Stepwise synthesis and structural characterization of the molybdenum-iron-sulfur {[MoFe3S4Cl2(Cl4cat)]2(.mu.2-S)2}6- and {[MoFe3S4Cl2(Cl4cat)]2(.mu.2-S)(.mu.2-OH}5- doubly bridged double cubanes obtained by the coupling of [MoFe3S4] clusters