is also dimeric; however, only two of the nitrogen atoms of the TMTA ligand coordinate to Li. The BH₄ groups bridge the two Li centers each with one H atom coordinating to two Li atoms, and two bind to a single Li atom. A totally different situation exists for NaBH₄·TMTCN, 7 (TMTCN = trimethyltriazacyclononane), which is tetrameric in the crystal. Only one hydrogen atom of the BH₄ group functions as a hydride bridge and binds to three Na centers. The molecule contains a Na₄B₄ heterocubane core. Thus, the different modes of the interaction of the BH₄ groups with the alkali metal atoms are determined by the number of donor atoms from the neutral amine ligand and the size of the cation. No definitive conclusion as to the structure of the amine solvates can be derived from IR and/or ¹¹B NMR spectra for the solution state. The crystallographic data are as follows. 1: a = 10.9939(5) A?, b = 9.9171(4) A?, c = 14.8260(8) A?, β= 94.721(3)°, V = 1611.0(1) A?³, monoclinic, space group P2(1)/n, Z = 4, R₁= 0.0823. 2: a = 10.121(1) A?, b = 12.417(2) A?, c = 13.462(3) A?, α = 83.189(2)°, β = 86.068(3)°, γ = 69.166(4)°, V = 1369.3(5) A?³, triclinic, space group P1?, Z = 2, R₁ = 0.0689. 3: a = 28.527(3) A?, b = 10.858(1) A?, c = 11.319(1) A?, V = 3505.7(6) A?³, orthorhombic, space group Fdd2, Z = 8, R₁ = 0.0502. 4: a = 7.591(3) A?, b = 15.325(6) A?, c = 8.719(4) A?, β = 99.80(2)°, V = 999.5(7) A?³, monoclinic, space group P2(1)/c, Z = 4, R₁ = 0.0416. 5: a = 14.68(1) A?, b = 11.830(7) A?, c = 16.960(8) A?, V = 2946(3) A?, orthorhombic, space group P2(1)2(1)2(1), Z = 8, R₁ = 0.0855. 6: a = 9.993(2) A?, b = 10.008(3) A?, c = 14.472(4) A?, β= 93.55(2)°, V = 1444.6(7) A?³, monoclinic, space group P2(1)/n, Z = 4, R₁ = 0.0455. 7: cubic, a = b = c = 13.859(5) A?, V = 2662(2) A?³, cubic, space group I4?3m, Z = 8, R₁ = 0.0871.

Full text of DOI:10.1021/ic990205c

4188
Inorg. Chem. 1999, 38, 4188-4196
Metal Tetrahydridoborates and Tetrahydridoborato Metalates. 23.1 Amine Solvates of
Lithium and Sodium Tetrahydridoborate†
Hans-Hermann Giese, Tassilo Habereder, Heinrich No1th,* Werner Ponikwar,
Steffen Thomas, and Marcus Warchhold
Institute of Inorganic Chemistry, University of Munich, Butenandtstrasse 5-13, Building D,
D-81 377 Mu¨nchen, Germany
ReceiVed February 19, 1999
A series of amine solvates of LiBH4 and NaBH4 have been prepared and characterized by IR and NMR spectroscopy
as well as by X-ray single-crystal structure determinations. LiBH4 crystallizes from pyridine as LiBH4‚3(py), 1,
in which the BH4 anion acts as a bidentate ligand. However, in the structure of LiBH4‚3py*, 2 (py* )
p-benzylpyridine), a tridentate BH4 group is observed. In contrast, LiBH4‚2(coll), 3 (coll ) 2,4,6-trimethylpyridine,
collidine), possesses only a bidentate tetrahydridoborate group, while a tridentate BH4 group is present in monomeric
LiBH4‚PMDTA, 4 (PMDTA ) pentamethyldiethylenetriamine). In contrast, NaBH4‚PMDTA, 6, is dimeric in
the solid state: three of the four H atoms of each BH4 group coordinate to the Na atoms; two form a double
bridge to two Na atoms while the third one is bonded only to one Na center. LiBH4‚TMTA, 5 (TMTA )
trimethylhexahydrotriazine), is also dimeric; however, only two of the nitrogen atoms of the TMTA ligand
coordinate to Li. The BH4 groups bridge the two Li centers each with one H atom coordinating to two Li atoms,
and two bind to a single Li atom. A totally different situation exists for NaBH4‚TMTCN, 7 (TMTCN )
trimethyltriazacyclononane), which is tetrameric in the crystal. Only one hydrogen atom of the BH4 group functions
as a hydride bridge and binds to three Na centers. The molecule contains a Na4B4 heterocubane core. Thus, the
different modes of the interaction of the BH4 groups with the alkali metal atoms are determined by the number
of donor atoms from the neutral amine ligand and the size of the cation. No definitive conclusion as to the
structure of the amine solvates can be derived from IR and/or 11B NMR spectra for the solution state. The
crystallographic data are as follows. 1: a ) 10.9939(5) Å, b ) 9.9171(4) Å, c ) 14.8260(8) Å, â ) 94.721(3)°,
V ) 1611.0(1) Å3, monoclinic, space group P2(1)/n, Z ) 4, R1 ) 0.0823. 2: a ) 10.121(1) Å, b ) 12.417(2) Å,
c ) 13.462(3) Å, R ) 83.189(2)°, â ) 86.068(3)°, γ ) 69.166(4)°, V ) 1369.3(5) Å3, triclinic, space group P1h,
Z ) 2, R1 ) 0.0689. 3: a ) 28.527(3) Å, b ) 10.858(1) Å, c ) 11.319(1) Å, V ) 3505.7(6) Å3, orthorhombic,
space group Fdd2, Z ) 8, R1 ) 0.0502. 4: a ) 7.591(3) Å, b ) 15.325(6) Å, c ) 8.719(4) Å, â ) 99.80(2)°,
V ) 999.5(7) Å3, monoclinic, space group P2(1)/c, Z ) 4, R1 ) 0.0416. 5: a ) 14.68(1) Å, b ) 11.830(7) Å,
c ) 16.960(8) Å, V ) 2946(3) Å, orthorhombic, space group P2(1)2(1)2(1), Z ) 8, R1 ) 0.0855. 6: a ) 9.993(2)
Å, b ) 10.008(3) Å, c ) 14.472(4) Å, â ) 93.55(2)°, V ) 1444.6(7) Å3, monoclinic, space group P2(1)/n, Z )
4, R1 ) 0.0455. 7: cubic, a ) b ) c ) 13.859(5) Å, V ) 2662(2) Å3, cubic, space group I4h3m, Z ) 8, R1 )
0.0871.
Introduction
for LiBH4‚O(Me)tBu,1 and even extended three-dimensional
arrays were observed.1 The structure of these and other solvates
depends both on the stoichiometry of the ether solvate and on
the number of oxygen atoms of the ligand. Therefore, it was of
interest to investigate more strongly complexing ligands than
ethers. Here we report on the synthesis and on the structure
determination of lithium and sodium tetrahydroborate complexes
with ligands such as pyridines, pentamethyldiethylenetriamine
(PMDTA), N,N′,N′′-trimethylhexahydrotriazine (TMTA), and
1,4,7-trimethyl-1,4,7-triazacyclononane (TMTCN).
Alkali metal tetrahydridoborates, and sodium tetrahydrido-
borate in particular, belong to an important class of hydridic
reducing reagents of wide applicability.2,3 One of the advantages
of NaBH4 is that it can be employed in aqueous or alcoholic
solutions due to its comparatively good solvolytic stability in
neutral and alkaline media, in contrast to LiBH4, which requires
an ether as a solvent. Solutions of LiBH4 in various ether
solvents contain contact ion pairs as exemplified by LiBH4‚
3THF even for the solid state,1 and various kinds of aggregations
have recently been determined. For example, LiBH4‚OEt2 forms
double-stranded chains.1,4 A similar situation has been found
Experimental Section
All experiments were performed in an atmosphere of dry dinitrogen
using Schlenk techniques. Solvents were made anhydrous by standard
procedures and were kept in an atmosphere of N2.
† Dedicated to Prof. Dr. M. F. Hawthorne, a pioneer in boron chemistry
and a good friend, on the occasion of his 70th birthday.
(1) Part 22. Giese, H. H.; Knizek, J.; No¨th, H.; Thomas, St. Eur. J. Inorg.
Chem. 1998, 941.
(2) Wiberg, E.; Amberger, E. Hydrides of the Elements of Main Group
I-IV; Elsevier Publ. Comp.: Amsterdam, London, New York, 1971.
(3) Hajos, A. Komplexe Hydride; VEB Deutscher Verlag der Wissen-
schaften: Berlin, 1966.
(4) Heine, A.; Stalke, D. J. Organomet. Chem. 1997, 542, 25.
(5) No¨th, H.; Wrackmeyer, B. NMR Spectroscopy of Boron Compounds;
NMR, Basic Principles and Progress, Vol. 14; Springer Verlag: Berlin,
Heidelberg, New York, 1978.
10.1021/ic990205c CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/25/1999

Amine Solvates of Lithium and Sodium Tetrahydridoborate
Inorganic Chemistry, Vol. 38, No. 19, 1999 4189
Materials. LiBH4 and NaBH4 were of commercial quality (Chemetall
GmbH) and were crystallized from ether and diglyme, respectively.
Coordinated ether was removed in vacuo. The amine ligands were also
of commercial grade and were used as supplied or after drying liquids
with zeolite beads. Elemental analyses were performed at the institute’s
microanalytical laboratory. The contents of alkali metal and amines
were determined by acidimetric titration and/or by AAS.
LiBH4‚(MeNCH2)3 (5). LiBH4 (0.66 g, 30 mmol) was suspended
in xylene (20 mL), and 1,3,5-trimethylcyclohexahydrotriazine (4.26 mL,
30.3 mmol) was added to the stirred suspension. Stirring was continued
for 2 h at ambient temperature. Then the suspension was kept at reflux
for 2 h. All solid material was removed from the hot solution by
filtration. On cooling the solution crystals of 5 separated (mp 89 °C
dec). Yield: 2.4 g of 5 (53%).
IR (cm-1, ν-BH region): 2154sh, 2225sh, 2271s, 2343s, 2367s. NMR
Physical Measurements. IR spectra were recorded with a Perkin-
1
Elmer FT 370 instrument using Nujol/Hostaflon suspensions. H, 7Li,
(C6D6): δ11B -39.3 (quintet, 1J(11B1H) ) 81 Hz); δ7Li (C6D6) 0.373.
13C, and 11B resonances were determined with a Bruker AP 200
instrument (11B), a JEOL X270 instrument (7Li,11B, 13C), or a JEOL
Anal. Calcd for C6H19N3BLi (150.99): C, 47.73; H, 12.68; N, 27.83;
Li, 4.60. Found: C, 46.73; H, 12.15; N, 26.57; Li, 4.23.
1
400 instrument (1H, 13C). TMS was used as internal standard for H
NaBH4‚(Me2NCH2CH2)2NMe (6). To a stirred suspension of NaBH4
(760 mg, 20 mmol) in hexane (20 mL) was added pentamethyldieth-
ylenetriamine (4.18 mL, 20 mmol). After the suspension was heated
to reflux for 10 min, toluene (15 mL) was added. This resulted in the
formation of a turbid solution. Insolubles were removed 3 h later by
filtration. Storing the solution at -14 °C yielded clear prismatic crystals
as well as needles. Yield: 3.08 g of 6 (73%), mp 55 °C. Solubility at
20 °C: 1.25 mol/L in toluene, 0.004 mol/L in hexane.
and 13C, while BF3‚OEt2 served as external standard for 11B, and an
7
aqueous solution of LiCl (1 M) served as external standard for Li.
LiBH4‚3NC5H5 (1). A saturated solution of LiBH4 in pyridine (30
mL) was prepared to which cyclohexane (10 mL) was added. The
precipitate which formed was removed by filtration. Cooling the
solution to 0 °C yielded needles of 1, mp 52-54 °C. Attempts to
dissolve 1 in C6D6 gave a solution consisting of ∼90% of H3B‚py and
10% of 1. However, if 1 was dissolved in a mixture of C6D6 and
pyridine (6:1), then 1 was the dominant species (>90%).
IR (Nujol, cm-1): 2316s, 2249vs, 2184s, 1852w, 1637w, 1507w,
1476s, 1393w, 1362w, 1331w, 1224m, 1179s, 1161s, 1140m, 1097s,
1076s, 1011s, 978m, 929s, 913s, 872w, 736w, 694w, 663w. The
following ratios were observed by 11B NMR spectroscopy. In hexane:
-7.3 (borane adduct, quartet, 1J(11B1H) ) 91 Hz) and -41.4 (6, quintet,
IR (cm-1, ν-BH region): 2354sh, 2309s, 2279s, 2245s. NMR for 1
1
in C6D6/NC5D5 (6:1): δ1H 1.4 (1:1:1:1 quartet, J(11B1H) ) 81 Hz;
septet, 1J(10B1H) ) 27 Hz); δ11B (pyridine) -41.9 (1, quintet, 1J(11B1H)
) 81 Hz), -10.4 (H3B‚NC5H5, quartet, 1J(11B1H) ) 98 Hz); δ7Li (C6D6)
0.7.
1
1J(11B1H) ) 81 Hz); ratio ) 1:1. In toluene: -7.4 (quartet, J(11B1H)
1
) 92 Hz) and -41.4 (quintet, J(11B1H) ) 81 Hz), ratio ) 1:10. δ1H
Anal. Calcd for C15H19N3BLi (259.08): C, 69.54; H, 7.39; N, 16.22;
B, 4.17; H-, 5.80. Found: C, 67.14; H, 7.33; N, 15.63; H-, 5.86.
LiBH4‚3NC5H4(p-CH2Ph) (2). LiBH4 (0.200 g, 9.18 mmol) was
suspended in toluene (10 mL), and p-benzylpyridine (10 mL) was
added. Stirring was continued for 2 h and insoluble material removed
by filtration. Colorless crystals of 2 separated from the filtrate on
standing for 5 days at 8 °C. The yield was not determined. No H3B‚
NC5H4(CH2Ph), mp 42-45 °C dec, was detected in the solution by
11B NMR. When 2 was dissolved in C6D6, p-benzylpyridine (4:1) about
40% of 2 had decomposed to give H3B‚NC5H4(CH2Ph), δ11B ) -12.1.
IR (cm-1, ν-BH region): 2331s, 2230s, 2209s, 2149sh. NMR (C6D6/
(C6D6) 3.67 (t, 2H), 2.59 (m, 4H), 2.48 (m, 4H), 2.21 (BH4); δ13C 58.3
(CH2), 56.9 (CH2), 46.0 (NMe2), 43.1 (NCH3).
Anal. Calcd for C9H27N3BNa (211.14): C, 51.20; H, 12.89; N, 19.90.
Found: C, 51.04; H, 13.04; N, 19.62.
NaBH4‚(MeNC2H4)3, 7. To NaBH4 (0.13 g, 0.33 mmol) in THF
(35 mL) was added (MeNCH2CH2)3 (0.1 mL, 0.5 mmol). On reducing
the volume of the solution to ∼8 mL, crystals separated at 8 °C within
several days. These were suitable for X-ray diffraction analysis. The
prismatic crystals melted at 155-157 °C with decomposition. The yield
was not determined.
1
p-benzylpyridine): δ1H (400 MHz) 1.4 (quartet, J(11B1H) ) 81 Hz;
IR (cm-1, ν-BH region): 2398m, 2304s, 2227m. NMR (in THF-
d8): δ1H -0.5 (1:1:1:1 quartet, 1J(11B1H) ) 81 Hz; septet, 1J(10B1H) )
27 Hz), 2.3 (s, 9H, Me), 2.6 (s, 12H, CH2); - δ13C 46.9 (CH2), 57.1
1
1
septet, J(10B1H) )27 Hz); δ11B -38.1 (quintet, J(11B1H) ) 81 Hz),
-12.1 (borane adduct, may be present); δ7Li (C6D6) 2.4.
Anal. Calcd for C36H37N3BLi (529.44): C, 81.67; H, 7.04; N, 7.94,
H-, 2.84. Found: C, 81.96; H, 7.16; N, 8.00; H-, 2.68.
1
(CH3); - δ11B -41.9 (quintet, J(11B1H) ) 81 Hz). Anal. Calcd for
C9H25N3BNa (209.14): C, 51.69; H, 12.05; N, 20.09; B, 5.17. Found:
C, 51.31; H, 11.79; N, 19.75; B, 5.00.
LiBH4‚2[2,4,6-(CH3)3NC5H2] (3). After preparation of a saturated
solution of LiBH4 in 2,4,6-trimethylpyridine and after removal of excess
LiBH4, methylcyclohexane (10 mL) was added to 50 mL of the LiBH4
solution. The solid that precipitated was removed by filtration. On
cooling the solution to -10 °C colorless prisms of 3 separated, mp
62-65 °C dec. On attempts to dissolve 3 in C6D6 only H3B‚NC5H2-
(Me)3 was present in solution (δ11B ) -16.8).
Crystallography. Data collection was performed with Mo KR
radiation employing a graphite monochromator at 193 K on a Siemens
P4 diffractometer equipped with a low-temperature device LT2 and a
CCD area detector. Crystals were transferred from the cold mother
liquor into precooled perfluoroether oil. The selected crystal was
mounted on a glass fiber and rapidly put on the goniometer head cooled
with a cold stream of N2. The sizes of the unit cells were calculated
from the reflections collected on 15 frames each per 5 different runs
and setting angles by changing æ by 0.3° for each frame. Data were
collected in the hemisphere mode of the program SMART6 with 10
s/frame exposure time. Two different ø settings were used and æ
changed by 0.3° per frame. Data on a total of 1290 frames for
compounds 1-3, 5, and 7 were collected and reduced with the program
SAINT.7 Data for compounds 4 and 6 were collected on a Siemens
R3m instrument in the ω/2θ mode. The structures were solved by direct
methods (SHELXTL).8 Non-hydrogen atoms were refined anisotropi-
cally. All H atoms of the BH4 groups were found in the difference
Fourier map. They were freely refined, and in the final cycles most of
them with fixed Ui parameters. Many crystals had only a weak
diffraction power. In this case several crystals were used, and the result
of the best data set is reported here.9 Relevant crystallographic data
and data referring to data collection and refinement are summarized in
Table 1. In the case of compound 6 it was difficult to locate and refine
IR (cm-1, ν-BH region): 2421m, 2330s, 2268s. NMR (2,4,6-
1
trimethylpyridine): δ11B -35.9 (quintet, J(11B1H) ) 82 Hz), -16.8
1
(quartet, J(11B1H) ) 98 Hz, borane-coll. Adduct may be present).
Anal. Calcd for C16H26N2BLi (264.14): C, 72.75; H, 9.92; N, 10.61.
Found: C, 67.31; H, 9.61; N, 10.01.
LiBH4‚(Me2NCH2CH2)2NMe (4). LiBH4 (0.45 g, 21 mmol) was
suspended in hexane (20 mL). To the stirred suspension was added
(Me2NCH2CH2)2NMe (4.31 mL, 21 mmol). Stirring was continued for
2 h, and then toluene (15 mL) was added and stirring continued for 1
h. After filtration, crystals of 4 separated from the solution within 1
week at a temperature of -14 °C. Yield: 2.78 g of 4 (68%).
IR (cm-1, ν-BH region): (in Nujol) 2324vs, 2284s, 2210vs, 2198s,
2181vs, 2163vs, 2102m; (in THF) 2323vs, 2272s, 2211vs, 2162s. NMR
(C6D6): δ1H 1.79 (3H, NMe), 2.08 (12H, Me), 2.13 (8H, CH2); δ13C
(THF) 44.6 (Me), 45.7 (Me), 53.4 (CH2), 56.8 (CH2); δ11B (toluene-
1
d8) -38.8 (quintet, J(11B1H) ) 81 Hz), -7.2 (quartet, amine-borane
(ratio 1:20); in toluene/hexane the ratio was 6:1); in ether only δ11B )
-38.3 was observed.
Calcd for C6H19LiBN3 (150.99): C, 47.73; H, 12.68; N, 27.83; Li,
4.60. Found: C, 46.73; H, 12.15; N, 26.57; Li, 4.23 (AAS).
IR (cm-1): ν ) 2154sh, 2225sh, 2271s, 2367s. NMR: (C6D6/
(6) SMART, Siemens Analytical Instruments, version 4, 1996.
(7) SAINT, Siemens Analytical Instruments, version 4, 1997.
(8) SHELXTL, Siemens Analytical Instruments, version 5, 1994.
1
TMS): δ7Li 0.373; δ11B -39.3 (quintet, J(11B1H) ) 81 Hz).

Table 1. Summary of Crystallographic Data as Well as Data Referring to Data Collection and Structure Refinement
1
2
3
4
5
6
7
empirical formula
fw
cryst size (mm)
cryst system
space group
a, (Å)
C15H19N3BLi
259.08
C36H37N3BLi
529.44
0.17 × 0.3 × 0.33
triclinic
P1h
10.121(1)
12.417(2)
13.462(3)
83.189(2)
86.068(3)
69.166(4)
1569.3(5)
2
C16H26N2BLi
133.08
0.20 × 0.20 × 0.20
orthorhombic
Fdd2
28.527(3)
10.8575(10)
11.3185(10)
90
90
90
3505.7(6)
16
1.009
C9H27N3BLi
195.09
C6H19N3BLi
150.99
C9H27N3BNa
211.14
C9H25BN3Na
209.14
0.2 × 0.3 × 0.35
cubic
0.15 × 0.2 × 0.45
monoclinic
P2(1)/n
10.9939(5)
9.9171(4)
14.8260(8)
90
94.721(3)
90
1610.96(13)
4
0.4 × 0.5 × 0.7
orthorhombic
P2(1)2(1)2(1)
14.683(12)
11.830(7)
16.960(8)
90
90
90
2946(3)
8
0.880
0.3 × 0.3 × 0.35
monoclinic
P2(1)/c
7.591(3)
15.325(6)
8.719(4)
90
99.80(2)
90
999.5(7)
4
0.3 × 0.3 × 0.4
monoclinic
P2(1)/n
9.993(2)
10.008(3)
14.472(4)
90
93.55(2)
90
1444.6(7)
4
I4h3m
13.859(5)
13.859(5)
13.859(5)
90
b, (Å)
c, (Å)
R, (deg)
â, (deg)
90
90
γ, (deg)
V, (Å3)
2661.6(17)
8
1.059
0.091
952
Z
F(calcd) (Mg/m3)
1.068
0.063
552
1.120
0.064
564
1.003
0.059
336
0.985
0.084
484
µ (mm-1
F(000)
)
0.057
1168
0.051
880
index range
-12 e h e 12,
-6 e k e 6,
-17 e l e 17
49.40
-7 e h e 12,
-16 e k e 16,
-16 e le 16
55.40
-33 e h e 33,
-13 e k e 13,
-14 e l e 13
52.74
0 e h e 16,
-13 e k e 0,
-19 e l e 19
47.14
-9 e h e 10,
-19 e k e 19,
-10 e l e 10
57.82
-6 e h e 10,
-1 e k e 11,
-15 e l e 2
46.10
-15 e h e 15,
-14 e k e 14,
-15 e l e 15
46.40
2θ (deg) max
temp (K)
193
193
193
223
193(2)
193
183(3)
reflns collected
reflns unique
reflns obsd (4σ)
R (int)
no. of variables
weighting schemea x/y
GOF
7087
1990
1256
0.0823
257
0.0538/0.1860
1.057
8950
4799
1499
0.1535
4600
1710
1416
0.0341
103
0.0403/2.2556
1.165
4823
4401
1961
0.0788
287
0.0985/2.7116
0.982
5535
2018
1723
0.0226
176
0.0330/0.2711
1.164
1828
1774
1261
0.0267
215
0.0994/2.2820
1.132
5847
376
347
0.0416
33
0.1030/5.9645
1.169
386
0.04700/0.0000
0.834
final R1 (4σ)
0.0522
0.1043
0.130
0.0689
0.1285
0.238
0.0503
0.1114
0.109
0.0855
0.2029
0.202
0.0416
0.0937
0.172
0.0527
0.1170
0.322
0.0871
0.2268
0.239
final wR2
largest residual peak (e/Å3)
a w-1 ) σ2Fo + (xP)2 + yP; P ) (Fo + 2Fc2)/3.
2
2

Amine Solvates of Lithium and Sodium Tetrahydridoborate
Inorganic Chemistry, Vol. 38, No. 19, 1999 4191
the H atoms bonded to atom B2. Moreover, one CH2CH2NMe2 unit of
the PMDTA ligand is disordered. The disorder refined with SOF )
0.5 for two configurations. In Figure 6 only one configuration is
represented.
Results and Discussion
Preparation. The amine complexes of LiBH4 and NaBH4
have been prepared by two different methods. The first method
is to add the amine ligand to a suspension of MBH4 (M ) Li,
Na) in a hydrocarbon. The suspension was then heated shortly
to reflux, insoluble material was then removed by filtration, and
the solvated MBH4 crystallized from the solution at low
temperature. When toluene was used as a solvent, the 11B NMR
spectrum showed the formation not only of MBH4‚nL but also
of L‚BH3 (quartet in the 11B NMR spectrum). However, when
the amine was used as a solvent, only small amounts of the
borane adduct were noted. Thus, there are two competing
reactions as shown by eqs 1 and 2. The second method starts
Figure 1. Molecular structure of compound 1 in the crystal. Thermal
ellipsoids are depicted at the 25% probability level. Selected atom
distances in Å: Li-N1 2.078(6), Li-N11 2.074(5), Li-N21 2.065(5),
Li‚‚‚B 2.401(7), Li-H(A) 2.06, Li-H(B) 1.90, B-H(A) 1.15, B-H(B)
1.07, B-H(C) 1.14, B-H(D) 1.11. Selected bond angles in deg: N1-
Li-N11 102.8(2), N1-Li-N21 105.1(2), N11-Li-N21 112.6(2),
N1-Li‚‚‚B 117.8(2), N11-Li-B 108.6(2), N21-Li-B 109.8(2),
H(A)-Li-H(C) 56.6, H(A)-Li-H(B) 108, H(B)-B-H(D) 97.4.
Interplanar angles in deg: N1 to C6/N11 to C16 87.9, N1 to C6/N21
to C26 96.1, N11 to C16/N21 to C26 62.1.
MBH4 + nL f (L)nMBH4
MBH4 + L f LiH + L‚BH3
(1)
(2)
(3)
NaBH4‚THF + nL f LnNaBH4 + THF
from NaBH4 in THF. On addition of the amine the coordinated
ether is replaced by the stronger amine base. Due to the
negligible solubility of NaBH4 in diethyl ether, THF had to be
used, although the solubility of NaBH4 in this solvent is also
quite low.
compound 2 with a µ1 -BH4 group features a different pattern
3
The following compounds have been obtained: LiBH4‚3(py)
(1), LiBH4‚py* (2), LiBH4‚2(coll) (3), LiBH4‚PMDTA (4),
LiBH4‚TMTA (5), NaBH4‚PMDTA (6), and NaBH4‚TMTACN
(7) (py ) NC5H5, py* ) NC5H4-p-CH2Ph, coll ) 2,4,6-NC5H2-
Me3, PMDTA ) MeN((CH2CH2)NMe2)2, TMTA ) (MeNCH2)3,
TMTACN ) (MeNCH2CH2)3).
which fits the prediction of three strong νBH bands (νBHt,
ν
s,asBH3). The IR spectrum of the mononuclear compound 4
resembles that of 2 with a µ1 -BH4 group. On the other hand,
3
the IR bands in the BH stretching region of compounds 5 and
6 are quite different in spite of the fact that the BH4 groups
contact the alkali metal centers in the same manner (2µ1 ,µ2 -
1
1
Spectroscopic Data. Toluene solutions of the solvates 1-7
exhibit 11B NMR signals with a 1:4:6:4:1 quintet structure as
expected for the presence of a BH4 group. There is little
difference in the chemical shifts, δ11B ranging from -35.9 (3)
to -41.9 (1 and 7). This is somewhat unexpected considering
the different bonding modes of the BH4 group, the ligand, and
the alkali metal atom, but would be in accord with a rather polar
interaction of the tetrahydridoborate unit with the alkali metal
cationic center. So, the bonding mode, which is truly different
in the solid state (vide infra) is not reflected in δ11B nor in the
BH4) the bonding of the BH4 group in compound 7 is
exceptional (vide infra) and different, e.g., from compound 2,
both having a BH4 group with approximate C3V symmetry. For
this case one expects three bands, and these are indeed found
for 7. We tentatively assign bands at 2390 and 2305 cm-1 to
terminal BH bonds and the band at 2226 cm-1 to the BHNa3
arrangement.
Molecular and Crystal Structures. LiBH4‚3(py), 1, crystal-
lizes from hexane/pyridine or from pyridine. All BH hydrogen
atoms were found in the difference Fourier synthesis, and those
bonding to the boron atom were freely refined. Figure 1 depicts
the structure of the molecule. There are no intermolecular
interactions in the solid state besides van der Waals H‚‚‚H
contacts.
1
coupling constant J(11B1H) for the solutions. In consonance
with this observation is the fact that the coupling constant
1J(11B1H) is 81((1) Hz for all of the compounds investigated.
For some of them, the 1H resonance also detects B-H bonding
by a 1:1:1:1 quartet for the 11BH4 protons, and a septet for the
10BH4 protons. Therefore, 11B NMR as well as 1H NMR
spectroscopy is no probe for testing any M-H-B bridge
bonding interactions. However, IR spectroscopy as a much better
time-resolved method may be able to discern between the
different modes of bonding between the BH4 groups and the
metal center as revealed by the structures in the solid state.
Regarding the pyridine type solvates 1-3, there is a close
The three Li-N distances are equal within the 3σ limit of
the esd (average 2.072 Å). As expected, the N-C bond lengths
are shorter than the C-C bonds, and it is noticeable that the
C-N-C bond angles are compressed (average 116.3°) while
the N-C-C bond angles are enlarged (average 123.8°). This
corresponds nicely with the bonding parameters of the free
ligand (C-N 1.340(1) Å, C-C 1.395(1) Å, C-N-C 116.8°.10
-
analogy between 1 and 3, which both contain a µ1 -BH4 group:
If we consider the BH4 group as a halide imitator (BH4 is
2
“isoelectronic” with F-),11 then the Li center is tetracoordinated
by three N atoms and the B atom of the BH4 group. However,
the arrangement of the ligands around the Li center deviates
there are two strong bands at 2304 and 2272 cm-1, while
(9) Supplementary data (excluding structure factors) for the structures
reported in this paper have been deposited with the Cambridge
Crystallographic Data Center as supplementary publication no. CCDC
124106-124112. Copies of the data may be obtained free of charge
on application to CCDC, 12 Union Road, Cambridge CB21EZ, U.K.
E-mail: deposit@ccdc.cam.ac.uk.
(10) Bak, B.; Hansen-Kygard, L.; Restrup-Andersen, J. J. Mol Spectrosc.
1958, 2, 361.
(11) This analogy was drawn by M. J. S. Dewar.

4192 Inorganic Chemistry, Vol. 38, No. 19, 1999
Giese et al.
Figure 3. Molecular structure of compound 3 in the crystal. Thermal
ellipsoids are shown at the 25% probability level. Selected atom
distances in Å: Li1-N1 2.050(3), Li1‚‚‚B1 2.252(6), B1-H1 1.17(2),
B1-H2 1.13(3). Selected bond angles in deg: N1-Li1-N1A 128.2(2),
N1-Li1-B1 115.9(1), N1-Li1-H1 105(1), H1-B1-H(1A) 108(2),
H2-B1-H(2A) 110(3), H(1A)-B1-H(2A) 106(1), C1-N1-C5
117.9(2), N1-C1-C2 122.6(2), C1-C2-C3 120.1(2), C2-C3-C4
117.3 (2). Interplanar angles in deg: N1 to C5/N(1A) to C(5A) 51.9,
H1-Li1-H1(1A)/N1 to C5 83.1, H1-Li1-H(1A)/N(1A) to C(5A)
96.9.
Figure 2. Molecular structure of compound 2 in the crystal. Thermal
ellipsoids are represented at the 25% probability level. Selected atom
distances in Å: N1-Li1 2.083(3), N2-Li1 2.107(3), N3-Li1 2.109(3),
Li1‚‚‚B1 2.279(4), B1-H1 1.15, B1-H2 1.11, B1-H3 1.12, B1-H4
1.23. Selected bond angles in deg: N1-Li1-N2 102.7(1), N1-Li1-
N3 99.0(1), N2-Li1-N3 105.3(1), N1-Li1‚‚‚B1 112.5(1), N2-
Li1‚‚‚B1 112.9(2), N3-Li1‚‚‚B1 121.9(2), H1-B1-H2 106, H2-B1-
H3 112, H3-B1-H4 114, H1-Li1-H2 52, H2-Li1-H4 40. Inter-
planar angles in deg: N1 to C5/N2 to C17 102.9; N1 to C5/N3 to C29
60.4; N2 to C17/ N3 to C29 69.9; N1 to C5/ C7 to C12 77.3; N2 to
C13/C19 to C24 90.7; N3 to C25/C29 to C33 74.7.
between B-H and Li-H bond lengths: the longer the B-H
bond, the longer the Li-H distance: the corresponding data
are 1.15 Å for B-H1 and 2.07 Å for Li-H1, 1.10 Å for B-H2
and 2.08 Å for Li-H2, and 1.13 Å for B-H4 and 2.26 Å for
Li-H4. The terminal B-H3 bond is 1.12 Å long.
from the ideal tetrahedral angle; the largest deviation is the bond
angle N1-Li-B1, 117.8(2)°. Thus, the BH4 group is not
symmetrically bonded to the Li center, due to the fact that the
Li(µ-H)2BH2 arrangement is asymmetric. The H-B-H bond
angle for the bridging H atoms is 112.8°, while that for the
terminal hydrogens is 97.4°. However, there is no clear
correlation between the B-H bond lengths for bridging versus
terminal hydrogens although the average B-H distance (1.09
Å) for the terminal hydrogen is 0.06 Å shorter than for the
bridging hydrogen atoms (1.15 Å). This conclusion, however,
must be taken with caution considering the inaccuracy of
determining B-H bond lengths and H-B-H bond angles by
X-ray diffraction.
In accordance with the triply bridging BH4 group,12 a short
Li-B atom distance is observed (2.279 Å). The interplanar
angles of the p-benzylpyridine ligand may give an explanation
why a µ1 -BH4 function is realized in 1 and a µ1 -BH4 group is
2
3
present in 2. These angles are 67.1°, 61.3°, and 103.9° (for
planes between AC/CE/EA with A ) pyridine N1, C ) pyridine
N2, E ) pyridine N3). The asymmetry of the molecule 2 results
not only from the asymmetry of the bonding of the BH4 groups
and the pyridines but particularly from the different orientation
of the phenyl rings with respect to the pyridine ring. The values
are 77.3° for pyridine N1, 90.7° for pyridine N2, and 74.7° for
pyridine N3. These data indicate that the phenyl groups are
differently oriented to their respective pyridine ring. However,
this is a typical phenomenon for the packing, because in solution
all benzylpyridine ligands are equivalent as disclosed by the
NMR data. Nevertheless, these different orientations of the
pyridine rings may be responsible also for the asymmetrically
bonded BH4 group.
One might expect that tris(p-benzylpyridine)lithium tetrahy-
droborate, 2, may have the same coordination for the Li center
as 1. This is, however, not the case as shown in Figure 2,
because the molecule possesses a µ1 -BH4 group coordinating
3
with three of its hydrogen atoms to the Li center. The average
Li-N distance is 2.10 Å, slightly longer than in 1, and the
average C-N-C bond angle is 115.2°, slightly smaller than in
1. It is somewhat surprising that the benzylpyridine ligands are
rather asymmetrically arranged about the Li center, bond angles
N-Li-N range from 99.0(1)° to 105.3(1)°, and consequently
the N-Li-B angles are also not uniform. Angles between
112.9(1)° and 121.9(2)° have been found. Moreover, the BH4
group does not bind symmetrically to the Li center, and the
BH4 group by itself is also not tetrahedral because the B-H
bond lengths vary from 1.11 to 1.15 Å and the H-B-H angles
from 105° to 113°. At least for 2, there seems to be a correlation
While pyridine and p-benzylpyridine form 1:3 adducts with
LiBH4, only two molecules of collidine (2,4,6-trimethylpyridine)
add to one molecule of LiBH4. This seems to be caused by the
steric effect of the two methyl groups in the 2,6-positions of
the pyridine ring.
The molecular structure of compound 3 is depicted in Figure
3. It reveals that the molecule contains a µ1 -BH4 group. This
2
(12) Edelstein, N. Inorg. Chem. 1981, 20, 297.

Amine Solvates of Lithium and Sodium Tetrahydridoborate
Inorganic Chemistry, Vol. 38, No. 19, 1999 4193
is an unexpected result in light of the structure of 2 and because
one might expect a µ1 -BH4 group because the number of N
3
ligands is reduced to two.
Molecule 3 possesses a crystallographically imposed 2-fold
axis with the Li and the B atoms lying in a C2 axis. Its Li-N
bond lengths (2.050(3) Å) fit well with many other Li-N
distances of coordinative function.13 In spite of the fact that the
BH4 group adopts a µ1 -bonding fashion, the Li‚‚‚B distance
2
falls into the range of a Li(µ1 -BH4) group.
3
A large asymmetry results for 3 because the N1-Li1-N1-
(A) bond angle is rather large, 128.2(2)°. However, if one
considers the BH4 anion as a halide imitator, then the coordina-
tion at the Li center is “trigonal planar”. However, the question
must be answered, why do we observe only a µ1 -BH4 instead
2
of µ1 -BH4 group. There is a small difference of only 0.04 Å
3
between the BH distance of bridging and terminal hydrogens,
and in this case the H-B-H bond angle involving the bridging
hydrogen atoms is smaller (108°) than for the terminal ones
(110°). However, it is noticeable that the Li-H1 distance is
only 1.83 Å, indicating a strong interaction.
Intermolecular contacts cannot be responsible for the µ1
2
function of the BH4 group and two neutral ligand molecules.
This suggests that the Li centers in 3 are sterically shielded,
making a µ1 -BH4 group obsolete. Intermolecular Li‚‚‚H
3
contacts result for Li with H(6C) and H(6AA) (2.60 and 2.69
Å, respectively) as well as intermolecular contacts of Li to two
H atoms of CH3 groups (H(6AA) and H(6CA)) with distances
of 2.69 and 2.60 Å. If we take this into account, then the atom
Li1 adopts hexacoordination.
Figure 4 demonstrates the molecular structure of LiBH4‚
PMDTA, 4. There are two independent molecules in the
asymmetric unit which differ not grossly in their molecular
parameters but definitely by their configuration.
The coordination about the Li atoms is the same as in 1, e.g.,
the Li atom is pentacoordinated (3N, 2H) or tetracoordinated
considering the BH4 group as a single ligand (3N, 1B). However,
due to the bidentate nature of the amine, the N-Li-N bond
angles are quite different, ranging from 83.8(6)° to 122.9(7)°
for the Li1 molecule and from 84.7(6)° to 124.3(6)° for Li2.
The coordination polyhedron is better described by a strongly
distorted tetrahedronstaking the N and B atoms into accounts
than by a distorted trigonal bipyramid (considering H atoms as
coordination partners). Moreover, the BH4 groups differ con-
siderably from a tetrahedral array because the H-B-H bond
angles vary from 96(4)° to 124(5)° for molecule Li1, and from
92(4)° to 135(5)° for molecule Li2. It is particularly noticeable
that the H-B-H bond angle for the bridging hydrogen atoms
is larger (125(4)° and 119(4)°) than for the terminal hydrogen
atoms (97(3)° and 92(4)°).
Figure 4. ORTEP plot of the molecular structure of 4. Thermal
ellipsoids are depicted at the 25% probability level. H atoms bound to
carbon atoms are removed for the sake of clarity. Selected atom
distances in Å: (molecule 1, top) Li1-N1 2.12(1), Li1-N2 2.14(2),
Li1-N3 2.14(1), Li1‚‚‚B1 2.29(2), Li1-H(1) 2.05(4), Li1-H(2)
2.04(4), B1-H(1) 1.07(4), B1-H(2) 1.07(4), B1-H(3) 1.22(4), B1-
H(4) 1.21(4), N1-C1 1.40(1), N1-C2 1.44(1), N1-C3 1.47(1), N2-
C5 1.446(9), N2-C4 1.48(1), N2-C6 1.48(1); (molecule 2, bottom)
Li2-N4 2.11(2), Li2-N5 2.14(2), N2-N6 2.12(2), Li2-H(5) 1.92(4),
Li2-H(6) 1.92(2), B2-H(5) 1.31(4), B2-H(6) 1.30(4), B2-H(7)
0.95(4), B2-H(8) 0.94(4), Li2‚‚‚B2 2.35(2). Selected bond angles in
deg: (molecule 1) N1-Li1-N2 85.5(6), N1-Li1-N3 122.9(7), N2-
Li1-N3 83.8(6), N1-Li1-B1 110.3(7), N2-Li1-B1 140.0(7). N3-
Li1-B1 113.5(7), H(1)-Li1-H(2) 55(2), H(1)-B1-H(2) 125(4),
H(3)-Li1-H(4) 97(3); (molecule 2) N4-Li2-N5 85.1(6), N4-Li2-
N6 124.3(7), N5-Li2-N6 84.7(6), N6-Li2-B2 110.7(8), N5-Li2-
B2 139.2(7), N4-Li2-B2 112.3(7), H(5)-B2-H(6) 68(2), H(5)-B2-
H(6) 110(3), H(7)-B2-H(8) 92(5).
orientation of the envelope conformations. The envelope atoms
C3/C7 and C12/C16 adopt an opposite orientation with respect
to the orientation of the MeN‚‚‚Li(B) group. The central nitrogen
atom is chiral, and therefore, two enantiomers had to be
expected.
Li-N distances in molecule Li1 are on average the same (2.13
Å) as found for Li2 (2.12 Å), in contrast to the Li-B distances,
which are 2.29(2) and 2.35(2) Å, respectively. The reason for
this difference is not easily rationalized, but corresponds with
shorter B-H (bridge) bonds for molecule Li1 (1.07, 1.07(4)
Å) compared to molecule Li2 (1.31, 1.30(4) Å). This reversed
situation holds also for the terminal B-H bonds (1.23, 1.21(4)
Å vs 0.95, 0.94(4) Å). It is obvious that a more precise
determination of BH bond lengths is needed.
Compound 5, lithium tetrahydridoborate 1,3,5-trimethyl-1,3,5-
triazacyclohexane (trimethylhexahydrotriazine ) TMTA) crys-
tallizes from a hot toluene solution. The X-ray structure analysis
shows that it is dimeric in the solid state. Figure 5 represents
an ORTEP representation of the molecule.
As can be noted from Figure 5, there is an inversion center
in the molecule which is also a crystallographic inversion center
situated at the midpoint between the two Li atoms. Most
surprising is the fact that only two of the three nitrogen atoms
of the TMTA molecules are used in coordination. The Li-N
distances to the coordinated atoms N2 and N3 are 2.156(2) and
2.217(2) Å, respectively, while the distance to the noncoordi-
nated atom N1 is 2.883(2) Å. There is also a comparatively
short Li1-C6 distance of 2.489 Å. The N2-Li1-N3 bond angle
is rather sharp, 64.49(7)°.
Figure 4 shows the difference of configurations between the
two independent molecules which results from the different
(13) For a summary of data see chapters 8 and 9 in the following: Sapse,
A.-M.; Schleyer, P. v. R. Lithium Chemistry. Theoretical and
Experimental OVerView; J. Wiley Interscience Inc.: New York,
Toronto, Singapore, 1995.

4194 Inorganic Chemistry, Vol. 38, No. 19, 1999
Giese et al.
Figure 5. Molecular structure of the dimeric compound 5. Thermal ellipsoids are drawn at the 25% probability level. Hydrogen atoms of the
TMTA ligand are not shown for the sake of clarity. Selected atom distances in Å: Li1-N2 2.156(2), Li1-N3 2.217(2), Li1-H2 2.08(1), Li1-H4
2.02(2), Li1-H(2A) 2.19(1), Li1-H(1A) 1.92(2), Li1‚‚‚C6 2.489(3), Li1‚‚‚B(1A) 2.395(3), B1-H1 1.14(2), B1-H2 1.15(2), B1-H3 1.12(2),
B1-H4 1.15(2). Selected bond angles in deg: N2-Li1-N3 64.49(7), N2-Li1-H4 90.8(4), N2-Li1-H2 132.1(4) N3-Li1-H4 104.5(4), N3-
Li1-H2 91.3(4), N3-Li1-H(2A) 170.8(4), N3-Li1-H(1A) 121.2(5), Li1-B1-Li1A 80.49(9), B1-Li1-B1A 99.51(9), H3-B1-H1 110(1),
H3-B1-H2 110(1), H3-B1-H4 111(1), H2-B1-H1 108(1), H2-B1-H4 109(1). Interplanar angles: N2-C6-N3/N2-C5-B3-C4 119.3,
C4-N1-C5/N2-C5-N3-C4 127.5; N2-C6-C3/C4-N1-C5 8.0.
Two BH4 groups bridge the two Li centers via six hydrogen
atoms; each BH4 unit acts as a 2µ1 ,µ21 ligand, one H atom per
1
BH4 group binds to two Li atoms via a single bridge, and two
H atoms bind via a single bridge to one Li center. This bonding
mode was first observed for [LiBH4‚TMEDA]2.14 Bond angles
at the boron atoms are very close to the tetrahedral angle. They
range from 108(1)° to 111(1)°. Therefore, they are all equivalent.
However, the bridging mode is reflected in this case by the B-H
distances. The shortest bond is to the terminal H atom H3
(1.12(2) Å), the largest to the H atom that coordinates to two
Li centers. However, all BH bonds can be considered to be of
almost equal lengths, esd’s being taken into account.
Bond angles at the Li atoms are, however, very different.
Two are practically equal (H(1A)-Li1-H(2A), 53(6)°, and
Figure 6. ORTEP representation of the molecular structure of dimeric
6. Thermal ellipsoid are represented at the 30% probability level. Only
the boron H atoms are depicted for the sake of clarity. Selected atom
distances in Å: Na1-N1 2.510(3), Na(1A)-B1 2.727(4), Na1-N2
2.597(2), Na1-N3 2.498(2), Na1‚‚‚B1 2.867(4), Na1-Na(1A) 3.639(2),
Na1-H(1) 2.58(3), Na1-H(2) 2.51(3), Na1-H(4A) 2.54(3), Na1-
H(2A) 2.49(3), Na1-H(1A) 2.49(3), B1-H(1) 1.10(3), B1-H(2)
1.13(3), B1-H(3) 0.97(3), B1-H(4) 1.07(3). Selected bond angles in
deg: N1-Na1-N2 70.79(9), N1-Na1-N3 120.8(1), N2-Na1-N3
71.95(8), N1-Na1-H(1A) 103.3(7), N1-Na1-H(4A) 114.5(6), N1-
Na1-H(2A) 145.8(7), N1-Na1-H(2) 84.8(7), N2-Na1-H(1) 152.0(6),
N2-Na1-H(2) 155.2(6), N1-Na1-B1 97.4(1), N2-Na1-B1 154.4a(1),
N3-Na1-B1 96.9(1), H(1)-B1-H(2) 110(3), H(1)-B1-H(3) 113(2),
H(1)-B1-H(4) 103(2), H(2)-B1-H(3) 114(2), H(2)-B1-H(4) 104(2).
H(4)-Li-H(2), 54.2°) while the bond angles H(2)-Li1-H(2A)
and H(2)-Li1-H(1) are 84.6(6)° and 108.5(6)°. In contrast,
the bond angles H(2)-Li1-N3 and H(2)-Li1-N2 differ
considerably, with 91.3(4)° and 132.1(4)°. Thus, the symmetry
about the hexacoordinated Li center is highly asymmetric.
Although there are little differences for the B-H bond lengths,
the Li-H distances vary considerably. The largest distance is
2.19(2) Å (Li1-H(2A)); the shortest is 1.92(1) Å for Li1-
H(1A).
Solvates of NaBH4. In contrast to LiBH4, it is much more
difficult to grow single crystals of amine solvates of NaBH4.
So far only two NaBH4 amine solvates, 6 and 7, provided single
crystals for an X-ray structure determination. Among these, only
a single pair, NaBH4‚PMDTA, 6, and LiBH4‚PMDTA, 4, can
be directly compared.
The amine solvate 6 crystallizes in the monoclinic space group
P21/n, Z ) 4. In the asymmetric unit there is only one molecule
of NaBH4‚PMDTA. This molecule is associated to dimers via
a crystallographic inversion center which is found at the
midpoint of the Na1-Na1A vector (3.639(2) Å) of the molecule
which is depicted in Figure 6. As can be noted, the Na centers
are coordinated to the three N atoms of the amine ligand and
to hydrogen atoms of two bridging BH4 groups. The Na-N
bond to the central N atom of the ligand (N2) is longer (2.597(2)
Å) than the Na1-N1 and Na1-N1A bonds (2.510(3) and
2.498(2) Å). In compound 6 the bridging B-H bond length is
better discriminated than in the other amine solvates, as the
terminal BH bonds (0.97 (3) Å) are shorter than the B-H bond
to the bridge. Taking all close Na-X distances into account,
the Na center is nonacoordinated.
The most surprising part of the structure of 6 is, at the
moment, that it does not resemble LiBH4‚TMEDA because
every BH4 group binds with two of its hydrogen atoms to two
sodium centers, and with one only to a single Na atom, while
the reverse is true for the Li compound. So the BH4 group has
to be classified as 2µ2 ,µ1 . The longest B-H bond (1.13(3) Å)
(14) Armstrong, D. R.; Clegg, W.; Colquhuon, H. M.; Daniels, J. A.;
Mulvey, R. E.; Stephenson I. R.; Wade, K. J. Chem. Soc., Chem.
Commun. 1987, 630.
1
1
is responsible for deviations from a tetrahedral array. Thus,

Amine Solvates of Lithium and Sodium Tetrahydridoborate
Inorganic Chemistry, Vol. 38, No. 19, 1999 4195
being 68.0(2)°. N-Na-B bond angles range from 100.9(2)° to
166.43(2)°. A second, somewhat surprising feature can be
recognized: the BH4 group supplies only a single hydrogen atom
for coordination to three Na centers. Thus, the BH4 groups act
as µ3 -bridging ligands. The other three H atoms of the BH4
1
group are “terminal”. If we consider the structure of 7 as derived
from a cubane, then the bridging hydrogen atoms occupy places
inside of the cubane.
Discussion
The seven structures of alkali metal tetrahydridoborates
solvated by pyridines or tertiary amines display four different
types of M‚‚‚BH4 interactions. Three types are already known,
the BH4 group coordinating as a µ12 and µ13 donor as well as a
µ2 ,2µ11 donor bridging two alkali metal centers. A new type is
1
found in the sodium tetrahydridoborate complex 7 where only
a single hydridic hydrogen atom is used for bridging to three
sodium centers (µ31 mode). Another unusual coordination mode
is observed in 5, where the amine ligand uses only two of its
three nitrogen centers for coordination with the Li atom.15
The effective radius of the BH4 group is closer to that of a
bromide than to that of a chloride.16 Therefore, the observed
structures for compounds 1-7 can be compared with those
found for halides LiX‚nL and NaX‚L (X ) Cl, Br; L ) amine
-
ligand). For this reason it is best to look at the BH4 anion as
a halide imitator. If we do so, then the Li centers in 1 and 2 are
tetracoordinated in contrast to 3 where the coordination number
would be 3. The first situation is found for LiX‚3(py),17,18 LiX‚
3(2,6-dimethylpyridine),19,20 and LiX-3-(4-tert-butylpyridine).15
While the Li-X distances (X ) BH4, Br, I) within each series
of LiCl and LiBr adducts do not alter significantly, this is not
the case for 1 and 2, where the Li‚‚‚B atom distance is
significantly shorter in 2, with its µ1 -BH4 group, in contrast to
3
1, which displays a µ1 -BH4 unit. This shortening is in accord
2
with Edelstein’s rule,12 and this is of course a good argument
-
for considering the BH4 group not simply as a pseudohalide
but rather as more than a “pseudohalide” and a rather specific
ligand.21,22 Lithium halide complexes of collidine have been
reported only for LiBr and LiI, the coordination compounds
having the composition (LiX)4‚6(coll).18
Moreover, the 2,6-lutidine (lut) adducts should also be
considered because they should exert the same steric effect as
2,4,6-trimethylpyridine (collidine). Reported lithium halide
complexes are [LiBr]4‚6(lut) and [LiI]4‚6(lut)23 having a four-
membered (LiX)2 ring structure in contrast to those with a (LiX)4
core. Thus, compound 3 has so far no pendant and is, at the
present time, unique. Since the Li1‚‚‚B atom distances can be
determined much more accurately than Li-H and B-H
distances, it is not easy to rationalize the observed Li‚‚‚H bridges
in compounds 1-3. While it is readily understood that the Li‚‚‚B
Figure 7. (a) The molecular structure of tetrameric 7 in the crystal.
Only boron-bonded hydrogen atoms are depicted for the sake of clarity.
The thermal ellipsoids are represented at the 20% probability level.
Selected atom distances in Å: Na1-N1 2.607(6), Na1‚‚‚B1 3.114(5),
N1-C2 1.440(6), N1-C1 1.457(9), B1-H1 1.14, B1-H2 1.26.
Selected bond angles in deg: Na(1A)-Na1-B1 166.3(2), B1-Na1-
B(1A) 88.9(2), Na1-B1-Na(1C) 91.2(2), N1-Na1-N(1A) 68.0(2),
N(1A)-Na1-B1 100.9(2), H1-B1-H2 115, H2-B1-H(2A) 104. (b)
The core frame of tetrameric 7.
Na1-H(1) is 2.58(3) Å, Na1-H(2) is 2.51(3) Å, and Na1-
H(4A) is 2.53(4) Å. The longest Na-H distance is to the doubly
bridging hydrogen atoms. However, the bonding of the two BH4
groups to the Na center is different as demonstrated by Na-B
distances of 2.727(4) and 2.867(4) Å (to Na1), respectively.
So far we were unable to prepare crystals of NaBH4‚TMTA
in order to compare its structure with that of the Li analogue.
However, sodium tetrahydridoborate 1,5,9-trimethyl-1,5,9-tri-
azacyclododecane, 7, was obtained as single crystals. The
structure determination revealed that it is present as a tetramer
in the solid state (Figure 7a). It is one of the few molecular
alkali metal compounds that crystallize in the cubic system, in
this case in space group I4h3m.
The most significant structural feature is that the Na and B
atoms form a cubane core which is slightly distorted, Na-B
distances being 3.114(6) Å while the Na-B-Na bond angles
are 91.2(2)° and B-Na-B bond angles 88.9(2)°. Each sodium
center is symmetrically coordinated by three nitrogen atoms with
a Na-N distance of 2.607(6) Å, the “bite” angle N-Na-N
(15) Raston, C. L.; Skelton, B, W.; Whitaker, C. R.; White, A. H. Aust. J.
Chem. 1988, 41, 341.
(16) Abrahams, S. C.; Kalnajy, J. J. Chem. Phys. 1954, 22, 1933.
(17) Raston, C. L.; Skelton, B. W.; Whitaker, C. R.; White, A. H. J. Chem.
Soc., Dalton Trans. 1988, 987.
(18) Raston, C. L.; Robinson, N. T.; Skelton, B. W.; Whitaker, C. R.; White,
A. H. Inorg. Chem. 1989, 28, 163.
(19) Skelton, B. W.; Whitaker, C. R.; White, A. H. Aust. J. Chem. 1990,
43, 755.
(20) Whitaker, C. R.; White, A. H. J. Chem. Soc., Dalton Trans. 1988,
991.
(21) Marks, T. J.; Kolb, J. R. Chem. ReV. 1977, 77, 263.
(22) No¨th, H.; Thomann, M. M.; Bremer, M.; Wagner, G. In Current Topics
in the Chemistry of Boron; Kabalka, G. W., Ed.; Special Publication
143; Royal Society of Chemistry: Cambridge, 1994; p 387.
(23) Raston, C. L.; Whitaker, C. R.; White, A. H. Inorg. Chem. 1989, 28,
163.

4196 Inorganic Chemistry, Vol. 38, No. 19, 1999
Giese et al.
atom distance is longer in 1 (2.402(7) Å for a µ1 -BH4 group)
LiBH4 complexes are better to be compared with the structures
of LiBr‚nL (L ) amine ligand) than with LiCl‚nL compounds.
The core of [NaBH4‚TMTACN]4 consists of a Na4B4 cubane
structure. Cubanes in alkali metal chemistry are not known for
lithium halides11 but are typical for alkali metal alkoxides
[MOR]4.26-28 However, the compound most akin to 7 is
[NaHBMe3]4, which possesses a Na4H4 core.29 Its hydrogen
atoms bridge to three Na centers as now found for 7. This is,
of course, electronically a favored arrangement fitting also with
a Na3H four-center two-electron bond. Nevertheless, there are
considerable differences in the structures of 7 and NaHBMe3.
In compound 7 the sodium centers are hexacoordinated, and,
therefore, this is a unique situation because only one out of four
hydridic hydrogen atoms of the tetrahydroborate group is used
in coordination.
2
than in 2 (µ1 -BH4 group, 2.279(4) Å), we observe a Li‚‚‚B
3
atom distance of 2.252(6) Å in compound 3 which corresponds
with that of 2, suggesting a tridentate µ1 -BH4 group. However,
3
in 3 there are only two collidine ligands presents; the coordina-
tion number, therefore, is smaller than in 1 and 2; and since
M-X bond length increases with increasing coordination
numbers, the short Li‚‚‚B distance observed for the µ1 -BH4
2
group in 3 can be rationalized. On the other hand, the Li‚‚‚B
distances of the two independent molecules of compound 4
suggest the presence of a µ1 -BH4 and a µ1 -BH4 group (2.29
2
3
and 2.35 Å) although the structure determination reveals
asymmetrically bonded µ1 -BH4 groups. As far as we know, 4
2
is the first mononuclear molecular amine solvate of LiBH4 so
far reported. LiBH4‚TMEDA is a binuclear molecular com-
pound,14 where the two Li centers are bridged by µ2 ,2µ1 -BH4
1
1
Conclusions
units. This bridging mode is not observed for dimeric NaBH4‚
PMDTA, 6, which shows 2µ2 ,µ1 -BH4 groups. Thus, the
Amine solvates of LiBH4 with pyridine, pentamethyldieth-
ylenetriamine, and trimethylhexahydrotriazine ligands are readily
accessible. However, depending on their mode of preparation
they lose a BH3 component by formation of an amine‚BH3
adduct. This cleavage, considered to be characteristic for
covalent tetrahydridoborates,9 seems to be also a feature for ionic
tetrahydridoborates. When these are transferred into small
molecules, this cleavage is even observed for NaBH4. For amine
solvates of lithium halides as well as for lithium tetrahydri-
doborate, the resulting coordination number at Li is a balance
between the ligand property of the BH4- anion, the number of
N atoms of the amine ligands coordinating to Li, and the steric
requirements of the amine ligand. The fewer nitrogen atoms
1
1
coordination number of Li in 4 is either 4 (3N, 1B) or 5 (3N,
2H), and that of Na in 6 is either 5 (3N, 2B) or 8 (3N, 5H), the
higher coordination being the consequence of the larger cation
radius of Na. On the other hand, the structure of (LiBH4‚
TMTA)231 resembles that of (LiBH4‚TMEDA)2. However, due
to the structure of the amine ligand, the N-Li-N bond angles
in 5 are rather small (70.8°, 71.9°) compared to that in (LiBH4‚
TMEDA)2 (74.6°).14 While compounds LiX‚TMEDA are dimer-
ic for X ) Br, I, the chloride forms a compound (LiCl)6-
(TMEDA)2 having a (LiCl)6 core formed by boat-shaped (LiCl)3
units staggered by Li‚‚‚Cl interactions between two six-
membered rings.24 The Li-Br and Li-I complexes are coun-
terparts for the structure of LiBH4‚TMEDA. However, in
contrast to monomeric LiBH4‚PMDTA, the halide LiBr‚
PMDTA proved to be dimeric in the solid state.25 The dimer of
LiBr‚PMDTA dissociates in the solution into the monomer; this
monomer should be comparable with the LiBH4 solvate 4.
Nevertheless, compound 4 has a simple structure in comparison
to (LiCl)4‚3PMDTA, which contains a four-membered (LiCl)2
ring in a chain structure.17 This again demonstrates that the
-
coordinate, the more likely is it that the BH4 group bridges
with three hydridic hydrogen atoms to the metal center or the
compound dimerizes. In the latter case it seems that the BH4
group in the bridging positions supplies two of its hydrogens
for each metal center, making the BH4 group a µ2 ,2µ11 ligand.
1
However, other coordinating modes in oligo- and multinuclear
metal tetrahydridoborates are possible as realized in the solid
state structure of Be(BH4)2 with its terminal µ1 -BH4 and
2
bridging 2µ1 -BH4 groups.30
2
(24) Raston, C. L.; Skelton, B. W.; Whitaker, C. R.; White, A. H. Aust. J.
Chem. 1988, 41, 1925.
(25) Hall, S. R.; Raston, C. L.; Skelton, B. W.; Whitaker, C. R.; White, A.
H. Inorg. Chem. 1983, 22, 4070.
(26) Schmidt-Ba¨se, J.; Klingebiel, U. Chem. Ber. 1989, 122, 815.
(27) Dippel, K.; Kewehols, N. K.; Jones, P. G.; Klingebiel, U.; Schmidt,
D. Z. Naturforsch. 1987, 42b, 1253.
Acknowledgment. We gratefully acknowledge the support
of this work by Chemetall AG and Fonds der Chemischen
Industrie. Moreover, we thank Mrs. E. Kiesewetter, Mr. D.
Lindscheid, and Mr. P. Meyer for recording many NMR spectra
and Dr. I. Krossing for the data collection of compound 5.
(28) Williard, P. G.; Sabrino, J. M. Tetrahedron Lett. 1985, 26, 3931.
(29) Bell, N. A.; Shearer, H. M. M.; Spencer, C. B. J. Chem. Soc., Chem.
Commun. 1980, 74.
(30) Lipscomb, W. N.; Marynick, D. J. Am. Chem. Soc. 1971, 93, 2322.
(31) It should added that a compound different from 5 of composition
(LiBH4)3‚2TMTA has also been isolated and characterized. Its structure
is quite different from that of 5 and will be reported shortly together
with other “chain” structures. See: Giese, H. H. Ph.D. Thesis,
University of Munich, 1998.
Supporting Information Available: Listings of data referring to
crystallography, structure solution and refinement, atomic coordinates,
full list of bond distances, bond angles, and thermal parameters, in
CIF format. This material is available free of charge via the Internet at
http://pubs.acs.org. Such files have also been deposited at the Cambridge
Crystallographic Data Center in CIF format.9
IC990205C