Two Complementary Approaches to Silicon-Supported Soluble [FeFe]-Hydrogenase Mimics

Abstract

Two series of silicon-supported [(μ-SCH2)2NRFe2(CO)6] (R = p-OHC6H4, 3, and (CH2)2OH, 4a, and (CH2)5OH, 4b) hydrogenase mimics, as well as octamethylsilsesquioxane (POSS)-based nanostructures containing eight [(μ-SCH2)2NRFe2(CO)6] moieties, have been prepared either by reaction of the corresponding silyl and silyloxy chlorides and compounds 3 and 4 or by CuAAC between propargyl derivatives of silyl and silyloxy chlorides and azide 14. Cycloaddition between POSS-derived azide 23b and [(μ-SCH2)2NRFe2(CO)6] (R = alkyne) complexes 24 and 26 is efficient, leading to POSS-based nanostructures containing eight [(μ-SCH2)2NRFe2(CO)6] units. All of the complexes prepared through this work were soluble in organic solvents and hence fully characterizable by spectroscopic media. The electrochemistry of the linear siloxanes 6a and 10 is similar, with a reduction wave around −1.7 V, which is characteristic of these [(μ-SCH2)2NRFe2(CO)6] entities. Complexes 6a and 6b lacking 1,2,3-triazole moieties were electrochemically stable in the presence of AcOH, showing a strong electrocatalytic wave at −2.2 V, while complexes 16 and 18 having the 1,2,3-triazole ring decomposed but were electrocatalytically active in the wave at −2.2 V. POSS-[(μ-SCH2)2NRFe2(CO)6] derivatives 25 and 27 show a strong irreversible reduction event and are deposited in the electrode either when adding AcOH or over time (successive voltammograms). 29Si NMR shows that the integrity of the silicon cage is not affected by time or AcOH addition. These synthetic protocols and electrochemical studies will be applied in the design of silicon-supported [(μ-SCH2)2NRFe2(CO)6] mimics.