Dinicotinamidium squarate

The crystal structure determination of the dinicotinamidium squarate salt, 2C₆H₇N₂O⁺·C₄O₄²⁻, is reported, with the squarate dianion residing on an inversion centre and the unique cation in a general position. Salt formation occurs by donation of two H atoms from squaric acid to the nicotin­amide base. The crystal packing is derived from three types of hydrogen bonding. The primary hydrogen bond involves a squarate anion O atom and an H atom of the protonated pyridine group of the nicotin­amide, with an N...O distance of 2.5760 (13) Å. The second hydrogen bond involves a second anion O atom and an amide H atom, with an N...O distance of 2.8374 (14) Å. Thirdly, an intermolecular interaction between two coplanar nicotin­amide moieties occurs between an amide O atom and a symmetry-related amide H atom, with an N1...O3 distance of 2.8911 (15) Å. These hydrogen bonds are also responsible for the planarity of the nicotin­amide moiety in the salt.

S1. Comment

It is known that hydrogen bonding is the most probable means of generating a supramolecular organic system (Lehn, 1995; Desiraju, 1989). These systems have been shown to have unique chemical and physical properties (Desiraju, 1995; MacDonald & Whitesides, 1994). Hydrogen bonding also plays a crucial role in molecular recognition (Goswami & Ghosh, 1997) and crystal-engineering research (Goswami et al., 1998). In particular, the mixing of different molecules with acid/base properties might make an important contribution to the predictability of the recognition process (Russell et al., 1994; Burchell et al., 2001). The design of highly specific solid state structures formed by hydrogen bonding finds significant application in organic chemistry, such as in the development of new optical, magnetic and electronic systems (Lehn, 1990). In this context, squaric acid and its anions are of potential interest because they are flat and rigid systems.

Squaric acid can crystallize in three forms, as shown in Scheme 1. It can crystallize as H₂SQ, as well as as HSQ⁻ or SQ^2- anions on deprotonation by amines. These three forms of squaric acid have been observed to crystallize by various types of hydrogen bonding, which are summarized in Bertolasi's work (Bertolasi et al., 2001). Squaric acid donates one or two H atoms (although this is not common) to planar aromatic bases, forming so-called low-barrier hydrogen bonds (LBHBs; Cassidy et al., 1999) or positive/negative charge-assisted hydrogen bonds [(+/-) CAHB; Gilli et al., 1996]. The present study reports an example of this type of hydrogen bonding, in which squaric acid donates two H atoms to a planar nicotinamide base, forming the title dinicotinamidium squarate salt, (I). \sch

The asymmetric unit of (I) contains one protonated nicotinamide cation and half of a centrosymmetric squarate anion (SQ²⁻). A view of the hydrogen-bonded structure of (I) and its numbering scheme are shown in Fig. 1. Both the squarate and the nicotinamidium moieties are planar and the dihedral angle between these planes is 87.54 (6)°. In (I), it is observed that the squarate anion is surrounded by four nicotinamide cations. This is formed such that each squaric acid gives two H atoms to each pyridine N atom of two trans nicotinamide moieties lying parallel to each other, since nicotinamide, like other organic bases, is protonated in acidic solution. Please check that this last sentence has been rephrased correctly.

Each O atom of the SQ²⁻ anion makes a contribution to the crystal packing (Fig. 2). The hydrogen bond between atom O2 [and O2ⁱ; symmetry code: (i) 1 − x, 2 − y, 1 − z] of the carboxylic acid and the protonated pyridine part of the nicotinamide is quite strong [N2—H2···O2 2.5760 (13) Å] and potentially belongs to the class of N2⁺—H···O low-barrier hydrogen bonds (Cassidy et al., 1999). This hydrogen bond connects two planar trans nicotinamidium moieties in parallel planes. This strong hydrogen bond is also the reason for the longer N2—H2 distance of 1.08 (2) Å. Atom O1 of the SQ²⁻ anion has an interaction with atom H1B of the amide part of the nicotinamide [N1—H1B···O1 2.8374 (14) Å]. There is also an intermolecular hydrogen bond that links the nicotinamide dimers [N1—H1A···O3 2.8911 (15) Å] and forms an eight-membered ring in the structure of (I). This hydrogen bond keeps the two nicotinamide moieties in the same plane.

The C3—C4 bond [1.4959 (17) Å] links the pyridine and amide moieties, and it is similar to the value found in a gas-phase electron diffraction study of nicotinamide [1.498 (8) Å; Takeshima et al., 2003]. A comparison of related bond distances involving the pyridine ring atoms of (I) with the values found in the above cited gas-phase study shows that the positive charge is localized on the pyridine N atom and does not have much effect on the resonance structure of pyridine. The skeletal structure of nicotinamide is non-planar, with a C5/C4/C3/N1 dihedral angle of 34° in the gas phase and 23° in a neutron diffraction study (Miwa et al., 1999). However, this same angle is 0.59 (19)° in the present study, indicating that the nicotinamide moiety in the title salt is almost planar, and this can be attributed to the effect of hydrogen bonding. The nicotinamide C═O bond length in the present salt is 0.017 Å longer than that in the gas phase, while the amide C—N bond length in the solid state is 0.021 Å shorter than that in the gas phase. These changes are close to the values observed for benzamide (Takeuchi et al., 1999) and they are ascribed to the effect of hydrogen bonding in the crystal structure.

S2. Experimental

Compound (I) was prepared by mixing nicotinamide and squaric acid in a 1:1 molar ratio in a mixed solution of methanol and water (50/50 v/v ?) with stirring at 333 K for 12 h. Crystals of (I) were obtained by slow evaporation of the solvent. The crystals were filtered, washed in water and methanol and dried in vacuo.

S3. Refinement

All H atoms were found in a difference map and their parameters were freely refined. C—H bond distances were found in the range 0.971 (14)–1.015 (16) Å and U_iso(H) values in the range 0.036 (4)–0.057 (5) Ų. The N1—H1B and N1—H1A bond distances were 0.880 (18) and 0.921 (17) Å, respectively, with U_iso(H) values of 0.045 (4) and 0.057 (5) Ų, respectively. The deviation of the N2—H2 bond distance [1.08 (2) Å, with U_iso(H) = 0.079 (6) Ų] from normal values was attributed to the strong N2···O2 hydrogen bond [2.576 (2) Å].

Computing details

Data collection: X-AREA (Stoe & Cie, 2002); cell refinement: X-AREA; data reduction: X-RED32 (Stoe & Cie, 2002); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPIII (Burnett & Johnson, 1996) and PLATON (Spek, 1996); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

	Fig. 1. A view of the ionic moieties of (I), illustrating the atom-numbering scheme and the hydrogen bonds. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. The hydrogen bonds are indicated by dashed lines [symmetry codes: (i) −x, y, 1/2 − z; (ii) 2 − x, 1 − y, 1 − z; (iii) 1 − x, 2 − y, 1 − z].
	Fig. 2. The three-dimensional structure of (I), linked by hydrogen bonds.

Dinicotinamidium squarate

Crystal data

2C6H7N2O^+·C4O4^2^−	F(000) = 372
Mr = 358.32	Dx = 1.548 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 6059 reflections
a = 13.2377 (15) Å	θ = 0.0–29.5°
b = 5.3623 (4) Å	µ = 0.12 mm^−1
c = 11.5142 (16) Å	T = 293 K
β = 109.822 (10)°	Plate, yellow
V = 768.90 (15) Å^3	0.48 × 0.31 × 0.08 mm
Z = 2

Data collection

Stoe IPDS 2 diffractometer	2140 independent reflections
Radiation source: fine-focus sealed tube	1497 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.000
Detector resolution: 6.67 pixels mm^-1	θmax = 29.5°, θmin = 1.6°
rotation method scans	h = −18→17
Absorption correction: integration (X-RED32; Stoe & Cie, 2002)	k = 0→7
Tmin = 0.938, Tmax = 0.985	l = 0→15
2140 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)] = 0.040	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.109	All H-atom parameters refined
S = 0.90	w = 1/[σ^2(Fo^2) + (0.0779P)^2] where P = (Fo^2 + 2Fc^2)/3
2140 reflections	(Δ/σ)max < 0.001
146 parameters	Δρmax = 0.23 e Å^−3
0 restraints	Δρmin = −0.23 e Å^−3

Special details

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
C1	0.57514 (9)	0.5697 (2)	0.50688 (11)	0.0334 (3)
C2	0.50007 (9)	0.6078 (2)	0.57371 (11)	0.0334 (3)
C3	0.90303 (9)	0.7921 (2)	0.41301 (11)	0.0324 (3)
C4	0.83192 (9)	0.9978 (2)	0.34299 (10)	0.0309 (2)
C5	0.73093 (9)	1.0438 (2)	0.34860 (11)	0.0337 (3)
C6	0.70581 (11)	1.3791 (3)	0.21238 (12)	0.0398 (3)
C7	0.80485 (11)	1.3401 (3)	0.20175 (13)	0.0412 (3)
C8	0.86845 (10)	1.1496 (3)	0.26781 (12)	0.0364 (3)
N1	0.86821 (9)	0.6474 (2)	0.48501 (11)	0.0394 (3)
N2	0.67171 (8)	1.2325 (2)	0.28508 (10)	0.0351 (2)
O1	0.66510 (7)	0.6548 (2)	0.51743 (10)	0.0455 (3)
O2	0.50341 (7)	0.7411 (2)	0.66475 (9)	0.0454 (3)
O3	0.99202 (7)	0.76427 (19)	0.40184 (9)	0.0428 (3)
H1A	0.9131 (14)	0.520 (4)	0.5253 (16)	0.057 (5)*
H1B	0.8025 (13)	0.656 (3)	0.4877 (15)	0.046 (4)*
H2	0.5959 (16)	1.261 (4)	0.2983 (19)	0.079 (6)*
H5	0.6987 (12)	0.940 (3)	0.4009 (14)	0.045 (4)*
H6	0.6548 (13)	1.517 (3)	0.1717 (15)	0.057 (5)*
H7	0.8306 (14)	1.450 (3)	0.1494 (16)	0.056 (5)*
H8	0.9396 (11)	1.118 (3)	0.2643 (13)	0.036 (4)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0261 (5)	0.0373 (6)	0.0391 (6)	0.0005 (5)	0.0142 (4)	0.0041 (5)
C2	0.0260 (5)	0.0378 (6)	0.0389 (6)	0.0010 (4)	0.0141 (4)	0.0020 (5)
C3	0.0282 (5)	0.0363 (6)	0.0351 (6)	0.0026 (5)	0.0140 (4)	0.0004 (5)
C4	0.0271 (5)	0.0350 (6)	0.0320 (5)	0.0003 (4)	0.0120 (4)	−0.0009 (5)
C5	0.0289 (5)	0.0374 (6)	0.0372 (6)	0.0012 (5)	0.0146 (4)	0.0018 (5)
C6	0.0382 (6)	0.0390 (7)	0.0400 (7)	0.0057 (5)	0.0103 (5)	0.0035 (5)
C7	0.0419 (7)	0.0437 (7)	0.0409 (7)	0.0003 (6)	0.0177 (5)	0.0079 (6)
C8	0.0318 (6)	0.0429 (7)	0.0387 (6)	0.0019 (5)	0.0174 (5)	0.0031 (5)
N1	0.0325 (5)	0.0436 (6)	0.0469 (6)	0.0071 (5)	0.0198 (5)	0.0128 (5)
N2	0.0272 (5)	0.0401 (6)	0.0383 (5)	0.0035 (4)	0.0115 (4)	−0.0010 (4)
O1	0.0290 (4)	0.0551 (6)	0.0579 (6)	−0.0090 (4)	0.0219 (4)	−0.0036 (5)
O2	0.0342 (5)	0.0557 (6)	0.0516 (6)	−0.0045 (4)	0.0213 (4)	−0.0136 (5)
O3	0.0319 (4)	0.0490 (6)	0.0546 (6)	0.0106 (4)	0.0239 (4)	0.0116 (4)

Geometric parameters (Å, º)

C1—O1	1.2422 (14)	C5—H5	1.015 (17)
C1—C2^i	1.4600 (17)	C6—N2	1.3337 (17)
C1—C2	1.4629 (16)	C6—C7	1.3735 (19)
C2—O2	1.2571 (16)	C6—H6	1.003 (18)
C2—C1^i	1.4600 (17)	C7—C8	1.3773 (19)
C3—O3	1.2367 (13)	C7—H7	0.982 (18)
C3—N1	1.3272 (16)	C8—H8	0.971 (14)
C3—C4	1.4959 (17)	N1—H1A	0.920 (19)
C4—C5	1.3823 (15)	N1—H1B	0.882 (16)
C4—C8	1.3889 (17)	N2—H2	1.08 (2)
C5—N2	1.3361 (16)
O1—C1—C2^i	136.56 (12)	N2—C6—C7	120.18 (12)
O1—C1—C2	134.20 (12)	N2—C6—H6	114.2 (10)
C2^i—C1—C2	89.24 (9)	C7—C6—H6	125.6 (10)
O2—C2—C1^i	136.41 (11)	C6—C7—C8	119.14 (12)
O2—C2—C1	132.83 (12)	C6—C7—H7	120.3 (10)
C1^i—C2—C1	90.76 (9)	C8—C7—H7	120.6 (11)
O3—C3—N1	122.62 (12)	C7—C8—C4	120.14 (11)
O3—C3—C4	118.72 (11)	C7—C8—H8	121.6 (9)
N1—C3—C4	118.65 (10)	C4—C8—H8	118.2 (9)
C5—C4—C8	118.12 (11)	C3—N1—H1A	116.5 (11)
C5—C4—C3	123.24 (11)	C3—N1—H1B	123.2 (11)
C8—C4—C3	118.65 (10)	H1A—N1—H1B	119.8 (16)
N2—C5—C4	120.45 (11)	C6—N2—C5	121.96 (11)
N2—C5—H5	117.0 (9)	C6—N2—H2	122.4 (12)
C4—C5—H5	122.5 (9)	C5—N2—H2	115.6 (12)
O1—C1—C2—O2	−0.1 (3)	C8—C4—C5—N2	−1.35 (19)
C2^i—C1—C2—O2	179.41 (19)	C3—C4—C5—N2	179.13 (11)
O1—C1—C2—C1^i	−179.54 (19)	N2—C6—C7—C8	−0.5 (2)
C2^i—C1—C2—C1^i	0.0	C6—C7—C8—C4	0.7 (2)
O3—C3—C4—C5	179.65 (12)	C5—C4—C8—C7	0.24 (19)
N1—C3—C4—C5	−0.59 (19)	C3—C4—C8—C7	179.78 (12)
O3—C3—C4—C8	0.14 (18)	C7—C6—N2—C5	−0.6 (2)
N1—C3—C4—C8	179.90 (12)	C4—C5—N2—C6	1.55 (19)

Symmetry code: (i) −x+1, −y+1, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N2—H2···O2^ii	1.08 (2)	1.51 (2)	2.5760 (13)	169 (2)
N1—H1A···O3^iii	0.920 (19)	1.974 (19)	2.8911 (15)	175.3 (16)
N1—H1B···O1	0.882 (16)	1.961 (17)	2.8374 (14)	171.8 (15)

Symmetry codes: (ii) −x+1, −y+2, −z+1; (iii) −x+2, −y+1, −z+1.