Tetra­methyl­ammonium borohydride from powder data

In the crystal structure of the title compound, C₄H₁₂N⁺·BH₄⁻, the tetra­methyl­ammonium cations are situated on special positions with site symmetry m2. The borohydride anions are situated on special positions with 4mm site symmetry and show rotational disorder around the fourfold axis.

Related literature

For details of the synthesis, see: Banus et al. (1952); King et al. (1956). For previous studies of the title compound, see: Harmon et al. (1974); Eckert et al. (2004). The isostructural compounds (CH₃)₄NClO₄ and (CH₃)₄NBF₄ were reported by McCullough (1964) and Giuseppetti et al. (1992), respectively. For applications of the title compound, see: Evans et al. (1988).

Experimental

Crystal data

• C₄H₁₂N⁺·BH₄⁻

• M_r = 88.99

• Tetragonal, P 4/n m m

• a = 7.9133 (2) Å

• c = 5.65696 (17) Å

• V = 354.24 (2) ų

• Z = 2

• Cu Kα radiation, λ = 1.54051, 1.54433 Å

• μ = 0.33 mm⁻¹

• T = 298 K

• cylinder, 18 × 1 mm

Data collection

• Bruker D8 Discover diffractometer

• Specimen mounting: quartz capillary

• Data collection mode: transmission

• Scan method: continuous

• 2θ_min = 8°, 2θ_max = 121°, 2θ_step = 0.012°

Refinement

• R_p = 0.014

• R_wp = 0.020

• R_exp = 0.007

• R_Bragg = 0.053

• χ² = 7.673

• 9220 data points

• 56 parameters

• 14 restraints

• H atoms treated by a mixture of independent and constrained refinement

Data collection: DIFFRACplus (Bruker, 2006); cell refinement: X-CELL (Neumann, 2003) and JANA2006 (Petricek et al., 2006); data reduction: DIFFRACplus; program(s) used to solve structure: JANA2006; program(s) used to refine structure: JANA2006; molecular graphics: CrystalMaker (Palmer, 2005); software used to prepare material for publication: publCIF (Westrip, 2010).

Supporting information

S1. Comment

Tetramethylammonium borohydride (I) and its derivatives have been used as selective reductors in organic chemistry (Evans et al., 1988) or as a source of hydrogen-rich BH₄^- anions for inorganic synthesis.

The structure of compound (I) has not been reported before; only the unit-cell parameters (a = 7.29 Å, c = 5.696 Å), the tentative space group (P4/n) (King et al., 1956) and several interatomic distances (Eckert et al., 2004) have been given. The structure presented here is isomorphous to the ambient temperature structures of (CH₃)₄NBF₄ (Giuseppetti et al., 1992) and (CH₃)₄NClO₄ (McCullough, 1964) and is composed of distinct (CH₃)₄N⁺ cations and BH₄^- anions.

The central atoms of the ions are separated by d(N1, B1) = 4.537 (4) Å, which compares well with the shortest N—B distances seen for fluoroborate (4.79 Å, s.u. not given in the paper) and N—Cl distances in perchlorate (4.86 Å, s.u. not given in the paper). Methyl groups in (I) are ordered (in contrast to (CH₃)₄NClO₄, where the hydrogen positions are disordered) and arranged in a staggered conformation like in (CH₃)₄NBF₄. This is explained by an increasing separation of cations as measured by d(N—N') distance of 5.5955 (2), 5.82 (s.u. not given in the paper), and 5.90 (s.u. not given in the paper) for (I), (CH₃)₄NBF₄, and (CH₃)₄NClO₄, respectively. The borohydride anions are centred at 2c (4 mm) site with B1 and H3 atoms at the fourfold symmetry axis, while H4 and H5 are used for the representation of disorder (four BH₄^- tetrahedra sharing vertex of H3 can be constructed). The B—H infrared absorption bands (δ_H—B—H = 1072 cm^-1, ν_B—H = 2225 cm^-1 and 2288 cm^-1) are very broad, up to ca 800 cm^-1 for stretching bands (!), which confirms H disorder of the BH₄^- anions in compound (I) (Harmon et al., 1974). In such arrangement the closest distances between hydrogen atoms of (CH₃)₄N⁺ and BH₄^- are: d(H1—H3) = 2.39 (2) Å, d(H2—H4) = 2.47 (3) Å and d(H2—H5)= 2.49 (3) Å, thus above the maximum range of a typical dihydrogen bond length of 2.2 Å.

Related literature

For details of the synthesis, see: Banus et al. (1952); King et al. (1956). For previous studies of the title compound, see: Harmon et al. (1974); Eckert et al. (2004). The isostructural compounds (CH₃)₄NClO₄ and (CH₃)₄NBF₄ were reported by McCullough (1964) and Giuseppetti et al. (1992), respectively. For applications of the title compound, see: Evans et al. (1988).

S2. Experimental

Compound (I) commercially available from Sigma-Aldrich (> 95%) has been used for powder XRD measurements without additional purification. The FTIR spectrum of compound (I) in KBr pellet has been recorded using Bruker Vertex 80v vacuum spectrometer.

S3. Refinement

The powder diffraction pattern of (I) was indexed with X-Cell (Neumann, 2003) in a tetragonal system of extinction class P4/nmm. The initial model of the structure of compound (I) was constructed according to the symmetry considerations, with all heavy atoms at special positions: N1 at 2 b (4m2), C1 at 8i (m), and B1 at 2c (4 mm). H1 and H2 (hydrogen atoms of the methyl groups) were placed at sites 8i (m), and 16k (1), respectively. An alternative structure with disordered methyl groups led to a worse Rietveld fit. Location of the hydrogen atoms of BH₄^- group was more problematic, as the tetragonal axis is incompatible with the symmetry alements of tetrahedron. The model containing four overlaping BH₄^- tetrahedra has been used for the Rietveld refinement performed using Jana2006 (Petricek et al. 2006). The following restraints have been aplied for the refinement: d(C1—H1), d(C1—H2) = 1.00 s.u. = 0.005 [Å], d(B1—H3) = 1.12, s.u. = 0.005 [Å], d(B1—H4), d(B1—H5) = 1.12, s.u. = 0.01 [Å]; a(Hi—C1—Hj), a(Hi—C1—N1) = 109.47, s.u.=0.01 [°], and a(H3—B1—H4), a(H3—B1—H5), a(H4—B1—H5) = 109.47, s.u.=0.01 [°]. The atomic displacement parameters (ADP) of hydrogen atoms were restricted according to the riding model to U_isoH1 = U_isoH2 = 1.2U_isoC1 and U_isoH3 = U_isoH4 = U_isoH5 = 1.5U_isoB1.

Computing details

Data collection: DIFFRACplus (Bruker, 2006); cell refinement: X-CELL (Neumann, 2003) and JANA2006 (Petricek et al., 2006); data reduction: DIFFRACplus (Bruker, 2006); program(s) used to solve structure: PLEASE SUPPLY; program(s) used to refine structure: JANA2006 (Petricek et al., 2006); molecular graphics: CrystalMaker (Palmer, 2005); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures

Fig. 1. A content of the unit cell of (I) viewed along axis c and showing the atomic labelling and 50% probabilty displacement ellipsoids [symmetry codes: (i) -x + 1,-y + 1,-z + 1; (ii) -x + 1/2,-y + 3/2,z; (iii) x + 1/2,y - 1/2,-z + 1; (iv) -y + 1,x + 1/2,-z + 1; (v) y,-x + 1/2,z; (vi) y - 1/2,-x + 1,-z + 1; (vii) -y + 3/2,x,z]. The H atoms are shown as spheres of arbitrary radius. Only one orientation of disordered BH4 anions is shown for clarity.

Fig. 2. The results of Rietveld refinement. Measured data are given as asterisks, the calculated profile as a solid line, and the difference profile as a solid line below. Vertical markers above the difference profile indicate the calculated Bragg reflection positions. The insert shows high-angle part magnified ca 18 times.

tetramethylammonium borohydride

Crystal data

C4H12N^+·BH4^−	F(000) = 104
Mr = 88.99	Dx = 0.834 Mg m^−3 Dm = 0.813 Mg m^−3 Dm measured by helium pycnometry (Banus et al., 1952)
Tetragonal, P4/nmm	Cu Kα radiation, λ = 1.54051, 1.54433 Å
Hall symbol: -P 4a 2a	µ = 0.33 mm^−1
a = 7.9133 (2) Å	T = 298 K
c = 5.65696 (17) Å	white
V = 354.24 (2) Å^3	cylinder, 18 × 1 mm
Z = 2	Specimen preparation: Prepared at 298 K

Data collection

Bruker D8 Discover diffractometer	Data collection mode: transmission
None monochromator	Scan method: continuous
Specimen mounting: quartz capillary	2θmin = 8°, 2θmax = 120.999°, 2θstep = 0.012°

Refinement

Rp = 0.014	14 restraints
Rwp = 0.020	2 constraints
Rexp = 0.007	All H-atom parameters refined
RBragg = 0.053	Weighting scheme based on measured s.u.'s w = 1/[σ^2(I) + 0.0016I^2]
χ^2 = 7.673	(Δ/σ)max = 0.009
9220 data points	Background function: 25 Legendre polynoms
Profile function: Pseudo-Voigt	Preferred orientation correction: none
56 parameters

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
H4	0.315 (2)	0.363 (4)	0.957 (3)	0.14971 (4)*	0.25
H2	0.1457 (17)	0.5963 (4)	0.2429 (17)	0.10060 (2)*
B1	0.25	0.25	0.8925 (14)	0.084 (5)
H3	0.25	0.25	0.702 (4)	0.14971 (4)*
C1	0.25	0.5959 (3)	0.3460 (5)	0.065 (2)
H1	0.25	0.492 (2)	0.4477 (19)	0.10060 (2)*
H5	0.25	0.119 (5)	0.957 (3)	0.14971 (4)*	0.25
N1	0.25	0.75	0.5	0.045 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
B1	0.100 (7)	0.100 (7)	0.053 (9)	0	0	0
C1	0.084 (4)	0.036 (3)	0.075 (4)	0	0	−0.013 (3)
N1	0.045 (4)	0.045 (4)	0.045 (7)	0	0	0

Geometric parameters (Å, º)

B1—H3	1.08 (2)	C1—H1	1.004 (15)
B1—H4	1.10 (3)	C1—H2	1.010 (12)
B1—H5	1.10 (4)	C1—N1	1.498 (3)
H1—C1—H2	109.5 (5)	H4—B1—H4^ii	109.5 (16)
H2—C1—N1	109.5 (4)	H4—B1—H5^iii	109.5 (12)
H2—C1—H2^i	109.5 (9)	C1—N1—C1^iv	108.91 (14)
H3—B1—H5	109.5 (12)	C1—N1—C1^v	109.75 (7)
H4—B1—H3	109.5 (10)

Symmetry codes: (i) −x+1/2, y, z; (ii) x, −y+1/2, z; (iii) y, −x+1/2, z; (iv) −x+1/2, −y+3/2, z; (v) y−1/2, x+1/2, −z+1.