Chemistry of molybdenum. Part 6. Synthesis, spectroscopic and redox properties of some eight-co-ordinate sulphur chelates, [MoIV(S2CNR2)n(acda)4–n] and [MoV(S2CNR2)n(acda)4–n]Br (n= 2 or 3), containing MoS8 chromophores (acda = 2-aminocyclopent-1-ene-1-carbodithioate)

Abstract

Eight-co-ordinated mixed-ligand complexes [Mo^IV(S₂CNR₂)_n(acda)_4–n]1a–1d and [Mo^V(S₂CNR₂)_n(acda)_4–n]Br 2a–2d(n= 2 or 3) containing predominantly σ-donor S₂CNR₂(R = Et or Pr) and π-acceptor acda (2-aminocyclopent-1-ene-1-carbodithioate) ligands have been synthesised and characterised by UV/VIS, magnetic, ESR and electrochemical studies. For the lower-energy ligand-to-metal charge-transfer bands of compounds 1a–1d the observed spectral intensity showed an almost linear increment with the number of co-ordinated acda ligands. The molybdenum(V) compounds 2a–2d are ESR active, and their frozen solutions (140 K) give rise to axial spectra with grossly identical features (g_II≈ 1.983, A_II≈ 52.5 × 10^–4 cm^–1: g_⊥≈ 1.984, A_⊥≈ 21.5 × 10^–4 cm^–1) consistent with the electron being localised in a metal-centred orbital of nearly constant composition for the entire series of compounds. Cyclic voltammetry indicates the existence of three molybdenum oxidation states [MoⁿS₈] with n=III, IV and V. The E_½ values (vs. saturated calomel electrode) are –0.40 and –0.18 V for the Mo^III→ Mo^IV and Mo^IV→ Mo^V couples respectively and independent of whether the starting compound is a molybdenum(IV), 1a, 1c, or a molybdenum(V) species 2a–2d. Of the two types of ligand systems present in [Mo(S₂CNR₂)_n(acda)_4–n]^m(m= 0 or +1), acda has been found to have a profound influence in controlling the redox and charge-transfer properties.