Generation of σ,π-furyl and thienyl ligands at di-iron centers via facile phosphorus–carbon bond cleavage: Synthesis and molecular structures of [Fe2(CO)6(μ-η1,η2-C4H3E){μ-P(C4H3E)2}] (E = O, S)

Abstract

Treatment of [Fe3(CO)12] with tri(2-furyl)phosphine (PFu3) or tri(2-thienyl)phosphine (PTh3) in dichloromethane at 40°C leads to facile phosphorus–carbon bond scission affording di-iron furyl- and thienyl-bridged complexes [Fe2(CO)6(μ-η1,η2-C4H3E){μ-P(C4H3E)2}] (1E=O, Fu; 3E=S, Th) in good yields, together with smaller amounts of the phosphine-substituted [Fe2(CO)5(μ-η1,η2-C4H3E){μ-P(C4H3E)2}{P(C4H3E)3}] (2E=O, 4E=S). When the same reactions were carried out at room temperature, small amounts of the tri-iron clusters [Fe3(CO)11{P(C4H3E)3}] (5E=O, 6E=S) were isolated in which the coordinated phosphine(s) remain intact. Thermolysis of [Fe3(CO)11{P(C4H3E)3}] at 40°C in dichloromethane gave [Fe2(CO)6(μ-η1,η2-C4H3E){μ-P(C4H3E)2}], which also undergo phosphine substitution under similar conditions. However, both of these processes are too slow to account for the reaction product ratios and yields observed in the room temperature reactions. In contrast, addition of catalytic amounts of Na+[Ph2CO] to 5 resulted in the rapid formation of 1. We therefore propose that these reactions may occur via a radical-initiated mechanism proceeding through the initial formation of the 49-electron radical anions [Fe3(CO)11{P(C4H3E)3}]. The crystal structures of [Fe2(CO)6(μ-η1,η2-Fu)(μ-PFu2)] (1), [Fe2(CO)5(μ-η1,η2-Fu)(μ-PFu2)(PFu3)] (2), [Fe2(CO)6(μ-η1,η2-Th)(μ-PTh2)] (3) and [Fe3(CO)11(PFu3)] (5) have been determined. The di-iron complexes all show the expected cis arrangement of three-electron donor ligands, short iron–iron distances consistent with a 34-valence electron count, and, in 2, the phosphine is coordinated to the π-bound iron atom and lies trans to the metal–metal bond. Close inspection of the bonding parameters within the Fe2C2E core reveals that these alkenyl species are quite different to those without electron-withdrawing substituents. The tri-iron cluster 5 has two independent molecules in the asymmetric unit. Each contains two bridging carbonyls and the molecules differ in the relative positions of these carbonyls and the coordinated phosphine ligand, the latter lying in the equatorial plane in both molecules.