Spectroscopic, magnetic and structural studies on the mixed-valence cuboidal clusters [Mo4E4(edta)2]3–(E = S or Se; H4edta = ethylenediaminetetraacetic acid) and [Mo4S4(H2O)12]5+

Abstract

The ⁷⁷Se NMR spectrum of the mixed-valence cluster [Mo₄Se₄(edta)₂]^3–(H₄edta = ethylenediamine-N,N,N′,N′-tetraacetic acid) was recorded and appears to be consistent with the presence in solution of a valence-trapped species Mo^III₃Mo^IV with a lifetime in excess of 10^–5 s. However this was not supported by ⁹⁵Mo and ⁷⁷Se NMR studies on other [Mo₄Se₄(edta)₂]^n–(n= 2 or 4) clusters. Overall it appears that multiple NMR signals arise from the presence in solution of species which differ slightly from the above formulae in the extent of co-ordination of H₂O and edta. The studies do not preclude the possibility of valence-trapped species with lifetimes shorter than 10^–6 s. A model in which intervalence electron exchange within the core clusters Mo₄E₄⁵⁺(E = S or Se) is slower than formation/decay (≈ 10⁹ s^–1) of ion-pair precursors is proposed to account for the biphasic kinetic behaviour previously observed for substitution of H₂O on [Mo₄S₄(H₂O)₁₂]⁵⁺. The crystal structure of a hydrated sodium salt of [Mo₄Se₄(edta)₂]^3– has been determined, and is compared to that previously obtained for [Mo₄S₄(edta)₂]^3–. Other techniques explored include magnetic moment measurements on a solid sample of [Mo₄S₄(H₂O)₁₂][pts]₅·14H₂O (pts = toluene-p-sulfonate) which show a Curie-like behaviour (χ=C/T) for an S=½ ground state, with a corresponding magnetic moment independent of temperature over the range 300–4.2 K. Metal–ligand stretching frequencies from Fourier-transform IR studies on the chloride salt of the same 5+ cluster have also been obtained.