N,N′-Bis(pyrazin-2-ylmeth­yl)pyrazine-2,3-dicarb­oxamide

Wang, Z.; Yang, Y.; Lin, H.

doi:10.1107/S2414314616015881

organic compounds

IUCrDATA

ISSN: 2414-3146

Volume 1| Part 10| October 2016| x161588

https://doi.org/10.1107/S2414314616015881

Open

access

N,N′-Bis(pyrazin-2-ylmethyl)pyrazine-2,3-dicarboxamide

Zhaodong Wang,^a ^* Yanshuang Yang ^a and Huanzhu Lin ^a

^aChongqing Key Laboratory of Environmental Materials & Remediation Technologies, Chongqing University of Arts and Sciences, Yongchuan, Chongqing 402160, People's Republic of China
^*Correspondence e-mail: ouwzdong@qq.com

Edited by J. Simpson, University of Otago, New Zealand (Received 5 October 2016; accepted 8 October 2016; online 14 October 2016)

The title compound, C₁₆H₁₄N₈O₂, was prepared by a condensation reaction between the dimethyl ester of pyrazine-2,3-dicarboxylic acid and an excess of 2-(aminomethyl)pyrazine. The molecule is approximately V-shaped with the central pyrazine ring joined through amide linkages to two pyrazin-2-ylmethyl substituents. In the crystal, molecules are linked by N—H⋯O, N—H⋯N and C—H⋯O hydrogen bonds, forming a three-dimensional framework.

Keywords: crystal structure; pyrazine-based ligand; amide ligand; hydrogen bonds.

CCDC reference: 1504885

Structure description

Pyrazine-based amide ligands have played a very important role in coordination chemistry (Cati et al., 2004 ; Cati & Stoeckli-Evans, 2004a ,b ; Klingele et al., 2007 ). The crystal structure of a symmetrical diamide ligand (Cati & Stoeckli-Evans, 2004c ) and its bi- and tetranuclear copper(II) complexes have also been reported (Cati & Stoeckli-Evans, 2004d ; Hausmann et al., 2003 ; Hausmann & Brooker 2004 ). In order to expand the research scope of this pyrazine-2,3-bisamide ligand system, we have prepared the title compound to investigate its potential use as a ligand in transition metal chemistry.

There are three pyrazine rings in the title compound (Fig. 1). The outer pair are linked to the central pyrazine ring by two CH₂–NH–C(O)–amide units, forming a V-shaped structure. The two outer pyrazine rings are inclined to the central ring by 48.98 (8) and 47.87 (8)°. In the crystal, N—H⋯O, N—H⋯N and C—H⋯O hydrogen bonds generate a three-dimensional framework (Table 1 Fig. 2). The coordination modes of the ligand system could be more flexible than those of previously reported pyrazine-based ligands due to the N atoms of the outer pyrazine rings. These could be used in a bridging sense with the potential to construct metal-organic frameworks.

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1⋯O2ⁱ	0.86	2.26	2.9940 (18)	144
N6—H6⋯N2ⁱⁱ	0.86	2.06	2.914 (2)	174
C4—H4⋯O1ⁱⁱⁱ	0.93	2.52	3.369 (2)	152
C5—H5⋯N7^iv	0.93	2.46	3.359 (3)	162
C14—H14⋯O2^v	0.93	2.56	3.236 (2)	130
Symmetry codes: (i) -x+1, -y+2, -z+2; (ii) x+1, y, z; (iii) -x, -y+1, -z+1; (iv) x-1, y-1, z; (v) -x+1, -y+2, -z+1.

Figure 1
The stucture of the title compound, showing the atom numbering with ellipsoids drawn at the 30% probability level

Figure 2
Crystal packing viewed along the a axis direction with hydrogen bonds shown as dashed lines.

Synthesis and crystallization

All reagents and solvents for the synthesis were purchased from commercial sources and used as received without further purification. The title compound was prepared by the condensation reaction of the dimethyl ester pyrazine-2,3-dicarboxylate (3.92g, 20mmol) with 2-aminomethylpyrazine (2.18g, 20mmol) under reflux in 15mL of methanol for 12h. Solvent was removed by rotary evaporation to give a light-yellow solid. Recrystallization from CH₃OH solution gave colorless crystals suitable for X-ray analysis.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2.

Table 2
Experimental details

Crystal data
Chemical formula	C₁₆H₁₄N₈O₂
M_r	350.35
Crystal system, space group	Triclinic, P $[\overline{1}]$
Temperature (K)	296
a, b, c (Å)	9.1166 (18), 10.022 (2), 10.781 (2)
α, β, γ (°)	116.951 (2), 97.130 (2), 102.195 (2)
V (Å³)	830.5 (3)
Z	2
Radiation type	Mo Kα
μ (mm⁻¹)	0.10
Crystal size (mm)	0.22 × 0.20 × 0.20

Data collection
Diffractometer	Bruker SMART APEXII CCD area-detector
No. of measured, independent and observed [I > 2σ(I)] reflections	3527, 3527, 2595
R_int	0.016
(sin θ/λ)_max (Å⁻¹)	0.653

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.041, 0.132, 1.05
No. of reflections	3527
No. of parameters	235
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.15, −0.17
Computer programs: APEX2 and SAINT (Bruker, 2013 ), SHELXT (Sheldrick, 2015a ), SHELXL2014 (Sheldrick, 2015b ) and OLEX2 (Dolomanov et al., 2009 ).

Structural data

CCDC reference: 1504885

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2414314616015881/sj4064sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2414314616015881/sj4064Isup2.hkl

Supporting information file. DOI: https://doi.org/10.1107/S2414314616015881/sj4064Isup3.cml

checkCIF report

Computing details top

Data collection: APEX2 (Bruker, 2013); cell refinement: SAINT (Bruker, 2013); data reduction: SAINT (Bruker, 2013); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

N,N'-Bis(pyrazin-2-ylmethyl)pyrazine-2,3-dicarboxamide top

Crystal data top

C₁₆H₁₄N₈O₂	Z = 2
M_r = 350.35	F(000) = 364
Triclinic, P1	D_x = 1.401 Mg m⁻³
a = 9.1166 (18) Å	Mo Kα radiation, λ = 0.71073 Å
b = 10.022 (2) Å	Cell parameters from 2164 reflections
c = 10.781 (2) Å	θ = 2.3–26.7°
α = 116.951 (2)°	µ = 0.10 mm⁻¹
β = 97.130 (2)°	T = 296 K
γ = 102.195 (2)°	Block, colourless
V = 830.5 (3) Å³	0.22 × 0.20 × 0.20 mm

Data collection top

Bruker SMART APEXII CCD area-detector diffractometer	2595 reflections with I > 2σ(I)
Radiation source: fine focus sealed tube	R_int = 0.016
Graphite monochromator	θ_max = 27.7°, θ_min = 2.2°
φ and ω scans	h = −11→11
3527 measured reflections	k = −13→11
3527 independent reflections	l = −13→13

Refinement top

Refinement on F²	0 restraints
Least-squares matrix: full	Hydrogen site location: mixed
R[F² > 2σ(F²)] = 0.041	H-atom parameters constrained
wR(F²) = 0.132	w = 1/[σ²(F_o²) + (0.0759P)² + 0.0242P] where P = (F_o² + 2F_c²)/3
S = 1.05	(Δ/σ)_max < 0.001
3527 reflections	Δρ_max = 0.15 e Å⁻³
235 parameters	Δρ_min = −0.17 e Å⁻³

Special details top

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

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
O1	0.44574 (13)	0.72355 (16)	0.68709 (12)	0.0573 (4)
O2	0.72074 (13)	1.03109 (13)	0.82972 (12)	0.0489 (3)
N1	0.35307 (13)	0.81613 (15)	0.88449 (13)	0.0423 (3)
H1	0.3740	0.8582	0.9766	0.051*
N2	−0.04468 (13)	0.59908 (14)	0.66043 (13)	0.0385 (3)
N3	0.0331 (2)	0.3695 (2)	0.7075 (2)	0.0799 (6)
N4	0.59054 (16)	0.72344 (19)	0.99377 (15)	0.0525 (4)
N5	0.88003 (14)	0.81968 (16)	0.94792 (13)	0.0432 (3)
N6	0.84472 (14)	0.86414 (16)	0.69834 (13)	0.0398 (3)
H6	0.8840	0.7905	0.6898	0.048*
N7	0.68896 (18)	0.96134 (17)	0.43602 (15)	0.0522 (4)
N8	0.48052 (19)	0.6643 (2)	0.32238 (17)	0.0695 (5)
C1	0.46175 (16)	0.77287 (18)	0.81589 (15)	0.0379 (4)
C2	0.20077 (16)	0.79296 (18)	0.80565 (17)	0.0424 (4)
H2A	0.2119	0.8171	0.7291	0.051*
H2B	0.1541	0.8659	0.8699	0.051*
C3	0.09378 (16)	0.62856 (17)	0.74138 (15)	0.0360 (3)
C4	−0.14353 (18)	0.45555 (19)	0.60466 (18)	0.0490 (4)
H4	−0.2414	0.4312	0.5482	0.059*
C5	−0.1042 (2)	0.3430 (2)	0.6286 (2)	0.0665 (6)
H5	−0.1766	0.2443	0.5879	0.080*
C6	0.1310 (2)	0.5138 (2)	0.7640 (2)	0.0610 (5)
H6A	0.2285	0.5379	0.8211	0.073*
C7	0.60761 (16)	0.78146 (17)	0.90534 (15)	0.0363 (3)
C8	0.7197 (2)	0.7138 (2)	1.0583 (2)	0.0603 (5)
H8	0.7136	0.6766	1.1230	0.072*
C9	0.8614 (2)	0.7568 (2)	1.03266 (18)	0.0517 (4)
H9	0.9465	0.7416	1.0757	0.062*
C10	0.75270 (16)	0.83414 (17)	0.88594 (14)	0.0334 (3)
C11	0.77139 (16)	0.91900 (18)	0.80107 (15)	0.0358 (3)
C12	0.86012 (18)	0.9254 (2)	0.60010 (17)	0.0454 (4)
H12A	0.9436	0.8978	0.5560	0.054*
H12B	0.8884	1.0391	0.6544	0.054*
C13	0.71396 (17)	0.86430 (18)	0.48330 (15)	0.0397 (4)
C14	0.5594 (2)	0.9103 (2)	0.33497 (19)	0.0607 (5)
H14	0.5376	0.9763	0.3013	0.073*
C15	0.4576 (2)	0.7646 (3)	0.2793 (2)	0.0645 (6)
H15	0.3686	0.7342	0.2084	0.077*
C16	0.6093 (2)	0.7170 (2)	0.42593 (19)	0.0561 (5)
H16	0.6294	0.6515	0.4609	0.067*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
O1	0.0428 (7)	0.0868 (9)	0.0372 (6)	0.0231 (6)	0.0054 (5)	0.0262 (6)
O2	0.0534 (7)	0.0522 (7)	0.0530 (7)	0.0304 (6)	0.0143 (5)	0.0292 (6)
N1	0.0275 (6)	0.0489 (8)	0.0373 (7)	0.0097 (6)	0.0023 (5)	0.0130 (6)
N2	0.0311 (6)	0.0396 (7)	0.0400 (7)	0.0083 (6)	0.0054 (5)	0.0178 (6)
N3	0.0610 (11)	0.0556 (10)	0.1300 (16)	0.0145 (9)	0.0086 (11)	0.0567 (11)
N4	0.0498 (8)	0.0739 (10)	0.0550 (9)	0.0270 (7)	0.0203 (7)	0.0436 (8)
N5	0.0369 (7)	0.0564 (8)	0.0423 (7)	0.0211 (6)	0.0057 (6)	0.0273 (7)
N6	0.0383 (7)	0.0510 (8)	0.0449 (7)	0.0226 (6)	0.0131 (6)	0.0311 (6)
N7	0.0629 (9)	0.0526 (9)	0.0490 (8)	0.0239 (7)	0.0090 (7)	0.0301 (7)
N8	0.0603 (10)	0.0770 (12)	0.0580 (10)	−0.0004 (9)	0.0033 (8)	0.0346 (9)
C1	0.0301 (8)	0.0415 (8)	0.0363 (8)	0.0077 (6)	0.0034 (6)	0.0170 (7)
C2	0.0281 (8)	0.0388 (8)	0.0516 (9)	0.0095 (6)	0.0037 (6)	0.0171 (7)
C3	0.0297 (7)	0.0378 (8)	0.0404 (8)	0.0123 (6)	0.0079 (6)	0.0186 (7)
C4	0.0351 (9)	0.0474 (10)	0.0492 (9)	0.0037 (7)	0.0023 (7)	0.0173 (8)
C5	0.0553 (12)	0.0404 (10)	0.0930 (15)	0.0037 (9)	0.0122 (11)	0.0304 (10)
C6	0.0423 (10)	0.0547 (11)	0.0892 (14)	0.0126 (8)	0.0000 (9)	0.0432 (11)
C7	0.0344 (8)	0.0415 (8)	0.0331 (7)	0.0147 (6)	0.0068 (6)	0.0176 (6)
C8	0.0671 (12)	0.0868 (14)	0.0607 (11)	0.0385 (11)	0.0240 (9)	0.0551 (11)
C9	0.0493 (10)	0.0729 (12)	0.0515 (10)	0.0335 (9)	0.0106 (8)	0.0400 (9)
C10	0.0306 (7)	0.0388 (8)	0.0303 (7)	0.0151 (6)	0.0042 (5)	0.0156 (6)
C11	0.0275 (7)	0.0416 (8)	0.0381 (8)	0.0122 (6)	0.0018 (6)	0.0203 (7)
C12	0.0424 (9)	0.0545 (10)	0.0485 (9)	0.0144 (8)	0.0123 (7)	0.0329 (8)
C13	0.0430 (9)	0.0454 (9)	0.0382 (8)	0.0173 (7)	0.0146 (7)	0.0241 (7)
C14	0.0647 (12)	0.0776 (14)	0.0540 (11)	0.0323 (11)	0.0099 (9)	0.0407 (10)
C15	0.0495 (11)	0.0973 (17)	0.0501 (11)	0.0189 (11)	0.0059 (8)	0.0422 (11)
C16	0.0582 (11)	0.0573 (11)	0.0551 (10)	0.0100 (9)	0.0067 (9)	0.0348 (9)

Geometric parameters (Å, º) top

O1—C1	1.2210 (17)	C2—C3	1.505 (2)
O2—C11	1.2291 (18)	C2—H2A	0.9700
N1—C1	1.3376 (19)	C2—H2B	0.9700
N1—C2	1.4479 (18)	C3—C6	1.377 (2)
N1—H1	0.8599	C4—C5	1.372 (3)
N2—C4	1.3328 (19)	C4—H4	0.9300
N2—C3	1.3333 (18)	C5—H5	0.9300
N3—C5	1.323 (3)	C6—H6A	0.9300
N3—C6	1.335 (2)	C7—C10	1.395 (2)
N4—C7	1.3312 (19)	C8—C9	1.375 (3)
N4—C8	1.336 (2)	C8—H8	0.9300
N5—C9	1.3327 (19)	C9—H9	0.9300
N5—C10	1.3347 (18)	C10—C11	1.507 (2)
N6—C11	1.3278 (19)	C12—C13	1.509 (2)
N6—C12	1.4523 (18)	C12—H12A	0.9700
N6—H6	0.8601	C12—H12B	0.9700
N7—C14	1.328 (2)	C13—C16	1.379 (2)
N7—C13	1.3311 (19)	C14—C15	1.359 (3)
N8—C15	1.326 (3)	C14—H14	0.9300
N8—C16	1.335 (2)	C15—H15	0.9300
C1—C7	1.5046 (19)	C16—H16	0.9300

C1—N1—C2	120.84 (13)	N4—C7—C10	121.72 (14)
C1—N1—H1	119.6	N4—C7—C1	116.92 (13)
C2—N1—H1	119.6	C10—C7—C1	121.09 (13)
C4—N2—C3	116.98 (14)	N4—C8—C9	122.50 (15)
C5—N3—C6	115.74 (17)	N4—C8—H8	118.8
C7—N4—C8	115.88 (14)	C9—C8—H8	118.7
C9—N5—C10	116.27 (13)	N5—C9—C8	121.79 (15)
C11—N6—C12	121.60 (13)	N5—C9—H9	119.1
C11—N6—H6	119.2	C8—C9—H9	119.1
C12—N6—H6	119.2	N5—C10—C7	121.65 (13)
C14—N7—C13	116.96 (15)	N5—C10—C11	117.86 (12)
C15—N8—C16	115.45 (17)	C7—C10—C11	120.38 (12)
O1—C1—N1	123.67 (14)	O2—C11—N6	124.70 (14)
O1—C1—C7	119.96 (14)	O2—C11—C10	119.93 (13)
N1—C1—C7	116.32 (13)	N6—C11—C10	115.38 (13)
N1—C2—C3	113.38 (13)	N6—C12—C13	113.24 (12)
N1—C2—H2A	108.9	N6—C12—H12A	108.9
C3—C2—H2A	108.9	C13—C12—H12A	108.9
N1—C2—H2B	108.9	N6—C12—H12B	108.9
C3—C2—H2B	108.9	C13—C12—H12B	108.9
H2A—C2—H2B	107.7	H12A—C12—H12B	107.7
N2—C3—C6	120.59 (14)	N7—C13—C16	120.23 (15)
N2—C3—C2	115.50 (13)	N7—C13—C12	116.23 (14)
C6—C3—C2	123.90 (14)	C16—C13—C12	123.54 (15)
N2—C4—C5	121.50 (16)	N7—C14—C15	122.10 (17)
N2—C4—H4	119.3	N7—C14—H14	119.0
C5—C4—H4	119.3	C15—C14—H14	119.0
N3—C5—C4	122.44 (17)	N8—C15—C14	122.35 (17)
N3—C5—H5	118.8	N8—C15—H15	118.8
C4—C5—H5	118.8	C14—C15—H15	118.8
N3—C6—C3	122.74 (17)	N8—C16—C13	122.89 (17)
N3—C6—H6A	118.6	N8—C16—H16	118.6
C3—C6—H6A	118.6	C13—C16—H16	118.6

C2—N1—C1—O1	5.8 (2)	C9—N5—C10—C11	174.14 (14)
C2—N1—C1—C7	−171.79 (12)	N4—C7—C10—N5	4.4 (2)
C1—N1—C2—C3	81.57 (18)	C1—C7—C10—N5	−169.43 (14)
C4—N2—C3—C6	0.5 (2)	N4—C7—C10—C11	−171.71 (14)
C4—N2—C3—C2	−178.48 (14)	C1—C7—C10—C11	14.5 (2)
N1—C2—C3—N2	−176.08 (12)	C12—N6—C11—O2	−5.5 (2)
N1—C2—C3—C6	5.0 (2)	C12—N6—C11—C10	174.84 (12)
C3—N2—C4—C5	−0.5 (2)	N5—C10—C11—O2	−125.27 (15)
C6—N3—C5—C4	0.7 (3)	C7—C10—C11—O2	50.96 (19)
N2—C4—C5—N3	−0.1 (3)	N5—C10—C11—N6	54.42 (18)
C5—N3—C6—C3	−0.6 (3)	C7—C10—C11—N6	−129.35 (15)
N2—C3—C6—N3	0.1 (3)	C11—N6—C12—C13	−77.41 (19)
C2—C3—C6—N3	178.94 (18)	C14—N7—C13—C16	1.2 (2)
C8—N4—C7—C10	−2.3 (2)	C14—N7—C13—C12	−178.84 (14)
C8—N4—C7—C1	171.75 (16)	N6—C12—C13—N7	148.37 (14)
O1—C1—C7—N4	−132.13 (17)	N6—C12—C13—C16	−31.7 (2)
N1—C1—C7—N4	45.6 (2)	C13—N7—C14—C15	−1.4 (3)
O1—C1—C7—C10	42.0 (2)	C16—N8—C15—C14	1.0 (3)
N1—C1—C7—C10	−140.35 (15)	N7—C14—C15—N8	0.3 (3)
C7—N4—C8—C9	−1.7 (3)	C15—N8—C16—C13	−1.2 (3)
C10—N5—C9—C8	−2.0 (3)	N7—C13—C16—N8	0.1 (3)
N4—C8—C9—N5	4.0 (3)	C12—C13—C16—N8	−179.88 (16)
C9—N5—C10—C7	−2.0 (2)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O2ⁱ	0.86	2.26	2.9940 (18)	144
N6—H6···N2ⁱⁱ	0.86	2.06	2.914 (2)	174
C4—H4···O1ⁱⁱⁱ	0.93	2.52	3.369 (2)	152
C5—H5···N7^iv	0.93	2.46	3.359 (3)	162
C14—H14···O2^v	0.93	2.56	3.236 (2)	130

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

Acknowledgements

The authors would like to thank the Talent Introduction Project of Chongqing University of Arts and Sciences (No. R2012CH12) and the Science and Technology Research Project of Chongqing Municipal Education Committee (No. KJ131215) for financial support.

References

Bruker (2013). APEX2 and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Cati, D. S., Ribas, J., Ribas-Ariño, J. & Stoeckli-Evans, H. (2004). Inorg. Chem. 43, 1021–1030. Web of Science CSD CrossRef PubMed CAS Google Scholar
Cati, D. S. & Stoeckli-Evans, H. (2004a). Acta Cryst. E60, m174–m176. Web of Science CSD CrossRef IUCr Journals Google Scholar
Cati, D. S. & Stoeckli-Evans, H. (2004b). Acta Cryst. E60, m177–m179. Web of Science CSD CrossRef IUCr Journals Google Scholar
Cati, D. S. & Stoeckli-Evans, H. (2004c). Acta Cryst. E60, o210–o212. Web of Science CSD CrossRef IUCr Journals Google Scholar
Cati, D. S. & Stoeckli-Evans, H. (2004d). Acta Cryst. E60, m180–m182. Web of Science CSD CrossRef IUCr Journals Google Scholar
Dolomanov, O. V., Bourhis, L. J., Gildea, R. J., Howard, J. A. K. & Puschmann, H. (2009). J. Appl. Cryst. 42, 339–341. Web of Science CrossRef CAS IUCr Journals Google Scholar
Hausmann, J. & Brooker, S. (2004). Chem. Commun. pp. 1531–1531. Google Scholar
Hausmann, J., Jameson, G. B. & Brooker, S. (2003). Chem. Commun. pp. 2992–2993. Web of Science CSD CrossRef Google Scholar
Klingele (née Hausmann), J., Boas, J. F., Pilbrow, J. R., Moubaraki, B., Murray, K. S., Berry, K. J., Hunter, K. A., Jameson, G. B., Boyd, P. D. W. & Brooker, S. (2007). Dalton Trans. pp. 633–645. Google Scholar
Sheldrick, G. M. (2015a). Acta Cryst. A71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Sheldrick, G. M. (2015b). Acta Cryst. C71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar

This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.