Low-spin manganese(II) and cobalt(III) complexes of N-aryl-2-pyridylazophenylamines: new tridentate N,N,N-donors derived from cobalt mediated aromatic ring amination of 2-(phenylazo)pyridine. Crystal structure of a manganese(II) complex

Abstract

Two new tridentate ligands of the type NH₄C₅NNC₆H₄N(H)C₆H₄(R) (R = H (HL¹) or CH₃ (HL²)) have been synthesized by the cobalt mediated direct phenyl ring amination of co-ordinated NH₄C₅NNC₆H₅. These bind to metal ions as monoanionic N,N,N-donors (L⁻), affording [Mn^IIL₂] and [Co^IIIL₂]ClO₄ complexes in very high yields. The compounds have low-spin electronic configurations. While the manganese complexes are paramagnetic with one unpaired electron (1.65–1.70 μ_B) the cobalt complexes are diamagnetic. Crystal structure determination of [Mn(L¹)₂] has revealed the presence of a distorted octahedral MnN₆ co-ordination sphere. The two aza nitrogens of the anionic tridentate ligands approach the metal centre closest with Mn–N(aza) ca. 1.89 Å. The other two Mn–N distances are: Mn–N(py), 2.00; Mn–N(amido), 1.95 Å. There is a significant degree of ligand backbone conjugation in the co-ordinated ligands which has resulted in shortening of the C–N bond distances and also in lengthening of the diaza (N=N) distances. In fluid solution, [MnL₂] species exhibit six line EPR spectra with low hyperfine constant (A = 75 G). In frozen dichloromethane–toluene (77 K), rhombic EPR spectra are observed, consisting of an isolated signal g₃ (ca. 1.91) and two relatively close signals g₁ and g₂ (ca. 2.06 and 2.03 respectively). Both manganese as well as cobalt complexes display multiple redox responses. The manganese complexes show Mn^II ⇌ Mn^III oxidation at ca. 0.38 V and the cobalt analogues display reversible Co^III ⇌ Co^II reduction at −0.40 V. Electrogenerated [Mn^IIIL₂]⁺ shows transitions in the near IR region.