Regiospecific oximato coordination at the oxygen site: ligand design and low-spin Mn^II and Fe^II/III species

Abstract

The (pyridylazo)oxime ligand (C₅H₄N)N=NC(=NOH)C₆H₄R(p), HRL (R = H, Me), has been synthesized with the objective of promoting the very rare mononuclear oximato-O coordination. The synthesis involves hydrazone nitrosation. HRL reacts with M(ClO4)₂·6H₂O, affording [M(RL)₂] (M = Mn, Fe), and X-ray structure determinations have indeed revealed the presence of the desired oximato-O coordination mode in the distorted octahedral MN₄O₂ sphere characterized by a 2-fold axis of symmetry relating the two tridentate ligands, all chelate rings being five-membered. The M-N and M-O distances are relatively short (1.84-1.99 Å), consistent with spin-pairing (M = Mn, S = 1/2; M = Fe, S = 0). The M-N bond length trend, Mn^II > Fe^II, is in accord with the larger radius of the low-spin MnII atom. The M-O length order is, however, Mn^II < Fe^II. This reversal occurs because of the rigidity of the ligand skeleton. Significant M-azo back-bonding is present (N=N length ~1.30 Å). [Mn(RL)₂] displays hyperfine-split axial EPR spectra (g|| ~1.98, g_⊥ ~2.06, A|| ~170 G, A_⊥ ~65 G) in frozen solution as well as in polycrystalline [Fe(RL)₂] lattices. The quasireversible Fe(III)/Fe(II) couple of [Fe(RL)₂] has E_1/2 ~0.7 V. The oxidized complex [Fe(RL)₂]PF₆ has been electrosynthesized. It is low-spin (S = 1/2) and displays axial EPR spectra (g|| ~1.98, g _⊥~2.14) in frozen solution. The Mn(II) → Mn(III) oxidation (1.0 V) in [Mn(RL)₂] is irreversible. Crystal data for [Mn(HL)₂]·CH₂Cl₂: crystal system monoclinic, space group I2/a, a = 14.142(7) Å, b = 10.721(7) Å, c = 16.933(10) Å, β = 93.01(4)°, Z = 4. Crystal data for [Fe(HL)₂]·CH₂Cl₂: crystal system monoclinic, space group I2/a, a = 14.254(6) Å, b = 10.742(6) Å, c = 16.719(8) Å, β = 92.35(4)°, Z = 4.