Dana S. Marlin

Coordination of carboxamido nitrogen to tervalent iron:insight into a new chapter of iron chemistry

Although coordination of carboxamido nitrogen to Fe(III) center has been assumed to be improbable, research work during the past few years has demonstrated that Fe(III) complexes with ligated carboxamido nitrogens can be readily synthesized. The Fe(III)–Namido bond distances lie in the range of 1.8–2.2 A in the various low spin and high spin Fe(III) complexes. These complexes are stable in aqueous media and their redox parameters indicate that the carboxamido nitrogens provide significant stability to the Fe(III) center.

Chemistry of iron(III) complexes of N,N′-bis(2-hydroxyphenyl)-pyridine-2,6-dicarboxamide: seven-coordinate iron(III) complexes ligated to deprotonated carboxamido nitrogens

Abstract As part of our continuing effort to develop the chemistry of iron(III) complexes of ligands that employ carboxamido nitrogens to bind the metal center, we have synthesized the designed ligand N,N′-bis(2-hydroxyphenyl)pyridine-2,6-dicarboxamide (POPYH4, Hs are dissociable phenolic and carboxamido hydrogens). Reactions of the completely deprotonated POPY4− with certain iron(III) starting materials in DMF afford pentagonal bipyramidal complexes of the type [Fe(POPY)X2]n− (X=1-MeIm, SCN−), in which the pentadentate POPY4− ligand occupies the equatorial plane. When smaller amounts of base are used, the partially deprotonated POPYH2 2− ligand gives rise to the tetrahedral bis complex [Fe(POPYH2)2]− in which only the phenolic oxygens of the ligand are bound to iron(III). With [Fe2OCl6]2−, one obtains [Fe(POPYH2)(Cl)]2O, a new species that contains a (μ-oxo)diiron(III) core. Here also, POPYH2 2− employs the phenolic oxygens to bind iron and acts as a bidentate ligand. The structures of these complexes have been determined by X-ray crystallography. The conditions for interconversions among the four complexes have also been elucidated. Both [Fe(POPY)X2]n− complexes with coordinated carboxamido nitrogens are stable in air and in aqueous solution. The redox properties of these species indicate that ligated carboxamido nitrogens provide enhanced stability to iron(III) centers.

Extended structures controlled by intramolecular and intermolecular hydrogen bonding: a case study with pyridine-2,6-dicarboxamide, 1,3-benzenedicarboxamide and N,N′-dimethyl-2,6-pyridinedicarboxamide

Abstract The small organic molecule pyridine-2,6-dicarboxamide, although known in the literature for some time, exhibits interesting and previously unexplored intermolecular and intramolecular hydrogen bonding both in solid state and in solution. With the aid of X-ray crystallography and variable-temperature NMR spectroscopy, we here demonstrate the presence of a very strong hydrogen bonding network for this molecule both in condensed state and solution. Furthermore, a novel extended hydrogen bonding graph-set has been derived for this molecule in crystalline state. Comparison of pyridine-2,6-dicarboxamide with 1,3-benzenedicarboxamide, where the intramolecular hydrogen bonding to the pyridine ring in the former has been removed, yields a different intermolecular hydrogen bonded structure in the solid state. A new graph-set has been determined for the extended structure of 1,3-benzenedicarboxamide in the solid state. In solution, 1,3-benzenedicarboxamide is shown to maintain a hydrogen bonding pattern that is weaker than that observed with pyridine-2,6-dicarboxamide. Replacement of one hydrogen on each carboxamide nitrogen of pyridine-2,6-dicarboxamide by a methyl group also alters the extended structure to a significant extent. In N , N ′-dimethyl-2,6-pyridinedicarboxamide, the three-dimensional hydrogen bonding pattern observed with pyridine-2,6-dicarboxamide all but collapses to one-dimensional chains.

Bis-(N,N′-bis[2-(2-pyridyl)methyl]pyridine-2,6-dicarboxamido)dicopper(II): spontaneous formation of a short double stranded helicate

Abstract In its deprotonated form, the ligand N,N′-bis[2-(2-pyridyl)methyl]pyridine-2,6-dicarboxamide (MePy3PH2, where the Hs represent the dissociable amide nitrogens) coordinates to Cu(II) as one strand of a short double helix. The crystal structure of the title complex, bis-(MePy3P)dicopper(II) [Cu2(MePy3P)2], has been determined. The binuclear structure of [Cu2(MePy3P)2] is in contrast to the monomeric Cu(II) complex [Cu(Py3P)] in which the ligand N,N′-bis[2-(2-pyridyl)ethyl]pyridine-2,6-dicarboxamide (Py3PH2) differs from MePy3PH2 by only a single methylene linker. The helical structure of [Cu2(MePy3P)2] has now been added to the rapidly growing number of spontaneously forming helical assemblies, for which crystal structures have been determined.

Spin States and Stability of FeIII Complexes of Ligands with Carboxamido Nitrogen and Phenolato Oxygen Donors

As part of our attempts to prepare FeIII complexes with “tunable” spin states, we have synthesized three novel six-coordinate FeIII complexes containing both carboxamido nitrogen (Namido) and phenolato oxygen (Ophen) donors in their ligand framework. Since it has already been shown that carboxamido nitrogen donors usually stabilize FeIII in a low spin configuration, while phenolato oxygens tend to stabilize FeIII in a high spin state, we have incorporated both donor groups in our designed ligands. The syntheses and structures of FeIII complexes with either one or two Namido and Ophen donors are reported in this paper. The properties of these complexes have been compared with the properties of analogous FeIII complexes. The competing effects of Namido and Ophen on the spin state of the FeIII center are discussed in terms of the stabilities of the overall complexes. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)