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

The (pyridylazo)oxime ligand (C5H4N)N=NC(=NOH)C6H4R(p), HRL (R = H, Me), ha s been synthesized with the objective of promoting the very rare mononuclea r oximato-O coordination. The synthesis involves hydrazone nitrosation. HRL reacts with M(ClO4)(2).6H(2)O, 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 charac terized by a a-fold axis of symmetry relating the two tridentate ligands, a ll chelate rings being five-membered. The M-N and M-O distances are relativ ely short(1.84-1.99 A), 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 Mn-II 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 similar to 1.30 Angstrom). [Mn(RL)(2)] displays hyperfine-split axial EPR spectra (g( parallel to) similar to 1.98, g(perpendicular to) similar to 2.06, A(parall el to) similar to 170 G, A(perpendicular to) similar to 65 G) in frozen sol ution as well as in polycrystalline [Fe(RL)(2)] lattices. The quasireversib le Fe(III)/Fe(II) couple of [Fe(RL)(2)] has E-1/2 similar to 0.7 V. The oxi dized complex [Fe(RL)(2)]PF6 has been electrosynthesized. It is low-spin (S = (1/2)) and displays axial EPR spectra (g(parallel to) similar to 1.98, g (perpendicular to) similar to 2.14) in frozen solution. The Mn(II) --> Mn(I II) oxidation (similar to 1.0 V) in [Mn(RL)(2)] is irreversible. Crystal da ta for [Mn(HL)(2)].CH2Cl2: crystal system monoclinic, space group 12/a, a = 14.142(7) Angstrom, b = 10.721(7) Angstrom, c = 16.933(10) Angstrom, beta = 93.01(4)degrees, Z = 4. Crystal data for [Fe(HL)(2)].CH2Cl2: crystal syst em monoclinic, space group 12/a, a =14.254 (6) Angstrom, b = 10.742(6) Angs trom, c 16.719(8) Angstrom, beta = 92.35(4)degrees, Z= 4.