Chemistry of molybdenum. Part 8. Syntheses, spectroscopy and electrochemical properties of molybdenum complexes containing dithioacid ligands

Abstract

Complexes cis-[MoVIO2(aacda)2][aacda = 2-(alkylamino)cyclopent-1-ene-1-carbodithioate; alkyl = propyl (pacda) or butyl (bacda)] react with acids HX (X = BF4 or PF6) to form species [MoVIO(aacda)3]X 1a–1d, which contain an MoVIO core. Reactivity of the terminal oxo ligand of this core towards electron-donor reagents has been investigated. Non-oxo monomeric molybdenum(V) compounds [MoV(acda)(aacda)3]BF42a, 2b and [MoV(abt)(aacda)3]BF42c, 2d can be obtained when 2-aminocyclopent-1-ene-1-carbodithioic acid (Hacda) and o-aminobenzenethiol (Habt) are used for oxygen abstraction. With triphenylphosphine, however, the products are non-oxo molybdenum(IV) compounds [MoIV(PPh3)(aacda)3]BF43a, 3b. In frozen solution [dimethylformamide–MeCN (1 : 10 v/v), 140 K], compounds 2a–2d show ESR spectra with axial symmetry. The g and A tensor parameters (g∥≈ 1.977, g⊥≈ 1.975, A∥≈ 51.5 × 10–4 cm–1 and A⊥≈ 19.5 × 10–4 cm–1) for this series of molybdenum(V) compounds are very similar to each other indicating that the unpaired electron is located in a metal-centred orbital of essentially identical composition. Complexes 2a and 2b display two consecutive reversible one-electron waves in their cyclic voltammograms at E½=–0.24 and –0.45 V vs. saturated calomel electrode (SCE), which correspond to MoV–MoIV and MoIV–MoIII redox couples respectively. The molybdenum(IV) compounds 3a and 3b are seven-co-ordinate with a spin-triplet ground state (µeff, 2.48 and 2.53) and show two distinct redox waves due to the interconversion of the three [Mo(PPh3)(aacda)3]z(z=+1, 0 or –1) species. Half-wave potentials, E1/2, for these processes are at –0.36 (MoIV–MoIII) and –0.90 V (MoIII–MoII)vs. SCE. The presence of the strongly π-acceptor phosphine ligand facilitates the electrochemical generation of relatively scarce molybdenum(II) species.