organic compounds\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

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Redetermination of para-amino­pyridine (fampridine, EL-970) at 150 K

CROSSMARK_Color_square_no_text.svg

aSchool of Chemical Sciences, National Institute for Cellular Biotechnology, Dublin City University, Dublin 9, Ireland, and bDepartment of Chemistry, 80 St George Street, University of Toronto, Ontario, Canada M5S 3H6
*Correspondence e-mail: john.gallagher@dcu.ie, alough@chem.utoronto.ca

(Received 16 March 2005; accepted 4 April 2005; online 16 April 2005)

The structure of fampridine (EL-970) or 4-amino­pyridine, C5H6N2, has been redetermined at 150 K. The room-temperature structure has been reported previously [Chao & Schempp (1977[Chao, M. & Schempp, E. (1977). Acta Cryst. B33, 1557-1564.]). Acta Cryst. B33, 1557–1564]. Pyramidalization at the amine N atom occurs in fampridine, with the N atom 0.133 (11) Å from the plane of the three C/H/H atoms to which it is bonded; the inter­planar angle between the pyrid­yl ring and NH2 group is 21 (2)°. Aggregation in the solid state occurs by N—H⋯N and N—H⋯π(pyridine) inter­actions with N⋯N and N⋯π(centroid) distances of 2.9829 (18) and 3.3954 (15) Å, respectively; a C—H⋯π(pyridine) contact completes the inter­molecular inter­actions [C⋯π(centroid) = 3.6360 (16) Å].

Comment

4-Amino­pyridine (fampridine) is used in the treatment of neurological ailments, such as multiple sclerosis (MS), with tests showing that fampridine improves motor function in MS patients (Schwid et al., 1997[Schwid, S. R., Petrie, M. D., McDermott, M. P., Tierney, D. S., Mason, D. H. & Goodman, A. D. (1997). Neurology, 48, 817-821.]). Related studies have utilized this small mol­ecule on episodic ataxia type 2 (EA2), as it functions as a potassium channel blocker (Strupp et al., 2004[Strupp, M., Kalla, R., Dichgans, M., Freilinger, T., Glasauer, S. & Brandt, T. (2004). Neurology, 62, 1623-1625.]). Our inter­est in para-amino­pyridine is to react it with aromatic carboxylic acids and ac­yl chlorides to generate new series of amide-based aromatic systems.

The structures of 2-, 3- and 4-amino­pyridine, NC5H4NH2, (I)–(III)[link], have been reported previously using data collected at room temperature on a Nonius CAD-4 diffractometer with Cu Kα radiation (Chao et al., 1975a[Chao, M., Schempp, E. & Rosenstein, R. D. (1975a). Acta Cryst. B31, 2922-2924.],b[Chao, M., Schempp, E. & Rosenstein, R. D. (1975b). Acta Cryst. B31, 2924-2926.]; Chao & Schempp, 1977[Chao, M. & Schempp, E. (1977). Acta Cryst. B33, 1557-1564.]), with corresponding Cambridge Structural Database (CSD; Version V6.26, February 2005 release; Allen, 2002[Allen, F. H., (2002). Acta Cryst. B58, 380-388.]) refcodes of AMPYRD, AMIPYR and AMPYRE. In the present study, we report the crystal structure of 4-amino­pyridine (fampridine), (IV)[link], at 150 K with greater precision than reported previously for (III) and comment on the inter­molecular hydrogen bonding for comparison with the related structures (I)–(III).

[Scheme 2]

In (I)–(III/IV), the primary donor (D) and acceptor (A) are the two NH2 donor H atoms and the pyridine N atom acceptor. Herein lies a mismatch in the number of strong donor and acceptors groups, although the aromatic CH groups (D) and the π-pyrid­yl system (A) can redress this imbalance and participate as weaker donor and acceptor groups in the hydrogen-bonding process.

[Scheme 1]

In (I) (Chao et al., 1975a[Chao, M., Schempp, E. & Rosenstein, R. D. (1975a). Acta Cryst. B31, 2922-2924.]), the primary hydrogen bonding consists of pairs of mol­ecules forming N—H⋯Npyridine hydrogen bonds in a cyclic array about inversion centres, with graph set R22(8) (Bernstein et al., 1995[Bernstein, J., Davies, R. E., Shimoni, L. & Chang, N.-L. (1995). Angew. Chem. Int. Ed. Engl. 34, 1555-1573.]) (H⋯N = 2.20 (3) Å, N⋯N = 3.071 (7) Å and N—H⋯N 171 (2)°]. Dimers stack into columns along the b axis direction, although there are no ππ stacking inter­actions of note. Further association of the dimers occurs via the second N—H donor as N—H⋯Npyridine inter­actions (about a twofold screw axis), forming a herringbone-type packing pattern in the [011] direction (inter­planar angle 58.9°), with H⋯N = 2.63 (3) Å, N⋯N = 3.416 (7) Å and N—H⋯N = 149 (2)°, and augmented by two C—H⋯π(arene) contacts per N—H⋯N inter­action. There are no other inter­actions of note apart from normal van der Waals contacts.

In (II) (Chao et al., 1975b[Chao, M., Schempp, E. & Rosenstein, R. D. (1975b). Acta Cryst. B31, 2924-2926.]), N—H⋯Npyridine inter­actions link mol­ecules along the a-axis direction in a head-to-tail fashion, thus generating infinite one-dimensional chains [H⋯N = 2.22 (3) Å, N⋯N = 3.123 (4) Å and N—H⋯N = 168 (2)°]. The second NH donor forms an inter­action with the amine N atom (lone pair of electrons), with H⋯N = 2.46 (3) Å, N⋯N = 3.336 (4) Å and N—H⋯N = 162 (2)°, thus linking the chains into a three-dimensional herringbone pattern, where each chain is surrounded by six others, and this process is augmented by two C—H⋯π(arene) inter­actions per mol­ecule of (II), e.g. H⋯C 2.81 and 2.87 Å, only one of which is depicted in the second scheme[link].

In (III) (Chao & Schempp, 1977[Chao, M. & Schempp, E. (1977). Acta Cryst. B33, 1557-1564.]), N—H⋯Npyridine inter­actions link mol­ecules in a head-to-tail manner, forming zigzag chains along the c-axis direction, with H⋯N = 2.14 Å, N⋯N = 3.007 Å and N—H⋯N = 159°. The second NH donor atom forms an N—H⋯π(pyrid­yl) inter­action with a symmetry-related chain, stacked antiparallel along the b-axis direction, with shortest H⋯C = 2.66 Å and N—H⋯C = 173°. The N—H⋯π(pyrid­yl) inter­action links each chain with two neighbouring chains, each consecutive NH donor alternately donating to either of the two π(pyrid­yl) groups. Thus, each chain is linked and effectively surrounded by four chains as the π(pyrid­yl) is also an acceptor of N—H inter­actions from two extra chains. Of note is that pyramidalization occurs at the amine N atom.

Thus, in structures (I)–(III), an Namine—H⋯Npyridine hydrogen bond forms and the remaining amine H-atom donor inter­acts in the crystal structure in one of three different ways, either via herringbone-type N—H⋯Npyridine inter­actions in (I), by N—H⋯Namine inter­actions in (II) or as N—H⋯π(pyrid­yl) inter­actions in (III), as depicted in the second scheme[link] above. It is pertinent to note that in the original reports the primary hydrogen bonding involving Namine—H⋯Npyridine was comprehensively discussed. However, only in (II) was the remaining (second) amine NH donor implicated in an inter­action and with the lone pair of electrons on the amine N atom. The authors further qualify this with the statement `although these distances are too long to be recognized as hydrogen bonds'.

In the present study of (IV)[link], the low-temperature structure of (III), the corresponding data are detailed in Tables 1[link] and 2[link]. Bond lengths and angles are similar but are determined to a higher degree of precision than reported for (III). In (IV)[link], amine atom N1 lies 0.133 (11) Å from the plane of atoms C1, H1A and H1B; this pyramidalization can also be observed by the three angles about N1 summing to 355.3°. The amine N1/H1A/H1B and the pyrid­yl NC5 group are twisted from coplanarity by 21 (2)°, while atom N1 is coplanar with the pyridine ring system; the H1A/H1B atoms lie 0.12 (2) and 0.22 (2) Å from this aromatic plane. The hydrogen-bonding distances in (IV)[link] (Table 2[link]) [values for (III) in brackets] are N⋯N 2.9829 (18) Å [3.007 Å], N⋯π(pyrid­yl) 3.3954 (15) Å [3.460 Å], and a contact between atom C3 and a neighbouring π(pyrid­yl) of 3.6360 (16) Å [3.704 Å].

In the isoelectronic compound aniline, C6H5NH2, (amino­benzene), studied at 252 K (CSD refcode BAZGOY), two strong donor groups and no strong acceptors are present (Fukuyo et al., 1982[Fukuyo, M., Hirotsu, K. & Higuchi, T. (1982). Acta Cryst. B38, 640-643.]). Aggregation in the solid state utilizes both NH donor groups as N—H⋯Namine [N⋯N = 3.180 (6) and 3.373 (5) Å] and N—H⋯π(phen­yl) inter­actions [N⋯Cg = 3.41 and 3.49 Å, H⋯N = 2.70 and 2.64 Å, and N—H⋯π(Cg) = 154 and 150°, where Cg represents the aromatic ring centroids]. The former two N—H⋯Namine inter­actions are similar to that observed in (II) above: the latter two are described as having H atoms that are `free from hydrogen bonding' (Fukuyo et al., 1982[Fukuyo, M., Hirotsu, K. & Higuchi, T. (1982). Acta Cryst. B38, 640-643.]). Such N—H⋯π(aromatic) inter­actions have received considerable attention in recent years both in chemistry and in biology (Desiraju & Steiner, 1999[Desiraju, G. R. & Steiner, T. (1999). The Weak Hydrogen Bond in Structural Chemistry and Biology. Oxford University Press.]). The distances and angles associated with these type of inter­actions in (III) and (IV)[link] are typical of the values reported in the literature.

[Figure 1]
Figure 1
A view of (IV)[link], with the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level.. H atom sphere radii are arbitrary.
[Figure 2]
Figure 2
A view of the two primary inter­actions in (IV)[link]. Atoms labelled with the suffixes # and * are at the symmetry-related positions (−[{1\over 2}] + x, [{1\over 2}] − y, 1 − z) and ([{1\over 2}] − x, 1 − y, [{1\over 2}] + z), respectively.
[Figure 3]
Figure 3
A view of the hydrogen-bonded chain generated by N—H⋯N hydrogen bonds and N—H⋯π(arene) inter­actions with a neighbouring chain.

Experimental

4-Amino­pyridine was purchased from Aldrich Chemical Company Ltd and recrystallized from CH2Cl2 prior to analysis.

Crystal data
  • C5H6N2

  • Mr = 94.12

  • Orthorhombic, P 21 21 21

  • a = 5.5138 (4) Å

  • b = 7.1866 (5) Å

  • c = 12.0459 (4) Å

  • V = 477.32 (5) Å3

  • Z = 4

  • Dx = 1.310 Mg m−3

  • Mo Kα radiation

  • Cell parameters from 2026 reflections

  • θ = 2.6–27.5°

  • μ = 0.08 mm−1

  • T = 150 (1) K

  • Block, colourless

  • 0.25 × 0.20 × 0.15 mm

Data collection
  • Nonius KappaCCD diffractometer

  • φ scan and ω scans with κ offsets

  • Absorption correction: multi-scan(SORTAV; Blessing, 1995[Blessing, R. H. (1995). Acta Cryst. A51, 33-37.])Tmin = 0.979, Tmax = 0.988

  • 3217 measured reflections

  • 662 independent reflections

  • 591 reflections with I > 2σ(I)

  • Rint = 0.030

  • θmax = 27.5°

  • h = −6 → 7

  • k = −9 → 9

  • l = −15 → 15

Refinement
  • Refinement on F2

  • R[F2 > 2σ(F2)] = 0.031

  • wR(F2) = 0.086

  • S = 1.10

  • 662 reflections

  • 89 parameters

  • All H-atom parameters refined

  • w = 1/[σ2(Fo2) + (0.0587P)2 + 0.0044P] where P = (Fo2 + 2Fc2)/3

  • (Δ/σ)max < 0.001

  • Δρmax = 0.17 e Å−3

  • Δρmin = −0.13 e Å−3

  • Extinction correction: SHELXL97

  • Extinction coefficient: 0.13 (4)

Table 1
Selected geometric parameters (Å, °)[link]

N1—C1 1.3597 (18)
C1—C2 1.409 (2)
C1—C6 1.403 (2)
C2—C3 1.378 (2)
C3—N4 1.345 (2)
N4—C5 1.346 (2)
C5—C6 1.375 (2)
N1—H1A 0.95 (2)
N1—H1B 0.94 (2)
N1—C1—C2 122.41 (13)
N1—C1—C6 121.33 (13)
C2—C1—C6 116.25 (13)
C1—C2—C3 119.37 (14)
N4—C3—C2 124.77 (15)
C3—N4—C5 115.25 (13)
N4—C5—C6 124.74 (14)
C5—C6—C1 119.61 (13)

Table 2
Hydrogen-bond geometry (Å, °)[link]

D—H⋯A D—H H⋯A DA D—H⋯A
N1—H1B⋯N4i 0.94 (2) 2.06 (2) 2.9829 (18) 167 (2)
N1—H1ACg1ii 0.95 (2) 2.61 (2) 3.3954 (15) 141 (2)
C3—H3⋯Cg1iii 1.03 (2) 2.76 (2) 3.6360 (16) 143 (2)
Symmetry codes: (i) [-x+{\script{1\over 2}}, -y+1, z+{\script{1\over 2}}]; (ii) [x-{\script{1\over 2}}, -y+{\script{1\over 2}}, -z+1]; (iii) [-x, y-{\script{1\over 2}}, -z+{\script{1\over 2}}].

All six H atoms bound to C and N were refined with isotropic displacement parameters with the four C—H bond lengths in the range 0.971 (19) to 1.028 (18) Å. Examination of the structure with PLATON (Spek, 2003[Spek, A. L. (2003). J. Appl. Cryst. 36, 7-13.]) showed that there were no solvent-accessible voids in the crystal structure. In the absence of significant anomalous scattering, Friedel pairs were merged prior to the final refinement cycles.

Data collection: KappaCCD Server Software (Nonius, 1997[Nonius (1997). KappaCCD Server Software. Windows 3.11 Version. Nonius BV, Delft, The Netherlands.]); cell refinement: DENZO–SMN (Otwinowski & Minor, 1997[Otwinowski, Z. & Minor, W. (1997). Methods in Enzymology, Vol. 276, Macromolecular Crystallography, Part A, edited by C. W. Carter Jr & R. M. Sweet, pp. 307-326. New York: Academic Press.]); data reduction: DENZO–SMN; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997[Sheldrick, G. M. (1997). SHELXS97 and SHELXL97. University of Göttingen, Germany.]); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997[Sheldrick, G. M. (1997). SHELXS97 and SHELXL97. University of Göttingen, Germany.]); molecular graphics: ORTEPIII (Burnett & Johnson, 1996[Burnett, M. N. & Johnson, C. K. (1996). ORTEPIII. Report ORNL-6895. Oak Ridge National Laboratory, Tennessee, USA.]) and PLATON (Spek, 2003[Spek, A. L. (2003). J. Appl. Cryst. 36, 7-13.]); software used to prepare material for publication: SHELXL97, ORTEX (McArdle, 1995[McArdle, P. (1995). J. Appl. Cryst. 28, 65-65.]) and PREP8 (Ferguson, 1998[Ferguson, G. (1998). PREP8. University of Guelph, Canada.]).

Supporting information


Computing details top

Data collection: KappaCCD Server Software (Nonius, 1997); cell refinement: DENZO–SMN (Otwinowski & Minor, 1997); data reduction: DENZO–SMN; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEX (Burnett & Johnson, 1996) and PLATON (Spek, 2003); software used to prepare material for publication: SHELXL97, ORTEX (McArdle, 1995) and PREP8 (Ferguson, 1998).

4-aminopyridine top
Crystal data top
C5H6N2? #Insert any comments here.
Mr = 94.12Dx = 1.310 Mg m3
Orthorhombic, P212121Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ac 2abCell parameters from 2026 reflections
a = 5.5138 (4) Åθ = 2.6–27.5°
b = 7.1866 (5) ŵ = 0.08 mm1
c = 12.0459 (4) ÅT = 150 K
V = 477.32 (5) Å3Block, colourless
Z = 40.25 × 0.20 × 0.15 mm
F(000) = 200
Data collection top
Nonius KappaCCD
diffractometer
662 independent reflections
Radiation source: fine-focus sealed X-ray tube591 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.030
φ scan and ω scans with κ offsetsθmax = 27.5°, θmin = 3.3°
Absorption correction: multi-scan
(SORTAV; Blessing, 1995)
h = 67
Tmin = 0.979, Tmax = 0.988k = 99
3217 measured reflectionsl = 1515
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.031All H-atom parameters refined
wR(F2) = 0.086 w = 1/[σ2(Fo2) + (0.0587P)2 + 0.0044P]
where P = (Fo2 + 2Fc2)/3
S = 1.10(Δ/σ)max < 0.001
662 reflectionsΔρmax = 0.17 e Å3
89 parametersΔρmin = 0.13 e Å3
0 restraintsExtinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
Primary atom site location: structure-invariant direct methodsExtinction coefficient: 0.13 (4)
Special details top

Experimental. ? #Insert any special details here.

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.

Planes data ###########

Least-squares planes (x,y,z in crystal coordinates) and deviations from them (* indicates atom used to define plane)

-2.7639(0.0029)x + 5.8598(0.0027)y - 3.4889(0.0068)z = 0.9722(0.0035)

* -0.0048 (0.0010) C1 * 0.0023 (0.0011) C2 * 0.0036 (0.0010) C3 * -0.0072 (0.0010) N4 * 0.0047 (0.0011) C5 * 0.0013 (0.0010) C6 0.0000 (0.0021) N1 - 0.1285 (0.0181) H1A -0.2253 (0.0231) H1B

Rms deviation of fitted atoms = 0.0044

-3.3817(0.0787)x + 5.6625(0.0835)y + 0.6624(0.3484)z = 3.2778(0.1869)

Angle to previous plane (with approximate e.s.d.) = 20.9(1.6)

* 0.0000 (0.0000) N1 * 0.0000 (0.0000) H1A * 0.0000 (0.0000) H1B

Rms deviation of fitted atoms = 0.0000

- 2.7837(0.0740)x + 6.0631(0.0710)y - 2.1990(0.1424)z = 1.6604(0.0959)

Angle to previous plane (with approximate e.s.d.) = 15.3(2.1)

* 0.0000 (0.0000) C1 * 0.0000 (0.0000) H1A * 0.0000 (0.0000) H1B 0.1331 (0.0106) N1 - 0.2686 (0.0221) N4

Rms deviation of fitted atoms = 0.0000

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
N10.0935 (2)0.45712 (19)0.56319 (10)0.0302 (4)
C10.0307 (3)0.4568 (2)0.46567 (11)0.0244 (4)
C20.0533 (3)0.3619 (2)0.37080 (11)0.0259 (4)
C30.0811 (3)0.3682 (2)0.27448 (13)0.0285 (4)
N40.2927 (2)0.4593 (2)0.26288 (10)0.0295 (4)
C50.3692 (3)0.5515 (2)0.35369 (12)0.0284 (4)
C60.2497 (3)0.5542 (2)0.45390 (12)0.0274 (4)
H1A0.228 (3)0.376 (3)0.5703 (14)0.034 (5)*
H1B0.020 (4)0.493 (3)0.6301 (18)0.054 (6)*
H20.204 (3)0.292 (3)0.3720 (12)0.030 (4)*
H30.026 (3)0.295 (3)0.2055 (15)0.036 (5)*
H50.523 (3)0.617 (3)0.3440 (14)0.031 (4)*
H60.309 (4)0.626 (3)0.5168 (15)0.037 (5)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
N10.0328 (7)0.0322 (7)0.0257 (7)0.0054 (7)0.0031 (5)0.0009 (6)
C10.0269 (7)0.0217 (7)0.0246 (7)0.0022 (7)0.0016 (5)0.0025 (6)
C20.0257 (7)0.0253 (7)0.0268 (7)0.0019 (7)0.0041 (6)0.0005 (6)
C30.0332 (8)0.0269 (7)0.0254 (7)0.0015 (8)0.0042 (6)0.0012 (6)
N40.0309 (7)0.0310 (7)0.0267 (6)0.0009 (6)0.0010 (5)0.0031 (5)
C50.0266 (7)0.0274 (7)0.0313 (8)0.0013 (7)0.0001 (6)0.0035 (6)
C60.0297 (8)0.0255 (7)0.0270 (7)0.0013 (8)0.0044 (6)0.0005 (6)
Geometric parameters (Å, º) top
N1—C11.3597 (18)N1—H1A0.95 (2)
C1—C21.409 (2)N1—H1B0.94 (2)
C1—C61.403 (2)C2—H20.971 (19)
C2—C31.378 (2)C3—H31.028 (18)
C3—N41.345 (2)C5—H50.978 (19)
N4—C51.346 (2)C6—H60.973 (19)
C5—C61.375 (2)
N1—C1—C2122.41 (13)C1—N1—H1B121.7 (14)
N1—C1—C6121.33 (13)C1—C2—H2121.3 (9)
C2—C1—C6116.25 (13)C3—C2—H2119.4 (9)
C1—C2—C3119.37 (14)C2—C3—H3120.4 (10)
N4—C3—C2124.77 (15)N4—C3—H3114.8 (10)
C3—N4—C5115.25 (13)N4—C5—H5114.4 (10)
N4—C5—C6124.74 (14)C6—C5—H5120.9 (10)
C5—C6—C1119.61 (13)C1—C6—H6118.4 (11)
H1A—N1—H1B115.5 (16)C5—C6—H6121.9 (11)
C1—N1—H1A118.1 (10)
N1—C1—C2—C3179.68 (13)C3—N4—C5—C61.3 (2)
C6—C1—C2—C30.5 (2)N4—C5—C6—C10.5 (2)
C1—C2—C3—N40.3 (2)N1—C1—C6—C5179.59 (14)
C2—C3—N4—C51.2 (2)C2—C1—C6—C50.4 (2)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1B···N4i0.94 (2)2.06 (2)2.9829 (18)167 (2)
N1—H1A···Cg1ii0.95 (2)2.61 (2)3.3954 (15)141.1 (15)
C3—H3···Cg1iii1.028 (18)2.76 (2)3.6360 (16)143.3 (15)
Symmetry codes: (i) x+1/2, y+1, z+1/2; (ii) x1/2, y+1/2, z+1; (iii) x, y1/2, z+1/2.
 

Acknowledgements

JFG thanks Ms Joyce McMahon for recrystallizing compound (IV). FPA, JFG and PTMK thank Dublin City University and the Department of Education, Ireland, for funding the National Institute for Cellular Biotechnology (PRTLI program, round #3, 2001–2008). AJL thanks the University of Toronto and NSERC Canada for funding.

References

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