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Determination of the Ni–Ni Bonding Strength in Metal-String Complexes Using Head-to-Head Nanorods and Electrochemical Surface-Enhanced Raman Spectroscopy

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journal contribution
posted on 01.03.2018, 00:00 authored by Bo-Han Wu, Li-Yen Hung, Jheng-Yang Chung, Shie-Ming Peng, I-Chia Chen
We report the bonding strength of nickel ions in trinickel extended metal atom chains (EMACs) and dinickel complexes using Raman, surface-enhanced Raman scattering (SERS), and electrochemical SERS (ECSERS). By using the redox ability of gold and silver nanoparticles during plasmonic excitation, the bonding strength and the valence state of metal ions can be determined. For dinickel complexes, we assign the Raman band at 322 cm–1 to Ni2+–Ni3+ stretch in [Ni2(TPG)4]­BF4 (TPG = N,N′,N″-triphenylguanidinate, [Ni2]5+) and 327 cm–1 for Ni2+–Ni1+ stretch of [Ni2]3+ moieties in Ni5(camnpda)4. For trinickel EMACs, no band is assigned to Ni3 symmetric stretch νNi3 sym in the neutral form Ni3(dpa)4X2 (dpa = dipyridyl amido and X = NCS, Cl). In the reduced form, the ECSERS curves show the band at 242 cm–1, which also appeared at gold nanoparticle SERS measurement, assigned to νNi3 sym for [Ni3]5+ core. The trinickel complexes were reduced by gold nanosphere, and this νNi3 sym band is further enhanced with SERS measurements when gold nanorods were used and the trinickel EMACs served as bridging compounds on both ends. On increasing the applied voltage in ECSERS to +1.3 V, complexes were oxidized and one additional band at 351 cm–1 appeared. This new band is assigned to νNi3 sym of [Ni3]7+ in [Ni3(dpa)4X2]+. Great vibrational frequency indicates that one electron from the metal σ* orbital instead of ligand was removed, leading to a three metal center bond. Distinct from the vibrational band wavenumber obtained in dinickel complexes, we confirm that [Ni3]5+,7+ has delocalized electronic structures.