684. Steric hindrance in Analytical Chemistry. Part II. The interaction of ferrous salts with 2-substituted 1 : 10-phenanthrolines

Abstract

Although 2-methyl-1 : 10-phenanthroline (pK = 5-42) is a stronger base than 1 : 10-phenanthroline (pK = 4-96) it forms weaker complexes with ferrous ions (log K , = 4-2, log K , = 3.7, and log K , = 2-9). This difference in behaviour is due to steric hindrance to co-ordination which increases with further substitution to such an extent that 2 : 9-dimethyl-1 : lo-phenanthroline will not form complexes with FeII although it does so with CuI.Tris-complexes predominate in the strongly red solutions formed by equivalent amounts of Fe++ and 1 : 10-phenanthroline. Corresponding solutions of Fe++ and 2-methylphenanthroline are feebly yellow and contain principally the bis-complex. That perchlorate ions precipitate tris-complexes from either solution can be explained quantitatively from considerations of the stability constants of the respective complex ions and the measured solubility products of their perchlorates.1 : 10-PHENANTHROLINE (" phenan " ; I ; R = R' = H) with ferrous salts gives a series of compounds containing the stable red ion Fe(phenan),++ which is of great analytical importance. The diamagnetism of its salts indicates that the organic components are held by octahedral d2sp3 hybrid orbitals. The coplanar resonating system formed by the three aromatic rings of each phenanthroline molecule and the five-membered ring which it forms with the central iron atom is thus repeated in three atomic planes at right angles, and the asymmetry of the resulting structure has recently been confirmed by a successful resolution (Dwyer and Gyarfas, J . PYOC. Roy. SOC. N.S.W., 1949, 83, 263). The considerable optical stability of the enantiomorphous tris-1 : 10-phenanthroline ferrous ions (t& = 121 min. at 15"; Dwyer, personal communication) is in keeping with the slow rate of exchange with the radio-nuclide 59Fe (it = 100 min.; Ruben, Kamen, Allen, and Nahinsky, J . Amer. Chem. SOC., 1942, 64, 2297) and the high over-all stability constant, since K3 = [Fe phenan,i+]/pei+][phenan]3 = 2 x 1O2I (Kolthoff, Lee, and Leussing, ibid., 1948, 70, 2348).On the other hand, Pfeiffer and Christeleit ( J . pr. Chem. 1938, 151, 127) commented on the fact that 2-methyl-1 : 10-phenanthroline (" mephenan " ; I ; R = Me, R' = H) forms sparingly soluble complex salts pe(mephenan)J (ClO,), and *-\ [Fe(mephenan)JS0,,12H20, which are only orange-coloured in contrast to <c\sd the deep red of the corresponding 1 : 10-phenanthroline complexes, and y-.,, in agreement with McCurdy and Smith (Artalyst, 1952, 77, 846) we have found that the introduction of a second methyl group adjacent to the other nitrogen atom produces a compound, 2 : 9-dimethyl-1 : 10-phenanthroline (I; R = R' = Me), which shows no tendency to form complexes with ferrous ions (cf. Case, J . A M Y . K ( ' 1

The Stability of Metallic Complexes of Two Dimethylphenanthrolines

“…In agreement with expectations, the increased basicity of the compounds investigated is paralleled by an increase in stability of the complexes investigated except, as noted elsewhere, when steric hindrance t o coordination about the metal atom occurs (5 …”

“…A number of studies t o determine the stability of metal complexes of the phenanthrolines have been published (1)(2)(3)(4)(5)(6)(7)(8), among the most complete of which are those of Irving and co-workers (2,4). In an extension of these studies the present investigation has sought formation curves, and hence formation constants, for the complexes formed by 5,6-and 4,7-dimethyl-1,lO-phenanthroline with a selected group of ions of the first transition series.…”

1H n.m.r. studies of palladium(II) complexes of 2,9-dimethyl-1,10-phenanthroline and 8-hydroxyquinoline

“…It should be noticed that [5] imposes the most steric strain of the three structures. In [5], none of the four ligand halves are equivalent, and in [6] there are three distinct chemical and magnetic environments for the halves of each ligand.…”

“…It should be noticed that [5] imposes the most steric strain of the three structures. In [5], none of the four ligand halves are equivalent, and in [6] there are three distinct chemical and magnetic environments for the halves of each ligand. The proposed structure [7] with the halide atom in the trigonal plane gives rise to two environments and indeed there are only two sets of signals observed in the n.m.r, spectra of the compounds.…”

“…The three possible geometries proposed (7) for these fivecoordinate compounds are shown in [5]- [7]. It should be noticed that [5] imposes the most steric strain of the three structures.…”