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The asymmetric unit of the title compound, 2C6H9N2 +·SO4 2−·H2O, contains two isomeric protonated amino­methyl­pyridine cations, a sulfate anion and a solvent water mol­ecule. The cations are in the iminium tautomeric form. In the crystal structure, inter­molecular O—H⋯O, N—H⋯O and weak C—H⋯O hydrogen bonds link the components into a three-dimensional network. Additional stabilization is provided by weak π–π stacking inter­actions, with centroid–centroid distances of 3.758 (2) and 3.774 (1) Å.

The title complex, [Cu2(C4H4O5)2(H2O)]n, has a two-dimensional layer structure. The Cu atom has a distorted octa­hedral (CuO6) environment and is coordinated by four carboxyl­ate group O atoms from three different oxydiacetate ligands in a planar arrangement and one half-occupancy water mol­ecule and an ether O atom in the axial positions. In the crystal structure, weak intra- and inter­molecular O—H⋯O hydrogen bonds help to stabilize the crystal packing. The structure has already been published [Whitlow & Davey (1975 ▶). J. Chem. Soc. Dalton. Trans. pp. 1228–1232]; this redetermination reports the structure with higher precision.

In the title compound, (CH6N3)[U(C6H4NO2)3O2], the uranyl group is coordinated by two O and two N atoms from two chelating picolinate ligands, and one O atom from a third picolinate ligand. The coordination environment of the UVI atom (N2O5) is distorted penta­gonal–bipyramidal. In the crystal, all amino groups are involved in the formation of N—H⋯O and N—H⋯N hydrogen bonds, which link cations and anions into layers parallel to the ac plane.

The asymmetric unit of the title compound, (C6H9N2)4[V4O12]·4H2O, contains half of a [V4O12]4− anion, two 2-amino-4-methyl­pyridinium, (2a4mpH)+, cations and two water mol­ecules. One water mol­ecule is disordered over two sets of sites with equal occupancies and the H atoms for this mol­ecule were not included in the refinement. The cation lies on an inversion center with four tetra­hedral VO4 units each sharing two vertices, forming an eight-membered ring. In the crystal, the components are linked by inter­molecular N—H⋯O hydrogen bonds, forming a one-dimensional network along [100]. Further stabilization is provided by weak inter­molecular C—H⋯O hydrogen bonds. In addition, π–π stacking inter­actions with centroid–centroid distances of 3.5420 (18), 3.7577 (18) and 3.6311 (19) Å are observed.

The asymmetric unit of the title salt, C6H9N2 +·C7H5O6S−, contains two crystallographically independent 2-amino-5-methylpyridinium cations and two sulfosalicylate anions. In the crystal structure, the sulfonate group of each 3-carb­oxy-4-hy­droxy­benzene­sulfonate anion inter­acts with the corresponding 2-amino-5-methyl­pyridinium cation via a pair of N—H⋯O hydrogen bonds, forming an R 2 2(8) ring motif. The ionic units are linked by N—H⋯O, O—H⋯O and C—H⋯O hydrogen bonds. Furthermore, the crystal structure is stabilized by π–π inter­actions between the benzene and pyridine rings [centroid–centroid distances = 3.5579 (8) and 3.8309 (8) Å]. There are also intra­molecular O—H⋯O hydrogen bonds in the anions, which generate S(6) ring motifs.

The asymmetric unit of the title compound, C6H9N2 +·C2F3O2 −, contains two independent 2-amino-4-methyl­pyridinium cations and two independent trifluoro­acetate anions. The F atoms of both anions are disordered over two sets of sites, with site occupancies of 0.50 (3) and 0.50 (3) in one of the anions, and 0.756 (9) and 0.244 (9) in the other. In the crystal, the cations and anions are linked into chains along the b axis by N—H⋯O hydrogen bonds and these chains are cross-linked by C—H⋯O hydrogen bonds, forming a two-dimensional network lying parallel to (101). The crystal structure is further stabilized by π–π inter­actions between the pyridinium rings [centroid–centroid distances = 3.5842 (13) and 3.5665 (16) Å].

The asymmetric unit of the title compound, (C6H9N2)4[BiCl6]Cl·H2O, contains four protonated 2-amino-6-methyl­pyridine (HAMP) cations and two-halves of two [BiCl6]3− anions, together with one water mol­ecule and one chloride anion. The BiIII atoms are hexa­coordinated by Cl atoms, forming distorted octa­hedral geometries. In the crystal structure, intra­molecular O—H⋯Cl and N—H⋯Cl, and inter­molecular O—H⋯Cl and N—H⋯O inter­actions link the mol­ecules into a three-dimensional network.

In the structure of the 1:1 proton-transfer compound of 4-methyl­pyridine (γ-picoline) with 4,5-dichloro­phthalic acid, C6H8N+·C8H3Cl2O4 −·H2O, determined at 200 K, the 4,5-dichloro­phthalate anions are bridged by the water mol­ecule through O—H⋯Ocarbox­yl hydrogen bonds, giving zigzag chains which extend along the c-axis direction. The 4-methyl­pyridinium cations are linked to the chains through single N—H⋯Owater hydrogen bonds and occupy the voids within the chains in the one-dimensional structure. The anions have the common ‘planar’ conformation with a short intra­molecular O—H⋯Ocarbox­yl hydrogen bond.

In the title salt, C6H9N2 +·C4H3O4 −, the dihedral angle between the pyridine ring and the plane formed by the hydrogen fumarate anion is 85.67 (6)°. Excluding the amino and methyl groups, the atoms of the cation are coplanar, with a maximum deviation of 0.005 (1) Å. In the crystal structure, the protonated N atom and the 2-amino group of the cation are hydrogen bonded to the carboxyl­ate O atoms of the anion via a pair of N—H⋯O hydrogen bonds, forming an R 2 2(8) ring motif. These motifs are further connected through N—H⋯O and C—H⋯O hydrogen bonds, leading to a supra­molecular chain along the c axis. These chains are further cross-linked via a pair of O—H⋯O hydrogen bonds involving centrosymmetrically related hydrogen fumarate anions, forming a two-dimensional network parallel to (101). These planes are further interconnected by O—H⋯O interactions into a three-dimensional network.

In the title mol­ecular salt, C6H9N2 +·C7H3N2O7 −, the 2-amino-5-methyl­pyridinium cation is essentially planar, with a maximum deviation of 0.023 (1) Å. There is an intra­molecular O—H⋯O hydrogen bond in the 3,5-dinitro­salicylate anion, which generates an S(6) ring motif. In the crystal, the protonated N atom and the 2-amino group are hydrogen bonded to the carboxyl­ate O atoms via a pair of N—H⋯O hydrogen bonds, forming an R 2 2(8) ring motif. Weak inter­molecular C—H⋯O inter­actions help to further stabilize the crystal structure.

In the title salt, C6H9N2 +·C5H7O4 −, the 2-amino-5-methyl­pyridinium cation is essentially planar, with a maximum deviation of 0.008 (1) Å. In the crystal, the protonated N atom and the 2-amino group are hydrogen bonded to the carboxyl­ate O atoms via a pair of N—H⋯O hydrogen bonds, forming an R 2 2(8) ring motif. The 4-carb­oxy­butano­ate anions are linked via O—H⋯O hydrogen bonds. The crystal structure is further stabilized by weak C—H⋯O inter­actions.

The asymmetric unit of the title compound, C6H9N2 +·C6H4NO2 −·0.63H2O, contains two crystallographically independent 2-amino-5-methyl­pyridinium cations, a pair of picolinate anions and two water mol­ecules, one with an occupancy of 0.25. Both the 2-amino-5-methyl­pyridine mol­ecules are protonated at the pyridine N atoms. In the crystal structure, the cations, anions and water mol­ecules are linked via N—H⋯O, N—H⋯N and O—H⋯O hydrogen bonds, as well as by C—H⋯O contacts, forming a chain along the b axis. In addition, weak π–π inter­actions are observed between pyridinium rings, with centroid–centroid distances of 3.5306 (13) Å.

The asymmetric unit of the title compound, (C13H11Cl2N2)2[Ni(C4N2S2)2], contains one-half of a centrosymmetric [Ni(mnt)2] anion (where mnt is maleonitrile­dithiol­ate or 1,2-dicyano-1,2-ethyl­enedithiol­ate) and an (E)-1-(3,4-dichloro­benzyl­ideneamino)-4-methyl­pyridinium cation. In the anion, the coordination around the Ni atom is a distorted square. In the cation, the aromatic rings are oriented at a dihedral angle of 7.81 (3)°. In the crystal structure, inter­molecular C—H⋯N hydrogen bonds link the cations and anions. π–π Contacts between the nickel dithiol­ene and pyridine rings and between the benzene and pyridine rings, [centroid–centroid distances = 3.682 (3) and 3.643 (3) Å, respectively] may further stabilize the structure.

The asymmetric unit of the title compound, (C6H9N2)2[Cu(C7H3NO4)2], contains half of a [Cu(pydc)2]2− (pydcH2 is pyridine-2,6-dicarb­oxy­lic acid) anion and one protonated 2-amino-4-methyl­pyridine (2a4mpH)+ counter-ion. The anion is a six-coordinated complex with a distorted CuN2O4 octa­hedral geometry around the CuII ion. N—H⋯O and C—H⋯O hydrogen bonds along with π–π contacts between the pyridine rings of the (2a4mpH)+ cations [centroid–centroid distance = 3.573 (2) Å] stabilize the crystal structure.

The asymmetric unit of the title compound, (C6H9N2)2[Ni(C4N2S2)2], contains one half of an [Ni(mnt)2]2− anion (mnt is maleonitrile­dithiol­ate or 1,2-dicyano­ethene-1,2-dithiol­ate) and one 1-amino-4-methyl­pyridinium cation. The NiII atom is located on an inversion centre. In the crystal structure, inter­molecular N—H⋯N hydrogen bonds link the mol­ecules.

The asymmetric unit of the title compound, (C13H13IN)[Ni(C6H4S2)2], contains half each of two centrosymmetric anions and a single cation in a general position. In the anions, the NiIII ions are surrounded by four S atoms in a distorted square-planar geometry. In the crystal, the anions exhibit two different packing modes by stacking along the a axis in face-to-face and side-by-side fashions. Inter­ionic C—H⋯S hydrogen bonds and π–π stacking inter­actions [centroid–centroid distance = 3.6947 (5) Å] are observed.

In the crystal structure of the title compound, (C6H9N2)2[CuBr4], the geometry around the Cu atom is inter­mediate between tetra­hedral (Td) and square planar (D4h). Each [CuBr4]2− anion is connected non-symmetrically to four surrounding cations through N—H⋯X (pyridine and amine proton) hydrogen bonds, forming chains of the ladder-type running parallel to the crystallographic b axis. These layers are further connected by means of offset face-to-face inter­actions (parallel to the a axis), giving a three-dimensional network. Cation π–π stacking [centroid separations of 3.69 (9) and 3.71 (1) Å] and Br⋯aryl inter­actions [3.72 (2) and 4.04 (6) Å] are present in the crystal structure. There are no inter­molecular Br⋯Br inter­actions.

In the title salt, C6H9N2 +·C8H5O4 −, the hydrogen phthalate anion is essentially planar, with a maximum deviation of 0.011 (2) Å. In the crystal structure, the protonated N atom of the pyridine ring and the 2-amino group of the cation are hydrogen bonded to the carboxyl­ate O atoms of the anion via a pair of N—H⋯O hydrogen bonds, forming an R 2 2(8) ring motif. In the hydrogen phthalate anion, there is a very strong, almost symmetric, intra­molecular O—H⋯O hydrogen bond, generating an S(7) motif [O⋯O = 2.382 (3) Å]. Furthermore, these two molecular motif rings are connected by a bifurcated N—H⋯(O,O) hydrogen-bonded motif R 1 2(4), forming a supra­molecular ribbon along the b axis. The crystal structure is further stabilized by π–π inter­actions between the cations and anions [centroid–centroid distance = 3.6999 (10) Å].

In the title mol­ecular salt, C6H9N2 +·C3H3O4 −, the cation is essentially planar, with a maximum deviation of 0.010 (3) Å. In the anion, an intra­molecular O—H⋯O hydrogen bond generates an S(6) ring and results in a folded conformation. In the crystal, the protonated NH group and the 2-amino group of the cation are hydrogen bonded to the carboxyl­ate O atoms of the anion via a pair of N—H⋯O hydrogen bonds, forming an R 2 2(8) ring motif. Weak inter­molecular C—H⋯O inter­actions help to further stabilize the crystal structure.

In the crystal structure of the title salt, C6H9N2 +·C7H5O6S−·H2O, the water mol­ecule acts as an acceptor of bifurcated N—H⋯O hydrogen bonds from the pyridinium H atom and one H atom of the 2-amino group, forming an R 2 1(6) ring. The 3-carb­oxy-4-hy­droxy­benzene­sulfonate anions self-assemble via O—H⋯O hydrogen bonds, leading to supra­molecular chains along the a axis. These chains and R 2 1(6) motifs are linked via O—H⋯O, N—H⋯O and C—H⋯O hydrogen bonds, forming a layer parallel to the ac plane. There is also an intra­molecular O—H⋯O hydrogen bond in the 3-carb­oxy-4-hy­droxy­benzene­sulfonate anion, generating an S(6) ring motif.

In the crystal structure of the title salt, C11H16NO2 +·Cl−, the cations and anions are linked by O—H⋯Cl hydrogen bonds. The structure is further stabilized by weak C—H⋯Cl hydrogen bonds.

In the crystal structure of the title solvated salt, C16H19N2 +·C8H5O7S−·CH3OH, the anions and the methanol solvent mol­ecules are linked by O—H⋯O hydrogen bonds. The cations and anions are packed as alternate layers parallel to (11). The crystal structure is further stabilized by a π–π inter­action between the pyridinium and benzene rings of the cations, with a centroid–centroid distance of 3.5492 (4) Å.

In the cation of the title compound, C18H23N2 +·C7H7O4S−·H2O, one ethyl group of the diethyl­amino unit is disordered over two sets of sites in a 0.665 (6):0.335 (6) ratio. The styrylpyridinium unit is nearly planar, with a dihedral angle between the pyridinium and benzene rings of 4.27 (8)°. In the crystal, the anion ring is almost perpendicular to the aromatic rings of the cation; the sulfonate-substituted benzene ring forms dihedral angles of 89.60 (8) and 89.37 (8)°, respectively, with the pyridinium and benzene rings of the cation. In the crystal, the three components are linked into a three-dimensional network by O—H⋯O and C—H⋯O hydrogen bonds. π–π inter­actions with centroid–centroid distances of 3.6999 (9) and 3.7106 (9) Å are also present.

The title ion-pair compound, (C7H7N2)2[Cu(C4N2S2)2], was obtained by the direct reaction of CuCl2·2H2O, disodium maleonitrile­dithiol­ate (Na2mnt) and 4-cyano-1-methyl­pyridinium iodide. The anion and one pyridinium cation lie entirely on a mirror plane, whereas for the other cation, a crystallographic mirror plane runs through the N and para-C atoms of the pyridine ring, the methyl C atom, and the cyano group. In the crystal, ions are linked into a three-dimensional network by C—H⋯N hydrogen bonds.

In the structure of the title salt, C6H9N2 +·C7H6NO2 −, the 4-amino­benzoate anions are linked to adjacent anions and 2-amino-4-methyl­pyridinium cations via N—H⋯O hydrogen bonds, forming a three-dimensional supra­molecular structure. The crystal structure also shows a weak C—H⋯O hydrogen bond between adjacent anions. Within the 4-amino­benzoate anion, the carboxyl­ate group is twisted by 14.0 (4)° with respect to the benz­ene ring.