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1.  p-Phenyl­enediamine and its dihydrate: two-dimensional isomorphism and mechanism of the dehydration process, and N—H⋯N and N—H⋯π inter­actions 
p-Phenyl­enediamine can be obtained as the dihydrate, C6H8N2·2H2O, (I), and in its anhydrous form, C6H8N2, (II). The asymmetric unit of (I) contains one half of the p-phenyl­ene­diamine mol­ecule lying about an inversion centre and two halves of water mol­ecules, one lying on a mirror plane and the other lying across a mirror plane. In (II), the asymmetric unit consists of one mol­ecule in a general position and two half mol­ecules located around inversion centres. In both structures, the p-phenyl­enediamine mol­ecules are arranged in layers stabilized by N—H⋯π inter­actions. The diamine layers in (I) are isostructural with half of the layers in (II). On dehydration, crystals of (I) transform to (II). Comparison of their crystal structures suggests the most plausible mechanism of the transformation process which requires, in addition to translational motion of the diamine mol­ecules, in-plane rotation of every fourth p-phenyl­enediamine mol­ecule by ca 60°. A search of the Cambridge Structural Database shows that the formation of hydrates by aromatic amines should be considered exceptional.
doi:10.1107/S010827011000541X
PMCID: PMC2855568  PMID: 20203409
2.  (Carbonato-κ2 O,O′)bis­(di-2-pyridyl­amine-κ2 N,N′)cobalt(III) bromide 
In the title compound, [Co(CO3)(C10H9N3)2]Br, a distorted octa­hedral coordination of the CoIII atom is completed by four N atoms of the two chelating di-2-pyridyl­amine ligands and two O atoms of the chelating carbonate anion. The di-2-pyridyl­amine ligands are nonplanar and the dihedral angles between the 2-pyridyl groups are 29.11 (9) and 37.15 (12)°. The coordination cation, which has approximate C 2 symmetry, is connected to the bromide ion via an N—H⋯Br− hydrogen bond. The ionic pair thus formed is further assembled into a dimer via N—H⋯O inter­actions about an inversion centre. A set of weaker C—H⋯O and C—H⋯Br− inter­actions connect the dimers into a three-dimensional network.
doi:10.1107/S1600536811008051
PMCID: PMC3100002  PMID: 21753946
3.  Phenazine–naphthalene-1,5-diamine–water (1/1/2) 
The asymmetric unit of the title compound, C12H8N2·C10H10N2·2H2O, contains one half-mol­ecule of phenazine, one half-mol­ecule of naphthalene-1,5-diamine and one water mol­ecule. The phenazine and naphthalene-1,5-diamine mol­ecules are located on inversion centers. The water mol­ecules serve as bridges between the naphthalene-1,5-diamine mol­ecules and also between the naphthalene-1,5-diamine and phenazine mol­ecules. The naphthalene-1,5-diamine and water mol­ecules are connected via N—H⋯O and O—H⋯N hydrogen bonds, forming a T4(2) motif. They are arranged into a two-dimensional polymeric structure parallel to (10) in which the water mol­ecule is a single donor and a double acceptor, whereas the amino group is a double donor and a single acceptor in the hydrogen bonding. These two-dimensional assemblies alternate with the layers of phenazine mol­ecules arranged into a herringbone motif. Each phenazine mol­ecule is hydrogen bonded to two water mol­ecules and thus a three-dimensional framework of hydrogen-bonded mol­ecules is generated.
doi:10.1107/S1600536809049009
PMCID: PMC2972038  PMID: 21578891
4.  5-Amino-1-naphthol 
In the title compound, C10H9NO, the amino and the hydr­oxy groups act both as a single donor and a single acceptor in hydrogen bonding. In the crystal, mol­ecules are connected via chains of inter­molecular ⋯N—H⋯O—H⋯ inter­actions, forming a two-dimensional polymeric structure resembling the hydrogen-bonded mol­ecular assembly found in the crystal structure of naphthalene-1,5-diol. Within this layer, mol­ecules related by a translation along the a axis are arranged into slipped stacks via π–π stacking inter­actions [inter­planar distance = 3.450 (4) Å]. The amino N atom shows sp 3 hybridization and the two attached H atoms are located on the same side of the aromatic ring.
doi:10.1107/S1600536809045152
PMCID: PMC2971357  PMID: 21578535
5.  Quinoxaline–3-amino­phenol–water (2/1/2) 
The asymmetric unit of the title compound, 2C8H6N2·C6H7NO·2H2O, contains two quinoxaline mol­ecules, one mol­ecule of 3-amino­phenol and two water mol­ecules which are hydrogen bonded to form a two-dimensional polymeric structure. Each of the symmetry-independent quinoxaline mol­ecules forms separate stacks of different symmetry. In one set of stacks, the mol­ecules are related by a screw axis and are slightly tilted [dihedral angle = 7.12 (1)°]. In the second set of stacks, adjacent mol­ecules are parallel and related by an inversion center [inter­planar distances = 3.376 (4) and 3.473 (4) Å].
doi:10.1107/S1600536808010568
PMCID: PMC2961097  PMID: 21202378

Results 1-5 (5)