The preparation of the tridentate chelating agents 2-(1,10- phenanthrolin-2-yl)imidazoline and 2-(1,10-phenanthrolin-2- yl)benzimidazole and their bis-ligand complexes with iron(II) and nickel(II) is described. The latter ligand coordinates as either a neutral or anionic tridentate. Data from the spectra of the nickel(II) complexes indicate that the ligands have field strengths in the iron(II) crossover region. The temperature dependence of the magnetism of the imidazoline iron(II) complex reveals a gradual, temperature- induced 5T2 ↔ 1A1 transition which is not complete within the experimental temperature range (83-363 K). Both the cationic and neutral iron(II) complexes of the benzimidazole derivative are essentially low-spin at room temperature but a significant increase in their magnetic moments at elevated temperatures indicates that a spin transition may be occurring in these compounds too.
The preparation of the tridentate chelating agents 2-(1,10- phenanthrolin-2-yl)thiazole, 2-(1,10-phenanthrolin-2-yl)-4-(2- pyridyl)thiazole 2-(1,10-phenanthrolin-2-yl)thiazolidine and 2-(1,10- phenanthrolin-2-yl)benzothiazole is described. Data from the spectra of the bis-ligand nickel complexes indicate that the ligands all have field strengths in the iron(II) crossover region. The temperature dependence of the magnetism of the bis-ligand iron(II) complexes reveals that, except for the complexes of the pyridylthiazole, a smooth, temperature-induced 5T2 ↔ 1A1 transition occurs in these compounds. For no complex is the transition complete within the experimental temperature range (83-363 K). The complex of the pyridylthiazole is high-spin throughout the range, the uncoordinated pyridyl group hindering the close approach of ligand and metal atom necessary for spin-pairing. The ability of the other ligands to induce a spin transition is primarily a consequence of distortions in the environment about the metal atom arising from coordination of the five- membered thiazole or related ring.
Bis-ligand iron(11) and nickel(11) complexes of the potentially tridentate chelating agents 2-(3-methyl-2-pyridylamino)-4-(2-pyridyl)thiazole and 2-(4-methyl- 2-pyridylamino)-4-(2-pyridyl)thiazole are described. The electronic spectra of the nickel(11) complexes show that the field strengths of the ligands are in the neighbourhood of the Crossover region for iron(11). The magnetic properties of the cationic iron(11) complexes are anomalous and consistent with a spin-equilibrium involving the nearly equi-energetic 5T2 and 1A1 states of the metal atom. The magnetism shows a strong dependence on the associated anion. 2-(6-Methyl-2-pyridylamino)-4- (2-pyridyl)thiazole is also described. In its bis-ligand iron(11) complexes it functions as a bidentate chelating agent and the complexes are high-spin. All three ligands may be deprotonated to yield inner complexes with iron(11) which in some instances have anomalous magnetic properties associated with the presence of a spin-equilibrium. 2-(6-Methyl-2-pyridyl)-4-(2-pyridyl)thiaole and its bis-ligand iron(11) and nickel(11) complexes are also described. The iron(11) complexes are high-spin and show normal temperature dependence of their magnetism.
Mono- and bis-ligand complexes of iron, cobalt, nickel and mono-ligand complexes of copper with the tridentate chelating agents 1,10- phenanthroline-2-carbo-thioamide and N-phenyl-1,10-phenanthroline-2- carbothioamide are described. Both ligands produce a strong field with iron(II) and the bis-ligand complexes of cobalt(II) and the former display a temperature-induced spin transition resulting in an anomalous temperature-dependence of their magnetism. In all the mono-ligand complexes the metal atom is believed to be six-coordinate. Both ligands may be deprotonated in their complexes and deprotonation of the N- phenyl derivative occurs spontaneously when it interacts with cobalt(II) or copper(II), the reaction with cobalt being accompanied by oxidation of the metal. Both ligands coordinate through the sulphur atoms of the thioamide group.
A series of substituted hydrazones derived from 1,lO-phenanthroline-2-carbaldehyde and their bis-ligand complexes with bivalent iron and nickel are described. The hydrazones show a gradation in field strength and this is reflected in the spin-state of the iron complexes. The methylhydrazone complex is essentially low-spin over the temperature range 83-363 K but the presence of some spin-free species at high temperatures is evident. Within the same range the dimethylhydrazone complex is essentially high-spin but undergoes significant spin-pairing and the phenylhydrazone complex displays a complete + 'A1 spin transition. This transition is very sharp, resulting in a pronounced change in the magnetism, and colour, of the complex within a few degrees. Complexes of the 2-pyridylhydrazone, the methylphenylhydrazone and the diphenylhydrazone are highspin over the entire experimental temperature range. IntroductionPrevious studies on derivatives of 1,lO-phenanthroline containing a donor group in the 2-position have shown that the field strength of the tridentate ligand, and consequently the spin state of the derived iron(11) complexes, is dependent on the nature of the substituent group. Thus 1,lO-phenanthroline-2-carboxylic acid1 and 1,lO-phenanthroline-2-carboxamide2 produce high-spin iron(11) complexes, while 1,l 0-phenanthroline-2-carbothioamide3 and 1,lO-phenanthroline-2-carboxamide oxime4 give low-spin iron@) complexes. It would be expected that greater control of the ligand field strength would be achieved by introducing non-coordinating substituents into the donor side-chain group. Steric or electronic effects due to these substituents may then allow a more gradual transition from high-spin to low-spin complexes. This approach was adopted to a limited extent earlier in a study of the complexes of the N-phenyl derivative of phenanthroline-2-carbothioamide but this, like the unsubstituted thioamide, was found to produce a low-spin iron(11) c~m p l e x .~ Some time ago Robinson and Busch6 examined the field-strengths of a number of substituted hydrazones of pyridine-2-carbaldehyde. These are related structurally to the bidentates 1,lO-phenanthroline and 2,2'-bipyridine. The spin-state of the
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