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Stoichiometric formation of open-shell [PtAu (SCHPh) ] via spontaneous electron proportionation between [PtAu (SCHPh) ] and [PtAu (SCHPh) ] 24
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Megumi Suyama, Shinjiro Takano, Toshikazu Nakamura, and Tatsuya Tsukuda J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06254 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 12, 2019
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Journal of the American Chemical Society
Stoichiometric formation of open-shell [PtAu24(SC2H4Ph)18]− via spontaneous electron proportionation between [PtAu24(SC2H4Ph)18]2− and [PtAu24(SC2H4Ph)18]0 Megumi Suyama,1 Shinjiro Takano,1 Toshikazu Nakamura,2 and Tatsuya Tsukuda1,3* 1
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. 2 Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan. 3 Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520, Japan. Supporting Information Placeholder (1S)2(1P)4 (Scheme 1b).9,10 The closure of the subshell has been explained in terms of Jahn-Teller distortion of the (MAu12)6+ core: one of the 1P orbitals aligned along the compressed axis is destabilized and constitutes the lowest unoccupied molecular orbital (LUMO).11 Lee and Jiang demonstrated that (MAu12)6+ can be electrochemically reduced to (MAu12)4+ with improved symmetry.11 The optical,6,11 photophysical,12 and magnetic7 properties of (MAu12)6+ are understood in terms of their electronic structures. We have recently found that the structures of the Au superatoms are affected by the interaction with hydride (H−). Phosphine-protected, oblate-shaped (Au9)3+ and (PdAu8)2+ superatoms with six electrons were converted to those with eight electrons via the binding of H− to the unligated central atom.13−15 Recent theoretical studies have suggested that H− can also be doped into a fully passivated (MAu12)6+ (M = Pt, Pd) superatoms in [MAu24(SR)18]0.16,17 Motivated by this theoretical prediction, we previously studied the reaction between [PtAu24(SC2H4Ph)18]0 ([PtAu24]0) and 1 eq of NaBH4 and found that the optical spectrum of the product was different from that of [PtAu24]2− produced by electrochemical reduction of [PtAu24]0.15 Base on this result, we proposed the formation of a hydride adduct, [HPtAu24(SC2H4Ph)18]− ([HPtAu24]−) with eight electrons.15 In the present study, we re-examined the reaction of [PtAu24]0 and NaBH4 (1 eq) using single-crystal X-ray diffractometry (SCXRD) and 1 optical, 1H NMR and electron paramagnetic resonance (EPR) spectroscopies. The results indicated that the product was not [HPtAu24]−, but [PtAu24(SC2H4Ph)18]− ([PtAu24]−) with seven electrons. Selective production of open-shell [PtAu24]− was ascribed to the electron transfer from [PtAu24]2− nascently formed by the reduction with H− to unreduced [PtAu24]0 remaining in the solution. We proposed that the dimer complex played a key role in the intercluster electron proportionation reaction. A highly purified sample of [PtAu24]0 was synthesized according to the previous report11 with some modifications.18 The structure determined by SCXRD is shown in Scheme 1b. Doubly charged anion [PtAu24]2− was obtained by the addition of a THF/EtOH solution of a large excess of NaBH4 (~200 eq) to the THF solution of [PtAu24]0 at room temperature. Upon
ABSTRACT: [PtAu24(SC2H4Ph)18]0 ([PtAu24]0) was fully and selectively converted to [PtAu24]−, having an open electronic structure with seven valence electrons, upon the addition of an equiamount of NaBH4. Stoichiometric production of [PtAu24]− by the reaction between an equal amount of [PtAu24]0 and [PtAu24]2− revealed that the above reaction proceeds via the spontaneous electron transfer (ET) from [PtAu24]2− nascently reduced by H–-mediated reduction to [PtAu24]0 remaining in the solution. Theoretical calculation suggested that the driving force of this novel ET reaction was the larger adiabatic electron affinity of [PtAu24]0 compared to that of [PtAu24]−, partly associated with reduction-induced relief of the Jahn-Teller strain. We propose that ET proceeds via the dimer complex of [PtAu24]2− and [PtAu24]0 formed through the aurophilic interaction between Au(I) sites in the surface layer. Ligand-protected gold clusters can be viewed as chemically modified superatoms, in which valence electrons are accommodated in atomic-like discrete orbitals designated as 1S, 1P, 1D, 2S, and so on.1−3 Representative thiolate (RS)-protected Au cluster [Au25(SR)18]− has an icosahedral Au13 core with a formal charge of +5 and takes a closed electron configuration of (1S)2(1P)6 (Scheme 1a). The (Au13)5+ superatomic core can be electrochemically oxidized up to (Au13)7+ with an electron configuration of (1S)2(1P)4. Doping of a foreign atom M affects the geometric and electronic structures of the superatoms. [MAu24(SR)18]0 (M = Pt, Pd)4,5,6 has an icosahedral (MAu12)6+ core7,8 and takes a subshell closed electron configuration of
Scheme 1. Structures of (a) [Au25(SR)18]− and (b) [MAu24(SR)18]0 (M = Pt, Pd). R ligands are omitted for clarity.
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Journal of the American Chemical Society mixing, the color of the solution quickly changed from green to yellowish green. The UV-Vis spectrum of the product quantitatively agreed with that of [PtAu24]2− produced by electrochemical reduction of [PtAu24]0.11 The final product was crystallized for the first time into black platelets by cation exchange with tetraoctylammonium (TOA) cations (Figure S1a)18. The SCXRD analysis (Figure 1a, left) revealed the presence of two (TOA)+ counter cations in the unit cell, confirming the charge state of −2. The bond length distributions and continuous symmetry measure (CSM), a measure of structural deformation from the perfect icosahedron (CSM = 0), (Figures S2a,c, Table S1)18 revealed that the PtAu12 core of [PtAu24]2− was closer to a perfect icosahedron than that of [PtAu24]0. The absence of hydride-doped products during the reaction between [PtAu24]0 and NaBH4 suggests that H− acted as the electron supplier according to: [PtAu24]0 + H− → [PtAu24]2− + H+ (1) The UV-Vis spectrum of [PtAu24]2− is compared with that of [PtAu24]0 in Figure 1b. Two-electron reduction by NaBH4 has also been reported for Au(I)-thiolate complex in the synthesis of Au clusters.19 The peak at ~1.1 eV of [PtAu24]0 assigned to the transition from HOMO-1 (nonsuperatomic orbital) to LUMO (1P)11 disappeared in [PtAu24]2−, but a new peak appeared at ~1.8 eV, which was assigned to HOMO (1P)[PtAu24]2– exhibited LUMO (1D) transition.11 photoluminescence at ~1.38 eV with a quantum yield (QY) of 0.43 % in contrast to [PtAu24]0 (QY:
