Mononuclear manganese(III) complexes of a heterodonor (N2OS) ligand containing thiolate-type sulfur: synthesis, structure, redox and spectroscopic properties

Abstract

Two series of five-co-ordinate high-spin (S= 2) mononuclear manganese(III) complexes have been prepared of general formulae [MnL(X)](X = Cl 1a, Br 1b, NCS 1c or N₃1d) and [MnL(B)]CIO₄[B = pyridine (py)2a, 3-methylpyridine (3Me-py)2b or 4-methylpyridine (4Me-py)2c] using methyl 2-[2-(salicylideneamino)ethylamino]cyclopent-1-ene-1-carbodithioate (H₂L) as tetradentate ligand with a (N₂OS)^2– donor set. The complexes have been characterised by a combination of IR and UV/VIS spectroscopy, magnetic measurements and electrochemical studies. Their UV/VIS spectra show two ligand-field transitions at ca. 685 (⁵B₁→⁵E) and 570 nm (⁵B₁→⁵B₂) along with a strong charge-transfer band in the near-UV region (ca. 440 nm), assigned as a phenolate O→Mn(d_π·) ligand-to-metal charge-transfer transition. Cyclic voltammograms of the complexes (except 1d) exhibit a one-electron quasi-reversible Mn^III–Mn^II reduction with E_½ close, to –0.12 V (vs. saturated calomel electrode). For 1a–1c irreversible Mn^III–Mn^IV oxidation is also observed. The compounds have an axially elongated square-pyramidal structure as dictated by the Jahn–Teller effect with ligands X or B occupying the axial position. Their electronic spectra and electrochemical properties show invariance with group X (or B) in the axial position. The crystal structure of complex 1b was solved by Patterson and Fourier methods followed by a least-squares refinement to a conventional R value of 0.037. The basal plane of the five-co-ordinated manganese centre is defined by N (two), S and O atoms, derived from the ligand L. The co-ordinated S atom behaves like a thiolate anion as reflected by the C–S bond distance 1.717(9)Å.