Chemistry of molybdenum. Part 7. Reactivity of terminal oxo-ligands in cis-[MoO2(acda)2](Hacda = 2-aminocyclopent-1-ene-1-carbodithioic acid) toward proton- and electron-donor reagents. Synthesis, redox chemistry and spectroscopic characterisation of neutral seven-co-ordinate catecholato and aryldiazenido compounds of molybdenum

Abstract

The reactivity of co-ordinated oxo-ligands in cis-[MoO₂(acda)₂](Hacda = 2-aminocyclopent-1-ene-1-carbodithioic acid) toward proton- and electron-donor reagents has been investigated. Molybdenum(VI) compounds [MoO(L)(acda)₂](L = catecholate, 2a; 4-tert-butylcatecholate, 2b; 3,5-di-tert-butylcatecholate, 2c; naphthalene-2,3-diolate, 2d; or tetrachlorocatecholate, 2e are obtained when catechols are used for oxo abstraction. Substituted phenylhydrazines on the other hand form non-oxodiazenido complexes [Mo(N₂C₆H₄X-p)(acda)₃](X = H, 3a; Me, 3b; or Cl, 3c). The compounds have been characterised by electronic, IR and ESR spectroscopy and electrochemistry. In the visible region the electronic spectra are dominated by a strong ligand-to-metal charge-transfer (l.m.c.t.) band for both series of compounds. Their electrochemistry has been studied in dimethylformamide at a platinum electrode. For the catecholato compounds 2a–2e a metal-based reduction (Mo^VI–Mo^V, E_pc) and a ligand-based oxidation (E_pa) each involving a single electron are observed. Substituents in the catecholato ring have a significant influence on the energy of the l.m.c.t. transitions (E_op) as well as on E_pc and E_pa. A linear relationship exists between E_op and ΔE(redox) where ΔE(redox)=E_pc–E_pa. The oxidised species derived from the 0/1 + redox couple of the diazenido series 3a–3c are ESR active with hyperfine spectra due to ^95,97Mo and ¹⁴N couplings. The electron-transfer orbital in this case is believed to be metal based with sufficient mixing from the diazenido nitrogen orbital.