Regiospecific oximato coordination at the oxygen site: ligand design and low-spin MnII and FeII/III species

Ganguly, Sanjib ; Karmakar, Soma ; Pal, Chandan Kumar ; Chakravorty, Animesh (1999) Regiospecific oximato coordination at the oxygen site: ligand design and low-spin MnII and FeII/III species Inorganic Chemistry, 38 (26). pp. 5984-5987. ISSN 0020-1669

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Official URL: http://pubs.acs.org/doi/abs/10.1021/ic9907011

Related URL: http://dx.doi.org/10.1021/ic9907011

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, MnII > FeII, is in accord with the larger radius of the low-spin MnII atom. The M-O length order is, however, MnII < FeII. 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 E1/2 ~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.

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Deposited On:26 Oct 2010 05:25
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