Proton reduction by phosphinidene-capped triiron clusters

Bis(phosphinidene)-capped triiron carbonyl clusters, including electron rich derivatives formed by substitution with chelating diphosphines, have been prepared and examined as proton reduction catalysts. Treatment of the known cluster [Fe3(CO)9(µ3-PPh)2] (1) various diphosphines in refluxing THF (for 5, toluene) afforded new clusters [Fe3(CO)7(µ3-PPh)2(κ2-dppb)] (2), [Fe3(CO)7(µ3-PPh)2(κ2-dppv)] (3), [Fe3(CO)7(µ3-PPh)2(κ2-dppe)] (4) [Fe3(CO)7(µ3-PPh)2(µ-κ2-dppf)] (5) moderate yields, together small amounts corresponding [Fe3(CO)8(µ3-PPh)2(κ1-Ph2PxPPh2)] (x = -C4H6-, -C2H2-, -C2H4-, -C3H6-, -C5H4FeC5H4-). The molecular structures complexes 3 5 established X-ray crystallography. Complexes 1–5 catalysts presence p-toluenesulfonic acid (p-TsOH) CH2Cl2. Cluster 1 exhibits two one-electron quasi-reversible waves at –1.39 V (ΔE 195 mV) –1.66 168 mV; potentials vs. Fc+/Fc). Upon addition p-TsOH unsubstituted shows a first catalytic wave –1.57 further processes –1.75 –2.29 V, each good current response. diphosphine-substituted are reduced more negative than parent 1. Clusters 2–4 exhibit an oxidation ca. +0.1 –1.6 V; for 4 conversion to redox active successor species is seen upon both reduction. show p-TsOH, exhibiting highest relative (icat/i0 ≈ 57) acid, albeit third process not observed 2 3. Addition dppf ligand diphosphinidene gave which exhibited single –1.95 three processes, all positive values compared 2–4. showed only weak activity p-TsOH. bonding was investigated DFT calculations, nature radical anion dianion discussed respect electrochemical data.