(Dibenzo-18-crown-6)(2-phenylamidopyridine)-potassium(I)

The title complex, [Rb(C11H9N2)(C20H24O6)], has potassium in an irregular eightfold coordination, with a hexadentate crown ether ligand and a chelating bidentate amido­pyridine ligand, each occupying one hemisphere of the coordination. The chelate KNCN ring is slightly folded, and the two rings of the amido­pyridine ligand are not coplanar, because of steric interaction of H atoms on the rings. The K—N(amido) bond is shorter than the K—N(pyridine) bond. The coordination of the crown ether to potassium is less symmetrical than that for the analogous rubidium complex, reflecting a poorer size match for K+ in this coordination site.

The title complex, [Rb(C 11 H 9 N 2 )(C 20 H 24 O 6 )], has potassium in an irregular eightfold coordination, with a hexadentate crown ether ligand and a chelating bidentate amidopyridine ligand, each occupying one hemisphere of the coordination.The chelate KNCN ring is slightly folded, and the two rings of the amidopyridine ligand are not coplanar, because of steric interaction of H atoms on the rings.The KÐN(amido) bond is shorter than the KÐN(pyridine) bond.The coordination of the crown ether to potassium is less symmetrical than that for the analogous rubidium complex, re¯ecting a poorer size match for K + in this coordination site.

Comment
In the preceding paper (Liddle & Clegg, 2004), we described the structure of the complex (dibenzo-18-crown-6)Rb(L), where HL is 2-phenylaminopyridine.We present here the structure of the analogous potassium complex, (dibenzo-18crown-6)K(L), (I).These complexes were prepared as part of a study of crown-ether-supported complexes of alkali metals with amide ligands, and were investigated speci®cally for comparison with the complexes with 18-crown-6 (Liddle et al., 2004;Liddle & Clegg, 2003).
Although the Rb complexes with the two different crown ethers are structurally very similar, there is a marked difference for the K complexes.Reaction of equimolar amounts of 18-crown-6, potassium hydride and HL leads to the polymeric complex [(18-crown-6)K(L) 2 K] I , even though this product has a 1:2:2 stoichiometry (Liddle et al., 2004).With dibenzo-18crown-6 instead of 18-crown-6, the title complex (with a 1:1:1 stoichiometry) is obtained, exactly analogous to the Rb case.
The dibenzo-18-crown-6 complexes of Rb and K are structurally similar in gross terms; there are subtle but signi®cant differences in detail.Fig. 1 shows the molecular structure of the title K complex, and selected geometric parameters are in Table 1.It consists of discrete neutral molecules with no special intermolecular interactions.The hexadentate crown ligand and the bidentate amide anion occupy the two coordination hemispheres of the potassium ion, giving irregular eightfold coordination, and the complex may be described as a contact ion pair.
The range of KÐO distances and the difference between the two KÐN distances are both somewhat greater than the corresponding values for the Rb complex, and this probably re¯ects the poorer ®t of the smaller potassium ion in the coordination site; it is generally recognized that 18-crown-6 and its substituted derivatives provide an ideal ®t for K + in the mean plane of the six O atoms rather than displaced from this plane.In the title complex, K lies 0.7830 (7) A Ê out of the oxygen mean plane (r.m.s.deviation 0.009 A Ê ), compared with a deviation of 1.0945 (6) A Ê for Rb in the analogous complex.Although all four OÐCÐCÐO aliphatic segments have a gauche conformation, as is expected for optimal chelation, the overall conformation of the crown in this complex differs from that in the Rb complex by conversion of one anti CÐCÐOÐ C linkage to gauche, the other eleven remaining anti (Table 1).The crown ligand is thus rather less symmetrical in the title complex than in the Rb complex.The two benzene rings are folded out of the oxygen mean plane, away from the amide ligand by 39.97 (6) and 8.52 (7) , in contrast to the angles of 22.93 (7) and 25.94 (7) towards the amide ligand in the Rb complex.
The two rings of the amide ligand have a dihedral angle of 49.91 (7) because of steric interaction of H atoms bonded to C4 and C11.The four-membered chelate ring (KNCN) is approximately planar, the dihedral angle between the KN 2 and CN 2 planes being 7.3 (2) , compared with only 0.3 (3) in the Rb complex, once again displaying the effects of greater geometrical strain from the size mismatch of the metal ion and its ligand set.

Experimental
Potassium hydride (0.04 g, 1.0 mmol) was added to a solution of 2-phenylaminopyridine (0.17 g, 1.0 mmol) and dibenzo-18-crown-6 (0.36 g, 1.0 mmol) in tetrahydrofuran (THF, 40 ml), to give a pale yellow precipitate.Volatile components were removed in vacuo and the remaining solid was washed with petroleum ether (3 Â 5 ml).Recrystallization from hot toluene containing a little hexamethylphosphoramide (HMPA) gave yellow crystals of (I) (yield 0.38 g, 67%).Chemical analysis results were satisfactory, and the 1 H and 13 C{ 1 H} NMR signals could be assigned on the basis of the crystal structure (Liddle, 2000).The molecular structure of (I), showing the atom labels and 50% probability displacement ellipsoids for non-H atoms.
We thank the EPSRC for ®nancial support.