A 3:1 Site-Differentiated [4Fe-4S] Cluster Immobilized on a Self-Assembled Monolayer

Abstract

A 3:1 site-differentiated [4Fe-4S] cluster is immobilized on a thiol-functionalized self-assembled monolayer (SAM) on Au(111) by thiol-thiolate exchange chemistry. Fe 2p signals observed by X-ray photoelectron spectroscopy support the presence of clusters at the SAM surface; further evidence comes from the detection of n-Bu 4 N + and cluster-derived species by secondary-ion mass spectrometry. The immobilizing interaction is sufficiently strong to allow the study of [4Fe-4S] clusters at solid-liquid interfaces. IntroductionThe widespread use nature makes of cubane-type clusters to mediate electron transfer in redox enzymes has led to detailed studies of synthetic analogues in solution. [1][2][3] On the other hand, studies of synthetic [4Fe-4S] clusters at solid surfaces are rare. Pickett and co-workers have immobilized [4Fe-4S] clusters by trapping them in an ionic polymer matrix on an electrode, demonstrating that the electrostatic interactions with the polymer have a large effect on the cluster redox potential. 4 In a later study, [4Fe-4S] clusters were immobilized by electropolymerization. Although the molecular nature of the clusters was lost, the technique elegantly created polyferredoxinlike materials. 5 Nonetheless, no strategy has yet been reported for the immobilization of [4Fe-4S] clusters onto solid surfaces in such a way that their molecular nature is preserved and each cluster is in a well-defined but readily variable environment. In an attempt to fill this void in [4Fe-4S] cluster chemistry, we report herein the first immobilization of synthetic [4Fe-4S] clusters on alkanethiol self-assembled monolayers (SAMs) 6,7 on Au(111) surfaces. Experimental MethodsGeneral. All glassware was treated with basic piranha solution (volume H 2 O:NH 3 (aq, conc.):H 2 O 2 (aq, conc.) ) 5:1: 1) at 70°C and rinsed with milli-Q grade H 2 O before use. Tools used for handling SAM samples were cleaned by sonication in acetone and toluene, respectively. 1-Decanethiol was purchased from Aldrich. All solvents were of p.a. quality. Solvents for [4Fe-4S] clusters were distilled over CaH 2 (DMF and CH 2 Cl 2 ) or Na/benzophenone (THF) and thoroughly degassed before use.Preparation of Self-Assembled Monolayers. 6 mM stock solutions of 1-decanethiol and 1,12-dodecanedithiol 8 in EtOH were prepared directly before use and mixed and diluted to yield a solution containing 0.9 mM 1-decanethiol and 0.1 mM 1,12-dodecanedithiol. The Au(111) on mica was prepared by evaporation of gold (99.99%) onto freshly cleaved mica (2.54 × 5.08 cm) in a home-built, fully automated thermal evaporator, until a thickness of 150 nm had been achieved. The gold substrates were cut into pieces of about 10 × 5 mm and flameannealed for 30 s with an H 2 flame directly before immersion into the thiol solution. After leaving the gold substrates in the solutions in the dark for 24 h, they were carefully washed for 30 s in ethanol, toluene, and 2-propanol, and spin-dried.Immobilization of [4Fe-4S] Clusters. SAM samples were placed in 0.1 mM solutions of (n-Bu 4 ...