Dihydrobis(methylamine)borate triiodide

Both cation and anion in the title compound, C₂H₁₂BN₂⁺·I₃^-, lie on a crystallographic mirror plane and are bound in the lattice by N-H...I^- hydrogen bonds, forming layers. Methyl-H-borane-H dihydride [-C-H([delta]⁺)...([delta]^-)H-B-] inter­actions between mol­ecules crosslink adjacent layers, giving `sandwich' stacking along the a axis.

S1. Comment

During studies investigating syntheses and structures of ammonia borane compounds and derivatives, crystals of the title compound, (I), were isolated. Although the reaction sequence is not confirmed, it is likely that the high reactivity of the B—I bond in the intermediate methylamine iodoborane (Nöth & Beyer, 1960) and the primary amine, methylamine borane, in a coordinating solvent (tetrahydrofuran), resulted in the formation of the diamino borane salt as described by Nöth et al. (1964). In the current case, employment of excess iodine led to crystallization of the triiodide salt.

Capitalized codenames hereafter are those of the Cambridge Structural Database (CSD; Version 5.27 with May 2006 updates; Allen, 2002). There are currently three amino borane compound structures in which both the B and the N atoms are di-protonated as here, namely ethylenediaminebis(borane) (EDABRO; Ting et al., 1972) and two cyanoboranes (FASJIT and LOYTAU; Vyakaranam, Rana, Zheng et al., 2002; Vyakaranam, Rana, Spielvogel et al., 2002b). By contrast there are many in which the boron is diprotonated and the nitrogen is singly protonated or present as a dimethylamine, for example the cyclic borazanes (e.g. DUJYOW; Narula et al., 1986) and linear boranes (e.g. BATCOO and IRITAE; Nöth & Thomas, 1999; Jaska et al., 2004).

The asymmetric unit of (I) contains the independent molecules shown in Fig. 1. Both cation and anion lie on a mirror plane (y = 1/4) in a neat packing arrangement discussed below. The B—N and C—N lengths are similar to those in EDABRO and the linear boranes but longer, as expected, than those in the cyano and cyclic borazanes. The triiodide anion I—I bond lengths are significantly different, following the trend observed when this anion is involved in N—H₂⁺···I₃⁻ interactions, e.g. 3.158 and 2.803 Å in GAFGUQ (Warden et al., 2004). The normal I—I distance in I₃⁻ (as shown by structures where the molecule has an enforced centre of symmetry) is 2.91–2.92 Å [e.g. 2.914 Å in PATVEM02 (Konarev et al., 2005)]. From a cursory study of 188 CSD entries for I₃⁻, it appears that the I—I—I bonding asymmetry generally reflects strong H⁺···I⁻ interactions in the lattice; there are exceptions, however, such as the values of 3.086 and 2.797 Å in HILLUJ (Grafe-Kavoosian et al., 1998). The intermolecular interaction distance between anions here is 3.5895 (7) Å compared with the expected 3.96 Å van der Waal distance. However, this interaction distance is typical and close to the minimum value reported so far in the CSD for this anion (3.55 Å; GUFNEA; Chandrasekaran et al., 2000).

Conventional N—H···I hydrogen bonds (Table 2 and Fig. 2) provide the main packing forces in the cell, building two-dimensional layers in the bc plane. Typical N—H···I₃⁻ hydrogen bonds have H···I distances and N—H···I angles ranging from 2.52 Å and 171° (RACNOY; Robertson et al., 1996) to 2.85 Å and 139° (GAFGUQ; Warden et al., 2004). In addition, dihydride methyl C—H···H-borane interactions (Table 2) cross-link adjacent layers to form a `sandwich stack', up the a axis (Fig. 2). The existence of intramolecular C—H···H—B close contacts, leading to stabilization against disproportionation, has been noted before by Custelcean & Jackson (2001). There are other crystallographic examples of such interactions: most commonly these involve ammonia borane (BH₃—NH₃) in compounds such as GIJPAC (Pears et al., 1988) with H···H distances of 2.21 and 2.25 Å or with the adjacent nitrogen singly protonated (e.g. LOKFAS; Amezcua et al., 1999; H···H = 2.26 Å). A CSD search with the boron bound to a C rather than the N atom shows that the H···H distance is usually longer and that the B atom is bonded to an electron-withdrawing group or atom, e.g. a metal atom such as Rh in MADMEK (Londesborough et al., 2004), with H···H distances of 2.38 and 2.35 Å. In our case, the elegant N—H···I hydrogen binding, whereby both ends of the I₃⁻ ion bind to different amine protons in the molecules, permits the dihydridic interactions to be available from both distance and steric points of view (as shown in Fig. 2).

S2. Experimental

Iodine (16.75 g, 0.066 mol) was added in small aliquots to an ice-cooled solution of methylamine borane (2.94 g, 0.066 mol) in tetrahydrofuran (100 ml). After stirring for 2 h, the solution was warmed to room temperature and stirred for a further 4 h. On removal of most of the solvent, needles of the title compound precipitated.

S3. Refinement

The H atoms on N1, N2 and B1 were positionally refined with U_iso(H) = 1.2U_eq(parent atom). Methyl atoms H1A and H2A were constrained to the mirror plane (y = 1/4); all the methyl H atoms were restrained to C—H = 0.98 (s.u.?) Å with refined isotropic displacement parameters. An alternative model (using the density setting AFIX 137 for the methyl H atoms) placed one H atom just off the mirror plane, indicating possibly minor disorder across the plane. As both gave identical final agreement parameters, the simpler model above was used.

Computing details

Data collection: SMART (Siemens, 1996); cell refinement: SAINT (Siemens, 1996); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP in WinGX (Farrugia, 1997) and Mercury (Macrae et al., 2006); software used to prepare material for publication: SHELXL97 (Sheldrick, 1997) and PLATON (Spek, 2003).

Figures

	Fig. 1. The asymmetric unit of (I) at the 50% ellipsoid probability level (ORTEP-3; Farrugia, 1997). The dashed line represents a hydrogen bond.
	Fig. 2. A ce l l view of (I), approximately down the c axis (Mercury; Macrae et al., 2006), showing the `sandwich' crystal packing along the a axis: dotted lines represent hydrogen-bonding interactions. For clarity, only one set of H and I atoms involved in hydrogen bonding are labelled. Symmetry codes: (i) x,1/2 − y,z (ii) −x, y + 1/2, −z + 1; (iii) x, y + 1, −1 + z.

Bis(methylaminedihydroborane) triiodide

Crystal data

C2H12BN2^+·I3^−	F(000) = 404
Mr = 455.64	Dx = 2.723 Mg m^−3
Monoclinic, P21/m	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yb	Cell parameters from 4246 reflections
a = 9.3387 (11) Å	θ = 2.4–33.0°
b = 7.1393 (8) Å	µ = 8.37 mm^−1
c = 9.4837 (10) Å	T = 93 K
β = 118.495 (5)°	Needle, red–purple
V = 555.70 (11) Å^3	0.58 × 0.13 × 0.06 mm
Z = 2

Data collection

Siemens SMART CCD area-detector diffractometer	2067 independent reflections
Radiation source: fine-focus sealed tube	1745 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.051
Detector resolution: 8.192 pixels mm^-1	θmax = 33.7°, θmin = 2.4°
ϕ and ω scans	h = −14→14
Absorption correction: multi-scan (SADABS; Blessing, 1995; Sheldrick, 1996)	k = −10→10
Tmin = 0.241, Tmax = 0.606	l = −14→14
6410 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.035	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.100	H atoms treated by a mixture of independent and constrained refinement
S = 1.10	w = 1/[σ^2(Fo^2) + (0.0505P)^2 + 0.986P] where P = (Fo^2 + 2Fc^2)/3
2067 reflections	(Δ/σ)max < 0.001
72 parameters	Δρmax = 3.82 e Å^−3
4 restraints	Δρmin = −1.97 e Å^−3

Special details

Experimental. Crystal decay was monitored by repeating the initial 10 frames at the end of the data collection and analyzing duplicate reflections. The standard 0.8 mm diameter collimator was used.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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

	x	y	z	Uiso*/Ueq
I1	0.32681 (5)	0.7500	0.01629 (4)	0.02095 (11)
I2	0.34934 (4)	0.7500	0.34885 (4)	0.01875 (11)
I3	0.36573 (5)	0.7500	0.65759 (4)	0.02141 (11)
N1	0.2427 (7)	0.2500	0.9279 (7)	0.0247 (10)
H1	0.302 (7)	0.162 (8)	0.951 (7)	0.030*
N2	0.2490 (6)	0.2500	0.6573 (7)	0.0245 (10)
H2	0.314 (7)	0.350 (8)	0.692 (6)	0.029*
C1	0.1394 (9)	0.2500	1.0104 (10)	0.0331 (15)
H1A	0.078 (8)	0.132 (5)	0.983 (8)	0.06 (2)*
H1B	0.191 (14)	0.2500	1.1277 (17)	0.07 (3)*
C2	0.1581 (9)	0.2500	0.4803 (8)	0.0331 (14)
H2A	0.096 (9)	0.134 (6)	0.438 (9)	0.08 (3)*
H2B	0.224 (12)	0.2500	0.425 (13)	0.08 (4)*
B1	0.1311 (8)	0.2500	0.7365 (9)	0.0258 (13)
H3	0.059 (7)	0.116 (8)	0.700 (7)	0.031*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
I1	0.01892 (19)	0.0246 (2)	0.01996 (19)	0.000	0.00980 (15)	0.000
I2	0.01280 (18)	0.0229 (2)	0.02067 (19)	0.000	0.00806 (14)	0.000
I3	0.02121 (19)	0.02343 (19)	0.01967 (19)	0.000	0.00983 (15)	0.000
N1	0.019 (2)	0.018 (2)	0.035 (3)	0.000	0.012 (2)	0.000
N2	0.016 (2)	0.021 (2)	0.030 (3)	0.000	0.006 (2)	0.000
C1	0.028 (3)	0.034 (4)	0.045 (4)	0.000	0.023 (3)	0.000
C2	0.027 (3)	0.036 (4)	0.023 (3)	0.000	0.001 (3)	0.000
B1	0.012 (3)	0.032 (4)	0.027 (3)	0.000	0.004 (2)	0.000

Geometric parameters (Å, º)

I1—I2	3.0592 (6)	N2—H2	0.89 (5)
I2—I3	2.8581 (6)	C1—H1A	0.98 (5)
N1—C1	1.504 (8)	C1—H1B	0.979 (17)
N1—B1	1.606 (9)	C2—H2A	0.98 (5)
N1—H1	0.79 (5)	C2—H2B	0.98 (12)
N2—C2	1.476 (9)	B1—H3	1.12 (6)
N2—B1	1.601 (9)
I3—I2—I1	179.232 (15)	N1—C1—H1B	120 (8)
C1—N1—B1	110.9 (5)	H1A—C1—H1B	102 (5)
C1—N1—H1	113 (4)	N2—C2—H2A	112 (5)
B1—N1—H1	107 (4)	N2—C2—H2B	116 (7)
C2—N2—B1	112.5 (5)	H2A—C2—H2B	101 (6)
C2—N2—H2	110 (3)	N2—B1—N1	108.0 (5)
B1—N2—H2	109 (4)	N2—B1—H3	108 (3)
N1—C1—H1A	107 (4)	N1—B1—H3	108 (3)
C2—N2—B1—N1	180.000 (2)	C1—N1—B1—N2	180.000 (3)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···I1^i	0.80 (6)	2.99 (6)	3.6651 (15)	144 (6)
N2—H2···I3	0.89 (6)	2.94 (6)	3.7320 (19)	149 (5)
C2—H2A···H3^ii	0.98 (5)	2.28 (9)	3.25 (6)	171 (6)

Symmetry codes: (i) x, y−1, z+1; (ii) −x, −y, −z+1.