Spontaneous Intercluster Electron Transfer X 2- + X 0 → 2 X - (X = PtAu 24 (SC n H 2 n +1 ) 18 ) in Solution: Promotion by Long Alkyl Chains.

In this work, we systematically investigated the ligand effects on spontaneous electron transfer (ET) between alkanethiolate-protected metal clusters in solution. The donor and acceptor clusters used were [PtAu(SCH)] ((C)) and [PtAu(SCH)] ((C)) (, = 2-16), which have icosahedral Pt@Au cores with eight and six valence electrons, respectively. The ET rate constant () from (C) to (C) in benzene exhibited a novel turnover behavior as a function of the total chain length + : the decreased with + in the range of 4-12, whereas it monotonically increased with + in the range of 12-32. Electrospray ionization mass spectrometry of the mixture of (C) and (C) detected the dimer complex (C)·(C), the relative population of which increased with + . The activation energy (), determined based on the Arrhenius plots for = , monotonically decreased with (≥ 6). Based on these results, we proposed that the promotion of ET by longer alkanethiolates was ascribed to two effects on the key intermediate (C)·(C): (1) elongation of the lifetime and (2) the contraction of the distance between (C) and (C) due to the stronger van der Waals interaction between the longer alkyl chains. Such alkyl-chain-promoted ET is specific to ultrasmall clusters in solution because a nonuniform ligand layer could be formed due to the large curvature of the cluster core.