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

Abstract

Complexes cis-[Mo^VIO₂(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 [Mo^VIO(aacda)₃]X 1a–1d, which contain an Mo^VIO core. Reactivity of the terminal oxo ligand of this core towards electron-donor reagents has been investigated. Non-oxo monomeric molybdenum(V) compounds [Mo^V(acda)(aacda)₃]BF₄2a, 2b and [Mo^V(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 [Mo^IV(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 E_½=–0.24 and –0.45 V vs. saturated calomel electrode (SCE), which correspond to Mo^V–Mo^IV and Mo^IV–Mo^III redox couples respectively. The molybdenum(IV) compounds 3a and 3b are seven-co-ordinate with a spin-triplet ground state (µ_eff, 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, E_1/2, for these processes are at –0.36 (Mo^IV–Mo^III) and –0.90 V (Mo^III–Mo^II)vs. SCE. The presence of the strongly π-acceptor phosphine ligand facilitates the electrochemical generation of relatively scarce molybdenum(II) species.