Spontaneous Intercluster Electron Transfer X²– + X⁰ → 2 X– (X = PtAu₂₄(SCnH2n+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 24 (SC n H 2 n +1 ) 18 ] 2– ( 8e (C n )) and [PtAu 24 (SC m H 2 m +1 ) 18 ] 0 ( 6e (C m )) ( n, m = 2–16), which have icosahedral Pt@Au 12 cores with eight and six valence electrons, respectively. The ET rate constant ( k ET ) from 8e (C n ) to 6e (C m ) in benzene exhibited a novel turnover behavior as a function of the total chain length n + m: the k ET decreased with n + m in the range of 4–12, whereas it monotonically increased with n + m in the range of 12–32. Electrospray ionization mass spectrometry of the mixture of 8e (C n ) and 6e (C m ) detected the dimer complex 8e (C n )· 6e (C m ), the relative population of which increased with n + m . The activation energy ( E a ), determined based on the Arrhenius plots for n = m, monotonically decreased with n (≥ 6). Based on these results, we proposed that the promotion of ET by longer alkanethiolates was ascribed to two effects on the key intermediate 8e (C n )· 6e (C m ): (1) elongation of the lifetime and (2) the contraction of the distance between 8e (C n ) and 6e (C m ) 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.