A series of high-spin octahedral 1 : 2 iron(I1r) complexes of Schiff bases derived from salicylaldehyde and aromatic amines and the 1 : 1 and 1 : 2 complexes of bis(pyridin-2-yl)-aza and bis(benzimidazol-2-yl)-aza and -thioether ligands have been isolated. The crystal structure of trichloro[bis(pyridin-2-ylmethyl)amine]iron(r11) has been determined. It contains two crystallographically independent molecules in the asymmetric unit cell. In each molecule iron(rI1) possesses a rhombically distorted octahedral co-ordination, constituted by all the three nitrogens of the facial ligand and three chloride ions. The effects of stereochemical and/or donor atom variations on the UV/VIS and EPR spectra and Fe"'-Fe" redox potentials and hence on the Lewis acidity of the iron(m) centre are discussed. The phenolate-to-iron(m) charge-transfer band energy of [FeL' ,]Cl [HL' = N-(pyridin-2-ylmethyl)salicylideneamine] is higher and its Fe"'-Fe" redox potential more negative than those of the corresponding 1 : 1 complex.

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Fourteen crystals involving various substituted organic acid molecules and 2-amino-4,6-dimethylpyrimidine (MP)/2-amino-4,6-dimethoxypyrimidine (MOP) were prepared and characterized by using single crystal X-ray diffraction. Among the 14 crystals, proton transfer from the carboxylic acid to pyrimidine base has occurred only in 5 crystals while the remaining were co-crystals. It is obvious that the R 2 2(8) ring motif occurs in co-crystals/salts when an acid interacts with a pyrimidine base. The present work has been focused on this aspect, confining the study to MP and MOP, where the results surely indicate that there are three main synthons, which regularly occur, linear heterotetramer (LHT), cyclic heterotetramer (CHT), and heterotrimer (HT). Among the whole lot, in both the present study and the literature, LHT is dominant as it occurs in 30 out of the existing 54 structures, while CHT and HT correspond to 9 each with the remaining structures forming newer synthons. Our results are also consistent with a recent paper in Crystal Growth and Design (2008, 8, 4031–4044), where LHT is predicted to be more stable among other synthons. Furthermore in the present study, attempts have been made to correlate the formation of salts/co-crystals using ΔpK a for both MP and MOP molecular complexes. The failure to explain MOP using ΔpK a also suggests that the use of Pka values in predicting co-crystals/salts could vary from system to system.

In the crystal structures of the two organic salts, namely, pyrimethamine hydrogen glutarate (1:1) 1 and pyrimethamine formate (1:1) 2, the pyrimethamine moieties are protonated at one of the nitrogen atoms of the pyrimidine rings. The carboxylate group of the respective anions (hydrogen glutarate and formate) interacts with the protonated pyrimidine moiety in a near linear fashion through a pair of N−H···O hydrogen bonds. The dihedral angle between the diaminopyrimidine and the p-chlorophenyl plane is 74.8(1)° in compound 1, and the corresponding value in compound 2 is 76.7(1)°. In both compounds, the pyrimidine moieties are centrosymmetrically paired through a pair of N−H···N hydrogen bonds. The 2-amino group of the one member of the pair and the 4-amino group of the other member are bridged by an O atom of the carboxylate group, using a pair of N−H···O hydrogen bonds. This combination of hydrogen bonds results in the complementary DADA (D = donor and A = acceptor in hydrogen bonds) arrays of quadruple hydrogen-bonding patterns. In compound 1, there are chains and ladders made up of the O−H···O, N−H···O, and C−H···N hydrogen bonds whereas compound 2 displays a three-dimensional network of hydrogen bonds.