A three-dimensional Al^III/Na^I metal–organic framework resulting from dimethyl­formamide hydrolysis

The three-dimensional metal–organic framework poly[bis(di­methyl­ammonium) [hexa-μ₂-formato-κ¹²O:O′-aluminium(III)sodium(I)]], {(C₆H₈N)₂[AlNa(HCOO)₆]}_n, was obtained serendipitously and has been characterized by X-ray diffraction. The product has arisen as a result of a hydrolysis reaction of dimethyl­formamide (DMF) and contains dimethyl­ammonium (DMA) cations included in structural voids form­ed by a three-dimensional [AlNa(HCOO)₆]⁻ network. This study provides evidence that, in the presence of traces of aluminium, DMF stored in a glass bottle can be hydrolysed to formate and dimethyl­amine with simultaneous extraction of Na⁺ cations from the glass. It also demonstrates that care must be taken regarding the metal and water content when DMF is not freshly distilled, since the hydrolysis of amide can occur.

S1. Comment

The crystal structure of the title compound, (I), contains a decomposition product of DMF and constitutes the first example of a highly symmetrical three-dimensional network built solely of heterometallic M–(µ₂-formato-O,O')–M' units. Compound (I) displays R3 symmetry, with Na^I and Al^III cations occupying special position sites 3a and 3b, respectively, a formate anion in a general position (18f) and two dimethylammonium (DMA) cations disordered around the threefold axis. The disorder model for each cation comprises both dimethylammonium C atoms lying on a threefold axis (Wyckoff notation 6c) and the NH₂ fragment in a general position (18f) statistically distributed around this axis. DMF hydrolysis leading to formic acid and DMA has been reported previously (Sletten & Jensen, 1973; Liu et al., 2003; Wang et al., 2004, 2006; Burrows et al., 2005; Clausen et al., 2005; Chen et al., 2006; Hawxwell & Brammer, 2006). In the resulting crystal structures, formic acid often serves as an anionic ligand in the construction of metal–organic frameworks and DMA acts as a counter-cation to balance the charge.

The structure of (I) contains DMA cations included in voids in the three-dimensional [NaAl(HCOO)₆]^- network. It is inferred that the source of Al^III cations is the aluminium foil and of Na^I cations is the glass container. Reports on the ability of formate solutions to extract Na^I cations have appeared in the literature previously (Alcock et al., 2006, and references therein).

Each Al^III and Na^I cation is surrounded by six formate anions in an octahedral arrangement (Fig. 1). The trans O—M—O angles are all 180°, while the cis O—M—O angles can be divided into two groups: those related by a threefold axis (smaller than 90° in the case of Al^III and larger than 90° in the case of Na^I), and their supplementary angles (Table 1). For these cis O—M—O angles, the maximum deviation in the bond angles from a perfect octahedral geometry amounts to 0.71° around Al^III and 0.84° around Na^I. There is one unique Al—O and one Na—O bond length (Table 1). Hence, the coordination polyhedron around each metal ion can be described as a trigonally distorted (MO₃O'₃) octahedron. Each formate anion connects two metal atoms in the anti–anti coordination mode, which results in infinite Al^III—O—C—O—Na^I chains forming a three-dimensional anionic network, similar to the reported homometallic coordination networks with formate as the only bridging ligand (Wang et al., 2004, 2006).

In the structure of (I), one can distinguish corrugated (001) anionic layers containing both metal atoms connected by the formate bridges (Fig. 1). These anionic layers are separated by the cationic [NH₂(CH₃)₂]⁺ layers. Viewed approximately along the [102] direction, a cubic network formed by heterometallic centres with interpenetrating channels occupied by DMA cations is observed (Fig. 2). The cross-section of the channels, measured between the midpoints of the Al^III and Na^I centres, has dimensions of 6.11 × 6.11 Å. A search of the Cambridge Structural Database (Version 5.29, November 2007; Allen, 2002) reveals that such a highly symmetric three-dimensional arrangement of heterometallic centres bridged solely by the formate units, each utilizing its two O atoms to join to only two metals, has not been observed previously. This type of high-symmetry arrangement has so far appeared only in homometallic formates. The heterometallic formates reported in the literature display lower crystal symmetry and contain some of the formate anions linked to more than two metal ions or doubly linked to the same metal centre (Alcock et al., 2006).

The electrostatic interactions between the anionic framework and the cationic solvent molecules are augmented by hydrogen bonding. The H atoms of the DMA cation are involved in the formation of hydrogen bonds with both O atoms of the formate anions. Hydrogen-bond parameters are presented in Table 2.

S2. Experimental

Crystals of (I) were obtained accidentally while storing DMF solvent in a glass bottle with an aluminium-covered cork.

S3. Refinement

All H atoms were placed in their idealized positions and refined as riding on their carrier atoms, with methyl C—H = 0.96 Å with U_iso(H)= 1.5U_eq(C), formate C—H = 0.96 Å with U_iso(H) = 1.2U_eq(C), and ammonium N—H = 0.90 Å with U_iso(H) = 1.2U_eq(N). Geometric restraints were applied to the N—C bonds and C···C intramolecular contacts within the DMA cation.

Computing details

Data collection: KM-4 Software (Kuma, 1997); cell refinement: KM-4 Software (Kuma, 1997); data reduction: KM-4 Software (Kuma, 1997) and XEMP (Siemens, 1989); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Mercury (Macrae et al., 2006); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

Fig. 1. The three-dimensional coordination polymer, (I), and the bridging function of the carboxylate group.

Fig. 2. The framework structure of (I), showing the DMA cations located in channels. In one of the channels one of the three different positions of the DMA cation is shown, and in the second channel the disorder of the DMA cation is shown. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. [Symetry codes: (i) 2/3 - x + y, 1/3 - x, 1/3 + z; (ii) 5/3 + y, 1/3 - x + y, 1/3 - z; (iii) 1 + x, y, z; (iv) 2/3 + x, 1/3 + y, 1/3 + z; (vi) -1/3 + y, 1/3 - x + y, 1/3 - z].

poly[bis(dimethylammonium) [hexa-µ₂-formato-κ¹²O:O'-aluminium(III)sodium(I)]]

Crystal data

(C2H8N)2[AlNa(HCO2)6]	Dx = 1.518 Mg m^−3
Mr = 412.27	Cu Kα radiation, λ = 1.54178 Å
Trigonal, R3	Cell parameters from 1265 reflections
Hall symbol: -R 3	θ = 9.9–68.0°
a = 8.251 (1) Å	µ = 1.83 mm^−1
c = 22.949 (3) Å	T = 296 K
V = 1353.0 (3) Å^3	Prismatic, colourless
Z = 3	0.20 × 0.20 × 0.10 mm
F(000) = 647.9

Data collection

Kuma KM-4 κ-geometry diffractometer	443 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.023
Graphite monochromator	θmax = 68.0°, θmin = 9.9°
ω scans	h = 0→9
Absorption correction: multi-scan (XEMP; Siemens,1989)	k = −9→0
Tmin = 0.635, Tmax = 0.833	l = −27→27
1164 measured reflections	2 standard reflections every 50 reflections
553 independent reflections	intensity decay: none

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)] = 0.034	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.104	H-atom parameters constrained
S = 1.07	w = 1/[σ^2(Fo^2) + (0.0658P)^2 + 0.8037P] where P = (Fo^2 + 2Fc^2)/3
553 reflections	(Δ/σ)max < 0.001
47 parameters	Δρmax = 0.39 e Å^−3
3 restraints	Δρmin = −0.21 e Å^−3

Special details

Experimental. The structure was solved by direct method and refined by a full-matrix least-squares technique based on F2 (SHELXL97).

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	Occ. (<1)
Al1	0.3333	0.6667	0.1667	0.0268 (4)
Na1	0.0000	0.0000	0.0000	0.0219 (4)
O1	0.3442 (2)	0.4831 (2)	0.11946 (7)	0.0379 (5)
C1	0.2160 (3)	0.3459 (3)	0.09229 (10)	0.0414 (6)
H1	0.0945	0.3311	0.0990	0.050*
O2	0.2358 (3)	0.2373 (3)	0.06147 (9)	0.0606 (6)
N1	0.5874 (9)	0.2473 (9)	0.0742 (3)	0.0545 (16)	0.3333
H1N	0.4687	0.2244	0.0740	0.065*	0.33333
H2N	0.5814	0.1353	0.0740	0.065*	0.33333
C2	0.6667	0.3333	0.1288 (2)	0.0673 (14)
H2A	0.7945	0.3616	0.1295	0.101*	0.33333
H2B	0.5993	0.2521	0.1607	0.101*	0.33333
H2C	0.6615	0.4465	0.1328	0.101*	0.33333
C3	0.6667	0.3333	0.0219 (2)	0.106 (3)
H3A	0.7978	0.3738	0.0214	0.158*	0.33333
H3B	0.6490	0.4384	0.0153	0.158*	0.33333
H3C	0.6042	0.2440	−0.0087	0.158*	0.33333

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Al1	0.0271 (5)	0.0271 (5)	0.0261 (7)	0.0135 (3)	0.000	0.000
Na1	0.0236 (6)	0.0236 (6)	0.0184 (8)	0.0118 (3)	0.000	0.000
O1	0.0371 (8)	0.0365 (8)	0.0391 (9)	0.0177 (7)	−0.0027 (6)	−0.0077 (6)
C1	0.0408 (12)	0.0354 (12)	0.0469 (13)	0.0181 (10)	−0.0062 (10)	−0.0089 (10)
O2	0.0613 (12)	0.0561 (12)	0.0616 (12)	0.0272 (10)	−0.0118 (10)	−0.0301 (9)
N1	0.050 (4)	0.041 (3)	0.074 (4)	0.024 (3)	−0.010 (3)	−0.004 (3)
C2	0.077 (2)	0.077 (2)	0.048 (3)	0.0385 (11)	0.000	0.000
C3	0.135 (4)	0.135 (4)	0.048 (3)	0.067 (2)	0.000	0.000

Geometric parameters (Å, º)

Al1—O1	1.8999 (14)	N1—H2N	0.9001
Na1—O2	2.4082 (18)	C2—H2A	0.9600
C1—O1	1.262 (3)	C2—H2B	0.9599
C1—O2	1.215 (3)	C2—H2C	0.9600
C1—H1	0.9600	C3—H3A	0.9600
N1—C3	1.380 (6)	C3—H3B	0.9600
N1—C2	1.428 (6)	C3—H3C	0.9601
N1—H1N	0.9000
O1—Al1—O1^i	180.0	C2—N1—H1N	106.8
O1—Al1—O1^ii	90.71 (7)	C3—N1—H2N	107.0
O1—Al1—O1^iii	90.71 (7)	C2—N1—H2N	107.0
O1—Al1—O1^iv	89.29 (7)	H1N—N1—H2N	106.8
O1—Al1—O1^v	89.29 (7)	N1—C2—H2A	108.0
O2—Na1—O2^vi	180	N1—C2—H2B	111.2
O2—Na1—O2^vii	90.84 (7)	H2A—C2—H2B	109.9
O2—Na1—O2^viii	89.16 (7)	N1—C2—H2C	109.6
O2—Na1—O2^ix	90.84 (7)	H2A—C2—H2C	109.9
O2—Na1—O2^x	89.16 (7)	H2B—C2—H2C	108.3
C1—O1—Al1	130.15 (16)	N1—C3—H3A	110.5
O2—C1—O1	126.0 (3)	N1—C3—H3B	110.7
O2—C1—H1	121.1	H3A—C3—H3B	109.8
O1—C1—H1	112.8	N1—C3—H3C	107.8
C1—O2—Na1	126.08 (18)	H3A—C3—H3C	109.8
C3—N1—C2	121.7 (5)	H3B—C3—H3C	108.2
C3—N1—H1N	106.8
Al1—O1—C1—O2	177.75 (19)	O1—C1—O2—Na1	−171.65 (18)

Symmetry codes: (i) −x+2/3, −y+4/3, −z+1/3; (ii) −x+y, −x+1, z; (iii) −y+1, x−y+1, z; (iv) x−y+2/3, x+1/3, −z+1/3; (v) y−1/3, −x+y+1/3, −z+1/3; (vi) −x, −y, −z; (vii) x−y, x, −z; (viii) −x+y, −x, z; (ix) y, −x+y, −z; (x) −y, x−y, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1N···O2	0.90	2.00	2.876 (7)	165
N1—H2N···O2^xi	0.90	2.30	3.059 (7)	142
N1—H2N···O1^xi	0.90	2.30	3.118 (6)	151

Symmetry code: (xi) −y+1, x−y, z.