Structures
of
NH B H 3
3
a n d
7
(NH ) BH Cl 3
2
2
C. E. NORDMAN, CURT REIMANN, and CHARLES R. PETERS
Downloaded by CALIFORNIA INST OF TECHNOLOGY on January 13, 2018 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch022
Department of Chemistry, University of Michigan, Ann Arbor, Mich.
A single-crystal x-ray diffraction study of NH B H and (NH ) BH CI was undertaken, to gain greater understanding of the reactions by which they may be prepared. The NH B H molecule contains a triangle of boron atoms having one side slightly shorter than the others. The NH group is attached to the boron atom opposite the shorter boron—boron bond and is pointed at about a 63° angle out of the boron plane. The (NH) BH Cl structure contains chloride ions lying very nearly at the points of a simple tetragonal lattice. The true unit cell, how ever, is orthorhombic and contains eight formula units of the compound. The (NH) BH ions have an angular Ν—Β—Ν configuration and are stacked in layers perpendicular to the pseudotetragonal c axis, interleaving the layers of chloride ions. 3
3 2
3
7
2
3
3
7
3
3
3
7
+
2
2
Tetraborane has been shown to form a stable diammoniate, B H - 2 N H , when allowed to react with ammonia i n ether solution at —78°C. (1). T h e diammoniate undergoes the following reaction when treated with anhydrous hydrochloric acid i n ether solution at low temperature (2) : 4
Et 0 2
B H - 2 N H , + HC1 + EUO 4
1 0
1 0
3
> (NH ) BH C1 + E^OBjHj + H 8
2
2
2
-78°C.
The chloride salt separates as a precipitate. T h e etherate i n turn, when treated with ammonia, undergoes the following reaction: NH
8
+ Et OB H 2
3
7
EtiO > N H B H + EW 8
8
7
-78°
The present single crystal x-ray study of ( N H ) B H C 1 and N H B H was under taken for the purpose of clarifying the structural chemistry of these reactions, and of the compounds themselves. 3
2
2
3
3
7
NH3B3H7 The room temperature modification of N H B H is disordered and transforms on cooling to an ordered phase (4). Single crystals of the low temperature modification, suitable for x-ray study, were grown from ether solution at — 45°C. and mounted for x-ray study on a cold stage. A total of 502 reflections were observed using the precession and Weissenberg methods and keeping the crystal specimen at about 3
3
7
-80°C.
The crystals are monoclinic with a = 10.40 ± 0.015 Α . ; b = 4.824 ± 0.006 Α . ; 204 BORAX TO BORANES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
NORDMAN ET AL.
Structures of ΝΗ Β Η 3
Ά
and (NH ) BH CI
Ί
3
2
205
2
c = 9.997 ± 0.012 Α . ; β = 115.2° ± 0.15° and belong to the space group P2 /n. T h e unit cell contains four formula units. A n hOl Patterson projection yielded approximate χ and ζ coordinates for the nitrogen and boron atoms and tentative y coordinates were found by trial and error. These parameters were then refined b y least squares methods using an I B M 650 computer. The hydrogen atoms i n the B H group were now located by means of three-dimensional Fourier difference syntheses. Ultimately a l l atomic coordinates, including those of the B H hydrogens, were refined b y least squares methods using individual thermal parameters for a l l atoms. T h e three hydrogen atoms of the ammonia group were not unambiguously located, presumably due to high thermal motion or disorder about the Β—Ν bond. T h e final value of the conventional R factor was 0.107 with all observed reflections included. 1
3
Downloaded by CALIFORNIA INST OF TECHNOLOGY on January 13, 2018 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch022
3
7
Figure 1. Left.
Electron
represented
Final difference Fourier synthesis
density
distribution
b y sections and
Right
7
near
in
ammonia-triborane
centers o f h y d r o g e n
atoms.
molecule Nitrogen
boron atoms a r e indicated schematically
Schematic drawing
of the same
molecule
Figure 1 shows the final difference Fourier synthesis and a schematic drawing of the molecule. Intramolecular interatomic distances and angles and their estimated standard deviations are given i n Table I . Table I. A.
Interatomic Distances a n d Bond Angles in NH3B3H7
B o n d Lengths, A . Ν—Β» Bi—BJ Bi—Βι B2—Bi
1.581 1.744 1.820 1.803 1.09 1.18 1.23 1.39 1.12 1.11 1.75 1.12 1.14
BI—HI B Iι — —H Β H ii
BB Ji — — HΗa*
Bj—Hi B*—He Bi—He Ba—HT
±0.003 ± 0.005 ±0.006 ±0.006 ±0.03 ±0.04 ±0.03 ±0.05 ±0.05 ±0.04 ±0.03 ±0.03 ±0.07
B . N o n b o n d e d Intramolecular Distances, A . 2.806 ± 0 . 0 0 7 Ν . . . Bi 2.861 ±0.005 Ν . . . Bt 2.18 ±0.04 Ν . . . He 2.28 ±0.07 Ν . . . HT 2.28 ±0.04 Β!... HT C.
B o n d Angles NB1B1 NBiBi Ν Β ι ( B plane)
111.0 115.3 117.2
±0.5° ±0.5° ±0.5°
The topology of the B H group apparently is that of a fragment of tetraborane (3), the boron triangle containing two bridge bonds, Β χ Η ^ and B H B , and one nonbridged bond B — B . There are, however, some significant differences. T h e Βχ—B distance is considerably longer than the value 1.712 A . found for the nonbridged distance i n tetraborane; at the same time the bridged B — B distance is 3
7
2
x
e
3
3
3
x
2
BORAX TO BORANES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
ADVANCES IN CHEMISTRY SERIES
206
shorter than any of the bridged distances i n B H , a l l of which are i n the range 1.84 ± 0 . 0 1 A . Another difference is the strong asymmetry of the B H B bridge bond. While the B — H distance is that of a Β—Η single bond, the B — H distance is about 0.4 A . longer than the Β—Η distance normally found i n B H B bridges. I n view of these departures from the geometry of the tetraborane molecule, an alternative description involving one bridge, B i H B , and a central three-center B i B B bond cannot be entirely ruled out. 4
1 0
2
3
e
3
2
e
2
3
e
2
3
(NH ) BH CI Downloaded by CALIFORNIA INST OF TECHNOLOGY on January 13, 2018 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch022
3
2
2
Considerable difficulty was experienced i n preparing a single crystal usable for x-ray work. M o s t common solvents either fail to dissolve the compound or de compose i t . Crystallization from liquid ammonia yielded a powder. A small number of crystals were finally grown from a solution i n diethylene glycol dimethyl ether layered with diethyl ether i n which the compound is insoluble. Only one of the crystals was found to be good enough for x-ray work and repeated attempts to grow more crystals failed. A total of 160 reflections were recorded, but no attempt was made to record a l l hkl reflections since the crystal began to deteriorate. ο
χ
Figure 2.
— -
α
Projection onto (001) of the ( N H ) B H C I structure 3
Arrangement
of
and
atoms
are
chlorine
boron
(small
(double
at intervals of 4 e A . "
2
circles),
circles).
nitrogen
2
(large
Electron density
f o r Β a n d N ; 10 e A . "
2
2
circles), contours
f o r CI.
Zero
contour omitted
The unit cell of ( N H ) B H C 1 is orthorhombic with a = 10.20 Α . ; b = 10.20 Α . ; c = 8.71 A . A few very weak and somewhat diffuse reflections calling for a doubling of the above c axis were ignored i n the structure determination. The diffraction pat terns taken parallel to the hkO net showed almost fourfold symmetry. T h e a p pearance of systematic absences, which were inconsistent with any space group extinction, led to the conclusion that the crystal was a twin consisting of fragments whose orientation differed b y 90 degrees about the c direction. This was supported by the observation that the reflections hkl and "khl" were i n a constant ratio when* 3
2
2
BORAX TO BORANES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
NORDMAN ET AL.
Structures of NH B H S
S
7
and (NH ) BH Cl s
2
2
ever, because of space group extinctions, these spots were not overlaps of reflections from the two fragments. I n this way the ratio of the sizes of the two fragments could be found and the F values could be determined for a l l reflections, including those which were overlaps of reflections from the two fragments. The space group is either Bba2 or Bbcm; the two are not distinguishable b y diffraction methods. T h e space group Bbcm requires the atoms to lie i n special positions. The observation that the diffracted intensities were particularly high for h, k, and I a l l even suggested that the chlorine atoms were located roughly on a simple tetragonal lattice having the repeat distances of one half those of the orthorhombic lattice. This was confirmed b y Fourier projections, which also showed that the Β and Ν atoms lie i n layers, parallel to ab, halfway between the chlorine layers. This arrangement is consistent with space group Bbcm; the boron and nitrogen atoms are at ζ = 0 and % and the CI atoms at χ = y = 0 ; ζ « 0.25. The parameters were refined b y several cycles of least squares, yielding a final value of R = 0.137. The atomic coordinates (x,y,z) are (0, 0, 0.264) for CI, (0.236, 0.064, 0) and (0.060, 0.237, 0) for N and N , and (0.212, 0.219, 0) for B . The projection onto (001) of the structure is shown i n Figure 2. T h e arrange ment of the chlorine atoms, and the Ν . . . C l distances, indicate that the compound is ionic—namely, [ ( N H ) B H + ] C 1 . T h e estimated standard deviations i n the interatomic distances are 0.04 A . for Β—Ν and 0.02 A . for Ν . . . C l distances. T h e two boron-nitrogen bonds i n the cation are therefore equal within experimental error, and also equal to the Β—Ν distance i n ammonia-triborane. 2
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107
x
3
2
2
2
_
Literature Cited (1) (2) (3) (4)
Kodama, Goji, Parry, R. W., J. Am. Chem. Soc. 79,1007(1957). Kodama, Goji, Parry, R. W., X V I Congr. IUPAC, Paris, July 1957. Moore, Ε. B., Dickerson, R. E., Lipscomb, W. Ν., J. Chem. Phys. 27, 209 (1957). Nordman, C. E., Acta Cryst. 10, 777 (1957).
BORAX TO BORANES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
